Note: Descriptions are shown in the official language in which they were submitted.
w oss/07422 t 69 7~g PCTrUS93/0998S
GASKET MATERIAL FOR USE IN PLATE AND
FRAME APPARATUS AND METHOD FOR MAKING AND USING SAME
RELATED APPLICATIONS
The present application is a continuation-in-part of copending
United States Patent Application Serial Number 050,903, filed April
20, 1993.
BACr.rROUND OF THE INVENTION
1. Field of the Invention
The present invention relates gasket and seal materials, and
especially gasket materials for use in sealing multiple layers of
an apparatus together to contain fluid flow therethrough, such as
in a plate and frame heat exchange or filter apparatus.
2. Description of Related Art
In order to maximize fluid flow though a filter or heat
exchange apparatus, multiple functional plates are often stacked in
series. In the case of a filter, the functional plates comprise
filter elements; in the case of heat exchange apparatus, the plates
comprise thin (e.g. 0.6 to 1.0 mm) thermally conductive material,
such as stainless steel. In either instance, a fluid seal must be
established and maintained between each of the plates to assure
that leakage does not occur around plates.
A plate and frame heat exchanger uses a large number of
plates, often ranging from 8-16 to 500 or more plates in a single
unit, to provide a mechanism for heat transfer. Generally, each of
the plates comprises an essentially rectangular element with upper
and lower ports therein. When the plates are stacked with their
faces parallel to each other and sealed along their edges, cells
are created with fluid flow directed through each of the cells,
across the face of the plates, from an upper port to a lower port
or visa versa.
Heat exchange is accomplished by stacking many plates in this
W095/07422 q,~6~ 2 PCTtUS93/09985
manner and establishing two distinct fluid paths through the heat
exchanger. A first path passes up and down the faces of the plates
in every other cell (e.g. passing from left to right in the
apparatus through the first, third, fifth, etc. cells); and an
counter-current second fluid path passes down and up the plates in
the alternating cells (e.g. passing from right to left in the
apparatus through the sixth, fourth, second cells).
The most important feature of any heat exchanger is its
successful ability to segregate and contain the fluids passing
through it. Clearly leakage or, worse, intermixing of fluids in
the exchanger compromises or completely destroys its proper
function. Moreover, where hazardous chemicals are handled by such
a system, any leakage can be disastrous. As a result, particular
attention is given to providing effective seals between each of the
plates of a heat exchanger.
A typical heat exchanger seal comprises a ring of elastomer
(e.g. butyl rubber, neoprene, ethylene-propylene diene monomer
(EPDM), etc.) or compressed sheet (e.g. asbestos or synthetic
fiber) that is mounted around the periphery of each plate and
around appropriate ports to assure proper fluid separation and
orientation. These gaskets are generally glued in place on the
plate and then the plates are stacked within a frame and torqued
down until a tight fit is created between each of the plates and
the intermediate gaskets.
Existing plate and frame gaskets have many serious drawbacks.
For instance, a sometimes difficult compromise must be struck
between a material which provides a tight seal and a material which
is adequately durable and chemical/heat resistant for long-term
use. Another common problem is that the gasket material is
required to conform to compensate for misaligned, bent, scored, or
otherwise defective plates.
One of the most serious problems with existing plate and frame
gasket materials is that many materials (e.g. asbestos or elastomer
glued in place with an epoxy or other adhesive) are extremely
difficult to remove and replace during routine maintenance. For
example, it is estimated that 1 to 1.25 man-hours are required for
the reconditioning of each plate in a medium size heat exchanger
employing elastomeric gasket material. For a heat exchanger with
w o ss/07422 ~ 7~ PCTrUS93/09985
over 100 plates, reconditioning of the entire heat exchanger is
clearly a massive undertaking. Additionally, materials such as
EPDM, neoprene, or butyl rubber can have a relatively short
operative life of only 6 to 9 months in harsh environments (e.g.
when subjected to excessive chemical or thermal attack).
Complicating both the installation and reconditioning of heat
exchanger gaskets is the use of texturing on the plates normally
employed in these systems. As is known, by corrugating or
otherwise texturing a heat exchanger plate, its surface area is
significantly increased, thus improving its ability to transmit
heat. Further, plate texturing also makes it quite difficult to
remove broken gasket material which can become stuck in the plate's
ridges.
Another complicating factor in sealing many plate and frame
apparatus is that the plates are often poorly supported within the
frames. "Non-optimum" plate and frame apparatus support the plates
through use of an ill-fitted connection between the plates and
guide rails. As a result, the plates must be carefully torqued
down to assure that proper alignment is maintained between the
plates.
A better system is offered in so-called "optimum" plate and
frame apparatus that use closely matched guide rails to help keep
the plates aligned during the installation procedure. Such
alignment is considered critical where significant compression must
occur during installation. Despite this improvement, care must
still be maintained to assure that the plates are evenly mounted.
It is important that the plates are kept in contact with each
other during service. Such contact is important for increasing
heat transfer along the corrugations, as well as compensating for
different pressures between plates and cyclic pumping actions which
can lead to flexing of the plates and mechanical fatigue. In this
regard, the proper placement and maintenance of the gasket material
is crucial. The gasket material must supply enough counterforce to
seal between the plates during installation; additionally, the
gasket material cannot cold flow or "creep" away from the
contacting plates by further thinning and widening during use--
which could lead to gaps and leakage.
One material that has superior heat and chemical resistant
w oss/07422 ~69~ ~ PCTrUS93/09985 ~
properties is polytetrafluoroethylene (PTFE). As a gasket, PTFE
has exhibited utility as a material for use in harsh chemical
environments which normally degrade many conventional metals and
polymeric materials. PTFE has a usable temperature range from as
high as 260C to as low as near -273C.
However, conventional non-porous PTFE gasket materials which
have been compression molded or extruded and then heated to a
temperature above 345C exhibit poor mechanical properties, such as
low tensile strength and low cold flow resistance. This limits the
use of such materials in areas requiring a measure of physical
strength and resistance to creep.
PTFE may be produced in an expanded porous form as taught in
United States Patent 3,953,566 issued April 27, 1976, to Gore.
Expanded polytetrafluoroethylene (ePTFE) is of a higher strength
than conventional PTFE, has the chemical inertness of conventional
PTFE, and has an increased temperature range of up to 315C in
service. An example of a porous expanded PTFE gasket material is
available from W. L. Gore & Associates, Inc., of Elkton, Maryland,
under the trademark GORE-TEX\ Joint Sealant.
Porous ePTFE joint sealants have proven to have excellent
seals in many applications. Unfortunately, due to the inherent
compression characteristics of this material, it generally requires
a relatively wide sealing surface and a significant clamping load
to provide a tight and stable seal between abutting surfaces (i.e.
whereby a wide, thin, fully densified gasket can be created). As a
result, ePTFE does not perform well in instances with narrow
sealing surfaces or requiring relatively thick gasket materials
since under compression creep can occur over time to distort the
gasket's proper placement. This is a serious constraint in
attempting to use this material in the relatively thick-gasketed
but high-compression environment of a plate and frame apparatus.
