DE4313513A1 - Magnetic recording medium with uniform charging across pigment surface - Google Patents

Abstract

Magnetic recording medium comprises, on a nonmagnetic substrate, a magnetic dispersion(s) contg. (i) a dispersant; and (ii) a pigment dispersed in a polymer binder, the pigment being precoated with a long-chain dispersant(s) (a) having a sterically stabilising effect and comprising lecithin, diamine, phosphoric ester of polyalkylene oxide contg. a polar gp. capable of absorption onto the pigment and a low mol.wt. codispersant (b) from solvated acids and bases, in amt. to enhance the electrostatic charge of the pigment surface, subsequently added. Also claimed is the prepn. of the recording medium where the pigment is precoated with (a) with subsequent addition of (b), followed by dispersion of the coated pigment in a polymeric binder and application of the dispersion to a non-magnetic substrate. Precoating of the pigment is pref. effected using lecithin or alkylated or ethoxylated diamine; (b) pref. comprises tetra-n-alkylammonium hydroxide, pentaalkylguanidine, pentaisoalkylguanidine, piperidine, ethanolic KOH soln., metal alkoxylate, Grignard cpd. or metal hydride; and pref. the pigment comprises CrO2 or gamma Fe2O3. Alternatively coating is effected using a long-chain phosphoric ester or a polyalkylene oxide acrylate contg. carboxylic acid; (b) comprises HCl, short-chain sulphonic acid, short-chain phosphonic acid, diethylhexyl phosphate or dibutyl phosphate; and pref. the pigment is a ferrite or metal type having an oxidic surface. Esp. (b) is used in amt. to provide a max. ESA value.

Description

The invention relates to a magnetic recording carrier, consisting of a non-magnetic layer carrier and at least one applied magneti rule dispersion containing finely divided magnetic Pig which disperses in a polymeric binder are as well as dispersing aids.

The development of magnetic recording media is alright for years in the direction of higher recording density and improved signal-to-noise ratio. To own this must improve the degree of filling of the magnetic Layer increased and a good surface smoothness by Ver Improvement of the dispersion and the orientation of the feintei soluble magnetic pigments, which are excellent magnetic Properties must be achieved.

Another requirement for magnetic recording media with high recording density is a Störteilchenfreie Layer. This is on the one hand only with magnetic Disper To reach sions that contain no agglomerates. Ande On the other hand, the filtration of the magnetic dispersion with filters of extremely small pore size (around 5 μm) necessary.

In order to achieve the desired dispersing properties, is an unconditional requirement that certain binders be used, which has both good binding properties as well as good dispersing properties for the magnetic Own pigment. Such dispersing compounds are hereinafter called dispersing resins. It will in the magnetic dispersions a certain proportion of polymeric binder, between 2 and 100%, by active  high or low molecular dispersion resins replaced, which a very good affinity for finely divided magnetic pigment to have. Here are among the dispersing resins chemical Compounds understood by polymerization, polycon condensation or polyaddition obtained from monomer building blocks and at least one polar for adsorption the surface of the magnetic pigment suitable group contain. This, however, the viscosity of the magneti under certain circumstances extremely increased, whereby Difficulties occur, the greater the fine the magnetic pigment is the part and the below be described in more detail.

In particular, magnetic dispersions containing finely divided pigments having a BET value of more than 35 m ² / g as ma gnetisches pigment can have a high yield point with the conventional formulation ingredients such as polymeric binder, which is why they are difficult to filter with special filtration units , Especially if the magnetic dispersion has a long life during production, which can occur for example at a Pro duktionsunterbrechung easily. If an increase in the freeness or pigment charge fluctuations leads to an increase in the secondary agglomerates, filter breakthroughs can occur, as a result of which the yield of strip material is drastically reduced.

There was no lack of attempts, the necessary homogeneity reach the dispersion.

