ANALYSIS OF THE INFLUENCE OF SHOCKPAD PROPERTIES ON THE ENERGY ABSORPTION OF ARTIFICIAL TURF SURFACES

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1 ANALYSIS OF THE INFLUENCE OF SHOCKPAD PROPERTIES ON THE ENERGY ABSORPTION OF ARTIFICIAL TURF SURFACES T. ALLGEUER 1, S. BENSASON 1, A. CHANG 2, J. MARTIN 2 AND E. TORRES 1 1 Dow Europe GmbH, Bachtobelstr. 3, Horgen, 8810, Switzerland, TAllgeuer@dow.com 2 The Dow Chemical Company, 2301 N Brazosport Blvd, Freeport, Texas, 77541, USA Sports surfaces as artificial turf or indoor sports floors see a rapid growth over the last years due to the improved player safety and game consistency provided by such surfaces. A big portion of such performance criteria are provided by the shock absorbency components integrated into such floorings in the form of granular infill or shock pads / underlays. The selection of such components obviously is a key factor for success, this work documents the efforts to develop performance models for different foam systems in order to provide appropriate selection tools. Different foam compositions were screened for their performance in stress response, elastic recovery and creep resistance as shockpads using lab tests in compression along with FQC (FIFA Quality Concept) tests both on the foams and on an artificial turf system. These properties were generally found to depend mainly on foam density and less on polymer density or degree of crosslinking. FQC test results demonstrate how a well designed shockpad can help in maintaining a uniform performance across the artificial turf surface overruling the effect of infill variations. 1 Introduction Artificial turf has come a long way in 42 years. From its beginnings in 1965 with the installation of the first artificial turf playing surface in the Houston (USA) Astrodome stadium, technology has revolutionized the quality of artificial playing surfaces making possible the exceptional performance of today s third generation artificial pitches. Most third generation pitches consist of a carpet with tufted polyethylene (PE) yarns, with an infill of ground styrene-butadiene rubber (SBR) in combination with silica sand, and are installed on prepared fields. There is currently a trend in using more environmentally friendly non-sbr infills for replacing the current SBR based solution. The performance of many of these non-sbr based infill solutions makes it necessary for employing an energy absorption layer, also called shockpad, underneath the carpet. Selection of appropriate foamed shockpad material, density and thickness, is guided in part by performance requirements on shock absorption as outlined in FIFA Quality Concept (FQC) for Artificial Turf. Moreover, design of new systems also requires consideration of long term durability and performance of the shockpads. Summarized herein are the initial findings of a scientific study of the influence of the foamed shockpad design parameters on turf system energy absorption performance. 1

2 2 2 Experimental Various foam candidates were subjected to a series of quasi-static lab tests in compression, along with standard FQC tests on energy absorption. 2.1 Materials A variety of closed-celled PE foams were selected to examine the effect of foam density and crosslinking. The former influences deformation properties significantly, while the latter could influence elevated temperature response. In addition, an ethylene-based crosslinked elastomer foam, along with a PU and an SBR shockpad were included. Samples are described in Table I. Specimens were labeled based on polymer type, foam density provided by the supplier (kg/m 3 ) and thickness (mm). In brackets, the foam type is specified as x for crosslinked, p for plastomeric and e for elastomeric. Table I: Foam Samples Uncrosslinked _ (kg/m 3 ) Crosslinked _ (kg/m 3 ) Elastomeric _ (kg/m 3 ) PE(p) PE(x,p) PE(x,e) PE(p) PE(x,p) SBR PE(p) PE(x,p) PU PE(p) PE(x,p) Compression Test Methods Stress-strain curves in compression were used to examine deformation behavior of the foams. Specimens of 5x5cm were loaded into an Instron at a strain rate of ca. 1 min -1. For experiments at 65 C an environmental chamber was used. Using the same set-up, hysteresis tests were performed with 10 cycles to examine the energy absorption and return characteristics of the foams. In this case, specimens were loaded to 0.39MPa, the stress estimated for foot impact of athletes based on work by Hennig et al [1]. Compressive creep and recovery tests were performed also with the same geometry for 12 hours at a stress of 0.16MPa, which simulates the pressure exerted by a light truck positioned on a field during maintenance or installation of an artificial turf surface. 2.3 FIFA Quality Concept Test Methods Measurements of force reduction (FR), energy restitution (ER) and vertical deformation (VD) of both shockpads and infill/carpet/shockpad systems were done as described by the FIFA Quality Concept Handbook of Test Methods for Football Turf March 2006 Edition [2]. Specifically, the new testing apparatus described in Annex A4 of the handbook was used.

