GB2038303A - Silica gel - Google Patents

Abstract

A toothpaste may contain as an abrading and polishing agent a silica gel having an average particle size of 1 to 30 microns and a) a surface area of 1 to 600 m<2>/g, b) a pore volume of 0.05 to 0.5 cm<3>/g, c) a product of surface area (in m<2>/g) x pore volume (in cm<3>/g) less than or equal to 240, d) a calculated pore diameter of 1.5 to 2.5 nm, and e) a water content of less than 25% by weight which is produced by gelling an aqueous silicate solution to form a silica hydrogel, washing the silica hydrogel to a purity of about 90 to 99% by weight SiO2 (based on the calcination loss-free substance) at a pH value below 6 and at a temperature of about 0 to 70 DEG C and grinding and drying the washed hydrogel so as to prevent ageing, the washing and drying conditions to prevent ageing being set so that the silica gel product has the desired properties. a

Description

SPECIFICATION Silica gel and process for preparing it The invention relates to novel silica gels and a novel process for the preparation of said silica gels having characteristics making them particularly suitable for use as cleaning, abrading and polishing agent, e.g. in toothpaste.

Much patent literature already exists regarding the use of fine-particles silicas as cleaning and polishing agents in tooth pastes. However, this prior art has not as yet clearly shown which characteristics a silica must have to be suitable as a polishing agent. Pyrongenic silicas, precipitated silicas and silica gels have been recommended having in each case different average particle sizes and which can have both a high and a low surface.

Thus, DAS 1,617,927 recommends silica xerogels with a particle size of 2 to 20 um and a surface area of at least 600m2/g. However, according to DOS 2,028,866 the surface area is not critical and can also be 300 to 500m2/g.

According to DOS 1,667,875 pyrogenic hydrophobic silicas are suitable which have a primary particle size of only approximately 0.01 to 0.03 pom. However, DOS 2,250,078 states that the particle size can also exceed 20 lem and the latter specification recommends silica xerogels with surface areas between 250 and 800m2/g, whose particle size is between 22 and 50, particularly 25 and 30 ttm.

DAS 2,446,038 attaches the greatest importance to the bulk density and shows that the abrasion capacity (determined as wire attrition) also increases with rising bulk density. However, it is not made apparent how the different bulk densities are obtained. In the examples, the surface area has no correlation with the abrasive action or with the bulk density.

The above statements show that hitherto no direct relationship has been found between the abrasion characteristics of silicas on the one hand and their other characteristics on the other. Not even the type of silica appears to be important because in addition to silicas of the gel type, also wet-precipitated silicas and pyrogenically produced silicas have been proposed.

According to the prior art, suitability within the class of silica gels is not even limited to completely dehydrated silica gels so-called xerogels, as is e.g. shown by DOS 2,704,504 which recommends gels with a water content of 15 to 35% having a particle size in the range 2 to 30 um. These water-containing gels have very good abrasion characteristics.

DOS 2,522,486 aims at silicas with a "low structure", which is understood to mean a low oil absorption and a high bulk density. Therefore, the pore volumes of these silicas are relatively low and this characteristic is considered important for the desired abrasion characteristics.

An optimum cleaning and polishing agent for toothpaste must ensure a given, relatively high degree of abrasiveness or cleaning capacity and thereby give the teeth a maximum brightess. In addition, the agent must give the toothpaste a favourable stable consistency, even during storage, must be compatible with the remaining toothpaste constituents and finally must not lead to corrosion of the packing material.

As is shown by the above discussion of the prior art, silica types have indeed been found which substantially fulfil these requirements. As is shown by DAS 1,617,927 and DOS 2,028,866, silica gels appear to generally produce a good polishing action in toothpastes. Other advantages are that transparent dentifrices can be produced (cf. U.S. Patent 3,538,230, DOS 2,250,078 and DOS 2,502,111) and that the compatibility with fluorides, added to dentifrices to prevent caries, is good (cf. DOS 2,153,821).

