Characterizing Viscoelastic Relaxation during Thermoset Post Cure Cycles
Characterizing viscoelastic relaxation during post cure cycles prevents internal stress lockup, ensuring long-term dimensional stability for structural thermosets.
Enthalpy Relaxation
Meaning
Physical ageing in amorphous materials occurs as the molecular structure slowly reorganizes toward a state of lower energy. Enthalpy relaxation is the heat flow release that accompanies this thermodynamic recovery when a polymer is stored below its glass transition temperature. This process reduces the volume and increases the density of the polymer over time.
Understanding this mechanism is necessary for predicting the dimensional stability of composite parts during storage.
Thermal Signature
Differential scanning calorimetry measures the excess energy absorbed by a polymer during reheating after storage. The material exhibits a characteristic endothermic peak at the glass transition, which represents the enthalpy relaxation that occurred during ageing. A larger peak indicates more extensive relaxation and a more aged polymer network.
Measuring this energy release helps engineers estimate the storage history of raw prepregs and resins.
Storage Limitation
Long shelf life is a common requirement for raw materials stored in production warehouses. Extended storage periods increase the enthalpy relaxation, making the resin stiffer and less tacky during layup operations. If this relaxation progresses too far, the material loses its processing characteristics and must be discarded.
This loss of capability creates a direct financial penalty for holding excessive inventory.
Processing Behavior
Moulding trials with aged resins show that physical relaxation influences flow behavior during initial heating. The aged polymer does not melt as predictably as a fresh batch, which can lead to voids and dry spots in the finished composite part. Controlling storage temperature slows the enthalpy relaxation rate and extends the usable life of the raw material.
This control ensures consistent polymer flow and high part quality on the factory floor. It reduces scrap rates and avoids costly tool cleaning cycles.
DiBenedetto Glass Transition Modeling in Diffusion Controlled Resins
DiBenedetto modeling governs vitrification prediction, preventing unreacted resin pockets and internal residual stress during structural composite cure cycles.