POLYMERS
Thermoplastics
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Thermoplastics soften when heated and
can be moulded into required shapes
when in this state. They will harden again
on cooling. By heating and cooling, they
can be softened and heated over and
over again.
Thermosetting plastics
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Thermosetting plastics undergo a
chemical change during moulding and
hardening and, therefore, cannot be
softened again by heating.
The structure of plastics
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The main raw material for plastics is
crude oil. The chemicals required are
obtained by refining the crude oil. These
chemicals are made from molecules
containing a no. of atoms. These small
molecules called monomers, react
together to form long chain molecules.
The chemical process of linking the
small molecules together is called
polymerisation. The product of
polymerisation is called a polymer.
Plastic additives
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Pigment – gives colour
Stabilisers – prevent degradation
Lubricants – makes moulding easier
Fillers – add strength to plastic eg wood
flour, glass fibre, cotton, etc.
Plasticiser – increases flexibility.
Polymer structure
The molecular structure of plastics can be;
 Linear chain,
 Branch chain,
 Cross-links.
Linear and Branch Chain
The bonding between
adjacent molecules is
secondary bonding or
Van der Waal forces of
attraction. These can be
weakened by heating.
Cross links
Primary bonding occurs with cross linking of adjacent
molecules. This results in a rigid, non-reversible
structure.
Polymerisation
The process of chemically producing plastics
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Addition polymerisation – chain growth.
Condensation polymerisation – step
growth.
Copolymerisation – different types of mers
are linked.
ADDITION POLYMERISATION
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The creation of long chain molecules by adding
large numbers of mers. Many thermoplastics
are produced in this way, eg polyethylene.
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Initiation – A catalyst (free radical) is added to
the C2H2 double bond. One bond is broken
and Carbon atoms link with other Carbon
atoms.
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Propogation - Long chain molecules form
quickly as the unpaired electron acts as a
radical for the next monomer.
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Termination – a second radical may end the
process
Condensation polmerisation
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The initial molecules are joined together
with small molecules, such as H2O,
being condensed out of the reaction.
Nylon and phenol formaldehyde are
produced in this way.
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Two phenol molecules are linked by the
formaldehyde molecule during the
reaction. The two phenols lose H atoms
and formaldehyde gives up an O atom.
Water is condensed out leaving a crosslink structure.
Co-polymerisation
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The joining of two different mers. This allows
for a diverse range of properties
Thermoplastics
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Can be moulded more than once.
Secondary bonds (Van der Waal forces) are
weak forces of attraction.
They are weakened by heating.
Linear or branch chain structure.
Common thermoplastics
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Low Density Polyethylene. Branch chain
polymer produces weak, soft and flexible
polymer. The most common polymer.
Used for buckets, bags, electrical cable,
cups, etc.
High Density Polyethylene. Linear chain
polymer which is stiff, strong and resists
chemicals. Used for bottle crates, barrels
and plumbing joints.
Polypropylene. Tough and rigid polymer
with high impact strength. Used for chairs,
helmets, hinged boxes and toys.
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Polystrene. Brittle in solid form and is
used for jugs, egg boxes, jar tops.
Expanded or foamed polystyrene is soft,
low density and a heat insulator and
used for coffee cups, packaging and
house insulation.
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Polyvinyl Chloride. Rigid uPVC is used
for gutters and doors. The plasticised
version is more flexible and used for
coats, suitcases, hose pipes, etc.
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Acrylic. PMMA can be transparent or
opaque and is brittle but machines well. It
is used for car lights, shop signs, safety
glasses, etc.
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Nylon. Produced by condensation
polymerisation, Nylon is hard wearing and
resistant to chemicals. Used for clothing,
nets, ropes, gears.
Thermosetting plastics
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Will only mould once.
Strong primary bonds are connected by
strong covalent bonds.
Cross-link process (curing) formed by
heat and pressure.
Non-reversible structure cannot be
softened by heating.
Common thermosets
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Polyester Resin. Polymerises at room
temperature with the addition of a
hardener. It is often reinforced with glass
fibre (GRP) for strengthening and is
used for boats, some vehicle bodies and
roofing.
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Epoxy resins. Cross-link polymers used
for adhesives.
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Polyurethanes. Has a wide range of
properties as structure can be adjusted
from soft ‘foam’ rubber to hard steering
wheels and paints.
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Phenol formaldehyde. Hard and brittle,
‘Bakelite’ has been used for electrical
fittings and saucepan handles.
Urea formaldehyde. Good electrical and
thermal properties and used for
electrical fittings and door handles.
Elastomers
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Produced by addition polymerisation.
Long chain structure, which is coiled,
giving elastic properties.
Natural rubber is produced from trees. It
can be vulcanised (cross-linked) by
adding sulphur.
Synthetic rubbers are manufactured with
a range of properties, eg ‘neoprene’.
Glass transition temperature below room
temp. changes from flexible to rigid solid.
Processing plastics
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Calendaring – produces sheets by
rolling into shape.
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Lamination – layers of materials (e.g.
paper, cloth) are bonded with a resin
into a strong solid structure, often with
heat and pressure.
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Foaming – expansion into sponge-like
material by a foaming agent.
Extrusion
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Produces tubes, rods and other shaped continuous
form lengths.
Heated polymer is fed into shaped die by a screw.
Injection Moulding
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A measured amount of molten thermoplastic is
driven by a ram past a heating system into the
mould.
The mould is split to allow finished object to be
removed after cooling.
Parts of an Injection Moulding
Machine
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A – split mould
B – heater
C – hopper
D – hydraulic
ram
E – torpedo
(spreader)
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Injection moulding produces accurate
and complex products with high quality
finish.
Production is fast with little waste.
Wide range of products including bowls,
buckets, containers, toys, electrical parts
and car parts.
Injection moulded parts can be
recognised by the distinctive circular
marks (5-10mm) caused by pins used to
remove object from it’s mould.
Blow Moulding
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Used to make bottles and hollow toys.
Air is blown into a plastic tube, called a parison,
to take the shape of the mould.
PVC and polythene are often used.
Blow moulding
Compression Moulding
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Thermoset plastics are shaped with heat
and pressure causing cross-linking.
The polymer can be in powder or slug
(cube) form.
Products such as electrical fittings,
saucepan handles and bottle tops are
often formed out of formaldehyde plastics.
High quality finishes are achieved with
only the removal of ‘flash’ (excess material
usually at the mould split) for finished
products.
Transfer Moulding
Thermoset polymers can be formed when a preset
amount of material is placed in a separate cavity
and heated. A plunger moves the material into the
shaped mould with high pressure.
Rotational Moulding
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A measured amount of polymer is placed in a
shaped mould cavity.
The mould is closed and rotated until the
polymer lines the mould and is then opened.
Plastic footballs are produced in this way.
Rotational moulding
Vacuum Forming
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Plastic sheet is clamped and heated.
Heat is removed and pattern raised.
Vacuum forces the sheet onto the pattern.
The sheet is removed and trimmed.