Although the cleaning action or more precisely the abrasive action has already been linked with the particle size and other structural data of the particular silicas, hitherto there has been no clear teaching as to how the abrasion action of a silica can be controlled. As a result, the particular silica has always had to be used in a specific concentration in order to set a given abrasiveness of the toothpaste. However, particularly high abrasion values cannot be reached, because the absorptivity of the formulations for porous, amorphous silicas is limited as otherwise the paste would become too rigid or too dry. If the silica concentration is changed to set a different abrasiveness, it becomes necessary at the same time to completely reformulate the entire formulation.Not only the consistency but also the storage stability, taste, compatibility and the like must be checked and adjusted, which is time consuming and complicated.

In addition, the particle size of the cleaning and polishing agent with an average value of about 1 Oum is already so large that it can be felt organoleptically by the user. Thus, a definite change to the particle size or the concentration leads to a definite displacement of the mouth feeling or sensation to the extent that the user thinks that it is completely different toothpaste. Therefore, it is not possible in practice to vary the abrasiveness of an existing toothpaste without modifying the other objectively or subjectively perceived propertias. For example, it is not possible to offer a given toothpaste formation with graded abrasiveness values for different user groups, e.g. young and old people.Such a procedure would in fact be particularly advantageous, because older people whose dentines are often exposed unprotected by gums require dentifrices with a lower abrasiveness.

The above-described difficulties are therefore particularly great, because it has not hitherto been possible to produce polishing and cleaning agents based on synthetic silicas which supply high or very high abrasion values and which in particular are suitable for use in dentrifices.

The main object of the invention is to provide silica gels and a process for the preparation of said silica gels, whose dentine abrasion action can be adjusted to given values practically independently of the particle size and within the normal range, independently of the concentration of use in the toothpaste. Afurther object of the invention is to provide silica gels with hitherto unachieved high abrasion values.

The invention therefore relates to silica gels with an average particle size of 1 to 30 um, characterized by the combination of the following features: a) a surface area (also termed "specific surface") of 1 to 600m2/g, b) a pore volume of 0.05 to 0.5cm3/g, c) a product of surface area (in m2/g) x pore volume (in cm3/g) of S 240, particularly S 200, d) a calculated pore diameter (also termed "mathematical pore diameter") of 1.5 to 2.5 nm, and e) a water content of less than 25% by weight.

Further the invention relates to a process for the preparation of silica gels with an average particle size of 1 to 30 cm by gelling aqueous silicate solutions and subsequent washing, drying and grinding to the desired particle size, characterized in that the silica hydrogel formed in washed to a purity of about 90 to 99% by weight SiO2 (based on the calcination loss-free substance) at pH values below 6 and at temperatures ofabout 0 to 70 C and is subsequently immediately dried, whereby to prevent ageing the washing and drying conditions are set in such a way that the silica gel has a surface area of 1 to 6Q0m2/g, a pore volume of 0.05 to 0.5 cm3/g, and a calculated pore diameter of 1.5 to 2.5 nm.

Further the invention relates to the use of a silica gel of the invention as cleaning, abrading angpolishing agent, particularly in dentrifices, and to a dentrifrice containing as an abrading and polishing agent a silica gel of the invention. A dentrifrice is a teeth-cleaning compositiosn, usually a toothpaste and will usually contain a humectant.

According to Ullmann's Encyklopadie der Technischen Chemie, 3rd edition, 1964, Vol 15, page 179, different silica gels can be obtained by washing hydrogels at different pH values. A distinction is made between microporous gel with a surface area of 600 to 800m2/g and a pore volume of approximately 0.3cm3/g and macroporous silica gel with a surface area of 250cm2/g and a pore volume of approximately 0.8cm3/g. Medium pored silica gels have values between those given above. By means of different pH values during washing and by using different washing temperatures, according to the prior art a continuous range of silica gels can be produced having rising surface areas with falling pore volumes.

