Note: Descriptions are shown in the official language in which they were submitted.
JO 96109879 ~ ~ ~ 7 ~ 6 9 PCTIUS94110942
A ~U._I~OIil'~ lJU~ E A~D
A ~ET~OD FOR FORMING SAME
This application is a continuation-in-part of
United States Patent Application Number 08/038,257,
which is incorporated herein by ref erence .
TEC~NICAL FIELD OF THE INVENTION
The present invention relates to a method of
forming a composite structure. More specifically,
the invention relates to a method for bonding a
porous medium and a support and drainage medium to
the surf ace of a substrate .
BACKGROUND OF THE INVENTION
Porous media are bonded to the surf ace of a
substrate for a wide variety of purposes. For many
applications, such as the formation of a resilient
or acoustically absorptive sur~ace, neither the
nature of the material which accomplishes the bond
nor the depth to which the bond penetrates the
porous facing is critical.
For a wide range of other applications, such as
the purif ication of pharmaceutical f luids or the
removal of bacteria from foods, e.g., milk and beer,
bonded assemblies which include finely porous filter
media secured to a solid substrate are used. Secure
bonding of the porous medium to a solid substrate is
particularly nP~ P~:S~y when the porous medium is
exposed during service to very high shear forces
which would disrupt an unsupported membrane.
Filtration applications also typically require
that the porous medium be bonded to the substrate
such that the fluid passing through the membrane is
provided with passageways through which it can f low
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as it issues from the membrane. Typically, the
passageway6 are grooves cut or cast into a plane
surface, the grooves being configured to drain
collectively into a central outlet port, which the
user connects to a receiver f or the f iltrate .
The porous medium may be secured to the
substrate by applying a layer of a viscous adhesive
to the substrate and then contacting the porous
medium with the adhesive layer. The use of a third
component, i.e., the adhesive, which could leach
into the filtrate, is very undesirable for many of
the applications described above. In addition, the
adhesive can often blind a substantial number of the
pores and alter the permeability of the medium.
Bonded assemblies may also be produced by
contemporaneously forming and integrally securing a
porous medium to the surface of a substrate. This
method, however, is 6everely limited by the
reguirement that the porous medium be precipitated
from a liquid suspension and secured to the
substrate in a single step: Some porous media,
which may be employed effectively in filter
applications, are not formed from liquid suspension.
For example, polytetrafluoroethylene (e.g. Teflon
TFE) is typically made as a powder, which is then
extruded to form a sheet, and the sheet is biaxially
stretched to f orm a porous membrane .
A filter membrane may also be secured to a
substrate by a method which involves the application
of a solvent to which the filter membrane is inert,
but which dissolves the substrate. The filter
membrane is saturated with the solvent, and then
contacted with the substrate. The contact of the
saturated membrane with the substrate dissolves a
portion of the substrate, which is then integrally
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~96l09879 PCrlUS94/10942
I
secured to the membrane after the solvent is
removed . This method has the severe f ault that it
may be ~CLL~ -ly difficult to maintain a uniform
distribution of solvent throughout the f ilter
membrane at the time it is applied to the substrate.
Simple dipping, or any procedure involving
manipulation of the wet membrane, invariably leaves
more solvent in some portions of the membrane than
in others. As a result, an excessively thick bond
may form in some areas of contact, while in other
areas the bonding between the membrane and the
substrate may be inadequate.
For many if not most applications, it is
important that the membrane be positioned precisely
at a specif ic location on the substrate . This is
difficult to do, because the prewetted membrane
quite generally is limp, i . e. has no rigidity, and
this difficulty is c, _u-.ded by the rapid
evaporation of the solvent, such that a significant
loss of solvent can occur in a few seconds.
Further, in the process described above, the
solvent is typically allowed to evaporate during the
dissolution and bonding process. The space within
any grooves, which may be present in the substrate,
is rapidly saturated by the vapor from a small
fraction of the solvent and, thus, the bulk of the
evaporation takes place at the exposed surface of
the filter membrane. As solvent evaporates from the
exposed surface, solvent from the rr~~;n~l~r of the
filter membrane migrates by capillarity through the
membrane to the exposed surface. Accordingly, the
solvent originally located in contact with the
substrate, which contains dissolved substrate in
solution, also evaporates from the exposed membrane
surface. In the process, dissolved substrate may be
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WO 96/~9879 PCrlUS94/10942
~774~5
deposited at the exposed surface of the filter
membrane. This is highly undesirable, as the pores
of the membrane may be at least partially clogged by
the deposited 6ubstrate, locally altering the pore
size and decreasing the peL --h; 1 ity of the
membrane .
Yet another problem exists with certain
supported membranes or porous media. For example,
the grooves or ~ h~nnPl q of some supported media tend
to be relatively wide. In such instances, the
portions of the porous medium superposed over the
rhi~nnPl ~ Of the substrate are unsupported. While
relatively thick porous media may not be
significantly affected in those regions which are
u~.~u~u, I ed, when thinner media or membranes are
used, large pressure differentials across the porous
medium tend to flex and/or distort the medium in the
regions of the ~-h~nnPl q. In those instances in
which the pressure differentials across the porous
medium are very high and the tensile strength of the
porous membrane is low, the unsupported portions of
the medium may not have sufficient pressure pulse
resistance to retain its structural integrity and
the porous medium may be breached. Although in some
instances a thicker porous medium with a higher
tensile strength may be employed, such an option may
not exist for many media. Furthermore, where
thicker, stronger media are available, the pressure
drop across the thicker porous medium may be too
high for the intended application. Another
alternative is to decrease the width and increase
the number of grooves in the surf ace of the
substrate in an attempt to obtain the same volume in
the grooves. ~owever, there are physical
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0 96109879 PCr/US94110942
21~7~69
limitations as to the number and depth of the
grooves which can be used.
.
