DESCRIPTION

[0001] The present invention relates to a method for the treatment of cords for the reinforcing layers of a pneumatic tyre.

[0002] In recent years, one of the priorities in the field of pneumatic tyre research has been that of reducing the noise that pneumatic tyres produce both inside and outside the passenger cabin.

[0003] This necessity clashes with the trend to reduce the weight of pneumatic tyres with the aim of lowering the rolling resistance. In fact, as is known to a person skilled in the art, reducing the weight of a pneumatic tyre, notwithstanding the improvement in terms of rolling resistance that this entails, leads, however, to an increase in the level of noise produced during operation.

[0004] As is known to a person skilled in the art, reinforcing plies are commonly used as a reinforcing material within the carcass, in the belts and in the cap- ply. In particular, said cap-ply is arranged between the belts and the tread belt.

[0005] For some time, part of the research within the sector of pneumatics tyres has been concentrated upon studying new reinforcing plies for the cap- plies of pneumatic tyre, with the aim of obtaining an improvement both in terms of rolling resistance and in terms of the noise produced.

[0006] In particular, a cap-ply has been made comprising a reinforcing ply composed of polyethylene terephthalate (PET) cords, wherein each thereof comprises a single filament having a linear density of between 500 and 3000 dtex. [0007] Cap-plies made using the above plies, even though they satisfy the requirements in relation to lower weight and less noise, have however some critical issues of a mechanical nature, in particular in relation to high speed durability.

[0008] The inventors of the present invention have produced a treatment for the single filament cords of the plies for cap-plies that, in addition to having advantages in relation to lower weight and a lower level of noise, also ensure the required levels of high speed durability.

[0009] The object of the present invention is a method for the treatment of polyethylene terephthalate cords for the reinforcing layers of a pneumatic tyre; said method comprising (a) an adhesive solution coating step, wherein the single filament cord is immersed in an adhesive solution; (b) a drying step, wherein the cord covered in adhesive resin is held within a drying oven at a temperature of between 120 and 180°C; and (c) a thermobonding step wherein the cord, proceeding from the drying step, is arranged within an oven at a temperature of between 230 and 260°C for a period of between 30 and 90 seconds; said method being characterized in that during said thermobonding step the single-filament cord is subjected to a twisting tension of between 8 and 12 mN/dtex.

[0010] Preferably, said thermobonding step is performed at a temperature of between 235 and 250° C.

[0011] Preferably, said thermobonding step endures for a period of between 40 and 80 seconds.

[0012] Preferably, each of said cords has a linear density of between 1000 and 2000 dtex.

[0013] Preferably, each of said cords has a twist factor of between 70 and 180; more preferably between 120 and 160. [0014] The twist factor is represented by the following formula Tf = a ^* Nt ^* T ⁰ · ⁵ , wherein a=8.513 ^* 10 ³ , Nt is the twist level (twists per meter) and T is the linear density (dtex) of the cord.

[0015] A further object of the present invention is a cap-ply made using plies comprising single-filament cords treated according to the method of the present invention.

[0016] A further object of the present invention is a pneumatic tyre comprising a cap-ply wherein the ply thereof comprises single-filament cords treated according to the method of the present invention.

[0017] The following are exemplary non-limiting embodiments given purely by way of illustration.

[0018] Four cap-plies (A - D) were made. Cap-ply A is a comparative example comprising a reinforcing ply made from multi-filament cords and represents the currently adopted solution. The cap-plies B and C comprise a reinforcing ply according to the present invention, whilst cap-ply D is a comparative example, insofar as the single-filament cords have been subjected to treatment wherein the characteristics thereof deviate from those reported within the attached claims.

[0019] In particular, the reinforcing ply of cap-plies B - C was made using single-filament cords of PET, characterized by a linear density within the range of between 1000 and 2000 dtex and a twist factor within the range of between 70 and 140.

[0020] In comparison, the reinforcing ply of cap-ply A was made, in accordance with the practice commonly adopted, using dual-filament PET cords. Each of the PET filaments has a linear density of 1100 dtex and a twist factor number of 104. [0021 ] Each of the cap-plies was made embedding the ply between two layers of rubber compound.

