Reactive polymer matrix composite materials generally have a high resistance to water transmission, although the extent depends very much on their formulation. Over the long-term, slow diffusion may lead to reversible equilibrium moisture contents of up to 10%. Eventually this can lead to the creation of pathways for ingress of deleterious species such as chloride ions, reduction in modulus and strength by acting as a plasticiser and degradation of the resin by hydrolysis. These effects are increased by air entrainment, due to poor mixing or compac- tion, and as temperatures are increased. Moisture ingress via the polymer, a porous substrate or exposed edges can lead to degradation of the adhesive bond.
The mechanism is not fully understood but probably involves plasticising and swelling of the polymer in the interfacial region and alkaline hydrolysis of the resin.