CA1062439A - Process for producing precipitated thickner silica - Google Patents
Process for producing precipitated thickner silicaInfo
- Publication number
- CA1062439A CA1062439A CA208,941A CA208941A CA1062439A CA 1062439 A CA1062439 A CA 1062439A CA 208941 A CA208941 A CA 208941A CA 1062439 A CA1062439 A CA 1062439A
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- CA
- Canada
- Prior art keywords
- silica
- alkali metal
- silicate
- acid
- weight
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09C—TREATMENT OF INORGANIC MATERIALS, OTHER THAN FIBROUS FILLERS, TO ENHANCE THEIR PIGMENTING OR FILLING PROPERTIES ; PREPARATION OF CARBON BLACK ; PREPARATION OF INORGANIC MATERIALS WHICH ARE NO SINGLE CHEMICAL COMPOUNDS AND WHICH ARE MAINLY USED AS PIGMENTS OR FILLERS
- C09C1/00—Treatment of specific inorganic materials other than fibrous fillers; Preparation of carbon black
- C09C1/28—Compounds of silicon
- C09C1/30—Silicic acid
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
- A61K8/00—Cosmetics or similar toiletry preparations
- A61K8/18—Cosmetics or similar toiletry preparations characterised by the composition
- A61K8/19—Cosmetics or similar toiletry preparations characterised by the composition containing inorganic ingredients
- A61K8/25—Silicon; Compounds thereof
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61Q—SPECIFIC USE OF COSMETICS OR SIMILAR TOILETRY PREPARATIONS
- A61Q11/00—Preparations for care of the teeth, of the oral cavity or of dentures; Dentifrices, e.g. toothpastes; Mouth rinses
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B33/00—Silicon; Compounds thereof
- C01B33/113—Silicon oxides; Hydrates thereof
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2002/00—Crystal-structural characteristics
- C01P2002/02—Amorphous compounds
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/12—Surface area
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/19—Oil-absorption capacity, e.g. DBP values
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/80—Compositional purity
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/80—Compositional purity
- C01P2006/82—Compositional purity water content
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- Health & Medical Sciences (AREA)
- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Life Sciences & Earth Sciences (AREA)
- Public Health (AREA)
- Animal Behavior & Ethology (AREA)
- General Health & Medical Sciences (AREA)
- Inorganic Chemistry (AREA)
- Veterinary Medicine (AREA)
- Birds (AREA)
- Epidemiology (AREA)
- Oral & Maxillofacial Surgery (AREA)
- Silicon Compounds (AREA)
- Cosmetics (AREA)
- Pigments, Carbon Blacks, Or Wood Stains (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
Precipitated silicas having a very high structure as defined by the ability of a silica material to hold water in its wet cake, high absorption, and very high surface area can be produced by controlling the rate of acidulation during their production between the levels of 1/x and 2/x hydrogen mols/
minute, where x is the weight ratio of sodium oxide of silica to the alkali metal oxide, and maintaining this same low rate during the entire course of the addition of acid to the solution of alkali metal silicate at temperatures of between 150 and 170°F.
The control of the index of refraction of the silica so produced is achieved by introduction of controlled amounts of adduct material such aluminum sulfate to the acid prior to its intro-duction to the silicate solution.
Precipitated silicas having a very high structure as defined by the ability of a silica material to hold water in its wet cake, high absorption, and very high surface area can be produced by controlling the rate of acidulation during their production between the levels of 1/x and 2/x hydrogen mols/
minute, where x is the weight ratio of sodium oxide of silica to the alkali metal oxide, and maintaining this same low rate during the entire course of the addition of acid to the solution of alkali metal silicate at temperatures of between 150 and 170°F.
The control of the index of refraction of the silica so produced is achieved by introduction of controlled amounts of adduct material such aluminum sulfate to the acid prior to its intro-duction to the silicate solution.
Description
~06;~:439 ~ 57153 The present invention relates to the novel silicic acid or silica pigments and, more particularly, to a method for producing synthetic precipitated silicas having new and unique properties.
As known in the art, commercially available silicas can broadly be divided into two basic categories. These categories are those produced from a liquid phase and those from a vapor phase process.
Liquid phase silicas include precipitated silicas produced by acidulating an alkali metal silicate with an acid such as sulfuric acid. Further liquid phase silicas are silica gels and colloidal silicas.
~-~ 10 Examples of prior techniques involving the acidulation of a silicate solution to produce precipitated silicas are disclosed in U. S. Patents 3,110,606 and 3,582,379.
Vapor process silicas, also called~fumed and pyrogenic silicas, are prepared by reacting silicon tetrachloride vapor with an oxygen-hydrogen gas at high temperatures to produce silica and hydrogen chloride. Pyrogenic silicas have high external surface areas and differ from other silicas (e.g., gels, precipitated silicas, etc,) prepared from the liquid phase process.
While precipitated silicas have been used in many applica-t1Ons, various properties thereof (such as low abrasiveness) render themunsuitable for many uses. For example, it is known that conventiional synthetic precipitated silicas are unsuitàble as polishing and abrasive agents in toothpaste compositions. See U. S. Patent 3,250,680 and German Patent 974,958. U. S. 3,538,230 specifically discloses that known amorphous silicas, such as precipitated silicas, pyrogenic silicas and aerogels are unsuitable for dentifrice use because of their initial small particle size and because of the ease in which they break down into smaller particle sizes.
