DE2145752C3 - Storage-stable preparations based on aqueous silicic acid solutions - Google Patents

Description

This invention relates to storage-stable preparations based on aqueous silica solutions containing a water-soluble organic polymer and silicon in an _{amount corresponding to an SiO 2} content of at least 0.2 percent by weight.

Preparations containing silica can be used instead of the generally burned Silica sol and alkyl silicate binders used as binders for the manufacture of ceramic molds will.

However, the silica gels fairly quickly in the preparations that have hitherto been available, which is why preparations of this type have so far not had any noteworthy practical technical use, e.g. B. as a binder, could find. Since preparations based on aqueous silica solutions, apart from their inadequate storage stability, represent products that can be used both as binders, especially for ceramic purposes, and for other purposes, e.g. B. in papermaking or for making carpets dirt-resistant, can be used with good success, the object of the invention is to provide preparations of the / Ut indicated at the outset which are distinguished from known preparations by better storage stability.

It has now been found that, surprisingly, the stability of silica solutions by certain water-soluble organic polymers can be improved.

In this context, it should be noted that that it is known to use water-soluble organic polymers, e.g. B. polyvinyl alcohols as organic binders or to use part of such binders in the production of ceramic moldings ("Ceramic Age" 34, Feb. 1960, pp. 25 to 26).

The invention thus relates to preparations of the type indicated at the outset, which are characterized in that are that the water-soluble organic polymer has a molecular weight of more than 4400 has.

Silica solutions forming the basis of storage-stable preparations of the invention are preferably prepared by bringing a dilute alkali metal silicate solution, usually a sodium silicate solution, into contact with a cation exchange resin in the hydrogen, as described, for example, in British patent specification 645 703. Here, a sufficient hui.τ proportion of alkali metal ions are removed so that there ·. I'product a pH less than 7 and preferably

ίο less than 6 at 15 ° C. More deta specialist known processes for the preparation of silica solutions, for example the acidification of a Alkali metal silicate solution with subsequent removal of the salt formed as a by-product, electrical

is osmosis of an alkali metal silicate solution or hydrolysis of silicon tetrachloride can also be used. Because of their inherent instability, it is difficult to obtain silica solutions _{that contain more than about 5 percent by weight SiO 2 , but SiO 2} concentrations of up to 8 or even 12 percent by weight can be achieved with the aid of special processes. For the production of preparations according to the invention, preference is given to

"For freshly prepared silica solution used, but a silica solution can also be used that has already been stored for a certain time, for example 1, 2 or 3 or even up to 5 days at room temperature.
Water-soluble organic polymers which can be used as stabilizers according to the invention include the polymers of vinyl-substituted heterocyclic nitrogen-containing compounds, for example N-vinylpyrrolidone, N-vinyl-C-alkylpyrrolidones, vinylpyridine-N-oxides,

Vinyl alkyl pyridine N-oxides and copolymers such compounds with each other or with other, preferably polar monomers, such as vinyl acetate, Alkyl acrylates and methacrylates and acrylonitrile. Specific examples of suitable water-soluble organic Polymers are

Polyvinylpyrrolidone,

Vinylpyrrolidone-vinyl acetate copolymers, polyvinyl-3-methylpyrrolidone,

Vinyl-3-methylpyrrolidone-vinyl acetate mixture

polymers,

Poly-2-vinylpyridine-N-oxide,
Poly-4-vinylpyridine-N-oxide,
4-vinylpyridine-N-oxide-vinyl acetate copoly-

merisate,

Poly-3-methyl-2-vinylpyridine-N-oxide,
Poly-ö-methyl ^ -vinylpyridine-N-oxide,
Poly-3-ethyl-6-vinylpyridine-N-oxide and
Poly-o-methyl ^ vinylpyridine-N-oxide

Methyl methacrylate copolymers.

Other water-soluble organic polymers that can be used are polyalkylene glycols, e.g. B. polyethylene glycols, polypropylene glycols and mixed polymeric glycols, which include both Containing ethylene and propylene units, sov / k water-soluble polyvinyl alcohols and water-soluble cellulose derivatives such as methyl cellulose.

The preferred water-soluble organic polymers are polyvinylpyrrolidone and poly-2-vinylpyridine-N-oxide and polyethylene glycols.

