EP0028796B1 - Coated magnetic metal or alloy pigments, process for their production, and their use - Google Patents

Description

The present invention relates to non-pyrophoric magnetic metal and alloy pigments which superficially contain a material based on silicic acid esters, a process for the production of these pigments and the use thereof for the production of magnetic recording media.

The constantly growing demand for ever higher quality materials for magnetic recording has led to the development of magnetic metal and alloy pigments, primarily based on iron, cobalt and / or nickel, in recent years. Compared to conventional iron oxide and chromium dioxide pigments, these are characterized by considerably larger energy products and allow the production of magnetogram carriers with a superior storage density while at the same time reducing the amount of magnetically active material used.

One of the reasons why these pigments have so far not been able to establish themselves on the market is their high chemical reactivity. Metal particles in pigment size have such a high affinity for oxygen that - if they are suddenly exposed to air at room temperature - they spontaneously convert to metal oxides with glowing (see Römpps Chemie-Lexikon, 7th edition, Franckh'sche Verlagshandlung, Stuttgart 1975, page 2852): They are pyrophoric and must therefore always be handled, transported and processed under inert conditions.

In the past there has been no lack of attempts to take appropriate measures to avoid this crucial disadvantage. It is known, for example, that the pyrophoric character can be removed from the metal particles by surface oxidation. As a result, however, a considerable part of the magnetically high-quality metal particle is converted into a magnetically low-value oxide, which is expressed in a decrease in the saturation magnetization and the remanence.

Attempts have also been made to protect metal particles against air oxidation by treatment with polymeric organic binders to form so-called masterbatches. Although this measure avoids the loss of metallic substance discussed above, it requires the use of large amounts of polymer if satisfactory environmental stability is to be achieved. However, the higher the polymer content in the masterbatch, the more problematic is its use for producing a magnetic recording medium that is highly filled with pigments. In addition, the polymer used for stabilization can lead to incompatibilities when the masterbatch is subsequently processed with binders of a different chemical structure. A pigment protected in this way can therefore only be used to a very limited extent.

The known protective measures also include treating the metal powder with polymer-forming compounds in such a way that polymer layers are produced on the metal particles.

The use of silane coupling agents in magnetic pigments is also known, for example from US Pat. No. 4,076,890 for improving the strength of magnetizable layers and from Japanese Offenlegungsschrift 51-104 594, which includes improving the stability and dispersibility of magnetic metal pigments in binders. Pigments treated with silane coupling agents cannot, however, be incorporated equally well in different binder systems.

From DE-C-893.566 it is known to embed carbonyl iron in binders made of ortho- or disilicates. As a result, the individual particles, the size above that of magnetic pigments, are held together in a composite and can thus be more easily compressed to form iron.

The present invention had for its object to provide non-pyrophoric magnetic metal and alloy pigments for recording purposes which do not have the disadvantages mentioned.

When dealing with this problem, it was surprisingly found that coated magnetic metal and alloy pigments with a coating of at least one orthosilica ester and / or its hydrolysates and / or condensates in an amount of 0.2 to 30% by weight, preferably 1 to 20 wt .-%, and particularly preferably from 2 to 15 wt .-%, are protected against the attack of atmospheric oxygen and moisture.

Furthermore, it has been found that this surface protective layer can contain corrosion inhibitors, which further increases the resistance of the pigments.

The metal particles protected in this way can be dispersed very well, and there are no compatibility problems with commercially available binders.

The present invention further describes a process for producing these pigments.

Pyrophoric magnetic metal and alloy powders which can be protected against attacks by atmospheric oxygen and moisture according to the teaching of the present invention are described in the literature. They consist essentially of iron, cobalt and nickel or of alloys, which these three ferromagnetic metals can form together. They may contain 0.1 to 10 wt .-% of one or more foreign elements, such as. As cadmium, lead, calcium, zinc, magnesium, aluminum, chromium, tungsten, phosphorus or boron. The pyrophoric metal and alloy pigments can also contain smaller amounts of water, oxides or oxide hydroxides. They can be produced by electrolytic deposition from the corresponding salt solutions on one Mercury cathode, by decomposing the corresponding metal carbonyls, by reducing the corresponding metal ions from their solutions with the aid of also reducing agents such as boranate, hypophosphite, etc., by reducing with gaseous reducing agents (usually hydrogen) from the corresponding oxides, oxide hydroxides, oxalates, formates, etc. Temperatures above 250 ° C or by one of the less common methods mentioned in the literature. They can be isometric or anisometric particles, which are either largely homogeneous or are more or less broken down into individual metal cores adhering to one another. Because of their superior properties as recording media, metal powders are preferred, the needle-shaped individual particles of which have on average no more than 5 pores and on average consist of no more than 2 metal cores. Such pyrophoric metal powders are described in European patent application EP-Ai-15485.

deren Hydrolysate einschließlich der Teilhydrolysate sowie die aus den Hydrolysaten bzw. Teilhydrolysaten gegebenenfalls unter Mitwirkung der monomeren, unverseiften Kieselsäureester gebildeten Kondensate.

