NL2020130B1 - Butyral based infill for Artifcial Turf - Google Patents

Abstract

A resilient infill for artificial turf, comprising tack resistant foam granules of a vinyl butyral comprising polymer. Also provided is a method for producing said infill and the use of said infill in artificial turf systems as well as the resulting artificial turf systems

Description

Butyral based infill for Artifcial Turf

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to resilient infill for artificial turf, in particular to foamed infill granules comprising a vinyl butyral comprising polymer. The invention also relates to a method for preparing said resilient infill granules, the use of said resilient infill granules in an artificial turf system and artificial turf systems comprising said resilient infill. 2. Description of the Related Art

Artificial turf systems are well known for various sporting and aesthetic purposes and have developed through a number of generations to their present form. In general, such systems seek to achieve the same characteristics as their natural counterparts although in certain areas these may have already been surpassed, at least in terms of predictability of behaviour.

Typical third generation turf systems comprise a backing layer with an upper surface and an infill layer of soft granules disposed between the fibres. The backing layer may consist of a woven fabric in which artificial grass fibres are tufted to provide pile fibres oriented in an upward position and fixed to the woven fabric by a backing layer of latex or polyurethane. Alternatively, the backing and the pile fibres can be produced simultaneously by weaving the carpet. Here there is considerable freedom for the position of the pile fibres and the backing structure.

Installation of the turf system typically involves providing a layer of loose sand, strewn between the upstanding turf fibres, which by its weight holds the backing in place and supports the pile in upward position. Onto this sand layer and also between the artificial turf fibres, soft elastomeric granules are strewn, forming a loose performance infill layer that provides the necessary sport perfoimance. These performance characteristics will be depend on the intended use but formost sports will include: rotational and lineair grip; force reduction; vertical ball bounce; and rotational friction. This performance can be further supported by applying a shock pad ore-layer directly under the backing layer. One system of this type has been described in UK patent application GB2429171.

There is an ongoing effort to develop new elastomeric granules that are usefull as loose performance infill. By carefully adjusting the properties of the granules that make up this infill material it is possible to further enhance the performance characteristics of the artificial turf comprising said granules. In addition the need to develop more sustainable technology also applies for artificial turf technology. In view of this, there is a need to develop new infill materials with a minimized environmental footprint. A promising route to improve the sustainability of infill material is to recycle or upgrade a waste stream in such a manner that it is suitable for use as performance infill. Such processes can however be difficult to accurately control, due to the variability of materials offered for recycling. A wide range of waste streams can be identified but at present, it is unclear as to which of the many available waste streams could lead to an infill having the enhanced properties expected by today's sportspersons.

BRIEF SUMMARY OF THE INVENTION

The invention relates to a resilient infill for artificial turf, comprising foam granules of a vinyl butyral comprising polymer, wherein the granules have a mean size of between 0.5 mm and 5 mm and wherein the granules are rendered tack resistant below 90°C.

In this context it is understood that resilient refers to the material property that the material can be deformed upon application of an external force and will spontaneously return to its original shape when the force is removed. This behavior can be fully elastic by which it is meant that the material returns to its original shape quickly when the force is removed and that the material substantially returns all energy which was needed to deform the material. However, preferably it is viscoelastic by which it is meant that the material only slowly returns to its original shape and that it does not give back all the energy needed to deform the material.

The use of viscoelastic resilient infill in artificial turf results in artificial turf also having viscoelastic behavior. The resulting artificial turf will dampen shocks, for instance those due to walking or running on it, without returning too much energy. It hence has a low energy restitution value. This combination of properties provides for a preferred user/player experience that is closer to that of natural turf.

The vinyl butyral comprising polymer is preferably polyvinyl butyral (PVB). The skilled person will understand that PVB always comprises monomers having hydroxyl and acetyl groups besides the monomers grafted with butyral groups. Typically PVB is produced starting from vinyl acetate which is polymerized to yield polyvinyl acetate. In a next step the polyvinyl acetate is saponified to polyvinyl alcohol (PVA). PVB is obtained by grafting the PVA with butyraldehyde (butyral). Said reactions are not complete and as a result there will always be some vinyl acetate and vinyl alcohol present. Suitable PVB comprises 7-35 wt.% vinyl alcohol, preferably 8-30 wt.% vinyl alcohol, more preferably 9-25 wt.% vinyl alcohol, even more preferably 10-20 wt.% vinyl alcohol, and most preferably 12- 18 wt.% vinyl alcohol. Suitable PVB comprises less than 10 wt.% vinyl acetate, more preferably less than 8 wt.% vinyl acetate, more preferably less than 6 wt.% vinyl acetate, more preferable less than 4 wt.% vinyl acetate and most preferably less than 2 wt.% vinyl acetate. In the art PVB is used to denote a polymer comprising at least 60 wt.% of vinyl butyral groups, more preferably at least 70 wt.% of polyvinyl butyral groups, more preferably at least 75 wt.% of polyvinyl butyral groups, more preferably at least 80 wt.% of polyvinyl butyral groups, even more preferably at least 85 wt.% of polyvinyl butyral groups and most preferably at least 90 wt.% of polyvinyl butyral groups.

