Magnetic recording medium - Patent Review 4637959

What is claimed is:

1. A magnetic recording medium comprising a non-magnetic substrate and a magnetic layer formed thereon composed mainly of magnetic powder and a binder, said binder containing a thermoplastic polyurethane-urea resin hardened by a polyisocyanate and having at least one hydrophilic polar group selected from the group consisting of the groups represented by general formulas: ##STR19## wherein M is hydrogen or alkali metal and M' is hydrogen, alkali metal or hydrocarbon group, the total concentration of urea groups and urethane groups in said resin being between 1.8 and 3 m mol/g, and the concentration ration between urea groups and ureathane groups in said resin being between 0.3 and 1.6.

2. The magnetic recording medium according to claim 1, wherein the thermoplastic polyurethane-urea resin is obtained by the polyaddition reaction of

(A) a long-chain diol having a molecular weight in a range of about 500 to 5000;

(B) a short-chain diol having a molecular weight in a range of about 50 to 500;

(C) an organic diamine;

(D) an organic diisocyanate; and

(E) a compound having at least one hydrophilic polar group selected from the group consisting of the groups represented by general formulas: ##STR20## wherein M is hydrogen or alkali metal and M' is hydrogen, alkali metal or hydrocarbon group.

3. The magnetic recording medium according to claim 1, wherein the thermoplastic polyurethane-urea resin is cured with a polyisocyanate curing agent.

4. The magnetic recording medium according to claim 1, wherein the concentration of hydrophilic polar group in said resin is between 0.01 and 1.0 m mol/g.

5. The magnetic recording medium according to claim 4, wherein the concentration of hydrophilic polar group in said resin is between 0.1 and 0.5 m mol/g.

6. The magnetic recording medium according to claim 1, wherein the number-average molecular weight of said resin is between 10000 and 100000.

7. The magnetic recording medium according to claim 6, wherein the number-average molecular weight of said resin is between 10000 and 60000.

8. The magnetic recording medium according to claim 1, wherein the softening point of said resin is not less than 80.degree. C.

9. The magnetic recording medium according to claim 8, wherein the softening point of said resin is not less than 100.degree. C.

10. The magnetic recording medium according to claim 1, wherein the glass transition point of said resin is not more than 0.degree. C.

11. The magnetic recording medium according to claim 10, wherein the glass transition point of said resin is not more than -10.degree. C.

12. The magnetic recording medium according to claim 2, wherein the weight ratio of the short-chain diol to the long-chain diol is not more than 3.

13. The magnetic recording medium according to claim 2, wherein the long-chain diol is a polyester diol, a polyether diol or a polyether ester diol.

14. The magnetic recording medium according to claim 13, wherein the polyester diol is obtained by the reaction of an aliphatic dicarboxylic acid, an aromatic dicarboxylic acid or a lower alcohol ester of the dicarboxylic acid with a glycol.

15. The magnetic recording medium according to claim 14, wherein the aliphatic dicarboxylic acid is succinic acid, adipic acid, sebacic acid or azelaic acid.

16. The magnetic recording medium according to claim 14, wherein the aromatic dicarboxylic acid is terephthalic acid or isophthalic acid.

17. The magnetic recording medium according to claim 14, wherein the lower alcohol ester of dicarboxylic acid is a methyl or ethyl ester.

18. The magnetic recording medium according to claim 14, wherein the glycol is ethylene glycol, 1,3-propylene glycol, 1,4-butylene glycol, 1,6-hexane glycol, diethylene glycol, neopentyl glycol or an ethylene oxide or propylene oxide adduct of bisphenol A.

19. The magnetic recording medium according to claim 2, wherein the long-chain diol is a polyester diol, a polyether diol or a polyether ester glycol.

20. The magnetic recording medium according to claim 19, wherein the polyester diol is obtained by the reaction of an aliphatic dicarboxylic acid, an aromatic dicarboxylic acid or a lower alcohol ester of the dicarboxylic acid with a glycol.

21. The magnetic recording medium according to claim 20, wherein the aliphatic dicarboxylic acid is succinic acid, adipic acid, sebacic acid or azelaic acid.

22. The magnetic recording medium according to claim 20, wherein the aromatic dicarboxylic acid is terephthalic acid or isophthalic acid.

