Distribution of Crosslinks in Vulcanized Blends

Abstract

Abstract All of the blend systems studied to date show a tendency to have an uneven distribution of crosslinks between the phases; this must be taken to be the norm, with instances of near-even crosslinking being the exception. When the crosslink distribution has been made to be near even, the properties investigated are generally improved. In blends of polar and nonpolar elastomers, there is a tendency for the polar elastomer to be more highly crosslinked than the less polar elastomer, because both the sulfur and accelerators partition in favor of the former. However, if one elastomer has a very high solubility parameter, the accelerator may partition in favor of the less polar elastomer. Provided this differential partition of the curatives is not extreme, the results can be beneficial. There is one recorded instance of a drastic reduction in overall crosslink density through very low crosslinking of one phase, and this may be ascribed to strong differential partition. Since crosslink distribution in blends of elastomers differing in solubility parameter appears to be controlled largely by partition of curatives and vulcanization intermediates, influence may be exerted by appropriate choice of accelerator(s). The crosslink distribution in blends of low and high unsaturation elastomers is extreme; at normal levels of curatives, the low unsaturation elastomer may be virtually uncrosslinked whilst the high unsaturation elastomer can have a higher crosslink density than is desirable. Modification of EPDM to either form an independent network in the EPDM phase or enforce the formation of sulfur crosslinks during vulcanization is effective. The level of crosslinking required in the EPDM phase in order to provide substantial improvements in properties of blends is modest, much lower than would be required to give acceptable properties in a single polymer vulcanizate. Blends of general purpose elastomers, such as NR and BR, do not differ significantly in either solubility parameter or degree of unsaturation and are generally considered not to be problematic. However, these often show an uneven distribution of crosslinks. Studies of the progress of crosslinking during vulcanization suggest that partition of curatives could play a role even here. The detailed view of crosslink distribution presented here is largely due to the success of a swollen-state NMR spectroscopy technique, and there is considerable scope for further advances in view of the list of combinations of elastomers which may be investigated in this way. The list is reproduced in Table IX, suitably updated to take account of recent developments. Further advances are expected; not just in respect of the investigation of new blend and curative systems, but also in the combination of the swollen-state NMR technique with other established procedures such as the use of chemical probe reagents to provide information on the type of sulfur crosslinks present in each phase of a blend.