Chemical Reactions Replace Simple Mixtures in Rubber Compounding

Rubber revolution

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Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.Cycle World

Since 1990 the compounding of tire tread rubber has undergone a profound change. As former Dunlop engineer Dave Watkins put it in 2012, “Before, tire compounds were mixtures. But today chemical reactions are the key to compounding.”

In former times, polymers, reinforcing carbon black, processing oils and waxes, tackifiers, and a suite of cure agents were charged into the top of a large and frightening machine called a Banbury mixer. By a process of vigorous shearing, driven by electric motors up to 4,000 hp, the Banbury distributed the carbon black and other mate­rials into the rubber to produce a uniform plastic mass, which could be forced through an extruder to produce “green” (meaning unvulcanized) tire tread.

Long-chain polymers such as styrene-butadiene rubber (SBR), butyl (BR), or natural rubber (NR) provide the necessary elasticity. Processing oils and waxes ease and accelerate mixing and provide control of softness. Tackifiers provide stickiness that allows tire elements to be assembled. Cure agents convert gooey rubber into an elastic solid by vulcanization, which employs heat and sulfur to chemically cross-link rubber chains to each other. Carbon black reinforces the rubber chains by attracting them to its surface via so-called “short-range forces.” Imagine a pot of steel bead chains of the kind used as light-pulls: The chains are free to slide over each other just as rubber chains are in unvulcanized rubber. The fine particles of carbon black are like myriad tiny magnets, mixed into the pot of bead chain—by attracting the chains to their surfaces, they reinforce the whole, improving abrasion, cut, and tear resistance. For many years, the progressive improvement of rubber required production of ever-finer carbon blacks and finding ways to uniformly mix such fine blacks into rubber without heating it so much that it begins to cure in the mixer (premature curing is called “scorch” in this business).

It’s easy to make durable hard rubber—that’s what taxis and delivery vans run on—but at some sacrifice of grip. What’s hard is to combine grip-enhancing softness (soft rubber more easily squeezes into intimate, large-area contact with pavement texture) with high-tensile strength (necessary to transmit high grip force into the tire). That drove the effort to use ever-finer carbon blacks.

Carbon black is not the only reinforcement used in rubber. In photos from before 1910, cars rolled on white tires, reinforced with zinc oxide. Another possibility has always been very fine silica, which is essentially common quartz sand. It had the problem that during mixing with rubber, it could clump together, resulting in unsmooth tread extrusions.

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Kevin Cameron By The Numbers.Cycle World

The rubber business is research-intensive. During the 1980s, means were discovered to “prime” the surface of silica or to terminate rubber chains with so-called “functional groups” that gave them affinity for a silica surface. This made it possible to chemically bond rubber to silica particles, rather than making do with the weaker forces that underlie the association of rubber chains with carbon black particles. The higher strength of such chemical bonding meant that relatively little silica could provide the desired durability. The result was rubber that remained soft at lower temperatures—just the thing for the winter rains of Europe. Silica chemistry now became the basis of racing rain tires. The 1992 Japanese GP was won in rain by Mick Doohan—the first such use of Michelin’s new silica rubber reinforcement technology. Soon after that, racing engineer Erv Kanemoto noted that Michelin was beginning to move some rain compounds onto dry tires.

The driving force behind this research was that until this time, high-grip rubber compounds depended on their internal friction, or “hysteresis,” for much of their grip, but government regulation was forcing vehicle makers to improve fuel mileage. The high rolling friction of high-hysteresis rubber was unacceptable. Would its grip have to be given up to reduce tire rolling resistance?

The answer was no. Thanks to something known as the “Payne effect” and the proper application of silica reinforcement, the internal friction of rubber could be made frequency dependent. At the low frequency of tire rolling flex (zero to 20 cycles per second) the new rubber could flex with low energy loss, giving reduced rolling resistance. But at the much higher stick-and-slip frequencies at which rubber generates grip against pavement, hysteresis remained at full strength, giving its usual higher dry and wet grip.

It’s easy to make durable hard rubber—that’s what taxis and delivery vans run on—but at some sacrifice of grip. What’s hard is to combine grip-enhancing softness with high-tensile strength.

In rubber reinforced only by carbon, straining the rubber caused some chains to be displaced from their positions on carbon particles, or even to be pulled right off, consuming energy that could not be immediately given back when the strain was relaxed. This internal energy loss was the mechanism of hysteresis, and in late 1950s auto racing, grippy high-hysteresis rubber gave higher corner speeds, but its high rolling resistance could cut top speed by 5 mph. This hysteresis was present at all frequencies.

But with rubber chemically bonded to silica particles, such rearrangement or release of rubber chains became possible only at the greater energy levels of high-frequency strain cycling. This gave silica-reinforced rubber compounds lower rolling resistance without losing the grip-boosting effect of hysteresis.

This was what Watkins meant when he said rubber compounding has become a process of chemical reactions rather than of simple mixtures prepared in Banbury mixers.

This is complex technology because changing one variable affects all others. Which silanol coupling agent for bonding rubber and silica interferes least with the mixing process? Which silanol gives best bonding between rubber chains and silica particle surfaces? Should the silica be primed with silanol before mixing, or can that extra step be eliminated by just pouring the silanol into the mixer? Every variable had to be worked out by testing at laboratory scale then validated at industrial scale.

Tires are so very much more than “round and black.” The technology hidden inside them can keep us upright and safe in conditions in which tires of an earlier era would have sent us sprawling.