Abstract:
A processable rubber stock is produced by the preparation of a silica-filled, vulcanized elastomeric compound comprising mixing an elastomer with an amorphous silica filler, from 0 to less than about 1% by weight based on said silica filler of bis[3-(triethoxysilyl)propyl]tetrasulfide, an alkylalloxysilane and a cure agent. A further processing aid comprising at least one of an ester of a fatty acid or an ester of a polyol is preferred. The elastomer is preferably a diene monomer homopolymer or a copolymer of at least one diene and at least one monovinyl aromatic monomer. By effecting vulcanization, a vulcanized elastomeric compound is produced containing good physical properties for use as tread stock for a pneumatic tire.

Description:
This application is a Divisional of U.S. Ser. No. 09/724,130, filed Nov. 28, 2000, now allowed, which is a Divisional of U.S. Ser. No. 08/893,864, field Jul. 11, 1997, now U.S. Pat. No. 6,221,943, issued Apr. 24, 2001 
    
    
     TECHNICAL FIELD 
     The subject invention relates to the processing and vulcanization of diene polymer and copolymer elastomer containing rubber stocks. More specifically, the present invention relates to the processing and vulcanization of diene polymer and copolymer elastomer-containing, silica-filled rubber stocks using a mixture of silanes as processing aids. 
     BACKGROUND OF THE INVENTION 
     In the art it is desirable to produce elastomeric compounds exhibiting reduced hysteresis when properly compounded with other ingredients such as reinforcing agents, followed by vulcanization. Such elastomers, when compounded, fabricated and vulcanized into components for constructing articles such as tires, power belts, and the like, will manifest properties of increased rebound, decreased rolling resistance and less heat-build up when subjected to mechanical stress during normal use. 
     The hysteresis of an elastomer refers to the difference between the energy applied to deform an article made from the elastomer and the energy released as the elastomer returns to its initial, undeformed state. In pneumatic tires, lowered hysteresis properties are associated with reduced rolling resistance and heat build-up during operation of the tire. These properties, in turn, result in lowered fuel consumption of vehicles using such tires. 
     In such contexts, the property of lowered hysteresis of compounded, vulcanizable elastomer compositions is particularly significant. Examples of such compounded elastomer systems are known to the art and are comprised of at least one elastomer (that is, a natural or synthetic polymer exhibiting elastomeric properties, such as a rubber), a reinforcing filler agent (such as finely divided carbon black, thermal black, or mineral fillers such as clay and the like) and a vulcanizing system such as sulfur-containing vulcanizing (that is, curing) system. 
     Previous attempts at preparing readily processable, vulcanizable, silica-filled rubber stocks containing natural rubber or diene polymer and copolymer elastomers have focused upon the sequence of adding ingredients during mixing (Bomal, et al.,  Influence of Mixing procedures on the Properties of a Silica Reinforced Agricultural Tire Tread , May 1992), the addition of de-agglomeration agents such as zinc methacrylate and zinc octoate, or SBR-silica coupling agents such as mercapto propyl trimethoxy silane (Hewitt,  Processing Technology of Silica Reinforced SBR, Elastomerics , pp 33-37, March 1981), and the use of bis[3-(triethoxysilyl)propyl] tetrasulfide (Si69) processing aid (Degussa, PPG). 
     The use of Si69 processing aid in the formulation of silica-filled rubber stocks has been successful, but generally requires a large amount of the additive, such as 10% by weight based on the weight of silica, in order to be effective. 
     Precipitated silica has been increasingly used as a reinforcing particulate filler in carbon black-filled rubber components of tires and mechanical goods. Silica-loaded rubber stocks, however, exhibit. relatively poor processability. 
     The present invention provides a mixture of silanes for use as processing aids for silica-filled rubber stocks, which greatly improve the processability and properties of the formulations and resulting vulcanized product. 
     SUMMARY OF INVENTION 
     It is therefore an object of the present invention to provide rubber stock processing aids which improve the processability of formulations of diene polymer elastomers with silica-filler. 
     It is another object of the present invention to provide a method for reducing the viscosity of silica-filled elastomeric vulcanizable compounds. 
     It is another object of the present invention to provide a method for enhancing the processability of silica-filled elastomeric vulcanizable compounds. 
     It is another object of the present invention to provide vulcanizable silica-filled elastomeric compounds having enhanced physical properties 
