Under the Direction of Dr. Stephen Michielsen)

ABSTRACT

VU, NGUYEN KHANH. Surface Modification of Meta–aramid to Enhance Dyeing. (Under the direction of Dr. Stephen Michielsen).

Well known for their high thermal and chemical stability, meta–aramid fibers play a very important role in high performance textiles, especially firefighter’s clothing.

Nonetheless, meta–aramid fibers also have limitations. Due to their high crystallinity and inertness, the coloration of these fibers is extremely difficult as confirmed by many references. Although many improvements have been made so far, the dyeing of meta–aramid fibers still requires high temperature and long duration to obtain good color strength and shade. Owing to this drawback, a surface modification step was implemented using the very popular industrial technique, pad–dry–cure, to modify the surface of meta–aramid fibers via grafting–to technique with poly(acrylic acid) (PAA). As an anionic polyelectrolyte, PAA facilitated the coloration of meta–aramid fibers with cationic dyes. A dyed fabric, whose K/S values can be considered industrially acceptable, was produced at room temperature (250C –

270C), and under neutral pH (=7) in 8 or 15 minutes (depending on selected dyeing technique). Dyeings had good crockfastness (both wet and dry), which proved the feasibility of the proposed treatment.

by Nguyen Khanh Vu

A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Fiber and Polymer Science

Raleigh, North Carolina

2018

APPROVED BY:

______Dr. Stephen Michielsen Dr. Donald B. Thompson Committee Chair

______Dr. Renzo Shamey Dr. Sam M. Hudson

DEDICATION

I would like to dedicate my dissertation to my wife (YÊN), my daughter (VÂN), my parents

(MINH and TRÚC), my sister (NGỌC) and my parents–in–law (KHÁNH and TUYẾT) as well as other relatives in my big family. Without their mental and physical support, it would have been impossible for me to fulfil this dream.

BIOGRAPHY

NGUYÊN was born in the biggest city in Viet Nam, Ho Chi Minh, in 1984. He finished his Bachelor of Science degree in Mechanical Engineering focusing on Textile

Technology in 2008 at Bach Khoa University (Formerly Ho Chi Minh City University of

Technology HCMUT). After two years of search, he got a chance to start a Master’s degree in high–tech textiles concentrating on multi–functional textiles at University of Minho,

Guimarães, Portugal. This program was funded by Erasmus Mundus Action 2 from 2010 to

2012. Subsequently, he returned to his undergraduate university and started working as a lecturer. In 2013, he won the 911–Fellowship (a nation–wide application process that takes a year to finalize) sponsored by the Viet Nam Government. Thanks to this fellowship, he was finally able to start his doctoral degree at North Carolina State University and earn the

Doctor of Philosophy title after a long and arduous journey.

iii

ACKNOWLEDGMENTS

It is a truism that the first person whom I have to thank is Dr. Stephen Michielsen.

Thanks to his guidance and directions upon professional knowledge, I could improve and widen my critical view of scientific world, especially in the area of fiber and polymer science. Without such support from Dr. Michielsen, it is not possible for me to achieve my title.

Immediately after, I would like to show my deep gratitude to all of those who have financially supported me so as I can spend my four years here at College of Textiles, North

Carolina State University. The first person is Dr. David Shafer, Assistant Dean at Graduate

School. Without his aid, I couldn’t have kept going with my doctoral degree. Next, Vietnam

International Education Cooperation Department, Ministry of Education and Training, Bach

Khoa University (Formerly Ho Chi Minh City University of Technology – HCMUT),

College of Textiles are organizations I would like to show my thankfulness. These institutions have put the first stones forming the pathway on which I could walk step by step to today’s achievement.

Additionally, I would like to send my gratefulness to companies who have sponsored materials (fabrics, chemicals) towards my research. Those are TenCate Protective Fabrics

(Nomex® IIIA fabric rolls) and M Dohmen USA Inc. (Good lightfastness basic dyestuff).

Their products play a huge role in my study.

Similarly, I would like to thank Ms. Birgit Andersen, Research Assistant and Lab

Manager, TECS; Mr. Jeffrey Krauss Pilot Laboratory Manager, ZTE and Ms. Teresa White,

Research Specialist, ZTE. They have taught me important skills from the very first day I started my research. Thanks to those skills I could develop and finish my work.

In the end, I am lucky to have many good friends around me in the College. I would like to say thank to all of those who ever helped, listened to, discussed and supported me during my student life here at College of Textiles.

TABLE OF CONTENTS

LIST OF TABLES ...... ix

LIST OF FIGURES ...... xiii

CHAPTER 1 – INTRODUCTION ...... 1

CHAPTER 2 – PRIOR ART ...... 4

2.1. META–ARAMID ...... 4

2.1.1. History, polymerization, and production ...... 4

2.1.2. Products produced from meta–aramid ...... 8

2.1.3. Applications [21] ...... 10

2.1.4. Coloration, finishing, and functionalization of meta–aramid ...... 12

2.2. POLY(ACRYLIC) ACID (PAA) [83] ...... 35

2.2.1. Definition ...... 35

2.2.2. Chemical structure and synthesis ...... 35

2.2.3. Physical properties ...... 36

2.2.4. Behavior in aqueous solution ...... 37

2.2.5. Applications ...... 38

CHAPTER 3 – MATERIALS ...... 52

3.1. NOMEX® IIIA FABRIC ...... 52

3.2. POLY(ACRYLIC ACID) – PAA ...... 52

3.3. TOLUIDINE BLUE O (TBO) ...... 52

3.4. GRAFTING OF PAA ONTO NOMEX® IIIA FABRIC ...... 53

3.4.1. Screening grafting procedure ...... 53

3.4.2. Modified grafting procedure – Design of Experiment ...... 53

3.5. QUANTIFICATION OF PAA GRAFTED ONTO NOMEX® IIIA FABRIC ...... 54

3.6. DYEING OF NOMEX® IIIA FABRIC USING BASIC DYES ...... 55

3.6.1. Preliminary dyeing procedure ...... 55

3.6.2. Modified dyeing procedure ...... 55

3.7. COLOR STRENGTH MEASUREMENT ...... 56

3.8. FABRIC HAND ...... 56

3.9. CROCKING FASTNESS ...... 57

3.10. WASHING ...... 58

CHAPTER 4 – RESULTS AND DISCUSSION ...... 59

PART I – PRELIMINARY RESULTS...... 59

4.1. QUANTITY OF PAA GRAFTED ONTO META–ARAMID FIBERS ...... 59

4.1.1. Standard curve of Toluidine Blue O (TBO) ...... 59

4.1.2. Amount of PAA grafted onto Nomex® IIIA ...... 60

4.1.3. Theoretical calculation of a monolayer of PAA grafted onto Nomex® IIIA

fiber ...... 61

4.2. MECHANISM OF GRAFTING PAA ONTO NOMEX® IIIA...... 63

4.2.1. Scheme 1 ...... 63

4.2.2. Scheme 2 ...... 64

4.3. K/S VALUES ...... 67

4.4. CROCKING FASTNESS ...... 69

4.4.1. Single thermal treatment ...... 69

4.4.2. With a 2nd thermal treatment ...... 69

PART II – FINAL RESULTS ...... 70

vii

4.5. DYEING MECHANISM ...... 70

4.6. COLOR UNIFORMITY ...... 72

4.6.1. Exhaust dyeing with an agitating bath ...... 72

4.6.2. Continuous dyeing with a padding mangle machine ...... 76

4.7. K/S VALUES ...... 80

4.7.1. K/S values of exhaust–dyed Nomex® IIIA samples ...... 80

4.7.2. K/S values of continuous pad–dyed Nomex® IIIA samples ...... 83

4.8. CROCKFASTNESS ...... 85

4.8.1. Exhaust–dyed Nomex® IIIA samples ...... 85

4.8.2. Improvement for crockfastness of Basic Blue 17 ...... 88

4.8.3. Pad–dyed Nomex® IIIA samples ...... 88

4.9. FABRIC HAND ...... 90

4.10. WASHING OF EXHAUST–DYED SAMPLES WITH BASIC BLUE 17 ...... 91

CHAPTER 5 – CONCLUSION ...... 92

CHAPTER 6 – FUTURE WORK ...... 94

REFERENCES ...... 96

APPENDICES ...... 104

viii

LIST OF TABLES

Table 1 History of development of Aramid Fibers [16]...... 5

Table 2 Company, Aramid Type, and Brand Names of Commercial Aramids [15]...... 6

Table 3 Filament yarns of Nomex® Aramid [24, 25]...... 9

Table 4 Nomex® paper and pressboard [24]...... 10

Table 5 Effectiveness of different carriers [31]...... 14

Table 6 Fastness of dyeing on fabrics of Nomex E–8 [31]...... 16

Table 7 Dye combinations for certain shades for protective clothing [32]...... 17

Table 8 Modified design of experiments for grafting of PAA onto Nomex® IIIA...... 54

Table 9 Statistically calculated of TBO's absorbance after different treating

conditions (PAA concentrations and curing duration)...... 61

Table 10 Crockfastness values (dry and wet) of dyed Nomex® IIIA samples...... 69

Table 11 Wet crockfastness values after 2nd thermal treatment at different

durations...... 69

Table 12 Dyed samples of PAA–grafted Nomex® IIIA cured at 2000C with

different PAA concentrations and curing durations using a shaking bath...... 73

Table 13 Dyed samples of PAA–grafted Nomex® IIIA cured at 2200C with

different PAA concentrations and curing durations using a shaking bath...... 74

Table 14 Dyed samples of PAA–grafted Nomex® IIIA cured at 2400C with

different PAA concentrations and curing durations using a shaking bath...... 75

Table 15 Basic Blue 41 dyed samples of PAA–grafted Nomex® IIIA cured at

2000C with different PAA concentrations and curing durations using a

padding mangle machine...... 77

Table 16 Basic Blue 41 dyed samples of PAA–grafted Nomex® IIIA cured at

2200C with different PAA concentrations and curing durations using a

padding mangle machine...... 78

Table 17 Basic Blue 41 dyed samples of PAA–grafted Nomex® IIIA cured at

2400C with different PAA concentrations and curing durations using a

padding mangle machine...... 79

Table 18 Color strength of pretreated Nomex® IIIA samples with a fixed PAA

concentration of 0.01 wt% and dyed with four different colors...... 81

Table 19 Numerical values of K/S max of dyed Nomex® IIIA samples with a fixed

PAA concentration of 0.01 wt% and dyed with four different colors...... 81

Table 20 Crockfastness values (dry and wet) of dyed Nomex® IIIA samples cured

at 2000C...... 86

Table 21 Crockfastness values (dry and wet) of dyed Nomex® IIIA samples cured

at 2200C...... 86

Table 22 Crockfastness values (dry and wet) of dyed Nomex® IIIA samples cured

at 2400C...... 87

Table 23 Molecular structures of Basic Blue 17, Basic Red 46, Basic Yellow 2, and

Basic Violet 16...... 87

Table 24 Crockfastness (wet and dry) values of Basic Blue 17 exhaust–dyed

Nomex® IIIA samples after a 2nd thermal treatment at 1800C...... 88

Table 25 Crockfastness (wet and dry) values of Basic Blue 41 (0.1 owf%) pad–dyed

Nomex® IIIA samples of different pretreating conditions...... 89

Table 26 Crockfastness (wet and dry) values of Basic Blue 41 (0.25 owf%) pad–

dyed Nomex® IIIA samples of different pretreating conditions...... 89

Table 27 Mean values of bending moduli obtained between two specific curvature

points (0.5 cm and 1.5 cm) going one way, and going the opposite way (–

0.5 cm and –1.5 cm) for both warp and filling directions...... 91

Table A.1 K/S–max values of exhaust–dyed Nomex® IIIA samples cured at 2000C...... 105

Table A.2 K/S–max values of exhaust–dyed Nomex® IIIA samples cured at 2200C...... 106

Table A.3 K/S–max values of exhaust–dyed Nomex® IIIA samples cured at 2400C...... 107

Table A.4 K/S–sum values of exhaust–dyed Nomex® IIIA samples as a function of

curing duration for PAA concentration of 0.01 (wt%) at different curing

temperatures...... 108

Table A.5 Maximal absorbance values (between 400 nm to 700 nm wavelengths) of

pad-dyed (0.1 owf%, Basic Blue 41) Nomex(R) IIIA samples...... 109

Table B.1 Bending rigidity of Nomex® IIIA samples (filling and warp directions)

cured at 2000C...... 110

Table B.2 Bending rigidity of Nomex® IIIA samples (filling and warp directions)

cured at 2200C...... 111

Table B.3 Bending rigidity of Nomex® IIIA samples (filling and warp directions)

cured at 2400C...... 112

Table B.4 Bending rigidity of Nomex® IIIA samples (filling and warp directions)

padded with 0.01% PAA then cured at different temperature in 3 minutes. ....113

Table B.5 Bending rigidity of Nomex® IIIA samples (filling and warp directions)

padded with 0.05% PAA then cured at different temperature in 3 minutes. ....114

Table C.1 Hue changing phenomenon in PAA–grafted Nomex® IIIA samples (0.1

wt% PAA, cured at 2400C in 3 minutes) dyed with Basic Blue 17 before

and after washing with AATCC 61–2A standard...... 115

Table C.1 (continued) ...... 116

xii

LIST OF FIGURES

Figure 1 Preparation of MPIA at (a) low and (b) high temperatures [15]...... 6

Figure 2 Wet spinning process of meta–aramid [22]...... 7

Figure 3 Nomex® honeycomb structure [27]...... 12

Figure 4 DSC results of m–aramid fiber after different DMSO treatment time at

60°C [57]...... 23

Figure 5 N–methyformanilide [61]...... 24

Figure 6 1–phenoxypropan–2–ol (left) and 2–phenoxyethanol (right) [63] ...... 24

Figure 7 Molecular structures of Kevlar® and Nomex® [68]...... 27

Figure 8 The synthesis route of copolymerization of MeDMA and PEO45 by atom

transfer radical polymerization [69]...... 28

Figure 9 K/S and exhaustion of UV–irradiated meta–aramid films [70]...... 29

Figure 10 Dye exhaustion (%) and K/S values on m–aramid fiber [73]...... 30

Figure 11 Comparison between plasma and wet processes [78]...... 31

Figure 12 SEM images of PMIA fibers treated by plasma under different amplitudes:

(a) untreated; (b) treated with 60s; (c) treated with 120s; (d) treated with

180s [76]...... 31

Figure 13 Schematic Illustration of Procedure for Fabrication of PMIA−PDA/Ag

Composite by Poly(dopamine)–Assisted [80]...... 32

Figure 14 SEM images of (a and b) silver coated PMIA fibers without

functionalization of dopamine and (c and d) silver coated PMIA–PDA

without exogenous reducing agent [80]...... 32

xiii

Figure 15 Illustration of Procedures for Preparing MPIA–PDA–KH560 Fibers (a)

and MPIA–(PDA+KH560) Fibers (b) [81]...... 33

Figure 16 ADMH grafting copolymerization and chlorination on the synthetic fibers

