JP3424759B2 - Magnetic recording media - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium such as a magnetic tape and a magnetic disk. [0002] Magnetic tapes, which are general-purpose magnetic recording media,
A floppy disk is made by dispersing needle-shaped magnetic particles having a major axis of 1 μm or less together with additives such as a dispersant, a lubricant and an antistatic agent in a binder solution to form a magnetic paint, and applying this to a polyethylene terephthalate film. It is made. Characteristics required for the binder of the magnetic layer include dispersibility, filling property, orientation of the magnetic particles, durability of the magnetic layer, abrasion resistance, heat resistance, adhesiveness to a non-magnetic support, and the like. However, binders play a very important role. As a binder for the magnetic layer, a mixture of an adipate-type or polycaprolactone-type polyurethane resin and nitrocellulose or a PVC-based copolymer has been mainly used. Regarding improvement of the dispersibility of magnetic particles, see JP-B-58-4
It is known from JP 1565 that it is effective to introduce a sulfonic acid metal base into polyurethane. [0003] In a magnetic recording medium, S
In order to improve the / N ratio (signal / noise ratio) and increase the recording density, the magnetic layer should be filled with magnetic particles that are more finely divided and highly oriented, and the surface of the magnetic layer should be smooth. It has been done. The smoother the surface of the magnetic layer, the higher the friction coefficient and the worse the running property and running durability of the magnetic tape. Therefore, a binder having good durability, abrasion resistance, heat resistance, and adhesion to a non-magnetic support is desired. Conventional binders for the magnetic layer are insufficient for these requirements. Polyurethane resin containing aromatic polyester diol as one component of high molecular weight polyol is excellent in mechanical strength, abrasion resistance, heat resistance, etc., and is in the direction of improvement for these requirements, but completely solves the problem Has not been reached. Polyurethane resin containing aromatic polyester diol as one component of high molecular weight polyol has a higher glass transition temperature than conventional polyurethane resin, so the effect of calendering to smooth the surface of the magnetic layer is reduced. In addition, the adhesion between the magnetic layer and the base film is deteriorated. Deterioration of the adhesiveness between the magnetic layer and the base film may cause the magnetic layer and the base film to peel off at the end face when slitting the magnetic tape. The missing magnetic powder causes clogging of the magnetic head or causes dropout. In order to improve the S / N ratio and increase the recording density of a magnetic recording medium, it is necessary to highly fill a magnetic layer with a high Hc magnetic powder such as a finer magnetic powder and a metal magnetic powder and to perform a high orientation. However, it is difficult to disperse these magnetic powders with a conventional binder, and poor dispersion not only lowers the electromagnetic conversion characteristics, but also increases the porosity in the magnetic layer. The increase degrades the running durability of the magnetic layer. Introducing a metal sulfonate group into polyurethane is extremely effective in improving the dispersibility of magnetic powder, but the viscosity of the magnetic paint may increase. Lower. An object of the present invention is to provide a binder having high dispersibility of magnetic powder, good adhesion to a non-magnetic support, good calendering properties, and low viscosity of a magnetic paint, and a good productivity of a magnetic recording medium. To enable the manufacture of Means for Solving the Problems As a result of intensive studies on the polyurethane resin, the present inventors have found that by using a polyurethane resin having a specific viscoelastic characteristic, the use of a polyurethane resin having a specific viscoelastic property and a segment adsorbed on magnetic powder can be improved. The inventors have found that the object of the present invention can be achieved by selecting a segment not to be used, and arrived at the present invention. That is, the present invention provides a magnetic recording medium in which a magnetic layer mainly containing a ferromagnetic powder and a binder is provided on a non-magnetic support, wherein the binder component includes:
A polyurethane resin comprising a high molecular weight polyol (A) having a molecular weight of 500 or more, an organic diisocyanate (B) and, if necessary, a polyol compound (C) having a molecular weight of 500 or less, wherein the high molecular weight polyol (A) is a sulfonic acid 20-600 equivalents of metal base / 1
0 ⁶ g multiplexer having aromatic polyester diol (A-1)
And a polyol (A-2) other than the (A-1), wherein the content of the aromatic polyester diol (A-1) is at least 20% by weight in the polyol (A), and a urethane group Concentration of 600 to 1500 equivalents / 10 ⁶ g
