TECHNICAL FIELD

The present invention relates to artificial turf with improved properties such as playing properties. The aim is to provide a more multifunctional fiber, saving on production and material costs on the one hand, and resembling advantageous properties of natural turf on the other hand. According to the invention a synthetic fiber is disclosed having amongst others, a tuned hydrophilicity, water transport ability, self-cooling, better bending properties, good resilience, longer life-time, reduced friction, anti- electrostatic, controlled biodegradability, anti-bacterial and/or having self-cleaning capability. Furthermore a method for regulating the temperature of an artificial turf is disclosed.

BACKGROUND

Artificial turf research is focused on the development of fibers for use in artificial lawns for user-friendly sports fields. It is known in the art to make a fiber with a core and a shell from different materials, herewith enabling a separation of the inner core and the outer material surface properties. In US 2010/0173102 an artificial turf fiber is disclosed comprising a core and a cladding, wherein the material for the cladding is tuned for hydrophilicity and where the material for the core is optimized for good bending recovery properties. The fiber is here obtained with a co-extrusion step. The use of co-extrusion for producing artificial turf fibers is however economically unattractive, in particular if only one beneficial property is being addressed. In literature more user friendly functionalities for artificial turf are taught on properties like better sliding and longer life time. Other artificial turf fibers are described in US2006/0093783 and US2003/0161996. Both disclosures describe fibers that are spliced in an axially-oriented direction.

SUMMARY

It is an object of the invention to enable and combine more beneficial properties, such as a tuned hydrophilicity, water transport ability, self-cooling, anti-electrostatic, reduced friction, longer life time, improved bending, good resilience, better sliding, UV-stable, flame retardant, biodegradable, anti-bacterial and self-cleaning capability by introducing pre, post and/or co-extrusion steps to obtain a cost efficient and more user friendly artificial turf.

According to the invention the artificial turf comprises at least one fiber, preferably having a core and a shell, characterized in that an outer surface of the fiber, preferably the shell, contains a material that has a contact angle with water of less than 90°. The surface comprises at least one axially oriented groove with a width of less than 500 micrometers, so that the groove is able to transport water over a height of at least a few cm.

The groove may also be formed by the provision on the outer surface of the shell of a ridge or a pair of ridges. Contrary to a groove that extends inwards into the surface, ridges are characterized by extending outward on the outer surface of the shell. By providing a ridges on the surface, the side of the ridge forms one of the sides of a groove, but now extending from the surface. Combinations of grooves and ridges are also possible. Grooves and ridges share the common characteristic that they have have a sharp corner less than 90° in which the capillary effect is present. Throughout the rest of the description and the claims these are together indicated as grooves, unless otherwise indicated.

It is an insight of the invention and surprisingly it has been found that even open grooves are able to transport water by some reduced form of capillary action. Rectangular grooves, round shaped (U-shape) grooves and V-shaped grooves have been found to work. With preference a groove should comprise one or more concave regions having a radius of curvature of at least 2 micrometers, preferably at least 5 micrometers, more preferably at least 10 micrometers and not more than 200 micrometers. U or V- shaped grooves having at its bottom sharp rounded corners have proven to be beneficial for the transport of water against gravity, especially a groove that is at least for a part triangularly shaped, having at its bottom a sharp corner less than 90°, preferably less than 75°, more preferably less than 60°, typically between 5° and 60° and with a preference between 20° and 60° and at its vertex a radius of curvature of at least 2 micrometers and not more than 200 micrometers. The angle of the sharp rounded corners are preferably measured as the angles of the walls of the groove. Grooves with a smaller mean width (cf. radius of curvature) have a substantially larger capillary action than wider grooves. However the flow resistance of grooves with a smaller width rapidly increases. Grooves with a mean width between 20 and 200 micrometer have proven to be able to transport enough water to have a significant cooling effect on the artificial turf by evaporation. The capillary action has been found to be inversely proportional with the value of the water contact angle (cf. wetting) of the shell material. Grooves with higher water contact angles can still transport water when grooves with a smaller width are present in the fiber.

