U.S. patent application number 10/382668 was filed with the patent office on 2004-09-09 for ptfe membranes and gaskets made therefrom.
Invention is credited to Bowen, Christopher, Dove, Kevin Edward, Jones, Carl, Minor, Raymond B..
Application Number | 20040173978 10/382668 |
Document ID | / |
Family ID | 32926937 |
Filed Date | 2004-09-09 |
United States Patent
Application |
20040173978 |
Kind Code |
A1 |
Bowen, Christopher ; et
al. |
September 9, 2004 |
PTFE membranes and gaskets made therefrom
Abstract
Improved expanded PTFE materials and improved gasket materials
made therefrom, the gaskets being capable of forming a seal with
greater bolt load retention than is possible with existing PTFE
gaskets. The expanded PTFE membranes of the invention can be
tailored to exhibit a matrix tensile strength in at least one
direction of at least 25,000 psi, a matrix tensile strength ratio
in two orthogonal directions of between 0.025 and 4, an orientation
index of 50.degree. or less, and a density of 2.0 g/cc or less. The
improved gaskets exhibit improved mechanical properties such as
high bolt load retention, low creep, high tensile strength, low
stress to seal and high crystallinity index.
Inventors: |
Bowen, Christopher; (Lincoln
University, PA) ; Dove, Kevin Edward; (Wilmington,
DE) ; Jones, Carl; (Baltimore, MD) ; Minor,
Raymond B.; (Elkton, MD) |
Correspondence
Address: |
W. L. Gore & Associates, Inc.
551 Paper Mill Road
P.O. Box 9206
Newark
DE
19714-9206
US
|
Family ID: |
32926937 |
Appl. No.: |
10/382668 |
Filed: |
March 6, 2003 |
Current U.S.
Class: |
277/650 |
Current CPC
Class: |
B32B 2307/732 20130101;
B29L 2031/265 20130101; Y10T 428/269 20150115; B32B 7/12 20130101;
F16L 23/22 20130101; B32B 2307/548 20130101; B29C 48/08 20190201;
B32B 3/26 20130101; B29C 48/475 20190201; B32B 2307/7242 20130101;
F16J 15/102 20130101; F16J 15/3284 20130101; B32B 27/322 20130101;
B32B 2307/54 20130101; B29K 2027/18 20130101; B32B 2307/546
20130101; B32B 27/205 20130101; Y10T 428/249978 20150401; B32B
2307/50 20130101; B32B 2307/714 20130101; F16J 15/104 20130101;
B32B 2307/552 20130101; B32B 2581/00 20130101; B32B 27/08 20130101;
B32B 2307/72 20130101; Y10T 428/215 20150115 |
Class at
Publication: |
277/650 |
International
Class: |
F16J 015/08 |
Claims
The invention claimed is:
1. A membrane comprising PTFE having a matrix tensile strength in
at least one direction of at least 25,000 psi; a matrix tensile
strength ratio in two orthogonal directions of between about 0.25
and 4; an orientation index of about 50.degree. or less; and a
density of about 2.0 g/cc or less.
2. The membrane of claim 1 having an orientation index of
40.degree. or less.
3. The membrane of claim 2 having an orientation index of
30.degree. or less.
4. The membrane of claim 3 having an orientation index of
20.degree. or less.
5. The membrane of claim 1 having a crystallinity index of 60% or
more.
6. The membrane of claim 5 having a crystallinity index of 65% or
more.
7. The membrane of claim 6 having a crystallinity index of 70% or
more.
8. The membrane of claim 1 having a density of about 1.0 g/cc or
less.
9. The membrane of claim 8 having a density of about 0.5 g/cc or
less.
10. The membrane of claim 1 having an enthalpy of at least about
9.0 Joules per gram (J/g).
11. The membrane of claim 10 having an enthalpy of at least 10.0
J/g.
12. The membrane of claim 11 having an enthalpy of at least 11.0
J/g.
13. A membrane comprising PTFE having a matrix tensile strength in
at least one direction of at least about 34,000 psi; a matrix
tensile strength ratio in 2 orthogonal directions of between about
0.25 and 4; and a density of about 2.0 g/cc or less.
14. The membrane of claim 13 having an orientation index of
50.degree. or less.
15. The membrane of claim 14 having an orientation index of
40.degree. or less.
16. The membrane of claim 15 having an orientation index of
30.degree. or less.
17. The membrane of claim 13 having a crystallinity index of 60% or
more.
18. The membrane of claim 17 having a crystallinity index of 65% or
more.
19. The membrane of claim 18 having a crystallinity index of 70% or
more.
20. The membrane of claim 13 having a density of about 1.0 g/cc or
less.
21. The membrane of claim 20 having a density of about 0.5 g/cc or
less.
22. The membrane of claim 13 having an enthalpy of at least about
9.0 Joules per gram (J/g).
23. The membrane of claim 22 having an enthalpy of at least 10.0
J/g.
24. The membrane of claim 23 having an enthalpy of at least 11.0
J/g.
25. A membrane comprising PTFE having a matrix tensile strength in
at least one direction of at least about 44,800 psi; a matrix
tensile strength ratio in 2 orthogonal directions of between about
0.25 and 4; and a density of about 2 g/cc or less.
26. The membrane of claim 25 having an orientation index of
50.degree. or less.
27. The membrane of claim 26 having an orientation index of
40.degree. or less.
28. The membrane of claim 27 having an orientation index of
30.degree. or less.
29. The membrane of claim 25 having a crystallinity index of 60% or
more.
30. The membrane of claim 29 having a crystallinity index of 65% or
more.
31. The membrane of claim 30 having a crystallinity index of 70% or
more.
32. The membrane of claim 25 having a density of about 1.0 g/cc or
less.
33. The membrane of claim 332 having a density of about 0.5 g/cc or
less.
34. The membrane of claim 25 having an enthalpy of at least about
9.0 Joules per gram (J/g).
35. The membrane of claim 34 having an enthalpy of at least 10.0
J/g.
36. The membrane of claim 35 having an enthalpy of at least 11.0
J/g.
37. A membrane consisting essentially of a single layer of PTFE
having a matrix tensile strength in at least one direction of at
least 25,000 psi; a matrix tensile strength ratio in 2 orthogonal
directions of between about 0.25 and 4; and a density of about 2.0
g/cc or less.
38. The membrane of claim 37 having an orientation index of
50.degree. or less.
39. The membrane of claim 38 having an orientation index of
40.degree. or less.
40. The membrane of claim 39 having an orientation index of
30.degree. or less.
41. The membrane of claim 37 having a crystallinity index of 60% or
more.
42. The membrane of claim 41 having a crystallinity index of 65% or
more.
43. The membrane of claim 42 having a crystallinity index of 70% or
more.
44. The membrane of claim 37 having a density of about 1.0 g/cc or
less.
45. The membrane of claim 44 having a density of about 0.5 g/cc or
less.
46. The membrane of claim 37 having an enthalpy of at least about
9.0 Joules per gram (J/g).
47. The membrane of claim 46 having an enthalpy of at least 10.0
J/g.
48. The membrane of claim 47 having an enthalpy of at least 11.0
J/g.
49. A gasket comprising PTFE, said gasket having: a thickness in
the range of about 1.5 mm to about 9 mm a compressibility of at
least about 40%, and a percent bolt load retention versus thickness
equal to or greater than the line connecting points A and B in FIG.
18 and defined in the following table:
12 Point % BLR at RT Thickness A 92% 1.5 mm B 52% 9 mm
50. The gasket of claim 49, wherein said gasket has a crystallinity
index of at least about 60%.
51. The gasket of claim 49, wherein said gasket has a tensile
strength in at least one direction of at least about 5,500 psi
(37.9 MPa).
52. The gasket of claim 49, wherein said gasket has a durometer of
about 85 or less.
53. The gasket of claim 49, wherein said gasket has a matrix
tensile strength of at least about 22,000 psi (151.7 MPa).
54. The gasket of claim 49, wherein said gasket consists
essentially of PTFE.
55. The gasket of claim 49, wherein said gasket has a percent bolt
load retention versus thickness equal to or greater than the line
connecting the points C and D in FIG. 18 and defined in the
following table:
13 Point % BLR at RT Thickness C 93% 1.5 mm D 70% 9 mm
56. The gasket of claim 55, wherein said gasket has a crystallinity
index of at least about 60%.
57. The gasket of claim 55, wherein said gasket has a tensile
strength in at least one direction of at least about 5,500 psi
(37.9 MPa).
58. The gasket of claim 55, wherein said gasket has a durometer of
about 85 or less.
59. The gasket of claim 55, wherein said gasket has a matrix
tensile strength of at least about 22,000 psi (151.7 MPa).
60. The gasket of claim 55, wherein said gasket consists
essentially of PTFE.
61. A gasket comprising PTFE having a nominal thickness of 1.5 mm,
a bolt load retention of at least about 91% and a compressibility
of at least 40%.
62. A gasket comprising PTFE having a nominal thickness of 2 mm, a
bolt load retention of at least about 90% and a compressibility of
at least 40%.
63. A gasket comprising PTFE having a nominal thickness of 3 mm, a
bolt load retention of at least about 85% and a compressibility of
at least 40%.
64. A gasket comprising PTFE having a nominal thickness of 6 mm, a
bolt load retention of at least about 65% and a compressibility of
at least 40%.
65. A gasket comprising PTFE having a nominal thickness of 1.5 mm,
a bolt load retention of at least about 91% and a durometer of 85
or less.
66. A gasket comprising PTFE having a nominal thickness of 2 mm, a
bolt load retention of at least about 90% and a durometer of 85 or
less.
67. A gasket comprising PTFE having a nominal thickness of 3 mm, a
bolt load retention of at least about 85% and a durometer of 85 or
less.
68. A gasket comprising PTFE having a nominal thickness of 6 mm, a
bolt load retention of at least about 65% and a durometer of 85 or
less.
69. A gasket comprising PTFE having a nominal thickness of 1 mm, a
bolt load retention of at least about 93%, a compressibility of at
least 40%, and a crystallinity index of at least about 60%.
70. A gasket comprising PTFE having a nominal thickness of 1 mm, a
bolt load retention of at least about 93%, a compressibility of at
least 40%, and a tensile strength in at least one direction of at
least about 5,500 psi (37.9 MPa).
71. The gasket of claim 61, wherein said gasket has a crystallinity
index of at least about 60%.
72. The gasket of claim 61, wherein said gasket has a tensile
strength in at least one direction of at least about 5,500 psi
(37.9 MPa).
73. The gasket of claim 62, wherein said gasket has a crystallinity
index of at least about 60%.
74. The gasket of claim 62, wherein said gasket has a tensile
strength in at least one direction of at least about 5,500 psi
(37.9 MPa).
75. The gasket of claim 63, wherein said gasket has a crystallinity
index of at least about 60%.
76. The gasket of claim 63, wherein said gasket has a tensile
strength in at least one direction of at least about 5,500 psi
(37.9 MPa).
77. The gasket of claim 64, wherein said gasket has a crystallinity
index of at least about 60%.
78. The gasket of claim 64, wherein said gasket has a tensile
strength in at least one direction of at least about 5,500 psi
(37.9 MPa).
79. The gasket of claim 65, wherein said gasket has a crystallinity
index of at least about 60%.
80. The gasket of claim 65, wherein said gasket has a tensile
strength in at least one direction of at least about 5,500 psi
(37.9 MPa).
81. The gasket of claim 66, wherein said gasket has a crystallinity
index of at least about 60%.
82. The gasket of claim 66, wherein said gasket has a tensile
strength in at least one direction of at least about 5,500 psi
(37.9 MPa).
83. The gasket of claim 67, wherein said gasket has a crystallinity
index of at least about 60%.
