In-Depth Analysis: The Scientific Logic and Industrial Value Behind Chemical Recycling of Waste Plastics

The "Plasticity" of Plastic Molecules: Innate Conditions for Recycling

Common plastics in our daily lives, such as polyethylene (PE), polypropylene (PP), and polystyrene (PS), are all polymer compounds composed of carbon and hydrogen elements. Their molecules consist of long chains of carbon and hydrogen atoms, and different plastics vary in the length, structure, and bonding patterns of these chains.

For example, polyethylene is composed of a large number of ethylene monomers (C₂H₄) connected by covalent bonds to form a linear long chain with a relatively simple and regular structure; polypropylene is polymerized from propylene monomers (C₃H₆), with methyl side chains attached to the molecular chain. This hydrocarbon-based molecular structure determines the relationship between plastics and petroleum. It should be noted that petroleum can be processed to obtain various hydrocarbons such as gasoline, diesel, lubricants, etc., and plastics themselves are products of petrochemical processes. Therefore, essentially, waste plastics have the potential to be converted into products similar to petroleum derivatives.

This hydrocarbon property makes the chemical recycling of waste plastics possible. Through specific chemical methods, we can break the covalent bonds in plastic molecules, decomposing the long hydrocarbon chains into fragments of different lengths. These fragments, after further processing, can be converted into products such as base oil, combustible gas, and carbon black.

The core process of chemical recycling: "deconstruction and reorganization" of molecular chains

The chemical recycling of waste plastics is a complex and precise process, with its core being the "decomposition" and "reorganization" of plastic molecular chains. This process mainly includes steps such as pretreatment, cracking, and separation and purification.

In the pretreatment stage, the collected waste plastics need to be sorted and crushed. Different types of plastics have varying chemical properties and recycling processes; sorting can improve recycling efficiency and product quality. Crushing breaks large pieces of plastic into smaller ones, increasing the contact area with the reaction medium, which is beneficial for the reaction process.

Pyrolysis is a key step in chemical recycling. It is the process in which plastic molecular chains break under the action of high temperature, high pressure, or catalysts. According to different reaction conditions, pyrolysis can be divided into thermal pyrolysis and catalytic pyrolysis. Thermal pyrolysis mainly relies on high temperature to break molecular chains, usually between 300-900°C; catalytic pyrolysis, on the other hand, lowers the activation energy of the reaction under the action of catalysts, allowing molecular chains to break at relatively lower temperatures while also improving the selectivity of target products.

During the cracking process, plastic molecular chains break according to certain patterns. Long-chain molecules first break into shorter fragments, which can further break down into even smaller molecules. Different reaction conditions affect the degree of chain scission and the distribution of products. For example, at lower temperatures (400-500°C) and with suitable catalysts, the formation of longer hydrocarbon chain fragments is favored. After separation and purification, these fragments can be used as base oils. When the temperature rises above 600°C, the molecular chains break more thoroughly, resulting in more short-chain hydrocarbons such as methane, ethane, and propane, which are the main components of combustible gases. In a high-temperature, oxygen-deficient environment, some plastic molecules undergo carbonization reactions, producing carbon black.

The separation and purification stage involves separating the mixture produced by cracking according to different properties such as boiling point and density, obtaining base oil, combustible gas, and carbon black with higher purity. This stage requires the use of various separation techniques such as distillation, adsorption, and filtration to ensure that the product quality meets the relevant standards.

Plastic Recycling: Different Types, Distinct Characteristics

Different types of plastics exhibit different characteristics in the chemical recycling process due to differences in their molecular structures and chemical properties.

Polyethylene (PE) is one of the easiest plastics to chemically recycle. Its molecular structure is simple, with a reasonable carbon-to-hydrogen ratio, making it prone to producing a larger amount of liquid products, namely base oils, during the cracking process. Studies show that 1 ton of waste polyethylene plastic can yield 600-700 liters of base oil through appropriate processes. The performance of these base oils is comparable to traditional petroleum-based base oils and can be widely used in fields such as lubricants and hydraulic oils.

The chemical recycling of polypropylene (PP) also holds high value. During pyrolysis, it generates a significant amount of propylene and other olefin compounds, which can be used not only as combustible gases but also as chemical raw materials for producing new plastics. In addition, the liquid products produced from the pyrolysis of polypropylene can be processed and used as base oil.

The molecular structure of polystyrene (PS) contains benzene rings, making it prone to generating styrene monomer during the cracking process, which is an important raw material for the production of polystyrene. At the same time, polystyrene can also produce a certain amount of combustible gas and carbon black through cracking.

Chemical recycling of polyvinyl chloride (PVC) is relatively complex because its molecules contain chlorine elements. During the pyrolysis process, chlorine is released in the form of hydrogen chloride, which can cause equipment corrosion and environmental pollution. Therefore, in the chemical recycling of PVC, dechlorination treatment is required first, which increases the cost and difficulty of recycling. However, with continuous technological advancements, significant progress has been made in the chemical recycling of PVC, with improved dechlorination efficiency, making its recycling possible.