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Key departmental publications, e.g. annual reports, budget papers and program guidelines are available in our online archive.

Much of the material listed on these archived web pages has been superseded, or served a particular purpose at a particular time. It may contain references to activities or policies that have no current application. Many archived documents may link to web pages that have moved or no longer exist, or may refer to other documents that are no longer available.

A National Approach to Waste Tyres

Commonwealth Department of Environment, 2001

6 Environmental Aspects of the Tyre and Rubber Industry

The debate on the environmental impact of tyres and other rubber products is generally dominated by the risks and impacts associated with above ground tyre stockpiles. These stockpiles are often visually prominent and the potential impacts from fires and the creation of breeding sites for mosquitoes and other vermin are well documented. However, the environmental impacts of rubber products extend well beyond these and appear through all of the stages in the life of the product. It is important to consider all of these impacts to ensure that waste management approaches do not simply result in the transfer of impacts to a different stage in the life cycle, or to a different environmental medium, and result in greater overall impacts.

For the purpose of this discussion the life cycle of rubber products is as illustrated in Figure 6.1. This illustration has been prepared for tyres but is also broadly applicable to other rubber products.

life cycle of a tyre

Figure 6.1 The life cycle of a tyre

When describing the environmental impacts of tyres and other rubber products it is generally only practical to discuss the impact in terms of potential. This is because local factors are critical to the realisation and significance of the potential. For example, tyres contain a range of toxic materials, but these materials are tightly bound in a stable matrix (vulcanised rubber). The rate at which these toxic substances are released to the environment is in most circumstances very low. The rate depends on local environmental factors and the magnitude of any impact is dependent on the sensitivity of the receiving environment and the presence of plants, animals or humans that may be affected. All of these factors vary from location to location, and therefore the impacts also vary.

The potential impacts at each stage of the life cycle are summarised in Table 6.1 and discussed in more detail in the following sections.

Table 6.1 Summary of the potential impacts at each stage of the life cycle of a tyre or other rubber product

Life stage Processes included Impacts
Raw material Natural rubber production
Synthetic rubber production
Steel and fabric production
Production of various other additives incorporated in tyres
Transportation
Resource depletion
Agriculture (for natural rubber)
Energy use
Greenhouse and other emissions
Solid and liquid wastes
Manufacture Production of the basic components (sheet extrusions, etc) from which the products are made
‘Building’ of the tyre or other rubber product
Vulcanising and finishing
Energy use
Greenhouse and other emissions
Solid and liquid wastes
Use Use of the product for its design application Tyres have a significant impact on the operation energy of vehicles resulting in energy use and emissions, dust from wear and tear
Recycling/Reuse Shredding
Crumbing
Energy/material recovery
Whole split or punched tyres
Energy use
Greenhouse and other emissions
Solid and liquid wastes
Disposal Disposal to land
Uncontrolled stockpiling
Disposal in landfill
Leachate to receiving environment
Fires
Free flow of landfill gas and leached compounds
Mosquitoes and other vermin
Visual impact
Erosion
Transportation Transport of raw materials
Transport of new tyres
Transport of used tyres to disposal or retreading
Transport of waste tyres
Energy use, greenhouse and other emissions
Noise

6.1 Raw materials

Tyres and other rubber products, including materials used for retreading, are made from a wide range of materials in various combinations. The range of materials, their source, applications and potential impacts is listed in Table 6.2.

The major raw material for a tyre is rubber. Rubber is produced either from natural sources or from petroleum and natural gas. Generally products are made from ‘compounds’ which are a blend of natural and various synthetic rubbers, fillers and other chemicals that impart desired characteristics such as wear resistance or resistance to oxidation. There are literally thousands of different compound recipes customised to meet the requirements of different products and manufacturing processes. Some authors have described rubber compounding as much an art as it is a science.

The two major rubber types, natural and synthetic rubber, are discussed below.

6.1.1 Natural rubber

The chemical form of natural rubber is a polymer of isoprene (2-methyl-1,3-butadiene). Natural rubber is predominantly sourced from the sap of the Hevea brasiliensis tree though, there are a number of other trees that also produce rubber such as the Ficus elastica, which is a native of the Congo, and Guayule, a desert scrub from Mexico and Arizona5. Hevea is a native of the Amazon basin and until about 1910 the majority of natural rubber was derived from trees growing wild in this region. Since then, plantations have been established around the world in suitable, generally tropical, climates.

