A large amount of plastic waste enters the ocean every year, threatening wildlife and harming biodiversity. Additionally, microplastics and plastic microfibers have been found in the air and urban drinking water, causing health and ecological issues. Common but highly unsustainable methods of plastic waste disposal include incineration and landfilling. The former is carbon-intensive and pollutes the air, while the latter may cause long-term environmental damage. Furthermore, many thermoplastic materials can be mechanically recycled into low-end but useful materials. However, mechanical recycling of plastics is often limited by the relatively low value of the resulting products. In order to continue utilizing plastics, effective recycling and upcycling strategies must be developed to ensure sustainability. Chemical recycling is a promising strategy that can recycle waste plastics into monomers, which can then be repolymerized in subsequent processes to produce high-quality products.
However, there are still several key issues with chemical recycling of plastics:
1. Commodity plastics cannot be selectively converted into monomers through traditional thermochemical methods
While some plastics with relatively low melting points can be easily depolymerized through traditional thermochemical methods such as pyrolysis, most polyolefins and polyesters cannot be selectively converted into their monomers because traditional methods lack precise control over the reaction process.
2. Traditional thermochemical depolymerization produces many by-products, making it difficult to obtain high-quality polymers from impure monomers
Due to the complex physicochemical transformations of reactants and intermediates, and the severe competitive side reactions during the continuous heating process of plastics, thermochemical depolymerization produces a series of by-products (e.g., gases, condensable hydrocarbons, aromatics, etc.) in addition to the desired monomers.
Therefore, separation technology plays an exceptionally important role in plastic chemical recycling pathways.
Chemical separation technologies empower chemical recycling by producing high-quality monomers and compounds, enhancing downstream processing possibilities, and creating opportunities for a circular economy.
In the following four areas, DODGEN separation technologies play a crucial role in the chemical recycling process:
1. Purification of monomers after depolymerization of PET/nylon.
2. Recovery of plastics and fibers after solvent recycling.
3. Recovery of PLA monomers.
4. Recycling of mixed plastics.
Details are as follows:
I. Combined Separation Technology for Purifying Monomers after PET Chemical Recycling
Distillation Technology: Recovery of monomers and solvents.
Melt Crystallization Technology: Purification of DMT/BHET.
II. Chemical Recycling of Polyamide Nylon
The coupling of distillation and layer melt crystallization technology is used to separate the mixture obtained after the depolymerization of polyamide nylon, resulting in high-purity caprolactam products, ensuring the production of high-quality polyamide for further polymerization.
Figure 3: Chemical Recycling Process of Polyamide Nylon
III. Solvent Recovery (Low Volatility Organic Polymers & Solvents)
• Separation of Polymers/Solvents
DODGEN’s evaporation technology is used to separate polymer/solvent mixtures with a wide viscosity range. It ensures low residue concentration for all thermoplastic polymers.
• Purification of Solvents
Recycled solvents require further purification for reuse in the dissolution stage. Evaporation, distillation, or liquid-liquid extraction technologies can be employed to remove impurities from the solvent.
IV. Polylactic Acid (PLA) Recycling
The depolymerized polylactic acid produces azeotropes that require powerful technical methods for separation.
V. Recycling of Mixed Plastics
The following diagram shows the recycling process for mixed plastics:
Figure 4: Process Flow for Mixed Plastic Recycling
Chemical fluid separation technology is a key challenge in plastic chemical recycling. It plays a crucial role in obtaining high-quality polymers, ensuring product quality, and improving the feasibility of material processing. DODGEN is the Chairman of the Degradable and Recycling Subcommittee of the Shanghai New Materials Association and has been deeply involved in this industry. DODGEN is eager to collaborate with companies focused on environmental protection and the circular economy to contribute to plastic recycling and utilization. We welcome partners from various fields to join us in advancing the commercialization of chemical recycling.
