Dispersion of unfractionated CO2-derived protein-rich microalgae (Chlorella sp. HS2) for ecofriendly polymer composite fabrication

This study investigates unfractionated protein-rich microalgae (Chlorella sp. HS2) (HS2) as a new CO2-derived biomass filler resource with which to develop an ecofriendly microalgae-based polymer composite. Unfractionated HS2 is mixed with poly(ethylene-vinyl acetate) (EVA) over wide range of concentrations ranging from 10 to 70 wt%. The dispersion of HS2 is analyzed based on morphological, rheological and mechanical measurements. Protein-rich HS2 has hydrophilic-hydrophobic surface due to the existence of chemical functional groups (C=O, N-H) caused by high protein content (51% protein), predicting compatibility with EVA with polar functional (C=O). Due to this compatibility, with 10-30 wt% of HS2, the composite shows a homogeneous micrometer-scale dispersion of HS2 in the EVA matrix (avg. diameter (D-avg) similar to 7 mu m). The composite maintains the dispersion of the HS2 without significant coalescence or network formation up to 50 wt% of HS2 (D-avg similar to 10 mu m). Correspondingly, the storage modulus (G' at 0.1 rad/s) of the composite increases linearly until the HS2 content reaches 40 wt%, after which it increases exponentially with an increase in the HS2 content. An EVA composite with 10-20 wt% HS2 shows increased ductility (from 1700% to 2000% elongation at break with 10 wt% HS2) without a decrease in the tensile strength due to the homogeneous dispersion. Even with higher concentration of HS2, the composite maintains its ductile behavior and retains its synergistic effect with EVA (similar to 500% elongation at break with 70 wt% HS2). The compatibility of HS2 with EVA and their hydrophilic surface delay agglomeration or percolation formation of HS2 cells in a polymer. This study suggests that protein-rich HS2 is a promising biomass filler that disperses in a polymer to the micrometer scale without additional chemical treatment.