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8/18/2019 Unit-4 High Polymers
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NARASARAOPETA ENGINEERING COLLEGE - NARASARAOPE ENGG.CHEM
Dr. P.V.Narayana Page
UNIT – IV
HIGH POLYMERS
1. INTRODUCTION
Polymers are ‘macromolecules’ built up by the linking together of a large
number of small molecules or units. Thus, small molecules which combine with each
other to form polymer molecules are termed monomers and the “repeat unit” in a
polymer is called as monomer.
For example, polystyrene is a polymer formed by linking together of a large
number of styrene monomer molecules. Similarly polythene is formed by the linking of
ethylene (C2H4) monomer molecules.
The number of repeating units (n) in a polymer is known as the “Degree of
polymerization” (DP). The process of formation of a polymer from its monomer units is
termed as Polymerization . Many polymers are naturally occurring like starch, cellulose
etc., and as many are synthetically made such as polystyrene, PVC, etc. The organic
polymers like starch, polyethene have carbon backbone, while the inorganic polymers
have atoms other than carbon, which have catenation property like silicon, sulphur,
phosphorous. E.g: silicates.
Oligo Polymers : Polymers with low degree of polymerisation are known as
oligo polymers, their molecular weight < 10000.
High Polymers : Polymers with high degree of polymerisation are known as
high polymers, and their molecular weight ranges from 10,000 - 2, 00,000.
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1.1. Nomenclature of polymers
Polymers consisting of identical monomer units are called homo-polymers and
monomers of different chemical unit structures are called hetero-polymers or co-
polymers.
-M-M-M-M-M-M-M-M- -M1-M2-M1-M2-M1-M2-M1-M2-
Homopolymer Hetero or copolymer
Based on the arrangement of monomeric units (structural units), copolymers can be
classified as:
Alternating copolymers: These polymers are formed by regularly altering the
two different monomeric units.
-M1 – M2 – M1 – M2 – M1 – M2 –
Statistical copolymer (Random copolymer): These are copolymers in which the
sequence of monomer units follows a statistical rule. The probability of finding a
given type of monomer unit, at a particular point in the chain is equal to the mole
fraction of that monomer unit in a chain.
-M1 – M2 – M2 – M2 – M1 – M2 – M1 – M1
Block copolymer: The copolymer consisting of two or more homopolymer
subunits linked through covalent bonds is called a block copolymer.
-M1 – M1 – M1 – M1 – M2 – M2 – M2 – M2 – M3 – M3 – M3 – M3-
1.2. Functionality: In the process of polymerization, for any molecule or unit to act as a
monomer, it must have at least two reactive sites or bonding sites for the extension of a
monomer to a dimer, trimer and ultimately a polymer. The number of such reactive
sites in the monomer is termed as its functionality.
Eg: In ethylene the double bond can be considered as site for two free valancies. Thus, ethylene is
considered to be bifunctional.
If the monomer has bifunctionality, it can only form a linear polymer. If the
functionality is more than two, the monomer has a chance to form cross linked
polymers having 2D or 3D structures. Based on functionality and the process of
polymerization, the polymer may be present in linear, branched or cross-linked (three-
dimensional) structure.
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2.STEREO-SPECIFIC POLYMERS
Tacticity: The stereo chemical placement of the asymmetric carbons in a polymer chain
is called tacticity. The differences in configuration or arrangement of functional groups
on the carbon backbone of the polymer (tacticity) affects the physical properties. Basedon the stereo chemical orientation of the atoms or groups at asymmetric carbons, the
polymers can be classified as
1. In a head-to-tail configuration, if the arrangement of functional groups are all on the
same side of the chain, it is called as an isotactic polymer. e.g., PVC
2. If the arrangement of functional groups is in an alternating fashion in the chain, it is
called syndiotactic polymer. e.g., gutta-percha.
3. If the arrangement of functional groups is at random around the main chain
without any regularity, it is called atactic polymer. e.g., polypropylene.
Fig.: Tacticity in the polymers
3.POLYMERIZATION
The chemical reaction by which the monomers are combined to form the
polymer is called polymerization.
Two different types of polymerization processes i) Addition or chain
polymerization and ii) Step or condensation polymerization.
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Addition or chain polymerization: The original monomeric molecule, usually,
contains one or more double bonds. In this addition polymerisation there is no
elimination of any byproduct. It is a reaction that yields a polymer, which is an exact
multiple of the original monomeric molecule. The polymer will have the same
chemical composition as the monomer.
Eg 1: Polyethylene is produced from ethylene.
n CH2=CH2 Heat/pressure/catalyst (CH2-CH2)n
Ethylene Polyethylene
Eg 2: PVC is produced from vinyl chloride.
n CH2=CH Heat/pressure/ catalyst (CH2-CH)n Cl Cl
Vinyl chloride Polyvinyl chloride
Condensation or step polymerization: Condensation or step-polymerization may be
defined as “a reaction occurring between monomers having simple polar functional
groups ( like -OH, COOH etc.,) forming a polymer by the elimination of small
molecules like water, HCl, ammonia etc. For example, hexamethylene diamine and
adipic acid condense to form a polymer, nylon 6:6.
