Abstract

Linear low density polyethylene (LLDPE) resins are polyethylene (PE) plastics with densities in a 0.915–0.925-g/cm³ range. They belong to the family of semicrystalline ethylene–-olefin copolymers, which are produced in catalytic polymerization reactions. Due to reduced crystallinity, all these copolymers have densities lower than that of the ethylene homopolymer, ~0.96 g/cm³. This family of PE resins includes medium density PE (MDPE, 0.940–0.925 g/cm³), LLDPE (0.925–0.915 g/cm³), and very low density PE (VLDPE, < 0.915 g/cm³). A variety of resins described by the general term LLDPE differ in several respects: the type of an -olefin used in copolymerization with ethylene, 1-butene, 1-hexene, 4-methyl-1-pentene, or 1-octene; the content of the -olefin in a copolymer, which varies from 1 mol% in MDPE to 2.5–3.5 mol% in LLDPE and to 10–15 mol% in VLDPE; the density and crystallinity of the resin, and the compositional uniformity of the copolymers.

The LLDPE resins are produced in industry with three classes of catalysts: Ziegler catalysts based on titanium compounds, several types of catalysts utilizing metallocene complexes, and Phillips catalysts based on chromium oxides. The resins are manufactured in gas-phase, solution, and slurry polymerization processes.

The LLDPE resins are major commodity products with a combined 2005 worldwide manufacture volume of ~15×10⁶ ton/year, ~25% of the overall polyethylene production. Film is the largest application market for LLDPE resins. The film is used to manufacture grocery sacks, bags for produce, trash bags, bags for merchandise packaging, garment, laundry and dry cleaning bags, and ice bags. The LLDPE film is also used for nonpackaging applications, such as industrial sheeting and agricultural mulch film. High clarity film produced with LLDPE resins manufactured with metallocene catalysts is widely used for food packaging and in numerous medical applications. Injection molding products represent the second largest market for LLDPE, mostly for housewares. These resins are also used for blow molding of bottles and drums; for rotational molding of toys, large containers, and tanks; for pipe and tubing; and for wire coating in electrical and telephone industry.

The VLDPE is a subclass of LLDPE resins; its density is < 0.915 g/cm³ and it is mostly used for food packaging of fresh produce, milk, meat, cheese, etc, as well as for the manufacture of multilayer coextruded film. These resins are also blended with other polyethylene and polypropylene-based resins to enhance clarity, sealability, and toughness of the materials.