U.S. patent application number 09/970159 was filed with the patent office on 2002-06-20 for form-in-place emi gaskets.
Invention is credited to Bunyan, Michael H., Kalinoski, John P..
Application Number | 20020076547 09/970159 |
Document ID | / |
Family ID | 22384234 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020076547 |
Kind Code |
A1 |
Kalinoski, John P. ; et
al. |
June 20, 2002 |
Form-in-place EMI gaskets
Abstract
A form in place conductive gasket is disclosed. The gasket,
foamed, gelled or unfoamed is made of one or more elastomer resins,
such as silicone urethane and/or thermoplastic block copolymers and
is either filled with a conductive filler and lined onto a desired
substrate or lined onto the substrate unfilled and then coated with
a conductive utrelayer, such as a silver filled elastomer or a
conductive flocked layer. A process and system for making the
gaskets are also disclosed.
Inventors: |
Kalinoski, John P.;
(Chelmsford, MA) ; Bunyan, Michael H.; (Derry,
NH) |
Correspondence
Address: |
ROPES & GRAY
ONE INTERNATIONAL PLACE
BOSTON
MA
02110-2624
US
|
Family ID: |
22384234 |
Appl. No.: |
09/970159 |
Filed: |
October 2, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09970159 |
Oct 2, 2001 |
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09478017 |
Jan 5, 2000 |
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09478017 |
Jan 5, 2000 |
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08967986 |
Nov 12, 1997 |
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08967986 |
Nov 12, 1997 |
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08421847 |
Mar 14, 1995 |
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08421847 |
Mar 14, 1995 |
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08119403 |
Sep 10, 1993 |
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Current U.S.
Class: |
428/328 ;
428/408; 428/425.8; 428/450 |
Current CPC
Class: |
Y10T 428/30 20150115;
Y10T 428/31609 20150401; Y10T 428/31605 20150401; Y10T 428/31681
20150401; Y10T 428/31786 20150401; F16J 15/064 20130101; F16J 15/14
20130101; Y10T 428/256 20150115; H05K 9/0015 20130101 |
Class at
Publication: |
428/328 ;
428/425.8; 428/408; 428/450 |
International
Class: |
B32B 005/16 |
Claims
What we claim:
1.) An EMI shielded substrate comprising: a.) a first electrically
conductive substrate; b.) a second electrically conductive
substrate adjacent to the first substrate; c.) a formed in place
electrically conductive gasket formed on and bonded to a
predetermined portion of the first substrate so as to provide an
electrical connection and EMI shielding between the first and
second substrates.
2.) The structure of claim 1 wherein the first substrate in an
enclosure and the second substrate is a cover for the
enclosure.
3.) The structure of claim 1 wherein the first and second
substrates are formed of an electrically conductive material
selected from the group consisting of metals, metal plated plastic,
metal/plastic laminate and composites, coated plastisols and
combinations thereof.
4.) The substrate of claim 1 wherein the gasket is formed of an
erastomer resin and is rendered electrically conductive by the
incorporation of one or more conductive fillers into the resin.
5.) The substrate of claim 1 wherein the gasket is formed of an
inner and outer layer, at least the outer layer being electrically
conductive.
6.) The structure of claim 1 wherein the gasket is formed of an
elastomer and one or more electrically conductive fillers.
7.) The structure of claim 6 wherein the fillers are selected from
the group consisting of 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 copper,
nickel or aluminum, for example, silver plated aluminum core
particles; noble-metal plated glass, plastic or ceramics such as
silver plated glass microspheres, noble-metal plated alumina or
noble-metal plated plastic microspheres; noble-metal plated mica;
and other such noble-metal conductive fillers. Non-noble
metal-based materials are also suitable, including 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; and non-metal materials such as carbon black and
graphite and the elastomer is selected from the group consisting of
EPDM copolymers, silicone rubbers, fluorosilicone rubbers, urethane
rubbers, nitrile rubbers, butyl rubbers, elastomeric
thermoplastics, and mixtures thereof.
8.) The structure of claim 1 wherein the gasket is formed of
silicone gel containing one or more electrically conductive
fillers.
9.) The structure of claim 1 wherein the gasket has a Shore A
hardness of from about 5 to about 60, a force/deflection value of
from about 0.2 pounds/inch to 15.0 pounds/inch, electrical
resistance value of from about 0.005 ohms to 0.1 ohms, a
compression set value of from about 5% to 50% and an EMI shielding
effectivenss of from about 10 dbs to about 120 dbs at a frequency
range from about 10 MHz to about 10 GHz.
