is a crystalline modification of high purity carbon. Graphite-filled PTFE has
one of the lowest coefficients of friction. It has excellent wear properties,
in particular against soft metals, displays high load-carrying capability in
high-speed contact applications and is chemically inert. It is often used in
combination with other fillers.
Purity: > 99%C
Particle size: < 75 um
is an alloy of copper and tin. Addition of high percentages of bronze powder to
PTFE results in a compound having high thermal conductivity and better creep
resistance than most other compounds. Bronze-filled PTFE is often used for
components in hydraulic systems, but is not suited for electrical applications
and is attacked by certain chemicals. Bronze has tendency to oxidise:
bronze-filled compounds should therefore be used fresh and containers should
always be kept closed. Some discoloration of the finished part during the
sintering cycle is normal and has no impact on its quality.
Low in phosphorus
Particle size: < 60um
Particle shape: Spherical (Bronze 60%)
disulfide adds to the hardness and stiffness of PTFE and reduces friction. It
has little effect on its electrical properties. It is quite inert chemically
and dissolves only in strongly oxidising acids. It is normally used in low
percentages and together with other fillers. Compounds containing molybdenum
disulfide need special attention during performing and sintering.
Purity: > 98%
Particle size: < 65um
or aluminium oxide is an excellent electrical insulator and is used to improve
mechanical properties of compounds used in high voltage applications. As it is
very hard, machining of the sintered part should be avoided whenever possible.
fluoride is a suitable filler for PTFE in uses where it comes in contact with
chemicals that attack glass, such as hydrofluoric acid and strong alkalis. High
purity grades of CaF2 are also used in electrical applications.
possible to pigment PTFE; using inorganic pigments that withstand the sintering
temperature of PTFE Pigments do not significantly charge the properties of
PTFE. Combinations of pigments and other fillers can be used.
recent years, polymeric fillers with sufficient heat stability to be used in
PTFE have become available. Some remarkable properties have been obtained with
polymer-filled compounds, particularly with respect to friction against soft
is mineral with a plate-like structure. During processing, the particles orient
themselves perpendicular to the pressing direction. This results in very low
shrinkage and low thermal expansion in the cross direction. Tensile properties
are poor, so that mica-filled compounds are only suitable for parts under
OF PTFE COMPOUNDS
reviewing the properties of PTFE compounds it is important to keep in mind that
the way a part is prepared effects its final properties. Perform pressures,
sintering cycles, shape and thickness of the specimen and direction of moulding
all play a part. Thus, the values that a processor measures on his finished
part may deviate from values quoted in the literature. For a number of tests, a
standard method for preparing the test sample has been described. Most tests
generally refer to ASTM methods.
virgin, unfilled PTFE and in PTFE compounds with spherical fillers, properties
are usually isotropic, i.e. the same in all directions. Irregular or
longitudinal fillers orientates themselves during compression moulding, so that
properties in the mould direction (MD) differ from these in the cross direction
(CD). In the following paragraphs both MD and CD properties are given where
table 1 (above)
AT LOW TEMPERATURES
most conventional plastics, which become brittle at cryogenic temperatures,
PTFE still has some ductility at-269°C, the boiling point of helium. As a
result PTFE can be used at extremely low temperatures, such as those in outer
space. Some properties have been summarised in table 2. Like most other
plastics, PTFE expands and contracts under the influence of heat and cold more
than metals, but the addition of fillers reduces its coefficient of thermal
AT HIGH TEMPERATURES/ THERMAL STABILITY
has excellent resistance of heat. It is capable of continuous service at 260°C
and can withstand temperatures up to 360°C for limited periods. At these
conditions, the extent of degradation still remains small. For example: at
390°C, the weight loss of PTFE is still less than 0.1 percent per hour. The
physical properties however, fall off with increasing temperature, and above
260°C mechanical properties become a limiting factor. Thus, the addition of fillers
to raise compressive strength and stiffness can be even more beneficial than at
room temperature. Fillers do not affect the heat stability of the PTFE itself.
Most fillers are stable up to 400°C. Molybdenum-disulfide-filled PTFE should be
sintered under special precautions, as the filler starts to oxidise at normal
has an extremely low vapour pressure (<10-5mbar at 1200C) and can be used
safely in vacuum. The same is true for most filled compounds. The exception is
compounds that contain graphite, as this material disintegrates under vacuum
conditions in the absence of traces of water.
coefficient of linear thermal expansion of unfilled PTFE is not constant over its
useful temperature range. A marked change in volume of 1.0 to 1.8% is evident
for PTFE resins in the transaction zone from 10 to 250°C. A part which has been
machined on either side of this zone will obviously change dimensions if
permitted to go through the zone. Thus, final operating temperature of a
precision part must be accurately determined. Measurement on a production basis
must allow for this volume change if the transition zone is traversed in either
manufacture or operation of the part. The expansion of PTFE compounds is
generally lower than that of unfilled PTFE (Tables 3 and 4). With some fillers
it is higher in the moulding direction (MD) than in the cross direction (CD).
To obtain compounds with the lowest thermal expansion, the use of mica or
carbon fibre filler is recommended.