This application is based on German Application DE 199 50 608.6, filed Oct. 21, 1999, which disclosure is incorporated herein by reference.
The present invention provides an organosilicon compound, a process for its preparation and its use.
It is known that sulfur-containing organosilicon compounds such as 3-mercaptopropyltrimethoxysilane, 3-mercapto-propyltriethoxysilane, 3-thiocyanatopropyltriethoxysilane or bis-(3-[triethoxysilyl]-propyl)tetrasulfane are used as silane bonding agents or reinforcing additives in oxidically filled rubber mixtures. The rubber mixtures are used, inter alia, for industrial rubber items and for parts of rubber tires, in particular for treads (DE 2 141 159, DE 2 212 239, U.S. Pat. Nos. 3,978,103, 4,048,206).
It is also known that the alkoxysilyl function, generally a trimethoxysilyl or triethoxysilyl group, reacts with silanol groups on the fillers, generally silica, during mixing-preparation and thus the silane becomes fixed onto the surface of the filler. Production of the filler/rubber bond then takes place during the vulcanization process, via the sulfur groupings on the fixed silane. Accordingly, the resulting properties of this type of vulcanizate, for a given constant amount of silane, depends critically on how high the coupling yield of the silane is and what network structure is produced. Furthermore, it is known that silanes with polysulfane functions such as, for example, bis-(3-[triethoxysilyl]-propyl)tetrasulfane, tend to participate in disadvantageous premature cross-linking during the mixing process, at appropriately high temperatures. Therefore, it is important that a maximum batch temperature of about 155xc2x0 C. is not exceeded when using these silanes.
The object of the present invention is to provide organosilicon compounds which have higher coupling yields, improved rubber properties and higher process reliability than the silanes known hitherto, when used as a bonding agent or reinforcing additive in rubber mixtures.
The invention provides an organosilicon compound of the general formula I
R1R2R3Sixe2x80x94R4xe2x80x94Sxe2x80x94Znxe2x80x94Sxe2x80x94R4xe2x80x94SiR1R2R3xe2x80x83xe2x80x83(I)
wherein
R1, R2, R3, independently, represent H, a halogen, a straight-chain or branched alkyl group or a straight-chain or branched alkoxy group and
R4 represents a straight-chain or branched alkylidene group.
The straight-chain alkyl groups may be methyl, ethyl, n-propyl, n-butyl-, n-pentyl- or n-hexyl groups. The branched alkyl groups may be iso-propyl, iso-butyl or tert-butyl groups. The halogen may be fluorine, chlorine, bromine or iodine. The alkoxy groups may be methoxy, ethoxy, propoxy, butoxy, isopropoxy, isobutoxy or pentoxy groups.
In the organosilicon compound in accordance with formula I, R1, R2, R3 preferably represent ethoxy and R4 preferably represents CH2CH2CH2 or isobutylidene.
The invention also provides a process for preparing the organosilicon compound of the general formula (I), characterised in that a mercaptan compound of the general formula (II)
R1R2R3Sixe2x80x94R4xe2x80x94Sxe2x80x94Hxe2x80x83xe2x80x83(II)
wherein
R1, R2, R3, independently, represent H, a halogen, a straight-chain or branched alkyl group or a straight-chain or branched alkoxy group and
R4 represents a straight-chain or branched alkylidene group,
is reacted with a zinc alcoholate. The reaction may be performed in alcoholic solution. The reaction may be performed at a temperature range of 20xc2x0 to 200xc2x0 C., preferably 50xc2x0 to 80xc2x0 C.
Zinc ethanolate may be used as the zinc alcoholate. Ethanol may be used to prepare the alcoholic solution. The zinc alcoholate may be prepared by reacting zinc chloride with sodium ethanolate in alcoholic solution. The zinc alcoholate may be reacted with double the molar amount of the mercaptan compound of formula (II) in alcoholic solution.
3-Mercaptopropyltriethoxysilane may be used as a mercaptan compound. In one embodiment, a compound of formula (II) with R1, R2, R3=ethoxy and R4=CH2CH2CH2, may be reacted with zinc ethanolate in ethanolic solution.
The organosilicon compound according to the invention is highly reactive and may be used in rubber mixtures.
Rubber mixtures which contain the organosilicon compound according to the invention as a bonding agent or as a reinforcing additive and the molded items which are produced after a vulcanization step, in particular pneumatic tires or tire treads, have a low rolling resistance with, simultaneously, good wet adhesion and high resistance to abrasion.
The invention also provides rubber mixtures, characterised in that they contain rubber, fillers, preferably precipitated silica, at least one organosilicon compound of formula (I) and optionally other rubber auxiliary substances.
The organosilicon compound of the formula (I) may be used in amounts of 0.1 to 15 wt. %, preferably 5-10 wt. %, with respect to the amount of filler used.
Natural rubber and/or synthetic rubber may be used as the rubber. Preferred synthetic rubbers are described, for example, in W. Hofmann, Kautschuktechnologie, Genter Verlag, Stuttgart 1980. The rubbers may be used individually or in combination. Anionic polymerized S-SBR rubbers with a glass transition temperature above xe2x88x9250xc2x0 C. and its mixtures with high-cis diene rubbers are used in particular for preparing motor vehicle tires.