` SllMN~RY OF TI~E INVENTION
An obj ect of the present invention is to
OV~ the drawbacks described above.
In a first aspect of the present invention, a
composite structure comprises a porous medium, a
substrate provided with at least one drainage
pathway, and a support and drainage medium
sandwiched between the porous medium and the
substrate. The porous medium, the support and
drainage medium and the substrate are bonded f ree of
any adhesive.
The ~mho~ i r^ntS of the invention represent a
considerable advance in the state of the art. As
indicated above, conventional elements are formed by
processes which either may not permit a preformed
porous medium to be secured to a substrate or, in
securing the porous medium to the substrate, may
substantially alter the porosity or permeability of
the medium.
The present invention also provides a unif ormly
bonded structure and a method of producing a bonded
structure which includes only the filter membrane,
the support and drainage medium and the substrate,
thereby avoiding the use of an adhesive component
which could leach into filtrate during use.
Further, this invention affords a method of
integrally securing a preformed porous medium, such
as a polytetrafluoroethylene membrane, to a support
and drainage medium and in turn to a substrate. In
addition, the present invention provides a method of
integrally securing a porous medium to a support and
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Wo 96109879 PCT/US94110942
~ ~7~9
,
`:
drainaqe medium and the latter to a substrate in a
manner that does not alter the pore structure or
substantially decrease the permeability of the
medium. This invention also permits a f ilter
membrane to be bonded to a support and drainage
medium and a substrate with minimal obstruction of
edgewise f low through those portions of the membrane
immediately adjacent the bonds. Thus, ~onding
between adjacent layers is uniform and the porous
medium is precisely located relative to the
substrate. In addition, blinding or blockage of the
pores of the porous medium is minimized and
significantly less than conventional elements. In
addition, the composite structures of the present
invention demonstrate is significantly i uve:d
edgewise f low of f luids through the media and
resistance to distortion or tearing when exposed to
large pressure differentials in either direction
across the porous medium, either in continuous or
pulsed form. As a result, the composite structures
of the present invention may be used in high shear
applications, such as in dynamic filtration and
cross-f low f iltration .
Another aspect of the present invention
provides a method of forming the composite structure
of the present invention in which a support and
drainage medium is bonded to both a substrate and a
porous medium on opposite surfaces of the support
and drainage medium. The process includes
contacting a porous medium with a first surface of a
support and drainage medium and contacting the
opposite surface of the support and drainage medium
with a surface of a substrate, where the surface of
the substrate has at least one f luid pathway .
Preferably the porous medium, substrate surface, and
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~774~
support and drainage medium are dry when they are
placed in contact. A bonding composition is then
introduced to the ~ t assembly formed from the
substrate, the support and drainage medium, and the
porous medium. The bonding composition at least
slightly dissolves a portion of the substrate
surface without dissolving the porous medium or the
support and drainage medium and the dissolved
portion of the substrate is contacted with the
support and drainage medium and the porous medium.
The bonding composition is then withdrawn,
preferably, in a direction opposite to that in which
it was introduced to the component assembly to form
an adhesive-free bond between the porous medium, the
support and drainage medium, and the substrate. A
preferred embodiment inrl~ldoc removing the bonding
composition from the component assembly preferably
by means of a vacuum.
In the methods of the present invention, the
bonding composition preferably includes two chemical
specie5 having controlled relative vapor pressures.
The methods generally include impregnating the
porous and support and drainage media with a bonding
composition comprising a first chemical species,
which is a solvent for the substrate, and a second
chemical species which is not a solvent for the
substrate (non-solvent species). Preferably neither
the f irst chemical species nor the second chemical
species is a solvent for the porous or support
media. The chemical species are preferably selected
such that when the bonding composition is removed by
a vacuum, the first chemical species evaporates
faster than the second chemical species.
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WO 96/09879 PCTIUS94/10942
2177~6~
These above described and other objects and
advantages o~ the present invention will be apparent
from the description of the invention which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an oblique view of a section of a
substrate of the present invention cut perpendicular
to grooves in the substrate surf ace .
Figure 2 is an oblique view of a section of a
composite structure of the present invention
including the section of the substrate of Figure 1.
DETAILED DESCRIPTION OF T~E INVENTION
The present invention provides a porous,
supported composite structure and a method for
bonding a porous medium and a support and drainage
medium to a substrate to f orm such a supported
composite structure. Preferably, each of the porous
medium and the support and drainage medium comprise
only a single layer, although each medium may
comprise two or more layers. More particularly, the
present invention is directed to a method of
integrally bonding a porous f ilter medium to one
surface of a support and drainage medium and the
opposite surface of the support and drainage medium
to a surface of a substrate and to a supported
composite structure produced thereby. Preferably
the bond between the porous medium, the support and
drainage medium, and the substrate is free of any
adhes ive .
The substrate may be any member having
3 0 suf f icient structural integrity to support the
porous medium and the support and drainage medium
and which can be bonded thereto by the method of the
pre~ent inventlon. ~ u~str,~te, which provid~
~O 96/09879 2 ~ 7 7 4 ~ 9 PCrrUS94/10942
support f or and def ines the conf iguration of the
support and drainage medium and the porous medium,
may be flexible, semi-flexible or rigid. Further,
the substrate inrl~ a material which at least
slightly dissolves in and is solvated by (i.e.
absorbs and/or is softened by) the bonding
composition. For example, the substrate preferably
includes a polymeric material, such as a
polyethersulfone, a polysulfone or a polyamide.
The substrate may be a solid structure. If the
purpose of the composite structure is to act as a
filter element, the substrate may include a
r--h~n; ~In or a pathway for draining fluid away from
the substrate surface. The drainage -^h~n; ~:m or
pathway may include one, but preferably a plurality
of rh;~nnF~ passages, or interconnecting pores in
the substrate surface. In the illustrated
~mho~;--nt, the drainage ---h~n; c:r includes at least
one groove and preferably a plurality of grooves,
which may be interconnected, in the substrate
surface. Typically, when used in a filter element,
the grooves are in fluid communication with a
filtrate outlet port, which may be coupled to a
receiver for filtrate passing through the porous
medium. The filter outlet port may also be used in
a preferred method of forming the composite
structure to introduce bonding composition to the
assembled substrate and media.