[82]...... 34

Figure 17 Poly(Acrylic) acid as a textile sizing agent [84]...... 35

Figure 18 Polymerization of tertbutyl acrylate and hydrolysis [83]...... 36

Figure 19 Dehydration of PAA to form cyclic anhydrides [85]...... 37

Figure 20 Illustration of different methods of preparing graft copolymers [92]...... 39

Figure 21 Two common approaches to membrane surface modification with

macromolecules. Shown are strategies that lead to covalently bound

polymer modifiers [94]...... 40

Figure 22 Modification of silica surface by “grafting–to” of poly(acrylic acid) [122]...... 45

Figure 23 Two different schemes to graft PAA onto silica particle [124]...... 46

Figure 24 Grafting reaction of cyclodextrin onto cellulose by the intermediate of a

polycarboxylic acid bearing more than three carboxylic functions [129]...... 47

Figure 25 The grafting of poly(acrylic acid) (PAAC) layers on n–heptylamine

(HApp) thin films via water–soluble carbodiimide (EDC/NHS) chemistry

[130]...... 48

Figure 26 Amidization of nylon 6,6 using EDC [9]...... 49

Figure 27 Amidization of a nylon 6,6 using NHS [9]...... 50

Figure 28 Molecular structure of Toluidine Blue O (TBO)...... 52

Figure 29 X–rite Color Spectrophotometer...... 56

xiv

Figure 30 Measuring principle of bending rigidity with Kawabata system (KES–F2)

[135]...... 57

Figure 31 AATCC Crockmeter...... 57

Figure 32 Standard curve of Toluidine Blue O...... 59

Figure 33 Absorbance of Toluidine Blue O in terms of initial percentage of PAA

solutions used for grafting to Nomex® IIIA...... 60

Figure 34 Circular cross–section of fiber with a singular coating layer...... 62

Figure 35 Amidization between carboxyl and primary amine...... 64

Figure 36 Amidization between anhydride and primary amine...... 65

Figure 37 Adsorption–grafting process of PAA onto nylon 66 [131]...... 66

Figure 38 K/S values of dyed Nomex® IIIA samples...... 67

Figure 39 Color strength of dyed Nomex® IIIA samples...... 68

Figure 40 Dried PAA–padded Nomex® IIIA...... 70

Figure 41 Cured PAA–padded Nomex® IIIA...... 71

Figure 42 Dyeing of PAA–grafted Nomex® IIIA...... 71

Figure 43 K/S–sum values of exhaust–dyed Nomex® IIIA samples with Basic Blue

17 as a function of curing durations for PAA concentration of 0.01 (wt%)

at different curing temperatures...... 83

Figure 44 K/S–max values of pad–dyed (0.1 owf%) Nomex® IIIA samples after

pre–cured at 2000C under different curing durations and PAA

concentrations ...... 84

Figure 45 K/S–max values of pad–dyed (0.1 owf%) Nomex® IIIA samples after

pre–cured at 2200C under different curing durations and PAA

concentrations ...... 84

Figure 46 K/S–max values of pad–dyed (0.1 owf%) Nomex® IIIA samples after

pre–cured at 2400C under different curing durations and PAA

concentrations ...... 84

Figure 47 K/S–sum values of Nomex® IIIA samples pad–dyed (0.1 owf%, Basic

Blue 41) as a function of PAA concentrations cured at different

temperatures in three minutes...... 85

Figure 48 Molecular structure of Basic Blue 41...... 87

xvi

CHAPTER 1 – INTRODUCTION

In heat resistant applications, the meta–structure in the aramid family of fibers is the most prevalent option. The first such fiber was produced by DuPont USA in the early 1960s, namely Nomex®. As described by Shishoo [1], meta–aramids have high–temperature resistance, moderate tenacity, and low modulus. The thermal degradation of meta–aramids typically begins at 3750C. The reason for the great thermal resistance is ascribed to the high bond dissociation energies of C–C and C–N bonds, which lead to the decomposition temperature of over 4000C. Besides, glass transition temperature of meta–aramid is quite high as well, 272–2750C resulting from hydrogen bonding and chain rigidity [2]. This advantage in performance turns out to be a disadvantage from an aesthetic viewpoint. The coloration of aramid fibers, in general, is extremely difficult. This is due, in part, to the crystallinity of the fiber. Nomex® can be produced in staple fiber, filament yarn, industrial paper, and pressboard forms. Both staple and filament yarns can be ordered undyed or producer–dyed; however, only limited colors can be obtained.

As with many other materials, textile products are experiencing fast developments where, thanks to technological innovations, a wide range of products with novel applications have been developed. Many of these value–added functions are closely related to textile surface properties. Commonly used techniques for the surface modification of textile materials are described by Wei [3]. Many methods have been devised to fulfill the current sustainability trends. Inventors have been trying to make textile processing less environmentally damaging. According to Sharma [4], the textile industry occupies the top position in the list of most polluting industries based on the volume and composition of discharged water. This is the major driver for the application of new benign technologies.

However, due to the investment cost, commerciallizability, complexity in utilization, etc.

traditional aqueous treatment is still the favored treatment in industry. During wet processing treatments, chemicals and reagents are deposited onto the surface of textiles (which may either be involved or not in chemical reaction with fibers) to change the product’s surface properties through which new properties are added. Better dyeability and improved dyeing yield are two typical objectives in textile wet processing [5-7].

In order to form covalent bonds between compounds with macromolecular weight substances, two possible approaches are applied which are known as “grafting to” and

“grafting from”. Three major techniques employed are chemical grafting, radiation grafting and photo–grafting [8]. According to Tobiesen and Michielsen [9], although many fibers have only a few graft sites, long polymer chains are able cover the entire surface of the substrate. If “grafting from” is applied, monomers will be deposited onto the surface and then polymerized under different activation sources such as plasma [10], UV radiation [11], and

γ–ray [12]. However, this approach takes a long time to implement and it is difficult or even impossible to determine the molecular weight of the created polymer. In the meantime, the

“grafting to” approach is much easier to carry out. For example Cai [13] successfully grafted polyacrylic acid onto nylon 66 to study the photostability of surface–bound dyes. The polymer used for the addition to the surface of the substrate can be fully characterized and known prior to grafting. The most common methods in the textile industry for the application of chemicals onto textile substrates are ‘pad–dry–cure’, UV irradiation and spraying as they are relatively easy and low cost finishing techniques [14].

In the current work, a chemical treatment process was carried out in order to provide primary dye sites onto a meta–aramid fabric via the pad–dry–cure technique. Specifically, poly(acrylic acid) (PAA) was padded onto Nomex® IIIA fabric then cured at high

temperature to trigger a surface grafting of PAA chains onto the fabric. An experimental design was constructed to implement a series of experiments using three different curing temperatures (2000C, 2200C and 2400C respectively) and three curing times (2, 3 and 4 minutes for each curing temperature) to determine the optimal pretreatment conditions

(chemical concentration, curing temperature, and curing duration) to deposit the highest quantity of required chemical onto Nomex® IIIA fabric. To evaluate the proposed procedure, a theoretical quantification of the grafted PAA was carried out and the results were then compared against the empirical values. Thereafter, pretreated meta–aramid fabric was colored by two different dyeing approaches, namely exhaust– and pad– dyeing processes, under both alkaline conditions at pH 10 and also neutral pH 7 to study their influences on color uniformity of dyed samples. Besides color uniformity, the correlation between the amount of grafted PAA and color strength was also investigated using K/S measurements.

Another important factor in dyeing is crockfastness (wet and dry) which was also assessed for dyed Nomex® IIIA fabric, using AATCC test method 8, to examine how strongly dye molecules were fixed onto the grafted surface. Finally, bending rigidity of pre–treated samples was measured with the Kawabata Evaluating System (KES F2) to determine how the proposed treatment technique affected fabric softness.

CHAPTER 2 – PRIOR ART

2.1. Meta–aramid

2.1.1. History, polymerization, and production

Nomex®, poly(m–phenylene isophthalamide) (MPIA), belongs to the polyamide family because it has the amide group (–CO–NH–) in its chemical structure but is as an aromatic polyamide. Because of the presence of aromatic backbones in the structure, aromatic polyamides possess better mechanical, thermal and chemical properties than the aliphatic polyamides (e.g. nylons). The US Federal Trade Commission (FTC) defines wholly aromatic polyamides as synthetic polyamides in which at least 85% of the amide groups are bound directly to two aromatic rings1 [15].

1Rules and regulations under the Textile Fiber Products Identification Act (http://www.ftc.gov/os/statutes/textile/rr-textl .pdf), Part 303.7 (Generic names and definitions for manufactured fibers), US Federal Trade Commission (FTC).

Table 1 History of development of Aramid Fibers [16].

Developer Base Year Event /Producer Polymer 1938 Commercialization of Nylon 1962 Introduction of Nomex® fiber DuPont Co., USA MPDI Discovery of anisotropic polymer by P.F. 1965 Flory 1970 Discovery of air–gap spinning 1971 Introduction of Fiber B DuPont Co., USA (i) PBA (ii) PPDT 1972 Introduction of Teijinconex Teijin Ltd., Japan MPDI Commercialization of Kevlar® DuPont Co., USA PPDT Akzo Chemicals Introduction of Twaron PPDT BF, Netherlands Rhone–Poulene, Introduction of Kernel® MPDI France Introduction of Fenilon® USSR MPDI Introduction of SVM fiber (formerly Polyhetero 1976 USSR Vniivlon) arylene 1978 Development of Arenka aramid fiber Introduction of HMO–50 (Technora) 1987 Teijin Ltd., Japan aramide fibre Commercialization of Twaron (formerly Toyobo, Japan 1988 Arneka) Introduction of PBO–HM Toyobo, Japan 1996 Introduction of Trevar (discontinued later) Kevlar 49 HS by new fiber technology 1997 DuPont Co., USA (NFT) p–aromatic 1998 Introduction of Armos Russia hydrocyclic copolyamide MPDI – Poly(m–phenylene isophthalamide); PBA – Poly(p–benzamide); and PPDT – Poly(p–phenylene terephthalamide)

Among the aromatic polyamides, Nomex® (registered by DuPont USA) or meta– aramid is the oldest. It was a breakthrough material, which was officially commercialized at the beginning of 1960s. Ever since that time the history of thermal and electrical insulation has turned a new page [17]. Besides Nomex®, there are several other commercial meta–

aramids, including Conex® (Teijin company, Japan) and Fenilon (the Soviet Union), but

Nomex® and Conex® are the most widely used [18].

Table 2 Company, Aramid Type, and Brand Names of Commercial Aramids [15].

Company Aramid Type Brand name Meta–aramid Nomex DuPont Para–aramid Kevlar Meta–aramid Teijinconex Teijin Para–aramid Twaron Copolymer ODA/PPTA Technora Kolon Industries Para–aramid Heracron Hyosung Para–aramid Alkex SRO Group Meta–aramid X–Fiber Yantai Meta–aramid Newstar Spandex Co. Para–aramid Taparan Woongjin Meta–aramid Arawin

The synthesis of aromatic polyamides can be done via two routes (Figure 1), (1) reaction between diacid chlorides and diamines at low temperatures and (2) direct polycondensation of aromatic diacids with diamines in solution at high temperatures

(Yamazaki–Higashi method). No matter which route is applied, polar aprotic solvents are frequently used (for instance HMPA, NMP, N,N–dimethylformamide (DMF), or N,N– dimethylacetamide (DMA) [15].

Figure 1 Preparation of MPIA at (a) low and (b) high temperatures [15].

The dry spinning process of MPIA has been discussed in detail by Sweeny [19] and

Carol [20]. During the procedure, the polymer solution is extruded through a die and then comes in contact with a hot air current (at 2250C) to evaporate the solvent. To improve the stability of the spinning solution, inorganic salts can be added (e.g. CaCl2, LiCl2) [20, 21]. In reality, complete evaporation of the solvent is avoided because the fiber should undergo a wet drawing process to improve its properties. The speed of the spinning process under this method is rather fast, in the order of one hundred meters per minute [21].

Figure 2 Wet spinning process of meta–aramid [22].

Alternatively, meta–aramids can be produced by a wet spinning process (Figure 2) where the dry polymer is dissolved at a low temperature in a 100% sulfuric acid to create an aramid dope. This dope is heated to 1000C to create a clear solution which will then be pressed through a spinneret submerged completely in a water bath with a high concentration of an inorganic salt to yield fibers. In this bath, the sulfuric acid solvent dissolving the polymer is removed leaving only the fibers, which are then drawn, dried and heat–set to obtain excellent mechanical behavior [2, 22]. Tai and his colleague [23] also invented a wet

spinning process specifically for meta–aramids where the spinning solution has a relatively high concentration of salt. In general, due to the high drag of the coagulation solvent, the production speed with this method is in the order of only ten meters per minute [21].

2.1.2. Products produced from meta–aramid

Meta–aramids can be manufactured into a variety of fiber products. They can be in staple or continuous filament yarns, industrial paper and pressboard.

2.1.2.1. Staple yarn

Nomex® staple and tow are used in yarn manufacturing. The fiber count is usually 1.5 or 2 denier per filament (dpf). In the meanwhile, staple fiber length is frequently from 1.5 to

6 inches and in some cases can even be cut with various lengths. Many different materials can be made from Nomex® fibers. Owing to their outstanding ability in thermal resistance as well as low flammability, Nomex® fibers are the first choice for protective apparel used in firefighting, military and sports uses. Furthermore, Nomex® when transformed into fabrics

(both staple and filament yarn) is used for industrial gas filtrations. However, when elemental halogens are present, Nomex® fibers degrade and PTFE or Teflon® fibers are used instead

[24].

2.1.2.2. Continuous filament yarn

Nomex® continuous filament yarns have low dyeability. The yarn’s fineness is usually in the range from 200 to 2400 denier with 2 dpf. Several types of 200 denier yarns are 430,

431, 432, 433 and 434. Those belonging to the 1200–denier group are Type 430, 431 and 432

yarns. All these filament yarns are frequently used in applications where the working condition involved high temperatures. Lately, several new yarn types, available as dyeable yarns or as specific producer colors, are also produced (as shown in Table 3).

Table 3 Filament yarns of Nomex® Aramid [24, 25].