23. The magnetic recording medium according to claim 20, wherein the lower alcohol ester is a methyl or ethyl ester.

24. The magnetic recording medium according to claim 20, wherein the glycol is ethylene glycol, 1,3-propylene glycol, 1,4-butylene glycol, 1,6-hexane glycol, diethylene glycol, neopentyl glycol or an ethylene oxide or propylene oxide adduct of bisphenol A.

25. The magnetic recording medium according to claim 19, wherein the polyether diol is obtained by the cleavage polymerization of a lactone.

26. The magnetic recording medium according to claim 25, wherein the lactone is .epsilon.-caprolactone.

27. The magnetic recording medium according to claim 19, wherein the polyether diol is polyether glycol.

28. The magnetic recording medium according to claim 27, wherein the polyether glycol is polyethylene glycol, polypropylene ether glycol or polytetramethylene ether glycol.

29. The magnetic recording medium according to claim 19, wherein the polyether ester glycol is a polyester glycol obtained by the reaction of a polyalkylene ether glycol with an aliphatic dicarboxylic acid or an aromatic dicarboxylic acid.

30. The magnetic recording medium according to claim 29, wherein the polyalkylene ether glycol is polyethylene glycol, polypropylene ether glycol or polytetramethylene ether glycol.

31. The magnetic recording medium according to claim 29, wherein the aliphatic acid is succinic acid, adipic acid, sebacic acid or azelaic acid.

32. The magnetic recording medium according to claim 29, wherein the aromatic acid is terephthalic acid or isophthalic acid.

33. The magnetic recording medium according to claim 2, wherein the short-chain diol is an aliphatic glycol or an aromatic diol.

34. The magnetic recording medium according to claim 33, wherein the aliphatic glycol is ethylene glycol, propylene glycol, 1,4-butylene glycol, 1,6-hexane glycol or neopentyl glycol.

35. The magnetic recording medium according to claim 33, wherein the aromatic diol is an ethylene oxide or propylene oxide adduct of bisphenol A or an ethylene oxide adduct of hydroquinone.

36. The magnetic recording medium according to claim 2, wherein the orgnaic diamine is an aliphatic diamine, an aromatic diamine or an alicyclic diamine.

37. The magnetic recording medium according to claim 2, wherein the organic diisocyanate is an aliphatic diisocyanate, an aromatic diisocyanate or an alicyclic diisocyanate.

38. The magnetic recording medium according to claim 2, wherein the compound having hydrophilic polar group is a diol having hydrophilic polar group, a diamine having hydrophilic polar group or a diisocyanate having hydrophilic polar group.

39. A magnetic recording medium comprising a non-magnetic substrate, a magnetic layer formed on one surface of said non-magnetic substrate and a back coating layer composed of a binder and non-magnetic pigment formed on the other surface of said non-magnetic substrate, said binder containing a thermoplastic polyurethane-urea resin hardened by a polyisocyanate and having at least one hydrophilic polar group selected from the group consisting of the groups represented by general formulas: ##STR21## wherein M is hydrogen of alkali metal and M' is hydrogen metal or hydrocarbon group, the total concentration of urea groups and urethane groups in said resin being between 1.8 and 3 m mol/g, and the concentration ratio between urea groups and urethane groups in said resin being between 0.3 and 1.6.

40. The magnetic recording medium according to claim 39, wherein the thermoplastic polyurethane-urea resin is obtained by the polyaddition reaction of

(A) a long-chain diol having a molecular weight in a range of about 500 to 5000;

(B) a short-chain diol having a molecular weight in a range of about 50 to 500;

(C) an organic diamine;

(D) an organic diisocyanate; and

(E) a compound having at least one hydrophilic polar group selected from the group consisting of the groups represented by general formulas: ##STR22## wherein M is hydrogen or alkali metal and M' is hydrogen, alkali metal or hydrocarbon group.

41. The magnetic recording medium according to claim 39, wherein the thermoplastic polyurethane-urea resin is cured with a polyisocyanate curing agent.

42. The magnetic recording medium according to claim 39, wherein the concentration of hydrophilic polar group in said resin is between 0.01 and 1.0 m mol/g.

43. The magnetic recording medium according to claim 42, wherein the concentration of hydrophilic polar group in said resin is between 0.1 and 0.5 m mol/g.

44. The magnetic recording medium according to claim 39, wherein the number-average molecular weight of said resin is between 10000 and 100000.

