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Granulate filler – Impulse generator on an artificial turf pitch
Playing fields with artificial turf continue to gain popularity. Decisive for the quality of an artificial turf playing surface are not only the yarns used, treatment of the surface and correct installation, but also the loose components such as sand and granulate fillers, which contribute a significant part towards the functional properties of a filled artificial turf sports system. Many different kinds of granules, varying in material properties and price, can be used as filler for an artificial turf pitch. To be successful in finding the best suited granules, these differences should be observed and compared to each other.
While sand as the lowest substrate level primarily provides a solid basis for the turf fibres, the granule filler material plays a greater role in controlling playing properties and is responsible for wear resistance. The quartz sand used is gained through surface mining of natural deposits and should have suitable properties for use in a sports application. As filler material for an artificial turf pitch the sand needs to pure and the grains should not have any sharp edges which could damage the polymer fibres of the turf. In general, the sand is not supplied by the artificial turf manufacturer, but can be obtained by the installer from regional traders.
The granule filler material on the other hand, comes into much closer contact with the athletes than the sand. When installed according to standard procedure, it ensures, in combination with the elastic layer, for impact absorbance and plays a major role for other important properties of the pitch such as ball rebound and rolling behaviour as well as the low risk of injury which is provided by modern artificial turf systems. The more differentiated the properties of a system are, the closer the properties of the granules are defined to suite the overall product. Not only are the sport-functional properties of the granules important here, but also for example, their resistance to UV radiation and weathering, flame retardant properties and no least, their suitability for sensitive environmental conditions. In addition, the visual appearance of a sports field can be defined through use of coloured granules and this can also have an influence on the amount of heat absorbed from the sun's ray by the playing surface.
The properties of an artificial turf pitch can be evaluated not only according to the material properties of the granules used in each case, but are also dependent on the grain shape and bulk density which differs according to the kind of material of which the granules are made and their shape. This makes the filling height a more conclusive factor than the weight / m².
During the years and with artificial turf playing surfaces gaining in importance, the number of different kinds of granules on the market has increased. While initially, granulate made of SBR recycling material was mainly used, there are now different kinds of product available which can be divided into five main categories. The differences can be clearly seen from the material and the price.
SBR granules are reported to have very good functional properties, especially the high elasticity, despite the inexpensive price. It is clearly less expensive to purchase than EPDM and TPE for example. The problem with SBR is however, that it is a recycling product (mainly from car tires), and that its origins are often not clear.
When purchased, it cannot be seen wither the material is already worn out, will quickly become brittle and then form debris and dust. Due to its black colour, an artificial turf pitch with SBR filler absorbs more heat than with other fillers and contains carbon black which can stain. If all these points are considered consciously beforehand, SBR filler granules can present a good and inexpensive solution when the material is obtained from a documented source with corresponding certification.
This variant of SBR is sheathed with coating of polyurethane (PUR) which is essentially intended to allow colouring of the basically black recycling granules. PUR is also used in the manufacture and installation of sports surfaces as highly abrasion-resistance surface sealant, but this property is not necessarily relevant in this application. With time and use of the artificial turf, the coating wears off and the original black colour becomes visible giving the pitch the same conventional properties gained from a purely SBR filler material.
Coated SBR material is therefore not as highly recommended for sports fields subjected to with intensive use or for an extended life expectancy.
EPDM is traditionally a component of many sports surface systems – for example in the elastic surface layers of hall flooring and artificial running track surfaces. EPDM is manufactured as a new material and modified to suit the special requirements placed on filler material. Due to its good elasticity, paired with its special resistance to weathering and the influence of chemicals, EPDM is the preferred filler material in many cases. In addition, EPDM can be modified with additives to suit the requirements of many different fields of application with very good ecological values.
Despite this, EPDM is not just EPDM. Problems have been encountered in the past with long-term stability, in particular with peroxide crosslinked EPDM filler granules. Sulphur crosslinked EPDM on the other hand, has been used for running tracks for several decades and has been a tried and proven component of filler material in use for more than ten years in sports facility construction. New developments have recently been created with regard to grain shape of this group of materials.
EPDM fibre granulate
EPDM fibre granules are a newly shaped infill fibre granules made 100% of sulphur crosslinked EPDM rubber. This especially UV stable material is given an irregular, natural fiber shape in a newly developed manufacturing process. The special factor is the organically shaped surface of the individual grains which is modelled on natural forms. This makes the material look more natural and also has a more pleasing feel when touched. It has a very long-lasting and proven material configuration. The soft surface structure and natural appearance create a pleasing running feeling, reduce grazing and increase the natural play behaviour. One example of this kind of filler is Bionic Fibre Infill made by the Melos Company, researched and developed in their in-house laboratories.
TPE granules are thermoplastic elastomers which have only been used for filler materials for a few years. Different grades of TPE granules can be modified according to the requirements on elasticity and ageing behaviour. Stabilisation of thermoplastic materials against the influences of temperature and ageing is however complex. For this reason, some manufacturers do not recommend use of TPE granulate in regions where high levels of UV radiation and high temperatures are to be expected. Use of highly stabilised TPE granules has its price – TPE is the most expensive type of granule used in these applications.
Natural infill material
Along with the tried and proven elastic filler materials, there is currently a trend towards use of natural filler material. Even though comprehensive long-term results are not yet available, it can be assumed that due to its natural origins, this kind of filler will exhibit a natural reaction to the influences of weathering.
Cork is the most often advertised natural filler material. It is obtained from the bark of cork trees and used as square-edged cut granulate. The low material weight reduces the cost per m², but also leads to high loss of material due to weathering. The cork can be washed out or float away after heavy rainfall.
At high temperatures cork can have a cooling effect thanks to its rest water content. In a dry state a high electrostatic charge and adhesion can be observed which is also caused by normal use and maintenance work. The bonding effect of the granules to the artificial turf fibres can increase the rolling resistance of the ball and reduce ball rebound. This static bonding effect can be nearly completely avoided by previous watering of the pitch before play so that the sport-functional properties of the pitch are not affected.
The various kinds of granules vary in some cases greatly in their material property profile and cost. In order to ensure that the correct decision is made during material selection for use in an artificial turf pitch, each individual product should be considered in detail. Along with product samples from manufacturers, visits to different artificial turf pitches using the various kinds of filler material can be recommended. In this case, an overall picture of the properties and condition of the different pitches can be made and players can also provide first-hand information about their experience with the materials in use.
Photo: Melos GmbH