Biomatter Plastics: Building Sustainable Materials with Biomacromolecules

Sustainable solutions designed to reduce the environmental harm of synthetic plastics often struggle to be both fully biobased and biodegradable. A considerable amount of biodegradable plastics does not entirely break down in natural environments, and despite significant efforts in investigating recycling processes, a large fraction of the plastic produced each year still enters the biosphere. To tackle this challenge, we have previously developed a completely biobased and backyard-compostable plastic alternative using only biological matter (biomatter), specifically spirulina and other algal biomasses, bypassing the need for extraction or energy-intensive processing. By applying heat and pressure, as-harvested biomatter transforms into a thermoformable material that when cooled is rigid and has mechanical properties comparable to polystyrene and polylactic acid. In this study, we delve into the self-bonding mechanism of biomatter during thermomechanical processing by creating a model system and varying the amounts of each component to emulate specific case studies of algal plastics. Specifically, we vary the ratios of pure carbohydrates, lipids, and proteins, physically blend them, and subject the resulting biopolymer composites to heated compression molding to produce materials that mirror the composition of our algal bioplastics. Mechanical testing and scanning electron microscopy are employed to evaluate how variations in macromolecular composition and the role of each macromolecule class affect the polymer morphology and mechanical properties of the produced biomatter analogues. By manipulating the protein-to-carbohydrate ratio, we compare the mechanical performance of these analogues to various algal species. Initially, we qualitatively assess the bonding mechanism during sequential reprocessing to isolate the effects of dynamic bonding. Subsequently, Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy, along with molecular dynamics simulations, are used to quantitatively examine both secondary and primary bonding interactions among different macromolecular components of the analogue composites. From these analyses, we propose a mechanism for self-bonding in biomatter plastics.