As already mentioned, the molecular weight of the water-soluble organic polymer must be higher be than 4400 and can for example be in the range before

4500 to 1000000. Preferably that is Molecular weight in the range from 5,000 to 200,000 and especially between 9,000 and 80,000. Particularly useful polyethylene glycols have a molecular weight in the range from 10,000 to 25,000 and particularly useful polymers and copolymers of heterocyclic nitrogen containing Vinyl compounds have molecular weights in the range of 35,000 to 50,000.

The weight ratio of SiO ₂ to water-soluble organic polymer in preparations of the invention is preferably in a range from 150: 1 to 2: 1, better 30: 1 to 3: 1 and in particular 15: 1 to 4: 1. The choice of the ratio depends primarily on the desired shelf life, because the stability of the preparations, i. H. the time required for gel formation during storage increases with the proportion of polymer. It has been found that preparations or silica solutions according to the invention which have ratios of 30: 1 to 3: 1, preferably 20: 1 to 8: 1, for example 15: 1 to 10: 1, are particularly useful and have a sufficient degree of stabilization while avoiding the use of an unnecessarily large amount of polymer.

The upper limit of the concentration of silica in a preparation of the invention such as this initially formed is determined by the difficulty noted above, concentrated silica solutions to manufacture. A typical stabilized formulation of the invention as initially formed may contain from 2 to 8 weight percent silica or less, usually from 2 to 10% Contain silicon dioxide. After stabilization, a certain concentration is possible, for example through Evaporation under reduced pressure or by reverse osmosis, whereby one contents up to possibly 10 or 12 percent by weight or even higher. For practical purposes it is it is desirable to obtain a silica concentration as high as that which is stable with reasonable storage stability is, so that preferred solutions at least 3, for example 3 to 8 and preferably at least 3,. \ contain, for example, 3.5 to 7 percent by weight silicon dioxide.

The pH of the solution should be less than 7.0, preferably less than 6.0 and in particular less be than 4.0, for example between 1.5 and 3.5.

The stability of the silica solutions is adversely affected by anions, which is why another A feature of preferred stabilized preparations of the invention is that the anion content is not is greater than 25 ppm, although in some cases up to 50 ppm can be tolerated. Deanionization can be effected by treating the silica solution with an anion exchange resin in the hydroxyl form either before or after stabilization.

The preparations of the invention can by adding the water-soluble organic polymer or a solution of the polymer in water or a water-miscible solvent to the aqueous Silica solution or vice versa are preferably prepared under such conditions that rapid mixing takes place. When the water-soluble organic polymer is added as such it is usually added as a finely divided powder. Suitable water-mix solvents are lower alkanols, e.g. B. ethanol or methanol, lower ketones, e.g. B. acetone, and ether, e.g. B. tetrahydrofuran and ethoxyethanol. Solutions of the water-soluble organic polymer used for the production of the preparations according to the invention, weight concentrations of, for example Have 0.5 to 50%. The polyalkylene glycols are expediently solid or as a solution, for example with

ίο a content of 5 to 50 percent by weight polymer added, while nitrogen-containing water-soluble organic polymers are preferred be added as a solution with a content of 0.5 to 8 percent by weight polymer.

! 5 A preferred embodiment <; according to the invention Preparations can be used as binders. These preparations are mixtures of (a) an inventively stabilized silica solution and (b) a silicon-containing binder as essential components.

The silicon-containing binder can, for example, be an Aquasol with an SiO _r content of 1 to 65 percent by weight, in particular 30 to 50 percent by weight, or an organosol with an SiO ₂ content of 1 to 50 percent by weight. The silicon-containing binder can be prepared by partial or total hydrolysis of an organic silicate such as an alkyl silicate or an alkyl polysilicate containing alkyl groups having 1 to 4 carbon atoms, e.g. B. ethyl or propyl radicals, are prepared in an aqueous alcoholic medium in the presence of a strong acid such as hydrochloric acid or sulfuric acid, according to British patents 926967 and 1 154 376 and British patent application 38 527/69. The silicon-containing binder can also be a mixture of a silica aquasol and a hydrolyzed organic silicate. The silicon-containing binder preferably has an SiO ₂ content of 5 to 55 percent by weight.

The preparations containing components (a) and (b) preferably have an SiO ₂ content of 2 to 45 and in particular 5 to 32 percent by weight.

The preferred preparations have a weight ratio of components (a) and (b) from 20:80 to 80:20, preferably 30:70 to 70:30 and in particular 40:60 to 60:40.

