Adhesives (glues)
by Chris Woodford. Last updated: June 25, 2022.
Have you ever stopped to think why glue
doesn't stick to its tube? Have you ever wondered why, when you open up a jam sandwich, there's
jam on both pieces of bread when you put it on only one slice to begin
with? If it's ever bothered you how adhesives work, and why they fail,
you're not alone. That question has taxed some of the world's best
minds since ancient times. Even after all these years, scientists still
don't fully understand how gluey substances make one thing stick to
another, though they've got some pretty good ideas. Let's take a
closer look!
Photo: Without adhesives, all kinds of everyday jobs would be
much more difficult. Adhesive bandages ("sticking plasters") work a bit like sticky tape: they use a pressure-sensitive adhesive on a plastic or textile backing. Historically, bandages like this used "natural" adhesives made from rubber and rosin. Today, they're more likely to use synthetic adhesives such as acrylic resins. These adhesives have to be sticky (but not so much that they rip your skin), water resistant, and
hypoallergenic (not causing an allergic reaction).
Old and new glues
Photo: PVA (polyvinyl acetate) is a typical household adhesive, commonly used for sticking wood together. Here's a small sample that I squirted out, next to its container.
According to historians and archeologists, adhesives have been used
for thousands of years—probably since Stone Age cave dwellers first applied bitumen (a
tarry substance used to surface highways) to stick flint axeheads to
the tops of their wooden hunting spears. In ancient times, people made
their glues from whatever they found in the world around them—such
things as sugar, fish skins, and animal products boiled in
water.
Artwork: Mucilage (a sticky substance, mostly obtained from plants) is a natural glue
that formed the basis of popular adhesives like Carter's Mucilage (pictured here in an 1871 product label).
Courtesy of US Library of Congress Prints and Photographs Division.
We still use some of these natural adhesives today, though
we're much more likely to use artificial adhesives made in a chemical
plant. It's obvious modern glues are chemical products from the
horrible names they have—polyvinyl acetate (PVA), phenol
formaldehyde (PH), ethylene vinyl acetate (EVA), and cyanoacrylate
("super glue") to name just four. Many modern adhesives are called synthetic resins for no good reason
other than that resin (a gooey substance found in pine trees and other
plants) was one of the first widely used adhesives.
Artwork: Flypaper is a simple way of trapping pesky insects on adhesive-coated paper.
Back in the 19th century, you could buy commercial fly paper like this "Sure Catch" (made by J. Hungerford Smith Co. of Rochester, NY, USA), but it was easy to make your own using sticky natural adhesives like molasses
or bird lime (itself made from tree fruits or bark). Photo courtesy of
US Library of Congress Prints and Photographs Division.
How forces make things stick
Knowing what something is called is a far cry from knowing how it
works. That was a lesson the Nobel-Prize-winning American physicist Richard
Feynman (1918–1988) often used to teach. So let's forget all about
adhesives, acetates, and acrylates and try to figure out why one thing
will stick to another. If you want a short answer, the word is "forces."
People stick to Earth's surface even though the planet is rotating
at high speed, and even there's no glue on the soles of our feet. The
reason is simply that gravity bonds us to the planet with enough force
to stop us whizzing off into space. But gravity isn't enough to keep us
permanently in place. If we supply bigger forces, for example by using
our muscles to move our legs and jump in the air, we can "unstick"
ourselves and go somewhere else. Life on Earth is a bit like being a
giant living Post-it® note—only with legs!
So you don't always need a blob of adhesive to stick things
together. That much is blindingly obvious whenever it rains on your
window.
Gravity tries to pull the water down to the bottom of the
glass, and
sooner or later it usually wins, but two interesting things try to stop
it. First, water molecules (two atoms of hydrogen and one atom of oxygen joined together) naturally stick to one another,
so they clump together in big droplets on the window. The forces that
make them do this are called cohesive
forces (and the process involved is called cohesion). Second,
the water droplets also stick to the glass without any help or glue.
Different forces are at work here known as adhesive
forces (the sticking
process is called adhesion). Now the cohesive forces must be bigger
than
the adhesive forces or the water wouldn't form droplets at all.
Instead, it would just spread out in a very thin layer on the
glass—much as oil does when you spread it on water. But the adhesive
forces are still pretty strong: some of the water droplets that stick
to your window are surprisingly big.
Artwork: Cohesive forces stick water drops together, while adhesive forces stick them to your window. These two types of forces pull upward on the bottom drop, helping it to resist the downward pull of its own weight.
