Magnetic recording media are extensively used as sound-recording tapes, video tapes, floppy disks, etc. Such the magnetic recording media comprise a nonmagnetic support having thereon a magnetic layer comprising ferromagnetic particles dispersed in a binder.
Magnetic recording media are required to have a high level of properties such as electromagnetic characteristics, running durability, and running property. That is, audio tapes for music recording/reproduction are required to have a higher degree of the ability for reproducing original sounds. Video tapes are required to have excellent electromagnetic characteristics (e.g., excellent ability for reproducing original images).
Magnetic recording media are thus required to have satisfactory running durability as stated above, simultaneously with such the-excellent electromagnetic characteristics. In order to obtain satisfactory running durability, the magnetic layer generally contains an abrasive material and a lubricant.
However, in order to obtain excellent running durability with an abrasive material, the addition amount thereof should be increased to some degree, and this results in a reduced packing density of ferromagnetic particles in the magnetic layer. In the case of employing an abrasive material having a large particle diameter in order to obtain excellent running durability, the magnetic layer tends to have abrasive material particles excessively projected from the magnetic layer surface. There is hence a problem that an improvement of running durability due to an abrasive material frequently results in deteriorated electromagnetic characteristics.
In the case of using a lubricant for improving running property as well as running durability, the addition amount thereof should be increased. However, the magnetic layer is apt to be more plasticized by the increased lubricant amount and this tends to reduce the durability of the magnetic layer.
It is a matter of course that the binder as one of the main components of a magnetic layer also plays an important role in improving durability and electromagnetic characteristics. The conventionally employed binder resins such as vinyl chloride resins, cellulosic resins, urethane resins, and acrylic resins have a problem that the magnetic layers containing these resins have poor abrasion resistance to contaminate members for magnetic-tape running such as guide rolls or guide poles.
A technique which is employed for improving the above problem is to use a hard binder to form a magnetic layer having a heightened hardness.
For example, a magnetic recording medium employing a binder comprising a polyester polyurethane resin or a polycarbonate polyurethane resin is described in JP-A-6-96437. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) Although urethanes having a urethane group content of 2 to 4 mmol/g are used in the examples of JP-A-6-96437, the content of a long-chain diol is not described therein and the content of OH groups is also unclear. In JP-B-6-19821 is described a magnetic recording medium employing a binder containing a urethane-urea having a total content of urethane and urea of 1.8 to 3.0 mmol/g. (The term "JP-B" as used herein means an "examined Japanese patent publication".) The polyurethane resin obtained in a resin synthesis example of JP-B-6-19821 has a long-chain diol content of 61% by weight, has a high urethane bond concentration, and excellent durability. However, the polyurethane resin gives a coating solution which has an increased viscosity and thereby has a reduced dispersibility, resulting in reduced electromagnetic characteristics.
Magnetic recording media employing as a binder a polyurethane resin formed by using a short-chain diol having a cyclic structure have been proposed. In JP-A-61-148626 is disclosed use of a polyester polyol containing 20% of bisphenol A. Although the urethane shown in the example of JP-A-61-148626 has a bisphenol A content of 13% by weight and a polyol content of 69% by weight, the dispersibility is low because the cyclic structure therein reduces solubility in solvents. In JP-A-1-251416 is described a polyurethane produced by using bisphenol A, which is a short-chain diol having a cyclic structure, as a chain extender and as a starting material for a polycarbonate polyol. Although the polyurethane shown in the example of JP-A-1-251416 has a bisphenol A content of 16% by weight and a polyol content of 63% by weight, it also has a problem that the dispersibility is low because the cyclic structure reduces solubility in solvents. In JP-B-7-21851 is described the use of a lactone-modified polyol containing bisphenol S. Although the polyurethane shown in the example of JP-B-7-21851 has a polyol content of 52% by weight and a bisphenol S content of 13% by weight, it has the same problem because of the cyclic structure having therein.
In U.S. Pat. No. 5,153,071 is described use of a polyurethane resin containing a polyether polyol having a cyclic structure and comprises as a diol an ethylene oxide or propylene oxide adduct of bisphenol A or hydrogenated bisphenol A (molecular weight: 250-3,000). However, the polyurethanes shown in the examples of U.S. Pat. No. 5,153,071 each has a polyol content of 70% by weight or higher and an ether content of 8 mmol/g or higher and hence gives a coating film which is too soft and has impaired durability to cause head stain, etc.
