光屏蔽剂对聚甲醛老化过程结构与性能的影响

采用炭黑和氧化锌两种光屏蔽剂对共聚甲醛进行耐老化改性。研究了改性前后聚甲醛在紫外光老化及热氧老化过程中的表面层及内部发生的微观物理化学变化，包括熔点、结晶度、表面羰基量及分子量变化等。结果表明，紫外光的照射主要导致聚甲醛试样表面层发生晶体破坏和无定形部分分子链降解，内层则主要发生非晶区重整结晶；热氧作用下聚甲醛发生氧化降解的同时整体较均匀地发生非晶区重整结晶，以上情况均导致聚甲醛结晶度上升、拉伸强度提高，但抗冲击韧性及断裂伸长率显著下降，如经过1000 h紫外光辐照后，聚甲醛表面层结晶度由42.06% 上升至 76.27%，而简支梁缺口冲击强度由6.74 kJ/m2 迅速下降至1.85 kJ/m2。通过添加1.5%~3%的炭黑或氧化锌可以有效地抑制紫外光对聚甲醛的降解和重结晶过程，从而提高材料老化后的性能保持率，但炭黑对抗氧剂过强的吸附力导致聚甲醛的热氧稳定性下降。

Effect of Light-shield Agents on the Structural and Properties of Ageing POM

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.