Method for preparing hydrophobic precipitated silica

The present invention is a method for the preparation of hydrophobic precipitated silicas which are useful as reinforcing fillers in rubber compositions. The method comprises a first step wherein an aqueous suspension of precipitated silica is contacted with an organosilicon compound in the aqueous phase in the presence of a catalytic amount of an acid to effect hydrophobing of the precipitated silica thereby forming an aqueous suspension of a hydrophobic precipitated silica. In the subsequent step, the aqueous suspension of hydrophobic precipitated silica is contacted with a water-immiscible organic solvent at a solvent to silica weight ratio greater than 5:1 to effect separation of the hydrophobic precipitated silica from the aqueous phase. In a preferred process, the hydrophobic precipitated silica has a surface area within a range of 100 to 750 m²/g.

Although hydrophobic precipitated silicas prepared by our claimed method are useful in many applications, such as reinforcing and extending filler in natural rubbers, thermal insulation and as filler in floatation devices, they are particularly useful as reinforcing fillers in silicone rubber compositions. It is well known that silicone rubbers formed from the vulcanization of polydiorganosiloxane fluids or gums alone generally have low elongation and tensile strength. One means for improving these physical properties involves incorporation of a reinforcing silica filler into the fluid or gum prior to curing. It is known to use treated pyrogenic silica as a reinforcing filler in such silicone rubbers. However, pyrogenic silica has the disadvantage of being expensive. Therefore, it would be advantageous to replace at least a portion of the pyrogenic silica used in such applications with a precipitated silica.

Attempts to make such a substitution have not been entirely successful for a number of reasons. Precipitated and pyrogenic silicas have the tendency to interact with polydiorganosiloxane fluids or gums to cause a phenomenon typically referred to as "crepe hardening". A great deal of effort has been made to treat the surface of these fillers with organosilanes or organosiloxanes to make the silica surface hydrophobic. This surface treatment reduces or diminishes the tendency of the compositions to crepe harden and also improves the physical properties of the resulting cured silicone rubber. Processes for treating precipitated silica have typically been difficult to conduct on a commercial scale. Our present method can economically be conducted on such a scale to make hydrophobic precipitated silica.

U.S. Patent 3,904,787 describes the treatment of a precipitated silica in aqueous suspension with an organohalosilane at a temperature within a range of 15 to 70°C. The hydrophobic precipitated silica is then filtered, washed, dried and tempered by heating in the range of 200 to 500°C.

U.S. Patent 4,015,031 discloses a process where a precipitated silica in powdered form is heated to a temperature of 200 to 300°C. with agitation to fluidize and then treated dropwise with an organosilane which is stable and boils below 300°C.

U.S. Patent 4,072,796 provides a process where an acidic wet suspension of precipitated silica at a temperature of 50 to 90°C. is hydrophobed with a prepolycondensed organohalosilane or a prepolycondensed mixture of organohalosilanes. The hydrophobed precipitated silica is thereafter filtered, washed, dried and tempered at 300 to 400°C.

U.S. Patents 4,208,316, 4,273,589 and U.S. 4,308,074 claim the treatment of dried precipitated silica with organosilicon compounds as hydrophobing agents in a ratio of 10:0.5 to 10:3. The product obtained thereby is tempered for 60 to 180 minutes, preferably 70 to 130 minutes, at a temperature of 200 to 400°C.

U.S. Patent 5,009,874 details a method for making a hydrophobic, essentially spheroidal precipitated silica, useful as a reinforcing filler in silicone elastomers. In a first step, the precipitated silica in aqueous suspension is hydrophobed with an organosilicon compound. In a second step a water immiscible organic solvent is added to effect separation of the hydrophobic precipitated silica from the aqueous phase. The water immiscible organic solvent is added to the process at a volume(L) to weight (Kg) ratio of silica of from 1 to 5 and preferably from 1.5 to 4.5. As used herein, "immiscible" implies that the organic solvent selected will not be uniformly blended or mixed with water.

