U.S. patent application number 10/831420 was filed with the patent office on 2005-10-27 for fluoropolymer barrier material.
Invention is credited to Hollenbaugh, Donald L. JR., Kennedy, Michael E..
Application Number | 20050238872 10/831420 |
Document ID | / |
Family ID | 35066243 |
Filed Date | 2005-10-27 |
United States Patent
Application |
20050238872 |
Kind Code |
A1 |
Kennedy, Michael E. ; et
al. |
October 27, 2005 |
Fluoropolymer barrier material
Abstract
A novel densified fluoropolymer article is described which has a
water vapor permeation of about 0.015 g-mm/m.sup.2/day or less, and
preferably has a matrix tensile strength of at least 10,000 psi in
two orthogonal directions. The articles are made by compressing
expanded porous PTFE at pressures, temperatures and times which
result in elimination of the pores, and subsequent stretching above
the crystalline melt temperature.
Inventors: |
Kennedy, Michael E.;
(Landenberg, PA) ; Hollenbaugh, Donald L. JR.;
(North East, MD) |
Correspondence
Address: |
GORE ENTERPRISE HOLDINGS, INC.
551 PAPER MILL ROAD
P. O. BOX 9206
NEWARK
DE
19714-9206
US
|
Family ID: |
35066243 |
Appl. No.: |
10/831420 |
Filed: |
April 23, 2004 |
Current U.S.
Class: |
428/336 ;
264/638; 428/421 |
Current CPC
Class: |
B32B 2307/518 20130101;
B29K 2027/18 20130101; B32B 27/08 20130101; B32B 15/085 20130101;
B32B 27/00 20130101; B32B 2307/54 20130101; B29C 55/023 20130101;
C08J 2327/18 20130101; B32B 27/322 20130101; Y10T 428/265 20150115;
Y10T 428/3154 20150401; B29C 55/005 20130101; B32B 2307/558
20130101; C08J 5/18 20130101; B32B 17/10 20130101; B32B 27/12
20130101; B32B 2307/581 20130101; B32B 2307/7246 20130101 |
Class at
Publication: |
428/336 ;
428/421; 264/638 |
International
Class: |
B28B 003/00; B32B
027/00 |
Claims
We claim:
1. An article comprising an expanded PTFE film having a water vapor
permeability coefficient of about 0.015 g-mm/m.sup.2/day or less
and a matrix tensile strength of at least 10,000 psi in two
orthogonal directions.
2. The article of claim 1, wherein said film has a matrix tensile
strength of at least 15,000 psi in at least one direction.
3. The article of claim 1, wherein said film has a matrix tensile
strength of at least 25,000 psi in at least one direction.
4. The article of claim 1, wherein said film has a water vapor
permeability coefficient of about 0.010 g-mm/m.sup.2 day or
less.
5. The article of claim 1, wherein said film has a water vapor
permeability coefficient of about 0.003 g-mm/m.sup.2/day or
less.
6. The article of claim 1, wherein said film further contains a
filler.
7. The article of claim 1, wherein said expanded PTFE film has a
thickness of 250 micrometers (.mu.m) or less.
8. The article of claim 1, wherein said expanded PTFE film has a
thickness of 150 micrometers (.mu.m) or less.
9. The article of claim 1, wherein said expanded PTFE film has a
thickness of 50 micrometers (.mu.m) or less.
10. The article of claim 1, wherein said expanded PTFE film has a
thickness of 10 micrometers (.mu.m) or less.
11. The article of claim 1, wherein said expanded PTFE film has a
thickness of 5 micrometers (.mu.m) or less.
12. The article of claim 1, wherein said expanded PTFE film has a
thickness of 1 micrometer (.mu.m) or less.
13. A laminate comprising: (a) an expanded PTFE film having a water
vapor permeability coefficient of about 0.015 g-mm/m.sup.2 day or
less and a matrix tensile strength of at least 10,000 psi in two
orthogonal directions; and at least one substrate.
14. The article of claim 13, wherein said film has a water vapor
permeability coefficient of about 0.010 g-mm/m.sup.2 day or
less.
15. The laminate of claim 13, wherein said substrate comprises a
fluoropolymer.
16. The laminate of claim 13, wherein said substrate comprises
PFA.
17. The laminate of claim 13, wherein said substrate comprises
FEP.
18. The laminate of claim 13, wherein said substrate comprises
THV.
19. The laminate of claim 13, wherein said substrate comprises
PTFE.
20. The laminate of claim 13, wherein said laminate comprises the
expanded PTFE film oriented between two substrates.
21. The laminate of claim 13, wherein said substrate comprises
PCTFE.
22. The laminate of claim 13, wherein said substrate comprises at
least one material selected from the group consisting of
polyurethane, polyethylene, polyamide, EVOH and PVDC.
23. The laminate of claim 13, wherein said substrate comprises at
least one pressure sensitive adhesive.
