Met 402 mod-6

MODULE 6

OUTLINES

Polymers

Ceramics

Why Plastics? Plastic products can be mass-produced & require less skilled

staff. Light weight, high weight to strength ratio, particularly when

reinforced Relatively low cost compared to metals & composites Corrosion resistance & generally waterproof Plastics require little or no finishing, painting, polishing etc Low electrical and thermal conductivity, insulator Easily formed into complex shapes, can be formed, casted &

joined. Wide choice of appearance, colors and transparencies

Disadvantages of using Plastics

Low strength

Low useful temperature range

Less dimensional stability over period of time (creep effect)

Aging effect, hardens and become brittle over time

Sensitive to environment, moisture and chemicals

Poor machinability

It can harm the environment when it is not being used

properly by humans

Additives in polymers

• To impart certain specific properties, polymers are usually compounded with additives.

• Additives improve polymers stiffness, strength, colour, weather ability, flammability and arc resistance for electrical applications.

Some examples are:1. Fillers2. Plasticizers3. Colorants4. Flammability5. Lubricants

Thermosetting plastics• The molecules of thermosetting

plastics are heavily cross-linked. They form a rigid molecular structure.

• The molecules in thermoplastics sit end-to-end and side-by-side.

• Although they soften when heated the first time, which allows them to be shaped they become permanently stiff and solid and cannot be reshaped.

• Thermoplastics remain rigid and non-flexible even at high temperatures.

Cross-linked molecules

Thermo sets: Behavior and Properties

During polymerization, the shape of the part is permanently set.

Curing is irreversible. Polymerization process takes place in 2 stages:1) molecules are partially polymerized into linear

chains2) cross-linking is completed under heat and pressure

Strength and hardness of thermo sets are not affected by temperatures or rates of deformation.

Some examples of thermo sets are:

Polyurethanes,

Vulcanized rubber

Bakelite,

Duroplast,

Urea-formaldehyde

Melamine resin

Epoxy resin

Polyamides

Applications---Thermo sets:

Phenolic is commonly used for circuit boards, automotive parts, handles for cutlery and ovens.

Epoxy is used in automotive equipment, electrical, sports equipment and adhesives.

PU (Polyurethane) is used as car seats, mattresses, cushions, diaphragms, gears, finishes and coatings.

Thermoplastics

• The molecules of thermoplastics are in lines or long chains

• The process of heating, shaping, reheating and reforming can be repeated many times.

Long chain molecules

Some major thermoplastics are;

1. Acetals2. Acrylics3. Acrylonitrile-butadiene-styrene4. Cellulosics5. Fluorocarbons6. Polyamides7. Polycarbonates8. Polyesters

Applications---Thermoplastics

PVC (polyvinyl chloride) is used in medical products, credit cards, cable insulation, packaging film, bottles, flooring and window frames.

PS (polystyrene) is used in cups, plates, tape cassettes and dairy product containers.

PP (polypropylene) is used in fibers, automotive parts, bottle crates, battery cases and food containers.

PET (polyethylene terephthalate) is used in food packaging, carpets and bottles.

LDPE (low density polyethylene) is used in flexible containers, cling film and plastic bags.

HDPE (high density polyethylene) is used in toys, bottles, automotive fuel tanks and piping.

Thermoplastic polymers Thermosetting polymers

(1) These soften and melt on heating.

These do not soften on heating but rather become hard in case prolonged heating is done these start burning.

(2) These can be remoulded recast and reshaped.

These can not be remolded or reshaped.

(3) These are less brittle and soluble in some organic solvents.

These are more brittle and insoluble in organic solvents.

(4) These are formed by addition polymerization.

These are formed by condensation polymerization.

(5) These have usually linear structures.Ex. Polyethylene, PVC, teflon.

These have three dimensional cross linked structures. Ex. Bakelite, urea, formaldehyde, resin.

1) Extrusion2) Injection molding 3) Structural foam molding4) Blow molding5) Rotational molding6) Thermoforming7) Compression molding8) Transfer molding9) Casting10) Process of reinforced plastics

Processing of Plastics

A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling.

Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous (e.g., a glass).

Ceramics now include domestic, industrial and building products , art objects & semiconductors

A wide-ranging group of materials whose ingredients are clays, sand and felspar.

Ceramics

Comparison metals v ceramicsCeramicsMetals

Ceramics Applications:

Refractories Sensors Capacitors Semiconductors The magnetic strip on a credit card Spark plug Cutting tools Die blanks Piston coating of Automobile engine

Ceramics materials

glasses

glass

Glass-ceram

ics

Clay product

s

Structural clay

products

White ware

refractories

fireclay

silica

basic

special

abrasives

Advance

ceramics

Classification of Ceramics

Based on chemical composition ceramic are classified as oxides, carbides, nitrides, sulfides and fluorides. Alumina ( Al2O3) Graphite Diamond (C) Silica (SiO2) Silicon Carbide (SiC) Tungsten Carbide (WC) Titanium Carbide (TiC) Silicon nitride (Si3N4) Titanium oxide (TiO2) Zirconia (ZrO2)

Glass in Buildings

Glass Containers

Refractory Brick

White ware: Bathrooms

Automotive Components in Silicon Carbide

Ceramic Brake Discs