TECHNICAL SECTOR

The present invention relates to a method for the construction of a pneumatic tyre.

PRIOR ART

As it is known, a pneumatic tyre comprises a toroidal carcass, which has two annular beads and which supports an annular tread. A tread belt is interposed between the carcass and the tread which comprises a number of tread plies. The carcass supports a pair of lateral sidewalls arranged between the tread and the beads. An innerliner is arranged within the carcass ply which is impermeable to air, constitutes an inner lining and has the function of retaining the air within the pneumatic tyre in order to maintain the inflation pressure of the pneumatic tyre itself over time.

Run-flat pneumatic tyres differ from traditional pneumatic tyres insofar as they meet, even at zero pressure, certain requirements regarding stability and safety at a well-defined maximum speed (for example 80 km/h) and for a limited number of kilometers (for example 80 km) . In run-flat pneumatic tyres each sidewall is reinforced in such a way as to increase the mechanical resistance to crushing, and therefore in such a way as to allow the pneumatic tyre to be used also when deflated at a limited speed and for journeys that are not too long. Typically, a run-flat pneumatic tyre is provided with a pair of annular reinforcing inserts that are arranged within the pneumatic tyre at the sidewalls of the pneumatic tyre itself. In particular, each reinforcing insert is interposed between the carcass and the innerliner.

The increase in weight of a run-flat pneumatic tyre due to the effect of the reinforced sidewalls in comparison to the weight of a pneumatic tyre of the traditional type is not, however, negligible. Furthermore it has been experimentally verified that under inflation conditions of the pneumatic tyre, the vertical rigidity of a run-flat pneumatic tyre with reinforced sidewalls is higher than the vertical rigidity of a pneumatic tyre of the traditional type. In recent years, whilst maintaining equal performance and vibration absorption capability, the development of pneumatic tyres has been directed towards a reduction in weight.

DESCRIPTION OF THE INVENTION

The aim of the present invention is to provide a method for the construction of a pneumatic tyre that is free from the disadvantages of the state of the art and that is, in particular, easy and inexpensive to implement.

According to the present invention, a method for the construction of a pneumatic tyre is provided, as set forth in the annexed claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the attached drawings, illustrating a non- limiting exemplary embodiment, wherein:

- Figure 1 is a schematic cross-section, with parts removed for clarity, of a pneumatic tyre constructed by means of a method derived according to the present invention;

- Figure 2 is an enlarged scale view of a detail of the pneumatic tyre of Figure 1 ;

- Figure 3 is a view of the pneumatic tyre of Figure 1 with the inserts deformed as a result of the deflation of the pneumatic tyre itself and to which a load is applied; and

- Figures 4 and 5 compare the vertical deflection of three different pneumatic tyres under differing inflation conditions and according to the applied load.

PREFERRED EMBODIMENTS OF THE INVENTION

In Figure 1, the number 1 denotes as a whole a pneumatic tyre of the run-flat type comprising a toroidal carcass 2, consisting of a single body ply 3.

On the opposite sides of the carcass 2 two annular beads 4 are arranged, each of which has a bead core 5 that is reinforced with a number of revolutions of a metallic wire and a bead filler 6. The body ply 3 extends up to the bead cores 5; according to a variant, the body ply 3 is partially folded onto itself (laterally presenting two mutually overlapping layers) in such a way as to surround the two bead cores 5.

The carcass 2 supports an annular tread 7; between the carcass 2 and the tread 7, a tread belt 8 is interposed, which comprises two tread plies 9. Each tread ply 9 comprises a number of cords (not shown) , which are embedded within a rubber belt, are arranged alongside one another with a given pitch and form an angle of inclination determined in relation to an equatorial plane of the pneumatic tyre 1.

Above the tread belt 8 (and therefore between the tread belt 8 and the tread 7) there is a reinforcing layer 10, which is commonly referred to as the "cap ply" and which constitutes a continuous green rubber ribbon that is internally reinforced with strands of nylon or similar which is longitudinally wrapped over the tread belt 8 and which has the function of protecting and containing the tread plies 9.

According to one embodiment, between the body ply 3 and the outer ends of the tread belt 8, two tread ply inserts (TPI, not shown) are interposed which have the function of a cushion .

