7.3.5 Mixing of blend compounds
Blend compounds are used widely to achieve combinations of properties not available from compounds based on a single rubber. All rubber blend compounds are immiscible at the molecular level and produce a phase morphology during mixing, usually at the micron level. The mixing of compatible rubbers, those with similar solubility parameters, does not normally present problems if their flow behaviour is also similar. In such cases a fine textured morphology is achieved quickly and the rate of filler dispersion remains as the controlling characteristic of the mixing process [13]. In contrast, mixing of compounds with incompatible and rheologically dissimilar rubbers presents substantial difficulties but may be necessary to achieve desired property combinations. Differences in both flow behaviour and solubility parameter influence the location of fillers and other additives during incorporation. The rubber with the lower viscosity will undergo more flow and consequently be able to encapsulate more filler [14]. This preferential incorporation will continue until filler loading in the initially lower viscosity rubber raises it to a viscosity similar to that of the other rubber. Strain rates in the incorporation zones of most mixers are low, so the traditional Mooney viscosity is a reasonable guide to incorporation behaviour. The preferential incorporation due to flow behaviour can be modified or reversed by differences of the affinities of the rubbers for the fillers and other additives. Carbon black, with its substantially non-polar surface, will incorporate preferentially into non-polar elastomers, such as NR and SBR, whereas silica and mineral fillers will favour polar elastomers. Coatings on fillers and additives are used to improve incorporation behaviour and to overcome problems of compatibility. If the measures available to improve the one-step mixing of blend compounds are inadequate, separate mixing of each rubber with a proportion of the compound ingredients, followed by blending of the sub-compounds thus formed, must be used. Provided a moderate level of filler dispersion is achieved in each sub-compound, transfer between phases does not occur [14]. In contrast, great care must be taken with selection of the crosslinking system to avoid migration between phases, both in mixing and subsequent processes [15].