Method for preparing hydrophobic precipitated silica

The present invention is a process for producing hydrophobic precipitated silicas which can be used 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 hydrophobization of the precipitated silica to thereby form an aqueous suspension of a hydrophobic precipitated silica , In the subsequent step, the aqueous suspension of hydrophobic precipitated silica is contacted with a non-water-miscible organic solvent in 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 method, the hydrophobic precipitated silica has a surface area within a range of 100 to 750 m 2 / g.

Although hydrophobic precipitated silicas prepared by our claimed process are useful in many applications, such as reinforcing and draining fillers in natural rubbers, thermal insulation, and as fillers in flotation devices, they are particularly useful as reinforcing fillers in silicone rubber compositions. It is well known that silicone rubbers formed by vulcanization of polydiorganosiloxane fluids or resins alone generally have low elongation and low tensile strength. One means of improving these physical properties involves incorporating a reinforcing silica filler into the liquid or resin before curing. It is known to use treated fumed silica as a reinforcing filler in such Sillcon rubbers. However, pyrogenic silica has the disadvantage of being very expensive. Therefore, it would be advantageous to replace at least a portion of the fumed silica used in such applications with precipitated silica.

Attempts to make such a replacement have not been completely successful for a number of reasons. Precipitated and fumed silicas tend to interact with the polydiorganosiloxane fluids or resins to cause a phenomenon typically referred to as "creping". Much effort has been devoted to treating the surface of these fillers with organosilanes or organosiloxanes to render the silica surface hydrophobic. This surface treatment reduces or reduces the tendency of the composition to creping and also improves the physical properties of the resulting cured silicone rubber. Methods of treating precipitated silica have typically been difficult to carry out on a commercial scale. Our present method can be carried out economically on such a scale as to produce 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 annealed by heating in the range of 200 to 500 ° C.

U.S. Patent 4,015,031 describes a process wherein a precipitated silica in powder form is heated with agitation to a temperature of 200 to 300 ° C with stirring and then treated dropwise with an organosilane which is stable and boiling below 300 ° C.

U.S. Patent 4,072,796 provides a process wherein an acid wet suspension of precipitated silica is hydrophobicized at a temperature of 50-90 ° C with a prepolycondensed organohalosilane or a prepolycondensed mixture of organohalosilane. The hydrophobized precipitated silica is then filtered, washed, dried and annealed at 300-400 ° C.

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

U.S. Patent 5,009,874 details a process for producing a hydrophobic, substantially spheroidal precipitated silica which can be used as a reinforcing filler in silicone elastomers. In a first step, the precipitated silica is rendered hydrophobic in aqueous suspension with an organosilicon compound. In a second step, a water-immiscible organic solvent is added to effect separation of the hydrophobized precipitated silica from the aqueous phase. The water immiscible organic solvent is fed to the process in a volume (L) to weight (kg) ratio of silica of 1: 5 and preferably 1.5: 4.5. As used herein, "immiscible" implies that the selected organic solvent will not uniformly mix with water.

The present invention is a process for producing 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 the formula R¹aHbSiX4-ab (1), and organosiloxanes described by the formula R²nSiO (4-n) / 2 (2) wherein each R¹ is an independently selected hydrocarbon radical having from 1 to 12 carbon atoms, each R² is independently selected from the group consisting of hydrogen, hydroxy and hydrocarbon radicals having from 1 to 12 carbon atoms, provided that at least 50 mole% of the R² substituents are hydrocarbon radicals, each X is independently selected from a halogen atom or Alkoxy groups of 1 to 20 carbon atoms, a is 1, 2 or 3, b is 0 or 1; a + b is 1, 2 or 3, provided that when b is 1 then a + b is 2 or 3; and n is an integer of 2 to 3, inclusive, to form an aqueous suspension of hydrophobic precipitated silica, and (B) contacting the prepared aqueous suspension of hydrophobic precipitated silica (A) with a non-water-miscible organic solvent a weight ratio of solvent to silica of greater than 5: 1, thereby effecting deposition of the hydrophobic precipitated silica from this aqueous suspension.

