U.S. patent application number 10/837337 was filed with the patent office on 2004-11-04 for preformed emi/rfi shielding compositions in shaped form.
This patent application is currently assigned to PRC-DeSoto International, Inc.. Invention is credited to Balladares, Adrian, Cosman, Michael A..
Application Number | 20040220327 10/837337 |
Document ID | / |
Family ID | 33435001 |
Filed Date | 2004-11-04 |
United States Patent
Application |
20040220327 |
Kind Code |
A1 |
Cosman, Michael A. ; et
al. |
November 4, 2004 |
Preformed EMI/RFI shielding compositions in shaped form
Abstract
Electrically conductive preformed compositions comprising
sulfur-containing polymers in shaped form and the use of preformed
compositions in shaped form to seal apertures are disclosed. The
preformed compositions can be used to seal an aperture having
EMI/RFI shielding effectiveness.
Inventors: |
Cosman, Michael A.;
(Valencia, CA) ; Balladares, Adrian; (El Segundo,
CA) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW,
GARRETT & DUNNER, L.L.P
1300 I Street, N.W.
Washington
DC
20005
US
|
Assignee: |
PRC-DeSoto International,
Inc.
|
Family ID: |
33435001 |
Appl. No.: |
10/837337 |
Filed: |
April 30, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60466981 |
Apr 30, 2003 |
|
|
|
Current U.S.
Class: |
524/779 ;
525/535 |
Current CPC
Class: |
C08K 7/06 20130101; C08K
9/02 20130101; C08L 81/04 20130101 |
Class at
Publication: |
524/779 ;
525/535 |
International
Class: |
C08L 11/12 |
Claims
What is claimed is:
1. A preformed composition in shaped-form comprising: a base
composition comprising at least one sulfur-containing polymer, and
at least one electrically conductive filler; and a curing agent
composition; wherein the preformed composition is capable of
shielding EMI/RFI radiation.
2. The preformed composition of claim 1, wherein the preformed
composition comprises 5 parts to 20 parts by weight of the curing
agent composition, and 100 parts by weight of the base
composition.
3. The preformed composition of claim 1, wherein the at least one
sulfur-containing polymer is present in an amount ranging from 10%
by weight to 50% by weight of the total weight of the base
composition.
4. The preformed composition of claim 1, wherein the at least one
sulfur-containing polymer is chosen from a polysulfide polymer, a
mercapto-terminated polymer, and a combination of a polysulfide
polymer and a mercapto-terminated polymer.
5. The preformed composition of claim 1, wherein the at least one
electrically conductive filler is present in an amount ranging from
40% to 80% by weight of the total weight of the base
composition.
6. The preformed composition of claim 1, wherein the at least one
electrically conductive filler comprises Ni fiber, and Ni-coated
graphite.
7. The preformed composition of claim 6, wherein the Ni fiber is
present in an amount ranging from 4% to 8% by weight of the total
weight of the base composition, and the Ni-coated graphite is
present in an amount ranging from 50% to 70% of the total weight of
the base composition.
8. The preformed composition of claim 1, further comprising at
least one corrosion inhibitor.
9. The preformed composition of claim 8, wherein the at least one
corrosion inhibitor inhibits galvanic corrosion.
10. The preformed composition of claim 8, wherein the at least one
corrosion inhibitor comprises calcium chromate.
11. The preformed composition of claim 8, wherein the at least one
corrosion inhibitor is present in an amount ranging from 3% by
weight to 7% by weight of the total weight of the base
composition.
12. The preformed composition of claim 1, further comprising at
least one adhesion promoter.
13. The preformed composition of claim 12, wherein the at least one
adhesion promoter comprises a phenolic adhesion promoter, a
mercapto-silane adhesion promoter, and an epoxy-silane adhesion
promoter.
14. The preformed composition of claim 12, wherein the at least one
adhesion promoter is present in an amount ranging from 1% by weight
to 6% by weight of the total weight of the base composition.
15. The preformed composition of claim 1, wherein the preformed
composition is curable at a temperature ranging from 10.degree. C.
to 30.degree. C.
16. The preformed composition of claim 1, wherein the preformed
composition is refrigerated prior to application.
17. The preformed composition of claim 1, wherein the cured
preformed composition exhibits a surface resistivity of less than
0.50 .OMEGA./.
18. The preformed composition of claim 1, wherein the curing agent
composition comprises a manganese dioxide curing agent.
19. The preformed composition of claim 18, wherein the manganese
dioxide is present in the curing agent composition in an amount
ranging from 25% to 75% by weight of the total weight of the curing
agent composition.
20. A method of sealing an aperture to provide EMI/RFI shielding
effectiveness comprising applying a preformed composition in
shaped-form comprising at least one sulfur-containing polymer, and
at least one electrically conductive filler to a surface associated
with an aperture to seal the aperture and provide EMI/RFI shielding
effectiveness.
21. The method of claim 20, wherein the performed composition
comprises a preformed composition according to claim 1.
22. The method of claim 20, wherein the surface is a surface of a
removable panel.
23. The method of claim 20, wherein the surface is a surface
adjacent to an opening.
24. The method of claim 20, wherein the aperture is on an aviation
or an aerospace vehicle.
25. The method of claim 20, further comprising applying an adhesion
promoter to at least one surface associated with the aperture prior
to application of the preformed composition.
26. The method of claim 20, further comprising applying a release
agent to a least one surface associated with the aperture prior to
application of the preformed composition.
