Rubber compositions comprising a specific crosslinking system

Abstract

The invention relates to a rubber composition based on at least: an elastomeric matrix, and a zinc oxide-free crosslinking system comprising 1 to 20 parts of a sulfur donor, said sulfur donor being selected from the group consisting of 1-phenyl-5-[(1-phenyl-1H-1,2,3,4-tetrazol-5-yl)disulfanyl]-1H-1,2,3,4-tetrazol of formula 1, 2,2′-dipyridyl disulfide of formula 2, 2,2′-dithienyl disulfide of formula 3, and mixtures of these compounds, Formula 1, Formula 2, Formula 3.

Description

Title of the invention: Rubber compositions comprising a specific crosslinking system Technical field

[0001] The present invention relates to rubber compositions, in particular to diene rubber compositions, comprising a crosslinking system containing little or no molecular sulfur. Prior art

[0002] It is known, and has been customary for many years, to use in tires rubber compositions whose elastomer matrix is ​​crosslinked with sulfur, this crosslinking is then called vulcanization.

[0003] It is known that the crosslinking of sulfur-vulcanized rubber consists of polysulfide bonds. These polysulfide bonds become thermally unstable with an increase in the number of sulfur atoms.

[0004] Those skilled in the art have been able to develop vulcanization systems over the years to make them more effective, for example by adding vulcanization accelerators, vulcanization activators, and possibly vulcanization retarders.

[0005] In these complex vulcanization systems, the skilled person has also been able to substitute at least part of the molecular sulfur with a sulfur donor with which it was easier to control the number of sulfur atoms in the polysulfide bonds between the elastomer chains. Among the sulfur donors that are usually found in rubber compositions based on an elastomer matrix, mention may be made in particular of 2-mercaptobenzothiazole disulfide ("MBTS"), caprolactam disulfide, thiuram polysulfide sulfur donors, such as tetrabenzylthiuram disulfide ("TBzTD"), tetramethylthiuram disulfide ("TMTD"), dipentamethylenethiuram tetrasulfide ("DPTT").

[0006] Those skilled in the art have also turned to other sulfur-free crosslinking systems such as, for example, systems based on peroxides or bisma-leimides.

[0007] For manufacturers of rubber compositions based on an elastomer matrix, the search for simplified crosslinking systems which nevertheless ensure effective crosslinking of the elastomer chains, in particular by polysulfide bonds, remains a constant concern.

[0008] Continuing its efforts in this research, the Applicant has demonstrated that the use of a crosslinking system free of zinc oxide and based on a molecule specific containing a disulfide bond, surprisingly allows the effective crosslinking of an elastomer matrix, particularly a diene one, with a level of crosslinking that is industrially satisfactory, within the usual time and temperature ranges, particularly in the absence of molecular sulfur.

[0009] The Applicant has also demonstrated that the presence of zinc oxide, often present in conventional vulcanization systems as a vulcanization activator, deteriorates the level of crosslinking obtained with such a specific molecule containing a disulfide bond.

[0010] The use of a specific molecule containing a disulfide bond has many advantages, including simplicity of formulation and better solubility of the sulfur donor in the elastomer matrix compared to molecular sulfur, which is often in powder form. It also makes it possible to consider this solution as an alternative to crosslinking with peroxides, the industrial management of which is always very delicate, particularly due to the self-decomposition temperatures of these compounds. Statement of the invention

[0011] Consequently, a first subject of the invention relates to a rubber composition based on at least: • an elastomer matrix, and • a crosslinking system free of zinc oxide and comprising from 1 to 20 pce of a sulfur donor, said sulfur donor being chosen from the group consisting of 1-phenyl-5-[(1-phenyl-1H-1,2,3,4-tetrazol-5-yl)disulfanyl]-1H-1,2,3,4-tetrazo le of formula 1, 2,2'-dipyridyl disulfide of formula 2, 2,2'-dithienyl disulfide of formula 3, and mixtures of these compounds

[0012] [Chem.l]

[0013] Formula 1,

[0014] [Chem.2] Formula 2,

[0015] [Chem.3] Formula 3.