The molecular weight of a condensation polymer is always less than the integral
multiple of their monomer units. Condensation polymerization is an intermolecular
combination, and it takes place through different functional groups (in the monomers)
having affinity for each other in a step-wise process. When monomers contain three
such functional groups, they may give rise to a cross-linked polymer.
Eg: Hexamethylene diamine and adipic acid condense to form a polymer, Nylon 6:6
n H2 N - (CH
2)
6 - NH
2+ n HOOC - (CH
2)
4 – COOH
Hexamethylene diamine Adipic acid
-(HN -(CH2)6-NH –C = O –(CH2)4 –C =O)- n
Nylon 6:6
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Differences between addition and condensation polymerization processes:
Addition polymerization Condensation polymerization
1. The functionality of a monomer is
the Π bond which is bifunctional
1. The functionality of a
monomer 2, 3 or any.2. The polymerization is by self
addition and is by chain
mechanism.
2. The polymerization is by
condensation which is slow
step wise.
3. No by products are produced. 3. By products of small molecules
like H2O, NH3 , CH3OH & HCl
are formed.
4.
The molecular weight of the
polymer is
sum of molecular weights of
monomer.
4.
The molecular weight of the polymer is
less than the sum of molecular weights
of monomers
5. The process is highly exothermic. 5. The process not exothermic.
6. An initiator is required for the
reaction.
6. A catalyst is required for the reaction
7. Thermoplastics are produced
Eg:polyethylene , PVC etc.
7. Thermosetting plastics are produced
Eg:Bakelite, urea formaldehyde
3.1. Mechanism of Addition Polymerisation
The mechanism of addition polymerisation can be explained by any one of the
following three types.1. Free radical mechanism
2. Ionic mechanism
3. Co-ordination mechanism
All the above mechanisms occur in three major steps namely,
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(i) Initiation
(ii) Propagation and
(iii) Termination
Free radical mechanism
(i) Initiation
It is considered to involve two reactions.
(a) First reaction involves production of free radicals by homolytic dissociation of an
initiator (or catalyst) to yield a pair of free radicals (I.).
I 2I.
Intiator Free radicals
Examples of some commonly used thermal initiators
Thermal initiator is a substance used to produce free radicals by homolytic
dissociation at high temperature.
70 --- 800 C
(i) CH3COO – OOC CH3 2(CH3COO).
Acetyl peroxide Free radicals
80---95o C
(ii) C6H5COO – OOCC6H5 2(C6H5COO).
Benzoyl peroxide Free radicals
(b) Second reaction involves addition of this free radical to the first monomer to
produce chain initiating species.
H H
I. + CH2 = C R – CH2 – C
X X
Free radical First monomer Chain initiating species
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(ii) Propagation
It involves the growth of chain initiating species by successive addition of large
number of monomers.
H H H H
R - CH2 - C. + n CH2 = C I –( CH2 – C_)n __CH2 – C
.
X X X X
Growing chain (living polymer)
The growing chain of the polymer is known as living polymer.
(iii) Termination
Termination of the growing chain of polymer may occur either by coupling
reaction or disproportionation.
a) Coupling (or) Combination
It involves coupling of free radical of one chain end to another free radical forminga macro molecule.
H H H H
R – CH2 – C. +
.C – CH2 –I I – CH2 – C – C – CH2 - I
X X X X
Dead polymer
b) Disproportionation
It involves transfer of a hydrogen atom of one radical centre to another radical
centre, forming two macromolecules, one saturated and another unsaturated.
H H H H H H H H
R – C – C. + .C – C – R R –C =C + H- C – C - R
H X X H X X H
Unsaturated saturatedmacromolecule macromolecule
The product of addition polymerisation is known as Dead polymer.
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2.Ionic Polymerisation
The addition polymerization that takes place due to ionic intermediate is called
ionic polymerization. Based on the nature of ions used for the initiation process ionic
polymerization classified into two types;
(a) Cationic polymerization and (b) Anionic polymerization
(a) Cationic polymerization : Cationic polymerization depends on the use of cationic
initiators whi ch include reagents capable of providing positive ions or H+ ions.
Typical examples are aluminum chloride with water (AlCl3+H2O) or boron
trifluoride with water (BF3+H2O). They are effective with monomers containing electron
releasing groups like methyl (−CH3) or phenyl (−C6H5) etc. They include propylene
(CH3−CH=CH2) and the styrene (C6H5−CH=CH2).
(i) Chain Initiation:
Decomposition of the initiator is shown as
BF3 + H2O → H+ + [HOBF3]
–
The proton (H+) adds to C – C double bond of alkene to form stable carbocation
[HOBF3] –
H+
+ CH2=CHX →CH3-+CHX [HOBF3]
–
(ii) Chain Propagation: Carbocation add to the C – C double bond of another monomer
molecule to from new carbocation.
