New strategies towards the next generation of skin-friendly artificial turf surfaces
Tay, Sock Peng
Date of Issue2016-10-28
School of Materials Science and Engineering
Institute for Sports Research; Loughborough University
The issue of skin friction related injuries has been one of the problems challenging the artificial sports turf industry. It has been identified by users as a major factor impeding acceptance of artificial turf at the professional level. However, information explaining the mechanisms for skin-turf abrasion is limited and little progress has been made, it appears, to derive an appropriate testing method for product approval or in evidence of improvement of the skin-friendliness of these products in sport surface surfaces. This research project focused on exploring the potential for improving the skin-friendliness of artificial turfs through a multi-faceted approach: identifying the contribution of the abrasive-components in modern artificial turf surfaces through mechanical testing; while critically evaluating currently available skin friction standards , evaluating strategies for polymer material modifications to reduce the skin-surface friction; and the designing of an appropriate bench-top set-up for the lab-based assessment of material skin-friendliness. The lack of understanding of skin-turf interaction was addressed by identifying the turf-component that has the greatest influence on the skin-turf friction – with the mechanical device used in the current industry standard. The ‘skin’-turf frictional profiles of a series of third generation (3G) turf surfaces were examined, in combination with independent measurements of the silicone ‘skin’ surface roughness pre- and post-friction testing. Results indicated that turf carpets without any infill material exhibited the highest frictional values while surfaces completely filled with either sand or rubber displayed similarly low frictional values, independent of infill type. Morphological measurements also showed the largest decrease in surface roughness for ‘skin’ samples tested on carpet-only surfaces, indicating a smoothening effect via abrasion. This abrading effect is alleviated with the addition of infill to the surface, with fully-filled surfaces having the least damage to the ‘skin’s. This unprecedented study suggests that the carpet may have the largest influence on the overall frictional behaviour of an artificial turf surface – narrowing down the turf component to be targeted when applying product improvements to address skin-friendly properties. The strategy of material surface modification was then employed, to study the effect of polyzwitterionic brushes on improving the skin-friendliness of the identified polypropylene substrate. To address the intended application for artificial turfs, a bench-top test was developed to investigate the frictional properties of the hydrated samples outside of commonly used aqueous environments, where an excess of lubricating water molecules is absent. Photo-grafted poly(sulfobetaine methacrylate) (pSBMA) brushes of various irradiation durations were prepared and the improvement in frictional properties was studied. Frictional measurements using silicone ‘skin’ tips, under both dry and hydrated surface conditions, showed that the applied modification was capable of forming a stable lubrication layer in the absence of excess water, significantly reducing the coefficient of friction by up to 78.8 %. The pSBMA brushes also provided the additional advantage of antifouling – exhibiting resistance towards pathogenic Staphylococcus aureus with almost zero surface colonization for well-grafted samples. The low ‘skin’-sample friction under ambient conditions and desirable fouling-resistance highlights the potential of pSBMA brushes as a modification strategy for achieving skin-friendly surfaces targeted at reducing the risk of skin abrasions. The tribological implications of counter-surface selection were investigated. Frictional assessments of the pSBMA-modified samples were carried out using standard steel tribo-tips, in addition to the ‘skin’ tips used. Measurements with the ‘skin’ tips showed that the hydrated pSBMA brushes were successful in reducing initial ‘skin’-sample friction though the effect diminishes with extended testing, attributed to the drying of the interfacial water. The standard steel tribo-tips were unable to reciprocate these results, returning consistently low frictional values regardless of extent of surface modification or hydration. These observations draw attention to the importance of counter-surface selection in frictional assessments, highlighting how appropriate test materials can identify characteristic surface properties while providing an interaction that simulates that of the intended application. The simple experimental set-up used may potentially be enhanced as an intermediate product qualification method in the manufacturing of skin-friendly artificial turf yarns.