Artificial Turf Fiber with Undulated Boundary Lines

The invention relates to the field of artificial turf, and more particular to artificial turf fibers.

Synthetic grass fields (or artificial turf) have been used for years to provide a surface that simulates natural grass. These synthetic grass fields have many benefits over natural grass and, in addition, can be installed and used in places that do not allow for natural grass fields, for example, in regions where it is particularly hot and dry. Artificial lawns, like artificial grass sport fields, require less maintenance and can

be used more intensively than lawns of natural grass. Although attempts are made to make the synthetic fibers used for the production of artificial turf look as natural as possible, for example by adding green pigments to the fibers or selecting the fiber cross-section to resemble the cross-section of certain grasses (as described e.g., in EP000003480344A1), in certain situations the artificial turf can still leave a visual impression that is clearly different from that of natural grass and appear “artificial” or “unnatural”.

In addition, an exact reproduction of certain types of grass can lead to the fibers being mechanically unstable or not being able to be produced in the desired form. Thus, the production of artificial turf fibers that give an artificial turf a natural appearance for as long as possible during the entire period of use still represents a major technical challenge.

US patent U.S. Pat. No. 10,793,973 B2 relates to a synthetic monofilament fiber for use in an artificial lawn which has multiple tapered elevations which are believed be associated with increased risk of skin abrasion, and an increased proneness to wear and tear and the associated generation of microplastic waste.

EP 1 950 350 A1 discloses various fibers, some of which have large bulbs at the center and on the ends. These fibers have stress points at the point the bulbs are connected to the fiber. As a result, these types of fibers have a tendency to fray or split along these stress points.

A further fiber is disclosed in U.S. Pat. No. 649,1991B2 which has a curved cross section with a series of flat, planar sections which may lead to decreased mechanical stiffness and other undesired properties.

Korean patent KR 10-1989-0002109 discloses a spinneret for manufacturing monofilaments for artificial turf. In cross-section, the spinneret has an outer periphery formed of continuously repeated triangles of the same size, and an inner periphery formed of continuously repeated curved parts of the same size, for forming a monofilament that by definition has a cross-section that is the same as the cross-section of the spinneret. Since the triangles and the curved parts are connected to neighboring triangles and curved parts for forming the outer periphery and inner periphery, respectively, then a slope (which may also be referred to as tangent line) is indeterminate at each connection point on each respective periphery. That is, the slope at each connection point between neighboring triangles and curved parts on each respective periphery is indeterminate, or in other words, the slope as measured at each point along the respective peripheries has a discontinuity at each connection point. If each point on each respective periphery is defined with respect to a cartesian coordinate system x-y to have coordinate (x,y), then the slope at each point is dy/dx.

It is an objective to provide for an improved artificial turf fiber and artificial turf comprising the same. The objectives underlying the invention are solved by the features of the independent claims.

In one aspect, an extruded artificial turf fiber includes first and second end portions, and a middle portion having a curved cross-sectional shape. The curved cross-sectional shape is defined by a first boundary line and a second boundary line opposite the first boundary line, where the first boundary line consists of first uninterrupted undulations and the second boundary consists of second uninterrupted undulations, and where either a width of the curved cross-sectional shape as measured between the first and second boundary lines is constant along a length of the curved cross-sectional shape, or the first uninterrupted undulations have a phase offset from the second uninterrupted undulations and/or the first and second uninterrupted undulations have different, equal or modulated spatial frequencies.

A modulated frequency of an undulation as used herein is a frequency that has been altered in accordance with a modulating function or scheme. The modulation may be described as a variation of the amplitude, frequency, and/or phase in accordance with a function or schema. The modulation may, for example, result in an increase or decrease of the undulation frequency and/or amplitude of the undulations of the first and/or second boundary line from one fiber profile end to the other.

The effect of a boundary line consisting completely of uninterrupted undulations may be beneficial because the entire boundary line of the fiber is free of planar areas, pointed elevations and pointed depressions.

This can provide a highly advantageous compromise between mechanical durability, wear resistance and a natural look and feel: the multiple undulations cause the fiber to scatter incident light and therefore appear dull, like most natural grass fibers.

As explained above, some prior art artificial turf fibers have a boundary line comprising a series of elevations or depressions to scatter incident light and provide a matt surface impression which is similar to the look of a natural grass fiber surface. However, some prior art fibers have fiber profile contours with multiple successive concave depressions or multiple successive convex elevations. Such an outline has several disadvantages: series of concave depressions result in thin, pointed protrusions. These can lead to a very rough surface, especially when using relatively hard, mechanically robust polymer material, which in turn can lead to skin damage. In addition, these pointed protrusions are subject to high mechanical stress, resulting in a large amount of material being abraded in a short period of time. This abrasion can end up in the environment as unwanted microplastic waste. Series of convex bumps in turn create thin, conical depressions. Dirt and unwanted germs can accumulate in these depressions and negatively affect the appearance and hygiene of the artificial turf. In addition, such conical depressions, especially if they are large, represent a mechanical weak point where the fibers can easily tear (splice) under mechanical stress.

