Effects of Thermomechanical Processing Conditions on the Morphology and Mechanical Properties of Spirulina Bioplastics

Abstract

The increasing consumption of non-degradable plastics and its harmful effects on the environment urgently call for the design and fabrication of degradable and renewably sourced materials. Recent studies have reported a mitigating strategy where materials derived from biological materials, or biomatter, are introduced as fillers in common plastic matrices, thus minimizing the use of non-renewable materials. For example, algae organisms, and more specifically spirulina microalgae, are a promising candidate for biomatter fillers as they can grow in a wide variety of aquatic environments and are already cultivated at industrial production rates as food supplements. However, the poor bonding between spirulina (and other plant-based biomatters) and common plastics often lead to the mechanical weakening of the produced biocomposites. The heavy use of chemical additives and multi-step processing necessary to circumvent these bonding obstacles have rendered the manufacturability of this filler-based strategy challenging.In this thesis, we focus on pure bioplastics made almost exclusively of spirulina, where biomatter is not used as a filler but instead composes the bulk matrix of the fabricated materials. Upon hot-pressing, i.e., compression-molding under the given temperature and pressure conditions, spirulina powder can self-plasticize and get transformed into a bioplastic material which is strong, modifiable by common manufacturing techniques, and degrades in common soil. In this work, commercially acquired spirulina powder was hot-pressed under varying temperature, pressure and time conditions and the resulting bioplastics were mechanically tested in bending. Their morphology was studied via scanning electron microscopy (SEM), thus enabling us to draw connections between processing conditions, microstructure, and macroscopic mechanical properties. First, the effect of time in the compression molding process was studied. We observed that above a threshold of 5-minutes press time, the mechanical properties reached a plateau, with no significant changes in longer press times. Therefore, when studying the effects of pressure and temperature, the 5-minute press time was kept constant. By scanning a broad range of processing temperatures (60-160 °C) and pressure loads (2 – 35 kN), the produced spirulina bioplastics were found to span a wide range of mechanical properties and morphologies. At the lower end of the temperature/pressure conditions, a low bending strength of 1.2 MPa was measured, as the specimen remained in a loosely bonded powder form with intact and distinct cells still visible in the SEM. The strongest bioplastics were pressed at 140 °C and 7 kN and had a bending strength of 25.5 MPa. The SEM shows a completely different morphology, with a smooth fracture surface showing no distinguishable spirulina cells. At the higher end of the pressing conditions, thermal degradation initiation was observed, with a reduction in strength to 17.4 MPa. In addition to pure-spirulina bioplastics, we also investigated the effects of introducing a plasticizer to further expand the attainable material properties. We used 1 - 30 wt% of a natural, plant derived plasticizer, sorbitol, which was compounded with the spirulina via twin-screw extrusion to create spirulina/sorbitol composites. The resulting powder was pressed using a given set of temperature, pressure and time conditions that were chosen to prevent the plasticizer from melting. For samples pressed at low temperature/pressure conditions, the addition of sorbitol showed strength increases of 2.4 times, elongation to break increases of 2.7 times, and toughness increases of 16 times. In addition to the morphological and mechanical properties, we used thermogravimetric analysis (TGA) to examine the thermal stability of powdered and pressed spirulina and conducted a soil degradation study to assess the biodegradability of our bioplastics.