In step (A) of our process, an aqueous suspension of a precipitated silica is hydrophobicized with an organosilicon compound. By "precipitated silica" we mean aggregated particles of colloidal amorphous silica that have never existed as a silica gel during their preparation. This precipitated silica is conveniently obtained by standard procedures known in the art for producing such silicas. The precipitated silica is normally prepared by combining an aqueous solution of a soluble metal silicate, usually an alkali metal silicate such as sodium silicate, and an acid to grow in a weakly alkaline solution colloidal particles which are further coagulated by the alkali metal ions of the resulting soluble alkali metal salt. Various acids used to facilitate the 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 additionally be treated by one or more procedures such as washing, filtering, drying, and elevated temperature heating prior to use in our claimed process. It is preferred that the precipitated silica has been washed to remove alkali metal ions. If desired, the precipitated silica of the present process prior to application in our claimed process may be subjected to shear to reduce the aggregated particle size and to improve the uniformity of the particle size distribution. The shearing force can z. B. by mechanical means such as a high-speed mixer or ultrasound.

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

The precipitated silica is added to step (A) of the present process as an aqueous suspension. The concentration of precipitated silica is not critical and is usually within a range of 5 to 90 wt%. It is preferred if this concentration is within a range of 10 to 50% by weight. Most preferred is a concentration of 10 to 30% by weight.

In our step (A), the aqueous suspension of the precipitated silica is contacted with one or more of the organosilicon compounds described by the formulas (1) and (2) in the presence of a catalytic amount of an acid. The acid catalyst is z. As a mineral acid such as hydrochloric, hydriodic, sulfuric, nitric, benzenesulfonic and phosphoric acid. For example, if the organosilicon compound is a chlorosilane, then the catalytic amount of the acid can be generated in situ by hydrolysis of the chlorosilane or by direct reaction of this chlorosilane with hydroxyl groups of the precipitated silica. In step (A), it is only necessary that the acid be present in an amount sufficient to effect the reaction of the organosilicon compound with the precipitated silica. In step (A) it is further preferred that the acidic catalyst ensures a pH of less than 6. More preferred is a pH of 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. For example, alcohols such as ethanol, isopropanol and tetrahydrofuran. As used herein, "miscible" solvents include that this solvent will mix uniformly with water.

The temperature at which step (A) is carried out is not critical and is within a range of 20 to 250 ° C. In general, it is preferred that step (A) be carried out at a temperature within a range of 30 to 150 ° C. Step (A) may be carried out at the reflux temperature of the water-miscible solvent or water-immiscible organic solvent, if any.

The surfactant or water-miscible solvent can be added in the presence or absence of any non-water-miscible organic solvent added to our process. Suitable surfactants include, for. B. anionic surfactants such as dodecylbenzenesulfonic acid, nonionic surfactants such as polyoxyethylene (23) lauryl ether and (Me 3 SiO) 2 MeSi (CH 2) 3 (OCH 2 CH 2) 7 OMe where Me is methyl and this means hereinafter and cationic surfactants such as N-alkyltrimethylammonium 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 R 1 is an independently selected hydrocarbon group having 1 to 12 carbon atoms. R¹ is either a saturated or unsaturated monovalent hydrocarbon radical. R¹ is also either a substituted or unsubstituted 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 of 1 to 12 carbon atoms wherein the functionality is e.g. B. Mercapto, disulfide, polysulfide, amino, carboxylic acid, carbinol ester or amido. A preferred organofunctional hydrocarbon radical is one which has disulfide or polysulfide functionality.

In formula (1), each X is independently selected from a halogen atom or an alkoxy group having 1 to 12 carbon atoms. When X is a halogen, it is preferable that it is chlorine. When X is an alkoxy radical, X is selected from methoxy, ethoxy and propoxy. It is preferred if each X is selected from chloro and methoxy.

In formula (2), each R 2 is independently selected from the group consisting of hydrogen, hydroxy and hydrocarbon radicals having from 1 to 12 carbon atoms, provided that at least 50 mole% of the R 2 substituents are hydrocarbon radicals. R 2 may be identical to R 1 as described above. The organosiloxanes described by formula (2) may e.g. B. be linear or cyclic in structure. The viscosity of these organosiloxanes is not limitative, ranging from that of a liquid to that of a resin. In general, higher molecular weight organosiloxanes will be cleaved by the acidic conditions in the present process, allowing them to react with the precipitated silica.

