JPS6362550B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to the production of amorphous precipitated silica useful for reinforcing silicone. Silicone rubbers reinforced with the silica products of this invention as the primary filler exhibit a water volume swell of not more than 4%, and preferably a wet dielectric strength of at least 400 volts/mil. It is an object of the present invention to provide a finely divided amorphous, precipitated, high structure silica for use as a primary filler for silicone rubber. That object is obtained by using a silica product according to claim 1 and a composition according to claim 8. The silica product according to claim 1 can be obtained by the process according to claims 16 and 20, the subclaims describing preferred embodiments of the invention. Unreinforced, vulcanized silicone elastomers have relatively low tensile strengths, and therefore they require reinforcement with particulate fillers to exhibit useful mechanical properties. Amorphous fumed silica, produced by the oxidation of silicon tetrachloride at high temperatures, is the primary reinforcing filler used in silicone rubber due to its high degree of purity, particle size and thermal stability. Amorphous precipitated silica, which is widely used as a filler in other organic rubbers such as styrene-butadiene rubber, is not widely used in silicone rubbers due to its high water and electrolyte content. Precipitated silica-filled silicone rubbers are generally inferior to fumed silica-filled silicone rubbers in edge strength and water swelling properties. Therefore, precipitated silica has been used primarily as a secondary filler in combination with fumed silica in silicone rubbers or in applications where electrical properties and water swelling are not important. Precipitated silica is most commonly prepared by combining an aqueous solution of an alkali metal silicate, usually sodium silicate, with an inorganic acid.
That is, it is produced by neutralizing with carbonic acid, hydrochloric acid or sulfuric acid. The level of impurities, such as electrolytes, in precipitated silica depends in large part on the raw materials used in the manufacturing process and the degree of cleaning of the product. Inorganic salts (electrolytes) of water-soluble alkali metals, such as sodium, generally constitute the largest single impurity in precipitated silica. These salts can be exemplified by sodium chloride, sodium sulfate, sodium carbonate and sodium bicarbonate, and such salts can be present in the precipitated silica alone or in various combinations and amounts. Sulfate salts appear to be particularly detrimental to the development of low values of percent water volume swell of silicone rubber. The above salts are generated by the acid-base reactions that occur during precipitation. Their presence in the silica used to reinforce silicone rubbers results in higher percent water swelling and lower dielectric strength values of such rubbers compared to silicone rubbers in which fumed silica is used as a reinforcing filler. Responsible. Normal cleaning of precipitated silica during the manufacturing process
Although some of the contaminating soluble alkali metal inorganic salts are removed, the level of such salts in the amorphous precipitated silica product is still between 1 and 4 or 5% by weight, e.g. -3% by weight. Further reduction in the level of soluble inorganic salts in the silica can be achieved by more extensive washing with electrolyte-free water or by passing an aqueous slurry of silica through a bed of ion exchange resin.
However, such methods add considerably to manufacturing costs. Now, we have developed amorphous, precipitated silica useful for reinforcing silicone rubber, reducing the amount of soluble inorganic salt impurities found in typical commercial products, such as sodium chloride and sodium sulfate, to very low levels, e.g. 0.6 wt. It has been found that it can be made without the additional water washes (or use of ion exchange resins) required to reduce the drop to below %. More specifically, treating the surface of precipitated silica with small amounts of water-soluble compounds of certain metals eliminates the adverse effects that the alkali metal soluble salts of chloride and sulfate that would otherwise be present in the silica have on water swelling. It has been found that offset and dielectric strength benefits are obtained when silica so treated is incorporated into silicone rubber. Metals that may be so useful are those that form relatively water-insoluble white or colorless sulfates, such as calcium and barium. The amount of metal required for the treatment of silica to produce the desired beneficial results is related to the amount of alkali metal inorganic salts, especially chlorides and sulfates, in the silica, and especially the amount of alkali metal present in the silica. , e.g. related to the total amount of sodium. GENERAL DESCRIPTION OF THE INVENTION In accordance with the present invention, there is provided a finely divided, amorphous, precipitated silica containing a small addition of metal cations that form relatively water-insoluble white or colorless sulfate salts. be done. The silica contains on an anhydrous basis, ie with bound water, at least 80 or 85%, preferably at least 90%, more preferably 93-97% by weight _SiO2 . The unmodified (free of added metal cations) amorphous precipitated silica is prepared by combining an aqueous solution of a soluble silicate, such as a sodium, lithium or potassium silicate, most commonly sodium silicate, with an inorganic mineral acid. , especially by reaction with carbonic acid, sulfuric acid or hydrochloric acid. This method is well described in US Pat. No. 2,940,830.
Such silicas contain varying amounts of bound water (about 2-5% by weight) and adsorbed water (about 3-7% by weight), the latter depending in part on the prevailing relative humidity. Adsorbed water is water that is removed from silica by heating at 105° C. for 24 hours at atmospheric pressure in a laboratory dryer. The bound water further calcinates the silica at temperatures e.g.