For some applications the problem of creep has been addressed
by providing an expanded PTFE core wrapped by a tape of porous
ePTFE. Commercial embodiments of such material are available from
W. L. Gore & Associates, Inc., under the designation GORE-TEX\
Valve Stem Packing, and Inertech, Inc., of Monterey Park,
California, under the designation INERTECH\ Valve Stem Packing.
These materials are suitable for use as a compression packing where
~ wo 9s/07422 ~7~ PCTrUS93/09985
they are confined within a defined volume. However, when used as a
gasket in an unconfined volume under a compressive load, these
materials exhibit undesirable creep characteristics (i.e.
continuing to thin and widen) over time, making them completely
unsuitable for use as gasket material in most plate and frame
apparatus.
As is demonstrated by United States Patent 5,160,773 issued
November 3, 1992, to Sassa, very successful use of a coated
expanded PTFE seal can be achieved in low compression applications,
such as in a "wiper" seal for moving surfaces with very low
clamping forces and low fluid pressures. In that case, the sealing
material comprises a PTFE felt encapsulated by a porous PTFE sheet
laminated to a melt-processible thermoplastic fluoropolymer.
Regretfully, where significant compression forces are applied,
deformation and undesirable creep is again experienced.
One suggestion for achieving the chemical resistance of PTFE
but limiting the amount of creep of the material is to coat a
generally creep-stable material such as synthetic rubber with a
coating of PTFE to provide chemical resistance. One example of
such a structure is presented in United States Patent 4,898,638
issued February 6, 1990, to Lugez. In this patent it is taught
that through a disclosed process one or more films of only
partially porous PTFE can be adhered to a rubber sheet to provide a
gasket material with chemical resistance. While this approach may
address some of the problems with existing plate and frame gasket
materials, the PTFE film can crack under the stresses of
compression, leading to exposure and failure of the core elastomer.
Further, it is believed that longer life and better thermal and
chemical resistivity are possible if an expanded PTFE material is
employed throughout the gasket.
As is disclosed in co-pending United States Patent Application
Serial Number 050,903, filed April 20, 1993, it has been determined
that a PTFE sealing material can be produced with limited long-term
creep by wrapping a core of elongated or expanded PTFE with a high
strength film of expanded PTFE. The high strength film is
resistant to deformation and stretching and serves to contain the
PTFE core in place within a compressed gasket. This material has
proven to be quite effective in sealing plate and frame heat
WO 95/07422 2~ 69~ ~9 PCT/US93/09985 ~
exchangers--providing thermal and chemical protection, long-life
and durability, and ease in replacement.
Despite the success of the above described material in sealing
plate and frame heat exchangers, it has a number of deficiencies.
Perhaps the greatest problem with the high strength film wrapped
PTFE material is that it requires extensive compression before
becoming properly seated within a plate and frame apparatus. A
typical gasket material with a rectangular cross-section generally
must be compressed in height down about 3:1 before proper seating
and sealing is established.
This seating problem is a very serious concern in an
application with many plates since a normal corresponding frame is
simply too small to contain all the plates and un-condensed gasket
material at one time. As a result, an installer must go through
the burdensome and time consuming procedure of installing and
compacting the plates and gasket materials in a number of batches.
This problem is vastly compounded in non-optimal plate and frame
apparatus where a large degree of movement of the plates in the
sealing process leaves too much room for plate distortion and
gasket shifting.
Accordingly, it is a primary purpose of the present invention
to provide a gasket material for a plate and frame apparatus that
provides an effective long-term seal under pressure, while being
durable, chemical and thermal resistant, non-contaminating, and
relatively easy to install.
It is another purpose of the present invention to provide a
gasket material for plate and frame apparatus that is readily
removed and replaced with minimal labor and expense.
It is still another purpose of the present invention to
provide a gasket material for plate and frame apparatus that
provides the benefits of expanded PTFE material, while avoiding the
problem of creep.
It is yet another purpose of the present invention to provide
a gasket material for plate and frame apparatus that can be readily
installed without requiring undue torque or plate movement.
It is a further purpose of the present invention to provide a
method for making and optimally using a gasket material with the
above properties.
w o 95/07422 7 ~7~ PcTrusg3/09985
These and other purposes of the present invention will become
evident from review of the following specification.
SUMMARY OF THE INVENTION
The present invention is an improved gasket material for use
in a variety of plate and frame apparatus, such as plate and frame
heat exchangers and filter units. The basic material of the
present invention comprises a core of elongated
polytetrafluoroethylene (PTFE) tightly wrapped in a high strength
film. When placed under high compression in a plate and frame
apparatus, the gasket material of the present invention has proven
to be highly resistant to cold flow or "creep," while providing all
the exceptional properties of PTFE material.
The preferred gasket material of the present invention
comprises an expanded PTFE core wrapped in a high strength PTFE
film and then pre-compressed to vastly reduce the time and effort
required to install the gasket material in a plate and frame
apparatus. Ideally, to aid in installation, the pre-compressed
gasket material includes a pattern or "footprint" which matches the
texture of an adjoining plate from the plate and frame device. The
inclusion of a conformable layer, such as a soft PTFE tape, on the
pre-compressed gasket further assists in establishing an improved
initial seal in the plate and frame device.
The gasket material of the present invention has numerous
benefits over previous plate and frame sealing material. Among the
improvements are longer life and greater durability in environments
of harsh chemicals and/or extreme temperatures. In addition, even
after an extended period of high compression, the gasket material
of the present invention releases very readily from a plate,
usually completely intact. This greatly improves plate
reconditioning time and effort.
DESCRIPTION OF THE DRAWINGS
The operation of the present invention should become apparent
from the following description when considered in conjunction with
WO 95/07422 ~ ~ ~ 9 PCTIUS93/09985 --
the accompanying drawings, in which:
Figure 1 is a three-quarter perspective view of a conventional
plate and frame heat exchanger;
Figure 2 is an elevational view of a conventional plate from a
plate and frame heat exchanger with a gasket material of the
present invention mounted thereon;
Figure 3 is a three-quarter isometric view of an uncompressed
gasket material of the present invention;
Figure 4 is a three-quarter isometric view of a fully
compressed gasket material of the present invention;
Figure 5 is a side elevational view of another embodiment of a
gasket material of the present invention, pre-compressed and
provided with a plate pattern on its bottom side and a conformable
sealing layer on its top side;
Figure 6 is a side elevational view of the embodiment of
gasket material shown in Figure 5 following full compression in a
plate and frame heat exchanger;
Figure 7 is a top plan view of a cord of gasket material of
the present invention cut to correct length for installation on a
plate;
Figure 8 is a top plan view of a cord of gasket material of
the present invention joined into a loop for installation on a
plate;
Figure 9 is a top plan view of a cord of gasket material of
the present inven~ion joined into a loop and shaped into correct
contours for installation on a plate.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a gasket material suitable for use in
a variety of applications, and especially in applications requiring
minimal cold flow or "creep." Of particular interest are plate and
frame apparatus (e.g. plate and frame filters or heat exchangers)
where multiple elements must be stacked together and then
- compressed.