[0022] In Table I the composition in phr is recorded of the compound used for the manufacture of the cap-ply.

Table I

[0023] The natural rubber is the cis-1 ,4-polyisoprene rubber of natural origin.

[0024] The vulcanization accelerator is TBBS (N-tert-butyl-2-benzothiazyl sulfenamide).

[0025] The cap-plies thus made were subjected to elastic modulus and durability measurements.

[0026] The elastic modulus of the cords was measured using a dynamic mechanical analyzer (DMA). The value is calculated based upon the storage modulus measured at a temperature range of between 30 and 160°C, under a preload of 10 N and a frequency of 52 Hz. The elastic modulus is calculated as the ratio between the dynamic force (N) and the dynamic twisting tension (%).

[0027] The durability assessment was based upon the fatigue method described below.

[0028] The organic material cords submerged in a rubber matrix in such a way as to produce parallel layers of cords. In this way a rubber sample for testing was produced. The rubber sample has a width of 50 mm, a length of 500 mm and a height of 5.5 mm. [0029] The number of cords is 50/50 mm; the distance between the cords is 2.5 mm; the distance between the center of the cords and the surface is 1.5 mm. Subsequently, the rubber sample is suspended from a pulley and a load of 50 Kg/inch (19.7 Kg/cm) is applied in the direction of the axis of the cords, and the twisting tension and compression force are applied cyclically with a frequency of 100 rpm, increasing the number of cycles until a tensile strength degradation of 80% is reached. The durability is then calculated based upon the number of cycles required in order to reach the above condition.

[0030] The cap-plies (A - D) above were used to make four tires (Pa - Pd) which differ from one another only insofar as the different cap-ply used.

[0031] The Pa - Pd pneumatic tyres were subjected to rolling resistance, noise and weight measurements.

[0032] The rolling resistance was measured according to the ASTM D5992 standard.

[0033] The noise measurement was performed within a semi-anechoic chamber with the dimensions: 8 m (Length) x 6 m (Width) x 3.5 m (Height), ISO 3744 certified (Sound Power Test).

[0034] The pneumatic tyre was loaded by means of a pneumatic tyre-carrier on a drum coated with a resin shell that simulates the asphalt used for external BPN (British Pendulum Number) approval tests. The measuring equipment was composed of 11 microphones spaced angularly in the same manner in order to evaluate the noise directivity. An algorithm (developed by the Applicant) makes it possible to calculate the predicted external BPN value (r ² =0.9) at 80 Km/h.

[0035] Table II reports the technical characteristics of the cords and of the method by which they are made, of the respective cap-plies and of the respective pneumatic tyres. In order to more clearly show the advantages of the invention, in Table II a few values are indexed to the relative value of the comparative example (cap-ply A), and some of the pneumatic tyre characteristics are indicated as a variation of the respective value of the pneumatic tyre Pa taken as a reference (Ref.).

Table II [0036] These measurements confirmed that the pneumatic tyres Pb - Pd: (i) have 2% lower rolling resistance compared to that of the pneumatic tyre Pa; (ii) produce 0.4 - 0.7 dB lower noise than that produced by the pneumatic tyre Pa; (iii) have less weight than the pneumatic tyre Pa. [0037] Furthermore, the values reported in Table II demonstrate how the method of the present invention, in addition to satisfying the demands in terms of weight and noise, ensure durability values that are comparable with those of the classic plies included in cap-ply A. In fact, the values in relation to cap-plies B and C, wherein the plies thereof were made according to the present invention, have high-speed durability values that are significantly higher than those of cap-ply D, wherein the plies thereof were made using a method that differs from the method according to the present invention. In particular, the cords of the ply in relation to cap-ply D, during the thermobonding step, were subjected to a lower twisting tension than that indicated as an essential feature in claim 1.

[0038] To summarize, the method, the object of the present invention, has the great advantage of producing plies that are capable of ensuring a decrease in weight and a decrease in the noise of the related pneumatic tyre without, for this reason, compromising the high-speed durability.