It is possible to adapt the properties of the PVB by including a small amount of an additional aldehyde, such as formaldehyde, propionaldehyde, pentanaldehyde, hexanaldehyde, or a higher aldehyde. These polymers are denoted as vinyl butyral comprising polymer whereby it is understood that vinyl butyral comprises at least 50 wt.% of the polymer, more preferably at least 60 wt.% of the polymer and most preferably at least 70wt.% of the polymer. Obviously PVB also belongs to the class of vinyl butyral comprising polymer. Depending on the amount of the additional aldehyde a skilled person might continue to address the polymer as PVB or would call the polymer a vinyl butyral comprising polymer. Reference in the following to PVB is intended to denote any of these vinyl butyral comprising polymers.

According to the invention, the resilient infill comprises foam granules that are rendered tack resistant. This is also intended to include granules that are treated to cause them to be tack resistant and also those that are tack resistant due to the initial choice of polymer (mix). In this context, tack resistant may be defined to comprise granules that are not tacky at temperatures below 90°C, more preferable below 80°C, more preferably below 70°C and most preferably below 65°C. Tackiness is normally assessed by heating a sample in a dish (about 5 mm layer thickness) overnight in an oven at a relevant temperature. The sample is subsequently cooled down and the granules are assessed. If the granules are still loose granules, or can be set free by slight agitation, then the sample is judged as not tacky. If, on the other hand, the granules form a cake this is judged as tacky or not tack resistant. The highest temperature where the pellets remain non-tacky is denoted as the tackiness temperature.

It is hence of importance that the granules are free flowing granules and thus that they do not stick to one another. This generally applies for the case that the granules are used as an infill for artificial turf as the desired properties may only be achieved when the granules are loose and free flowing. It however may also apply during storage of the granules before being used as infill as it is simply not possible to use the material if it comprises lumps of multiple granules.

In an embodiment the resilient infill for artificial turf, comprising foam granules of a vinyl butyral comprising polymer is characterized in that it is non-tacky and/or non-sticky even after storage. Hence it is generally very important that the granules are not sticky or tacky under all use and storage conditions. Additionally it is preferred that the granules of the resilient infill have a high melting point or more preferably no melting point. The skilled person will understand that a material not having a melting point will, upon increasing the temperature, combust before it melts. Preferably the melting point is above 90°C, more preferably above 100°C, more preferably above 110°C, and most preferably above 120°C.

In an embodiment the vinyl butyral comprising polymer is a cross-linked vinyl butyral comprising polymer. Cross-linking of PVB is known in the art and the skilled person knows how to control the degree of cross-linking by controlling the amount of cross-linking agent or by any other means known in the art. Any crosslinking agent that reacts with the polyvinyl alcohol groups on the polymer can be used. Examples of cross-linking agents and their inclusion levels are described below.

Cross-linking of PVB reduces the tackiness at a certain temperature and results in PVB with a higher tackiness temperature. In an embodiment the entire granule comprises cross-linked PVB. However, if desired it is also possible to reduce the tackiness of a PVB granule by only cross-linking the surface of the granule. Such cross-linking is denoted as skin cross-linking. In this context it is understood that the cross-linked skin or surface must have a certain thickness which is preferably less than 2 mm, more preferably less than 1 mm and most preferably less than 0.5 mm. In relative terms it is preferred that the thickness of the skin is less than 25% of the diameter of the granule, more preferably less than 20% of the diameter ofthe granule, even more preferably less than 15% of the diameter of the granule and most preferably less than 10% ofthe diameter ofthe granule.

In an alternative embodiment the tackiness ofthe granules can be reduced using a chemical additive such as a slip additive. Slip additives can be added during production ofthe infill granules and will migrate to the surface of the granule after production. As a result, a slip additive may form a film on the surface which causes a change of the surface parameters including a reduction of the tackiness. Such an additive can be added to the PVB melt during production which will result in a granule having the additive incorporated throughout. Alternatively such an additive can be added in such a manner that it is only present in the skin of the granule. Examples of slip additives are given below.

