DK144331B - PROCEDURE FOR THE PREPARATION OF A SPECIFIC AMORFT COUNTED SILICON Dioxide PIGMENT WITH VERY HIGH STRUCTURE - Google Patents

Description

(19) DENMARK

| ® (12) PUBLICATION (n> 144331 B

DIRECTORATE OF THE PATENT AND TRADEMARKET SYSTEM

(21) Application No. 5121/74 (51) | nt.CI.3 C 01 B 33/18 (22) Filing day 27 · sep. 1974 C 09 C 1/30 (24) Race day 27 · sep. 1974 (41) Aim. available Apr 4 1975 (44) Presented 22. f eb. 1 982 (86) International application no. - (86) International filing day - (85) Transfer day - (62) Master application no. -

(30) Priority 5 * Oct. 1975, 402927, US

(71) Applicant J.M. HUBER CORPORATION, Locust, US.

(72) Inventor of Satish Kumar Wason, US.

(74) Associate Company Chas. Hude.

(54) Process for producing a particularly high amorphous precipitated silicon® dioxide pigment with very high structure.

The present invention relates to a process for producing a particularly high amorphous precipitated silica pigment, a wet cake moisture exceeding 85%, an oil absorption greater than 200 cm / 100 g, a surface area greater than 250 m / g suitable for use as a thickening agent in dentifrices, providing an aqueous reaction medium containing from 10 to 20% by weight of an alkali metal silicate having a weight ratio - SiC₂ / l₂O, called X, from 2.4 to 3.3, where M is an alkali metal, after which the reaction medium is heated to from 71 to 77 ° C and an acidifier is added and the precipitated pigment is finally isolated, dried and comminuted.

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As is well known, commercially available silica can be broadly divided into two basic categories. These categories are those produced by a liquid phase process and those produced by a vapor phase process.

Liquid phase silicon dioxides include precipitated silica produced by acidifying an alkali metal silicate with an acid such as sulfuric acid. Other liquid phase silicas are silica gels and colloidal silica. Examples of the prior art involving acidification of a silicate solution to prepare precipitated silica are disclosed in U.S. Patents Nos. 0,110,606 and 3,582,379.

Precipitated silica has been used for many purposes, but various properties of them (such as low abrasion) make them unsuitable for certain purposes. For example, it is known that conventional synthetic precipitated silica is unsuitable as a polishing and abrasive agent in dentifrices. See U.S. Patent No. 3,250,680 and German Patent No. 974,958. Specifically, US Patent No. 3,558,230 discloses that known amorphous silica such as precipitated silica, pyrogenic silica and aerogels are unsuitable for toothpaste use due to their initially small particle size and the ease with which they decompose into smaller particles.

In order to further expand the usefulness of precipitated silica, new techniques have been developed in recent years for the production of silica pigments with new and special properties, for example, to make them particularly suitable for use in dentifrices and as a substitute for phosphate polishes.

Thus, U.S. Patent Application No. 285,966 discloses a process for preparing silicic acid pigments which can be used as abrasive or polishing agent in toothpaste, wherein a salt or electrolyte: is used to prepolymerize the alkali metal silicate before its acidification with an acid.

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U.S. Patent No. 3,110,606 discloses the precipitation of a silica pigment of a dilute sulfuric acid silicate solution. The patent discloses only acidification of calcium silicate and other Group II metal silicates, but not acidification of sodium silicate or other alkali metal silicates.

The present invention provides a further improvement in the preparation of precipitated silica pigments which involves acidification of an alkali metal silicate with an acid. The invention is based on the discovery that silicon dioxide pigments with improved properties can be produced by carefully controlling the rate of acidification. Specifically, it has been found that very high structure silica pigments (as defined below), high oil absorption and very high surface area can be produced by controlling the acidification and maintaining it at a very low rate during the acid addition to the solution of the alkali metal silicate. In this way, the precipitation is effected under homogeneous environments and so that the final particle size is controlled. The resultant product has the above-mentioned improved properties which make the product particularly suitable for use as a thickening agent in toothpastes, as a matting agent in paints and varnishes and the like.