Rubber trees start yielding at about 5 years and reach peak production at around 15 years after which production gradually decreases until, at about 30 years, the tree is replaced. Yields from plantations are generally above 2,000 kg/ha per year. The wood from Hevea has a variety of uses such as in furniture, for construction, and charcoal.

The bulk of natural rubber production is by small landholders with plot sizes of only a few hectares. Large plantations or estates have declined in importance as a result of a range of historical, technological and economic factors. Both the first and second world wars were a stimulus for the development of synthetic rubbers to supplement supplies of natural rubber, which were disrupted by military action. Wild price variations during and after the war, and also concerns in Europe and America about the dependence on rubber supplies from foreign sources, provided further impetus for the development of alternatives. Low prices and difficulties in competing in price, quality and consistency keep natural rubber prices low, making the viability of large plantations marginal6.

Though natural rubber plantations are a cause of habitat loss, plantations provide environmental benefits particularly in relation to their ability to lock up carbon dioxide (CO2), the major greenhouse gas. Nevertheless, low prices for natural rubber place considerable financial stress on rubber producers, which can result in poor environmental practices being followed. Notwithstanding concerns about habitat loss, according to IRRDB7, rubber plantations compare well in terms of biomass, fertility and fauna and flora, though the values used in the analysis for biomass in native jungle seem very low.

6.1.2 Synthetic rubber

There are a number of types of synthetic rubber with various physical and chemical characteristics. Among the most widely used are styrene-butadiene rubbers, ethylene-propylene rubbers, butyl rubbers, acrylic elastomer, and silicone rubbers8. Like natural rubber all are polymers, synthetic rubbers are sourced from various hydrocarbons, which are blended and reacted under controlled conditions to form the polymers.

More than half of the world's synthetic rubber is styrene-butadiene rubber (SBR) made from styrene and butadiene monomers which are abundant in petroleum. Three quarters of all the SBR made goes into tyres. The rest goes into products such as footwear, sponge and foamed products, waterproofed materials, and adhesives.

Ethylene-propylene rubbers are used in rubber membranes for roofing, agriculture, and water distribution. With modification, they can be used in radiator and heater hoses, brake components, pond and ditch liners, agriculture silos, tank linings, wire and cable, gaskets, and washers.

Butyl rubbers are used in tyre inner tubes and other products that require a good barrier against gases. The thermal stability of these rubbers makes them suitable for use in automotive radiator hoses. Their ozone resistance makes them appropriate for electrical insulation and for weather resistance. Their ability to absorb shock is earning them wide application in automotive suspension bumpers. These rubbers also have a few disadvantages: they are incompatible with many natural and synthetic rubbers, they tend to pick up foreign matter and impurities, and they lose elasticity at low temperatures.

Acrylic elastomers are used in applications such as spark plug boots, ignition wire jacketing, and hoses where oil resistance is crucial. They are not suited for normal tyre use, however, because they have little resistance to abrasion at low temperatures.

Silicone rubbers perform exceptionally well in O-ring and sealing applications. Many types of wire and cable are insulated with these rubbers, which will burn to an ash yet still function as an insulator. Their resistance to moisture makes them good for outdoor applications. Because they are odourless, tasteless, and non-toxic, they are used for gas masks, food and medical-grade tubing, and some surgical implants. Their use is limited by the high cost of manufacture.

Table 6.2 The potential environmental impacts of materials used in tyres and other rubber products

