Swedish Corrosion Institute Roslasgv. 101, hus 25 SE-10405 Stockholm Sweden
Keywords: acrylic waterbornes, zinc, galvanized steel
The implications of all this paint chemistry for coating-substrate interactions, and hence coating-substrate compatibility, is not well known. The subject is of more than academic interest: experience from the field shows that waterborne acrylic paints on galvanized surfaces sometimes perform extremely well but sometimes degrade quickly, for unknown reasons.
The aim of this study was to examine the compatibility of various ferrous substrates with acrylic waterborne resins. Substrates were: hot-rolled steel, abrasive-blasted to Sa2½; phosphated cold-rolled steel; hot-dipped galvanized; and zinc-aluminum coated steel (galvalume). Paints with different coating compositions were studied.
Previously unexposed samples were also run in the salt spray (ASTM B117) test, to see if the different environment could provide any information. Blistering below the scribe was observed on samples run in the salt spray; the blisters seem to follow the lines of run-off liquid from the scribe. For two of the coatings, much smaller blisters could be seen scattered across the entire sample; but below the scribe, blisters were much larger and concentrated in the lines of the run-off liquid. Because of the rapid dissolution of zinc in the wet conditions of the salt spray, the run-off liquid below the scribe is expected to be rich in zinc. It is believed that the dissolved zinc is crucial in saponification of the acrylic coating and hence the blisters; but the mechanism is not known.
Theories for why the zinc caused this degradation of the acrylics include:
Either the initial adhesion to the zinc was poor (a polymer composition problem) or the zinc substrate was not wetted (a paint formulation problem). In either case, water and oxygen can reach the zinc surface and initiate corrosion of the zinc. This causes an alkali environment at the zinc-paint interface, causing saponification of the polymer (2, 3).
The zinc is causing further crosslinking of the binder, depending on which monomers are used. Crosslinking goes too far and the coating becomes brittle. Weathering stresses such as temperature change and wetting/drying cycles break down the cured coating.
Zn++ ions destroyed the coulombic stability of the dispersion, leading to flocculation. The paint is in effect destroyed after being applied but before a coherent film can form.
Zinc ions, together with organic acids in the paint (from additives, rather than the binder), form a zinc soap which in turn breaks down the cured paint.
It was not possible, using grazing-angle FTIR, to establish what had happened; infrared results seem to indicate that an alkali environment is necessary to the degradation of the coatings; but the role which the zinc plays is not clearly defined.
The conclusion drawn is not that waterborne acrylic coatings are unsuitable for galvanized substrates, but that further work is necessary to understand the interactions of zinc and waterborne acrylic polymers.
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