This water is removed by heating at 1200℃. Silica is 5-100 nanometers, usually 5-40,
More commonly, they can have ultimate particle sizes in the range of 15 to 30 nanometers. Preferably the ultimate particle size is about 20 nanometers. The BET surface area of silica, measured using nitrogen gas, can range from 75 to 600 square meters per gram (m ² /g), but usually ranges from 75 to 250 m ² /g, more usually from 120 to 600 m 2 /g. The range is 160m ² /g. The BET method for measuring surface area is described in Journal of the American Chemical Society, Vol. 60, p. 304 (1930). The amorphous precipitated silica treated according to the invention is a reinforcing silica, that is, it is useful as a reinforcing filler or pigment for organic solids, especially in rubber compositions for use in tires and shoe soles. Such reinforcing pigments are also
Technically designated as high structure silica due to the high percentage of water (sometimes designated as structured water) associated with the filter cake obtained during the manufacture of reinforced precipitated amorphous silica. High structure silicas are those in which the percent water in the filter cake is about 70-85% by weight. less than 70% by weight, e.g. about 50-70
Silica that retains the weight percent is technically designated as low structure silica. For example, US Patent No.
See No. 4157920 (column 4, lines 29-40). Total structural water content is an important property of silica pigments and is directly related to the functional end-use properties of the silica. Accordingly, high structure silicas find major end use as reinforcing fillers in elastomers and rubbers because they exhibit physical properties such as high structure, low valley wear, and low packing density. In contrast, low structure silicas are not useful as reinforcing fillers because they exhibit physical properties of low structure, high abrasion, and high packing density. Low structure silica has particular utility as an abrasive or polishing agent for toothpaste compositions. The highly structured amorphous precipitated silicas of the present invention for use in reinforcing silicone rubber typically have a water-soluble sulfate content of less than about 2% by weight, preferably less than 1% by weight, less than about 1% by weight, preferably about 0.5% by weight. It has a water-soluble chloride salt content of less than about 0.05-5% by weight, preferably less than about 2% by weight, more preferably less than 1% by weight of alkali metals, such as sodium oxides. The combined total content of soluble sulfate and chloride salts and alkali metal oxides, such as sodium oxide, in such silica is preferably in the range of about 1-3% by weight, more preferably less than about 2.5% by weight. . The levels of chloride and sulfate salts and sodium oxide are determined by analysis of chlorine, sulfur and sodium by X-ray fluorescence spectroscopy. The amounts of chlorine, sulfur and sodium are respectively sodium chloride and
Shown on the basis of precipitated silica as sodium sulfate and sodium oxide. The amount of metal treating the surface of the reinforced precipitated silica is
The amount of alkali metals, such as sodium (usually expressed as metal oxides) as well as chlorine and sulfur, present in the untreated precipitated silica, in particular the total amount of alkali present therein, is determined. According to the invention, when incorporated as the main silica filler in silicone rubber, the amount of silica does not exceed 4%, preferably less than 2%, more preferably 1%.
A sufficient amount of the selected metal is added to the surface of the untreated precipitated silica to yield a silica product that yields a reinforced silicone rubber having a water volume swell of less than 100 mL. In one embodiment, at least about 1 molar equivalent of metal based on the alkali metal (calculated based on the atomic weight of each element or common stable oxide thereof) is used to treat the silica. Typically one or more molar equivalents of metal, such as about 1:1 to 2:
A molar equivalent ratio of metal to alkali metal of 1, more typically from about 1:1 to 1.5:1 is used. As used herein, the "total amount" of alkali metals present in silica shall mean the sum of all alkalis present, i.e., the sum of the analyzes for alkalis expressed as sulfate and chloride salts and as oxides. . Metals that may be useful in the practice of this invention are those that form white or colorless sulfate salts that are relatively water insoluble. Typically such metal sulfate salts have a solubility of less than 1 g per 100 cubic centimeters of cold (0-30°C) water, preferably less than 0.75 g, more preferably less than 0.3 g. Typical such metals include calcium, barium, lead, silver,
Contains strontium and titanium. Mixtures of such metals can also be used if desired.
The amount of metal added to the precipitated silica can be determined by X-ray fluorescence spectroscopy and is usually expressed as metal oxide. In calculating the amount of metal cations to be added, it is possible to take into account the amount of such metal cations to be added already present in the precipitated silica from impurities in the reagents (including water) used to prepare the silica. can. Calcium and barium are preferred as added metal cations. Calcium is the more preferred metal that can be used to treat silica surfaces because of its relative cost, availability, and low molecular weight. The amount of calcium present on the silica, expressed as oxide (CaO), is at least
It varies from 0.45 to about 3% by weight, more usually from about 0.5 to 3%, more typically from about 0.5 to 2%.
The amounts of other metals used, such as barium, are equivalent on a molar basis to the statements regarding calcium. On a weight basis, the amounts of other metals used are proportional to the ratio of their molecular weight to calcium.
For example, the amount of barium required is approximately 3.4 (137/40) times the weight basis amount of calcium required. Similarly, the amount of lead, silver, strontium and titanium required on a weight basis is approximately 5.2 of the amount required for calcium.