Filament Type Grade Denier dtex Twist Luster Color # Regular 200 220 0 100 Bright Natural Regular 1200 1330 0 600 Bright Natural 430 Regular 1600 0 Bright Natural Regular 2400 0 Bright Natural Olive 432 Regular 200 220 100 Color–sealed green 433 Regular 200 220 100 Color–sealed Sage green Low 150 100 Bright Dyeable N104 Crystallinity 200 100 Bright Dyeable Producer N101 Regular 200 100 colored Producer N102 Texturized 900 400 colored

2.1.2.3. Paper and pressboard

Nomex® paper types comprise 410, 411, 412, 414, and 418 and are provided in different thicknesses and densities depending on the final application. Nomex® paper types are most widely used in the electrical industry, for instance armature slot insulation, wire wrap, phase insulator, wedges, lead insulations, end laminates, bushings, coil wrappers and interleaving, and crossover tubing and end caps in motors and generators. Some are also used in transformers, appliances, and for military applications [24].

Nomex® pressboard is another form of meta–aramid fiber. There are several types including 992, 993 and 994, whose densities, thicknesses, physical and electrical properties

are quite diverse (Table 4). These pressboards are popularly exploited as spacers and barriers in transformers and end lams in motors [24].

Table 4 Nomex® paper and pressboard [24].

Thickness Type Density, g/cc End–use mil mm 410 2 – 30 0.05 – 0.8 0.7 – 1.2 Electrical insulation 411 5 – 23 0.13 – 0.58 0.3 Electrical insulation 412 1.5 – 5 0.04 – 0.13 0.7 – 0.9 Honeycomb structure 414 3.4 – 15 0.09 – 0.38 0.9 – 1.0 Electrical insulation 418 3 – 14 0.08 – 0.36 1.0 – 1.1 High–voltage electrical insulation 419 7 – 13 0.18 – 0.33 0.5 High–voltage electrical insulation 992 63, 125 1.6, 3.2 0.5 Low–density pressboard 993 40 – 240 1.0 – 6.0 0.7 – 0.9 Medium–density pressboard 994 40 – 380 1.0 – 9.6 1.1 – 1.5 High–density pressboard

2.1.3. Applications [21]

Meta–oriented aramid fibers can provide a good option to a broad domain of applications where their core values canbe maximized. In practice, protection and industrial applications are the two principal areas for use of meta–aramid fibers.

2.1.3.1. Protection

The most famous and prevalent usage of meta–aramid fibers is in protective garments particularly developed to prevent workers from hazardous risks of heat and flame. Specific areas of application include oil industry, electrical workers, and firefighter suits. In the first two cases, direct contact with flame and fire rarely occur but firefighters often have to go into

the fire to do their job. Thus, garments with low shrinkage property are required. In order to minimize the shrinkage of the garment, blending with para–aramid fibers improves the performance of the meta–aramid protective clothes that naturally have low shrinkage.

In addition to functional performance, aesthetics of the product is another great concern. The coloration of the fabric is in fact a major requirement. To provide color for the fabrics, some are piece dyed which brings about a good range of colors; while some are dope–dyed or producer dyed fibers which provide better light fastness properties. With protective garments, fibers are used in staple form then processed into yarns via conventional spinning technologies for cotton and wool. When the material is used for construction of racing driver or pilot suits, filament yarns are utilized.

2.1.3.2. Industrial applications

Aircraft seat covers should prevent fire from spreading to their proximity. For such applications meta–aramid fibers are normally blended with flame retardant wool fibers [26].

With good resistance to both chemicals and heat, these fibers are a good solution for hot air filtration. Moreover, its exceptional thermal stability makes this fiber a widely used needle felt filter media for high temperature baghouse operation. The automotive sector also has certain applications where such fibers may be needed. Because of their decent fatigue properties, meta–aramid fibers, in filament form, are used for mechanical reinforcement of elastomeric hoses and belts.

Figure 3 Nomex® honeycomb structure [27].

Not only in the textile industry but in the paper industry meta–aramids also play an important role. During the preparation of paper, short fibers may be combined with meta– aramid binding particles, namely “fibrids”. This type of paper has good electrical insulation even under exposure to a constant temperature of 2200C. Furthermore, Pinzelli and Loken

[28] designed and developed specific papers (Nomex® 412) into honeycomb structures

(Figure 3) taking advantage of the lightweight, high stiffness and flame retardancy of meta– aramids to manufacture parts for airplanes [27].

2.1.4. Coloration, finishing, and functionalization of meta–aramid

2.1.4.1. Coloration

It has been 50 years since the introduction of Nomex® to the market in 1967. This meta–aramid fiber can be considered as the most important milestone in the fibrous thermal resistant material. The foundation of this success is based on the works of Wilfred Sweeney

(1926–2011) who was awarded the Lavoisier Medal in 2002 [29]. A number of enhancements have been applied to this aromatic polyamide to improve its performance

including the coloration of the fiber, which is still one of the most difficult aspects of processing this fiber.

Generally, dye molecules diffuse into non–crystalline or low order regions of a fiber.

This implies that fibers whose polymer chains are highly oriented (e.g. meta–aramid, para– aramid, etc.) may be very difficult to dye. Solutions devised to overcome such problems include the utilization of plasticizing agents, application of certain dyes which have higher diffusion ability into fibers, or the use of dyeing machinery that contain pressurized chambers

[30].

The very first procedure to dye Nomex® was introduced by two researchers working at

Du Pont Co., Wilmington, Delaware, USA [31]. The Nomex® used in that study was the E–8 which is normally used for aircraft furnishings. Schumm and Cruz [31] tested many carriers in the dyeing of Nomex® to determine the most appropriate solution. Table 5, shows a list of carriers and the strength of the dyed fiber expressed in K/S value. K/S increases with an increase in dyeing strength. Results indicate that the three best carriers are Chemocarrier

KD5W, Latyl Carrier A, and β–Naphthol.

Table 5 Effectiveness of different carriers [31].

Carrier K/S of dyed fiber Control 0.06 Anisole 0.70 Acetamide 0.72 Biphenyl 0.72 Benzamide 0.85 Benzyl Alcohol 1.12 Methyl Salicylate 1.61 Dimethyl Terephthalate 1.70 Carolid 50 1.72 Tanavol 1.73 Acetanilide 1.76 Benzanilide 2.49 Dimethyl Isophthalate 2.74 Salicylaldehyde 3.40 Cindye DAC–888 5.61 Chemocarrier KD5W 9.01 Latyl Carrier A 9.27 β–Naphthol 9.89

However, β–Naphthol was excluded due to uncontrollable bleeding in repeated scours meanwhile, Latyl Carrier A is not recommended for liquor–circulating machines thus

Chemocarrier KD5W (an emulsifiable liquid) was the best candidate which gave both good shades and had good compatibility with the equipment. The optimal concentration of KD5W was 10 g/l, at pH 4 – 5 (by using acetic acid) and the fabric was dyed for 2 hours at 2500F.

Cationic dyes with good fastness to washing, dry cleaning and rubbing are listed in Table 6.

Around one year later, Evan and Schumm [32] published another procedure to dye

Nomex® 450, a well–established fiber for production of nonflammable apparel. The carrier was a mixture of emulsifiable benzaldehyde, called TLF–2791, which has good biodegradability and low toxicity. The electrolyte used was sodium nitrate at 25 g/l, which did not increase the total process cost significantly. To attain good shade reproducibility, dyeing was done at pH 2.5 and at 2500F. The reason is that Nomex® T–450 physical

properties are little affected in the pH range of 2 to 9. In addition, pH lower than 3 is a standard practice for all shades in which Cl Basic Green 6 is used. The fabric could be dyed in piece, stock, and yarn forms. Some combinations of dyes to produce certain shades are shown in Table 7.

Another attempt to dye meta–aramid fibers was carried out by Richardson and Walck in 1970 [33]. In that work, the authors used a copolymer of poly(m–phenylene isophthalamide) and poly[N,N’–m–phenylene bis(meta amino benzamide)isophthalamide] to produce tows which were then cut into staple fibers 2 inches in length. The cut fibers were, in turn, converted into pads and then dyed at a pressure of 20 p.s.i.g. The dye bath comprised a basic dye and an organic dye carrier. The outcome revealed that spun copolymer fibers without super atmospheric pressure had low dye pickup even with the highest amount of Na–

SMPD (sodium salt of 2,4–diaminobenzene sulfonic acid). Thus, besides the pressure, they found that a dye assistant and an adequate amount of Na–SMDP were required. Furthermore, it was recommended that the filament tow must not be heated while taut.

Table 6 Fastness of dyeing on fabrics of Nomex E–8 [31].

Crocking Xenon Arc Modified ASTM Dry Washing (20 Hrs.) D620–S7T (192 Clean IIIA SC SC Hrs.) SC SC Dry Wet rating rating Sevron Yellow 4 – 3 5 – 4 5 – 4 5 – 4 5 5 MFW Sevron Yellow 4 – 3 5 – 4 5 5 5 5 3RL Astrazon Yellow 3 4 5 – 4 5 – 4 5 – 4 5 7GLL Calcozine Acrylic 4 5 – 4 4 5 – 4 5 – 4 5 Yellow G Sevron Brown YL 4 5 – 4 4 5 – 4 5 – 4 5 Sevron Orange G 4 4 4 5 – 4 5 – 4 5 Astrazon Orange R 5 – 4 5 – 4 5 – 4 5 – 4 5 – 4 5 Astrazon Orange 4 4 4 5 – 4 5 – 4 5 RRL Astrazon Orange 5 – 4 4 4 5 – 4 5 – 4 4 3RL Astrazon Golden 4 5 – 4 4 5 – 4 5 5 Yellow GLD Lyrcamine Lt. Fast 5 – 4 5 – 4 4 5 – 4 5 – 4 4 Orange JL Astrazon Red BBL 5 – 4 5 – 4 4 5 – 4 5 – 4 5 Genacryl Red RL 5 – 4 5 4 4 5 – 4 5 Basacryl Red GL 5 – 4 5 5 – 4 5 – 4 5 – 4 5 Sevron Pure Blue 4 – 3 5 – 4 5 5 – 4 5 5 Sevron Blue 5G 4 – 3 3 4 4 5 5 Astrazon Blue BG 4 4 – 3 5 – 4 5 – 4 4 5 Deorlene Br. Blue 4 5 – 4 4 4 5 – 4 5 BR Calcozine Acrylic 4 4 4 5 – 4 5 – 4 5 Blue HU Astrazon Olive 5 – 4 5 – 4 5 – 4 5 – 4 5 – 4 5 Green BL Astrazon Black R 4 3 4 – 3 5 – 4 5 – 4 5

Table 7 Dye combinations for certain shades for protective clothing [32].

CI No. Brown Olive Navy Black Basic Yellow 53 X Basic Yellow 25 X Basic Orange 22 X Basic Red 29 X X X Basic Blue 54 X X Basic Green 6 X X X X Light (Fade–O–meter) Carbon Ar Shade Change 20 Hrs. 4 4 – 3 4 4 40 Hrs. 4 – 3 3 4 3

Pretreatment of polyaramid has also been suggested as a solution to dyeability of the fiber. Hermes [34] suggested a thermal treatment of shaped articles made of polyaramid, where at first the articles would pass through a hot bath of a high boiling point organic liquid

(e.g glycols, glycol ethers, solvents or solvent blends) at atmospheric pressure. Thereafter, the pretreated articles would be dyed in another hot bath containing a conventional organic dyestuff (dissolved or dispersed) in a high boiling point organic liquid. This process could either be batchwise or continuous and shaped articles could be yarns (staple or filament), tow or fabric, etc. The merit of this invention was the short treatment time, which was favored by industry. Another improvement was done by Preston and Hofferbert [35]. In their research, the authors pointed out the drawback from prior art was the poor tensile properties of dyed polyaramids. In order to overcome all demerits from previous studies, Richardson and Walck

[33] used pyridine for the aqueous dyeing of polyaramids. Preston and Hofferbert [35] confirmed that the usage of pyridine, to a certain degree, had provided good depth of shade and clean shade on bright fibers, had retained good fiber tensile properties and given adequate dyed lightfastness. An advantage of pyridine was its water solubility, therefore, after the dyeing process it was completely removed resulting in excellent lightfastness of dyed fabrics as confirmed by the authors. Moore and Weigmann [36] examined the chemical

energy provided from suitable solvents (dimethylformamide (DMF), dimethylacetamide

(DMAc) and dimethyl sulfoxide (DMSO)) as a means of modifying fiber structure [37] to help improve the dyeability.

To improve the affinity of basic dyes for high molecular aromatic polyamides, Nischk and his colleagues [38] invented a method to modify the molecular structure of polyamides with disulfone imide. Although there were no specific descriptions confirming the approach could be applied to meta–aramids, the results showed positive signals. To determine the improvement of dyeability, treated fibres were dissolved in 25 cc. of dimethyl acetamide and the extinction values at 475 mμ compared with pure dimethyl acetamide were measured in a photometer. The extinction values of modified polyamide were always higher than that of unmodified homopolycondensate product. The claimed novel polyamide comprised a conventional high molecular weight aromatic polyamide and 30 to 100 molar percent of the disulfone imide segments.

Swelling the polyaramid is another solution to overcome the dyeability difficulties of the fiber. In 1984 Kelly realized that preceding endeavors to color aromatic aramid substrates normally required high temperature (212oF or higher), but resulted in poor wash fastness and color fading under exposure to light [39]. To overcome these issues, he swelled polyaramid fibers and introduced into those fiber substances that could form strong bonds with anionic dyes into the fiber. These substances were amines or substituted amines (both aliphatic and aromatic). For convenience both tasks were done at the same time, thus the swelling solvent had to dissolve the amines as well (e.g. dimethylformamide, dimethylsulfoxide and dimethylacetamide). The detailed explanation of this successful patent was provided by

Moore and Weigmann [36]. Wolf and his co–workers [40] chose a different way to insert

color into polyaramids. In this invention, just dry–spun filament yarns went through a dyebath (containing either cationic or anionic dyestuff) prior to or during the stretching stages in the production line. The claimed advantage was that the aramid filament yarns had not been modified with any acid or basic groups. The proposed temperature in this invention was preferably in the range of 500C – 800C to give the best outcome. The solvents used were polar solvents which included dimethylacetamide, N–methyl pyrrolidone, dimethylformamide or hexamethyl phosphoric acid tris amide.

In order to have a continuous or semi–continuous dyeing process for meta–aramid fibers in combination with the enhancement of flame resistance, a series of patents [41-43] have been issued to researchers from Burlington Industry Inc. in Greensborough, North

Carolina, USA. In all of the suggested procedures, the meta–aramid materials went through solutions containing swelling agents before being treated with dyes and flame retardants.

They could also use this process to add flame retardants to improve the substrates flame resistance. The LOI for Nomex® T–455 fabric, dyed and flame–retardant treated from this invention, reached 39% compared to 26.6% for the greige Nomex® T–455 fabric. Printing of

Nomex® fabric by treating N–cyclohexyl–2–pyrrolidone prior to the application of printing paste was as well executed [44] and shaped articles made of meta–aramid fibers could be printed to improve flame resistance [45]. However, Riggins and Hauser [46] pointed out that these inventions [41-43] did not use conventionally available machinery and therefore the process had to be improved with common machinery and thus N–cyclohexyl–2–pyrrolidone was used to promote diffusion. Moreover, this process was specifically suitable for meta– aramids only and few improvements were obtained when applied to para–aramid (e.g.