Preferably, the substrate has at least one
planar surface in which the groove(s) are formed.
The grooves may be spaced from the edges of the
substrate, ~ f;n;n~ flats on the planar surface
between the grooves and the edges . In a pref erred
embodiment, the substrate is formed as a sheet or
plate having opposing planar surfaces. The
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WO 96/09879 PCr/US94/10942
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groove(s) or recessed portion(s~ may be formed in
one or, preferably, both planar surfaces with each
grooved surface bonded to one surface of a support
and drainage medium and a porous medium bonded to
the opposite surface of the support and drainage
medium .
The porous medium is pref erably a porous
structure that may be employed as a f ilter medium
and is preferably formed from a polymeric resin but
may include any material capable of f orming a porous
structure. The substance(s) from which the porous
medium, as well as the support and drainage medium
are formed are sufficiently chemically dissimilar to
the substrate material in a particular composite
structure so as to have no signif icant solubility in
the bonding composition employed in the present
invention. Thus, the same substance is most
preferably not used to form a substrate and a
support and drainage medium and/or a porous medium
in the same composite supported structure.
The porous medium may comprise any one of a
number of materials, including f ibrous media made by
a variety of means including melt blowing,
Fourdrinier deposition, or air laying. The porous
medium may also comprise porous membrane media made
by a variety of means including (i) introducing a
solution of a resin in a relatively good solvent
into a solution which is a relatively poor solvent
for the resin, e.g., as described in U.S. Patent
4,340,479, (ii) by preparing a solution of a resin
in a mixture of two solvents, one of which is a
better solvent with a relatively higher vapor
:.ULe compared with the second solvent, and
allowing the solvents to evaporate, thereby forming
a porous film, or (iii) as in the case of Teflon
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/0 96/09879 PCTIUS94110942
~7 469
membranes, by precipitating a suspension of f inely
particulate PTFE, which is then hot compressed to
f orm a sheet in which the particles are bonded to
each other, followed by stretching the sheet to form
the membrane.
In a preferred ~mhorl;r-ntl the porous medium
may comprise a miC:LU~ULUUS filter medium, such as a
mi~:L~l~ULUUS fibrous matrix or a mi~:LU~ULUUs
membrane. The method of this invention is
particularly useful for securing a mi~:LU~ULUUS
f ilter medium to a substrate.
Exemplary porous media may include
f luoropolymers, polyamides, polyethersulf ones,
acrylic polymers, polyesters, or cellulose ester.
Preferably, the porous medium includes
po ly ( viny l idene di f luor ide ), p o lytetraf luoroethy l ene
or a nylon, such as nylon-46, nylon-6, nylon-66 or
nylon-610. For example, miuLu~oLu~ls filter media
may be prepared using polyamides following the
procedure of U.s. Patent 4,340,479, using
poly(vinylidene difluoride) following the procedure
of U. S . Patents 4, 341, 615 and 4, 774 ,132, using
polytetrafluoroethylene following the procedure of
U. S . Patents 3, 953, 566 and 4, 096, 227, or using a
polyethersulfone following the pLuceduL~ of
copending U.S. application Serial No. 07/882,473.
When used as filter media, the porous media of
the present invention are not restricted to any
particular pore sizes but will depend on the
particular materials being filtered. However, the
porous media preferably have pore ratings ranging
from about 10 nanometers to about lo ~m or more,
preferably from about o. 04 ~m to about 5 ~m.
The support and drainage medium or layer is
preferably formed from a very open material,
Wo 96/09879 PCr/US94/10942
2~7469
allowing fluid to flow laterally and to uniformly
distribute the fluid acros6 the downstream surface
of the porous medium. Thus, the support and
drainage layer pref erably has a very low edgewise
flow resistance. The support and drainage medium to
some extent supplements and provides similar
functions to some of those provided by the
substrate. Thus, the support and drainage layer
provides the ability to conduct liquid away from the
downstream surface of the porous medium toward the
drainage pathways of the substrate. In addition,
the support and drainage layer provides additional
DLLu~_LuLcll integrity or strength to the porous
medium. Thus, while the substrate provides most of
the support to the porous medium, in those locations
where the porous medium would not be supported by
the substrate , e . g ., because of recesses in the
substrate underlying the porous medium, the support
and drainage medium provides support which increases
the resistance to deformation of the porous medium
into the recessed portions of the substrate. Such
support by the support and drainage medium is
effective in preventing undue distortion and/or
breaches of the porous medium under sustained high
pL~SDULe~ conditions or short term pressure pulses in
which the pressure drop across the medium involves a
significant force on the media. The support and
drainage medium pref erably provides support and
resists undue distortion and/or breaching of the
porous medium at pressure drops in the forward
f iltration f low direction across the membrane of at
least about 25 psid, more preferably of at least
about 75 psid, and most preferably of more than
about lOO psid.
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~7~7 ~69
The support and drainage medium should, like
the porous medium, be substantially insoluble in the
solvent system used as the bonding composition to
bond the substrate to the support and drainage
medium as well as to the porous medium. The support
and drainage medium should also not be unsuitably
affected in any manner by contact with the bonding
composition or any liquid medium which it may
contact in use.
