Effect of dual functional ionic liquids on the thermal degradation of poly(vinyl chloride)

The short-term thermal stability of PVC on addition of functional ionic liquids (ILs) based on phosphonium and pyridinium cations, and a docusate anion, was studied using FTIR, TGA-MS and TGA-FTIR. The thermal stability of PVC plasticized with these ILs is reduced and the activation energy for thermal degradation for the first step of PVC degradation lowered, relative to neat PVC. This feature is not dependent on the thermal stability of the IL itself, as their addition to neat PVC did not result in increased thermal stability of the resin. Analysis of the gases evolved during thermal degradation showed that there is no variation in the mechanism of PVC degradation in the presence of ILs, the only difference observed is that HCl is evolved at lower temperatures for PVC-ILs samples. The ILs had no effect on the second step of the thermal degradation of PVC. The decrease in the thermal stability of PVC-ILs is associated with two different effects: firstly, the anion weakens the CH bond while the cation forms complexes and weakens the CCl bond; second, the IL interaction with the polymer chains partly substitutes the interaction between the polymer chain and heat stabilizers (e.g. stearates) added to the PVC compound. In this way, the latter tends to be expelled from the bulk polymer, thus not being available for PVC stabilization at high temperature. Heat stabilizer migration is related to the solvating power of the ILs: the higher the solvating power, the higher the migration of heat stabilizer, the lower the thermal stability of the polymer.