Rubber is one of the harder materials to recycle because of how it’s made. During manufacturing, rubber undergoes a process called vulcanization, which creates strong chemical bonds (sulfur crosslinks) between polymer chains. Those bonds make rubber durable and elastic, but they also make it resistant to simply melting down and reshaping the way you can with plastic or glass. Still, several proven methods exist for recycling rubber, from grinding it into reusable crumb to breaking it back down into fuel and raw carbon.
Why Rubber Is Difficult to Recycle
Most recyclable materials can be melted and reformed. Rubber can’t. The sulfur bonds created during vulcanization lock the material into a permanent shape. To recycle it, those bonds either need to be physically broken apart through grinding, chemically reversed through a process called devulcanization, or the rubber needs to be decomposed entirely through heat. Each approach produces a different end product with different uses.
Globally, only about 42% of end-of-life tires are recovered as materials and another 15% are burned for energy, based on a survey of 45 countries. Two-thirds of all discarded tires worldwide end up in landfills or illegal dumps. In the United States alone, roughly 317 million waste tires are thrown away each year, and about 75% of those go to landfills. The European Union has done significantly better: approximately 95% of end-of-life tires were collected for recycling in 2019, with 55% going to material recovery and 40% used as fuel in cement kilns and power plants.
Mechanical Grinding: The Most Common Method
The most widely used recycling method is mechanical, meaning the rubber is physically shredded and ground into smaller and smaller particles. The process starts with large shredders that cut tires or rubber products into rough chunks. Steel wires and fabric fibers are separated out magnetically or by screening. Then the chunks move to grinding equipment that reduces them further into what’s known as crumb rubber, with particles sieved to specific sizes depending on the intended use.
There are two main approaches to grinding. Ambient grinding happens at room temperature using high-powered mills that tear the rubber apart through friction and mechanical force. The particles produced tend to have rough, irregular surfaces, which actually helps them bond with other materials later. Cryogenic grinding takes a different approach: liquid nitrogen cools the rubber to extremely low temperatures, making it brittle. The frozen rubber shatters cleanly when ground, producing smoother, more uniform particles that can be sieved down to very fine sizes (under 63 micrometers in some processes). Cryogenic grinding uses more energy for cooling but produces a more consistent product.
What Crumb Rubber Becomes
The largest single use for crumb rubber is in road construction. When mixed into asphalt at concentrations of 4% to 20%, crumb rubber improves the pavement’s resistance to cracking and deformation. Rubberized asphalt has been shown to outperform conventional asphalt under identical conditions, with a longer overall lifespan. It also reduces road noise. Beyond roads, crumb rubber goes into playground surfaces, athletic tracks, rubber mats, shoe soles, and as infill for artificial turf fields.
Devulcanization: Reversing the Chemistry
Devulcanization aims to do what grinding cannot: break the sulfur bonds that make rubber rigid while leaving the main polymer chains intact. If successful, the result is a material that can be re-vulcanized and shaped into new rubber products, much closer to true recycling than grinding alone.
Several methods can accomplish this. Microwave devulcanization uses targeted microwave energy to break sulfur-to-sulfur and carbon-to-sulfur bonds in the crosslinks. The challenge is precision. Too much energy will destroy the polymer backbone itself, so the process parameters need careful control. Chemical agents can assist by reacting with the broken bonds to stabilize them, preventing new crosslinks from forming immediately.
Biological devulcanization is a newer approach that uses bacteria or fungi to selectively attack sulfur bonds on the rubber surface. Certain microorganisms convert sulfur crosslinks into elemental sulfur or sulfate compounds while leaving the main polymer chains unchanged. One wood-rotting fungus, for instance, has been shown to oxidatively break down sulfide bonds in vulcanized natural rubber sheets, reducing the crosslinked network density. Researchers have also isolated strains of bacteria from the genera Streptomyces, Actinoplanes, and Methylibium that can degrade both natural rubber and synthetic versions of the same polymer. These biological methods are gentler and more selective than heat or chemical treatments, though they currently work slowly and primarily on rubber surfaces.
Pyrolysis: Breaking Rubber Down With Heat
Pyrolysis heats rubber in the absence of oxygen, decomposing it into three usable products. Typical yields from tire pyrolysis break down roughly as follows: 34% to 42% fuel oil, 35% to 40% solid char (recovered carbon black), and 10% to 30% synthetic gas. The fuel oil can substitute for diesel or other petroleum products. The recovered carbon black can partially replace virgin carbon black in new tire manufacturing and other rubber goods, though it typically needs upgrading to match the quality of freshly produced carbon black. The gas produced during pyrolysis is often burned on-site to power the pyrolysis reactor itself.
Pyrolysis is energy-intensive and requires specialized industrial facilities, but it handles rubber that’s too contaminated or degraded for mechanical recycling. It’s particularly useful for tires that have been stockpiled for years in landfills.
What You Can Do With Rubber at Home
Most curbside recycling programs do not accept rubber of any kind. Tires are the one rubber product with a well-established recycling path: tire retailers and auto shops typically accept old tires (sometimes for a small fee), and many municipalities run periodic tire collection events. In the U.S., most states have laws requiring tire retailers to take back old tires at the point of sale.
For other rubber items like garden hoses, rubber gloves, rubber-soled shoes, or yoga mats, options are more limited. Some specialty recyclers accept these products by mail. Shoe brands including Nike operate take-back programs that grind old athletic shoes into material for playground and track surfaces. Bike inner tubes can be sent to companies that repurpose them into bags, wallets, and other goods. For small quantities of household rubber, repurposing is often the most practical route: old tires become planters or swings, inner tubes become tie-downs or jar openers.
Environmental Concerns With Recycled Rubber
Recycled rubber products aren’t without environmental trade-offs. Zinc is the primary concern. It makes up about 1% of tire rubber by weight, and leaching studies have found that shredded rubber mulch releases high concentrations of zinc into water, especially in acidic or warm conditions. Other metals like selenium, lead, and cadmium appear in tire leachate at much lower levels, and metals like chromium, copper, and lead were below detection limits in most tests.
Organic compounds are a smaller but acknowledged concern. Lab tests have detected carbon disulfide, toluene, phenol, and benzothiazole in scrap tire leachate, though reported levels fell well below regulatory limits. Volatile organic compounds released in vapor form from ground rubber have also been measured, but the concentrations under normal environmental conditions (moderate temperature, neutral pH) appear to pose limited risk based on available studies. The practical takeaway: recycled rubber mulch in gardens or playgrounds isn’t an acute health hazard, but using it near waterways or in areas with acidic soil increases the potential for zinc to leach into groundwater.