Tire technology--recent advances and future trends.

Advances in tire materials, tire constructions and tire technologies have led to new products and the development of new market segments. Tire manufacturing technology has progressed in parallel with tire construction technology so that tires are now designed not only to meet specific performance targets, but also to enable improved manufacturability, i.e., more efficient, lower cost and more uniform production.

Increasing automobile manufacturers' requirements and ever growing customer expectations have resulted in the evolution of new product technology. This fascinating product will still continue to develop to accommodate new applications, safety, health and environmental issues, advantages of novel materials like nano composites, plasma surface modified carbon black, development of computer simulation techniques and, finally, to develop a cybernetic or thinking tire.

Materials

In order to support and meet this demand, an all around development has taken place on the material front too, be it an elastomer; new generation nano-filler; surface modified or plasma treated filler; reinforcing materials like aramid, polyethylene naphthalate (PEN) and carbon fiber; nitrosamine-free vulcanization and vulcanizing agents; antioxidants and antiozonants; a series of post-vulcanization stabilizers; environmentally friendly process oil; etc.

Polymers/elastomers

Today, there is a worldwide huge crunch for natural rubber, and the rapidly rising NR prices are a major concern for all tire 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 tires and an increasing demand in China. In the future, usage of more synthetic rubber and the partial replacement of NR by synthetic polyisoprene is expected to rise. Even though natural rubber is traded above $2 per kg, it is still the first choice for radial truck tire manufacturers because of its excellent physical and mechanical properties, and better adhesion to steel cord (ref. 1).

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 tires. Solution SBR is prepared by solution polymerization using Ziegler-Natta catalyst. Solution SBR has no organic fatty acid and there is more control of micro-structure (tailor-made product). This results in better physical properties like lower rolling resistance and better wet grip, which is a most stringent requirement for ultra high performance (UHP) and other tires (ref. 2).

Oil extended butadiene rubber prepared using neodymium catalyst has very high cis content, very high molecular weight and narrow molecular weight distribution. This polymer exhibits better physical properties and higher abrasion resistance, but it is difficult to process (ref. 3). Recently, Bridgestone has announced the development of a synthesis technology that chemically bonds a high cis butadiene rubber (cis BR) with silica at its molecular chain end, resulting in a better performing, more resistant tire (ref. 4). Bridgestone has generated two technologies that, when integrated, led to the development of the end-functionalized high cis butadiene rubber with reactivity to silica. The first creates a polymer chain of high cis BR with an active site at it chain end, by redeveloping the polymerization catalyst that converts the monomer molecules to a polymer chain. The second relates to the chemical conversation of the chain-end active site to the silica-reactive functionality, by reaction with a carefully designed functionalizing agent. Bridgestone said if the new compound, with nonporoustech (nanostructure-oriented properties control technology), is used in a tire tread, it should improve flexibility in cold conditions, yet retain stiffness for warm conditions.

Reinforcing fillers

Carbon black is incorporated into the polymer to achieve several requirements. The most stringent requirements are good tread wear, low rolling resistance and superior traction. Out of these parameters, if one improves, another will deteriorate. These three inter-related properties are sometimes assigned to a so-called magic triangle (figure 1). In recent years, the worlds' leading carbon black manufacturers have come up with innovative grades to meet our conflicting demands.

In the recent past, a lot of development has taken place in reinforcing filler technology. These developments include the introduction and usage of improved grades of silica, development of dual fillers, nanotechnology for improved tire performance, introduction of biofillers such as corn starch in tire treads (eco friendly tires, Goodyear), introduction of polymeric fillers, and ground rubber as a recycling material and eco friendly filler.

[FIGURE 1 OMITTED]

The use of highly dispersible silica, together with silanes and high vinyl solution SBR, has met all critical magic triangle requirements like good dry and wet traction, reduced tread wear and low rolling resistance. Highly dispersible, high surface area silica offers nearly a 40% reduction of rolling resistance and approximately 30% reduction of heat build-up.