2.7, 2.2 and 1.2 times. The correlation between alkali metals such as sodium, sulfur, and chlorine in precipitated silica and percent water swelling can be determined by the well-known technique of multiple regression analysis. When the metal used is calcium, a linear model describing the percent water volume swelling as a function of the ionic form of such element is: /0 water swelling ≡S= C ₁ [Cl ^- ] + C ₂ [SO ₄ ⁼ ] +C ₃ [Na ⁺ ] + C ₄ [Ca ⁺⁺ ] (1), where C ₁ , C ₂ , C ₃ and C ₄ are numerical regression coefficients, [Cl ^- ], [SO ₄ ⁼ ] , [Na ⁺ ] and [Ca ⁺⁺ ] are expressed as sodium chloride (NaCl), sodium sulfate (Na ₂ SO ₄ ), sodium oxide (Na ₂ O) and calcium oxide (CaO), respectively. Weight percent of ions. Equation (1) above can be used to evaluate the percent water volume swelling of silicone rubber reinforced with high structure silica. In the above formula, the numerical coefficients of C ₁ , C ₂ , C ₃ and C ₄ can be varied. C ₁ varies from 8 to 12, for example
10, C ₂ varies from 1 to 3, for example 2, C ₃ is 9.5
For example, C ₄ changes from −5.5 to −
It changes to 8.5, for example -7°C. Therefore, increasing the chloride, sulfate and/or sodium ion content of the precipitated silica increases the percent water volume swelling of silicone rubber containing such modified silica, and increasing the calcium ion content in the precipitated silica increases the percent water volume swelling of silicone rubber containing such modified silica. The percent water volume swelling of silicone rubber containing the modified silica is reduced. Concentrations (by weight) of chloride, sodium and sulfate anions, expressed as sodium chloride, sodium oxide and sodium sulfate, respectively, and calcium cations, expressed as calcium oxide, in precipitated silica for a calculated percent water swelling of 4% or less. The relationship with the concentration of ions can be expressed by the ratio: 4-C ₁ [Cl ^- ]-C ₂ [SO ₄ ⁼ ]-C ₃ [Na ⁺ ]/C ₄ [Ca ⁺⁺ ] (2) , C ₁ , C ₂ , C ₃ in the formula,
C ₄ , [Cl ^- ], [SO ₄ ⁼ ], [Na ⁺ ] and [Ca ⁺⁺ ] are as described above. When using metals other than calcium, the coefficient C ₄ is the ratio of molecular weights, i.e. 40/
Multiplied by M (M is the atomic weight of the other metal).
The above ratio (2) is 4 for the volumetric swelling of water in silicone rubber.
It can be used to evaluate the amount of metal cations, e.g. Ca ⁺⁺ , required to obtain % or less. The precipitated silicas of this invention provide silicone rubber with mechanical and electrical properties that approximate those achievable with fumed silicas. In particular, silicone rubbers having a percent low volume swelling of less than 4, often less than 3, can be obtained using the precipitated silicas of the invention. Silicone rubbers with percent water volume swellings of 1% or less were obtained using the precipitated silicas so treated. The electrical properties of silicone rubber are typically described by the dielectric strength (wet and dry) of the rubber.
Wet dielectric strength values of at least 400 volts/mil for dielectric strength, such as at least 435 volts/mil, are desired for applications requiring good electrical properties. Dielectric strength and percent water volumetric swelling are not parallel properties, ie, a low value of volumetric swelling does not necessarily correspond to a high value for dielectric strength. However, in general, when the metals mentioned above are added to the surface of precipitated silica in the amounts mentioned above, both the dielectric strength and the percent water volume swelling of silicone rubbers incorporating such silicas decrease compared to silicone rubbers incorporating untreated silicas. It will be advantageous. The reinforced high structure silica of the present invention can be produced by treating an amorphous, high structure precipitated silica with an aqueous solution of a compound of the metal. For example, a precipitated silica filter cake can be dispersed in an aqueous solution containing a metal compound in solution and washed with a solution of the metal compound, or a silica powder can be dispersed in an aqueous medium substantially free of electrolytes, e.g. It can be recovered by filtration and the filter cake washed with or dispersed in an aqueous solution of the metal compound. Other equivalent methods known to those skilled in the art of treating solid fine powder particles with aqueous metal cation-containing solutions can also be used. The above processing,
That is, washing, dispersion, etc. can be repeated several times as necessary to treat the silica with the desired amount of the selected metal. Metal cations of metal compounds, such as calcium or barium, are adsorbed onto the surface of the silica. It is believed that the adsorbed metals exist as polybasic cations or very small positively charged colloidal metal oxide particles. The above treatment can be accomplished by modifying the recovery step in the conventional wet process of producing amorphous hydrated precipitated silica. In the above method,
An aqueous slurry of crude precipitated silica is produced. The crude settled slurry is washed one or more times with water and then separated from the aqueous suspension medium, for example by filtration. The wet silica cake, such as the filter cake, is dried and ground into a finely divided product. According to one embodiment of the invention, the wet silica, e.g. filter cake, recovered from the aqueous slurry is washed once or more times with water containing metal cations, and if desired then with distilled water or deionized water. Wash with water. The treated and washed silica is dried and ground by methods known in the art. In other embodiments, an aqueous slurry of crude precipitated silica is diluted (1
times or more times) to wash the silica,
Dilute the washed silica with an equal volume of water containing metal cations. Although the concentration of silica in the resulting slurry can vary, it should be sufficiently diluted to provide intimate contact between the silica particles and the water or metal cation-containing water. Typically the slurry has a silica concentration of about 5% by weight. The diluted slurry is filtered and the filter cake is washed with at least once distilled or deionized water, or if necessary with water containing metal cations. The washed filter cake of treated silica is then dried and ground. Thus, the crude precipitated silica product can be washed successively with distilled or demineralized water until the chloride and sulfate ions are reduced to the above-mentioned common commercial range; A subsequent wash with water containing metal cations places sufficient metal cations on the silica to provide a precipitated silica that, when incorporated into a silicone rubber, yields a material with a low percent water volume swell. Alternatively, the crude silica is washed or slurried with an aqueous metal cation-containing solution such that soluble chloride and sulfate salts are removed from the silica while the desired level of metal cations is maintained or precipitated on the silica. be able to. The concentration of various ions on the silica can be determined by X-ray fluorescence spectroscopy of dried samples of the treated silica. Aqueous metal cation-containing solutions can be prepared by dissolving the metal cation-containing compound (to the extent of its solubility) in a diluent or wash solution. The amount of such liquid used will often be based on the amount of soluble salts of chlorides and sulfates and alkali metals present in the silica and less than 3 or 4%, preferably 2% when incorporated into the silicone rubber.
