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Thickness and weight of synthetic turf fibres
There are two possibilities for determining fibre thickness: maximum thickness and average thickness of the fibres.
Neither the FIFA nor the standard EN 15330-1, provide information on this point. Functional, usual and sufficient is determination of the thickness using a micrometre gauge (fig. 1). To do this, shaped fibres (C or scythe shaped) are carefully flattened. Measuring accuracy with this method is 0.001mm. Special care is required in the case of shaped fibres with burling/lugs on both sides of the fibres (fig. 2) so that the fibres are not squashed, giving wrong results.
The average fibre thickness is determined according to DIN SPEC 18035-7. To do this, the fibre cross-section must be microscopically recorded on a scale of 1:100 and then measured optically in 100 cross-sections. The average value of these measurements gives the average fibre thickness. If the fact can be ignored that carrying out 100 measurements on a fibre width of 1.0 – 1.5 mm makes no sense, a well-defined value which can be gained using a generally accessible regulation is at least obtained.
The question now arises as to what significance this finding has for the performance of the turf fibre. Observing the market for sport turf, it would appear that the fibre thickness is very important as the manufacturers try to outbid themselves with fibre thicknesses: from original values of 0.100 mm values have today risen to 0.360 mm while the width of the individual pile fibres has remained between 1.0 to 1.5 mm. From a technical point of view the increase in size of fibre thickness has resulted in the fibres becoming more rigid. This in turn has an effect on the abrasiveness of the pile layer on players' skin: the stiffer the fibres, the rougher the surface. This fact is however, ignored when advertising the properties of the pile fibres. In fact, it is claimed that thicker fibres are stronger and a lower tendency to split. This may be true for a certain time after the manufacture of the synthetic turf surface. The maximum thickness is not however the most important factor. The distribution of thickness over the cross-section (from a technical point of view, the section modulus of the cross-section) is more important. In many cases, an increased thickness is only a thickened strand and not the whole cross-section of the pile fibre. The capability of the fibre, however, to maintain a certain tension and give the pile layer an appearance similar to that of grass, despite the effects of UV radiation and mechanical stressing, is a question of the composition of the fibre; the type of polyolefin and the additives.
Looking at it from this aspect, it is understandable why manufacturers are not very accurate with regard to details about the fibre thickness (excuses are easy to find without a binding regulation to determine the thickness). As a concrete example a fibre with a product specification of 0.300 mm was tested. An actual maximum thickness of only 0.260 mm was found. The difference is explained by the fact that the manufacturer did not measure the thickness using a micrometre, but evaluated the fibre cross-section including its curvature. This procedure is, of course, questionable.
As a rule in the textile industry not fibre thickness but the weight of fibres per 10,000 m is used to characterise the fibre. The unit for this is called [dtex = g/10'000m]. A dtex value given actually refers to single fibres although with regard to the manufacture of synthetic turf, the weight of all fibres in one tuft is given, i.e. the actual fibre weight is multiplied by 6 or 8 (number of single fibres in the tuft). The dtex value has no significance for performance and has simply a descriptive function.
The pile layer weight in [g/m2] / fibre length is important for evaluation of the thickness of the pile layer (nap) and therefore, the offered/delivered "volume of grass". The pile layer weight should not be confused with the original pile weight. While the first value involves only the fibres above the basic fabric, the original pile weight includes the whole pile fibre volume, i.e. also the part of the fibres which run underneath the basic fabric (and which therefore, have nothing to do with the performance of the pile layer).
Foto: fotolia.com//Fotograf: Gerhardt Seybert