Finely, Divided, Precipitated Silica With High Structure, Method of Its Preparation and Use

The present invention relates to a finely divided silica with high structure, a method of preparing the silica and the use of the silica as a delustering agent in films of the paint, lacquer, varnish and enamel type.

It is known that the delustering capacity of a silica depends on various factors such as e.g. the type of silica, the particle size, the refraction index and also on the vehicle system. Particle form and particle-size distribution of the secondary particles as well as the total effective specific particle volume are of particular significance.

A number of other requirements are placed on delustering silicas in addition to high efficiency, expressed by the reduction of the degree of glossiness in comparison to a non-matte paint film. Thus, for example, the drying behavior of the film coatings should not be adversely affected, no excessive thickening of the paint coating system should occur and the scratch resistance of the film coatings should not be lowered by the silica added. The behavior in suspension of the silica is an important point. The tendency of silica to settle and to form hard sediment which can only be stirred up again with difficulty can be prevented or at least improved by a number of measures. Thus, for example, DE-AS 15 92 865 describes the impregnation of silica during the preparation process by means of wax emulsions.

The addition of synthetic silica as delustering agent is known. See for example the publications:

Numerous methods of preparing synthetic silica are also known (cf. Ferch in "Chem.-Ing.-Techn." 48, pp. 922-33 (1976)).

According to German Patent 24 14 478, aerogel-like delustering agents can be obtained by the structuring of pyrogenically prepared silica. To this end, pyrogenic silica is moistened with alkaline adjusted water, ground and dried. This procedure of preparing delustering agents is complicated and expensive.

The delustering agent according to German Patent 24 14 478 is an excellent delustering agent. However, it has the disadvantage that it thickens the film compositions too heavily. A solvent must therefore be added in addition for working with a spray gun to facilitate application of the films to a surface to be coated.

A wax-coated precipitated silica acid is prepared according to the process described in DE-PS 15 92 865 and is used as delustering agent. This delustering agent has the disadvantage, however, that the paints prepared with it exhibit an undesirable blue bloom on dark surfaces.

It is an object of the present invention to provide a finely divided precipitated silica of high structure with certain desired characteristics and especially to achieve a great savings of solvent while retaining the desired delustering action.

Another object of the invention is to prepare a waxcoated delustering agent which does not exhibit a blue bloom and has a better delustering action.

In more particular detail, an object of the present invention is to provide finely divided precipitated silica with a high structure and the following characteristics:

In order to determine the particle size with a Coulter counter, approximately 0.5 g silica acid are dispersed in 50 ml isotonic solution of common salt (0.5% NaCl and 0.089% Na.sub.4 P.sub.2 O.sub.7. 10 M.sub.2 O in distilled water) with a magnetic stirrer and subsequently treated for 1 minute with supersonics (200 watts). This suspension is added to 200 ml isotonic solution of common salt and agitated.

A measuring capillary tube is immersed into the agitated suspension and an electric field is applied thereto. When the particles pass through the measuring capillary tube, the electric field is varied as a function of the particle size.

It is still a further object of the invention to provide a method for preparing finely divided precipitated silica with a high structure processing the following characteristics:

The method of the invention is characterized by forming a mixture of water and sodium silicate which is then heated under agitation to a temperature of 70.degree. to 80.degree. C. Concentrated sulfuric acid is then dosed into this mixture until half of the alkali (sodium) present has been neutralized. The reaction mixture is treated by a shearing unit and, optionally, the temperature raised at the same time to 86.degree..+-.5.degree. C. Concentrated sulfuric acid is added after a waiting period of 30 to 120 minutes at a rather high rate of addition until the pH of the silica suspension produced is 3.0 to 3.5. The silica suspension is optionally diluted with water and the coarse portion optionally separated by a centrifugal pump and a hydrocyclone. The silica is filtered off by known filter devices and the resulting silica filter cake is washed free of sulfate. Then, the silica filter cake is redispersed with the addition of water using an agitator unit to form a suspension with a solid content of 80.+-.10 g/l. Alkyl dimethylbenzyl ammonium chloride is optionally added to this suspension, and the suspension obtained in this manner is spray-dried. The recovered dried product can then be ground, if desired.

In carrying out the present invention, silica particles are prepared with particle diameters in a particle-size range of 1 to 6 .mu.m. Particles smaller that 1 .mu.m are ineffective for delustering purposes and generally cause an undesirable thickening of the film vehicle. On the other hand, particles which are too large result in a disadvantageous roughness of the surface in the film coating. The object is to control appropriate sizes during the precipitation reaction and to retain chosen particle sizes by suitable measures until the finished product is obtained. In order to assure that all particles find the same conditions of growth, the entire charge of water glass starting material is put into a receiver at the beginning of the process for production. The addition of sulfuric acid is performed in two steps. In the first step, the addition of acid is measured in such a manner that the silica begins to flocculate after the end of the first stage of the addition of the acid. During this growth phase, in order to prevent an excessive particle growth, shearing is performed in addition to the agitation. The addition of acid remains interrupted until the desired particle spectrum has been achieved. Subsequently, the remaining alkali content of the water glass is neutralized in a second stage, during which shearing is continued. After a slightly acidic pH has been achieved, the precipitation reaction is terminated.

