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Volume 2 Extended Abstract 12

 Submitted 26th August 1999

Degradation of Acrylic Coatings on Galvanized Steel

 Amy Forsgren, Niklas Steihed

Swedish Corrosion Institute Roslasgv. 101, hus 25 SE-10405 Stockholm Sweden

Keywords: acrylic waterbornes, zinc, galvanized steel

Introduction

 Waterborne paints are not simply solventborne paints in which the organic solvent has been replaced with water; the paint chemist must design an entirely new system from the ground up. Additives to keep the pigment from clumping, for example, may be completely different for a dispersion in a polar liquid such as water than in a nonpolar organic solvent. The same can be said for the chemicals added to make the pigments integrate with the binder, so that pigment particles are not simply occupying voids in the binder. The binder itself most frequently exists as a dispersion in waterborne paints. To keep the solid polymer in a stable and even dispersion, rather than in clumps at the bottom of the paint can, requires some creative use of additives. And of course, additives may be used to prevent flash rusting of the steel before enough water has evaporated for the polymer to form a coherent film which can provide corrosion protection.

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.

Results and Discussion

 Samples were subjected to a weakly-accelerated outdoor exposure in Stockholm for two years, wherein the samples are sprayed twice-weekly with 3% NaCl solution. Interestingly, the acrylic paints all performed worse on galvanized steel than on carbon steel. Undercutting from a scribe and edges occurred on the carbon steel; the same coatings, on galvanized steel, degrade completely and flake off from the entire sample surface. This contradicts previous experience of waterborne acrylics at the Swedish Corrosion Institute and elsewhere (1).

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.

    References

  1. Rendahl, B. and A. Forsgren. "Field Testing of Anticorrosion paints at Sulphate and Sulphite Mills". KI Rapport 1997:6E. Stockholm: Korrosionsinstitutet (Swedish Corrosion Institute), 1997. ISSN: 0348-7199.
  2. Billmeyer, F.W. Textbook of Polymer Science, third ed. John Wiley & Sons, New York. ISBN 0-471-03196-8. p 388. 1984.
  3. Bentley, J. "Organic Film Formers" , chapter in Paint and Surface Coatings Theory and Practice, ed. R. Lambourne. Ellis Horwood Limited, Publ. Chichester (Great Britain). ISBN 0-85312-692-5. 1987.

    Acknowledgements

The authors would like to thank Sten Palmgren of SCI for painting and exposing the samples, and performing the adhesion tests. The assistance of Akzo-Nobel, International, Teknos Tranemo and Dickursby paint companies in supplying coating samples is gratefully acknowledged. The authors are also greatly indebted to Siegfried Riemann, Alain Garzon, Kris Peeters and Robert Krasnansky (all of Rohm & Haas) for discussions of acrylic film formation and degradation.

 

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