Method and apparatus for continuously producing a semi-finished product for the production of self-sealing tyres

Abstract

An apparatus, and relative method, for producing a semi-finished product with a strip of sealing compound for building tires, having at least one unsaturated styrene thermoplastic elastomer, one bonding resin and one cross-linking agent are described.

Claims

1. A method for producing a semi-finished product comprising a strip of sealing compound for building tyres, the sealing compound comprising at least the following components: (a) at least one unsaturated styrene thermoplastic elastomer; (b) at least one bonding resin; (c) at least one cross-linking agent; the method comprising: feeding the components of the sealing compound in a continuous mixer comprising an inner chamber, and mixing the components while they advance in the continuous mixer until the sealing compound is obtained; subsequently continuously feeding the sealing compound from the continuous mixer directly to a buffer device having a respective inner chamber and coupled in series with structural continuity to the continuous mixer, downstream of the latter, in a manner such that the two respective inner chambers are physically connected without interruption; subsequently advancing the sealing compound along the buffer device, absorbing possible flow rate pulses of the sealing compound coming from the continuous mixer, in a manner so as to continuously feed the sealing compound to a gear pump having a respective inner chamber and a gear system housed in the respective inner chamber, and coupled with structural continuity to the buffer device downstream of the latter, in a manner such that the two respective inner chambers are physically connected without interruption; and subsequently continuously feeding, by means of the gear pump, the sealing compound to an extrusion head coupled directly and with structural continuity to the gear pump, in a manner so as to obtain, in output from said extrusion head, said strip of sealing compound with predetermined transverse size.

2. The method as claimed in claim 1, wherein a temperature of the sealing compound at an outlet of the extrusion head is greater than or equal to 60 C. and/or less than or equal to 160 C. and wherein a flow rate of the sealing compound is greater than or equal to 30 kg/h and/or less than or equal to 5000 kg/h.

3. The method as claimed in claim 1, wherein the sealing compound further comprises at least one diene elastomer and/or at least one plasticising agent and/or at least one reinforcement filler and/or at least one peptising agent.

4. The method as claimed in claim 1, wherein the sealing compound comprises from 20 to 100 phr of at least one unsaturated styrene thermoplastic elastomer, from 0 to 80 phr of at least one synthetic or natural diene elastomer, from 20 to 200 phr, preferably from 30 to 150 phr, of at least one bonding resin, from 0.1 to 6 phr of at least one cross-linking agent.

5. The method as claimed in claim 1, wherein the buffer device reduces the amplitude and/or the frequency of the instantaneous flow rate pulses in its inner chamber with respect to the amplitude and/or frequency of the pulses in the inner chamber of the continuous mixer.

6. The method as claimed in claim 1, wherein the sealing compound, in a passage from the continuous mixer to a first portion of the inner chamber of the buffer device, proximal to the continuous mixer, undergoes a fall of pressure.

7. The method as claimed in claim 1, wherein the buffer device lower a temperature of the sealing compound during its travel along its respective inner chamber, the difference between the temperature at an inlet and that at an outlet of the buffer device being greater than or equal to 5 C.

8. The method as claimed in claim 1, wherein said strip has a profile, taken along an orthogonal section, having two opposite greater sides with length greater than a mutual maximum distance, wherein the method further comprises: conveying said strip away from the extrusion head, continuously unwinding a service film in a manner such that said strip exiting from the extrusion head is abutted against said service film, continuously unwinding a structural film in a manner such that said structural film adheres to said strip, and continuously unwinding a first and a second elongated elements made of an elastomeric compound in a manner such that said elongated elements at least partially adhere to said strip, thereby producing the semi-finished product that further comprises: the service film adherent to one of said greater sides, the structural film adherent to said strip on a side opposite said service film, and the first and the second elongated elements made of the elastomeric compound, in partial adhesion to said strip on mutually opposite sides of said strip and at least partially arranged transverse to said greater sides.

9. A process for producing self-sealing tyres, comprising: building, vulcanising and moulding green tyres, wherein the building of the green tyres includes producing a semi-finished product comprising a strip of sealing compound in accordance with the method as claimed in claim 1.

