(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)
Polymer Emulsion Paints and Media
AN EMULSION Is the suspension of tiny solids in a liquid. Milk is an example of an emulsion, and most solids, if made small enough, will tend to remain in suspension. A polymer is a larger molecule made of smaller and simpler chemical units most often arranged in a chainlike formation. A polymer emulsion is the suspension of polymers in a liquid. As the liquid evaporates, the suspended polymer solids come closer together until they touch and combine to form larger chains and eventually a film. A paint can be made by pigmenting a polymer emulsion. The type of polymer used determines the type of paint or medium acrylic polymers for acrylic paints and vinyl polymers for vinyl paints.
Polymer emulsion paints, by comparison with oil paints and watercolors, have a short history, which began in late 1948 with the development of polyvinyl acetate emulsion (PVA), commonly known as white glue. However, PVA was too sensitive to water and heat, and the paints made from it were not durable. The acrylic polymer emulsions now used in artists’ paints are a byproduct of the attempt to develop a new type of house paint during the early 1950s. (Today, however, the amount of actual acrylic binder in house paint is often quite low. In some states it can be as low as 20 percent and still be called acrylic paint. This would be too low to use in fine artwork.) The first artists’ acrylic polymer paint became readily available in North America around 1963 and in Europe about two years later.
Manufacturing of Acrylics
The manufacture of polymer emulsion paints is a balancing act. Mixing dry pigment directly into a polymer emulsion rarely creates a usable paint. Several additives are needed to produce a workable paint. The polymer solids must be coated with emulsifiers to prevent them from binding together before the liquid evaporates. Dispersants are necessary to keep the pigments that are added from clumping together and/or settling out of the liquid. Antifoaming agents are needed to prevent foaming during the application of a paint so that the dried paint film does not have a craterlike surface. Wet-edge agents, such as ethylene glycol (very poisonous) or propylene glycol (less effective, but nontoxic), are used to regulate the drying time, allowing sufficient time for mixing and applying the paint. Thickeners are used to transform the milky quality into a paintlike consistency. And last but not least important, a preservative is added.
The type of paint film that is formed depends on the specific polymer formulation used. For example, the Rohm and Haas Company, the supplier of acrylic emulsion to all paint manufacturers in North America and many parts of the world, offers several varieties of 100 percent acrylic emulsion (of which 44 and 47 percent of the emulsion is composed of resin solids). Rhoplex is the name it has given to its acrylic emulsion and each formulation is assigned its own number. The most commonly used emulsions that are dispersed in water are AC-22, AC-33, AC-34, AC-35, and AC-235. (There are also acrylic solutions such as Acryloid F-10, which is 40 percent resin solids in mineral thinner which are used in the manufacture of paint varnishes and acrylic paints that can be thinned with mineral spirits.) AC-22 has good flow and leveling properties, but is less durable than the other formulas. AC-33 was the first formula used in the arts and is still commonly used. AC-34 was designed for outdoor use on wood, but tends to be slightly more brittle. AC-235, the improved version of AC-35, is used to give paint a thicker quality and would be used in impasto painting.
Since each of these formulas has desirable qualities as well as undesirable qualities, most paint manufacturers blend the various polymers like chefs to obtain, what is in their opinions, the best working characteristics. A blend of more than one polymer is referred to as co-polymers; virtually all artist acrylic and vinyl paints are co-polymers.
The physical process of making acrylics resembles the process one would more expect to see in a pastry shop than in a paint mill. Some manufacturers do not use rollers to grind the paint mixture as would be done with oil paints, but simply mix the ingredients together. Mixed paints are less desirable for airbrushing and for watercolors where fine dispersion is important.
Paint Film Characteristics
After the liquid of a polymer emulsion evaporates, such as with an acrylic polymer, a tough waterproof film forms. Although the chains formed by polymers are broken by ultraviolet light, there is such an overwhelming number of these chains that little or no visible damage occurs from exposure to indoor lighting. Visible damage can occur, however, when the emulsion is exposed to large amounts of direct sunlight outdoors, or when the emulsion has been thinned excessively before application and then exposed to direct sunlight. If a polymer emulsion is applied to a nonoily, absorbent surface, it will remain permanently attached. Although polymer films are quite flexible, there are situations where they can crack. Cracking can occur when a polymer emulsion is overloaded with particulate matter, such as additional pigment or sand, or when a significant amount of polymer is washed out of a paint mixture during the painting process. And an extremely absorbent painting ground can draw out enough polymer from a polymer paint to cause cracking.
The color range of polymer paints is limited for two reasons. The first concerns pH. Alkaline-sensitive pigments cannot be used in the manufacture of acrylics because acrylic emulsions are alkaline. Acid-sensitive pigments cannot be used in vinyl paints because vinyl emulsions are acidic. The second reason is that in polymer emulsions, subtle differences between similarly colored pigments often cannot be seen and are therefore pointless to manufacture.
