Rubber pieces for recycling

Sustainability in the Rubber Cycle

04/21/2023
By Amber Nicole Swindall

If you hear the word rubber, you may be more likely to envision a pile of old, discarded tires than to imagine a Southeast Asian tree plantation or an Amazonian rainforest. This widely used resource, however, comes straight from nature.

Most of the world’s natural rubber is derived from the Pará rubber tree plant, Hevea brasiliensis. The latex extracted from this tree’s trunk is the primary material used in producing tires. Recently, an alternative—a North American desert shrub called the guayule (Parthenium argentatum)—has come to the attention of rubber producers and may become very important to the tire industry.

Unlike latex from the Pará rubber tree, the guayule’s version doesn’t contain allergenic proteins that can cause life-threatening allergy symptoms in some people, so this desert shrub is already proving to be very important in protecting human health.

US Ground Rubber Market 2015 chart
2015 Ground Rubber Market data from the U.S. Rubber Manufacturers Association shows what ground rubber is commonly used for in the United States.

The words latex and rubber are often used interchangeably, but they actually represent different things. Latex is a milky liquid produced by plants. It contains water, proteins, sugar, rubber particles, and other chemicals. Manufacturers can concentrate latex so that its rubber particles coagulate to form a solid material, natural rubber (the particles from both the Pará rubber tree and the guayule consist almost entirely of a molecule called cis-1, 4-polyisoprene). Latex gloves are just one example of a product made from this material. When you combine natural rubber with the synthetic version, you get a product that has better flexibility and resilience, necessary characteristics for the manufacture of truck and airplane tires.

People often ask how harmful rubber is to the environment. In truth, tire rubber can be quite detrimental if it is not recycled. Because of an overabundance of used tires in our landfills, environmental service companies work to reclaim, recycle, reuse, and repurpose used tires:

  • they reclaim old, unusable tires, collecting them so as not to overload landfills, pollute waterways, or deplete air quality;
  • they recycle them, recirculating all tire parts into the economy and providing such raw materials as crumb rubber, which is used for playground and sports surfaces;
  • they reuse them, grading the scrap tires and reselling high-quality ones; and
  • they repurpose them, creating safe, eco-friendly products and materials from downgraded tires.

By using these environmental approaches, recyclers help eliminate hazards associated with burning piles of tires, improve and minimize economic concerns, mitigate health risks, and work toward complete sustainability. Recycling reduces the amount of space that tires take up in landfills, decreases toxins that harm the environment, increases economic advantages, and creates sustainability by lowering energy consumption.

US scrap tire stockpiles chart
2016 Scrap Tire Stockpiles data from the U.S. Rubber Manufacturers Association show how the amount of these stockpiles has significantly decreased over the past decade.

Fortunately, there are several ways to break down tire rubber’s compounds for recycling purposes. Some of the most common methods consist of sorting, devulcanization, freezing, and tire pyrolysis.

Some recycling companies first clean and cut down end-of-life tires (ELTs) to a uniform size. To do so, they feed the ELTs into a granulator, fragmenting them into the smaller pieces that we call crumb rubber. A single passenger tire can generate 10–12 lbs. of this material. Crumb rubber (also called ground tire rubber or GTR) is the smallest and highest-end use of recycled rubber. Charles Goodyear, who invented vulcanization— the use of high heat to make rubber stronger—was the first person to recycle vulcanized rubber by grinding down old tires more than 150 years ago.

Toxic smoke stemming from tire fires.
Toxic smoke stemming from tire fires can pollute air, soil, and groundwater for many months and is very costly to clean up.

Crumb rubber still contains polyester and steel fragments or fibers, so additional processing is needed to remove these impurities and to create a finer crumb size, or mesh. Recyclers refer to this key step as sorting or ambient grinding, and they manage this while they shred the tires. They use magnets to draw out the steel fibers, and they employ other tools and techniques too, including screens that are shaken, low-vacuum suction, and wind sifters that help sort out the polyester fibers. Crumb rubber is a much more versatile material for manufacturers to use than whole tires, and it can range in size from particles that are almost dustlike to fragments that are roughly one-half inch in size. The smaller the rubber particle, the more steel and fiber has been removed.

For other recycling companies, devulcanization offers an advantage because it allows them to reformulate the rubber, making it suitable for new products, provided that the process does not degrade the material but instead preserves its insulation and elasticity. Rubber devulcanization methods rely on different types of energy: thermal (from conventional heat or microwave radiation sources); mechanical (from shear forces); or chemical.

GTR (ground tire rubber) reclaiming vs. devulcanization
GTR (ground tire rubber) reclaiming vs. devulcanization

These techniques are patented but are not classified by the energy source used; rather, they are characterized by the equipment employed and by the recipe, or specific process, involved. The goal of any of these methods is to preserve the original microstructure of the rubber and to allow it to maintain a relatively high molecular weight. Thus far, none of the devulcanization techniques has proven to be commercially viable, so there is room for new patents and innovation.

Other companies recycle rubber by freezing it or processing it cryogenically, using liquid nitrogen. Once the rubber is frozen below -80 degrees Celsius, it becomes very brittle. The frozen rubber is then smashed at a hammermill to create granules. These granules can be used to create new rubber products.

Charts showing recent research carried out on rubber recycling.
((A) The recent research carried out on rubber recycling, reflected in scientific papers and patents; (B) countries’ contributions to the development of rubber recycling (data according to Scopus, available on 1/28/2022; key terms: rubber recycling, rubber reclaiming, and rubber devulcanization).

The final recycling method is called tire pyrolysis, and it involves melting the rubber without oxygen present, which avoids the possibility of combustion or the release of toxic fumes. Through this process, the rubber decomposes into oil, gases, and char. Once the polymers in the rubber break down into small molecules, the molecules then vaporize. These gases are extracted through a condensing system that transforms them into iquid oil. Iron, steel, ceramics, and chemical industries use this liquid oil to fuel their systems.

Disposal of waste rubber remains a significant environmental issue because the material takes several years to degrade and large quantities of it are produced. Vulcanized rubbers are extensively used in a wide range of applications (mainly in tire industries) because of their mechanical strength, excellent durability, abrasion resistance, and low cost. Reprocessing these tires is tricky because of their crosslinked structures and the various additives in their composition, and also because the rubber is elastic, insoluble, and an infusible thermoset material that cannot be directly reprocessed to return it back to its natural state. The good news is that there are cost-effective methods that can be used for recycling waste rubber from scrap tires.

The most environmentally friendly recycling method is shredding and grinding waste tires into GTR and using the particulate material as a filler in thermosets, virgin rubbers, or thermoplastics (especially recycled resins) to produce thermoplastic elastomer (TPE) compounds. The TPE market is expected to grow significantly soon, due to increased demand for green and low-cost compounds obtained from waste polymers. However, the lack of literature about the thermal, dynamic mechanical, and aging behavior of these compounds highlights a crucial need for more research on the use and quality of these compounds.

 

Amber Nicole Swindall Amber Nicole Swindall is a graduate student studying Engineering Technology and Occupational Health and Safety at Middle Tennessee State University. Her hobbies include exploring the great outdoors by skiing, hiking, swimming, biking, running, paddleboarding, and boating.

 

This article was originally published in AWIS Magazine. Join AWIS to access the full issue of AWIS Magazine and more member benefits.