Metal Soap Greases - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 57

A good monolayer lubricant is not only characterized by a low shear strength for
the bonds it forms with an asperity but also that the shear strength should not sig-
ni¿ cantly increase (ideally should decrease) under a compressive load. A solid ¿ lm
lubricant should have high compressive strength and it should form an interface of low
shear strength at an asperity contact [7].
This review has intended to summarize the studies on metal soap grease for easy
understanding. Metal polymer and metal-metal contact lubrication in use are discussed,
and classi¿ cation, properties, colloidal, and morphological behaviors of metal soaps
were introduced. The composition researches of grease regarding base oil additive and
metal soaps were reviewed from literature with respect to tribological, rheological,
and morphological point of view with some examples. Finally CaF 2 and antimony
dithiocarbamide additives were reviewed as additives for the metal soap grease.
6.2 LUBRICATION OF POLYMER-METAL CONTACTS
6.2.1 Polyolefins
One of the underlying rationales for incorporating comonomers into the backbone of
polyethylene is to modify the long and short chain branching distribution and alter the
polymer morphology. Copolymerization can result in softer resins with better optical
properties, improved tensile and impact strengths, superior low temperature charac-
teristics, and enhanced heat sealability. Predictably, it also increases inherent surface
friction and tack, making these materials more difficult to process and convert [8]. Lu-
bricants can tie up catalyst residues, usually calcium stearate, stearate, and ethylene-
bis-stearamide waxes are sometimes used in processing of fine powdered polefines,
erucamides preferred for films and mouldings, floropolymer 'alloy' give better use of
polymer [9].
6.2.2 Polyvinyl Chloride (PVC)
The PVC Lubricants are classified as “internal” and “external” according to their
chemical structure. They are internal if they reduce melt viscosity, are soluble in
PVC, provide little metal release and/or have little effect on fusion or promote fusion.
They are classified if they provide metal release and are not soluble in PVC [10]. The
stearates are used broadly as viscosity stabilizers particularly where low temperature
profiles are required. Metallic stearates are inexpensive and able to modify melt vis-
cosity without delaying fusion and are among the widely used internal lubricants in
polymers. Among the disadvantages are that they are supplied as fine powders and
so can cause a dust hazard and nuisance in processing. They also tend to exude from
the polymer compound, leaving a surface residue and are not generally suitable for
mouldings that will require secondary finishing such as printing, painting, bonding,
or electroplating [9].
Practical experience shows that external lubrication using oil and grease is more
effective than internal lubrication in reducing friction and wear of plastics. It was
found that they act as surfactants and slip layers but do not ¿ t the “internal and ex-
ternal” model by synergy of lubricants with fatty acid salts [11]. Later it was shown
that À ow properties are mainly affected by the concentration of paraf¿ n wax, while
the characteristics of fusion may be greatly inÀ uenced by calcium stearate [12]. Slip