Within the body ply 3, over the entire available surface, an innerliner 11 is arranged which is impermeable to air, constitutes an inner lining and has the function of maintaining the air within the pneumatic tyre 1 in order to retain the inflation pressure of the pneumatic tyre 1 itself over time.

The body ply 3 supports a pair of sidewalls (SW) 12 arranged externally to the body ply 3, arranged below the tread 7 and extending up to the beads 4.

According to a first variant, the sidewalls 12 are partially and externally clad with mini sidewalls (MSW, not shown) which are typically arranged at the same level as the tread 7, and therefore, within a contact zone, they are arranged side by side with the tread 7 itself. Finally, according to a further variant, the body ply 3 also supports a pair of abrasion gum strips (AGS, not shown) arranged externally below the sidewalls 13 and at the beads 4.

Finally, the pneumatic tyre 1 comprises a pair of annular inserts, which mirror one another and are indicated with 13. The inserts 13 are arranged on the opposite sides of the carcass 2, are arranged within the body ply 3 (in other words, they are arranged on the opposite side of the body ply 3 in relation to the sidewalls 12), within an area substantially comprised between the tread 7 and the beads 4.

The inserts 13 are implemented in order to increase the mechanical resistance to crushing, and therefore in order to allow the pneumatic tyre 1 to be used even when deflated (at a limited speed and for journeys that are not excessively long) .

According to a preferred variant shown in Figures 1, 2 and 3, the inserts 13 are externally attached to the innerliner 11 (i.e. the inserts 13 are directly attached to the innerliner 11 from the opposite side of the innerliner 11 in relation to the body ply 3) . According to a second variant, not shown, the inserts 13 are interposed between the innerliner 11 and the body ply 3 (i.e. the inserts 13 on one side are in direct contact with the innerliner 11 and on the opposite side are in direct contact with the body ply 3) .

Each insert 13 is provided with a respective inner surface 14 that is intended to implement the bonding with the innerliner 11 and having a shape that is complementary to the shape of the body ply 3 comprised between the tread 7 and each bead 4.

Furthermore, each insert 13 is provided with a respective outer surface 15 facing the inside of the pneumatic tyre 1 (i.e. facing the other insert 13) . Each outer surface 15 is a concave surface, in particular with the concavity facing the body ply 3. In Figure 2 the tangent to the outer surface 15, at a point P of maximum thickness of the insert 13, is indicated with the letter t.

Furthermore, in Figure 2, the upper edge (at the tread 7) formed by the outer surface 15 and the inner surface 14 is indicated with Ei, whilst the lower edge (at the bead 4) formed by the outer surface 15 and the inner surface 14 is indicated with E ₂ . The two edges Ei and E ₂ are connected by means of a segment A in Figure 2.

In Figures 1 and 2 the following quantities are also indicated:

- H maximum height of the entire pneumatic tyre 1 (from the upper end of the tread 7 to the lower end of the bead 4) at a reference internal pressure for a reference rim;

- v volume of each insert 13; and

- V total volume of the cavity defined between a rim 16 and the pneumatic tyre 1 and without the inserts 13.

In particular, the reference rim is defined in accordance with what has been established by the industrial standards that are valid for the region wherein the pneumatic tyre is manufactured or used, such as, for example, the E.T.R.T.O. {European Tyre and Rim Technical Organisation) standards. Furthermore, the internal reference pressure corresponds to the air pressure value at maximum load capacity.

The ratio of the segment A and the height H is instead comprised between 35% and 80%. In particular, the ratio between the segment A and the height H is preferably comprised between 45% and 50%.

Finally, the ratio of the volume v of the single insert 13 and the volume V is comprised between 3% and 30%. In particular, the ratio of the volume v and the volume V is preferably comprised between 5% and 10%.

