NO143163B - PROCEDURE FOR THE MANUFACTURE OF AMORPHE, DEPOSITED SILICONE ACID PIGMENTS - Google Patents

Description

The present invention relates to a method for the production of synthetic precipitated silicon dioxides with a unique combination of physical and chemical properties.

It is well known that precipitated silicic acid pigments can be produced by acidifying an aqueous silicate solution with an acid such as sulfuric acid, hydrochloric acid etc. Examples of

such methods, which include an acidification of a silicate solution for the production of silicon dioxide-containing pigments, are described, among other things, in US patents 3,110,60 6

and 3,582,379. It is also known that the properties or characteristics of the products, and such products are often referred to as silicon dioxide, are dependent on the specific reaction conditions used, e.g. the pH at which the smearing takes place, the reaction temperature, etc. Nevertheless, previously known pigments are characterized by the following properties: strong structure, high wet cake moisture content, high oil absorption, low wear resistance, high surface area and low density in the packed state. On the basis of such characteristics

such as high oil absorption, high surface area, etc., such pigments have been used as reinforcing pigments in various types of rubber, in the production of paper, as moisture-regulating agents and the like.

However, the high wet cake moisture content is not an advantage, as the drying and filtering speeds are increased so that the total costs of the production of the product are increased. It can e.g. it is mentioned that in a normal production of silicic acid pigments (as these are defined above) both produc-

tet's moisture content in the wet cake (after filtering the precipitated reaction mass) be approx. 82%. This means that you can only gain approx. 18 parts dry pigment from 100 parts wet cake.

Furthermore, the low abrasiveness of known types of silicon dioxide and silicate pigments will make them unsuitable for many purposes.

It is e.g. it is well known that common synthetic precipitated silica types are suitable as polishing agents and abrasives in toothpaste compositions. See e.g. German patent 974.9 58,

French patent 1,130,627, British patent 995,351, Swiss patent 280,671 and US patent 3,250,680. US patent 3,538,230 be-

writes in particular that known amorphous silicon dioxides such as precipitated silicon dioxide, pyrogenic types of silicon dioxide and aerogels are unsuitable in toothpastes because they have too small a particle size and because they easily break down into small particles which result in poor cleaning ability.

In toothpaste, abrasive or polishing agents are intended to remove stains, food residues and bacterial coatings from the tooth surface. Ideally, the polishing agent should provide maximum cleaning action at acceptable abrasive levels and must be compatible in amounts of from 15 to 50% with the other ingredients in the toothpaste. Examples of known polishing agents include aluminum oxides, thermosetting resins (eg melamine-formaldehyde resins), zirconium silicates and various phosphate salts or compounds such as 3-tricalcium orthophosphate. Specific examples of phosphate polishing agents are described in US patents 3,169,096, 3,359,170 and 3,442,604.

The present invention relates to the production of finely divided precipitated types of silicon dioxide which, due to their new and unique properties, can be used as a polishing agent in toothpaste compositions. The invention is based on the discovery that precipitated silicon dioxides with high abrasiveness and with superior fluoride compatibility and cleaning properties equivalent to those of the aforementioned phosphates can be produced by simultaneously adding an acid and an alkali metal silicate to an aqueous reaction medium containing an electrolyte, such as sodium sulfate, after which a fine distribution of a finely divided pigment is produced at a constant pH. Important process variables include the SiC>2/Na2O molar ratio in the alkali metal silicate and the initial pH of the reaction medium. With regard to the latter, the pH in the aqueous reaction medium can be adjusted before adding the acid and the silicate so that it is the same or somewhat similar to the pH at which the sweep is to be carried out. As described in more detail below, the salt or electrolyte added to the reaction medium will preferably be a metal salt of the acid used for the acidification. If you e.g. If sulfuric acid is used, the electrolyte should consist of a metal sulphate such as sodium sulphate. Preferred electrolytes include sodium, lithium, potassium and ammonium sulfate, while the acid is

sulfuric acid. If sodium silicate is acidified with sulfuric acid,

sodium sulfate is the preferred electrolyte.

As mentioned above, the product that is obtained has a unique combination of physical and chemical properties which include high abrasiveness and high cleaning ability which corresponds to that known from known phosphate polishing agents or other cleaning agents. This discovery is unexpected and it forms the basis of the present invention.

