A Staff Report from the Straight Dope Science Advisory Board

How does glue work?

August 1, 2006

Dear Straight Dope:

This is a question I have often pondered . . . how does glue work? What makes it bind things together?

We're not entirely sure.

That's a little surprising, I know. Glues (more properly called adhesives) have been around for thousands of years. Beeswax and tar were among the earliest--there's an extensive archeological record of their use. Later came adhesives derived from plants and animals--for example, carpenter's glue made from slaughterhouse leftovers such as hides and bones. Modern manufactured adhesives arrived on the scene around 1910, when phenol formaldehyde adhesives were developed for making plywood. Next came acrylic adhesives, cyanoacrylates (also known as "super glue"), epoxies, and so on.

A good adhesive has excellent properties of adhesion (the ability to stick to the surfaces to which it's applied) and cohesion (the ability to stick to itself). When you pull apart something that's been glued together and the glue comes right off the pieces, that's an adhesive failure. If the glue itself splits apart, leaving glue on either side of the joint, then that's a cohesive failure:

Adhesion and cohesion are both important for an adhesive to work. For example, while tar has high adhesion (as anyone who has worked on a roof can testify), it's not so hot at cohesion. While it can be used for some low-strength applications, it's not really a high-strength adhesive.

Now to your core question. Research continues to this day on exactly what happens when two objects stick together. There's no universally accepted theory, and given the variety of adhesives more than one process may be at work. It's generally agreed that adhesion occurs at the molecular level, the chief processes involved being Van der Waals forces, ionic bonding, covalent bonding, and metallic bonding. The last three types of bonding are the result of chemical reactions that don't have much to do with glue, so we'll confine our discussion to Van der Waals forces, with a little excursion into mechanical bonding thrown in.

Van der Waals forces come into play when molecules are polarized--that is, they have a positive end and a negative end. The following graphic shows the polarity of a water molecule due to the angle of 104.5° between the two hydrogen atoms. Because the hydrogen atoms are skewed to one side, one end of the molecule has a weakly positive charge and the other has a weakly negative charge. Since opposites attract, the positive side of one water molecule will be attracted to the negative side of a neighboring one. That's the Van der Waals force.

For Van der Waals forces to work, there must be close contact between the two surfaces being joined--a separation of only a few angstroms. (An angstrom is one ten-billionth of a meter.) At that range the Van der Waals and other molecular forces interact strongly and the adhesive and the glued surfaces bond together. To ensure close contact, the adhesive must have the ability to wet the surfaces of the parts properly--it must spread and flow such that it maintains the maximum contact area possible, and it should spread thinly to avoid being subject to cohesive failures.

According to recent research, Van der Waals forces explain how the lizards known as geckos can stick to so many surfaces in a seemingly impossible manner. Specifically, the tiny hairs on the gecko's feet (called setae) are split at the microscopic level into "as many as 1,000 branches, whose spatula-shaped tips are only 200 nanometers wide." As a result, even though the Van der Waals forces acting on an individual tip is small, the adhesion of a billion or so tips adds up to enough force to let the gecko stick to anything.

Persuasive as all this sounds, other scientists doubt that Van der Waals forces alone fully explain how glue works. They attribute at least some of the strength of adhesives to mechanical bonding, also known as mechanical locking. The idea here is that all surfaces, even seemingly smooth ones, are actually rough at the molecular level, and that an adhesive achieves some of its strength by flowing into the hidden valleys and imperfections of the surfaces to be bonded. When the adhesive hardens, the two parts are mechanically locked together, like so:

In mechanical bonding as with Van der Waals forces, an adhesive needs to be able to spread finely into the microscopic roughness of the surfaces, wet the tiny cracks properly, and allow trapped air to escape. The main difference is that mechanical bonding will work at distances that would render Van der Waals forces useless.

There are several other theories of adhesion that I ought to mention briefly. The electrostatic theory arose in part from the observation that some adhesives generate static sparks when pulled apart, while other items will cling together when placed next to each other, especially in the case of plastics. The theory proposes that as the joint forms, electrostatic charges develop between the two joined surfaces and between the surfaces and the adhesive, and these electrical forces help hold the joint together. But electrostatic forces are believed to be only a supplemental source of joint strength, not the primary one.

Adhesion can also occur when both the adhesive and the parts being joined have long chains of molecules that are chemically soluble in each other. In this case, it's believed that some of the long molecule chains at the surfaces will diffuse or interlink physically into each other, working like dog hair into the fibers of your couch. Again, this is not thought to be a primary source of strength in adhesive joints. Still another type of adhesion involves the presence of tiny air bubbles--in 1999, French researchers presented a theory in Discover magazine that instant adhesives (like that used in Scotch-brand tape and Post-It notes) actually work by creating numerous microscopic bubbles each having a partial vacuum in them, which act as suction cups.

Lastly, some glues such as those used for plastic hobby models chemically weld pieces together. These glues slightly melt and diffuse into the parts being joined. This can make a very solid joint, but at the cost of changing the structure and shape of the pieces being joined, so they're not suitable for some uses. But they're fine for assembling model ships.

To summarize, the primary force at work in most cases of adhesion is most likely Van der Waals forces resulting from polarized molecules, coupled with some degree of mechanical locking. The proper amount of surface roughness seems key in making a strong adhesive joint--while a rough surface is useful because it has a large surface area and thus a high potential for mechanical locking, it's difficult to clean, allowing impurities and trapped air to weaken the joint. A smooth surface allows for easy cleaning and liberation of air but reduces opportunities for mechanical locking. Each glue has an optimal level of roughness and certain types of surfaces to which it's best suited, so follow the manufacturer's recommendations to make sure your adhesives adhere.

References:

Petrie, E.M., Handbook of Adhesives and Sealants, 2000

Kunzig, Robert, "Why Does It Stick? - the hypothesis that push-on adhesives use bubbles to create vacuum," Discover, July 1999.

Ben-Ari, Elia, "Geckos Yield Their Sticky Secrets," BioScience, October 2002.

Kellar Autumn, Metin Sitti, Yiching A. Liang, Anne M. Peattie, Wendy R. Hansen, Simon Sponberg, Thomas W. Kenny, Ronald Fearing, Jacob N. Israelachvili, and Robert J. Full. "Evidence for van der Waals adhesion in gecko setae." Edited by Thomas Eisner, Cornell University, Ithaca, NY, and approved July 9, 2002, Proceedings of the National Academy of Sciences of the United States of America.

Parmley, Robert O., Standard Handbook of Fastening and Joining, 1989.

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