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Unit – 2
Polymers & Composites
1. Polymers – Introduction -a) Definition –monomer, polymer, degree of freedom, functionality, oligo polymer, High polymer, isotactic, atactic, syndiotactic, Homopolymers, Heteropolymers. b) Types of polymerization – Addition, condensation, co-polymerization – comparison c) Mechanism of Free radical polymerization – 3 step process Step1-Initiation (Radical and Chain initiating species formation) Step2 -Propagation (Living polymer formation) Step3 -Termination (By coupling and disproportionation) 2. Plastics a) Classification – i)based on thermal property – Thermo / Thermoset plastics ii) Based on usage – Commodity and Engineering plastics b) Prepration, properties and applications of i) PVC ii) Teflon iii) Poly carbonate iv) Poly Urethane v)PET vi) Nylon 3. Rubber a) Raw rubber – Preparation - problems of raw rubber – Vulcanisation – Difference between raw and vulcanized rubber b)Synthetic rubber – Preparation, properties, uses of i) SBR ii) Butyl rubber 4. Composites a) Definition – properties - Types - Polymer matrix / Metal matrix / Ceramic matrix composties b) Fibre Reinforced Plastics (FRP) – Types of FRP - Applications of FRP
The word polymer is derived from Greek words “poly” means much, many and mers means units or parts.
INTRODUCTION:
Polymers are generally macromolecules formed by the repeated linking of large number of small molecules.
Polymers are widely used in:
Automobiles
Defence
Electrical goods and
Computer components etc.
Polymers are
POLYMERS
macromolecules (giant molecules of higher molecular weight) formed by the repeated linking of large number of small molecules called monomers.
Example:
Polyethylene is a polymer formed by the repeated linking of large number of ethylene molecule.
nCH2=CH2 (CH2--CH2 )
Ethylene(monomer) Polyethylene(polymer)
n
Monomer is a micromolecule (small molecule) which combines with each other to form a polymer.
MONOMER
Ethylene: CH
Example:
2=CH2
POLYMERISATION
Polymerisation is a process in which large number of small molecule(called monomers) combine to give a big molecule(called a polymer) with or without elimination of small molecules like water.
The number of repeating units (n) in a polymer chain is known as the degree of polymerisation.
DEGREE OF POLYMERIZATION:
5CH
Example:
2=CH2 -CH2-CH2-CH2-CH2-CH2- CH2-CH2-CH2-CH2-CH2
In this example, five repeating units are present in the polymer chain.So; the degree of polymerisation is 5.
-
Polymers with low degree of polymerisation are known as oligo polymers, their molecular weight ranges from 500-5000 Daltons.
(i)OLIGO POLYMERS:
Polymers with high degree of polymerisation are known as high polymers ,their molecular weight ranges from 10,000-2,00,000 Daltons.
(ii)HIGH POLYMERS:
NOMENCLATURE OF POLYMERS
1. If the polymer chain contains same type of monomer, it is “ Homo polymer”.
e.g PVC structure : A – A – A- A- A-A -A
2. If the polymer chain contains different type of monomer, it is “Hetero polymer”.
e.g Nylon A-B- A-A-A-B-A
3.Number of Reactive sites or Bonding sites or functional groups present in a polymer is called ‘ Functionality’.
e.g CH2 = CH2
CH
, The double bond is acting as two reactive site, So, Ethylene functionality is 2.
2
│
– OH In glycerol three –OH groups present. So, functionality = 3
CH – OH
│
CH2
– OH
If F = 2, they form linear chain structure.
If F=3, they form branched structure.
If F≥ 4, then they form complexed 3D structure.
4. Orientation of monomers in a polymer chain is called “Tacticity”. If the groups are in same orientation, it is isotactic. If they are random it is “atactic”. If they are arranged in alternative fashion, it is syndiotactic.
