Details
Material hierarchy: Polymer - Thermoplastic - Polyolefin - Polyethylene
So-called conventional low-density polyethylenes (LDPE) are produced in high-pressure reactors. Such polymers have highly branched structures with moderate crystallinity (50-65%).
Low-density polyethylenes are quite flexible, with high impact strength and relatively low heat resistance (maximum recommended service temperature is 140-175°F), although grades are available with heat resistance up to 200°F.
The high-pressure route also is used to make copolymers of ethylene with polar monomers such as vinyl acetate or ethyl acrylate. The EVAs and EEAs that are produced have low-temperature flexibility and are used in tough films and as a component of multilayer constructions for low-temperature heat sealing. Acid copolymers (with acrylic or methacrylic acid) are used for their hot tack and adhesive properties. Neutralized with metallic ions, these materials become clear ionomers, used in coating applications.
Linear polyethylenes, as the name suggests, have very little branching along the polymer chains. The polymerization of linear polyethylenes at low pressures has been used to manufacture high-density resins since the mid-1950s. Methods for making linear low-density polyethylenes (LLDPE) did not become commercial until the 1970s.
Commercial LLDPE resins are made in gas-phase reactors; the polymers typically contain up to 10% alpha olefin comonomers (butene, hexene, methyl pentene, or octene). The reactor output is in granular form, but normally is extruded and pelletized to incorporate additive systems.
LLDPE polymers, with little long-chain branching, have much greater elongation than LDPE. Their higher tear, tensile, and impact strength, along with improved resistance to environmental stress cracking, allow stronger products to be produced with less material. This has been particularly important in film markets, where considerable downgauging has been accomplished. Ultra- and very low-density polyethylenes (ULDPE and VLDPE) are essentially synonymous designations for linear polyethylenes with densities down to 0.880 g/cu cm. Produced in gas-phase reactors, they are finding application as impact modifiers for other polyolefins and in film and sheet markets.
Common Applications
Traditional markets for LDPE are in packaging films, extrusion coating of paper, wire and cable coating, injection molding, and pipe and tubing. Since the introduction of linear low-density polyethylene, conventional LDPE has been gradually displaced in some of these areas.