depending on the amount of metal cation, such as calcium, required to produce a treated amorphous, precipitated high structure silica that yields a product with a water volume swell of less than 1%, such as less than 1%. In preferred embodiments, the treated high structure silica has a percent water volume swelling of less than 2%, more preferably less than 1%, and a wet dielectric strength of at least 400 volts/mil. The metal compounds used to form the wash water or dilution water containing metal cations are metal oxides or hydroxides of the selected metal, or metal compounds that hydrolyze to form metal oxides when mixed with water. It can be any other inorganic or organic compound of the metal. Preferably, the solution of metal compounds does not introduce other anionic species into the silica, such as sulfate and chloride, as such anions tend to increase the percent aqueous volume swelling. Furthermore, care should be taken to ensure that solutions of metal compounds do not introduce significant amounts of cationic species other than those specified, since they can adversely affect wet dielectric strength or percent water volume swelling. It is. For example, aluminum tends to reduce wet dielectric strength. Examples of compounds that can be used include: ionizable inorganic oxides, hydroxides or metal salts and ionizable metal salts of _C1 - _C4 carboxylic acids, such as calcium oxide, calcium hydroxide, barium carbonate, barium hydroxide, Includes barium oxide, calcium acetate and calcium formate. The treated precipitated silica of the present invention is incorporated into the silicone elastomer gum in an amount sufficient to reinforce the silicone rubber when vulcanized, ie, in a reinforcing amount. Typically, the amount of treated precipitated silica used will range from about 10 to 100, more usually 20 to 70, preferably 30 to 60 parts of silica per 100 parts of silicone elastomer gum. Any silicone gum known in the art that is vulcanized, ie, crosslinked, to silicone rubber with a free radical generator can be used as the silicone elastomer gum. Examples of silicone gum include methyl,
Includes vinyl, phenyl, methyl vinyl, methyl phenyl and fluorinated silicone gums. The free radical generators used to catalyze the vulcanization (crosslinking) of silicone gums are organic peroxides and gamma or high energy electron radiation. Typical commonly used organic peroxides are benzoyl peroxide, bis(2,4-dichlorobenzoyl) peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and t-butylperoxybenzoate. and dicumyl peroxide. Peroxide typically ranges from 0.2 to 100 parts of gum.
Used in an amount of 1.0 part. In addition to settling fillers and free radical generators, silicone gums contain processing aids (0 to 12 parts/100 parts of gum).
%), silane additives (0-1 parts/100 parts gum), colorants, heat stabilizers, plasticizers, etc. may be included. All ingredients (except peroxide) are mixed until homogeneous in equipment such as a Banbury internal mixer or a Sigma mixer. The mixture is then cooled, if necessary, and peroxide catalyst is added. The catalyst-added mixture is then vulcanized in a manner known in the art. Typically, the mixture is cured at about 170°C for 10-15 minutes. Post-vulcanization at about 250°C for 1 to 4 hours is also used. The following examples are intended as illustrations of the invention and its preferred embodiments. All parts and percentages in the examples are by weight unless otherwise indicated. Example 1 Aqueous slurry containing approximately 100 g of crude precipitated high structure silica produced by acidifying an aqueous solution of sodium silicate with carbon dioxide20
100% stainless steel container and distilled water.
Diluted with 80%. A settling aid (medium molecular weight polycationic polymer) in an amount of 0.4% by weight based on _SiO2
was added to the tank and the slurry was heated to 70°C. The slurry was allowed to stand at 65-70°C overnight. The next morning, the supernatant liquid in the tank was drained, leaving approximately 3 ml of settled slurry. A second time, distilled water was added to the tank to a total volume of 100 ml. Heat the slurry to 70℃ while stirring.
It was aged for 6 hours at that temperature. Thereafter, the second supernatant liquid was removed and the tank was again filled with distilled water. The slurry was stirred and heated to 70°C while it was being stirred and left at 65-70°C overnight. The next morning, the third supernatant was discharged, and the pH of the remaining sediment slurry was adjusted to 3.7 with 5N sulfuric acid. Heat the acidified slurry to 95℃ and add 1
Time aged. Heat aged slurry (this is 6.0
) was filtered to obtain 17,500 grams of filter cake. The filter cake was reslurried and diluted with distilled water 61 saturated with calcium oxide. The slurry was stirred while being heated to 70°C and aged for 30 minutes. The hot slurry was filtered and the filter cake was reslurried to a total volume of 100 with calcium oxide saturated distilled water. The slurry had a pH of 9.9, which was lowered to 5.5 with dilute sulfuric acid and then aged at 70°C for 30 minutes. The hot slurry was filtered and the filter cake was dried in a rotary dryer at 140°C. The dried silica had a BET surface area of 138 m ² /g. The pH of the 5% silica slurry was 6.8.
Silica is modeled for chloride, sulfate, sodium and calcium ions.
X using XRD-410 automatic X-ray spectrograph
Analyzed by line fluorescence spectroscopy. The spectrograph uses a dual target x-ray tube (tungsten/chromium) operating at 60 kilovolts and 50 milliamps for excitation of the small and trace element spectrum. The dried silica was found to contain 2.43% sodium sulfate, 0.00% sodium chloride, 0.49% sodium oxide, and 1.18% calcium oxide. 60 parts of the above dried precipitated silica was added to methyl vinyl silicone gum (SWS silicone) having a specific gravity of 0.98.