84. The gasket of claim 67, wherein said gasket has a tensile
strength in at least one direction of at least about 5,500 psi
(37.9 MPa).
85. The gasket of claim 68, wherein said gasket has a crystallinity
index of at least about 60%.
86. The gasket of claim 68, wherein said gasket has a tensile
strength in at least one direction of at least about 5,500 psi
(37.9 MPa).
87. A gasket comprising PTFE wherein said gasket has a nominal
thickness of 3 mm; a tensile strength in at least one direction of
at least 4,500 psi (31.0 MPa); a bolt load retention of at least
85%; and a stress to seal equal to or less than 3500 psi (24.1
MPa).
88. The gasket of claim 87, wherein said tensile strength in at
least one direction is at least 5,500 psi (37.9 MPa).
89. The gasket of claim 87, wherein said tensile strength is at
least 6,000 psi (41.4 MPa).
90. The gasket of claim 87, wherein said tensile strength is at
least 7,000 psi (48.3 MPa).
91. The gasket of claim 87, wherein said gasket comprises multiple
PTFE membranes.
92. The gasket of claim 87, wherein said gasket comprises at least
one rigid layer.
93. The gasket of claim 87, wherein said rigid layer comprises
PTFE.
94. The gasket of claim 87 in the form of a form-in-place tape.
95. A gasket comprising PTFE wherein said gasket has a nominal
thickness of 1.5 mm; a tensile strength in at least one direction
of at least 4,500 psi (31.0 MPa); a bolt load retention of at least
91%; and a stress to seal equal to or less than 3500 psi (24.1
MPa).
96. The gasket of claim 95, wherein said tensile strength in at
least one direction is at least 5,500 psi (37.9 MPa).
97. The gasket of claim 95, wherein said tensile strength is at
least 6,000 psi (41.4 MPa).
98. The gasket of claim 95, wherein said tensile strength is at
least 7,000 psi (48.3 MPa).
99. The gasket of claim 95, wherein said gasket comprises multiple
PTFE membranes.
100. The gasket of claim 95, wherein said gasket comprises at least
one rigid layer.
101. The gasket of claim 95, wherein said rigid layer comprises
PTFE.
102. The gasket of claim 95 in the form of a form-in-place
tape.
103. A gasket comprising PTFE, said PTFE having a tensile strength
in at least one direction of at least about 5,500 psi (37.9 MPa)
and a compressibility of at least about 40%.
104. The gasket of claim 103, said gasket having a tensile strength
in at least one direction of at least about 6,000 psi (41.4
MPa).
105. The gasket of claim 103, said gasket having a tensile strength
in at least one direction of at least about 7,000 psi (48.3
MPa).
106. The gasket of claim 103, wherein said gasket comprises
multiple PTFE membranes.
107. The gasket of claim 103, wherein said gasket comprises at
least one rigid layer.
108. The gasket of claim 107, wherein said rigid layer comprises
PTFE.
109. A gasket comprising PTFE, said PTFE having a tensile strength
in at least one direction of at least about 5,500 psi (37.9 MPa)
and a durometer of 85 or less.
110. The gasket of claim 109, said gasket having a tensile strength
in at least one direction of at least about 6,000 psi (41.4
MPa).
111. The gasket of claim 109, said gasket having a tensile strength
in at least one direction of at least about 7,000 psi (48.3
MPa).
112. The gasket of claim 109, wherein said gasket comprises
multiple PTFE membranes.
113. The gasket of claim 109, wherein said gasket comprises at
least one rigid layer.
114. The gasket of claim 109, wherein said rigid layer comprises
PTFE.
115. A gasket consisting essentially of PTFE, said gasket having a
bolt load retention of at least about 85% for a nominal 3 mm
thickness.
116. A gasket consisting essentially of PTFE, said gasket having a
bolt load retention of at least about 91% for a nominal 1.5 mm
thickness.
117. A gasket consisting essentially of PTFE, said gasket having a
matrix tensile strength of at least about 22,000 psi (151.7
MPa).
118. The gasket of claim 117, wherein said matrix tensile strength
is at least about 25,000 psi (172.4 MPa).
119. A gasket consisting essentially of PTFE, said gasket having a
creep of less than 5.0% and a nominal thickness of 1.5 mm.
120. A gasket comprising PTFE, said gasket having a compressibility
of at least about 40%, a creep of less than 5% and a nominal
thickness of 1.5 mm.
121. A gasket comprising PTFE, said gasket having a durometer of 85
or less, a creep of less than 5% and a nominal thickness of 1.5
mm.
122. A gasket comprising PTFE, said gasket having a nominal
thickness of 1.5 mm, a tensile strength in at least one direction
of at least about 5,500 psi ((37.9 MPa) and creep of less than
5%.
123. The gasket of claim 122, said gasket having a tensile strength
in at least one direction of at least about 6,000 psi (41.4
MPa).
124. The gasket of claim 122, said gasket having a tensile strength
in at least one direction of at least about 7000 psi (48.3
MPa).
125. A gasket comprising PTFE, said gasket having a nominal
thickness of 1.5 mm, a bolt load retention of at least about 91%
and a stress to seal of less than or equal to 3500 psi (24.1
MPa).
126. A gasket comprising PTFE, said gasket having a nominal
thickness of 3 mm, a bolt load retention of at least about 85% and
a stress to seal of less than or equal to 3500 psi (24.1 MPa).
127. A gasket comprising PTFE, said gasket having a nominal
thickness of 1.5 mm, a bolt load retention of at least about 91%
and a stress to seal of less than or equal to 1500 psi (10.3
MPa).
128. A gasket comprising PTFE, said gasket having a nominal
thickness of 3 mm, a bolt load retention of at least about 85% and
a stress to seal of less than or equal to 1500 psi (10.3 MPa).
129. A gasket comprising PTFE, said gasket having a nominal
thickness of 6 mm, a bolt load retention of at least about 65% and
a stress to seal of less than or equal to 1500 psi (10.3 MPa).
130. A gasket comprising PTFE, said gasket having a nominal
thickness of 1.5 mm, a creep of less than 5% and a stress to seal
of less than or equal to 3500 psi (24.1 MPa).
131. The gasket of claim 130, said gasket having a stress to seal
of less than or equal to 1500 psi (10.3 MPa).
132. A gasket comprising PTFE, said gasket having a nominal
thickness of 3 mm, a tensile strength in at least one direction of
at least 5,500 psi (37.9 MPa) and a stress to seal of 3500 psi
(24.1 MPa) or less.
133. The gasket of claim 132, said gasket having a tensile strength
in at least one direction of at least 6000 psi (41.4 MPa).
134. The gasket of claim 132, said gasket having a tensile strength
in at least one direction of at least 7000 psi (48.3 MPa).
135. The gasket of claim 132, said gasket having a stress to seal
of 1500 psi (10.3 MPa) or less.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to improved expanded PTFE
materials and to improved gaskets made therefrom, the gaskets being
capable of forming a seal with greater bolt load retention than is
possible with existing PTFE gaskets.
BACKGROUND OF THE INVENTION
[0002] A wide variety of gaskets are known for use in sealing
applications. Expanded polytetrafluoroethylene (PTFE) is widely
used today as a gasket material. As disclosed in U.S. Pat. No.
3,953,566 to Gore, this material has numerous properties making it
highly desirable as a gasket. These properties include being
readily compressible and conformable, being chemically resistant,
having relatively high strength, and being far less prone to creep
and loss of sealing pressure than non-expanded, non-porous PTFE
alone.
[0003] In many sealing applications, the gasket is used to seal the
junction between flanges, such as between pipes. In such
applications, expanded PTFE is a desirable material for the gaskets
because the expanded PTFE gasket can be placed between the flanges,
and the flanges can then be pressed together with the application
of force, such as by tightening of bolts. This application of force
compresses the expanded PTFE. As the expanded PTFE is compressed,
its initial pore volume is reduced, thus densifying the expanded
PTFE. Particularly with metal-to-metal flanges, it is possible to
apply sufficient force (or "stress") to the flanges to fully
densify the expanded PTFE. Thus, in at least part of the expanded
PTFE gasket, the pore volume is reduced to substantially zero, such
that a fluid contained within the pipes is prevented from leaking
between the flanges by the densified, non-porous PTFE gasket, which
seals the flanges.
[0004] In many applications, particularly when harsh chemicals are
used which would readily break down the metal or the metal could
contaminate the chemical which is being transported or housed, it
is common to use glass-lined steel, glass, or fiberglass reinforced
plastic ("FRP") piping and vessels. Because this equipment is so
often used with extremely harsh chemicals, there is great desire to
use PTFE gaskets to seal the connecting flanges of this equipment
because of the well known extraordinary chemical resistance of
PTFE. Unfortunately, non-expanded, non-porous PTFE gaskets are
generally not conformable enough to effectively seal this type of
equipment. In the case of glass-lined steel flanges, although there
is a relatively smooth finish, there is often a large amount of
unevenness or lack of flatness associated with the flanges. This
unevenness or lack of flatness requires the gasket to have to
conform to large variations around the perimeter as well as between
the internal and external diameter of the flange in order to create
an effective seal. Thus, a non-expanded, non-porous PTFE gasket is
not conformable enough to seal many of these applications.
[0005] Because expanded PTFE is so conformable, it would be
desirable to use expanded PTFE to seal these commonly uneven
flanges. Unfortunately, in many of these applications it is not
possible to apply sufficient force to the flanges to create enough
gasket stress to fully densify the expanded PTFE gasket to create
an effective seal. For example, glass-lined steel piping flanges,
glass flanges, or FRP piping flanges may deform, fracture, or break
upon the application of a high amount of stress. Thus, in these
applications, an expanded PTFE gasket may not be completely
densified to reach a non-porous state, and therefore does not
become leak proof, because the maximum stress that can be applied
to the flanges without breaking them is not sufficient to so
densify the gasket.
[0006] U.S. Pat. No. 6,485,809, in the name of Minor et al.,
teaches a low stress to seal gasket construction comprising a
multilayer, unitary gasket including at least one inner layer of
expanded PTFE disposed between a first substantially air
impermeable outer layer and a second substantially air impermeable
outer layer, and a substantially air impermeable region bridging
the first and second substantially air impermeable layers. By "low
stress to seal" is meant a gasket which provides a substantially
air tight, or air impermeable, seal upon the application of a
relatively low stress (i.e., a stress below that required to fully
densify a porous expanded PTFE gasket, generally less than about
20,700 kPa (3000 psi)). The Minor et al. gasket forms a
substantially air impermeable seal when compressed at low stress.
This patented construction overcomes many challenges in creating a
desired low stress to seal gasket. However, improvements to such a
construction are still desirable.
[0007] While expanded PTFE materials exhibit much better
performance than non-expanded, non-porous PTFE in gasket
applications, it still exhibits some propensity to creep, or flow,
under load. Thus, expanded PTFE materials with improved performance
characteristics are desirabe.
[0008] It has been taught that PTFE structures can be stretched
above the melt temperature to impart improved properties to the
PTFE. U.S. Pat. No. 2,776,465, to Smith, for example, teaches
stretching non-expanded PTFE materials at temperatures above about
325.degree. C. to achieve higher tenacity and modulus in the
resulting structures.
[0009] U.S. Pat. No. 5,814,405, to Branca et al., teaches heating
an expanded amorphously locked article to a temperature above the
crystalline melting temperature of the PTFE and stretching in at
least the direction orthogonal to the direction of stretch carried
out below the melt temperature. The resulting articles exhibit a
microstructure which can be characterized as having highly
elongated nodes with aspect ratios greater than 25 to 50,
preferably greater than 150. Such membranes also exhibit high air
flow and high strength. These membranes are useful as filters in
filtration devices, as scaffolding for holding reactive or
conductive fillers and as support layers in composite
constructions.