[0016] Another subject of the invention is a rubber article comprising at least one rubber composition in accordance with the invention. Summary of the invention

[0017] The invention, described in more detail below, has as its subject at least one of the embodiments listed in the following points:

[0018] 1. Rubber composition based on at least: • an elastomer matrix, and • a crosslinking system free of zinc oxide and comprising from 1 to 20 pce of a sulfur donor, said sulfur donor being chosen from the group consisting of 1-phenyl-5-[(1-phenyl-1H-1,2,3,4-tetrazol-5-yl)disulfanyl]-1H-1,2,3,4-tetrazo le of formula 1, 2,2'-dipyridyl disulfide of formula 2, 2,2'-dithienyl disulfide of formula 3, and mixtures of these compounds

[0019] [Chem.4]

[0020] Formula 1,

[0021] [Chem.5] Formula 2,

[0022] [Chem.6] Formula 3.

[0023] 2. Rubber composition according to the preceding embodiment, in which the sulfur donor is selected from the compound of formula 2, the compound of formula 3, and mixtures of these compounds.

[0024] 3. Rubber composition according to any one of the preceding embodiments, in which the sulfur donor level is in the range of 2 to 10 pce, preferably 3 to 8 pce.

[0025] 4. Rubber composition according to any one of the preceding embodiments, which composition is free of molecular sulfur or contains less than 0.2 pce.

[0026] 5. Rubber composition according to any one of the preceding embodiments, which composition is free of molecular sulfur.

[0027] 6. Rubber composition according to any one of the preceding embodiments, wherein the elastomer matrix comprises at least one diene elastomer.

[0028] 7. Rubber composition according to any one of the preceding embodiments, wherein the elastomer matrix comprises natural rubber.

[0029] 8. Rubber composition according to any one of the preceding embodiments, in which the elastomer matrix comprises natural rubber as the majority diene elastomer.

[0030] 9. Rubber composition according to any one of the preceding embodiments, wherein the elastomer matrix consists essentially of natural rubber, preferably consists of natural rubber.

[0031] 10. Rubber composition according to any one of the preceding embodiments, further comprising a charge.

[0032] 11. Rubber composition according to any one of the preceding embodiments, further comprising a reinforcing filler, the reinforcing filler being selected from the group consisting of carbon black, silica and the mixture of carbon black and silica.

[0033] 12. Rubber composition according to the preceding embodiment in which the rate reinforcing filler is included in a range from 20 to 200 pce.

[0034] 13. Rubber composition according to any one of the preceding embodiments 11 to 12, in which the reinforcing filler is predominantly a carbon black.

[0035] 14. Rubber composition according to any one of the preceding embodiments 11 to 12, in which the reinforcing filler is mainly silica.

[0036] 15. Rubber article comprising at least one rubber composition in accordance with any one of the preceding embodiments, preferably the rubber article being chosen from the group consisting of a tire, a semi-finished article for a tire, a shoe sole, a rubber track, an anti-vibration support, a conveyor belt, a timing belt, a transmission belt, a hose, a floor covering, a seal, a molded mechanical part.

[0037] Definitions

[0038] The expression "composition based on" means a composition comprising the mixture and / or the in situ reaction product of the different constituents used, some of these constituents being able to react and / or being intended to react with each other, at least partially, during the different phases of manufacture of the composition; the composition thus being able to be in a totally or partially crosslinked state or in a non-crosslinked state.

[0039] By the expression "part by weight per hundred parts by weight of elastomer" (or pce), it is meant, within the meaning of the present invention, the part, by mass per hundred parts by mass of elastomer.

[0040] In this document, unless expressly indicated otherwise, all percentages (%) indicated are percentages (%) by mass.

[0041] On the other hand, any interval of values ​​designated by the expression "between a and b" represents the range of values ​​from more than a to less than b (i.e., excluding limits a and b), while any interval of values ​​designated by the expression "from a to b" means the range of values ​​from a to b (i.e., including the strict limits a and b). In the present document, when an interval of values ​​is designated by the expression "from a to b", the interval represented by the expression "between a and b" is also and preferably designated.