24. The laminate of claim 13, wherein said substrate comprises at
least one textile.
25. The laminate of claim 13, wherein said substrate comprises
metal.
26. The laminate of claim 13, wherein said substrate comprises
glass.
27. The laminate of claim 13, wherein said substrate comprises an
inorganic sheet.
28. A method of making a polytetrafluoroethylene (PTFE) film having
a water vapor permeability coefficient of about 0.015
g-mm/m.sup.2/day or less, the method comprising the steps: (a)
preparing of a biaxially expanded PTFE film; (b) densifying said
expanded PTFE film; (c) stretching the densified expanded PTFE film
at a temperature exceeding the crystalline melt temperature of
PTFE.
29. The method of claim 28, wherein the expanded PTFE film is
sintered prior to step (b).
30. The method of claim 28, wherein said biaxially expanded PTFE
film comprises two or more plies of expanded PTFE.
31. The method of claim 28, wherein said steps are carried out in a
continuous manner.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a fluoropolymer barrier
material, preferably comprising a dense polytetrafluoroethylene
sheet or film, which exhibits very low water vapor permeation and
improved tensile properties in both the length and width dimensions
(i.e., directions), and to processes for manufacture of said
barrier which include a combination of densification, sintering,
and stretching of polytetrafluoroethylene.
BACKGROUND OF THE INVENTION
[0002] The challenge of locating a thermally stable polymer film
with excellent barrier properties as well as good mechanical
properties for use in a broad range of applications has led
researchers in varied directions. Both monolithic and
multi-component, multi-layer films have been constructed; however,
to date, no suitable materials have been available which provide
the unique combination of thermal stability, strength, thinness
and, most importantly, barrier properties as demonstrated by
resistance to water vapor permeation.
[0003] One attempt to solve this problem is taught in U.S. Pat. No.
6,465,103 B1, to Tsai et al., which is directed to highly oriented
multilayer films produced by coextruding or laminating at least one
layer of PCTFE (polychlorotrifluoroethylene) fluoropolymer, at
least one layer of a polyolefin homopolymer or copolymer and an
intermediate adhesive layer of a polyolefin having at least one
functional moiety of an unsaturated carboxylic acid or anhydride
thereof. The polyolefin layer allows the fluoropolymer layer to be
stretched up to ten times its length to orient the fluoropolymer
film and increase mechanical properties and water vapor properties
of the film. Commercially available films of this construction are
sold under the trade name ACLAR.RTM. by Honeywell Corporation.
However, limitations exist with respect to these materials,
including the presence of the polyolefin and adhesive layers which
contribute undesirable thickness to the final film and added cost
during processing. Moreover, these films have limited chemical and
temperature resistance (e.g., maximum thermal stability reported
for ACLAR.RTM. films is about 215.degree. C.) and limited water
vapor permeation resistance.
[0004] Other materials have also been evaluated for suitability in
demanding barrier applications. For example, a polyvinylidene
chloride (PVDC) copolymer film sold by the Dow Chemical Company
(Midland, Mich.) under the trade name SARAN is widely known as a
barrier film for protecting foods against oxygen, moisture and
chemical attack, as well as other barrier applications. However,
this PVDC film has limited chemical and temperature range (i.e.,
melt temperature of about 160.degree. C.) and limited water vapor
permeation resistance.
[0005] The advantage of using polytetrafluoroethylene (PTFE) in
harsh chemical environments and over a broad range of temperatures
is well known. PTFE has exhibited utility as a material for use in
harsh chemical environments where other polymers quickly degrade.
PTFE also has a useful temperature range from as high as
260.degree. C. to as low as near -273.degree. C. However, PTFE is
characterized by poor mechanical properties such as low tensile
strength, poor cold flow resistance or creep resistance, poor
cut-through and abrasion resistance and a general poor mechanical
integrity that precludes its consideration in many materials
engineering applications.
[0006] Low porosity PTFE articles have been made in the past
through use of a skiving process in which solid PTFE films are
split or shaved from a thicker preformed article. These articles
are characterized by low strength, poor cold flow resistance, and
poor load bearing capabilities in both the length and width
directions of the film. Processes including paste extrusion of PTFE
fine powder have also been used to produce low porosity PTFE
articles, however they are also characterized by relatively poor
mechanical characteristics. Attempts have also been made to
strengthen low porosity PTFE films by stretching in the length
dimension. Strength gains are minimal and, by the nature of the
process, are achieved in only a single dimension, thus greatly
minimizing the utility of the film.
[0007] A PTFE material, specifically, expanded
polytetrafluoroethylene, may be produced as taught in U.S. Pat. No.
3,953,566. This porous expanded polytetrafluoroethylene (ePTFE) has
a microstructure consisting of nodes interconnected by fibrils. It
is of higher strength than unexpanded PTFE and retains the chemical
inertness and wide useful temperature range of unexpanded PTFE.
