DE3007218A1 - MAGNETIC RECORDING MEDIUM WITH SELF-CROSS-LINKED LATEX BINDER, METHOD FOR THE PRODUCTION THEREOF AND STABLE DISPERSION OF THE LATEX Binder - Google Patents

Description

M 4189

Minnesota Mining and Manufacturing Co., 3M Center, Saint Paul, Minnesota 551ol, V. St. A.

Magnetic recording medium with self-crosslinked latex binder

Process for its preparation and stable dispersion of the latex binder

A typical magnetic recording medium consists of a backing member that has a layer of magnetizable Contains particles in a polymeric binder. To apply the magnetizable layer, the binder is usually in organic solvent, which, however, will pollute the atmosphere if not recovered after volatilization will. The recovery of organic solvents can in turn pose the risk of explosion and fire.

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Such problems are kept to a minimum by water-based binders, such as those of LJS-PSS 3 o23 123, 3,795,539 and 3,9ol,816. Described by these patents only US-PS 3,9ol 816 crosslinkable aqueous binders, the crosslinking is currently considered necessary, to have fully satisfactory physical properties, such as resistance to blocking and wear of the magnetic Recording tape.

Magnetic recording tapes made using aqueous binders are, as far as is known, not on the market.

The invention relates for the first time to technically accessible magnetic recording media with a coating of magnetizable media Particles in a cross-linked binder applied from water. The aqueous latex used for application The coating used has self-crosslinking properties, but it can both itself and its dispersion with magnetizable particles can be stored longer at normal room temperatures and shipped commercially. Magnetizable particles dispersed in the aqueous latex can be at least as convenient and economical as Binder based on organic solvents are drawn up. The raw materials for the latex are readily available

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Trade available at a reasonable cost. The strength against Abrasion and blocking of the coatings is excellent. Surprisingly, a smooth recording surface can be achieved without difficulty. In addition, if one considers the widely used biaxially oriented Polyethylene terephthalate film backing uses the physical properties of water-based coatings significantly improved over such solvent-based coatings, because there are low molecular weight components extracted from the film base by organic solvents migrate to the surface of the recording layer and crystallize there. The latex on water The basis shows little or no tendency to extract such low molecular weight components from polyethylene terephthalate film substrates.

Non-magnetizable inorganic particles commonly found in Coatings used on magnetic recording media such as carbon black and alumina are also effective to be dispersed in the aqueous latex.

The stable, coatable dispersion of inorganic particles in a water-based latex is new and will be used here presented for the first time. The latex has a self-crosslinking copolymer made from the monomers (in parts by weight)

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- Io -

(1) 100 parts of an alkyl acrylate and / or alkyl methacrylate having 1 to 8 carbon atoms in the alkyl group or a mixture of alkyl acrylates and / or alkyl methacrylates with an average of up to 8 Carbon atoms in the alkyl group,

(2) 1 to 2o parts of acrylic acid, methacrylic acid and / or itaeonic acid and

(3) 1 to 15 parts of glycidyl acrylate and / or glycidyl methacrylate on.

The manufacture of a coating on a magnetic recording medium comprises dispersing inorganic parts in the aqueous latex, drawing the dispersion onto a support and heating to dry the coating. The copolymer of the latex is during the drying step cross-linked to the coating. These monomers are chosen so that the copolymer is in the coating after crosslinking has a second glass transition temperature, T, of -5 ° to 60 ° C.

Crosslinking could begin during emulsion polymerization of the latex and progress to an undesirable extent when the latex is exposed to high temperatures. Around

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To avoid this, the monomers are preferably copolymerized at 5o to 60 ⁰ C. If no further signs
a reaction are present, polymerization initiator can additionally be added, after which the mixture is ^{heated at 60 to 80 °} C. for about 1 hour in order to ensure essentially complete conversion of the monomers.

If less than two parts of the copolymerizable acid or less than two parts of the glycidyl acrylate and / or glycidyl methacrylate be used to produce the copolymer, the copolymer can be used when drying the coating does not crosslink adequately by itself, so it is appropriate to use an additional crosslinking agent.

Use of more than about 11 to 13 parts of the glycidyl acrylate and / or glycidyl methacrylate could cause problems with the stability of the aqueous latex in admixture with it
Cause iron oxide particles and especially with iron particles, on the other hand, however, the aqueous latex should remain sufficiently and suitably stable for several months at ordinary storage temperatures, both before and after mixing with inorganic particles. 5 to 10 parts are preferred.

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The use of more than about 16 to 18 parts of the copolymerizable Acids could create difficulties in the manufacture and handling of the aqueous latex. the The best results have been achieved with 10 to 15 parts of acrylic acid and / or methacrylic acid.

