ABSTRACT GUPTA, AMIT. Improving UV resistance of high strength fibers. (Under the guidance of Dr. Abdelfattah M. Seyam and Dr. Gary N. Mock.) High strength fibers such as Zylon®, Kevlar®, and Vectran® are characterized by their high strength to weight ratio. They are used to produce high performance products such as cables and ropes, preforms for composites, bulletproof vests, cut resistance articles, firefighter gear, airbags for passenger cars, and tendons for giant scientific balloons. This work starts with a discussion on the properties and uses of the above mentioned high strength fibers followed by a critical review of the published research in the area of UV degradation of textile structures made from different polymeric materials as well as high strength fibers. The physiochemical aspects of polymer photo degradation, the factors affecting polymer photo degradation, the experimental aspects of photo degradation, and the photo degradation of some commonly used polymers have been discussed. It was found in literature that high strength fibers degrade upon exposure to high energy UV radiation. This represents a serious impediment to the use of high strength fibers in a number of end uses where their high strength to weight ratio could be very advantageous. Finally a discussion on photo stabilization of polymers is also provided. The overall goals of this research were to investigate options for the creation of techniques to improve the resistance of high strength fibers to UV and establish a better understanding of the mechanism of the UV instability for high strength fibers. In this work it has been shown that unprotected Zylon® and Vectran® yarns lose more than 70% of their strength after six days of UV exposure in an Atlas weatherometer, while Kevlar® yarn loses more than 15% of its strength. From our work, the results of the effect of UV exposure time on the strength loss of high strength fibers, and the mechanism of strength loss for Zylon® yarn as a result of UV degradation have been reported. Characterizing techniques like X-ray diffraction, ATR-FTIR spectrophotometry, molecular weight measurements, and SEM micrographs have been used to understand the photo chemical phenomena taking place when this yarn is exposed to high energy UV radiation using an artificial light source. Keywords: Ultraviolet degradation, ultraviolet resistance, high strength fibers, chemical coatings, sheathings. IMPROVING UV RESISTANCE OF HIGH STRENGTH FIBERS by AMIT GUPTA A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Master of Science DEPARTMENT OF TEXTILE ENGINEERING, CHEMISTRY, AND SCIENCE RALEIGH May 2005 APPROVED BY: Dr. Abdelfattah M. Seyam Dr. Gary N. Mock Chair of advisory committee Co-chair of advisory committee Dr. Samuel M. Hudson Dr. Thomas Theyson Member of advisory committee Member of advisory committee Dr. Jan Genzer Member of advisory committee Biography Amit Gupta is currently a Master’s student at the department of Textile Engineering Chemistry & Science, College of Textiles, North Carolina State University, NC, USA. He received his bachelor’s degree in textile technology from the department of Textile Technology, Indian Institute of Technology, New Delhi, India. He is currently working on the area of photo degradation and stabilization of high strength fibers. He would like to continue working in the field of polymer science by pursuing a doctoral degree in fiber & polymer science. ii Acknowledgements I would like to express my most sincere appreciation to my advisor and co-advisor, Dr. Abdelfattah M. Seyam and Dr. Gary N. Mock, for their advice, guidance, support, encouragement and many hours of helpful discussions, which made this research possible. I would also like to thank the other committee members for their excellent support, Dr. Samuel M. Hudson, Dr. Jan Genzer who is my minor representative from the department of chemical engineering and Dr. Thomas Theyson. Special thanks to Dr. Theyson for helping in the preparation of chemically coated yarn samples at Goulston Technologies. A particular appreciation is extended to Ms. Birgit Andersen for training me on the use of analysis instruments, to Dr. Jan Pegram for setting up the tensile tester, and to Mr. Hai Bui for helping in preparation of the sample holders for yarns for exposure in the weatherometer. Special thanks to Dr. Magdi Said of NASA for providing the yarn samples and his invaluable suggestions in completion of this project and to NASA for funding this research. iii Table of Contents Page LIST OF TABLES …………………………………………………… viii LIST OF FIGURES …………………………………………………. xix LIST OF ABBREVIATIONS ……………………………………….. xxiii 1. Introduction ………………………………………………………. 