CH3-+CHX [HOBF3]
–+ CH2=CHX → CH3-CHX- CH2-
+CHX [HOBF3]
–
CH3-CHX- CH2-+CHX [HOBF3]
–+ n CH2=CHX →
CH3-CHX-(CH2-CHX)n- CH2-+CHX [HOBF3]
–
Living Polymer
(iii) Chain Termination: Reaction is terminated by coupling of negative ion to
carbocation.
CH3-CHX-(CH2-CHX)n- CH2-+CHX [HOBF3]
– →
CH3-CHX-(CH2-CHX)n-CH2-CHX-OH + BF3
Dead polymer
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(b) Anionic Polymerization
Anionic polymerization depends on the use of anionic initiators which include
reagents capable of providing negative ions. Typical catalysts include sodium in liquid
ammonia, alkali metal alkyls, Grignard reagents and triphenylmethyl sodium[(C6H5)3C-Na]. They are effective with monomers containing electron withdrawing
groups like nitrile (–CN) or chloride (- Cl), etc They include acrylonitrile
[CH2=CH(CN)], vinyl chloride [CH2=CH(Cl)], methyl methacrylate
[CH2=C(CH3)COOCH3], etc.,
(i) Chain Initiation:
Decomposition of the initiator is shown as
R
-
Na
+
+ CH2=CHX→ R
-CH2-
-
CHX + Na
+
(ii) Chain Propagation: Carbocation add to the C – C double bond of another monomer
molecule to from new carbocation.
R-CH2--CHX + CH2=CHX → R- CH2-CHX-CH2-
-CHX
R- CH2-CHX-CH2--CHX + n CH2=CHX → R- CH2-CHX-(CH2-CHX)n-CH2-
-CHX
Living Polymer
(iii) Chain Termination: Reaction is terminated by addition of H+.
R- CH2-CHX-(CH2-CHX)n-CH2--CHX +H+→ R - CH2-CHX-(CH2-CHX)n-CH2-CH2X
Dead Polymer
3.Coordination Polymerization
It usually involves transition-metal catalysts. Here, the "active species" is a
coordination complex, which initiates the polymerization by adding to the monomer’s
carbon-carbon double bond. The most important catalyst for coordination
polymerization is so-called Ziegler-Natta catalyst discovered to be effective for alkene
polymerization. Ziegler-Natta catalysts combine transition-metal compounds such aschlorides of titanium with organometallic compounds [TiCl3 with Al(C2H5)3]. An
important property of these catalysts is that they yield stereoregular polymers when
higher alkenes are polymerized, e.g., Polymerization of propene produces produces
polypropene with high selectivity.
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(i) Chain Initiation:
Cat-R + CH2=CHX → Cat-CH2-CHX-R
(ii) Chain Propagation :
Cat-CH2-CHXR+ CH2=CHX → Cat- CH2-CHX-CH2-CHX-R
Cat- CH2-CHX-CH2-CHX-R +nCH2=CHX → Cat- CH2-CHX-(CH2-CHX)n-CH2-CHX-R
Living Polymer
(iii) Chain Termination: by the addition of HX
Cat- CH2-CHX-(CH2-CHX)n-CH2-CHX-R + HX
Cat-X+ CH3-CHX-(CH2-CHX)n-CH2-CHX-R
Dead Polymer
4. PHYSICAL AND MECHANICAL PROPERTIES OF POLYMERS
4.1. Physical Properties
Crystallinity: The degree of structural order arrangement of polymeric molecules is
known as crystallinity. Crystallinity favours denser packing of molecules, thereby
increasing the intermolecular forces of attractions. This accounts for a sharp and highersoftening point, greater rigidity and strength. The polymers with low degree of
symmetry and with long repeating units are partially crystalline and are amorphous in
structure. The crystalline polymer units have packing close to each other through
intermolecular forces. Completely crystalline polymers are brittle. The crystallinity
influences properties like solubility, diffusion, hardness, toughness, density and
transparency of polymers.
Amorphous state: Random arrangement of molecules, less intermolecular forces lead to amorphous nature of a polymer. So they can be moulded into a desired shape. Both
thermosetting and thermoplastic polymers can exist in amorphous state.
4.2. Mechanical Properties:
Strength: The polymer chains adjacent to each are held together by weak
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intermolecular forces. The strength of intermolecular forces can be increased by either
increasing chain length or molecular weight or the presence of polar groups (-OH, -
COOH, -OMe, -COOR, -X). The lower molecular weight polymers are quite soft and
gummy. High molecular weight polymers are tough and heat resistant. The cross linked
polymer chains are strongly linked to each other by strong covalent bonds, which cause
greater strength, toughness, brittleness and low extensibilities. The strength of the
polymer is characterized by the stress and strain curve. Strength of the polymers also
depends on the shape of the polymer.
Eg: In PVC, large size chlorine atoms are present. The strong attractive forces restrict the
movement of molecules and so PVC is tough and strong.
b) Elastic character:. Elasticity is the relaxation to original shape after removal of
applied stress. Polymers like nylon, having this stretching nature are called elastomers.