27. The method of claim 20, wherein the sealed aperture exhibits
shielding effectiveness from 1 MHz to 18 GHz.
Description
[0001] Applicants claim the right to priority under 35 U.S.C.
.sctn. 119(e) based on U.S. Provisional Patent Application No.
60/466,981, filed Apr. 30, 2003, entitled "PREFORMED EMI/RFI
SHIELDING COMPOSITIONS IN SHAPED FORM," and which is expressly
incorporated herein by reference in its entirety.
FIELD
[0002] The present disclosure relates to preformed compositions in
shaped form and the use of preformed compositions for sealing
apertures. The present disclosure further relates to preformed
compositions in shaped form exhibiting EMI/RFI shielding
effectiveness, and the use of such preformed compositions for
sealing apertures.
Introduction
[0003] Electromagnetic interference can be defined as undesired
conducted or radiated electrical disturbance from an electrical or
electronic source, including transients, which can interfere with
the operation of other electrical or electronic apparatus. Such
disturbance can occur at frequencies throughout the electromagnetic
spectrum. Radio frequency interference ("RFI") is often used
interchangeably with electromagnetic interference ("EMI"), although
RFI more properly refers to the radio frequency portion of the
electromagnetic spectrum usually defined as between 10 kilohertz
(KHz) and 100 gigahertz (GHz).
[0004] Electronic equipment is typically enclosed in a housing. The
housing can serve not only as a physical barrier to protect the
internal electronics from the external environment, but also can
serve to shield EMI/RFI radiation. Enclosures having the ability to
absorb and/or reflect EMI/RFI energy can be employed to confine the
EMI/RFI energy within the source device, as well as to insulate the
source device or other external devices from other EMI/RFI sources.
To maintain accessibility to the internal components, enclosures
can be provided with openable or removable accesses such as doors,
hatches, panels, or covers. Gaps typically exist between the
accesses and the corresponding mating surfaces associated with the
accesses that reduce the efficiency of the electromagnetic
shielding by presenting openings through which radiant energy may
be emitted. Such gaps also present discontinuities in the surface
and ground conductivity of the housing and in some cases, may
generate a secondary source of EMI/RFI radiation by functioning as
a slot antenna.
[0005] For filing gaps between the mating surfaces of the housing
and removable accesses, gaskets and other seals can be used to
maintain electrical continuity across the structures, and to
exclude environmental degradants such as particulates, moisture,
and corrosive species. Such seals can be bonded or mechanically
attached to one or both of the mating surfaces and can function to
establish a continuous conductive path by conforming to surface
irregularities under an applied pressure.
[0006] Conventional processes for manufacturing EMI/RFI shielding
gaskets include extrusion, molding, and die-cutting. Molding
includes the compression or injection molding of an uncured sealant
or thermoplastic material into a certain configuration which is
then cured to a final shape. Die-cutting includes the forming of a
gasket from a cured polymeric material which is cut or stamped
using a die into a certain configuration. Form-in-place ("FIP")
processes are also used for forming EMI/RFI shielding gaskets
wherein the FIP process includes the application of a bead of a
viscous, curable, electrically-conductive composition in a fluent
state to a surface that is subsequently cured-in-place by the
application of heat, atmospheric moisture, or ultraviolet radiation
to form an electrically-conductive, EMI/RFI shielding gasket.
[0007] Electrical conductivity and EMI/RFI shielding effectiveness
can be imparted to polymeric gaskets by incorporating conductive
materials within the polymer matrix. The conductive elements can
include, for example, metal or metal-plated particles, fabrics,
meshes, fibers, and combinations thereof The metal can be in the
form of, for example, filaments, particles, flakes, or spheres.
Examples of metals include copper, nickel, silver, aluminum, tin,
and steel. Other conductive materials that can be used to impart
EMI/RFI shielding effectiveness to polymer compositions include
conductive particles or fibers comprising carbon or graphite.
Conductive polymers such as polythiophenes, polypyrroles,
polyaniline, poly(p-phenylene) vinylene, polyphenylene sulfide,
polyphenylene, and polyacetylene can also be used.
[0008] In addition to providing continuous electrical conductivity
and EMI/RFI shielding effectiveness, in certain applications it is
desirable that gaskets or seals to surfaces exposed to the
environment, such as in aviation and aerospace vehicles, not lead
to corrosion of the metal surfaces. When dissimilar metals and/or
conductive composite materials are joined in the presence of an
electrolyte, a galvanic potential is established at the interface
between the dissimilar conductors. When the interfacial seal is
exposed to the environment, particularly under severe environmental
conditions such as salt fog or salt fog containing a high
concentration of SO.sub.2, corrosion of the least noble of the
conductive surfaces can occur. Corrosion may lead to a degradation
in the EMI/RFI shielding effectiveness of the seal. Mechanisms
other than galvanic potentials, for example, crevice corrosion, may
also compromise the electrical and mechanical integrity of the
enclosure.
[0009] Polysulfide polymers are known in the art. The production of
polysulfide polymers is characterized by Fettes and Jorzak,
Industrial Engineering Chemistry, November 1950, on pages
2,217-2,223. The commercial use of polysulfide polymers in the
manufacture of sealants for aerospace applications has long been
known and commercially used. For example, polysulfide sealants have
been used to seal an aircraft body because of the high tensile
strength, high tear strength, thermal resistance, and resistance to
high ultraviolet light. Polysulfide sealants have been used to seal
aircraft fuel tanks because of the resistance to fuel and adhesion
upon exposure to fuel.