Next time it rains, watch how the water behaves. See how
the rain naturally clumps into droplets (because of cohesion), which
remain on the glass (because of adhesion). The drops fall down the
window only when they're too heavy for the adhesive forces to keep them
in place (when the gravitational force pulling them down is greater
than the adhesive force holding them up). Notice how they run down the
window in distinct tracks, with droplets following existing, watery
paths. That's because the water drops that are falling are trying
harder to
stick to the water that's already there rather than to the glass
(cohesion at work again). Why does the rain form those streaky channels? Because as drops fall down
the glass, cohesive forces tear some of the water molecules away from
passing drops, leaving behind droplets that are small enough to stick to the glass
(adhesion again).
Adhesive and cohesive forces in glues
Artwork: Adhesive and cohesive forces both play a part in sticking things together.
What does all this have to do with adhesives? Adhesive and cohesive
forces are also at work in glues. Let's say you want to stick together
two bits of wood, A and B, with an adhesive called C. You need three
different forces here: adhesive forces to hold A to C,
adhesive forces to stick C to B, and cohesive forces to hold C together
as well. The first two are pretty obvious: the glue has to stick to
each of the materials you want to hold together. But the glue also has
to stick to itself! If that's not obvious,
think about sticking
a training shoe to the ceiling. The glue clearly has to stick both to
the training shoe and to the ceiling. But if the glue itself is weak,
it doesn't matter how well it sticks to the shoe or the ceiling
because it will simply break apart in the middle, leaving a layer of
glue behind on both surfaces. That's a failure caused when the adhesive
forces
are greater than the cohesive ones and the cohesive forces aren't big
enough to overcome the pull of gravity.
Jam sandwiches may not be the first thing to spring to your mind
when you think about adhesives, but the jam is working as a kind of
glue. It's made of sugar and water: a classic adhesive recipe used
since ancient times. If you use fairly strong bread, you can pick up a
jam sandwich by just one corner of one slice and the whole thing will
stay together in your hand—thanks to the jammy glue. Jam has pretty
high cohesive forces (that's why jam can be hard to dig out of the jar
with your knife), but its adhesive forces are high too. If you butter
two pieces of bread and cover one slice with jam, then close up
the sandwich, then peel it apart, you'll find there's some jam left on
both surfaces. As you pull apart the sandwich, you'll find the jam
breaking itself in two in lots of little strands. That's because the
adhesive forces are stronger
than the cohesive ones. Your jam sandwich "fails" due to a failure of
cohesion.
Photo: When you put spread on a single slice of bread, make a sandwich, then peel the sandwich apart,
you'll find there's some spread on both slices. This ground-breaking scientific experiment demonstrates a catastrophic cohesive failure of the spread as a glue. Unlike most experiments, it also tastes good.
This illustrates another important point about glue: adhesive is a relative term. Whether something
glues effectively or not depends on the size of the forces it has to hold against.
You can easily "glue" a glass of water to a coaster if the bottom of the glass is wet
and the coaster is light. That's because the adhesive and cohesive forces involved—holding
the coaster to the glass—are greater than the coaster's own weight. But you can't use water
to glue a coaster to a block of wood or a lump of metal. You can't glue yourself to the ceiling
with water, though an insect might be able to manage it.
How do cohesive forces work?
Now we know that adhesives work through adhesive and cohesive
forces, we need to understand a bit more about how those forces
themselves work. Let's start with cohesive forces.
As you can discover in our main article about the magic of water, water molecules join
together with one another because they're not symmetrical. One end has a
slight positive charge, the other end has a slight negative charge, and
the positive and negative ends of different molecules snap together
like the opposite ends of magnets. That's a kind of electrical or
electrostatic bonding. In metals, the atoms are strongly held together
in a rigid crystal structure called a lattice (a bit like scaffolding
or a climbing frame with atoms at the joins and invisible bars holding
them together). You can easily separate
one "piece" of water from another (by lifting some out with a spoon):
the cohesive forces are quite weak. But you can't easily separate one
bit of iron from another (with a spoon or anything else) because the
cohesive forces are incredibly strong.
Water and iron are both pretty useless as glues, but for quite
different reasons. Water could be an excellent glue because it sticks
quite well to other substances (such as glass), but its cohesive forces
are incredibly weak. You can stick paper to the wall by wetting it
first, but you can usually peel it off quite easily too. When you peel,
you're breaking the weak cohesive forces that hold one water molecule
to another. Iron is no good as a glue because it's too preoccupied with
sticking to itself to stick to anything else. All its forces are
occupied internally, fixing one iron atom to another in a strong
cohesive structure. There's nothing it can use to attach itself to
other objects: its adhesive forces are virtually nonexistent.
