Effect of time‐temperature path of cure on the water absorption of high Tg epoxy resins

Abstract

AbstractThe effect of different time—temperature paths of cure on the water absorption of high Tg epoxy resins has been investigated. The resins were cured isothermally for different times, with the following results: as extent of cure increased, the glass transition temperature (Tg) increased, the room temperature (RT) modulus decreased, the RT density decreased, the RT diffusion coefficient appeared to decrease, and the RT water absorption increased. The decrease in RT density is related to an increase in free volume, which controls the amount of water absorbed. A qualitative model accounts for the increase in RT free volume with increasing cure. The model is based on a restricted decrease of free volume on cure due to the rigid molecular segments in the cured resin systems. The sorption isotherms can be characterized by the dual mode theory at low activities but at high activities the sorption is complicated by penetrant clustering. A thermodynamic approach, independent of the absorption model, can correlate sorption data at different temperatures. The diglycidyl resin was also cured for extended times at three temperatures, in an effort to achieve full cure at each temperature. For these, the higher the cure temperature, the lower the RT density, which could result from the lower initial density of materials cured at higher temperatures. The equilibrium water absorption increased with increasing cure temperature, consistent with the decrease in RT density. The systems studied were a diglycidyl ether of bisphenol A cured with an aromatic tetrafunctional diamine, trimethylene glycol di‐p‐aminobenzoate (Tg∞ = 156°C), and a triglycidyl ether of tris(hydroxyphenyl)methane cured with the same amine (Tg∞ = 268°C).