To the contrary, a shape with a contour consisting of uninterrupted undulations according to embodiments the invention has the advantage that the incident light is diffusely scattered, so that a matt, natural surface impression is created, without having to accept problems regarding the risk of injury, microplastics, hygiene or mechanical integrity of the fibers. In a fiber cross section of a fiber according to embodiments of the invention, all depressions and indentations of the fiber surface are rounded, or in other words, a tangent line may be formed or defined (i.e., a tangent line is determinate) at each point on boundary lines that define the fiber in cross section, thereby minimizing the risk of splicing, the risk of skin burns, the generation of microplastic and the accumulation of dirt and debris. That is, if each point on the boundary lines is defined with respect to a cartesian coordinate system x-y to have coordinate (x,y), then the slope at each point is dy/dx, and according to an embodiment of the present invention, the boundary lines have a continuous slope as measured at each point along the boundary lines. In other words, the boundary lines have no discontinuities in slope.

A further benefit may be that the extrusion process can run true to shape. This means that the shape of the fiber cross section essentially matches the shape of the extrusion die profile opening. As the boundary line of the fiber profile is free of pointed protrusions or indentations, also the extrusion die profile is free of such pointed protrusions or elevations. As a consequence, the formation of speed differences of the extruded polymer mass during extrusion which may result in deformed fibers may be prevented.

The width of the curved cross-sectional shape as measured between the first and second boundary lines being constant along a length of the curved cross-sectional shape, advantageously result in an increase in mechanical stability of the fiber (e.g., increase the fiber's elasticity, or in other words, its ability to stand up again and resume its original curved cross-sectional shape after the fiber is repeatedly trampled down).

The first uninterrupted undulations having a phase offset from the second uninterrupted undulations and/or the first and second uninterrupted undulations having equal spatial frequencies imply that the width of the fiber is not constant. For example, the fiber may be thicker at the center of the fiber than at the distal portions of the fiber close to the ends. This may provide the fiber a more natural appearance, as many natural grass species also have a thickened, comparatively stiff central portion and more flexible, thinner arms. A fiber with a curved profile, i.e., a fiber with a concave side and a convex side, will have a concave-side outline that is shorter than the outline of the convex side. If both sides have equal spatial frequencies, the fiber width cannot be constant, as the curvature will introduce an offset between the undulations on both sides.

According to other examples, the first and second uninterrupted undulations different, e.g. modulated spatial frequencies. This may more faithfully represent the natural appearance of grass fibers.

The width of the fiber may hence not be constant in some embodiments of the invention.

According to some examples, the first and second ends of the fiber have a radius of curvature being at least as large as (or larger as) the smallest radius defining the undulations of the boundary line of the other parts of the cross-sectional shape.

For example, the ends may have a radius of curvature being at least 5% larger, e.g., at least 10% larger, e.g., at least 15% larger, e.g., 25% larger than the smallest radius defining the undulations of the boundary line of the other parts of the cross-sectional shape.

This may be beneficial as the ends will have a curvature based on a radius which is at least as large, and possible larger, than the smallest radius defining one or more of the undulations of the boundary line of other fiber parts, e.g., the center.

This may be beneficial as it protects the fiber against abrasion and also eases the manufacturing process: filigree fiber ends may result in a strongly reduced flow rate of the polymer matrix at the respective portions of the extrusion nozzle opening, which may result in a significant deviation of the shape of the extruded fiber from the shape of the extrusion nozzle opening. A further advantage of the above-mentioned fiber end curvature is that incident light is diffusely scattered even when it falls on the ends of the fibers. This is because a large radius of curvature at the fiber ends ensures that the incident light hits a relatively wide surface at the ends, so that the light scattering behavior of the fiber surface at its ends is similar to the scattering behavior at its wide inside and outside surfaces. With artificial turf, due to the industrial manufacturing process, there is always a risk that the synthetic lawn optics will depend on the viewing angle, as the fibers can have an unnatural-looking uniform orientation or distribution, for example. Because the ends have a comparatively large curvature radius, the light is scattered similarly at the ends as in the wide side and it is less noticeable if the majority of the fibers should have the same orientation.

According to some examples, the fiber has thickenings at the fiber ends. The widths (or in the case of circularly-shaped fiber ends, the “diameter”) of each of the thickenings is thicker than the thickest part of the middle portion of the fiber. For example, the middle portion of the fiber may have a width (i.e., a thickness) that is constant along a longitudinal direction of the fiber, or a width that is variable (i.e., non-constant) along the longitudinal direction.

