Polymer producers have responded to the changing tyre industry expectations, their innovations take into account a more reactive functionality within polymer architecture, and this is closely matched by increasingly greater degrees of filler surface chemistry. As a result, tyre compound properties may now be tailored to meet quite specific tyre performance targets.

Tyre processing methods also underwent progressive stages of modernisation in recent years, for example, more sophisticated reactive mixing technology together with high-speed extrusion systems allowing for direct extrusion onto the building drum become established as cost effective production routes for many of the major producers.

These advancements however bring about their own conflicts, processing pathways involving multiple stages, often with the ability to rework compound or adjust rheological properties by additional time or energy input during the process are no longer viable options for most tyre producers. It has thus become necessary to achieve processability through an increasingly narrow operating window.

Formulations optimised to achieve peak tyre performance in most cases also tend towards more challenging processing characteristics. This is to be expected; the use of high and narrow molecular weight reactive polymers alongside fillers having high surface area and chemistry, often in loadings above that of the polymer are the norm. The trend towards usage of high loading of plasticiser and resinous materials to adjust tyre tread grip and traction response all contribute to a less forgiving processing nature. Often those compounds that are highly reinforced appear the most fragile during processing and poor green strength with an easily tearing or crumbly compound appearance are often-discussed processability issues. By contrast, the use of high loadings of resins and plasticisers, for example in winter or high-performance tyre formulations, results in compounds that during processing can more resemble chewing gum than tyre treads!

The use of process additive chemicals in an attempt to overcome the processing limitations observed gives rise to further conflicts; Lubricant additives might improve compound surface appearance; however, green strength will probably further reduce due to the unwanted softening effect. The same is true for release additives where lower tack unfortunately remains at a higher value than the also lowered tensile strength of the compound. Filler dispersion is often targeted by additives, however higher loading of fillers mandate equally higher additive loadings, we should more accurately discuss loading as “parts per hundred of filler” not rubber, and under the appropriately higher additive loading, the risk of interference with vulcanisation properties or even additive migration leading to surface bloom become realistic concerns.

Conflicting performance characteristics

The development team at Schill + Seilacher has recognised the need to decouple conflicting performance characteristics found within conventional process additive chemistries. As a result, innovation within our Struktol® range offers tyre compounders opportunities to achieve processability without compromise.

Reduced viscosity leading to better extruder flow properties and improved surface appearance, whilst at the same time achieving an increased compound green strength can be realised by use of Struktol HT 300, a new generation of reactive process additive.

An extract of key processing and property influence in a typical highly silica filled sSBR tread compound are highlighted in the following data.

Significantly lowered Mooney viscosity as well as better mixer batch off with reduced sticking to the mixer rotor and gate with Struktol HT 300 are observed.

An increase in compound green strength was obtained by the addition of Struktol HT 300; this is the opposite of expectation for conventional process additive chemistry, where reduced viscosity is obtained. In addition, the filler dispersion as evidenced by a reduction in the so-called “Payne Effect” as tested in uncured compound by means of RPA strain sweep, is also improved.

Lab extrusion trials, using cold feed extruder demonstrate improved surface appearance and lower compound pressure achieved by use of Struktol HT 300, both desirable processing conditions.      

Physical properties are also acceptable, with a progressive increase in tensile strength an elongation and maintenance of stiffness with loading of 6 phr of process additive, only at higher loading of 12 phr would a balancing slight reduction in process oil be required.

Compound hardness remained unchanged alongside improved wear resistance, as measured by DIN abrasion loss testing, even when using higher loading of process additive, are important aspects.

The ability to decouple the relationship between lubrication, important for improved rheological behaviour, and the maintenance of strength and stiffness in both the uncured and vulcanised condition is only possible with such new and innovative class of process additive. This departure from conventional thinking offers the tyre compounder significant degree of freedom to retain the benefits in terms of easier processability without sacrifice of key tyre performance properties.

In this example, the use of Struktol HT 300 prioritised green strength alongside reduced viscosity. By contrast, our new Struktol HT 250 decouples release from other properties, especially effective for winter tyre tread; compound stickiness is resolved without compromise of viscoelasticity.

Ensuring that migration and ultimately bloom within rubber compounds is kept to the lowest level is important for final article aesthetics, for tyres additional considerations arise; they are composite structures, therefore it is imperative that chemicals do not migrate across boundary layers in an uncontrolled manner, which could result in changed behaviour or interfacial adhesion failure over time. In order to limit migration, the compatibility, solubility and concentration of chemicals are carefully considered. However, one method of ensuring long-term stability involves chemically binding the additive within the vulcanisation network.

At Schill + Seilacher, we have achieved this degree of crosslinking capability for a number of new-generation Struktol process additives. Their usage allows the compounder to avoid completely the risk of migration and bloom due to additive presence.

Photographs of two vulcanised rubber sheets based on the same formulation. On the left-hand side, evidence of typical surface bloom, which may occur due to migration of a conventional process additive, on the right, containing reactive additive Struktol HT 600 as replacement, it can be seen that bloom was eliminated.

This technology also opens tremendous opportunities to “fix” process additives in place within the respective component, the role of additives withinthe cross-linking mechanism may additionally lead to vulcanisate performance characteristics.

                                                                                  One interesting tyre related example involves the development of a superior tyre curing bladder performance, here we have developed new reactive plasticisers called Struktol HT 815 and Struktol HT 820, their use is directed towards resin-cured butyl rubber. This combination of polymer and curing system provides for superior heat resistance with excellent flex fatigue resistance and is used as the basis for tyre curing bladders. Here the replacement of widely used castor oil as plasticiser with new Struktol HT 800 series product leads to a significantly improved bladder performance life, with greater stability in viscoelastic properties. Reduced stiffening of the bladder, due to lower degree of plasticiser migration translated into a much lower flex-cracking rate, especially after high temperature steam ageing.

