A study on improving the comfort of artificial grass in sports

A study on improving the comfort of artificial grass in sports University of Twente Course Surfaces for Comfort and Touch Student Robert Burgers s1017969 Teaching staff Dr. Ir. Emile van der Heide Dr. Xiangqiong Zeng June 1, 2014

Abstract The goal of this report is to research artificial grass for sports and to give recommendations about improving its surface in order to enhance comfort. The report starts off with an analysis of artificial grass in general, different types of artificial grass, its benefits and drawbacks as opposed to natural grass and environmental influences on it. The most important environmental influence is water, for it can lubricate the surface and thereby reduce friction. There are three types of artificial grass: water filled (1G) pitches that are used for field hockey, sand filled (2G) pitches that are used for field hockey and possibly soccer training, and rubber granule filled (3G) pitches that are suitable for soccer and rugby. After the analysis, the emphasis lies on the latter (3G) type of surface; the one that is designed to be in contact with human skin. The comfort of this surface is studied by researching what the difference is in injury rate and -severity between 3G pitches and natural grass. It turned out that although many studies claim that natural grass causes more (severe) injuries, professional soccer players experience that 3G grass is more strongly related to injuries; they very clearly prefer natural grass. The reasons for this are: - Greater surface stiffness - Greater surface friction - Larger metabolic cost Thereafter, it is researched what causes this different surface stiffness, friction and metabolic cost. Greater surface friction is highlighted as a problem because it causes most discomfort, 3G grass more easily causes friction burns. It turns out the cause for this is the greater durability of the artificial surface. Natural grass breaks, as does ground, when forces are high. The energy of a sliding is therefore absorbed by the ground and not by the skin. In artificial grass, the opposite is true: apart from the rubber granules displacing (like ground) and polyethylene fibres bending, the fibres will not break. In the players words, the surface does not give and because of friction energy is absorbed by the skin, causing discomfort. Hereafter, it is researched what possibilities there are to decrease the friction of 3G artificial grass. This can be done in three ways: - Decrease friction coefficient of fibre material - Distribute force better - Make use of a lubricant Making use of a lubricant seemed to be the best option to reduce friction without compromising durability of the surface. A biomimetic approach was used to generate two ideas that revolved around surface lubrication with water. The first idea was to have the strands retain some water through capillary action by making small holes in the strands. The second idea was to have the surface collect water from (foggy) air, just like the Namib beetle does. Drawbacks are that the first idea only has a temporary efect, water will evaporate. The second idea will probably be more difficult to realise and can be realised via a coating or surface treatment. However, this surface will probably be compromised by abrasion. Eventually, the best option seemed to be making synthetic strands out of a hydrophilic material that collects the optimal amount of water from rain and the air by itself. The strands will then be moist in many circumstances, significantly reducing friction and increasing comfort. Drawbacks of this could be that the surface will be less easy to keep and the material might not be as resilient as it currently is. Further research will have to be done to find out if this material is available and indeed suitable.

Table of Contents 1. Introduction 1 2. Project Approach 1 3. Artificial Grass 2 3.1. The rise of artificial grass 2 3.2. Resistance to artificial grass 3 3.3. Advantages and disadvantages 4 3.4. Classification of artificial grass 6 3.5. The functioning of an artificial pitch 8 3.6. Environmental influences 8 4. Comfort of Artificial Turf 9 4.1. Injuries 9 4.2. Causes of injury 10 5. Improvements to Artificial Grass 10 5.1. Decrease coefficient of friction 10 5.2. Distribute force more evenly 11 5.3. Make use of lubricant 11 5.4. Biomimetic approach 12 5.4.1. Differences 12 5.4.2. Imitate shape 13 5.4.3. Hydrophilic surface 13 6. Conclusion 14 7. References 15 Appendices 17 Appendix 1 17