The concentrated sulfuric acid used for purposes of the present invention is conventional and, for example, is in the range of D=1.75 to 1.85 and 90 to 97% by weight.

The silica suspension is worked up and conditioned in the customary manner. There is the possibility that hard and coarse particles are produced in the course of the reaction by local excess acidifying. These undesired particles can be removed from the suspension with the aid of a hydrocyclone. The filtration of the suspension is performed e.g. with a plate-and-frame press in which the filter cake is washed free of sulfate. The washed filter cake is redispersed in water, a cationic tenside is optionally added and the product is then spray-dried. The cationic tenside (surfactant) brings about a displacement of the water from the particle surface thereby even in the aqueous phase, which suppresses to a great extent the shrinking process which takes place during the drying process. In this manner, it is possible to prevent the particles from clumping together to rather large, solid formations during drying. Depending on the desired degree of fineness, the silica can be used as is or additionally ground. As a result of the procedure described herein, a light grinding is sufficient, that is, only little grinding energy need be expended in order to disagglomerate the particles again.

The precipitated silica of the present invention can be used as delustering agent in a wide variety of film coatings such as paints, varnishes, lacquers and the like.

It has the advantage that no additional solvent need be used.

In order to improve the sedimenting behavior of the silica products in coating film compositions, an impregnation with emulsions is carried out in accordance with the disclosure of DE-PS 15 92 865, incorporated herein by reference.

A still further object of the invention is to provide a precipitated silica which is coated by an emulsion and prepared from the precipitated silica in accordance with the present invention. In this aspect of the invention, the coating can take place according to the known method of DE-PS 15 92 865 which corresponds to U.K. Patent 1,236,775, the entire disclosure of which is relied on and incorporated by reference.

The coated precipitated silica exhibits a carbon content of 1 to 8% by weight in addition to the same physical and chemical characteristic data of the uncoated precipitated silica.

In one embodiment of the invention, the precipitated silica can be coated by a silicon oil emulsion.

This precipitated silica can be used as delustering agent in paints, varnishes, lacquers and the like.

In a further particular embodiment of the invention, the precipitated silica can be coated by a polyethylene wax emulsion.

This precipitated silica product can be used as delustering agent in paints, varnishes, lacquers and the like.

It is especially advantageous that the precipitated silica coated with the wax emulsion does not generate a blue bloom in the paint surfaces.

All manner of film forming components can be combined with the delustering agents of the invention.

The following examples serve to illustrate the details of the present invention.

The method of the invention is carried out in known apparatuses.

The core of the precipitation apparatus used herein is a hard-rubberized, double-jacketed vessel with a volume of 120 liters which is provided with an agitator mechanism. Potential agitators suitable for the invention are e.g. anchor mixers, straight-arm paddle agitators or turbines. The reaction vessel can be heated with oil as heat carrier and thermostatted via the double jacket. A discharge pipe or tube is fixed to the bottom of the reaction vessel which pipe comprises a branch line in front of the bottom discharge valve. This branch runs to a shearing unit (Dispax reactor) with which the content of the reaction vessel can be rotated. The rotated precipitation suspension can be reintroduced into the top of the reaction vessel via the pipeline fixed to the pressure side of the shearing unit. The addition of water glass takes place either via a dosing pump from a storage vessel or directly from a vat by a vat pump. The sulfuric acid is dosed from a storage vessel by a dosing pump.

The precipitation suspension is pumped for filtration with the aid of a positive-displacement pump into a plate-and-frame press on which the filter cake produced is washed free of sulfate with water.

The washed filter cake is either placed on metal sheets and dried in a drying oven or is dispersed in water and spray-dried.

The dried silica can be subsequently ground. The product dried in a drying oven must generally be pre-ground in a toothed-disk mill before the fine grinding by means of a pin mill or air-jet mill. The spray-dried product can either be used directly or subjected to a fine grinding.

The efficiency of the non-coated precipitation silica prepared according to Examples 1-6 is compared in a black baking varnish with the product prepared according to DE-PS 24 14 478. In addition to the solvent requirement, the degree of glossiness according to Lange at a reflection angle of 60.degree. and the grindometer value according to Hegman are evaluated.