And a dynamic recording medium having a dynamic loss modulus of 10 ⁹ dyne / cm ² or more measured at 110 Hz and a temperature range of 40 ° C. or more. The high molecular weight polyol (A) constituting the polyurethane resin used in the present invention comprises an aromatic polyester diol (A-1) containing a sulfonic acid metal base and a polyol (A-1) other than the (A-1). 2) wherein (A-2) is poorly compatible with polyol (A-1) which is liquid at 20 ° C. and does not contain a metal sulfonate group, and (A-1)
-1) It contains a polyol having a smaller cohesive force. The carboxylic acid component of the aromatic polyester diol (A-1) used in the present invention includes terephthalic acid, isophthalic acid,
Orthophthalic acid, 1,5-naphthalic acid, 2,6-naphthalic acid, 4,4′-diphenyldicarboxylic acid, 2,2′-
Examples include aromatic dicarboxylic acids such as diphenyldicarboxylic acid and 4,4′-diphenylether dicarboxylic acid, and particularly, terephthalic acid, isophthalic acid, orthophthalic acid,
2,6-Naphthalic acid is preferred. The glycol component is ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentaneddiol, 1,6-hexanediol, and 3-methyl-diol. 1,5-pentanediol, neopentyl glyco-
, Diethylene glycol, dipropylene glycol,
2,2,4-trimethyl-1,3-pentaneddiol,
Cyclohexane dimethanol, neopentyl hydroxypivalate, bisphenol A ethylene oxide and propylene oxide adducts, hydrogenated bisphenol A ethylene oxide and propylene oxide adducts, 1,9-nonane Diol, 2-methyloctanediol, 1,10-dodecanediol, 2-butyl-2-ethyl-1,3propanediol, tricyclodecanedimethanol and the like,
Of these, ethylene glycol, neopentyl glycol, cyclohexane dimethanol, neopentyl hydroxypivalate, 2-butyl-2-ethyl-
1,3 propanediol and tricyclodecane dimethanol are preferred. As raw materials for introducing a sulfonic acid metal base into an aromatic polyester diol, 5-sodium sulfoisophthalic acid, 5-potassium sulfoisophthalic acid, sodium sulfoterephthalic acid, 2-sodium sulfo-1,4-butane Diol, 2,5-dimethyl-3
Examples thereof include dicarboxylic acids and glycols such as -sodium sulfo-2,5-hexanediol, and 5-sodium sulfoisophthalic acid and 5-potassium sulfoisophthalic acid are preferred. The amount of the sulfonic acid metal base in the aromatic polyester diol is in the range of 20 to 600 equivalent / 10 ⁶ g. If the amount is less than 20 equivalents / 10 ⁶ g, the dispersibility of the magnetic powder is insufficient. If the amount exceeds 600 equivalents / 10 ⁶ g, the viscosity of the polyurethane resin solution or the magnetic paint becomes high, and the handling and the production of the magnetic recording medium become difficult. More obstacles. A particularly preferred range is from 60 to 400 equivalents / 10 ⁶ g. The high-molecular-weight polyol constituting the polyurethane resin used in the present invention may be a polyol which is liquid at 20 ° C. and does not contain a metal sulfonate other than the aromatic polyesterdiol (A-1) containing a metal sulfonate. Polyol (A-2) having poor compatibility with (A-1) and lower cohesive strength than (A-1) is used. The polyol (A-2) which is liquid at 20 ° C. that does not contain a metal sulfonate group has a melting point of 20 ° C. or less or is an amorphous aliphatic polyester, alicyclic polyester, Ether and polycarbonate are exemplified. Specific examples of aliphatic or alicyclic polyesters having a melting point of 20 ° C. or lower or amorphous include adipic acid, azelaic acid, sebacic acid, β
-Methyladipic acid, 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,2-
Cyclohexanedicarboxylic acid, 4-methyl-1,2-cyclohexanedicarboxylic acid, dicarboxylic acid such as dimer acid and propylene glycol, neopentyl glycol,
3-methyl-1,5-pentanediol, neopentyl hydroxypivalate, 2-butyl-2-ethyl-1,3 propanediol, diethylene glycol,
There are combinations of side chains such as triethylene glycol or glycols having an ether bond, and linear glycols such as ethylene glycol, 1,4-butanediol, and 1,6-hexanediol are liquid at 20 ° C. May be included. Examples of the polyether include amorphous polyethers such as polypropylene glycol and poly (3-methyl-1,3-propylene glycol). Also, amorphous polycarbonates such as poly (diethylene glycol carbonate) and the like can be mentioned. In the polyol (A-2), (A-
Examples of the polyol having poor compatibility with (1) and having a lower cohesive force than (A-1) include a case where the molecular weight increases in a solid at room temperature and a case where the difference in solubility parameter from (A-1) is large. . Although the metal sulfonate has an adsorption ability to magnetic powder, an aromatic polyester diol (A-1) containing the metal sulfonate is combined with a liquid high molecular weight polyol (A-2) having a small cohesive force. By obtaining polyurethane, the interaction between the polyurethane chains to which the magnetic powder is adsorbed is reduced, and as a result, the viscosity of the magnetic paint is reduced, and the magnetic powder filling property in the magnetic layer is improved.
