The effect of pressure on carbon black/elastomer powders. IV. Comparison of press‐cured powders containing vulcanizing agents with mill‐mixed compounds

Abstract

AbstractSBR latex was compounded with six different types of carbon black (SRF, FEF, HAF, SAF, HEB, and HCC‐I) and vulcanization ingredients in aqueous dispersion by means of a dispersing agent. The particle size of the carbon black agglomerates in aqueous dispersion was estimated from turbidity and light scattering measurements. It was found that the size of the aggregates is by far larger than the average particle size of the SBR‐latex used. The latex mixture containing carbon black and vulcanization ingredients was spray‐dried to a fine‐sized powder of an average particle size of 0.25 mm; this powder was subsequently vulcanized without the action of shear forces at a pressure of 200 bar. It was the aim of this study to answer the question whether latex‐stage compounded vulcanizates can be reinforced without applying shear forces, if only the distribution of the carbon black in the compound is sufficiently good. The tensile strength was used for judging reinforcement; the quality of the carbon black dispersion was estimated by means of the Cabot rating method. The carbon black dispersion in the compounded powders proved to be excellent with fine‐sized carbon blacks and was in the order of A 1 and B 2 of the Cabot scale. Scanning electron micrographs of the fracture zones and transmission electron micrographs on ultrathin cuts showed, however, differences in the microdispersion of the carbon blacks between powder‐and mill‐mixed compounds. A comparison of the mechanical properties of powder‐and mill‐mixed compounds yielded partially comparable tensile values for the powder compounds up to 20 MN/m2, but strikingly high hardness, elongation set and a pronounced stress‐softening after repeated deformation (Mullins‐effect).