Plastic Slides

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S. Venkannah Faculty of Engineering 1

MATERIALS SCIENCE- MECH 2121 - PLASTICS.

The term plastics has been applied to those synthetic

non-metallic materials that can be made sufficiently

fluid to be shaped readily by casting, molding orextruding and that may be subsequently hardened to

preserve the desired shape.

POLYMERIC MATERIALS: Synthetic large

molecules are made by joining together thousands of

small molecular units known as monomers. Addition polymerization Condensation polymerization

Examples of main additives:

Antistatic agents.

Coupling agents

Extenders

Fillers

Flame retardants.

Lubricants

Pigments

Plasticisers

Reinforcement.

Stabilizers

Catalysts.

There are two important classes of Plastics

(a) THERMOPLASTIC MATERIALS

e.g of thermoplastics :Acrylics, ABS, Polyethylene

,PVC, Polystyrene, Nylon, Cellulose acetate, Acetal,

Polycarbonate, Poly methyl mathacrylate (PMMA),

PTFE, Polyamides, Polyesters, Polyimides,

Polysulfones and Polypropylene.


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Thermoplastic are subdivided into crystalline

(ordered) or amorphous (random) structure.

(b)THERMOSETTING PLASTICS

A thermosetting plastic is produced by a chemical

reaction which has two stages. Thermosets are quite

rigid materials and their mechanical properties are

not heat sensitive.

Examples are Alkyds, Aminos, Silicones, Phenol

formadehyde, Melamine formaldehyde, Urea

formaldehyde, Epoxies and some Polyesters.

PLASTICS AVAILABLE TO ENGINEERS,

Engineering plastics

Thermosets

Thermoplastics

Composites

Structural foam

Elastomers

Polymer alloys

Liquid crystal polymers

Main properties of plastics:


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Light weight

Wide range of colors

Resistance to deterioration by moisture

Fairly good resistance to attack by inorganic acids, saltsand bases

Plastics in many cases are non-flammable, self

extinguishing, and burn very slowly

Can be transparent, translucent or opaque

Good dimensional stability

Good sound absorption properties

Good thermal and electrical insulator

Good tensile strength

Can be easily be fixed in position

Good resistance to peeling

Quite durable

Easy maintenance of plastic surfaces

Factors affecting the properties of plastics:

Branching : Copolymerization: Compounding polymers: . Blending of plastics:

Commercial forms available:

Plastic sheets

Plastic tiles

Plastic laminates

Plastic panels

Plastic pipes


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Building Blocks used for certain desired properties:

Material Property Enhancement

Polycarbonate, Acrylic,

ABS, Ethylene-propylenerubber

Impact strength

Polyphenylene oxide,

polycarbonate,

polysulfone, polyimide

Heat resistance, tensile

strength, stiffness

PVC, chlorinated

polyethylene

Flame retardancy

PVC, Nylon, Polyester

(PET and PBT)

Chemical resistance

Nylon, Ethylene vinyl

alcohol

Barrier properties

Polystyrene, ABS,

Acrylic

Processability

SELECTION OF PLASTICS:

(i) Mechanical properties

(ii) Corrosion susceptibility and degradation.

(iii) Wear resistance and frictional properties

(iv) Special properties, e.g, thermal, electrical,optical and magnetic properties, damping

capacity.

(v) molding and/or other methods of fabrication.

(vi) Total cost attributable to the selected materials

and manufacturing route.


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Degradation:

thermal aging Stress whitening: Crazing

Physical or Chemical attack:

Oxidation

Environmental stress cracking:

SPECIAL PROPERTIES:

Thermal properties-Plastics tend to be rigid and brittleat low temps. and soft and flexible at high temps.

Impact resistance is improved at elevated temps.,

while mechanical properties are reduced and

destructive actions of external agents to which they

are sensitive are accelerated.

In general moderate temps. are required for storage

of plastics over long periods; low temps. are to be

avoided because of the low temp. brittleness of most of

the plastics; and high temps. should be avoided

because of rapid loss of mechanical properties

,volatization of plasticisers and the susceptibility of alarge number of plastics to distortion.

Electrical properties: Traditionally plastics have

established themselves in applications which require

electrical insulation.


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The thermal and electrical properties of some plastics

e.g. polyacetylene, Polyaniline, Polypynole,

Polythiophene, can be increased by doping. The

electrical conductivity also increases with absorptionof moisture.

Effects of light: Prolonged exposure to sunlight will

affect adversely all plastics with the exception of

tetrafluoroethylene(teflon).

Flammability: Critical Oxygen Index(COI) is defined

as the minimum concentration of Oxygen, expressed

as volume per cent, in a mixture of Oxygen and

Nitrogen that will just support combustion under the

conditions of test.

Effects of Oxygen: Organic plastics are nearly all

subject to oxidation when exposed to the atmosphere.

The process is accelerated by high temps. and light

but over long periods of time, oxidative deterioration

may take place at room temp.

Effects of moisture:Some polymers such as nylon areable to absorb water and makes the polymer more

plastic.


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::::EXTRUSION

INJECTION MOLDINGTHERMOFORMING

CALENDERING

EXTRUSION STRETCH BLOW MOULDING:

ROTOMOLDING:

EXPANDABLE BEAD MOLDING:

REACTION INJECTION MOLDING

COMPRESSION MOLDING

TRANSFER MOULDING.

BONDING OF THERMOPLASTICS:

Fusion bonding Solvent bonding Friction bonding Induction bonding Ultrasonic bonding Vibration bonding

MOLDING REINFORCED THERMOSETS.

HAND LAYUP

MATCHED DIE MOLDING

RIGIDIZED VACUUM FORMING

PULTRUSION

FILAMENT WELDING.


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CASTING:

a) Conventional casting method

b) Centrifugal castingc) Potting and encapsulation

d) Foam molding and casting

COLD FORMING:

The important considerations are:

The material must be sufficiently ductile at roomtemperature

The materials deformation must be nonrecoverable

SOLID PHASE FORMING:

RECYCLING OF PLASTICS

BIODEGRABLE PLASTICS:

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