Avoiding Sticky Situations: The Science of Adhesives

Adhesives play a pretty important role in our everyday lives; even if we aren’t always aware of them, they’re present in the world all around us. Whether it’s the wood glue holding our tables and chairs together, or the duct tape that we haphazardly throw on something in a desperate attempt for a quick fix, adhesives have contributed a lot to modern society. But how exactly do these miracle sticky substances work? Let’s take a look at the science behind adhesion and different types of adhesives.

Micro Photo of Glue

Let’s begin with the history of adhesives. The first uses of adhesive substances may date all the way back to Ancient Egypt, where they used natural glue made from animal collagen. Other ancient societies used things like tar and tree resin. Other natural substances, like beeswax, have been used as adhesives for many years as well.
But what exactly are these adhesive substances, and what makes them so sticky? An adhesive can broadly be defined as “a substance capable of holding materials together by surface attachment.” They are usually a liquid, solid, or paste and attach to (or stick) materials in three ways:

  • mechanically
  • chemically and/or
  • electrostatically

Their degree of stickiness can change depending on the combination of these interactions.

The most recognizable type of adhesive is tape, which is a pressure-sensitive adhesive (PSA), a type of polymer that has a very high viscosity and some elastic characteristics. This means that the PSA has a high resistance to flowing, like pancake syrup, and can resist outside forces, retaining its shape if it is moved like a rubber band. Despite being a solid, PSAs have some liquid characteristics, so they will “wet” a surface when applied to it, but they will also resist separation when stressed because of their elasticity. The resulting stickiness is enhanced and removable due to this “viscoelastic” characteristic.
There are two major interactions that contribute to PSA’s stickiness: mechanical (the wetting process) and electrostatic (Van der Waals forces). Wetting means a solid adhesive can spread across and be absorbed into the material to which it is being applied. Think of water beading up (non wet) on the surface of a freshly waxed car versus spreading out on a wooden table (wet). Wetting typically occurs because the surface tension of a substance is lower than the surface on which it is. Because solid adhesives have a low surface energy, they are able be absorbed into the material, allowing for the surface molecules of the adhesive to flow easily into the pores of the material across a wide area.
As this is happening, the tape is also electrostatically binding to the material with the creation of Van der Waal’s forces- weak attractions between usually neutral molecules whose charges are not evenly distributed, creating a dipole moment. These charges, or polarities, enable the molecules to form bonds with other polar molecules. The molecules of PSAs exhibit these dipole moments and consequently create corresponding dipole moments in the molecules of the surface to which they are binding. Therefore, all it takes is a little pressure in order to force these physical bonds between the material and the adhesive. This also means, however, that no chemical reaction takes place and it is strictly a physical phenomenon.
Glue, on the other hand, is a chemical and sometimes mechanical interaction which has stronger bonds. When you use glue to stick two materials together, the chemistry of the glue can change so that the glue molecules permanently bind to themselves (hardening). This means that these bonds are irreversible; if an object breaks after glue has been applied to it, the same glue cannot be used to put it back together. The bonds formed by glue are stronger and more permanent than the bonds formed by tape and other similar adhesives.
Although we have come a long way from using beeswax and animal collagen as adhesives, we are still searching for more and better adhesive materials. Niels Holten-Andersen, John Chipman Assistant Professor of Materials Science and Engineering at MIT, is currently researching the adhesive powers of aquatic animals. “Mussels and shellfish produce fibers that allow them to stick to ships and rocks and anything they want to grow on,” Holten-Andersen says. Understanding how these organisms create adhesive fibers could prove helpful in creating synthetic glues that will work underwater, and could have numerous medical applications, such as for organ transplants. Clearly, adhesives are a force to be reckoned with!

Brinson, H. F. Adhesives and Sealants. S.l.: ASM International, 1990. Print.