When a stabilized silica solution of the invention or a preparation containing the components

(a) and (b), as described above, contains as binders for the production of a ceramic material or object, for example a ceramic mold shell for metal casting, is used, the manner of use is essentially the same as in the case of conventional silicon-containing binders of the type indicated under (b) above. For the production of a ceramic casting mold shell, slurry and stucco or Layers of plaster applied to a model,

to wherein the slurries are mixtures of a particulate refractory material and a, expediently silicon-containing binders, e.g. B. a silica sol or a hydrolyzed alkyl silicate or - according to the invention - a stabilizing

6j th silica solution or binder preparation of the type described above can be used. The strength of the ceramic product after drying and burning depends on the ratio of connective

medium (as SiO ₂ ) to refractory material in the slurry. The optimum ratio will vary with the type and degree of refractory material and whether the slurry is used as the primary or secondary coating, but is usually in a range of 1: 7 to 1: based on weight. 35. In the manufacture of a ceramic casting mold, each slurry and stucco layer is allowed to gel and usually partially dry before the next is applied, the gelation can, if desired, be accelerated by applying a coating in an alkaline atmosphere containing a preparation of the invention as a binder such as ammonia.

Slurries of particulate refractories Materials in a stabilized silica solution or binder preparation of the invention can can also be used for cast coating for the production of metal bars. Metal bars are poured into molds that are flat shells of metal, usually cast iron, in their bases remain and are generally referred to as teams. If the surface of the If the team is not protected in any way, it suffers from rapid erosion, which in turn is due to it the irregular surfaces thereby formed causes the ingot to stick to the combination. It is therefore customary to provide the team with a protective covering, for which, as stated above, the slurries described above are excellent.

It has also been found that treated with a stabilized silica solution of the invention Paper has better grip (anti-slip properties) than untreated paper.

It was also found that with a stabilized according to the invention, optionally a surface-active Agent-containing silica solution treated carpets have better dirt-repellent properties than untreated carpets.

The invention is illustrated by the following examples.

In some experiments the silica solution was first treated with hydrochloric acid.

With the exception of the three experiments 19, 20 and 21, the polyvinylpyrrolidone solution became the silica solution in each case added. In experiments 19 to 21 the procedure was reversed, i.e. the procedure was reversed. H. the silica solution added to the polyvinylpyrrolidone solution.

Further details of the experiments can be found in Table 1.

Table 1 example 1

45

This example describes the production of preparations or silica solutions according to the invention.

4 1 aqueous sodium silicate solution (specific weight 15.5 ° C / 15.5 ° C = 1.042; SiO ₂ : Na ₂ O = -3.3: 1; 4.0 percent by weight SiO ₂ ) were through a with an anion exchange resin in the H ⁺ -form filled column and the resulting silica _{solution (SiO 2} content = 3.6 percent by weight) percolated under pressure from a Watson-Marlow flow regulator through a column filled with a cation exchange resin in the OH form, whereby a chloride-free silica solution is obtained obtained, the pH of which was 3.0.

Solutions of polyvinylpyrrolidone (molecular weight (J ₀ weight approximately 44,000) in water having a pH of 3.0-4.0 were mixed with separate (100 ml) portions of the chloride-free silica solution at room temperature vigorously stirred with a magnetic stirrer. The pH of the silica solutions did not change when mixed with the polyvinylpyrrolidone solutions. When mixed, turbidity occurred immediately.

Mil 100 ml of silica solution water Other Gelation mixed material (ml) additions time of Ver 2.0 Product search Polyvinyl 1.5 0 at 25 C No. pyrrolidone 1.5 0 (Weeks) (G) 1.0 0 5 1 0.03 0.5 0 4th 2 0.03 4.0 0 4th 3 0.03 4.0 0 4th 4th 0.03 2 ml N / 4th 5 0.03 3.0 10 HCl 7th 6th 0.06 2.5 0 2 7th 0.06 2.0 0 6.0 0 6th 8th 0.06 4.5 0 5 9 0.06 4.5 0 5 10 0.06 4.5 0 10 11th 0.09 2 ml N / 9 12th 0.09 3.0 10 HCl 9 13th 0.09 2.5 0 2 14th 0.09 9.0 0 6.0 0 H 15th 0.09 6.0 0 8th 16 0.09 6.0 0 23 17th 0.18 8.8 0 21 18th 0.18 6.0 0 20th 19th 0,! 8 0 28 20th 0.24 32 21 0.26 > 35 22nd 0.30

Except for Runs 3 and 13, the silica solution was used within 8 hours of their preparation from sodium silicate solution. the Experiments 3 and 13 were performed using silica solution that had been in place for 2 days was previously made from sodium silicate solution.

