The overall market for wood finishes -- coatings which, unlike paint, do not completely mask the wood’s appearance -- has been growing, along with the demand for environmentally preferable wood finishes.
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Many of the products used to coat wood around the home have traditionally contained volatile organic solvents and other toxic chemicals that have been linked to poor indoor air quality and possible health impacts. But because of increasing public awareness of indoor air pollution and chemical toxicity in household and building products, manufacturers of wood coatings are offering many more products that are low in volatile organic compounds (VOCs) and formulated without aromatic solvents, heavy metals, or cancer-causing chemicals. These products are healthier for the users of these products – namely, workers on new home construction projects and do-it-yourself homeowners – as well as the occupants of buildings where these finishing products have been applied. Woodfinish manufacturers have also stepped up the production of “green” products because of the rising interest among developers in obtaining Leadership in Energy and Environmental Design (LEED) certification for new buildings. Sales of waterborne coatings (including paints and wood finishes) now dominate the national market.
What is a Wood Finish?
A finish is a liquid, paste, or gel that can be spread thinly onto wood. There are two basic types of wood finish: those that form a film or coating on the wood and those that penetrate the wood surface. Film finishes, which cure hard and can be built up in layers, include varnish, shellac, lacquer, water- and latex-based semitransparent stains, and solid-color stains. Penetrating finishes are oil-based and don’t cure to a hard film. These include oil finishes, such as tung and linseed oil, and oil-based stains.
Wood finishes, rather than paints, are often used to showcase the natural beauty of wood grain, such as on floors and interior trim. Outside, wood finishes are used on decks and siding.
Wood finishes can include a number of components:
- Pigments or dyes are used in wood stains to add color and hide flaws.
- Resin is the natural or synthetic film-forming component of these finishes. (Resins are also called binders, especially when used in stains or paints, because they hold – or bind – the pigment to the wood surface.) Resins include acrylics, vinyls, alkyds, cellulosics, epoxies, polyurethanes, and oils. The type of resin determines the finish’s hardness, flexibility, and resistance to stains, solvents, and water.
- Solvents and/or thinners are used to maintain the finish in liquid form. Solvents are needed to dissolve resins. Thinners are used to reduce viscosity (thickness) of the liquid. Some finishes contain both solvents and thinners. Solvent-based coatings typically use organic solvents such as alcohols, ketones, glycol ethers, petroleum distillates (mineral spirits, toluene, xylenes, and naphtha), and turpentine. The solvent in a waterborne coating is usually a glycol ether, and water is used as a thinner.
- Various additives are used, in smaller amounts, to adjust drying time, prevent fungus and mildew growth, act as thickeners, etc. For centuries, varnish (a catch-all term for clear wood finishes) was made from resins collected from natural products such as tree saps or insect secretions and mixed with linseed or other natural oils. To create the final product, the mixture was thinned with turpentine. More recently, synthetic resins derived from coal tar and petroleum began to replace the natural resins, and petroleum distillates became the most commonly used solvents. The petroleum-based products in the synthetic resins helped increase durability. But, beginning in the 1970s, increased concerns over the impact of certain chemicals on human health and the environment accelerated a trend toward the development of high-solids (lower-VOC) finishes and water-based products. A resurgence in the use of pure oils and shellacs containing natural ingredients marks another recent trend in wood finishes.
This article covers varnishes, shellacs, lacquers, natural oils, water-based wood finishes, and wood stains.
Varnishes. Technically, all resin-and-solvent mixes are varnishes. Shellac is a spirit varnish—it uses distilled spirits (denatured alcohol) as a solvent, while spar varnish uses a drying-type vegetable oil (typically tung oil) combined with phenolic resin. In today’s common usage, “varnish” means a mix of drying oil and a natural or artificial resin that is cooked (often with an inert-metal catalyst) to make a clear finish that is typically used indoors. Varnish, which cures by chemical reaction (polymerization), is known for good resistance to heat, solvents, and water. Alkyd and polyurethane varnishes, however, may contain high VOCs. Water-based varnishes using polyurethane or acrylic polymers have been developed that emit lower VOCs.
Shellacs. In its pure form, shellac is a natural resin secreted by the lac insect, found mostly in India and Thailand. Shellac cures as its solvent, alcohol, evaporates. Although it creates a brilliant shine, shellac’s uses are limited because of its susceptibility to damage from liquids and heat. Shellac is useful for touch-ups because it bonds well to most other finishes. Shellac is also used as a sealer and under-coat with lacquer or varnish and polyurethane. It may not be compatible as a sealer under certain polyurethanes, though, because of the natural wax it contains.
Lacquers. Lacquer is a clear finish best suited for accenting wood grain. It dries fast and is very durable. The most common type, nitrocellulose lacquer, uses a binder made from a natural polymer, cellulose, which is derived from wood or cotton. Lacquer thinner is a blend of solvents, which may include ketones and esters, alcohols, and fast-evaporating hydrocarbons such as toluene or xylene. Like shellac, lacquer cures by evaporation. Without a clear protective coating, it is easily scratched and susceptible to water damage. Another drawback to lacquer is its typically high solvent content.
