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A STUDY OF OPTIMIZING PROCESSES FOR METALLIZED TEXTILE DESIGN APPLICATION
GUO RONGHUI
Ph.D
The Hong Kong Polytechnic University
2010
THE HONG KONG POLYTECHNIC UNIVERSITY
INSTITUTE OF TEXTILES AND CLOTHING
A STUDY OF OPTIMIZING PROCESSES FOR METALLIZED TEXTILE DESIGN APPLICATION
GUO RONGHUI
A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy
January 2010
CERTIFICATE OF ORIGINALITY
I hereby declare that this thesis is my own work and that, to the best of my knowledge and belief, it reproduces no material previously published or written, nor material that has been accepted for the award of any other degree or diploma, except where due acknowledgement had been made in the text.
______(Signed)
Guo Ronghui
______(Name of student)
ABSTRACT
Metallized textiles generally have many special properties including heat insulation, anti-static, electromagnetic interference (EMI) shielding, UV radiation screen, anti-bacterial, radar reflectivity, as well as intelligent usage. EMI shielding effectiveness (SE) is one of the important properties of metallized textiles and is closely related to their surface resistance. Based on a comprehensive literature review of electroless plating technology, the thesis presents a systematic investigation to optimize the process of electroless plating with nickel and copper on polyester fabric
aiming to achieve functional textiles. The potential prospects of electroless plating in
textile applications put forth the objective of this study. The purpose of this research is
to find an optimum electroless plating process in order to obtain relatively low surface
resistance, and improve functional properties and appearance of nickel-plated and
copper-plated polyester fabrics. The work consists of five parts, i.e. (i) to review the performance of metallized textiles and developments of electroless plating on textiles;
(ii) to investigate thermodynamics and kinetics of electroless plating on polyester fabric; (iii) to optimize electroless nickel and copper plating processes and conditions;
(iv) to investigate the micro-structures and properties of metallized textiles; and (v) to employ optimum electroless plating processes for textile surface modification.
There are many parameters affecting electroless plating process, including the
I chemical concentration, pH value and bath temperature of the solutions.
Thermodynamic investigation reveals that electroless nickel and copper plating on
polyester fabric is feasibile. The kinetic models of electroless nickel and copper
plating on polyester fabric have been established. The optimum processes of
electroless nickel and copper plating on polyester fabric have been developed to
achieve lower surface resistance with the aid of full factorial design (FFD) and
response surface design (RSM) methods. The results indicate that the NiSO4 concentration and temperature of the bath in the plating process are most important factors influencing surface resistance of electroless nickel-plated polyester fabric.
However, NiSO4 concentration and pH of the plating bath are most significant factors
affecting electroless copper plating.
The nickel/copper multi-layer plated polyester fabric (nickel deposited on the copper-
plated polyester fabric) is developed to enhance the anti-corrosive properties of
copper-plated polyester fabric. The micro-structures and properties of nickel and
copper, and nickel/copper multi-layer plated polyester fabrics have been studied. In
the case of electroless nickel plating, the nickel deposit layer becomes more uniform
and continuous when prepared at higher NiSO4 concentration (the experimental
NiSO4 concentration from 6 g/l to 15 g/l) and higher bath temperature (the
experimental bath temperature from 50℃ to 80℃). As for the electroless copper
plating, the surface morphology of the copper deposits indicates that the average
diameter of the particles is increased with the rise of NiSO4 concentration and pH.
II The surface morphology of nickel/copper multi-layer deposits reveals the presence of
ultra-fine nodules and the deposits are compact and uniform in size.
There is a decrease in surface resistance and an increase in EMI SE with respect to the
rise of Ni2+ concentration and bath temperature for electroless nickel plating; and
surface resistance decreases and EMI SE increases with the rise of Ni2+ concentration
and pH of the plating solution for electroless copper plating on polyester fabric. With
the same deposit weight, the EMI SE of nickel/copper-plated fabric is greatly higher
than that of the nickel-plated fabric, but slightly lower than that of the copper-plated
fabric. However, the anti-corrosive property of nickel/copper-plated fabrics is
significantly superior to the copper-plated fabrics, but slightly inferior to the
nickel-plated fabric.
The overall results suggest that electroless nickel, copper and nickel/copper multi-layer plated polyester fabrics have potential applications as EMI shielding materials. In addition, the electroless nickel/copper-plated polyester fabric increases the corrosion resistance of copper-plated polyester fabric. Design application effects have been explored by the controlling plating conditions. The electroless plating parameters play an important role in the metallized fabric appearance based on their influences on color and texture. This study has developed optimization productions of electroless plating of nickel, copper and nickel/copper multi-layer on polyester fabric in terms of the functional and aesthetic effects.
III PUBLICATIONS ARISING FROM THE THESIS
Referred Journal Publications
1. R. H. Guo, S. Q. Jiang, C. W. M. Yuen, M. C. F. Ng, G. H. Zheng，Influence of Nickel Ions for Electroless Ni–P Plating on Polyester Fabric, Journal of Coating Technology and Research (SCI), 2009, In Press.
2. R. H. Guo, S. Q. Jiang, C.W.M. Yuen, M.C.F. Ng, Microstructure and Electromagnetic Interference Shielding Effectiveness of Electroless Ni–P Plated Polyester Fabric, Journal of Materials Science: Materials in Electronics (SCI), 2009, 20: 735–740.
3. R. H. Guo, S. Q. Jiang, C.W.M. Yuen, M.C.F. Ng, Effect of Cu Content on the Properties of Ni-Cu-P Plated Polyester Fabric, Journal of Applied Electrochemistry (SCI), 2009, 39: 907–912.
4. R. H. Guo, S. Q. Jiang, C.W.M. Yuen, M.C.F. Ng, Surface Characterisation of Electroless Copper-plated Polyester Fibres, Surface Engineering (SCI), 2009, 25 (2): 101-105.
5. R. H. Guo, S. Q. Jiang, C.W.M. Yuen, M.C.F. Ng, An Alternative Process for Electroless Copper Plating on Polyester Fabric, Journal of Materials Science: Materials in Electronics (SCI), 2009, 20: 33–38.
6. R. H. Guo, S. Q. Jiang, C.W.M. Yuen, M.C.F. Ng, Temperature Dependence of Electroless Ni–P Deposition on Polyester Fabric, Surface Review and Letters (SCI), 2008, 15 (5): 587–593.
IV 7. R. H. Guo, S. Q. Jiang, C.W.M. Yuen, M.C.F. Ng, Surface Characterisation of Electroless Silver-plated Polyester Fibres, Research Journal of Textile and Apparel, 2007, 11 (3): 48-53.
8. S. Q. Jiang, C. W. M. Yuen, L. Zhang, R. H Guo, and P. S. R. Choi, Characteristics of Silver-plated Silk Fabric with Plasma Pre-treatment, Fibers and Polymers (SCI), 2009, 10 (6): 791-796.
9. S. Q. Jiang, R. H. Guo, Effect of Polyester Fabric through Electroless Ni-P Plating, Fibers and Polymers (SCI), 2008, 9(6): 755-760.
10. S.X. Jiang, W.F. Qin, R.H. Guo, L. Zhang, Surface Functionalization of
Nanostructured Silver-coated Polyester Fabric by Magnetron Sputtering, Surface
and Coatings Technology, 2010, 204(21-22): 3662-3667.
11. S.Q. Jiang, X.M. Tao, C.W.M. Yuen, C.W. Kan and R.H. Guo, Characterization of Electroless Ni-P Plated Polyester Fabric, Research Journal of Textile and Apparel, 2008, 12 (4): 52-60.
12. S. X. Jiang, R. H. Guo, G. H. Zheng, Crystal Structure and Properties of Electroless Silver Plating on Polyester Fabric, Electroplating & Finishing, 2009, 28(7): 22-24 (Chinese).
Book Chapters
1. S. Q. Jiang, R. H. Guo, Chapter 5: Modification of textile surfaces by electroless deposition, Surface modification of textiles, edited by Q Wei, Woodhead Publishing Limited, 2009, pp. 236-286.
V Referred Conference Papers
1. R. H. Guo, S.Q. Jiang, C.W.M. Yuen, M.C.F. Ng, G.H. Zheng, Influence of Nickel Content on Properties of Electroless Ni-P Plated Polyester Fabric, European Materials Research Society 2009 Spring Meeting, Strasbourg, France, June, 2009.
2. R. H. Guo, S.Q. Jiang, C.W.M. Yuen, M.C.F. Ng, Effect of Ni2+ Concentration in Electroless Copper Plating on Polyester Fabric, 2009 National Conference for Doctoral Candidates, Shanghai, May, 2009.
3. R. H. Guo, S.Q. Jiang, C.W.M. Yuen, M.C.F. Ng, Effect of Ni2+ Concentration in Electroless Copper Plating on Polyester Fabric, International Conference on Fibrous Materials 2009, The Fiber Society 2009 Spring Conference, Shanghai, May, 2009.
4. R. H. Guo, S.Q. Jiang, C.W.M. Yuen, M.C.F. Ng, Microstructure and Properties of Electroless Ni-Cu-P Plating on Polyester Fabric, 86th Textile Institute World Conference, Hong Kong, November, 2008.
5. R. H. Guo, S.Q. Jiang, C.W.M. Yuen, M.C.F. Ng, Study of Microstructure and Properties of Electroless Ni-Cu-P Alloy Plated Polyester Fabric，The 9th China Electroless Plating Conference, Zhuhai, September, 2008.
6. S. Q. Jiang, R. H. Guo, G. H. Zheng, Characterization of Silver-coated Polyester Fabric between Magnetron Sputtering and Electroless Plating, The 10th International Symposium on Sputtering and Plasma Process (ISSP 2009), Japan, July, 2009.
VI 7. S. Q. Jiang, W. F. Qin, C. K. Kan, R. H. Guo, L. Zhang, Properties of Sputtering-Deposited Ag Film on Silk, The 10th International Symposium on Sputtering and Plasma Process (ISSP 2009), Japan, July, 2009.
8. S. Q. Jiang, G. X. Yuan, J. Yip, R. H. Guo, L. Zhang, Design Application of Silver-coated Lingerie through Sputtering Technology, International Conference on Fibrous Materials 2009, The Fiber Society 2009 Spring Conference, Shanghai, May 2009.
Exhibitions
1. The art of Fashion-Experimental Textiles exhibited in Design Museum, University
of California, Davis, 13 May-18 July, 2010.
2. Metallic sound exhibited in The Bonington Gallery, Nottinghom Trent University,
Nottinghom, Britain, 18 January-17 February, 2010.
3. Fabric Applications- Textiles & Fabrics in the Age of Design exhibited in The Fashion Gallery, The Hong Kong Polytechnic University, Hong Kong, September -October, 2009.
4. Metallized fabric design- From Lausanne to Beijing: 5th international Fiber Art Bienneale exhibited in Tsinghua University, Beijing, 25 November-25 December, 2008.
5. JiangMTex-Fashion Art of Kinor Jiang exhibited in The Fashion Gallery,, The Hong Kong Polytechnic University, Hong Kong, 22 September -17 October, 2008.
VII 6. Yan Sui, Metallized fabric design in Design Shibori: International Students Juried Exhibition in “Ecole Nationale Superieure Des Arts Decoratifs”, Paris, 27 October-8 November, 2008.
Awards
1. R. H. Guo, S.Q. Jiang, C.W.M. Yuen, M.C.F. Ng, “Effect of Ni2+ Concentration in Electroless Copper Plating on Polyester Fabric”, was awarded Excellent Paper by 2009 National Conference for Doctoral Candidates, Ministry of Education of the People’s Republic of China, Shanghai, May, 2009.
2. R. H. Guo, S.Q. Jiang, C.W.M. Yuen, M.C.F. Ng, “Effect of Ni2+ Concentration in Electroless Copper Plating on Polyester Fabric”, was awarded Best Academic Report by 2009 National Conference for Doctoral Candidates, Ministry of Education of the People’s Republic of China, May, 2009.
3. S.Q. Jiang, X.M. Tao, C.W.M. Yuen, C.W. Kan and R.H. Guo, “Characterization of Electroless Ni-P Plated Polyester Fabric”, was awarded Excellent Paper by Research Journal of Textile and Apparel, Hong Kong Institute of Textile and
Apparel, February, 2009.
VIII
ACKNOWLEDGEMENTS
First and foremost, I would like to express my sincerest thanks to my chief supervisor,
Dr Jiang Shou-xiang Kinor, for giving me the opportunity to work on this interesting
project. I am grateful for his patient guidance, invaluable advice and sustained interest
throughout the whole research study. His enthusiasm in scientific pursuits and
indefatigable dedication to research study inspired me. His comments and encouragement gave me much confidence in solving the problems.
I am greatly indebted to my Co-supervisor, Professor Yuen Chun-wah Marcus, for his guidance and suggestions in my research. Also, I greatly thank him for guiding me to study scientific research methodology and systematic thoughts. His comments on the writing of this thesis and proofreading of the whole thesis are highly appreciated. I would also like to express my sincere thanks to my Co-supervisor, Dr Ng Manching,
Frankie, for his support and kind guidance throughout the research work. His kindness and warm encouragement brought me great confidence.
Research facilities and technician support from our department are gratefully acknowledged. I would like to express my gratitude to numerous members and laboratory technicians in our department for their assistance in the experimental
IX works.
Special thanks to my husband and son for their encouragement and support
throughout the completion of the study. I would also like to thank my parents and parents-in-law for their love, and encouragement. Without their love, understanding, and support, no achievement in my career can become reality.
Finally, I also thank the financial support provided by The Hong Kong Polytechnic
University in the form of a postgraduate scholarship.
X LIST OF CONTENTS
ABSTRACT...... I
PUBLICATIONS ARISING FROM THE THESIS...... IV
ACKNOWLEDGEMENTS ...... IX
LIST OF CONTENTS...... XI
LIST OF FIGURES ...... XV
LIST OF TABLES ...... XXI
LIST OF ABBREVIATIONS ...... XXIII
Chapter 1 Introduction...... 1
1.1 Background...... 1
1.2 Objectives ...... 4
1.3 Research Methodology ...... 5
1.4 Project Significance and Values ...... 7
1.5 Structure of the Thesis ...... 9
Chapter 2 Literature Review ...... 12
2.1 Introduction...... 12
2.2 Metallized Textiles...... 12
2.2.1 Radiant Heat Reflection...... 13
2.2.2 Conductivity/Anti-static Property...... 15
2.2.3 Electromagnetic Interference (EMI) Shielding...... 16
2.2.4 Ultraviolet (UV) /Infrared (IR) Protection...... 18
2.2.5 Anti-bacterial Property...... 19
2.2.6 Intelligence...... 21
2.2.7 Radar Reflectivity ...... 23
2.2.8 Decorative Application ...... 24
2.3 Metallizing Techniques...... 29
2.4 Electroless Plating and Development in Textile ...... 34
XI 2.4.1 Background of Electroless Plating...... 34
2.4.2 Applications of Electroless Plating...... 35
2.4.3 Development of Electroless Plating in Textiles...... 37
2.5 Research Gap ...... 41
2.6 Summary...... 45
Chapter 3 Justification of Research Methodology...... 46
3.1 Introduction...... 46
3.2 Mechanism of Electroless Plating...... 46
3.3 Research Field Justification ...... 51
3.3.1 Substrate Fabrics...... 51
3.3.2 Electroless Nickel and Copper Plating ...... 53 3.3.2.1 Electroless Nickel Plating (ENP)...... 54 3.3.2.2 Electroless Copper Plating (ECP)...... 58 3.3.2.3 Electroless Plating Bath Parameters ...... 61
3.4 Fundamental Methodology ...... 69
3.5 Summary...... 74 Chapter 4 Thermodynamics and Kinetics of Electroless Nickel and Copper
Plating ...... 75
4.1 Introduction...... 75
4.2 Experimental...... 75
4.2.1 Materials and Reagents...... 75
4.2.2 Experimental Procedure...... 76
4.3 Analysis...... 79
4.3.1 Thermodynamics...... 79
4.3.2 Effects of Plating Parameters on Plating Rate ...... 86 4.3.2.1 Electroless Nickel Plating...... 86 4.3.2.2 Electroless Copper Plating...... 91
4.3.3 Establishment of Kinetic Models...... 97 4.3.3.1 Electroless Nickel Plating...... 100 4.3.3.2 Electroless Copper Plating...... 105
XII 4.3.4 Verification of Models...... 108
4.4 Summary...... 111
Chapter 5 Optimization of Electroless Nickel and Copper Plating ...... 113
5.1 Introduction...... 113
5.2 Experimental Design...... 113
5.2.1 Full Factorial Design (FFD)...... 114
5.2.2 Response Surface Method (RSM) ...... 118
5.3 Results and Discussion ...... 121
5.3.1 Full Factorial Design...... 121 5.3.1.1 Pareto Plot of Effect...... 121 5.3.1.2 Main Effect Plot...... 123 5.3.1.3 Interaction Effect Plot...... 126
5.3.2 Response Surface Design...... 128
5.3.3 Experimental Validation of the Optimized Condition...... 137
5.4 Summary...... 138 Chapter 6 Characterization of Electroless Nickel-plated and Copper-plated
Polyester Fabrics...... 140
6.1 Introduction...... 140
6.2 Characterization ...... 140
6.3 Results and Discussion ...... 142
6.3.1 Electroless Nickel Plating...... 142 6.3.1.1 Deposit Composition ...... 142 6.3.1.2 Surface Morphology ...... 145 6.3.1.3 Crystal Structure ...... 149 6.3.1.4 Surface Resistance ...... 152 6.3.1.5 EMI Shielding Effectiveness ...... 155
6.3.2 Electroless Copper Plating...... 159 6.3.2.1 Deposit Composition ...... 159 6.3.2.2 Surface Morphology ...... 163 6.3.2.3 Crystal Structure ...... 168
XIII 6.3.2.4 Surface Resistance ...... 172 6.3.2.5 EMI Shielding Effectiveness ...... 174
6.4 Summary...... 176 Chapter 7 Characterization of Nickel/Copper Multi-layer Plated Polyester
Fabric ...... 179
7.1 Introduction...... 179
7.2 Experimental...... 179
7.2.1 Procedure ...... 179
7.2.2 Characterization ...... 180
7.3 Results and Discussion ...... 181
7.3.1 Deposit Composition ...... 181
7.3.2 Surface Morphology ...... 183
7.3.3 Crystal Structure ...... 185
7.3.4 Surface Resistance ...... 187
7.3.5 EMI Shielding Effectiveness ...... 188
7.3.5 Anti-corrosive Property ...... 189
7.4 Summary...... 194
Chapter 8 Textile Design Application of Electroless Plating...... 197
8.1 Introduction...... 197
8.2 Performance of Plated Fabrics...... 198
8.2.1 Electroless Nickel-plated Fabrics ...... 198
8.2.2 Electroless Copper-plated Fabrics ...... 201
8.2.3 Electroless Nickel /Copper Plated Fabrics...... 208
8.2.4 Analysis...... 211
8.3 Design Application ...... 213
8.4 Summary...... 219
Chapter 9 Conclusions and Suggestions for Future Research...... 221
9.1 Conclusions...... 221
9.2 Suggestions for Future Research ...... 227 References...... 230
XIV LIST OF FIGURES
Figure 1-1 Scheme of research activities...... 5 Figure 2-1 Heat protection suit for workers in the steel industry [22]...... 15 Figure 2-2 The Oricalco shape memory alloy shirt [81]...... 22 Figure 2-3 Silver-plated polyester fabric [93] ...... 25 Figure 2-4 Stainless series (Nuno corporation, Reiko Sudo) [82]...... 26 Figure 2-5 Spring 2009 Fashion Trend Metallic Color [94]...... 27 Figure 2-6 Metallized clothes [94]...... 28 Figure 2-7 Warehouse metallic bag [94]...... 28 Figure 2-8 Schematics of the vacuum deposition [89] ...... 31 Figure 2-9 Schematics of sputtering [49]...... 32 Figure 2-10 Schematics of electroplating process [89]...... 33 Figure 2-11 (a) Silver-plated, (b) copper-plated and (c) nickel-plated polyester fabrics with design method [152] ...... 41 Figure 3-1 Structure of the tin–palladium agglomerate and mechanism of dissolution [168] ...... 48 Figure 3-2 Schematic illustration of the basic components of electroless deposition cell [167] ...... 49 Figure 3-3 Chemical structure of polyethylene terephthalate (PET)...... 52 Figure 4-1 The potential pH diagram for (a) the nickel-phosphorus-water system, and (b) the copper-phosphorus-water system [170, 201]...... 82 Figure 4-2 Effect of nickel sulfate concentration on the plating rate of nickel plating ...... 87 Figure 4-3 Effect of sodium hypophosphite on the plating rate of nickel plating 88 Figure 4-4 Effect of sodium citrate on the plating rate of nickel plating...... 89 Figure 4-5 Effect of pH of the plating solution on the plating rate of nickel plating ...... 90 Figure 4-6 Effect of bath temperature on the plating rate of nickel plating ...... 91 Figure 4-7 Effect of copper sulfate concentration on the plating rate of copper
XV plating ...... 92 Figure 4-8 Effect of nickel sulfate concentration on the plating rate of copper plating ...... 93 Figure 4-9 Effect of sodium hypophosphite concentration on the plating rate of copper plating...... 94 Figure 4-10 Effect of sodium citrate concentration on the plating rate of copper plating ...... 95 Figure 4-11 Effect of pH of the plating solution concentration on the plating rate of copper plating ...... 96 Figure 4-12 Effect of bath temperature on the plating rate of copper plating ...... 97 Figure 5-1 Pareto chart of the standardized effects for the electroless (a) nickel
plating and (b) copper plating with (A) concentration of NiSO4·6H2O, (B)
concentration of NaH2PO2·H2O, (C) concentration of Na3C6H5O7 ·2H2O, (D) pH and (E) bath temperature...... 123 Figure 5-2 Main effect plot for the effect of processing parameters on the electroless (a) nickel plating and (b) copper plating...... 125 Figure 5-3 Interaction plot for the effect of processing parameters on the electroless (a) nickel plating and (b) copper plating...... 127 Figure 5-4 Response surface plots (a) and contour plots (b) of the electroless
nickel plating as a function of NiSO4 concentration and bath temperature 135 Figure 5-5 Response surface plots (a) and contour plots (b) of the electroless
copper plating as a function of NiSO4 concentration and pH...... 137 Figure 6-1 The equipment of EMI shielding effectiveness measurement ...... 142 Figure 6-2 Deposit composition of the nickel-plated polyester fabrics with
different NiSO4 concentrations...... 144 Figure 6-3 Deposit composition of the nickel-plated polyester fabrics with different bath temperatures ...... 145 Figure 6-4 SEM micrographs of (a) untreated polyester, and nickel-plated
polyester fibers treated with different NiSO4 concentration of (b) 6.0g/l, (c) 9.0g/l, (d) 12.0g/l and (e) 15.0g/l...... 147
XVI Figure 6-5 SEM micrographs of nickel-plated fibers treated at (a) 50℃, (b) 60℃, (c) 70℃ and (d) 80℃...... 148 Figure 6-6 SEM micrographs of nickel-plated fibers obtained under the optimal condition ...... 149 Figure 6-7 XRD patterns of the electroless nickel-plated polyester fabrics with
different NiSO4 concentrations: (a) 6g/l, (b) 9g/l, (c) 12g/l and (d) 15g/l ..150 Figure 6-8 XRD patterns of the electroless nickel-plated polyester fabrics at (a) 50℃, (b) 60℃, (c) 70℃ and (d) 80℃...... 151 Figure 6-9 XRD patterns of the electroless nickel-plated polyester fabric obtained under the optimal condition ...... 151
Figure 6-10 Surface resistance of the nickel-plated fabrics with different NiSO4 concentrations ...... 154 Figure 6-11 Surface resistance of the nickel-plated fabrics with different bath temperature ...... 154 Figure 6-12 EMI shielding effectiveness of the electroless nickel-plated polyester
fabrics with different NiSO4 concentrations: (a) 6g/l, (b) 9g/l, (c) 12g/l and (d) 15g/l...... 157 Figure 6-13 EMI shielding effectiveness of the nickel-plated polyester fabrics at (a) 50 ℃, (b) 60 ℃, (c) 70 ℃ and (d) 80℃...... 158 Figure 6-14 EMI shielding effectiveness of the nickel-plated polyester fabric obtained under the optimal condition ...... 159 Figure 6-15 Composition of copper deposits on polyester fabric with different
NiSO4 concentration ...... 161 Figure 6-16 Composition of copper deposits on polyester fabric at different pH ...... 163 Figure 6-17 SEM micrographs of the copper-plated polyester fabrics with
different NiSO4 concentrations: (a) 0.8 g/l, (b) 1.0 g/l, (c) 1.5 g/l and (d) 2.0 g/l ...... 165 Figure 6-18 SEM micrographs of the copper-plated polyester fibers treated in the bath at different pH: (a) 8.5, (b) 9.0, (c) 9.5 and (d) 10...... 166
XVII Figure 6-19 SEM micrograph of the copper-plated polyester fibers obtained under the optimal condition ...... 167 Figure 6-20 Schematic diagram of copper crystallization process of electroless deposition showing (a) nucleation and diffusion of nuclei, (b) copper granule formation and (c) the larger copper grain and granule growth ...... 168
Figure 6-21 XRD patterns of the copper-plated fabrics with different NiSO4 concentrations: (a) 0.8g/l, (b) 1.0g/l, (c) 1.5g/l and (d) 2.0g/l ...... 170 Figure 6-22 XRD patterns of the copper-plated fabrics at different pH: (a) 8.5, (b) 9.0, (c) 9.5 and (d) 10...... 171 Figure 6-23 XRD patterns of the copper-plated fabric under the optimal condition ...... 172 Figure 6-24 Surface resistance of the copper-plated polyester fabrics with
different NiSO4 concentrations ...... 173 Figure 6-25 Surface resistance of the copper-plated polyester fabrics at different pH...... 174 Figure 6-26 EMI shielding effectiveness of the copper-plated fabrics with
different NiSO4 concentrations: (a) 0.8 g/l, (b) 1.0 g/l, (c) 1.5g/l and (d) 2.0g/l ...... 175 Figure 6-27 EMI shielding effectiveness of the copper-plated fabrics at different pH: (a) 8.5, (b) 9.0, (c) 9.5 and (d) 10...... 175 Figure 6-28 EMI shielding effectiveness of the copper-plated fabric obtained under the optimal condition ...... 176 Figure 7-1 Deposit composition of (a) nickel, (b) copper and (c) nickel/copper multi-layer plated polyester fabrics ...... 183 Figure 7-2 SEM micrographs of (a) nickel-plated, (b) copper-plated and (c) nickel/copper multi-layer plated polyester fibers ...... 185 Figure 7-3 XRD patterns of (a) nickel-plated, (b) copper-plated and (c) nickel/copper multi-layer plated polyester fabrics...... 186 Figure 7-4 Surface resistance of nickel-plated, copper-plated and nickel/copper- plated polyester fabrics ...... 188
XVIII Figure 7-5 EMI shielding effectiveness of (a) nickel-plated, (b) copper-plated and (c) nickel/copper-plated polyester fabrics...... 189 Figure 7-6 The polarization curves of (a) nickel-plated, (b) copper-plated and (c) nickel/copper multi-layer plated polyester fabric in 3.5% NaCl solution ..193 Figure 8-1 Design results of of the electroless nickel-plated fabric with different
NiSO4 concentrations: (a) 6g/l, (b) 9g/l, (c) 12g/l and (d) 15g/l...... 199 Figure 8-2 Design results of the electroless nickel-plated fabric at different bath temperatures: (a) 50℃, (b) 60℃, (c) 70℃ and (d) 80℃...... 200 Figure 8-3 Design results of of the electroless nickel-plated (a) Green-Fabric and (b) Black-Fabric...... 201 Figure 8-4 Design results of the electroless copper-plated fabric with different
NiSO4 concentrations: (a) 0.8g/l, (b) 1.0g/l, (c) 1.5g/l and (d) 2.0g/l...... 202 Figure 8-5 Design results of the electroless copper-plated fabric at different pH of the solution on : (a) 8.5, (b) 9.0, (c) 9.5 and (d) 10...... 203 Figure 8-6 Results of plating time on the copper-plated fabric exposed to the plating bath at pH 8.5 and 85℃: (a) 0min, (b) 10min, (c) 15min, (d) 20min, (e) 25min, (f) 30min and (g) 35min...... 205 Figure 8-7 Schematic illustration of the formation of brochantite crystals on the copper surface ...... 206 Figure 8-8 Illustration of the reactions involved in the brochantite formation during patination ...... 206 Figure 8-9 Design results of the electroless copper-plated: (a) Green-Fabric and (b) Black-Fabric...... 208 Figure 8-10 Design results of the electroless nickel/copper multi-layer plated fabric with different plating time of nickel: (a) 6 min, (b) 9min, (c) 12min and (d) 15min...... 209 Figure 8-11 Results of the electroless nickel/copper plated: (a) Green-Fabric and (b) Black-Fabric...... 211 Figure 8-12 Electroless nickel-plated and copper-plated Green-Fabrics...... 215 Figure 8-13 Copper patina on the copper-plated Green-Fabric ...... 217
XIX Figure 8-14 (a) Copper-plated and (b) nickel-plated and copper-plated Black-Fabrics ...... 218
XX
LIST OF TABLES
Table 4-1 Bath composition and operating condition for electroless nickel plating ...... 77 Table 4-2 Bath composition and operating condition for electroless copper plating ...... 78 Table 4-3 Logarithmic relationship between the factors and plating rate for electroless nickel plating...... 104 Table 4-4 Logarithmic relationship between the factors and plating rate for electroless copper plating...... 107
Table 4-5 Comparison of experimental values (vexp) with calculated values (vcal) by kinetic models for electroless nickel plating...... 109
Table 4-6 Comparison of experimental values (vexp) with calculated value (vcal) by kinetic models for electroless copper plating...... 110 Table 5-1 Factors and levels for two level factorial design ...... 115 Table 5-2 Design matrix and experimental data obtained from the 25 full factorial design for the electroless nickel and copper plating ...... 117 Table 5-3 Levels of the factors tested in the central composite design ...... 120 Table 5-4 Experimental design and results of the central composite design ...... 120 Table 5-5 Analysis of variance (ANOVA) and coefficient estimates of the central composite second-order polynomial model for the electroless nickel plating ...... 131 Table 5-6 Analysis of variance (ANOVA) and coefficient estimates of the central composite second-order polynomial model for the electroless copper plating ...... 133 Table 6-1 The full width at half maximum of diffraction peak of the electroless nickel-plated fabrics...... 152 Table 6-2 The mol ratio of Ni/ (Cu+Ni) in films deposited in various plating baths ...... 161
XXI Table 7-1 Full width at half maximum and grain size of nickel, copper and nickel/copper multi-layer coatings...... 187 Table 7-2 Corrosion characteristics of the electroless nickel-plated, copper-plated and nickel/copper-plated polyester ...... 194
XXII LIST OF ABBREVIATIONS
ANOVA analysis of variance ASTM American society for testing and materials CCD central composite design DOE design of experiments ECP electroless copper plating EDX energy dispersive X-ray spectroscopy ELP electroless plating EMI electromagnetic interference ENP electroless nickel plating FCC face-centered cubic FFD full factorial design PET polyethylene terephthalate RFI radio frequency interference RSM response surface method SE shielding effectiveness SEM scanning electron microscopy UV ultraviolet XRD X-ray diffraction (X-ray diffractometer)
XXIII Chapter 1 Introduction
1.1 Background
Scientific advancements made in various fields have undoubtedly increased the quality and value of human life. However, it should be recognized that the technological developments have also exposed us to greater risks and danger of being affected by unknown physical, chemical and biological attacks. In addition, our bodies continue to be exposed to hazards from radiation. Fortunately, simple and effective means of protection from most of these hazards are available. The cloth always acts as the final protection layer to keep the body off these hurts [1].