The specific surface areas of the fresh silica and that obtained from experiments 18 and 19 Products were made using one of Sears, Anal. Chem. 1956, 28, 1981 Titration method determined. The following results were obtained:

specific surface
(nvVg SiO ₂ ) Silica 1312
1291
1271 Product from experiment 18
Product from experiment 19

S "

Example 2

This example describes the production of a further preparation according to the invention.

Experiment 18 of Example 1 was repeated on a larger scale to produce 3 liters of preparation. The product had the following properties:

IO

«5

20th

47 nanometers (nm)

Appearance cloudy

SiO ₂ content (percent by weight). 3.4
SiO ₂ / polyvinylpyrrolidone

Weight ratio 20/1

pH at 20 ^° C. 3.0

Specific surface area (titration) 1271 m ² / gSiO ₂

Specific surface after 8 weeks

at room temperature 1271 m ² / gSi0 ₂

Gel time (storage at

Room temperature) 20 weeks

Particle size determination
by means of the scattered light method

Weight average

knife (90 'spread) ..
Weight average

knife (by Dissy- * S

metry) 130 nm

Weight average

knife (according to the room

plot method) 184 nm

These results indicate that the particles are not spherical.

Example 3

This example describes the manufacture of a ceramic mold shell using the Preparation of example 2.

A ceramic mold shell was made using a slurry of 4 ml of a nonionic wetting agent (condensation product from Octylphenol and ethylene oxide), 8 ml of n-octanol, 2375 g of zirconium sand powder with a particle size of 15 to 70 μπι and 800 ml of the preparation of Example 2 produced. A wax model (11.43 x 2.54 x 0.32 cm) was made dipped into the slurry, then with fine calcined kaolin clay powder with a grain size of 0.19-0.55mm coated and the coating allowed to air dry for 1 hour. Then were on analogous way four more layers of coarse calcined kaolin clay (grain size 0.55 to 1.17 mm) applied one after the other. The 5-layer casing was then allowed to air dry for 24 hours and then fired at 1000 ° C for 1 hour.

Example 4

The preparation described in example 2! 1500 ml) was mixed with Saikaaquasol (1500 ml), which had an SiO ₂ content of 30 percent by weight, a density of 1.204 and a specific surface area of 250 m ² / g SiO ₂ , to form a preparation which had an SiO ₂ content of 17.7 percent by weight

Example 5

This example describes the manufacture of a ceramic mold shell using the Preparation of Example 4.

A first and a second slurry, the composition of which is given in Table II, were prepared from the preparation of Example 4 (hereinafter referred to as A). For comparison purposes, silica aquasol, as was used in Example 4, was diluted with water to give a silica aquasol (hereinafter referred to as B) with an SiO ₂ content of 17.7 percent by weight, from which a first and a second slurry with the one also from tables according to the composition.

Table II Final viscosity Second viscosity First composition IBug 4 B Mug4B 118 sec. 115 sec. A (500 ml) Slurry Wetting agent *) composition (4 ml) n-octanol A (500 ml) (8 ml) Wetting agent *) Zircon sand **) (2 ml) (2.5 kg) 31 sec. n-octanol 30 sec. B (500 ml) (4 ml) Wetting agent *) Kaolin clay ***) (4 ml) (800 g) n-octanol B (500 ml) (8 ml) Wetting agent *) Zircon sand **) (2 ml) (800g) n-octanol (4 ml) Kaolin clay ***) (800 g)

*) As in example 3.
") As in example 3.

***) Calcined kaolimony powder with a grain size of approx 50 to 75; im

The slurries made from A had a shelf life of approximately 2 weeks.

Using the slurries from Table II, wax models (11.43 × 2.54 0.32 cm) coated by dipping the model in one of the slurries and then either fine or coarse burnt kaolin clay (see Example 3). Then each coating was left air dry for 2 hours before applying the next coat. The layered bowls were then allowed to dry in the air for 24 hours

The wax was then removed from the trays and the flexural strength made from it ceramic strip determined by a process in which increasing pressure is applied to the center of a 7.62 cm long field of the 2.54 cm wide kera

«0 mix strip is applied until the strip breaks. The thus their flexural strength ceramic strip to be tested have previously been fired at 1000 ⁰ C and had a thickness of 0.5 to 0.75 cm Description of the tested Gußfornwbalen un <flexural strength obtained are given in Table II. The given flexural strengths are in each case average values from measurements on six test strips.

