I. INTRODUCTION

Manufacturing of tire and other rubber products involves vulcanization process, an irreversible reaction between the elastomer, sulfur, and other chemicals producing crosslinks between the elastomer molecular chains and leading to the formation of a three-dimensional chemical network. The crosslinked elastomers are solid, insoluble, and infusible thermoset materials. This makes the direct reprocessing and recycling of used tires and waste rubbers impossible. Therefore, the environmental problems caused by used tires and other waste rubber products have become serious in recent years. In fact, Charles Goodyear, who invented the sulfur vulcanization process more than 150 years ago [1], was also the first to initiate efforts to recycle cured rubber wastes through a grinding method [2]. Even after so many years of efforts in recycling, the development of a suitable technology to utilize waste rubbers is an important issue facing the rubber industry.

According to a recent survey of the Scrap Tire Management Council of the Rubber Manufacturers Association, approximately 281 million scrap tires were generated in the United States in 2001 alone [3]. The market for scrap tires is consuming about 77.6% of that total amount [3] while the rest is added to an existing stockpile of an estimated 300 million scrap tires located around the United States [4]. These stockpiled tires create serious fire dangers and provide breeding grounds for rodents, snakes, mosquitoes, and other pests, causing health hazards and environmental problems [5]. Moreover, the major...