The resilient infill is usually present on top of a sand layer and between the upstanding turf fibres. In order to fit between the fibres and to give the required performance, the particles have a mean size of between 0.5 mm and 5 mm, more preferably between 1 mm and 3 mm, even more preferably between 1.5 and 2.5 mm, and most preferably between 1.8 and 2.2 mm. It is hereby understood that for generally spherical or cylindrical particles, the size of the particles refers to the diameter of the particles. For irregularly shaped particles, the skilled person will understand that although reference is given to the mean size of the granules being between specific limits, a number of different procedures may be used to determine this size. In the present context, this value is given according to ASTM C136 / C136M -14 “Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates". Not all of the particles in the infill layer need to meet the specification given to the granules. In terms of size, the number of particles within the defined size range may be given by D90 for the upper limit and D50 for the lower limit, more preferably D30 or even D20 for the lower limit (wherein the skilled person understands that a Dxx limit refers to xx% of the particles meeting the relevant limit).

According to an embodiment of the invention, the foam granules comprising the resilient infill have a bulk density of between 0.1 kg/L and 0.65 kg/L, preferably of between 0.2 kg/L and 0.5 kg/L, more preferably between 0.25 kg/L and 0.35 kg/L and most preferably between 0.25 kg/L and 0.35 kg/L. Hereby it is understood that the bulk density refers to the mass of a bulk amount of foam granules divided by the bulk volume of the granules, wherein the bulk volume of the granules refers to the total volume a bulk amount of granules occupies. Alternatively it is possible to define a granular density which is the weight of a granule divided by the volume that it occupies. The average granular density of the resilient infill is preferably between 0.2 kg/L and 1.2 kg/L, more preferably between 0.4 kg/L and 0.9 kg/L, more preferably between 0.45 kg/L and 0.65 kg/L, and most preferably between 0.45 and 0.55 kg/L. It will be understood that the density of the vinyl butyral comprising polymer is typically about 1.1 kg/L. The relevant granular density ranges refer to foamed granules. Hence when a polymer has a density of 1 kg/L and the granule has a granular density of 0.5 kg/L, the foam granule comprises about 50 vol.% of gas, wherein vol.% is used to denote volume %. It is further understood that the average granular density is preferably number weighted.

According to an embodiment of the invention, the foam granules comprising the resilient infill deform rather easily upon exertion of an external force. Preferably the Young’s modulus is between 1 MPa and 10 GPa, more preferably between 0.01 GPa and 1 GPa, more preferably between 0.02 GPa and 0.1 GPa, and most preferably between 0.03 and 0.08 GPa. Alternatively the ease of deformation can be expressed using the Shore A hardness scale. On this scale, it is preferred that the granules have a hardness lower than 50, more preferably lower than 30, more preferably lower than 20 and most preferably lower than 15. Alternatively it is preferred that the granules have a Shore A hardness of between 5 and 50, more preferably between 8 and 30, and most preferably between 10 and 20.

Alternatively the modulus of the granules can be assessed by placing an amount of granules in a fixed volume and determining the deformation upon application of a pressure. A suitable setup comprises a steel cylinder of internal diameter 71 mm which is filled with 200 ml of infill granulate and a steel disc that exactly fits inside the cylinder. The steel disc is used to compress the infill in the cylinder at a rate of 50 mm/min (determined using a tensile tester). The displacement of the disc is recorded at a maximum compressive force of 500 N. In this test a typical elastic infill, such as SBR (granulated tyre infill), has a displacement of about 13 mm. A typical hard infill such as HDPE mini-granulate has a displacement of less than 2 mm and a high-performance infill, such as the commercially available So.F.ter “Holo” infill, has a displacement of about 9 mm. The foamed infill comprising a vinyl butyral comprising polymer of the current invention can be produced in such a manner to give desired displacement properties. Typically the displacement of the infill according to the invention is between 3 mm and 11 mm when measured using the above described test. Alternatively the displacement is between 5 mm and 9 mm and in another alternative it is between 6 mm and 8 mm when measured using the above described test.

In one embodiment, the foam granules comprising the resilient infill have a substantially cylindrical shape. The ends of the cylinder might be pinched as a result of the manufacturing process, such as the cutting of the material into granules. Preferably the ratio of the height of the cylinder over the diameter of its circular base - or the projection of the circular middle of a substantially cylindrical granule on the base area - is between 0.5 and 5, more preferably between 1 and 3, most preferably between 1.5 and 2.5. In another embodiment the foam granules have a substantially spherical shape. Preferably such a granule has a medium to high sphericity. Preferably the granule having a cylindrical shape or the granule having a spherical shape has a sphericity greater than 0.5 or greater than 0.7 or even greater than 0.9, wherein sphericity is defined as the ratio of the diameter of a sphere of equal volume to the granule to the diameter of the circumscribing sphere.

