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Polymers

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Title: Polymers


1
  • Polymers
  • Substances containing a large number of
    structural units joined by the same type of
    linkage.
  • These substances often form into a chain-like
    structure.
  • Polymers in the natural world have been around
    since the beginning of time.
  • Starch, cellulose, and rubber - possess
    polymeric properties.
  • Man-made polymers studied since 1832.
  • Today, the polymer industry has grown to be
    larger than the aluminum, copper and steel
    industries combined.

2
  • WHAT ARE POLYMERS?
  • Tiny molecules strung in long repeating chains
    form polymers.
  • Why should you care?
  • Our body is made of them. DNA, the genetic
    blueprint that defines people and other living
    things, is a polymer.
  • The proteins and starches in the foods we eat,
    the tires on our bikes and cars, the wheels on
    skateboards and skates.
  • Surrounded by polymers every day, everywhere we
    go.
  • Another great reason to learn about polymers.
  • Understanding their chemistry enables in wisely
    using them.
  • Once familiar with the varieties of polymers that
    people make, such as plastics, we can recycle
    many of them and use them again.
  • Thats good for the environment.

3
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4
  • Polymers at Home

4
2
What makes all these different? Each connects
with a different kind of human-made polymer that
we encounter in our homes every day.
1
3
5
  • Water-resistant paints and varnishes derive from
    a family of synthetic polymers called acrylics.
    You can also paint yourself warm with acrylics
    Spun acrylics find their way into fiberfill
    jackets and bedtime comforters.

6
  • In 1907 Leo Baekeland patented a revolutionary
    new material.
  • Could mold it at high temperatures and it would
    retain its shape when cooled, could dye it with
    brilliant colors.
  • Baekeland named it Bakelite after himself.
  • Soon everything from telephones and radios to
    auto parts, furniture, and jewelry was being made
    from Bakelite.
  • In a cover story on Leo Baekeland in 1924, Time
    magazine proclaimed that in a few years
    Bakelite will be embodied in every mechanical
    facility of modern civilization. From the time
    that a man brushes his teeth in the morning with
    a Bakelite handled brush, until the moment he
    falls back on his Bakelite bed ...all that he
    touches, sees, uses, will be made of this
    material of a thousand uses.

7
  • World War II pushed plastics production into
    high gear.
  • Japanese submarines made it impossible for Allied
    nations such as Great Britain and the United
    States to import latex, the basis of most natural
    rubber, from Asian plantations.
  • Industrial chemists rose to the challenge,
    devising economical means of producing synthetic
    rubber in huge volumes.
  • They also created new polymers for use in
    airplanes, ships, and tanks under fire.
  • Silk without silkworms? Practically. The
    plastic nylon replaced the silk in hosiery in
    1938.
  • Many of the airborne troops in World War II
    floated to earth beneath nylon parachutes.
  • Other synthetic fibers such as polyester made the
    fashions of the 1970s possible.

8
  • Natural rubber from latex, made balls that could
    bounce.
  • But it became hard and brittle when it got too
    cold, a sticky mess when it got too warm.
  • In 1839 Charles Goodyear discovered that latex
    heated with sulphuror vulcanizedwould remain
    elastic at a wide range of temperatures.
  • Sulphur made bridges between the long chain
    polymers in rubber to keep them from sliding past
    one another or contracting into knots
  • Carriages, cars, trucks, and buses have traveled
    billions of miles on tires made from vulcanized
    rubber and synthetic substitutes.

9
  • Polystyrene foam - made into cartons to protect
    eggs or into packing peanuts to cushion fragile
    objects for shipping.
  • Insulates - as cups and coolers to keep the warm
    ones warm and the cold ones cold.
  • Placed behind walls and ceilings in homes,
    polystyrene foam helps keep the weather outside
    at bay.
  • Chlorofluorocarbons (CFCs), containing both
    chlorine and fluorine, were sometimes used to
    make foam products. This was found to damage the
    earths protective ozone layer-hence phased out
    their use in the creation of foam packaging and
    most other types of polystyrene foam.

10
Polymers in Nature
  • Everything we see in nature-
  • What do all these have in common?
  • They contain polymers!
  • You can find different plants, animals, and
    natural objects that make or contain polymers.
  • You can build a miniature world of your own.

11
  • Rosin
  • Dead wood and pulp from pine trees contain a
    polymer called rosin, which is used to make
    varnish and soap. Violinists rub rosin on the
    horsehairs in their bows to make them slide
    smoothly across the strings. Gymnasts and
    baseball players use rosin to improve their
    grips.
  • Animal Horns
  • Antelope, buffalo, sheep, cattle, and rhinos all
    have horns. Unlike a deers antlers, made of
    bone, horns are made of the polymer keratin.
  • Parts of ours are made of keratin too
    -ingredient in our hair and fingernails. Keratin
    in the outermost layer of our skin makes it
    waterproof like other mammals, so one doesnt get
    waterlogged the moment he dives in the pool.

12
  • Range of applications
  • Far exceeds that of any other class of material
    available to man.
  • Extend from
  • adhesives, coatings, foams, and packaging
    materials to textile and industrial fibers
  • composites, electronic devices, biomedical
    devices, optical devices, and precursors for many
    newly developed high-tech ceramics.