Currently, textile finishing is of the critical importance in improving textile products by imparting them with multi-functional properties such as anti-bacterial property, ultraviolet (UV) protection, and heat protection. Textile coating is a finishing process that covers one or both sides of a substrate with a finish or layer of protective materials. For decorative and functional purposes, many different coating methods are employed to treat textiles [2]. Textile metallization is one of the coating processes in textile treatments. It is well known that the metallic particles that are deposited on textile surfaces to create metal-coated fabrics can produce fabrics with unique appearance and functional properties. Metallized textile is a kind of new material that
1 has attracted the attention of scientists and engineers due to its special properties such
as electrical conductivity, radio frequency interference (RFI) and electromagnetic
interference (EMI) shielding, anti-static and anti-bacterial properties, UV radiation
screen, heat insulation and radar reflectivity [3, 4]. In particular, metallized textiles have the appearance of brightness that can create beautifully reflective and lustrous
images. Therefore, metallized textiles are very popular in both the contemporary
consumer market and technical applications [5, 6].
Nowadays, the developed metallizing techniques include laminating with metal foil,
sputtering, vacuum deposition, electroplating and electroless plating. Among them, electroless plating is probably the preferred way to produce metallized textiles as it is very flexible and suitable for almost any fiber substrate. Its advantages are low cost, easy formation of a continuous and uniform coating, and it can be applied at any step of the textile production such as yarn, stock, fabric or clothing [7]. It can produce complex shapes without significantly altering other properties such as density, flexibility and handle of the substrate [7].
In electroless plating, metallic deposition is formed as a result of chemical reaction
between a reducing agent and metal ions present in the plating solution. During
electroless plating, the electroless bath typically comprises an aqueous solution of
metal ions, complexing agents, reducing agents and stabilizers, operating at a specific
temperature and pH ranges. The rate of plating and properties of coatings depend on a
number of factors such as the type and concentrations of the plating components and
2 operation conditions. Since the discovery of electroless plating, some of the properties
of these coatings have received considerable research attention. Most of these
properties are dependent on the coating structure [8]. The properties of metallized
textiles have also been the focus of attention of a number of researchers [6, 9-12] over
the years. However, the coatings of metallized textiles depend on the weight of the
metal layer which varies with different ratios of compositions with respect to solution
concentrations, reaction duration, temperature and other parameters. Since the
metallized textile design effects also depend on treatment conditions, thus it is important to study the optimization of electroless plating process. Optimization of the
deposition process can provide useful information for industries to obtain greater
quality and stable plating. However, an extensive review of the literature reveals that no report is available for the optimization of the process parameters of electroless plating to maximize the properties and decorative effects.
Modern textile design aims to pursue an ideal combination of technologies, aesthetics
and functions. Metallized textile design has attracted much attention for potential utilization. Using metallic materials is an important trend in textile innovation since the 1970s. Metallized design has commonly employed various metals such as yellow gold, white silver, red bronze, grey nickel and brilliant titanium. Multi-layer coating can improve the properties of metallized textiles and produce different colors from pure metal. However, there is little literature reporting the electroless multi-layer plating on fabric, and the effects of the plating parameters concerned on the
3 micro-structure, properties and appearance of the plated fabric.
1.2 Objectives
Metallized textiles are very promising for functional and decorative applications, though there are still many challenges for the commercial products. The aim of the research is to investigate the optimization of electroless nickel and copper plating, nickel/copper multi-layer plating processes, and micro-structure, properties and design effects of the plated fabric in order to meet the requirement of high technical and apparel applications.
The main objectives for this project are summarized as follows:
(1) To establish kinetic models for metallized textile functional and design
applications, and determine the optimization of electroless nickel and copper
plating processes;
(2) To investigate the mechanism and the effects of plating parameters on
micro-structure and properties of the electroless plated fabrics;
(3) To develop the nickel/copper multi-layer plated fabrics by using electroless
plating for functional and design applications;
4 (4) To develop multi-functions of metallized textiles so as to meet the demands of
consumer and industry.
1.3 Research Methodology
In order to achieve the objectives outlined above, the six elements of systematic research activities are carried out as shown in Figure 1-1. The six elements listed in this project outline different stages of systematic research activities.
Literature review
Thermodynamics and kinetics of electroless nickel and copper plating on polyester fabric
Optimization of electroless nickel and copper plating on polyester fabric
Characterization of electroless nickel, copper and nickel/copper multi-layer plated polyester fabrics
Design applications of electroless nickel, copper and nickel/copper multi-layer plated fabrics
Conclusions
Figure 1-1 Scheme of research activities
5
The performance of metallized textiles, metallizing technologies, electroless plating and development in textiles has been traced in the literature review, and the mechanism of electroless plating has been introduced. Based on the study of materials and process parameters of electroless plating, the exploitation of optimizing electroless plating processes has emerged. It is assessed that the combination of both metal materials and plating parameters can create the functional and decorative metallized textiles.
In view of high conductivity of copper and anti-corrosive properties of nickel as well as different color effects of two metals, copper and nickel, and nickel/copper multi-layer are thus chosen as coating metals in this project. In terms of high stability exposed to acid and alkali solution, polyester also selected as substrate material. In the present study, the hypophosphite-based electroless nickel and copper plating processes are involved. Thermodynamics and kinetics of electroless nickel and copper plating on polyester fabric are proposed.
Conductivity and EMI SE are the most important properties of the electroless plated fabric. According to the preliminary study, the full factorial design (FDD) and response surface method (RSM) are adopted in the optimization of plating processes for nickel and copper plating on polyester fabric in order to obtain a minimal surface resistance of metallized fabric and design effects.
6
Based on the effect of plating parameters on the micro-structure and properties, the
deposit composition, surface morphology and crystal structure of electroless nickel,
copper and nickel/copper plated textiles are studied. Surface resistance, EMI shielding effectiveness (SE) and anti-corrosive properties are evaluated. The appearance of metallized textiles obtained under different conditions is investigated, and the metallized textile design is developed using innovative techniques.
1.4 Project Significance and Values
Electroless plating has a relatively long history and is well established. Electroless
plating on textiles has attracted much attention in recent years. With the increasing
demands from the consumers and the society, simple electroless plating is unable to
cope with these demands since the properties and appearance of metal-plated fabric depend on deposition conditions. To apply this technique in industries properly, an optimization process suitable for metal deposition on fabric is required. Hitherto, there is no detailed study to optimize processes for electroless nickel and copper plating on fabric. There is a practical significance to establish the optimal electroless plating processes which can provide useful information for industries to obtain high value-added products. The major benefit of this project is to find out the cost-effective and optimal electroless deposition conditions in textile industries, associated with multi-functional properties and beautiful appearance. Additionally, the present study
7 is to fill up the gaps of relationship between the micro-structure and properties of metallized textile products as well as kinetic mechanism of metal plating on fabric.
The present project has developed multi-layer plating techniques for multi-functions and textile design applications. The originality of the research is to exploit the electroless nickel/copper multi-layer plating on fabric in order to overcome the disadvantage of single-layer metal-coated fabric. It can process innovative fabrics with aesthetic effect. This research explores not only the multi-layer technical feasibility, but also an unusual metallized fabric design. It is actually the result of combining technology and materials. There is a practical significance to extend complex metallic color in the area of textile design innovation.
The major significance and values of the project are identified and summarized as follows:
(1) The present study will provide a practical optimal process of metallizing which
will contribute to the cost-effective technology and bring a great benefit to the
metallized textile industry;
(2) This project is anticipated to develop metal multi-layer plating for improving
functional properties and appearance of textiles;
(3) The current research will provide a process for obtaining the metallized textiles
with multi-functional properties, which will add values to the specific products in
the textile field.
8 (4) Greater commercial value will be gained based on the outcome of the optimizing
process for metallized textile design, leading to high potential in the metallized
textile manufacturing and better properties of the product.
1.5 Structure of the Thesis
Chapter 1 gives a brief introduction to the background of metallized textiles and
historical development of metallizing techniques. It also highlights the advantages of
electroless plating technique in preparing metallized textiles, and the importance of optimizing the electroless plating and multi-layer plating on fabric processes. The problem involved puts forth the objective of this research and research methodology.
Finally, the originality and significance of this project are stated.
Chapter 2 reviews the performance and design applications of metallized textiles.
Fabrication technologies are covered in this chapter, and the development of
electroless plating in fabrics is also reviewed in the later part of this chapter.
The mechanism of electroless plating and materials of electroless plating are
described in Chapter 3. Deposition parameters of electroless plating are described and
fundamental methodology is proposed.
Chapter 4 focuses on thermodynamic possibility of electroless nickel and copper on
polyester fabric. In addition, the kinetic models used for the electroless nickel and
9 copper plating on polyester fabric are established.
The optimization of electroless nickel and copper plating processes on polyester
fabric to obtain minimal surface resistance are investigated with Minitab model in
Chapter 5. Optimal conditions of nickel plating and copper plating are obtained.
Chapter 6 is an extension of Chapter 5. In this chapter, the micro-structure and
properties of nickel-plated and copper-plated polyester fabrics are studied in terms of
different deposition parameters significantly influencing the surface resistance in
accordance with standard test method. Deposit composition, surface morphology and crystal structure are discussed, and surface resistance and EMI SE are assessed.
Chapter 7 presents the characteristics of electroless nickel/copper multi-layer plated polyester fabric when compared with the nickel-plated and copper-plated polyester fabric. Their deposit composition, surface morphology and crystal structure are investigated. Surface resistance, EMI SE and corrosive resistance are also evaluated.
Chapter 8 reports the design applications of electroless nickel and copper plating as well as the nickel/copper multi-layer plating on fabrics. Color, pattern and texture are conducted by combining materials and design conditions. The influence of deposition conditions on metallized textile design creation is described. Design collections with different colors and reflective appearance are obtained.
10
Finally, Chapter 9 provides the general conclusions and some suggestions on the possible future work are also discussed.
11 Chapter 2 Literature Review
2.1 Introduction
Fabrics are metallized for several reasons including better insulation, enhanced light
or RF reflectivity, electrical conductivity, or a shiny metallic appearance. This chapter
reviews the literature regarding the metallized textiles, metallizing techniques and the development of electroless plating in textile modification. It begins with an
introduction to the metallized textiles followed by a survey of metallizing technologies and the development of electroless plating in textiles.
2.2 Metallized Textiles
Metallized textiles refer to textiles with a thin covering of metal [13]. Metallized fabrics provide good abrasion resistance, light reflectivity over extended time, wear
resistance and mechanical strengthening [14, 15]. In particular, metallized textiles
possess many functional properties including radiant heat reflection, electrical
conductivity, RFI and EMI shielding, anti-static and anti-bacterial properties, UV
radiation screen, radar reflectivity and intelligence as well as decorative effects.
These innovations in metallized textiles have led to many applications including
12 ironing board covers, window insulation, pipe wrap and high-tech protective suits
[16], which can be used in industries. Their practical application is to protect people from electromagnetic pollution such as the potentially harmful waves from mobile phones. Metallized textiles have been considered in various applications for the defense, electrical and electronic industries as well as medical field [17].
2.2.1 Radiant Heat Reflection
All textile materials are almost black substances and the reflective ratio is below 10 percent. The highly polished metallized surface can reflect over 90% of the radiant heat [18]. Since the metallized textiles such as aluminized fabrics have high surface reflection, they are preferred for light and radiant heat protection [14, 19, 20]. The metallized textiles involving resistive heating is resulted from the passage of an electric current through the metal-coated panels of fabric [21]. It is reported that the aluminized and gold coated fabrics can reflect up to 90% to 100% of radiant heat respectively [14]. As a result, the metallized textiles are used for protection against intense radiant heat of short duration [18, 22, 23].
The heat protection property of metallized fabrics is used for protective clothing, particularly protective garments for firemen and others exposed to high temperatures including workers in steel mills [24]. This property can also be used for making heated garments, gloves and blankets [21]. The application is exemplified by the
13 polyester suits bonded with aluminum by means of vacuum deposition. They are used by firefighters and workers in the steel industry for protection against blast furnace radiation and molten metal splash as shown in Figure 2-1 [13, 16]. The development of these suits has been stimulated by the widespread withdrawal of asbestos as a heat-resistant material [16, 25, 26]. Another application is that metallized textile conserves the body heat of space-walking astronauts which will be lost rapidly by radiation in cold, airless space [22]. Additionally, metallized textiles are used to clothe athletes temporarily after strenuous activity such as marathon running in order to reduce the radiation of body heat and guard against chilling [27].
The largest end-use in this category is the pleated window shade [13, 28]. Metallized textiles find their application primarily in curtains and blinds [13], but also in pleated window shades, especially in the USA. The metallized surface, facing outside, reflects solar heat during the summer and radiant heat back into the room during the winter
[16, 22]. The high reflection values of this unique fabric make roller blinds a solution for all situations where light intensity or solar heat is a problem [28].
14
Figure 2-1 Heat protection suit for workers in the steel industry [22]
2.2.2 Conductivity/Anti-static Property
Fibers made from synthetic polymers are very low in electrical conductivity and tend to generate static electrical charges very easily as a result of motion. Cotton and wool goods dissipate their electrical charges due to their moisture content. However, under very dry conditions, they can also have static problems [22]. The conductivity of common textile fibers is of the order of 10-13 ohm· cm-1. Increasing the conductivity to the range of 10-3 to 10-10 ohm· cm-1 is usually adequate for the dissipation of static
charge [7].
Textile metallization can significantly improve the electrical conductivity of these
textiles and also their anti-static properties. The presence of metal helps to conduct
electricity so that the static shock can be reduced on non-conductive textiles. It is
15 reported that the surface conductivity of textiles coated with silver, copper and nickel shows a significant increase as compared to the uncoated ones [29, 30]. Development of copper-, nickel-, and silver-coated fibers was reported for anti-static applications [6,
7, 9, 31]. Jiang et al [10] explored the conductivity of chemical silver-plated cotton and polyester, and the results showed that the resistance of cotton and polyester was changed from ∞ to 14.9 and 15.5 MΩ, respectively. Wei et al [32] stated that polypropylene and polyamide exhibited a surface resistance less than 100 Ω/ cm after the silver sputtering coating. Bertuleit [33] revealed that in carpet manufacture, the addition of 0.05% silver is sufficient to obtain normal anti-static qualities.
Hence, the electrically conductive metal coated yarns and fibers are used as anti-static elements in carpets; furnishing fabrics; and in industrial applications such as filter fabrics, needlefelts, webbing conveyor belt fabrics and balloon nets [33]. They are also used in workwear, clean-room garments, electronic industry, operating theatre clothing, and anti-explosion wear [33].
2.2.3 Electromagnetic Interference (EMI) Shielding
Nowadays, electromagnetic radiation is becoming the fourth kind of public space pollution in addition to noise, water and air pollution. EMI is a well-known problem for commercial and scientific electronic instruments, antenna systems and military electronic devices. As a result, human beings are looking not only for the healthy
16 electromagnetic environments, but also for daily work and particularly proper
communications. It is obvious that there is a critical need to develop effective and
practical EMI shielding materials and their associated applications [34, 35].
In the past, metal foils were often used for electromagnetic shielding. However, foils
are more expensive to manufacture and tend to crinkle or tear after being flexed or
stressed [13]. Metallized textile screening materials have been known since the end of
the 1980s [36]. In general, textiles which are coated with aluminum, copper, silver
and nickel are the important types of material for preventing EMI [37-43]. It was
estimated that these metallized fabrics could provide 95-97% of the shielding than
those which would be provided by an equivalent mass of metal in foil form [31].
In recent years, many researchers have focused on studying the EMI shielding
properties of metallized fabrics and obtained good results [36, 42, 44]. When
combining the basic physical properties of industrial fabrics with a highly conductive
metallized surface, the metallized textile can achieve the attenuated electromagnetic
wave [45] and produce high-quality EMI shielding [46]. The high EMI SE of a typical
metal is due to its high reflectance [39]. Various coating materials have different EMI
SE with a thicker coating film having a higher EMI SE [41]. The study indicates that copper-plated woven fabrics have an 80-90dB shielding [31], implying that these fabrics can be used in the advanced electronic products and defense applications [12,
47, 48]. Non-woven fabric with 70% Texment loading and 30% deposition of copper
17 and nickel shows a very good microwave reflection and a transmission loss of over
65dB in the 8-12GHz range [7]. Cu, Cu/Ni, or Sn/Cu on non-woven nylon gave a
shielding between 56-90dB in the frequency range of 100 to 10,000 MHz. Shielding
of over 40dB has been obtained in the range of 90 GHz to 500 MHz using the
silver-coated nylon fabric [33] developed with the Statex TM system of coating for
EMI shielding application.
Metallized textile is also emerging as a material of choice for EMI/RFI shielding of
sensitive equipment, particularly in defense and aerospace. Metallized textiles-based
shields are used in wide-ranging products for the computer, telecommunications,
aerospace, defense, medical, general electronic, and automotive industries [46]. The
conducting fabric can be tailored to produce the ready-to-use adhesive tapes, curtains, bags and so on [49]. Undergarments which are even made from the metallized fabric can provide personal shielding against the low-frequency electromagnetic radiation
[16]. Targeted markets include pregnant women who operate computers and those who work around high voltage lines and microwave environments [16].
2.2.4 Ultraviolet (UV) /Infrared (IR) Protection
UV radiation falls into three categories namely (1) longer
wavelength/low-energy-fraction UV-A (λ ＝ 320–400 nm), (2) shorter
wavelength/high-energy fraction UV-B (λ＝280–320 nm), and (3) the highest energy
18 fraction UV-C (λ<280 nm). Too much exposure to the shorter wavelength component of solar UV radiation, i.e. UV-B, can result in the greatest skin damage. Accordingly, the awareness of the need for protection against harmful UV radiation is rapidly increasing [50].
Metallized textiles present a considerable improvement in UV and IR radiation. It is reported that fabric coated with a metal such as silver, aluminum, copper and nickel can offer excellent protection from ultraviolet radiation as indicated by the Ultraviolet
Protection Factor rating of 50+ [6, 9-11, 51]. Zhou and Zhao announced that no more than 1% of ultraviolet radiation could traverse the aluminum-plated fabric, and only
2% of infrared radiation could pass through the aluminum-plated fabric [3]. Wei et al observed that the uncoated non-woven fabric showed the transmittance of about 60% in the wavelength range of 300–600 nm. However, when the non-woven fabric was coated with a metal, the transmittance dropped gradually to about 15% in the wavelength range of 300 to 200 nm, indicating the UV shielding effect of the material
[30]. Metallized textiles provide a high degree of reflection of infrared radiation from the human body [22].
2.2.5 Anti-bacterial Property
The control of microorganisms on textile fabrics extends into diverse areas such as the hospital environment and everyday household. Although textiles are wholly made of
19 natural fibers or synthetic fibers, neither natural fibers nor synthetic fibers have a
strong resistance to bacteria or pathogenic fungi. Hence, various anti-bacterial finishes
and disinfection techniques have been developed for all types of textiles [52, 53].
It is a well-known fact that some metals, especially silver and copper, exhibit toxic effects on microorganisms [54-56]. Silver, being a non-toxic metal, has enjoyed reputation for anti-microbial properties and has been known for centuries [57].
Anti-bacterial property of silver on fabric has been investigated by many researchers
[4, 55, 56, 58-64]. The results indicated that the anti-bacterial properties were
improved as the film thickness was increased [63, 65]. As the coating thickness
exceeded 1 nm, the reduction percent of both tested bacteria- Staphylococcus aureus
and Escherichia coli reached 100 % [63, 65]. Furthermore, the silver-coated cotton
fabric after being washed 10 times has a slightly lower anti-bacterial activity, but it
can still retain >60% [66].
The application of silver nano-particles for imparting anti-microbial capabilities
properly to textile has recently received a growing interest from both the academic
and commercial fields [66-69]. Silver-coated fabric has been widely used in various
biomedical fields such as wound dressing materials, body wall repairs, augmentation
devices, tissue scaffolds, and anti-microbial filters [70-73]. Since silver is an aseptic,
thus the silver-coated fabric can be placed directly on open wounds. Swabs placed on
top of the fabric can soak up the wound secretions through the fabric, and so new skin
20 can form under the silver coated fabric without any risk of being ingested [33].
Silver-coated fabrics can also assist in the healing of bone breaks, and provide conductivity for electrical therapy [16, 18, 22, 58, 74-78]
2.2.6 Intelligence
Intelligent textiles can be described as smart textiles that are able to sense and react to environmental conditions or stimuli such as those from mechanical, thermal, chemical, electrical, magnetic or other sources [79, 80].
Metallized textiles have the ability to memorize a pre-determined shape for intelligence. Shape Memory Alloys, usually nickel and titanium, can regain their original shape when heated [81]. The sleeve fabric is programmed to shorten immediately as the room temperature rises. The screwed Ti-Ni alloy blended fabric will back to its original shape with a heat treatment. [74, 82]. A lightweight alloy, containing 50% titanium and originated in the aerospace industry, is used for medical applications such as heart valve surgery. This knowledge has been transferred to clothing by the companies such as the Italian Corpo Novo, in the production of an
‘intelligent’ shirt as shown in Figure 2-2.
21
Figure 2-2 The Oricalco shape memory alloy shirt [81]
Metallized textiles are potential candidates of smart textiles, having good characteristics such as good conductivity, flexibility, free tailoring and easy sewing.
These characteristics yield great possibilities to be used in manufacturing the textile-integrated level of smart wear [83]. Cho [84, 85] reported that the Cu/Ni electroless metal plated fabric could be used for the construction of signal
transmission lines and keypads. Electroless copper plating on nylon/spandex
stretchable fabric was investigated to pursue the feasibility of using the conductive
electrode pad materials for electrotherapy [86]. The results of electrocardiogram
signals generated from the copper sputtered electrodes showed a big potential as
textile-based electrodes [87]. By applying electrodes on the coated materials, the
system will be sensitive for any touches, which is useful for different applications,
e.g., the guarding of shop windows, exhibition and museums, without wiring, alarms
and security guards at windows and doors; sensors for different processes and noise.
22 Notably, the metal-coated non-woven becomes a surface sensitive medium, ready for developing a new kind of surface sensor [15].
2.2.7 Radar Reflectivity
Electromagnetic waves reflect or scatter from any large change in the dielectric or diamagnetic constants. This means that a solid object in air or a vacuum, or other significant change in atomic density between the object and what is surrounding it, will usually scatter radar or radio waves. Metallized textiles are capable of reflecting electromagnetic radiation and can act as a radar target by giving strong echo [88].
One of the main applications of these fabrics is for making life-saving devices for locating persons marooned in high seas during an emergency, and foldable radar fabric reflectors for life rafts. The movement of meteorological balloons is tracked by providing targets of the metalized textiles installed on them. Bertuleit [33] mentioned that the silver-coated mesh fabrics “marinized” with special protective coatings were being used in the manufacture of flexible, foldable radar reflectors in use on life saving islands and sailing boats. In defense applications, the target banner fabric, which is made of mono-filament yarn, e.g. polyethylene, nylon and viscose containing the metallized threads of duraliminum or silver, is towed behind an aircraft at a distance for practicing surface-to-air firing practice by soldiers after locating the same on radar.
23
2.2.8 Decorative Application
Metals have had a long history in textile decorations. The incorporation of metal into
textiles dates back to the Roman era, when they were mainly used for decorative purposes. Gold and silver were used for decorating the clothing of kings, leaders and nobilities [5, 89]. During historic ages, gold and silver were used for decorating the textiles of kings and aristocrats. The 18th century Nigel Atkinson used the
heat-reactive ink encrusted with gold leaf on cotton velvet [90]. The modern era of metallic textiles started in the 1930s with gold and silver coated fabrics [89]. Since
then, the technological advances in materials science have brought a new look to
metallic fabrics. With its unique appearance and texture, metallized textiles are now
widely applied to both clothing and interior decoration [89]. Metallized textiles pursue
a combination of functional properties and novel effects.
Metallized textile has the characteristics of luxurious and glimmering appearance
which can generate fashionable effects [91, 92]. Hence, it serves to mark the wearer’s
wealth and status in the community. Many kinds of metals such as aluminum,
titanium, platinum, steel, gold, copper, nickel and silver are frequently applied to
render modern effects. These metals have become the attractive research field in both
aesthetic and functional applications. Aluminum, copper, steel and nickel are the
metals most commonly used for inclusion in textile constructions. Precious metals
24 such as gold and silver are eternally tokens of fortune. Bright in color and stable chemical consistency, they are often used to adorn luxurious costumes as shown in
Figure 2-3 [74, 93].
Figure 2-3 Silver-plated polyester fabric [93]
Fashion designers have discovered the dramatic effects created by combining the metal such as copper or aluminum with the more traditional fabrics of silk, wool, and polyester and so on. As little as 1% of metal in a polyester gauze can create a silk-like shiny fabric. However, using 50% of aluminum can create a fabric with a shape memory quality that will also offer protection against abrasion and wear [74].
The development of new materials and new technologies not only makes it possible to improve the quality of textile products, but also creates enormous room for innovative design. Design and technology, in spite of their difference in the mode of thinking and
25 practice, have been closely connected in the history of textile development [93]. The
designer, Reiko Sudo with Nuno in Japan, has experimented the stainless steel onto
polyester fabrics by sputtering technique to create different looks and textures as
shown in Figure 2-4 [74, 82].
Figure 2-4 Stainless series (Nuno corporation, Reiko Sudo) [82]
Metallized textiles have played an important role in the development of fashion, regardless of their aesthetic value and practical function. The introduction of metal
adds a fashion element and charming vigor to fabrics. Design, which is beyond simple
appearance and decoration, is exerting unique advantages and showing a great
potential for further development [93]. Advanced metallic fabrics have led to new
developments and innovations in textile design field. Therefore, contemporary
metallic fabrics combine both high performance and high function [91]. Metallized
textiles have a big potential in textile design to achieve brilliant performance.
26 In the Spring 2009, the designers embraced metallic and mixed them with bright colors as shown in Figure 2-5 [94]. Prada and Marc Jacobs both used metal as a main point of interest in their spring collections. On the other hand, many designers used sequins and embroidery to display metallic shades.
Figure 2-5 Spring 2009 Fashion Trend Metallic Color [94]
Metallic clothes and accessories have also gone a touch softer for the spring of
2009. Metallic Spandex Fabric has a deep glittery shimmer and a silky smooth feel with 4-way stretch. It is perfect for costumes and dance wear as shown in Figure 2-6
[92].
27
Figure 2-6 Metallized clothes [94]
Metallic handbag is ultra-hot in 2009. For instance, glimmering gold and silver bags
are popular in the world as shown in Figure 2-7.
Figure 2-7 Warehouse metallic bag [94]
Metallized textiles are new fashion fabrics. They look very special and have their own metallic sparkle and crinkle effect. Consumers are increasingly becoming more adventurous in curiosity and taste to drive the design industry in different endeavors.
28 Technological advances are making these innovations possible [95]. Nowadays, the
metallized textiles are incorporated into spotlight, particularly at the high end of fashion due to its look and feel. Fashion world is getting heated up with metallic fabrics of gold, silver and bronze on the runways. It carries a promising market for metallic accent in the shades of brass, silver and gold.
2.3 Metallizing Techniques
Metallizing techniques are employed to place metals on substrate surfaces for
aesthetic or functional purposes. Metallizing technologies for textiles have a long
history and have undergone tremendous revolutions. In ancient times, the tinsel yarns
used to add glitter to fabrics were made by flattening thin wire or sheets of noble
metals. It was not until the twentieth century that the fundamental technologies were
radically changed. The techniques of lamination were developed between the world
wars using aluminum foil and bonding agents of synthetic textiles. By the 1930s,
aluminum foil strips were coated on both sides by cellulose acetate-butyrate. However,
these yarns had poor compatibility with the more flexible and extensible textile yarns
[22]. After the Second World War, the advent of vacuum metallizing further
revolutionized the industry, in conjunction with the development of polymer fibers.
Importantly, with the development of vapor-deposited aluminized polyester in the
1960s, 1mm wide strips of these films were used as yarns resulting in a much
improved flexibility.
29
Although metallized processing technology is new in the textile field, it has a great potential for application to garments for both functional and decorative uses. Various methods have been developed for textile materials coated with metals including vacuum deposition, sputtering, electroplating and electroless plating [18].
Vacuum deposition is a physical process for preparing metallized textiles. In this process, the substrate to be coated is placed in a vacuum chamber over a set of crucibles containing the metal to be coated in the form of a powder/wire as shown in
Figure 2-8 [89]. The crucible is heated by resistance heating to melt the metal.
Substrate is passed over a cooled drum placed over the crucible. The metal atoms rising out of the molten metal hit the surface of the substrate and condense into solid metal as it passes over the crucible. Continuous metal film coatings can be formed on just about any surface including film, fiber or fabric, with the thicknesses ranging from microns to millimeters. A few metals can be vacuum evaporated with the most common one being aluminum, copper, silver and gold [7, 16, 18, 26, 41, 60, 96].
30
Figure 2-8 Schematics of the vacuum deposition [89]
Sputtering has opened up new possibilities for producing metallized textiles. The
equipment consists of a vacuum chamber containing an inert gas, usually argon, at
10-3 to 10-1 torr as shown in Figure 2-9. The chamber is equipped with a cathode
(target) which is the source of the coating material, and an anode which acts as a
substrate holder. Application of an electrical potential of the order of 1000Vdc
between the two electrodes produces a glow discharge. A flow of current occurs due
to the movement of electrons from cathode to anode. The electrons ionize the argon
gas to form argon ions which are accelerated toward the cathode at a high speed due
to high electric potential. The bombardment of the energetic ion on the target results
in a transfer of momentum. If the kinetic energy of the striking ion is higher than the
binding energy of the surface atoms of the material of the target, atoms are dislodged
or sputtered from its surface by a cascade of collisions. The sputtered atoms and ions condense on the substrate to form a thin film of coating [23, 97]. Metals including
31 silver, copper, gold, platinum, nickel and zinc have been deposited on fabric by
sputtering method [29, 32, 36, 37, 56, 59, 63-65, 98-102].
Figure 2-9 Schematics of sputtering [49]
Metal particle can be deposited on the conductive fabric by means of electroplating.
The electroplating process is performed in an electrolytic cell which contains an electrolyte and two electrodes as shown in Figure 2-10. Anode is a coating metal, while cathode is the part to be coated. Under a low-voltage current, the metal ion
existed in the electrolyte arrives at the cathode and is deposited on fabric.
Electroplating is applied to carbon and graphite fibers only as it requires a conductive
substrate. Electroplating can be only used on the non-conductive materials when it is
conducted through pre-treatment [89].
32
Figure 2-10 Schematics of electroplating process [89]
Although the above metallizing technologies have advantages, yet they have obvious disadvantages. Vacuum deposition is a relatively inexpensive method that produces metallic coated fibers. Unfortunately, this process has several limitations. There is adhesion between the metal and fiber creating an unstable construction. In addition, the process is difficult and the fibers so produced have low resistance to corrosion and wear [103]. Electroplating is comparable with vacuum deposition, but only possible for conductive fibers. Furthermore, textile materials metallized by an electrochemical method are looking bad. They have hard touch and the adhesion of coating to a substrate is poor. Due to their high price and difficult processability, they are hardly used by the textile industry [7]. The latest sputtering technology has improved the quality, especially the coating thickness control. This method has such advantages as being uniform and compact, having stronger bonding between the film and its substrate, and it is environmentally friendly [4, 104, 105]. However, the process is
33 expensive and the rate of deposition is low [106]. According to the literature,
electroless plating is an interesting alternative and is already available on the market.
The metal layers are thin enough to be undamaged in the textile processing to form
the final products [107].
2.4 Electroless Plating and Development in Textile
2.4.1 Background of Electroless Plating
The term “electroless plating (ELP)” is used to describe the methods of depositing
metals by means of chemical reduction. This method is termed “autocatalytic” or
self-catalyzing which refers to their ability to plate onto their own deposits to build up
metallization thickness. ELP dated back to 1835 with the reduction of silver salts by
reducing aldehydes. Progress in the field remained slow until 1946. The modern
electroless plating began with the rediscovery by Abner Brenner and Grace Riddell
[108] who accomplished nickel plating onto steel without an applied current. Since
then, this technique has been greatly studied and developed with a large number of
applications [109].
The growth of ELP is traceable to: (1) the discovery that some alloys produced by
ELP, notably nickel phosphorus, have unique properties; (2) the growth of the
electronics industry, especially the development of printed circuits; and (3) the large-scale introduction of plastics and other types of substrates benefiting from ELP
34 to meet many types of engineering requirements.
