JPS6321613B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to coatings comprising fine particles, such as magnetizable particles, and a binder on a substrate, and primarily relates to coatings for magnetic recording media. In the technical field of applying coatings such as magnetic particles dispersed in elastomeric binders,
A dispersant is normally used to disperse the particles, typically in an amount of 10 to 50% by weight of the binder composition.
As the dispersant remains in the coating, it tends to have a plasticizing effect that weakens the effectiveness of the binder. Therefore, the coating exhibits poor abrasion resistance and eventually flakes off wear debris. Furthermore, the dispersant causes the migration of low molecular weight substances in the coating and their leaching onto the recording surface. Dispersants are also used to apply coatings containing fine particles that are not magnetizable, such as the conductive carbon black of the windability control backside coating of many magnetic recording tapes. These and other coatings involving fine particle dispersions in elastomeric binders all suffer from the aforementioned problems. US Pat. No. 3,490,945 discloses polyurethane binders for magnetic recording media that are themselves dispersants. The polyurethane binder is a moisture cured diisocyanate terminated prepolymer containing surface active tertiary or quaternary nitrogen. Although this patent should have solved the above problems in principle, it is believed that to date no elastomer has been commercially available that both acts as a high quality binder and effectively disperses inorganic particles. This Slovinsky patented polyurethane binder is unique because it is impractical to formulate the binder under dry conditions and keep it dispersed under a dry atmosphere, and because moisture curing results in some foaming. It is thought that it was not commercially available. The present invention is believed to relate to the first commercially viable magnetic recording media having a coating of uniformly dispersed magnetizable particles in a binder that does not require a dispersant. This binder is believed to be novel in itself and effectively disperses acicular gamma Fe ₂ O ₃ particles without the aid of a dispersant. However, for more difficult to disperse particles, such as magnetizable particles such as fine magnetite and chromium dioxide particles, it may be desirable to add small amounts of dispersant. This new binder also effectively disperses non-magnetic particles commonly used in magnetic recording media, such as carbon black and aluminum oxide. The present invention is also useful in providing coatings for abrasive sheet materials and other applications. Briefly, the novel binder of the present invention comprises a hydroxyl-terminated polyurethane block copolymer comprising polyester or polyether segments, each segment having an average molecular weight of 200 to 3000, at least 10 mole percent of the segments. comprises a short chain aliphatic segment containing tertiary and/or quaternary nitrogen, and a symmetrical unbranched aromatic urethane segment linked to said polyester or polyether segment and short chain aliphatic segment. . The term "short chain" shall mean chains having an average of 12 carbon atoms or less, which are unbranched or have methyl or ethyl side chains. A larger average chain length or longer side chains will impair the otherwise high toughness of the block copolymer. If the average molecular weight of the polyester or polyether segment is less than 200, it tends to lack the flexibility necessary for a good binder, while if it exceeds 3000, the polyester or polyether segment is undesirable. It tends to exhibit crystallinity. Therefore, 400~
A range of 2000 is preferred. A particularly preferred symmetrical unbranched aromatic diisocyanate is p,p'-diphenylmethane diisocyanate. Other diisocyanates include p
-xylene diisocyanate, 1,4-bis(2
-isocyanatoethyl)benzene and 1,5-
There is naphthalene diisocyanate. The novel polyurethane block copolymers of the present invention are prepared by (a) interreacting an excess of soluble p,p'-diphenylmethane diisocyanate with a hydroxyl-terminated polyester or polyether having an average molecular weight of 200 to 3000 to form an isocyanate capped copolymer; (b) forming a mixture of isocyanatecapped polyurethane and unreacted diisocyanate; It may be prepared by a continuous process with chain extension to form a soluble hydroxyl terminated polyurethane block copolymer. Optionally, it may include a subsequent step of (c) quaternizing some or all of the tertiary nitrogen. Generally, this additional step is irreparably costly. 60-100 mol% of aliphatic diol used in step (b)
If it contains a tertiary amine, the novel block copolymer may be softer than desired for use as a binder for coating magnetic recording media. Some of the short chain diols used in step (b) are
