Monitoring the initial phase of a slow curing epoxy through the dynamic viscoelastic properties

The isothermal curing of an epoxy mixture of diglycidyl ether of bisphenol‐A and diglycidyl ether of 1,4 butanediol was monitored through the dynamic viscoelastic properties. Measurements were made at a frequency of 37.7 s−1 using a Weissenberg Rheogoniometer Model R‐17 fitted with a piezoelectric load cell and interfaced with a microcomputer. Cures were conducted at four temperatures, 15.6 ± 0.2°C to 40.1 ± 0.2°C with the cure time varying from 35 to 11 hours, respectively. Additional measurements of the steady shear viscosity, up to three hours after cure initiation, were conducted using a Weissenberg Rheogoniometer Model R20. These results show that in the early stages of the cure cycle, the fluids show low elasticity and constant viscosity. Also, independent of temperature, the relationship, η′ = 0.05 η″, holds, although it is not maintained near the gel point. This implies that initially, the longest relaxation time is independent of the extent of reaction and the temperature.