Rubber composition

Abstract

The invention relates to a rubber composition based on at least: - an elastomer matrix comprising at least one butyl rubber; - a filler comprising at least one reinforcing filler and a non-reinforcing filler; and - a crosslinking system; wherein the crosslinking system comprises from 0.1 phr to 8 phr of a sulfur donor comprising N,N'-caprolactam disulfide; and wherein the crosslinking system comprises neither soluble sulfur nor insoluble sulfur, or comprises less than 0.6 phr of soluble sulfur, of insoluble sulfur or of a mixture of soluble sulfur and insoluble sulfur. This composition is particularly suitable for producing the inner liner for tyres.

Description

[0001]Title of the invention: Rubber composition Technical field The field of the invention is that of rubber compositions for rubber products, in particular rubber compositions comprising butyl rubber. Prior art In the field of manufacturing rubber compositions, the skilled person knows that it is important to allow crosslinking of the rubber compositions within industrially acceptable times, while preserving a minimum safety period ("scorch time") during which the compositions can be shaped without risk of premature crosslinking ("scorch"). A constant objective of manufacturers of rubber products is to guarantee a sufficient scorch time while maintaining the crosslinking time. It turns out that finding a good balance between these two properties is not easy because improving the scorch time often leads to increasing the crosslinking time.In this research, patent document JP2004-018682A1 has previously proposed to combine with a diene elastomer the combination of a cobalt salt of an aliphatic or alicyclic carboxylic acid and an aromatic carboxylate. It was thus found that when a cobalt salt of an aliphatic carboxylic acid and a benzoate are used in combination with a diene elastomer, the scorch time is prolonged while the vulcanization time is shortened. Manufacturers of rubber articles are always looking for solutions to improve the curing properties of their rubber compositions or to shift their equilibrium. Therefore, there is always a need for rubber compositions that have an improved scorch time while maintaining the crosslinking time within an industrially acceptable range.Statement of the invention Continuing its research, the Applicant has found a rubber composition which makes it possible to meet this need. In particular, the Applicant has found a rubber composition which has a significantly longer scorching time while maintaining an acceptable crosslinking time in terms of industrial productivity compared to a control rubber composition.Thus, a first subject of the invention is a rubber composition based on at least: - an elastomer matrix comprising at least one butyl rubber; - a filler comprising at least one reinforcing filler and one non-reinforcing filler; and - a crosslinking system comprising a sulfur donor comprising N,N'-caprolactam disulfide; and in which the crosslinking system comprises neither soluble sulfur nor insoluble sulfur, or comprises less than 0.6 phr of soluble sulfur, insoluble sulfur or a mixture of soluble sulfur and insoluble sulfur. Another subject of the invention is a finished or semi-finished rubber product comprising a rubber composition in accordance with the invention. Summary of the invention The invention, described in more detail below, has as its subject at least one of the embodiments listed in the following points: 1.Rubber composition based on at least: - an elastomer matrix comprising at least one butyl rubber; - a filler comprising at least one reinforcing filler and one non-reinforcing filler; and - a crosslinking system; wherein the crosslinking system comprises from 0.1 phr to 8 phr of a sulfur donor comprising N,N'-caprolactam disulfide; and wherein the crosslinking system comprises neither soluble sulfur nor insoluble sulfur, or comprises less than 0.6 phr of soluble sulfur, insoluble sulfur or a mixture of soluble sulfur and insoluble sulfur. 2. Rubber composition according to embodiment 1, wherein the elastomer matrix comprises at least 50 phr of butyl rubber. 3.Rubber composition according to embodiment 1 or embodiment 2, in which the elastomer matrix comprises at least one other diene elastomer chosen from the group consisting of polybutadienes (abbreviated as "BR"), natural rubber (NR), synthetic polyisoprenes (IR), butadiene copolymers, isoprene copolymers (other than butyl rubber) and mixtures of these elastomers, preferably natural rubber. 4. Rubber composition according to any one of the preceding embodiments, in which the elastomer matrix comprises at least one other elastomer in a content of less than 50 phr. 5. Rubber composition according to any one of the preceding embodiments, in which the reinforcing filler predominantly comprises a carbon black. 6.Rubber composition according to any one of the preceding embodiments, in which the reinforcing filler comprises a carbon black in a total amount of greater than 50% by weight per 100% by weight of the reinforcing filler. 7. Rubber composition according to any one of the preceding embodiments, in which the reinforcing filler consists essentially of carbon black. 8. Rubber composition according to any one of the preceding embodiments, which comprises from 10 to 60 phr of carbon black, preferably from 20 to 45 phr. 9. Rubber composition according to any one of the preceding embodiments, in which the non-reinforcing filler predominantly comprises a kaolin, preferably in a amount greater than 50% by weight per 100% by weight of the non-reinforcing filler. 