By intensive grinding of the magnetic dispersion leaves to achieve a certain degree of homogeneity, by un different strong entanglement or sintering the pigment agglomerate is the more easily splittable pigment However, already heavily overpowered before the simmered Pigment needle content is sufficiently crushed. Insbeson  In the case of metal pigments, it is possible to stabilize them against oxidation sierende surface layer to be destroyed.

In DE-OS 39 05 910 it is proposed during the fine dispersion of the magnetic dispersion to add just as much dispersing aid in several stages as the pigment surface is newly formed by the dispersing process. From DE 10 05 754 it is known, for lowering the viscosity of the dispersion built up with conventional binders, to add certain phosphoric acid esters. Furthermore, from DE-OS 25 35 277 and US 4 533 565 known, the surface of the magnetic Pig ments with certain compounds such as polymerizable alkylene oxides or compounds with polar groups bele conditions before they are added to the magnetic dispersion. In DE 22 50 384 are for a magnetic Aufzeich tion carrier, which contains as a magnetic pigment CrO ₂ , described as dispersing alkylarylsulfonic acids alone or in admixture with phosphoric acid esters or with alkoxylated alkylphenols.

In the German application P 40 34 747 are low molecular weight Mono- or diesters of phosphoric acid described which used in combination with a dispersing resin wherein the dispersing resin is at least one polar, for Adsorption on the magnetic pigment capable group ent holds.

From several publications by F. M. Fowkes, in particular in "Colloids and Surfaces in Reprographic Technology", ACS Symposium Series 200, Am. Chem. Soc. (1982) p. 307, "Poly Adsorption and Dispersion Stability ", ACS Symposium Series 240, Am. Chem. Soc. (1984), p. 331, "Ceramic Powder Science and Technology; Advances in Ceramics ", Vol. 21 (1987), P. 411, "J. Adhesion Sci. Techn.", Vol. 1 (1987), No. 7.  It is known that the electrostatic repulsion to Stabi tion of dispersions in organic media. For this purpose, an ionic interaction of the dispersant assumed with the pigment surface (acid-base-alternating Effect) and an excess of unadsorbed Dis pergator leads to a charge generation at the pigment surface. The formed corresponding acids or Bases, solvated as a diffuse ionic layer, stabilize the charge build up on the pigments. On the other hand is ever the person skilled in the magnetic field that a Excess of unadsorbed dispersant detrimental Properties on the mechanical, magnetic and position tion properties of the magnetic recording medium has.

Therefore, the object was a magnetic recording carrier of the generic type mentioned above, in which in the preparation of the dispersion an optimal Equal charge of the pigment surfaces and thereby an increase increase in dispersion stability, a low yield point and a flat course of the τ-D diagram (shear stress / shear speed), resulting in good mechani and magnetic properties of the recording medium leads. According to the invention, the object has been achieved with a magnetic recording media with the characterizing Part of claim 1 features. Further single units of the invention will become apparent from the dependent claims and the description.

The essence of the invention is that the first Surface of the magnetic pigments with a steric stabilizing acting long-chain dispersant occupied and that in a second step, a low molecular weight Codispergator is added for magnetic dispersion, selected from solvatable acids or bases, wherein the amount of codispergator is such that it has a  Charge reinforcement produced on the pigment surface.

The choice of the sterically stabilizing dispersant so How the codispergator depends on whether the magnetic Pigments strengthen acidic or basic surface groups respectively.

In the case of magnetic pigments such as CrO ₂ or γ-Fe ₂ O ₃ , the pre-assignment of the magnetic pigments is preferably effected by long-chain basic or amphoteric dispersants, for example by lecithins and / or alkylated or ethoxylated amines, in particular tertiary amines, diamines or polyamines. As low-molecular codis pergatoren solvatable bases are added in a second stage, for example, quaternary ammonium compounds, in particular tetra-n-alkylammonium hydroxide, penta-al kyl-guanidine, penta-isoalkyl-guanidine, where as Alkylgrup pen methyl-bis-octyl groups in question , 1,2,2,6,6-penta methylpiperidine, ethanolic KOH solution or others.