~,~
~L~6243~
In recent years and to further expand the utility of precipitated silicasJ prior art workers have developed new tech-niques for producing silica pigments having new and uni~ue prop-erties; e.g., render them particularly suitable for use in tooth-paste compositions and as a replacement for phosphate polishing -; ,.
agents. - --Thus, there is disclosed in U.S. Patent No. 3,977,893 a process for producing siliceous pigments which may be used as an abrasive or polishing agent in toothpaste wherein a salt or 10 electrolyte is employed to pre-polymerize the alkali metal --~
silicate prior to its acidulation with an acid.
In accordance with a further innovation, there is dis-closed in U.S. Patent No. 3,928,541, a process for producing pre-cipitated silica pigments that have high abrasiveness and a relative cleaning ability e~uivalent to high grade phosphates used as polishing agents in toothpastes. In accordance with teaching of this patent, the pigments are produced by the simultaneous introduction of a solution of an alkali metal silicate and an acid into an aqueous receiving medium (i.e., water) and maintaining the pH of the aqueous receiving medium constant during the pre-cipitation thereof. ~ ~
In summary, the present invention provides a further improvement in the production of precipitated siliceous pigments involving the acidulation o an alkali metal silicate with an acid.
In its broadest aspects, the invention is based on the discovery that silica pigments having improved properties and character- ~ ;
istics can be produced by carefully controlling the rate of the acidulation. More specifically, it has been found that silica pigments having very high structure (as defined hereinafter), high oil absorption and a very high surface area can be produced by controlling the acidulation and maintaining same at a very low rate during the course of its (the acid) addition to the solution of the ~, `'; ;' '" .
6~439 alkali metal silicate, In this manner, the precipitation is effected under a homo~eneous environment and such that the ultimate particle size is controlled. The resulting product has the above noted improved characteristics which render the product particularly suitable for use as a thickening agent in toothpaste formulations, as a flattening agent in paints and varnishes and the like.
In this regard and before turning to the more precise details of the invention, precipitated silicas have not been used as thickening and gelling agents in toothpaste. In general when phosphates are employed as the polishing or abrasi~e agent, a thickening agent, such as gum tragacanth, has been employed. See U.S. Patent 2,588,992.
In more recent years, the use of precipitated silicas ~such as disclosed in the aforementioned patent applications) as polishing or abrasive agents has created the need for, and the general use of, siliceous materials as thickeners. Specific examples of siliceous thickeners or gelling agents are disclosed `
in U.S. Patent 3,538j230. The thickeners disclosed in the latter include "Cab-O-Sil: ~a trademark for a pyrogenic silica) and "Syloid 244" ~a trademark for an aerogel).
Thus, an object of the present invention is to provide a process for producing a finely divided precipitated silica that may be employed as a thickening and/or gelling agent for denti-frices.
The products of the invention, as will be described in detail hereinafter, have a controlled refractive index, a very high structure and may also be used as reinforcing agents, flattenin~ agents in paints and varnishes, as flow conditioner and the like.
The manner in which the foregoing and other objects are ~-achieved in accordance with the present invention will be better understood in view of the following detailed description which .:
discloses particularly advantageous method and composition ~ ::
embodiments for illustrative purposes.
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"` ~062439 In this regard, in the practice of the process of the invention, a solution of an alkali metal silicate is first charged to a reaction vessel (equipped with suitable heating and agitation equipment) with the silicate solution then being heated to a temperature in the range of from 150 to 170F, preferably in the range of from about 160 to 165F. The silicate solution may, of course, be prepared in the reaction vessel itself.
As used herein, the term "alkali metal silicate" includes all the common forms of alkali silicates, as for example ~etasilicate, dysilicates and the like. Water-soluble potassium silicates and sodium silicates are particularly advantageous, with the latter being preferred.
In general, sodium silicates are effective in any composition in which the weight ratio of SiO2 to Na20 is from about 1.0 to 4Ø However, particularly advantageous and thus preferred results are obtained if the SiO2/Na20 weight ratio is in the range of from about 2.5 to 3.3.
After the solution has been heated to the above-noted tempera-ture, the acidulating agent or acid is added at a very slow rate and in the range of from 200 to 350 ml/min. In conventional processes involving the - acidulation of a silicate, the rates of addition have in general been very high and on the order of 500 ml/min. In the present invention the rate of addition must not exceed ~at any stage of acidulation) a rate i-n excess o~
or above about 350 ml/min.
The acid is preferably a strong mineral acid, such as sulfuric acid, nitric acid, and hydrochloric ac~d. However, other acids including organic acids or salts thereof (for example, acetic acid, forlnic acid, carbonic acid, ammonium carbonate, etc.) can be employed.
The acid is preferably added as a dilute solution thereof with preferred results being obtained if the acidic solution comprises from about 8 to 25% by weight acid, based on the total weight of the solution.
1062~39 Particularly advantageous results are obtained if the silicate concentration of the initial silicate solution is on the order of from about lO to 20% by weight, based on a total weight of the silicate solution.
