JPH0226285B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a magnetic recording medium such as a magnetic tape having a magnetic layer in which magnetic powder is dispersed in a binder. Polyurethane-based binders for magnetic powder in magnetic tapes have excellent wear resistance and are used in various tapes. For example, there is a mixed system of thermoplastic linear polyurethane and thermosetting low molecular weight urethane resin.In this system, the thermosetting component provides wear resistance and heat resistance, and the thermosetting component provides coating properties (magnetic properties). Improves paint consistency) and tackiness. However, this mixed system has been found to have the following serious drawbacks. (1) Since it is composed of a polymeric thermoplastic component and a low-molecular thermosetting component, these two components phase separate to form each domain when cured.
This results in so-called non-uniform hardening. As a result, the mechanical strength of the coating film deteriorates. (2) Mainly for video tapes, the temperature is high at around 50°C.
A high Tg value (glass transition temperature) is required, but conventional urethanes have a low molecular weight and generally have a small Tg value, which deteriorates the running properties of the tape. Therefore, the urethane component, which has excellent abrasion resistance, cannot be increased much in the coating composition, and can only be contained at around 30% at most. For this reason, the wear resistance is not good either. (3) Urethane curing agent (isocyanate compound)
does not react 100% and crosslinks are not effectively formed. This seems to be due to the fact that the curing agent cannot act homogeneously due to the above-mentioned phase separation of both components. The present invention provides a magnetic recording medium having a magnetic layer in which magnetic powder is dispersed in a binder, wherein the binder has an OH value of 20 to 100 and an average molecular weight of 10,000.
With the formula: The present invention relates to a magnetic recording medium characterized in that the main component thereof is a crosslinked structure obtained by crosslinking a polymeric polyurethane polyol having repeating units represented by the formula with an isocyanate compound. In addition, Japanese Patent Publication No. 47-28044 discloses that polyurethane having an OH group at the molecular end, which is obtained by extending the molecular chain of polyester with isocyanate, is reacted with isocyanate, and this reaction product is used to make magnetic tapes. It is disclosed for use as a binder in paints. However, the polyurethane used in the above-mentioned Japanese Patent Publication No. 47-28044 is merely a polyurethane having an OH group at the end of the molecule, and the high-molecular polyurethane polyol used in the present invention is, as detailed below, It is foreign. That is, at present, only the polyurethane having an OH group at the molecular end as disclosed in the above-mentioned Japanese Patent Publication No. 47-28044 is commercially available. Since such polyurethane has OH groups only at the ends, even if it is crosslinked with an isocyanate compound as in the above-mentioned Japanese Patent Application Laid-open No. 47-28044,
Because the number of crosslinks is small and the crosslinked structure is linear, a three-dimensional crosslinked structure is not generated.
Only those with low Tg values can be obtained. On the other hand, since polyurethane has excellent wear resistance, it is desirable to increase the proportion of polyurethane in the binder composition. However, the above-mentioned special public service
In the case of Publication No. 28044, only polyurethane with a low Tg value can be obtained, so if the proportion of polyurethane is increased, the Tg value of the binder will decrease and the running properties of the magnetic tape will deteriorate. The proportion of polyurethane in the binder composition cannot be increased significantly because the tape will stick. In contrast, the binding agent according to the present invention has a