For example, the diameter of each of the thickenings of the end portions may be at least 5%, e.g., at least 10%, e.g., at least 15%, e.g., 25% thicker than the thickest part of the middle portion of the fiber. For example, the diameter of the thickening may be the width of the thickening measured along a line perpendicular to the curved longitudinal axis of the fiber.

This feature may have similar beneficial effects like the use of the above-mentioned use of fiber end undulation having a radius of at least a certain size. It is possible, however, that the boundary line of the thickened ends comprises multiple undulations and hence cannot be described by the size of a single curvature radius.

Applicant has observed that thickenings at the fiber end portions may increase the mechanical stability of the fiber and increase its ability to stand up again after the gras was trampled down. As the thickenings are rounded, the damage caused by abrasion at the fiber end portions is reduced compared to fibers lacking a rounded thickening at the fiber arms.

According to some examples, the curved cross-sectional shape has an outer, convex boundary line and an inner, concave boundary line. At least 70%, in particular at least 80%, e.g., 100% of the undulations of the outer boundary line are defined by first circles () having the same first diameter. At least 70%, in particular at least 80%, e.g., 100% of the undulations of the outer boundary line are defined by second circles having the same second diameter.

For example, the first and second diameters can be identical or similar, wherein a similar diameter lies in a range of plus or minus 10% of the other diameter.

The largely uniform wave shape of the bounding line may have the advantage that there are no particularly deep wave valleys where the fiber thickness is reduced to such an extent that weak points are created at which the fiber tears open under mechanical load. The risk of splicing is thereby further reduced. Likewise, there may not exist particularly high protrusions which may be particularly prone to wear and tear. This may further help to prevent the generation of microplastic waste.

According to some further examples, the fiber comprises a thickening at its center which forms a rounded protrusion to at least one side of the fiber. The curvature of the protrusion is defined by a circle having a radius selected such that a ratio of the said radius to the radius of the first circles is in the range of 1.40 to 1.80, in particular 1.50 to 1.70, in particular 1.55 to 1.68.

According to some further examples, the radius of curvature of the fiber ends is selected such that a ratio of the radius of curvature of the fiber ends to the radius of the first circle is in the range of 1.40 to 1.80, in particular 1.50 to 1.70, in particular 1.55 to 1.68.

According to a further example, the cross-section of the fiber is shaped like the arc of a segment of a circle. This circle is referred herein as the “fiber profile circle” and the radius of this circle the “fiber profile circle radius”. According to some embodiments, the ratio of the width of the fiber profile and the fiber profile radius is in the range of 1.40 to 1.80, in particular 1.50 to 1.70, in particular 1.55 to 1.68.

Applicant has observed that this ratio provides for a particularly “natural” look of a synthetic yarn made of respective fibers. Without the intention to be bound by any theory, applicant believes that this effect may be the result of said ratio value range approximately representing the “golden ratio”.

If one divides a stretch of an elongated object into two parts, of which the smaller part relates to the larger part as the larger part relates to the whole, then one speaks of the so-called ‘golden ratio’. In this case, the relationship of the larger part to the smaller part is: 1.618 [ . . . ] and also the ratio of the whole to the larger part is 1.618 [ . . . ]. In a circle, this “golden ratio” corresponds to an angle of 137.5 degrees. And this is exactly the most common arrangement of leaves and flowers around a plant stem in nature. For example, the golden ratio can be found in the arrangement of leaves and inflorescences of many plants. In these plants, the angle of two successive leaves divides the full circle in the ratio of the golden section. For example, the petals of the rose are arranged according to the golden ratio.

According to some examples, the fiber comprises a nucleating agent.

This may have the advantage of further increasing the surface roughness, because the nucleating agent may induce or boost the formation of polymer microcrystals at the surface of the fiber during or after the extrusion process.

For example, the nucleating agent may be a substance or substance mixture selected from a group comprising: talcum; kaolin (also known as “China clay”); calcium carbonate; magnesium carbonate; silicate: aluminum silicate and; as e.g. sodium aluminosilicate (in particular zeolites of natural and synthetic origin); amorphous and partially amorphous silica and mixed morphologies hereof, e.g. fumed silica; silicic acid and silicic acid esters; e.g. tetraalkyl orthosilicate (also known as orthosilicic acid ester) aluminum trihydrate; magnesium hydroxide; meta- and/or polyphosphates; and coal fly ash (CFA); coal fly ash is a fine recovered e.g. from coal-fires of electric generation power plants; wherein the organic nucleating agent consists of one of the following items or a mixture thereof: 1,2-cyclohexane dicarbonic acid salts (also known as main component of “Hyperform®”); in particular calcium salts of the 1,2-cyclohexane dicarbonic acid; benzoic acid; benzoic acid salt; the benzoic acid salt may be, in particular, an alkaline metal salt of the benzoic acid (e.g. sodium and potassium salts of the benzoic acid); and an alkaline earth metal salt of the benzoic acid (e.g. magnesium and calcium salts of the benzoic acid); sorbic acid; and sorbic acid salt.