10.) An EMI shielded substrate comprising: a.) a substrate having
an electrically conductive surface; b.) a cover for the substrate,
the cover having an electrically conductive surface which
corresponds to and is in register with the conductive surface of
the substrate; and c.) a formed in place electrically conductive
gasket formed on and bonded to a predetermined portion of the
condudtive surface of the substrate or cover so as to provide an
electrical connection and EMI shielding between the substrate and
cover upon the mating of the cover to the substrate.
11.) The substrate of claim 10 wherein the substrate and cover are
formed of an electrically conductive material selected from the
group consisting of metals, metal plated plastic, metal/plastic
laminates and composites, electrically conductive coated plastics,
and combinations thereof.
12.) The substrate of claim 10 wherein the gasket is formed of an
elastomer resin and is rendered electrically conductive by the
incorporation of one or more conductive fillers into the resin.
13.) The substrate of claim 10 wherein the gasket is formed of an
inner and outer layer and wherein at least the outer layer being
electrically conductive.
14.) A form-in-place EMI gasket comprising a composition formed of
a silicone resin, one or more conductive fillers, a curing agent
for the resin wherein the composition, when mixed and applied to a
substrate, will create a form stable, form-in-place gasket capable
of providing EMI shielding of from about 20 dBs to about 80 dBs
over a frequency range from 10 MHz to 10 GHz.
15.) The gasket of claim 14 wherein the silicone resin is a
silicone gel.
16.) An EMI gasket comprising a composition formed of: a.) a first
component which is a primary polymer having end groups that are
capable of chemically reacting with each other in the presence of
moisture to form a derivative polymer having a longer average chain
length than said primary polymer; b.) a second component which is a
noncross-linked elastomer that is not substantially chemically
reactive with itself or with said first component; and c.) a third
component which is one or more electrically conductive fillers,
wherein when said first, said second, said third components are
intimately mixed, said composition, when maintained in the absence
of moisture and other active hydrogen donor materials, being
readily extrudable and otherwise conventionally moldable
thermoplastic composition but, upon exposure to moisture, becoming
essentially thermoset.
17.) The gasket of claim 16 wherein the first component has a
polyester backbone and isocyanate or trialkoxysilyl end groups, and
the second component is selected from the group consisting of a
styrene-isoprene-styrene block polymer, a styrene-butadiene-styrene
block polymer, an isoprene homopolymer, a polyvinyl chloride,
polyisobutylene, styrene-ethylene-butylene-styrene, or
ethylene-propylene rubber and the third component is selected from
the group consisting of noble metal fillers; noble metal-plated
noble metals; noble metal-plated non-noble metals; noble-metal
plated glass, plastic or ceramics noble-metal plated mica;
non-noble metals, non-noble metal-plated non-noble metals; and
non-metal materials such as carbon black and graphite and
combinations thereof.
18.) A system for forming EMI shielded enclosures comprising: a.) a
support platform; b.) a compound applicator nozzle located above
and in register with the platform; c.) a supply of electrically
conductive compound connected to the applicator nozzle; and d.) a
drive mechanism for moving the nozzle or platform relative to each
other in one or more directions of travel.
19.) The system of claim 18, further comprising a second drive
mechanism.
20.) The system of claim 18 further comprising a curing chamber for
curing of the compound after application.
21.) The system of claim 20 wherein the chamber is selected from
the group consisting of hot air ovens, infrared ovens, light
chambers, moisture chambers or combinations thereof.
22.) The system of claim 18 further comprising a computer for
controlling preprogrammed, respective movements of the nozzle
relative to the table so as to create a predetermined gasket
configuration.
23.) A process for forming a form-in-place conductive EMI shielding
gasket or a substrate comprising the steps of: a.) providing a
substrate to be gasketed; b.) providing a supply of conductive
gasket material; c.) applying the material to the substrate's
surface in a predetermined pattern; and d.) curing the material in
place upon the substrate.
24.) The process of claim 23 further comprising applying a primer
to the substrate before step (c).
25.) The process of claim 23 wherein the gasketing material is
formed of a conductive filled resin, the resin being selected from
the group consisting of silicone, urethanes, thermoplastic
elastomers and mixtures thereof; the fillers being selected from
the group consisting of noble metal fillers; noble black and
graphite and combinations thereof; and the substrate is an
electrically conductive material selected from the group consisting
of metals, metal composites, metal coated plastics and metal
laminates.
26.) The process of claim 23 wherein the curing occurs via a curing
agent, a cross linking agent, heat, light, moisture or combinations
thereof.