ELP is an effective process to deposit metal film on the surface of dielectric substrate
without the use of electrical energy. Unlike electroplating where the externally
supplied electrons act as reducing agent, metallic deposition in ELP, is formed as a result of chemical reaction between a reducing agent and metal ions present in the
solution [26, 110, 111]. Electroless plating method has some advantages as follows
[79, 112]:
(1) It is very flexible and suitable for almost any substrate;
(2) Deposition is low cost and simple, i.e. it is enough to immerse the pretreated
product in the electroless plating solution;
(3) Coatings have uniform thickness irrespective of the shape of the product to be
plated;
(4) It allows the fibers to achieve high performance without significantly altering
other properties such as density, flexibility and handle of the substrate.
2.4.2 Applications of Electroless Plating
One of the most appealing aspects of ELP is the wide variety of metals that can be deposited. ELP technique has been developed for various metals including Ag [113,
114], Cu [115-117], Ni [118, 119], Co [120, 121], Au [122, 123], Pt [124, 125], Pd
35 [123, 126], Zn [127], Cr [128], Sn [129] and Sb [130]. It is not difficult to deposit two
or more metals at a time and this method is known for deposition of more than 50
alloys of different qualitative compositions, mostly based on nickel, cobalt and copper
[110]. The most successful commercial ELP systems today include copper and nickel
in the form of binary, ternary or quaternary alloys. More common electroless
deposited alloys today include Ni-P, Ni-B, Cu, Cu-P, Co-P, Ni-Sn-P and Ni-Cu-P.
Furthermore, ELP provides the ability to deposit metals on the conductive, semi-conductive, and non-conductive materials such as polymer [131, 132], plastic
[133] and glass [134]. This valuable process can coat not only the electrically conductive materials including metals but also insulators like plastics, rubber and fabrics.
Since the inception of ELP, it has been the subject of research interest. In the past two decades, emphasis has been shifted to the studies of its properties and applications.
The co-deposition of particulate matter or substance within the growing film has led to a new generation of electroless composite coatings, and many of them possess
excellent wear and corrosion resistance.
The low coating rates with these can provide a better reflectivity of the plated surfaces
and many more applications. Coatings can be tailored for desired properties by
selecting the composition of the coating alloy/composite/metallic to suit specific
36 requirements. The market for these coatings is expanding fast as the potential
applications are on the rise. By the controlled chemical reduction reaction, the
electroless coating chemistry has emerged as one of the leading growth areas in
surface engineering and metal finishing. Materials formulated by ELP have
applications in the electronics, aerospace, mining, salvage and repair, chemicals,
hydraulics, automotive medical, dental and pharmaceutical, printing, battery and
textile industries [135]. In recent years, there has been a growing interest in the
application of the metallized polymer films for the electronics-based industries such as integrated circuits, packing, printed circuit boards and sensors [136].
2.4.3 Development of Electroless Plating in Textiles
The “selective” nature of electroless deposition makes it appealing for the preparation
of metallized textiles since it can potentially lead to the production of a large volume of multi-functional and decorative textiles at relatively low costs. The functional applications of electroless metal-plated textiles with a variety of metals have been researched for use in areas such as fungicides, bactericides, armour, electrodes, anti-static devices, radio frequencies, radar reflectors and home decoration. The main application fields of metallized textiles include EMI/RFI shielding, anti-static garments, UV radiation protection, infrared reflectors and lightning strike protection
[31].
37 Some researchers have suggested a process to achieve the conductive fabric by using the electroless silver plating [6, 9, 10, 33]. All kinds of non-conducting fabrics such as cotton, silk, polyester, nylon, aramid and polyamide have been used to manufacture the metallized textiles by means of the electroless silver plating method. Many application functions have been improved including anti-bacterial properties, anti-static properties and UV radiation shielding [6, 9, 10]. Mechanical performance such as color changes in electroless silver-plated fabrics after rubbing and washing, has been assessed according to standard testing methods. The results show that the rubbing fastness of the silver-plated fabric is relatively poor with respect to the commercial requirement. In addition, the color of silver-plated cotton and polyester fabrics slightly change after washing, i.e. the tested fabrics are slightly paler than the original silver-plated specimens. This confirms that some silver particles are lost during washing. The unstable single-used ammonia silver plating solutions are employed in preparing silver-plated fabric. It is reported that the electroless silver plating solution tends to self-decompose and so it is difficult to obtain the thick silver-plated fabric.
Due to the high conductivity of copper, the electroless copper plating is currently used for manufacturing conductive fabrics [137-140]. Electroless copper plating on fabrics for EMI SE has been studied by some researchers [12, 31, 38, 44, 45, 138, 141].
When the copper weight on the fabric is 40 g/m2, the EMI SE of the copper-coated polyester fabrics is more than 85 dB at the frequencies ranging from 100 MHz to 20
38 GHz [12]. In addition, the effects of pretreatment, sensitization, activation and
electroless deposition processes on the properties of the electroless copper-plated fabric have been investigated [141]. In order to achieve an improvement in the electroless copper plating effect, plasma, supercritical carbon oxide and chemical etching pretreatments have been introduced to enhance the hydrophilic and absorption properties of fabrics prior to electroless copper plating [11, 139, 142, 143]. The copper layer shows a higher adhesion to fabrics and is homogeneously deposited on the surface of fabrics after plasma pretreatment [44]. Han et al. [38, 45] investigated the effect of chemical etching pretreatment conditions, such as (1) catalyst concentration in the scouring bath, (2) etching temperature and (3) time on the EMI
SE and physical properties of the copper-plated fabric [38, 45]. The results indicated that polyester (PET) fabric treated without etching did not show EMI SE at any measured frequencies. It was found that KCl was the better catalyst activation than the commonly used SnCl2. The EMI SE of copper plated fabrics rises as the KCl is
increased up to 1:8 molar ratio of PdCl2: KCl and then decreases with further addition.
As the catalyzation temperature is increased from 25 to 40℃, the EMI SE and air
resistance of the copper-plated fabrics will increase, whereas their EMI SE is reduced
to about zero when the activation temperature exceeds over 40℃. The physical
properties including tensile extension and drape stiffness of the copper-plated fabrics are higher than those of the untreated PET. The investigation also revealed that a better plating film with more complete coverage could be obtained by adopting the ultrasonic irradiation method than the conventional electroless metal plating.
39
Owing to its wide range of technological applications, electroless nickel plating has a great potential in the textile industry [142, 144]. The possibility of applying an electroless nickel plating process of textile has been explored [11, 16, 145, 146].
Electroless nickel plating of textile materials have provided special functions such as anti-static properties, electrical conductivity, UV radiation protection, water repellency and brilliant decorative effects [79, 147, 148]. It can be applied in different areas of the textile industry to confer different functional properties [89]. Electroless nickel-plated fabrics can be used in military applications by allowing the camouflage to be lightweight and more breathable and adaptable to different climates. Fabrics with electroless nickel plating can also resist microbial attacks and they are effective for wall coverings [149]. The use of electroless nickel-plated fabric in shielding against EMI and RFI has gained importance over the past few years [89]. In order to achieve an improvement in the plating effect, plasma treatment has been introduced to alter the surface morphology of woven and non-woven polyester fabrics prior to electroless nickel plating so as to enhance its hydrophilic and absorption properties
[150, 151]. The results indicate that the plasma treatment can improve the performance of nickel-plated fabrics as reflected by their tensile strength, UV protection, fabric weight as well as wrinkle recovery [51, 147]. There is also a slight improvement in crocking fastness of all the nickel-plated polyester fabrics after plasma treatment. However, there is no influence on the performance of color fastness to laundering.
40
In 2006, Jiang has discovered that electroless plating not only resulted in conferring functional properties but also provided chemical process for textile design [6]. As an innovative technology for the metallized textile design, the color of the fabric is effectively changed to metallic color with gleaming effect after electroless plating.
The silver, copper and nickel plating processes that change the appearance of standard fabrics and innovative aesthetic effects as shown in Figure 2-11 have been obtained
[152].
(a) (b) (c)
Figure 2-11 (a) Silver-plated, (b) copper-plated and (c) nickel-plated polyester fabrics
with design method [152]
2.5 Research Gap
Based on the extensive literature review shown above, the following knowledge gaps
are identified:
All of copper-plated fabrics were prepared using formaldehyde as the reducing agent.
41 Formaldehyde has some disadvantages such as carcinogen and irritation to human body [108, 153, 154]. Moreover, high pH destroys the chemical structure of fabrics.
Electroless copper plating using sodium hypophosphite as the reducing agent to replace formaldehyde is attractive because of its low pH, low cost and relative safety when compared with the high pH formaldehyde-based solutions [155-157]. Therefore, the hypophosphite bath does not give off harmful vapor and can deposit copper at a lower pH value [157] without destroying fabric substrate. Electroless copper plating on polyester fabrics using hypophosphite as the reducing agent was investigated only by Gan et al [12]. He reported the effect of the concentrations of nickel ions and the additive K4Fe (CN) 6 in the plating solution as well as the pH of the plating bath on the deposition process. However, there are no reports about the optimization of electroless copper plating on fabric using hypophosphite as the reducing agent.
The optimization of the electroless nickel plating process used for textile applications was investigated using the orthogonal array testing strategy [146]. Limited number of studies has focused on the optimization of process parameters and responses in the case of electroless nickel plating processes on fabrics. To apply this technique to industrial use properly, optimization of the effectiveness of the nickel particle deposition on fabric is required.
Electroless nickel and copper plating solutions are relatively stable and the thickness of the plated fabric can be controlled. Nickel reduction by hypophosphite and the
42 copper (II) reduction by formaldehyde, the autocatalytic nature of these processes, thermodynamic and kinetic aspects of different substrates have been studied in the last few decades in a large number of works [116, 158, 159]. It is known that the deposition behavior is related to substrates. However, there is little literature concerning the thermodynamics and kinetics of electroless nickel and copper plating on fabrics using hypophosphite as the reducing agent. Many parameters including bath composition, bath pH value and plating temperature affect the electroless plating process. Nevertheless, the previous studies were limited to a discussion about the effect of one particular process parameter on the plating rate and kinetics of electroless plating on fabrics.
All functional properties of the deposits depend to a greater or lesser degree on the
composition of the bath and the conditions of the deposition [160]. Most of the
properties of coating are structure-dependent and the structure depends on the
composition of the deposits. This is controlled by adjusting the plating bath
constituents/conditions [161]. Since the discovery of electroless coatings, the
properties and appearance of electroless plated fabrics have received a considerable
attention. Although there have been some studies on the properties of electroless
nickel and copper deposits [157, 162-165], there have been a few reports on the study
of electroless nickel and copper plating on fabrics so far [10-12, 45, 146]. In addtion,
the effect of deposition parameters on the micro-structure and properties of electroless
nickel-plated and copper-plated fabrics have not been revealed.
43
Copper has a good electrical conductivity and is less expensive. Recently, due to its
susceptibility to corrosion and oxidation, it has been reported that the corrosive
property of copper film can be improved when the copper thin layer is usually coated
along with other protection metal [166]. Owing to the fact that both nickel and copper
have very good compatibility, the inner copper layer can be covered by an outer
nickel layer homogeneously. The outer nickel layer can be used to prevent the
oxidization of copper. For example, a metal coating consisting of 0.9μm of copper
with only 0.1μm overcoat of nickel can prevent the copper from oxidizing [21]. For
metalizing and multi-layer coating, nickel coated on the surface of copper deposits has been chosen as it appears to be the best compromise between price, conductivity, mechanical properties and corrosion resistance [7]. Therefore, nickel/copper multi-layer plating can (i) improve the properties, (ii) keep the stability of the copper performance and (iii) produce different colors from pure metal. However, only few papers report the micro-structure and properties of the electroless nickel/copper multi-layer plated fabric.
In addition, there is a lack of literature about the design application of electroless nickel, copper and nickel/copper multi-layer plated fabrics obtained under different conditions.
Therefore, a systematic scientific study needs to be carried out to fill the knowledge
44 gaps.
2.6 Summary
The foundation for the research of the metallized textile background is reported with metallized textiles, metallizing techniques and application developments of electroless plating in textile. Metallized textiles are very popular in terms of functional performance and decorative textiles, which are correlated with high performance and aesthetic requirement. Electroless plating is a novel technique applied to textile materials to efficiently produce fabrics with functional properties and a brilliant appearance. It involves the application of an ever-widening range of metals to an ever-widening range of fabrics.
Based on the extensive literature review shown above, the following knowledge gaps are identified: (i) there is no report about thermodynamics and kinetics of electroless plating on fabric; (ii) few literatures are focused on the optimization of electroless plating processes on fabric; (iii) the research works about micro-structure and properties of the electroless plated fabric are limited; and (iv) there is no report about the electroless multi-layer plating on fabric. The above knowledge gaps are the indispensable elements for the study of the metalized textile by means of electroless plating.
45 Chapter 3 Justification of Research Methodology
3.1 Introduction
Based on the problems mentioned in the literatures and also the research gaps discussed in Chapter 2, the research methodology will be discussed in this chapter. In order to understand the electroless plating (ELP) process, the mechanism of ELP is introduced. The substrate fabrics of ELP, and the deposition parameters such as metal ions, reducing agents, complexing agents, stabilizing agents, pH and temperature are reviewed. According to the general investigation of ELP, the fundamental methodology is described in this chapter.
3.2 Mechanism of Electroless Plating
The mechanism of ELP is a result of the chemical reduction based on the reducing agent used in the processing solution. The metallic phase that appears in such reactions may be obtained either in the bulk of the solution or as a precipitate in the form of a film on a solid surface [110]. The deposition of metallic layer onto the non-conducting material such as a polymer substrate by the electroless process implies the previous creation of catalytic sites on the non-conducting surface to initiate the chemical reaction [167]. These sites usually consist of chemisorbed
46 palladium nuclei. Palladium chemisorption can be carried out by two different
procedures. One of them is the two-step process corresponding to the sensitization
and activation. During sensitization, active sites are produced by dipping the substrate
in a reductive solution. SnCl2 is a sensitizer because it favors nucleation and inlay of
precious metallic catalyzer particles. The sensitization bath usually contains SnCl2 and hydrochloric acid (HCl). Sometimes, a small quantity of tin is added into sensitization solution in order to prevent tin (II) ions from oxidization. The activation step consists of providing catalysts. The substrate is immersed in a bath containing precious metal ions, i.e. usually palladium ions. The global reaction is:
22++ 40 + Sn+→+ Pd Sn Pd Equation 3-1
Another method for palladium chemisorption is the one-step process where the
substrate is activated in the mixed PdCl2–SnCl2 colloidal solution. An acceleration
step is necessary to remove the tin layer and then make the catalyst active for further
metal deposition [168]. Reactions that occur and lead to the beginning of the metal
coating can be illustrated by the following equilibrium and also Equation 3-1 [168].
2+ 2+ - 2+ 2+ Pd +Sn +4Cl → Sn (PdCl4) Equation 3-2
Figure 3-1 shows the dissolution of tin-palladium agglomerate in activation [168].
Particles with negative charges are adsorbed on the substrate surface.
47
Figure 3-1 Structure of the tin–palladium agglomerate and mechanism of dissolution
[168]
Metal particles are deposited on the catalyzed surface of the substrate after this substrate is placed in an electroless plating bath. The electroless deposition of a metal from an aqueous solution of metal salt has an electrochemical mechanism, i.e. both oxidation and reduction (redox) reactions, involving the transfer of electrons between
reacting chemical species. The oxidation of a substance is characterized by a loss of
electrons, while reduction is distinguished by a gain of electrons. Furthermore,
oxidation describes an anodic process, whereas reduction indicates a cathodic process.
An electrochemical model used for the process of electroless metal deposition is
suggested on the basis of the mixed-potential theory of deposition processes.
According to the mixed-potential theory of electroless deposition, the overall reaction
can be decomposed into one reduction reaction, i.e. the cathodic partial reaction [167], and one oxidation reaction (the anodic partial reaction). The reduction and oxidation reactions are described as follows:
M z+ +→ze M (reduction) Equation 3-3
48 and
RedOxme→+ (oxidation) Equation 3-4
where Mz+ is a metal ion, Red is a reducing agent, M is a metal atom, Ox is a oxidation product of the reducing agent Red, e is electron, z and m are the number of electrons that are transferred in the redox reaction.
Hence, the overall reaction of electroless deposition can be explained by Equation 3-5, which is the outcome of the combination of two different partial reactions as shown in
Equations 3-3 and 3-4. The schematic illustration of the basic components of electroless deposition cell is shown in Figure 3-2.
M z+ +→+Re dMOx (overall) Equation 3-5
Figure 3-2 Schematic illustration of the basic components of electroless deposition
cell [167]
49
In a steady state, these oxidation and reduction reactions occur simultaneously such
that no net current, i.e. movement of electrons is generated [167]. At that point, the
current being generated by the anodic reaction exactly equals the current come from
the cathodic reaction. This current is directly related to the rate of the electroless
plating reactions [169].
The progression of these redox reactions is governed by the thermodynamic and
kinetic considerations. According to a thermodynamic perspective, the redox potential
of the reducing agent, i.e. anodic reaction, must be more negative than that of the
metal ion, i.e. cathodic reaction in order for ELP to proceed [170]. This may be seen more clearly through the change in the standard Gibbs free energy which is governed
by the equation below [135]:
ΔG° = -nFΔE° Equation 3-6
where n is the number of electrons that are transferred in the redox reaction, F is
Faraday’s constant, and ΔE° is the difference of the cathodic standard potential (metal being reduced) and the anodic standard potential (reducing agent) [135].
When the anodic potential is more negative than the cathodic potential, the ΔE° is positive and the ΔG° is negative [135, 171]. The negative ΔG° indicates that the
50 reaction will proceed spontaneously at a given temperature and pressure [135, 171].
Hence, the reaction is thermodynamically favorable which can lead to spontaneous, homogenous redox reactions in the bulk solution. In order to minimize thermal decomposition, it is necessary to use reactions that are kinetically inhibited [172].
This implies that the activation energy must be high enough so that the redox reaction does not readily occur in the absence of an external catalys, i.e. in the bulk solution
[173]. However, in the presence of a catalytic surface, lower activation energy pathways become accessible, leading to redox reactions that occur only on the catalytic sites [173, 174]. Under such condition, the substrate must have sufficient catalytic activity to oxidize the reducing agent at a reasonable rate [169].
3.3 Research Field Justification
3.3.1 Substrate Fabrics
The metallized substrate may be of virtually any geometry. Woven or knitted fabrics,
and various types of non-woven product, may be used as electroless plating substrates.
Textile structures are complex three-dimensional constructions. The properties of
these textile structures are determined by the particular properties of constituent fibers
and the constructions of yarns and fabrics [175]. The compositions plated include
cotton, silk, polyester, polyimide, polypropylene, arimide, polyamide and acrylic [31].
Research into synthetics is crucial and there has been a huge change in the way these
51 materials are perceived [176]. Polyester is the most extensively used synthetic textile
fiber, making up a quarter of the world’s fiber production. Introduced in the 1950s,
the success of this versatile fiber has been due to the wide range of performance
factors, textural variations and surface handles that it offers [74].
Although there are many types of polyester, the term "polyester" as a specific material
most commonly refers to polyethylene terephthalate (PET). PET is the condensation
product of terephthalic acid and ethylene glycol. Polyester is a polymer with a chain
of repeating units, and the individual units are held together by ester linkages as
shown in Figure 3-3.
Figure 3-3 Chemical structure of polyethylene terephthalate (PET)
Polyester has many advantages including inexpensive cost; superior strength and resilience; lightweight; hydrophobicity. It has an unusually high melting point and is resistant to dyes, solvents and most chemicals; stain resistance; stretching and shrinking resistance; quick drying; wrinkle, mildew and abrasion resistance. It also retains heat-set pleats and creases and is easy to launder. It has good fade resistance, particularly when protected from UV radiation and it is noted to retain its shape. In addition, polyesters have excellent stability toward light, oxygen, water and many
52 chemicals [177]. Moreover, polyester coatings, especially the cross-linked ones,
exhibit excellent flexibility or even elasticity, and they also show very good impact
and scratch. It has good adhesion properties, especially to metals, combined with
good corrosion protection and weather resistance [177]. It can be highly transparent
and colorless but thicker sections are usually opaque and off-white. It exhibits static cling tendencies and is frequently used in fabrics that give the appearance of being bright and shiny.
Polyester fibers are extensively used for interior and architectural purposes, screens, fiber filling and industrial textiles. Woven polyester fabrics are used in apparel and home furnishings such as bed sheets, table sheets, curtains and draperies. Similarly,
industrial polyesters are used in tyre reinforcements, ropes, fabrics for conveyor belts,
safety belts, coated fabrics and plastic reinforcements with high energy absorption.
Polyester fibers are also used for stuff pillows, comforters and cushion padding.
Based on the above advantages, polyester fabric is widely used as substrate in
electroless plating.
3.3.2 Electroless Nickel and Copper Plating
Electroless copper and electroless nickel are the most common systems
commercialized today [178]. A typical deposition mechanism of electroless plating
53 with different metals including copper and nickel, and electroless plating parameters
are described as follows.
3.3.2.1 Electroless Nickel Plating (ENP)
Due to its excellent corrosion and wear resistance, good electrical conductivity and high hardness, ENP is the most important and widespread plating process used in industries [160] for corrosion protection [179-181], electromagnetic interference
(EMI) shielding [182-184], and other surface finishes [185-187].
ENP is a chemical reduction process in which the nickel ions are catalytically reduced
by reducing agents. The typical components of an ENP bath are (a) a source of nickel
ions, (b) a reducing agent, (c) a complexing agent, (d) a stabilizer/inhibitor and (e)
source of energy. In an electroless nickel bath, nickel chloride or nickel sulfate are
used as a source of nickel. Other nickel salts such as nickel acetate are used for
limited applications. A chemical reducing agent in an aqueous solution is the driving
force for the reduction of nickel ions. The most common reducing agents used in the electroless plating of nickel are sodium hypophosphite, sodium borohydride, dimethylamine borane and hydrazine. Among them, sodium hypophosphite is the predominant reducing agent used for the electroless plating of nickel. It is a strong
reducing agent and very soluble in water. The autocatalytic reaction occurs in both acidic and alkaline pH illustrated in Equations 3-7 and 3-8 [135, 160]:
54 2+− − + Ni++→+++22 H22 PO H 2 O Ni 2 H 23 PO H 2 2 H (acid bath) Equation 3-7 or
22+−− − Ni++→++ H22 PO32 OH Ni HPO 3 H 2 O (alkaline bath) Equation 3-8
The reaction takes place on catalytical active surfaces. In addition to metallic nickel,
+ - hydrogen ions (H ), molecular hydrogen and orthophosphate ions (H2PO3 ) are also formed. The formation of hydrogen ions causes the bath to become more acidic.
Since the discovery of the ENP, many studies have been performed to elucidate the
ENP process [135, 188-190]. However, the ENP process is so complicated that the exact mechanism is still far from being fully understood. Many parameters including bath compositions, stabilizers, pH, and plating temperature affect the ENP process.
Regarding the chemical reactions that occur in the hypophosphite reduced electroless nickel plating solutions, two mechanisms have been proposed [161, 191].
(1) The atomic hydrogen mechanism, where atomic hydrogen is released as a result of the catalytic dehydrogenation of hypophosphite molecule adsorbed at the surface, is illustrated in Equation 3-9 [192].
−− HPO22+→ HO 2 HPO 23 +2 Had Equation 3-9
At the catalytic surfaces, the adsorbed atomic hydrogen will then reduce nickel ions:
55
2++ Ni+→+22 Had Ni H Equation 3-10
The formation of elemental phosphorus is thought to be due to secondary reaction between hypophosphite ion and atomic hydrogen as shown in Equation 3-11:
−− HPO22+→++ Had P HO 2 OH Equation 3-11
The evolution of hydrogen gas, which always accompanies the electroless plating of nickel, is attributed to the following reaction where two hydrogen atoms recombine:
2HHad → 2 Equation 3-12
The adsorbed active hydrogen reduces nickel at the surface of the catalyst.
Simultaneously, some of the adsorbed hydrogen reduces a small amount of hypophosphite ions at the catalytic surface to form water, hydroxyl ion and phosphorus. Since most of the hypophosphite ions present are catalytic, thus they will be oxidized to orthophosphate and gaseous hydrogen causing low efficiency of electroless nickel solutions for alloy coating while the deposition of nickel and phosphorus continues. Therefore, the deposit from electroless nickel plating is an alloy of nickel and phosphorus.
56 (2) The electrochemical mechanism, showing the catalytic oxidation of the
hypophosphite ions yields electrons at the catalytic surface, and in turn reduces
nickel and hydrogen ions as illustrated in the following:
An oxidation reaction releases electrons formed by the reaction between water and hypophosphite:
−−+ HPO22+→ HO 2 HPO 23 ++22 H e Equation 3-13
Reduction reactions utilize the electrons formed in the oxidation reaction:
Ni2+ +→2 e Ni Equation 3-14
+ 22HeH+→2 Equation 3-15
−+ HPO22++→+22 H e P HO 2 Equation 3-16
According to the electrochemical mechanism, the evolution of hydrogen gas that takes place during nickel deposition is due to the secondary reaction.
The ENP bath using hypophosphite ion as the reducing agent produces a binary alloy composed of nickel and phosphorus (Ni–P) [193]. According to the content of phosphorus, the electroless Ni–P can be classified into three types, viz. low (1–3%P), medium (4–7%P), and high (7%P and above) phosphorus [194]. In accordance with
Graham [195], there is a difference between the components deposited from acid and alkaline electroless baths. Those containing less phosphorus deposited from alkaline media have a greater electrical conductivity.
57 The properties and micro-structures of electroless nickel coatings depend on the amount of phosphorus alloyed in the deposit [196, 197]. The electroless process used for nickel coatings produces the deposits having a microcrystalline, amorphous or fully crystalline nature over a wide range of compositions [198]. Based on XRD studies, Graham [195] reported that the deposits containing 5% P were microcrystalline in nature. However, Kazuyuki and Ueno [199] showed the structure of low phosphorous (<7%P) deposits to be crystalline. In the case of high phosphorous deposits (>7%P), Goldenstein et al [200] revealed an amorphous structure with a ‘liquid-like’ disorder of the atoms with a random distribution of phosphorus throughout the coating.
3.3.2.2 Electroless Copper Plating (ECP)
Due to high conductivity of copper, the ECP is an important industrial process with a wide variety of applications [201]. The first commercial application of ECP was reported by Cahill in 1957. EC coatings are usually used before electroplating on plastics, ceramics polymers and other non-conducting materials, providing that a conductive base is used for subsequent plating. ECP technology has advanced rapidly in recent years including a widespread application in areas such as (1) through-hole plating in the printed circuit boards, (2) decorative plating of household utensils and in the automotive industry, (3) electromagnetic interference shielding of electronic components, and (4) conductive traces in electronic interconnection devices and integrated circuit manufacturing [201]. Consequently, ECP baths have been widely
58 studied.
The most important compositions of ECP bath are metal ions and a reducing agent.
These baths generally utilize a divalent copper salt such as copper sulfate (CuSO4) for the metal ions [201, 202]. Various common reducing agents have been used in electroless copper baths such as formaldehyde, dimethylamine boron, borohydride and hypophosphite. Among these representative reducing agents, formaldehyde is the most favored one utilized as the reducing agent [201, 203]. Since formaldehyde is a stronger reducing agent under basic conditions [204], thus increasing the pH should result in an increase in the rate of plating. Indeed, several authors have reported that pH > 11 is necessary to maintain the feasible rates of copper deposition for plating processes [188, 204]. If pH < 9.5 is used, the deposition process becomes very slow and may even stop altogether [201]. However, if the pH is increased above 14, the bath becomes unstable [201]. Formaldehyde has some disadvantages such as carcinogen and irritation to human body [108, 153, 154]. Moreover, high pH destroys the chemical structure of fabrics.
The hypophosphite bath does not give off harmful vapor or destroy fabric substrate and can deposit copper at a lower pH value [157]. ECP employing hypophosphite as a reduction agent should become the electroless copper bath of the future because the solution has wide operation parameters, long solution life, and will not release harmful formaldehyde vapors. However, the hypophosphite-based ECP process is
59 complicated because copper is not a good catalyst for the oxidation of hypophosphite, resulting in little or no plating on a pure copper surface. The nickel serves to catalyze the oxidation of hypophosphite enabling the continuous copper deposition [156, 205,
206]. Several papers have studied the electroless copper solutions using hypophosphite as reducing agent [156, 207-210]. These studies are focused mainly on the effect of additives on the properties of the copper deposits or the application of electroless copper solutions to the fabrication of printed circuit boards.
An overall reaction in a ECP solution is listed in Equations 3-8 and 3-17 [165]:
22+−− − Cu++→++ H22 PO32 OH Cu HPO 3 H 2 O Equation 3-17
Generally, there is a diproportionation reaction of phosphorous in the hypophosphite ion as assumed in the following equation [206]:
−−2 322HPO22→+ P HPO 3 + HO 2 Equation 3-18
Since the reaction is a side one, thus the concentration of phosphorus present in the film is rather low.
The plating rate and composition of the deposit are two main characteristics of any
ELP bath used. The effects of the individual operating parameters on these
60 characteristics have been the focus for many research activities. The plating rate, the properties of coated components and the structural behavior of deposits mainly depend upon the plating bath constituents and conditions.
The micro-structure characterization of metallic coatings prepared by electroless methods is important because it allows the prediction of some of their physical, chemical and mechanical properties. Furthermore, this characterization can provide information for the growth mechanisms of coatings. The understanding of these mechanisms permits the optimization processing variables for obtaining coatings with different desired chemical, physical and mechanical properties. In the previous reports concerning the electroless process, a great effort was made to determine the effect of the processing variables on the final properties of the deposited coatings, especially to develop specific surface characteristics such as morphology, micro-structures and color. There exist some experimental results that show how deposition parameters affect the physical and chemical characteristics of the films. For instance, changes in the pH (concentration level) of the baths can modify, the coating micro-structure from banded to laminar or acicular forms, as well as the distribution of the metals deposited simultaneously [211]. Therefore, the chemical compositions of the baths and deposition conditions have a great influence on the growth habits of the coatings.
3.3.2.3 Electroless Plating Bath Parameters
Despite the growing degree of ELP applications, there is a lack of a fundamental
61 understanding of the chemistry and physics of electroless plating. In fact, the influence of various processing parameters on the resultant coating are still not well understood [135]. Hence, it is important to discuss the experimental parameters that affect deposition kinetics. These parameters describe the solution in which deposition takes place. The components of the electroless plating bath include an aqueous solution of metal ions, reducing agents, complexing agents and bath stabilizers, operating under the specific condition of concentration, temperature and pH [135].
The rate of electroless plating and the properties of the deposits are controlled by these conditions. Each process parameter has its specific role on the process that influences the process response variables.
Metal Ion Sources
Industrially-feasible metal ions are typically inexpensive, water-soluble metal salts
[211]. However, due to environmental concerns, certain hazardous salts such as those containing cyanide are generally avoided [211]. In the cases where the purity of the coating is essential, the hydroxide-based metal salts will be used as they do not contain heteroatomic anions that might be incorporated into the deposited metal coating [211]. Some of the commonly used metal ion sources such as AgNO3, CuSO4,
NiSO4, CoSO4, Na3Au(SO3)2, PdCl2, H2PtCl6 and Na2Pt(OH)6 [201]. The effect of metal ions source on the plating rate, deposit micro-structure and properties were investigated by some researchers [193, 212]. It is reported that high concentration of metal ions can ruin the stability of solution by the deposition of rough film on the
62 surface of substrate. However, the low concentration of the solution has a slow plating reaction [155, 211]. As for the ENP, the influence of the nickel concentration on the plating rate in the pyrophosphated baths was investigated and the results showed that the rate was only slightly enhanced by increasing the nickel ion concentration in the pyrophosphate bath [211].
Reducing Agents
Reducing agents donate electrons to the metal that is being reduced. Typical reducing agents include hypophosphite, formaldehyde, alkali borohydrides, dialkylamine borane and hydrazine [26, 169]. There are several factors influencing the selection of reducing agent. In order for the process to be catalytic, the standard redox potential of the reducing agent must be more electronegative than that of the metal being reduced.
