Cyclic oligomers in saturated polyesters

To get an in-depth understanding of the structure-property relationship in saturated polyester resins, the reactivity of various diols and diacids as well as reaction conditions were studied. Adipic acid (AA), phthalic anhydr ide (PA), isophthalic acid (TPA), terephthalic acid (TPA), 2-butyl-2-ethyl- 1,3-propanediol (BEPD) and 2,2-dimethyl-1,3-propanediol (NPG) were addresse d. It was found that in addition to linear oligomers with carboxyl and/or h ydroxyl end groups cyclic oligomers were detected. Cyclic dimers of TPA and NPG are known to migrate to the coating surface causing "blooming". The un derstanding of this phenomenon and the monomers capable of forming cyclic d imers are the subject of this study, where we combined molecular modeling a nd analytical characterization. The probability of the formation of macrocy clic structures was shown to decrease in the order: BEPD-PA > BEPD-AA great er than or equal to BEPD-IPA > BEPD-TPA. The temperature dependence was fou nd to be small. (C) 2000 Elsevier Science Ltd. All rights reserved.