Description
BACKGROUND OF THE INVENTION
[0001] EMI shielding gaskets are used on electronic equipment to
provide protection against interference from electromagnetic
energy, including radio frequency interference (RFI) and more
broadly all bands of interference commonly called electromagnetic
interference (EMI). The shielding has an electrically conductive
element, be it a wire mesh, conductive filler or conductive
plating, coating or fabric which prevents external EMI from
interfering with an electronic device and/or protects other
adjacent electronic devices from EMI emitted by an electronic
device.
[0002] Typically, EMI gaskets are prepared in one of three
configurations: linear, die cut or compression molded. By linear,
it is meant as an extrusion, molding, etc. of a defined, straight
length. By die cut, it is meant that a gasket configuration is
formed from a conductive sheet material which is cut by a die to
the desired shape, such as round, square, etc. By compression
molded, it is meant that the gasket configuration is formed by
placing uncured elastomer which may contain conductive filler or a
conductive mesh, into a specifically designed mold which is then
subjected to compression (pressure) and then cured to cause the
elastomer to assume the desired gasket configuration.
[0003] All three methods have disadvantages especially when used to
form complex multidirectional or multiaxial gaskets, such as may
occur in devices with a number of compartments that each need to be
shielded, from each other as well as the external environment.
Moreover, the problems are even more critical on smaller devices,
such as cellular phones, notebook computers and other hand held
devices, where the diameter of the gasket becomes very small and
the ability to manufacture and attach such gaskets securely becomes
very difficult and labor intensive.
[0004] Using linear gasketing material to form complex
multiaxis/multidirectional gaskets (e.g. either x and y or in the
x, y and z planes), is difficult, time consuming and costly. Each
gasket portion must be hand cut and bonded to the adjacent portions
of other linear gaskets and then bonded or secured in position upon
the substrate.
[0005] Die cutting of conductive sheet stock will work in many
instances especially in two plane (e.g. flat; x,y) applications,
provided that each portion of the gasket is wide enough and/or
thick enough to be self supportive. Die cutting parts however
results in significant waste of the sheet stock because the
material is typically a crosslinked resin such as silicone or
polyurethane. This is not acceptable as it drives up the cost of
such parts unacceptably. Further as diecutting is a rough process,
the sheet stock needs to be fairly stiff and self supportive which
is opposite that desired by the gasket user (i.e. soft and
flexible).
[0006] Compression molding is slow and again generates scrap in the
form of flash which must be removed. Further, each gasket design
must use a specifically designed mold, making the process expensive
for all but large volume stock items.
[0007] A form in place EMI gasket and system for forming complex
multiaxis/multidirectional EMI gaskets which generates a minimum of
scrap, which forms the gasket in place and requires no special
tooling is desired. The present invention provides such a
system.
OBJECTS AND SUMMARY OF THE INVENTION
[0008] The present invention relates to a form-in-place EMI gasket
and a system for forming such a gasket using a table and/or
dispenser that are capable of moving in multiaxial directions
relative to each other and the substrate to be gasketed. The
invention also relates to a process of providing a conductive
elastomer, forming it in place along a desired gasket configuration
so as to create form-in-place EMI gasket.
[0009] An object of the present invention is an EMI shielded
substrate comprising:
[0010] a.) a first electrically conductive substrate;
[0011] b.) a second electrically conductive substrate adjacent to
the first substrate;
[0012] c.) a formed in place electrically conductive gasket formed
on and bonded to a predetermined portion of the first substrate so
as to provide an electrical connection and EMI shielding between
the first and second substrates.
[0013] A further object of the present invention is an EMI shielded
substrate comprising:
[0014] a.) a substrate having an electrically conductive
surface;
[0015] b.) a cover for the substrate, the cover having an
electrically conductive surface which corresponds to and is in
register with the conductive surface of the substrate; and
[0016] c.) a formed in place electrically conductive gasket formed
on and bonded to a predetermined portion of the conductive surface
of the substrate or cover so as to provide an electrical connection
and EMI shielding between the substrate and cover upon the mating
of the cover to the substrate.
[0017] Another object of the present invention is to
[0018] (CLAIM 16 .dwnarw.)
[0019] An EMI gasket comprising a composition formed of:
[0020] a.) a first component which is a primary polymer having end
groups that are capable of chemically reacting with each other in
the presence of moisture to form a derivative polymer having a
longer average chain length than said primary polymer;
[0021] b.) a second component which is a noncross-linked elastomer
that is not substantially chemically reactive with itself or with
said first component; and
[0022] c.) a third component which is one or more electrically
conductive fillers, wherein when said first, said second, said
third components are intimately mixed, said composition, when
maintained in the absence of moisture and other active hydrogen
donor materials, being readily extrudable and otherwise
conventionally moldable thermoplastic composition but, upon
exposure to moisture, becoming essentially thermoset.
