CA1059705A - Amorphous precipitated siliceous pigments and improved process for producing such pigments - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process for producing precipitated silicic acid pigments having new and improved properties is disclosed. The pigments are produced by the simultaneous introduction of a solution of an alkali metal silicate and an acid, such as sulfuric acid, into an aqueous receiving medium which contains a salt or electrolyte.
The pH of the aqueous receiving medium is adjusted prior to the introduction of the acid and silicate so that it is the same or equal to that pH at which the precipitation of the pigment is effected. The electrolyte is preferably a metal salt of the acid used for acidulation of the silicate. The acid and silicate are added to the receiving medium at a rate such that the preci-pitating pH is maintained constant. The products of the invention are characterized by their high abrasiveness and have a relative cleaning ability or scale (RCS) equivalent to high grade phosphates used as polishing agents in toothpastes and a superior fluoride compatibility to that of said phosphates. The silicates used in the process are preferably those which are partially polymerized and have a SiO2/Na2O ratio of at least 2Ø

Description

1 1059705 1~ 57155 The present invention relates to silicic acid pig~ents and to an unique process for producin~ synthetic precipitated silicas havin~ an unique combination of physical and chemical properties.
As ~nown in the art, precipitated silicic acid pigments can be prepared by the acidulation of an aqueous silicate solution with ~n acid such as sulfuric acid, hydrochloric acid, etc.
Examples of prior $echniques which involve the acidulation o~
silicate solution to produce siliceous pigments are dis~losed in U.S. patents 3,110,606 and 3,582,379. In general it is k~o~n that the nature or characteristics of the above discussed products, sometimes herein re~erred to as silicas, depend on the speciiic reaction conditions employed, as ~or example, the precipitating pH, the reaction temperature, etc. Notwithstanding this fact, prlOr KnOWn pi~m~n~ are cnaracteri~e~ by, ~n~ h~ve, the follo~ g properties: high structure, high wet cake moisture content, high oil adsorption, low valley abrasion, high surface area and . low pack density. In this regard, and due to properties such a~
high oil adsorption, high surface area, etc., the pigments have been widely used as reinforcing pigments in rubber, in the manu-facture of paper, as moisture conditioners and the like~
¦ The high wet cake moisture content is disadvanta~eous j however, in that the dryin~ and iiltration rates ar~ increased, ¦ thus increasing the overall cost of the final product. For ¦ ~ example, in tle conventional production o~ silicic acid pi~ments ~as defined above) the wet cake moisture content of the product * i~

~ 97~5 (following filtration o~ the prec~plt~ted reaction ~
approximately 82Z~ This means that there can be recoYered only 18 parts of dry pigments from 100 parts of wet cake.
Further~ the low abrasiveness of the known silica and silicate pigments renders them unsuitable for many uses. For example it is well known that conventional synthetic precipitated silicas are unsuitable as polishing and ahrasive agents in tooth-paste compositions. See German patent 974~958; French patent 1~130,627; British patent 995~351; Swiss patent 280~671 and U. S~ patent 3,Z50,680. U. S. Patent ~538,230 speci~ically discloses that known amorphous silicas ~uch as precipitated silicas,pyro~enic silicas and aerogels are unsuitable for dentifrice use because of their initial small particle size and because oi the ease in which they breakdown into small particles sizes which result in poor cleaning ability. t In this regardJ a primary ~unction of abrasive or polishing agents in toothpaste is to remove stains, food debris and bacterial plaque from the tooth suriace. Ideally the polishing agent should provide maximum cleaning action at acceptable abrasive levels and m~st be compatiable at loadings of from about lS% up to 50% with the other toothpaste ~ormula ingredients.
Examples of known polishing agents include aluminas, tl~ermo-; -'setting resins (as e.g. melamine-formaldehyde resins), zirconium silicates and various phosphate salts or compounds such as beta tricalcium orthophosphate. Specific examples of phosphate polishing agents are disclosed in UO S. patents 3,169,096; 3~359~170 and ,44a,604.