A A A A A A B B A B A B A B A
│ │ │ │ │ │ │ │ │ │ │ │ │ │ │
M -M - M - M - M - M - M - M - M –M M - M - M – M - M
│ │ │ │ │ │ │ │ │ │ │ │ │ │ │
B B B B B B A A B A B A B A B
(Iso tactic) (Atactic) (syndiotactic)
TYPES OF POLYMERISATION
1. Addition 2. Condensation 3. Copolymerisation
:
No Addition Polymerisation Condensation Polymerisation
1 Eg. PVC Eg. Nylon 6,6
2 Otherwise known as “Chain growth Polymerisation”.
Otherwise known as “Step wise Polymerisation”.
3 Monomers are adding together to form polymers.
Monomers are condensed to form polymer.
4 No elimination of other molecules. Elimination of smaller molecules occur.
5 At least one multiple bond presence is essential condition.
Monomers must have two or more functional groups.
6 Homo polymers are formed. Hetero polymers are formed.
7 Thermoplastics are formed. Thermo set plastics are formed.
8 Molecular weight of the polymer is the integral multiple of monomers.
Need not be so.
9 Monomers disappear slow and steadily. Monomers disappear at the initial stage of the reaction.
10 Longer processing time is needed. Longer time is essential.
e.gAddition Polymerisation:
n(CH2 = CH2 ) -( CH2 - CH2 -)
Ethylene Polyethylene
n
Condensation Polymerisation
n H2N - (CH2)6 – NH2 + n HOOC – (CH2)4
Hexa methylene diamine Adipic acid
– COOH
[ - HN - (CH2)6 – NH - OC – (CH2)4 – CO - ]n
Nylon 6,6
Co-Polymerisation
1. It is a special kind of polymerisation, otherwise known as “Joint polymerisation”.
The product is known as ‘Co-polymers’. It is used to alter the hardness, strength, rigidity of the monomers.
e.g SBR synthesis
n CH2 = CH - CH = CH2 + n CH2
= CH
( 75% butadiene) (25% Styrene)
[ CH2 - CH = CH - CH2 - CH2 - CH -]
n
(Styrene – Butadiene Rubber)SBR
O
O
PLASTICS
Plastics are high molecular weight organic materials, that can be moulded into any desired shape by the application of heat and pressure in the presence of a catalyst.
DEFINITION:
Plastics were discovered and then developed based on trial and error method. The present time is sometime referred as plastic age, because the use of polymeric material is peculated in large variety of applications. This rapid growth has taken place only in the last 50 years. It was man’s desire to develop plastics for engineering applications to replace, glass, metal, ceramic, wood and other materials of constructions. Since plastics possess the following advantages, they have wider applications.
• They are light in weight.
ADVANTAGES OF PLASTICS:
• They possess low melting point. • They can be easily moulded and have excellent finishing. • They possess very good strength and toughness. • They possess good shock absorption capacity. • They are corrosion resistant and chemically inert. • They have low co-efficient of thermal expansion and possess good thermal and
electrical property. • They are very good water-resistant and possess good adhesiveness.
• Softness.
DISADVANTAGES OF PLASTICS:
• Embrittlement at low temperature. • Deformation under load. • Low heat-resistant and poor ductility. • Combustibility. • Polymers tend to degrade upon exposure to heat and uv-radiation. • Polymers are non bio-degradable.
CLASSIFICATION OF PLASTICS
I) On usage
CLASSIFICATION OF PLASTICS ARE BASED
II) On structure
Based on usage Plastics are divided into two categories
I)CLASSIFICATION OF PLASTICS BASED ON USAGE:
1.General purpose plastics(commodity)
2.Engineering plastics (or) high performance plastics
General purpose plastics have low to medium mechanical properties. They are used for manufacture of commodity items .They account for 80-85% of the total polymer production
1.GENERAL PURPOSE PLASTICS:
They have low use temperature therefore cannot be used at high temperature
PROPERTIES OF GENERAL PURPOSE PLASTICS:
They have low abrasion resistance and poor dimensional stability
They are mostly crystalline with low glass transition temperature(Tg
) (or) they are glossy (or) amorphous polymer
Engineering materials are a group materials obtained from high polymer resin
2.ENGINEERING PLASTICS:
They are mainly used to replace conventional material like metal , wood , glass and cermaics
Not only engineering plastics can replace metals but they can also be used along with metals
High load bearing characteristics
CHARACTERISTICS (OR) PROPERTIES OF ENGINEERING PLASTICS:
High mechanical strength
High dielectric constants
Readily moldable properties into complicated shapes
High abrasion resistance
Very good rigidity
Good dimensional stability
Fairly good thermal stability
Light weight
High performance properties
They can be used alone (or) in conjunction with metals ,ceramics or glasses etc
APPLICATIONS:
They find application in demanding areas like automobiles ,defense , electrical and electronics,telecommunication,textiles,satellites,robots,computer components
Plastics are classified depending upon the type of resin used for the manufacture of plastics.