Since the main component is a polyurethane polyol having a predetermined number of OH groups (20 to 100 OH groups) and an average molecular weight of 10,000 or more that has been crosslinked and cured with a crosslinking agent (isocyanate compound), it has no effect on the conventional mixed binders mentioned above. All shortcomings can be overcome as described below. (1) Since it mainly consists of a crosslinked structure of high molecular weight polyurethane polyol, even if it is mixed with other high molecular weight thermoplastic resins, since both components are polymers, they will mix homogeneously and will not separate. Does not harden. Therefore, homogeneous curing is possible and the mechanical strength of the coating film is improved. (2) Specify the OH groups of the polyol to be in the range of 20 to 100 and the average molecular weight to be 10,000 or more, so that a large number of crosslinks are formed through the large number of OH values present in the side chains. As a result, a crosslinked structure of high molecular weight polyurethane polyol can be effectively formed, and therefore the Tg value is significantly improved. Therefore, since the proportion of polyurethane polyol in the binder composition can be increased (in some cases accounting for the total amount of binder), both the abrasion resistance and the tape running properties are significantly improved. vice versa,
If the OH value of the polyol is less than 20, the crosslinking density will be too low and the Tg value will be small, and if it exceeds 100, the hydrophilicity due to the OH group will be too high and the solubility in the coating composition will be low. If the crosslinking density becomes too high, gelation may occur, which is undesirable. (3) Since the polyurethane polyol is mixed homogeneously with the thermoplastic resin, the curing agent of the polyol reacts efficiently and sufficiently. (4) Since the polyol is polyurethane-based, it has good abrasion resistance as mentioned above, and since polyurethane has a high molecular weight and is thermoplastic, it has good viscosity and less tack. This is the reason why the above-mentioned crosslinked structure can be used alone and its proportion can be increased even in a mixed system. The above polyurethane polyol can be synthesized by the method described below. Commercially available polyols have a low molecular weight and only 2 to 3 functional groups (OH groups). On the other hand, the polyol of the present invention is made of, for example, a triol that is polycondensed to have a high molecular weight and multifunctionality. Triols as starting materials include polyols such as trimethylolethane, trimethylolpropane, hexanetriol, and glycerol; organic dibasic acids such as phthalic acid, adipic acid, dimerized linoleic acid, and maleic acid; butadiol, (poly) It may be an ester triol obtained by reacting a diol such as propylene glycol, ethylene glycol, diethylene glycol, or polyoxyethylene glycol. The molecular weight of this ester triol is preferably 500 to 2,500 in order to adjust the OH group of the polyurethane polyol to 20 to 100. The basis for this molecular weight range corresponds to the reason for limiting the OH group of the polyol; if it exceeds 2500, the OH group of the polyol becomes small and the Tg value becomes low.
Moreover, if it is less than 500, it is undesirable in terms of solubility of the polyol in the paint and gelation. To synthesize the desired polyurethane polyol with an average molecular weight of 10,000 or more, the above ester triol is mixed with 2,
4-Toluene diisocyanate (TDI), 4,
4'-Diphenylmethane diisocyanate (MDI)
It is preferable to react with a diisocyanate compound such as. An example of this reaction is shown below. Here, the starting material triol is the above-mentioned low-molecular-weight ester-based triol, and one in which the reactivity of the three hydroxyl groups is not equivalent (i.e., one having a primary OH group and a secondary OH group in the molecule) is ideal, but at present only hexanetriol satisfies this requirement. Therefore, even if the reactivity of OH is apparently equivalent, due to the effects of steric hindrance, triols are not actually equivalent, such as trifunctional triols YT101 (molecular weight 1000, manufactured by Asahi Denka Co., Ltd.) and YT102 (molecular weight 800, manufactured by Asahi Denka Co., Ltd.). (manufactured by). The molecular structure of this triol is as follows. When synthesizing this polyurethane polyol,
Actually, both reaction components are reacted in a solvent such as ethyl acetate or methyl ethyl ketone, which is a good solvent for urethane. Typical examples of catalysts used in urethane synthesis are amine catalysts and organotin catalysts, such as dibutyltin dilaurate, which has a lubricant and polymer decomposition prevention effect in addition to catalytic activity. Note that the diisocyanate reacted with the triol linearly extends the chain of the triol to form a stretchable polymeric linear polyurethane, so it can be said to be a desirable chain extender. Other diisocyanates other than TDI and MDI include 2,6-tolylene diisocyanate, p-phenylene diisocyanate, diphenylmethane diisocyanate, m-
