|Datasheets show the typical physical properties of filled PTFE. These values refer to molded material and have been determined according to the listed methods.|
PTFE is reinforced with glass fibres, the percentage varying between 5 and 40%. The added glass fibre improves the wear properties and, to a minor degree, also the deformation strength under load while leaving substantially unchanged the electrical and chemical characteristics. Glass itself, has a rather poor resistance against alkalis and is easily attacked by hydrofluoric acid. The coefficient of friction is slightly increased and for this reason, graphite is sometimes added to compensate this side effect.
Carbon is added to the PTFE in a percentage by weight between 10 and 35%, along with small percentage of graphite. Also, the carbon tends to improve to a considerable degree, wear and deformation strength, while leaving practically unchanged the chemical resistance, but substantially modifying the electrical properties.
Bronze, when used as filler, is added in percentages of weight between 40 and 60%. Bronze filled PTFE has the best wear properties, remarkable deformation strengths and good thermal conductivity, but poor electrical characteristics and chemical resistance.
The percentages used vary between 5 and 15%. Graphite lowers the coefficient of friction and is, therefore, often added to other types of filled PTFE for improving this property. It improves the deformation under load, strength and, to a minor degree, the wear properties.
Molybdenum sulfide, though decreasing the coefficient of friction, is sometimes preferred to graphite. Some metal powders (stainless steel, nickel, titanium), in consideration of their particular resistance to chemical agents, is sometimes used as fillers for PTFE, even though their wear resistance, with respect to bronze, are inferior. The metal oxides, added to other fillers, give better wear properties.
The contact between two sliding
surfaces, because of the inevitable friction generated in
the contact zone, results in a certain wear whose
magnitude depends on load, speed and time of sliding
R = KPVT
where, expressed in the
measuring units of table:
The value of the factor PV after which the coefficient of wear loses its linear behavior, assuming remarkable values with the system passing from weak to strong wear condition, is known as "PV limit". This PV limit and the wear factor are, therefore, characteristic parameters of each material. In practice, however, it can be easly perceived, the wear factor and the PV limit of the same filled material can vary also with the nature, the hardness and the surface finish of the other contact "partner" with the presence, or not, of cooling and/or lubricating fluids.
The PV limit indicated in table 2, increases with sliding speed, with other factors like the geometry of the sliding system and the temperature. The values given in table refer to dry friction.
PTFE, like most other
plastic materials, has no "elastic zone" where
the ratio load/deformation (Youngs modulus) has a constant
value. This ratio load/deformation depends upon the time
of application of the load and the ensuing deformations;
this phenomenon is known as "creep", and at the
removal of the load, there is only a partial return of
the deformation to the original state ("elastic
recovery"), so that we are always in the presence of
a "permanent deformation".
|The thermal expansion of filled PTFE is in general inferior to that of unfilled PTFE and always greater in the direction of the molding than crosswise. The thermal conductivity is superior to that of unfilled PTFE, particularly when using fillers having a high thermal conductivity of their own. Filled PTFE therefore have, better thermal properties than the unfilled ones.|
|These properties depend to a large degree upon the nature of the filler. Only PTFE filled with glass fibre possess good dielectric properties, even though different from those of unfilled PTFE. For example, the volume and surface resistivity, the dielectric constant and the dissipation factor vary lagerly with the variation of the humidity and frequency.|
listed on stock shapes tables refer to virgin PTFE.
For filled PTFE shall be given on request.
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