“UVC” yellowing resistance test for polyurethane
Most of the polyurethane resin is prepared by the reaction of MDI (diphenylmethane diisocyanate) containing benzene ring with polyester or polyether polyol, which belongs to aromatic polyurethane resin; Some polyurethane resins are prepared by the reaction of benzene-free HDI (hexamethylene diisocyanate) and IPDI (isophorone diisocyanate) with polyester or polyether polyols, which are aliphatic polyurethane resins. The cost of aromatic polyurethane products is relatively low, and the mechanical properties are good, but the light is easy to change color and the aging resistance is poor; Aliphatic polyurethane products have higher raw material costs and mechanical properties are not as good as aromatic products, but their light fastness is good and it is not easy to yellow. The photodegradation of polyurethane includes two formation mechanisms: the first mechanism, after the polyurethane absorbs light with a wavelength greater than 340 nm, the methylene group on the MDI oxidizes to form an unstable hydroperoxide, which in turn forms the chromophore monoquinimide structure, which causes the polyurethane to turn yellow, and then further oxidizes to form a diquinonimide structure, and finally becomes amber.
In the second mechanism, after the polyurethane absorbs light with a wavelength of less than 340 nm, Photo-Fries rearrangement occurs, generating aromatic amines, which are further degraded to produce yellowing. Aliphatic polyurethane resin does not contain double bond structures such as benzene rings, so aliphatic polyurethane products have good light resistance and are not easy to yellow.