Meaning
Catalyzed redistribution of alkoxy groups between different polyester chains during melt processing alters the molecular weight distribution and copolymer structure of the material. This transesterification process, often called ester exchange, occurs spontaneously at elevated temperatures when different polyesters are co-extruded or blended. It can transform a physical blend of two distinct homopolymers into a block copolymer and eventually into a fully randomized copolymer.
The reaction rate depends heavily on temperature, residence time, and the presence of residual catalyst residues from the polymerization stage.
Reaction Equilibrium
Thermodynamic forces drive the reaction toward a randomized state where the entropy of the polymer chains is maximized. During this ester exchange, the exchange of ester linkages does not alter the total number of polymer chains or the average molecular weight. Instead, the process changes the sequence length of the repeating units, which directly influences the crystallization behavior and thermal transitions of the resulting blend.
Organic titanates or antimony compounds are frequently employed to speed up or control these molecular interactions.
Industrial Compounding
Moulders and compounders exploit this mechanism to create customized barrier resins or impact-modified blends with tailored properties. For instance, blending polycarbonate with polybutylene terephthalate can yield a tough, chemical-resistant alloy if the level of exchange is kept within strict limits. Controlling the residence time in the extruder barrel determines the extent of copolymerization, which directly dictates the morphology of the blend.
Material Degradation
Excessive reaction time can lead to a complete loss of crystallinity, making the moulded part brittle and prone to warpage. This degradation occurs when the block structures randomize too far, preventing the chains from packing into organized crystals. Adding phosphorus-based stabilizers helps deactivate residual catalysts, stopping the reaction and preserving the desired blend structure.
