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!

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Bekaert has launched a sustainability label, inhera, to identify its top-performing solutions designed to help customers accelerate progress towards net-zero targets. The label is applied to products that meet defined criteria, including alignment with the EU Taxonomy Regulation, carbon-emission reduction, improved resource efficiency and support for circularity. Eight solutions currently carry the label.

The company said the initiative responds to rising expectations for transparency and credible sustainability claims. It presents inhera as a means of offering clearer information on the impact of its solutions across sectors including construction, automotive and energy.

The name reflects Bekaert’s stated commitment to integrating sustainability across its operations. Ann-Françoise Versele, Vice-President Sustainability, said: “Sustainability is inherent in everything we do at Bekaert. Through our products and processes, we aim to leave a legacy of sustainability for future generations. Our partners who choose inhera solutions are choosing to accelerate sustainability and create real impact.”

The eight labelled solutions include Ampact copper magnet wire for high-voltage electric-motor stators, which the company says delivers a 10 per cent reduction in direct CO₂ emissions. Flexisteel hoist ropes for elevators are designed to cut daily energy use by up to 36 per cent compared with traditional steel ropes.

Elyta Ultra and Mega Tensile tyre reinforcements provide up to 16 per cent CO₂ savings per tyre and support circular reuse. High-tensile steel cores for overhead power lines reduce steel consumption by more than 30 per cent.

Bezinal Vineyard Plus coatings, made with low-carbon raw materials, deliver more than 50 per cent CO₂ reduction and 14 percent lower life-cycle greenhouse gas emissions. Bezinox non-magnetic armouring wire for submarine power cables is engineered to extend cable life and reduce operating temperatures.

Next-generation hose-wire reinforcement uses 28 per cent less wire and 5 per cent less rubber per hose, resulting in up to 70 percent lower CO₂ emissions, while subsea cable armouring wire made from at least 70 per cent recycled steel offers more than 50 per cent CO₂ savings and is fully recyclable at end-of-life.

Bekaert said additional products will be added to the inhera label as further innovations are developed.

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Yokohama Rubber has reported new findings from an international research programme aimed at combating leaf fall disease, a fungal infection that has disrupted natural rubber production in Indonesia, the world’s second-largest supplier.

The results were presented on 9 December at the fifth Joint Coordinating Committee Meeting held at Universitas Indonesia. The initiative, formally titled “Development of Complex Technologies for Prevention and Control of Rubber Tree Leaf Fall Diseases”, forms part of the Science and Technology Research Partnership for Sustainable Development, a scheme run by the Japan Science and Technology Agency and the Japan International Cooperation Agency with support from Japan’s foreign and education ministries.

The project brings together Japanese industry, government and academic institutions, including the national research agency RIKEN, with Indonesian partners. Its objective is to maintain output from smallholders, who account for more than 90 per cent of Indonesia’s natural rubber production. The programme is pursuing several approaches: pesticide-based disease control; the development of disease-resistant clones; and early detection using satellite and drone imagery.

Yokohama Rubber, which uses natural rubber as a principal raw material for tyres, has been involved since the project’s launch in 2020. The company began assessing pesticide effects on rubber quality in 2024, following a screening process. Field tests on large plantations have shown that correctly applied pesticides do not impair the properties of raw or vulcanised rubber. Yokohama Rubber is now contributing to research on how such treatments may affect smallholders.

The group said its “Procurement Policy for Sustainable Natural Rubber” incorporates support for participants across the supply chain, including small-scale farmers. It expects its role in the project to aid the sustainable production of natural rubber and help stabilise smallholder incomes. Yokohama Rubber is also an official partner in a separate SATREPS initiative on using rubber seeds to develop environmentally focused products intended to mitigate global warming and plastic pollution.

Under its sustainability theme, “Caring for the Future”, the company has emphasised links between its commercial activities and broader environmental and social objectives.