The inserts 13 are produced from a polymer matrix foam. Preferably, the polymer matrix foam is of the closed- cell type. According to a first variant, the polymer matrix foam is a rubber-based foam. According to a further variant, the polymer matrix foam is a polyurethane foam. The density of the polymer matrix foam is between 50 kg/m ³ and 800 kg/m ³ . Preferably, the density of the polymer matrix foam is comprised between 200 kg/m ³ and 700 kg/m ³ ; in more detail, the density of the polymer matrix foam is preferably comprised between 250 kg/m ³ and 600 kg/m ³ . The empty/full ratio of the polymer matrix foam is between 35% and 95%; preferably the empty/full ratio of the polymer matrix foam is comprised between 50% and 90%. The Poisson's ratio of the polymer matrix foam assumes a positive value of between 0 and 0.5. Preferably, the Poisson's ratio of the polymer matrix foam assumes a positive value of between 0.2 and 0.45.

The modulus of elasticity (or Young's modulus) of the polymer matrix foam assumes values of between 0.3 and 9 MPa. Preferably, the modulus of elasticity (or Young's modulus) of the polymer matrix foam assumes values of between 0.5 and 6 MPa.

In Figure 3, the inserts 13 are deformed as a result of the deflation of the pneumatic tyre 1 and of the application of a load; under these conditions, the inserts 13 act as air springs, wherein the air trapped in the cells of the polymer matrix foam provides support for the pneumatic tyre 1 even at zero pressure, for example in the case of a puncture. It was experimentally verified that the inserts 13 of the type described above make it possible to obtain a reduction in weight of the insert 13, which corresponds to a reduction in the weight of the entire pneumatic tyre compared to a run- flat pneumatic tyre of the traditional type with reinforced sidewalls, in such a way as to maintain, with respect to the latter, a similar mechanical resistance to crushing (for example by means of a pair of annular reinforcing inserts) .

In addition, as shown in Figure 4, it has been experimentally demonstrated that under standard inflation conditions of the pneumatic tyre 1, the vertical rigidity of a run-flat pneumatic tyre of the traditional type with reinforced sidewalls (RFT) is increased compared to the vertical rigidity of a pneumatic tyre of the traditional type (STD-NO-RFT) ; in other words, with the same applied load, the deformation that a pneumatic tyre of the traditional type is subjected to is greater than the deformation that a run-flat pneumatic tyre of the traditional type with reinforced sidewalls is subjected to. The vertical rigidity of a pneumatic tyre fitted with inserts 13 (FOAM-RFI) increases in comparison to a conventional pneumatic tyre (STD-NO-RFT) by a lesser extent than a pneumatic tyre with reinforced sidewalls (RFT) ; this means that, for a given applied load, the deformation that a pneumatic tyre with reinforced sidewalls (RFT) is subjected to is less than the deformation that a pneumatic tyre fitted with inserts 13 is subjected to.

Under conditions of zero pressure, as a result of the deflation of the pneumatic tyre 1, the vertical rigidity of a run-flat pneumatic tyre of the traditional type with reinforced sidewalls is instead substantially the same as a pneumatic tyre provided with inserts 13 (and considerably higher than the vertical rigidity of a pneumatic tyre of the traditional type) .

The following describes the method for the construction of a pneumatic tyre 1 provided with the inserts 13 of the type herein described and intended to increase the mechanical resistance to crushing.

In the case wherein the inserts 13 are attached to the innerliner 11, the method firstly provides for the bonding of the tread ply inserts (if present) to the stretched body ply 3 and the incorporating of the reinforced bead cores 5 and the bead fillers 6. The body ply 3 is subsequently wound around a forming drum (not shown) in order to give the body ply 3 an annular shape, overlapping the two opposite ends of the body ply 3. Subsequently, the innerliner 11, which has the function of rendering the pneumatic tyre 1 impermeable, is applied to the inner surface of the body ply 3 (i.e. the opposite surface in relation to any tread ply inserts) . Finally the formation of the green pneumatic tyre 1 is completed by also bonding the other components to the body ply 3, such as the sidewalls 12 and the tread 7. Once the assembly of the green pneumatic tyre 1 has been completed, the latter is subjected to a vulcanization process in special molds in order to thereby obtain a semi-finished pneumatic tyre 1. The semi-finished pneumatic tyre 1, in order to complete the production process, is subjected to a further processing step wherein the inserts 13 are attached to the innerliner 11.