The process according to the invention is characterized in that the acidifying agent is a dilute aqueous solution of from 8 to 25% by weight of a strong mineral acid which is added at a rate of 200 to 350 ml / minute, thereby obtaining from 1 / x to 2 / x mol of hydrogen ions. per. and that an adduct of from 3 to 10% by weight of a water-soluble aluminum salt is added to the acidifying agent prior to its introduction into the reaction medium.

Precipitated silica has not been used as a thickening agent or gelling agent in toothpaste. When phosphates are used as a polishing or abrasive agent, a thickening agent such as tragacanth has generally been used, cf. U.S. Patent No. 2,588,992.

4 144331 In recent years, the use of precipitated silica as polishing or abrasive agents has created the need for and the general use of silicon-containing materials as thickening agents. Specific examples of silicon-containing thickeners or gelling agents are described in U.S. Pat. Patent No. 3,538,230. The thickening agents described therein include "Cab-O-Sil" (pyrogenic silicon dioxide) and "Syloid 244" (aerogel).

As further described below, the products made according to the invention have regulated refractive index, a very high structure and can also be used as reinforcing agents, paint and lacquer matting agents and as flow regulating agents and the like.

The process of the invention is better understood by the following detailed description which describes particularly advantageous embodiments of the method and composition.

By. in the practical embodiment of the process according to the invention, a solution of an alkali metal silicate is first charged into a reaction vessel (equipped with suitable heating and stirring means), after which the silicate solution is heated to a temperature in the range of 71 to 77 ° C, preferably in the range of 71 to 74 ° C. Of course, the silicate solution can be prepared in the reaction vessel itself.

As used herein, the term "alkali metal silicate11 encompasses the common forms of alkali silicates, e.g., metasilicate, disilicates and the like. Water-soluble potassium silicates and sodium silicates are particularly advantageous and the latter preferred.

After the solution has been heated to the above temperature, the acidifier or acid is added at a very slow rate in a range of 200 to 350 ml / minute. In conventional methods involving acidification of a silicate, the addition rates have generally been very high and of the order of 500 ml / minute.

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According to the present invention, the rate of addition must not exceed (at any stage of the acidification) a rate of 350 ml / min.

The acid is preferably a strong mineral acid such as sulfuric acid, nitric acid and hydrochloric acid. Other acids, including organic acids or salts thereof (e.g. acetic acid, formic acid, carbonic acid, ammonium carbonate, etc.) may be used.

As will be seen from the above, the starting materials include the alkali metal silicate and the acid. Significant process variables include the SiO2 / Na2O ratio, the reaction temperature and the rate of acid addition to the solution of the alkali silicate.

It has also been found that the refractive index of the product (as well as the other aforementioned properties) can be carefully controlled and maintained if an adduct material such as aluminum sulfate is premixed with the acid prior to its introduction into the silicate solution. Therefore, prior to acidification, the aluminum sulfate (preferably as a solution thereof) is premixed with the acid such that its concentration is on the order of 3 - 10% aluminum sulfate based on the total weight of the acid. The adduct is a water-soluble aluminum salt such as aluminum sulfate, but other salts such as magnesium sulfate can be used. As mentioned, the adduct is mixed with the acid and the mixture of acid and metal salt is then used to acidify the aqueous solution of the alkali metal silicate.

As mentioned above, the products made according to the invention have a very high structure. Thus, it may be mentioned at this location that the term structure used in the present specification should include and is defined as the ability of a silica material to hold water as a wet cake, i.e., check that the precipitate has been filtered properly. Commercially available precipitated silica contains a high percentage of water (i.e., on the order of 75-85%) and is referred to as high structure silica. Materials that hold water in the range of 65 to 75% are commonly referred to as medium structure silica. Silicon dioxides having a moisture content of the wet cake exceeding 85% and up to 95% and more are referred to as very high structure silica.

The present invention relates to the production of very high structure silicon dioxides made under carefully controlled acidification conditions.

As previously mentioned, it is well known that precipitated silica is prepared by acidifying an aqueous alkali metal silicate with a suitable acidifying agent until a final pH of about 5 to 6 is obtained. The silica precipitate is then filtered, washed free of salts, dried and dried. ground to the desired degree of fineness.

