Microbial Synthesis Of Bacterial Cellulose And Used In The Preparation Of Composite Materials

Obtained from fermentation by the Acetobacter Xylinum, bacterial cellulose (BC) presents a uniform ultrafine-fibre network structure. Compared with the plant cellulose, BC displays higher purity, crystallinity, Young's modulus and tensile strength. BC, therefore, has been considered as one of the most promising renewable sources for various applications. In this thesis, we have first optimized the fermentation process of BC in both lab-scale and medium-scale experiments to increase the yield of BC. In the second part of my research, the application of BC as biomaterial has been explored by blending with poly lactic acid (PLA).In the first part of fermentation research, we have studied in detail the effects of various carbon sources and pH in the fermentation medium on the yield of BC. In the lab-scale experiments, the optimal fermentation conditions were found as following:sucrose 1.5%, glucose 2.5%, pH 5.5-6.0. When adopting these optimal conditions in static-fermentation, we have found addition of 1% ethanol can further increase the resulted BC yield. In the medium-scale experiments with the above-mentioned optimal fermentation conditions, BC yield was found to be 3.3 g/L under stirring medium in a 100 L fermentation tank. In order to maintain the optimal fermentation conditions, glucose as carbon source and ammonia as pH adjuster were added into the culture. The BC yield in medium-scale fermenter can be further improved to 3.68 g/LTo explore the bio-applications of our BC by taking full advantage of its tensile strength, in the second part of my thesis, we have prepared PLA/BC composites by heat pressed molding. In the molding process, the dried BC membrane was sandwiched between two layers of powered PLA before heat-pressing. Two kinds of dried BC membranes, namely, heat-dried BC membrane and freeze-thaw dried BC membrane were used. The results show that the addition of heat-dried BC membranes to PLA matrix can improve its tensile strength obviously. For example, the tensile strength of PLA/BC (80/20) composite was increased to 70.7MPa, which is 30% improvement of pristine PLA (52MPa). However, PLA/BC composites with freeze-thaw dried BC membranes harvested with reduced tensile strength.We have also explored the preparation of PLA/BC composites by solution-dissolved PLA heat-pressed molding. The PLA was first dissolved in chloroform and the heat-dried BC membranes were soaked in PLA chloroform solution. The resulted compounds were further processed with heat-pressed molding. Similar results were found, i.e. PLA/BC composites achieved higher tensile strength than pure PLA. For instance, the tensile strength of PLA/BC composite was nearly 70MPa with addition of 20% BC.To increase the elongation at break, the Glycerol, poly ethylene glycol (PEG) and acetyl tributyl citrate were used as plasticizer. The results show that the acetyl tributyl citrate was the best plasticizer. With the content of acetyl tributyl citrate increased to 15%, the elongation at break was 200%. With the acetyl tributyl citrate increased, the tensile strength decreased dramatically. When the cellulose was added, the tensile strength can keep in the high tensile strength.In summary, we have optimized the fermentation process of BC in both lab-scale and medium-scale. The bio-applications of BC were further studied by compounding with PLA.