1. Introduction The use of artificial grass to replace natural grass is becoming increasingly predominant in today s world. Especially in sports, different sports pitches use one of the many different types of artificial grass because of the superior qualities it has over natural grass. It for instance does not require any sunlight or mowing and it can be used more often. However, when it comes to comfort, artificial grass is still no match for a well maintained natural grass turf. This report aims to provide a proposal to enhance the comfort, i.e. reduce the discomfort of artificial grass by altering the surface properties. It therefore is focused on artificial grass that is designed to be in contact with human skin, such as artificial grass in sports like soccer or field hockey. The report starts off with a description of several different kinds of artificial grass. Thereafter, it is investigated what types of discomfort or injuries typically result from using the turf, after which the underlying reasons for this discomfort are investigated. This eventually leads to a number of possible solutions and a final conclusion. 2. Project Approach The central research question of this project is: How can the comfort of artificial grass be further improved by altering the surface properties? A number of research questions have been formed that will be answered in this order in the following report. These questions are: 1. What is artificial grass? 1.1. What is it used for? 1.2. Which types are there? 1.3. What are the properties of artificial grass? 1.4. How is the performance of artificial grass influenced by the environment? 2. Which use problems occur with artificial grass? 3. What causes the discomfort with artificial grass? 4. Which use problems can be tackled by altering the surface properties? 5. How can the surface be further improved? 5.1. General approach 5.2. Biomimetic approach 1

3. Artificial grass Artificial grass, or artificial turf, is a synthetic surface that imitates natural grass. It was originally used for sports fields to replace natural grass but nowadays is also used in playgrounds, gardens and for landscaping public areas (fig 1, 2 and 3). Currently 13 out of 32 National Football League (NFL) teams in the USA use artificial grass. Also it became much used for outside fields in Canada because of the cold climate which is not ideal for maintaining natural grass. Due to the climate, all eight Canadian Football League stadiums have artificial grass. 2 3.1. The rise of artificial grass The first artificial grass was used in the 1960 s. The main reason for its use was maintenance; maintaining a natural grass pitch in a sports stadium can be very difficult and expensive. Tall or covered stadiums limit the amount of sunlight the grass can absorb, which it needs to stay healthy. Other than that, intense use of a natural grass pitch can damage it, after which it takes time to recover. The quality of natural grass pitches can also be compromised by environmental influences such as rain, heat, cold, heavy sunlight, animals, insects and weeds. A drenched football pitch is for instance easily damaged when using it (fig 4). The first stadium in which artificial grass was installed was the Astrodome in Houston, Texas. The reason for this was that the natural grass pitch had severely deteriorated during the season. The dead grass and dirt was even being painted green. This first artificial grass brand, patented in 1965 was originally called Chemgrass but was soon rebranded to Astroturf in 1966, since it was heavily promoted by its use in the Astrodome stadium. The use of first generation artificial turfs spread through North America in stadiums and indoor sports fields in the 1970 s. It was improved to a point that it was used for field hockey in the 1976 Olympic Games in Montreal, Canada. 1 Over the years new and improved types of artificial turf were developed, the so called 2G, 3G and 4G (section 1.4). Figure 1: Decorative Artificial Grass Figure 2: Artificial grass soccer field Figure 3: Piece of artificial grass Figure 4: Damaged natural grass field 1 Artificial Grass For Sport. Victoria State Government Department of Planning and Community Development. pp 19 2 The History of Artificial Grass http://www.artificialgrassdirect.com/about-us/history-artificial-grass, (May 2014) 2

3.2. Resistance to artificial grass Nowadays there are many other manufacturers of artificial grass and many different types exist. It is used all over the world because of its advantages over natural grass. Major advantages are the consistent surface (no holes), low maintenance of the field and more intense playability; it can be used more frequent than natural grass. Despite all the improvements made with the current artificial grass it is not preferred over natural grass by everyone. For instance in football in Europe, artificial grass is being used ever more to cut costs while maintaining a certain level of performance. Especially amateur clubs install the turf for maintenance reasons. However, a well maintained natural grass pitch is still preferred in general; artificial grass pitches are even banned from the English Premier League football competition (Fig 5). This is because artificial turf performs and is experienced differently than a grass pitch. Artificial pitches tend to make the ball bounce higher than a natural grass pitch does and running on synthetic surface also feels different than running on natural grass. The smell of grass is also missing and on a hot day a synthetic pitch generally heats up more than a natural grass pitch does. The greatest disadvantage of artificial grass is that the surface can cause injuries: the grass fibres, which are made out of a resilient material, more easily create abrasions and cuts than natural grass does. 3 For these reasons and possibly for the conservative nature and traditions of some sports like football (e.g. no video assistance), of which in England there is a strong culture of, artificial grass is avoided. Some venues even have sliding natural grass fields that can be slid outside the stadium to absorb sunlight and air, like the Veltins Arena in Gelsenkirchen, Germany or the University of Phoenix Stadium in the US (Fig 6). It seems that natural grass is preferred, but only when it is in good condition. This is only guaranteed for wellmaintained pitches without frequent use like the ones in football stadiums. Public grass pitches are for instance rarely without holes or bald spots without grass. It can be concluded that currently artificial grass is especially suited for pitches with frequent use, in certain geographic locations (climate), venues that want to cut maintenance costs or indoor pitches. Figure 6: Veltins Arena movable natural grass field Figure 5: News article about a continued ban on artificial grass 3 Artificial Grass for Sport, pp 22-23 3

