E&MU: Tire Makers Refine Compounds to Improve Durability, Mileage

By Eric St.John, Features Editor

This story appears in the May/June 2012 issue of Equipment & Maintenance Update, a supplement to the May 14 print edition of Transport Topics. Click here to subscribe today.

Recent advances in rubber compounding processes have helped make tires more durable and fuel efficient, according to manufacturers who compare the techniques to the culinary arts.

“It’s a bit like cooking,” said Roger Stansbie, director of engineering for Continental Commercial Vehicle Tires the Americas, Fort Mill, S.C. “If you want the resulting compound to last a bit longer in processing, you add a bit more processing aids. So you add a little pinch here and a little pinch there. That’s the way [compounders] learn their art.”

“It’s as much an art as it is a science,” said Walter Weller, vice president of sales for Double Coin tires, a brand of the China

Manufacturing Alliance, Monrovia, Calif. “Every manufacturer guards those recipes vigilantly.”

Compounding is the process used by manufacturers to instill certain characteristics into all the different sections of the tire — such as the tread, sidewalls, inner liner, steel cords and bead area, which is the section of the tire that seats on the wheel and help prevent the truck from sliding when making a turn.

“You have a different compound on the bead area, a different compound on the sidewall, a different compound for the belt packages, a different compound for the undertread, that little area between the belt package and the tread, and then a different compound for the tread,” said Rick Phillips, director of commercial sales for Yokohama Tire Corp., Fullerton, Calif. 

“Years ago, you might have just one compound, and premium tires would all have the same compounds,” he added. “Now, the engineers have learned how to build characteristics into [the compounds], so what is better for a steer tire might not be the same characteristic needed for a drive tire. So they are more application-specific in the compounding now.”

For example, Stansbie said that about 12 to 15 different compounds can go into the tread — such as natural and synthetic rubber, sulfur, carbon black, silica, plasticizers (wax and oil), antioxidants and zinc oxide, among others. The compound also plays a role in tread life, he said.

“There are other factors involved [in tread life] such as the number of ribs, rib width, tread depth, radius — that sort of thing,” he said. “But the compound is probably one of the main features of tread performance — meaning [better] mileage or traction or handling.”

“When you look at the compounding from 10 years ago, it only moved the needle a little bit at the gas pump,” said Phillips, who added that the current process has made “huge strides” in improving the durability and fuel efficiency of tires.

Jeff Pyle, manager of truck tire development group for Michelin North America, who works out of the company’s office in Laval, Québec, credits government regulations for the improvements.

“With SmartWay, greenhouse-gas and fuel-efficiency regulations, rolling resistance is becoming much more important. It’s kind of a game-changer in truck tires,” he said. “This will push us further to find compounds that have a low hysteresis, which is what leads to rolling resistance.”

Pyle said hysteresis is a way of measuring the loss of energy.

“Let’s say you have a ball that you are going to bounce against a table. If it’s perfectly elastic — in other words, no hysteretic losses — you would bounce the ball off the table from three feet, then when it bounced back up to you it would bounce exactly three feet. There would be no energy losses, no hysteretic losses. That’s called perfectly elastic.”

However, Pyle continued, “if you have a plastic-substance ball [such as Silly Putty], and you try to bounce it from the same 3-foot elevation, the ball would not bounce off the table at all. It would have such a high hysteresis that all of the energy from dropping it would be absorbed by the ball itself, and it would not rebound. . . . It just sticks to the table.”

“So,” he concluded, “a tire is somewhere in between that [low and high level of hysteresis], and the closer you can get to the perfectly elastic case, the less hysteretic losses you have.”

To achieve the desired results, however, manufacturers said they have to make sure the compounding of one section of the tire meshes with the compounding of all the other sections.

“On one end of the spectrum, you’ve got characteristics that make the tire very fuel- efficient. What that does is, it lowers the rolling resistance of the tire. On the other end of the spectrum is the compound that makes it resistant to cutting and chipping,” Phillips said.

“The more you make a tire fuel-efficient, the less resistance it has to cutting and chipping, and . . . [when] you make a tire more resistant to cutting and chipping, the less fuel-efficient it’s going to be. . . .  So, the key is you want to develop a balance with compounding; you have to match the compounding with the rest of the tire.”

Pyle said, “When you start talking about rolling resistance and tread rubber, you can start talking about potential trade-offs in other performances. The compounding technology that’s used can make a difference in not having to make significant trade-offs.”