As will be seen from the ahove~ the starting materials include the alkali metal silicate and the acid. Significant process variables include the SiO2/Na20 weight ratio, the reaction temperature and the rate of the acid addition to the solution of the alkali silicate.
In this regard, 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. Thus, prior to the acidulation, the aluminum sulfate (preferably as a solution thereof) is premixed with the acid such that its concentration is on the order of about 3 to 10% aluminum sulfate based on total weight of the acid. While the adduct material is preferably a water-soluble aluminum salt, such as aluminum sulfate, other salts such as magnesium sulfate may be employed. As indicated, the adduct is premixed with the acid and the acid/metal salt mixture is then used for acidulating the aqueous solution of the alkali metal si1icate. -As indicated above, the products of the invention have a very high structure. Thus at this point, it may be noted that as used herein the term "structure" is intended to include, and is defined as, the ability of a silica material to hold water in its wet cake; i.e., after the precipitate has been filtered. Commercially available precipitated silicas hold a high percentage of water (i.e., in the neighborhood of 75 to 85%) and are known and referred to as high structure silicas.
Materials holding water in a range of 65 to 75% are generally referred to as medium structure silicas. Silicas having wet cake moistures in excess of 85% and up to 95% and higher are referred to as very high structure silicas.
106243~
The present invention is d;rected to the production of very high structure, thickener silicas produced under carefully controlled acidulation conditlons.
As previously noted, lt is well known in the art that precipitated silicas are prepared by acidulating an aqueous alkali metal silicate with a suitable acidulating agent until a final pH of from about 5.6 to 6.0 is obtained. The silica precipitate is then filtered, washed free of salts, dried, and milled to desired degree of fineness.
The objects of the present invent;on are to provide an improved process for thè preparation of very high structure, thickener silicas; to provide a process for controlling the relative sizes and uniformity of the primary particles; and to provide unique silicas having improved functionality in toothpaste compositions and other use areas.
In this regard and turning now to further details of the invention, commercially available sodium silicate solutlons may be B represented by the for~ula Na20-x ~ ~ wherein x stands for the weight ratios of SiO2/Na20. Since the molecular weights of Na20 and SiO2 are substantially the same, it is common practice to use weight ratios for sodium silicates. Water glass is the most commonly used silicate for preparing siliclc acid pigments. This product has a weight ratio of 3.22 which means that it has 3.22 parts of SiO2 and one part alkali;
i.e., Na20.
In the prior art, it has been customary to acidulate a sodium sillcate wherein the value of x was in the range of from about 3.0 to 3.4. In this prior practice, the acid was merely introduced in the sodium silicates solution until the desired final pH was obtalned, with the product then being recovered using the conventional procedures.
~L062~39 In the present invention, the acidulation is very carefully controlled, particularly at the outset of the reaction and at that point at which the precipitation initially occursi i.e., wherein the value of x is approximately 4.2.
The invention will be further illustrated by the following examples which set forth particularly advantageous method embodiments.
While the examples serve to illustrate the present invention, they are not intended to limit it thereto.
_ .
In this experiment 10 gallons of a sodium silicate solution having an SiO2/Na20 weight ratio of 2.6 was added to a stirred reaction vessel and the solution was heated to 165F. The concentration of the silicate solution was 13.3X. Sulfuric acid of 11.4% concentration and aluminum sulfate solution (1.4 lbs/gal.) were premixed in the ratio of lOO to 7 by volume. This mixed acidulating agent was then added very slowly to the reactor at a rate of 300 ml/min. until a final pH of 5.9 was obtained. The product of this example had very high structure, a wet cake moisture of 88%, an oil absorption of 240 cc/100 g and a surface area of 250 m2/g.
The procedure of Example 1 was repeated except that the rate of addition of the premixed acidulating agent was 250 ml/m~n.
The product of this example had very high structure, a wet cake mois-ture of 87%, an oil absorption of 230 cc/100 g and a surface area of 275 m2/g.
-The procedure of Example 1 was repeated except that the rate of addition of ~he acidulating agent was 275 ml/min. The product of this example had very high structure, a wet cake moisture of 86%, an oil absorption of 220 cc/100 9 and a surface area of 265 m2/g.
The procedure of Example 1 was repeated except that the rate of acidulating agent was maintained at 225 ml/min. up to the initiation of precipitation at an x value of 4.2. The product of this example had very high structure, a wet cake moisture of 89X, an oil absorption of 250 cc/100 9 and a surface area of 280 m2/g.
As a control example, lO gallons of a sodium silicate solution having an SiO2/Na20 weight ratio of 2.6 was added to a stirred reaction vessel and the solution was heated to.l65~F. The concentra-tion of the silicate solution was 13.3~ Sulfuric acid of 11.4% concen-tration was added to the reactor at a rate of 500 ml/min. until a final pH of 5.9 was obtained. The product of this controi;example was only of high structure with a wet eake moisture of 84%, an oil absorption of 180 cc/100 g and a surface area of 150 m2/g.
