CA1079057A

CA1079057A - Stabilization of chromium dioxide magnetic pigments

Info

Publication number: CA1079057A
Application number: CA241,679A
Authority: CA
Inventors: Manfred Ohlinger; Dieter Schaefer; Paul Deigner; Eduard Schoenafinger; Eberhard Koester; Werner Stumpfi; Juergen Amort; Claus-Dietrich Seiler; Heinz Nestler; Otto Ambros
Original assignee: Dynamit Nobel AG
Current assignee: Dynamit Nobel AG
Priority date: 1974-12-18
Filing date: 1975-12-11
Publication date: 1980-06-10
Also published as: NL7514524A; GB1527760A; IT1060452B; JPS5186796A; FR2295096A1

Abstract

ABSTRACT OF THE DISCLOSURE: Chromium dioxide magnetic pig-ments can be stabilized against the loss of their magnetic properties by treating them, in alcoholic aqueous suspension, with a silane or a hydrolysis product thereof, so that when the pigment is isolated from the suspension medium the particles bear at least partially hydrolyzed silane on their surfaces, and then drying the magnetic pigment at from 50 to 200°C.

Description

1~79~57 OOZ. 31,049~519 STABILIZATION OF CHROMIUM DIOXIDE MAGNETIC PIGMENTS
The present invention relates to the manufacture and use of chromium dioxide magnetic pigments which have improved stability to the loss of their magnetic properties under adverse climatic conditions, and give ma~netic recording media with similarly improved stability.
The improvement o~ the stability of chromium dioxide magnetic pigments by sur~ace treatment has been disclosed. For example, it is known to treat chromium dioxide pigments, in aqueous suspen-sion, with a solution of a reducing agent, such as a sulfite solu-tion, whereby a protective layer o~ chromium(III) oxide or hydratedchromium(III) oxide i8 produced on the pigment surface. The pigment i8 then wa~hed, to remove the sulfate or sulfite~ and is dried. The stability of the resulting magnetic pigments depends on the thickness --of the resulting protective layer o~ chromium(III) oxide, but because Or the high residual induction required for magnetic tapes of high quality, the layer must not be very thick. The resulting chromium dioxide pigment~, and the magnetic tapes manufactured therewith, do not exhibit the requisite level o~ stability under adverse climatic conditions. The same applie~ to chromium dioxide pigments which have been stabilized in accordance with the procedure described in U.S.
Patent 3,686,031, i.eO by applying a protective layer of an inorganic metal phosphate; inadequate stabilization is achieved.
It is an object of the present invention to provide magnetic v~

.

~O~g~S7 recording media, based on chromium dioxide magnetic pigments, ` exhibiting improved stability against the loss of their magnetic properties under adverse climatic conditions, without a significant decrease in the residual induction of the magnetic layer of the recording media.

It is a further object of the present invention to provide suitable chromium dioxide magnetic pigments for incorporation in magnetic layers when manufacturing such improved magnetic recording media.

We have found that these objects are achieved by stabilizing chromium dioxide magnetic pigments against the loss of their magnetic properties by treating the chromium dioxide pigments, suspended in an al-ooholic, aqueous alcoholic or aqueous medium, with a silane of the formula R Si(oR') n 4-n where R is an organic radical of 1 to 18 carbon atoms, R' is an aliphatic radical of 1 to 8 carbon atoms, and n is an integer from 1 to 3,or with its hydrolysis productin the presence ofthe suspended chromium dioxide pigment, the pH of the suspension being from 2 to 6 or from 7.5 to 9.5 and the amount of silane or silane hydrolysis product being from 0~5 to 40% by weight based on the amount of the chromium dioxide magnetic pigments, so that after isolating the chromium dioxide pigments from the alcoholic and/or aqueous medium, the surfaces of the particles bear at least partially hydrolyzed silane, and drying the treated chromium dioxide pigments at from about 50 to 200C. the suspension containing from about 5 to 50~ by weight of dry chromium dioxide~

In the above formula, R' is an aliphatic radical especially of 1 to 4 carbon atoms and n is especially the integer 1~ Spray-drying has proved particularly suitable. It is particularly advantageous to .