For example, sodium hypophosphite is a practical reducing agent for nickel since its redox potential is -1.57 V in alkali solution [135]. It is reported that the hypophosphite ion concentration has a pronounced effect upon the plating rate [211]. The rate of plating in the alkaline electroless nickel solutions is proportional to the hypophosphite concentration. Increasing the concentration of hypophosphite can rapidly enhance the plating rate. However, an excessive amount of hypophosphite will cause the bath to become unstable and the deposited surface is rough [193].
Complexing Agents
63 Complexing agents in the electroless plating bath perform dual functions. A complexing agent maintains control over the free metal salt ions available to the solution, thereby preventing a spontaneous chemical reaction between the metal ion and reducing agent in the bulk of the solution. It also acts as a buffer to control the pH.
A complexing agent is an electron donating group that displaces the hydrated water molecules in order to occupy the metal ion coordination sites in aqueous solutions
[135]. When the coordination site of a metal ion is complex, the site will not be reactive as it is no longer coordinated with a weakly bound water molecule. Since the electroless deposition only takes place at vacant or free coordination sites, the concentration of complexing agents present in an electroless plating bath is very important. When a complexing agent (L) is in solution, the free and complex sites will reach a state of equilibrium as illustrated in Equation 3-19. The corresponding equilibrium rate constant is often referred to the instability constant, as shown in Keq –
Equation 3-20, while its reciprocal is the stability constant, β, as shown in Equation
3-21 [135].
zn+− zmn − MmLML+→m Equation 3-19
znm+− zmn − KMLMLeq= []/ m Equation 3-20
zn−+− z n m β ==1/KMLMLeq m / [ ] Equation 3-21
64 where Mz+ is a metal ion and Ln- is a complexing agent. m is the number of the complexing agent. Keq and β are instability constant and stability of metal complexes respectively.
Various complexing agents form a range of metal complexes with differences in the stability constants. Metal complexes with low stability constants yield a large quantity of free metal ions as compared to the complexes with high stability constants. Since only free coordination sites are available for reduction reactions, thus smaller values of Keq (larger beta) will result in fewer free sites, causing the rate of plating to be slower. A balance is required as higher concentration of complexing agent will essentially halt the ELP process [212]. Higher concentration of complexing agent also tends to slow down the electroless reaction [212, 213]. The downside of introducing a complexing agent is that it usually substantially reduces the plating rate. Therefore, the concentration of a complexing agent should be carefully considered so as to have a sufficient amount of free metal ions available for the reaction [155, 211].
Generally, these complexing agents are organic acids or salts of organic acids, including glycolic, citric and tartaric acids or their salts [211]. The common complexing agents are ammonia (NH3), sodium citrate, sodium potassium tartrate
(Rochelle salt), and ethylenediamine-tetraacetic acie (EDTA) [211].
Stabilizing Agents
65 The stabilizers act as an inhibitor to stop the potential spontaneous deposition of electroless bath. ELP baths are inherently unstable and the presence of active nuclei such as dust or metallic particles can lead over a period of time to processes similar to homogeneous reduction. To overcome this problem, a stabilizer is added in small concentration so that it can be adsorbed onto the active nuclei and shields them form the solution.
There are four different classes of stabilizing agents namely (1) molecules containing
- - Group VI elements (S, Se), (2) molecules containing oxygen (IO3 , MoO4 ), (3) molecules containing heavy metals (Pb2+, Sb3+), and (4) organic acid molecules that are unsaturated (maleic acid) [211].
The stabilizer plays an important role during the electroless deposition. If the stabilizer content is too low, the bath becomes unstable as accompanied by the onset of homogeneous chemical reaction over the entire bulk of the solution rather than the controlled autocatalytic reaction over the specimen surface. On the other hand, too high a stabilizer concentration will result in the reaction getting inhibited [213-216].
The effect of thiourea, succinic acid and lead acetate on the formation and characteristics of the electroless Ni–P deposits obtained from an acidic hypophosphite reduced electroless nickel bath is addressed by Baskaran et al [217]. The rate of plating of electroless Ni–P coating is found to be a function of concentration of these additives. Addition of these additives also causes a change in phosphorus content of
66 the deposits. The electroless Ni–P coatings obtained in the presence of thiourea and succinic acid exhibit a nodular feature with a typical cauliflower like structure. The size of the nodules is relatively less in the latter case. In contrast, the electroless Ni–P coating obtained in the absence of additives and in the presence of 1 ppm of lead acetate is relatively smooth [214, 217].
pH
The change in pH will affect the rate of plating and the composition of the deposit. As the deposition reaction in the bath involves the production of either protons or hydroxyl ions, thus the pH of the bath will be altered during the deposition. Although some complexing agents can act as buffers, the additional pH buffers are sometimes necessary to maintain the desired pH [170]. A buffer has to be added to control the pH.
In general, the pH of a bath can have a large impact on the standard potential of the reducing agent. For example, the standard potential for both formaldehyde oxidation and hydrazine oxidation varies by ~1 V due to pH changes [170]. The oxidation of the reducing agent can result in the production of H+ or OH-, which can further change the solution pH [170].
The maintenance of a relatively narrow pH range can ensure a constant plating rate
[218]. Higher pH level has quicker deposition while lower pH level makes the reaction become slower. In addition, too high the pH destroys the chemical structure of fabric. The electroless copper and nickel baths are characterized by a plating rate
67 that firstly increases and passes through a peak, and then begins to decrease as a function of pH [155, 219-221]. Changes in pH can lead to changes in the composition of the resultant metal coating , e.g. phosphorous content in the Ni coatings [197]. The most pronounced effect of lowering the solution pH is on the composition of the deposit and hence on its properties. Lowering the pH leads to an increase in the phosphorous content of the deposit [196, 222]. The pH should be controlled so that an optimum value is maintained.
Temperature
The temperature of the ELP bath has been reported to be the most important parameter affecting the rate of plating [211]. Temperature initiates the reaction mechanism which can control the ionization process in the solution and charge transfer process from source to substrate. It was reported that the bath temperature affected the reaction rate strongly as well as the plating efficiency. The temperature influences the reaction rate through its effect on the reactive activity. Most of the reactions involved in the overall process of electroless plating are endothermic for all bath types at all pH conditions. The higher the bath temperature, the larger the reaction activity and the reaction rate will be [223]. As for the Ni baths irrespective of the reducing agent used, temperature is typically exponentially related to the rate of plating [211], indicating the typical Arrhenius behavior. However, if the temperature of the bath becomes too high, the bath will become unstable [155, 219].
68 Temperature also has an effect on the deposit composition and hence its properties
[213]. In acidic ENP baths, the deposit becomes poor in phosphorus as temperature is increased. The ideal uniform and dense metal films can be obtained at appropriate temperatures.
In addition, the deposition time, loading and bath agitation can also affect ELP.
Coating thickness increases with the prolonged increasing coating time, which is followed by parabolic rate law [197]. With an increase of coating time, the corrosion rate of the nickel-coated specimen is decreased [224]. The loading must be restricted within a certain content because too much substrate may induce self-decomposition of the plating solution. In order to achieve an integrated and homogeneous coating, it is also very important to restrict the proper loading. Finally, increasing the bath agitation may enhance the rate of plating by decreasing the external mass transfer limitations, thereby enabling the rate of diffusion to exceed the rate of electroless plating [160].
3.4 Fundamental Methodology
The present study seeks to optimize the ENP and ECP processes on fabric in terms of both technical investigation and aesthetic effects which involve materials, technologies and design. The originality of this project lies in optimizing the deposition processes and developing the nickel/copper multi-layer plating technology.
Design applications are proposed to create new metallized fabric effects.
69
First of all, it is very important to investigate the possibility of deposition reaction.
The thermodynamics of ENP and ECP on polyester fabric is studied. The effect of deposition parameters including bath compositions, pH and bath temperature on the plating rate and kinetic models which contribute to the optimization processes of nickel and copper plating is thoroughly discussed.
Conductivity and EMI SE are the most important properties of the electroless plated fabric. To obtain the excellent properties of metallized fabric, the plating solutions must be maintained under appropriate plating solution conditions. The components of the electroless nickel and copper plating baths include an aqueous solution of metal ions, reducing agents and complexing agents, operating under the specific conditions of concentration, temperature and pH. The properties of deposits are actually controlled by these conditions. Each particular electroless bath has an optimum range at which the bath should be operated. Traditionally, the optimizing variables during assay development has been performed in a series of one-factor-at-a-time experiments
[225]. This method does not account for the combined effect of all the influential factors since other factors are maintained arbitrarily at a constant level. In addition, it is time consuming and requires a large number of experiments to determine the optimum levels of the production medium [226]. However, the limitations of the single-factor optimization process can be overcome through the use of statistical experimental designs such as the two-level factorial design and response-surface
70 method (RSM) [227]. Statistical design of experiments (DOE) method, on the other hand, is more efficient to investigate the effect of several factors at one time, and has been used for decades at various phases of an optimization strategy. In this respect, factorial design and response surface design are the important tools to determine the optimal process conditions.
In order to have a better understanding of the role of each process parameter, interactions among the process parameters and optimization of process parameters as well as responses, a statistical analysis is thus essential. In this study, a statistically designed experiment known as “full factorial design” is used to evaluate the effect of deposition parameters including nickel sulfate concentration, hypophosphite sodium concentration, citric sodium, pH and temperature, on the surface resistance of electroless nickel-plated and copper-plated polyester fabrics. Initial screening of the ingredients is performed to understand their significance in the surface resistance, and then a few important ingredients are selected for further optimization. Different types of statistical method are available for such optimization experiments. The two-level factorial design is well established and widely used in the statistical design technique for screening deposition parameters. Once the important factors are identified, the next objective is to determine which actual values of these variables will result in a response value that is near to the optimum.
RSM can investigate the individual variables and the interactions of the variables
71 simultaneously. RSM was developed by Box and Wilson in 1951 to aid the improvement of manufacturing processes in the chemical industry [228]. RSM is used to optimize the parameters of a process when the function that describes it is unknown.
The procedure involves fitting a function to the given data and then using optimization techniques to obtain the optimal parameters [229]. The basis of RSM is in the form of an experimental design and the most frequently used is the central composite design (CCD) for the prediction and verification of model equation together with the optimization of the response as the function of the independent parameters. CCD, which is a rotatable experimental design with uniform precision, is used to collect data to fit the second order response surface model, and the optimal values of process parameters are determined for deposit properties. Therefore, RSM with CCD is adopted in the present study. The computations are carried out using a
Minitab software package. For fitting a second-order response model, a CCD is preferred since the interaction parameter estimates were not completely confounded
[228]. A desirability function is then employed in addition to the simultaneous optimization for the electroless nickel and copper plating. The reliability of the predicting response surface equation is evaluated using the Fisher’s F-ratio test.
Micro-structure of electroless coatings is important in determining their properties.
Deposition conditions, i.e. bath compositions, pH and temperature. determine the deposit structure and these, in turn, are reflected in the properties of the deposit. The studies of such three-way relationships are more important in metal finishing than in
72 the case of electroless plating [160]. By modifying the operating conditions, e.g., pH, temperature and compositions of plating bath, it is possible to alter the micro-structure and thus the properties of the coating [160]. Hence, controlling the deposition parameters is of significant importance to obtain an optimum coating. The present study also aims to investigate the effect of bath parameters on the deposit composition, surface morphology, crystal structure, surface resistance and EMI SE of the electroless coated fabric. These microstructure and properties are measured by EDX,
SEM, XRD, four-probe equipment and network analyzer according to the standard test methods. Furthermore, the deposit composition, surface morphology, crystal structure, surface resistance, EMI SE and anti-corrosive property of the electroless
Ni/Cu multi-layer coated fabric are compared with the nickel-plated and copper-plated polyester fabrics having the same deposit weight.
An integrated system of optimizing process for the electroless plating on fabric is summarized as: (i) thermodynamics and kinetic models for the electroless nickel and copper deposition on polyester fabric will be proposed; (ii) optimal condition will be created; (iii) the effects of deposition parameters on the plating rate, micro-structure and properties will be revealed; (iv) nickel/copper multi-layer plating will be developed for functional and decorative effect; and (v) the combination of electroless single-layer and multi-layer coating will used to develop the metallized textile design, and innovative metallized fabric has been collected.
73 3.5 Summary
The mechanisms of electroless plating, fabric substrates, ENP and ECP, and deposition parameters have been reviewed. It is confirmed that the properties of metallized textiles depend mostly on the deposition conditions. Based on the background study, the substantial technical problems of the electroless plating on fabrics have been revealed. The present available electroless plating does not provide an optimum process for the electroless plating on fabric. Subsequently, the fundamental methodology is proposed. Factorial design and RSM are selected for optimizing the methods used for the electroless plating on fabric. Functional properties and design effects also depend on deposition parameters. The originality of the present research is to develop multi-layer coating and design application of the plated fabric. A system of optimizing processes suitable for the metalized textile design application by means of ELP has been determined and summarized as: (i) the possibility of deposition reaction in thermodynamics and kinetics; (ii) optimizing processes of the ENP and ECP on fabric using the design of experiments; (iii) micro-structure and properties of the nickel, copper and nickel/copper multi-layer plating on fabric; and (iv) design applicaton of electroless nickel, copper and nickel/copper multi-layer plating on fabric.
74 Chapter 4 Thermodynamics and Kinetics of Electroless
Nickel and Copper Plating
4.1 Introduction
In this chapter, the thermodynamics of the electroless nickel and copper plating on polyester fabric is assessed. Accordingly, the effects of individual process parameters, including the concentration of nickel sulfate, copper sulfate, sodium hypophosphite, and sodium citrate, pH and bath temperature, on the plating rate of nickel and copper plating on polyester fabric are evaluated in order to provide the appropriate ranges of the plating condition for optimizing process. The kinetic study of the electroless process is conducted to establish the models for the nickel and copper plating rate with the aid of the additional experiments.
4.2 Experimental
4.2.1 Materials and Reagents
A 100% polyester plain weave fabric with white color (fabric density: 47 (warp)/cm ×
40 (weft)/cm, fabric weight: 84 g/m2) was used as the substrate material. All the chemicals used were of analytical grade. Nickel sulfate (NiSO4·6H2O), copper sulfate
(CuSO4·5H2O), sodium hypophosphite (NaH2PO2·H2O), sodium citrate
75 (Na3C6H5O7·2H2O), sodium hydroxide (NaOH), boric acid (H3BO3), tin chloride
(SnCl2·H2O), palladium chloride (PdCl2), and hydrochloric acid (HCl) and deionized water were used during solution preparation.
4.2.2 Experimental Procedure
In the present study, the electroless nickel plating and copper plating were carried out via a multi-step process including pre-treatment, sensitization, activation, electroless deposition, post-treatment to stop nickel and copper reduction, rinsing and drying.
All the fabric samples underwent pre-treatment before electroless plating. They were washed with 5% detergent at room temperature for 20 minutes and then rinsed thoroughly in deionized water. Surface sensitization was conducted by immersing the samples into an aqueous solution contained 10 g/l stannous chloride and 40 ml/l hydrochloric acid (38%) at 25°C for 10 minutes. The samples were then rinsed in deionized water and activated by immersing them in a solution containing 0.5 g/l palladium chloride and 20 ml/l hydrochloric acid (38%) at 25°C for 10 minutes. They were rinsed again in a large volume of deionized water for more than five minutes to prevent contamination of the plating bath. The samples were subsequently immersed in the electroless nickel and copper plating baths for 20 minutes respectively. Five of the most important parameters for electroless nickel plating and six of the most important parameters for electroless copper plating were varied while the others were
76 kept constant for coating deposition. Tables 4-1 and 4-2 indicate the electroless bath compositions and operating conditions used for the deposition of electroless nickel plating and copper plating respectively. Electroless plating was carried out using nickel sulfate as the source of nickel, copper sulfate as the source of copper, sodium hypophosphite as the reducing agent, sodium citrate as the complexing agent and boric acid as the buffer agent. The concentration of buffer agent used in baths was constant. Stabilizing agents were not used in this study because the plating bath was stable under the operating condition within the variable range. The pH of the plating bath was adjusted by using the sodium hydroxide aqueous solution.
Table 4-1 Bath composition and operating condition for electroless nickel plating
Bath composition Operating condition
NiSO4·6H2O (g/l) 6-21 pH 7.0-11.0
NaH2PO2·H2O (g/l) 10-50 Temperature (℃) 55-85
Na3C6H5O7·2H2O (g/l) 10-50 Time (min) 20
H3BO3 (g/l) 30
77 Table 4-2 Bath composition and operating condition for electroless copper plating
Bath composition Operating condition
CuSO4·5H2O (g/l) 4-12 pH 8.5-11.5
NiSO4·6H2O (g/l) 0.8-3.0 Temperature (℃) 60-90
NaH2PO2·H2O (g/l) 15-60 Time (min) 20
Na3C6H5O7·2H2O (g/l) 15-60
H3BO3 (g/l) 30
In the post-treatment stage, the samples were rinsed in deionized water at 40°C for 20 minutes and then dried in an oven at 60°C. All the plated samples were conditioned in accordance with the standard practice for conditioning and testing textiles (ASTM
D1776-04) before measurement.
The plating rate (v: mg/(cm2·h)) of the electroless plating was determined by the gravimetrical method and expressed in terms of the weight gain per unit of plating area and time after the deposition. An analytical balance (Shimadzu BX300 meter) with a precision of 0.1 mg was employed to weigh the plated samples. The plating rate was calculated according to the following formula [230]:
()60MM−× v = t 0 Equation 4-1 Ats ×
where v is the plating rate, t (min)is the plating duration, Mt (mg) is the mass of the object plated for a length of time, M0 (mg) is the initial weight of substrate and As
78 (cm2) is the surface area of specimen.
To reduce experimental error, 5 specimens were coated for each experimental condition. The final results were the average value of 5 measurements. The experimental error was reproducible within an error of ±5%.
4.3 Analysis
4.3.1 Thermodynamics
Electroless plating consists of the auto-catalytic reduction of metal ions with an appropriate reducing agent, leading to uniform coverage of the substrate. Electroless nickel and copper plating consist of the selective reduction of nickel ions and copper ions at the surface of a catalytic polyester fabric by immersing in an aqueous solution containing the sodium hypophosphite reducing agent. Nickel ions present in the electroless nickel plating bath and copper ions present in the electroless copper plating bath are reduced by the electrons generated through the oxidation of hypophosphite ions.
The electrochemical processes which can theoretically occur on the fabric surface are shown as follows [135]:
The cathodic partial reaction:
79 Ni2+ +→2 e Ni E0 = -0.25V Equation 4-2 Ni2+ /Ni
Cu2+ +→2 e Cu E0 = 0.34V Equation 4-3 Cu2+ /Cu
The anodic partial reaction:
In acid solution,
−−+0 HPO22+→ HO 2 HPO 23 ++22 H e E- − = -0.50V Equation 4-4 HPO/23HPO 22
In alkaline solution
−−2 − 0 HPO22+→322 OH HPO 3 ++ HO 2 e E− - = -1.57V Equation 4-5 HPO22/HPO 3
Based on the theoretical principles already mentioned, the redox potential for this process has to be more negative than that for the coating metal. In the system of the electroless nickel and copper plating using hypophosphite as the reducing agent, it is required that hypophosphite can reduce nickel ions and copper ions. When hydrogen ions (H+) and hydroxide ions (OH-) is involved in the reaction in aqueous solution system, the redox potential of the reducing agent (E = -0.50V or -1.57V) (anodic reaction) is more electronegative than that of copper (E = +0.34 V) and nickel (E =
-0.25 V) (cathodic reaction). Hence, the redox potentials of the reactants are affected by pH of the solution. The best method to know about the possibility of the reduction of nickel ions and copper ions by hypophosphite is to investigate the redox potential pH diagram of the metal ions and reducing agent. The combined potential pH diagrams for the nickel-water and phosphorus-water system and the copper-water and phosphorus-water system are shown in Figure 4-1 [170, 201]. It is obvious that the
80 ranges of potential and pH with respect to various ions, oxides and pure metals. are thermodynamically stable.
- From Figure 4-1, it is found that the potential of reducing agent (H3PO2 or H2PO2 ) is always lower than that of the nickel ions and copper ions. Hence, nickel ions and copper ions can be reduced by hypophosphite at any pH level. However, changes in pH can lead to the precipitation of metal in the bulk solution. The by-products of the anodic process may also react with the metal to form insoluble salts, e.g. nickel phosphite or copper phosphite. To overcome this problem, the complexing agents are added to the bath in order to maintain the metal ion in the solution. These chemicals depress the free metal ion concentration below that corresponding to the solubility product of the metal salt. A complexing agent also allows the bath to be operated at higher pH values. In addition, the difference between the redox potentials of the reducing agent and metal should not be very large in order to obtain thin metal films without spontaneous metal deposition in the solution. The addition of a complexing agent to a deposition solution can reduce the difference between the redox potentials of the reducing agent and metal because of the decrease in the metal redox potential due to complex formation.
81 (b) (a)
pH
Figure 4-1 The potential pH diagram for (a) the nickel-phosphorus-water system, and
(b) the copper-phosphorus-water system [170, 201]
Therefore, sodium citrate is used a complexing agent in the present study. Citrate ions, nickel ion or copper ion can form complex ion in the solution as shown in Equations
4-6 and 4-7 [135]. The equations show that the amount of free Ni2+ and Cu2+ decreases with respect to the increase in sodium citrate concentration in the solution.
23+− − Ni+→()[()] C657 H O Ni C 657 H O Equation 4-6
23+− − Cu+→()[()] C657 H O Cu C 657 H O Equation 4-7
Sodium citrate has different complexing ability with nickel ion or copper ion. The formation constants of nickel (II) ion and copper (II) ion with sodium citrate are 1014.3 and 1018.0 [231]. The complexing agent present in the electroless nickel and copper
82 solution shifts the redox potentials of Cu2+/Cu and Ni2+/Ni toward more negative values as given by the Nernst equation.
0 RT EE=−22++lnβ =− 0.67 V Equation 4-8 NiL()/ II Ni Ni/ Ni2F Ni
0 RT E =−EV22++lnβ =− 0.19 Equation 4-9 CuL()/ II Cu Cu/ Cu2F Cu
3- where L stands for C6H5O7 ;
− − NiL(II) and CuL(II) are [(Ni C657 H O )]and [(Cu C657 H O )] respectively;
0 2+ 2+ E Ni /Ni and ENiL(II)/Ni are the standard redox potential of Ni /Ni and the
redox potential of NiL(II)/Ni respectively;
0 2+ 2+ E Cu /Cu, ECuL(II)/Cu are the standard redox potential of Cu /Cu and the redox
potential of CuL(II)/Cu respectively;
2+ 2+ βNi and βCu are the formation constants of nickel (II) and copper(II)
complexes respectively;
F, R and T are Faraday constant (=96485.3399C·mol-1), gas constant
(=8.314J/K·mol) and temperature respectively.
In alkaline solution
−−2 − HPO22+→322 OH HPO 3 ++ HO 2 e
83 α 2− 0 RT HPO3 EE22−−=+ −− ln HPO322// H PO HPO 322 H PO 3 2F αα− − HPO22 OH 0.059 1 =+E0 lg HPO2−−/ H PO 3 Equation 4-10 322 2 α OH − =−2.81 + 0.09pH ( V )
0 2- - 2- - where E HPO3 /H2PO2 and E HPO3 /H2PO2 are the standard redox potentials and redox
2- - potentials of HPO3 /H2PO2 respectively.
The redox potential of NiL (II) / Ni and CuL (II) / Cu is still more positive than that of
- - HPO3 /H2PO2 in alkaline solution. The difference of the cathodic standard potential
(metal being reduced) and the anodic standard potential (reducing agent) is as follows.
Δ=EE2-+− −E/ =2.14-0.09pH (V)>0 Equation 4-11 Ni/HPO Ni23 H 22 PO
Δ=EE2-+− −E/ =2.62-0.09pH (V)>0 Equation 4-12 Cu/HPO Cu23 H 22 PO
Positive ΔE indicates that the reaction is thermodynamically favorable, leading to spontaneous, homogenous redox reactions.
However in the bath using hypophosphite as reducing agent, the catalytic activities of metals for the oxidation of hypophosphite decrease in the following order: Au > Ni >
Pd > Co > Pt > Cu [155]. Hence, the reaction of electroless nickel plating in the hypophosphite-type baths can be carried out without a catalyst. It is obvious that the reaction of electroless copper plating in the hypophosphite-type baths is impossible
84 without a catalyst. The Ni2+ ions have been used as catalysts for electroless copper plating in the present study. A very small amount of co-deposited nickel present in the copper deposit serves to catalyze the oxidation of hypophosphite enabling the continuous copper deposition. Therefore, the deposits obtained often consist of Cu-Ni alloy rather than pure copper.
As for the electroless copper plating, the standard redox potential of Cu2+/Cu is more positive than that of Ni2+/Ni. The use of a complexing agent not only stabilizes the solution, but can also bring the deposition potential of the two ions closer together by shifting the potentials to different amounts, making the co-deposition possible.
Although the formation constants of copper (II) complexes are higher than that of nickel (II) complexes, the redox potential of CuL (II) / Cu is more positive than that of NiL (II) / Ni in the solution. When the Pd-activated fabric substrate is immersed into the electroless copper solution, the copper (II) ions are preferentially reduced with respect to nickel ions. In addition, the concentration of deposited nickel is constrained by the displacement reaction with copper (II) [165]
Cu22+++→ Ni Cu + Ni Equation 4-13
The redox reaction in Equation 4-13 is thermodynamically possible due to differences in the redox potentials of Ni2+/Ni and Cu2+/Cu. The nickel particles return to the solution again. As a result, the concentration of co-deposited nickel in the deposit is
85 low and so it is beneficial to electroless copper plating.
4.3.2 Effects of Plating Parameters on Plating Rate
4.3.2.1 Electroless Nickel Plating
The composition of the plating bath used for electroless nickel plating on polyester fabric included nickel sulfate as the source of nickel, sodium hypophosphite as the reducing agent and sodium citrate as the complexing agent. The levels of any four of the five factors were kept constant while the other one was studied in order to investigate its effect on the nickel plating rate. This investigation was repeated with each factor.
Composition of the nickel plating bath
(1) Nickel Sulfate
The effect of nickel sulfate concentration on the plating rate is shown in Figure 4-2.
There is a nearly linear relationship between the nickel ion concentration and the plating rate. It can be seen that the rate rises with respect to the increase in nickel sulfate concentration. Nickel sulfate is the main contributor for the nickel ions of the deposit. When the nickel sulfate concentration is decreased, the amount of nickel reduced by the reducing agent becomes small, causing the plating rate to drop. As the
2+ - nickel sulfate concentration is increased, the Ni :H2PO2 ratio will also rise, making the release of Ni2+ ions from the complexing ion group increase gradually. These
86 nickel ions are reduced by the electrons released from the reducing agent to become neutral nickel atoms which will deposit on the catalyzed fabric substrate. The plating
2+ - rate, therefore, increases with the Ni : H2PO2 ratio. However, the solution will decompose when the concentration of nickel ions is approximately 21 g/l.
Figure 4-2 Effect of nickel sulfate concentration on the plating rate of nickel plating
(2) Sodium Hypophosphite
Figure 4-3 shows the effect of NaH2PO2 concentration on the plating rate. The plating rate rises as the concentration of NaH2PO2 increases. A similar phenomenon was observed by Li and An [193]. The reducing power of the bath increases when the concentration of reducing agent is raised, i.e. the number of the neutral nickel atoms reduced from the reducing agent increases causing the plating rate to increase.
However, when the reducing agent is in excess, i.e 50 g/l NaH2PO2, the stability of the bath deteriorates or even decomposes, resulting in the decrease in the number of reduced neutral Ni atoms and making the plating rate drop.
87
Figure 4-3 Effect of sodium hypophosphite on the plating rate of nickel plating
(3) Sodium Citrate
Variation of the plating rate as a function of the complexing agent (sodium citrate) concentration is presented in Figure 4-4. The present experiments have shown that deposition does not proceed if the concentration of sodium citrate is less than 10 g/l because of the precipitate formed during the heating of the bath. It is evident that the plating rate drops as the sodium citrate is increasing. Similar results in the electroless nickel plating have been observed by other researchers [221, 232, 233]. However, if the complexing agent of the plating bath is increased to 50 g/l the bath stability will become better, but the plating rate drops. Low plating rate can slow down the initiation of the electroless nickel on the fabric surface, thereby reducing the coating.
88
Figure 4-4 Effect of sodium citrate on the plating rate of nickel plating
Operating condition
(1) pH
The pH of the electroless nickel plating solution is an important factor affecting the rate of hydrogen evolution. Figure 4-5 shows the relationship between the plating rate and pH value. The plating rate increases with the rise of pH initially but decreases after reaching the maximum at pH 8.5. It is found that the deposit is difficult to form when the pH is less than 7. However, when the pH reaches 11, the bath will decompose. The plating rate approaches the maximum at the pH 8.5.
The phenomenon is explained by Equation 3-8 such that the greater the OH- in the solution, the more the deposition of nickel will be. At low pH, the concentration of
OH- ion present in the solution is low. When the pH is less than a certain value, there will be no deposit of nickel atoms formed at all. As the pH is raised, the reaction is
89 shifted to the right of Equation 3-8. As a result, the plating rate increases. When the pH reaches a certain value, e.g. 8.5, two aspects will become more significant as the pH is raised continuously. On the one hand, according to the reaction mentioned in
Equation 3-8, hydroxide ions will affect the plating rate kinetically; partically the plating rate is faster at higher pH. On the other hand, hydroxide ion can also affect the plating rate thermodynamically. The complexing ability is sensitive to the pH of the bath. Usually, the higher the pH, the stronger the complexing ability will be. Hence, it is more difficult to carry out the reduction of nickel (II) complex ion thermodynamically at high pH. Therefore, it is possible to select an optimum pH for the electroless nickel plating. Both of these actions can cause the plating rate to decrease, and so this is the reason to explain why the plating rate presents a maximum at a particular pH value.
Figure 4-5 Effect of pH of the plating solution on the plating rate of nickel plating
(2) Bath Temperature
90 The temperature of the plating bath significantly affects the rate of the deposition process [193]. The variation of plating rate with the bath temperature is shown in
Figure 4-6. Below 50℃ , it will take considerately more time for the reaction to start and the plating rate is also very low. As the temperature is raised, the plating rate increases accordingly. However, when the temperature of the bath is increased to
85℃ , it is very difficult to maintain the pH of the plating bath. This is due to the fact that the chemical composition of the bath becomes unstable and tends to decompose at higher temperatures [234, 235]. In order to maintain the stability of the plating bath, it is necessary to carefully control the bath temperature.
Figure 4-6 Effect of bath temperature on the plating rate of nickel plating
4.3.2.2 Electroless Copper Plating
The electroless copper plating bath contained a lot of components including copper sulfate, sodium hypophosphite, nickel sulfate and citrate sodium as metal source,
91 reducing agent, catalyst and the complexing agent respectively. The plating rate can be affected by the concentration of copper ions, nickel ions, hypophosphite ions and complexing agent as well as pH and temperature. When five of the six factors are fixed as constants, the other factor will be studied to evaluate its effect on the copper plating rate, respectively.
Compositions of the copper plating bath
(1) Copper Sulfate
The effect of the concentration of copper sulfate, on the plating rate is shown in
Figure 4-7. It is found that the plating rate is enhanced with respect to the increase in the concentration of copper sulfate in the bath. If the concentration of CuSO4 is higher than 12 g/l, the plating bath becomes unstable.
Figure 4-7 Effect of copper sulfate concentration on the plating rate of copper plating
92 (2) Nickel Sulfate
Nickel ions are required in the solution to maintain the plating rates because nickel has the catalytic activity suitable for the oxidation of hypophosphite. As reflected by the pattern of the curve, the effect of concentraion of Ni2+ ions on the plating rate as shown in Figure 4-8 is similar to that of the effect of copper sulfate concentration on the plating rate as shown in Figure 4-7. The deposits can not be obtained when the
NiSO4 concentration is below 0.8 g/l. The plating rate increases at higher concentration of Ni2+ in the bath. If the content of Ni2+ ions is too high, e.g 3 g/l
NiSO4, the plating rate will be excessively high and the shape of the plated film is rough.