Description

(1) Such description will be set forth hereinbelow with reference to the enclosed drawings, provided for exemplifying and hence non-limiting purposes, in which:

(2) FIG. 1 schematically shows, in radial half-section, a self-sealing tyre for vehicle wheels;

(3) FIG. 2 shows a partial perspective view of a semi-finished product comprising two elongated elements made of elastomeric material and intended to form part of the self-sealing tyre of FIG. 1;

(4) FIG. 3a shows a schematic plan view, partially by functional units and partially in cross section, of a first portion of an apparatus according to the present invention;

(5) FIG. 3b shows a lateral schematic view, partially by functional units, of a second portion of an apparatus according to the present invention;

(6) FIG. 4 shows, in a qualitative manner and with arbitrary scales, the profiles of temperature and flow rate along an apparatus according to the present invention.

(7) With reference number 1, a self-sealing tyre for vehicle wheels is indicated in FIG. 1; such self-sealing tyre generally comprises a carcass structure 2 comprising at least one carcass ply 3 having respectively opposite end flaps engaged with respective annular anchoring structures 4, possibly associated with elastomeric fillers 4a, integrated in the zones 5 normally identified with the name beads. The carcass ply 3 comprises a plurality of textile or metallic reinforcement cords arranged parallel to each other and at least partially covered by a layer of elastomeric material.

(8) The carcass structure 2 is associated with a belt structure 6 comprising one or more belt layers, situated in radial superimposition with respect to each other and with respect to the carcass ply 3, having typically metallic reinforcement cords.

(9) In radially outer position with respect to the belt structure 6, a tread band 7 is applied that is made of an elastomeric compound, like other semi-finished products constituting the tyre 1.

(10) Respective sidewalls 8 made of an elastomeric compound are also applied in axially outer position on the lateral surfaces of the carcass structure 2, each extended from one of the lateral edges of the tread band 7 up to the respective annular structure for anchoring to the beads 5.

(11) In radially inner position with respect to the carcass 2, a layer of substantially air-impermeable elastomeric material or so-called liner 9 is also preferably provided.

(12) In the embodiment illustrated in FIG. 1, the tyre 1 is of the type for automobiles; nevertheless, the present invention can also be applied to tyres of the type intended for heavy vehicles or motor vehicles.

(13) The self-sealing tyre 1 also comprises a sealing compound layer 10 arranged at a crown zone of the tyre 1 and in radially inner position with respect to the liner 9. The sealing compound layer 10 is extended over the entire circumferential extension of the tyre 1. The sealing compound layer 10 has a maximum thickness t1 substantially at the middle plane X of the finished tyre 1, i.e. moulded and vulcanised, and thins towards the axial ends of the crown zone (FIG. 1). Preferably, said maximum thickness t1 is comprised between about 2.5 mm and about 6 mm.

(14) The sealing compound 10 for example comprises from 20 phr to 100 phr of at least one unsaturated styrene thermoplastic elastomer, from 0 to 80 phr of at least one synthetic or natural diene elastomer, from 0.1 to 6 phr of at least one cross-linking agent, from 20 to 200 phr, preferably from 30 phr to 150 phr, of at least one bonding agent, from 10 phr to 200 phr, preferably from 20 phr to 60 phr of plasticising agent (oil or liquid polymer), and preferably from 1 to 40 phr, preferably from 5 to 30 phr, of at least one reinforcement filler. Preferably the sealing compound can also comprise from about 1 phr to about 20 phr of at least one homogenising agent. Preferably the sealing compound can also comprise from 0.05 phr to 5 phr of at least one peptising agent.

(15) According to a preferred embodiment, the unsaturated styrene thermoplastic elastomer is a styrene polymer selected from among styrene/butadiene/styrene (SBS), styrene/isoprene/styrene (SIS), styrene/butadiene/isoprene/styrene (SBIS) block copolymers and mixtures thereof, also optionally comprising the corresponding diblock thermoplastic elastomers, such as styrene-butadiene (SB) and styrene-isoprene (SI). Particularly preferred are the styrene/isoprene/styrene block copolymers or mixtures of one or more unsaturated styrene thermoplastic elastomers containing at least 50% styrene/isoprene/styrene block copolymers. Particularly preferred are styrene/isoprene/styrene block copolymers with a content of styrene equal to or less than 20%, more preferably comprised between 14% and 20%.