1079~S7 heat the substantially dry chromium dioxide magnetic pign~-ts for several h~urs, eg. from 2 to 10 hours, at from 50 to 200C. and preferably akout 70 to 170C, preferably with the pigments in the form of a thin layer.
Though the heat treatment may be carried out in air or under an inert gas abmDsphere, use of the latter, eg. a nitrogen atmosphere, has proved par-ticularly advantageous.

m e present invention also relates to the use of the chrcmdum dioxide magnetic pigments, stabilized as above, for ~he manufacture~of ~ ~~
magnetic reo~rding media, and to magnetic recording media which - . . .. .
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1079~57 O.ZO 31,049/519 contain chromium dioxide magnetic pi~ments, stabilized as above, in the magnetic layerO
Advantageous alcoholic media in which the chromium dioxide magnetic pigments may be suspended have proved to be lower aliphatic alcohols and especially monohydric alcohols of 1 to 4 carbon atoms, egO methanol, ethanol, propanol or isobutanol, as well as appropriate alcohol/water mixtures which preferably contain at least 40 percent by volume of alcohol. Water alone may also be used as the solvent if a silane of the above formula which îs substantially water-soluble, or becomes substantially water-soluble or compatible with water as a result o~ hydrolysis, is used.
The suspension, which i8 of from 5 to 50 percent strength, advantageously ~rom 15 to 50 percent strength, and especially from 20 to 35 percent strength, by weight,is brought, by the addition o~ an acid, e~O acetic acid, or a base, eg. aqueous ammonia, to a slightly acid or slig~tly basic pH, advantageously to a pH of from a~out 1 ~o 6, especiall~ from 2 to 6, or from 705 to 9J 5; a pH o~ ~rom 8 to 9 has proved to be particularly advantageousO

In an advantageous embodiment, the silane is hydrolyzed in the suspension of the chromium dioxide pigments, in general by adding the silane to the said suspension, which is at a pH within the above ran~es or is suasequently brought to the appropriate pR by admixture of an acid or base~ The suspension is then advan- -tageously ~tirred for some time, preferably for from 2 to 10 hours, and pre~erably at from 20 to 80C, in order to complete the hydrolysis.
! Suitable silanes which can be hydrolyzed are those of the rormula RnSi(OR~)4-n where R is especially an aliphatic radical of 3 to 18 carbon atoms, -- -'' . : ' .:
, , OOZ. 31,049/519 and preferably of 3 to 8 carbon ~toms~ especially an appropriate hydrocarbon radical or a saturated or unsaturated radical inter-rupted by oxygen atoms in the carbon chain, egO a propyl, isobutyl, n-butyl~ hexyl and 2-ethylhexyl radical, an acryloxyalkyl or meth-acryloxyalkyl radical, a glycidyloxyalkyl radical or some other radical containing a functional group, or an aromatic radical, eg.
phenyl, R' is an aliphatic radical Or 1 to 4 carbon aboms, eg.
methyl and ethyl, which may advantageously be interrupted by oxygen atoms in the carbon chain, egO a methoxyethoxy radical or a radical R-0-(CH2-CH2-0)x, where R is alkyl of 1 to 4 carbon atoms and x is an integer of from 1 to 3 (silanes with the latter meaning of R' being particularly suitable for treating chromium dioxide in aqueous suspension), and n is an integer from 1 to 3, especially lo The use of n-butyl-trimethoxysilane and isobutyl-trimethoxysilane is pre~erred The hydrolysi~ of the silanes f;rst gives a product of the formula Rnsi(H)4 n~ :
but condenRation products thereo~ may also be produced depending on the hydrolysis conditionsO
The amount of silane or silane hydrolysis product generally used for treating the chromium dioxide ma~netic pigments i9 advantageously from 0~5 to 40 percent by wei~ht, and especially from 3 to 15 percent by weight 3 based on the amount of the chromium dioxide magnetic pig-ments. The suspension adva~geously contains from about 15 to 50 per-cent by weight, and especially from about 20 to 35 percent by weight, of dry chromium dioxideO
Chromium dioxide magnetic pigments which may be used are the conventional, preferably acicular, pigmen's of ferromagnetic crystal-line chromium(IV) dioxideO The~e pigments may already bear a protectiv~
surface layer before application of the silanes in accordance with the invention. For example, chromium dioxide magnetic pi~ments which, ~ollowing reduction of the pigment surface~ already carry a thin protective layer of chromium(III) oxide or hydra ~ chromium(III) oxide, are very suitableO