The following may be used as fillers:
carbon blacks, which are prepared by the lamp black, furnace black or channel black process and have a BET surface area of 20 to 200 m2/g,
highly disperse silicas prepared, for example, by precipitation from silicate solutions or by flame hydrolysis from silicon halides, with specific surface areas of 5 to 1000 m2/g, preferably 20 to 400 m2/g (BET surface area) and with primary particle sizes of 10 to 400 nm, optionally also as mixed oxides with metal oxides such as Al, Mg, Ca, Ba, Zn and titanium oxides,
synthetic silicates such as aluminum silicate, alkaline earth silicates such as, for example, magnesium silicate or calcium silicate, with BET surface areas of 20 to 400 m2/g and primary particle diameters of 10 to 400 nm,
natural silicates such as kaolin and other naturally occurring silicas,
glass fibers and glass fiber products (mats, ropes) or glass microbeads.
The rubber mixtures may contain synthetic rubber and silica as a filler. Highly disperse silicas, prepared by precipitation from silicate solutions, with BET surface areas of 20 to 400 m2/g are preferably used, in amounts of 10 to 150 parts by weight, with respect to 100 parts by weight of rubber.
The fillers mentioned may be used individually or as a mixture.
In a particularly preferred embodiment of the rubber mixture, 10 to 150 parts by weight of a pale filler, optionally together with 0 to 100 parts by weight of carbon black, with respect to 100 parts by weight of rubber, and 0.1 to 15 parts by weight, preferably 5 to 10 parts by weight of a compound of formula (I), with respect to 100 parts by weight of the filler used, may be used to prepare the mixtures.
The organosilicon compounds according to the invention may be used either in the pure form or attached to an inert organic or inorganic support. Preferred support materials may be silica, natural or synthetic silicates, aluminum oxide or carbon black. The organosilicon compound according to the invention may be used on its own or combined with other organosilicon compounds, in particular monofunctional alkylalkoxysilanes.
Rubber auxiliary products which may be used are reaction accelerators, reaction delayers, antioxidants, stabilizers, processing aids, plasticizers, waxes, metal oxides and activators such as triethanolamine, polyethylene glycol or hexanetriol which are well-known in the rubber industry.
The rubber auxiliary substances may be used in conventional amounts which are governed, inter alia, by the ultimate application. Conventional amounts may be 0.1 to 50 wt. %, with respect to rubber. The organosilicon compounds may be activated by adding sulfur and accelerators before the actual cross-linking reaction. This activation may take place during the vulcanization step. Suitable vulcanization accelerators may be mercaptobenzthiazoles, sulfenamides, guanidines, thiurams, dithiocarbamates, thioureas and thiocarbonates. The vulcanization accelerator and sulfur or peroxides may be used in amounts of 0.1 to 10 wt. %, preferably 0.1 to 5 wt. %, with respect to rubber.
Use of organosilicon compounds according to the invention in rubber mixtures results in advantages in the static and dynamic vulcanizate data as compared with mixtures according to the prior art. This is shown in particular by a higher tensile strength, a higher 300% stress modulus, and an improved 300%/100% stress modulus reinforcing ratio. In addition, mixtures according to the invention exhibit a decreased build-up of heat (Goodrich flexometer test), which indicates a positive hysteresis behavior, and an advantageously low loss factor tan xcex4(60xc2x0 C.), which correlates with the rolling resistance value.
The invention also provides a process for preparing rubber mixtures, characterized in that rubber is mixed with fillers, at least one organosilicon compound of formula (I) and optionally other rubber auxiliary substances.
The organosilicon compounds according to the invention and the fillers may preferably be incorporated at bulk temperatures of 100xc2x0 to 200xc2x0 C., but may also be incorporated later at lower temperatures 40xc2x0 to 100xc2x0 C., for example together with other rubber auxiliary substances. Mixing the constituents may be performed in conventional mixing equipment such as rollers, internal mixers and mixer extruders.
Vulcanization of rubber mixtures according to the invention may take place at temperatures of 100xc2x0 to 200xc2x0 C., preferably 130xc2x0 to 180xc2x0 C., and optionally under a pressure of 10 to 200 bar. Rubber vulcanizates according to the invention may be used for molded items, for example pneumatic tires, tire treads, cable sheathing, hoses, drive belts, conveyer belts, roller coverings, tires, soles of shoes, sealing rings and damping elements.
The use of organosilicon compounds according to the invention as bonding agents or reinforcing additives in rubber mixtures leads to much higher coupling yields, and a correspondingly improved set of rubber properties, than known silanes. Organosilicon compounds according to the invention do not exhibit the known tendency to premature cross-linking of the unaccelerated mixture at high mixing temperatures. Thus, much higher processing temperatures can be tolerated with greater process reliability. Activation of the sulfur-functional group can take place only during vulcanization with the addition of sulfur and accelerator.