By Sunish Vadakkeveetil, Mehran Shams Kondori, and Saied Taheri

Center for Tire Research (CenTiRe), Virginia Tech

 

 

 

 

 

 

 

 

Rubber, mainly because of its viscous nature, is a widely used material for most contact applications such as, seals, tyres, footwear, wiper blades, bushings etc. The material possesses the property of both a liquid (viscous) and a solid (elastic). Hence, rubber frictional losses at the contact interface is classified into three mechanisms as shown in Figure 1. Hysteresis  (μ_hys ) – Energy dissipated due to internal damping of rubber caused by undulation in the surface. Adhesion (μ_adh ) – Due to intermolecular or Vander Waals attraction at the contact interface. It vanishes in the presence of contaminants or lubricants on the surface. Viscous (μ_visc ) – Due to hydrodynamic resistance caused by the fluid in the contact interface. It mainly occurs under the presence of lubricant or fluid in between the contact interface.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Friction as a concept has evolved, as shown in Figure 2 from a simple empirical relation, developed by Amonton’s (1699) and Columb (1785) to more complex representations by considering these different mechanisms of friction. Initial experimental observations by Bowden and Tabor [1] observed the microscopic behaviour of the contact and obtained that the real area of contact is only a part of the nominal contact area. Grosch & Schallamach [2] performed experimental observation to determine the influential factors and obtain a relation between temperature and velocity-dependent friction to frequency-dependent viscoelastic behaviour. Savkoor[3] considers the frictional losses due to adhesive mechanism at the contact interface using a rudimentary theory where the interaction is considered as a series of processes from the growth of contact area in the initial stage to initiation and propagation of crack in the final stage.

Heinrich [4] developed an analytical representation to estimate the hysteretic component of friction by considering the energy losses at the contact interface to the internal damping of rubber from the undulations of the surface. The energy loss thus obtained is related to the frictional shear stress by the energy relation given by Eq. (2).

            ΔE=∫d^3 x dt u ̇ . σ (1)

           σ_f=ΔE/(A_0 v t)     (2)

Persson and Klüppel [5] extended the theory to consider the effect of the surface roughness by assuming the surface to behave as a fractal nature and obtaining the total energy loss being the sum over the different length scales. Klüppel considers the GW theory to consider the contact mechanics where Persson developed a stochastic based contact mechanics theory assuming the rubber deformations to follow the surface asperities, the results are as shown in Figure 3. To consider the actual deformation profile of rubber, an affine transformation approach [6] is considered to obtain the actual deformation of rubber contact. The results are as shown in Figure 4.

 

 

 

 

 

 

 

In addition to analytical methods, computational approaches are also considered to estimate deformation behaviour of a rubber block on a rough substrate (Figure 5). The numerical model [7] is validated using indentation experiment and compared against a single asperity model as shown in Figure 6. This is later being extended to obtain friction and wear characteristics of rubber at the contact interface by considering the deformations at the contact interface and obtaining the frictional force [5], [8].

 

 

 

 

 

 

 

 

 

 

Figure 6: FE Model Of Single Asperity Model & Comparison Of Results With Experimental & Analytical Approach

Wear is mainly due to the frictional shear stress generated at the contact interface leads to energy dissipation at the rubber – substrate contact interface that is either transformed into heat or responsible for crack initiation and propagation eventually leading to material removal. The major contribution of the wear occurs either due to the interaction of smooth asperity and rubber surface (adhesive wear), Figure 7 (a) instantaneous tearing of rubber by sharp asperities (abrasive wear), Figure 7 (b) or due to repeated cyclic contact stress (fatigue wear, Figure 7 (c)).

Due to the importance and complexity of the wear problem, it has been a vital topic of interest studied by many researchers [2]. Numerical techniques and empirical approaches have seen their light in the midst of the expensive and cumbersome experimental observations [9], [10]. Archard’s law states that “the volume rate of wear (W) is proportional to the work done by the frictional forces” as given by Eq. (3), where τ_f is the frictional shear stress and v is sliding velocity.

            W∝τ_f  v       (3)

 

 

 

 

 

 

 

 

 

 

 

In the case of road surfaces, the removal of rubber particles can be considered as a process of nucleation and propagation of crack like defects until it is detached to form a wear particle, as shown in Figure 8. Based on this mechanism of crack propagation, a physics-based theory assuming the crack propagates (Figure 9 & Figure 10) from already present defects or voids on the rubber surface was considered and then later compared with experimental methods performed using Dynamic Friction Tester (Figure 11) [6], [11], [12]. Future studies are being performed using analytical and computational approached to estimate the wear characteristics of a rubber material considering damage mechanics [8] and crack propagation theory considering the effect of surface roughness. An experimental technique is also being developed based on the Leonardo Da Vinci concept to experimental test the friction and wear characteristics of a rubber block under pure sliding.

