By Jenny Lam
If you’ve ever used glue, you’ve probably wondered why glue doesn’t stick to the sides of its containers. Or maybe you've at least heard someone else ask the question. Some may initially think that the plastic of the container plays a role, but glue’s sticky behavior is mainly due to its chemistry and exposure to air.
Glue is a liquid adhesive that dries hard to hold materials together. The glue we commonly use today is technically not a true glue; true glue is made from organic compounds, such as collagen taken from the bones, muscles, and other tissues of animals. In the past, glue was primarily made from horses, but glues today are known as synthetic “glues” or adhesives made with polyvinyl acetate emulsions.
The most well-known type of glue, PVA glue, is commonly referred to as Elmer’s glue. PVA glue is a solution of polyvinyl acetate in water. The water acts as a solvent that keeps the glue in a liquid form while it’s in the container. When one puts glue on a surface, the water is exposed to air and evaporates, leaving the polyvinyl acetate behind to cross-link, hardening into a solid mass in a process known as mechanical adhesion. In a closed bottle, the water is unable to evaporate out of the bottle and causes the glue to be sticky. If one leaves the cap off of a bottle of glue, the water would eventually evaporate, leaving the rest of the glue to become stickier.
Superglue works a little differently, as it is made of cyanoacrylates rather than polyvinyl acetate. Cyanoacrylates react with small amounts of water to produce an intermediate ion that can further react with more cyanoacrylates, resulting in a long polymer chain. Superglue’s stickiness comes from the strength of these polymers, as they are difficult to tear apart. Intermolecular forces between the glue and the surfaces it is sticking to effectively hold the items one is trying to glue together. Superglue gains more strength when its polymer chains form cross-links with each other. The water vapor in the air is enough to react with the superglue’s cyanoacrylates, so superglue has to be stored in a tightly-sealed container to prevent water vapor from coming into contact with the glue. In superglue, water causes the glue to become sticky, whereas it helps PVA glue stay in a liquid form and prevent it from sticking to its container. Cyanoacrylates are particularly interesting because it is not only used for repair or creating arts and crafts projects. A less toxic version of superglue, 2-octyl cyanoacrylate, can be used to temporarily seal wounds, especially in combat situations.
Besides glue and superglue, there are many other types of adhesives, such as glue sticks, which are made of plant-starch-based adhesives, and glue guns, which heat up a copolymer of ethylene and vinyl acetate to melt it. Together, all of these types make up a diverse family of adhesives and rely on the forces of chemistry to operate properly.
Questions:
1. What is mechanical adhesion?
PVA glue uses mechanical adhesion to stick surfaces together. It is when an adhesive flows into the pores of the surfaces, holding them together by interlocking. This means that rough surfaces are able to be fixed easily with glue, but a surface that is too rough can lead to insufficient filling of the structures on the surface. This differs from another common type of adhesion called specific adhesion, in which the molecules of two surfaces form bonds such as hydrogen bonds. In mechanical adhesion, there are no intermolecular forces involved.
2. What is the role of water in glue?
Water plays an important role in liquid adhesives. In PVA glue, water is the solvent that keeps the polymers from forming cross-links and hardening. When the glue is applied to a surface, the water evaporates, allowing the rest of the glue to harden and stick to the surface. In superglue, exposure to water (such as water vapor) causes the cyanoacrylates to form cross-links and harden. In superglue, water has the opposite effect compared to PVA glue. PVA glue bottles are designed to keep water from leaving, whereas superglue containers are designed to limit contact with water.
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Image Credit:
Jenny Lam
No changes were made to the following image, SuperGlue - Guatemala | fumadaestrategica.com/ | Flickr, License: Creative Commons — Attribution-ShareAlike 2.0 Generic — CC BY-SA 2.0