Issue Date: November 16, 2009
Stretching Tires' Magic Triangle
During the presidential campaign of 2008, then-Sen. Barack Obama (D-Ill.) suggested that U.S. drivers could save as much energy through proper tire inflation and car maintenance as oil companies could gain by drilling for oil from new sources offshore.
Obama’s opponent, Sen. John McCain (R-Ariz.), belittled the idea and even handed out tire pressure gauges with “Obama’s Energy Plan” printed on the side. FactCheck.org, an online project at the University of Pennsylvania’s Annenberg Public Policy Center that checks the accuracy of claims made during campaigns, delivered a verdict: Proper tire inflation might save a lot of energy cheaply and quickly, but it can’t quite deliver the volume of energy that offshore drilling can.
The political theater highlighted the importance of tires in the energy picture. Overcoming the resistance that tires encounter when they roll accounts for 20% of the fuel used in the average car, according to the French tire manufacturer Michelin. By itself, rolling resistance is responsible for a startling 4% of worldwide carbon dioxide emissions from fossil fuels, Michelin says.
Tire makers and their raw material suppliers have been eyeing lower rolling resistance as a way to boost fuel economy. But they are constrained by a principle in the tire world called the “magic triangle of tire technology,” which holds that an improvement to rolling resistance has to come at the expense of wet-road grip and durability.
“Optimizing the rolling resistance while still respecting the other functions that a tire needs to deliver is really the key to our work,” says Forrest Patterson, technical director for car and light-truck replacement tires at Michelin. Armed with new forms of silica and even nanomaterials, chemical companies are trying to make that work easier. As consumers become more environmentally aware and as fuel-efficiency regulations become increasingly stringent, chemical firms see a lucrative market for “green” tires, which deliver on environmental performance via increased fuel efficiency.
The main proponent of green tires has been the auto industry, which wants to boost fuel economy to meet consumer demand and government fuel-economy standards, says Paul A. Ita, principal of Notch Consulting, a market research firm in Amherst, Mass. Green tires are “a relatively painless way for car makers to squeeze a mile or two per gallon out of the car,” he says.
In the tire aftermarket, Ita notes, a future driver for growth might be new labeling—something Europe will require in 2012—that will disclose how tires perform in fuel economy, wet grip, and noise level. Similar labels are also being considered in the U.S. by the National Highway Traffic Safety Administration. But they would show tread life instead of noise performance.
The key to rolling resistance is a property called tan δ, according to Avraam I. Isayev, a rubber expert and professor of polymer engineering at the University of Akron. The property is defined as the ratio of the energy dissipated during dynamic stretching of a material to the energy released when it relaxes back to its normal position. A metal spring has a tan δ of close to zero, meaning it loses very little energy when it is pulled and let go. “Other materials, like rubber, rubber filled with carbon black, or rubber filled with silica, will have losses due to molecular and rubber-filler interface motions” and would have a higher tan δ, Isayev says.
Tires contain materials such as carbon black and silica fillers, as well as oils and other additives, surrounded by a matrix of polybutadiene and styrene-butadiene rubber. These components are interconnected through vulcanization, organosilane coupling agents, or mechanical forces. “The more movement you have because of loose ends or because of friction in the system, the more you are going to generate heat,” Michelin’s Patterson says. “The root cause of rolling resistance is the way the material functions at its microstructure.”
According to Isayev, in the temperature range of 50–70 °C, rolling resistance correlates directly with tan δ: The lower the tan δ, the lower the rolling resistance. Reducing the tan δ in this temperature range, however, also reduces the tan δ at lower temperatures. But good wet grip corresponds to a higher tan δ at lower temperatures. This is why the wet-grip and rolling-resistance corners of the magic triangle are hard to pull apart.
