Low density polyethylene (LDPE)

Low density polyethylene (LDPE)

August 23, 2024

wanplas

Low-density polyethylene, also known as high-pressure polyethylene (LDPE), is the lightest variety of polyethylene resin. It is milky white, tasteless, odorless, non-toxic, and matte waxy particles. It has good softness, extensibility, electrical insulation, transparency, easy processing, and certain air permeability. It has good chemical stability and is resistant to alkali and general organic solvents.

Overview

High pressure-low density polyethylene (HP-LDPE, abbreviated as LDPE) has been industrialized for more than 70 years. Although the variety and output of polyethylene have made great progress with the discovery and development of olefin polymerization catalysts, high pressure polyethylene still occupies an important position. Ethylene is polymerized into high molecular weight polyethylene by free radical mechanism under high temperature and pressure, which has the following characteristics:

The ethylene polymerization process is a highly exothermic process. The polymerization heat of ethylene is about 93.5 kJ/mol (or 3.3 kJ/g). The specific heat capacity of ethylene is 2.51~2.85 J/(g·℃) at 235 MPa and 150~300℃. If the reaction heat cannot be removed in time, the temperature will rise by 12~13℃ for every 1% of ethylene polymerization. If the temperature is too high, it will also cause ethylene decomposition.
Under high pressure, ethylene has been compressed to a gas-tight phase state with a density of 0.5 g/mL, which is similar to an incompressible liquid. At this time, the distance between ethylene molecules is significantly shortened, thereby increasing the probability of collision between free radicals and ethylene molecules, so polymerization reactions are likely to occur.
At high temperatures, the free radical activity of the growing chain is high, and chain transfer reactions are likely to occur. The resulting polymer is a linear structure with many branches. Usually, there are 20 to 30 branches per 1,000 carbon chain atoms, the crystallinity is 45% to 65%, the density is 0.910 to 0.925 g/mL, it is light, flexible, and has good low temperature resistance and impact resistance.

Related categories

Linear low-density polyethylene (LLDPE) is a substantially linear polymer (polyethylene) with a significant amount of short-chain branching, typically produced by the copolymerization of ethylene with long-chain olefins. Linear low-density polyethylene differs structurally from conventional low-density polyethylene (LDPE) in that long-chain branching is absent. The linearity of LLDPE results from the different manufacturing processes of LLDPE and LDPE. Typically, LLDPE is produced at lower temperatures and pressures by the copolymerization of ethylene with higher alpha-olefins such as butene, hexene, or octene. The copolymerization process produces LLDPE polymers with a narrower molecular weight distribution than conventional LDPE and, combined with the linear structure, has significantly different rheological properties.

Production method

Low-density polyethylene can be divided into high-pressure method and low-pressure method according to the polymerization method. According to the type of reactor, it can be divided into kettle method and tubular method. Ethylene is used as raw material and sent into the reactor. Under the action of initiator, the polymerization reaction is carried out under high pressure compression. After the material coming out of the reactor is removed from the unreacted ethylene by the separator, it is melt-extruded into granules, dried, blended, and sent for packaging.

Both LDPE and LLDPE have very good rheological properties or melt flowability. LLDPE is less shear sensitive because it has a narrow molecular weight distribution and short chain branches. During shear processes (such as extrusion), LLDPE maintains a greater viscosity and is therefore more difficult to process than LDPE of the same melt index. In extrusion, LLDPE’s lower shear sensitivity allows for faster stress relaxation of the polymer molecular chains, and thus reduces the sensitivity of physical properties to changes in the blow ratio.

In melt extension, LLDPE generally has a lower viscosity at all strain rates. That is, it will not strain harden as LDPE does when stretched. LDPE shows a dramatic increase in viscosity as the rate of deformation of polyethylene increases, which is caused by molecular chain entanglement.

This phenomenon is not observed in LLDPE because the lack of long chain branches in LLDPE keeps the polymer from entanglement. This property is extremely important for film applications because LLDPE films are easier to make thinner films while maintaining high strength and toughness. The rheological properties of LLDPE can be summarized as “rigid in shear” and “soft in extension”. Film extrusion equipment and conditions must be modified when replacing LDPE with LLDPE. The high viscosity of LLDPE requires a more powerful extruder and provides higher melt temperature and pressure. The die gap must be widened to avoid reduced production due to high back pressure and melt fracture.

The “soft when stretched” property of LLDPE is a disadvantage in the film blowing process. The blown film bubble of LLDPE is not as stable as that of LDPE. The conventional single-lip air ring is sufficient for the stability of LDPE. The unique bubble of LLDPE requires a more sophisticated double-lip air ring for stability. Cooling the internal bubble with a double-lip air ring increases bubble stability while increasing film production capacity at high production rates. In addition to better cooling of the bubble, many film manufacturers use blending with LDPE to enhance the LLDPE solubility. In principle, LLDPE extrusion can be completed on existing LDPE film equipment when the concentration of LLDPE in the LDPE blend reaches 50%. When processing 100% LLDPE or LLDPE-rich blends with LDPE, using conventional LDPE extruders, equipment must be improved.

Depending on the life of the extruder, improvements may require widening the die gap, improving the air ring, modifying the screw design for better extrusion, and, if necessary, increasing the motor power and torque. For injection molding applications, generally no equipment modification is required, but processing conditions must be optimized. Roto-molding requires that the LLDPE be ground into uniform particles (35 mesh). The process involves filling the mold with powdered LLDPE, heating and biaxially rotating the mold to evenly distribute the LLDPE. After cooling, the product is removed from the mold.

