NL7908956A - SILIC ACID GEL, METHOD FOR THE PREPARATION THEREOF, AND USE - Google Patents

Description

-70t 70 8828

Silica gel, process for its preparation, and use,

The invention relates to new silicic acid gels, as well as to a new process for preparing these silicic acid gels, which exhibit properties which make them suitable for use as cleaning, grinding and polishing agents, e.g. in tooth-5 pastes particularly suitable.

Sr already has extensive patent literature on the use of finely divided silicas as a cleaning and polishing agent in toothpastes. On the basis of this prior art, it has not hitherto become clear what characteristics a selic acid must have to be suitable as a polishing agent. Pyrogenic silicas as well as precipitating silicas and silicic gels are recommended, each having different average particle sizes and having both a large and a small surface area.

For example, DE-AS 1,617 * 927 recommends silica xerogels with a particle size of 2 to 20 µm and a surface area of at least 600 µm / g. According to DE-OS 2.028.866 sou 2, however, the surface is not critical and can also be 300 to J00 m / g.

According to BE-OS 1,667,875, pyrogenic hydrophobic silicic acids would be useful whose primary particle size is only about 0.01 to 0.03 yam. On the other hand, in BS OS 2,250,078 it is said that the particle size is also more than 20 µm Vg-n; there, silica xerogels with ƒ 2 surfaces between 250 and 800 m / g are recommended, the particle size of which is between 22 and 50, in particular 25 - 30 µm.

DE-AS 2,446,038 places the greatest weight on the shake density and shows that the abrasion power (determined as wire abrasion) with increasing shaking weight also increases 79 0 8 S 5 5% -2. However, it does not become clear how the different shaking weights can be obtained. In the examples, the surface shows no correlation with the sanding effect or with the shaking weight.

It follows from the above that a direct relationship between the abrasion properties of Kielic acids on the one hand and the other key figures on the other hand has apparently not been found so far. At one time, the type of silicic acid appears to be essential since, in addition to such gel types, wet-deposited silicas and pyrogen-generated silicas have been found suitable.

Within the class of silicic acid gels, suitability according to the prior art is also not limited to practically completely dehydrated silicic gels, the so-called xerogels, such as e.g. DE-OS 2,704,504 shows that gels with a water content of 15 to 35% recommend which exhibit a particle size in the range of 2 to 30 µm; These water-containing gels exhibit very good abrasion properties.

DE-OS 2,522,486 contemplates "low structure" silicas, which should be understood to mean low oil uptake and high shaking density. Accordingly, the pore volumes of these silicas are relatively small, and this property is considered essential for the desired abrasion properties.

An optimal cleaning and polishing agent for toothpastes must guarantee a certain, relatively high degree of abrasion or cleaning power and thereby give the teeth the greatest possible shine. In addition, the agent must give the toothpaste a favorable consistency, which is also stable during storage, with which the other toothpaste components are tolerated and, finally, must not lead to corrosion of the packaging material. As discussed above, the state of the art. The technique shows that, although types of silicic acid have been found that correspond to 7908956 • + - 3 - t

these requirements essentially meet. Silica gels seem to have a very good general polishing effect in toothpastes, as can be seen from DE-AS 1,617,927 and DE-OS 2,028,886. Other advantages are that transparent dental care compositions can be prepared (compare Am. Os. 3, 538,230, DE-OS

2,250,078 and 2,502,111) and that the tolerance to fluorides added to dental caries to prevent caries is good (compare DE-OS 2,153 * 821).

Although the cleaning action and the more precise sanding effect have already been associated with the particle size and other structure numbers of the silicas, there is as yet no clear teaching on how to influence the sanding effect of the silicic acid. As a result, the silicic acid present in each case must be used at a very specific concentration in order to set a certain abrasive power of the toothpaste. It is true that particularly high abrasion values cannot be achieved at all, because the absorption capacity of the formulations for porous amorphous silicas is limited, since otherwise the pastes become too stiff or too dry. If the concentration of the silicas is changed in order to set a different abrasive power, a complete reformulation of the total formulation is thereby also required. Sr not only the consistency, but also the storage stability, the taste, the tolerability and the like have to be retested and adjusted, which is time consuming and complicated.