[0008] However, ePTFE is porous and hence cannot be used as a
barrier layer to low surface tension fluids since such fluids with
surface tensions less than 50 dyne-cm pass through the pores of the
membrane. Compressed ePTFE articles are taught in U.S. Pat. No.
3,953,566 in which a platen press was used to densify a thin sheet
of ePTFE with and without heat. However, cold flow occurred in the
press, non-uniform parts resulted and a density of over 2.1 g/cc
was not achieved. The ePTFE sheet used in U.S. Pat. No. 3,953,566
was stretched or strengthened in only one direction and, hence, the
utility of the finished article was severely limited.
[0009] Similarly, U.S. Pat. No. 4,732,629, to Cooper et al.,
describes a method of increasing the cut-through resistance of a
PTFE insulated conductor. Unsintered PTFE was expanded and
compressed and then applied to a conductor. However, densities of
2.1 g/cc or greater were not achieved, and the resultant tensile
strengths of the finished article were not reported for either the
length or width directions.
[0010] U.S. Pat. No. 5,061,561 to Katayama describes a method to
produce high density fibers from ePTFE; however, the method yielded
an article that is significantly different from this invention and
applicable only to fine filaments and not to sheets.
[0011] In U.S. Pat. No. 5,374,473 to Knox et al., a method is
described for producing articles of densified ePTFE by placing two
or more layers of porous ePTFE inside a heat and pressure stable
flexible container, evacuating gas from the chamber, subjecting the
chamber to a pressure of 150 to 350 psi and temperature from
368-400.degree. C., then cooling the container while reducing
pressure. The resulting densified structure is described as useful
in such barrier applications as pump diaphragms when laminated to a
flexible backer. While the Knox et al. materials exhibit improved
barrier properties in the applications described, the methods and
articles taught are limited to making thin, flexible PTFE films
with uniformly good barrier properties (e.g., a water vapor
permeation coefficient on the order of 0.10 g-mm/m.sup.2/day).
[0012] U.S. Pat. No. 5,792,525, to Fuhr et al., teaches forming
creep resistant articles which are dimensioned from a stock
material of one or more layers of expanded polytetrafluoroethylene
which have been densified. The densified expanded PTFE material
exhibits remnants of a fibril and node structure, and the resultant
article is resistant to creep at high temperatures and under high
loads. The stock material is preferably formed in the manner taught
in U.S. Pat. No. 5,374,473, to Knox et al., described earlier
herein. The shaped articles are then formed by any suitable method
such as a heat forming process or a machine forming process.
Compression molding and lathing are specifically described as
shaping methods. Fuhr et al. does not teach or suggest the
capability of forming thin PTFE films with good barrier
properties.
[0013] WO 02/102572 A1 is directed to PTFE resin blow molded
articles and resin blow molding methods. The PTFE starting material
is drawn by blow molding at a temperature at or above the
temperature at which PTFE begins to melt, which is a temperature
where both crystalline and non-crystalline regions are present in
the PTFE, to form a non-porous structure. From the teachings, this
method and product are subject to significant variations in
processing and product properties, and trial and error is necessary
to determine the drawing temperature and draw ratio for each batch
of material. In addition, significant limitations in material size
and material strength would result based on the processing
techniques taught.
[0014] Two products currently available from W.L. Gore and
Associates, Inc. include a dense fluoropolymer film exhibiting
barrier properties. The first product comprises a PTFE barrier
layer bonded between two porous PTFE layers. The second product
comprises a PTFE barrier layer bonded on one side to a
thermoplastic layer such as FEP (fluoroethylene propylene), PFA
(perfluoroacrylate) or THV (a polymer of tetrafluoroethylene,
hexafluoropropylene and vinylidene fluoride). The barrier layer in
these commercial products is a film of high water vapor resistance
(i.e., low water vapor permeation) PTFE having good tensile
properties in the orthogonal directions of width and length. It
would be understood by an artisan of skill in the art that barrier
performance and bulk density of a material are positively
correlated. This barrier layer has a bulk density of 2.11 g/cc or
greater, is substantially free of pores, has a matrix tensile
strength of 10,000 psi or greater in both the width and length
directions, and has a water vapor permeation coefficient of 0.018
g-mm/m.sup.2/day. While these materials have been successfully
implemented in a number of applications requiring flexible, thin
materials with good chemical resistance and water vapor permeation
resistance, a need still exists for materials with further improved
performance for even more demanding barrier applications.
