Investigating curing-induced internal stress in epoxy coatings

Researchers have studied the effects of epoxy binder, curing agent, filler, initial solvent concentration, curing temperature, and relative humidity on the curing-induced internal stress in epoxy coatings.

A red coating is peeling off a wooden board.
The mitigation of coating delamination is one of the important challenges for epoxy coatings.  Image source: analogicus - Pixabay (symbol image).

Curing-induced internal stresses in epoxy coatings are highly influenced by the type and concentration of product ingredients and the conditions applied. In the new work, the effects on the internal stress development of the epoxy/crosslinker chemistry, curing temperature, relative humidity, filler conditions, and initial solvent concentration, are studied. An elevated curing temperature (35 or 45 °C) resulted in a smaller elastic modulus and, despite an accelerated curing reaction and a higher final reactant conversion, only a slight increase, around 0.2 MPa, in the average internal stress. An increased relative humidity (from 35 to 60 %), also resulted in a smaller elastic modulus and less volumetric shrinkage and internal stress. However, at 90 % relative humidity, the internal stress, due to an enhanced final reactant conversion, was higher than observed at 60 % RH.

Guidelines to avoid internal stress

The presence of either BaSO4 or CaCO3 filler in the formulation reduced the final reactant conversion and volumetric shrinkage, but resulted in a higher elastic modulus and internal stress. When the solvent concentration increased from zero to 20 vol%, the final conversion, although extra volumetric shrinkage was introduced by the solvent evaporation, increased from 0.72 to 0.85, while the internal stress decreased from 1.25 to 0.6 MPa. The study presents guidelines to optimise coating formulations and curing conditions to avoid internal stress.

The study has been published in Progress in Organic Coatings, Volume 173, December 2022.

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