Good quality coatings have been made using methyl acrylate, ethyl acrylate, and n-butyl acrylate as Alkyl acrylate, preferably two of these, used to optimize tape properties. Isobutyl acrylate, t-butyl acrylate and the propyl acrylates should be considered the same Alkyl acrylates may be useful, but the propyl acrylates are not readily available. The alkyl methacrylates have tend to produce higher T than the alkyl acrylates and tend to be brittle. Hence it is preferred to use the methacrylates to be used only in connection with the acrylates.

It is believed that by mixing two of the copolymers as T differs widely, the obtained magnetic recording tape has less changes in work performance shows as a function of temperature changes. If such mixtures are used, the T of one of the copolymers after crosslinking in the coating essentially below -5 ° C, e.g. up to about -2o ° to -5o ° C, if the T of the other copolymer after crosslinking is relative

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is high, e.g. around 40 to 60 ° C. Mixtures of copolymers tapes with such low and high T values have produced unusually good strength against show the blocking.

Up to about 50% by weight of the alkyl acrylate and / or alkyl methacrylate can be replaced by acrylonitrile; this tends to reduce the toughness and therefore the wear resistance to improve; Io to 40% are preferred. Methacrylonitrile is also suitable. However, the toxic nature of acrylonitrile and methacrylonitrile implies additional costs With regard to the device to protect the people who make the tape.

Another preferred copolymerizable monomer is 2-hydroxyethyl acrylate. If it is used as a substitute for up to 25% by weight of the alkyl acrylate and / or alkyl methacrylate, it shows the tendency, the frictional properties of the recording layer to improve. 2-hydroxyethyl methacrylate and 2-hydroxypropyl acrylate and methacrylate are also readily available and have the same beneficial effects.

Other copolymerizable monomers, such as vinyl chloride, vinyl acetate, Butadiene, styrene, isoprene, diallyl phthalate and acrylamide can also be used in limited quantities,

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which are chosen so that they do not have any drastically changed properties bring forth. Methacrylic acid ester of polyethylene glycol with an average molecular weight of 4oo and 75o have been used up to about 12% by weight of the alkyl acrylate and / or to replace alkyl methacrylate without in differences became apparent in the results.

A dispersion of inorganic particles in the aqueous latex can be used in the same manner as a solvent-based coating be applied. The dispersion is self-thickening and provides a dense, uniform coating, without the need for a defoaming device.

Crosslinking of the interpolymer appears to be essentially complete when the latex is dried. By doing As the crosslinking is still incomplete, it progresses slowly during the storage of the windings of the finished Tape continues at room temperature and can be accelerated by exposing the rolls of the finished tape to elevated levels Temperatures such as 60 to 70 ° C for about 2 weeks. As long as the glycidyl acrylate or glycidyl methacrylate and copolymerizable ones If acid is present within the ranges given above, the copolymer is not highly crosslinked.

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As soon as the magnetizable coating is dry, a second aqueous latex coating can be applied without the risk of destroying the underlying layer.

The particle size of the copolymer of the aqueous latex is preferably less than 0.25 μm in order to ensure good coalescence to choose.

In the following batches, all parts of the data are parts by weight.

Aqueous latex A

The following premix was made in a mixing vessel under nitrogen:

Parts by weight

Deionized water 4oo

Emulsifier 16

n-butyl acrylate loo

Methyl acrylate 18ο

2-hydroxyethyl acrylate 56

Methacrylic acid 4o

Glycidyl methacrylate 24

t-dodecyl mercaptan o, 2

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In a glass-lined polymerization reactor, 200 parts of the premix and 400 parts of water were placed under nitrogen. Everything was agitated and heated to 35 C and mixed with 0.32 parts of potassium persulfate and 0.24 parts of sodium metabisulfite. The exothermic reaction raised the temperature to 60 ⁰ C and the reaction cooled to 55 ° C and the remaining premix was added at a rate that holding the temperature between 55 ° C and 60 C (about
Io parts per minute). When all of the premix was added, 0.08 parts potassium persulfate and 0.4 parts sodium metabisulfite were added; the approach
was heated to 70C for 1 hour, then cooled and filtered. The aqueous latex A obtained had a pesticide content
of 33.6 %, a pH of 4.8 and an inherent viscosity of 1.2
dl / g in dimethylformamide at 0.15 g / dl. The latex contained less than 0.1% unreacted monomer.

Gel swelling tests in dimethylformamide showed that the mixed polymer of latex A was self-crosslinking. The calculated

T of the copolymer was 4 ° C.
y

Aqueous latex B

The following premix was taken in a mixing kettle
Nitrogen produced.