The foam granules comprising the resilient infill may have roundness values of greater than 0.5 or greater than 0.7 or even greater than 0.9, wherein roundness is defined as the ratio of the average radius of curvature of the corners and edges of the granule to the radius of the maximum sphere that can be inscribed. The skilled person will be well aware that the properties of the granule will depend on the roundness, the sphericity and/or the shape of the cylinder.

According to the invention the foam granules comprise gas pockets. In one embodiment the relative volume of gas pockets is at least 10 vol.%, more preferably at least 20 vol.%, even more preferably at least 30 vol.%, even more preferably at least 40 vol.%, even more preferably at least 50 vol.% and most preferably at least 60 vol.%. In an embodiment the foam granule comprises an open celled foam, however most preferably the foamed granule comprises a closed celled foam. Using foam granules has multiple benefits. Firstly, it is the most convenient manner to make a viscoelastic granule from a non-viscoelastic polymer or to improve the viscoelastic behavior of an already viscoelastic polymer. Furthermore, a foam granule uses less polymer to fill the same volume compared to a solid, non-foamed granule. Hence the foam granule is more sustainable and more economical than the solid, non-foamed granule is.

In another embodiment the granules are hollow granules, wherein it is understood that hollow granules can either have an open or a closed cavity inside. Figure 2 shows a drawing of a cylindrically shaped hollow granule which serves illustrative purposes. In a preferred hollow granule the ratio ofthe internal radius describing the size ofthe cavity (n) over the external radius describing be particle size (r2) is at least 0.4, more preferably 0.5, even more preferably 0.6, even more preferably 0.7 and most preferably 0.75.

An alternative manner to limit the amount of expensive or less-sustainable polymer is to include a filler into the matrix of the granules of the infill. In a preferred embodiment the filler is included into the polymeric matrix of a foam granule. The filler can be introduced during production in an amount of up to 50 wt.%, more preferably up to 40 wt.%, even more preferably up to 30 wt.%, even more preferably up to 20 wt.%, and most preferably up to 10 wt.%. Suitable materials include inorganic fillers, such as talcum or calcium carbonate (CaCCh) in the form as chalk or ground calcium carbonate (GCC). Alternatively it is beneficial to use a filler comprising alumina trihydrate (ATH) or magnesium hydroxide (MDH) as these compounds also improve the flame retardant properties ofthe infill.

In a preferred embodiment ofthe invention, the granules ofthe infill comprising vinyl butyral comprising polymer additionally comprise one or more thermoplastic materials. These materials may be selected from the group comprising: polyethylene (PE, LDPE, LLDPE, MDPE, HDPE), polypropylene (PP), polyamides (PA), polyurethane PU), polystyrene (PS), polyolefin elastomers (POE), polycarbonate (PC), polyethylene terephthalate (PET), polyethylene isosorbide terephthalate (PEIT), polyethylene furanoate (PEF), polyhydroxy alkanoates (PHA), polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), polybutylene succinate (PBS), polybutylene adipate co-terephthalate (PBAT), polybutylene terephthalate (PBT), polycaprolactone (PCL), phenol formaldehyde (PF) polypropylene carbonate (PPC), polytrimethylene terephthalate (PTT), polyvinyl chloride (PVC), polyvinyl alcohol (PVOH), thermoplastic starch (TPS) and combinations ofthe above. Mixtures may also include thermoplastic elastomers (TPE) e.g. mixtures of TPE with PE where the PE content is greater than 30%, or TPEs with a high content of polystyrene.

Said granules comprising vinyl butyral comprising polymer and one or more thermoplastic materials comprise at least 50 wt.% ofthe vinyl butyral comprising polymer, more preferably at least 60 wt.% ofthe vinyl butyral comprising polymer, more preferably at least 70 wt.% ofthe vinyl butyral comprising polymer, even more preferably at least 80 wt.% of the vinyl butyral comprising polymer and most preferably at least 90 wt.% ofthe vinyl butyral comprising polymer.

It is understood that granules according to the invention comprise vinyl butyral comprising polymer. In a preferred embodiment the molecular weight ofthe polymer is at least 5.000 g/mol, more preferably at least 25.000 g/mol and most preferably at least 50.000 g/mol. Alternatively the molecular weight ofthe polymer is between 5.000 g/mol and 500.000 g/mol, more preferably between 25.000 g/mol and 500.000, even more preferably between 50.000 g/mol and 500.000 g/mol, and most preferably between 50.000 g/mol and 250.000 g/mol.

