November/December 2010
Cryogenic crumb rubber is hot, driven by its popularity as artifical turf infill. Though there’s more type than evidence of its superiority to ambient crumb in that application, producers believe the material shows great potential in other, larger markets.
By Anne C. Logue and Rachel H. Pollack
U.S. tire processors turned about 1.1 billion pounds of scrap tires into ground rubber in 2007, according to data from the Rubber Manufacturers Asso-ciation (Washington, D.C.). Most processors that produce this category of scrap tire product start with whole or partial tires, then grind and shred them to create chips that can range in size from a few inches across to less than 1/4 inch. Some companies further process these tire chips into granulated particles—called ground or crumb rubber—by sending the tire chips through an additional series of mills and screens. A smaller proportion of companies do cryogenic processing, in which they use liquid nitrogen to rapidly freeze the partially shredded tire rubber, making it brittle enough to shatter into pieces that range in size from small granules to ultrafine powders.
New entrants into the tire recycling industry might be drawn to cryogenic processing because it’s sexy new technology with applications in the world of professional and big-time college sports. Some wonder, however, whether those markets are large and stable enough to justify the investment in the processing equipment. One producer of cryogenic rubber powder, on the other hand, has its sights set much higher.
Dissecting demand
For years, many tire processors had a supply-side incentive for their work, as governments wanting to eliminate ugly and hazardous tire piles offered them grants or the opportunity to bid on contracts for access to the material. With tire stockpiles down 87 percent from 1990 to 2007, according to the RMA, and the remaining tire piles concentrated in seven states, there is no longer a huge excess available to the marketplace. New entrants into the field are often surprised to find that the front-end revenues don’t always cover expenses; the profits come from processing the material and selling it to end markets.
The RMA tracks those end markets in its biennial report “Scrap Tire Markets in the United States,” which it most recently produced in 2009 using 2007 data. Though more than half of all recovered tires—54 percent—get burned for fuel, ground rubber applications are the second-most-prevalent category of use. This market grew 43 percent from 2005 to 2007. Within the ground rubber category, sports field applications are growing the most rapidly, up 67 percent in that two-year period. Despite that rapid growth, the total sports surfacing category, which encompasses artificial turf fields, running tracks, and playground surfaces, is still only 27 percent of the ground rubber market. This is second to molded or extruded products (37 percent of the market) and ahead of four other segments, each holding about 9 percent of the ground rubber market: playgrounds, mulch, and animal bedding; asphalt rubber; automotive applications; and exported ground rubber.
Tire processors and those looking to enter the industry should keep these numbers in mind as they consider investing in processing technology. “The entire scrap tire market is demand-pulled, not supply-side driven,” says Michael Blumenthal, RMA vice president. Some argue over which processing method is best, ambient or cryogenic, but what’s best depends on how the buyer will use the material. “We’ve seen a lot of new entrants into this market get seduced by the technology,” Blumenthal says, but “one [technique] does not hold any advantage over the other” in absolute terms because it all depends on what characteristics the buyer wants. “You’re not producing rubber for your benefit. You’re producing rubber for the benefit of someone else.”
Companies interested in cryogenic processing need to consider whether their customers want rubber processed that way. What makes predicting demand somewhat difficult is that customers will specify ambient or cryogenic crumb, but they don’t always explain the reasoning behind that choice, says Tim Leighty, director of quality at Liberty Tire Recycling (Pittsburgh), which has both ambient and cryogenic facilities. “Many of the specifications are proprietary and are not disclosed to us,” he says. “We don’t know why they want [what they want], only that they do.” At least one company, Lehigh Technologies (Tucker, Ga.), is working to build demand with an applications laboratory and a staff of engineers and chemists who work to convince manufacturers that cryogenically processed rubber powder from recycled tires is an economically and environmentally preferable material for their products.