Any suitable woven or nonwoven material having
a relatively coarse porosity and a relatively low
edgewise f low resistance compared to the porous
medium can be used for the support and drainage
medium, with nonwoven materials being generally
preferred. Typically, such support and drainage
media have air permeabilities from about 70 scfm to
about 1500 scfm at ~ inch water. Preferably, the
support and drainage medlum used in the present
invention should have a tensile strength of at least
about 75 lbs/in2 (5.2 kgs/cm2) and a thickness of
about 1 to about 20 mils ~about 25 microns to about
0 . 51 mm), more preferably from about 3 to about 10
mils. The air permeability of the support and
drainage medium is preferably at least about 70
scfm/ft2 of airflow at one-half inch of water, most
preferably about 100 to 300 scfm/ft2. Natural fibers
or polymeric materials may be employed to f orm the
support and drainage medium, with certain polymeric
materials being preferred. Preferably the support
and drainage medium is formed from a polyolefin,
such as polypropylene, or a polyester, such as
polyethylene terephthalate. However, polyamides,
aramids and glass f ibers may also be used .
Preferred nonwoven support and drainage media
include a polyethylene terephthalate polyester web
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2177469
available from HiroseD as 05TH15, a polypropylene
web available from Hirose~ as HOP30H, and spunbonded
polypropylene media available from Midwest
Filtration Company as Unipro.
Woven support and drainage media are less
pref erred than nonwoven media because they typically
are thicker and have a less uniform surface and high
edgewise flow resistance. However, woven support
and drainage media typically have greater strength
than nonwoven media so they may be pref erable f or
substrates with wide grooves or in high pressure
environments. A preferred woven material is a
polyester mesh available from Tetko~ as mesh no. 7-
105/52 .
As shown in Figure l, the portion of the
substrate 10 between adjacent grooves 12 is referred
to hereinafter as the crest 11. The grooves 12 may
have any suitable conf iguration, such as a
semicircular configuration, a V-shaped configuration
or the generally U-shaped configuration shown in
Figure 1. Similarly, the crests 11 may have a
variety of conf igurations, such as the apex of the
angle between closely-spaced V-shaped grooves or,
more preferably, a flat planar surface between more
widely spaced grooves. The crests 11 constitute
much of the substrate surface to which the support
and drainage medium 13 and the porous medium 5 are
secured .
As shown in Figure 2, the porous medium 15, the
support and drainage medium 13, and the substrate
are bonded together, pref erably by dissolved
substrate free of any adhesive. The support and
drainage medium 13 is integrally secured to the
substrate surface 14 by forming a bond between the
crests 11 and the support and drainage medium 13.
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~17~4~
In the composite structure of the present invention,
the porous medium 15 is also integrally bonded to
the support and drainage medium 13 by a bond formed
between the surface portions 16 of the two media.
Typically, the dissolved substrate material which
bonds the substrate 10 to the support and drainage
medium 13 and the support and drainage medium 13 to
the porous medium 15 extends across the thickness of
the support and drainage medium 13 from one bonded
surface to the other bonded surface. Thus, to
provide an effective bond, the thir~knc~ of the
6upport and drainage medium 13 is preferably less
than about 20 mils ( . 51 mm) . The depth of
penetration of the bond 16 into the porous medium 15
may preferably be a very small fraction of the
thickness of the porous medium 15, as this permits
the portion of the porous medium 15 above the crests
11 to function effectively without blinding by
allowing edgewise flow. This is however, much less
of a problem with the composite structure of the
present invention as compared with conventional
elements which lack the support and drainage medium
of this invention.
An embodiment of the present invention also
comprises a method for bonding a porous medium and a
support and drainage medium to a substrate which
includes, for example, contacting a porous medium
with one surface of a support and drainage medium
and the opposite surface of the support and drainage
3 0 medium with a surf ace of a substrate so that the
support and drainage medium is sandwiched between
the porous medium and the substrate. Preferably
this is done when both media and the substrate
surface are dry. The support and drainage medium
and at least a portion of the porous medium are then
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impregnated with a bonding composition which at
least slightly dissolves the substrate surface
without dissolving the porous medium or the support
and drainage medium.
Initially, the c Ls of the composite
structure ( e . g ., the substrate , the support and
drainage medium, and the porous medium) are located
with the support and drainage medium sandwiched
between the porous medium and the substrate. In
composite structures in which the substrate includes
~luid pathways or grooves on opposing surfaces, the
substrate is sandwiched between two support and
drainage media, and the first support and drainage
medium, the substrate, and the second support and
drainage medium are, in turn, sandwiched between two
porous media . Each surf ace of the substrate then
contacts one surface of a support and drainage
medium while the opposite surface of the support and
drainage medium contacts one surface of a porous
medium.
The components of the composite structure are
placed between clamping plates of a rigid, liquid
impervious material, such as aluminum. In many
instances, it is desirable to place a porous pad
between the clamping plate and the porous medium in
contact with the outer surface of the porous medium.
The material from which the porous pad is formed has
properties, including the non-solubility in bonding
composition, similar to those of the support and
drainage medium used in the present invention. The
thickness of the pad, however, is preferably much
greater than that of the support and drainage medium
used in a particular application so the porous pad
may contain a greater volume of the bonding
composition. For example, the porous pad may
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~ 77~69
contain from 1 to 30 times the volume of the bonding
composition compared to the porous medium and the
support and drainage medium. Materials suitable for
use as a porous pad include the same materials used
for support and drainage media. In addition, a
coarse mesh may be positioned between the clamping
plate and the porous pad. The apparatus, as well as
many of the techniques, used to form the composite
structure of the present invention are analogous to
those described in commonly owned, co-pending
International Application No. PCT/US94/03104 filed
23 March 1994.
Once the coarse mesh, porous pad(s) and the
components of the composite structure are placed
between the clamping plates, the clamping plates are
activated to ~ ess the entire assembly. In a
preferred ~mho~ t, pressure is applied to force
the coarse mesh, the porous pad, the porous medium,
the support and drainage medium, and the substrate
together, compressing the assembly and ensuring that
the porous medium, the support and drainage medium,
and the substrate are in f irm contact . The pressure
applied by the clamping plates varies with the
nature of the substrate and the media used. The
pressure suitably ranges from bare contact to about
100 psi. Preferably the pressure is about 2 to
about 25 psi and most preferably is about 5 to about
7 psi.