'', . " , ' '' . 10~79057 OOZ~ 31,049/519 We assume that in the treatment according to the invention the silanes applied and/or their hydrolysis products react at least partially with polar groups on the surfaces of the particles and the subsequent drying and heating, with elimination of water, promotes condensation of the hydrolysis productsD
The chromium dioxide magnetic pigments stabilized in accordance with the invention may be processed, by conventional methods, into dispersîons of the pigments in binders and solvents, for use in producing magnetic layers for magnetic recording media. For the production o~ such magnetic layers, the ratio of magnetic pigment to binder or binder mixture is in general from 2 to 10 parts by weight, and especially from 3 to 5 parts by weight, of magnetic pigment per part by weight of binder or binder mixturec The conventional binders for magnetic dispersions may be used, eg. vinyl chloride/vinyl acetate copolymers and their commercially available hydrolysis products containing from about 5 to 20 percent by weight of vinyl alcohol units, copolymers of vinyl chloride with lower esters of maleic acid or ~umaric acid, polyvinyl~ormals, copoly-amides, mixtures of ela~tomeric polyester-urethanes, which are prac-tically free ~rom i80cyanate groups, with vinyl chloride~vinyl acetatecopolymers, vinylidene chloride/acrylonitrile copolymers, polyesters or phenoxy resins, such as are described in German Printed Application No. 1,282,700, U~S0 Patent 3,144,352, German Printed Application No. 1,269,661 and German Printed Application NoO 1,295,011, and especially mixture~ of a) from 15 to 80 parts by weight of a soluble, thermoplastic polyester-urethane, which is partically free from hydroxyl groups, and is manu- -I ~actured from an aliphatic dicarboxylic acid of 4 to 6 carbon atoms, i eg. adipic acid, an aliphatic diol of 4 to 10 carbon atoms, eg.
30 1,4-butanediol, and a diisocyanate of 8 to 20 carbon atoms, eg.

4,4'-diisocyanato-diphenylmethane, and -b) from 20 to 85 parts by weight of a polyvinylformal which contains at least 65 percent by weight, pre~erably at least 80 percent by weight - : . . . . .. ..
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107g857 O.Z. 31,049/519 of vinylformal unitsO Very suitable polyvinylformals contain from 5 to 13 percent by weight of vinyl alcohol units, from 7 to 15 per-cent by weight of vinyl acetate units and from 80 to 88 percent by weight of vinylformal units and preferably have a viscosity of from 50 to 120 cp, measured at 20C on a solution of 5 g of polyvinyl-~ormal in 100 ml of a 1 : 1 phenol/toluene mixture.
Suitable polyester-urethanes are described, eg~, in German Printed Application NoO 1,106,959; particularly suitable products are those manufactured using branched glycols of ll to 12 carbon atoms, egO neopentylene glycol, as the sole glycol or as a mixture with linear glycols, egO ethylene glycol or 1,4-butanediol.
Conventional rigid or flexible base materials may be used as the non-magnetic and non-magnetizable bases, in particular films of linear polyesters, eg~ polyethylene terephthalate, which are generally from 5 to 50~u thick and in particular from about 10 to 35~u thick, Preparation of the magnetic coatings may be carried out in a conventional manner~ The magnetic dispersion prepared ~rom the magne-tic pigment and the binder solution in the presence of dispersing agents and other additives in dîspersing apparatus, e.g. a tube mill or a stirred ball mill, is advantageously filtered and applied to the non-magnetizable base in one or more thin layers or to a base already carrying another magnetic coating, using conventional coating equip-ment, e.g. a knife coater. Usually, orientation o~ the magnetic particles i8 effected be~ore the fluid mix has dried on the base, drying being advantageously carried out for from 2 to 5 minutes at temperatures o~ ~rom 50 to 90~.
The magnetic coatings may be subjected to a conventional sur-face treatment, e.gO calendering in which the coated material is passed between polished rollers, with the optional application of pressure and optional heating preferably at temperatures of from 60 to 80C. Following this treatment the thickness of the magnetic coating is generally from 3 to 20~u and preferably from 8 to 15/u.

