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So much for standard linear polymers. They have many desirable properties but often they are not rigid or hard enough for a specific adhesive function. To add rigidity, we make sure that the polymer chains form bonds, “crosslinks” between themselves, hence we call them crosslinked polymers (Figure 2.16).

Figure 2.16 In a linear polymer, each chain is, in principle, independent (in practice they tangle like spaghetti). In a crosslinked polymer, chains are chemically linked to each other, giving very different properties.

To create a crosslinked polymer, whichever polymerization system we choose, the key is to make sure that the basic units to be polymerized contain extra functionality. If a standard unit is termed 1-functional, we can make 2-, 3-, 4-functional equivalents. A typical epoxy system might start with a 2-functional epoxy and a 3-functional amine (Figure 2.17). This will create a somewhat complex network. A 3-functional epoxy and a 4-functional amine would create an even more complex network.

Figure 2.17 Multifunctional starting materials can form complex crosslinked networks.

This sort of trick is especially important in acrylate systems. It happens to be easy to make 2-, 3-, 4-, 5- and 6-functional acrylates. We will see that mixtures of these systems are the key to many adhesive systems, from nail polish to dental fillings.

For these crosslinking processes we often say that the system “cures” (or “sets”) rather than “polymerizes”. It makes no difference which word is used: the process of setting solid or becoming fully cured is a polymerization.