Pyle said Michelin focuses on “a balance of performances where we don’t emphasis a particular [characteristic] at the expense of another.”

Among elements in the tire, Stansbie said plasticizers such as wax and oil make the compound easier to process, while antioxidants prevent cracking, mostly in the sidewall compound because that’s where the tire flexes the most, but also in the tread. Carbon black acts as filler and adds both rigidity and color to the tire. Another, silica, has properties comparable to carbon black, he said.

Sulfur is also a key ingredient, he said. When making a new tire, manufacturers must “use enough sulfur so that there is some sulfur remaining in the base compound when you put a precured tread on it,” Stansbie said. Sulfur makes the polymer chains within the tire more rigid, he said, and is important for tires that are retread.

“If you didn’t have enough sulfur residing there, then you don’t get the correct chemical bond to retain the tread during the retread stage,” he said.

Weller added, “As far as durability is concerned, there are a variety of compounds through the tire that affect durability from a tread-wear standpoint as well as from a casing durability standpoint. There are different compounds that promote a cooler running tire, which impacts retreadability in the casing,” he said.

“For truck tires, compounding is important because they usually have to be retreaded multiple times,” Pyle said. “The casing has to be durable through multiple lives, and there are certain attributes about the mix’s characteristics that allow you to measure the durability of those compounds.

“Common mixing and component manufacturing equipment can be used for multiple products, but they can be carefully tailored for different sized tires and the components sized for each individual tire,” said Mike Manges, communications manager for commercial tires at the Goodyear Tire & Rubber Co., Akron, Ohio. “Some materials can be used across many product lines, whereas others can be used in perhaps only a few products. Large off-the-road tires may have some materials that are different from those used on small trucks in an over-the-highway application.”

Mixed in with all of these ingredients, of course, is rubber — both natural and synthetic —the premier ingredient in tires.

“On a truck [tire’s tread], natural rubber accounts for about 35% of the weight,” said Jackie Pobiega, Continental’s commercial vehicles tires communication manager.

Stansbie added that, aside from the tread on Classes 7 and 8 truck tires, Continental uses synthetic rubber on the other parts of the tire.

He said that about 75% of the rubber is natural and 25% is synthetic.

“For the larger truck tires, natural rubber has a unique property, in that it handles the heat and stress imposed upon the truck tire as it is going down the road,” Stansbie said. “Synthetic rubber . . . doesn’t have the same properties as natural rubber. It’s impossible to replace natural rubber, because it has unique properties, especially for deep-treaded tires and for rolling resistance.”

Stansbie said that tires for smaller trucks, Classes 4 through 6, usually have more synthetic rubber than natural rubber. But, he added, with the current emphasis on improving rolling resistance and decreasing greenhouse gas emissions, more and more of the tires for smaller trucks are being made with more natural rubber — as they were before synthetic rubber began to be used so heavily in those lower classes.

Indeed, the tire’s application plays an important role in how it is compounded.

“Every tire has a unique combination of compounds, so you use different compounds on a steer tire than you would on a drive tire and on a trailer tire because there are different dynamics [for] the application,” Phillips said.

When discussing the mixing process, Stansbie returned to the cooking analogy.

“[There’s] a master batch, which would be like your basic dough,” he said. “One can create [that] basic mix and then add other ingredients to make it suitable for various components, which require specific properties depending on their specific function within the tire.  So the final mix would consist of a master batch and then additional components to make the final compound.”

After that, Stansbie said, the final compound is run through mixers that he called mills, “which basically exercise the rubber — and that can be two or three passes through the mills, depending on how elastic you want the rubber to be when you start to produce it.”

Goodyear tires are made in an “energy-intensive mixing operation in which batches of up to several hundred pounds may be prepared from many raw ingredients in a matter of a few minutes,” he said. “Often multiple mix stages are used to incorporate, distribute and disperse all of the ingredients.”

“A rubber compound is just like a recipe that you would [use to] cook something,” said Phillips. “So you add a lot of ingredients in and you mix them.”

Phillips said Yokohama runs its compounded batch through the mill just once.

“What we [came up with in 2005] is a single-step mixing process where we’re able to get the exact ingredients we need and just run it through the mixing process once, and it comes up with a lot more consistent compound,” he said. “The dispersion of all the molecules in the rubber compound are more evenly dispersed, and it’s a cleaner, better compound.”