-1734 cm -1 -narrow band characteristic for the carbonyl group. Together with the narrow bands of 1163 cm -1 , 1096 cm -1 (secondary) and 1049 cm -1 (primary) characteristic of the acyl and alkoxy groups, they indicate the presence of an ester group, and thus the production of a polyester -1709 cm -1 -narrow band, overlapping with the band of 1734 cm -1 , together with the broad band of 3466 cm -1 , they indicate the presence of free acid groups in the structure of the polyester.