JPS61104329A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a magnetic recording medium having excellent durability, etc. by using a magnetic powder binder consisting of a vinyl chloride copolymer having an SO3H group, etc. and having the OH/CH absorption ratio of an IR absorption spectrum within a specific range, polyurethane resin having an SO3M group and polyisocyanate. CONSTITUTION:The magnetic layer formed by dispersing the magnetic powder into the resin binder consisting of 20-80wt% the copolymer having the specific compsn. range of vinyl chloride contg. the groups of >=1 kinds among SO3H, SO3Na, -SO2, phosphoric acid and ester thereof at about 1-500g equiv./10<4>g in the copolymer range and having the intensity of 3,450cm<-1> OH absorption wavelength of the IR absorption spectrum and 0.2-0.7 OH/CH absorption ratio at 2,930cm<-1> absorption wavelength of CH, vinyl ester alkylcarboxylate, copolymerizable monomer such as maleic acid and the saponified matter of the vinyl ester alkylcarboxylate, 20-80wt% the polyurethane resin having SO3M (M is an alkaline metal) in the molecule at about 10-1,000g equiv./10<4>g range and 5-30wt% polyisocyanate is provided on a substrate. The excellent magnetic recording medium is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a magnetic recording medium, and more specifically, it provides a magnetic coating film with improved dispersibility and surface properties of magnetic powder, and with good electromagnetic conversion characteristics. The present invention also relates to a magnetic recording medium that has good storage stability and running properties at high temperatures and high humidity.

BACKGROUND ART In general, magnetic recording media such as magnetic tapes are made of y-IFe.
It is made by applying a magnetic paint in which magnetic powder such as No. 106 is dispersed in a high molecular weight resin binder component onto a support such as a polyester film. The properties of magnetic coatings are not only related to physical properties such as running properties, but also to magnetic properties such as the dispersibility of magnetic powder, and these are largely controlled by resin binder components and additives. . Therefore, various resin binder components have been proposed so far. As one of these, conventional vinyl chloride/vinyl acetate/vinyl 7/1/coal copolymer has been widely used as a binder resin component for magnetic tapes. The characteristics of this copolymer are that it is relatively inexpensive, easily soluble in solvents, and has good compatibility with other resins, and because it has hydroxyl groups in its molecules, it has good affinity with magnetic powder and disperses it. It has advantages such as improved magnetic properties, and provides excellent magnetic properties such as degree of orientation and maximum residual magnetic flux density.

However, as magnetic powder particles have become finer in recent years, dispersion of finely differentiated magnetic powders is no longer always satisfactory with these resins alone.As a result of considering increasing the amount of hydroxyl groups in this base copolymer, we found that dispersion It has been found that it is appropriate to introduce the group until the absorption ratio of 0H10H in the infrared absorption spectrum becomes 0.7 or more. However, copolymers with an increased amount of hydroxyl groups have a high glass transition temperature of 70°C or higher, which makes it difficult to process the surface and improve the surface properties of the magnetic layer after calendering. There is a drawback.

In view of this point, the present inventors first dispersed magnetic powder by mixing a resin component having a glass transition point of 65°C or less with the vinyl chloride/vinyl acetate/vinyl alcohol copolymer as a binder component. It has been discovered that it is possible to improve the surface properties of the coating film without impairing its properties, and also to improve the gloss after calendering (Japanese Patent Application No. 56-9
No. 4,016).

However, this time, due to the lowered glass transition point, there was a drawback that the storage stability and runnability under high temperature and high humidity conditions were lowered. In order to improve these shortcomings,
A copolymer consisting of vinyl chloride, an alkyl carboxylic acid vinyl ester, another monomer copolymerizable with vinyl chloride, and a saponified product of the alkyl carboxylic acid vinyl ester, whose infrared absorption spectrum OH/an A material with an absorption ratio of 0.2 to 0.7 is used as a binder component,
It is also believed that by reducing the amount of saponified substances and suppressing K, the increase in the glass transition point can be suppressed, and that the decrease in the dispersibility of magnetic powder due to the decrease in saponified substances is compensated for by other monomers that can be copolymerized with vinyl chloride. As a result, improvements in the degree of orientation, maximum residual magnetic flux density, etc.
6-182, 254, etc.).

