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BACKGROUND OF THE INVENTION
Gaskets and seals perform the primary function of preventing the flow of
fluids past moving or stationary mechanical interfaces. Gaskets are
normally employed with stationary interfaces. The normal requirements for
gaskets as set forth in the Handbook of Mechanical Packings and Gasket
Materials published by the Mechanical Packing Association, New York, New
York, include mainly low creep, resistance to chemical attack and
conformability. While many gasket materials are available, none has been
developed that satisfies all of these requirements to the degree that is
desired in commercial practice. It is particularly difficult to combine
low creep and high conformability. Elastomeric materials that conform well
to rough surfaces, tend to creep under high pressures. Metal gaskets, in
the other extreme, resist high pressure creep, but have poor
conformability. Also, they are very susceptible to corrosion.
Seals commonly prevent the flow of fluids past sliding mechanical
interfaces, such as rotating shapes, and have additional requirements over
and above those of gaskets. Seals must have resistance to wear and must
possess a low coefficient of friction. Another requirement in many seal
applications is porosity which is essential to retain fluid lubricants. In
such applications there is the need to retain this porosity under
compression. Of the many seal materials available, including packing
materials, none satisfactorily meets all requirements desired in
commercial practice.
OBJECTS OF THE INVENTION
It is the primary object of this invention to provide a composite system
useful both for gaskets and seals which can be adjusted to provide for
desired combinations of resistance to creep, resistance to chemical
attack, good conformability resistance to high temperature flow, and
flexibility. A further object of the invention is to provide a unique
method of making such composite systems. Another object of the present
invention is a method of sealing and gasketing by the use of the novel
structures of the present invention.
BRIEF DESCRIPTION OF THE INVENTION
Gaskets and seals of this invention are composite structures in which
particulate materials are interconnected and intertwined with fibrils of
polytetrafluoroethylene. The amount, size and composition of the
particulate materials employed in the present invention may be selected to
give desired combinations of properties for gasket and/or seal
applications. The gasket and seal structures according to the invention
include a fluid medium of specified viscosity commonly either a
lubricating grease or a liquid which can set to produce a stronger
product. Gaskets and seals as defined are applied to mechanical interfaces
by conforming the structures to the surface and then compressing the
structure while it is in place.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a gasket or seal structure having
particulate material of platelette structure in the uncompressed state
showing particulate material such as flake graphite 1 interconnected and
intertwined with fibrils of polytetrafluoroethylene 2.
FIG. 2 is a cross-sectional view of the same structure as shown in FIG. 1
but in a compressed state illustrating how particles 1 consolidate to form
a denser structure.
FIG. 3 is a cross-sectional view of an interlocking structure according to
the invention in the uncompressed state wherein dendritic particles 3 are
employed.
FIG. 4 is a cross-sectional view of the structure of FIG. 3 in a compressed
state in which particulate material 3 has interlocked to form a structure
that is highly resistant to creep.
DETAILED DESCRIPTION OF THE INVENTION
The structures of this invention are uniquely suited for use as gaskets and
seals. In their uncompressed state, as illustrated in FIGS. 1 and 3, the
structures are very flexible and compliant. This properly enables them to
conform to irregular or mismatched surfaces. When compressed, as shown in
FIGS. 2 and 4, the particulate material interlocks. Interlocking can be
enhanced by using irregularly shaped particles 3 as shown in FIGS. 3 and
4. Such a structure is extremely resistant to flow or creep under
compression. Through the selection of materials, particles can be made to
contact each other and effect particle-to-particle bonds which further
enhance resistance to creep, which is essential in gaskets used at high
temperatures and pressures. The gasket and seal materials are normally in
the form of sheets but may also be in the form of rods, squares or other
objects.
When porosity is desired to hold lubricants for the sealing of sliding
surfaces, the ratio of particulate material to polytetrafluoroethylene
(hereafter PTFE) can be increased to the point where there is insufficient
PTFE to fill spaces between compressed particles 1 and 3. Under these
circumstances, void spaces will be retained even when the sealing material
is compressed. The ratio of particulate material to PTFE can be adjusted
to give the desired levels of porosity needed for lubrication. In this
manner, the seals of this invention are pliable in their uncompressed
state for conforming to mating surfaces, yet resist creep when comressed
because the particles interlock but still remain porous to retain
lubricant.