45. The magnetic recording medium according to claim 44, wherein the number-average molecular weight of said resin is between 10000 and 60000.

46. The magnetic recording medium according to claim 39, wherein the softening point of said resin is not less than 80.degree. C.

47. The magnetic recording medium according to claim 46, wherein the softening point of said resin is not less than 100.degree. C.

48. The magnetic recording medium according to claim 39, wherein the glass transition point of said resin is not more than 0.degree. C.

49. The magnetic recording medium according to claim 48, wherein the glass transition point of said resin is not more than -10.degree. C.

50. The magnetic recording medium according to claim 40, wherein the weight ratio of the short-chain diol to the long-chain diol is not more than 3.

51. The magnetic recording medium according to claim 39, wherein the magnetic layer is composed mainly of magnetic powder and a binder.

52. The magnetic recording medium according to claim 39, wherein the magnetic layer is composed of ferromagnetic metal thin film.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic recording medium and, more particularly, to a magnetic recording medium having a coating layer in which a binder having improved properties is used.

2. Brief Description of the Prior Art

So far, the vinyl chloride-vinyl acetate copolymers, cellulose derivatives or polyester resins have been used extensively as the binder for the magnetic recording medium. The thermoplastic polyurethane resin has also been used for improving wear resistance or controlling coating properties of the magnetic layer of the recording medium.

On the other hand, the magnetic recording medium is required to satisfy the demand for high density recording. As a result thereof, attempts have been made towards reducing the size of the ferromagnetic powders to be filled in the magnetic layer and towards improved smoothness of the surface of the magnetic layer.

However, with improved surface smoothness of the magnetic layer, its contact area is increased, so that its running performance and durability are affected, while its anti-blocking performance is also drastically lowered. Above all, since the conventional binder has a low softening point and inferior heat resistancy, when the magnetic recording medium in the tape form is taken up and stored on a reel under elevated temperatures or for a prolonged time, it is liable to become tightly affixed to the non-magnetic substrate neighboring to the magnetic layer. Thus, due to the resulting exfoliation of the magnetic layer, the properties of the magnetic recording medium are not exhibited satisfactorily. In addition, an increase in the specific surface resulting from comminution of ferromagnetic powders not only results in the drastically lowered dispersibility of the powders in the binder and deteriorates surface gloss and filling properties, but also renders it difficult to prevent powder debris or tape injury or to provide sufficient running durability or electromagnetic properties or characteristics that are critical to the magnetic recording medium.

Hence, in order to elevate heat resistancy of the thermoplastic polyurethane resin and thereby improve anti-blocking of the magnetic recording medium, it has been proposed to use the thermoplastic polyurethane resin as the binder for the magnetic layer of the recording medium, wherein the ratio of the low molecular weight diol in the thermoplastic polyurethane resin is increased for elevating the concentration of the urethane groups in the molecule.

The thermal properties of thermoplastic polyurethane resins may be improved in general by increasing the concentration of the urethane groups. That is, the higher the concentration of the urethane groups in the molecule, the higher the softening point of the thermoplastic polyurethane resin and the lower its glass transition temperature. However, with increase in urethane group concentration in the thermoplastic polyurethane resin, it becomes undesirably insoluble in general-purpose solvents such as ketones, alcohols, esters, aromatic hydrocarbons or aliphatic hydrocarbons, it being only soluble in solvents of higher toxicity such as dimethylformamide or tetrahydrofuran. Moreover, dimethylformamide or tetrahydrofuran used as solvent for a magnetic paint for formation of the magnetic layer may corrode surface portions of the substrate on which the paint is applied, or any surface portions or materials with which it may be brought into contact, so that creases or wrinkles are partially caused or, in some cases, these portions may become dissolved. Therefore, there is a limitation on improvement by elevating the concentration of urethane groups in the thermoplastic polyurethane resin.

Furthermore, even when the thermoplastic polyurethane resin with elevated concentration in the urethane groups is used as the binder for the magnetic layer, it has only negligible effects in improving the dispersibility of the comminuted ferromagnetic powders in the binder.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a magnetic recording medium with a layer containing a thermoplastic polyurethane-urea resin having improved heat resistance, anti-blocking and durability as a binder.

It is another object of the present invention to provide a magnetic recording medium with a layer containing a binder further having improved compatibility with other resin and a solvent.