A preferred embodiment comprises a pair of mirrored shaped grooves that together form a common deepened surface with a width that can vary e.g. from 50 micrometer up to many hundreds of micrometer. The deepened hydrophilized surface has a number of advantages: - it will not wear or suffer from mechanical abrasion, even when a thin hydrophilic coating has been applied, -it serves as the cooling/ evaporation area of the water that has been transported via the grooves, - it can mechanically lock the coating or shell material inside this area even when this material has been abrased from other exposed parts of the fiber. With preference the grooves are triangular shaped, such that the direction of a vertex is not orthogonal to the fiber surface, but sideways oriented, adding more strength to the fiber (Figure 4). An additional advantage in the sideways orientation is that there is less risk that the fiber splices form the bottom of the groove while at the same time an improved flow and evaporation can be achieved.

To avoid cutting injuries during use of the artificial turf field it has been found to prefer grooves having at its top rounded corners or convex areas. Grooves having at least a convex area with a radius of curvature of at least 20 micrometer up to 2000 micrometer have proven to be very functional. It has also been found that the top corner of the grooves has little influence on the water transport capability.

Grooves which hold or absorb a higher volume of water are preferred, because of the increased heat conductance and heat capacity of the fiber. Without the water holding grooves much friction heat will be generated during a sliding movement causing skin burns and scrapes. This is because artificial turf can't take up the same amount of heat as natural grass does in a short time. The water absorption capability of the grooved fibers is therefore an important parameter for not only controlling the temperature of the artificial turf through evaporation, but also to absorb the friction heat due to sliding. With preference a fiber therefore comprises at least 2 grooves with a mean width larger than 100 μπι. In the case of smaller grooves the fiber comprises at least 10 grooves with a mean width larger than 20 μπι. Fibers used in artificial turf typically have dimension in the range of 1500 micrometer wide by 200 micrometers thick (resembling natural grass). The number of grooves and the sum of their average widths is more or less limited thereby. Thus the number of grooves may vary between one and 100 grooves per fiber and the sum of the average width of the grooves may vary between 50 and 1000 micrometer. In one embodiment, the width of the grooves at the surface is not more than 80% of the breadth of the fiber, more preferably not more than 60%, more preferably not more that 50%. There is a lower limit for the sum of the average width of at least 3% of the breadth of the fiber, preferably at least 5%, more preferably at least 10%. In preferred embodiments, the sum of the mean width of the axially oriented groove is between 3 and 70 % of the breadth of the fiber, more preferably between 5 and 60% and most preferably between 10 and 50 %. The fiber may be provided with multiple grooves of varying width. Grooves may be provided on one or on both sides of the fiber and may be positioned such that the bottom corner of the groove on one side is adjacent to the bottom corner of the groove on the opposite sides, i.e. the grooves alternate from one side to the other. The depth of a groove typically is in the order of not more than 50 micrometer, preferably not more than 40 micrometer. In embodiments the fiber has a depth to breadth ratio of not less than 2 with a higher ratio such as 3, 4 or 5 preferred since a broader groove has a positive effect on the evaporation.

The fiber may comprise a core material (core) and a shell material (shell). They may have the same composition, but they may also have a different composition.

Whereas for the core of the fiber materials can be selected for e.g. a longer life time, improved bending and a good resilience, it will be clear that for the shell other material selection criteria may hold, such as preserving for a long time an optimum water contact angle (e.g. between 10° and 30°) to enable water transport. It is known that most polymers become more hydrophobic in time with water contact angles above 60°, also grooves may easily get clogged with organic contaminations both issues limiting sufficient water transport. According to the invention measures have been taken to auto-regenerate the water contact angle and to prevent organic contamination. The artificial turf comprising at least one fiber may be one base material, but may also, and preferably be, a two base material resulting in having a fiber having a core and a shell. Core and shell can be made of similar base materials, but can also be made of different base materials. In certain embodiments, the shell base material comprises particles, which may differ in amount or type with respect to possible particles in the core material. Suitable base material for both shell and core are hydrocarbon polymers, such as polyethylene and/or polypropylene which can be independently chosen as a base material. Of course other thermoplastic hydrophilic materials such as polycarbonate, polyolefins, polyamides, polyethyleneglycol, polyvinylalcohol and polyesters such as polylacticacid are equally possible to use as a core and/or shell material. By providing the fiber with a core and a shell, it becomes possible to provide each part of the fiber with a location specific functional property, which property does not need to be present elsewhere in the fiber, or needs to be present only to a distinctly reduced degree. The required properties may also be obtained by adding functional particles, or mixtures of particles with different functionalities to either the core base material or the shell base material. The base materials can however also be completely equal and may contain functional particles, which makes it possible to use a single die extrusion process.