The inserts 13 are produced during a processing step that is independent of the semi-finished pneumatic tyre 1 forming process. In particular, the inserts 13 are obtained by means of an extrusion step or else the molding of the polymer matrix foam.

According to a first variant, the inner surfaces 14 are attached to the innerliner 11 by means of heat bonding. In other words, the inserts 13 are attached to the innerliner 11 before the hardening of the semi-finished pneumatic tyre 1 is completed. According to a preferred variant, the heat bonding between the inserts 13 and the innerliner 11 is performed by means of the application of a substantially uniform layer of adhesive material.

According to a second variant, the inner surfaces 14 are attached to the innerliner 11 by means of cold bonding. In other words, the inserts 13 are attached to the innerliner 11 once the hardening of the semi-finished pneumatic tyre 1 has been completed. According to a preferred variant, the cold bonding between the inserts 13 and the innerliner 11 is performed by means of the application of a substantially uniform layer of an adhesive substance.

It is evident that this second variant makes it possible to render the construction method universal and easily adaptable to any type of pneumatic tyre 1. The inserts 13 that are intended to increase the mechanical resistance to crushing can be applied to any type of pneumatic tyre 1 and, in essence, it is possible to transform any pneumatic tyre 1 into a run-flat type of pneumatic tyre 1 with improved stability and safety characteristics, even at zero pressure.

In contrast, in the case wherein the inserts 13 are interposed between the innerliner 11 and the body ply 3, the method firstly provides for the bonding of the tread ply inserts (if present) to the stretched body ply 3 and the incorporating of the reinforced bead cores 5 and the bead fillers 6. The polymer matrix material that will constitute the inserts 13 is applied to a forming drum (not shown) . The innerliner 11, which has the function of rendering the pneumatic tyre 1 impermeable, is also applied to the forming drum. According to a first variant, the innerliner 11 is applied to the polymer matrix material that will constitute the inserts 13. Alternatively, the polymer matrix material that will constitute the inserts 13 is interposed between the innerliner 11 and, during a later step, the inner surface of the body ply 3 (in other words, first the innerliner 11 and then the polymer matrix material that will constitute the inserts 13 is applied on the forming drum) . Finally, the body ply 3 is wound around the forming drum in order to give the body ply 3 an annular shape, overlapping the two opposite ends of the body ply 3.

Finally the formation of the green pneumatic tyre 1 is completed by also bonding the other components to the body ply 3, such as the sidewalls 12 and the tread 7. Once the assembly of the green pneumatic tyre 1 has been completed, the latter is subjected to a vulcanization process in special molds in order to thereby obtain a finished pneumatic tyre 1. According to a first variant, the foaming reaction of the polymer matrix material interposed between the innerliner 11 and the body ply 3, in order to allow for the formation of the inserts 13, is completed at the same time as the rubber matrix vulcanization process.

According to a further embodiment, the foaming reaction of the polymer matrix material may alternatively be completed at a different time (previous or subsequent) to that of the completion of the actual vulcanization step of the green pneumatic tyre 1.

In particular, in the case wherein the foaming reaction of the polymer matrix material is faster than the polymer matrix material vulcanization step, the method provides, in sequence: a polymer matrix material extrusion step (whilst still flat and not folded) ; a pre- vulcanization step, wherein the polymer matrix material foaming reaction is performed and, simultaneously, partial vulcanization of the polymer matrix material (the pre- vulcanization step is preferably conducted with pressure values of between 0 and 500 kPa) ; a step of applying the insert on the forming drum prior to the application of the body ply 3; and a completion step for the vulcanization of the polymer matrix material in special molds in order to obtain a finished pneumatic tyre 1.

In the case wherein the foaming reaction of the polymer material is slower than the polymer matrix material vulcanization step, the method provides, in sequence: a polymer matrix material extrusion step (whilst still flat and not folded) ; a step of applying the insert around a forming drum before the application of the body ply 3; a vulcanization step, wherein the polymer matrix material foaming reaction is activated and, simultaneously, vulcanization of the polymer matrix material in special molds; and a post-vulcanization step in order to be able to complete the polymer matrix material foaming reaction, which is conducted at pressures that are lower than the pressures of the vulcanization step.