As opposed to this, the silica gels prepared according to this invention and which are characterised by an approximately constant calculated pore diameter D=OF .103 = 1.5 - OF to 2.5 nm particularly 1.8 to 2.2nm (PV= pore volume in cm3/g; SA= surface in m2/g) and a product PV x SA S 300, particularly S 240 do not belong to the correlation group of known silica gels and instead form a separate class. Admittedly, the pore volumes are correlated with the surface area, but in the opposite way, i.e. with decreasing surface area the pore volumes decrease.These results also show that the surface area along is not adequate for characterising silica gels and instead the pore volume must additionally be taken into consideration for clearly describing the silica gels according to the invention.

The silica gels formed are washed at pH below 6. Thus, the starting and the effluent washing liquid is acid of pH below 6.

Preferably, the silica gels are washed at low pH values, particularly below 3 and at low temperatures of approximately 0 to 70"C, particularly 0 to 60"C until a purity level of 99.9% by weight SiO2, particularly 96 to 99.7% by weight SiO2, is obtained. The main impurity removed comprises sodium sulphate. As the silica gel is developed from a fully dispersed silica sol, whose colloid particles during gelling still have a very low molecular weight of approximately 6000, an important part for the control of the further development is played by the temperature and washing speed.In the case of a given pH, these parameters determine the time during which the low molecular weight silica polycondenses to higher molecular weights, so that the SiO2 gel structure reaches ever further advanced stages in the sense of the degree of polymerisation which, after drying, are generally recognised by a decreasing surface area. In the process according to the invention, a particularly early development stage is sought in this sense, because very "young" silica gels are to be produced. Thus, in addition to the low pH value and a not quite complete purity, low treatment temperatures are required and the treatment period overall must not be too long. For example, a very long treatment period at 20"C can be equivalent to a much shorter treatment at for example 75"C with regard to the development state of the silica gel.

Thus at a very low pH value and a very low treatment temperature the maximum treatment period would be about 48 hours. Of course, much shorter treatment periods are required at higher pH values and/or higher treatment temperatures.

For reaching the necessary low product of surface area and pore volume of below 300, it is therefore important according to the invention to prevent ageing during the washing of the silica hydrogel and this can be achieved by corresponding reduction of the treatment temperatures or shortening of the treatment.

Preferably, pH values below 4 and in particular below 3 are used.

For the silica gels according to the invention, the product of surface area, expressed in m2/g and pore volume, expressed in cm3/g is less than 300, normally less than 240 and preferably less than 200. Typical values are 108 (surface area of 450 m2/g and pore volume of 0.24 cm3/g) or 52 (surface area of 326 m2/g, pore volume of 0.16 cm3/g). The lower the value for the product, the greater the abrasiveness of the silica gel. In addition, the abrasion capacity rises with the particle size.

According to the invention, the silica hydrogel is preferably washed semi-continuously in such a way that after a corresponding interval the washing water is replaced.

Drying and grinding can be carried out in either order or simultaneously, the drying being carried out immediately after washing or after grinding. Fluid energy mills, particularly steam jet mills, are eminently suitable for grinding the silica gels. It is possible to combine grinding with a partial or substantially complete drying, such as is e.g. described in German Patent 1,036,220. The water content of the silica gels produced according to the invention is preferably below 25% by weight. Usually it is at least 0.1% by weight and a preferred range is 0.1 to 15% by weight.

The drying to a water content of below 25% by weight has to be carried out as quickly as possible, the maximum drying period being about 2 hours while the preferred drying in fluid energy mills and particularly steam jet mills lasts only a few seconds. A slower drying, e.g. due to use of re-circulated air or storage at room temperature for more than 24 hours which can also be defined as slow drying, does not lead to the silica gels of the invention. As shown in the examples suitable drying temperatures are, for example 140 to 180"C (inlet temperature of hot air) but other temperatures as for example below 100"C, can also be used.

It is not yet possible to explain fully the success of the invention. However, it is probable that with an increasingly young development state of the silica gels, ever lower surface areas are obtained, because the structure still comprises silica units which are only slightly cross-linked and easily deformable. The tension occurring during drying therefore has a particularly pronounced effect, so that the complete structure collapses and a considerable part of the originally present surface area is lost. This theory is supported by the fact that the pore volume also decreases the younger the development state. As from a given development stage (polycondensation and cross-linking level) the gels clearly reach a stability such that no further extreme shrinkage is possible and no such dense structure can be obtained.

