Seeking new options

In the context of legislative demand for lower rolling resistance and compulsory tyre labelling, engineers are facing incredible technological challenges. The European tyre labelling regulations aim to lower carbon emissions, noise pollution and save fuel through use of quality tyres.
Initially the programme only requires that manufacturers benchmark and certify their tyres on three performance parameters: Noise Pollution, Rolling Resistance and Wet Grip. The minimum quality standards based on these would be made more stringent over time.
“All these requirements have opened up the opportunities to the tyre manufacturers to work with different new materials,” says Technical Director at JK Tyre Vijay Kumar Misra told Polymers & Tyre Asia in an interview.
“Materials which flex more and bounce back more slowly exhibit greater rolling resistance than materials which flex less, such as steel or materials which bounce back more quickly, such as silica,” he explains.
Low rolling resistance tyres often incorporate silica in their tyre tread compounds to reduce energy loss without compromising traction. Other factors which affect the amount of rolling resistance include tyre inflation pressure, tread compound, tread design and temperature.
Each component of a tyre has different requirements. Around 40% of a high-performance tyre consists of different types of synthetic rubbers. Solution SBR is one of the synthetic rubbers which deliver a better bond with silica. It helps reduce the friction between the filler particles and as a result improves the rolling resistance.

Way forward

Commenting on the technological trends that he sees in the use of natural rubber and special synthetic fillers in the production of eco-friendly tyres, Misra said that today, there is a worldwide huge crunch for natural rubber, and the rapidly rising NR prices are a major concern for all tyre manufacturers.
“The worldwide shortage of NR is arising mainly due to production cuts in Malaysia and shifting plantations more towards palm oil, the growing usage of NR in radial tyres and an increasing demand in China,” h says.
“In the future, usage of more synthetic rubber and the partial replacement of NR by synthetic polyisoprene are expected to rise,” he says.
Even though natural rubber is traded above US$2 per kg, it is still the first choice for radial truck tyre manufacturers because of its excellent physical and mechanical properties, and better adhesion to steel cord.
Of the available synthetic rubbers, styrene butadiene rubber (SBR) possesses the greatest balance of functional qualities in the widest range of applications. Today, solution SBR is widely replacing emulsion SBR in passenger radial tyres.
“With silica and hybrid filler we have been able to make tyres which are more durable, extremely low on rolling resistance and easy to recycle. I think combination of functional polymers with activated silica is the way forward for future,” Misra says.
Regarding the new compounds and tread-design features that would become integral in upcoming tyres that are designed to offer better rolling resistance, wet grip, and lower wear and noise under new regulations, he said that approximately 4-7% of a car’s fuel consumption is spent overcoming the tyres’ rolling resistance, which is the energy lost from the tyres rolling against the road.
Developing high-efficiency tyres that minimise rolling resistance while develop high-efficiency tyres that minimise rolling resistance and maintaining traction, better handling, greater durability and offer higher fuel economy goals would contribute to higher fuel economy goals. Advanced computational tools, like finite element analysis (FEA), are being applied in designing tyres to isolate regions where energy losses are greatest due to rolling resistance.
The results of these analyses will be useful in developing new tyre designs with features such as special tread patterns, lighter yet stronger materials, reduced sidewalls in order to minimise flexing and higher pressure.
The new regulations demand greener tyres, typically tyres with low rolling resistance which consume less fuel and consequently emit less carbon dioxide and the industry has to rise to the challenges.
The labelling legislation requires all the tyres to be rated according to their rolling resistance, wet grip and tire wear. In places like Europe, the noise generated is just as important as tyre wear in North America.
Companies have to comply with at least the minimum criteria on all the three aspects of rolling resistance, wet grip and noise generation in Europe, whereas in North America, they have to ensure wear resistance, as consumers prefer tyres that have better green efficiency and safety ratings.