• R bedeutet darin einen gegebenenfalls verzweigten und/oder durch Sauerstoffbrücken ein- oder mehrfach unterbrochenen Alkyl-, Cycloalkyl- oder Alkenylrest mit 1 bis 20 Kohlenstoffatomen oder einen Aryl- oder Aralkylrest. Der Alkyl- bzw. Alkenylrest kann linear aufgebaut sein ; bevorzugt wird jedoch ein mindestens einmal verzweigter Kohlenwasserstoffrest. Beispiele hierfür sind die Reste Isopropyl, t-Butyl, 3-Methylbutyl, 2-Ethyl-hexyl, Octadecyl, Oleyl. Beispiele für Cycloalkylreste sind Cyclopentyl, Cyclohexyl oder Cycloheptyl. Der Arylrest kann ein Phenylrest sein. Bevorzugt wird indes ein Alkyl-substituierter Phenylrest (Aralkylrest), wobei besonders bevorzugt ist, wenn sich der oder die Alkylreste in o-Stellung zur Esterbindung befinden. Beispiele hierfür sind die Reste o-Tolyl, o,o'-Xylyl- oder o,o'-Diethylphenyl. Beispiele für andere Aralkylreste sind Benzyl, 1-Phenylethyl oder 2-Phenylethyl.
• R' stellt eine Alkylgruppe mit 1 bis 4 Kohlenstoffatomen dar. Der Alkylrest R' kann verzweigt sein ; bevorzugt wird jedoch ein linearer unverzweigter Kohlenwasserstoffrest wie Methyl oder Ethyl.
• n bedeutet eine Zahl von 0 bis 4, bevorzugt 1 bis 3.

Silicic acid esters which perform the protective function according to the invention are in particular compounds of the general formula

their hydrolysates including the partial hydrolysates and the condensates formed from the hydrolysates or partial hydrolysates, optionally with the participation of the monomeric, unsaponified silicic acid esters.

• R therein denotes an alkyl, cycloalkyl or alkenyl radical having 1 to 20 carbon atoms or an aryl or aralkyl radical which is optionally branched and / or interrupted one or more times by oxygen bridges. The alkyl or alkenyl radical can have a linear structure; however, a hydrocarbon radical which is branched at least once is preferred. Examples of these are the residues isopropyl, t-butyl, 3-methylbutyl, 2-ethylhexyl, octadecyl, oleyl. Examples of cycloalkyl radicals are cyclopentyl, cyclohexyl or cycloheptyl. The aryl radical can be a phenyl radical. However, an alkyl-substituted phenyl radical (aralkyl radical) is preferred, it being particularly preferred if the alkyl radical (s) are in the o-position to the ester bond. Examples of these are the residues o-tolyl, o, o'-xylyl- or o, o'-diethylphenyl. Examples of other aralkyl radicals are benzyl, 1-phenylethyl or 2-phenylethyl.
• R 'represents an alkyl group with 1 to 4 carbon atoms. The alkyl radical R' can be branched; however, a linear unbranched hydrocarbon radical such as methyl or ethyl is preferred.
• n is a number from 0 to 4, preferably 1 to 3.

Silica esters of the specified type can be prepared in a variety of ways according to the prior art, cf. W. Noll, Chemistry and Technology of Silicones, 2nd edition, Verlag Chemie, Weinheim 1968, page 554 ff. For the intended use according to the invention, production processes are preferred which allow the esters to be completely free of acidic impurities (especially hydrogen halides) without great effort. to maintain, as these are known to promote the corrosion of metal.

For the purposes of the present invention, the surface of the metal particles can be coated with only one ester, its hydrolyzate and / or condensate. However, the surface layer can also be formed from two or more silicic acid esters, their hydrolyzates or cohydrolysates and / or condensates or cocondensation. It is therefore not absolutely necessary that the silicic acid esters used for the invention are uniform compounds. Rather, it is entirely possible to use reaction mixtures that may be produced directly for stabilization during their production. This eliminates the need for sometimes very complex and expensive cleaning.

Instead of the monomeric orthosilica esters, it is also possible from the outset to use their reaction products with sufficient amounts of water to coat the metal particles in order to completely hydrolise them.

wobei R einen gegebenenfalls verzweigten und/oder durch Sauerstoffbrücken ein- oder mehrfach unterbrochenen Alkyl-, Cycloalkyl- oder Alkenylrest mit 1 bis 20 Kohlenstoffatomen oder einen Aryl- oder Aralkylrest oder ein Wasserstoffatom bedeuten kann. R' steht für eine Alkylgruppe mit 1 bis 4 Kohlenstoffatomen oder ist gegebenenfalls gleich R. o und p sind Zahlen von 0 bis 50, bevorzugt von 1 bis 35.The incomplete hydrolysis produces polysilicic acid esters, which can be described, for example, by the following formulas:

where R may mean an alkyl, cycloalkyl or alkenyl radical having 1 to 20 carbon atoms or an aryl or aralkyl radical or a hydrogen atom which may be branched and / or interrupted one or more times by oxygen bridges. R 'stands for an alkyl group with 1 to 4 carbon atoms or is optionally equal to R. o and p are numbers from 0 to 50, preferably from 1 to 35.

