182 Lubricant Additives: Chemistry and Applications
Its chemical inertness makes it useful in cryogenic to moderate operating temperatures and in a
variety of atmospheres and environments. Operating temperatures are limited to ∼260°C due to
the decomposition of the polymer.
One consideration in using PTFE is the cold weld property of the material. This could eliminate
its use for some applications where extreme pressure is encountered. Such pressure may result in the
destruction of the polymer particle and in the lubrication failure, as the PTFE congeals and fails to
remain intact on the rubbing surface.
PTFE fi nds many uses in bonded fi lm lubrication at ambient temperature. These applications
include fasteners, threading compounds, and chain lubrication and engine oil treatments. PTFE is
widely used as an additive in lubricating greases and oils, for both industrial and consumer applica-
tions (see Table 6.6 for basic properties).
Although diffi cult to accomplish due to the low surface energy of PTFE, colloidal dispersions of
PTFE in oil or water can be produced. This is useful for applications requiring the stable suspension
of PTFE particles in the lubricating medium such as for crankcase oil or hydraulic oil. The nature
and feedstock of the PTFE infl uence the ability to create a stable, unfl occulated dispersion, which
is necessary for effective lubrication.
6.3 PREPARATION FOR LUBRICANT APPLICATION
For a lubricant to be effective, the solid has to be applied in a manner that provides an effective
interface between the mating substrates that require wear protection or lubrication. Dry-powder
lubrication can be used, but it is limited in its scope of application. In other words, the dry powder
can be sprinkled onto the load-bearing substrate. By a combination of the rubbing action from slid-
ing and the natural adhesion properties of the solid lubricant, some measure of attachment to the
substrate will occur by burnishing to provide lubricating protection [10]. MoS 2 seems to function
particularly well from this manner of application, as it has an effective burnishing capability.
The use of free powder has limitations. The fi lms tend to have a short duration of service since
adhesion is usually insuffi cient to provide any longevity for a continuous application. The use of
dry powder also makes it diffi cult in many circumstances to accurately apply the lubricant to the
place intended, with the possible exception of tumbling metal billets for achieving a coating over
phosphated substrates.
This can be overcome by the use of bonded fi lms. Bonded fi lms will provide a strong adhesion to
the substrate requiring protection. It also allows for a more controlled rate of fi lm wear, which depends
on the properties of the bonding agent and the fi lm thickness of the bonded fi lm. Bonded fi lms can be
achieved by a number of ways, all by use of secondary additives that promote a durable and longer-
lasting fi lm. The intended application will dictate the appropriate type of bonding agent. For applica-
tions of continual service, resin and polymer bonding agents are typically used. These include phenolic
resins, acrylics, celluloses, epoxies, polyimides, and silicones. Some of the binders such as epoxies are
TABLE 6.6
Typical Physical Properties of PTFE
Property Value
Coeffi cient of friction (ASTM D1894) 0.04–0.1
Dielectric constant 2.1–2.4
Hardness 50–60 Shore D
Melting point 327°C
Service temperature Up to 260°C
Specifi c gravity 2.15–2.20