Shown in Figure 1 is a conventional plate and frame heat
exchanger 10. The heat exchanger 10 comprises a fixed end frame
WO 95/07422 ~ PCT/US93/09985
12, a movable end frame 14, multiple plates 16 mounted between the
two end frames 12, 14, compression bolts 18a, 18b, 18c, 18d, 18e,
18f, 18g, 18h spanning between the two end frames 12, 14, and
compression nuts 20a, 20b, 20c, 20d holding the moveable end frame
14 in tight contact with the plates 16.
In order to establish gas or liquid fluid flow through the
heat exchanger 10, upper ports 22a, 22b and lower ports 24a, 24b
are provided in the end plates 12, 14 corresponding to ports in
each of the plates 16. Threaded studs 26 are provided around each
of the ports 22, 24 to accommodate fluid line attachments (not
shown).
Shown in Figure 2 is a representative heat exchanger plate 16
for use in a heat exchanger. As has been noted, the plate includes
upper ports 28a, 28b and lower ports 30a, 30b corresponding to
ports 22, 24 in heat exchanger 10. In order to increase contact
area between the plate and fluid flowing over its surface 32, a
series of corrugations 34 or other texturing is provided. The
plate's edge 36 is likewise corrugated with a groove 37 typically
provided for attachment of a gasket material 38. In addition to
increasing surface area, as has been explained, the corrugations
contact one another to help support the plates in order to resist
plate fatigue and improve mechanical integrity of the unit.
Since these types of plates come in a very wide variety of
sizes, shapes and textured patterns, it should be understood that
considerable customizing of gasket material may have to be
performed to fit gaskets to different models of plate and frame
devices. Fortunately, as will be evident from the following
description, the gasket material of the present invention is fully
adaptable to such customizing.
Gasket material 38 of the present invention is installed on
the plate to establish a fluid flow path across it. In the
embodiment shown, when this plate is attached against an identical
plate an outer perimeter gasket 38a forms a cell 40 communicating
fluid flow between upper port 28b and lower port 30b. Fluid flow
will pass between the surfaces of the two plates through various
channels created between corresponding corrugations 34 in the two
plates.
Gasket material 38b, 38c is also installed around ports 28a
WO 95t07422 2~.G~ ~ 10 PCT/US93/09985--
and 30a to direct alternate (i.e. countercurrent) fluid flow pass
this plate into a fluid flow path established across the next plate
stacked in series. The gasket material 38b, 38c around ports 28a,
30a serve to assure that fluid leakage does not occur from the heat
exchanger and that fluid does not intermix with the fluid in cell
40. As a further assurance against fluid mixture, each of the port
gaskets 28a, 30a includes a vent 41a, 41b to allow release of fluid
to the atmosphere and away from cell 40 in the case of some failure
in either port gasket 38b or 38c.
In operation, the gasket material 38 is installed on each of
the plates 16 to create two distinct fluid paths through the heat
exchanger 10. By way of example, a first path passes up and down
the faces of the plates in every other cell (e.g. passing from
upper port 22b left to right through the apparatus through every
other cell (e.g. odd numbered cells) and out a lower port (not
shown) in the moveable end plate 14); and an counter-current second
fluid path passes down and up the plates in the alternating cells
(e.g. passing from an upper port (not shown) in the moveable end
plate 14 right to left in the apparatus through the alternative
cells (e.g. even number cells) and out lower port 24a).
Once the gasket material 38 has been correctly positioned in
this manner on each of the plates 16, the plates are stacked in
series and mounted between the frames 12, 14. In the apparatus
shown, the heat exchanger has been provided with guide rails 42, 44
running the length of the apparatus. Slots 46, 48 are provided in
each of the plates 16 corresponding to these rails 42, 44. By
aligning the slots in the plates along the guide rails such that
lateral movement of the plates is not feasible, "optimal" alignment
of the plates is assured during installation and use.
In order to establish a tight seal between the plates, once
they are aligned between the frame ends 12, 14 in the manner
described, each of the compression nuts 20 are tightened down along
compression bolts 18. Care must be exercised to assure that the
plates are evenly torqued down in this manner, with a limited
amount of torque applied to any one bolt at a time.
One embodiment of the gasket material 50 of the present
invention is shown in Figure 3. This material comprises a core 52
of elongated, or preferably porous expanded,
W o 9s/07422 ~ 69 ~ PCT~US93/09985
polytetrafluoroethylene (PTFE) tightly wrapped in a high strength
film 54, such as a highly oriented film of expanded PTFE. It has
been determined that the high strength film wrap serves to contain
the PTFE core and prevent it from creeping even when placed under
extensive compression and heat cycling. The goal here is to
prevent substantial lateral flow of the PTFE core under stress. In
this manner a desired height-to-width ratio in the compressed
gasket is maintained such that compressive force continues to be
shared between the plates and the gasket material and is not
relieved from the gasket.
Preferably, the core material is prepared by paste extrusion
of PTFE fine powder to form a rod or beading by methods and
equipment known in the art. The paste extruded rod or beading is
then expanded to form a flexible porous structure of nodes
interconnected by fibrils by stretching it according to the process
taught in United States Patent 3,953,566 to Gore. The paste
extruded PTFE rod or beading is stretched in the longitudinal
direction an amount in the range 2:1 to 25:1, preferably an amount
in the range 3:1 to 12:1, depending on the strength and
compressibility properties desired in the core material.
Prior to wrapping, the elongated porous PTFE core material 52
has a surface shape that permits the film 54 to be wrapped in
continuous contact with the surface of the core material. For use
as a gasket in a plate and frame apparatus, preferably, the
elongated porous PTFE core material 52 is wrapped in a circular
cross-section and then the wrapped material is molded to establish
a rectangular cross-section for installation. Alternatively, the
core may also be wrapped in virtually any shape having no recessed
surfaces (e.g. rectangular, oval, square, triangular, etc.). More
complex shapes, e.g., surfaces with depressions or projections, can
be formed after the core material has been wrapped.
The elongated PTFE core may contain a particulate filler. The
term "particulate" is meant to include particles of any aspect
ratio and thus includes particles, chopped fibers, whiskers, and
the like. The particulate filler may be an inorganic filler which
includes metals, semi-metals, metal oxides, carbon, graphite, and
glass. Alternatively, the particulate filler may be an organic
filler, which includes polymeric resins. Suitable resins include,
WO 95/07422 ~69rl 12 PCT/US93/09985--
for example, polyether ether ketone (PEEK), fluorinated ethylene
propylene (FEP), copolymer of tetrafluoroethylene and
perfluoro(propylvinyl ether)(PFA), and other similar high melting
polymers.
