Quantification of internal stress and cracking in ambient temperature curing thermoset coatings

For metallic structures, such as ships, wind turbine towers, and bridges, epoxy coatings are widely used as anticorrosive coatings. However, curing-induced internal stress can provoke cracks in coatings at an early stage, thereby causing degradation and failure of anticorrosive coatings.

In the present project, we investigated the development of internal stress in epoxy coatings, as well as the premature crack formation, as a result of the internal stress during curing.

The first investigation was on the development of internal stress in a solvent-free epoxy coating under conditions of ambient curing. Using a beam deflection method, the average internal stress was quantified, while simultaneously monitoring the degree of cure, the elastic modulus, and the relative volumetric shrinkage. For analysis, attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, dynamic mechanical thermal analysis (DMTA), and an optical 3D profilometer were used.

As expected, due to crosslinking reactions, the curing transients of the coating (i.e., elastic modulus, relative volumetric shrinkage, and internal stress) began to build after vitrification. The curing-induced internal stress was strongly influenced by the coating elastic modulus and the rate of the volumetric shrinkage. Taking into account the diffusion-controlled post-vitrification effect, the modified Kamal-Sourour model successfully simulated the degree of cure. In addition, to simulate the coating elastic modulus, the volumetric shrinkage, and the internal stress as a function of conversion, engineering models were used and simulations found to be in good agreement with experimental data.Using the same characterization techniques as for the solvent-free epoxy coating, the influence on curing-induced internal stress of binder and curing agent chemistry, pigmentation, initial solvent content, curing temperature, and relative humidity was studied. Epoxy resins and curing agents with a lower functionality or reactivity were found to reduce the curing-induced internal stress. Furthermore, despite a reduction in the final conversion and the volumetric shrinkage was observed, the presence of rigid BaSO₄ and CaCO₃ fillers in the coating resulted in a higher internal stress. In addition, due to its smaller volumetric shrinkage and internal stress sensitivity, CaCO₃ filler was superior to BaSO₄ in providing strength to the coating. As the solvent concentration increased from zero to 20 vol. %, a reduced internal stress, despite extra volumetric shrinkage was  introduced by the solvent evaporation, could be achieved. Compared to ambient curing (23 ℃), only a slight increase (around 0.2 MPa) was seen in the coating cured at an elevated temperature (35 or 45 ℃). Increasing the relative humidity (from 35 to 60 %) resulted in a smaller internal stress due to a swelling effect. However, at an even higher relative humidity (90 %), due to an enhanced final reactant conversion, the internal stress was higher than at 60 %.

In addition to the study of curing-induced internal stress development, the premature crack formation in a solvent-containing novolac epoxy coating was investigated. The method applied was tensile testing, supported by crack morphology characterization using optical and scanning acoustic microscopy.
Despite the measured internal stress was much smaller than the coating strength, and a sharp decrease observed in the stress-strain curve was sufficient to estimate the crack susceptibility, premature cracks initiated and propagated in the curing coatings. The residual solvent, due to the plasticizing effect from the residual solvent, resulted in a reduced curing-induced internal stress and crack susceptibility. However, because of continued evaporation, coupled with a higher final reactant conversion and a larger internal stress at long reaction times, a greater risk of cracking was expected. The presence of rigid pigments (BaSO₄ and CaCO₃) resulted in an earlier development of a high crack susceptibility.
In summary, a promising method to quantify the curing-induced internal stress and crack susceptibility in curing epoxy coatings has been proposed. Moreover, the results confirm the negative effects of internal stress and provide formulation insights and curing guidelines for reduction of internal stress and premature crack formation in epoxy coating