Thermal Stability and Melt Processing Rules for Polyhydroxyalkanoate Resins
Polyhydroxyalkanoate processing demands strict moisture control below 200 ppm and residence times under three minutes to prevent cis-elimination chain scission.
Thermal Stabilization
Meaning
Chemical additives added to polymer formulations prevent thermal-oxidative degradation during high-temperature melt processing and long-term service life. This thermal stabilization prevents the chain scission and crosslinking that occur when the polymer melt is exposed to heat and oxygen. The treatment maintains the mechanical properties of the resin during compounding and moulding.
Additive Chemistry
Primary and secondary antioxidants work together to scavenge free radicals and decompose hydroperoxides before they can attack the polymer chains. In polyesters or polyolefins, effective thermal stabilization requires a combination of hindered phenols and phosphites. This combination protects the polymer during both the short, high-heat extrusion step and the subsequent injection moulding phase.
Without these additives, the polymer would undergo rapid yellowing.
Process Control
Melt flow index stability shows the effectiveness of the additive package during processing. If thermal stabilization is insufficient, the melt flow index will rise or fall dramatically during extrusion, indicating polymer degradation. Moulders monitor torque and viscosity to ensure that the resin has been properly stabilized.
This monitoring prevents costly downtime caused by degraded material clogging the nozzle.
Mechanical Retention
Finished parts retain their tensile and impact properties even after long exposure to heat. This retention prevents premature failure of the component when it is used in under-hood automotive applications.