Production characteristics

A series of equipment surrounding the polymerization unit, such as compressors, reactors, separators, pipelines, pumps, etc., are required to be able to be used under ultra-high pressure of more than 100 MPa. Even some of the equipment in the separation and recovery processes are required to operate at 100-350 MPa. Therefore, there are many difficulties in the entire process both in terms of equipment and operation.
The polymerization heat of ethylene is much higher than that of other monomers. In the polymerization reaction, the polymerization rate reaches 10%-20% or even 30%-40% in an instant. Therefore, how to remove the polymerization heat has become an important issue in the process flow and is also one of the keys to improving the single-pass conversion rate and reducing energy consumption.
The viscosity of the polymer product in the reaction system is very high, and polymers are easily deposited on the inner walls of the kettle reactor in the kettle process and the tubular reactor in the tubular process.
There are also certain difficulties in transporting molten polymers. Both reaction pressure and temperature affect the viscosity of the product, so it is necessary to pay close attention to controlling temperature and pressure.
How to effectively remove the low molecular weight polyethylene wax contained in the circulating ethylene coming out of the high-pressure separator is also a problem. To solve these problems, various companies have developed a variety of production processes. According to the type of reactor, it can be divided into two categories: tubular process and kettle process. The main feature of the tubular reactor is that the logistics flows in the tube in a plunger-like manner without backmixing; the reaction temperature varies along the length of the reaction tube, so the reaction temperature has a peak, so the molecular weight distribution of the obtained polyethylene is relatively wide. In the kettle reactor, the materials can be fully mixed, so the reaction temperature is uniform, and the operation can be divided into zones so that each reaction zone has a different temperature, thereby obtaining polyethylene with a narrower molecular weight distribution.

The inner diameter of the tubular polymerization reactor is usually a slender high-pressure alloy steel pipe with an inner diameter of 2.5~2.7cm. In order to increase the single-line production capacity, the pipe diameter is increased to 5.0~7.5cm. The ratio of diameter to length is 1:250~1:40000, and the tubular reactor is 900~1500m long. The reaction pressure is about 200~350MPa, the temperature is 250~330℃, the fluid velocity is 10~15m/s, the single-pass conversion rate is 20%~34%, and the maximum single-line production capacity is 100,000 tons/year. The shape of the kettle reactor has two specifications: slender and short and fat. The ratio of the inner diameter to length of the slender polymerization reactor is 1:20~1:4, while the ratio of the inner diameter to length of the short and fat type is 1:4~1:2. The reaction pressure is usually lower than that of the tubular process, at 110~250MPa, the temperature is 130~280℃, the single-pass conversion rate is 20%~25%, and the maximum single-line production capacity is 180,000 tons/year. The motor that drives the agitator is generally installed inside the reactor to reduce the difficulty in designing the shaft seal of the agitator. With the development of mechanical sealing technology, the motor can also be installed outside the reactor. The reactor has been further enlarged. The reactor of ICI Company has a volume of 1000L, while the reactor of CdF Company in France has a volume of 1600L, which is one of the largest reactors in the world. According to statistics, 55% of the high-pressure polyethylene produced in the world is produced in tubular reactors, and the remaining 45% is produced in tank reactors.

Performance

The main characteristics of low-density polyethylene are as follows:

The film is slightly milky white and transparent, and is soft. Its strength is lower than that of high-density polyethylene, but its impact strength is higher than that of high-density polyethylene.
Cold-resistant, low-temperature-resistant and high-temperature-resistant. Thicker films can withstand the sterilization process of immersion in 90°C hot water.
It has good moisture-proof performance, stable chemical properties, and is insoluble in general solvents.
It has high air permeability, so when used as packaging for easily oxidizable foods, the storage period of its contents should not be too long.
The oil resistance is poor, and the product can slowly swell. When packaging oily foods, the food will have a rancid smell after long-term storage.
Long-term exposure to ultraviolet rays and heat will cause aging, affecting its physical and dielectric properties.
The melting point is 110~115℃, and the processing temperature is 150~210℃. If in an inert gas, the temperature can reach 300℃ and remain stable. However, the melt is prone to degradation when in contact with oxygen.

Application Areas

Application range of low-density polyethylene: Suitable for food packaging such as seasonings, cakes, sugar, candied fruit, biscuits, milk powder, tea, fish floss, etc.; pharmaceutical packaging such as tablets and powders; fiber product packaging such as shirts, clothing, knitted cotton products and chemical fiber products; daily chemical packaging such as washing powder, detergent, and cosmetics. Since the mechanical properties of single-layer PE film are poor, it is usually used as the inner layer of composite packaging bags, that is, the heat-sealable substrate of multi-layer composite film.

Development Prospects

Polyethylene is a crystalline polymer. According to its production method, it can be divided into high-pressure polyethylene, medium-pressure polyethylene, and low-pressure polyethylene, and correspondingly obtain low-density polyethylene (LDPE), medium-density polyethylene (MDPE) and high-density polyethylene (HDPE). Since linear low-density polyethylene has better performance than ordinary low-density polyethylene, it has a faster development speed and tends to replace low-density polyethylene.

Polyethylene plastics for automobiles account for 5-6% of the total amount of automobile plastics, ranking fifth after polyvinyl chloride, ABS, polypropylene, and polyurethane. Polyethylene is mainly used to manufacture air ducts and various storage tanks. In recent years, the amount of polyethylene used in automobiles has basically not increased. However, the trend of lightweighting has promoted the plasticization of fuel tanks, with high molecular weight high density polyethylene (HMWH-DPE) as the main material. Europe has officially used plastic fuel tanks in automobiles. The Federal Republic of Germany achieved the industrialization of plastic fuel tanks earlier. Japan’s research and development work has progressed rapidly, but the automotive industry has taken a cautious attitude towards its industrialization, paying special attention to the developments in the United States. The polyethylene used in the automotive industry is basically medium and low pressure polyethylene.