Photo: Sticky tape (also called Scotch® tape and Sellotape® after two well-known brands)
is simply a pressure-sensitive adhesive on a convenient, transparent, film backing.
How do adhesive forces work?
Now for the real question: what makes a gluey substance stick to
something else? You may be surprised to hear that there's no single,
simple answer—but that's not so surprising if you consider how many
different types of glue there are and how many different ways in which
we can use them. For each different glue, and each different surface we
use it on, scientists think a combination of different factors are at
work holding the two together. But the plain truth is: no-one exactly
what's going on in every case.
Artwork: Four theories of how things can stick. Clockwise from top left: 1) Adsorption is a surface sticking effect caused by small, attractive forces between the adhesive (yellow) and the substances it's sticking (red and blue). 2) Chemisorption involves chemical bonds forming between the adhesive (orange) and the substances it's sticking together. 3) Diffusion sticks two things together when molecules cross the boundaries from one into the other and vice-versa. 4) Mechanical adhesion happens when a glue (green) fills the space between two substances and the cracks inside them, creating a strong physical bond.
One of the main factors is called adsorption.
When you spread adhesive, it wets the surface you apply it to. Lots of
very weak electrostatic forces between the glue molecules and the
molecules in the surface (called van der Waals forces for the physicist
Johannes
Diderik van der Waals (1837–1923) who discovered them) hold the two
things together. For adhesives to
work well like this, they have to spread thinly and wet the surfaces
very well. There's no actual chemical bond between the glue and the
surface it's sticking to, just a huge number of tiny attractive forces.
The glue molecules stick to the surface molecules like millions of
microscopic magnets.
In some cases, adhesives can make much
stronger chemical
bonds with the materials they touch. For example, if you use certain
glues on certain plastics, the glue and the plastic actually merge
together to
form a very strong chemical bond—they effectively form a new chemical
compound at the join. That process is called chemisorption.
Adsorption and chemisorption are chemical
connections
between the glue and the surface. Glues can also form physical (mechanical)
bonds with the surface they're sticking to. Suppose the surface is
porous (full of holes). The glue can seep into those holes and grip
through them, like a climber's fingers grabbing holes in a rock face.
That's called the mechanical
theory of adhesives.
Another theory of how glues work suggests the adhesive can diffuse
into the surface and vice-versa, with molecules swapping over at the
join and mingling together. This is called the diffusion
theory.
How do Post-it® notes work?
So what about that little Post-it® note stuck to your wall? How does that work?
Look at the back of a sticky note using an electron microscope
and you'll see not a continuous film of adhesive but lots of microscopic glue bubbles,
known as microcapsules, which are about 10–100 times bigger and much weaker than the glue
particles you'd find lazing around on normal sticky tape.
When you push a Post-it® onto a table, some of these
relatively large sticky capsules cling to the surface, providing just enough adhesive force to hold the weight of
the paper in the little note. Every time you attach and peel off the note, dust and dirt attach to the adhesive
capsules, so they progressively lose their stickiness. But since there are so many capsules of all different sizes, a Post-it® note does go on sticking for quite a while.
Photo: Post-it® notes attach themselves with help from lots of "microcapsules" (tiny microscopic bubbles
of adhesive) on the reverse, which are much larger than the glue particles on conventional sticky tape.
Why doesn't glue stick to the tube?
Photo: Epoxy glues are made of two substances that become sticky only when you mix them.
Often they're packaged in a pair of syringes joined together, like this.
Adhesives are designed to work when they leave the tube—and not
before. Different adhesives achieve this in different ways. Some are
dissolved in chemicals called solvents
that keep them stable and non-sticky in the tube. When you squeeze them
out, the solvents quickly evaporate in the air or get absorbed by the
surfaces you're sticking to, freeing the adhesives themselves to do
their job. Plastic modeling glue works like this. It contains
molecules of polystyrene in an acetone solvent. When you squeeze the
tube, the glue spurts out and you can usually smell the very strong
acetone as it evaporates. Once it's gone, the
polystyrene molecules lock together to make strong chemical bonds. Glue
doesn't smell when it's dry because all the solvent has vanished into
the air. Some
glues (such as synthetic, epoxy resins) have to be mixed together
before they work. They come in two different tubes, one containing the
synthetic resin and the other containing a chemical that makes the
resin harden. The two chemicals are useless by themselves but, mixed
together, form a tough, permanent adhesive.
Photo: 1) Stick adhesives are solvent-free and
very safe to use. 2) Spray-on adhesives often contain harmful solvents and it's a
good idea to wear a safety mask or use them outdoors.
