What Is Paint And How Is Paint Made?
To most people, paint is the colour on the walls of their home, the colour of their car, boat or caravan.
Paint is more than just the colour though; it is a material that is applied as a liquid and dries by a variety of chemical processes to a solid.
We apply paint for:
Decoration
Protection
Identification
Sanitation
What is paint made of?
Paint typically consists of pigment, resin, solvent and additives:
Pigment - to provide colour, hiding and control gloss.
Pigments are usually divided into two groups. One called ‘Prime Pigments’ includes pigments such as Titanium Dioxide (white), Chrome Green Oxide, Yellow and Red Iron Oxides, etc. The other group of pigments is called ‘Extender Pigments’ and includes Calcite (Calcium Carbonate), Talc (Magnesium Silicate), Mica, Barytes (Barium Sulphate), etc.
Resin – the binder to hold the pigment particles together and provide adhesion to the surface painted.
Waterborne paints most often use acrylic emulsion polymers as binders. These come in a wide variety of types and combinations. Common acrylic polymer types are based on monomers such as methyl methacrylate and butyl methacrylate. Traditionally, lower cost paints have been formulated on PVA (Poly Vinyl Acetate) binders.
Solventbased resins come in a very wide range of types. The most common solventbased resins are termed ‘alkyd resins’ that are normally used in enamel paints. Urethane alkyds often used in clear varnishes. Protective coating resins include types such as Epoxy, Urethane, Polysiloxane and Moisture Cured Urethane.
Solvent – to act as a carrier for the pigments and resin – the solvent may be organic (such as Mineral Turps) or water.
Additives – to enhance certain properties such as ease of brushing, mould resistance, scuff resistance, drying and sag resistance.
Manufacturing process
Our paint is generally manufactured in batch lots from 200 litres for special products and colours to 10,000 litres for mainline white products.
The manufacturing process involves five critical parts:
Part 1 – 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. For batches larger than about 4000 litres the vats are loaded onto load cells connected to accurate electronic scales. The scales allow addition of ingredients with a measurement accuracy of +/- 5 kg in 20,000 kg. Where greater accuracy of small additions is required highly accurate floor scales are used.
Part 2 – Mill-base preparation and pigment dispersion
Pigments are powders of typically small size that tend to stick together to form clumps or agglomerates. These must be broken down into separate particles that must then be wetted by resin and additives to stop them sticking together again. This is the process of dispersion.
High speed mixers are used for combining materials and dispersing most pigments. These machines rotate stainless steel serrated discs of up to 60cm diameter, at up to about 1000 rpm. The largest machine also has rotating scraper blades to ensure material doesn’t stick to the sides of the mill-base vat.
Pigments are added slowly, from containers of 10 kg up to 1000 kg, to a portion of the liquid paint components, with the mixer running, to form the mill-base.
Certain pigments are more difficult to disperse and require ball milling, bead milling or bar milling.
Ball mills are used for small batches of difficult to disperse mill-bases. Ball mills are large porcelain lined rotating drums containing golf ball sized porcelain balls. Rolling and tumbling of the balls provides sufficient force to break up agglomerates.
Bead mills are used for large batches and can maintain semi-continuous output. Operation is similar to a ball mill but the vessel is smaller and balls are 3mm diameter zirconium dioxide beads, and mixing is at a higher speed producing more rapid output.
Bar mills are especially useful for highly viscous (thick) mill-bases. The mill-base is forced through a small clearance between a rotating water cooled roller and a bar pushed against the roller.
Part 3 – Let-down
In a separate, larger, vat the rest of the paint (resin, solvent and additives) is combined and mixed. This is called the let-down. When the let-down and the mill-base are completed, the mill-base is added with stirring to the let-down. At this stage, if required by the formulation, any final additions are made and added in.
Part 4 – Finished product and in process laboratory testing
Product quality is monitored throughout the manufacturing process by the Product Verification Laboratory. Critical ingredients are tested before manufacturing starts. The mill-base may be tested for dispersion; if necessary further processing may be required. The let-down may be tested to ensure it is sufficiently mixed.
The completed batch (mill-base plus let-down and any final additions) is thoroughly tested by the Product Verification Laboratory. These final tests evaluate properties such as degree of dispersion, viscosity (consistency), density, hiding, tint strength and colour, application, dry time, gloss and dry film appearance.
Part 5 – Canning
When testing is completed the batch is passed for canning. During canning two samples are taken; a retain sample, which is stored in case it is needed for future reference, and a Final Inspection sample. The Final Inspection sample is tested in the Resene Final Inspection Laboratory to guarantee conformance to relevant standards and specifications, such as APAS, Environmental Choice, etc. After this final stage of testing is complete the batch is passed for use in the warehouse and dispatch to branches as required.
How does paint work?
The simplest types of paint are lacquers that form a film by evaporation of solvent.
Waterborne paints, such as Resene Hi-Glo, are usually based on emulsion resins that consist of trillions of tiny resin particles, about 1/100th of the diameter of a human hair. As the water in these paints evaporates the resin and pigment particles get closer and closer together until they begin to touch each other. When the resin particles touch each other and the pigments, they stick together and fuse into a tough elastic solid, which we recognise as the paint film.
Solventborne enamel paints, such as Resene Super Gloss, are based on alkyd resin dissolved in solvent (mineral turps). When the solvent evaporates the first stage is the formation of a tacky lacquer. The alkyd resin progressively reacts with oxygen from the atmosphere and polymerises to form a hard, tough coating.
Two component protective coating paints are unreactive on their own, but when mixed together undergo a chemical reaction. The chemical reaction takes a few hours (depending upon temperature) and results in an extremely tough, hard coating with great adhesion. These paints generally require specialist surface preparation and application, and are often used in extreme environments.
The boundaries between waterborne paints and solventborne enamels, and other reactive coatings, are becoming blurred as new technologies develop. For example, Resene Enamacryl and Resene Lustacryl consist of waterborne emulsion resins and chemistry that produces polymerisation of the dried film similar to solventborne enamels.
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