Brochure: Carbon Additives for Polymer Compounds - Timcal Graphite

Incorporation of graphite powder into a thermoplastic
polymer will generally result in a
reduction in the friction coeffi cient (with the exception
of PTFE) but rarely improves the wear
resistance. This behaviour is illustrated in the
two graphs, which show the mean friction coeffi
cient and specifi c wear rate for a stainless
steel ball (ø = 5 mm) rubbing on discs of graphite
fi lled polystyrene and polyamide at constant
load (32.5 N) and speed (0.03 m/s). The specifi c
wear rates of the graphite-polymer composites
were calculated from the diameters of the wear
tracks and the contact geometry.
In the case of polystyrene, addition of 30–50%
of a high purity macrocrystalline synthetic
graphite (T 75), reduced both friction and wear
rate. With polyamide however, addition of a
graphite similar to T 75 reduced the friction
coeffi cient, but caused a slight increase in the
wear rate, with the fi ner particle size powder
(KS 6) giving the better result. In the case of
low density polyethylene and polypropylene,
graphite incorporation causes both an increase
in friction and wear.
The results described above are thought to be
related to the strength of adhesion at the polymer-graphite
interface, which depends upon
the wettability of the powder by the molten polymer,
powder surface area to volume ratio, surface
chemistry, etc. In simple terms, polystyrene
shows a strong affi nity for the graphite surface,
while polyolefi ns show a weak affi nity. Interfacial
adhesion increases with increasing powder
surface area to volume ratio, or decreasing particle
size.
For this reason relatively fi ne graphite powders
(95%