Concepts regarding high-pressure polyethylene

High-pressure polyethylene is not high-density polyethylene. Based on the pressure in the production equipment, polyethylene can be divided into high-pressure polyethylene (with a reaction pressure of around 100–250 MPa, produced through free-radical polymerization, and resulting in low-density polyethylene) and low-pressure polyethylene (with a pressure generally below 2 MPa, produced through coordination polymerization, and resulting in high-density polyethylene or linear low-density polyethylene). Based on product density, it can be divided into low-density polyethylene (density 0.91–0.925 g/cm3), high-density polyethylene (0.941–0.970 g/cm3), and medium-density polyethylene, which falls between the two (density 0.926–0.940 g/cm3).

What I know else is this: high-pressure polyethylene refers to LDPE, which is also low-density polyethylene； Low-pressure high-density polyethylene is HDPE ； Another type is low-pressure linear low-density polyethylene LLDPE. Hehe, it’s more or less the same as what was said above: lol

juyi* Polyethylene is a thermoplastic resin obtained by the polymerization of ethylene. □ It is abbreviated as PE. In industry, this also includes copolymers of ethylene and small amounts of □-olefins. Polyethylene is odorless and non-toxic; it has a waxy texture. It boasts excellent low-temperature resistance (with a minimum operating temperature of -70 to -100°C), good chemical stability, and resistance to the corrosion of most acids and bases (except those with oxidizing properties). It is insoluble in common solvents at room temperature, has low water absorption, and exhibits excellent electrical insulation properties ； However, polyethylene is very sensitive to environmental stresses (chemical and mechanical effects), and it has poor resistance to thermal aging. The properties of polyethylene vary depending on the type, primarily depending on its molecular structure and density. Different production methods can yield products with varying densities (0.91–0.96 g/cm³). Polyethylene can be processed using the standard molding methods for thermoplastics (see plastic processing). It has a wide range of applications, mainly used in the manufacture of films, containers, pipes, filaments, wires and cables, as well as daily household items; it can also serve as a high-frequency insulating material for televisions, radars, and other devices. With the development of the petrochemical industry, polyethylene production has grown rapidly, accounting for about 1/4 of the total plastic production. In 1983, the total global production capacity for polyethylene was 24.65 Mt, with the capacity of plants under construction amounting to 3.16 Mt. History: In 1933, the British company Bunneman Chemical Industries discovered that ethylene could be polymerized under high pressure to produce polyethylene. This method was industrialized in 1939 and is commonly known as the high-pressure method. In 1953, K. Ziegler in West Germany discovered that ethylene could also be polymerized at lower pressures using TiCl₄-Al(C₂H₅)₃ as a catalyst. This method was put into industrial production by the German company Hoesch in 1955, and is commonly known as low-pressure polyethylene. In the early 1950s, Phillips Petroleum in the United States discovered that ethylene could be polymerized into high-density polyethylene under medium pressure using chromium oxide-silicoalumina gel as a catalyst, and industrial production was established in 1957. In the 1960s, DuPont in Canada began producing low-density polyethylene using solutions of ethylene and □-olefins. In 1977, Union Carbide and Dow Chemical Company developed low-density polyethylene using the low-pressure process, which was known as linear low-density polyethylene; among these methods, Union Carbide’s gas-phase process was the most significant. Linear low-density polyethylene has properties similar to those of low-density polyethylene, while also possessing several characteristics of high-density polyethylene. Coupled with its low energy consumption during production, it has developed very rapidly and has become one of the most notable new synthetic resins. The core technology of the low-pressure method lies in the catalyst. The TiCl□-Al(C□H□)□ system invented by Ziegler in Germany was the first generation of catalysts for polyolefins; it had low catalytic efficiency, yielding only a few kilograms of polyethylene per gram of titanium. In 1963, the Belgian company Solvay developed the second generation of catalysts using magnesium compounds as carriers; these catalysts achieved a catalytic efficiency that allowed tens of thousands to hundreds of thousands of grams of polyethylene to be produced per gram of titanium. Using a second-generation catalyst also eliminates the post-treatment step of removing catalyst residues. Later, highly efficient catalysts based on the gas-phase method were developed. In 1975, the Italian company Montedison developed a catalyst that allowed for the direct production of spherical polyethylene without the need for granulation; this catalyst was known as the third-generation catalyst and represented another breakthrough in the production of high-density polyethylene. Classification: There are various methods of classification, with the main one being based on density (Figure 1: Properties of polyethylene). ① High-density polyethylene is an opaque white powder; after granulation, it takes the form of milky-white particles. Its molecules have a linear structure with little branching, making it a typical crystalline polymer. Its mechanical properties are all superior to those of low-density polyethylene; its melting point is higher than that of low-density polyethylene, at around 126–136°C, while its embrittlement temperature is lower, at around -100–-140°C. ②Low-density polyethylene is a colorless, translucent pellet with long side chains in its molecules, and the molecules are not arranged closely together. ③Linear low-density polyethylene typically contains only short side chains in its molecules; its mechanical properties lie between those of high-density and low-density polyethylene. Its melting point is 15°C higher than that of ordinary low-density polyethylene, it has better low-temperature resistance as well, and