Paint
by Chris Woodford. Last updated: June 27, 2020.
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.
Photo: Paint for protection: Painting the anchor on the aircraft carrier USS John C. Stennis
to protect it from seawater. Photo by Dakota Rayburn courtesy of
US Navy.
What is paint?
Paint is protection—much more than just color in a tin or a tube. 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. For example, ceramic substances
can be added to paints to improve their strength and durability.
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.
Do paints harm your health?
Paints can contain a variety of harmful chemicals, posing not just a health risk to the
people who use them but also to those who may be exposed to them for years afterward—and the wider environment.
Lead (a toxic heavy metal with a variety of
health impacts) was widely used as a paint pigment and additive until environmentally aware countries started banning it or restricting its use (the
United States CPSC banned it in 1977, with the European Union following suit in its
1989 directive 89/677/EEC). Of course, that wasn't the end of lead in paint or the problems it posed: buildings and other things painted with lead continue to pose a risk for years afterward as the paint flakes off and turns to potentially toxic dust. And though banned in some parts of the world, lead paint still frequently crops up in newspaper scare-stories about badly painted children's toys; in 2007, for example, the giant Mattel Corporation had to recall almost a million toys covered in lead paint made in China.
As recently as 2018, over 40 years after its 1977 ban, the US CPSC recalled over 31,000 rubber critter toys because of concerns over "levels of lead that exceed the federal lead paint ban."
Lead isn't the only harmful substance used in paints. Some of the solvents used in paints are VOCs (volatile organic compounds), which evaporate to make localized air pollution and can have a variety of long-term health impacts. Fortunately, many modern "latex paints" now use water as the solvent and a synthetic polymer (plastic) as the binder and are relatively safe compared to old paints.
Finding safer paints
Photo: Making paints safer. This can is labeled "Minimal," indicating that it has less than 0.29 percent VOCs. These days, I consciously pick paints like this, which give off almost no smell.
Paint manufacturers have made impressive efforts to reduce the harmful solvents in their products
over the last few years. In Europe, this trend has been driven partly by consumers (who don't like
foul-smelling paints) and partly by legislation:
European law (EU directive 2004/42/EC) set a maximum limit of 30g per litre of VOCs on water-based paints and related products.
If you look carefully on modern European paint cans (or on the data sheets you find
online), most now reveal whether they are low or high-odor and typically even specify the percentage
of VOCs they contain on a simple scale:
- Minimal VOC: Less than 0.29 percent.
- Low VOC: 0.3–7.99 percent.
- Medium VOC: 8–24.99 percent.
- High VOC: 25–50 percent.
- Very high VOC: Over 50 percent.
A typical household water-based emulsion would likely fall into the second category, while a very low-odor
emulsion might even make the first. Oil-based gloss paints generally rank in the medium or high category, while
metallic radiator paints would score "high" or perhaps even "very high."
Disposing of paint safely
Disposing of old or unwanted paint is also a problem: according to the US Environmental Protection Agency, roughly 10 percent of the paint we buy is never actually used
(that's a whopping 65–69 million gallons in the United States alone). If you simply tip it down a drain, you're adding potentially toxic chemicals to wastewater, risking water pollution in
rivers and oceans, harming aquatic or marine life—and potentially harming human life too when the water cycles its way back into our drinking water supplies. Here are a few sites that give tips on disposing of old paint more safely:
How paints are made
Photo: Watercolor paints are made by dispersing pigments in gum arabic (the binder), which is soluble in water.
Although there are many different types of paint, they are broadly
all made the same way. First, the pigment is prepared. If it's made
from a metal salt such as titanium dioxide, it'll be dug from the
ground as a mineral ore, so it will need to be refined in various
ways to remove impurities. (Having pure pigment chemical is essential
to ensure the final paint has a uniform color.) The pigment chemical
might start off as a lump of rock, so it needs to be ground into a
very fine powder. It may also need to be physically or chemically
treated to change its color in subtle (or not so subtle ways). It
might be roasted, for example, to make it darker. Once it's been
ground to a powder, the pigment is mixed with the binder by a huge,
industrial machine that works a bit like a giant food mixer, and
solvent and additives are added as necessary. That's not the end of
the process, however. Because it's vital that each sample of a
particular paint looks exactly the same color as every other sample,
the mixed paint has to be sampled and compared with previous batches.
If the color isn't exactly right, the factory workers add extra
pigments. Extra solvents are added if the paint is too thick. Once
the paint is the right color and consistency, it can put into cans,
bottles, tubes, or other containers and shipped to the stores.
