Rolling Spaghetti

Rolling spaghetti

TDC

EVERY TIME I BOIL SPAGHETTI, I THINK about rubber. Rubber is a mass of writhing, whirling, tangled long-chain molecules, and the spaghetti strands in the pot look like my imagination's picture of rubber. The analogy needs help because, in a finished rubber product, the rubber molecules are pinned to each other—cross-linked in many places by chemical bonds. This converts the sticky, liquid mass into an elastic solid. So I amend the analogy by imagining the spaghetti drained, sitting in a dish, beginning to stick to itself everywhere. If I wait too long, it will become a solid mass-as vulcanized (cross-linked) rubber is.

Rubber is vulcanized or cured by a complicated proprietary chemistry of accelerators, scorch-delay agents, and so on, but the basic idea is to heat the uncured rubber with sulfur. A high tem perature or long heating produces many sulfur cross-links and an increasingly hard product-something like my ACE pocket comb, which bears the words "hard rubber." Tires are cured in a mat ter of minutes at about 315 degrees F in steam-heated metal molds. The insides of the tires are simultaneously cured by steam inflation of rubber bags.

Rubber is useful in tires because it is elastic-when stretched or otherwise deformed, then released, it snaps back, resuming its former shape and dimen sions. An analogy for the rubber mole cules is the jump-rope that used to be enjoyed by children before they were immobilized by television. The faster the rope is whirled, the more tension there is in it, tending to draw the ends toward each other.

What we feel as temperature is actu ally molecular agitation, and in the case of rubber's long molecules, this can take (among others) the form of jumprope-like whirling between cross-link sites. Like the jump-rope, this whirling tries to draw the ends nearer each other. When we stretch the rubber, untold tril lions of these tiny, whirling jump-ropes are pulled endwise, flattening the whirling loops. Naturally, the centrifu gal force of the "rope" opposes our pull. When we let go, the molecular whirling draws all the little ropes up short again.

This elasticity is wonderful-it allows soft tire-tread rubber to squish into pavement irregularities, "keying" to them to generate traction. Elasticity also allows the tire tread to flatten lo cally to form a footprint of large area. Normal solids like steel could never tolerate these large deformations, but rubber happily flexes away hour after hour. Elasticity requires some modifi cation in tires. Specifically, it needs damping, just as suspension systems do. Damping in rubber is complicated, but a large part of it is supplied by oil that is a part of the tread compound. All sorts of effects can be created by vary ing the viscosity and chemistry of this so-called "extender" oil. When Ron Pierce won a 250 race in 1975 despite having his rear tire covered in transmis sion oil, I expressed amazement that he hadn't fallen. The nearby Goodyear en gineer shrugged, "What's so amazing? After all, a third of the tire is oil."

To prevent my spaghetti from becom ing a solid mass, I pour on olive oil and stir it. With rubber, this stirring takes place in a Banbury mixer which may be driven by a thousand-horsepower electric motor. A pair of nubbly rotors kneads and shears the green, uncured rubber, thoroughly mixing all ingredients.

Not all of rubber's strength comes from sulfur cross-links. Carbon is mixed into tread rubber as well, to form secondary bonds with the rubber mole cules. Remember that carbon is widely used as a purifying agent (in charcoal filters, for example) because so many substances are adsorbed onto its sur face. Rubber is one such substance. Carbon's association with rubber makes the rubber stronger without making it uselessly hard. The method used to pro duce the carbon particles determines the degree and type of chemical activity of their surface. Rubber compounded with chemically pure "graphitized" car bon has reduced durability.

Kevin Cameron

The motorcyclist wants high grip, but unless his ambition is a last-ditch quali fying effort at Daytona, the rider also wants durability-those qualifying gumballs only go three laps. This means that the rubber must combine maxi mum softness, for keying, with enough tensile strength and cut-resistance to have a reasonable lifetime.

Rubber can be made as strong as you like by vulcanizing it more thoroughly, but elasticity and other valuable proper ties are lost. That may be a good way to make high-mileage taxicab tires, but it's a poor choice for softness and grip. The jump-ropes have to be kept long and free to whirl, yet must be pinned strongly together in enough places to produce the needed strength.

For many years, a way to improve the strength/softness compromise has been to divide chemically active carbon more finely, and thoroughly mix this superfine carbon into rubber. This in creases the gross area of engagement between the carbon and the rubber. The surface area of this carbon is measured in acres per pound! The secondary bonds with carbon strengthen the rub ber without ruining its other properties.

Ultra-soft rubber compounds are used in racing rain tires, where the presence of water cools and lubricates the rubber. Often the weather clears, the track dries, and rain tires heat up and turn to grease. But today, some former rain compounds have become dry-tire compounds, thanks to tire-construction methods that reduce operating temperature. One development in rain compounding has been the use of ultra-fine silica as a partial replacement for carbon. Silica with bridging com pounds can now form strong chemical bonds with rubber molecules, so less of it need be used to achieve what carbon would otherwise do. Maybe this inter feres less with the whirling jump-ropes, preserving elasticity, while the greater bond energy delivers plenty of tensile strength. I suspect that rubber engineers can now control not only the total popu lation of cross-links, but have better sta tistical control over their spacing.

Somewhere in all this, the spaghetti analogy was permanently lost-we had to eat it before it got cold.