The amount of PTFE used to manipulate the properties of the structures of
this invention varies between about 5 and about 35 volume percent
depending upon the total volume of solids percent, although concentration
beyond these limits can be used for special effects. In general, going
above 35 volume percent PTFE reduces the amount of interlocking filler
material during compression unless large dendritic particles are used. For
maximum resistance to creep in high pressure gasket applications, the
preferred range of PTFE is between 8 and 17 volume percent. This allows
particles to come close enough together to permit unencumbered
interlocking. For seal applications requiring higher tensile strength and
low coefficients of friction, the preferred range of PTFE is from 18 to 35
volume percent.
The preferred material for forming fibrils to bond particulate material
together as shown in FIGS. 1 and 3 is fibrillatable PTFE of the type
designated Teflon 6C manufactured by E. I. DuPont de Nemours & Co. Any
polymeric material that can be fibrillated by random mechanical shearing
can be used. Besides having the property of being fibrillatable, PTFE is
very resistant to chemical attack and can be used up to 450.degree. F. The
preparation of PTFE is well known and is illustrated by U.S. Pat. Nos.
2,510,112, 2,587,357 and 2,685,707. Typical particle size of the PTFE
useful in the present invention ranges from about 50 to about 560 microns
although larger or smaller particles are useful under certain
circumstances.
The selection of a specific particulate material is based upon service
requirements. When low friction is required, dry lubricants such as flake
graphite as depicted in FIGS. 1 and 2 or molybdenum disulfide can be used.
Typically, the dry lubricant particles are between 10 and 44 microns over
their major axes, though larger and smaller particles can be used.
Alternatively low friction bearing materials in particulate form can be
used. Included in this class of materials are the lead and copper base
alloys or lead and copper alone.
Other particulate materials can be employed including the general class of
metals, intermetallic compounds, ceramics salts and plastics. Mixtures of
one or more of such particulate materials can be used for special effects.
Generally, the specific particulate material will be chosen for the use
intended keeping in mind that particulate materials which are very hard
may cause excessive wear where moving surfaces are to be sealed.
This selection of particulate material thus depends upon the combination of
friction and resistance to chemical attack that is needed and low friction
materials are preferred for sealing applications. Graphite is more
resistant to chemical attack than the metal base filler materials.
When greater resistance to creep or mechanical shear under compression is
needed equiaxial particles such as ground artificial graphite, ground
ceramics or metal powders can be used. Dendritic particles such as shown
in FIGS. 3 and 4 are available in stainless steel, iron and other metal or
glass powders made by atomization of molten materials. Electrolytic metals
also are dentritic. For very small dentritic powders carbonyl nickel can
be used.
Particle sizes of particulate materials useful in the present invention may
range from about 0.1 to 200 microns although larger and smaller particle
sizes are possible.
To promote particle-to-particle bonding under compression, soft metal
particles that readily cold weld such as copper, nickel, aluminum and lead
or alloys thereof can be used. Under the forces and movement generated
during compression, particulate filler materials come into moving contact
which effects bonding, thereby rigidizing the structure.
Dendritic particles of the type shown in FIGS. 3 and 4 have the further
advantage that they impart elasticity to the compressed structure. This is
needed in gaskets to accommodate changes of the distance between surfaces
that are confining the gasket. Under these circumstances dendritic
material made from atomized alloys, such as spring steels or bronzes, are
used.
Increased elasticity and additional resistance to creep and wear can be
obtained by incorporating fibers of metals, plastics, graphite or glass
into the structure along with the PTFE and other filler material. Fibers
can be from 1/16 inch up to 1/2 inch long or more with a
length-to-diameter ratio of over 4 to 1. The fiber content can comprise
from 0 to 100% of the filler content.