A silicone rubber was prepared by mixing 100 parts of Polymer Co., Ltd.-C-155) and 6 parts of a plasticizer. 45%
0.5 part of powdered peroxide with active peroxide [2,5-dimethyl-2,5-di(tert-butylperoxy)hexane] was added and the mixture was press-vulcanized for 15 minutes at 170°C. The rubber was heated at 250℃ in a laboratory furnace.
Vulcanized after hours. The resulting silicone rubber was tested for aqueous volume swelling by ASTM method D-471 and dielectric strength by ASTM method D-149. The percent volumetric swelling obtained was 1.0%. The dielectric strength obtained was 408 volts/mil (dry) and 427 volts/mil (wet). Example 2 Precipitated high structure silica obtained by filtering an aqueous slurry of precipitated silica prepared by neutralizing an aqueous solution of sodium silicate with carbon dioxide and then post-acidifying with sulfuric acid to a pH of 3.7.
14000g was slurried in 20ml of distilled water. Then 10 g of distilled water containing 33.3 g of concentrated sulfuric acid was added to the stirred slurry. Heat the stirred slurry to 70℃,
Aged for 30 minutes. The aged slurry (which had a pH of 2.8) was filtered. The filter cake was reslurried in 30 ml of distilled water and the slurry was heated to 70°C and aged for 30 minutes. The hot slurry (which had a PH of 3.3) was filtered. The filter cake was reslurried in 30 g of distilled water saturated with calcium oxide. The slurry was heated to 70°C and aged for 30 minutes. The aged slurry (with a pH of 8.5) was filtered and the filter cake was dried at 140°C in a rotary dryer. The final dry product had a BET surface area of 133 m ² /g. The 5% slurry had a PH of 7.3. The dried silica product was analyzed for sulfate, chloride, sodium and calcium ions by X-ray fluorescence spectroscopy according to the method described in Example 1. The result obtained is sodium sulfate 0.07
%, sodium chloride 0.01%, sodium oxide 0.56
% and calcium oxide 1.34%. Dry silica product, silicone gum mixture
It was incorporated into the silicone gum in the manner and amount described in Example 1, except that it was press cured at 170 DEG C. for 10 minutes instead of 15 minutes. Silicone gums were tested for water volume swelling and dielectric strength. The results obtained were a water volume swelling of 0.4% and a dielectric strength of 419 volts/
mil (dry) and 421 volts/mil (wet). Example 3 (Comparison) Three lots of untreated precipitated high structure silica produced by a conventional wet process were mixed with powdered peroxide of 0.8
were separately incorporated into the silicone gum in the amounts and manner described in Example 1, except that a portion of each was used. Press vulcanization of silica-silicone gum mixture at 170℃ for 10 minutes.
It was then vulcanized into silicone rubber by post-vulcanization in a 250°C oven for 1 hour. The rubber was tested for water volume swelling. The table is a table of data obtained for this sample.

Table: The data for Examples 1, 2, and 3 show that when a precipitated silica with the defined relationship of chloride, sulfate, sodium, and calcium ions described herein is incorporated into a silicone rubber, the percent water volume swelling is 4.
Indicates that it is less than Example 4 Test A An aqueous slurry of crude precipitated high structure silica produced by neutralizing an aqueous solution of sodium silicate with carbon dioxide was washed extensively with laboratory tap water to reduce the inorganic salt content to a typical was reduced to a level significantly lower than that obtained in commercially produced samples. The washed slurry was filtered. Approximately 20 Kg of the resulting filter cake was dried at 140°C in a rotary dryer. The dried silica product was analyzed for sulfate, chloride, calcium and sodium ions by the method described in Example 1. This dry silica product
The BET surface area was 156 m ² /Kg. The results are shown in the table. Test B 20 Kg of the precipitated silica filter cake prepared in Test A was reslurried with 55 kg of an aqueous solution saturated with calcium oxide. Dilute calcium oxide solution. Remove excess calcium oxide in distilled water (calcium oxide
1.48g/). The slurry was stirred for 1 hour and allowed to settle. The supernatant calcium oxide containing solution was used in this and subsequent tests.
It was used as described here. The reslurry precipitated silica filter cake was stirred for 1 hour and left over the weekend. Then stir the slurry,
Approximately 1/3 (25) was filtered. The filter cake was washed with calcium oxide containing solution 6. The washed filter cake was dried in a rotary dryer at 140°C. The dry product had a BET surface area of 129 m ² /g. The pH of the 5% by weight slurry of dried pigment was 9.2.
The dried silica product was analyzed for sulfate, chloride, sodium and calcium ions using the method described in Example 1. The results are shown in the table. Test C The remaining two-thirds (approximately 50) of the precipitated silica slurry diluted with aqueous calcium oxide solution from Test B was allowed to settle for 6 hours. Discard the resulting supernatant (20) and
The residual sedimentation volume (approx. 30) was diluted with 20 of a saturated calcium oxide-containing aqueous solution. The slurry was stirred and about 1/2 (25) was filtered. The resulting filter cake was washed with a saturated calcium oxide-containing aqueous solution 6 and the washed silica was dried in a rotary dryer at 140°C. The BET surface area of the dry product was 125 m ² /g. A 5% slurry of dried product had a PH of 9.35. The dried silica product was analyzed for sulfate, chloride, sodium and calcium using the method described in Example 1. The results are shown in the table. Test D The residual slurry 25 from Test C was allowed to stand overnight.