[0023] These and other objects of the invention will become clear
from the following description and claims.
DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 illustrates in planar view a preferred configuration
of a form-in-place conductive EMI gasket, having a complex physical
structure comprising a series of elongated sections in the x, y
axis, on a desired substrate.
[0025] FIG. 2 shows in crosssection a preferred embodiment of the
gasket of FIG. 1.
[0026] FIG. 3 shows in planar view a preferred system for
forming-in-place an EMI gasket, according to the present
invention.
[0027] FIG. 4 shows another embodiment of a system of the present
invention in crosssection.
[0028] FIG. 5 shows a crosssection of another preferred embodiment
of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The present invention relates to a form-in-place EMI
gasket.
[0030] Such gasekts are useful when positioned between two adjacent
substrates such as a boxed cover, so as to form an electrical
bridge or continuity between the two substrates and thereby prevent
or reduce the potential for EMI.
[0031] FIG. 1 shows a preferred embodiment of the present
invention. The form in place conductive gasket 1 is mounted on a
preselected section or sections of a substrate 2 which is to be
shielded. In FIG. 1, the substrate 2 is a modular enclosure formed
of two compartments 3, 4 separated by a wall 5. Such an enclosure
can be a cellular phone carcass, a switching box, hard disk drive
case, etc. As can be seen, the gasket 1 is formed in place along
the edges of the enclosure which will mate with a cover (not
shown).
[0032] FIG. 2 shows the conductive form in place gasket 11 in
crosssection as mounted to the enclosure 12. In this embodiment,
the area of the enclosure to be gasketed 13 has a shoulder 14
against which the gasket is formed. Other embodiments may not have
the shoulder 14, being flat instead, or may use other locating
devices such as undercuts, dovetails, channels, etc. into which or
against which the gasket may be formed.
[0033] The form in place gasket overcomes many of the problems of
the prior approaches. It eliminates the necessity to form the
gasket and then apply it in a separate step as occurs with the
linear or die cut gaskets. It reduces waste as occurs with die cut
or compression molded gaskets by being a flashless or scrapless
process. It is less labor intensive than linear or die cut gaskets
as there is no hand assembly of complex gasket shapes or mounting
of the gaskets into place. Further, there is no need for the
manufacture of specialized dies or molds which are useful for only
one gasket configuration. Instead, the gasket can be applied to any
substrate, in any configuration, easily and in a cost effective
manner with a minimal investment for tooling. Moreover, with the
use of preprogrammable application equipment, one can store an
infinite number of gasket configurations which can be called up and
used quickly and repeatedly without the necessity to manufacture a
specific die or a mold.
[0034] Lastly, it allows for the exact placement of very small
diameter gaskets (e.g. 0.010 inch diameter or less) which i-s
difficult to achieve will all but compression molding.
[0035] The word "elastomeric" should be given its usual meaning
given the purpose for which the invention is intended. The
elastomer bases used in the invention can be-thermosetting resins;
e.g. resins which to cross-link and subsequently cure either at a
critical temperature or in the presence of a curing agent/catalyst
such as peroxide, photoinitiator, moisture, etc. Any flexible
thermosetting elastomer base is suitable for use in the invention,
such as EPDM copolymers, silicone rubbers, fluorosilicone rubbers,
urethane rubbers, nitrile rubbers, butyl rubbers, and mixtures
thereof.
[0036] Elastomeric thermoplastics may also be used. Thermoplastic
rubbers, such as various block copolymers (KRATON.RTM. rubber,
NORPRENE.RTM. resin or SANTOPRENE.RTM. resin) are particularly
useful. Omission of other elastomers is not meant to specifically
exclude their use in the invention. Certain physical or electrical
requirements of the application for which the gaskets are intended
may dictate that particular elastomeric compositions be used.
[0037] The-selected material should be sufficiently viscous and/or
form stable so that it does not slump, sag or run between the time
of application and the time of curing. It may be in the form of a
paste, a caulk, a gel or viscous fluid. Alternatively, when the
material has a fast curing cycle or creates an initially stable
material such as a gel or a skinned or foam structure upon
application, the material as applied can be a relatively thin or
non-viscous fluid.
[0038] In addition, the selected resin should form a soft,
resilient, compression set resistant gasket even with the addition
of relatively highloadings of conductive fillers, if used.