3.3. Advantages and disadvantages The previous sections already mentioned a few advantages and drawbacks of artificial grass over natural grass fields in sports. A more complete list is given below: 4 Advantages - High quality consistent surface - Relatively low maintenance - More resistant to bad weather conditions - Can be played on intensively (3-4 times hours of use) - Requires no water (except for wet fields) - Can reduce impact on player s joints by placing shock pads - Cleaner to play on (no mud) - Multi-sport usability (e.g. 2G pitches) use are lower. This is the factor that makes artificial pitches relatively low cost. How much an artificial pitch can be used on average in comparison to how much a natural pitch can be used varies. Some sources say it is three to four times more hours of use (AGFS) and some say ten (Cranfield Study). This has to do with different types of artificial grass. Figure 7 gives an overview of costs over 10 and 25 years of natural and artificial pitches. In the long run an artificial turf is less costly per hour of use. Disadvantages Costs - More abrasive surface - Heat retention / reflection - High initial cost - High repair costs - Upgrade or replacement needed every eight to fifteen years - May require fencing to protect facility - Perception of negative environmental and health impact Looking at these arguments almost all drawbacks of artificial grass have to do with costs. It seems that artificial pitches are much more costly than natural grass pitches. This is true; installing one is more expensive than a natural grass pitch. Maintaining an artificial grass pitch is about as costly as maintaining a natural one according to a Cranfield University survey in 2008 on 2G grass: 8000 pounds versus 7500 pounds annually (Maintaining Synthetic Turf: Sand Filled Systems, 2008, pp4). 5 These figures vary per source. Figure 7: Cost of ownership of natural and synthetic turf pp. 148 Heat retention Synthetic pitches do not absorb heat as well as natural pitches do. Especially the rubber granules of 3G pitches retain heat, which can lead to a 40% hotter sports environment. Children are more at risk from heat stress than adults, since they are shorter and closer to the ground. However on a windy day this is not a problem. 6 The difference is made in the amount of hours played on the field. Because artificial pitches can be used more intensively their maintenance costs per hour of 4 Artificial Grass for Sport, pp 21-22 5 Maintaining Synthetic Turf: Sand Filled Systems (2008) pp 3-4 6 Artificial Grass for Sport, pp. 53 4

Health and environment One of the arguments against artificial grass is that it might form a health hazard due to toxics in the yarn or rubber granules that release because of wear. Most rubber granules are made from recycled tyres in which there can be traces of toxics such as heavy metals. However, studies reveal that these levels are well within standards. The amount of take up of substances or particles through breathing, ingestion and body contact is also small. One might think that artificial pitches are bad for the environment but it has its advantages and disadvantages. The benefits of artificial pitches are that they take up less space per playing hour and require no water. In many areas they are beneficial in this respect. Also an artificial pitch keeps about 20.000 tyres out of landfills. However at the end of the lifecycle of an artificial pitch it is brought to a landfill or burned. This is an issue the industry is working on, for instance since 2013 the company Astroturf fully recycles used carpets. The carpets are ground up, mixed with virgin components and melted so they may be used to construct for instance plastic warehouse pallets. 7 7 Astroturf unveils full field recycle program http://www.astroturf.com/astroturf-unveils-full-fieldrecycling-program/, (May 2014) 5