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Parts by weight

Ethyl acrylate 144

Acrylonitrile 4o

Acrylic acid io

Glycidyl methacrylate 6

t-Dodecy! mercaptan o, l

Under nitrogen, 100 parts of the premix with 400 parts of deionized water and 8 parts of emulsifier were introduced into the polymerization reactor. After stirring and heating to 30 ° C., 0.4 parts of potassium persulfate, 0.134 parts of sodium metabisulfite and ο, οοΐ parts of iron (II) sulfate heptahydrate were added. After the exotherm to 66 ° C. to 7 ° C., the batch was cooled to 5 ° C. and the remaining premix was added; the temperature was allowed to rise to 65-70C. The batch was distilled with steam in order to remove unreacted monomer, cooled and filtered. The aqueous latex B obtained had a solids content of 38%, a pH of 3.4 and an inherent viscosity of 1.4 dl / g in methyl ethyl ketone at 0.15 g / dl. The latex contained less than 0.5% unreacted monomer.

Gel swelling tests as with latex A showed that the copolymer of latex B was self-crosslinking. The calculated T of the copolymer was 3 ° C.

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Further aqueous latexes were prepared in the same way as the aqueous latex B, except that steam distillation was used omitted and the following monomers used.

η-butyl acrylate η-butyl methacrylate Methacrylic acid glycidyl methacrylate calculated T.

more watery Latex C Water latex D 85 - - 85 Io Io 5 5 -4o ° C. 40 ° C example 1

A grinder was filled with 100 parts of needle-shaped particles with an aspect ratio of 5: 1 and an average length of 0.4 µm, 2 parts of a dispersant, 2 parts of alumina, 2 parts of NN-dimethylaminoethanol and parts of deionized water. After the resulting slurry was milled through to uniformity, 91.7 parts ( diluted to 32.43% pesticides) of Aqueous Latex B and 3 Tenile lubricants were added to make a dispersion which filtered, deaerated and doctored on biaxially oriented polyethylene terephthalate film backing was drawn up. The wet coating was passed through a magnetic field to move the needle-shaped particles longitudinally

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direction, and heated in an oven to temperatures not exceeding 95 ° C to avoid the volatile Abort levels. After polishing, the coated backing was cut to standard widths.

The magnetizable coating was 9.65 µm thick, had a squareness of 0.75, a B of 93o Gauss (93 mT) and H _c of 325 Oersted (2587 A / m). The resistance to blocking was excellent when measured according to GSA-FSS Interim Federal SpecificaLion WT-ool5 72 Section 4.4.8 of May 17, 1972 with the exception that the test was carried out at an elative humidity of 80 % at 66 ° C. This excellent resistance to blocking indicates that the copolymer of latex B has been crosslinked. The tape also exhibited excellent resistance to abrasion, as demonstrated by a signal loss of only 2.7 dB after more than 6.75 hours of stopping motion on a Sony U-Matic video carrier.

Example 2

A grinder was filled with 100 parts of the needle-shaped gamm-Fe ₂ O ₃ parts of Example 1, two parts of a dispersant, 2 parts of NN-dimethylaminoethanol and 120 parts of deionized water. After uniformly milling, there were 75.5 parts (33.6 % solids) of Aqueous Latex A and 3 parts

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Added lubricant to obtain a dispersion that deaerates, filters, rotogravures on a corona -primed, biaxially oriented polyethylene terephthalate film wound, magnetically oriented, dried, polished and cut to standard widths.

The magnetizable coating had a thickness of 6 µm, a squareness of o.79, a B of 1,000 Gauss (0.1 T), and an H of 347 Oersted (2.762 A / m). The blocking resistance was excellent at 66 ° C / 80 % relative humidity as tested in Example 1.

The tape exhibited abrasion resistance at least equivalent to the best audio recording tapes on the market and was much better than many of these tapes.

Example 3

After a part of the aqueous latex A was stored at room temperature for about one month, it became a dispersion produced from acicular gamma-FepCU particles as in Example 2, however, only 73.75 parts of latex A were used. After this dispersion had been stored for 124 days, it was raised as in Example 2. The magnetizable coating had a thickness of about 7.8 μm, a right

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angularity of ο.77, a B _r of 1225 Gauss (122.5 mT), an H _c of 3o8 Oersted (2,452 A / M) and excellent resistance to blocking and abrasion. This band was in his Q ^u ality to that of Example 2 completely equivalent.