In addition, the particle size of the cleaning and polishing agent having an average value around 10 µm is already so large that it is perceived organoleptically by the user. A clear change in the particle size or concentration therefore leads to a strong shift in the mouthfeel, with the result that the consumer believes that he has a completely different toothpaste in front of him. It is therefore practically not possible to use a dental 7908956 already introduced on the market

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- 4 - to vary the paste only with respect to its abrasive power without also changing the other objective or subjectively perceived properties, e.g. it is impossible to use a particular toothpaste composition with stepped abrasive values for different use groups, e.g. young and old people. In itself, such a case would be particularly desirable since older persons, whose teeth are often exposed unprotected by the gums, need dental care products with lower abrasive power.

The difficulties outlined above are therefore particularly great, since it has hitherto not been possible to prepare polishing and cleaning agents based on synthetic silicas which yield high or very high abrasion values and which are especially suitable for use in dental care products.

The object of the invention is primarily to propose silicic acid gels and processes for the preparation of these silicic acid gels, the dentin abrasion effect of which can be adjusted to certain values, which is almost independent of the particle sizes and within the use window independent of the application concentration in the toothpaste. Moreover, it is also possible to prepare silicas with hitherto unattainable high abrasion values with the method according to the invention.

The invention now relates to a silica gel with an average particle size of 1 to 30 µm, which is characterized by the following / combination of characteristics! 2 a) a surface area of 1 to 600 m / g, b) a pore volume of 0.05 to 0.5 cm ^ / g, 2 30 c) a product of surface area in m * / g and pore volume in cm ^ / g 240 and preferably 200, d) an arithmetic pore diameter of 1.5 to 2.5 nm, and e) a water content of less than 25% by weight.

The invention further relates to a method 7908956 a - 5 - τοογ the preparation of Tan silicic gels with an average particle size of Tan 1 to 30 by gelling Tan aqueous silicate solutions and subsequently washing, drying and stroking to the desired particle size, which method is characterized by this 5 that the wormed silica hydrogel has a pH value below 6 and temperatures of Tan about 0 to 70 ° G to a purity of Tan about 90 to 99> 9 gsw. ^ SiOg (based on the loss-free substance) and then immediately to a water content below 25 wt.% Dry, with the washing and drying conditions being adjusted so as to avoid aging, such that the silica gel has a surface area of 1 to 600 m / g and a pore volume of 0.05 to 0.5 cm 2 / g and exhibits an arithmetic pore diameter of 1.5 to 2.5 nm.

Finally, the invention also relates to the use of the silicic acid gels according to the invention as a cleaning, grinding and polishing agent, in particular in dental care agents.

According to Ilmann's Sncyklopadie der Technischen Chemie, 3 *

Auflage (1964), Vol. 15, p. 719, various silica gels can be obtained by washing hydrogels at different pH values. A narrow pore gel with a surface of 600 to S00 m ^ / g and a pore volume around 0.3 cm ^ / g is distinguished from a large pore silica gel with a surface of 250 m ^ / g and a pore volume around 0.9 cm ^ / g. / g. Mid-porous seed gels are in between these two values. Due to the different ρΞ values during washing and using different washing temperatures, a continuous spectrum of silicic gels can be prepared according to the prior art, which have increasing surfaces with decreasing pore volume.

On the other hand, the pebble geese prepared according to the invention belong, which is due to an approximately constant arithmetic V “V 1θ3 pore diameter D = -. »1.5 to 2.5 nm, in particular 2.8 to 2.2 nm (? V = pore volume in csr / g; 0? = Area in mVés) and sen P-sdukt OF x PV £ 300 , in particular, ^ 240 7908955

Am - 6 - * are not characterized in the correlation series of the "known silica gels, but they form a class of their own. Although the pore volume also correlates with the BET surface, however, the pores-5 volumes decrease in reverse manner with decreasing BET surface. Moreover, these results show that to characterize silica gels the specific surface alone is by no means sufficient, but that the pore volume "moreover must be taken into account in order to unambiguously describe the silica gels to be obtained according to the invention.

