Woodworker's Journal 2009-33-5, страница 62

Finishing Hotline

Just because a finish is called "water-based" does not mean it is solvent free. Are water-based products greener than others?

Shellac is another natural, insect-made product that has a very favorable profile. The resin itself is not only renewable, but so harmless as to be edible. The most common solvent for it is ethanol, or grain alcohol, the same alcohol that appears in beer, wine and hard liquor. While ethanol is technically a VOC, it is a slow enough ozone generator that many contend it should be made exempt. In any case, it is relatively safe for the finisher, who at worst runs the risk of becoming inebriated from huffing too many fumes.

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 parachlorobenzotrilluoride (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 clangers 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 bccomc 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.

Water-based Coatings

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 docs 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 n-methyl-2-pyrollidone (NMP) or gamma butyrolactone (BLO) as well as glycol ethers, both of which are also considered VOCs and HAPs.

Finishing continues on page 64 ...

Formaldehyde in Finish

For the record, formaldehyde shows up in only one group of finishes we woodworkers use: the so-called catalyzed and pre-cat lacquers and conversion varnishes. It is not in shellac, lacquer, oil varnish, water-based lacquers, oil- or water-based polyurethane or even two-part automotive polyurethanes or polyesters.

Cabinetmakers who use catalyzed finishes are justifiably concerned about their workers and generally insist they wear protective gear when spraying these finishes. However, there is little concern about the end-user of the finished product.That's because the formaldehyde dissipates within a month. By the time it comes into your home, it is no longer offgassing formaldehyde.

It's worth noting that some interior plywood offgasses formaldehyde, depending on the type of glue used. Sealing the plywood with a film-forming finish blocks its release. Thus, while one type of finish can release formaldehyde, almost any other finish can block it.

62