What is Surface Tolerant Epoxy Coating?
The history of Surface Tolerant Epoxy Coating
The 1960’s and early 70’s
In the 1960’s and early 1970’s, the predominant heavy duty protective coating system was inorganic zinc silicate primer with an epoxy topcoat. Some well-known characteristics of zinc rich primers were that they were ideally suited to the vast new construction boom, which supplied petrochemical, oil exploration, pulp and paper, and other heavy industrial markets. After a productive, extended life cycle of 6, 8, 10 years or more, the epoxy topcoat typically chalked to a degree where underlying zinc was directly exposed. In some locations, such as the edges of structural members, nuts, bolts and weld beads, the zinc was depleted and active rusting occurred. This led to the need for effective, heavy-duty maintenance coatings, which could be utilized without stopping productive capacity in different operating areas.
At the time, surface tolerant coatings were mainly limited to single package materials with good wetting characteristics (alkyds, epoxy esters, chlorinated rubber, etc.) Most of these products did not compare in overall chemical resistance to the original epoxy topcoat. Conversely, the two component epoxies, epoxy-phenolics and coal tar epoxies with excellent chemical resistance were normally poor surface tolerant products that still required abrasive cleaned surfaces.
A team of industrial chemists and engineers responded to the marketing and sales guidelines, which sought product(s) to meet the following criteria:
- Tolerant of poor surface preparation.
- Compatible with intact existing aged coatings.
- Suitable for use under wide environmental conditions.
- Suitable with varying application methods (brush, roller, etc.)
- High chemical resistance equal to or better than existing high build epoxy polyamides.
Based on experience, the use of aluminum pigments with coal tar epoxy resins demonstrated some feasibility to this type of approach. However, coal tar was still not suitable in its wetting characteristics.
This led to significant improvements on wetting resins and additives to maintain chemical resistance of aluminum pigmented coal tar, in addition to increasing solids and minimizing or eliminating the “bleed through” tendency inherent in coal tar type materials. The original Carbomastic 15, often copied but never duplicated, was originally released to the field in 1974 to begin an unparalleled success story for stopping corrosion cold.
The use of surface tolerant coatings in the 1980’s grew exponentially, as dids did the growth in the number of coatings manufacturers who were supplying them. The appearance of surface tolerant coatings or mastics changed immensely during this time. Originally 90% volume solids, aluminum filled epoxy coatings now range from 75-100% volume solids and have unlimited color availability.
Adhesion of these coatings to marginally prepared substrates is a function of the resin system. Typically, the majority of these coatings use the same epoxy resin. It is a 100% solid epoxy resin that must be reacted with an amine to cure. It is the amine and wetting additives that determine the adhesion characteristics of the coating. The amines can be polyamides, amino amines, aromatic amines, aliphatic amines or cycloaliphatic amines. These high solids coatings dry only through chemical reactions and generally require an overnight cure before handling.
The performance of surface tolerant coatings is a function not only of the resin system but the pigmentation of the coating. As previously mentioned, there is a rather broad spectrum of resin combinations that impact potlife, flexibility, chemical resistance, abrasion resistance and others. Surprisingly, the pigmentation can also greatly affect the performance of an epoxy coating.
As we have seen, the epoxy mastics were traditionally aluminum filled. The laminar nature of the aluminum pigment provides exceptional resistance properties because the aluminum platelets form a very tight labyrinth that is difficult for chemicals to penetrate. As the level of aluminum pigmentation increases, the packing of the aluminum plate becomes denser, and tthus increases the performance of the coating. Many aluminum-filled mastics actually appear as a gray coating because the level of aluminum is relatively low. This is done to reduce the cost of the coating. The brightness of an aluminum mastic is directly related to the aluminum content and therefore its performance.
Colored mastics are composed of the same resin systems, but the pigmentation system is different. Organic pigments are angular and do not have the ability to pack like the aluminum flakes. Chemicals have a more direct line to the substrate and can potentially affect the coating at a faster rate. Pigment combinations are more critical in the colored mastics, and the resin system plays an important role in the performance of the coating.
The resin systems are modified with extender resins or plasticizers which enhance the volume solids of the coating. These plasticizing resins are generally 100% solids and low in viscosity. The quality of these resins range fromform high performance to non-reactive resins. The non-reactive resins are lower in cost but they also detract fromform the performance of the coating. Reactive extender resins become an integral part of the coating and do not decrease the coating performance.
Due to advances in resin technology, surface-tolerant coatings are now also available in other generic classes. Surface tolerant polyurethanes and acrylics are available today with strong potential growth for the 1990’s. The coatings industry has a high demand for this technology, and surface tolerant coatings will play an increasingly more critical role in maintenance coating systems.