The increasing price of crude oil has built up the pressure on the tire and automobile industries to develop low rolling resistant tires with better traction. Filler with a combination of carbon and silica, with a coupling agent, (dual filler technology) shows low rolling resistance with better traction and skid resistance in tire tread compounds. Carbon black developed by a plasma process and nano structure black are new significant developments in filler technology.

Carbon black-silica dual phase fillers reduce hysteresis while maintaining or improving abrasion resistance. This system is less expensive, as the coupling agent requirement is less, and produces a semi-conductive product compared to a full silica filler system. The dual phase filler is less abrasive to the processing equipment compared to the usage of silica filler alone. But, the use of dual phase filler increases the cost of a compound compared to traditional carbon black.

Carbon black by a plasma process replaces the incomplete combustion with direct splitting of the hydrocarbon feed stock oil, using plasma, into hydrogen and carbon black. It produces carbon black of pre-determined characteristics.

Nano-structure black is a family of new carbon blacks characterized by a rough surface and enhanced filler-polymer interaction (figure 2). It hinders the slippage of polymer molecules along the rough nano-structure surface and reduces the hysteresis significantly. This type of black ideally meets truck tire requirements, as it provides improved tread wear in addition to low hysteresis.

Silica filler and clay are hydrophilic in nature, while most of the polymers are hydrophobic in nature. This generates a huge surface energy difference at the polymer-filler interface and results in macro-phase separation. Coupling agents were found to be one of the alternative ways to improve reinforcement by silica, however this requires very lengthy and multistage mixing technology and consuming energy. The efficiency of coupling agents is very low in the presence of other rubber chemicals. Recently developed functionalized fillers overcome this. Functionalized filler is a new category of fillers in which the surface chemistry of the filler is changed for better polymer-filler interaction. This is a suitable way to improve the degree of reinforcement of rubber and polymer-filler interaction by providing functionality to the filler surface, which is stable at normal mixing temperature, but can react with the rubber matrix during vulcanization. This leads to grafting of filler particles on the rubber matrix, with great improvement of physical properties. The shape of the aggregate plays an important role in the distribution of filler in the rubber matrix, and controlled aggregate size distribution (ASD) also helps in processing. This can be achieved through chemical surface modification of carbon black (ref. 5)

[FIGURE 2 OMITTED]

Nano particles and nano composites

Developments in nanocomposites are a significant development of the 21st century. Nowadays, the market value of nanocomposites is $75 million and is expected to soar to $250 million by 2010. The rapid growth in this field will definitely change the material demand scenario for the tire industry to a large extent in coming years. Polymer nanocomposites are organic-inorganic hybrid materials where the inorganic phase like nano silica, nano zinc oxide and nano clay are distributed in nanoscale within the organic polymer matrix. Increased surface area of nano-fillers produces better interfacial polymer-nanofiller interaction. Polymer nanocomposites show a very high degree of reinforcement; high thermal stability (ref. 6); improved impermeability to gases (ref. 7), vapor and liquids; good optical clarity (ref. 8) in comparison to conventionally filled composites; and flame retardancy with reduced smoke emission (ref. 9).

Fillers with extremely high aspect ratios (1,000-10,000), such as carbon nano-tubes, have a much lower percolation threshold (lower amount is required for equivalent reinforcement). Therefore, it might be possible to formulate a compound that both conducts and has all the other advantages of the 'green' tire by the incorporation of relatively small amounts of carbon nano-tubes into the silica filled compound.

Plasticizers/oils

Mineral oils, also known as extender oils, are a wide range of a minimum 1,000 different chemical components and are used for the reduction of compound cost and improved processing behavior (ref. 10). They are also used as a plasticizer for improved low temperature properties and improved rubber elasticity. Basically, they are a mixture of aromatic, naphthenic, paraffinic and polycyclic aromatic (PCA) materials. Almost 75% of the extender oils are used in the tread, sub-tread and shoulder; 10-15% are used in the sidewall; approximately 5% are used in the innerliner and less than 10% are used in the remaining parts of a typical passenger car radial tire. In total, one passenger tire can contain up to 700 grams of oil.