The amount of silicic acid ester which must at least be used in order to deprive the metal particles of the pyrophoric character depends on the particle size, the specific surface area (BET nitrogen adsorption method) and the porosity of the pigments. The degree of reduction of the pigments also goes down: Materials that contain smaller amounts of oxides or oxide hydroxides can - with comparable particle size, BET surface area and pore content - be stabilized with smaller amounts than completely reduced metal powders. With the help of simple test trials, a specialist can easily determine the minimum amount. However, since in most cases it is not only important to deprive the metal particles of their pyrophoric character, but rather to retain the magnetic parameters when stored in the air over a longer period of time, treatment with a somewhat larger amount is required anyway Make silicic acid esters as the minimum amount, especially since the surface protective layer has no adverse effect in the later processing.

The coated magnetic metal and alloy pigments claimed therefore contain at least one ortho-silicic acid ester of the type mentioned, its hydrolyzate and / or condensate in an amount of 0.2 to 30% by weight, preferably 1 to 20% by weight, and particularly preferably 2 to 15% by weight. Although the magnetic metal and alloy pigments claimed in claims 1 and 2, even when they are embedded in the commercially available binder of a magnetic recording medium when processed, already show a remarkably high resistance to electrochemical corrosion processes, an even higher one can be used for some purposes Resilience may be desirable.

It has been found that this is achieved with magnetic metal and alloy pigments which contain one or more corrosion inhibitors in addition to the silica ester coating. The metal powders treated in this way as such show increased stability against atmospheric oxidation in atmospheres of different relative atmospheric humidity.

Corrosion inhibitors which can be used are the compounds which are available and active according to the prior art, for. B. higher amines, aldehydes, alcohols or ketones, amidines, guanidines, heterocyclic compounds with nitrogen and / or oxygen and / or sulfur as heteroatoms (urotropin, pyrazoles, imidazoles, imidazolines, oxazoles, isoxazoles, thiazoles, isothiazoles, triazoles, triazines, Pyridines and the corresponding benzene-fused systems such as benzimidazoles, benzothiazoles, benzotriazoles, quinolines or isoquinolines, quinazolines and others) sulfur-nitrogen compounds, organic acetylene derivatives, organic nitro compounds, phosphonic acids and phosphonocarboxylic acids and their derivatives and salts, nitrogen-containing phosphonic or phosphonocarboxylic acids and their derivatives and salts, organic and inorganic salts of carboxylic acids such as acetates, benzoates, cinnamates, salicylates and other, organic and inorganic phosphates (polyphosphates), phosphites and sulfonates, etc. Of course, suitable mixtures of these agents can also be used.

Magnetic metal and alloy pigments in which the corrosion inhibitors in the silica ester layer are compounds from the series of the benzotriazoles, the benzothiazoles, the benzimidazoles, the guanidines, the amidines and / or the metal salts of the carboxylic acids and in an amount of 0 are particularly preferred , 01 to 5 wt .-%, preferably from 0.1 to 4 wt .-%, and particularly preferably from 0.5 to 3 wt .-%, based on metal.

The stabilizing protective layer can be applied to the pyrophoric magnetic metal and alloy pigments by treating them with the silicic acid esters previously converted into a finely divided state by evaporation or atomization in suitable apparatus (fluidized bed reactor, rotary tube, etc.), optionally with the participation of water vapor and / or catalytically active substances and, if necessary, at temperatures higher than room temperature.

However, it is preferred to carry out the stabilization in the liquid phase in such a way that the untreated pigments in suspensions in organic and / or aqueous medium are treated with at least one orthosilicic acid ester and / or polysilicic acid ester and optionally at least one corrosion inhibitor, then separated from the organic and / or aqueous phase and dried at temperatures between 50 and 250 ° C.

Lower alcohols such as methanol, ethanol, isopropanol, lower ketones such as acetone, butanone (2) and lower ethers such as tetrahydrofuran, 1,4-dioxane are well suited as organic media. Since the magnetic metal and alloy pigments are self-igniting before stabilization and are only converted into a thermodynamically metastable state by the treatment, the treatment will be carried out in organic / aqueous media or in water for safety reasons. If necessary, auxiliaries can be added which facilitate the suspension of the pigments (dispersants) and / or promote the hydrolysis or condensation of the ester. These are, for example, condensed phosphates, cationic, anionic, amphoteric or nonionic wetting agents or surfactants, acidic or alkaline substances or substance mixtures, which can be in free or immobilized form, pH-stabilizing substances such as buffer mixtures.

The pyrophoric magnetic metal or alloy pigment is suspended in the organic and / or aqueous medium by methods known per se. Silicic acid esters and optionally corrosion inhibitors can be added at any time before, during or after the preparation of the suspension of the liquid phase, in which case the addition is expediently carried out directly after the suspension. The pigment content of the suspension is generally between 5 and 50% by weight.

In many cases it has proven to be advantageous to leave the pigment in suspension for some time before it is then separated off and dried at temperatures of 50 to 250 ° C., preferably 80 to 150 ° C. During this period - preferably half to five hours - the suspension is stirred at room temperature or temperatures higher than room temperature, in particular at 20 to 90 ° C.

It is particularly preferred to combine the separation of the treated magnetic metal and alloy pigments from the organic and / or aqueous medium and their drying with one another by subjecting the suspension to spray drying. The temperature of the drying gas can be up to 500 ° C. The spray-dried, coated pigments can be subjected to an additional heat treatment at the temperatures indicated above and, if appropriate, can be transferred from an inert gas atmosphere to an oxygen-containing atmosphere.

