Polyester film for magnetic recording tape

Claim 1

A polyester film, comprising 0.05-1% by weight of particles consisting essentially of calcium and phosphorus, formed by the reaction of a calcium compound, a trivalent phosphorus compound and a pentavalent phosphorus compound during polymerization, and in which the number, H 2 , of the secondary interference fringes (number/mm 2 ) and the average center-line roughness, R a , (μm) satisfy the following relationships: H.sub.2 <25 (1) 0.001≦R.sub.a ≦0.025 (2) R.sub.a >2×10.sup.-4 H.sub.2 +0.01 (3) and wherein the total amount of the calcium compound is 0.3-1 mol % based on the total amount of the acid components in the polyester film, the molar ration of the phosphorus compound to the calcium compound of 0.7-3, the amount of calcium and phosphorus per particle is about 1-15% by weight, and the molar ratio of the pentavalent to the trivalent phosphorus compound is 1-20, and wherein the trivalent phosphorus compound is selected from the group consisting of phosphorus acid, trialkylesters of phosphorus acid, and partial alkylester of phosphorus acid, and the pentavalent phosphorus compound consists of a mixture of phosphoric acid and a trialkylester of phosphoric acid and a partial alkylester of phosphoric acid.