|It is estimated that that there are 2-3 billion scrap tyres stockpiled in the US, a situation that is mirrored in countries round the world. But as new US and EU legislation to ensure their environmentally safe disposal comes into force, the tyre recycling industry is experiencing tremendous growth. There are now a variety of technical solutions to the problem of recovering rubber recyclate from steel radial tyres, and industry is finding an increasing number of applications for its use.
Scrap vehicle tyres make a significant contribution to the generation of waste. For instance, the rate of scrap tyre generation in industrialized countries is approximately one passenger car tyre equivalent (PTE, or 9 kg) per capita per year. Furthermore, it is estimated that an additional 2-3 billion scrap tyres are stockpiled on unregulated or abandoned piles throughout the US, a figure which represents the cumulative scrap tyre generation of approximately 10 years. For EU Member States, it is reasonable to assume illegal or semi-legal scrap tyre accumulation in the same order of magnitude.
In response to the environmental problems and health hazards posed by the countless uncontrolled and abandoned scrap tyre piles around the world, most industrialized countries have put in place a legal framework to address this issue. Regulations vary from country to country, but the main thrust of such legislation is to require the removal of abandoned piles, provide for the environmentally safe disposal of newly generated waste tyres and also support new applications for tyre-derived material through the use of government grants.
Firemen tackle a tyre fire in Oranienburg, near Berlin, Germany, in April 2002.
Photo: Feuerwehr Velten, Germany
The most noticeable problem associated with large tyre piles is the fire hazard they present. Once a tyre pile catches fire, it is very hard, if not impossible, to extinguish. In some instances, tyre piles have been burning for several months with the black fumes being visible for many miles. Diseases such as encephalitis and dengue fever have also been reported around scrap tyre piles, particularly in areas with warmer climates which are an ideal breeding ground for disease-carrying mosquitoes.
It is fair to say that rubber recycling – in one form or another – is as old as the industrial use of rubber itself. In 1910, natural rubber cost nearly as much as silver, and it thus made perfect sense to reuse as much as possible of this valuable commodity. During this time, the average recycled content of all rubber products was over 50%.By 1960, the recycling content in rubber products dropped to around 20%. In the subsequent decades, cheap oil imports, the more widespread use of synthetic rubber and the development of steel-belted radial tyres have all contributed to a steady decline in rubber recycling.
As of 1990, the established tyre and rubber industry used only around 2% recycled material. However, in the past decade the tyre recycling industry has experienced a tremendous growth, thanks to a legal framework requiring the safe disposal of scrap tyres, the availability of reliable size reduction technologies and the emergence of innovative, economically viable applications for recycled rubber.
Scrap tyre disposal statistics
Statistical data on scrap tyre generation and disposal are published by a number of organizations, most prominently the US Rubber Manufacturers’ Association and the European Tyre Recycling Association (ETRA). Figures 1 and 2 give an overview of the current routes of disposal for scrap tyres in the US and Europe, while Table 1 presents the amounts directed through each source (though these figures should be regarded only as approximate values).
|‘In industrialized countries, the equivalent of one passenger car tyre is disposed of per person per year’
TABLE 1. Scrap tyre disposal for 2000-2001 in EU Member States and the US. Source: European Tyre Recycling Association (ETRA), US Rubber Manufacturers’ Association; compilation and unit conversion by Kurt Reschner
||Tyres disposed (tonnes)
||Tyres disposed (tonnes)
|Used tyre export
|Note: these figures do not include retreaded tyres
The different disposal methods are looked at below.
FIGURE 1. Scrap tyre disposal routes in the US, 2000-2001
FIGURE 2. Scrap tyre disposal routes in the Europe, 2000-2001
While uncontrolled tyre fires cause substantial air and ground pollution, the incineration of whole tyres or tyre chips in industrial furnaces is environmentally safe. The calorific value of tyre-derived fuel (TDF) exceeds that of coal, while the sulphur content is in the same order of magnitude or even lower.The use of TDF as a fuel supplement in cement kilns, paper mills or power plants is a perfectly reasonable use for scrap tyres, if the safe disposal of large amounts of scrap tyres is the primary objective. However, a closer look at the energy invested into the production of tyre rubber indicates that reusing the material for originally intended (or related) purposes is the preferred option, both environmentally and economically.
