# Patent

## Title
Rubber blends containing silicic acid and comprising additives containing sulphur

## Abstract

The present invention relates to a silica-containing rubber mixture produced from at least one rubber, one sulphur-containing alkoxysilane, one crosslinking agent, one filler, and optionally further rubber auxiliary products, wherein this mixture comprises from 0.1 to 15 parts by weight, based on 100 parts by weight of rubber used, of a polysulphide additive of the formula (I)

A-S—(S)x—S—B  (I)

    where x is 0, 1, 2, 3 or 4,
    and A and B, independently of one another, are an optionally derivatised carboxyalkyl, carboxyaryl or hydroxyaryl group,
    and comprises from 0.1 to 15 parts by weight, based on 100 parts by weight of rubber used, of 1,6 -bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).

## Background
The present inversion relates to silica-containing rubber mixtures which comprise sulphur-containing additives, to processes for the production of this rubber mixture, to use thereof and to rubber vulcanisates produced therefrom and to an additive mixture for rubber.
Many proposals have been devised for successful production of tyres with reduced rolling resistance. DE-A2 255 577 and 4 435 311, EP-A1 0 0670 347, and also U.S. Pat. No. A4,709,065 have described particular polysulphidic silanes as reinforcing additives for silica-containing rubber vulcanisates. However, the use of the polysulphidic silanes described in those documents as reinforcing additives for silica-containing rubber vulcanisates have the disadvantage that in order to achieve acceptable processability it is necessary to use relatively large quantities of the expensive polysulphidic silanes, and that hardness is unsatisfactory.
Other additional substances have been proposed for improving the processability of silica-containing rubber mixtures, for example fatty acid esters, fatty acid salts or mineral oils. The additional substances mentioned have the disadvantage that they increase flowability, but at the same time reduce the moduli at relatively high elongation (e.g. from 100% to 300%) or else reduce the hardness of the vulcanisates, and some of the reinforcing effect of the filler is therefore lost. Insufficient hardness or stiffness of the vulcanisate results in unsatisfactory running performance of the tyre, particularly in curves.
An increase in the quantity added of the reinforcing filler increases the hardness of the vulcanisate, but the higher viscosity of the mixture is disadvantageous for processability, and the same applies to a reduction in the quantity of the plasticizing oil.
EP 1 134 253 describes polyether additives for silica-containing rubber vulcanisates which do not exhibit the abovementioned disadvantage of reducing the modulus. However, the person skilled in the art requires a usage quantity of 8% by weight of the product, based on the rubber, in order to increase the Shore A hardness value by 3 units. The low modulus at 300% elongation is disadvantageous.
EP 0 489 313 describes additives with good mechanical properties and with improved hysteresis performance. However, the examples reveal only slight, or no, increase of Shore A hardness in comparison with the prior art, bis[3-(triethoxysilyl)propyl]tetrasulphide according to German Offenlegungsschrift 2 255 577, and therefore no improvement of interaction between polymer and filler.
EP 1 000 968 moreover uses bis[3-(triethoxysilyl)propyl] tetrasulphides in combination with a specific reversion stabilizer in SBR, where the 300-modulus values are very low and therefore inadequate.
EP 0 791 622 B1 describes a rubber composition with at least one diene-based elastomer, filler composed of silica and of carbon black, and also with a silica coupling agent, selected from

    (i) tetrathiodipropanol polysulphide mixture or
    (ii) combination of tetrathiodipropanol polysulphide and bis(3-trialkoxysilylalkyl) polysulphide. In particular, the quantity of tetrathiodipropanol polysulphide is markedly greater than the quantity of bis(3-trialkoxysilylalkyl)polysulphide, and this is not advantageous economically because the tetrathiodipropanol polysulphide is relatively expensive. In addition, the said mixture exhibits very low tensile strength values. It can be concluded that the said mixture is too soft (as confirmed by the Shore A values measured), as reflected in relatively poor running performance of the tyre, and also reduced lifetime.


It is an object of the present invention to provide rubber mixtures which comprise a specific combination of additional substances which do not impair the flowability of rubber mixtures and which provide vulcanisates produced therefrom with good properties, in particular in respect of rolling resistance, abrasion and wet skid performance in tyres and the hardness or stiffness of the vulcanisate, with the possibility of resultant improvement in the running performance of tyres.
Surprisingly, it has now been found that, in combination with sulphur-containing alkoxysilanes 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6) and certain sulphur-containing additives do not adversely affect the flowabiiity of rubber mixtures and lead to vulcanisates with good dynamic performance, relatively short scorch time, good hardness/stiffness, markedly improved rolling resistance and, in particular, less abrasion. The full vulcanization time is moreover markedly improved.
It is believed that the synergistic effect results from improved interaction between polymer and filler. The invention therefore provides rubber mixtures produced from at least one rubber, one sulphur-containing alkoxysilane, one crosslinking agent, one filler, and optionally further rubber auxiliary products, and also at least one sulphur-containing additive of the formula (I)

A-S—(S)x—S—B  formula (I)

where x is 0, 1, 2, 3 or 4,

    A is a moiety






and

    B is a moiety






where

    R1 to R4 are identical or different and are hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or a group —CH2—OR5, —CH2—CH2—OR5, —NHR5, —COR5, —CH2COOR5where R5═hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or C1-C6-acyl, and
    R6 to R7 are identical or different and are hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or a group —CH2—OR5, —CH2—CH2—OR5, —NHR5, —COR5, —COOR5, —CH2COOR5, where R5═hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or C1-C6-acyl and
    y and z independently of one another are 0, 1 or 2, or
    A and B independently of one another are one of the radicals






in which R1 to R2 are identical or different and are hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or a group —CH2—OR5, —CH2—CH2—OR5, —NHR5, —COR5, —CH2COOR5where R5═hydrogen, C1C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or C1-C6-acyl, and from 0.1 to 15 parts by weight, based on 100 parts by weight of rubber used, of l,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).

