Entrepreneurs were recycling and reclaiming vulcanized rubber products long before the development of the automobile and the advent of the pneumatic tire. Then once the motor car revolution got underway early in the 20th century, all major US tire companies operated commercially viable rubber reclamation facilities. These factories used mechanical energy, heat and/or chemicals to convert worn-out tires into bits of “devulcanized” rubber by processes that ruptured polymeric chains and sulfur cross-links. With residual steel and fabric removed, the resulting rubber bits could be reformulated and re-vulcanized into usable products with measurable but tolerable degradation of physical properties. For example, Firestone operated its Xylos reclaim subsidiary from 1916 to 1972, recycling end-of-life tires and factory scrap into less elastic rubber fragments. These were then used in molded goods, and at low levels in tread compounds to reduce cost. Reclaimed rubber consumption spiked during WWII in the USA, when rubber was a vital war commodity, and remained as high as 300,000 tons annually as late as 1960.
With the post-WWII development of synthetic rubber, the economics of processing reclaim were adversely affected, with the recycled product sometimes being more expensive than virgin rubber. Additionally a large market for reclaim – molded rubber goods – was largely displaced by less costly plastics such as polyethylene and polyvinyl chloride, which were used in wire insulation, battery cases and garden hoses, among other products. Consequently the rubber reclaim business entered a downward spiral, resulting in bankruptcies or financial restructurings of many such companies over the past few decades.
That seems to be changing today as larger rubber chips and somewhat smaller rubber crumb from reclaim find use in civil engineering applications such as rubberized asphalt and as tire derived fuel. However, rubber bits and granules free of impurities must be broken down into smaller particles for incorporation into suitable tire compounds. Grinding is the most common method to reduce particle size and increase surface area, and can normally be achieved by ambient temperature or cryogenic grinding. Cryogenic processing tends to be more expensive than purely mechanical methods due to the cost of liquid nitrogen, but does provide the preferred smaller particles and smoother surfaces than alternative methods. Recycled rubber crumb is typically 45-50% elastomer and 25-30% carbon black, with the remainder being chemical additives, unwanted impurities and reaction products formed during vulcanization.
Lehigh Technologies refers to its pulverized rubber particles as MRP (micronized rubber powder) to distinguish the product from run-of-the-mill crumb rubber, and manufactures powders from 80 mesh (~0.177mm), down to 300 mesh (~0.050mm). For reference, individual carbon black particles are three orders of magnitude smaller and usually vary in size from 20-200nm, with aggregates in cured-tire compounds perhaps 5-10 times larger depending on structure and surface area. Lehigh Technologies recently declared that more than 300 million tires had been placed in service using its MRP material. Bridgestone touts the use of 5% ground rubber from scrap tires in the tread compound of its Ecopia EP 422 line. From these numbers, it appears that low levels of fine powdered rubber can be used in selected tire components to achieve tire performance levels close or equal to tires made from 100% virgin material. What seem to be missing from powdered rubber technology are rigorous technical standards governing physical and chemical properties, as are in place with other tire compound additives.
Worn-out tires are now being discarded at the rate of 15 million tons per year. Pessimists view these scrap tires as an environmental burden, while optimists view them as a worthwhile resource. But these apparently contrary views are not incompatible. The recent uptick in using recycled rubber in the tire industry addresses environmental concerns while potentially lowering the cost of tires. These are positive trends for both producers and consumers, but may not be enough to placate activists who lament that tire producers are still too dependent on fossil fuels. It is worth noting that the economics of recycling paper, plastic and rubber is extremely sensitive to the price of petroleum. One hopes that despite recent swings in the price of energy and raw materials, the global tire industry will continue to meet its environmental and regulatory responsibilities by using previously processed rubber – critics and costs notwithstanding.