The invention relates to a loose infill material, ideal for example as a filler material of artificial grass on synthetic playing fields, to a process for producing it and to a machine suitable for performing the process.

The artificial grass 10 of the playing field - see also US 4 735 825 - is usually formed as in fig. 1. On a suitably prepared laying surface 12 an elastic/draining synthetic base under-turf 14 is laid which artificial grass with threads 20 is laid on. As ballast for the grass it is covered with a layer of silica sand 16.

To prevent the sand 16 from causing abrasions to the players, it is covered with what is called "performance infill", a layer of various organic (coconut fibres, turf etc) and/or synthetic (rubber granules) and/or thermoplastic materials.

One infill material widely used on account of the fact that it does not rot and absorbs impact well is the granular rubber made for example by milling used tyres. The tyre is normally reduced to granules of a maximum dimension of 0.8-2.5 mm, coloured green for example and then spread over the synthetic playing field.

The main objective of the invention is to reduce the cost of the filler material, in particular of the performance infill, even more particularly of the granular material. A further purpose is to improve the capacity of the filler material to absorb impact, increasing the comfort and/or safety of the user.

These objectives are achieved by a process according to claim 1. The invention also relates to a machine which performs the method and produces the material, defined in claim 6.

The advantages of the invention will be evident from the following description of a preferred embodiment of the process and the machine, with reference to the appended drawings wherein:

Fig. 1 shows a cross section of a known synthetic playing field; Fig. 2 shows a granule;

Fig. 3 schematically shows the processing of the granule;

Fig. 4 shows the granule obtained by the process,

Fig. 5 schematically shows the working members of a granule producing machine.

The process which the invention relates to is used to produce filler material starting from synthetic granules. In a green-economy perspective of recycling highly pollutant materials, rubber granular material made by mincing old tyres (a material otherwise disposed of as waste) is preferred. Obviously, the rubber to be processed or synthetic starting material may be of different origin.

A starting granule 30 is shown in fig. 2, and usually has maximum dimensions of 2.5 - 4 mm. The invention may however be applied to granules of other dimensions.

The granule 30 is stretched - see fig. 3- on opposite sides with different tangential velocities, indicated as Fl and F2. As a result the granule 30 changes shape: it becomes longer and expands, and acquires an "open" shape as shown in fig. 4 by reference numeral 20.

Compared to the granule 30 the granule 40 is the same weight but has a greater volume so that for the same cost a larger playing area may be filled. The expanded structure of the granule 40 confers significant and greater shock absorbent properties, giving the infill surface excellent play ability and dynamic response.

The speeds Fl, F2 may be equiverse or in opposite directions, it being sufficient to apply to the granule 30 friction forces tending to elongate /stretch it.

The expansion effect of the granule 30 is accentuated if, before or during stretching, its surface is incised, for example with cuts 32. This option has proved very advantageous because the final volume of the granule 40 is even greater with even greater impact absorption properties. By way of precaution it is preferred to cool the granule 30 before and/or during stretching. If the granule 30 is rubber, its stretching may result in sufficient overheating to cause fires. One advantageous variant has proved to be spraying or atomizing a fluid, such as water, on the granule 30 to remove heat.

A preferred embodiment of the machine producing the granule 40 comprises moving members adapted to stretch opposite sides of the granules with different tangential velocities. The machine is schematically shown in Fig. 5 and may comprise, for example, as moving members, two rotating cylinders 50, 52, rotating on parallel axes to rotate with equal angular velocities and having different diameters. The granules 30 are made to pass between the cylinders 50, 52.

Preferably the step of cutting the granule is combined with the stretching step and non-smooth cylinders with a ridged surface or in general such as to mechanically act on the granule 30 so as to cause surface cracks or defects, are used.

It is preferred to fit the machine with means of cooling the granules to avoid, as said, the risk of spontaneous combustion. Among the various options, it has been found simple in terms of construction and highly efficient to fit the machine with:

• means 56 (such as nozzles) for spraying or atomizing a fluid 54 on the granules to remove heat from them, and/or

• hollow cylinders inside which water or coolant liquid is made to circulate,.

The granules are thereby cooled during the phase of maximum stress and heat generation.