Sticking Together. Steven Abbott

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      Figure 2.13 When we use superglue, the polymerization of the cyanoacrylate proceeds via a series of anions (negative charges) instead of radicals.

      The key difference is that for radical systems the initiation comes either from molecules such as dibenzoyl peroxide that like to break down into radicals with temperature (thermal initiators), or via molecules (photoinitiators) that are split into radicals via the energy of UV light. For anionic polymerization, we need a “base” (the opposite to an acid) which can be a negatively charged HO⁻ group from water or an amine group. Common “accelerators” for cyanoacrylates are water, water made basic with sodium hydroxide or sodium bicarbonate, or a solution of an amine. The cyanoacrylates can also polymerize via radicals, so tubes of glue often contain a radical inhibitor to avoid the risk of them going solid before the tube has been opened for the first time by the user.

      A different type of polymerization occurs when molecules are set up to react repeatedly at each end to create a polymer chain. One common type of reaction, mentioned earlier, mixes an acid with an alcohol to create an ester. If you mix a di-acid with a di-alcohol, the “condensation polymerization” can carry on indefinitely, to create a polyester (Figure 2.14).

      Figure 2.14 This is a condensation polymerization where A–A reacts with B–B to from A–AB–B, and where the A and B end groups of the new molecule are free to carry on reacting.

      Or if you start with a cyclic molecule called an epoxy, an alcohol group can react with one side of the triangle, creating a new alcohol group that can further react (Figure 2.15).

      Figure 2.15 The anion RO⁻ reacts with the triangular epoxy, opening the ring to create another O⁻, which can react further. In epoxy adhesives the epoxy rings react with amines, a much faster reaction.

      This is one form of epoxy adhesive. If a di-epoxy is mixed with a molecule containing a di-amine then these react rapidly to form a polymer. A dual tube epoxy has the di-epoxy in one tube and the di-amine (plus some tri-amine to create crosslinks) in the other.

2.11 CROSSLINKED POLYMERS

      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.

2.12 WATER AS THE INITIATOR

      The beauty of polymerization is that you just need to get it going and then it carries on, in principle, until all the monomer is used up. In practice, polymerizations terminate when they meet impurities or another reacting chain, so we need a moderate number of initiators to get a good conversion. For superglues, urethane glues and silanes the first step of the polymerization reaction can be initiated by water molecules. This is wonderfully convenient and is an element of the popularity of these types of adhesive. It is also frustratingly inconvenient.

       It is convenient because there is usually enough moisture in the air or sitting on the surface of the adherends to start things off. No need to add any other chemicals (“activators”). If the atmosphere or surface is too dry, a huff of moist breath or a spritz with water does the job.

       It is inconvenient because opening the adhesive package to squeeze out the adhesive allows at least a few water molecules to get into the bulk of the adhesive, which will eventually go solid.

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