**Recycling of Scrap Tires**

Ahmet Turer

*Middle East Technical University, Civil Engineering Dept. Turkey* 

#### **1. Introduction**

As Rachel Louise Carson (1907-1964) successfully noted in her phrase "The human race is challenged more than ever before to demonstrate our mastery - not over nature but of ourselves", we are challenged to find ways to produce more energy, reduce our waste production while minimizing use of limited natural resources. Although recycling of materials has a history going back to the times of Plato BC400 and collecting scrap bronze & metals in Europe in pre-industrial times (Wikipedia, 2011), the demand roar for raw materials in the 19th and 20th centuries with industrial development caused cheaper alternative of reusing scrap material rather than mining them out. Interestingly, 21st century's major driving force has additional items on top of the existing reasons of using recycled material, such as reducing consumption of limited natural resources and lowering carbon dioxide emissions against the greenhouse effect. The increasing demand for energy production and dealing with larger amounts of waste contaminating the nature, forces mankind to find innovative ways to deal with the produced pollutant waste, emit lesser amounts of CO2, and generate more energy. Recycling of scrap tires turns out to be a perfect match for the recent requirements of the 21st century. This chapter discusses various ways of recycling scrap tires and how they relate to the recent energy, material, and nature needs of our times.

Recycling of scrap tires until the 1960's in the US can be taken as an example; about half of the manufactured automobile tires used to be recycled since only synthetic or natural rubber was used in the tire manufacturing process and tires could have been directly used without major processing. Recycling of used tires was further encouraged by the fact that these materials were also expensive. The increasing use of the synthetic rubber, however, lowered the manufacturing costs and reduced need for recycling. Moreover, the development of steel belted tires in the late 1960's was almost the end of tire recycling since additional processing of tires was needed. Consequently, by 1995, the rate of rubber recycling fell to only 2% [Reschner].

Highway construction industry is a big alternative market for recycling scrap tires. Many studies have been carried out on crumb rubber modified asphalt. In 1995, it was required by all federal states in the U.S. to fund paving projects with tire modified asphalt. After that, the consumption rate of wasted tires in modified asphalt projects was increased, and in some states a maximum recycling rate of 20% was reached [Sheehan]. Other methods to gain the raw material and energy available inside scrap tires are further discussed under each

Recycling of Scrap Tires 197

product by applying steam heat under pressure, for four to six hours at 132 Celsius (270

Following the discovery of vulcanization, manufacturers began producing tires from solid rubber which yielded a strong material to resist cuts and abrasions. Although this was a great progress, the tires were too heavy and rigid. In order to decrease vibration and improve traction, Robert W. Thomson, first produced the pneumatic rubber tire which consisted of rubber filled in with air. His idea could not a commercialized since it was introduced too early for its time. John Boyd Dunlop from Ireland, who did not know about Thomson's earlier invention, once more introduced the pneumatic tire to the market in 1888. This time, pneumatic tire caught the public's attention because bicycles were becoming

extremely popular and the lighter tire provided a much better ride (Rubberis, 2011).

In early 19th century, manufacturers started producing vehicle tires comprising two parts, i.e., an inner part and an outer part. The inner part, called the inner tube, contained compressed air and the outer part was a casing protecting the inner tube and providing the tire with a better grip. An important element of the outer part were the layers called plies which were made of rubberized fabric cords embedded in the rubber and they were strengthening the casing. They were known as bias-ply tires because the cords in a single ply run diagonally from the beads on one inner rim to the beads on the other rim. The orientation of the cords change from ply to ply so that the cords crisscross each other

The steel-belted radial tires were first produced in 1948 by the Europeans. In those first tires, the ply cords radiate at a 90 degree angle from the wheel rim. Together with this, a belt of steel fabric that wrapped the circumference of the tire reinforced its casing. Radial tire ply cords are composed of nylon, rayon or polyester. The advantages of radial tires include longer tread life, better steering and less rolling resistance. On the other hand, radials have a harder riding quality, and are about twice as expensive as the tires without radials. The

Massive disposal sites of scrap tires is common in many cities of modern times as about 1 scrap tire is produced per person every year. The stored used tires slowly degrade under the effects of solar radiation as well as rusting of steel takes place. Degraded material would slowly contaminate soil and underground water over years. The disposal sites waiting under the sun for extended periods of time might catch on fire either by accident or because of bottles or broken glass focusing sunlight. Tires burn with thick black smoke and heat, quickly spreads over the whole disposal area, and leaves oily residue contaminating the soil. Such fires are difficult to put off and generates significant amount

One of the overseen problems of scrap tire disposal yards is that these areas become breeding places for rodents and mosquitoes. Stagnant water that collects inside tires is a suitable breeding place for mosquitoes. Elimination of scrap tire disposal sites by proper recycling would also have secondary advantages of eliminating disposal related problems.

production sequence of steel-belted radial tires is briefly illustrated in Fig. 2.

**3. Scrap tire disposal related problems** 

Fahrenheit) degrees (Goodyear, 2011).

(Rubbentire, 2011).

of air pollution.

heading below. The outcomes of scrap tire recycling are not only limited by easy access to cost-efficient material such as rubber and steel, but also have positive effects on the environment: Recycling of scrap tires on a global scale can drastically reduce waste yards, soil and atmospheric contamination caused by dump yards and large scale tire fires.

Fig. 1. Scrap tire storage areas and fires.
