Background Polyethylene (PE), one of the most widely used commodity plastics, is transitioning from a single life-cycle system to a multiple life-cycle (MLC) recycling system. However, repeated recycling inevitably causes material degradation, driving PE from higher-quality to lower-quality applications along cascading pathways and raising concerns about its long-term supply–demand evolution. Conventional material flow analysis (MFA) can effectively quantify the movement and accumulation of materials, but generally provides limited resolution regarding quality transitions and material redistribution across successive recycling cycles. A systematic understanding of these cascading effects is therefore essential for evaluating the long-term sustainability of PE recycling.

Methods We developed an MLC cascading-effect analysis model to characterize PE cascading pathways and their evolution in China. Unlike conventional MFA, the proposed framework explicitly tracks quality-dependent material flows across multiple recycling cycles by integrating sector-specific cascading pathways, maximum allowable secondary PE content, quality transitions, and utilization constraints. The model captures the accumulation of secondary PE in lower-quality applications and identifies potential saturation and spillover that may emerge over successive cycles. We further conducted a scenario analysis to evaluate the effects of recycling rate improvement, cascading pathway optimization, chemical recycling, and eco-design on long-term PE circularity and primary material demand.

Results The results show that only 23.1% of China's consumed PE was recycled in 2025, with high- and medium-quality secondary PE accounting for less than 1.0%. As recycling cycles progress, secondary PE progressively accumulates in low-quality and inferior-quality products, indicating that a higher recycling rate does not necessarily translate to higher-quality circularity. This accumulation causes spillover from 2030 onward, reaching 0.968 million metric tons by 2060. Scenario analysis shows that increasing the recycling rate alone cannot overcome downcycling. Although an increased recycling rate expands the utilization of secondary PE, the spillover rate will reach 25.3% by 2060 as increasing amounts of secondary PE are redirected toward lower-quality applications. In contrast, a combined strategy centered on chemical recycling, cascading pathway optimization, and eco-design mitigates quality degradation and improves the circulation of secondary PE. By 2060, this strategy reduces primary PE consumption by 62.6%, increases the closed-loop recycling rate by over 35.0%, and decreases the spillover rate by 20.3%.

Conclusions These results demonstrate that the long-term performance of PE recycling depends not only on recycling volumes, but also on material quality evolution and the distribution of secondary PE across successive life cycles. By extending conventional MFA from aggregate material flow accounting to quality-dependent cascading analysis, the proposed MLC framework provides a means to reveal hidden accumulation and spillover risks that might be overlooked when recycling rates are considered alone. The findings highlight the importance of coordinating recycling targets with quality-preserving and quality-restoring technologies, optimized cascading pathways, and upstream eco-design. Such an integrated perspective can support evidence-based policy formulation and help guide the transition toward a more efficient, resilient, and circular plastics economy in China.