The silicic acid gels are washed at a pH value below 6, ie the pH value of the fresh as well as the used wash water is below 6.

Preferably the silica gels are washed at low pH values, in particular at values below 3 and at low temperatures -15 from about 0 to 70 ° 0, in particular 0 to 60 ° C, to a purity of 90 "to 99 > 9 wt.% SiOg, in particular 96 to 99.7 g. Wt. SiOg has been reached The impurities to be removed consist mainly of sodium sulfate, since the silica gel develops from a fully dispersed silica sol, the colloidal particles of which gelling still shows a very low molecular weight of the order of 6000, temperature and speed play an important role in the control of further developments, since these parameters determine the time during which the low molecular weight silica is higher molecular weight, condenses, whereby the SiOg gel skeleton in the sense of the degree of polymerization reaches progressively progressed stages which (after drying) generally decrease specific surface can be recognized. In the process according to the invention, a particularly early developmental state is sought in this sense, since very "young" silica gels must be prepared. Consequently, in addition to the low pH value and incomplete purity, low treatment temperatures are required, whereby also the treatment time in total should not be too long 7908956 * - 7 - Μ è, eg a very long treatment time at 20 ° C with regard to the result with regard to the development state of the silica gel with a significantly shorter treatment at eg 75 ° C are equivalent.

Accordingly, the maximum treatment time at a very low pH value and a very low treatment temperature will be about 48 hours. Of course, at higher PI values and / or higher treatment temperatures, much shorter treatment times are required.

Accordingly, according to the invention, in order to achieve the required low products of specific surface area and pore volume of less than 300, it is essential according to the invention to avoid aging when washing the silicic acid hydrogel, which is achieved by an appropriate reduction of the treatment temperatures or shortening of treatment is possible. Preferably, pE values below 4> especially below 3%.

applied. Torch pore volumes of less than 0.4 cmyg are preferred.

For the silicic acid gels of the invention, the surface area product, expressed in m / g, and specific pore volume expressed in cm 2 / g, is less than 300, preferably even less than 200. Typical values are 108 (surface 450 m / g and a pore volume of 0.24 cnr / g) or 52 (surface of 326 m / g, pore volume of 0.16 cnr / g). The lower the value for the product, the greater the abrasion performance of the silica gel. The abrasion power further increases with the particle size.

According to the invention, the washing of the silicic acid hydrogel preferably takes place semi-continuously, in such a way that the washing water is renewed each time after a suitable standing time. Gas jet mills, in particular vapor jet mills, are particularly suitable for grinding the silica particles. It is also possible to combine the grinding with a partial or almost complete drying such as this e.g. in DS-PS 1.036.220 after 7903956 fe - 8 - der ia ". The water content of the silica gels prepared according to the invention is preferably below 25% by weight.

Drying to a water content of less than 25% by weight should take place as soon as possible. The maximum drying time is about 2 hours, while the preferred drying in gas jet mills and in particular vapor jet mills takes only a few seconds. Dry more slowly, e.g. the use of circulating air or storage at room temperature for more than 24 hours, which can also be described as slow drying, does not lead to the silicic acid gels according to the invention. As can be seen from the examples, suitable drying temperatures are e.g. at 140 - 180 ° C (entering temperature of the hot air), however other temperatures, for example, below 100 ° C can also be used.

It is admittedly not yet possible to give a complete explanation of the course of the implementation of the method according to the invention. It is likely, however, that increasingly smaller surface values are observed with increasing younger developmental condition of the silica gels, because the structure still consists of little cross-linked silica units and is easily deformable. The shrinkage stress occurring during drying therefore has a particularly strong effect, so that the entire structure collapses and a significant part of the specific surface area originally present is lost. This interpretation is supported by the fact that with the younger developmental state the pore volume also decreases. At a certain stage of development (polycondensation and cross-linking degree), the gels apparently achieve such stability that excessive shrinkage is no longer possible and a skeleton so dense is no longer obtained.

