Meaning
Chemical degradation processes in polymers occur when water molecules break the covalent bonds within the molecular chain at high temperatures. The occurrence of hydrolytic scission leads to a reduction in molecular weight, which significantly weakens the mechanical properties of the finished part. This reaction is the primary reason why hygroscopic resins must be dried to very low moisture levels before injection molding.
Even a small amount of residual water can trigger this breakdown during the melting phase.
Molecular Breakdown
Cleaving the polymer chains turns long, entangled molecules into shorter fragments. As hydrolytic scission progresses, the viscosity of the melt drops, often leading to flashing or inconsistent part weights. The shortened chains can no longer provide the impact resistance or tensile strength required by the product design.
Degradation Rate
Temperature and moisture concentration determine the speed at which the chemical bonds are destroyed. While a material might be stable at room temperature, the risk of hydrolytic scission increases exponentially as it reaches the processing temperature of the machine. Monitoring the dew point in the dryer is the best defense against this type of material failure.
Material Brittleness
Finished parts affected by this chemical change often fail under low stress or during assembly. Because hydrolytic scission is an irreversible process, the damaged material cannot be restored by further drying or reprocessing. Quality tests for intrinsic viscosity are used to confirm that the polymer has maintained its structural integrity.