[0010] A need has existed, however, for improved expanded PTFE
materials with enhanced properties to meet the ever-increasing
demands for improved gasketing performance, as well as for other
high performance applications.
[0011] Particularly, while it is desirable to provide low stress to
seal characteristics in a gasket, it is also important from a
performance perspective for a gasket to exhibit good bolt load
retention, which is a measure of the resistance to stress
relaxation of the gasket material. The amount of leakage associated
with a gasketed assembly is dependent on the amount of compressive
load, also known as bolt load, on the gasket. "Bolt load
retention," as used herein, is intended to refer to the retention
of the compressive load supplied to a gasket through a pair of
flanges from the tightening of the bolts or clamps used to fasten
the pair of flanges. Typically, the higher the bolt load on a
gasket the lower the leakage will be from that gasketed assembly.
PTFE gaskets (i.e., greater than about 50% PTFE, by weight) are
prone to creep and stress relaxation when subjected to a
compressive load. Reducing the amount of creep and stress
relaxation in the gasket material results in higher bolt load
retention by the gasket. Higher bolt load retention in the gasket
produces a tighter seal with less leakage over the life of the
gasket.
[0012] Thus, what has been desired for many years is an
easy-to-use, highly chemically resistant gasket which can
effectively conform to flange surfaces and sustain a high bolt load
retention to maintain a tight seal over the life of the gasket.
Accordingly, a purpose of the present invention is to provide not
only such high performance gaskets, but also to provide improved
expanded PTFE materials which can be used to achieve these
goals.
[0013] Further, it has been desired to provide a highly chemical
resistant, highly conformable gasket which not only has the ability
to seal at low loads, but also sustains a high bolt load retention
even when subjected to high loads. Such a universal gasket could be
effectively used for both low load applications such as glass-lined
steel, glass and FRP piping and vessels, as well as for high load
applications such as with metal piping and vessels.
[0014] These and other purposes of the present invention will be
provided herein.
SUMMARY OF THE INVENTION
[0015] The present invention concerns improved expanded PTFE
membranes and gasketing materials and gaskets incorporating these
improved PTFE membranes.
[0016] The improved expanded PTFE membranes exhibit improved
combinations of mechanical properties which were heretofore not
achieved by the teachings of the prior art. Specifically, PTFE
membranes of the invention can be tailored to exhibit a matrix
tensile strength in at least one direction of at least 25,000 psi
(172 MPa), with a matrix tensile strength ratio in two orthogonal,
or perpendicular, directions of between 0.025 and 4, and an
orientation index of 50.degree. or less. Densities of these
membranes are 2.0 g/cc or less, more preferably 1.9 g/cc or less,
more preferably 1.8 g/cc or less, even more preferably 1.7 g/cc or
less, more preferably 1.6 g/cc or less, more preferably 1.5 g/cc or
less, more preferably 1.0 g/cc or less and most preferably 0.5 g/cc
or less. In an alternative embodiment, the membranes may exhibit a
matrix tensile strength in at least one direction of at least
34,000 psi (234 MPa), more preferably at least 44,800 psi (309
MPa), a matrix tensile strength ratio between 0.025 and 4 and a
density of 2.0 g/cc or less. The membranes can further exhibit a
crystallinity index greater than 50%, more preferably greater than
60%, and most preferably greater than 68%; and an enthalpy of at
least 9.0 J/g, more preferably at least 10.0 J/g, and most
preferably at least 11.0 J/g. Orientation indices of less than
50.degree., more preferably less than 40.degree., even more
preferably less than 30.degree., and most preferably less than
20.degree. can also be achieved in the unique membranes of the
invention.
[0017] Expanded PTFE membranes of the present invention exhibiting
such combinations of improved mechanical properties are prepared by
ram extruding a lubricated PTFE powder to form a tape, then
calendering, drying, stretching in the machine, or longitudinal,
direction at temperatures at or above 325.degree. C., then
stretching perpendicular to the machine direction (i.e., the
transverse direction) again at temperatures at or above 325.degree.
C. while constraining the tape from shrinking in the longitudinal
direction, then subjecting the stretched tape to a 380.degree. C.
heat zone while constrained. A unique feature of the present
invention is that by varying the degree of calendering of the tape,
and by varying the stretch temperatures, stretch rates and stretch
ratios for both directions, membranes which are surprisingly strong
and which have very balanced strengths can be produced. In
addition, higher crystallinity and lower orientation indices can be
achieved. Suitable processing variations for achieving membranes
with these enhanced properties are contained in the Examples
herein.
[0018] It was discovered that the improved PTFE membranes of this
invention can be incorporated into gasket materials and gaskets
exhibiting improved mechanical properties such as high bolt load
retention, low creep, high tensile strength, low stress to seal and
high crystallinity index. Moreover, it was discovered that PTFE
gaskets exhibiting the highest bolt load retention and lowest creep
characteristics ever produced could be constructed using these
improved PTFE membranes. As used herein for convenience, the term
"gasket" is intended to refer to materials in sheet, tape or
finished (i.e., cut or shaped, such as annular, etc.) gasket
form.
[0019] In one embodiment, the gasket material of the present
invention comprises a multilayer construction comprising PTFE
membrane. For example, the gasket material may be formed by
wrapping the PTFE membrane onto a mandrel to a desired thickness,
heating to unify the layers into a unitary multilayer construction,
then cutting the construction off the mandrel to yield a sheet of
gasket material. Depending on the desired end application, gaskets
may be cut or configured from the gasket material to any specified
dimensions.
[0020] Alternatively, the gasket materials of this invention may be
cut from the mandrel to form a continuous, form-in-place gasket,
such as is described in U.S. Pat. No. 5,964,465. In a further
embodiment, the gasket materials of the present invention may be
incorporated into a low stress to seal gasket construction such as
that described in U.S. Pat. No. 6,485,809. In a further embodiment,
the gasket materials of the present invention may be incorporated
into a gasket construction such as that described in U.S. Pat. No.
5,879,789.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The present invention is described herein in conjunction
with the accompanying drawings, in which:
[0022] FIGS. 1a and 1b are scanning electron micrographs (SEMs)
taken at 1000.times. and 4000.times., respectively, of the membrane
made in accordance with Example 1.
[0023] FIGS. 2a and 2b are SEMs taken at 1000.times. and
4000.times., respectively, of the membrane made in accordance with
Example 2.
[0024] FIGS. 3a and 3b are SEMs taken at 1000.times. and
4000.times., respectively, of the membrane made in accordance with
Example 3.
[0025] FIGS. 4a and 4 are SEMs taken at 1000.times. and
4000.times., respectively, of the membrane made in accordance with
Example 4.
[0026] FIGS. 5a and 5b are SEMs taken at 1000.times. and
4000.times., respectively, of the membrane made in accordance with
Example 5.
[0027] FIGS. 6a and 6b are SEMs taken at 1000.times. and
4000.times., respectively, of the membrane made in accordance with
Example 6.
[0028] FIGS. 7a and 7b are SEMs taken at 1000.times. and
4000.times., respectively, of the membrane made in accordance with
Example 7.
[0029] FIGS. 8a and 8b are SEMs taken at 1000.times. and
4000.times., respectively, of the membrane made in accordance with
Example 8.
[0030] FIGS. 9a and 9b are SEMs taken at 1000.times. and
4000.times., respectively, of the membrane made in accordance with
Example 9.
[0031] FIG. 10 is a three-quarter perspective view of one
embodiment of a gasket or gasket material of the present
invention.
[0032] FIG. 11 is an elevational view of one embodiment of an
apparatus which may be used to produce gaskets of the present
invention.
[0033] FIG. 12 is an elevational view of another embodiment of an
apparatus which may be used to produce gaskets of the present
invention.
[0034] FIG. 13 is a top view of a gasket according to an exemplary
embodiment of the present invention.
[0035] FIG. 14 is a side cross-sectional view of the gasket of FIG.
13;
[0036] FIG. 15 is an exploded side cross-sectional view of a
portion of the gasket of FIG. 14.
[0037] FIG. 16 is an elevational view of another embodiment of a
gasket construction of the present invention.
[0038] FIG. 17 is an elevational view of a further embodiment of a
gasket construction of the present invention.
[0039] FIG. 18 is a graph showing plots indicating unique
characteristics of bolt load retention versus thickness exhibited
by gaskets of the present invention.
[0040] FIG. 19 is a graph of l vs. 2.theta. scans used in
determining the crystallinity index characteristics of materials of
the present invention.
[0041] FIGS. 20 and 21 are representative azimuthal scans used in
determining the crystallinity index characteristics of materials of
the present invention.
[0042] FIG. 22 is a photograph showing the apparatus used for
determining the stress to seal values of materials of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0043] The present invention concerns improved expanded PTFE
membranes and gasketing materials and gaskets incorporating these
improved PTFE membranes.
[0044] Referring to FIGS. 1a and 1b, there is shown a scanning
electron micrograph (SEM) at 1000.times. and 4000.times.,
respectively, of an exemplary membrane of the present invention
made in accordance with Example 1, herein. The membrane is observed
to have a structure of small nodes (less than 1 .mu.m in diameter)
connected by randomly arranged fibrils.
[0045] Referring to FIGS. 2a and 2b, there is shown an SEM at
1000.times. and 4000.times., respectively, of a membrane of the
present invention made in accordance with Example 2, herein. The
membrane is observed to have a structure of elongated nodes running
in parallel connected by fibrils which are largely perpendicular to
the major node axis.
[0046] Referring to FIGS. 3a and 3b, there is shown an SEM at
1000.times. and 4000.times., respectively, of a membrane of the
present invention made in accordance with Example 3, herein. The
membrane is observed to have a structure of small nodes (less than
2 .mu.m) connected by randomly arranged fibrils.
[0047] Referring to FIGS. 4a and 4b, there is shown an SEM at
1000.times. and 4000.times., respectively, of a membrane of the
present invention made in accordance with Example 4, herein. The
membrane is observed to have a structure of elongated nodes running
in parallel connected by fibrils which are largely perpendicular to
the major node axis.
[0048] Referring to FIGS. 5a and 5b, there is shown an SEM at
1000.times. and 4000.times., respectively, of a membrane of the
present invention made in accordance with Example 5, herein. The
membrane is observed to have a structure of small nodes (less than
about 5 .mu.m for the major axis) connected by randomly arranged
fibrils.
[0049] Referring to FIGS. 6a and 6b, there is shown an SEM at
1000.times. and 4000.times., respectively, of a membrane of the
present invention made in accordance with Example 6, herein. The
membrane is observed to have a structure of elongated nodes running
in parallel connected by fibrils which are largely perpendicular to
the major node axis.
[0050] Referring to FIGS. 7a and 7b, there is shown an SEM at
1000.times. and 4000.times., respectively, of a membrane of the
present invention made in accordance with Example 7, herein. The
membrane is observed to have a structure of small nodes (less than
5 .mu.m for the major axis) connected by fibrils arranged largely
in one direction.
[0051] Referring to FIGS. 8a and 8b, there is shown an SEM at
1000.times. and 4000.times., respectively, of a membrane of the
present invention made in accordance with Example 8, herein. The
membrane is observed to have a structure of elongated nodes running
in parallel connected by fibrils which are largely perpendicular to
the major node axis.
[0052] Referring to FIGS. 9a and 9b, there is shown an SEM at
1000.times. and 4000.times., respectively, of a membrane of the
present invention made in accordance with Example 9, herein. The
membrane is observed to have a structure consisting essentially of
very fine elongated nodes (less than about 2 .mu.m in width)
running in parallel connected by fibrils which are largely
perpendicular to the major node axis.