Kevlar®). One year later, another patent [47] was disclosed that improved the previous one

[46] and deployed conventional pressure jet dyeing machines. In this upgraded procedure,

Riggins and Hauser provided manufacturers with an ability to simultaneously dye meta– aramid fibers while improving their fire resistance without lessening the fiber’s inherent strength. Additionally, the task ensured a high level of fire retardancy with an LOI of 37% –

44% as claimed by Cates [42, 43]. Later, Johnson [48] devised another process to impart both color and fire retardancy onto Nomex® substrates. The work related to a thermosol/pad process where a “neat” solution of fire retardant contained the required amount of disperse/acid dyestuff. This solution was then padded at room temperature onto the substrate and thereafter heated (3500F to 3900F) for 10 seconds to 2 minutes. Treated fabrics/fibers were rinsed by a halogenated hydrocarbon (perchloroethylene). The process was also applicable in print form using a paste or via immersion of the whole fabric into a hot bath containing necessary chemicals. Thermal treatment and rinsing steps were exactly similar for different routes chosen. This work claimed to confer better color fastness and fire resistance for treated Nomex®. The fire retardant used was cyclic phosphonate esters, which swell the

Nomex® fibers to create openings for the insertion of dyestuff molecules. The author also suggested the utilization of common chemicals such as N–methyl pyrrolidone, dimethylsulfoxide (DMSO) and dimethylacetamide to improve the swelling ability of the solution. Another work [49] involved coloring MPDI material with acid dyes via printing operation. In this method, a solution of diamine salt (hexamethylenediamine dihydrochloride) and a surfactant were imbibed into never–dried MPDI fibers to confer better printing and/or overprinting properties.

Ghorashi [50] introduced another method to dye tows of crystalline poly(meta– phenylene isophthalamide) fibers or filaments with water–insoluble dyes. First, an aqueous

dispersion of water–insoluble dye was padded onto the tow. Second, the tows were steam heated at a temperature higher than dye–activation temperature but lower than glass transition temperature of the MPIA. This temperature can reach up to 1650C so the dye can gradually diffuse into the MPID fibers. In short, the task was carried out at low temperature and over a short period (no longer than 30 minutes). More importantly, no swelling agents or carriers were used. By modifying dry spinning stages, Headlinger and collaborators [51-53] successfully obtained patents for the creation of low shrinkage meta–aramid yarns with good coloration values. The extruded filaments after going through the quenching step were conditioned in a solution at 300C or up to 1000C during which the drawing was also implemented. The filaments were thereafter washed, dried and heat treated up to 3000C in 0.5 to 5 seconds. This process was claimed to be continuous and darkened the color shade.

A common nuisance encountered with colored wholly aromatic polyamides (including meta type) is that the light resistance is quite weak, especially when a basic dye is used. In order to overcome this problem, prior to the dyeing process, wet or dry spun aromatic polyamide fibers should be treated by dipping in a UV–shielding solution. Nonetheless, during the dyeing process utilizing carriers, these “transporters” will shed the UV–shielding substances. Another approach was the use of alkylbenzene sulfonic acid onium salt as a dyeing assistant but this was considered costly in practice. Finally, pigments resistant to fading under light could be selected but this can result in loss of production time due to aggreation of pigment crystals, difficulty in small–lot production and restricted hues (selected color is dependent on market trend and has to be stored in large quantities. If color change is demanded, the extrusion equipment needs a thorough and complete cleaning) [54]. Lately, a

Japanese research group [55] disclosed a patent which claims to have overcome all the

aforementioned disadvantages by inserting a highly hydrophobic UV–absorber into the meta–aramid fiber so as the final fiber will have good light–resistance and can be dyed under carrier dyeing procedure without shedding of light stabilizer.

In addition to industrial patents, research–scale experiments were carried out in order to generate better color shades on the meta–aramid substrate. The role of a carrier, benzyl alcohol, in the dyeing of meta–aramid, with a cationic dye has been studied [56]. The researchers proposed that initially, benzyl alcohol swells the fiber’s structure, which facilitates the invasion of water molecules. These water molecules are then attached to the polar groups of the fiber and simultaneously with the carrier diminish the interaction of polymer chains in the amorphous zones. The interesting fact was that there must be a synergism between water molecules and the carrier to have the best effect. The utilization of swelling agents can be considered as the ubiquitous approach for the coloration of aramid fibers in general. Huapeng et al. [57] dipped meta–aramid fibers in dimethyl sulfoxide

(DMSO), as a swelling agent, and investigated the dyeability of pretreated fibers with disperse dye. The swelling agent used was confirmed to reduce the meta–aramid fiber’s

0 0 “estimated” glass transition temperature Tg from about 265 C to 170 C after 90–minute dipping with DMSO (Figure 4). Besides, due to the breakage of hydrogen bonds and the rearrangement of rigid chains (slippages), the dye uptake improved tremendously as the meta–aramid fibers treated with longer time and higher temperature.

Figure 4 DSC results of m–aramid fiber after different DMSO treatment time at 60°C [57].

Amines were also potential candidates for the pretreatment of meta–aramid fibers in advance of the dyeing stage. At first, it was claimed that they decrease the fiber’s Tg which facilitates the diffusion of basic dye. However, more reasoning proved that voids formed on/in the aramid fiber by the amines could strongly enhance the dyeability of the basic dye.

Furthermore, the dye was confirmed to bond with the –NH– on the surface of the void or pores only and this is independent of Tg. However, the treatment had certain influences on the fiber physical properties (rearrangement of polymer chains owing through shrinkage, hydrolysis of polymer chains under high amine concentrations, lower Tg) [58, 59]. A very recent doctoral dissertation [60] introduced a new eco–friendly swelling agent, N– methyformanilide (Figure 5), which greatly improved the dye uptake of basic and disperse dyestuff by swelling the molecular structure and reducing the Tg. Moreover, this carrier improved the thermal resistance of treated meta–aramid fibers (although the treated fiber begins to thermally decompose sooner than the pristine fiber), preserved the surface chemical

composition, and maintained the weight and physical properties of the original fibers [61].

This outcome is in compliance with that found in an investigation done by some Korean researchers where they found that compared to 1–phenoxypropan–2–ol, better K/S values were obtained with N–methylformanilide [62].

Figure 5 N–methyformanilide [61].

A group from China examined a new carrier, 2–Phenoxyethanol (Figure 6), which is confirmed to have an analogous structure to 1–phenoxypropan–2–ol, but is much less hazardous. The work successfully improved the dye uptake of basic dyes from 12% to 100% with a close relation with the amount of the swelling agent employed. The rubbing fastness was not affected by this carrier but wash–fastness was slightly reduced.

Figure 6 1–phenoxypropan–2–ol (left) and 2–phenoxyethanol (right) [63]

Researchers have also tried to color Nomex® fibers with vat dyes [64]. Dyeing behaviors of vat dyes on meta–aramid fibers were found to be similar to those on cellulose but with lower color yield, which was ascribed to the fiber’s high level of crystallinity.

However, this drawback can be ameliorated by careful control of dyeing conditions, vat dye selection and the usage of auxiliaries such as a swelling agent. Among the three vat dyes used, meta–aramid fabric dyed with C.I. Vat Green 1 (1% dye concentration) possessed a satisfactory tolerance of reflectance spectrum of forest green (Korean military standard with near infrared spectra result) and could thus be exploited in the military for camouflage purposes. The vat dyes also did not change the thermal and mechanical behaviors of dyed

Nomex®, and while washing and perspiration fastness were acceptable, rubbing and light fastness were not good enough for commercialization.

Following the global tendency toward eco–friendly finishing processes which aim to reduce the amount of water involved in production lines, several studies chose supercritical carbon dioxide as the medium for conveying dye molecules into meta–aramid fibers [65, 66].

Disperse dyes were used and in order to achieve good color depth and color strength, a carrier, CINDYE DNK, was suggested [66]. Alternative solutions involved dyeing at 1500C and 30 MPa [65]. Moreover, the chemical and physical properties of dyed materials were slightly altered but fastness to washing, light, and rubbing was not consistently good among the studies.

2.1.4.2. Finishing and functionalization

In a thorough review by Volokhina [67], it was reported that the production of meta– aramid fibers had developed dramatically during the second half of the last century as a thermostable material. With the ability to withstand high temperature (up to 200 – 2500C for long duration and 300 – 3500C for short duration) and not to burn in air, these fibrous materials are usually exploited in textile articles aiming to provide safety to human activities.

In the market, more than 18,000 tons/year of meta–aramid fibers manufactured belong to

DuPont, i.e. more than 90% of worldwide production. This was explained by the low–cost raw materials and availability of stock for the large industrial scale production. Nevertheless, for textile applications, meta–aramids still pose a number of drawbacks:

 Thermal shrinkage at high temperature

 Insufficiently fire resistance where oxygen is abundant

 Low resistance to photodegradation (UV radiation) and insufficient weather

resistance.

 Difficulty in dyeing via conventional methods

 Expensive

Within the scope of this work, a summary of many researches focusing on surface modification of meta–aramid to enhance the fiber’s properties was completed. Amongst the constraints existing in meta–aramid, improvement in dyeability is one of the most interesting targets of researchers. It is widely known that the molecular structure of aramid fibers, especially those of Kevlar® and Nomex® have a very high level of crystallinity due to the good stacking of the molecular chains (Figure 7).

Figure 7 Molecular structures of Kevlar® and Nomex® [68].

Utilizing the pad–dry–cure technique, Han & Jaung [69] synthesized a diblock copolymer (PEO45–MeDMA) and used this copolymer to modify the surface of meta–aramid

They successfully improved the dyeability with acid dyes (both leveling and milling types).

The diblock copolymer was formed via atom transfer radical polymerization (ATRP) (Figure

8).

The influences of pH and dye concentrations were considered and the data they obtained confirmed that the pretreated samples gave superior color strength and wash fastness to those of pristine samples. They ascribed these results to the strong ionic bonds between the cationic nitrogen atoms present in the copolymer chains and the anionic acid dyes.

Figure 8 The synthesis route of copolymerization of MeDMA and PEO45 by atom transfer radical polymerization [69].

Some researchers consider UV irradiation in the presence of ozone to be a cheap, clean, eco–friendly and energy saving textile process [70], and hence has been selected to modify the surface of meta–aramid film [71]. The surface roughness and O1s/C1s ratio were both raised. The enhanced wettability of the film due to improved roughness and higher surface energy was given as the major reason leading to the good cationic dyeability of UV– irradiated meta–aramid. The K/S values and exhaustion rate increased linearly as the UV energy increased (Figure 9).

Figure 9 K/S and exhaustion of UV–irradiated meta–aramid films [70].

UV/O3 treatment was also attempted by Yuanyaun and Jinho [72]. They found enhanced cationic dyeability of the UV/O3–irradiated meta–aramid fabric. A sequential process of traditional swelling agent plus electrolyte dyeing after UV/O3 pre–treatment was also studied [73]. They investigated three types of dyes (disperse, acid and basic) and found that cationic dyes had the highest dye exhaustion (Figure 10). Many parameters were studied including pH, types of electrolytes; the concentration of swelling agent used, and dye concentration.

Figure 10 Dye exhaustion (%) and K/S values on m–aramid fiber [73].

Using plasma discharge for surface modification is considered as a sustainable, greener and cleaner process being pursued by textile industry. A comparison between plasma processing and traditional textile wet processing is provided in Figure 11. A myriad of textile materials can be pre–treated with plasma technology, for instance, wool, cotton, polyester, and polyamide. A detailed review of benefits that can be obtained with plasma has been done lately [74]. However, reports of using plasma for meta–aramid fibers are quite few. In dielectric barrier discharge (DBD) plasma, a discharged particle within the plasma field etches the fabric surface as well as implanting oxygen–containing species (O–C; O=C;

O–C=O) which caused an enhancement of roughness and thus wettability [75]. Zhang et al.

[76], in subsequent research tried to use DBD plasma to alter the properties of Nomex® III instead of Nomex® IIIA but found little difference between these two types on meta–aramid.

The surface of Nomex® fibers under different plasma amplitudes were altered as shown in

Figure 12. Horrocks and colleagues exploited atmospheric plasma to create an inorganic coating with functionalized clay to improve the flash–fire resistance for fire resistant meta– aramid and cotton fabrics [77].

Figure 11 Comparison between plasma and wet processes [78].

Figure 12 SEM images of PMIA fibers treated by plasma under different amplitudes: (a) untreated;

(b) treated with 60s; (c) treated with 120s; (d) treated with 180s [76].

In addition to coloration, providing novel functions for meta–aramid is also of interest to researchers. These include sustainability and bio–mimicry [79]. Several researchers have successfully coated silver onto the inert surface of polymetaphenylene isophthamide (PMIA) fibers [80] (Figure 13). SEM images of these fibers revealed significant differences between fibers treated with polydopamine prior to silver plating and those without the pretreatment.

The polydopamine pretreated PMIA fibers had a uniform coating of dispersed discrete silver

nanoparticles which resulted in a low electrical resistivity (0.61 mΩ·cm for a 490 nm thick coating, Figure 14)

Figure 13 Schematic Illustration of Procedure for Fabrication of PMIA−PDA/Ag Composite by Poly(dopamine)–Assisted

[80].

Figure 14 SEM images of (a and b) silver coated PMIA fibers without functionalization of dopamine and (c and d) silver coated PMIA–PDA without exogenous reducing agent [80].

Another group grafted an epoxy functionalized silane (KH560) onto the polydopamine pretreated surface of meta–aramid (PMIA) [81]. They claimed a 62.5% improved interfacial bonding between the epoxy–grafted meta–aramid fibers and a rubber matrix. Although the authors described two different routes (Figure 15), they did not clarify the advantages or disadvantages between the two.

Figure 15 Illustration of Procedures for Preparing MPIA–PDA–KH560 Fibers (a) and MPIA–(PDA+KH560) Fibers (b)

[81].

Another group grafted a chlorinated analog of 5,5–dimethyl–3–(triethoxysilylpropyl) hydantoin (BA–1) onto Nomex® to produce regenerable self–decontaminating textiles for protection against chemical warfare agents [81]. Similarly, N–halamide was grafted onto

Nomex® and other synthetic fibers via a pad–dry–cure approach, to impart antimicrobial activity after chlorination (Figure 16) [82].

Figure 16 ADMH grafting copolymerization and chlorination on the synthetic fibers [82].