[0039] Preferred elastomers that meet the requirements above
include silicone rubbers, whether foamed or unfoamed;
silicone-gels, whether foamed or unfoamed, typically such gels are
relative soft silicone rubber which may have been extended with
oils or plasticizers or which are only lightly crosslinked;
polyurethanes, especially the prepolymer type of urethane in which
the prepolymer is capped or terminated with an isoacyanate group
which when exposed to an activating agent (typically a hydroxy
containing group), such as water, amines or alcohols cause the
prepolymer to a crosslink; elastomeric thermoplastic rubbers such
as DYNAFOAM.RTM. and NORPRENE.RTM. from Norton Co.; SANTOPRENE.RTM.
resin from Monsanto, and KRATON.RTM. rubber from Shell Oil. These
thermoplastics generally comprise at least a block copolymer, such
as SBS or SIS rubber with or without other polymers (polyethylene,
polystyrene, etc.) and/or oils or plasticizers. Additionally,
various blends of the elastomers can be used as well.
[0040] Such polymers are generally known and widely available, see
e.g. U.S. Pat. Nos. 4,931,479, 4,643,924 and EP Patent Application
0326704A.
[0041] An EMI gasket can be formed of a composition as taught by EP
Patent Application 0326704A by using a two component polymer
system, one which is thermoplastic in the nature, the other being
thermoset when exposed to moisture or active hydroxyl groups.
[0042] a first component which is a primary polymer having end
groups that are capable of chemically reacting with each other in
the presence of moisture to form a derivative polymer having a
longer average chain length than said primary polymer, such as an
isocyanide capped polyester prepolymer;
[0043] a second component which is noncross-linked elastomer that
is not substantially chemically reactive with itself or with said
first component in the presence of moisture, such as a block
copolymer e.g. styrene-butadiene-styrene block copolymers, and
[0044] a third component which is one or more electrically
conductive fillers wherein the first, the second, and the third
components are intimately mixed, and the composition, when
maintained in the absence of moisture and other active hydrogen
donor materials, form a readily extrudable and otherwise
conventionally moldable or coatable thermoplastic composition but,
upon exposure to moisture, becoming essentially thermoset.
[0045] A preferred silicone gel is known as SYLGARD J27 (Parts A
and B) available from Dow Corning Corporation. It can be mixed with
one or more conductive fillers to form an EMI material.
[0046] The gasket may be rendered electrically conductive either
through the use of a conductive filler incorporated into the
elastomer base and/or the use of an electrically conductive outer
layer formed over a core which may be conductive or
nonconductive.
[0047] The fillers that are used to impregnate elastomers to make
them electrically conductive are well-known in the art. Examples of
these fillers include but are not limited to 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 copper, nickel
or aluminum, for example, silver plated aluminum core particles or
platnium plated copper particles; noble-metal plated glass, plastic
or ceramics such as silver plated glass microspheres, noble-metal
plated alumina or noble-metal plated plastic microspheres;
noble-metal plated mica; and other such noble-metal conductive
fillers. Non-noble metal-based materials are also suitable,.
including 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; and non-metal
materials such as carbon black and graphite combinations of the
fillers to meet the desired conductivity, hardness and other
parameters desired for a particular application.
[0048] The shape and size of the electrically conductive fillers is
not critical to the present invention. The fillers may be of any
shape that is generally used in the manufacture of conductive
materials, including spherical, flake, platelet, irregular or
fibrous (such as chopped fibers). In making gaskets in accordance
with the invention it is preferred that the particle shape be
spherical, substantially spherical or irregular. Flake or platelet
shaped fillers are preferred when used in forming an outer
conductive coating for the foam-in-place gasket.
[0049] The particle size of the electrically conductive fillers can
be within the range normally used for fillers in conductive
materials. Generally the particle size of the one or more fillers
is from about 0.250 .mu. to about 2501.mu., preferably from about
0.250 .mu. to about 75 .mu., and most preferably from about 0.2501
.mu. to about 60 .mu..
[0050] The amount, or loading, of the one or more electrically
conductive fillers in the conductive elastomeric material used in
the present invention can vary over a wide range, as long as the
electrically conductive filler is present in an amount sufficient
to provide EMI/RFI shielding properties. Generally loading of the
filler particles in the conductive elastomeric material is from
about 10 to about 80 volume percent, preferably from about 20 to
about 66 volume percent.
[0051] When a conductive outer layer is used to provide the
conductivity to the gasket, it may be in the form of a plating, a
coating or a film. Plated layers, such as silver plating, are not
preferred as the platings tend to be rigid and crack under
compression. Films, such as a conductive filled polyethylene or
polyimide, may be used.