If no tertiary amine is present, the polyurethane initially made in step (a) is chain-extended with a portion of the diol containing tertiary nitrogen in the presence of excess isocyanate, and then the polyurethane made in step (a) is chain-extended in the presence of excess isocyanate with a portion of the diol containing no tertiary nitrogen. For ease of operation, it is preferred to carry out partial chain extension to produce a soluble hydroxyl terminated polyurethane block copolymer. Therefore,
Preferably, the diol contains 10 to 60 mol % of tertiary nitrogen and relatively 90 to 40 mol % of the diol is free of tertiary nitrogen. Preferred nitrogen-free short chain diols are straight chain aliphatic diols having 2 to 6 carbon atoms, especially 1,4-butanediol, and one or two methyl or ethyl side diols, such as neopentyl glycol. The corresponding diol with a chain. The short chain aliphatic diol may contain one cycloaliphatic group such as cyclohexanedimethanol. Preferred diols containing tertiary amines are similar to the nitrogen-free short chain diols described above except for the tertiary amine. N-methyldiethanolamine is preferred. Preferred hydroxyl-terminated polyethers contain multiple methylenes (-
CH ₂ −) Contains _X units. Preferred hydroxyl terminated polyesters are those consisting essentially of multiple methylene ( _-CH2- ) _X units linked together by ester bonds. When the polyurethane block copolymer described above is dissolved in a sufficiently volatile vehicle to form a coatable dispersion of fine magnetic particles and/or other particles, the dispersion can be prepared without the addition of any dispersant. can be coated onto a substrate to provide a smooth, uniform coating. The dispersion may also contain other materials such as lubricants commonly used in coatings of magnetizable particles to reduce friction with the magnetic head. It is often desirable to add the novel polyurethane block copolymer crosslinking agent to the coatable dispersion immediately before its use. Such cross-linking tends to provide a tougher, more durable, more heat and solvent resistant coating, as is known in the art, and is particularly useful for magnetizable coatings that come into contact with magnetic heads at high speeds. This is especially important. Generally, the magnetizable coating includes a large weight percentage of magnetizable particles and a small weight percentage of binder. The binder may contain additional polymeric materials such as phenoxy resins (e.g. Union Carbide PKHH, a thermoplastic copolymer of equivalent amounts of bisphenol A and epichlorohydrin) that give a somewhat harder coating. good. Other useful such materials are vinyl chloride polymers, such as Union Carbide VROH and VAGH, which are partially hydrolyzed vinyl chloride/vinyl acetate copolymers. The volatile vehicle for the coatable dispersion is:
It should primarily contain non-aromatic polar solvents such as dimethylformamide, tetrahydrofuran or cyclohexanone. Aromatic solvents such as toluene, which are often used to make magnetic recording media, may cause the binder system to gel prematurely. Examples 1-7 below illustrate the polyurethane block copolymers of this invention and Examples 8-9 illustrate the preparation of representative coatings using those block copolymers. In each example, "part" means part by weight. Example 1 42.2 parts of 4,4'-diphenylmethane diisocyanate (hereinafter referred to as "MDI") were dissolved in 70 parts of tetrahydrofuran solvent in a stainless steel pressure reactor equipped with a paddle stirrer. After heating to 65℃,
36 parts of polytetramethylene ether glycol (hereinafter referred to as "PTMEG") having an average molecular weight of 650 was added. With continued stirring, the mixture was heated to about 125° C. until the NCO content stabilized and an additional 136 parts of tetrahydrofuran were added. Temperature about 80℃
3.6 parts of N-methyldiethanolamine (hereinafter referred to as "MDEA") was added while maintaining the temperature, and 30 minutes later, 7.5 parts of 1,4-butanediol (hereinafter referred to as "BD") were added. Then set the temperature to 130
The reaction was continued until the NCO content was below 0.3%. The inherent viscosity of the polyurethane elastomer obtained was 0.75 (measured in tetrahydrofuran). This batch was then diluted with tetrahydrofuran to a 17% solids content;
The solution was hereinafter referred to as "Product of Example 1." Example 2 The product of Example 1 in the reactor of Example 1 under nitrogen atmosphere
0.89 parts of methyl iodide was added to 100 parts at 22°C over 24 hours with gentle mixing to quaternize the tertiary nitrogen of the diol segment. Examples 3-7 Analogously to Example 1 and a third portion of the product of Example 6
Polyurethane block copolymers of Examples 3 to 7 were prepared by quaternizing the polyurethane nitrogen in the same manner as in Example 2 to produce the product of Example 7. The materials used in Examples 3-7 are shown in Table 1.

