Tires easier to make with Italian-made production process
Improved easy-to-work-with polymers: not easy to reconcile these two features, but a group of Eni-Versalis researchers seems to have succeeded in doing so, adding enhanced sustainability to the compound
The use of natural rubber is very ancient and goes back to the Central American civilization of the Olmecs, who used it to make footballs. The Aztecs also used it to waterproof containers and fabrics, but for the first scientific explanation we had to wait until the Age of Enlightenment and a treatise written by François Fresneau in 1751. The rest is recent history, with Charles Goodyear who by developing vulcanization kick-started one of the oldest industries in the world. Although natural rubber is no longer the main component found in tire compounds, the manufacturing process of a product requires that the raw compound takes a predetermined form before vulcanization.
The technical term "processability" includes much which cannot be scientifically understood, but rheological properties have been recognized as one of the most important features in evaluating the processing behaviours of raw rubbers. The study "Newly designed easy processing SSBR for optimum performance in rubber compounding", conducted by Fabio Bacchelli and other researchers at Versalis (Eni group) exposes new processes that facilitate the creation of compounds while maintaining quality and performance. Furthermore, these processes show further improvements in the characteristics of the compound using additives of vegetable origin, with significant effects on the sustainability of the process. The researchers report new discoveries in polymerization techniques largely responsible for the creation of new elastomers.
New but not revolutionary
However, despite these achievements in developing new materials, there has been a lag in the rational study of methods of processing elastomers and a tendency to polymerize and compound rubbers to fit speeds and dimensions of existing equipment. Emulsion polymerized SBR are widely used in rubber mixing, due to a good filler incorporation and efficient processing of the compound. In the emulsion copolymerization of butadiene and styrene with a free-radical initiator there is an inherent mechanism of chain transfer, which results in the formation of branches. This polymerization mechanism shows very poor flexibility in terms of molecular architecture, however broad molecular weight distribution and branches promote filler incorporation and processability.
It’s a matter of weight and more
A brief aside: the molecular weight distribution is a bit like an inventory that says: in polymer XY there is a 10% of molecules weighing 15, another 10% weighing 20 and so on. The molecular weight is related to the length of the different polymer chains and the weight distribution changes the general chemical-physical properties of the compound. The weight distribution is said to be "narrow" if the various molecular weights are not very different from each other. Solution polymerization (Sbr) instead allows a better control of the microstructure and therefore better characteristics, in terms of hysteresis and mechanical properties, of the vulcanized products. These Sbr have an almost linear structure and this, together with the "narrower" distribution of the molecular weight, does not benefit the processing (higher Mooney viscosity, early onset of extruded distortion, etc.).
An Italian-based research
Versalis (ENI), using a proprietary industrial production technology, is developing a new family of continuous sSBR containing a controlled amount of chain branching, specifically designed to optimise processing and performance at the same time.
Dry SBR grades have been tested in ASTM D-3185 formulation (carbon black based). The easy processing polymer (SOL R C2525 EP) was compared with a high performance batch grade (SOL R 71420, carbon black functionalised) and a highly processable emulsion grade (SBR 1502). As expected, the high performance SOL R 71420, a batch polymerised grade characterised by narrow molecular weight distribution and high molecular weight, shows excellent hysteretic properties, but stiff tensile behaviour and much higher Mooney compound. The easy processing polymer SOL C3555T, produced with the same technology used for SOL R C2525 EP, was compared with the high performance batch grade SOL R 74618
Easy-to-process compounds and sustainability
According to the previous investigation, a significant improvement in terms of Mooney compound (lower) can be observed for the easy processing sSBR with only a slight worsening of hysteretic and mechanical properties. The beneficial effect of the easy processing technology on extrudability is depicted in the figure, where the results of a Garvey extrusion test are reported. In this case, a compound fully based on emulsion rubber (SBR 1739) was used as reference. And further improvements are possible. To further optimise the trade-off between processability and performance, with the advantage of improving carbon footprint and sustainability, Versalis (ENI) offers another tool, developed in the frame of a go-green strategy. Commercialized under the name Matrilox, a bio-sourced plasticiser is produced through a bio-refining process by Matrìca, a joint venture between Versalis and Novamont, a major player in the bio-plastics market. The investigated PF801R grade has been specifically designed for the tyre and rubber industry with the aim of partially or totally replacing process oils of fossil origin. Using the already described formulation, a compound containing SOL R C3555T was prepared, where 35% of the TDAE oil is replaced with Matrilox. The positive introduction of the bio-additive in terms of compound properties is evident in the beneficial effect to hysteresis, Mooney compound, abrasion and other properties is clearly observed, confirming the interesting combination of easy processing technology and bio-sourced ingredients.