Upgraded aromatic polyester material in powder form

A process for preparing a polyester in powder form by upgrading the molecular weight of an aromatic polyester or a mixture of aromatic polyesters with an initial molecular weight of about 10,000 to a product possessing a relative viscosity of at least 1.8 as measured in m-cresol at 31.degree. C. which comprises:

• reacting said polyester with at least one reagent selected from the group consisting of (1) diisocyanates represented by the formula R(NCO).sub.2 and at least one difunctional compound in which the functionality is selected from the group consisting of --OH, --NH, --COOH and mixtures thereof;
• and (2) the reaction products of said diisocyanate and said difunctional compound alone or with at least one other difunctional compound;
• effecting said reaction in a polar aprotic solvent at temperatures of 100.degree.-180.degree. C., the relative proportions of the reactants being such that the total number of equivalents of isocyanate groups is substantially equal to the total number of equivalents of --OH, --NH, and/or --COOH reactive sites present in the polyester and difunctional compounds, the difunctional compound being present in the amount of 1 to 50 percent by weight based on the weight of said polyester, and R represents a divalent number selected from the group consisting of alkylene, cycloalkylene, alkenylene, cycloalkenylene, arylalkylene, arylene, biarylene, and all the R groups are not required to be the same and thereafter removing said solvent.