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.
Thermal Scission Rate
Meaning
The speed at which chemical bonds within a polymer backbone break due to thermal energy limits the maximum processing temperature and residence time that a resin can withstand without degrading. A high thermal scission rate during extrusion leads to a rapid drop in molecular weight as the long chains are broken into shorter fragments. This degradation mechanism is particularly severe in polymers processed near their thermal limits, where the heat generated by mechanical shear compounds the thermal load.
The balance between chain building and chain breakage determines the final melt viscosity of the polymer.
Degradation Kinetics
Random cleavage of the polymer backbone occurs when the thermal energy exceeds the dissociation energy of the carbon-carbon or carbon-oxygen bonds. This process is quantified by the thermal scission rate, which increases exponentially with temperature according to the Arrhenius relationship. In polyester processing, this degradation competes directly with the polymerization reaction, setting an upper limit on the achievable molecular weight.
Processing Influence
Excessive residence time in the injection moulding barrel or extruder increases the amount of degraded material in the melt. If the thermal scission rate is high, the polymer’s molecular weight distribution widens, which leads to a loss of melt strength and causes problems like drooling at the nozzle. These issues make it difficult to maintain consistent cycle times and part dimensions.
Resin Stabilization
Antioxidants and thermal stabilizers are added to the resin to intercept the free radicals generated during bond cleavage. These additives reduce the effective thermal scission rate by terminating the degradation cascade before it can spread along the polymer chain. In regrind applications, these stabilizers prevent the recycled material from turning brittle during re-extrusion.