     The foregoing objects, together with the advantages thereof over the existing art, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed. 
     The present invention provides a process for the preparation of a silica-filled, vulcanized elastomeric compound comprising: mixing an elastomer with an amorphous silica filler, from 0 to less than about 1% by weight based on said silica filler of bis[3-(triethoxysilyl)propyl]tetrasulfide, an alkylalkoxysilane and a cure agent; and, effecting vulcanization. Preferably, the elastomer is a diene monomer homopolymer or a copolymer of a diene monomer and a monovinyl aromatic monomer. 
     The present invention further provides a vulcanizable silica-filled compound comprising an elastomer, a silica filler, from 0 to less than about 1% by weight based on said silica filler of bis[3-(triethoxysilyl)propyl]tetrasulfide (Si69), an alkylalkoxysilane and a cure agent. Preferably, the elastomer is styrene butadiene rubber, optionally containing a carbon black filler. The compound is more readily processable during mixing, due to the use of the mixture of silane processing aids. 
     The present invention further provides a pneumatic tire comprising tread stock vulcanized from the inventive vulcanizable silica-filled compound. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In general, the present invention provides a means to reduce the level of Si69 needed to obtain good physical properties in a cured rubber stock containing silica as a filler. In addition, the present invention further provides maintenance of the processability of the compounded stock, as measured by Mooney viscosity, at the same level as achieved with high levels of Si69. 
     The present invention utilizes the presence of an alkylalkoxysilane as a silica hydrophobating agent, such that minimal amounts of Si69 are needed to obtain good processability, and yet still give good physical properties. According to the invention, therefore, a less costly silane can be substituted for the majority or all of the Si69 that would be normally used without any loss of processability or properties. Additionally, remilling can be eliminated, and the cure of the rubber stock is not dependent on the high sulfur level present in the Si69. 
     The silica-hydrophobating agents useful according to the present invention include those alkylalkoxysilanes of the formula (R 1 ) 2 Si(OR 2 ) 2  or R 1 Si(OR 2 ) 3 , wherein the alkoxy groups are the same or are different; each R 1  independently comprising C1 to about C18 aliphatic, about C6 to about C12 cyclo-aliphatic, or about C6 to about C1 aromatic, preferably C1 to about C10 aliphatic, about C6 to about C10 cyclo-aliphatic, or about C6 to about C12 aromatic; and each R 2  independently containing from one to about 6 carbon atoms. Representative examples include octyltriethoxy silane, octyltrimethoxy silane hexyltrimethoxy silane, ethyltrimethoxy silane, propyltriethoxy silane, phenyltrimethoxy silane, cyclohexyltrimethoxy silane, cyclohexyltriethoxy silane dimethyldimethoy silane, i-butyltriethoxy silane, and the like. Of these, octyltriethoxysilane is preferred. 
     According to the present invention, polymerized elastomer is compounded in the rubber stock, e.g., polybutadiene, polyisoprene and the like, and copolymers thereof with monovinyl aromatics such as styrene, alpha methyl styrene and the like, of trienes such as myrcene. Thus, the elastomers include diene homopolymers, A, and copolymers thereof with monovinyl aromatic polymers, B. Exemplary diene homopolymers are those prepared from diolefin monomers having from 4 to about 12 carbon atoms. Exemplary vinyl aromatic polymers are those prepared from monomers having from 8 to about 20 carbon atoms. Examples of conjugated diene monomers and the like useful in tile present invention include 1,3-butadiene, isoprene, 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene and 1,3-hexadiene, and aromatic vinyl monomers include styrene, α-methylstyrene, p-methylstyrene, vinyltoluenes and vinylnaphthalenes. The conjugated diene monomer and aromatic vinyl monomer are normally used at the weight ratios of about 90:10 to about 55:45, preferably about 80:20 to about 65:35. 
     Preferred elastomers include diene homopolymers such as polybutadiene and polyisoprene and copolymers such as styrene butadiene rubber (SBR). Copolymers can comprise from about 99 to 20 percent by weight of diene units and from about 1 to about 80 percent by weight of monovinyl aromatic or triene units, totaling 100 percent. The polymers and copolymers of the present invention may have the diene portion with a 1,2-microstructure contents ranging from about 10 to about 80 percent, with the preferred polymers or copolymers having 1,2-microstructure contents of from about 25 to 65 percent. The molecular weight of the polymer that is produced according to the present invention, is preferably such that a proton-quenched sample will exhibit a gum Mooney viscosity (ML 4 /212° F. ) of from about 2 to about 150. The copolymers are preferably random copolymers which result from simultaneous copolymerization of the monomers, as is known in the art. Also included are nonfunctionalized cis-polybutadiene, ethylene-propylene-diene monomer (EPDM), emulsion SBR and natural rubber. 
