Both interior and exterior architectural coatings fall into two general categories: latex or water-based paint or oil-or alkyd-based paint.
Contractors and do-it-yourselfer’s select latex for its good performance and ease of use. Latex moved from 30% to 35% of paint sales in the 1970’s to over 80% of architectural coatings sold today.
While the pigments in latex and oil-based paints are often the same, different carriers and binders give latex and oil-based paints different characteristics. The vehicle or carrier is the volatile part of the paint that evaporates as the paint dries. The carrier in latex paints can be water with some solvent — typically glycols and glycol ethers are used as coalescing agents. In oil- based paint organic solvents usually from petro-chemicals are used with mineral spirits being prevalent.
The carrier keeps the pigments and the binders in suspension until the paint is applied to a surface. The binder is the body of the paint, the permanent part of the paint that remains when the paint dries. As the name implies, binders help the paint to adhere to a surface.
In oil-based paint, binders can be synthetic resins, derived from petroleum, including alkyds, polyurethanes, and silicones. Natural oils include linseed, soybean, safflower, tung, and cottonseed oils.
Water-soluble binders include acrylic resins, polyvinyl acetate, and styrene butadiene. The best latex paints are 100% acrylic, followed by vinyl acrylics and polyvinyl acetates.
In oil and latex paints titanium dioxide is the primary pigment used for its hiding properties. Pigment extenders include: silica, calcium carbonate, dolomite, talc, clay, mica, barite, and gypsum. Pigment extenders minimize the settling of the paint solids and impart gloss. Extender pigments are also bulking agents that influence scrub, stain and chalking resistance. Paints other than white also contain color pigments.
Most people buy latex paint because painting projects using latex omit less odor and cleanup with soap and water, but latex has many other positive characteristics. In the damp Pacific Northwest, latex is preferred because it can be painted over a damp surface. Oil is a non-breathable surface sealer while latex breathes allowing moisture to escape. The peeling and cracking that occurs when moisture is trying to escape from a surface coated with oil-based paint is eliminated with latex paint.
Latex paint, especially acrylic latex, is less prone to fading or yellowing than oil-based paint. Chalking is a characteristic of oil-based paint. This self-cleaning process causes oil-based paint to lose much of its color intensity.
Even after oil-based paint is dry to the touch it continues to dry until the paint starts to crystallize and crack. Latex paint remains flexible. Because latex paint is not as sensitive to alkali as oil-based paint it can be used over cement and plaster.
Oil-based paint can be applied at a lower temperature than latex’s 50-degree Fahrenheit minimum temperature but oil-based paint will never dry in high humidity. Latex paint is dry to the touch in minutes and ready for a second coat in about four hours. Oil-based paints are dry to the touch in six to eight hours but need twenty-four hours before re-coating. The longer the drying time the greater the chance of insects and dust penetrating the wet surface.
Latex paint is also easier to apply as it flows off of the brush or roller. Oil-based paint is sticky and must be stretched out and worked into the surface. The chief advantage of using oil-based paint over latex is that the oil-based paint requires less surface preparation and adheres to dirtier, shinier, or more weathered surfaces than latex paint. It is more resistant to abrasion.
Containing anywhere from 50% to 90% water, the environmental impact from water-based paint is much less than oil-based paint. Jim Quinn, supervisor of Metro Regional Government’s household hazardous waste program notes, “Oil-based paint is considered hazardous waste under State and Federal regulations. It costs Metro over $7 a gallon to bulk and dispose of the oil-based paints left-over from household applications.” Household leftover oil-based paint is sent to a cement kiln in the Midwest for energy recovery.
Because latex paint formulations are compatible Metro can reblend the useable latex paint it receives from households and some business. Selling this paint back to the public at cost covers production expenses. Re-using the resources preserves habitat, prevents pollution, and conserves energy, water, and landfill space.
Since 1992 Metro has produced over a half million gallons of reblended latex paint suitable for interior or exterior application. Filtered to industry standards the paint has been brushed, rolled, and sprayed on wood, masonry, concrete and vinyl and metal siding. Tested for performance and environmental considerations the latex is available in 13 colors. Call Metro Recycling Information at 503-234-3000 for a color brochure and money-saving coupon. Or consult Metro’s website www.metro-region.org/paint for a color chart and test results.
Paulette Rossi is a Certified Master Recycler living in Portland, OR.