Polyethylenes

A group of polyolefin polymers derived from ethylene by polymerization by heat and pressure. Polyethylene plastics are one of the lowest-cost and most widely used plastics. As a group, they are noted for toughness, excellent dielectric strength, and chemical resistance. Another outstanding characteristic is their low water absorption and permeability, which is the reason for their wide use in sheet form as moisture barriers. They are white in thick sections, but otherwise the range varies from translucent to opaque. They feel waxy. The many available types, ranging from flexible to rigid materials, are classified by density (specific gravity) into three major groups: low density, 0.910 to 0.925; medium density, 0.926 to 0.940; high density, 0.941 to 0.959. The variations of properties among these three groups are directly related to density. As density increases, polymer cross-bonding or branching and chrystallinity increase. Thus stiffness, tensile strength, hardness, and heat and chemical resistance increase with density in polyethylenes. Low-density polyethylenes are flexible, tough, and less translucent than high density grades. High-density grades, often called linear polyethylene grades, are stronger, more rigid, and have high creep resistance under load, but they have lower impact resistance. Typical uses of low-density polyethylenes include blow-molded bottles and containers, gaskets, paintbrush handles, and flexible-film packaging. High-density grades are used for wire insulation, beverage cases, dishpans, toys, and the film used for boil-in-bag packaging. In general, polyethylenes are not used in load-bearing applications because of their tendency to creep. However, a special type, high-molecular-weight polyethylene, is used for machine parts, bearings, bushings, and gears.


Polyethylenes can be blended or combined with other monomers—propylene, ethyl acrylate, and vinyl acetate—to produce copolymers to improve such properties as stress-crack resistance and clarity and to increase flexibility. They can also be modified by exposure to high-energy radiation, which produces cross-linking and thereby increases heat resistance and stiffness. Zetafax resin, of Dow Chemical Corp., is an ethylene acrylic resin. It has good adhesion to metals and high chemical resistance. Zaetabon is a Zetafax-coated metal tape for cable shielding. The Ultrathene UE630-81A, of U.S. Industrial Chemicals Co., is an ethylene vinyl acetate copolymer for rubberlike packaging film. The Aircoflex resins, of Air Reduction Co., are similar copolymers for use as pigment binders, paper, and textile coatings and adhesives. They have higher molecular weight and better stability than polyethylene.


Polythene and Alathon are names for polyethylene of DU Pont, in the forms of molding powder, rod, sheet, tubes, foil for packaging, and paper coatings. Agilene, of the American Agile Corp., is polyethylene. The plastic can be cross-linked by irradiation, and irradiated polyethylene parts become thermoset and have increased strength, toughness, and higher heat resistance. Irrathane , of the General Electric Co., is irradiated polyethylene. The plastic can also be cross-linked chemically by heating with carbon black and a diperoxide. For piping, this method increases strength, improves weather resistance, and eliminates stress cracking. Irrathene tape SPT, of this company, is a flexible irradiated polyethylene self-sealing insulating tape for corona-resistant electric cables. It withstands temperatures to 260°F (127°C).


Fortilex A, of the Celanese Corp., used for rigid chemical piping, is polyethylene of specific gravity 0.96, having a linear crystalline structure. It has a tensile strength of 4,500 lb/in2 (30MPa), flexural strength of 5,500 lb/in2 (37MPa), Rockwell hardness R40, and will withstand operating temperatures above 400°F (204°C). For piping and wire covering. Polyethylene is also compounded with small amounts of carbon black to give high resistance to weathering. Fortiflex F-087 has a specific gravity of 0.938 and transverse elongation of 600%. It is used for packaging film and takes printing ink well. Microthene ML708, oh U.S. Industrial Chemicals Co., is a high-density powder of 350 mesh for rotational molding of thin walled parts. Dylan, of the Koppers Co., is a low pressure linear polyethylene used for polyethylene fibers, with a density of 0.95, a tensile strength of 3,500 lb/in2 (23MPa), elongation of 225%, and softening point at 255°F (123°C). The fibers and fabrics are marketed under trade names. Reevon, of Reeves Bros., Inc., is an upholstery fabric woven of polyethylene monofilament. Polyethylene foam is light in weight, has negligible water absorption, and is used in sheet and film for thermal insulation, and for wire insulation. Orthofoam and Metafoam, of Ludlow Papers, are polyethylene foams in sheets from 0.016 to 0.035 in (0.041 to 0.089 cm) thick. The low-density film has a tensile strength to 1,500 lb/in2 (9MPa), and a high-density material has a tensile strength to 12,000 lb/in2 (82MPa) with elongation of 2.5 to 5%. Polyethylene DGDA-2580, of the Union Carbide Corp., for extruded insulation on electric cables, gives uniformly dispersed closed cells so that the material has about 30% gas by volume. Extruded coatings have a smooth surface, a tensile strength of 2,800 lb/in2 (18MPa), and a dielectric constant of 1.5 to 1.7.