     Initiators known in the art such as an organolithium initiator, preferably an alkyllithium initiator, can be employed to prepare the elastomer. More particularly, the initiators used in the present invention include N-lithio-hexamethyleneimine, organolithium compounds such as n-butyllithium, tributyltin lithium, dialkylaminolithium compounds such as dimethylaminolithium, diethylaminolithium, dipropylaminolithium, dibutylaminolithium and the like, dialkylaminoalkyllithium compounds such as diethylaminopropyllithium and the like, and trialkyl stanyl lithium, wherein the alkyl group contains 1 to about 12 carbon atoms, preferably 1 to about 4 carbon atoms. 
     Polymerization is usually conducted in a conventional solvent for anionic polymerizations such as the various cyclic and acyclic hexanes, heptanes, octanes, pentanes, their alkylated derivatives, and mixtures thereof. Other techniques for polymerization, such as semi-batch and continuous polymerization may be employed. In order to promote randomization in copolymerization and to increase vinyl content, a coordinator may optionally be added to the polymerization ingredients. Amounts range between 0 to 90 or more equivalents per equivalent of lithium. The amount depends upon the amount of vinyl desired, the level of styrene employed and the temperature of the polymerizations, as well as the nature of the specific polar coordinator employed. 
     Compounds useful as coordinators are organic and include those having an oxygen or nitrogen hetero-atom and a non-bonded pair of electrons. Examples include dialkyl ethers of mono and oligo alkylene glycols; “crown” ethers; tertiary amines such as tetramethylethylene diamine (TMEDA); THF; THF oligomers; linear and cyclic oligomeric oxolanyl alkanes, such as 2-2′-di(tetrahydrofuryl) propane, di-piperidyl ethane, hexamethylphosphoramide, N-N′-dimethylpiperazine, diazabicyclooctane, diethyl ether, tributylamine and the like. Details of linear and cyclic oligomeric oxolanyl coordinators can be found in U.S. Pat. No. 4,429,091, owned by the Assignee of record, the subject matter of which is incorporated herein by reference. 
     Polymerization is usually begun by charging a blend of the monomer(s) and solvent to a suitable reaction vessel, followed by the addition of the coordinator and the initiator solution previously described. Alternatively, the monomer and coordinator can be added to the initiator. The procedure is carried out under anhydrous, anaerobic conditions. The reactants are heated to a temperature of from about 10° to 150° C. and are agitated for about 0.1 to 24 hours. After polymerization is complete, the product is removed from the heat and terminated in one or more ways. 
     To terminate the polymerization, a terminating agent, coupling agent or linking agent may be employed, all of these agents being collectively referred to herein as “terminating agents”. Certain of these agents may provide the resulting polymer with a multifunctionality. That is, the polymers initiated according to the present invention, carry at least one amine functional group as discussed hereinabove, and may also carry a second functional group selected and derived from the group consisting of terminating agents, coupling agents and linking agents. 
     Examples of terminating agents according to the present invention include those commonly employed in the art, including hydrogen, water, steam, an alcohol such as isopropanol, 1,3-dimethyl-2-imidazolidinone (DMI), carbodiimides, N-methylpyrrolidine, cyclic amides, cyclic ureas, isocyanates, Schiff bases, 4,4′-bis(diethylamino) benzophenone, and the like. Other useful terminating agents may include those of the structural formula (R 1 ) a  ZX b , wherein Z is tin or silicon. It is preferred that Z is tin. R 1  is an alkyl having from about 1 to about 20 carbon atoms; a cycloalkyl having from about 3 to about 20 carbon atoms; an aryl having from about 6 to about 20 carbon atoms; or, an aralkyl having from about 7 to about 20 carbon atoms. For example, R 1  may include methyl, ethyl, n-butyl, neophyl, phenyl, cyclohexyl or the like. X is a halogen, such as chlorine or bromine, or an alkoxy (—OR 1 ), “a” is from 0 to 3, and “b” is from about 1 to 4; where a+b=4. Examples of such terminating agents include tin tetrachloride, (R 1 ) 3 SnCl,(R 1 ) 2 SnCl 2 , R 1 SnCl 3 , and R 1 SiCl 3  as well as methyltriphenoxysilane (MeSi(OPh 3 )). 
     The terminating agent is added to the reaction vessel, and the vessel is agitated for about 1 to about 1000 minutes. As a result, an elastomer is produced having an even greater affinity for silica compounding materials, and hence, even further reduced hysteresis. Additional examples of terminating agents include those found in U.S. Pat. No. 4,616,069 which is herein incorporated by reference. It is to be understood that practice of the present invention is not limited solely to these terminators inasmuch as other compounds that are reactive with the polymer bound lithium moiety can be selected to provide a desired functional group. 