According to some examples, 0.01%-3.0% by weight of the artificial turf fiber consists of the nucleating agent. preferably, 0.2%-0.4% by weight of the artificial turf fiber consists of the nucleating agent. This is a comparatively low amount. Nevertheless, applicant has observed that this small amount is sufficient to achieve a diffuse light scattering that is almost indistinguishable from the light scattering on natural grass. It is possible to use only very small amounts of the nucleating agent, because the diffuse scattering is not only caused by the crystals on the fiber surface, but also by the undulations of the fiber profile. Using only very small amounts of the nucleating agent (or none at all) may be beneficial as the crystalline portions induced by the nucleating agent at the surface and within a fiber may increase the brittleness of the fiber, thereby increasing the tendency to break or splice.

According to some examples, the cross-section of the fiber is shaped like the arc of a segment of a circle, or an arc of segment of an ellipse, or an arc of a segment of a horseshoe, or an arc of a segment of a U, or an arc of a segment of a Q. Applicant has observed that a cross-section of the fiber being shaped like an arc of a segment of an ellipse, an arc of a segment of a horseshoe, an arc of a segment of a U, or an arc of a segment of a 22 may have the advantage of providing fibers which are particularly robust against the flattening of the fiber during production or use. It has been observed that small curvatures do not always recover their original shape (as produced during the extrusion process) after being compressed or flattened during transport through rollers and spinnerets or when subjected to a weight, e.g. the weight of a player or a ball. To the contrary, a strong curvature as observed in a segment of a horseshoe, a segment of a U, or a segment of a 22, provides an intrinsic elasticity and ability to recover the original shape. The use of fiber profiles with a boundary line that is curved like a circular segment arc can have the advantage that light falling from different directions is scattered homogeneously because the curvature of the fiber profile is the same when viewed from all directions. This also means that the light reflected by the artificial turf looks the same when viewed from different angles. As a result, even a synthetic turf that has a too uniform orientation of the fibers due to the manufacturing process does not have any artificial dependence of the optical impression on the viewing angle.

According to some other examples, the cross-section of the fiber is shaped like a catenary. The catenary is a particular type of arced curve which is particularly robust against mechanical stress.

Using an artificial turf fiber having a cross-section shaped like a catenary may strengthen the ability of a downed fiber to straighten up quickly. With artificial turf, the problem exists that synthetic fibers that have been depressed by the ball or the players need several minutes or even hours to straighten up again. In some cases, the fibers do not straighten at all. This has the disadvantage that the footprints of the players are visible on the turf for a longer period of time, because the bent-down, essentially horizontally oriented fibers reflect the light differently. The footprints in artificial turf are therefore visible for a certain time as highly reflective, bright, shiny areas. This not only looks unnatural and unattractive; it can even lead to spectators and players being dazzled in strong sunlight. By using a fiber profile having a cross-section shaped like a catenary, the fiber becomes particularly mechanically stable. The tendency of the fiber to buckle under low loads is reduced, and the ability of the fiber to quickly straighten up again after a temporary load-induced buckling increases. Thus, this special shape not only supports the mechanical robustness of the fibers, but also has the particular effect of giving the turf a more natural-looking overall appearance, as footprints are no longer as strong or visible for as long.

According to some examples, the fiber comprises a thickening at its center (i.e., at a center of the middle portion of the fiber).

This may help increasing the mechanical strength of the fiber and to increase the ability of the fiber to quickly straighten up again after a temporary load-induced buckling.

For example, the thickening at its center can be formed such that a round bulge (or protrusion) is formed towards the outer surface of the fiber, wherein the thickening does not lead to a bulge (or protrusion) towards the inner surface of the fiber.

According to some examples, the fiber has a width (w) measured as a straight line connecting the first and second ends of 0.7 to 2.5 mm, in particular of 0.9 to 1.5 mm.

According to some examples, the undulations are formed such that the fiber has at least 6, in particular at least 7, in particular 7-11, e.g., 9 round bulges on its outer surface, and/or such that the fiber has at least 6, in particular 6-10, e.g., 8 round bulges on its inner surface.

According to some examples, the majority of undulations form consecutive pairs of a round bulge and a round indentation, wherein each pair has a length of 0.10 mm to 0.30 mm, e.g. 0.10 to 0.20 mm.

According to some examples, at least 70% of the undulations of the inner surface and at least 70% of the outer surface have the same or a similar undulation length, wherein an undulation length is similar to a given length if it differs no more than plus or minus 10% from said given length.