The present invention relates to an artificial grass built up of fibres that consist of a core and a cladding, which core and which cladding are composed of a plastic material.

Such a type of artificial grass is known per se from European patent EP 0 996 781 granted to the present applicant, which evolved out of WO 99/04074, in which a polyamide-containing yarn is used for fabricating artificial grass, wherein the yarn, besides polyamide, also contains a polyolefin compound selected from the group consisting of polypropylene, LLDPE and a block copolymer of polypropylene and polyethylene. In addition to the method of manufacturing a yarn that is known therefrom, wherein the polymer is extruded so as to form monofilaments, which are subsequently processed into bands, with several bands being twisted to form a yarn, a method is disclosed in which the yarn is obtained by means of a co-extrusion process. When such an extrusion process is used, the cladding consists of polyamide, whilst the core consists of one of the aforesaid plastics.

From International application WO 2004/106601 there is known a yarn for an artificial lawn in which the yarn is a so-called tape filament consisting of a core layer and two outer layers of a material different from that of the core layer, wherein the core layer contains polyester and/or polyolefin material and wherein the outer layers contain high-density polyethylene.

Japanese patent publication JP 2003 342848 discloses a yarn for an artificial lawn that consists of a conjugated yarn formed of a resin composition comprising 70-99 wt.% nylon (polyamide) and 1-30 wt.% polyethylene as the inner layer and polyethylene as the outer layer.

A well-known phenomenon with artificial grass, in particular if the artificial grass is used for playing soccer thereon, is the occurrence of burns when players make a sliding tackle. This problem does not occur with natural grass, which natural material has a high a water content and which feels soft upon contact, so that the friction with the skin will be high. This high friction is disadvantageous from the viewpoint of skin damage, but because of the fact that the natural grass will wear away slightly upon contact, burns do not occur upon contact with natural grass. In addition to that it can be noted that natural grass is soft and will grow again after having worn away, in which connection it can furthermore be noted that the relatively soft nature of natural grass also contributes to favourable resilient properties.

On the other hand, there is generally a controversy when selecting the materials to be used for artificial grass. The material used for the artificial grass is a soft material, for example, which has a large elastic range. Such a property makes for a good resilience, but also a high degree of friction, which latter aspect will lead to excessive adhesion of the skin to the artificial grass when a sliding tackle is made, and thus to inevitable damage to the skin, which is undesirable. Said materials are evaluated as unsatisfactory as regards the heat development that takes place when a sliding tackle is made, in spite of the fact that because of the high degree of friction the period of contact between the skin and the artificial grass remains limited. The material used for the artificial grass may also be a hard material having a small elastic range, which leads to unsatisfactory properties as regards resilience but also to less friction. The reduction as regards the degree of friction will lead to less skin damage. In addition to that, said hard materials are evaluated as satisfactory as regards the heat development during sliding tackles, because the period of contact is longer.

It is an object of the present invention to provide a special type of artificial grass which imitates the advantageous properties of natural grass, in particular as regards resilience and behaviour during sliding tackles, as much as possible.

Another object of the present invention is to provide an artificial grass which exhibits a high degree of durability and which can be composed of commercially available materials.

According to the present invention, the artificial grass as referred to in the introduction is characterized by the characterising portion of claim 1.

One or more of the above objects are achieved by using such a type of artificial grass. Because a soft material is used for the core, favourable properties as regards resilience can be obtained, without the aforesaid disadvantageous surface properties playing a part. Moreover, because the cladding is built up of a relatively hard material, use is advantageously made of the relatively favourable surface properties, viz. a low degree of skin damage as a result of the low adhesion to the skin. Moreover, the unfavourable properties as regards resilience are minimised by using a hard material for the cladding.

The terms "soft" and "hard" as used in the present description must be interpreted in relation to each other, in which connection it is in particular important that the material for the cladding is harder than the material that is used for the core. The soft material for the core will have an elasticity modulus (according to ASTM D638) of maximally 500 MPa, preferably maximally 300 MPa. The hard material for the cladding will have an elasticity modulus (according to ASTM D638) of minimally 500 MPa, preferably minimally 750 MPa.

In a special embodiment, the material for the cladding has preferably been selected from the group consisting of polyethylene compounds, polypropylene and fluor polymers or combinations thereof, wherein the polyethylene compounds have been selected from the group consisting of high density polyethylene (HDPE), medium density polyethylene (MDPE), low density polyethylene (LDPE), linear low-density polyethylene (LLDPE) and anhydride modified polyethylene compounds.

In a special embodiment of the present invention, the cladding of the artificial fibre is relatively thin, viz. the thickness of the cladding is such that its volume makes up 1-40%, preferably 1-20%, of the volume of the entire fibre. Such a small thickness for the cladding makes it possible to make advantageous use of the high specific heat properties of the core material, which will lead to a reduced occurrence of burns.

The artificial grass according to the present invention is further characterized in that the artificial grass fibre has been obtained by means of a co-extrusion process, wherein the material for the core and the material for the cladding are extruded simultaneously, with the cladding being in direct contact with the core.

According to a preferred embodiment of the present artificial grass, the material for the core is polyamide and the material for the cladding is an anhydride modified HDPE.

The present invention furthermore relates to an artificial lawn built up from an artificial grass as described above.

In a special embodiment, a mixture of, for example, polyamide, in particular polyamide 12, and a modified polyethylene can be used for the core.

To prevent delamination of the core and the cladding, a compatibility agent has been added to the material for the core and/or to the material for the cladding, or to both materials, which compatibility agent enhances the bond between the material for the core and the material for the cladding. The compatibility agent is a copolymer of ethylene and methyl acrylate, with the methyl acrylate content in particular amounting to less than 50%, preferably about 30%.