Influence of mold temperature on the properties of wastewater-grown microalgae-based plastics processed by injection molding

Nowadays there is a need for sustainably produced degradable bio-based plastics to lessen the high dependence on non-renewable petrochemical resources. Therefore, this study evaluates the development of bio-based plastics from Desmodesmus sp. and Tetradesmus obliquus biomass cultivated in wastewater. Also, it evaluates the effects of mold temperature over the mechanical and viscoelastic properties of the bioplastics by dynamical mechanical analysis (DMA), tensile tests, and water uptake capacity (WUC). Results prove that an increase in injection molding temperature resulted in an increase in the viscoelastic properties (elastic moduli, G′, from oscillatory torsion tests increased from 3.8 · 107 ± 0.2 Pa to 6.0 ± 0.1 Pa and from 2.2 · 107 ± 0.2 Pa to 3.3 ± 0.1 Pa for Desmodesmus sp. and T. obliquus, respectively) and a decrease in WUC (from 865.9 ± 44.6% to 447.5 ± 28.7%, and from 393.6 ± 18.6% to 219.2 ± 5.3% for Desmodesmus sp. and T. obliquus, respectively), suggesting a direct relationship between protein cross-linking. Desmodesmus sp. based bioplastics showed higher maximum stress and WUC than T. obliquus based bioplastics, while T. obliquus based plastics showed higher strain at break. These differences could be related to the high cell wall rigidity of Desmodesmus sp. which might induce the formation of lower toughness probes with higher swelling capacity. Moreover, its higher protein content (31%) may lead to stronger molecular interactions which explained a higher σmax (28.8 MPa). On the other hand, T. obliquus induced compact, rigid, and tough plastics that kept their structure after water immersion. Additionally, a certain cell wall disruption may take place when processing T. obliquus based plastic, which may favor cross-linking to a larger extent than in Desmodesmus sp. system. The bio-based plastics developed may provide high-value and sustainable solutions for the by-products generated in wastewater treatment.