The silica gels produced by the process of the invention have a number of important advantages: 1. The dentine abrasion action of the cleaning and polishing agent for toothpaste can be adjusted freely in the ranges which are of practical interest during the actual production of the silica gels. Thus, on using the agent according to the invention, in an existing, proven formulation, the abrasive agent concentration and particle size need not be changed. The dentine abrasion action of the toothpaste is adjusted by selecting the suitable RDA value, which is in turn obtained by the corresponding process parameters. If desired two different silica gels of the invention, having different surface areas and/or pore volume can be included in the toothpaste.

2. As cleaning and polishing agents, the silica gels according to the invention have an unusually high cleaning action, as is shown by RDA values of at least 200, usually in the range 200 to 300, but possibly above. If desired, the concentration in the dentifrice can be reduced. Furthermore, translucent toothpastes can be prepared having a much higher dentine abrasion action than hitherto possible.

3. According to the invention, the silica gels can be produced with a residual water content up to about 25% by weight, without loss of abrasive action. This makes possible formulations with a lower solids content, which experience has shown leads to a better fullness of flavour and a faster development of the flavour.

4. Finally, the silica gels according to the invention which may optionally still contain water permit the production of toothpastes characterised by a very good storage stability and compatibility with fluorides.

As will be further shown by the following examples, the process of the invention in practice permits the production of silica gels with predetermined abrasion values. It is particularly advantageous for the use in dentifrices that as a result the toothpaste manufacturer can be provided e.g. with a silica gel having both a high and a low abrasion capacity with otherwise coinciding characteristics, so that he is in a position through a corresponding combination of only two silica gels to set the desired abrasion value, without any other change to the dentifrice formulation being necessary.

In order to obtain the desired toothpaste consistency, another silica gel of a lower particle size and/or a silica aerogel of particle size 1 to 10 microns can be admixed with the silica gel according to the invention, whereby said additional silica gel has a good thickening action, but virtually no abrasive action.

Hereinafter, the invention is illustrated by means of examples.

Dentine abrasion is always defined as the RDA value (radioactive dentine abrasion), determined by means of the process described in DAS 2,028,866 with a reference standard of 100 for calcium pyrophosphate (cf.

J.J. Hefferren in J.Dental Research 55,563-573, 1976 and "procedure for dentifrice analysis" of the Missouri Analytical Laboratories, St. Louis, U.S.A.). In the present application "specific surface" and "surface area" mean the surface area determined according to the method of Brunauer, Emmet and Teller (BET method), which is given in m2/g. The "pore volume" is determined according to the nitrogen method and are given in cm3/g. This refers to the pore volume in the pore diameter range < 600 according to the Kelvin equation, measured with nitrogen at 96.7% of the N2 saturation pressure (cf.E.P.Barrett et al, J.Am.Chem.Soc, 73, 373,1951).

Example 1 Comminuted samples of freshly gelled silica gel with an SiO2 content of 18% by weight and a sulphuric acid excess corresponding to a normality of 0.425 were immediately washed by water changes at a temperature of 50 to 65"C until the varyingly high pH values were reached or until given conductivity was reached as a measure for the purity obtained. The silica gels were then immediately dried to water contents below 3% by weight on a perforated screen by treating with hot air at 140 to 1800C, followed by ultra-fine comminution in an impact plate mill to average particle sizes of about 14 cm (median volume) measured with the Coulter counter. The characteristics of the gels obtained such as pH value, pore volume, surface area, etc were determined prior to comminution.

The data obtained for the silica gels are given in Table 1.

