Transcript THERMOPLASTICS & THERMOSETTING PLASTICS

Course: B.E (EEE) – Part Time Subject: APPLIED SCIENCE
Unit: V
PLASTICS
Dr. K. SIVAKUMAR
Dept. of Chemistry, SCSVMV University
Syllabus: Plastics
Thermoplastics
Thermosetting plastics
Processing of plastics
Injection moulding
Blow moulding
Extrusion processes
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THERMOPLASTICS
&
THERMOSETTING PLASTICS
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Thermoplastics (resins): Thermoplastics soften when
heated and subsequently melt. When cooled they
become hard and rigid once again.
Example: Polyethylene, PVC, Nylon
Uses: Bags, Mugs, Toys
Thermosetting plastics (resins): Thermosetting resins
do not become softy on heating & they never melt
once set.
Example: Bakelite
Uses: Electric switches, Telephone parts, Cooker
handles
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Moulding constituents of plastic:
Higher polymer material is mixed with 4 to 10
ingredients each of which imparts some useful
property to the moulded products.
Some are,
Resins: Holds the different constituents together
Example: Linear polymers with low molecular
weights.
Resins form 40-50% of moulding mixture
Plasticisers: To increase the plasticity and flexibility
Example: Castor oil, Dibutylphthalate, Alkyl & Aryl
Phosphate etc.,
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Fillers: To impart valuable service properties such as
strength, resistance, finish and workability.
Reducing cost, shrinkage on setting and brittleness.
Example: Wood flour, asbestos, china clay, talc,
gypsum, saw dust.
Fillers form 60% of plastics
Lubricants: To facilitate the moulding operations.
Gives glossy finish
Example: Waxes, soaps, Oleates, stearates
Catalyst/accelerators:
To
accelerate
the
polymerisation (added only in thermosetting resins)
Example: Hydrogen peroxide, Benzoyl peroxide,
Metals (Ag, Cu & Pb)
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Difference between Thermoplastics & Thermosetting plastics
S.No.
THERMOPLASTICS
THERMOSETTING PLASTICS
1.
Formed by addition Polymn.
Formed by condensation Polymn.
2.
Long chain linear polymers
Three dimensional network
structure joined by strong covalent
bonds
3.
Soften on heating and stiffen on
cooling
Do not soften on heating
4.
Can be remoulded
Cannot be remoulded
5.
Usually soft, weak & less brittle
Usually hard, strong & more brittle
6.
Can be reclaimed from wastes
Cannot be reclaimed from wastes
7.
Soluble in organic solvents
Insoluble in organic solvents
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INJECTION MOULDING (for Thermoplastics):
•Materials of definite quantity at Hi Temp. is injected into a cold mould under P
•In cold mould, the polymer solidifies
•Polymer is fed as granules, pellets / powder through hopper
•Polymer get softened in horizontal cylinder
•Hydraulic piston pushes the molten material into mould
•Uniform spreading inside the hot cylinder is ensured by torpedo
•Molten plastic is injected through a nozzle into moulded cavity
Mould:
•Consists of movable & stationary parts
•Molten plastic is injected at a P of
15000kg/cm2
•Proper flow of material is ensured by
preheating the mould to appropriate T
•Mould is cooled by circulating water
and then opened to eject the moulded
article
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BLOW MOULDING (for thermoplastics); [for hollow plastic articles]
•To produce soft drink bottles, containers using Polyethylene, PVC, Polystyrene
•PARISON – hot softened thermoplastic made by injection / extrusion moulding
•Parison is placed inside a two piece hollow mould (as 2 halves)
•Parison kept between two halves encloses a blowing pin at one end
•Other end pinches and closes
•Parison is blown using pressurized air through the blow pin
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BLOW MOULDING: Continued……
•Parison kept under hot condition is inflated and expands until it comes in
contact with inner area of the cold hollow mould
•Under pressure
•Parison acquires the shape of hallow cavity of mould
•Mould is cooled and the thermoplastic article is removed from the mould by
opening the mould
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EXTRUSION MOULDING:
•For producing common plastic materials
•eg: films, filaments, hoses, sheets & coating wires and cables with PVC
•Plastic material as powder / granules is fed into a cylinder thru hopper
•Cylinder is with the provision for heating
•Hot plastic is pushed thru cylinder by helically revolving screw
•Due to friction the Temperature increases
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EXTRUSION MOULDING: (Continues….)
• Plastic material passes through three zones
1. Feed zone
2. Compression zone
3. Metering zone
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EXTRUSION MOULDING: (Continues….)
• In compression zone, powder charge is melted by heating element
• It is compressed by revolving screw
• Polymer flows at a constant rate due to helical movement of screw
•
Pressure in this zone enables plastic to be in melted stage & enters the die
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EXTRUSION MOULDING: (Continues….)
• From die moulded plastic is coming out in desired profile
• Extrudated material from die is hot and hence it is cooled rapidly to
avoid any deshaping
• Cooling by passing extrudate over a conveyor belt through which
tubes of cold water/cold air are passed
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Good Luck!
Dr. K. SIVAKUMAR
Department of Chemistry
SCSVMV University
[email protected]
+91 98423 61378
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