Abstract
The solvent uptake in equilibrium of a highly cross-linked epoxy o-cresol novolac resin in water, isopropanol, and heptane was experimentally measured and modeled with the perturbed-chain statistical association fluid theory (PC-SAFT) equation of state. As suggested in the literature, PC-SAFT was combined with a network term, which takes additional elastic forces into account. The model parameters of the epoxy resin were generated by fitting them to the measured solvent uptake in pure substances and to the density of the epoxy resin, which provided a very good agreement with the experimental data. Furthermore, the solvent uptake in the mixtures isopropanol/water and isopropanol/heptane was predicted in very good agreement to the experimental data. For the first time, a thermodynamic model was developed to calculate the solvent uptake in an epoxy resin.
Highlights
Epoxy resins offer a wide range of applications such as protective coatings, encapsulations, and lightweight structures.[1−3] In most industrial applications, epoxy resins are used as highly cross-linked thermosetting polymers.[1−3] Of the epoxy resins sales volume, more than 75% are glycidyl ether derivatives of bisphenol A (DEGBA resins).[1]
The pure component perturbed-chain statistical association fluid theory (PC-SAFT) parameters of the epoxy resin, the epoxy resin network parameters, and the binary interaction parameters between the epoxy resin and the three different solvents water, isopropanol, and heptane have been fitted to the solvent uptake in the pure components and to the densities of the epoxy resin at 25 and 100 °C
A highly cross-linked ECN epoxy resin was modeled with the PC-SAFT equation of state by combining it with the network term of Miao et al.,[15] which takes additional elastic forces because of the solvent uptake into account
Summary
Epoxy resins offer a wide range of applications such as protective coatings, encapsulations, and lightweight structures.[1−3] In most industrial applications, epoxy resins are used as highly cross-linked thermosetting polymers.[1−3] Of the epoxy resins sales volume, more than 75% are glycidyl ether derivatives of bisphenol A (DEGBA resins).[1] For applications where the thermal or chemical resistance of DEGBA resins is too low, often epoxy resins based on novolacs (phenolformaldehyde condensates) are used.[1,4] The lowest molecular weight member of the novolacs is bisphenol F, which is the backbone for resins based on glycidyl ether derivatives of bisphenol F (DEGBF resins).[1,3] Another type of novolac resins are epoxy phenol novolac resins (EPN resins) and epoxy o-cresol novolac resins (ECN resins).[1,3,4] Both offer a higher thermal stability than the DEGBA and DEGBF resins because of their higher cross-link density.[1] If EPN and ECN resins are compared with each other, ECN resins have higher costs but higher stability and lower moisture uptake.[1] The chemical structure of a cross-linked ECN epoxy resin is shown in Figure 1.1 ECN epoxy resins are multifunctional epoxy resins with an epoxide functionality between 2.5 and 5.5 that are usually cured with multifunctional novolacs.[1,4] Because the degree of polymerization of the novolac is in the same range as the epoxide functionality, the used stoichiometric ratio of epoxy resin to curing agent is about 1:1 to obtain a high crosslink density and a maximum stability and a minimum moisture uptake.[1,4]
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