The present invention is a method for preparing a hydrophobic precipitated silica. The method comprises: (A) contacting an aqueous suspension of a precipitated silica with (1) a catalytic amount of an acid and (2) an organosilicon compound selected from the group consisting of organosilanes described by formula R¹_aH_bSiX_4-a-b (1) and organosiloxanes described by formula R²_nSiO_(4-n)/2 (2) where each R¹ is an independently selected hydrocarbon radical comprising 1 to 12 carbon atoms, each R² is independently selected from the group consisting of hydrogen, hydroxy and hydrocarbon radicals comprising 1 to 12 carbon atoms, with the proviso that at least 50 mole percent of the R² substituents are hydrocarbon radicals, each X is independently selected from halogen atom or alkoxy radicals comprising 1 to 12 carbon atoms, a=1, 2 or 3; b=0 or 1; a+b=1, 2 or 3, with the proviso that when b=1 then a+b=2 or 3; and n is an integer from 2 to 3 inclusive, to form an aqueous suspension of a hydrophobic precipitated silica; and (B) contacting the produce aqueous suspension of hydrophobic precipitated silica of (A) with a water-immiscible organic solvent at a solvent to silica weight ratio greater than 5:1 to thereby effect separation of the hydrophobic precipitated silica from said aqueous suspension.

In step (A) of our method, an aqueous suspension of a precipitated silica is hydrophobed with an organosilicon compound. By "precipitated silica", we mean aggregated particles of colloidal amorphous silica that have not at any point existed as a silica gel during their preparation. This precipitated silica is conveniently obtained by standard methods known in the art for producing such silicas. The precipitated silica is normally made by combining an aqueous solution of a soluble metal silicate, ordinarily an alkali metal silicate such as sodium silicate and an acid so colloidal particles grow in a weakly alkaline solution and are further coagulated by the alkali metal ions of the resulting soluble alkali metal salt. Various acids used to facilitate formation of the precipitated silica include mineral acids, such as hydrogen chloride and sulfuric acid and/or carbon dioxide.

Precipitated silicas prepared by such methods may be additionally treated by one or more procedures such as washing, filtering, drying and heating at elevated temperature prior to use in our claimed method. Preferred is when the precipitated silica has been washed to remove alkali metal ions. If desired, the precipitated silica of the present method may be subjected to a shearing force to reduce aggregate particle size and to improve uniformity of the particle size distribution, before use in our claimed method. The shearing force may be applied, for example, by a mechanical means such as a high-speed mixer or ultrasound.

The BET surface area of the precipitated silica used in this method is not critical and is generally within a range of 50 to 1000 m²/g. However, a preferred silica for use in our method, particularly when the precipitated silica is to be used as a reinforcing filler in rubber compositions, is within a range of 100 to 750 m²/g.

The precipitated silica is added to step (A) of the present method as an aqueous suspension. The concentration of precipitated silica is not critical and is normally within a range of 5 to 90 weight percent.

Preferred is when said concentration is within a range of 10 to 50 weight percent. Most preferred is a concentration of 10 to 30 weight percent.

In our step (A), the aqueous suspension of precipitated silica is contacted with one or more of the organosilicon compounds described by formulas (1) and (2) in the presence of a catalytic amount of an acid. The acid catalyst is, for example, a mineral acid such as hydrochloric, hydroiodic, sulfuric, nitric, benzene sulfonic and phosphoric acid. When the organosilicon compound is, for example, a chlorosilane, then the catalytic amount of the acid may be generated in situ by hydrolysis of the chlorosilane or the direct reaction of said chlorosilane with hydroxyl groups on the precipitated silica. In step (A), it is only necessary that the acid be present in an amount sufficient to effect reaction of the organosilicon compound with the precipitated silica. In step (A), it is also preferred that the acid catalyst provide a pH of less than 6. More preferred is a pH less than 3.

During step (A) it may be desirable to add a surfactant or water-miscible solvent to facilitate the reaction of the organosilicon compound with the precipitated silica. Suitable water-miscible solvents include, for example, alcohols such as ethanol, isopropanol and tetrahydrofuran. As used herein, "miscible" solvents imply that said solvent will blend or mix uniformly with water.

The temperature at which step (A) is conducted is not critical and is within a range of 20 to 250°C. Generally, it is preferred that step (A) be conducted at a temperature within a range of 30 to 150°C. Step (A) can be conducted at the reflux temperature of the water-miscible solvent or water-immiscible organic solvent when present.

The surfactant or water-miscible solvent may be added in the presence, or absence, of any water-immiscible organic solvent added to our method. Suitable surfactants include, for example, anionic surfactants such as dodecylbenzene sulfonic acid, nonionic surfactants such as polyoxyethylene(23)lauryl ether and (Me₃SiO)₂MeSi(CH₂)₃(OCH₂CH₂)₇OMe where Me is and hereafter denotes methyl and cationic surfactants such as N-alkyltrimethyl ammonium chloride.