3.4. Classification of Artificial grass Artificial grass can be realised in various forms and combinations. It can be classified in a few general types: 1G, 2G, 3G and future 4G artificial grass. There are many possibilities that lie between these categories by varying the strand length (pile-height), infill type, and density of the strands. 8 1 st generation Artificial Grass (water infill) First generation artificial grass is composed of low pileheight high-density fibres made of nylon strands. It is suitable for sports like field hockey and tennis. A major disadvantage of these fields is that the surface can easily cause friction burns unless played on a wet surface (fig X). It is therefore not suitable for sports like rugby where players make frequent contact with the surface. 2 nd generation Artificial Grass (sand infill) 2G artificial grass is characterised by medium pile height, lower density fibres of 20 to 35 mm of which the space in between is filled with sand. Currently it is made out of polypropylene. It is a versatile and durable turf that can be used for both field hockey and football. However, it is not suited for rugby in general and football on a competitive level. The turf was first installed by the English football club Queens Park Rangers in 1981. Some other clubs followed but the turf was not a success; players got injured by the abrasive surface and the ball behaved poorly on the turf. This eventually led to a ban on all forms of artificial turf in all professional football in England that is still in force. Figure 8: 1G water field pitch (top) and 2G sand filled pitch (bottom) Figure 9: 2G sand filled pitch configuration 8 Artificial Grass for Sport. Pp. 19-21 Figure 10: 1G, 2G and 3G pitch configurations. Notice the diferent lengths of strands and different fill-materials. 6

3 rd generation Artificial Grass (rubber granule infill) 3G artificial grass is composed of long pile height low density fibres of 35 to 65 mm. The space in between is filled with a layer of sand and a layer of rubber granules. The polyethylene fibres are less rigid than the fibres of 1G and 2G turfs which makes the surface less abrasive. The rubber infill allows for damping of shocks and allows shoes with studs to be used. 3G turfs are therefore suitable to create fields for rugby and football in which players slide over the surface or fall to the ground. The surface was developed in the late 1990s and is being recognised as a suitable surface for these sports. 4 th generation Artificial Grass (future types) 4G refers to the next generation of artificial grass that is currently under development. This could for instance be a surface for football use without infill. This generation has not been defined yet, but some sellers might refer to their product as 4, 5, or 6G for promoting reasons. Figure 11: 3G pitch in use. Notice the rubber granules that are in the air. Different configurations Figure X displays different configurations of artificial grass for different sports and gives a good impression of their dimensions. An enlarged version of this figure along with additional information such as suitability for certain sports is in appendix 1. Figure 12: 3G pitch configuration Figure 13: Different configurations of artificial pitches, suitable for different sports. Rubber crumb type (3G), Sand type (2G) and Water type (1G) 7

3.5. The functioning of an artificial pitch Synthetic pitches are comprised of four main components (fig X): Carpet - Carpet with fibres - Infill - Shockpad - Asphalt or stone base The synthetic fibres provide the cushioning surface necessary for field sports, especially in 1G and 2G fields. It also provides traction so players do not slip and the ball does not roll too far. Infill A square meter of 2G turf holds about 20 to 25 kilograms of sand. The sand infill thereby adds weight to the carpet to keep it in place. Other than that it supports the fibres to stay upright. It fills the space in between the bunches of strands, otherwise they would flatten (Cranfield pp5). 9 3.6. Environmental influences The performance of artificial grass pitches can be influenced by various factors. Rain is by far the most influential one for it has a large influence on friction. Much like on a natural grass pitch, the friction coefficient of the surface of an artificial grass pitch is lower when it is wet. The ball will move faster and further, and the same goes for making slidings on a 3G pitch. The surface becomes more slippery in the rain, but players do not slip easily if their shoes have studs. Puddles should not form on an artificial pitch because of the drainage system. When conditions are very dry the ball will move more slowly and the surface will be more abrasive. A soccer player has to kick the ball harder when he passes the ball over the ground. Therefore, and because there is more friction between the shoes and the surface a drier field can be more tiresome. In 3G pitches, the rubber granules play a big role in providing the cushioning surface and have a role in traction; they create a surface that can be better penetrated or indented with studs, much like a natural pitch. Shockpads Shockpads are placed beneath the carpet to absorb impacts. In some 3G pitches that have rubber granules to absorb shocks they may be optional. Asphalt / stone base The asphalt base provides a strong and flat surface for the carpet. It also allows rainwater to be drained away from the pitch. Figure 14: Detailed configuration of 2G pitch 9 Maintaining Synthetic Turf: Sand Filled Systems (2008) pp. 5 8