13
  • Applications
  • Industry
  • Automobile parts, windshields for fighter
    planes, pipes, tanks, packing materials,
    insulation, wood substitutes, adhesives, matrix
    for composites, and elastomers
  • Agriculture and Agribusiness
  • Polymeric materials -in and on soil to
    improve aeration, provide mulch, and promote
    plant growth and health.
  • Medicine
  • Many biomaterials, - heart valve
    replacements and blood vessels, are made of
    polymers like Dacron, Teflon and polyurethane.
  • Consumer Science
  • Plastic containers of all shapes and sizes
    are light weight and economically less expensive
    than the more traditional containers.
  • clothing, floor coverings, garbage disposal
    bags, and packaging.
  • Sports
  • Playground equipment, various balls, golf
    clubs, swimming pools, and protective helmets

14
  • Medical Therapeutic apheresis -a treatment
    process that enables substances which cause
    disease to be safely removed from the blood while
    it is outside the body.
  • A Germany-based medical technology company
    Fresenius Medical Care developed a technology
    called DALI (Direct Adsorption of Lipoproteins)
    especially for the treatment of patients with
    severe lipometabolic disorders. It enables LDL
    cholesterol, also known as bad cholesterol
    because of its influence on vascular
    calcification, to be extracted from the blood.
  • -An adsorber filled with a special material
    electrostatically bonds the LDL cholesterol. For
    the housing of the adsorber system, a
    fracture-resistant plastic was required.
    Makrolon 2458 developed by Polycarbonates
    Business Unit of Bayer Material Science AG.
  • This polycarbonate is sufficiently tough and
    stiff, which protects it from becoming easily
    damaged in the often hectic everyday hospital
    environment.
  • Withstands the pressurized hot-steam
    sterilization of the DALI adsorber, where
    temperatures reach at least 121 C for over 20
    minutes.

15
 
Plastics in the medical technology sector
                                                            
  • Polycarbonate adsorber housings
  • Robust in everyday hospital use, suitable for
    hot-steam sterilization
  • The housing of the DALI adsorber system is made
    from the fracture-resistant polycarbonate
    Makrolon 2458. This withstands pressurized
    hot-steam sterilization, where temperatures reach
    at least 121 C for over 20 minutes.

In the DALI treatment, the patient's blood is
removed from an arm vein and passed through the
adsorber where the LDL cholesterol sticks to the
adsorber globules. The cleaned blood reenters the
patient's body via another arm vein
16
  • A further benefit of the polycarbonate is its
    high transparency which allows continuous visual
    monitoring of the blood treatment by hospital
    personnel and therefore enhances patient safety.
    Makrolon 2458 meets the requirements of the
    American standard US-Pharmacopeia, Class VI,
    relating to the biological compatibility of
    plastics. Like all medical technology products
    from Bayer MaterialScience, it also complies with
    international standard ISO 10993-1 regarding the
    biocompatibility of plastics that are in contact
    with body fluids and tissue for up to 30 days.

17
  • Future Trends
  • Nature has used biological polymers as the
    material of choice, Mankind chose polymeric
    materials as the choice material.
  • From the Stone Age, through the Bronze, Iron, and
    Steel Ages into its current age, the Age of
    Polymers.
  • An age in which synthetic polymers are and will
    be the material of choice.
  • Potential for exciting new applications in the
    foreseeable future.
  • Areas as conduction and storage of electricity,
    heat and light, molecular based information
    storage and processing, molecular composites,
    unique separation membranes, revolutionary new
    forms of food processing and packaging, health,
    housing, and transportation.
  • Polymers will play an increasingly important role
    in all aspects of our life.
  • The large number of current and future
    applications of polymeric materials has created
    need for persons specifically trained to carry
    out research and development in Polymer Science
    and Engineering-
  • can expect to achieve both financial reward and
    personal fulfillment.

18
  • Scientific Principles
  • The field is so vast and the applications so
    varied
  • Important to understand how polymers are made and
    used
  • There are over 60,000 different plastics
    knowledge of this important field can truly
    enrich our appreciation of this wonder material.
  • Companies manufacture over 30 million tonnes of
    plastics each year, spend large sums on RD, and
    more efficient recycling methods.
  • Some of the scientific principles involved in the
    production and processing of these fossil fuel
    derived materials known as polymers are

19
  • Polymerization Reactions
  • The chemical reaction in which high molecular
    mass molecules are formed from monomers is known
    as POLYMERIZATION
  • Two basic types of polymerization
  • chain-reaction (or addition) polymerization.
  • step-reaction (or condensation) polymerization.

20
1. Chain-Reaction (addition) Polymerization
  • A three step process involving two chemical
    entities.
  • The first, known simply as a monomer, can be
    regarded as one link in a polymer chain. It
    initially exists as simple units. In nearly all
    cases, the monomers have at least one
    carbon-carbon double bond.
  • Initiation
  • Propagation
  • Termination

Ethylene is one example of a monomer used to
make a common polymer
21
  • The other chemical reactant is a catalyst.
  • In chain-reaction polymerization, the catalyst
    can be a free-radical peroxide added in
    relatively low concentrations. A free-radical is
    a chemical component that contains a free
    electron that forms a covalent bond with an
    electron on another molecule. The formation of a
    free radical from an organic peroxide is
  • In this chemical reaction, two free radicals
    have been formed from the one molecule of R2O2.
  • With the chemical components identified, a look
    at the polymerization process

22
  • Step 1 Initiation
  • The first step, initiation, occurs when the
    free-radical catalyst reacts with a double bonded
    carbon monomer, beginning the polymer chain. The
    double carbon bond breaks apart, the monomer
    bonds to the free radical, and the free electron
    is transferred to the outside carbon atom in this
    reaction.

