Biodegradable copolymers of 3-hydroxybutyrate-co-3-hydroxyhexanoate (Nodax™), including recent improvements in their mechanical properties

A general review is first provided on the properties of this important member of the class of polyhydroxyalkanoate copolymers now being actively investigated as environmentally friendly materials. This soon to be commercialized material is already finding numerous applications and has the advantage of biodegradability, but its mechanical properties have not yet been fully optimized. In recent work, pre-orientation of uncrosslinked and crosslinked Nodax films was found to improve their moduli, tensile strengths, and yield stresses, as well as increasing (or at least maintaining) their toughness. These improvements presumably resulted from increases in crystallinity with orientation, a conclusion supported by increases in the intensity of X-ray diffraction patterns of the pre-oriented samples relative to those of the unoriented ones. Pre-orientation techniques could therefore be very useful in processing such materials for industrial applications. Addition of organoclay to Nodax was found to significantly improve thermal stability. These nanocomposites had structures ranging from exfoliated layers of clay at low contents to intercalated ones at higher contents. These changes in structure were investigated through X-ray diffraction peaks, with shifts in the peak toward lower values indicating intercalation and decreases in intensity of the clay peak indicating exfoliation. In the case of this reinforcing filler, the major advantage was increased moduli, but at the cost of decreased values of the elongation at break and toughness. Some of these disadvantages might be avoided by the use of other layered fillers, such as mica or graphite. Also, there are obviously other improvements possible in such layered nanocomposites, including decreases in flammability, and increases in solvent resistance and barrier properties.