Although ample efforts have been carried out to improve dyeability of meta–aramid fibers, the results obtained are not adequate and the fiber physical properties have been compromised. Surface modification has been tried for various finishing applications using

UV/O3 or plasma treatments but these methods were found to be intricate and costly. Even with the easiest technology, e.g. pad–dry–cure, the attempted preparation steps were still complex. The successes obtained for finishes via surface modification suggest that a pad– dry–cure process may be able to enhance dyeability of meta–aramid fibers.

2.2. Poly(acrylic) acid (PAA) [83]

2.2.1. Definition

Poly(acrylic acid), also referred to as poly(2–propenoic acid) is a polymer of acrylic acid each of whose monomer units possesses a carboxylic group. Figure 17 shows the molecular structure of PAA, which is often used as a textile sizing agent.

Figure 17 Poly(Acrylic) acid as a textile sizing agent [84].

2.2.2. Chemical structure and synthesis

For every two carbons in the polymer backbone of PAA, there are two carboxylic groups (–COOH). Thus the density of negative charges will be high if all carboxylic groups dissociate. Both PAA and its sodium salt, poly(sodium acrylate) (NaPAA), are commonly selected as water–soluble negative electrolytes and even as a food additive owing to its low toxicity. PAA can be manufactured via radical polymerization and results in a broad distribution of molar mass. In order to obtain narrower distribution of molar mass, PAA can also be synthesized by the hydrolysis of a narrow molecular weight distribution poly(tertbutyl acrylate) (Figure 18). PAA can even be obtaind with a dispersity index of approximately 1.1.

Figure 18 Polymerization of tertbutyl acrylate and hydrolysis [83].

2.2.3. Physical properties

PAA is a colorless, glass–like solid substance at room temperature with a glass

0 transition temperature Tg of 106 C. The chemical is soluble in water, alkali water, alcohols, formamide, and dimethylformamide. Theta solvents, in which polymer chains behave as ideal chains, of PAA are reported as 1,4–dioxane and 0.2 M aqueous HCl.

The properties of PAA are sensitive to pH, cross–linking by dehydration, strong interactions with polar surfaces, etc. The level of water absorption is better for PAA compared to its counterpart, polymethacrylic acid (PMAA) in which it is difficult to obtain complete anhydrous form. Upon heating to 2000C, dehydration will occur between meso– dyads to form cyclic anhydrides (Figure 19) [85].

Figure 19 Dehydration of PAA to form cyclic anhydrides [85].

If the molecular chains are free from anhydride groups, PAA will be completely water soluble although it is always soluble in slightly basic water. Thus it can be used as thickening agent of aqueous solutions to improve suspensions and dispersions, a flocculating agent, and ion exchange resins. Moreover, with the ability to interact with metal surfaces via the carboxylic groups, PAA is also used in certain adhesive formulations [85].

2.2.4. Behavior in aqueous solution

Under neutral pH value in both water and 1,4–dioxane, PAA will not completely

– + dissociate into –COO and H which has acidity comparable to acetic acid (CH3COOH), a typically weak acid. At room temperature, the polymer chains in 1,4–dioxane will be close to random coils because of the highly rotatable single C–C bond. The C∞ (characteristic ratio) of

PAA in 1,4–dioxane at 3000C will be 6.7.

C∞, from Dr. Flory’s book [86], is defined as the ratio of the mean–square distance of an element from the center of gravity of a nonlinear polymer to that of the corresponding linear polymer of the same polymerization. This C∞ is calculated from /(2n*L2), where r2 and L are the unperturbed mean–square end–to–end distance of the polymer and the length of the C–C bond of the main chain, respectively.

2.2.5. Applications

2.2.5.1. Printing thickener

In textiles, one of the most popular applications of PAA is as a printing paste for reactive dyes as an alternative to the widely used sodium alginate. Major reasons for this are high price and low availability of the latter. Ibrahim et al. [87] compared different PAA– based resins (polyacrylic acid (PAA), polyacrylic acid/polyethylene glycol–1000

(PAA/PEG–lOOO), polyacrylic acid/polyethylene glycol–6000 (PAA/PEG&OO), and polyacrylic acid/polymethacrylic acid (PAA/PMA)) with sodium alginate resin. The results showed that the depth of prints and other properties follow the following tendency

(PAA/PMA) > (PAA/PEG – 1000) > sodium alginate > PAA/PEG–6000 > PAA. In an attempt to improve printing pastes, Abo‐Shosha et al. [88] applied free radical polymerization to create adducts between PAA with either Gum Arabic (GA) or Dexy 85 (D, a commercial dextrin) to form PAA/GA1, PAA/GA2, PAA/D1, and PAA/D2, (where 1 and 2 refer to the low and high LR, respectively). They extended this study by combining PAA with either karaya gum or tamarind seed gum [89]. Both the attained adducts proved to be excellent thickeners for reactive and acid printing on wool, silk and nylon 6 therein color strength of all the adducts are better than that of native gums. They found that adducts with tamarind seed gum are better than adducts with karaya gum. Optimally detailed recipes to print wool, silk and nylon 6 with reactive dyes by using such novel thickeners were provided

[90].

2.2.5.2. Graft unit

2.2.5.2.1. Grafting techniques

There are two major groups for wet surface modifications, physical and chemical interaction with the fibers. Grafting approaches belong to the latter one [91]. To form graft copolymer chains, there will be three major routes: (i) grafting through, (ii) grating to, and

(iii) grafting from (Figure 20). Each technique has its own drawback but the common disadvantage is the steric hindrance of the reactive center which influences the grafting efficiency for all three routes [92].

Figure 20 Illustration of different methods of preparing graft copolymers [92].

In ‘grafting from’ technique, the polymerization will be initiated from the substrate surface via anchored initiators (normally covalently bonded). If a high grafting density is desired such as in brush formation, ‘grafting from’ is a useful technique [93]. However, with

‘grafting from’ it is difficult to characterize the resulting polymers. On contrary, with

‘grafting to’ technique, the polymer properties can be fully characterized prior to grafting.

Figure 21 depicts major characteristics between the two routes.

Figure 21 Two common approaches to membrane surface modification with macromolecules. Shown are strategies that lead to covalently bound polymer modifiers [94].

2.2.5.2.2. Surface modification with PAA using ‘grafting–from’

To polymerize PAA chains on the surface of substrates, initiators or activators must first be anchored at the surface to start the polymerization. Gamma irradiation from 60Co has been used to graft PAA onto polytetrafluoroethylene (PTFE) [95]. The PTFE–g–AA fibers proved to be a good adsorbent material for the uptake of Er(III). Similarly, PAA was successfully grafted onto expanded PTFE (ePTFE) membrane for the improvement of bioactivity of ePTFE facial prosthesis. Nonetheless, the grafting process affected the mechanical properties of the grafted membranes [96]. UV irradiation was selected to graft

PAA onto polyamide where the photo–initiator was Anthraquinone–2–sulfonate sodium, an acid dye. An interesting result was that after the photo grafting step, the photo–initiator was claimed to be covalently bonded to the polyamide substrate although the specific bonding between photo–initiators, polymerized monomers and the polymers substrate were not fully understood [97]. Electron beam pre–irradiation has been attempted as well for the graft polymerization of PAA onto poly(vinylidene fluoride) (PVDF) film [98] and polyester fabric

[99]. Plasma has also attracted interest from researchers for the grafting onto many substrates for instance Kevlar 29 fiber to improve surface wettability and adhesion [100], polypropylene fiber [101], and PTFE film to modify the electrical properties [102].

Galactosylated PET films were achieved through –COOH bridges after sequential steps of plasma pretreatment then UV–induced graft polymerization of PAA onto the films [103].

Treatments using super critical carbon dioxide CO2 [104] and ozone [105] have also been attempted. With a global propensity of green, sustainable treatments of textile materials are gaining much attention, nevertheless, novel techniques for the surface modification such as

ultrasound, ultraviolet irradiation, gamma irradiation, plasma treatment and ion beam continues to confront challenges of large scale manufacturing [106].

Conversely, although the ecologically adverse impacts of wet pre–treatments to modify the surface of polymers [107] they are still frequently used in industry [91]. For grafting of

PAA onto different types of polymers, benzoyl peroxide (BzO2) is a popular initiator.

Polyester multifilament fibers have been treated with BzO2 solution and acrylic acid monomers [108]. Many parameters, temperature, time, monomer and initiator concentration, type of solvent and alcohol, were investigated to study the grafting yield. The moisture regain of the grafted polyester was increased due to the amount of –COOH groups. The grafted fibers were more densely packed after grafting because the PAA chains filled the free volume within the fiber structure even though no swelling agents were used. Observing the cross– section of dyed fiber under optical microscope, it was realized that the dye molecules couldn’t go further than a quarter of the fiber’s radius. This was considered to be a barrier effect caused by the PAA chains when grafting reached 8 – 9%. In different research, a polyester surgical suture was grafted with PAA using BzO2 and displayed antimicrobial performance thanks to the of PAA to fix antibiotics [109]. Analogous results have also been found in case of polyamide 6 [110, 111] and polyacrylonitrile (PAN) [112]. With polyamide

66, the more PAA grafted onto yarns, the lower the water contact angle and the higher elongation at break [113]. Zhikang Xu et al. [114] altered the surface of fibrous polypropylene fibers used in a microporous membrane. The grafting yield reached the maximum amount at 700C with toluene as the dissolving solvent for acrylic acid monomers.

More interestingly, the addition of a cross–linker divinylbenzene improved the grafting degree 1.5 times as compared with no divinylbenzene. Nonetheless, the grafting of PAA onto

the porous fibers clogged the pores of the membrane due to swelling of PAA. Similar successes with PP have also been confirmed [115, 116].

Besides benzoyl peroxide, several other catalysts have been studied for grafting PAA onto the substrates based on ‘grafting–from’ principle. Ghosh and Das [117] used NaH2PO4

(via esterification) and K2S2O8 (via radical polymerization) separately or in combination by pad–dry–cure technique at neutral pH (pH 7) to graft PAA onto bleached cotton fabric.

Cotton was simultaneously altered at the surface and inner core by a formaldehyde–free procedure with improvements in physical and mechanical properties such as weight gain or tex value, appearance, texture and flexibility, crease resistance, moisture regain and basic dye uptake (K/S) value. PAA was also grafted onto cellulose and cellulosic fibers by different methods. A group of investigators from Mexico first treated cellulose microfibers and continuous cellulose filaments with a vinyl–terminated ethoxy silane then polymerized PAA chains using potassium persulfate (KPS) as the initiator [118]. Utilizing the same initiator a researcher claimed success in growing PAA chains on henequen cellulose microfibers with vinyl epoxides that contain a terminal double bond (1,2–epoxy–5–hexene and 1,2–epoxy–7– octene) [119], while another one obtained the same result on ultrafine cellulose fiber by either ceric ion initiated reaction or methacrylation of the cellulose [120]. Ammonium persulfate was also used as an initiator for grafting of PAA from chitosan beads [121].

2.2.5.2.3. Surface modification with PAA using ‘grafting–to’

When the ease of processing is considered, ‘grafting–to’ technique should be used because with ‘grafting–to’, the polymer can be isolated, purified, and well–studied in advance [94] then quickly grafted to the surface. PAA was grafted onto the surface of porous silica particles after treatment with aminopropyltriethoxysilane (APS) [122]. The silica particles with –OH functional groups reacted with APS at 1100C in the presence of toluene as a catalyst to anchor the amino end groups of the silane–based substance. This amino group was able to form amide linkage with the carboxylic group –COOH on the PAA. Proposed reaction scheme is depicted in Figure 22. Grafting of PAA has been successful with a ceramic membrane that had undergone functionalization with a silane (γ– methacryloxypropyl trimethoxy silane) [123]. In a different context, PAA was grafted onto silica particles under two different routes [124].

Figure 22 Modification of silica surface by “grafting–to” of poly(acrylic acid) [122].

In the first route, PAA modified the silica particles by reacting 1,1’– carbonyldiimidazole with 3–aminopropyl–functionalized silica particles, and in the second route, PAA reacted with 3–glycidoxypropyl–functionalized silica particles. Both routes are expressed in Figure 23. Non–porous P2 glass beads could also be grafted with PAA using

APS and glycidoxypropyltrimethoxysilane (GPS) as coupling agents [125]. Besides directly

grafting PAA chains onto certain substrates, Sudre et al. [126] at first treated a silicon substrate with an epoxy terminated silane (3–glycidoxypropyltrimethoxysilane), thereafter poly(tert–butyl acrylate) (PtBuA) was grafted on by spin–coating/thermal annealing technique. Finally, the poly(tert–butyl acrylate) brush was converted into PAA by a pyrolysis step.

Figure 23 Two different schemes to graft PAA onto silica particle [124].

Lately, diazonium salt has been utilized as a coupling agent in surface modification

[127]. Amongst many studies implemented, PAA has been successfully grafted onto a variety of substrates including metals (Au, Zn, Ti, and stainless steel), glasses, carbon nanotubes, and PTFE. The core mechanism of such grafting was proposed to be the redox activation of appropriate aryl diazonium salts with iron powder acting as a catalyst [128]. Based on this approach, PAA was grafted onto acrylonitrile–butadiene–styrene (ABS) polymers thereon an electrodeless plating of copper or nickel could be achieved opening a new, efficient palladium– or chrome–free procedure in the plating industry.

In order to bond cyclodextrin onto cotton and wool fabrics, polycarboxylic acids have been utilized as the linking agents (Figure 24) [129]. The investigators compared three polycarboxylic substances, 1,2,3,4–butanetetracarboxylic acid, citric acid, and PAA. The process was carried out using the pad–dry–cure technique. For cotton fabric, the grafting reaction happened firstly between polycarboxylic acid with cyclodextrin and secondly between polycarboxylic acid and cotton. Among the three polycarboxylic acids used as the crosslinking agent, PAA has the highest fixation of cyclodextrin onto cotton. Similar trend was found for the study with wool fabric.

Figure 24 Grafting reaction of cyclodextrin onto cellulose by the intermediate of a polycarboxylic acid bearing more than three carboxylic functions [129].

For the immobilization of proteins, PAA, was grafted onto n–heptylamine (HApp) thin film which was plasma deposited onto a silicon wafer via radio–frequency glow discharge followed by N–hydroxysuccinimide (NHS), and 1–ethyl–3–(3–(dimethylamino)propyl) carbodiimide (EDC) mediated amidiztion (Figure 25) [130].

Figure 25 The grafting of poly(acrylic acid) (PAAC) layers on n–heptylamine (HApp) thin films via water–soluble carbodiimide (EDC/NHS) chemistry [130].

By applying the same combination of catalysts (EDC/NHS), nylon 6,6 was modified with PAA through the amine end groups on the surface of nylon [9]. At first, EDC catalyzes the formation of O–acylisourea which in turn reacted with the primary amine group on nylon

6,6 to form an amide linkage. Furthermore, an acid anhydride can be formed as the second proton attached to the activated urea and the newly formed acid anhydride to establish another amide bond and recreate another carboxylate. Every reaction steps are visualized in

Figure 26.