II)CLASSIFICATION OF PLASTICS BASED ON STRUCTURE:
A resin is a basic binding material, which undergoes polymerization reaction during the molding of plastics.
Plastics or Resins are classified into 2 types
1. Thermoplastic Resin
2. Thermosetting Resins (or) Thermostats
S.no Thermoplastic Resins Thermo setting resins
1 They are formed by Addition polymers They are formed by condensation polymers
2 They consist of linear long chain polymers They consist of three dimensional network structure
3 All the polymer chains are held together by weak vanderwaal’s forces
All the polymer chains are linked by strong covalent bonds
4 They are weak ,soft, and less brittle They are strong ,hard and more brittle
5 They soften on heating and harden on cooling
They do not soften on heating
6 They can be remolded They cannot be remolded
7 They have low molecular weight They have high molecular weights
8 They are soluble in organic solvents They are insoluble in organic solvents
9 Ex PVC ,Polyethylene Ex Bakelite ,Polyester
STRUCTURE OF THERMO PLASTIC RESINS:
STRUCTURE OF THERMO SETTING RESINS:
PROPERTIES AND USES OF ENGINEERING PLASTICS
No Name Properties Uses 1 PVC 1.Colourless , odourless
powder. 2. Affected by Organic chlorinated acids. 3. It is degraded by high temperature and radiations.
1.Pipes 2Electrical wire covering 3.Table cloth 4.Adhesives
2 TEFLON 1. Except Fluorine, it is chemically inert. 2.Withstands up to 3500
3. Electrical insulators. C
1. In chemical carrying pipes 2.Gaskets in cookers 3. Electrical switchboards.
3 POLY CARBONATE
1. Withstand very high temperatures. 2. They are having high transparency.
1. In sterilizable bottles. 2. Film industry – Camera, Photography films 3. Transparent bottles
4 POLY URETHANE
Used at Subzero (-ve) temperatures.
1.Oceanography 2.In defense 3.High altitude mountains
5 PET 1.High stretch resistance 2. High wrinkle resistance 3. Unbreakable 4. Acid proof
1. PET jars, bottles 2.Helmets 3.Terylene fabrics 4.Textile , wool industry
6 POLY AMINDES
1.Flexibililty 2.Elasticity 3.Elongatable property
1. Tooth brush bristles 2.Automobile gears 3.Textile industry 4. Nylon ropes
PREPARATION OF SOME IMPORTANT ENGINEERING PLASTICS 1. PVC – POLY VINYL CHLORIDE Step 1 – Acetylene is treated with Hydrochloric acid at 60-800
C in presence of some metallic chloride catalyst. It forms Vinyl chloride.
CH≡CH + HCl MCl / 60-800C CH2
│ = CH
Cl (Acetylene) (Vinyl chloride)
Step 2 – Vinyl chloride, in presence of Hydrogen peroxide undergoes polymerisation to form Poly vinyl Chloride. CH2 = CH H2O2 CH2
│ │ CH
Cl Polymerisation Cl
n
Vinyl Chloride PVC
• PVC is colourless, odorless and chemically inert powder
PROPERTIES:
• It is insoluble in inorganic acids and alkalies,but soluble in hot chlorinated hydrocarbons such as ethyl chloride
• It undergoes degradation in presence of heat or light.