[0010] Polysulfide sealants are generally applied to a surface by
extrusion using a caulking gun. Such a process can be efficient for
permanent panels installed on an airframe. However, extruding a
sealant to seal apertures in and/or on an airframe such as those
associated with access doors or panels can require a significant
amount of additional effort. To extrude an uncured sealant, the
interior perimeter of the access door opening is masked and the
exterior perimeter of the access door is coated with a release
agent prior to extruding the sealant to the masked area of the
access door opening to avoid sealing an access door shut. The
access door is put in place and clamped down to force the excess
uncured sealant around the access door. The sealant is then cured
and the excess sealant is trimmed away. This process is time
intensive and can add significant labor to servicing aircraft with
many access doors. Some aircraft can have as many as a hundred or
more access doors that are used to cover sensitive electronic
equipment or fittings that must be periodically accessed and
resealed.
[0011] Accordingly, it is desirable to provide a method for sealing
access doors, for example those in an airframe of an aviation or
aerospace vehicle, that does not require masking, reduces trimming
and/or is not as labor and time intensive as the conventional
extrusion method for sealing the access doors.
[0012] Electrically conductive sealants that exhibit EMI/RFI
shielding effectiveness are commercially available. For example,
PRC-DeSoto International, Inc. (Glendale, Calif.) manufactures
several class B electrically conductive sealants specifically
developed for aviation and aerospace applications. For example,
PR-2200 Class B electrically conductive sealant is an electrically
conductive polythioether sealant that meets the requirements of MMS
327 (Boeing St. Louis Military Material Specification) test
methods. These two-part, nickel-filled sealants comprise a
polythioether polymer, PERMAPOL P-3.1, and are not corrosive when
used on aluminum alloys or between dissimilar metals. However,
commercially available sealants such as exemplified by the PR-2200
product are not provided as a preformed composition.
[0013] Therefore, it is further desirable to provide a method for
sealing access doors to provide effective EMI/RFI shielding and
cause minimal corrosion to conductive surfaces in environments
encountered in aviation and aerospace applications that does not
require masking, reduces trimming and/or is not as labor and time
intensive as is the conventional extrusion method for sealing the
access doors.
Summary
[0014] In accordance with embodiments of the present disclosure,
preformed compositions in shaped form comprising a base composition
comprising at least one sulfur-containing polymer, and at least one
electrically conductive filler; and a curing agent composition;
wherein the preformed composition is capable of shielding EMI/RFI
radiation, are provided.
[0015] In accordance with embodiments of the present disclosure,
methods of sealing an aperture to provide EMI/RFI shielding
effectiveness comprising applying a preformed composition in
shaped-form comprising at least one sulfur-containing polymer, and
at least one electrically conductive filler to a surface associated
with an aperture; and curing the preformed composition to seal the
aperture and provide EMI/RFI shielding effectiveness, are
disclosed.
[0016] Additional embodiments of the disclosure are set forth in
the description which follows, or may be learned by practice of the
embodiments of the present disclosure.
DESCRIPTION OF VARIOUS EMBODIMENTS
[0017] In certain embodiments of the present disclosure, preformed
compositions in shaped form suitable for sealing apertures, for
example, elongated apertures in or on the body of an aircraft,
comprises at least one sulfur-containing polymer, and at least one
electrically conductive filler. The term "preformed" refers to a
composition that can be prepared into a particular shape for ease
of packaging, storage, and/or application. A composition that is
preformed can be reshaped into any shape, either intentionally, or
as a result of shipping and/or handling. The term "shaped form"
refers to a configuration such that the thickness of the preformed
composition is substantially less than the lateral dimension and
includes tapes, sheets, and cut-out or gasket forms. The "shaped
form" can be, for example, in the form of a tape meaning a narrow
shape, strip, or band that can be stored as a roll, coil, or strip.
A "shaped form" can also be die-cut to the dimensions of an
aperture to be sealed.
[0018] The term "sealant," "sealing," or "seal" as used herein
refers to compositions that have the ability to resist atmospheric
conditions such as moisture and temperature and at least partially
block the transmission of materials such as water, fuel, and other
liquids and gasses. Sealants often have adhesive properties, but
are not simply adhesives that do not have the blocking properties
of a sealant. The term "elongated aperture" as used herein refers
to an opening in which the length is at least three-times the
width.
[0019] Preformed sealant compositions of the present disclosure can
be prepared by blending an electrically conductive base
composition, and a curing agent composition. A base composition and
a curing agent composition can be prepared separately, blended to
form a sealant composition, and preformed to a particular shape. A
conductive base composition can comprise, for example, at least one
sulfur-containing polymer, at least one plasticizer, at least one
adhesion promoter, at least one corrosion inhibitor, at least one
electrically non-conductive filler, at least one electrically
conductive filler, and at least one adhesion promoter. A curing
agent composition can comprise, for example, at least one curing
agent, at least one plasticizer, at least one electrically
non-conductive filler, and at least one cure accelerator. In
certain embodiments, 5 to 20 parts by weight of a curing agent
composition are blended with 100 parts by weight of a base
composition, and in certain embodiments, 8 to 16 parts by weight of
curing agent composition are blended with 100 parts by weight of a
base composition to form an electrically conductive sealant
composition.