Reactive Struktol plasticisers, HT 815 and HT 820 exhibit a significantly reduced flex cracking rate when compared to the widely used castor oil, which readily migrates from the bladder; as a result, significant extension of bladder service life is possible.

New reactive process additives, with tailored functionality to closely match the specific chemistry of polymers, fillers and cure systems are actively developed. These innovative products under the Struktol brand offer tyre compounders a more comprehensive toolkit in order to tailor compound performance to meet tyre performance demands. The conflicts of property versus processability diminish and new possibilities emerge!

Kraton Corporation, a leading global producer of speciality polymers and high-value biobased products derived from by-products of pine wood pulping, has received an International Sustainability and Carbon Certification (ISCC) PLUS certification for its manufacturing facility in Niort, France.

Kraton can now supply 100 percent ISCC PLUS-certified AMS and AMS Phenolics resins using a mass balance attribution approach thanks to the certification. Kraton's dedication to promoting the biobased and circular economy is further demonstrated by the Niort facility's accreditation. Kraton is committed to lowering carbon emissions and accelerating the shift to a more sustainable future by empowering clients to include more environmentally friendly products into their supply chains.

An independent multi-stakeholder project, the International Sustainability and Carbon Certification (ISCC) is a leading certification method that guarantees accurate mass-balancing bookkeeping and reporting of renewable and recycled materials across the supply chain.

Lana Culbert, Kraton Pine Chemicals VP of Marketing, said, “Our SYLVARES™ and SYLVATRAXX™ brands feature a range of high-performance AMS and AMS Phenolics resins used in speciality adhesives and tyre applications. Our ISCC PLUS journey began in 2021 with the certification of our Sandarne, Sweden facility. Earning this certification for our Niort plant marks a significant milestone, further expanding our portfolio of sustainable solutions.”

Sumitomo Rubber Industries, Ltd. has said that it has clarified the process of rubber breakdown, a crucial element that affects tyre longevity, through joint research with Professor Gert Heinrich at Dresden University of Technology (Saxony State, Germany).

In a special session called ‘Combining Physics, Chemistry & Engineering of Rubber: A Symposium in Honor of Charles Goodyear Medalist Gert Heinrich’, which was held in honour of the Professor's medal-winning achievement, the findings were presented in an invited talk at the American Chemical Society, Rubber division, ACS 2025 Spring Technical Meeting held in Orlando, Florida, US, on 6 and 7 March. Professor Heinrich is a member of the Leibniz Institute of Polymer Research Dresden, which took part in the collaborative effort.

Tyre durability is determined by cracking processes, which result in the development of cracks and rips in a rubber substance. The endurance of rubber was previously mostly assessed using tearing tests, but there were still numerous unanswered concerns regarding the microscale structural alterations at the crack tip. This effort identified a factor that determines the start and propagation of cracks in rubber by analysing the force applied to a fracture tip using a simulation approach. In rubber, a crack tip undergoes dilatation deformation to create voids, or microvoids within the item. The fracture gets worse as the voids expand and come together. It was also discovered that the tension focused on the fracture tip is lessened when voids emerge.

The German Rubber Industry Association (Wdk) is advocating for a thorough and legally enforceable regulation of the end-of-waste status for Germany and the whole of Europe. The association is pusing for the circular economy to be strengthened for tyres.

Valuable secondary raw materials are still legally regarded as waste since there are no consistent standards for the end-of-waste status, according to Stephan Rau, Technical Director of the WDW. This restricts the market integration of sustainable recycled materials, impedes investments and creates legal uncertainties. “We call for a comprehensive and binding end-of-waste regulation for Germany and the whole of Europe to strengthen the recycling sector, facilitate investment in modern technologies, and sustainably reduce the CO₂ footprint of the rubber and tire industry,” he said.

In order to adhere to the prescribed waste hierarchy and stop illicit exports, Rau also emphasised that lawmakers must quantitatively and openly document waste tyre flows. He added that waste tyres should only be gathered by approved disposal companies and pre-sorted based on their intended use.

On the important role of mechanically recycled tyre granules and powder, Rau said, "This is an important secondary raw material that is already used in a variety of durable and safe products. A clearly defined end to its waste status must be made more economically viable. To ensure the market success of tyre granules, a risk-based approach to assessing chemical ingredients is essential. Binding limit values ​​must be assessed using a migration analysis, i.e. based on their bioavailability. Second, the use of tyre granule products must become mandatory in the construction industry and infrastructure projects. The goal is clear: Recycling must be economically viable."

Kuraray, a global speciality chemicals company and one of the largest suppliers of industrial polymers and synthetic microfibres, has received ISCC PLUS certification for its thermoplastic elastomers SEPTON and HYBRAR as well as Liquid Rubber produced at the Company’s Kashima Plant at Ibaraki Prefecture.

ISCC PLUS makes sure that certified products, such biomass and recycled raw materials, are appropriately handled throughout their supply chains, including the manufacturing process, by applying the mass-balance technique.

In the five years preceding up to its centenary in 2026, Kuraray is carrying out its medium-term management strategy, PASSION 2026. In order to achieve the long-term Kuraray Vision 2026 of being a speciality chemical company that grows sustainably by integrating new foundational platforms into its own technologies and contributing to customers, society and the planet, the company will keep growing its range of products that improve the natural and living environments.