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 Association of Natural Rubber Producing Countries (ANRPC) has released its Monthly NR Statistical Report for February 2025.

A statement from the organisation says that although NR (natural rubber) prices fluctuated significantly this month, they were nevertheless on the rise as compared to the prior month. The market's upward trend may be attributed to a number of important variables, including the US tariff policies, the EUDR's deferral and the robust demand from the tyre sector.

The report further highlights that China saw a spike in demand after the holidays, which was fuelled by an increase in downstream tyre manufacturing. According to recent reports from ANRPC member nations (AMC), changes in India's 2024 production estimates are expected to contribute to a marginal 0.4 percent rise in worldwide NR output in 2025 over 2024. Furthermore, the 2025 demand prediction indicates a modest increase of 1.7 percent.

GRP Limited has commenced commercial crumb rubber production at its new manufacturing unit in Solapur, Maharashtra.

The company invested approximately INR 250 million in the new facility, with funding sourced from borrowings ( INR 180 million ) and internal accruals ( INR 70 million ). The new unit has an annual production capacity of 31,875 metric tonnes of crumb rubber.

According to the filing to the BSE, the new manufacturing facility is strategically positioned to meet growing demand across various sectors, including reclaim rubber, tyre pyrolysis, tyre manufacturing, road surfacing and rubber goods production.

Located in the MIDC Industrial Area in Chincholi, Solapur, the unit began commercial production on  24 March  2025. The facility represents a significant expansion for GRP Limited, with the company noting that it is a new manufacturing location with no existing production capacity.

Covestro, one of the world’s leading manufacturers of high-quality polymer materials and their components, has announced that it will open an automated laboratory for optimising coating and adhesive formulations to provide better support to its customers.

Formulations for coatings and adhesives including Covestro binders and crosslinkers will be tested in the new facility. Properties like hardness, adhesion, opacity, gloss or durability are guaranteed by their astute selection. The qualities of the finished product are determined by the mix of these formulas, which usually include seven to 15 components. Standard formulas are typically employed due to the large number of conceivable combinations that arise. The computer-aided design of test series and automation have also made it possible for the new laboratory to conduct longer test series.

The new facility can operate 24/7 with a goal to conduct tens of thousands of tests annually. In terms of quantity, diversity, accuracy and testing speed, this establishes a new benchmark. A significant quantity of structured data is produced by the automated laboratory. As a result, the body of information regarding formulation options and contributing factors will expand quickly. To further enhance formulations, the gathered data is assessed using specialised machine learning algorithms in conjunction with measurement data from previous experiments. In order to create a self-learning system, artificial intelligence is also employed to forecast future experiments based on property goals and concurrently validate them in the automated laboratory.

Apart from creating 1K and 2K systems based on water and solvent, the automated laboratory also conducts a variety of material tests on the applied films, formulations and raw materials. To replicate product use under application settings, application can even be conducted in a lab setting with varying climates. The quick transfer of created samples to more specialised testing labs is another benefit of Covestro's ongoing laboratory digitisation. This facilitates the addition of market-specific test findings to the datasets and speeds up the identification of pertinent relationships.

Thomas Büsgen, head of the laboratory, said, "With our automated laboratory, we can work together with our customers on the future of coatings and adhesives. Because it operates almost completely autonomously and learns from our existing knowledge and data lake as well as newly generated data, it makes the process of optimising and developing formulations many times more efficient and precise. This allows us to optimise existing formulations faster or even develop completely new formulations for and together with our customers. We can say: we are reaching a new level of modern research.”

Martin Merkens, Head of Sales & Market Development EMLA in Covestro's Coatings and Adhesives business entity, said, "Our new automated laboratory gives us more possibilities for testing formulations. It relieves our specialised laboratories of their standard tasks and can analyse samples more systematically. This allows us to focus our expertise and experience even more on customer-specific topics or try approaches we couldn't have implemented otherwise. This will particularly help us in the area of circular economy: Alternative raw materials, for example bio-based or recycled materials, can be tested faster and evaluated for their properties in the final product.”

Covestro has successfully completed the modernisation of its TDI (Toluene Diisocyanate) plant in Dormagen, Germany. The facility was formally commissioned by Covestro as it unveiled its new goal of boosting production energy efficiency during an event attended by over 60 visitors from the commercial, political and personnel sectors, including Oliver Krischer, North Rhine-Westphalia Environmental Minister.

By using 80 percent less energy than traditional methods, the modernised plant reduces CO2 emissions by 22,000 tonnes annually. This is made possible by a new reactor that is over 20 metres high and weighs more than 150 tonnes. It produces steam using the reaction energy that is created. Covestro began the modernisation process in the summer of 2023. Over 3.5 kilometres of new pipes, over 14 kilometres of cables and hundreds of new pieces of equipment, valves and monitoring devices were placed throughout the facility. Through its federal subsidy programme for energy and resource efficiency, the Federal Ministry for Economic Affairs and Climate Action (BMWK) has provided assistance for this modernisation.

The company targets a 20 percent reduction in CO2 emissions from energy use per tonne of product by 2030 compared to 2020, underlining energy efficiency as a key component in reaching operational climate neutrality by 2035. With a 300,000-tonne-per-year capacity, the Dormagen TDI facility stands as a shining example of how to successfully convert current manufacturing facilities to be more energy-efficient.

Dr Philip Bahke, Head of the North Rhine-Westphalia Site Network, said, "The successful completion of this project shows that climate protection and competitiveness can go hand in hand. The modernised plant sets new standards in energy efficiency and underlines our path toward climate-neutral production. In light of persistently high energy costs, the project significantly strengthens the competitiveness of TDI production in Europe. My special thanks go to the entire project team, who professionally implemented this complex modernisation during ongoing operations."

Dr Christine Mendoza-Frohn, Head of Sales Performance Materials for the EMEA and LATAM regions, said, "In Dormagen, we operate Europe's largest TDI production plant. With the successful modernisation, we can now offer our customers TDI with an even better carbon footprint. This supports our customers in achieving their own sustainability goals and strengthens our position as a reliable partner for climate-neutral and circular solutions."