Elastomers are slightly cross linked, amorphous and rubber like polymers. In the
absence of deforming forces these polymers have peculiar chain configuration of
irregularly coiled ‘snarls’. So the polymer is amorphous due to random arrangement.
When they are stretched cross-links begin to disentangle and straighten out.
c) Plastic deformation: This is found in thermoplastics. These polymers have structure
which is deformed under heat or pressure. This property is used to process them into
desired shape. Due to weak inter molecular forces, these polymers show permanent
deformation at high temperature and pressure. The Vander wall forces are weak in a
linear polymer at high temperature and result in ‘slippage’. The plasticity of a polymer
decreases with temperature.
5. PLASTICS
Plastics are polymers which can be moulded into any desired shape by the
application of heat and pressure in the presence of a catalyst. They undergo
permanent deformation under stress termed as plasticity. The terms plastic and resin
are used synonymously. Plastics are obtained by mixing a resin with other
ingredients to impart special engineering properties. These are characterized by light
weight, good thermal and electrical insulation, corrosion resistance, chemical
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resistance, adhesive nature, low cost, high abrasion resistance, dimensional stability,
strength, toughness and impermeability to water. A plastic material should have
sufficient rigidity, dimensional stability and mechanical system at room temperature
to serve as a useful article. It may be moulded to shape by application of reasonabletemperature and pressure.
Types of Plastics
Thermoplastics
These are linear, long chain polymers, which can be softened on heating and
hardened on cooling reversibly. Their hardness is a temporary property and it
changes with the raise or fall of temperature. They can be reprocessed.
Examples: Polyethylene (PE), Polypropylene (PP), Polyvinyl chloride (PVC),polystyrene (PS), Nylons, Poly tetra fluoro ethylene (PTFE) etc.
Thermosetting Plastics
These polymers, during moulding get hardened and once they are solidified,
cannot be softened i.e, they are permanently set polymers. During moulding, these
polymers acquire three dimensional cross-linked structure, with strong covalent
bonds. Thermosets once moulded cannot be reprocessed.
Examples: Polyester (terylene), Bakelite, epoxy- resin (araldite), Melamine, urea-formaldehyde resin etc.
Difference between thermoplastic and thermosetting resins
Thermoplastic resins Thermosetting resins
1. They are formed by addition
polymerization
1. They are formed by condensation
polymerization
2. They consist of linear long chain
polymer
2.
They consist of three dimensional
network structure
3. All the polymer chains are held
together by weak vanderwaals
forces
3. All the polymer chains are held
together by strong covalent bonds
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4. They are weak, soft & less brittle4. They are strong, hard & more
brittle
5. They soften on heating & harden on
cooling5. They do not soften on heating
6. They can be remoulded 6. They cannot be remoulded
7. They have low molecular weights 7. They have high molecular weights
8. They are soluble in organic solvents8. They are insoluble in organic
solvents
5.1.Compounding of a plastic
A high polymeric material is mixed with 4 to 10 ingredients during fabrication,each of which these ingredients either discharge a useful function during moulding
or impart some useful property to the finished article. This is called a mix. Some of
the main types of compounding ingredients are:
(1) Resin or a binder; (2) Plasticizers; (3) Fillers; (4) Plasticizers / Lubricants; (5)
Catalysts or accelerators; (6) Stabilizers.
Resin or a binder: The product of polymerization is a resin, which forms the major
portion of the body of the plastic. It also holds the different constituents together. Thebinders used may be natural or synthetic resin or cellulose derivatives. Resin forms
the major part of the plastic and determines the types of treatment needed in the
moulding operations.
Plasticizers: These are the materials that are added to resins to increase their
plasticity and flexibility. Their action is considered to be the result of the
neutralization of part of the intermolecular forces of attraction between macro
molecules. They decrease the strength and chemical resistance. They impart greaterfreedom of movement between the polymeric macro molecules of resins. Most
commonly used plasticizers are vegetable oils (non-drying type),camphor, esters (of
Stearic, Oleic or phthalic acids) and some phosphates ( tricresyl phosphate, tributyl
phosphate, tetra butyl phosphate and triphenyl phosphate)
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Fillers: Fillers are added to give to the plastic better hardness, tensile strength,
opacity, finish and workability. They reduce the cost, shrinkage on setting and
brittleness. They are also added to impart special characters to the product. The
percentage of fillers is up to 50% of the total moulding mixture.
Eg: a). Carborundum and mica are added to provide extra hardness b). Barium salts are
added to make plastic impervious to X- rays. c ). Addition of asbestos provides heat and
corrosion resistance. Most commonly used fillers are wood flour, asbestos, china clay,
talc, gypsum, metallic oxides like ZnO, PbO and metal powders like Al, Cu, Pb etc.
The fillers which enhance mechanical strength are reinforcing fillers.
Eg: Addition of carbon black to natural rubber, increase its strength to 40% and also
enhances its abrasion resistance.
Plasticizers/Lubricants: Lubricants like waxes, oils, soaps are employed to make the
moulding of plastic easier. They impart a glossy finish to the products. They also
prevent the plastic material from sticking to the fabricating equipment. They make
moulding easier and impart glossy flawless finish to the product. Commonly used
lubricants are waxes, oils, stearates, oleates and soaps.
