Two kinds of light-shield agents of ZnO and carbon black were used to improve the ageing resistance of polyacetal (POM). Changes in exterior and interior characteristics, such as melting point, crystallinity, carbonyl and molecular weight of POM under UV-irradiated or thermal-oxidative ageing were investigated via DSC, FTIR and Ubblohde type viscosity in relation to molecular weight. The results showed that UV radiation mainly led to crystal destruction and molecular degradation on the surface and recrystallization on the interior part of the sample. The restructuring and crystallizing of the amorphous phase occurred during thermal-oxidative ageing. Both changes induced crystallinity and tensile yield strength increase but toughness and elongation at break decrease，for example, after UV irradiation for 1000 h, the crystallinity of polyacetal exterior part increased from 42.06% to 76.27%,while the charpy impact strength declined quickly from 6.74 kJ/m2 to 1.85 kJ/m2. Adding 1.5%~3% carbon black or ZnO could maintain the properties of POM by means of restraining the molecular degradation and recrystallization attributed to UV-irradiation. However, the adsorption to antioxidant of carbon black might accelerate thermal-oxidative degradation of POM.