Michelin researchers succeeded in defying the magic triangle in 1992 when they added precipitated silica to tire treads in place of carbon black, according to Timothy A. Okel, senior research associate for elastomer applications at the precipitated silica maker PPG Industries. Tire makers matched highly dispersible silica with a coupling agent, bis(triethoxysilylpropyl)tetrasulfide, to attach it to the rubber, he says.
The innovation allowed tire makers to reduce tan δ in the temperature range important for rolling resistance while also increasing it in the range for wet grip. “What was discovered was the capability of pushing out two of those ends of the triangle while maintaining one of them,” Okel says. Other experts, however, have noted that silica-based tires are not as resistant to abrasion as the original carbon-black-based treads.
These green tires had a rolling resistance of up to 30% less than that of the carbon-black-containing radial tires that preceded them. The new technology brought rolling resistance down to about 10 kg per metric ton, which results in a force equal to driving up a 1% gradient.
To put this advance in perspective, Patterson cites Michelin data showing that solid rubber tires, the norm until the 1890s, had a rolling resistance of 30 kg per metric ton. The first pneumatic tires cut that down to 25 kg per metric ton. Tire cord construction, introduced around the time of World War I, reduced resistance by another 20%. And radial tires, which came on the scene in the 1950s, brought it down to 15 kg per metric ton.
Since tire makers started incorporating silica in the early 1990s, their understanding of how to use it has gradually evolved, enabling them to lower rolling resistance even further. Both chemistry and design come into play in Michelin’s latest generation green tire, the Energy Saver A/S, which the company says is 8% more fuel efficient than other tires in its class and offers better wet-braking performance. Michelin’s competitor, Goodyear Tire & Rubber, says its new Assurance Fuel Max tire offers 4% better fuel efficiency than its previous Assurance tire while retaining wear and traction properties. Fuel Max tires will come standard on the Chevrolet Volt hybrid vehicle and the latest iteration of the Ford Fusion.
The tire industry has turned into a huge market for precipitated silica makers, Ita notes, and with the continued interest in green tires, the importance of this material to the tire industry is ever increasing. Tire manufacturing consumes about 485,000 metric tons of silica globally each year. Of that, about 215,000 metric tons goes into the treads of green tires. The rest is employed for other uses such as adhesion promotion. Before the recession hit the brakes on tire sales, Ita says, the market for silica in green tires was growing at about 9% per year.
With strong growth and continued interest from automakers and tire companies, chemical makers have incentives to create more ways of boosting tire performance. They have been responding by again trying to expand the magic triangle.
One of the frontiers of innovation is developing silicas that yield better tire properties by providing higher reinforcement of the rubber, says Christophe Geffray, the market development manager for Perkasil precipitated silicas for tires at W.R. Grace. The company does this with micropores that increase the surface area of the silica particles and thus the contact between them and the rubber.
There is a trade-off, though. “Most of the time, the more you increase the surface area, the more difficult it is to disperse this product,” Geffray says. And dispersion, he notes, is a critical technical issue for precipitated silica, which makes up nearly 40% of the compounded rubber in today’s green tire treads.
Dispersion relates directly to the rolling-resistance properties of the tire, Akron’s Isayev says. “You need to obtain uniform distribution of particles within the matrix of the elastomers,” he explains. “If you disperse better, you get less energy loss.” For example, Goodyear says the breakthrough in its Assurance Fuel Max tire was a polymer blend for dispersing the silica within the rubber matrix.
But by and large, experts say, coupling agents haven’t changed much over the years. In the October 2009 issue of the German polymer technical publication Kunststoffe International, Werner Obrecht, a project manager at Lanxess, opines that most of the emphasis in silicas in recent years has been on processing technologies and getting the most out of existing ingredients. “Even the bifunctional organosilanes, which were crucial for dispersing the silica and coupling it to the rubber matrix, are virtually unchanged since the 1970s,” he wrote.
Chemical companies do have development activities under way in that area. Several years ago, Momentive Performance Materials introduced its NXT family of silanes, meant to reduce the number of mixing steps in the tire manufacturing process. And last year, organosilane maker Dow Corning teamed up with silica maker Rhodia to develop new silica and silane combinations for fuel-efficient tires.