A lot of studies since 1970 have been done and have, more or less, proved the carcinogenic potential of the PCAs. Concern about the toxicity of highly aromatic oil has led to the usage of non-toxic types of extender oil. These non-toxic oils are mostly DAE (distillate aromatic extract), TDAE (treated distillate aromatic extract) and MES (mild extraction solvates).

Existing extender oils can be replaced by RAE (residual aromatic extract), hydrogenated naphthenic oils (HNAP) and blends of highly immobile asphaltenic hydrocarbons. Low molecular weight polymer also acts as a process aid during mixing and processing, and is a potential candidate that can replace the conventional process oil. It gets cured along with the rubber and offers better performance.

Reinforcing materials/fabrics

There are several fibers currently used in carcass reinforcement. High modulus low shrinkage (HMLS) polyester (PET) is typically used in most radial passenger tires and offers a good mix of properties. Nylon is used extensively in bias ply tires where strength is paramount. Rayon is the reinforcement cord of choice in Europe for high performance tires, due to its ability to maintain mechanical properties at high temperature. Aramid fiber offers ultra high performance properties, but at a high cost. This cost limits the usage of aramid to high-end luxury and race car tires.

The performance properties of PEN (polyethylene naphthalate) present opportunities for replacement of rayon or polyaramid in carcass construction. The use of PEN cord in these applications is currently being evaluated in both Asia and Europe. PEN has a demonstrated acceptable flexural fatigue equivalent to PET and rayon. It has equivalent toughness to rayon, which is important for sidewall impact resistance. PEN's superior mechanical properties also afford opportunities to use less fiber in carcass construction, enabling production of lighter weight, more fuel-efficient tires.

In radial tires, the carcass provides reinforcement along the radial direction. To provide reinforcement in the rolling direction, belts are necessary (ref. 11). Steel is the traditional material of choice, but there has been a drive to replace it with lighter-weight synthetic cords. Aramids offer a synthetic alternative, but their cost has been prohibitive. PEN has good fatigue at low twist and superior compressive modulus compared to aramid. This can be translated into a significant reduction in tire weight and improvement in fuel usage. In addition, the use of a polyester fiber like PEN offers greater opportunity to recycle used tires.

The cap ply is wound over the shoulders or the entire structure of the belt to provide a compressive force, which resists the centrifugal forces created in the belt at high speed. Nylon 6,6 has been typically used for cap ply applications, due to its high retractive force at elevated temperatures. PEN cord has been demonstrated to offer similar properties to nylon at high temperature and is an excellent replacement material. Honeywell (formerly Allied Signal) has commercialized a high tenacity/high modulus PEN fiber under the trade name Pentex. In January 1999, Pirelli launched a new tire reinforced with cap made from Pentex in its Dragon EVO Corsa line of radial motorcycle tires. PEN's significantly higher modulus vs. nylon or rayon (360 vs. 50 and 125 g/d. respectively) gives it the dimensional stability required for these high performance uses.

Bridgestone, the largest tire company in Japan and second largest in the world, is also using PEN cord replacing nylon cap in its Regno GR7000 tire. In addition to the dimensional stability, the rigidity that PEN brings to the tire prevents noise generated by the road surface from being transmitted to the car. Tests show that these PEN containing tires reduce road noise by as much as 30 percent, making the tires ideally suited for high-end luxury cars, such as the Lexus.

Post vulcanization stabilizer 

The reversion characteristics of natural rubber are of great concern. A lot of novel chemicals have been introduced to increase the reversion resistance efficiency of NR. Some examples of these are a zinc soap activator (Struktol A73), a silane coupling agent (Si-69), an anti-reversion agent--Perkalink 900, and post-vulcanization stabilizers--Duralink HTS and Vulcuren KA 9188. These materials will enhance the life of the tire and enable the use of more retreading, thereby reducing the material demand (refs. 12-14).