[0053] The mechanical properties of the present expanded PTFE
(ePTFE) membranes can be tailored to exhibit unique combinations of
mechanical properties not achieved in the prior art. For example, a
membrane comprising a single layer of ePTFE can be made having a
matrix tensile strength in at least one direction of at least
25,000 psi, with a matrix tensile strength ratio in two orthogonal
directions of between 0.025 and 4 and a density of 2.0 g/cc or
less.
[0054] PTFE membranes of the invention can have a matrix tensile
strength in at least one direction of at least 25,000 psi, with a
matrix tensile strength ratio in two orthogonal directions of
between 0.025 and 4, an orientation index of about 50.degree. or
less, and a density of 2.0 g/cc or less. In more preferred
embodiments, the membranes have a matrix tensile strength in at
least one direction of at least 34,000 psi, and more preferably at
least 44,800 psi and even lower densities, as noted earlier
herein.
[0055] Membranes of the invention preferably exhibit an orientation
index of 50.degree. or less. The orientation index is a
characterization of the degree of directional molecular alignment
of the PTFE chains making up the node and fibril structure.
Orientation indices of less than 40.degree., more preferably less
than 30.degree., and most preferably less than 20.degree. may be
achieved in the present invention.
[0056] The membranes of the present invention can also exhibit a
crystallinity index of at least 60%. The crystallinity index is a
measure of the degree of molecular order of the PTFE chains making
up the node and fibril structure. This crystallinity index can also
be characterized as inversely proportional to the number of
crystalline defects present. Crystallinity indices of the membranes
of the present invention may be tailored to be greater than about
65%, and most preferably greater than about 70% or greater.
[0057] Enthalpies of the expanded PTFE membranes of the present
invention are typically at least about 9.0 J/g, more preferably at
least about 10.0 J/g, and most preferably at least about 11.0
J/g.
[0058] The gasket materials of the present invention comprise PTFE
membranes, preferably in a stacked or layered configuration, having
a thickness which is tailored to meet the specific needs. For
convenience, gaskets will be referred to herein as having a
particular nominal, or approximate, thickness, which is consistent
with the manner in which commercially available gasket thicknesses
are reported. Typically, gasket manufacturers make gaskets of a
particular thickness and label the material commercially with both
nominal metric and English units. For example, a nominal 1.5 mm
gasket will also be referred to as a nominal {fraction (1/16)} inch
gasket. Actual ranges of thickness can vary .+-.20% around these
nominal reported thicknesses. For purposes of this invention,
whenever nominal thicknesses are reffered to, both nominal English
and nominal metric units are encompassed. For example, gaskets of
nominal thickness of 1 mm ({fraction (1/32)} inch), 1.5 mm
({fraction (1/16)} inch), 3 mm (1/8 inch), 6 mm (1/4 inch) and 9 mm
(3/8 inch) are most commonly available.
[0059] Shown in FIG. 10 is one embodiment of a gasket material 24
of the present invention. While the present invention is not
limited to the number of layers shown in this figure, this
exemplary gasket material 24 of the present invention comprises
multiple layers 26, 28, 30, 32, and 34. Layers 26, 28, 30, 32, and
34 are self-adhered to each layer of which they are in contact,
having exposed outside surfaces 26 and 34.
[0060] The gasket material 24 of the present invention can be
produced by wrapping PTFE membranes on a mandrel, stacking
membranes or other suitable formation techniques. As the term is
used herein, a PTFE membrane may comprise a single membrane layer
or, alternatively, multiple membrane layers having the same or
different characteristics, provided that at least one of the layers
is PTFE. For example, the membrane may comprise one or more layers
of ePTFE membrane having the same or different membrane properties.
Additionally, the PTFE membrane may comprise, for example, one or
more additional layers having a chemical composition other than
PTFE. The thickness of each PTFE membrane or layer may be
approximately 0.0005 to 0.50 inch (0.013 to 12.7 mm), and
preferably approximately 0.001 to 0.02 inch (0.025 to 0.51 mm). The
gasket material may be formed in virtually any quadrilateral
cross-sectional dimension of importance to sealing applications and
may optionally be filled with particulate fillers or coated with
polymeric materials prior to production of the gasket material.
[0061] FIG. 11 shows one embodiment of an apparatus 36 for forming
the PTFE gasket elements of the present invention. The apparatus
comprises two metal drums 38 and 40, having the capability for
heat, a payoff 48 consisting of multiple payoff arms 50, 52, 54,
56, and 58 (not limited to this quantity), and a take-up 60.
[0062] In order to form a gasket material 24 of the instant
invention, coiled lengths of PTFE 64, 66, 68, 70, and 72 (again,
not limited to this quantity) are fed between drums 38 and 40 from
pay-offs 50, 52, 54, 56, and 58. The outside surfaces of PTFE
lengths 64 and 72 come into direct contact with drums 38 and 40
during formation of the gasket material. Drums 38 and 40 are
typically of a surface that will not promote sticking and
preferably are heated to maintain a temperature of between 300 and
450.degree. C. The pressure, temperature, and speed applied by
drums 38 and 40 should be such that the layers of PTFE 64, 66, 68,
70, and 72 being fed between them self adhere to one another,
respectively, as they pass between the drums, thereby forming a
gasket material 74 containing multiple layers of PTFE
concentrically rolled and adhered to themselves. The tension
supplied by the pay-off arms 50, 52, 54, 56, 58 and the take-up 60
should be controlled in such a way that the material before and
after the drums remains substantially taut and does not droop.
[0063] FIG. 12 shows another embodiment of an apparatus 80 for
forming the PTFE gasket material 24 of the present invention. This
apparatus comprises a pay-off 82, idler drum 84, which, among other
things, acts as a positioning control, take-up 86, and a mandrel 90
with pins 91 for restraining the material.
[0064] In order to form the PTFE element of the present invention,
a coiled length of PTFE 88 membrane is fed from the pay-off 82,
over the idler drum 84, and onto the mandrel 90. The PTFE 88 is
layered upon itself as the mandrel is rotated by the take-up 86.
When the required number of mandrel rotations or PTFE layers 92
have been delivered, the PTFE 88 is cut or severed and the mandrel
is removed from the take-up 86. The mandrel 90 containing the PTFE
layers 92 is heated at a temperature and through an adequate time
duration such that the layers of PTFE 92 will self-adhere to each
other, as described in more detail below. After the heating, the
mandrel and PTFE are cooled and the gasket material is cut from the
mandrel.
[0065] In an alternative embodiment, the gasket may be constructed
in accordance with the teachings of U.S. Pat. No. 6,485,809 B1, the
subject matter of which is specifically incorporated herein by
reference, wherein the gasket includes at least one inner layer of
expanded PTFE disposed between a first substantially air
impermeable outer layer and a second substantially air impermeable
outer layer. The gaskets further include a substantially air
impermeable region located, for example, on an edge thereof or
bridging two sections thereof. For example, as shown in FIGS.
13-15, a gasket 20 of the present invention may be constructed
comprising a single chamber 21 with substantially air impermeable
region 13 disposed on the inner periphery of gasket 20. Chamber 21
is formed of an inner layer 15 of expanded PTFE sandwiched by outer
layers of substantially air impermeable layers 140.
[0066] Shown in FIG. 16 is an alternative embodiment of a gasket 10
which may be formed having the novel features of the present
invention, and this construction is based on teachings in U.S. Pat.
No. 5,879,789, in the names of Dolan et al, the subject matter of
which is specifically incorporated herein by reference. This gasket
10 comprises external layers 12, 14 of a porous, expanded PTFE
membrane of the present invention and a core material 16 of a
higher rigidity PTFE membrane. Layer 12, 14 may be attached to the
core material 16 in any suitable manner including by using an
adhesive material, through melting, or other bonding method. The
rigidity of the gasket is supplied by core 16. This material should
be sufficiently stiff that the gasket will not "flop" when held on
edge. Moreover, the core 16 comprises a fluoropolymer material that
has the same chemical properties as the external layers 12, 14.
Preferably, the core material 16 is an expanded PTFE layer that has
a higher rigidity than the outer layers 12, 14. Once core 16 is
formed, each of the external layers 12, 14 are positioned around
the core 16 and the structure is then laminated together into the
gasket material 10 of the present invention.
[0067] A further embodiment of the present invention is shown in
FIG. 17, also based on the teachings of U.S. Pat. No. 5,879,789. In
this embodiment, the gasket sheet material 18 comprises: outer
layers 20, 22 of conformable PTFE material; rigid inner layers 24,
26 of rigid PTFE material attached to each of outer layers 20, 22,
respectively; and a center layer 28 of conformable PTFE attached
between each of the rigid inner PTFE material layers 24, 26.
Although the properties of each of the layers may be modified to
satisfy specific performance characteristics to the sheet 18, for
most applications the conformable layers 20, 22, and 28 should
comprise a flexible expanded PTFE material, such as that previously
described. In the embodiment shown, outer layers 20, 22 are
approximately 0.006" (0.15 mm) wide, and center layer 28 is
approximately 0.034" (0.86 mm) wide. Similarly, the rigid inner
layers 24, 26 comprise a densified expanded PTFE material, such as
the core 16 previously described.
[0068] The improved gaskets of the present invention comprising
PTFE desirably have a thickness in the range of about 1.5 mm to
about 9 mm, a compressibility of at least about 40%, and a percent
bolt load retention versus thickness equal to or greater than the
line connecting points A and B in FIG. 18 and defined in the
following table:
1 Point % BLR at RT Thickness A 92% 1.5 mm B 52% 9 mm
[0069] In addition, gaskets of the present invention can exhibit a
compressibility of at least 40%, a durometer of 85 or less, and a
crystallinity of at least a bout 60%.
[0070] Alternatively, gaskets of the present invention may have a
percent bolt load retention versus thickness equal to or greater
than the line connecting the points C and D in FIG. 18 and defined
in the following table:
2 Point % BLR at RT Thickness C 93% 1.5 mm D 70% 9 mm
EXAMPLES
[0071] The present invention will now be described in conjunction
with the following examples which are intended to illustrate the
invention, not to limit it. In the examples, the following test
methods were used.
[0072] Test Methods
[0073] DSC Measurement
[0074] Differential Scanning Calorimetry (DSC) was performed to
determine the crystalline state of the material both before and
after the transverse expansion process. The characterization before
transverse expansion is performed to determine the level of heat
conditioning that was applied through the machine direction
expansion process. The equipment used to perform these measurements
was a TA Differential Scanning Calorimeter (Model # 2920),
calibrated using an appropriate standard, with an attached
refrigerated cooling unit (TA RCS Model #991100.901).
[0075] Sample preparation was accomplished by first cutting (using
a scalpel or razor blade) a sample weighing approximately 10
milligrams out of the center of the ePTFE membrane web. The balance
used to weigh the sample was a Mettler AT20 Electronic
Microbalance. Once cut, the sample was then folded over on itself
such that it fits into an aluminum sample pan (TA P/N 990999.901).
The sample was placed into the pan, the supplied lid was placed on
top of the sample and the lid was crimped in place using a sample
press (TA Model #900680.902). This sample was then placed into the
DSC testing chamber to be tested. The reference material that was
used for this series of measurements was an empty aluminum sample
pan and lid.
[0076] Once the sample and reference pan were inserted into the
testing chamber, the atmosphere in and around the test chamber was
purged with nitrogen gas. The testing cycle was set up to
equilibrate at -20 degrees Celsius and then ramped to 60 degrees
Celsius at a rate of 10 degrees Celsius per minute. At the
conclusion of the test, an energy plot output is created showing
the measured heat flow in watts/gram (i.e. the measured output
normalized to the actual mass of the ePTFE sample being tested)
versus the temperature during the scan. Using a program by TA
Instruments, Inc. called Universal Analysis 2000 (version 3.0G,
build 3.0.0.93), a baseline is generated by drawing a line between
the measured data at zero and 45 degrees Celsius. The area between
the measured data and the baseline is then integrated to give an
enthalpy value in Joules/gram. This enthalpy value is used to
describe the state of the ePTFE article being examined and is
generally between 5 and 15 Joules/gram.