Once the assembly is clamped together, a
bonding composition is il,LL~,duced to the clamped
assembly to bond the porous medium, the support and
drainage medium, and the substrate. The bonding
composition preferably is free of any adhesive and
comprises a mixture of at least two chemical
species, the f irst chemical species being a good
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WO 96/09879 PCrlUS94/~0942 ~
21 77~gg
solvent for a substrate and the second chemical
species being a non-solvent for the substrate.
Preferably, neither the solvent species nor the non-
solvent species is a solvent for the support and
drainage medium or the porous medium. The starting
constitution of the bonding composition may vary
from 100% solvent species and 0% non-solvent species
to about 10% solvent species and 90% non-solvent
species by weight . More pref erably, the starting
composition is in the range from about 70% solvent
species and 30% non-solvent species to about 30%
solvent species and 70% non-solvent species.
Exemplary chemical species which may be used as
a solvent species include but are not limited to
halogenated hydrocarbons, such as methylene chloride
or chloroform. Preferably, the solvent species
includes methylene chloride. Exemplary chemical
species which may be used as the non-solvent species
include but are not limited to alcohols and
hydrocarbons. Preferably, the non-solvent species
is methanol, cyclopentane, polymethyl pentane.
Exemplary bonding compositions for bonding a
polyamide, a poly (vinylidene fluoride) or a
polytetraf luoroethylene porous medium and a
polyester or polyethylene support and drainage
medium to a polyethersulfone or polysulfone
substrate include mixtures of methylene chloride as
the solvent species and methyl alcohol, polymethyl
pentane, or cyclopentane as the non-solvent species.
The bonding composition may be introduced to
the clamped assembly in any suitable manner. In a
preferred method, the bonding composition may be
introduced f irst to the substrate whence it f lows to
and contacts the support and drainage medium,
ultimately impregnating and saturating the support
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2~L~7~69
and drainage medium. The bonding composition then
contacts at least the surface of the porous medium
contacting the support and drainage medium.
Preferably the bonding composition also saturates
the porous medium, the porous pad, and the coarse
mesh .
As indicated above, the substrate is provided
with a ~ ni~ or a pathway for draining filtrate
away from the substrate surface, such as rh~nnPl ~ or
grooves . These rh;~nn~l A or grooves may also serve
as the path of the bonding composition along the
substrate to the support and drainage medium and the
porous medium and hence to the porous pad and the
coarse mesh. Frequently, particularly when the
substrate is a rigid solid material of significant
thickness, the substrate is also provided with a
f iltrate outlet port or permeate port which is in
f luid communication with the grooves in the
substrate surface. When such a port is provided in
the substrate, the bonding composition may be
introduced to the substrate, for example, using a
syringe or a syringe pump and/or an appropriate
f itting in the port . While the orientation of the
clamped assembly does not appear to be critical,
during the introduction of the bonding composition
when the bonding composition is introduced from the
direction of the substrate and f lows to the porous
medium, it is generally preferred to orient the
clamped assembly in a vertical position with the
3 0 permeate port at the top .
The impregnated porous medium, the impregnated
support and drainage medium, and the substrate
surface are maintained in contact until the
substrate surface is at least slightly solYated by
or slightly dissolved in the bonding composition and
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WO 96/09879 PCT/US94/10942
~774~9
the di6601ved 6ub6trate contact6 the 6upport and
drainage medium and the porou6 medium. The bonding
compo6ition and the di6601ved 6ub6trate wicks along
the 6upport and drainage medium between the porous
medium and the cre6t6 and f lat6 of the 6ub6trate and
into the porou6 medium. Preferably, the
characteri6tics of the bonding composition are
6elected or adju6ted to obtain a 6atisfactory degree
of bonding during a hold period of at least about 15
to 25 6econd6. ("Hold period" refer6 to the time
during which the porou6 medium, the impregnated
6upport and drainage medium, and the substrate are
maintained in contact from the introduction to the
removal of the bonding composition. ) Still longer
hold periods, such as from about 100 to about 150
6econd6 or more, are more preferred.
The optimum duration of the hold period i6
preferably detPrminPd empirically for a 6pecific
bonding compo6ition. Generally, the bond 6trength
increases but the p~ -~hi 1 ity of the porou6 medium
decrea6e6 with increasing hold time. For example,
the final compo6ite 6tructure may be tested by
passing water therethrough in the normal filtration
flow direction (i.e., from the porous medium through
the 6upport and drainage medium to the 6ub6trate) in
order to determine what percentage of the
permeability of the porous medium ha6 been 106t.
Thi6 percentage becomes higher a6 the hold period i6
increa6ed and more of the di6601ved sub6trate wick6
into the porous medium. The compo6ite 6tructure may
al60 be te6ted by f lowing water in the reverse
filtration flow direction, in order to ~ptprminp the
.ULe: at which the porou6 medium i6 duely
di6torted or 6eparate6 from the 6upport and drainage
medium and/or the latter from the 6ub6trate.
-- 20 --
O 96/09879 ~! 1 7 ~ 4 ~ !~ PCr~uss4nOs42
Several SpP~-;r-n~ can be made using a given bonding
composition and various hold periods. The test data
derived from these specimens may then be used to
select an optimum hold period.
The optimum hold period varies greatly
t~erPn~9; ng on the particular chemical species used to
prepare the bonding composition. Because the
bonding composition may be compounded using a
combination of an aggressive solvent species with a
non-solvent species, the degree of solvency of the
substrate in the bonding composition, and hence the
hold period required, may be adjusted by varying the
proportions of the two species.