23
  • Step 2 Propagation
  • Propagation, is a repetitive operation in
    which the physical chain of the polymer is
    formed. The double bond of successive monomers is
    opened up when the monomer is reacted to the
    reactive polymer chain. The free electron is
    successively passed down the line of the chain to
    the outside carbon atom.

24
  • This reaction is continuous because the energy in
    the chemical system is lowered as the chain
    grows.
  • Thermodynamically speaking, the sum of the
    energies of the polymer is less than the sum of
    the energies of the individual monomers.
  • Simply put, the single bounds in the polymeric
    chain are more stable than the double bonds of
    the monomer.

25
  • Step 3 Termination
  • Termination occurs when another free radical
    (R-O.), left over from the original splitting of
    the organic peroxide, meets the end of the
    growing chain.
  • This free-radical terminates the chain by
    linking with the last CH2. component of the
    polymer chain.
  • This reaction produces a complete polymer
    chain. Termination can also occur when two
    unfinished chains bond together.

26
These termination types are as below.
Other types of termination are also possible.
This exothermic reaction occurs extremely fast,
forming individual chains of polyethylene often
in less than 0.1 second. These polymers have
relatively high molecular weights. branches or
cross-links with other chains also may occur
along the main chain.
27
  • 2. Step-Reaction (condensation)Polymerization
  • Another common type of polymerization.
  • This method produces polymers of lower
    molecular weight than chain reactions and
    requires higher temperatures to occur.
  • Unlike addition polymerization, step-wise
    reactions involve two different types of
    di-functional monomers or end groups that react
    with one another, forming a chain.
  • Condensation polymerization also produces a
    small molecular by-product (water, HCl, etc.).
  • EgFormation of Nylon 66, a common polymeric
    clothing material, involving one each of two
    monomers, hexamethylene diamine and adipic acid,
    reacting to form a dimer of Nylon 66.

28
The polymer could grow in either direction by
bonding to another molecule of hexamethylene
diamine or adipic acid, or to another dimer. As
the chain grows, the short chain molecules are
called oligomers. This reaction process
theoretically can continue until no further
monomers and reactive end groups are available.
The process is relatively slow and can take up
to several hours or days. This process breeds
linear chains that are strung out without any
cross-linking or branching, unless a
tri-functional monomer is added.
29
  • Polymer Chemical Structure
  • The monomers in a polymer can be arranged in a
    number of different ways.
  • Both addition and condensation polymers can be
    linear, branched, or cross-linked. Linear
    polymers are made up of one long continuous
    chain, without any excess appendages or
    attachments. Branched polymers have a chain
    structure that consists of one main chain of
    molecules with smaller molecular chains branching
    from it. A branched chain-structure tends to
    lower the degree of crystallinity and density of
    a polymer. Cross-linking in polymers occurs when
    primary valence bonds are formed between separate
    polymer chain molecules.
  • Chains with only one type of monomer are known as
    homopolymers. If two or more different type
    monomers are involved, the resulting copolymer
    can have several configurations or arrangements
    of the monomers along the chain.
  • The four main configurations are depicted below

30
Copolymer configurations
31
  • Polymer Physical Structure
  • Segments of polymer molecules can exist in two
    distinct physical structures.
  • CRYSTALLINE or AMORPHOUS forms.
  • Crystalline polymers are only possible if there
    is a regular chemical structure (e.g.,
    homopolymers or alternating copolymers), and the
    chains possess a highly ordered arrangement of
    their segments. Crystallinity in polymers is
    favored in symmetrical polymer chains, but never
    100. These semi-crystalline polymers possess a
    rather typical liquefaction pathway, retaining
    their solid state until they reach their melting
    point at Tm.

32
  • Amorphous polymers do not show order.
  • The molecular segments are randomly arranged and
    entangled.
  • - Do not have a definable Tm due to their
    randomness. At low temperatures, below their
    glass transition temperature (Tg), the segments
    are immobile and the sample is often brittle.
  • As temperatures increase close to Tg, the
    molecular segments begin to move. Above Tg, the
    mobility is sufficient (if no crystals are
    present) that the polymer can flow as a highly
    viscous liquid.
  • The viscosity decreases with increasing
    temperature and decreasing molecular weight.
  • There can also be an elastic response if the
    entanglements cannot align at the rate a force is
    applied (as in silly putty). This material is
    then described as visco-elastic.
  • In a semi-crystalline polymer, molecular flow is
    prevented by the portions of the molecules in the
    crystals until the temperature is above Tm. At
    this point a visco-elastic material forms.
  • These effects are as in the specific volume
    versus temperature graph.