~05~7~5 In s~mmary the present invention relates to the product- .
ion of finely divided precipitated silicas which~ because of their new and unique properties~ can be used as a pollshing a~ent in $oothpaste composition~. In its broadest aspect, the invention is based on the discovery that high abrasion precipitated silicas, ;
having superior fluoride compatibility and cleaning properties equivalent to the aforementioned phosphates~ can be produced by the simu~aneous introduction of an acid and an alkali metal silicate into an aqueous reaction medium which contains an electrolyte~ such as sodium sulfate, and thereafter effecting the precipitation of the ~inely divided pigment at a constant pH. Significant proce~s variables include the SiO2/Na20 mole r~tio of the alkali metal silicate and the initial pH o~ the reaction med~a. With regard to the latter, the pH of t~e aqueous reaction medium must be ad~usted prior to the introduction of the acid and sillcate so that it is tne same or equal lo ~nal p~ al wn:lcn ~ne pr~cipilalion OI ~ne ¦pigment is carried out. As will be discussed in more detail here-inbelow, the salt or electrolyte employed or added to the reaction medium at the outset is preferably a metal salt of the acid used for acidulation~ For example, i~ sulfuric acid is employed~ the electroly$e w~uld comprise a metal sulfate such as sodium sulfate.
Preferrcd electlolytes comprise sodium, lithium~ potassium, and ammonium sulfate when the acid is sulfuric acid. If sodium j~silicate is acidulated ~ith sulfuric acid then sodium sulfate ¦ would be the preferred electrolyte.
As indicated, the product of the illention has an unique combination of physical and chemical properties including _ 4 _ ~0~;97(~5 high abrasion and a cleaning abillty or action equival~nt to that of known phosphate and other cleaning or polishing agents. This was truly unexpected and9 ~n part, embodies the concept or discovery upon which the instant invention is based.
It is accordingly a general object of the present invention to provide a process for producing precipitated silicic acid pigments having an unique combination of physical and chemical properties.
Another and more part~cular object is to provide im-proved synthetic amorphous precipitated silicas which have particular utility for use as an abrasive or polishing agent ~or 11 dentifrice compositions.
¦ A futher object is to provide a novel process ~or producing precipitated amorphous sllicas which ~ave unexpected properties as compared to conventionally known precipitated pigments, such properties includi~g low structure, high abrasion, high pack density, and high cleaning action in toothpastes.
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 adval~geous method and composition 'lembodiments for illustrative purposesO
As discussed above silicic acid plgments are conven-tionally produced by the acidulntion of an aqueous silicate solution with an acid~ In other words a silicate solution is charged to a reactor with the solution then being acidulated by he introduction of an acid. More recently however (aAd before _ 5 _ ~L059705 turning to the specific details of the invention) there has been developed, as disclosed in U.S, Patent No. 3,928,541, a process for producing siliceous pigments having improved properties wherein a salt or electrolyte is employed to prepolymerize an alkali metal silicate solution prior to its acidulation with an acid. In accordance with the teachings of this application an alkali or alkaline earth salt is first added to a solution of the alkali metal silicate, e.g., sodium silicate, with the latter being pre-polymerized by the addition of the salt. The said silicate solution is then heated to a temperature in the range of from lOO~F to 200F, preferably on the order of from about 150-175F. The acidulating agent or acid, e.g., sulfuric acid, is next charged to the reactor until the precipitation of the silicate is complete. The reaction or precipitation is carried out at a pH in the range of from about 6.5 to llØ At the end of the precipitation, an excess of the acid is preferably added to bring the pH of the pigment within the range of from about 5.5 to 6.5 and the reaction mass is filtered, washed and dried. In a first method embodiment disclosed by this application, the entire solution of the salt induced polymerized silicate is initially charged to reactor. In another embodiment one-half to two-thirds of the silicate is initially charged to the reactor with the ; remaining silicate being added with the acid.
In contrast, in accordance with the present in~ention an aqueous receiving media, i.e., water, is first charged to the reactor. The electrolyte is then added to the water and the pH