(16) Such copolymers are for example sold with the name of Europrene SOL T190, T9133 by Polimeri Europa, Vector 4113, 4114 by Dexco Polymers, Kraton D1111, D1112 and D1107J by Kraton.

(17) According to a preferred embodiment, the synthetic or natural elastomer included in the sealing compound can be selected from among those commonly used between the elastomeric materials that can be cross-linked with sulfur or peroxides, which are particularly adapted for tyre production, i.e. from the elastomeric polymers or copolymers with an unsaturated chain having a vitreous transition temperature (Tg) generally below 20 C., preferably in the range of 0 C. to 110 C. These polymers or copolymers can be of natural origin or they can be obtained by means of polymerisation in solution, polymerisation in emulsion or polymerisation in gaseous phase of one or more conjugated diolefins, optionally mixed with at least one comonomer selected from among monovinylarenes and/or polar comonomers in a quantity not greater than 60% by weight. The conjugated diolefins generally contain from 4 to 12, preferably from 4 to 8 carbon atoms and they can be selected, for example, from the group comprising: 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, 3-butyl-1,3-octadiene, 2-phenyl-1,3-butadiene, or mixtures thereof. 1,3-Butadiene or isoprene are particularly preferred. The polar comonomers that may be used can be selected, for example, from among: vinylpyridine, vinylquinoline, acrylic acid esters and alkyl acrylic acid esters, nitriles, or mixtures thereof, such as methyl acrylate, methyl methacrylate, ethyl methacrylate, acrylonitrile or mixtures thereof. Preferably, the synthetic or natural elastomer included in the sealing material can be selected, for example, from among: cis-1,4-polyisoprene (natural or synthetic rubber, preferably natural rubber), 3,4-polyisoprene, polybutadiene (in particular polybutadiene with a high 1,4-cis content), isoprene/isobutene copolymers that are possibly halogenated, 1,3-butadiene/acrylonitrile copolymers, styrene/1,3-butadiene copolymers, styrene/isoprene/1,3-butadiene copolymers, styrene/1,3-butadiene/acrylonitrile copolymers, or mixtures thereof.

(18) The bonding agents advantageously used in the present invention can be selected in the group of hydrocarbon resins having an average numeric molecular weight comprised between several hundred and several thousand and which provides stickiness when the resin is mixed with natural or synthetic rubber.

(19) As resins, various types of synthetic resins can be used. The aforesaid average numeric molecular weight (Mn) can be measured according to techniques known in the art, such as for example by gel permeation chromatography (GPC). In particular, as bonding agents the following can be used: hydrocarbon resins, phenol-based resins, carbon-based resins, xylene-based resins and natural resins such as resins with rosin base or terpene-based resins.

(20) Examples of commercial hydrocarbon resin products comprise aromatic petroleum-based resins such as PETCOAL produced by Tosoh Co., Ltd.; C5/C9 hydrocarbon-based resins such as PETROTACK produced by Tosoh Co.; C5 hydrocarbon-based resins such as Escorez 1102 (produced by ExxonMobil).

(21) Examples of phenol-based resins comprise resins with alkylphenol-formaldehyde base, and derived resins modified with rosin, resins with alkylphenol-acetylene base, alkylphenol and terpene-phenol modified resins. Specific examples indicated by trademark comprise commercial products such as RESIN SP-1068 (produced by SI GROUP Inc.) which is a octylphenol-formaldehyde resin, and KORESIN (produced by BASF Company) which is a p-t-butylphenol-acetylene resin.

(22) Examples of carbon-based resins include coumarone-indene resins. Specific examples comprise commercial products, mentioned by trademark, such as NOVARES C resins (produced by RUTGERS CHEMICAL GmbH) which are synthetic coumarone-indene resins (such as NOVARES C10, C30, and C70).

