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.
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According to historians and archeologists, adhesives have been usedfor thousands of years—probably since Stone Age cave dwellers first applied bitumen (atarry substance used to surface highways) to stick flint axeheads tothe tops of their wooden hunting spears. In ancient times, people madetheir glues from whatever they found in the world around them—suchthings as sugar, fish skins, and animal products boiled in water.
We still use some of these natural adhesives today, thoughwe"re much more likely to use artificial adhesives made in a chemicalplant. It"s obvious modern glues are chemical products from thehorrible names they have—polyvinyl acetate (PVA), phenolformaldehyde (PH), ethylene vinyl acetate (EVA), and cyanoacrylate("super glue") to name just four. Many modern adhesives are called synthetic resins for no good reasonother than that resin (a gooey substance found in pine trees and otherplants) 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 itworks. That was a lesson the Nobel-Prize-winning American physicist RichardFeynman (1918–1988) often used to teach. So let"s forget all aboutadhesives, acetates, and acrylates and try to figure out why one thingwill stick to another. If you want a short answer, the word is "forces."
People stick to Earth"s surface even though the planet is rotatingat high speed, and even there"s no glue on the soles of our feet. Thereason is simply that gravity bonds us to the planet with enough forceto stop us whizzing off into space. But gravity isn"t enough to keep uspermanently in place. If we supply bigger forces, for example by usingour 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 agiant living Post-it® note—only with legs!
So you don"t always need a blob of adhesive to stick thingstogether. That much is blindingly obvious whenever it rains on yourwindow.Gravity tries to pull the water down to the bottom of the glass, andsooner or later it usually wins, but two interesting things try to stopit. 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 thatmake them do this are called cohesiveforces (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 adhesiveforces (the stickingprocess is called adhesion). Now the cohesive forces must be biggerthanthe adhesive forces or the water wouldn"t form droplets at all.Instead, it would just spread out in a very thin layer on theglass—much as oil does when you spread it on water. But the adhesiveforces are still pretty strong: some of the water droplets that stickto 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 howthe rain naturally clumps into droplets (because of cohesion), whichremain on the glass (because of adhesion). The drops fall down thewindow only when they"re too heavy for the adhesive forces to keep themin place (when the gravitational force pulling them down is greaterthan the adhesive force holding them up). Notice how they run down thewindow in distinct tracks, with droplets following existing, waterypaths. That"s because the water drops that are falling are tryingharder tostick 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 downthe glass, cohesive forces tear some of the water molecules away frompassing drops, leaving behind droplets that are small enough to stick to the glass(adhesion again).
Adhesive and cohesive forces in gluesArtwork: Adhesive and cohesive forces both play a part in sticking things together.
What does all this have to do with adhesives? Adhesive and cohesiveforces are also at work in glues. Let"s say you want to stick togethertwo bits of wood, A and B, with an adhesive called C. You need threedifferent forces here: adhesive forces to hold A to C,adhesive forces to stick C to B, and cohesive forces to hold C togetheras well. The first two are pretty obvious: the glue has to stick toeach of the materials you want to hold together. But the glue also hasto stick to itself! If that"s not obvious,think about stickinga training shoe to the ceiling. The glue clearly has to stick both tothe 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 ceilingbecause it will simply break apart in the middle, leaving a layer ofglue behind on both surfaces. That"s a failure caused when the adhesiveforcesare greater than the cohesive ones and the cohesive forces aren"t bigenough to overcome the pull of gravity.
Jam sandwiches may not be the first thing to spring to your mindwhen you think about adhesives, but the jam is working as a kind ofglue. It"s made of sugar and water: a classic adhesive recipe usedsince ancient times. If you use fairly strong bread, you can pick up ajam sandwich by just one corner of one slice and the whole thing willstay together in your hand—thanks to the jammy glue. Jam has prettyhigh cohesive forces (that"s why jam can be hard to dig out of the jarwith your knife), but its adhesive forces are high too. If you buttertwo pieces of bread and cover one slice with jam, then close upthe sandwich, then peel it apart, you"ll find there"s some jam left onboth surfaces. As you pull apart the sandwich, you"ll find the jambreaking itself in two in lots of little strands. That"s because theadhesive forces are strongerthan the cohesive ones. Your jam sandwich "fails" due to a failure ofcohesion.
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 somethingglues 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 wetand the coaster is light. That"s because the adhesive and cohesive forces involved—holdingthe coaster to the glass—are greater than the coaster"s own weight. But you can"t use waterto glue a coaster to a block of wood or a lump of metal. You can"t glue yourself to the ceilingwith water, though an insect might be able to manage it.
How do cohesive forces work?