Products

With all-around development, the expectations of tire customers have also grown. Now, customers are more demanding and looking for better mileage (tread wear), lower heat build-up, better ride and handling (dry and wet traction) and environmental friendliness (i.e., low rolling resistance, reduced noise, more durability and less pollution). Keeping customers' expectations in mind, major tire manufacturers have introduced several innovative products. The development of super single tires, run flat technology, active wheel systems, the Tweel tire, solid tires (closed cellular PU fires) and multi air chamber fires are among the major path-breaking achievements in the recent past of the tire industry.

Super single tire

The super single fire is a fire that replaces two fires, increases freight volume by lowering the floor level, decreases total fire weight and rolling resistance (RR), decreases tire waste and reduces materials (figure 3).

Active wheel system

In an active wheel system, each active wheel is equipped with an electric motor. The electric motor not only runs the wheel, but it also can be used to slow and stop it, so traditional disc or drum brakes might eventually be eliminated. By using electric motors to turn the wheels, the large heavy transmission and differential become obsolete.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Polyurethane tire

Solid polyurethane tires can have weight equal to or less than a pneumatic tire and tube, strength and durability, and with wear resistance exceeding that of pneumatic rubber tires by up to 300 percent or a life span of up to four times that of the normal rubber tire. A wide range of PU tires can be developed with pressures ranging from 30 to 175 psi to provide a riding quality and firmness normally associated with rubber pneumatic tires, can be fully molded with a thick self-skin and can be snap-fitted on standard rims with inexpensive mounting tools. Competitive pricing makes these tires affordable compared to regular rubber tires and puncture-proof tubes. In tests performed by an independent laboratory, tire performance was shown to be equal to or better than pneumatic tires. Micro-cellular polyurethane tires are recyclable, with worn-out material being used for such applications as doormats, truck bed liners, racetracks and non-pneumatic tires. The micro-cellular tire will be one of the biggest revolutions in the field of tire technology. A PU tire made of micro-cellular PU foam consists of hundreds of thousands of microscopic air cells, both open and closed, trapped in a dense and very tough PU rubber matrix. The result is a puncture-proof PU tire. Properties of PU rubber can be easily changed from rigid to flexible; hence it becomes an excellent alternative to the traditional pneumatic tire. In PU tires, each micro-cell acts as an independent cushioning agent, and a puncture will not cause the whole tire to go fiat. In addition, these microscopic air ceils will regain their original size, shape and form once the sharp object is removed from the tire, returning the rider to the original comfort and grip of the journey. According to Huntsman Polyurethanes, several companies have conducted trials on micro-cellular PU bicycle tires. As per a January 2006 Smithers report, Amrityre had "positive results" from both independent laboratory and field-testing of its proposed polyurethane tread stock for tire retreading.

Run flat tire

The spare tire is a heavy and, in many cases, unnecessary weight to be carried around in the vehicle. Tires are becoming more reliable and durable, but punctures can still occur. Companies have been studying techniques for eliminating the spare tire for many years. Most of the major tire companies now have products that will run for a defined safe distance and speed when the air is lost. Self-supporting tires with heavy sidewall inserts have required compounders to formulate new recipes that are stiff enough to support the load, but also resistant to the excessive temperatures that are generated when the tire over-deflects. Ultra high tensile steel cords replace or supplement the conventional polyester or rayon carcass of run-flat car tires. The downside of this insert philosophy, which is not required for a very high percentage of the tire's life and in most cases never at all, is that it is a heavy weight addition and detracts from rolling resistance and ride comfort (figure 4).

Other run-flat systems have been developed that incorporate a separate internal support ring of novel materials and designs, which prevents the tire from over-distorting and, hence, dislodging from the rim.

Tire pressure monitoring system (TPMS) for run flat tires

The market is still reluctant to adopt these run-flat tires until there is an integral system that warns the driver of pressure loss. Many new 'smart' developments are hitting the markets that monitor tire temperatures and pressure. This type of monitoring is also beneficial to fleet operators who need to measure and maintain tire operating pressure to minimize tire wear and fuel consumption.

