Upgraded aromatic polyester material in powder form

Claim 1

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° 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) 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°-180° 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.