TABLE 2. Specific energy values of tyre-related materials
|Energy required to manufacture a tyre
|Energy required to produce tyre rubber compound
|Energy content of tyre-derived fuel (TDF)
|Energy consumed to produce crumb rubber from tyres
As shown in Table 2, the energy recovered from TDF is just a fraction of the energy invested into the production of tyre rubber. This correlation is clearly reflected in the market prices for TDF (US$30-50 per tonne) and crumb rubber from scrap tyres (US$180-300 per tonne).
|‘Reusing tyre rubber for its originally intended purpose is the preferred option, both environmentally and economically’
The traditional tyre and rubber manufacturing industry currently uses only 2-5% post-consumer recycled rubber. The low recycled content in conventional rubber products does not tell the whole story, however. Effective size reduction methods and innovative applications for crumb rubber led to a significant increase in the use of tyre-derived crumb rubber in the past decade. Some important applications are discussed below, in the section ‘Common applications for recycled rubber’.
Landfilling and stockpiling
Many landfill operators are hesitant to accept whole scrap tyres because tyres are awkward to handle and difficult to compact. It is not uncommon for tyres to work their way to the top of a landfill even years after closure, thus causing costly damages to the landfill cover.Most states in the US have enacted legislation that restricts or even bans the disposal of tyres in landfills. Similarly, the EU Landfill Directive restricts this route of disposal by 2006.
Civil engineering applications
Tyre-derived products, mostly 50-mm (2-inch) tyre chips, can be used to replace conventional construction material, such as road fill, gravel, crushed rock or sand. The benefits of using tyre chips instead of conventional construction materials are, amongst others, reduced density, improved drainage properties and good thermal insulation. The projects listed below are examples of the successful use of scrap tyre chips in civil engineering applications:
- lightweight fill for embankments and retaining walls
- leachate drainage material at municipal solid waste landfills
- alternative daily covers at municipal solid waste landfills
- insulating layers underneath road surfaces and behind retaining walls.
Civil engineering applications of scrap tyres are expected to become more widespread as more and more applications can be proven to be technically viable and economically advantageous.
Used tyre export
The reuse of a waste product for its originally intended purpose is considered to be one of the most environmentally sound options for waste management; the vibrant international trade in used tyres is a clear indication that this preference is also very profitable. It is a fair assumption that at least 10% (probably more than officially reported) of scrap tyres generated in industrialized countries are sold as used tyres, especially to Eastern Europe, Africa and Latin America.The downside of sending scrap tyres from industrialized countries to less-developed regions is that the receiving countries end up with a disproportionate number of scrap tyres. Since these countries do not usually have the legal framework or the infrastructure to provide for the environmentally safe disposal of scrap tyres, large uncontrolled dumps are likely to accumulate there.
Innovative new uses for scrap tyres in developing countries include this trash bin made from an old tyre, seen in Thailand
A number of EU Directives are expected to significantly impact on the way scrap tyres are disposed of in the coming decade. The three most important legislative changes are discussed briefly below.
Landfill Directive (1999/31/EC)
There is some argument among legal experts as to when this EU Directive will become effective; however, it is likely that the landfilling of tyres will no longer be possible in EU Member States by 2006. If fully implemented, the Landfill Directive will have a significant impact on the manner in which tyres are disposed of in Europe, and new routes of disposal will have to be found for at least 30% of newly generated scrap tyres. In some Southern European countries such as Greece, Portugal and Spain where most (if not all) scrap tyres are currently landfilled, this law will have a dramatic impact.
End-of-Life Vehicle (ELV) Directive (2000/53/EC)
This Directive was ratified by the EU Member States in 2002. Its objective is to reduce the amount of waste generated by vehicles, and to facilitate reuse and recycling of end-of-life vehicles. By 2006, at least 85% of an ELV by weight has to be reused or recovered, and by 2015 this percentage will increase to 95%.At present, metals are the only significant materials from ELVs being recycled. In order to achieve the mandated reuse rates, non-metal components will also have to be recycled or recovered. As waste tyres represent up to 5% of an ELV by weight, and are comparatively easy to dismount and recover, salvage yard operators will most likely increase their efforts to recycle tyres from ELVs.
Waste Incineration Directive (2000/76/EC)
The Waste Incineration Directive was adopted into national law in the EU Member States in 2002, with the aim of preventing or limiting emissions from incineration and co-incineration of waste. The Directive sets more stringent emission standards for a number of pollutants including dust, HCl, HF, NOx, dioxins and heavy metals. Since thermal recovery in cement kilns and power plants is one important route for disposal of scrap tyres, the Waste Incineration Directive may compel some current users of tyre-derived fuel to refurbish their emission control systems. Although this Directive is not thought to have a significant impact on the incineration or co-incineration of waste tyres, scrap tyre pyrolysis and gasification operations may not be able to meet the set criteria for minimum operating temperatures and total organic content (TOC) in the bottom ash.
