Topic 6 : PLASTIC
INTRODUCTION
What are polymeric materials?

Polymers are organic materials made of very large molecules
containing hundreds of thousands of unit molecules called
“mers” linked in a chain-like structure (repeated pattern)

Polymers are characterized by:


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Low density materials (replace metals such as steel, aluminium
etc)
Versatility in synthesis – processing – properties relationship
Raw materials and processing are cost-effective
Recycling is possible and practical
Applications of Polymers
Bottles extrusion
blow molding
PP
Polycabonate roof
Classification of Polymers
POLYMERS
NATURAL
SYNTHETIC
e.g; wood, cotton, leather, skin,
hair
ELASTOMER
(RUBBER)
THERMOPLASTICS
PE, PVC, PP, PS
THERMOSETS
Epoxy, phenolic
resins
Chemical Composition of Polymers
 Polymers are classified into:
1. Homopolymers
 Only 1 type of repeat unit
Chemical Composition of Polymers
2. Copolymers
 At least 2 types of repeat unit
Molecular Chain
Structure of polymers consists of a large number of repeating
molecules to form a long flexible chain.
Thermoplastics: PVC, acrylic,
polyethylene
Polyethylene
1. Linear
2. Branched
Rubbers
3. Cross-linked
1
Strength increases
Thermostetting
polymers
4. Network
4
 Thermoplastics - Linear or branched polymers in
which chains of molecules are not interconnected to
one another.
 Thermosetting polymers - Polymers that are heavily
cross-linked to produce a strong three dimensional
network structure.
 Elastomers (Rubbers) - These are polymers
(thermoplastics or lightly cross-linked thermosets)
that have an elastic deformation > 200%.
Thermoplastics and Thermosettings
 Polymers are basically divided into:
1. Thermoplastics

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Solids at room temperature that are melted or softened by heating, placed
into a mould and then cooled to give the desired shape
Can be recycled
2. Thermosets


Can be either liquids or solids at room temperature that are place into a
mould and then heated to cure (set) or harden, thus giving the desired
shape and solid properties
Thermosets cannot be reshaped by heating
Thermoplastic materials
Thermoplastic
Uses
Mechanical properties
Polyethylene(PE)
Packaging bags, containers, piping,
chemical equipment, coating for
cables and wires.
Tough and flexible at temperatures.
Good dimensional stability. Easily
moulded. Good resistance to common
solvents.
Polypropylene(PP) Moulding for hospital and laboratory Similar properties to polythene,
equipment; chemical plant.
better heat resistance to chemical
attack. Tough, rigid and light in
weight.
Polystyrene(PS)
Refrigerator trays, boxes and many A tough, dense plastic-hard and rigid.
articles of household hollowware. Good dimensional stability. Moulds
Toys, display figures, etc.
with high surface gloss.
PE
Thermoplastic materials
Thermoplastic
Uses
Mechanical properties
Polyvinyl Chloride Mouldings. Imitation leather cloth, Soft, flexible and rubbery. Good
(PVC)
table cloths, raincoats; etc. Piping dimensional stability. Good resistance
and electrical cable covering.
to water, acids, alkalis and most
common solvent.
Polymethyl
Methacrylate
(Perspex)
Lenses, dentures, telephones, knobs Excellent transmission of light. Strong
and handles. Aircraft windows, and rigid, easily scratched. Attacked
sinks, building panels.
and many organic solvents.
Polyamides (Nylon) Plastic gears, valves, bearings, cams. Very strong and tough. Good
Raincoats. Climbing ropes, fishing resistance
to
abrasion.
Good
lines and textiles.
dimensional stability. Deteriorates
with long exposure to outdoor.
Thermoset materials

Thermoset parts are made from polymer resins that are capable of forming
chemical crosslinks.

