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181
The cross-linking of the resin is, of course, not carried out until it is
in situ in the finished product. This will take place by heating the resin at
elevated temperatures with a catalyst, e.g. triethanolamine and metal
octoates. The selection of the type and amount of resin has a critical
influence on the rate of cure and on the properties of the finished resin.
Properties
The general properties of the resins are much as to be expected.
They have very good heat resistance but are mechanically much weaker
than the corresponding organic cross-linked materials. This weakness may
be ascribed to the tendency of the polymers to form ring structures with
consequent low cross-linking efficiency and also to the low intermolecular
forces.
High phenyl content resins are compatible with organic resins of
the P-F, U-F, M-F, epoxy-ester and oil-modified alkyd types but are not
compatible with non-modified alkyds. Silicone resins are highly water
repellent.
The resins are good electrical insulators, particularly at elevated
temperatures and under damp conditions.
Applications
Laminates
Methyl-phenylsilicone resins are used in the manufacture of heat-
resistant glass-cloth laminates, particularly for electrical applications. The
glass cloth is first cleaned of size either by washing with hot
trichloroethylene followed by hot detergent solution or alternatively by
heat cleaning. The cloth is then dipped into a solution of the resin in an
aromatic solvent, the solvent is evaporated and the resin is partially cured
by a short heating period so that the resin no longer remains tacky. Resin
pick-up is usually in the order of 35-45% for high-pressure laminates and
25-35% for low-pressure laminates.
The pieces of cloth are then plied up and moulded at about 170°C
for 30-60 minutes. Whilst flat sheets are moulded in a press at about 7
MPa pressure, complex shapes may be moulded by rubber bag or similar
techniques at much lower pressures 0.1 MPa if the correct choice of resin
is made. A number of curing catalysts have been used, including
triethanolamine, zinc octoate and dibutyl tin diacetate. The laminates are
then given a further prolonged curing period in order to develop the most
desirable properties.
182
The properties of the laminate are dependent on the resin and type
of glass cloth used, the method of arranging the plies, the resin content and
the curing schedule.
A number of different resins are available and the ultimate choice
will depend on the end use and proposed method of fabrication. For
example, one resin will be recommended for maximum strength and
fastest cures whilst another will have the best electrical properties. Some
may be suitable for low-pressure laminating whilst others will require a
moulding pressure of 7 MPa.
Of particular importance are the electrical properties of the
laminates. The dielectric constant is normally in the range 3.6-4.4 and
decreases with an increase in resin content. The dielectric properties are
reasonably constant over a fair range of temperature and frequency.
The mechanical properties of the laminates are somewhat poorer
than observed with phenolic and melamine laminates. Tensile and flexural
strength figures are typically about 20% less than for the corresponding PF and M-F materials and about 60% of values for epoxy laminates.
Silicone-asbestos laminates are inferior mechanically to the glassreinforced laminates and have not found wide commercial use. Interesting
laminates have, however, been introduced based on mica paper and it is
expected that their use will increase.
Silicone laminates are used principally in electrical applications
such as slot wedges in electric motors, terminal boards, printed curcuit
boards and transformer formers. There is also some application in aircraft,
including use in firewalls and ducts.
Moulding compositions
Compression moulding powders based on silicone resins have
been available on a small scale from manufacturers for a number of years.
They consist of mixtures of a heat-resistant fibrous filler (e.g. glass fibre or
asbestos) with a resin and catalyst. Non-fibrous inorganic fillers may also
be included. They may be moulded, typically, at temperatures of about
160°C for 5-20 minutes using pressures of 7-30 MPa. Post-curing is
necessary for several hours in order to develop the best properties.
Materials currently available suffer from a short shelf life of the order of 36 months but have been used in the moulding of brush rings holders,
switch parts and other electrical applications that need to withstand high
temperatures. They are extremely expensive and are of even greater
volume cost than PTFE.
183
Miscellaneous applications
Like the fluids, the silicone resins form useful release agents and
although more expensive initially are more durable. The resin is applied in
solution form and the coated surface is then dried and the resin cured by
heating for about two hours at 200-230°C. The bakery industry has found a
particular use for these materials in aiding the release of bread from baking
pans.
Resins, usually in a partially condensed form, are used to provid e a
water-repellent treatment for brickwork and masonry. Methylphenylsilicone resins are used as coatings for eletrical equipment and in the
impregnation of class H electrical equipment. Dimethylsilicone fluids are
also used as water-repellent coatings for class A or class В insulation.
The heat resistance and water resistance of the resins are attractive
properties for surface coatings but the poor scratch resistance of the
materials has limited applications of straight silicone resins.
