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PLASTICS
Mr. Hart UWC Maastricht Diploma HL/SL
Why use plastics
•
Plastic are easily formed materials.
•
The advantage to the manufacturer is that plastic products can be massproduced and require less skilled staff.
•
Plastics require little or no finishing, painting, polishing etc. Plastic is
referred to as a self-finishing material. Particular finishes can be achieved
at relatively low cost.
•
Plastics can be easily printed, decorated or painted.
•
Plastics are corrosion resistant, and generally waterproof although
certain types of plastics such as UPVC can become brittle and it is possible
for the sun’s rays to cause the colour of the plastic to fade. It becomes
bleached.
•
Plastics are lighter than metals, giving deeper sections for a given
weight, and hence stronger sections.
Origins of Plastics - synthetic plastics.
•
The main source of synthetic plastics
is crude oil.
•
Coal and natural gas are also used.
•
Petrol, paraffin, lubricating oils and
high petroleum gases are bi-products,
produced during the refining of crude
oil.
•
These gases are broken down into
monomers. Monomers are chemical
substances consisting of a single
molecule.
•
A process called Polymerisation
occurs when thousands of monomers
are linked together. The compounds
formed as called polymers.
•
Combining the element carbon with
one or more other elements such as
oxygen, hydrogen, chlorine, fluorine
and nitrogen makes most polymers.
Natural Plastics
•
Natural ‘plastic products’ occur in such things as animals’ horns, animals’
milk, insects, plants and trees.
•
Animals horns - Casein (glue)
•
Animals milk - Formaldehyde (glue)
•
Insects - Shellac (French polishing)
•
Plants - Cellulose (table tennis balls), Cellulose acetate (cloth, photographic
film, handles), Cellophane (wrapping), Bitumen (roads, flat roofs)
•
Trees - Latex (rubber)
Thermoplastics
•
There are a wide range of
thermoplastics, some that are rigid and
some that are extremely flexible.
•
The molecules of thermoplastics are in
lines or long chains with very few
entanglements. When heat is applied
the molecules move apart, which
increases the distance between them,
causing them to become untangled.
This allows them to become soft when
heated so that they can be bent into all
sorts of shapes.
•
When they are left to cool the chains
of molecules cool, take their former
position and the plastic becomes stiff
and hard again. The process of
heating, shaping, reheating and
reforming can be repeated many
times.
Long chain molecules
Thermoplastics and Plastic
Memory
Each time a thermoplastic is reheated it will try and return to its
original shape, unless it has been damaged due to overheating or
overstretching. This property is called plastic memory.
This is why a shape formed in thermoplastic becomes flat when
reheated.
MELT / FREEZE cycle
Thermosetting plastics
•
The molecules of thermosetting
plastics are heavily cross-linked.
They form a rigid molecular
structure.
•
The molecules in thermoplastics
sit end-to-end and side-by-side.
•
Although they soften when heated
the first time, which allows them to
be shaped they become
permanently stiff and solid and
cannot be reshaped.
•
Thermoplastics remain rigid and
non-flexible even at high
temperatures. Polyester resin
and urea formaldehyde are
examples of thermosetting
plastics.
Cross-linked molecules
Expanded polystyrene
• This is used for
disposable food
packaging,
disposable cups, heat
insulation and
protective
packaging for
electrical equipment.
• Image: Protective
packaging
Clear Acrylic (Perspex)
• It was first used to
make aircraft
canopies. It is ten
times more impact
resistant than glass.
• Image: Perspex top
of a container
Polystyrene
• Polystyrene is used to
make plates, cutlery
and model kits.
• It is stiff hard and
comes in a wide
range of colours.
• Image: cup and
saucer
Nylon
• Nylon is hard, tough, selflubricating, has a high
melting point and has
very good resistance to
wear and tear.
• It has been used to make
clothing, bearings and
propellers.
• Image: A nylon castor
(wheel).
PVC
• The rigid type is used to
make pipes, guttering
and roofing. It is very
lightweight and is
resistant to acids and
alkalis.
• The plasticised type is
used for suitcases,
hosepipes, electrical
wiring and floor
coverings.
• Image: plumbing U-bend
Polythene
• High-density
polythene has been
used to manufacture
milk crates, bottles,
buckets, bowl and
gear wheels.
• It is stiff, hard, can be
sterilised and is
dense.
Injection Moulding of Plastics
•
1. Granules of plastic powder (note the
plastics listed above) are poured or fed into
a hopper which stores it until it is needed.
•
2. A heater heats up the tube and when it
reaches a high temperature a screw thread
starts turning.
•
3. A motor turns a thread which pushes the
granules along the heater section which
melts then into a liquid. The liquid is forced
into a mould where it cools into the shape (in
this case a DVD storage unit).
•
4. The mould then opens and the unit is
removed.
Further examples. . .
EXAMPLES:
Acrylic:
This is a hard plastic
that can be clear
and has good optical
properties (can be
used for lighting).
Light up signs
Display stands
Acrylic - hard, clear, good optical properties (can be used
for lighting) easily machined, polishes well.
It can be easily
machined and
polishes well to give
a good finish.
EXAMPLES:
High Impact
Polystyrene
(HIPS):
This plastic is
light but can still
be hard and
stiff.
CD case
Audio tape casing
High Impact Polystyrene (HIPS) - Light, hard, stiff, water
resistant, can be transparent or coloured.
It is water
resistant and
can be
transparent or
coloured.
EXAMPLES:
High Density
Polythene
(HDPE):
This plastic can
be easily
coloured and is
ideal for toys.
Utensils
Toys
High Density Polythene (HDPE) - stiff, strong plastic, can
be easily coloured and is often used for toys.
It is also stiff and
strong.
EXAMPLES:
(ABS):
This plastic is
good for
products that
need high impact
strength, need to
be tough, scratch
and chemical
resistant.
Household Appliances
Acrylonitrile butadienestyrene (ABS) - high impact
strength, tough, scratch and chemical resistant.
QUIZ…
WHICH TYPE OF
THERMOPLASTIC…
Acrylic, HIPS, HDPE or ABS?
ABS
HDPE
HIPS
ABS
HDPE
Acrylic
Acrylic
THERMOPLASTICS - EVALUATION QUESTIONS:
1) What are thermoplastics?
2 ) Why do we use them?
3 ) When wouldn’t we use them?
4) identify one example of:
HIPS
Acrylic
HDPS
ABS
and give reasons why it is used.