Cross linking is the interconnection of polymer chains by chemical bridges or bonds, for example under the action of radiation, in order to make a network of higher molecular mass with different physical-chemical properties from the initial polymer, e.g. insolubility in solvents;
Cross linking causes a thermosetting resin to go from the paste form to the solid state. Cross linking is a final product of polymerisation and is irreversible.
In thermosetting polymers, cross linking corresponds to the formation of a three-dimensional macromolecular network.
The polymer passes from a paste form (viscoelastic) to a rigid, elastic and infusible solid state. The gel state is the beginning of cross linking. The cross linking mechanism differs according to the resin but only applies to multifunctional molecules.
It can be by:
• thermal or catalytic activation of reactive sites (phenolic resins of the resol type);
• cross linking by the addition of a reactive monomer in the resin (polyester) ;
• reaction with a hardener (epoxides).
The vulcanising of rubber is an example of cross linking.