Magnetic recording medium comprising magnetic particles and a specified polyurethane resin

Claim 1

A magnetic recording medium comprising a non-magnetic support and a magnetic layer formed thereon, said magnetic layer comprising ferromagnetic particles and a resinous binder; wherein said resinous binder comprises a polyurethane resin containing at least a high molecular weight polyol (A) having a number average molecular weight in the range of from about 500 to about 10,000 and a diisocyanate (B) as its constituents; said high molecular weight polyol (A) comprising about 30% or greater by weight of a first polyester polyol (a 1 ) containing an aromatic dicarboxylic acid as its acid component and having a glass-transition temperature of 20° C. or more and having a number average molecular weight in the range of 500 to 10,000 and a second polyol (a2) having a glass-transition temperature of -20° C. or less, the content ratio of the polyol (a 1 ) to the polyol (a2) being in the range of 90:10 to 30:70, wherein said polyurethane resin optionally contains a low molecular weight compound (C) having a number average molecular weight of less than 500; wherein said polyurethane resin has at least one hydrophilic polar group consisting of -SO 3 M 1 wherein M 1 is a hydrogen atom or an alkali metal; wherein said polyurethane resin has hydroxyl groups at both of its molecular terminals, at least one of said hydroxyl groups being derived from the second polyol (a 2 ) or the low molecular weight compound (C), and wherein said polyurethane resin has a urethane bond concentration in the range of 500 to 1,500 equivalents/10 6 g.