Unit IV MSB Polymers

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MSB/KH; Dept. of Chemistry; GITAM, HYD Page 1

POLYMERISATION:

It may be defined as the process of linking or

joining together small molecules like monomers to

make large molecules.

1. Additional polymerization or Chain polymerization:

This polymerization yields an exact multiple of

basic monomeric molecules. This monomeric

molecule contains one or more double bonds. By

intermolecular rearrangement of these double

bonds makes the molecule bifunctional. In this

polymerization process light, heat and pressure or

catalyst is used to breakdown the double covalent

bonds of monomers.

2. Condensation polymerization or Step polymerization:

May be defined as “a reaction occurring between

simple polar-group-containing monomers with the

formation of polymer and elimination of small

molecules like water, HCl, etc. For example,

hexamethylene diamine and adipic acid condense

to form a polymer, Nylon6:6.

Free Radical Addition Polymerisation:-

Additional polymerization is a chain reaction

converting of a sequence of three steps. Initiation,

propagation and termination.

a. Initiation step is considered to involve two

reactions. The first is the production of free

radicals, usually, by the homolytic dissociation of

an initiator (or catalyst) to yield a pair of radicals R’.

I R._………(1)

(Initiator) (Free radicals)

The second part of initiation under the addition of

this radical to the just moment molecule (M) to

produce the chain initiating species M1.

R._{+ M M1 ……….(2)}

Free radical monomer

Thus the polymerization of monomer CH2=CH2

taken in the form.

b. Propagation step: Consists of the growth of M1 by

successive additions of large numbers of monomer

molecules according to equation.

(2)

c. Termination step: At some time, the propagation

polymer chain steps growing and terminates.

(i) Coupling :-

R-(CH2-CH2)n-CH2-CH2. + .CH2-CH2-(CH2-CH2)n'-R

R-(CH2-CH2)n+n'+2-R

(ii) Disproportion:-

which a hydrogen atom of one radical centre is

transferred to another radical centre. This results

in the formations of two polymer molecules, are

saturated and one unsaturated e.g.

R-(CH2-CH2)n-CH2-CH2. + .CH2-CH2-(CH2-CH2)n'-R

R-(CH2-CH2)n-CH2-CH3 + R-(CH2-CH2)n-CH=CH2

What is Plastic?

Plastics are the materials that show the property of

plasticity and can be moulded into any desired

shape and dimensions by the application of heat

and pressure. Plastics having variety of properties

are in use in present applications. The properties

are low thermal and electrical conductivities, easy

to fabricate, low specific gravity etc. The plastics

can be fabricated for large number of colours and

can be used for decorative purpose. Plastics can be

used to produce complicated shapes and accurate

dimensions very cheaply by moulding process.

Plastics are generally used for making automobile

parts, goggle, telephones, electrical instruments,

optical instruments, household appliances etc.

plastics having high wear resistance properties can

be used for making gears, bearings etc.

Merits of Plastics

1. Plastics have good shock absorption capacity

compared with steel.

2. Plastics r high abrasion,chemical resistance &

high corrosion resistance compared to metals.

3. Mounding, machining, drilling etc. can be easily

done on plastic materials.

6. Plastics are light in weight having specific gravity

from 1 to 2, 4.

7. Plastics can be made according to the order like

hard, soft, rigid, tough, brittle, malleable etc.

8. Fabrication of plastics into desired shape and

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Polyethlene :-

It is also known as polythene.it is preparecd by the

process of addition polymerization of ethyulene.

They are two types

1. Low density Polyethylene (LDPE)

It is prepared by polymerizing ethylene at high

pressures of 1000 to 5000 atm and at 250C in

presences of free radical initiator (O2).

Properties:

1. During the propagation steps, large number of

chain transfer reactions results in the formation of

branched structures. These branches are mainly

short chain and occasionally long chain. These

branches are mainly short branched chain. These

branches do not allow the molecules to pack

efficiently and hence its density is low (about 0.91

-0.925 gm/cm3_).