The supernatant (approximately 5 ml) was removed and replaced with 5 ml of a saturated calcium oxide-containing aqueous solution. The resulting slurry was stirred and filtered. The filter cake was washed with a saturated aqueous calcium oxide solution 6 and the washing product was placed in a rotary dryer.
Dry at 140°C. The BET surface area of the dry product is
It was 124m ² /g. The PH of the 5% slurry of dried pigment was 9.4. The dried silica product was analyzed for sulfate, chloride, sodium and calcium ions as described in Example 1. The results are shown in the table. Example 5 Example 1 using the dry silica products of Tests A, B, C and D of Example 4, respectively, except that the silica-silicone gum-plasticizer-peroxide mixture was press-vulcanized at 170° C. for 10 minutes. Silicone rubber was produced by the method described. Example 1 of the resulting silicone rubber
Tested for water volume swelling and wet dielectric strength (WDS) according to the ASTM method described. The results obtained are shown in the table.

The data for Examples 4 and 5 show that the addition of calcium ions to precipitated silica pigments significantly reduces the percent water volume swelling and significantly improves the wet dielectric strength of silicone rubbers incorporating calcium-treated precipitated silicas. Additionally, the data shows that extensive washing of the precipitated high structure silica to reduce the total salt content to low levels helps to reduce the percent water volume swelling to values below 4%. Example 6 Test A Precipitated silica filter cake produced in Test A of Example 4
20Kg was slurried again with distilled water containing 3.36g of barium oxide (hereinafter referred to as saturated barium oxide solution). The resulting slurry was stirred for 1 hour and allowed to stand overnight. The next morning, the slurry was stirred and 1/3 (approximately 25) of the slurry was filtered. The resulting filter cake was washed with saturated barium oxide solution 6 and dried at 140° C. in a rotary dryer. The dry product is 123m ² /g
It had a BET surface area. The 5% slurry of dry silica had a PH of 9.1. Example 1 of dry silica product
Analyzed for sulfate, chloride, sodium, calcium and barium ions as described.
The results are shown in the table. Test B The remaining 2/3 of the slurry from Test A in this example (approximately 50)
was allowed to settle for 6 hours. The supernatant liquid (approximately 18 ml) was removed and replaced with a saturated barium oxide aqueous solution 18 ml. The resulting mixture was stirred and 1/2 (approximately 25) was filtered. The filter cake was washed with saturated barium oxide solution 6 and dried in a rotary dryer at 140°C. The surface area of the dry product was 116 m ² /g. The pH of the 5% slurry of dried product was 9.5. The dried silica product was treated with sulfate, chloride, sodium,
Analyzed for calcium and barium ions. The results are shown in the table. Test C The remaining 25 of the slurry from Test B was allowed to settle over the weekend. The supernatant liquid (approximately 7) was removed and replaced with a saturated barium oxide aqueous solution (7). The resulting mixture was stirred and filtered. The filter cake was washed with an aqueous barium oxide solution 6 and the washed product was dried at 140° C. in a rotary dryer. The surface area of the dry product was 111 m ² /g.
A 5% slurry of dried product had a PH of 9.6.
The dried product was treated with sulfate, chloride,
Analyzed for sodium, calcium and barium ions. The results are shown in the table. Example 7 Silicone rubbers were prepared in the manner described in Example 1 using the dried silica products of Tests A, B and C of Example 6, respectively. The resulting silicone rubber was tested for water volume swelling and wet dielectric strength. The results are shown in the table.

TABLE The results of Examples 6 and 7 show that treatment of precipitated silica pigments with barium ions reduces the percent water volume swelling and improves dielectric strength of silicone rubber incorporated with barium-modified precipitated silica. Example 8 Approximately 40 Kg of precipitated high structure silica filter cake obtained by filtration of an aqueous slurry of precipitated silica prepared by neutralization of an aqueous solution of sodium silicate with carbon dioxide and subsequent acidification to a pH of 3.7 with sulfuric acid. Slurry again with distilled water and dilute to 150. The resulting slurry was stirred for 30 minutes and the slurry was heated to 70°C and allowed to settle overnight. The first supernatant was removed from the sedimentation volume (approximately 80) and distilled water was added to bring the total volume to 150. The resulting slurry was stirred, heated to 70°C and allowed to settle for 6 hours. The second supernatant was removed from the settling volume (approximately 80) and distilled water was added to bring the total volume to 150. The slurry was stirred, heated to 70°C and allowed to settle overnight. The third supernatant was removed leaving a settled slurry of about 80%, which had a solids content of about 7%. Test A The final slurry 10 was filtered and the filter cake was dried in a rotary dryer at 140°C. This dry product is 146
Surface area of m ² /g and PH of 7.6 (5% slurry)
It had It was analyzed for sulfate, chloride, sodium and calcium ions using the method described in Example 1. The results are shown in the table. Test B The final slurry 10 with a pH of 6.8 was
Diluted with (C ₂ H ₃ O ₂ ) _{2.H 2} O 110 g/300 ml of calcium acetate aqueous solution. The resulting slurry
Stirred for 30 minutes and had a pH of 6.8. The slurry was filtered and the filter cake was dried in a rotary dryer at 140°C. The dry product has a surface area of 139 m ² /g and
It had a PH of 7.3 (5% slurry). Example 1
It was analyzed for sulfate, chloride, sodium and calcium ions as described. The results are shown in the table. Example 9 Silicone rubbers were prepared in the manner described in Example 1 using each of the dried silica products of Tests A and B of Example 8. The resulting silicone rubber was tested for water volume swelling and wet dielectric strength. The results are shown in the table.