But Isayev doesn’t dispute Obrecht’s assertion. Although a lot of innovative organosilane chemistry has been done in the lab, he says, only a few new molecules that improve tire properties have been released commercially in recent years, probably because the development and manufacturing required is too expensive.
Grace’s Geffray says his company looks to the silica itself to improve dispersion, namely by altering its surface chemistry. He won’t disclose how this is accomplished but says it’s done at the particle surface level when the precipitated silica is drying.
In a new technology that it launched earlier this year, PPG adds coupling and compatibilizing functionality to the silica particles themselves. One of the aims of this new Agilon technology is to make manufacturing easier, Okel says. Similar to what Momentive’s NXT silanes do, the Agilon technology eliminates a number of mixing steps in the tire manufacturing process. It circumvents the need to react the organosilanes and the silicas together when mixing the tire rubber.
At the same time, Okel says, the company was able to achieve better performance and expand the magic triangle. “You can double the impact of the current conventional system,” he claims. That’s “another 6% increase in fuel efficiency while also having the ability to improve tread wear and traction.” The new technology transforms the normally polar silica into a less polar material, Okel explains, helping it interact better with the rubber. An improved coupling agent and a silica core are other features of the Agilon system, he adds.
Rubber maker Lanxess aims to expand the magic triangle beyond the limits of what silica and organosilanes can do by launching a material never before seen in tire tread. Called Nanoprene, the material is a nanogel particle with a highly cross-linked core and an outer coating of hydroxyl groups. These groups interact with the polar ingredients in the rubber compound, such as silica, via dipole interactions and hydrogen bonding. Lanxess’ Obrecht compares it to a “spice” that improves the rubber formulation.
The company says Nanoprene improves the abrasion resistance, grip, and rolling resistance of tires, pushing out all three corners of the magic triangle simultaneously. Lanxess recently began commercial production of the material. Its first customer, Toyo Tire & Rubber, will use it in winter tires.
Akron’s Isayev has seen claims like these about the magic triangle before. “I’m not so sure,” he says, noting that only testing and real-life use of the tires will prove whether or not they are true.
But a tire company can put all the energy-saving, magic-triangle-defying technology in the world in a tire’s tread, and it would all go to waste if a tire is underinflated. ExxonMobil Chemical, which makes halobutyl rubber for tire inner liners, has developed two new materials to help tires retain air.
One of them is the newly introduced Exxcore DVA resin. DVA stands for dynamically vulcanized alloy. ExxonMobil makes the material via reactive extrusion of nylon with a brominated, copolymerized elastomer of isobutylene and p-methylstyrene (BIMSM). “Exxcore DVA has the unique property of having the low permeability of a plastic with the flexibility and low-temperature durability of a rubber,” says Vanessa Talbot, butyl global marketing manager at ExxonMobil. The air permeability of the resulting film is as little as one-tenth of its other halobutyl rubber inner liners, the company says.
ExxonMobil also has pilot-scale facilities making a composite of BIMSM and organically modified nanoclay. This material has air retention properties that exceed those of halobutyl rubbers by about 50%. That’s not nearly as much improvement as that offered by Exxcore DVA, but the new material can be mixed and cured in the same process as conventional inner liners. It can even be made thinner, saving on weight, materials, and rubber-mixing throughput. Exxcore DVA, in contrast, is a film that is applied to the tire with an adhesive.
But ExxonMobil’s new materials are for more than making President Obama happy: They will help ensure that the chemistry going into green tires is not wasted. Talbot points out that the newfangled tires made with precipitated silica and other materials are tested with proper inflation. Some 40% or so of real-world motorists, she says, are driving around on underinflated tires. “It does not matter what tread technology is being utilized,” she says. “Rolling resistance is not at its optimum if a tire is losing air.”
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