Material Source Application Potential impacts
Natural rubber Natural rubber is predominantly obtained from the sap of the Hevea brasiliensi tree. The proportion of natural rubber to total rubber has been declining steadily over the past several decades and currently makes up about 30% to 40% of the total rubber used. Loss of habitat in tropical forests - there are approximately 9.5 million ha of rubber plantation.
Impacts of agricultural practices on local environments.
Impacts from transportation to markets.
Impacts from processing including odour.
Synthetic rubber All of the synthetic rubbers are made from petrochemicals. This makes up approximately 60 to 70% of the total rubber used. Resource depletion of petroleum.
Energy consumption, emissions and waste during manufacture.
Steel cord and beading including the coating materials and activators,copper/tin/zinc/ chromium The steel is premium grade and is only manufactured in a limited number of plants around the world due to the high quality requirements. Steel is used to provide rigidity and strength to the tyres.
In a passenger tyre steel cord makes up about 15% by weight.
Impacts during production and transportation.
Leaching of metals during disposal.
Issues with difficulty in recycling.
Other reinforcing fabrics Predominantly sourced from petrochemicals. Used for structural strength and rigidity. Makes up about 5% of a radial tyre. Impacts during production and transport.
Carbon black Generally sourced from petroleum stock. Imparts durability and wear resistance and resistance to degradation.
Makes up about 28% of a passenger tyre. The % is higher in the rubber that make up the wearing surfaces.
Impacts during production and transport.
Zinc oxide
Zinc is added to provide resistance to UV degradation, control vulcanisation and enhance blending.
Zinc oxide makes up about 1.2% of a passenger tyre.
Impacts during manufacture and disposal.
Impacts due to leach/emission from waste tyres.
Sulphur (including compounds) Sulphur is used to vulcanise the rubber. Makes up about 1% of a passenger tyre. Impacts during production.
Impacts during combustion for energy recovery.
Other additives and solvents9:age resistors, processing aids, accelerators, vulcanising agents, softeners and fillers The other additives are used in the various rubber compounds to modify handling manufacturing and end-product properties. The additives make up about 8% by weight of a passenger tyre. Impact associated with manufacture and transportation.
Emissions during manufacture.
Impacts associated with use and disposal of the solvents.
Emissions from tyres in use, during recycling and in final disposal.
Recycled rubber Recovered from used tyres or other rubber products. Used in some rubber compounds in the manufacture of ‘new’ rubber products and retread materials. Impacts from energy use in production.

 


5Others include Castilla spp. and Manihot spp. in Tropical America, Funtumia elastica and Landolphia spp, in Africa, Ficus elastica in Asia. There are also some rubber-bearing species of Compositae, Taraxacum (USSR), Solidago spp. (USA). In all there are over 200 species of rubber bearing plants.

6International Rubber Research and Development Board, web address www.irrdb.org

7ibid

8 Comptons Encyclopaedia, web address http://www.comptons.com/encyclopedia/ARTICLES/0150/01578240_A.html)

9 The list of additives is extensive and includes xylene, benzene, petroleum naphtha, chlorinated solvents (for example 1,1,1-trichloroethane), polycyclic aromatic hydrocarbons, anthracene, phenanthrene, benzo(a)pyrene, phenols, amines, oil, acids and alkalis (eg, sodium hydroxide), polychlorinated biphenyls, halogenated cyanoalkanes, processing aids, and plasticisers. Extenders: oils (carbon chain length approximately C20), activated ester, anti-oxidants based on amines (for example dimethylamine), para-phenylenediamine based on substituted phenols and dibenzyl disulphide. Accelerators: MBS (2-morpholino-thio-benzothiazole-sulphenamide), N-oxydiethylene-benzothiazole-sulphenamide, ziram,1,3-diphenylguanidine,N,N’-diphenylthiourea,disulfiram,2,2’-dithiobis (benzothiazole), TBBS (N-tertiary-butyl-benzothiazole-sulphenamide), stearic acid, magnesium oxide (used in neoprene). Stain protectors: 2,5 di-tert-pentylhydroquinone, 4,4’-dithiodimorpholine. Retarders: organic acids (for example salicylic acid) used in the past N-cyclohexyl-thiophthalimide (CTP). Waxes: paraffin waxes (carbon chain length C18-50). Tackifying resins. Hardeners: phenol derivatives, polymerised products, carbon chain length >C500. Desiccants: calcium oxide. Peptisers: 2,2’-dibenzamidediphenyl disulphide, zinc, dibenzamidediphenyl disulphide, dicyclohexyl carbodiimide. Colouring pigments: iron oxide, antimony pentasulphide. Flame retardants: zinc, metaborate dihydrate, sodium borate.

(Source: UK Environment Agency (undated), Tyres in the environment)