TABLE 1 Test Washing Washing pH of pH of Conduc- Residual Surface Pore Pore Average RDA No. temp duration washing hydro- tivity moisture area vol vol. Coulter Value C (h) water gel of wash- (% by (m/g) (cm/g) x counter ing water weight) surface particle water area size ( m) (S/cm) at the end of washing 1 65 12 3.2 - 0.0014 < 3 700 0.40 280 14.7 166 2 50 20 3.0 - 0.0014 < 3 715 0.39 279 15.2 145 3 65 10 3.2 - 0.0014 < 3 670 0.37 248 14.1 145 4 50 7 5.2 5.4 - < 1 550 0.32 176 15.3 256 5 50 6 3.2 6.3 - < 1 560 0.31 174 15.2 275 Whereas in the comparative tests 1 to 3 RDA values of only 145 to 166 were obtained, the process according to the invention led to products with RDA values up to 275.

It is clear that the RDA values increase in inverse proportion to the product of surface area and pore volume.

Example 2 In a further series of tests, silica gels were washed in the same way by water changes in rapid sequence to different purities or at low pH values to different conductivities, but the immediately following comminution was performed in a steam jet mill. The silica gel was in part supplied to the mill in the form of hydrogel with a water content above 60% so that in the corresponding samples drying was performed simultaneously with grinding, However, in accordance with example 1, some of the samples were initially dried to xerogel and then micronised in the steam jet mill. Throughout the tests, a particle size of about 4am, measured with the Coulter counter was sought. The water contents of the micronisates which were finally obtained were between 7.1 and 24.8% by weight.There were cases where a xerogel again absorbed moisture during steam jet grinding so that the water content was subsequently 13.8% by weight and also that a hydrogel was not completely dried, so that the end product still had a water content of 21.6 or 24.8% by weight. The RDA values of all samples were measured. The data obtained for the samples are given in Table 2.

TABLE 2 Test Washing Washing pH of pH of Conduct- Purity Residual No. temp. durat- Washing hydro lvity of (% moisture Test ion(h) water gel washing (SiO2) (% by Material Coulter Surface Pore Porevol. RDA No. water weight supplied counter area vol x surface Value water) to mill. average (m/g) (cm /g) area part.

Size ( m) at the end of washing (S/cm) 6 50 6 1.8 3.0 1.4x10-2 99.7 8.0 Hydrogel 4.0 590 0.31 183 112 7 50 1.5 1.7 2.8 8.9x10-2 97.1 8.4 Hydrogel 2.3 552 0.31 171 116 8 50 6 1.8 2.9 1.5x10-2 99.9 7.1 Xerogel 5.5 538 0.26 140 138 9 50 1.5 1.7 2.8 2.9x10-2 97.1 21.6 Hydrogel 3.5 450 0.24 108 157 10 50 1.5 1.7 2.8 2.9x10-2 97.1 7.5 Hydrogel 4.2 362 0.17 81.6 211 11 50 1.5 1.7 2.7 3x10-2 96.7 5/.8 Xerogel 3.85 362 0.16 52 363 12 50 1.5 1.7 2.7 3.0x10-2 96.7 \13.8 Xerogel 5.5 326 0.16 52 316 13 60 5.5 3.2 3.2 8.9x10-4 99.8 12.0 Hydrogel 3.8 565 0.28 158 90 14 60 5.5 3.2 3.7 8.9x10-4 99.8 9.9 Hydrogel 5.9 596 0.31 185 90 15 60 5.5 3.2 3.7 8.9x10-4 99.8 24.8 Hydrogel 5.3 548 0.26 142 124 This example also revealed that the RDA values increase in inverse proportion to the product of the surface area and the pore volume, i.e. that the dentine abrasion action is controlled by this property of the gel.

Examples 1 and 2 also shown that very high RDA values (275,316,363) can be obtained, which was not to be expected on the basis of the.prior art. These two examples also show that the particle size has a significant influence on the dentine abrasion action, as is stressed by comparing the values of examples 1 and 2.

Example 2 particularly shows that at least for the silica gels according to the invention with a low product of surface area and pore volume, very considerable values for dentine abrasion are obtained, even if the average particle size is as low as 2.3 um. According to the prior art, such a pronounced dentine abrasion action could not be expected for such a low particle size, because hydrated silicas having an average particle size in the range 2 to 20 um are recommended.

The fact that micronised silica gels of Example 2 have significant and in part high water contents clearly does not impair the dentine abrasion action. Finally, it is very surprising that in both examples those cleaning and polishing agents having the highest RDA values have the smallest specific surface.

