Tires are highly specialized goods that have been designed and built to be very durable and provide years of dependable use under the most demanding punishment. Tires are composed of a composite of materials (rubber, oil, steel, fabric and chemicals) that are combined under extreme heat and pressure.
The properties that make tires flexible and durable make them difficult to recycle. Like a cake, once it is baked it is nearly impossible to break it down into its original components. Instead, most recycling relies on mechanical means to break tires into smaller and smaller pieces. As they are broken down the steel and bits of fabric are separated, in the end leaving a relatively pure secondary material made up of small bits of blended rubber.
Over the years the number and variety of uses and applications for recycled rubber has grown. Recycled rubber can be used as a feed stock in the production of new rubber materials but it is essentially filler. New tire manufacturers have been trying for years to find a way of incorporating recycled rubber back into tires. Unfortunately only a small percentage can be added before performance begins to be effected.
The range of uses of scrap tires and rubber from scrap tires include:
whole and cut tire uses, shredded tires (i.e., pieces of rubber larger than 15 mm in diameter, nominally about 50 mm in diameter);
granulated rubber, usually called crumb (i.e., finely ground rubber 15 mm in diameter and less, usually steel-free); and,
extruded products made from rubber crumb, rubberized asphalt and energy- recovery, usually called tire-derived fuel or “TDF” (not really recycling but a popular end-use elsewhere).
Some of the applications for these materials are briefly listed below. Please note that some of these uses are covered by various patents or may be protected by trademarks.
Whole and cut tires:
Whole tires can be used baled or bolted together and used in civil engineering applications, such as breakwaters and erosion barriers.
Cut tires are used to make mats, animal feeders, fencing, composters, planters, blasting mats, etc. This use also includes rubber parts punched from cut tires.
Large tire shreds have been successfully used as lightweight fill material in roadbed and embankment stabilization projects. The material is also used as a leachate drainage material in sealed landfills to enhance drainage.
Medium-sized rubber crumb (5-10 mm) has become popular as a material for playgrounds to provide protection to children from falls and injuries. This same material size-range is being used in equestrian training arenas (usually mixed with sand) to provide a secure and safer footing for horses. It is also being used in golf courses and sports fields to protect turf and provide better drainage.
Fine rubber crumb is a feedstock to a range of manufacturing processes.
A full range of consumer products is manufactured from recycled rubber from tires. The main applications are where the rubber crumb is mixed with a binding agent and molded into products: rubber mats, paving stones and blocks, curb stops, railroad crossings, automotive parts, solid wheels, fatigue mats and truck-box liners.
Loose rubber crumb is also being made into animal mattresses for dairy cattle.
Ultra fine rubber crumb can be made into sealants, soaker hoses, carpet underlay, and partially re-vulcanized rubber reclaim.
Recycled rubber crumb has been used in asphalt in two different ways: as a filler or aggregate replacement and as a binding agent. Rubberized asphalt has shown enhanced wear and traction performance, as well as minimizing the wear-related road rutting and frost deformation of roads in cold climates.
A similar application is the pour-in-place surfaces for playgrounds and running tracks that use fine rubber crumb and a binding agent.
Energy recovery (TDF):
Both whole and shredded tires are used extensively in other parts of the world in energy recovery. Controlled combustion of tires can be used to recover the energy in the hydrocarbons, but there are a number of environmental and economic, as well as social, issues related to this use.
The “holy grail” of tire recycling is devulcanization — the production of raw rubber (as well as oils and carbon blocks) from scrap tires. While this is theoretically possible and many claim to have produced devulcnized rubber from tires, there are no commercially viable processes in operation. There are many companies that produce various forms of partially devulcanized rubber that can be used as raw materials in manufacturing processes, but in most cases the rubber is an amalgam that does not have the same properties as the original rubber. It is kind of like trying to un-bake a cake. Once you have mixed all of the ingredients together and cooked them, it is nearly impossible to separate them again.