It is clear that the method herein described can find advantageous application even in the case wherein the inserts 13 are attached to innerliner 11 by applying the polymer matrix material directly to the innerliner 11 (as opposed to the inner surface of the body ply 3) . In this case as well, during the vulcanization process, the foaming reaction of the polymer matrix material applied to the innerliner 11 in order to allow for the formation of the inserts 13 takes place.

According to a further variant, the pneumatic tyre 1 is at least partially devoid of the innerliner 11 at the pair of sidewalls 12; the inserts 13, being impermeable to air, constitute an inner lining with the function of retaining the air within the pneumatic tyre 1 in order to retain the inflation pressure of the pneumatic tyre 1 over time. In other words, the innerliner 11 is at least partially absent at the inserts 13.

According to a first variant, the innerliner 11 extends within the body ply 3 at the tread 7 and at the beads 4. Preferably, the inserts 13 are attached on the inside of the cavity to the body ply 3 and to the innerliner 11 in such a way as to overlap, at least partially, the end portions of the innerliner 11 itself in order to ensure perfect air tightness.

According to another variant, the innerliner 11 extends within the body ply 3 at the tread 7 while the inserts 13 are arranged within the body ply 3 below the tread 7 extending up to the beads 4. Preferably, the inserts 13 are internally attached to the body ply 3 and to the innerliner 11 in such a way as to overlap, at least partially, at the end portion of the innerliner 11 in order to ensure perfect air tightness.

In this case, the method for the construction of the pneumatic tyre 1 firstly provides for the bonding of the tread ply inserts (if present) to the stretched body ply 3 and the incorporating of the reinforced bead cores 5 and the bead fillers 6. Subsequently, the innerliner 11 is applied to the inner surface of the body ply 3 within the central zone and, optionally, at the beads 4. The body ply 3 is subsequently wound around a forming drum (not shown) in order to give the body ply 3 an annular shape by overlapping the two opposite ends of the body ply 3 and in such a way as to obtain a green pneumatic tyre.

Finally the formation of the green pneumatic tyre 1 is completed by also bonding the other components to the body ply 3, such as the sidewalls 12 and the tread 7. Once the assembly of the green pneumatic tyre 1 has been completed, the latter is subjected to a vulcanization process in special molds in order to thereby obtain a semi-finished pneumatic tyre 1. In order to complete the production process, the semi-finished pneumatic tyre 1 is subjected to a further processing step wherein the inserts 13 are internally attached to the body ply 3 and, according to a preferred variant, at least partially overlap at the ends of the innerliner 11.

The inserts 13 are produced during a processing step that is independent of the semi-finished pneumatic tyre 1 forming process. In particular, the inserts 13 are obtained by means of an extrusion step or else the molding of the polymer matrix foam.

According to a first variant, the inner surfaces 14 are internally attached to the body ply 3 by means of heat bonding. In other words, before the hardening of the semifinished pneumatic tyre 1 is completed, the inserts 13 are internally attached to the body ply 3. According to a preferred variant, the heat bonding between the inserts 13 and the body ply 3 is performed by means of the application of a substantially uniform layer of adhesive material .

According to a second variant, the inner surfaces 14 are internally attached to the body ply 3 by means of cold bonding. In other words, once the hardening of the semifinished pneumatic tyre 1 has been completed, the inserts 13 are internally attached to the body ply 3. According to a preferred variant, the cold bonding between the inserts 13 and the body ply 3 is performed by means of the application of a substantially uniform layer of an adhesive substance .

According to a variant, the pneumatic tyre 1 is completely devoid of the innerliner 11 at the pair of sidewalls 12. The body ply 3 is manufactured in such a way as to be provided, over the entire surface available on the inside of the cavity, with a layer of rubber that replaces the innerliner 11. In particular, the rubber layer is impermeable to air and defines an inner lining that is capable of retaining the air within the pneumatic tyre 1 in order to maintain the inflation pressure of the pneumatic tyre 1 itself over time. The inserts 13 are attached on the inside of the cavity to the body ply 3 (in particular, at the rubber layer) in order to further improve the air tightness. As described previously, the inserts 13 are hot or cold bonded to the rubber layer.