The impact of heat and humidity on unplasticized poly(vinyl chloride)

Abstract

The thermal degradation at 60 °C and 80 °C of unplasticized poly(vinyl chloride), PVC, was comprehensively investigated through the application of numerous spectroscopic techniques, as well as contact angle measurements (CA), dynamic mechanical analysis (DMA), and size-exclusion chromatography (SEC). To study the effect of relative humidity (RH) on the deterioration of unplasticized PVC, two regimes of accelerated degradation experiments were selected: low RH (max. 20 % RH) and high RH = 60 %, which corresponds to usually the highest RH in heritage institutions equipped with an HVAC system. Nuclear magnetic resonance (NMR) analysis revealed no substantial alterations in the material during its degradation for up to 20 weeks. This finding suggests that no significant branching or crosslinking occurred, which was confirmed by SEC measurements. Notable changes were observed in the Raman and UV–Vis spectra, indicative of the formation of conjugated carbon-carbon double bonds through dehydrochlorination. The formation of polyenes was responsible for the yellowing of samples, characterized with a CIELab color analyzer. Notwithstanding, the aforementioned changes did not lead to a notable decline in the mechanical properties, as evidenced by DMA measurements. Electron paramagnetic resonance (EPR) spectroscopy demonstrated the formation of stable radicals even at 60 °C, and in the sample degraded at 80 °C the presence of radicals was evident. This indicates that a radical degradation mechanism cannot be excluded even at such low temperatures, prevailing at higher relative humidity values. A summary of employed methods was prepared as a guideline for heritage scientists, considering the invasiveness and destructiveness of the techniques and their outcomes.