Ultra Low and Zero VOC Coatings
Nitrocellulose lacquer, long the darling of furniture manufacturers, contains about 75% solvent, almost all of which is both a VOC and a HAP. Several strategies have emerged to lower that number, sometimes to zero. You can now buy zero VOC lacquer, which oddly enough still contains about 75% solvent. The difference is that it is formulated with exempt VOCs, meaning those that are not ozone generators. Typically, the solvent package is a mixture of parachlorobenzotrifluoride (PCBTF) and acetone, the latter of which appears less harmful to us than some of the solvents it replaces. Thus, zero VOC lacquer may actually be safer to use, not just less ozone generating.
A totally different approach, and one that works particularly well with two-part cross-linking coatings (a specific chemical formulation), is to raise the solids and lower the solvent level. These so-called ultrahigh solids coatings can range from 85% solids to almost 100% solids, the latter containing little or no solvent whatsoever. Typically, these are either chemically cured polyesters or UV cured acrylics and polyesters. Most are designed to be sprayed and are sold only to the industry, not to hobby woodworkers.
Even at that, their safety is a bit of a mixed bag. While they contain almost no VOCs, they carry dangers to the workers indoors. That’s because they often emit free monomers that are hazardous. Monomers are resins, not solvents, but the molecule is so small that it can become airborne, and thus can be breathed in. As a result, those who use such coatings generally suit up in hooded coveralls with clean air pumped in to the hood.
Contrary to popular belief, water-based coatings are not water-soluble, nor do they contain water-soluble resins. That’s a good thing. If they did, you would be able to remove any evaporative water-based finish with water after it dried. Imagine wiping a counter with a wet sponge and having the finish wipe off. If not water-soluble, then what are they?
Water-based coatings are those in which some of the solvents of a traditional coating have been replaced with water. Almost any type of coating can be made water-based; there are water-based lacquers, shellacs, polyurethanes, one and two-part cross-linking coatings, and even water-based UV cured coatings.
Water-based does not mean that the coating contains no solvents. It merely means some of the solvent has been replaced by water. The majority of clear water-based finishes contain glycol ethers, all of which are considered VOCs. While some are quite harmless, others are hazardous to humans, including a very common one, EB (sold under the trade name Butyl Cellosolve), which is believed to be a teratogen (a substance that can cause birth defects). Water-based polyurethanes typically contain nmethyl-2-pyrollidone (NMP) or gamma butyrolactone (BLO) as well as glycol ethers, both of which are also considered VOCs and HAPs.
Still, water-based coatings contain far lower amounts of VOCs than their solvent-based counterparts. For example, a typical nitrocellulose lacquer will contain upwards of 75% solvent, most of which is VOCs, while its water-based counterpart will contain only 10% or less. There are even zero VOC water-based coatings.
Pros and Cons
Although water-based coatings are lower in VOCs, clear water-based finishes are also a bit weaker. For instance, water-based polyurethane has about the same abrasion resistance as oil-based, but it has lower solvent, chemical and heat resistance. One could make the argument that if the finish does not hold up as long and you must refinish more often, the extra solvent involved in early refinishing offsets any savings you made initially. Of course, refinishing also means more time and money spent. But there is a way around that. More and more, single-component, self-curing water-based coatings are being offered to kitchen cabinetmakers, and they match oil-based polyurethanes in durability. They’ve long been offered as floor finishes and are slowly becoming more popular for furniture as well.
While water-based coatings are generally considered greener, there are aspects to them that make them less green, at least in some minds. The resins used in water-based coatings are typically synthesized from petroleum, a nonrenewable resource that has a fairly high carbon footprint, thanks to the energy needed to extract and ship it. By contrast, the resins in oil-based varnish and polyurethane are made from modified linseed, tung or soy oil, all renewable-source plant oils grown right here in the U.S. The tradeoff is natural, renewable, local resins with a higher dose of VOC solvents versus synthetic, foreign imported petroleum-derived resins with a lower dose of VOC solvents. As I said, this is not an easy equation.
Danish oil, teak oil, spar varnish and the whole field of oil-based varnishes and polyurethanes, both liquid and gel, make up the next category. In general, these finishes are relatively safer for us than for the environment. They contain ozone-generating VOC solvents, and thus are not good for the environment. However, the ones they contain, such as mineral spirits, are fairly safe for us, at least in the limited amounts to which we are exposed. Their high solids content, often 45% or so, means what solvents they do contain are in more limited amounts than what you would find in lacquer, for example.
Although the solvents are petroleum-based, the resin portion is made from natural seed oils, usually linseed, soya or tung. Such oils are renewable crops grown here in the U.S., and they require little processing to go from raw oil to resin. What’s better is that these coatings are almost never sprayed, but rather are applied either by brush, or by wiping on and off, as in the case of Danish oils and gel polyurethanes. Those application methods have a very high transfer efficiency, which means very little is wasted and only a minimum of solvent is used.