Example 4

Magnetic recording tapes were made as in Example 2, except that a mixture of 30 parts (33.6 % solids) aqueous latex C and 45 parts (34.1 % solids) aqueous latex D was used in place of the aqueous latex A.

The magnetizable coating had a thickness of 5.4 µm, one Rectangularity of o.77, a B of 1175 Gauss (117.5 mT) and an H of 322 Oersteds (2,663 A / m). Resistance to blocking and wear and tear were acceptable.

Example 5

The magnetizable material of this example were acicular fine iron particles with an average length of 0.3 μm a mean diameter of 0.05 µm and 187 emu / g. A 120 ml quickie grinder was placed in an inert atmosphere with 100 parts of the needle-shaped fine iron, 4 parts of dispersant and 87 parts of deionized water.

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After grinding for about 1 hour to obtain a uniform Paste had been ground, the grinder was opened to the atmosphere and washed with 86 parts of deionized water, 43 parts of aqueous latex A and 4 parts of lubricant were added, after which grinding was restarted for about 1 minute; to obtain a coatable dispersion. This was filtered, vented and primed with a squeegee on an air plasma biaxially oriented polyethylene terephthalate film mounted, magnetically oriented, dried, polished and closed Standard widths cut.

The magnetizable coating had a thickness of 3.7 µm, a squareness of 0.69, a B of 296o Gauss (296 mT) and an H _c of 900 Oersteds (7,164 A / m). Blocking and abrasion resistance were excellent.

The acicular fine iron in the tape of this example was 163 emu / g, showed 17 % oxidation during processing. The same order of magnitude of oxidation is found when the fine iron is used in solvent-based binder formulations.

Example 6

A tape was produced as in Example 5, but before the spreadable dispersion was drawn on, this 4 days in one

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glass vessel closed with a cap has been left to stand,
90 % of the space was taken up by air. The needle-shaped fine iron in the tape was 158 emu / g and showed 3%
additional oxidation. This indicates that the applicable
Dispersion for at least several days during manufacture
was storable without significant impairment.

Example 7

A grinder was filled with 100 parts of conductive carbon black, 20 parts of dispersant and 36o parts of deionized water. After grinding to uniformity, the filling was in
given a blade mixer and added 99o parts of aqueous latex A and 24oo parts of deionized water. After thorough mixing, it was filtered, deaerated and rotogravure coated onto a corona-primed biaxially oriented polyethylene terephthalate film and dried to obtain a conductive underlayer 2 µm thick
of Example 2 applied. Testing of the obtained magnetic recording tape showed that the undercoat caused bleeding of static charges.

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Despite the relatively large amount of heat in the evaporation of water, less energy is required in the process of drying the coating compared to the usual system with
Solvent recovery that requires a high volume of air to minimize fire and explosion. In order to drive off water, the takes in the output stages
The hot air used for drying has a limited amount of water and can therefore be heated further and used again in the initial stages. A heat exchanger can transfer the heat
for further use without the risk of fire or explosion
to save.

Dr.Ro/Be

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Claims (1)