33
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34
Members of the Polymer Family
  • Separated into two different groups depending on
    their behavior when heated.
  • Polymers with linear molecules are likely to be
    thermoplastic.
  • These are substances that soften upon heating
    and can be remolded and recycled. They can be
    semi-crystalline or amorphous.
  • The other group of polymers is known as
    thermosets. These are substances that do not
    soften under heat and pressure and cannot be
    remolded or recycled. They must be remachined,
    used as fillers, or incinerated to remove them
    from the environment.

35
Thermoplastics
  • Generally carbon containing polymers synthesized
    by addition or condensation polymerization.
  • This process forms strong covalent bonds within
    the chains and weaker secondary Van der Waals
    bonds between the chains.
  • Usually, these secondary forces can be easily
    overcome by thermal energy, making thermoplastics
    moldable at high temperatures.
  • Thermoplastics will also retain their newly
    reformed shape after cooling.
  • Applications of thermoplastics include parts for
    common household appliances, bottles, cable
    insulators, tape, blender and mixer bowls,
    medical syringes, mugs, textiles, packaging, and
    insulation.

36
  • Thermosets
  • Have the same Van der Waals bonds that
    thermoplastics do.
  • Also have a stronger linkage to other chains.
  • Strong covalent bonds chemically hold different
    chains together in a thermoset material.
  • The chains directly bonded to each other or
    bonded through other molecules. This
    "cross-linking" between the chains allows the
    material to resist softening upon heating.
  • Thermosets must be machined into a new shape if
    they are to be reused or they can serve as
    powdered fillers.
  • Difficult to reform, but have many distinct
    advantages in engineering design applications
    including
  • High thermal stability and insulating properties.
  • High rigidity and dimensional stability.
  • Resistance to creep and deformation under load.
  • Light-weight.
  • Applications for thermosets include epoxies
    (glues), automobile body parts, adhesives for
    plywood and particle board, and as a matrix for
    composites in boat hulls and tanks.

37
Unit Operations in Polymer Processing
  • Thermoplastic and thermoset melt processes may be
    broken down into
  • Preshaping
  • Shaping
  • Shape Stabilization

38
Unit Operations in Polymer Processing
  • Preshaping steps
  • Solids handling and conveying most processes
    usually involve feed in particulate form
  • Plastication The creation of a polymer melt from
    a solid feed.
  • Mixing often required to achieve uniform melt
    temperature or uniform composition in compounds
  • Pumping The plasticated melt must be
    pressurized and pumped to a shaping device
  • Shaping
  • The polymer melt is forced through the shaping
    devices to create the desired shape.
  • The flow behavior (rheology) of polymer melts
    influences the design of the various shaping
    devices, the processing conditions and the rate
    at which the product can be shaped.
  • Shape stabilization
  • Involves the solidification of the polymer melt
    in the desired shape, through heat transfer

39
Polymer Processing
  • Five basic processes to form polymer products or
    parts.
  • They are
  • Injection molding,
  • Compression molding,
  • Transfer molding,
  • Blow molding, and
  • Extrusion
  • Compression molding and transfer molding are used
    mainly for thermosetting plastics.
  • Injection molding, extrusion and blow molding are
    used primarily with thermoplastics.

40
Injection Molding
  • Common process for forming plastics- involves
    four steps
  • Powder or pelletized polymer is heated to the
    liquid state.
  • Under pressure, the liquid polymer is forced into
    a mold through an opening, called a sprue. Gates
    control the flow of material.
  • The pressurized material is held in the mold
    until it solidifies.
  • The mold is opened and the part removed by
    ejector pins.
  • Advantages of injection molding include rapid
    processing, little waste, and easy automation.
  • Molded parts include combs, toothbrush bases,
    pails, pipe fittings, and model airplane parts.

41
Diagram of injection molding
42
Injection Molding
  • Injection molding is the most important process
    used to manufacture plastic products. It is
    ideally suited to manufacture mass produced parts
    of complex shapes requiring precise dimensions.
  • It is used for numerous products, ranging from
    boat hulls and lawn chairs, to bottle cups. Car
    parts, TV and computer housings are injection
    molded.
  • The components of the injection molding machine
    are the plasticating unit, clamping unit and the
    mold.

43
Injection Molding Cycle
  • Injection molding involves two basic steps
  • Melt generation by a rotating screw
  • Forward movement of the screw to fill the mold
    with melt and to maintain the injected melt under
    high pressure
  • Injection molding is a cyclic process
  • Injection The polymer is injected into the mold
    cavity.
  • Hold on time Once the cavity is filled, a
    holding pressure is maintained to compensate for
    material shrinkage.
  • Cooling The molding cools and solidifies.
  • Screw-back At the same time, the screw retracts
    and turns, feeding the next shot in towards the
    front
  • Mold opening Once the part is sufficiently cool,
    the mold opens and the part is ejected
  • The mold closes and clamps in preparation for
    another cycle.

