plastics technology
DESCRIPTION
plastic technologyTRANSCRIPT
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Technology
Plastics
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What are plastics?
A diverse group of materials which have chain-like
molecular structures
Most plastics are produced from petrochemical
products mostly by the polymerisation process
There are natural plastic, e.g. rubber and cellulose
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Low-density non-load-bearing construction material
Not subject to corrosion, but they may be degraded by
the action of direct sunlight
PVC (polyvinyl chloride) is an example of plastic, used
in :
pipes,
electrical cable insulation,
windows
Plastics: Key properties
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Typical polymerisation process
Ethylene >>>>>>>> polyethylene (polythene)
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Another typical polymerisation process
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Summary: Polymerisation process
Monomer >>>>>>>> Polymer
Examples
Ethylene >>>>>>>> polyethylene
(polythene)
Viny chloride >>>>>>>> Polyvinyl chloride
Styrene >>>>>>>> Polystyrene
Tetrafluoroethylene >>>>>>>> Polytetrafluoroethyne
(PTFE)
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Condensation polymerisation
Process of incorporating oxygen or nitrogen atoms into the
backbone of the macromolecular chains
Example: Polyester (resins) and polyamides nylons)
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Branched chains
Polymer chains may be STRAIGHT or BRANCHED,
affecting the closeness of packing of the chains and
therefore the bulk density of the materials
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Copolymers
Two or more different monomers are polymerised
together
The properties of the copolymer will be dependent upon
whether the two components have joined together in
alternating, random or block sequences
More complex plastics can be produced
Styrene-Acrylonitrile and Butadiene-Styrene rubber
combines to produce Acrylonitrile Butadiene Styrene
(ABS)
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Random, alternate and block
copolymers
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Crystallinity
Most polymers consist of amorphous randomly-
orientated molecular chains
Stretching of the polymer in one direction may causes
an alignment of the molecular chains.
Alignment may create crystalline regions by solidification
of simple close packing of polymers such as
polyethylene, leading to partial formation of crystalline
regions and an associated anisotropy
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Crystallinity in polymers
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Plastics: Classes
Polymers are normally categorised in respect of their
physical properties as either :
Thermoplastic
Thermosetting, or
Elastomeric
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Thermoplastics
Thermoplastics soften upon heating. The process is
reversible and the plastic resets on cooling
The material is unaffected by repeating the cycle,
providing that excessive temperatures, which would
cause polymer degradation, are not applied
Many thermoplastics are soluble in organic solvents,
whilst others swell by solvent absorption
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Thermosetting plastics
Have a three-dimensional cross-linked structure
Formed by the linkage of adjacent macromolecular chains
They are not softened by heating
Can also be produced mixing two components, such as a
resin and a hardener
They are usually solvent-resistant and harder than
thermoplastics
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Thermosetting plastics: Cross-linking
- Essentially this is the
macromolecular component
- Cross-links the
liquid resin into a thermoset plastic
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Elastomers
Long-chain polymers with helical or zig-zag molecular chains.
The helical chains are free to straighten when the material is
stretched, then recover when the load is removed
The degree of elasticity depends on the extensibility of the
polymeric chains
When sulfur is added to rubber, the vulcanisation process will
restricts its movement by locking together adjacent polymer
chains
Cross-linking is required to ensure that an elastomeric returns
to its original form when the applied stress is removed
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Elastomers: Effect of cross-linking
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Additives
Plasticisers are frequently incorporated into plastics to
increase their flexibility
Fillers such as chalk, sand, china clay or carbon black
are often added to plastics to reduce costs, improve fire
resistance or fire capacity
Dyes and pigments may be added to the monomer or
polymer
Stabilizers are added to absorb ultraviolet light which
otherwise would cause degradation
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Degradation of plastics
Attributed to the breakdown of the long molecular chains
In the case of PVC, degradation is loss of plasticiser
Surface stress cracks may develop where degradation has
caused cross-linking, resulting in embrittlement of the
surface
Discolouration occurs through the production of molecular
units with double bonds, usually causing a yellowing of the
plastic
Task 1
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Degradation of plastic
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Fire properties of plastic
All plastics are combustible, producing noxious fumes and smoke
Carbon monoxide may be produced
Plastics containing nitrogen, such as polyurethane foam, generate
hydrogen cyanide (danger!)