Catalyst/Accelerator: These are added to thermosetting plastic, during mouldingoperation, to accelerate the polymerization of fusible resin, into cross-linked infusibleform.
Eg: Catalysts used for compounding include H2O2, benzoyl peroxide, acetyl sulphuric
acid, metals like Ag, Cu, and Pb; metallic oxides like ZnO, NH3 and its salts.
Stabilizers: They improve the thermal stability during polymerization and further
processing. Vinyl chloride shows a tendency to undergo decomposition and
discoloration at moulding temperature. Hence, during moulding, heat stabilizers are
used. Commonly used stabilizers
a) Opaque moulding compounds like salts of lead (viz. white lead, litharge, lead
chromate, red lead etc.)b) Transparent moulding compounds like stearates of lead, Cd and Ba.
6.6. Colouring materials: Color and appeal are very important for commercial high
polymer goods. Commonly used coloring materials are organic dye stuffs and opaque
inorganic pigments.
Eg: Carbon black, anthraquinones(yellow), azodyes(yellow,red),phthalocyanins (green).
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5.2. Fabrication of Plastics into Articles
The fabrication of plastic into commercial goods is done by moulding
Moulding: Moulding is an important method of fabrication of plastic. The moulding of
the plastic is done around a metal insert so that the finished product has a metal part
firmly bonded to the plastic.
Commonly used moulding methods are
(i). Compression moulding
(ii). Injection moulding
(iii). Transfer moulding
(iv). Extrusion moulding
(i) Compression moulding:
This method is applied to both thermoplastic and thermosetting resins. A
predetermined quantity of ingredients required for the plastic are filled between the
two half- pieces of the mould. Heat and pressure are then applied as per required
standards. The cavity gets filled with fluidized plastic. The halves of the mould are
closed slowly. Final curing (the time required for the plastic to set in the shape) is done
either by heating (for thermosetting) or cooling (for thermoplastic). These moulded
articles are then taken out by opening the parts of the mould.
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(ii) Injection moulding:
This method is applicable for thermoplastic resins. The moulding plastic powder is fed
into a heated cylinder through a hopper and 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 has been cured sufficiently, half of the mould is opened to allow the
removal of the finished article without any deformation. Heating is done by oil or
electricity.
Advantages:
This is most widely used method because of its high speed of production, low
mould cost, very low loss of material and low cost. There is a limitation of design of
articles to be moulded, because large number of cavities cannot be filled
simultaneously.
(iii) Transfer moulding: This method is useful for moulding of thermosetting plastics.
The powdered compounding material to be moulded is placed in a heated chamber,
maintained at a minimum temperature, where powder just begins to become plastic.
This is then injected through an orifice into the mould by a plunger, working at high
pressure. Due to the friction developed at the orifice, the temperature rises to the extent
that the moulding powder becomes liquid and flows quickly into the mould. This is
then heated up to curing temperature for setting. This is then heated up to curing
temperature for setting.
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Advantages: The plasticized mix flows into cavity in highly plasticized condition and
hence very delicate articles can be handled without distortion or displacement. Thick
pieces can also be cured completely and uniformly. Non attainable shapes by
compression moulding can be obtained. The article produced is free from flow marks.
Finishing cost of fabricated article is almost low and blistering of the goods is almost
eliminated..
(iv) Extrusion moulding:
This method is mainly used for continuous moulding of thermo plastics into
materials of uniform cross-section like tubes, rods, sheets, wires, cables etc.. The
thermoplastic ingredients are heated to plastic state ( a semi solid condition) and then
pushed by means of screw conveyor into a die, having the shape of the article to be
fabricated. The plastic mass gets cooled due to atmosphere exposure or artificially by
air jets or a spray of water.
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6. SOME INDIVIDUAL POLYMERS
6.1. Polyethylene or PE
Polyethylene is most commonly used polymer, produced by the polymerization
of ethylene in presence of a catalyst. By using free radical initiator (benzoyl peroxide) at
80-1250C low density polythene (LDPE) with density of 0.92g/cc is produced, while by
using an zieglar-natta catalyst like tri ethyl aluminium,, high-density polythene
(HDPE) with density 0.965g/cc is obtained.
n CH2=CH2 Heat/pressure/benzoyl peroxide (CH2-CH2)n
Ethylene Polyethylene
Preparation:Low density polythene (LDPE) is produced from ethylene by free radical addition
polymerization.
n CH2=CH2 Heat/pressure/benzoyl peroxide (CH2-CH2)n
Ethylene LDPE
High density polythene (HDPE) is produced from ethylene by Zieglar Natta
addition polymerization. In the polymerization of HDPE the catalyst is usually based
on titanium tetrachloride/aluminium alkyl (e.g., diethylaluminium chloride).
n CH2=CH2 Heat/pressure/benzoyl peroxide (CH2-CH2)n
Ethylene Titanium tetrachloride/Tri alkyl aluminium
HDPE
Properties:
1. Polyethylene is a rigid, waxy, white, translucent, non polar solid material with
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good electrical insulation property. It is a soft flexible polymer.