Particulate fillers, when used, are selected to impart or
enhance certain properties in the core or wrapping film according
to the application in which the composite gasket material of the
invention will be used. For example, they can be used to impart or
enhance properties such as electrical conductivity and thermal
conductivity, and can also be used to modify compressibility and
dimensional stability properties of the composite gasket material.
Particulate fillers can be used in concentrations as high as 90
volume percent, but are more generally used in the concentration
range 10-70 volume percent.
The particulate filler and PTFE fine powder may be combined
using conventional dry mixing methods after which they can be
formed to provide the core material of the invention by the process
taught in United States Patent 3,953,566 to Gore. Alternatively,
the particulate filler may be mixed with PTFE in aqueous dispersion
and coagulated together to form a wet mixture of solids. The water
is removed from the mixture by standard drying methods and the
mixture further processed in the same manner as dry mixed
materials.
The high strength film 54 is preferably a porous expanded PTFE
film as produced by the process taught in United States Patent
3,953,566 to Gore. By stretching a paste-formed PTFE sheet in one
or more directions, a porous expanded polytetrafluoroethylene film
having high strength is produced. The high strength porous PTFE
film may be made by stretching uniaxially, either in longitudinal
or transverse direction; or biaxially, in both longitudinal and
transverse directions, sequentially or simultaneously. The film is
preferably uniaxially stretched in the longitudinal direction an
amount in the range 2:1 to 150:1, more preferably an amount in the
range 2:1 to 80:1.
Longitudinal direction as used herein indicates the planar
direction of manufacture of the film; transverse direction
indicates the planar direction normal to the direction of
manufacture.
~ 49
13
For most plate and frame applications, the preferred core comprises
an expanded PTFE with a density of 1.1 g/cc (within a range of 0.9 to 1.2
g/cc) after being wrapped and shaped, which has general pre-installed
dimensions of about 7.6-8.9 mm by 10.2-12.7 mm in cross section. For this
use, à dual film layer is used comprising an inner film and an outer film
coaxially wrapped around the core. Prior to installation on the core, the
preferred inner film is about 51 micrometer (2 mil) thick and about 25.4 mm
(1 inch) wide, and has a tensile strength of 212.7 MPa and a modulus of
elasticity at 2% strain of about 7212 MPa, the preferred outer film is about
152 mm (6 mil) thick and 38 mm ~1.5 inches) wide, and has a tensile
strength of about 19.9 MPa and a modulus of elasticity at 2% strain of about
590 MPa.
To assist in retaining this gasket material in place once installed, it is
preferred that a thin coating of adhesive be applied to the gasket material
and/or the groove 37 in the plate. The ideal adhesive comprises a
composite adhesive material comprising a pressure sensitive adhesive layer
(e.g. rubber or acrylic) applied to either side of a woven or non-woven
carrier sheet (e.g. MYLAR~) polyester). The choice of adhesive is
application specific and depends upon the chemicai and temperature
conditions under which the gasket is to be employed. The adhesive should
have good holding properties against both expanded PTFE and metal or
plastic.
Additionally, it is very desirable that the adhesive be easily removed
from the plate for reconditioning of the plate. For example, an adhesive
layer of styrene-butadiene rubber (SBR) on both sides of a MYLAR
polyester carrier sheet can be quickly removed from the plate by merely
pulling on the carrier sheet. Any adhesive residue can be wiped off the
plate with a solvent such as acetone or rubbing alcohol.
Ideally, the high strength PTFE film 54 is a colllposite film comprising
a high strength porous expanded PTFE film adhered to a thin layer of melt-
processible thermoplastic fluoropolymer. By thin is meant a thickness of 30
micrometers or less, preferably 20 micrometers or less, and more preferably
10 micrometers or less. The expanded layered composite film is produced
in the following manner.
PTFE fine powder, which may be combined with the same particulate
21697~9
14
filler materials and prepared as described above, is mixed with a
hydrocarbon extrusion aid, usualiy an odorless mineral spirit, to form a
paste. The paste is compressed into a billet and subsequently extruded
through a die in a ram-type extruder to form a coherent planar sheet. The
coherent PTFE sheet, with or without particulate filler materials, is optionallycalendered and then dried by volatilizing the hydrocarbon extrusion aid with
heat. Evaporation of the hydrocarbon extrusion aid results in the PTFE
sheet having a small degree of porosity. The resulting porous PTFE sheet
is now ready to be combined with a melt-process ~1~ thermoplastic
fluoropolymer film and the combined sheets expanded together. However,
if a highly porous ~xpanded PTFE film is desired, the porous PTFE sheet
may be preliminarily expanded by stretching it at 200 - 300C about 1.5 to 5 - ~
times its original length prior to combining it with the melt-processible
thermoplastic fluoropolymer.
The porous PTFE sheet is combined with the melt-processible
thermoplaslic fluoropolymer film by placing the melt-processible film on the
porous PTFE sheet and heating the combination to a temperature between
the melt point of the melt-processih'~ fluoropolymer and 365~C. The porous
PTFE sheet is kept under tension when heated thereby maintaining its
dimensions while the melt-processible fluoropolymer layer is combined with
it. As the porous PTFE sheet is heated to a temperature above the melt
point of the melt-processi'~le fluoropolymer layer, the melt-processible
fluoropolymer layer in contact with the porous PTFE sheet at least partially
melts and flows onto the surface of the porous PTFE sheet thereby forming
a colllposil~ precursor, i.e., a coated porous PTFE sheet ready to be
ex~anded.
The coated porous PTFE sheet may be expanded according to the
method taught in United States Patent 3,953,566 to Gore. The temperature
range at which expansion of the coated porous PTFE sheet is performed is
between a temperature at or above the melt point of the melt-processible
thermoplastic fluoropolymer layer and a temperature at or below the melt
point of PTFE. The coated porous PTFE sheet may be stretched uniaxially,
either in a longitudinal or transverse direction; or biaxially, in both
longitudinal and transverse directions, se~uentially or
AMENDED S~E~
~ W095/07422 15 2169749 PCTIUS93/09985
simultaneously. It may be stretched in one or more steps.
