Biocomposites from bacterial cellulose

Abstract

Bacterial cellulose has high strength and crystallinity, and as such has been suggested as a strategically-useful material that could form the reinforcement phase in composites. In addition to any property improvement, if a biodegradable matrix is reinforced with bacterial cellulose, the entire system should be biodegradable. However, bacterial cellulose is not easily dispersed and thus has not been widely used in such composites. Investigations into the production of bacterial cellulose composites where the cellulose reinforcement component is evenly dispersed is therefore of benefit. Bacterial cellulose can be produced in high quantities by the bacterium Gluconacetobacter xylinus in various media and under various reaction conditions, however altering the growth conditions has been shown to change the yield and properties of the resulting cellulose. Poly-3-hydroxybutyrate (PHB) is a bioplastic that has been hypothesised as a material that could replace traditional plastics, however it is very stiff and brittle. It is possible that its use in a composite with an effective reinforcing phase could improve these properties. Therefore this material was selected as a matrix material to be blended with bacterial cellulose. The examination of growth conditions in this work led to a methodology by which high amounts of cellulose with high crystallinity could be obtained. In addition, methods were determined by which to achieve modified cellulose fibrils. These modifications included cellulose fibrils produced with PHB physically attached to the surface. Solution blending and melt blending techniques were investigated as ways of producing PHB/bacterial cellulose composites. Solution blending was found to produce composites with well dispersed bacterial cellulose, however melt blending was found to degrade samples. It was found that solution blends using cellulose in a ground powder form did not achieve improved properties; however a composite with cellulose in its fibrillar form achieved improved tensile strength and modulus. PHB/bacterial cellulose composites with cellulose in its fibrillar form were produced by dispersing the cellulose fibrils by sonication. Sonication was investigated as a method of harvesting and dispersing bacterial cellulose fibrils in various solvents, including chloroform which could directly dissolve PHB, however only small weights of individual fibrils were obtained in this way. The composite with improved mechanical properties contained 2 wt% cellulose; however it was found that these improvements were observed only if the cellulose was retained in a hydrated never-dried state. Investigations into the biodegradability of PHB and a PHB/bacterial cellulose composite revealed that the composite degraded at a greater rate than neat PHB. This indicates that a bacterial cellulose reinforcement phase is able to assist a PHB matrix to degrade at a faster rate when submerged in compost. It is apparent that techniques can be developed to use bacterial cellulose successfully to confer strength to composites when used as a reinforcing material, as well as increasing the rate of biodegradation of a PHB matrix. Composites of these materials should therefore be considered in the design of biodegradable materials.