Effects of ultraviolet radiation exposure on vinyl ester resins: characterization of chemical, physical and mechanical damage
Introduction
In recent years, there has been increasing interest in the use of fiber-reinforced polymer (FRP) composites in building and construction due to the many advantages offered by composite materials. Among these advantages are excellent corrosion and fatigue resistance and high strength-to-weight ratio. However, the outdoor environment contains elements that are destructive to organic polymers, such as moisture, acid rain, temperature cycling, and ultraviolet (UV) radiation. UV radiation, in particular, is known to be highly damaging to organic polymeric materials.
The primary objective of this study was to investigate the chemical and mechanical effects of UV radiation on vinyl ester, a thermosetting polymer commonly used in building and construction applications. Effects of exposure to UV radiation generated by a 1000 W xenon arc source were characterized by tensile testing, atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Nanoindentation was performed with the AFM on exposed and unexposed specimens to measure changes in hardness and Young's modulus in the exposed surface. The AFM was also used to monitor changes in the surface morphology at the nanoscale. These mechanical, chemical, and microscopy analyses were used to obtain an understanding of how this resin system degrades under exposure to UV radiation, and to determine the effects of UV exposure on bulk tensile properties.
Section snippets
Specimen preparation
All specimens were fabricated from Derakane 411-350 PA1 vinyl ester resin, cured with 0.03 mass
Tensile testing
Stress–strain curves for specimens tested after 0, 1000, and 4000 h of exposure are presented in Fig. 5(a)–(c). The unexposed material exhibited slightly ductile behavior, displaying a yield point and a small degree of plastic deformation. Little, if any, statistically significant changes in the tensile properties were observed after 1000 h of exposure; the stress-strain curves still exhibited yielding and plastic deformation. However, after 4000 h of irradiation, there was a distinct
Summary
Exposure to UV radiation can significantly affect the bulk tensile properties of a vinyl ester resin matrix. The ultimate tensile properties such as ultimate strain and specific toughness were sensitive to degradation, with up to a 40% decrease in the ultimate strain and a 60% decrease in specific toughness after 4000 h of exposure in an integrating-sphere-based exposure chamber. The hardness and modulus, as measured by an AFM indentation technique, both increased after 1000 h of exposure, but
Acknowledgements
The authors would like to thank Eric Byrd and Ned Embree of NIST, and Frank Cromer of the Chemistry Department at Virginia Tech, for their assistance in this research.
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