10. Rubber composition according to any one of the preceding embodiments, in which the non-reinforcing filler consists of kaolin. 11.Rubber composition according to any one of the preceding embodiments, which comprises from 10 to 80 phr of kaolin as non-reinforcing filler, preferably from 10 to 70 phr and more preferably from 20 to 70 phr. 12. Rubber composition according to any one of the preceding embodiments, in which the total content of the filler is greater than or equal to 10 phr, preferably greater than or equal to 20 phr, and less than or equal to 110 phr. 13. Rubber composition according to any one of the preceding embodiments, in which the total amount of N,N'-caprolactam disulfide is greater than 50% by weight per 100% by weight of the sulfur donor. 14. Rubber composition according to any one of the preceding embodiments, in which the sulfur donor contains N,N'-caprolactam disulfide as the sole sulfur donor. 15.A rubber composition according to any one of embodiments 1 to 13, wherein the sulfur donor comprises a sulfur donor other than N,N'-caprolactam disulfide. 16. A rubber composition according to the preceding embodiment, wherein the sulfur donor other than N,N'-caprolactam disulfide is an alkylphenol disulfide, N,N'-dimorpholine disulfide, or a combination thereof. 17. A rubber composition according to any one of the preceding embodiments, wherein the level of sulfur donor comprising N,N'-caprolactam disulfide is greater than or equal to 0.1 phr and less than or equal to 5 phr. 18. A rubber composition according to any one of the preceding embodiments, wherein the level of sulfur donor comprising N,N'-caprolactam disulfide ranges from 0.1 to 4 phr, preferably from 0.2 to 3 phr. 19.A rubber composition according to any one of the preceding embodiments, wherein the N,N'-caprolactam disulfide content ranges from 0.1 to 4.0 phr, preferably from 0.2 to 3 phr, more preferably from 0.2 to 2. 20. A rubber composition according to any one of the preceding embodiments, wherein the N,N'-caprolactam disulfide content ranges from 0.2 to 1 phr. 21. A rubber composition according to any one of the preceding embodiments, wherein, when present, the total amount by weight of soluble sulfur, insoluble sulfur, or the mixture of soluble sulfur and insoluble sulfur is less than that of the sulfur donor. 22. A rubber composition according to any one of the preceding embodiments, wherein the crosslinking system comprises neither soluble sulfur nor insoluble sulfur. 23.A rubber composition according to any one of the preceding embodiments, wherein the crosslinking system further comprises at least one vulcanization accelerator. 24. A rubber composition according to any one of the preceding embodiments, wherein the crosslinking system further comprises at least one vulcanization accelerator selected from the group consisting of sulfenamide type accelerators, thiazole type accelerators, thiuram type accelerators, dithiocarbamate type accelerators, dithiophosphate type accelerators, thiourea type accelerators and xanthate type accelerators and mixtures thereof. 25. A rubber product comprising a rubber composition according to any one of embodiments 1 to 24. 26. A rubber product according to embodiment 25, which product is a tire. 27.Rubber product according to embodiment 26, which product is a tire whose inner rubber comprises the rubber composition according to any one of embodiments 1 to 24. Definitions The expression "composition based on" means a composition comprising the mixture and / or the in situ reaction product of the various constituents used, some of these constituents being able to react and / or being intended to react with each other, at least partially, during the various 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. The expression "part by weight per hundred parts by weight of elastomer" (or pce) means, within the meaning of the present invention, the part, by mass per hundred parts by mass of elastomer. In the present invention, unless expressly indicated otherwise, all the percentages (%) indicated are percentages (%) by mass.On the other hand, any range 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 range 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 invention, when a range of values ​​is designated by the expression "from a to b", the range represented by the expression "between a and b" is also and preferably designated. When a "majority" compound is referred to, it is understood, within the meaning of the present invention, that this compound is the majority among the compounds of the same type in the composition, i.e., that it is the one which represents the largest quantity by 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 that 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, is meant a mass proportion of more than 50%; when the compound represents 100% by mass, it is also referred to as "majority". The compounds mentioned in the description may be of fossil or biosourced 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. Detailed description of the invention 1. Elastomer matrix By "elastomer matrix" is meant all the elastomers in the composition. 