In an embodiment of the invention, the infill granules comprise cross-linked vinyl butyral comprising polymer. The skilled person understands that, although a granule consisting of cross-linked polymer molecules can be viewed as one large molecule, the above-mentioned molecular weights correspond to those of the vinyl butyral comprising polymer as were it not cross-linked. This is a normal procedure in the art as the molecular weight of all polymer including cross-linker molecules is not considered a useful parameter.

Alternatively the vinyl butyral comprising polymer can be defined by its melt flow index (MFI) -measured using the ISO 1133 test; protocol D; 190°C; 2.16 kg - of between 0.5 and 10 dg/min, preferably between 0.7 and 7 dg/min, more preferably between 1 and 5 dg/min, even more preferably between 1.2 and 3.5 dg/min, and most preferably between 1.5 and 3 dg/min.

The vinyl butyral comprising polymer can be cross-linked using a cross-linking agent. Any crosslinking agent that reacts with the polyvinyl alcohol groups on the polymer may be used. Examples of such agents are melamine, phenolic resins, dialdehydes, di-isocyanates, epoxy grafted polymer and anhydride grafted polymer. Preferably the inclusion level of this cross-linking agent is between 1wt.% and 15wt.%, more preferably between 1wt.% and 10wt.%, even more preferably between 2wt.% and 7wt.%. In a preferred embodiment, the cross-linking agent is a maleic anhydride grafted polymer, more preferably a maleic anhydride grafted polyethylene. Preferred commercially available cross-linking agents are sold as Fusabond® (ex Dupont), wherein Fusabond® E226 is highly preferred, and as Orevac® (ex Arkema), wherein Orevac® 18341 is highly preferred.

In an embodiment the resilient infill comprises a slip additive. Known examples of slip additives are long-chain, fatty acid, amide waxes, like oleamide and urecamide wax, and silicone gums such as Genioplast Pellet S (ex. Wacker Silicones). A typical inclusion level of a slip additive is 100 ppm - 2000 ppm, more preferably 200 ppm - 1500 ppm, even more preferably 400 ppm - 1200 ppm, even more preferably 600 ppm - 1100 ppm and most preferably 700 ppm - 1000 ppm, wherein ppm refers to parts per million which corresponds to mg of slip additive per kg of polymer.

In an embodiment the resilient infill comprises a plastomerto improve processability as well as durability of the resilient infill. Preferably the inclusion level of the plastomer is below 25 wt.%, more preferably below 20 wt.%, even more preferably below 15 wt.%, and most preferably below 10 wt.%. Alternatively the inclusion level of the plastomer is between 1-25 wt.%, more preferably between 5-15 wt.%, most preferably between 5-10 wt%. In a preferred embodiment the plastomer is an ethylene octane copolymer. Commercially available plastomers include those available under the Queo® brand (ex Borealis), wherein Queo® 8207 and 8210 are preferred, and those available under the Affinity® brand (ex DOW Chemical Company). It is understood that for the purpose of the current invention the terms plastomer and plasticizer have the same meaning and are thus interchangeable.

It is understood that the resilient infill comprising foam granules has pockets of gas trapped in the solid polymer matrix. Said air pockets can be created by directly injecting a gas during production.

Preferred gasses are inert gasses such as carbon dioxide or nitrogen. Alternatively the pockets of gas can be created using a blowing agent. In the art physical and chemical blowing agents are known. Physical blowing agents - such as hydrocarbon, CO2 and nitrogen - are typically introduced as a liquid and are in-situ converted to a gas by a phase transition. Chemical blowing agents produce a gas as a result of a chemical reaction that usually occurs at elevated temperature. In general there are two families of chemical blowing agents: endothermic blowing agents mainly based on bicarbonate which release predominantly CO2 gas, and exothermic blowing agents based on components like azodicarbonamide which release a mixture of CO2, nitrogen and Nf-Lgas. Forthe current invention exothermic blowing agents are preferred as it has been found that they are better controllable and yield a more homogeneous foam.

In the art the term foaming agent is used for chemicals that aid in the formation of a foam. The class of foaming agents includes blowing agents as well as other agents helpful forthe formation of a foam such as stabilizing agents and/or surfactants.

It is understood that the unreacted blowing agent as it is added during production of the resilient infill is chemically different from the reacted blowing agent which is present in the granules. In the art a foam granule that has been produced using a chemical blowing agent and comprises the reaction products of the blowing agent - or perhaps even comprising some of the reaction products as the gas has been replaced by another gas - is still denoted as a foam granule comprising that chemical blowing agent.