It is a further object of the present invention to provide a magnetic recording medium with a layer containing a binder further having improved dispersibility of a pigment, an inorganic filler or the like.

According to an aspect of the present invention, there is provided a magnetic recording medium comprising a non-magnetic substrate and a magnetic layer formed thereon composed mainly of magnetic powder and a binder, said binder containing a thermoplastic polyurethane-urea resin having at least one hydrophilic polar group selected from the groups represented by general formulas: ##STR2## wherein M is hydrogen or alkali metal and M' is hydrogen, alkali metal or hydrocarbon group.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventors conducted many researches with a view to overcoming these deficiencies and found that the thermoplastic polyurethane-urea resin in which hydrophilic polar group or groups are introduced as side chain or side chains thereof, or a cured product of the resin obtained upon reaction between the polyisocyanate curing agent and the resin, has such an advantage that the magnetic layer of the magnetic recording medium prepared thereby has improved anti-blocking and improved dispersibility of the ferromagnetic powders, and that, in addition, the resin is readily soluble in general-purpose solvents. Such finding has led to completion of the present invention, which resides in a magnetic recording medium having a non-magnetic substrate and a magnetic layer formed thereon and consisting essentially of ferromagnetic powders and a binder. It is characterized in that a thermoplastic polyurethane-urea resin obtained by reacting long-chain diols with the molecular weight ranging between about 500 and about 5000, short-chain diols with the molecular weight ranging between about 50 to about 500, organic diamines, organic diisocyanates and compounds containing hydrophilic polar groups, is contained as a binder in the magnetic layer, and in that a cured product obtained by reacting the aforementioned thermoplastic polyurethane-urea resin and a polyisocyanate curing agent is contained as the binder in the magnetic layer.

The thermoplastic polyurethane-urea resin has a characteristic that both urethane bonds and urea bonds are contained in the molecule of the resin, and in that hydrophilic polar group or groups are introduced to a side chain or side chains of the molecule.

The aforementioned urethane and urea bonds play an important role in improving thermal properties of the binder and provide for lowering the glass transition temperature of the resin and elevating its softening point which is a measure of the heat resistancy of the resin. In addition, these bonds are effective in improving anti-blocking and maintaining stable physical properties of the magnetic layer over a wide range of temperature. Thus the introduction of the urea groups as well as that of urethane groups is effective in markedly elevating the thermal properties of the resin. More importantly, the introduction of urea groups gives a resin which is readily soluble in a solvent system consisting essentially of a combination of the aforementioned solvents, that is, ketones, alcohols, esters, aromatic hydrocarbons and aliphatic hydrocarbons. Moreover, the concentration of the polar groups (urethane and urea groups) in the thermoplastic polyurethane-urea resin molecules can be rendered higher than that in the ordinary thermoplastic polyurethane resin, thus resulting in the improved interaction among the neighboring molecules and improved durability and coating properties of the resulting magnetic layer. Thus, by using the thermoplastic polyurethane-urea resin as the binder for the magnetic recording medium, there may be provided a magnetic recording medium which is superior in anti-blocking and durability.

The combined concentration of the aforementioned thermoplastic polyurethane-urea resin is preferably 1.8 to 3.0 m mol/g. With the concentration less than 1.8 m mol/g, the softening point of the resin is lowered and anti-blocking is not improved. With the concentration exceeding 3.0 m mol/g, the resin becomes insoluble in general-purpose solvents, it being soluble only in dimethylformamid or the like toxic solvents. On the other hand, the ratio of urea group concentration to urethane group concentration is preferably in the range of 0.3 to 1.6. With the ratio less than 0.3, the resin is insoluble in general-purpose solvents and, with the ratio in excess of 1.6, the glass transition temperature of the resin becomes higher.

Since the hydrophilic polar groups introduced as side chain into a thermoplastic polyurethane-urea resin exhibit superior affinity with the ferromagnetic particles mixed into the magnetic layer, it is highly effective in dispersing these ferromagnetic powders into the binder so as to deal successfully with the increased specific surface of the ferromagnetic powders. In addition, the introduction of the hydrophilic polar groups into the thermoplastic polyurethane-urea resin contributes markedly to improved dispersibility of the ferromagnetic powders and to improved physical properties of the magnetic layer. Thus the introduction of the hydrophilic polar groups renders it possible that the ferromagnetic powders and the binder interact directly with one another so that the magnetic layer exhibits superior strength and durability.