4. Comfort of Artificial Turf Multiple researches have been carried out to find out whether synthetic pitches cause more injury than natural pitches. Many studies tell that the incidence rate per hour of play is not higher than on natural grass, although other studies tell a different story. Most information is based on 3G pitches in soccer, which are also called Football Turf (FT). Therefore this part focuses on 3G turf in soccer. 4.1. Injuries According to a FIFA research that compared two under 17 soccer tournaments there was little difference in the incidence, nature or cause of injuries. 10 A study on Saudi national team players tells that the incidence rate on natural grass is higher than on 3G artificial grass, 56.1 versus 37.9 injuries per 1000 hours of match and training play. 11 Another study comparing about 500 professional players in Europe confirms this: 2.42 versus 2.94 injuries per 1000 training hours per person, and 19.60 versus 21.48 injuries per 1000 match hours for AG and NG respectively. This difference is however small. Interestingly, the risk of ankle sprain was increased on artificial grass: 4.83 versus 2.66 per 1000 match hours. 12 Discomfort Despite the figures in these researches, a study about the perception of 99 professional Major League Soccer players (USA and Canada) on the risk of injury reveals that they believe 3G turf is associated with greater risk. 13 94% believed this, and in North America soccer is played much on 3G pitches. The participants played about a third of their career of 3G pitches and the rest on natural grass. Although this research did not present any figures on injury rate, it surprisingly reveals that players experience 3G turf more negatively than NG. The players identified three surface related factors: - Greater surface stiffness - Greater surface friction - Larger metabolic cost Some of the players statements are: Football Turf doesn t give like grass. If a foot gets caught in, it is more dangerous because the turf can t dig up to release the foot. Temperature on hot days zaps and dehydrates the body In figure X some key phrases corresponding to the three surface factors in injuries. Figure 15 and 16: Data on injury frequency and severity of Saudi players 11 (above) and players in Europe 12 (below). 10 Artificial Grass for Sport, pp. 39 11 M. Almutawa et al. (2014) The incidence and nature of injuries sustained on grass and 3rd generation artificial turf: A pilot study in elite Saudi National Team footballers, pp. 49. 12 Ekstrand et al. (2009) Risk of injury in elite football played on artificial turf versus natural grass: a prospective twocohort study Figure 17: Key injury related comments on artificial grass 13. 13 C. Poulos et al. (2014). The perceptions of professional soccer players on the risk of injury from competition and training on natural grass and 3rd generation artificial turf 9

It is not hard to imagine some of the statements of these players. Should a shoe get stuck in the turf while turning this could lead to serious injury. There is no give in the turf because it does not break like grass and ground do. However, it is strange that the outcomes of the Major League Soccer study do not correspond with the studies that say AG leads to less or the same amount of injuries. Also, no mention of minor injuries due to abrasion was given. 4.2 Causes of injury Players believe that the risk of injury on 3G turf is increased by three factors: greater surface stiffness, greater surface friction and larger metabolic cost. The larger metabolic cost is caused by increased friction; the ball then needs to be kicked harder and shoes grip on more into the surface. Also larger surface stiffness could play a part in this, too little damping might make running more strenuous. The surface stiffness is created by a combination of rubber granules, the shockpads and to a lesser extent the fibres. The surface friction is provided by the strands and rubber granules. When passing a ball, the strands provide most of the friction. They are flexible and allow to ball to rest on the rubber granules. These provide a ground-like surface for studs to grip in to. When a sliding is made, rubber granules are displaced, which alleviates pressure on the skin. The relatively hard strands are somewhat flexible but stay intact and in place, which causes friction burns. Natural grass is much softer and can break; therefore it cannot impose that much pressure on the skin. Because the strands on a 3G surface are durable, they do not give and can cause injury. Therefore, there is a contradiction between durability and stress in artificial grass. The surface of an NG pitch would more easily break when stress is increased. 5. Improvements to Artificial Grass Of the problems mentioned in the previous section, greater surface friction is the greatest obstacle of AG. The durable strands cause pressure peaks that lead to friction burns and shoes might get stuck in the surface, leading to injury. Furthermore, greater friction causes greater metabolic cost. Surface stiffness seems like an issue that can already be solved with proper use of rubber granules and shock pads. According to the TRIZ problem solving method, there is no solution for the contradiction between the features 11: stress or pressure, and 16: durability of a nonmoving object. However, there might be some solutions to reduce friction, which are: - Decrease friction coefficient of fibre material - Distribute force better - Make use of a lubricant 5.1 Decrease coefficient of friction Decreasing the friction coefficient can be done in three ways, by: - Changing the microstructure - Finishing the surface - Applying a coating The strands in 3G turf are made of a polyolefin which primarily contains polyethylene and polypropylene, and is relatively smooth 14. It seems to be the right material for the strands. Whether the microstructure or surface should be altered is unclear because not much information on this topic is available. Other than that, a coating could be applied to lower the friction coefficient. The backing of artificial grass is coated with polyurethane or latex, the strands are however not coated. If a coating like Teflon (PTFE) would be used on the strands they might become less abrasive. A durable coating would be a valid option as long as it is not a health or environmental hazard, and is not too costly. 14 Artificial Grass for Sport, pp. 22 10

