Modeling the ultraviolet photodegradation of rigid polyurethane foams

Before the Montreal Protocol of 1987 and the subsequent phasing-out of chlo rofluorocarbons (CFCs) in industrial applications, rigid polymer foams were made using these compounds as secondary blowing agents. The CFCs remain tr apped in the gaseous part of the cellular foam structure, and once discarde d these foams constitute a significant reservoir for the environmental rele ase of ozone-depleting chemicals. Environmental degradation of the foam acc elerates this process. Of particular interest in this work is the degradati ve effect of ultraviolet (UV) light on rigid polyurethane foams. Foams were subjected to accelerated weathering conditions and then viewed with a scan ning electron microscope. The thin cell membranes near the foam surface deg rade when exposed to UV light, leaving only a network of polymer struts tha t offers negligible resistance to the escape of CFCs or any other gases con tained within. This effect has been reproduced qualitatively through simula ted weathering of a computer-generated foam structure. If enough is known a bout the optical properties and photosensitivity of the polymer foam, this simulation technique can be used to estimate the rate of weathering penetra tion in any situation where photodegradation is the primary concern.