Turf infills

The technology behind the small rubber granules used in synthetic grass systems

The use of tiny fractions of recycled car tyres in combination with a longer fibre length made the introduction of synthetic grass for football possible. The infill material contributes to the interaction between the player or ball, and the field, as well as the look and safety of the field.

The infill material contributes to the safety and playing
characteristics of the field

Their use even gave the name to the product, as today people in the industry refer to “rubber infilled synthetic grass systems”. Thousands of these fields have been installed globally but concerns over pollution by the rubber granulate kept pace with its increasing acceptability. In The Netherlands, where the level of acceptance of rubber infill synthetic grass systems is perhaps the highest in the world, test institutes, manufacturers and installers jointly conducted a study.

The wide variety of infill material available today is almost as large and diverse as the variety of different fibres. Initially particles of crushed car tyres (SBR) were used but over the years several other types of infill were introduced. Besides the variety in which crushed car tyres are now available (coated, uncoated, cryogen crushed or ambient crushed), the industry now even uses recycled crushed shoe soles. And then you have EPDM infill material as well as TPE infill material, both developed specifically for their use in synthetic sports fields.

The introduction of these materials became possible after questions were raised about the environmental friendliness of SBR rubber. Studies had shown that zinc and polycyclic aromatic hydrocarbons (PAK), which are used in the production process of car tyres, tend to leak from the material, thereby forming a potential hazard for player safety and the environment.

The Netherlands is not the first country where questions have been raised. The Italian FA decided earlier this year, for the time being, to no longer allow black SBR to be used in synthetic turf football fields. This decision also provided a solution for a problem that, perhaps, is a bigger issue in a Mediterranean country; the temperature on the field. Due to its black colour SBR infill material had a negative influence on the temperature of the field during the summer season.

Every third generation synthetic grass field has a
layer of infill which is up to 2-3cm thick

Furthermore Scandinavian countries have prohibited the use of this material in indoor sports facilities for years now, to prevent the formation of smoke during a fire. In addition, if SBR is used in an indoor facility the wind is unable to blow away toxic fumes generated by the materials from which SBR is made.

The purpose of the study was to generate unbiased information about the environmental and health issues concerning rubber granulate made from crushed car tyres. In the absence of proper standards, criteria from similar industries were used to test the material. To test the health friendliness of SBR rubber, the criteria of the German TÜV directive for toys was used. All samples tested were well within the limits of this directive. Finding criteria to test the leaking of heavy metals however, turned out to be a bigger challenge.

An average synthetic turf field only has 2-3cm rubber infill but regulations used in Holland to test whatever material is used in the sub-base (such as for houses or synthetic grass systems), required a layer of 20cm. The outcome of the test showed that when a layer of 20cm was tested, the leaking of zinc did exceed the allowed quantity. However with an average layer of only 2-3cm, which is normally used in synthetic grass fields, leakage was within the permitted standard.

The Dutch study only applied to the situation in The Netherlands. “In other countries there is no discussion like we had in The Netherlands about SBR infill rubber,” says Erik van Swinderen from test institute ISA-Sport. But with the acceptance of synthetic grass increasing further, the study might be of use to set further quality criteria, at least for the time being. “The international car tyre industry already has a directive that by 2009 heavy metals may no longer be used for car tyres. And by 2010 the use of PAKs is also prohibited, something which in western Europe is already commonplace. Winter tyres particularly hardly use PAKs anymore,” says Bert Kösters from granulate manufacturer Granuflex. “However, it might take a while before tyres with heavy metals and PAKs will have been completely replaced globally.”

The study has answered many questions that were raised following a newspaper publication in The Netherlands. But it also raises new questions. Not much is known yet about the consequences of skin contact lasting for over 30 seconds. And with SBR granulate only used since 2000 in The Netherlands, it is also unclear what the behaviour of the material will be over a period of years. Despite the conflict of interest, the partners who conducted the study have decided to do further research on these issues. The outcome is expected over the course of this summer.