When the bonding composition is introduced to
the clamped assembly, some parts of the porous
medium, the support and drainage medium and the
substrate are unavoidably wetted by the bonding
composition before other parts. For example, if the
substrate, support and drainage medium, and porous
medium being bonded are quite large, some parts may
be exposed for as much as 15 seconds or more longer
than other parts. If the bonding composition is
selected or compounded such that the hold period is
about 15 seconds, then some parts of the porous
medium in contact with the support and drainage
medium may have been exposed for twice as long as
others. This may lead to uv~:Ll,u.lding of one section
of the resulting composite structure with the flow
of filtrate through the composite :,~. U~;~ULe
inhibited locally, while another section may fail in
the reverse f iltration f low mode .
A bonding composition comprising a mixture of
chemical species makes possible relatively longer
hold periods. As noted above, the advantage of
longer hold periods is that the effect of the
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WO 96l09879 PCr/US94/10942
~177~g
dirferential wetting which can occur during the
filling operation is minimized. When compared, for
example, with the same 15 second wetting
differential of the preceding paragraph, the use of
a mixture of ~h~m;t~:~l species composition for which
bonding is optimized by a 150 second hold period
reduces the difference between the longest and
shortest total time during which any part of the
substrate is in contact with the impregnated porous
medium prior to f lushing to about 1096 of the hold
period .
After the hold period, the bonding compositlon
is then withdrawn or removed from the clamped
a6sembly. For example, the bonding composition may
be withdrawn in a direction opposite to that in
which it was introduced, i . e., from the porous
medium through the support and drainage medium and
then through the substrate, e.g. along the filtrate
pathway and out the f iltrate port . P.s soon as the
desired hold period between the porous medium, the
support and drainage medium and the substrate has
been reached, the bonding composition contained in
the coarse mesh and in the porous pad may be rapidly
f lushed through the porous medium and the support
and drainage medium, preferably in the direction of
the substrate and out of the clamped apparatus via
the grooves and the f iltrate port of the substrate .
For example, the bonding solution may be removed by
applying air or other gas LIL~:S--UL- at the coarse
3 0 mesh or the porous pad or by applying a vacuum to
the side of the porous medium closest to the
substrate, e.g., by applying a vacuum to the
substrate outlet port. The application of pressure
or vacuum may then be continued until the now bonded
structure is dry ( i . e ., the residual bonding
-- 22 --
~0 96/09879 PCTIUS94J10942
469
composition has been evaporated), in the one case by
evaporation into the introduced gas and in the other
by evaporation of the solvent into the vacuum. As
the solvent species is evaporated from the bonding
composition, the dissolved substrate precipitate6
and 601idifies within the support and drainage
medium and the porous medium and on the surface of
the substrate, mechanically entangling and
generating a strong, secure bond between the
substrate, the support and drainage medium, and the
porous medium.
The rapid flushing of bonding composition from
the coarse mesh and the porous pad through the
porous medium towards the substrate is benef icial,
as it removes some of the dissolved substrate from
the porous medium. Allowing all of the dissolved
substrate to remain in place could unduely blind the
porous medium and partially obstruct filtrate flow
in the porous medium.
It is desirable to reduce as much as possible
dif f erences in exposure time in this stage between
one part of the bonded surface and another. This
may be accomplished in part by applying a high
degree of vacuum at the conclusion of the hold
period, thereby rapidly removing the bonding
composition by evaporation as the bonding
composition is being flushed through the porous
medium. The effectiveness of this E.Ic,cedul t: is,
however, hampered by the absorption of heat during
vaporization which cools the chemical species
contained in the bonding composition, reducing their
vapor pressure and the ef f ective pumping rate .
I~owever, the non-solvent species may be selected to
have a lower vapor pressure than the solvent species
of the bonding composition, preferably by about 10%
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WO 96/09879 2 1 7 7 ~ ~ 9 PCr/[JS94/10942
or more at ambient temperature. When the vacuum is
applied, the solvent species is removed faster than
the non-solvent species, thereby decreasing the
cu~cel.LL<ltion of the solvent species in the residual
bonding composition. Preferably, the starting
constitution of the bonding composition is chosen
such that the residual bonding composition becomes a
non-solvent for the substrate after a very short
period of evaporation, thereby preventing any
further dissolution of the substrate and limiting
the time during which dissolution of the substrate
occurs to a very short period after exposure to the
vacuum, which may be as short as about 5 seconds or
less .
Various alternative methods also embody the
present invention. As one alternative method,
bonding composition may be introduced from the
direction of the coarse mesh through the porous pad
to the porous medium and then through the support
and drainage medium to the surface of the substrate.
As with the method described immediately above, the
bonding composition is allowed to remain in the
porous and the support and drainage media, both of
which are impregnated, and in contact with the
surface of the substrate for a suitable ~old period.
As another ~mho~l;r L, the bonding composition may
be removed from the compressed assembly in the
direction of the coarse mesh or porous pad using a
yLes~uL~ differential, for example, by applying a
vacuum at the coarse mesh or the porous p d.
-- 24 --
96109879 ~ l 7 7 ~ 6 9 PCrllJS94/10942
Test Methods
The f ollowing non destructive test methods are
conducted on composite structures comprising
mi~.:L ~p~L ~JUS membrane f ilter elements made by methods
of this invention. The filter elements are
generally tested in a leak-tight assembly having
seals which separate the upstream side from the
downstream side of the f ilter element .
Bond Strenqth (Reverse pressure): The bond
strength between individual components and the
adjacent c~ _rlPnt to which they are bonded in the
composite structure, i.e., the porous medium, the
support and drainage medium, and the substrate, can
be determined by applying pressure in the reverse
f iltrate f low direction . Pressure is increased
incrementally until the bond between the porous
medium, the support and drainage medium, and the
substrate fails. If no evidence of bond failure is
observed at 5 psi for a 60 second dwell time, the
relative bond strength is deemed acceptable.