Further, a polyol having a small cohesive force with an aromatic polyester diol has relatively poor compatibility, and the peak width of the dynamic loss elastic modulus increases based on the polyol. for that reason,
Even if the glass transition temperature of the polyurethane is high, the adhesion of the magnetic paint to the base film is good, and the calenderability is also good. Such an effect is that the polyurethane resin used in the present invention has a dynamic loss elastic modulus of at least 10 ⁹ dyne / cm ^{2 in} a temperature range of at least 40 ° C. or more measured at 110 Hz in the range of −20 to 100 ° C. Becomes more noticeable. Aromatic polyester diols (A-1) containing a metal sulfonate group of a high molecular weight polyol (A) constituting the polyurethane resin used in the present invention and (A)
Poorly compatible with polyol (A-1) which is liquid at 20 ° C. which does not contain a sulfonic acid metal base other than -1);
-1) The polyol (A-2) having a lower cohesive force is preferably used in a weight ratio of 10: 1 to 1: 4, more preferably in a range of 10: 1.5 to 1: 3. When the content of the polyol (A-2) is less than 20% by weight in the total polyol (A), the above-mentioned effect of the present invention is hardly exhibited, and the width of the temperature range showing a dynamic loss elastic modulus of 10 ⁹ dyne / cm ² or more is obtained. It becomes messy. The organic diisocyanate component of the polyurethane resin used in the present invention includes 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, p
Phenylene diisocyanate, diphenylmethane diisocyanate, m-phenylene diisocyanate, hexamethylene diisocyanate, tetramethylene diisocyanate, 3,3'-dimethoxy-4,4 '-Biphenylenediisocyanate, 1,5-naphthalenediisocyanate, 2,6-naphthalenediisocyanate, 3,
3'-dimethyl-4,4'-diisocyanate, 4,
4'-diisocyanatodiphenyl ether, 1,5-
Xylylene diisocyanate, 1,3-diisocyanate
Tomethylcyclohexane, 1,4-diisocyanatomethylcyclohexane, 4,4'-diisocyanatocyclohexane, 4,4'-diisocyanatocyclohexylmethane, isophorone diisocyanate and the like. A polyol compound having a molecular weight of 500 or less, which is used as necessary, adjusts the urethane group concentration in the polyurethane resin to impart a toughness specific to the polyurethane resin. It has the effect of increasing the crosslinking density. For example, ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentaneddiol, 1,6-hexanediol, neopentyl glycol , Diethylene glycol, dipropylene glycol
2,2,4-trimethyl-1,3-pentanedio-
, Cyclohexane dimethanol, neopentyl hydroxypivalic acid ester, ethylene oxide adduct and propylene oxide adduct of bisphenol A,
Diol compounds such as ethylene oxide adducts and propylene oxad adducts of hydrogenated bisphenol A,
Trifunctional or higher functional polyol compounds such as trimethylolpropane, glycerin, pentaerythritol, dipentaerythritol and the like, especially neopentyl glyco-
, Neopentyl hydroxypivalate, trimethylolpropane, and glycerin are preferred. The polyol compound having a molecular weight of 500 or less is
There is an effect of adjusting the urethane group concentration and imparting a toughness specific to the polyurethane resin, but the urethane group concentration in the polyurethane resin is 600 to 1500 equivalent / 10 ⁶ g, preferably 700 to 1300 equivalent / 10 ⁶ g. . If the urethane group concentration falls below this range, the cohesive force of the resin becomes too small, and the running durability and heat resistance deteriorate.
