Thermal aging of High Tenacity Polyvinyl Alcohol yarns
Résumé
A both multiscale and multi-techniques approach was developed to characterize the thermal aging in air at 70, 90, 100 and 120 °C of High Tenacity Polyvinyl Alcohol (HT-PVAl) yarns, and to elucidate the consequences of thermal aging on the chemical structure, crystalline morphology and several physico-chemical and mechanical properties of practical interest. First of all, FTIR and UV–Vis spectrometries showed that the main degradation products are isolated (1656 cm−1) and conjugated (1592 cm−1) carbon-carbon double bonds, and a large variety of carbonyl products such as: esters (1757 cm−1), aldehydes (1737 cm−1), carboxylic acids (1720 cm−1), ketones (1707 cm−1), but also polyenones (1630 cm−1). Possible mechanistic schemes were then proposed to explain the formation of all these different degradation products. Moreover, no change in the physico-chemical properties of the amorphous phase was detected by modulated differential scanning calorimetry and dynamic vapor sorption. In contrast, the decrease in both melting temperature and crystallinity ratio with exposure time clearly indicates a “nibbling” of the crystalline phase during thermal aging, thus highlighting a chemical attack preferentially localized at the amorphous/crystal interface for this complex anisotropic material. This heterogeneous degradation is responsible for the loss of fracture properties in tension of HT-PVAl yarns. Fairly simple relationships between the structural variables determined at the different scales of analysis were established. As an example, a linear correlation was found between the changes in fracture properties and the changes in crystalline morphology.
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