What makes an adhesive stick?

by Jerome Laux

The answer is—it depends on the adhesive. There are many forms of adhesives, and this article discusses some of those typically used with wood.

To start: consider the surface of the wood. Here is a picture of the side grain of a piece of sapele.

See all those nice fibers and rough places as well as voids? Lots of nice sticking places for your adhesive to stick to. Here is a picture of the end grain.

Polyurethanes

One-part (i.e. Gorilla Glue) Polyurethanes have a somewhat long urethane molecule that has an end isocyanate group dissolved in a suitable solvent. Water or a suitable hydroxyl group comes along and activates it. Ideally you get a nice, long bulky chain (the growing chain often cross-links with another chain giving it bulk). This can intermesh with protruding wood fibers (aren't you glad you sponged it with a little water to raise the grain?). If you are very lucky, it chemically binds with the molecules making up the wood fibers.

Two-part - In these, you have a polyol component, which can consist of long chain polyalcohols. This is usually polyester based or acrylic based. You have an isocyanate component, which can consist of TDI (toluene di-isocyanate), MDI or more complex isocyanates. The isocyanate component reacts with the polyol component creating a nice long bulky chain. It can intermesh with the wood fibers and hopefully react with some of the molecules making up the wood fibers.

PVAC Adhesives

These are typically emulsions or dispersions of polyvinyl acetate (PVAC) spheroids in water, polyvinyl alcohol and a few other ingredients. Think Elmer's yellow or white or Titebond when you think of PVAC. As the water from the glue evaporates, the PVAC spheres coalesce into a film, which intermeshes with the wood fibers.

As many people use PVAC adhesives, here is a photomicrograph of a PVAC adhesive on wood.

Casein based adhesives

Again; these are solutions of a material (casein, a protein) in water. Similar to PVA in how it acts. Long bulky chains of molecules are attracted to the wood fibers and intermesh with them as the water dries.

Hide glues

Yet again, typically solutions of a material (collagen and similar proteins) in water. Hide glues hold on to water a little better, which is why they have a longer working life. Long bulky chains of molecules are attracted to the wood fibers and intermesh with them.

The two previous adhesives are solutions of polymers, typically in water. The polymer chains are already formed. As the water goes away, the polymer sets and intermeshes the wood fibers and down into the pores of the wood.

Acrylates

Cyanoacrylates are typically one-part, room-temperature-curable adhesives that are available in a range of viscosities. Examples of this are glues such as Hot Stuff which is a low viscosity glue with a water-like consistency, Super T with a medium viscosity or Special T with a thick consistency. When pressed in a thin film between two pieces of wood or sprayed with a chemical activator, cyanoacrylates cure rapidly to form a rigid bond with most woods.

Cyanoacrylates typically tack cure within one minute (or less) and achieve full bond strength within 24 hours. These types of adhesives are familiar to the public under brand names such as Super Glue, Loctite, and Krazy Glue.

Cyanoacrylate adhesives are cyanoacrylate esters, of which methyl and ethyl cyanoacrylates are the most commonly used in adhesive formulation. Cyanoacrylates polymerize in the presence of a weak base such as water. When the adhesive contacts a surface, trace amounts of water or hydroxyls from the wood fibers on the surface neutralize the stabilizer in the adhesive, resulting in the rapid polymerization of the adhesive. If there is not enough water present or if you desire a more rapid polymerization, you may use an activator. The cyanoacrylate adhesives form bulky polymers, which intermesh with the wood fibers. Cyanoacrylates may also seep into the wood and wood pores and extend out from these pores into the interface matrix. The thinner the monomer (or glue), the more likely it is to seep into the wood. Just remember to use enough to give you a uniform polymer matrix.

There are also some modified acrylate adhesives; which incorporate rubber modifiers to give a more flexible, less rigid bond. This is really nice when joining dissimilar materials such as metal and wood or glass and wood.

Epoxy Adhesives

Epoxy adhesives are typically two-part adhesives consisting of a polyamine or base component and an epoxy component. The reaction of the two components forms the polymer matrix. Due to the large number of hydroxyl groups present in these types of adhesive, they typically have excellent bonding to glass as well as wood. The reaction between the two parts gives you a nice large bulky polymer chain, which can incorporate some of the wood fibers. There are two -part epoxy putty sticks, which consist of the two parts incorporated into a single stick. Pinch off what you need and mix (knead) it thoroughly (great for filling those accidental holes). There is some bonding between the epoxy adhesive and the wood.

Surface preparation

How smooth should your surface be? Obviously, not glass smooth. You want to have enough wood fibers interpenetrating the adhesive polymer matrix to give you a good, resistant bond. As you can tell from the information above, it is the long bulky chains or spheres intermeshing with the wood fibers and down into the pores that does the trick of gluing the wood together.

Wood problems

Some woods will give you problems. An oily wood doesn't play well with glue that has a water carrier. You might have to devote extra time in your surface preparation and even think about rinsing the surface with thinner or alcohol to get rid of some of the oil. Sappy or resinous woods may also cause adhesive problems, so be aware of them.

Solvents, including water may affect the adhesive bond. Be aware which solvents you are contemplating to use and use a compatible adhesive.

Thanks to the technical guys from Rohm & Haas for the education afforded by one of their seminars at work. Thanks too, to Bill Tindall for his advice on PVAC adhesives.