In another embodiment of the present invention, it was discovered that when
a grease was incorporated into the seal and gasket materials of this
invention, a very elastic, slippery PTFE base structure was produced. The
presence of the grease uniformly distributed in the voids of the
structures shown in FIGS. 1 and 3 caused the structure to be extremely
flowable under compressive stress. Such a property is particularly useful
in gaskets requiring exceptionally high degrees of conformability in order
fully to occupy the void space between two surfaces to be sealed.
The liquid lubricant (e.g., grease) filled structure of this invention also
has exceptional lubricating properties, falling in between those of dry
lubricants and greases. This property makes it useful for seals requiring
a high degree of lubricity and also for lubricating bearings.
The grease content in the structure was found to be effective between about
0.1 to about 60 volume percent. At amounts of about 40 to 60 volume
percent, a maximization of fibrillation of PTFE during production is
achieved whereas at lower volume percentages, for example 3 to 30%,
flowability of the seal and gasket material under compression is improved
although the extent of fibrillation enhancement will be less than at the
higher percentages.
Fluid lubricants which are essentially semi-solids yet flow under stress at
the temperatures involved, preferably greases, useful in the present
invention, have a viscosity falling within the range of about 0.5 to about
1,000 poises and preferably between about 10 to about 200 at temperatures
between about 10.degree. and 150.degree. C.
Incorporation of the fluid lubricant uniformly within the structures of the
present invention is accomplished by admixing the fluid lubricant with the
particulate material and PTFE during the mechanical working step. Except
for the presence of the fluid lubricant along with the PTFE and
particulate material, mechanical working to obtain fibrillation is carried
out in the same manner as described in U.S. Pat. No. 3,864,124. In this
connection, it was found that when lubricants of the specified viscosity
are included with the particulate material and PTFE during the working
step, processing is made easier and higher amounts of PTFE can be
utilized, while still obtaining the fibrillation essential to a flexible
structure with good green strength, than is possible in the process of
aforesaid U.S. Pat. No. 3,864,124.
This effect was quite surprising because the process of U.S. Pat. No.
3,864,124 was found effective, with relatively low volume amounts of PTFE,
in the substantial absence of liquid lubricants. The process of U.S. Pat.
No. 3,864,124 is in turn contrasted with the earlier processes described
in U.S. Pat. Nos. 3,556,161 and 3,281,511 wherein a lamellae structure was
obtained which mechanically entrapped particulate material. This earlier
process utilized liquids of low viscosity such as kerosene during
processing whereas in the present invention, fluid lubricants of
substantially higher viscosity are employed.
While it is not entirely understood, and applicants do not mean to be bound
thereby, it is believed that the relatively high viscosity, semi-solid,
fluid lubricants when present during the working step of the present
invention actually aid in the fibrillation of the PTFE. This is in
contrast to the low viscosity liquids suggested in U.S. Pat. Nos.
3,556,161 and 3,281,511 which result in a lamellae structure, rather than
a fibrillated structure, apparently due to interference with the
fibrillation effect. The enhancing of the fibrillation effect when using
relatively high viscosity lubricants in accord with the present invention
permits an easier working while at the same time allowing the use of
higher amounts of PTFE than in said U.S. Pat. No. 3,864,124 and yet the
desired fibrillated structure which is needed for good strength,
pliability and cloth-like character is still obtained.
Fluid lubricants of the specified viscosity useful in the present invention
are preferably greases of the type well known in the art for lubricating
purposes. Such fluid lubricants normally comprise a synthetic, mineral,
animal or vegetable oil having the desired viscosity per se or more
commonly are greases containing a thickening agent such as a metallic soap
to obtain the desired viscosity. Typical oils include petroleum or mineral
oils, polyglycols (e.g., polyethylene and polypropylene glycols),
phosphate esters, chlorinated or fluorinated hydrocarbons (e.g.,
chlorinated biphenyls), dioctyl sebacate, diethyl hexyl azelate, silicate
esters and the like. Thickening agents, present in an amount sufficient to
achieve the desired viscosity (e.g., 0.5-20 percent by weight) include
lithium, calcium and sodium soaps, lime, silica and other thickening
agents well known in the art such as polymer particles insoluble in the
oil employed.