2. It exhibits chemical resistance to strong acids, alkali and salt solutions at room
temperature but attacked by oils, organic solvents especially kerosene.
3. Polyethylene crystallizes very easily due its highly symmetrical structure. Thedegree of crystallization varies from40-95% depending on the number of
branching in the polymeric chain.
4. Commercially polyethylenes are sub divided into three groups based on itsdensity.
i) Low Density Polyethylene LDPE ; ii) Medium density Polyethylene; iii) High
Density Polyethylene(HDPE)
5. It is resistant to atmospheric gases, moisture and UV light
Engineering applications: PE is used for making high frequency insulator parts, bottle
caps, packing materials, tubes, coated wires, tank linings in chemical plants and
domestic appliances.
6.2. Poly vinyl chloride or PVC
It is a thermoplastic polymer and is obtained by the free radical addition
polymerization of vinyl chloride in the presence of benzyl peroxide or hydrogen
peroxide. In PVC the mass of chlorine is 57% of the total mass of the polymer. Vinyl
chloride is obtained by treating acetylene with HCl at 60-800C in the presence of metal
oxide catalyst.
Preparation: Preparation of PVC involves the following two steps.
1 Step: Vinyl chloride is prepared by treating acetylene with hydrogen chloride at
60 - 80°C in the presence of metal chloride as catalyst.
II Step: Polyvinyl chloride is obtained by heating water emulsion of vinyl chloride
in presence of benzoyl peroxide (or) hydrogen peroxide under pressure.
Heat/pressure/ H2O2
n CH2=CH -(- CH2-CH -)-n
Cl Cl
Vinyl chloride Polyvinyl chloride
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Properties:
1. PVC is colorless, non – inflammable and chemically inert powder. It is strong butbrittle.
2. It is resistant to ordinary light, atmospheric gases, moisture, inorganic acids and
alkalis, but undergoes degradation in heat or UV light.
3. It is soluble in hot chlorinated hydrocarbons like ethyl chloride
4. Pure resin possesses a high softening point.
5.
It has greater stiffness and rigidity compared to polyethylene.
Engineering applications:
1. It is widely used as a synthetic plastic.
2. Rigid PVC is used for making sheets, light fittings, safety helmets, refrigerator
components, tyres, and cycle and motor cycle mudguards.
3.
Plasticized PVC is used in making continuous sheets viz., table cloths, raincoats,curtains etc.,
4.
Used in injection moulding of articles like toys, tool – handles, radio –
components, chemical containers, conveyor belts etc.
6.3. Bakelite
It is prepared by condensing phenol with formaldehyde in presence of
acidic/alkaline catalyst. The initial reaction results in the formation of non polymeric
mono, di and tri methylol phenols depending on the reactant ratio. These compounds in
the first stage react to form a linear polymer, Novolac. Novolac in the second stage
undergoes further reaction with these linear polymers to form cross linking and bakelite
plastic resin is produced.
All these stages in a step wise manner are shown in the reaction below,
ultimately giving the cross linking polymer, bakelite.
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Stage :1
Stage : 2
Stage : 3
Properties:
1. Phenolic resins ( like bakelite) set to rigid, hard, strong, scratch-resistant,
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infusible, water-resistant, insoluble solids, which are resistant to non-oxidizing
acids, salts and many organic solvents.
2. But these are attacked by alkalis, because of the presence of free hydroxyl
group in their structures.3. They possess excellent electrical insulating character.
4. They are good anion exchange resins capable of replacing anions with –OH
groups.
5. They are good adhesives, corrosion resistant and resistant to atmospheric
gases, moisture and UV light.
Engineering applications:
The phenol-formaldehyde resins are extensively used1. for making electric insulator parts like switches, plugs, switch-boards, heater
handles, etc.
2. for making moulded articles like telephone accessories, cabinets for radioand television.
3. for impregnating fabrics, wood and paper.
4. as adhesives (e.g., binder) for grinding wheels.
5.
in paints and varnishes.
6.
as hydroxyl group exchanger resins in water softening7. for making bearings, used in propeller shafts for paper and rolling mills.
9. Elastomers
Rubbers, also known as elastomers are high polymers, having elastic
property i.e.; the ability to regain their original shape after releasing the stress.
They have temporary deformation in their physical structure on application of
stress of more than 600 elastic units. Thus, a rubber can be stretched to 4 to 10
times its original length. The elasticity of rubber is due to its coiled structure.
Elastomers are expected to have the following characteristics.
1. They have elasticity i.e.; it can be stretched by applying stress and can regain
original shape and dimension by releasing the stress.
2. They have very low inter chain attraction forces.
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3. They have coiled structure.
4. They can absorb water.
5. They has low chemical sensitivity
6. At high temperature they become sticky.
Elastomers are classified into two types 1) Natural rubber 2) Synthetic rubber
9.1. Natural rubber
Natural rubber consists of basic material latex (cell sap) , which is a dispersion of
isoprene. During the treatment, these isoprene molecules polymerizes to form, long-
coiled chains of cis-polyisoprene.
