Product-specific kinetics reveal effect of epoxy crystallization on thermoset thermal degradation

• Published in: Polymer (Guilford) 2024-05, Vol.301 (1), p.127003, Article 127003
• Main Authors: Dwyer, Derek B., Gallego, Nidia C., Isbill, Sara, Miskowiec, Andrew, Niedziela, J.L.
• Format: Article
• Language: English
• Subjects: Bisphenol F; Crystallized epoxy resin; EGA-MS; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Product-specific kinetics; Thermoset
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/j.polymer.2024.127003

Crystallization is a common problem for epoxy resins, which are ubiquitous in industrial and commercial use. Integration of crystallized epoxy monomers into cured thermosets has been shown to alter the thermosets’ final mechanical properties. However, no studies have investigated the impact of these crystals on the thermal stability of the thermoset. Here we investigate the degradation kinetics of a bisphenol F–based epoxy thermoset with and without crystallized monomers using Product-Specific Kinetic (PSK) analysis coupled with Evolved Gas Analysis-Mass Spectrometry (EGA-MS). PSK analysis revealed significant differences in evolved product ion kinetics, suggesting changes in the degradation kinetics between thermoset configurations. It was concluded that early stages of degradation are influenced most by crystal presence due to the high concentration of unreacted epoxy monomers and lower cross-linking density of the cured network. After post-cure annealing, significant changes are observed in the degradation kinetics of thermosets without crystal inclusions. Conversely, post-cure annealing procedures of crystal integrated thermosets showed little change in the thermoset degradation kinetics across all conversion extents. These findings suggest that post-cure annealing of thermosets with crystals present at the onset does not alter the cross-linking density of the polymer network enough to significantly change the degradation kinetics. We hypothesize this is because the excess monomers from the melted crystals are unable to find suitable reaction sites for complete binding into the polymer network, which has direct implications for the material properties and final thermal stability of the thermoset. [Display omitted] •Two sources of phenol formation during degradation were identified using PSK.•Partially bound monomers structures identified in thermosets.•Crystal integration increases partially bound monomer content after post cure.•Differences in char formation from crystal integration were identified.•Presence of crystals mitigates improvements in network structure via post-curing.