The amount of the hydrophilic polar groups introduced into the thermoplastic polyurethane-urea resin is preferably in the range of 0.01 to 1.0 m mol/g and more preferably in the range of 0.1 to 0.5 m mol/g. With the amount of the hydrophilic polar groups less than 0.1 m mol/g, the groups are not sufficiently effective in improving dispersibility of the ferromagnetic powders. With the amount in excess of 1.0 m mol/g, intermolecular or intramolecular aggregation may take place so that the dispersibility of the ferromagnetic powders is lowered and/or the ordinary or general-purpose solvents are unable to be used with the resin.

It should be noted that the number-average molecular weight of the thermoplastic polyurethane-urea resin is preferably 10000 to 100000 and more preferably 10000 to 60000. With the number-average molecular less than 10000, the film forming capacity of the resin becomes insufficient. With the number-average molecular weight higher than 60000, the mixing, transport and coating steps during the preparation of the magnetic paint may be affected adversely.

The softening temperature of the thermoplastic polyurethane-urea resin is preferably higher than 80.degree. C. and more preferably higher than 100.degree. C. With the softening temperature lower than 80.degree. C., the properties of the resin are closer to those of the conventional thermoplastic polyurethane resin so that it becomes impossible to improve physical properties and anti-blocking of the thermoplastic polyurethane-urea resin.

It should also be noted that the glass transition temperature of the polyurethane-urea resin is preferably lower than 0.degree. C. and more preferably lower than -10.degree. C. With the glass transition temperature higher than -10.degree. C., the transition temperature of the physical properties approaches undesirably to ambient temperature.

The method of preparing a thermoplastic-urea resin employed in the magnetic recording medium of the present invention is now explained.

The thermoplastic polyurethane-urea resin is obtained by a polyaddition reaction of long-chain diols, short-chain diols, organic diamines and organic diisocyanates. The polyaddition reaction is carried out by a prepolymer method in which a mixture of long-chain diols and short-chain diols is previously reacted with organic diisocyanates to give a prepolymer having an isocyanate group or isocyanate groups at the terminal or terminals thereof and the organic diamine is added to the resulting prepolymer to permit chain prolongation and introduction of an urea group or urea groups.

The long-chain diol used for preparation of the thermoplastic polyurethane-urea has a molecular weight of approximately 500 to 5000 and is classified for example into polyester diols, polyether diols and polyetheresterglycols. The polyester diols may include lactonic polyester diols obtained by ring opening polymerization of lactones such as e-caprolactone, or polyester diols obtained by reacting aliphatic dicarboxylic acids such as succinic acid, sebacic acid or azelaic acid, aromatic dicarboxylic acids such as terephthalic acid or isophthalic acid or esters thereof with lower alcohols with ethylene glycol, 1,3-propyrene glycol, 1,4-butylene glycol, 1,6-hexane glycol, diethylene glycol, 1,6-hexane glycol, diethylene glycol, neopentyl glycol, an ethylene oxide adduct of bisphenol A or a mixture thereof. The polyether diols may for example include polyalkylene ether glycols such as polyethylene glycol, polypropyrene ether glycol, polytetramethylene ether glycol, or a polyether glycol, which is a copolymer thereof. The polyether ester glycols may for example be obtained by reacting aliphatic or aromatic dicarboxylic acid with the aforementioned polyalkylene ether glycol as polyol component. If the molecular weight of the long-chain diol is too small, the concentration of urethane groups of the resulting thermoplastic polyurethane-urea resin becomes too high so that the resin becomes less pliable and less soluble in solvents. This is not desirable when the resin is to be used as the binder for the magnetic recording medium. On the other hand, when the molecular weight of the long-chain diol is too large, the contents of long-chain diols in the resin become too high so that the concentration of the urethane groups is too low, resulting in the lower wear and heat resistancy of the resin.

The short-chain diols employed for preparation of the thermoplastic polyurethane-urea resin has a molecular weight approximately equal to 50 to 500, and may for example include aliphatic glycols such as ethylene glycol, propyrene glycol, 1,4-butylene glycol, 1,6-hexane glycol or neopentyl glycol, or aromatic diols such as ethylene or propyrene adduct of bisphenol A or ethylene oxide adduct of hydroquinone, these being used either singly or in combination at any desired mixture ratio depending on the desired properties of the polyurethane-urea resin.