As the above examples illustrate,the production of silica pigments of very high structure ~s made possible by a carefully controlled acidulation to and about the point of lnitial precipitation at an x value of 4.2 in the case of sodium silicate of weight ratio (x value) SiO2/Na20 of 2.6 and 13.3~ concentration. By controlling the acidulation to and about the x value of 4.2 for as long a time as practically possible, it is possible to control the ultimate silica particles and precipitate them under homogeneous environments with controlled micelle formation during the precipitation process. This controlled process then yields a silica of improved characteristics and functionality when compared with silica produced by prior art processes.
1~62~39 When the findings illustrated by the above examples are correlated with related findlngs, it is found possible to note that mol ratio and weight ratio of SiO2/M20, where M is an alkali metal such as sodium, potassium and the like, is inversely proportional to the number of mols of hydrogen ions provided by the acidulating agent per unit time to provide the controlled micelle formation dwring the precipitation of the silica required in a process in accord with the present invention.
For the above examples employing a sodium silicate of weight ratio (x value~ SiO2/Na20 of 2.6, the preferred rate of acidula-tion is from 200 to 350 ml/min. or from approximately 0.4 ~ol to 0.8 mol of hydrogen ions per minute.
In general, the des~red rate of acldulation of an alkali metal silicate of mol (or weight) ratio x = SiO2/M20, where M is an alkali metal, is from [H+]/x to 2tH+]/x, where ~H+~ is the mol concen-tration of hydrogen ions to be added per minute.
The silicas of the present invention was produced in two 1500 pound lots to confirm the datà and effectiveness of the product of Examples 1 - 4. These runs resulted in materials with the following properties:
Run and Product A: Oil Absorption: 237 c~/100 9 Surface Area: 308 m /9 Run and Product B: Oil Absorption: 222 cc/100 9 Surface Area: 259 m2/g The surface area of Product A was higher than that of Product B because it was finished at a lower final pH than was Product B.
The thickening efficiency of Products A and B was compared in typical humectant systems used in dentifrices.
~06Z439 Two humectant systems were used. One designated "C"
comprising a 70X sorbitol solution sold as SORBO by Atlas Chemical, Inc.
~ilmington, Delaware, and another designated "D" comprising a blend of 45 parts of a 70% sorbitol solution and 15 parts of a 95% glycerol solution~
Products A and B were dispersed by stirring in a known weight of each of humectants "C" and "D" until a free flowing powder was obtained. Silica of Example 5 was also dispersed in humectants "C" and "D" for comparison.
The control silica retained a maximum of 280% of "C" and 270% of "D." Product A retained a maximum of 300% of "C" and 296% of "D." Product B retained a maximum of 294% of "C" and 296% of "D."
As can be seen, the siliras of the present invention have superior thickening or drying-up capacity for typical humectants to the prior art silica.
Thus, the very high structure silicas of the present invention can be used as gelling and th;ckening agents in a variety - of dent;frice and toothpaste compositions.
Silicas of the present invention can also be used as reinforcing agents, flatting agents in paint and varnished, as a carrier, as flow conditioners, as a liquid drying agent and the like.
For flatting application, 10 grams of silica ~which was airmilled) of the instant invention was mixed with 350 grams of the nitrocellulose lacquer (conforming to Military Specification MIL-L-10287A - amendment 2, Type II, of issue 27 August 1959) and mixed for 3 minutes using the low speed setting of the Hamilton-Beach #30 mixmaster. The lacquer containing dispersed silica was tested for Hegman fineness of grind (5.50) and cleanliness of grind.
~L~62439 ..,,.;.
The lacquer containing dispersed silica was mixed with no lacquer and additional lacquer to prepare stock solution containing 10%, 3.5~, and 1.75% by weight of vehicle solids. A draw down of various stock solutions (containing 10%, 3.5% and 1.75% silica in lacquer) was made on carrara glass using a #34 wire wound coatings application rod.
Carrara glass draw downs were allowed to dry for 45 minutes under dust-free conditions. Using the above method, draw downs were also made from stock solutions containing the silica developed via the prior art processes.
Using the Gardner multi-angle gloss meter, the gloss and sheen values of the various draw downs were measured at 60 and 85, respectively. These values were compared with measured values obtained when a prior art silica was dispersed in the lacquer.
Silicas of the present invention result in cleaner Hegman grinds and exhibit better clarit~ when dispersed in the lacquer. The better clarity is attributed to the fact that the silicas of the present invention are of unifonm particle sizes and favorable structures.
Flatting data below suggests that the novel silicas of the present invention exhibit lower gloss and sheen values than the control.
Lower gloss and sheen values are preferred and advantageous for paint flatting application.
FLATTING DATA
60 Gloss 85 Sheen 10% 3.5% 1.75%10% 3.5% 1.75%
Silica via Exa~ple 1 8 29 44 28 61 74 Silica via Example 5 10 38 54 37 72 82 (control)~
Although preferred embodiments of the present invention are described in detail above, the illustrative description is not intended 1~6243~
to limit or restrict the invention disclosed to those skilled in the art and the invention is thus declared to cover all changes and modifications of the above disclosure which do not constitute departures from the spirit and scope of the claimed invention.
As known in the art, commercially available silicas can broadly be divided into two basic categories. These categories are those produced from a liquid phase and those from a vapor phase process.