     Quenching is usually conducted by stirring the polymer and quenching agent for about 0.05 to about 2 hours at temperatures of from about 30° to 120° C. to ensure complete reaction. Polymers terminated with a functional group as discussed hereinabove, can be subsequently quenched with alcohol or other quenching agents as described hereinabove. 
     Lastly, the solvent is removed from the polymer by conventional techniques such as drum drying, extruder drying, vacuum drying or the like, which may be combined with coagulation with water, alcohol or steam, thermal desolventization, or any other suitable method. If coagulation with water or steam is used, oven drying may be desirable. 
     The elastomeric polymers can be utilized as 100 parts of the rubber in the treadstock compound or, they can be blended with any conventionally employed treadstock rubber which includes natural rubber, synthetic rubber and blends thereof. Such rubbers are well known to those skilled in the art and include synthetic polyisoprene rubber, styrene/butadiene rubber (SBR), polybutadiene, butyl rubber, Neoprene, ethylene/propylene rubber, ethylene/propylene/diene rubber (EPDM), acrylonitrile/butadiene rubber (NBR), silicone rubber, the fluoroelastomers, ethylene acrylic rubber, ethylene vinyl acetate copolymer (EVA), epiclorohydrin rubbers, chlorinated polyethylene rubbers, chlorosulfonated polyethylene rubbers, hydrogenated nitrile rubber, tetrafluoroethylene/propylene rubber and the like. When the polymers discussed hereinabove are blended with conventional rubbers, the amounts can vary widely with a range comprising about 5 to about 99 percent by weight of the total rubber. It is to be appreciated that the minimum amount will depend primarily upon the degree of reduced hysteresis that is desired. 
     According to the present invention, amorphous silica (silicon dioxide) is utilized as a filler for the diene polymer or copolymer elastomer-containing vulcanizable compound. Silicas are generally classed as wet-process, hydrated silicas because they are produced by a chemical reaction in water, from which they are precipitated as ultrafine, spherical particles. 
     These primary particles strongly associate into aggregates, which in turn combine less strongly into agglomerates. The surface area, as measured by the BET method gives the best measure of the reinforcing character of different silicas. For silicas of interest for the present invention, the surface area should be about 32 to about 400 m 2 /g, with the range of about 100 to about 250 m 2 /g being preferred, and the range of about 150 to about 220 m 2 /g being most preferred. The pH of the silica filler is generally about 5.5 to about 7 or slightly over, preferably about 5.5 to about 6.8. 
     Silica can be employed in the amount of about 1 part to about 100 parts per 100 parts of polymer (phr), preferably in an amount from about 5 to about 80 phr. The useful upper range is limited by the high viscosity imparted by fillers of this type. Some of the commercially available silicas which may be used include: Hi-Sil® 215, Hi-Sil® 233, and Hi-Sil® 190, all produced by PPG Industries. Also, a number of useful commercial grades of different silicas are available from De Gussa Corporation, Rhone Poulenc, and J. M. Huber Corporation. 
     The polymers can be compounded with all forms of carbon black in amounts ranging from about 2 to about 50 parts by weight, per 100 parts of rubber (phr), with about 5 to about 40 phr being preferred. The carbon blacks may include any of the commonly available, commercially-produced carbon blacks but those having a surface area (EMSA) of at least 20 m 2 /gram and more preferably at least 35 m 2 /gram up to 200 m 2 /gram or higher are preferred. Surface area values used in this application are those determined by ASTM test D-1765 using the cetyltrimethyl-ammonium bromide (CTAB) technique. Among the useful carbon blacks are furnace black, channel blacks and lamp blacks. More specifically, examples of the carbon blacks include super abrasion furnace (SAF) blacks, high abrasion furnace (HAF) blacks, fast extrusion furnace (FEF) blacks, fine furnace (FF) blacks, intermediate super abrasion furnace (ISAF) blacks, semi-reinforcing furnace (SRF) blacks, medium processing channel blacks, hard processing channel blacks and conducting channel blacks. Other carbon blacks which may be utilized include acetylene blacks. Mixtures of two or more of the above blacks can be used in preparing the carbon black products of the invention. Typical values for surface areas of usable carbon blacks are summarized in the Table I hereinbelow. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Carbon Blacks 
               