Blends with alkyd or other organic resins have, however, been
prepared and these show heat resistance intermediate between those of the
organic resins and the silicones. Of particular interest is the use of siliconeorganic resin blends filled with aluminium powder for the coating of metal
chimneys and furnace doors. At the operating temperatures the resins are
destroyed, leaving a layer of aluminium film.
1.7.4 Fluorine-containing Polymers: Polytetrafluoroethylene
The high thermal stability of the carbon-fluorine bond has led to
considerable interest in fluorine-containing polymers as heat-resistant
plastics and rubbers. The first patents, taken out by IG Farben in 1934,
related to polychlorotrifluoroethylene (PCTFE) (Figure 48 (a)), these
materials being subsequently manufactured in Germany and the United
States. PCTFE has been of limited application and it was the discovery of
polytetrafluoroethylene (PTFE) (Figure 48 (b)) by Plunkett in 1938 which
gave an impetus to the study of fluorine-containing polymers.
The inability to process PTFE by conventional thermoplastics
techniques has nevertheless led to an extensive search for a meltprocessable polymer but with similar chemical, electrical, non-stick and
low-friction properties. This has resulted in several useful materials being
marketed, including tetrafluoroethylene-hexafluoropropylene copolymer,
poly(vinylidene fluoride) (Figure 48 (d)), and, most promisingly, the
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copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether. Other
C
C
F
F
Cl
F
C
C
F
F
F
F
C
C
H
H
H
F
C C
H
F
H
F
(a)
(b) (c)
(d)
fluorine-containing plastics include poly(vinyl fluoride) and polymers and
copolymers based on CTFE.
The fluororubbers also form an important class of speciality
elastomers and although the market is dominated by the vinylidene
fluoride-hexafluoro-propylene copolymers a wide range of materials has
been produced over the past 40 years.
Figure 48 - (a) Polychlorotrifluoroethylene (PCTFE),
(b) Polytetrafluoroethylene (PTFE), (c) Poly(vinyl fluoride),
(d) Poly(vinylidene fluoride)
World-wide capacity for fluoropolymers in the late 1990s has been
estimated at 90000 t.p.a. divided roughly equally between Western Europe,
North America and the rest of the world. This total is dominated by PTFE
although this has decreased from about 80% of the total in 1980 to about
60% in the late 1990s.
In addition to the presence of stable С–F bonds, the PTFE molecule
possesses other features which lead to materials of outstanding heat
resistance, chemical resistance and electrical insulation characteristics and
with a low coefficient of friction. It is today produced by a number of
chemical manufacturers such as Du Pont (Teflon), ICI (Fluon), Hoechst
(Hostaflon TF), Rhone-Poulenc (Soreflon), Montecatini (Algoflan), Nitto
Chemical-Japan (Tetraflon) and Daikin Kogyo-Japan (Polyflon).
Preparation of monomer
Tetrafluoroethylene was first prepared in 1933. The current
commercial syntheses are based on fluorspar, sulphuric acid and
chloroform.
The reaction of fluorspar (CaF2) and sulphuric acid yields
hydrofluoric acid:
CaF2 + H2SO4 → CaSO4 + 2HF
Treatment of chloroform, obtained by reacting methanol and
chlorine, with the hydrofluoric acid yields monochlorodifluoromethane,
also used as a refrigerant, which is a gas boiling at -40.8°C.
185
CHCl3 + 2HF → CHClF2 + 2HCl
The monochlorodifluoromethane may be converted to
tetrafluoroethylene by pyrolysis, for example by passing through a
platinum tube at 700°C.
2CHClF2 → CF2=CF2 + 2HCl
Other fluorine compounds are produced during pyrolysis, including
some highly toxic ring structures. Since very pure monomer is required for
polymerization, the gas is first scrubbed to remove any hydrochloric acid
and then distilled to separate other impurities. Tetrafluoroethylene has a
boiling point of -76.3°C. For safe storage under pressure the oxygen
content should be below 20 ppm. Traces of compounds which react
preferentially with oxygen such as 0.5% dipentene, benzaldehyde or
methylmethacrylate may be added as stabilizers.
Polymerization
Pure uninhibited tetrafluoroethylene can polymerize with violenc e,
even at temperatures initially below that of room temperature. There is
little published information concerning details of commercial
polymerization. In one patent example a silver-plated reactor was quarterfilled 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. The reactor was
closed and evacuated, and 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.
PTFE is made commercially by two major processes, one leading
to the so called “granular” polymer and the second leading to a dispersion
of polymer of much finer particle size and lower molecular weight. One
method of producing the latter involved the use of a 0.1% aqueous
disuccinic acid peroxide solution. The reactions were carried out at
temperatures up to 90°C. It is understood that the Du Pont dispersion
polymers, at least, are produced by methods based on the patent containing
the above example.
