|Title of Invention||
SELF-LUBRICATING PLASTICS MATERIAL FOR SEALING ELEMENTS
|Abstract||The present invention relates to a plastics material with self lubricating action which is particularly suitable for the production of sealing elements for reciprocating compressors, said material comprising a wear-resistant polymer matrix, preferably made from polyketone, within which are dispersed microcapsules containing a lubricating fluid. The microcap sules incorporated into the polymer matrix break up as a result of friction with the contact surface of the sliding partner, resulting in the escape of the lubricating fluid and a consequent reduction in friction.|
|Full Text||SELF-LUBRICATING PLASTICS MATERIAL FOR SEALING
The present invention relates to a self-lubricating
plastics material for sealing elements.
In particular, the present invention relates to a
plastics material with self-lubricating action which is
in particular suitable for the production of sealing
elements for reciprocating compressors.
As is known, reciprocating compressors are equipped
with a piston which is moved axially within a cylinder
in order to compress a gas. Generally, the piston of
reciprocating compressors is equipped with annular
sealing elements arranged coaxially relative to the
axis of the piston and the cylinder and accommodated in
a seat formed in the side wall of the piston itself.
It is furthermore known that the piston sealing
elements or rings are subject to wear as they slide
along the cylindrical cavity.
With the aim of limiting this wear, efforts are made to
minimise friction between the sliding surfaces by
making use of lubricants in liquid or powder form.
Despite the sliding occurs in the presence of
lubricants, wear impairs the integrity of the sealing
element over time, such that, after an initial period
of service, the compressor is no longer capable of
achieving the highest pressures.
This disadvantage is a particularly serious problem for
compressors with high operating pressures in which the
sealing ring is required to maintain, when in motion,
an elevated pressure differential.
In order to minimise wear between components which are
in contact during sliding, use has been made of new
wear-resistant materials in the manufacture of sealing
Among non-metallic materials, one typical material
currently used for producing sealing elements is
tetrafluoroethylene or plastics blends which contain
Said resin, which is abbreviated as PTFE, is widely
used due to its low modulus of elasticity, ease of
handling, sealing properties and low coefficient of
It has, however, been found that sealing elements made
from PTFE resin, especially sealing rings for
reciprocating pistons, have a tendency to undergo
permanent deformation if subjected to stress for
extended periods. In particular, under operating
conditions of elevated pressure and temperature, PTFE
sealing rings not only degrade prematurely, but are
also subject to permanent deformation along the
dividing line to an extent such as to impair their
Moreover, since the temperature of the sliding surfaces
in inadequately lubricated reciprocating compressors
may rise to values in excess of 100°C, said PTFE
sealing elements are often subject to extrusion when
subjected to elevated pressure loads. Especially when
the coupling members which, during sliding, must come
into contact with the sealing elements are made from
steel with low thermal conductivity, the temperature of
the sliding surfaces tends to rise excessively due to
the build-up of heat in the steel, even if the
lubricant transmits heat adequately to the coupling
When coupling members are made from aluminium alloy
equipped with an anticorrosion lining, heat is not
transmitted due to the reduced surface roughness. Under
these conditions, structural damage may occur, not only
to the cylinder lining but even to the aluminium
Under certain service conditions/ using PTFE in the
production of sealing elements may thus prove somewhat
unsatisfactory since it requires reinforcement with
other fibres, additives and fillers, the addition of
which is, however, also not without disadvantages.
At present, in addition to PTFE, other non-metal-based
materials with a low coefficient of friction, such as
PEEK and PBS, are thus used to produce sealing
In particular, PEEK is a highly wear-resistant
material, even in applications with elevated operating
loads and elevated pressures.
However, it has been found that using PEEK as the sole
constituent of the sealing elements in reciprocating
compressors may result in excessive wear of the
cylinder liner sleeves.
This disadvantage has been partially overcome by means
of the addition of appropriate lubricating fillers
between the sliding surfaces.
