Solid State Polymerization Kinetics and Equilibrium in PET
Solid state polymerization elevates PET intrinsic viscosity above melt limits by driving byproduct removal below the polymer melting point under inert flow.
PET Resin
Meaning
Saturated semicrystalline polyester formed by the polycondensation of purified terephthalic acid and monoethylene glycol provides a high-strength, barrier-efficient thermoplastic raw material for packaging, strapping, and fibre manufacturing. This polymer features an ester-linked backbone that balances mechanical rigidity, chemical resistance, clarity, and recyclability. The term pet resin governs the raw virgin or post-consumer polymer pellets supplied for industrial conversion, excluding formulated thermoset polyesters, elastomeric polyesters, and chemically modified engineering polybutylene terephthalate grades.
Polymer Chemistry
Industrial production of this material proceeds through liquid-phase esterification followed by melt-phase polycondensation, generating a base polymer with moderate molecular weight. Solid-state polymerization subsequently advances pet resin to higher intrinsic viscosity levels for demanding applications. Thermal properties include a glass transition temperature around seventy-eight degrees Celsius and a crystalline melting point near two hundred and fifty degrees Celsius.
These physical transition boundaries define the tight processing windows required during drying, injection plasticization, and stretch blow moulding operations.
Rheological Windows
Processing behaviour depends strictly on chain preservation through melt operations. Hydrolytic degradation occurs rapidly if pet resin enters an injection barrel with moisture content above thirty parts per million, causing chain scission that destroys melt strength. The resulting drop in intrinsic viscosity produces wall thinning, drop impact weakness, and brittle preform necks.
Moulders run desiccant drying hoppers at temperatures between one hundred and sixty and one hundred and eighty degrees Celsius for four to six hours to ensure complete moisture extraction without causing pellet discoloration.
Market Sourcing
Commercial purchasing divides this material into broad commodity classifications based on intrinsic viscosity, comonomer content, and optical purity. Post-consumer recycled resin blends directly with virgin pet resin to satisfy packaging sustainability targets, though recycled flakes introduce color shift, lower intrinsic viscosity, and trace contamination risks. Solid contamination causes gate blockages in multi-cavity injection moulds, while volatile contaminants accelerate mold deposit formation and require frequent tooling cleaning cycles.
Tracking batch-to-batch intrinsic viscosity variance remains the primary quality barrier in preventing scrap during high-speed container conversion.