Thermal degradation behaviour of poly(oxymethylene):: 1.: Degradation and stabilizer consumption

The influence of processing, physical ageing and UV-irradiation on the degradation behaviour of heat-stabilized poly(oxymethylene) (POM) homo- and copolymers was studied by means of thermogravimetric analysis (TGA). For that purpose, injection moulded specimens have been processed, followed by subsequent reprocessing up to six times, aged by oven storage at T = 140 degreesC for up to eight weeks and UV-irradiated in a xenon tester. It is well known that homo- and copolymers undergo degradation in different ways due to their specific chemical structure. TGA indicates a random scission of the main chain as the initiation mechanism of degradation. The subsequent process depends on the material condition and the degradation treatment (pure thermal loading or UV-irradiation). As observed by gel permeation chromatography (GPC), multiple processing does not influence the molecular weight distribution of the materials. TGA/MS (mass spectrometry) investigations prove formaldehyde formation and continuous stabilizer consumption as the main processes during the ageing procedures, which result in some cases in a strong reduction in thermal stability of the homopolymer. In addition, a second mechanism is proposed occurring during thermal loading, which leads to an increase in thermal stability of some main chains to a degree similar to or even superior to the initial pellet state condition. As expected, the copolymers are more resistant against thermal loading. Thermomechanical as well as thermal loading decrease the onset temperature, changes in the degradation mechanism cannot be proved. UV-irradiation causes comparable effects than in the homopolymers. (C) 2004 Elsevier Ltd. All rights reserved.