Plastics Technology

Technology

Plastics

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

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

Typical polymerisation process

Ethylene >>>>>>>> polyethylene (polythene)

Another typical polymerisation process

Summary: Polymerisation process

Monomer >>>>>>>> Polymer

Examples

Ethylene >>>>>>>> polyethylene

(polythene)

Viny chloride >>>>>>>> Polyvinyl chloride

Styrene >>>>>>>> Polystyrene

Tetrafluoroethylene >>>>>>>> Polytetrafluoroethyne

(PTFE)

Condensation polymerisation

Process of incorporating oxygen or nitrogen atoms into the

backbone of the macromolecular chains

Example: Polyester (resins) and polyamides nylons)

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

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)

Random, alternate and block

copolymers

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

Crystallinity in polymers

Plastics: Classes

Polymers are normally categorised in respect of their

physical properties as either :

Thermoplastic

Thermosetting, or

Elastomeric

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

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

Thermosetting plastics: Cross-linking

- Essentially this is the

macromolecular component

- Cross-links the

liquid resin into a thermoset plastic

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

Elastomers: Effect of cross-linking

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

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

Degradation of plastic

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

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

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

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

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

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

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

Examples

Practical applications of plastic products in the

construction industry

Damp Proof Course (DPC)



Rebar Chair and spacers



Rebar Chair and spacers contd.



Hydrophobic strips for Constriction joints

Drainage pipes - Ultraviolet Radiation may cause discolouration



Drainage/sewer pipes - Ultraviolet Radiation, causing discolouration



Fire Degradation of plastics

Before

After


Before



After


Before


During

firing


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Plastics Technology

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