The effects of heat treatment and ageing on the mechanical properties of rigid PVC

摘要

Indoor profiles made of unplasticized PVC, both new and more than 20 years old, were annealed at temperatures in the range 50-190 ℃ for periods varying from 1 h to 143 days. The effects were examined mainly by elongation at break measurements and differential thermal analysis (DSC). A dramatic decrease in elongation at break was observed upon heat treatment at various temperatures both below and above glass transition temperature (T_g). However, when the new and old materials were re-extruded, the original values of elongation at break were obtained again. A decrease of elongation at break in samples annealed at temperatures below T_g was explained by the effect of physical ageing, while a decrease of elongation at break in samples annealed at temperatures above T_g was explained by the annihilation of the orientation in the materials. This hypothesis also explains why the original elongation at break was not attained by any heat treatment but by re-extrusion only. The DSC thermograms of the untreated, extruded materials were composed of an exothermal peak immediately above T_g, which continuously passed over into a broad melting endotherm. This exothermal peak remained after heat treatment at temperatures below T_g but disappeared in the DSC thermograms when the materials were heat treated above T_g. The exothermal peak was resurrected by re-extrusion only. Our interpretation of the DSC results was that extrusion causes orientation in the material, producing mesomorphic structures with chains aligned but not packed. Those structures can easily crystallize when heating above T_g, resulting in the crystallization peak in DSC.