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ABSTRACT
Ultraviolet light with a primary wavelength of 254 nanometers (0.01 mil), referred to as UVC, is being used to kill microbes in the air and on the surfaces of coils/drain pans in HVAC systems and on the surfaces of food processing equipment. UVC also is being combined with catalytic surfaces to remove volatile organic compounds from the air. Since many organic compounds present in HVAC systems and processing equipment may be exposed to the UVC light due to poor shielding or design, the potential of material degradation leading to component failure, and possibly, total system malfunction is significant. Standardized methods for exposing, then evaluating, nonmetallic materials have been developed for UVA and UVB but not UVC. Consequently, a research program consisting of a literature search and the UVC exposure of numerous polymers and nonmetallic compounds representative of common materials present in HVAC systems was performed. The following paper describes the results of the literature search performed to identify the photodegradation mechanisms of various polymers exposed to UVC, the photoreactors capable of performing long-term, accelerated UVC exposure tests of multiple samples and the analytical techniques best-suited for evaluating the photodegradation suffered by the UVC exposed samples.
INTRODUCTION
The literature search presented herein describing the different degradation mechanisms of the materials and nonmetallic components typically subjected to long-term UVC [254 nanometers (nm), 0.01 mil] exposure in HVAC systems is a summary of the project final report (Kauffman 2011). Two books published by J. Rabek (1995 and 1996) were the basis for the conducted literature search and contain thousands of references on research performed to study polymer photodegradation mechanisms. Although the majority of the reported UV studies were performed with UVA [320-400 nm (0.013 - 0.016 mil)] and UVB (280 - 320 nm (0.011 - 0.013 mil)], a significant number of the studies explored the effects of UVC on polymer degradation. The results of the conducted literature search were broken down into the following subcategories for discussion.
1. UVC absorption by the fresh polymeric surface
2. Photodegradation reaction mechanism
3. Effects of UV wavelength on degradation products
4 . Effects of contaminants and additives on degradation rate
5 . Effects of UV irradiance on degradation rate (Reciprocity Law)
6. Correlation between laboratory and HVAC UVC results