SUMMARY OF THE INVENTION
[0015] One objective of the present invention is to provide
improved fluoropolymer barrier materials exhibiting enhanced
barrier properties, including excellent water vapor permeation
resistance (reported as the water vapor permeation coefficient of
the material) and strength, which have not heretofore been achieved
in the prior art. It will be appreciated by those of skill in the
art that the water vapor permeation resistance of a given material
is a strong indication of permeation resistance to a wide variety
of permeants, and the present invention is in no way limited
thereby. Such improved barrier properties are valuable in many
applications where resistance to a variety of permeants under
aggressive conditions is desirable. The present invention provides
improved fluoropolymer barrier materials, preferably comprising
dense PTFE sheets or films, with water vapor permeation
coefficients of about 0.015 g-mm/m.sup.2/day or less, more
preferably about 0.010 g-mm/m.sup.2/day or less, and even more
preferably as low as about 0.003 g-mm/m.sup.2/day or less. As noted
earlier herein, important benefits to fluoropolymers comprising
PTFE materials include a resistance to harsh chemical environments
where other polymers quickly degrade and a useful temperature range
from as high as 260.degree. C. to as low as near -273.degree.
C.
[0016] Another objective of this invention is to enhance the
utility of such fluoropolymer barrier materials, preferably
comprising dense PTFE sheets or films, by providing improved
tensile strengths in orthogonal (i.e., the length and width, etc.)
directions. This improvement is of utility in applications
requiring improved flex life, load bearing, impact and rupture
resistance, notch propagation resistance, cut-through resistance,
and abrasion resistance. Improved tensile strengths in both the
length and width directions can be achieved in dense PTFE sheets
without the need for reinforcing materials that compromise the
chemical performance of the finished article. This invention
provides for a fluoropolymer barrier material with not only lower
water vapor permeation, but also greater tensile strengths in
orthogonal directions and greater toughness, along with the
excellent chemical and thermal characteristics of traditional dense
PTFE sheets or films. Sheets and films of the invention can be made
in unusually thin form.
[0017] Thus, this invention now provides fluoropolymer barrier
materials which have the unique combination of thermal stability,
strength, thinness and, most importantly, excellent barrier
properties. Desirable thicknesses of the barrier materials of the
present invention are on the order of 3 mm or less, more preferably
0.5 mm or less, and even more preferably as low as 18 .mu.m and
down to about 2 .mu.m or less. Preferred tensile strengths of the
materials of the present invention are on the order of at least
10,000 psi in both the width and length directions (i.e., in two
orthogonal directions), more preferably at least 15,000 psi in at
least one direction, and most preferably at least 25,000 psi in at
least one direction. Throughout this document the terms width and
length are respectively analogous to the x and y directions.
Barrier properties of the novel fluoropolymer materials of the
present invention as demonstrated by resistance to water vapor
permeation are about 0.015 g-mm/m.sup.2/day or less, more
preferably about 0.010 g-mm/m.sup.2/day or less, and even more
preferably as low as about 0.003 g-mm/m.sup.2/day or less.
[0018] This invention is directed to both products and processes.
The processes are processes for making sheets or films of high
density PTFE, high density filled PTFE, and composites of high
density PTFE and other materials, as desired, with low water vapor
permeation and with high tensile strength in both the length and
width directions.
[0019] These processes comprise compressing the sheet or sheets of
porous ePTFE, either on an appropriate batch press, such as a
platen press, or alternatively, in a continuous manner by
compressing between rollers or other suitable compression equipment
at a linear speed and at a pressure sufficient to substantially
eliminate the pores, and at a temperature above usual room
temperature (about 20.degree. C.). The resultant dense material is
subsequently stretched above the crystalline melt temperature of
PTFE.
[0020] In one preferred aspect, the product is a sheet comprising
high density PTFE having improved permeation properties and
improved tensile properties. Specifically, the product has a water
vapor permeation coefficient of about 0.015 g-mm/m.sup.2 day or
less, more preferably about 0.010 g-mm/m.sup.2/day or less, and
even more preferably as low as about 0.003 g-mm/m.sup.2/day or
less, and has a matrix tensile strength of at least 15,000 psi in
at least one direction.
[0021] In another aspect of the invention, the product may comprise
a sheet of high density PTFE incorporating at least one filler and
having the improved barrier properties and other properties
described.
[0022] In another preferred embodiment, the product is a sheet
comprising a low permeation PTFE film laminated to another
substrate. Lamination can be achieved by adhering or co-joining
other films, e.g., by thermally, chemically or mechanically bonding
the materials. Specifically, this other substrate may include one
or more fluoropolymer sheets or films such as FEP, PFA, PTFE, THV
and other suitable fluoropolymers. Similarly, other polymer
substrate materials may include, but are not limited to,
polyurethanes, polyethylenes, polyamides, ethylene vinyl alcohol
(EVOH), polyvinylidene chloride (PVDC), and the like. Further, the
substrate may be metallic, glass, an inorganic sheet, pressure
sensitive adhesive(s), etc. Various laminated structures may be
made which facilitate or enhance further bonding to additional
layers (e.g., textiles, or the like). The barrier component of this
product has a water vapor permeation coefficient of about 0.015
g-mm/m.sup.2/day or less, more preferably about 0.010 g-mm/m.sup.2
day or less, and even more preferably as low as about 0.003
g-mm/m.sup.2 day or less. Even more preferred structures
incorporate a material with these superior barrier properties along
with a tensile strength of 10,000 psi or greater in both the x and
y directions.
