Sustainability has been in the news a lot lately. Whether it is smog alerts in Hong Kong or warnings about islands of plastic in the Pacific, we are inundated with stories about how waste shapes our world. A lot of these problems seem overwhelming, but humans have an amazing knack for turning a problem into a resource.
Synthetic rubber is a perfect example. It’s difficult to manage because of the large volume of waste, durability of the material, and the hazards involved in handling and storage. Until recently, no one had the technology or infrastructure to deal with it; rubber stockpiles were standard, despite the hazards.
Decades of concerted effort have culminated in effective rubber recycling programs. Now your old tires can find new life as playground surfaces, floor mats, or even impact resistant parking stops.
No solution is perfect, but the success of rubber recycling can give us hope for better management of other non-biodegradable waste.
We generate A LOT of used rubber
Rubber recycling is synonymous with waste tire management, and for good reason — old tires are far and away the most common source of waste rubber. Passenger cars, trucks, airplanes, and off-road vehicles all use and abuse rubber tires during operation.
Tire service life can be extended by retreading, but not indefinitely. Eventually, end of life tires (ELTs) are taken off the road. From there, they are either recycled, stockpiled, or sent to the landfill.
The sheer volume of waste tires is intimidating. Approximately 242 million ELTs are generated annually in the U.S., with another 40 million from Mexico and 35 million from Canada. That’s over 317 million ELTs every year from North America alone.
Rubber stockpiles used to be a big problem
Most of the ELTs in developed countries are recycled, but that’s a fairly recent development. Prior to the 21st century, very few countries had the collection and processing infrastructure to recycle rubber on a large scale. The only possible destinations for ELTs were the landfill or a dedicated tire stockpile.
A constant influx of used tires could quickly exceed landfill capacity, so most regions stockpiled them for future use.
While theoretically more responsible than a landfill, standing ELT stockpiles are problematic. They discourage development of surrounding land, encourage illegal dumping, and even harbor disease-bearing mosquitoes and vermin. The negative economic and health impacts alone are cause for concern, but the most serious issue is flammability – tire stockpiles are essentially mountains of fuel waiting for a spark.
The negative economic and health implications are certainly cause for concern, but the most serious issue is flammability — tire stockpiles are essentially mountains of fuel waiting for a spark.
The longer a stockpile sits, the more likely it is to ignite, and when it does the consequences are severe.
Tire fires are notoriously toxic and difficult to put out. The thick, black smoke they generate negatively impacts air quality for weeks, and soil and ground water for a lot longer.
Because ELTs are non-biodegradable, the problem doesn’t diminish over time — they are a persistent environmental hazard. Site remediation can minimize the damage, but at considerable expense. The most cost effective option is preemptive clean up and recycling.
Recycled rubber has unique and useful properties
Environmental concerns aside, failure to recycle ELTs is a missed opportunity for a useful secondary raw material. Ground rubber has unique properties that make it better than traditional materials in certain applications.
- Shock absorption
- Sound absorption
- Abrasion and crack resistant
Recycling has become the new standard
Thankfully, recycling is becoming more frequent as capable facilities get up and running. The recycling effort is responsible for nearly eliminating the massive stockpiles of the 1990’s. As of 2015, 93% of the peak stockpile volume in the U.S. has been cleaned up and recycled3. Similar cleanups have taken place in most developed countries.
The utilization rate for recycled scrap in the U.S. has remained above 80% for the past decade3. The majority is still utilized as tire-derived fuel, but that trend is shifting towards ground rubber and related products. The most common uses are asphalt, playground mulch, sports surfaces, and other molded rubber products.
How rubber is recycled
Unprocessed ELTs are large, unwieldy, and usually covered in dirt. Before even approaching a useful form, they need to be cleaned and cut down to a uniform size.
ELTs are fed into a granulator, where they are fragmented into smaller pieces collectively known as crumb rubber. A single passenger tire can generate 10-12 pounds.
Rough crumb rubber is more manageable that whole tires, but not all that useful. It still contains fragments of reinforcing fabric and wire. Depending on the end market, additional processing may be necessary to remove those impurities and create a finer crumb size, or mesh.
“Mesh” is a term for the number of holes per inch that the crumb rubber can pass through. The finer the crumb, the higher the mesh count.
Recyclers use a few different processes to increase mesh. The two most common are ambient grinding and cryogenic processing. Wet grind is a rarer process used to produce a much finer mesh. The final mesh makes the crumb rubber suitable for different applications.
Ambient ground rubber undergoes a series of shear and compression processes at ambient temperatures. The resulting crumb is irregular.
In cryogenic processing, frozen rubber is fractured in a highspeed hammer mill. The process produces fine mesh with smooth sides and easy flow.
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Crumb rubber has many innovative uses.
Processing technology and manufacturing capacity have been in development since the 1990s, and have made some important breakthroughs. Due to the effort, crumb rubber markets are steadily growing. The rise in popularity is partially driven by simple economics—recycled rubber is relatively inexpensive—but that is far from the full story.
Thanks to its strength, durability, and impact absorption, recycled rubber can often do the job better than traditional materials.
Parking stops are a simple example. Traditionally, parking stops are made from concrete: extremely heavy, inflexible, and prone to cracking under impact.
Recycled rubber parking stops are better at absorbing impact, a property that extends their service life much farther than the traditional concrete alternative. The lighter weight also makes them easier (and less expensive) to install.
Recycled rubber is now a standard material for a wide variety of applications.
- playground surfaces
- athletic tracks
- synthetic turf
- textured paints
- non-slip surfaces
- automobile floor mats
- waterproofing silo and roof liners
- speed bumps and humps
- shock absorbing pads for machinery
- garbage cans
- United States Environmental Protection Agency. “Scrap Tires: Handbook on Recycling Applications and Management in the U.S. and Mexico.”
- United States Environmental Protection Agency. “Scrap Tire Cleanup Guidebook.”
- Rubber Manufacturers Association. “2015 U.S. Scrap Tire Management Summary”
- Title: photographer name, CC BY 2.0, via Flickr
- Tire pile for rubber recycling: vagawl, CC BY 2.0, via Flickr
- Synthetic rubber tire stockpile: Tim Parkinson, CC BY 2.0, via Flickr
- Tire stockpile fire in Westley, CA: Wisconsin Department of Natural Resources, CC BY-ND 2.0, via Flickr
- Somerset Tire Fire aerial view: Wisconsin Department of Natural Resources, CC BY-ND 2.0, via Flickr
- Playground surface crumb rubber: Ian D. Keating, CC BY 2.0, via Flickr