44
Injection Molding Cycle
  • The total cycle time is tcycletclosingtcooling
    tejection.

45
Molding Processes
  • Molding techniques for polymers involve the
    formation of three-dimensional components within
    hollow molds (or cavities)
  • Injection Molding
  • Thermoforming
  • Compression Molding
  • Blow Molding
  • Rotational Molding

46
Compression Molding
  • This type of molding was among the first to be
    used to form plastics. It involves four steps
  • Pre-formed blanks, powders or pellets are placed
    in the bottom section of a heated mold or die.
  • The other half of the mold is lowered and is
    pressure applied.
  • The material softens under heat and pressure,
    flowing to fill the mold. Excess is squeezed from
    the mold. If a thermoset, cross-linking occurs in
    the mold.
  • The mold is opened and the part is removed.
  • For thermoplastics, the mold is cooled before
    removal so the part will not lose its shape.
    Thermosets may be ejected while they are hot and
    after curing is complete. This process is slow,
    but the material moves only a short distance to
    the mold, and does not flow through gates or
    runners. Only one part is made from each mold.

47
Compression Molding
  • Compression molding is the most common technique
    for producing moldings from thermosetting
    plastics and elastomers.
  • Products range in size from small plastic
    electrical moldings and rubber seals weighing a
    few grams, up to vehicle body panels and tires.
  • A matched pair of metal dies is used to shape a
    polymer under the action of heat and pressure.

48
  • Transfer Molding
  • This process is a modification of compression
    molding. It is used primarily to produce
    thermosetting plastics. Its steps are
  • A partially polymerized material is placed in a
    heated chamber.
  • A plunger forces the flowing material into molds.
  • The material flows through sprues, runners and
    gates.
  • The temperature and pressure inside the mold are
    higher than in the heated chamber, which induces
    cross-linking.
  • The plastic cures, is hardened, the mold opened,
    and the part removed.
  • Mold costs are expensive and much scrap material
    collects in the sprues and runners, but complex
    parts of varying thickness can be accurately
    produced.

49
  • Blow Molding
  • Blow molding produces bottles, globe light
    fixtures, tubs, automobile gasoline tanks, and
    drums. It involves
  • A softened plastic tube is extruded
  • The tube is clamped at one end and inflated to
    fill a mold.
  • Solid shell plastics are removed from the mold.
  • This process is rapid and relatively inexpensive

50
Blow Molding
  • Blow molding produces hollow articles that do
    not require a homogeneous thickness distribution.
  • HDPE, LDPE, PE, PET and PVC are the most common
    materials used for blow molding. There are three
    important blow molding techniques
  • Extrusion blow molding
  • Injection blow molding
  • Stretch-blow processes
  • They involve the following stages
  • A tubular preform is produced via extrusion or
    injection molding
  • The temperature controlled perform is transferred
    into a cooled split-mould
  • The preform is sealed and inflated to take up the
    internal contours of the mould
  • The molding is allowed to cool and solidify to
    shape, whilst still under internal pressure
  • The pressure is vented, the mold opened and the
    molding ejected.

51
Extrusion Blow molding
  • In extrusion blow molding, a parison (or tubular
    profile) is extruded and inflated into a cavity
    with a specified geometry. The blown article is
    held inside the cavity until it is sufficiently
    cool.

52
Injection Blow Molding
  • Injection blow molding begins by injection
    molding the parison onto a core and into a mold
    with finished bottle threads. The formed parison
    has a thickness distribution that leads to
    reduced thickness variations throughout the
    container. Before blowing the parison into the
    cavity, it can be mechanically stretched to
    orient molecules axially (Stretch blow molding).
    The subsequent blowing operation introduces
    tangential orientation. A container with biaxial
    orientation exhibits higher optical clarity,
    better mechanical properties and lower
    permeability.

53
  • Extrusion
  • This process makes parts of constant cross
    section like pipes and rods. Molten polymer goes
    through a die to produce a final shape. It
    involves four steps
  • Pellets of the polymer are mixed with coloring
    and additives.
  • The material is heated to its proper plasticity.
  • The material is forced through a die.
  • The material is cooled.
  • An extruder has a hopper to feed the polymer and
    additives, a barrel with a continuous feed screw,
    a heating element, and a die holder. An adapter
    at the end of an extruder blowing air through an
    orifice into the hot polymer extruded through a
    ring die produces plastic bags and films.

54
The Single Screw Plasticating Extruder
  • Regions 1, 2, 3 Handling of particulate solids
  • Region 3 Melting, pumping and mixing
  • Region 4 Pumping and mixing
  • Regions 34 Devolatilization (if needed)

55
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56
Cast Film Extrusion
  • In a cast film extrusion process, a thin film is
    extruded through a slit onto a chilled, highly
    polished turning roll, where it is quenched from
    one side. The speed of the roller controls the
    draw ratio and final film thickness. The film is
    then sent to a second roller for cooling on the
    other side. Finally it passes through a system of
    rollers and is wound onto a roll.
  • Thicker polymer sheets can be manufactured
    similarly. A sheet is distinguished from a film
    by its thickness by definition a sheet has a
    thickness exceeding 250 mm. Otherwise, it is
    called a film.