However, in recent years, the particle size of magnetic powder has progressed further, and when these vinyl chloride copolymers are used alone as a binder resin component, the dispersibility of the magnetic powder becomes unsatisfactory, resulting in insufficient adhesive strength. , problems such as storage stability and running properties of magnetic tapes at high temperatures and high humidity remained.
A copolymer consisting of an alkylcarboxylic acid vinyl ester, another polymer copolymerizable with vinyl chloride, and a saponified product of an alkylcarboxylic acid vinyl ester, the OH/cH absorption ratio of its infrared absorption spectrum is If a resin binder containing 0.2 or more, (1) a rubber binder, and (ii) a polyisocyanate curing agent is used, it has excellent dispersibility compared to a binder consisting of the above-mentioned copolymer alone, and can be easily calendered. It has been found that processing is easy, the surface properties of the coating film are improved, and storage stability and runnability at high temperature and high humidity are also improved.However,
Even with a binder made of such a three-component resin system, completely satisfactory dispersibility cannot be obtained with recent ultrafine magnetic powders, and furthermore, the physical properties of the coating film, such as adhesive strength,
It was also recognized that the rubber binder added for the purpose of improving running properties, etc., on the other hand, led to a decrease in the dispersibility of the magnetic powder.

Problems to be Solved by the Invention It is an object of the present invention to particularly improve the unsatisfactory dispersibility associated with the above-mentioned ultrafine magnetic powders, which have been found in the prior art, and to improve surface properties and to improve electromagnetic properties. It is an object of the present invention to provide a magnetic recording medium which has good conversion characteristics and good storage stability and runnability at high temperature and high humidity.

Means for Solving the Problems The above object is achieved by the present invention having the following configuration. That is, the present invention provides: (1) sulfonic acid, sulfonic acid) 17 rum,
A vinyl chloride copolymer having a sulfone, phosphoric acid or phosphate ester group, or a mixture thereof; (1) a polyurethane resin having -SO3M (M represents an alkali metal) in the molecule; and (2) a polyurethane resin. This magnetic recording medium is made by fixing magnetic powder dispersed in a resin binder made of isocyanate to a support.

The vinyl chloride copolymer used in the present invention is composed of vinyl chloride, an alkyl carboxylic acid vinyl ester, another monomer copolymerizable with vinyl chloride, and a saponified product of the alkyl carboxylic acid vinyl ester. It is a copolymer with an OH/cH absorption ratio of 0.2 to 0.7 in the infrared absorption spectrum. In such a vinyl chloride copolymer, preferably,
Based on the weight of the copolymer, vinyl chloride is 50 to 9
0% by weight, alkyl carboxylic acid vinyl ester is 5-4
0fKi, other monomers copolymerizable with vinyl chloride are L
It constitutes 0 to 5.0% by weight. Alkylcarboxylic acid “□
The saponified product of vinyl ester is an essential component of the copolymer, and if it is too large, the glass transition point will rise, which is undesirable.On the other hand, if it is too small, the object of the present invention will not be achieved. Not done.

Examples of the alkyl carboxylic acid vinyl ester and saponified product used in the production of the vinyl chloride copolymer used in the present invention include vinyl acetate, vinyl propionate, vinyl versatate, and saponified products thereof. Other monomers copolymerizable with vinyl chloride used in the present invention include maleic acid, fumaric acid, methacrylic acid, and esters thereof, and acrylic acid and esters thereof.

When saponifying the magnetic powder, it is necessary to suppress the influence on the deterioration of the dispersibility of the magnetic powder to some extent, so it is necessary to make the OV-absorption ratio of the infrared absorption spectrum 0.2 or more as described above. In order to improve surface properties, it is necessary to set the 0vOII absorption ratio of the infrared absorption spectrum to 0.7 or less. In addition, in the infrared absorption spectrum shown here, O
H absorption is -1 at 3.450, OH absorption is 2.930 cm
-1, and 0VCH is its intensity ratio.

In the present invention, if the proportion of sulfonic acid, sodium sulfonate, sulfone or phosphate ester groups, or mixtures thereof contained in the vinyl chloride copolymer is too large, the strength of the coating film on the tape surface will deteriorate and powder will form. A fall occurs. On the other hand, if the amount is too low, the contribution to improving the dispersibility of the magnetic powder will be reduced, and the amount will be about 1 to 500. g
A range of equivalents/104I is suitable. That is, if the ratio is less than about 11 equivalents/10'i, the contribution to dispersibility will be low, and if it is more than 500 g equivalents/10'·I, the coating will become weak and powder will fall off.