Polyethylene film has high resistance to oils, greases, and fatty acids; it also has good tear strength and fold endurance, and the light weight gives a large area per pound. It is thus widely used for packaging. Tenite 161M, of Eastman Chemical Products, Inc., is an extruding grade giving a film with a density of 0.923, tensile strength of 2,200 lb/in2 (14MPa), and elongation of 600%. When polyethylene film is irradiated and stretched biaxially it can be shrunk as much as 20% in all directions by applying a blast of hot air or dipping in water at a temperature of 180°F (82°C), and such films are used for packaging meats and poultry where a tight, close fit is desired. Cryovac L, of W. R. Grace & Co., is a film of this kind. High-density polyethylene has a high concentration of hydrogen atoms which are capable of slowing down or stopping fast neutrons, and sheets made with a small amount of boron to stop also the low-energy neutron are used for atomic shielding where light weight is necessary. Panelyte sheet, of the St. Regis Paper Co., for this purpose, is made of Petrothene 100, of the U.S. Industrial Chemicals Co., a polyethylene containing 2% boron.


Polyethylene rubbers are rubberlike materials made by cross-linking with chlorine and sulfur, or they are ethylene copolymers. Chlorosulfonated polyethylene is white spongy material. It has chlorine atoms and sulfonyl chloride groups spaced along the molecule. It is used to blend with rubber to add stiffness, abrasion resistance, and resistance to ozone, ans also for wire covering. Hypalon S-2, of Du Pont, is this material. Plaskon CPF200, of Allied Chemical Corp., is a chlorinated polyethylene containing about 73% chlorine. It is used for coatings and has high resistance to acids and alkalies. Ethylene-propylene rubber, produced by various companies, is a chemically resistant rubber of high tear strength. The ethylene butadiene resin of Phillips Petroleum Co. can be vulcanized with sulfur to give high hardness and wide temperature range. For greater elongation a terpolymer with butane can be made.


Polyethylene of low molecular weight is used for extending and modifying waxes, and also in coating compounds especially to add toughness, gloss, and heat-sealing properties. Epolene N-11, of the Eastman Chemical Products, Inc., for blending with waxes, has a molecular weight of 1,500, a density of 0.925, and a softening point at 103°C. Epolene N, used in paste polishes, has a molecular weight of 2,500 to 3,000. Epolene LVE, used in paper and textile coatings, is a low-density polyethylene with a molecular weightof 1,500, but Epolene HDE, used for self-polishing floor waxes to add hardness to the film, has the same molecular weight but a high-density, 0.956. Such materials are called polyethylene wax, but they are not chemical waxes. They can be made emulsifiable by oxidation, and they can be given additional properties by copolymerization with other plastics. Elvax, of Du Pont, is such a copolymer of ethylene with vinyl acetate. It is compatible with vegetable and paraffin waxes, and when added to these waxes it increase adhesiveness, gloss, toughness, and heat sealing. Wax polyethylene compounds for paper coatings may be sold under trade names. Ladcote, of the L. A. Dreyfus Co., is such as a compound. Chemetron 100, of the Chemetron Corp., is a modified ethylene wax. It is an ethylene stearamide, and comes as powder of beads. It improves luster, pigment, dispersion, and mar resistance in lacquers. The polyethylene waxes are microcrystalline and have sharper melting points than the ethylene waxes. They are more costly, but have high luster and durability. Polybutylene plastics are rubberlike polyolefins with superior resistance to creep and stress cracking. Films of this resin have high tear resistance, toughness, and flexibility, and are used widely for industrial refuse bags. Chemical and electrical properties are similar to those polyethylene and polypropylene plastics. Polymethyl pentene is a moderately crystalline polyolefin plastic resin that is transparent even in thick sections. Almost optically clear, it has a light transmission value of 90%. Parts molded of this plastic are hard and shiny with good impact strength down to —20°F (—29°C). Specific gravity (0.83) is the lowest of any commercial solid plastic. A major use is for molded food containers for quick frozen foods that are later heated by the consumer.


Application:


·         Wire coverings
·         Radio-frequency components
·         Containers (bucket, tableware)
·         Packaging materials
·         Insulating barriers

        Polyethylene Low Density ( LDPE )
        Polyethylene Medium Density ( MDPE )
        Polyethylene High Density ( HDPE )