Polyester composition and process for producing the same

A process for producing a polyester composition comprising the steps of:

• esterifying or interesterifying an acid component comprising a principal acid moiety consisting of terephthalic acid or its ester-forming derivative with a diol component at an elevated temperature;
• polycondensing the esterification or interesterification product to provide a polyester resin, at least one stage between the initial stage of the esterification or interesterification step and the middle stage of the polycondensation step, (a) 0.1 to 20 molar %, based on the molar amount of the principal acid moiety, of a first additional acid moiety consisting of at least one member selected from the aromatic dicarboxylic acid compounds of the formulae (I) and (II):
• ##STR25## wherein R represents a member selected from the group consisting of a hydrogen atom and alkyl radicals having 1 to 2 carbon atoms, being added to the principal acid moiety, and at least one stage between the final stage of the esterification or interesterification step and the middle stage of the polycondensation step, a second additional acid moiety consisting of at least one member selected from the group consisting of (i) 0.01 to 20 molar %, based on the molar amount of the principal acid moiety, of at least one member selected from the aromatic monocarboxylic acid components of the formulae (III) and (IV):
• ##STR26## wherein R is as defined above, and (ii) 0.005 to 0.15 molar %, based on the molar amount of the principal acid moiety, of at least one member selected from the group consisting of alkali metal salts and alkaline earth metal salts of organic carboxylic acids, being added to the principal acid moiety;
• and
• dispersing 0.01 to 3%, based on the weight of the polyester resin, of titanium dioxide particles having an average size of 0.5 .mu.m or less and a particle size distribution ratio [.gamma.] of 2.3 or less, which particle size distribution ratio [.gamma.] is determined in a cumulative weight distribution of the particles fractionated by means of a centrifugal precipitation method and is defined by the equation:
• [.gamma.]=D.sub.25 /D.sub.75
• wherein D.sub.25 represents a size of the fractionated precipitated particles at a cumulative weight of 25% based on the entire weight of the precipitated particles and D.sub.75 represents a size of the fractionated precipitated particles at a cumulative weight of 75% based or the entire weight of the precipitated particles, and containing agglomerates consisting of a plurality of titanium dioxide primary particles and having a size of 1.5 .mu.m or more, in a content of 0.5% or less based on the entire weight of the titanium dioxide particles, into the esterification or interesterification reaction mixture and/or the polycondensation reaction mixture.