Effect of service temperature on structure and mechanical properties of polyamide 6 & 66 tyre cords

The effect of service temperature on chemical structure and mechanical properties of polyamide 6 & 66 tyre cords was studied over the broad range of 50–200 °C and for a period of 16 h. The heat treatment of cords at below 100 °C and above 120 °C was found to reduce their tensile properties considerably. The changes in properties above 120 °C were caused by increase in width of the molecular weight distribution curve as ascertained by gel permeation chromatography (GPC) studies and increase in irregularity of the polymeric chains as ascertained by birefringence studies; this appears to be due to the fact that, by raising the temperature, both chain folding and chain scission occur. Since there was deformation of the amorphous regions as ascertained by FTIR spectroscopy and birefringence studies, the changes in properties below 100 °C were attributed mainly to the effectiveness of thermal oxidation and annealing in the amorphous phase. In other words, the degree of crystallinity increased, the tyre cord became brittle, and breaking load and elongation at break were decreased. The lower reduction of tensile properties at an intermediate temperatures of 100–120 °C was caused by the lower polydispersity and irregularity in the polymeric chains, in comparison with higher temperatures and less crystallinity than lower temperature treatments.