Examples of suitable organosilicon compounds include diethyldichlorosilane. Allylmethyldichlorosilane, Methylphenyldichlorosilane, phenylethyldiethoxysilane, 3,3,3-trifluoropropylmethyldichlorosilane, trimethylbutoxysilane, sym-diphenyltetramethyldisiloxane, trivinyltrimethylcyclotrisiloxane, octamethylcyclotetrasiloxane, hexaethyldisiloxane, pentylmethyldichlorosilane, Divinyldipropoxysiklan, vinyldimethylchlorosilane, vinylmethyldichlorosilane, vinyldimethylmethoxysilane, Inimethylchlorsilan, hexamethyldisiloxane, hexenylmethyldichlorosilane, hexenyldimethylchlorosilane, dimethylchlorosilane, Dimethyldichlorosilane, mercaptopropylmethyldimethoxysilane, bis {3- (triethoxysilyl) propyl} tetrasulfide, polydimethylcyclosiloxanes having 3 to 20 dimethylsiloxy units and preferably 3 to 7 dimethylsiloxy units, and polymethylsiloxy or hydroxydimethylsiloxy endblocked polydimethylsiloxane polymers having a viscosity within a range of 1 to 1000 mPa · s at 25 ° C.

The amount of organosilicon compound added to our process is that sufficient to adequately hydrophobe the precipitated silica and provide a hydrophobic precipitated silica suitable for its intended use. In general, the organosilicon compound should be added to our process in an amount such that there is at least 0.04 organosilyl unit per SiO 2 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 necessary to saturate the precipitated silica will suitably act as a solvent for the process.

In our step (B), a water immiscible organic solvent is added to a solvent to silica weight ratio greater than 5: 1 to effect precipitation of the hydrophobic precipitated silica from the aqueous suspension. In a preferred method, step (A) of our method is performed first and then followed by step (B). However, in our claimed process, the water-immiscible organic solvent may be added in advance, 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 phase of the organic solvent.

For our invention, any organic solvent that is immiscible with water is used. Suitable water-immiscible organic solvents include low molecular weight siloxanes such as hexamethyldisiloxane, octamethylcycotetrasiloxane, diphenyltetramethyldisiloxane and trimethylsiloxy endblocked polydimethylsiloxane fluids. When a siloxane is used as the solvent, it can serve as both a solvent and a reactant with the precipitated silica. Further, 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 diethyl ether or dibutyl ether; Halogenated hydrocarbon solvents such as methylene chloride, chloroform, ethylene chloride and chlorobenzene, and ketones such as methyl isobutyl ketone.

The amount of non-water-miscible organic solvent added to our claimed process provides a solvent to silica weight ratio of 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 not to form a true precipitate. For solvent to silica weight ratios greater than 5: 1, the hydrophobic precipitated silica precipitates into the organic solvent phase thereby causing separation from the aqueous suspension. The upper limit to the amount of non-water miscible solvent added to our process is limited only by economic considerations such as solvent cost, solvent recovery or disposal and capacity costs of the equipment. It is preferred if the weight ratio of solvent to silica is greater than 6: 1. More preferred is a weight ratio of solvent to silica of from 6: 1 to 10: 1.

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

In our step (B), the water-immiscible organic solvent is added to this process, thereby causing deposition 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, if desired, can be used without further treatment. Alternatively, the hydrophobic precipitated silica can be washed to reduce impurities. The hydrophobic precipitated silica is then recovered from the solvent, dried and further treated by such methods as heating.

example 1

Precipitated silica was rendered hydrophobic with dimethyldichlorosilane. To a 100 ml flask was added 26 g of Degussa ™ FK320DS precipitated silica (98% SiO 2, BET surface area 175 m² / g, tap density of 75 g / l, manufactured by Degussa AG, Frankfurt / Main, 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 dropwise to the stirred suspension over a period of 3 minutes. Then, the suspension was refluxed with stirring for 3 minutes. To the cooled suspension was added 200 ml of toluene. The resulting two-phase system was stirred to convert 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 three times with 300 ml of distilled water. Remaining 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 for carbon content by CHN analysis using a Perkin Elmer ™ Model 2400 CHN Elemental Analyzer (Perkin Elmer Corporation, Norwalk, CT). The dried hydrophobic precipitated silica was determined to contain 5% by weight of carbon.

Example 2

Precipitated silica was hydrophobicized with octamethylcyclotetrasiloxane. To a 100 ml flask was added 26 g of Degussa ™ FK320DS precipitated silica, 150 g of distilled water, 64 g of isopropanol and 28 g of concentrated hydrochloric acid and 9.6 g of octamethylcyclotetrasiloxane. The resulting aqueous suspension was stirred for 5 minutes and then heated at reflux for 30 minutes. To the cooled suspension was added 200 ml of toluene. The resulting two-phase system was stirred to convert 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 three times with 300 ml of distilled water. Remaining 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. It was determined that the dried hydrophobic precipitated silica contained 6.8% by weight of carbon.