             
          
           
               
                   
                 ASTM Designation 
                 Surface Area (m 2 /g) 
               
               
                   
                 (D-1765-82a) 
                 (D-3765) 
               
               
                   
                   
               
             
          
           
               
                   
                 N-110 
                 126 
               
               
                   
                 N-220 
                 111 
               
               
                   
                 N-339 
                 95 
               
               
                   
                 N-330 
                 83 
               
               
                   
                 N-351 
                 74 
               
               
                   
                 N-550 
                 42 
               
               
                   
                 N-660 
                 35 
               
               
                   
                   
               
             
          
         
       
     
     The carbon blacks utilized in the preparation of the rubber compounds of the invention may be in pelletized form or an unpelletized flocculent mass. Preferably, for more uniform mixing, unpelletized carbon black is preferred. 
     The reinforced rubber compounds can be cured in a conventional manner with known vulcanizing agents at about 0.2 to about 5 phr. For example, sulfur or peroxide-based curing systems may be employed. For a general disclosure of suitable vulcanizing agents one can refer to Kirk-Othmer,  Encyclopedia of Chemical Technology , 3rd ed., Wiley Interscience, N.Y. 1982, Vol. 20, pp. 365-468, particularly “Vulcanization Agents and Auxiliary Materials” pp. 390-402. Vulcanizing agents can be used alone or in combination. 
     Vulcanizable elastomeric compositions of the invention can be prepared by compounding or mixing the elastomeric polymer with silica, optionally carbon black, the silica-hydrophobating agent according to the present invention, a minimal amount of Si69, and other conventional rubber additives including for example, fillers, plasticizers, antioxidants, curing agents and the like, using standard rubber mixing equipment and procedures. 
     GENERAL EXPERIMENTAL 
     In order to demonstrate the preparation and properties of silica-filled, diene elastomer containing rubber stocks prepared according to the present invention, styrene butadiene rubber (SBR) polymers were prepared and were compounded using the formulations set forth in Tables II and III below. 
     Test results for the Control, C-A, using the Si69 processing aid only, and Examples 1-3 using silane processing aids according to the invention in Formulation A, are reported in Table II. 
     
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Formulation A for the Partial Replacement of Si69 
               
               
                 and Physical Test Results 
               
             
          
           
               
                 Material 
                 Amount (phr) 
               
             
          
           
               
                 Example No. 
                 C-A 
                 1 
                 2 
                 3 
               
               
                   
               
             
          
           
               
                 SBR 
                 100 
                 100 
                 100 
                 100 
               
               
                 Oil 
                 20 
                 20 
                 20 
                 20 
               
               
                 Silica 
                 60 
                 60 
                 60 
                 60 
               
               
                 Carbon Black 
                 6 
                 6 
                 6 
                 6 
               
               
                 Stearic Acid 
                 2 
                 2 
                 2 
                 2 
               
               
                 Wax 
                 0.75 
                 0.75 
                 0.75 
                 0.75 
               
               
                 Si-69 
                 5.4 
                 0.6 
                 0.6 
                 0.6 
               
               
                 Silane (Type) 
                 — 
                 Octyl 
                 Methacroyl 
                 Dimethyl 
               
               
                   
                   
                 Trimethoxy 
                 Trimethoxy 
                 Dimethoxy 
               
               
                 Silane (Amount) 
                 0 
                 4.71 
                 4.99 
                 3.62 
               
               
                 Tackifier 
                 3.5 
                 3.5 
                 3.5 
                 3.5 
               
               
                 Antioxidant 
                 0.95 
                 0.95 
                 0.95 
                 0.95 
               
               
                 Sulfur 
                 1.4 
                 1.4 
                 1.4 
                 1.4 
               
               
                 Accelerators 
                 2.4 
                 2.4 
                 2.4 
                 2.4 
               
               
                 Zinc Oxide 
                 3 
                 3 
                 3 
                 3 
               
               
                 Physical Properties 
               
               
                 ML 1 + 4  @ 100° C. 
                 93.7 
                 84.7 
                 93.3 
                 88.8 
               
               
                 Tensile 
                 2913 
                 2216 
                 2476 
                 2834 
               
               
                 (psi) @ 23° C. 
               
               
                 Tensile 
                 1239 
                 954 
                 1122 
                 1294 
               
               
                 (psi) @ 100° C. 
               
               
                 % Elong. at break, 
                 444 
                 603 
                 504 
                 551 
               
               
                 23° C. 
               
               
                 % Elong. at break, 
                 262 
                 407 
                 342 
                 365 
               
               
                 100° C. 
               
               
                 Ring Tear 
                 191 
                 198 
                 179 
                 223 
               
               
                 (lb/in) @ 100° C. 
               
               
                 Dispersion Index, % 
                 72.9 
                 76.1 
                 84 
                 84.3 
               
               
                   
               
             
          
         
       
     
     Test results for the Control, C-B, using the Si69 processing aid only, and Examples 4-7, using silane processing aids according to the invention in Formulation B, are reported in Table III. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 Formulation B for the Partial Replacement of Si69 
               
               
                 and Physical Test Results 
               
             
          
           
               
                 Material 
                 Amount (phr) 
               
             
          
           
               
                 Example No. 
                 C-B 
                 4 
                 5 
                 6 
                 7 
               
               
                   
               
             
          
           
               
                 SBR 
                 75 
                 75 
                 75 
                 75 
                 75 
               
               
                 BR 
                 25 
                 25 
                 25 
                 25 
                 25 
               
               
                 Oil 
                 41.25 
                 41.25 
                 41.25 
                 41.25 
                 41.25 
               
               
                 Silica 
                 80 
                 80 
                 80 
                 80 
                 80 
               
               
                 Carbon Black 
                 8 
                 8 
                 8 
                 8 
                 8 
               
               
                 Stearic Acid 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Wax 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
               
               
                 Si-69 
                 7.2 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
               
               
                 Silane (Type) 
                 — 
                 Propyl 
                 3- 
                 Octyl 
                 i-Butyl 
               
               
                   
                   
                 Triethoxy 
                 Chloropropyl 
                 Triethoxy 
                 Triethoxy 
               
