The binder is the actual film forming component of paint. It is the only component that must be present. Sometimes you will hear the term “vehicle” applied to the ingredients in paint, but this is an ambiguous term sometimes used to refer to the solvent or sometimes the binder. The binder imparts adhesion, binds the pigments together, and strongly influences such properties as gloss potential, exterior durability, flexibility, and toughness. Binders include synthetic or natural resins such as alkyds, acrylics, vinyl-acrylics, vinyl acetate/ethylene (VAE), polyurethanes, polyesters, melamine resins, epoxy, or oils.
Binders can be categorized according to drying, or curing mechanism. The four most common are simple solvent evaporation, oxidative cross linking, catalyzed/cross linked polymerization, and coalescence. There are also less common curing mechanisms. Drying and curing are two different processes. Drying generally refers to evaporation of the solvent or thinner. Curing refers to polymerization of the binder. Depending on the chemistry and composition, any particular paint may undergo either or both processes. So some paints only dry, some paints dry and then cure, and there are those that do not depend on drying for curing.
Paints that dry by simple solvent evaporation and contain a solid binder dissolved in a solvent are known as lacquers. The term lacquer originates from the Portuguese word for lacre, a type of resin excreted from certain insects. The use of lacquer paints can be traced back to ancient China, Japan, and India. These “lacquers” were derived from both insects as well as the T. vernicilflurrm or “varnish tree.” In modern usage, lac-based varnishes are referred to as shellac, while lacquer paint refers to other polymers dissolved in volatile organic compounds (VOCs), such as nitrocellulose, and later acrylic compounds dissolved in lacquer thinner. Lacquer thinner is a mixture of several solvents typically containing butyl acetate and xylene or toluene. The classic Floquil was a type of xylene based lacquer which is why it had a characteristic smell that differed from the typical lacquer thinner.
In a general sense, lacquer is a clear or colored varnish that dries by solvent evaporation, and sometimes a curing process as well, that produces a hard, durable finish in any sheen level from ultra matte to high gloss and that can be further polished as required. A solid film forms when the solvent evaporates, and because the film can re-dissolve in solvent, lacquers are not suitable for applications where chemical resistance is important. Classic nitrocellulose lacquers fall into this category. Quick-drying solvent-based lacquers that contain nitrocellulose, a resin obtained from the nitration of cotton and other cellulostic materials, were developed in the early 1920s, and extensively used in the automobile industry for 30 years. Prior to their introduction, mass produced automotive finishes were limited in color, with Japan Black being the fastest drying and thus most popular. General Motors Oakland automobile was the first (1923) to introduce one of the new fast drying nitrocellulose lacquers, a bright blue, produced by DuPont under their Duco trade name. These lacquers are also used on wooden products, furniture primarily, and on musical instruments and other objects. The nitrocellulose and other resins and plasticizers are dissolved in the solvent, and each coat of lacquer dissolves some of the previous coat. These lacquers were a huge improvement over earlier automobile and furniture finishes, both in ease of application, and in color retention.
The preferred method of applying quick-drying lacquers is by spraying, and the development of nitrocellulose lacquers led to the first extensive use of spray guns. Nitrocellulose lacquers produce a very hard, durable finish that can be polished to a high sheen. Performance varies by formulation, but lacquers generally tend to have better UV resistance and lower corrosion resistance than comparable systems that cure by polymerization or coalescence. Drawbacks of these lacquers include the hazardous nature of the solvent, which is flammable, volatile and toxic. Lacquer grade of soluble nitrocellulose is closely related to the more highly nitrated form which is used to make explosives. They are generally not toxic after about a month after the solvent has completely evaporated and any polymers have cured.
Lacquers using acrylic resin, a synthetic polymer, were developed in the 1950s. Acrylic resin is colorless, transparent thermoplastic, obtained by the polymerization of derivatives of acrylic acid. Acrylic is also used in enamel paints, which have the advantage of not needing to be buffed to obtain a shine. Enamels, however, are slow drying. The advantage of acrylic lacquer is its exceptionally fast drying time. The use of lacquers in automobile finishes was discontinued when tougher, more durable, weather and chemical resistant two-component polyurethane coatings were developed.
Due to health risks and environmental considerations involved in the use of solvent-based lacquers, much work has gone in to the development of water-based lacquers. These lacquers are considerably less toxic and more environmentally friendly, and in many cases, produce acceptable results. More and more water-based colored lacquers are replacing solvent-based clear and colored lacquers in the automobile and other similar industrial applications. Water based lacquers are used extensively in wood furniture finishing as well.
