Thermodynamic Limits of Solid State Ester Transesterification
Solid-state ester transesterification reaches absolute thermodynamic limits when glycol partial pressure matches local chemical potential in amorphous channels.
Ester Interchange Equilibrium
Meaning
Reversible chemical reaction states where the rate of transesterification equals the rate of the reverse reaction govern the molecular weight distribution of polyesters during melt processing. When polyethylene terephthalate is heated, the ester interchange equilibrium establishes a dynamic balance between the polymer chains and the low-molecular-weight byproducts. Displace of this balance is necessary to drive the reaction forward and build higher molecular weight.
If the reaction is not managed, the polymer will quickly reach a plateau where no further chain growth occurs.
Reaction Mechanism
Transesterification involves the exchange of organic groups between an ester and an alcohol. In a closed system, the ester interchange equilibrium prevents any net increase in polymer length because chain breakdown occurs at the same rate as chain formation. Catalysts are added to increase the rate at which this state is achieved.
Processing Output
Extrusion of virgin resin blended with regrind shifts the chemical balance due to the introduction of different moisture levels and end-group concentrations. The ester interchange equilibrium is altered by the thermal history of the recycled material, which often leads to molecular weight degradation. This shift causes a reduction in the melt viscosity of the polymer during moulding.
Thermal Management
Removing the reaction byproducts, such as ethylene glycol, is the primary method used to force the reaction past its natural limit. In vacuum reactors, extracting these volatile compounds shifts the ester interchange equilibrium toward longer chain lengths by preventing the reverse reaction from taking place. If the vacuum pressure drops, the byproducts accumulate, which causes the polymer to undergo depolymerization and results in weak, brittle moulded parts.