LEGO strategy to upcycle waste polyethylene into high-performance materials

Polyethylene (PE), as one of the most produced and widely used plastics, faces challenges in functionalization and chemical degradation due to its chemical inertness. Meanwhile, the environmental pressure from large amounts of waste PE is increasingly severe.

Traditional PE functionalization methods often rely on physical blending, polar monomer copolymerization, or chemical grafting, which frequently encounter problems such as poor compatibility, difficult synthesis, limited functionality, or decreased mechanical properties. Achieving high-value and multifunctional recycling of PE is a significant challenge currently facing the circular economy of plastics .

To address the above issues, Academician Yu-Zhong Wang's team at Sichuan University proposed the "LEGO" strategy, which achieves the upgrading and recycling of PE in two steps. First, PE is controlled to be oxidized and degraded into oligomers with active end groups (ADOPE-CHO), and then chemically reconstructed with different functional modules (such as flame retardant/UV shielding modules and antistatic/dyeable modules) through dynamic imine bonds to construct a multifunctional material with a dynamic cross-linked network.

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The dynamically cross-linked multifunctional material exhibits excellent mechanical properties and solvent resistance. The tensile strength reaches up to 27 MPa, nearly four times that of the original PE. Simultaneously, it demonstrates low swelling and mass loss rates in most solvents.

The constructed flame-retardant/UV-shielding material has a limiting oxygen index of up to 27%, is self-extinguishing in air, and its peak heat release rate is reduced by up to 73% compared to the original PE. The formation of combustible small alkane molecules is significantly suppressed during thermal decomposition, and its char formation ability is also significantly improved, with a residual mass of up to 33% at 700 °C. It can form a stable char layer during combustion, thus providing thermal and oxygen insulation. Furthermore, this material can achieve full-band UV shielding.

The surface resistivity and volume resistivity of the obtained antistatic/dyeable material are reduced by 2-3 orders of magnitude compared to the original PE, meeting the standards for electrostatic dissipative materials. The material can be dyed deep blue with methylene blue solution, and the color fastness is good, with the dye not being soaked out. Different colors can be achieved by adjusting the concentration or type of dye solution.

Since PE itself lacks reactive functional groups, its functionalization is usually achieved through blending with functional fillers, which results in significant polarity differences, poor compatibility, and decreased mechanical properties after blending modification. This degradation-dynamic crosslinking reconstruction method not only enables modular construction of multifunctional materials but also significantly enhances the mechanical properties of the material.

Based on the dynamic changes of imine bonds under thermal stimulation and their dissociation ability in acidic organic solutions, the material can be physically recycled through multiple thermal processes, or it can be chemically recycled by complete degradation under acidic conditions.

In summary, this work utilizes a "LEGO" strategy to transform waste PE into a high-performance, multifunctional, and recyclable new material, achieving an upgraded recycling process from "waste plastic" to "high-value material." This strategy offers high flexibility, enabling further functionalization based on specific needs, and provides new insights into the functional modification and high-value recycling of polyolefin plastics.

This research was recently published in CCS Chemistry. Chengfeng Shen, a doctoral student at Sichuan University, is the first author of this paper, and Professor Shimei Xu and Academician Yu-Zhong Wang are the corresponding authors. This research was supported by the National Natural Science Foundation of China.