Tuesday, December 28, 2010

Polypropylene Plastics

  An important group of synthetic plastics employed for molding resins, film, and texture fibers. Developed in 1957 in Italy and Germany, they are produced as polypropylene by catalytic polymerization of propylene, or may be copolymers with ethylene or other material. Propylene is a methyl ethylene, CH3CH:CH2, produced in the cracking of petroleum, and also used for making isopropyl alcohol and other chemicals. The boiling point is —48.2°C. It belongs to the class of unsaturated hydrocarbons known as olefins, which are designated by the word ending –ene. Thus propylene is known as propene as distinct from propane, the corresponding saturated compound of the group of alkanes from petroleum and natural gas. These unsaturated hydrocarbons tend to polymerize and form gums, and are thus not used in fuels although they have antiknock properties.

In polypropylene plastics the carbon atoms linked in the molecular chain between the CH2 units have each a CH3 and the H attached as side links, with the bulky side groups spiraled regularly around the closely packed chain. The resulting plastic has a crystalline structure with increased hardness and toughness and a higher melting point. This type of stereo symmetric plastic has been called isotactic plastic. It can also be produced with butylene or styrene, and the general term of the plastics is polyolefins. Copolymers or propylene are termed polyallomers.

Polypropylene is low in weight. The molded plastic has a density of 0.910, a tensile strength of 5,000lb/in2 (34MPa), with elongation of 150% and hardness of Rockwell R95. The dielectric strength is 1,500 volts per mil (59 X 106­ volts per meter), dielectric constant 2.3, and softening point 150°C. Blown bottles of polypropylene have good clarity and are nontoxic. The melt flow is superior to that of ethylene. A unique property is their ability in thin sections to withstand prolonged flexing. This characteristic has made polypropylene popular for “living hinge” applications. In test, they have been flexed over 70 million times without failure.

The many different grades of polypropylenes fall into three basic groups: homopolymers, copolymers, and reinforced and polymer blends. Properties of the homopolymers vary with molecular-weight distribution and the degree of crystallinity. Commonly, copolymers are produced by adding other types of olefin monomers to the propylene monomers to improve properties such as low-temperature toughness. Copolymers are also made by radiation grafting. Polypropylenes are frequently reinforced with glass or asbestos fibers to improve mechanical properties and increase resistance to deformation at elevated temperatures.

Tenite polypropylene, of Eastman Chemical Products, Inc., is used for molded parts, film, fibers, pipe, and wire covering. The polypropylene film of the Avisun Corp., called Olefane, used for packaging, has a specific gravity of 0.89. It is resistant to moisture, oils, and solvents, is crystals clear and is flexible. It withstands temperature to 250°F (121°C). The 0.001-in (0.003-cm) film has 31,000 ft/lb (20,830 m/kg). Dynafilm 200, of U.S. Industrial Chemical Co., is polypropylene laminated with polyethylene to give easy heat sealing for packaging. The 0.001-in (0.003-cm) film has a strength 3,000 lb/in2 (20MPa) and 400% elongation. Dynafilm 300 has the appearance, feel, and machine-handling properties of a Cellophane film. It consists of oriented polypropylene coated on both sides with vinyl acetate. The film comes in thickness from 0.0008 to 0.0012 in (0.0020 to 0.0030 cm) and heat-seals at 100°F (38°C)

Polypropylene fiber was originally produced in Italy under the name of Merkalon. Unless modified, it is more brittle at low temperatures and has less light stability than polyethylene, but it has about twice the strength of high-density linear polyethylene. Monofilament fibers are used for filter fabrics, and have high abrasion resistance and a melting point at 310°F (154°C). Multifilament yarns are used for textiles and rope. Polypropylene rope is used for marine hawsers, will float on water, and does not absorb water like Manila rope. It has a permanent elongation, or set, of 20%, compared with 19% for nylon 11% for manila rope, but the working elasticity is 16%, compared with 25% for nylon and 8% for Manila. The tensile strength of the rope is 59,000 lb/in2 (406MPa). Fine-denier multifilament polypropylene yarn for weaving and knitting dyes easily and comes in many colors. Chlorinated polypropylene is used in coatings, paper sizing, and adhesives. It has good heat and light stability, high abrasion resistance, and high chemical resistance.

Application:

        ·         Washing machines (Rotors washing tub)
        ·         Battery cases
        ·         TV set and tape recorder housings
        ·         Terminal bed
        ·         Wiring accessories

Polypropylene General

Molding Properties

       ·                 Drying temperature                                        : —
       ·                 Drying time                                                    : —
       ·                 Injection forming cylinder temperature            : 200~300°C
       ·                 Injection molding tool temperature                  : 20~90°C
       ·                 Injection molding pressure                              : 703~1410kgf/cm­­2
       ·                 Compression molding temperature                  : 171~288°C
       ·                 Compression molding pressure                       : 0.35~0.70kgf/cm2
       ·                 Molding shrinkage rate                                   : 1.0~2.5%
       ·                 Specific gravity                                               : 0.90~0.91density
                JIS  : K6911,K7122
                A.S.T.M   : D792

Mechanical Characteristics

       ·               Tensile strength                                               : 210~400kgf/cm2
                JIS            : K6911,K7113
                A.S.T.M   : D638
       ·              Tensile elongation                                             : 100~800%
       ·              Compressive strength                                       : 260~562kgf/cm2
                JIS            : K7208
       ·              Flexure strength                                                : 352~492kgf/cm2
                JIS            : K7203
       ·             Izot impact strength                                            : 2.2~110kgf/cm2
                JIS            : K7110,K7111
       ·            Rockwell hardness                                              : R50~110
                JIS            : K7202
                A.S.T.M   : D785

Thermal Characteristic


       ·         Heat resistance temperature (continous)                     : 88~115°C
       ·         Deflection temperature under load
                 a)                  Bending stress18.6kgf/cm2                   : 45.9~59.8°C
                 b)                  Bending stress 4.6kgf/cm2                    : 103~130°C
            JIS            : K7206,K7207
            A.S.T.M   : D648


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