Acrylic Polymer Emulsion Paints
Golden Artist’s Arcylic, Liquitex, made by the Binney and Smith Company, and Atelier, a lesser-known acrylic paint made by Chroma Acrylics, are examples of the differing formulations of acrylic paints in the marketplace today. Two different formulations are used for Liquitex, one type for the paint that comes in tubes, and the other for the paint that is available in jars. The paint in tubes is designed to have a thicker consistency and to leave brushstrokes the way oil paint can. The paint that comes in jars is designed to behave more the way an enamel paint would; the paint film tends to level out and the brushstrokes tend to disappear. Most other manufacturers’ jars of acrylic paint are made with the same formulation as their tubes. Thinning tube paints will rarely achieve the same effect as Liquitex that comes in jars, and you usually end up with a thinned paint that still leaves brushstrokes.
Atelier is an example of a new type of acrylic for which updated formulations are used. It is designed to appeal to the impasto painter, who desires texture, thickness, and that heavier oil paint appearance. No matter how a manufacturer decides to formulate the consistency of the colors, additives or media are always supplied to alter the consistency to suit the artist’s wishes.
Acrylic Polymer Solution Paints
There are certain formulations of acrylic polymer, such as Acryloid B-72 manufactured by Rohm and Haas, that form a solution, rather than a emulsion, in mineral spirits. Magna Color, made by Bocour Company, is an example of a paint made with this solution as the binder. Its consistency and general working characteristics are like oil paint, but it dries like acrylic. However, this type of paint has never attained even half the popularity that acrylic emulsion paints have.
Acrylic Polymer Media
A number of auxiliary products are available for use in acrylic painting. They are designed to prepare a surface for painting, to slow the drying time of the paints, or to enable the artist to create texture on a surface.
Polymer Gel is a thickening agent that may be added to a polymer medium. For example, sodium polyacrylate can be added to acrylic polymer emulsions. Gels allow for an impasto style of painting with polymer paints. The more gel used, the more transparent the paint film. Gels are resistant to cracking and are excellent as adhesives for collage.
An Acrylic Retarder is either a gel or an additive that evaporates without a trace. It is designed to slow the drying time of acrylic polymer emulsion paints. The recommended amount of retarder to be used varies from manufacturer to manufacturer and if specified amounts are exceeded, the paint film will not form properly. Most retarders, when mixed properly with a color, will slow the drying time to approximately three hours. This allows areas of a painting to be blended or reworked in much the same way as can be done with oil paint.
Acrylic Modeling Paste is a unique product. It is a waterborne, puttylike substance that dries matte and opaque. It is an acrylic polymer medium mixed with marble dust and titanium dioxide. This product is used to build up textured surfaces and sculptured reliefs on absorbent surfaces. If it is to be applied to a flexible surface it must be mixed half and half with gel medium or cracking may result. When this paste is dry it may be carved, sanded, and painted with either waterborne or oil paints. Acrylic polymer paints may be mixed directly into it. Acrylic Polymer Gesso is not the same as the traditional gesso formulation of white pigment and chalk mixed with hide glue. Although both are designed to create a primed surface for painting, the traditional gesso can only be used on a rigid support, such as a wood panel, since it is not the least bit flexible and will crack if applied to anything that moves or bends. Acrylic polymer gesso is a combination of gypsum, or chalk, titanium dioxide, and just enough acrylic polymer medium to keep the undiluted mixture from cracking on a flexible support, such as canvas.
Acrylic polymer gesso can be used to prepare any absorbent surface to receive either polymer emulsion paints or oil paints. Recent investigation seems to indicate that acrylic gesso may be more resistant to chemical attack by the pollution in city air than traditional gesso for rigid supports or the traditional combination of rabbit-skin glue and lead white for canvas supports. Today, acrylic polymer gesso has all but replaced all traditional gessos.
Although acrylic polymer gesso can be applied without thinning, it is much easier to work with if it is thinned. Because it has a minimum of acrylic polymer to provide a very absorbent painting surface, it can crack if diluted only with water and applied to a flexible and highly absorbent surface. It is therefore wise to add some acrylic polymer medium whenever water is added. Excellent results can be obtained using a mixture of 25 percent polymer medium, 25 percent water, and 50 percent acrylic polymer gesso for priming canvas.
Artists have reported that some brands of less expensive acrylic gesso may be more subject to cracking. The problem occurs when the gesso is not properly formulated-when it contains too much particulate matter (pigment and filler) in relation to the acrylic emulsion. As the gesso dries, the overloading of pigment prevents the acrylic resin particles from forming a complete paint film, and the resulting paint film is weak and brittle. Problems such as these teach two important lessons-read the instructions on the jar, and combine acrylic polymer gesso and acrylic polymer paint made only by the same manufacturer.
Acrylic Polymer Varnishes
Unprotected polymer paint films are not very durable because they are relatively soft compared to oil paint films and can be marred easily as the result of abrasion. They are also more porous and the pigments and paint film can be affected by air pollution. A varnish can, however, provide the necessary protection. A varnish can also be used to control the surface appearance in terms of matte and gloss.
