Method of manufacturing polyester granules with improved photochemical and thermal stability, and its application

A process for production of a heat-stable and light-fast product of polyester comprising at least 85% by weight of polyethylene terephthalate units, wherein said product contains from 0.2 to 0.5% by weight, based on a total amount of said product, of a UV absorber having a melting point of from 135 to 145° C., said process comprising the steps of:

• a) mixing said UV absorber and a colorant with a polyester powder at room temperature to form a starting mixture, wherein said polyester powder comprises at least 85% by weight polyethylene terephthalate and said colorant is selected from the group consisting of perinone and anthraquinone dye compounds having a melting point greater than 180° C.;
• b) metering said starting mixture into a cooled feed zone of a twin-screw extruder;
• c) melting, kneading and homogenizing said starting mixture in said twin-screw extruder to form polymer strands;
• d) subsequently cooling the polymer strands in a water bath to form cooled strands and cutting the cooled strands to form a compound;
• and
• e) then melt-spinning said compound to form said product;
• whereby said UV absorber is incorporated into said product.