(23) Examples of natural resins are the rosin resins and the terpene resins, which can be used as they are or modified: examples of these classes are the DERCOLYTE terpene resins produced by DRT, the resins derived from rosin acids DERTOLINE, GRANOLITE and HYDROGRAL produced by DRT.

(24) Examples of xylene-based resins comprise the xylene-formaldehyde resins.

(25) The aforesaid bonding agents can be used on their own or mixed together. Suitable cross-linking agents are sulfur or molecules containing sulfur, in the presence of compounds containing zinc and fatty acids, or peroxides. Specific examples of molecules containing sulfur usable as cross-linking agents in the sealing materials for the obtainment of self-sealing tyres are elementary sulfur, thiurams, such as tetraisobutyl thiuram disulfide or tetrabenzyl thiuram disulfide), or dithiophosphates, such as zinc dibutyldithiophosphate, or dithiocarbamates, such as zinc dimethyl dithiocarbamate, together with ZnO or compounds containing zinc, fatty acids and sulfenamides, such as N-tert-butyl-2-benzothiazyl sulfenamide (TBBS) or N-cyclohexyl-2-benzothiazyl sulfenamide (CBS), or thiazoles, such as 2,2-dithiobis-(benzothiazole) (MBTS).

(26) Specific examples of peroxides usable as cross-linking agents in the sealing materials for the obtainment of self-sealing tyres are organic peroxides such as dicumyl peroxide (DCP), 2,5-dimethyl-2,5-di(tert-butyl-peroxy)hexane (DBPH), bis-(2,4-dichlorobenzoyl) peroxide (DCBP), di-tert-butyl-peroxide.

(27) Preferably a peroxide, still more preferably 2,5-dimethyl-2,5-di(tert-butyl-peroxy)hexane (DBPH) is used as cross-linking agent.

(28) A specific example of usable DBPH is a mixture of 45% DBPH with calcium carbonate and silica sold with the name Luperox 101 XL45 by Arkema.

(29) The quantity of peroxide is preferably comprised between about 0.1 phr and about 6 phr.

(30) The presence of peroxide or sulfur or another cross-linking agent allows the partial chemical cross-linking of the sealing composition during the vulcanisation of the tyre in a manner so as to improve the chemical sealing characteristics of the sealing material layer.

(31) At least one reinforcement filler can be advantageously added to the abovementioned cross-linkable elastomeric composition, in a quantity generally from 0 phr to 120 phr, preferably from 20 phr to 90 phr. The reinforcement filler can be selected from among those commonly used for cross-linked products, in particular for tyres, such as carbon black, silica, aluminium, aluminosilicates, calcium carbonate, kaolin or mixtures thereof. Carbon black, silica and mixtures thereof are particularly preferred. According to a preferred embodiment, said carbon black reinforcement filler can be selected from among those which have a surface area not less than 20 m2/g (determined by STSAstatistical thickness surface areaaccording to ISO 18852:2005).

(32) In radially inner position with respect to the sealing compound layer 10 and in direct contact with said sealing compound 10 layer, a structural film 11 is arranged which can be made of thermoplastic polymer and/or thermoplastic-elastomeric polymer and/or elastomeric polymer.

(33) With thermoplastic polymer it is intended a polymer comprising linear chains or chains with few branches, not bonded with each other (i.e. not cross-linked): it is sufficient to increase the temperature in order to bring such polymer to a viscous state.

(34) The Thermoplastic Elastomers (TPE), or thermoplastic rubbers, are a class of copolymers or a polymer mixture (usually one plastic and one rubber) with both thermoplastic and elastomeric properties. While most elastomers are thermosetting, the TPEs are instead relatively easy to use in the production, for example, through injection moulding. The thermoplastic elastomers show the advantages typical both of plastics and rubbers. The main difference between the thermosetting elastomers and the thermoplastic elastomers is the type of cross-linking of their structures. Indeed, the cross-linking is an essential structural factor that contributes to conferring high elastic properties.

(35) The structural film 11 is self-supporting, and can be made of an elastomeric thermoplastic material by means of a block copolymer such as a thermoplastic polyurethane and/or made of polyamide and/or made of polyester. The self-supporting film 11 is extended, like the sealing compound layer 10, for the entire circumferential extension of the tyre 1 and has a width, i.e. a transverse extension, slightly less than the transverse extension of said layer 10.