Conversely, if the urethane group concentration increases beyond this range, the cohesive force between the non-adsorbed segments increases, causing a decrease in the filling property of the magnetic powder, an increase in the viscosity of the magnetic paint, and a deterioration in the calenderability. The molecular weight of the polyurethane resin used in the present invention is from 5,000 to 80000, preferably from 10,000 to 50,000. If the molecular weight is less than 5000, the mechanical strength is insufficient, and the running durability is poor. Molecular weight 8
If it exceeds 0000, the solution viscosity increases, and workability and dispersibility of magnetic powder, abrasive, carbon black and the like deteriorate. In the present invention, in addition to the polyurethane resin, it is desirable to add another resin and / or mix a crosslinking agent for the purpose of adjusting flexibility, improving cold resistance, heat resistance, and the like. Examples of other resins include a vinyl chloride resin, a cellulose resin, a polyester resin, an epoxy resin, a phenoxy resin, a polyvinyl butyral, an acrylonitrile-butadiene copolymer, and a polyurethane resin other than the present invention. On the other hand, examples of the curing agent include a polyisocyanate compound, an epoxy resin, a melamine resin, a urea resin, and an acid anhydride. Among these, a polyisocyanate compound is particularly preferable. The ferromagnetic particles used in the magnetic layer of the magnetic recording medium of the present invention include γ-Fe ₂ O ₃ and γ-Fe ₂
Mixed crystal of O ₃ and Fe ₃ O ₄ , γ-Fe ₂ O coated with cobalt
₃ or a ferromagnetic oxide such as Fe ₂ O ₄ or barium ferrite, or a ferromagnetic alloy powder such as Fe—Co or Fe—Co—Ni. The magnetic recording medium of the present invention includes
In addition, if necessary, a plasticizer such as dibutyl phthalate or triphenyl phosphate, dioctyl sodium sulfosuccinate, t-butylphenol polyethylene ether, sodium ethylnaphthalene sulfonate, dilauryl succinate, zinc stearate, soybean lecithin, silicone oil And various kinds of antistatic agents can also be added. According to the magnetic recording medium of the present invention, an aromatic polyester diol (A-1) containing a metal sulfonate group having a high ability to adsorb to magnetic powder as a binder component of a magnetic layer and a metal sulfonate are used. Since it is a polyurethane resin containing a base containing no polyol at 20 ° C. or having poor compatibility with (A-1) and having a low cohesive force (A-2) than (A-1) as a high molecular weight polyol component, The use of a polyol (A-2) component having a small cohesive force reduces the interaction between the polyurethane molecular chains adsorbing the magnetic powder, and as a result, reduces the viscosity of the magnetic paint and improves the dispersibility of the magnetic powder. Thus, the filling property of the magnetic layer is improved. Further, aromatic polyester diol and a polyol such as a liquid having a small cohesive force (A
-2) is relatively poor in compatibility, so that polyol (A-
The dynamic loss elastic modulus based on 2) increases in a wide temperature range. Therefore, even if the glass transition temperature of the polyurethane is high, the adhesion to the base film is good, and the calenderability is also good. The present invention will be specifically illustrated by the following examples. In the examples, “parts” means “parts by weight”. Synthesis Example 1 of Polyurethane Resin 1 Polyester diol (A) 1 shown in Table 1 was placed in a reaction vessel equipped with a thermometer, a stirrer, a reflux condenser, and a distillation tube.