【table】

TABLE The polyester used in Examples 4 and 5 was polyepithilone caprolactone with an average molecular weight of 830. The bioforms of Examples 1 and 3-5 were spincast and cut using ASTM test methods.
Test specimens were prepared as described in E111-61 and D638-76. The test results are shown in Table 2.

[Table] Example 8 Manufacture of magnetic recording tape The following charges were used. Part by weight of charge A Acicular gamma Fe ₂ O ₃ particles 110 Product of Example 1 32.4 Aluminum oxide 1.65 B Tetrahydrofuran 77.1 C Carbon black 4.9 Product of Example 1 13.75 D Product of Example 1 93.6 Tetrahydrofuran 75.9 E Product 93.6 Tetrahydrofuran 75.9 The acicular particles had an average length of 0.8 micrometers, an average diameter of 0.1 micrometers and a surface area of 12.2 m ² /g, determined by nitrogen adsorption. Charge A was milled for 20 minutes in a ball mill containing 1930 parts of 1.6 cm stainless steel balls, then Charge B was added.
Before adding Charge C and before adding each subsequent charge, kneading was continued until a smooth coated dispersion was obtained, when additional tetrahydrofuran was added to reduce the viscosity to about 50 centipoise. continued. Throughout the kneading process, keep the temperature around 30°C.
I made sure not to exceed ℃. The final dispersion was coated with a gravure coater onto a 36 micrometer thick biaxially oriented polyethylene terephthalate film. Immediately before its coating, a 60% solids solution of 3.3 parts lubricant and crosslinker
4.63 parts were added to the final dispersion with stirring. The crosslinker solution consisted of an adduct of toluene diisocyanate and trimethylolpropane in a solvent consisting primarily of 25/15 ethyl glycol acetate/xylol. After drying in an oven, the magnetic coating was supercalendered to a surface smoothness of about 0.18 micrometers peak-to-peak. The resulting magnetic coating had the following properties. Thickness: 14 micrometers Coercive force: 345 Oersted Br 824 Gauss φr 0.72 line/cm φr/φmax 0.81 The obtained magnetic recording tape was cut into a width of 12.7 mm.
Stored at 60°C and 95% relative humidity. Rolls of tape were kept at 22° C. and 50% relative humidity for 24 hours approximately every two weeks and then tested using an IBM 3420, Model 8 computer tape transport. The tape was run in contact with the head at a speed of 5.08 m/sec until 732 meters of tape had passed the head, then the tape was stopped and the tape and head were visually inspected for defects. No defects were observed until after the roll of tape had been stored for 75 days, at which point a faint residue formed on the head during a run test. Of the many commercially available tapes that were previously subjected to this test, the best tapes were those with visible residue after 35 to 55 days. Example 9 Back side coating of magnetic recording tape The following charge was used. Part by weight of charge A Tetrahydrofuran 2.81 Toluene 2.35 Product of Example 3 30.38 Carbon black 2.50 B Tetrahydrofuran 9.28 Toluene 1.85 C Tetrahydrofuran 24.66 Toluene 2.74 D Crosslinker solution of Example 8 0.60 After kneading charge A in a sand mill for 1 hour ,
The same proportion of solvent as Charge C was added to reduce the viscosity to 50 centipoise. Charge B was then added and mixing continued for about 4 hours until a smooth coated dispersion was obtained. Charge C was then added and kneaded for an additional approximately 30 minutes to form a dispersion that provided a smooth coating. Charge D was added to the dispersion with stirring and immediately coated onto the back side of a magnetic recording tape having a biaxially oriented polyethylene terephthalate film backing. After the coating was dried to a thickness of 2-3 micrometers, the coating was effective as a roll control backside coating. The carbon black used in Example 9 had an average particle size of 30
Surface area measured in nanometers and nitrogen absorption method
It was 1000m ² /g. Other carbon blacks that have been effectively used in the present invention include Vulcan
XC-72 and Shawinigan, with average particle sizes of 29 and 42 nanometers, and surface area, respectively.