Figure 26 Amidization of nylon 6,6 using EDC [9].

N–hydroxysulfosuccinimide (NHS) provides another route to form amide bond between PAA and nylon 6,6. Under mild condition, aminoacyl ester will be formed which will subsequently be attacked by the primary amine end group of nylon 6,6 resulting in formation of an amide link and regenerated NHS (Figure 27).

Figure 27 Amidization of a nylon 6,6 using NHS [9].

Based on the obtained data, the author confirmed that EDC was a better amidization accelerator than NHS. When both agents were used in combination, due to the stability and the steric hindrance present in the EDC/NHS–activated carboxyl group, access to the amine groups was hindered as compared with EDC–activated ones.

Thompson [131] could graft PAA onto the surface of nylon 6,6, by either solution grafting or adsorption grafting using 4–(4,6–dimethoxy–1,3,5–triazin–2–yl)–4–methyl– morpholinium chloride as the coupling agent. The PAA–grafted nylon 6,6 was then

successfully modified with different amines. She was able to form a superhydrophobic surface mimicking the lotus effect by grafting 1H,1H–perfluorooctylamine or octadecylamine to poly(acrylic acid) chains which had already been bonded on nylon 6,6 woven fabric surface. Water contact angles of 168° were achieved [132].

In a quick recap, since the appearance of meta–aramid in the market (1962), coloration has always been a challenge. Solutions have been studied and disclosed which generally include modification of polymer structures, different designs of carrier and surface modification of polymers. Modification of polymer structures are usually not simple and quite time consuming to find appropriate monomers meanwhile carriers even after many years of investigation continue to have deleterious effects on the physical properties of the final fibers. Amongst the three options, surface modification is in most favor since it does not harm the bulk properties and offers possible prompt processing. With meta–aramid specifically, most of recent research relating to surface modification has focused on improving the interface between the fibers and matrix in composite materials. Some work attempted using several novel techniques such as UV radiation or plasma but these approaches are expensive and mostly performed at laboratory scale. Those constraints have motivated the objective of changing the surface of meta–aramid fiber utilizing an easy and quick method (pad–dry–cure) so it can be dyed at mild conditions (room temperature, neutral pH) with acceptable (or adequately good) color fastness for certain applications.

CHAPTER 3 – MATERIALS

3.1. Nomex® IIIA fabric

The fabric used in this work was a greige woven fabric made of Nomex® IIIA (Type

462) yarns (warp and weft). These constituent yarns are a 38/2 plied type consisting of 1.5– denier fibers. Type 462 Nomex® is a combination of 93% m–aramid fibers, 5% p–aramid

(Kevlar) and 2% D–140 antistatic fibers (mostly used for firefighter apparel) [133]. The fabric has a basis weight of 139 g/m2 (4.1 oz/yd²). There were no sizing agents used for the fabric and only a small amount of self–emulsifiable wax applied to the fabric which was removed through a washing process using 0.5 g/L of surfactant (Invadine DA), 1.0 g/L TSP at 800C (1760F) in 10 minutes.

3.2. Poly(acrylic acid) – PAA

The poly(acrylic acid) – PAA was purchased from Sigma–Aldrich®. It had an average molecular weight Mw of approximately 450,000 g/mol with impurities ≤ 0.5% benzene. This polymer was used as received without any further preparations.

3.3. Toluidine Blue O (TBO)

This basic dye was purchased from Aldrich Sigma® and used as received without any further preparation steps. The molecular structure is shown in Figure 28.

Figure 28 Molecular structure of Toluidine Blue O (TBO).

3.4. Grafting of PAA onto Nomex® IIIA fabric

3.4.1. Screening grafting procedure

PAA was simply dissolved in distilled water to produce PAA solution of different concentration 2, 3, 4 (wt%). The fabric samples were then padded with these solutions to

100% wet pick up and left to air dry at room temperature prior to the curing stage at 2400C for 2, 3, 4 minutes accordingly. Thereafter, the samples are washed thoroughly with water to remove unfixed PAA molecules.

3.4.2. Modified grafting procedure – Design of Experiment

Screening data showed that larger amounts of PAA applied to Nomex® IIIA fibers resulted in more dye sites deeper color when dyed fabrics with Toluidine Blue O (section 4.3 and 4.7). However, all samples treated with 2%, 3%, 4% of PAA respectively gave harsh hand which is not desirable. Therefore, a modified design of experiment for the grafting of

PAA at lower concentrations was carried out as described in Table 8.

Table 8 Modified design of experiments for grafting of PAA onto Nomex® IIIA.

PAA Curing time Temperature concentration (minutes) (0C) (w%) 200 2 220 240 200 0.01 3 220 240 200 4 220 240 200 0.05 3 220 240 200 0.1 3 220 240 200 0.5 3 220 240 200 1.0 3 220 240

3.5. Quantification of PAA grafted onto Nomex® IIIA fabric

In order to measure the amount of PAA grafted onto the Nomex® IIIA fabric, an indirect method using TBO was selected [98, 103, 134]. 30 mL of an aqueous solution of 30

µM TBO, adjusted to pH 10 by sodium hydroxide NaOH, was poured into 0.1 grams of

PAA–grafted fabric samples and held for 6 hours at 370C in an agitating bath. Finally, the samples were rinsed with NaOH solution pH 10 to remove unfixed dye. Under alkaline environment, ionic complexes will be formed between the –COOH groups and the basic dye molecules. To determine the amount of TBO attached, desorption of the dye was carried out by putting the dyed samples into 50% (v/v) acetic acid solution and the amount of TBO in

the resulting solution was determined by measuring the optical density of the extracted dye solution at the wavelength of 633 nm with a UV–vis spectrophotometer. Based on the assumption that 1 mol of TBO would bond to 1 mol of the COOH group on the PAA molecular chains, the quantity of PAA was measured indirectly.

3.6. Dyeing of Nomex® IIIA fabric using basic dyes

3.6.1. Preliminary dyeing procedure

An exhaustion dyeing process was utilized for the coloration of treated Nomex® IIIA samples in this work. After being grafted, all the samples were cut into squares of various weights, dipped in dyeing solutions of pH 10 at a ratio of 300mL/g of fabric, and placed in a shaking bath at 370C for 1 hour.

Dyeing was also performed in a pad–dyeing process using 1% owf TBO solutions of

200 mL and a wet pick up of 100% with the padding speed of 1.5 m/min.

3.6.2. Modified dyeing procedure

Following the screening stage, a modified pad–dye process was designed. The new conditions were pH 7, temperature of 250C – 270C, 15–minute exhaustion time and dye application levels of 0.1% and 0.25% owf. Pad–dyed samples were produced and compared with exhaust–dyed ones.

3.7. Color strength measurement

Color strength of the dyed fabrics was measured using the X–rite Color Spectrometer

(Figure 29). The reflectance was measured in the UVD65, 10 mm of area view and 4–spot measurement mode and the K/S values were obtained from the Kubelka–Munk equation K/S

= (1 – R)2/2R, where R is the reflectance of the fiber, K the coefficient of absorption of the dye, and S the coefficient of scattering [65].

Figure 29 X–rite Color Spectrophotometer.

3.8. Fabric hand

Engineered fabrics should be sufficiently flexible for its application. The stiffness of treated fabrics was evaluated through bending rigidity (both warp and weft) using Kawabata system (KES–F2). The principle of this measurement is described in Figure 30.

Figure 30 Measuring principle of bending rigidity with Kawabata system (KES–F2) [135].

3.9. Crocking fastness

The dry and wet crocking fastness of the dyed fabric samples was measured using an

AATCC Crockmeter (Figure 31) according to the AATCC Test Method 8 Colorfastness to

Crocking.

Figure 31 AATCC Crockmeter.

3.10. Washing

To examine the wash fastness of cationic dye molecules on pre–grafted Nomex® IIIA fabric, dyed samples underwent a washing process following AATCC 61 – 2A standard for one cycle. This is an accelerated washing process whose one cycle is claimed to be equivalent to 5 conventional washing cycles. The temperature was 490C for a 45 minute was cycle.

CHAPTER 4 – RESULTS AND DISCUSSION

PART I – PRELIMINARY RESULTS

4.1. Quantity of PAA grafted onto meta–aramid fibers

4.1.1. Standard curve of Toluidine Blue O (TBO)

The indirect technique applied for the calculation of the amount of PAA grafted onto the surface of Nomex® IIIA requires a standard curve of absorbance of TBO relating TBO concentration and its absorbance (Figure 32).

Figure 32 Standard curve of Toluidine Blue O.

4.1.2. Amount of PAA grafted onto Nomex® IIIA

With the assumption that the molar ratio between TBO dye molecule and COO– functional groups on the surface, the amount of PAA grafted onto Nomex® IIIA can be deduced from the absorbance of the extracted TBO. The relationship between the absorbance of TBO and the treating conditions i.e. PAA concentration and curing duration is sketched

(Figure 33). The treatment conditions which yielded the highest absorbance of TBO were 4% of PAA solution and 3–minute curing at 2400C (with the maximal error percentage of 1%).

The obtained highest value of TBO’s absorbance, in addition to the formula provided in

Figure 32, can be converted to the concentration of PAA actually grafted onto the surface of

Nomex® IIIA fabric as given in the last row in Table 9.

Figure 33 Absorbance of Toluidine Blue O in terms of initial percentage of PAA solutions used for grafting to Nomex®

IIIA.

Table 9 Statistically calculated of TBO's absorbance after different treating conditions (PAA concentrations and curing duration).

4.1.3. Theoretical calculation of a monolayer of PAA grafted onto Nomex® IIIA fiber

Assume that the cross–section of Nomex® IIIA fiber is circular (Figure 34).

Nomex® IIIA has fiber density of ~1.38 g/cm3, fiber linear density of 1.5 D and the molecular weight of PAA is MwPAA = 450000 g/mol. Then the fiber radius is Then the fiber radius is

1.5푔 1 1.5 1 1 ∗ = R2 * π  R =√ ∗ ∗ = 6.2 10–4 cm = 6.200 µm 900000푐푚 1.38푔/푐푚^3 900000 1.38 휋

If a single monolayer of PAA molecules grafts to the surface in a layer of thickness t, equal to the radius of gyration of PAA:

푛 ∗ 푙2∗ 퐶 t = s = √ ∞ = 18 nm 6

Where s is the radius of gyration, n is the number of sigma bonds in the back bone of the polymer chains (450000/72 * 2 = 12500 bonds), C∞ is the characteristic ratio (= 6.7)

[136] and l is the length of a C–C sigma bond (1.54 Å). Using multiangle x–ray photoelectron spectroscopy (XPS), Dr. Kimberlee Thompson measured the thickness of the same PAA on nylon 66 to be 5 nm at 78% the coverage [131]. Assuming the true thickness of the coating to lie between 5 nm ≤ t ≤ 18 nm, the weight of PAA an the fiber is just the relative annular area, as shown in Figure 34. The annular area can be calculated as follow

0.195 x 10–8 cm2 ≤ [(R + t)2 – R2] * π ≤ 0.224 * 10–8 cm2

Since the density of PAA is 1.410 g/cm3, the weight of ring w of PAA per centimeter of fiber length should be 0.275 * 10–8 g ≤ w ≤ 0.315 * 10–8 g. Then a single monolayer of PAA on

Nomex® IIIA fiber corresponds to 0.165–0.189 % owf.

Figure 34 Circular cross–section of fiber with a singular coating layer.

By comparing this calculated amount of PAA required to form a monolayer on

Nomex® IIIA and the levels used in Table 9, it seems a large excess of PAA was used in the initial experiments. To test this, a second set of experiments were performed with reduced

PAA concentrations in a modified design of experiment (section 3.4.2) where the range of

PAA was 0.01 wt% to 1 wt%. This resulted in better color uniformity than the preliminary studies. Detailed discussions will be provided in section 4.6.

4.2. Mechanism of grafting PAA onto Nomex® IIIA

4.2.1. Scheme 1

The major raw materials used to produce meta–aramid fibers are diacids or diacid chlorides and diamines. After polymerization is complete, there are few chain–end functional groups, but sufficient for grafting high molecular weight polymers. Amongst the three raw materials, amines are the most promising groups for reacting with the carboxylic groups present on PAA. Such reaction will form an amide linkage similar to the backbone of meta– aramid fibers (Figure 35).

Figure 35 Amidization between carboxyl and primary amine.

4.2.2. Scheme 2

PAA possess a special property during thermal exposure. Under high temperature, the polymer will self–adjust its molecular structure forming anhydrides. Depending on the temperature range, the amount of anhydride will be different. In this work, the curing temperature was 2400C in 2, 3, and 4 minutes respectively the quantity of anhydride formed should be in range of 3% – 5% of the carboxylic acid groups in PAA. [137]. The anhydride group is a quite reactive functional group which will likely react with the amine chain–end groups to form an amide linkage (Figure 36).

Figure 36 Amidization between anhydride and primary amine.

These two proposed mechanisms were proposed by Kimberly Thompson [131] who showed that she could graft PAA onto nylon 6,6 fiber through covalent bonds of amide linkages using static contact angle and multi–angle X–ray photoelectron Spectroscopy (XPS) to verify the presence of PAA even after extensive extraction procedures performed to remove any non–grafted PAA. To form amide linkages between COO– of PAA and primary amine NH2 of nylon 6,6 fiber she used a condensing agent, 4–(4,6–dimethoxy–1,3,5–triazin–

2–yl)–4–methyl morpholinium chloride (DMTMM). The grafting process is depicted in

Figure 37.

Figure 37 Adsorption–grafting process of PAA onto nylon 66 [131].

In Dr. Thompson’s treatment, the surface grafting was driven through a chemical

– activation by a condensing agent – DMTMM. After bringing two species COO and NH2 closed to each other, DMTMM was added to drive the reaction. For the present study, instead of using a chemical catalyst, thermal energy was applied. It is well known that direct amidization between a carboxylic acid and a primary amine will be difficult unless high temperature are used (at least 1800C [138]). The temperatures used here were 2000C to

2400C. Therefore, the formation of a covalent bond between PAA chains and Nomex® IIIA was highly likely. Several other researches have published similar cases [139-141].

Within this section, both experimental and theoretical analysis of PAA have been calculated and compared. In the next section, the dyeing of grafted PAA will be discussed.

4.3. K/S values

Based on the data shown in Figure 38 the sample treated with 3% of PAA and cured at

4 minutes gave the best color strength. The amount of PAA grafted onto the surface of meta– aramid fibers was highest with this treatment as described in section 4.1. The highest amount of PAA means the highest number of dye site (–COO–) leading to the highest concentration of dye molecules could be bonded.

Figure 38 K/S values of dyed Nomex® IIIA samples.

