Bio-composite of bacterial poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) reinforced with vegetable fibers

Composites of a bacterial copolyester poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), P(3HB-co-3HH), with flax fibers were investigated. A satisfactory reinforcing effect was observed in composites prepared by compression molding and using long fiber mats (long fiber composites, LFC). On the other hand, composites obtained by batch mixing fibers with molten polymer resulted in poor mechanical properties, owing to drastic fiber shortening during processing (short fiber composites, SFC). In order to improve fiber–matrix adhesion in SFC, fibers chemically modified at the surface (by acetylation or by short-chain-PEG grafting) were also used. The best results were obtained with surface acetylated fibers. In the flax fiber composites the crystallization rate of P(3HB-co-3HH) remarkably increased compared with that of the plain polyester. The fibers displayed a nucleating effect on P(3HB-co-3HH) crystallization, whose magnitude depended on fiber surface chemistry. This feature was confirmed by the appearance of transcrystallinity in isothermal crystallisation experiments run in a hot stage of a polarized optical microscope.