Sulfonic acid, sulfoneffisu)! jum, sulfone,
The amount of the bigol chloride copolymer having one or more phosphoric acid or phosphoric acid ester groups is 20 to 80% by weight, preferably 30 to 70% by weight, based on the total resin content in the binder. range.

In the polyurethane resin having -8o, M groups in the molecule used in the present invention, if the proportion of -8o, M groups is too small, the contribution to improving the dispersibility of the magnetic powder will be small; on the other hand, if it is too large, In some cases, curing with polyisocyanate becomes insufficient.

Generally, the proportion of -805M groups is about 10 to 1. OO0
, 9 equivalents/104 & is suitable. That is, the ratio is IOF total/10
If it is smaller than 4g, the contribution to dispersibility is low and 1.00
If it is larger than 0 g/10' g, there will be no reactivity with polyisocyanate as a curing agent, which is unfavorable for the physical properties of the coating film. The amount of the So and M group-containing polyurethane resin used is suitably in the range of 20 to 80% by weight, or 30 to 70% by weight, based on the total resin content in the binder.

The So, M group-containing polyurethane resin used in the present invention is
It is obtained by the reaction of a polyhydroxy compound and a polyinsyanate. In this case K is used, a polyhydroxy compound partially or completely containing a sulfonic acid metal base. Also used are polyester polyols consisting of a carboxylic acid component, a glycol component and a dicarboxylic acid component and having a sulfonic acid metal base. It is the carboxylic acid or the dicarboxylic acid or the moiety of both that has the sulfonic acid metal group. Examples of the dicarboxylic acid and carboxylic acid component include terephthalic acid, isophthalic acid, orthophthalic acid, aromatic dicarboxylic acid, aromatic oxycarboxylic acid, succinic acid, adipic acid, azelaic acid, and sebacic acid. Glycol components include ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-bentanediol, neopentyl glycol, diethylene glycol, 1,6-hexanediol, and dipropylene glycol. , 2,2.4-) dimethyl-1,3-bentanediol, 1,4-cyclohexane dimetatool, ethylene oxide adduct and propylene oxide adduct of bisphenol, ethylene oxide adduct and propylene oxide adduct of hydrogenated bisphenol, Examples include polyethylene glycol, polypropylene glycol, and polytetraglycol. Further, triols and tolaols such as trimethylolethane, trimethylolpropane, glycerin, and pentaerythritol may be used in combination. There may be one or more types of polyhydroxy compounds having the above-mentioned sulfonic acid metal bases. The polyisocyanate used to form the polyurethane resin of the present invention includes 2,
4-tolylene diisocyanate, 2.6-tolylene diisocyanate, p-phenylene diisocyanate, diphenylmethane diisocyanate, m-phenylene diisocyanate, hexamethylene diisocyanate, tetramethylene diisocyanate Isocyanate and 3,3'-dimethoxy-4
, 4'-biphenylene diisocyanate, and 1.3-su7
Talen diisocyanate, 4.41-diphenylene diphenyl ether, p-xylylene diisocyanate, m
-xylylene diisocyanate, 1,3-diisocyanatemethylcyclohexane, 1,4-diincyanatomethylcyclohexane, 4,4'-diisocyanatedicyclohexane, 4,4'-diisocyanatedicyclohexylmethane, inphorone diisocyanate, and the like. A polyurethane resin can be obtained by reacting a polyhydroxy compound and a polyisocyanate in a solvent or in the absence of a solvent by a known method. The molecular weight of such polyurethane resin is preferably in the range of s,ooo to 100,000.

Examples of the polyisocyanate curing agent used in the present invention include bifunctional incyanates and trifunctional isocyanates such as tolylene diisocyanate, diphenylmethane diisocyanate, and hexane diisocyanate. Furthermore, any polyisocyanate that can be used as a curing agent can be used. The weight ratio of these polyisocyanates to the total resin content is 5 to 30.
5 Preferably add 10 to 25%. Commercially available polyisocyanate compounds used for this purpose include products such as "Coronate L", "Coronate HL" and "Coronate 2036" manufactured by Nippon Polyurethane Kogyo Co., Ltd.