The main source of natural rubber is the latex of the “Hevea brasiliensis”. More than
95% of the rubber is obtained from Hevea brasiliensis. Natural rubber obtained from
Hevea brasiliensis is a cis- polymer of isoprene (2-methyl. 1,3 – butadiene). The
polyisoprene in natural rubber is in long coiled chain form, responsible for its
elasticity.
nCH2= C – CH – CH2 -(-CH2 -C = CH – CH2-)-n
CH3 CH3
Isoprene Polyisoprene
Processing of Rubber : Latex obtained from tapping of the tree is diluted to contain
25 to 40% of rubber. Then filtered to eliminate any impurities like bark or leaves
present in it. Then natural rubber is coagulated to soft white mass by addition of acetic
acid or formic acid. The coagulated white mass is washed. The coagulum is treated as
below:
(a) Crepe rubber: The coagulum is allowed to drain for about 2 hours. It is then passed
through a creping machine and the spongy coagulum is converted into a sheet,
dried in air for 5 to 10 days at about 50oC. The sheets posses an uneven rough
surface resembling a crepe paper .
(b) Smoked rubber: Coagulation is carried out in long rectangular tanks fitted with
metal plates. Diluted latex is poured into these tanks to which dilute acetic acid or
formic acid is added and the mixture is stirred thoroughly. The tanks are kept
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undisturbed for about 16 hrs. After inserting the partition plates into the grooves,
the coagulum forms into tough slabs between the plates. The slabs are passed
through a series of rollers, so as to give ribbed pattern to the final rubber sheet. The
sheets are then hung for about 4 days in a smoke chamber, at a temperature
between 40-50oC.
9.2. Gutta-Percha
This is another type of natural rubber obtained from the mature leaves of
dichopsis gutta and palgum gutta trees. The mature leaves are ground carefully; treated
with water at about 70 oC for half an hour and then poured into cold water, when gutta
perch floats on water surface and is removed by extraction with CCl4. After the
evaporation of the solvent, it is extruded in a sheet form by passing between two rollers.
Properties
a. Gutta-percha is tough and horny at room temperature but turns soft at about
100oC.
b. It is soluble in chlorinated and aromatic hydrocarbons, but not in aliphatichydrocarbons.
c. Gutta percha is used in the manufacture of submarine cables, golf ball
covers, tissue for adhesive and surgical purposed.
Engineering applications
1) Dentists use it to make temporary fillings.
2) It is used in conjunction with Balata resin, in conveyor belts.
9.3. Draw backs of natural rubber
1) It is soft at high temperature, brittle at low temperatures, weak and has poortensile strength.
2) It has a high water absorption capacity, swells in water.
3)
It dissolves in mineral oils, acids, bases and non-polar organic solvents likebenzene.
4) It is attacked by oxidizing agents including atmospheric oxygen and becomessticky.
5) It undergoes permanent deformation when stretched.
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10. Vulcanization
When rubber is heated with sulphur at temperature of 100-140 oC , sulphur
combines chemically at the double bonds of the different chains of rubber and produces
three dimensional crossed linked rubber, which over comes all the drawbacks of naturalrubber. This vulcanized rubber does not melt on heating. This is the fundamental
difference between a thermoplastic and rubber. The extent of stiffness of vulcanized
rubber depends on the amount of sulphur added. For example., a tyre rubber may
contains 3-5 % of sulphur, but a battery case rubber may contain as much as 30%
sulphur. Vulcanization provides cross linking of sulphur atoms between the adjacent
chains of rubber.
The amount of sulphur used for ordinary soft rubber is 0.5 to 5% where as for
hard rubber it is 32 to 35% of the rubber. The other vulcanizing agents used include Se,Te, benzoyl chloride, tri nitro benzene, alkyl phenol sulphides, H2S, MgO, benzoyl
peroxide etc.
CH3 CH3
….-CH2 – C = CH – CH2 – C = CH – CH2-...
+
….-CH2 – C = CH – CH2 – C = CH – CH2-...
CH3 CH3
Vulcanization +S
CH3 CH3
….-CH2 – C - CH – CH2 – C - CH – CH2-...
S S S S (sulphur cross links)
….-CH2 – C - CH – CH2 – C - CH – CH2-...
CH3 CH3
Vulcanization of raw rubber with sulphur as vulcanizing agent
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Advantages of Vulcanization
Vulcanization transforms the weak, thermoplastic rubber into a strong and toughrubber.
1. The working temperature range is – 10 oC to 100 oC.
2.
The tensile strength increases. (2000 kg/cm3)
3. The water absorptivity decreases.
4. The article made from vulcanized rubber returns to the original shape when the
deforming load is removed, i.e., the resilience power is increased.
5. The vulcanized rubber becomes resistant to organic solvents like CCl4, benzene, fats
and oils, however it swells in these solvents
6. It becomes resistant to abrasion, ageing and reactivity with oxygen & ozone.
7.
