Ester Bond Hydrolysis Kinetics in Molten Polyethylene Terephthalate
Polyethylene terephthalate ester bonds hydrolyze rapidly in melt above 270 °C, requiring drying below 30 ppm moisture to prevent intrinsic viscosity collapse.
Auto-Catalysis
Meaning
Chemical reaction acceleration takes place when a byproduct of the primary reaction actively stimulates further identical occurrences within the material matrix. While standard polymer breakdown relies on ambient oxygen or mechanical shear, auto-catalysis represents an internal transition where the chain molecules participate in their own degradation sequences. Acidic species formed during initial chain cleavage lower the reaction thresholds for surrounding polymers.
Feedback Cycle
Polymer chains in compostable resins show higher sensitivity to this phenomenon because their functional groups are susceptible to proton acceleration. The generated acidity from a single rupture event spreads into the surrounding melt and initiates multiple secondary failures. Moulders encounter erratic viscosity levels when small quantities of degraded material remain in the barrel from a previous run.
Consistent flow requires neutral conditions.
Process Stability
High temperatures and excessive residence times typically provide the energy needed to cross the barrier into self-sustaining reaction. Once these catalysts reach a saturation threshold, the structural integrity of the resin collapses in seconds. Chemical buffers are added to specialty grades to intercept the protons and delay the arrival of this phase.
Part brittleness and yellowing appear as the first observable signs of this transition in clear parts. Thermal limits decrease as the reaction progresses. Virgin material prevents this drift better than aged resin.
Contamination Margin
Virgin polymer behaves differently than regrind batches because the recycled material often enters the hopper with an accumulated density of reactive sites. Blending regrind in percentages above recommended levels causes thermal instability during the injection sequence.