• It is used in the production of pipes, cables insulations, tables covers and rain coats, etc. USES:
• It is also used for making sheets, which are employed for tank-linings, light fittings, refrigerators components, etc.
2. TEFLON (Poly Tetra Flouro Ethylene –PTFE) Step – 1 When Chloro Difluoro methane is heated, it forms Tetra flouro ethylene is formed. CaF2+H2SO4 CaSO4
CH
+2HF
4+3Cl2 CHCl3
CHCl
+3HCl
3 + 2HF SbF3 CHClF2
+ 2HCl
2CHClF2 heating CF2 = CF
2
Step 2 - Tetra fluoro ethylene , in presence of Benzoyl peroxide (Be2O2
) , polymerized to form PTFE.
n (CF2 = CF2 ) Be2O2 CF2 - CF2
polymerisation
n
(Tetra flouro ethylene) (Teflon – PTFE)
• Teflon is extremely tough, flexibile material possessing high softening point (about
PROPERTIES:
3500
• It possesses extremely good electrical and mechanical properties. c).
• It is chemically resistant towards all chemicals (except hot alkali metal and hot fluorine).
• It is used as a very good electrical insulating material in motors, cables, transformers, electrical fittings.
USES:
• It is also used for making gaskets, packing’s, pump parts, tank linings, etc. • It is also used for making non-lubricating bearing, chemical carrying pipes, etc.
3. POLY CARBONATE ( Lexan / Merlan)
They are prepared by interaction of diphenyl carbonate with bisphenol-A.
PREPARATION:
• They have high impact and tensile strength over wide range of temp.
PROPERTIES:
• They are soluble in organic solvents and alkali.
• They possess good dimensional stability, stiffness, transperancy etc.
•
• They are used for making electrical insulators ,housing apparatus , plugs , sockets , switches , sterilizable , transparent containers , camera , photographic films , hair –drier bodies , baby bottles ,safety windows in prison and jewellery shops, etc.
USES:
4.POLY URETHANE ( Perlon – u)
• It is obtained by the reaction of 1,4 –butanediol with 1 , 6-hexamethylene di – isocyanate
• It possesses excellent flexibility, toughness even at sub-zero temperature. PROPERTIES:
• It is characterized by excellent resistance to abrasion and solvents.
• Polyurethanes are used as coatings, films, foams adhesives and elastomers. USES:-
• They are also used in defence, oceanographic research, mountaineering. 5. POLY ETHYLENE TERYPTHALATE (PET)
HO – (CH2)2
- OH + HOOC - - COOH
Ethylene glycol Terypthalic acid
(-O – (CH2)2 – O – C - - C - )n + 2 H2
║ ║
O
O O
(PET)
O
O
• Amorphous nature PROPERTIES:
• High clarity • High heat distortion temp. • Flame retardance • Good electrical property
• Solar collectors USES:
• Safety devices • Making synthetic fiber like terylene,dacron. • Used in safety helmets, aircraft battery boxes
6.POLYAMIDES (nylon):
Nylon- 6:6 is obtained by the polymerisation of adipic acid with hexamethylene diamine . 1) NYLON- 6:6:
n H2N- (CH2)6- NH2 +nHOOC –(CH2)4- COOH -------- [HN-(CH2)6-NHCO-(CH2)4-CO]n+2nH2
Hexamethylene diamine Adipic acid nylon6:6(polyhexamethylene adipate)
O
It is prepared by self –polymerization of caprolactam 2)NYLON-6:
It is prepared by self-condensation of ω -Aminodecanoic acid . 3)NYLON-11:
• Nylons are translucent and high melting polymers. PROPERTIES:
• They possess high temperature stability and good abrasion resistance. • They are insoluble in common organic solvents and soluble in phenol and formic acid. • They possess low coefficient of friction. • Kevlar’s (aromatic polyamide similar to nylon, but instead of aliphatic chain, benzene
ring will be present) they have high temperature resistant.
