Direct melt extrusion of polyhydroxyalkanoate solvent-rich gels after polymer extraction and melt processing with integrated solvent recovery

Melt extrusion was evaluated for polyhydroxyalkanoate (PHA)-solvent gels. Gels in 2-butanol were produced with polyhydroxybutyrate (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) co-polymer blends. Gels were formed by extracting PHA from dried biomass, or by dissolving it in 2-butanol, at up to 135 °C followed by cooling the solutions to room temperature. Excess solvent was pressed from the gel to make a compact gel cake. Cakes with less than 95 wt.% 2-butanol were grated into a granulate that could then be consistently fed and melt-processed in a twin-screw extruder. Gels showed significantly reduced PHA melt temperatures. For example, PHB melting temperature was reduced from 180 °C (neat polymer) to 115 °C in a 60 wt.% 2-butanol gel. The PHA molecular weight decomposition rates were independent of 2-butanol presence, and they were reduced 6-fold from 180 to 160 °C. Gel extrusion accomplishes polymer drying, solvent recovery, and melt processing altogether. More than 98 wt.% solvent removal with 77 wt.% solvent recovery was achieved using a vacuum pump and condenser attached to one extruder vent port. Extrudate quality was consistent, but it was influenced by the selected die temperature and barrel conditions that controlled gel residual solvent content at the die. PHA gel extrusion was characterized, and experiments are reported herein. Process efficiencies (solvent recovery and specific mechanical energy), and extrudate qualities were assessed. Insights were gained with thermogravimetry, differential scanning calorimetry, rheology and microscopy. The outcomes suggest benefits from directly combining solvent-based PHA recovery from biomass with gel-based PHA formulation and melt-processing into bioplastic articles. • PHA recovery from dried biomass was seamlessly integrated with melt processing. • Drying solvent rich PHA gels before melt extrusion processing is not required. • Consistent quality extruded pellets were produced alongside solvent recovery. • Specific mechanical energy and production output rates were improved.