The Cold, Hard facts
Cryogenic processing involves freezing partially processed tire chips with liquid nitrogen until they’re brittle, then feeding them through equipment that shatters them into smaller particles and liberates any remaining fiber or wire. Lehigh Technologies, for example, starts with tire chips of about 1/4 inch to 1/2 inch in diameter from which most fiber and metal have been removed. Liquid nitrogen at -320 degrees F is then used to “freeze” the rubber, making it brittle, like glass. The chips then get sucked into a rapidly spinning turbine, which shatters the chips into tiny particles in an array of sizes. Liberty Tire Recycling’s three cryogenic processing facilities start with chips roughly 1 inch to 3 inches across and use a hammermill to shatter the chips after they’re frozen.
The tire processing industry measures the crumb by the mesh size—the number of holes or openings per linear inch of screen through which the material can fit—per the ASTM International E11 standard. An 80-mesh crumb, for example, can fit through a screen that has 80 holes per linear inch. With current technology, 90 percent of cryogenically processed material can fit through an 80-mesh screen on the first pass, according to Air Products (Allentown, Pa.), a developer of cryogenic grinding technology. Ambient crumb, in contrast, would require multiple passes through milling and separating equipment to reach an 80 mesh particle size. At Lehigh Technologies, the patented turbo mill produces particles that range from 40 mesh (slightly finer than sand) to 300 mesh (about the consistency of talcum powder). The system then brings the powder back up to room temperature to prevent condensation and “classifies” the material, or sorts it by mesh size. The entire process takes less than 15 minutes. Liberty’s cryogenic facilities produce crumb sizes ranging from -8 mesh to -40 mesh. The process takes eight to 11 minutes from infeed to qualified size, Leighty says.
The tire processing industry has long held that cryogenic processing produces crumb rubber particles with a different form and structure, or morphology, than crumb rubber produced at ambient temperatures. Ambient processing creates crumb that has a rough, irregular surface, which gives the particles an overall greater surface area that’s conducive to binding together via an adhesive agent, industry insiders say. Cryogenic processing produces particles with smoother surfaces and less surface area. Under a microscope, a cryogenic particle will look like a faceted gemstone, Leighty explains, while an ambient particle will look more like river gravel, riddled with irregular, curved edges.
That dichotomy is accepted as fact throughout the industry, but documentation of the difference is hard to find. Leighty confirms that he has seen different surface textures between the two types of crumb when he worked at other tire processing facilities earlier in his career, before he joined Liberty Tire, and RMA’s Blumenthal reports having seen pictures showing the difference between the two as well. Lehigh Technologies has conducted its own tests, however, with intriguing results. The company took samples of its own cryogenically processed material and a sample of ambient crumb and photographed them through an electron microscope. “We found there’s not a significant difference” between the two, says Tom Rosenmayer, the company’s vice president of technology. The cryogenic particles didn’t necessarily have less surface area—in fact, Lehigh can adjust its cryogenic processing equipment to produce powders with more or less surface area depending on what the customer demands. The primary difference was uniformity versus variety. Lehigh Technologies found that the cryogenic particles it tested were more uniform in appearance across the sample, whereas the ambient sample “had a variety of particles. Some had a lot of texture on the surface, but most of them looked like the cryogenic [particles],” he says. The company doesn’t know if the ambient crumb samples it tested were representative of all ambient crumb, Rosenmayer notes.
Liberty Tire’s Leighty agrees that cryogenic processing produces more consistent, or uniform, morphology, for several reasons. First, he says, “it’s a misnomer to lump all ambient materials into the same category” because processing techniques can vary widely. Ambient tire processing equipment might use friction, pressure, shearing, pulling, or cutting to size-reduce the material and remove metal and fiber, he says. Each technique could affect the product’s morphology. Further, if the processing technique used blades, were the blades sharp or dull? That could have an impact as well. Both Rosenmayer and Leighty say there’s a lack of data on the subject of crumb rubber morphology.