The silicic gels prepared by the process according to the invention show a number of important advantages! 1) The dentin abrasion effect of the cleaning and polishing agent for toothpastes can be arbitrarily adjusted in the practically interesting areas in the preparation of the silicic gels in the preparation of the invention. The existing abrasive composition, which has proven favorable in itself, should not be changed, the dentifrice concentration and the particle size The dentin abrasion effect of the toothpaste is adjusted by selecting the appropriate RDS value, which in turn can be obtained by the corresponding process parameters.

2) The silicic acid gels according to the invention, as a cleaning and polishing agent, optionally exhibit unusually strong cleaning action, as the RDE values in the range of 200 to 300 and above make clear. Therefore, if desired, the concentration in the dental care agent can be reduced. In addition, transparent toothpastes can be prepared to a much greater degree of denitine abrasion than hitherto possible.

3) The silicic acid gels can be prepared according to the invention without loss of abrasion with a certain residual water content up to about 25% by weight, which allows for less solid-rich formulations, which, according to experience, are improved by a better aroma fullness and faster aroma. development characteristics.

4) Finally, toothpastes can be prepared with the optionally still-containing silicic acid gels according to the invention, which are characterized by very good storage stability and compatibility with a set of fluoride.

As the following examples further illustrate, practically silica gels with predetermined abrasion values can be achieved with the aid of the method according to the invention.

For the use of dental care agents it is particularly advantageous that the toothpaste preparation therefore contains, e.g. each time a silica gel with a high and low abrasion capacity can be made available with further corresponding properties, so that it is capable of practically achieving the desired abrasion by suitable combination of only two silica gels of the series. 79 0 8 95 § «10 ·· é .

value without having to change the further composition of the dental care product.

In order to obtain the desired consistency of the toothpaste, another silica gel of smaller particle size can be mixed with the silica gel according to the invention, which has a good thickening effect, but practically no abrasion effect.

The invention is further illustrated by the following examples.

The dentin abrasion is in each case indicated as RDE (radioactive dentin abrasion), which was determined by means of the method described in DE-AS 2.028.866 at a reference standard of 100 for calcium pyrophosphate (compare JJ Hefferren in J. Dental Research 5-3-73 (197 °) and 15 Procedure for Dentifrice Analysis of the Missouri Analytical

Laboratories, St. Louis, V.St.A.). Within the framework of the present application, surface is always understood to mean the surface determined according to the method of 3runauer, Emmet and Teller (BET method), which is indicated in m / g. The "pore volumes" were determined by the nitrogen method and are expressed in cm 2 / g. This refers to the pore volume in the pore diameter range ^ 600 i according to the Kelvin equation determined with nitrogen at 69.7% of the N ^ saturation pressure (compare EP Barrett et al., J. Am. Chem. Soc. 22 » 375 (1951))

Example I

Miniaturized samples of freshly gelled silica gel with an SiOg content of 18 wt.% And a sulfuric acid excess according to a normality of 0.425 were immediately washed by water exchange at a temperature of 50 - 65 ° C until reaching differently high pH values, respectively. until reaching certain conductivity values as a measure of the purity achieved. The silica gels were then immediately dried on a perforated grid by treatment with hot air from HO to 180 ° C to a water content of less than 3% by weight and in

78 0 8 9 5 S

- 11 - a hammer mill finely divided to average grain sizes of approximately 14 µm (volume average) determined with the Coulter counter. Before the gels were obtained, the properties were determined before comminution, such as the pH value, pore volume, surface, etc.

The data of the obtained silica gels are shown in Table A.

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While in the comparative tests 1 to 3 only JDA values of 145-166 were reached, the process according to the invention produces products with high RDA values, up to 275

Note that the RDA values increase inversely proportional to the surface product and the pore volume.