[0052] Preferably, the outer conductive layer is some form of
conductive coating. More preferably it is a conductively filled
resilient coating. Such coatings can and preferably are based upon
the same elastomer resin that is used to form the inner layer.
Preferred coatings include silicone, polyurethane, acrylic and
epoxy resins filled with one or more of the conductive fillers in
the same size range and amounts as described above.
[0053] Other fillers and ingredients may also be added to the
elastomer base if desired. Such fillers include microwave absorbing
materials, thermally conductive fillers, inert or reinforcement
fillers such as silicas and pigmentation fillers. Additionally,
curing agents, cross linking agents, flame retardants, diluents,
solvents or dispersion aids, etc., may be added as is well known in
the art to form the desired conductive elastomeric material. In
addition the elastomers may additionally comprise other compounds,
such as plasticizers, extender oils, softeners, tackifiers
catalysts, blowing agents or other agents that impart desired
properties to the cured gasket.
[0054] Typically, the gasket should have a SHORE A hardness (as
measured by ASTM standards) of between 5 and 60, preferably 5 and
50 and more preferably 5 and 40. The properties of the gasket will
vary depending upon the resin chosen, whether it is foamed or not,
the amount of filler contained therein and the other constituents
(oils, plasticizers, reinforcing fillers etc.) that may be
added.
[0055] A typical form in place gasket should have a low closure
force, e.g a force of less than about 5 pounds/linear inch
preferably less than 3 pounds, and more preferably less than 1
pound to deflect the gasket sufficently to ensure proper electrical
continuity between the two adjacent substrates.
[0056] The gasket should be capable of being dispersed by automated
equipment (if so desired) in diameters ranging from about 0.010 to
0.25 inch wide and aspect ratios as from about 0.25 to 1 to about 3
to 1.
[0057] EMI Shielding effectiveness should be at least 10 dBs,
preferably at least 20 dBs over a range of frequencies from about
10 MHz to 10 GHz. More preferably, it should provide an EMI
Shielding effectiveness of from about 20 dBs to 80 dBs over a
frequency range of from about 10 MHz to 10 GHz. Shielding
effectivenss will of course vary with the amount of conductive
material present, the deflection imposed upon the gasket and the
test method used. All values above assume a typical loading of
conductive materials as referenced above, with at least 10%
deflection, preferably 10 to 50% deflection and standard MIL spec.
test procedures.
[0058] The process of applying such form-in-place conductive
elastomers preferably includes the use of automated equipment such
as robotic applicators, such as x-y, x-y-z and other such multiaxis
or rotational type of applicators; hand applicators such as
caulking guns; transfer applicators and other such processes.
[0059] Preferably, the process relates to the formation of an
elastomer which is capable of being formed in place, applying the
elastomer to a substrate along a predetermined pathway and curing
the elastomer in place.
[0060] If desired or required due to the elastomer resin selected
and/or its adhesion to a certain substrate, a bonding agent or
primer may be used.
[0061] For example, some silicone compositions are known to have
poor Adhesion properties, especially to metal substrates. A primer,
such as a silane, silicate ester, a cyanurate. or a silicone based
adhesive may be used to cause the silicone composition to adhere to
the metal substrate.
[0062] One preferred process is to use a stationary support or
table to which the substrate to be gasketed is fixed in place. A
movable applicator, such as a programmable x-y or x-y-z nozzle,
which is connected to a supply of form in place elastomer, is
positioned adjacent and preferably above the substrate and then
caused to travel along a predetermined path, applying the elastomer
to the portion of the substrate over which it travels in a desired
amount. The elastomer is then cured.
[0063] Alternatively, the nozzle may be stationary and the table
may be caused to move in two (x-y), three (x-y-z) or more planes of
movement.
[0064] In a further embodiment, both the nozzle and the table may
move in one or more planes relative to each other. One example is
where the nozzle moves in two planes (x-y) and is rotational as
well and the table is capable of vertical (z) movement.
[0065] Other variations and embodiments can be used as well.
[0066] A typical system for performing the process is shown in FIG.
3 wherein a support platform or table 21 has a substrate to be
gasketed 22 mounted upon it. An applicator, such as the lining
dispensor 23 is located over the platform 21. The dispenser is
connected to a supply (not shown) of form in place elastomer via a
conduit 24.
[0067] The dispensor 23 is capable of moving in at least two planes
of motion relative to the platform, such as in the x and y axes.