Preferably the blowing agent is included during the production of the resilient infill in an amount of between 0.1-2 wt.%, more preferably between 0.2 and 1 wt.% and even more preferably in an amount of between 0.4-0.8 wt.%. Preferably the chemical blowing agent produces a harmless gas that naturally occurs in air, such as CO2. Preferably the blowing agent is Hydrocerol CT3084® (exClariant). Alternatively any other method of processing the polymers of the invention into a foam is also part of the invention. This includes, but is not limited to, mechanical frothing.

The resilient infill according to the invention can be produced using a range of techniques. In a preferred embodiment, the resilient infill is an extrudate that has been produced using an extruder. Using an extruder has a multitude of advantages such as their availability, their size, the combination with mixing and other technology and the possibility to stretch or shear the polymer melt such that the polymer molecules orient in the direction of the stretch or shear. Hence in an embodiment the resilient infill comprises oriented polymer molecules. If beneficial the resilient infill can be produced using coextrusion. In this process a feed of different materials is pushed through a single die. For example a first extruder can provide a feed of PVB and a second extruder can provide a feed of cross-linked PVB. Both feeds are combined in such a manner that a cylinder with a PVB center and a cross-linked PVB outer layer is formed. This cylinder is subsequently cut - directly at the die or after the formation of a strand - to form granules with a PVB core and a cross-linked PVB outer layer. It will be understood that other polymers may be used forthe outer layer, e.g. in order to render the granule tack free while still enjoying the benefits of the use of PVB forthe core.

Alternatively the resilient infill can be produced by forming a foamed melt and cutting or grinding the foamed melt into granules ofthe desired size. This normally yields rectangular granules, or at least granules with rather sharp edges that have different bulk properties compared to round granules.

In an embodiment the resilient infill comprises a mixture of different granules to achieve the desired properties ofthe infill. This mixture can be a mix of 2 or more resilient foam granules wherein the granules differ in density (e.g. the volume of gas pockets in the continuous polymeric material differs). In another embodiment rectangular foam granules are mixed with round, cylindrical and/or spherical granules. Beneficially hollow granules can be added. In another embodiment, the resilient infill ofthe invention is mixed with smooth, hard granules that are characterized by a Shore D value of more than 50 and which have a mean size of between 0.5 mm and 5 mm. Preferably said smooth, hard granules are substantially spherical. According to an embodiment, the infill mixture comprises between 70 vol.% and 50 vol.% ofthe infill according to the invention and between 30 vol.% and 50 vol.% respectively of said smooth, hard granules. More preferably the infill mixture comprises about 60 vol.% ofthe infill according to the invention and about 40 vol.% of said smooth, hard granules. In this context, the volumetric percentages (vol.%) indicate the percentages of granules and soft infill particulates used to constitute the mixture, and relate to bulk volumes defined prior to mixing.

The resilient infill for artificial turf according to the invention can be produced by mixing vinyl butyral comprising polymer with suitable additives, such as a cross-linking agent, a slip additive, a plastomer, and/or a blowing agent, subsequently extruding the polymer / mixture through a die and cutting the product to the desired size or shape. The vinyl butyral comprising polymer is preferably PVB. In this method the cross-linking agent is preferably a maleic anhydride grafted polyethylene at an inclusion level of between 1-10 wt.%. Preferably the plastomer is an ethylene octene polymer that has been included in a level of between 0-25 wt.%. The blowing agent can be any suitable blowing agent, for instance a suitable exothermic blowing agent.

Preferably the mixing takes place in the extruder. This is common practice in the art and the skilled person knows which extruder, mixing screws (single or double screws that are co- or counterrotating) and settings should be used for optimal results. In a preferred embodiment the extrusion is performed in or coupled to a face-cut granulator wherein the extrudate is cut at the die. Examples of such a granulator include underwater granulators, wherein the extrudate is cut at a die which is submerged in a fluid or water bath, as well as water-ring granulators or air-cooled granulators. In addition it is possible to produce the infill according to the invention using a device whereby the extruder and the granulator are not directly coupled, such as is the case in a strand granulator.

The resilient infill ofthe invention can beneficially be used to provide an infill layer in an artificial turf system. Such an artificial turf system can beneficially be used in the construction of a pitch for sports such as, for example, field hockey, football, American football or rugby. Hence the invention also extends to artificial turf systems comprising the resilient infill ofthe invention.

