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.

 

The Recycling and Economic Development Initiative of South Africa (REDISA) called out the country’s waste tyre recycling system a ‘ticking time bomb’. The country with an estimated population of about 62 million has more than 13 million registered vehicles including roughly eight million passenger cars and generates an estimated 200,000–250,000 tonnes of waste tyres from road vehicles alone each year.

This has created a major environmental and waste-management challenge alongside rising vehicle ownership.

Commenting on the issue, Executive Director of Operations at REDISA Stacey Jansen told Tyre Trends, “Waste tyre management in South Africa has, in effect, collapsed since the Waste Management Bureau under the Department of Forestry, Fisheries and the Environment (DDFE) took over in 2017. The effect is overfull depots posing significant fire risks including the dumping and burning of tyres illegally causing harmful chemicals to seep into groundwater and causing severe air pollution.”

“Economically, a huge opportunity is being missed, in that a structured management programme geared towards recycling can not only create jobs but also contribute to the circular economy as a whole. This was precisely what REDISA did between 2013 and 2017,” she added.

She also stated that internal research has shown that a functional waste plan for just 13 waste streams could raise South Africa’s GDP growth by 1.5 percentage points. For a country struggling with unemployment and stagnation, this is an avenue that must be pursued.

REDISA alleges serious governance failures within the DFFE and the Waste Management Bureau. The first problem is that no dependable data exists.

“We all know that there is a problem, but we don’t know the extent of it. The department’s figures and reports are filled with inconsistencies and errors and this impacts any effective decision-making on how to fix the issue of waste tyre management,” said Jansen.

Secondly, she argues that there does not seem to be a realisation that the government cannot handle waste tyre management on its own as it does not have the expertise, technology or experience.

Thirdly, more headline-grabbing issues such as conservation and climate, which are important, of course, receive a lot of attention. But ground-level interventions such as waste management, while not as media-friendly, offer real and relatively immediate ways to address environmental and economic problems, she stated.

THE BOMBARDING

The Biesiesvlei depot fire in 2023 caused extensive environmental damage. Alluding to the lessons learned from the incident, Jansen said, “This is a question perhaps best posed to the DFFE. Since that disaster, we have not seen a country-wide response that puts the safety of citizens and the environment first. If something isn’t done on a national scale, more depots will burn, releasing extremely toxic pollutants into the air.”

Moreover, the auctioning of nearly R100 million (USD 5–5.5 million) worth of unused pre-processing equipment has been called an ‘admission of failure’ by REDISA. Commenting on this, Jansen said, “We wish the government could tell us how they ended up idle. Either they bought the wrong equipment or they were unable to deploy it. The right decisions were clearly not made by the leadership in the department.”

Moreover, the exclusion of small businesses and micro-collectors from the current system has also impacted tyre collection, illegal dumping and rural employment.

According to Jansen, from 2013 to 2017, REDISA managed waste tyres in South Africa. In a short space of time, it built 22 tyre collection centres, employed more than 3 000 people and created 226 small waste enterprises.

This was all funded by a management fee levied on plan subscribers (producers and importers) as part of the approved Industry Waste Tyre Plan. In February 2017, following a legislative change, the state imposed an environmental levy, which replaced the fee REDISA was collecting. The levy is still being collected today, but the producers and the citizens are not seeing their money channelled into effective waste tyre management.

In fact, more than half of the money collected is going into the general tax fund. The result has been job losses, mostly in urban areas.

REDISA also claimed that the government underspent on tyre transport due to lack of storage space. Answering how does this contradiction affect the integrity of the waste tyre management system, she said, “The department admits this underspend and gives the reason in its latest annual report. They are silent on the consequences, but it can only lead to illegal dumping and burning of tyres. If you drive by almost any informal settlement or urban fringe in South Africa, you will see dumped tyres. And this could be transformed into an asset under the right system.”

CLEAR VIEW

During her interaction, Jansen encouraged citizens and journalists to visit waste tyre depots in their communities and see if they adhere to safety standards viz-a-viz 6-metre fire breaks between heaps, 8-metre gaps to buildings and fences, maximum heap size of 10 metre x 20 metre and more.

Collectors and transporters regularly complain to REDISA that the situation at the overfull depots and dumps have worsened so much since 2017 and that they are deeply concerned.

Questioning the sustainability of the current approach, Jansen said that generating nearly 70,000 waste tyres every day makes an over-reliance on storage depots deeply flawed. “This is not sustainable at all. The only outcome will be increased air pollution, contaminated groundwater and heightened fire risks. It is an attempt to apply a band-aid to the problem without addressing its root cause,” she said.

Jansen was equally critical of the DFFE’s decision to issue tenders for 32 new depots covering close to one million square metres. According to her, the move signals more than a stop-gap response. “I would describe it as an acknowledgement of defeat and clear evidence of an inability to effectively address tyre recycling in South Africa,” she added.

Reflecting on South Africa’s earlier leadership in circular tyre waste management, Jansen said restoring that position would not require sweeping policy or structural reforms. “The DFFE does not need new frameworks or radical changes. What is required is leadership that acknowledges the scale of the crisis and a willingness to return to a model that has already proven its worth, the internationally recognised REDISA model,” she said.