97CI~;

of the water-electrolyte media is adjusted such that it is the same at which the precipitation of the pigment is to be carried out. In the practice of the invention the precipitating pH should be in the range of 8 to 10.4. Thereafter, aqueous solutions of the acid and silicate (as to be discussed in more detail herein-below) are added simultaneously in a manner and at a rate such that the reaction or precipitation is carried out at the present, constant pH. After the pigment has been completely precipitated and depending upon the ultimate or intended use of the pigment the pH can be reduced or adjusted by the addition of acid, etc. While the product of the recently developed process of U.S. Patent No.
3,928,541 has high abrasiveness and can be advantageously used in dentifrice compositions, it has unexpectedly been discovered that products produced in accordance with the method of the instant invention have significantly improved cleaning ability or action, with the latter being the same as or ec~ual to that of high grade phosphate toothpaste polishing agents. In addition and as pre-viously noted the products, when used :;n toothpaste formulations, have a fluoride compatibility (i.e., the fluoride is available in its soluble form and not converted e.g., to an insoluble salt, etc.) that is superior to, and not subject to the disadvantages of, known phosphates, such as calcium pyrophosphate.
As will be seen from the above and turning now to further specific details in the practice of the invention, the starting materials include an electrolyte; an alkali metal silicate and an acid. As used herein the term alkali metal silicate includes all the common ~orms of alkali silicates, as for example, metasilicates, disilicates and the like. Water soluble potassium silicates and sodium silicates are particularly advantageous Because of their 1(~597~5 relatively low cost, sodium silicates are preferred. Sodium silicates are effective in any composition in which the mole ratio of the SiO2 to Na2O is from about l.0 to 4. In this regard commercially available sodium silicate solutions are more or less polymerized depending on their silica to sodium oxide (SiO2/Na2O) ratios. For example, sodium metasilicate solution (mole ratio unity) known to be predominately monomeric in character while water glass (mole ratio 3.3) is both monomeric and polymeric in character. As the silica to sodium oxide mole ratio of sodium silicate increases, so does the polymer to monomer ratio of its silicate anions. While sodium silicates having a SiO2/Na2O mole ratio of from 1 to 4 may be employed, particularly advantageous results are obtained if the SiO2/Na2O ratio is in the range of from about 2.0 to 3.3, or more preferably from about 2.0 to 2.8.
Although the commercially available silicate solutions may be more or less polymerized depending on their silica to sodium oxide (SiO2/Na2O) ratios, it has also been found that particularly ad~antageous and preferred results are obtained if a sulfate salt or electrolyte is added to the silicate solution prior to its simultaneous introduction with the acid into the water-electrolyte reaction or receiving medium. In other words the silicate solution itself can be pre-polymerized as disclosed in U.S. Patent 3,928,541. In addition, and in lieu of simply mixing the salt with the silicate solution, the silicate itself may be a product, wherein silica is reacted with a hydroxide in the presence of a sulfate slat to produce a polysilicate.
While the acidulating agent or acid is preferably a ~` Il` 1059705 strong mineral acid, such a~ sul~uric acid, nitric acid and hydro~
chloric acid, it should be understood that other acids~ includlng organic acids, as for example, acetic acid, ~ormic, or carbonic acld, and ~alts of carbonic acid such as ammonium carbnoate, can be employed. As noted above, the acidulating agent and silicate should preferably be added dil~te solutions thereof~ Preferred results are obtained i~ the acidic solution is from about 10 to 25% by weight acid based on the total weight of the solution.
Particularly advantageous and thus pre~erred results are obtained i~ the concentration oi the silicate ~olution is on the order 0~ ¦
~ro~ about 1.0 to about 2.5 pounds per gallon.
As known in the art, the ter~ electrolyte refer~ to ionic or molecular substances which, when in solution, break Id own or disassociate to form ions or charged particles. As used ¦herein (and re~erring to the eleckrolyte added to the water prior to the introduction of the acid and sil;cate solutions) the term electrolyte is intended to have its common meaning. However in the practice of the invention, and again as discussed above,;
the electrolyte should be compatible with the acid and si1ieate.
Thus if sodium silicate and sulfuric acid are employed the pre- i ferred electrolyte would be sodium sul~ate. If hydrochloric acid is employed as the acid ~again using ~odium silicate) then the electrolyte would be sodium chloride. I~ the silicate-acid jcomb~llation is potassium silicate and sulfuric acid, then the electrolyte would be potassium sulfate, etc. Further examples ~ electrolytes include sodium nitrate, sodium acetate and the like. It should be noted however t~t the electrolyte must not be a salt that would produce a water insoluble by-product with - D -the precipitatin~ pigment. An example oi thi~ is calcium suliate.

