title: Enhancing the functionality of biobased polyester coating resins through modification with citric acid.
abstract: Citric acid (CA) was evaluated as a functionality-enhancing monomer in biobased polyesters suitable for coating applications. Model reactions of CA with several primary and secondary alcohols and diols, including the 1,4:3,6-dianhydrohexitols, revealed that titanium(IV) n-butoxide catalyzed esterification reactions involving these compounds proceed at relatively low temperatures, often via anhydride intermediates. Interestingly, the facile anhydride formation from CA at temperatures around CA's melting temperature ( T m = 153 degrees C) proved to be crucial in modifying sterically hindered secondary hydroxyl end groups. OH-functional polyesters were reacted with CA in the melt between 150 and 165 degrees C, yielding slightly branched carboxylic acid functional materials with strongly enhanced functionality. The acid/epoxy curing reaction of the acid-functional polymers was simulated with a monofunctional glycidyl ether. Finally, the CA-modified polyesters were applied as coatings, using conventional cross-linking agents. The formulations showed rapid curing, resulting in chemically and mechanically stable coatings. These results demonstrate that citric acid can be applied in a new way, making use of its anhydride formation to functionalize OH-functional polyesters, which is an important new step toward fully biobased coating systems.
authors.affiliation: Laboratory of Polymer Chemistry, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands.
authors.forename: Bart A J
authors.initials: BA
authors.lastname: Noordover
authors.forename: Robbert
authors.initials: R
authors.lastname: Duchateau
authors.forename: Rolf A T M
authors.initials: RA
authors.lastname: van Benthem
authors.forename: Weihua
authors.initials: W
authors.lastname: Ming
authors.forename: Cor E
authors.initials: CE
authors.lastname: Koning
mesh_terms: D019343:Citric Acid / Q000138:chemical synthesis*; D020099:Coated Materials, Biocompatible / Q000138:chemical synthesis*; D011091:Polyesters / Q000138:chemical synthesis*; D012117:Resins, Synthetic / Q000138:chemical synthesis*
chemical_list: D020099:Coated Materials, Biocompatible; D011091:Polyesters; D012117:Resins, Synthetic; D019343:Citric Acid