The process according to the invention is normally carried out with the greatest possible exclusion of oxygen in an inert gas atmosphere (“inert gas atmosphere” is also understood here to mean vacuum), as a rule nitrogen, in order to avoid oxidation of the magnetic metal and alloy pigments. Only after drying or tempering has been carried out is the coated pigment released into the air. If necessary, the inert gas atmosphere can be gradually replaced by an air atmosphere by metered addition of oxygen or air. In this way it can be ensured that any active centers which are still present and which would oxidize immediately if atmospheric oxygen is directly admitted and which could act as an initial ignition source for the entire amount of pigment due to the energy released in the process can be passivated without risk. This passivation can of course also be carried out with gas mixtures containing oxygen other than air.

It should be pointed out that it is extremely surprising even for the person skilled in the art that the method described leads to success despite the use of protic media, without the magnetic metal and alloy pigments suffering in the process. The saturation magnetization drops by at most the percentage that the non-magnetic protective cover makes up by weight in the stressed pigment. The apparently stronger drop in the remanent magnetization, which was partly observed on powder samples, is due to texture and packing influences and is not reflected in the band remanence.

The non-pyrophoric coated magnetic metal and alloy pigments according to the invention can be processed according to the known methods to dispersions of the pigments in binders and solvents, which can then be used to produce the actual magnetic recording media. Their excellent dispersion behavior is noticeable in both organic and aqueous systems. No intolerance was observed. Rather, it was unexpectedly found that in the preparation of the dispersions the amount of binder can be reduced without disadvantageous consequences by the proportion which corresponds in weight to the stabilizing protective layer on the pigments, an advantage which should not be underestimated, especially when high pigment fill levels are required.

In principle, the binders customary in the prior art for the production of magnetogram carriers can be used in conjunction with the coated magnetic metal and alloy pigments according to the invention, such as, for. B. copolymers of vinyl chloride with vinyl acetate including the variants containing carboxyl, hydroxyl or epoxy groups, copolymers of vinyl chloride with acrylic acid esters or methacrylic acid esters including the versions containing hydroxyl groups accessible by partial saponification, copolymers of vinyl chloride with unsaturated OH-containing monomers Acrylonitrile or vinylidene chloride, copolymers of acrylic acid esters with acrylonitrile, with vinylidene chloride or with styrene, and the analog copolymers of methacrylic acid esters, copolymers of vinylidene chloride or butadiene with acrylonitrile, polyvinyl fluoride, polyvinyl acetals such as polyvinyl formal or polyvinyl butyrals, polyurethane elastomers, polyamide-based resins, polyesters, polyester resins, polyesters Binders based on synthetic rubbers, unsaturated polyester resins, phenoplasts, aminoplasts, epoxy resins in combination with hardeners, reactively modified alkyd resins, polyethers modified with isocyanate groups, Binder based on polyurethane prepolymers with reactive isocyanate groups. Suitable mixtures of these binders can of course also be used.

When creating the pigment dispersions, additives can be added at a suitable time, as are usually used in the production of magnetic recording media: dispersants, lubricants, plasticizers, fungicides, aging stabilizers, antistatic agents, non-magnetizable fillers or pigments.

Coated magnetic metal and alloy pigments according to the present invention can be used to manufacture any type of magnetic recording media, such as B. tapes, video tapes, instrument tapes, computer tapes, flexible and rigid magnetic cards, flexible magnetic disks, rigid magnetic disks, drum storage.

Such magnetic information carriers are used for image and sound recording, for storing measurement data, as external storage for data processing systems, for archiving purposes, for word processing systems, for identification systems such as ID cards, credit cards, tickets.

The invention is explained in detail below by means of examples, which, however, in no way limit the subject of the application. The magnetic data given in the examples were measured in a field of 3.5 KOe.

example 1

20 g of a needle-shaped pyrophoric metal pigment consisting essentially of iron (I ^H c: 772 Oe, Br / p: 1 102 Gg ^-1 cm ³ , 4πl _s / ρ: 1 855 Gg ^-1 cm ³ , Br / 4πl _s : 0.59) were dispersed in 200 g of distilled water using a mixed siren. During this process, 4 g of tetraethyl silicate were added. The suspension was filtered and the filter cake was dried in a stream of nitrogen at 105 ° C. for 10 hours. After cooling, a sample was taken to the air and pulverized. The magnetic powder data of this sample were measured immediately (A) and after ten days' storage in air in a thin layer at 25 ° C and 65% relative humidity (B):

Example 2

3.7 of the product obtained after separating off the low boilers (residual pressure 20 mbar, bottom temperature up to 95 ° C.) of the reaction of 1 mol of tetraethyl silicate with 1 mol of n-octadecanol were mixed in a mixture of 120 g of butanone (2) and 30 g of distilled water solved. Using a dissolver, 15 g of a needle-shaped pyrophoric metal pigment consisting essentially of iron (I ^H c: 1 126 Oe, Br / p: 1 233 Gg- ¹ cm ³ , 4πl _s / p: 1 950 Gg ^-1 cm ³ , Br / 4πl _s : 0.63) dispersed in this solution. After the resulting suspension had been stirred at 65 ° C. for 75 minutes, solvents and cleavage products from the ester hydrolysis or condensation were evaporated on a rotary evaporator at a residual pressure of 30 mbar and temperatures up to 80 ° C. The granular material was subjected to a heat treatment at 90 ° C under an inert gas for one hour. A sample of the cooled product was rubbed into the air. The magnetic powder data determined immediately on this sample (A) and after ten days' storage in air in a thin layer at 25 ° C. and 65% relative atmospheric humidity (B) were:

