Abstract
The transition from a linear, fossil-based polymer economy to a circular bio-economy is critical for mitigating resource depletion and greenhouse gas emissions. This study provides a rigorous comparison of two biomass-to-plastic pathways: a biochemical route (PLA via enzymatic hydrolysis) and a thermochemical route (bio-PE via gasification and MTO). Based on Aspen Plus simulations and a "lifetime-adjusted" lifecycle assessment framework, we evaluate the environmental performance of these routes in the transition from linear to circular systems. Unlike standard "cut-off" methods, the lifetime-adjusted model accounts for virgin make-up and molecular retention across multiple recycling cycles. Results indicate that at current 15% recycling rates, PLA exhibits the lowest global warming potential due to significant biogenic carbon sequestration. However, as recycling rates reach 75%, process efficiency becomes the dominant factor; the precise biochemical recycling of PLA continues to outperform mechanical HDPE recycling, whereas the energy-intensive thermochemical bio-PE route loses its competitive advantage. Our sensitivity analysis reveals that LCA modeling choices significantly diverge at intermediate recycling rates, resulting in different values for environmental impact. Ultimately, while bioplastics serve as a vital agent for CO2 sequestration during the transition to circularity, long-term sustainability necessitates a shift toward "recyclable-by-design" materials.