Influence of sulfur in pyrolytic carbon black on in-rubber properties

Sustainability of elastomeric materials and products is a key issue nowadays in the rubber industry. There are different paths to make rubber more sustainable, of which one is recycling of used rubber and re-use in new rubber products. One of the commonly used technologies is pyrolysis, resulting in gas, oil and a solid residue. The latter consists mainly of carbon, besides silica and zinc compounds. This blend of different materials is investigated and compared to virgin carbon black N660, with the aim to elucidate the causes for the differences in properties and reinforcing behavior of these two types of filler.
An obvious difference with consequences for the reinforcing behavior is the presence of carbonaceous residues and silica. However, this does not explain the disparity between uncured compounds showing even higher polymer-filler interactions compared to N660, while in cured rubber, they exhibited lower mechanical properties. One possible explanation is the presence of sulfur in pyrolytic carbon black. However, the question is if this sulfur is still active. This was confirmed in our study: The sulfur is able to interact with the rubber matrix causing pre-scorch or chemical polymer-filler coupling as indicated by specific cure characteristics and bound rubber content. Elemental mapping of this recovered filler demonstrates a high concentration of sulfur deposited on the silica surface as well: sulfur-carbon-silica compounds are formed. These findings can explain the contradiction of increased polymer-filler interaction and at the same time deterioration of the strength properties of the material.