In step (A), the precipitated silica is reacted with one or more organosilicon compounds described by formulas (1) and (2). In formula (1), each R¹ is an independently selected hydrocarbon radical comprising 1 to 12 carbon atoms. R¹ is either a saturated or unsaturated monovalent hydrocarbon radical. R¹ is also either a substituted or non-substituted monovalent hydrocarbon radical. R¹ is selected from alkyl radicals such as methyl, ethyl, propyl, t-butyl, hexyl, heptyl, octyl, decyl and dodecyl; alkenyl radicals such as vinyl, allyl and hexenyl; substituted alkyl radicals such as chloromethyl, 3,3,3-trifluoropropyl and 6-chlorohexyl; and aryl radicals such as phenyl, naphthyl and tolyl. R¹ is also selected from an organofunctional hydrocarbon radical comprising 1 to 12 carbon atoms where, for example, the functionality is mercapto, disulfide, polysulfide, amino, carboxylic acid carbinol ester or amido. A preferred organofunctional hydrocarbon radical is one having disulfide or polysulfide functionality.

In formula (1), each X is independently selected from halogen atom or alkoxy radicals comprising 1 to 12 carbon atoms. When X is a halogen, it is preferred that it be chlorine. When X is an alkoxy radical, X is selected from methoxy, ethoxy and propoxy. Preferred is where each X is selected from chlorine atoms and methoxy.

In formula (2), each R² is independently selected from the group consisting of hydrogen, hydroxy and hydrocarbon radicals comprising 1 to 12 carbon atoms, with the proviso that at least 50 mole percent of the R² substituents are hydrocarbon radicals. R² can be the same as R¹ as described above. The organosiloxanes described by formula (2) can be, for example, linear or cyclic in structure. The viscosity of these organosiloxanes is not limiting and range from that of a fluid to a gum. Generally, higher molecular weight organosiloxanes will be cleaved by the acidic conditions of the present method allowing them to react with the precipitated silica.

Examples of useful organosilicon compounds include diethyldichlorosilane, allylmethyldichlorosilane, methylphenyldichlorosilane, phenylethyldiethoxysilane, 3,3,3-trifluoropropylmethyldichlorosilane, trimethylbutoxysilane, sym-diphenyltetramethyldisiloxane, trivinyltrimethylcyclotrisiloxane, octamethylcyclotetrasiloxane, hexaethyldisiloxane, pentylmethyldichlorosilane, divinyldipropoxysilane, vinyldimethylchlorosilane, vinylmethyldichlorosilane, vinyldimethylmethoxysilane, trimethylchlorosilane, hexamethyldisiloxane, hexenylmethyldichlorosilane, hexenyldimethylchlorosilane, dimethylchlorosilane, dimethyldichlorosilane, mercaptopropylmethyldimethoxysilane, bis{3-(triethoxysilyl)propyl}tetrasulfide, polydimethylcyclosiloxanes comprising 3 to 20 dimethylsiloxy units and preferably 3 to 7 dimethylsiloxy units and trimethylsiloxy or hydroxydimethylsiloxy endblocked polydimethylsiloxane polymers having a viscosity within a range of 1 to 1,000 mPa·s at 25°C.

The amount of organosilicon compound added to our method is that sufficient to adequately hydrophobe the precipitated silica to provide a hydrophobic precipitated silica suitable for its intended use. Generally, the organosilicon compound should be added to our method in an amount such that there is at least 0.04 organosilyl unit per SiO₂ unit in the precipitated silica. The upper limit of the amount of organosilicon compound added to our process is not critical since any excess in the amount required to saturate the precipitated silica will suitably act as a solvent for the method.

In our step (B), a water-immiscible organic solvent is added at a solvent to silica weight ratio greater than 5:1 to effect separation of the hydrophobic precipitated silica from the aqueous suspension. In a preferred method, step (A) of our method is first conducted and then followed by step (B). However, in our claimed method the water-immiscible organic solvent can be added before, simultaneously with or after the addition of the organosilicon compound of step (A). In the first two situations above, the conversion of the precipitated silica to a hydrophobic precipitated silica is accompanied by a phase separation in which the hydrophobic silica separates into the organic solvent phase.