The aforementioned organic diamines may include aliphatic diamines such as tetramethylene diamine or hexamethylene diamine, aromatic diamines such as m-phenylene diamine, p-phenylene diamine, 2,4-tolylenediamine, 2,6-tolylenediamine, m-xylylenediamine, p-xylylenediamine, diphenylmethanediamine, 3,3'-dimethoxy-4,4'-biphenylenediamine, 3,3'-dimethoxy-4,4'-biphenylenediamine, 3,3'-dimethyl-4,4'-biphenylenediamine, 4,4'-diaminodiphenylether, 1,5-naphthalenediamine or 2,4-naphthalenediamine, or cycloaliphatic diamines such as 1,3-diaminomethylcyclohexane, 1,4-diaminomethylcyclohexane, 4,4'-diaminodicyclohexylmethane or isophoronediamine.

The organic diisocyanates may include aliphatic diisocyanates such as tetramethylene diisocyanate or hexamethylene diisocyanate, aromatic diisocyanates such as m-phenylene diisocyanate, p-phenylene diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, diphenylmethane diisocyanate, 3,3'-dimethoxy-4,4'-biphenylene diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanate, 4,4'-diisocyanate diphenyl ether, 1,5-naphthalene diisocyanate or 2,4-naphthalene diisocyanate or cycloaliphatic diisocyanates such as 1,3-diisocyanate methyl cyclohexane, 1,4-diisocyanate methyl cyclohexane, 4,4'-diisocyanate dicyclo hexylmethane or isophorone diisocyanate.

When the polyisocyanate curing agent is used in combination with the aforementioned thermoplastic polyurethane-urea resin, there can be obtained a magnetic recording medium with excellent wear-resistancy. Any polyisocyanate curing agents so far known and used in the art, such as "Coronate L" (manufactured by Nippon Polyurethane Kogyo KK) or "Desmodule L" (manufactured by Bayer AG), may be used as aforementioned polyisocyanate curing agents. These polyisocyanate curing agents can be used in accepted amounts.

In the above described reaction, the molar ratio of the short-chain diols to the long-chain diols is preferably less than 3. If the molar ratio is too large, the concentration of the urethane groups becomes too high so that the resulting polyurethane-urea resin is undesirably not soluble in the aforementioned general-purpose solvents used for the preparation of the magnetic paint. When the straight chain diols such as ethylene glycol, 1,4-butylene glycol or 1,6-hexane glycol are employed as short-chain diols, the aforementioned molar ratio is preferably less than 1 and more preferably less than 0.5. When ethylene or propyrene oxide adducts of bisphenol A or branched short-chain diols such as neopentylglycol are used as short-chain diols, the aforesaid molar ratio may be increased as compared to the case of using the straight chain diols because the resin exhibits an improved solubility. However, the aforementioned molar ratio higher than 3 is not desirable because the solubility of the resin is correspondingly lowered.

In the preparation of the thermoplastic polyurethane-urea resin employed in the present invention, polyester diols, and above all polybutylene adipate, polyhexamethylene adipate or polycaprolactonediol among the aforementioned compounds are most preferred as long-chain diols having the molecular weight of approximately 500 to 5000. Likewise, branched short-chain diols and above all neopentyl glycol among the aforementioned compounds are most preferred as short-chain diols having the molecular weight of approximately 50 to 500. On the other hand, isophorone diamine is most preferred among the aforementioned compounds as the organic diamines, while 4,4-diphenylmethane diisocyanate and isophorone diisocyanate are most preferred among the aforementioned compounds as the organic diisocyanates.

The methods of polyaddition reaction adopted in the preparation of the thermoplastic polyurethane-urea resin employed in the present invention may include a melt polymerization according to which the reaction is carried out in the molten state, and solution polymerization according to which the reaction is carried out in a solution of the raw materials as referred to hereinabove in an inert solvent such as ethyl acetate, methylethylketone, acetone or toluene or a mixture thereof. For the manufacture of the thermoplastic polyurethane resins which are employed in a solution in a solvent in many cases, such as the binder for the magnetic recording medium, the solution polymerization is preferred. It is particularly preferred that the melt polymerization is carried out during the preparation of the prepolymer and, prior to the chain prolongation reaction, the solution polymerization of the prepolymers is carried out in a solution thereof in the inert solvent.