Tweel--the tire of the future

Michelin's new innovation, the Tweel, is a single unit consisting of four pieces, including the hub, polyurethane spokes, a shear band surrounding the spokes and tread band. Its high lateral stiffness improves cornering. The Tweel can be engineered to give five times the lateral stiffness as a pneumatic tire without any loss in ride comfort. It is impervious to nails on the road. The tread will last two to three times as long as today's radial tires, and can be retreaded again. The Tweel's hub functions as it would in a normal wheel. A rigid attachment connects the hub to the axle. The polyurethane spokes are flexible to help absorb road impacts. The shear band surrounding the spokes effectively takes the place of the air pressure, distributing the load. The tread is similar in appearance to a conventional tire (ref. 15) (figures 5 and 6).

Multi air chamber tire

Bridgestone announced in a news release in February 2006 the development of a multi air chamber tire, which has three separate chambers acting as pressure vessels (ref. 16).

[FIGURE 5 OMITTED]

The all-new type of tire contour was born from extensive R&D efforts with a view to the future of tires. The inside of the tire consists of a main chamber in the center and sub-chambers on both sides. The air pressure in each of the chambers can be controlled independently, making it possible to adjust the stiffness balance between the tread area and the left and right sidewalls. This ensures ideal contact in any condition, and thus, a more stable and comfortable drive. It also means performance can be adjusted in line with customer needs, such as comfort or maneuverability.

[FIGURE 6 OMITTED]

Also, as the tire is divided into three parts, even if the air pressure in one of the chambers drops to 0 kPa, caused by a nail puncture, for instance, the remaining chambers will be able to support the weight of the tire, so it will be able to continue to operate for a specified distance (figure 7).

Manufacturing technology

Over a few decades, tire manufacturing technology has undergone a series of changes and the tire has become a high technology product. There are some differences in manufacturing technologies adopted by different companies to achieve these conflicting demands based on their technical competency. However, the major players are trying to develop modular manufacturing systems, e.g., C3M (carcasse, monofill, moulage at mechanique) by Michelin, MIRS (modular integrated robotized system) by Pirelli, BIRD (Bridgestone innovative rational development) by Bridgestone, or IMPACT (integrated manufacturing precision assembly cellular tech) by Goodyear.

Future challenges in material requirements

Performance requirements, environmental issues and availability/cost of the material will mainly drive material requirements in the future. In order to face a huge tire wastage problem causing a major hazard in the environment, future development in rubbery materials will be focused on development of thermoplastic polymers so that used polymers could be recovered by thermal treatment and separation, biological degradable by radiation or addition of a chemical into the rubber compound that could be activated by exposure to radiation, and development of a bio-polymer.

Solution SBR with a star branched structure, alpha methyl styrene grades of SBR and high styrene ESBR are replacing conventional SBR grades. Neodymium catalyst based BR, epoxidized NR and synthetic IR as a substitute for NR are other general-purpose rubbers getting continuous popularity due to different reasons. Polyurethane usage in solid tires (first commercialization by Amerityre passed FMVSS tests), usage of EPDM with improved filled intake/odor related issues--"Insite" technology with metallocene catalysts, and increased popularity of bromobutyl in place of chloro are other major development in elastomers to meet growing performance expectations.

In reinforcing materials, double-dipped polyesters for improved tire durability, plasma treated yams for improved bonding in the tire, increased usage of aramid fabric as belt and application of PEN are the areas where manufacturers are showing interest. The introduction of new styles of steel wire geometry for improved rubber to metal adhesion and new steel wire coating formulations for improved rubber to metal bonding are other focused areas of development.

[FIGURE 7 OMITTED]

A major breakthrough in reinforcing materials can come with the development of nanofibers that can be aligned in a polymer matrix and give high strength to the polymer. This can reduce the capital equipment requirement and recycling problem. Successful development of a molecular architecture where crystallization of planes within the polymer structure would give reinforcement to a flexible amorphous phase could produce a tire without fabric reinforcement.