The coated porous PTFE sheet forms a porous expanded PTFE film
as it is stretched. The expanded PTFE film is characterized by a
series of nodes interconnected by fibrils. As the coated porous
PTFE sheet is expanded to form the high strength porous expanded
PTFE film, the melt-processible thermoplastic fluoropolymer layer
adhered to it is carried along the surface of the expanding sheet
while in a melted state, thereby becoming progressively thinner and
forming a thin melt-processible thermoplastic fluoropolymer layer
on the porous expanded PTFE sheet. The thin melt-processible
fluoropolymer layer has a thickness of 30 micrometers or less. The
thin melt-processible fluoropolymer layer preferably has a
thickness of one half, more preferably one tenth, of the
thermoplastic fluoropolymer film's original thickness. For
example, a thermoplastic fluoropolymer film originally having a
thickness of 25.4 micrometers (1 mil) could produce a thin
thermoplastic fluoropolymer layer having a thickness as low as
about 2.5 micrometers (0.1 mil) or less after expansion of the
porous PTFE sheet into the porous expanded PTFE article.
The means for heating the porous expanded PTFE sheet may be
any means for heating commonly known in the art including, but not
limited to, a convection heat source, a radiant heat source or a
conduction heat source. The conduction heat source may be a heated
surface such as a heated drum, roll, curved plate, or die. When a
conduction heat source is used as the means for heating the coated
porous expanded PTFE sheet, the uncoated surface of the sheet
should be against the conduction heat source so to prevent sticking
and melting of the melt-processible fluoropolymer layer upon the
conduction heat source.
Thermoplastic fluoropolymers which are of utility as the melt-
processible thermoplastic fluoropolymer layer have melt points of
342`C or less. They include copolymer of tetrafluoroethylene and
hexafluoropropylene (FEP), copolymer of tetrafluoroethylene and
perfluoro(propylvinyl ether)(PFA), homopolymers of
polychlorotrifluoroethylene (PCTFE) and its copolymers with TFE or
VF2, ethylene-chlorotrifluoroethylene (ECTFE) copolymer, ethylene-
tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), and
polyvinylfluoride (PVF). Thermoplastic fluoropolymers are
WO95/07422 c~ 16 PCTl 593/09985--
preferred as the melt-processible thermoplastic fluoropolymer since
they are similar in nature to PTFE, having melt points near the
lowest crystalline melt point of PTFE, and therefore are relatively
high temperature thermoplastic polymers. Thermoplastic
fluoropolymers are also relatively inert in nature and therefore
exhibit resistance to degradation from many chemicals.
When applied under sufficient temperature and/or pressure, the
melt-processible thermoplastic fluoropolymer film can act as an
adhesive to adhere the high strength porous expanded PTFE film to
the surfaces of other materials.
The expanded layered composite film 54 is wrapped on the core
of elongated PTFE 52 so that the thin layer of melt-processible
thermoplastic fluoropolymer contacts the core of elongated
polytetrafluoroethylene 52. The composite film layer is then
heated to cause the thin layer of melt-processible thermoplastic
fluoropolymer to at least partially melt and adhere to the core of
elongated PTFE core 52.
The PTFE film 54 may be wrapped on the core 52 in any desired
manner. For instance, the film 54 can be wrapped on the core 52 in
a helically so that the film forms a helical seam on the composite
gasket material. Alternatively, the high strength film 54 may be
wrapped on the core 52 in a longitudinal manner so that the film
forms a longitudinal seam on the composite gasket material.
While the film 54 may be wrapped on the core 52 by hand, it is
preferred that the wrapping is accomplished through the use of
high-speed mechanical wrapping apparatus, such as a conventional
tape-wrap machine used to wrap dielectric tape layers on
conductors. One such machine is disclosed in United States Patent
3,756,004 to Gore. The tape wrap machine applies a degree of back
tension to the high strength film as it wraps it in a helical
fashion around the core which applies a compressive force to the
core and thereby somewhat densifies the core in the process. The
degree of back tension applied to the high strength film may be
varied so that the density of the core and final dimensions of the
assembly may also be varied.
Densification (i.e. reduction in porosity) of the core results
in no change to the tensile strength or tensile modulus properties
which were developed in it by the expansion process, however,
~ wo ss/07422 21 6 9 74 ~ PCTrUS93/09985
densification has a substantial effect on the flex and compressive
characteristics of the material. By partially densifying the core
and then constraining it by wrapping it with the high strength
film, control over the amount of deformation required to densify it
fully when in service can be exercised. In other words, a
composite gasket material is produced such that a compressive load
sufficient to provide an excellent seal can be applied to the
composite gasket material with relatively little movement together
of the sealing surfaces. Thus, the composite gasket material of
the invention can provide a much thicker gasket that covers a much
smaller sealing surface area than can be obtained from existing
PTFE gasket materials having lower density or strength.
Alternatively, other means for densifying the expanded PTFE
core can be used prior to the application of the high strength film
to the PTFE core. Other means for densifying porous expanded
polytetrafluoroethylene include compression by platen presses,
grooved or flat calender rolls, and reducing or forming dies.
The high-strength porous expanded PTFE film wrapped upon the
elongated PTFE core imparts a substantially increased measure of
circumferential strength and restraint to the PTFE core. The
result is a composite gasket material with a reduced tendency to
creep (i.e. a gasket material that has much greater resistance to
becoming thinner and wider under steady compressive loads when
compared to a PTFE gasket without the high strength wrap).
Alternatively, a second porous expanded PTFE film, which
likewise may be coated with a melt-processible thermoplastic
fluoropolymer as described above, may be wrapped upon the first
high strength film. The second wrapped film can have tensile
properties which provide additional strength and creep resistance
to the composite gasket material or, alternatively, can have lower
tensile strength and tensile modulus properties than the first
wrapped film in order to enhance sealing surface conformability of
the gasket material.
The following examples disclosing processes and products
according to the present invention are illustrative only and are
not intended to limit the scope of the present invention in any
way.
WO 9S/07422 ~ PCTIUS93/09985
18
TEST DESCRIPTIONS
TENSILE TEST
To determine the tensile properties of the high strength
porous expanded polytetrafluoroethylene film, a 2.54 cm (1.0 inch)
wide by 20.3 cm (8.0 inches) long sample of the film is obtained.
Thickness of the film is determined with a snap micrometer gauge
and width of the film is determined with a linear gauge. A
constant rate-of-jaw-separation machine (Instron testing machine,
Model 1122) is used to test samples to break. The gauge length of
the specimen is 10.16 cm (4.0 inches). The strain rate employed is
2.54 cm/min (1.0 inch/min). Samples are tested to break. The
tensile modulus at 2% extension and maximum stress are calculated
and recorded as described in ASTM Standard Test Method D 882-91.
A population of five to eight samples is averaged to give each
value listed herein.
GASKET FLOW T~ST
Two sections of gasket material each 12.7 cm (5 inches)in
length are obtained. The samples are mounted, in parallel
alignment approximately 20 cm (8 inches) apart, between two 25.4 cm
(10 inches) square rigid flat platens. An initial compressive load
of 8.01 kN/linear cm (1800 lbf/linear in) is applied to the
samples. The samples remain compressed for a period of 10 minutes
at a temperature of 200`C. The compressive load is reduced by
creep of the samples during the 10 minute compression period. No
effort is made to maintain a constant load.
