Meaning
Material resistance to high-temperature degradation defines the processing and environmental limits of thermoplastic resins. The property known as thermal stability represents the ability of a polymer to retain its molecular weight, mechanical properties, and appearance when subjected to heat. This resistance is critical during melt processing, where the resin is heated above its melting point to be molded or extruded.
It is influenced by the polymer’s chemical structure and the presence of stabilization additives.
Analytical Evaluation
Laboratory techniques like thermogravimetric analysis and oxidation induction time measurements are used to quantify this resistance. In thermal stability testing, a polymer sample is heated under a controlled atmosphere, and its weight loss is monitored as a function of temperature. A high onset temperature of degradation indicates a resin that can withstand high-temperature processing without undergoing chain scission.
These tests are documented on material datasheets to guide processors in selecting resins for specific thermal environments.
Processing Signification
Extruder barrel settings and cycle times must be optimized to match the thermal limits of the resin. If the processing temperature exceeds the thermal stability limit of the polymer, rapid degradation can occur, leading to a loss of melt viscosity and the generation of gas bubbles. This degradation results in parts with low impact strength and cosmetic defects like splay marks or discoloration.
Molders use stabilizers and limit residence times to protect the polymer during melt processing.
Material Formulation
Incorporating protective additives is the primary method used to improve the resistance of polymer formulations. Sourcing resins with effective stabilizer packages is essential for applications that require multiple processing runs or long-term heat exposure. For example, secondary phosphite stabilizers are added to primary phenolic antioxidants to provide synergistic protection against thermo-oxidative degradation during extrusion.
This compounding strategy is particularly important for recycled resins, where the thermal history has already consumed a portion of the original stabilizers. It ensures that the recycled material can be processed safely without sacrificing the quality of the final product.