PBS is currently used as an alternative in the
production of sealing elements since said polymer is
relatively resistant to elevated temperatures and is
capable of forming sulfides with a lubricating action.
These properties make it particularly suitable for dry
PBS does, however, exhibit the disadvantage of not
having elevated thermal conductivity, which restricts
the use thereof under operating conditions with
In order to overcome this disadvantage and to increase
mechanical strength, PBS is used in combination with
additives and fillers which allow the dissipation of
It has in fact been found that, in reciprocating
compressors, polymer-based' sealing elements essentially
operate due to the transfer layer present on the metal
surface of the cylinder liner sleeve. This phenomenon
results in low friction polymer-polymer sliding. When
said transfer layer is absent, high friction metal-
polymer contact occurs, resulting in premature wear of
the sealing ring.
Thus, in the absence of force-feed lubrication, the
performance of conventionally used sealing elements is
substantially determined by the presence or otherwise
of said transfer layer.
It is thus obvious that it would be worthwhile to be
able to make use of sealing elements made from
materials with a low wear rate which are capable of
continuously forming a transfer film which reduces the
friction force between the sliding parts.
One of the general objects of the present invention is
to overcome or to reduce substantially the
disadvantages of the prior art which result in
premature wear of sealing elements.
Another object of the present invention is to provide a
non-metal-based material for the production of sealing
elements in reciprocating compressors which has a low
wear rate and does not require force-feed lubrication.
A further object of the invention is to provide a self-
lubricating plastics material for sealing elements in
reciprocating compressors, which material has good wear
resistance properties, even under conditions with
elevated pressure loads and elevated temperatures. .
Another object is to provide a plastics material for
the production of sealing elements which are
substantially not subject to permanent deformation
under sliding conditions without lubricant.
In the light of said and other objects which will
emerge more clearly below, a first aspect of the
present invention provides a self-lubricating material
which is particularly suitable for sealing elements and
comprises a wear-resistant polymer matrix in which are
dispersed microcapsules containing a lubricating agent.
Advantageously, said wear-resistant polymer matrix
comprises one or more thermoplastic and/or
thermosetting resins exhibiting a low wear rate even
under elevated pressure and temperature conditions.
According to a preferred embodiment of the invention,
said polymer matrix comprises one or more polyketones,
advantageously aromatic polyketones, it being
preferred, among said polyketones, to use
polyetherether ketone (PEEK).
According to another embodiment, base components of
said polymer matrix which may be used are thermoplastic
or thermosetting resins having elevated wear resistance
under operating conditions with elevated loads, such
as, for example, polytetrafluoroethylene (PTFE) and
polybutadiene-styrene (PBS), individually, together
with one another or blended with other polymers.
The polymer matrix of the material of the invention may
also contain further substances capable of imparting
greater resistance to frictional wear.
For example, the polymer matrix may comprise additives
and/or fillers which assume the function of increasing
the thermal conductivity of the material of the
invention in order efficiently to dissipate the heat
generated by any friction between the sliding parts.
The material of the invention may advantageously also
contain fibres with elevated mechanical strength and
elevated resistance to deformation and wear.
According to one embodiment, said polymer matrix
furthermore provides the incorporation of a hard phase
and/or a transfer film in order to reduce friction
between the sliding partners.
The essential feature of the material of the invention
is the presence of lubricating microcapsules dispersed
within said polymer matrix1.
For the purposes of the present invention, lubricating
microparticles are intended to mean encapsulated
lubricating particles and multiparticles, homogeneous
fluids or encapsulated lubricating multilayer materials
and in general lubricating agents incorporated in
Suitable lubricating agents are lubricating oils, such
as for example organic, natural or synthetic oils.
Particularly suitable oils are lubricating oils which
are low in acidity and resistant to elevated operating
According to a preferred embodiment, the lubricating
oil incorporated into said microcapsules exhibits
viscosity values within the range between 20 and 250
cSt, measured at a temperature of approx. 40°C.
