DE2145752B2 - Storage-stable preparations based on aqueous silica 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, inter alia, instead of the generally known Silica sol and alkyl silicate binders used as binders for making ceramic molds will.

However, the silica gels fairly quickly in the preparations known up to now, which is why so far Such preparations have no noteworthy practical-technical use, e.g. B. as a binder, could find. Since preparations based on aqueous silica solutions, apart from theirs Inadequate storage stability, represent products that act both as binders, in particular for ceramic purposes, as well as for other purposes, e.g. B. in papermaking or dirt-proof Equip before. Tcppichen, in itself with good Applying success, the invention is based on the object of preparing preparations of the types specified at the outset Kind to make available, which is compared to known preparations by better Excellent 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 it is known to use water-soluble organic polymers, z. B. polyvinyl alcohols as organic binders or components of such binders in the To use 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 become preferably prepared by using a dilute alkali metal silicate solution, usually a sodium silicate solution, with a cation exchange resin in the hydrogen form, such as in the British Patent 645,703 described brings in contact. A sufficiently high proportion will be used the alkali metal ions are removed so that the product has a pH of less than 7 and preferably

ίο less than 6 at 15 ° C. More to the expert 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

osmosis of an alkali metal silicate solution or hydrolysis of silicon tetrachloride can also be used will. Because of their inherent instability, it is difficult to make silica solutions. to obtained that is greater than about 5 percent by weight

μ SiO ₂ contain, but with the help of special processes SiO ₂ concentrations of up to 8 or even 12 percent by weight can be achieved. For the production of preparations according to the invention, preference is given to

'' used freshly prepared silica solution,

however, it is also possible to use a silica solution which has already been stored for a certain time, for example 1, 2 or 3 or even up to 5 days at room temperature.
To water-soluble organic polymers that

can be used as stabilizers according to the invention may 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, Alk \! Acr> late and methacrylate and acrylonitrile. Specific Examples of suitable water-soluble organic polymers are

Polyvinylpyrrolidone,

Vinylpyrrolidone-VinylaceUtmischpolymerisiMe, Polyvinyl-3-methylpyrrolidon,
Vinyl-S-methylpyrrolidone-vinyl acetate mixture

pulymerisate,

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

Methyl methacrylate copolymers.

Other water-soluble organic polymers that can be used are polyalkylene glycols, z. B. polyethylene glycols, polypropylene glycols and mixed polymeric glycols, which include both Contains ethylene and propylene units, as well as water-soluble polyvinyl alcohols and water-soluble « Cellulose derivatives such as methyl cellulose.

The preferred water-soluble organic poly merisate are polyvinylpyrrolidone, poly-2-vinylpyri Din-N-oxide and polyethylene glycols.

The molecular weight of the water-soluble organic polymer must, as already mentioned, be high than 4400 and can for example be in the range voi

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 of 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, ie the time required for formation during storage, increases with the proportion of the polymer. It has been found that preparations or silica solutions according to the invention which have ratios from 30: 1 _M to?: 1, preferably from 20: 1 to 8: 1, for example 15: 1 to 10: 1, are particularly useful and have a sufficient degree of stabilization have, while avoiding the use of an unnecessarily large amount of polymer. _2S

The upper limit of the concentration of silica in a preparation of the invention such as this initially formed is given by the above Difficulty determined, 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 Vei steam 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 3.5, 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 laning 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 Sa is usually added as a finely divided powder. Suitable water-miscible 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 a content of 5 to 50 percent by weight polymer added, while containing nitrogen water-soluble organic polymers, preferably as a solution with a content of 0.5 to 8 percent by weight Polymer are added.

A preferred embodiment of preparations according to the invention can be used as a binder will. These preparations are mixtures of (a) a silicic acid solution stabilized according to the invention and (b) a silicon-containing binder as essential ingredients.

