PURPOSE: A very flat polyester film, obtained by forming a coating layer, consisting of a specific composition e.g. polyurethane, etc., and having many small protrusions on at least one surface of a polyester film, having improved slipperiness and suitable as a material for magnetic tapes, etc.
CONSTITUTION: At least one surface of a polyester film before completing the crystal orientation, e.g. polyethylene terephthalate, is coated with an aqueous dispersion of a composition containing (A) a polyurethane containing 0.5W15wt% one or more of carboxylate groups, sulfonate groups, half sulfate groups, e.g. carboxylic acid ammonium salt groups, etc., (B) a low-molecular weight polyolefin wax, e.g. polyethylene wax having 1,000W10,000 average molecular weight, and (C) a surface roughening material having ≤0.15μm average particle diameter, e.g. polystyrene or silica, at 98/2W40/60 weight ratio of the total of the components (A) and (B) to the component (C) and 98/2W40/60 weight ratio of the components (A) to (B) and dried. The film is then drawn and heat- treated to form a coating film having 0.002W0.01μm center line average height.
MIURA SADAMI
KANAI TAMAKI