The invention provides an improved process for producing silicon dioxide as very high structure thickeners, a process which regulates the relative sizes and uniformity of the primary particles and provides unique silicon dioxides which have improved functionality in toothpastes and in other applications.

Commercially available sodium silicate solutions may be represented by the formula Na2Q, xSO2, where x is the SiO2 / Na2O weight ratio. Since the molecular weights of Na 2 O and SiO 2 are substantially the same, it is common practice to use weight ratios for sodium silicates. Water glass is the most commonly used silicate for the production of silicic epigments. This product has a weight ratio of 3> 22, that is, it has 3.22 parts SiO 2 and 1 part alkali, i.e. Na20.

In the prior art it has been common to acidify a sodium silicate in which the value of x was in the range of approx. 3> 0 to 3.4. By this prior art, the acid was simply introduced into the sodium silicate solution until the desired final pH was reached, after which the product was recovered using conventional methods.

In the present process, the acidification is very carefully controlled, especially at the beginning of the reaction and at the point where the precipitation first occurs, i.e. where the value of x is approx. 4.2.

7 U4331 x changes during acidification as sodium oxide leaves the sodium silicate so that the ratio of silica to sodium oxide grows to a point of approx. 4.2.

The invention is further illustrated by the following examples which show particularly advantageous embodiments of the method. The examples serve to illustrate the invention, but not to limit it.

Example 1

In this experiment, 38 liters of sodium silicate solution having a SiO 2 / Na 2 O weight ratio of 2.6 was added to a stirred reaction vessel and the solution was heated to 74 ° C. Concentrational of the silicate solution was 13.3%. Sulfuric acid with a concentration of 11.4% and aluminum sulphate solution (0.17 kg / liter) was premixed in a ratio of 100 to 7 by volume. This mixed acidifying agent was then added very slowly to the reactor at a rate of 300 ml / minute until a final pH of 5.9 was obtained. The product in this example had a very high structure, a moisture content of the wet cake of 88%, an oil absorption of 240 cm 2/100 g and a surface area of 250 m 2 / g.

Example 2

The procedure of Example 1 was repeated except that the rate of addition of the premixed acidifying agent was 250 ml / minute. The product of this example had a very high structure, a wet cake moisture of 87%, an oil absorption of 230 cm 2 / g

ISLAND

and a surface area of 275 m / g.

Example 3

The procedure of Example 1 was repeated except that the rate of addition of the acidifying agent was 275 ml / minute. The product in this example had a very high structure, a wet cake moisture of 86%, an oil absorption of 220 cm 2/100 g, and a surface area of 265 m 2 / g.

Example 4

The procedure of Example 1 was repeated except that. the rate of addition of the acidifying agent was maintained at 225 ml / minute up to the initiation of the precipitate at a x value of 4.2. The product in this example had a very high structure, a wet cake moisture of 89%, an oil absorption of 250 cm 2/100 g and a p surface area of 280 m / g.

Example 5

As a control example, 38 liters of a sodium silicate solution having a SiO 2 / Na 2 O ratio of 2.6 was added to a stirred reaction vessel and the solution was heated to 74 ° C. The concentration of the silicate = solution was 13.3%. Sulfuric acid with a concentration of 11.4% was added to the reactor at a rate of 500 ml / minute until a final pH of 5.9 was reached. The product in this control example had only high structure and a wet cake moisture of 84%, one

~ Z O

oil absorption of 180 cnr / 100 g and a surface area of 150 mosquitoes.

As the above examples illustrate, the production of very high structure silica pigments is possible by carefully controlled acidification to approx. the point at which initial precipitation occurs at an x value of 4.2, in the case of sodium silicate having a weight ratio (x value) SiO 2 / Na 2 O of 2.6 and a concentration of 13.3%. By regulating the acidification to and around an x value of 4.2, as long as practicable, it is possible to regulate the final silica particles and precipitate them under homogeneous environments with controlled micelle formation during the precipitation process. This controlled process then provides a silica with improved properties and functionality compared to silica produced by the known methods.

When the results illustrated by the above examples are compared with related findings, it is possible to note that the molar ratio and the weight ratio of SiC₂ / l₂O where M is an alkali metal such as sodium, potassium and the like are inversely proportional with the number of moles of hydrogen ions supplied by the acidifying agent per time unit, to provide the controlled micelle formation during the precipitation of silica by the process of the invention.

