Welding of filled sintered polytetrafluoroethylene

A method for welding together filled sintered polytetrafluoroethylene components. The ends of the components to be welded are carefully tapered to form reverse mated surfaces and then assembled in position to be welded. The actual welding step is carried out under heat and pressure followed by cooling under pressure. The process is particularly useful in the fabrication of very large gaskets or other very large planar objects.

FIELD OF THE INVENTION 
This invention relates to the welding of a plurality of filled sintered 
polytetrafluoroethylen (hereinafter PTFE) components to one another and 
more particularly to an improved process for such welding whereby strong 
welds are obtained without substantial harm to the physical properties of 
the material being welded. 
BACKGROUND OF THE INVENTION 
Ever since the introduction of PTFE to the market many years ago, 
fabricators have encountered problems with joining the material to itself 
or to other materials. These problems have been even more evident when 
attempting to join sintered PTFE elements. During the ensuing years, a 
number of bonding and/or welding processes have been developed. For 
example, U.S. Pat. No. 3,207,644 to Hobson et al. describes a welding 
process whereby pure PTFE elements are joined by subjecting them to heat 
and pressure followed by water quench. U.S. Pat. No. 4,701,291 to Wissman 
discloses a complex process for welding pure PTFE components in a mold and 
utilizing a bath of molten metal or salt. Neither of these references 
makes any representation that its process is applicable to filled sintered 
PTFE components and neither shows any recognition that filled sintered 
PTFE material presents different processing problems than pure PTFE. 
Other bonding or welding processes call for enhancing the integrity of 
joints by the use of an intermediate layer of fluorine-containing resin 
such as fluorinated ethylene propylene (FEP) or perfluoroalkoxy (PFA) 
resin. Examples of such methods are described in U.S. Pat. Nos. 4,211,594 
to Freitag et al., 4,073,856 to Chu and 2,833,686 to Sandt. Other prior 
art patents describe processes for fusing unsintered PTFE surfaces. Among 
this group are U.S. Pat. Nos. 3,645,820 to Clary, 4,283,448 to Bowman and 
4,364.884 to Traut. 
These and other methods have been useful for joining homogeneous PTFE, but 
experience reveals that such methods are not satisfactory for joining 
filled sintered PTFE components. Sometimes tensile strength across the 
bond is lower than that of the main body of material, sometimes the 
surfaces in the area of the weld are poor and sometimes, chemical and/or 
temperature resistance is reduced. For example, a silica filled PTFE 
material welded with an FEP intermediate layer was found to have a tensile 
strength across the weld of about 1350 psi whereas the silica filled PTFE 
material itself has an average tensile strength of about 2000 psi or more. 
In another trial, the method of U.S. Pat. No. 3,207,644 was employed with 
a silica filled PTFE and the tensile strength across the weld was about 
1300 psi and elongation was severely reduced. For reason or reasons as yet 
unknown, the presence of filler(s) in PTFE materials inhibits the 
obtaining of satisfactory welds. 
SUMMARY OF THE INVENTION 
These problems are overcome by the present invention whereby welded joints 
in filled, sintered PTFE materials are obtained without significant 
adverse effect on the strength, dimensions or other important 
characteristics of the material in the area of the weld. The improved 
welding process includes the steps of preparing carefully mated and 
tapered surfaces on each of the components to be welded and subsequent 
heating and cooling under pressure. 
Thus, it is an object of the invention to provide a relatively simple and 
effective process for joining or welding together filled sintered PTFE 
components. 
It is a further object to provide a welding process which does not have a 
significant adverse effect on the properties of such PTFE components. 
Another object of the invention is to provide a welding process which will 
not result in significant thinning of the filled sintered PTFE material in 
the vicinity of the weld. 
