Biodegradable natural composites. I. Processing and properties

This paper presents an investigation of the properties of composites of bacteria-produced polyesters reinforced with wood cellulose. Although cellulose fibers improved the strength and stiffness of the polyhydroxybutyrate (PHB), the samples were very brittle. The impact strength and elongation at break of these composites were significantly improved when PHB copolymers with increased amounts of hydroxyvalerate (HV) were used as the matrix. Dynamic mechanical properties of PHB copolymers of varying compositions and of cellulose-filled composites were investigated. The introduction of cellulose resulted in a decreased loss factor owing to restrictions of the chain mobility in the amorphous phase. An improvement was observed in the dynamic modulus, which was seen to be greatest at elevated temperatures. An excellent dispersibility of cellulose fibers was achieved in the PHB matrix as compared with such synthetic matrices as polystyrene or polypropylene. The degree of dispersibility was strongly affected by processing conditions. The defibrillation observed on extracted fibers suggests a possible hydrolysis of cellulose by crotonic acid formed in situ as a result of the thermal decomposition of the PHB matrix.