1.1 Butyl rubber According to the invention, the elastomer matrix comprises a butyl rubber.As is known, butyl rubber is understood to mean a copolymer of isobutylene and C4-C6 diene, preferably isoprene (abbreviated IIR), as well as the halogenated, preferably chlorinated or brominated, versions of this type of copolymer. According to the invention, butyl rubber is also understood to mean a mixture of butyl rubbers. Generally, butyl rubbers contain from 1 to 5 mol% of diene unit, in particular isoprene. Halogenated butyl rubbers are obtained by halogenation, in particular by chlorination or bromination of copolymers of isobutylene and C4-C6 diene, in particular isoprene. The halogen content in the halogenated butyl rubber is preferably within a range of from 1 to 4% by weight relative to the weight of the butyl rubber. Preferably, according to the invention, the butyl rubber is a halogenated butyl rubber, preferably chlorinated or brominated, more preferably a brominated butyl rubber.More preferably, the butyl rubber is a copolymer of isobutylene and halogenated isoprene (XIIR), preferably chlorinated (CIIR) or brominated (BIIR). Preferably, the butyl rubber useful for the purposes of the invention is a copolymer of isobutylene and brominated isoprene (BIIR). By extension of the definition of butyl rubber, the butyl rubber may also be a terpolymer of isobutylene, paramethylstyrene and halogenated paramethylstyrene, preferably brominated, marketed under the names Bromobutyl or Exxpro from Exxon or X BUTYLTM BB 2030 from Arlanxeo. Advantageously, the butyl rubber is the majority elastomer of the elastomer matrix. For this purpose, preferably, the level of butyl rubber in the composition according to the invention is at least 50 phr, preferably varies in a range from 50 phr to 100 phr, preferably from 60 phr to 100 phr, more preferably from 65 phr to 100 phr. 1.2 Other elastomer The elastomeric matrix of the composition according to the invention may contain one or more diene elastomers different (hereinafter referred to as "other diene elastomer" for the sake of simplification of wording) from the butyl rubber used in the context of the present invention. Advantageously then, the composition according to the invention contains in a minor manner one or more other different diene elastomers. For example, the other diene elastomer may be chosen from the group of highly unsaturated diene elastomers consisting of polybutadienes (abbreviated as "BR"), natural rubber (NR), synthetic polyisoprenes (IR), butadiene copolymers, isoprene copolymers (other than butyl rubber) and mixtures of these elastomers.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. According to a variant of the invention, the other diene elastomer may be a natural rubber. The other diene elastomer may be modified, i.e. either coupled and / or star-shaped, or functionalized, or coupled and / or star-shaped and simultaneously functionalized. Thus, the other diene elastomer can be coupled and / or star-shaped, for example by means of a silicon or tin atom which links the elastomer chains together.According to an alternative embodiment of the invention, the composition comprises the other diene elastomer with a content of less than 50 phr, preferably within a range from 0 to 40 phr, preferably from 0 to 30 phr, more preferably from 0 to 20 phr. The elastomeric matrix may also contain, in a minor manner, any type of synthetic elastomer other than diene, or even polymers other than elastomers, for example thermoplastic polymers. Preferably, the elastomeric matrix does not contain any synthetic elastomer other than diene or any polymer other than elastomers or contains less than 10 phr, preferably less than 5 phr. 2. Fillers and coupling agents By filler is meant here any type of filler, whether reinforcing or non-reinforcing. The rubber composition of the invention is based on a filler comprising at least one reinforcing filler and one non-reinforcing filler.Preferably, the total rate of the filler is greater than 10 phr, more preferably greater than or equal to 20 phr, and less than or equal to 110 phr. 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. All carbon blacks are suitable as carbon blacks, in particular the blacks conventionally used in tires. Among these, we will particularly mention the reinforcing carbon blacks of the 100, 200, 300 series, or the 500, 600 or 700 series blacks (ASTM D-1765-2017 grades), such as for example blacks N115, 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). By "reinforcing inorganic filler" is meant here any inorganic or mineral filler, whatever its color and origin (natural or synthetic), also called "white" filler, "light" filler or even "non-black" filler as opposed to carbon black, capable of reinforcing on its own, without any means other than an intermediate coupling agent, a rubber composition intended for the manufacture of tires.As is known, certain reinforcing inorganic fillers may be characterized in particular by the presence of hydroxyl groups (-OH) on their surface. Suitable reinforcing inorganic fillers include mineral fillers of the siliceous type, preferably silica (SiO2) or of the aluminous type, in particular alumina (Al2O3). 