2. Due to same reason low crysstallinity of LDPE is

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3. Non polar polymer have no strong intermolecular

forces and back bone is flexible. Hence they have

low softening temperature (115ºC)

4. LDPE is chemically inert and has good chemical

resistance.

5. As non-polar it has good electrical insulation

properties.

6. It is tough and flexible even at low temperatures.

Applications

Used in preparation of squeeze bottles

particularly for detergents, moulding toys, ink tubs,

pens and films for general packaging and carry

bags.

Limitations :-

1. Due to low density and crystallinity, LDPE are not

suitable for load bearing applications.

2. LDPE is permeable to gas molecules because in the

amorphous zones the free volume and segmentak

movements facilitate the passage of small

molecules. Hence LDPE is not suitable for

manufacturing of pipes for distribution of gas.

2. High density Polyethylene (HDPE)

Preparation :-

(a) Using Zeigler- Natta Catalyst ethylene is

polymerized under 6-7 atom pressure at 60-70C in

the presences of TiCl4 + Al(C2H5)3 as catalyst.

(b) Using CrO3 supported on silica alumina, ethylene is polymerized at 35 atm and temperature of 60-200C.

Properties:-

a. HDPE molecules are linear and their packing is

easy. Hence HDPE has high density

(0.95-0.97gm/Cm3_{) and more percentage crystallinity}

(80-90%). Its Softing temperature is 135ᴼC higher

than LDPE

b. HDPE has excellent chemical resistance and

good electrical insulation properties.

c. It is free from odour and toxicity.

d. It is more stiff, hard and possesses greater tensile strength compared to LDPE.

Applications:-

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2. Because of better oxidation and UV-resistance,

HDPE is used for manufacture of food tubs,

industrial containers and overhead tanks

3. Bottles for milk, household chemicals and drug

packing are also made from it.

4. It can also used for domestic water and gas piping.

Polyvinyl Chloride (PVC) or Rigid PVC :-

Preparation:-

It is prepared by heating vinyl chloride in the

presence of small amount of benzoyl peroxide as

catalyst under pressure in an autoclave.

Cl

n HC2 CH

Cl _n Vinyl Chloride

Polyvinyl chloride benzoyl peroxide

Properties :-

1. Presences of Cl atom cause an increase in the

interchange attraction, which increase the

hardness and stiffness of polymer.

2. Softening temperature is 148°C.

3. PVC is colourless, odourless and Non-inflammable.

4. It has superior chemical resistance, oil resistance

and resistance to weathering

Applications:-

1. It is used in acid recovery plants and in plants for

handling hydrocarbons, due to its good chemical

resistance to chemicals.

2. PVC has great potential importance to building

industries since it has good weather resistances.

3. PVC is less brittle, lighter, has greater design

flexibility, better clarity, oil resistance and barrier

properties compared to glass.

Plasticized PVC:-

It is obtained by adding plasticizers such as

Dioctyl phthalate (DOP) Dibutyl phthalate, tricresyl

phthalate etc., to rigid polymers.

Applications:-

It is used in making sheets, pipes, rain coats, hand

bag, table clothes, plastic dolls, floor covering ,

electrical insulation and coating on electrical

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Polystyrene (Styron):-

Preparation:-

Free radical polymerization of polystyrene in

the presences of benzyl peroxide gives polystyrene.

H2

C CH

Styrene

Polystyrene benzoyl peroxide

n

Properties:-

1. Presence of bulky phenyl groups PS chains are

not efficient packed.

2. Its softening temperature is low (82-100°C)

3. Good optical properties - transparent

polymer-high refractive index (1.592)-polymer-high brilliance.

4. Due to chain stiffening effect of benzene ring, PS

is hard and brittle.

5. It has good chemical resistance, oil resistances

and electrical insulation characterization.

