It's always been the tire industry's ``Catch 22'': Reduce a tire's rolling resistance, you sacrifice traction; improve traction, you have more rolling resistance and poorer fuel economy. The solution: precipitated silica.
The ``discovery'' a few years ago that using precipitated silica-as a reinforcing agent instead of or in combination with carbon black-can yield significant reductions in rolling resistance has caused major changes in the tire industry. Tire companies began accelerating their research and development in this field, and introduced products utilizing the technology.
Meanwhile, suppliers of silica and related materials have shown their confidence in the viability of the material with a wave of expansions, activity mirrored by specialty synthetic rubber producers.
This development's importance to the tire industry is substantial. About 14 percent of a car's fuel consumption results from its tires' rolling resistance, according to Groupe Michelin. Since nearly 75 percent of a tire's rolling resistance comes from the tread, finding a low hysteresis polymer for the tread compound is critical to reducing rolling resistance.
Michelin claims its technology reduces rolling resistance 35 percent from a comparable conventional tire-a 5-percent reduction in fuel consumption. Its development involves ``refined tire profiles and structural designs, coupled with tread compounds that utilize custom-tailored base polymers.''
The company began volume production of ``green tires'' in mid-1991 at plants in Europe and the U.S., initially for original equipment fitments in North America.
The firm also began offering green tires for the European aftermarket under the ``Energy'' name. Michelin claims it can apply its low rolling resistance technology to just about any of its passenger car tire lines, including performance and snow tires.
Silica has long been used in the rubber industry as a reinforcing agent for shoe soles. But it has found little application in tires or mechanical rubber goods, in part because of dispersibility problems, according to Siegfried Wolff, a Degussa A.G. retiree, considered by many to be the guru of silica reinforcement chemistry.
Because of the earlier problems, each of the precipitated silica expansion projects announced lately has emphasized the improved dispersability of its product.
Blending these newer-generation silicas with more recently developed solution-SBR and poly-butadiene types and tailored carbon blacks significantly reduces the compromises, observers said.
For now, the primary coupling agent used in the silica-rubber compound is silane. But research is being stepped up by users and suppliers to find alternatives, according to Pirelli Group.
The industry has yet to make a wholesale change to precipitated silica; the cost of producing silica-reinforced tires is as much as 12 percent higher than using conventional formulations. Still, silica producers are rushing to meet expected future demand, with 100,000 metric tons of new capacity now under construction.
Only a relatively small portion of passenger tire production has been shifted so far to silica reinforcement, although the pace of change is accelerating, said Rudolf Lotz, director of applied technology at Degussa.
Considering the potential impact of lowered rolling resistance on truck and bus tires, it's only a matter of time before silica finds its way into this segment, effectively doubling the potential market, Mr. Lotz said.
It took the breakthrough of silica used with solution-SBR to virtually eliminate the wet grip/rolling resistance compromise. And using polybutadiene at concentrations up to about 30 parts per hundred also can improve treadwear, without seriously affecting the other characteristics, said a recent Bayer A.G. report.
Nearly all the major manufacturers now have a silica-based product on the market, especially at OE level. Some are keeping their options open and continuing to investigate new developments in activated carbon blacks and specialty synthetic rubbers, such as high vinyl SBRs and BRs.
A wholesale shift to S-SBR away from emulsion SBR-predicted with the beginning of S-SBR production about 15 years ago-has not yet occurred. SR makers still are unsure about the return on investment, and automakers, while demanding less weight and reduced rolling resistance, won't accept the added cost entirely.
Similarly, tire companies are not in a position to absorb the increased costs, leaving the demand-supply situation unresolved.
Another approach to the rolling resistance question is Continental A.G.'s development of the ``ContiEcoContact CP'' performance tire line introduced this summer in Europe, with a similar winter tire scheduled to come out soon.
Earlier, Conti had attempted to tackle not only the rolling resistance problem but the environmental question in general by developing a lighter weight tire using higher percentages of renewable resources, i.e., natural rubber. The ``EcoContact'' has met with mixed reviews, but Conti continues to work on the concept.
Also taking a lightweight approach, but in a more radical direction, is Sumitomo Rubber Industries Ltd. Its SP Reifenwerke GmbH unit in Germany has developed an ultra-lightweight performance tire featuring aramid-fiber belts and beads.
The new Dunlop-brand tire is supposed to be 30 percent lighter than a comparable steel-belted version, and was conceived initially to meet OE demands for weight reduction and, more recently, as a means to reduce rolling resistance.
Michelin and Goodyear also have spoken publicly of their efforts to pare weight from their tires, both to meet OE weight requirements and to help reduce rolling resistance.
Goodyear cites several changes in tire design for the improvements: ``efficient'' mold shape; lighter tire weight; optimal placement of tread components; and proprietary polymer developments.
Recent Goodyear R*&*D efforts include work on utilizing the benefits of solution-SBR, silica, carbon black, other specialty polymers and their many combinations-all at a minor price premium.
Goodyear's first attempt at a lower-rolling-resistance tire for the aftermarket-its Invicta GFE (for Greater Fuel Economy), launched in 1991-was heralded by the company as a ``leapfrog in fuel efficiency.'' But the concept proved extremely difficult to market to the buying public, and test marketing was suspended.
Conti earlier this year displayed a prototype 425/65R22.5 ``super single'' truck tire with a three-layer aramid fiber casing that reduced the tire's weight by 44 pounds vs. an all-steel version.
It used steel belts and beads, but Akzo Fibers B.V., the aramid supplier, said aramid fiber belts and beads ``might be feasible'' because of the high modulus and dimensional stability of the latest generation ``Twaron'' aramid.
Akzo maintains replacing steel with aramid would not only reduce weight, but would lower rolling resistance, increase carcass life and improve retreadability. The higher initial cost would thus be offset by enhanced performance and durability characteristics.
Another entry into the rolling resistance sweepstakes is the so-called ``Janus'' tire concept, brought to market earlier this year by Slovakia's Matador A.S.
Unveiled more than 20 years ago, it replaces traditional steel belts with a solid but ``rippled'' flexible steel or synthetic band.
The tire design-named for German engineer Jonny Janus who holds most of its relevant pat-ents-is said to provide lateral stiffness while being flexible in the rolling direction. In theory, this reduces tread-area squirm and therefore energy-robbing heat.
The concept has been studied at one time or another by nearly all the major tire makers and rejected for various reasons. Mr. Janus is now working with Matador and the former East German tire maker Pneumant Reifen A.G. to bring the design to market.
Just how important the rolling resistance issue might become was evident at last year's Frankfurt motor show, where energy-efficient ``hybrid'' concept cars were featured by at least six carmakers.
Using electric or internal combustion power, these vehicles are generally small, designed for city use or short-haul trips. Because of the power choice, they must be aerodynamic and their tires must have low rolling resistance.
While these prototypes are unlikely to become the norm in the next few years, they do reflect advanced thinking by the vehicle makers, who in turn are trying to anticipate buyer attitudes.
While fuel economy helps sell cars, the same doesn't appear to apply to tires in the aftermarket-at least not quite yet.
Though tires can account for one-sixth of a new car's fuel economy, studies show the average consumer is unaware of this, or ignores it at tire-buying time.
Unlike performance or wet grip, reduced rolling resistance doesn't lend itself to creative advertising. Selling rolling resistance as a desirable performance criterion is being left to the tire dealer.