Although structures according to the present invention having the fluid
lubricant present within the structure form very useful sealing and
gasketing materials, particularly where sliding surfaces are involved, it
is also possible to extract the fluid lubricant from the resulting
structure after fibrillation has occurred, followed if necessary by
compressing the structure such as by rolling to achieve the desired
thickness. By this method, a final product is obtained which has a higher
degree of compressibility because the presence of the fluid lubricant
during working minimizes interlocking of the particulate material.
Moreover, the fluid lubricant also increases the amount of fibrillation so
that the resulting structure after extraction has a greater number of
intertwined fibrils which interconnect and entrap the particulate material
than is obtained when fluid lubricant is not present during the working
step. Such fibrils include those which interconnect particles, that is,
are adhered to two or more particles as well as those which entrap
particles, that is, wind over and around particles without necessarily
adhering to any particles. It is possible to utilize relatively high
amounts of fluid lubricant during initial processing, for example 40 to 60
volume percent, to maximize fibrillation, and then to extract some but not
all of the fluid lubricant to reduce the amount present in the final
product to about 3 to 30 volume percent so that flowability under
compression is retained.
Extraction of all or part of the fluid lubricant can be achieved by any
suitable means such as by the use of well-known solvents for the
particular lubricant employed. Illustrative solvents include petroleum
hydrocarbons such as kerosene, naptha gasoline and related fractions,
toluene, benzene, hexane, etc. as well as chlorinated hydrocarbons such as
carbon tetrachloride, methylene chloride, and the like. Other solvents
include alcohols (methanol, ethanol, isopropanol), ketones (acetone,
methylethyl ketone), aldehydes (acetaldehyde) and the like. The specific
choice of solvent will, of course, depend upon the particular fluid
lubricant to be extracted or dissolved from the fibrillated structure.
To effect a stiffening of the final structure in situ liquids whose
viscosity falls within the above limits and are capable of setting up due
to evaporation or polymerization can be used in place of or in addition to
the fluid lubricants. Included in this embodiment of the invention are
epoxies, acrylates, silicates, etc. The stiffening liquids may be in the
form of a monomer mixture which polymerized after fibrillation, a polymer
which either cross-links or stiffens by solvent or dispersing medium
removal after fibrillation, an inorganic stiffening agent such as a
silicate which stiffens by chemical reaction or evaporation of solvent or
dispersing medium, or any other of the well-known mixtures which can be
stiffened. Thickening agents can be employed to obtain the desired
viscosity.
Stiffening in situ augments interlocking of particles in increasing the
resistance of seals and gaskets to creep after they have conformed to
close the space to be sealed.
As stated earlier, the preferred method that can be used to make the gasket
and seal materials of this invention is described in U.S. Pat. No.
3,864,124 issued Feb. 4, 1975. It entails mechanically working measured
quantities of PTFE and filler materials to obtain a uniform dispersion,
with or without the presence of a fluid medium having the stated
viscosity. The mixture may then be hot rolled several times with folding
of the powders to double the thickness between each rolling step.
Temperatures between 60.degree. C. to 150.degree. C. are adequate. After
the powders become coherent due to fibrillation of the PTFE, the resultant
sheet is rolled in the following manner 4 to 10 more times or more to
develop the desired level of strength:
After each pass, the sheet is folded upon itself to form two layers. This
is rotated 90.degree. and rerolled. In this manner the PTFE is fibrillated
in orthogonal directions. Fibrils typically have a ratio of length to
diameter of 10 to 1 to 100 to 1 or greater.
After the sheet is rolled to desired thickness, which can vary from 0.002
up to 0.25 inches, it is cut to desired shapes. To further increase the
tensile strength of the structure, it can be sintered near the melting
point of the PTFE in the range of 330.degree. to 390.degree. C. to
accomplish sintering of the PTFE. Exact sintering conditions are provided
by the manufacturers of PTFE.
