A paint is composed of pigments, solvents, resins, and various additives. The pigments give the paint color; solvents make it easier to apply; resins help it dry; and additives serve as everything from fillers to antifungicidal agents.
Hundreds of different pigments, both natural and synthetic, exist. The basic white pigment is titanium dioxide, selected for its excellent concealing properties, and black pigment is commonly made from carbon black. Other pigments used to make paint include iron oxide and cadmium sulfide for reds, metallic salts for yellows and oranges, and iron blue and chrome yellows for blues and greens.
Solvents are various low viscosity, volatile liquids. They include petroleum mineral spirits and aromatic solvents such as benzol, alcohols, esters, ketones, and acetone. The natural resins most commonly used are lin-seed, coconut, and soybean oil, while alkyds, acrylics, epoxies, and polyurethanes number among the most popular synthetic resins. Additives serve many purposes. Some, like calcium carbonate and aluminum silicate, are simply fillers that give the paint body and substance without changing its properties. Other additives produce certain desired characteristics in paint, such as the thixotropic agents that give paint its smooth texture, driers, anti-settling agents, anti-skinning agents, defoamers, and a host of others that enable paint to cover well and last long.
Manufacturing process
The manufacturing process involves five critical parts:
Accurate measurement of ingredients
Ingredients are typically measured by weight on scales, and in some cases by volume in calibrated vats and graduated measuring containers.
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Preassembly and Premix The first step in the manufacturing process is preassembly and premix. In this step, the liquid raw materials (e.g., resins, solvents, oils, alcohols, and/or water) are "assembled" and mixed in containers to form a viscous material to which pigments are added. The pigment and liquid mixture forms a thicker material, which is then sent to the grinding operations. At this stage, the particles in the concentrate are rather large (250 pm) and not consistently mixed. The premix stage results in the formation of an intermediate product which is referred to as the base or mill base. With further processing, this base with high pigment concentration may become any one of a variety of specific end products.
Equipment selection Premixing is necessary to keep the pigment in suspension in the resin, alcohol, solvent, and oil mixture and to supply the dispersion equipment with a consistently mixed material. A wide variety of equipment may be used in the premix process. Choosing which to use depends in part on batch size. Drum-sized batches made in the drum itself is blended with a Portable Mixer which clamps onto the rim of the drum. These mixers normally have a three or four blade impeller and is electric. Materials made in portable mix tanks is blended using larger, permanent high-speed dispersers or variable-speed mixers fitted with propellor agitators.
Pigment Grinding or Milling The incorporation of the pigment into the paint to yield a fine particle dispersion is referred to as pigment grinding or milling. This process occurs in three stages (i.e., wetting, grinding, and dispersion) which may overlap in any grinding operation.
Wetting:
To wet the pigment particles, the wetting agent, normally a surfactant, must displace all contaminants (e.g., air, moisture, and gases) adsorbed on the surface of the pigment particles. The wetting process actually begins in the premix step, when the pigment is charged to the liquid vehicle.
Grinding:
Grinding is the mechanical breakup and separation of the pigment particle clusters into isolated primary particles. Dispersion is the movement of the wetted particles into the body of the liquid vehicle to produce a permanent particle separation.
The goal of pigment grinding is to achieve fine, uniformly-ground, smooth, round pigment particles which are permanently separated from other pigment particles. The degree to which this is realized determines the coating effectiveness and permanency of the paint. Grinding equipment must work effectively with the vehicle to accomplish this end. The machinery used by elite paints is described in the following sections.
Roller mills
Roller mills may have from one to five rolls which grind pigments into vehicles. Elite paints uses conventional three-roll mills. The premixed pigmented paste is charged to the space between the feed and center rolls called the feed bank. End plates prevent the material in the feed bank from spilling out the sides. The mill base is carried into the feed nip region by the inward rotation of the feed and center rolls which are turning at different speeds. Some of the material remains in the feed bank while another portion transfers through the feed nip to the underside of the rolls. Here the material splits. Part transfers to the center roll while the remaining portion stays on the feed roll to return to the feed bank. The material that was transferred to the center roll passes through the apron nip, after which a second split takes place. One amount remains with the center roll, returning to the feed nip, while the other transfers to the apron roll where it is removed from the roller mill by the takeoff apron. As the material moves through both the feed and apron nips, it is subjected to very high shear. This shearing action serves to disperse the pigment throughout the vehicle, while the nip space determines the degree of this dispersion. Roller mills are labor intensive, requiring highly skilled operators. Their lack of speed and high operating cost make them unsuitable for large-volume production. The use of roller mills is confined to the manufacture of very high-quality paints and inks and viscous pigmented products which require fine dispersion and clean color.
Ball mills
Ball mills are cylindrical containers mounted horizontally and partially filled with either pebbles or ceramic, glass, or metallic bails which serve as the grinding media. Paint components, either in raw material or in premix form, are charged to the mill through a top chute. The ball mill and its contents then rotate about the horizontal axis at a rate sufficient to lift the grinding media to one side and then cause them to cascade to the lower side. The tumbling action results in pigment dispersion. Ball mills contain steel, alumina, iron, or nickel balls and have an interior surface of alloy steel or another metallic liner.
Attritors
An attritor is a stationary, vertical, cylindrical grinding tank fitted with a centralized, rotating agitator shaft to which are attached evenly-spaced spokes. The spokes extend into the ball media and mill base mixture which fills the attritor during the milling process. As the spokes rotate through the attritor tank contents, they agitate the ball charge. The agitation provides the required shear and impact to effectively disperse the pigment into the vehicle. They operate on a batch or on a continuous process basis and usually contain small ceramic or steel balls (i.e., 1/4 inch diameter). An attritor achieves pigment dispersion approximately three times faster than a ball mill, but requires constant supervision. Attritors can also handle higher viscosity materials than a ball mill.
Product Finishing
Final product specifications are achieved in the product finishing step which consists of three intermediate stages: thinning, tinting and blending.
Thinning (letdown)
Material letdown, or thinning, is the process by which a completed mill base dispersion is let down or reduced with solvent and/or binder to give a coating which is designed to provide a durable, serviceable film that is easily applied to the substrate. The volume of the paint may increase significantly at this point depending on the final product specifications.
Tinting
Tinting is the process of adjusting the color of completed mill base dispersions. Normally, an operator will collect a sample of the paint or ink once it exits the milling equipment. This sample will be taken to the laboratory and compared to the desired color standard. Various combinations of pigments, solvents, resins, and pastes are added to the material to meet the color requirements .
Product Filling
The final step in paint manufacturing is product filling operations. After the material has been blended, it is transferred from the blend tanks into containers for product shipment. The transfer step normally involves product filtration. Filtering Filtering acts to screen out impurities (e.g., dust, gelled resin, and pigment aggregates) and to enhance the quality and uniformity of the product. In the case of media mills, filters prevent the grinding media from exiting the mill and entering shipment containers.
The finished paint product is then pumped into the canning room. For the standard 8 pint (3.78 liter) paint can available to consumers, empty cans are first rolled horizontally onto labels, then set upright so that the paint can be pumped into them. A worker pours paint into cans and places lids onto the filled cans, and the cans pass along a conveyor belt to be arranged in concentric circles on a circular platter.