[0042] When we refer to a "majority" compound, we mean, within the meaning of the present invention, that this compound is the majority among the compounds of the same type in the composition, that is to say that it is the one which represents the greatest quantity in mass among the compounds of the same type. Thus, for example, a majority elastomer is the elastomer representing the greatest mass relative to the total mass of the elastomers in the composition. In the same way, a so-called majority filler is the one representing the greatest mass among the fillers in the composition. For example, in a system comprising a single elastomer, this is the majority within the meaning of the present invention; and in a system comprising two elastomers, the majority elastomer represents more than half of the mass of the elastomers. On the contrary, a "minority" compound is a compound which does not represent the largest mass fraction among the compounds of the same type. Preferably, by majority, we mean a mass proportion of more than 50%; when the compound represents 100% by mass, it is also referred to as "majority".

[0043] The compounds mentioned in the description may be of fossil or bio-sourced origin. In the latter case, they may be, partially or totally, derived from biomass or obtained from renewable raw materials derived from biomass. In the same way, the compounds mentioned may also come from the recycling of materials already used, that is to say they may be, partially or totally, derived from a recycling process, or obtained from raw materials themselves derived from a recycling process. This concerns in particular polymers, plasticizers, fillers, etc. 1. DETAILED DESCRIPTION OF THE INVENTION 1.1 Elastomeric matrix

[0044] The rubber composition is based on an elastomer matrix, among other components.

[0045] By "elastomer matrix" is meant all the elastomers in the composition.

[0046] The elastomer matrix of the composition according to the invention may contain one or more different elastomers, in particular one or more different diene elastomers.

[0047] By diene elastomer (or indistinctly rubber) is meant, in a known manner, an elastomer consisting at least in part (i.e., a homopolymer or a copolymer) of diene monomer units.

[0048] The term diene elastomer capable of being used in the context of the present invention is particularly understood to mean:

[0049] a) any homopolymer of a diene monomer, conjugated or not, having from 4 to 18 carbon atoms,

[0050] b) any copolymer of a diene, conjugated or not, having from 4 to 18 carbon atoms and at least one other monomer. The other monomer may be ethylene, an olefin or a diene, conjugated or not.

[0051] Suitable conjugated dienes are conjugated dienes having from 4 to 15 carbon atoms, in particular 1,3-dienes, such as in particular 1,3-butadiene and isoprene.

[0052] Suitable olefins are vinylaromatic compounds having from 8 to 20 carbon atoms and aliphatic α-monoolefins having from 3 to 12 carbon atoms.

[0053] Suitable vinylaromatic compounds include, for example, styrene, ortho-, meta-, para-methylstyrene, the commercial mixture "vinyl-toluene", para-tert-butylstyrene, preferably styrene.

[0054] Suitable aliphatic α-monoolefins include, in particular, acyclic aliphatic α-monoolefins having from 3 to 18 carbon atoms.

[0055] Preferably, the composition of the invention comprises an elastomer matrix based on at least one diene elastomer. The diene elastomer is chosen from the group consisting of polybutadienes (BR), synthetic polyisoprenes (IR), natural rubber (NR), butadiene copolymers, isoprene copolymers and mixtures thereof. Such copolymers may for example be selected from the group consisting of butadiene-styrene copolymers (SBR), isoprene-butadiene copolymers (BIR), isoprene-styrene copolymers (SIR), isoprene-butadiene-styrene copolymers (SBIR), butadiene-acrylonitrile copolymers (NBR), butadiene-styrene-acrylonitrile copolymers (NSBR) or a mixture of two or more of these compounds.

[0056] According to a variant of the invention, the elastomer matrix of the composition according to the invention comprises natural rubber. Optionally, the elastomer matrix of the composition according to the invention comprises natural rubber as the majority diene elastomer. Then, the rubber composition according to the invention preferably comprises more than 50 pce of natural rubber.

[0057] According to a variant of the invention, the elastomer matrix of the composition is essentially made up of natural rubber, more preferably consists of natural rubber. 1.2 Crosslinking system

[0058] The rubber composition according to the invention is based on a crosslinking system free of zinc oxide and comprising from 1 to 20 phr of a sulfur donor for crosslinking, which sulfur donor is chosen from the group consisting of 1-phenyl-5-[(1-phenyl-1H-1,2,3,4-tetrazol-5-yl)disulfanyl]-1H-1,2,3,4-tetrazole of formula 1, CAS No.: 5117-07-7, 2,2'-dipyridyl disulfide of formula 2, CAS No.: 2127-03-9, 2,2'-dithienyl disulfide of formula 3, CAS No.: 6911-51-9, and mixtures of these compounds,

