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Mapping, emissions and environmental and health assessment of chemical substances in artificial turf

2 Artificial turf pitches – Materials and chemical substances

2.1 Possible constituents

For artificial turf pitches, plastic fibres made of polyethylene (PE), polypropylene (PP) and polyamide (PA) are used (The Grass Yarn & Tufters Forum, 2006). These are attached to a perforated mesh of PP or polyester. This is coated with a latex-based adhesive which is subsequently cured. Rubber granules or thermoplastic polymers are used for the elastic infill, but there is also a product on the market which is based on natural fibres. Sand is used to weigh down the grass. The sand could be coated with an elastomer or PE. Additionally, an elastic sublayer is usually included under the artificial turf. This could typically be a coarse rubber fraction from tyres glued together using polyurethane, but other types are also available in the market.

Below follows a brief description of the substances which can be expected to leach from artificial turf pitches on the basis of the chemical structure of the plastic or rubber type used as well as the additive chemicals used to stabilise the molecular chains against degradation due to weather.

For rubber, degradation products may be present as a result of vulcanisation. The volume and the chemical structure are often unpredictable, because they depend on the vulcanisation system used, and the time and temperature.

With wear and tear, rubber dust may be produced, which can be a source of exposure via skin or respiratory passages.

2.1.1 Chemical substances in the artificial turf mats

The existing literature (The Grass Yarn & Tufters Forum, 2006) states that the artificial turf fibres are primarily produced from polyethylene (PE) or polypropylene (PP), but that nylon (polyamide) is also an option. The same source indicates that PE is used exclusively for football pitches.

Antioxidants are usually added to PE and PP in order to improve weather resistance, which is otherwise already good due to the saturated carbon chain structure. The antioxidants are typically organic phenolic structures with a relatively high molecular weight in order to prevent evaporation. Organic phosphites are usually added as auxiliaries, as this provides a synergistic protective effect against oxidative degradation.

Furthermore, UV stabilisers are added to protect against light degradation. Typical UV stabilisers are of the HALS type (Hindered Amine Light Stabilisers). Some UV stabilisers are contain zinc, e.g. Tinuvin 494 from Ciba Speciality Chemicals.

The fibres are coloured green. Some of the green colourants can be based on metallic complexes (copper), or they can be of the azo colourant type, of which some, e.g. yellow, are known to be potentially carcinogenic. The green colour is produced by mixing yellow and blue colours.

Previous studies carried out by the Danish Technological Institute show that volatile short-chain hydrocarbons often degas from PE as well as PP. It was thus expected beforehand that the artificial turf mats would degas volatile, short-chain hydrocarbons in a similar manner. It was expected that there might also be softeners in the latex adhesive used, e.g. in the form of phthalates.

2.1.2 Chemical substances in the elastic infill

According to the existing literature (The Grass Yarn & Tufters Forum, 2006), the overwhelming majority of the artificial turf pitches laid are based on recycled, granulated tyres (98%).

Information exists (T.V. Pedersen, 2007) that one of the disadvantages of using the granules is the black colour, and that, especially in wet weather, the rubber easily sticks to clothes, footwear and skin.

Another disadvantage is the smell of rubber, which is most predominant in hot weather, however. The smell can be suppressed by watering.

There are alternative options when choosing materials. You can either use coated granules from tyres in order to reduce the migration of substances from the granules, or you can use EPDM rubber granules and thermoplastic elastomers (TPE) or mixed granules. Coated sand is also mentioned as an option. Infill material based on natural fibres also exists. Finally, an infill material is marketed under the designation industrial rubber. According to the supplier, this is surplus rubber from the production of window piping.

The problem with several of the alternatives to granulated car tyres is that the price level of the infill materials is increasing.

According to (The Grass Yarn & Tufters Forum, 2006), coated tyre granules are four times as expensive as the base raw material.

According to the same reference, if you switch to EPDM or TPE, the price increases further to eight to 14 times, respectively, compared to natural rubber granules (The Grass Yarn & Tufters Forum, 2006). Since the price of an artificial turf football pitch using tyre granules is approx. DKK 5 million, substituting the tyre granules results in a considerably increased installation price. This is assuming that the figures are correct, which is questionable on the basis of information from suppliers to the Danish market.