At the end of the compression period the samples are recovered
and the distance around the perimeter (Pf)of the compressed sample
is measured (in a direction perpendicular to the long axis of the
sample). The Pf measurement is compared to an initial perimeter
measurement (Pj) of the sample taken in like manner prior to
testing and the increase reported as Gasket Flow (GF) according to
the formula:
GF, percent = (Pf - Pj / Pj) x 100
-
~ w o ss/07422 ~ ~ ~ 7~ PcTrusg3/0998s
ExamPle 1
A composite gasket material of the instant invention was
produced in the following manner:
A 0.0127 mm (0.5 mil) FEP tape (50A available from E. I.
duPont de Nemours & Co.) was laminated to a porous PTFE sheet
- through the introduction of enough heat to melt and attach the FEP
sheet to the porous PTFE sheet as follows:
The combined sheets were first longitudinally stretched ar.
amount 1.5:1 at a temperature of approximately 330`C over a heated
curved platen, and then further longitudinally stretched an amount
1.5:1 in a second heated zone at a temperature of approximately
340`C, thus forming a high strength composite film having a total
amount of expansion of 2.25:1. The composite film was subsequently
heated at a temperature of 335`C in a third heated zone at a
stretch ratio of 1:1 so that no additional expansion occurred.
Subsequently, the composite film was slit lengthwise and
helically wrapped upon a core of porous expanded
polytetrafluoroethylene beading that had not been previously
subjected to an amorphous locking process. The high strength
composite film was wrapped so that 1/2 of the film was overlapped
on the previously applied wrap.
Prior to wrapping the porous expanded polytetrafluoroethylene
beading had a density of about 0.3 g/cc and an outside diameter of
17.8 mm (0.70 inch). Back tension was applied on the composite
film so that when the wrapping of the beading was completed, the
outside diameter of the wrapped beading was reduced to 12.2 mm
(0.48 inch).
The wrapped beading was passed through an oven at about 405`C
to amorphously lock the high strength expanded
polytetrafluoroethylene film and to melt the FEP layer, thus
adhering the composite film to the porous expanded
polytetrafluoroethylene beading.
A second layer of the high strength composite film was wrapped
upon the wrapped gasket material described above and amorphously
locked as the previously applied first layer. Back tension was
applied on the composite film so that when the wrapping of the
beading was completed, the outside diameter of the wrapped beading
was reduced to 11.7 mm (0.46 inch).
WO 9S/07422 ~69~ 20 PCT/US93/09985 ~
The result was a composite gasket material of the instant
invention.
Tensile properties of high strength composite film prepared as
described in Example 1 were tested as described above. Tensile
strength was 19.87 MPa (2882 psi) and 2% secant tensile modulus was
589.7 MPa (85520 psi). The composite gasket material of Example 1
was tested by the Gasket Flow Test described above and the results
shown in Table 1.
ExamPle 2
A second example of the composite gasket material of the
instant invention was produced in the following manner:
A 0.0254 mm (1.0 mil) FEP tape (100 A available from E. I.
duPont de Nemours & Co.) was laminated to a porous PTFE sheet,
which had been preliminarily stretched an amount 1.9:1 at a
temperature of about 250`C, through the introduction of enough heat
to melt and attach the FEP sheet to the porous PTFE sheet as
follows:
The combined sheets were first longitudinally stretched an
amount 2:1 at a temperature of approximately 330`C over a heated
curved platen, and then further stretched an amount 10:1 in a
second heated zone at a temperature of approximately 340`C, thus
forming a high strength composite film having a total amount of
expansion of about 38:1. The composite film was subsequently
heated at a temperature of 335`C in a third heated zone at a
stretch ratio of 1:1 so that no additional expansion occurred.
Subsequently, the composite film was slit lengthwise and
helically wrapped upon a core of porous expanded
polytetrafluoroethylene beading that had not been previously
subjected to an amorphous locking process. The high strength
composite film was wrapped so that 1/2 of the film was overlapped
on the previously applied wrap.
Prior to wrapping the porous expanded polytetrafluoroethylene
beading had a density of about 0.3 g/cc and an outside diameter of
17.8 mm (0.70 inch). Back tension was applied on the composite
film so that when the wrapping of the beading was completed, the
outside diameter of the wrapped beading was reduced to 13.7 mm
(0.54 inch).
WO 95/07422 21 ~16C,~ PCT/US93/09985
The wrapped beading was passed through an oven at about 405`C
to amorphously lock the high strength expanded
polytetrafluoroethylene film and to melt the FEP layer, thus
- adhering the composite film to the porous expanded
polytetrafluoroethylene beading.
- A second layer of the high strength composite film was wrapped
upon the wrapped gasket material described above and amorphously
locked as the previously applied first layer. Back tension was
applied on the composite film so that when the wrapping of the
beading was completed, the outside diameter of the wrapped beading
was reduced to 13.3 mm (0.52 inch).
The result was a composite gasket material of the instant
invention.
Tensile properties of high strength composite film prepared as
described in Example 2 were tested as described above. Tensile
strength was 173.7 MPa (25200 psi) and 2% secant tensile modulus
was 5838 MPa (846700 psi). The composite gasket material of
Example 2 was tested by the Gasket Flow Test described above and
the results shown in Table 1.
ExamPle 3
A third example of the composite gasket material of the
instant invention was produced in the following manner:
A 0.0254 mm (1.0 mil) FEP tape (100 A available from E. I.
duPont de Nemours & Co.) was laminated to a porous PTFE sheet,
which had been preliminarily stretched an amount 1.9:1 at a
temperature of about 250`C, through the introduction of enough heat
to melt and attach the FEP sheet to the porous PTFE sheet as
follows:
The combined sheets were first longitudinally stretched an
amount 2:1 at a temperature of approximately 330`C over a heated
curved platen, and then further stretched an amount 20:1 in a
second heated zone at a temperature of approximately 340`C, thus
forming a high strength composite film having a total amount of
expansion of about 76:1. The composite film was subsequently
heated at a temperature of 335`C in a third heated zone at a
stretch ratio of 1:1 so that no additional expansion occurred.
Subsequently, the composite film was slit lengthwise and
WO 95/07422 ~,~69~ 22 PCT/US93/09985
helically wrapped upon a core of polytetrafluoroethylene beading
that had not been previously subjected to an amorphous locking
process. Prior to wrapping the porous expanded
polytetrafluoroethylene beading had a density of about 0.3 g/cc and
an initial outside diameter of 17.8 mm (0.7 inch).
The high strength porous expanded polytetrafluoroethylene
film in the form of the composite film was wrapped so that 1/2 of
the film was overlapped on the previously applied wrap. Back
tension was applied on the composite film so that when the wrapping
of the beading was completed, the outside diameter of the wrapped
beading was reduced to 12.2 mm (0.48 inch).