Yet another object is to provide large gaskets having substantially uniform 
properties and characteristics at all locations within the body of such 
gaskets.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, FIG. 1 shows portions of a pair of filled 
sintered polytetrafluoroethylene (hereinafter PTFE) components 10, 20 each 
having an overlap portion 12, 22 intended to be formed and mated to the 
overlap portion of the other component. The overlap portions are then 
tapered to an angle B, as shown in FIG. 2, by cutting or grinding or some 
other suitable forming method to form reverse mated surfaces 14, 24 having 
a length not substantially less than 1/2 and preferably in the range of 
about 3/4 to 1". The presently preferred forming method is grinding which 
provides closer control and greater accuracy of the surfaces 14, 24. Angle 
B is selected in relation to the component thickness so as to taper the 
entire length of overlap portions 12, 22 thus producing tapered overlap 
portions 12', 22' with feathered edges 16, 26. In PTFE gasket 
applications, the components to be joined may typically be 1/16" or 1/8 
thick and have reverse mated surfaces of about 3/4 to 1" in length. If 
thinner component material is used, Angle B becomes smaller in order to 
maintain the desired length of the reverse mated surfaces. For example, 
Angle B should be on the order of 8.degree. with 1/8 thick components and 
about 4.degree. with 1/16" thick material. Each component that is to be 
welded to another is prepared by tapering its overlap portion in the same 
manner so as to produce pairs of reverse mated surfaces 14, 24 as shown in 
FlG. 2. 
As shown in FlG. 3, once the tapered overlap portions 12', 22' are 
prepared, the components to be joined are assembled with their reverse 
mated surfaces 14, 24 adjacent to one another and placed in a press (not 
shoWn) between heated platens 30. Sheets of aluminum foil 32 are used 
adjacent to the components 10, 20 being welded and steel sheets 34 are 
placed on the other side of the aluminum foil sheets to form a 
multilayered sandwich. The steel sheets help to maintain smooth surfaces 
on the material being welded and the aluminum foil prevents the PTFE 
material from bonding to the steel sheets. Good results have been obtained 
with temper- hardened aluminum foil of 0.001" thickness and stainless 
steel sheets on the order of 1/16" thickness. The entire sandwich is 
clamped to a portion of the press body or a supporting table and the press 
is closed to apply pressure P to hold the reverse mated surfaces 14, 24 in 
relative position while heat is applied to the degree and time necessary 
to fuse the mated surfaces together. Press bars 38 are used to direct the 
heat and pressure to the desired location at and adjacent to the joint 
being welded. Preferably the press bars will cover a segment on the order 
of 3" wide, including and extending on either side of the reverse mated 
surfaces 14, 24. The preferred initial pressure on some typical material 
compositions is on the order of 100 to 150 psi and the preferred press 
temperature is on the order of about 650.degree. F. to 700.degree. F. As 
the temperature of the filled sintered PTFE components increases, they 
expand and the pressure within the press increases, typically to a level 
on the order of 400 to 500 psi. With a typical 1/8 thick component and a 
preheated press, total time in the press to fuse the components is on the 
order of 3 minutes. Thinner components will fuse more quickly, while 
thicker ones will require a longer press time. 
Upon completion of the fusing or heating step, the resultant fused assembly 
is cooled under pressure. Preferably, the assembly is transferred quickly 
from the heated or fusing press to a cooling press which is then closed to 
apply and maintain a pressure during the cooling step less than the 
initial pressure during the heating step. Generally, a pressure on the 
order of about 5 to 30 psi will be sufficient. Both the heating and the 
cooling steps may be carried out in the same press if necessary or 
desired. 
Many filler materials are used in making PTFE components with the choice 
being made to best accommodate the desired properties and characteristics 
of the finished material. Among suitable fillers are carbon, graphite, 
silica, a variety of clays and microspheres of glass or other material. 
According to the variety and quantity of fillers used in the PTFE 
components, the optimum and maximum allowable pressures for heating and 
for cooling will vary to a degree. If excessive pressure is applied during 
the heating process or during the cooling process, there will be an 
unacceptable degree of flow, distortion or thinning in the area of the 
joint as compared to the remaining area of the components. In PTFE gasket 
materials, a very important result of using fillers is that there is a 
markedly reduced potential for cold flow when such gaskets are placed in 
service. However, since different fillers produce different results, some 
experimentation will be needed for any given composition to determine the 
optimum pressures during the heating and cooling steps of the present 
welding process. Table 1 illustrates some of the variations in thickness 
which ca occur with different press pressures during the heating or 
cooling steps with a silica filled material. For example, one test weld 
(Sample No. 5) of 1/8 thick material was made with an initial pressure 
slightly over 1000 psi and even though a strong weld was achieved, the 
thickness of the area under pressure was reduced more than 0.010". Such a 
thickness variation is not acceptable in typical gasket applications. Each 
of the samples shown in Table I attained maximum pressure within three (3) 
minutes and each passed standard ASTM F147 gasket flex test with no 
evidence of delamination. Samples 1 and 2 produced unsatisfactory surfaces 
believed to result from improper alignment of the reverse mated surfaces, 
excessive material flow and/or differences in thickness of components 
prior to welding. With some material compositions, it may be advisable to 
make use of side restraints in at least the hot press to prevent lateral 
flow of the PTFE composition. Table II shows the results of standard 
gasket tests for 1/8" thick silica filled PTFE gasket material and 
compares the test results across a welded joint with the results of the 
same tests on the same body of material, but in areas adjacent to the 
areas subjected to the process of the invention. All of the tests follow 
standard ASTM procedures with the exception of the Blowout Test which is a 
procedure of the assignee. 