[0059] [Chem. 7] ! . N

[0060]

[0061] Formula 1, [Chem. 8] S. s Formula 2, [Chem.9]

[0062]

[0063]

[0064]

[0065] Formula 3. Preferably, the sulfur donor may be chosen from the compound of formula 2, the compound of formula 3, and mixtures of these compounds. The crosslinking system of the rubber composition is free of zinc oxide and preferably comprises from 2 to 10 pce, more preferably from 3 to 8 pce of a sulfur donor among those corresponding to formulas 1, 2, 3 and the mixtures of these compounds, preferably among those corresponding to formulas 2, 3 and the mixtures of these compounds. The use of a sulfur donor from among those corresponding to formulas 1, 2, 3 and mixtures of these compounds, preferably those corresponding to formulas 2 and 3 and mixtures of these compounds, in the absence of zinc oxide, in the rubber composition according to the invention makes it possible to achieve, surprisingly, effective crosslinking in the absence of molecular sulfur. By "effective crosslinking", it is meant that the crosslinking performance expressed in base 100 is greater than half the crosslinking of a reference composition whose crosslinking system is a conventional vulcanization system based on molecular sulfur, N-cyclohexyl-2-benzothiazyl sulfenamide ("CBS"), zinc oxide and stearic acid, according to a molecular sulfur / CBS mass ratio of 0.5 / 0.5 and according to a zinc oxide / stearic acid mass ratio of 5 / 1.5. Effective crosslinking corresponds to a crosslinking performance greater than 50.

[0066] The crosslinking performance is characterized on the cured rubber compositions by a rheometry measurement at 200°C, with a stress at a frequency of 1.67 Hz and with an imposed deformation of 2.8%. The crosslinking performance is evaluated by calculating the difference between the maximum torque and the minimum torque.

[0067] In the description, molecular sulfur is understood to mean sulfur whether in so-called soluble form or in so-called insoluble form. The soluble and insoluble forms of sulfur that can be used to crosslink rubber compositions are well known to those skilled in the art.

[0068] Thus, according to preferred variants of the invention, the rubber composition is free of molecular sulfur or contains less than 0.2 phr. More preferably, according to these variants, the rubber composition is free of molecular sulfur. By "free of molecular sulfur", it is meant, within the meaning of the present invention, that the rubber compositions do not comprise molecular sulfur or, if they do contain it, this compound is in trace or impurity form.

[0069] The crosslinking system of the composition according to the invention may contain, in a known manner, various primary or secondary vulcanization accelerators. Any compound capable of acting as a vulcanization accelerator for elastomers, particularly diene elastomers, may be used as a (primary or secondary) vulcanization accelerator, in particular thiazole-type accelerators and their derivatives, sulfenamide-type accelerators, thiuram-type accelerators, dithiocarbamate-type accelerators, dithiophosphate-type accelerators, thiourea-type accelerators and xanthate-type accelerators.Examples of such accelerators include, but are not limited to, the following compounds: 2-mercaptobenzothiazyl disulfide (abbreviated as "MBTS"), N-cyclohexyl-2-benzothiazyl sulfenamide ("CBS"), N,N-dicyclohexyl-2-benzothiazyl sulfenamide ("DCBS"), N-tert-butyl-2-benzothiazyl sulfenamide ("TBBS"), N-tert-butyl-2-benzothiazyl sulfenimide ("TBSI"), tetrabenzylthiuram disulfide ("TBZTD"), zinc dibenzyldithiocarbamate ("ZBEC"), l-phenyl-2,4-dithiobiuret ("DTB"), zinc dibuthylphosphorodithioate ("ZBPD"), zinc 2-ethylhexylphosphorodithioate ("ZDT / S"), bis O,O-di(2-ethylhexyl)-thiophosphonyl disulfide ("DAPD"), dibutylthiourea ("DBTU"), zinc isopropyl xanthate ("ZIX") and mixtures of these compounds.

[0070] According to the invention, the crosslinking system of the rubber composition is free of zinc oxide. In other words, the crosslinking system of the rubber composition does not contain zinc oxide or if it does, this compound is in trace or impurity form. 1.3 Charges

[0071] The composition of the invention may further comprise at least one filler or a mixture of fillers.