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 CTAB 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. Any type of precipitated silica may be used, in particular highly dispersible precipitated silicas (known as "HDS"). Examples include the silicas described in applications WO03 / 016215-A1 and WO03 / 016387-A1.Among the commercial HDS silicas, we can notably 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 the company 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 the company PPG. As reinforcing inorganic filler, mention may also be made of mineral fillers of the aluminous type, in particular alumina (Al2O3) or aluminum (oxide)hydroxides, or reinforcing titanium oxides.Those skilled in the art will understand that, as a replacement for the reinforcing inorganic filler 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 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.In this presentation, 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 the NF ISO 5794-1 standard, annex 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]. For inorganic fillers such as silica, for example, CTAB specific surface area values ​​were determined according to 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. For carbon blacks, the STSA specific surface area is determined according to ASTM D6556-2016.To couple the reinforcing inorganic filler to 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 the inorganic filler (surface of its particles) and 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 inorganic filler and a second functional group capable of interacting with 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 an inorganic filler and a second functional group comprising a sulfur atom, said second functional group being capable of interacting with the diene elastomer. 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 TESPD, marketed under the name "Si75" by the company Evonik, polyorganosiloxanes, mercaptosilanes, blocked mercaptosilanes, such as "NXT-Silane" marketed by the company Momentive. More preferably, the organosilane is a polysulfurized organosilane.Of course, mixtures of the coupling agents described above could also be used. The coupling agent content 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 coupling agent content represents from 0.5% to 15% by weight relative to the amount of reinforcing inorganic filler. This content is easily adjusted by a person skilled in the art according to the content of reinforcing inorganic filler used in the composition. According to a variant of the invention, as reinforcing filler, carbon black is used at a content ranging from 10 to 60 phr. Indeed, beyond this content, the penalty in terms of rigidity of the composition is too great for certain applications. Preferably, the carbon black content ranges from 20 to 50 phr, and more preferably from 20 to 45 phr.Carbon black may advantageously constitute the only reinforcing filler or the majority reinforcing filler, that is to say that the reinforcing filler preferably comprises more than 50% by weight of carbon black relative to the total weight of the reinforcing filler. Of course, a single carbon black or a blend of several carbon blacks of different ASTM grades may be used. Carbon black may also be used in blend with other reinforcing fillers and in particular reinforcing inorganic fillers as described above, and in particular silica. In this case, the total level of reinforcing filler is preferably within a range from 10 phr to 60 phr. According to a variant of the invention, carbon black constitutes the only reinforcing filler of the composition according to the invention. The filler of the composition according to the invention also comprises a non-reinforcing filler.As non-reinforcing filler, mention may be made, for example, of phyllosilicates such as kaolin, talc, mica, graphite, clays or modified clays ("organo clays"), bentonite, chalk. According to the invention, a non-reinforcing filler content greater than or equal to 10 phr is preferably used, preferably varying from 10 to 80 phr, more preferably from 10 to 70 phr and even more preferably from 20 to 70 phr. Kaolin may advantageously constitute the only non-reinforcing filler or the majority non-reinforcing filler, that is to say that the non-reinforcing filler preferably comprises more than 50% by weight of kaolin relative to the total weight of the non-reinforcing filler. According to a variant of the invention, the composition comprises kaolin at a rate greater than or equal to 10 pce, preferably ranging from 10 to 80 pce, more preferably from 10 to 70 pce and even more preferably from 20 to 70 pce.The composition according to the invention may then comprise a non-reinforcing filler other than kaolin with a rate preferably ranging from 0 to 50 phr, preferably from 0 to 40 phr. Very preferably according to this embodiment variant of the invention, the composition does not comprise other non-reinforcing fillers and kaolin constitutes the only non-reinforcing filler of the composition. According to a preferred embodiment variant of the invention, the rubber composition comprises carbon black at a rate varying from 10 to 60 phr, preferably from 20 to 50, more preferably from 20 to 45 phr, and kaolin at a rate varying from 10 to 80 phr, preferably from 10 to 70 phr and even more preferably from 20 to 70 phr. Preferably still the filler of the rubber composition consists of carbon black and kaolin, present in the composition at these same rates, as the only fillers. 