O.Z. 31,01~9/519 The chromium dioxide magnetic pigments stabilized in accordance with the invention exhibit excellent dispersibility and processability.
It is surprising that, despite the usually high mechanical stresses to which the pigments are subjected during dispersing, they give magnetic recording media which, in a hot humid climate, exhibit a degree of stability against the loss of their magnetic properties, especially the residual induction, which is many times greater than that shown by the corresponding untreated chromium dioxide magnetic pigmentsO
In the following Examples and Comparative Experiments, parts and percentages are by weight, unless stated otherwise. Parts by volume bear the same relation to parts by weight as the liter to the kilogram.

100 parts of a filter cake, obtained after conventional surface reduction o~ chromium dioxide with sulfite and comprising about 40 parts o~ chromium dioxide pigment and about 60 parts of water, are suspended in 50 parts o~ methanol using a high-speed stirrer.
4.8 parts of iRobutyltrimethoxysilane are added and the suspension is then brought to a pH of 8.6 by adding aqueous ammonia and is stirred for a further 2 hours at room temperature to achieve almost complete hydrolysis of the silane to the silanol. The suspension is then fed to a nitrogen-operated spray tower where the magnetic pigment is dried, the gas temperature being from about 130 to 230C, One part of the batch is used without additional heat after-treatment (Example 1 A) ~or the manufacture of magnetic tapes.
A second part Or the batch is first heated, spread out in a thin la~er, ~or 4 hour~ at 100C in air (Example 1 B), and a third part o~ the batch is first heated for 8 hourB at 100C in air tExample 1 C) U~ing each of the chromium dioxide magnetic pigments obtained and a chromium dioxide magnetic pigment from the same CrO2 production batch, but obtained - without the silane treatment according to the invention - by spray-drying of the moist pigment filter cake -,. . .
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07 9 5~
OOZ. ~1,049t519 used to produce the stabilized pigment (Comparative Experiment I), magnetic tapes are produced by the same method. For this purpose, about 35 parts of each pigment, 002 part of stearic acid, o.8 part of isopropyl myristate, 27 parts of a solution, o~ about 13% strength, o~ an elastomeric polyester-urethane derived from adipic acid, 1,4-butanediol and 4,4'-diisocyanatodiphenylmethane in a mixture o~ equal parts of tetrahydro~uran and dioxane, 20 parts of a 13% strength solution o~ a polyvinyl~ormal in a 1 : 1 tetrahydro-~uran~dioxane mixture and 36 parts o~ the latter solvent mixture are dispersed ~or several days in a tube mill containing steel balls and the dispersion is then filtered through a paper filter, under pressure, and coated onto polyethylene terephthalate films ~-~
which are about 25/u thicko The coated films are passed through a magnetic field to orient the magnetic particles, dried at from about ~: .
70 to 90C, and then passed between rollers heated to about 80C.
The magnetic properties o~ the 4 di~ferent magnetic tapes : ~:
obtained are then measured (compare Table l)o It is ~ound that the stabilizing treatment, according to the invention, does not signi-~icantly reduce the residual lnduction of the magnetic layer of the tape.
To determine the stability o~ the magnetic tapes against the 1088 of their magnetic properties under adverse climatic conditions, samples o~ the tapes are stored in a conditioning cabinet at 65C
and 95% relative humidity, and the time it takes for the saturation - -:
magnetization of a sample to ~all to 90% of the value before the envîronmental test is determinedO This time (in days) is given as Ago in Table 1. From the number of days it is possible to derive a relative factor Frel = Ago f the treated material/A9O 0~ the untreated material, and this ~actor appears in the last column of Table lo It is readily apparent that the heat a~ter-treatment can substantially increase the stab:lizing e~fectO