Furthermore are suitable:

• Metal alkoxylates, for example aluminum secbutylate, Aluminum tert.butylate, aluminum ethylate, aluminum iso propylate, potassium tert -butylate, potassium ethylate, potassium me thylate, sodium ethylate, sodium methylate, sodium tert.amyl lat,
• - Grignard compounds, for example methylmagnesium chloride, methylmagnesium iodide, methylmagnesium bromide,  
• - Metal hydrides, in particular NaBH ₄ sodium borohydride, Na triumtetrahydridoborat (sodium boronate) or the corre sponding Li or K compounds, LiAlH ₄ lithium aluminum hydride, Lithiumtetrahydridoaluminat (lithium alanate), LiH, NaH, KH in oil, as suspension lithium hydride, NaNH ₂ in toluene as a suspension (sodium amide), LiNH ₂ potassium tri-sec. butylborohydride, K-Selectride.

In the case of magnetic pigments with a basic surface, for example ferrites, in particular co-ferrites and metal oxides with an oxidic surface, for example an Al ₂ O ₃ layer or a MeSiO ₃ layer or mixtures thereof, long-chain dispersants other than the aforementioned combination are also suitable for the pre-assignment of phosphoric acid esters such as the known from the prior art Gafac or Lutensit or a carboxylic acid-containing Polyalkylenoxidacrylat suitable. Also suitable as codispergator are solvatable acids such as HCl, HClO ₄ , short-chain phosphonic or sulfonic acid or diethylhexyl phosphate or dibutyl phosphate.

The selection of the codispersants is based on the determined ESA signal, as explained below. The higher the Ab absolute value of the ESA signal, the more effective the codis pergatoren, and this depends, among other things, on how theirs Dissociation in the following organic solution is obtained. The optimized codispersant amount can be determined by titration of the dissolved codispergator in determine the magnetic pigment suspension. This is chooses the amount for which a maximum ESA value results, This usually involves a larger concentration range for the codispergator.  

Binder for the magnetic recording according to the invention carriers are, for example, copolymers of vinyl chloride, Vinyl acetate and vinyl alcohol, copolymers of vinylidene chloride and acrylonitrile, polyvinylacetals such as polyvinylformals, Polyester / polyurethanes, polycarbonate / polyurethanes, polyure than- or polyether elastomers, phenoxy or epoxy resins, as well as their mixtures and the already mentioned Dispersing resins.

As solvent for all aforementioned polymeric bandage For example, tetrahydrofuran, dioxane, di methylformamide, cyclohexanone, methyl ethyl ketone, toluene, Methyl isobutyl ketone and others, optionally also in Mixture, are used.

Of course, the above-mentioned Dispersierhilfs medium also combined with other dispersing aids be such as cephalin, fatty acid amines or -diamines, fatty acid amides or diamides, fatty acids relate or their ammonium salts, ethoxylated fatty acids optionally, aliphatic or aromatic, optionally ethoxy phosphoric esters, sulfosuccinic acid esters, sorbi tanester, sulfonates, fatty alcohol sulfates and others.

Additional recipe ingredients can vary depending on the desired Properties of the dispersion or the finished layer Ver find a turn. In particular, lubricants such as for example, fatty acids or fatty acid derivatives, silicone oils, Paraffins, waxes, fluorinated oils, dispersions of poly tetrafluoroethylene. Possible other additives are for example As plasticizers, abrasives, crosslinkers and given if crosslinking catalysts, viscosity-regulating Fabrics and more.  

After introduction of the pre-assigned and pre-dispersed pigments in the dispersion is the An Homo on a mixing unit under high shear forces geniated, for example, on a kneader of a colloid mill, a ball mill or an attritor can be done. Subsequently, the fine dispersion takes place with addition of the codispergator, for example, on a sand mill whose Mahlintensität by variation of the grinding media size and filling, the speed and the paint throughput controlled can be.