[0072] Any type of filler may be used, whether reinforcing or non-reinforcing. For example, the composition of the invention may comprise a mixture of reinforcing and non-reinforcing fillers, or may comprise only one or more reinforcing fillers, or may comprise only one or more non-reinforcing fillers.

[0073] Among the non-reinforcing fillers well known to those skilled in the art, mention will be made in particular of those chosen from the group consisting of microparticles of natural (chalk) or synthetic calcium carbonates, synthetic or natural silicates (such as kaolin, talc, mica, vermiculite), aluminosilicates (clay, bentonite, montmorillonite), glass microbeads, glass flakes and a mixture of these compounds. Zinc oxide is not considered to be a filler within the meaning of the present invention, nor as a non-reinforcing filler within the meaning of the present invention, nor as a reinforcing filler within the meaning of the present invention.

[0074] Preferably, the composition of the invention may further comprise a reinforcing filler. Preferably, the total content of the reinforcing filler is within a range from 20 phr to 200 phr, advantageously from 25 phr to 180 phr, in particular from 35 phr to 160 phr. Those skilled in the art will be able to adapt the total content of reinforcing filler according to the relevant use of the composition of the invention.

[0075] Any type of so-called reinforcing filler, known for its ability to reinforce a rubber composition, may be used, for example an organic filler such as carbon black, an inorganic filler such as silica or a mixture of these two types of fillers. Such a reinforcing filler typically consists of nanoparticles whose average size (by mass) is less than one micrometer, generally less than 500 nm, most often in a range from 20 to 200 nm, in particular and more preferably in a range from 20 to 150 nm. Preferably, the reinforcing filler when it is present in the composition of the invention is chosen from the group consisting of carbon black, silica and the mixture of carbon black and silica.

[0076] Suitable carbon blacks are all carbon blacks, in particular the blacks conventionally used in tires or their treads. Among the latter, mention will be made more particularly of carbon blacks reinforcing tires. series 100, 200, 300, or series 500, 600 or 700 blacks (ASTM D-1765-2017 grades), such as blacks NI 15, N134, N234, N326, N330, N339, N347, N375, N550, N683, N772, N774). These carbon blacks can be used in the isolated state, as commercially available, or in any other form, for example as a support for some of the rubber additives used. The carbon blacks could, for example, already be incorporated into the diene elastomer, in particular isoprene, in the form of a masterbatch (see, for example, applications WO97 / 36724-A2 or WO99 / 16600-A1).

[0077] The silica used may be any reinforcing silica known to those skilled in the art, in particular any precipitated or pyrogenic silica having a BET specific surface area and a CT AB specific surface area both less than 450 m2 / g, preferably within a range from 30 to 400 m2 / g, in particular from 60 to 300 m2 / g.

[0078] Any type of precipitated silica can be used, in particular highly dispersible precipitated silicas (called "HDS"). Examples include the silicas described in applications WO03 / 016215-A1 and WO03 / 016387-A1. Among the commercial HDS silicas, it is possible in particular to use the silicas "Ultrasil ® 5000GR", "Ultrasil ® 7000GR" from the company Evonik, the silicas "Zeosil ® 1085GR", "Zeosil® 1115 MP", "Zeosil® 1165MP", "Zeosil® Premium 200MP", "Zeosil® HRS 1200 MP" from the company Solvay. As non-HDS silica, the following commercial silicas can be used: silicas "Ultrasil ® VN2GR", "Ultrasil ® VN3GR" from Evonik, silica, silicas "Hi-Sil EZ120G(-D)", "Hi-Sil EZ160G(-D)", "Hi-Sil EZ200G(-D)", "Hi-Sil 243LD", "Hi-Sil 210", "Hi-Sil HDP 320G" from PPG.

[0079] Those skilled in the art will understand that, as a replacement for the silica described above, a reinforcing filler of another nature could be used, provided that this reinforcing filler of another nature is covered with an inorganic layer such as silica, or else has functional sites on its surface, in particular hydroxyl sites, requiring the use of a coupling agent to establish the bond between this reinforcing filler and the elastomer, in particular diene elastomer. By way of example, mention may be made of carbon blacks partially or completely covered with silica, or carbon blacks modified with silica, such as, without limitation, the "Ecoblack®" type fillers of the CRX2000 series or the "CRX4000" series from Cabot Corporation.

