Polymeric Materials Complete

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Polymeric Materials

Asma Ashraf

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Contents

The history of plastics

Introduction

Additives

General properties of polymeric materials

Properties and applications of elastomers

Properties and application of typical

thermoplastics Properties and applications of typical

thermosetting plastics

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History

Plastics do not exist naturally

Their manufacturing started after the development ofchemical industry and in particular the oil refiningindustry which provided raw materials for many of

these plastics. The first man-made plastic was Parkesene which

was a substitute of traditional bone ware (1854).

The second plastic was Bakelite which expanded the

electrical engineering industry (1907). A mass production of many different polymers started

after 1930 till the present modern and fullydeveloped polymer industry.

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Polymers

There is an ever increasing number of synthetic,polymeric materials available under the popularname ofPlastics.

The name plastic comes from the fact that during themoulding process by which they are shaped, they arereduced to a plastic condition by heating them.

There are three main groups of polymeric or plasticmaterials:

Thermosetting plastics (thermosets)

Thermoplastics

Elastomers

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Thermosetting plastics (Thermosets)

This group of polymeric materials undergoes

chemical change during the moulding process and

can never be softened again by reheating.

These materials are generally hard, rigid and brittle. Example is melamine formaldehyde.

The strength of Thermosetting plastics can be

greatly increased by reinforcing them with fibrousmaterials.

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Thermoplastics

They become soft and can be reheated each

time they are heated.

They are not so rigid as thermosetting

plastics but they are tough.

For example PVC

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Elastomers

The elastomers or rubbers are cross-linkedpolymeric materials in which there are notsufficient cross links to make them as rigid asthe thermosetting plastics, but just sufficient to

make them return to their original dimensionswhen the deforming load is removed.

Thermosets show little elongation under stressbut elastomers are capable of extreme elastic

deformation at low levels of stress (upto1000%).

Unlike metals strain is not proportional to stress.

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Crystallinity in Polymers

Most polymers are amorphous

The crystallinity of a polymeric material is

defined as the ratio between the mass of

crystallites and the total mass of the materialbeing considered. For example a material having

80% crystallinity will consists of 80% crystallite

structure and 20% amorphous structure. A material with high crystallinity will be more

dense than the material with a low crystallinity.

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Crystallinity in Polymers

Increasing the crystallinity of a material:

Increase, the melting point of material and, insteadof softening gradually with increasing temperature,it will exhibit a sharp melting point.

Increases the resistance of material to theabsorption of water and solvent attack.

Prevents the penetration of plasticizers and thisreduces the ultimate elongation of the material.

Makes the material more impervious to gases andthis may be useful in food packaging nd protectivecoatings

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Effect of crystallinity on ultimate tensile strength

and percentage elongation of a polyethylene

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Glass Transition Temperature

The temperature at which a polymeric materialchanges from being rigid and brittle to being flexibleand rubbery is called glass transition temperature (Tg).

It is less well defined than the melting temperature

(Tm) and is difficult to determine. Below the glass transition temperature the polymeric

materials show a relatively high tensile modulus, withlittle extension and a high level of rigidity.

Above the glass transition temperature the tensilemodulus is lower and the extension is veryconsiderably increased.

Glass transition temperature is unaffected bycrystallinity.

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Effect of glass transition temperature on themechanical properties of a typical thermoplasticmaterial.

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Effect of temperature on polymer

application

Since polymeric materials with a high crystallinityhave a well defined melting point, they can be hotformed (moulded) above this temperature and coldformed between their Tg and Tm , when they will be

solids but soft and flexible.

For example polyethylene with 95% crystallinity has aTg of -120

OC and a Tm of +138OC. Thus it is soft and

flexible over a wide temperature range.

The maximum service temperature is usually taken asapproximately 85% of the melting point, which in thiscase is 120OC.

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For amorphous polymeric materials

Amorphous polymeric materials are usuallymoulded or formed above glass transitiontemperature where they are soft, but are used

below this temperature where they are rigid. For example rigid PVC is an amorphous polymer

with a Tg of 87OC.

It is normally softened by hot blast of air.

Amorphous plastics do not have a well-definedmelting temperature, the service temperature istaken as 85% of the Tg.

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Additives

The appearance and performance of mostplastics and elastic polymers can be improvedby the use of various additives. Some types

of additives are: Plasticizers

Fillers

Stabilizers

Colorants

Antistatic agents

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Plasticizers

These are added to polymeric material to:

reduce their rigidity and brittleness

And to improve their flow properties

There are two main groups of plasticizers:

Primary plasticizers

Secondary plasticizers

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Primary plasticizers

These are used to reduce the Vander Waals force

between adjacent molecular chains and allow

greater mobility between adjacent polymerchains.