1BERLIN33 8MUNICH80 Auguste-Viktoria-Strasse 65 ρ. ηιιΟΛυΐ / Γ: ί Γ> Λ r> T M π: η Pienzenauerstraße 2 Pat. Dr. Ing.Ruschke Ur. KUOOMISt & KAK I NhK Pat.-Anw.Dipl.-lng. _{o ΛΤ} _ _ΝΙ _. »Ι.«. · «·, Τ r-Pat.-Anw Dipl.-ing. OlafRuschko PATENTANWÄLTE Hans Ε. Ruschke Tel. (030) 8 26 38 95/8 26 44 81 BERLIN - MUNICH ^Τβ '"C ⁸⁹ ) ^{98 03 24} ' ^ ^ra Telegram address: Telegram address: Quadrature Berlin Quadrature Munich TELEX: 183786 TELEX: 522767 M 4189 Claims l "i Stable, coatable dispersion of inorganic particles in a latex on an aqueous basis, which is suitable for the production of coatings for magnetic recording media, characterized by a self-crosslinking copolymer of the monomers (in parts by weight) (1) 100 parts of at least one alkyl acrylate or alkyl methacrylate with 1 to 8 carbon atoms in the alkyl group, (2) about 1 - 20 parts of at least one acid from the group: acrylic acid, methacrylic acid and itaconic acid, and (3) about 1-15 parts of at least one compound from the group formed by glycidyl acrylate and glycidyl ethacrylate, these monomers being chosen so that the copolymer has a T of -5.degree. C. to 60.degree. C. after crosslinking. 2. Dispersion according to claim 1, characterized in that up to about 5o wt .-% of the alkyl acrylate and / or alkyl methacrylate are replaced by at least one monomer from the group formed by acrylonitrile and methacrylonitrile. 030036/0840 3. Dispersion according to claim 1, characterized in that up to about 25 wt .-% of the alkyl acrylate and / or alkyl methacrylate are replaced by 2-hydroxyethyl acrylate. 4. Dispersion according to one of claims 1 to 3, characterized in that this latex also contains a second self-crosslinking copolymer made from the same monomers, which, however, are chosen so that the second copolymer has a T of -5 ° C to -5o after crosslinking ° C. 5. Dispersion according to claim 4, characterized in that the first-mentioned copolymer has a T of 40 ° C to 6o C after crosslinking and the second copolymer has a T of ~ 20 ° C to -5o ° C after crosslinking. 6. Dispersion according to one of claims 1 to 5, characterized in that the monomers (1) are selected from the group: methacrylate, ethyl acrylate and n-butyl acrylate. 7. Dispersion according to one of claims 1 to 6, characterized in that these monomers have 5 to 15 parts of at least one acid from the group acrylic acid / methacrylic acid and 3 to 8 parts of at least one monomer from the group glycidyl acrylate / glycidyl methacrylate. 030036/0840 8. Dispersion according to one of claims 1 to 7, characterized in that the inorganic particles are magnetizable. 9 * A process for the preparation of the coating of a magnetic A _u fzeichnungsmediums, wherein inorganic particles are dispersed in a latex aqueous-based, the dispersion is coated on a substrate and is then heated to dry the coating, characterized in that the latex aqueous-based a latex is used, which is a self-crosslinking copolymer of the monomers (in parts by weight) (1) 100 parts of at least one alkyl acrylate or alkyl methacrylate with 1 to 8 carbon atoms in the alkyl group, (2) about 1 - 20 parts of at least one compound from the group: acrylic acid, methacrylic acid and itaconic acid, and (3) about 1-15 parts of at least one compound selected from glycidyl acrylate and glycidyl methacrylate Group, and that the latex is crosslinked during the step of drying the coating, these monomers being so selected be that the copolymer has a T of -5 ° C to 60 ° C after crosslinking. Io. Process according to Claim 9, characterized in that the particle size of the copolymer of the aqueous latex is smaller 030036/0840 than ο, 25 to is. 11. The method according to claim 9 or Io, characterized in that the inorganic particles used are magnetizable. 12. Magnetic recording medium consisting of a base which contains a layer of inorganic particles in a crosslinked latex binder, characterized in that the binder is a crosslinked copolymer of the monomers (in parts by weight) (1) 100 parts of at least one alkyl acrylate or alkyl meth- acrylate with 1 to 8 carbon atoms in the alkyl group, <2) about 1 - 2o parts of at least one compound from the Group: acrylic acid, methacrylic acid and itaconic acid, and (3) about 1-15 parts of at least one compound from the group consisting of glycidyl acrylate and glycidyl methacrylate, contains, the monomers being chosen so that the crosslinked copolymer has a T of -5 ° C to 60 ° C. 13. Magnetic recording medium according to claim 12, characterized in that up to about 5o wt .-% of the alkyl acrylate and / or alkyl methacrylate are replaced by at least one member from the group formed by acrylonitrile and methacrylonitrile. 030036/0840 14. Magnetic recording medium according to claim 12, characterized in that up to about 25 wt .-% of the alkyl acrylate
and / or alkyl methacrylate are replaced by 2-hydroxyethyl acrylate. 15. Magnetic recording medium according to one of claims 12, 13 or 14, characterized in that this binder also has a second crosslinked copolymer made of the same
Contains monomers which, however, are chosen so that the second crosslinked copolymer has a T of -5 C after crosslinking to -5o C. 16. A magnetic recording medium according to claim 15, characterized in that _t the first-mentioned cross-linked copolymer after crosslinking of a T 4o ° C to 6o C and the second cross-linked copolymer according to Vernet & mg of a T -2O C to -5o C. 17. Magnetic recording medium according to one of claims 12, 13 and 14, characterized in that these monomers (1) are selected from the group: methyl acrylate, ethyl acrylate and n-butyl acrylate. 18. Magnetic recording medium according to one of claims 12 to 17, characterized in that these monomers 5 to 030036/0840 30Q7218 15 parts of at least one compound selected from the group consisting of acrylic acid and methacrylic acid and 3 to 8 parts at least a compound from that of glycidyl acrylate and glycidyl methacrylate existing group included. 19. Magnetic recording medium according to one of claims 12 to 18, characterized in that these inorganic particles are magnetizable. 030038/0840