 

References:

[1]       D. Bowden, F. P., & Tabor, The friction and lubrication of solids. Oxford university press., 2001.

[2]       A. Gent and J. Walter, The Pneumatic Tire, no. February. 2006.

[3]       A. R. Savkoor, “Dry adhesive friction of elastomers: a study of the fundamental mechanical aspects,” 1987.

[4]       H. Gert, “Hysteresis friction of sliding rubber on rough and fractal surfaces,” Pochvozn. i Agrokhimiya, vol. 25, no. 5, pp. 62–68, 1990.

[5]       S. Vadakkeveetil, “Analytical Modeling for Sliding Friction of Rubber-Road Contact,” Virginia Tech, 2017.

[6]       A. Emami and S. Taheri, “Investigation on Physics-based Multi-scale Modeling of Contact, Friction, and Wear in Viscoelastic Materials with Application in Rubber Compounds,” Virginia Tech, 2018.

[7]       S. Vadakkeveetil, A. Nouri, and S. Taheri, “Comparison of Analytical Model for Contact Mechanics Parameters with Numerical Analysis and Experimental Results,” Tire Sci. Technol., p. tire.19.180198, May 2019.

[8]       S. Vadakkeveetil and S. Taheri, “MULTI – LENGTH SCALE MODELING OF RUBBER TRIBOLOGY FOR TIRE APPLICATIONS,” Virginia Tech, 2019.

[9]       K. R. Smith, R. H. Kennedy, and S. B. Knisley, “Prediction of Tire Profile Wear by Steady-state FEM,” Tire Sci. Technol., vol. 36, no. 4, pp. 290–303, 2008.

[10]    B. W. and R. N. D. Stalnaker, J. Turner, D.Parekh, “Indoor Simulation of Tire Wear: Some Case Studies,” Tire Sci. Technol., vol. 24, no. 2, pp. 94–118, 1996.

[11]    A. Emami, S. Khaleghian, C. Su, and S. Taheri, “Comparison of multiscale analytical model of friction and wear of viscoelastic materials with experiments,” in ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE), 2017, vol. 9.

[12]    M. Motamedi, C. Su, M. Craft, S. Taheri, and C. Sandu, “Development of a Laboratory Based Dynamic Friction Tester,” in ISTVS 7th Americas Regional Conference, 2013.

 

Continental is accelerating its transition towards a circular economy by systematically increasing the use of renewable and recycled materials in its tyres. The company, which averaged a 26 percent sustainable material share in 2024, has set an ambitious target to raise this to at least 40 percent within five years. This strategy involves not only internal innovation but also actively encouraging its supply chain to develop and provide more sustainable raw materials.

A critical area of development is finding green alternatives for reinforcement materials like steel and textiles, which are essential for tyre safety, durability and performance. These materials can constitute over 18 percent of a passenger car tyre, and even more in commercial vehicle tyres. Continental is already integrating recycled steel and is pioneering the use of polyester yarn made from recycled PET bottles. Depending on the tyre size, the carcass of a single passenger car tyre can incorporate the equivalent of up to 15 bottles. This recycled polyester, developed with partner OTIZ, is verified to cut CO₂ emissions by approximately 28 percent compared to conventional materials and is already featured in production tyres like the UltraContact NXT.

The company's sustainable material portfolio extends beyond reinforcements. It includes synthetic rubber derived from used cooking oil, bio-based resins from waste streams and silica obtained from rice husk ash. Complementing these material advances is a commitment to greener manufacturing processes. Together with Kordsa, Continental has developed COKOON, an adhesion technology that bonds textiles to rubber without harmful chemicals. In a move to uplift the entire industry, this innovative solution has been made available to all tyre manufacturers as a free, open-source license, demonstrating Continental's broader commitment to fostering industry-wide sustainability.

