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.
Specific Heat Capacity
Meaning
Thermodynamic property defining the amount of heat energy required to raise the temperature of a unit mass of polymer by one degree Celsius governs the heat transfer dynamics of melting and solidification cycles. Measured in joules per gram-kelvin under standard ASTM or ISO protocols, this variable dictates how much energy an extruder must supply to plasticize resin and how much heat a mold must extract to freeze a part. The measurement scope covers thermal energy storage over designated temperature ranges, excluding enthalpy contributions from phase change latencies unless reported as apparent thermal capacity.
Thermal Transitions
Enthalpy curves for polymers shift dramatically across physical transition boundaries. For an amorphous resin like polystyrene, specific heat capacity climbs gradually across the glass transition without sudden spikes. Semicrystalline polymers like polyethylene and polypropylene exhibit a large, sharp peak in apparent heat capacity at their melting points, reflecting the latent heat of fusion required to break down crystalline lamellae.
Injection moulders rely on differential scanning calorimetry curves to determine actual thermal energy requirements across the entire heating and cooling profile.
Processing Energetics
Barrel heating, screw drive power, and mold chiller sizing depend directly on this thermodynamic variable. Resins with elevated specific heat capacity demand more screw mechanical power to reach melt temperature and require significantly longer cooling times inside the tool before ejection. A moulder shifting from an unfilled polycarbonate to a high-capacity semicrystalline polyolefin must extend cooling cycles or lower coolant temperatures to prevent part ejection warping.
Inadequate heat removal leaves parts too soft, leading to ejector pin push-through and post-moulding shrinkage variations.
Moulding Economics
Cycle time estimation on technical injection tooling relies on accurate thermal property data. Virgin resins deliver consistent specific heat values, but glass fiber additions, mineral fillers, and metal flake loadings substantially lower the compound thermal capacity per unit mass. Regrind additions alter crystallization kinetics, creating slight shifts in the apparent specific heat curve during solidification.
Moulders monitor mold surface temperature rise to verify that heat extraction rates balance the energy delivered by the molten plastic, ensuring dimensionally stable parts across twenty-four-hour continuous production runs.