DESCRIPTION

The invention relates to a process for manufacturing a noise reducing layer. Furthermore, the invention relates to a tyre comprising the noise reducing layer housed in the inner cavity of the tyre.

One of the noises produced by a tyre in use is the resonance noise of the cavity generated by the vibration of air under pressure inside the inner cavity of the tyre.

In particular, the rolling of the tyre on a surface with a macro-roughness excites the air inside the tyre. The column of air trapped in the tyre has its own natural modes.

Its first natural mode, called cavity mode, ranges between 180 and 250 Hz for a typical passenger car tyre, depending on the size of the envelope. When the column of air starts resonating, it creates acoustic pressure on the wheel, which causes it to vibrate. The wheel then transmits its vibrations to hub of the wheel, then to the passenger compartment through structure-borne propagation. The cavity noise perceived inside the car usually resembles the one of a panpipe and it is known as cavity resonance noise. To reduce this type of noise, the use of a porous material applied to the surface of the impermeable layer of the inner cavity of the cavity, so that the cavity resonance noise is reduced by the porous material, has long been known. The porous material works by reducing the section available to the propagating pressure waves and contributes by dissipating the pressure waves.

In case the sealing action and the noise reducing action need to be combined in one single tyre, its inner cavity has to simultaneously house both the sealant layer and the noise reducing layer. Obviously, taking into account the mechanism with which the sealant layer operates, experimented solutions have adopted a sealant layer arranged in contact with an air impermeable layer and a foam material layer arranged, in turn, in contact with the sealant layer on the opposite side relative to the air impermeable layer.

Herein and below, for "air impermeable layer" we mean a layer substantially impermeable to air, and used in tubeless tyres in order to make sure that the air inside the carcass remains as much as possible under pressure.

These solutions turned out to be ineffective because, as prior art documents (e.g. WO2017/076531) pointed out, the foam material physically interacts with the sealant layer and forbid the flowability of the sealant.

Therefore, there is a strong need to rely on a solution that allows manufacturers to use, in the inner cavity of a tyre, both the sealant layer and the noise reducing element, leaving, at the same time, the respective sealing and noise reducing properties unchanged.

The inventors of this invention offer an effective noise reducing layer that can be used alone or together with a sealant layer, without suffering from the above-mentioned drawback of the prior art.

The subject-matter of the invention is a process for manufacturing a noise reducing layer designed to be housed inside the cavity of a tyre; said process being characterized in that it comprises:

- a stacking step, during which a stack of fibre- reinforced rubber or pre-preg layers is created, in which between one layer and the other there is interposed a plurality of anti-sticking elements spaced apart from one another; said anti-sticking elements being arranged in a staggered position along the two faces of a same fibre- reinforced rubber or pre-preg layer;

- a vulcanization step, during which said stack (1) of fibre-reinforced rubber or pre-preg layers is subjected to a compacting pressure and to a fibre-reinforced rubber or pre-preg vulcanization temperature; the anti-sticking elements being in a liquid state.

Hereinafter by "anti-sticking element" we mean a material that, when placed between two rubber layers, prevents said rubber layers, during the vulcanization step, from being subjected to chemical bonds, thus being glued to one another.

The use of the anti-sticking element in the liquid state is preferred as it ensures a quicker and more effective removal thereof at the end of the manufacturing of the fibre- reinforced rubber or pre-preg structure.

The anti-sticking elements are arranged between the rubber layers and are preferably spaced apart from one another by a same distance and are staggered by half said distance along two opposite faces of a same fibre-reinforced rubber or pre-preg layer.

By so doing, a honeycomb fibre-reinforced rubber or pre-preg structure can be obtained.

Said compacting pressure preferably ranges from 1.5 to 2.5 MPa.

Preferably, the fibre-reinforced rubber or pre-preg structure comprises a fibre fabric consisting of a plurality of cords parallel to one another; more preferably, the fibre- reinforced rubber comprises a fibre fabric with a net structure; even more the fibre-reinforced rubber or pre-preg comprises a fibre fabric with a net structure in which the cords intersect one another at a right angle.

The fibre fabric with a plurality of cords parallel to one another or, even better, with a net structure in which the cords intersect one another at a right angle is preferred as, during the manufacturing of the fibre-reinforced rubber or pre-preg structure, it ensures a better structural resistance when it undergoes a great compression during the vulcanization step.

Preferably, said fibre is Nylon.

Preferably, the fibre-reinforced rubber or pre-preg structure has a honeycomb structure.

A further subject-matter of the invention is a pneumatic tyre comprising a tread, a carcass defining an inner cavity and a noise reducing layer, which is arranged in said inner cavity in the area of the tread; said tyre being characterized in that the noise reducing layer comprises a fibre-reinforced rubber or pre-preg structure with through cells; said through cells of having a cross area ranging from 100 to 20000 mm ² ; said fibre-reinforced rubber or pre- preg structure having a height ranging from 15 to 40 mm.