57
Sheeting Dies
  • One of the most widely used extrusion dies is
    the coat-hanger or sheeting die. It is used to
    extrude plastic sheets. It is formed by the
    following elements
  • Manifold evenly distributes the melt to the
    approach or land region
  • Approach or land carries the melt from the
    manifold to the die lips
  • Die lips perform the final shaping of the melt.
  • The sheet is subsequently pulled (and cooled
    simultaneously) by a system of rollers

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Blown Film Extrusion
  • Film blowing is the most important method for
    producing Polyethylene films (about 90 of all PE
    film produced)
  • In film blowing a tubular cross-section is
    extruded through an annular die (usually a spiral
    die) and is drawn and inflated until the frost
    line is reached. The extruded tubular profile
    passes through one or two air rings to cool the
    material.
  • Most common materials LDPE, HDPE, LLDPE

59
Coextrusion
  • In coextrusion two or more extruders feed a
    single die, in which the polymer streams are
    layered together to form a composite extrudate.

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Process Thermoplastic (TP) or Thermoset (TS) Advantages Disadvantages
Inj TP, TS It has the most precise control of shape and dimensions, is a highly automatic process, has fast cycle time, and the widest choice of materials. It has high capital cost, is only good for large numbers of parts, and has large pressures in mold (20,000 psi).
Comp TS It has lower mold pressures (1000 psi), does minimum damage to reinforcing fibers (in composites), and large parts are possible. It requires more labor, longer cycle than injection molding, has less shape flexibility than injection molding, and each charge is loaded by hand.
Trans TS It is good for encapsulating metal parts and electronic circuits. There is some scrap with every part and each charge is loaded by hand.
Blow TP It can make hollow parts (especially bottles), stretching action improves mechanical properties, has a fast cycle, and is low labor. It has no direct control over wall thickness, cannot mold small details with high precision, and requires a polymer with high melt strength.
Extru TP It is used for films, wraps, or long continuos parts (ie. pipes). It must be cooled below its glass transition temperature to maintain stability.
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Product Shaping / Secondary Operations
EXTRUSION
Final Product (pipe, profile)
  • Secondary operation
  • Fiber spinning (fibers)
  • Cast film (overhead transparencies,
  • Blown film (grocery bags)

Shaping through die
  • Preform for other molding processes
  • Blow molding (bottles),
  • Thermoforming (appliance liners)
  • Compression molding (seals)

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Annular (Tubular) Dies
  • In a tubular die the polymer melt exits through
    an annulus. These dies are used to extrude
    plastic pipes. The melt flows through the annular
    gap and solidifies at the exit in a cold water
    bath.

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Profile dies
  • Profiles are all extruded articles having
    cross-sectional shape that differs from that of a
    circle, an annulus, or a very wide and thin
    rectangle (such as flat film or sheet)
  • To produce profiles for windows, doors etc. we
    need appropriate shaped profile dies. The
    cross-section of a profile die may be very
    complicated

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Secondary Shaping
  • Secondary shaping operations occur immediately
    after the extrusion profile emerges from the die.
    In general they consist of mechanical stretching
    or forming of a preformed cylinder, sheet, or
    membrane. Examples of common secondary shaping
    processes include
  • Fiber spinning
  • Film Production (cast and blown film)

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Fiber Spinning
  • Fiber spinning is used to manufacture synthetic
    fibers. A filament is continuously extruded
    through an orifice and stretched to diameters of
    100 mm and smaller. The molten polymer is first
    extruded through a filter or screen pack, to
    eliminate small contaminants. It is then extruded
    through a spinneret, a die composed of multiple
    orifices (it can have 1-10,000 holes). The fibers
    are then drawn to their final diameter,
    solidified (in a water bath or by forced
    convection) and wound-up.

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Fiber Spinning
  • Melt spinning technology can be applied to
    polyamide (Nylon), polyesters, polyurethanes and
    polyolefins such as PP and HDPE.
  • The drawing and cooling processes determine the
    morphology and mechanical properties of the final
    fiber. For example ultra high molecular weight
    HDPE fibers with high degrees of orientation in
    the axial direction have extremely high stiffness
    !!
  • Of major concern during fiber spinning are the
    instabilities that arise during drawing, such as
    brittle fracture and draw resonance. Draw
    resonance manifests itself as periodic
    fluctuations that result in diameter oscillation.

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Thermoforming
  • Thermoforming is an important secondary shaping
    operation for plastic film and sheet. It consists
    of warming an extruded plastic sheet and forming
    it into a cavity or over a tool using vacuum, air
    pressure, and mechanical means. The plastic sheet
    is heated slightly above the glass transition
    temperature for amorphous polymers, or slightly
    below the melting point, for semi-crystalline
    polymers. It is then shaped into the cavity over
    the tool by vacuum and frequently by plug-assist.

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Thermoforming
  • Thermoforming is used to manufacture refrigerator
    liners, shower stalls, bathtubs and various
    automotive parts.
  • Amorphous materials are preferred, because they
    have a wide rubbery temperature range above the
    glass transition temperature. At these
    temperatures, the polymer is easily shaped, but
    still has enough melt strength to hold the
    heated sheet without sagging. Temperatures about
    20-100C above Tg are used.
  • Most common materials are Polystyrene (PS),
    Acrylonitrile-Butadiene-Styrene (ABS), PVC, PMMA
    and Polycarbonate (PC)

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  • Recycled polymers Eg A typical park.
  • Recycling gives new life to the things we use.
  • It can - conserve valuable resources landfill
    space, energy, raw materials.
  • But recycling also takes effort. One place to
    start is looking at the recycling codes on
    different packages. The numbers and letters by
    the triangle will help to sort plastics for
    recycling.
  • Get trash to the recycling center as well.
  • Community offers curbside recycling. If not,
    maybe we need to set up a recycling center near
    us.