Paints that cure by oxidative cross linking are generally single stage coatings. When applied, the exposure to oxygen in the air starts a process that crosslinks and polymerizes the binder component. Classic alkyd enamels would fall into this category. These paints typically use a petroleum-based solvent which is why they are often referred to as oil-base or oil paint. Oil based paints were the first type of paint used in plastic modeling. Alkyd enamels, dry and cure from the inside to the outside. Sometimes this process can take weeks. Oxidative cure coatings are catalyzed by metal complex driers such as cobalt naphthenate.
The newest types of paint are acrylics. The origin of these paints can be traced back to the early work of Rohm and Haas. Otto Rohm and Otto Haas were German immigrants who formed a small Pennsylvania based company. Following WWI, they were able to concentrate on Rohm’s earlier acrylic polymerization work and develop prototypes of what was to become known as Plexiglas.
One of their biggest challenges was synthesizing acrylic monomers; the single molecule building blocks used to form polymers. Producing these monomers was expensive, time-consuming and generated relatively low yields. These difficulties limited potential markets, especially in areas of coatings and molding powders. Rohm would work on this problem until his death in 1939. Haas’s quest to develop an efficient and economical synthesis of acrylic monomers began in earnest in 1946 and would take three years.
In 1951, Haas authorized the construction of a new plant in Houston, Texas. The plant began producing acrylate monomers in 1952. The first paint emulsion, Rhoplex AC-33, appeared a year later. AC-33 was the critical binder of these new paint formulations. Rhoplex AC-33 and the other acrylic emulsions that Rohm and Haas would develop in the years ahead have some amazing qualities. Emulsion polymers are prepared in water and stabilized with surfactants, molecules that are hydrophilic (“water-loving”) in one segment and hydrophobic (“water-hating”) in the other. Emulsion polymers begin forming when a free radical, acting as an initiator, breaks a double bond between two carbon atoms in an acrylic monomer, starting a reaction that causes as many as 10,000 monomer units to bind together into a polymer chain. As these chains take shape, they grow into submicron-sized spheres. Within each sphere there are about 300 acrylic polymer chains.
As the spheres disperse in the solution, the water changes color: turning chalky white with a consistency similar to skim milk. These spheres are incredibly small. A 200-gram sample of an acrylic emulsion, about one-tenth the amount used in a typical gallon of paint, contains more of these submicron-sized spheres than there are stars in the Milky Way. The polymers created in the emulsion are easy to handle and formulate. But perhaps their most amazing characteristics are their extremely high molecular weights, which approach one million. Because of this, as the water evaporates the polymers coalesce into a tough acrylic film. These films, depending on how they are prepared, can have the tackiness of “fly paper” or the hardness of glass. In addition, for the creative chemist, emulsion polymers offer a number of “dials to turn” to optimize properties. These include backbone composition, molecular weight, functional monomers, morphology and surface chemistry.
Acrylic emulsions offer another important advantage: they’re environmentally friendly. As Rhoplex AC-33 and other acrylic emulsions supplanted solvent-based paint components, their water-based, non-toxic, non-flammable nature reduced harmful emissions and enhanced worker safety.
Acrylic binders are used in all forms of paints so that you see acrylic lacquers and acrylic enamels (petroleum based). These were the most common types of automobile paints in use until the introduction of the two-stage (color coat, clear coat) catalyzed paint systems. These acrylic paints used special solvents to reduce the acrylic binders. Anyone who has shot these acrylic formulations is familiar with the characteristic odor of the thinners. In modeling, our acrylic paints are generally water or alcohol based.
Paints that rely on catalyzed/cross linked polymerization are two part urethanes and epoxy type paints. They require that a catalyst be added to the paint. These types of paints are most commonly used in automotive finishes. They are highly toxic and should be used with care.
Latex paint is a water-borne dispersion of sub-micrometre polymer particles. The term "latex", in the context of paint, simply means an aqueous dispersion. Latex rubber (the sap of the rubber tree that has historically been called latex) is not an ingredient. These dispersions are prepared by emulsion polymerization. Latex paints cure by a process called coalescence where first the water, and then the trace, or coalescing, solvent, evaporate and draw together and soften the latex binder particles and fuse them together into irreversibly bound networked structures, so that the paint will not redissolve in the solvent water that originally carried it. The residual surfactants in latex paint as well as hydrolytic effects with some polymers cause the paint to remain susceptible to softening and, over time, degradation by water.
In addition to these broad categories of paint, each paint company may use additives to improve or enhance certain qualities of their paints. So no paint is generic, and it is important to keep this in mind when mixing and thinning paints.
Paints For Model Building
Of course we are all free to paint our models with whatever we choose. Depending on the material that your model is constructed from, you might indeed want to use a paint that was not specifically formulated for model builders. In this regard, wood and metal come readily to mind. When I was building 1/12th scale cars and motorcycles, I was almost always using automobile paints and catalyzed clear coats. In hindsight, not a particularly smart choice. Today, I would most likely use something different.