In certain embodiments an additional hydrophilic coating can be provided to the shell in the grooves, preferably in a post-extrusion step, e.g. by applying a vapor, spray or solution coating of polyethylene glycol, polyvinyl alcohol, or a low temperature sol/gel metal-oxide coating. Such coating can also be provided later. The coating can be applied very thin and will not suffer from wear inside the grooves. In a further embodiment, the shell material can be hydrophilized to promote adhesion of the coating to the shell by grafting one or more reactive groups on the shell material, for example maleic acid anhydride is able to react with polyethylene (as the shell/core material), thereby rendering it more hydrophilic.

The base materials of the core and shell can be not exactly identical, but similar, such as hydrocarbon mixtures with a similar glass transition temperature. This makes it possible to obtain co-extruded fibers in an easy cost effective way. The melt extrusion of medium and high molecular weight polymers, such as hydrocarbon polymers into the desired (flat) fiber shape structures is accomplished by well-known procedures wherein a single or a double rotating screw pushes a viscous polymer melt through an extrusion die. The required particles can be added to the outer shell after passage through the die, e.g. when the shell base material is still hot and with preference before stretching the fiber to increase e.g. the strength and bending recovery. The particles can be deposited to the shell in powder form or by passing through one or more baths with the required powder in suspension form. Particles with a size between 2 and 200 nanometer have proven to perform very well, and do not have a negative impact on the required (sharp) geometries of the grooves and vertices with curvatures of 2 micrometer or more.

In a further embodiment according to the invention, the water contact angle can be lowered by the addition of hydrophilic particles to the shell material. With preference the surface of the particles should contain a sufficient number of polar groups, such as hydroxyl groups, that are able to bind water molecules and lower the surface energy. These particles can be molecules of organic origin, such as described in (WO2005111281A1), ionomers, or with preference they belong to the class of metal and semi-metal oxides. Many metal oxide particles are known to have hydrophilic properties, but some of them have also good flame retardant, anti-electrostatic and/or self-cleaning properties. In another embodiment, the surface may be coated with quaternary ammonium ions to lower the contact angle A specific desire for artificial turf is the ability to have 1) a low wettability, when the playfield is not used, such as overnight to inhibit evaporation, 2) a moderate wettability on cloudy days to lower friction, and 3) a high wettability when the sun shines to cool the turf by evaporation and to have increased sliding and reduced skin burning properties. To meet this desire it is preferred that the artificial turf is capable of adjusting hydrophilicity and hence the capillary rise of water with an increasing temperature/light intensity to achieve this. This can be achieved by the incorporation of zinc oxide and/or titanium oxide particles in the artificial turf fibers. Subjecting the turf to light can increase the hydrophilicity of the turf and hence increase its wettability. With preference according to the invention zinc oxide and/or titanium oxide particles are being applied. Zinc oxide has a band gap of 3.5 eV and UV light with a wavelength beneath 350 nm is able to alter the ZnO particles to a super-hydrophilic state. Also upon UV irradiation mild chemical radicals are being created that have mild antibiotic and self-cleaning properties.Likewise titanium dioxide particles can be used or blended with zinc oxide. UV treatment of Ti0 ₂ particles, especially from the anatase form are known to create strong electron-hole radicals that are able to create hydroxyl groups. Adding these Ti0 ₂ particles can thus induce an enhanced tunable hydrophilicity. According to the invention also the life time of the fiber itself can be tuned with these particles. The strong radicals can also cut the C-C organic bonds of the fiber molecules itself, which is a great advantage in biodegradable artificial turf applications. With preference a Ti02 particle powder with a mean particle size smaller than 50 nanometer is being employed. It has been found that this very fine powder becomes more transparent and the normally white pigment effect of titanium dioxide is strongly diminished. Likewise the white pigment effect of Ti02 can be tempered by adding carbon black powder. An advantage of this combined black/white dispersion is that the UV light can herewith be tuned to have a certain penetration depth, herewith controlling the life time of the fiber. The man skilled in the art has now tools to choose the further right pigments to control the demanded color of the fiber for the intended application. Titanium dioxide incorporating a small amount (< 1%) of manganese in the crystal lattice can also be employed in the shell material. This allows absorbed UV energy to be dissipated, virtually eliminating the generation of free radicals. Manganese at the surface of the particle can scavenge free radicals that have been generated. This may significantly extends polymer lifetime under solar exposure. On the other hand titanium dioxide, when spiked with nitrogen ions or doped with metal oxide like tungsten trioxide will have increased photocatalyst properties under either visible or UV light and can be used to promote self-cleaning properties of the fiber, especially preventing the build-up of organic contamination in the grooves. It is also an insight according to the invention that relative many injuries result from a high friction between the artificial turf fibers and the player, because the conventional fibers are smooth and dry and have thus a large friction contact area with the injured parts of the player. The injuring energy transfer is directly proportional with the contact area. Reducing this contact area with low friction particles of a specific size and surface density will be disclosed. The invention teaches the use of PTFE (polytetrafluoroethylene) nanoparticles with a mean particle size between 20 and 2000 nanometer and with a mean surface coverage between 0.1 and 10%. The amount of particles in the fiber varies between particles with about 0.05 to about 5 percent by volume of the (shell) base material. Preferably quasi- spherical particles are used having a preferred weight average particle size of from about 0.1 to about 50 micrometers. Particles having fluorinated molecules have been tested to have extremely low adhesion forces with respect to natural and synthetic materials or attributes of the players. Especially friction forces between skin, cloths or shoes have been tested advantageously. The coefficient of friction of PTFE varies within the range 0.02 - 0.1, and does not depend on the environment. It is as low in oxidizing, as in non-oxidizing moist and dry atmospheres encountered at the artificial turf play field. Also due to non-stick properties of polytetrafluoroethylene there is very small difference between the static and dynamic coefficients of friction. Likewise, of course in some cases also non- or partly fluorinated particles can be employed in the shell material.

