Polytetrafluoroethylene is one of the fluoro Polymers

2.  The presence of stable C-F bonds leads to
PTFE with outstanding
› Heat resistance
› Chemical resistance
› Electrical insulation characteristics
› Low coefficient of friction

3.  Tetrafluoroethylene was first prepared in 1933
by RUFF and BRETSCHNEIDER
› By decomposition of Tetrafluoromethane by electric
arc.
 The most widely used method is based on –
Pyrolysis of Chlorodifluoromethane(CHClF2)
(Also called HCFC-22 or R-22)
 fluorspar (mineral of calcium fluoride,CaF2) ,
sulphuric acid and chloroform.

4. Step 1: HF preparation
CaF2 + H2S04  CaS04 + 2HF
Step 2: Preparation of Chloroform
CH4+3Cl2  CHCl3+3HCl
Step 3: Chlorodifluoromethane preparation
CHCl3 + 2HF  CHClF2 + 2HCl (SbF3 Catalyst)
Step 4: Pyrolysis
2CHClF2  CF2=CF2 + 2HCl

5. • Since very pure monomer is required for
polymerisation, the gas is first scrubbed to
remove any hydrochloric acid and then
distilled to separate other impurities(By-
products that are formed during the
reaction).

6. • Free Radical Polymerisation is the
technique used to produce PTFE.
• A silver-plated reactor was quarter-filled with a
solution consisting of 0.2 parts ammonium
persulphate, 1.5 parts borax and 100 parts
water, and with a pH of 9.2.
• In reactor 30 parts of monomer were let in. The
reactor was agitated for one hour at 80°C and
after cooling gave an 86% yield of polymer.
Pressure: 10-20 atm

7.  Another Methods :
› Decomposition of TFE under the influence of an electric
arc.
› Emulsion Polymerisation carried out by method using
peroxide initiators e.g. H2O2 (Hydrogen peroxide) and
ferrous sulphate. In some cases oxygen is used as
initiator.
› Dispersion polymerization can be used to obtain a
colloidal dispersion of PTFE particles (Ø0.25 micron) in
water. The dispersion can be concentrated and used for
spraying articles.
NOTE: All the above reactions are exothermic in
nature and therefore if not controlled properly can
lead to explosion.

8.  Polytetrafluoroethylene(PTFE) is a linear polymer free
from any significant amount of branching.
 The fluorine atoms is larger than those of hydrogen
 As a consequence the molecule takes up a
twisted zigzag shape, with the fluorine atoms
packing tightly in a spiral around the carbon-
carbon skeleton.
 This compact interlocking of the fluorine atoms
leads to a molecule of
 Great stiffness
 High crystalline melting point
 Thermal stability

9.  The carbon-fluorine bond is very stable.
 Further when two fluorine atoms are attached to a
single carbon atom there is a reduction in the C-F
bond distance ,as a result bond strengths is very
high and thus PTFE has a very high heat stability,
even when heated above its crystalline melting point of
327°C.
 Because of its high crystallinity and incapability of
specific interaction, there are no solvents at room
temperature.
 Its mechanical strength remains unchanged over a wide
range of temperatures(-100 to 300⁰C)
 Unlike other thermoplastics, PTFE cannot be processed
by melt extrusion.

10.  BASIC PROPERTIES OF PTFE
› Colour : Transparent-Transluscent
› Tensile strength :26-36 Mpa
› Density :2.15-2.20 g/cm3
› Water Absorption :0.005 %
› Crystallinity :92-98 %
› Glass transition Temperature :127⁰C
› Melting Temperature :326⁰C
› Working Temperature Range : -100 to +300 ⁰C
› Thermal Conductivity :0.25 W/mk
› Melt Viscosity :1010-1011 poise

12. Advantages of PTFE
› Tough but have only moderate tensile strength (26MPa)
› flexible, non-resilient
› Excellent resistance to heat, chemicals and to the passage
of an electric current.
› It remains ductile in compression at temperatures as low as
-269°C.
› Coefficient of friction is unusually low and stated to be
lower than that of any other solid
› Very high oxygen index (95%)

13. › Has good weathering resistance but is degraded by high-
energy radiation
› The polymer is not wetted by water and does not measurably
absorb it
› The permeability to gases is low
› Water vapour transmission rate being approximately half that of
low-density polyethylene
› The non-adhesive properties.
› They are Self extinguishing.

14. Limitations
 High cost.
 Low strength (26 MPa) and stiffness
 Cannot be melt processed
 Poor radiation resistance.
 The polymer has a low dielectric constant

15.  Because of the high processing temperatures there
are few pigments suitable for use with PTFE. A
number of inorganic pigments, particularly the
› cadmium compounds
› iron oxides
 The resistance of FTFE to creep can be improved by
blending in up to 25% of glass or asbestos fibre
 Alumina, silica and lithia may be incorporated to give
compounds of improved dimensional stability coupled
with good electrical insulation properties.

16.  Molybdenum disulphide and graphite improve
dimensional stability without losing the low
coefficient of friction
 The use of barium ferrite will produce a material that
can be magnetised.
 The incorporation of titanium dioxide serves to
increase the dielectric constant
 Certain compounds of boron increase the resistance to
neutron bombardment.

17.  Because of its chemical inertness over a wide temperature
range it is used in
› seals
› gaskets
› Packings
› valve and pump parts
› laboratory equipment.
 Its excellent electrical insulation properties lead to its use in
› wire insulation
› in insulating transformers
› in hermetic seals for condensers
› In laminates for printed circuitry
 Because of low coefficients of friction or non-adhesive
characteristics
› coating other metal objects (for Non-Stick equipments)
› In clothing fabrics

19.  The high melt viscosity of the PTFE is the
reason why it is not processed by
conventional processes.
 Mostly the technique of free sintering is used
to process PTFE.
› This simply involves heating the preform in an oven
at about 380°C for a time of 90 minutes plus a further
60 minutes for every 0.25 in (0.65cm) thickness. For
example a sample 0.5 in (1.25 cm) thick will require
sintering for 3.5 hours.
› After sintering, the moulding is cooled
› Thicker sections are usually cooled under pressure

20.  But for more intricate shapes
› Machining
› Coining (involves stamping a sintered moulding
of the same weight and approximate dimensions
as the finished part at 320°C)
 PTFE are also Extruded by
› Ram extruders
› Screw extruders
But the process is very slow and requires the
extruders with very high aspect ratio(≈90)

21.  Product Manufacturing:
› PTFE film may be obtained by peeling the
layers from a pressure sintered ring
› PTFE mouldings can be machined by either
Convetional or CNC machines
› PTFE Tapes- here the lubricant selected is a
non-volatile oil. The preform is placed in the
extruder and the extrudate is then passed
between a pair of calender rolls at about 60-
80°C. Sintering is carried out after fabrication.
 The current most important application of
unsintered tape is in pipe-thread sealing.

22. › PTFE Dispersions are used to coat metal
surfaces and fabrics either by
 Spray coating or
 Dip coating

23.  Brydson J.A. ”Plastics material” 7th edition
 Dr. Vijay Kumar Mathur, CIPET-Lucknow