[0077] Tensile Strength (Calendered Tape)
[0078] Mechanical testing of the unexpanded calendered tapes was
accomplished in a similar manner to that used for ePTFE membranes.
The major differences are that the sample geometry used here is a
0.635 cm wide by 10.16 cm long rectangular sample, the gauge length
is 1.27 cm and the crosshead speed (pulling speed) is 2540 mm/min.
The data analysis and matrix tensile strength calculations are
identical to that shown for membranes, described below.
[0079] Matrix Tensile Strength (For Membranes)
[0080] Sample preparation was accomplished by using a die punch to
cut 165 mm long by 15 mm wide rectangular samples out of the ePTFE
membrane web. The membrane web was placed on the cutting table such
that it was free from wrinkles in the area where the sample was to
be cut. The 165 mm.times.15 mm die was then placed on the membrane
(generally in the center 200 mm of the web) such that its long axis
is parallel to the direction that will be tested. The directions
quoted in this publication were measured in the machine direction
(parallel to the direction of travel during processing) and the
transverse direction (perpendicular to the direction of travel
during processing). Once the die was aligned, pressure was applied
to it to cut through the membrane web. Upon removal of this
pressure, the rectangular sample for testing was inspected to
ensure it was free from edge defects which may impact the tensile
testing. At least 3 samples in the machine direction and three
samples in the transverse direction was cut to characterize the
membrane web. Once samples were prepared, they were measured to
determine their mass (using a Mettler-Toledo analytical balance
model AB104) and their thickness (using a Kafer FZ1000/30 snap
gauge). Each sample was subsequently tested to determine its
tensile properties using an Instron 5500 tensile tester running
Merlin Series 1.times.software (version 7.51). The samples were
inserted into the tensile tester and held using Instron Catalog
2702-015 (rubber coated face plate) and 2702-016 (serrated face
plate) grip plates such that each end of the sample was held
between one rubber coated and one serrated face plate. The pressure
applied to the grip plates was approximately 50 psi. The gauge
length between the grips was set at 50 mm and the crosshead speed
(pulling speed) was set to a speed of 508 mm/min. A 0.1 kN load
cell was used to carry out these measurements and data was
collected at a rate of 50 points/sec. The laboratory temperature
was between 68 and 72 degrees Farenheit to ensure comparable
results. Finally, if the sample happened to break at the grip
interface, the data was discarded. At least 3 samples in the
machine direction and three samples in the transverse direction
were successfully pulled (no slipping out of or breaking at the
grips) in order to characterize the membrane web.
[0081] The data analysis and calculations were performed with the
Merlin software or any other data analysis package. First, the
maximum load able to be supported by the sample during the tensile
test was located. This maximum load was then normalized to the
sample physical properties (thickness and density) via the
following equation to calculate the matrix tensile strength. 1 MTS
= F max * ( o * l 100 * m )
[0082] where: MTS=Matrix tensile strength in Mpa
[0083] F.sub.max=maximum load measured during test (Newtons)
[0084] .rho..sub.o=theoretical density for PTFE (2.2 grams/cc)
[0085] l=sample length (cm)
[0086] m=sample mass (grams)
[0087] Density for Membranes
[0088] To measure the density of the membrane material examples of
the present invention and the comparative examples, property data
measured on the tensile samples was used. As noted above, the 165
mm.times.15 mm samples were measured to determine their mass (using
a Mettler-Toledo analytical balance model AB104) and their
thickness (using a Kafer FZ1000/30 snap gauge). Using this data, a
density can be calculated with the following formula: 2 = m w * l *
t
[0089] where: .rho.=density (g/cc)
[0090] m=mass (g)
[0091] w=width (1.5 cm)
[0092] l=length (16.5 cm)
[0093] t=thickness (cm)
[0094] Density for Gaskets
[0095] To measure the density of the gasketing material examples of
the present invention and of the comparative examples, specimens
having dimensions of about 25.4 mm in length and 25.4 mm in width
were cut from the example sheets. The length, width, thickness and
mass of each specimen were measured and recorded. The density of
the specimen was determined from the following calculation:
Density (g/cc)=mass (g)/volume (cc)
[0096] where volume (cc)=length (cm).times.width
(cm).times.thickness (cm)
[0097] Tensile Strength for Gaskets
[0098] Tensile strength was determined by tests performed in
accordance with procedures outlined in ASTM D638-00 and ASTM
F152-95 test procedures. Test specimens having dimensions in
accordance with ASTM Standard D638 -00 (Standard Test Method for
Tensile Properties of Plastics) for a Type I specimen were cut from
the gasketing material examples. Specimens were cut in the
direction defined as the machine direction (MD) and transverse
direction (TD) of the sheets. If the orientation of the sheet was
not known or relevant, one set of test specimens was cut in line
with one of the edges of the sheet and the second set was cut at
90.degree. to the first set. The width and thickness of the
specimens were measured at the narrow section of the specimens and
recorded.
[0099] The specimens were tested using an Instron test machine with
a 10 kN load cell. The extension rate was set at 12 inches/minute
(305 mm/minute) and the initial jaw separation was set to 4 inches
(102 mm). The Instron test machine automatically recorded the load
and extension data. From the test data the tensile strength was
determined by dividing the maximum load achieved during the test by
the initial cross sectional area of the narrow region of the
specimen.
Tensile Strength (TS)=maximum load/cross sectional area
[0100] The tensile strength reported is the higher tensile strength
measured in two orthogonal directions for a given sample.
[0101] Matrix Tensile Strength
[0102] The Matrix Tensile Strength (MTS) of the gasket materials
consisting essentially of expanded PTFE was calculated using the
following equation:
MTS=TS.times.(2.2/.rho.)
[0103] where:
[0104] TS=tensile strength,
[0105] .rho.=density of the gasket material,
[0106] and 2.2 is the density of solid PTFE.
[0107] Crystallinity Index and Orientation Index
[0108] Wide angle x-ray scattering was used to determine
crystallinity index and orientation indicies of the PTFE component
for gaskets and membranes. All measurements were made in
transmission mode using a Rigaku R-Axis IV Image Plate X-ray
Analyzer mounted on a Rigaku Ultra 18 kW rotating anode x-ray
generator a graphite monochromator and a 0.3 mm pinhole collimator.
Operating conditions on the generator for all experiments were 50
kV and 200 mA. Radiation type was Cu K.sub..alpha..sub..
Sample-to-detector distance was set at 125-135 mm, and calibrated
using a silicon powder standard. Two-dimensional image data was
processed using Rigaku R-Axis image processing software, to obtain
radial (l vs. 2.theta.) scans and azimuthal (l vs. .phi.) scans.
The azimuthal scans were collected by integration over the angular
range from 2.theta.=17.5.degree. to 2.theta.=19.0.degree. in
increments of .DELTA..phi.=1.0.degree. to determine the orientation
in the 100 plane of polytetrafluoroethylene, which is related to
the orientation in the direction of the fiber axis. The radial
scans were typically collected by integration over the angular
range from 2.theta..about.0.degree. to 2.theta..about.55.degree. in
increments of .DELTA.2.theta.=0.044.degree..
[0109] Membrane samples were prepared by cutting and stacking
strips of the membrane approximately 2.54 cm wide. All strips in
the stack were aligned in the same direction relative to the
machine direction and transverse direction of the original membrane
sample. Gasket samples were prepared by cutting pieces
approximately 0.5 cm by 0.5 cm square from larger sheets with edges
aligned approximately parallel to the machine direction and
transverse direction, and then sectioning the samples by slicing
them parallel to their face to reduce the total sample thickness to
.about.0.6-1.0 mm. All samples were measured with the x-ray beam
parallel to the direction normal to the sheet of the gasket or
membrane which is perpendicular to both MD and TD, yielding planar
orientation and planar crystallinity information. For convenience,
membrane samples were measured with the MD positioned horizontally,
and gasket samples were measured with the MD positioned
vertically.
[0110] The crystallinity index was obtained by peak fitting of l
vs. 2.theta.scans using Jade 6.1 commercial XRD processing software
from Materials Data, Inc. The scans were read into the software
program and fit without further processing. The fitting procedure
was as follows. The profile fitting range was limited to
2.theta.=12 .degree.-22.0.degree., and a linear background was
defined such that it coincided with the measured intensity at
2.theta.=12.degree. and 2.theta.=22.degree.. This background is
necessary to take into account intensity contributions due to air
scattering and detector read noise that was determined from "blank"
scans to be linear over the range of fitting. Two initial peaks
were inserted at .about.18.2.degree. and .about.16.5.degree.,
approximating positions of the 100 peak of the crystalline fraction
and the amorphous peak, respectively, and then the profile fitting
function was used to fit the peaks to the scan. The Pearson VII
function was used to fit both peaks. The procedure was to first
unify the shape and skew of both peaks and simultaneously fit a
linear background and the Height, 2.theta. position, full-width at
half-maximum intensity (FWHM), shape and skew of both peaks,
followed by further refinement of the profiles by first releasing
the condition of unifying the peak skew and then releasing the
condition of unifying the peak shape. Using this fitting sequence,
highly reproducible fits of both the amorphous and crystalline
peaks were obtained with Residual Error of Fit values of 2.5% or
less. A typical fitted profile is shown in FIG. 19. The
crystallinity index was then calculated from the area under the
fitted 100 crystalline peak (A.sub.100) and the area under the
fitted amorphous peak (A.sub.amorphous) according to equation
[1]:
Crystallinity
Index={A.sub.100/(A.sub.100+A.sub.amorphous)}.times.100% [1]
[0111] The crystallinity index was determined in this way for both
membrane samples and gasket samples.
[0112] The orientation was quantified from l vs. .phi. azimuthal
scans using two different parameters, the orientation angle index
and the azimuthal intensity ratio index. Representative azimuthal
scans are shown in FIGS. 20 and 21. It should be noted that the
appearance of four intensity peaks in the azimuthal scan, as shown
in FIG. 21, represents crystalline orientation along two different
directions, for example, in both MD and TD. We believe this
characteristic to be unique to our single-layered membrane
materials.
[0113] For each azimuthal scan, intensity values are defined that
represent the peak intensities (l.sub.peak), the minimum measured
intensity (l.sub.min), and the background intensity for the
azimuthal scan (l.sub..phi.-bkg). The background intensity for the
azimuthal scan is extracted from the l vs. 2.theta. integrated
intensity scans, and includes amorphous contributions, air scatter,
and contributions from detector read noise. The amorphous
contribution is assumed to be unoriented for the purposes of
calculating l.sub..phi.-bkg. l.sub..phi.-bkg is determined from the
average intensities of data points in the l vs. 2.theta. scan
falling into the ranges immediately preceding and following the
range in which the azimuthal intensity is calculated. More
specifically, l.sub..phi.-bkg is equal to the average of the
averages of intensity values corresponding to data points in the l
vs. 2.theta. scan with
17.0.degree..ltoreq.2.theta..ltoreq.17.5.degree. and
19.0.degree..ltoreq.2.theta..ltoreq.19.5.degree., normalized to
account for the summation of intensities in the azimuthal scan for
all 2.theta. values falling in the azimuthal scan integration range
(2.theta.=17.5 .degree.-19.0.degree.) and the division of the total
intensity integrated over the full 360.degree. azimuthal range into
360 .DELTA..phi.=1.0.degree. values. The minimum measured
intensity, l.sub.min, is defined as the average of the lowest
numerical intensity value corresponding to each of the two lowest
intensity minima observed on the l vs. .phi. plots, after
artificially low intensity values corresponding to physical
blocking by the beam stop were removed from the data sets. The peak
intensities, l.sub.peak, were simply chosen to be equal to the
largest numerical intensity values corresponding to the observed
intensity peaks on the l vs. .phi. plots.