5.2. Distribute force more evenly Looking at the macrostructure in figure 18, it seems the strands have fairly sharp edges. This is especially true for the tops where the strands have been cut. A possible improvement might be to make the strands slightly thicker and to round off the edges, then the pressure peaks might be lower. With increased thickness comes increased strength, and the material might be made a bit more elastic by altering additives. In figure 19 two commonly used forms of strands are displayed. 5.3. Make use of lubricant A lubricated surface decreases its friction coefficient. Water filled pitches are dressed with a very shallow layer of water for this purpose, and they store the water. In contrast, 3G pitches are designed to efficiently irrigate the water away from the surface. When it is raining or shortly after a rain, the strands are still lubricated with water although they are hydrophobic. This increases comfort when sliding on the surface. Figure 18: 3G artificial grass Therefore, a lubricant like water could be sprayed on before play. This will however cost extra effort and increase water consumption. A better option might be to make the strands hydrophilic, so they accumulate water that will decrease friction. Also, little holes could be made in the strands that might store water by means of capillary action. The same material can then be used. When force is applied the water will be squeezed out of the strands and lubricate the sliding. This could also be done by letting only some of the strands fulfil this function. Another option is to have the rubber granules accumulate water, but this might not be as effective as having the more abrasive strands doing this. Damp granules might cool off the skin of a sliding player so they might reduce friction burns. Either of these two options, lubricated strands or granules, seems a good improvement as long as they do not reduce durability. A disadvantage of these solutions might be that the ease of cleaning of the turf is decreased because water and dirt is then accumulated in the material. Figure 19: Commonly used forms of strands for artificial grass 11

5.4. Biomimetic Approach In this section, some of the differences between synthetic and natural grass are highlighted. Thereafter two ideas will be presented to lubricate artificial grass from a biomimetic approach. One aims to gain some of grass attributes by imitating its shape and the other imitates the hydrophilic surface of the Namib beetle. 5.4.1. Differences Natural grass is thinner and more flexible than synthetic grass. On top it is more easily bent because of the flat shape. At the root the shape is circular and the grass is less flexible. In between these areas, the flat shape is gradually folded into a semi-circle (fig 21). When grass is dipped into water and the held upright, most of the water quickly slides down to the ground. Interestingly the half circle shape seems to trap a small drop of water because of capillary action. This could be a temporary advantage of natural strands over synthetic ones; more water is trapped in the surface (fig 20). However, to combat this effect, natural grass has a tiny piece of material within the half circle to keep water out (fig 21). It is designed to efficiently transport water downwards with its hydrophobic surface. Figure 20: Drops of water on grass. Most drops run off the side, some gets caught in the semi-circle (right). Figure 21: Drawing of a strand of natural grass. Notice the change in shape. Figure 23: Unsuccesful shapes of synthetic grass Figure 22: Natural grass 12