It becomes better electrical insulator.
8. It can be easily manipulated into desired shape.
Compounding of rubber
1) Accelerators: These are meant for catalyzing the vulcanization process, thus
reducing the time required for vulcanization and maintain the vulcanization
temperature. The inorganic accelerators include lime, magnesia, litharge and white
lead, where as the organic accelerators are complex organic compounds such as
aldehydes and amines. Sometimes, ZnO can acts as an accelerator activator.
2) Antioxidants: These substances retard the deterioration of rubber by light and
air. These are complex organic amines like phenyl naphthyl amine, phenolic substance
and phosphates.
3) Reinforcing agents: These are usually added to give strength, rigidity andtoughness to the rubber
and may form as much as 35% of the rubber. The commonly used reinforcing agents are
carbon black, ZnO, MgCo3, BaSO4, CaCO3 and some clays.
4) Fillers: The function of the fillers is to alter the physical properties of the mix to
achieve simplification of the subsequent manufacturing operations, or to lower the cost
of the product.
5) Plasticizers (or) softeners: These are added to impart great tenacity and adhesion to
the rubber. The most commonly used plasticizers are vegetable oils, waxes, stearic acid,
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rosin etc.
6) Coloring agents: These are added to impart desired colour to the rubber.
TiO2, lithophane - hite Ferric oxide - Red
Lead chromate - ellow Carbon black - Black
Chromiumtrioxide - Green Ultra marine - Blue
Engineering applications of rubber: The major application of rubber is in making tyres
and tubes. It is also used in making belts for transport, material handling, tank inner
lining in chemical plants where corrosive materials are stored. Rubber sandwiches are
used in machine parts as gaskets to reduce vibrations. Foamed rubber is used in making
cushions, mattresses and paddings.
11. SYNTHETIC RUBBERThe natural rubber sources are not sufficient and could not supplement the needs of
automobile industry. An attempt was made to synthesize rubber, but rubber like
materials were synthesized to supplement the needs of various industries. These
materials synthesized by various processes are called elastomers. The artificially
prepared polymer, which has elastomeric property, is known as synthetic rubber. There
are several types of synthetic rubbers available and used on commercial grade.
11.1. SBR (Styrene – Butadiene Rubber) or BUNA -S
It is a copolymer of about 75% butadiene and 25% styrene. Hence it is called asstyrene rubber.
Preparation : It is produced by the copolymerization of butadiene, CH2=CH–CH=CH2
(about 25% by weight) and styrene, C6H5CH = CH2 (75% by weight), in presence of
sodium as catalyst.
nCH2 = CH – CH = CH2 + nCH2 =CH
C6H5
Butadiene styrene
Na -(-CH2 - CH = CH - CH2 - CH2 – CH-)-n
C6H5
Styrene butadiene rubber (Buna-S-Rubber)
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Properties
1) It has excellent abrasion resistance and high load bearing capacity.
2) A reinforcing filler (carbon black) is essential to achieve good physicalproperties.
3)
It is a good electrical insulator
Uses: It is used for lighter – duty tyres, hose pipes, belts, moulded goods, unvulcanized
sheet, gum, floorings, rubber shoe soles and electrical insulation cables, chemical plant
inner linings etc.
11.2. BUNA-N or Nitrile Rubber (NBR)
Nitrile rubber is the copolymer of butadiene and acrylonitrile. Bu – stands forButadiene, N-stands for acrylonitrile.
nCH2 = CH – CH = CH2 + nCH2 =CH
CN
Butadiene acrylonitrile
-(-CH2 - CH = CH - CH2 - CH2 – CH-)-n
CN
Nitrile butadiene rubber (Buna-N-Rubber)
Properties
1) Because of the presence of –CN group in the structure BUNA-N possess excellent
resistance to heat, sunlight, oils, acids and salts and less resistant to alkalis than
natural rubber.
2) It is a strong and tough polymer with light weight
3) BUNA-N is also vulcanized with sulphur
Engineering Applications
1) It is used for making conveyor belts, aircraft components.
2) BUNA-N is extensively used for fuel tanks, gasoline hoses, creamery equipment,
and automobile parts.
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3)
11.4. Polysulphide rubber (or) Thiokol rubber (GR-P)
This is synthesized by the copolymerization of sodium polysulphide (Na2S4) and
ethylene dichloride and during the reaction NaCl gets eliminated.
Properties
1. The properties of the material depend upon the length of aliphatic group and
number of sulphur atoms. It possesses strength and impermeability to gases and
low abrasion resistance.
2. Thiokol is resistant to swelling, oils, solvents and fuels.
3. Thiokol is inert to fuels, lubricating oils, gasoline and kerosene.
Uses
1.
It is used for coating fabrics, for making life rafts and jackets.2. It is used for making gaskets, diaphragms and seals in contact with solvent and
for printing rolls.
3. It is used for lining hoses for gasoline and other transport pipes
4. Liquid Thiokol can be used to make tough solvent resistant temperature liquid
compounds which are used as liners for aircraft.
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