The increasing use of PET materials has led to a rise in PET waste, making efficient recycling solutions essential. Researchers have developed several methods to depolymerize, purify, and recover PET waste. PET recycling is broadly divided into physical and chemical methods.
1. Physical Recycling
Physical recycling uses mechanical processes to melt and re-pelletize PET waste. It produces recycled PET (rPET). This approach yields lower-quality PET compared to new materials, often with reduced viscosity and a darker color. So it limits its use to low-grade fiber applications. Additionally, this method has high energy demands, reducing its cost-effectiveness.
Some common foreign physical recycling technologies and their characteristics are as follows:
| Technology | Advantages | Disadvantages |
| Buhler | High clarity; specialized pelletizing for moisture removal Requires high-quality PET feed without contaminants like PVC | Starlinger Uses minimal nitrogen, modular design Low production capacity |
| OHL | Efficient impurity removal with screw technology | Low and inconsistent output |
| EREMA | High vacuum and self-friction process | High costs and limited capacity |
| URRC | Alkaline cleaning for surface impurities | High material loss |
2. Chemical Recycling
Chemical recycling breaks down PET into its original monomers, allowing for high-quality PET production. Major methods include methanolysis, glycolysis, and hydrolysis.
2.1 Methanolysis
Methanolysis uses methanol as a catalyst to break down PET into dimethyl terephthalate (DMT) and ethylene glycol (EG). These products can be re-used to make new PET. Methanolysis is ideal for continuous production but has high costs due to its complex separation process.
2.2 Glycolysis
Glycolysis involves ethylene glycol at low temperature and pressure to produce bis(hydroxyethyl) terephthalate (BHET). It can undergo further polymerization. This method operates under mild conditions and has lower catalyst costs. So it makes commercial applications in various enterprises.
2.3 Hydrolysis
Hydrolysis uses water or acids to break down PET, yielding terephthalic acid (TPA) and ethylene glycol (EG). While efficient, hydrolysis often requires high temperatures and pressures. It increases costs and limits scalability.
2.3.1 Acid Hydrolysis
- Process: PET reacts with strong acids (e.g., sulfuric acid) at 200–250°C to yield TPA and EG.
- Pros/Cons: Fast PET breakdown but high wastewater and salt pollution; acid corrosion increases costs.
2.3.2 Alkaline Hydrolysis
- Process: Using strong bases like sodium hydroxide (NaOH) to break PET at moderate temperatures (60–120°C).
- Pros/Cons: Produces pure TPA but requires further processing. Base waste is challenging to treat and increases corrosion costs.
2.3.3 Neutral Hydrolysis
- Process: High-pressure water (200–260°C, 1.5–2.1 MPa) reacts with PET, producing TPA and EG.
- Pros/Cons: No waste byproducts, but energy-intensive and yields low-purity TPA, needing further refinement.
2.4 Ammonolysis
This process decomposes PET into terephthalamide under mild conditions with ammonia or amines. Although cost-effective and safe, it has limited commercial applications due to slow reaction rates and low yield.
3. New Chemical Depolymerization Techniques
Newer methods seek to overcome the limitations of traditional techniques:
- Microwave-Assisted Depolymerization: Speeds up reactions and saves energy by activating PET molecules. But it is still under research.
- Super/Subcritical Fluid Depolymerization: Uses supercritical fluids for efficient PET breakdown with minimal waste. But it requires high costs and equipment.
- Ionic Liquid Depolymerization: Uses ionic liquids as catalysts for mild, fast reactions. However, high costs and scalability remain barriers.
- Enzyme-Catalyzed Depolymerization: Enzymes from bacteria or fungi can degrade PET in environmentally friendly ways. Its low production remains a challenge.
Effective PET recycling demands methods that are cost-effective, low-energy, high-yield, and eco-friendly. Traditional methods like acid and alkaline hydrolysis are industrially established but expensive. Innovative approaches like microwave or enzyme-catalyzed methods hold promise but need further development for broader use.
4.The rPET Market and Its Development Trends
The market for recycled PET (rPET) is growing as people and companies increasingly choose sustainable and recyclable products. In 2021, the global rPET market was valued at around $9.4 billion, and it’s expected to expand at an annual rate of 7.4%, potentially reaching $18.46 billion by 2030.
4.1 Regional Market Share and Growth in the Asia-Pacific Region
In 2020, Asia-Pacific dominated the rPET market, holding 45% of the total share. Factors like affordable land and labor have made this region attractive for production, with emerging economies like China and India seeing a rise in rPET manufacturing.
4.2 Key Uses of rPET
Globally, the main uses for rPET products include:
In China, rPET is mostly used to make polyester fibers, which account for over 80% of its applications. However, its use in food packaging remains limited.
4.3 Policies and Initiatives Driving Recycling
Currently, close to 90% of plastic products worldwide aren’t recycled, which makes tackling plastic waste urgent. Governments and organizations are rolling out policies to promote recycling. For example, China has introduced waste classification and plastic recycling policies to push for better recycling systems. In Europe, the Circular Plastics Alliance has brought together nearly 300 organizations to promote plastic recycling, with a goal to recycle over 10 million tons annually by 2025.
4.4 High Demand for Food-Grade rPET
Food-grade rPET has become highly profitable for exports. Unlike other recycled plastics, food-grade rPET often costs more than virgin PET because of high processing costs and growing demand. In the EU, regulations now require that recycled PET must make up at least 30% of PET bottles by 2030. Major brands like Coca-Cola have committed to using 100% recycled PET in their bottles over time. This growing demand has led to food-grade rPET prices exceeding those of virgin PET by €200 to €700 per ton, even reaching twice the cost of virgin food-grade PET, at around ¥20,000 per ton.
4.5 Recycling Methods and Future Directions
Currently, physical and chemical recycling methods are the main ways to process rPET. Physical recycling produces fiber-grade PET with lower quality, while chemical methods can achieve the high viscosity needed for bottle-grade PET. Moving forward, the “bottle-to-bottle” approach, where recycled bottles are used to make new bottles, will likely lead the industry as demand for high-quality, sustainable packaging rises.