Size reduction technology
Tyres are built to be tough and durable. The very properties that ensure a long service life and a safe ride make size reduction both difficult and costly. Since the steel-belted radial tyre has become commonplace since the 1970s, grinding scrap tyres into steel- and fibre-free crumb rubber requires fairly complex and expensive machinery.
The purpose of size reduction is twofold:
- to liberate steel and fibre from rubber
- to process the rubber fraction into a saleable particle size.
The typical product yield from scrap tyres is shown in Table 3.
TABLE 3. Typical product yield from scrap tyres
||Yield by tyre type
||Earth mover tyres
|Fibre and scrap
|Scrap tyres become rubber granules for asphalt rubber
|Currently, 3000 million tyres are produced worldwide each year, and demand is increasing. One problem that arises is how to deal with all the tyres that are mechanically damaged or have a worn profile.The largest and most modern scrap tyre recycling plant in Europe is based at Asamer Holding’s tyre recycling facilities in the upper Austrian town of Gmunden. Covering an operating area of 20,000 m2, up to 40,000 tonnes of scrap tyres can be processed each year in an economical and environmentally friendly manner, in a three-process operation. The complete development and construction of the innovative recycling technology, which started operation in winter 2002, has been undertaken by MeWa Recycling Maschinen und Anlagenbau, Germany.
Granulating line at the Asamer plant. Photo: MeWa
In the first process, truck, car and tractor tyres are pre-shredded into strip-like pieces, to a size of 100 mm x 150 mm. A conveyor feeds two large bunkers with a total volume of over 2000 m3, where the pre-shredded truck tyres and car tyres are stored, temporarily separated from each other.
The second process occurs in several granulating lines. The end product of this is a largely textile- and steel-free rubber granulate, less than 3 mm in size. High-value products are made from this granulate for a range of different manufacturers. These products are used in the following areas, amongst others:
- sports field building, such as track surface construction
- traffic calming and noise control products (e.g. noise barrier)
- floor surfacing for schools and kindergartens
- industrial construction
The steady advancement of the range of applications for rubber granulate secures a future market for the scrap tyre resource.
|One key market will be the use of tyre granulate in the upper asphalt layer of roads, the so-called deterioration layer, as aggregate. Experience with this application in the US over the last 10 years shows that a doubling of the lifetime of the deterioration layer can be achieved at only 20% aggregate fraction. At the same time, the use of aggregate has an extremely positive effect on noise level, aquaplaning behaviour regarding water run-off and vehicle braking and deceleration. In co-operation with the local road construction authority, Asamer has already installed appropriate proving grounds for testing aggregate in Austria.
Tyre granulate output from the Asamer plant. Particles are 1-2 mm in size. Photo: MeWa
|The plant also contains a further production area, where a third process is due to be carried out, with rubber granulate used in the production of a high-value rubber powder. At a temperature of -120°C, the granulate will become glass-hard, and can then be ground to a fineness of 50-250 µm (0.05-0.25 mm) in special mills. The technology used will guarantee a high purity for the produced rubber powder. There is a demand for this powder from a variety of industrial and chemical processes, for the production of compounds and anti-corrosives. There is also a steady development of new product applications for this rubber powder.
The first operation runs of the pre-shredding and granulating processes have already successfully started. The fine-milling steps were put into operation in the first quarter of 2003.
Whenever scrap tyres are disposed of in a controlled manner, they are, by and large, first shredded into 50-mm (2-inch) tyre chips. Tyre shredding is a mature technology, with reliable machines being offered by a number of reputable companies throughout North America and Western Europe. The most common machine used for tyre shredding is a rotary shear shredder with two counter-rotating shafts that operate at low speed (20-40 rpm) and high torque.
Inside the cutting chamber of a scrap-tyre shredder
|‘The very properties of tyres that ensure a long service life and a safe ride make size reduction both difficult
Some operators remove the steel beads from truck tyres prior to shredding. Debeading significantly reduces wear and tear in the shredder and in consecutive size reduction machines such as granulators or cracker mills. While steel beads represent only 10-15% of a truck tyre by weight, it is probably fair to state that the 25-mm (1-inch) thick steel beads cause as much as 70% of the wear and tear in the shredder and the consecutive grinding machines.