(36) Preferably, before shaping, said self-supporting film 11 made of elastomeric thermoplastic has a thickness t2 comprised between about 10 m and about 100 m.

(37) The structural film 11 can be thermoplastic, made of polyamide or polyester. Preferably, said self-supporting thermoplastic film, before shaping, has a thickness less than 50 m.

(38) Preferably, said self-supporting thermoplastic film, before shaping, has a thickness comprised between about 10 m and about 30 m.

(39) Preferably, in the finished tyre, the structural films have thicknesses ranging from 10% to 40% less than the thickness measured before shaping.

(40) Preferably, in the finished tyre, said self-supporting film has a puncture strength less than about 30N.

(41) The sealing compound layer 10 and the self-supporting film 11 form a sealing complex 12. The sealing complex 12, when a sharp-pointed element (such as a nail) penetrates into the tyre and traverses the sealing compound layer 10 and the self-supporting film 11, is able to adhere to the object inserted therein and can also flow inside the hole when such object is removed, in such a manner sealing the hole itself and preventing the outflow of air from the tyre. The sealing complex 12 can be easily perforated by the sharp-pointed element while maintaining a deformability and a stickiness such to contribute to the transfer of the sealing compound during the expulsion of the sharp-pointed element. The tyre 1 also preferably comprises two elongated elements made of elastomeric material 13, each arranged at a circumferential edge of the sealing complex 12. An axially inner portion 13a of each elongated element made of elastomeric material 13 is partially superimposed on the sealing complex 12 and is arranged in radially inner position with respect to said sealing complex 12. An axially outer portion 13b of each elongated element made of elastomeric material 13 lies in direct contact with the liner 9. By axially inner portion 13a, it is intended a portion closer to a middle plane X of the tyre 1 with respect to the axially outer portion 13b.

(42) More in detail, the axially inner portion 13a in turn has an axially inner zone applied directly on the structural film 11 and an axially outer zone applied directly on one surface of the sealing compound layer 10. Indeed, the sealing compound layer 10 has an axial extension greater than the axial extension of the structural film 11. It follows that each elongated element made of elastomeric material 13 is in direct contact both with the sealing compound layer 10 and with the structural film 11.

(43) The building of a green precursor of a tyre 1 as described above, comprising the sealing complex 12, is preferably actuated by means of assembly of respective semi-finished products on one or more forming supports, not illustrated.

(44) The carcass structure and the belt structure are generally made separately from each other in respective work stations, in order to be mutually assembled at a later time.

(45) The sealing complex 12 corresponds to a semi-finished product 50, its precursor in the green tyre.

(46) The semi-finished product 50, as an example partially shown in FIG. 2, comprises a sealing compound strip 51 (precursor of the sealing compound 10) having a profile (shown in FIG. 2 in perspective view) taken along the orthogonal section, having a first horizontal direction 52, directed along the maximum size, and a second height direction 53, orthogonal to the horizontal direction. As an example, such profile has trapezoid shape.

(47) Preferably the semi-finished product also comprises a service film 54 in order to allow the handling of the sealing compound 51 and which is removed before the application of the semi-finished product on the building drum. Preferably the service film adheres to the larger base of the trapezoid. Preferably the semi-finished product 50 also comprises a structural film 55 (precursor of the aforesaid structural film 11) adherent to the continuous strip 51 on the side opposite the service film 54, at the smaller base of the trapezoid.

(48) Preferably the semi-finished product 50 also comprises a first and a second elongated element 56 made of an elastomeric compound (precursors of the aforesaid elongated elements 13), in partial adhesion to the continuous strip 51 on mutually opposite sides of the continuous strip. Preferably each elongated element has, on the orthogonal section, a first terminal portion 56a arranged in contact with the structural film 55 on the side opposite the strip di sealing material, a second terminal portion 56b arranged in contact with the service film 54 and an intermediate portion 56c arranged in contact with the sealing material strip (at the oblique sides).

(49) The strip of sealing material 51 has a thickness t3 along said height comprised between about 2.5 mm and about 6 mm.

