Mechanism of moisture retention in aerospace epoxies

Epoxy resins used in composite manufacturing are intentionally prepared with an excess of epoxy compared to hardener. This is done to ensure that the hardener is completely reacted during cure and is not allowed to remain in the network and cause plasticisation. A mixture with an epoxy—hardener ratio of 1 1 would not be able to fully cure due to the mobility arguments given in this chapter. The epoxy excess means that there will be a significant number of unreacted and relatively polar functional groups present in the cured 3D network. 

3D network will contain enclosed areas where no further reactions are possible, leading to the so-called free volume. 

There are a number of models which can be used to predict the effect that absorbed diluents, in particular water, have on a cured polymeric resin. One of the most powerful of these is Group Interaction Modelling (GIM), a continuum-type model with a set of versatile input parameters based on the number and type of chemical functional groups present in the network. This allows the complex chemistty of amine-cured epoxy resins to be catered for whilst retaining the speed afforded by using a set of linked constitutive equations of state for property prediction. 

GIM is particularly suitable for predicting the change in polymer properties observed upon introduction of a diluent into a thermosetting resin. Firsdy, the model 

The fundamental equation of state central to GIM is a modified Lennard—Jones potential function which describes the interaction energy between adjacent polymers, E. This function has powers of 6 and 3 instead of the normal 12 and 6 because volume, V, is proportional to the square of the interchain separation distance, r. In a polymer, the chain length is significantly larger than r and is therefore assumed to be invariant. coh refers to the zero point cohesive energy and Vq is the zero point volume. 

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