[0114] For the two highest intensity peaks in each l vs. .phi.
scan, the full-width at half the maximum peak intensity is
determined from the difference of the angles corresponding to the
intensity values closest to l.sub.peak/2 on either side of the
peak. These two FWHM values are averaged to obtain the Orientation
Angle Index. The Azimuthal Intensity Ratio is defined by equation
[2]:
Azimuthal Intensity
Ratio=(l.sub.min-l.sub..phi.-bkg)/(l.sub.peak-l.sub..p- hi.-bkg)
[2]
[0115] The Azimuthal Intensity Ratio was calculated for the two
highest intensity peaks present in each l vs. .phi. scan and
averaged to give the Azimuthal Intensity Ratio Index. This index
gives a measure of the fraction of the crystalline portion of the
material that is oriented in a given direction relative to the
fraction of the crystalline portion of the material that has random
orientation. It will take a value of approximately zero when the
crystalline fraction of the material is fully directionally
oriented and l.sub.min.about.l.sub..phi.-bkg, and a value of
approximately one when the material is fully randomly oriented and
l.sub.peak.about.l.sub.min.
[0116] Bolt Load Retention
[0117] The Bolt Load Retention test measures the amount of
compressive load on a gasket sample over a period of time and
through a thermal cycle. The thermal cycle consists of a ramp
segment from room temperature to a specified elevated temperature,
a specified dwell time at the elevated temperature, and a cool down
segment back to room temperature. The load on the test gasket
varies through the test based on the stress relaxation properties
of the gasket material.
[0118] Annular test specimens were cut to the ANSI 2".times.150 lb.
class ring gasket dimensions with an outer diameter of 4.125"
(104.8 mm) and inner diameter of 2.375" (60.3 mm). The test gaskets
were placed between two blind, carbon steel ANSI 2".times.150-lb
class flanges with a surface finish of 250 RMS. The gasket/flanges
were loaded into a platen press. The platen press had platens with
electric cartridge heaters and a load capacity of 60,000 pounds.
The flanges were compressed to an initial loading of approximately
46,100 pounds at a uniform rate over a three minute period. After
the initial load was reached, the temperature of the platens was
increased to 100.degree. C. The temperature was held for at least
four, and up to eight hours. At the end of the dwell time at
elevated temperature, the platens were allowed to cool to room
temperature. Once the platens reached room temperature
(approximately 23.degree. C.) the test was complete and the gasket
was removed from the press and flanges.
[0119] The load on the gasket was measured and recorded
electronically throughout the test. The percent bolt load retention
(% BLR) is defined as the final load on the gasket at room
temperature divided by the initial load and multiplied by 100:
% BLR=(final load at room temperature/initial load).times.100%.
[0120] The results of the bolt load retention test provide a means
for comparing the creep and stress relaxation properties of
different gasket materials. Since creep and stress relaxation in
polymeric materials is dependent on mass, test specimens were
produced and tested having a thickness that spanned the nominal
commercially available gasket thicknesses, e.g., ranging from 1 mm
to 6 mm. In order to produce the different thickness test specimens
from the inventive examples, layers of the ePTFE membrane were
removed from the gasket material sheets prior to cutting the
annular rings to produce specimens with a thickness less than the
original thickness of the sheet material described in the inventive
examples. To produce test specimens having a thickness greater than
that of the original thickness of the gasket materials of the
inventive samples, the annular rings were stacked on top of each
other until the desired thickness was reached with membrane layers
being removed as necessary. The thickness and mass of the test
specimen were measured and recorded prior to conducting the bolt
load retention test.
[0121] Stress to Seal Test
[0122] The "stress to seal" of a gasket, also known as the y-value,
is the amount of compressive stress required on a gasket to provide
an initial air tight seal at a specified internal pressure and
ambient conditions. FIG. 22 shows the apparatus 101 used for
determining the stress to seal values on the example gasket
materials. An air tight, or air impermeable, seal is defined here
as substantially no leakage as indicated by the fluid levels in the
manometer of the test apparatus. These test procedures were
developed based on the test method of ASTM F37-00, Standard Test
Methods for Sealability of Gasket Materials.
[0123] For this test, annular test gasket samples 102 were cut from
the example gasket material sheets where the annular test gaskets
had an inner diameter of approximately 2.38 inches (60.5 mm) and an
outer diameter of approximately 4.13 inches (105 mm). The thickness
and mass of each test gasket sample were measured and recorded. The
stress on the gaskets was determined by dividing the load on the
gasket by the initial gasket area as defined by the nominal inner
and outer diameters.
[0124] The test gasket was centered on the lower test platen 103.
The upper test platen 104 was then placed on top of the test
gasket, being careful not to move the test gasket from its centered
position. The test platens had surface finishes of at least as
smooth as RMS 32. The gasket/test platen assembly was then placed
between upper platen 105 and lower platen 106 in a platen press and
centered on the press platens over a load indicating transducer. A
pre-selected initial load was applied to the gasket at a uniform
rate over an approximately 1 minute period. The initial loads were
selected as starting loads based on experience with various gasket
materials. The initial load was reapplied to the gasket after
waiting one minute to account for initial relaxation of the gasket
material. Compressed air was supplied to the test gasket via the
air regulator 107 and valves 108, 109, and 110. The air line 111
was connected to the fitting on the lower test platen. Valves 108,
109, and 110 were opened fully. The regulator 107 was adjusted
until the gage 112 displayed a pressure of 30 psig (207 kPa). Valve
107 was then closed: At this point, the manometer 113 levels were
checked to make sure they were equal to insure zero pressure
differential in the system. Valve 109 was then closed to begin
testing for leakage. Leakage was identified by a change in the
manometer fluid levels.
[0125] If leakage was present, the air pressure was released from
the system and the load on the gasket was increased. Compressed air
was again supplied to the gasket and the manometer was checked for
indications of leakage. These steps were repeated at increasing
loads until a load was reached where no leakage occurred. Once a
load was reached where no initial leakage occurred, the air
pressure and load were maintained on the test gasket for a period
of 30 minutes to insure that there was substantially no leakage.
The "stress to seal" value was reached when there was substantially
no change in the manometer levels after the 30 minute time
period.
[0126] Creep For Gaskets
[0127] The creep of the gasket material examples was measured
following the test procedures defined in DIN 28090-2. Test gaskets
were cut from the example sheets having an inner diameter of 49 mm
and an outer diameter of 92 mm and a nominal thickness of 1.6 mm.
The DIN28090-2 test procedures test the deformation characteristics
of gasket materials and specifically creep. Creep is defined in the
ASTM International standard F 118-97 as a transient stress-strain
condition in which the strain increases as the stress remains
constant. The stress being applied to the test gasket in the DIN
28090-2 procedures is compressive. Therefore, a change in strain
under the compressive stress due to creep in the gasket material
results in a decrease in the thickness of the test gasket.
[0128] The test fixture for the DIN 28090-2 test consisted of a
computer controlled hydraulic press with a force measurement cell,
two independent heating plates and a gasket height caliper. The
gaskets were tested between a pair of adaption plates which were
constructed to apply the compressive load on a surface having an
inner diameter of 55 mm and an outer diameter of 75 mm and surface
finishes of Rz<6.3 .mu.m. The gaskets were subjected to a
preload of 1 Mpa for 1 minute. Then, a mainload of 25 Mpa was
applied for 5 minutes. The load was then returned to 1 Mpa for 5
minutes. The mainload was reapplied and held for 16 hours. During
the second mainload, the temperature was increased to 150.degree.
C. at a rate of 5 K/min and held for the duration of the 16 hour
dwell.
[0129] The percent creep is defined as the percent change in
thickness of the gaket sample at the end of dwell time of the first
application of the mainload and the end of the dwell time of the
second application of the mainload.
[0130] Durometer Hardness
[0131] The durometer of the various gasket material examples was
measured using a PTC Instruments Type A Durometer, model #306L,
from Pacific Transducer Corp., Los Angeles, Calif. Test samples
having a nominal thickness of 1/8 inch (3 mm) were cut from the
example gasketing material sheets and were tested in three
locations at approximately 0.5 inches (12.7 mm) apart. The
durometer tester was placed on the sample and a light hand force
was applied to the tester until the base of the tester contacted
the surface of the test sample. The durometer reading (Shore A
scale) was read from the dial gage on the tester and recorded. The
thickness of each test sample was also measured and recorded.
[0132] Compressibility
[0133] Compressibility of the example gasket materials was
determined by compressing a test sample to a specified stress for a
short period of time. The procedures described below were developed
based on the test procedures defined in ASTM F 36-99, Standard Test
Methods for Compressibility and Recovery of Gasket Materials.
[0134] Annular test specimen were cut from the example gasket
material sheets such that the annular specimen had an inner
diameter of approximately 0.5 inches (12.7 mm) and an outer
diameter of approximately 1 inch (25.4 mm) with a nominal thickness
of {fraction (1/16)} inch (1.5 mm (for gaskets of thicker
dimension, layers were removed to reach this nominal thickness).
The initial thickness of each test specimen was measured and
recorded. The test specimen was placed on the lower platen of the
test fixture and centered. A solder plug, approximately 0.25 inches
(6.4 mm) long, was placed on the lower platen inside of the inner
diameter of the test specimen. The solder was used to capture the
compressed thickness of the specimen under load. The solder has no
appreciable recovery in thickness after the load is removed.
Therefore, the thickness of the solder plug after the load was
removed equaled the compressed thickness under the major load. The
test consisted of three segments. The first segment was a pre-load
segment where the test specimen was loaded to approximately 500 psi
(3.4 MPa) for 15 seconds. The second segment consisted of the
application of the major load of approximately 2500 psi (17.2 MPa)
stress for 60 seconds. The third segment consisted of a return of
the load to the pre-load level of approximately 500 psi (3.4 MPa)
for 60 seconds. After the test was complete, the final thickness of
the test specimen and solder plug was measured and recorded. The
percent compressibility of a material was calculated from the
equation:
Compressibility (%)=(initial thickness-compressed
thickness)/initial thickness.times.100.
[0135] Membrane Comparative Examples
[0136] Membranes of the prior art were obtained and tested. Prior
art membranes A and B were made in accordance with the teachings of
U.S. Pat. No. 3,953,566 and prior art membranes C and D were made
in accordance with the teachings of U.S. Pat. No. 5,476,589.
3TABLE 1 Longitudinal Transverse Orientation MTS MTS Index Density
Prior Art (MPa) (MPa) (degrees) (g/cc) A 49 107 71 0.44 B 123 132
180 0.4 C 173 133 50 0.7 D 792 91 n/a 0.48 (n/a = not
available)
Example 1
[0137] An expanded PTFE membrane was produced by practicing the
following steps in sequence:
[0138] 1. Blending PTFE fine powder (PTFE 601A, DuPont, Wilmington,
Del.) with a lubricant (Isopar K, Exxon, Houston, Tex.) in the
proportion of 130 cc/lb
[0139] 2. Compressing the lubricated powder into a cylindrical
shape
[0140] 3. Paste extruding at a reduction ratio of 135:1
[0141] 4. Calendering to a thickness of 0.018 inch
[0142] 5. Drying the tape in a convection oven set to 210.degree.
C.