1 2. Literature Review ………………………………………………… 3 2.1 High Strength Fibers …………………………………………. 3 2.1.1 Zylon® ……………………………………………... 4 2.1.2 Vectran® …………………………………………… 5 2.1.3 Kevlar® ……………………………………………. 7 2.2 Physiochemical aspects of photo degradation ………………….. 9 2.2.1 Photostability of polymers ………………………….. 10 2.2.2 General mechanism of photo oxidative degradation of polymers ………………………………………… 10 2.3 Factors affecting photo degradation …………………………... 13 2.3.1 Role of polymer morphology ………………………... 13 2.3.2 Role of temperature ………………………………… 16 2.3.3 Role of polymer manufacturing and processing ……… 18 2.3.4 Effect of moisture ………………………………….. 20 iv 2.3.5 Role of oxygen ……………………………………... 21 2.3.6 Influence of wavelength ……………………………. 24 2.4 Practical aspects of photo degradation ……………………….. 27 2.4.1 UV/VIS lamps and radiation source devices ………… 27 2.4.2 Photo degradation procedure ………………………. 28 2.4.3 Application of UV/VIS and IR spectroscopy in the study of polymer oxidation ………………………………… 29 2.4.4 Viscosity measurements …………………………… 29 2.4.5 Mechanical testing of polymer (plastic) samples …….. 30 2.4.6 Study of weathering of polymers …………………… 31 2.5 Photo degradation of homo chain polymers …………………… 33 2.5.1 Polyolefins ………………………………………… 33 2.5.2 Polyacrylonitrile …………………………………… 35 2.5.3 Polyacrylates and polymethacrylates ……………….. 35 2.6 Photo degradation of hetero chain polymers ………………….. 36 2.6.1 Polyamides and polyaramids ……………………….. 36 2.6.2 Polyesters …………………………………………. 38 2.6.3 Polyurethanes …………………………………….. 39 2.7 Strength loss in high strength fibers upon exposure to high-energy radiation ……………………………………….. 40 v 2.8 Photo stabilization of polymers: fiber protection for improved performance …………………………………………… 44 2.8.1 Antioxidants ……………………………………… 45 2.8.2 Photostabilizers …………………………………… 46 2.8.3 Pigments …………………………………………. 47 3. Research objectives ……………………………………………… 48 4. Experimental …………………………………………………….. 51 4.1 Materials …………………………………………………… 51 4.2 Application of UV protective systems ………………………… 51 4.3 UV exposure ……………………………………………….. 55 4.4 Development of sample holder for UV exposure ……………….. 56 4.5 Tensile testing ………………………………………………. 59 4.6 Fiber characterization for Zylon® …………………………… 61 4.7 Statistical analysis ………………………………………….. 62 4.8 Exposure of yarns in high altitude conditions ………………….. 63 5. Results & Discussion ……………………………………………… 72 5.1 Tensile properties ……………………………………………. 72 5.2 Variability test for Zylon® ……………………………………. 82 5.3 Effect of number of layers in covering protection technique component ……..…………………………………………. 83 vi 5.4 Weight add-on calculations for the coverings ………………….. 87 5.5 Crystallinity and crystal thickness ……………………………… 91 5.6 Molecular weight (Mw) …………………………………………. 94 5.7 Scanning Electron Microscopy ………………………………….. 97 5.8 Fourier Transform Infrared – Attenuated Total Reflectance Spectroscopy ……………………………………….. 101 5.9 Effect of temperature and gases generated in the weatherometer ….. 104 5.10 Mechanism of UV degradation of Zylon® ………………………… 104 5.11 Results for yarns exposed in high altitude conditions ……………… 106 5.11.1 Tensile properties ……………………………………… 106 5.11.2 Crystallinity and crystal thickness ……………………… 107 5.11.3 Molecular weight (Mw) ………………………………… 112 5.11.4 Scanning Electron Microscopy …………………………. 113 5.11.5 Fourier Transform Infrared – Attenuated Total Reflectance Spectroscopy …………………………………………. 117 6. Conclusion ………………………………………………………….. 120 7. Future Work ……………………………………………………….. 124 8. References …………………………………………………………. 126 9. Appendix ………………………………………………………….. 136 vii List of Tables Table 1 Properties of Zylon® fiber ……………………………………… 4 Table 2 Properties of 166.67 Tex/300 filament yarns from Vectran® HS and M fibers …………………………………………………………. 6 Table 3 Properties of different grades of Kevlar® fibers ………………….. 8 Table 4 Comparison of physical properties for Zylon® HM, Vectran® HS, and Kevlar® 49…………………………………………………… 8 Table 5 Strength of 13 mm rope from Kevlar® 49 in terms of exposure period to Florida sun ……………………………………………………. 44 Table 6 Yarn specifications and source …………………………………. 51 Table 7 List of finish/covering applications for different high strength yarns ……………….………………………………... 54 Table 8 Basis weight and thickness of covering films ………………….. 55 Table 9 Yarn specifications and source ………………………………... 64 Table 10 Effect of storage time on high performance yarn strength ……… 72 Table 11 Test of effect of storage time on tensile strength of Zylon® ……… 73 Table 12 Analysis of variance for tensile strength of Zylon® ……………... 74 Table 13 Test of effect of storage time on tensile strength of Vectran® …… 75 Table 14 Analysis of variance for tensile strength of Vectran® …………. 75 Table 15 Test of effect of storage time on tensile strength of Kevlar® …… 76 Table 16 Analysis of variance