Figure 4-8 Effect of nickel sulfate concentration on the plating rate of copper plating
(3) Sodium Hypophosphite
It is found that increasing the concentration of hypophosphite can rapidly enhance the
93 plating rate as shown in Figure 4-9. However, an excessive amount of hypophosphite
(60 g/l NaH2PO2) will cause the bath to become unstable and the deposited surface is also rough.
Figure 4-9 Effect of sodium hypophosphite concentration on the plating rate of copper
plating
(4) Sodium Citrate
In Figure 4-10, it is also found that as the concentration of the complexing agent is increasing, the plating rate will decrease. The complexing agent can affect the plating rate and quality of the deposited layer [156]. If the concentration of citrate is too high, e.g. 60 g/l, the electroless copper deposition reaction will stop. On the contrary, copper hydroxide will be produced, and the bath can not be properly operated if the concentration of citrate is insufficient. Hence, it is important to maintain the concentration of citrate in a suitable quantity.
94
Figure 4-10 Effect of sodium citrate concentration on the plating rate of copper
plating
Operating condition
(1) pH
Bath pH is another important operating parameter in the electroless copper plating. It affects both anodic and cathodic reactions as well as the nature of the complex species.
The effect of pH on the plating rate is shown in Figure 4-11. It is impossible to get any deposit from the plating solution with the pH less than 8.5. When the pH is raised from 8.5 to 10.5, the plating rate will increase accordingly. It is well known that the oxidation of hypophosphite can be enhanced by the presence of hydroxyl ions [155].
If the pH is higher than 11.5, the copper plating bath becomes unstable. The similar behavior is also observed in the electroless Ni–Co–P alloys plating [134] and electroless Ni–Fe–P alloys plating [236].
95
Figure 4-11 Effect of pH of the plating solution concentration on the plating rate of
copper plating
（2）Bath Temperature
The hypophosphite-type bath must be operated at higher temperature which is advantageous to the reaction [237]. It is found that increasing the operating temperature will enhance the plating rate as shown in Figure 4-12. However, the high bath temperature exceeding 90℃ will cause the copper surface to become rough and make the bath decompose.
96
Figure 4-12 Effect of bath temperature on the plating rate of copper plating
4.3.3 Establishment of Kinetic Models
From the results of effect of plating parameters on the plating rate, it is found that the concentration of nickel sulfate, sodium hypophosphite and sodium citrate, pH and temperature influence the plating rate of electroless nickel plating on polyester fabric.
The concentration of copper sulfate, nickel sulfate, sodium hypophosphite and sodium citrate, pH and temperature also affect the plating rate of electroless copper plating on polyester fabric. Therefore, the plating rate of electroless nickel and copper plating on fabric are determined by all these deposition parameters. Kinetic investigation aims to predict the plating rate in terms of different concentration of components, and operation condition.
The aim of chemical kinetics is to predict the rate of chemical reaction and to delineate the path or paths by which the reactants proceed to products, i.e. the reaction
97 mechanism. Usually, reaction rate is determined under the condition of constant temperature, preferably at several temperatures. In a typical kinetic experiment, the data are the records of concentrations of reactant and product species taken at various times and constant temperatures [211, 238].
Theoretical expression for reaction rate as function of the concentrations of reactants and products is a differential equation of the form:
dC Rate=±i = f( C , C ,..., C ) Equation 4-14 dt AB N
where t denotes the time, and Ci is the time-dependent concentration of the ith species
(+ for product, - for reactant) that is followed to determine the reaction rate.
Generally, the concentrations of the reactants are known at the moment of mixing.
The progress of the reaction is then studied as a function of time by any number of available techniques. Similar experiments may then be performed by using different initial concentrations of the reactants at different temperatures.
The rate of a reaction may be determined by measuring the rate of decrease in the concentration of a particular reactant, or conversely, the rate of increase of by-products. In the case of electroless plating, the rate of plating is most commonly determined by measuring the rate of weight gain of the deposit in mg/cm2 ·h, or the
98 rate of deposit thickness increase in μm/hr.
The next step is to express the resultant data obtained from the rate measurements in a simple mathematical form; i.e. an expression represented by the function f in Equation
4-14 is needed for the reactants and products. Usually, the relationship is expressed as:
dCA Rate=± = k' Cαβγ C C ... Equation 4-15 dt ABc
where k’ is a numerical proportionality constant called the rate constant;
CA, CB and Cc, etc. refer to the concentrations of the chemical species A, B, C
and so forth, present in the reacting system at time t;
α, β and γ etc. are called the reaction orders of the respective chemical species.
In terms of this empirical rate equation, the particular reaction is said to be α order with respect to A, β order with respect to B etc.; and α + β + γ… is the overall reaction order. The individual time derivatives dCA/dt, dCB/dt, dCc/dt are referred to as the rate of the reaction with respect to species A, B and C respectively.
The dominant variable in most reactions is the temperature. Many reactions occurring close to room temperature double their rates for each 10℃ rise in temperature. It is noted that 100℃ is considered close to room temperature. The expression of the rate
99 constant, k’, in Equation 4-15 is given in the form:
Ea kk'=− exp( ) Equation 4-16 1 RT
where k1 is the frequency factor, Ea is the activation energy, T is the absolute temperature °K, and R is the universal gas constant.
According to the empirical rate law, Equation 4-15 can now be written as:
dC Ea Rate==A k[ C ]αβ [ C ] ...[ C ] γ exp( − ) Equation 4-17 dt1 aB N RT
4.3.3.1 Electroless Nickel Plating
Based on those variables that affect the plating rate, an the plating rate equation for nickel plating can be derived from Equations 4-18 and 4-17 [239, 240].
Ni23+−++ H PO C H O −− +→ OH 22 657 Equation 4-18 23−− Ni++ P HPO365722 + C H O + H + H O
Ea vkNiHPOOHCHO=−[23+−−− ]αβγδ2-ε [ ] [ ] [ ][HPO ] exp( ) Equation 4-19 n 1226573 RT
2 where vn is plating rate of electroless nickel deposition, mg/ cm ·h;
Ea is activation energy, J/mol;
T is plating temperature, K;
100 R is gas constant, R = 8.314 J/ (mol·K);
k1, k2 are rate constants;
2+ - - 3- 2- [Ni ], [H2PO2 ], [OH ], [C6H5O7 ] and [HPO3 ] are concentrations of
- NiSO4·6H2O, NaH2PO2·H2O, OH , Na3C6H5O7 ·2H2O and Na2HPO3
respectively, mol/L;
α, β, γ, δ and ε stand for the reaction orders.
Put Equation 4-19 into logarithmic form to yield:
Ea lgvk=+α lg[ Ni23+− ] + β lg[ HPO ] + γ ( pH − 14) + δ lg[ CHO − ] +ε lg [HPO2- ] − n 2226573 2.303RT Equation 4-20
+ where k2=lgk1, and lg(H ) = -pH.
According to Equation 4-20, the reaction orders, α, β, γ, δ and ε, and the activation energy, Ea, can be evaluated:
∂ lg v ()n = α Equation 4-21(a) ∂lg[]Ni2+ q
∂ lg vn ()− u = β Equation 4-21(b) ∂lg[]HPO22
∂ lg v ()n = γ Equation 4-21(c) ∂pH w
∂ lg vn ()3− x = δ Equation 4-21(d) ∂lg[]CHO657
101 ∂ lg vn ()2- y = ε Equation 4-21(e) ∂lg[HPO3 ]
∂ lg v Ea ()n =− Equation 4-21(f) ∂(1/TR )z 2.303
The subscripts, q, u, w, x, y and z, denote each set of variables held constant for the particular partial derivative.
The values of certain reaction orders can be ascertained, though not rigorously, to form the discussion concerning the effect of the respective variables on the plating rate. As the phosphate accumulates in the bath, the relatively insoluble nickel phosphate will commence to precipitate. When these phosphite-induced phenomena are occurring, the soluble phosphite has little or no effect on the plating rate. In other
- words, the partial derivation, Equation 4-21 (e), vanishes, i.e., ε = 0 and [HPO3 ] = 1.
The remaining reaction orders, α, β, γ and δ can be determined by plotting lgv versus lg (concentration of species). The slopes of the straight lines obtained in these plots are the reaction orders.
Equation 4-19 can be simplified to: Ea vkNiHPOOHCHO=−[23+−−− ]αβγδ [ ] [ ] [ ] exp( ) Equation 4-22 n 122657 RT
The activation energy, Ea is computed from the slope of the straight lines obtained in the lgv versus 1/T plots, i.e.:
102 ∂ lg v Ea ()n =− =slope ∂(1/TR ) 2.303
Ea = 2.303(8.314J mol-1 K -1 ) (slope)
In order to simply computations, Equation 4-22 can be put in the following form: EaEaEa vkNiHPOOHCHO=−−[23+−−− ]αβγδ [ ] [ ] [ ] exp( )exp( )exp( ) n 122657 RTR333 333 R Equation 4-23
Since Ea Ea exp( )exp(−= ) 1 333RR 333 Thus after rearranging: Ea T − 333 vkNiHPOOHCHO= [23+−−− ]αβγδ [ ] [ ] [ ] exp[ ( )] Equation 4-24 n 222657333RT
Ea where kk=−exp( ) 21 333R
Taking the logarithm of Equation 4-24 to get:
Ea T − 333 lgvk=+αβ lg[ Ni23+− ] + lg[ HPO ] + γ ( pH − 14) + δ lg[ CHO − ] − ( ) n 3226572.3RT T Equation 4-25 where k3= lg k2
It can be found from Equation 4-25 that the relationship between any factor and the plating rate should be linear when the other factors remain unchanged. The slopes of
103 the straight lines are α, β, γ, δ and -Ea/ (2.303R) respectively. The straight line plots for the reaction orders are extrapolated to the point where the concentrations of the appropriate components are unity. At this point, the respective plating rates can be read from the ordinate. The slope value and linear correlation coefficient of each logarithmic relation are graphically obtained and listed in Table 4-3. The data in the second column of Table 4-3 exhibit that all the six logarithmic relations basically are linear.
Table 4-3 Logarithmic relationship between the factors and plating rate for electroless
nickel plating
Logarithmic relation Linear correlation Slope or Ea
coefficient
2+ lg vn - lg [Ni ] 0.997 α=0.765
- lg vn - lg [H2PO2 ] 0.991 β=0.444
lg vn - lg pH, when pH 0.917 γ=0.242
<8.5
lg vn – lg pH >8.5 -0.964 δ=-0.079
3- lg vn - lg [C6H5O7 ] -0.955 ε=-0.579
-1 lg vn – 1/T -0.946 Ea=20.16 kJ·mol
Substituting the slope values and Ea value into Equation 4-24 to get:
104 20160T − 333 vkNi= [23+−−− ]0.765 [ HPOOHCHO ] 0.444 [ ] 0.242 [ ] -0.579 exp[ ( )] n 222657333RT when pH <8.5 Equation 4-26 and 20160T − 333 vkNi= '[23+−−− ]0.765 [ HPOOH ] 0.444 [ ] -0.079 [ CHO ] -0.579 exp[ ( )] n 222657333RT when pH >8.5 Equation 4-27
The rate constants, k2 and k2’, can be computed by solving Equations 4-26 and 4-27,
3 -2 when T = 60℃ (333K). Therefore the plating rate constants of k2 = 1.58×10 l cm
-1 -2 -1 h and k2’=26.52 l cm h can be obtained by the plating rates corresponding to different operation conditions at 60℃ . Substituting k2 and k2’ into Equations 4-26 and
4-27 obtains the plating rate equations of electroless nickel plating as follows:
T − 333 vNiHPOOHCHO=×1.58 1032 [+−−− ]0.765 [ ] 0.444 [ ] 0.242 [ 3 ] -0.579 exp[7.28( )] n 22 657 T when pH <8.5 Equation 4-28 T − 333 vNiHPOOHCHO= 26.52[23+−−− ]0.765 [ ] 0.444 [ ] -0.079 [ ] -0.579 exp[7.28( )] n 22 657 T when pH >8.5 Equation 4-29
4.3.3.2 Electroless Copper Plating
Similarly, the relationship between the plating rate of electroless copper plating and the investigated factors can be expressed as follows [239, 240]:
22++ − 3 − − Cu++ Ni H22 PO + C 657 H O + OH → 23−− Equation 4-30 Cu+++ Ni P HPO365722 + C H O + H + H O
105 22++αβ − γ − δ 3 − ε vkCuNiHPOOHCHOfTc = 122657[][][][][ ]() Equation 4-31 EaEaEa vkCuNiHPOOHCHO=−−[22++ ]αβ [ ] [ − ] γ [ − ] δ [ 3 − ] ε exp( )exp( )exp( ) c 122657RTR343 343 R
Equation 4-32 Ea T − 343 v= k[ Cu22++ ]αβ [ Ni ] [ H PO − ] γ [ OH − ] δ [ C H O 3 − ] ε exp[ ( )] c 222657343RT Equation 4-33
2 where vc is plating rate of electroless copper deposition, mg/ cm ·h;
α, β, γ, δ and ε are constants; Ea kk=−exp( ) . 21 343R
Taking the logarithm of Equation 4-33 to get: Ea lgvk=+α[Cu]+β lg2+ lg[ Ni23+− ] +γδε lg[ HPO ] + ( pH − 14) + lg[ CHO − ] − c 3226572.3RT
Equation 4-34 where k3= lg k2
It can be found from Equation 4-34 that the relationship between any factor and the plating rate should be linear when the other factors remain unchanged. The slopes of the straight lines are α, β, γ, δ, ε and -Ea/ (2.3R) respectively. The slope value and linear correlation coefficient of each logarithmic relation are graphically obtained and listed in Table 4-4. The data in the second column of Table 4-4 exhibit that all the six
106 logarithmic relations basically are linear.
Table 4-4 Logarithmic relationship between the factors and plating rate for electroless
copper plating
Logarithmic relation Linear correlation Slope or Ea
coefficient
2+ lg vc - lg [Cu ] 0.996 α=0.745
2+ lg vc - lg [Ni ] 0.986 β=0.436
- lg vc - lg [H2PO2 ] 0.890 γ=0.330
lg vc - pH 0.984 δ=0.243
3- lg vc - lg [C6H5O7 ] 0.945 ε=-0.424
-1 lg vc – 1/T -0.984 Ea=14.82kJ·mol
Substituting the slope values and Ea value into Equation 4-33 to get: 14820T − 343 vkCu= [22++ ]0.745 [ Ni ] 0.436 [ HPO − ] 0.330 [ OHCHO − ] 0.243 [ 3 − ] -0.424 exp[ ( )] c 222657343RT Equation 4-35
4 -2 -1 Therefore, the plating rate constants of k2 = 1.16×10 l cm h is calculated by the plating rates corresponding to different operation conditions at 70℃ . Substituting k2 into Equation 4-33 obtains the plating rate equations of electroless copper plating as follows:
107 T − 343 vCuNiHPOOHCHO=×1.16 1042 [++ ]0.745 [ 2 ] 0.436 [ − ] 0.330 [ − ] 0.243 [ 3 − ] -0.424 exp[5.20( )] c 22 657 T
Equation 4-36
4.3.4 Verification of Models
The proof tests of the kinetic models were experimentally carried out through 20 sets of test with different conditions in order to check their accuracy and practicability.
The plating rate (vcal) calculated by the models was compared with those of the experimentally measured plating rates (vexp). The results of the electroless nickel and copper plating listed in Tables 4-5 and 4-6 respectively show that most of the calculated values (vcal) are close to the experimental values (vexp). Their relative errors are within the range of 0% to ±5% except those of the second and fourteenth level groups for the electroless nickel plating, and those of the second, ninth and seventeenth groups for the electroless copper plating. The overall results obtained confirm a good consistency of the models with the experiment.
108 Table 4-5 Comparison of experimental values (vexp) with calculated values (vcal) by
kinetic models for electroless nickel plating
Run NiSO4 NaH2PO2· pH Na3C6H5O7 T vexp vcal Errors
2 2 ·6H2O H2O (g/l) ·2H2O (g/l) (℃) (mg/cm (mg/cm (%)
(g/l) h) h)
1 6 15 8 20 60 6.29 6.16 2.11
2 7 20 8.5 20 60 9.63 10.41 -7.49
3 8 25 9 20 60 11.90 11.34 4.94
4 9 30 9.5 20 60 12.78 12.32 3.73
5 10 15 10 20 60 9.35 8.95 4.47
6 11 20 8 20 60 10.61 11.11 -4.50
7 12 25 8.5 25 60 14.97 15.31 -2.22
8 13 30 9 25 60 14.89 15.66 -4.92
9 14 15 9.5 25 60 10.94 11.18 -2.15
10 15 20 10 25 60 12.65 12.23 3.43
11 16 25 8 25 60 15.10 14.46 4.43
12 6 30 8.5 25 70 12.43 12.09 2.81
13 7 15 9 25 70 8.54 8.92 -4.26
14 8 20 9.5 25 70 10.77 10.25 5.07
15 9 25 10 25 70 11.49 11.31 1.59
16 10 30 8 30 70 12.39 12.13 2.14
17 11 15 8.5 30 70 12.27 12.69 -3.30
109 Run NiSO4 NaH2PO2· pH Na3C6H5O7 T vexp vcal Errors
2 2 ·6H2O H2O (g/L) ·2H2O (g/l) (℃) (mg/cm (mg/cm (%)
(g/l) h) h)
18 12 20 9 30 70 13.23 13.77 -3.92
19 13 25 9.5 30 70 15.19 14.75 2.98
20 14 30 10 30 70 14.80 15.45 4.20
Table 4-6 Comparison of experimental values (vexp) with calculated value (vcal) by
kinetic models for electroless copper plating
Run CuSO4 NiSO4 NaH2PO2· pH Na3C6H5O7 T vexp vcal Error
·5H2O ·6H2O H2O (g/l) ·2H2O (K) (mg/c (mg/c s
(g/l) (g/l) (g/l) m2 h) m2 h) (%)
1 4 0.8 20 9 20 70 3.98 4.15 -4.10
2 5 1.2 25 9.5 25 70 8.05 8.55 -5.85
3 6 1.6 30 10 30 70 15.05 14.434.30
4 7 2.0 35 10.5 35 70 24.24 23.25 4.26
5 8 0.8 20 9 20 70 6.87 7.14 -3.78
6 9 1.2 25 9.5 25 70 13.68 13.263.17
7 10 1.6 30 10 30 70 21.62 21.122.37
8 4 2.0 35 10.5 35 70 13.95 13.58 2.72
9 5 0.8 20 9 20 70 5.90 5.53 6.69
10 6 1.2 25 9.5 25 70 9.36 9.78 -4.29
110 Run CuSO4 NiSO4 NaH2PO2· pH Na3C6H5O7 T vexp vcal Error
·5H2O ·6H2O H2O (g/l) ·2H2O (K) (mg/c (mg/c s
(g/l) (g/l) (g/l) m2 h) m2 h) (%)
11 7 1.6 30 10 30 70 16.68 16.193.03
12 8 2.0 35 10.5 35 70 22.87 23.35 -2.06
13 9 0.8 20 9 20 70 8.33 8.58 -2.91
14 10 1.2 25 9.5 25 80 17.22 16.643.49
15 4 1.6 30 10 30 80 10.43 10.97-4.92
16 5 2.0 35 10.5 35 80 21.67 20.98 3.29
17 6 0.8 20 9 20 80 7.94 7.35 8.03
18 7 1.2 25 9.5 25 80 12.56 12.761.57
19 8 1.6 30 10 30 80 20.02 20.75-3.52
20 9 2.0 35 10.5 35 80 31.09 32.51 -4.37
4.4 Summary
In this chapter, the thermodynamics and kinetics of the electroless nickel and copper plating on polyester fabric are examined. The results showed that the electroless nickel and copper deposited on polyester fabric could be carried out thermodynamically. The plating rates of nickel and copper were correlated with the major plating composition and condition. The effects of plating parameters including the concentration of nickel sulfate, sodium hypophosphite and sodium citrate, pH and
111 temperature of the plating bath on the plating rate of electroless nickel plating were evaluated. Furthermore, the effects of copper sulfate, nickel sulfate, sodium hypophosphite, sodium citrate, pH and temperature of the plating bath on the plating rate of electroless copper plating on polyester fabric wer reported. As for the electroless nickel and copper deposition, the plating rates enhanced with respect to the increase in the concentration of nickel sulfate, copper sulfate and sodium hypophosphite, pH and bath temperature. However, the plating rates decreased with respect to the increase in sodium citrate. The kinetic models, i.e. the nickel and copper plating rate equations of the self-catalysis reaction, were established and proved to be consistent with the practical measurements through experimental verification.
112 Chapter 5 Optimization of Electroless Nickel and Copper
Plating
5.1 Introduction
EMI SE is one of the important properties of metallized textiles and is closely related to their surface resistance. Although low surface resistance is not a criterion for EMI shielding, higher electrical conductivity will result in higher EMI shielding because of the reflection effect. Therefore, by determining the surface resistance, their EMI SE can be estimated. The purpose of this chapter is to develop an electroless plating solution in order to obtain a relatively low surface resistance. The present study involves the optimization of different parameters affecting surface resistance. The objective of the present work is to apply the statistical methods to optimize the surface resistance of the electroless nickel and copper plating on polyester fabric. Several important factors of surface resistance were screened by the full factorial design (FFD) and the optimum deposition conditions were obtained by response surface method
(RSM) with desirable function.
5.2 Experimental Design
Statistical experimental designs are applied in two steps. The first step is to identify
113 the factors that have a significant effect on the objective of the optimization using
FFD. The second is to optimize the significant deposition parameters obtained from
FFD by using a central composite design (CCD) in RSM. The experimental design and statistical analysis of the data are done by using the Minitab 15 software (Minitab
Inc., Pennsylvania, USA).
5.2.1 Full Factorial Design (FFD)
To identify the important electroless deposition parameters, a factorial design has been employed for each complete trial or replication of the experiment, and all the possible combinations of different levels of the factors have been investigated. For any experiment which have k factors, each at only two levels, i.e. high (+1) and low
(−1), they are known as 2 level factorial design (2k). The number of experimental run required to complete a replicate is given by 2 × 2 ×. . .× 2 = 2k where k is the number of factor, thus giving it its name [241, 242]. In the present study, according to the results of Chapter 4, the five factors which were considered to affect the electroless nickel plating and copper plating were (1) NiSO4·6H2O concentration, (2)
NaH2PO2·H2O concentration, (3) Na3C6H5O7·2H2O concentration, (4) pH and (5) temperature. The H3BO3 concentration and deposition time were kept at 30 g/l and 20 minutes respectively, while the CuSO4·5H2O concentration was kept at 10g/l for the electroless copper plating. Coded values were used since experimental parameters had different units and ranges in the experimental domain making the regression analysis
114 impossible to be performed. Coded variables were forced to range from −1 to +1 so that they would response more evenly and the unit of the parameters was irrelevant.
The equation commonly used for coding is as follows:
xii=−()/XX0 Δ X i Equation 5-1 where Xi stands for the real value of variable;
X0 is the real value of the Xi at the center point;
△Xi is step change in Xi;
xi represents the coded value of the variable; i = 1, 2, 3.
The experimental levels were selected for these variables according to the results of
Chapter 4, indicating that an optimum could be found within the level of parameters being studied. The factors and levels of parameters used for the experimental design are shown in Table 5-1.
Table 5-1 Factors and levels for two level factorial design
Variables Factors Levels
Electroless nickel plating Electroless copper plating
-1 1 -1 1
A NiSO4·6H2O (g/l) 12 18 1.0 2.0
B NaH2PO2·H2O (g/l) 20 30 20 40
C Na3C6H5O7·2H2O 10 20 10 30
(g/l)
115 D pH 7.5 8.5 9.5 10.5
E T ()℃ 70 80 70 80
A 25 (32) two level full factorial design was employed and analyzed by the statistical software of Minitab release 15. The detailed experimental designs with the coded values of five parameters are shown in Table 5-2.
A four-point probe method was used for measuring the surface electrical resistance
(Rs) of the deposited films as described in ASTM F 390 (SDY-4D four point-support probe produced by GuangZhou Semiconductor Institute). Rs is considered to be the resistance of a square sample (1cm×1cm), and the unit of Rs is commonly expressed as ohm per square or Ω/sq. The surface resistance was taken as the response
(dependent variable) in the experimental design (Y). These experiments were performed in a randomized order according to the run number that was arranged by the software. The response values were the mean of three duplicate measurements.
After processing the experimental trials, the surface resistance of different trials was plotted and the sum of the surface resistance values of each trial was subsequently calculated. By computing the sum of the surface resistance values of each trial with the help of the Minitab software, the dominant factors of the results could then be assessed by the Pareto chart, main effect interaction plot and interaction effect of the five different factors. The factors which were identified as important or significant were then investigated more thoroughly in the subsequent experiment.
116
Table 5-2 Design matrix and experimental data obtained from the 25 full factorial
design for the electroless nickel and copper plating
Surface resistance (mΩ Run A B C D E /sq) Electroless Electroless nickel copper plating plating 1 -1 -1 -1 -1 -1 1919.4 898.14
2 1 -1 -1 -1 -1 1187.9 403.1
3 -1 1 -1 -1 -1 1753.9 992.69
4 1 1 -1 -1 -1 841.35 395.18
5 -1 -1 1 -1 -1 1904.2 1142.95
6 1 -1 1 -1 -1 901.2 485.69
7 -1 1 1 -1 -1 1232.6 810.54
8 1 1 1 -1 -1 810.1 363.58
9 -1 -1 -1 1 -1 1886.4 258.61
10 1 -1 -1 1 -1 502.5 98.62
11 -1 1 -1 1 -1 1401.2 184.3
12 1 1 -1 1 -1 720.4 58.18
13 -1 -1 1 1 -1 1705.4 370.15
14 1 -1 1 1 -1 995.6 85.25
15 -1 1 1 1 -1 1814.6 216.93
16 1 1 1 1 -1 925.8 103.17
117 Surface resistance (mΩ Run A B C D E /sq) Electroless Electroless nickel copper plating plating 17 -1 -1 -1 -1 1 1352.9 589.42
18 1 -1 -1 -1 1 720.4 262.74
19 -1 1 -1 -1 1 1281.2 743.65
20 1 1 -1 -1 1 360.19 329.54
21 -1 -1 1 -1 1 1163.7 1325.39
22 1 -1 1 -1 1 412.54 224.81
23 -1 1 1 -1 1 1085.2 446.57
24 1 1 1 -1 1 725.6 293.26
25 -1 -1 -1 1 1 902.5 109.25
26 1 -1 -1 1 1 385.24 98.23
27 -1 1 -1 1 1 673.6 143.64
28 1 1 -1 1 1 301.52 60.15
29 -1 -1 1 1 1 1325.3 181.4
30 1 -1 1 1 1 349.82 63.31
31 -1 1 1 1 1 1254.3 103.15
32 1 1 1 1 1 340.26 89.25
5.2.2 Response Surface Method (RSM)
The deposition parameters that significantly affect the surface resistance are
118 optimized by CCD. The CCD experimental design is preferred and widely used for fitting a second order model represented as follows:
2 Yb=+0 ∑∑ bXii + bX iii + ∑ bXX iji j Equation 5-2
where Y is the response;
X represents the variables of the system;
i and j are design variables;
b represents model coefficients that are estimated using a least square fit of the
model to the experimental results obtained during the design runs.
The concentration of NiSO4 and temperature of plating bath for the electroless nickel plating, as well as the concentration of NiSO4 and pH of plating bath for the electroless copper plating were respectively left to be optimized after the full factorial experiments. To define the optimum setting of these two factor levels, a 25 CCD was selected for two experimental variables, each at two levels with four replicates at the center points which required 12 experiments to be performed to fit the response data into the second-order model. The four replicates at the center points were carried out in order to estimate the pure error variance. The statistical software Minitab release 15 was used to design and analyze the experiment. The level and ranges chosen for the factors were shown in Table 5-3. The low, middle, and high levels of each variable were designated as -1.414, -1, 0, 1 and 1.414 respectively. The experimental design
119 and results of the central composite design are shown in Table 5-4.
Table 5-3 Levels of the factors tested in the central composite design
Factor Levels of factors Designated -1.414 -1 0 1 1.414
Electroless NiSO ·6H O (g/l) 4 2 10.76 12 15 18 19.24 Ni plating T 67.9 70 75 80 82.1
Electroless NiSO ·6H O (g/l) 4 2 0.8 1.0 1.5 2.0 2.2 Cu plating pH 9.3 9.5 10 10.5 10.7
Table 5-4 Experimental design and results of the central composite design
Trial number Variables/levels Surface resistance (mΩ/sq)
A B Electroless Electroless nickel plating copper plating 1 -1 -1 1479.4 698.14
2 1 -1 854.9 293.1
3 -1 1 917.6 214.15
4 1 1 366.47 77.25
5 -1.414 0 1145.6 502.43
6 1.41 0 456.72 113.68
7 0 -1.414 1087.6 525.86
8 0 1.414 549.24 97.35
9 0 0 591.2 203.68
10 0 0 530.8 269.59
11 0 0 558.4 240.15
120 12 0 0 505.5 223.92
5.3 Results and Discussion
5.3.1 Full Factorial Design
Thirty-two experiments were carried out according to the conditions fixed by the experimental design, and the results are shown in Table 5-2.
5.3.1.1 Pareto Plot of Effect
The factor that has the greatest effect on the surface resistance can be determined by examining the data with the graphic capabilities of a bar chart called the Pareto chart.
In this chart, the factor with the greatest effect is placed in the upper position, while the factor with the rest of the effects in descending order is placed in the lower position [243].
The Pareto charts of the standardized effects at p = 0.05 for the electroless nickel and copper plating are presented in Figure 5-1 (a) and (b) respectively. For the electroless nickel plating, the result shows that the concentration of NiSO4·6H2O and temperature present an absolute value higher than 183.0 (p=0.05) which is significant as it is located at the right of the red line. This is due to the fact that the model with a p-value less than 0.05 (95% confidence) is considered to be statistically significant. However, the value of concentration of NaH2PO2·H2O and Na3C6H5O7 ·2H2O, and pH value are
121 located at the left of the red line, which is not significant. Additionally, concentration of NiSO4·6H2O has the greatest effect on the surface resistance, which is followed by bath temperature, pH, concentration of NaH2PO2·H2O and Na3C6H5O7 ·2H2O, respectively.
Similarly, for electroless copper plating, pH value and concentration of NiSO4·6H2O are significant. However, the value of concentration of NaH2PO2·H2O and
Na3C6H5O7 ·2H2O, and bath temperature are not significant. In addition, pH value of bath has the greatest effect on the surface resistance, which is followed by concentration of NiSO4·6H2O, bath temperature, concentration of NaH2PO2·H2O and
Na3C6H5O7 ·2H2O, respectively.
