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[dbnd]O, N-H) caused by high protein content (51% protein), predicting compatibility with EVA with polar functional (C[dbnd]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) ~ 7 µm). The composite maintains the dispersion of the HS2 without significant coalescence or network formation up to 50 wt% of HS2 (D_avg ~ 10 µm). 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 (~ 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.