Tyres are rather complex objects, mainly because they are the combination of very different materials: elastomers, fabrics of various types, metal cords, fillers and special rubber liners. Advances in technique and technology make these heterogeneous components, able to perform in different situations and safe even after years and tens of thousands of kilometres. Academic and industrial research is ongoing, and proposes new elements to further improve tyres, with an increasing emphasis on reducing their environmental impact. This is where the study, conducted by researchers at the Milan and Turin Polytechnics and supported by Pirelli, focuses on a bonding agent between silica and elastomers in the compound, called Janus. This molecule can be obtained from natural raw materials with by-products consisting practically only of water and super low carbon emissions.
The importance of silica and its mixing properties
Silica is one of the most widely used fillers in modern tyres, making up 30-45% of the compound along with carbon black. Silica, in particular, has proven capable of reconciling almost apposite characteristics, such as better grip at low temperatures and less internal dissipation, which decreases rolling resistance. In a nutshell, compounds with a high silica content remain softer in cold climates, improving grip in difficult conditions, while managing to dissipate less energy than traditional compounds loaded with carbon black. The use of silica is therefore particularly interesting in the creation of high-performance tyres, but it doesn’t come without any issues. Tyre grade silica is an amorphous (non-crystalline) form of silicon oxide, a polar and hydrophilic substance. It follows that, unlike carbon black, it is poorly compatible with the polymer matrix of a tyre compound. Therefore, the action of an agent aimed at facilitating the bonding process between silica and rubber polymers is required: ethoxy-silane compounds are generally used for this purpose.
Sulphur “bridges” and the role of silica
These elements, an example of which is bis(triethoxysilylpropyl) tetrasulphide abbreviated to Tespt, reacts with the surface of the silica and its polysulphide “bridge” is involved in the curing reaction, and forms a solid covalent bond to the polymer chains of the compound. The result is a significant improvement in the mechanical properties of elastomers. The highly dispersible tyre grade silica available on the market is produced by dissolution in an alkaline watery environment and precipitation by acidification: this material therefore features a pH that makes it one of the acidic substances. Vulcanisation accelerators are alkaline products, as are the by-products in the reaction itself: these ingredients are neutralised by the acidity of the silica and this produces a slowdown in the reaction kinetics.
The productive transformation induced by silica
The use of large quantities of silica in tread compounds has led to important repercussions on several aspects of tyre production technology. The different temperature control requirements necessary to manage the chemical reactions in the compound has led to changes in both mixing equipment and procedures. The different reactions of compounds with a high silica content also needed a change in the chemical curing cycles. The tendency of silica to re-aggregate, despite its reaction with silanes, rapidly increases the viscosity of the compound after mixing to the point of making it unworkable, hence transport and storage procedures had to be changed. What happens is reminiscent of the behaviour of water-oil emulsions, in which the two fluids separate over time: it would therefore be interesting to find sustainable coupling agents that create solid bonds between the silica and the elastomers in the compounds.
The Janus molecule
The study by the researchers at the Milan and Turin Polytechnics highlights some critical points of Tespt, which is also an oil derivative. As mentioned above, it is widely recognised that the bonding reaction between Tespt and elastomer chains has limited efficiency; it is likewise known that the reaction of Tespt's SiOR groups with silica releases ethanol, which is a rather undesirable by-product. Therefore, the solution would be to use an alternative bonding agent in tyre rubber compounds, ideally of biological origin. Such a compound should be reactive with the sulphur-based system used in vulcanisation, and a candidate for this role is serinol pyrrole - SP, which has the formula s2-(2,5-dimethyl-1H - pyrrol-1-yl)-1,3-propanediol. SP (whose symmetry has earned it the nickname 'Janus molecule') is a derivative of glycerol, an important co-product of biodiesel production and is one of the substances that could be produced in a biorefinery; serinol can also be obtained from renewable sources. Examples of the use of serinol pyrrole, e.g. as reinforcing fillers in elastomeric composites in combination with carbon black, have been reported in specialised publications, but no studies are reported on its behaviour as a bonding agent for silica with unsaturated polymer chains, hence the purpose of this research.
In search of confirmation
The silica/SP mixture was obtained simply by mixing and heating the physical elements and was analysed by various methods such as solvent extraction and X-ray photoelectron spectroscopy. The mixture was then used in compounds based on poly(styrene-co-butadiene) (S-SBR, synthetic rubber) and poly(1,4-cis-isoprene) from Hevea Brasiliensis, which is the NR natural rubber; only the SP silica compound was used as filler. Compounds using TESPT as a silica coupling agent and others without a coupling agent were also prepared for comparison. An important result was that the tan delta value (loss factor, used as an index of the hysteresis of a compound and thus of the internal friction that determines rolling resistance) was similar in the compound with SP and in that with TESPT. The structure of the cross-links was also similar, while the dynamic modulus of elasticity in the compound with SP appeared slightly higher. The compound with the silica/SP compound showed higher deformations given the same stress compared to the compound with TESPT together with a higher elongation at break. Other experimental results show that SP is able to establish covalent bonds with both silica and the unsaturated elastomer, making it a viable candidate to replace TESPT in high-silica compounds.
Focus on sustainability
Furthermore, the synthesis is a relatively simple reaction and can take place directly in silica: the preparation of the silica-bonding agent mix could therefore take place at the same site where the elastomer composite is prepared. This operation is also scalable to the level of mass production, with a significant reduction in the carbon footprint of tyre production technology because other intermediate steps and transport of materials would be unnecessary. The elimination of ethanol as a by-product is also good for the environment, as it is generally burnt and responsible for releasing greenhouse gases. The findings of the study thus highlight the effectiveness of SP as a coupling agent for silica, and the authors believe that their work could pave the way for the industrial use of these compounds, whose environmental impact appears to be significantly low.
Pirelli manifests a keen interest for Janus, the two-sided molecule that promises to improve sustainability and tyre performance