Example 3 In accordance with the procedure of Example 1, silica hydrogels were produced, but only half the samples were dried immediately after washing, hilt the other half was stored covered at ambient temperature for 6 or 7 days. These samples were only dried after being stored for this period. The data obtained are given in Table 3.

TABLE 3 Test Washing pH of hydrogel Total Storage Drying Pore Surface Pore No. temp. after each of 4 washing time type vol. area volume washing steps time(h) (days) (cm/g) (mg/g) x surface I. II. III. IV. area 16 2.3 3.2 3.8 5.8 7 - Fresh 0.34 590 204 air 17 " " " " " 6 " 0.58 730 423 50 C Re18 - 3.2 3.8 5.8 8 - circu- 0.73 627 458 lated air 19 " " " " " 6 0.84 580 487 Sample 17 dried after storage belongs to the class of known silica gels according to the prior art, whereas without ageing silica gel 16 according to the invention was obtained. It is of particular interest that during the ageing of the young hydrogel there was a higher surface area in the beginning, corresponding to the passage from one class to the other.According to general experience with silica gels according to the prior art, higher surface areas change into lower surface areas if correspondingly effective treatments are made more intensive or are extended.

Finally, it is known that drying in a higher water vapour-saturated atmosphere leads to a minor shrinkage of the silica gel. Sample 18 was dried under such ambient conditions (re-circulated air under closed conditions). The formation of a silica gel according to the invention was prevented despite the drying which was immediately performed. Storage for 6 days additionally acted in the same direction (sample 19). The silica hydrogel was produced in completely identical manner to samples 16 and 17.

Example 4 According to Example 1, silica gels were produced, but washing was carried out at varyingly high temperatures, accompanied by the addition of ammonia, so that prior art gels were obtained with surface areas in the range 400 to approximately 650m2/g and pore volumes above 0.75 cm3 g. After drying according to Example 1, the xerogels obtained were ground to average particle sizes of about 5 um in a steam jet mill.

Finally, the dentine abrasion action of the micronisates obtained was measured. The data obtained are given in Table 4.

TABLE 4 Test Moisture Particle pH Pore Surface Surface RDA No. (% water) size value vol. area area x Coulter (cm3,9) (m2;g) pore counter volume (item) 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 4p0 460 14 It is apparent that, despite surface areas of 510 or 633m2 g, these prior art micronised silica gel only supply low RDA values (19,47), because they are not "young" silica gels.

Example 5 Using-the silica gel prepared according to the invention of test 10 of Example 2, a toothpaste was prepared in accordance with the following form ulation : TABLE 5 % by weight Silica gel of test 10, example 2 10 Commerical sillica filler (50 % water)+ 10 Commerical aerogel+ 6.5 70 % sorbitol 35 Saccharin 0.2 Titanium dioxide pigment 1.0 Sodium lauryl sulphate t 1.5 Sodium carboxymethylcellulose ) 1.6 50% NaOH 0.5 Peppermint flavour 1.0 Water 32.7 + no dentine abrasion action.

A toothpaste with a good consistency was obtained, which when stored for more than 6 months was very flavourably evaluated. It had a pH value of 7.8 and a density of 1.25g/cm . As a mean value of threemeasurements, the RDA value in the paste was 143.

At the concentration of 10% water-containing cleaning and polishing agent the dentine abrasion action obtained is rated very high. In order to obtain a comparison in this respect, 10 different commerical products were investigated in 1977. The chemical composition of the abrasive and the RDA values found in these pastes are given in the following Table. There were several samples of certain of these commerical products.