3.Crosslinking system The rubber composition according to the invention is based on a crosslinking system. The crosslinking system of the rubber composition according to the invention comprises from 0.1 phr to 8 phr of a sulfur donor comprising N,N'-caprolactam disulfide. According to variants of the invention, the total level of sulfur donor in the crosslinking system of the rubber composition according to the invention is within a range varying from 0.1 phr to 5 phr, preferably from 0.1 phr to 4 phr, more preferably still from 0.2 phr to 3 phr, more preferably still from 0.2 phr to 2 phr. According to variants of the invention, N,N'-caprolactam disulfide is the majority sulfur donor in the crosslinking system of the composition. Preferably then, the total amount of N,N'-caprolactam disulfide (DTDC) is greater than or equal to 50% by weight, preferably greater than or equal to 70% by weight per 100% by weight of the sulfur donor.According to preferred variants of the invention, the sulfur donor of the crosslinking system of the rubber composition essentially consists of N,N'-caprolactam disulfide (DTDC). In other words, according to this variant, the sulfur donor of the crosslinking system of the rubber composition does not comprise any other sulfur donor than N,N'-caprolactam disulfide or N,N'-caprolactam disulfide is the only sulfur donor of the crosslinking system. The crosslinking system of the rubber composition may also comprise a sulfur donor other than N,N'-caprolactam disulfide, for example an alkylphenol disulfide (abbreviated as "APDS", for example para-(tert butyl) phenol disulfide), N,N'-dimorpholine disulfide or a combination thereof.The sulfur donor of the crosslinking system of the rubber composition according to the invention may further comprise at least one elastomeric binder (for example, EPDM / ethylene vinyl acetate (EVA, EVM)) and dispersants, preferably the total amount of the elastomeric binder and dispersants is greater than 0% by weight and less than 50% by weight, in particular less than or equal to 30% by weight per 100% by weight of the sulfur donor. According to variants, the level of N,N'-caprolactam disulfide in the rubber composition according to the invention is greater than or equal to 0.1 phr, more particularly greater than or equal to 0.2 phr, and less than or equal to 5 phr, preferably less than or equal to 4 phr, more preferably still less than or equal to 3 phr, more particularly still less than or equal to 2 phr.Thus, according to variants, the level of N,N'-caprolactam disulfide in the rubber composition according to the invention varies, for example, in a range from 0.1 to 5 phr, or even from 0.1 to 4 phr, preferably from 0.2 to 3 phr, more preferably from 0.2 to 2 phr. According to preferred variants of the invention, the level of N,N'-caprolactam disulfide in the rubber composition according to the invention varies, for example, from 0.2 to 1 phr. In this range, a significant and unexpected increase in the scorching time is observed in certain cases, without, however, having too much of an impact on the crosslinking time and while maintaining it industrially acceptable. Furthermore, the crosslinking system of the rubber composition according to the invention does not comprise either soluble sulfur or insoluble sulfur, or comprises less than 0.6 pce, particularly less than 0.4 pce, of soluble sulfur, insoluble sulfur or a mixture of soluble sulfur and insoluble sulfur.Preferably, when present, the total amount by weight of soluble sulfur and / or insoluble sulfur is less than that of the sulfur donor. According to a preferred variant, the crosslinking system of the rubber composition according to the invention comprises neither soluble sulfur nor insoluble sulfur. The crosslinking system of the rubber composition according to the invention may, according to certain variants, comprise a crosslinking agent other than a vulcanizing agent. Such a crosslinking agent is, for example, a peroxide, a bismaleimide, or one of their mixtures.The crosslinking system of the rubber composition according to the invention may, according to certain variants, comprise a vulcanization accelerator, in particular and in a known manner 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. Also optionally, a known vulcanization activator such as zinc oxide, stearic acid or equivalent compound such as stearic acid salts and transition metal salts, guanidine derivatives (in particular diphenylguanidine), or known vulcanization retarders may be used. 4.Various additives: 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) or mixtures thereof, tackifying resins, processing agents, pigments, protective agents such as anti-ozone waxes, chemical anti-ozonants, antioxidants. 5.Manufacture of rubber compositions: 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 thermomechanical 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 of between 110°C and 200°C, preferably between 135°C and 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 between 40°C and 100°C, a finishing phase during which the crosslinking system comprising N,N'-caprolactam is incorporated.The final composition thus obtained is then calendered, for example, in the form of a sheet or plate, in particular for laboratory characterization, or extruded in the form of a semi-finished (or profiled) rubber or a finished rubber product. The crosslinking of the composition can be carried out in a manner known to those skilled in the art, for example at a temperature between 130°C and 200°C, under pressure. The rubber product comprising at least in part a rubber composition according to the invention described above can be used in various applications.For example, a rubber composition according to the invention can be used for the manufacture of tires, shoe soles, rubber tracks (treads or pads), anti-vibration mounts, conveyor belts, timing belts, transmission belts, hoses, floor coverings, seals, molded mechanical parts. When the rubber composition according to the invention is