The purpose of the invention is to provide synthetic, amorphous, precipitated silicon dioxides which can especially be used as abrasive or polishing agents in toothpaste compositions, which pigments have unexpected properties compared to previously known precipitated pigments, e.g. such properties as include low strength, high abrasiveness, high density in the packed state and high cleaning power in toothpastes.

As mentioned above, silicic acid pigments are usually prepared by acidifying an aqueous silicate solution with an acid.

In other words, a silicate solution is added to a reactor which is then acidified by adding an acid. More recently, however, as stated in US Patent No. 3,928,541, a method for the production of silicon dioxide-containing pigments with improved properties has been described in which a salt or an electrolyte is used to prepolymerize an alkali metal silicate solution before it is acidified with an acid. In accordance with this technique, an alkali or alkaline earth metal salt is first added to a solution of the alkali metal silicate, e.g. sodium silicate, after which this is prepolymerized by the addition of the salt.

Said silicate solution is then heated to a temperature

in the range from 37.7 to 93°C, preferably in the range from 65

to 80°C. The acidulant or acid, e.g. sulfuric acid,

is then added to the reactor until complete elimination of the silicate is obtained. The reaction or precipitation is carried out at a pH in the range from 6.5 to 11.0. At the end of the sweep, an excess of acid is preferably added to bring the pH of the pigment in the range from 5.5 to 6.5, after which the reaction mass is filtered, washed and dried. In a first out-

procedure described in the above-mentioned patent application, the entire solution of the salt-induced polymerized silicate becomes

put the reactor. In another embodiment, from half to two-thirds of the silicate is first added to the reactor, while the rest of the silicate is added together with acid.

In contrast to this, in the present invention one will first add an aqueous receiving medium, e.g. water, to the reactor. The electrolyte is then added to the water, and the pH of the aqueous electrolyte-containing media is then adjusted so that it becomes the same as that at which the precipitation of the pigment is carried out. In accordance with the present invention

otherwise, the pH should preferably be in the range from 8 to 10.4. Then, aqueous solutions of the acid and the silicate (as will be described in more detail hereinafter) are added simultaneously in such a manner and at such a rate that the reaction or precipitation occurs at the predetermined constant pH. After the pigment has been completely precipitated and somewhat dependent

of the use of the pigment, the pH is reduced or adjusted by the addition of acid. While the product from the aforementioned US patent no. 3,928,541 has high abrasiveness and can advantageously be used in toothpaste compositions, it has unexpectedly been discovered that the product

is prepared in accordance with the present process, has significantly improved cleaning ability or action, and this ability or action is the same or similar to that found in high-quality phosphate polishing agents for toothpastes.

In addition to this, when used in toothpaste compositions, the products have very good fluoride compatibility (i.e. the fluoride which is available in soluble form and which is not converted, for example, into an insoluble salt), and which also does not have the disadvantages that has in known phosphates, such as calcium pyrophosphate.

According to the present invention, it is provided

a process for the production of amorphous, precipitated silicic acid pigments with improved chemical and physical properties including high abrasiveness, suitable for use in toothpaste compositions, providing an aqueous reaction medium containing an electrolyte; adds an acid and an alkali metal silicate simultaneously to the aqueous reaction medium, continues the addition of the acid and the silicate to the aqueous reaction medium in such a way as to maintain a substantially constant pH until the precipitation of the pigment is substantially complete, where-

after recovering the precipitated pigment from the aqueous reaction medium, and this method is characterized by the fact that before the addition of the alkali metal silicate the pH is adjusted to

the aqueous medium to a predetermined value corresponding to that at which said pigment is to be precipitated.

The above-mentioned abrasiveness is measured in mg thread loss as follows

it appears in example 1 below.

