SOLID SILICA COUPLING AGENT COMPOSITES IN SILICA RUBBER

Abstract: Silica containing rubber compositions such as those utilized in various components of tires contain a solid silica coupling agent binder composite.

Description: SOLID SILICA COUPLING AGENT COMPOSITES IN SILICA RUBBER Field of the Invention The present invention relates to a silica coupling agent composite containing a solid binder which is utilized in rubber compositions containing silica. The use of a solid silica coupling agent composite has ease of handling benefits such as minimization of agglomeration, enhanced dispersion, the promotion of adhesion between silica and rubber, and ease of identification of the composite. Background of the Invention Coupling agents are often utilized to connect silica, a reinforcing agent, to a polymer chain such as a sulfur vulcanizable rubber chain. In recent years, organosilanes have become popular coupling agents used in rubber compositions. One component of the coupling agent reacts with the surface of a silica and another component reacts with the rubber thereby linking or bridging them as through covalent bonding. The organosilanes which are currently employed commercially present numerous problems in the practicality of their use. Commonly, organosilanes, such as bis(3-triethoxysilypropyl)tetrasulfιde are in liquid form. Examples of such silanes include SI-69 sold by the Degussa Corporation and A1289 sold by Witco. Organosilanes are also available as a "supported grade", in which organosilanes are mixed with carbon black in various ratios, such as X50-S available from Degussa Corp. Frequently, these materials are troublesome to handle. With both the liquid form and supported grades, it is difficult to achieve accurate measurements and often waste is significant due to the materials adhering to equipment, containers, and the like. Supported grades of organosilane tend to be dusty, hard to disperse and look like other types of carbon blacks. In addition, the dust resulting from handling of the product may be undesirable. Summary of the Invention The present invention relates to rubber compositions containing silica and a composite comprising a silica coupling agent and a binder. A solid binder is preferred such as a wax, as well as optionally, a polyolefin. The use of the solid silica coupling agent composite is desirable in various tire components such as a tread when it contains silica therein. The composites of the present invention fully melt into a composition, such as a rubber composition, and results in improved dispersion. Such a composite is clean, environmentally friendly, and easily identifiable. Embodiments of the Invention An important aspect of the present invention is the addition of a generally solid composite to a silica reinforced rubber composition. The composite comprises a substantially homogenous mixture of a silica coupling agent and a binder. Generally any solid shape can be utilized, such as pellets, cubes, spheres, and the like, with pellets being preferred. Generally, any conventional silica coupling agent can be utilized including those which have a functional or reactive group which can react with silica and also another functional or reactive group which can react with the rubber, particularly, a sulfur vulcanizable rubber which contains unsaturated carbon to carbon bonds. The silica coupling agent thus acts as a connecting bridge between the silica and the rubber. Suitable functional groups include amino. vinyl, epoxy. mercapto. chloro. bromo. and iodo. methacryl and methacryoyl. glycidoxyl, nitroso. imido. octyl. oligomeric. ureido. polysulfide groups, and the like. Preferred coupling agents of the present invention include organosilanes of the formula: R 1 R 4 I I R 2 — O— Si— (CH 2 ) — (S) n — (CH 2 ) m — Si— O— R 5 O O R 3 R 6 wherein n is an integer of from 2 to about 7; wherein k and m. independently, is from 0, i.e. non-existent, to about 3. and wherein R 1 through R , independently, is H or an alkyl group having from 1 to 12 carbons. Examples of such organosilane coupling agents include: 2.2'-bis(trimethoxysilylethyl)disulfιde; 3 ,3 '-bis(trimethoxysilylpropyl)disulfιde ; 3.3'-bis(triethoxysilylpropyl)disulfide; 2.2'-bis(triethoxysilylpropyl)disulfide; 2.2'-bis(tripropoxysilylethyl)disulfιde; 2.2'-bis(tri-sec.butoxysilylethyl)disulfιde 2.2'-bis(tri-t-butoxyethyl)disulfide: 3.3'-bis(triethoxysilylethyl tolylene)disulfιde: 3.3'-bis(trimethoxysilylethyl tolylene)disulfιde: 3.3'-bis(triisopropoxypropyl)disulfιde: 3.3'-bis(trioctoxypropyl)disulfιde: 2.2'-bis(2'-ethylhexoxysilylethyl)disulfιde: 2.2'-bis(dimethoxy ethoxysilylethyl)disulfide. 