Silica-Enhanced Rubber Compounds and You

Wait, there's sand in my tire?

It seems like every tire on the market lately is touting their new “silica-enhanced compound.” Wait, what? Is there sand in my tires? What is it about silica that makes it so seemingly magical that literally every maker out there has some kind of proprietary silica blend in their rubber? And why does every tiremaker have to keep their blend a secret somewhat more closely guarded than nuclear codes?

If you do any research on silica as a tire additive, the first thing you may find is that every source of information on the interwebs will probably tell you something different. Silica increases wear resistance but decreases grip. Silica increases grip but decreases wear resistance. Silica decreases rolling resistance but requires the blood of faeries. That sort of thing. The thing about silica is that it is, in a manner of speaking, magical. Silica has properties that when blended with tire rubber, allows tire engineers to decrease rolling resistance while increasing grip, breaking some rules that used to be thought unbreakable. So here's what silica does, and why there really is sand in your tires, but no faerie blood. That's what the powdered unicorn horn is for...

A tire's specialized rubber compound is a mix of many different materials, particularly forms of both natural rubber and synthetic rubber. Fillers are used both to help bond the different rubbers together and to create various effects in the resulting compound, whether softening or hardening the rubber. These fillers include such materials as petroleum oils and carbon black. Since these are major pollutants, many tire companies have been searching for ways to replace both additives with something a little more eco-friendly.

Tire engineers first began experimenting with silica as an alternative filler in tire rubber in the 1970's as a way of attempting to lower rolling resistance and get better fuel mileage from their tires. At first they found that adding silica definitely lowered rolling resistance, but at the expense of also lowering grip. Then they tried a mixture of pure silica and a substance called silane, which is a hydrosilicate, or silica with hydrogen bonded to it at the molecular level. That has done the trick.

To understand the miraculous effects of the silica-silane mixture, one must understand that since the development of pneumatic tires, engineers have lived by a simple and immutable law – soft tire compounds get more grip, but wear faster and have high rolling resistance, while harder compounds wear slower and have lower rolling resistance, but get less grip. The inevitable tradeoffs that engineers must make between grip, rolling resistance and treadwear is known as the “magic triangle.” To properly balance these properties for a specific tire has been the goal of every tire engineer who has ever mixed a compound.

The issue is in the physical property known as hysteresis. Hysteresis is a measurement of how much energy an object returns when rebounding from a deformation. A good example of this is to imagine dropping a Superball and a hockey puck from identical heights. The Superball bounces back to nearly the height at which it was dropped, because it returns nearly all of the energy from the impact with the ground. This is considered low hysteresis. On the other hand, the hockey puck barely bounces at all, because it loses a great deal of energy by not deforming and rebounding. This is high hysteresis.

Most of the rolling resistance of a tire comes from the way in which it deforms and rebounds as the tire revolves under load, which is known as a low frequency distortion. If the tire compound has low hysteresis at low frequencies, it rebounds like a spring and loses less energy, meaning greater fuel economy. On the other hand, tire grip is determined by how the rubber compound distorts around the unevenness of the road surface, which is known as a high frequency distortion. If the tire has high hysteresis at high frequencies, it conforms to the tiny gaps in the road rather than “bouncing” and gives better grip.

When tire engineers began using silica and silane together as a filler material, they came to understand that the silica-silane compounds definitely lowered rolling resistance, but in complete opposition to the magic triangle, they also enhanced grip while keeping wear constant. Somehow, the use of silane allows both natural and synthetic rubber to bond together much tighter at a molecular level, and produces a rubber compound that has both low hysteresis at low frequencies and high hysteresis at high frequencies, allowing tire engineers to literally both have and eat their cake. The magic triangle has been blown to smithereens by the magic compound. According to a paper on this issue in the journal Rubber World: “The use of silica can result in a reduction in rolling resistance of 20% and can also improve wet skid performance by as much as 15%, substantially improving braking distances at the same time.”

Silica also provides substantial benefits when used in winter and all-season tires. Silica-silane compounds remain much more flexible at low temperatures, making them ideal for winter tire compounds, and producing low rolling resistance winter tires with the same miraculous grip and wear resistance. Together with new techniques of cutting siping patterns, this has produced a revolution in the tire industry that has basically destroyed all the old rules and set everything we used to know on its ear.

The other big issue to solve with silica-enhanced compounds has been the difficulty and high price of deriving pure silica from sand for use in these compounds. It appears that Goodyear has made a breakthrough in that area recently by figuring out how to derive pure silica from the ashes of burnt rice husks. What will they think of next?