Acrylic polymer solution varnishes, which are dissolved in mineral spirits, are the best protection for acrylic paint films. They are more durable, forming a harder protective film than acrylic emulsion varnishes, which are waterborne. Acrylic polymer solution varnishes also provide better protection against air pollutants because they seal the painting’s surface more thoroughly.
An acrylic polymer emulsion varnish is the same as an acrylic polymer emulsion used as medium, except that the varnish contains a hardener. Unless specified by the manufacturer, an acrylic varnish should not be used as a painting medium. Most manufacturers discourage the thinning of acrylic varnishes, and if diluted with more than 50 percent water, the clarity of the dry varnish film can be seriously affected.
The application of a varnish should be smooth and consistent, and areas that might require more varnish should not be reworked. Reapplying a varnish to a partially dry area can result in streaking, evident brush strokes, and a milky appearance. It is best to wait the hour or two until the varnish dries, and then apply a second coat. Matte varnishes are particularly susceptible to this problem.
The painting surface shine can be regulated not only by choosing a matte or gloss varnish, but also by using a mixture of the two. A matte varnish, despite the hardeners added to it, is not as durable as a gloss varnish. And the matte surface can be rubbed or polished to a shine.
Permanency of Acrylic Paints
Oil paint has been used for several hundred years, and its range of permanency is well established. Although oil paint is subject to some yellowing and cracking over the centuries, it is still a highly durable medium. Acrylic polymer paint, on the other hand, has been in use only for several decades. Its range of permanency is not established, but is instead implied through accelerated aging tests. Although aging tests have often proven to be quite accurate, they can never take into account all the variables. For example, acrylic and vinyl polymers seem to be more vulnerable to weakening from exposure to ultraviolet light (clear acrylic sheeting used in displays and picture framing does yellow in time from the ultraviolet light in sunlight and fluorescent light) and sulfur air pollutants than previously thought. This is not to say that such paints are unsafe, but rather that all the data are not yet in and there is still some uncertainty as to the actual longterm stability.
As an example of this uncertainty, Binny and Smith rate its most permanent colors in Liquitex, artists’ acrylic emulsion paints, as having slight or no color changes after the equivalent of one hundred years of indoor museum exposure. Many of these same colors in oil paint often carry a rating that is equivalent to two hundred or more years before any visible changes occur. Oil paint with ratings of seventy five to one hundred years are often classed only as durable. Again, the point here is not that acrylics will not last as long as oil paint, but rather that no one can yet say, even with accelerated aging tests.
Oil Paints and Acrylic Paints
There is a longstanding debate over whether it is safe to use oil paint over acrylics because they are made with different paint vehicles. New research seems to indicate that these two types of paint film do adhere quite well to each other, but only more complete testing will tell if the original optimism about the ability of oil paint to bind to acrylic emulsions is justified. Acrylic paint, however, should never be applied over oil paint.
Many people have noticed that oil paint seems to look “richer” than acrylic paint. This is partly due to the fact that oil paint vehicles can hold more pigment than can acrylic emulsions. Also, due to cost, most companies that manufacture acrylic paints use less pure grades of pigment when making colors such as cadmium red and cadmium yellow. (At present, Liquitex’s cadmium colors, for example, are made with cadmium-barium pigment instead of pure cadmium. The manufacturer claims that a switch will be made to pure cadmium in the near future.) One company, Winsor & Newton, is now marketing a line of acrylic colors that are made from chemically pure pigments. It uses pure cadmium instead of the cadmium-barium pigment that most other companies use. Whether it makes a great deal of difference if pure pigments are used in acrylic paints seems to be a matter of personal taste. For instance, different types of black pigment such as Mars black and ivory black, which are easily distinguished from one another in an oil paint vehicle, look similar when ground into an acrylic emulsion because of differing refractive properties of oil and acrylic media.
Hazards of Acrylic Paints
The polymer emulsion itself has hazards of its own that must be considered. Although most major manufacturers have replaced the highly toxic ethylene glycol (wet-edge agent) and toxic preservatives with less hazardous ones in their polymer emulsions, they cannot be considered safe. Some of the polymers evaporate with the water and can be inhaled. Although the amounts are not significant in most painting situations, there are many painters who still do stain painting and color field painting, which involve the use of large amounts of acrylics, and the artist stands directly over the drying paint without any protection.
There are painters and small manufacturers making polymer paints who are not aware of the hazards involved in the manufacturing process and how they can be passed along to other users of their paints. For example, the raw acrylic polymer emulsion used in the manufacture of paint contains significant levels of highly toxic and volatile monomers that must be reduced or eliminated for safe use before handling. Not only can the vapors be hazardous, but mural painters should know about the experience of a well-known mural painter who was hospitalized for acrylic and heavy metal poisoning. She was working outdoors on a mural for the Olympics. The heat of the day apparently opened the pores of her skin, allowing a significant amount of acrylic polymer and pigment to be absorbed. Although polymer paints are not especially dangerous, they are not as safe as the prevailing opinion, and appropriate precautions are necessary.