DESCRIPTION

The present invention relates to a bladder for use in pneumatic tyre vulcanization molds.

As is known to a person skilled in the art, the pneumatic tyre production process provides that after the assembly step, in which the various portions of the pneumatic tyre are assembled together, the green tire (the pneumatic tyre that has not yet been vulcanized) thus obtained is subjected to a vulcanization step. The pneumatic tyre vulcanization step is generally performed inside a mold that confers the final shape to the pneumatic tyre.

During the vulcanization step, the green pneumatic tyre is compressed between the walls of the mold by means of the action of a bladder that, by swelling, pushes from the inside the pneumatic tyre against the walls of the mold.

As it may seem obvious, the bladder must be made using a material which is elastic but, at the same time, is able to withstand the mechanical and thermal stresses that the vulcanization step implies, and which is capable of ensuring several usage cycles of the bladder itself. To date, one of the most widely used materials for the manufacture of the bladder is butyl rubber. Despite the fact that butyl rubber has excellent resistance to mechanical stress , heat and elongation, nonetheless it causes adhesion problems with the internal parts of the pneumatic tyre during the vulcanization step . In fact, once the conditions for vulcanization are obtained, the butyl rubber remains glued to the internal parts of the green tire , i . e . , the innerliner layer .

To obviate this inconvenience , antiadhesive substances are normally used, which are appl ied to the surface of the bladder and to the inner surface of the pneumatic tyre . The use of these antiadhesive substances necessarily entails both an issue of production nature , due to the need for an additional step for their application thereof , as well as , with regards to speci fic technologies , to their removal thereof , and an issue of chemical nature , due to the possible contamination of the innerliner layer .

The prior art document JP2010221505 describes a bladder to be used in vulcanization molds for pneumatic tyres that does not require antiadhesive substances to be applied to the surface of the bladder itself , nor to the inner surface of the pneumatic tyre . The proposed solution consists in providing a bladder comprising several layers of rubber, wherein the furthest out layer contains a silicone rubber and one or more inner layers contain a rubber other than silicone rubber . However, a bladder consisting of multiple rubber layers has the disadvantage of having poor construction efficiency and involves the possibility of detachment phenomena between the different layers due to the materials ' incompatibility

Therefore, a need was felt to have a bladder made of a single material that could withstand the thermal and mechanical stresses and which, at the same time, would not adhere to the inner parts of the pneumatic tyre and, therefore , would not require the use of antiadhesive substances .

The inventors of the present invention have manufactured a bladder using rubber compounds based upon organopolysiloxanes that are capable of satisfying the aforementioned requirement .

The obj ect of the present invention is a bladder for a pneumatic tyre vulcani zation mold made from a rubber compound; said bladder being characteri zed in that it comprises a single layer of a rubber compound comprising 100 phr of a cross-linkable polymeric base consisting of an organopolysiloxane , wherein the maj ority of organic groups is methyl ; from 20 to 100 phr of silica ; and from 1 to 3 phr of a vulcani zing agent belonging to the class of peroxides .

Preferably, said organopolysiloxane is methyl vinyl silicone . More preferably, the methyl vinyl silicone has a vinyl content of between 0 . 01 and 0 . 5% . Preferably, said rubber compound comprises from 0 to

20 phr of a thermoplastic elastomer. More preferably, said thermoplastic elastomer is selected from styrene-butadiene- styrene (SBS) , styrene-ethylene-butadiene-styrene (SEBS) , styrene-ethylene-propylene-styrene (SEPS) copolymer, styrene-isoprene (SIS) copolymer, thermoplastic polyester (TPC) , thermoplastic vulcanizates (TPV) , still more preferably it is styrene-ethylene-butadiene-styrene (SEBS) .

Preferably, said rubber compound comprises from 40 to 70 phr of silica.

Preferably, said silica has a surface area (BET) of greater than 150 m ² /g.

Preferably, said rubber compound comprises from 2 to 10 phr of anti-structuring agents included in the class of alpha-mega-siloxanediols or the class of alkoxysilanols.

Preferably, said vulcanizing agent belonging to the class of peroxides is di-tert-butyl 1 , 1 , 4 , 4-tetramethyl tetramethylene diperoxide.