Applications

1. Polystyrene is used in making house ware (storage

containers), bottle caps, brushes, combs,

refrigerator parts etc.,. It is also used in preparing

audio cassettes, CDs, DVDs.

Polyamides:-

Polyamides are the polymers obtained by

condensation of acids and amines and contain

amides [- C-NH- ] groups, e.g., Nylon -6, Nylon –

6,6, Nylon 6,10. Etc.,

Nylon-6:-

It is obtained by prolong heating of caprolactum at

260 -270 C. caprolactum is obtained by Beckmann

rearrangement of Cyclohexanone oxime.

O N OH

NH O

H

N CH2 C 5

O

n

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Nylon 6,6:-

It is prepared by condensation reaction of

hexamethylenediamine and adipic acid in the ratio

of 1:1.

H₂N HC2 NH₃ _HOOC _CH₂ _COOH 4

H

N HC2 HN C

O H2 C C

O

6 4

n Hexamethylenediamine Adipic acid

Nylon - 6:6

Nylon 6,10:-

Condensation of hexamethylenediamine and

sebacic acid.

H2N

H2

C NH3 HOOC CH2 ₄COOH

H N

H2

C HN C

O H2

C C

O

6 4

n Hexamethylenediamine sebacic acid

Nylon - 6:10 Properties :-

The structures of nylon are linear, the molecular

chains are held together by H-bonds. Thus have

hish crystallinity, high melting point, electicity,

toughness and retention of good mechanical

properties upto 125ºC. They are also sterilzable.

Since nylon is polar polymers, they have good

hydrocarbon resistance.

Application:-

1. Nylon 6,6 is primarily used for fibers-socks,

undergraments, carpets etc.,

2. It is used in mechanical engineering applications

like gears, bearings, bushes etc., Nylon moving

parts may be frequently operated without

lubrication.

3. Nylon 6,6 is mainly used for making airbags,

apparel, tires, ropes, Durable hair combs etc.,

Kevlar

Kevlar is similar in structure to nylon-6,6 except

that instead of the amide links joining chains of

carbon atoms together, they join benzene rings. The

two monomers are benzene-1,4-dicarboxylic acid

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MSB/KH; Dept. of Chemistry; GITAM, HYD Page 8 C C OH HO O O NH H N C C O O n

H2N NH2

n

Kevlar

Kevlar is a very strong material - about five times as

strong as steel, weight for weight. It is used in

bulletproof vests, in composites for boat

construction, in lightweight mountaineering ropes,

and for lightweight skis and racquets - amongst

many other things.

Polycarbonates (PC) [lexan, Merlon]:-

Polycarbonates are prepared by reaction of

bisphenol-A with phosgene in the presences of a

base(pyridine).

HO C

CH3

OH

n + nClCOCl

O C CH3 CH3 O n + 2nHCl base

20-300_C

Properties:-

1. Polycarbonates have good mechanical

properties.

2. They have high impact strength

3. They are resistant to water and many organic

compounds

Uses :-

Polycarbonates are used to make safety goggles,

safety shields, telephone parts and boat propellers.

Bakelite:-

Synthesis:-

When phenol and formaldehyde are heated in the

excess of phenol with an catalyst such as HCl,

H2SO4 or oxalic acid, a linear polymer is formed

which is thermoplastic in nature and is known as

Novolac.

Novalac on further heating in the presence of

hexamethylenetetramine, Novolac produces 3D

cross-linked networked thermosetting polymer

which is known as Bakelite.

The hexamethylenetetramine decomposes into

formaldehyde and ammonia, which is used as

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OH

+ H O

H

OH CH2OH

monometylol p henol

OH OH OH OH

C O H H

H2C CH2

Novolac

(C H2)6N4

Bakelite

Hexa methyl ene 6 H C

O

H 4NH3

OH OH OH

H2

C

H2

C

CH2 CH2 CH2

OH OH OH

BAKELITE

Uses:-

1. Novolac (thermoplastic resin) is used in fixing brass

holders to electric bulbs, adhesives in fixing doors,

bonding cardboard paper etc.,

2. Bakelite are hard, rigid and with stand very high

temperature and hence are used in missile nose

cones. They have very good electrical insulating

character. Because of their good dielectric properties,

they are also used in paints and Varnishes as

hydrogen-exchanger resins in water softening.