The subsequent coating of the non-magnetic layer Carrier with the magnetic dispersion is carried out according to the State of the art, for example by means of reverse roll coater, Screen printing knife pourer or extruder caster.

As a support film can be made of polyester, such as polye thylene terephthalate, polyolefins such as polypropylene, Cellu losederivate such as triacetate, polycarbonates or rigid layer carrier of non-magnetic metals such as aluminum or ceramic materials are used.

Further processing of coated materials, such as the surface smoothing by calendering, cutting and con make up in a known manner.

From the examples are order of approach as well as amount of used dispersants for pre-assignment of the pigment top surface as well as the subsequent use of the codispersants to see.  

As already explained above, a charge build-up on ma magnetic pigment through the use of charge-generating low molecular weight codispersants, consisting of solvati targeted bases or acids brought about. The there with additionally effective electrostatic repulsion of the magnetic pigments leads to a considerable Increasing the dispersion stability.

In the context of the present invention has now been found that the charge ratios of the magnetic pigment by means ESA measurement (electrokinetic sound amplitude), to be followed to determine the optimal amount of codispergator. The measuring principle of the ESA measurement is that two Elek electrodes with a high-frequency alternating voltage, preferably 1 MHz from a power generator. The charged dispersed particles and their opposite Charged surroundings are due in the alternating electric field their different mass inertia different degrees accelerated. This leads to a one-sided predominant Solvent absorption and thus to build up a pressure wave, whose frequency coincides with the AC frequency. The effect is greater the greater the charge of the disperse As the accelerating force in the elek tric field with the particle charge increases. The stability and absorption properties of concentrated dispersions depend significantly on the net surface charge of the disper gelled magnetic pigments.

A suitable device for measuring the ESA is sold by the company Matec Instruments Inc., USA. Between the electrodes of the apparatus, a dispersion was introduced consisting of 2% by weight of the magnetic pigment, which was pretreated according to the invention with a long-chain dispersant and admixed with a low molecular weight codispergator, dispersed in 100 parts by weight of the solvent THF. It was found that a sufficient charge generation on the magnetic pigment, and consequently a very good Disper sion stability achieved if the described ESA measurement revealed an absolute value of at least 10 m × × uPA volts ^-1, measured at 1 MHz. Accordingly, the addition amount of codispergator, which can be very small, was controlled so as to obtain or exceed the above value, as shown in the examples below. The sign of the obtained value of the ESA measurement is positive or negative, depending on the sign of the generated charge on the pigment surface.

Examples 1 to 8

A magnetic dispersion of the following composition 1 was prepared, wherein after addition of the dispersant 1 Stirred for 30 minutes and then the codispergator and the further additives were added and 6 hours in a stirred ball mill in the presence of ceramic Mahlkör pern of 0.6-0.8 mm diameter was finely ground. to closing

• a) as described above, the ESA value of the dispersion and the rheological data (τP, τ500) measured.
  τp is the yield stress (Pa)
  t500 is the shear stress (Pa) measured at 500 s ¹ shear rate.
• b) A wet swab with a wet film thickness of 50 μm optically measure the gloss of a glass plate.

Composition 1 Parts by weight CrO₂ (BET = 30 m² / g, _I H _c = 52 kA / m² 100 Dispersant (s) 1 2.8 Codispergator 0.15 Vinyl chloride copolymer 3.4 Solvent (tetrahydrofuran, cyclohexanone) 119

Table 1 shows the results, with examples 2, 4, 6 and 8 have the composition according to the invention, while Examples 1, 3, 5 and 7 are comparative examples are which do not correspond to the present invention.

Table 1

The terms mean:
L = highly purified lecithin
B = tetra-n-butylammonium hydroxide
ED = ethoxylated alkyl-propylene-diamine
AD = alkyl propylene diamine.

Examples 9-14

A magnetic dispersion of the following Composition 2 was prepared as described above.