Phenyl diisocyanate, hexamethylene diisocyanate, butylene-1,4-diisocyanate, octamethylene diisocyanate, 3,
3'-dimethoxy-4,4-biphenylene diisocyanate, 1,18-octadecamethylene diisocyanate, polymethylene diisocyanate, naphthalene-2,4-diisocyanate,
3,3'-dimethyl-4,4'-biphenylene diisocyanate, 1-methoxyphenylene-2,4-
Diisocyanate, diphenylene-4,4-diisocyanate, 4,4'-diisocyanate diphenyl ether, naphthalene-1,5-diisocyanate, diisocyanate-dicyclohexyl-
Examples include methane, p-xylene diisocyanate, m-xylene diisocyanate, diphenylene diisocyanate hydride, diphenylmethane diisocyanate hydride, and the like. In the synthesis of polyurethane polyol, the quantitative ratio of triol and diisocyanate is a molar ratio of 0.7≦triol/diisocyanate≦
A value of 1.5 is desirable. If this molar ratio is less than 0.7, the OH value becomes low and the crosslinking density decreases → Tg
Moreover, gelation is likely to occur due to the branching reaction caused by the diisocyanate. Furthermore, if this molar ratio exceeds 1.5, the molecular weight of the polyol becomes low and the Tg value also decreases. The synthesized polyurethane polyol is then
It is blended into a magnetic coating material for coating and forming a magnetic layer of a catalytic recording medium such as a magnetic tape, and is crosslinked and cured by a curing agent (isocyanate compound) in the coating material after coating. The crosslinking agents used here include the above-mentioned diisocyanates, polymethylene polyphenyl isocyanate, benzene triisocyanate, toluene-2,4,6-triisocyanate, 3-methyl-4,6,4'-triisocyanate diphthalate, etc. Enylmethane, 2,4,4'-triisocyanate-diphenyl, 2,4,4'-triisocyanate-diphenyl ether, etc. may be used. Alternatively, a long chain hydrocarbon having an NCO group at the end, a substituted hydrocarbon having an NCO group at the end,
Reactive polyisocyanates or prepolymers containing terminal NCO groups can also be used. However, in order to effectively form a network structure of a crosslinked structure of polyol, it is desirable to use a polyisocyanate compound of triisocyanate or higher as a curing agent among the above-mentioned isocyanates. In this curing reaction, the quantitative ratio of polyurethane polyol and curing agent is preferably 0.2≦curing agent/polyol≦0.9 in terms of weight ratio. If this ratio is less than 0.2, there will be fewer crosslinks and Tg will decrease.
If it exceeds 0.9, the urethane will be cured too much and will lose its elasticity, and the excess curing agent will react with water, making the coating hard and brittle. This quantitative range is based on the case where polyurethane polyol is used alone as a binder, and when a mixed system with a thermoplastic resin is used as a binder as described later, it is desirable that curing agent/polyol ≦0.5. , 0.2 or less is sufficient. In addition, the desirable proportion of polyurethane polyol in magnetic paint is 40 to 100 PHR.
If it is less than 40 PHR, the wear-resistant effect of urethane will be insufficient. The more polyurethane polyol, the better. Due to the presence of the crosslinked structure of polyurethane polyol produced in this curing reaction, the cured coating film becomes
It was found that the Tg was several tens of degrees higher than in the case of conventional urethane systems, and the peak of viscoelastic loss, which is one parameter of homogeneity, was sharpened, and the tensile strength was also improved. These effects are advantageous for magnetic tapes, and can improve wear resistance such as reducing the amount of powder falling off. In addition, when it is required to further improve the Tg of the coating film, for example to about 50°C, a mixture of the above-mentioned polyurethane polyol crosslinked structure and a thermoplastic polymer compound is used as the binder. Usable thermoplastic polymer compounds include vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinyl acetate-maleic acid copolymer, and vinyl chloride-vinylidene chloride copolymer. Polymer, vinyl chloride-acrylonitrile copolymer, acrylic ester-acrylonitrile copolymer, acrylic ester-
Vinylidene chloride copolymer, methacrylic acid ester-vinylidene chloride copolymer, methacrylic acid ester-styrene copolymer, phenoxy resin, vinylidene chloride-acrylonitrile copolymer, styrene-butadiene copolymer, polyester resin, alkyd resin, epoxy Examples include resin. However, these thermoplastic resins may be used in combination with other thermoplastic resins commonly used as binders for magnetic paints (however, this resin alone has no effect on increasing Tg). Other thermoplastic resins include thermoplastic polyurethane resins, butadiene-acrylonitrile copolymers, acrylonitrile-butadiene-
