New Technological Solutions for Recycling Spent Tire Rubber

Introduction

Used tires pose both a serious public health and an environmental threat. The disposal of spent tires is already an increasing problem for the European Union, the number of car and truck tires scrapped every year is projected to be around 250 million, representing about 2.6 million tons of tires. Moreover, spent tires represent the main part of the total amount of the waste rubber material, which is produced every year in the world. In fact, with respect to the annual total global production of rubber material (16–17 million tons), approximately 65% is used for the production of tires. In the most industrialized countries, tires are usually dumped in landfills or left in the open air, generating significant environmental disturbances as non-biodegradable residues and dangerous situations like the risk of fire. There are several solutions to recycle waste tire rubbers: in any case, the first processing step is grinding; each tire is cut in small parts for successive processing. At present, the best solution for tire recycling seems to be the energy reclaim by combustion of the rubber as there are some factories (e.g., cement kilns) which need high energies for their processes. The use of shredded tires for energy reclaim in cement kilns allows to solve the regulatory problem of the tire manufacturers as the cement kilns are able to burn almost all the rubber coming from the spent tires but this solution is absolutely not sustainable or environmentally conscious. In fact, the rubber from spent tires cannot be used to produce other tires but it is not true that it cannot be used in any other manufacturing process. Even if it is politically right, it seems to be ethically wrong to burn the rubber only to remove the tires from the landfills. It is always possible to reclaim energy from the rubber by combustion and it would be environmentally correct to use this solution when other technological solutions are not practicable.

Many scientific studies deal with the production of rubber modified concretes, and scrap tire powders can be used as fillers for many other structural materials. Fattuhi and Clark (1996) proposed the fabrication of cement-based, mortar, and concrete using various proportions of rubber made by shredding scrap tires. They found that rubber type had only marginal effect but, in any case, density and compressive strength were reduced by addition of rubber. In the same year Cecich et al. (1996) investigated engineering properties of shredded tires: they used the waste materials as lightweight backfill for retaining structures. In comparison with the conventional retaining structures filled with the sands, using shredded tires can cause a substantial cost reduction and an increase in the factor of safety.

Dealing with new recycling technologies, Holst, Stenberg, and Christiansson (1998) discussed that micro-organisms able to break sulfur-sulfur and sulfur-carbon bonds can be used to de-vulcanize waste rubber in order to increase binding upon vulcanization with virgin rubber. Ishiaku, Chong, and, Ismail (1999) determined the optimum of De-Link R (a de-vulcanizing agent) suitable for recycling the rubber powder. Smith et al. (2001) investigated the cryogenic mechanical alloying as a viable strategy by which to produce highly dispersed blends composed of thermoplastics and tire. Hernandez-Olivares et al. (2002) investigated mechanical behavior under static and dynamic load of concrete filled with small volumetric fractions of crushed tire rubber and polypropylene short fibers. In the same year Fukumori et al. (2002) create a new technology to recycling tires. They used a modular screw to crush into small pieces and devulcanized the waste rubbers in order to obtain a continuous recycling technology applicable to the new tire rubber compounds. Sulkowski et al. (2003) obtained a rubber waste-polyurethane composite and investigated the influence of the polyurethane resin on hardness, elasticity, glass transition temperature and thermal stability of composites. In the same year Franzis (2003) reinforced bituminous binders used in the construction of flexible pavements with crumb rubber produced from waste tires. It was shown that the binder has the potential to be used as an all-weather wearing course in flexible roads.