Information is thus provided that some of the alternative infill materials are only two-to-four times more expensive than pure tyre granules. From one supplier to the Danish market, the following approximate prices were stated for 120 tonnes of infill in the types of infill used for the pitches: Granulated car tyres: DKK 180,000, coated granules from car tyres: DKK 500,000, EPDM, TPE and coir: DKK 850,000. If the density of EPDM compared to granulated car tyres is taken into consideration, the price of EPDM increases to DKK 1,100,000.

Information was provided, however (T.V. Pedersen, 2007), that by using fourth-generation structures with foamed cross-linked polyethylene (PEX) pads it is possible to achieve competitive prices for alternative infill materials made from EPDM rubber.

2.1.2.1 Granulated tyres

In connection with the “Emission and health evaluation of PAHs and aromatic amines in car tyres” project (Danish Environmental Protection Agency, Kortlægning nr. 54, 2005), detailed descriptions were given of the raw rubber types forming part of the recipes for passenger cars tyres as well as for truck tyres.

The raw rubber types are natural rubber (NR), styrene butadiene rubber (SBR) and butadiene rubber (BR). SBR rubber is the main component in tyre treads.

In addition to carbon black (soot), aromatic oils, zinc oxide, stearic acid, antioxidants and antiozonants as well as sulphur and accelerators form part of the recipe. The accelerators contain nitrogen and sulphur and, when heated, can emit carbon disulphide and split off amines, of which several can be nitrosamine-forming. In addition to zinc, rubber granules made from discarded car tyres can emit copper and chromium from the steel cord used to reinforce the tyres. The accelerators used are typically based on benzothiazole, which can be split off during vulcanisation. The antiozonants are predominantly 6-PPD [N(1,3-di-methyl-butyl)N´phenyl-p-phenylene diamine], but other p-phenylene diamine-based antiozonants, e.g. IPPD (N´-isopropyl-N´phenyl-p-phenylene diamine) are also used. The Norwegian studies (C. Dye et al., 2005; C. Dye et al., 2006; T.S.W. Plesser, 2004; T. Sanner, 2006) also observed emission of phthalates as well as long-chain alkylphenols from the rubber granules. The phthalates could originate from adhesives and the alkylphenols from reactive resins used in the vulcanisation of butyl rubber types (J.S. Dick, 2001). Butyl rubber forms part of tyres as an airtight layer.

2.1.2.2 EPDM rubber

EPDM rubber consists of carbon chains constructed from the monomers ethylene and propylene and a diene component, typically norbornene, built in as a side chain. The advantage of this polymeric structure is great weather resistance, making it unnecessary to add antiozonants to EPDM rubber. The volume of antioxidants can also be reduced. EPDM can be subjected to either peroxide or sulphur vulcanisation. In both cases, zinc oxide may be a constituent, but in greater volume in the sulphur vulcanisation. In sulphur vulcanisation, the usual accelerators based on nitrogen and sulphur are used. For the peroxide vulcanised types, organic peroxides are used, typically dicumyl peroxide which splits off acetophenone during vulcanisation. Other types may split off tert-butyl alcohol. The softeners used for EPDM are predominantly naphthenic oils with a relatively low aromatic content. Triallyl cyanurate is used in peroxide vulcanised EPDM rubber as a cross-linking regulator.

2.1.2.3 Industrial rubber

Municipalities and football clubs that do not want tyre granules specify instead that they want infill in the form of industrial rubber.

In principle, industrial rubber is all rubber, as all technical rubber products are produced industrially. One supplier calls industrial rubber a surplus product from the rubber industry, originating from window piping production.

Based on the supplier’s information alone, it is not possible to assess the chemical composition of the granules.

2.1.2.4 TPE

TPE is an abbreviation of thermoplastic elastomers. SEBS is a typical thermoplastic elastomer based on a Styrene Ethylene Butadiene Styrene structure. TPE is distinct from rubber in that it is not vulcanised. Instead, a mesh structure is formed by the styrene segments forming crystalline domains. Since the chain structure is saturated, SEBS is characterised by good weather resistance, as was described for EPDM rubber. The emission of chemical substances from SEBS is predicted to be limited, because no vulcanisation chemicals are used as is the case for rubber.

2.1.2.5 Natural fibre-based infill

A single supplier offers a natural fibre-based infill material. This is based on coir fibres.

 

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Version 1.0 October 2008, © Danish Environmental Protection Agency