Liquid phase silicas include precipitated silicas produced by acidulating an alkali metal silicate with an acid such as sulfuric acid. Further liquid phase silicas are silica gels and colloidal silicas.
~-~ 10 Examples of prior techniques involving the acidulation of a silicate solution to produce precipitated silicas are disclosed in U. S. Patents 3,110,606 and 3,582,379.
Vapor process silicas, also called~fumed and pyrogenic silicas, are prepared by reacting silicon tetrachloride vapor with an oxygen-hydrogen gas at high temperatures to produce silica and hydrogen chloride. Pyrogenic silicas have high external surface areas and differ from other silicas (e.g., gels, precipitated silicas, etc,) prepared from the liquid phase process.
While precipitated silicas have been used in many applica-t1Ons, various properties thereof (such as low abrasiveness) render themunsuitable for many uses. For example, it is known that conventiional synthetic precipitated silicas are unsuitàble as polishing and abrasive agents in toothpaste compositions. See U. S. Patent 3,250,680 and German Patent 974,958. U. S. 3,538,230 specifically discloses that known amorphous silicas, such as precipitated silicas, pyrogenic silicas and aerogels are unsuitable for dentifrice use because of their initial small particle size and because of the ease in which they break down into smaller particle sizes.
~,~
~L~6243~
In recent years and to further expand the utility of precipitated silicasJ prior art workers have developed new tech-niques for producing silica pigments having new and uni~ue prop-erties; e.g., render them particularly suitable for use in tooth-paste compositions and as a replacement for phosphate polishing -; ,.
agents. - --Thus, there is disclosed in U.S. Patent No. 3,977,893 a process for producing siliceous pigments which may be used as an abrasive or polishing agent in toothpaste wherein a salt or 10 electrolyte is employed to pre-polymerize the alkali metal --~
silicate prior to its acidulation with an acid.
In accordance with a further innovation, there is dis-closed in U.S. Patent No. 3,928,541, a process for producing pre-cipitated silica pigments that have high abrasiveness and a relative cleaning ability e~uivalent to high grade phosphates used as polishing agents in toothpastes. In accordance with teaching of this patent, the pigments are produced by the simultaneous introduction of a solution of an alkali metal silicate and an acid into an aqueous receiving medium (i.e., water) and maintaining the pH of the aqueous receiving medium constant during the pre-cipitation thereof. ~ ~
In summary, the present invention provides a further improvement in the production of precipitated siliceous pigments involving the acidulation o an alkali metal silicate with an acid.
In its broadest aspects, the invention is based on the discovery that silica pigments having improved properties and character- ~ ;
istics can be produced by carefully controlling the rate of the acidulation. More specifically, it has been found that silica pigments having very high structure (as defined hereinafter), high oil absorption and a very high surface area can be produced by controlling the acidulation and maintaining same at a very low rate during the course of its (the acid) addition to the solution of the ~, `'; ;' '" .
6~439 alkali metal silicate, In this manner, the precipitation is effected under a homo~eneous environment and such that the ultimate particle size is controlled. The resulting product has the above noted improved characteristics which render the product particularly suitable for use as a thickening agent in toothpaste formulations, as a flattening agent in paints and varnishes and the like.
In this regard and before turning to the more precise details of the invention, precipitated silicas have not been used as thickening and gelling agents in toothpaste. In general when phosphates are employed as the polishing or abrasi~e agent, a thickening agent, such as gum tragacanth, has been employed. See U.S. Patent 2,588,992.
In more recent years, the use of precipitated silicas ~such as disclosed in the aforementioned patent applications) as polishing or abrasive agents has created the need for, and the general use of, siliceous materials as thickeners. Specific examples of siliceous thickeners or gelling agents are disclosed `
in U.S. Patent 3,538j230. The thickeners disclosed in the latter include "Cab-O-Sil: ~a trademark for a pyrogenic silica) and "Syloid 244" ~a trademark for an aerogel).
Thus, an object of the present invention is to provide a process for producing a finely divided precipitated silica that may be employed as a thickening and/or gelling agent for denti-frices.
The products of the invention, as will be described in detail hereinafter, have a controlled refractive index, a very high structure and may also be used as reinforcing agents, flattenin~ agents in paints and varnishes, as flow conditioner and the like.
The manner in which the foregoing and other objects are ~-achieved in accordance with the present invention will be better understood in view of the following detailed description which .:
discloses particularly advantageous method and composition ~ ::
embodiments for illustrative purposes.
'~'.'' ;''~, : ~' ' ' .',, .-~ ~", "'.' ', ' ':,'~ ,,' ~; ~
. '' '" .
~-,," ~,.
';'" ' ' ~
~''.
'"" ' : . ' .
:~ . ,:
"` ~062439 In this regard, in the practice of the process of the invention, a solution of an alkali metal silicate is first charged to a reaction vessel (equipped with suitable heating and agitation equipment) with the silicate solution then being heated to a temperature in the range of from 150 to 170F, preferably in the range of from about 160 to 165F. The silicate solution may, of course, be prepared in the reaction vessel itself.