Figure 39 shows the K/S maximum values obtained from different treatment conditions. At 2 and 3 minute curing durations, there is a very clear dependence of color strength upon the concentration of PAA grafted onto the meta–aramid fibers. However, at 4– minute curing treatment, this trend was reversed. This could be due to variability in measurements of the color strengths. The exhaust dyeing samples were very small (0.1 gram each), thus the color strength measurements performed only once for each sample.

Figure 39 Color strength of dyed Nomex® IIIA samples.

4.4. Crocking fastness

4.4.1. Single thermal treatment

Based on the above results, Nomex® IIIA fabric samples were treated with 4% PAA owf, curing time of 3 minutes at 2400C and dyed with 1% owf TBO solution, also by pad dyeing process. Both dry and wet crocking fastness test were done and values are shown in

Table 10.

Table 10 Crockfastness values (dry and wet) of dyed Nomex® IIIA samples.

Dyed then heated at Dyed then heated at Dyed samples 1700C in 1 minute 1300C in 3 minutes Dry 4 – 5 4 – 5 4 – 5 Wet 1 – 2 1 – 2 1 – 2

4.4.2. With a 2nd thermal treatment

To improve the wet crockfastness of dyed samples, a second thermal treatment (1800C in 3 minutes) was applied to them. Due to the unknown thermal degradation temperature of

TBO, a range of temperatures and durations were conducted as in Table 11.

Table 11 Wet crockfastness values after 2nd thermal treatment at different durations.

Temperature 170 175 180 185 (0C) Duration 1 2 3 1 2 3 1 2 3 1 2 3 (minutes) Wet All dyestuff 2 – 3 2 – 3 2 – 3 3 3 – 4 crockfastness degraded

PART II – FINAL RESULTS

4.5. Dyeing mechanism

Nomex® IIIA samples were padded with PAA solutions after which they were dried and the morphology of those pre–treated samples are as depicted in Figure 40.

Figure 40 Dried PAA–padded Nomex® IIIA.

In the next step, dried PAA–padded Nomex® IIIA samples went through a curing step at a high temperature during which a chemical reaction occurs, namely amidization.

Figure 41 displayed that PAA chains have been covalently bonded to Nomex® IIIA fibers through amide linkages –NHCO– (purple squares). Besides, not all –COOH functional groups will be depleted, and these –COOH groups (brown circles) will be available dye sites for a subsequent coloration procedure.

Figure 41 Cured PAA–padded Nomex® IIIA.

Finally, a basic dye solution at pH 7 (neutral) was used for dyeing of treated Nomex®

IIIA samples. At neutral pH, –COOH groups dissociate into –COO– because pKa of PAA is

4.5 – 4.75 [142]. Due to the opposite charges between –COO– and dye molecules of basic dyes, ionic bonds will be formed and dye molecules are attached to the fiber surface (Figure

42).

Figure 42 Dyeing of PAA–grafted Nomex® IIIA.

4.6. Color uniformity

4.6.1. Exhaust dyeing with an agitating bath

From preliminary results, the PAA–treated fabric was stiff. The model of coating the fibers with a monolayer of PAA indicated a lower concentration may be adequate. Thus a series of lower PAA concentrations were selected with an expectation of reduced stiffness. In this series, PAA concentrations were as low as 0.01 wt%. Even this concentration exhibited dyeing efficiency and also avoided stiffness. Using the coating model adapted nylon 6,6

[131] discussed above, 0.1 wt% concentration should be sufficient. When dissolved in D.I water (pH~7), the solution was dilute enough for PAA chains to fully expand due to ionization [143, 144]. This would increase the size of PAA chains causing lower adsorption of PAA chains onto the surface of Nomex® fiber. At a concentration of 0.01 wt% in neutral pH~7, PAA chains could be adsorbed evenly throughout the surface without excessive chain overlap, resulting in better uniformity of grafted surface which, in turn, allows cationic dye molecules to equally bonded to available dye sites. This could explain why color uniformity remarkably improved for lower PAA concentrations.

Visual tabulated samples from Table 12, Table 13, and Table 14 clearly show that the color uniformity obtained was satisfactory. With Basic Red 46, Basic Yellow 2 and Basic

Violet 16 samples, color uniformity could be maintained up to PAA concentration of 0.1%.

At PAA concentration of 0.5% and higher, color uniformity couldn’t be maintained.

Unlike the other basic dyes, Basic Blue 17 (TBO) exhibited color uniformity only up to

PAA concentration of 0.05%. In order to crosscheck the situation, another basic blue was used, Basic Blue 41, with the same outcome.

Table 12 Dyed samples of PAA–grafted Nomex® IIIA cured at 2000C with different PAA concentrations and curing durations using a shaking bath.

Pretreating Basic Blue 17 Basic Red 46 Basic Yellow 2 Basic Violet 16 conditions

2 min

0.01% PAA 3min

4min

0.05% PAA 3 minutes

0.1% PAA 3 minutes

0.5% PAA 3 minutes

1% PAA, 3 minutes

Table 13 Dyed samples of PAA–grafted Nomex® IIIA cured at 2200C with different PAA concentrations and curing durations using a shaking bath.

Pretreating Basic Blue 17 Basic Red 46 Basic Yellow 2 Basic Violet 16 conditions

2 min

0.01% PAA 3min

4min

0.05% PAA 3 minutes

0.1% PAA 3 minutes

0.5% PAA 3 minutes

1% PAA, 3 minutes

Table 14 Dyed samples of PAA–grafted Nomex® IIIA cured at 2400C with different PAA concentrations and curing durations using a shaking bath.

Pretreating Basic Blue 17 Basic Red 46 Basic Yellow 2 Basic Violet 16 conditions

2 min

0.01% PAA 3min

4min

0.05% PAA 3 minutes

0.1% PAA 3 minutes

0.5% PAA 3 minutes

1% PAA, 3 minutes

4.6.2. Continuous dyeing with a padding mangle machine

The exhaust dyeing process using a shaking bath appeared to have a problem in controlling the color uniformity of dyed samples. In an attempt to overcome the non– uniformity, a continuous dyeing procedure with a padding mangle machine was conducted.

Furthermore, a padding machine can provide a much higher productivity in addition to its dominance in the textile industry. In this section, Basic Blue 41 (0.1 owf% and 0.25 owf%) was applied to Nomex® III fabric treated with PAA. The results are presented in Table 15.

All of these Nomex® IIIA samples have a much more uniform blue color.

This can be explained by better control of the dye process. In the shaking bath, mixing is not uniform. At high PAA concentration, the dye will ionically bond to grafted and non– grafted PAA. When the non–grafted PAA is washed away, the resulting fabric is nonuniform.

At low PAA concentrations (0.01 – 0.05% owf), all of the dye ionically bonds to grafted

PAA, which does not wash off. With a mangle padding machine, dye molecules are applied uniformly. Thus, when the non–grafted PAA is washed away, the final product is still uniform. Another advantage with the padding mangle machine is that the dyeing step took only 7 – 8 minutes to achieve similar or even better results which required 15 minutes with the shaking bath technique. The padding mangle dyeing results are presented in Table 15 to

Table 17

Table 15 Basic Blue 41 dyed samples of PAA–grafted Nomex® IIIA cured at 2000C with different PAA concentrations and curing durations using a padding mangle machine.

Pretreating 0.1 owf% 0.25 owf% conditions

2 min

0.01% PAA 3min

4min

0.05% PAA 3 minutes

0.1% PAA 3 minutes

0.5% PAA 3 minutes

1% PAA, 3 minutes

Table 16 Basic Blue 41 dyed samples of PAA–grafted Nomex® IIIA cured at 2200C with different PAA concentrations and curing durations using a padding mangle machine.

Pretreating 0.1 owf% 0.25 owf% conditions

2 min

0.01% PAA 3min

4min

0.05% PAA 3 minutes

0.1% PAA 3 minutes

0.5% PAA 3 minutes

1% PAA, 3 minutes

Table 17 Basic Blue 41 dyed samples of PAA–grafted Nomex® IIIA cured at 2400C with different PAA concentrations and curing durations using a padding mangle machine.

Pretreating 0.1 owf% 0.25 owf% conditions

2 min

0.01% PAA 3min

4min

0.05% PAA 3 minutes

0.1% PAA 3 minutes

0.5% PAA 3 minutes

1% PAA, 3 minutes

4.7. K/S values

4.7.1. K/S values of exhaust–dyed Nomex® IIIA samples

Although preliminary results have shown that the highest color strength was obtained with PAA concentration of 3% and 4–minute curing duration but the fabric hand was harsh after the treatment. To reduce the harshness lower concentrations of PAA were used as described above. In Section 4.5.1 the color uniformity obtained with different colors were tabulated. However, different curing temperatures (2000C, 2200C, 2400C) and curing durations (2, 3 and 4 minutes) for PAA were tested to determine the influence of these parameters on color strength of dyed Nomex® IIIA samples.

The measured color strength for each dye is given in Table 18. At first, K/S values for each color used are quite similar even with various curing durations. Numerical data of such

K/S values are tabulated in Table 19 where the maximum variations between K/S values

(difference between the highest and lowest measured values) for each curing temperature under different treating conditions are given.

Table 18 Color strength of pretreated Nomex® IIIA samples with a fixed PAA concentration of 0.01 wt% and dyed with four different colors.

Table 19 Numerical values of K/S max of dyed Nomex® IIIA samples with a fixed PAA concentration of 0.01 wt% and dyed with four different colors.

For red, yellow and violet, the observed maximum variations were approximately 0.1 or less. The situation is different with Basic Blue 17 whose highest maximum variations calculated was approximately 0.2. In addition, at 2400C, the K/S value decreases for longer curing time with Basic Blue 17. A similar trend may occur for Basic Violet 16 although it is less obvious. This could be attributed to the thermodegradation of the dye at these high

temperatures. In general, it can be concluded that there are no significant influences of curing durations on the amount of PAA grafted onto Nomex® IIIA samples in this process but 3 minutes will be recommended because this curing time provided a high consistency of K/S values observed with all four colors tabulated in Table 18. An analogous conclusion can be deducted with other curing temperatures used. However, at a curing temperature of 2200C,

K/S values expressed the best regularity with different curing durations thus this should be the optimal temperature being selected.

With regards to the dependence of color strength as a function of PAA concentrations, measured K/S values for all four colors at different treating conditions altogether exhibited similar trend as discussed in section 4.3. This means that higher concentrations of PAA will definitely yield higher K/S values. At 0.01% PAA concentration, K/S values were the lowest and they kept ascending when PAA concentrations rose gradually to 1% despite of curing durations (2, 3 or 4 minutes). Twelve detailed graphs will be shown in the Appendix A

(Table A.1, Table A.2 and Table A.3)

As can be observed from Figure 43 with Basic Blue 17, there is no consistent trend of

K/S with the grafting temperature or time, as also seen for Basic Blue 41, Basic Red 46,

Basic Yellow 2 and Basic Violet 16 shown in Table A.4

Figure 43 K/S–sum values of exhaust–dyed Nomex® IIIA samples with Basic Blue 17 as a function of curing durations for

PAA concentration of 0.01 (wt%) at different curing temperatures.

4.7.2. K/S values of continuous pad–dyed Nomex® IIIA samples

From three graphs in Figure 44, Figure 45, and Figure 46, the K/S values follow the same pattern for exhaust–dyed samples and for pad–dyed samples. As seen in those graphs, there are bathochromic shifts in K/S values therefore the integrated K/S values were used in the analysis in Figure 47 where the K/S values as a function of PAA concentration. For PAA concentration over 0.05%, the K/S values increased linearly with PAA concentration. These results are summarized in Appendix A (Table A.4). The main difference between the exhaust dyeing and pad–dyeing processes is that pad–dyeing reduced dyeing time from 60 minutes to

8 minutes.

Figure 44 K/S–max values of pad–dyed (0.1 owf%) Nomex® IIIA samples after pre–cured at 2000C under different curing durations and PAA concentrations

Figure 45 K/S–max values of pad–dyed (0.1 owf%) Nomex® IIIA samples after pre–cured at 2200C under different curing durations and PAA concentrations

Figure 46 K/S–max values of pad–dyed (0.1 owf%) Nomex® IIIA samples after pre–cured at 2400C under different curing durations and PAA concentrations

Figure 47 K/S–sum values of Nomex® IIIA samples pad–dyed (0.1 owf%, Basic Blue 41) as a function of PAA concentrations cured at different temperatures in three minutes.

4.8. Crockfastness

4.8.1. Exhaust–dyed Nomex® IIIA samples

Values of crockfastness (wet and dry) from various pretreatment conditions were considerably improved for PAA concentrations of 1% owf compared to those from preliminary results as can be seen from Table 20, Table 20, and Table 21. The fabric stiffness is also reduced.

Molecular structures of five basic dyes (Blue 17, Red 46, Yellow 2, and Violet 17) and

Blue 41 used for this work are displayed in Table 22 and Figure 48.

Table 20 Crockfastness values (dry and wet) of dyed Nomex® IIIA samples cured at 2000C. Dry Wet Basic Basic Basic Basic Basic Basic Basic Basic Blue Red Yellow Violet Blue Red Yellow Violet 17 46 2 16 17 46 2 16 2min 4 – 5 4 – 5 4 – 5 4 – 5 2 – 3 4 – 5 4 – 5 4 – 5 0.01% 3min 4 – 5 4 – 5 4 – 5 4 – 5 2 – 3 4 – 5 4 – 5 4 – 5 PAA 4min 4 – 5 4 – 5 4 – 5 4 – 5 3 4 – 5 4 – 5 4 – 5 0.05% PAA 3 minutes 4 – 5 4 – 5 4 – 5 4 – 5 3 4 – 5 4 – 5 4 0.1% PAA 4 – 5 4 – 5 4 – 5 4 – 5 3 4 – 5 4 – 5 4 3 minutes 0.5% PAA 4 – 5 4 – 5 4 – 5 4 – 5 3 4 4 – 5 3 – 4 3 minutes 1% PAA, 4 – 5 4 – 5 4 – 5 4 – 5 3 4 4 – 5 3 3 minutes

Table 21 Crockfastness values (dry and wet) of dyed Nomex® IIIA samples cured at 2200C.

Dry Wet Basic Basic Basic Basic Basic Basic Basic Basic Blue Red Yellow Violet Blue Red Yellow Violet 17 46 2 16 17 46 2 16 2min 3 – 4 4 – 5 4 – 5 4 – 5 2 4 – 5 4 – 5 3 – 4 0.01% 3min 3 – 4 4 – 5 4 – 5 4 – 5 2 – 3 4 – 5 4 – 5 3 – 4 PAA 4min 3 – 4 4 – 5 4 – 5 4 – 5 2 4 – 5 4 – 5 3 – 4 0.05% PAA 3 minutes 3 – 4 4 – 5 4 – 5 4 – 5 2 4 4 – 5 3 – 4 0.1% PAA 4 – 5 4 – 5 4 – 5 4 – 5 2 4 4 3 3 minutes 0.5% PAA 4 – 5 4 – 5 4 – 5 4 – 5 2 – 3 3 – 4 4 2 – 3 3 minutes 1% PAA, 4 – 5 4 – 5 4 – 5 4 – 5 2 3 4 2 – 3 3 minutes

Table 22 Crockfastness values (dry and wet) of dyed Nomex® IIIA samples cured at 2400C.