Example 3

3a 10.4 g of 2-ethylhexyloxytriethoxysilane were dissolved in a mixture of 315 g of ethanol and 135 g of distilled water. The pH was adjusted to 4.5 by adding acetic acid and the solution was stirred at room temperature for 1 hour. Then 50 g of a pyrophoric iron pigment (I ^H c: 714 Oe, Br / p: 973 Gg ^-1 cm ³ , 4πl _s / p: 1 864 Gg ^-1 cm ³ , Br / 4πl _s : 0.52) were removed using a Dissolver dispersed in this solution. The pigment suspension was stirred at 50 ° C. for 1 hour and then fed into a laboratory spray dryer (IRA mini spray drying system HO) which was operated with nitrogen (drying gas temperature between 180 and 200 ° C.). The spray-dried pigment was still Annealed for 1 hour under nitrogen at 100 ° C, before a sample of the finely divided material was placed in air. The magnetic powder data of this sample were measured immediately (A) and after ten days' storage in air in a thin layer at 25 ° C and 65% (B) or 95% (C) relative humidity:

3b 8.6 g of 2-ethylhexyloxytriethoxysilane were dissolved in a mixture of 160 g of ethanol and 65 g of distilled water. The pH was adjusted to 4.5 by adding acetic acid and the solution was stirred at room temperature for 1 hour. In the meantime, a suspension of 50 g of the same pyrophoric iron pigment as in Example 3a was prepared in a mixture of 155 g of ethanol and 70 g of distilled water, in which 1 g of 1,2-pentamethylene benzimidazole had previously been dissolved. With the aid of an intensive stirrer, this suspension was diluted with the silane solution and the procedure continued as in Example 3a. The measurement of the magnetic powder data was also carried out as described in Example 3a and gave:

3c 8.6 g of 2-ethylhexyloxytriethoxysilane and 1 g of Preventol CI 6 (ethoxylated 2-mercaptobenzimidazole, Bayer AG) were dissolved in a mixture of 315 g of ethanol and 135 g of distilled water. After the solution was stirred at room temperature for 1 hour, 50 g of the same pyrophoric iron pigment as in Example 3a was suspended therein. The further procedure was the same as in Example 3a. Measurement of the magnetic powder data of the stabilized pigment showed:

3d 8.6 g of 2-ethylhexyloxytriethoxysilane and 1 g of 1-hydroxybenzotriazole were dissolved in a mixture of 315 g of ethanol and 135 g of distilled water. 50 of the same pyrophoric iron pigment as in Example 3a were suspended in this solution. The further procedure was the same as in Example 3a, with the exception that the suspension was used for spray drying immediately after preparation. Measurement of the magnetic powder data of the stabilized pigment showed:

3e As example 3c, except that 1 g of cyclohexyliminocarbonic acid dimorpholide was used instead of 1 g of ⁹ Preventol CI 6. Measurement of the magnetic powder data of the stabilized pigment showed:

3f As in Example 3b, except that 1 g of isophoronediamine-biscaprolactamamidine was used instead of 1 g of 1,2-pentamethylenebenzimidazole. Measurement of the magnetic powder data of the stabilized pigment showed:

• 1 g Metallpigment wurde in 2,5 cm³ einer 12 gew.-%igen Lösung eines Polyvinylchlorid/Polyvinylacetat-Copolymerisats in gleichen Teilen Ethyl- und Butylacetat, welche außerdem 4 Gew.-% (bezogen auf Pigment) eines Dispergiermittels enthielt, auf einem Muller bei 6 kp Belastung mit 175 Umdrehungen dispergiert. Die erhaltene Lackprobe wurde mit einem 90 µ-Filmzieher auf eine 30 µ, starke Polyesterfolie aufgezogen, in einem Magnetfeld ausgerichtet und anschließend getrocknet.

In order to assess the corrosion resistance in the strip, short test strips were produced with the stabilized pigments and for comparison with the untreated pyrophoric iron pigment:

• 1 g of metal pigment was dissolved in 2.5 cm ^{3 of} a 12% by weight solution of a polyvinyl chloride / polyvinyl acetate copolymer in equal parts of ethyl and butyl acetate, which also contained 4% by weight (based on pigment) of a dispersant Muller dispersed at 6 kp load with 175 revolutions. The paint sample obtained was drawn onto a 30 μ thick polyester film using a 90 μ film puller, aligned in a magnetic field and then dried.