Perm~hilitv (Flow ~P): The effective
permeability of a porous medium can be det~rm; nf~-l by
measuring the flow of water as a function of applied
pressure. Using water at ambient t~"~ Lu~e, which
has been previously passed through a 0.04 ,ILm filter,
the filtrate flow and permeability is measured in
the f orward f iltrate f low direction at 2 . 5, 5 . 0, and
lO . 0 psi . The data is reported as an average f low
rate in units of mil/min/psi.
PorositY Bubble Point Test: Each membrane
filter element is tested for porosity by using a
bubble point test as described in ASTM F316-86.
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Wo 96/09879 PCrlUS94/10942
2~L774~9
Forward Pressure/Temperature Rating: The
relative forward ~es--uLe/temperature rating of a
porous medium bonded to a substrate can be obtained
by applying ~Les_uLa in the filtrate forward flow
direction, at a given t~ ,_LCI-UL~, until the porous
medium begins to yield or gross failure is observed.
Hot water filtered through a 0.1 ~m filter is pumped
through the composite structure at 60 psi yL~s. uLe
and 9 0 C f or 3 0 mlnutes .
F~AMPT,F~
7le 1:
A l/4-inch thick, 16 inch diameter semi-
circular injection molded polysulfone disk served as
the substrate. The semicircular disk was provided
with a series of concentric grooves on both sides
that drain to a single central channel and permeate
port. A wet laid polyester nonwoven fibrous web
available under the trade designation HIROSE 05TH15
was used as the support and drainage medium. An
ULTIPOR~ N66 polyamide 0 . 45 ~m rated microporous
membrane available from Pall Corporation, East
Hills, New York served as the porous medium. The
6upport and drainage medium and porous medium were
cut to dimensions such that once the support and
drainage medium and the porous medium were each
positioned on and adjacent the substrate,
respectively, the entire grooved area of the
substrate was covered, extending past the peripheral
grooves onto the flats by 0.300 inch (7.6 mm). The
3 o substrate was sandwiched between a layer of support
and drainage medium and a layer porous medium on
both sides, with the support and drainage medium
layers positioned closest to the substrate. This
-- 26 --
O 96/09879 ;~ 1 7 7 ~ 6 9 PCT/US94/10942
layered assembly was then placed between a pair of
porous pads consisting of 10 layers of spunbond
polypropylene nonwoven available under the trade
designation LUTRASIL LSVP 688 and a pair of 3/4"
Alllminllm plates and clamped at 7 psi clamping
pressure. The permeate port, which extended out
from the clamping plates, was equipped with an O-
ring sealed stainless steel f ixture to permit the
bonding composition to be injected and evacuated
from the permeate port. The bonding composition
consisted of a solvent/non-solvent mixture of 5496
methylene chloride and 46% cyclopentane, by weight.
With the clamped assembly in the vertical position,
150 ml of bonding composition was injected quickly
through the permeate port with a glass syringe,
filling the grooves of the substrate, the support
and drainage media, the porous media and the porous
pads with bonding composition and ~YrPl l in~ the air.
Once the bonding composition was injected, a hold
time of 120 seconds was initiated. After 120
seconds elapsed, the excess bonding composition was
evacuated by applying vacuum f or 15 minutes at the
permeate port. After 15 minutes, evacuation was
discontinued and the composite structure comprising
a f ilter element was removed from the clamped
plates. The bond strength was tested by applying a
reverse pressure of 5 psi for 60 seconds at the
permeate port with no evidence of membrane failure,
indicating that the porous medium was integrally
bonded to the substrate. The flow ~P and bubble
point were det~rmin~d to be 1215 ml/min/psi and 31.5
psi, respectively. The flow QP and the bubble point
of the porous medium prior to bonding were
de~ormin~-i to be 1550 ml/min/psi and 31.0 psi,
respectively. Thus, the composite structure
-- 27 --
WO 96l09879 PCrlUS94/10942
~1 77~9
retained 78% of the porous medium effective
permeability with no substantial alteration in pore
size as a result of the bonding method described. r
~Y~ ele 2
A 1/4-inch thick, 6" diameter, circular
injection molded polysulfone disk of the type used
in Example 1 but having a series of concentric
grooves on only one side and draining to a single
central channel and permeate port served as the
substrate. As in Example 1, a wet laid polyester
nonwoven f ibrous web available under the trade
designation HIROSE 05TH15 served as the support and
drainage medium, and an ULTIPOR N66~D polyamide O . 45
rated mi-:lvpo~vuS membrane available from Pall
Corporation served as the porous medium. The
support and drainage medium and porous medium were
cut to dimensions 6uch that once the support and
drainage medium and porous medium were positioned on
the substrate, the entire grooved area of the
substrate was covered, extending past the peripheral
grooves onto the ~flats by O . 300 inch. The support
and drainage medium was sandwiched between the
substrate and the porous medium on the grooved side,
with the support and drainage medium positioned
closest to the substrate. This layered assembly was
then positioned between a pair of 1/8" aluminum
plates, a porous pad consisting of 10 layers of a
spunbond polypropylene nvll..Jvl:~l available under the
trade designation LUTRASIL LSVP 688 was placed
between the porous medium and one of the clamping
plates and the assembly was clamped together at 8
psi clamping pressure. The permeate port, located
on the ungrooved side of the substrate, was eguipped
with an interference fit polypropylene luer fitting
-- 28 --
~096l09879 ~ ~ 7 ~ ~ 6 ~ PCrNS94/10942
to permit the bonding composition to be injected and
evacuated from the permeate port. The bonding
composition consisted of a solvent/non-solvent
mixture of 54% methylene chloride and 46%
cyclopentane, by weight. With the clamped assembly
in the horizontal position and the grooves in the
upper surface of the substrate, 45 ml of bonding
composition were inj ected quickly through the
permeate port with a glass syringe, filling the
grooves of the substrate, the support and drainage
medium, the porous medium, and the pad with bonding
composition and expelling the air. Once the bonding
composition was injected, a hold time o~ 150 seconds
was initiated. After 150 seconds elapsed, the
excess bonding was evacuated by applying a vacuum
for 2 minutes at the permeate port. After 2
minutes, evacuation was discontinued and the
composite :,Lru~:LuL~ comprising a filter element was
removed from between the clamped plates. Flow l~P
and bubble point of the composite structure were
measured at 260 ml/min/psi and 31 psi, respectively.