In JP-A-61-190717 is described use of a polyurethane resin produced by using a poly(tetramethylene glycol) and a polycaprolactone polyol. However, since the polyurethanes shown in the examples of JP-A-61-190717 each has a polyol content of 70% by weight or higher, these polyurethanes also give coating films which are too soft and have impaired durability to cause head stain, etc.
In JP-B-6-64726 is described a polyurethane resin obtained by reacting an isocyanate-terminated prepolymer with a branched polyester polyol. However, the polyurethane shown in the synthesis example of JP-B-6-64726 has an OH group content of 8.2.times.10.sup.-5 eq/g and gives a solution which has an increased viscosity and is thereby reduced in the dispersibility. Furthermore, due to use of a branched polyol, the resin has a reduced strength, resulting, for example, in impaired suitability for repetitions of running.
In U.S. Pat. No. 5,254,404 is described a magnetic recording medium employing a binder produced from a compound having at least one OH group at both terminals and a polyisocyanate. However, there is in U.S. Pat. No. 5,254,404 only a description that a polyester polyol is used. The resin described in U.S. Pat. No. 5,254,404 has a reduced strength due to the branched polyester polyol, resulting, for example, in impaired suitability for repetitions of running.
In JP-A-62-82510 is described a binder comprising a polyurethane resin in which the number of molecular terminals in a main chain and branches, is 3 or more on the average and at least two terminals have a primary hydroxyl group. Although the polyurethane resins shown in the examples of JP-A-62-82510 are produced by using a polyester polyol, the resins are insufficient in strength, suitability for repetitions of running, etc.
As described above, according to the prior art references described above, the polyurethane resins and polyurethane-urea resins used as binders for magnetic recording media are generally produced by using a long-chain diol having a hydrophilic segment, such as a polyester, polyether, or polycarbonate. The polyurethane resins shown in the examples given in those references each has a long-chain diol content of 25% by mole or higher.
However, since the polyurethane resins and polyurethane-urea resins described above each has a hydrophilic segment, the affinity thereof for organic solvents is reduced and the hydrophilic polar groups are apt to cause aggregation. As a result, the extension of molecular chains in organic solvents tends to be reduced. Thus, the hydrophilic segments function to reduce the dispersibility of fine ferromagnetic particles.
When used for producing polyesters, the long-chain diols having a hydrophilic segment have a problem that the ester groups are susceptible to hydrolysis to impair storage stability. When used for producing polyethers, those long-chain diols give polyethers which have a low T.sub.g, are too soft, and have low strength, such as those produced from poly(tetramethylene ether glycol), poly(propylene glycol), or poly(ethylene glycol).
A magnetic recording medium is well-known which has a magnetic layer formed on a nonmagnetic layer to thereby have a reduced magnetic-layer thickness. For attaining higher-density recording, it has become necessary to form a magnetic layer having a smaller thickness and containing even finer ferromagnetic metal particles. In addition, the surface properties of the nonmagnetic layer itself on which a thin magnetic layer is formed are coming to exert greater influences on the surface properties of the magnetic layer. Although an attempt was made to improve the surface properties of a nonmagnetic layer by using nonmagnetic particles having a reduced particle size, this technique has a problem that such fine nonmagnetic particles have reduced dispersibility like ferromagnetic metal particles and this leads to impaired surface properties. In the case where a suitable kind of binder is selected for ensuring the dispersibility of nonmagnetic particles, there is a problem that the binder is insufficient in the strength necessary for holding the nonmagnetic particles and, as a result, the nonmagnetic layer peels off at the tape edges. In the case where soft magnetic particles are used with any of the conventional binders to form a lower layer for improving the electromagnetic characteristics of the magnetic layer to be formed thereon, the soft magnetic particles show insufficient dispersibility due to the magnetically attractive force among the particles to pose the same problem as the above.
As described above, a binder in which nonmagnetic particles and soft magnetic particles have excellent dispersibility and which has both of excellent hardness (i.e., a high T.sub.g and a high Young's modulus) and toughness (elongation) and has excellent durability is desired. However, any of the polyurethane resins described above is unable to sufficiently meet these requirements. Any person skilled in the art has failed to find out a binder capable of satisfying those requirements and optimum nonmagnetic-particle conditions suitable for the binder.