A specific demand for artificial turf is the ability to tune the resilience properties. According to the invention for this silicon dioxide particles are known to be hydrophilic but also to improve the bending recovery of many plastics. These particles can thus be added both to the shell and the core material.

The invention in one aspect also pertains to a method for transporting water through a fiber comprising providing a fiber as described herein. The invention further pertains to a method for controlling the wettability and/or temperature and/or evaporation of water and/or of an artificial turf by providing an artificial turf comprising fibers as described herein. By proving an artificial turf with the fibers containing grooves as described herein, the capillary action of the grooves will transport the water, at least over a height of several centimeters, depending on the design, number, width and the structure of the grooves and the presence of hydrophilicity- improving elements such as the herein described particles or coatings. By controlling the transport of the water, the wettability can be regulated as well as the rate of evaporation of the water, which in its turn has an effect on the temperature of the artificial turf. Using the methods described herein in the examples, the skilled person is capable with some routine experimentation to determine the evaporation surface needed the exact design (in terms of number of grooves, particles and coatings) to come to an artificial turf that meets the needs of the user in a specific area. For instance, in a moderate climate with less sun a different turf may be optimal compared to a moist tropical climate with intense sunlight.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: FIG. 1-4 shows schematic embodiments of a cross section of the shell of an artificial turf fiber and

FIG. 5 shows attained heights (against gravity) for transport of water along a groove with a given width.

FIG. 6 shows a SEM image of an array of grooves in a fiber.

FIG. 7 shows the surface temperature of different fiber samples during IR irradiation. FIG. 8 shows the decrease of the contact angle for three different temperatures.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS Fig. 1 schematically depicts a cross section part of the shell of an artificial turf fiber 1 according to the invention with four different grooves 2 with a mean width 3 and a mean depth 4. Semi-circular, rectangular and triangular grooves are depicted, the latter also with a vertex having a concave (less sharp) shape 5. The concave corner 5 avoids the formation of micro shear/crack forming in the shell material.

In Fig. 2 the grooves are filled with water 6 showing a hollow meniscus 7. All grooves with a mean width less than 500 micrometer are capable in transporting water against gravity against a height of at least a few cm. Triangular shapes having at its bottom a sharp corner much less than 90°, typically between 30° and 80° and at its vertex a radius of curvature of at least 5 micrometers and not more than 50 micrometers have proven to function better that semi-circular and square shaped grooves.

