Environmental degradation is the most important problem that we are facing today. Its origin lies in the growing population, rapid industrialization, and huge quantities of solid waste refusal, which is generated daily. To alleviate this environmental degradation, it has become imperative to use appropriate technologies for recovering energy from non-conventional sources, like municipal and/or industrial wastes, refused plastics and used tires, etc. The disposal of these organic solid wastes from human activity is a growing environmental problem for modern society.
Some hazards are overlooked, and one of them is scrap tires, which are one of the most common and important hazardous solid wastes all over the world. Without good management, treatment of scrap tires can threaten not only our environment but public health as well. For example – run-off from scrap tire-generated fires can contaminate groundwater and surface water. Scrap tire sites are often also an ideal habitat for the breeding of disease-carrying insects.
With the continued worldwide increase in the production of automobiles and trucks, the generation rate of waste tires is increasing dramatically. For example, the world generation of used tires in 2005 was over 2.5 million tons in North America, 2.5 million tons in Europe, and 0.5 – 1.0 million tons in Japan. More than 1 million tons of waste tires were generated in USA, Europe, and Japan in 2007. It is estimated that globally one billion used tires arise each year.
Tires are built to be tough and durable, the properties that ensure a safe ride and long service life, make scrap tire disposal a difficult task. Waste tires are recalcitrant to natural degradation. The vulcanized rubber consists of long-chain polymers (polyisoprene, polybutadiene, and styrene-butadiene copolymers) that are cross-linked with sulfur bonds and are further protected by antioxidants and antiozonants that resist degradation. Combustion of tires produces toxic gases that contain carcinogenic and mutagenic chemicals. So, tires incineration requires an expensive air emission control system.
RECYCLING/REUSING WASTE TYRES
The properties of used tires (such as elasticity, stability in air atmosphere, and high level of humidity) have led to several alternative applications like in
- Agriculture: as weights for silage cover sheets,
- In landscaping: as erosion protection for dam walls and slopes,
- Inshore protection: as breakwaters,
- In harbors and docks: as dock bumpers and ship fenders,
- In the fishing industry: as artificial reefs for fish breeding,
- In household and communities: as bumpers in garages, playground equipment,
- Shoemaking: soles, heels, and straps of sandals can be made from tire materials,
- Road construction: tires are mixed with asphalt cement forming asphalt rubber, asphalt rubber filler costs 40 % more than conventional material.
Retreating used tires is the most preferable way of making use of old tires. However, two aspects might be of negative impact on this process, which are:
(a) The acceptance of retreated materials by the customers: people tend to think that “retreads” don’t have the same quality compared to new tires. All investigations have shown that the retreads show similar qualities as the new tires.
(b) The availability of old tires with good quality: in countries where tires are valuable products compared to the average income, people want to drive a tire as long as possible, then tires are often driven until the threads or other internal layers become visible. These tires are no longer used in the re-manufacturing process.
Nevertheless, these measures – alternative applications and retreads – are not capable of dealing with the massive numbers of waste tires being produced. Thermal valorization is emerging as a possible solution for reprocessing huge amounts of this material. The three main applied technologies for thermochemical valorization are pyrolysis (degradation in the absence of oxygen), gasification (degradation in the presence of water vapor), and combustion (degradation in the presence of oxygen). To manage the increasing amount of waste tires, many studies have been carried out on these materials by converting them into a useful source of raw materials or useful sources of energy (incineration, distillation, gasification, and pyrolysis).
Pyrolysis of tires involves the thermal degradation of the tire rubber at a temperature of 300 – 900 °C in an inert atmosphere. The pyrolysis of waste tires has received increasing attention since the process conditions may be optimized to produce high-energy oil, gas, and residual char in addition to the steel casing of tires. Steel is recycled back into the iron or steel industry. The other three products are used as energy sources (fuel) or/and chemical resources. Pyrolytic products of waste tires have high calorific values which means the three products could be used as good fuel.
As tire combustion faces serious problems related to harmful emissions, pyrolysis appears as a process that allows the management of toxic compounds.
UNIQUE INNOVATIONS FOR SUSTAINABILITY
At tire manufacturer Continental, the topic of sustainability is currently high on the agenda. In the process, they call into question every tire component and replace it, if necessary, with more environmentally compatible materials. For instance, an innovative process has been introduced to return waste rubber to the production cycle, enabling rubber from end-of-life truck tires to be re-utilized during retreading.
The latest product to join the Continental line-up is a special tire for hybrid vehicles that features a 30 percent drop in rolling resistance compared to a standard tire. With this rubber fitted, hybrid models can reduce the distances covered with the help of their internal combustion engines and increase the stretches travelled in electric mode.
One aspect of Continental’s sustainability activities that has been more in the public eye is the “dandelion tire”. Here the company is cooperating with the Fraunhofer Institute for Molecular Biology and Applied Ecology, IME. The objective is to use natural latex obtained from the roots of the dandelion as a commercially viable substitute for natural latex from rainforest plantations. The dandelions can even be cultivated on land that is unsuitable for food crops so creating “plantations beside the tire plants” in Central Europe makes both economic and ecological sense. The short transportation distances mean a substantial drop in CO2 emissions; monocultures of rubber trees in rainforest regions can be reduced, and the tire manufacturer can gain a degree of immunity from the volatile prices on the global rubber market.