Paragon II

In a further series of experiments, silica gels were washed in the same manner by water changes in rapid succession at different purities, respectively at low pH values, at 10 different conductivity levels, but the subsequent reduction was carried out in a vapor jet mill. The silica gel was partly supplied to the mill in the form of a hydrogel with a water content of more than 60 fo, so that the drying of the corresponding samples took place simultaneously with the grinding. Some of the samples were, however, also dried to xerogel according to Example I, but then also micronized in the steam jet mill.

Throughout the test series, a grain size of about 4 yam, determined with the Coulter counter, was aimed for. The water content of the micronizates finally obtained were between 7.1 and 24.8 wt. Both the case occurred when a xerogel absorbed moisture again during the vapor jet grinding, so that the water content was subsequently 13.8 wt. 6 and 24.8 weight percent respectively. The HDA values were measured on all samples. Obtained values of the samples are summarized in Table B.

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Also to. this example shows that the HDA values are inversely proportional to the product from the surface and increased pore volume, i.e. the dentin abrasion effect is controlled by this property of the gel. Examples I5 and II furthermore show that very high HDA values (275, 316, 363) can be obtained, which was not to be expected according to the state of the art. Furthermore, both examples show that the particle size has an important influence on the dentine abrasion effect, as can be seen from a comparison of the value between example I and example II. In particular in example II it appears that at least for the silica gels according to the invention with a lower product from the surface and pore volume still important value for the dentin abrasion are obtained, even when the average particle sizes assume such small values as 2.3 µm. According to the prior art, such a small particle size could not be expected to have such a pronounced dentin abrasion effect, since dehydrated silicon dioxide with a particle spectrum of 2 to 20 µm is recommended.

Since the micronized silica gels of Example II exhibit noticeable and partly important water contents, the dentin abrasion effect apparently does not affect. Finally, it is very surprising that in both examples it is precisely those cleaning and polishing agents which have the highest HDA values which have the smallest area.

Example III

Following the procedure of Example I, silica hydrogels were prepared, however, only half of the samples were dried immediately after washing, while the other half was stored covered for 6 or 7 days at room temperature. After these storage periods, these samples were dried in the same manner. The numbers obtained are summarized in the following Table G.

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Sample 17 dried after storage belongs to the class of the known silica gels according to the prior art, while the silica gel according to the invention 16 was formed without aging. It is particularly interesting that the aging of the young hydrogel actually first creates a higher specific surface area, corresponding to the transition from one class to another. According to the general experience with silica gel prior art, higher transitions to lower specific surfaces when correspondingly effective treatments are intensified or extended.

Finally, it is known that drying in a high atmosphere saturated with water vapor leads to a small shrinkage of the silica gels. Sample 18 was dried at such circulating air conditions (drying cabinet, closed). Despite the drying carried out immediately, the formation of a seed gel according to the invention was prevented. The six-day storage works incidentally in the same direction (sample 19). Hst silica hydrogel was completely analogous to that prepared in samples 16 and 17.

20 Example 17

Silica gels were prepared according to Example I, however washing was carried out at various high temperatures with the addition of ammonia, so that prior art gels were obtained with surfaces in the region of 400 to about 650 m / g and pore volumes greater than 0 .75 cm 3 / g. After drying according to Example I, the xerogels obtained were ground in a vapor jet mill to average grain sizes of about 5. Finally, the dentin abrasion effect of the obtained micronizates was determined. The numbers obtained are summarized in Table D below.

7908956>.

- 18 -

TABS! D

Taste moisture grain, pH, pores, surface EST x REA

No. ($ H-0) size value volume plain PY

2 3 2

Coulter (cnr / g) (m / g) 5 (ym) ___ 20 1.5 5.3 7.0 0.75 633 475 47 21 5.0 5.75 5.6 0.98 510 500 19 22 2 .7 5.85 5.5 1.15 400 460 14

These micronized silica gels are seen in the prior art despite surfaces of 510, respectively

633 m2 / g give only low RDA values (19 »47), as it does not concern the class of" young "pebble gels, Yoorbeeld Y

A toothpaste was prepared in accordance with the following recipe with the silica gel of Test 10 of Example 2 prepared according to the invention.