More preferably, said through cells have a cross area ranging from 500 to 9000 mm ² ; even more preferably said cells have a cross area ranging from 1000 to 5000 mm ² .

Preferably, the fibre-reinforced rubber or pre-preg structure has a height ranging from 20 to 35 mm.

Preferably, the fibre-reinforced rubber or pre-preg structure comprises a fibre fabric consisting of a plurality of cords parallel to one another; more preferably, the fibre- reinforced rubber or pre-preg comprises a fibre fabric with a net structure; even more preferably, the fibre-reinforced rubber or pre-preg comprises a fibre fabric with a net structure in which the cords intersect one another at a right angle.

Preferably, the fibre is Nylon.

Preferably, the fibre-reinforced rubber or pre-preg structure has a honeycomb structure.

Preferably, said noise-reducing layer comprises foam material layer placed to cover a surface of said grid facing the inner cavity. Preferably, said foam material is made of polyurethane.

Preferably, said noise-reducing layer comprises a membrane or plate placed to cover a surface of said grid facing the inner cavity. More preferably, said membrane or plate is perforated. Even more preferably, said perforated membrane or plate is made of expanded polypropylene.

Preferably, the pneumatic tyre comprises a sealant layer placed between an air impermeable layer and said noise reducing layer.

Hereinafter you can find the description of embodiments of the invention, by mere way of example, with the aid of the accompanying figures, wherein:

- figure 1 shows a semi-finished product of the fibre- reinforced rubber structure according to the invention; and

- figure 2 shows the fibre-reinforced rubber structure according to the invention.

- figure 3 shows a first embodiment of the invention;

- figure 4 shows the embodiment of figure 3 with the presence of a sealant layer;

- figure 5 shows a second embodiment of the invention;

- figure 6 shows a third embodiment of the invention;

- figure 7 shows a fourth embodiment of the invention;

The example below uses a fibre-reinforced rubber structure consisting of a 6.6 Nylon fabric with a net structure in which the cords intersect one another at a right angle and of a rubber layer which incorporates said fabric and whose composition in phr is shown in Table I.

Table I

Six layers made of the above-mentioned fibre-reinforced rubber and with a thickness of 1.5 mm were stacked on top of one another so as to form a stack according to figure 1, where it is indicated with number 1. A plurality of anti-sticking elements 2 were interposed between one layer and the other, so as to avoid, during the vulcanization, the creation of a chemical bond between two adjacent layers and, hence, a gluing thereof. In particular, the anti-sticking elements used is a silicon spray which is marketed with the trade name "RESINA P8".

The stack 1 was inserted into a suitable mould, where it was subjected to a pressure of 2 MPa and to a temperature of 145°C for an amount of time of 30 min.

After the vulcanization step, the stack 1 was opened by pulling the two end layers in opposite directions, thus obtaining the fibre-reinforced rubber structure 3 according to the invention.

The fibre-reinforced rubber structure 3 is a honeycomb structure with cells with a hexagonal shape, each having a cross area of 900 mm ² and a height of 30 mm.

As can be evident to a skilled man of the art, the above method can be effectively repeated using a pre-preg material.

In figure 3, number 4 indicates, as a whole, a tyre according to the invention. The tyre 4 comprises a tread 5, a carcass 6 defining an inner cavity 7 of the tyre 4, an inner-liner 8 facing the inner cavity 7 and designed to make sure that the air contained in the inner cavity 7 remains under pressure, and the fibre-reinforced rubber structure 3 arranged inside the cavity 7 in order to attenuate the resonance cavity noise generated by the operating tyre.

The fibre-reinforced rubber structure 3 comprises a honeycomb structure defining a plurality of through cells.

In the other figures (3 - 7) the parts of the respective tyres identical to those of the tyre 4 will be indicated with the same numerical reference and will not be described again.

Figure 4 shows a tyre 11 that differs from that of figure 3 only by the presence of a sealant layer 12.

Figure 5 shows a tyre 13 that differs from that of figure 3 only by the presence of a membrane 14 placed to cover a surface of said grid facing the inner cavity 7.

Figure 6 shows a tyre 15 that differs from that of figure 3 only by the presence of a perforated membrane 16 placed to cover a surface of said grid facing the inner cavity 7.

The materials and the thickness of membrane 14 and the perforated membrane 16 can be decided according to purposes. For example, it can be made of expanded polypropylene—and are 3 mm-thick.

Figure 7 shows a tyre 17 that differs from that of figure 3 only by the presence of a foam material layer 18 made of polyurethane and placed to cover a surface of said grid facing the inner cavity 7.

When a grid structure of the invention is laid down in the tyre cavity, it can work as the noise reducer. Additionally, when put the grid structure on the sealant layer surface, it covers only a minimal fraction of the sealant area, typically less than 5%. That means that the sealant is free from impediments in the remaining 95% of the area. When a puncture occurs in the area where the grid structure of the invention is present, sealing performances are still guaranteed by the repairing sealant flow coming from the surrounding cells free surface areas.