3 3 3 Results and Discussion 3.1 Compression Test Results Compressive stress-strain behavior was measured in order to highlight the effects of foam density, crosslinking, material type, with select results illustrated in figure 1. The curves feature typical regimes of linear elasticity, an intermediate region indicative of elastic collapse, followed by a steep densification zone, as reviewed by Ashby [3]. Clearly, with decreasing foam density, the foams enter densification region at lower stresses. In light of stress levels expected in use, the density of the foam thus has to be picked such that the foam does not deform much beyond the plateau region. Also evident from the figure is that crosslinking does not influence load-deflection curves significantly, as would be expected considering the insensitivity of the modulus to crosslinking at ambient conditions. Figure 1. Compressive stress-strain behavior of selected foams at 20 C Cyclic tests were performed in stress-controlled mode to simulate repeated deformations applied by players, estimated at 0.39MPa. Such tests provide insights into recovery characteristics, as well as hysteretic energy loss in each deformation cycle. The area under the loading curve, energy input, is indicative of energy absorption and the area under the unloading curve is the energy output. The ratio of output to input energy should be indicative of energy restitution in dynamic tests defined in the FQC. As shown in figure 2, the ratio increased after the first cycle and in subsequent cycles it tended to plateau. Thus, the response exhibits an initial breaking-in period followed by stable performance. Higher energy output/input ratio was generally found for lower density foams which became more compliant and the effect of crosslinking was minor. When temperature was increased to 65 C, a shift toward higher energy output/input was found, likely a consequence of increased compression leading to densification of foams.

4 4 Figure 2. Energy output/input for selected foams at 20 and 65 C Compressive creep measurements shown in figure 3 were performed at 0.16MPa and 65 C, as an estimate for highest service temperature. Samples were kept under load for 12h followed by a period of 2h recovery. For the lower density foams, the effect of crosslinking was not significant. However, PE(p) foam did not reach a plateau indicating a relatively higher creep resistance, thanks to the lower initial deformation produced by the load. The strain recovery lead to similar conclusions. Figure 3. Compressive creep behavior at 0.16MPa and 65 C for (a) uncosslinked and (b) crosslinked foams, and (c) recovery thereafter 3.2 FIFA Quality Concept Test Results FIFA Quality Concept (FQC) tests, specifically force reduction (FR), energy restitution (ER) and vertical deformation (VD), have been used to quantify the energy absorption performance of selected foams and artificial turf systems. FIFA 1-star classification specifies for the system an FR range of 55-70% and 4-9mm for VD. While currently there is no specification on ER, low values from 30 to 45% are desired. Figures 4 to 6 show the average of the 4 th and 5 th impact results. The T-lines found above the FR and VD bars show the starting point (result from 1 st impact) as an indication of how the properties of the shockpad degraded after repeated impacts.

5 Figure 4 shows the FQC results measured directly on shockpads. Samples are grouped as uncrosslinked, crosslinked and plastomeric vs elastomeric. Also to show the effect of pre-compression or severe usage, tests were performed with selected shockpads pre-compressed between two heated rolls, denoted with ** in figure 4. Note that elastomeric foams did not deform permanently with the same method. The difference between uncrosslinked and crosslinked samples at same foam density, although present, is not significant. FR increases and ER decreases with increasing foam density while VD does not change significantly. The key finding is the parity of performance of plastomeric and elastomeric foams despite thinner gauge for the elastomeric sample. 5 Figure 4. FR, ER and VD measured on different shockpads according to FQC Tests Figure 5 shows the FR, ER and VD results for the set of foams above, now measured as part of an artificial turf system. In this case, the system was made with 20mm of coated sand infill evenly scattered in a 35mm pile height PE-yarn carpet and was same in all tests. Coated sand was used because it has low energy absorption capability when compared to SBR, thus the role of a shockpad is accentuated. Tested as part of the system, the difference between uncrosslinked and crosslinked shockpads is attenuated. Furthermore, for these low density foams, FR falls outside the FIFA 1-star requirement and ER is high. The higher density plastomeric and elastomeric foams do fall within the FIFA 1-star requirement. To determine the importance of the infill versus the shockpad, the best shockpad from figure 4, namely PE(x,e) , was used in systems with different infill heights. The same coated sand infill used previously was compared against SBR infill, which is best-in-class for energy absorption. In all cases, the free pile height was 15mm. The results plotted in figure 6 show how the use of shockpads makes the energy absorption properties of a field uniform regardless of the infill height. Given a good shockpad design, one could think of infill-less pitches being used in the future as shown by the results in figure 4 and figure 6 for PE(x,e) sample, whereby most of the energy could be absorbed by the shockpad.

6 6 Figure 5. FR, ER and VD measured on systems with different shockpads according to FQC Tests Figure 6. FR, ER and VD measured on systems with different infill types/heights according to FQC Tests 4 Conclusions Different foamed shockpads have been evaluated and compared for their shock absorption properties with the objective to determine the key resin and foam parameters that influence the performance. Lower density PE foams do not appear suitable for shockpads in the application, and crosslinking does not show a performance advantage within the density range studied. Foam density is the key parameter for performance. Ongoing is a study to explore the role of crosslinking in higher density foams. Moreover, elastomeric foams outperform plastomeric ones allowing for thinner shockpads at similar shock absorption performance. The results of this work provide evidence that shockpads highly influence the energy absorption property of an artificial surface, sports surfaces with no or non-shock absorbing infill can be designed and based on raw material design and foam characteristics, desired performance characteristics can be predicted. It also became evident that only a limited correlation exists between the quais-static lab tests and the performance relevant system tests. References [1] EM Hennig et al (1995). J. Applied Biomechanics. 11 (3) [2] FIFA Quality Concept (2006). Handbook of Test Methods for Football Turf. p. 49

7 [3] LJ Gibson et al (1997). Cellular Solids Structure and Properties. Cambridge 7