Technology challenges

New expressways and highways increase demands for more speed capability, low cost/high mileage and energy efficient tires. With the rapid growth of mining and infrastructure industry in developing countries, there is a high demand for OTR tires. New tire technology has to face demand arising out of changes in modern vehicles with faster speed, safety regulations, mechanization in agriculture and construction work, continuous crude oil price increases and aggressive competition.

Technology needs to be geared up for growing radialization that could reach up to even 90-95% in developing countries, H and V rated tires would emerge as brand builders, ride and handling, noise level, rolling resistance, etc., as unique selling propositions, and wet skid becoming a mandatory requirement. Mileage, coupled with high performance, would be the future challenge, and radialization would come faster. Truck radials are to grow in inter-nodal and bus segments. LT radials are used in short haul and hilly terrains. Uniformity, balance, wet skid, etc., may become mandatory.

The bias segment will face challenges imposed by radials. This segment would demand more mileage with high load carrying capacity and at higher speeds. Rising fuel cost will increase the cost pressure--thus high mileage, fuel efficiency and better retreadability will be the unique selling propositions. Mechanization will increase demand for farm and OTR tires. The emergence of high HP tractors leading to larger tire sizes, and radial technology improving mileage and cut/stubble resistance will be the key challenges.

Future developments

Lower aspect ratio for automobile tires, higher load carrying capacity and reduced size for truck tires would be key sectors for development. Farm and off-road tires will face demands for higher speeds, more ride comfort and enhanced traction. Development of advanced higher strength reinforcements will lead to lower weight and reduced thickness for the tire.

Advancement in tire technology is expected to continue to take advantage of emerging technologies, to meet further customer demands and to accommodate new applications. Different tires will be introduced in the front and rear positions of automobiles as a result of the different loadings. Unsymmetrical tread designs will have different patterns on the outer and inner halves of the tread, thus accommodating the different contact forces on the two halves of the footprint, especially during cornering.

Reduction in tire development time and improvement in the quality continue to advance by the use of computers. Customer demands for enhanced performance will lead to further customizing of tire designs for particular applications. Different vehicles may require different tires, fine-tuned to the specific features of each vehicle and its suspensions. New designs may include "zoned" treads with different materials used in the shoulders and in the crown area to optimize the overall performance, or zone tread designs with a relatively wide and deep circumferential groove in the middle of the tread to facilitate water removal and prevent aquaplaning (ref. 17). The development of a tire able to run some distance after air loss will continue to be pursued to eliminate the requirement of a spare tire. Pressure warning devices, which provide a safety alarm and may give the driver enough warning to be able to drive to the nearest aid station, are another research area.

Conclusion

The pneumatic tire, which already performs remarkable functions at a very modest cost, will continue to develop and further enhance its value to the customer.

This article is based on a paper presented at a meeting of the Rubber Division, ACS (www.rubber.org).

References

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(2.) D.C. Blackley, Synthetic Rubbers: Their Chemistry and Technology," Applied Science Publishers Limited, London, 1983.

(3.) ibid.

(4.) ICIS News, Houston, dated July 21, 2006.

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(12.) N.R. Kumar, A.K. Chandra and R. Mukhopadhyay, J. Mat. Sci., 32, 3,717, (1997).

(13.) P.P. Chattaraj, A.K. Chandra, R. Mukhopadhyay and Th. Abraham, Kaut. Gummi Kuns., 44, No. 6, 555 (1991).

(14.) R.N. Datta, A.M. Schotman, P.J.C. Van Haeren, A.J.M. Weber and F.H. Van Wijk, Rubber Chem. Technol., 69, 727, (1996).

(15.) News Clippings, Motoring Channel Staff, "Web Wombat," dated February 11, 2005.

(16.) News release, Bridgestone, Tokyo, Feb. 28, 2006.

(17.) R.A. Ridha and W.W. Curtiss in Rubber Products Manufacturing Technology, ed. A.K. Bhowmick, M.M. Hall and H. A. Benarey, Marcel Dekker Inc., NY, 1994.

Arup K. Chandra, Apollo Tyres Ltd. (www.apollotyres.com)