[0020] In certain embodiments, two-component curable compositions
are preferred to the one-component curable compositions because the
two-component compositions provide the best rheology for
application and exhibit desirable physical and chemical properties
in the resultant cured composition. As used herein, the two
components are referred to as the base composition, and the curing
agent composition. In certain embodiments, the base composition can
comprise polysulfide polymers, polythioether polymers, oxidizing
agents, additives, fillers, plasticizers, organic solvents,
adhesion promoters, corrosion inhibitors, and combinations thereof.
In certain embodiments, the curing agent composition can comprise
curing agents, cure accelerators, cure retardants, plasticizers,
additives, fillers, and combinations thereof.
[0021] In certain embodiments, sulfur-containing polymers useful in
the practice of the present disclosure include polysulfide polymers
that contain multiple sulfide groups, i.e.,--S--, in the polymer
backbone and/or in the terminal or pendent positions on the polymer
chain. Such polymers are described in U.S. Pat. No. 2,466,963
wherein the disclosed polymers have multiple --S--S-- linkages in
the polymer backbone. Other useful polysulfide polymers are those
in which the polysulfide linkage is replaced with a polythioether
linkage, i.e.,
--[--CH.sub.2--CH.sub.2--S--CH.sub.2--CH.sub.2--].sub.n--
[0022] where n can be an integer ranging from 8 to 200 as described
in U.S. Pat. No. 4,366,307. The polysulfide polymers can be
terminated with non-reactive groups such as alkyl, although in
certain embodiments, the polysulfide polymers contain reactive
groups in the terminal or pendent positions. Typical reactive
groups are thiol, hydroxyl, amino, and vinyl. Such polysulfide
polymers are described in the aforementioned U.S. Pat. No.
2,466,963, U.S. Pat. No. 4,366,307, and U.S. Pat. No. 6,372,849,
each of which is incorporated herein by reference. Such polysulfide
polymers can be cured with curing agents that are reactive with the
reactive groups of the polysulfide polymer.
[0023] Sulfur-containing polymers of the present disclosure can
have number average molecular weights ranging from 500 to 8,000
grams per mole, and in certain embodiments, from 1,000 to 5,000
grams per mole, as determined by gel permeation chromatography
using a polystyrene standard. For sulfur-containing polymers that
contain reactive functional groups, the sulfur-containing polymers
can have average functionalities ranging from 2.05 to 3.0, and in
certain embodiments ranging from 2.1 to 2.6. A specific average
functionality can be achieved by suitable selection of reactive
components. Examples of sulfur-containing polymers include those
available from PRC-DeSoto International, Inc. under the trademark
PERMAPOL, specifically, PERMAPOL P-3.1 or PERMAPOL P-3, and from
Akros Chemicals, such as THIOPLAST G4.
[0024] A sulfur-containing polymer can be present in the conductive
base composition in an amount ranging from 10% to 40% by weight of
the total weight of the conductive base composition, and in certain
embodiments can range from 20% to 30% by weight. In certain
embodiments, wherein a sulfur-containing polymer comprises a
combination of a polysulfide polymer and a polythioether polymer,
the amount of polysulfide polymer and polythioether polymer can be
similar. For example, the amount of polysulfide polymer and the
amount of polythioether polymer in a base composition can each
range from 10% by weight to 15% by weight of the total weight of
the conductive base composition.
[0025] Preformed compositions of the present disclosure comprise at
least one curing agent for curing the at least one
sulfur-containing polymer. The term "curing agent" refers to any
material that can be added to a sulfur-containing polymer to
accelerate the curing or gelling of the sulfur-containing polymer.
Curing agents are also known as accelerators, catalysts or cure
pastes. In certain embodiments, the curing agent is reactive at a
temperature ranging from 10.degree. C. to 80.degree. C. The term
"reactive" means capable of chemical reaction and includes any
level of reaction from partial to complete reaction of a reactant.
In certain embodiments, a curing agent is reactive when it provides
for cross-linking or gelling of a sulfur-containing polymer.
[0026] In certain embodiments, preformed compositions comprise at
least one curing agent that contains oxidizing agents capable of
oxidizing terminal mercaptan groups of the sulfur-containing
polymer to form disulfide bonds. Useful oxidizing agents include,
for example, lead dioxide, manganese dioxide, calcium dioxide,
sodium perborate monohydrate, calcium peroxide, zinc peroxide, and
dichromate. The amount of curing agent in a curing agent
composition can range from 25% by weight to 75% by weight of the
total weight of the curing agent composition. Additives such as
sodium stearate can also be included to improve the stability of
the accelerator. For example, a curing agent composition can
comprise an amount of cure accelerator ranging from 0.1% to 1.5% by
weight based on the total weight of the curing agent
composition.
[0027] In certain embodiments, preformed compositions of the
present disclosure can comprise at least one curing agent
containing at least one reactive functional group that is reactive
with functional groups attached to the sulfur-containing polymer.
Useful curing agents containing at least one reactive functional
group that is reactive with functional groups attached to the
sulfur-containing polymer include polythiols, such as
polythioethers, for curing vinyl-terminated polymers;
polyisocyanates such as isophorone diisocyanate, hexamethylene
diisocyanate, and mixtures and isocyanurate derivatives thereof for
curing thiol-, hydroxyl- and amino-terminated polymers; and,
polyepoxides for curing amine- and thiol-terminated polymers.
