Linear low density polyethylene, structurally different from general low density polyethylene, because there are no long chain branches. The linearity of LLDPE depends on the different production and processing processes of LLDPE and LDPE. LLDPE is usually formed by the copolymerization of ethylene and higher alpha olefins such as butene, hexene or octene at lower temperature and pressure. The LLDPE polymer produced by the copolymerization process has a narrower molecular weight distribution than general LDPE, and at the same time has a linear structure that makes it have different rheological properties.
melt flow properties
The melt flow characteristics of LLDPE are adapted to the requirements of the new process, especially the film extrusion process, which can produce high quality LLDPE products. LLDPE is used in all traditional markets for polyethylene. Enhanced stretch, penetration, impact and tear resistance properties make LLDPE suitable for films. Its excellent resistance to environmental stress cracking, low temperature impact resistance and warpage resistance make LLDPE attractive for pipe, sheet extrusion and all molding applications. The latest application of LLDPE is as a mulch for landfills and linings for waste ponds.
Production and Characteristics
The production of LLDPE starts with transition metal catalysts, especially of the Ziegler or Phillips type. New 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 and butene in a solution phase reactor. Hexene and ethylene are polymerized in a gas phase reactor. The LLDPE resin produced in the gas phase reactor is in particulate form and can be sold as a powder or further processed into pellets. A new generation of super LLDPE based on hexene and octene has been developed by Mobile, Union Carbide. Companies such as Novacor and Dow Plastics launched. These materials have a large toughness limit and have new potential for automatic bag removal applications. Very low density PE resin (density below 0.910g/cc.) has also appeared in recent years. VLDPES has flexibility and softness that LLDPE cannot achieve. The properties of resins are generally reflected in melt index and density. The melt index reflects the average molecular weight of the resin and is primarily controlled by the reaction temperature. Average molecular weight is independent of molecular weight distribution (MWD). Catalyst selection affects MWD. Density is determined by the concentration of comonomer in the polyethylene chain. The comonomer concentration controls the number of short chain branches (the length of which depends on the comonomer type) and thus controls the resin density. The higher the comonomer concentration, the lower the resin density. Structurally, LLDPE is different from LDPE in the number and type of branches, high-pressure LDPE has long branches, while linear LDPE has only short branches.
processing
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 a reduced sensitivity of physical properties to changes in blow-up ratio. In melt extension, LLDPE varies under various strains Generally have lower viscosity at speed. That is, it will not strain harden when stretched like LDPE. Increase with the deformation rate of polyethylene. 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 easily 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”.
Post time: Oct-21-2022