Pareto chart of the standardized effects （Response is surface resistance (mΩ/sq)，
Alpha = .05，only 30 largest effects shown Factor Name 183.0 AA BB A E CC CD ABCD DD D EE B ACDE AE BDE ACE AB ACD BCDE DE BC BE
Term ABCDE BD BCE C ABCE AD ABE CE ADE ABC CDE ABD AC BCD 0 100 200 300 400 500 600 700 800 Standardized effect (a)
122 Pareto chart of the standardized effects （Response is surface resistance (mΩ/sq)，
Alpha = .05，only 30 largest effects shown） Factor Name 138.8 AA D BB A AD CC E DD ABC BC EE BCD B AB ABCD ABDE ABCE DE ABD ABCDE C
Term AC AE BD ABE ACDE BCE BCDE ACE BDE CDE ACD CD ADE CE 0 100 200 300 400 500 Standardized effect (b)
Figure 5-1 Pareto chart of the standardized effects for the electroless (a) nickel
plating and (b) copper plating with (A) concentration of NiSO4·6H2O, (B)
concentration of NaH2PO2·H2O, (C) concentration of Na3C6H5O7 ·2H2O, (D) pH and
(E) bath temperature
5.3.1.2 Main Effect Plot
A main effect is a plot of the mean response values at each level of a design parameter
[244]. It indicates the relative strength of effect of various factors. The sign of the main effect indicates the direction of the effect. Figure 5-2 shows the main effect plots
(average of the duplicate data) for the surface resistance of the electroless nickel and copper plating on polyester fabric. It is obvious that the concentration of NiSO4·6H2O and NaH2PO2·H2O, pH and temperature of the solution have a negative effect while the concentration of Na3C6H5O7 ·2H2O has a positive effect, on the surface resistance in the variation range of each variable selected for the present study. The surface
123 resistance decreases as there is an increase in the concentration of NiSO4·6H2O and
NaH2PO2·H2O, pH and temperature of the solution. However, the surface resistance increases as the concentration of Na3C6H5O7·2H2O increases. Furthermore, in the main effect plot if the line representing a particular parameter is near horizontal, then the parameter has no significant effect. On the other hand, a parameter for which the line has the highest inclination will have the most significant effect. As for the electroless nickel plating, it is very obvious in Figure 5-2 (a) that the parameters A
(concentration of NiSO4·6H2O) and E (bath temperature) are the most significant parameters while the parameters B (concentration of NaH2PO2·H2O), C
(concentration of Na3C6H5O7 ·2H2O) and D (pH) have a relatively less significant influence. With regard to the electroless copper plating, Figure 5-2 (b) shows that the parameters A (concentration of NiSO4·6H2O) and D (pH) are the most significant parameters while the other parameters have a relatively less significant influence on the surface resistance in the variation range of each variable selected for the present study.
124 Main effect plot (data means) for surface resistance
A B C 1400
1200
1000
800
600 -1 1 -1 1 -1 1 D E 1400
1200
1000 Surface resistance (mΩ/sq) 800
600 -1 1 -1 1 (a)
Main effect plot (data means) for surface resistance
A B C 600
500
400
300
200
-1 1 -1 1 -1 1 D E 600
500
400
Surface resistance (mΩ/sq) 300
200
-1 1 -1 1 (b)
Figure 5-2 Main effect plot for the effect of processing parameters on the electroless
(a) nickel plating and (b) copper plating
125 5.3.1.3 Interaction Effect Plot
An interaction plot is a graphical tool which plots the mean response of two factors at all possible combinations of their settings [245]. The interaction effects of the factors affecting the surface resistance of the electrolesss plating are demonstrated in the interaction plots which can show how different factors interact. As far as the interaction plots are concerned, estimating an interaction means determining the non-parallelism of parameter effects. Hence, if the lines are non-parallel, it is an indication of interaction between two factors, and if the lines cross, strong interaction occurs between parameters [244]. Parallel lines indicate that there is no interaction between two factors. The corresponding interaction effect plots between the process parameters of the electroless nickel plating and copper plating are shown in Figure
5-3. In the case of the electroless nickel plating, the result shows that there is a strong interaction between the parameters C and D. In addition, there is a moderate interaction between the parameters B and C, between B and D, between B and E, between D and E, and between C and E. However, there is no interaction effect of other combination of factors, i.e. the lines are more or less parallel. As for the electroless Cu plating, from Figure 5-3 (b) shows that there is a strong interaction between B and C. Furthermore, there is a moderate interaction between the parameters A and B, between A and D, between B and D, and between C and D, while there is no interaction effect of other combination of factors.
126 Interaction plot (data means) for surface resistance
-1 1 -1 1 -1 1 -1 1
(a) 1500 A -1 A 1000 1
500 A -1 1500 B 1-1 B 1000 A 1 500 -1 1 B 1500 C -1A-1 C 1000 -11 1 1 500 C B -1 1500 D-1 -11 1 D 1000 1
500
E
Interaction plot (data means) for surface resistance (b) -1 1 -1 1 -1 1 -1 1 800 A -1 A 400 1 0 A 800 B -1-1 B 400 1 1 B A 0 800 -1C-1 -11 1 C 400 B1 -1 A 0 C 800 -1D1-1 -11 1 D 400 1
0
E
Figure 5-3 Interaction plot for the effect of processing parameters on the electroless (a)
nickel plating and (b) copper plating
127 Based on the present analysis, it is clear that the parameters A (concentrations of
NiSO4) and E (bath temperature) are the most influencing parameters for the electroless nickel plating on polyester fabric. However, the parameters A
(concentrations of NiSO4) and D (pH) are the most influencing parameters for the electroless copper plating on polyester fabric. Therefore, the two variables, i.e. concentration of NiSO4 (A) and temperature (E) as well as concentration of NiSO4 (A) and pH (D), were selected and used for further optimization of the electroless nickel and copper plating on polyester fabric in order to achieve a minimum surface resistance by CCD in RSM respectively. As for the electroless nickel plating, the concentration of NaH2PO2·H2O, concentration of Na3C6H5O7 ·2H2O and pH in the following CCD experiments were set at their middle levels of 25 g/l, 15 g/l and 8.0 respectively. With regard to the electroless copper plating, the concentration of
NaH2PO2·H2O, concentration of Na3C6H5O7 ·2H2O and temperature in the following
CCD experiments were set at their middle levels of 30 g/l, 20 g/l and 75℃ respectively.
5.3.2 Response Surface Design
Following screening, CCD was employed to determine the optimal levels of the two selected factors that significantly affected the surface resistance. The variables showing the significant influencing parameters in the two-level factorial design were selected and further optimization was achieved by using CCD. The CCD in RSM was
128 employed to investigate the interactions among the significant factors, i.e. concentration of NiSO4 and temperature for the electroless nickel plating, and concentration of NiSO4 and pH for the electroless copper plating, and determine the exact optimal values of the individual factors. The respective low and high levels with the coded levels for the factors were defined. The design matrix and the corresponding results of RSM experiments to determine the effects of two independent variables are shown in Table 5-4. Multiple regression analysis was used to analyze the data obtained as shown in Tables 5-5 and 5-6. A second-order polynomial equation was derived from the regression analysis as follows:
2 2 Yn = 546.48 -268.73A -226.45B +151.04A +159.67B +18.34AB Equation 5-3
where Yn was the predicted response for electroless nickel plating;
A and B were coded values for concentration of NiSO4 and temperature for
the electroless nickel plating.
2 2 Yc= 234.34-136.46A -163.23B +39.57A +41.34B +67.04AB Equation 5-4
where Yc was the predicted response for electroless copper plating;
A and B were coded values for concentration of NiSO4 and pH for the
electroless copper plating.
129 The results of the second-order response surface model in the form of analysis of variance (ANOVA) for the electroless nickel plating are given in Table 5-5. The
ANOVA results indicate that the quadratic regression to produce the second-order model is significant. The lack-of-fit test is insignificant (P = 0.059), and only 5.9% of the total variations can not be explained by the model (R2 = 97.08%). The value of the adjusted determination coefficient (adjusted R2 = 94.64%) is also high to advocate a high significance of the model. This suggests that the model accurately represents the data in the experimental region. It also indicates that second-order terms are sufficient and higher-order terms are not necessary.
The results also showed that among the independent factors, the parameters A (NiSO4) and B (temperature) have significant effects on the surface resistance. The negative coefficient of them shows a linear effect on the decrease in surface resistance. The quadratic term of the two factors also has significant effects and the interaction between the parameters A and B has no significant effect.
130 Table 5-5 Analysis of variance (ANOVA) and coefficient estimates of the central
composite second-order polynomial model for the electroless nickel plating
Source of Degree of Sum of Mean F-value P-value
variation freedom squares square
Regression 5 1247060 249412 39.86 0.000
Linear 2 98796 493982 78.94 0.000
Square 2 25775 128875 20.60 0.002
Interaction 1 1346 1346 0.22 0.659
Lack of fit 3 33477 11159 8.23 0.059
Pure error 3 4067 1356 -- --
Total 11 1284604 ------
Source of Degree of Coefficient Standard t-value P-value
variation freedom estimate error
Intercept 1 546.48 39.55 13.817 0.000
A 1 -268.73 27.97 -9.609 0.000
B 1 -226.45 27.97 -8.097 0.000
A2 1 151.04 31.27 4.831 0.003
B2 1 159.67 31.27 5.107 0.002
AB 1 18.34 39.55 0.464 0.659
R2 = 0.9708; R2 (adjust) = 0.9464; 5% significance level.
131 As for the electroless copper plating, the statistical significance of Equation 5-4 was checked by F-test, and the ANOVA used for the response surface quadratic model is summarized in Table 5-6. The model with the F-value of 130.08 implies that the model is significant. The P-value is used as a tool to check the significance of each coefficient, which is necessary to understand the pattern of the mutual interactions between the best factors. The smaller the P-value, the bigger the significance of the corresponding coefficient will be [246]. The P-value which is also very low (P =
0.000) indicates the significance of the model. The global fit of the model can be checked by the determination coefficient R2. If R2 is calculated to be 0.9909, this indicates that 99.09% of the variability in the response can be explained by the model.
Normally, a regression model having the R2-value higher than 0.9 is considered as having a very high correlation [247]. The present R2-value, therefore, reflects a very good fit between the observed and predicted responses, and it is considered reasonable to use the regression model to analyze the trends of the responses. The lack of fit P-value of 0.668 implies that the lack of fit is not significant relative to the pure error. The significance of the regression coefficients is tested by a Student’s t-test. The regression coefficients and the corresponding P-values used for the model are presented in Table 5-6.
The results showed that among the independent factors, the parameters A
(concentration of NiSO4) and B (pH) as well as the interaction between A and B had significant effects on the surface resistance. The quadratic term of the two factors has
132 no a significant effect.
Table 5-6 Analysis of variance (ANOVA) and coefficient estimates of the central
composite second-order polynomial model for the electroless copper plating
Source of Degree of Sum of Mean F-value P-value
variation freedom squares square
Regression 5 397579 79516 130.08 0.000
Linear 2 362134 181067 296.20 0.000
Square 2 17470 8735 14.29 0.005
Interaction 1 17975 17975 29.40 0.002
Lack of fit 3 1343 448 0.58 0.668
Pure error 3 2325 775
Total 11 401246
Source of Degree of Coefficient Standard t-value P-value
variation freedom estimate error
Intercept 1 234.33 12.362 18.956 0.000
A 1 -136.46 8.741 -15.611 0.000
B 1 -163.23 8.741 -18.673 0.000
A2 1 39.57 9.773 4.049 0.007
B2 1 41.34 9.773 4.230 0.005
AB 1 67.04 12.362 5.423 0.002
R2 = 0.9909; R2 (adjust) = 0.9832; 5% significance level.
133 The interaction and optimal level of the variables are determined by plotting the response surface curves and contour plot. The shape of the contour plots indicates whether the mutual interaction between the variables is significant or not. The circular contour plot of the response surface suggests that the interaction is negligible between the corresponding variables. On the other hand, an elliptical or saddle nature of the contour plot indicates that the interaction between the corresponding variables is significant [248]. The contour plot and response surface curves of the electroless nickel plating were generated as shown in Figure 5-4, depicting the interaction between two variables by keeping the other variables at their zero levels for the surface resistance. These 3D and their respective contour plots provided a visual interpretation of the interaction between two factors, and facilitated the location of optimum experimental conditions. The oval shape in the minimum region suggested that the effect of bath temperature on the surface resistance was less predominant than of the concentration of NiSO4 as shown in Figure 5-4. In contrast to the temperature, the concentration of NiSO4 had a major influence on the surface resistance in the minimum region. According to Equation 5-3, the optimal values of A and B were estimated to be 0.84 and 0.66 respectively. Correspondingly, their actual values were
17.52g/l and 78.3℃ for the concentration of NiSO4 and bath temperature respectively.
The predicted minimal surface resistance of the electroless nickel-plated polyester fabric was 358.88 mΩ/sq under the optimum condition.
134 (a)
2000
1500 Surface resistance (mΩ/sq) 1000
1 500 0 B -1 -1 0 A 1
Isogram graph of surface resistance vis NiSO4 and T (b) Surface resistance 1.0 (mΩ/sq) < 450 450– 600 0.5 600– 750 750– 900 900– 1050 1050– 1200
B 0.0 1200– 1350 1350– 1500 1500– 1650 -0.5 1650– 1800 > 1800
-1.0
-1.0 -0.5 0.0 0.5 1.0 A
Figure 5-4 Response surface plots (a) and contour plots (b) of the electroless nickel
plating as a function of NiSO4 concentration and bath temperature
135 The 3D response surface curves and contour plots of the electroless copper plating on polyester fabric are shown in Figure 5-5. The oval shape in the minimum region suggested that the effect of the concentration of NiSO4 on the surface resistance was less pre-dominant than pH with as shown in Figure 5-5. In contrast to the concentration of NiSO4, the pH of plating bath had a major influence on the surface resistance in the minimum region. The model predicted that the optimal values of test factors in the coded units were A=0.22 and B=1.85. As for these values, the concentration of NiSO4 and pH were 1.72 g/l and 10.9 respectively. The minimal predicted surface resistance of the copper-plated fabric was 68.34 mΩ/sq.
(a)
1000 Surface resistance (mΩ/sq) 500
1
0 0 B -1 -1 0 A 1
136 Isogram graph of surface resistance vis NiSO4 and pH (b) Surface resistance 1.0 (mΩ/□) < 150 150– 231 0.5 231– 312 312– 393 393– 474 474– 555
B 0.0 555– 636 636– 717 717– 798 -0.5 798– 879 > 879
-1.0
-1.0 -0.5 0.0 0.5 1.0 A
Figure 5-5 Response surface plots (a) and contour plots (b) of the electroless copper
plating as a function of NiSO4 concentration and pH
5.3.3 Experimental Validation of the Optimized Condition
To confirm the model adequacy for predicting the minimal surface resistance, three additional experiments under the optimum deposition conditions were performed. In the case of the electroless nickel plating, the mean value of surface resistance was
372.82 mΩ/sq, which was close to the theoretical predicted value of 358.88 mΩ/sq.
The excellent correlation between the predicted and measured values verified the model validation and the existence of an optimal point, indicating that the model was adequate for obtaining the optimum value in the range of the studied parameters. The final deposition parameters optimized with RSM were (1) concentration of
137 NiSO4·6H2O= 17.52 g/l; (2) concentration of NaH2PO2·H2O= 25 g/l; (3) concentration of Na3C6H5O7 ·2H2O= 15 g/l; (4) pH= 8.0 and (5) temperature= 78.3℃ respectively.
With regard to the electroless copper plating, the mean value of the surface resistance was 72.58 mΩ/sq, which was in excellent agreement with the predicted value of 68.34 mΩ/sq. This indicated that the optimization was reliable in the present study. The final deposition parameters optimized with RSM was (1) the concentration of
NiSO4·6H2O= 1.72g/l; (2) concentration of NaH2PO2·H2O= 30 g/l; (3) concentration of Na3C6H5O7 ·2H2O= 20 g/l; (4) pH= 10.9 and (5) temperature= 75℃ respectively.
5.4 Summary
In this chapter, the statistical experimental design was proved to be an effective tool to optimize the deposition parameters for the electroless nickel and copper plating on the polyester fabric. The statistical analysis of factorial design, RSM and desirability function could help screen several significant deposition parameters and found out the optimum deposition parameters for the electroless nickel and copper plating in a quick and economical way.
Based on the experimental design study, it was found that each of the factors, i.e. the concentration of NiSO4·6H2O, NaH2PO2·H2O and Na3C6H5O7·2H2O, pH and temperature had its own contributing effect to the surface resistance of the electroless
138 nickel and copper plated fabrics. In addition, each factor had an interaction effect with the others, affecting the surface resistance. It was confirmed that the surface resistance of the electroless nickel plating on fabric was predominantly influenced by the concentration of NiSO4 and bath temperature of plating bath. However, the surface resistance of the electroless copper plating on fabric was mostly influenced by the concentration of NiSO4 and pH of plating bath.
The experimental results obtained from the response surface method, which was subjected to regression analysis and plotted as contour diagrams, were effectively used to study the effects of the key variables on the surface resistance. Furthermore, the combined statistical approaches were useful for determining the conditions which could give the minimal surface resistance. The overall results showed that the optimized deposition parameters so obtained were composed of (1) 17.52 g/l
NiSO4·6H2O, 25 g/l NaH2PO2·H2O and 15 g/l Na3C6H5O7·2H2O at pH 8.0 and 78.3℃ for the electroless nickel plating, and (2) 1.72 g/l NiSO4·6H2O, 30 g/l NaH2PO2·H2O and 20 g/l Na3C6H5O7·2H2O at pH 10.9 and 75℃ for the electroless copper plating, respectively. In the experimental validation of the optimized conditions, the mean surface resistances of 372.82 mΩ/sq and 76.58 mΩ/sq were obtained. Since the experimental results were in good agreement with the predicted values, thus the results obtained could also serve as a basis for further study with a large scale of production for the electroless nickel-plated and copper-plated fabrics.
139 Chapter 6 Characterization of Electroless Nickel-plated and
Copper-plated Polyester Fabrics
6.1 Introduction
In this chapter, the micro-structure and properties of electroless nickel-plated and copper-plated fabrics were investigated under different deposition conditions which significantly influenced the surface resistance. The effects of NiSO4 concentration and temperature of plating bath for the electroless nickel plating as well as the effects of
NiSO4 concentration and pH of plating bath for the electroless copper plating on deposit compositions, surface morphology, crystal structure, surface resistance and
EMI SE of the plated polyester fabric were discussed. The micro-structure and properties of the electroless nickel-plated and copper-plated fabrics obtained under the optimal conditions were also revealed.
6.2 Characterization
A field emission scanning electron microscope (SEM, JSM-6335F) was used to characterize the surface morphology of the nickel and copper deposits. The composition of deposits was determined by an energy dispersive X-ray (EDX) analyzer attached by a scanning electron microscope (SEM). Three measurements
140 were conducted on each specimen and their average values were taken. The crystal structures of the deposits were analyzed using the X-ray diffractometer (XRD,
Brucker D8 discover). The radiation used was Cu Kα with a wavelength of 1.5418 Å.
The patterns were recorded at a tube voltage of 40 kV and a tube current of 40 mA, applying a fixed scan rate of 2◦ per minute in the angular range of 30–100◦. The intensity and 2θ of the peaks obtained for the experimental samples were compared with the standard values of Joint Committee Powder Diffraction Standards (JCPDS).
EMI SE was tested using the coaxial cable method according to ASTM D 4935-99.
The set-up as shown in Figure 6-1 consisted of a SE tester which was connected to an
Agilent-E8363A vector network analyzer. Standard attenuators with total attenuation of 100 decibels (dB) were tested to ensure that the dynamic measurement ranging up to 100 dB was valid for all the sample measurements. The frequency was scanned from 10MHz to 18 GHz. The attenuation under transmission and that under reflection were measured. The former was equivalent to the SE.
141
Figure 6-1 The equipment of EMI shielding effectiveness measurement
The EMI SE value expressed in dB is calculated from the incident to the transmitted power ratio of the electromagnetic wave as shown in Equation 6-1:
PE SE ==10log11 20log Equation 6-1 PE22
where SE is EMI shielding effectiveness (decibels, dB);
P1 (E1) and P2 (E2) are the incident power (incident electric field) and the transmitted power (transmitted electric field) respectively [17].
6.3 Results and Discussion
6.3.1 Electroless Nickel Plating
6.3.1.1 Deposit Composition
142 Controlling the composition of electroless nickel alloys is a relevant consideration for the design of the eventual material properties. Most of the properties of the coating are structure-dependent. The structure, in turn, is mainly dependent on the phosphorus content [196, 249]. Hence, controlling the phosphorus content is of significant importance to obtain an optimum coating. By modifying the operating conditions, it is possible to alter the phosphorus content and thus the density and structure of the coating [197, 250].
Figures 6-2 and 6-3 show the weight percent of nickel and phosphorus in nickel deposits at different nickel ion concentration and bath temperature respectively. The nickel content rises and phosphorus content decreases as the nickel ion concentration is increased as shown in Figure 6-2. The percent of nickel increases from 88.97% to
91.64% when the NiSO4 concentration is ranged from 6.0 to 15.0g/l. The phosphorus content in the deposits drops from 11.03% to 8.36%. The effects of NiSO4 concentration on nickel and phosphorus are shown in Equations 3-9, 3-10 and 3-11
[135].
High NiSO4 concentration can promote the deposition of nickel but at the same time, the amount of hypophosphite will reduce and the phosphorus co-deposition is inhibited.
As shown in Figure 6-3, the percent of nickel present in the deposit decreases slightly
143 while the percent of phosphorus increases slightly with respect to the rise of temperature of reaction. Similar dependence of deposit composition on the bath temperature was also found by Wang et al [182].
Figure 6-2 Deposit composition of the nickel-plated polyester fabrics with different
NiSO4 concentrations
144 Figure 6-3 Deposit composition of the nickel-plated polyester fabrics with different
bath temperatures
The effect of temperature on the electromotive force (E) of nickel and phosphorus is shown in Equations 6-2 and 6-3, respectively [182].
RT α 2+ EE=+0 ln Ni Ni22++// Ni Ni Ni Equation 6-2 2F α Ni
α − 0 RT HPO22 EE−−=+ln 2 Equation 6-3 HPO22// P HPO 22 P F αα P OH −
In general, higher temperature can increase the electromotive force which has an effect on the reaction. For instance, a higher reaction rate can promote nickel plating.
However, the deposition rate of phosphorus increases more quickly than that of nickel with respect to the rise of temperature. Consequently, the percent of phosphorus present in the deposit increases at higher temperature of reaction, while the percentage of nickel decreases. The weight percent of nickel and phosphorus is 91.83% and
8.17%, respectively under the optimal deposition condition.
6.3.1.2 Surface Morphology
SEM images of the untreated and the nickel-plated polyester fibers with different
NiSO4 concentrations and bath temperatures are shown in Figures 6-4 and 6-5 respectively. It is obvious that the surface of polyester fibers has changed significantly
145 after nickel plating. It can be seen that the surface of the sample is fully covered by the deposits and the nickel coating is compact. The electroless nickel plating layer exhibits a nodular feature with a typical cauliflower-like structure. The nickel deposits have different surface morphology, i.e. their nodule sizes will increase but the nodule number will decrease with respect to the increase in NiSO4 concentration as shown in
Figure 6-4 [251]. This indicates that the nickel nodule gradually grows by the agglomeration or coalescence with the neighboring nodule when the NiSO4 concentration is increased.
Similarly, the SEM micrographs of the nickel-plated polyester fibers with different bath temperatures (Figure 6-5) exhibit that the nodule size of nickel deposits increases with respect to the rise of temperature. According to the polymorphous transformation kinetics shown in Equation 6-7 [252],
D = exp(-Q/kT), Equation 6-4
where D is particle size;
Q is activation energy;
k is the Boltzmann constant;
T is temperature.
The equation indicates that particle size is a function of temperature, i.e. the particle
146 size increases with respect to the rise of the temperature. Additionally, the surface morphology of the deposits is not compact at low temperature as shown in Figure 6-5.
On the contrary, the deposits become more compact at higher temperature.
(a)
(b) (c)
(d) (e)
Figure 6-4 SEM micrographs of (a) untreated polyester, and nickel-plated polyester
147 fibers treated with different NiSO4 concentration of (b) 6.0g/l, (c) 9.0g/l, (d) 12.0g/l
and (e) 15.0g/l
(a) (b)
(c) (d)
Figure 6-5 SEM micrographs of nickel-plated fibers treated at (a) 50℃, (b) 60℃, (c)
70℃ and (d) 80℃
Surface morphology of the electroless nickel-plated polyester fabric obtained under the optimal condition is shown in Figure 6-6. It is found that the deposit is uniform and defect free with no pores or cracks. It is composed of compact fine nodules with a uniform size accompanied by some coarser cauliflower nodules. Those nodules consist of a number of very fine sub-micro nodules which are resolved at higher magnification. The formation of agglomerative coating nickel layer around the
148 polyester fabrics is due to the aggregation of the nano-sized nickel particles with high surface energy originally produced at the polyester /solution interface.
Figure 6-6 SEM micrographs of nickel-plated fibers obtained under the optimal
condition
6.3.1.3 Crystal Structure
Figures 6-7 and 6-8 present the XRD patterns of the nickel-plated fabrics with different NiSO4 concentrations and bath temperatures respectively. Figure 6-9 shows the XRD pattern of the nickel-plated fabric obtained under the optimal condition. It is evident from these patterns that only a single broad peak at around 2θ of 45º is seen for all the deposits which have an amorphous structure. The reflections corresponding to the (111) plane of a face-centered cubic phase of nickel can be observed. This is in correspondence with other research results that an electroless nickel deposit will become amorphous when the phosphorus content is above 7% [217, 253]. It is in good agreement with the above-mentioned deposit composition, suggesting that the coating with high phosphorous content is amorphous. The result indicates the metallization forms the amorphous alloys with high P content which are flexible metallic coatings with a low degree of crystallization. Theoretically, a disorder in the atomic
149 arrangement manifests itself as a broad peak in the XRD patterns [249]. The mechanism of the formation of the electroless coatings indicates that during the electroless nickel deposition, a random capturing of P on Ni atoms occurs such that the variation in the rate of segregation of Ni and P atoms determines the crystal-structure of the resultant coating. Between these two atoms, the diffusion rate of P is relatively small as compared to that of Ni [254]. Hence, the electroless nickel coating with a higher P content involves the movement of more P atoms from a given area per unit time during deposition to achieve the segregation of Ni and P [217, 253].
Since the required P segregation is very large, thus it prevents the nucleation of the face-centered cubic Ni phase, resulting in the formation of amorphous structure in the plating condition.
Figure 6-7 XRD patterns of the electroless nickel-plated polyester fabrics with
different NiSO4 concentrations: (a) 6g/l, (b) 9g/l, (c) 12g/l and (d) 15g/l
150
Figure 6-8 XRD patterns of the electroless nickel-plated polyester fabrics at (a) 50℃,
(b) 60℃, (c) 70℃ and (d) 80℃
Figure 6-9 XRD patterns of the electroless nickel-plated polyester fabric obtained
under the optimal condition
It is observed that the intensity of the nickel peaks enhances with respect to the increase in NiSO4 concentration and temperature. This is caused by the increase of
151 weight in the deposits for the same deposition time. Additionally, the full width at half maximum (FWHM) of the diffraction peak of the electroless nickel-plated fabrics with different NiSO4 concentrations and bath temperatures is obtained from the
Gaussian fitting as shown in Table 6-1. The results indicate that FWHM of the diffraction peak is smaller at higher NiSO4 concentration and temperature, implying that the crystallization of the deposits increases with respect to the rise of NiSO4 concentration and temperature.
Table 6-1 The full width at half maximum of diffraction peak of the electroless
nickel-plated fabrics
NiSO4 concentration (g/l) FWHM (°) Temperature (℃ ) FWHM (°)
6 4.176 50 5.880
9 4.032 60 5.075
12 3.878 70 3.847
15 3.582 80 3.570
6.3.1.4 Surface Resistance
The surface resistances of the nickel-plated fabrics with different NiSO4 concentrations and bath temperatures are shown in Figures 6-10 and 6-11 respectively.
It is observed that there is a decrease in the surface resistances of the Ni deposits with respect to the increase in NiSO4 concentration and bath temperature. The difference in
152 the surface resistance is mostly attributed to different weight of Ni deposits, P content, particle size and crystallization of the deposits. It is well known that the surface resistance of the nickel-plated fabrics is correlated with the weight of nickel coating on the fabrics. In the present study, the deposition rate and weight of the films will increase with respect to the increase in NiSO4 concentration and temperature. When considering the fact that the surface resistance is inversely proportional to the weight, thus the lower surface resistance of the sample fabricated is mostly due to the higher weight which is resulted from higher deposition rate. Since the weight of the deposits fabricated can be controlled, thus the surface resistance value intended to be achieved can be realized. In addition, the electrical conductivity of the coated polyester fabric can also be affected by the phosphorus content present in the coating because different P contents will result in different micro-structure of the alloy film. When the phosphorus content decreases below a certain limit, the uniform film with lower phosphorus content has a reasonably high crystallization and electrical conductivity.
Additionally, the decrease in resistance resulted from the increased crystallite size is due to the reduction in defects and the decreasing contribution of electron scattering of the grain boundaries and interfaces in the film [255]. As a result, the lower resistance of the sample prepared with different NiSO4 concentrations is primarily attributed to the increase of the weight in the deposits, the reduction of P content, and the increase in the particle size and crystallization. Although the P content slightly increases with respect to the rise of temperature as shown in Figure 6-3, yet the reduction of surface resistance of the nickel-plated fabrics with different bath
153 temperatures is mostly due to the increase in the weight of the deposits as well as the increase in the particle size and crystallization.
Figure 6-10 Surface resistance of the nickel-plated fabrics with different NiSO4
concentrations
Figure 6-11 Surface resistance of the nickel-plated fabrics with different bath
temperature
154 As mentioned in Chapter 5, the surface resistance of the electroless nickel-plated fabric obtained under the optimal condition is 372.82 mΩ/sq.
6.3.1.5 EMI Shielding Effectiveness
When the electromagnetic waves transmit to the surface of a shielding material, there are three mechanisms to attenuate the wave: (1) the reflection attenuation (R) on the surface of the shielding material; (2) the absorption attenuation (A) in the body of the shielding material; and (3) the multi-reflection attenuation (B) in the body of the shielding material. After the waves penetrate the shielding material, the total electromagnetic shielding effect (SE) can be calculated by the following equations
[256]:
SE=R+A+B Equation 6-5
μ f R =−168 10lg(r ) (far field plane wave) Equation 6-6 σ r
1/2 Atf=1.314 (μσrr ) Equation 6-7
where SE is EMI shielding effectiveness;
R is reflection attenuation on the surface of the shielding material;
A is absorption attenuation in the body of the shielding material;
B is multi-reflection attenuation in the body of the shielding material;
t stands for the thickness of deposits in centimeter;
f stands for the frequency of electromagnetic wave (Hz);
155 μr stands for relative magnetic conductivity;
σr stands for relative electrical conductivity (compared to pure Ni).
Generally speaking, electrical shielding material has a good electrical conductivity, and is usually used in high impedance electrical field. The electrical shielding effect lies on the reflection attenuation R. Magnetic shielding material has a good magnetic conductivity, and magnetic shielding effect lies on the absorption attenuation A. A value is the main influencing factor under the condition of high frequency, and R value is the main influencing factor under the condition of low frequency [257].
According to Equations 6-6 and 6-7, a good electromagnetic shielding material should have both high electrical and high magnetic conductivities in order to get the best shielding effect in a wide range of frequency. In usual practice, the SE value lower than 30 dB is regarded as a poor shielding effect. The general industrial requirement of shielding effect is 30–60 dB and military requirement is 60–120 dB [257].
Figure 6-12 shows the EMI shielding effectiveness (SE) of the nickel-plated polyester fabrics with different NiSO4 concentrations. The result shows that the EMI SE ranges from 20 to 40 dB at the frequency of 2 to 18GHz. There is a tremendous increase in the EMI SE of the nickel-plated polyester fabrics with respect to the rise of NiSO4 concentration. According to the Schelkunoff theory [258], better conductivity of the fabrics leads to higher SE. The increase in the EMI SE is mostly attributed to the decrease in the surface resistance of the nickel deposits in the plated fabrics. When the
156 concentration of NiSO4 is below 9 g/l, the EMI SE value is below 30dB. However, when the concentration of NiSO4 is above 12 g/l, the EMI SE value is above 30dB.
The result suggests that the nickel-plated polyester fabric can meet the general electromagnetic shielding demands when the concentration of NiSO4 is above 12 g/l.
Figure 6-12 EMI shielding effectiveness of the electroless nickel-plated polyester
fabrics with different NiSO4 concentrations: (a) 6g/l, (b) 9g/l, (c) 12g/l and (d) 15g/l
The EMI SE of the nickel-plated polyester fabrics with different temperatures of the plating bath is shown in Figure 6-13. The result indicates that the EMI SE of the nickel-plated polyester fabric enhances at higher temperature. In addition, EMI SE of the nickel-plated polyester fabric ranges from 35 to 60 dB at the frequency of 2 to
18GHz. This suggests that the nickel-plated polyester fabric can meet the general electromagnetic shielding demands at all temperatures.