               
                   
                   
                   
                 Triethoxy 
                   
               
               
                 Silane 
                 0 
                 5.5 
                 6.42 
                 7.39 
                 5.88 
               
               
                 (Amount) 
               
               
                 Tackifier 
                 3 
                 3 
                 3 
                 3 
                 3 
               
               
                 Antioxidant 
                 1.17 
                 1.17 
                 1.17 
                 1.17 
                 1.17 
               
               
                 Sulfur 
                 2.8 
                 2.8 
                 2.8 
                 2.8 
                 2.8 
               
               
                 Accelerators 
                 2.4 
                 2.4 
                 2.4 
                 2.4 
                 2.4 
               
               
                 Zinc Oxide 
                 1.7 
                 1.7 
                 1.7 
                 1.7 
                 1.7 
               
             
          
           
               
                 Physical Properties 
               
             
          
           
               
                 ML 1 + 4  @ 
                 64.8 
                 69.2 
                 96.1 
                 53.8 
                 93.9 
               
               
                 100° C. 
               
               
                 Tensile (psi) @ 
                 2497 
                 2268 
                 2566 
                 2400 
                 2513 
               
               
                 23° C. 
               
               
                 Tensile (psi) @ 
                 1453 
                 1278 
                 1693 
                 1280 
                 1379 
               
               
                 100° C. 
               
               
                 % Elong. at 
                 487 
                 614 
                 544 
                 612 
                 649 
               
               
                 break, 23° C. 
               
               
                 % Elong. at 
                 386 
                 486 
                 487 
                 467 
                 499 
               
               
                 break, 100° C. 
               
               
                 Ring Tear 
                 190 
                 270 
                 245 
                 262 
                 298 
               
               
                 (lb/in) @ 
               
               
                 100° C. 
               
               
                 Dispersion 
                 93.1 
                 80.5 
                 95.7 
                 87.9 
                 93.3 
               
               
                 Index, % 
               
               
                   
               
             
          
         
       
     
     A series of tests were conducted, in which the Si69 processing aid was omitted and insoluble sulfur was added, while processing Formulation B with 2 phr octyl-triethoxy silane, and 4 phr sorbitan oleate. Test conditions and results are reported for Examples 8-17 and the Control (no added insoluble sulfur), C—C, in Table IV, below. As illustrated in Table IV, the degree of cure of the composition, expressed as the 300% Modulus (psi) and/or the molecular weight between crosslinks (M c , g/mol), improved as the amount of additional sulfur increased until the 300% Modulus and/or the M c  are well known in the art to be indicators of the state of cure of a vulcanized elastomeric composition. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE IV 
               
               
                   
               
               
                 Physical Properties of Formulation B with 2 phr Octyl-Triethoxy Silane, 
               
               
                 4 phr Sorbitan Monooleate, and Insoluble Sulfur without Si69 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Sample 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
                 16 
                 17 
                 C—C 
               
               
                   
               
               
                 Insoluble S (phr) 
                 1.4 
                 1.7 
                 2 
                 2.3 
                 2.6 
                 2.9 
                 3.3 
                 3.7 
                 4.1 
                 4.5 
                 0 
               
               
                 Total S (phr) 
                 2.8 
                 3.1 
                 3.4 
                 3.7 
                 4 
                 4.3 
                 4.7 
                 5.1 
                 5.5 
                 5.9 
                 1.4 
               
               
                 Physical Test Results 
               
               
                 ML 1+4 /100° C. 
                 84 
                 81.9 
                 80.7 
                 78.9 
                 78.5 
                 103.6 
                 101.8 
                 99.5 
                 99.8 
                 101.7 
                 75.7 
               
               
                 Monsanto Cure @ 171° C. 
               
               
                 ML 
                 13.6 
                 14.7 
                 13.4 
                 13 
                 12.8 
                 18 
                 18.2 
                 18 
                 17.8 
                 18.2 
                 116 
               
               
                 MH 
                 33.4 
                 34.8 
                 37.1 
                 37 
                 38.3 
                 46.5 
                 48.3 
                 50.6 
                 50.8 
                 53.9 
                 37.37 
               
               
                 ts2 
                 2:54 
                 2:48 
                 2:41 
                 2:47 
                 2:44 
                 2:42 
                 2:34 
                 2:30 
                 2:28 
                 2:29 
                 2:30 
               
               
                 tc90 
                 10:51 
                 9:50 
                 9:42 
                 9:28 
                 9:15 
                 12:05 
                 11:36 
                 11:11 
                 10:29 
                 11:11 
                 10:01 
               
               
                 Ring Tensile @ 24° C. 
               
