Meaning
Rate and mechanism of chemical breakdown in a polymer when exposed to heat provide the data needed to set safe processing limits. Thermal degradation kinetics describe how temperature and time interact to shorten polymer chains or cause cross-linking. This information is essential for preventing the loss of mechanical properties during the injection or extrusion cycle.
Residence Time
Duration that the resin spends in the heated barrel determines the total thermal load it must withstand. Thermal degradation kinetics show that a material might survive a high temperature for a short time but fail at a lower temperature if held there too long. This is why dead spots in a barrel or runner system lead to black specks or brittle parts.
Reaction Pathway
Chemical changes like oxidation or side-group stripping occur as the polymer reaches its decomposition threshold. Thermal degradation kinetics involve different energies for each type of reaction, which can be measured using thermogravimetric analysis. Some polymers release corrosive gases when they break down, which can damage the mould and the machine.
Material Recovery
Ability to use regrind is limited by the amount of thermal damage accumulated during each heat cycle. Thermal degradation kinetics help to determine the maximum percentage of recycled material that can be blended with virgin resin without compromising the final product. A part moulded from degraded resin will show reduced impact strength and may exhibit yellowing or other color shifts.
Testing the melt flow index of the regrind compared to the virgin resin provides a simple measure of how much degradation has occurred. This data ensures that the production process remains within the safety margins defined by the material supplier.
