"In other instances, bubbles can form in
coatings due to cooler temperatures or
high relative humidity during application.
When application occurs during cooler
temperatures or elevated humidity, the
drying and curing of the coating film is
slowed considerably. As a result, the
proper release of solvents from the film
does not occur. Consequently, if the next
coating layer is applied too soon, the solvent in the underlying film becomes trapped and bubble formation can occur."
Unfortunately it is a commonly held and incorrect belief that the drying rate of a coating containing only organic solvents is effected by RH. Unlike waterborne coatings, the drying of solvent-borne coatings is not effected to a significant degree by RH. High RH can result in blushing if moisture condensation occurs as a result of evaporative cooling of a solvent-borne coating. I suppose in theory it might be possible that someone could paint over the condensed moisture and hence blistering could occur. However, this is not the mechanism presented in the paper. The described blister mechanism can only occur in latex coatings and then only if coalescent solvent diffusion is retarded by a relatively low temperature (warm enough for water evaporation but not coalescent diffusion) and the coating is subsequently exposed to a sudden and significant temperature increase. This is especially true if the coating contains too much coalescent and if the coating is applied too thick. The failure mechanism is altogether different for latex and waterborne epoxy coatings that have been topcoated too soon after application at high RH. The retained water retards curing. In the case of a latex this means that proper film formation (coalescence) may not occur. The epoxy may cure but will have very poor mechanical properties. An analogous situation occurs when a solvent-borne epoxy is cured without sufficient ventilation. The relatively high partial pressure of evaporated solvent can retard further solvent evaporation from the film leading to a cured coating with poor mechanical properties (high internal stress/brittle/shear related adhesion failures).
Comment from kv g, (10/15/2014, 8:04 AM)
Could you explain the cases of blisters formed from the bare metal (sweep blasted surface of hot dip galvanised) noticed after the final coat curing (System is 2 coat High Build Epoxy follwed with one coat PU). Please respond.
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