TABS! E

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Silica gel from experiment 10, example II 10 20 Silica filler from the trade 10 (5096 HO) *

Airgel from the market 6.5

Sorbitol, 70 $ 35

Saccharin 0.2 25 Titanium dioxide pigment 1.0

Na lauryl sulfate 1.5 ITa carboxymethyl cellulose 1.6

MaOH, 50 $ 0.5 peppermint flavor 1.0 50 water 32.7 s no dentin abrasion / 908056 - 19 - 2r, a good consistency toothpaste was obtained, which received a very good rating in a storage test longer than 6 months. She showed a pH value of 7.8 and a density of 1.25 g / cm. The BDA value in the pasta was 5 average from three determinations 143.

At the application concentration of 10 ^ of this water-based cleaning polishing agent, the dentin abrasion effect obtained can be seen as very high. To obtain a comparison in this respect, 10 different market products were examined in the space 1977. The chemical composition of the abrasive and the BDA values determined on these pastes are shown in the table below. Some of the commercial products were represented by multiple samples.

15 TABLE · E

Toothpaste abrasive RDA

1 SiO 2 107

SiO 2 101

Si02 115 20 2 Si02 13 3 Sx02 104 4 Si02 13 5 A1203 138 6 A1203 144 25 7 CaCO3 50

CaCO3 49.

8 DCPs 117 dcp 95 9 'Ι, Μ.Ρ.χχ 63 30 10 DCP / CaC03 49 je s dicalcium phosphate xx = Ua-metaphosphate, insoluble 790395? - 20 -

In the toothpaste containing the gel according to the invention, the abrasive content is 9.3 $ (dry matter) and the highest RDA value found on the market is set. According to experience, the market products have a higher, partly importantly higher abrasive content.

79 0 8 95®

Claims (10)

Silica gel with an average particle size of Tan 1 to 50 µm, characterized by the following combination of characteristics: a) a surface area of 1 to 600 m / g, b) a pore volume of 0.05 to 0.5 cm 3 / g, C) a product of area in m ^ / g and pore volume in cm ^ / g ^ 240, preferably 200, d) an arithmetic pore diameter of 1.5 to 2.5 nm, and e) a water content of less than 25 wt. $. 2. Process for preparing silicic acid gels having an average particle size of 1 to 30 µm by gelling aqueous silicate solution and then washing, drying and grinding to the desired particle size, characterized in that the silica hydrogel formed is dissolved at pH values below 6 and at temperatures from about 0 to 10 ° C to a purity of about 90 to 99% by weight 9 wt. SiOg (based on the loss-on-ignition) and immediately thereafter to a water content of less than 25 wt. dries, the washing and drying conditions being adjusted so as to avoid aging, so that the silica gel has a surface area of 1 to 600 m / g and a pore volume of 0.05 to 0.5 µg / g as well as exhibits an arithmetic pore diameter of 1.5 to 2.5 nm. Process according to claim 2, characterized in that the washing of the silica hydrogel is carried out semi-continuously. 4. Process according to claims 2-5, characterized in that the silicic acid hydrogel formed is washed at pE values below 5 and temperatures from 0 to 60 ° C. Process according to claims 2-4, characterized in that the grinding and drying are carried out in a gas jet mill, preferably a vapor jet mill. 6. Process according to claims 2-5, characterized in that the grinding is carried out first and subsequently dried continuously in a suitable temperature. 7108956 λ - 22 - The use of the silicic acid gel according to claim 1 or "prepared according to the method of claims 2 -6, as a cleaning, grinding and polishing agent, in particular in dental care products. The use according to claim 7, not characterized in that the silica gel has an RSA value of at least about 200. 9. Use according to claim 7> with the. characterized in that two of the silicic gels of different surfaces and / or pore volumes are used to set the desired dentin abrasion effect in dental care agents. Use according to claims 7-9 *, characterized in that a silica airgel with an average particle size of about 1 to 10 µm is added to adjust the desired toothpaste consistency, 7908956