Whether it’s because of the morphology differences, the more uniform product, or the desire for ultrafine powders, there’s no doubt that certain buyers are requesting cryogenically processed crumb and willing to pay more for the material. That’s important because “there are some pretty extraordinary costs associated with cryogenic rubber,” Leighty says. The cost varies with the type of equipment needed, the desired finished particle size, and the anticipated production volume, says Kimberly King, industry engineer for Air Products, but systems range in price from about $200,000 to more than $1 million. Factoring into the high cost, Leighty says, is the need for hardened steel or stainless steel equipment that can handle the low temperatures, as well as the fact that there are “no real turnkey systems” available, so each one is unique. Add to that the cost of the nitrogen gas. Some generators consume 3/4 to 11/2 pounds of liquid nitrogen per pound of crumb rubber generated, at a price around 10 cents a pound, Leighty says. Liberty’s cryogenic plants consume nitrogen even on days when a facility is not processing rubber, Leighty adds, because the nitrogen reverts from a liquid to a gas in the storage tanks and must be vented. These costs can be steep for a plant that does not have enough supply or demand to be in production every day. Despite the costs, Lehigh Technologies considers its process very efficient. “We believe our process has a smaller carbon footprint than other cryogenic or ambient processes,” says Dave Petroni, the company’s vice president for operations.
The Turf Market
Artificial turf is driving much of the current demand for cryogenic crumb rubber, says Bert Darling, president of Rooster Rubber, a tire recycler in Kansas City, Mo., that uses both ambient and cryogenic processing techniques. The Synthetic Turf Council (Atlanta) reports that North America now has 5,500 multiuse synthetic fields, including half of all National Football League team fields. More than 1,000 artificial-turf fields were installed in 2009 alone. Schools and athletic organizations are switching from natural to artificial turf even though it’s more expensive, Darling says, because it’s low maintenance: Artificial-turf fields don’t need pesticides, never need to be closed for mowing, and can return to use after bad weather more quickly.
Most entities installing turf prefer to use cryogenic crumb rubber for the fill, Darling says, because they believe it has superior drainage capabilities. Also, he says, ambient particles are more likely to trap air beneath them and float above water on a field, creating a blotchy appearance. His company uses only cryogenic crumb rubber for turf infill, but other companies use ambient crumb, a mix of ambient and cryogenic crumb (“cryambient”), or a mix of crumb rubber and sand. Proof of the superior performance of cryogenic crumb in artificial turf infill is hard to come by, however, and industry participants have their own theories about why the material has caught on in this market.
Artificial turf companies might favor cryogenic crumb rubber because the company that pioneered the use of crumb rubber infill in playing fields uses that material, says Bill Schomburg, a consultant based in Springfield, Ill., who advises educational institutions on artificial turf installations. That company installs fields at Division 1 colleges and universities, he notes—the ones with the largest and most high-profile athletics programs. Further, he says, in the early days, some ambient rubber processors had quality-control problems. “When we talk about the difference between ambient and cryogenic, we’re often talking about the quality of the producer,” he says. But ambient producers seem to be closing the quality gap: Recently, he’s found that many ambient producers keep their equipment well maintained and use tight quality control to produce crumb with the consistent size and low dust levels that turf installers prefer.
The low dust level is what Leighty believes is the real issue. Studies he has seen indicate that drainage “has more to do with the percentage of fines that have or have not been removed” than the processing technique, he says. If the crumb is full of tiny, static-charged rubber particles that are “a couple hundred microns” in size, “it could form a semi-impervious barrier” that slows drainage. Turf infill is one of the largest markets for Liberty’s cryogenic material, he says, and the company provides the crumb in a range of sizes that fit each specific installation. If the user is mixing the crumb with sand, for example, “we’ll try to use a gradation of rubber that’s similar in size and distribution to the sand,” whether it’s -8 to 14 mesh, -10 to 20, or -14 to 30. Rooster Rubber sells two size ranges of crumb to an artificial turf company, 10 to 14 mesh and 14 to 30 mesh.
The choice of material often comes down to price. In the artificial turf market, “a penny or two a pound can sometimes drive a financial decision” if the field designer has the latitude to choose, Leighty says. A Division 1 college that plays televised bowl games might spend top dollar for the product it perceives as having the highest quality, but a public school district might not have that luxury. A standard-size artificial-turf football field uses about 240,000 pounds of crumb rubber, Schomburg says, so the pennies add up to several thousand dollars per field.