After adding and dissolving 00 parts and 100 parts of toluene, toluene 2
Zero parts were distilled and the reaction system was dehydrated by azeotropic distillation of toluene / water. After cooling to 60 ° C., 26.5 parts of diphenylmethane diisocyanate were added, and the mixture was heated at 80 ° C. for 2 hours. Parts were added. After reacting at 80 ° C. for 6 hours, 2 parts of neopentyl glycol was added, and the mixture was further heated for 2 hours, and 121.3 parts of toluene, 121.3 parts of methyl ethyl ketone, and 4 parts of trimethylolpropane were added, and 8 parts of
The reaction was performed at 0 ° C. for 4 hours to obtain a polyurethane resin 1 having a solid content of 30%. Table 1 shows the properties of the polyurethane resin 1. In Table 1, the solution viscosity was 25 ° C., the number average molecular weight was measured by gel permeation chromatography using a tetrahydrofuran solution, and the dynamic viscoelasticity was measured at 110 Hz. The unit of the solution viscosity is poise. In Table 1, the term “dynamic viscoelasticity” represents the temperature at the inflection point of the dynamic storage elastic modulus (E ′) and the temperature range where the dynamic loss elastic modulus (E ″) exceeds 10 ⁹ dyne / cm ² (initial temperature). And end temperature). Synthesis Examples 2 to 6 of Polyurethane Resin Comparative Synthesis Examples 1 to 6 In the same manner as in Synthesis Example 1 of polyurethane resin, a polyurethane resin solution was obtained from the raw materials shown in Table 1. Table 1 shows the properties of the obtained polyurethane resin. However, in Comparative Synthesis Example 1, polycaprolactone solid at 20 ° C. was used instead of the polypropylene glycol used in Synthesis Example 1. Comparative Synthesis Example 2 has the same urethane group concentration as Synthesis Example 3, but the molecular weight of polytetramethylene glycol is different, and Synthesis Example 3 has poor compatibility with polyester A (even if the urethane-forming reaction proceeds). The solution was cloudy), and in Comparative Synthesis Example 2, the compatibility was better than in Synthesis Example 3.
The solution became transparent due to the progress of the urethanization reaction. In Comparative Synthesis Example 3, polytetramethylene glycol having a molecular weight of 1,000 was used in place of the polytetramethylene glycol having a molecular weight of 2,000 used in Synthesis Example 3. Comparative Synthesis Example 3
In those urethane group concentration is outside the scope of the claims 1640 equivalents / 10 ⁶ g, Comparative Synthesis Example 4, the concentration of metal sulfonate group in the aromatic polyester diol is 14 eq / 1
0 ⁶ g outside the scope of the appended claims, the Comparative Synthesis Example 5 Aromatic
Patent for ratio of aliphatic polyester diol to liquid polyol
In Comparative Synthesis Example 6, the dynamic loss elasticity is out of the required range.
Patented for temperature range width exceeding 10 ⁹ dyne / cm ²
It is outside the scope of the requirements. [Table 1] Abbreviations in the table are as follows. Raw material composition, number average molecular weight Polyester AT / I / DSN // EG / NPG (50/47/3 // 50/50 molar ratio) MW = 2500 PPG-2000 Polypropylene glycol MW = 2000 PPG-1000 Polypropylene glycol MW = 1000 Polyester B Poly (3-methyl-1,5-pentane adipate) MW = 4000 Polyester CT / I / DSN // EG / NPG (50/40/10 // 50/50 molar ratio) MW = 2000 Polyester D Poly (3-methyl-1,5-pendane adipate) MW = 2000 Polyester ET / I / DSN // TCD / CHDM / EG (50 / 49.2 / 0.8 // 30/40/30 molar ratio) MW = 2200 PCL-2000 Polycaprolactone MW = 2000 Polyester FT / I / DSN // EG / NPG (50 / 49.7 / 0.3 // 50/50 molar ratio) MW = 2500 NPG Neopentyl glycol TMP Trimethylolpropane MDI 4,4'-Diphenylmethane diisocyanate PTG -2000 polytetramethylene glycol MW = 2000 PTG-1000 polytetramethylene glycol MW = 1000 where T: terephthalic acid I: isophthalic acid DSN: dimethyl 5-sodium Musulfoisophthalic acid EG: Ethylene glycol TCD: Tricyclodecane dimethylol CHDM: Cyclohexane dimethanol Example 1 A composition having the following composition ratio was dispersed in a ball mill for 48 hours, and then a polyisocyanate compound, coronate MR (manufactured by Nippon Polyurethane Industry) as a curing agent
Parts, 0.05 parts of stearic acid and 0.05 parts of butyl stearate were added as a lubricant and mixed for 1 hour to obtain a magnetic paint. This was applied on a 15 μm-thick polyethylene terephthalate film so that the thickness after drying was 4 μm, and dried while applying a magnetic field of 2000 Gauss.