Table HI

Preparation ungcn.
those used to manufacture the.
Aufschliim were used

For making the shell
coatings used

Flexural strength

after before burning | Burn

at 1000 C

1 coating first

Slurry / finer

Kaoiinton +

4 layers second

Slurry / finer

Kaolin clay

1 coating first

Slurry / finer

Kaolin clay +

4 layers second

Slurry / coarser

Kaolin clay

1 coating first

Slurry / finer

Kaolin clay +

4 layers second

Slurry / finer

Kaolin clay

1 coating first

Slurry / finer

Kaolin clay +

4 layers second

Slurry / coarser

Kaolin clay

kp / cnr

7.3

3.3

12.1

7.7

kp / cm ²

24.5

12.6

37.1

28.5

These results show that preparation A of preparation B in the manufacture of ceramic Mold shells is superior.

Example 6

This example describes the manufacture of a ceramic mold shell using the first and second slurries of Example 5 containing Preparation A and the preparation a metal casting using these molds.

A total of 6 layers were applied to a wax model, a first layer from the first Slurry and fine kaoi clay and five more layers of second slurry and coarse kaolin clay. Each coating was allowed to dry for 2 hours before the next applied became. The layered casing was last dried in a wind tunnel for 4 days.

Then the wax was removed from the casting mold in a steam autoclave and the mold was kept at 1000.degree ! Hour burned.

A cast stainless steel was performed using a mold temperature of 1000 ^c C and a temperature of the molten metal of 1,600 ⁰ C. The casting was satisfactory and free from mold deposits, cracks and deformations.
5

Example 7

This example describes the production of a preparation according to the invention.

ίο An aqueous sodium silicate solution (specific weight 15.5 / 15.5 = 1.073; SiO ₂ : Na ₂ O = 3.3: 1; SiO ₂ content = 6.6 percent by weight) by a cation exchange resin in the H ⁺ -Form filled column and the resulting silica solution (specific weight 1.038) then percolated under pressure through a column filled with an anion exchange resin (in the OH form), a chloride-free silica solution with a pH of 3.1 and a specific gravity of 1.037

was obtained. 6 ² / j 1 of the silicic acid solution thus obtained were mixed with IV ₃ 1 of an aqueous solution of polyethylene glycol (10 percent by weight; molecular weight 20,000) was added at room temperature under vigorous stirring, the stabilized silicic acid solution thus obtained had a SiO ₂ content of 5.9 weight percent and a storage time of more than one year

Example 8

This example describes the use of a preparation according to the invention for the production of Paper with improved grip.

An aqueous silica solution stabilized with polyethylene glycol (molecular weight 20,000) and having an SiO ₂ content of 4 percent by weight and in which the weight ratio of SiO ₂ to polyethylene glycol was 20: 1 was prepared. This solution (0.5 ml) was mixed with water (1 ml) and the mixture was sprayed onto a piece of kraft paper (22.8 x 7.6 cm).

The application ratio was 0.012 g mixture / dm ² . The angle of slip of the treated paper was 31, while that of the untreated paper was W on average.

Example 9

This example describes the use of a

Preparation according to the invention for improving the dirt-repellent properties of carpets.

It was an aqueous silica solution according to

Example 8 made. This solution (33 g) was mixed with a wetting agent (0.2 ml) and the mixture sprayed onto carpet samples. A mixture of sieved vacuum cleaner protection (350 g) and carbon black (50 g) in an amount of 0.27 g / dm ^{2 was then added} to the

Rubbed in carpet samples. The carpet samples were then shaken and cleaned. The with the invention Afterwards, parts of the carpet that had been pre-treated were much cleaner than others, parts used for comparison that have not been pretreated

had been.

Claims (3)

Patent claims: 1. Storage-stable preparations based on aqueous silica solutions containing a water-soluble organic polymer and silicon in an _{amount corresponding to an SiO 2} level of at least 0.2 percent by weight, characterized in that the water-soluble organic polymer has a molecular weight of more than 4,400 2. Preparation according to claim 1, characterized in that it is used as a water-soluble organic Polymer is a polyethylene glycol with a molecular weight in the range of 10,000 contains up to 25,000. 3. Preparation according to claim 1, characterized in that that they are a polymer or copolymer as a water-soluble organic polymer a vinyl substituted heterocyclic one. Nitrogen-containing compound with a molecular weight in the range of 35,000 to 50,000.