3.3'-bis(methoxyethoxypropoxysilylpropyl)disulfιde: 3.3'-bis(methoxy dimethylsilylpropyDdisulfide: 3.3'-bis cyclohexoxy dimethylsilylpropyl)disulfιde; 4.4'-bis(trimethoxysilylbutyl)disulfιde: 3.3'-bis(tripropoxysilyl-3-methylpropyl)disulfιde: 3.3'-bis(trimethoxysilyl-3-methylpropyl)disulfide: 3.3'-bis(dimethoxy methylsilyl-3-ethylpropyl)disulfιde; 3.3'-bis(trimethoxysilyl-2-methylpropyl)disulfide: 3.3'-bis(dimethoxyphenylsilyl-2methylpropyl)disulfide; 3,3'-bis(trimethoxysilylcyclohexyl)disulfιde; 12.12'-bis(trimethoxysilyldodecyl)disulfιde; 12.12'-bis(triethoxysilyldodecyl)disulfide; 18.18'-bis(trimethoxysilyloctadecyl)disulfιde; 18,18'-bis(methoxydimethylsilyloctadecyl)disulfide; 2,2.'-bis(trimethoxysilyl-2-methylethyl)disulfιde; 2.2'-bis(triethoxysilyl-2-methylethyl)disulfide; 2,2'-bis(tripropoxysilyl-2-methylethyl)disulfide: and 2,2'-bis(trioctoxysilyl-2-methylethyl)disulfide. Highly preferred organosilane silica coupling agents include bis(3-triethoxysilylpropyl)tetrasulfιde, bis (3-triethoxysilylpropyl)disulfide. and combinations thereof. Other suitable organosilane coupling agents include methyltriethoxysilane; methyltrimethoxysilane: vinyltriethoxysilane: vinyltrimethoxysilane:vinyl-tris (2methoxyethoxysilane): vinvltriacetoxvsilane; gammamethacryloxypropyltrimethoxysilane; gamma-methacryloxypropyl-tris- (2-methoxyethoxy)silane: beta-(3.4-epoxycyclohexyl)ethyltrimethoxysilane: gammaglycidoxypropyltrimethoxysilane; gamma- mercaptopropyltrimethoxysilane; gamma mercaptopropyltriethoxysilane: gamma-aminopropyltriethoxysilane; gamma-aminopropyltriethoxysilane; gamma-aminopropyltriethoxysilane: aminoalkyl silicone solution: modified aminoorganosilane; gammaaminopropyltrimethoxysilane: N-beta-(aminoethy 1 )- gammaaminopropyltrimethoxysilane: modified aminoorganosilane: triaminofunctional silane. and the like. Whenever silane modified elastomers are utilized, that is elastomers having silane functional groups as part of the polymer, proportionately smaller amounts of the above coupling agents are utilized because of the existence of the silica coupling agents contained within the elastomer. The amount of the silica coupling agent based upon 100 parts by wt. of silica is generally from about 0.1 to about 20, desirably from about 0.5 to about 15, and preferably from about 1.0 to about 10 parts by wt. A function of the binder is to act as a carrier for the silica-coupling agent. As stated herein, the term "binder" comprises all components in the composite except the silica- coupling agent. The binder has a higher melting point than the silica coupling agent, and imparts the solid form to the composite at ambient temperatures, for example, about 70°F (21 °C). The binder can be a wax (preferred), a thermoplastic polymer, a compatibilizing agent, a wetting agent, a fatty acid, ethylene vinyl acetate, a stabilizer, and the like as well as combinations thereof. Any natural, synthetic or petroleum based wax can be utilized in the binder. Examples include, but are not limited to, paraffin, microcrystalline. carnauba and beeswax. Microcrystalline wax is preferred. Any thermoplastic polymer can be utilized in the binder of the present invention, such as a polyolefm made from monomers having from 2 to 6 carbon atoms. Thermoplastic polymers are generally optional and are utilized as a blend with the wax or other component. Preferably the thermoplastic polymer is oxidized polyethylene, which helps promote compatibility between the silica coupling agent and the binder components. The amount of the binder is generally from about 25 to about 90. desirably from about 28 to about 75. and preferably from about 32 to about 55% by weight based upon a total weight of the silica coupling agent and the binder. While oxidized polyethylene can be utilized alone, it is generally utilized as a minor portion in combination with a wax. The binder composite is formed by combining the silica coupling agent and the binder in a liquid phase where the components can be blended to form a homogeneous mixture. The mixture is processed through conventional methods into a desired usable form such as pellets, spheres or the like. One such procedure for creating the composite can be found in U.S. Patent No. 5,621,032. A preferred composite utilized in the present invention is available from Elastochem known as "EF(TESPT)-60". This composite contains bis(3-triethoxysilylpropyl) tetrasulfide in a binder of wax and oxidized polyethylene. This composite is generally free of mineral tillers and carbon black. The amount of the silica coupling agent composite can vary, but generally is from about 0.2 to about 30. desirably from about 3 to about 25. and preferably from about 6 to about 18 parts by weight based upon 100 parts by weight of all silica utilized within the rubber composition. The silica in the rubber composition generally can be any type of silica such as fumed, hydrated and preferably is precipitated silica. Advantages of using silica include reduced rolling resistance in tires and hence improved gasoline mileage of the vehicle. Suitable silicas generally have a BET surface area, as measured utilizing nitrogen gas, of from about 40 to about 600 and preferably from about 50 to about 300 square meters per gram. The actual BET method