Impact of fermentation residues on the rheological and structural properties of PHBV produced from cheese whey and olive mills wastewaters

Abstract

In addition to their poor processability with conventional plastic processing techniquesi , polyhydroxyalkanoates (PHA) are expensive when compared to other plastics. PHA recovery and purification take a strong share on the final product cost, but the impact of fermentation residues on PHA properties relevant for processing and final product properties is not sufficiently documented in the literature. Here we report on the rheological, thermal and structural properties of two types of polyhydroxy(butyrate-co-valerate) (PHBV) produced from mixed microbial cultures fed with cheese whey (CW) or olive mills wastewaters (OMW). The two types of PHA are PHBV block copolymers as demonstrated by distinct melting peaks and glass transitions in the differential scanning calorimetry (DSC) curves. PHBV have different amounts of HV units, namely 11% and 18% for OMW and CW, respectively. PHBV were extracted and purified using different downstream routes, enabling the recovery of polymers with different purity levels ranging from 100% to 89%. Rotational rheometry indicates that fermentation residues affect the thermal stability and the mechanical spectra of the melts: a reinforcing effect of the residues balances the viscosity drop associated with thermal degradation. However such improvement comes at the cost of depressed melt processability, as suggested by the difficulty in assessing the melt flow index of samples with largest amounts of residues. Fermentation residues act as nucleating agents thus improving the polymer crystallinity measured with wide angle Xrays diffraction (WAXD). Both DSC and rotational rheometry show that residues affect the crystallization kinetics (primary and secondary) at high and lower temperatures. Results will be benchmarked with a commercial PHBV.