Preferably, it is capable of moving in three planes of motion (x,
y, z) and may also be rotational soaks to accommodate changes in
the height or the angle of the substrate 21 over which it passes
and applies the elastomer to form a gasket 25 at a desired
position.
[0068] FIG. 4 shows another typical system in which the dispensor
26 and the table 27 move relative to each other. Also in this
instance, the nozzle has two supply lines 28, 29 which allows for
the use of two component systems such as urethanes or the
introduction of a gaseous component (air, CO.sub.2, nitrogen) into
the elastomer just before application so as to form a foamed
structure. The table 27 to which the substrate 30 is mounted is
moved in one or more directions (x, y and/or z) by a drive
mechanism as represented by box 31. The nozzle is moved via a
similar drive mechanism 32.
[0069] One method of forming the form in place gasket is to mix a
silicone rubber preferably in the form of a gel with conductive
filler in an amount sufficient to provide EMI shielding. The
mixture is then mixed with additional silicone rubber and/or a
curing agent or catalyst and then added to a syringe applicator
mounted on an x-y applicator arm. The material is then dispersed
along a peripheral edge of a substrate, such as a cellular phone
housing where it cures in place.
[0070] Another method is to form a nonconductive elastomer gasket
layer, such as by the process described above and then forming a
conductive outer layer over the nonconductive core via spraying,
coating, painting or dipping a conductive outer layer onto the
core. FIG. 5 shows such an embodiment. The gasket 40 is contained
in a channel 41 formed in a substrate 42. The inner layer 43 is
covered at least partially with a conductive outer layer 44. The
inner layer 43 is preferably nonconductive. However, if desired, it
could be conductive, containing e.g. carbon black as a filler.
[0071] Alternatively, the nonconductive core can be coated with an
adhesive layer which is then flocked with conductive fibers, as
taught by U.S. Pat. No. 5,115,104, which is incorporated herein in
its entirety.
[0072] The gasket may be cured by any of the mechanisms commonly
used for the selected polymer so long as the cure does not
adversely affect the slump properties of the gasket between
application and cure, and/or the physical or electrical properties
of the cured gasket.
[0073] Some elastomers such as prepolymer based polyurethanes are
basically selfcuring in that once the reaction between the
isocyanate group and hydroxy group begins, it typically continues
until one or both groups are expended.
[0074] Other elastomers, such as some silicone and thermoplastic
rubbers use chemical curing agents such as peroxide, sulfur, zinc
or amines and/or heat to crosslink and cure the resin.
[0075] Photocurable resins may also be used via the incorporation
of a light sensitive curing agent or photoinitiator which upon
exposure to a certain wave length of light (UV, etc.) causes the
resin to crosslink and cure.
[0076] Some resins use heat to cure. In this instance in order to
expedite the cure of the form in place gasket, one may warm the
substrate before, during or after application (especially if it is
metal) in order to hasten the cure and avoid problems of a cool
substrate coating as a heat sink and drawing away heat from the
resin. Alternatively, the resin can be heated, such as in the
nozzle just before application. The use of hot air or infrared
heaters adjacent the substrate may also be used.
[0077] Hot melt resins such as those based upon KRATON rubbers
typically need to cool in order to set. Affirmative cooling of the
substrate in this instance may be useful. Those hot melts which
contain a crosslinking agent may actually need to be kept at an
elevated temperature (albeit it below the melting point of the
resin) in order to crosslink.
[0078] The description above, and that which follows, is only meant
to describe one particularly advantageous embodiment of the present
invention and as such is only meant to illustrate, not limit
it.
EXAMPLE 1
[0079] A conductive particle filled solid silicone form-in-place
gasket was made of a two component silicone system, mixed in a
ratio of 1 part A to 1 part B.
[0080] Part A contained:
[0081] 22.4 parts of silicone resin
[0082] 77.6 parts of silver plated glass spheres, (avg.
[0083] size 30-50 microns)
[0084] Part B contained:
[0085] 22.3 of silicone resin
[0086] 0.4 of a catalyst for the resin
[0087] 77.3 of silver plated glass spheres
[0088] (30 to 50 microns average).
[0089] Part A and Part B were mixed separately by hand until each
was homogenous. Then equal parts were added and mixed by hand until
homogenous.
[0090] The mixed material was added to a 10 cc syringe with a
needle tip of 0.033 diameter. The syringe was mounted onto a
dispensing head of a CAM-A-LOT Model 1818 x-y
Positioning/Dispensing System. The material was forced out of the
syringe via air pressure of about 90 psi in programmed patterns
onto an aluminum flange (3 inches diameter, 0.25 inch thickness)
which was mounted on a stationary table. The sample was cured in a
hot air circulating oven for 60 minutes at 100.degree. C.
