Colour and protection: dyes, pigments, paints and coatings: Paints, coatings and inks: formulation, manufacture and markets

5.1 A paint is pigment dispersed in a binder, carried by a solvent

Shreve defines liquid paint as a dispersion of finely divided pigment in a liquid made of a resin or binder and a volatile solvent; the liquid part is called the vehicle. Kent lists the constituents of a solvent-borne coating more fully: resin (binder), cross-linker, rheology control agent, catalyst, pigment, flow additive, solvents, and weathering stabilisers where needed. The binder forms the film and holds everything to the surface; without it, Shreve says, the pigment would not cling. The cross-linker joins binder molecules into a network as the film cures, and the catalyst speeds that. Rheology agents make it thin under a brush but thick on the wall, so it does not sag.

The oldest binders are drying oils such as linseed, soybean, castor and tung, which cure by absorbing oxygen at their double bonds and cross-linking, helped by 1 to 2% of driers (metal soaps). Alkyd resins, made from phthalic anhydride, polyhydric alcohols such as glycerine, and fatty acids, are synthetic versions that displaced most oil varnishes. In latex paints the binder is a dispersion of polymer particles in water, such as polyvinyl acetate or acrylic, which coalesce into a film as the water leaves. Shreve gives a typical latex paint as 35% pigment and filler and about 21% film formers, so roughly 44% is water and additives. Kent's chapter on structural adhesives states a rule that fits coatings too: the material must be liquid to wet the surface, then become a solid, usually cross-linked, polymer.

The key formulating ratio is pigment volume concentration (PVC): pigment volume divided by pigment plus non-volatile binder volume. Shreve's ranges run from 25 to 35% for gloss paints to 50 to 75% for flat paints. Above a critical PVC the binder cannot fill the gaps between particles, and the film turns porous and weathers faster.

• Shreve (1984), Ch 24 Surface-Coating Industries · pdf pp. 443-445, 448, 451
• Kent (2012), Ch 29 Pigments, Paints, Polymer Coatings, Lacquers, and Printing Inks · pdf p. 1123
• Kent (2012), Ch 14 The Chemistry of Structural Adhesives · pdf pp. 558-559

5.2 Solvent-borne, water-borne, powder, high solids and radiation cure

The main technical story in coatings since the 1970s is the escape from solvent. Solvents evaporate as volatile organic compounds (VOCs), which react with nitrogen oxides in sunlight to form ground-level ozone, or smog. Kent reports that before 1970 most paints carried well over 5 lb of VOC per US gallon; major industrial paints were later limited to about 3.5 lb, and California's South Coast district now allows wood stains, sealers and roof coatings only 50 g per litre. Converting, 1 lb per gallon is 454 g in 3.785 litres, or 120 g per litre, so the fall is from over 600 g/L to about 420 g/L in general industry and to 50 g/L at the strict end.

Formulators have four escape routes. Water-borne coatings replace most solvent with water; Shreve says more than 70% of US decorative paints were water-based by the early 1980s. High-solids coatings use lower-molecular-weight resins, so less solvent is needed to reach spraying viscosity. Kent notes radiation-cure oligomers of 100 to 1,000 g/mol need little or no solvent. Powder coatings contain no solvent at all: dry resin and pigment powder is sprayed electrostatically or applied in a fluidised bed, then fused in an oven. Kent gives growth of 7 to 9% a year, but cure needs 300 to 350 F (about 150 to 175 C), and Shreve notes colour problems and wastage. Radiation-cure coatings (ultraviolet or electron beam) polymerise in seconds without ovens.

Solvent is not dead. Kent reports solvent systems still made up 47% of the US paint market, because many formulators simply substituted permitted solvents or raised solids. Regulation also raised the cost of staying in business: smaller firms lacking the capital for compliance were forced into consolidation, and US coatings companies fell from about 2,000 in 1975 to fewer than half that.