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  • Recycling Today's Challenge, Tomorrow's Reward
  • Overview
  • Consumer waste poses a challenge to everyone.
  • Waste solid materials can be grouped into the
    following categories
  • metals - aluminum, steel, etc.
  • glass- clear, colored, etc.
  • paper - newsprint, cardboard, etc.
  • natural polymers- leather, grass, leaves, cotton,
    etc.
  • synthetic polymers - synthetic rubbers,
    polyethylene terephthalate, polyvinyl chloride,
    etc.

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  • Plastics constitute between 14 and 22 of the
    volume of solid waste.
  • One possible answer to this problem is recycling.
  • In 1990, 1 to 2 of plastics, 29 of aluminum,
    25 of paper, 7 of glass, and 3 of rubber and
    steel as post consumer wastes were recycled.
    Obviously, increasing the amount of plastics
    recycled would appear to be the answer. However,
    a major handicap in the reuse of plastics is that
    reprocessing adds a heat history, degrades
    properties and makes repeat use for the same
    application difficult. For example, the 58 gram,
    2-liter polyethylene terephthalate (PET) beverage
    bottle consists of 48 g of PET, the rest being a
    high density polyethylene (HDPE) cup base, paper
    label, adhesive, and molded polypropylene (PP)
    cap. The cup base, label, adhesive and cap are
    contaminants in the recycling of the PET.

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  • contaminants issue in plastic recycling, plastic
    products designed "reuse-friendly". Products
    made with recyclability as a viable means for
    disposal. PET for cost effective recycling.
    plastic beads are being used to remove paint from
    aircraft employing a "sand blasting" type method.
    In place of harsh, environmentally unfriendly
    chemical solvents use.
  • Another reason for not discarding plastics is the
    conservation of energy. The energy value of
    polyethylene (PE) is 100 of an equivalent mass
    of 2 heating oil. Polystyrene (PS) is 75, while
    polyvinyl chloride (PVC) and PET are about 50.
    With the energy value of a pound of 2 heating
    oil at 20,000 B.T.U., land filling plastics
    results in a waste of energy. Some countries,
    notably Japan, tap into the energy value of
    plastic and paper with waste-to-energy
    incinerators.

73
  • Another factor is the economic trend of
    progressively increasing tipping fees at
    landfills. As the cost of land filling of solid
    waste increases, so does the incentive to
    recycle. When the cost of land filling exceeds
    the cost of recycling, recycling will be a
    practical alternative to land filling.
  • Tipping fees, the charge to the waste hauler for
    dumping a load of solid waste, have been
    increasing regularly. Municipalities have imposed
    restrictions and/or have banned the startup of
    new landfills within their boundaries. As an
    example, 50 of New Jersey's solid waste is
    shipped out of state for landfill burial.
  • These factors led to certain recommendations by
    the United States Environmental Protection
    Agency. EPA's recommendations are source
    reduction, recycling, thermal reduction
    (incineration), and land filling. Each of these
    is not without its problems. Source reduction
    calls for the redesigning of packaging and/or the
    use of less, lighter, or more environmentally
    safe materials. The trade-off could mean reduced
    food packaging with the possibility of higher
    food spoilage rates. There would be fewer
    plastics, but more food in solid waste to be
    disposed. Whatever disposal method is chosen, the
    choice is complex. Whatever the costs, the
    consumer will bear them.

74
  • Today, consumers are using more products and,
    therefore, producing more solid waste. As time
    goes by, we find ourselves with less space to put
    this waste. Eighty percent of all solid waste is
    buried in landfills. Today there are one third
    fewer landfills in operation than the 18,500
    available a decade ago, making land-filling much
    more expensive.
  • The amount which synthetic polymers contribute to
    the weight of solid waste will continue to go up
    as the use of plastics increases

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  • Recycling of Different Plastics
  • PET (Poly Ethylene Terephthalate)
  • In 1989, a billion pounds of virgin PET were used
    to make beverage bottles of which about 20 was
    recycled. Of the amount recycled, 50 was used
    for fiberfill and strapping. The reprocessors
    claim to make a high quality, 99 pure,
    granulated PET. It sells at 35 to 60 of virgin
    PET costs.
  • The major reuses of PET include sheet, fiber,
    film, and extrusions. When chemically treated,
    the recycled product can be converted into raw
    materials for the production of unsaturated
    polyester resins. If sufficient energy is used,
    the recycled product can be depolymerized to
    ethylene glycol and terephthalic acid and then
    repolymerized to virgin PET.

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  • HDPE (high density polyethylene)
  • Of the plastics that have a potential for
    recycling, the rigid HDPE container is the one
    most likely to be found in a landfill. Less than
    5 of HDPE containers are treated or processed in
    a manner that makes recycling easy. Virgin HDPE
    is used in opaque household and industrial
    containers used to package motor oil, detergent,
    milk, bleach, and agricultural chemicals.
  • There is a great potential for the use of
    recycled HDPE in base cups, drainage pipes,
    flower pots, plastic lumber, trash cans,
    automotive mud flaps, kitchen drain boards,
    beverage bottle crates, and pallets. Most
    recycled HDPE is a colored opaque material, that
    is available in a multitude of tints.