It appears from the above that the starting compounds include an electrolyte, an alkali metal silicate and an acid. The term alkali metal silicate includes all known forms

of alkali silicates, e.g. metasilicates^ disilicates and the like. Particularly advantageous are water-soluble potassium silicates and sodium silicates. Due to relatively affordable prices, sodium silicates are preferred. Sodium silicates are effective in any composition where the molar ratio SiO2 to Na20 varies from 1.0 to 4^ In this respect, commercially available sodium silicate solutions are more or less polymerized depending on their silicon dioxide to sodium oxide ratio (SiO^/Na20). Thus, e.g. sodium metasilicate solution (molar ratio one) is essentially monomeric in nature, while water-glass (molar ratio 3.3) is both monomeric and polymeric in nature. As the silicon dioxide to sodium oxide mole ratio in the sodium silicates increases, the polymer to monomer ratio will increase for its silicate anions. While sodium silicates with a SiC>2/Na2O molar ratio of 1 to 4 can be used, particularly advantageous results are obtained if the SiO2/Na2O ratio is in the range from 2.0 to 3.3, more preferably in the range from 2.0 to 2.8.

Although commercially available silicate solutions may be more or less polymerized depending on their silicon dioxide to sodium oxide (SiO 2 /Na 2 O) ratio, it has been found that particularly advantageous and preferred results are obtained if a sulfate salt or electrolyte is added to the silicate solution before it is added with the acid to water - the electrolyte reaction medium. In other words, the silicate solution itself can be prepolymerized as described in US patent 3-928,541. In addition to this, instead of simply mixing the salt with the silicate solution, the silicate 1 itself can be a product as described in US patent no. 3,838,192 where silicon dioxide reacts with a hydroxide in the presence of a sulphate salt to thereby produce a polysilicate.

While the acidulant or acid is preferably a strong mineral acid such as hydrochloric acid, nitric acid and hydrochloric acid, it is also understood that other acids can be used such as organic acids, for example acetic acid, formic acid or carbonic acid can be mentioned, or salts of carbonic acid such as ammonium carbonate . As mentioned above, the acidifying agent and the silicate should preferably be in the form of dilute solutions. Preferred results are obtained if the acid solution has a content of from 10 to 25% by weight of acid based on the total weight of the solution. Particularly advantageous and thus preferred results are obtained if the concentration of the silicate solution is of the order of magnitude from 110 to 270 g/1.

The term electrolyte refers to ionic or molecular compounds that disassociate in solution so that ions or charged particles are formed. As used in the present invention, however, the electrolyte must be compatible with the acid and the silicate. Thus, if sodium silicate and sulfuric acid are used, the preferred electrolyte should be sodium sulfate. If one uses hydrochloric acid as the acid and again uses sodium silicate, then the electrolyte should be sodium chloride. If the silicate-acid combination is potassium silicate and sulfuric acid then the electrolyte should be potassium sulfate etc. Other examples of electrolytes include sodium nitrate, sodium acetate and the like. However, it should be noted that the electrolyte must not be a salt which will give a water-insoluble by-product with the precipitating pigment. An example of such is calcium sulfate. Thus, a calcium salt must not be used together with sulfuric acid. The amount of electrolyte should be in the range of 3 to 15% by weight, based on the weight of water initially added to the reaction. Usually, the temperature used during the precipitation is not critical, and can be the same as in the above-mentioned known methods. IN

in a preferred embodiment, the receiving medium should contain an electrolyte which is heated to a temperature in the range from 37 to 83°C before adding the silicate and the acid.

It appears from the above that no special equipment is necessary in the present method. However, the reactor should be equipped with heating devices, e.g. a heating jacket, to maintain the desired reaction temperature and should also have sufficient piping arrangements to produce strong currents in the body of liquid so as to avoid zones with a high concentration of the incoming reactants.

It is desirable to bring the reactants together as quickly as possible, as this promotes uniformity in the resulting products.

One should also provide storage vessels. connected to the reaction vessel, and the lines between these vessels should be equipped with current regulating devices. The reaction vessel can also be equipped with an outgoing line leading to a filter which can be of a conventional design known to everyone. After precipitation, the filtered mass is washed and dried. Such steps can also be carried out in commonly known equipment, and as such do not form one. part of the present invention.

Toothpaste compositions containing the manufactured product can, in the form of a paste, contain moisturizing compounds and binders so that the paste has a glassy consistency and good flowability. Glycerin, sorbitol, corn syrup, glucose and the like can be used as carriers. Examples of binders include gum tragacanth, sodium carboxymethyl cellulose and the like. The above substances and materials are well known and the same applies to specific compositions and ingredients for toothpaste compositions, and they are e.g. described in numerous publications such as US patents 2,994,642 and 3,538,230.