Examples of polyepoxides include hydantoin diepoxide, Bisphenol-A
epoxides, Bisphenol-F epoxides, Novolac-type epoxides, aliphatic
polyepoxides, and epoxidized unsaturated resins, and phenolic
resins. The term "polyepoxide" refers to a material having a
1,2-epoxy equivalent greater than one and includes monomers,
oligomers, and polymers.
[0028] A preformed sealant composition can comprise at least one
compound to modify the rate of cure. For example, cure accelerants
such as dipentamethylene/thiuram/polysulfide mixture can be
included in a sealant composition to accelerate the rate of cure,
and/or at least one cure retardant such as stearic acid can be
added to retard the rate of cure and thereby extend the work life
of a sealant composition during application. In certain
embodiments, a curing agent composition can comprise an amount of
accelerant ranging from 1% to 7% by weight, and/or an amount of
cure retardant ranging from 0.1% to 1% by weight, based on the
total weight of the curing agent composition. To control the cure
properties of the sealant composition, it can also be useful to
include at least one material capable of at least partially
removing moisture from the sealant composition such as molecular
sieve powder. In certain embodiments, a curing agent composition
can comprise an amount of material capable of at least partially
removing moisture ranging from 0.1% to 1.5% by weight, based on the
total weight of the curing agent composition.
[0029] In certain embodiments, preformed compositions of the
present disclosure can comprise fillers. As used herein, "filler"
refers to a non-reactive component in the preformed composition
that provides a desired property, such as, for example, electrical
conductivity, density, viscosity, mechanical strength, EMI/RFI
shielding effectiveness, and the like.
[0030] Examples of electrically non-conductive fillers include
materials such as, but not limited to, calcium carbonate, mica,
polyamide, fumed silica, molecular sieve powder, microspheres,
titanium dioxide, chalks, alkaline blacks, cellulose, zinc sulfide,
heavy spar, alkaline earth oxides, alkaline earth hydroxides, and
the like. Fillers also include high band gap materials such as zinc
sulfide and inorganic barium compounds. In certain embodiments, an
electrically conductive base composition can comprise an amount of
electrically non-conductive filler ranging from 2% to 10% by
weight, based on the total weight of the base composition, and in
certain embodiments, can range from 3% to 7% by weight. In certain
embodiments, a curing agent composition can comprise an amount of
electrically non-conductive filler ranging from less than 6 percent
by weight, and in certain embodiments ranging from 0.5% to 4% by
weight, based on the total weight of the curing agent
composition.
[0031] Fillers used to impart electrical conductivity and EMI/RFI
shielding effectiveness to polymer compositions are well known in
the art. Examples of electrically conductive fillers include
electrically conductive noble metal-based fillers such as pure
silver; noble metal-plated noble metals such as silver-plated gold;
noble metal-plated non-noble metals such as silver plated cooper,
nickel or aluminum, for example, silver-plated aluminum core
particles or platinum-plated copper particles; noble-metal plated
glass, plastic or ceramics such as silver-plated glass
microspheres, noble-metal plated aluminum or noble-metal plated
plastic microspheres; noble-metal plated mica; and other such
noble-metal conductive fillers. Non-noble metal-based materials can
also be used and include, for example, non-noble metal-plated
non-noble metals such as copper-coated iron particles or nickel
plated copper; non-noble metals, e.g., copper, aluminum, nickel,
cobalt; non-noble-metal-plated-non metals, e.g., nickel-plated
graphite and non-metal materials such as carbon black and graphite.
Combinations of electrically conductive fillers can also be used to
meet the desired conductivity, EMI/RFI shielding effectiveness,
hardness, and other properties suitable for a particular
application.
[0032] The shape and size of the electrically conductive fillers
used in the preformed compositions of the present disclosure can be
any appropriate shape and size to impart EMI/RFI shielding
effectiveness to the cured preformed composition. For example,
fillers can be of any shape that is generally used in the
manufacture of electrically conductive fillers, including
spherical, flake, platelet, particle, powder, irregular, fiber, and
the like. In certain preformed sealant compositions of the
disclosure, a base composition can comprise Ni-coated graphite as a
particle, powder or flake. In certain embodiments, the amount of
Ni-coated graphite in a base composition can range from 40% to 80%
by weight, and in certain embodiments can range from 50% to 70% by
weight, based on the total weight of the base composition. In
certain embodiments, an electrically conductive filler can comprise
Ni fiber. Ni fiber can have a diameter ranging from 10 .mu.m to 50
.mu.m and have a length ranging from 250 .mu.m to 750 .mu.m. A base
composition can comprise, for example, an amount of Ni fiber
ranging from 2% to 10% by weight, and in certain embodiments, from
4% to 8% by weight, based on the total weight of the base
composition.
[0033] Carbon fibers, particularly graphitized carbon fibers, can
also be used to impart electrical conductivity to preformed
compositions of the present disclosure. Carbon fibers formed by
vapor phase pyrolysis methods and graphitized by heat treatment and
which are hollow or solid with a fiber diameter ranging from 0.1
micron to several microns, have high electrical conductivity. As
disclosed in U.S. Pat. No. 6,184,280, carbon microfibers, nanotubes
or carbon fibrils having an outer diameter of less than 0.1 micron
to tens of nanometers can be used as electrically conductive
fillers. An example of graphitized carbon fiber suitable for
conductive preformed compositions of the present disclosure include
PANEX 3OMF (Zoltek Companies, Inc., St. Louis, Mo.), a 0.921 micron
diameter round fiber having an electrical resistivity of
0.00055.OMEGA.-cm.
