The use of anhydride linkages to increase the glass transition temperatures of polymers containing carboxyl end groups: A perspective in powder coatings

A method to increase the glass transition temperature of carboxyl terminated low molecular weight cycloaliphatic polyesters through anhydride linkages is described. Low molecular weight carboxyl terminated cycloaliphatic polyesters, poly(1,4-cyclohexylidenecyclohexane-1,4-dicarboxylate) (PCCD), were synthesized by the polycondensation reaction of cis/trans-1,4-cyclohexane dicarboxylic acid and cis/trans-1,4-cyclohexane dimethanol. The corresponding poly(ester-anhydrides)s were obtained using a mixed anhydride method in melt where the low molecular weight carboxyl terminated cycloaliphatic polyesters were chain extended at the terminal carboxylic acid end groups through reversible anhydride linkages. The poly(ester-anhydrides)s as expected had higher glass transition temperature than the corresponding parent oligomers. PCCD oligomers with no anhydride linkages and PCCD based poly(ester-anhydrides)s with anhydride linkages at comparable molecular weights, had similar glass transition temperatures. However, the latter potentially offer higher reactive group concentration thanks to the reactive anhydride groups. This work was carried out as a proof of concept to show that the glass transition temperature of carboxyl terminated oligomers can be increased by increasing the molecular weight but without sacrificing the reactive group concentration. This approach, the authors suggest, may potentially be used in the design of new carboxyl functional base resins, especially for powder coatings, where both the acid number and glass transition temperature are important.

► Increasing the glass transition temperature of difunctional carboxyl terminated aliphatic polyesters.
► Mixed anhydride method is used to synthesize poly(ester-anhydrides).
► 31P NMR method for carboxyl and hydroxyl end group analysis.
► Glass transition temperature increase through anhydride linkages was observed.