Examples include acrylic acid copolymers, acrylonitrile-butadiene-methacrylic acid copolymers, polyvinyl butyral, and cellulose derivatives. Note that other known materials can be used when producing a catalyst recording medium using the crosslinked polyurethane polyol structure of the present invention. For example, magnetic powders include γ-Fe ₂ O ₃ , Fe ₃ O ₄ , γ
−Interphase of spinel structure between Fe ₂ O ₃ and Fe ₃ O ₄ , Co
γ−Fe ₂ O ₃ doped with , Co doped
Fe ₃ O ₄ , Co-doped intermediate phase, CrO ₂ , barium ferrite, alloy particles (e.g. Fe-Co,
Co−Ni, Fe−Co−Ni, Fe−Co−B, Fe−Co−
Examples include Cr-B, Mn-Bi, Mn-Al, Fe-Co-V), iron nitride, and mixtures thereof. In addition, lubricants such as olive oil, dispersants such as lecithin,
Anti-wear agents such as Al ₂ O ₃ and SiO ₂ and antistatic agents such as carbon black may be added. Solvents used during paint preparation include ketones (e.g. acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone), alcohols (e.g. methanol, ethanol, propanol, butanol),
Esters (e.g. methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, ethylene glycol acetate monoethyl ether), glycol ethers (e.g. ethylene glycol-dimethyl ether, ethylene glycol-monoethyl ether, dioxane), aromatic hydrocarbons (e.g. ethylene glycol-dimethyl ether, ethylene glycol-monoethyl ether, dioxane) For example, it may be benzene, toluene, xylene), fatty hydrocarbons (eg hexane, heptane), nitropropane, or mixtures thereof. Non-magnetic bases for applying magnetic paint include polyesters such as polyethylene terephthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, polycarbonate, polyvinyl chloride, polyimide, metals such as aluminum and copper, paper, etc. It's good to be relaxed. Next, methods for measuring various equivalences in the present invention will be explained. (1) Measurement of OH value In this measurement, OH groups are generally acetylated with acetic anhydride in pyridine, and p-toluenesulfonic acid was used as a catalyst for this acetylation. Specifically, after reacting for 2 to 3 hours, first add 2 ml of water and shake well.
Furthermore, 10 ml of pyridine/water = 3/1 was added. Next, after leaving it at room temperature for 5 minutes, dilute it by adding 60 ml of butanol/toluene = 2/1.
Titrated with a methanol-potassium hydroxide solution (the indicator was phenolphthalein). This titration amount was defined as _VP . On the other hand, titrate a blank solution containing no sample in the same manner as above, and calculate this titration amount.
It was set as V _B. The OH value is expressed by the following formula. OH value = (V _B - V _P ) x 28.05 x f/grams of sample ± acid number (however, f: factor of 1/2N methanol-potassium hydroxide solution, acid number: neutralize 1 g of sample) (2) Measurement of viscoelasticity The viscoelasticity of the cured product is measured using TBA
(Torsional Braid Analysis). The measurement temperature range was -50°C to 100°C, and the temperature increase rate was 1.25°C/min. (3) Swelling test A cured film sample was immersed in methyl ethyl ketone for two days and nights to swell, and its weight was measured to determine the ratio of the solvent to the cured film in the swollen volume. Weight before swelling W ₀ and dry weight after swelling W ₁
and the elution amount (W ₀ −W ₁ /W ₀ ×100%) was determined. When the swelling property is low, there are many crosslinks and the crosslink density is high. (4) Measurement of molecular weight distribution Molecular weight distribution was measured by GPC (gel permeation chromatography).
In this case, adjust the sample concentration to 0.5% by weight,
Tetrahydrofuran was used as a solvent. Hereinafter, the present invention will be explained in more detail based on specific examples. Polyol Synthesis Results and Discussion First, trifunctional triol YT101 and toluene diisocyanate (TDI) were reacted under the conditions shown in Table 1 below, and the products were evaluated.

[Table] According to this table, it can be seen that good results are obtained when YT101 and TDI are used in equal moles. It was also found that gelation tends to occur when this molar ratio is less than 0.7 and when the solution concentration is 20% or more. This No. 1 polyol does not gel, has good solubility in ethyl acetate, and has low viscosity.
It seems to be close to a model polymer. This No.1
The molecular weight of the polyol was measured by GPC method, but
It was found that it has a high molecular weight of over 10,000.
