CCS355 Neural Network & Deep Learning Unit II Notes with Question bank .pdf
Polymers
1. Linear, Branched, and Cross-linked Polymers
Linear structure – chain-like structure
Characteristic of thermoplastic polymers
Branched structure – chain-like but with side branches
Also found in thermoplastic polymers
Cross-linked structure
Loosely cross-linked, as in an elastomer
Tightly cross-linked, as in a thermoset
6. Effect of Branching on Properties
Thermoplastic polymers-
Always possess linear or branched structures, or a mixture
of the two
Branches increase disorder among the molecules,
which makes the polymer:
Stronger in the solid state
More viscous at a given temperature in the plastic or
liquid state
7. Effect of Cross-Linking on Properties
Thermosets possess a high degree of cross-linking,
while elastomers possess a low degree of cross-linking
Thermosets are hard and brittle, while elastomers are
elastic and resilient
Cross-linking causes the polymer to become chemically
set
The reaction cannot be reversed
The polymer structure is permanently changed; if heated, it
degrades or burns rather than melt
8. Mechanical Properties of Thermoplastics
Low modulus of elasticity (stiffness)-lower than metals
and ceramics
Low tensile strength, hardness than metals or ceramics
Greater ductility on average
9. Important thermoplastic resins
Commercially imp thermoplastic resins include natural
resins and resins derived from biopolymers and
sunthetic resins
Natural Resins- resins such as copal, amber, rosin,
kauri, dammar and mastic, used in varnishes.
When the plants and trees are wounded they exude
resins known as balsams as a protective measure.
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10. Natural resin polymers are hard with low thermal
conductivity and low dielectric constant
They find use in electrical insulation and binders
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11. Cellulose derivatives
Naturally occurring cellulose- is a polymer of
glucose, a polyhydroxy molecule represented by
general formula (C6H10O5)n
Most widely used cellulose derivatives
Cellulose esters- cellulose nitrate and cellulose acetate
Cellulose ethers -methyl cellulose and ethyl cellulose
-
These derivatives have film forming capability but the
films are not adherent. They also tend to wrinkle
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12. POLY VINYL CHLORIDE (PVC)
When vinyl chloride is heated in an autoclave under
press in the +nce of initiator like H2O2 to yield PVC
nCH2=CH → -(-CH2-CH-)n-
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ClCl Cl
C C C C C C
HHH
HHHHHH
Polyvinyl chloride (PVC)
Cl Cl
13. PVC is colorless and odorless powder,
chemically inert, non-inflamable, exhibit high
resistance to light, atmospheric oxygen, acids
and alkalis
It is soluble in chlorinated HC (ethyl chloride),
tetrahydrofuran (THF) and cyclohexanone
Commercial PVC known under different trade
name tygon, vinylite, velon, geon etc is a hard
and stiff amorphous plastic attributed to its
strong intermolecular forces but brittle
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Pure Polyvinyl Chloride powder
POLY VINYL CHLORIDE (PVC)
14. Manufacture of pipes, tubes, tank linings, safety
helmets, refrigerator components, light fittings, trays,
cycle and motor cycle mudguards
In the manufacture of flexible films or sheets of
varying thickness (0.1-8mm) required for making
table clothes, curtains, raincoats
Also used for wire coating and insulation of eletric
cables and manufacture of upholstery
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POLY VINYL CHLORIDE (PVC)
15. Polytetrafluoroethylene (PTFE)
PTFE is a synthetic fluoropolymer of
tetrafluoroethylene that finds numerous applications
The most well known brand name of PTFE is Teflon
PTFE is hydrophobic: neither water nor water-
containing substances wet PTFE, as fluorocarbons
demonstrate moderate dispersion forces due to the
high electronegativity of fluorine.
Has the lowest coefficients of friction against any solid
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18. PTFE
PTFE is used as a non-stick coating for pans and other
cookware.
It is very non-reactive because of the strength of C–F
bonds, and so it is often used in containers and pipework
for reactive and corrosive chemicals.
It is used as lubricant- reduces friction, wear, and energy
consumption of machinery.
