Wednesday, December 29, 2010

Polystyrene

           Often referred to as styrene resin, it is used for molding, in lacquers, and for coatings, formed by the polymerization of monomeric styrene, which is a colorless liquid of the composition C6H5CH:CH2, specific gravity 0,906, and boiling point 145°C. it is made from ethylene, and is ethylene with one of the hydrogen atoms replaced by a phenyl group. It is also called phenyl ethylene and vinyl benzene. As it can be made by heating cinnamic acid, C6H5CH:CHCO2H, an acid found in natural balsams and resins, it is also called cinnamene. In the form of vinyl toluene which consists of mixed isomers of methyl styrene, the material is reacted with drying oils to form alkyd resins for paints and coatings.

The polymerized resin is a transparent solid very light in weight with a specific gravity of 1.054 to 1.070. The tensile strength is 4,000 to 10,000 lb/in2 (27 to 68MPa), compressive strength 12,000 to 17,000 lb/in2 (82 to 117MPa), and dielectric strength 450 to 600 volts per mil (18 to 24 X 106 volts per meter). Polystyrene is notable for water resistance and high dimensional stability. It is also tougher and stronger at low temperatures than most other plastics. It is valued as an electrical insulating material, and the film are used for cable wrapping.

When produced from methyl styrene, parts have a hardness to Rockwell M83, with tensile strengths to 8,900 lb/in2 (61MPa), and have a stiffness that makes them suitable for such products as cabinets and housings. Dielectric strength is also high, above 800 volts per mil (32 X 106 volts per meter), and the resin is thus used for electronic parts. The heat distortion temperature is 215°F (101°C).

Styrenes are subject to creep. Therefore the long-term bearing strength (over 2 weeks) is only about one-third the short-time tensile strength. Since their maximum useful service temperature is about 160°F (71°C), their use is restricted chiefly to room-temperature application. Because of their low cost and ease of processing, polystyrene are widely used for consumer products. The impact grades and glass-filled types are used quite widely for engineering parts and semistructural application. Also, polystyrene foams are highest in volume use of all the foam plastics. Because of good processing characteristic, polystyrenes are produced in a wide range of forms. They can be extruded, injection-, compression-, and blow molded, and thermoformed. They are also available as film sheet and foam.

Polystyrenes can be divided into the following major types: general-purpose grades, the lowest in cost, are characterized by clarity, colorability and rigidity. They are applicable where appearance and rigidity, but not toughness, are required. Common uses are wall tiles, compact cases, knobs, brush backs, and container lids. Impacts grades of polystyrene are produced by physically blending styrene and rubber. Grades are generally specified as medium, high, and extra-high. As impact strength increases, rigidity decreases. Medium-impact grades are used where a combination of moderate toughness and translucency is desired, for example, in such products as containers, closures, and small radio cabinets. High-impact polystyrenes have improved heat resistance and surface gloss. They are used for refrigerator door liners and crispers trays, containers, toys, and heater ducts in automobiles. The extra-high-impact grades are quite low in stiffness, and their use is limited to parts subject to high-speed loading.

Styrene can be polymerized with butadiene, acrylonitrile, and other resins. In terpolymer, acrylonitrile-butadiene-styrene (abbreviated ABS), is one of the common combinations. Styrene-acrylonitrile (SAN) has excellent resistance to acids, bases, salts, and some solvents. It also is among the stiffest of the thermoplastics, with a tensile modulus of 400,000 to 550,000 lb/in2 (2,757 to 3,791MPa). Styrene resins for molding are now marketed under a wide variety of trade names, with or without fillers and reinforcing agents. Many of these are copolymer resins, or are modified with plasticizer or cross-linking agents. Victron, of the U.S. Rubber Co., is a clear transparent polystyrene. Lustron, of Monsanto, is polystyrene in various grades, and Stymer is a polystyrene resin for sizing textiles. Piccotex, of the Pennysylvania Industrial Chemical Corp., is styrene copolymer in solid form soluble in mineral spirits for use in paints, coatings, and adhesives. Styron and Styraloy, of the Dow Chemical Co., are polystyrene molding resins, and Tyril, of this company, originally called Styrex, is a styrene-acrylonitrile copolymer.

Loalin, of the Catalin Corp., is a polystyrene with a specific gravity 1.05 to 1.07. it is crystal clear, and will take light pastel colors. In the clear form in transmits 90% light. It is water-resistant and has a dielectric strength of 500 to 700 volts per mil ( 1,970 to 2,758 volts per meter). It is not affected by alcohol, acids, or alkalies, but is soluble in aromatic hydrocarbons. It is preferably injection-molded. Exon 860, of the Firestone Plastic Co., is a soft grade of polystyrene that molds easily into products of high flexibility. The molded material has a tensile strength of 6,000 lb/in2 (41MPa) with elongation of 50%, Rockwell hardness of R100, and the dielectric strength of 510 volts  per mil (20 X 106 volts per meter). Fibertuff, of the Koppers Co., marketed in pellets for injection molding, is 60% polystyrene and 40% glass fiber. Molded parts have a specific gravity of 1.33, a tensile strength of 11,000 lb/ in2 (75MPa), heat distortion point of 220°F (104°C), and high impact resistance.

Styrene-butylene resins are copolymers that mold easily and produce thermoplastic products of low water absorption and good electrical properties. They have strength equal to the vinyls with grater elongation. Foamed polystyrene is available in blocks and heavy sheets for thermal insulation. It weighs about 1 lb/ft3 (16 kg/m3) and is rigid. Flexible styrene foam is also made into very thin sheets for wrapping frozen foods. It is grease-resistant and a good insulator, and is low in cost. Styrene is know best known for its use in synthetic rubbers, but the difference between resins and rubbers is chiefly in flexibility.

Applications:

·       Refrigerators (internal box)
·       Electric fans (blade)
·       Measuring instrument cases
·       Circuit component frames
·       Plastic models

 Polystyrene General

Molding Properties

         ·            Drying temperature                                            : —
         ·            Drying time                                                        : —
         ·            Injection forming cylinder temperature                : 170~280°C
         ·            Injection molding tool temperature                      : 20~60°C
         ·            Injection molding pressure                                  : 703~2110kgf/cm­­2
         ·            Compression molding temperature                      : 129~204°C
         ·            Compression molding pressure                           : 70.3~703kgf/cm2
         ·            Molding shrinkage rate                                       : 0.4~0.7%
         ·            Specific gravity                                                   : 1.03~1.05density
               JIS            : K6911,K7122
               A.S.T.M   : D792

Mechanical Characteristics


         ·           Tensile strength                                               : 350~840kgf/cm2
               JIS            : K6911,K7113
               A.S.T.M   : D638
         ·           Tensile elongation                                           : 3~4%
         ·           Compressive strength                                     : 809~1120kgf/cm2
               JIS            : K7208
         ·            Flexure strength                                             : 562~984kgf/cm2
               JIS            : K7203
         ·             Izot impact strength                                       : 1.4~2.2kgf/cm2
               JIS            : K7110,K7111
         ·            Rockwell hardness                                         : M60~75
               JIS            : K7202
               A.S.T.M   : D785

Thermal Characteristic

         ·                     Heat resistance temperature (continuous)          : 65.3~76.5°C
         ·                     Deflection temperature under load
                          a)                  Bending stress18.6kgf/cm2               : 104°C
                          b)                  Bending stress 4.6kgf/cm2                : 82~110°C
                JIS            : K7206,K7207
                A.S.T.M   : D648








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