Cure Terms Glossary

Functionality

The functionality of a molecule is the number of sites it has for bonding to other molecules in the reactions of polymerisation or cure. In addition reactions to double bonds (e.g., vinyl polymerisation), the functionality is twice the number of double bonds in the molecule (i.e. each C=C can link to two other molecules). In stepwise reactions (e.g., condensation polymerisation), the functionality is equal the number of functional groups in the molecule. For a molecule to participate in polymerisation, or chain extension, the functionality must be at least two. Purely difunctional ingredients create linear polymers, e.g.,

In the case of conjugated dienes, the two double bonds work in concert to give an overall functionality of two for each pair of double bonds, e.g.,

For network formation, the average functionality of the mix must be higher than two. Thus in chain-growth processes, the ingredient mix must contain reactants with more than one double bond in the molecule (e.g., divinylbenzene, ethylene glycol dimethacrylate, etc.). In step growth processes, some tri- or polyfunctional ingredients must be present. The higher the average functionality at the start, then the higher the likely crosslink density - and the earlier that gelation will occur.

Where the starting material is itself a polymer, and the repeating units along the backbone provide the active sites for cure (e.g. see cis-polybutadiene above), then some exceptionally high functionalities are possible. The risk of premature cure with diene rubbers (scorch in rubber technology) is a constant challenge. But this is not just a problem in rubbers, as the need to balance ease of processing with a suitably rapid cure - whilst achieving the required property profile - is a universal requirement. Processing behaviour and product performance are both critically dependent on functionality.