It is preferable to use, as polysulphide additive, at least one of the compounds of the formula (II):




in which

    x is 0, 1,2, 3 or 4, particularly preferably 2.


It is preferable to use, as polysulphide additive, at least one of the compounds of the formula (IIa),




in which x is 0, 1, 2, 3 or 4, particularly preferably 2.

It is preferable to use, as polysulphide additive, at least one of the compounds of the formulae (III), (IIIa), (IIIb).




in which x is 0, 1, 2, 3 or 4, particularly preferably 2.

It is preferable to use, as polysulphide additive, at least one compound of the formula (IV),




particularly preferably L-cystine, CAS No.: 56-89-3.

The addition of the rubber additive 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6) to be used according to the invention, and also of at least one sulphur-containing additive of the formula (I), and also the addition of, optionally, other additional substances preferably takes place in the first portion of the mixing process when temperatures of the composition are from 120 to 200° C.; however, it can also take place later at lower temperatures (from 20 to 120° C.), for example with sulphur crosslinking agent and/or with accelerator. The form in which the additive of the invention is added here can be directly that of a mixture of the components 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6) and at least one sulphur-containing additive of the formula (I), or can be that of the individual components.
Other preferred polysulphide additives are listed in the Examples.
It is preferable that the silica-containing rubber mixture of the invention comprises at least one SBR rubber and at least one BR rubber.
It preferably comprises at least one SBR rubber and at least one BR rubber, in a ratio by weight SBR:BR=from 60:40 to 90:10.
It can preferably also comprise at least one NR rubber.
It is preferable that it comprises at least one SBR rubber and at least one BR rubber and at least one NR rubber in a ratio of at least 60 and at most 85 percent by weight of SBR, based on rubber, and at least 10 and at most 35 percent by weight of BR, based on rubber, and at least 5 and at most 20 percent by weight of NR, based on rubber.
Both natural rubber and synthetic rubbers are suitable for the production of the rubber mixtures of the invention and of the rubber vulcanisates of the invention. Preferred synthetic rubbers are described by way of example in W. Hofmann, Kautschuktechnologie [Rubber technology], Genter-Verlag, Stuttgart 1980.
They encompass inter alia

    BR— polybutadiene
    ABR— butadiene/C1-C4-alkyl acrylate copolymer
    CR— polychloroprene
    IR— polyisoprene
    SBR— styrene/butadiene copolymers with styrene contents of from 1 to 60% by weight, preferably from 20 to 50% by weight
    IIR— isobutylene/isoprene copolymers
    NBR— butadiene/acrylonitrile copolymers with acrylonitrile contents of from 5 to 60% by weight, preferably from 10 to 50% by weight
    HNBR— partially hydrogenated or fully hydrogenated NBR rubber
    EPDM— ethylene/propylene/diene copolymers,

and also mixtures of these rubbers.



It is preferable that the silica-containing rubber mixture according to the invention comprises from 0.3 to 7 parts by weight of one or more polysulphide additives of the formula (I) or of any of the following formulae derived therefrom, as listed in the claims, based on 100 parts by weight of rubber used and from 0.3 to 7 parts by weight of 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.; 151900-44-6), based on 100 parts by weight of rubber used.
It is preferable that the quantity of sulphur-containing alkoxysilane is greater than or equal to the quantity of the polysulphide additive of the formula (I).
It is preferable to use the sulphur-containing alkoxysilane in a ratio by weight of from 1.5:1 to 20:1, particularly from 5:1 to 15:1, in relation to the polysulphide additive of the formula (I).
The rubber mixture of the invention preferably comprises from 0.5 to 5 parts by weight, based on 100 parts by weight of rubber used, of a polysulphide additive of the formula (I), and preferably comprises from 0.5 to 5 parts by weight of 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6), based on 100 parts by weight of rubber used.
The present invention further provides rubber vulcanisates which can be produced from the rubber mixtures of the invention.
The present invention further provides a process for the production of filled rubber vulcanisates, which is characterized in that

    i) at least one robber is mixed with
    ii) from 10 to 150% by weight, preferably from 30 to 120% by weight, based on rubber (i), of filler and
    iii) from 0.1 to 15% by weight, preferably from 0.3 to 7% by weight, based on rubber (i), of polysulphide additives of the formula (I) and
    iv) from 0.1 to 15% by weight, preferably from 0.3 to 7% by weight, of 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6), based on rubber (i) where temperatures of the composition are at least 120° C. and shear rates are from 1 to 1000 sec (exp.−1), preferably from 1 to 100 sec (exp.−1) and the mixture is then vulcanized conventionally after addition of further vulcanization chemicals.


