Processing and Application of LLDPE1

Both LDPE and LLDPE have excellent rheology or melt flow. LLDPE has less shear sensitivity because of its narrow molecular weight distribution and short chain branches. During shearing (eg extrusion), LLDPE retains a greater viscosity and is therefore more difficult to process than LDPE with the same melt index. In extrusion, the lower shear sensitivity of LLDPE allows for faster stress relaxation of the polymer molecular chains, and thus reduces the sensitivity of physical properties to changes in blow-up ratio. Usually has a lower viscosity at the speed. That is, it will not strain harden when stretched like LDPE. With the increase of polyethylene deformation rate. LDPE shows a surprising increase in viscosity, which is caused by entanglement of molecular chains. This phenomenon is not observed in LLDPE because the lack of long chain branches in LLDPE keeps the polymer free from entanglement. This property is extremely important for thin film applications. Because LLDPE films can be easily made into thinner films while maintaining high strength and toughness. The rheology of LLDPE can be summarized as“Rigidity in shear”and“soft when extended”. Film extrusion equipment and conditions must be modified when replacing LDPE with LLDPE. The high viscosity of LLDPE requires more power of the extruder. And provide higher melt temperature and pressure. The die gap must be widened to avoid reduced throughput due to high back pressure and melt fracture. The general die gap size of LDPE and LLDPE is O. 024~0． 040 in. and 0. 060-0. 10in. LLDPE“soft when extended”The characteristic is a disadvantage in the blown film process. Blown film bubbles of LLDPE are not as stable as those of LDPE. The general single-lip wind ring is enough for the stability of LDPE. The unique bubble of LLDPE requires a more complete double-lip air ring to stabilize. Cooling the inner bubble with a double-lip air ring increases bubble stability while increasing film throughput at high production rates. In addition to better cooling of the film bubbles, many film production plants use the blending method with LDPE to enhance the LLDPE solution. In theory, the extrusion of LLDPE can be done on existing LDPE film equipment. When the concentration of LLDPE in the LDPE blend up to 50%. When processing 100% LLDPE or LLDPE-rich blended materials with LDPE, general LDPE extruders are used, and equipment must be improved. According to the life of the extruder, the requirements for improvement may be to widen the die gap, improve the air ring, modify the screw design for better extrusion, and increase the motor power and torque if necessary. For injection molding applications, equipment modifications are generally not required, but processing conditions need to be optimized. Rotational molding requires LLDPE to be ground into uniform particles (35 mesh). Processing involves filling a mold with powdered LLDPE, heating and biaxially rotating the mold to distribute the LLDPE evenly. The product is removed from the mold after cooling. LLDPE has penetrated most traditional markets for polyethylene, including film, molding, pipe, and wire and cable. Anti-seepage mulch is a newly developed LLDPE market mulch, a large extruded sheet used as landfill and waste pool liner to prevent leakage or contamination of surrounding areas. Some of the film markets for LLDPE, such as the production of bags, garbage bags, elastic packaging, industrial liners, towel liners and shopping bags, take advantage of the resin's improved strength and toughness. Clear films, such as bread bags, have been dominated by LDPE because of its better haze. However, a blend of LLDPE and LDPE will improve the strength, penetration resistance and stiffness of the LDPE film without significantly affecting the clarity of the film. Injection molding and rotational molding are the two largest molding applications for LLDPE. The resin's superior toughness and low temperature, impact strength are ideally suited for waste bins, toys and refrigerated appliances. In addition, LLDPE's high resistance to environmental stress cracking makes it suitable for injection molded lids that come into contact with oily foods, rotomolded waste containers, fuel tanks, and chemical tanks. The market for applications in pipe and wire and cable coatings is smaller, where the high burst strength and environmental stress crack resistance offered by LLDPE suffice. Currently, 65%-70% of LLDPE is used to make films.