The relative proportions of the three resin binder components used to prepare the magnetic paint in the present invention are generally 20-80% by weight of the polyurethane resin, 5-30% by weight of the polyisocyanate curing agent, and 5-30% by weight of the polyisocyanate curing agent, based on the total resin content. The polymer is the balance, and preferably the polyurethane resin is 30 to 7
0 heavy view%, polyinsyanate curing agent 10-25ml
% and copolymer is 70-30% by weight.

The magnetic powder used does not constitute a characteristic part of the present invention, and magnetic particles such as γ-iron oxide, Oo-containing γ-iron oxide, and ferroalloy, which have been conventionally used in magnetic recording media, can be used. can.

However, the present invention uses ultrafine powder (for example, 40m in BET value).
279 or more, preferably 45m" or more (7g or more), the effect is particularly great.

According to the present invention, sulfonic acid, vinyl chloride copolymer,
By containing sodium sulfonate, sulfone, phosphoric acid, and phosphate ester groups, the increase in the glass transition point of the copolymer can be suppressed without reducing the proportion of the saponified product of the alkyl carboxylic acid vinyl ester. ,
In this way, surface processing becomes easier with improved dispersibility, and the use of a polyurethane resin having -SO3M groups in the molecule eliminates the decrease in dispersibility seen with conventional rubber binders, and furthermore, it can be used as a curing agent. By using polyisocyanate in combination, improvements in the physical properties of the magnetic coating film can also be obtained.

The present invention will be further explained below with reference to Examples.

Example 1 The following composition: Dispersant
5. li + lubricant
5f! Magnetic powder (00 containing r-yeloj BIT value 25m”/fl) 200
g polyinocyanate) 101
A composition having the following properties was charged into a ball mill, and an organic solvent was added thereto at 300f! The mixture was then mixed for 48 hours and spread onto a 14μ thick polyester film with a thickness of 5.0μ (
12.65 ttt*
I cut it into 5 pieces (1/2 inch) wide to make magnetic tape.

The thus obtained magnetic tape was tested for residual magnetic flux density Er (G), y-8/n, and Envelope 1 running durability. The results obtained are shown in Table 1.

Furthermore, the relationship between the BET value of the magnetic powder and the obtained y-S/N value is shown in FIG.

Example 2 Composition below: Dispersant
5I lubricant
5! I magnetic powder (Co-rFe,
06 EE'I' value 35fi2/g) 200
A magnetic tape was prepared in the same manner as in Example 1 using a composition having p-polyinsyanate 10/. The magnetic tape was subjected to the same test as in Example 1, and the results are shown in Table 1 and plotted with FIG. 1.

Example 3 This example shows the effect of the present invention by varying the proportions of other monomers copolymerizable with vinyl chloride and saponified alkyl carboxylic acid vinyl esters in a vinyl chloride copolymer. Find out how it affects

The following composition: Dispersant 5g Lubricant
5! i magnetic powder (Co-rFe206 EET
A magnetic tape was prepared in the same manner as in Example 1 using a composition having a value of 45/9) 200 g and 10 g of polyisocyanate. The magnetic tape was subjected to the same test as in Example 1, and the results are shown in Table 1 and plotted in Figure 1 (copolymer: male content = 2.0%, OH/cH absorption ratio = 0. 5). and,
For the magnetic tape of this example, the surface properties (glossiness) after coating are plotted in plot 2, and the surface properties (glossiness) after calendering are shown in FIG.

Comparative Example Composition of 1° or less: Polyurethane resin 20g dispersant
5.1i [Lubricant
5g magnetic powder (Co-containing - FegOa BET
Value 45m”/9) 2001 Polyinsyanate
Example 1 using a composition having 10II
A magnetic tape was made in the same way. The magnetic tape was subjected to the same test as in Example 1, and the results are shown in Table 1.

Composition of Comparative Example 2° or less: Polyurethane resin 20.9 Dispersant 5y Lubricant
5I magnetic powder (Co-containing r -F e heavy OB B
A composition having an ET value of 45 m''/! l) 200.9 polyisocyanate) lOg was used.
- A magnetic tape was produced in the same manner as in Example 1. The magnetic tape was subjected to the same test as in Example 1, and the results are shown in Table 1.

