Carbon Black (CB) is a versatile product that is used in plastic products, in inks, paints and rubber articles, especially tyres, which absorb most of its global production. The challenge in recovering CB used in rubber items stems from the problem of recycling the rubber itself and this has led to it being thought of as a disposable product. However, the need to reduce the industry’s environmental impact has led to an increased commitment to studies on the possibility of recovering carbon black from end-of-life rubber products. In this article, we report the results of a study, funded by the Artis research laboratory, and the conclusions reached by of a joint experiment carried out by Michelin and Bridgestone, research aimed at measuring the properties of recovered Carbon Black (rCB) and the need to define standards for this material.
How does rCB behave?
As sustainability is fast becoming a central topic for the rubber industry, recovered carbon black (rCB) has emerged as a candidate to replace traditional fossil-based fillers. Indeed, carbon black is produced from the incomplete combustion of vegetable fats and oils as well as from oil products such as tar (either obtained from coal or ethylene). The process of recovering rCB from rubber (the tyre industry alone accounts for about 70 per cent of global CB production) thus appears capable of significantly reducing the carbon footprint of the rubber industry. Since rCB is a new class of rubber filler, we first need to understand which of its properties will influence the performance of the products using it. Research by C. J. Norris (Murfitts Industries), A. López Cerdán (Artis) and Pieter ter Haar (Circtec) shows the importance of aggregate size distribution and silica, carbon and organic residue content in the production of a consistent rCB with predictable properties.
Crucial parameters
A vital characteristic of carbon black is the Aggregate size distribution (Asd), a measure of how much carbon black tends to aggregate. Where it is present, Carbon Black comes in colloidal form, i.e. as an aggregation of elementary nodules: in fact, the single particle, which is between 10 and 300 nanometres in size, is practically very short-lived as it tends to bind with others, forming aggregates with larger dimensions, up to 500 nm. Aggregates also join together, forming clusters that can exceed 100 micrometres in size. However, every commercial carbon black particle has the same size, which is not the case with rCB due to the non-uniformity of the “raw material”, i.e. end-of-life tyres: the Asd is therefore a very indicative quantity of the properties of rCB. If to classify CB knowing the structure index (expressed in cubic centimetres/100 grams of product) and the surface area (square metres per gram) may be enough, this is not the case for rCB. Besides the already mentioned Asd, the study identifies other values needed to characterise recovered carbon black: silica, carbon residues and organic residues, substances that are “inherited” from the tyres that were used to produce rCB. The study highlights which properties of rCBs should be controlled and monitored to ensure consistent product performance, hence the need for the recovered carbon black industry to adopt new testing procedures. It has been shown, for example, that a significant Asd, typical in rCB, leads to a substantial reduction in the quasi-static properties of the compound (the difference between the elastic modulus of the rubber for small and large deformations) compared to CB with similar colloidal properties. A large Asd is considered one of the dominant factors for the disparity between the reinforcement potential of rCB and CB of similar surface area and structure level, and this parameter is therefore recommended as one of the main characterisation tools for rCBs. The conclusion of the matter was that the properties of an rCB can be adapted to different applications by controlling the production process: leaving a small amount of organic residue, for example, improved the dispersion properties and tensile strength.
Michelin and Bridgestone together for recovered Carbon Black
Recovered Carbon Black is looked upon with interest in view of reducing the carbon footprint of tyre making. In 2022, Michelin and Bridgestone published a white paper reporting the results of a joint initiative to increase the use of recovered carbon black. Among the goals Bridgestone and Michelin intend to achieve is to work with rCB suppliers and other stakeholders to define standards, grades and specifications for this filler. The initiative aims to raise new rCB suppliers’ awareness of the quality and performance requirements for the tyre industry and to create a common language between manufacturers and rCB users to support the growth of the industry. The stakes are high: it is estimated that globally one billion tyres, or about 30 million tonnes of material, reach the end of their useful life each year. Although many other initiatives exist to recover and reuse materials from ELTs, significant difficulties remain in achieving material circularity on a scale that allows for effective tyre circularity: today, less than 1% of the carbon black used globally in the production of new tyres is recycled. Recovered carbon black represents an opportunity to reduce the tyre industry's dependence on petrochemicals, and the use of recovered carbon black in the production of new tyres could reduce CO2 emissions by up to 85% compared to virgin materials.
Classifying recovered Carbon Black
By 2030 Bridgestone and Michelin expect the rCB market to reach 1 million tonnes/year, but a strong commitment is needed as early as next year. As seen above and highlighted by Martin Wolfersdorff, rCB is a mixture of different products, some of which are related to raw material obtained from end-of-life tyres. Thus, one finds “soft” grade CBs such as those classified as N7xx, N6XX, N5xx series and hard “tread-grade” CBs of the N3xx, N2xx, N1xx series. Inorganic ash, mainly zinc compounds and silica, along with carbon residues are also found. The ratio between these different substances depends not only on the original tyre design but also on the degree of wear: pyrolysis on a batch of relatively new car tyres could produce an rCB with a high silica content. Treatments can also trigger some differences, which mainly concern the level of pyrolysis completion (affecting volatile hydrocarbons) and carbon residues. Following a great deal of work, an initial classification was reached, based on Astm standards (American Society for Testing and Materials) and shown in the table. Grade A corresponds to three subgrades while grade D has yet to be defined; other ratings are listed in another table, but it is already interesting to see how much raw material can influence the characteristics of the final product. Much still needs to be done to reach a definitive classification and scalable manufacturing processes, not to mention the need to define criteria for environmental protection, safety and consequent registration within the Reach regulations for the use of chemical compounds.
Recovered Carbon Black, the challenge of sustainability
