Ali Ansarifar, a retired senior lecturer from Loughborough University in the UK, believes is time to leave traditions behind and embrace a more environmentally friendly approach to the use of chemical additives in rubber formulation.
There have been two major technological developments over the years in processing rubber. They were sulfur curing or sulfur vulcanization for shape retention and stability by forming stable chemical covalent crosslinks between the rubber chains, and use of solid fillers, for example carbon black and synthetic silica and silane, to reinforce or improve mechanical and dynamic properties of rubber.
However there are two fundamental issues with the aforementioned materials. Petroleum-based carbon black is toxic and silica/silane systems are expensive. In addition, often the reaction of silane with silica during mixing with rubber can be problematic. The reaction of sulfur with rubber even at elevated temperature is slow and inefficient, and to achieve high level of efficiency in sulfur vulcanization, organic accelerators often in combination with zinc oxide as a primary activator, and stearic acid as secondary activator, are added with sulfur. These chemicals are essential for improving the curing process, but their excessive use is having an adverse impact on human and animal health as well as the environment.
Studies in recent years have shown that the use of chemical curatives in sulfur vulcanization can be reduced by a significant margin and mineral fillers such as kaolin (China clay) can replace carbon black and silica/silane systems in rubber reinforcement, offering benefits to rubber properties. Research by some workers has shown that natural oils can replace petroleum-based processing oils in rubber, offering environmental benefits.
All the indications are that a green future for rubber is on the horizon, but the rubber industry still faces some major challenges. One factor delaying this transformation to a more sustainable supply chain is adherence to traditional methods of preparing rubber formulations. Since Charles Goodyear discovered in 1839 that heating raw rubber with sulfur modified the rubber to retain its shape, there has been an increasing trend to use more chemical curatives in rubber compounds to achieve more efficiency of the curing process to reduce processing time for rubber articles.
Indeed, using a wide range of chemicals in combination with sulfur at elevated temperature has produced a highly efficient vulcanization effect but excessive use of the chemical curatives is harmful to health, safety and the environment. New European chemicals policy Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) and various legislations for environment and safety are restricting the use of these harmful chemicals in rubber.
Despite recent advances in research, which have shown that chemical curatives can be reduced significantly in sulfur vulcanization without compromising efficiency of cure, and imposition of various regulations and legislations, which demand lesser use of these chemicals, the progress has been very slow. All the new scientific research suggests that the traditional methods of sulfur vulcanization are no longer viable and there must be a change in the way chemicals are used in vulcanization.
A similar argument applies to the use of solid fillers. Mineral fillers have proved to be highly efficient in rubber reinforcement but there is little evidence that rubber formulation scientists are replacing carbon black and silica/silane systems with mineral fillers in rubber compounds. Once again, this is not due to a lack of scientific data that shows excellent effects of mineral fillers, but rather it is due to a tendency to adhere to the more traditional solid fillers for rubber reinforcement.
It seems that a major step toward a green future for rubber is possible if a more environmentally friendly approach to the use of chemical curatives and solid fillers can be implemented in place of the traditional methods.
The traditional methods of sulfur vulcanization using so many chemicals and rubber reinforcement by harmful and expensive solid fillers have served us well in the past but it is time to leave traditions behind and embrace a more environmentally friendly approach to the use of chemical additives in rubber formulation.
by Ali Ansarifar, retired senior lecturer, materials department, Loughborough University, UK
Interested in the latest rubber developments? Don’t miss numerous presentations on the subject at this year’s Tire Technology Expo Conference, including a paper from Dr Katrina Cornish, professor, The Ohio State University, titled ‘Valorization and scalability of alternative natural rubber through tailoring properties‘, part of Stream 1 – Tire Technology for the Vehicle Revolution on Wednesday 26 February.