Linear low-density polyethylene (LLDPE) is a molecular structure formed by copolymerization of ethylene and a small amount of α-olefin on the main chain of linear ethylene with very short comonomer branches.
Linear low-density polyethylene is a non-toxic, tasteless, odorless milky white particle with a density of 0.918~0.935g/cm³. Compared with LDPE, it has higher softening temperature and melting temperature, and has the advantages of high strength, good toughness, high rigidity, good heat resistance and cold resistance. It also has good resistance to environmental stress cracking, impact strength, tear strength and other properties, and is resistant to acids, alkalis, organic solvents, etc., and is widely used in industry, agriculture, medicine, health and daily necessities.
Structural features
Linear low-density polyethylene (LLDPE) resin is known as the third generation of polyethylene. In addition to the properties of general polyolefin resins, it has particularly excellent tensile strength, tear strength, environmental stress cracking resistance, low temperature resistance, heat resistance and puncture resistance.
Although LLDPE and the previous LDPE belong to the same density range, their molecular structures are different, their melt rheological behaviors are also different, and therefore their basic physical properties and molding processing characteristics are also different.
Structurally, LLDPE differs from HDPE only in the number of short chain branches. HDPE has fewer short chain branches, which ultimately depends on its application. The density of LLDPE and LDPE is between 0.91g/cm³ and 0.925g/cm³. This higher crystallinity also increases the melting point of LLDPE by 10 to 15% compared to LDPE. Higher tensile strength, penetration resistance, tear resistance and increased elongation are the characteristics of LLDPE, making it particularly suitable for film production. Even impact resistance and tear resistance can be greatly improved if hexene or octene is used instead of butene as a comonomer. The longer side chains of hexene and octene resins act like “knot” molecules between the chains, improving the toughness of the compound. The resin produced using cycloolefin metal derivative catalysts will have unique properties. These are similar to LLDPE produced using Ziegler catalysts.
In terms of transparency, LLDPE has similar disadvantages to LDPE: the turbidity and gloss of LLDPE film are poor, mainly because its higher crystallinity causes the surface roughness of the film. The transparency of LLDPE resin can be improved by blending with a small amount of LDPE.
Production
Linear low-density polyethylene uses ethylene as the main raw material and a small amount of α-olefins (such as 1-butene-1, 1-octene, etc.) through high-pressure or low-pressure gas-phase fluidized bed polymerization under the action of a catalyst. The structural formula is -[CH2-CH2]n-. The reacted material is granulated, dried, and sent for packaging.
The production of LLDPE begins with transition metal catalysts, typically of the Ziegler or Phillips type. Newer processes based on cycloolefin metal derivative catalysts are another option for LLDPE production. The actual polymerization reaction can be carried out in solution and gas phase reactors. Typically, octene is copolymerized with ethylene in a solution phase reactor with butene. Hexene is polymerized with ethylene in a gas phase reactor. The LLDPE resin produced in a gas phase reactor is in granular form and can be sold as a powder or further processed into pellets. Linear low density polyethylene is usually characterized by melt index and density. The melt index reflects the average molecular weight of the resin and is mainly controlled by the reaction temperature. The average molecular weight is independent of the molecular weight distribution (MWD). The choice of catalyst affects the MWD. The density is determined by the concentration of the comonomer in the polyethylene chain. The comonomer concentration controls the number of short chain branches (whose length depends on the type of comonomer) and thus the resin density. High pressure LDPE has long chain branches, while linear LDPE has only short chain branches.
Processing
Both LDPE and LLDPE have excellent rheological or melt flow properties. LLDPE is less shear sensitive because of its narrow molecular weight distribution and short chain branches. LLDPE retains a greater viscosity during shearing and is therefore more difficult to process than LDPE of the same melt index. In extrusion, LLDPE’s lower shear sensitivity results in faster stress relaxation of the polymer chains and, as a result, less sensitivity of physical properties to changes in the blow ratio. In melt extension, LLDPE generally has a lower viscosity at a variety of strain rates. This means that it will not strain harden as LDPE does when stretched.
As the deformation rate of polyethylene increases, LDPE shows a dramatic increase in viscosity, which is caused by molecular chain entanglement. This phenomenon is not observed in LLDPE because the lack of long chain branches in LLDPE prevents the polymer from entanglement. This property is extremely important for film applications. LLDPE film is 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”. When LLDPE is used instead of LDPE, film extrusion equipment and conditions must be modified. The high viscosity of LLDPE requires a higher power 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 general die gap sizes for LDPE and LLDPE are 0.024~0.040 and 0.060~0.10 respectively.
The “softness when stretched” property of LLDPE is a disadvantage in the blown film process. The blown film bubble of LLDPE is not as stable as that of LDPE. The conventional single-lip air ring is sufficient for stabilizing LDPE. The unique bubble of LLDPE requires a more sophisticated double-lip air ring for stability. Using a double-lip air ring to cool the internal bubble increases bubble stability and improves film production capacity at high production rates. When using LLDPE or LLDPE-rich blends with LDPE, conventional LDPE extruders must be modified. Depending on the life of the extruder, the required modification may be to widen the die gap, but the 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.
Packaging, storage and transportation
The product is packed in polyethylene heavy-duty film bags, and the outer packaging is polypropylene woven bags, with a net weight of 25kg per bag. The storage warehouse should be kept clean, dry, cool and well ventilated. It can be transported by train, car, ship, airplane, etc. It can be transported as non-dangerous goods. The transportation vehicle should be kept clean and dry, and no sharp objects such as nails should be allowed. During storage and transportation, attention should be paid to fire prevention, waterproofing, sun protection, dust prevention and pollution prevention. Iron hooks should not be used during loading and unloading.
Applications
LLDPE has penetrated most traditional markets for polyethylene, including film, molding, pipe, and wire and cable. Leakage-proof ground film is a newly developed LLDPE market. Ground film, a large extruded sheet, is used as a liner for landfills and waste pools to prevent leakage or contamination of surrounding areas.
Some of the film markets for LLDPE, such as bags, trash bags, elastic packaging, industrial liners, towel liners, and shopping bags, take advantage of the improved strength and toughness of this resin. Transparent film. The penetration resistance and stiffness of LDPE film are not significantly affected by the film’s clarity. Injection molding and rotational molding are the two largest molding applications for LLDPE. The superior toughness and low temperature, impact strength of this resin are ideal for waste containers, toys, and refrigerated appliances. In addition, LLDPE’s high environmental stress cracking resistance makes it suitable for injection molded lids that come into contact with oily foods, and rotational molded waste containers, fuel tanks, and chemical tanks. There is a smaller market for pipe and wire and cable coatings, where the high burst strength and environmental stress cracking resistance provided by LLDPE meet the requirements. 65% to 70% of LLDPE is used to make films.
The LLDPE polymer produced by the copolymerization process has a narrower molecular weight distribution than general LDPE, and its linear structure gives it different rheological properties. The melt flow characteristics of LLDPE meet the requirements of new processes, especially the film extrusion process, which can produce high-quality LLDPE products. LLDPE is used in all traditional markets for polyethylene, and has enhanced elongation, penetration, impact and tear resistance. Its excellent resistance to environmental stress cracking, low-temperature impact and warping make LLDPE attractive for pipe, sheet extrusion and all molding applications. The latest application of LLDPE is as a ground film for the lining of waste landfills and waste liquid pools.