Has high resistance to wear and deformation under load
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19. PTFE
Already has high strength
compressive strength of PTFE products can also be further
enhanced by addition of fillers such as asbestos, glass fibers,
graphite etc
This polymer has exceptionally high softening point
(M.P. 327 °C)
A good electrical insultor used in wires, cables, motors
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21. PC are thermoplastic polysters having functional groups linked
together by carbonate groups (-O-(C=O)-O) in a long molecular
chain and commercially known as MERLON, LEXAN.
Most common type of polycarbonate plastics are obtained by
diphenyl carbonate with bis-phenol A (2,2-bis-(4-hydroxyphenyl)-
propane) to give bis phenol A groups linked by carbonate groups
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POLYCARBONATE
22. PC have high tensile and impact strength over a wide
range of temperatures
Polymers are highly transparent to visible light and
have better light transmission characteristics than many
types of glasses
They are soluble in organic solvents and alkalis
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POLYCARBONATE
24. Synthesis of Polycarbonate
Alternative Route24
From BPA and diphenyl carbonate:
(HOC6H4)2CMe2 + (C6H5O)2CO → -[OC(OC6H4)2CMe2]-n
Diphenyl carbonate Polycarbonate
+
Bisphenol A (BPA)
+ 2 C6H5OH
25. APPLICATIONS
PC are used for making MOULDED domestic wares,
housing for apparatus and electrical insulation
Used in manufacture of sunglasses/eyeglass lenses
CR-39 is a specific polycarbonate plastic with good optical and
mechanical properties
CDs, DVDs, nalgene bottles and food storage
containers
They can be laminated to make bullet proof glass
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26. PHENOLIC RESINS (PHENOPLAST)
PHENOLIC RESINS are derived from the condensation reaction
between phenol or phenolic derivatives (e.g. resorcinol) and
aldehydes (formaldehyde and furfural)
Commercially imp. As well as the erliest phenolic resins known as
bakelite is obtained by condensation pol. of phenol with
formaldehyde
Linear poly. As well as highly crosslinked 3-D network structure can
be obtained by varying the proportion of phenol and
formaldehyde as well as the nature of catalyst
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27. If the phenol to formaldehyde (P/F) ratio is greater
than 1, a linear polymer is obtained with an acid
catalyst
On the other hand with P/F ratio less than 1 and with a
basic catalyst a 3D network polymer is obtained
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28. Phenol reacts with formaldehyde initially to form methylol
derivatives in the o- and p- positions which undergo polymerization
in the presence of acid catalyst to yield methylene bridged linear
polymeric resin called NOVOLAC resin with m.w. in the range of
about 1000 corresponding to about 10 phenyl residues
NOVOLAC resin can undergo further polymerization to yield 3D
polymers only when access formaldehyde is added and a basic
catalyst is used
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29. Resole resin is obtained by condensing phenol with access of
formaldehyde (P/F < 1) in the presence of basic catalyst
The reaction is exothermic and hence require cooling
Viscosity of the mix increases indicating the formation of
polymer
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30. Condensation product water is removed by suction to yield a
thermoplastic A- stage resin soluble in organic solvents
A- stage resin is powdered and necessary fillers, colorants,
lubricants are added then additional amount of formaldehyde
required is added in the form of hexamethylenetetramine which
decomposes to form HCHO and NH3 during the final step
NH3 acts as a curing catalyst
Mixture is rolled in heated mixing equipments to facilitate the
continuation of the polymerization reaction to yield B-stage resin
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31. Resin is cooled and cut into required form
B-stage resin is nearly insoluble in organic sovents but
can be fused with the application of heat and pressure
B-stage resin is moulded into the final desired finished
product and during moulding the highly crosslinked C-
stage polymer (bakelite is formed)
Finished product is removed from the mould and cant
be reshaped or reclaimed in any manner as the
polymer has been permanently cured by heat settings
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32. Properties and applications
Phenolic Resins have good adhesive and bonding
properties
After heat setting exhibit high resistance to heat,
flammability, abrasion, water, chemicals and
solvents
However they are susceptible to attack by alkalis
They are hard and infusible with good dielectric
properties
Fillers are used during molding of these resins to
reduce cost and also to impart specific properties
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33. Common fillers- wood powder, asbestos, cotton rags
etc
Phenolic resins find use in the manufacture of
electric insulation parts such as switches, plugs, switch
boards, handles for electrical appliances
Molded parts for automobiles, PCBs and consumer
electronics and bearing in propeller shafts for rolling
mills and paper mills
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