We are using polyurethane adhesives in various places on our frame, while it is not being used structurally, mostly gap filling / joining insulation boards, I was wondering how it's structure gets damaged by UV exposure. I dimly recall from sleepy materials science lectures about 40 years back that surface changes of plastics resulting from various sources can penetrate beyond the skin although I cannot recall the process. Does this happen in PU adhesives or are the UV degradation effects limited to the outer surface? Any ideas?