The microcapsules used for the purposes of the present
invention may be spherical, symmetrical or irregularly
According to one embodiment, the capsules have an
average diameter within the range between 5 and 500
Advantageously, said microcapsules comprise a shell of
wax or of a polymer material, preferably
polyoxymethylene urea, which is abbreviated as PMU.
Apart from the lubricating agent, the microcapsules may
contain selected additives depending upon the intended
application. In particular for use under elevated
pressure conditions, microelements such as zinc, boron
and mixtures thereof may be incorporated.
Advantageously, the lubricating microcapsules are
uniformly dispersed within the polymer matrix in such a
manner as to achieve contents by weight of between 2
and 30 wt.%.
The capsules containing the lubricating fluid may be
produced using various microencapsulation technologies,
such as dry spraying, prilling, coacervation, with soft
alginate beads and in situ polymerisation.
The various lubricant encapsulation technologies are
used depending upon the required dimensions of the
lubricating particles and upon the. ultimate use of the
plastics material of the invention.
Using the dry spraying process, for example, it is
possible to encapsulate the oils in capsules of
dimensions as small as 5-30 microns.
In the prilling process, which is usually used to
produce capsules of dimensions between 1 and 100
microns, the lubricant to be encapsulated is first of
all introduced into a molten wax or other polymer
matrix, then sprayed into droplets and cooled to
solidify them. The resultant microcapsules act as a
shell for the lubricant contents. Microcapsules
produced by prilling release the lubricant under
pressure or, if desired, by selecting polymers with an
appropriate melting point, after exposure to a
Using the coacervation method, the lubricant may also
be enclosed in capsules of a diameter within the range
between 25 and approx. 300 microns.
In simple coacervation, the walls of the capsules are
typically made from gelatine, . polyvinyl alcohol,
methylcellulose, polyvinylpyrrolidone and other
In complex coacervation, the capsule walls are produced
using systems based on gelatine-acacia copolymers.
Among the various technologies which are available, in
situ polymerisation is preferred for the production of
the microcapsules because it makes it possible to
produce a strong polymer shell, preferably of urea-
formaldehyde copolymer (PMU), around the drop of
lubricating liquid. Encapsulation in a PMU shell is
typically an emulsion process, in which an emulsion of
the material to be encapsulated is prepared in an
By way of example, microcapsules containing lubricant
produced using the method described in US patent
5,112,541 may be used for the purposes of the present
Once produced, the microcapsules are incorporated into
the polymer matrix, preferably by moulding, for example
by means or compression or injection moulding.
Temperatures within the range between 260 and 350°C are
conveniently used during moulding.
Compression moulding is advantageously performed within
a closed mould in order to permit uniform heating and
pressurisation of the composite material.
According to one embodiment, the mould is pressurised
when cold for example to 1.5-2.5 t in order to expel
the air from the mould. The mould is placed in a
preheated press. The temperature of the press
conveniently depends upon the melting point of the
polymer material used. Approx. 80-90% of the selected
pressure temperature can be achieved before application
of the load. The load is thus generally applied to
values of between 250 and 1500 kg with time and
pressure increments for a total period of approx. 10-15
minutes. The final load is maintained while the mould
is allowed to cool to ambient temperature.
According to another embodiment, the injection moulding
method is used, with low processing temperatures or
short heating and cooling cycles.
It has been found that using the self-lubricating
material of the invention as the base constituent for
sealing elements minimises the transfer layer required
for self-lubrication by providing, due to the
microcapsules, a replenishable source of lubricant.
Furthermore, its use surprisingly reduces the wear rate
of the sealing element, so minimising the risk of
surface wear of the sliding partner and increasing the
service life of the compressor.
According to another aspect, the present invention
provides a sealing element or. packing comprising the
self-lubricating material described above.
Advantageously, said sealing element is a sealing ring
for a reciprocating compressor piston.
