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A magnetic recording medium is disclosed. The medium is comprised of a non-magnetic support having coated thereon a magnetic recording layer and a backing layer on the surface opposite the magnetic recording layer. The backing layer contains carbon black and the surface treated calcium carbonate dis

A magnetic recording medium is disclosed. The medium is comprised of a non-magnetic support having coated thereon a magnetic recording layer and a backing layer on the surface opposite the magnetic recording layer. The backing layer contains carbon black and the surface treated calcium carbonate dispersed in a binder. The calcium carbonate is surface treated so as to make the particles hydrophobic. Including the carbon black and the calcium carbonate in the backing layer provides a recording medium with improved durability and a reduced amount of drop out.

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1. A magnetic recording medium comprising a nonmagnetic supporting having coated thereon a magnetic recording layer comprising a ferromagnetic powder and a binder and a backing layer on the surface opposite to the magnetic recording layer, said backing layer containing (1) carbon black having an ave

1. A magnetic recording medium comprising a nonmagnetic supporting having coated thereon a magnetic recording layer comprising a ferromagnetic powder and a binder and a backing layer on the surface opposite to the magnetic recording layer, said backing layer containing (1) carbon black having an average particle size in the range of about 10 mμ to about 150 mμ and (2) surface treated hydrophobic calcium carbonate. 2. A magnetic recording medium as claimed in claim 2, wherein the carbon black has an average has an average particle size in the range of 30 mμ to 120 mμ. 3. A magnetic recording medium as claimed in claim 2, wherein the carbon black has an average particle size in the range of 50 mμ to 100 mμ. 4. A magnetic recording medium as claimed in claim 1, wherein the calcium carbonate has an average particle size of about 10μ or less. 5. A magnetic recording medium as claimed in claim 4, wherein the calcium carbonate has an average particle size in the range of 0.01 to 5μ. 6. A magnetic recording medium as claimed in claim 5, wherein the calcium carbonate has an average particle size in the range of 0.05 to 1μ. 7. A magnetic recording medium as claimed in claim 1, wherein the mixing ratio by weight of the calcium carbonate to carbon black is in the range of 5:95 to 95:5. 8. A magnetic recording medium as claimed in claim 7, wherein the mixing ratio by weight of calcium carbonate to carbon black is in the range of 10:90 to 90:10. 9. A magnetic recording medium as claimed in claim 8, wherein the mixing ratio by weight of calcium carbonate to carbon black is in the range of 30:70 to 70:30. 10. A magnetic recording medium as claimed in claim 1, wherein the surface treated calcium carbonate contains a surface treating agent in an amount of about 0.001 to about 10 parts by weight based on 100 parts by weight of calcium carbonate. 11. A magnetic recording medium as claimed in claim 10, wherein the surface treated calcium carbonate contains a surface treating agent in an amount of 0.05 to 5 parts by weight based on 100 parts by weight of calcium carbonate. 12. A magnetic recording medium as claimed in claim 1, wherein the surface treated calcium carbonate contains a surface treating agent in an amount of 0.1 to 3 parts by weight based on 100 parts by weight of calcium carbonate. 13. A magnetic recording medium as claimed in claim 10, wherein the surface treating agent is a natural surfactant. 14. A magnetic recording medium as claimed in claim 10, wherein the surface treating agent is a nonionic surfactant. 15. A magnetic recording medium as claimed in claim 10, wherein the surface treating agent is a cationic surfactant. 16. A magnetic recording medium as claimed in claim 10, wherein the surface treating agent is an anionic surfactant. 17. A magnetic recording material as claimed in claim 10, wherein the surface treating agent is an amphoteric surfactant. 18. A magnetic recording medium as claimed in claim 13, wherein the natural surfactant is selected from the group consisting of saponin, lecithin and resin acid. 19. A magnetic recording medium as claimed in claim 18, wherein the natural surfactant is lecithin or resin acid. 20. A magnetic recording medium as claimed in claim 19, wherein the natural surfactant is resin acid. 21. A magnetic recording medium as claimed in claim 14, wherein the nonionic surfactant is selected from the group consisting of an alkylene oxide, a glycerol and a glycidol. 22. A magnetic recording medium as claimed in claim 21, wherein the nonionic surfactant is an alkylene oxide. 23. A magnetic recording medium as claimed in claim 15, wherein the cationic surfactant is selected from the group consisting of a higher alkylamine, a quaternary ammonium salt and a heterocyclic compound. 