As will be apparent to one skilled in the art, other methods of
mechanically working a mixture of PTFE and appropriate filler materials
may be employed and such methods are illustrated in U.S. Pat. No.
3,864,124 (e.g., ball milling, stretching, elongating, and the like).
Moreover, the filler materials useful in the present invention encompass
those materials mentioned in the aforesaid U.S. Pat. No. 3,864,124.
Gaskets are cut in configurations to match the sealing surfaces of the
interface to be sealed. Ancilar rings with bolt holes are the most
frequent configurations of gasket used.
Packings for sealing rotating shafts can be made by cutting or rolling the
materials of this invention into strips, the diameter and length of which
depend upon the diameter of shaft and size of packing cavity.
When in the unsintered state, materials of this invention form coherent,
homogeneous structures when compressed. This phenomenon results in packing
strips being converted into shear resistant, integral seals when
compressed. Identity of individual strips is lost.
This reforming phenomenon also allows fabrication of seals and gaskets by
compression molding. By this method the fibrillated materials of this
invention are pelletized or chopped to give a free flowing material. This
in turn is flowed into a die and compressed to desired shapes, either with
or without heat depending upon properties required by the pressed part. At
higher concentrations of PTFE, compression molding can be carried out
under conditions approaching those used for compression molding PTFE. At
lower concentration levels of PTFE, higher pressures are
required--sometimes approaching those used for compressing metal and
ceramic powders.
The reforming property of unsintered PTFE filler materials permits the
fabrication of composite structures in which a reinforcing mesh such as
screen is embedded with PTFE-filler materials. This is made by laminating
the screen between two sheets of the unsintered gasket material by hot
rolling. Mesh material can be made from filaments of metals such as
stainless steel, glass or polymeric materials such as nylon or polyester.
The size of the mesh is dependent upon the thickness of the laminated
gasket thickness. Both woven and nonwoven meshes can be used. In general
the thickness of the mesh should constitute over 50 percent of the total
thickness of the gasket. Under compression by rolling the sheets of
unsintered gasket material conform closely to and surround the screen to
embed it within the resulting laminate, intimately contacting the screen
with the sheets. After compression, an essentially unitary structure
results with the screen completely embedded within the structure with no
visible evidence of an interface between the two sheets which have been
laminated.
EXAMPLE 1
A stainless steel-PTFE gasket was made by the following procedure: 64 grams
of minus 325 mesh stainless steel powder was dry mixed with 4.4 grams of
Teflon 6C manufactured by E. I. DuPont de Nemours & Co. in a two quart
ball mill using 1/2 inch steel balls, for 30 minutes. The mixture was
separated from the steel balls and hot rolled by the procedure previously
described to form a sheet 0.031 inches thick.
A washer 2.06 inches OD and 1.30 inches ID was cut from this sheet. The
washer was placed between two flat 3.00 inch diameter plattens and
compressed at successively higher loads. For comparison, unreinforced,
sintered PTFE sheet 0.030 inches thick was tested in the same manner.
Results are tabulated below.
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Load % Reduction
Material in Tons in Thickness
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Stainless steel PTFE
2 10
4 16
6 16
8 23
10 23
12 29
14 26
16 29
18 29
20 32
PTFE (.030 inch
2 10
initial thickness)
4 13
6 23
8 23
10 30
12 37
14 37
16 40
18 43
20 43
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These data show that at 12 tons load, particles of stainless steel
interlocked and arrested further deformation of the gasket. The PTFE on
the other hand continued to deform.
EXAMPLE 2
A PTFE-graphite material for sliding seals was prepared as follows: 17.6
grams of minus 325 mesh flake graphite and 4.4 grams of 6C Teflon were
made into 0.070 thick sheet by the method described in Example 1.