Thus a calcium salt eould not be usçd with sul~uric acid. The ~ es;~
amount o~ the electrolyte used should~be i~ the ran~e of from 3 ko 15% by w~ight, based on the weight of the water (receiving media) initially added to the reaction. In general the temperat~re employed in precipitating the pig~ent of the inventlon is not critical and is the same as in the above discussed known processes.
In a preferred embodiment the receiving medium containing the electrolyte is heated to a temparature ~n the range of from 100F to 200 F prior to the introduction of the silicate and acid. ~
As should be readily appreciated by thos~ skilled in the j art, no special equipment i~ required in the method herein des-¦crlbed. In this re~ard, however, the reactor should be equipped with heating means, e.g., a stea~ jacket, in order to maintain the desired reaction temperature and should h ve adequate agitatlng means to prod~ce a strong backflow on the body o~ the liquid and thus avoid zo~es of high concentration of the incoming reactants. It is desirable to bring the reactants together so as to produce an instantaneous reaction of all material being fed to the fullest extent reasonably possible, as such promotes uni~ormity of the resultin~ products. Storage vessels (for the reactants) connected to the reactlon vessel through lines fitted with flow control means may also be provided. ~he reaction vessel may be equipped with an outlet line leading to a filter which may be of conventional desi~n. After precipitation, the filtered mass is washed and dried. Such steps may also be ¦~conducted in nventional equipment, it bulng understood, of - 10 - .

10597~5 course, that same do not form a part oi the present invention.
I~ the pigments of the invention are used in toothpaste compositions, the dentifrice (if in the form o~ a paste) may contain humectant materials and binders to give the dentifrice a smooth texture and good flowability. Glycerine, sorbitol, corn syrup, glucose and the like may be used as carriers. Examples of binders include gum tragacanth, sodium carboxymethylcellulose and the like. The above materials as well as specific f~rmulations and ingredients of toothp~ste compositions are well kno~vn in the art and are disclosed in numerous publications and e.g., in U.S~
patents 2~994,642 and 3~538~230 Before turnin~ to specific examples~ it may be noted that as used herein the term "structure" is intended to include, and is defined as , the ability o~ a silica material to hold water in its wet cake. ~hen silicas, such as the a~orementioned conventional precipitated silicas~ hold a high percentage of water, i.e. 9 in the neighborhood of 75% to B5%,they are known and referred to as lligh structure. silicas. ~aterials holding less than 75% and preferably in tbe neighborhood of from about 50% to 70~ water in their wet cake are referred to as low structure silicas.
1 The invention will be further illustrated by the follow-¦¦ing examples which set forth particularly advantageous method land composition embodiments. While the examples serve to illustrate the present invention, they are not intended to limit it theretoO

