Polymerization Process, Structural Characteristics, and the Fundamental Raw Material Status of Polyethylene in the Plastics Industry

The large-scale production, low cost, and good processing adaptability of polyethylene resin make it a pillar material of the modern plastics industry, supporting the development of numerous sectors including packaging, agriculture, construction, electronics, and automobiles.

As the most widely used synthetic resin globally, polyethylene's polymerization process and molecular structure determine the diversity and adjustability of its product properties. Polyethylene production uses ethylene as a monomer, and polymerizes into high molecular chains under the action of a catalyst. Depending on the polymerization pressure, it is mainly divided into two processes: high-pressure and low-pressure. The high-pressure process produces low-density polyethylene (LDPE) at reaction pressures as high as 1500 to 3000 atmospheres, employing a free radical polymerization mechanism. The molecular chains have more branches, resulting in lower density and better flexibility. The low-pressure process produces high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) at reaction pressures of only 1 to 5 atmospheres, using Ziegler-Natta or chromium-based catalysts. The molecular chains have fewer branches, a regular structure, higher density, higher crystallinity, and better rigidity and strength. The powder after polymerization is granulated into granular resin for easy transportation and processing.

The structural characteristics of polyethylene determine its performance characteristics. Polyethylene is composed of carbon and hydrogen elements, with saturated molecular chains, good chemical stability, and resistance to acids, alkalis, and organic solvents. The crystallinity of polyethylene can be adjusted through polymerization processes and comonomers. Crystallinity affects the material's density, hardness, heat resistance, and barrier properties. Low-density polyethylene (LDPE) has lower crystallinity, more flexible molecular chains, and better transparency, making it suitable for films and flexible products. High-density polyethylene (HDPE) has higher crystallinity, greater rigidity, and better heat resistance, making it suitable for containers and pipes. Linear low-density polyethylene combines the advantages of both, exhibiting high tensile strength and puncture resistance. The molecular weight and distribution of polyethylene also affect its processing and mechanical properties. Resins with a wide molecular weight distribution have better flowability and are suitable for extrusion molding; resins with a narrow molecular weight distribution have higher impact strength and are suitable for injection molding.

Polyethylene's fundamental role as a raw material in the plastics industry is reflected in its huge production volume and wide range of applications. Polyethylene resin can be processed into various products through various molding processes such as blow molding, injection molding, extrusion, and rotational molding. Blow-molded films are the largest application area for polyethylene, used to make various packaging bags, agricultural films, and garbage bags. Injection-molded products include containers, toys, household utensils, and automotive parts. Extruded products include pipes, profiles, wire and cable insulation, and geomembranes. Hollow blow-molded products include various bottles, barrels, oil tanks, and large storage tanks. Polyethylene can also have its performance improved and its applications expanded through modification technologies such as crosslinking, foaming, and blending. The large-scale production, low cost, and good processing adaptability of polyethylene resin make it a pillar material of the modern plastics industry, supporting the development of numerous sectors including packaging, agriculture, construction, electronics, and automobiles.