TABLE The results of Examples 8 and 9 demonstrate that calcium acetate can be used to treat well-washed precipitated silica. Although acetate ion appears to slightly increase the water volume swelling, its wet dielectric strength also increased. In the following example, low structure precipitated silica was prepared according to the procedure of US Pat. No. 3,960,586. Example 10 (Comparison) Sodium sulfate (4.2 Kg) was added to 100 containers containing 37.85 g of tap water to give a reaction medium containing 10% sodium sulfate. With a SiO ₂ /Na ₂ O weight ratio of 2.6 and a concentration of 2 lbs/gal, 7%
An aqueous sodium silicate solution containing sodium sulfate was added to the reaction medium until the pH was 9.0. The reaction medium was then heated to 65 °C (149 °C) and sodium silicate and 11.4% sulfuric acid were added at 756 °C, respectively.
ml/min and 453 ml/min into the reaction medium,
A constant precipitation pH of 9.0 was maintained. After 30 minutes, the sodium silicate addition was stopped. Excess sulfuric acid was added until the pH of the slurry reached 5.4. Slurry at 77℃
(171〓) and aged for 20 minutes. The resulting slurry 16 was filtered. The filter cake is about silica
Contained 1000g. The filter cake was reslurried with 12 ml of a saturated aqueous calcium oxide solution containing approximately 0.84 g of calcium. The reslurry suspension was stirred for 30 minutes and filtered. The filter cake was reslurried with saturated calcium oxide, stirred for 30 minutes, and filtered. This filter cake sample contained CaO, 1.58% and _Na2O ,
It was recognized that it contained 1.46%. Since the sodium level was higher than 1%, the filter cake was diluted with distilled water.
Re-slurry the mixture and stir for 30 minutes. Filter the slurry and place the filter cake in a forced draft oven at 105℃.
It was dried overnight. The dry processed product is 37m ² /g
BET surface area, PH of 9.9 (5% slurry) and analysis of: NaCl, <0.01 wt%; _{Na2SO4 ,} 0.10
Weight %: Na ₂ O, 1.25% by weight and CaO, 1.57% by weight. They were obtained by X-ray fluorescence spectroscopy. 60 parts of the dry silica product was added to methyl vinyl silicone gum (SWS Silicone Corp-C-155).
100 parts of plasticizer, 6 parts of plasticizer, 0.5 part of powdered peroxide [2,5-dimethyl-2,5-di(t-butylperoxy)hexane] with 45% active peroxide and mixed at 170°C for 10 Press vulcanize the mixture for 250℃ for min.
A silicone rubber was produced by post-vulcanization for 1 hour. It was apparent that the silica pigment did not adequately reinforce the silicone gum, as the silicone gum-silica mixture remained free-flowing and did not form a rubbery mixture. The resulting silicone rubber was tested for water volume swell, wet dielectric strength and certain rubber properties. The percent volume swelling obtained was 4.9%. The wet dielectric strength was 394 volts/mil. The rubber properties of the resulting silicone rubber were compared to silicone rubbers made using fumed silica, conventional high structure precipitated silica, and high structure extended washed precipitated silica. The results are shown in the table.

[Table] The data in the table shows that low structure silica produced by the method described in U.S. Patent No. 3,960,586
We show that silicone rubber produces silica that has very low hardness, low elongation resistance, and very low tensile strength and is therefore not useful as reinforcing silica.

Claims (1)

Claims: 1. A finely divided amorphous, precipitated high structure silica consisting essentially of at least 85% by weight SiO ₂ on an anhydrous basis, less than 2% by weight of a water-soluble alkali metal sulfate salt; less than 1% by weight of water-soluble alkali metal chloride salts and 0.05-5% by weight of alkali metal oxides, the total amount of the alkali metal salts and oxides in the silica is about 1-5% by weight; on the surface an oxide of a metal selected from the group consisting of calcium, barium and mixtures of oxides of such metals in at least an equimolar amount of said metal based on the total amount of alkali metal present in the silica. precipitating on the silica in sufficient amount to provide
Silica, wherein said amount of metal is such that the silicone rubber reinforced with said silica as a predominant silica filler exhibits a volumetric swelling in water of not more than 4%. 2 The molar equivalent ratio of metal and alkali metal is approximately 1:
1-2:1 silica according to claim 1. 3. The silica according to claim 1 or 2, wherein the alkali metal is sodium. 4. The silica contains less than 2% by weight of soluble sulfate salts, less than 0.5% by weight of soluble chloride salts, and less than 2% by weight of alkali metal oxides, and the total amount of said alkali metal salts and oxides in said silica is about 1 to 3% by weight, and the water volume swelling of silicone rubber is 2%.
The silica according to claim 3, wherein the silica is less than or equal to 5 The metal is calcium, chloride, sulfate,
_The ^{relationship between} _the ^{concentrations} _of ^sodium _and calcium ionic species _is as follows _: , C ₂ , C ₃ and C ₄ are each from 8 to 12
up to, 1 to 3, 9.5 to 14.5 and −
It is a numerical coefficient that varies from 5.5 to −8.5, and Cl ^- ,
SO ₄ ⁼ , Na ⁺ and Ca ⁺⁺ are the weight percentages of said ions in the silica expressed as sodium chloride, sodium sulfate, sodium oxide and calcium oxide). of silica. 6. The silica of claim 5, wherein the silica has a BET surface area of about 120-160 m ^<2> /g. 7. The silica of claim 5, wherein the amount of calcium present in the silica is about 0.5 to 3% by weight expressed as oxide. 8 Silicone elastomer gum that can be vulcanized with a free radical generator and 1.0 to 100 parts of silicone gum.