157
Figure 6-13 EMI shielding effectiveness of the nickel-plated polyester fabrics at (a)
50 ℃, (b) 60 ℃, (c) 70 ℃ and (d) 80℃
The EMI SE of the nickel-plated fabric obtained under the optimal condition is about
60 dB at the frequency ranging from 2 to 18 GHz as shown in Figure 6-14. Generally, the conductive materials with 40–50 dB of shielding effectiveness can prevent the electromagnetic interference emitted from 90% of the current commercial electronic devices. Hence, the electroless nickel-plated fabric is practically useful for many EMI shielding application requirements.
158
Figure 6-14 EMI shielding effectiveness of the nickel-plated polyester fabric obtained
under the optimal condition
6.3.2 Electroless Copper Plating
6.3.2.1 Deposit Composition
The deposit composition strongly depends on the deposition conditions. Copper is the dominant component within the copper coating. The composition of the coatings suggests that only a small quantity of nickel and phosphorus is formed within the bulk of the coatings. The deposit of the electroless copper plating using hypophosphite as the reducing agent is actually Cu–Ni-P alloy.
The chemical composition of the electroless copper deposits with different nickel ion concentrations in the alkaline plating bath is shown in Figure 6-15. The deposit layers with different copper contents ranging from 95.13% to 92.24% are obtained. The
159 higher copper content in the deposits is believed to be due to the higher reduction potential of copper than that of the nickel and phosphorus. According to the electrochemistry, the standard reduction potential of copper, nickel, and hypophosphite in basic solution are equations 4-2 and 4-3, and shown as follows
2−− 0 H22PO+→ e P +2 OH [E25°C = -1.82V]. Equation 6-8
Therefore, both Cu2+ and Ni2+ can be reduced to copper and nickel by hypophosphite in alkaline solution, and the reduced trend of Cu2+ is higher than that of Ni2+. In other
2+ - 2+ words, Cu can obtain electrons from H2PO2 more easily than Ni . On the other hand, phosphorus is not observed in the deposits. The phenomenon is explained by the fact that the reduction potential of phosphorus is relatively lower than that of copper and nickel, thereby inhibiting the reduction of hypophosphite [259].
When the nickel sulfate concentration is increased in the solution, the copper content in the coating will decrease whereas the amount of nickel will increase slightly. The copper content decreases from 95.13% to 92.24% but reversely the nickel content in the deposits increases from 4.87% to 7.76% as the mol ratio of NiSO4/(CuSO4 +
NiSO4 ) in the plating bath increases from 0.076 to 0.237. Table 6-2 shows the mol ratio of Ni/ (Cu + Ni) in the coating and in the plating bath concerned. The mol ratio of Ni/(Cu + Ni) in the coating is smaller than that in the plating bath implying that the
Cu2+ ions are easier to be reduced to form atoms and then attach to the film surface
160 than the Ni2+ ion in the plating baths.
Figure 6-15 Composition of copper deposits on polyester fabric with different NiSO4
concentration
Table 6-2 The mol ratio of Ni/ (Cu+Ni) in films deposited in various plating baths
1 2 3 4 5 mol ratio of Ni/ (Cu+Ni) in plating bath 0.076 0.095 0.143 0.190 0.237 mol ratio of Ni/ (Cu+Ni) in the coatings 0.051 0.060 0.065 0.074 0.084
Figure 6-16 shows the deposit composition of the electroless copper-plated polyester fabrics at different pH. The percent of copper in the deposits increases from 76.99% to 95.83% when the pH of the solution is raised from 8.5 to 10.5. On the contrary, the relative amount of nickel decreases from 21.26% to 4.17% and the percent of phosphorus also decreases from 1.76% to 0.7% when pH is changed from 8.5 to 9.0.
161 However, phosphorus is not observed when pH is at 9.5, 10.0 and 10.5.
In general, the reducing ability of sodium hypophosphite increases with respect to the rise of pH. This is due to the fact that higher pH can help generate more hydrogen followed by more nickel and copper in the coating as described in Equations 3-9, 3-10 and 6-9 below:
2++ Cu+→+22 Had Cu H Equation 6-9
With increasing pH, the secondary reaction usually occurred between the hypophosphite and hydrogen to form the elemental phosphorus is inhibited as shown in Equation 3-11.
At higher pH, the reaction shown in Equation 3-9 is facilitated to proceed forward by decreasing the concentration of the hypophosphite ions in the solution. Consequently, the amount of phosphorus present in the deposit decreases. The interaction between
2+ − Cu and H2PO2 is more favored at higher pH than that of lower pH. However, the pH of the solution has little effect on the reduction of Ni2+.
162
Figure 6-16 Composition of copper deposits on polyester fabric at different pH
The percent of copper present in the deposits under the optimal condition is 94.62% and the relative amount of nickel is 5.38%, but phosphorus is not observed.
6.3.2.2 Surface Morphology
The surface morphology of the copper deposits was investigated using SEM. The
SEM micrographs of the copper-plated polyester fibers with different nickel ion concentrations are shown in Figure 6-17. It is obvious that the particles size increases at higher nickel ions concentration. At the same time, it is found that the deposited film is composed of loosely stacked copper with uniform distribution. With more careful observation, it can be seen that some particles aggregate together to form larger granules. When the concentration of NiSO4 is 0.8 g/l, the surface of copper sheet is in the form of granular structure but the size of the granules composing of the aggregates is in the range of 100–150 nm as shown in Figure 6-17 (a). However, when
163 the concentration of NiSO4 is increased to 1.0g/l, more granules with the average size of 150–200 nm are formed as shown in Figure 6-17 (b). When the concentration is further increased to 1.5 g/l and 2.0 g/l respectively, it is found that the average size of the granules becomes even larger but the number of the porous structure decreases accordingly as shown in Figure 6-17 (c) and (d). The result indicates that the copper crystal grains with higher NiSO4 concentration are bigger than those with lower
NiSO4 concentration. This suggests that the effect of NiSO4 concentration on the growth of copper crystal is positive. In addition, higher NiSO4 concentration possibly can accelerate the growth of copper crystal growth but inhibit the nucleation of copper crystal.
(a) (b)
164 (c) (d)
Figure 6-17 SEM micrographs of the copper-plated polyester fabrics with different
NiSO4 concentrations: (a) 0.8 g/l, (b) 1.0 g/l, (c) 1.5 g/l and (d) 2.0 g/l
Figure 6-18 shows the SEM images of the copper-plated polyester fibers obtained from different pH of the plating solution. The SEM micrographs of copper deposits reveal that the deposits are composed of granules with different shapes and sizes. The surface morphology of the copper deposits largely depends on the bath pH. In the copper bath at pH 8.5, fine nano-particles formed on the polyester fibers are arranged randomly. As the bath pH is increased to 9.0, their shape gradually transforms from a nano-particle to a nano-block shape. It is obvious that particles size increases with the respect to the rise of pH in the plating solution. The deposition behavior of the electroless copper deposit can be described in the following steps. In the sensitization process, there are many nano-sized tin particles deposited on the surface of the polyester fiber substrate. During the activation process, the palladium particles with a larger size and less quantity than tin emerge on the surface of substrate. In the electroless plating process, the copper particles can deposit at the sites not only near palladium, but also at a distance from the palladium particles. The copper particles
165 consist of tiny grains and their shape is roughly spherical in the initial stage. When the copper particles grow large enough to touch each other in the horizontal direction, they will merge to form larger particles and become nano-blocks in shape. This is probably due to the hindrance of hydrogen and bubbles. As a result, the copper deposit generally contains a bottom layer of small particles beneath the layer of larger grains [260].
(a) (b)
(c) (d)
Figure 6-18 SEM micrographs of the copper-plated polyester fibers treated in the bath
at different pH: (a) 8.5, (b) 9.0, (c) 9.5 and (d) 10
The surface morphology of the copper-plated fibers obtained under the optimal condition show that the deposits are composed of the aggregated particles with the
166 size ranging from 200 to 300nm as shown in Figure 6-19.
Figure 6-19 SEM micrograph of the copper-plated polyester fibers obtained under the
optimal condition
A schematic diagram of copper deposition is shown in Figure 6-20. The copper ions, which are homogeneously distributed over the gap, are rapidly reduced to form nuclei at high temperature within the localized area between the substrate and the heated plate as shown in Figure 6-20 (a). In Figure 6-20 (b), the reduced nuclei diffuse across the narrow gap to the surface of substrate by thermal diffusion and convection. The continuous deposition is carried out through the 3-dimensional self stacking of nuclei to form copper granules. This process not only enhances the diffusion of nuclei and ions, but also induces the heterogeneous nucleation of new nuclei at the interface between the copper nucleus and solution. Moreover, the high temperature can induce the growth of small copper granules and grains by means of agglomeration or coalescence with the neighboring granules and grains during the reaction process. The agglomerated grains and granules correspond to the decrease of boundary density as shown in Figure 6-20(c) [261].
167
Figure 6-20 Schematic diagram of copper crystallization process of electroless
deposition showing (a) nucleation and diffusion of nuclei, (b) copper granule
formation and (c) the larger copper grain and granule growth
6.3.2.3 Crystal Structure
Figures 6-21 and 6-22 show the XRD patterns of the copper-plated polyester fabrics with different nickel ion concentrations and pH of the plating bath respectively. The four major strong characteristic peaks of the copper-plated polyester samples at
2θ=43.4°, 50.3°，74.2° and 90.1° correspond to the crystal faces of (111), (200), (220) and (311) of copper respectively. The XRD patterns identified by the PDF card of the
JADE-SCAN software reveal that the deposited copper film exhibits a characteristic face-centered cubic crystalline structure, implying that the copper-plated polyester fabrics have a perfect conductivity property. The characteristic peaks of nickel are not observed in the XRD diagram. It is probably due to very low content of nickel in the coating, which is in good agreement with the above-mentioned deposit composition.
In addition, it is worth noting that the copper films formed under different conditions were very pure without any detectable impurity such as CuO or Cu2O. This is
168 probably due to the fact that higher NiSO4 concentration and pH can make the solubility of O2 gas decrease in the bath [262], resulting in the generation of higher driving force to enhance the reduction of copper. As a result, the formation of copper oxide is prevented. High Cu (111)/ Cu (200) ratio is observed in the copper deposits, indicating that the copper film manifests the preferred (111) orientation. The copper film with a higher (111) texture is preferred due to its higher reliability against electro-migration and lower electrical resistance [263]. The ratio of Cu (111)/ Cu (200) is 3.06 for 0.8g/l as shown in Figure 6-21 (a). Figures 6-21 (b) and (c) show that the
Cu (111)/Cu (200) ratio rises to 3.37, 3.48 and 3.56 as the NiSO4 concentration is increased to 1.0g/l, 1.5g/l and 2.0g/l respectively. The ratio of Cu (111)/ Cu (200) is
2.93 at pH 8.5 as shown in Figure 6-22 (a). Figures 6-22 (b)-(d) show that the Cu
(111)/Cu (200) ratio rises to 3.07, 3.24 and 3.60 as the pH is increased to 9.0, 9.5 and
10.0 respectively. The intensity of the copper peaks increases with respect to the rise of the NiSO4 concentration and pH, which is due to the growth of the copper particles.
The crystallinity of the copper-plated polyester fabrics is also strengthened with increasing NiSO4 concentration and pH.
Based on the XRD results, the crystal size of the coatings can be determined from the broadening of the diffraction peak from the (111) planes by employing the Scherrer formula as expressed by Equation 6-10 [264]:
nλ D = Equation 6-10 B cosθ
169
where D is the crystal size;
λ is the X-ray wavelength corresponding to Cu Kα radiation (0.154056nm);
θ is the diffraction angle;
B is the full width half maximum (FWHM) of the diffraction peak at 2θ;
n is the Scherrer constant as 0.89 [17].
According to the Scherrer equation, the average sizes of copper particles are 26.68nm,
27.85nm, 30.70nm and 32.57nm with respect to the Cu (111) main peak for 0.8g/l,
1.0g/l, 1.5g/l and 2.0g/l NiSO4 respectively. Furthermore, the average sizes of copper particles are 21.42nm, 25.41nm, 26.36nm and 28.01nm with respect to the Cu (111) main peak for the pH of 8.5, 9.0, 9.5 and 10.0 respectively. Therefore, the results indicate that the particles grow at higher NiSO4 concentration and pH.
Figure 6-21 XRD patterns of the copper-plated fabrics with different NiSO4
concentrations: (a) 0.8g/l, (b) 1.0g/l, (c) 1.5g/l and (d) 2.0g/l
170
Figure 6-22 XRD patterns of the copper-plated fabrics at different pH: (a) 8.5, (b) 9.0,
(c) 9.5 and (d) 10
Figure 6-23 shows the XRD pattern of the copper-plated polyester fabric obtained under the optimal condition. The result indicates that the deposited copper film exhibits a characteristic face-centered cubic crystalline structure. The ratio of Cu
(111)/ Cu (200) is 3.52 and the average size of copper particles is 33.25nm.
171
Figure 6-23 XRD patterns of the copper-plated fabric under the optimal condition
6.3.2.4 Surface Resistance
The surface resistance of the copper-plated fabrics with different NiSO4 concentrations and pH of the plating bath is shown in Figures 6-24 and 6-25 respectively. It is observed that the surface resistance of the copper deposits decreases with respect to the increase in NiSO4 concentration and pH. Although nickel content in copper deposits increases, the decrease in resistance is mostly attributed to the increase in the weight of copper deposits and particle size in the present study. On the other hand, the reduction of surface resistance at higher pH is due to the increase in the weight of copper deposits, decrease of in the nickel content and the increasing particle size in the deposits. In addition, the presence of defects, hydrogen entrapment in the films and film discontinuity are the major factors contributing to the observed increase in the surface resistance of the plated films. The result also indicates that the surface resistance of the electroless copper deposits is higher than that of the bulk
172 copper as measured by a four-point probe because the nickel atoms co-exist in the copper coatings. This can be explained by the fact that nickel has a high electrical resistivity, and the nickel atoms in the copper lattice increase the number of crystal defects in the copper deposits. The resistance rate of the two metals concerned at 25℃ is as follows: Cu=1.750 ×10－8 Ω m; Ni=6.84 ×10－8Ω m [257].
The surface resistance of the electroless copper-plated polyester fabric obtained under the optimal condition is 76.58 mΩ/sq.
Figure 6-24 Surface resistance of the copper-plated polyester fabrics with different
NiSO4 concentrations
173
Figure 6-25 Surface resistance of the copper-plated polyester fabrics at different pH
6.3.2.5 EMI Shielding Effectiveness
Figures 6-26 and 6-27 show the EMI SE of the copper-plated fabrics with different concentrations of NiSO4 and pH respectively. The EMI SE of the copper-plated polyester fabrics ranges from 30 to 60 dB at the frequency of 2 GHz to 18GHz with different NiSO4 concentrations and pH. Due to a decrease of the surface resistance, the EMI SE increases with respect to higher NiSO4 concentrations and pH. The EMI
SE of the copper-plated polyester fabric obtained under the optimal condition ranges from 70 to 73dB (SE>99.999%) at the frequency of 2GHz to 18GHz as shown in
Figure 6-28. The result shows that the plated polyester fabric has good shielding effectiveness, and can be used in some special application such as advanced electronic products and national defense field.
174
Figure 6-26 EMI shielding effectiveness of the copper-plated fabrics with different
NiSO4 concentrations: (a) 0.8 g/l, (b) 1.0 g/l, (c) 1.5g/l and (d) 2.0g/l
Figure 6-27 EMI shielding effectiveness of the copper-plated fabrics at different pH:
(a) 8.5, (b) 9.0, (c) 9.5 and (d) 10
175
Figure 6-28 EMI shielding effectiveness of the copper-plated fabric obtained under
the optimal condition
6.4 Summary
The micro-structure and properties of the electroless nickel-plated and copper-plated polyester fabrics affected by the two most important influence factors, viz., (1) concentration of nickel sulfate solution and bath temperature for the electroless nickel plating, and (2) concentration of nickel sulfate solution and pH for the electroless copper plating, were studied in the present project. The deposit composition, surface morphology and crystal structure were investigated using SEM, EDX and XRD respectively. Surface resistance and EMI SE were also evaluated. Based on the experimental results and analysis, the following conclusions were drawn:
In the case of the electroless nickel plating, the nickel content in the coating increased
176 when the NiSO4 concentration was raised, but the phosphorus content decreased. On the contrary, the nickel content in the coating decreased whereas the phosphorus content increased slightly with respect to the rise of temperature. It was found that the nickel deposit layer became more uniform when prepared with higher NiSO4 concentration and bath temperature. Furthermore, the particle size and crystallinity enlarged with respect to the increase in the NiSO4 concentration and temperature. The
XRD patterns indicated that all the deposits were in amorphous phase. In addition, the surface resistance decreased and the EMI SE increased mostly due to the heavier weight and bigger particle size of the nickel deposit layer with respect to the rise of
NiSO4 concentration and bath temperature respectively. It was confirmed that the surface resistance of the plated fabric was 372.82 mΩ/sq, the EMI SE was about 60 dB at the frequency of 2GHz to 18 GHz.
As for the electroless copper plating, the results showed that at a higher NiSO4 concentration, the copper content present in the coating decreased. On the contrary, the nickel content increased slightly but the phosphorus was not observed. On the other hand, the copper content present in the coating increased, whereas the nickel content and phosphorus decreased with respect to the rise of pH. The surface morphology of the copper deposits showed that the particle size increased with respect to the rise of NiSO4 concentration and pH. The XRD patterns indicated that the copper-plated polyester fabrics were crystalline. In addition, there was a decrease in the surface resistance and an increase in the EMI SE with respect to the rise of Ni2+
177 concentration and pH of the solution as a result of gaining a greater weight in the deposits. The surface resistance of the plated fabric obtained was 76.58 mΩ/sq, the
EMI SE was more than 70 dB at the frequency of 2GHz to 18 GHz. The overall results suggested that the electroless nickel-plated and copper-plated polyester fabrics had a great potential application as an EMI shielding material.
178 Chapter 7 Characterization of Nickel/Copper Multi-layer
Plated Polyester Fabric
7.1 Introduction
In this chapter, the nickel/copper multi-layer plating on polyester fabric was prepared according to the optimal nickel and copper plating condition obtained in Chapter 5.
With regard to the nickel-plated and copper-plated polyester fabrics, the composition and micro-structure of nickel/copper multi-layer deposits were investigated. The surface resistance, EMI SE and corrosion resistance of the nickel/copper multi-layer plated fabric were also studied.
7.2 Experimental
7.2.1 Procedure
The electroless nickel/copper multi-layer-plated polyester fabric was prepared as follows. In this process, copper first deposited on the polyester fabric surface followed by the deposition of nickel on the copper coating to produce the final multi-layer metal-coated fabric. The plating bath composition and operating procedure of the electroless nickel and copper deposition were the optimal conditions as discussed in
Chapters 4 and 5. The weight of copper and nickel deposits was 32.56 g/m2 and 17.42
179 g/m2 respectively. In order to compare the micro-structure and properties of nickel/copper multi-layer coating, the electroless nickel-plated and copper-plated polyester fabrics with the same deposit weight of 50 g/m2 were prepared according to the optimal combination of the process parameters.
7.2.2 Characterization
The coating mass gain of deposits was measured by means of the weight on electronic scale (Shimadzu BX300 meter). It can be calculated using the following equation:
GG− W = 0 Equation 7-1 S
where W is the weight of the deposits in unit of area (g/m2);
G is the weight after plating;
G0 is the weight of untreated fabric before plating;
S is fabric area for deposition.
Potentiodynamic polarization corrosion tests were used to study the general corrosion resistance of the samples at room temperature using an electrochemical workstation
(Versa STAT 3, METEK) with analysis software. In the three-electrode system used in the potentiodynamic polarization test, the specimen was the working electrode with platinum foil as the counter electrode and saturated calomel (SCE) as the reference
180 electrode. All the surfaces of the sample were sealed so as to leave only one surface with the dimensions of 1 cm×1 cm exposed to the 3.5% NaCl solution. Before the experiment, each specimen was allowed to stabilize in 3.5% NaCl solution. The potentiodynamic polarization curves were obtained by scanning from -1000 mV below the open circuit potential (OCP) to 1000 mV at a scanning rate of 100 mV s−1.
The corrosion potentials Ecorr and corrosion current densities icorr were determined by
Tafel extrapolation with CorrWare provided by the working system. To ensure the reproducibility of the results, three replicate experiments were carried out.
7.3 Results and Discussion
7.3.1 Deposit Composition
The composition of the nickel, copper and nickel/copper multi-layer coatings was analyzed with an energy dispersive X-ray analysis (EDX). Figure 7-1 shows the typical elemental composition of the electroless nickel, copper and nickel/copper multi-layer coatings. The results indicated that the copper coating consisted of
93.12% copper and 6.88% nickel. The elemental composition of Ni deposits was found to be 90.98% Ni and 9.02% P. In the case of nickel/copper multi-layer plated deposits, the measured composition was found to have 54.51% Cu, 39.90% Ni and
5.59% P as shown in Figure 7-1(c). The carbon and oxygen elements of polyester substrate were not detected through nickel, copper and nickel/copper multilayer coatings, indicating that the polyester was coated evenly by the nickel, copper and
181 nickel/copper multi-layer coating. As compared to the copper coating, the nickel content increased and phosphorus was a new element. This indicates the formation of nickel film on the polyester fabric via the copper inter-layer. The weight content of copper in the nickel/copper film was lower than that of the copper coating as reflected by the thin covering of nickel. It was inferred that the multi-layer metal-coated fabric was successfully prepared by coating nickel on the copper plated polyester fabric.
(a)
(b)
182 (c)
Figure 7-1 Deposit composition of (a) nickel, (b) copper and (c) nickel/copper
multi-layer plated polyester fabrics
7.3.2 Surface Morphology
The surface morphology of nickel/copper multi-layer deposits was studied using SEM and compared with the nickel and copper deposits. Figure 7-2 shows the SEM micrographs of the nickel, copper and nickel/copper multi-layer coatings. The nickel deposits obtained under optimum conditions are compact and exhibit larger nodule size feature with a typical cauliflower like structure as shown in Figure 7-2 (a). Figure
7-2 (b) shows that the hemispherical nano-blocks clusters can be observed on the surface of copper deposits. The deposits are uniform and loose, and some lumps are clearly observed. Some irregular nano-blocks with the size ranging from 50nm to
200nm appear in the image. The SEM micrograph analysis of the deposits shows that the surface is smooth and dense with no obvious surface damage.
183
The SEM image of a typical nickel/copper film shown in Figure 7-2 (c) suggests that the films have typical spherical nodular structures with good uniformity and dense coverage. The main difference between the surface morphology of nickel and nickel/copper is the shape of the nodules. The nodules of nickel coatings are in the form of “elliptical” shape and agglomerated nodules, while the spherical nodules are observed in the nickel/copper coatings. In contrast, the surface of nickel/copper coating becomes very smooth and compact as shown in Figure 7-2 (c). The grains composed of fine nodules uniform in size are refined and the gaps between them are very small. The observed randomness in the distribution of spherical nodules with the size ranging from 100–150 nm is due to the partially amorphous structure present in the nickel/copper film grown in the perpendicular direction of the fabric surface. This is also interpreted as acceleration in nucleation rate.
(a) (b)
184 (c)
Figure 7-2 SEM micrographs of (a) nickel-plated, (b) copper-plated and (c)
nickel/copper multi-layer plated polyester fibers
7.3.3 Crystal Structure
The diffraction patterns of the electroless nickel, copper and nickel/copper deposits are shown in Figure 7-3, and the corresponding peak position, FWHM and grain size are given in Table 7-1. The diffraction pattern of the nickel-coated polyester only exhibits a single broad peak centered 2θ at 44.5° corresponding to the Ni (111) phase with a grain size of 2.28 nm, indicating the amorphous nature of the coatings as shown in Figure 7-3 (a). This is in correspondence with the knowledge that an electroless nickel deposit will become amorphous when its P% is above 7% [217,
253]. The intensity of the Ni (111) peak is so low that the quantity of Ni crystal can be neglected. In Figure 7-3 (b), the four major strong characteristic peaks of the copper-plated polyester sample at 2θ=43.4°, 50.3°，74.2°and 90.1° correspond to the crystal faces of (111), (200), (220）and (311) of copper respectively. There is no impurity peaks being observed, indicating the high purity of the final products. In
185 Figure 7-3 (c), only the characteristic peaks of copper are observed and there is no indication of nickel phase formation in the nickel/copper film. It is found that peak broadness has been increased as compared to the peak obtained for copper deposit.
Similar observations have also been reported by Lu and Zangari [265]. The presence of nickel in the film is supposed to cause the segregation of nickel atoms in the grain boundaries in order to block the grain boundary diffusion path for copper diffusion.
Decrease in grain size or higher value of FWHM is probably due to the presence of Ni atom and P atom in face-centered cubic copper matrix [265, 266], which would have influenced the formation of crystalline structure [267]. This can be supposed that the prepared nickel coating on the copper-plated fabric is amorphous, and so it may be helpful to improve the corrosion resistance of the copper deposits [180].
Figure 7-3 XRD patterns of (a) nickel-plated, (b) copper-plated and (c) nickel/copper
multi-layer plated polyester fabrics
186 Table 7-1 Full width at half maximum and grain size of nickel, copper and
nickel/copper multi-layer coatings
Type of coating FWHM Grain size (nm)
nickel 3.714 2.28
copper 0.332 25.48
nickel/copper 0.397 21.31
7.3.4 Surface Resistance
The surface resistances of nickel-plated, copper-plated and nickel/copper-plated fabric are shown in Figure 7-4. It is observed that the surface resistance of the nickel deposit is the highest, while the surface resistance of the copper-plated fabric is the lowest.
However, the surface resistance of nickel/copper-plated fabric is medium due to different electrical resistivity of copper and nickel [257]. Ni deposits with higher P content tend to be amorphous leading to lower conductivity. This is because the conductivity of the amorphous electroless nickel deposits coating is poor than that of crystalline. When the deposits change from amorphous structure for nickel deposits to crystal structure for copper deposits, the relative electrical conductivity has an obvious increase. This might be attributed to the grain growth in copper films with high crystallinity. However, in the crystal deposits, the electrical conductivity of nickel/copper deposits is lower than that of the copper deposits. This is due to the fact that nickel/copper deposits have higher P content and therefore lower crystalline than
187 the copper deposits. The experimental results show that the nickel/copper multi-layer coating has good electrical conductivity.
Figure 7-4 Surface resistance of nickel-plated, copper-plated and nickel/copper-
plated polyester fabrics
7.3.5 EMI Shielding Effectiveness
Figure 7-5 shows the EMI SE of nickel-plated, copper-plated and nickel/copper multi-layer plated polyester fabrics. The result reveals that under the same weight of the deposits, the EMI SE of the nickel-plated polyester fabric only ranges from 45-50 dB at the frequency of 2 to 18GHz. Nevertheless, it is found that there is a tremendous increase in the EMI SE of polyester fabric coated with copper. On the other hand, the
EMI SE of the nickel/copper multi-layer coated fabric is slightly lower than that of copper-coated fabric but greatly higher than that of the nickel-coated sample. This
188 indicates that there is no significant difference in EMI SE between the nickel/copper-plated and copper-plated polyester fabrics due to interface reflection between nickel and copper deposits. A deposit of 50 g/m2 of the nickel/copper multi-layer plated polyester fabric will provide about 55 to 60dB of shielding which will be sufficient for 95% of the current commercial applications.
Figure 7-5 EMI shielding effectiveness of (a) nickel-plated, (b) copper-plated and (c)
nickel/copper-plated polyester fabrics
7.3.5 Anti-corrosive Property
Potentiodynamic polarization measurements were employed to evaluate the effect of coatings on the corrosion resistance characteristics of the nickel-plated, copper-plated and nickel/copper multi-layer-plated polyester fabrics. The potentiodynamic polarization curves obtained for the nickel, copper and nickel/copper coatings in 3.5%
189 NaCl solution at room temperature are presented in Figure 7-6. The corrosion potential Ecorr and corrosion current density icorr were calculated from the intersection of the cathodic and anodic Tafel curves, and the extrapolated cathodic and anodic polarization curves were tabulated as shown in Table 7-2.
As for all the polarization curves, active reaction dominates the anodic side and oxygen reduction dominates the cathodic side. The potentiodynamic curves of nickel, copper and nickel/copper multi-layer coatings have similar shapes. The corrosion potentials of all of the samples are in the range of −520 to −400mV SCE. It is found that the corrosion potential of the electroless plated nickel shifts to a more positive direction when compared with those of the electroless plated copper and nickel/copper. The corrosion current of nickel is also almost smaller than that of the electroless plated copper and nickel/copper at the same potential. Table 7-2 shows that nickel deposit has the most positive corrosion potential, the least corrosion current density and the highest polarization resistance. Therefore, the corrosion resistance of nickel deposit is the best. It is reported that phosphorus is enriched on the surface layer by the preferential dissolution of nickel. The passive barrier layer of
- H2PO2 formed by the reaction of the enriched P with water will block the supply of water to the electrode surface and prevent the hydration of nickel. This is considered to be the first step to form either soluble Ni2+ species or a passive nickel film [224]. It can be concluded from Figure 7-6 that the anti-corrosion ability in 3.5% sodium chloride solution is electroless nickel plated > electroless plated nickel/copper >
190 electroless plated copper. Generally, the improved corrosion resistance obtained for electroless the nickel/copper and nickel coatings is due to the enrichment of phosphorous on the electrode surface [268, 269].
When compared, it is obviously seen that the corrosion potentials (Ecorr) of the nickel coating and the nickel/copper multi-layer coating are −299 and −337 mV respectively.
Corrosion potential is increased by 38 mV and the coating moves the corrosion potential of the polarization of the nickel-plated fabric in the noble direction. On the contrary, the corrosion current density of nickel coatings is 1.06×10-7 A /cm2 which is somewhat lower than that of the nickel/copper coating, i.e. 3.61×10-7 A /cm2. It is evident that the nickel coating layer provides an effective protection more than the nickel/copper coating. It is well known that the chemical resistance of an electroless alloy deposit, to a large extent, is determined by its structure [129]. For example, the amorphous structure has an excellent corrosion resistance but the crystalline structure is not so good. As a matter of fact, the corrosion resistance characteristics of the electroless nickel and nickel/copper coatings are governed primarily by their phosphorous content. In general, the electroless deposition of nickel and nickel/copper deposits can be classified into three categories according to the phosphorus content, i.e. low (1–3 %), medium (4–7 %) and high (7 % and above) phosphorus deposits
[194, 270]. The nickel or nickel/copper deposits with different phosphorus contents have different physicochemical properties [257]. When the phosphorus contents are in low and medium levels, the deposited electroless plating nickel or nickel/copper is a
191 merely mixture of amorphous and micro-crystalline nickel. However, when the phosphorus content is high, a full amorphous micro-structure can be formed [257]. In the present study, the electroless nickel coatings are in amorphous structure, whereas the electroless nickel/copper coatings are in crystalline structure. According to Lee
[271], this superior corrosion resistance of nickel coatings is also attributed to the absence of crystalline defects, and their chemically homogeneous single-phase nature.
This in turn ensures the formation of a uniform and highly protective passive film.
The possible reason is that the nano-structure should accelerate corrosion by forming much more micro-electrochemical cells between the huge amount of grain boundaries and the matrix. As a result, the amorphous structure should exhibit inherently a high corrosion resistance because of its extreme structural homogeneity without preferential corrosion paths like grain boundaries or other structural defects [270,
272].
When compared with the copper coatings, the corrosion potential Ecorr of nickel/copper coatings shows a significant positive shift to −337 mV. The corrosion
-6 2 current density icorr decreases evidently from 4.67×10 A/cm of the copper deposit to 3.61×10-7 A/cm2 of the electroless nickel/copper layer. This implies that the anodic dissolution reaction of the nickel/copper coating is restrained, thereby effectively decreasing the corrosion sensibility of the coated sample in NaCl solution.
The results demonstrate that the electroless nickel coating does provide a good protection for the copper coatings. This behavior is expected because the nickel
192 deposition on copper has a surface conditioning effect on refining the crystal grains as well as producing a dense and uniform structure, leading to the increase in the corrosion resistance. This is probably due to the fact that the existence of Ni and P obviously improves the corrosion resistance of Cu. As a result, the crystal structure of the deposited nickel layer on copper has a lower corrosion rate than that of the deposited copper.