Nylon are used for making filament for ropes, bristles for tooth brushes and films,etc USES:-
Nylon -6 and nylon -11 are mainly used for moulding purposes for gear, bearings, etc… Nylon 6:6 is used for fibers, which is used in making socks, dresses, carpets, etc…
Rubber is a high polymer with basic qualities of elasticity and non-crystallinity. They are known as elastic + polymer = elastomers.
Types – i) Raw (natural) rubber ii) Synthetic rubber
Natural rubber preparation:
1. When we cut the bark of rubber tree, latex milk is coming out. It is collected in containers. 2. Latex contain major portion of water (70%) and 30% rubber as isoprene C5H8
3. To get colloidal rubber, as coagulant, we are adding acetic acid. units.
4. The colloidal rubber is dried in air or passing smoke. The previous one is called ‘dried rubber’ and another one is ‘smoked rubber’. 5. Then it is made as sheets using rollers. Vulcanisation: To remove the defects of rubber, we are adding sulphur to rubber and heating at 100 – 1400
The defect of rubber is mainly due to the non-cross linked isoprene structure of rubber. But, the added sulphur attacks the double bond of isoprene units and converts the non-cross linked structure into a cross linked structure. So, the defects of natural rubber are removed.
C under high pressure. This process is called Vulcanisation.
If 3 -5 % sulphur is added, it is soft rubber. They are used in tyres.
If more than 30% Sulphur is added, it is called ‘hard’ or ‘ebonite rubber’. They are used in acid battery cases.
Isoprene C5H8
CH
Poly isoprene
3 CH3 CH
│ │ │
3
CH2 = C – CH = CH2 –CH2 – C= CH–CH2 –CH2 – C = CH – CH2
–
–CH2 – C= CH–CH2 –CH2 – C = CH – CH2
│ │
–
CH3 CH
( Rubber defects due to non-cross linked structure)
3
Adding sulphur during vulcanization alters the structure as follows:
CH3 CH
│ │
3
–CH2 – C– CH–CH2 –CH2 – C – CH – CH2
│ │ │ │
–
S S S S
│ │ │ │
–CH2 – C– CH–CH2 –CH2 – C – CH – CH2
│ │
–
CH3 CH
3
Differences between raw and vulcanized rubber
No Raw rubber Vulcanised rubber
1 Soft and sticky during summer Not Soft and sticky during summer
2 Hard and brittle during winter Not Hard and brittle during winter
3 Swells in oil Does not Swell in oil
4 Absorbs high amount of water Does not Absorb high amount of water
5 Affected by organic and inorganic acids
Not Affected by organic and inorganic acids
6 Easily undergoes oxidation Does not easily undergo oxidation
7 Poor life time High life time
8 Tensile strength is low (200 kg/cm2 Tensile strength is high (2000 kg/cm) 2)
9 Used between 10 – 60 0 Used between - 40 to 100 C 0C
10 E.g) Raw rubber E.g) SBR , Butyl rubber
SOME IMPORTANT SYNTHETIC RUBBER
1
.SBR – Styrene Butadiene Rubber – Buna -S
n CH2 = CH - CH = CH2 + n CH2
= CH
( 75% butadiene) (25% Styrene)
[ CH2 - CH = CH - CH2 - CH2 - CH -]
n
(Styrene – Butadiene Rubber )SBR
1. For vulcanization, it needs little amount of sulphur but more amount of accelerators. properties:
2. High tensile strength. 3. Not Soft and sticky during summer 4. Not Hard and brittle during winter 5. Not affected by organic and inorganic acids.
Tyres – Belts – Gaskets – Shoes – Tank linings Uses:
2.
CH
BUTYL RUBBER – (GR-I) rubber
│
3
x [ H2C = C (CH3)2 ] + y [CH2 = C – CH = CH2
CH
]
│
3
[ H2C – C (CH3)2 ] x [CH2 – C = CH –CH2] y
O
O
Properties:
1. Low permeability to air 2. High tensile strength. 3. Not Soft and sticky during summer 4. Not Hard and brittle during winter 5. Not affected by organic and inorganic acids.
Uses:
Inner tubes of automobile tyres - Belts – Gaskets – Shoes – Tank linings
