Chapter 5: Strong Adhesion

The most obvious way to make a strong bond is by using a very strong adhesive. This is usually a bad idea because strength is often linked to brittleness. If a crack starts to break the adhesive interface, in a brittle material it just carries on. Instead, the key to strong adhesion is dissipation of crack energy. So it is often a weaker material, or one with fewer strong bonds across the interface, that gives better adhesion. Rather than “strength”, the key word for strong adhesion is “entanglement”. Once we understand that dissipation via entanglement is the key to adhesion, we can arrange our system so that attempts to break it cause the maximum stretching and sliding of bonds across the interface. By going through the main classes of adhesives (cyanoacrylates, epoxies, urethanes, wood glues) we see how they each accomplish the mission via various tricks. We find out why most general-purpose adhesives say “Will not stick to polyethylene” and why many of our adhesives contain some forms of silicon-based chemistry. If you have ever been puzzled by those complicated ingredients listed on a tube of adhesive, we have an instant guide. And we find that the biggest barrier to a great general-purpose adhesive is us, the users.