Moulding Constituents:-

3. Usually high-polymer material is mixed with 4 to 10

ingredients, which imparts useful function during

moulding or useful property to the finished artifact.

The main ingredients are

4. Resin – is a binder, which holds different constituents

together. Usually thermosetting resins of linear and

low molecular weight polymer are used due to their

fusibility and mould able nature. The conversion of

this fusible form into cross-linked infusible form take

place, during moulding or in presence of catalysts

etc.,

5.Plasticizers-are materials that are added to resins

to increase their plasticity and flexibility. Plasticizers

reduces the intermolecular forces of attraction an d

imparts great freedom of movement between the

polymeric macromolecules of resins- increasing the

flexibility and plasticity of compounded material at

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resistance. Ex Vegetable oil, camphor, esters,

phosphates

6. Fillers-are added to give final plastic better

hardness, tensile strength, opacity, finish and

workability. It also reduces the cost, shrinkage on

setting and brittleness. Ex. Carborundum, quartz,

asbestos, chain clay, wood-flour, gypsum carbon

black etc.,

7. Lubricants- waxes, oils, stearate, oleate and soaps

are employed to make the moulding of plastic easier

and impart a flawless, glossy finish to the products.

8. Catalysts or Accelerators- are added only in the case of

thermosetting plastics to accelerate the

polymerization of fusible resin, during moulding

operation, into cross-linked infusible form.

9. Stabilizers-to improve the thermal stability. Common

stabilizers are (i)opaque moulding compounds- lead

salts e.g. white lead, lead chromate, litharge, red lead

(ii) Transparent moulding-stearate of lead, cadmium

and barium.

10.Colouring materials- Colour appeal is often of prime

importance in high-polymer artifacts. The main

colouring materials are organic dyestuffs and opque

inorganic pigments.

MOULDING OF PLASTICS INTO ARTICLES

Moulding is an important method to convert

fabricating plastics into desired-shapped article and

is done in several ways as given below.

A) Compression moulding

B) Injection Moulding

C) Transfer Moulding

D) Extrusion Moulding.

A). Compression moulding : This method is applied to both

thermoplastic and thermosetting resins. The

predetermined – quantity of plastic ingredients in

proper proportions get fill between the two half –

pieces of mould, which are capable of being moved

relative to each other. Then high pressures and

temperatures are applied to complete the curing

process. The cavities get filled with fluidized plastic.

Two halves are closed very slowly. Finally, curring is

done either by heating (in case of thermosetting) or

cooling (in case of thermoplastics). After curing the

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Injection Moulding : This method is applicable mainly to

thermoplastic resins. In this method the moulding

plastic powder is fed into a heated cylinder, from

where it is injected at a controlled rate into the tightly

– locked mould by means of a screw arrangement or

by a piston plunger. The mould is kept cold to allow

the hot plastic to cure and become rigid. When the

material have been cured sufficiently, half of the

mould is opened to allow the injection of the finished

article without any deformation, etc. heating is done

by oil or electricity. The temperatures used are 90ᵒC to

260ᵒC.

Advantages :

This method is the most widely used for moulding

of thermoplostics, because of high speed

production, low mould cost (since fever cavities are

needed), very low loss of material and low finishing

cost.

C) Transfer Moulding : It is a method, which used the

principle of injection moulding for thermosetting

materials. In this, the moulding powder is placed in

a heated chamber. Maintained at the minimum

temperature at which the moulding powder just

begins to become plastic. This plastic material is

then injected through an orifice into the mould by a

plunger, working at a high pressure. Due to the

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temperature of the material, at the time of ejection

from the orifice, rises to such an extent that the

moulding powder becomes almost liquid and

consequently it flows quickly into the mould, which

is being heated up to curing temperature required

for setting. The moulded article is then ejected

mechanically.