Dr Matthias Haufe, Head of Material Development and Industrialization, Continental Tires, said, “We are not reinventing the wheel – but we are reinventing the tyre, with more sustainable materials and more environmentally compatible production processes. It’s not just about the rubber itself. We also focus on the materials that give the rubber its shape and make tyres stable and safe. Recycled steel and polyester yarn made from recycled PET bottles are important for more sustainable tyre production. Our goal is to use at least 40 percent renewable and recycled materials in our tyres within five years. Every alternative material brings us an important step closer to this goal. When it comes to sustainability, it’s not just the materials we switch to, but also those we deliberately do without.”

Pyrum Innovations AG has officially announced that it will break ground on its next wholly-owned recycling facility in Perl-Besch on 14 November 2025. This new facility is a landmark project for the company, designed to be its largest to date and more than double its existing recycling capacity by processing in excess of 22,000 tonnes of used tyres each year.

The financial framework for this expansion is already taking shape. The project is supported by a diversified funding strategy that includes drawing on a EUR 25 million credit line from BASF and a committed debt financing term sheet from a major European bank. Finalising the package is contingent upon an agreement with Saarland authorities regarding land costs. Crucially, securing the Perl-Besch financing will unlock access to further substantial funding, including a second loan tranche from BASF, paving the way for additional projects in the company's rollout plan.

From a technical and logistical perspective, the Perl-Besch plant will be a state-of-the-art operation. It will be constructed on a 25,000-square-metre site in the strategically important border triangle of Germany, France and Luxembourg. The integrated facility will comprise a shredder plant, three next-generation Pyrum reactors, its own power plant and a grinding and pelletising plant. Insights gained from the existing plant in Dillingen are being directly applied to optimise construction and commissioning, aiming for a faster ramp-up to full production. The site was selected for its superior logistical advantages, offering direct connections to the Moselle River, railway lines and a nearby motorway to efficiently manage material flows from across Europe.

This new facility is central to Pyrum's financial roadmap, with the company projecting it will reach break-even upon its commissioning in 2027. Achieving this milestone is anticipated to create significant momentum and provide a solid foundation for the accelerated rollout of the company's broader project pipeline.

Pascal Klein, CEO, Pyrum Innovations AG, said, “Now that all the legal formalities have finally been clarified – development plan, planning permission and access to the site – we can hardly wait for things to visibly get underway. In the background, planning is already well advanced: The site has been prepared, numerous plant components with long delivery times – so-called long leads – have been ordered and the architect’s tenders for the ground work are underway. During construction, we will also benefit from the experience we have gained from the expansion of our main plant in Dillingen, so we are planning to start production in Perl-Besch in 2027.”

Capital Carbon, a brand under India's Rathi Group, has successfully commissioned its new greenfield Recovered Carbon Black (rCB) facility in Gummidipoondi, Tamil Nadu. This development dramatically boosts the group's total rCB manufacturing capacity to 20,000 metric tonnes per year, a significant rise from its previous 5,000-tonne capacity.

The group distinguishes itself through complete vertical integration, handling the entire process from shredding end-of-life tyres to pyrolysis. This operation transforms waste into valuable materials, including rCB, fuel oil, steel wires and pyrolytic gas. The company utilises this gas for process heating, while the carbon char is either refined into rCB or supplied to cement plants as a sustainable energy source.

Ravi Rathi, Director, Rathi Group, said, "As Recovered Carbon Black gains wider acceptance, the industry continues to prioritise quality and consistency – and that's exactly what we've focused on addressing.”

Xingda Steel Tyre Cord Co., Ltd, one of the world’s largest manufacturers of rubber reinforcement materials, has issued a public statement informing that the Brazilian Trade Remedies Authority has closed its antidumping probe into steel tyre cord imports from China without imposing any duties. The investigation was formally terminated on 14 October 2025.

Xingda actively participated in the proceedings with the assistance of its legal teams. Through its cooperation and technical submissions, the firm successfully demonstrated that its export activities to Brazil involved no unfair trade practices. This conclusion follows a preliminary finding from January 2025, which had already determined a negative dumping margin of -33.5 percent for Xingda.

The company has expressed its approval of the resolution, viewing it as a validation of its commitment to transparent and fair commercial operations in the international market.

The company statement read: “Throughout the investigation, XINGDA actively cooperated with the Brazilian authorities through its legal representatives in China and Brazil. By combining full transparency, extensive collaboration, and the dedicated technical efforts of its team, XINGDA was able to demonstrate and substantiate to the Brazilian authority the complete absence of any unfair trade practices in its exports to Brazil. XINGDA confirms itself the integrity of its commercial practices and its unwavering commitment to fair competition and to its valued customers.”