               
                 100% Modulus, psi 
                 188 
                 184 
                 209 
                 194 
                 227 
                 212 
                 267 
                 256 
                 284 
                 326 
                 318 
               
               
                 300% Modulus, psi 
                 494 
                 485 
                 592 
                 556 
                 667 
                 670 
                 792 
                 765 
                 872 
                 988 
                 1150 
               
               
                 Tensile str, psi 
                 1798 
                 1550 
                 1814 
                 1548 
                 1769 
                 1842 
                 2120 
                 1757 
                 1925 
                 2076 
                 2809 
               
               
                 % Elongation 
                 724 
                 657 
                 641 
                 613 
                 594 
                 601 
                 591 
                 538 
                 527 
                 510 
                 556 
               
               
                 Break energy, lbs/in 2   
                 5273 
                 4203 
                 4835 
                 4000 
                 4445 
                 4569 
                 5196 
                 4034 
                 4343 
                 4580 
                 6596 
               
               
                   
               
               
                 Sample 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
                 16 
                 17 
                 C 
               
               
                   
               
               
                 Ring Tensile @ 100° C. 
               
               
                 100% Modulus, psi 
                 131 
                 151 
                 191 
                 215 
                 187 
                 210 
                 231 
                 255 
                 286 
                 292 
                 268 
               
               
                 300% Modulus, psi 
                 333 
                 381 
                 519 
                 566 
                 532 
                 621 
                 656 
                 730 
                 846 
                 833 
                 661 
               
               
                 Tensile str, psi 
                 905 
                 1062 
                 1233 
                 1162 
                 983 
                 1156 
                 1017 
                 878 
                 1079 
                 1042 
                 1263 
               
               
                 % Elongation 
                 649 
                 652 
                 592 
                 529 
                 500 
                 500 
                 441 
                 340 
                 375 
                 364 
                 364 
               
               
                 Break Energy, lbs/in 2   
                 2612 
                 3070 
                 3306 
                 2850 
                 2310 
                 2849 
                 2172 
                 1565 
                 1984 
                 1906 
                 2092 
               
               
                 Ring Tear @ 171° C., ppi 
                 250 
                 217 
                 228 
                 230 
                 201 
                 247 
                 216 
                 201 
                 192 
                 221 
                 276 
               
               
                 Pendulum Rebound 65° C. 
                 33.6 
                 35 
                 32.4 
                 37.6 
                 40.2 
                 37.2 
                 40.2 
                 37.6 
                 41.2 
                 41.4 
                 53.6 
               
               
                 Wet Stanley London, (#/std) 
                 56/53 
                 54/53 
                 56/53 
                 53/53 
                 57/53 
                 60/54 
                 62/54 
                 63/54 
                 64/54 
                 63/54 
               
               
                 Shore A @ 24° C. 
                 68 
                 67 
                 68 
                 68 
                 69 
                 71 
                 71 
                 73 
                 73 
                 72 
                 65 
               
               
                 Dispersion Index #1 
                 85.6 
                 85.5 
                 86.5 
                 87.1 
                 88 
                   
                   
                   
                   
                   
                 59.4 
               
               
                 Specific Gravity 
                 1.184 
                 1.186 
                 1.189 
                 1.188 
                 1.189 
                 1.195 
                 1.197 
                 1.199 
                 1.199 
                 1.21 
                 1.202 
               
               
                 Rheometries @ 7% strain 
               
               
                 tan δ @ 65° C. 
                 .1978 
                 .1924 
                 .1807 
                 .1858 
                 .1789 
                 .1697 
                 .1662 
                 .158 
                 .1583 
                 .1503 
                 .1839 
               
               
                 Δ G′ @ 65° C., MPa 
                 4.884 
                 6.201 
                 6.133 
                 5.937 
                 6.117 
                 7.747 
                 8.845 
                 9.295 
                 9.552 
                 10.041 
                 6.88 
               
               
                 Tensile Retraction 
               
               
                 Mc × 10 −3  g/mol 
                 20.9 
                 20.0 
                 17.5 
                 17.4 
                 16.3 
                 15.4 
                   
                   
                 13.2 
                 12.3 
                 12.2 
               
               
                   
               
             
          
         
       
     
     A further series of tests were conducted, in which Formulation B, described in Table III, was processed with added sulfur and a processing aid comprising 1.5 phr octyl-triethoxy silane, 0.5 phr Si69, and 4 phr sorbitan oleate. Test conductions and results are reported for Examples 18-22 in Table V, below. Table V illustrates a progressive improvement in the degree of cure of the composition, expressed as the 300% Modulus (psi) and/or the molecular weight between crosslinks M c , g/mol), as the total amount of added sulfur is progressively increased. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE V 
               
             
             
               
                   
               
               
                 Physical Properties of Formulation B with 1.5 phr Octyl-triethoxysilane, 
               
               
                 4 phr Sorbitan, 0.5 Si69 and Insoluble Sulfur 
               
             
          
           
               
                 Sample 
                 18 
                 19 
                 20 
                 21 
                 22 
               
               
                   
               
             
          
           
               
                 Insoluble S (phr) 
                 2.8 
                 3.2 
                 3.6 
                 4 
                 4.4 
               
               
                 Total S (phr) 
                 4.2 
                 4.6 
                 5 
                 5.4 
                 5.8 
               
               
                 Physical Test Results 
               
               
                 ML 1 + 4 /100° C. 
                 81.9 
                 83.6 
                 84.2 
                 86.3 
                 80.8 
               
               
                 Monsanto Cure @ 171° C. 
               