24. A magnetic recording medium as claimed in claim 23, wherein the cationic surfactant is a quaternary ammonium salt. 25. A magnetic recording medium as claimed in claim 16, wherein the anionic surfactant has an acidic group. 26. A magnetic recording medium as claimed in claim 25, wherein the acidic group is a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, a sulfate group or a phosphate group. 27. A magnetic recording medium as claimed in claim 26, wherein the acidic group is a carboxylic group or a sulfonic acid group. 28. A magnetic recording medium as claimed in claim 17, wherein the amphoteric surfactant is selected from the group consisting of an amino acid, an amino sulfonic acid and a sulfate or phosphate of an amino-alcohol. 29. A magnetic recording medium as claimed in claim 28, wherein the amphoteric surfactant is an amino acid. 30. A magnetic recording medium as claimed in claim 10, wherein the surface treating agent is a high molecular weight resin having a polar group. 31. A magnetic recording medium as claimed in claim 30, the resin is a vinyl chloride-vinyl acetate copolymer having --COOH and/or --OH or nitrocellulose. 32. A magnetic recording medium as claimed in claim 31, wherein the resin is a vinyl chloride-vinyl acetate copolymer having --COOH and/or --OH. 33. A magnetic recording medium as claimed in claim 32, wherein the resin is a vinyl chloride/vinyl acetate/maleic acid copolymer. 34. A magnetic recording medium as claimed in claim 10, wherein the surface treating agent is lignin. 35. A magnetic recording medium as claimed in claim 1, wherein the amount of the calcium carbonate is about 1 to about 500 parts by weight based on 100 parts by weight of the binder. 36. A magnetic recording medium as claimed in claim 35, wherein the amount of the calcium carbonate is 5 to 200 parts by weight based on 100 parts by weight of the binder. 37. A magnetic recording medium as claimed in claim 36, wherein the amount of the calcium carbonate is 10 to 100 parts by weight based on 100 parts by weight of the binder. 38. A magnetic recording medium as claimed in claim 1, wherein the backing layer comprises a binder selected from the group consisting of a copolymer of vinyl chloride and vinyl acetate, a copolymer of vinylidene chloride and acrylonitrile, a copolymer of vinyl chloride and vinylidene chloride, a urethane elastomer, a copolymer of acrylonitrile and butadiene and a cellulose derivative. 39. A magnetic recording medium as claimed in claim 38, wherein the binder is selected from the group consisting of a copolymer of vinyl chloride and vinyl acetate, a copolymer of vinylidene chloride and acrylonitrile, a urethane elastomer and a cellulose derivative. 40. A magnetic recording medium as claimed in claim 1, wherein the carbon black and calcium carbonate are dispersed in a binder having a softening temperature of 150° C. or less, an average molecular weight in the range of 10,000 to 200,000 and a polymerization degree of about 200 to 2,000. 41. A magnetic recording medium as claimed in claim 40, wherein the binder includes a curing agent having a molecular weight of 200,000 or less. 42. A magnetic recording medium as claimed in claim 41, wherein the curing agent is selected from the group consisting of epoxy resins, polyurethane curing type resins, alkyd resins, silicone resins, acryl type reactive resins, mixtures of a high molecular weight polyester resin and an isocyanate prepolymer, mixtures of a copolymer of methacrylate and a diisocyanate prepolymer, mixtures of polyester poloyl and polyisocyanate, mixtures of a low molecular glycol/a high molecular diol/triphenyl methane triisocyanate, polyamine resins, polyisocyanates and mixtures thereof. 43. A magnetic recording medium as claimed in claim 42, wherein the curing agent is selected from the group consisting of epoxy resins, polyurethane curing type resins, alkyd resins and polyisocyanates. 44. A magnetic recording medium as claimed in claim 1, wherein the backing layer has a dry thickness of about 0.05 to about 5μ. 45. A magnetic recording medium as claimed in claim 44, wherein the backing layer has a dry thickness of 0.1 to 3μ. 46. A magnetic recording medium as claimed in claim 45, wherein the backing layer has a dry thickness of 0.2 to 2μ. 47. A magnetic recording medium as claimed in claim 1, wherein the calcium carbonate has an average particle size of about 10μ or less, wherein the mixing ratio by weight of calcium carbonate to carbon black is in the range of 5:95 to 95:5, wherein the surface treated calcium carbonate contains a surface treating agent in an amount of about 0.001 to about 10 parts by weight based on 100 parts by weight of calcium carbonate and wherein said calcium carbonate is surface treated by contacting the same with said surface treating agent prior to forming the backing layer. 48. A magnetic recording medium as claimed in claim 47, wherein said backing layer further comprises an organic polymer binder. 49. A magnetic recording medium as claimed in claim 48, wherein said calcium carbonate is contacted with said surface treating agent by mixing the same in the presence of a solvent.