used for the manufacture of tires, it can advantageously compose the inner rubber of a tire. The inner rubber ("innerliner" in English) of the tire is, in a known manner, a layer of rubber designed to prevent the flow of air and maintain the high air pressure of a tire. EXAMPLES OF EMBODIMENTS OF THE INVENTION The following examples illustrate the invention; however, the latter cannot be limited to these examples alone.In order to confirm the effect of the invention, rubber compositions (A1 to A3, B1 and C1: examples according to the invention, and TA, TB and TC: respective controls) were used. Each of the rubber compositions is based on an elastomer matrix comprising a butyl rubber, where appropriate combined with natural rubber (abbreviated as "NR"), a filler comprising carbon black (as reinforcing filler) and kaolin (non-reinforcing filler), and N,N'-caprolactam disulfide or alkylphenol disulfide as sulfur donor or sulfur in insoluble form. Each of the formulations of the rubber compositions is presented in Table 1 with the quantity of the different products expressed in phr. Compositions A1 and B1 are identical. 1.Preparation of rubber compositions The tests are carried out as follows: the charge composed of carbon black and kaolin, the plasticizing system if applicable, the butyl rubber and the natural rubber, as well as various other possible ingredients except the crosslinking system, are successively introduced into an internal mixer, filled to 70% by volume and with an initial tank temperature of approximately 60°C. Thermomechanical work (non-productive phase) is then carried out in one step, lasting a total of approximately 3 to 4 minutes, until a maximum "fall" temperature of 150°C is reached. The mixture thus obtained is recovered, cooled and then the crosslinking system comprising sulfur, alkylphenol disulfide or N,N'-caprolactam disulfide is incorporated on an external mixer (homo-finisher) at 30°C, mixing everything (productive phase) for an appropriate time (for example between 5 and 12 min).The compositions thus obtained are then calendered in the form of plates (thickness of 2 to 3 mm). 2. Measurement of the scorching time (or fixing time) and the crosslinking time The measurements are carried out before crosslinking at 130°C, in accordance with the French standard NF T 43-005. The evolution of the consistometric index as a function of time makes it possible to determine - the scorching time of the rubber compositions, assessed in accordance with the aforementioned standard by the parameter t5 (case of a large rotor), expressed in minutes, and defined as being the time necessary to obtain an increase in the consistometric index (expressed in MU) of 5 units above the minimum value measured for this index. - the crosslinking time applied, t90, which is the time necessary for the torque of the composition to reach 90% of the maximum torque of the composition at the end of recording the evolution of the torque as a function of time.The composition torques are measured at 130°C with an oscillating chamber rheometer, according to DIN 53529 - part 3 (June 1983). 3. Compositions and results Table 1 A1 A2 A3 B1 TC C1 Composition TA TB 1phr 0.5phr 2phr 1phr 1phr NR (1) 10.00 10.00 10.00 10.00 10.00 10.00 BIIR (2) 90.00 90.00 90.00 90.00 90.00 90.00 70.00 70.00 BIIR (3) 30.00 30.00 Kaolin (4) 30.00 30.00 30.00 30.00 30.00 30.00 20.00 20.00 Carbon black (5) 30.00 30.00 30.00 30.00 30.00 30.00 40.00 40.00 Processing agent (6) 3.00 3.00 3.00 3.00 3.00 3.00 2.00 2.00 Resin (7) 2.50 2.50 Sulphur donor (8) 0.40 0.80 Sulphur donor (9) 1.00 0.50 2.00 1.00 1.00 MBTS (10) 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 Stearic acid (11) 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 ZnO (12) 1.00 1.00 1.00 1.00 1.00 1.00 0.70 0.70 Insoluble sulfur (13) 1.25 Table 2 Composition T. A A1 A2 A3 T B B1 T CC1 DTDC rate (with APDS) 1phr 0.5phr 2phr (with S) 1phr (with APDS) 1phr Scorching time 10 24 25 24 15 24 13 30 t5(minutes) Increase in scorching time control 14 15 14 control 9 control 17 (minutes) Crosslinking time t90 (minutes) 19 30 27 31 23 30 21 33 Increase in scorching time control 11 8 12 control 7 control 12 (minutes) The results in Table 2 demonstrate that the compositions according to the invention (Ai, Bi and Ci) have a scorching time (t5) significantly longer than the scorching time (t5) of the control compositions (TA, TB and TC). More particularly, by comparing the compositions TA (control with 0.4 pce APDS) and A2 (according to the invention with 0.5 pce of DTDC), the roasting time is significantly and unexpectedly extended in view of the close sulfur donor levels. In addition, Table 2 shows that the examples (Ai, Bi and Ci) according to the invention have a crosslinking time (t 90) which remains acceptable from an industrial productivity point of view and whose increase compared to the respective controls (T A , T B and T C ) remains lower than the gain on the roasting time. More particularly, by comparing the compositions T A (control with 0.4 pce APDS) and A2 (according to the invention with 0.5 pce of DTDC) the scorching time is significantly extended without having too much impact on the crosslinking time, while maintaining it favourable from an industrial point of view. In conclusion, the rubber composition according to the invention makes it possible to ensure an improved (longer) scorching time while maintaining an industrially acceptable crosslinking time or without degrading it beyond what is industrially acceptable.