A further object of the present invention is a pneumatic tyre vulcanization mold comprising a bladder according to the present invention.

Still a further object of the present invention is a pneumatic tyre manufactured with a vulcanization step that uses a mold comprising a bladder according to the present invention . The following are three examples of embodiments of the invention for illustrative and non-limiting purposes only .

Three organopolysiloxane-based compounds were produced, wherein the compositions thereof , in phr, are given in Table I .

Table I

The methyl vinyl silicone 1 used has a vinyl content equal to 0 .2% .

The methyl vinyl silicone 2 used has a vinyl content equal to 0 . 1% .

The TPE used is styrene-ethylene-butylene-styrene ( SEBS ) .

The silica used has a surface area (BET ) equal to 170 m ² /g . The compounds of Table I, were processed according to the classical procedure and were subjected to a vulcanization step at a temperature of between 160 and 200°C for a period of between 5 and 10 minutes and a pressure of greater than 3.5 Mpa. After the vulcanization step, the compounds were subjected to a post-vulcanization step within a ventilated air oven at a temperature ranging from 190 to 200°C for a period of less than or equal to 4 hours.

Once the vulcanization and post vulcanization steps had been implemented, the rubber was subjected to mechanical tests. Tables II and III give the values from the mechanical tests. In particular, the following were measured: M 100% and M 300% measured according to the ISO 37 standard (2017 edition) ; elongation at break measured according to the ISO 37 standard (2017 edition) ; tear strength measured according to the ASTM D624-C standard (2012 edition) ; Shore A hardness measured according to the ISO 48-4 standard (2018 edition) ; dynamic stiffness measured according to the ISO 4664-1 standard (2011 edition) ; De Mattia crack growth at 1000 and 6000 cycles, measured according to the ISO 132 standard (2017 edition) .

Table II

Table I I I

The data reported in Tables I I and I I I demonstrate how the selected compounds are able to produce a bladder, wherein the mechanical characteristics thereof are capable of satisfying the necessary requirements in order to be used in a mold for pneumatic tyres .

In particular, the bladder for tire manufacturing should ensure heat resistance and elongation to maximize its service life . For this reason, the physical properties of the bladder compound need to be balanced taking into account that during service life one of the main failure modes is due to an increase on the modulus that leads to crack formation . The selected compounds provide satisfactory values of elongation at break together with improved flex properties (crack growth) , and satisfactory values of Shore Hardness A and tear strength to resist the repeated cycles. The values of dynamic stiffness of all compounds ensure the prevention of buckling effects.

An adhesion test was performed in order to check the adhesion of the bladder to the inner portions of the pneumatic tyre, wherein the rubber portions made using the aforementioned compounds were placed in contact with an innerliner rubber and subsequently subjected to vulcanizing conditions within a vulcanization mold. It has been verified for all examples of the invention, by means of a torque test according to the ASTM 1876 standard (edition 2015) , that the rubber made using the compounds of the present invention does not exhibit any phenomenon regarding adhesion to the innerliner rubber.

In addition, a bladder according to the invention was subjected to aging using ozone according to the ISO 1431 standard (2012 edition) . The M 100% of the bladder was measured using a dynamometer, before and after being subjected to a known and constant concentration of ozone, under isothermal conditions and for a predetermined period of time . Under the aforementioned conditions , a retention module index was calculated o f between 5 and 30% .

The ozone aging tests gave excellent results , which can be explained by the absence of double bonds within the polymer that constitutes the bladder .

From the above description, it follows that the inventors of the present invention have developed a new bladder for pneumatic tyre vulcani zation molds that is capable of avoiding the use of antiadhesive substances . As a result, the bladder made of silicone rubber, which is the obj ect of the present invention, compared to the prior art bladder made totally or partially of butyl rubber, has advantages both in terms of productivity, by avoiding the step of applying the antiadhesive substances themselves , and in terms of the integrity of the innerliner layer . Furthermore, the bladder which is the obj ect of the present invention has an important advantage in terms of sustainability, insofar as it involves eliminating the use of releasing agents and, consequently, reduces the environmental impact of these materials .