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These are monomers of a compatible but inertmaterial without polar groups.

These are added to provide mechanicalseparation of the polymer chains in the sameway that a lubricant separates a shaft from itsbearing.

Separation of the polymer chain in this mannerreduces the Vander Waals force of attractionbetween them

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Internal Plasticization

Small amounts of plasticizers are added during

polymerization. Separation of polymer chain is

achieved during polymerization process.

PVC is a rigid and brittle plastic material. But it

can be made flexible for raincoats, insulation of

electric cables etc by the addition of 15% of vinyl

acetate as a secondary plasticizer during

polymerization.

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External plasticization

This is more common method of plasticization.

The plasticizer is added after polymerization.

The plasticizer acts as a lubricant and fills the voidsbetween the polymer chains.

Only amorphous zones will be treated in this waybecause crystalline materials will not absorbsufficient plasticizer.

However in case of crystalline polymers the presenceof plasticizer reduces the degree of crystallinity andalso reduces the glass transition Temperature Tg

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Fig

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Fillers

They have influence on the properties of moldingsproduced for any given polymeric material.

They increase the impact strength and reduceshrinkage during molding.

Fillers are essential in thermosetting moldingpowders and may be present in quantities up to 80%by weight.

In thermoplastics their amount is up to 25% byweight. The exceptions are thermoplastic floor tileswhich may contain up to 40% calcium carbonate asfiller.

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Some filler materials and their

properties

Filler Material Properties

Glass fiber Good electrical insulation properties

Wood flour; calcium carbonate Low cost, low strength

Asbestos Heat resistant ( health hazards)

Aluminium powder High mechanical strength

Shredded paper/cloth Good strength, electrical insulation

properties

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Selection of filler Material

A filler material is selected according to the

properties required. However all fillers must:

Have a low moisture absorption rate

Not adversely affect the color or surface finish of

the product

Not cause abrasive wear in the processing

equipment Be capable of being wetted by the resin.

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Stabilizers

Stabilizers reduce degradation of polymeric materialand prevent it from environmental attack.

They are used in plastics where the material have towithstand environmental attack over a long period oftime such as window frames etc.

Degradation of polymeric materials occurs when theyare exposed to heat, sunlight or weathering.

Such degradation is usually accompanied by

color change

Deterioration of mechanical properties

Cracking and surface crazing

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Colorants

Three categories of colorants are: Dyestuffs

Organic pigments

Inorganic pigments

Dyestuffs Are usually aromatic organic chemicals which are soluble in

variety of solvents.

they absorb light selectively to produce their characteristiccolors.

They are suitable for coloring transparent and translucentpolymers such as polystyrene

Organic dyestuffs have only a limited color stability whenexposed to sunlight, and they may also degrade at moldingtemperatures of some high temperature polymers.

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Colorants

Organic pigments They are opaque and cannot be used to color transparent

plastic materials.

They are usually used in opaque plastic products.

Their light and heat stability is superior to dyestuffs

Inorganic pigments They are based on metal oxides and salts

Have greatest opacity

Superior light and heat stability. e.g.

Titanium oxide used in non-toxic white plastics and paints Iron oxides used to provide yellows and red colors

Carbon black used to produce black color and is also used asultraviolet radiation absorber

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Antistatic Agents

These are included to increase surface

conductivity so that static charges can leak

away.

This prevents the attraction of dust particles

They reduce the risk of explosion in hazardous

environments caused by the spark associated

with an electric discharge. It also prevents electric shocks when synthetic

fabric materials are handled in very dry climate.

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General properties of Polymeric

Materials

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Electrical Insulation All polymeric materials exhibit good electrical

insulation properties. But they have low heatresistance and are soft.

Strength/weight Ratio Polymeric materials vary in strength considerably

Some polymers e.g. Nylon are comparable withWeak metals.

All polymeric materials are much lighter than any of

the metals used for engineering purpose. Some polymeric materials are being used in place of

metals because of their comparable strength/weightratio.

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Corrosion Resistance

All polymeric materials are inert to most

inorganic chemicals and can be used in

environments which are hostile even to mostcorrosion-resistant metals.

Synthetic rubber is superior to natural rubber

since it os not attacked by oils and greases.

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Degradation of Polymers

Temperature has much greater effect on polymericmaterials than it has on metals and substantially reducesthe fatigue resistance of material.

Polymeric materials heat up internally when subjected torapidly alternating stresses. The greater the stress and themore rapidly it alternates, the greater will be thetemperature rise.