5.4.2. Imitate shape Attempts could be made to imitate the real, more complex shape of grass to acquire the advantage of collecting some water. There have already been attempts to imitate the curved shape of grass but these proved to be much less durable according to simulations 15. The strands would fail at the points indicated by the arrows (fig 23). Imitating the real shape of grass, narrow below and broad on top will be difficult to realise with the existing tufting method. The section of the strands is homogenous because they are made from one very long cord. Altering the section over the length of the strand seems too complex and is also not necessary. A better way should be found to collect and trap water in the surface of artificial grass. Figure 24: Ideas for strands to retain water via capillary action A possible way to temporalily capture rainwater (before it evaporates) is displayed in figure 25. The strands are punctured and the holes should retain some water. In dry conditions this will not be of use, unless some water is sprayed on the surface beforehand. A disadvantage of this configuration is that the holes might increase friction. The sketches in figure 24 display another option: a hollow artificial strand made from two strands that can contain water which will be forced out when a sliding is made. This will however be much more difficult to manufacture and might rupture more easily. 5.4.3. Hydrophilic surface A good way to collect water would be to have the surface collect water from the air. Just like the wings of the Namib beetle do (fig 26). This beetle seems to have a smooth surface and the micro surface collects water from foggy air. If artificial grass could collect water from the air it would become a bit moist and friction would become reduced. This solution might be reached by means of a surface treatment or coating. Figure 25: Punctured strand that retains drops of water idea 15 B. Kolgjini (2012). Structure and long term properties of polyethylene monofilaments for artificial turf applications pp. 20 Figure 26: Namib beetle 13

6. Conclusion Artificial grass is a resilient surface that needs to withstand heavy impacts over a long period of intense use. At the same time, it needs to provide a comfortable surface to play sports on, to for instance make slidings. Because artificial grass is made to be very durable, the surface does not give and relatively much of the energy put into a sliding is absorbed by the skin, in contrast to natural grass which breaks. The durable but abrasive surface is therefore also the greatest disadvantage of artificial grass. Currently used 3G artificial grass has proved to be a great improvement in this respect; slidings can be made by which soft rubber granules are displaced and energy is absorbed. However, the relatively hard synthetic strands, that are necessary for grip, stay largely in place and create friction burns. This is why you never see soccer players make knee slides on a synthetic pitch (fig 27). Several ways to improve the surface i.e. reduce friction have been discussed. The friction coefficient of the material could be reduced by: changing the microstructure, finishing the surface or applying a coating. Also, force could be distributed more evenly by for instance making thicker, more elastic strands. Finally, friction could be reduced by adding a lubricant. Of these solution directions, probably most will have already been optimized to a large extent, e.g. strand thickness and microstructure. Furthermore, strands are currently not coated. They are made from a homogeneous material so they can be gradually worn down and replaced after about 15 years. A coating would probably become damaged easily by studs. Probably the most effective way to decrease friction without compromising durability is adding a lubricant. Wet artificial grass fields are far less abrasive. A few solutions have been presented to lubricate the surface from a biomimetic approach. One aims to retain water in the surface, the other collects water from humid air. The former only offers a temporary advantage in mild climates, because otherwise the water will eventually evaporate. But if the strands or (some of the) granules could retain some water this would be a useful attribute. The latter solution, collecting water from the air, is even more useful. However, creating such a surface via a surface treatment or coating might be difficult. Also, in climates with dry air it might not have any effect. An added disadvantage is that the hydrophilic surface would probably be abraded off by intensive use. To conclude, the best option to improve artificial grass would be to use a more hydrophilic material that will accumulate a certain amount of moisture by itself, from rain and air. At the same time it has to be durable, although it will need to cope with less friction force when the surface is moist. Therefore it might not have to be as resilient as the strands that are used currently. The material should be designed thusly, that it collects the optimal amount of moisture to increase comfort and create the best experience. Figure 27: Liverpool player Luis Suarez making a knee slide. Notice the damage done to the field, which prevented injury. 14

