Artificial Grass Infill

The artificial turf and synthetic turf infill is applied in several layers: normally it begins with a layer of sand to act as a ballast layer to stabilize the surface by its weight, followed by a layer of elastomeric granules made of:

• SBR/NR (recycled tire granulates, black),
• PUR coated SBR,
• EPDM (pigmented special granules) of various density and hardness
• TPE

There are also synthetic grass surface systems in which a mix of mineral and elastomeric granules is applied between the 1st and the 2nd layer of infill.

In respect to function and monetary concerns, so-called SBR granules are by far the most advantageous as fake grass infill. Although this provides a second use for the rubber from recycled car tires, there is a public concern on this regarding health and the environment. It is possible that its black color is viewed as visually unpleasant or, in indoor applications, the transient strong rubber smell is distasteful. This is hard to say as color is assessed differently by different people and the smell can be greatly reduced if the facility is well ventilated (inflated roof, windows or forced-air ventilation system). The experimental study revealed that the health and environmental concerns are not founded, but rise from unsubstantiated rumor or sales oriented disinformation.

A few years ago, it was observed that artificial grass pile fibers of artificial turf surfaces filled with SBR granules “changed color” from the original green to grey-green within 3 to 6 months. From within the industry it is reported that this was due to an incompatibility of the fiber stabilizers and components within the SBR. This problem was resolved by an adjustment to the stabilization process of the artificial grass pile fibers.

SBR granules can be improved in both color and ecological respect if they are covered with pigmented PUR binder. However, the effect of this improvement is gradual and reasonable progress towards the environmental aspects is questionable.

In addition to recycled tire material EPDM recyclate is also offered in black or grey. These granules are made of scrap which is collected from the production of gasket material (technical scrap). Although these materials are primarily of good quality, the inclusion of inferior waste within these products must be factored in if considering their environmental compatibility. If such products are used, the origin of the granules must be investigated and evidence of impeccable quality determined.

With newly manufactured EPDM granules, the color appearance of the artificial turf and synthetic grass surface is improved; however the monetary cost is significant. Since these granules are manufactured directly for the infill of pile layers – not a recycled product – the properties of the granules can be specifically and uniformly designed according to the application. EPDM-granules are fully cross linked materials which can be either sulphur or peroxide cured. Sulphur cured materials have been in use since the 1980’s in synthetic track surfaces and have been proven to be weather stable. Peroxide cured materials are a relatively new development. Even with virgin EPDM products failures such as lumping or hardening of the granules have been observed: (reason: use of off-spec components instead of prime quality).

Granules made from thermoplastic elastomers are relatively new in sport surface construction (about 2 years). The materials used in sport surfaces are divided into TPE-V and TPE-S groups. TPE-V materials are blends of EPDM and a thermoplastic polyolefin, which are partially cross linked. Here, the stabilization and cross linking method strongly influences the weatherability of the product and its elasticity. Also, the only advantage over fully cross linked EPDM materials is recyclability. TPE-S materials are a blend of a styrene copolymer and a thermoplastic polyolefin. These materials are physically cross linked, no chemical reaction is needed. The weatherability is especially good, if SEBS is used as base polymer. Since there are many options in the design of the granules great care in finding the optimal system is needed. Also, many materials are labeled TPE only, thus special consideration and a thorough assessment of their quality is required.

Occasionally, the production technique of rubber granules is discussed: crushing of raw material at normal temperature or in deep frozen condition (cryogenic granules). The advantage of cryogenic granules is viewed as their better size definition (i.e. very little dust). However, with modern production techniques normally milled granules do not fall behind in this respect.

It is difficult to control the precise amount of infill during installation of filled turf layers. This is crucial though as the performance of the surface is highly dependant on the
thickness of the layer. This layer-depth characteristic cannot easily be determined since the surface of the infill layers are below the pile layer surface (not possible visually; with caliper by individual point measurement only). Thus, a situation often occurs when too much sand is initially applied the required amount of rubber granules cannot be applied (pile layer too full; min. 15mm free fiber length must stand out from the infill). Therefore, special attention must be paid to the application of the sand. To do this, the pouring-in process must be performed with specially designed sand spreaders which control the amount of infill and brush the sand / granules into the pile layer. It is especially important to fill in the sand in multiple passes in order to prevent
pile fibers being bent and trapped under the sand.