Advantages :

1) Since the mix flows into the mould cavity in a

highly plasiticized condition, so very delicate

articles may be handled without distortion or

displacement. Fine wires and glass fibers may be

inserted into the mould.

2) Article produced is free from Flow Marks

3) Even Thick pieces curve almost completed and

unifromly.

4) Owing to uniform and through cure, shrinkage

and distortions are at a minimum and hence, the

mechanical strength and density of fabricated

pieces is higher

5) Finishing cost of Fabricated article is almost

entirely eliminated

6) Mould cost is less, since it involves very low

abrasive action.

D) Extrusion: This method is used mainly for

continuous moulding of thermoplastic materials

into articles of uniform cross – section like tubes,

rods, strips, insulated electric cables. In this

method, the material of the required composition is

forced by a screw conveyer into a die, having the

required outer shape of the article to the

manufactured. Hence, the finished product that is

cooled by atmospheric exposure or by blowing air or

by spraying water. A long conveyer carries away

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Engineeering plastics and their Uses

Engineering plastics or performance plastics are a

group of materials obtained from high resins, which

posses

1. Plasticity at some stage of their processing

2. High load – bearing characteristics

3. High mechanical strength

4. High dielectric constant

5. Readily mouldable characteristics into

complicated shapes

6. Rigidity ,

7. High abrasion resistance

8. Dimensional stability

9. Good thermal stability

10. Light weight

11.High performance properties which permits

them to be used in the same manner as metals,

alloys and ceramics

Engineering plastic not only replaces metal they

can also use along with metals. They meet the

crucial needs of the day with greater efficiency and

reduction in both density and weight. They are

finding applications in demanding areas like

automobiles, defense and electronics,

telecommunications, textiles, satellite, robots,

mountaineering, computer components etc., where

so far only metal or ceramics have been found

suitable.

Polyamides or nylons are easily mouldable, tough,

strong, and resistant, good chemical resistant, and

of low coefficient of friction. Aromatic polyamides

(kevlars) are high temperature resistant. They find

applications in gears, automobile tyres, watch

straps, unlubricated bearings electric mountings,

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Polycarbonates (like lexan, merlan) possess high

impact and tensile strengths over a wide range of

temperatures, good demensionl stability, stiffness,

transparency etc, they are used for electircals –

industrial plugs, sockets, switches, crash helmets,

cooking utensils covers, bodies, sterilizable

transparent containers, camera and binocular

bodies, solar collectors, telephone booths,

photographic films, etc.,

Polyurethanes: possess excellent flexibility,

toughness, even at subzero temperature, excellent

resistance to abrasion and solvents. They are used

in defence, oceanographic research mountaineering

plastic foam etc.,

Solid silicons possess good electrical insulation

properties and outstanding heat-resistance,

chemical inert ness good water repellency,

resistance to weathering effects etc., they are used

most voltage insulators high thermally stable

moulding and high temperature insulting foams.

Polyacetals possess low coefficient of friction,

stiffness, resistance to creep, excellent fatigue

endurance, etc., They find use in gears, link in

chains bearings pump impellers, conveyor belts

etc.,

Teflons possess extrme toughness, very high

softening temperature, excellent chemical

resistances low coefficient of friction, extremely

good mechanical properties, very good electrical

insulators etc., electrical insulating materials in

moters, cables, transformers, electrical fittings, for

making non-lubricating bearings, chemical-carry

pipes tubing’s, etc.,

INORGANIC POLYMERS

• They are gaint-molecules composed of atoms, except carbon.

• Atoms in these polymers are linked together by covalent bonds.