TABLE 6 Toothpaste Abrasive RDA 1 SlO2 107 SiO2 101 SiO2 115 2 SlO2 13 3 SiO2 104 4 SiO2 13 5 Al203 138 6 Al203 144 7 CaCO3 50 CaCO3 49 8 DCP+ 117 DCP 95 9 I.M.P.++ / 63 10 DCP/CaCO3 49 + = dicalcium phosphate ++ = insoluble sodium metaphosphate

Claims (24)

1. A silica gel having an average particle size of 1 to 30 microns and a) a surface area of 1 to 600 m2/g, b) a pore volume of 0.05 to 0.5 cm3/g, c) a product of surface area (in m2/g) x pore volume (in cm3/g) less than or equal to 240, d) a calculated pore diameter (as hereinbefore defined) of 1.5 to 2.5 nm, and e) a water content of less than 25% by weight.

2. A silica gel according to claim 1 in which the product of surface area x pore volume is less than or equal to 200.

3. A silica gel according to claim 1 or 2 in which the product of surface area x pore volume is equal to or greater than 50.

4. A silica gel according to claim 1, 2 or 3 in which the water content is from 0.1 to 15% by weight.

5. A silica gel according to claim 1, 2,3 or 4 wherein the mathematical pore diameter is from 1.8 to 2.2.

6. A silica gel according to claim 1 substantially as described in any one of tests 4 to 16 of Examples 1 to 3.

7. A process for the preparation of a silica gel having an average particle size o 1 to 30 microns, which comprises gelling an aqueous silicate solution to form a silica hydrogel, washing the silica hydrogel to a purity of about 90 to 99 % by weight SlO2 (based on the calcination loss-free substance) at a pH value below 6 and at a temperature of about 0 to 70"C, and in either order or simultaneously drying the washed hydrogel quickly, and grinding it, the drying being carried out immediately after washing or after grinding, so as to prevent ageing, the washing and drying conditions to prevent ageing being set so that the silica gel product has a surface area of 1 to 600 m2/g, a pore volume of 0.05 to 0.5 cm3/g and a calculated pore diameter (as hereinbefore defined) of 1.5 to 2.5 nm.

8. A process according to claim 7, wherein the washing conditions are selected so that the silica gel has a surface area of up to 600 m' g and a pore volume of up to 0.4cm5 g.

9. A process according to claim 7 or 8, wherein the silica hydrogel is washed semi-continuously.

10. A process according to claim 7,8 or 9, wherein the silica hydrogel is washed at a pH value below 3 and at a temperature of 0 to 60cC.

11. A process according to any one of claims 7 to 10, wherein the grinding and drying are performed in a fluid energy mill.

12. A process according to claim 1 wherein the fluid energy mill is a steam jet mill.

13. A process according to any one of claims 7 to 12, wherein grinding is performed before drying, the grinding being followed immediately by continuous drying.

14. A process according to any one of claims 7 to 13, wherein the silica gel is dried to a water content below 25% by weight.

15. A process according to claim 7 substantially as described in any one of tests 4 to 16 of Examples 1 to 3.

16. A silica gel produced by a process claimed in any one of claims 7 to 15.

17. A dentifrice containing as an abrading and polishing agent a silica gel claimed in any one of claims 1 to 6 or in claim 16.

18. A dentifrice according to claim 17 containing a humectant.

19. A dentifrice according to claim 17 or 18 wherein the silica gel has an RDA value of at least 200.

20. A dentifrice according to claim 17, 18 or 19 containing two silica gels having different surface areas and or pore volumes.

21. A dentifrice according to any one of claims 17 to 20 which also contains a silica aerogel of average particle size 1 to 10 microns.

22. Use of a silica gel according to any one of claims 1 to 6 and 16 as a cleaning, abrading or polishing agent.

23. Use according to claim 22 for dentrifrices.

24. Process for the preparation of silica gels with an average particle size of 1 to 30 um, by gelling aqueous silicate solutions and subsequent washing, drying and grinding to the desired particle size, characteristic in that the silica hydrogel formed is washed to a purity of about 90 to 99 O by weight SiO2 (based on the calcination loss-free substance) at pH values below 6 and at temperatures of about 0 to 70"C and is subsequently immediately dried, whereby to prevent ageing the washing and drying conditions are set in such a way that the silica gel has a surface area of 1 to 600 m2 g, a pore volume of 0.05 to 0.5 cm3 g and a calculated pore diameter of 1.5 to 2.5 nm substantially as hereinbefore described.