[0143] 6. Longitudinally stretching the tape between two banks of
rollers separated by a 325.degree. C. heat zone
[0144] 7. Transversely stretching the longitudinally stretched tape
in a 335.degree. C. heat zone, while constraining the tape from
shrinking in the longitudinal direction
[0145] 8. Subjecting the stretched tape to a 380.degree. C. heat
zone while constrained.
[0146] The stretch rates for the longitudinal and transverse
directions were 39.7%/sec and 56.2%/sec, respectively. The stretch
ratios for the longitudinal and transverse directions were 6.05:1
and 16.1:1, respectively.
[0147] Tensile strengths were measured for calendered tape samples
utilizing a strain rate of 20000%/min. Tensile strengths of
membrane samples were measured using a strain rate of 1016%/min.
The dry calendered tape had tensile strengths in the longitudinal
and transverse directions of about 5520 psi (38 MPa) and 1760 psi
(12.1 MPa), respectively. The final membrane had matrix tensile
strengths in the longitudinal and transverse directions of about
223 MPa and 200 MPa, respectively. The membrane density was 0.27
g/cc.
[0148] The orientation index was 42.8.degree. and the crystallinity
index was 61.2%.
Example 2
[0149] Another membrane was made in accordance with the process
described in Example 1 with the following exceptions. The stretch
ratios for the longitudinal and transverse directions were 4.1:1
and 7.6:1, respectively.
[0150] The calendered tape process conditions were the same,
yielding approximately the same longitudinal and transverse
strengths. The final membrane had matrix tensile strengths in the
longitudinal and transverse directions of about 186 MPa and 175
MPa, respectively. The membrane density was 0.38 g/cc. The
orientation index was 27.62.degree. and the crystallinity index was
55.06%.
Example 3
[0151] Another membrane was made in accordance with the process
described in Example 1 with the following exceptions. The stretch
ratios for the longitudinal and transverse directions were 4.75:1
and 15.3:1, respectively.
[0152] The calendered tape process conditions were the same,
yielding approximately the same longitudinal and transverse
strengths. The final membrane had matrix tensile strengths in the
longitudinal and transverse directions of about 185 MPa and 250
MPa, respectively. The membrane density was 0.28 g/cc. The
orientation index was 50.33.degree. and the crystallinity index was
61.95%.
Example 4
[0153] Another membrane was made in accordance with the process
described in Example 1 with the following exceptions. The stretch
ratios for the longitudinal and transverse directions were 8:1 and
17.2:1, respectively.
[0154] The calendered tape process conditions were the same,
yielding approximately the same longitudinal and transverse
strengths. The final membrane had matrix tensile strengths in the
longitudinal and transverse directions of about 245 MPa and 287
MPa, respectively. The membrane density was 0.32 g/cc. The
orientation index was 20.27.degree. and the crystallinity index was
68.67%.
Example 5
[0155] Another membrane was made in accordance with the process
described in Example 1 with the following exceptions. The
lubrication ratio was 120 cc/lb. The stretch ratios for the
longitudinal and transverse directions were 4.75:1 and 16.1:1,
respectively.
[0156] The resultant dry calendered tape in this example had
tensile strengths in the longitudinal and transverse directions of
about 5860 psi (40.4 MPa) and 1840 psi (12.7 MPa), respectively.
The final membrane had matrix tensile strengths in the longitudinal
and transverse directions of about 203 MPa and 231 MPa,
respectively. The membrane density was 0.28 g/cc.
Example 6
[0157] Another membrane was made in accordance with the process
described in Example 1 with the following exceptions. The
lubrication ratio was 120 cc/lb. The stretch ratios for the
longitudinal and transverse directions were 7.35:1 and 22.4:1,
respectively.
[0158] The resultant dry calendered tape in this example had
tensile strengths in the longitudinal and transverse directions of
about 5860 psi (40.4 MPa) and 1840 psi (12.7 MPa), respectively.
The final membrane had matrix tensile strengths in the longitudinal
and transverse directions of about 235 MPa and 294 MPa,
respectively. The membrane density was 0.38 g/cc. The orientation
index was 20.35.degree. and the crystallinity index was 70.02%.
Example 7
[0159] Another membrane was made in accordance with the process
described in Example 1 with the following exceptions. The stretch
ratios for the longitudinal and transverse directions were 8:1 and
7.1:1, respectively.
[0160] The calendered tape process conditions were the same,
yielding approximately the same longitudinal and transverse
strengths. The final membrane had matrix tensile strengths in the
longitudinal and transverse directions of about 310 MPa and 77 MPa,
respectively. The membrane density was 0.28 g/cc. The orientation
index was 27.34.degree. and the crystallinity index was 61.51%.
Example 8
[0161] Another membrane was made in accordance with the process
described in Example 1 with the following exceptions. The stretch
ratios for the longitudinal and transverse directions were 1.5:1
and 10.4:1, respectively. The tape was calendered to a thickness of
0.008 inch.
[0162] The dry calendered tape had tensile strengths in the
longitudinal and transverse directions of about 9620 psi (66.3 MPa)
and 1400 psi (9.7 MPa), respectively. The final membrane had matrix
tensile strengths in the longitudinal and transverse directions of
about 90 MPa and 280 MPa, respectively. The membrane density was
0.55 g/cc. The orientation index was 16.12.degree. and the
crystallinity index was 57.17%.
Example 9
[0163] Another membrane was made in accordance with the process
described in Example 1 with the following exceptions. The stretch
ratios for the longitudinal and transverse directions were 6.7:1
and 14.3:1, respectively.
[0164] The calendered tape process conditions were the same,
yielding approximately the same longitudinal and transverse
strengths. The final membrane had matrix tensile strengths in the
longitudinal and transverse directions of about 251 MPa and 258
MPa, respectively. The membrane density was 0.31 g/cc. The
orientation index was 35.88.degree. and the crystallinity index was
66.77%.
[0165] Process and membrane property data pertaining to Examples 1
through 9 appear in the following tables:
4TABLE 2 Lubricant Thickness of Calendered Tape Calendered Tape
Level Calendered Longitudinal MTS Transverse MTS Ex. (cc/lb) Tape
(inch) (psi) (psi) 1 130 0.018 5520 1760 2 130 0.018 5520 1760 3
130 0.018 5520 1760 4 130 0.018 5520 1760 5 120 0.018 5860 1840 6
120 0.018 5860 1840 7 130 0.018 5520 1760 8 130 0.008 9620 1400 9
130 0.018 5520 1760
[0166]
5TABLE 3 Stretch Temperature Longitudinal Longitudinal Transverse
Transverse Heat (deg. C) Stretch Stretch Rate Stretch Stretch Rate
Setting Ex. Long.; trans. Ratio (%/sec) Ratio (%/sec) Temp
(.degree. C.) 1 325; 335 6.1 39.7 16.1 56.2 380 2 365; 380 4.1 26.8
7.6 59.3 380 3 350; 350 4.8 32.4 15.3 55.2 380 4 354; 350 8.0 50.7
17.2 91.2 380 5 350; 350 4.8 32.4 16.1 55.7 380 6 354; 350 7.4 49.1
22.4 92.5 380 7 325; 335 8.0 50.7 7.1 54.8 380 8 361; 380 1.5 13.2
10.4 31.3 380 9 351; 350 6.7 43.4 14.3 65.3 380
[0167]
6TABLE 4 Final Final Final Membrane Membrane Membrane Orientation
Crystallinity Long. MTS Trans. MTS Density Index Index Ex. (MPa)
(MPa) (g/cc) (degrees) (%) 1 223 200 0.27 42.8 61.2 2 186 175 0.38
27.62 55.06 3 185 250 0.28 50.33 61.95 4 245 287 0.32 20.27 68.67 5
203 231 0.28 n/a n/a 6 235 294 0.38 20.35 70.02 7 310 77 0.28 27.34
61.51 8 90 280 0.55 16.12 57.17 9 251 258 0.31 35.88 66.77
[0168] Gasket Comparative Examples
[0169] Gaskets of the prior art were obtained and tested for
mechanical properties. The test results appear in the following
tables:
7TABLE 5 Matrix Tensile Tensile Crystallinity Thickness Density
Strength Strength Index Prior Art (mm) (g/cc) (MPa) (MPa) (%) 1/8
inch GR 3.38 0.57 36.9 142.3 47.48 sheet.sup.1 {fraction (1/16)}
inch GR n/a n/a n/a n/a 47.48 sheet.sup.1 1/4 inch GR n/a n/a n/a
n/a 47.48 sheet.sup.1 1 mm GR n/a n/a n/a n/a 47.48 sheet.sup.1
Inertex.sup.2 3.37 0.89 19.4 48.0 59.36 3 mm Blue 3.07 1.62 n/a n/a
n/a Gylon.sup.3 1.5 mm Blue 1.48 1.61 17.9 n/a n/a Gylon.sup.3
.sup.1GORE-TEX GR .RTM. sheet gasketing, W. L. Gore &
Associates, Inc., Elkton, MD .sup.2INERTEX .RTM. SQ-S Gasket Sheet,
Inertech, Inc., Monterey Park, CA .sup.3Blue Gylon .RTM. Gasketing,
Style 3504, Garlock .RTM. Sealing Technologies, Palmyra, NY
[0170]
8TABLE 6 Bolt Load Retention (%) Compressi- Stress 1 mm; 1.6 mm;
3.2 mm; bility to Seal Prior Art 6.4 mm Creep (%) (Mpa) 1/8 inch GR
n/a; n/a; 79.05; n/a n/a n/a 22.1 sheet.sup.1 {fraction (1/16)}
inch GR n/a; 90.70; n/a; n/a 6.1 58.5 19.0 sheet.sup.1 1/4 inch GR
n/a; n/a; n/a; 62.3 n/a n/a n/a sheet.sup.1 1 mm GR 92.8; n/a; n/a;
n/a n/a n/a n/a sheet.sup.1 Inertex.sup.2 n/a; n/a; 66.2; n/a n/a
n/a 24.8 3 mm Blue n/a; n/a; 52.3; n/a n/a n/a 12.4 Gylon.sup.3 1.5
mm Blue n/a; 71.4; n/a; n/a n/a 15.7 15.7 Gylon.sup.3
Example 10
[0171] A gasket material was constructed utilizing the membrane of
Example 4. Approximately 143 layers of the ePTFE membrane were
wrapped on a stainless steel mandrel such that the longitudinal
direction of the membrane was oriented circumferentially around the
mandrel. The mandrel had an outer diameter of approximately 600 mm
and a length of approximately 1370 mm.
[0172] The membrane layers were secured at the ends of the mandrel
to restrain the membrane from shrinking at elevated temperatures.
The membrane and mandrel were placed in a forced air oven set at
365.degree. C. for approximately 45 minutes. After removal from the
oven and allowed to cool, the ePTFE material was circumferentially
cut at both ends of the mandrel and then cut along the length of
the mandrel. The expanded PTFE was then removed from the mandrel in
the form of a sheet.
[0173] The tensile strength of the gasket material in the machine
direction was 53.3 MPa (MTS=186.3 MPa). The tensile strength of the
gasket material in the transverse direction was 64.5 MPa (MTS=225.2
MPa). Thus, the reported tensile strength (highest) of the gasket
material was 64.5 MPa (MTS=225.2 MPa). The average thickness and
density of the gasket material, as produced, were 2.0 mm and 0.63
g/cc, respectively. The crystallinity index of the gasket material
was 69.5%.