According to another aspect, the present invention
provides the use of a self-lubricating material of the
type described above according to the attached claims
According to a further aspect, the present invention
provides a method for reducing the friction or wear of
elements which are adjacent to or in contact with one
another when in motion, in which method at least one
portion of the adjacent surfaces of said elements
comprises a self-lubricating material of the type
According to a preferred embodiment, the invention
provides the use of said self-lubricating material as a
sealing element for a reciprocating compressor in order
to reduce wear and/or appreciably to improve service
life, in particular for dry sliding applications.
Further features and advantages associated with the
self-lubricating plastics material according to the
present invention will emerge more clearly from the
following description, which is provided merely by way
of non-limiting example, with reference to the attached
diagrams, in which:
Figure 1 is a schematic diagram of the mode of
operation of a conventional sealing element for a
piston in a reciprocating compressor;
Figure 2 is a schematic diagram of a side-section of a
sealing element of the invention and the sliding
Figure 3 shows bar charts comparing the coefficients of
friction and wear for a known wear-resistant material
and a material according to one embodiment of the
Figure 4 shows bar charts comparing coefficients of
wear for a known PEEK-based wear-resistant material and
a PEEK-based material incorporating the self-
lubricating microcapsules according to one embodiment
of the invention.
With reference to Figure 1, a conventional sealing ring
1 made from PEEK resin is shown accommodated in a
lateral seat 3 of a piston 2 in a reciprocating
compressor (not shown). The piston 2 moves with a
reciprocating motion, sliding along an internal sleeve
4 of a cylinder (not shown). The sliding motion between
the two sliding partners takes place due to the sliding
layer 5 deposited on the metal surface of the sleeve 4.
This layer or film allows a reciprocating motion with a
low coefficient of friction. Absence of the transfer
layer 5 results in higher friction due to metal-resin
With reference to Figure 2, reference numeral 10
indicates a cross-section of an embodiment of a sealing
element made from self-lubricating material of the
The sealing element 10 comprises a polymer matrix 11 of
PEEK, in which are uniformly dispersed microcapsules 12
filled with lubricating oil. Figure 2 furthermore
schematically illustrates the sliding partner 13 of the
sealing element 10, which, under the pressure load
indicated with reference numeral 14, slides along the
sliding surface in the direction indicated by the arrow
identified with the reference numeral 15. The
microcapsules 12 filled with fluid lubricant are
transformed into ruptured microcapsules 16 by the shear
force. The release of fluid from the ruptured
microcapsules 16, which may also be effected thermally,
lubricates the sliding surfaces, reducing the
coefficient of friction and wear.
Figure 3 shows four bar charts (21-24) which summarise
the results of comparative testing of wear stated in
terms of a wear coefficient as factor K
(in3min/ft/lb/hr)xl09; (shaded column 22 and dotted
column 24) and of coefficient of friction, stated in µ
(clear columns 21 and 23).
Columns 21-24 show the comparative behaviour data
arising from sliding of:
1) a polymer designated Ultem 1000 (columns 21 and
22), a standard product of General Electric
2) a polymer material based on Ultem 1000 with
microcapsules incorporated at a rate of 10 wt.%,
produced according to one embodiment of the method
of the invention (columns 23 and 24),
against tempered steel.
The microcapsules incorporated into the Ultem 1000
resin contain a low viscosity oil according to one
embodiment of the invention.
The wear tests whose results are summarised in the bar
charts were performed under conditions which provide a
sliding speed of 300 ft/min ('1.524 m/s), a pressure
load of 200 psi (13.8 bar) and a test duration of 20
hours "run-in" and,80 hours "steady state".
On the basis of the results from the tests performed,
it is clear that the wear rate against steel of the
plastic Ultem 1000. incorporating microcapsules produced
according to an embodiment of the method of the
invention, is reduced by a good three orders of
magnitude, while friction is reduced by one order of
magnitude, relative to the prior art resin "Ultem 1000.