In Fig. 3 triangular shaped grooves 2 are depicted with a convex (rounded) area 8 in between with a radius of curvature of at least 20 micrometer up to 2000 micrometer or more, to smoothen the top of the grooves with the outer surface, to avoid cutting injuries.

Fig. 4 depicts cross sections of a preferred embodiment. A pair of triangular shaped grooves 2 is mirrored with respect to each other and together forms a common deepened surface with a width 10 that can vary from 50 micrometer up to a few mm. A rounded vertex 5 to avoid micro-tear/crack formation and a rounded convex area 9 to prevent injuries are depicted. Example 1 Water transport along the fiber

Fig. 5 shows a graphic presentation of different experiments of a hydrophilized shell with grooves having a different width and different bottom profiles:

• half spherical grooves, ■ U-shaped grooves with a slightly rounded bottom, and ▲ V-shaped triangular grooves with a bottom angle of 30° and a relatively sharp vertex according to the invention (see also fig. 1). All grooves were able to transport water very well when the contact angle for water was less than 60°, although the transport of water in semi-circular grooves was significantly slower. V-shaped grooves were able to transport water against gravity to a height of 10 centimeter with a larger mean velocity (typically »lcm/sec) than comparable U-shaped grooves (typically between 0.1 and 1 cm/sec seconds) with the same mean width of 100 micrometer.

Example 2 Cooling effect of the fiber

Fig. 6 shows a SEM image of an array of grooves in a polyester fiber that has been used to measure the cooling effect of the fibers according to the invention. The grooves have an U-shape with a depth of 100 μπι and a width of 90 μπι and on the upper side it comprises a convex area with a radius of curvature of about 80 μπι to prevent cutting injuries. At the bottom of the groove there is a concave curvature of about 5 μπι to avoid micro-crack formation. The grooves in the surface of the fibers have been hydrophilized via a covalent linker method leading to one type (■) with a water contact angle of 56°, and a second type (A) with a water contact angle of 18°. Next a hydrophilized fiber without grooves and two type of hydrophilized fibers with grooves are placed standing upright in a petri dish with water at room temperature before a shield. A reference digital thermometer (X) is placed at the shield. A 250 W IR lamp is situated at 25 cm before the fibers to provide thermal heat by IR irradiation. In Fig. 7 is shown the temperature of the fibers in time as measured with a directed IR thermometer. We see that without hydrophilic grooves the temperature of this fiber type (·) raises up to more than 70 °C. The temperature of the fibers with the hydrophilic grooves (■ , A) remains much lower; around 35-40 °C. The temperature of the shield (X) raises to 50 °C during the experiment, indicating that transport and evaporation of water from the hydrophilized grooves contributes significantly to the cooling power of the artificial turf according to the invention for more than four hours. Example 3 Surface modification of the fiber

A fiber with a content of at least 50% Polylacticacid (PLA) having an initial contact angle of 85-90° is subjected to hydrolysis of the PLA ester groups with a mild caustic solution of 0.1 M at different temperatures. The decrease of the contact angle in time is depicted in Fig. 8 for three different temperatures (■ : 26°C, ·: 50°C,▲: 80°C). At elevated temperature the water contact angle decreases substantially within 10-50 minutes. Due to the hydrolysis of the ester group hydrophilic carboxyl and polar alcohol end groups are obtained. Both of these groups can be further used for further surface modification to make e.g. a bond with a permanent hydrophilic group and/or to obtain another functionality. The coupling of e.g. specific quaternary ammonium groups may give the following functionalities to the grass: antibiotic, herbicidal, antistatic, disinfectant. For this the PLA is first treated with a dimethylaminopropylamine solution under mild conditions, forming a direct bond of the amine groups with the carboxyl groups of the present ester groups in the PLA. Next under mild conditions a butylchloride solution is added to terminate the dimethylamine group with a butyl group. Herewith a quaternary ammonium endgroup is formed. The specific functionality, such as sufficiently hydrophilic or antibiotic, can be obtained by exchanging the butylchloride with octylchloride, etc. The fiber including the grooves can herewith be provided with many types of an enduring protective coating in an environmental friendly manner. Likewise particles, can be added, such as titanium oxide, zinc oxide, colloidal silver or silica with specific groups, to provide the appropriate functionality according to the invention. It will be clear that the invention can easily be extended by the man skilled in the art and is therefore not restricted to the disclosed examples for artificial turf.