[0023] The process is a process for making sheets of high density
PTFE, filled PTFE, or PTFE laminates with improved water vapor
permeation and improved tensile strength in both the x and y
directions. One such process comprises:
[0024] (a) densifying at least one sheet of expanded porous PTFE or
a bundle of layered sheets, in either sintered or unsintered form,
according to the teachings of Knox et al., U.S. Pat. No.
5,374,473,
[0025] (b) preheating the densified PTFE above the crystalline melt
temperature of PTFE, and
[0026] (c) stretching the heated PTFE membrane in the width
direction, the length direction, or both the width and length
direction, either sequentially in either order or simultaneously,
at a rate of at least 1 percent per second, more preferably at
least 3 percent per second, more preferably at 5 percent per second
or greater, and at a stretch ratio of greater than 4:1. It should
be appreciated that interactions of the mechanical properties of
the precursor and the stretch rate and/or stretch ratio for
stretching performed above the crystalline melt temperature can
impact the barrier performance of the resulting material, as
demonstrated in more detail in the Detailed Description and
Examples herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a chart of the temperature and pressure conditions
for densification of the membranes of Example 1.
[0028] FIG. 2 is a differential scanning calorimetry (DSC) scan of
a sample made in Example 1.
[0029] FIG. 3 is a graph showing water vapor permeation
coefficients for a material of the invention and for a range of
commercially available materials.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The objectives of this invention are accomplished by a
process involving, in an initial step, expanding a
polytetrafluoroethylene (PTFE) sheet or sheets and compressing said
sheet or sheets in a direction normal to the x-y plane in order to
achieve a bulk PTFE density of 2.11 g/cc or greater, such as is
described in U.S. Pat. No. 5,374,473 to Knox et al. ("Knox '473").
After compression, in a further processing step, the compressed
sheet(s) are heated to a temperature above the crystalline melt
temperature of PTFE and subsequently stretched. The resultant sheet
has greater tensile strength in the direction of stretch than the
compressed precursor from which it was made, has improved barrier
properties as demonstrated by increased water vapor permeation
resistance, and has reduced thickness, increased width and/or
increased length as dictated by the stretching operation performed.
This aspect of the invention is novel in that no one has heretofore
made a PTFE material with this unique combination of
properties.
[0031] The sheets, or films, of expanded PTFE were made in
accordance with the teachings of U.S. Pat. No. 3,953,566. In one
embodiment of the invention, an aliquot of the
polytetrafluoroethylene fine powder (PTFE 601A, DuPont, Wilmington,
Del.) was obtained and subsequently combined with a lubricant
(Isopar K aliphatic hydrocarbon, Exxon, Houston, Tex.). After
blending, the lubricated powder was compressed into a cylindrical
pellet and heat treated for a period of 18 hours. The pellet was
then extruded through a rectangular die at a reduction ratio of
70:1. The direction of paste extrusion is referred to as the y, or
machine, direction. The resulting tape was then dried. The dried
PTFE tape was then expanded in the y-direction between heated drums
at a linear rate of greater than 10%/second, a drum temperature of
about 225.degree. C. and a stretch amount equal to about 400%. The
tape was then expanded in the x-direction at a linear rate greater
than 10%/second, a temperature of about 295.degree. C. and a
stretch amount equal to about 700%. It is understood that this
expansion may be conducted in either direction or both directions,
either sequentially or simultaneously, utilizing a pantograph
machine or continuously on a tenter frame or similar machine.
Suitable expansion ratios may vary significantly, such as from 1:1
to 100:1, or greater and at varying expansion rates.
Representative, but in no way limiting, expansion rates and ratios
are included in the examples that follow. The films are next
compressed in accordance with the teachings of Knox '473.
[0032] The densified films were then stretched at temperatures
exceeding the crystalline melt temperature of PTFE. Stretch ratios
as high as 12:1 were achieved at stretch rates including, but not
limited to, 5% per second. It is understood that this stretching
process may be conducted in either direction, both directions
either sequentially or simultaneously utilizing a pantograph
machine or continuously on a tenter frame or similar machine. More
specifics are noted in individual examples.
[0033] It is believed that stretch ratios as high as 12:1, or
greater, in both the x and y directions are achievable, and one
skilled in the art realizes that the limitations associated with
stretch amount are a function of the original compressed precursor.
More specifically, stretch ratio is believed to be limited by the
original mechanical properties and thickness of the compressed
precursor. The thickness of the compressed precursor directly
impacts the ability to achieve high stretch amounts as when the
compressed precursor is stretched at a temperature above the
crystalline melt temperature of the ePTFE, the bulk density of the
compressed precursor is increased. The stretching results in a
reduction in unit weight and thickness. A significant increase in
the matrix tensile strength of the sheet or sheets is also
observed. As the following examples illustrate, matrix tensile
strengths of greater than 80,000 psi were achieved with an increase
in the bulk density of the PTFE sheet or sheets, as demonstrated by
the reduction in water vapor permeation. It is also believed that
greater matrix tensile strengths can be achieved through greater
amounts of stretch.