230 and 65 m ² /g.

Claims (1)

[Scope of Claims] 1. A coating containing a binder containing a polyurethane block copolymer and fine particles on a substrate, wherein the block copolymer is hydroxyl-terminated and each segment has an average molecular weight of 200 to 3000. a short-chain aliphatic segment in which 10 to 100 mol% of the segment contains tertiary and/or quaternary nitrogen; A coating comprising a symmetrical unbranched aromatic urethane segment. 2. A magnetic recording medium comprising a backing member, a binder containing a polyurethane block copolymer, and at least one coating containing fine particles, wherein the block copolymer is hydroxyl terminated, and each segment has an average molecular weight. 200 to 3000 short chain aliphatic segments, in which 10 to 60 mol% of the segment contains tertiary and/or quaternary nitrogen;
and A magnetic recording medium comprising a p,p'-diphenylmethane urethane segment connected to the polyester or polyether segment and the short chain aliphatic segment. 3. The magnetic recording medium according to claim 1 or 2, further characterized in that each of the polyester or polyether segments has an average molecular weight of 400 to 2000. 4 10 to 60 mol% of short chain aliphatic segments are N-
The magnetic recording medium according to any one of claims 1 to 3, further characterized in that it is a residue of methyldiethanolamine. 5. Claims 1 to 4 further characterized in that the remainder of the short chain aliphatic segment is the residue of a straight chain aliphatic diol having 2 to 6 carbon atoms.
The magnetic recording medium according to any one of paragraphs 1 and 2. 6 Add polyurethane copolymer to 40~
A soluble hydroxyl-terminated polyurethane block copolymer is produced by further chain extension by mixing with an amount of short chain aliphatic diol having no tertiary nitrogen to provide 90 mole percent, and the polyurethane block copolymer is The particles and a sufficiently volatile vehicle are mixed in an amount in which the weight of the fine particles exceeds the weight of the copolymer to form a coatable dispersion, the dispersion comprising the fine particles and a binder being deposited on a substrate. In a method of applying a coating to a substrate, the polyurethane copolymer is reacted in solution with an excess of a soluble symmetrical unbranched aromatic diisocyanate and a hydroxyl-terminated polyester or polyether having an average molecular weight of 200 to 3000 to form an isocyanate. (b) converting the mixture into a short-chain aliphatic diol, of which 10 to 100 mole percent contains tertiary nitrogen; A process characterized in that it is produced by chain extension with a group diol to form a soluble hydroxyl-terminated polyurethane block copolymer. 7. Following step (b), further (c) quaternizing at least some of the tertiary nitrogen of the block copolymer. 7. The method of claim 6, further comprising the steps of: 8 Less than 100 mol % of the diol contains tertiary nitrogen, and in step (b) the isocyanate-capped polyurethane made in step (a) is treated with less than 100 mol % of the diol containing tertiary nitrogen in the presence of excess isocyanate. 7. The method of claim 6, further characterized in that the hydroxyl-terminated copolymer is first chain-extended with a portion of the diol having no tertiary nitrogen and then further chain-extended with a portion of the diol that does not have a tertiary nitrogen. . 9. The method according to any one of claims 6 to 8, further characterized in that the fine particles are magnetic particles. 10. Claim 6, further characterized in that in step (d), a crosslinking agent for the polyurethane block copolymer is added to the polyurethane block copolymer after mixing with the fine particles to form a dispersion that can be coated. 9. The method according to any one of items 9 to 9.