Example: Silicones (Poly Silaxanes),

Polyphosphazine, Polysulphur nitride (not in

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SILOCONES:

Silicone resins contain alterantive Silicon-Oxygen

structure

The reaction product is fractionally distilled to get

different Organo-Silon chlorides, which are then

polymerized by controlled hydrolysis.

1. Dimethyl-Silicon Dichloride is “bifunctional” can

give very long chain polymer

2. Trimethyl silicon chloride: ‘monofunctional’ and

hence chain stopper. This is used in limit the chain

length

3. Monomethyl silicon chloride: Trifunctional; gives

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Characteristic of Silicones:

i. Depends on the various alkyl halides use

Silicones are liquids, viscous liquids, rubber-like

and solid in nature

ii. Because of Silicon-Oxygen bond they exhibits

outstanding stability at high temperatures

iii. They are good water-resistance, good oxidation

stability but their chemical resistance lower than

other plastics

iv. Their specific gravity ranges from 1.03 to 2.1

v. Their physical properties are much less effective

by variation in temperature

vi. They are non-toxic in nature

Types of Silicon and their Uses:

1. Liquid Silicones or Silicon oils: Relatively low molecular

weight silicones, generally of dimethyl silicones.

They posses great wetting-power for metal, low

surface tension and show very small change in

viscosity with temperature

Uses: High temperature lubricants, antifoaming agents, water-repellent finishes for lather and

textiles, heat transfer media. Also used in cosmetics

and polishes

2. Silicon greases: Modified silicon oils, obtained by

filler like silica, carbon black, lithium soap etc.

Uses: Used as lubricants in situation where very high and very low temperatures are encounter.

3. Silicon Rubbers: Obtained by mixing high molecular

weight dimethyl silicon polymers with fillers (silicon

dioxide) and peroxide containing curing agents.

Peroxide cause the formation of dimethyl bridge

(cross-link) between methyl groups of adjacent

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Properties:

i. They possess exceptional resistance to prolonged

exposure to sunlight, weathering, most common

oils, boiling water, dilute alkalis and acids.

ii. They remain flexible in the temperature range of

90-250ᵒC, hence used in tyres of fighter aircrafts,

since they prevent the damage on landing.

iii. Upon decomposition they converted into

non-conducting silica (SiO2), instead of carbon tar

Uses:

i. As a sealing materials in search-lights and in

aircraft engines

ii. Manufacturing of tyres for fighter aircrafts

iii. For insulating the electrical wiring in ships

iv. In making lubricants, paints and protective

coatings and water-proofing

v. Adhesive in electronic industries, insulation for

washing mechines, electric blankets for iron board

covers, artificial heat valves and padding for plastic

vi. For making boots for use at very low

temperature as they less affected by temperature

variations: Neil Armstrong used silocone rubber

boots when he walked on the moon

4. Solid silicon resins: are cross-linked silicons,

obtained from b- and tri-functional silicon halides

Properties: They are thermosettings, possess good

electrical insulating properties and outstanding

heat-resistance, water-repellent, chemically hight

thermal stability

Uses : For making high voltage insulators, high

temperature insulating foams

Poly Phosphazenes:-

Phosphazenes were initially termed phosphonitrilic

polymers. Later on, the new term was used to

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double bonds (=ene) which are always present in these polymers. They are thus ‘unsaturated PN

compounds’ containing phosphorus, mostly in +V

state. They are usually represented as

Uses

Both the cyclic and non-cyclic polyphosphazenes

have great oxidative, thermal & radiative stabilities.

Their water repellency, solvent resistance, flame

resistance, retention of flexibility at low

temperatures and their dielectric properties make

them very useful for a variety of modern

technological purposes. These are used for making:

i) rigid plastics, plastic films, expanded foams

ii) fuel hoses, gaskets

iii) O-ring seals for use in extremely cold climate

(e.g. in high flying aircrafts or in vehicles for Arctic

type climate)

iv) metal coatings & wire insulation

v) composite materials together with asbestos, glass