Before describing the examples, it should be noted that the term "structure" as used herein means the ability of a silica-containing material to retain water in the form of a wet cake.

When silicon dioxides, as mentioned above for ordinary precipitated silicon dioxides, can hold a high content of water, e.g. of the order of magnitude from 75 to 85%, then these are usually designated as high-structure silicon dioxides. Silicas that hold less than 75%, preferably in the range of 50 to 70% water in the form of a wet cake, are often referred to as low-structure silicas.

The following examples illustrate the invention.

Example 1

In this experiment, dry sodium sulphate was added to 40 l of water in an 800 liter reactor, so that the sodium sulphate concentration in the reaction medium was 10%. The pH of the medium was adjusted to 9.0 by adding sodium silicate. The reaction medium was then heated to 65°C. Sodium silicate was then added with a SiC>2 to Na2O molar ratio of 2.5 and with a concentration of

220 g/l along with sulfuric acid with an 11.4% concentration to the reaction medium at a rate of 756 ml/min. and 453 ml/min. respectively, so that a constant precipitation

pH of 9.0. The sodium silicate solution used in this example also contained 7% sodium sulphate which had been added to the solution before this was added to the reactor. After 30 minutes

the precipitation was complete. A surplus of

acid until the pH of the suspension was 5.4. The reaction suspension was heated at 77°<: for 20 minutes and then filtered, washed, dried and ground in the usual manner. The product produced in this example had a wet cake moisture content of 51%, a surface area of 173 m 2 /g, a packed density of 0.57 g/cm 3 and an abrasiveness (mg thread loss) of 70.2. It was further found that the product had a relative cleaning power (RCS) of 100, which is the same as the phosphate polishing agent described in US patent 3,359,170,

and also had superior fluoride stability.

Example 2

Example 1 was repeated except that the reaction temperature was maintained at 80°C while the precipitating pH was 10.0. The boiling temperature was 93°C. The reaction mixture was worked up

in a regular way. The product's properties correspond to those stated in example 1.

Example 3

Example 1 was repeated except that the reaction temperature was maintained at 88°C while heating the reaction mixture at 93°C. The mixture was processed in the usual way. The product's properties correspond to those found in example 1.

Example 4

In a series of samples the procedure of Examples 1 to 3 was repeated except that the precipitating pH, although kept constant in each sample, "was varied" from 8*0 to 10.4. The results were essentially the same except from finding that the specific properties could be regulated within predetermined limits by changing the pH. All products were low in moisture content and in the form of a wet cake, they were low in structure and had relatively low oil absorption and high abrasiveness compared to a control where ordinary precipitated silica was prepared by neutralizing 125 g/l of sodium silicate (40 1) with 11.4% sulfuric acid. The abrasiveness of the latter product was 2.5 compared to up to 167.8 (Example 3) for products prepared according to the present invention.

Example 5

The procedure from examples 1-4 was repeated except that nitric acid, hydrochloric acid, acetic acid and formic acid were used instead of sulfuric acid. The results were substantially the same as in Examples 1 to 4. The corresponding salts (ie sodium nitrate, sodium chloride etc.) were also used as the electrolyte first added to the reactor (instead of the sodium sulfate).

Example 6

In a series of samples, the same procedure as in examples 1 to 5 was used, except that aqueous sodium silicates with a molar ratio (SiO2/Na2O) in the range from 1 to 3 were used instead of the 2.5 silicate from examples 1 to 5. The results were generally significant the same as in Examples 1 to 5, except that it was found that using alkali metal silicates with a SiO 2 /Na 2 O molar ratio in the range of 2.0 to 2.8 gave products with superior properties (as defined above).

Example 7

The procedure from Examples 1 to 6 was repeated except that the salt used as the electrolyte in the aqueous receiving medium varied in concentration from 3 to 15% by weight (in increments of about 5%). The results were essentially the same as in the above examples).

Example 8

The procedure from Example 1 was repeated except that the sodium silicate solution did not contain any sulfate before it was added to the reactor. It was found that although the grinding ability was relatively high compared to the control (see example 4), the cleaning effect was approx. 85 (RCS) compared to 100 for the product from Example 1.

