Meaning
Chemical exchange reactions occur when the ester groups of one polymer chain react with the hydroxyl or ester groups of another chain, altering the molecular weight distribution of the melt. This ester interchange is prevalent in polyesters like polyethylene terephthalate and polybutylene terephthalate during high-temperature compounding or co-extrusion. It can lead to the formation of random copolymers from blends of homopolyesters, which changes the crystallization behavior and melt flow characteristics of the resin.
Chemical Reaction
The kinetics of ester interchange depend heavily on the processing temperature and the presence of residual catalyst from the polymerization stage. In the melt state, polymer chains are highly mobile, allowing the transesterification reaction to proceed rapidly. This process can be accelerated by acidic or basic impurities, which act as catalysts.
If left unchecked, the reaction can cause a substantial shift in the melting temperature of the blend, complicating the injection molding window.
Structural Impact
When molders process blends of different polyester resins, ester interchange can homogenize the mixture by creating block or random copolymers. While this can improve the compatibility of the two phases, it often reduces the overall crystallinity of the material. Lower crystallinity can lead to longer cooling times in the mold and reduced stiffness in the finished part.
However, controlled transesterification can be used to tailor the barrier properties of films used in packaging applications.
Thermal Stabilization
Stabilizers are often added to the resin formulation to inhibit ester interchange by deactivating residual polymerization catalysts. Phosphite compounds are commonly employed to bind with transition metal catalysts, ensuring that the polymer blend maintains its structural integrity and processing behavior throughout subsequent recycling steps.