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10'79057 o.z. 31,04~/519 200 parts of a chromium dioxide pigment (magnetic properties of the powder, measured at 160 kiloamps/m : Hc : 39.1 (kiloamps/m);
remanence : 4403 (nTm3g); saturation magnetization : 81l.1 (nTm3/g);
tap density : 10293 (g/cm3)) are susnended in a mixture of 440 parts of water and 200 parts o~ methanol, using a high-speed stirrer. The pH is brought to 8.9 by adding aqueous ammonia, 20 parts of iso-butyltrimethoxysilane are then added and the suspension is refluxed for 1 hour whilst stirringO After having cooled to room temperature, whilst stirring, the suspension is filtered and the pigment which has been separated of~ is rinsed with 100 parts of methanol. The chromium dioxide magnetic pigment is dried by prolonged heating at 45C followed by heating ~or 4 hours at 100C.
Magnetic tapes are produced, as described in Example 1, using the (treated) pigments and the untreated pigments (Comparative Experiment II) and their properties are measured. The results are given in the ~ollowing Table:
, Table 2 MR Mm Hc ~go Frel.
tmT) (mT) (kilo- ~days) (from A
! . . . amps~m) 90 .
Comparatîve Experiment II 146 170 3808 5.8 Example 2 155 180 38.3 25.7 4.43 The procedure o~ Example 2 is followed except that ethanol is used in place of methanol in the pigment suspension. The magnetic tape exhibits the ~ollo~ing properties:

~ 150 Mm ~ 174 Hc = 38~4 Ago = 12.1 F = 2.1.

100 parts of chromium dioxide pigment are suspended in 322 parts of methanol and 38 parts of isobutyltrimethoxysilane and 40 parts of 2% strength aqueous acetic acid are added whilst stirring.

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. . . ,: . : . . - . - - -10790s7 O.Z. 31,059/419 The mixture i~ stirred ~or 1/2 hour at room temperature and the pigment is then filtered O~r and dried ~or 4 hours at 100C.
Magnetic tapes produced from these pigments as described in Example 1 and sub~;ected to the environmental test have an Ago value of 14.3 days The procedure followed is as in Example 4 except that the suspension is stirred ~or 4 hours at room temperature to bring about the hydrolysisO The Ago value of the magnetic tapes produced as described in Example 1 using the pigment obtained from the suspension is 1507 daysO
EXAMPLES 6 to 8 lQ 3 separate batches are prepared using high-speed stirrers, each batch consisting o~ 200 parts o~ chromium dioxide pigment ~ originating ~rom the same production batch suspended in 700 parts - of technîcal-grade ethanol~
EXAMPLE 6: 32 parts of vinyltrimethoxysilane are added to the 1st batch, and the pH of the suspension is brought to 405 with acetic acid.
EXAMPLE 7: 32 parts o~ ~-methacryloxypropyl-trimethoxysilane are added to the 2nd batch, and the pH of the suspension is brought to 3.7 with acetio acid~
EXAMPLE 8: 32 parts of ~-glycidyloxypropyltrimethoxysilane are added to the 3rd batch.
All 3 batches are stirred ~or 3 hours at room temperature;
therea~ter, the pigment is in each case filtered o~f, washed with ethanol and dried in vacuo at 70Co Magnetic tapes are produced using the pigments treated in this wa~, and the untreated pigment ~rom the same production batch, as de~cribed in Example 1, and are subjected to the environmental test (c~0 Example l)o Compared to the tape produced with untreated pigment, the other tapes have Ago values improved by the ~ollowing ~actors:

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1079057 o.z. 31,059/419 Example 6: Frel = 2.87;
xample 7: FrelO 20 69;
Example 8: Frel = 20 910 EXAMPLES 9 and 10 a) Stabilization o~ chromium dioxide magnetic pigment In each Or two stirred vessels, 300 parts of a finely divided chromium dioxide magnetic pigment are suspended in 700 parts of distilled water by means o~ high-speed stirrers. 90 parts of n-propyltrimethoxyethoxysilane are then added to each suspension.
The pH o~ one suspension (Example 9) i5 brought to 9.1 by adding aqueous ammonia while stirring, whereas the pH of the other suspension is brought to 302 by adding acetic acid (Example lO)o Both suspensions are stirred thoroughly for a further 2 hours 10 at room temperature~ The two batches are then ~iltered separately, -and the treated magnetic pigments are each dried ~or 4 hours at , about 100Co ,, b) Produ¢tîon o~ magnetic tapes using chromium dioxide magnetic pigments stabilized as above (Example 9 and 10) and using chromium dioxide which has not been treated with silane.
Using-each Or the ohromium dioxide magnetic pigmen~ stabilized '' as described under a) (Example~ 9 and 10) and using a chromium dioxide magneti¢ pigment ~rom the same CrO2 production batch, but obtained - without treatment with silane in accordance with the invention - by spray-drying o~ the moist pigment filter cake used to produce the stabilized pigment (Comparative Example III) t magnetic tapes are produced by the process described in Example 1.
Measurement o~ the residual induction Or the magnetic coatings Or the 3 difrerent magnetic tapes obtained shows that this property is not lowered by stabilization in accordance with the invention.
In the environmental test, the tapes have the following Ago values: , Example 9: 1805 days Example 10: 10,4 days , -Co,mparative,Experiment III tuntreated): 515 days. ,~' - - 12 - ~

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Process for stabilizing a chromium dioxide magnetic pigment against the loss of its magnetic properties, wherein the chromium dioxide pigment in particle form, suspended in an alcoholic, aqueous-alcoholic or aqueous medium, is treated with a silane of the formula RnSi(OR')4-n where R is an organic radical of 1 to 18 carbon atoms, R' is an aliphatic organic radical of 1 to 8 carbon atoms and n is an integer from 1 to 3, or with its hydrolysis product, in the presence of the suspended chromium dioxide pigment, the pH of the suspension being from 2 to 6 or from 7.5 to 9.5 and the amount of silane or silane hydrolysis product being from 0.5 to 40% by weight based on the amount of the chromium dioxide magnetic pigments, so that after isolating the chromium dioxide pigment from the alcoholic, aqueous-alcoholic or aqueous medium, the surfaces of the chromium dioxide particles bear at least partially hydrolyzed silane, and the treated chromium dioxide pigment is dried at from about 50 to 200°C, the suspension containing from about 5 to 50% by weight of dry chromium dioxide.

2. A process as claimed in claim 1, wherein the treated suspended chromium dioxide magnetic pigment is spray-dried.

3. A process as claimed in claim 1, wherein the substantially dry treated choromium dioxide pigment is heated for several hours at from about 70 to 200°C.

4. An improved magnetic recording medium wherein a magnetic layer of from about 3 to 20 µ contains a chromium dioxide magnetic pigment stabil-ized with a silane of the formula RnSi(OR')4-n where R is an organic radical of 1 to 18 carbon atoms, R' is an aliphatic organic radical of 1 to 8 carbon atoms and n is an integer from 1 to 3, or with its hydrolysis product.