To enable a more precise statement, several sample tapes were made from each pigment sample and stored for 48 hours at room temperature. Then tape samples with a homogeneously smooth and closed surface were introduced into a climatic chamber in which there was a constant temperature of 55 ° C. and a relative air humidity of 95%. The state of the magnetic layer was assessed at 35x magnification in incident light. The results of the climatic tests are summarized in the following table:

Example 4

10 g of a needle-shaped pyrophoric metal pigment consisting essentially of iron (l ^H c: 844 Oe, Br / p 1 182 Gg- ¹ cm ³ , 4πrl _s / p: 1 917 Gg ^-1 cm ³ , Br / 4πl _s : 0 , 62) were suspended in an emulsion of 1.9 g of dodecyloxytriethoxysilane in a mixture of 50 g of 1,4-dioxane and 30 g of distilled water by means of a Kotthoff stirrer. The suspension was stirred at 40 ° C for 3 hours. It was then filtered off and the filter cake was dried in a stream of nitrogen at 150 ° C. for 8 hours. A sample of the cooled product was taken to the air and crushed. The magnetic powder data of this sample were measured immediately (A) and after ten days' storage in air in a thin layer at 25 ° C and 65% relative humidity (B):

Example 5

4.4 g of o-methylphenoxytriethoxysilane were dissolved in a mixture of 80 g of isopropanol and 20 g of distilled water. The pH of the solution was adjusted to 9 with ammonia water. Then 15 g of an acicular pyrophoric metal pigment consisting essentially of iron (I ^H c: 1 313 Oe, Br / p: 1 137 Gg- ¹ cm ³ , 4πl _s / p: 1 935 Gg- ¹ cm ³ , Br / 4πl _s : 0.59) dispersed therein with the aid of a mixing siren. The pigment suspension was stirred at 85 ° C for 1 hour. It was filtered off and the filter cake was dried in a stream of nitrogen at 130 ° C. for 9 hours. After cooling, a sample was rubbed in air. The magnetic powder data measured immediately (A) and after ten days' storage in air in a thin layer at 25 ° C. and 65% relative atmospheric humidity (B) on this sample were:

Example 6

• 25,7 Gew.-% Kieselsäuretetraethylester
• 43,3 Gew.-% 2-Ethylbutyloxytriethoxysilan
• 22,9 Gew.-% Bis(2-Ethylbutyloxy) diethoxysilan
• 6,8 Gew.-% Tris(2-Ethylbutyloxy) ethoxysilan
• 0,9 Gew.-% Ethanol
  und
• 0,4 Gew.-% 2-Ethylbutanol

wurden in einer Mischung aus 6 750 g Ethanol und 2 250 g destilliertem Wasser gelöst. Der pH-Wert der Lösung wurde durch Zugabe von Essigsäure auf 4,2 eingestellt. Nachdem die Lösung 90 Minuten bei Raumtemperatur gerührt worden war, wurden 1 000 g eines nadelförmigen, im wesentlichen aus Eisen bestehenden pyrophoren Metallpigments (I^Hc: 1 050 Oe, Br/p : 1 182 Gg-¹ cm³, 4al_s/p : 1 910 Gg-¹ cm³, Br/4πl_s: 0,62) mit Hilfe eines Dissolvers hierin dispergiert. Die resultierende Suspension wurde 2 Stunden bei 45 °C gerührt und dann in einen Laborsprühtrockner (IRA-Mini-Sprühtrocknungsanlage HO) eingespeist, der mit Stickstoff betrieben wurde (Trocknungsgastemperatur zwischen 180 und 200 °C). Das sprühgetrocknete Pigment wurde noch 2 Stunden bei 110 °C unter Stickstoff getempert. Nach dem Erkalten wurde eine Probe des pulverförmigen Produkts an die Luft gebracht. An dieser Probe wurden sofort (A) und nach zehntägiger Lagerung in dünner Schicht bei 25 °C und 65 % relativer Luftfeuchte (B) folgende Magnetwerte gemessen :

285 g of a mixture consisting of:

• 25.7% by weight of tetraethyl silicate
• 43.3% by weight of 2-ethylbutyloxytriethoxysilane
• 22.9% by weight bis (2-ethylbutyloxy) diethoxysilane
• 6.8% by weight of tris (2-ethylbutyloxy) ethoxysilane
• 0.9 wt% ethanol
  and
• 0.4% by weight of 2-ethylbutanol

were dissolved in a mixture of 6 750 g ethanol and 2 250 g distilled water. The pH of the solution was adjusted to 4.2 by adding acetic acid. After the solution had been stirred for 90 minutes at room temperature, were 1000 g of an acicular, substantially consisting of iron pyrophoric metal pigment (I ^H c: 1050 Oe Br / p: 1 182 ^GG-1 cm ^3, 4al _s / p : 1 910 Gg- ¹ cm ³ , Br / 4πl _s : 0.62) dispersed therein with the aid of a dissolver. The resulting suspension was stirred at 45 ° C. for 2 hours and then fed into a laboratory spray dryer (IRA mini spray drying system HO) which was operated with nitrogen (drying gas temperature between 180 and 200 ° C.). The spray-dried pigment was annealed for 2 hours at 110 ° C under nitrogen. After cooling, a sample of the powdered product was brought into the air. The following magnetic values were measured on this sample immediately (A) and after ten days of storage in a thin layer at 25 ° C. and 65% relative atmospheric humidity (B):

• a) Physikalisch trocknendes Bindemittelsystem :
  • 236 g des stabilisierten Metallpigments wurden in einer Lösung von 53 g Polyvinylchlorid/Polyvinylacetat-Copotymerisat, 4,5 g Siliconöl und 6 g Dispergiermittel in 440 g einer Mischung aus gleichen Teilen Ethyl- und Butylacetat dispergiert und 3,5 Stunden in einer Perlmühle (Mahlkörper : Glaskugeln mit 1, 2 und 3 mm Durchmesser) gemahlen. Danach wurde filtriert, der Lack auf eine 12 µ, starke Polyesterfolie aufgetragen und das im Lack dispergierte Eisenpigment in einem Magnetfeld ausgerichtet. Anschließend wurde die Magnetschicht getrocknet. Die Schichtdicke im trockenen Zustand betrug ca.

