Noordover B A J, et al. Biomacromolecules, 2007, 8(12), 3860-3870.
Citric acid (CA) was evaluated as a functional enhancement monomer in bio-based polyesters for coating applications. It was found that OH-functional polyesters reacted with CA in the melt between 150 and 165 °C to produce slightly branched carboxylic acid functional materials with strong reinforcing functionality.
Preparation procedure for CA-modified OH-functional polyesters
· Linear hydroxy-functional polyesters synthesized from isosorbide and succinic acid were reacted with citric acid (CA) to introduce carboxylic acid end groups. For a typical procedure, 52.9 g of an OH-functional polymer (Mn = 2200 g/mol, acid value = 1.5 mg KOH/g, hydroxyl value = 65.0 mg KOH/g) was combined with a 1.1 molar excess of CA, calculated based on the OH groups determined through titration. The temperature was gradually raised to 170 °C (Tmax), resulting in a clear melt. The mixture was then stirred for 2.5 hours under an inert gas atmosphere, followed by vacuum processing for an additional 3 hours. The clear melt was subsequently removed from the reactor and allowed to cool.
· For 13C NMR analysis, the hydroxyl-functional succinic acid/isosorbide copolyester (entry I in Table 2) was reacted with 13C-labeled citric acid (denoted as CA-1,5-13C2). The ratio of OH groups (measured by titration) to CA-1,5-13C2 was set at 1:1.1, accounting for about 20 wt % of CA-1,5-13C2. Throughout the reaction, samples were collected to track changes in molecular weight, polydispersity, and conversion of the polyester resin's OH groups (via 1H NMR) as well as the carboxylic acid groups in CA (via 13C NMR).