Structure and properties
Polytetrafluoroethylene is a linear polymer free from any
significant amount of branching:
186
C
F
F
F
F
C
n
Whereas the molecule of polyethylene is in the form of planar
zigzag in the crystalline zone this is sterically impossible with that of PTFE
due to the fluorine atoms being larger than those of hydrogen. As a
consequence the molecule takes up a twisted zigzag, with the fluorine
atoms packing tightly in a spiral around the carbon-carbon skeleton. A
complete turn of the spiral will involve over 26 carbon atoms below 19°C
and 30 above it, there being a transition point involving a 1% volume
change at this temperature. The compact interlocking of the fluorine atoms
leads to a molecule of great stiffness and it is this feature which leads to the
high crystalline melting point and thermal form stability of the polymer.
The intermolecular attraction between PTFE molecules is very
small. The polymer in bulk does not thus have the high rigidity and tensile
strength which is often associated with polymers with a high softening
point.
The carbon-fluorine bond is very stable. Further, where two
fluorine atoms are attached to a single carbon atom there is a reduction in
the С–F bond distance from 1.42 Å to 1.35 Å. As a result bond strengths
may be as high as 504 kJ/mole. Since the only other bond present is the
stable С–С bond, 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. At temperatures
approaching the melting point certain fluorinated liquids such as
perfluorinated kerosenes will dissolve the polymer.
The properties of PTFE are dependent on the type of polymer and
the method of processing. Th e polymer may diff er in particle si ze and/or
molecular weight. The particle size will influence ease of processing and
the quantity of voids in the finished product whilst the molecular weight
will influence crystallinity and hence many physical properties. The
processing techniques will also affect both crystallinity and void content.
The weight average molecular weights of commercial polymers
appear to be very high and are in the range 400000 to 9000000. ICI report
that their materials have a molecular weight in the range 500000 to
5000000 and a percentage crystallinity greater than 94°C as manufactured.
Fabricated parts are less crystalline. The degree of crystallinity of the
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finished product will depend on the rate of cooling from the processing
temperatures. Slow cooling will lead to high crystallinity, with fast cooling
giving the opposite effect. Low molecular weight materials will also be
more crystalline.
It is observed that the dispersion polymer, which is of finer
particle size and lower molecular weight, gives products with a vastly
improved resistance to flexing and also distinctly higher tensile strengths.
These improvements appear to arise through the formation of fibre-like
structures in the mass of polymer during processing.
General properties
PTFE is a tough, flexible, non-resilient material of moderate tensile
strength but with excellent resistance to heat, chemicals and to the passage
of an electric current. It remains ductile in compression at temperatures as
low as 4K (-269°C).
PTFE is an outstanding insulator over a wide range of temperature
and frequency. The volume resistivity exceeds 1020 Ωm and it appears that
any current measured is a polarization current rather than a conduction
current. The power factor is negligible in the temperature range -60°C to
+250°C at frequencies up to 10
10
Hz. The polymer has a low dielectric
constant similarly unaffected by frequency.
The chemical resistance of PTFE is exceptional. There are no
solvents and it is attacked at room temperature only by molten alkali
metals and in some cases by fluorine. Treatment with a solution of sodium
metal in liquid ammonia will sufficiently alter the surface of a PTFE
sample to enable it to be cemented to other materials using epoxy resin
adhesives.
Although it has good weathering resistance, PTFE is degraded by
high-energy radiation. Exposure to a dosage of 70 Mrad will halve the
tensile strength of a given sample. The polymer is not wetted by water and
does not measurably absorb it. The permeability to gases is low, the water
vapour transmission rate being approximately half that of low-density
polyethylene and poly(ethylene terephthalate).
Processing
PTFE is normally available in three forms:
(1) Granular polymers with median particle size of 300 and 600
μm.
(2) Dispersion polymer obtained by coagulation of a dispersion. It
188
consists of agglomerates with an average diameter of 450 μm made up of
primary particles 0.1 μm in diameter.
(3) Dispersions (latices) containing about 60% polymer in particles
with an average diameter of about 0.16 μm.
The exceptionally high melt viscosity above the melting point
(about 1010-1011 poises at 350°C) prevents the use of the usual techniques
for processing thermoplastics. In the case of granular polymers, methods
allied to those used with ceramics and in powder metallurgy are employed
instead. In principle this involves preforming the powder, usually at room
temperature, sintering at a temperature above the melting point, typically at
about 370°C, and then cooling.
Additives
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 and ultramarines, may,
however, be used.
The resistance of PTFE to creep can be improved by blending in up
to 25% of glass or asbestos fibre using PTFE dispersions as mentioned in
the previous section. By the same technique alumina, silica and lithia may
be incorporated to give compounds of improved dimensional stability
coupled with good electrical insulation properties. Molybdenum disulphide
and graphite improve dimensional stability without losing the low
coefficient of friction whilst the use of barium ferrite will produce a
material that can be magnetised. The incorporation of titanium dioxide
serves to increase the dielectric constant whilst certain compounds of boron
increase the resistance to neutron bombardment.