PVC produces hydrochloric acid
Some plastics have a high surface spread of flame and produce
burning droplets
Some plastics are self-extinguishing
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MATERIALS BEHAVIOUR IN FIRE
Thermoplastics
1 Polythene/polypropylene Melts and burns readily
2 Polyvinyl chloride Melts, does not burn easily, but emits
Smoke and hydrogen chloride
3 Polytrafluoethylene (PTFE) Does not burn, but at high temperatures
It evolves toxic fumes Ethylene Tetrafluoroethylene (ETFE)
4 Polymethyl methacrylate Melts and burns rapidly, producing
droplets of flaming material
5 Polystyrene Melts and burns readily, producing dense
black smoke and droplets of flaming material
6 Acrylonitrile Butadiene Styrene
(ABS) copolymers
Burns readily
7 Polyurethane The foam burns readily producing highly toxic
fumes including cyanide and its variables
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MATERIALS BEHAVIOUR IN FIRE contd.
Thermosetting plastics
8 Phenol formaldehyde
Melamine formaldehyde
Urea formaldehyde
Resistant to ignition, but produces noxious
fumes including ammonia
9 Burns producing smoke, but flame-retarded
grades are available
10 Rubber
11 Neoprene Better fire resistance than natural rubber
Burns readily producing black smoke and
Sulphur dioxide
Glass reinforced polyester (GRP)
Elastomers Task 2
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Strength
Plastics have a good tensile strength to weight ratio
They have a low modulus of elasticity which renders them
unsuitable for most load-bearing situations, although,
some – e.g. glass-fibre reinforced polyester (GRP) can be
used for some limited load-bearing applications
Thermoplastics are subject to creep under ambient
conditions
Task 3
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Thermal expansion
The thermal expansion of most plastics is high, and plastics
in general have a high coefficients of linear expansion.
However, the expansion of glass-fibre reinforced polyester
(GRP) is low, and is similar to that of aluminum
Typical coefficients of linear expansion are :
Polythene (HD) 110-130,
Polypropylene 110,
ABS 83-95,
PVC 40-80,
GRP 20-35 X 10-6 deg C-1
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Thermal & Moisture movement
Attention must be paid to careful detailing of plastics, to
allow for adequate thermal movement
Most plastics are resistant to water absorption, and
therefore do not exhibit moisture movement
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Thermoplastics
1 Polythene (low density) Damp Proof Course (DPC), Damp Proof Membrane
(DPM), vapour checks, roof sarking
2 Polythene (high density) Cold-water tanks, cold-water plumbing
3 Polypropylene Pipework and fittings, drainage systems, water tanks,
WC systerns,
4 Polybutylene Hot and cold-water pipework and fittings
5 Polyvinyl chloride (PVC-U) Rainwater goods drainage systems, pipes and fittings,
underground services, window/door frames, garage
door, conservatories, translucent roofing sheets
Plastics: Typical uses in construction
Material Examples of Plastics in construction
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Examples
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Practical applications of plastic products in the
construction industry
Damp Proof Course (DPC)
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Practical applications of plastic products in the
construction industry
Rebar Chair and spacers
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Practical applications of plastic products in the
construction industry
Rebar Chair and spacers contd.
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Practical applications of plastic products in the
construction industry
Hydrophobic strips for Constriction joints
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Drainage pipes - Ultraviolet Radiation may cause discolouration
Practical applications of plastic products in the
construction industry
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Drainage/sewer pipes - Ultraviolet Radiation, causing discolouration
Practical applications of plastic products in the
construction industry
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Fire Degradation of plastics
Before
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After
Fire Degradation of plastics
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Before
Fire Degradation of plastics
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Fire Degradation of plastics
After
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Fire Degradation of plastics
Before
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Fire Degradation of plastics
During
firing
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Fire Degradation of plastics
After
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Questions?