Comparative Example 36 The following composition: Polyurethane resin 20. ! i1 dispersant
5I lubricant
5g magnetic powder (Co-containing r-Feg
A magnetic tape was prepared in the same manner as in Example 1 using a composition having 10 g of 200 p polyinsyanate (Oa EET value 25 m2/g). The magnetic tape was subjected to the same tests as in Example 1, and the results are shown in Table 1 and plotted in FIG.

Composition of Comparative Example 4° or less: Polyurethane resin 20g Dispersant 5g Lubricant
5g magnetic powder (CO containing r-Fe106 BE
A magnetic tape was prepared in the same manner as in Example 1 using a composition having 10 g of 2001 polyisocyanate (T-direction: 35 m"/g). The magnetic tape was subjected to the same tests as in Example 1, and the results are shown in Table IK. and is plotted in FIG.

Comparative Example 5 In this comparative example, tests were conducted with various proportions of other monomers copolymerizable with vinyl chloride and saponified alkyl carboxylic acid vinyl esters in the vinyl chloride copolymer.

The following composition: Polyurethane resin 201/Dispersant 5g Lubricant 5I Magnetic powder (Co containing r-re2og BET value 45m2
/,! 9) 200 g polyinsyanate
A magnetic tape was prepared in the same manner as in Example 1 using a composition having a molecular weight of 10.9. The magnetic tape was subjected to the same test as in Example 1, and the results are shown in Table 1.
and plotted in Figure 1 (copolymer: maleic acid amount:
2.0, oVaH absorption ratio = Q, 5). The surface quality (gloss) of the magnetic tape in this example after coating was
The surface properties (glossiness) after calendering are shown in FIG.

Composition of Comparative Example 6° or less: Polyurethane resin 209 dispersant
5g lubricant
5f! Magnetic powder (c, containing γ-Fegos B
ET value 45/i) 2009 polyinsyanate
Example 1 using a composition with 10 g
A magnetic tape was made in the same way. The magnetic tape was subjected to the same tests as in Example 1, and the results are shown in Table 1.

Composition of Comparative Example 7° or less: Vinyl chloride-vinyl acetate copolymer 20I (maleic acid amount: 2.0%) (OH/cm absorption ratio: O, S) Polyurethane resin 20.9 (80,
Na group: 5oo1 equivalent/10'F) Dispersant
5y lubricant
5g magnetic powder (Co-containing-F820s B
A magnetic tape was prepared in the same manner as in Example 1 using a composition having F; X' value of 45/g) and 10 g of zoo,y polyisocyanate. The magnetic tape was subjected to the same test as in Example 1, and the results are shown in Table 1.

As is clear from Table 1 and FIG. 1, 5O6Na group-containing vinyl chloride, alkylcarboxylic acid vinyl ester, other monomers copolymerizable with vinyl chloride, and alkylcarboxylic acid vinyl ester sulfonate A magnetic recording medium made of a copolymer as a constituent component, a polyurethane resin containing an SOaM group (M is an alkali metal or a hydrogen atom), and a polyisocyanate as a curing agent component has surface properties, maximum residual magnetic flux density, y -S/N is improved.

Furthermore, the magnetic recording medium of the present invention has improved electromagnetic conversion characteristics compared to a binder in which 80@Na is contained in either a chloride and (d)v-based or polyurethane resin. (See Example 3 and Comparative Example 6.7) As is clear from FIG.
It has a great effect on the dispersion of Similar results were also obtained with methacrylic acid and fumaric acid as monomers.

Similar results were obtained when sulfonic acid, sulfone, phosphoric acid, or phosphoric acid ester groups were contained in place of S Ql and Na.

Further, FIGS. 2 and 3 show the surface properties of Example 3 and Comparative Example 5 after coating and after calendering. Second
As is clear from Figures 3 and 3, the monomer (maleic acid) and 80. Those containing Na groups at the same time are 5OaN
Those containing & group but not containing monomer (maleic acid), and those containing monomer 80. The glossiness is clearly improved compared to magnetic coatings whose resin component is one that does not contain INa groups or one that does not contain either of them. From FIG. 3, as the O\cH absorption ratio increases, the gloss after coating increases. Similarly, the gloss after calendering increases as the 0VaH absorption ratio increases, but it decreases when the oVan absorption ratio exceeds 0.5 due to the relationship with the glass transition point. With the binder according to the present invention, the gloss after coating and the gloss after processing are OV■The absorption ratio is 0.2
It can be seen that those with a value of 0.7 are excellent in both the gloss after coating and the gloss after processing.