Artificial turf systems comprising the resilient infill ofthe invention beneficially meet the requirements as set by athlete organizations. For example, the Fédération Internationale de Football

Association (FIFA) has published a Handbook of requirements for football turf together with a handbook of test methods for football turf. The handbook of test methods describes many useful methods among which there are methods for determining wearing, weathering, ball roll and rebound. Of particular relevance for the current invention are the determination of shock adsorption (section 11) and the determination of energy restitution (section 13). The shock absorption of an artificial turf system according to the invention, as measured in a field test (FIFA handbook of test methods, October 2015 edition, section 11) is preferably between 50 % and 80 %, more preferably between 55 % and 75 %, even more preferably between 60 % and 70 % and most preferably between 62 % and 68 %. Alternatively the shock adsorption is at least 50 %, more preferably at least 55 %, even more preferably at least 60 %, and most preferably at least 65 %.

The energy restitution of an artificial turf system according to the invention, as measured using the test in section 13 of the FIFA handbook of test methods, October 2015 edition, is preferably lower than 50 %, more preferably lower than 45 %, even more preferably lower than 40 %, even more preferably lower than 35 % and most preferably lower than 30 %. Preferably the energy restitution is higher than 10 %, more preferably higher than 15 %, more preferably higher than 20 %, and most preferably higher than 25 %.

The resilient infill layer can be present at a depth that is sufficient to adequately support the pile fibres over a substantial portion of their length and will depend on the length of these fibres and the desired free pile. In a preferred embodiment, the resilient infill layer has a depth of at least 10 mm. This may correspond to at least the depth of a typical stud being used forthe intended sport. In other embodiments, the resilient infill layer may be present to a depth of at least 20 mm or even to a depth of greater than 30 mm. It will be understood that the final depth will also depend upon whether the resilient infill layer is the only layer on the substrate supporting the pile fibres or if there are multiple separated infill layers present. In the latter case it is furthermore understood that multiple infill layers are only separated on newly build pitches with artificial turf. Upon using the pitch the infill will mix to some extend creating at least a diffuse middle layer comprising both infill materials. In an embodiment, the pile fibres may be at least 40 mm in length or even at least 50 mm in length. Depending on the nature of the sport, they may extend at least 10 mm or at least 15 mm or even more than 20 mm above the level of the infill.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will be better appreciated upon reference to the figures.

Figure 1 shows a cross-section through an artificial turf system 10 according to an embodiment of the present invention. The turf system 10, comprises a stabilised sub-base 12, optionally a resilient layer 13, a woven artificial turf substrate 14 having upstanding pile fibres 16, a stabilising sand layer 17 and a resilient infill layer 18 and 19. In an embodiment the resilient infill consists of one type of foam granules, in which case 18 and 19 represent the same granule. In another embodiment, the resilient infill comprises foam granules of differing properties or comprises foam granules mixed with other infill granules. In this case 18 and 19 indicate the use of granules having different properties.

Figure 2 shows a schematic representation of a hollow, cylindrically shaped granule.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Example 1 PVB infill granulate was extruded on a Berstorff 50 mm twin screw extruder equipped with a melt pump and an underwater granulating system. The formulation consisted of 95% PVB, 0.5% Hydrocerol CT3084 foaming agent masterbatch, 2% UV stabilizer masterbatch and 2% beige colour masterbatch. The resulting granulate had a granular density of approximately 0.75 kg/L. A sample of the these granules was collected for further testing.

Visco-elastic behavior was assessed using the shock absorption test according to the FIFA FQP Handbook of Test Methods 2015 was performed. A sample of artificial grass carpet “MX Elite” (ex Greenfields) was placed on a concrete floor and filled with 30 mm infill material. One sample was filled with the PVB infill granule described above, and a second reference sample was filled with granulated car tyres (known in the art as “SBR"). Subsequently the test was performed according to the prescribed procedure.

Table 1: Results of the shock adsorption test of the PVB infill granulate of the example.

Table 2: Results of the shock adsorption test of the SBR infill.

The results show that the energy restitution (ER) for the material ofthe invention is significantly lower than for the reference material and hence that the artificial grass comprising the PVB infill will resemble natural grass better than the artificial grass comprising the SBR infill.

Example 2 A series of 10 formulations was extruded on a Berstoff ZE26 Basic twin screw extruder, which was equipped with a strand die and strand granulator, so the appearance ofthe product could be evaluated. The details of the formulations are presented together with the extruder RPM in Table 3.

After the experiment the extruded granules were appraised by an expert and the results of this appraisal is presented in Table 4. The amount of crosslinking can be influenced by the amount of Fusabond crosslinker or by the residence time in the extruder (controlled by the extruder RPM - higher RPM results in a lower crosslinking). Note that a strand granulator is sensitive for strand break, but when the materials were granulated using a face-cut granulator this had no influence.