[0034] This invention is novel in that for the first time an
extremely thin, high PTFE bulk density film with extraordinarily
low water vapor permeation coefficients and high tensile strengths
in both the x and y directions can be produced. For example,
preferred thicknesses of less than 250 .mu.m, more preferably less
than 150 .mu.m, even less than 50 .mu.m, and most preferably even
less than 10 .mu.m, can be obtained. It is not intuitively obvious
that one can stretch a dense PTFE material and produce a finished
article with lower water vapor permeation, increased strength and
without reduction in bulk density.
[0035] The novel processing technology detailed above has enabled
the fabrication of a new, unique and novel PTFE sheet. As will be
further described in the following examples, this new material is a
PTFE with a water vapor permeation coefficient of about 0.015
g-mm/m.sup.2/day or less, more preferably about 0.010
g-mm/m.sup.2/day or less, and even more preferably as low as about
0.003 g-mm/m.sup.2/day or less. In addition, these materials
preferably have a matrix tensile strength in both the x and y
directions of at least 10,000 psi, more preferably at least 15,000
psi in at least one direction, and most preferably at least 25,000
psi in at least one direction. This material may be produced in an
array of lengths and widths, and thicknesses as low as
3.5.times.10.sup.-5- inches (0.9 .mu.m) or less have been achieved.
In addition, the novel PTFE sheet may be filled with one or more
fillers or incorporated in a composite sheet or sheets.
[0036] From a processing perspective this technology is unique and
affords a means of overcoming prior limitations to producing low
permeability films. In summary, the value of this processing
technology is the ability to significantly lower the water vapor
permeation of the sheet or sheets, significantly increase its
matrix tensile strength and reduce thickness.
[0037] The following examples are not intended to limit the scope
of this process or the materials that result therefrom.
Test Methods and Process Metrics
[0038] Water Vapor Permeability Testing and Water Vapor Permeation
Coefficient Determination
[0039] Determination of the water vapor permeability of the
materials was carried out in accordance with ASTM F-1249 by MOCON,
Inc. (Minneapolis, Minn.).
[0040] Specifically, the instrument used to test the water vapor
permeation of the materials was a MOCON Permatran W 3/31
(MOCON/Modern Controls, Inc., Minneapolis, Minn.). The permeant
used was 100% RH water vapor (49.157 mmHg), the carrier gas was
100% nitrogen, dry, at ambient pressure and the temperature at
which the test was carried out was 37.8.degree. C.
[0041] Test samples were cut to approximately 10 cm by 10 cm,
affixed in the instrument diffusion cell and conditioned according
to the instructions for the MOCON Permatran W 3/31. Water vapor
transmission rate, or water vapor permeability, was reported by the
instrument in g/m.sup.2/day.
[0042] The water vapor permeation coefficient of each sample was
calculated by multiplying the water vapor transmission rate by the
thickness of the test sample. Results are reported as
g-mm/m.sup.2/day.
[0043] Matrix Tensile Strength Testing
[0044] All specimens were tested according to ASTM D 882-90. A 20
in./min. (508 mm/min.) cross-head speed, 2 inch (51 mm) gauge
length and rectangular specimen of at least 5 inches (127 mm) in
length were employed.
[0045] The quantity matrix tensile is a means of expressing the
maximum load developed during the test as a function of the
cross-sectional area of material in the specimen. This provides a
means of accurately comparing tensile strengths among PTFE-based
specimen of varying density or porosity by normalizing the stress
at maximum load with respect to the cross-sectional area of the
PTFE within the sample.
[0046] Specifically:
Matrix Tensile (psi)=Max load (lb.)/X-sect area PTFE
(in..sup.2)
[0047] where, Max load=Maximum load specimen generates during
testing
x-sect area PTFE
(in..sup.2)=gperft/(12.times.P.times.2.54.sup.3)=gperft.t-
imes.2.3.times.10.sup.-3
[0048] where gperft=specimen unit weight in grams per 1 foot
P=Mean intrinsic density of PTFE=2.18 g/cc
Therefore Matrix Tensile (psi)=Max Load
(lb.)/(gperft.times.2.3.times.10.s- up.-3)
Similarly, Matrix Tensile (Mpa)=Matrix Tensile
(psi).times.6.89*10.sup.-3
[0049] Differential Scanning Calorimetry (DSC)
[0050] This test is performed using a TA Instruments Q1000 DSC and
TA Instruments standard aluminum pans and lids for DSC. A TA
Instruments Sample Encapsulation Press was used to crimp the lid to
the pan. Weight measurements were performed on a Sartorius MC 210P
microbalance.