It was passed through a calender roll at 0 ° C. and a linear pressure of 200 Kg / cm to smooth the surface of the magnetic layer to prepare a magnetic tape. After leaving the obtained magnetic tape at 60 ° C. for 1 day,
Slit to inches. The surface gloss of the magnetic layer before calendering and the squareness ratio of the obtained tape were measured. The surface roughness of the magnetic layer before and after calendering was also measured. Furthermore,
The adhesion between the magnetic layer and the base film, the filling property of the magnetic layer, and the viscosity of the magnetic paint were measured by the methods described below. In addition, 100 ° C at 10 ° C and 40 ° C on a commercially available VTR deck
The abrasion state of the magnetic layer after the round running was observed. Table 2 shows the results. Solution of polyurethane resin obtained in Synthesis Example 1 10 parts (30% solution of MEK / toluene = 1/1) Magnetic powder (Cobalt-coated ferrite BET 45 m ² / g) 12〃 Cyclohexanone 5〃 Toluene 5〃 MEK 5〃 Alumina (Average particle size: 0.05μ) 0.5〃 The evaluation method is as follows. Magnetic Layer Adhesive cellophane tape made of Nichiban was adhered to the adhesive magnetic layer, and the magnetic tape was peeled off at 180 ° at a speed of 100 mm / min. The measurement temperature was 20 ° C. Magnetic layer filling property A precisely weighed magnetic tape was immersed in silicone oil and subjected to ultrasonic treatment for 10 minutes. After wiping off the silicone oil adhering to the surface of the magnetic tape, the weight was precisely weighed. Since the silicone oil enters the voids in the magnetic layer, the filling rate of the magnetic layer is worse as the weight increase due to the silicone oil immersion is larger. The viscosity of the magnetic paint at 25 ° C. was measured at 12 rpm using a B-type viscometer manufactured by Tokyo Keiki. Examples 2 to 8 Comparative Examples 1 to 8 In the same manner as in Example 1, Synthesis Examples 2 to 6 and Comparative Synthesis Examples 1 to
A magnetic tape was prepared using the polyurethane resin of No. 6.
Tables 2 to 5 show the evaluation results. [Table 2] [Table 3] [Table 4] [Table 5] In Tables 2 to 5, PVC-1 is a PVC copolymer (MR-100 manufactured by Zeon Corporation). Abrasion state of magnetic layer ：: No scratch is observed on the magnetic layer. Δ: Scratches are observed in the magnetic layer but few. ×: A flaw is clearly observed in the magnetic layer. As is clear from the above Examples and Comparative Examples, the high molecular weight polyol is an aromatic polyester diol (A-1) containing a sulfonic acid metal base and a high molecular weight polyol containing no sulfonic acid metal base. By using a polyurethane resin containing a polyol (A-2) which is poorly compatible with polyol (A-1) which is liquid at ℃ and has a lower cohesive force than (A-1) as a binder for magnetic particles. Excellent dispersibility of magnetic powder, low viscosity of magnetic paint, good productivity, and obtained magnetic recording medium has excellent running durability, abrasion resistance, adhesion to magnetic layer and non-magnetic support. Also, the calenderability of the magnetic layer is good.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G11B 5/70

Claims (1)

(57) [Claim 1] In a magnetic recording medium provided with a magnetic layer mainly containing a ferromagnetic powder and a binder on a non-magnetic support,
As the above-mentioned binder component, a high molecular weight polyol (A) having a molecular weight of 500 or more, an organic diisocyanate (B) and, if necessary, a polyol compound (C) having a molecular weight of 500 or less.
The polyurethane resin comprises an aromatic polyester diol (A-1) in which the high molecular weight polyol (A) has a sulfonic acid metal base content of 20 to 600 equivalents / 10 ⁶ g, and the (A-1) ), The combined amount of the aromatic polyester diol (A-1) is at least 20% by weight in the polyol (A), and the urethane group concentration is 600 to
A magnetic recording medium having a temperature range of not less than 40 ° C., which is 1500 equivalent / 10 ⁶ g, and has a dynamic loss elastic modulus measured at 110 Hz of 10 ⁹ dyne / cm ² or more.