[0091] The flange was cooled and placed in an Instron machine in a
modified ASTM D-575 Compression Testing fixture. Electrodes were
placed into the flange opposite facing aluminum surface and the
sample was compressed at a rate of 0.005 inch/minute to a total
compression of 50% of the original gasket height. During
compression, stress, strain and resistivity values were
recorded.
[0092] Following the compression testing, the flange was removed
and bolted to a second flange (with no gasket) until the gasket was
compressed to 50% its original height. The assembly was heated in a
hot air circulating oven for 22 hours at 85.degree. C. The sample
was removed, disassembled and allowed to cool and recover for 30
minutes. The gasket height was remeasured and compression set was
calculated as:
% set=Original height-Final height Original height-Deflected
height
[0093] The results of those tests are shown in Table 1.
EXAMPLE 2
[0094] A conductive particle filled foamed silicone gasket was
prepared and tested in the manner set forth in Example 1.
[0095] The Components of Example 2 were:
[0096] Part A
[0097] 21.6 Silicone RTV Foam
[0098] 75.7 Silver Powder (325 mesh)
[0099] 2.7 Toluene
[0100] Part B
[0101] 21.4-Silicone RTV Foam
[0102] 74.9-Silver Powder
[0103] 1.1-Catalyst
[0104] 2.6-Tolune
[0105] The results are shown in Table 1.
EXAXPLE 3
[0106] A conductive coating over a nonconductive underlayer
form-in-place gasket was prepared as follows:
[0107] The underlayer was prepared and applied as taught in Example
1. The underlayer or core comprised:
1 TABLE 1 Electrical Resistance (ohms) at stated Compression
FORCE/DEFLECTION (lb/inch) Force/Deflection (lb/inch) Set EXAMPLE
10% 20% 30% 40% 10% 20% 30% 40% (%) #1 .5 1.5 2.7 4.3 .060 .020
.011 .007 13 #2 .7 2.2 4.0 6.2 .050 .010 .007 .002 50 #3 .3 1.3 2.5
4.0 .063 .024 .018 .016 41 #4 2.4 4.9 8.4 13.7 .037 .016 .010 .007
30
[0108] Part A
[0109] 84.2-Silicone RTV Foam
[0110] 10.5-Cab-o-Sil (silica)
[0111] 5.3-Toluene
[0112] Part B
[0113] 80.8-Silicone RTV Foam
[0114] 4.0-Catalyst (Sil gard 184B)
[0115] 10.1-Cab-o-Sil (silica)
[0116] 5.1-Toluene
[0117] After formation and cure, a conductive coating formed of
silicone RTV, catalyst, solvent and silver coated glass conductive
filler was applied by brush to the outer surfaces of the
underlayer.
[0118] The coating was comprised of two parts:
[0119] Part A
[0120] 11.5-RTV Silicone
[0121] 4.71-Silver Powder
[0122] 11.8-Silver Flake
[0123] 29.6-Toluene
[0124] Part B
[0125] 100 RTV Silicone
[0126] The Parts were mixed in a ratio of 100 Part A to 1.21 Part B
(by weight).
[0127] The results are shown in Table 1.
EXAMPLE 4
[0128] A stock, linear piece of conductive gasket, known as
CHO-SEAL 1350 gasket, available from Chomerics, Inc. and formed of
a conductively filled silicone rod, 0.060 inch in diameter was
tested for compression and resistivity values. The results are
shown in Table 1.
EXAMPLE 5
[0129] A conductive flocked form-in-place was formed of a urethane
under layer applied and cured as described in Example 1. The outer
layer of the urethane under layer was coated with a flocking
adhesive which in turn was covered by a silver plated nylon flock
as taught by U.S. Pat. No. 5,115,104 which is incorporated by
reference in its entirety. The flocked gasket was placed in a hot
air circulating oven for 10 minutes at 200.degree. F. to cure the
adhesive. The flocked gasket was found to provide EMI shielding
over a wide range of frequencies.
EXAMPLE 6
[0130] A urethane form-in-place conductive gasket was prepared,
assembled and tested according to the procedures of Example 1.
[0131] The gasket was formed of:
[0132] 100 grams-urethane prepolymer
[0133] 3 grams-activator
[0134] 360 grams-silver powder
[0135] 1.5 grams-silica (as a reinforcing filler)
[0136] The gasket was applied to and adhered to a substrate. The
gasket was found to provide adequate EMI shielding over a wide
range of frequencies.
* * * * *