For the determination of the degree of glossiness, which is a measure for the delustering power of the tested delustering silica, the gloss meter according to B. Lange, which is often used in Germany, is used. The Lange gloss meter uses an angle of 45.degree. as incident and reflection angle. The measured degrees of glossiness are indicated in percents. The smaller their value, the better the delustering capacity of the silica tested, or, in other words, the less delustering agent needs to be used in order to achieve a quite specific degree of gloss.

The determination of grindometer value is carried out with the aid of a grindometer. The grindometer value, which is measured in .mu., is a measure for the coarsest particles located in the finished, sprayable film composition mixture after the delustered silica has been stirred in. It can be related to the formation of specks in the dry paint film. The feared and undesired "spray grain" can be recognized with the aid of the grindometer.

The paint used has the following composition:

7.5 parts by weight paint prepaste Tack.RTM. 1

47 parts by weight fatty-acid modified alkyd resin 60% in xylene (Alftalat AR 481 m)

24 parts by weight melamine resin Maprenal MF 800 55% in isobutanol

6 parts by weight butanol

10.5 parts by weight xylene

1 part by weight silicon oil Baysilon paint additive OL 17 1% in xylene.

2.6 parts by weight product are worked in at a time. The working in is carried out by ten-minute agitation with a wing agitator at 2000 rpms. The paint is sprayed onto metal sheets in a dry layer approximately 30 .mu.m thick, air-dried and annealed 30 min. at 180.degree. C.

The values determined can be gained from table 2 below.

66 kg Water and 21 kg soda water glass (d=1.35 g/cm.sup.3 ; ratio SiO.sub.2 :Na.sub.2 O=3.3) are added into the precipitation vessel and the mixture heated under agitation to 75.degree. C. Concentrated sulfuric acid (d=1.83) is dosed into the precipitation mixture at a rate of 1.45 l/h. After 25 minutes of precipitation time, the shearing unit (Dispax reactor) is switched on. Shortly after the end of the addition of acid, silica begins to flocculate and the temperature of the precipitation is raised to 85.degree. C. The supply of acid is interrupted for 20 minutes (waiting stage). After 50 minutes, the further addition of acid takes place with 1.8 l/h over a period of 15 minutes. The silica suspension produced exhibits a pH of 3.4 thereafter. The shearing unit is then cut off.

The suspension is diluted with 28 l water and conveyed by means of a centrifugal pump with 4.5 bars supply pressure onto a hydrocyclone. The ratio of coarse material suspension to fine material suspension is 1:12.

The fine material suspension is passed over a plate-and-frame press and washed sulfate-free.

The washed filter cake is redispersed using an Ultra-Turrax with the addition of water in such a manner that a suspension of 80 g/l is produced. An alkyl dimethylbenzyl ammonium chloride (BARQUAT.RTM.) is added to the suspension, so that 0.8 g/l active substance is present in the suspension.

This suspension runs through a sieve with a mesh size of 120 .mu.m, which retains coarse foreign particles. The suspension is spray-dried immediately thereafter. The atomization takes place by means of a two-fluid nozzle.

The dried product has the following physical and chemical properties:

Precipitated silica is prepared as described in Example 1 with the sole difference that the time of the interruption of the addition of acid (equal to the waiting stage) is varied.

The physical and chemical data of the silica products obtained are ser forth in Table 1 below.

A precipitated silica is prepared as described in Example 1. In contrast to Example 1, the heating to 85.degree. C. during the second phase of the addition of acid is eliminated. The waiting stage is 30 minutes.

The physical and chemical data of the precipitated silica obtained in this was set forth in Table 1 below.

A precipitated silica is prepared as described in Example 1. In contrast to Example 1, the waiting stage is 60 minutes. After the end of precipitation, 3.4 kg of an emulsion of silicon oil are added into the precipitated silica suspension.

The emulsion is prepared as follows:

0.24 parts by weight Emulan AF are dissolved in 80 parts by weight water. 20 parts by weight Baysilon oil.RTM. AC 3031 are added under dispersion with an Ultra-Turrax.

The work-up of the precipitated silica suspension takes place as described in Example 1. The spray-dried precipitated silica is subsequently ground in an air-jet mill.

The physical and chemical data for this product are set forth in Table 3 below.

A precipitated silica is prepared as described in Example 1. The waiting stage is 90 minutes. 45 l of the fine material suspension obtained is compounded with 625 g of a wax emulsion under agitation. The wax emulsion is prepared in an autoclave which can be heated with vapor and is provided with a dispenser. 4.8 parts by weight of an alkyl polyglycol ether (Marlowet.RTM. GFW) are first dissolved in 81.0 parts by weight water at approximately 100.degree. C. Then, 14.2 parts by weight low-pressure polyethylene wax are added and heated to 130.degree. C. When 130.degree. C. has been reached, the dispenser is turned on and the mixture dispersed for 30 minutes. During this time, the temperature is maintained between 130.degree. C. and 140.degree. C. After the dispenser has been turned off and the mixture cooled off to approximately 110.degree. C., the finished emulsion is discharged.

