Thermal Stabilization of Polyhydroxyalkanoates by Washing with Acid

Abstract: The main goal of this project was to investigate how the thermal stability of two different types of polyhydroxyalkanoates; poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate) were affected when washing them with acid solution. Initially, four different acids were used: citric acid, hydrochloric acid, formic acid and acetic acid. Citric acid gave the best results for both biopolymers and was further investigated with respect to different concentrations and washing times to find the optimal parameters. Experiments where deionized water was used as the washing solution, and experiments where citric acid was mixed into the samples were also investigated. The analyzing techniques used to evaluate the results was thermogravimetric analyze, differential scanning calorimetry, rheometer and size exclusion chromatography. The degradation temperatures of as-received P(3HB) and P(3,4HB) powder were 279℃ and 247℃, respectively, determined with TGA. After the citric acid wash (1 mM, washing time 30 min), the degradation temperatures increased with 16℃ (296℃) for P(3HB) and with 48℃ (295℃) for P(3,4HB). Higher degradation temperatures could not be reached with higher concentrations or longer washing times. However, further investigation of P(3HB) showed that a concentration of only 0.05 mM and a washing time of 45 s was enough to raise the degradation temperature to 296℃. For P(3,4HB), a concentration of 1 mM and a washing time of 45 s was required to reach a degradation temperature of 295℃. By washing P(3HB) with deionized water for 30 minutes, the degradation temperature became 294℃, and for P(3,4HB) it became 271℃. Due to the high increase in degradation temperature, more washing times were investigated. For P(3HB), a washing time of only 45 s was enough to raise the degradation temperature to 294℃, and for P(3,4HB) 5 min was required to reach 271℃. When citric acid was mixed into the homopolymer, the degradation temperature decreased with 27℃, and for the copolymer the degradation temperature stayed the same. The increase in degradation temperature by washing the biopolymer powder indicates a significant increase in the thermal stability.