The fatigue resistance of polymeric materials and theirgeneral strength and toughness can be adversely affectedby presence of organic substances such as soaps,detergents and alcohols.

UV radiation and the presence of ozone can cause somepolymeric materials to degrade and fail in fatigue at alower then normal value.

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Properties and applications of

elastomers

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Elastomers are substances which allow extremereversible extensions to take place at normaltemperatures.

Natural rubber is an example of elastomers.

Applications of rubbers Resilient floor coverings

Footwear

Vehicle tires

Joint sealants

Expansion joints

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Vibration insulation and isolation Shock absorbers

Anti-vibration machines and engine mountings

Sound insulation

Distortional systems Changing shapes such as belts, rollers and tires

Seals and gaskets

Protective systems

Protection against abrasion Protection against corrosion

Electrical insulations

Protective clothing, gloves, aprons, boots etc.

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Some commercials Elastomers

Acrylic rubbers excellent resistance to oils, oxygen, ozone and UV radiation

Butyl rubber Impervious to gases and is used as a vapor barrier.

Highly resistant to outdoor weathering and UV radiations Nitrile rubber

excellent resistance to oils and solvents

Used for hose linings air craft fuel tank linings

Resistant to Refrigerant gases

Polychloropren rubbers Resistant to oils, solvents, abrasion and elevated

temperatures.

fire resistant

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Some commercials Elastomers

Polyisoprene high tensile strength

Easily attacked by solvents, oils and ozone

Degrades rapidly in sunlight

Polysulfide rubber Low mechanical strength

High weathering resistance

Good bonding properties

Silicone rubbers Low tensile strength

Working temperature range is -80OC to +235OC

Used for high temperature seals

Space vehicles and aircraft satellites37


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Properties and application of

Thermoplastics

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Polyethylene

Most versatile and widely used plastic material

Remains tough and flexible over a wide temperaturerange and has good dimensional stability

Easily molded and is used in a wide range of domesticgoods

Used commercially for water piping, chemicalequipment and electrical insulation

Resistant to most solvents and has good weatheringproperties

It degrades when exposed to strong sunlight unless itcontains UV filter such as carbon black.

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Polystyrene

Tough, dense plastic which is hard and rigid

and has good dimensional stability

High surface gloss

Good mechanical properties but it tends to

be brittle

Attacked by petrol and other solvents

Used for ceiling tiles and refrigerator trays

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Polyvinyl Chloride (PVC)

Unplasticized PVC is hard and tough To form and mold rigid PVC, it has to be heated above

its glass transition Temperature (87OC) to make it softand flexible

When plasticized, it becomes flexible and rubbery andis used for water proof clothing Hose pipes

Electric cables insulation

Chemical tank lining

It offers good resistance to attack by water, acids,alkalis and most chemicals

but it becomes brittle with age.

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Polytetraflouroethylen (PTFE)

High cost Suitable for manufacturing tough moldings and as a

non-stick, anti-friction coating

It doesnt burn and is neither attacked by any known

reagent Good electrical insulator and it has lowest co-efficient

of friction of any known solid

widely used for fuel hoses, gaskets and tapes and as a

non-stick coating for cooking utensils Also used as a lining for chemical equipment because

of its resistance to chemical attack

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Nylons

Strong, tough and flexible

Good resistance to abrasion

Their dimensional stability and electrical

insulation properties are affected by waterabsorption

Good resistance to most common solvents

but are not good weather resistant Nylons are easily extruded and drawn into

fibers.

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Polyesters

They can be thermoplastic or thermosetting

Thermoplastic polyesters have good

dimensional stability

resistant to most organic solvents but they are

attacked by acids and alkalis.

Excellent insulation properties and are used as

dielectric in capacitor

Polyester films and fibers are used in textiles

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Polycarbonates

Good impact strength Good heat resistance

Good dimensional stability

Good electrical insulation properties

Good optical properties

high scratch resistance

Applications are:

Electrical insulators

Capacitor dielectrics

Lightweight spectacle lenses

Aircraft components

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Properties and Applications of

Thermosetting Plastics

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Phenol-formaldehyde

Phenolic resins are never used by themselvesbut in conjunction with fillers and otheradditives which reduce the inherent brittleness,improve the mechanical and electricalproperties

Applications are:

Electrical insulators

Electrical plugs and sockets Handles

Clutch and brake lining

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Urea-formaldehyde

Hard, brittle, rigid and scratch resistant

Never used by itself but in conjunction with

fillers and additives

Resistant to most solvents and detergents

Good electrical insulation properties

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Epoxides

Used for bonding glass fiber fillers

Resistant to water and most reagents

Have excellent insulation properties

Documents

Polymeric Materials Complete