[0174] A 1 mm nominal thickness gasket of this Example (i.e.,
layers removed to achieve this nominal thickness) was measured to
have a bolt load retention value of 93.35%. A 1.5 mm nominal
thickness gasket of this Example (layers removed) was measured to
have a bolt load retention value of 93.17% and a stress to seal
value of 18.4 MPa. The 3 mm nominal thickness gasket of this
Example (formed with stacked layers) was measured to have a bolt
load retention value of 88.5%. A 6 mm nominal thickness gasket of
this Example (formed with stacked layers) was measured to have a
bolt load retention value of 83.95%. Compressibility was measured
to be 63.3%. The thickness of the samples used to measure
compressibility were, on average, 1.69 mm. Creep was measured to be
3.1%. The thickness of the creep sample was 1.84 mm. Durometer was
measured to be 67. The thickness of the durometer sample was 2.92
mm.
Example 11
[0175] Another gasket material was made in accordance with the
process described in Example 10 with the following exceptions.
Approximately 130 layers of the membrane of Example 3 were
circumferentially wrapped around the mandrel.
[0176] The tensile strength of the gasket material in the machine
direction was 43.9 MPa (MTS=146.4 MPa). The tensile strength of the
gasket material in the transverse direction was 50.3 MPa (MTS=167.7
MPa). Thus, the reported tensile strength (highest) of the gasket
material was 50.3 MPa (MTS=167.7 MPa). The average thickness and
density of the gasket material, as produced, were 3.3 mm and 0.66
g/cc, respectively. The crystallinity index of the gasket material
was 62.7%.
[0177] The 1 mm nominal thickness gasket (layers removed) was
measured to have a bolt load retention value of 93.25%. The 3 mm
nominal thickness gasket (layers removed) was measured to have a
bolt load retention value of 86.77% and a stress to seal value of
22.5 MPa. The 6 mm nominal thickness gasket (stacked layers) was
measured to have a bolt load retention value of 69.40%.
Compressibility was measured to be 61.0%. The thickness of the
samples used to measure compressibility were on average 1.69 mm.
Durometer was measured to be 66. The thickness of the durometer
sample was 2.82 mm.
Example 12
[0178] Another gasket material was made in accordance with the
process described in Example 10 with the following exceptions.
Approximately 110 layers of the membrane of Example 1 were
circumferentially wrapped around the mandrel.
[0179] The tensile strength of the gasket material in the machine
direction was 42.7 MPa (MTS=177.3 MPa). The tensile strength of the
gasket material in the transverse direction was 42.4 MPa (MTS=176.0
MPa). Thus, the reported tensile strength (highest) of the gasket
material was 42.7 MPa (MTS=177.3 MPa). The average thickness and
density of the gasket material, as produced, were 2.3 mm and 0.53
g/cc, respectively. The crystallinity index of the gasket material
was 60.83%.
[0180] The 1 mm nominal thickness gasket (layers removed) was
measured to have a bolt load retention value of 93.10%. The 1.5 mm
nominal thickness gasket (layers removed) was measured to have a
bolt load retention value of 91.33% and a stress to seal value of
20.2 MPa. The 3 mm nominal thickness gasket (stacked layers) was
measured to have a bolt load retention value of 86.03%.
Compressibility was measured to be 64.9%. The thickness of the
samples used to measure compressibility were on average 1.59 mm.
Durometer was measured to be 67. The thickness of the durometer
sample was 2.74 mm.
Example 13
[0181] Another gasket material was made in accordance with the
process described in Example 10 with the following exceptions.
Approximately 40 layers of the membrane of Example 7 were
circumferentially wrapped around the mandrel. Also, a different
mandrel size was used. Both the outer diameter and length of the
mandrel were approximately 600 mm.
[0182] The tensile strength of the gasket material in the machine
direction was 54.6 MPa (MTS=230.9 MPa). The tensile strength of the
gasket material in the transverse direction was 13.4 MPa (56.7
MPa). Thus, the reported tensile strength (highest) of the gasket
material was 54.6 MPa (MTS=230.9 MPa). The average thickness and
density of the gasket material, as produced, were 2.3 mm and 0.52
g/cc, respectively.
[0183] The 1.5 mm nominal thickness gasket (layers removed) was
measured to have a bolt load retention value of 90.90%. The 2 mm
nominal thickness gasket (layers stacked) was measured to have a
stress to seal value of 20.2 MPa. The 3 mm nominal thickness gasket
(layers stacked) was measured to have a bolt load retention value
of 87.45%. Durometer was measured to be 51. The thickness of the
durometer sample was 3.56 mm.
Example 14
[0184] Another gasket material was made in accordance with the
process described in Example 10 with the following exceptions. A
mandrel having an outer diameter of 600 mm and a length of 600 mm
was used. Two different membranes were used in the construction of
the sheet. Approximately 22 layers of the membrane of Example 7 and
approximately 22 layers of the membrane of Example 8 were
circumferentially wrapped around the mandrel. One payoff spool was
positioned above the other such that both membrane types were fed
together onto the mandrel, thereby resulting in alternating layers
of the membranes.
[0185] The tensile strength of the gasket material in the machine
direction was 32.7 MPa (MTS=138.3 MPa). The tensile strength of the
gasket material in the transverse direction was 28.8 MPa (MTS=121.8
MPa). Thus, the reported tensile strength (highest) of the gasket
material, as produced, was 32.7 MPa (MTS=138.3 MPa). The average
thickness and density of the gasket material, as produced, were 1.9
mm and 0.52 g/cc, respectively.
[0186] The 1.5 mm nominal thickness gasket was measured to have a
stress to seal value of 23.8 MPa. The 3 mm nominal thickness gasket
(layers stacked) was measured to have a bolt load retention value
of 87.07%.
Example 15
[0187] Another gasket material was made in accordance with the
process described in Example 10 with the following exceptions.
Approximately 250 layers of the membrane of Example 6 were
circumferentially wrapped around the mandrel having both an outer
diameter and length of approximately 600 mm.
[0188] The tensile strength of the gasket material in the machine
direction was 44.1 MPa (MTS=183.1 MPa). The tensile strength of the
gasket material in the transverse direction was 61.1 MPa (MTS=253.5
MPa). Thus, the reported tensile strength (highest) of the gasket
material was 61.1 MPa (MTS=253.5 MPa). The thickness and
crystallinity index of the gasket material, as produced, were 3.13
mm and 73.6%, respectively.
[0189] The 3 mm nominal thickness gasket was measured to have a
bolt load retention value of 91.87% and a stress to seal value of
24.8 MPa. The 6 mm nominal thickness gasket was measured to have a
bolt load retention value of 79.0%. Durometer was measured to be
67. The thickness of the durometer sample was 3.30 mm.
Example 16
[0190] A gasket material was constructed in accordance with the
teachings of U.S. Pat. No. 6,485,809 to Minor et al utilizing the
membrane of Example 5 and an expanded PTFE sheet which had been
calendered to reduce the porosity therein. The reduced porosity
expanded PTFE sheet was produced by calendering three layers of an
ePTFE membrane between two rollers separated by a fixed gap. The
expanded PTFE membrane used to produce the reduced porosity
expanded PTFE sheet was produced in accordance with the teachings
of U.S. Pat. No. 3,953,566, to Gore. The thickness and density of
the expanded PTFE membrane were 0.082 mm and 0.4 g/cc,
respectively. After calendering the three membrane layers, the
reduced porosity expanded PTFE sheet had a thickness and density of
0.045 mm and 1.85 g/cc, respectively. One layer of the sheet was
wrapped around a stainless steel mandrel having an outer diameter
of approximately 1000 mm and a length of approximately 1600 mm.
[0191] Approximately 104 layers of the membrane of Example 5 were
then wrapped over top of the previously wrapped reduced porosity
expanded PTFE sheet. A second layer of the reduced porosity ePTFE
sheet was then wrapped over the top layer of the ePTFE membrane.
Another approximately 110 layers of the ePTFE membrane of Example 5
were wrapped over top of the second nearly nonporous ePTFE sheet.
The layers were secured at the ends of the mandrel to prevent them
from shrinking at elevated temperatures. The layers of ePTFE were
sintered in a forced air oven set to 365.degree. C. for
approximately 55 minutes. After cooling, the ePTFE material was
circumferentially cut at both ends of the mandrel and then cut
along the length of the mandrel and removed in the form of a
composite sheet. The composite sheet was separated into two sheets
by peeling apart the composite sheet at the interface between the
second reduced porosity expanded PTFE sheet layer and the second
approximately 110 layers of the membrane of Example 5. The sheet
comprising the two outer layers of the reduced porosity expanded
PTFE sheets and the approximately 104 layers of the membrane of
Example 5 constituted the gasket material of the present
example.
[0192] After being cut from the gasket material, the samples for
the Bolt Load Retention and Stress to Seal test were compressed
along the inner diameter as taught in U.S. Pat. No. 6,485,809 to
Minor et al to complete the low stress to seal gasket construction.
The 3 mm nominal thickness gaskets were measured to have a bolt
load retention of 88.5% and a stress to seal of 9.7 MPa. The
durometer was measured to be 73. The thickness of the durometer
sample was 2.54 mm.
Example 17
[0193] The gasket material of the present example comprised the
approximately 110 layers of ePTFE membrane separated from the
composite sheet described in Example 16.
[0194] The tensile strength of the gasket material in the machine
direction was 45.4 MPa (MTS=146.9 MPa). The tensile strength of the
gasket material in the transverse direction was 51.8 MPa (MTS=167.5
MPa). Thus, the reported tensile strength (highest) of the gasket
material was 51.8 MPa (MTS=167.5 MPa). The average thickness and
density of the gasket material were 2.5 mm and 0.68 g/cc,
respectively.
[0195] The 3 mm nominal thickness gasket (layers stacked) was
measured to have a bolt load retention of 87.64%. The durometer was
measured to be 69. The thickness of the durometer sample was 2.36
mm.
[0196] Gasket material property data pertaining to Examples 10
through 17 appear in the following tables:
9TABLE 7 Tensile Matrix Tensile Stress to Seal Thickness Density
Strength Strength (MPa) 1.5 mm; Ex. (mm) (g/cc) (MPa) (MPa) 2 mm;
3.0 mm 10 2.0 0.63 64.5 225.2 18.4; n/a; n/a 11 3.3 0.66 50.3 167.7
n/a; n/a; 22.5 12 2.3 0.53 42.7 177.3 20.2; n/a; n/a 13 2.3 0.52
54.6 230.9 n/a; 20.2; n/a 14 1.9 0.52 32.7 138.3 23.8; n/a; n/a 15
3.1 0.53 61.1 253.5 n/a; n/a; 24.8 16 2.4 n/a n/a n/a n/a; 9.7; n/a
17 2.5 0.68 51.8 167.5 n/a; n/a; n/a
[0197]
10TABLE 7a Thickness for Durometer Bolt Load Retention (%) Ex.
Durometer (mm) 1 mm; 1.5 mm; 3 mm; 6 mm 10 67 2.92 93.35; 93.17;
88.50; 83.95 11 66 2.82 93.25; n/a; 86.77; 69.40 12 67 2.74 93.10;
91.33; 86.03; n/a 13 51 3.56 n/a; 90.90; 87.45; n/a 14 n/a n/a n/a;
n/a; 87.07; n/a 15 67 3.3 n/a; n/a; 91.87; 79.00 16 73 2.54 n/a;
n/a; 88.5; n/a 17 69 2.36 n/a; n/a; 87.64; n/a
[0198]
11TABLE 8 Crystallinity Thickness Thickness for Index for Creep
Compressibility Compressibility Ex. (%) Creep (mm) (%) (mm) 10 69.5
3.1 1.84 63.3 1.69 11 62.7 n/a n/a 61.0 1.69 12 60.83 n/a n/a 64.9
1.59 13 n/a n/a n/a n/a n/a 14 n/a n/a n/a n/a n/a 15 73.6 n/a n/a
n/a n/a 16 n/a n/a n/a n/a n/a 17 n/a n/a n/a n/a n/a
* * * * *