This document discusses various types of plastics, additives, and production processes. It describes the molecular structures of polymers as linear, branched, or cross-linked. It also explains the three main polymerization reactions - addition, condensation, and copolymerization. Finally, it outlines several common plastic materials and processing methods like injection molding, blow molding, and vacuum forming.

1. POLYMERS

2. PLASTIC ADDITIVES
 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.

3. POLYMER STRUCTURE

4. The molecular structure of plastics can be;
 Linear chain,
 Branch chain,
 Cross-links.

5. 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.

6. Cross links
Primary bonding occurs with cross linking of adjacent
molecules. This results in a rigid, non-reversible
structure.

7. POLYMERISATION
 The process of chemically producing plastics.
 Addition polymerisation – chain growth.
 Condensation polymerisation – step growth.
 Copolymerisation – different types of mers
are linked.

8. ADDITION POLYMERISATION
 The creation of long chain molecules by adding
large numbers of mers. Many thermoplastics are
produced in this way, eg polyethylene.
 Initiation – A catalyst (free radical) is added to the
C2H2 double bond. One bond is broken and Carbon
atoms link with other Carbon atoms.
 Propogation - Long chain molecules form quickly as
the unpaired electron acts as a radical for the next
monomer.

9.  Termination – a second radical may end the
process

10. CONDENSATION
POLYMERISATION
 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.
 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 cross-link structure.

12. COPOLYMERISATION
 The joining of two different mers. This allows for a
diverse range of properties

13. THERMOPLASTICS
 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.

14. Common thermoplastics
 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.

15.  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.
 Polyvinyl Chloride. Rigid uPVC is used for
gutters and doors. The plasticised version
is more flexible and used for coats,
suitcases, hose pipes, etc.

16.  Acrylic. PMMA can be transparent or
opaque and is brittle but machines well. It
is used for car lights, shop signs, safety
glasses, etc.
 Nylon. Produced by condensation
polymerisation, Nylon is hard wearing and
resistant to chemicals. Used for clothing,
nets, ropes, gears.

17. THERMOSET PLASTICS
 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.

18. Common thermosets
 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.
 Epoxy resins. Cross-link polymers used for
adhesives.
 Polyurethanes. Has a wide range of
properties as structure can be adjusted from
soft ‘foam’ rubber to hard steering wheels
and paints.

19.  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.

20. ELASTOMERS
 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.

21. PROCESSING PLASTICS
 Calendering – produces sheets by rolling into
shape.
 Lamination – layers of materials (eg paper,
cloth) are bonded with a resin into a strong
solid structure, often with heat and pressure.
 Foaming – expansion into sponge-like
material by a foaming agent.

22. Extrusion
 Produces tubes, rods and other shaped continuous
form lengths.
 Heated polymer is fed into shaped die by a screw.

23. Injection Moulding
 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.

24.  A – split mould
 B – heater
 C – hopper
 D – hydraulic
ram
 E – torpedo
(spreader)
Parts of Injection Moulding Machine

25.  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.

26. Blow Moulding
 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.

27. Compression Moulding

28.  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.

29. 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.

30. Rotational Moulding
 A measured amount of polymer is placed in a
shaped mould cavity.
 The mould is closed and rotated until the
ploymer lines the mould and is then opened.
 Plastic footballs are produced in this way.

31. Vacuum Forming
 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.