For practical applications it is important that the granules are not tacky as this might cause the formation of lumps during storage or transport as well as during application as infill. To assess the tackyness, a granule sample was taken from every extruded batch and heated in an oven for 1 hour in a flat dish (in a layer of approx. 5 mm thick), at varying temperatures. The samples were then cooled down and checked for mobility ofthe granules. The oven temperature where the granules start adhering to each other was noted and is identified as the tackiness temperature in Table 5.

Table 3: Compositions used in trials 1-10.

Table 4: Assessment of the granule and observations during extrusion.

Table 5: Assessment oftackyness temperature.

Claims (27)

An elastic litter material for an artificial grass field, comprising foam granules of a vinyl butyral-containing polymer, wherein the granules have an average size of between 0.5 and 5 mm and wherein the granules are made resistant to sticking. The elastic infill material according to claim 1, wherein the granules have an average size of between 1 and 3 mm, preferably of between 1.5 and 2.5 mm. The elastic litter material according to claim 1 or 2, wherein the granules are stick-resistant below 90 ° C, preferably below 80 ° C. The elastic infill material according to claim 1,2 or 3, wherein the vinyl butyral-containing polymer is a cross-linked vinyl butyral-containing polymer. The elastic infill material according to claim 1,2, 3 or 4, wherein the foam granules contain a slip additive and preferably wherein the slip additive is a silicone gum. Elastic bedding material according to any of claims 1-5, wherein the granules have a bulk density of between 0.1 and 0.5 kg / L, preferably of between 0.2 and 0.4 kg / L, or more preferably between 0.25 and 0.35 kg / L. The elastic infill material according to any of claims 1-6, wherein the shape of the foam granules is substantially cylindrical and wherein the ratio of the height of the cylinder to the diameter of its round base is between 0.5 and 5. The elastic infill material according to any of claims 1 to 7, wherein the foam granules contain gas bubbles and wherein the relative volume of the gas bubbles is at least 0.1, preferably at least 0.3. The elastic infill material according to any of claims 1-8, wherein the granules are closed-bubble granules. The elastic litter material according to any of claims 1 to 9, wherein the granules have a melting point of at least 100 ° C. The elastic litter material according to any of claims 1 to 10, wherein the vinyl butyral-containing polymer has a melt flow index of between 0.5 and 10. The elastic litter material according to any of claims 1 to 11, wherein the elastic litter material contains between 1-10% by weight of a crosslinking aid, preferably between 2-7% by weight of a crosslinking aid. The elastic litter material according to claim 12, wherein the crosslinking aid is a maleic anhydride-grained polymer, preferably maleic anhydride-grained polyethylene. The elastic litter material according to any of claims 1-13, wherein the litter material contains less than 25% by weight of a plastomer, preferably less than 15% by weight of a plastomer, and more preferably less than 10% by weight of a plastomer contains. The elastic infill material according to claim 14, wherein the plastomer is an ethylene octene copolymer. The elastic litter material according to any of claims 1 to 15, wherein the litter material contains between 0.2 and 1 wt% blowing agent, the blowing agent preferably being exothermic. The elastic litter material according to any of claims 1 to 16, wherein the litter material is an extrudate. The elastic litter material according to any of claims 1 to 17, wherein the vinyl butyral-containing polymer is oriented. A method of making elastic infill foam granules for artificial turf comprising: - providing a vinyl butyral-containing polymer, - melting and optionally mixing the vinyl butyral-containing polymer with one or more additives, - extruding the mixture through a mold and cutting of the product to obtain granules, further comprising providing a component to the melt, extrudate or granules which adjusts the sticking behavior such that the granules become stick resistant. The method of claim 19, wherein the mixing takes place in an extuder. The method of claim 20, wherein the extruder is coupled to a die cutting ("facecut") granulator wherein the product is cut at the die. The method of claim 19, 20 or 21, wherein the additives comprise one or more of a cross-linked adjuvant, a slip additive, a plastomer, a blowing agent or a foaming agent. The method of claim 20, 21 or 22, wherein the foam is formed by direct gas injection. An elastic litter material available according to any of claims 19-23. Use of the elastic infill material according to any of claims 1-18 or claim 24 as an infill material for an artificial grass field system. An artificial grass field system comprising litter material, wherein the litter material comprises the elastic litter material of any of claims 1-18 or claim 24. An artificial grass field system according to claim 26, wherein the energy refund is lower than 40%, preferably lower than 35%.