Stock material or miscellaneous articles – Composite – Of fluorinated addition polymer from unsaturated monomers
Reexamination Certificate
1999-03-03
2001-02-27
Sanders, Kriellion (Department: 1714)
Stock material or miscellaneous articles
Composite
Of fluorinated addition polymer from unsaturated monomers
Reexamination Certificate
active
06194074
ABSTRACT:
The functioning of an aircraft is dependent on numerous mechanical, hydraulic and electronic systems. Access to these systems, for example for maintenance purposes, can often only be made from the outside or inside of the aircraft by removing panels. In order to protect the systems from environmental influences such as water ingress, a reliable panel sealant is required.
In the past two-component sealants have been mixed and applied to the aircraft structure with a gun or a spatula. Controlled ambient conditions, a limited pot life as well as long curing times (usually 24 hours but can be longer depending on air humidity and temperature) are characteristic of this method. Polysulphide is an example of such a sealant material. Furthermore, the sealants are difficult to remove when they have to be replaced. The replaced sealants have to be disposed after use with special waste techniques for poisonous waste.
W. L. Gore & Associates have developed a range of sealant tapes sold under the name of SKYFLEX® which overcome these problems. They have none of the above limitations and do not require the use of protective measures during installation. The sealant tapes are described in more detail in the German Patent Application DE-A-37 26 853 (Forrest et. al.).
The SKYFLEX® sealant tapes are manufactured from expanded polytetrafluoroethylene (ePTFE). This material is chemically inert and not affected by any common chemical. It functions over a wide temperature range (−240° C. to +260° C.), does not age and is weather and UV resistant. The ePTFE used in SKYFLEX® sealant is manufactured by the GORE® expansion process. The result is a material which combines the above advantages with high pressure resistance. In addition the material conforms perfectly to irregularities in sealed surfaces.
As well as in aircraft the sealant tapes are currently finding application in other systems such as cars and trains in which reliable panel sealants are required. All of these systems also suffer from corrosion problems around the panels.
FIGS. 1 and 2
illustrate these problems.
FIG. 1
shows a top view two metal parts
10
a,
10
b
made in this example of aluminium alloy between which is placed a sealant
20
. At the boundary between the metal parts
10
and the sealant
20
, joins
30
a
and
30
b
are formed. The joins
30
a
and
30
b
behave like a crevice
30
(such as illustrated in
FIG. 2
) on the metal surface but in macroscopic dimensions.
In
FIG. 2
a side view of the metal crevice
30
is depicted. Within the join
30
oxygen diffusion is hindered. The hindered diffusion of oxygen leads to a depletion of oxygen within a lower or inner part
40
of the join
30
. As a result the inner part
40
of the join
30
shows a reduced electrochemical potential compared with an upper or outer region
50
of the join
30
which is fully covered with aluminium oxide. This difference in electrochemical potential initiates an electrochemical reaction. In the inner part
40
of the join
30
, aluminium ions will be generated. In a second reaction, these ions will be hydrolysed which causes the generation of acid. The existence of acid within the join
30
accelerates the metal dissolution which in turn further accelerates the corrosion.
Corrosion is reduced in two component sealants by adding corrosion-inhibiting substances such as chromates or phosphates to the two component sealant. These corrosion-inhibiting substances are, however, mostly very toxic and have the disadvantage that they need to be dispersed within the sealant mass. They are only effective in a limited area and experiments have shown that with time they tend to lose their activity and dissipate out of the sealant mass and thus the anti-corrosion properties of the sealant diminish. The corrosion-inhibiting substances work by increasing the electrochemical potential of the metal surface and thus leading to a temporary passivation of the surface.
The U.S. Pat. No. 4,028,324 (Tuschner et al.), assigned to United Technologies Corporation, illustrates a method for improving the chemical resistance of a sealant for a phosphoric fuel cell. The sealant used is an extruded seal material, the composition of which consists essentially of ePTFE, a halogenated oil, 0-5% by weight of a halogenated solvent and a filler stable in phosphoric acid and compatible with PTFE. This sealant is manufactured by mixing PTFE, fluorocarbon oil, solvent and filler in a mixer. After the components are mixed, it is allowed to dry and the solvent evaporates. The compound is afterwards extruded into the form of tape by means of a ram extruder.
Because of environmental concerns, the teachings of this patent are, however, nowadays of limited relevance. The use of CFCs is increasingly being avoided due to their known detrimental effects on the ozone layer in the atmosphere.
The taught manufacturing method also has a number of disadvantages as the addition of PFPE to PTFE prior to the extrusion of PTFE is only possible in a limited way. Firstly, the PFPE must completely saturate the PTFE which requires a low surface energy and viscosity of PFPE (i.e. less than 24 mN/m). Secondly the proportion of PFPE in relation to PTFE can only be chosen in a limited range. If the proportion is too low, for example under 30%, then the material can no longer be extruded as the extrusion pressure required is too high. If the share of PFPE is too high, then the extrusion pressure falls. However, the extruded material is then not adequately stable to allow its use as sealants. A further disadvantage is that Tuschner mentions that the material needs only to be stable to 240° C. (400° F.). However, expanded PTFE requires sintering at temperatures of above 250° C. Thus the teachings of Tuschner cannot be applied to the manufacture of expanded PTFE with PFPE.
In WO-A-92/21715 (Chung) assigned to the Donaldson Company Inc. the use of ePTFE which has been treated with a non-functionalised perfluoropotyether (PFPE) fluid prior to or after expansion as a mechanical seal is described. The inventor of this patent application recognised that the system described in this application offered improved radiation resistance properties. However, there is no hint in the patent application that sealant material made from this material could furthermore be resistant to corrosion.
Similarly JP-A-61-163944 (Omori) assigned to Daikin Industry Ltd. teaches the incorporation of a non-functionalised PFPE compound into ePTFE for use as a diaphragm in a sensor. This patent application also does not teach the anti-corrosion properties of the PFPE/PTFE system.
The incorporation of both functionalised and non-functionalised PFPE into ePTFE is also taught in DE-A-43 08 369 (Bürger) assigned to W. L. Gore & Associates. This patent application teaches that the system possesses improved oleophobic properties. However, the disclosure fails to teach that this system also shows anti-corrosion properties.
The object of the invention is therefore to provide a sealant with improved anti-corrosion properties.
A further object of the invention is to provide an improved manufacturing method for a sealant with improved anti-corrosion properties.
A further object of the invention is to provide a sealant made of microporous material which incorporates corrosion inhibitors within the body of the microporous material.
This object is achieved by providing a sealant material which before use consists essentially of a) 30-90% by weight porous base material and b) 10-70% by weight corrosion inhibitor having the requirements that it is compatible with said porous material. The words “by weight” as used in the current patent refer to a weight of the sealant or of the corrosion inhibitor as a proportion of the total weight of the sealant plus corrosion inhibitor. This combination of materials is non-toxic and is easy to apply between two metal parts, requiring no special tools and no setting time. Furthermore the sealant material is easy and cheap to manufacture requiring no special mixing steps prior to the expansion of the polytetr
Burger Wolfgang
Holbein Reinhold
Rüsch Hans-Jörg
Walter Wolfgang
Wiesemann Amadeus
Sanders Kriellion
Wheatcraft Allan M.
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