Ambient grinding technology
The schematic in Figure 3 shows a typical ambient scrap tyre recycling plant. The process is referred to as ‘ambient’ because all size reduction steps take place at or near ambient temperatures, i.e. no cooling is applied to embrittle the rubber particles.In the plant layout shown in Figure 3, the tyres are first processed using a preliminary shredder (A). The tyre chips then enter a granulator (B), where the chips are reduced to a size of less than 10 mm (0.38 inches), while liberating most of the steel and fibre from the rubber granules. Upon leaving the granulator, steel is removed magnetically, and the fibre fraction is removed by a combination of shaking screens and wind sifters (C, F).
FIGURE 3. Schematic of an ambient scrap tyre recycling system.
Source: CIMP, France
While there is some demand for 10 mm rubber granules, most applications call for finer mesh material, typically in the range of 0.6-4.0 mm (5-30 mesh).For this reason, most ambient grinding plants operate a number of consecutive grinding steps (D).The machines most commonly used for fine grinding in ambient plants are:
- secondary granulators
- high-speed rotary mills
- extruders or screw presses
- cracker mills
Ambient grinding is the preferred technology if relatively coarse crumb rubber material – i.e. larger than 0.6 mm (30 mesh) – is being produced.
Cryogenic grinding technology
This processes is referred to as ‘cryogenic’ because whole tyres or tyre chips are cooled to a temperature of below -80°C (-112°F). Below this ‘glass transition temperature’, rubber becomes nearly as brittle as glass, and size reduction can be accomplished by crushing and breaking. Cryogenic size reduction of rubber requires less energy and fewer pieces of machinery than ambient size reduction. Another advantage of the cryogenic process is that steel and fibre liberation is much easier, leading to a cleaner end product. The drawback, of course, is the additional operating expense for liquid nitrogen (LN2).
FIGURE 4. Schematic of a cryogenic scrap tyre recycling system.
Source: Recovery Technologies, Inc., Canada
The cryogenic process shown in Figure 4 begins with preliminary shredding, which is largely the same as in ambient plants. The tyre chips are then cooled in a continuously operating freezing tunnel (B) to below -120°C, and then dropped into a high-rpm hammer mill (C). In the hammer mill, chips are shattered into a wide range of particle sizes. Because the rubber granules may be damp upon leaving the hammer mill, the material is dried (E) before classification into different, well defined particle sizes (F). A secondary cryogenic grinding step (G) is required to produce fine rubber powder.
|‘Aside from savings in material costs, adding recycled rubber to the virgin rubber compound offers
Common applications for recycled rubber
Crumb rubber as a filler in virgin rubber compounds
Many tyre manufacturers add recycled material into their compounds. Aside from the savings in material costs, adding crumb rubber to the virgin rubber compound offers the following processing advantages:
- better mixing properties and improved stability form for uncured parts
- improved degassing during the vulcanization process
- improved mould release
- reduced cure times
Since crumb rubber from scrap tyres consists of a random mix of compounds (depending on manufacturer, type of tyre, etc.) there is an upper limit as to how much recycled material can reasonably be used in a new tyre without compromising quality, safety and performance characteristics. This limit is commonly thought to be at around 5% for passenger car tyres, but significantly higher for less safety-critical products.
Athletic surfaces and playground covers
When combined with a moisture-curing urethane binder, crumb rubber can be poured into place using conventional paving equipment to form an elastic and resilient surface cover. This technique is widely used for running tracks, tennis courts, and similar athletic surfaces and playground covers. This application has been very successful in the past decade, and is expected to continue to grow in coming years.
As the supply of crumb rubber has become more reliable, a number of recycled products made by simple compression moulding have been developed and successfully introduced onto the market. Most of these products are high-volume, low-tech products, such as rubber paving blocks, livestock mats, railroad crossings, removable speed bumps and gymnasium mats.
Devulcanization is sometimes compared to turning breadcrumbs into fresh dough. In chemical terms, devulcanization means returning rubber from its thermoset, elastic state back into a plastic, mouldable state, and is accomplished by selectively severing the sulphur bonds in the molecular structure. This processing step enables rubber manufacturers to use a much lager percentage of recycled material without compromising quality, appearance or performance characteristics.Table 4 briefly explains important devulcanization methods, including newly developed technologies.
