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Profitable Reclamation of Used Tires

The disposal of worn out tires for cars and trucks (and other waste vulcanized rubber materials) represents an environmental problem of global concern. In the US alone, over 2 billion discarded tires are stockpiled in open dumps where they blight the landscape, provide breeding places for mosquitoes and other undesirable insects and represent fire hazards that, once ignited, are almost inextinguishable and produce massive amounts of air and groundwater pollution. Many states have passed legislation and regulations designed to mitigate or control this problem but the stockpiles of unused materials continue to grow in size and number in the absence of a viable disposal means.

Obviously, a means for economically reusing the materials in spent tires would convert this huge and continuously growing environmental problem into a useful resource, and would provide profitable businesses and jobs. It would potentially also offer opportunities for significant reductions in the importation of rubber and fabricated rubber products, and could provide substantial savings in energy consumption in the manufacture of rubber based products.

Many attempts have been made to establish such waste rubber utilization businesses, involving a variety of technologies, but none have provided a solution to the overall problem. Spent tires have been used as fuel (corrosion, pollution), been subjected to high temperature pyrolysis and distillation (cost, low value product, pollution), been ground up and used in paving asphalt (cost, low performance, quality), made into agglomerated blocks (low value, high cost), etc. A very modest sized industry is involved in shredding tires, separating the fiber and metal components from the rubber particles (rubber crumb) and selling the components. This is a relatively low profit margin, low market size business that has not progressed. This industry has not been able to keep up with the rate at which spent tires are generated. Clearly, higher value products must be made for recycling to be sustainable.

Recovery of usable rubber material offers such an opportunity. Such recovery is complicated by the fact that the virgin rubber used in the tire or industrial rubber products is vulcanized during manufacture. Vulcanization involves a chemical reaction in which sulfur bonds are formed between adjacent molecules of rubber to stiffen and strengthen the finished product. These sulfur bonds are very strong and difficult to break without destroying the rubber like properties of the material. A number of attempts have been made to establish businesses based on technology to do this but none have proven viable. Most of them have been based on a "devulcanization" technology developed by Dr. Leonid Rappoport many years ago when he was Head, Innovation Research, at the USSR Institute for Rubber (and Polymer) Chemistry. This technology was developed to process Russian-made tires of the time, which were based on different rubber compositions than those generally used today.

Recognizing the need for a different and improved technology for today's vulcanized materials, Dr. Rappoport, now an American citizen and working with POLYMERight, Inc., has invented a new process based on a devulcanization agent called DB-26. When intimately mixed with rubber crumb and passed through an extruder under controlled temperature and pressure conditions, the sulfur bonds are broken with a minimal breaking down of the rubber molecules. The result is a "devulcanized" product that now can be reprocessed and vulcanized to make new rubber products. The current prices for virgin natural rubber are at the very low end of their normal range, and are in the order of 30-35/lb. Devulcanized rubber, which can replace a high percentage of the virgin natural and/or synthetic rubber in the formulations used to make non-tire products, can be made profitably at an estimated 28/lb. or less, depending upon scale of the operation. For virgin synthetic rubber, current prices run from 50 to $1.00 or more per pound so economics of replacement of these materials are even much more favorable.

A typical formulation of rubber compound for manufacture of non-tire products includes additions of carbon black, and various plasticizers to provide the desired end product properties. The devulcanized rubber product, however, already contains effective quantities of these components. For this reason, addition of these materials is greatly reduced, or perhaps eliminated, depending upon the formulation required. Carbon black is manufactured from the partial combustion of natural gas or oil and sells for approximately $1.00 per pound, and so recovery and reuse of important quantities of this material will mean very substantial energy and cost savings.

An important cost in the preparation of formulations for the fabrication of rubber goods is the result of the large amounts of electrical energy consumed in the intensive mastication of the rubber mix to assure the intimate mixing, throughout the rubber, of the plasticizers, sulfur and carbon black. In fact, excesses of these materials are normally used to assure that sufficiently intimate mixes are achieved. Since the devulcanized material is already well mixed, and the devulcanization process provides further mixing, the formulation for revulcanization of the recovered rubber does not require as much mastication as with virgin materials. The energy savings possible because of this are very important to the cost of the fabricated product.

The new devulcanization technology has been demonstrated on a laboratory scale and piloted in small extruders with good results. It now must be scaled up to larger equipment, the equipment design and operating conditions for optimum property retention checked out, and fabricated rubber products made with the reclaimed rubber tested and proven in use. Since standard types of equipment are used, and fabricating technology is not changed, final development should be relatively quickly accomplished. Manufacture of the DB-26 has been demonstrated in multi-drum quantities. An economic model has been prepared and favorable economics projected. The cost for a larger pilot-scale demonstration of the devulcanization technology will be on the order of magnitude of $200,000, while the cost of a 12,000,000 lb./yr. commercial plant has been estimated to be approximately $ 3 million. In areas with large stockpiles of discarded tires nearby, much larger plants can be justified with even better economics.