[0051] One pan and lid were weighed on the balance to 0.01 mg
precision. Using a 6.0 mm die punch, enough discs of the test
sample material were added to the pan to constitute 6 mg, again
recorded to 0.01 mg precision. These values were entered into the
Thermal Advantage control software for the Q1000. The lid was
placed on the pan and was crimped using the press. Care was taken
to ensure that no sample material was caught in the crimp between
the lid and the pan. A similar pan for reference was prepared, with
the exception of the sample article, and its weight was also
entered into the software. The pan containing the sample article
was loaded onto the sample sensor in the Q1000 and the empty pan
was loaded onto the reference sensor. The samples were then
subjected to the following thermal cycling steps:
[0052] 1) Equilibrate at -50.00.degree. C.
[0053] 2) Ramp 20.00.degree. C./min to 360.00.degree. C.
[0054] 3) Isothermal for 5.00 min
[0055] 4) Mark end of cycle
[0056] 5) Ramp 20.00.degree. C./min to -50.00.degree. C.
[0057] 6) Mark end of cycle
[0058] 7) Ramp 20.00.degree. C./min to 420.00.degree. C.
[0059] 8) Mark end of cycle
[0060] 9) End of method
[0061] Data was analyzed, unaltered, using Universal Analysis 2000
v.3.9A from TA Instruments. Data from the scan indicated in step 7
were analyzed.
EXAMPLES
Example 1
[0062] A 240 lb. aliquot of PTFE fine powder (PTFE 601A, DuPont,
Wilmington, Del.) was combined with 44.16 lb. of lubricant (Isopar
K, Exxon, Houston, Tex.), subsequently blended, compressed into a
cylindrical pellet, and thermally conditioned for 18 hours at a
temperature of 49.degree. C. The cylindrical pellet was then
extruded through a rectangular die at a reduction ratio of 70:1.
The resultant tape was then dried in order to remove the
lubricant.
[0063] The dried PTFE tape was then expanded in the y-direction
between heated drums at a linear rate of greater than 10%/s, a drum
temperature of 225.degree. C. and stretch amount equal to 400%. The
tape was then expanded in the x-direction at a linear rate greater
than 10%/s, a temperature of about 295.degree. C. and stretch
amount equal to 700%. The resulting product was an unsintered ePTFE
membrane.
[0064] In order to determine the effect of sintering on the water
vapor permeability of membranes, a portion of this unsintered ePTFE
membrane was restrained and subsequently sintered so that two
membranes, one sintered and one unsintered, could be subjected to
further processing. By "sintering" is meant subjecting the material
to a temperature above the crystalline melt temperature of PTFE.
Sintering of the one membrane was accomplished by exposing the
membrane to a temperature of 375.degree. C. for 220 seconds by
passing it through an oven. No additional expansion was imparted
during the sintering operation.
[0065] The two resulting membrane precursors, one sintered and one
unsintered, were densified according to U.S. Pat. No. 5,374,473
Knox, et al. Specifically, four plies of the unsintered membrane
with a nominal thickness of 0.013 inch (330 .mu.m) and five plies
of the sintered membrane with a nominal thickness of 0.008 inch
(203 .mu.m) were placed between two caul plates in an autoclave bag
assembled from polyimide film (DuPont's KAPTON.RTM.). The assembly
was placed in an autoclave (Vacuum Press International Series 24),
vacuum was drawn in the bag and the pressure and temperature of the
autoclave were gradually raised based upon the temperature and
pressure conditions summarized in FIG. 1. The resultant compressed
ePTFE sheets, one sintered and one unsintered, were approximately
0.010 inch thick. Samples of each of the sintered and unsintered
forms of this intermediate PTFE were tested for water vapor
permeation and were found to be 0.1 and 0.127 g-mm/m.sup.2/day,
respectively.
[0066] The resultant compressed articles were then placed in a
pantograph machine wherein the material was heated above the
crystalline melt temperature of PTFE by exposure to air temperature
of about 370.degree. C. for a period of 20 minutes. The samples,
while still heated, were then stretched in the x-direction or
simultaneously in both the x and y-directions at stretch amounts of
up to 1100% and a stretch rate of 5% per second for each direction.
Processing conditions are summarized in Table 1. As shown in Table
1, for each of the sintered and unsintered membranes, two melt
stretch conditions were performed. Four test specimens were
analyzed from each combination of sintering condition and melt
stretch processing. Note that a two-pass process wherein the result
of the first melt stretch operation was used as the precursor of
the second stretch was used to produce these embodiments.
[0067] The samples were then subjected to Water Vapor Permeability
testing using the procedure described above herein. Table 2
summarizes the water vapor permeation coefficients, matrix tensile
strengths, thicknesses and percent crystallinity for the various
samples.
[0068] A sample processed according to processing condition "D"
identified in Table 2 was evaluated thermally using Differential
Scanning Calorimetry, and the resulting scan for this sample is
shown in FIG. 2.