Therefore, the synthesis conditions for polyol were determined as follows. YT101/TDI (molar ratio): 1 Concentration of reaction system: about 10% Catalyst concentration: 0.02% Reaction temperature: 60°C Next, the molar ratio of YT101/TDI was increased as shown in Table 2 below, and the The concentration of OH groups present was increased. This made it possible to increase the amount of urethane bonds or allophanate bonds generated during the curing reaction. Note that η is the intrinsic viscosity of the synthetic polyol.

[Table] According to this, the OH values of each sample are close to the theoretical values, so it seems that the chain of the triol has been extended as expected. Next, tryol instead of YT101 mentioned above.
A polyol was synthesized using YT102 ( _W = 800). This is due to the functional groups (OH
The aim was to improve the mechanical properties by increasing the number of groups). The results are shown in Table 3 below.

[Table] Since the OH value of sample No. 4 almost corresponds to the theoretical value, it seems that chain elongation occurred as expected. Also, from the value of η, the molecular weight of the polyol is larger when YT101 is used. Physical Properties of Polyol Crosslinked Structures (1) In the case of a polyol-based binder The above-mentioned polyols No. 1 and 2 were cured with a curing agent, and the viscoelasticity was measured by TBA. As a result, the data shown in Table 4 below was obtained.

[Table] In this case, when the ratio of curing agent and polyol is 0/10, only the synthetic polyol remains,
When the ratio is 3/10, the curing agent is approximately equivalent to urethane,
When it is 6/10, the curing agent is equivalent to allophanate reaction, and when it is 9/10, it is when an excessive amount of curing agent is added in consideration of other reaction mechanisms (for example, reaction between the curing agent and H ₂ O). . From this result, the following knowledge can be obtained. That is, as the amount of curing agent added increases, Tg shifts to the higher temperature side. To state this further,
Tg=-38℃ for No.2a system without adding curing agent
and than the No.1a system with YT101/TDI=1.00.
The lower Tg is due to the lower molecular weight of the polyol. However, in the case of No. 2b to d with addition of hardening agent, Tg increases systematically,
The values remained lower than those of Nos. 1b to d, indicating that the molecular weight of the polyol produced greatly influences the Tg of the cured product. Next, sample No. 4 (triol is YT102)
and sample No. 5 (triol is 20g of YT101,
Comparing the viscoelasticity measurement results of systems in which each polyol was cured with a curing agent (YT101/TDI = 1.00, catalyst concentration 0.02%), the Tg of both systems increased as the amount of curing agent increased. When no curing agent is used, the polyol obtained from YT102 has a lower Tg than that obtained from YT101. This seems to be because the former polyol has a large number of functional groups in one molecule but a small molecular weight. In the system to which a curing agent was added, No. 4, which has a larger number of functional groups, showed a higher Tg, indicating that the higher the urethane bond concentration, the higher the Tg. However, when the amount of curing agent is 1.5 equivalents or more, the rate of increase in Tg decreases rapidly, but it seems that the urethane reaction occurs at 0 to 1.0 equivalents, and the allophanate reaction occurs at 1.0 to 2.0 equivalents, and this is due to the difference in reaction mechanism. Therefore, it was found that the contributions to Tg differ from each other. (2) In the case of a VAGH-polyol mixed binder A mixed system of VAGH (vinyl chloride-vinyl acetate-vinyl alcohol copolymer) and polyol is used as a binder, and the viscoelasticity is measured by varying the mixing ratio. did. As a result, it was found that as the proportion of VAGH increases, Tg shifts to the higher temperature side, and the loss modulus also becomes one sharp peak. This seems to indicate that both components have good compatibility. Next, the change in Tg depending on the amount of curing agent was measured, and the results shown in Table 5 below were obtained.

[Table] According to this table, it can be seen that in the VAGH-polyol mixed system, Tg clearly increases with the amount of curing agent, which indicates that the crosslink density increases. Furthermore, since the tendency of increase in Tg is similar to that in the case of the polyol alone system described above, it can be said that the polyol acts dominantly on Tg in both of these systems. What is important from the above results is that by selecting the amount of curing agent, it is possible to increase the proportion of polyol even if the binder is a mixed system, and therefore the urethane component increases, which significantly improves the wear resistance. That's true. From the point of view of Tg, the ratio of polyol is, for example, VAGH: polyol = 2:
Even if it is as large as 8, a sufficient practical Tg can be obtained.
