To the rescue: recovered carbon blacks
Chris Norris, Analytical Services Manager at ARTIS, UK, explains work on conducting a global recovered carbon black benchmarking programme, assessing their composition and in-rubber performance in relation to conventional carbon blacks.
During RubberCon 2014, in Manchester, Dr Akutagawa of Bridgestone Corporation conveyed the importance of sourcing raw ingredients from alternative sources to satisfy the predicted growth in tyre production. Recycling of tyres, through processes such as pyrolysis, potentially offers such alternative sources while assisting with the environmental issues associated with end-of-life tyres. The use of pyrolysis to recover carbon black filler is nothing new, with a number of ventures sporadically offering material to the marketplace over a period of several decades. It is clear that recovered carbon black (rCB) has historically been treated and marketed as conventional carbon black, which partly explains the short-lived nature of a number of these pyrolysis ventures. Such an approach does not account for the unique properties of rCB. To overcome this issue, and to challenge negative market perceptions, UK-based materials consultancy firm ARTIS recently conducted a global benchmarking programme to better understand the quality and properties of current state-of-the-art rCB materials.
Firstly, the structure level and surface area of the rCB samples were considered, as these colloidal properties are used as key indicators of the reinforcing potential of a rubber filler. The data generated demonstrates that the rCB samples fall in the colloidal space between N550 and N330 carbon blacks (carcass and tread grades) – a whole tyre feedstock will contain a blend of carbon black grades. Based on these measurements alone, rCB has historically been marketed as a drop-in replacement for N550 or even N330 carbon black.
Characterisation of in-rubber performance, through use of a model Styrene-Butadiene Rubber formulation, conclusively demonstrated that simply regarding rCB as a drop-in replacement for carbon blacks of similar colloidal properties is not appropriate. Rheological, physical and dynamic characteristics class rCB as a semi-reinforcing filler, so it may find use in applications currently occupied by N600 and N700 series carbon blacks.
Incorrect marketing has often led to rCB being used within inappropriate formulations, which has been a significant contributor to the lack of approvals attained. The other major challenge to these recycled materials revolves around dispersion within a rubber matrix. For conventional carbon black, a number of post-reactor processes are required to produce the finished beaded product. Such refining steps have often been overlooked in the pyrolysis industry, with the resultant materials having very poor dispersability. Poor dispersion negatively impacts on processing, surface appearance, physical properties and fatigue performance. Evaluation of current rCB showed that post-reactor refinement is still being overlooked by some, with others, such as carbon clean tech (in Germany) and Reklaim (in the USA), producing commercially available materials that have dispersion ratings approaching those of the conventional carbon blacks.
To better understand the issues with dispersability and the disparity between colloidal and in-rubber properties, the composition and surface activity of rCB needs to be considered.
The deposition of the carbonaceous residue is the dominating factor in controlling surface activity and dispersion of rCB. The net result is to fuse a number of primary aggregates together, which, without sufficient post-reactor processing, produces a material of very large rCB aggregate size and, consequently, the poor dispersion of many of the unrefined products. In addition, masking of the active sites of the original carbon black has a significant impact on filler-filler and filler-polymer interactions. This is best demonstrated through assessment of the strain dependency of rCB filled compounds, via dynamic mechanical analysis.
When mixed in the rubber matrix, carbon black aggregates have a tendency to associate with each other to form agglomerates, owing to van der Waals type attraction forces between particles. At low dynamic strains, these filler-filler interactions contribute to the compound stiffness. As the strain increases, the carbon black network is progressively disrupted, eventually leading to a plateau at high dynamic strains where there is no contribution from filler-filler interactions. This is commonly referred to as the Payne effect. The difference between the high and low strain elastic modulus (∆E’ = E’0-E’∞) provides a measurement of the networking efficiency of a given filler system. The commercial carbon blacks generate an R2 value of 0.99, confirming the linear relationship between networking efficiency and surface area.
The correlation between networking efficiency
and surface area for the rCB samples (R2 = 0.95) suggests the level of filler-filler interactions is not influenced by the non-carbon species present, such as silica. If it is accepted that the carbonaceous residues formed on the carbon black surfaces are dictating surface chemistry, then it is likely that such residues will also form on the surfaces of the inorganic components, imparting the same surface chemistry. To some extent, this negates concerns over slight variances in feedstock having a significant impact on performance.
Although the data presented is far from exhaustive, understanding the fundamental characteristics will only help in recognising the potential of rCB as an alternative, green source of raw materials.
Identification of suitable applications for refined commercially available rCB products, such as those offered by carbon clean technology and Reklaim, will inevitably lead to greater product approvals.
Carbon blacks generated from pyrolysis processes are known to contain the following:
- Original carbon black – The loadings and colloidal properties of the carbon black contained within the feedstock will have an influence on the performance of the rCB.
- Carbonaceous residue – Volatile species formed during the pyrolysis process have a tendency to form condensate on the surfaces of carbon black, which will carbonise as the process progresses, forming a layer of carbonaceous residues.
- Inorganic materials – Ash content typically ranges from 10–25wt%, the vast majority of which is accounted for by silica filler and zinc oxide. The variances in ash content are a reflection of the feedstock used at each location or process. For example, the use of a typical European tyre tread compound as the feedstock will likely result in a higher rCB ash content (due to increased use of silica filler) over that derived from a North American tyre tread.
Challenges that need to be addressed to increase rCB utility:
- Classification – ARTIS is currently working with ASTM sub-committee D24.67 (sustainability) to classify rCB and identify the correct parameters that will differentiate between refined and poor quality recyclate. Ultimately, this will allow potential users to make informed decisions regarding their source(s) of carbon black.
- Characterisation – Combined evidence of several studies by ARTIS suggests that both surface area and structure levels of rCB are being overestimated. New or revised testing protocols may be necessary to better characterise rCB.
- Applications – Potential 100% replacement for N700–N500 series has been demonstrated for some applications. The generation of application data is essential, given the disparity between colloidal and in-rubber properties. ARTIS has conducted a number of studies demonstrating the suitability of refined rCB for applications such as tyre inner-liner and sidewalls. For more information, email Chris Norris, Chris.Norris@artis.uk.com