[0080] In the present disclosure, the BET specific surface area is determined by gas adsorption using the Brunauer-Emmett-Teller method described in "The Journal of the American Chemical Society" (Vol. 60, page 309, February 1938), and more precisely according to a method adapted from standard NF ISO 5794-1, appendix E of June 2010 [multipoint volumetric method (5 points) - gas: nitrogen - vacuum degassing: one hour at 160°C - relative pressure range p / po: 0.05 to 0.2].

[0081] Also, the CT AB specific surface area values ​​were determined according to standard NF ISO 5794-1, annex G of June 2010. The process is based on the adsorption of CTAB (N-hexadecyl-N,N,N-trimethylammonium bromide) on the "external" surface of the reinforcing filler.

[0082] For carbon blacks, the STSA specific surface area is determined according to ASTM D6556-2016.

[0083] To couple the silica to the elastomer, in particular the diene elastomer, it is possible to use, in a well-known manner, an at least bifunctional coupling agent (or bonding agent) intended to ensure a sufficient connection, of a chemical and / or physical nature, between silica (surface of its particles) and the elastomer, in particular the diene elastomer. In particular, at least bifunctional organosilanes or polyorganosiloxanes are used. By "bifunctional", is meant a compound having a first functional group capable of interacting with the silica and a second functional group capable of interacting with the elastomer, in particular the diene elastomer. For example, such a bifunctional compound may comprise a first functional group comprising a silicon atom, said first functional group being capable of interacting with the hydroxyl groups of a silica and a second functional group comprising a sulfur atom, said second functional group capable of interacting with the elastomer, in particular the diene elastomer.

[0084] Preferably, the organosilanes are chosen from the group consisting of polysulfurized organosilanes (symmetrical or asymmetrical) such as bis(3-triethoxysilylpropyl) tetrasulfide, abbreviated to TESPT, marketed under the name "Si69" by the company Evonik, or bis-(triethoxysilylpropyl) disulfide, abbreviated to TES PD, marketed under the name "Si75" by the company Evonik, polyorganosiloxanes, mercaptosilanes, blocked mercaptosilanes, such as, for example, "NXT-Silane" marketed by the company Momentive. More preferably, the organosilane is a polysulfurized organosilane.

[0085] Of course, mixtures of the coupling agents previously described could also be used.

[0086] The content of coupling agent in the composition of the invention is advantageously less than or equal to 20 phr, it being understood that it is generally desirable to use as little as possible. Typically the level of coupling agent represents from 0.5% to 15% by weight relative to the quantity of silica. This level is easily adjusted by a person skilled in the art according to the level of silica used in the composition.

[0087] According to a variant of the invention, the reinforcing filler is mainly carbon black, preferably it comprises more than 50% (>50%) by weight of carbon black relative to the total weight of reinforcing filler. Optionally according to this variant, the reinforcing filler may also comprise silica, as well as a coupling agent. Alternatively, according to this variant of the invention, the reinforcing filler may consist of carbon black as the only reinforcing filler.

[0088] According to another variant of the invention, the reinforcing filler is predominantly a silica filler, preferably it comprises more than 50% (>50%) by weight of a silica relative to the total weight of the reinforcing filler. Optionally according to this variant, the reinforcing filler may also comprise carbon black. According to this option, the carbon black is used at a rate less than or equal to 20 phr, more preferably less than or equal to 10 phr (for example the rate of carbon black may be in a range from 0.5 to 20 phr, in particular from 1 to 10 phr). In the indicated ranges, the coloring (black pigmenting agent) and anti-UV properties of carbon blacks are benefited from, without otherwise penalizing the typical performances provided by silica. 1.4. Various additives

[0089] The rubber compositions in accordance with the invention may also comprise all or part of the usual additives and processing agents, known to those skilled in the art and usually used in rubber compositions, such as, for example, plasticizers such as liquid plasticizers (for example: oils), solid plasticizers (for example: hydrocarbon resins having or not having tackifying resin characteristics) or mixtures thereof, pigments, protective agents such as anti-ozone waxes, chemical anti-ozonants, antioxidants. 1.5. Manufacture of rubber compounds