As used herein, the term "alkali metal silicate" includes all the common forms of alkali silicates, as for example ~etasilicate, dysilicates and the like. Water-soluble potassium silicates and sodium silicates are particularly advantageous, with the latter being preferred.
In general, sodium silicates are effective in any composition in which the weight ratio of SiO2 to Na20 is from about 1.0 to 4Ø However, particularly advantageous and thus preferred results are obtained if the SiO2/Na20 weight ratio is in the range of from about 2.5 to 3.3.
After the solution has been heated to the above-noted tempera-ture, the acidulating agent or acid is added at a very slow rate and in the range of from 200 to 350 ml/min. In conventional processes involving the - acidulation of a silicate, the rates of addition have in general been very high and on the order of 500 ml/min. In the present invention the rate of addition must not exceed ~at any stage of acidulation) a rate i-n excess o~
or above about 350 ml/min.
The acid is preferably a strong mineral acid, such as sulfuric acid, nitric acid, and hydrochloric ac~d. However, other acids including organic acids or salts thereof (for example, acetic acid, forlnic acid, carbonic acid, ammonium carbonate, etc.) can be employed.
The acid is preferably added as a dilute solution thereof with preferred results being obtained if the acidic solution comprises from about 8 to 25% by weight acid, based on the total weight of the solution.
1062~39 Particularly advantageous results are obtained if the silicate concentration of the initial silicate solution is on the order of from about lO to 20% by weight, based on a total weight of the silicate solution.
As will be seen from the ahove~ the starting materials include the alkali metal silicate and the acid. Significant process variables include the SiO2/Na20 weight ratio, the reaction temperature and the rate of the acid addition to the solution of the alkali silicate.
In this regard, 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. Thus, prior to the acidulation, the aluminum sulfate (preferably as a solution thereof) is premixed with the acid such that its concentration is on the order of about 3 to 10% aluminum sulfate based on total weight of the acid. While the adduct material is preferably a water-soluble aluminum salt, such as aluminum sulfate, other salts such as magnesium sulfate may be employed. As indicated, the adduct is premixed with the acid and the acid/metal salt mixture is then used for acidulating the aqueous solution of the alkali metal si1icate. -As indicated above, the products of the invention have a very high structure. Thus at this point, it may be noted that as used herein the term "structure" is intended to include, and is defined as, the ability of a silica material to hold water in its wet cake; i.e., after the precipitate has been filtered. Commercially available precipitated silicas hold a high percentage of water (i.e., in the neighborhood of 75 to 85%) and are known and referred to as high structure silicas.
Materials holding water in a range of 65 to 75% are generally referred to as medium structure silicas. Silicas having wet cake moistures in excess of 85% and up to 95% and higher are referred to as very high structure silicas.
106243~
The present invention is d;rected to the production of very high structure, thickener silicas produced under carefully controlled acidulation conditlons.
As previously noted, lt is well known in the art that precipitated silicas are prepared by acidulating an aqueous alkali metal silicate with a suitable acidulating agent until a final pH of from about 5.6 to 6.0 is obtained. The silica precipitate is then filtered, washed free of salts, dried, and milled to desired degree of fineness.
The objects of the present invent;on are to provide an improved process for thè preparation of very high structure, thickener silicas; to provide a process for controlling the relative sizes and uniformity of the primary particles; and to provide unique silicas having improved functionality in toothpaste compositions and other use areas.
In this regard and turning now to further details of the invention, commercially available sodium silicate solutlons may be B represented by the for~ula Na20-x ~ ~ wherein x stands for the weight ratios of SiO2/Na20. Since the molecular weights of Na20 and SiO2 are substantially the same, it is common practice to use weight ratios for sodium silicates. Water glass is the most commonly used silicate for preparing siliclc acid pigments. This product has a weight ratio of 3.22 which means that it has 3.22 parts of SiO2 and one part alkali;
i.e., Na20.
In the prior art, it has been customary to acidulate a sodium sillcate wherein the value of x was in the range of from about 3.0 to 3.4. In this prior practice, the acid was merely introduced in the sodium silicates solution until the desired final pH was obtalned, with the product then being recovered using the conventional procedures.
~L062~39 In the present invention, the acidulation is very carefully controlled, particularly at the outset of the reaction and at that point at which the precipitation initially occursi i.e., wherein the value of x is approximately 4.2.
The invention will be further illustrated by the following examples which set forth particularly advantageous method embodiments.
While the examples serve to illustrate the present invention, they are not intended to limit it thereto.
_ .
In this experiment 10 gallons of a sodium silicate solution having an SiO2/Na20 weight ratio of 2.6 was added to a stirred reaction vessel and the solution was heated to 165F. The concentration of the silicate solution was 13.3X. Sulfuric acid of 11.4% concentration and aluminum sulfate solution (1.4 lbs/gal.) were premixed in the ratio of lOO to 7 by volume. This mixed acidulating agent was then added very slowly to the reactor at a rate of 300 ml/min. until a final pH of 5.9 was obtained. The product of this example had very high structure, a wet cake moisture of 88%, an oil absorption of 240 cc/100 g and a surface area of 250 m2/g.
The procedure of Example 1 was repeated except that the rate of addition of the premixed acidulating agent was 250 ml/m~n.