[0034] The average particle size of an electrically conductive
filler can be within a range useful for imparting electrical
conductivity to a polymer-based composition. For example, in
certain embodiments, the particle size of the one or more fillers
can range from 0.25 microns to 250 microns, in certain embodiments
can range from 0.25 microns to 75 microns, and in certain
embodiments can range from 0.25 microns to 60 microns. In certain
embodiments, preformed composition of the present disclosure can
comprise Ketjen Black EC-600 JD (Akzo Nobel, Inc., Chicago, Ill.),
an electrically conductive carbon black characterized by an iodine
absorption of 1000-11500 mg/g (J0/84-5 test method), and a pore
volume of 480-510 cm.sup.3/100 gm (DBP absorption, KTM 81-3504). In
certain embodiments, an electrically conductive carbon black filler
is Black Pearls 2000 (Cabot Corporation, Boston, Mass.).
[0035] In certain embodiments, electrically conductive polymers can
be used to impart or modify the electrical conductivity of
preformed compositions of the present disclosure. Polymers having
sulfur atoms incorporated into aromatic groups or adjacent to
double bonds, such as in polyphenylene sulfide, and polythiophene,
are known to be electrically conductive. Other electrically
conductive polymers include, for example, polypyrroles,
polyaniline, poly(p-phenylene) vinylene, and polyacetylene. In
certain embodiments, the sulfur-containing polymers forming a base
composition can be polysulfides and/or polythioethers. As such, the
sulfur-containing polymers can comprise aromatic sulfur groups and
sulfur atoms adjacent to conjugated double bonds such as
vinylcyclohexene-dimerc- aptodioxaoctane groups, to enhance the
electrical conductivity of the preformed compositions of the
present disclosure.
[0036] Preformed sealant compositions of the present disclosure can
comprise more than one electrically conductive filler, and the more
than one electrically conductive filler can be of the same or
different materials and/or shapes. For example, a preformed sealant
composition can comprise electrically conductive Ni fibers, and
electrically conductive Ni-coated graphite in the form of powder,
particles or flakes. The amount and type of electrically conductive
filler can be selected to produce a preformed sealant composition
which, when cured, exhibits a sheet resistance (four-point
resistance) of less than 0.50 .OMEGA./, and in certain embodiments,
a sheet resistance less than 0.15 .OMEGA./. The amount and type of
filler can also be selected to provide effective EMI/RFI shielding
over a frequency range of from 1 MHz to 18 GHz for an aperture
sealed using a preformed sealant composition of the present
disclosure.
[0037] Galvanic corrosion of dissimilar metal surfaces and the
conductive compositions of the present disclosure can be minimized
or prevented by adding corrosion inhibitors to the composition,
and/or by selecting appropriate conductive fillers. In certain
embodiments, corrosion inhibitors include strontium chromate,
calcium chromate, magnesium chromate, and combinations thereof.
U.S. Pat. No. 5,284,888 and U.S. Pat. No. 5,270,364 disclose the
use of aromatic triazoles to inhibit corrosion of aluminum and
steel surfaces. In certain embodiments, a sacrificial oxygen
scavenger such as Zn can be used as a corrosion inhibitor. In
certain embodiments, the corrosion inhibitor can comprise less than
10% by weight of the total weight of the electrically conductive
preformed composition. In certain embodiments, the corrosion
inhibitor can comprise an amount ranging from 2% by weight to 8% by
weight of the total weight of the electrically conductive preformed
composition. Corrosion between dissimilar metal surfaces can also
be minimized or prevented by the selection of the type, amount, and
properties of the conductive fillers comprising the preformed
composition.
[0038] In certain embodiments, preformed compositions of the
present disclosure comprise plasticizers such as phthalate esters,
chlorinated paraffins, hydrogenated terphenyls, partially
hydrogenated terphenyls, and the like. A preformed composition can
comprise more than one plasticizer. The amount of plasticizer in
the base composition can range from 0.1% to 5% by weight based on
the total weight of the base composition, and in certain
embodiments, can range from 0.5% to 3% by weight. The amount of
plasticizer in the curing agent composition can range from 20% to
60% by weight of the total weight of the curing agent composition,
and in certain embodiments, can range from 30% to 40% by
weight.
[0039] In certain embodiments, preformed compositions further
comprise an organic solvent, such as a ketone or an alcohol, for
example methyl ethyl ketone, and isopropyl alcohol, or a
combination thereof.
[0040] In certain embodiments, preformed compositions of the
present disclosure comprise adhesion promoters such as, for
example, phenolic resin, silane adhesion promoter, and combinations
thereof. Adhesion promoters can facilitate adhesion of the
polymeric components of the preformed sealant composition to a
substrate, as well as to the electrically non-conductive and
electrically conductive fillers in the sealant composition. In
certain embodiments, a conductive base composition can comprise
form 0.15% to 1.5% by weight of a phenolic adhesion promoter, from
0.05% to 0.2% by weight of a mercapto-silane adhesion promoter and
from 0.05% to 0.2% by weight of an epoxy-silane adhesion promoter.
The total amount of adhesion promoter in the base composition can
range from 0.5% to 7% by weight, based on the total weight of the
base composition.
