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					Plastics
Plastics Background
   Definition the term plastics,
    as it is commonly used today
    refers to a large group of
    synthetic materials which are
    made from a number of
    common substances such as
    coal, salt, oil, natural gas,
    cotton, wood, and water
   Simple chemicals are pulled
    from the material to produce
    monomers. Which are capable
    of reacting with one another,
    are produced.
   Monomers are then built up in
    a chainlike molecules called
    polymers
Types of Plastics
 Developments have produce over 40
  groups or families of plastics. Each with
  its own characteristics, such as light
  weight, heavy weight, heat-resistant,
  hard, soft, clear, or opaque.
 Two general classifications
       Thermoplastics
       Thermosetting plastics
Thermoplastics
   Become soft when heated and hard when cooled,
    regardless of the number of times the process is
    repeated
       This is due to the fact that linear molecular chains can
        move together when heated and cooled.
       Types of plastics – acrylics, celluloses, polyethylene,
        polyvinyl chloride (PVC), polystryrene, polyallomers,
        polycarbonates, polyimide, polypropylene, polysulfone,
        phenylene oxide nylons, methyl pentenes, ionomer,
        fluroplastics, acetal and acrylonitrile butradiene styrene
        (ABS)
Thermosetting plastics
   Are also chainlike in
    molecular structure but the
    curing and hardening
    process involves the
    formation of cross-links
    between molecules in
    adjacent chains. Resulting
    in complex chains that are
    no longer free to move.
       Types – alkyds, aminous
        (urea and melamine
        formaldehyde),
        diallylphthalate (DAP),
        epoxies, furan, phenolics,
        polyesters, polyurethane,
        and silicones
Production of Plastics
   Process of uniting monomers to form
    polymers is known as polymerization
       Accomplished by either a condensation or an
        addition process
   Raw material can come in any of the
    following forms
       Granules
       Powder
       Beads
       Liquid resins
Production of Plastics
   The raw material is then produced into the following forms
       Sheets
       Films
       Tubing
       Rods
       Solid or semisolid shape
   Additions to plastics
       Plasticizers – added for workability
       Fillers – are added to produce bulk
       Fibers – added for strength and durability
       Hardeners – added to induce setting
   Plastics products are formed by a number of methods –
    injection molding, blow molding, expandable bead molding,
    compression molding, transfer molding, rotational
    modeling, form modeling, extrusion thermoforming,
    laminating, casting, and calendaring
Injection Molding
   Measured amounts of
    powder or granules are fed
    into the intake end of the
    heated barrel of a molding
    machine. And forced
    forwarded by a ram.
   Heated to temperatures of
    300 to 650 c
   Pressure from 5000 to
    40000 psi
   Thermoplastics and
    thermosets are used.
Blow Molding
   Three processes
       Extrusion blow molding – an extruder produces a hollow
        tube called a parison which is captured between the two
        halves of a hollow mold. The mold is close and air is blown
        into the tube forcing it to expand.
       Injection blow molding – process is same except the tube
        (parison) is injected molded on a steel rod and the rod is
        removed prior to closing the mold
       Stretch blow molding – involves, stretching the parison in
        the axial direction before blowing.
   Polyethylene is typically used, but PVC, polystyrene, and
    polypropylene are also used.
   Bottles and water cans are typical shapes produced
Expandable Bead Molding
   Process used to produce lightweight products of
    polystyrene foam
   Small granules together with a small amount of
    an expanding agent are placed in a rolling drum
    and steam heated
   Heat softens the granules and the expanding
    agent enlarges their size
   Cooled and transferred to a mold
   Again heated to 135 c and until they fused
    together
   Flotation equipment, shipping containers, rigid
    insulation are typical products
Compression Molding
   Simplest form of molding
   A measured amount of powder is place in a
    heated mold then closed
   Heat and pressure are applied to the plastic
    material which melts and flows to all parts of
    the mold
Transfer Molding
   Differs from
    compression molding
    in that in the transfer
    molding process the
    molding compound is
    liquefied outside the
    molding chamber
   Transfer to the mold
    under pressure
   There the forming and
    setting takes place
Rotational Molding
   Used to form hollow units with complex shapes
    and heavy walls
   A pre measured amount of powder or liquid
    resin is placed in the bottom half of a cold
    mold and the mold is closed
   Mold is heated and rotated. Then allowed to
    cooled
Form Molding
 Made by mixing an expanding agent with
  either granules or powder and then
  heating it
 Heat melts the plastic and causes the
  formation of a gas which expands the
  molten material into foamed structure
 Quickly cooled to set the material
 Insulation is made in this manner
Extrusion Thermoforming
 Is used to produce materials which have a
  constant cross section
 Heat material is pushed through a die
Laminating
   Bonding together a number of layers of
    materials to form a single sheet.
Casting
   Liquid plastic are poured into molds and
    set
Calendaring
 Form of extrusion
 Plastic material is fed to a pair of revolving
  rollers which extrude a thin film or sheet
  between them
Properties of Plastics
   Transparency (acrylics, methyl pentenes, polycarbonates, ionomer,
    polysulfone)
   resistance to discoloration (acrylics)
   good resistance to weathering (acrylics, epoxy, silicone, polyvinyl chloride)
   good dimensional stability (fluoroplastics, polycarbonates, phenylene,
    oxide, polyvinyl chloride, urea, melamine)
   toughness (Cellulosics, ionomer, nylone, phenoxies, polycarbonates,
    polyethylene)
   high impact resistance (acrylics, ethyl cellulose, polypropylene, polyimide,
    polycarbonates)
   abrasion resistance (nylons, polyallomers, polyimide, polypropylene)
   low moisture absorption (fluoroplastics, phenylene oxide, polypropylene,
    diallyl phthalate, silicone)
   ductility (phenoxies, acrylics, nylons)
   good adhesive qualities (epoxy, phenolics, amino resins)
   good resistance to chemicals (nylons polyethylene, phenylene oxide,
    polypropylene, polysulfone, diallylphthalate)
Additives for plastics
   not all properties of plastics are positive ones, additives are
    necessary to reinforce another property.
       Antioxidants
       antistatic agents
       coloring agents
       coupling agents
       Fillers
       reinforcing agents
       flame retardants
       foaming agents
       Lubricants
       Plasticizers
       Preservatives
       processing aids
       ultraviolet stabilizers
Plastics in Construction
   Structural Applications
       For use in structural applications plastics need to be
        reinforced
       Glass fiber, jute, cotton, sisal, asbestos, and synthetic
        and metallic fibers are used
       Flat sheets – glazing
       Thermoformed shapes – roof shapes, curtain wall
       Glass fiber structural shapes – used where corrosion is a
        problem (like steel members)
       Plastic foam boards – decking, roofing
       Fabrics – balloons over a pool
       Foamed insulation – insulating materials
Nonstructural applications
   resilient flooring – vinyl tile, vinyl-cushioned flooring,
    linoleum, cork tile, rubber flooring
   Non resilient flooring – plastic terrazzo – plastic topping –
   carpets – tufted carpet – woven carpet - loomed carpet –
    knitted carpet – needle punched carpet
   interior finishes – wall coverings
   moisture control – vapor barriers
   plastic pipe – pvc
   caulking and sealing compounds
   paints and varnishes
   lights and sunscreens
   adhesives
   material coatings

				
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posted:1/25/2013
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