Comparison of LLDPE and LDPE

Linear low-density polyethylene (LLDPE) differs structurally from conventional low-density polyethylene (LDPE) because it lacks long-chain branching. The linearity of LLDPE depends on the different production and processing methods of LLDPE and LDPE. LLDPE is typically produced by copolymerizing ethylene with higher alpha-olefins such as butene, hexene, or octene at lower temperatures and pressures. The LLDPE polymer produced by this copolymerization process has a narrower molecular weight distribution than conventional LDPE and its linear structure gives it different rheological properties.

Melt Flow Characteristics

The melt flow characteristics of LLDPE are suitable for new processes, especially film extrusion, which can produce high-quality LLDPE products. LLDPE is used in all traditional polyethylene markets. Its enhanced tensile strength, puncture resistance, impact resistance, and tear resistance make LLDPE suitable for films. Its excellent environmental stress crack resistance, low-temperature impact resistance, and warp resistance make LLDPE attractive for pipe and sheet extrusion and all molding applications. The latest application of LLDPE is as a geomembrane for lining landfills and waste liquid ponds.

Production and Properties

The production of LLDPE begins with transition metal catalysts, particularly Ziegler or Phillips type catalysts. 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 in a solution phase reactor, while butene and hexene are polymerized with ethylene in a gas phase reactor. The LLDPE resin produced in the gas phase reactor is in granular form and can be sold as powder or further processed into pellets. A new generation of ultra-LLDPE based on hexene and octene has been introduced by companies such as Mobil, Union Carbide, Novacor, and Dow Plastics. These materials have a high toughness limit and show new potential in automatic bag dispensing applications. Very low-density PE resins (density below 0.910 g/cc) have also appeared in recent years. VLDPES possesses flexibility and softness that LLDPE cannot achieve. The properties of the resin are generally reflected in its melt index and density. The melt index reflects the average molecular weight of the resin and is primarily controlled by the reaction temperature. The average molecular weight is independent of the molecular weight distribution (MWD). Catalyst selection affects the MWD. ​​Density is determined by the concentration of the comonomer in the polyethylene chain. The comonomer concentration controls the number of short branches (whose length depends on the type of comonomer), thus controlling 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 fewer short branches and is therefore a higher-density material. The physical properties of LLDPE are controlled by its molecular weight, MWD, and density. Whether LLDPE is superior to LDPE ultimately depends on its application.

Uses of LLDPE and LDPE

Generally, LLDPE is used to produce more rigid products in all applications, although according to the ASTM standard for low-density materials, the density of both LLDPE and LDPE is between 0.91 and 0.925. LLDPE forms a higher crystalline structure because of the absence of long branches. The greater crystallinity of LLDPE results in a more rigid product. This higher crystallinity also increases the melting point of LLDPE by 10-15°C compared to LDPE. Higher tensile strength, puncture resistance, tear resistance, and increased elongation are characteristics of LLDPE, making it particularly suitable for film production. If hexene or octene is used as a comonomer instead of butene, even impact strength and tear resistance can be significantly improved. For a given resin with the same melt index and density, hexene and octene LLDPE resins show up to a 300% improvement in impact and tear properties. The longer side chains of hexene and octene resins act like “knots” between the chains, improving the toughness of the compound. Resins produced using metallocene catalysts based on cyclic olefins will possess unique properties. These include a narrower molecular weight distribution (MWD), improved comonomer distribution, and better film transparency, sealability, and impact strength, similar to LLDPE produced with Ziegler catalysts. However, in terms of transparency, LLDPE exhibits similar drawbacks to LDPE. LLDPE films have poor haze and gloss, mainly due to the higher crystallinity causing surface roughness. The transparency of LLDPE resin can be improved by blending it with a small amount of LDPE.