Paint
by Chris Woodford. Last updated: April 15, 2023.
If you don't like it, paint over it. It's certainly true that paint has the power to change things. You can brighten up a room with a colorful picture in much the same way that you can make your house look more attractive with a fresh coat of paint. This is probably why we think of painting as a kind of "alchemy"—a way of using chemicals to change something we don't like into something we do. But have you ever stopped to think what paint is or how it works, what chemicals it contains and what they actually do? Let's take a closer look!
Photo: An artist's oil painting palette is as much a chemistry set as a playful collection of colors. "Artistic" paints are much more about color than anything else. If you're painting the outside of your home, it's a whole different story: here, you're much more concerned with applying a surface protective coating to wood or metal, which needs to look attractive only as a secondary consideration.
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Contents
What is paint?
Paint is protection—much more than just color in a tin or a tube.
Photo: Paint for protection: Painting the hull of the USS Blue Ridge to protect it from seawater. Photo by Patrick Semales courtesy of US Navy and DVIDS.
You don't necessarily paint things to make them look better. You paint the outside of your home to keep the rain out. We paint cars and bicycles partly to make them look good, but also to stop the metal inside them from going rusty. On airplanes and space rockets, paint is one of the things that protects the structure when air, rushing past at high speed, creates friction and heat.
What sort of chemicals are inside paint?
You might think paint is just a color chemical dissolved in a liquid to make it spread, but it's a bit more than that. Most paints actually have three main components called the pigment, the binder, and the solvent. (The binder and solvent are sometimes collectively called the vehicle.) There are also typically a number of additives to improve the paint's properties in various ways, depending on where and how it's going to be used.
Artwork: Paint is made of a pigment, a binder, and a solvent. The binder holds the pigment together; the solvent turns the binder and pigment into a thinner, easier-to-spread fluid.
Pigments
The pigment is the color chemical in a paint. It looks a certain color because it reflects some wavelengths of light and absorbs others (see our article on light for an explanation of how colors work). Traditionally, metal compounds (salts) are used to create different colors so, for example, titanium dioxide (a bright white chemical often found in sand) is used to make white paint, iron oxide makes yellow, red, brown, or orange paint (think of how iron turns rusty red), and chromium oxide makes paint that's green. Black (arguably not a color) comes from particles of carbon (think what your burned toast looks like and you're getting close to a color chemical known as "carbon black"). Different pigments are mixed together to make paint of any color you can imagine.
Photo: You might think white paint doesn't contain any color so it doesn't need any pigment. Actually, it needs as much pigment as any other paint. Titanium white paint is so-called because it's made with titanium dioxide pigment.
Binders
Pigments are typically solids, so you couldn't use them to paint by themselves. They'd be difficult to apply, they wouldn't spread evenly, they wouldn't stick to paper or a wall, and they'd wash straight off if they got wet. That's why paints also contain substances called binders. Their job is to glue the pigment particles to one another, but also to make them stick to the surface you're painting. Some binders are made from natural oils such as linseed oil, but most are now made from synthetic plastics (originally they were made from rubber, which is why we still talk about "latex paints" today). Visualize the binder as an invisible skin of plastic with a colorful pigment dispersed through it and you can see just how a paint gives a layer of protection.
Solvents
Mix a pigment and a binder and you get a thick gloopy substance that's difficult to spread. Ever tried painting a wall with treacle? That's what using a pigment and a binder is like. It's the reason why paints have a third major chemical component called the solvent. As its name suggests, a solvent is something that dissolves something else. The solvent's job is to make the pigment and binder into a thinner and less viscous (more easily flowing) liquid that will spread evenly (that's why paint solvents are sometimes called thinners). Once the paint has spread out, the solvent evaporates into the air, leaving the paint evenly applied and dry beneath it. When you apply a really nasty paint and there's a smell lingering for days while it dries, that's the solvent evaporating into the air.
Photo: Gloss paint uses oil-based solvents so it spreads evenly. It's usually much thicker and more opaque than water-based emulsion and the oily solvents have a powerful smell that can linger for days afterward.
Water is the best-known and most versatile solvent we have and it's widely used in water-based paints, including emulsions (for walls) and watercolor paints (for paintings). When you paint a picture with watercolors, you're using water as a solvent to dissolve some pigment on your brush that you can easily spread on the paper. One big advantage of water-based paints is that they're relatively easy to clean up if you spill them (and generally they wash out of clothes).
Other paints (including oil and gloss paints) use solvents made from strong organic (carbon-based) chemicals extracted from petroleum, such as naptha. If you leave paints sitting in tins and jars, gravity gradually separates them into their different chemical components. Typically you find the solvent sitting on top as a reasonably clear, thin fluid with the binder and pigment making up a thick, opaque sludge underneath. That's why it's always important to stir tins of paint before you use them. Oil-based paints are harder to clean up if you spill them on things like clothes or carpet. (Water won't be much use; you'll need an organic solvent to dissolve them, such as white spirit.)
Additives
Apart from the pigment, binder, and solvent, most paints also have chemical additives of various kinds. Some of these are designed to protect the paints themselves—to stop them from degrading in ultraviolet light (from sunlight), repeated heating and cooling in summer and winter, rain (which is slightly acidic), or from the harsh environmental conditions you sometimes find in places like factories and chemical plants. For example, ceramic substances can be added to paints to improve their strength and durability.
Often additives have very specific jobs to do. Fluorescent pigments added to paints make them glow in the dark. Additives in paint designed for outdoor use can help to make things waterproof and rustproof, protect against frost or sunlight, and keep them free of mold and mildew.
How paints dry
If you've ever painted the walls and windows of your home, you'll be well aware that different paints "dry" in different ways. The kind of water-based emulsion paints you use on walls dry more quickly (in perhaps 2–4 hours), whereas the oil-based gloss paints you use on window frames seem to dry much more slowly (16–24 hours). What's the difference? There are two quite different drying processes going on.
Water-based paints literally dry in a two-step process: the watery solvent evaporates and the binder starts to do its job, tugging the pigment particles together to form a solid coating.
Photo: When water-based paints dry, the water quickly evaporates and the binder and pigment particles pull together, forming a hard, uniform coating.
Oil paints work differently: they "cure". In other words, the oils they contain react with oxygen in the air to form cross links that harden them (a bit like the way cross-links form in rubber when it vulcanizes (hardens, through the addition of sulfur and heat).
Photo: When oil-based paints "dry", oxygen in the air helps to form cross-links between the paint polymer molecules, making them harder from the outside in a much slower process.
This cross-linking is what makes the skin on gloss oil paint as it "dries" from the outside in—and why drying is such a long process. Even when a surface skin has formed on oil paint, there can still be uncured paint just underneath. That's why you really need to allow anything from a few days to a week or more for your gloss-painted windows to dry before you close them very tightly or put things down on a newly glossed surface.
Photo: Oily gloss paint "dries" by forming a surface skin that gradually hardens. Oily gloss "dries" by curing (reacting with oxygen in the air) and hardening. Unlike with water-based paint, there is no water in the paint as a solvent at the start of the process. Interestingly, however, the curing process is an example of what's called condensation polymerization, in which water is formed and given off as the paint hardens. If you disturb the outer skin too quickly, you'll find there's still "wet" paint underneath. Here I'm poking the skin off a thin patch of white gloss paint with a screwdriver and you can see the paint is still "wet" beneath it even a week later.
