November/December 1995
Tire manufacturers have traditionally eschewed the use of recycled rubber in new tires, but new technologies give hope that closed-loop, tire-to-tire recycling is becoming a more realistic possibility.
By Lynn R. Novelli
Lynn R. Novelli is a writer based in Russell, Ohio.
On the surface, it seems simple: Recycle old tires into new tires.
But in practice, the idea has been largely dismissed by tire manufacturers as economically unfeasible and technically impractical, if not outright impossible.
The problem is that the rubber in scrap tires has significantly different properties than virgin rubber. The reason? Vulcanization—the sulfur curing process used in manufacturing tires that increases the strength and elasticity of natural rubber, creating new chemical bonds that are difficult to break. This chemical difference has heretofore prevented crumb rubber—the pulverized ground rubber recovered from scrap tires—from being used to any sizable extent as a substitute for virgin rubber in tire manufacturing. In fact, tire makers say they have only been able to use small amounts of crumb rubber as filler in new tires, accounting for 1 to 2 percent of a tire's overall composition.
But that vulcanization obstacle could become less of an issue, thanks to new technologies being developed to allow higher percentages of scrap tire rubber in the production of new tires. If they work, these technological developments could have a dramatic impact on the scrap tire market. According to one industry estimate, adding 10 percent recycled rubber to every new passenger tire would create demand for about 370 million pounds of recycled rubber annually, the equivalent of about 30 million scrap tires.
Upping that figure to 20 percent—which initial tests have shown to be realistic—would mean consumption of about 60 million scrap tires a year, or about a quarter of the estimated 250 million scrap tires generated annually in this country.
Then again, achieving those targets won't be easy, if they can be achieved at all. Not only are there problems related to tire chemistry, but there's the challenge of overcoming the tire industry's long-established skepticism of using recycled rubber in new tires.
Recycling the Mechanical Way
One new technology that could help resolve the tires-to-tires recycling problem is called solid-state shear extrusion (SSSE) pulverization, a process that is "completely different than the usual mechanical grinding," says Klementina Khait, manager of the Polymer Reclamation Center at Northwestern University 's Basic Industry Research Laboratory (Evanston, Ill.), which is developing the technology.
The centerpiece of the SSSE process is a twin-screw extruder that applies high shear force and pressure to reduce ¼-inch rubber feedstock to a very fine, uniform, high-quality powder. "We call it a mechano-chemical process because, as a result of the mechanical manipulation, some chemical changes occur in the rubber, and some of the sulfur bonds are ruptured," Khait explains.
The partially devulcanized rubber powder has mechanical properties "far superior" to those of crumb rubber, Khait claims, noting that the powder produced from scrap tires has excellent tensile and tear strength. "The high value of its mechanical properties opens up the possibility of it being used in new tires in much higher percentages than crumb rubber can be used," she states. Just how much recycled rubber powder can be added to new tires with no compromise in quality has yet to be determined by research, however, she points out.
Though the SSSE process is still in the laboratory stage, scale-up to commercial size is planned over the next two years. And several tire manufacturers pressed an interest in testing the equipment, the technology, and the powder in new tire manufacturing, Khait reports.
A Chemical Approach
While the SSSE process is designed to partially devulcanize scrap tire rubber through mechanical means, another new technology aims to achieve that feat chemically.
In this process, developed and patented by STI -K Polymers (Kuala Lumpur, Malaysia), crumb rubber is mixed with a chemical reactant called De-Link that reportedlydissolves up to 80 percent of the sulfur bonds in vulcanized rubber products. The end product, named De-Vulc, is a devulcanized rubber sheet that is said to have up to 75 percent of the tensile properties of natural rubber.
"This is a major breakthrough," says Fred Siesseger, president of STI -K Polymers America Inc. (Washington, D.C.), an STI -K subsidiary that is commercializing the technology in North America . "For the first time, vulcanized rubber can be brought back to its original state and regain the bulk of its original properties."
Among its attributes, De-Link can chemically break rubber bonds in five to 10 minutes and is "environmentally neutral," Siesseger claims. And the resulting product, De-Vulc, he reports, costs a fraction of the price of virgin rubber, can be relinked with other polymers by applying heat, and can be used in virtually any natural rubber applicationwithout compromising product quality, he says.
STI -K has made one major step toward commercializing the technology by forming a work arrangement with Baker Rubber Inc. (South Bend, Ind.), a leading scrap tire and rubber recycler. In the arrangement, Baker Rubber processes scrap tires to crumb rubber, then STI -K runs the material through its proprietary De-Link devulcanizing process. (STI -K will also handle the sales and marketing of the De-Vulc rubber.)
Initially, STI -K plans to produce 2,000 tons of De-Link a month—enough to produce 456,000 tons annually of De-Vulc. Where will it all go? "The most immediate, easily penetrable market is industrial rubber products such as automotive hoses and belts that don't require a long period of extensive testing," Siesseger says.
STI -K's longer-term marketing objective is to find a home for De-Vulc in new tire manufacturing, but gaining entree into that market will take time due to the lengthy safety and performance testing tires must undergo, he explains.