(50) The structural film 55 before the incorporation of the semi-finished product 50 in the precursor of the tyre 1 and the shaping of the same has a thickness t4 less than 50 m and preferably comprised between about 10 m and about 30 m.

(51) The obtainment of the carcass structure first provides for the continuous production of the semi-finished product 50 in accordance with the present invention, which is subsequently orthogonally cut into sections of suitable length and wound around a radially outer surface of a building drum, maintaining the structural film 55 in radially more internal position. Opposite end flaps of the semi-finished product 50 are mutually joined by means of an adhesive type, for example.

(52) The liner 9 and carcass ply or plies 3 are applied on the semi-finished product 50 to form a so-called carcass sleeve, typically substantially cylindrical. The annular structures 4 for anchoring to the beads 5 are fit or formed on the opposite end flaps of the carcass ply or plies 3, which are subsequently turned up around the annular structures 4 themselves in a manner so as to enclose them in a kind of noose.

(53) On a second drum or auxiliary drum, a so-called external sleeve is obtained comprising the belt layers 6 applied in mutual radial superimposition, and possibly the tread band 7 applied in radially outer position with respect to the belt layers 6. The external sleeve is then drawn by the auxiliary drum in order to be coupled to the carcass sleeve. For such purpose, the external sleeve is arranged coaxially around the carcass sleeve, after which the carcass ply or plies 3 are shaped according to a toroidal configuration by means of mutual axial approaching of the beads 5 and simultaneous insertion of pressurised fluid inside the carcass sleeve, in a manner so as to determine a radial expansion of the carcass plies 3 until they are made to adhere against the inner surface of the external sleeve.

(54) Following the building of the green tyre 1, a moulding and vulcanisation treatment is executed, aimed to cause the structural stabilisation of the tyre 1 by means of cross-linking of the elastomeric compounds as well as impart on the tread band 7 a desired tread pattern and to impart at the sidewalls 8 possible indicia. After vulcanisation, the sealing compound layer 10 has the desired characteristics of deformability and stickiness (which can already be possessed by the sealing material strip 51).

(55) FIGS. 3a and 3b schematically show an apparatus 100 embodiment for producing a semi-finished product 50 comprising a sealing compound strip 51.

(56) The apparatus 100 comprises a continuous mixer 101 structured for receiving the components of the sealing compound and to continuously mix them until the sealing compound is obtained. Schematically indicated in FIG. 3a is a hopper 102 at one end of the mixer for feeding the components of the sealing compound. However, the mixer can comprise (and typically comprises) more than one feed mouth or hopper, for example distributed along the main extension direction F (which defines the direction of flow of the compound being processed). As an example, the continuous mixer comprises at least one feed mouth at the upstream end of the mixer for the masterbatch (i.e. a premixed compound, comprising for example at least the unsaturated styrene thermoplastic elastomer, but still lacking some components), and, more downstream (not shown), a feed mouth for the bonding resin, one or more feed mouths for the plasticising agent (oil) and a feed mouth for the cross-linking agent. The feeding of the components to the continuous mixer typically occurs by means of a continuous metering system (not shown), arranged upstream of the plant 100 itself.

(57) The continuous mixer 101 comprises at least one inner chamber 103 within which at least one worm screw 104 rotates, for example, or a plurality of screws rotates. The mixing of the material occurs during its travel along the major extension direction F of the inner chamber 103. The continuous mixer is mainly provided with mixing portions 105 capable of conferring high shear stress to the compound, and with possible redistribution portions. The continuous mixer is also provided with conveyor portions 107 for conveying the material being processed from one longitudinal end to the other of the inner chamber.

(58) In one embodiment example, the continuous mixer is a co-rotating twin-screw mixing extruder Maris TM58HT, having diameter (D) 58 mm and ratio between length (L) and diameter (D) equal to L/D=56.

(59) The apparatus 100 comprises, downstream of the continuous mixer 101, a buffer device 110 and, downstream of the buffer device, a gear pump 120, both provided with a respective inner chamber 111 and 121.