Production of magnetic tapes:

• a) Physically drying binder system:
  • 236 g of the stabilized metal pigment were in a solution of 53 g of polyvinyl chloride / polyvinyl Acetate copolymer, 4.5 g of silicone oil and 6 g of dispersant dispersed in 440 g of a mixture of equal parts of ethyl and butyl acetate and ground for 3.5 hours in a bead mill (grinding media: glass balls with a diameter of 1, 2 and 3 mm). It was then filtered, the paint was applied to a 12 μ thick polyester film and the iron pigment dispersed in the paint was aligned in a magnetic field. The magnetic layer was then dried. The layer thickness in the dry state was approx.

Magnetic measurements on the tape showed:

The data in the corresponding band with unstabilized pigment are given in brackets. Both bands have similar volume fill factors with regard to the magnetically active material.

b) Crosslinking binder system:

275 g of the stabilized metal pigment were dissolved in a solution of 14.4 g of polyvinyl chloride / polyvinyl acetate copolymer, 5 g of acrylonitrile / butadiene copolymer, 19.4 g of polyester urethane, 92.4 g of hydroxyl-containing polyacrylate and 8.5 g of dispersant Dispersed mixture of 231 g of tetrahydrofuran, 115.5 g of methyl isobutyl ketone and 51.7 g of cyclohexanone. The mixture was then milled for 3.5 hours in a bead mill (grinding media: glass balls with a diameter of 1, 2 and 3 mm). 30 minutes before the end of the milling period, 19.4 g of a polyfunctional aliphatic isocyanate were added to crosslink the binder system. It was then filtered, the paint was applied to a 12μ thick polyester film and the iron pigment dispersed in the paint was aligned in a magnetic field. The magnetic lacquer was then dried. Its thickness in the dry state was approx. 6 µ.

Magnetic measurements on the tape showed:

c) Aqueous, physically drying binder system:

240 g of the stabilized metal pigment were suspended in 175 g of distilled water, which contained 8% by weight (based on pigment) of a dispersant. A mixture of 200% of a 50% dispersion of an acrylate / styrene copolymer and 10 g of butyl diglycol acetate was added to the suspension and the entire batch was milled for 3.5 hours in a bead mill (grinding media: glass balls with a diameter of 1, 2 and 3 mm). It was then filtered, the paint was applied to a 12 μ thick polyester film and the iron pigment dispersed in the paint was aligned in a magnetic field. The magnetic layer was then dried. The layer thickness in the dry state was approx. 6 µ.

Magnetic measurements on the tape showed:

Example 7

5.8 g of the product obtained after the volatile components had been stripped off (residual pressure 18 mbar, bottom temperature up to 175 ° C.) of the reaction of 1 mol of tetraethyl silicate with 1 mol of 9-octadecen-1-ol were adjusted to pH 6.8 with ammonia Emulsified mixture of 270 g isopropanol and 180 g distilled water.

Using a dissolver, 50 g of a needle-shaped, essentially iron pyrophoric metal pigment (I ^H C: 852 Oe, Br / p: 1 108 Gg ^-1 cm ³ , 4πl _s / ρ: 1 847 Gg- ¹ cm ³ , Br / 4πl _s : 0.60) dispersed in this emulsion. The suspension was stirred for 45 minutes at 50 ° C. and then spray-dried under inert conditions at a drying gas temperature of about 200 ° C. (IRA mini spray drying system HO). A sample of the powdery product was placed in air. The magnetic powder data determined immediately on this sample (A) and after ten days' storage in air in a thin layer at 25 ° C. and 65% relative atmospheric humidity (B) were:

Example 8

13 g of 2-ethoxyethyloxytriethoxysilane. were emulsified in a mixture of 135 g of ethanol and 315 g of distilled water. Then 50 g of an acicular pyrohoric iron pigment (I ^H C: 999 Oe, Br / p: 1 005 Gg- ¹ cm ³ , 4πl _s / ρ: 1 771 Gg- ¹ cm ³ , Br / 4πl _s : 0.57) dispersed in this emulsion. The resulting suspension was stirred for half an hour at room temperature and then fed into a laboratory spray dryer (IRA mini spray drying system HO), the drying gas of which nitrogen had a temperature between 200 and 220 ° C. A sample of the spray dried pigment was placed in the air. The measurement of the magnetic powder data immediately (A) and after ten days' storage in air in a thin layer at 25 ° C and 65% relative humidity (B) showed:

Example 9

75 g of a needle-shaped pyrophoric metal pigment consisting essentially of iron (I ^H C: 845 Oe, Br / p: 1 150 Gg- ¹ cm ³ , 4πl _s / ρ: 1916 Gg- ¹ cm ³ Br / 4πl _s : 0, 60) were suspended using a dissolver in an emulsion of 12.8 g of cyclohexyloxytriethoxysilane in a mixture of 264 g of isopropanol and 396 g of distilled water. The emulsion had previously been adjusted to a pH of 5 by adding ammonia water. The metal pigment suspension was stirred for a further 30 minutes at 75 ° C. and then subjected to spray drying under nitrogen (IRA mini spray drying system HO, drying temperature approx. 200 ° C.). A sample of the cooled product was placed in air. The magnetic powder data of this sample were measured immediately (A) and after ten days' storage in air in a thin layer at 25 ° C and 65% relative humidity (B):

Example 10

330 g of 2-ethylbutyloxytriethoxysilane were reacted at 70 ° C. in the presence of an immobilized acid as a catalyst with 18 g of distilled water, dissolved in 60 g of ethanol. The cooled reaction mixture was filtered and then freed from low-boiling constituents at a residual pressure of 30 mbar and a maximum temperature of 70 ° C.

