Epoxy-amine resins are widely used as anti-corrosive coatings. However, water pathways eventually form throughout these coatings, resulting in corrosion. Given that the massive global costs of corrosion, it is imperative to understand the failure mechanisms of these coatings.
Epoxy-amine systems composing of the epoxy monomer DER331 (bisphenol-A) and the amine crosslinking agent m-Xylylenediamine (MXDA) were created to investigate the potential causes of this failure mechanism. In order to more closely mimic industrially-made coatings, samples of these epoxy-amine systems were made with varying levels of epoxy, with solvents (3:1 ratio mix of xylene and butanol) and with the homopolymerizing agent 2,4,6-Tris(dimethylaminomethyl)phenol (DMP-30).
Cure kinetics, final conversion and final glass-transition temperature ($T_g$) measurements were measured via differential scanning calorimetry (DSC). Network properties of the cured systems were measured vis small-angle neutron scattering (SANS) and positron-annihilation lifetime spectroscopy (PALS). Solvent uptake measurements were then taken, the results from which were related to those from the previous two investigations to determine how different networks affected uptake and why.
DSC measurements showed DMP-30 to raise the conversion of epoxy to a maximum and the homopolymerized network to have a slightly higher crosslink density than the epoxy-amine network. DMP-30 also raised the rate of conversion in epoxy-amine formulations. SANS measurements showed samples cured with solvents displayed a heterogeneous network. Solvent uptake was also shown to be greater than samples cured without solvents. It was deduced that solvents separated from epoxy-amine during cure before evaporating, resulting in ‘voids,’ areas of low crosslink density into which solvents can easily enter. A greater conversion rate leads to greater uptake, believed to be due to greater void sizes in the network. For non-solvent formulations, solvents must probe the polymer network. Uptake for these formulations is therefore dependent on crosslink density and the density of hydrogen-bonding sites.