Figure 7-6 The polarization curves of (a) nickel-plated, (b) copper-plated and (c)
nickel/copper multi-layer plated polyester fabric in 3.5% NaCl solution
193 Table 7-2 Corrosion characteristics of the electroless nickel-plated, copper-plated and
nickel/copper-plated polyester
Type of coating Ecorr icorr
(mV vs. SCE) (A/cm2)
nickel -299 1.07×10-7
copper -511 4.67×10-6
nickel/copper -337 3.61×10-7
7.4 Summary
An electroless plating technique was used to prepare the nickel/copper multi-layer film on polyester fabric. When compared with the properties of the electroless nickel and copper coatings, the deposition composition, surface morphology, crystal structure, surface resistance, EMI SE and corrosion resistance of the nickel/copper coating were evaluated and discussed. The conclusions are summarized as follows:
Based on the results obtained, the following conclusions can be drawn:
(1) The nickel coating was composed of 90.98% Ni and 9.02% P; and the copper coating contained 93.12% Cu and 6.88% Ni. On the other hand, the nickel/copper coating consisted of 54.51% Cu, 39.90% Ni and 5.59% P.
(2) It was found that the nickel coating exhibited some coarse nodules which were
194 gathered in a micro-scale cauliflower shape, while the copper coating demonstrated nano-block clusters. However, the nickel/copper deposit surface morphology revealed the presence of ultrafine nodules. They were compact, uniform in size and smooth without any detectable defects or porosity.
(3) XRD analyses revealed that the nickel deposits were in an amorphous phase. The structures of the copper coating and nickel/copper coating were typical crystalline.
However, FWHM was broadened when nickel was coated on the copper-plated fabric when compared with the copper coating.
(4) The sequence of surface resistance of the plated fabrics was summarized as follows: nickel coating> nickel/copper coating>copper coating. The EMI SE sequence was adverse due to different micro-structure, i.e. various P content. When compared, the EMI SE of the nickel/copper-plated fabrics was close to 55-60 dB with the frequency ranging from 2 to 18 GHz. Hence, the nickel/copper multi-layer plated fabric could meet the general electromagnetic shielding demand.
(5) The potentiodynamic measurements indicated that the nickel coating exhibited a better anti-corrosion ability in 3.5% sodium chloride solution than those of the electroless plated nickel/copper and copper. High phosphorus content of the nickel coating on polyester fabric resulted in an enhanced corrosion resistance property. The results demonstrated that the coating of nickel obviously could improve the corrosion
195 resistance of the copper-plated fabric. The decrease in corrosion rate was mainly attributed to the high P content, low micro-porosity and smooth morphology of the nickel/copper coating.
In conclusion, the electroless nickel/copper-plated fabric is the best compromise to satisfy the requirements of both corrosion resistance and EMI SE in most the applications.
196 Chapter 8 Textile Design Application of Electroless Plating
8.1 Introduction
According to the previous experimental results, both functional properties and unique metallic appearance was obtained. These nickel-plated and copper-plated polyester fabrics had a metallic color with reflective effect and smooth handle. The purpose of this chapter is to investigate the images of color, shape and texture on the electroless nickel, copper and nickel/copper multi-layer plated fabrics. This chapter also describes the appearances of the nickel, copper and nickel/copper multi-layer plated fabrics produced by different plating conditions, e.g. different nickel sulfate concentrations and temperatures for the electroless nickel plating and various nickel sulfate concentrations and pH for the electroless copper plating as mentioned in
Section 5.3.1. Different thickness of nickel deposit on the copper-plated fabrics was investigated for the nickel/copper multi-layer. A range of color of the nickel, copper and nickel/copper plated fabrics was developed based on preliminary study. Design applications of the electroless nickel and copper plating on fabrics using the optimizing process developed in Section 5.3.2 are explored. The electroless nickel-plated and copper-plated fabric designs combining design elements, technology and materials were conducted on two types of fabrics with collections being presented.
197 8.2 Performance of Plated Fabrics
The textile designs of the electroless nickel, copper and nickel/copper plating were inspired by the traditional dyeing techniques. In the present study, two types of fabrics namely Green-Fabric (50% polyester and 50% nylon) and Black-Fabric (28% cotton,
29% vinylon, 30% polyester and 13% nylon) were used. Design method, namely tie-dye and material shielding were chosen to create various shapes. The fabrics were gathered together and masked at the designed areas with thread, wooden sticks or stainless steel clips to achieve the purpose of creating pattern with different shapes on the experimental fabrics. When the covered materials were removed, the original color pattern of the masked area was presented with no metal plating while the metallic color appeared at the areas without shielding.
8.2.1 Electroless Nickel-plated Fabrics
The design results of the electroless nickel-plated Green-Fabric with different NiSO4 concentration and bath temperature are shown in Figures 8-1 and 8-2 respectively. In each photograph, both the covered and uncovered areas can be found representing the areas where the fabric is covered and uncovered. The results showed that the
Green-Fabric changed to black color throughout the process, while the light grey appearance was observed on the surface of the plated fabrics. The intensity of brightness increased at a higher NiSO4 concentration and bath temperature due to the rise of the plating rate of nickel plating (see Section 4.3.2), resulting in the increment
198 of the weight of nickel deposits. However, when the NiSO4 concentration was increased to 21 g/l and the bath temperature was raised to 85℃, the plating bath tended to be unstable with the dark grey color being observed on the surface of the fabric. Owing to the high thermal stability of metallic nickel, the nickel-plated fabric thus remained grey after being treated at high temperature.
(a) (b)
(c) (d)
Figure 8-1 Design results of of the electroless nickel-plated fabric with different
NiSO4 concentrations: (a) 6g/l, (b) 9g/l, (c) 12g/l and (d) 15g/l
199 (a) (b)
(c) (d)
Figure 8-2 Design results of the electroless nickel-plated fabric at different bath
temperatures: (a) 50℃, (b) 60℃, (c) 70℃ and (d) 80℃
Electroless nickel plating was carried out on two different kinds of fabrics under the optimal condition. Complementary materials such as metal clips and wooden sticks were employed to generate various shapes. A variety of images were created after the electroless nickel plating as shown in Figure 8-3. The plated fabric displayed light grey in the uncovered area and the repeated line shape was also created after nickel plating. The plated fabric exhibited the reflective effect after the electroless nickel plating treatment. Figure 8-3 also indicates that the smooth Green-Fabric exhibited more shining effects than the Black-Fabric after the nickel plating. The results
200 suggested that the substrate of fabrics could influence the metal-plated effects.
(a) (b)
Figure 8-3 Design results of of the electroless nickel-plated (a) Green-Fabric and (b)
Black-Fabric
8.2.2 Electroless Copper-plated Fabrics
The design results of the electroless copper-plated polyester fabric obtained with different NiSO4 concentrations and pH of the plating bath are shown in Figures 8-4 and 8-5 respectively. The copper-plated fabrics generally showed red color except under the condition of 0.8g/l NiSO4 and pH 8.5 where the plated fabric exhibited brownish red and red purple respectively. These phenomena could be explained by the fact that the plating rate of copper was so slow that some metallic copper were oxidized in the plating bath. The color effects could be enhanced with respect to the increase in NiSO4 concentration and pH. As a result, the shape was clear and the plated fabric showed the smooth handle.
201 (a) (b)
(c) (d)
Figure 8-4 Design results of the electroless copper-plated fabric with different NiSO4
concentrations: (a) 0.8g/l, (b) 1.0g/l, (c) 1.5g/l and (d) 2.0g/l
(a) (b)
202 (c) (d)
Figure 8-5 Design results of the electroless copper-plated fabric at different pH of the
solution on : (a) 8.5, (b) 9.0, (c) 9.5 and (d) 10
Copper showed the inexhaustible color effects by adjusting various plating treatment conditions. In order to obtain different colors in the present study, a high temperature solution was used to speed up the weathering of copper so as to achieve copper patina which was the product of corrosion on copper. Before carrying out each coloring operation, the solution needed to be kept at the temperature of around 85℃ and pH
8.5. It was obvious that the color obtained depended largely on the treatment condition. Different conditions of the electroless copper plating created different color effects in the uncovered areas as shown in Figure 8-6. The color of the copper-plated fabric freshly prepared was red as shown in Figure 8-6 (a), but it turned deep rust red after 10 minutes as shown in Figure 8-6 (b). After 15 minutes the color of the copper-plated fabric turned red-purple and then black brown as shown in Figure 8-6
(c) and (d) respectively. The color of the copper-plated fabric turned to yellow-green, blue-green and then blue after 25, 30 and 35 minutes. The blue color of the copper-plated fabric showed that a significant corrosion of the copper content had
203 taken place.
(a) (b)
(c) (d)
(e) (f)
204 (g)
Figure 8-6 Results of plating time on the copper-plated fabric exposed to the plating
bath at pH 8.5 and 85℃: (a) 0min, (b) 10min, (c) 15min, (d) 20min, (e) 25min, (f)
30min and (g) 35min
The overall results indicated that copper patina was formed after the copper-plated fabric had been exposed to the copper plating bath at high temperature, i.e. 85℃.
After a continuous exposure to the plating bath, the copper-plated fabric rapidly turned to the more familiar copper color due to the formation of a thin surface oxide.
On further exposure, the color was darkened to brown and then to black as the oxide grew in thickness. These changes in color were due to the formation of cuprite which had the chemical formula of Cu2O wherein the copper was in the +1 oxidation state
[273]. Subsequently, there was a formation of the green layer on the top of the oxide layer by further reaction with the trace atmospheric impurities so that the copper was in the +2 oxidation state. The external green layer commonly consisted of brochantite,
CuSO4 3Cu(OH)2, and the inner layer of cuprite, Cu2O [274].
The observation of the formation of the corrosion products can be schematically
205 illustrated in Figure 8-7. This has led to the following model for the patina formation as shown in Figure 8-8. Cuprite is formed on the copper surface by the oxidation reaction of copper [275, 276].
20.5Cu+→ O22 Cu O Equation 8-1
Figure 8-7 Schematic illustration of the formation of brochantite crystals on the
copper surface
Figure 8-8 Illustration of the reactions involved in the brochantite formation during
patination
The oxidation rate of Equation 8-1 is controlled by the diffusion of Cu+ through the
206 cuprite layer, and is equivalent to the dissolution/oxidation rate of cuprite as shown in
Equation 8-2. Surface copper ions originated from the cuprite are further oxidized from Cu+ to Cu2+ in the presence of a layer of surface water via the two partial electrochemical reactions as shown in Equation 8-2 [276].
Cu++→+ Cu2 e Equation 8-2 − 0.5OHOe22++→ 2 2 OH
Brochantite is precipitated by the reaction shown in Equation 8-3 when the pH in the water becomes sufficiently alkaline
22++ − Cu++ SO44263() OH → CuSO Cu OH Equation 8-3
Brochantite thus precipitates from an aqueous surface layer formed from an oxidative dissolution of some cuprite.
Two kinds of the copper-plated fabrics with red color obtained under the optimal condition created some fine patterns as shown in Figure 8-9. The figure exhibited the red color of the copper-plated fabrics in the uncovered areas of both fabrics whereas black color appeared in the covered area of Green-Fabric. However, for Black-Fabric shallow red color was observed in the covered area where the fabric was not soaked in the plating bath, while the original black color appeared in the covered area as a result of the penetration of a large number of the electroless plating solution into this. This
207 was mainly due to the effects of sensitization, activation and plating solution on the fabrics. Figures 8-9 shows the metallic copper color surface on both fabrics exhibiting delicate forms in the fabrics. The figure indicates that the two copper-plated specimens exhibited some clear edges of patterns with red color. This evidence verified the shielding ability of the masking metal plating on the fabric. The plated
Green-Fabric had a smoother surface than the plated Black-Fabric. The contrast of the plated Green-Fabric was stronger than the plated Black-Fabric.
(a) (b)
Figure 8-9 Design results of the electroless copper-plated: (a) Green-Fabric and (b)
Black-Fabric
8.2.3 Electroless Nickel /Copper Plated Fabrics
The design results of the electroless nickel/copper multi-layer plating on the polyester fabric with different plating times of nickel and various color effects are shown in
Figure 8-10. The light grey with some red color was observed after the copper-plated fabric was immersed in the electroless nickel plating bath for 6 min as shown in
208 Figure 8-10 (a). The grey ingredients increased but the red ingredients decreased with the respect to the prolonged plating time because more deposition of nickel would increase thickness of nickel, leading to the enhancement of nickel grey for a longer plating time. In addition, it was found that the smooth and reflective surface appeared in all the plated fabrics.
(a) (b)
(c) (d)
Figure 8-10 Design results of the electroless nickel/copper multi-layer plated fabric
with different plating time of nickel: (a) 6 min, (b) 9min, (c) 12min and (d) 15min
Figures 8-11 shows the surface transition from red to light grey on the Green-Fabric and Black-Fabric with the clear patterns and subtle shapes. The color was changed
209 gradually from light red to light grey after some nickel particles deposited on of the copper-plated fabric. These areas were emerged from the patterned surface providing a clear contrast with the covered areas. Their colors changed deeply when they were overlapped with the fabrics during metal plating. The design created in Figure 8-11 produced different results when immersing the fabrics in the nickel plating bath for different times. The final pattern showed the clear shape and subtle colors of both fabrics. Area A of the copper-plated fabric was not immersed in the nickel plating bath, the fabric still exhibited red color. On the other hand, Area B on the nickel/copper plated fabrics showed light red color with little grey after the plated fabric was immersed in the nickel plating solution for 5 min. However the plated fabrics show light grey in Area C after the fabric was immersed in the nickel plating solution for 10 min, indicating that the copper-plated fabric was uniformly covered with nickel particles. It was obvious that metal color was an important visual factor in this design. Many small covered areas could preserve the black color pattern on the grey or shallow red background. The color of the designed fabrics ranged from black to red and grey tone for the Green-Fabric, and also from shallow red to red and grey tone for the Black-Fabric.
A shape is a visually perceived area created by either an enclosing color change or defining the outer edge. The applied materials are usually used to produce the simple geometric forms. The lines are created by irregular or regular structures. They are capable of forming infinite forms which are made quickly to convey all sorts of
210 moods and feelings. As shown in Figures 8-11, both the plated fabrics demonstrated the nickel/copper multi-layer plated fabric combining the parallel lines and modified colors harmoniously.
(a) (b)
Figure 8-11 Results of the electroless nickel/copper plated: (a) Green-Fabric and (b)
Black-Fabric
8.2.4 Analysis
The nickel-plated fabrics obtained showed a uniform light grey color, while the copper-plated fabrics indicated different color effects under various conditions. The nickel/copper multi-layer plated fabric also exhibited different colors with various thickness of nickel deposit on the copper-plated fabric. The results suggested that nickel was stable to heat but copper was not stable to heat and easy to be oxidized.
Copper is well known for its red but under the influence of moisture and air containing carbon dioxide and sulfur compounds, it rapidly discolors. Depending on
211 the exposure period and conditions, the discoloration will show a yellow or brown to black shade [277, 278].
With regard to metal, patina is a coating of various chemical compounds such as oxides or carbonates formed on the surface during exposure to the elements
(weathering). The electroless plating results suggest that copper is a suitable material for surface treatment to produce a variety of colors, ranging from dark red to black and blue. The final color depends on the solution composition, immersion time and operator skill.
Coloring is primarily an art and so practical experience is necessary to develop the skill required to produce uniform finishes consistently. Copper is colored chemically to enhance the image of a product, provide an under-coating for the subsequent organic finishes, and reduce light reflection in optical systems. Chemical coloring produces a thin layer of chemical compound on the surface of the base metal. This layer retains some of the characteristics of the metal surface prior to coloring such as smooth and lustrous or dull [279, 280]. The blue-green patina of copper is a protective coating that is aesthetically pleasing. All the colors which are the favorites for designers can manifest the class and nobility. The color change of the copper-plated fabric provides the prospect of wide applications of metallized textiles.
The various patterns were obtained by the material shielding method, and the result of
212 shielding areas showed the color of the fabrics. The varieties of images were created after conducting the electroless nickel, copper and nickel/copper multi-layer plating on two different types of fabrics. Both fabrics had shining metallic color surfaces. The substrates of fabrics could influence the metal-plated effects, for example, the smooth
Green-Fabric exhibited more shining effects than the Black-Fabric after the plating of nickel, copper and nickel/copper.
The present study has discovered the relationship between the electroless plating condition and the design results of plated fabrics. The influence of plating condition on the results of nickel, copper and nickel/copper plated fabrics was also investigated.
It was obvious that the electroless plating could change the original appearance of the fabric through the deposition of metal particles on the fabric surface. The final pattern was well defined using many details such as lines and forms created by the design methods. By combining the electroless plating technology with nickel, copper and nickel/copper, there is a great potential development of textile design depending on the techniques of masking and operational condition.
8.3 Design Application
In the fashion world, new technological textiles have made a significant impact with their unique look, handle and performance. In the last two decades, momentous advances in textile technology have provided all areas of fashion with futuristic
213 fabrics that are functional as well as beautiful [82]. Many of the leading textile designers and manufacturers determine the fabric look, texture and performance through the advanced techniques. Furthermore, fashion designers are frequently adopting new technologies as they can transform a traditional textile by updating it for a contemporary look to ensure a wide appeal [90]. Many textile practitioners creating fabrics for fashion are interested in innovative techniques to create exceptional textures [90]. Electroless plating can endow fabric with irregular look and texture by using the traditional techniques to give the fabric a unique identity.
The color, shape and texture account for the three essential elements of textile design.
Color not only deals with visual perception, but also acts a response to the psychological refraction of light. Color is used to create moods and enhance the design theme of the final products. It is a powerful element in decorative textiles which provide atmosphere and color emotion of either warmth or coolness. For example, red has a warming and calming effect on people, while grey has a cold effect. Color of grey produced by the nickel plating and red produced by the copper plating were important visual effects in the present study as shown in Figure 8-12.
Light grey was uniformly formed on the Green-Fabric by means of the electroless nickel plating and light red appeared on the Black-Fabric by means of the electroless copper plating in the uncovered areas. Both the grey and red color had a shining effect on the surface of fabric. However, the covered areas of the Green-Fabric were finally changed to black color, thereby producing an ideal background. The phenomenon was
214 attributed to the strong effect of plating bath on fabric. The Black-Fabric color contrasted with the brilliant grey nickel and red copper, resulting in creating a beautiful and rich visual effect.
Figure 8-12 Electroless nickel-plated and copper-plated Green-Fabrics
The characteristic of colors depends on the uniqueness of the operative procedure.
Different areas of the fabric shown in Figure 8-13 were treated for different periods of time. As shown in Figure 8-13(a), the lower area of the fabric turned to black brown which became darker after the plating process was conducted for 15 minutes. Figure
8-13(b), (c) and (d) show that the areas treated for 25 minutes turned to golden yellow,
215 while the areas for 30 minutes and 35 minutes changed to green and blue in the middle area respectively. These designs exhibited a gradual change of color appearing at different areas for the same piece of fabric. The change in color reinforced the potential of the electroless copper plating for rendering the alternative and innovative textile designs. All the plated fabric exhibited a reflective and lustrous surface. It is obvious that the sharply contrasting values attract attention and the use of light color against dark or vice versa can create the illusion of size difference and reflect three dimensional effects.
Unrestricted patterns were obtained on the Green-Fabric by means of the tie-dye and material covering methods. The various shapes were formed based on the design operation which enhanced the versatile features of the fabric. The formation of final shapes after the electroless nickel and copper plating was dependent on the design patterns using the shielding tools which were closely related to the design operation.
The nickel-plated and copper-plated Green-Fabrics showed very clear patterns on the surface of fabric. These plated fabrics could be manufactured in a large scale with three-dimensional shapes and flexibility. There were boundless potential depending on the design operation and application.
A variety of images were created after conducting the electroless nickel and copper plating on the Green-Fabric. The plated fabric showed a smooth and reflective texture with beautiful appearance and tactile qualities. As a result, the unusual and innovative
216 fabrics could be created by means of electroless plating. There was a minor change in the handle of the nickel-plated and copper-plated fabrics when compared with the original fabrics.
(a) (b)
(c) (d)
Figure 8-13 Copper patina on the copper-plated Green-Fabric
Design collections obtained from the deposition of nickel and copper on the
Black-Fabric are shown in Figure 8-14. Grey nickel, red copper, shallow red, green and black colors were observed from these plated fabrics. The Black-Fabric changed to shallow pink after the sensitization and activation treatments. The color change was due to effect of acid solution on the fabric. The uncovered area of the fabric changed
217 to grey and red colors after the nickel and copper plating. However, the covered area of fabric was changed to green color as a result of the penetration of only small amount of the electroless plating solution into this, but turned to the original black after the fabric was soaked in the plating bath. The formation of final shapes and colors in the covered areas depended on the intensity of pressure generated by the tied tools. Owing to the fact that the metallized fabric appearance depended on the texture of the substrate, thus the Black-Fabric had more rough surface effect when compared with the Green-Fabric. Different treatment conditions produced various value effects, e.g. the red color of Black-Fabric shown in Figure 8-14 (b) was more shining after the plating treatment than that shown in Figure 8-14 (a). The explanation was that less catalytic palladium was dispersed on the surface of the fabric, leading to slow metal plating. As a result, copper was not very uniformly deposited on the fabric. In addition, it was more difficult to obtain copper patina in the Green-Fabric than the
Black-Fabric.
(a) (b)
Figure 8-14 (a) Copper-plated and (b) nickel-plated and copper-plated Black-Fabrics
218 8.4 Summary
The effects of the electroless nickel and copper plating on the black and
Green-Fabrics were investigated for the metallized fabric design application under different plating conditions. The results indicated that nickel and copper showed a light grey and red color, respectively. However, the copper-plated fabric illustrated various color effects through different treatment. Copper patina was produced on the copper-plated fabric when the fabric was exposed to the plating bath. The red, brown red, black red, golden yellow, blue and green colors were formed on the copper-plated fabric, creating a lot of interesting color effects. The nickel/copper multi-layer plated fabric indicated that the color changed from red to grey color with respect to an increase of thickness in the nickel deposits. All the fabrics had shining surface and clear shape after the electroless plating. However, the Green-Fabric had more shining effect than the Black-Fabric. The present study laid the foundation for the electroless nickel, copper and nickel/copper plated fabric design.
According to the results of design application, some advantages were revealed in the current metallizing fabric design including (i) nickel and copper design images were obtained by changing the plating condition; (ii) rich colors appeared in the copper-plated fabric; (iii) nickel/copper multi-layer design with grey and red color effects was developed; (iv) a variety of decorated textiles with shining surface have been achieved after conducting the electroless nickel, copper and nickel/copper plating on two different kinds of fabrics; (v) electroless plating operation and design
219 creation might produce delicate patterns; and (vi) the design images were influenced by many factors such as metal, plating condition, fabric and design method.
220 Chapter 9 Conclusions and Suggestions for Future Research
9.1 Conclusions
The aim of the study was to evaluate the optimization of the electroless nickel and copper plating for the polyester fabric. Thermodynamics and kinetics of the electroless nickel and copper plating for the polyester fabric were also proposed. The effects of process parameters on the plating rate were investigated, and the kinetic models used for the electroless nickel and copper plating on the polyester fabric were established. Plating processes were optimized to obtain good properties by using the method of experimental design. The effect of the plating parameters on the micro-structure and properties of the electroless nickel-plated and copper-plated fabric were also studied. The mechanism of relationship between the micro-structure and properties of the electroless plated fabric was revealed. The electroless nickel/copper multi-layer plated fabric was developed for obtaining good EMI shielding effectivenss and anti-corrosive properties. Several conclusions representing some significant contributions to the knowledge of the relevant scientific areas have been drawn and summarized as follows:
1. The study has established kinetic models for metallized textile functional
and design applications, and determined the optimization of electroless
221 nickel and copper plating processes.
The thermodynamic analysis of equilibrium in the electroless nickel and copper plating showed the possibility of reducing the nickel ions and copper ions to form metal particles which would deposit on the fabric in alkaline solutions. The instantaneous nickel and copper plating rate depended largely on the plating parameters. As for the electroless nickel and copper plating, the plating rates enhanced with respect to the increase in the concentration of nickel sulfate and sodium hypophosphite, pH and bath temperature. However, the plating rates decreased with respect to the increase in the sodium citrate. The kinetic models, i.e. the plating rate equations for nickel and copper deposition were established and proved to be consistent with the practical measurements through the experimental verification. The results are summarized in the following:
For the electroless nickel plating when pH <8.5: T − 333 vNiHPOOHCHO=×1.58 1032 [+−−− ]0.765 [ ] 0.444 [ ] 0.242 [ 3 ] -0.579 exp[7.28( )] n 22 657 T
when pH >8.5: T − 333 vNiHPOOHCHO= 26.52[23+−−− ]0.765 [ ] 0.444 [ ] -0.079 [ ] -0.579 exp[7.28( )] n 22 657 T
222
For the electroless copper plating
T − 343 vCuNiHPOOHCHO=×1.16 1042 [++ ]0.745 [ 2 ] 0.436 [ − ] 0.330 [ − ] 0.243 [ 3 − ] -0.424 exp[5.20( )] n 22 657 T
(the details of the quotations refer to Sections 4.3.3)
The optimized electroless plating method could provide useful
information for the textile industries to obtain good properties and stable
plating. Based on the experimental results of the optimizing processes, it
was found that each of the factors, i.e., the concentration of NiSO4·6H2O,
NaH2PO2·H2O and Na3C6H5O7 ·2H2O, pH and temperature, had its own
contributing effect on the surface resistance of the electroless nickel and
copper plating on polyester fabric. It was concluded that the surface
resistance of the electroless nickel plating was predominantly influenced
by the concentration of NiSO4 and temperature of the plating bath. On
the other hand, the surface resistance of the electroless copper plating
was significantly influenced by the concentration of NiSO4 and pH of the
plating bath. The optimized plating parameters obtained were composed
of the concentrations of 17.52 g/l NiSO4·6H2O, 25 g/l NaH2PO2·H2O and
15 g/l Na3C6H5O7 ·2H2O at pH 8.0 and 78.3℃ for the electroless nickel
plating, and the concentration of 1.72 g/l NiSO4·6H2O, 30 g/l
NaH2PO2·H2O and 20 g/l Na3C6H5O7 ·2H2O at pH 10.9 and 75℃ for the
223 electroless copper plating respectively. These results provided some
important information for a large scale of production. Optimization of
the electroless plating was a cost-effective means of producing
metallized fabric with good properties.
2. The research has investigated the mechanism and the effect of plating parameters on micro-structure and properties of the electroless plated fabrics.
The micro-structure and properties of the electroless nickel-plated and
copper-plated polyester fabrics were investigated when these two most
important influential factors were changed. The mechanism concerning
the effect of the parameters on the micro-structure and properties was
proposed. With regard to the electroless nickel plating, the nickel deposit
layer became more uniform when prepared at higher NiSO4
concentration and bath temperature. The decrease in surface resistance of
the plated materials and increase in EMI SE were due to the greater
weight and bigger particle size with respect to the rise of NiSO4
concentration and bath temperature respectively. As for the electroless
copper plating, the surface morphology of the copper deposits showed
that the particle size increased with respect to the rise of NiSO4
concentration and pH. There was a decrease in the surface resistance and
224 an increase in the EMI SE with respect to the rise of the Ni2+
concentration and pH of the plating solution as a result of greater deposit
weight, larger particle size, lower P content and high crystallization.
These results suggested that the electroless nickel and copper plated
polyester fabrics had greater potential applications as an EMI shielding
material. The EMI SE could meet different requirements according to the
selection of appropriate plating conditions.
3. The nickel/copper multi-layer plated fabrics by using electroless plating for functional and design applications have been developed.
The nickel/copper multi-layer film formed on the polyester fabric was
developed. Deposit composition, surface morphology, crystal structure,
surface resistance, EMI SE and corrosion resistance of nickel/copper
coating were thoroughly investigated. The surface morphology of
nickel/copper deposit revealed the presence of ultrafine nodules and the
deposits so formed were compact and uniform in size. The EMI SE of
the nickel/copper plated fabric was higher than that of the nickel-plated
fabric, but lower than that of the copper-plated fabric. On the other hand,
the anti-corrosive property of the nickel/copper plated fabrics was better
than the copper-plated fabrics, but worse than nickel-plated fabrics.
Therefore, the electroless nickel/copper plated fabric was the appropriate
225 material to meet the requirements of both corrosion resistance and EMI
SE in most of the textile applications.
The effects of different plating conditions on the electroless nickel,
copper and nickel/copper multi-layer plated fabrics were investigated for
the metallized fabric design applications. The plated fabric had a shiny
effect and clear shape after the electroless plating. Copper patina formed
on the copper-plated fabric could create very interesting color effects and
textures. The process of designing the nickel, copper and nickel/copper
multi-layer plated fabrics could provide more information about the
necessary treatment conditions for different fabrics. It was believed that
the electroless plating had a great potential for designing decorative
textiles with visual effects. Furthermore, the metallic shining surfaces
could offer more imaginative effects. It was concluded that the diverse
color appearance could be managed by controlling the deposition
condition. Design methods and deposition parameters did play an
important role in metallizing the fabric design due to their influence on
textures and colors.
4. Multi-functions of metallized textiles have been developed to meet the demands of consumer and industry.
The electroless plated fabrics had a unique quality suitable not only for
226 the consumer demand but also for the functional applications. Based on
the experimental results, the metallized fabrics with the property of
electromagnetic waves shielding could also be used as decorative
textiles.
9.2 Suggestions for Future Research
In the present study, the optimization processes have been developed for the electroless nickel and copper plating and multi-layer plating. In general, optimization can offer an effective approach to achieving very high quality of metallized fabrics.
Some research opportunities have been revealed and identified in the present study.
The following recommendations would be suggested to make the electroless plating on textiles more perfect in the future.
(1) Hitherto, only nickel, copper and nickel/copper multi-layer plating on fabric had been conducted in the present study due to the time constraint. It is well known that different metals may create various metal-plated fabric effects. It is highly recommended to conduct more metallic multi-layer plating and alloy plating so that they may be employed for future application. A wider range of metal materials can be used for the electroless plating. The substrate material should cover a wide range of fabrics.
227 (2) The processes of sensitization and activation process can affect the properties and appearance of the electroless plated fabric. Due to the time constraint, the optimization of sensitization and activation processes has not been evaluated in the present study, and so they should be considered in future studies.
(3) Fastness, particularly the wash fastness can evaluate durability of the plated polyester fabrics. Hence, fastness measurement of the plated fabrics should be involved in future research. In addition, in the present study, the plated metal particles are found to have only a moderate adhesive strength on the surface of fabrics.
Pre-treatment such as chemical etching and plasma treatment can be employed to improve the adhesive strength before the electroless plating. Other forms of binding materials may also be used to improve the durability as they will help enhance the affinity between the metal particles and fibers.
(4) It is well known that mechanical properties such as hardness, fastness and adhesive strength, depend on a number of factors such as chemical composition of the resultant coatings, their morphology and structure. For electroless nickel plating, it is commonly recognized that phosphorous content in nickel coating will determine the coating’s structure. When the phosphorus content of electroless nickel coatings increases, the coating structure transforms from crystalline to a mixture of nanocrystalline and amorphous phases, and finally to full amorphous phase. High phosphorous nickel coatings have outstanding corrosion resistance, but their hardness
228 and wear resistance are lower than their low phosphorous counterparts. The films with amorphous and crystalline structure have different mechanical performance. Surface roughness also has a large impact on the mechanical properties. Therefore, relationship between mechanical performance and chemical composition of the resultant coatings, their morphology and structure, would be systematically investigated.
(5) Design application of the metallized fabric can exhibit various color effects and textures under different electroless plaating conditions. It is obvious that coloration process can widen the design effects of the metallized textiles. However, only a limited variety of colors have been obtained in the present study. The coloration process may be involved in the future studies so as to enrich the fabric color.
Coloration of the metallized textiles may be carried out in order to produce multi-color effects and meet the growing demands for the decorative textiles. In addition, coloration will also provide a process for obtaining higher value-added products and bringing greater benefits to the metallized textile industries.
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