For the above examples, where a sodium silicate having a weight ratio (x value) of SiC> 2 / Na 2 O of 2.6 is used, the preferred acidification rate is from 200 to 350 ml / minute or from approx. 0.4 mole to 0.8 mole of hydrogen ions per minute.

In general, the desired rate of acidification of an alkali = metal silicate having a molar ratio (or weight ratio) x = SiOg / l 2 O, where M is an alkali metal, from [H +] / x to 2 [H +] / x, where [ H +] is the molar concentration of hydrogen ions to be added per minute.

The silicon dicksides of the invention were prepared in 2 portions of 675 kg to confirm the data and efficiency of the product of Examples 1 to 4. These experiments resulted in materials having the following properties: "2

Experiment and product A: Oil absorption 237 cnr / 100 g

ISLAND

Surface area 308 m / g

Experiment and product B: Oil absorption 222 cm 2/100 g

Surface area 259 m 2 / g.

The surface area of product A was higher than that of product B because it was finished at a lower final pH than product B.

The thickening effect of products A and B was compared in typical moisture binding systems used in dentifrices.

Two moisture-binding systems were used. One designated "C *" comprised a 70% sorbit solution marketed as *, SORB0 "by Atlas Chemical, Inc., Wilmington, Delaware, and another designated" D "comprising a 45 part mixture of a 70% sorbit solution and 15 parts of a 95% glycerine solution.

Products A and B were dispersed by stirring in a known amount of weight of each of the moisture-binding agents C and D until a free-flowing powder was obtained. The silica of Example 5 was also dispersed in the moisture binding agents C and D for comparison.

The control silicon dioxide retained a maximum of 280% of C and 270% of D. Product A retained a maximum of 300% C and 296% D. Product B retained a maximum of 294% C and 296% D.

As will be seen, the silica nitrides produced in accordance with the invention have better thickening and drying properties for typical moisture-binding agents than previously known silica.

Thus, the silicon dioxides prepared according to the invention with very high structure can be used as gelling agents and thickening agents in many different toothpastes and dentifrices.

Silicon dioxides made in accordance with the invention can also be used as agents, nutrients, matting agents in paints and varnishes and as a carrier, a flow regulating agent and as a liquid desiccant and the like.

For matting purposes, 10 g of silica (which was air-milled) according to the invention were mixed with 350 g of nitrocellulose varnish (corresponding to military specification MIL-L-10287A - Addition 2, Type II, issued August 27, 1959) and mixed for 3 minutes using a Hamilton-Beach # 30 low speed mixer. The lacquer containing dispersed silica was examined for Hegman grinding fineness (5.50) (ASIM sample D 1316 part 27 (1975)) and cleanliness of grinding.

The lacquer containing dispersed silica was mixed with additional lacquer to prepare stock solutions containing 10%, 3.5% and 1.75% solids, respectively. A coating of various stock solutions (containing 10%, 3.5% and 1.75% silica in the lacquer) was made on Carrara glass using X = 144331 of a # = 34 wirewound applicator bar. Carrara glass coatings were allowed to dry for 45 minutes under dust-free conditions. Using the above method, coatings were also made with stock solutions containing silica prepared by known methods.

Using the Gardner multi-angle gloss gauge, the gloss and gloss values of the different areas were measured at 60 ° and 85 °, respectively. These values were compared with measured values obtained when previously known silica was dispersed in the lacquer.

Silicon dioxides made according to the invention result in cleaner Hegman mills and exhibit better clarity when dispersed in the varnish. The better clarity is attributed to the fact that the silicon dioxides of the invention have uniform particle size and favorable structure.

The matting data given below suggests that the novel silica according to the invention exhibits lower gloss and gloss values than the control. Lower gloss and gloss values are preferred and are advantageous for matting paint.

Matter data _60 ° gloss 85 ° gloss 1096 3.5% 1.75% 10% 3.5% 1.75%

Silica according to Example 8 29 44 28 61 74

Silica according to ex.5 10 38 54 37 72 82 (control)