Quantitative relationships between the parameters of thermal degradation of polyvinyl chloride and the loss of properties

Thermal degradation of polyvinyl chloride (PVC) has been extensively studied by a host of eminent researchers, both in terms of its mechanism, and effects. Arrhenius-type relationships have been proposed to correlate the loss of hydrogen chloride (HCl) with temperature and duration of thermal degradation. However, the rate of dehydrochlorination, does not always correlate with the loss of polymer properties.; The purpose of this research was to obtain quantitative relationships between the parameters of thermal degradation of PVC, and the loss of mechanical, thermal, and optical properties of the polymer. The parameters of thermal degradation considered in this study were temperature, and heating time. The affected properties considered in this study were color, tensile strength, Izod impact strength, flexural modulus, and heat deflection temperature at 264 psi.; Test specimens of PVC containing 1, 3, and 5 phr of dibutyltin bis (isooctyl thioglycolate) and barium-cadmium stearate were prepared. The rigid specimens for mechanical, and thermal testing were prepared by dry blending, extrusion and injection molding, whereas the specimens for optical testing were formulated as plastisols. These test specimens were subjected to five gradually increasing temperatures; 350, 360, 370, 375, and 380{dollar}spcirc{dollar}F for five different heating times; 30, 60, 120, 240, and 480 minutes. The selected test properties were determined in appropriate tests before and after degradation.; 'Percentage property retention' was defined as the ratio of the value of the test property after a given heating time (post-degradation) to the original value of the test property (pre-degradation). The lowest percentage retention value of the test property acceptable for service purposes was assumed as 66% for mechanical properties. The lowest heat deflection temperature acceptable for service purposes was assumed to be 120{dollar}spcirc{dollar}F. The highest Gardner color number acceptable for service purposes was assumed to be 12.; Percentage property retention (test property value in case of color and heat deflection temperature) was plotted against logarithm of heating time for each value of temperature. A family of five curves was obtained in this way for each concentration of each heat stabilizer. From these plots, the values of log heating time corresponding to 66% property retention (or the acceptable value for service purposes) at each temperature were determined. These were then plotted against the reciprocal of absolute temperature.; The plots thus obtained gave quantitative relationships between the temperature of thermal degradation and the time duration of thermal degradation of PVC based on the deterioration of tensile strength, flexural modulus, Izod impact strength, heat deflection temperature, and color. All these relationships showed a typical Arrhenius-type character.