5 g of the product thus obtained were emulsified in a mixture of 88 g of methanol and 352 g of distilled water. In this emulsion 50 g of a needle-shaped, essentially iron pyrophoric metal pigment (I ^H C: 714 Oe, Br / p: 973 Gg ^-1 cm ³ , 4πl _s / p: 1 864 Gg ^-1 cm ³ , Br / 4πl _s : 0.52) suspended. The suspension was stirred for one hour at room temperature and then spray-dried at a drying gas temperature between 200 and 220 ° C. under inert conditions (IRA mini spray drying system HO).

A sample of the fine-particle material in air had the following magnetic powder data immediately after production (A) or after ten days' storage in air in a thin layer at 25 ° C. and 65% relative atmospheric humidity (B):

Example 11

und 0,6 Gew.-% Ethanol wurden in Gegenwart einer immobilisierten Säure als Katalysator bei 70 °C mit 17 g destilliertem Wasser, gelöst in 80 g Ethanol, umgesetzt. Anschließend wurde das Reaktionsgemisch abgekühlt, filtriert und dann bei einem Restdruck von 30 mbar und einer Temperatur von maximal 70 °C ausgeheizt.397 g of a mixture consisting of:

and 0.6% by weight of ethanol were reacted in the presence of an immobilized acid as a catalyst at 70 ° C. with 17 g of distilled water, dissolved in 80 g of ethanol. The reaction mixture was then cooled, filtered and then heated at a residual pressure of 30 mbar and a maximum temperature of 70 ° C.

50 g of an acicular, essentially iron pyrophoric metal pigment (I ^H C: 1 094 Oe, Br / p: 1 086 Gg- ¹ cm ³ , 4πl _s / ρ: 1 742 Gg- ¹ cm ³ , Br / 4πl _s : 0.62) were dispersed in an emulsion of 6 g of the product so obtained in 440 g of distilled water. The suspension was evaporated to dryness on a rotary evaporator at a residual pressure of 30 mbar and temperatures up to 150 ° C. and the granular material was cooled under nitrogen. A sample of the cooled product was rubbed off and placed in air. The magnetic powder data determined immediately on this sample (A) and after ten days' storage in air in a thin layer at 25 ° C. and 65% relative atmospheric humidity (B) were:

Claims (7)

1. Coated magnetic metal and alloy pigments having a coating of at least one ortho-silicate and/or hydrolysates thereof and/or condensates thereof in a quantity of from 0.2 to 30 % by weight, preferably from 1 to 20 % by weight and most preferably from 2 to 15 % by weight.

2. Coated magnetic metal and alloy pigments according to claim 1, characterised in that the ortho-silicate is a compound corresponding to the general formula

wherein R denotes an alkyl, cycloalkyl or alkenyl group with 1 to 20 C-atoms which may be branched and/or interrupted one or more times by oxygen bridges or it denotes an aryl or aralkyl group, R' denotes an alkyl group with 1 to 4 C-atoms which may be branched, and n = 0 to 4, preferably 1 to 3.

3. Coated magnetic metal and alloy pigments according to one of the claims 1 or 2, characterised in that the coating in addition contains one or more corrosion inhibitors.

4. Coated magnetic metal and alloy pigments according to claim 3, characterised in that the corrosion inhibitors are compounds from the series of benzotriazoles, benzothiazoles, benzimidazoles, guanidines, amidines and/or the metal salts of carboxylic acids and are present in a quantity of from 0.01 to 5 % by weight, preferably from 0.1 to 4 % by weight and most preferably from 0.5 to 3 % by weight, based on the metal.

5. Coated magnetic metal and alloy pigments according to one of the claims 1 to 4, obtainable by treatment of the untreated pigments in suspension in organic and/or aqueous medium with at least one ortho-silicate and/or polysilicate and optionally at least one corrosion inhibitor, separation of the organic and/or aqueous phase, drying at temperatures of from 50 to 250 °C and optionally transfer of the pigments from an inert gas atmosphere into an atmosphere containing oxygen.

6. Process for the preparation of coated magnetic metal and alloy pigments according to one of the claims 1 to 5, characterised in that the untreated pigments are treated in suspension in organic and/or aqueous medium with at least one-ortho-silicate and/or polysilicate and optionally at least one corrosion inhibitor and subsequently separated from the organic and/or aqueous phase and dried at temperatures of from 50 to 250 °C and optionally transferred from an inert gas atmosphere into an atmosphere containing oxygen.

7. Use of the coated magnetic metal and alloy pigments according to one of the claims 1 to 5 for the preparation of magnetic recording media.