TABLE I 
__________________________________________________________________________ 
Results of Welding Experiments 
with Silica Filled Sintered PTFE 
__________________________________________________________________________ 
Sample No. 1 2 3 4 5 6 
Heating Time, min. 
3 3 3 3 3 3 
Initial (Press) 
222.2 111.1 111.1 388.8 1111.1 
111.1 
Pressure, psi 
Maximum Pressure, psi 
400 450 450 550 1260 450 
Cooling Time, min. 
4-5 4-5 4-5 4-5 4-5 4-5 
Cooling Pressure, psi 
10 10 10 10 10 444 
Average Thickness of 
Each Component (10) - 
.122 .118 .124 .128 .125 .124 
Before Welding (20) - 
.124 .121 .124 .130 .125 .124 
Joint Thickness - 
Range of 6 .120 to .122 
.117 to .120 
.121 to .123 
.122 to .123 
.114 to .116 
.112 to .113 
Measurements 
__________________________________________________________________________ 
TABLE II 
______________________________________ 
Comparison of Measured Properties 
of Silica Filled Sintered PTFE 
Material 
Adjacent 
Across 
to Weld 
Welded 
Area Joint 
______________________________________ 
Average Tensile Strength, psi 
2197 2083 
ASTM F152 
Average Elongation, % 
321 232 
ASTM F152 
Average Compressibility, % 
12.5 8.9 
ASTM F36 
Average Recovery, % 42.5 48.0 
ASTM F36 
Creep Relaxation, % 58.7 51.1 
ASTM F38 
Average Sealability, ml/hr 
2.25 1.5 
ASTM F37 
Average Blowout @ 500.degree. F., psi 
4467 4373 
______________________________________ 
Various other joint designs and bonding processes have been tried, but none 
have obtained the results of the present process and joint design. Instead 
of the reverse mated surfaces, components with a step joint were subjected 
to pressurized heating and cooling steps, but the resulting joints failed 
the standard ASTM F147 gasket flex test at those portions of the joint 
that were in alignment with the direction of the press pressure 
(perpendicular to the finished surface) and exhibited a tensile strength 
of about 950 psi across the weld. Only those joints which have a uniform 
and positive pressure applied at all points of the mated surfaces during 
welding and cooling exhibit consistently uniform and high quality. It also 
seems to be important to mate the opposing surfaces very carefully, 
especially at the feathered edges, if a smooth surface is to be obtained. 
Also, the thickness of each component to be joined should be very nearly 
equal to one another. Failure to exercise the necessary care Will produce 
joints which are not completely fused and/or which have irregular 
surfaces. 
The process of the invention may be particularly useful, for example, in 
fabricating large gaskets. At present, filled sintered PTFE gasketing 
sheet material is available in sizes up to 60" square, but there 
frequently is a need for gaskets with larger dimensions. Thus, referring 
to FIG. 4, there is shown a gasket 40 made of N (in this instance, four) 
components 42 welded to one another at joints 44. If linear dimension "d" 
is slightly less than 60", then an annular gasket on the order of 80" in 
diameter may be made by welding together four (4) such components by 
carrying out the steps of the invention a total of N times. If a still 
larger diameter gasket is desired, all that is required is to use a larger 
number N of components, each subtending a smaller arc. FlG. 5 shows a 
cross-section through a joint 44 of gasket 40 to illustrate the fuse line 
between reverse mated surfaces as described in detail in the previous 
description of FIGS. 2 and 3. If, instead of an endless product, a 
two-ended product is desired, the same process may be used except that the 
total number of weldments will be N-1 as compared to N components. 
While preferred embodiments of the invention have been shoWn and described 
in detail, other modifications will be readily apparent to those skilled 
in the art of processing and bonding PTFE resins. Thus, the preceding 
specification should be interpreted as exemplary rather than as limiting 
and the scope of the invention is defined by the following claims.