It is preferable that the addition of the inventive polysulphide additives of the formula (I), and also the addition of 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6) takes place in the first portion of the mixing process where temperatures of the composition are from 100 to 200° C. and shear rates are as mentioned, but it can also take place later at lower temperatures (from 40 to 100° C.), for example together with sulphur and accelerator.
The form in which the polysulphide additives of the formula (I) and 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6) are added to the mixing process can mutually independently be either pure form or else a form absorbed on inert, organic or inorganic carriers. Preferred carrier materials are silica, natural or synthetic silicates, aluminium oxide and/or carbon blacks.
It is also possible that the polysulphide additives of the formula (I) are added to the mixing process in the form of a mixture with 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).
For the purposes of this invention, silica-containing fillers that can be used for the rubber mixture and rubber vulcanisates of the invention are the following fillers:

    
    
        fine-particle silica, produced for example by precipitation from solutions of silicates or flame hydrolysis of silicon halides with specific surface areas of from 5 to 1000 m2/g, preferably from 20 to 400 m2/g (BET surface area) and with primary particle sixes of from 10 to 400 nm. The silicas can optionally also take the form of mixed oxides with other metal oxides, such as Al, Mg, Ca, Ba, Zn, Zr, Ti oxides.
        Synthetic silicates, such as aluminium silicate, alkaline earth metal silicates, such as magnesium silicate or calcium silicate, with BET surface areas of from 20 to 400 m2/g and primary particle size of from 10 to 400 nm,
        natural silicates, such as kaolin and other naturally occurring silicas,
        glass fibres and glass-fibre products (mats, strands) or glass microbeads.
    
    


