Understanding Waterborne Chemistry
A discussion of the role of water in waterborne formulas, with explanations of why water can't re-dissolve fully cured finishes. June 18, 2010
Question
Can waterborne finishes be dissolved by water they dry? The way I understand it is that waterborne finishes consist of finishing material that is thinned in a water solution, but will not be dissolved by water once it has dried. Am I correct?
Forum Responses
(Finishing Forum)
From contributor J:
I don't know enough chemistry to say whether such finishes are technically solutions or suspensions or something else, but you're generally correct. They won't re-dissolve after they've cured.
From contributor P:
No, they can't be re-dissolved by water. But some of them will absorb water and get milky white spots. You don't need to understand chemistry to understand the properties of finishes.
From contributor K:
The water in water-based finish has a similar role as the lacquer thinner in nitrocellulose lacquer, or paint thinner in paint or urethanes. It provides a medium for the resins that is thin enough to be sprayed easily (and acts to keep the resins from curing). There is a substantial difference in the chemistry, though. As for the water-based urethanes, I suspect that there are the same general differences. The water (thinner) evaporates. In the process the resins will cross-link at the molecular level to form a different material than what was sprayed on. Once cured, the finish will not dissolve with water contact.
However, just as in nitro lacquer, prolonged contact with water will damage the finish. Lacquer thinner and other solvents will also damage it. Think of water-based as reduced- (VOC) solvent materials. If you normally spray nitro lacquer, it is important to know that the atmospheric conditions (mainly cold, heat, and humidity) that work for nitro can cause problems with water-based. Also some of your visual clues are different. I have used both, although it has been a while since I have used the water-based products. When I did use them, I frequently employed the spray and pray technique.
From the original questioner:
After doing some more research on water based finishes I have come to the same conclusion as you have described. All the text I have read is in line with your explanation, namely that in the case of water based solutions. Water is just a thinner/dilutent that suspends the already dissolved resin component of the finish (which might be urethane, acrylic or a combination of the two). If I understand it correctly solvents are products that can dissolve another material, while dilutents (or thinners) are products that can reduce the viscosity (or thin out) another liquid. In other words; a solvent turns a solid into a liquid, while a thinner makes a liquid "more liquid". In some cases, as with water based finishes, the thinner is unable to dissolve the components and can only thin it once it is dissolved by something else. This brings me to the conclusion that water based solutions are coalescing solutions, which dry by means of evaporation and reaction (cross linking) combination. Again this is in line with your explanation.
From contributor R:
Actually waterbornes are an emulsion. A solution is when the resins are completely dissolved in the liquid medium. An emulsion is a liquid made up of unblendable substances, like oil and vinegar. In this case it consists of mainly of polymers, coalescing solvent and water. Waterbornes dry by evaporation. As the water evaporates the concentration of coalescing solvent increases and melt the polymer latexes together. Yes the dry film is very resistant to water.
From the original questioner:
I think the way you explained it is more or less how I understood it, except for the fact that it is technically an emulsion, rather than a thinned out liquid. This makes more sense now that I think about.