With reference of Figure 4, bar charts (31 and 32) are
shown which summarise the' results, stated in terms of
the factor K (in3min/ft/lb/hr)x109, of friction testing
carried out on:
1) a sealing element of PEEK (Victrex PEEK 4 50G from
GE Corporation, dotted column 31)
2) a second sealing element of PEEK with incorporated
microcapsules of Gargoyl lubricant in a quantity
equivalent to approx. 10% of the weight thereof
(hatched column 32) according to an embodiment of
both against tempered steel.
The results summarised in the bar charts 31 and 32
show, for the sealing element according to the present
invention, a significant reduction in the coefficient
of friction of approx. one order of magnitude.
1. A self-lubricating plastics material for sealing
elements/ comprising a wear-resistant polymer matrix in
which are dispersed microcapsules containing a
2. A material according to claim 1, characterised in
that said polymer matrix comprises a polyketone.
3. A material according to claim 2, characterised in
that said polyketone is an aromatic polyketone.
4. A material according to claim 3, characterised in
that said aromatic polyketone is polyetherether ketone
5. A material according to claim 1, characterised in
that said polymer matrix comprises a resin selected
from among polybutadiene-styrene (PBS),
polytetrafluoroethylene (PTFE) and mixtures thereof.
6. A material according to any one of the preceding
claims 1-5, characterised in that said microcapsules
comprise a shell of polyoxymethylene urea (PMU).
7. A material according to any one of the preceding
claims 1-6, characterised in that said microcapsules
have an average diameter of between 5 and 500 µ.
8. A material according to any one of the preceding
claims .1-7, characterised in that said microcapsules
are dispersed in said polymer matrix in a ratio by
weight of between 2 and 30 wt. %.
9. A material according to any one of the preceding
claims 1-8, characterised in that said lubricant
incorporated in the microcapsules is an oil which is
low in acidity.
10. A material according to any one of the preceding
claims 1-9, characterised in that said lubricant is a
fluid lubricant which has a viscosity within the range
between 20 and 250 cSt.
11. A material according to any one of the preceding
claims 1-10, characterised in that said lubricant
further comprises an additive or filler to increase
mechanical strength or thermal conductivity.
12. A material according to claim 11, characterised in
that said additive is a microelement selected from the
group consisting of zinc, boron and mixtures thereof.
13. Use of a material according to any one of the
preceding claims 1-12 for reducing friction.
14. Use of a material according to any one of the
preceding claims 1-12 for reducing wear on adjacent
surfaces of elements in motion.
15. Use of a material according to any one of claims
1-12 as a self-lubricating material.
16. Use of a material according to according to any one
of the preceding claims 1-12 as a self-lubricating
sealing element with a reduced wear rate.
17. Use according, to claim 16 in which said sealing
element is a sealing ring for a piston in a
18. A method for reducing the friction or wear of
adjacent elements in motion, in which one of the
adjacent surfaces of said sliding elements comprises a
self-lubricating material according to any one of the
preceding claims 1-12.
19. A method according to claim 18 in which one element
of the sliding pair is based on metal.
The present invention relates to a plastics material with self
lubricating action which is particularly suitable for the
production of sealing elements for reciprocating compressors,
said material comprising a wear-resistant polymer matrix,
preferably made from polyketone, within which are dispersed
microcapsules containing a lubricating fluid. The microcap
sules incorporated into the polymer matrix break up as a
result of friction with the contact surface of the sliding
partner, resulting in the escape of the lubricating fluid
and a consequent reduction in friction.
|Indian Patent Application Number||850/KOLNP/2004|
|PG Journal Number||43/2011|
|Date of Filing||18-Jun-2004|
|Name of Patentee||NUOVO PIGNONE HOLDING S.P.A.|
|Applicant Address||VIA F. MATTEUCCI, 2, I-50127 FIRENZE|
|PCT International Classification Number||F04B39/00;C09K3/10|
|PCT International Application Number||PCT/EP2002/14850|
|PCT International Filing date||2002-12-30|