(60) The buffer device is arranged and structured for receiving the sealing compound from the continuous mixer and for feeding the gear pump (at the same time absorbing possible flow rate pulses of the compound coming from the continuous mixer). The buffer device is preferably coupled in series, with structural continuity, to the continuous mixer in a manner such that the two respective inner chambers 103 and 111 are physically connected without interruption. For example, the buffer device 110 is directly flanged to the continuous mixer 101, as is shown as an example in FIG. 3a.

(61) Preferably the buffer device is a single-screw extruder, such as for example with Maris trademark having diameter (D) equal to 70 mm and ratio between length (L) and diameter (D) equal to L/D=10.

(62) The single-screw extruder comprises a helical single conveyor screw 112 housed in the respective inner chamber 111 and configured for pushing the material along the flow direction F, exerting thereon a substantially zero or very low mixing.

(63) The gear pump 120 (e.g. Maag having internal diameter of the chamber equal to 50 mm) has a gear system 122 housed in the respective inner chamber 122, and is coupled with structural continuity to the buffer device 110 downstream of the latter, in a manner such that the two respective inner chambers are physically connected without interruption. For example, the gear pump 120 is directly flanged to the buffer device 110, as is shown as an example in FIG. 3a.

(64) The apparatus 100 also comprises an extrusion head 130 coupled directly and with structural continuity to the gear pump 120, downstream of the latter, in a manner such to receive in input the sealing compound from the gear pump and deliver in output the sealing compound in the form of said strip 51 with predetermined transverse size. As an example, the extrusion head is a flat thermostated head with output section 200 mm5 mm.

(65) The apparatus 100 preferably comprises a conveyor device 140 (e.g. a conveyor belt) placed downstream of the extrusion head in a manner so as to receive the strip 51 therefrom and convey it away from the extrusion head, as is shown as an example in FIG. 3b.

(66) Preferably the apparatus comprises a first reel 150 on which the service film 54 is wound for its continuous unwinding on the conveyor device in a manner such that the strip 51 in output from the extrusion head is abutted against the service film 54.

(67) Preferably the apparatus comprises a second reel 160 on which the structural film 55 is wound for its continuous unwinding on the conveyor device in a manner such that the structural film adheres to the strip 51 on the side opposite the service film 54.

(68) Preferably the apparatus 100 comprises a third and/or a fourth reel 170 (only one reel is shown in FIG. 3), on each of which a respective elongated element is wound made of an elastomeric compound 56 for their continuous unwinding on the conveyor device 140 in a manner such that each elongated element at least partially adheres to the strip 51 on mutually opposite sides thereof.

(69) The apparatus 100 can also comprise a cooling station (not shown) arranged downstream of the conveyor device 140. The cooling station can be a further conveyor belt or a further section of the conveyor belt of the conveyor device, at which suitable fans are arranged for air cooling.

(70) The apparatus 100 can comprise a series of drums (not shown) arranged downstream of the cooling station for the festoon arrangement of the semi-finished product 50, in order to create a buffer for the semi-finished product 50.

(71) Preferably the apparatus 100 comprises a collection station (not shown) arranged downstream of the cooling station and/or of the series of drums, configured for winding the semi-finished product on suitable collection reels possibly together with a service belt.

(72) During use, the apparatus 100 implements the aforesaid method for producing the semi-finished product 50 comprising the sealing compound strip 51 for building tyres.

(73) The method comprises: feeding the components of the sealing compound into the continuous mixer 101, and mixing the components while they advance in the continuous mixer until the sealing compound is obtained (having the desired uniformity and characteristics of the present invention); continuously feeding the sealing compound from the continuous mixer directly to the buffer device 110; advancing the sealing compound into the buffer device in a manner so as to continuously feed the sealing compound to the gear pump 120; continuously feeding, by means of the gear pump, the sealing compound to the extrusion head 130, in a manner so as to obtain in output therefrom the sealing compound strip 51 with predetermined transverse size.