The product of this example had very high structure, a wet cake mois-ture of 87%, an oil absorption of 230 cc/100 g and a surface area of 275 m2/g.
-The procedure of Example 1 was repeated except that the rate of addition of ~he acidulating agent was 275 ml/min. The product of this example had very high structure, a wet cake moisture of 86%, an oil absorption of 220 cc/100 9 and a surface area of 265 m2/g.
The procedure of Example 1 was repeated except that the rate of acidulating agent was maintained at 225 ml/min. up to the initiation of precipitation at an x value of 4.2. The product of this example had very high structure, a wet cake moisture of 89X, an oil absorption of 250 cc/100 9 and a surface area of 280 m2/g.
As a control example, lO gallons of a sodium silicate solution having an SiO2/Na20 weight ratio of 2.6 was added to a stirred reaction vessel and the solution was heated to.l65~F. The concentra-tion of the silicate solution was 13.3~ Sulfuric acid of 11.4% concen-tration was added to the reactor at a rate of 500 ml/min. until a final pH of 5.9 was obtained. The product of this controi;example was only of high structure with a wet eake moisture of 84%, an oil absorption of 180 cc/100 g and a surface area of 150 m2/g.
As the above examples illustrate,the production of silica pigments of very high structure ~s made possible by a carefully controlled acidulation to and about the point of lnitial precipitation at an x value of 4.2 in the case of sodium silicate of weight ratio (x value) SiO2/Na20 of 2.6 and 13.3~ concentration. By controlling the acidulation to and about the x value of 4.2 for as long a time as practically possible, it is possible to control the ultimate silica particles and precipitate them under homogeneous environments with controlled micelle formation during the precipitation process. This controlled process then yields a silica of improved characteristics and functionality when compared with silica produced by prior art processes.
1~62~39 When the findings illustrated by the above examples are correlated with related findlngs, it is found possible to note that mol ratio and weight ratio of SiO2/M20, where M is an alkali metal such as sodium, potassium and the like, is inversely proportional to the number of mols of hydrogen ions provided by the acidulating agent per unit time to provide the controlled micelle formation dwring the precipitation of the silica required in a process in accord with the present invention.
For the above examples employing a sodium silicate of weight ratio (x value~ SiO2/Na20 of 2.6, the preferred rate of acidula-tion is from 200 to 350 ml/min. or from approximately 0.4 ~ol to 0.8 mol of hydrogen ions per minute.
In general, the des~red rate of acldulation of an alkali metal silicate of mol (or weight) ratio x = SiO2/M20, where M is an alkali metal, is from [H+]/x to 2tH+]/x, where ~H+~ is the mol concen-tration of hydrogen ions to be added per minute.
The silicas of the present invention was produced in two 1500 pound lots to confirm the datà and effectiveness of the product of Examples 1 - 4. These runs resulted in materials with the following properties:
Run and Product A: Oil Absorption: 237 c~/100 9 Surface Area: 308 m /9 Run and Product B: Oil Absorption: 222 cc/100 9 Surface Area: 259 m2/g The surface area of Product A was higher than that of Product B because it was finished at a lower final pH than was Product B.
The thickening efficiency of Products A and B was compared in typical humectant systems used in dentifrices.
~06Z439 Two humectant systems were used. One designated "C"
comprising a 70X sorbitol solution sold as SORBO by Atlas Chemical, Inc.
~ilmington, Delaware, and another designated "D" comprising a blend of 45 parts of a 70% sorbitol solution and 15 parts of a 95% glycerol solution~
Products A and B were dispersed by stirring in a known weight of each of humectants "C" and "D" until a free flowing powder was obtained. Silica of Example 5 was also dispersed in humectants "C" and "D" for comparison.
The control silica retained a maximum of 280% of "C" and 270% of "D." Product A retained a maximum of 300% of "C" and 296% of "D." Product B retained a maximum of 294% of "C" and 296% of "D."
As can be seen, the siliras of the present invention have superior thickening or drying-up capacity for typical humectants to the prior art silica.
Thus, the very high structure silicas of the present invention can be used as gelling and th;ckening agents in a variety - of dent;frice and toothpaste compositions.
Silicas of the present invention can also be used as reinforcing agents, flatting agents in paint and varnished, as a carrier, as flow conditioners, as a liquid drying agent and the like.
For flatting application, 10 grams of silica ~which was airmilled) of the instant invention was mixed with 350 grams of the nitrocellulose lacquer (conforming to Military Specification MIL-L-10287A - amendment 2, Type II, of issue 27 August 1959) and mixed for 3 minutes using the low speed setting of the Hamilton-Beach #30 mixmaster. The lacquer containing dispersed silica was tested for Hegman fineness of grind (5.50) and cleanliness of grind.
~L~62439 ..,,.;.
The lacquer containing dispersed silica was mixed with no lacquer and additional lacquer to prepare stock solution containing 10%, 3.5~, and 1.75% by weight of vehicle solids. A draw down of various stock solutions (containing 10%, 3.5% and 1.75% silica in lacquer) was made on carrara glass using a #34 wire wound coatings application rod.