In carrying out the reaction, organometallic compounds such as organotin compounds, e.g. stannous octylate, dibutyltin dilaurate, or tertiary amines such as N-methyl morpholine or triethylamine may be added as catalyst. In order to increase the stability of a product, an antioxidant, an ultraviolet ray absorbing agent, a hydrolysis preventive agent or the like may be added in necessary amounts.

The hydrophilic polar groups are introduced into the thermoplastic polyurethane-urea resin. The hydrophilic polar groups may include ##STR3## wherein M represents a hydrogen atom or an alkali metal and M' a hydrogen atom, an alkali metal or a hydrocarbon residue.

The methods of introducing these hydrophilic polar groups into the thermoplastic polyurethane-urea resin may include

(i) employing a compound containing hydrophilic polar groups as one of the raw materials for the preparation of the thermoplastic polyurethane-urea resin; and

(ii) denaturing thermoplastic polyurethane-urea resin containing two or more -OH groups with a compound or compounds containing hydrophilic polar groups.

The compounds containing hydrophilic polar groups employed in the method (i) may include diols containing hydrophilic polar groups, diisocyanates containing hydrophilic polar groups, and diamines containing hydrophilic polar groups. These compounds are polymerized with other raw materials to form a part of the high polymeric chain of the thermoplastic polyurethane-urea resin, as a result of which the hydrophilic polar groups are introduced into the thermoplastic polyurethane-urea resin.

The aforementioned diols containing hydrophilic polar groups may include diols containing phosphoric acid ester groups represented by the general formula ##STR4## wherein R.sub.1 represents alkylene groups with 2 to 6 carbon atoms with or without substituents, R.sub.2 represents alkylene groups with 1 to 6 carbon atoms, with or without substituents, and R.sub.3, R.sub.4 represent alkyl groups with 1 to 6 carbon atoms.

The diols containing the hydrophilic polar groups may also be diols containing --SO.sub.3 M groups, where M represents a hydrogen atom or an alkali metal. The diols containing --SO.sub.3 M groups may be obtained by reacting a carboxylic acid component not containing --SO.sub.3 M groups, a glycol component and a dicarboxylic acid component containing --SO.sub.3 M groups.

The carboxylic acid component not containing the --SO.sub.3 M groups may be enumerated by aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid or 1,5-naphthalic acid, aromatic oxycarboxylic acids such as p-oxybenzoic acid or p-(hydroxyethoxy) benzoic acid, aliphatic dicarboxylic acids such as succinic acid, adipic acid, sebacic acid or dodecanedicarboxylic acid, and tri- and tetracarboxylic acids such as trimellitic acid, trimesic acid and pyromellitic acid.

The aforementioned glycol component may be enumerated by ethylene glycol, propyrene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, neopentylglycol, diethylene glycol, dipropyrene glycol, 2,2,4-trimethyl-1,3-pentanediol, 1,4-cyclohexanedimethanol, ethylene oxide adduct and propyrene oxide adduct of bisphenol A, ethylene oxide adduct and propyrene oxide adduct of hydrogenated bisphenol A, polyethyleneglycol, polypropyrene glycol and polytetramethylene glycol. Tri- and tetraols such as trimethylolethane, trimethylolpropane, glycerin and pentaerythritol may also be used in conjunction with the aforementioned glycol component.

The dicarboxylic acid component not containing the --SO.sub.3 M groups may be enumerated by 5-sodium sulfo-isophthalic acid, 5-potassium-sulfo-isophthalic acid, 2-sodium-sulfo-terephthalic acid and 2-potassium-solfo-terephthalic acid.

The aforementioned organic diisocyanates containing the hydrophilic polar groups can be obtained by reacting polyisocyanate compounds having three or more functional groups with a compound or compounds containing the hydrophilic polar groups.

Among these polyisocyanate compounds, there are known trifunctional compounds known under the trade names of "Desmodule L" (manufactured by Bayer AG) and "Coronate L" (manufactured by Nippon Polyurethane Co. Ltd.). In general, the polyfunctional polyisocyanate compounds may be obtained by addition reaction of polyols and polyisocyanates.

The po