its resistance to environmental stress cracking is several dozen times greater than that of ordinary low-density polyethylene. Furthermore, based on the production method, polyethylene can be classified into low-pressure polyethylene, medium-pressure polyethylene, and high-pressure polyethylene (Table 1: Classification of PE by production method). Different production methods result in varying densities and melt indices (which indicate fluidity) for polyethylene (Figure 2: Density and melt index of polyethylene produced by different methods). Based on molecular weight, it can be divided into low-molecular-weight polyethylene, medium-molecular-weight polyethylene, and ultra-high-molecular-weight polyethylene (Table 2: Classification of PE by molecular weight). Production methods are divided into three types: high-pressure method, low-pressure method, and medium-pressure method. The high-pressure method is used to produce low-density polyethylene; it was developed early on, and polyethylene produced by this method accounts for about 2/3 of the total polyethylene production to this day. However, with the advancement of production technologies and catalysts, its growth rate has **fallen behind that of the low-pressure method**. In terms of its implementation methods, the low-pressure process includes the slurry method, the solution method, and the gas-phase method. The slurry method is mainly used to produce high-density polyethylene, while the solution method and the gas-phase method can not only produce high-density polyethylene but also, by adding comonomers, produce medium- and low-density polyethylene, also known as linear low-density polyethylene. In recent years, various low-pressure process technologies have developed rapidly. The medium-pressure method is still used only by Philips to this day, primarily for producing high-density polyethylene. The high-pressure method is a process in which oxygen or peroxides are used as initiators to polymerize ethylene into low-density polyethylene. After secondary compression, ethylene enters the reactor (Figure 3: High-pressure polyethylene production process), where it polymerizes into polyethylene under pressures of 100–300 MPa and temperatures of 200–300°C, with the aid of an initiator. The reaction mixture is then separated under reduced pressure to recover the unreacted ethylene for reuse, while the molten polyethylene is extruded and granulated after the addition of plastic additives. (See the color illustration of the high-pressure polyethylene production plant.) The polymerization reactors used include tubular reactors (with a tube length of up to 2000 m) and batch reactors. The one-pass conversion rate for the tubular process is 20% to 34%, with an annual production capacity of 100 kt per line. The one-pass conversion rate for the kettle process is 20%–25%, with an annual production capacity of 180kt per unit. The low-pressure method includes three types: the slurry method, the solution method, and the gas-phase method. Except for the solution method, the polymerization pressure is below 2 MPa in all cases. The general steps include catalyst preparation, ethylene polymerization, polymer separation, and granulation. ①Polyethylene produced by the slurry method is insoluble in solvents and appears in a slurry form. Slurry polymerization features mild conditions and is easy to operate; alkyl aluminum is commonly used as an activator, hydrogen as a molecular weight regulator, and batch reactors are typically employed. The polymer slurry coming out of the polymerization reactor passes through a flash evaporator and a gas-liquid separator before reaching the powder dryer, and then it is granulated (Figure 4: Low-pressure polyethylene slurry process). The production process also includes steps such as solvent recovery and solvent purification. By using different combinations of polymerization reactors in series or parallel, products with varying molecular weight distributions can be obtained. ②The solution method of polymerization takes place in a solvent; however, both ethylene and polyethylene are soluble in this solvent, resulting in a homogeneous solution as the reaction system. The reaction temperature (≥140°C) and pressure (4–5 MPa) are high. It is characterized by a short polymerization time and high production capacity; it can produce polyethylene of high, medium, and low densities, allowing for good control over the properties of the product ； However, the polymers obtained by the solution method have a lower molecular weight, a narrow molecular weight distribution, and a lower solid content. ③In the gas-phase method, ethylene is polymerized in its gaseous state, usually using a fluidized-bed reactor. There are two types of catalysts: chromium-based and titanium-based. They are added in quantified amounts to the bed from storage tanks, and a high-speed ethylene circulation is used to maintain bed fluidization and remove the heat generated by the polymerization reaction. The produced polyethylene is discharged from the bottom of the reactor (Figure 5: Low-pressure polyethylene gas-phase process). The pressure in the reactor is about 2 MPa, and the temperature is 85–100°C. The vapor phase method is the primary approach for producing linear low-density polyethylene. It eliminates processes such as solvent recovery and polymer drying, and reduces investment costs by 15% and operating costs by 10% compared to the solution method. 