Are multiple paint coating systems becoming unnecessary?
Conventional wisdom says three coats - sometimes more - is the minimum requirement for corrosion protection on structures exposed in marine atmospheres. It hasn't always been so.
Just how did the coatings industry get to the point where both equipment owners and coating suppliers support the notion that a three-coat system is more desirable than a single coat of inorganic zinc? Any chemistry school graduate knows that a single coat of zinc primer is all that's really needed if the goal is simply to prevent rust.
In Ameron's 20 years of researching industrial and marine protective coatings, it has been interesting to see how expectations for coatings performance have evolved.
Simple corrosion resistance is not enough. Other factors include new methods for applying coatings, shorter curing times, temperature considerations, lower labor costs, longer life between applications, stricter emissions regulations, appearance over time, and others.
Twenty years ago we visited several offshore platforms that had a single cast of the original inorganic zinc primer. Even after 20-25 years service from the zinc primer, there was less than 0.5% corrosion. To improve aesthetics and resistance to chemicals and abrasion, an epoxy or vinyl topcoat was added.
Unfortunately, that extra coat often "pinholed" over inorganic zinc and chalked in an offshore salt atmosphere. This was aesthetically unacceptable, and so it became common industry practice to recommend a third coating be applied to add gloss, promote color retention, and ensure even longer life. When applied over epoxy, the recommendation was aliphatic polyurethane; over vinyl, a coat of vinyl-acrylic became standard practice.
Soon it seemed logical to conclude that if three coats were a good barrier to the detrimental effects of a marine environment, then four or more would be even better. The results of accelerated coatings tests published by independent labs, manufacturers, and the owners themselves added credence to this theory. Even today there are coatings industry customers who insist on specifying four coats for their equipment.
In our current global economy, with the cost of everything from labor to capital continually rising, a practical question begs answering: "Have multiple coat systems matched the 20-25 year performance benchmark set by a single coat of inorganic zinc two decades ago?"
With few exceptions, platforms sporting multiple coat systems - such as zinc primer/epoxy mid-coat/urethane topcoat - have had to be recoated in as few as five years, and within 12 years, after the original system was applied. Is there a way to improve on long-term performance while reducing the number of coats that are required in order to cut application costs? What would the technology to do so look like?
One two-coat system
Several years ago the authors met a barge owner who had developed a non-conventional coating procedure. After abrasive blasting his barges, he would apply a coat of waterborne inorganic zinc and then topcoat the zinc film with vinyl. He showed us several barges on which his two-coat system had been applied 5-10 years previously. They were in excellent condition. In fact, his two-coat system outperformed the three-coat epoxy system on other barges he owned by a factor of three.
The secret of his two-coat system was a topcoat of 30% solids vinyl diluted 50% with thinner. On closer inspection of the two-coat system, several hypotheses became evident to explain why it worked:
- The diluted vinyl penetrated and filled the air pockets in the inorganic zinc film.
- The total system was very abrasion-resistant.
- The two coats together forged a system that was considerably stronger than each individual coating was seperately.
- The inorganic zinc fully cured.
The discovery that a two-coat system could perform equal to or better than a three-coat system seemed revolutionary. Unfortunately, environmental considerations and strict VOC regulations intervened. Vinyls were on their way out, and the two-coat idea died on the board. The industry went back to high-solids epoxies and aliphatic polyurethanes over inorganic zincs - a three-coat system at minimum. This remained the norm until the advent last year of epoxy polysiloxane coating.
Coatings that incorporate engineered siloxanes, as they are being called, have demonstrated durability, corrosion resistance, excellent adhesion, and high resistance to weathering under many diverse conditions.
This new class of chemistry has an inorganic backbone of silicon-oxygen, rather than the carbon-based structure of most present-day coatings. Polysiloxanes have by themselves some very interesting characteristics: they are a room temperature cure ceramic-like coating with ultra high heat and acid resistance.
When combined with other chemistries such as epoxy, they deliver quantum improvements in performance over typical epoxies.
An epoxy polysiloxane coating has better corrosion resistance than the best epoxy mastic, and provides longer term weathering resistance than the best aliphatic polyurethane. The epoxy polysiloxane coating is low in viscosity, high in solids (about 90%) and environmentally compliant. Epoxy polysiloxane adhesion to substrates is as good as many glues (over blasted steel, its adhesive strength is 2700 psi, or 190 Kg/sq cm).
Coating application time can be reduced from six days for a three-coat system (inorganic zinc/epoxy/aliphatic polyurethane) down to two-and-a-half days using the two-coat system of inorganic zinc/epoxy polysiloxane.
Epoxy polysiloxane has low viscosity and penetrates microscopic crevices. Most high-gloss finish coats cannot be applied directly over inorganic zinc - but the epoxy polysiloxane can.
The combined characteristics of adhesion, color and gloss retention, abrasion resistance, and moisture and humidity resistance offered by this new category of heavy-duty coatings yield at a to-coat system that is equal to, or even better than, the best inorganic zinc/epoxy/ polyurethane.
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