               
                 ML 
                 13.15 
                 13.2 
                 13.15 
                 13.82 
                 12.48 
               
               
                 MH 
                 41.84 
                 44.62 
                 44.62 
                 46.58 
                 46.98 
               
               
                 ts2 
                  2:50 
                  2:44 
                 2:43 
                 2:35 
                 2:38 
               
               
                 tc90 
                 10:15 
                 10:12 
                 9:12 
                 9:24 
                 8:59 
               
               
                 Ring Tensile @ 24° C. 
               
               
                 100% Modulus, psi 
                 273 
                 291 
                 326 
                 341 
                 408 
               
               
                 300% Modulus, psi 
                 935 
                 994 
                 1112 
                 1158 
                 1452 
               
               
                 Tensile str, psi 
                 2323 
                 2183 
                 2112 
                 2012 
                 2497 
               
               
                 % Elongation 
                 582 
                 537 
                 483 
                 461 
                 460 
               
               
                 Break Energy, lbs/in 2   
                 5760 
                 5099 
                 4545 
                 4164 
                 5130 
               
               
                 Ring Tensile @ 100° C. 
               
               
                 100% Modulus, psi 
                 251 
                 251 
                 287 
                 307 
                 311 
               
               
                 300% Modulus, psi 
                 826 
                 798 
                 933 
                 1030 
                 998 
               
               
                 Tensile str, psi 
                 1326 
                 1215 
                 1255 
                 1229 
                 1113 
               
               
                 % Elongation 
                 444 
                 428 
                 388 
                 350 
                 329 
               
               
                 Break Energy, lbs/in 2   
                 2720 
                 2439 
                 2306 
                 2069 
                 1800 
               
               
                 Ring Tear @ 171° C., psi 
                 240 
                 230 
                 201 
                 219 
                 206 
               
               
                 Pendulum Rebound 65° C. 
                 37.2 
                 39 
                 42.8 
                 39.4 
                 42.4 
               
               
                 Wet Stanley London (#/std) 
                 64/53 
                 61/53 
                 64/53 
                 65/53 
                 65/53 
               
               
                 Shore A, @ RT 
                 72 
                 71 
                 72 
                 74 
                 73 
               
               
                 Specific Gravity 
                 1.195 
                 1.196 
                 1.197 
                 1.197 
                 1.202 
               
               
                 Rheometries @ 7% strain 
               
               
                 tan δ 65° C. 
                 0.1577 
                 0.1528 
                 0.1444 
                 0.1384 
                 0.1533 
               
               
                 Δ G′ @ 65° C., MPa 
                 6.89 
                 6.798 
                 6.676 
                 6.285 
                 7.789 
               
               
                 Tensile Retraction 
                 12.6 
                 12.4 
                 11.1 
                 10.4 
                 9.7 
               
               
                 Mc, × 10 −3  g/mol 
               
               
                   
               
             
          
         
       
     
     The present invention can thus further utilize the presence of an ester of a fatty acid or an ester of a polyol as a processing aid to replace the silane Si69 to give equal processability of the vulcanizable compound, and better hot tear strength and lower hysteresis of the vulcanized rubber stock, without loss of the other measured physical properties. 
     The further processing aid, such as the preferred sorbitan oleate, is air stable and does not decompose. The sorbitan oleate is lower in cost and more storage stable than Si69, and when used with a silica filler and a silane terminated polymer, gives similar reduction of ML 4 , and tan δ with an increase in tear strength. 
     The additional processing aids useful according to the present invention include esters of fatty acids or esters of polyols. Representative examples include the sorbitan oleates, such as sorbitan monooleate, dioleate, trioleate and sesquioleate, as well as sorbitan esters of laurate, palmate and stearate fatty acids, and the polyoxyethylene derivatives of each, and other polyols, including glycols such as polyhydroxy compounds and the like. Of these, sorbitan monooleate is preferred. 
     It is therefore demonstrated that the present invention provides a means for improving the processability of formulations of diene polymer elastomers with silica-filler, reducing the viscosity of silica-filled elastomeric vulcanizable. It is further demonstrated that the present invention provides vulcanizable silica-filled elastomeric compounds having enhanced physical properties. 
     It should be appreciated that the present invention is not limited to the specific embodiments described above, but includes variations, modifications and equivalent embodiments defined by the following claims.