For our invention, any organic solvent immiscible with water is employed. Suitable water-immiscible organic solvents include low molecular weight siloxanes such as hexamethyldisiloxane, octamethylcyclotetrasiloxane, diphenyltetramethyldisiloxane and trimethylsiloxy endblocked polydimethylsiloxane fluids. When a siloxane is employed as solvent, it may serve both as a solvent and as a reactant with the precipitated silica. In addition, suitable water-immiscible organic solvents include aromatic hydrocarbons such as toluene or xylene; heptane and other aliphatic hydrocarbon solvents; cycloalkanes such as cyclohexane; ethers such as diethylether or dibutylether; halohydrocarbon solvents such as methylene chloride, chloroform, ethylene chloride and chlorobenzene; and ketones such as methylisobutylketone.

The amount of water-immiscible organic solvent added to the claimed method provides a solvent to silica weight ratio greater than 5:1. At solvent to silica weight ratios of less than 5:1, the hydrophobic precipitated silica tends to flocculate in the solvent and to not form a true precipitate. At solvent to silica weight ratios greater than 5:1, the hydrophobic precipitated silica precipitates into the organic solvent phase thereby effecting separation from the aqueous suspension. The upper limit for the amount of water-immiscible solvent added to our method is limited only by economic considerations, such as solvent cost, solvent recovery or disposal expense and equipment capacity. Preferred is when the weight ratio of solvent to silica is greater than 6:1. Even more preferred is a weight ratio of solvent to silica of 6:1 to 10:1.

It is preferred that the water-immiscible organic solvent has a boiling point below 250°C. to facilitate its removal from the hydrophobic precipitated silica. However, the boiling point of the water-immiscible organic solvent is not critical since the solvent may be removed from the hydrophobic silica by filtration, centrifuging or other suitable means.

In our step (B), the water-immiscible organic solvent is added to said method thereby effecting separation of the hydrophobic precipitated silica from the aqueous suspension. The hydrophobic precipitated silica is then recovered in the organic solvent phase to provide a product which may be used, if desired, without further treatment. Alternatively, the hydrophobic precipitated silica may be washed to reduce contaminates. The hydrophobic precipitated silica is subsequently recovered from the solvent, dried and further treated by such methods as heating.

Example 1

Precipitated silica was hydrophobed with dimethyldichlorosilane. Into a 100 ml flask was added 26 g of Degussa™ FK320DS precipitated silica (98% SiO₂, BET surface area 175 m²/g, tapped density 75 g/l, manufactured by Degussa AG, Frankford/Main, Federal Republic of Germany), 150 g of distilled water and 51 g of isopropanol. The resulting aqueous suspension was stirred for 5 minutes and then 11 g of dimethyldichlorosilane was added drop-wise to the stirring suspension over a 3 minute period. Then the suspension, with stirring, was heated to reflux for 30 minutes. To the cooled suspension was added 200 ml of toluene. The resulting two-phase system was stirred to transfer the hydrophobic silica to the toluene phase. The aqueous phase was separated from the toluene phase in a separatory funnel. The toluene phase containing the hydrophobic precipitated silica was washed 3 times with 300 ml of distilled water. Residual water was removed from the washed toluene phase by azeotropic distillation, followed by distillation to remove the toluene. The recovered hydrophobic precipitated silica was dried in an oven at 130°C for 24 hours. The dried hydrophobic precipitated silica was analyzed by CHN analysis using a Perkin Elmer™ Model 2400 CHN Elemental Analyzer (Perkin Elmer Corporation, Norwalk, CT) for carbon content. The dried hydrophobic precipitated silica was determined to comprise 5 weight percent carbon.

Example 2

Precipitated silica was hydrophobed with octamethylcyclotetrasiloxane. Into a 100 ml flask was added 26 g of Degussa™ FK320DS precipitated silica, 150 g of distilled water, 64 g of isopropanol, 28 g of concentrated hydrochloric acid and 9.6 g of octamethylcyclotetrasiloxane. The resulting aqueous suspension was stirred for 5 minutes and then heated to reflux for 30 minutes. To the cooled suspension was added 200 ml of toluene. The resulting two-phase system was stirred to transfer the hydrophobic silica to the toluene phase as a precipitate. The aqueous phase was separated from the toluene phase in a separatory funnel. The toluene phase containing the hydrophobic precipitated silica was washed 3 times with 300 ml of distilled water. Residual water was removed from the washed toluene phase by azeotropic distillation, followed by distillation to remove the toluene. The recovered hydrophobic precipitated silica was dried in an oven at 130°C for 24 hours. The dried hydrophobic precipitated silica was analyzed by CHN analysis as above for carbon content. The dried hydrophobic precipitated silica was determined to comprise 6.8 weight percent carbon.