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Australia has released its first national specification for the use of crumb rubber asphalt on local roads, a move intended to give councils clearer guidance on designing and maintaining light-to-medium-duty networks and to strengthen domestic recycling demand for end-of-life tyres.

The Crumb Rubber Modified Dense Graded Asphalt (CRM DGA) Model Specification for light to medium duty roads was published by the Australian Flexible Pavement Association (AsPA) following collaboration with Tyre Stewardship Australia. The document offers standardised technical requirements for councils, which manage about 75 percent of the national road network — roughly 675,000km of streets and community-level infrastructure.

Existing asphalt standards were largely developed for higher-order state roads, leaving local governments to interpret specifications that did not reflect lower traffic loads or the environmental conditions typical of suburban and regional networks. The new model specification aims to close that gap by setting guidance aligned with the factors that most influence degradation on council roads, such as surface ageing and weather exposure.

The specification promotes the use of crumb rubber modified binders, which can extend pavement life under light-to-medium-duty conditions. Incorporating recycled rubber also aligns with broader circular-economy policies across Australia’s states and municipalities, which are seeking to reduce landfill and illegal dumping while supporting domestic tyre-recycling capacity.

AfPA said the CRM DGA Model Specification V1.0, dated October 2025, is publicly available. It includes requirements for mix design and materials, construction processes such as compaction and temperature control, and performance testing suited to council networks. It also offers practical guidance on integrating reclaimed asphalt pavement content.

Local governments seeking case studies and procurement tools on crumb rubber applications can access Tyre Stewardship Australia’s Crumb Rubber Resource Centre for further technical and project information.

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Flexsys has created what it says is the tyre industry’s first practical and scalable alternative to 6PPD, marking a major step toward replacing a chemical used for decades but now under regulatory pressure.

The company said the new antidegradant is the result of several years of research and testing with federal laboratories, independent scientific groups and tyre makers. Early results show the material could match the performance and safety of 6PPD while avoiding the environmental risks linked to 6PPD-quinone, a transformation product identified in 2020.

Flexsys said the new chemistry provides the short- and long-term protection needed to stop tyres cracking or ageing. It is also designed to fit into existing rubber compounds with minimal changes, which could help manufacturers adopt it quickly. The company added that the product meets environmental and regulatory benchmarks, including criteria set by the Washington State Department of Ecology.

Importantly, the new molecule is not part of the “PPD” family, meaning it does not form quinone during use. Flexsys said this would remove the environmental impact associated with 6PPD-quinone. The company is also using many of the same intermediate chemicals already used in 6PPD production. This could allow manufacturers to rely on existing factory assets and speed the shift to the new technology.

“This achievement reflects our unwavering commitment to responsible innovation, built on decades of expertise in tire protection chemistry,” said Carl Brech, Chief Executive Officer of Flexsys. “Our solution is formulated to deliver the performance and reliability that tire makers expect and is designed for future environmental and regulatory standards.”

6PPD has been essential to tyre durability for 50 years. But studies published in 2020 showed that 6PPD-quinone could harm aquatic species, including coho salmon. Regulators and tyre producers have been looking for a safer option since then. Flexsys said its new antidegradant meets this challenge without reducing tyre safety.

“Our team set out to develop a next-generation antidegradant that meets the tire industry’s highest performance standards without compromising tire safety, while also reducing toxicity,” said Neil Smith, Chief Technology and Sustainability Officer. “I could not be more proud of the perseverance and dedication of the Flexsys R&D team. Our group has been highly motivated by both the technical challenges of this project as well as the positive societal impact that this work will ultimately have.”

Flexsys acknowledged support from the Sustainable Polymers Tech Hub in Akron, Ohio, part of the U.S. EDA Tech Hubs programme.

The company is now working on process optimisation to allow large-scale production. It is also in discussions with regulators around the world to secure approvals for commercial use. Testing with tyre makers is continuing.

“Flexsys is helping set the direction of the tire industry for the coming decades with this development,” Brech said. “We will continue to work tirelessly to bring this breakthrough to the market as soon as possible.”

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