As the number of crosslinks increases, the stiffness of the material also
increases. Thus, many thermosets are typically stiffer and more brittle than
thermoplastics

The impact toughness can be increases by adding fillers or reinforcements
(also increase strength)

Thermosets include:

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Phenolics (PF)
Amino plastics (UF and MF)
Polyester thermosets (TS)
Epoxies (EP)
Thermoset polyamides
Thermoset materials
Thermoset
Uses
Mechanical properties
Phenol
formaldehyde
Electrical equipment, handles buttons, Filled with fibrous materials to
radio cabinets,
engine ignition increase strength. Absorbs water but
equipment,
cheap
jewellery, resistant to alcohol, oils and most
ornaments.
common solvents.
Epoxides
Sold as resins and syrup. Used as Commonly used with glass fibre for
adhesives for gluing metal. Laminates increased strength. Good adhesion to
(with glass fibre): boat hulls, table top. metals. Good resistance to water and
most solvents.
Melamine
formaldehyde
As syrups and resins : surface coating Properties are generally similar to
of papers and textiles , laminates. those of urea formaldehyde but with
Mouldings : electrical equipment, table improved resistance to heat.
ware.
The advantages of using plastics as
engineering materials:
 Good electrical insulation
 Excellent heat resistance
 Shock – resistant
 Good resistance to attack by acids and alkalis.
 Self-lubrications.
The disadvantages of using plastics:
 Low range temperatures operation (150-600 oF).
 Adversely affected by prolonged exposure to ultra-violet
light.
 Not suitable for outdoors.
 Ductile in low temperatures.
 Deteriorate with atmospheric moisture and temperatures.
Processing of Plastics
 Plastics can be formed from granular and pallets into shape product
such as sheet, rods, extruded sections, pipe, or finished moulded
parts.
 Process used depends on whether the plastic is a thermosetting or
thermoplastic.
 Among of the processes:
 Injection moulding
 Extrusion moulding
 Compression moulding
 Transfer moulding
 Blow moulding
Injection Moulding
 Widely used process of forming thermoplastic materials.
 The basic process involves six major steps in the moulding
cycle:
 The hopper is loaded with granular plastic materials.
 Heat is applied to the plastic until it becomes soft enough to
flow.
 The softened plastic is forced through a nozzle into the mould
cavity.
 When cool, the halves of the mould are separated.
 The part is ejected from the mould.
 Gates connecting the product to the runner system are
removed.
Processing Plastics - Molding
 Compression and transfer molding
 thermoplastic or thermoset
Adapted from Fig. 15.23,
Callister 7e. (Fig. 15.23 is from
F.W. Billmeyer, Jr., Textbook of
Polymer Science, 3rd ed.,
John Wiley & Sons, 1984. )
Compression Moulding

This process was the first to be used to form plastics. It involves four steps:
1.
Pre-formed blanks, powders or pellets are placed in the bottom section of a
heated mould or die.
The other half of the mould is lowered and is pressure applied.
The material softens under heat and pressure, flowing to fill the mould. Excess
is squeezed from the mould. If a thermoset, cross-linking occurs in the mould.
The mould is opened and the part is removed.
2.
3.
4.
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When thermoplastics are used, the mould is cooled before removal so the part
will not lose its shape.
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When thermosets are used, they 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.
Extrusion Moulding

This process makes parts of constant cross section like pipes and rods. liquid
polymer goes through a die to produce a final shape. It involves four steps:
1. Pellets of the polymer are mixed with colouring and additives.
2. The material is heated to its proper plasticity.
3. The material is forced through a die.
4. The material is cooled.
Blown-Film Extrusion
Adapted from Fig. 15.26, Callister 7e.
(Fig. 15.26 is from Encyclopædia
Britannica, 1997.)
Transfer Moulding
 Transfer moulding is most generally used for thermosetting
plastics.
 Method is like compression moulding in that the plastic is cured
into an infusible state in a mould under heat and pressure.
 In transfer moulding the plastic is heated to a point of plasticity
before it reaches the mould and forced into a closed mould by
means of a hydraulically operated plunger resulting in lower cavity
pressure.
Blow Moulding Process
 The blow moulding of thermoplastic materials began during
World War 11.
 Blow moulding is intended for use in manufacturing hollow
plastic products.
Blow Moulding Process
 The basic steps of blow moulding process are:
 Melt the material
 Form the molten resin into a tube or parison
 Enclose the hollow parison in the blow mould
 Inflate the parison inside the mould
 Cool the blow-moulded part
 Remove the part from the mould
 Trim flash, as needed