• Kent (2012), Ch 29 Pigments, Paints, Polymer Coatings, Lacquers, and Printing Inks · pdf pp. 1119-1124, 1126
• Shreve (1984), Ch 24 Surface-Coating Industries · pdf pp. 442, 459

5.3 Making paint is mixing, milling and tinting in batches

Shreve stresses that the operations used to make paint are wholly physical. The chemistry happens upstream, in making pigments and resins, and later as the film dries. His flowsheet for a mass-production plant uses gravity. On the top floor, pigments and vehicle are weighed on platform scales and mixed in a heavy mixer rather like a dough kneader with sigma-shaped blades. This pre-mix wets the pigment powder with binder.

On the floor below, the paste is ground. Grinding here mostly means dispersion: breaking clusters of pigment particles apart and coating each with binder, not making the particles themselves smaller. Ball, pebble, sand and roll mills (including three-roll mills) and high-speed dispersers are used, chosen to suit pigment and vehicle; no one machine suits all. Shreve says the step needs much skill to give a smooth product at reasonable cost.

Next the concentrated grind is let down: thinned with the rest of the vehicle and solvent, and tinted to the target shade, in agitated tanks holding several thousand litres. It is strained through screens, centrifuges or pressure filters to remove undispersed pigment, then fed by gravity to machines that fill, label and pack automatically. Latex paint follows a similar order: dispersant and ammonia in water, pigments ground in, then film formers, preservative and antifoam, with the latex stirred in slowly near the end.

Two facts shape the economics. First, variety: Shreve notes that a complete paint factory uses upwards of 2,000 raw materials and makes ten times as many products, so no single yield describes it. Second, formulation is still largely empirical. Many trial batches are needed to hit a set of properties, which is why statistical test design and automated colour-matching matter; Shreve credits them with cutting matching time and keeping pigment use, and so cost, under control.

• Shreve (1984), Ch 24 Surface-Coating Industries · pdf pp. 445-448
• Dryden (1997), IIIC Paints and Varnishes (including Pigments) · pdf pp. 324-325

5.4 Industrial coatings, decorative paint and printing inks

Shreve separates trade-sale or architectural coatings, sold for buildings and furniture and applied on site, from industrial coatings applied to goods as they are made. The two compete on different things. Decorative paint is sold through shops on colour, brand and distribution. Industrial coatings are engineered systems sold to a factory line, often with the supplier built into it: Kent describes PPG winning an exclusive contract at Nissan's Mississippi plant for everything from pretreatment to clear-coat.

A car body shows how layered an industrial system is. The steel first gets a corrosion-resistant primer by cathodic electrodeposition, or e-coat: the body is dipped in a water-borne epoxy dispersion and made the cathode at 250 to 425 V, and paint deposits until the insulating film itself stops the current, giving a very even coat. Then come a primer surfacer, a coloured basecoat and a clear-coat. Adding Kent's thicknesses, the stack is about 105 to 145 micrometres.

Kent's US data for 2010 show architectural paint at 652 million gallons, OEM product finishes at 331 and special purpose at 171, so decorative was about 56% of volume. Dryden says decorative paint was about 70% of the Indian market in the mid-1990s, the reverse of developed countries, where industrial was 70%. He expected cars to shift the mix: an economy car takes 10 to 12 litres of paint against 1.8 litres for a scooter.

Printing inks are close cousins. Kent describes them as a colorant, an insoluble pigment or a dye, in a vehicle, applied in films of only 2 to 30 micrometres. Letterpress and offset lithographic inks are viscous pastes that dry by absorption, oxidation or UV cure; flexographic and gravure inks are fluid and dry mainly by evaporation, which is where water-based inks have advanced most.

• Shreve (1984), Ch 24 Surface-Coating Industries · pdf pp. 442, 459-461
• Kent (2012), Ch 29 Pigments, Paints, Polymer Coatings, Lacquers, and Printing Inks · pdf pp. 1119-1120, 1124-1128
• Dryden (1997), IIIC Paints and Varnishes (including Pigments) · pdf p. 325
• Dryden (1997), IB Indian chemical industry · pdf p. 25

See it in the table

Check your understanding

In paint manufacture, what is the main purpose of the milling or grinding step?

A coating has 3.5 lb of VOC per US gallon. Roughly how many grams per litre is that?

According to Dryden, what share of total cost do raw materials make up in paint manufacture?