7. References 1 Artificial Grass for Sport. Victoria State Government Department of Planning and Community Development. pp. 19 2 The History of Artificial Grass http://www.artificialgrass-direct.com/about-us/history-artificial-grass, (May 2014) 3 Artificial Grass for Sport, pp. 22-23 4 Artificial Grass for Sport, pp. 21-22 5 Maintaining Synthetic Turf: Sand Filled Systems (2008) pp 3-4 6 Artificial Grass for Sport, pp. 53 7 Astroturf unveils full field recycle program http://www.astroturf.com/astroturf-unveils-full-field-recycling-program/, (May 2014) 8 Artificial Grass for Sport, pp. 19-21 9 Maintaining Synthetic Turf: Sand Filled Systems (2008) pp. 5 10 Artificial Grass for Sport, pp. 39 11 M. Almutawa et al. (2014) The incidence and nature of injuries sustained on grass and 3rd generation artificial turf: A pilot study in elite Saudi National Team footballers, pp. 49. 12 Ekstrand et al. (2009) Risk of injury in elite football played on artificial turf versus natural grass: a prospective twocohort study 13 C. Poulos et al. (2014). The perceptions of professional soccer players on the risk of injury from competition and training on natural grass and 3rd generation artificial turf 14 Artificial Grass for Sport, pp. 22 15 B. Kolgjini (2012). Structure and long term properties of polyethylene monofilaments for artificial turf applications pp. 20 Images Figure 1 FAQ's. (n.d.). Retrieved May 2014, from Totally Green Synthetic Grass Solutions: http://www.totallygreensyntheticgrass.com/faqs.html Figure 2 10 years artificial grass. (n.d.). Retrieved May 2014, from Aojian Artificial Grass: http://www.china-artificialgrass.com/productinfo.php?id=76 Figure 3 Artificial Grass Suppliers. (n.d.). Retrieved May 2014, from CC Grass US: http://ccgrassus.wordpress.com/ Figure 4 Fullerians Rugby Club. (n.d.). Retrieved May 2014, from Watford Observer: http://m.watfordobserver.co.uk/news/11002740.rugby_club_counting_costs_after_pitch_is_damaged/ Figure 5 Williamson, L. (n.d.). Conference clubs veto artificial turf. Retrieved May 2014, from Daily Mail: http://www.dailymail.co.uk/sport/football/article-2548253/conference-clubs-veto-artificial-turf-despiteincreasing-postponements.html Figure 6 Schalke 04's wonderful stadium. (n.d.). Retrieved May 2014, from http://architectism.com/shalke-04swonderful-stadium-veltins-arena/ Figure 7 Artificial Grass for Sport, pp. 148 Figure 8 B. Kolgjini (2012) Structure and long term properties of polyethylene monofilaments for artificial turf applications pp. 7 Figure 9 Cranfield Guidelines to Maintaining Synthetic Turf)pp. 5 Figure 10 Kolgjini (2012) Structure and long term properties of polyethylene monofilaments for artificial turf applications pp. 3 15

Figure 11 sv-schalkhaar-c3-albatross-c1-de-fotos. (n.d.). Retrieved May 2014, from UVV Albatross: http://www.uvvalbatross.nl/sv-schalkhaar-c3-albatross-c1-de-fotos/ Figure 12 Artificial Grass for Sport, pp. 20 Figure 13 (n.d.). Artificial Surfaces for Outdoor Sport. Pp. 24 Figure 14 (n.d.). Artificial Surfaces for Outdoor Sport. Pp. 7 Figure 15 M. Almutawa et al. (2014) The incidence and nature of injuries sustained on grass and 3rd generation artificial turf: A pilot study in elite Saudi National Team footballers, pp. 49. Figure 16 Ekstrand et al. (2009) Risk of injury in elite football played on artificial turf versus natural grass: a prospective two-cohort study Figure 17 C. Poulos et al. (2014). The perceptions of professional soccer players on the risk of injury from competition and training on natural grass and 3rd generation artificial turf Figure 18 Artificial grass for rugby. (n.d.). Retrieved May 2014, from Prestige Sports Pitches: http://www.prestigesportspitches.co.uk/index.php/rugby-pitches/artificial-grass-for-rugby/ Figure 19 B. Kolgjini (2012). Structure and long term properties of polyethylene monofilaments for artificial turf applications pp. 20 Figure 23 B. Kolgjini (2012). Structure and long term properties of polyethylene monofilaments for artificial turf applications pp. 20 Figure 26 namib desert beetle motivates biochemist develop self-filling water bottle. (n.d.). Retrieved May 2014, from French Tribune: http://frenchtribune.com/teneur/1214676-namib-desert-beetle-motivates-biochemistdevelop-self-filling-water-bottle Figure 27 Stop with the knee sliding. (n.d.). retrieved May 2014, from Daily Mail: http://www.dailymail.co.uk/sport/football/article-2518039/stop-knee-sliding-fun-goal-celebrations--adam- Crafton.html Appendices Appendix 1 (n.d.). Artificial Surfaces for Outdoor Sport. pp. 24 16

Appendix 1 Different artificial pitch configurations and their suitability for different sports. 17