Brochure: Carbon Additives for Polymer Compounds - Timcal Graphite

TYpICAL AppLICATIonS FoR TIMREX® GRApHITE And CokE
18
Self lubricating
polymers
The choice of a polymer-based self lubricating
solid for a particular application depends mainly
upon the operating conditions of: temperature,
chemical environment and the maximum values
of pressure (p) and sliding speed (v). For each
polymer or composite material, a pv limit is quoted,
which corresponds to the pressure times the
sliding speed at which the material fails, either
due to unacceptable deformation, or to the high
frictional energy dissipated causes surface melting,
softening and excessive wear.
The pv limit of a polymeric material may be increased
by increasing its mechanical strength
(resistance to deformation), thermal conductivity
(reduction in surface temperatures)
and by decreasing friction (reduces frictional
heating). In practice, thermoplastics (with the
exception of PTFE) are mainly used as pure
solids, since their wear resistance and frictional
coefficient, are satisfactory for most applications.
Solid lubricant fillers or fibre reinforcement
(glass fibres, carbon fibres, textiles) are
only employed under the more extreme conditions
of load and speed.
The major polymers employed as self lubricating
solids/composites, are illustrated below.
Graphite powder is widely used in polymer
composites, either alone or in combination with
reinforcing fibres, PTFE or various inorganic
fillers, e.g. mica, talc (bottom, right table). Applications
include gears, dry sliding bearings,
seals, automotive and micro-mechanical parts.
The properties of graphite which favour its use
in polymer composites are:
• low friction lamellar solid
(reduces friction);
• tendency to form a transfer film on the
countersurface
(assists in wear reduction, particularly when
graphite is applied as water based dispersion
i.e. LB 1300);
• high thermal conductivity
(decreases temperature rise due to frictional
heating);
• electrical conductivity
(prevent build-up of static charge which may
be a problem in some cases);
• chemically inert
(used in conjunction with PTFE in corrosive
environments);
• high thermal stability
(favours use in high temperature applications,
e.g. polyimide graphite composites
may be used up to 350°C).