Effects of the Invention When a binder having the composition described above is used in the coating layer of a magnetic recording medium, the drawbacks that occurred with conventional binders are greatly improved, and the dispersibility and surface properties are improved, and the binder is resistant to high temperature, high humidity, and low temperature conditions. This has had a significant effect on the running reliability of the vehicle.

[Brief explanation of the drawings] Figure 1 shows BETs for Examples 1 to 3 and Comparative Examples 3 to 5.
It is a graph which shows the relationship between a value and y-S/N. FIGS. 2 and 3 show the surface properties (glossiness) after coating and the surface properties (glossiness) after processing of the magnetic recording media for Example 3 and Comparative Example 5, respectively. Figure 1 Ishitake i end, e) S E T 4fL (m17g)
Procedural Amendment Document January 1985 (1985) Commissioner of the Patent Office Manabu Shiga Indication of the case 1988 Patent Application No. 221967 Name of the invention Relationship to the case of the person who amended the magnetic recording medium Name of the patent applicant (
306) TDC Co., Ltd. Address: Oil and Fat Industry Hall, 3-13-11 H-Motobashi, Chuo-ku, Tokyo Detailed description of the invention in the specification subject to amendment Details of drawing amendment As shown in Attachment 1 Specification, page 12 bottom Correct "traor" in line 5 to "tetraor". 2 Change “sO1Nm group” on page 17, line 10 to “so, N
Corrected to "a group". 5. The heading "CH10H absorption ratio" in Table 1 on page 24 is corrected to "ob/CH absorption ratio". 4. Replace Figure 2 with attached Figure 2. 5. Replace Figure 3 with attached Figure 3. Figure 2 0 0.2 0.4 0.6 0.8
1.00H/CHaL yield w Figure 3 0 0.2 0.4 0.6 0.
8 1.00H/CHOL4 Nui Procedural Amendment February 18, 1985

Claims (1)

[Scope of Claims] 1. (i) It has one or more types of sulfonic acid, sodium sulfonate, sulfone, phosphoric acid, or phosphate ester groups, and has an OH/CH absorption ratio of 0.2 to 0 in the infrared absorption spectrum. .7, the copolymer is made of vinyl chloride, an alkyl carboxylic acid vinyl ester, another monomer copolymerizable with vinyl chloride, and a saponified product of the alkyl carboxylic acid vinyl ester. (ii) a polyurethane resin having a -SO_3M group (M represents an alkali metal) in the molecule, and (iii) a resin binder consisting of a polyisocyanate, in which magnetic powder is dispersed, and this is used as a support. A fixed magnetic recording medium. 2. The copolymer contains 50 to 90% by weight of vinyl chloride, 5 to 40% by weight of an alkyl carboxylic acid ester, and 1.0% of a monomer copolymerizable with vinyl chloride, based on the weight of the copolymer. 2. The magnetic recording medium according to claim 1, comprising ~5.0% by weight and the remainder of a saponified product of an alkyl carboxylic acid ester. 3. Copolymer of one or more of sulfonic acid, sodium sulfonate, sulfone, phosphoric acid, or phosphoric acid ester group 10^
3. The magnetic recording medium according to claim 1, wherein the copolymer has an equivalent weight of about 1 to 500 g per 4 g. 4. The polyurethane is about 10 to 1000 g equivalent/10
The magnetic recording medium according to claim 1, having a -SO_3M group in the range of ^4g. 5. The magnetic recording medium according to claim 1 or 4, wherein the polyurethane resin is a polyurethane resin, a polyester resin, or a mixture thereof. 6. Based on the weight of the total resin in the resin binder,
Claims 1 to 10 in which the copolymer is present in an amount of 20 to 80% by weight, the polyurethane resin having a -SO_3M group is present in an amount of 20 to 80% by weight, and the polyisocyanate is present in an amount of 5 to 30% by weight.
The magnetic recording medium according to any one of Item 5.