Already, the firm is working closely with Ford Motor Co. (Dearborn, Mich.) and has met with the major tire manufacturers in the United States . All of these parties "are actively involved in evaluating the technology and the product," Siesseger says, but he notes that no agreements have yet been reached.
The results of some recent tests conducted in Europe , however, could help De-Vulc make faster headway in the new tire market. These tests reportedly indicate that passenger tires containing up to 20 percent De-Vulc from scrap tires can pass high-speed tests, thus meeting European standards for performance. "We hope these results are enough to kick-start things in the United States and get the tire companies interested in doing some testing," says Siesseger.
Mission Impossible?
While tire manufacturers agree that environmentally sound, economically feasible methods for increasing consumption of scrap tires are needed, some assert that neither the SSSE process nor STI -K's De-Link approach meet both criteria. "There are several new processes out there for reclaiming rubber from used tires," says Alan Halbert, a spokesman for Bridgestone/Firestone Inc. (Nashville). "Some of them are exciting, but none of them is commercially feasible."
Andy Eastman, chairman of the scrap tire task force at Goodyear Tire & Rubber (Akron, Ohio), points out that the major tire manufacturers, including Goodyear, have been researching the use of varying amounts of recycled rubber in tires for at least 20 years and have not found it feasible from a quality or economic standpoint. "The solutions we see that are viable and competitive are primarily in the area of fuel," he asserts.
Currently, in fact, energy recovery is the largest use for scrap tires in the United States , consuming about 56 million tires a year as a supplemental or dedicated fuel bycement kilns, pulp and paper mills, electric utilities, and tire-to-energy facilities. The crumb rubber industry, in contrast, consumes only about 4 million scrap tires annually, for use in such applications as mats, truck bed liners, and athletic surfaces.
While the SSSE and De-Link processes aim to expand the markets for powder and crumb rubber, Eastman and many of his peers remain skeptical, asserting that these technologies are not the breakthroughs their supporters claim. In fact, even though the SSSE and De-Link processes can reportedly produce recycled rubber with up to 75 percent of the properties of virgin rubber, the response from tire manufacturers and compounders is that that's just not good enough.
Without a doubt, it's a difficult task for recycled rubber to live up to the qualities of its virgin counterpart. A big part of the problem is that "tires are not like glass or plastics or metal," says Michael H. Blumenthal, executive director of the Scrap Tire Management Council (Washington, D.C.). "Tire chemistry is incompatible with closed-loop recycling. Even if you can break the sulfur bonds—and I'm not sure you can—you don’t get the pure rubber that needs to go into a new tire."
Halbert agrees, adding, "No process can return vulcanized rubber to its natural state, with its natural properties intact. What you get is not rubber, it's some other unpredictable compound that doesn't have rubber's properties and is unreliable in bonding."
Indeed, vulcanization is only part—albeit a large part—of what complicates tire-to-tire recycling, and devulcanization is therefore only part of the solution. Another part of the equation is that when tires are made, virgin rubber is bonded with other polymers to create 30 to 60 different blends for use in different parts of those tires.
Since both the SSSE and De-Link technologies make a product from all parts of a scrap tire, "the end result is an average of all those different polymers," Eastman points out. Such a product doesn't live up to the quality standards of tire manufacturers, he asserts, noting, "To give tires the highest qualities, all components that go into their manufacture need to be the best, not an average."
Furthermore, Halbert states, manufacturing realities make the new technologies impractical. As he explains, “You’re talking about net energy input, not to mention the handling of potentially hazardous chemicals. These considerations, plus the extra processing required, will mean the recycled product will be more expensive than the product made from virgin rubber.”
Taking It to the Street
Not that all tire manufacturers are writing off these new processes and the idea of using scrap rubber in new tires. Michelin (Clermont-Ferrand, France), for instance, has clearly expressed an interest in devulcanization, giving added credibility to the pursuit of devulcanizing scrap tire rubber and reusing the material in new tires. "We're involved in research along those lines," says Doug Bell, director of corporate administration for Michelin North America (Greenville, S.C.). Recently, in fact, the firm completed tests with Ford Motor Co. in which 10-percent crumb rubber was used in new tires with reportedly no compromise in quality or performance.
Adding weight in scrap rubber's favor, Bell notes that “prices on the raw materials market have increased substantially in recent years, so devulcanizing could possibly-be an alternative to virgin rubber.” He also points out that distribution and processing costs will play a role in the economics of substituting devulcanized rubber for virgin.
So, will the SSSE and De-Link processes make significant gains in introducing recycled rubber into the new tire market? The consensus is it's too early to tell. As Blumenthal states, "Laboratory success means little in terms of marketplace realities."
In the longer term, he asserts, "the proof of these technologies will be in the success or failure of their end products. They have a rubber product produced from scrap tires. Now they have to prove they can do something with it that the market wants.”
Tire manufacturers have traditionally eschewed the use of recycled rubber in new tires, but new technologies give hope that closed-loop, tire-to-tire recycling is becoming a more realistic possibility.