The warning signs are no longer theoretical. Idle equipment, expanding depots and rising illegal dumping point to a system drifting further from circularity. Without decisive leadership and a return to proven, accountable models, South Africa risks compounding environmental damage, economic loss and public health threats, allowing a ticking time bomb to keep counting down.

Ecolomondo Corporation, a leading Canadian innovator in sustainable scrap tyre recycling technology, has engaged August Brown, LLC as an independent risk advisor. This appointment supports the planning stages for a new facility in Shamrock, Texas. The firm will conduct a validation of the project's business plan and risk management approach, a step taken in preparation for marketing the green bond that will finance the development.

The proposed Texas site will feature a six-reactor plant, replicating the company’s proprietary, modular Thermal Decomposition Process (TDP) technology currently operating at its Hawkesbury, Ontario, facility but with triple capacity. This expansion follows the successful commercialisation of Ecolomondo’s proprietary TDP technology. Local support has been secured through the Shamrock Economic Development Corporation, along with a 136-acre industrial site and long-term feedstock agreements intended to supply ongoing operations.

August Brown's role will begin with a comprehensive feasibility study examining business, market and financial risks. A subsequent phase will focus on engineering, technology validation and project execution risks. This independent review process aims to improve transparency and strengthen confidence among potential bondholders and project partners. The project represents the next phase in the company's growth strategy, replicating its proven modular technology on a larger scale.

Eliot Sorella, Executive Chairman, Ecolomondo, said, “Independent validation of our technology, projected operations and financial model for our planned Shamrock Facility is an essential step that resonates strongly with investors, lenders and potential joint-venture partners.”

The Wacker Group is showcasing two new silicone-based impact modifiers, GENIOPERL W37 and GENIOPERL W38, at the JEC World composites exhibition. These additives are engineered to enhance the mechanical properties of thermosetting resins such as epoxies and vinyl esters. Their specialised molecular structure, built on functional silicone, facilitates a distinct phase separation within the resin matrix. This process creates tiny elastomeric domains that increase toughness and help prevent composite materials from fracturing under stress. Sustainability was a key consideration in their design, leading to a notably reduced cyclics content. Both modifiers disperse readily with simple mixing equipment, maintain their effectiveness even at low concentrations and do not compromise the material’s inherent strength, viscosity or thermal resistance. The company is located at booth 5N142 at JEC World, taking place in Paris from 10 to 12 March 2026.

GENIOPERL W37 is specifically formulated to boost impact resistance in low-temperature environments. It is recommended for use at concentrations between two and eight percent by weight, a level at which it has minimal impact on the resin’s viscosity or the cured product’s glass transition temperature. Achieving optimal dispersion requires processing temperatures of at least 50 degrees Celsius. Similarly, GENIOPERL W38 also improves impact strength at very low temperatures when used within the same dosage range. It offers the added benefit of containing anti-foaming agents, making it particularly suitable for casting processes conducted under reduced pressure.

A third major highlight at the Wacker booth will be POWERSIL Resin 710, a silicone compound developed for components that must endure extreme heat. This material can be processed using compression moulding, pressure gelation or injection moulding. Parts manufactured from it meet the criteria for thermal class R, signifying their ability to withstand prolonged exposure to temperatures reaching 220 degrees Celsius. As an alternative to high-performance polymers like PTFE and PEEK, POWERSIL Resin 710 provides excellent electrical insulation, mechanical strength and UV stability. It is solvent-free, has a low viscosity for easier processing and is available in both peroxide-curing and catalyst-curing versions.

Wacker’s exhibition will also feature a range of other specialised products for the composites industry. These include SILRES silicone resins for enhancing electrical insulation and flame retardancy, HDK pyrogenic silica for precise rheology control, VINNAPAS low-profile additives to reduce shrinkage and GENIOSIL organofunctional silanes for promoting adhesion and treating fillers and fibres.

Sri Trang Agro-Industry Public Company Limited (STA) has formally committed to the Science Based Target Initiative for Industrial Greenhouse Gas Reduction towards Net Zero (Phase 3), organised by Thailand Greenhouse Gas Management Organization (TGO) in collaboration with the Center of Excellence in Eco-Energy, Faculty of Engineering, Thammasat University. This declaration positions the company among 16 leading Thai organisations committed to embedding scientifically validated climate targets throughout their operations and supply networks.

STA has established a target to cut Scope 1 and 2 emissions by 23 percent by 2030, using 2024 as its reference point, with the ultimate ambition of reaching net zero by 2050. These goals directly support the international objective of capping global warming at 1.5 degrees Celsius. Beyond direct emissions, the company is enhancing its rubber and teak plantations to function as carbon sinks, generating certified credits while supplying raw materials. This strategy aligns with its net zero pathway and responds to the European Union’s Corporate Sustainability Due Diligence Directive, which promotes heightened corporate environmental accountability.

By embracing this initiative, STA underscores its vision of evolving into a low-carbon, fully integrated natural rubber enterprise. The company aims to reconcile commercial growth with ecological and social stewardship, thereby aiding Thailand’s wider shift towards a sustainable, low-carbon future.