Identifying the chemical gel point of epoxy-fumed silica composites using optimally windowed chirp measurements

Abstract

The chemical gel point that occurs in the resin matrix of thermoset composites is crucial to the design of manufacturing process parameters. However, the formation of a physical network of filler material can affect the viscoelastic response of the composite so strongly that conventional rheological indicators of the chemical gel point (like power-law stress relaxation) are no longer observed. Additionally, many industrially relevant thermoset composites have a small linear viscoelastic region, limiting the utility of the high-strain multiwave measurement approach that was developed to monitor the frequency-dependent behavior of rapidly evolving materials. Here, we pair frequency-dependent properties obtained by low-strain Optimally Windowed Chirp (OWCh) measurements with existing rheology-conversion relationships to apply time-cure superposition to the loss tangent of epoxy-amine resins filled with unreactive particles. We show that this advanced rheological approach allows us to track the relative change in the relaxation time, providing another way to identify the elusive chemical gel point of these thermoset composites. The results allow us to assess the applicability of several gel point criteria to crosslinking composites, including the G'-G'' crossover, frequency-independent tan δ, peak in tan δ, and divergence of the relaxation time. Investigation of a resin with a weak filler network reveals all of these gel point criteria, with the frequency-independent tan δ providing the best agreement with the easily identifiable gel point of the neat resin. For resins with a strong physical filler network, frequency independence of tan δ does not occur, but the divergence of the relaxation time matches the gel point of the neat resin and the peak in tan δ.