[0090] The rubber compositions of the invention are manufactured in suitable mixers, using two successive preparation phases according to a general procedure well known to those skilled in the art: a first thermo-mechanical working or kneading phase (sometimes referred to as the "non-productive" phase) on a suitable mixer (for example a "Banbury" type mixer) at high temperature, up to a maximum temperature in a range from 110°C to 200°C, preferably in a range from 135°C to 185°C, followed by a second mechanical working phase (sometimes referred to as the "productive" phase) at a lower temperature, typically below 120°C, for example in a range from 40°C to 100°C, a finishing phase during which the crosslinking system free of zinc oxide and comprising the sulfur donor is incorporated, said sulfur donor being chosen from those corresponding to formulas 1, 2, 3 and mixtures of these compounds,preferentially being chosen from those corresponding to formulas 2, 3 and mixtures of these compounds.,

[0091] The final composition thus obtained is then calendered, for example, in the form of a sheet or a plate, in particular for characterization in the laboratory, or still extruded in the form of a semi-finished (or profile) rubber or a finished rubber product.

[0092] The crosslinking of the composition can be carried out in a manner known to those skilled in the art, for example at a temperature in a range from 130°C to 200°C, under pressure. 1.6 Article

[0093] Another subject of the present invention is a rubber article comprising at least one rubber composition according to the invention described above. Preferably all or part of the rubber article comprises at least one rubber composition of the invention described above. The rubber article can be used in various applications. For example, a rubber composition according to the invention can be used for the manufacture of rubber articles such as tires, semi-finished products for tires (treads, sidewalls, etc.), shoe soles, rubber tracks (treads or pads), anti-vibration mounts, conveyor belts, timing belts, transmission belts, hoses, floor coverings, seals, molded mechanical parts. 2. EXAMPLES OF CARRYING OUT THE INVENTION

[0094] The following examples illustrate the invention; however, the latter cannot be limited to these examples alone.

[0095] In order to confirm the effect of the invention, rubber compositions (C1, C3 and C5: compositions according to the invention, C2, C4 and C6: comparative compositions, T0 and T1: control compositions) were used. Each of the rubber compositions is based on an elastomer matrix consisting of natural rubber (abbreviated as "NR"), and either a sulfur donor corresponding to one of formulas 1, 2 and 3, with or without zinc oxide, or a conventional vulcanization system based on molecular sulfur. Each of the formulations of the rubber compositions is presented in Table 1 with the quantity of the different products expressed in pce. 2.1 Preparation of rubber compositions

[0096] All tests were conducted with 100 pce natural rubber (NR) to which the sulfur donor was added at the same molar quantity of disulfide bond SS (0.018 mol) regardless of the sulfur donor used. When present, the metal oxide is ZnO at a level of 5 pce.

[0097] The reference mixture is a composition vulcanized using a molecular sulfur / CBS couple, in a ratio of 0.5 / 0.5 in pce, zinc oxide and stearic acid in a ratio of 5 / 1.5 in pce, known to those skilled in the art.

[0098] The sulfur donor compounds, zinc oxide, were introduced during the second phase of mechanical work (described as the "productive" phase) on a cylinder tool at 50°C with a friction coefficient of 0.9 and a cylinder gap of 1 mm.

[0099] Once the mixtures were made, they were characterized during cooking by a rheometry measurement at 200°C, stressed at a frequency of 1.67 Hz and with an imposed deformation of 2.8%. The crosslinking is evaluated by calculating the difference between the maximum torque and the minimum torque (delta torque). The establishment of this performance on a base of 100 is made in comparison with the molecular sulfur / CBS reference according to a ratio of 0.5 / 0.5 in pce and zinc oxide and stearic acid according to a ratio of 5 / 1.5 in pce to which the value of 100 was assigned. 2.2 Results

[0100] The results are shown in the table which also includes a summary of the compositions tested.