Subsequently, the dispersion was treated with Pall profile filters the pore size 5 microns and filtered in an extruder  Foundry on a 15.2 microns thick 66 cm wide polyethylene Terephthalatfolie with a dry film thickness of 2.3 microns potted and calendered after drying. The Pigmentvo lumenkonzentration was 48%.

The magnetic recording medium was at half inch width longitudinally cut and the relevant mechanical and ma magnetic characteristics of the magnetic layer were measured. The results are shown in Table 2.

Composition 2 Parts by weight CrO₂ 85 Co-doped γ-Fe₂O₃ 15 Polyester-polyurethane (molecular weight 80 000) 14.1 Vinyl chloride copolymer 6.1 diisocyanate 3.3 Dispersant 1 2.4 Codispergator 0.15 fatty acid 0.6 fatty acid ester 0.8 Solvent (tetrahydrofuran, cyclohexanone) 209

Table 2

Example 9 (comparative example) does not have the Invention proper composition, all other examples 10-14 ent speak the invention.

Examples 15-17

A magnetic dispersion of the following composition tion 3 was prepared as described above.

Composition 3 Parts by weight Co-ferrite (BET 42 m² / g, _I H _c 54 kA / m) 100 Vinyl chloride copolymer 6 Polyester polyurethane 14 diisocyanate 3.3 Dispersant 1 4 Codispergator 0.15 fatty acid 0.6 fatty acid ester 0.8 Solvent tetrahydrofuran 180

The dispersion was skimmed as in Examples 1-8 This was further processed and gave the in the table 3 results.

Table 3

The terms mean:
G = penta-alkyl guanidine
P = 1,2,2,6,6-pentamethylpiperidine.

Example 18

A magnetic dispersion of composition 4 was prepared as described above and as in Example 1-8 further processed.

Composition 4 Parts by weight Fe pigment (BET 56 m² / g, _I H _c 120 kA / m) 100 Polyester polyurethane 8th Vinyl chloride copolymer 8th diisocyanate 2 Dispersant 1 (AD) 2.0 Codispergator (B) 0.2 Fatty acid mixture C 14 - C 18 2 Solvent tetrahydrofuran 250

The dispersion gave the following results:

Claims (4)

1. Magnetic recording medium, consisting of a nonmagnetic substrate and at least one magnetic dispersion applied thereon, comprising finely divided magnetic pigments which are dispersed in a polymeric binder and dispersing agents, characterized by a pre-emplacement of the magnetic pigments with at least one long-chain sterically stabilizing dispersant, selected from lecithins, diamines, phosphoric esters or polyalkylene oxides having at least one polar group capable of adsorption on the pigment and by subsequent addition of at least one low molecular weight codispergator selected from solvated acids or bases, the amount thereof for generating an increase in the electrostatic charge on the Pigment surface is sufficient. 2. A magnetic recording medium according to claim 1, since characterized in that the pre-assignment by lecithins and / or alkylated or ethoxylated diamines and that the codispergator is selected from tetra-n-al kylammonium hydroxide, penta-alkyl-guanidine, penta-iso alkyl-guanidine, piperidine, ethanolic KOH solution, Metal alkoxylates, Grignard compounds and / or metal hydrides. 3. Magnetic recording medium according to claim 1, characterized in that the surface coverage by long-chain phosphoric acid ester and / or carboxylic acid-containing Polyalkylenoxidacrylat done and that the codispergator is selected from HCl, HClO ₄ , short-chain sulfonic acid, short-chain phosphonic acid, diethylhexyl phosphate or dibutyl phosphate. 4. A method for producing a magnetic Aufzeich tion carrier according to claims 1 to 3, characterized in that the addition amount of codispergator is such that the ESA measurement gives an absolute value of at least 10 mPas × m × vol ^-1 , measured at 1 MHz in a dispersion consisting of 2 wt.% Magnetic pigment based on 100 weight percent solvent.