In this paper a detailed study in investigating the effect of the chain flexibility in epoxy-amine crosslinked network is done. In order to introduce flexibility into the crosslinked network a homologous series of four aliphatic diamine curing agents varying only in the chain length and having a constant functionality (f=4) is taken and cured stoichiometrically with aromatic epoxy (f=2) resin. For each of the cured mixture the viscoelastic master curve and corresponding shift factors were determined. It is found the introduction of flexibility shifts the viscoelastic curves by 5 decades with respect to frequency scale. This shift in the viscoelastic curve is modeled with a parameterized Havriliak-Negami model for the master curve. The free volume contribution for the changes in the coefficient of thermal expansion at T g is also determined.
A thermoset coating that is applied to an elastic substrate will develop residual stresses during curing because of polymerization shrinkage of the resin. This shrinkage only partly contributes to the residual stresses because, before gelation, the stresses relax completely. In this study, we developed explicit analytical expressions for the curing efficiency factor, the residual stresses, and the resulting warpage. We did this by assuming that after gelation, the material was in its rubbery state and that viscoelastic effects were absent. A difference between the free and constrained warpages during curing was made. The analytical warpage models were shown to give results comparable to those of the numer-ical calculations with a fully curing-dependent viscoelastic material model. Furthermore, for the first time, accurate analytical expressions for the stress-free temperature and stress-free strain were obtained. With these expressions, the effect of curing shrinkage on the residual stresses could easily be incorporated into existing (numerical) stress analysis without the need for extensive curing-dependent viscoelastic material models.
A complete vapor pressure model based on a micromechanics approach is developed in this paper. The model can be extended to calculate the initial vapor pressure as traction loading subjected to the interfaces after the delamination. The impact of the vapor pressure induced expanison on the material’s deformation is discussed.
A detailed study investigating the relation between the initial resin composition and mixing ratio on the viscoelastic behavior was performed. The resin system, containing a difunctional aromatic epoxy (DER 332), was mixed with three aliphatic amines containing functionalities of 2, 3, and 4 in different stoichiometric mixing ratios and was cured. For each of the cured mixtures, the viscoelastic master curve and corresponding shift factors were determined. Depending on the crosslink density, the viscoelastic curves shifted by about 12 decades with respect to the frequency scale. This shift in the viscoelastic curve was predicted by the combination of the Miller-Macosko theory for crosslink density with a parameterized Havriliak-Negami model for the master curve. A single set of 9 parameters turned out to be sufficient to describe all viscoelastic data.
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