Other fillers that can be used are carbon blacks. The carbon blacks to be used here are produced by way of example by the lamp-black process, furnace-black process or gas-black process and have BET surface areas of from 20 to 200 m2/g, examples being SAF, ISAF, IISAF, HAF, FEF, or GPF carbon blacks.
Preferred quantities used of the polysulphide additives of the formula (I) in the rubber mixtures of the invention are from 0.3 to 7%, based on rubber.
One particularly preferred variant consists in the combination of silica, carbon black and polysulphide additives of the formula (I), The ratio of silica to carbon black in this combination can be varied as desired. From the point of view of tyre technology, preference is given to a silica: carbon black ratio of from 20:1 to 1.5:1.
Sulphur-containing silanes that can be used for the rubber vulcanisates according to the invention are preferably bis(triethoxysilylpropyl) tetrasulphane and the corresponding disulphane and 3-triethoxysilyl-1-propanethiol or silanes such as Si 363 from Evonik, Germany or silane NXT or NXT Z from Momentive (previously GE, USA), where the alkoxy moiety is methoxy or ethoxy where quantities used are from 2 to 20 parts by weight, preferably from 3 to 11 parts by weight, calculated in each case as 100% strength active ingredient and based on 100 parts by weight of rubber. However, it is also possible to use a mixture made of the said sulphur-containing silanes. Liquid sulphur-containing silanes can have been absorbed on a carrier to improve ease of metering and/or ease of dispersion (dry liquid). Active ingredient content is from 30 to 70 parts by weight, preferably from 40 to 60 parts by weight, for every 100 parts by weight of dry liquid.
The rubber vulcanisates according to the invention can comprise other rubber auxiliary products, for example reaction accelerators, antioxidants, heat stabilisers, light stabilizers, antiozonants, processing aids, plasticizers, tackifiers, blowing agents, dyes, pigments, waxes, extenders, organic acids, retardants, metal oxides, and also activators, such as triethanolamine, polyethylene glycol, hexanetriol, where these are known to the rubber industry.
The quantity used of the rubber auxiliary products is conventional and depends inter alia on the intended purpose of the vulcanisates. Conventional quantities, based on rubber, are from 0.1 to 30% by weight.
The following are used as crosslinking agents: peroxides, sulphur, magnesium oxide, zinc oxide, and the known vulcanisation accelerators can also be added to these, for example mercaptobenzothiazoles, -sulphenamides, thiurams, thiocarbamates, guanidines, xanthogenates and thiophosphates. Preference is given to sulphur.
The quantities used of the crosslinking agents and vulcanization accelerators are about 0.1 to 10% by weight, preferably 0.1 to 5% by weight, based on rubber.
As mentioned above, it is advantageous to add antioxidants to the rubber mixture to counteract the effect of heat and oxygen. Suitable phenolic antioxidants are alkylated phenols, styrenated phenol, sterically hindered phenols such as 2,6-di-tert-butylphenol, 2,6-di-tert-butyl-p-cresol (BHT), 2,6-di-tert-butyl-4-ethylphenol, sterically hindered phenols containing ester groups, sterically hindered phenols containing thioether, 2,2′-methylenebis(4-methyl-6-tert-butylphenol) (BPH), and also sterically hindered thiobisphenols.
If discoloration of the rubber is not of importance, it is also possible to use aminic antioxidants, e.g. mixtures of diaryl-p-phenylenediamines (DTPD), octylated diphenylamine (ODPA), phenyl-α-naphtyhlamine (PAN), phenyl-β-naphthylamine (PBN), preferably those based on phenylenediamine. Examples of phenylenediamines are N-isopropyl-N′-phenylenediamine, N-1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine (6PPD), N-1,4-dimethylpentyl-N′-phenyl-p-phenylenediamine (7PPD), N,N′-bis(1,4-dimethylpentyl)-p-phenylenediamine (77PD).
Among the other antioxidants are phosphites such as tris(nonylphenyl)phosphite, polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ), 2-mercaptobenzimidazole (MBI), methyl-2-mercaptobenzimaidazole (MMBI), zinc methylmercaptobenzimidazole (ZMMBI). The phosphites are generally used in combination with phenolic antioxidants. TMQ, MBI and MMBI are mainly used for NBR types which are vulcanized peroxidically.
Ozone resistance can be improved by using antioxidants known to a person skilled in the art, such as N-1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine (6PPD), N-1,4-dimethylpentyl-N′-phenyl-p-phenylenediamine (7PPD), N,N′-bis(1,4-dimethylpentyl)-p-phenylenediamine (77PD), enol ethers or cyclic acetals.
Processing aids are intended to act between the rubber particles and to counteract frictional forces during the mixing, plastification and shaping process. Processing aids which can be present in the rubber mixture according to the invention are any of the lubricants conventionally used for the processing of plastics, for example hydrocarbons, such as oils, paraffins and PE waxes, fatty alcohols having from 6 to 20 carbon atoms, ketones, carboxylic acids, such as fatty acids and montanic acids, oxidised PE wax, metal salts of carboxylic acids, carboxamides and carboxylic esters, for example with the following alcohols: ethanol, fatty alcohols, glycerol, ethanediol, pentaerythritol, and long-chain carboxylic acids as acid component.
The rubber mixture can be crosslinked not only with sulphur accelerator systems but also with peroxides.
Examples of crosslinking agents that can be used are peroxidic crosslinking agents such as bis (2,4-dichlorobenzyl) peroxide, dibenzoyl peroxide, bis(4-chlorobenzoyl) peroxide, 1,1-bis (tert-butylperoxy)-3,3,5-tremethylcyclohexane, tert-butyl perbenzoate, 2,2-bis(tert-butylperoxy)butene, 4,4-di-tert-butyl peroxynonylvalerate, dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, tert-butylcumyl peroxide, 1,3-bis-(tert-butylperoxyisopropyl) benzene, di-tert-butyl peroxide and 2,5-dimethyl-2,5-di(tert-butylperoxy)hex-3-yne.
It can be advantageous to use, alongside the said peroxidic crosslinking agents, further additions which can be used to increase crosslinking yield: a suitable example here being triallyl isocyanurate, triallyl cyanurate, trimethylolpropane tri(meth)acrylate, triallyl trimellitate, ethylene glycol dimethacrylate, butanediol dimethacrylate, trimethylolpropane trimethacrylate, Zn diacrylate, Zn dimethacrylate, 1,2-polybutadiene or N,N′-m-phenylenedimaleimide.
Another crosslinking agent that can be used is sulphur in elemental soluble or insoluble form or sulphur donors.
Examples of sulphur donors that can be used are dimorpholyl disulphide (DTDM), 2-morpholino-dithiobenzothiazole (MBSS), caprolactam disulphide, dipentamethylenethiuram tetrasulphide (DPTT), and tetramethylthiuram disulphide (TMTD).
For the sulphur-vulcanization of the rubber mixture according to the invention, it is also possible to use further additions which can be used to increase crosslinking yield. In principle, however, it is also possible to use sulphur or sulphur donors alone for crosslinking.