[0101] [Tables 1] T0 Tl Cl C2 C3 C4 C5 C6 NR(1) 100.00 100.00 100.0 0 100.00 100.0 0 100.00 100.00 100.00 Stearic acid (2) 1.50 1.50 0.00 0.00 0.00 0.00 0.00 0.00 Zinc oxide (3) 0.00 5.00 0.00 5.00 0.00 5.00 0.00 5.00 Molecular sulfur (4) 0.50 0.50 0.00 0.00 0.00 0.00 0.00 0.00 CBS (5) 0.50 0.50 0.00 0.00 0.00 0.00 0.00 0.00 Sulfur donor of formula 1 (6) 0.00 0.00 6.50 6.50 0.00 0.00 0.00 0.00 Sulfur donor of formula 2 (7) 0.00 0.00 0.00 0.00 4.04 4.04 0.00 0.00 Sulfur donor of formula 3 (8) 0.00 0.00 0.00 0.00 0.00 0.00 4.20 4.20 Delta torque (base 100) 35 100 276 128 76 10 246 24

[0102] (1) NR: natural rubber;

[0103] (2) Stearic acid: Stearic acid: Stearin marketed by the company Uniquema under the name “Pristerene 4931”

[0104] (3) Zinc oxide: industrial grade zinc oxide marketed by the company Umicore

[0105] (4) Molecular sulfur: soluble sulfur,

[0106] (5) CBS: Cyclohexyl-benzothiazyl sulfenamide accelerator of the company AKROCHEM

[0107] (6) Sulfur donor of formula 1, i.e. 1 -Phenyl-5- [( 1 -phenyl-1 H-1,2,3,4-tetrazol-5-yl)disulfanyl] -1 H-1,2,3,4-tetrazole, CAS number: 5117-07-7, 354.5 g / mol

[0108] (7) Sulfur donor of formula 2, i.e. 2,2'-dipyridyl disulfide, CAS No.: 2127-03-9, 220 g / mol

[0109] (8) Sulfur donor of formula 3, i.e. 2,2'-dithienyl disulfide, CAS No.: 6911-51-9, 230.5 g / mol

[0110] Compositions C1, C3, and C5 with the respective use of the sulfur donors of formula 1, formula 2, and formula 3 show effective crosslinking activity with a performance greater than 50 on a 100 basis. Compositions that additionally use zinc oxide (compositions C2, C4, and C6) show a decrease in crosslinking activity compared to the use of non-additive sulfur donors.

Claims

Claims

1. Rubber composition based on at least: • an elastomer matrix, and • a crosslinking system free of zinc oxide and comprising from 1 to 20 pce of a sulfur donor, said sulfur donor being chosen from the group consisting of 1-phenyl-5-[(1-phenyl-1H-1,2,3,4-tetrazol-5-yl)disulfanyl]-1H-1,2,3,4-tetrazole of formula 1, 2,2'-dipyridyl disulfide of formula 2, 2,2'-dithienyl disulfide of formula 3, and mixtures of these compounds [Chem. 10] Formula 1, [Chem. 11] Formula 2, [Chem. 12] Formula 3.

2. A rubber composition according to claim 1, wherein the sulfur donor is selected from compounds of formula 2, compounds of formula 3, and mixtures of these compounds.

3. Rubber composition according to any one of the preceding claims, in which the level of the sulfur donor is in a range from 2 to 10 phr, preferably in a range from 3 to 8 phr.

4. A rubber composition according to any preceding claim, which composition is free of or contains less than 0.2 phr of molecular sulfur.

5. A rubber composition according to any preceding claim, which composition is free of molecular sulfur.

6. A rubber composition according to any preceding claim, wherein the elastomer matrix comprises at least one diene elastomer.

7. A rubber composition according to any preceding claim, wherein the elastomeric matrix comprises natural rubber.

8. A rubber composition according to any preceding claim, wherein the elastomer matrix comprises natural rubber as the majority diene elastomer.

9. A rubber composition according to any preceding claim, wherein the elastomer matrix consists essentially of natural rubber.

10. A rubber composition according to any preceding claim, further comprising a filler.

11. A rubber composition according to any preceding claim, further comprising a reinforcing filler selected from the group consisting of carbon black, silica and the mixture of carbon black and silica.

12. Rubber composition according to the preceding claim, in which the level of reinforcing filler is within a range from 20 to 200 phr.

13. A rubber composition according to any one of the preceding claims 11 to 12, wherein the reinforcing filler is predominantly carbon black.

14. A rubber composition according to any one of the preceding claims 11 to 12, wherein the reinforcing filler is predomi- silica.

15. A rubber article comprising at least one rubber composition according to any one of the preceding claims.