Now we know that adhesives work through adhesive and cohesiveforces, we need to understand a bit more about how those forcesthemselves work. Let"s start with cohesive forces.As you can discover in our main article about the magic of water, water molecules jointogether with one another because they"re not symmetrical. One end has aslight positive charge, the other end has a slight negative charge, andthe positive and negative ends of different molecules snap togetherlike the opposite ends of magnets. That"s a kind of electrical orelectrostatic bonding. In metals, the atoms are strongly held togetherin a rigid crystal structure called a lattice (a bit like scaffoldingor a climbing frame with atoms at the joins and invisible bars holdingthem together). You can easily separateone "piece" of water from another (by lifting some out with a spoon):the cohesive forces are quite weak. But you can"t easily separate onebit of iron from another (with a spoon or anything else) because thecohesive forces are incredibly strong.
Water and iron are both pretty useless as glues, but for quitedifferent reasons. Water could be an excellent glue because it sticksquite well to other substances (such as glass), but its cohesive forcesare incredibly weak. You can stick paper to the wall by wetting itfirst, but you can usually peel it off quite easily too. When you peel,you"re breaking the weak cohesive forces that hold one water moleculeto another. Iron is no good as a glue because it"s too preoccupied withsticking to itself to stick to anything else. All its forces areoccupied internally, fixing one iron atom to another in a strongcohesive structure. There"s nothing it can use to attach itself toother 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 tosomething else? You may be surprised to hear that there"s no single,simple answer—but that"s not so surprising if you consider how manydifferent types of glue there are and how many different ways in whichwe can use them. For each different glue, and each different surface weuse it on, scientists think a combination of different factors are atwork holding the two together. But the plain truth is: no-one exactlywhat"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 ofvery weak electrostatic forces between the glue molecules and themolecules in the surface (called van der Waals forces for the physicistJohannesDiderik van der Waals (1837–1923) who discovered them) hold the twothings together. For adhesives towork well like this, they have to spread thinly and wet the surfacesvery well. There"s no actual chemical bond between the glue and thesurface it"s sticking to, just a huge number of tiny attractive forces.The glue molecules stick to the surface molecules like millions ofmicroscopic magnets.
In some cases, adhesives can make muchstronger chemicalbonds with the materials they touch. For example, if you use certainglues on certain plastics, the glue and the plastic actually mergetogether toform a very strong chemical bond—they effectively form a new chemicalcompound at the join. That process is called chemisorption.
Adsorption and chemisorption are chemicalconnectionsbetween the glue and the surface. Glues can also form physical (mechanical)bonds with the surface they"re sticking to. Suppose the surface isporous (full of holes). The glue can seep into those holes and gripthrough them, like a climber"s fingers grabbing holes in a rock face.That"s called the mechanicaltheory of adhesives.
Another theory of how glues work suggests the adhesive can diffuseinto the surface and vice-versa, with molecules swapping over at thejoin and mingling together. This is called the diffusiontheory.
How do Post-it® notes work?Photo: Post-it® notes attach themselves with help from lots of "microcapsules" (tiny microscopic bubblesof adhesive) on the reverse, which are much larger than the glue particles on conventional sticky tape.
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 microscopeand 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 glueparticles you"d find lazing around on normal sticky tape.When you push a Post-it® onto a table, some of theserelatively large sticky capsules cling to the surface, providing just enough adhesive force to hold the weight ofthe paper in the little note. Every time you attach and peel off the note, dust and dirt attach to the adhesivecapsules, 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.
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 notbefore. Different adhesives achieve this in different ways. Some aredissolved in chemicals called solventsthat keep them stable and non-sticky in the tube. When you squeeze themout, the solvents quickly evaporate in the air or get absorbed by thesurfaces you"re sticking to, freeing the adhesives themselves to dotheir job. Plastic modeling glue works like this. It containsmolecules of polystyrene in an acetone solvent. When you squeeze thetube, the glue spurts out and you can usually smell the very strongacetone as it evaporates. Once it"s gone, thepolystyrene molecules lock together to make strong chemical bonds. Gluedoesn"t smell when it"s dry because all the solvent has vanished intothe air. Someglues (such as synthetic, epoxy resins) have to be mixed togetherbefore they work. They come in two different tubes, one containing thesynthetic resin and the other containing a chemical that makes theresin harden. The two chemicals are useless by themselves but, mixedtogether, form a tough, permanent adhesive.
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Photo: 1) Stick adhesives are solvent-free andvery safe to use. 2) Spray-on adhesives often contain harmful solvents and it"s agood idea to wear a safety mask or use them outdoors. Photo courtesy of Defense Imagery.