(74) As an example, by using the devices indicated as an example above, the method provides for a flow rate of the sealing compound equal to 150 kg/h. The work conditions of the continuous twin-screw mixer 101 (TSE) are the following: rotation velocity of the screws 300 rpm, torque 35%, pressure equal to 10 bar. The work conditions of the single-screw extruder 110 are the following: rotation velocity of the screws 50 rpm, torque 10%, pressure 10 bar. The work conditions of the gear pump 120 are the following: rotation velocity of the screws 25 rpm, torque 15%, pressure at the output 50 bar. The temperature of the compound at the outlet of the extrusion head is equal to 110 C.

(75) The extruded strip 51 is received by the belt 140 and is inserted between the two films 54 and 55 at its lower and upper surfaces and delimited at the sides by the two elongated elements 56 that have the function of sealing the flaps. The semi-finished product 50 then advances to the cooling station where it is cooled in a manner such that it is hardened and becomes suitable for storage and use in building the tyre 1. For the purpose of the cooling, the semi-finished product is festooned on the series of drums, which have the function of continuing to collect the incoming semi-finished product from the line during the reel change in the collection station. Finally, the semi-finished product 50 reaches the collection station where it is wound on the collection reels together with a service belt.

(76) FIG. 4 shows, in qualitative form and with arbitrary scales, the progression of the temperature T (dashed line 401) of the sealing compound and the progression of the instantaneous flow rate Q (solid line 402) of sealing compound along the apparatus 100. The horizontal axis represents, in arbitrary scale, the position L along the apparatus 100. Indicated in brief in the graph are the sections of the apparatus 100 corresponding to the various devices constituting the same, by means of the use of the corresponding reference numbers. By instantaneous flow rate it is intended the flow rate of compound through the section of the apparatus at an arbitrary instant.

(77) As can be observed from FIG. 4, the buffer device 110 in accordance with the present invention carries out the function of controlling the temperature of the compound, and hence its viscosity, through a cooling profile. In addition, it effectively absorbs the flow rate pulses (in terms of instantaneous frequency and/or amplitude) typically introduced by the mixer (which, if they reached the gear pump and/or the extrusion head, would cause a corresponding pulsing of the transverse size). In such a manner, the buffer device is capable of continuously filling the chamber of the gear pump and with a flow rate that is at most slowly variable.

(78) The gear pump 120, for its part, receives the material from the buffer device and ensures the volumetric stability and the thermal control of the material for the correct feeding of the extrusion head, also absorbing the possible slow flow rate pulse transmitted by the buffer device.

(79) The result is a sealing compound strip 51 with the geometric characteristics sufficiently precise and stable for being used directly in a semi-finished product 50 (i.e. with requiring further shaping processing).

(80) As reported above, before arriving at the method and apparatus according to the present invention, the Applicant implemented a first comparative arrangement in which the flat thermostated extrusion head with output section 200 mm5 mm was coupled directly, with structural continuity, to the outlet of a co-rotating twin-screw mixing extruder Maris TM58HT, having diameter (D) 58 mm and ratio between length (L) and diameter (D) equal to L/D=56, and a second comparative arrangement in which the aforesaid extrusion head was coupled directly, with structural continuity, to a gear pump Maag with internal diameter equal to 50 mm, which in turn was coupled directly, with structural continuity, to the outlet of the aforesaid continuous twin-screw mixer.

(81) With respect to the aforesaid first and second comparative arrangement, the Applicant has found significant improvements in the sealing material strip produced by one embodiment example of the present invention, in which a single-screw extruder Maris having diameter (D) equal to 70 mm and ratio between length (L) and diameter (D) equal to L/D=10 was introduced in the second comparative arrangement, interposed with structural continuity between the aforesaid continuous twin-screw mixer and the aforesaid gear pump.

(82) In particular the Applicant has empirically established that the variance (intended as the interval of maximum variability) of each transverse size of the sealing compound strip produced according to the aforesaid embodiment of the present invention resulted, given the same flow rate, about a third of the variance introduced by the first arrangement and about half the variance introduced by the second arrangement.

(83) In addition, the Applicant has empirically established that the temperature of the sealing compound at the outlet of the extrusion head in the aforesaid embodiment of the present invention resulted, given the same flow rate, about 20 C. lower than the output temperature in the first arrangement and about 10 C. lower than the output temperature in the second arrangement.