It is clear from the above-mentioned examples that the method according to the present invention produces silicon dioxides with a lower wet cake moisture content, lower structure, lower oil absorption, higher density in the packed state and higher abrasiveness than is found in commonly known products. In addition to this

(and this is really unexpected) the new products had a cleansing effect when used in toothpaste which is very high and very similar to what is found for known phosphates. The cleansing effect was approx. 50% higher than in products manufactured as described in US patent 3,928,541. The abrasiveness of the latter products and products manufactured according to the present invention are similar.

The method according to the present invention also leads to silicon dioxides with lower production costs than normally precipitated silicon dioxides. For example, it can be mentioned that the average wet cake moisture content (examples 1-8) for silicon dioxides produced via the present method is approx. 53% in contrast to 82% for ordinary silicon dioxide. This means that you can recover 47 parts of dry silicon dioxide from 100 parts of wet cake. Via the new method, 29 parts more dry silica can thus be obtained, and this corresponds to an increase of (29/18) x 100, or approx. 160%. The new method results in silicon dioxide with better drying and filtration speed and, consequently, significantly lower production costs than precipitated silicon dioxide produced in the usual known way.

It appears from the above that the present invention provides a simplified method for the production of silicic acid pigments with new and unique properties. You can also make use of modifications: As it e.g. is described in US patent no. 3,928,541, the refractive index in the precipitated pigment can be regulated by adding an adduct element (e.g. aluminium, magnesium or the like), thereby obtaining

an abrasive or polishing agent for a clear translucent

or transparent toothpaste composition. In such an embodiment, the acid is premixed with a solution of the adduct material (e.g. aluminum sulfate), after which the acid-metal-salt mixture is used to acidify the alkali metal silicate.

Blind trial

Example 1 above was repeated as the pH value for the reaction medium was adjusted during the precipitation. The following method was used.

40 liters of sodium sulphate solution were added to a 120 liter reactor. The concentration of sodium sulfate in the reaction medium was 10%. The pH value of the reaction medium was not adjusted to pH 9. Now sodium silicate with a molar ratio SiO2/Na2O of 2.5 and with a concentration of 220 g/l,

as well as sulfuric acid solution with a concentration of 11.4% added at the same time to the reactor. The rate of addition of the silicate was 756 ml/min. and the acid addition rate was controlled to precipitate a product with a pH of 9.0. The sodium silicate solution used in this blind experiment also contained

7% sodium sulfate. The reaction slurry was heated as indicated in Example 1. The product was filtered, washed, dried and ground in a conventional manner. The following data were obtained:

This blind test shows that when the pH value is adjusted during precipitation instead of before precipitation, the resulting product has a lower surface area. This surface area will make the product less desirable as an ingredient in a toothpaste composition.

Claims (6)

1. Process for the production of amorphous, precipitated silicic acid pigments with improved chemical and physical properties including high abrasiveness, suitable for use in toothpaste compositions, where one provides an aqueous reaction medium containing an electrolyte; adds an acid and an alkali metal silicate simultaneously to the aqueous reaction medium, continues the addition of the acid and the silicate to the aqueous reaction medium in such a way that a substantially constant pH is maintained until the precipitation of the pigment is substantially complete, after which the precipitated pigment is recovered from the aqueous reaction medium, characterized in that prior to the addition of the alkali metal silicate adjusts the pH of the aqueous medium to a predetermined value corresponding to that at which said pigment is to be precipitated. 2. Method according to claim 1, characterized in that said acid is selected from sulfuric, nitric and hydrochloric acid; the alkali metal silicate is selected from sodium, potassium and lithium silicate and the electrolyte consists of a sodium, potassium or lithium salt of the acid. 3. Method according to claim 1, characterized in that the alkali metal silicate has a SiC^/X^O molar ratio in the range 2.0-3.3 and where X is selected from sodium, potassium and lithium. 4. Method according to claim 1, characterized in that the precipitating pH is kept in the range 8-10.4. 5. Method according to claim 1, characterized in that the electrolyte is added to the aqueous reaction medium in amounts varying from 3 to 15% by weight based on the weight of the reaction medium. 6. Method according to claim 1, characterized in that the concentration of the solution of the silicate is kept in the range of 110-270 g/l and the acid solution has a concentration in the range of 10-25% by weight acid based on the total weight of the acid solution.