Rectangular pieces were placed between a flat metal block and a steel
wheel 13/8 inches in diameter with its face being 5/16 inch wide pressing
the PTFE-graphite composite. A load of 90 lbs. was applied to the block
and the wheel was rotated to give a sliding velocity of 12 ft./min. While
operating, the dynamic coefficient of friction was determined from normal
and tangential forces. The time required to wear through the PTFE-graphite
composite was recorded. The results in comparison with commercial
unreinforced PTFE and commercial glass reinforced PTFE evaluated in the
same way are compared below:
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Coefficient of
Material Friction Time to Failure
______________________________________
Sintered PTFE
.070 7 minutes
Glass filled PTFE
.071 21 minutes
Graphite-PTFE of
.070 150 minutes
this invention
______________________________________
This indicates that the graphite-PTFE structure had much greater resistance
to wear than unfilled, sintered PTFE or glass filled PTFE while retaining
a low coefficient of friction.
EXAMPLE 3
Eighteen grams of minus 325 mesh flake graphite, 4 grams of PTFE and 12
grams of Mobilplex 47 lubricating grease manufactured by Mobil Oil Company
were ball milled as described in Example 1. This mixture was then
mechanically worked by cross-rolling as described in Example 1. The sheet
was rolled to 0.035 inch thickness and tested as described in Example 1.
The results are tabulated below:
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Load % Reduction
Material in Tons in Thickness
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Graphite-PTFE 2 61
with grease 4 77
8 84
PTFE 2 5
4 15
8 18
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The presence of the grease imparted a high degree of flowability under
pressure compared to that of PTFE alone.
EXAMPLE 4
A structure containing 4 grams of PTFE, 18 grams of minus 50 mesh glass
powder and 12 grams of grease (as in Example 3) was prepared by the method
given in Example 3. The grease was removed by extraction for 60 minutes
with a triethylene chloride solvent.
The resultant sheet was folded and then rolled at 90.degree. C. down to
0.031 inch thickness. A tough, pliable sheet was produced.
When tested by the procedure given in Example 1, the following results were
obtained:
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Load % Reduction
Material in Tons in Thickness
______________________________________
Glass-PTFE 2 13
4 13
6 19
8 23
10 19
12 23
14 26
16 26
18 26
20 26
PTFE (.030 inch
2 10
initial thickness)
4 13
6 23
8 23
10 30
12 37
14 37
16 40
18 43
20 43
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EXAMPLE 5
A structure containing 4 grams of PTFE, 18 grams of minus 325 mesh glass
powder and a sodium silicate solution made by Sauereisen Company in
Pittsburgh, Pennsylvania and designated Sauereisen 14 was ball milled and
rolled by the procedure given in Example 1. Care was taken to avoid drying
by rolling between 60.degree. and 90.degree. C. The final thickness was
0.035 inch. The sheet was tough and pliable. A gasket was cut from this
and clamped between two flanges. After 48 hours, the pliable gasket was
transformed into a hard gasket that was made more resistant to compression
flow by the setting up of the silicate solution.
The gasket and seal materials of the present invention are typically used
to prevent or minimize the passage of a fluid medium through an interface
or juncture of two surfaces. The fluid medium is normally a liquid and/or
a gas, and the seal and gasket material serves to overcome the inability
or lack of economics to machine the abutting surfaces to tolerances
necessary for precise alignment which per se would prevent or minimize
passage of the fluid medium. Alternatively, the gasket and seal materials
are used where the desired tolerance cannot be maintained during use
(e.g., where due to wear, the close tolerances cannot be maintained
suitably close to prevent or minimize fluid medium passage).
In accord with the present invention, the surfaces to be sealed may be
stationary with respect to one another or they may move relative to each
other. Typical uses for the seals and gaskets of the invention include
abrasion seals and compression seals in jet engines, pumps, motors,
chemical process equipment, pistons, valves, flanges, pipes, vacuums, and
the like. In the environment of use, the temperatures may vary greatly
from below freezing to considerably above say up to 3100.degree. F.
With respect to the sealing and gasketing materials, they normally are
placed between the surfaces to be gasketed or sealed and the surfaces are
then moved into their desired relationship and spacing which serves to
compress the material and effect the desired sealing and gasketing.
Dendritic as employed in this application includes particles sufficiently
irregular such that they interlock upon compression.
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