Therefore, in order to maintain a high Tg and increase wear resistance, it is desirable that the proportion of polyol in the mixed system be 50% or more. In either case, the VAGH percentage can be reduced.
Furthermore, even if there is a large amount of polyol, its dispersibility in solvents is good because it has a high molecular weight. In addition
VAGH has rigidity and has little opportunity to associate with the above-mentioned curing agent, so even if the above mixed system and curing agent are mixed, the curing agent easily reacts with the flexible polyol side, resulting in hardening of the polyol. The reaction will proceed satisfactorily. Furthermore, when the above-mentioned curing type binder was used, it was found from the results of the swelling test that the crosslink density increased. In both the polyol single system and the curing system, as the amount of curing agent increased, the elution amount decreased, that is, the crosslinking density increased. At the same amount of curing agent, as the amount of VAGH increased, the ratio of the solvent to the cured film in the swelling volume increased, but this was due to the difference in the swelling properties of VAGH and urethane to methyl ethyl ketone, and the crosslinking due to the increased concentration of OH groups. This seems to be due to an increase in temperature. Since the elution amount in the mixed system is much smaller than in the single system, it is thought that the crosslinking reaction progresses more homogeneously. Application to Magnetic Tape An example of actually creating a magnetic tape based on the present invention will be described. First, the following composition was prepared. γ-Fe ₂ O ₃ acicular magnetic powder (particle size: long axis =
0.2-0.4μ, acicular ratio = 8-10, σ _s = 73.0emu/g)
100 parts by weight Lecithin 2 parts by weight Methyl ethyl ketone (solvent) 80 parts by weight This composition was placed in a ball mill, mixed and dispersed for 15 hours, and then a binder solution having the following composition was added, followed by mixing in the ball mill for 24 hours. Vinyl chloride-vinyl acetate copolymer [Vinyrite VYHH manufactured by Union Carbide, vinyl chloride: vinyl acetate = 87:13 (weight ratio)] 5 parts by weight polyurethane polyol (average molecular weight ≒
33000: produced by the reaction of YT101 with a molecular weight of 1000 and TDI) 20 parts by weight Cyclohexanone (solvent) 120 parts by weight After this mixing, polyisocyanate (curing agent)
A magnetic coating material was prepared by adding and mixing 25 parts by weight of Dismodule L (trade name, manufactured by Bayer AG). Next, this magnetic paint was applied onto a polyester film with a thickness of 20μ so that the film thickness after drying would be 6μ, and after orientation by applying a magnetic field, it was heated and dried.
A magnetic tape was thus obtained. The magnetic coating film of this magnetic tape had a B _s (saturated magnetic flux density) of 1630 Gauss and had high packing properties because the magnetic paint did not have sufficient viscosity (low viscosity). Additionally, the amount of powder falling off was significantly reduced to 1.30mg due to the cured polyurethane component in the magnetic coating. The measurement of powder falling was performed by sliding the tape with #1000 sandpaper at a constant speed and a constant load to wear it out.
The amount of powder falling was determined from the difference in weight before and after sliding a 10 cm long tape (1/4 inch width). In the above example, the properties of the magnetic tape obtained when the blending ratio of vinyl chloride-vinyl acetate copolymer (VAGH) and polyol were changed are shown in the table below.
6. For comparison, the characteristics of a magnetic tape using a binder made of vinyl chloride-vinyl acetate copolymer (VAGH) and a conventional linear polyurethane (Nipporan, manufactured by Nippon Polyurethane Co., Ltd.) are also shown.

[Table] From the results, it can be seen that Samples 6 to 9 have good B _s and powder removal due to the presence of polyol.

Claims (1)

[Claims] 1. In a magnetic recording medium having a magnetic layer in which magnetic powder is dispersed in a binder, the binder comprises:
It has an OH value of 20 to 100, an average molecular weight of 10,000 or more, and the formula: A magnetic recording medium characterized in that the main component thereof is a crosslinked structure obtained by crosslinking a polymeric polyurethane polyol having repeating units represented by the formula with an isocyanate compound.