Applications
The use of PTFE in a great diversity of applications may be
ascribed to the following properties:
(1) Chemical inertness.
(2) Exceptional weathering resistance.
(3) The excellent electrical insulation characterist ic s.
(4) The excellent heat resistance.
(5) The non-adhesive properties.
(6) The very low coefficient of friction.
Because of its chemical inertness over a wide temperature range it
is used in a variety of seals, gaskets, packings, valve and pump parts and in
laboratory equipment.
189
Its excellent electrical insulation properties lead to its use in wire
insulation, in valve holders, in insulated transformers, in hermetic seals for
condensers, in laminates for printed ciruitry and for many other
miscellaneous electrical applications.
PTFE is used for lining chutes and coating other metal objects
where low coefficients of friction or non-adhesive characteristics are
required. Because of its excellent flexing resistance, inner linings made
from dispersion polymer are used in flexible steam hose. A variety of
mouldings are used in aircraft and missiles and also in other applications
where use at elevated temperatures is required.
Because of its high volume cost PTFE is not generally used to
produce large objects. In many cases, however, it is possible to coat a
metal object with a layer of PTFE and hence meet the particular
requirement.
One significant development in recent years has been the
widespread treatment of clothing fabrics to give a measure of water and
stain resistance.
1.7.5 Tetrafluoroethylene-Hexafluoropropylene Copolymers
These materials were first introduced by Du Pont in 1956 and are
now known as Teflon FEP resins. (FEP = fluorinated ethylene-propylene.)
Subsequently other commercial grades have become available (Neoflon by
Daikin Kogyo and Teflex by Niitechim, Russia). These copolymers may be
regarded as the first commercial attempt to provide a material with the
general properties of PTFE and the melt processability of the more
conventional thermoplastics.
The commercial polymers are mechanically similar to PTFE but
with a somewhat greater impact strength. They also have the same
excellent electrical insulation properties and chemical inertness.
Weathering tests in Florida showed no change in properties after four
years. The material also shows exceptional non-adhesiveness. The
coefficient of friction of the resin is low but somewhat higher than that of
PTFE.
The maximum service temperature is about 60°C lower than that of
PTFE for use under equivalent conditions. Continuous service at 200°C is
possible for a number of applications. The polymer melts at about 290°C.
Injection moulding and extrusion may be carried out at
temperatures in the range of 300-380°C. The polymer has a high melt
190
viscosity and melt fracture occurs at a lower shear rate (about 102 s-1) than
with low-density polyethylene (about 103 s-1) or nylon 66 (about 105 s-1).
Extruders should thus be designed to operate at low shear rates whilst large
runners and gates are employed in injection moulds.
The advantage of being able to injection mould and extrude these
copolymers has perhaps had a less marked effect than might have been
expected. This is because the fabrication of PTFE has been developed by
firms closely related to the engineering industries rather than by the
conventional plastics fabricators. The PTFE fabricators, because they do
not normally possess conventional injection moulding and extrusion
machines, would see no obvious advantage in melt processability. At the
same time the conventional plastics fabricators, if they wished to enter the
field of fabricated fluorine-containing thermoplastics, would have to
modify their existing machinery in order to cope with the processing
temperatures and high melt viscosity. In spite of these retarding influences
the use of FEP copolymers has grown steadily.
At the present time they are used for a variety of electrical and
chemically resistant mouldings, for corrosion-resistant linings, for
coatings, for flexible printed circuits and for wire insulation. One particular
growth area arising from the inherent flame retardancy has been for wire
and cable insulation, particularly for data networks and for optical fibre
insulation.
In the mid-1980s Hoechst introduced a related material, Hostaflon
TFB, a terpolymer of tetrafluoroethylene, hexafluoropropene and
vinylidene fluoride.
1.7.6 Tetrafluoroethylene-Ethylen e Copol ym ers (ET FE)
A second melt-processable copolymer containing
tetrafluoroethylene residues was introduced by Du Pont in 1972 as Tefzel.
This material is similar in many properties to the tetrafluoroethylenehexafluoropropylene copolymers but claimed to have exceptional abrasion
resistance for a fluorine-containing plastics material. It also has very high
impact strength. Unlike PTFE it cross-links during irradiation. It also
differs from PTFE in that glass fibre actually reinforces the polymer,
giving tensile strengths as high as 85 MPa.
This copolymer has proved particularly suitable for wire and cable
insulation, with many grades being rated at 155°C for 20000 h continuous
exposure. It is extensively used in electrical systems for aircraft,
underground railways, computers, telecommunications installations and