of measuring the surface area is described in the Journal of The American Chemical Society, Volume 60, page 304 (1930). The ultimate particle size of the silica is generally from about 0.1 to about 100. and desirably from about 5 to about 50 nanometers as measured by an electron microscope although smaller or larger particles can exist. The amount of the silica generalh ranges from about 5 to about 100 . desirably from about 15 to about 80. and preferably from about 25 to about 60 parts by weight per 100 parts by weight of total tire component rubber. Commercially available silicas which can be utilized in the present invention include silicas commercially available from PPG Industries under the Hi-Sil trademark such as designations 190. 210. 233, 243. etc.; silicas from Rhone-Poulenc such as Zl 165MP and Z165GR; silicas available from Degussa AG such as VN2 and VN3; and silicas from Akzo Chemical. The rubber composition of the present invention is made from natural rubber, at least one conjugated diene monomer, or from a conjugated diene and one or more vinyl-substituted aromatic monomers, and optionally from ethylene and propylene monomers, or ethylene-propylene and a non- conjugated diene (i.e., EPDM rubber.) The conjugated diene monomers have a total of from 4 to 10 carbon atoms with examples including 1,3-butiene, isoprene, 1,3-pentadiene, 2,3-dimethyl-l,3-butadiene, 2 -methyl- 1,3-pentadiene, 2.3-dimethyl- 1,3-pentadiene. 2-phenyl- 1.3 -butadiene, and 4.5-diethyl-l,3- octadiene. The one or more vinyl-substituted aromatic monomers have a total of from 8 to 12 carbon atoms such as 1 - vinylnaphthalene, 3-methylstyrene (p- methyl styrene), 3,5-diethylstyrene, and the like with styrene being preferred. Preferred tread rubber compositions generally include natural rubber (cis- 1 ,4- polyisoprene). synthetic polyisoprene. styrene butadiene rubber, modified styrene-butadiene rubber, butadiene rubber, modified butadiene rubber, and the like. Often the rubber is oil extended. The rubber composition of the present invention such as the tread can be compounded by methods and procedures well known to the rubber compounding art and contain various conventional additives in suitable amounts. For example, carbon black, extra conductive carbon black, as well as other types of carbon black, curing aids such as sulfur, sulfur containing compounds and the like are utilized. Vulcanizing accelerators which can be used in the present invention include amines, disulfides. guanidines. thioureas. thiazoles, thiurams. sulfenamides. dithiocarbamates. and the like. Other additives include various oils such as aromatic, naphthenic. or paraffmic; various antioxidants such as various phenylenediamines: various antiozonants; various aliphatic acids such as steric acid: zinc oxide: various waxes such as micro crystalline waxes; various peptizers. starch, and the like. Various fillers can also be utilized such as clay, for example kaolin, clay, and the like. The rubber composition containing the silica coupling agent composite of the present invention can be utilized in any application wherein it is desirable to use silica coupling agents in a form that is easier to handle, compound and weigh. Specific applications thus include various components in a tire such as a tire tread, tire sidewall, tire casing, carcass plies sub-treads, and the like, and preferably is utilized in a tread. The silica coupling agent composite can also be utilized in many other applications such as in hoses, belts, wire cables, shoe soles, and wherever silica reinforced rubber is utilized. The solid composites of the present invention are a good alternative when compared to a liquid or supported grades of silica coupling agents. The composite is environmentally friendly as it leaves no appreciable residue in containers or handling equipment and therefore less of the product is wasted. Since it is a solid, no stratification or loss to dust collectors will occur. The composite also offers improved batch to batch consistency and is readily visually distinguished from blends containing carbon black with other materials. The following examples serve to illustrate, but not to limit, the present invention. Examples Two examples were prepared utilizing silicone containing styrene-butadiene rubber solution and the same were compounded with EF(TESPT)-60 and EF(DISS) 60. respectively, the tetrasulfide and disulfide forms of this bis(3-triethoxysilylpropyl) manufactured by Elastochem Inc. and which contains a binder of a wax and oxidized polyethylene. The recipes of Examples 1 and 2 along with a control are set forth in Table I as well as the physical properties thereof. i TABLE 1 I The Control and Example 1 were prepared as follows: The first mixing stage involved adding the polymer, half of the carbon black, silica coupling agent and all other ingredients except the accelerators and sulfur. It was mixed in a Banbury at a temperature of 300°F for