Meaning
Chemical breakdown of the linkages that hold a polyester chain together occurs through a reaction with water molecules. Widespread ester bond cleavage is the primary mechanism of degradation in materials like polyethylene terephthalate and polycarbonate. This reaction splits a long, strong molecule into two shorter, weaker fragments.
Hydrolytic Mechanism
Water attacks the carbonyl carbon atom in the ester group, leading to the permanent separation of the chain. This process of ester bond cleavage is accelerated by high temperatures and the presence of acidic or basic impurities in the resin. Each break releases a new carboxyl group which can further catalyze the reaction.
Once the chain length drops below a critical threshold, the polymer loses its ability to form a cohesive solid.
Material Property
Physical properties of the plastic deteriorate as the average length of the molecular chains decreases. Following significant ester bond cleavage, the material loses its ability to withstand impact and becomes prone to stress cracking. The resulting parts often fail during assembly or shortly after entering service.
Process Control
Strict moisture limits are the only way to prevent this chemical failure during the injection moulding cycle. Resins must be dried to levels below fifty parts per million to ensure that ester bond cleavage does not occur in the melt. Maintaining a dry hopper is the most effective defense against molecular degradation.