Examples of suitable additions which can be used to increase crosslinking yield are dithiocarbamates, thiurams, thiazoles, sulphenamides, xanthogenates, bi- or polycyclic amines, guanidine derivatives, dithiophosphates, caprolactams and thiourea derivatives.
Examples of equally suitable additions are: diamine zinc diisocyanate, hexamethylenetetramine, 1,3-bis-(citraconimidomethyl)benzene and also cyclic disulphanes.
Preference is given to the sulphur accelerator system in the rubber mixture according to the invention.
In order to reduce flammability and to reduce smoke generation during combustion, the rubber mixture composition according to the invention can also comprise flame retardants. An example of a flame retardant used is antimony trioxide, phosphoric esters, chloroparaffin, aluminium hydroxide, boron compounds, zinc compounds, molybdenum trioxide, ferrocene, calcium carbonate or magnesium carbonate.
The rubber vulcanisate can also comprise other synthetic polymers, acting by way of example as polymeric processing aids or impact modifiers. The said synthetic polymers are selected from the group consisting of the homo- and copolymers based on ethylene, propylene, butadiene, styrene, vinyl acetate, vinyl chloride, glycidyl acrylate, glycidyl methacrylate, acrylates and methacrylates having alcohol components of branched or unbranched C1-C10-alcohols. Particular mention may be made of polyacrylates having identical or different alcohol moieties from the group of the C4-C8-alcohols, particularly of butanol, hexanol, octanol and 2-ethylhexanol, polymethyl methacrylate, methyl methacrylate-butyl acrylate copolymers, methyl methacrylate-butyl methacrylate copolymers, ethylene-vinyl acetate copolymers, chlorinated polyethylene, ethylene-propylene copolymers, ethylene-propylene-diene copolymers.
The rubber vulcanisate according to the invention can be used for producing foams. For this, chemical or physical blowing agents are added. Chemical blowing agents that can be used are any of the substances known for this purpose, for example azodicarbonamide, p-toluolsulphonyl hydrazide, 4,4′-oxybis(benzenesulphonyl hydrazide), p-toluenesulphonylsemicarbazide, 5-phenyltetrazole, N,N′-dinitrosopentamethylenetetramine, zinc carbonate or sodium hydrogencarbonate, and also mixtures comprising these substances. An example of a suitable physical blowing agent is carbon dioxide or halogenated hydrocarbons.
The vulcanization process can take place at temperatures of from 100 to 200° C., preferably from 130 to 180° C., optionally under a pressure of from 10 to 200 bar.
The blending of the rubber with the filler and with the polysulphide additives of the formula (I) can be carried out in/on conventional mixing assemblies, for example rolls, internal mixers and mixing extruders.
The rubber vulcanisates according to the invention are suitable for producing mouldings with improved properties, e.g. for producing cable sheathing, hoses, drive belts, conveyor belts, roll coverings, tyres, shoe soles, sealing rings and damping elements.
An important factor in the processing of rubbers is that the rubber mixture initially prepared with the additives has low flow viscosity (Mooney viscosity ML 1+4/100° C.), so that it is easy to process. In many applications, the intention is that the vulcanization process which follows (for example at 170° C., t95) for the rubber mixture is to proceed as rapidly as possible with exposure to heat, in order to restrict the cost of time and of energy.
The shaping-dependent scorch time (for example t5) is intended to be within a narrow range of times, in particular >300 seconds and <500 seconds.
It is preferable that the loss factor tan delta of a vulcanisate produced from the silica containing rubber mixture according to the invention under heating conditions 170° C./t95 is <0.13 at 60° C. and that the Shore A hardness thereof is simultaneously >60 at 23° C., and it is particularly preferable that the loss factor tan delta is <0.12 at 60° C. and that the shore A hardness is simultaneously >60 at 23° C. The 300 modulus value of the vulcanisate is >12 MPa, preferably >15 MPa, in particular >20 MPa.
It is preferable that the loss factor tan delta of a vulcanisate produced from the silica-containing rubber mixture under heating conditions 170° C./t95 is less than 0.12 at 60° C. and that its scorch time is simultaneously greater than 300 seconds, hut less than 1000 seconds.
It is preferable that the loss factor tan delta of a vulcanisate produced from the silica-containing rubber mixture under heating conditions 170° C./t95 is less than 0.12 at 60° C. and that its full vulcanisation time is simultaneously less than 1000 seconds, preferably less than 500 seconds.
It is preferable that the scorch time of a vulcanisate produced from the silica-containing rubber mixture under heating conditions 170° C./t95 is greater than 300 seconds and that its full vulcanisation time is simultaneously less than 500 seconds.
The invention further provides the use of the silica-containing rubber mixture of the invention for the production of vulcanisates and rubber mouldings of any type, in particular for the production of tyres and tyre components.
The invention further provides an additive mixture comprising one or more polysulphide additives of the formula (I) and 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).
It is preferable that the additive mixture comprises from 10 to 90 parts by weight of one or more polysulphide additives of the formula (I), based on 100 parts by weight of additive mixture, and from 10 to 90 parts by weight, based on 100 parts by weight of additive mixture, of 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).
It is preferable that the ratio by weight of one or more polysulphide additives of the formula (I) to 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6) in the additive mixture of the invention is from 10:90 to 90:10, in particular from 25:75 to 75:25.
The automobile industry has been searching for cost-effective ways of reaching the target of no more than 130 g/km of CO2 emission, at least since the European Union has been concerned with limits for carbon dioxide emission from cars. Low-rolling-resistance tyres are of substantial importance here. They reduce fuel consumption by requiring less energy for deformation during freewheeling.
In order that the reduction of rolling resistance is not achieved at the cost of other important properties, the requirements relating to wet grip and rolling noise are also simultaneously defined. A first indication of wet skid performance and roiling resistance is givers by the loss factor tan delta. This should be as high as possible at 0° C. (good wet skid resistance) and as low as possible at from 60 to 70° C. (low rolling resistance). The hardness of a rubber vulcanisate gives a first indication of its stiffness.