The wrapped beading was passed through an oven at about 405`C
to amorphously lock the high strength expanded
polytetrafluoroethylene film and to melt the FEP layer, thus
adhering the composite film to the porous expanded
polytetrafluoroethylene beading.
A second layer of the high strength composite film was wrapped
upon the wrapped gasket material described above and amorphously
locked as the previously applied first layer. Back tension was
applied on the composite film so that when the wrapping of the
beading was completed, the outside diameter of the wrapped beading
was reduced to 11.9 mm (0.47 inch).
The result was a composite gasket material of the instant
invention.
Tensile properties of high strength composite film prepared as
described in Example 3 were tested as described above. Tensile
strength was 212.7 MPa (30850 psi) and 2% secant tensile modulus
was 7212 MPa (1046000 psi). The composite gasket material of
Example 3 was tested by the Gasket Flow Test described above and
the results shown in Table 1.
Example 4
A fourth example of the composite gasket material of the
instant invention was produced in the following manner:
A 0.0254 mm (1.0 mil) FEP tape (100 A available from E. I.
duPont de Nemours & Co.) was laminated to a porous PTFE sheet,
which had been preliminarily stretched an amount 1.9:1 at a
temperature of about 250`C, through the introduction of enough heat
WO9SI07422 23 6~ 9 PCT/US93/09985
to melt and attach the FEP sheet to the porous PTFE sheet as
follows:
The combined sheets were first longitudinally stretched an
~ amount 2:1 at a temperature of approximately 330`C over a heated
curved platen, and then further stretched an amount 10:1 in a
- second heated zone at a temperature of approximately 340`C, thus
forming a high strength composite film having a total amount of
expansion of about 38:1. The composite film was subsequently
heated at a temperature of 335`C in a third heated zone at a
stretch ratio of 1:1 so that no additional expansion occurred.
Subsequently, the composite film was slit lengthwise and
helically wrapped upon a core of porous expanded
polytetrafluoroethylene beading that had not been previously
subjected to an amorphous locking process. Prior to wrapping the
porous expanded polytetrafluoroethylene beading had a density of
about 0.3 g/cc and an initial outside diameter of 17 . 8 mm (0. 7
inch).
The high strength porous expanded polytetrafluoroethylene
film in the form of the composite film was wrapped so that 1/2 of
20 the film was overlapped on the previously applied wrap. Back
tension was applied on the composite film so that when the wrapping
of the beading was completed, the outside diameter of the wrapped
beading was reduced to 12.2 mm (0.48 inch).
The wrapped beading was passed through an oven at about 405`C
25 to amorphously lock the high strength expanded
polytetrafluoroethylene film and to melt the FEP layer, thus
adhering the composite film to the porous expanded
polytetrafluoroethylene beading.
The result was a composite gasket material of the instant
invention.
Tensile properties of high strength composite film prepared as
described in Example 4 were tested as described above. Tensile
strength was 212.7 MPa (30850 psi) and 2% secant tensile modulus
was 7212 MPa (1046000 psi). The composite gasket material of
Example 4 was tested by the Gasket Flow Test described above and
the results shown in Table 1
9,~9 . .
24
Comparative F~ample 1
For comparative purposes a section of commercially available
wrapped porous polytetrafluoroethylene gasket material, Inertex 318" Valve
Stem Packing, was obtained and tested as described in the examples
5 above. A section of the tape wrapped around the core was unwound and
sarr~ples were given the tensile test as described above except that the
sample width was 1/2 inch. The results are also shown in Table 1.
T~RI F 1
Film Film
Tensile Modulus ~ Gasket
Strength 2% strain Flow
Fxample (MPa) iMPa) r%!
19.9 590 70
2 173.4 5838 1 5
1 5 3 212.7 7212 29
4 173.4 5838 40
Comp. Ex. 1 6.6 79 137
As should be evident from the above examples, the basic film
wrapped material of the present invention provides a very distinct
20 improvement over PTFE sealing materials and thus can be effectively
utilized as a plate and frame sealing material. However, as has been
mentioned, this material continues to have some deficiencies. One very
limiting characteristic of this basic material is that it must be compressed
approximately 3:1 in the plate and frame apparatus during assembly.
25 Often times, the frame that the plates are compressed between is not long
enough to fit all of the plates that are gasketed with this much thicker
sealant, requiring burdensome compression in batches.
More debili~ting, however, is the problem of plate shifting. There is
significant "travel" or compression of the plates when the plate pack is
- 30 assembled together with thicker wrapped PTFE sealant. If the plate and
frame device does not have a suitable guide bar to provide "optimal"
AMENGED S~ T
l69~ig .
25
packing (i.e. rigidly fixed to prevent slidihg or bending of the plates), the
plates sealed with the basic wrapped gasket material are prone to shifting
and sliding. This is further aggravated by the high compressive forces that
are required to adequately compress the gasket If even one plate shifts
S out of alignment, a leak will form. As a result, use of the basic wrapped
gasket material, instdlled in this fashion, is limited to those applic~lions
where risk of debili~ .,g plate movement is mini",al. Examples of such
applications include: plate and frame apparatus with thicker plates;
"optimal" plate and frame apparatus with suitable guide bar designs; and
plate and frame apparatus with tighter control over plate movement (e.g.
those with smaller plates, less than 100 plates, and/or suffcient bolting
capacity).
In order to address these limitations, a further embodiment of the
present invention is shown in Figure 4. As has been noted, even with a
high strength film wrap, the gasket material of the present invention
undergoes a significant decrease in thickness before reaching sufficient
density and compression to assure creep stability. Once formed in place,
the contours of the gasket will achieve a pattern complementary to the
texture of the plate to which it is attached. One such pattern is shown on
the gasket 56 in Figure 4, comprising a series of projections 58 and
indentations 60 corresponding to the corrugated texture of a plate to which
the gasket was attached. From a typical starting density of 1.0 to 1.3 g/cc,
the fully co",pressed gasket normally achieves a density of about 1.8-1.9
g/cc.
It has been determined that i"~lalldlion of the gasket material of the
present invention can be greatly enhanced by pre-compressing the gasket
material prior to installation. In its simplest form, the gasket material is
mounted on a plate or other mold containing the desired texture and then
compressed under pressure to impart the desired contours to the material,
such as is shown in Figure 4. Once formed in this manner, the gasket
material and plate can then be installed, or the material may then be
removed from the mold and installed on a similarly textured plate.
One method to perform this procedure employs a hydraulic press
capable of generating a compressive force of about 32 to 45 megagrams
(35 to 50 tons) or more. The gasket material is installed on one plate, or
AMENDcj S,~LET
21 69 7~ r
26
between two plates, in a conventional manner and then compressed to
impart at ieast an initial reduced thickness to the material. Spacer bars or
similar stops should be provided on either side of the plate to prevent the
press from over compressing the material or damaging the plate/mold.
