Transcript Slide 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.
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.
•That’s good for the environment.
4
Polymers at Home
2
3
What makes all these different? 1
Each connects with a different kind of
human-made polymer that we encounter in
our homes every day.
• 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.
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.”
•
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.
• 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 sulphur—or
“vulcanized”—would 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.
• 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 earth’s protective ozone layerhence phased out their use in the creation of
foam packaging and most other types of
polystyrene foam.
Polymers in Nature
Everything we see in natureWhat 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.
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 deer’s 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
doesn’t get waterlogged the moment he dives in the
pool.
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.
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
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 fractureresistant 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.”
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
• 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 USPharmacopeia, 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.
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.
•
•
•
•
•
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 R&D,
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:
Polymerization Reactions
•
The chemical reaction in which high
molecular mass molecules are formed from
monomers is known as POLYMERIZATION
•
Two basic types of polymerization:
1. chain-reaction (or addition) polymerization.
2. step-reaction (or condensation) polymerization.
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 carboncarbon double bond.
• Initiation
• Propagation
• Termination
Ethylene is one example of a monomer
used to make a common polymer
• 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
• Step 1: Initiation
The first step, initiation, occurs when the freeradical 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.
• 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.
• 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.
• 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.
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.
• 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.).
Eg:Formation 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.
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.
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 chainstructure 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:
Copolymer configurations
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.
• 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.
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 semicrystalline 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.
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.
• 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.
Unit Operations in Polymer Processing
Thermoplastic and thermoset melt processes may be broken down into:
• Preshaping
• Shaping
• Shape
Stabilization
Introduction
Chee 390
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
Introduction
Chee 390
Polymer Processing
Five basic processes to form polymer products or
parts.
They are:
1.
2.
3.
4.
5.
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.
Injection Molding
Common process for forming plastics- involves four steps:
1. Powder or pelletized polymer is heated to the liquid state.
2. Under pressure, the liquid polymer is forced into a mold
through an opening, called a sprue. Gates control the flow
of material.
3. The pressurized material is held in the mold until it
solidifies.
4. 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.
Diagram of injection molding
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.
Introduction
Chee 390
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.
Introduction
Chee 390
Injection Molding Cycle
 The total cycle time is: tcycle=tclosing+tcooling+tejection.
Introduction
Chee 390
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
Introduction
Chee 390
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, crosslinking 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.
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.
Introduction
Chee 390
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 crosslinking.
• 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.
•
•
•
•
•
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
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.
Introduction
Chee 390
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.
Introduction
Chee 390
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.
Introduction
Chee 390
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.
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 3+4: Devolatilization (if needed)
Introduction
Chee 390
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.
Introduction
Chee 390
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
Introduction
Chee 390
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 crosssection 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
Introduction
Chee 390
Coextrusion
In coextrusion two or more extruders feed a single die, in which the
polymer streams are layered together to form a composite extrudate.
Introduction
Chee 390
Process
Inj
Comp
Trans
Blow
Extru
Thermoplastic (TP)
or Thermoset (TS)
Advantages
Disadvantages
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).
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.
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.
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.
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.
Product Shaping / Secondary Operations
EXTRUSION
Final Product (pipe, profile)
Shaping
through die
Secondary operation
Fiber spinning (fibers)
Cast film (overhead
transparencies,
Blown film (grocery bags)
Preform for other molding
processes
Blow molding (bottles),
Thermoforming (appliance
liners)
Compression molding
(seals)
Introduction
Chee 390
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.
Introduction
Chee 390
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
Introduction
Chee 390
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)
Introduction
Chee 390
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 110,000 holes). The fibers are then
drawn to their final diameter, solidified
(in a water bath or by forced
convection) and wound-up.
Introduction
Chee 390
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.
Introduction
Chee 390
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.
Introduction
Chee 390
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 20-100°C above Tg are used.
• Most common materials are Polystyrene (PS), Acrylonitrile-ButadieneStyrene (ABS), PVC, PMMA and Polycarbonate (PC)
Introduction
Chee 390
• 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.
• 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.
• 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.
• 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.
• 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.
• 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
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.
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.
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.
• .
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.
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.
• 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
• 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 multimillion 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.
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, fastfood sandwich container
License plate holders, golf course and septic tank
drainage systems, desk top accessories, hanging files,
food service trays, flower pots, trash cans
SUPER PLASTICS
The substance (classed as an organic semiconductor) consists of snowflakeshaped 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.
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, eg:Hoechst 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.