PET Drying Windows and the Hydrolysis a Moulder Absorbs
Excess residual moisture causes instant ester chain hydrolysis in molten PET, dropping intrinsic viscosity and destroying mechanical part strength.
Autocatalytic Degradation
Meaning
Degradation processes in polymers often accelerate once they begin because the initial reaction byproducts act as catalysts for further breakdown. Rapid autocatalytic degradation occurs when acidic or metallic species released during thermal stress trigger additional molecular fragmentation throughout the melt. This mechanism is particularly dangerous in materials like polyoxymethylene where the buildup of formic acid leads to a sudden loss of mechanical properties.
Kinetic Acceleration
Chemical reactions following this pattern show a slow initial induction phase followed by a sudden increase in velocity. In the context of autocatalytic degradation, the accumulation of carboxyl groups or volatile monomers reduces the activation energy required for subsequent bond breaking. Stable resins suddenly liquefy or char when the internal concentration of these catalysts reaches a critical threshold.
This exponential increase in reaction rate makes the process difficult to stop once the induction period ends.
Processing Impact
Molders encounter severe issues when residence times in the barrel exceed the stability window of the material. Once autocatalytic degradation starts, the melt viscosity drops unpredictably and the resulting parts exhibit gas voids or brittle failure. Adjusting the screw speed or lowering the heater band temperature becomes ineffective once the chemical cycle sustains itself.
Stabilizer Function
Preventive strategies involve the addition of chemical scavengers that neutralize acidic byproducts before they can trigger further reactions. Effective autocatalytic degradation control requires high concentrations of thermal stabilizers or metal deactivators in the resin formulation. A well stabilized grade provides a wider processing window for complex moulding cycles.