Dry Wet Basic Basic Basic Basic Basic Basic Basic Basic Blue Red Yellow Violet Blue Red Yellow Violet 17 46 2 16 17 46 2 16 2min 3 – 4 4 – 5 4 – 5 4 – 5 2 – 3 4 4 – 5 3 – 4 0.01% 3min 3 – 4 4 – 5 4 – 5 4 – 5 2 4 4 – 5 3 – 4 PAA 4min 3 – 4 4 – 5 4 – 5 4 – 5 2 – 3 4 4 – 5 3 – 4 0.05% PAA 3 minutes 3 – 4 4 – 5 4 – 5 4 – 5 2 4 4 – 5 3 0.1% PAA 3 – 4 4 – 5 4 – 5 4 – 5 2 3 – 4 4 3 3 minutes 0.5% PAA 3 – 4 4 – 5 4 – 5 4 – 5 3 3 4 3 3 minutes 1% PAA, 3 – 4 4 – 5 4 – 5 4 – 5 3 3 4 3 3 minutes

Table 23 Molecular structures of Basic Blue 17, Basic Red 46, Basic Yellow 2, and Basic Violet 16.

Basic Blue 17 (Toluidine Blue O) Basic Red 46

Basic Yellow 2 Basic Violet 16

Figure 48 Molecular structure of Basic Blue 41.

4.8.2. Improvement for crockfastness of Basic Blue 17

In an attempt to increase crockfastness further, a second thermal treatment was applied for 3 minutes at 1850C. These results shown in Table 24 reveal improvement in both wet and dry crockfastness values of Basic Blue 17.

Table 24 Crockfastness (wet and dry) values of Basic Blue 17 exhaust–dyed Nomex® IIIA samples after a 2nd thermal treatment at 1800C.

Dry Wet Curing temperature 200 220 240 200 220 240 (0C) 2min 4 4 – 5 4 – 5 3 – 4 3 – 4 3 – 4 0.01% 3min 4 4 – 5 4 – 5 3 3 – 4 3 – 4 PAA 4min 4 4 – 5 4 – 5 3 – 4 3 – 4 3 – 4 0.05% PAA 4 – 5 4 – 5 4 – 5 3 3 3 3 minutes 0.1% PAA 4 – 5 4 – 5 4 – 5 3 3 3 3 minutes 0.5% PAA 4 – 5 4 – 5 4 – 5 3 – 4 3 3 3 minutes 1% PAA, 4 – 5 4 – 5 4 – 5 3 – 4 3 3 3 minutes

4.8.3. Pad–dyed Nomex® IIIA samples

Pad–dyed Nomex® IIIA samples exhibited better crockfastness than exhaust–dyed samples for both 0.1 owf% and 0.25 owf% (values shown in Table 25 and Table 25)

Table 25 Crockfastness (wet and dry) values of Basic Blue 41 (0.1 owf%) pad–dyed Nomex® IIIA samples of different pretreating conditions.

0.1 owf%

Dry Wet Curing temperature 200 220 240 200 220 240 (0C) 2min 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 0.01% 3min 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 PAA 4min 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 0.05% PAA 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 3 minutes 0.1% PAA 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 3 minutes 0.5% PAA 4 – 5 4 – 5 4 – 5 4 – 5 4 4 3 minutes 1% PAA, 4 – 5 4 – 5 4 – 5 4 – 5 4 4 3 minutes

Table 26 Crockfastness (wet and dry) values of Basic Blue 41 (0.25 owf%) pad–dyed Nomex® IIIA samples of different pretreating conditions.

0.1 owf%

Dry Wet Curing temperature 200 220 240 200 220 240 (0C) 2min 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 0.01% 3min 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 PAA 4min 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 0.05% PAA 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 4 3 minutes 0.1% PAA 4 – 5 4 – 5 4 – 5 4 4 4 3 minutes 0.5% PAA 4 – 5 4 – 5 4 – 5 3 – 4 3 – 4 3 – 4 3 minutes 1% PAA, 4 – 5 4 – 5 4 – 5 3 – 4 3 – 4 3 – 4 3 minutes

4.9. Fabric hand

This current work has applied a polymer, Poly(acrylic acid) – PAA, as a coating layer on Nomex® IIIA fabric. As mentioned in the literature review section, one of the most prevalent applications of PAA is a thickening agent or sizing agent. This means that the more

PAA used, the stiffer the end use product will be. With the present study, PAA also plays a role of providing dye sites to facilitate coloration using basic dyes. In order to have deeper color strength, higher PAA concentrations must be higher accordingly. However, as seen in the preliminary stage, PAA concentrations of 2%, 3% and 4% were certainly inappropriate due to stiff fabric hand. By lowering PAA concentrations down to the range of 0.01% to 1%, a number of good results have been achieved. Fabric hand has been assessed through bending rigidity using Kawabata evaluation system KES F2 for two PAA concentrations, 0.01% and

0.05% which exhibit excellent results for both exhaust–dyeing and pad–dyeing techniques.

All detailed graphs of fabric hand are provided in Appendix B.

Curing durations (2, 3 and 4 minutes) had no effect on the bending rigidity (Table B.1,

Table B.2, and Table B.3). At a PAA concentration of 0.05%, fabric hand became stiffer which can be subjectively perceived by touching. This meant that curing durations are not a serious concern for fabric hand but increased PAA concentrations has negative influence on fabric hand.

Likewise, the curing temperatures (2000C, 2200C and 2400C) had no effect on the stiffness as seen in Table 27.

Table 27 Mean values of bending moduli obtained between two specific curvature points (0.5 cm and 1.5 cm) going one way, and going the opposite way (–0.5 cm and –1.5 cm) for both warp and filling directions.

Warp direction 0.01% PAA 0.01% PAA 0.01% PAA 0.05% PAA Curing 2 minutes 3 minutes 4 minutes 3 minutes temperature Bending rigidity (gf.cm2/cm) 2000C 0.0802 0.0821 0.0828 0.1012 2200C 0.0885 0.0948 0.0913 0.1484 2400C 0.1062 0.0918 0.0983 0.1269 Control 0.0736 (no treatment) Filling direction 0.01% PAA 0.01% PAA 0.01% PAA 0.05% PAA Curing 2 minutes 3 minutes 4 minutes 3 minutes temperature Bending rigidity (gf.cm2/cm) 2000C 0.0631 0.0672 0.0624 0.0789 2200C 0.0719 0.0829 0.0721 0.1079 2400C 0.0727 0.087 0.0759 0.0838 Control 0.0603 (no treatment)

4.10. Washing of exhaust–dyed samples with Basic Blue 17

Washfastness is another property that important for textile dyeing. Several samples were washed by utilizing the AATCC 61–2A standard. Since these samples were surface dyed with ionic dyes in this work, dye molecules are expected to be removed during washing. However, all washed samples still exhibited a good level of color uniformity but a totally different hue from the same, unwashed fabrics. All samples are shown in the

Appendix C.

CHAPTER 5 – CONCLUSION

Meta–aramid is considered as one of the most difficult fibers to dye hence, a study of how to facilitate the dyeability of meta–aramid fiber was carried out in the present work.

Several findings are:

1. A pretreatment procedure using pad–dry–cure technique has been successfully

applied in which PAA has been grafted onto the surface of meta–aramid fiber

(Nomex® IIIA in this research) to create dye sites which are readily available for the

dyeing using basic dyes. The optimal parameters were 2200C for curing temperature, 3

minutes for curing time and 0.01 wt% of PAA concentration which could yield in good

K/S values and color uniformity. The dyeing temperature was 250C – 270C, pH value is

neutral (pH7) and dyeing duration is 7 – 8 minutes or 15 minutes depending on dyeing

technique selected (pad–dyeing or exhaust–dyeing method). In past work, dyeing

temperature is always ~1200C at acidic pH values for 60 minutes to obtain final results.

Furthermore, crockfastness (wet and dry) were as good as conventional dyeing process.

This is a great improvement over current practice.

2. To have a deeper color of dyed samples, PAA concentrations need to be increased.

Empirically, higher concentrations of PAA increased color depth but 1% of PAA was

found be the limit due to unacceptable increased stiffness of the fabrics. Final results of

Basic Red 46, Basic Yellow 2, and Basic Violet 16 were good but Basic Blue 17 was

not as good and requires more studies. Moreover, PAA concentration also directly

causes fabric hand to be stiffer which is not desired for final product thus a compromise

between color depth and fabric hand must be taken into account.

3. Two dyeing techniques were conducted to evaluate the proposed pretreatment, exhaust–dyeing using a shaking bath and pad–dyeing using a padding mangle machine.

The former method did well up to 0.05 wt% of PAA used for three colors (Basic Red

46, Basic Yellow 2, and Basic Violet 16) with color uniformity and crockfastness but struggled with Basic Blue 17. Therefore, a second thermal treating step was needed to improve dyeing with Basic Blue 17. However, a second thermal treatment will increase the cost and time. On the contrary, a padding mangle machine has proven to be a convenient and efficient tool for dyeing these fabrics. Both color uniformity and crockfastness achieved were at least comparable to current industrial performance.

Especially, all for basic dyestuff used (Basic Red 46, Basic Yellow 2, Basic Violet 16, and Basic Blue 17) had good color uniformity and crockfastness with all PAA concentrations grafted. This continuous dyeing method is strongly recommended and is widely used in the industry for other processes.

CHAPTER 6 – FUTURE WORK

Even though a new pretreatment process has been designed and conducted to facilitate the coloration of meta–aramid fiber through this work, there are still several areas that should be examined in future.

First, as a fabric is dyed, it is always desired to have a range of shades of a specific color. This current work has proven that dye meta–aramid (Nomex® IIIA) can be dyed easily at mild conditions but a wide range of different shades has not been achieved. The major problem for deep shaded lies in the coating layer of PAA which stiffens the fabric increasingly as more PAA used. Hence, it is highly suggested to find an alternative agent which can have similar functions like PAA but can be added more to the substrate without detrimentally affect the fabric hand.

Second, an approach to increase PAA concentrations without increasing stiffness should be investigated.

Third, for basic dyes (Basic Red 46, Basic Yellow 2, Basic Violet 16, and Basic Blue

17) were studied in this work however, they were selected randomly without taking into account of their chemical structures, and properties. 3 out of 4 dyestuffs exhibited similarly good results but Blue Basic 17 did not perform as well. Further studies should be performed to determine the chemical characteristics that lead to good dyeing performance.

Finally, although current work relates to the coloration of Nomex® IIIA fibers, the scientific basis is the modification of surface chemistry. However, if the coating is made of biopolymer, tissue engineering can be a potential area. This process can also be applied for water filtration as another chemical can be grafted to this coating. Furthermore, instead of using a basic dye, a basic UV absorber can be used as well to protect Nomex® IIIA fibers

from UV degrading. And last but not least, the current treating process may be applicable to fibers by applying the treatment at the spin–finish stage for synthetic fibers.

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APPENDICES

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Appendix A

Table A.1 K/S–max values of exhaust–dyed Nomex® IIIA samples cured at 2000C.

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Table A.2 K/S–max values of exhaust–dyed Nomex® IIIA samples cured at 2200C.

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Table A.3 K/S–max values of exhaust–dyed Nomex® IIIA samples cured at 2400C.

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Table A.4 K/S–sum values of exhaust–dyed Nomex® IIIA samples as a function of curing duration for PAA concentration of 0.01 (wt%) at different curing temperatures.

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Table A.5 Maximal absorbance values (between 400 nm to 700 nm wavelengths) of pad-dyed (0.1 owf%, Basic Blue 41)

Nomex(R) IIIA samples.

Curing temperature 200 220 240 (0C)

Maximal absorbance

2min 0.4558 0.4607 0.4744 0.01% 3min 0.4661 0.4873 0.4548 PAA 4min 0.4674 0.4868 0.4885

0.05% PAA 0.4636 0.5123 0.5614 3 minutes

0.1% PAA 0.4675 0.5905 0.5834 3 minutes

0.5% PAA 0.4953 0.6456 0.6805 3 minutes

1% PAA, 0.5256 0.6934 0.739 3 minutes

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Appendix B

Table B.1 Bending rigidity of Nomex® IIIA samples (filling and warp directions) cured at 2000C.

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Table B.2 Bending rigidity of Nomex® IIIA samples (filling and warp directions) cured at 2200C.

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Table B.3 Bending rigidity of Nomex® IIIA samples (filling and warp directions) cured at 2400C.

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Table B.4 Bending rigidity of Nomex® IIIA samples (filling and warp directions) padded with 0.01% PAA then cured at different temperature in 3 minutes.

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Table B.5 Bending rigidity of Nomex® IIIA samples (filling and warp directions) padded with 0.05% PAA then cured at different temperature in 3 minutes.

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Appendix C

Table C.1 Hue changing phenomenon in PAA–grafted Nomex® IIIA samples (0.1 wt% PAA, cured at 2400C in 3 minutes) dyed with Basic Blue 17 before and after washing with AATCC 61–2A standard.

Sample 001 – 30 μM, 600mL Basic Blue 17 Sample 003 – 30 μM, 500mL Basic Blue 17 Exhaust– dyed at 370C, 3 hours, pH 10 Exhaust– dyed 370C, 1 hour, pH 10 4.41 grams Nomex® IIIA 4.53 grams Nomex® IIIA

Sample 004 – 60 μM, 200mL Basic Blue 17 Sample 009 – 60 μM, 200mL Basic Blue 17 Exhaust– dyed at 370C, 30 minutes, pH 7 Exhaust– dyed at 370C, 1 hour, pH 7 4.57 grams Nomex® IIIA 4.21 grams Nomex® IIIA

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Table C.1 (continued)

Sample 007 – 60 μM, 200mL Basic Blue 17 Sample 008 – 60 μM, 200mL Basic Blue 17 Pad–dyed at 270C, 15 minutes, pH 7 Exhaust–dyed at 370C, 30 minutes, pH 7 4.84 grams Nomex® IIIA 4.19 grams Nomex® IIIA

Sample 010 – 60 μM, 200mL Basic Blue 17 Sample 015 – 30 μM, 710mL Basic Blue 17 Exhaust–dyed at 370C, 30 minutes, pH 7 Exhaust–dyed at 370C, 3 hours, pH 10 4.56 grams Nomex® IIIA 4.24 grams Nomex® IIIA

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