The silicon-containing binder can, for example, be an Aquasol with an SiO ₂ 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 complete 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 sol and a hydrolyzed organic silicate. The silicon-containing binder preferably has an SiO _r 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 especially 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 a binder for the production of a ceramic material or Object, for example a ceramic mold shell for metal casting, is used, corresponds the manner of use is essentially that of conventional silicon-containing binders, the above under (b) specified type, applied. So are used to manufacture a ceramic mold shell alternately slurry and stucco or Layers of plaster applied to a model, 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 stabilized 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. While the optimal ratio will vary with the type and nature of the refractory material and whether the slurry is used as the primary or secondary coating, it will normally be in the range of 1: 7 to 1: by weight. 35. In the manufacture of a "ceramic mold shell, each slurry and stucco layer is allowed to gel and usually partially dry before applying the next, the gelation being accelerated, if desired, by applying a coating in an alkaline atmosphere containing a formulation of the invention as a binder, like ammonia.

Slurries of particulate refractories in a stabilized silica solution or binder preparation of the invention can also be used for cast coating for manufacture used by metal ingots. Metal ingots are poured into molds that are made from shallow shells Metal, usually cast iron, are preserved in their bases and generally as harnesses are designated. If the surface of the team is not protected in any way, it suffers from rapid erosion, which in turn due to the irregular surfaces formed by it Sticking of the ingot to the team. It is therefore common to use the team with a To provide protective covering, for which, as indicated above, the slurries described above 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.

example 1

This example describes the production of preparations or silica compounds according to the invention 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 was added to the silica solution in each case. In the tests 19 Ws 21 the procedure was reversed, i.e. the silica solution added to the polyvinylpyrrolidone solution.

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

Table 1

45

solutions.

4 1 aqueous sodium silicate solution (specific weight 15.5C / 15.5 ° C = 1.042; SiO ₂ : Na ₂ O = 3.3: 1; _so 4.0 percent by weight SiO ₂ ) were 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 was obtained, whose pH was 3.0.

Solutions of polyvinylpyrrolidone (molecular weight approximately 44,000) in water that have a pH from 3.0 to 4.0 were mixed with separate (100 ml) portions of the chloride-free silica solution at room temperature mixed. The reaction mixtures were stirred vigorously with a magnetic stirrer. The pH the silica solutions did not change when mixed with the polyvinylpyrrolidone solutions. Turbidity occurred immediately upon mixing.

With 100 ml of silica solution water Other Gelling 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 11 0.09 2 ml N / 9 12th 0.09 3.0 10 HCI 9 13th 0.09 2.5 0 2 14th 0.09 9.0 0 6.0 0 8th 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 0.18 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 surfaces of the fresh silica and that obtained from Experiments 18 and 19 Products were made using one of Sears, Anal. Chem. 1 < > 56, 28, 1981, described titration method determined. It became the following Get results:

Silica

Product from experiment 18
Product from experiment 19

specific surface

1312 1291 1271

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:

Appearance cloudy

SiO ₂ content (percent by weight). 3.4 Si0 ₂ / polyvinylpyrrolidone

Weight ratio 20/1

pH at 20 ^° C. 3.0

Specific surface (titration) .... 1271 m ² / gSi0 ₂ specific surface after 8 weeks

at room temperature 1271 m ZgSiO ₂

Gel time (storage at

Room temperature) 20 weeks

Particle size determination using the scattered light method

Weight average

diameter (90 ° scatter) ... 47 nanometers (nm) weight average

knife (by Dissy-

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 produced using a slurry of 4 ml of a nonionic wetting agent (condensation product of octylphenol and ethylene oxide), 8 ml of n-OctanoU, 2375 g of zirconium sand powder with a particle size of 15 to 70 μm and 800 ml of the preparation of Example 2. A wax model (11.43 x 2.54 x 0.32 cm) was dipped into the slurry, then coated with fine calcined kaolin clay powder with a grain size of 0.19 to 0.55 mm and the coating allowed to dry in air for 1 hour . Then four further layers of coarse calcined kaolin clay (grain size 0.55 to 1.17 mm) were applied one after the other in an analogous manner. The casing was then 5schichtige to air dry for 24 hours and then allowed to baked for 1 hour at 1000 ⁰ C.

Example 4

The preparation (1500 ml) described in Example 2 was mixed with silica aquasol (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 mVgSiO 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 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 same from Table II can be seen composition were prepared.