1TABLE 1 Process Conditions for Samples of Example 1 Transverse
Machine Direction Direction Melt Melt Example Stretch Stretch
1.sup.st Pass 1.sup.st Pass 2.sup.nd Pass 2.sup.nd Pass ID (TDMS)
(MDMS) PTFE TDMS MDMS TDMS MDMS Number Ratio Ratio Precursor Ratio
Ratio Ratio Ratio A-1 9 3 Non-Sintered 3 3 3 1 A-2 9 3 Non-Sintered
3 3 3 1 A-3 9 3 Non-Sintered 3 3 3 1 A-4 9 3 Non-Sintered 3 3 3 1
B-1 12 3 Non-Sintered 3 3 4 1 B-2 12 3 Non-Sintered 3 3 4 1 B-3 12
3 Non-Sintered 3 3 4 1 B-4 12 3 Non-Sintered 3 3 4 1 C-1 9 3
Sintered 3 3 3 1 C-2 9 3 Sintered 3 3 3 1 C-3 9 3 Sintered 3 3 3 1
C-4 9 3 Sintered 3 3 3 1 D-1 12 3 Sintered 3 3 4 1 D-2 12 3
Sintered 3 3 4 1 D-3 12 3 Sintered 3 3 4 1 D-4 12 3 Sintered 3 3 4
1
[0069]
2TABLE 2 Properties of Samples of Example 1 Water Permeation
Tensile Coefficient Strength - Tensile Example (g * mm/ Length
Strength - ID - m{circumflex over ( )}2/day) Direction Width
Direction Thickness A-1 .0042 30420 psi 71360 psi 9.0 .mu.m A-2
.0056 32500 67600 9.0 A-3 .0042 31800 64920 9.0 A-4 .0040 30190
68940 9.0 B-1 .0037 26890 88880 8.5 B-2 .0024 24760 91910 8.5 B-3
.0021 24870 91660 8.5 B-4 .0024 28780 86910 8.5 C-1 .0054 29000
63670 14.0 C-2 .0091 30440 63800 14.0 C-3 .0103 29340 65340 14.0
C-4 .0054 28970 62540 14.0 D-1 .0043 24640 78060 13.0 D-2 .0043
25790 76910 13.0 D-3 .0039 23610 80460 13.0 D-4 .0036 26500 75310
13.0
Example 2
[0070] A laminate of dense PTFE and perfluoroacrylate (PFA) was
made in the following manner. Specifically, a PTFE material was
made according to the processing conditions noted for the "A"
samples described in Example 1, using a two-pass stretching
operation. Following the second pass stretching, the heater was
removed from the pantograph. While the stretched PTFE barrier film
remained on the pantograph pin body, a PFA film (Part No. 100 LP,
0.001 inch (25 .mu.m) thick, from DuPont, Wilmington, Del.) was
placed on one side of the PTFE barrier film. The two films were
then subjected to a temperature of 370.degree. C. for 5 minutes to
form a laminate of PTFE barrier film and PFA.
Example 3
[0071] An unsintered expanded PTFE material was made and densified
according to U.S. Pat. No. 5,374,473 Knox, et al. as described in
Example 1, except that only a single ply of the unsintered material
was subjected to the densification step. The resulting densified
material was then stretched according to the processing conditions
"A" described in Example 1. The resulting PTFE barrier film had a
thickness of 0.1 mils (2.5 .mu.m) and a water vapor permeation
coefficient of 0.007 g-mm/m.sup.2/day.
Example 4
[0072] An unsintered expanded PTFE material was made and densified
according to U.S. Pat. No. 5,374,473 Knox, et al. as described in
Example 1, except that only two plies of the unsintered material
were subjected to the densification step. The resulting densified
material was then stretched according to the processing conditions
"B" described in Example 1. The resulting PTFE barrier film had a
thickness of 0.1 mils (2.5 .mu.m) and a water vapor permeation
coefficient of 0.003 g-mm/m.sup.2/day.
COMPARATIVE EXAMPLE
[0073] In an attempt to roughly assess the relative water vapor
permeation coefficient of the material of the present invention
versus commercially available fluoropolymer materials, a series of
commercially available fluoropolymer films was evaluated. Four
samples of each of the following films were sent to MOCON, Inc. for
determination of water vapor permeation coefficient, and the
results are shown graphically in FIG. 3:
3 Material Source ACLAR ULTREX .RTM. 2000 film Honeywell
Corporation ACLAR ULTREX .RTM. 3000 film Honeywell Corporation
DEWAL 200T .RTM. film (2 mil) DeWAL Industries DEWAL 220T .RTM.
film (2 mil) DeWAL Industries DEWAL 502T .RTM. film (2 mil) DeWAL
Industries FEP (2 mil) DuPont PFA (2 mil) DuPont
[0074] Comparison was made in FIG. 3 with sample D-4 of Example
1.
* * * * *