Claims

CLAIMS 1. Rubber composition based on at least: - an elastomer matrix comprising at least one butyl rubber; - a filler comprising at least one reinforcing filler and one non-reinforcing filler; and - a crosslinking system; wherein the crosslinking system comprises from 0.1 phr to 8 phr of a sulfur donor comprising N,N'-caprolactam disulfide; and wherein the crosslinking system comprises neither soluble sulfur nor insoluble sulfur, or comprises less than 0.6 phr of soluble sulfur, insoluble sulfur or a mixture of soluble sulfur and insoluble sulfur.

2. Rubber composition according to claim 1, wherein the elastomer matrix comprises at least 50 phr of butyl rubber. 3.Rubber composition according to any one of the preceding claims, in which the reinforcing filler comprises predominantly a carbon black, preferably at a rate greater than 50% by weight per 100% by weight of the reinforcing filler.

4. Rubber composition according to any one of the preceding claims, which comprises from 10 to 60 phr, preferably from 20 to 45 phr of carbon black.

5. Rubber composition according to any one of the preceding claims, in which the non-reinforcing filler comprises predominantly a kaolin, preferably at a rate greater than 50% by weight per 100% by weight of the non-reinforcing filler.

6. Rubber composition according to any one of the preceding claims, which comprises 10 to 80 phr, preferably from 10 to 70 phr, of kaolin as non-reinforcing filler. 7.A rubber composition according to any preceding claim, wherein the total amount of N,N'-caprolactam disulfide is greater than 50% by weight per 100% by weight of the sulfur donor.

8. A rubber composition according to any preceding claim, wherein the sulfur donor contains N,N'-caprolactam disulfide as the sole sulfur donor.

9. A rubber composition according to any one of claims 1 to 7, wherein the sulfur donor comprises a sulfur donor other than N,N'-caprolactam disulfide.

10. A rubber composition according to any one of the preceding claims, wherein the level of sulfur donor comprising N,N'-caprolactam disulfide varies from 0.1 to 5 phr, preferably from 0.2 to 3 phr.

11. A rubber composition according to any one of the preceding claims, wherein the level of N,N'-caprolactam disulfide varies from 0.1 to 4, preferably from 0.2 to 3 phr, more preferably from 0.2 to 2 phr.

12. A rubber composition according to any one of the preceding claims, wherein the level of N,N'-caprolactam disulfide varies from 0.2 to 1 phr. 13.A rubber composition according to any preceding claim, wherein, when present, the total amount by weight of soluble sulfur, insoluble sulfur, and the mixture of soluble sulfur and insoluble sulfur is less than that of the sulfur donor.

14. A rubber product comprising a rubber composition according to any one of claims 1 to 13.

15. A rubber product according to claim 14, which product is a tire, the inner rubber of which comprises the rubber composition according to any one of claims 1 to 13.

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