Flow ~P and bubble point of the ~nh~n~ porous
medium were measured at 313 ml/min/psi and 31 psi,
respectively. Thus, the composite structure retained
83% of the porous medium effective p~ hil i ty with
no alteration in pore size. The bond strength was
tested by applying a reverse pressure of 5 psi for
60 seconds at the permeate port with no evidence of
membrane failure, indicating that the porous medium
was integrally bonded to the substrate.
F le 3
A polytetraf luoroethylene O . 2 ~ rated
mi~-,~u~ .us membrane, serving as the porous medium,
was bonded with a polyester nonwoven support and
-- 29 --
WO 96/09879 PCr/US94110942
21~7469
, ,
drainage medium and a polysulfone disk of the type
and by the general method described in Example 2.
The bond strength was tested by applying a reverse
pre6sure of 5 psi for 60 at the permeate port with
no evidence of membrane failure, indicating that the
porous medium wa6 integrally bonded to the
substrate. The bubble point ~LC:S5ULC: of the porous
medium mea6ured in alcohol before and after bonding
was 15.6 and 15.5 psi, respectively, indicating no
alteration in pore size. The flow ~P of the
composite structure was measured at 194 ml/min/psi.
F~r~nle 4
An ULTIPOR3 N66 polyamide o. 45 ~Lm rated
microporous membrane available from Pall Corporation
served as the porous medium, and a woven polyester
mesh available under the trade designation TETKO
PeCap 7-105/52 served as the support and drainage
medium. The porous medium and the support and
drainage medium were bonded to a polysulfone disk of
the type and generally by the method described in
Example 2. A volume of 32 ml of bonding composition
was injected into the permeate port and a hold time
of 200 seconds was used. After 200 seconds elapsed,
the excess bonding composition was evacuated by
applying a vacuum for 3 minutes at the permeate
port. After 3 minutes, evacuation was discontinued
and the composite ~,LLu.i~uLe was removed from between
the clamped plates. The bond strength was tested at
a reverse pressure of 5 psi for 60 seconds at the
permeate port with no evidence of membrane failure,
indicating that the porous medium was integrally
bonded to the substrate. Flow P and bubble point
of the composite structure element were measured at
246 ml/min/psi and 31 psi respectively. Flow ~P and
-- 30 --
0 96/09879 ;~ s 6 9 PCr/US94/10942
bubble point of the ~nh~nd~d porous medium were
measured at 314 ml/min/psi and 31 psi, re6pectively.
Thus, the composite ~>LLu~i~uLa retained 79% of the
porous medium effective pP --hi 1 ity with no
alteration in pore size.
ExamPle 5
A composite structure was prepared as described
in Example 2 and tested to establish a forward
pressure/temperature rating. The composite
structure was exposed to 90C filtered deionized
water at 60 psi forward pressure for 30 minutes.
The bond strength was also tested at a reverse
pressure of psi for 60 seconds applied at the
permeate port with no evidence of membrane failure,
indicating that the porous medium remained
integrally bonded to the substrate . The f low ~P and
bubble point of the composite structure before the
exposure were measured at 250 ml/min/psi and 30. o
psi, respectively. After the exposure, the flow ~P
and bubble point were measured at 270 ml/min/psi and
30.5 psi respectively, indicating no significant
alteration in permeability or pore size. No
evidence of membrane yield was observed.
As shonw in the previous disclosure and
examples, a composite structure embodying the
present invention has many advantages, in
particular, the composite structure may comprise a
highly superior filter element. For example, filter
element is capable of withstanding reverse pressures
of 5 psi and forward pLes~uL~ drops across the
porous mediium of at least about 25 psid. Further,
the bond between the porous medium, the support and
drainage medium, and the substrate is ~-Yrr~ n~ly
strong, enabling the porous medium to withstand
-- 31 --
W0 96/09879 ~ l ~ 7 4 ~ 9 PCrlUS94/10942
sheer rates of about 5000 per second. Not only is
the bond ~YreP~l;n~ly strong bu the p~ --hi 1 ;ty of
the bonded porous media remains ~Y~-ee~l; n~ly high.
For example, p, -h; l; ty of the bonded porous
medium is at least about 50% of the r~ - -h; 1 i ty of
the 11nhnn~led porous medium. In addition, the
porosity o~ the bonded porous medium is
substantially unchanged from the porosity of the
nnhnn~9 porous medium.
Another advantage of f ilter elements embodying
the present invention is that the bond between the
porous medium, the support and drainage medium, and
the substrate is f ormed without any adhesive by the
solidification of the dissolved substrate.
Consequently, there is no adhesive to leech into the
filtrate and the bond is not effected by aggressive
chemicals unless the chemicals are capable of
attacking the substrate. Further, where the
substrate i6 formed from a high temperature
polymeric material, the bond remains intact at
elevated temperatures until the sof tening point of
the substrate is reached. For a high temperature
polymer such as polysulfone, operating and
sterilization temperatures may be as great as 250
degrees celcius.
Although the present invention has been
described in terms of exemplary ~mhQ-l; r -ntS, it is
not limited to these F~rhorl;~~nts Alternative
embodiments, examples, and modifications which would
still be ~nc ~:s~d by the invention may be made by
those skilled in the art, particularly in light of
the foregoing teachings. Therefore, the following
claims are intended to cover any alternative
o~ nts, examples, modi~ications, or equivalents
-- 32 --
~O 96109879 ~ 4 6 ~ PCr/US94110~42
which may be included within the spirit and scope of
the invention as def ined by the claims .