POLYSULPHIDE ADDITIVE EXAMPLES
Example 1




    Apparatus: 500 ml four-necked flask with thermometer, dropping funnel with pressure equalization, reflux condenser with gas-discharge attachment (bubble counter) and tubing, stirrer,
    Initial charge: 91.75 g=0.75 mol of methyl 3-mercaptopropionate (Acros, ≧98%) 250 ml of cyclohexane (analytical grade, from Merck, dried over molecular sieve)
    Feed: 51.15 g=0.375 mol of disulphur dichloride (≧99%, from Merck)


In the apparatus flushed with nitrogen, dried cyclohexane and methyl 3-mercaptopropioniate are introduced. When the methyl 3-mercaptopropionate is fully dissolved, the disulphur dichloride is added dropwise in the course of about 1 hour at a temperature of 5-10° C., during which nitrogen is passed through the mixture. The metering rate should be set such that a temperature of 10° C. is not exceeded.
After the end of the reaction, stirring is continued, still with nitrogen being passed through, at room temperature overnight.
The reaction solution is subsequently concentrated by evaporation on a Rotavapor at 50° C. and the product is dried to constant weight in a vacuum drying oven at 60° C.

    Yield: 108.4 g (95.6%) of a polysulphide mixture of the idealised formula






Example 2




    Apparatus: 2000 ml four-necked flask with thermometer, dropping funnel with pressure equalization, reflux condenser with gas-discharge attachment (bubble counter) and tubing, stirrer, gas-inlet tube
    Initial charge: 118.0 g=0.75 mol of mercaptobenzoic acid (Aldrich, ≧99%) 900 ml of toluene (analytical grade, from Aldrich, dried on a molecular sieve)
    Feed: 51.15 g=0.375 mol of disulphide dichloride (≧99%, from Merck)


In the apparatus flushed with nitrogen, dried toluene and mercaptobenzoic acid are introduced. The suspension present is then admixed dropwise with the disulphur dichloride over the course of about 1 hour at a temperature of 0-5° C., with nitrogen being passed through. The metering rate should be set such that a temperature of 5° C. is not exceeded.
After the end of reaction, stirring is continued, with nitrogen being passed through, at room temperature overnight.
The reaction solution is subsequently filtered through a D4 frit and the filter product is rinsed 2× with about 200 ml of dried toluene. The product is dried in a vacuum drying oven at room temperature (about 25° C.).

    Yield: 144.6 g (104.1%) of a polysuiphide mixture of the idealized formula






Example 3




Results

The examples below provide further explanation of invention, but there is no intention that the invention be restricted thereby.
The following rubber formulations, listed in Table 1, were selected for the tests. Unless otherwise stated, all numeric data are based on “parts per hundred rubber” (phr).
The following rubber mixtures were produced in a 1.5 L internal mixer (70 rpm), start temperature 80° C., mixing time: 5 minutes. Sulphur and accelerator were finally admixed on a roll (temperature: 50° C.).







TABLE 1





Rubber formulation



















Reference
Rubber formulation 1
Rubber formulation 2





BUNA CB 24
30
30
30


BUNA VSL 5025-1
96
96
96


CORAX N 339
6.4
6.4
6.4


VULKASIL S
80
80
80


TUDALEN 1849-1
8
8
8


EDENOR C 18 98-100
1
1
1


VULKANOX 4020/LG
1
1
1


VULKANOX HS/LG
1
1
1


ROTSIEGEL ZINC WHITE
2.5
2.5
2.5


ANTILUX 654
1.5
1.5
1.5


SI 69
6.4
6.4
6.4


VULKACIT D/C
2
2
2


VULKACIT CZ/C
1.5
1.5
1.5


CHANCEL 90/95 GROUND
1.5
1.5
1.5


SULPHUR


Vulcuren

1
1


Example 1

1


Example 2


1













Non-inventive comparisons
Comparison 1
Comparison 2
Comparison 3





BUNA CB 24
30
30
30


BUNA VSL 5025-1
96
96
96


CORAX N 339
6.4
6.4
6.4


VULKASIL S
80
80
80


TUDALEN 1849-1
8
8
8


EDENOR C 18 98-100
1
1
1


VULKANOX 4020/LG
1
1
1


VULKANOX HS/LG
1
1
1


ROTSIEGEL ZINC WHITE
2.5
2.5
2.5


ANTILUX 654
1.5
1.5
1.5


SI 69
6.4
6.4
6.4


VULKACIT D/C
2
2
2


VULKACIT CZ/C
1.5
1.5
1.5


CHANCEL 90/95 GROUND
1.5
1.5
1.5


SULPHUR


Example 1
1


Example 2

1


Vulcuren


1












BUNA CB 24
BR
Lanxess Deutschland GmbH


BUNA VSL 5025-1
SBR
Lanxess Deutschland GmbH


CORAX N 339
Carbon black
Degussa-Evonik GmbH


VULKASIL S
Silica
Lanxess Deutschland GmbH


TUDALEN 1849-1
Mineral oil
Hansen&Rosenthal KG


EDENORC 18 98-100
Stearic acid
Cognis Deutschland GmbH


VULKANOX 4020/LG
N-1,3-dimethybutyl-N-phenyl-p-
Lanxess Deutschland GmbH



phenylenediamine


VULKANOX HS/LG
Polymerized 2,2,4-trimethyl-1,2-
Lanxess Deutschland GmbH



dihydroquinoline


ROTSIEGEL ZINC
Zinc oxide
Grillo Zinkoxid GmbH


WHITE


ANTILUX 654
Light-stabilizer wax
RheinChemie Rheinau GmbH


Si 69
bis(triethoxysilylpropyl)
Evonik Industries



tetrasulphide


VULKACIT D/C
1,3-Diphenylguanidine
Lanxess Deutschland GmbH


VULKACIT CZ/C
N-cyclohexyl-2-benzothiazole-
Lanxess Deutschland GmbH



sulphenamide


CHANCEL 90/95
Sulphur
Solvay Deutschland GmbH


GROUND SULPHUR


Vulcuren
1,6,bis(N,N-dibenzyl-
Lanxess Deutschland GmbH



thiocarbamoyldithio)hexane
















TABLE 2







Collation of results
















Rubber
Rubber


Parameter
Unit
DIN
Reference
formulation 1
formulation 2















Mooney viscosity
[MU]
53523
95
125
147


(ML 1 + 4)


Mooney scorch at
sec
acc. to
1253
346
461


130° C. (t5)

ASTM D




5289-95


Full vulcanization at
s
53529
1417
246
290


170° C./t95


Shore A hardness at
[Shore A]
53505
66
65
65


23° C.