approximately 3 minutes. During the second mixing stage, the rest of the carbon black was added and mixed in a Banbury at approximately 300°F for about 2 minutes. The subsequent remill stage was mixed at temperatures of approximately 260°F for about 1 minute. The final mixing stage involved adding all of the accelerators and sulfur to the batch, and mixing at a temperature of approximately 200°F for 1 minute. The rubber stock was sheeted out into preferred (conventional) shapes (test samples) and cured at 170°C for 15 minutes. After curing the samples, the rubber was tested for viscoelastic properties set forth in Table 1. Example 2 had the same recipe as Example 1 except a disulfide organosilane coupling agent was utilized. This allowed the recipe to be formulated at a slightly higher temperature during the first stage i.e. 330°F instead of 300°F. The second stage mixing temperature was 300°F whereas the third stage mixing temperature was only 200°F. A fourth stage mixing step was not required since use of the disulfide coupling agent allowed a higher first stage mixing temperature. The elimination of a single stage in a mixing or a compounding operation is a notable advantage. As apparent from Table 1 , the properties utilizing the solid silica coupling agent composites of the present invention yielded very similar properties as the control which utilized a carbon black supported grade silane coupling agent. While similar properties were achieved utilizing the composite of the present invention, notable improvements were achieved with regard to the above stated advantages such as a dust free environment, non appreciable residues left in the handle equipment, mixing containers, and the like. While in accordance with the patent statutes the best mode and preferred embodiment have been set forth, the scope of the invention is not limited thereto, but rather by the scope of the attached claims.

Claims: WHAT IS CLAIMED IS: 1. A tire component, comprising; at least one rubber composition containing silica therein; and a solid silica coupling agent composite comprising a silica coupling agent and a solid binder. 2. The tire component according to Claim 1, wherein the amount of said silica coupling agent present in the composition is from about 0.1 to about 20 parts by weight per 100 parts by weight of silica. 3. The tire component according to either Claim 1 or Claim 2, wherein said solid binder comprises a wax. a thermoplastic polymer, a compatibilizing agent, a wetting agent, a fatty acid, ethylene vinyl acetate, a stabilizer, or combinations thereof; and wherein the amount of said solid binder is from about 25% to about 90% by weight based upon the total weight of said silica coupling agent composite. 4. The tire component according to any of Claims 1-3, wherein said silica coupling agent comprises an organosilane having the formula R 1 R 4 R 2 — O— Si— (CH 2 ) — (S)n— (CH 2 ) n — Si— O— R 5 I j O O R 3 R 6 wherein n is an integer of from 2 to about 7. wherein k and m. independently, is from 0 to 3. and wherein R 1 through R 6 . independently, is H. or an alkyl having from 1 to 12 carbon atoms, and wherein the amount of silica is from about 5 to about 100 parts by weight per 100 parts by weight of said rubber. 5. The tire component according to Claim 4, wherein the amount of said silica coupling agent is from about 1.0 to about 10 parts by weight per 100 parts by weight of said silica, wherein said silica coupling agent is bis(3-triethoxysilylpropyl)tetrasulfide, or bis(3-triethoxylsilylpropyl)disulfide, or combinations thereof, and wherein said rubber is natural rubber, synthetic polyisoprene, styrene-butadiene rubber, modified styrene-butadiene rubber, butadiene rubber, modified butadiene rubber, or combinations thereof. 6. The tire component according to Claim 5, wherein said binder is a blend of a wax and oxidized polyethylene. 7. The tire component according to any of Claims 1-6, wherein said silica is present in an amount of from about 5 to about 100 parts by weight of a silica per 100 parts by weight of said tire component rubber. 8. The tire component according to any of Claims 1-7, wherein said silica coupling agent has least one reactive group comprising an amino. vinyl, epoxy. mercapto, chloro. bromo. iodo. methacryl. methacryoyl. glycidoxyl. nitroso. imido. octyl. oligomeric, ureido. or polysulfide. 9. A tire component composition according to any of Claims 1-8. wherein the silica coupling agent is bis (3-triethoxysilylpropyl) tetradisulfide. bis(3-triethoxysilylpropyl) disulfide. or combinations thereof. 10. The tire component of any of Claims 1 -9. wherein the tire component is a tire tread or subtread or a tire carcass.

Inventors: FOUST JOY A, O'BRISKIE WILLIAM J, FOUST, JOY, A., O'BRISKIE, WILLIAM, J.
Applicants: BRIDGESTONE FIRESTONE RES INC, BRIDGESTONE/FIRESTONE RESEARCH, INC.
Application year: 2000
Main classification: C08K5
IPC codes: B60C1, C08K3, C08K5, C08L21, C08L101