Characterisation of Crosslinks in Vulcanised Rubbers: From Simple to Advanced Techniques
Crosslink density is a valuable property that affects the functional performance of a vulcanized rubber. It is generally determined by the solvent swelling method. The presence of filler can affect the results of crosslink density determination. While the information on the total crosslink density is useful, when more than one type of crosslinks are present in a vulcanized rubber, further characterization work is necessary to understand the contribution of these crosslinks to the performance properties of the rubber. Therefore, the ionic and covalent crosslinks of carboxylated nitrile butadiene rubber and natural rubber can be quantified individually using solvents of different polarities. By using suitable chemical probes, such as thiolate ion, this technique also allows the separate determination of the mono- di- and polysulphidic crosslinks in vulcanized rubbers. The crosslink density can also be determined using the Mooney-Rivlin equation based on stress-strain data of vulcanized rubbers. By swelling vulcanized rubbers in styrene, polymerizing the styrene, and observing the microtomed rubbers under a transmission electron microscope, crosslinks that formed within a latex particle, or intra-particle crosslinks, and those of inter-particle crosslinks can be visualized and determined semi-quantitatively. This characterization technique is useful for one to understand the extent of inter-particle and intra-particle crosslinking reactions.