[0041] In certain embodiments, a base composition can be prepared
by batch mixing at least one sulfur-containing polymer, additives,
and/or fillers in a double planetary mixer under vacuum.
[0042] Other suitable mixing equipment includes a kneader extruder,
sigma mixer, or double "A" arm mixer. For example, a base
composition can be prepared by mixing at least one
sulfur-containing polymer, plasticizer, and phenolic adhesion
promoter. After the mixture is thoroughly blended, additional
constituents can be separately added and mixed using a high shear
grinding blade, such as a Cowless blade, until cut it. Examples of
additional constituents that can be added to a base composition
include corrosion inhibitors, non-conductive fillers, electrically
conductive fiber, electrically conductive flake, and silane
adhesion promoters. The mixture can then be mixed for an additional
15 to 20 minutes under a vacuum of 27 inches of mercury or greater
to reduce or remove entrapped air and/or gases. The base
composition can then extruded from the mixer using a high-pressure
piston ram.
[0043] A curing agent composition can be prepared by batch mixing a
curing agent, additives, and fillers. In certain embodiments, 75%
of the total plasticizer such as partially hydrogenated terphenyl
and an accelerant such as a dipentamethylene/thiuram/polysulfide
mixture are mixed in a single-shaft anchor mixer. Molecular sieve
powder is then added and mixed for 2 to 3 minutes. Fifty percent of
the total manganese dioxide is then mixed until cut in. Stearic
acid, sodium stearate, and the remaining plasticizer are then mixed
until cut in followed by the remaining 50% of the manganese dioxide
which is mixed until cut in. Fumed silica is then mixed until cut
in. If the mixture is too thick a surfactant may be added to
increase wetting. The curing agent composition is then mixed for 2
to 3 minutes, passed over a three-roll paint mill to achieve a
grind, and returned to the single-shaft anchor mixer and mixed for
an additional 5 to 10 minutes. The curing agent composition can
then be removed from the mixer with a piston ram and placed into
storage containers and aged for at least 5 days prior to combining
with a base composition.
[0044] A base composition and a curing agent composition are mixed
together to form a preformed sealant composition. A base
composition and a curing agent composition can be combined in the
desired ratio using meter mix equipment fitted with a dynamic mix
head. Pressure from the meter mix equipment forces the base and
curing agent compositions through the dynamic mix head and an
extrusion die. In certain embodiments, a preformed composition is
extruded into a laminar form such as a tape or sheet. A preformed
composition in sheet form can be cut to any desired shape such as
defined by the dimensions of an aperture to be sealed. In certain
embodiments, the shaped form can be coiled, with release paper
separating each ring for packaging purposes. The shaped form can be
refrigerated by placing the shaped form on a bed of dry ice and
placing another layer of dry ice over the shaped form. The shaped
form can be refrigerated immediately after mixing the base
composition and the curing agent composition. The shaped form can
remain exposed to the dry ice for 5 to 15 minutes and then placed
at a storage temperature of -40.degree. C. or lower. The term
"refrigerated" refers to reducing the temperature of the preformed
composition so as to retard and/or stop the curing of the preformed
composition. In certain embodiments, the preformed composition in
shaped form is refrigerated below -40.degree. C.
[0045] For sealing an aperture, the temperature of the preformed
composition is raised to a use temperature ranging from 4.degree.
C. to 32.degree. C. (40.degree. F. to 90.degree. F.) prior to
application to one or more surfaces associated with the aperture.
This is done such that the preformed composition reaches use
temperature for no more than 10 minutes prior to application.
[0046] In certain embodiments, the preformed composition in shaped
form can be used to seal an aperture between a removable access
panel and a surface adjacent to the perimeter of an opening in an
aircraft fuselage. Adhesion promoter is first brushed on the
perimeter of the access panel opening after the surface has been
cleaned with a cleaning solvent such as DESOCLEAN (PRC-DeSoto
International, Inc.). The surface of the access panel is then
cleaned and coated with a release agent prior to applying the
preformed composition. The preformed composition in shaped form is
manually applied to the surface adjacent to the perimeter of the
access panel opening, to the surface adjacent to the perimeter of
the access panel, or to both. The access panel is then positioned
against the surface adjacent to the opening and clamped down to
force the excess preformed composition around the edges of the
access panel. Excess preformed composition is easily removed by
using, for example, a flat surface. Excess preformed composition
can be removed either prior to curing or after the preformed
composition has cured, and preferably after the preformed
composition cures.
[0047] The integrity, moisture resistance, and fuel resistance of
the seal resulting from application of preformed compositions of
the present disclosure can be evaluated by performing the tests
identified in specification MMS 327. An acceptable seal will be
tight and resistant to moisture and aircraft fuel.
[0048] It is noted that, as used in this specification and the
appended claims, the singular forms "a," "an," and, "the" include
plural referents unless expressly and unequivocally limited to one
referent. Thus, for example, reference to "a filler" includes one
or more fillers. Also it is noted that, as used herein, the term
"polymer" refers to polymers, oligomers, homopolymers, and
copolymers.
[0049] For the purposes of this specification and appended claims,
unless otherwise indicated, all numbers expressing quantities of
ingredients or percentages or proportions of other materials,
reaction conditions, and so forth used in the specification and
claims are to be understood as being modified in all instances by
the term "about." Accordingly, unless indicated to the contrary,
the numerical parameters set forth in the following specification
and attached claims are approximations that may vary depending upon
the desired properties sought to be obtained by the present
disclosure.
