Polyester film having improved slipperiness

Claim 1

A magnetic recording tape comprising a substrate with a magnetic coating thereon, said magnetic recording tape having good transportability and high abrasion resistance and being free of a back lubricant layer, said substrate consisting of a biaxially oriented polyester film having a kinetic friction coefficient at a tape speed of 25 cm/sec in the range from 0.05 to 0.25, and having at least one rough surface with protrusions integrated thereto, said protrusions having a maximum size of 1 micron in height and having an average peak-to-valley height of not more than about 0.55 micron and being originated from an insoluble inert substance having a representative particle diameter of not more than 0.55 micron incorporated into said polyester in an amount ranging from 0.08 to 2% by weight, wherein the height denoted by h (micron) in the range 1.0≧h>0.75, is not more than 14/mm 2 , 0.75≧h>0.5, is from 5 to 150/mm 2 , and 0.5≧h, is not less than 40/mm 2 , and wherein the insoluble inert substance consists essentially of not more than 8% by weight of a fraction having particle diameters, expressed as d in microns, 2.5≧d>1.5 and from 40 to 80% by weight of a fraction of 0.5≧d, said film having an abrasion resistance grade of at least 2 when transported at a speed of 25 cm/sec at 25° C. and 60% relative humidity with said rough surface in contact with an SUS 27 stationary rod wrapped with tissue paper having a surface roughness of 0.35, an outer diameter of 20 mm φ with a wrapping arc angle of θ=(152/180) π radius (152°), said abrasion resistance grade being defined as follows: Grade 1: free from white powder; Grade 2: a small amount of white powder; Grade 3: white powder speckled all over the tissue paper; Grade 4: white powder more than Grade 3; Grade 5: white powder covering the whole surface of the tissue paper.