Asphalt and Beyond
One ground rubber market with significant unrealized potential is asphalt rubber. Federal and state government tests have concluded that incorporating ground rubber into asphalt can improve a road’s crack resistance, life span, smoothness, and water dispersion and reduce road noise. Though five states use asphalt rubber and several more are testing the material, the RMA report notes that the market did not grow significantly from 2005 to 2007, and it still faces substantial barriers to further growth. (For more on asphalt rubber, read “Where the Rubber Meets the Road” in the November/December 2009 Scrap.)
Federal highway specifications call for ambiently produced crumb in asphalt rubber projects, says Serji Amirkhanian, an independent consultant and former director of the Asphalt Rubber Technology Service at Clemson University (Clemson, S.C.). The assumption, Amirkhanian says, is that because ambient rubber particles have more edges to them, they will form better bonds with the aggregate, whereas cryogenic rubber particles would not form a strong bond or make a good driving surface. His research shows that this might not be true, however. “I believe you can use cryogenic, but you have to pay attention to some details,” he says. Asphalt using cryogenic crumb would need a higher proportion of rubber to binder to get the same performance. The resulting product would be “greener” because it would contain a higher proportion of recycled material, but it does not have any known performance advantages over asphalt made with ambient crumb, he says, and it might cost more to produce.
Lehigh’s Petroni suspects that asphalt rubber requires ambient crumb because ambient crumb producers first advocated its use many years ago. “Once it’s in the specs, it’s hard to change,” he says. Lehigh had its first sale of cryogenic crumb into the asphalt rubber market this year, Rosenmayer says, for an interstate highway project in Georgia, and he hopes that it will become a significant market for the company. “We’re in the process of proving that our product works” for this application “and growing it from there,” he says.
RMA research indicates that the next big ground-rubber product could be powders, Blumenthal says, which are used in pigments, extruded products, and roofing materials. Both ambient and cryogenic systems can produce powders, though it seems easier to get consistent, fine sizing using cryogenic processing, he says. Enough product development research is taking place on rubber powders that the product is on the agenda of an RMA conference being held in November.
Lehigh Technologies sells its powders to manufacturers who make products including new tires, sealants and coatings, plastics, and industrial rubber products. Tires are a huge potential market, Rosenmayer says. The RMA report notes that new tires contain less than 5 percent recycled rubber by weight. It doesn’t predict any growth in that market, however, because tire manufacturers’ tests in the past have shown that adding more recycled content can reduce tire performance on several measures. Lehigh Technologies’ technical center “works with its customers to develop high-performance tires with higher levels of recycled content by using advanced micronized rubber powder and optimized formulations,” Rosenmayer says.
Looking ahead, Lehigh Technol-ogies sees a wide range of potential applications for its cryogenic rubber powder. “Synthetic rubber, natural rubber, and plastics are all polymers made of hydrocarbon atoms linked together” in different molecular chains, Rosenmayer explains, thus recycled rubber is a potential substitute for virgin material in almost any polymer product. “Most of our engineering work is in polyolefins, like polyethylene or polypropylene,” Rosenmayer says. The company has a grant from the National Science Foundation (Arlington, Va.) to explore the use of micronized cryogenic powder rubber in polypropylene, for example. Lehigh’s technical center can compound, extrude, and mold plastics made with cryogenic crumb to test how they perform compared with those made from virgin materials. “The plastics market is a huge opportunity for us,” he says.
No one expects the introduction of a category-busting, revolutionary new product using crumb rubber anytime soon, or if they do, they aren’t talking. Still, the research continues. “We’re just at the beginning,” says Elizabeth Searcy, Lehigh Technologies’ vice president for marketing, of the market for cryogenically processed powder rubber. “As industry is becoming more and more interested in green materials,” the company expects interest in its work to grow.
Ann C. Logue is a Chicago-based freelance writer. Rachel H. Pollack is editor of Scrap.
Cryogenic crumb rubber is hot, driven by its popularity as artifical turf infill. Though there’s more type than evidence of its superiority to ambient crumb in that application, producers believe the material shows great potential in other, larger markets.