For a gasket material with an expanded PTFE core and a high strength
film wrap of PTFE, a typical compression procedure comprises applying
approximately 214 kg pe~ linear centimeter (1,200 Ibs per linear inch) of
force to the seaiant for a period of approximately 5 seconds, with or
without heat. Less force is required if heat is applied to the gasket
material, such as through use of a heat mold plate. Generally, the force
applied to a core of expanded PTFE wrapped with an expanded PTFE film
should be at least 89 to 143 kg per linear centimeter(500 to 800 Ibs per
Iinear inch).
Ideally the gasket material will be compressed enough to decrease
significantly the amount of travel experienced during installation and to
provide a "footprint" of the adjoining plate on to the gasket to help prevent
shiFting of the plates. However, densification should not be so great that
further compression and fitting of the gasket cannot occur during actual
installation. As such, a density of about 1.6 to 1.8 glcc should be sought.
Shown in Figure 5 is a cross sPction of a gasket material 62 which
has been partially compressed in this manner. In this case, the gasket 62
has been compressed on its lower face 64 to impart a series of
indentations 66 to the gasket corresponding to corrugated texturing of a
mounting plate. The gasket has not been fully densified and in this case
its top surFace 68 remail)s planar, allowing for further customized fitting
once i"s~alled This material has been pre-compressed to approximately
50% of its original Ll, :k.less.
To create an even better seal once installed, the embodiment of
- Figure 5 also includes a confor")able sealing layer 70 which is installed on
the gasket 62 after pre-compression. Preferably, the conformable sealing
layer 70 comprises a strip of low density, expanded PTFE s~aling tape
with thickness of about 0.5 to 1.0 mm. A suitable tape including a self-
adhesive layer on one side, permi~ting quiclc and easy installation, is
commercially available from W. L. Gore ~ Associates, Inc., of Elkton,
Maryland, under the trademark GORE-TEX(19 Gasket Tape. This tape
- AMEI~DED S~tEET
I 21 69 7~
27
becomes a conformable member of the composite sealant once installed
and fully compressed in place. At this point, there is provided a dense
base material which is adequately seated against a lower plate to which it
is pre-molded, and a low density (e.g. 0.4 g/cc) top sealing layer which
5 seats easily into a complementary plate during installation.
Figure 6 is illustrative of the composite gasket 62 of Figure 5 once
installed in place. As can be seen, the sealing layer 70 compresses and
densifies in close contact with the gasket material 62, while filling in slight
differences between the plates (e.g. the raised areas of the sealing layer
70 shown in Figure 6). The sealing layer 70 also creates a seal between a
dense base gasket material and an adjoining plate with far less force than
would be required without such a layer.
The function of the gasket tape strip on the slightly compressed
gasket is at least threefold: -
1. It allows a seal to be created with much lower bolt force such that
hot fluids can be held within the plate and frame apparatus to soften the
gasket before final compression of the final seal with the compression
bolts. Much higher bolt forc~s are required to effect this initial seal without
the gasket tape strip, and, thus, would promote plate shifting.
Furthermore, some plate and frame devices may not c~r~hle of achieving
enough sealing pressure without the aid of a conformable layer;
2. It allows the adjoining plate to imprint its "footprint" into the
sealant at low bolt forces, thus, again mini",i,i,)g the potential for plate
shifting; and
3. It allows greater conformability of the seal assembly.
Assembly of this embodiment of the present invention in "optimum"plate and frame apparatus, which are not prone to plate shifting, involves
uniform tightening of the compression bolts to draw the unit together
30 slowly and compress each gasket between the plates approximately 2.3
mm (0.09"). The sealant is seated against the plates and densified to
required levels to insure long-term mechanical reliability.
Assembly of the gasket in "non-optimum" plate and frame apparatus
(i.e. those that are prone to plate shifting) involves uniform tightening of
35 the compression bolts to compress the plates toward each other
hl~lEI~DED SltEET
~ ~1 69 7~
28
approximately 0.5 mm (0.02") to initially seat the sealant. Since much less
force is required to compress each gasket 0.5 mm (0.020"), shifting
problems are minimized. At this point, the plates are seated against the
gaskets, yet the gasket core still requires further compression to insure
long-term mechanical reliability. To prevent plate shifting during this
subsequent compression, hot fluid should be fed into the plate and frame
apparatus to soften the sealant. The bolts are then uniformly tightened
until the gasket is adequately compressed. The bolt force required to
compress the gaskets the finai 1.8 mm (0.070") is much lower with the
sealant warm than when at room te",pe(dlure.
In all forms of the present invention the gasket material may be
provided in a variety of forms to solve specific sealing needs. Figure 7
illustrates a cord gasket material 72, with typical dimensions of 12.7 mm
wide, 7.6 mm thick. The cord gasket material can be provided in
continuous lengths, such as on a spool, to allow it to be cut to size for
particular i":,talld~ion demands.
While the cord 72 can be cut to provide specialized sealing, such as
around ports 28a, 30a in the plate of Figure 2, for most plate and frame
uses the cord gasket material 72 is joined to itself to form a continuous
loop 74 like that shown in Figure 8. The joint 76 is then connected
together by simply splicing the ends. One such splicing technique
comprises cutting the ends with a 25.4 mm (1 inch) minimum scive cut,
joining the ends together, w, appil1g the joined ends with a tape (which
ideally should be similar or identical to the film w, dppi. 19 the gasket
material), and then heat setting the tape in place with a mold press.
The flexibility of the PTFE core and the resistance of the gasket
material to cold flow allows the loop 74 of gasket material to be shaped
and ,etairled in various posilions for ins~alldlion in a plate and frame
apparatus. One such shaped position is shown in Figure 9.
The gasket material of the present invention provides significant
improvements in the durability, longevity, chemical and thermal resistance,
and ease in installation of gasket material for use in plate and frame
devices. Moreover, the nature of expanded PTFE allows the material to
release very easily and intact from the plate, even a~ter it has been
installed under heavy pressure for a
AMENDED SltFET
21 ~ 7~g
WO 95/07422 PCT/US93/09985
29
.
long period of time.
For most applications, the gasket material is removed by
merely prying the material away from the plate and pulling the rest
~ of the material intact away from the plate. Residue adhesive, if
employed, can be stripped by removing an adhesive carrier sheet
and/or by wiping with a suitable solvent. As a result of the
simplicity of this procedure, plates can be almost instantaneously
cleaned of old gasket material and adhesive in a matter of minutes,
compared to an average of over an hour of clean up time per plate
with conventional gasket materials.
While particular embodiments of the present invention have
been illustrated and described herein, the present invention should
not be limited to such illustrations and descriptions. It should
be apparent that changes and modifications may be incorporated and
embodied as part of the present invention within the scope of the
following claims.