TABLE 4. Important devulcanization methods
|Thermal reclaim process
||Rubber is exposed to elevated temperatures over an extended period of time in order to break the sulphur bonds as well as the polymer ‘backbone’. This process was first patented by H.L. Hall in 1858, but is not widely used today due to environmental concerns and relatively severe degradation of the material. There are some commercial applications in Asia and Eastern Europe.
||Vulcanized rubber is exposed to intense mechanical work (mastication) in order to selectively break the sulphur bonds in the polymer matrix. Mechanical devulcanization does not alter the chemical composition is any way and yields material with excellent physical properties. The first commercial applications are in Western Europe.
|Devulcanization with ultrasound
||Technically speaking, this is a special form of mechanical devulcanization. The results of the first research, conducted at the University of Akron, are fairly encouraging.
||Fine rubber powder is exposed to an aqueous suspension with bacteria such as thiobacillus, rhodococcus and sulfolobus that consume sulphur and sulphur compounds. This process is technically viable, but economically questionable due to the complexity of the process.
Commercial applications of rubber-modified asphalt (RMA) road surface date back to the 1960s, and were first introduced by Charles McDonald in Arizona. Apart from recycled tyre rubber, virgin SBS (styrene-butadiene-styrene) block co-polymers are often added to the asphalt mix in order to improve the performance characteristics.The main advantages of RMA can be summarized as follows:
- thermal cracking (caused by frost) and rutting (softening of the road surface on hot summer days) can be reduced with one and the same asphalt mix; RMA is particularly useful in areas with extreme climates, i.e. high temperatures in summer and severe frost in winter
- severely cracked road surfaces can be resurfaced with RMA or with a stress-absorbing membrane interlayer (SAMI), because the elastic properties of a SAMI significantly reduces reflective cracking
- due to lower maintenance costs and increased durability, the life-cycle cost of RMA is significantly lower when compared to conventional asphalt road surfaces
- traffic safety is increased due to better de-icing properties, as is skid resistance, and fewer construction sites are required
Two stretches of Interstate 40 near Flagstaff, Arizona, US. Both surfaces were laid in 1990 and the pictures above were taken eight years later, in 1998. While the conventional surface (left) is already severely cracked, the RMA surface (right) is in much better shape. Photos: George Way, Arizona Department of Transportation
The legislative changes in the US and the EU will drastically change the manner in which tyres are disposed of in the coming decade. Most prominently, the landfill ban will compel the industry to find new disposal routes for 20-30% of the annual scrap tyre generation. A large portion of the tyres currently landfilled are likely to be used for energy recovery, since cement kilns, paper mills and power plants have the capacity to utilize large amounts of TDF.
|‘The ban on landfilling of tyres will compel the industry to find new disposal routes for 20-30% of the annual scrap tyre generation’
In the longer term, applications for recycled rubber will also gain significance.
Two applications deserve special mention. The first of these is the asphalt market; given the enormous size of this market and the proven technical and economic benefits of RMA, this application has excellent potential for growth. In fact, in the US, this market segment grew from 43,000 tonnes per year in 1995 to 131,000 tonnes per year in 2001. In Europe, the use of crumb rubber in RMA applications is still in a state of infancy, but similar growth rates appear very likely.
The second technology with good growth potential is mechanical devulcanization. The ability to devulcanize rubber without damaging the polymer ‘backbone’ now makes it possible to truly close the loop in the rubber industry. Based on the excellent savings potential for rubber manufacture, this technology is likely to become widely accepted in future, especially for the processing of higher-value rubber compounds and factory scrap.
Bibliography and sources
- Morton, Maurice. Rubber Technology. Van Nostrand Reinhold. 1987.
- Snyder, Robert H. Scrap Tyres – Disposal and Reuse. Society of Automotive Engineers. 1998.
- Brown, C. J., Brown, D. A., Hodgkinson, N. M., Watson, W. F.’ ‘The waste problem – Cured’. In Tyre Technology International. June 2001.
- Way, George B. Flagstaff I-40 Asphalt Rubber Overlay Project: Nine Years of Success. Paper presented to the Transportation Research Board. August 1999.
- Fifth Annual Report of the Used Tyre Working Group. July 2001.
- Scrap Tyre and Rubber Users Directory 2002. Recycling Research Institute.
- US Scrap Tyre Markets 2001. Rubber Manufacturers’ Association. December 2002.
|KURT RESCHNER is an independent consulting engineer active in the field of scrap tyre disposal and rubber recycling.
Fax: +49 30 306 2218
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