30% of the investment for traditional high-pressure methods, and 1/6 of the operating costs. Thus, it developed rapidly. However, the gas-phase method needs further improvement in terms of product quality and variety. The medium-pressure method involves using a chromium-based catalyst supported on silica gel to polymerize ethylene at medium pressure in a loop reactor, thereby producing high-density polyethylene. Processing and applications: It can be processed using methods such as blow molding, extrusion, and injection molding, and is widely used in the production of films, hollow products, fibers, and various household items. In actual production, to improve the stability of polyethylene against ultraviolet rays and oxidation, as well as to enhance its processing and performance characteristics, small amounts of plastic additives must be added. Commonly used UV absorbers include o-hydroxydiphenylmethane or its alkoxy derivatives, etc., while carbon black is an excellent UV shield. In addition, antioxidants, lubricants, colorants, and others are also added, further expanding the range of applications for polyethylene. More than half of the total production of low-density polyethylene in film form is manufactured by blow molding; such films possess good transparency and a certain level of tensile strength, and they are widely used as packaging materials for various foods, clothing, pharmaceuticals, fertilizers, industrial products, as well as in agricultural applications (see the color illustrations of polyethylene film greenhouses). It can also be processed by extrusion into composite films for packaging heavy items. Since 1975, high-density polyethylene film has also been developed; it features high strength, low-temperature resistance, moisture resistance, as well as good printability and processability. The most common use of linear low-density polyethylene is also in the production of films. It possesses greater strength and toughness than low-density polyethylene, as well as better resistance to puncture and rigidity. Although its transparency is lower, it is still slightly better than that of high-density polyethylene. Furthermore, a polyethylene coating can also be extruded and applied onto paper, aluminum foil, or other plastic films to create polymer composite materials. Hollow products: High-density polyethylene has high strength, making it suitable for use in hollow products. It can be used to manufacture containers such as bottles, barrels, tanks, and troughs by blow molding, or large-scale containers like tank cars and storage tanks by casting. Polyethylene pipes can be produced by the extrusion method; high-density polyethylene pipes have high strength and are suitable for use underground. The extruded sheets can be further processed. High-density polyethylene can also be converted into low-foam plastics using foam extrusion and foam injection methods, for use as tabletops and building materials (see Polymer Materials for Construction). Fibers are known as polyethylene in China; they are generally produced by spinning low-pressure polyethylene into synthetic fibers. Polyethylene is mainly used to produce fishing nets and ropes, or it can be spun into short fibers for use as stuffing, and it is also employed in industrial acid- and alkali-resistant fabrics. Ultra-high strength polyethylene fibers (with strengths of 3–4 GPa) have now been developed and can be used in bulletproof vests, as well as in composite materials for use in automobiles and marine applications. Miscellaneous items: Miscellaneous items produced by injection molding include daily-use goods, artificial flowers, turnover boxes (see the color illustration of polyethylene food turnover boxes), small containers, and parts for bicycles and tractors. High-density polyethylene should be used in manufacturing structural components. Polyethylene modification: The modified variants of polyethylene mainly include chlorinated polyethylene, chlorosulfonated polyethylene, cross-linked polyethylene, and blended modified variants. Chlorinated polyethylene is a random chloride obtained by partially replacing the hydrogen atoms in polyethylene with chlorine. Chlorination is carried out under the initiation of light or peroxides, and the aqueous suspension method is primarily used in industry for production. Due to differences in the molecular weight and distribution of the polyethylene precursor, as well as its degree of branching, the degree of chlorination after chlorination, the distribution of chlorine atoms, and the residual crystallinity, chlorinated polyethylene with properties ranging from rubbery to rigid plastic-like can be obtained. Its main use is as a modifier for polyvinyl chloride to improve its impact resistance. Chlorinated polyethylene can also be used as an electrical insulating material and a floor covering. Chlorosulfonated polyethylene is obtained when polyethylene reacts with chlorine containing sulfur dioxide; as a result, some of the hydrogen atoms in the molecule are replaced by chlorine and small amounts of sulfuryl chloride (—SO₂Cl) groups. The main industrial method is the suspension process. Chlorosulfonated polyethylene exhibits good resistance to ozone, chemical corrosion, oils, heat, light, wear, as well as high tensile strength; it is an elastomer with excellent overall properties, suitable for use in manufacturing components of equipment that come into contact with food. Cross-linked polyethylene is obtained by using radiation methods (such as X-rays, electron beams, or ultraviolet light) or chemical methods (polyperoxides or silicone cross-linking) to transform linear polyethylene into a networked or three-dimensional form of cross-linked polyethylene. Among them, the silicone crosslinking method features a simple process and low operating costs, and the molding and crosslinking steps can be carried out separately; therefore, blow molding and injection molding are suitable for use. Cross-linked polyethylene exhibits significantly improved heat resistance, environmental stress cracking resistance, and mechanical properties compared to polyethylene, making it suitable for use in large-scale pipes, cables and wires, as well as rotational molding products. Blending modification of polyethylene: By blending linear low-density polyethylene with low-density polyethylene, it can be used to manufacture films and other products, and the performance of these products is better than that of low-density polyethylene. Blending polyethylene with EPDM can produce thermoplastic elastomers with a wide range of applications. Bibliography: C.E. Hilsner*et al., I. Skested (eds.), translated by Tang Shipeng et al.: Polymerization Processes, Chemical Industry Press, Beijing, 1984. (C.E.Schildknecht and I.Skeist, Polymerization Processes, John Wiley & Sons, New York, 1977.)　　　　　　　　　　　　　　(Tang Shipeng, Song Xian□)

The two are not the same concept; it was clearly explained on the second floor

Thank you to the original poster for posting this, and thank you to everyone who responded – it has allowed me to learn so much.

The 4th floor explanation was very detailed; I learned a lot from it