100 parts, on a substantially anhydrous basis, of at least 85% by weight SiO ₂ , less than 2% by weight of a water-soluble alkali metal sulfate salt, less than 1% by weight of a water-soluble alkali metal chloride salt, and from about 0.05% to an alkali metal oxide. 5% by weight, and the total amount of salts and oxides of said alkali metals in said silica is about 1-5% by weight, and on its surface consists of a mixture of calcium, barium and oxides of such metals. reinforcement, wherein an oxide of a metal selected from the group is precipitated on the silica in an amount sufficient to provide at least an equimolar amount of said metal based on the total amount of alkali metal present in the silica;
and an amorphous precipitated high structure silica, said amount of metal being such that the silicone rubber produced upon vulcanization of said silicone elastomer gum composition exhibits a water volume swelling of not more than 4%. A certain composition. 9. The silicone elastomer composition of claim 8, wherein the alkali metal is sodium and the ratio of molar equivalents to alkali metal is about 1:1 to 2:1. 10 The metal is calcium, and the relationship between the concentrations of chloride, sulfate, sodium, and calcium ionic species is as follows: 4-C ₁ [Cl ^- ]-C ₂ [SO ₄ ⁼ ]-C ₃ [Na ⁺ ] /C ₄ ≦ [Ca ⁺⁺ ] (In the formula, C ₁ , C ₂ , C ₃ and C ₄ are each from 8 to 12
up to, 1 to 3, 9.5 to 14.5 and −
It is a numerical coefficient that varies from 5.5 to −8.5, and Cl ^- ,
SO ₄ ⁼ , Na ⁺ and Ca ⁺⁺ are the weight percentages of said ions in the silica expressed as sodium chloride, sodium sulfate, sodium oxide and calcium oxide. silicone elastomer composition. 11. The silicone elastomer composition of claim 10, wherein the amount of calcium present in the silica is about 0.5 to 3% by weight expressed as oxide. 12. A silicone elastomer composition according to claim 8 or 10, wherein the amount of silica used is in the range of 20 to 70 parts of silica per 100 parts of silicone gum. 13. A silicone rubber composition comprising the vulcanized silicone elastomer gum according to claim 8. 14. A silicone rubber composition comprising a vulcanized silicone elastomer gum according to claim 10, further characterized in that it has a water volume swell of less than 2%. 15. The vulcanized silicone rubber composition of claim 14 further characterized in that it has a wet dielectric strength of at least 400 volts/mil. 16 In a method for producing amorphous precipitated high structure silica by acidifying an aqueous alkali metal silicate solution with an inorganic acid, the total amount of alkali metal in the precipitated silica is measured, and in response to the measurement, the precipitated silica is On the surface of the silica, a metal oxide of a metal selected from the group consisting of calcium, barium and mixtures of oxides of such metals is added, based on the total amount of alkali metals present in the silica. the metal is precipitated in an amount sufficient to provide a molar amount on said silica, and the amount of metal is such that the silicone rubber reinforced with said precipitated silica as a predominant silica filler exhibits a water volume swelling of not more than 4%. A method for producing a precipitated high structure silica useful in reinforced silicone rubber, the metal oxide being precipitated from an aqueous solution of an ionizable oxide, hydroxide or _C1 - _C6 carboxylate of said metal. 17. The method of claim 16, wherein the alkali metal is sodium and the molar equivalent ratio of metal to sodium is about 1:1 to 2:1. 18. The method of claim 17, wherein the metal oxide is precipitated onto the precipitated silica by slurrying the silica in an aqueous solution of an ionizable oxide or hydroxide of the metal. 19. The method of claim 17, wherein the metal oxide is precipitated onto the precipitated silica by washing the silica wet cake with an aqueous solution of an ionizable oxide or hydroxide of the metal. 20 In a method for producing amorphous precipitated high structure silica by acidifying an aqueous sodium silicate solution with an inorganic acid, the concentrations of chloride, sulfate, and sodium ionic species in the silica are measured, and In response, an oxide of calcium is deposited on the surface of the precipitated silica from an aqueous solution of an ionizable oxide or hydroxide of calcium according to the following relationship: 4-C ₁ [Cl ^- ]-C ₂ [SO ₄ ⁼ ]- C ₃ [Na ⁺ ]/C ₄ ≦ [Ca ⁺⁺ ] (where C ₁ , C ₂ , C ₃ and C ₄ are each from 8 to 12
is a numerical coefficient varying from 1 to 3, from 9.5 to 14.5, and from −5.5 to −8.5, and Cl ⁻ ,
SO ₄ ⁼ , Na ⁺ and Ca ⁺⁺ are the weight percentages of said ions in the silica expressed as sodium chloride, sodium sulfate, sodium oxide and calcium oxide). A method for producing precipitated high structure silica useful for reinforcing silicone rubber. 21 The amount of calcium present on the silica is approximately 0.5 to 3% by weight, calculated as calcium oxide.
21. The method according to claim 20. 22. The method of claim 20 or claim 21, wherein the calcium oxide is precipitated onto the precipitated silica by slurrying the silica in an aqueous solution of calcium oxide or hydroxide. 23. The method of claim 20 or claim 21, wherein the calcium oxide is precipitated onto the precipitated silica by washing the silica wet cake with an aqueous solution of calcium oxide or hydroxide.