Evaluation of the Thermal Stability of Thermoplastic Bio-Polyesters and the Effect of Thermal Stabilizers Using Multi-Step Torque Rheometry Tests

Stabilizing thermoplastic polymers against thermal degradation is an important aspect that must be addressed during material development and becomes critical in the case of bio-polymers, which often reveal reduced thermal stability and a narrow processing temperature window. Herein, we propose a new methodology to analyze and compare the thermal stability of thermoplastic materials, exampled by several types of bio-polyesters, such as aliphatic PBS and PBSA, aliphatic-aromatic PBAT and PBST, and amorphous PHBV, and evaluate the impact of thermal stabilizer on their processability and thermal stability. The proposed method relies on multi-step torque rheometry experiments that involve controlled cycling of the tested material under varied thermal conditions, shear forces, and processing times to acquire and evaluate the changes in flow behavior of the sample after its processing. By monitoring polymer melt behavior and comparing the changes before and after repetitive processing steps, we can gain valuable insights into the material performance and stabilizing efficiency of additives. The thermal stability of polymers and the efficiency of thermal stabilizers can be assessed by means of the relative change in temperature-normalized torque, ∆τ%, measured after different processing steps. Significantly, we demonstrate that the obtained ∆τ% values correlate with changes in the molar mass of neat polymers as a result of their processing. The proposed approach enables a semi-quantitative evaluation of the thermal stability of various polymers and the study of the efficiency of thermal stabilizers and their performance, providing a robust strategy for optimizing compound formulations, particularly regarding the optimal fractions required.