300 modulus
MPa
53504
15
21
21


Elongation at break
%
53504
349
327
303


Tensile strength
MPa
53504
19
19
21


Abrasion
mm3
53516
74
64
65


Wet skid performance
—

0.463
0.386
0.437


(tan d (0° C.))


Rolling resistance
—

0.133
0.116
0.111


(tan d (60° C.))









Surprisingly, as shown by the results in Table 2, it was found that the scorch time (130° C./t5) measured in the Inventive Examples (rubber formulation 1 and rubber formulation 2) was significantly shorter when comparison was made with the reference. Mechanical properties such as tensile strength, elongation at break and in particular 300 modulus are very good. The vulcanisates tested show good wet skid performance and markedly improved rolling resistance in relation to the reference (tan delta at 0° C.>0.35 and tan delta at 60° C.<0.13) and likewise very advantageous abrasion values (<70 mm3). There was a marked improvement in full vulcanization time.







TABLE 2a







Collation of results of comparisons















Comparison
Comparison
Comparison


Parameter
Unit
DIN
1
2
3















Mooney viscosity (ML 1 + 4)
[MU]
53523
82
92
106


Mooney scorch at 130° C. (t5)
sec
acc. to
1244
1228
551




ASTM D




5289-95


Full vulcanization at 170° C./t95
s
53529
1617
1315
262


Shore A hardness at 23° C.
[Shore A]
53505
72
73
65


300 modulus
MPa
53504
17
18
19.5


Elongation at break
%
53504
346
308
313


Tensile strength
MPa
53504
20
18
20.5


Abrasion
mm3
53516
95
93
78


Wet skid performance (tan d (0° C.))
—

0.444
0.452
0.434


Rolling resistance
—

0.168
0.154
0.112


(tan d (60° C.))









Surprisingly, as shown by the results in Table 2 and 2a, the scorch time (130° C./t5) measured in the Inventive Examples was found to be significantly shorter than in the Comparative Examples. The vulcanisates of the invention show better 300 modulus values and excellent rolling resistance. The very advantageous abrasion values (<70 mm3) of the Inventive Examples are particularly surprising. Abrasion was improved by 20% in the Inventive Examples when comparison is made with Comparative Example 3. Measured on the basis of the abrasion values from Comparative Examples 1 and 2, the improvement potential achieved with the Inventive Examples is more than 40%.
Testing of the Rubber Mixture and of the Vulcanisates:
Mooney Viscosity Measurement:
Viscosity can be determined directly from the resisting force exerted by the rubbers (and rubber mixtures) while they are processed. In the Mooney shearing-disc viscometer a grooved disc is surrounded above and below by sample substance and is rotated at about two revolutions per minute in a beatable chamber. The force required for this purpose is measured in the form of torque and corresponds to the respective viscosity. The specimen is generally preheated to 100° C. for 1 minute; the measurement takes a further 4 minutes, while the temperature is held constant.
The viscosity is given together with the respective test conditions, an example being ML (1+4) 100° C. (Mooney viscosity, large rotor, preheat time and test time in minutes, test temperature).
The viscosities of the rubber mixtures specified in Table 1 are measured by means of a Mooney shearing-disc viscometer.
Scorch Performance (t5 Scorch Time):
The same test can also be used as described above to measure the “scorch” performance of a mixture. The temperature selected in this Patent is 130° C. The rotor runs until, after the torque value has passed through a minimum, it has risen to 5 Mooney units relative to the minimum value (t5). The greater the value (the unit here being seconds), the slower the scorch (high scorch values here).
Rheometer (Vulcameter) 170° C./t95 Full Vulcanization Time:
The progress of vulcanization in a MDR (moving die rheometer) and analytical data therefor are measured in accordance with ASTM D5289-95 in a MDR 2000 Monsanto rheometer. Table 2 collates the results of this test.
The time at which 95% of the rubber has crosslinked is measured as the full vulcanization time. The temperature selected was 170° C.
Determination of Hardness
In order to determine the hardness of the rubber mixture according to the invention, milled sheets of thickness 6 mm made of the rubber mixture were produced according to formulations from Table 1. Test specimens of diameter 35 mm were cut from the milled sheets, and the Shore A hardness values were determined for these by means of a digital Shore hardness tester (Zwick GmbH & Co. KG, Ulm).
Tensile Test:
The tensile test serves directly to determine the loading limits of an elastomer. The longitudinal elongation at break is divided by the initial length to give the elongation at break. The force required to reach certain stages of elongation, mostly 50, 100, 200 and 300%, is also determined and expressed as modulus (tensile strength at the given elongation of 300%, or 300 modulus).
Table 2 lists the test results.
Dyn, Damping:
Dynamic test methods are used to characterise the deformation performance of elastomers under loadings which change periodically. An external stress changes the conformation of the polymer chain.
This measurement determines the loss factor tan delta indirectly by way of the ratio between loss modulus G′′ and storage modulus G′.

