Chemical Education Today
From Past Issues
Rubber Reclamation by Kathryn R. Williams
“Rubber the Remarkable, the material of over three thousand good uses—to be without rubber is a dismaying thought, even a little frightening, since nothing else will quite do.” This quote from Williams Haynes’ and Ernst Hauser’s Rationed Rubber and What to Do About It appeared in the November, 1942 issue of J. Chem. Educ. (1). The need for rubber was especially severe at the time, due to the combined effects of wartime needs and the Japanese occupation of the major producers of natural rubber. Hauser amplified these words of warning in his address to the New England Association of Chemistry Teachers (2). Citing a 1942 report from the national Rubber Survey Committee, Hauser indicated a natural rubber deficit of over 200,000 tons for 1943, about 25% of the total need, and pointed to three ways to alleviate the problem: cultivation of natural rubber in the southwestern U.S., development of synthetic rubber, and reclamation of used rubber. Efforts to domesticate guayule, a rubber-producing plant suitable for arid climates, produced only 1400 tons of rubber by war’s end, although the research was key to other projects on U.S. cultivation of natural rubber later in the 20th century (3). On the positive side, readers hardly need to be informed of the triumph of the rubber synthesis industry. Only a year after his grim address, Hauser wrote, “Today we are assured that our production of synthetic rubber will shortly reach and even surpass [prewar] figures. This is a miraculous achievement; a lasting monument to the ingenuity and resourcefulness of our chemists and engineers.” (4) Hauser was also aware of the importance of reclamation to the rubber emergency. In his New England address, he indicated that “reclaimed rubber still remains the first line of defense as far as the civilian goes,” but added that the recycled material “is not comparable with virgin rubber in its physical properties…” and that “the nationwide speed limits…should do a great deal to prolong the use of a tire made entirely of reclaim.” (2) Aside from a brief segment in Fisher’s article, “Natural and Synthetic Rubbers” (5), there has been only one J. Chem. Educ. article devoted to rubber reclamation. In “Reclaimed Rubber” (6), Gilbert Trimble described the history and technology of the reclamation process—a fitting look backward in honor of the current Earth Day theme. After the discovery of vulcanization in 1839 by Charles Goodyear and (separately) by Englishman Thomas Hancock, rubber became a vital contribution to the industrial revolution and the transportation industry. The inventors also realized the importance of recovering vulcanized scrap. But although both men obtained patents for recycling methods, neither was very useful. The second half of the 19th century witnessed attempts by several other workers to develop a reclamation process. However, 1899 marked the beginning of the reclamation industry. In that year, Arthur Marks patented his alkali process, which remained in use well into the 20th century. www.JCE.DivCHED.org
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Figure 1 shows the flowchart for the Marks process for tire reclamation. The rather complicated series of steps comprised three key phases: scrap preparation, digestion, and milling. Preparation involved removal of all metal components (e.g., the beading where the tire meets the wheel rim, metal valves, and metal objects picked up during tire usage) and mechanical chopping/grinding to produce small pieces of rubber. Of course, rubber was not the only strategic material in short supply during the war. Trimble made special note of the metal, which was saved and recovered as part of the overall reclamation process. The chemistry occurred in the digestion phase. Although the key reactant was ca. 5% NaOH solution, various oils and softeners were also added. The rubber/alkali/softener slurry was sealed in an agitator and treated with pressurized steam for 10–20 hours to hydrolyze cellulose tire cord and convert the rubber to a soft, sticky mass. Although the digestion did
Figure 1. Flow chart for rubber reclamation by the Marks Process. (JCE 1942, 19, 420).
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Journal of Chemical Education
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Chemical Education Today
From Past Issues not destroy the rubber/sulfur bonds, any unreacted sulfur from the original vulcanization process remained in the liquor. After washing to remove the liquor and then partial drying, the rubber proceeded to the final stage, called milling. There, large rollers removed lumps and converted the mass to thin sheets, which were pressed together to form finished slabs about one inch thick, as shown in Figure 2. In the first half of the 20th century, recycling was a key factor in the rubber industry, because of the scarcity and high price of virgin rubber (about the same as silver on a weight basis) (7, 8). However, the successful development of synthetic rubber changed the situation completely. In the first half of the century, rubber products contained an average of about 50% recycled material. This figure dropped to 20% by 1960, and by 1995 only 2% of rubber was recycled. Of course, the used rubber had to be put somewhere, and most of it ended up in huge stockpiles of scrap tires— eyesores to be sure, but also safety and health hazards. Stagnant water pools trapped in tires serve as breeding areas for carriers of West Nile Virus and other mosquito-borne diseases. Another major threat is fire. Once ignited, tires are very difficult to extinguish, and the smoke is extremely detrimental to the environment, as major fires in Ohio and California proved in 1999 (7, 8). Faced with mounting piles of scrap tires, many states passed legislation to regulate tire disposal by the end of the 20th century (9). Today, the majority of worn tires leave the road for second careers as crumb rubber or as TDF (tire-derived fuel). Recycling companies market shredded and ground rubber for use as fill in civil engineering projects and mulch for playgrounds, combined with urethane for molded products, and as rubber-modified asphalt (RMA) for road surfaces. Consumption as tire-derived fuel ranks as the largest single fate (almost 50% in 2000) of scrap tires in the U.S. and EU, in scenarios such as cement kilns, paper mills, and power plants. Although better than stockpiling, combustion can hardly be considered an effective use, since only a fraction of the energy consumed in tire manufacture is reclaimed (7, 8). Unfortunately, efforts to recycle tires to chemically useful forms have met with limited success. Depolymerization reactions require catalysts, which are easily fouled by the many additives in tires. Another chemical treatment involves pyrolysis (i.e., heating in the absence of oxygen) to decompose the rubber to hydrocarbon gases, carbon black, and oils. Although these products are marketable, they are often inferior to those available from other sources. Surely chemists “can” and will find ways to overcome these obstacles to make rubber recycling more commercially effective and environmentally favorable.
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Figure 2. Finished slabs of reclaimed rubber. (JCE, 1942, 19, 426).
Literature Cited 1. Haynes, Williams; Hauser, Ernst A. Rubber the Remarkable, J. Chem. Educ. 1942, 19, 509. 2. Hauser, Ernst A. Our Rubber Problem, J. Chem. Educ. 1943, 20, 203–205. 3. Ray, Dennis T. Guayule: A Source of Natural Rubber, http:// www.hort.purdue.edu/newcrop/proceedings1993/v2-338.html (accessed Sep 2006). 4. Hauser, Ernst A. Synthetic Rubber and Plastics, J. Chem. Educ. 1944, 21, 15–17. 5. Fisher, Harry L. Natural and Synthetic Rubbers, J. Chem. Educ. 1942, 19, 522–530. 6. Trimble, Gilbert K. Reclaimed Rubber, J. Chem. Educ. 1942, 19, 420–427. 7. Ohio Department of Natural Resources. Recycling Tires, http:/ /www.ohiodnr.com/recycling/awareness/facts/tires (accessed Sep 2006). 8. Reschner, Kurt. Scrap Tire Recycling, http://www.entireengineering.de/str/en.html (accessed Sep 2006). 9. Sikora, Mary. Tire Recycling Legislation—More Important Than Ever. http://www.tireindustry.org/features/ recycling_legislation.asp (accessed Sep 2006).
Kathryn R. Williams is in the Department of Chemistry, University of Florida, PO Box 117200, Gainesville, FL 326117200;
[email protected] Vol. 84 No. 2 February 2007
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