Compound elements

“However, legislations, while proving beneficial for one set of materials, have dampened the market prospects of other types of materials. The choice of material used largely depends on the kind of performance that is expected from the tyres,” Misra comments.
Since lowering rolling resistance and improving wet grip has been the main objective of tyre manufacturers, materials such as solution styrene butadiene rubber (SBR) and silica are expected to cannibalise the more conventional materials like emulsion SBR and carbon black.
Traditionally, tyre manufacturers have often had to reduce the rolling resistance at the expense of wet grip. However, more advanced fillers such as silica have helped improve the rolling resistance and wet grip simultaneously, while solution SBR with silica has reduced rolling resistance.
With respect to compound, hardness, heat generation, wear resistance and hysteresis loss are some of the important design factors to modify the tyre rolling resistance, wet grip and noise. Void ratio, block size, side density of tread pattern and sidewall stiffness, tread radius, tread depth, belt width are some of the tyre construction functions which affect the rolling resistance, wet grip, noise of the tyre.
In recent years, the awareness of the conservation of resources and energy has increased globally with a focus on countermeasures for global warming and environmental protection.
Tyres are the only components of automobiles that make contact with the ground, and they make possible the fundamental automotive operations of driving, rolling and stopping by transferring the drive force of the engine to the surface of the ground. It is not too much to say that responding to the demands for greater performance in fuel economy without losing these fundamental performance properties has been the most important challenge for tyre manufacturers.

Energy saving

“With the dissemination of energy-saving tyres, terms such as ‘low fuel consumption’ and ‘rolling resistance’, which were technical terms, have come to be recognized by the general public. In addition, there have been requirements for energy-saving performance in tyres not only for passenger cars, but also for large vehicles such as trucks and buses in recent years,” Misra points out.
Energy-saving performance is achieved through a variety of improvements and innovations from the fundamental structure of the tyre to various types of materials, but among these the role played by the tread, which is the part coming into contact with the surface of the ground, is particularly important.
“This means providing polymer materials that give energy-saving performance and road surface grip, which is related to safety, in the trade-off between these two properties. And this is an area where the most is expected of the manufacturers of polymers for tyres,” he stresses.
The need for reducing fuel consumption leading the market is to be more and more demanding for the performances improvements, which cannot be achieved with E-SBR, making an inevitable gradual trend towards the use of S-SBR.
New generation Solution Polymerisation SBR, S-SBR, synthesized by living anionic polymerisation; has polar functional groups in the polymer backbone, aimed at developing new silica compound grades.
“By introducing polar functional groups and increasing the polarisation of the polymer itself, we can expect to have a good direct effect on the affinity for the hydrophilic silica,” he notes.
“With silica compounding, we cannot expect physical binding as is seen between carbon and rubber components. Therefore, the introduction of polar functional groups to the polymer chain is even more important, as are silane coupling agents. With these developments, future years are for the S-SBR and Silica compound in tyre tread application,” he feels.

New fibres

Regarding polyethylene naphthalate (PEN) tyre-cord’s supremacy in spite of the availability of new cheaper green substitutes, Misra says that being a new generation polymer, PEN is a high- performance member of the polyester family.
“Its unique chemical structure renders it useful as fibres. PEN has a modulus that is five times that of nylon, two-and-a-half times that of polyester, and double that of rayon. It also has very good dimensional stability—low elongation and low shrinkage,” he asserts.
Today, the primary environmental concerns in the tyre industry are automotive fuel efficiency, tyre recyclability and the impact of manufacturing processes on the environment. “PEN, being a lightweight reinforcement with improved properties and it allows tyre weight reduction and reduced rolling resistance to increase fuel economy.
“PEN, with a high Tg, shows excellent behaviour up to 150°C. Since this temperature is beyond what could be reasonably expected in actual tyre performance, PEN could not only be an excellent replacement for rayon, but could also provide the opportunity for tyre designs which are not possible with current reinforcements,” he feels.
Both the fibre and tyre industries are becoming more dynamic, with product improvements happening at a more rapid pace. This is a positive environment for PEN fibre.
Another positive sign is that the tyre industry is slowly evolving from a commodity towards a specialty industry. The trend to redefine value as the performance of component systems rather than looking at individual component cost/performance ratios will also help a higher performance/price reinforcement like PEN.
With these advantages, it is difficult to replace the PEN with new cheaper green substitute for high- performance tyre application, Misra concludes.