Scrap tires reclaiming

Tires, natural mbber tubes, and butyl tubes are the main sources of scrap and reclaim (see Elastomers, synthetic-polyisoprene). Specialty reclaim materials are made from scrap siUcone, chloroprene (CR), nitrile— butadiene (NBR), and ethylene—propjlene—diene—terpolymer (EPDM) mbber scraps (see... 

The United States generates ca 250 x 103 scrap tires per year. In 1977, ca 521 of reclaimed mbber and 14,400 t of cmmb mbber were produced from these tires (40). In 1980, ca 4050 t of reclaimed and crumb mbber were used in asphalt—mbber compounds, which is less than 5% of the recycled mbber. Except for a reduction in reclaimed mbber, the 1986 production rates were higher an estimated 25,200 t of mbber were used in asphalt. 

Overall, burning tires or TDF in cement kilns appear to be an economically satisfactory and environmentally sound way of not only disposing scrap tires, but also reclaiming their fuel value. 

The number of scrap tires in Japan in 1977 was estimated to be approximately h J million with a total weight of 550 thousand tons. Although some of them are reused as reclaimed rubber, recapped tires, etc., it is difficult to recycle all of the scrap tires by the usual methods, therefore, there is a demand for new recycling systems for scrap tires. 

However, it is believed that incineration of strap tires is not the most desirable long-run approach when the total natural resource situation is considered. Scrap tires, a discarded resource rich in hydrocarbons, should be returned to the production cycle as useful materials, such as reclaim, reinforcing carbon black, and carbon compounds that could be chemical feed stocks. This last approach would conserve gas and oil reserves. 

Up to the time of World War 11, scrap tires contained only natural rubber. Reclaiming processes were well established, and a reasonable proportion (20%) of reclaim was accepted in compounding of new tires. (Frequently, the percentage was much more in other rubber products.) In 1941 the consumption of reclaim was 32% of the consumption of new rubber. 

Embankment or fill Coal fly ash Mineral processing wastes Nonferrons slags Reclaimed asphalt pavement Reclaimed concrete Scrap tires... 

Reclaiming is a procedure in which the scrap tire rubber or vulcanized rubber waste is converted, using mechanical and thermal energy and chemicals, into a state in which it can be mixed, processed, and vulcanized again. The principle of the process is devulcanization. In devulcanization, it is assumed that the cleavage of intermolecular bonds of the chemical network, such as carbon-sulfur and/or... 

Waste materials such as municipal solid waste, scrap tires, and waste plastics have traditionally been placed in sanitary landfills. However, with landfill space rapidly decreasing in the United States and worldwide, an alternative disposal method for these waste materials becomes imperative. The recycling of solid wastes is a challenging problem, with both economic and environmental constraints. Recently, two broad approaches have been attempted to reclaim solid wastes. The first approach relies on thermal or catalytic conversion of waste materials into fuel and valuable chemical feedstocks. Examples of this approach include gasification, pyrolysis, depolymerization, and liquefaction. The second approach relies on the physical recovery of valuable ingredients in the waste materials. 

Nippon Zeon estimated that the break-even cost of its tire pyrolysis pilot plant was 0.25 per tire (29,30). One study indicates that pyrolysis of tires and other polymers should be considered as a means for disposing of scrap within environmental constraints. A plant processing 81,000 t/yr of scrap could be profitable, based on sales of reclaimed products (31). 

Vehicle tire scrap containing polyisoprene rubber, SBR, and butadiene rubber was devulcanized by low-temperature phase-transfer catalyst. Both the devulcanizing agent composition and the process were patented. The novelty of this process lies in the use of low-temperature phase-transfer catalyst and a process temperature lower than 150" C. The devulcanized rubber of this invention is distinguishable from conventional reclaimed rubber in that the devulcanized rubber is substantially free from polysulfide crosslinks, which are selectively broken during the process with negligible main chain scission. 

When it comes to finding more complex systems of reusing the scrap rubber powder, recovery alternatives include pyrolysis, gasification, tire derived fuel, reuse and recycling, rubber reclaim, and retreading [3]. 

As already said in the Preface, a question arose as to the appropriateness of having a chapter in this book devoted to the cure of rubbers and to their properties concerned with the recovery of scrap rubber obtained from old tires. However, the amount of old tire rubber is so large that it could be considered as a raw material, and reclaiming will become a necessity when crude oil and natural rubber experience shortages. This problem is so vast that only a book in itself could describe all the possibilities of reusing. Nevertheless, a large number of opportunities has been identified, and the reclaiming processes based on the cure of these scrap rubbers have been considered. 

Worn-out tires and scraps and trimmings of other vulcanized products constitute the raw material for reclaimed rubber. Therefore a good reclaiming process must not only turn the rubber soft and plastic but also must remove reinforcing cords and fabrics that may be present. There are a number of commercial processes [43] for rubber regeneration (1) alkali digestion process, (2) neutral or zinc chloride digestion process, (3) heater or pan process, and (4) reclaimator process. 

A mechanical or reclaimator process (LaGrone, 1986 Szilard, 1973 Klingensmith, 1991 Leyden 1991) has been used for the continuous reclaiming of whole tire scrap. Fine mbber crumb (typically 30 mesh) mixed with various reclaiming oils is subjected to high temperature with intense mechanical working in a modified extmder for reclaiming the mbber scrap. 

There are certain technical limitations in the devulcanization of rubbers, and vulcanization is, in fact, not truly reversible (Pryweller, 1999). The partial devulcanization of scrap rubber will result in a degradation of physical properties. In many cases, this may limit the amount of substitution levels in high-tech applications such as passenger tires. But it can provide the compounder of less stringent products with an excellent low-cost rubber that can be used as the prime rubber or at very high substitution levels. According to Franta (1989), reclaim cannot be used for tread compounds in tires because every addition may decrease their resistance to wear. However, this statement has not been checked in case rubber devulcanized without an addition of chemicals. Considerable amounts of reclaim are consumed for carcasses of bias ply tires for cars if the compounds are of NR for carcasses of radial tires no reclaim is added. On the other hand, reclaim is added to compounds for bead wires and it may also be added to sidewalls. Within the framework of direct recycling options a number of applications for GRT outside the rubber industry have been proposed. Such applications include the use as a tiller in asphalt for the surface treatment of roads and as a rubberized surface for sport facilities. 

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