Carrara glass draw downs were allowed to dry for 45 minutes under dust-free conditions. Using the above method, draw downs were also made from stock solutions containing the silica developed via the prior art processes.
Using the Gardner multi-angle gloss meter, the gloss and sheen values of the various draw downs were measured at 60 and 85, respectively. These values were compared with measured values obtained when a prior art silica was dispersed in the lacquer.
Silicas of the present invention result in cleaner Hegman grinds and exhibit better clarit~ when dispersed in the lacquer. The better clarity is attributed to the fact that the silicas of the present invention are of unifonm particle sizes and favorable structures.
Flatting data below suggests that the novel silicas of the present invention exhibit lower gloss and sheen values than the control.
Lower gloss and sheen values are preferred and advantageous for paint flatting application.
FLATTING DATA
60 Gloss 85 Sheen 10% 3.5% 1.75%10% 3.5% 1.75%
Silica via Exa~ple 1 8 29 44 28 61 74 Silica via Example 5 10 38 54 37 72 82 (control)~
Although preferred embodiments of the present invention are described in detail above, the illustrative description is not intended 1~6243~
to limit or restrict the invention disclosed to those skilled in the art and the invention is thus declared to cover all changes and modifications of the above disclosure which do not constitute departures from the spirit and scope of the claimed invention.
Claims (5)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of producing a unique, amorphous, pre-cipitated silica pigment having very high structure, a wet cake moisture in excess of 85%, an oil absorption in excess of 200cc/
100 g, a surface area in excess of 250 m2/g and having improved chemical and physical properties suitable for use as a thickener in dentifrice compositions, said method comprising the steps of:
providing an aqueous reaction medium containing 10 to 20% by weight of an alkali metal silicate selected from sodium and potassium silicate having an SiO2/M2O weight ratio, x, of from 1.0 to 4.0, where M is an alkali metal selected from sodium and potassium; heating said reaction medium to from 150°F to 170°F;
and adding an acidulating agent at a rate of from l/x to 2/x mols of hydrogen ions per minute.
100 g, a surface area in excess of 250 m2/g and having improved chemical and physical properties suitable for use as a thickener in dentifrice compositions, said method comprising the steps of:
providing an aqueous reaction medium containing 10 to 20% by weight of an alkali metal silicate selected from sodium and potassium silicate having an SiO2/M2O weight ratio, x, of from 1.0 to 4.0, where M is an alkali metal selected from sodium and potassium; heating said reaction medium to from 150°F to 170°F;
and adding an acidulating agent at a rate of from l/x to 2/x mols of hydrogen ions per minute.
2. The method of claim 1 wherein the reaction temperature is from 160°F to 165°F and said alkali metal sili-cate is sodium silicate of weight ratio, x, from 2.4 to 3.3 and said acidulating agent is a dilute solution of from 8 to 25% by weight of a strong mineral acid
3. The method of claim 2 wherein said reaction temp-erature is 165°F and said alkali metal silicate is sodium silicate of weight ratio, x, 2.5 ? 1 and said acidulating agent is sulfuric acid added at a rate of from 200 to 350 ml/minute.
4. The method of claim 1 including adding an adduct of from 3 to 10% by weight of a water-soluble aluminum salt to said acidulating agent prior to its introduction to said reaction medium.
5. The method of claim 4 where said aluminum salt is aluminum sulfate.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US40292773A | 1973-10-03 | 1973-10-03 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1062439A true CA1062439A (en) | 1979-09-18 |
Family
ID=23593839
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA208,941A Expired CA1062439A (en) | 1973-10-03 | 1974-09-11 | Process for producing precipitated thickner silica |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS604129B2 (en) |
| BR (1) | BR7408074D0 (en) |
| CA (1) | CA1062439A (en) |
| DK (1) | DK144331C (en) |
| FI (1) | FI62129C (en) |
| GB (1) | GB1478387A (en) |
| ZA (1) | ZA745778B (en) |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2159580A5 (en) * | 1971-11-04 | 1973-06-22 | Sifrance |
-
1974
- 1974-09-10 GB GB3940374A patent/GB1478387A/en not_active Expired
- 1974-09-11 CA CA208,941A patent/CA1062439A/en not_active Expired
- 1974-09-11 ZA ZA00745778A patent/ZA745778B/en unknown
- 1974-09-20 FI FI2756/74A patent/FI62129C/en active
- 1974-09-27 DK DK512174A patent/DK144331C/en not_active IP Right Cessation
- 1974-09-30 BR BR8074/74A patent/BR7408074D0/en unknown
- 1974-10-03 JP JP49114287A patent/JPS604129B2/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| AU7334874A (en) | 1976-03-18 |
| ZA745778B (en) | 1975-10-29 |
| FI62129B (en) | 1982-07-30 |
| BR7408074D0 (en) | 1975-07-22 |
| DK144331B (en) | 1982-02-22 |
| FI62129C (en) | 1982-11-10 |
| JPS604129B2 (en) | 1985-02-01 |
| DK512174A (en) | 1975-06-09 |
| DK144331C (en) | 1982-07-19 |
| FI275674A (en) | 1975-04-04 |
| GB1478387A (en) | 1977-06-29 |
| JPS5064197A (en) | 1975-05-31 |
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