Professor Zhang Fan of Sichuan University: Upcycling Waste Commercial Polymers into High-Performance Polyarylates Using Plastic Capping Agents

Abstract: Commodity polymers are widely present in our society, and due to their versatile properties, they have replaced many inorganic and metal-based materials. However, their functionality largely depends on the addition of various components known as additives, which makes it challenging to effectively recycle the polymer portion of plastic materials. Therefore, developing efficient chemical recycling strategies for commodity polymers and additives is essential to promote the direct utilization of recycled monomers and additives without further purification. Here, the authors developed a strategy to upcycle two waste commodity polymers, polycarbonate (PC) and polyethylene terephthalate (PET), into polyaryl esters, a high-performance transparent engineering plastic. By combining highly active metal-free ionic liquid catalysts and a two-stage interfacial polymerization technique with variable temperature control, polyacrylate film materials were successfully prepared from directly utilized active plastics derived from recycled monomers. These materials exhibit excellent thermal properties (Tg = 192.8℃).

Figure 1 Schematic diagram of conventional and upcycling routes for waste PET/PC

a. The conventional practice of chemical recycling of waste PET and PC, including solvent dissolution, purification, and re-polymerization

b. The work of this article is about directly utilizing PC and PET to convert into r-PAR impurities

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Introduction: The Dilemma and Recycling Challenges of PC and PET

Plastic is the cornerstone of modern society, with an annual global production exceeding 500 million tons, but the recycling rate is less than 15%. A large amount of waste plastic is disposed of through landfill, incineration, or random dumping, not only causing a waste of resources but also releasing toxic substances (such as bisphenol A), threatening the ecological environment and human health. Traditional mechanical recycling methods are inefficient and have difficulty handling mixed plastics containing additives. How to achieve efficient and environmentally friendly plastic recycling has become an urgent problem that the global scientific community needs to solve.

PC and PET are the two most widely used types of commodity plastics. PC is extensively used in electronic casings, optical discs, but its degradation may release the endocrine disruptor bisphenol A (BPA); PET is commonly found in beverage bottles, textiles, but mechanical recycling can lead to performance degradation. Although existing chemical recycling technologies can depolymerize plastics into monomers, they face bottlenecks such as additive interference and high purification costs.

Figure 2 Catalytic hydrolysis of PC under mild conditions

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Technological Breakthrough: Dual Innovation in Catalysis and Polymerization

Recently, a team led by Professor Zhang Fan and Researcher Zhang Gang from Sichuan University has collaboratively developed an innovative technology for the synergistic recycling and upcycling of waste PC and PET into high-performance transparent engineering plastic, polyarylate (PAR) (Figure 1). This process directly utilizes impurities in the recycled monomers as end-capping agents, achieving the transformation of waste into valuable resources. The related research findings have been published in the top academic journal Nature Communications.

Figure 3 Catalytic methanolysis of waste PET and PC and analysis of recovered BPA monomer

2.1. Efficient Metal-Free Ionic Liquid Catalysts

The research team designed a new type of ionic liquid catalyst [TBDH]Ac, which activates the carbonyl bonds in methanol and plastics through a synergistic effect, significantly improving the efficiency of methanolysis. Experiments show that this catalyst can completely degrade PC within 4 hours at 100°C, with monomer recovery rates as high as 98% (BPA) and 99% (DMT), and the catalyst can be recycled more than 5 times, demonstrating excellent stability.

2.2. Two-stage interface aggregation technology

Traditional methods require high-cost purification of the recovered monomer to remove additives (such as the capping agent PTBP), while this study innovatively developed the "temperature-controlled two-stage polymerization" technology:

First stage (5–10°C): Initiate the polymerization reaction at low temperatures, utilizing the low water solubility of PTBP to inhibit its interference.

Second stage (15-18°C): The temperature rise dissolves PTBP, which directly participates in the reaction as a capping agent, both avoiding impurity effects and improving material performance.

2.3. Performance Verification: Regenerated Materials Rivaling Commercial Products

The regenerated polyarylate (r-PAR) prepared by this method exhibits excellent performance:

Thermal stability: Glass transition temperature (Tg) reaches 192.8°C, initial thermal decomposition temperature 470°C, comparable to commercial PAR (U-100).

Transmittance: The film transmittance in the 400–450 nm wavelength band reaches over 85%, suitable for optical devices.

Flame retardancy: Passed UL-94 V-0 level test, with a burning time of only 2.8 seconds, far exceeding the V-1 level of PC.

Recyclability: r-PAR can be depolymerized and repolymerized again to achieve closed-loop recycling.

Figure 4 Upgrading Waste PC and PET Together into High-Performance Engineering Plastics—Polyarylates

Figure 5 The laboratory demonstrates large-scale waste commercial plastics and fibers entering r-PAR and its closed-loop cycle,

as well as the transparency and flame retardancy tests of r-PAR film material.

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Environment and Economic Benefits: Low-Carbon Pathways and High Value-Added

3.1. Life Cycle Assessment (LCA)

Compared to traditional plastic management methods, this technology reduces the global warming potential (GWP) by 26%, decreases freshwater ecotoxicity by 37%, and significantly lowers the risk of human toxicity (especially with a 53% reduction in BPA recycling).

3.2. Economic Feasibility

Based on an annual production of 10,000 tons of r-PAR, the added value per ton of product is as high as $8,049 (approximately RMB 58,000), and the product performance is benchmarked against commercial U-100 (market price $10,000/ton). Large-scale production can significantly reduce catalyst and energy costs, and mitigate market fluctuations.

Figure 6 LCA and input-output analysis of co-cycled waste PE and PC, and LCA of the closed-loop cycle of PAR