Table II

«5 first 20th A (500 ml) viscosity Second viscosity Slurry Wetting agent *) Mug 4 Β! Slurry Mug4B) composition (4 ml) composition n-octanol 118 sec. 115 sec. ²⁵ (8 ml) Zircon sand **) A (500 ml) (2.5 kg) Wetting agent *) B (500 ml) (2 ml) Wetting agent *) n-octanol (4 ml) (4 ml) n-octanol 31 sec. Kaolin clay ***) 30 sec. (8 ml) (800 g) 35 zircon sand **) B (500 ml) (800g) Wetting agent *) (2 ml) n-octanol (4 ml) Kaolin clay ***) (800 g)

·) As in example 3.
* ·) As in example 3.

· **) Calcined kaolin clay powder with a grain size of approx 50 to 75 μm.

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 x 0.32 cm) coated by dipping the model in one of the slurries and then either fine or coarse burnt kaolin clay cf. Example 3)

applied. The coating was then allowed to air dry for 2 hours before the next coating was applied. The 5-layer trays were then air dried for 24 hours calmly.

The wax was then removed from the shells and the flexural strength of egg made from it ceramic strips according to a process determined! with increasing pressure on the center 7.62 cm long field of the 2.54 cm wide kera

Mix the strip is applied until the strip

breaks. The one to be tested for its flexural strength

ceramic strips were previously ^{fired at 1000 r} <and had a thickness of 0.5 to 0.75 cc

Details of the tested mold shells un

the flexural strength obtained are given in Table Γ. The given flexural strengths are sir average values from measurements on secl test strips.

409523/3

Table III

Preparations,
those used to manufacture the
Notices were used

For making the shell
coatings used

before burning

kp, cm-

24.5

12.6

37.1

B 1 coating first 7.3

Slurry / finer
Kaolin clay +
4 layers second
Slurry / finer
Kaolin clay

B 1 coating first 3.3

Slurry / finer
Kaolin clay +
4 layers second
Slurry / coarse kaolin clay

1 coating first 12.1 slurry / finer
Kaolin clay +
4 layers second
Slurry / finer
Kaolin clay

1 coating first 7.7 slurry / finer
Kaolin clay +
4 layers second
Slurry / coarse kaolin clay

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

Example 6

Flexural strength

me

the burning

at 1000 "C

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 kaolin 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 5-layer 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 heated at 1000 ° C Hour burned.

A casting of stainless steel was carried out using a mold temperature of 1000 ° C and a temperature of the molten metal of 1,600 ⁰ C. The casting was satisfactory and free of mold deposits, wrinkles, cracks and deformations.

Example 7

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

An aqueous sodium silicate solution (specific weight 15.5715.5 ^s = 1.073; SiO ₂ : Na ₂ O = 3.3: 1; SiO ₂ content = 6.6 percent by weight) was filled ^{with a cation exchange resin in the H + form} 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 weight of 1.037

was obtained. 6 ^{_2/3 of} 1 of the silicic acid solution thus obtained 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 silica solution obtained in this way has an SiO ₂ content of 5.9 percent by weight and a storage time of more than a year.

Example 8

28.5 30 This example describes the use of a preparation according to the invention for the production of Paper with improved grip.

It was an aqueous, with polyethylene glycol (molecular weight 20,000) stabilized Kieselsäurelösun ^; with an SiO ₂ content of 4 percent by weight, in which the weight ratio of SiO ₂ to polyethylene glycol was 20: 1. This solution (0.5 ml) was mixed with water (1 ml) and the mixture sprayed onto a piece of kraft paper (22.8 x 7.6 cm). The application ratio was 0.012 g mixture / dm ² . The slip angle of the treated paper was 31. while that of the untreated paper averaged 19.

Example 9

This example describes the use of a preparation according to the invention to improve the dirt-repellent properties of carpets.

An aqueous silica solution according to Example 8 was prepared. 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 dirt (350 g) and carbon black (50 g) was then rubbed into the carpet samples ^{in an amount of 0.27 g / dm 2.} The carpet samples were then shaken and cleaned. The parts of the carpet pretreated with the preparation according to the invention were then much cleaner than other parts used for comparison that had not been pretreated.

Claims (3)

Patent claims: 1. Storage-stable preparations based on aqueous silica solution 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, 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 it is used as a water-soluble organic Polymer a polymer or copolymer of a vinyl-substituted, heterocyclic, Nitrogen containing compound having a molecular weight in the range of 35,000 contains up to 50000.