[0050] Embodiments of the present disclosure can be further defined
by reference to the following examples, which describe in detail
the preparation of compositions of the present disclosure and
methods for using compositions of the present disclosure. It will
be apparent to those skilled in the art that modifications, both to
materials and methods, may be practiced without departing from the
scope of the present disclosure.
EXAMPLE 1
[0051] Example 1 provides an electrically conductive preformed
composition in shaped form exhibiting EMI/RFI shielding
effectiveness. The following materials were mixed in the
proportions according to Table I to provide an electrically
conductive base composition: PERMAPOL P 3.1 polythioether polymer
from PRC-DeSoto International, Inc., THIOPLAST G4 polysulfide
polymer from Akcros Chemicals (New Brunswick, N.J.), phenolic resin
adhesion promoter from PRC-DeSoto International, Inc, and HB-40
modified polyphenyl plasticizer from Solutia, Inc. (St. Louis,
Mo.). Using a high shear grinding blade (Cowless blade), the
following materials were individually added and blended until cut
in: calcium chromate corrosion inhibitor (Wayne Pigment Corp.,
Milwaukee, Wis.), hydrophobic fumed silica (R202, from
Aerosil/Degussa, Diamond Bar Calif.), Ni fiber (30 .mu.m diameter,
500 .mu.m length; from Intramicron, Birmingham, Ala.), Ni-coated
graphite (I) (60% Ni-coated graphite; from Novamet, Wyckoff, N.J.),
Ni-coated graphite (II) (60% Ni-coated graphite; from Sulzer
Metco/Ambeon, Switzerland), mercapto silane adhesion promoter
(Silane A189; GE Specialty Materials, Wilton, Conn.), and epoxy
silane adhesion promoter (Silane A187; GE Specialty Materials,
Wilton, Conn.).
1TABLE I Electrically Conductive Base Composition Material Weight
Percentage PERMAPOL P 3.1 Polythioether Polymer 11.92 THIOPLAST G4
Polysulfide Polymer 12.04 Phenolic Resin 0.63 HB-40 Plasticizer
1.14 Calcium Chromate 3.69 Silica 5.23 Ni Fiber 6.98 Ni-coated
Graphite (I) 29.08 Ni-coated Graphite (II) 29.08 Silane Adhesion
Promoter (mercapto) 0.10 Silane Adhesion Promoter (epoxy) 0.10
[0052] Separately, the following materials were mixed in the
amounts according to Table II to form a curing agent composition:
manganese dioxide from EaglePicher (Phoenix, Ariz.), partially
hydrogenated terphenyl, stearic acid, fumed silica, sodium stearate
from Witco Chemicals, molecular sieve powder to remove excess
moisture from the curing agent, and
dipentamethylene/thiuram/polysulfide mixture from Akrochem
Corporation (Akron, Ohio) to accelerate the cure. The curing agent
composition was allowed to set or age for at least 5 days before
combining with the base composition.
2TABLE II Curing Agent Composition Material Weight Percent
Manganese Dioxide 54.59 Partially Hydrogenated Terphenyl 35.92
Stearic Acid 0.60 Fumed Silica 2.00 Sodium Stearate 0.73 Molecular
Sieve Powder 0.70 Dipentamethylene/Thiuram/Polysulfide Mixture
5.46
[0053] One hundred parts by weight of the electrically conductive
base composition according to Table I, and 10 parts by weight of
the curing agent composition of Table II were combined to prepare
the electrically conductive preformed composition. After thorough
mixing and degassing, the electrically conductive preformed
composition thus formed was extruded into a tape form and
refrigerated at -40.degree. C.
[0054] The surface adjacent to the perimeter of an aircraft access
panel was first coated with low VOC epoxy primer according to
specification MMS-423 and cured. The surface was cleaned and then
coated with adhesion promoters PR-148 or PR-184 from PRC-DeSoto
International, Inc. The access panel was made from titanium alloy
conforming to AMS-T-9046. After the refrigerated electrically
conductive preformed composition equilibrated to use temperature,
4.degree. C. to 32.degree. C. (40.degree. F. to 90.degree. F.), the
electrically conductive preformed composition in tape form was
manually applied to the surface adjacent to the perimeter of the
access panel. The access panel was put in place to cover the access
opening and clamped down, forcing the excess electrically
conductive preformed composition around the edges of the access
panel to fill the aperture. Excess electrically conductive
preformed composition was easily removed. After 3 to 4 hours at a
temperature of 4.degree. C. to 32.degree. C. (40.degree. F. to
90.degree. F.), a tight seal, resistant to moisture and aircraft
fuel, resulted.
[0055] The cured sealant exhibited a sheet resistance (four-point
probe) of less than 0.50 .OMEGA./. Seals to apertures between an
aluminum test fixture and a carbon/epoxy lid exhibited shielding
effectiveness from 1 MHz to 200 MHz when tested in an anechoic
chamber. Similarly sealed apertures also exhibited shielding
effectiveness from 0.1 GHz to 18 GHz when tested in a stirred mode
chamber.
[0056] Other embodiments of the present disclosure will be apparent
to those skilled in the art from consideration of the specification
and practice of the invention disclosed herein. It is intended that
the specification and examples be considered as exemplary only,
with the true scope and spirit of the present disclosure being
indicated by the following claims.
* * * * *