The polyethylene wax used is characterized by the following characteristics:

The precipitated silica suspension is worked up as described in Example 1. The spray-dried precipitated silica is then ground in an air-jet mill.

The physical and chemical data for this product are set forth in Table 3 below.

A precipitated silica is prepared as described in Example 1. The waiting stage is 90 minutes. 45 l of the fine material suspension obtained is compounded with 860 g of a wax emulsion under agitation. The wax emulsion is prepared in an autoclave which can be heated with vapor and is provided with a dispenser apparatus. 4.8 parts by weight of an alkyl polyglycol ether (Marlowet.RTM. GFW) are first dissolved in 81.0 parts by weight water at approximately 100.degree. C. Then, 14.2 parts by weight low-pressure polyethylene wax are added and heated to 130.degree. C. When 130.degree. C. has been reached, the dispenser is turned on and the mixture dispersed for 30 minutes. During this time, the temperature is maintained between 130.degree. C. and 140.degree. C. After the dispenser apparatus has been turned off and the mixture cooled off to approximately 110.degree. C., the finished emulsion is discharged.

The polyethylene wax used has the following characteristics:

The precipitated silica suspension is worked up as described in Example 1. The spray-dried precipitated silica is then ground in an air-jet mill.

The physical and chemical data are set forth in Table 3 below.

The technical applications capabilities of the precipitated silicas obtained according to examples 7,8 and 9 are compared in three test-paint formulations with a delustering agent prepared according to DE-PS 15 92 865. The data is compiled in Table 4 below.

The determination of the degree of glossiness is performed with gloss-measuring devices according to Lange and Gardner (ASTM D 523-53 T). According to Lange, the incidence and reflection angles are 45.degree., according to Gardner 60.degree. and 85.degree.. The grindometer value is determined according to ISO 1524 in black stoving lacquer.

The text-paint formulations and the method of procedure are described below.

A) Black stoving lacquer

5 g precipitated silica as the delustering agent are stirred into 100 g paint with a wing agitator at 2000 rpms for 8 minutes. The viscosity of the mixture is set with xylene to a discharge time of 20 seconds (Ford beaker, DIN 4 mm nozzle).

The paint is sprayed onto metal sheets in a dry layer approximately 30 m thick, air-dried and annealed 30 min. at 180.degree. C.

B) Polyester paint (UP paint)

6.5 g precipitated silica as the delustering agent are added to 100 g of this paint mixture before processing and dispersed 8 minutes at 2000 rpms with a wing agitator. The viscosity of the mixture is adjusted with ethyl acetate to a run-off time of 20 seconds (Ford beaker, DIN 4 mm-nozzle). The paint mixture is applied in layer thicknesses of approximately 80 .mu.m.

C) DD paint

10.8 g delustering agent and 36 g Desmodur.RTM. L/75% in ethyl acetate are added to 100 g of the above mixture and dispersed 8 minutes at 2000 rpms with a wing agitator. The mixture is adjusted with ethyl acetate to a run-out time of 18 seconds (DIN beaker, 4 mm nozzle according to DIN 53211). The application is performed in layer thicknesses of 30-40 .mu.m.

Table 4 shows that a distinct improvement of the delustering action can be determined in all test paint formulations over the state of the art.

The trade designations used in the examples have the following meanings:

Characteristic: Non-plasticizied, isobutyl-etherified melamine formaldehyde resins.

Polyether-modified methyl polysiloxane

Chemical characterization polysiloxane diol

Thixotropic, unsaturated polyester resin (paraffin type), reactive

Octa-Solingen cobalt is a dry substance which contains 16% Co in addition to 2-n ethyl hexanoic acid (C.sub.8 H.sub.16 O.sub.2).

Aerosil.RTM. 200 is a pyrogenically prepared silicic acid with the following physical and chemical characteristic values:

1) according to DIN 53 194

2) according to DIN 55 921

3) according to DIN 53 200

4) according to DIN 53 580

5) in relation to the substance annealed 2 hours at 1000.degree. C.

6) in relation to the substance dried 2 hours at 105.degree. C.

7) HCl content is a component of the annealing loss

NC-Chips E 730 is a collodion cotton [c.c. (wool), pyrocellulose, soluble nitrocellulose, pyroxyline]

Shellsol.RTM. A is a carbon solvent rich in aromatic hydrocarbons with the following data:

Aromatic polyisocyanate

The term "DIN" stands for German industrial standard.

As will be apparent from the above, many different film forming components such as synthetic resins and mixtures with or without pigments are suitable for combination with the finely divided precipitated silica delustering agent of this invention. Further modifications and variations will be apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto.

German priority application P 38 15 670.9-41 is incorporated and relied on herein.