## Claims

1. A silica-containing rubber mixture comprising:
a rubber,
a sulphur-containing alkoxysilane,
a crosslinking agent,
a filler,
0.1 to 15 parts by weight, based on 100 parts by weight of the rubber, of a polysulphide additive of the formula (I)


A—S—(S)x—S—B  (I)


where x is 0, 1, 2, 3 or 4,
A is a moiety




and
B is a moiety




where
R1 to R4 are identical or different and are hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or a group —CH2—OR5, —CH2—CH2—OR5, —NHR5, —COR5, —COOR5, —CH2COOR5, where R5=hydrogen, C1-C6-alkyl, C5- C6-cycloalkyl, C6-C10-aryl or C1-C6-acyl,
R6 R7 are identical or different and are hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or a group —CH2—OR5, —CH2—CH2—OR5, —NHR5, —COR5, —COOR5, —CH2COOR5, where R5=hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or C1-C6-acyl, and
y and z independently of one another are 0, 1 or 2, or
A and B independently of one another are one of the radicals




in which R1 and R2 are identical or different and are hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or a group —CH2—OR5, —CH2—CH2—OR5, —NHR5, —COR5, —COOR5, —CH2COOR5, where R5=hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or C1-C6-acyl, and
0.1 to 15 parts by weight, based on 100 parts by weight of the rubber, of 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).

2. The silica-containing rubber mixture according to claim 1, wherein the polysulphide additive is a compound of the formula (II),



where x is 0, 1, 2, 3 or 4.

3. The silica-containing rubber mixture according to claim 1, wherein the polysulphide additive is a compound of the formula (IIa),



where x is 0, 1, 2, 3 or 4.

4. The silica-containing rubber mixture according to claim 1, wherein the polysulphide additive is a compound of the formula (III), (IIIa), (IIIb),



where x is 0, 1, 2, 3 or 4, particularly preferably 2.

5. The silica-containing rubber mixture according to claim 1, wherein the polysulphide additive is a compound of the formula (IV)

6. The silica-containing rubber mixture according to claim 1, wherein a quantity of sulphur-containing alkoxysilanes is greater than or equal to the quantity of polysulphide additives of the formula (I).

7. The silica-containing rubber mixture according to claim 1, wherein the rubber comprises at least one SBR rubber and at least one BR rubber.

8. The silica-containing rubber mixture according to claim 7, wherein the rubber also comprises at least one NR rubber.

9. The silica-containing rubber mixture according to claim 1, wherein the mixture comprises 1 to 15 parts by weight of one or more sulphur-containing alkoxysilanes, based on 100 parts by weight of the rubber, and a ratio by weight of sulphur-containing alkoxysilane to polysulphide additive of the formula (I) is 1.5:1 to 20:1.

10. The silica-containing rubber mixture according to claim 1, wherein the mixture comprises 0.3 to 7 parts by weight of the polysulphide additives of the formula (I), based on 100 parts by weight of the rubber, and 0.3 to 7 parts by weight, based on 100 parts by weight of the rubber, of 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).

11. to the silica-containing rubber mixture according to claim 1, wherein the mixture comprises 0.5 to 5 parts by weight, based on 100 parts by weight of the rubber, of the polysulphide additive of the formula (I) and 0.5 to 5 parts by weight, based on 100 parts by weight of the rubber, of 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).

12. The silica-containing rubber mixture according to claim 1, wherein the filler comprises one or more inorganic and/or organic fillers, where a quantity of the fillers is 50 to 200 parts by weight, based on 100 parts by weight of the rubbers.

13. The silica-containing rubber mixture according to claim 12, wherein the filler is selected from the group consisting of oxidic fillers, silicatic fillers, carbon black fillers, and mixtures of these.

14. The silica-containing rubber mixture according to claim 13, wherein the filler is selected from the group consisting of precipitated silicas and/or silicates with specific surface area of 20 to 400 m2/g.

15. A method for producing vulcanisates or rubber mouldings, the method comprising vulcanizing the silica-containing rubber mixture according to claim 1.

16. Vulcanisates or rubber mouldings comprising silica-containing rubber mixtures according to claim 1.

17. A process for the production of the silica-containing rubber mixture according to claim 1, the process comprising mixing the rubber, the sulphur-containing alkoxysilane, the crosslinking agent, the filler, the at least one polysulphide additive, and the 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).

18. An additive mixture comprising:
10 to 90 parts by weight, based on 100 parts by weight of additive mixture, of 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6), and
10 to 90 parts by weight of one or more polysulphide additives of the formula (I), based on 100 parts by weight of additive mixture


A-S—(S)x—S—B  (I)


where x is 0, 1, 2, 3 or 4,
A is a moiety




and
B is a moiety




where
R1to R4 are identical or different and are hydrogen, C1-C6-cycloalkyl, C6-C10-aryl or a group —CH2—OR5, —CH2—CH2—OR5, —NHR5, —COR5, —COOR5, —CH2COOR5, where R5=hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or C1-C6-acyl,
R6 to R7 are identical or different and are hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C5-C10-aryl or a group —CH2—OR5, —CH2—CH2—OR5, —NHR5, —COR5, —COOR5, —CH2COOR5, where R5=hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or C1-C6-acyl, and
y and z independently of one another are 0, 1 or 2, or
A and B independently of one another are one of the radicals




in which R1 and R2 are identical or different and are hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C6-C10-aryl or a group —CH2—OR5, —CH2—CH2—OR5, —NHR5, —COR5, —COOR5, —CH2COOR5, where R5=hydrogen, C1-C6-alkyl, C5-C6-cycloalkyl, C5-C10-aryl or C1-C6-acyl.