Paint technology has advanced a long way since 1867, when the first ready-mixed paints were patented in the United States. Before that, one of the most common wood coatings was a mixture of slaked lime and chalk known as whitewash. Today, homeowners have their choice coatings with every color of the spectrum and sheens that vary from full shiny gloss to dead flat. Paint chemistry can be a complex topic, but you don't need to know all the details about what goes into a can of paint to make good paint choices. Having a handle on the basics of the manufacturing process, however, does give you an edge when you're reading product labels.
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What's in a Can of Paint?
The paints you use to touch up living room walls and that you use to protect outdoor woodwork fall into the general category of architectural paint products, as opposed to industrial or specialty products. Architectural paints all contain three general ingredients in varying proportions: pigments, binders and a solvent. Many products also include additives included to enhance performance in some way, such as to control mold, improve spreadability, or modify sheen.
Pigments provide the color in paint. Common pigments include titanium dioxide, which is white, chrome green oxide, yellow and red iron oxide, zinc oxide, and carbon black. Pigments are powders that tend to clump together, and manufacturers use various methods to mill and separate them prior to adding them to the paint mixture.
Binders provide the adhesion in paint, allowing the applied coat to hold together without cracking. Binders are resins that may be derived from plants, which is the source of traditional alkyd resins; or they can be synthesized in laboratories. Binders can be water-borne, which is true of acrylic polymers; or they can be carried in a non-aqueous solvent. Urethane and epoxy polymers are both solvent-borne binders. One of the least expensive (and durable) water-based binders is polyvinyl acetate (PVA), which is typically used on paints for drywall.
Solvents make paint "flow." Until the mid-twentieth century, all paints were oil-based, which means the solvent was something other than water. In most cases, the binder in early paint was turpentine. The development of water-soluble acrylic polymers introduced the market to a whole new range of products that have all but replaced oil-based paints for most residential applications.
Additives improve the basic product. Some chemicals, such as calcium carbonate, refract incident light and give the paint a matte finish. Others, such as zinc phosphate, inhibit rust formation; and some, such as talc (magnesium silicate), improve spreadability. Additives typically make up a very small percentage of the total volume of a can of paint.
The Manufacturing Process
Paint manufacture involves several discrete stages. In the first, the manufacturer carefully weighs out all the ingredients for the batch being prepared. The pigment is prepared in a second process known as the mill-base. The manufacturer mills the pigment and wets it in a small amount of solvent and resin to create a non-binding liquid that will disperse evenly into the final paint mixture. During the third stage, known as let-down, the binder, resin and additives are mixed with the solvent, and the pigment is added. The paint is now ready to be evaluated for color, consistency, drying time, sheen and film durability, and a small amount of the finished product is set aside as a reference. Finally, during the canning stage, the paint is transferred to cans and sent out to distributors.
Two Important Percentages
Every paint product is a mixture of solid pigments and binders and a solvent, and is categorized by measurement known as volume solids (VS). The VS is the ratio of the volume of the solid material in the paint to the total volume of the product, multiplied by 100. In general, the higher the VS rating of a product, the better the paint will cover and the more durable will be the film. A good way to compare the qualities of two different products is to compare their VS percentages. Pound for pound, the product with the higher number is the better product.
Pigment Volume Concentration (PVC) is another important measurement used to judge paint quality. PVC is the ratio of pigments to binder; a higher number indicates a higher concentration of pigments. It is the binders that make paint glossy, so a higher concentration of pigments cuts the sheen and flattens the finish. Pigments are less durable than binders, so a high PVC also indicates a paint that won't wear as well as one with a low PVC. High-PVC paints are usually reserved for walls and ceilings in living rooms and other areas in which wear isn't an issue.
Volatile Organic Compounds (VOCs)
When painting indoors, you'll want to use a product designed for interior use, and certain states, such as California, regulate the amount of VOCs these products can contain. Low- and no-VOC paints are always water-based products, because it's the solvents in oil-based paints that produce the bulk of the noxious fumes. These paints can contain such harmful chemicals as benzene, toluene and formaldehyde.
Even water-based paints can off-gas, so if you're sensitive to VOCs, check the label and purchase only no-VOC products. If you notice odors coming from your fresh paint, you can remediate them in several ways. One way is to open windows to provide ventilation, and another is to distribute bowls of vinegar or baking soda around the room. The odor typically goes away by itself in a day or two.