Natural oils. Linseed and tung oils are penetrating finishes that cure by absorbing oxygen from the air, a process that strengthens the finish. Linseed oil is extracted from flax seeds. Boiled linseed oil used in wood finishing has been mixed with chemical additives to hasten the drying time. Tung oil is derived from the nuts of trees that are native to Asia. These oils are durable, water-resistant, and easy to apply but require several coats and are slow-drying. Natural oils that have been heat-treated to increase gloss and hardness and reduce curing time are called polymerized oils.
Water-based. Water-based finishes are actually made up of droplets of solvent-based finish, usually acrylic or polyurethane, and a solvent, usually a glycol ether, with water functioning as a thinner. Water-based finishes cure by coalescing: the droplets of finish move closer together and interlock as the water evaporates. Water-based finishes offer minimal solvent fumes, easy cleanup, and good scuff resistance, but they may raise the wood grain and offer only moderate resistance to water, heat, and solvents.
Wood Stains. A wood stain is a finish for wood that contains a dye or pigment. Stains typically impart a semi-transparent or opaque coating and are designed to change the color of a surface without concealing the grain pattern or surface texture. More and more homeowners, builders, and architects are turning to penetrating stains instead of paints, especially for exterior use. Unlike paints, which form a film on the surface, these stains soak into the wood, accenting the wood grain rather than hiding it. Stains become part of the wood, which helps to prevent the cracking, peeling, chipping, or blistering that commonly occurs with paints.
Exterior stains. Exterior stains are used primarily on wood siding, shingles, decks and outdoor structures, and furniture. They are available in latex- and oil-based formulas. Oil-based semi-transparent stains offer the best protection of wood fully exposed to weather; they can also be used on new or weathered wood, or wood previously finished with other penetrating finishes, without extensive surface preparation.
If a solid color is desired, latex opaque stains are preferable over oil-based opaque stains, since they are more flexible, have better color retention, and are less prone to mildew. However, extensive surface preparation is required for proper adhesion, and latex stains in general should not be used on structures exposed to direct sunlight.
Interior stains. Interior stains, used for furniture and woodwork, come in either pigmented or dye categories. Both can have an oil, synthetic, or water base. Pigmented stains color the wood with the same type of pigments used in paint. They range in color from almost clear to semi-transparent. They are easily applied with a brush or a rag, and are then wiped off to control the depth of the stain. An oil or polyurethane finish is often mixed with the stain, so the do-it-yourselfer can complete the staining and finishing job in one step.
Health and Environmental Concerns
As organic materials in finishes evaporate during the drying/curing process, VOCs are released into the air, affecting indoor and outdoor air quality. The American Lung Association reports that VOCs and their byproducts can produce a number of physical problems, including eye and skin irritation, lung and breathing problems, headaches, nausea, muscle weakness, and liver and kidney damage. VOC levels can be ten times higher indoors than outdoors, with numbers rising up to 1,000 times higher immediately following application of a new coat of finish.
Outside, VOCs released into the atmosphere can combine with each other, or with other substances in the air, to create new chemical compounds, such as ground-level ozone. Ozone is a major component of smog, which causes negative health and environmental impacts when present in high concentrations at ground level. VOCs are considered air pollutants, and the amount that can be released for a given amount of solids is now regulated in many areas.
Wood finishes may also contain other toxic ingredients. For example, 1,4-dioxane and acetaldehyde are suspected carcinogens. N-methyl pyrrolidinone is a reproductive toxin. And aromatic solvents such as toluene and xylene can cause a number of health problems if inhaled. All of these examples are currently reported as ingredients in some wood-finishing products. Lead, a well-known toxic metal, had been used as a metallic drier in oil and resin varnishes; lead has since been replaced by other metals such as cobalt and zinc. Metals may still be problematic because of their environmental toxicity and persistence.
Phthalates, used as plasticizers, are of concern because they may potentially affect human and animal hormone systems. Ozone-depleting compounds, such as the solvent 1,1,1-trichloroethane, have been phased out of most products.
Wood finishes that include a preservative, such as a fungicide, must be registered under the Federal Insecticide, Fungicide, and Rodenticide Act.
Over the past few years, regulation under the Clean Air Act and consumer demand for low-VOC finishes have led to the creation of a variety of new products. Many penetrating finishes, such as semi-transparent stains, have low solids content (pigment, oils, polymers) and are being reformulated to meet low-VOC regulations. To meet the VOC requirements, these reformulated finishes may contain higher solids content, reactive diluents (dilutants or thinners), new types of solvents and/or cosolvents, or other non-traditional substitutes. These low-VOC requirements favor film-forming formulations over products that penetrate the wood surface, since traditional wood stains were formulated to penetrate the wood, and the new formulations that meet the VOC requirements may not penetrate as well.
Another way to decrease air emissions from wood finishes is to change the formulation to a water-based coating. The new water-based products achieve a dramatic improvement over solvent-based finishes in terms of VOC emissions and human comfort and health. Companies that have successfully switched to water-based coatings have worked closely with their suppliers to determine the best water-based formula for their specific uses.
(This material was adapted with permission from Green Seal's Choose Green Report.)
Green Seal is developing a new standard and certification for stains and finishes. For more information:
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