~05970S

EXA~IPLE l In this experiment dry sodium sulfate was added to 10.0 gallons of water in a 200 gallon reactor such that the sodium ' sulfate concentration in the reactlon medium was 10%. The pH
of the reaction medium was adjusted to 9.0 by the addition o~
sodium silicate. The reaction medium was then heated to 150F.
Sodium silicate having a SiO2 to Na20 mol ratio o~ 2.5 and a concentration of 2.0 pounds/gallon and sulfuric acid of 11.4%
concentration were then ad~ed to the reaction medium at the rate o~ 756 ml/min and 453 ml/minute respectively so that a constant precipitation pH of 9.0 was maintained. The sodi~
silicate solution employed in this Example also contained 7%
sodium sulfate which was added to the solution prior to its introduction into the reactor. After 30 minutes the precipitation was complete. Excess acid was added until a slurry pH of 5~4 was reached. The reaction slurry was d;gested at 170F for 20 minutes and then ~ilte~ed, washed, dried and milled in the conventional manner. The product produced in this Example had a wet cake moisture content of 51%; a surface area of 173 m2/g;
pack density of 35.6 lbs/cu.~t. and a valley abrasion (mg wire loss) of 70.2. It was further ~ound that the produ t had a relative cleanillg scale (RCS) of 100, which was the same as the phospllate polishin~ agent disclosed in U. S. patent 3,359,170, and I had superior fluoride stability.

Example 1 uas repeated except that the reaction tem-perature was maintained at 175F and the precipitation pH was 10Ø Digestion temperature was 199F. The batch was otherwise ¦processcd in thc conventional manncl. The properties of the product wcre s~ st~ntially tl~c same as in Example 1.

~597~5 EXA~IPL~ 3 Example 1 was repeated except thtt the reaction tem-perature was ma.intained at 185F and the batch was digested at 199F. The batch was otherwise processed in the conventional manner. The properties of the product were substantially the same as in Example 1 EXA~IPLE 4 In a series of tests the general procedure of Examples 1-3 were repeated except that the precipitating pH, although held constant in each test, was varied from 8.0 to 10.4. The results were substantially the same except that it was found that the specific properties could be controlled within pre- !
determined limits by changing thc pH. In any event the products had a low wet cake moisture content, were o~ low structure~ had relallvely lOW Oll a~SOrpllOnS ana nl~ler valley a~raslon as compared to a control wherein conYentional precipitated silica was prepared by neutralizing 1.24 lbs/gal sodium silicate ~10 gal.) with 11~4% sulfuric acid. The valley abrasion of the la*ter was foand to be 2.5 as compared to up to 167.8 (Example 3) of the products of the invention.

The ~eIleral procedures o~ Example 1-4 were repeated i except that nitric acid, hydrochloric acid~ acetic acid and formic acid were substituted for the sulfuric ac~d. The results were substantially the same as in Examples 1-40 The correspondin~
salts (eOg., sodium nitrate, sodium chloride~ etc.) were also used as the electrolyte initially charged to the reactor (in lieu of tbe d1um sulfate), - 13 - ;
I 1, 1~ 10597(15 EX~MPLE G
In a series of tests the general procedures of Examplcs 1-5 were repeated except that aqueous sodium silicates havin~
mol ratios (SiO2/Na20~ in the range o~ from 1 to 3 were substituted for the 20 5 silicate of Examples 1-50 The results were generally the same as in Examples 1-5 except that it was ~ound that the us~ of alkali metal silicates ha~ng a SiO2~Na20 mol ratio within the range of from about 2.0 to 2.8 resulted in superior properties (as defined hereinabove), The procedures o~ Examples 1-6 were repeated except that the salt employed as the electrolyte in the aqueous receivin~
medium was varied ~rom 3 to 15% by weight (by increments of about 5%~0 The results were substantially the same as in the above ~xamples. ' The procedure of Example 1 was repeated except that the sodlum silicate solution did not contain any sulfate prior to its introduction into the reactor. It was found that the while the valley abrasion was relatively high as compared to th~
control ~see Example 4) the cleaning action was about 85 (RCS) as compared to 100 for the product of Example 1.

