Property changes in polyoxymethylene (POM) resulting from processing, ageing and recycling

The degradation processes initiated by thermo-mechanical and thermo-oxidative loading as well as exposure to ultraviolet irradiation weathering were examined in commercially available semi-crystalline polyoxymethylene (POM), using predominantly thermo-analytical methods. With reference to different injection moulding conditions and moulding geometries (such as loudspeaker grilles and safety-belt components) it is demonstrated that POM-copolymer chain is not affected, even under higher shear stresses and complicated moulding geometries, if an appropriate additive is used. Thermogravimetric Analysis (TGA) was used to observe stabiliser consumption and further degradation, whereas investigations of melt-flow index and molar mass show effects correlated to late-term changes of the molar mass distribution. Mass-spectrometry investigations performed on parallel to the thermogravimetric analysis identified formaldehyde and carbon dioxide as the main degradation products. Weathering by ultraviolet irradiation results in damage similar to the thermo-oxidative impact. Generally, the copolymers are less sensitive to thermo-mechanical and thermo-oxidative degradations than the homopolymers due to the chain modification. Furthermore, reprocessing results in a decrease of the elongation at break and thermo-oxidative induced degradation proceeds faster for the additional UV-stabilised materials. (C) 2007 Elsevier Ltd. All rights reserved.