What is LLDPE plastic film and what are its main uses?
Production and characteristics The production of LLDPE starts with transition metal catalysts, especially of the Ziegler or Phillips type. The new process based on cycloolefin metal derivative catalysts is another option for LLDPE production. The actual polymerization reaction can be carried out in solution and gas phase reactors. Generally, octene and ethylene are copolymerized in a solution phase reactor, butene. Hexene and ethylene are polymerized in a gas-phase reactor.
The LLDPE resin produced in the gas-phase reactor is in the form of granules and can be sold as powder or further processed into pellets. A new generation of ultra-LLDPE based on hexene and octene has been produced by Mobile and Union Carbide. Companies such as Novacor and Dow Plastics launched. These materials have great toughness limits and have new potential in the application of automatic bag removal. Very low density PE resin (density below 0.910g / cc.) Has also appeared in recent years. VLDPES has flexibility and softness that LLDPE cannot reach. The characteristics of resins are generally reflected in the melt index and density. The melt index can reflect 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 MWD. The density is determined by the concentration of the comonomer in the polyethylene chain. The comonomer concentration controls the number of short branches (the length of which depends on the comonomer type) to control the resin density.
The higher the comonomer concentration, the lower the resin density. Structurally, LLDPE differs from LDPE in the number and type of branches, high-pressure LDPE has long branches, while linear LDPE only has short branches. Structurally, LLDPE differs from HDPE only in the number of short branches. HDPE has a small number of short branches, so it is a higher density material. The physical properties of LLDPE are controlled by its molecular weight, MWD and density. LLDPE is superior to LDPE and ultimately depends on its use. Generally, LLDPE is used in all applications to produce more rigid products, although according to the ATSM standard for low-density materials, the density of LLDPE and LDPE is between 0.91 and 0.925. LLDPE forms a higher crystalline structure because there are no long branches. LLDPE's greater crystallinity results in higher rigidity products. This higher crystallinity also increases the melting point of LLDPE compared to LDPE by 10-15 ° C. Higher tensile strength, penetration resistance, tear resistance and increased elongation are the characteristics of LLDPE, making it particularly suitable for film making.
If hexene or octene are used instead of butene as the comonomer, even impact resistance and tear resistance can be greatly improved. For a given resin at the same melt index and density, the hexene and octene LLDPE resins improved to 300% in impact and tear properties. The longer side chains of hexene and octene resin act like "knotted" molecules between the chains, improving the toughness of the compound. The production of resins with cycloolefin metal derivative catalysts will have unique properties. Narrower MWD, improved comonomer distribution, better film transparency, sealing and impact strength, which are similar to LLDPE produced with Ziegler catalysts. In terms of transparency, LLDPE has similar disadvantages as LDPE. The turbidity and gloss of LLDPE film is not good, mainly because its higher crystallinity causes the film surface roughness. The transparency of LLDPE resin can be improved by blending with a small amount of LDPE. Processing Both LDPE and LLDPE have excellent rheological or melt flow properties. LLDPE has less shear sensitivity because it has a narrow molecular weight distribution and short branches. During shearing (eg extrusion), LLDPE maintains 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 makes the stress of the polymer molecular chain relax faster, and thus the sensitivity of the physical properties to the change of the inflation ratio is reduced in the melt extension, LLDPE in various The strain rate usually has a lower viscosity. In other words, it will not produce strain hardening during stretching like LDPE. As the deformation rate of polyethylene increases. LDPE shows a surprising increase in viscosity, which is caused by molecular chain entanglement. This phenomenon cannot be observed in LLDPE because the lack of long-chain branches in LLDPE prevents the polymer from becoming entangled. This property is extremely important for thin film applications. Because LLDPE film is easy to make thinner film while maintaining high strength and toughness. The rheology of nLLDPE can be summarized as "rigid when sheared" and "soft when stretched". When replacing LLDPE with LLDPE, film extrusion equipment and conditions must be modified. The high viscosity of LLDPE requires greater power of the extruder. And provide higher melt temperature and pressure. The die gap must be widened to avoid a reduction in production due to high back pressure and melt fracture. The general die gap size of LDPE and LLDPE are respectively O. 024 ～ 0. 040 in. And 0. 060-0. 10in.
The "softness of stretch" of LLDPE is a disadvantage during the film blowing process. The blown film bubble of LLDPE is not as stable as LDPE. The general single-lip wind ring is sufficient for the stability of LDPE. The unique bubble of LLDPE requires a more perfect double-lip wind ring to stabilize. Cooling the internal bubble with a double-lip air ring can increase the bubble stability and at the same time increase the film production capacity at high productivity. In addition to better cooling of the film bubble, many film production plants use the blending method with LDPE to enhance the melting path of LLDPE. LLDPE extrusion can be completed on the existing LDPE film equipment. When it reaches 50%. When processing 100% LLDPE or LLDPE-rich blended materials with LDPE, the general LDPE extruder is used, and equipment must be improved. According to the life of the extruder, the 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, there is generally no need to improve equipment, but processing conditions need to be optimized. Rotational molding requires LLDPE to be ground into uniform particles (35 mesh). The process includes 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. Applications LLDPE has penetrated most of the traditional markets of polyethylene, including films, moldings, pipes and wires and cables. The anti-leakage mulch is a newly developed LLDPE municipal mulch, a large-scale extruded sheet used as a waste landfill and waste pool liner to prevent leakage or contamination of the surrounding area. Some of LLDPE's film markets, such as production bags, trash bags, elastic packaging, industrial liners, towel liners and shopping bags, all take advantage of this resin with improved strength and toughness. Transparent films, such as bread bags, have been dominated by LDPE because of its better turbidity. However, blends of LLDPE and LDPE will improve strength. Penetration resistance and LDPE film stiffness without significantly affecting film transparency.