From the above Examples it is clear that the process of the invention results in silicas of lower wet cake moistnre, lower structure3 lower oil absorption, hi~her pack density and hi~her valley abrasion than the co~ventional products. In ~0597Q~;
addition (and this was truly unexpected) the cleaning ac-tion of the new products when used in toothpaste were found to be very high and equal to known phosphates. The cleaning action was also approximately 50~ higher than the products produced in accordance with the recently developed process. The valley abrasion of the latter and the products of the invention are similar.
The new process also leads to silicas of lower pro-cessing costs than regular precipitated silicas. For example, the average wet cake moisture (Examples 1 thru 8) of silicas produced via the new process is approximately 53~ as opposed to 82% for regular silica. This means 47 parts of dry silica from 100 parts of wet cake can be recovered. Thus, via the new process 29 parts of more dry silica or an increase of (29/18) x 100 or about 160% is obtained. The new process results in silicas of better drying and filtration rates and hence significantly lower processing costs than the precipitated silicas produced by the conventional process.
From the above it will be seen that the present invention provides a truly remarkable and simplified process for producing silicic acid pigments having new and unique properties.
While preferred method and compositions embodiments have been disclosed for illustrative purposes it should be understood that the invention is not limited thereto. For example, as disclosed in U.S. Patent No. 3,928,541, the refractive index of the precipitated pigment can be controlled by the addition of an adduct element (such as aluminum, magnesium and the like) to provide an abrasive or polishing agent for a clear translucent or transparent toothpaste composition. In this embodiment, the acid is premixed with a solution of the adduct material (e~gO, aluminum sulfate) and the acid-metal salt mixture is then used for acidulating the alkali metal silicate.

`!

l l

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for producing amorphous, precipitated silicic acid pigments having improved chemical and physical properties including high abrasiveness and high cleaning action, said method comprising the steps of: providing an aqueous reaction medium containing 3 to 15% by weight of water, of an electrolyte, adjusting the pH of the aqueous medium to a predetermined value in the range 8 to 10.4 equal to that at which the said pigment is to be precipitated;
simultaneously introducing an acid and an alkali metal silicate into said aqueous reaction; continuing the addition of the said acid and said silicate to said aqueous reaction medium in a manner such that a substantially constant pH is maintained and until the precipitation of said pigment is substantially complete and recovering the precipitated pigment from the aqueous reaction medium.

2. The method in accordance with claim 1, wherein said acid is selected from the group consisting of sulfuric acid, nitric acid and hydrochloric acid; said alkali metal silicate is selected from the group consisting of sodium silicate, potassium silicate and lithium silicate and said electrolyte comprises sodium, potassium or lithium salt of said acid.

3. The method in accordance with claim 1, wherein said alkali metal silicate has a SiO2/X2O mol ratio in the range of from about 2.0 to 3.3 wherein X is selected from the group consisting of sodium, potassium and lithium.

4. The method in accordance with claim 1, 2 or 3, wherein said electrolyte is added to the aqueous reaction medium in an amount in the range of from 3 to 15% by weight based on the weight of the reaction medium.

5. The method in accordance with claim 1, 2 or 3, wherein the concentration of the solution of said silicate is in the range of from about 1.0 to 2.5 pounds per gallon and the acidic solution is from about 10 to 25% by weight acid based on the total weight of acidic solution.

6. The method in accordance with claim 1, 2 or 3, wherein an adduct material selected from the group consisting of the water soluble salts of aluminum and alkaline earth metals is combined with said acid to thereby control the refractive index of the precipitated pigment and to form a metal silicate pigment having particular utility for use as an abrasion and gelling agent in clear toothpaste compositions.

7. An oral dentifrice composition providing good cleaning and polishing of the teeth, said composition having therein a polishing and abrasive ingredient comprising an amorphous, precipitated silicic acid pigment produced by the simultaneous introduction of dilute solutions of an acid and an alkali metal silicate into an aqueous reaction medium containing an electrolyte, said reaction medium containing 3 to 15% by weight water and having an initial pH adjusted in the range 8 to 10.4 and equal to that at which the precipitation of said pigment is effected, said acid and silicate solutions being introduced into said aqueous reaction medium at a rate such that said precipitation is carried out at a constant pH.

8. A composition as claimed in claim 7, wherein said precipitated pigment further comprises an adduct material sel-ected from the group consisting of aluminum and an alkaline earth metal, said adduct serving to control the refractive index of said precipitated pigment and thereby rendering the said composition translucent.