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  • PVC (polyvinyl chloride)
  • There is much controversy concerning the
    recycling and reuse of PVC due to health and
    safety issues. When PVC is burned, the effects on
    the incinerator and quality of the air are often
    questioned. The Federal Food and Drug
    Administration (FDA) has ordered its staff to
    prepare environmental impact statements covering
    PVC's role in landfills and incineration. The
    burning of PVC releases toxic dioxins, furans,
    and hydrogen chloride. These fumes are
    carcinogenic, mutagenic, and teratagenic. This is
    one of the reasons why PVC must be identified and
    removed from any plastic waste to be recycled.
  • .

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LDPE (low density polyethylene)
  • LDPE is recycled by giant resin suppliers and
    merchant processors either by burning it as a
    fuel for energy or reusing it in trash bags.
    Recycling trash bags is a big business. Their
    color is not critical, therefore, regrinds go
    into black, brown, and to some lesser extent,
    green and yellow bags.

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  • PS (Polystyrene)
  • PS and its manufacturers have been the target of
    environmentalists for several years. The
    manufacturers and recyclers are working hard to
    make recycling of PS as common as that of paper
    and metals. One company, Rubbermaid, is testing
    reclaimed PS in service trays and other utility
    items. Amoco, another large corporation,
    currently has a method that converts PS waste,
    including residual food, to an oil that can be
    re-refined.

80
  • Currently, PVC is used in food and alcoholic
    beverage containers with FDA approval. The future
    of PVC rests in the hands of the plastics
    industry to resolve the issue of the toxic
    effects of the incineration of PVC.
  • PVC accounts for less than 1 of land fill waste.
    When PVC is properly recycled, the problems of
    toxic emissions are minimized. Various recyclers
    could reclaim PVC without the health problems.
    Uses for recycled PVC include aquarium tubing,
    drainage pipe, pipe fittings, floor tile, and
    nonfood bottles. When PVC is combined with other
    plastic waste it is used to produce plastic lumber

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  • A potential use as plastic lumber.
  • Recycled plastic is mixed with wood fibers
    and processed into a replacement for lumber. The
    wood fibers would have become land fill if not
    reused. The end product is called Biopaste. This
    is expected to eventually become a multi-million
    dollar enterprise. R D continue to improve this
    product.
  • Recycling is a cost effective means of dealing
    with consumer plastic waste. Research to reduce
    the cost of recycling needs to continue.
    Recycling of plastics is not going to reach the
    level of the recycling programs of paper and some
    metals until lower cost, automatic methods of
    recycling are in place. Fortunately, the
    solutions to these problems are not beyond the
    scope of our technology or our minds.

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Resin Name Common Uses Examples of Recycled Products
(PET or PETE) Soft drink bottles, peanut butter jars, salad dressing bottles, mouth wash jars Liquid soap bottles, strapping, fiberfill for winter coats, surfboards, paint brushes, fuzz on tennis balls, soft drink bottles, film
(HDPE) Milk, water, and juice containers, grocery bags, toys, liquid detergent bottles Soft drink based cups, flower pots, drain pipes, signs, stadium seats, trash cans, re-cycling bins, traffic barrier cones, golf bag liners, toys
(PVC-V) Clear food packaging, shampoo bottles Floor mats, pipes, hoses, mud flaps
(LDPE) Bread bags, frozen food bags, grocery bags Garbage can liners, grocery bags, multi purpose bags
(PP) Ketchup bottles, yogurt containers, margarine, tubs, medicine bottles Manhole steps, paint buckets, videocassette storage cases, ice scrapers, fast food trays, lawn mower wheels, automobile battery parts.
(PS) Video cassette cases, compact disk jackets, coffee cups, cutlery, cafeteria trays, grocery store meat trays, fast-food sandwich container License plate holders, golf course and septic tank drainage systems, desk top accessories, hanging files, food service trays, flower pots, trash cans
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SUPER PLASTICS
The substance (classed as an organic
semiconductor) consists of snowflake-shaped
molecules and can be used in a variety of light-
emitting forms from mobile phone displays to food
packaging. It will also be possible to use the
material to light up wallpaper in a variety of
colours as an alternative to traditional overhead
lighting. The material is also so flexible and
durable that it could be applied to clothing in
everything from school uniforms to sports gear.
Semi conducting plastic can amplify light -
making it one thousand times brighter. This work
could, in the future, make the internet faster.
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  • The Future
  • Recycling is a viable alternative to all other
    means of dealing with consumer plastic waste.
  • In response to the problem of mixed plastic
    waste, a coding system has been developed and
    adopted by the plastic industry. The code is a
    number and letter system. It applies to bottles
    exceeding 16 ounces and other containers
    exceeding 8 ounces. The number appears in the 3
    bent arrow recycling symbol with the abbreviation
    of the plastic below the symbol.
  • Western European companies, egHoechst and Bayer,
    have entered the recyclable plastic market with
    success. With a high tech approach, they are
    devising new methods to separate and handle mixed
    plastics waste.
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