U.S. patent application number 10/903462 was filed with the patent office on 2005-03-24 for electrically conductive pressure sensitive adhesives, method of manufacture, and use thereof.
Invention is credited to Bessette, Michael D., Paul, Sankar K..
Application Number | 20050062024 10/903462 |
Document ID | / |
Family ID | 34193169 |
Filed Date | 2005-03-24 |
United States Patent
Application |
20050062024 |
Kind Code |
A1 |
Bessette, Michael D. ; et
al. |
March 24, 2005 |
Electrically conductive pressure sensitive adhesives, method of
manufacture, and use thereof
Abstract
An adhesive, comprising a pressure-sensitive adhesive
composition; and carbon nanotubes in an amount effective to render
the pressure-sensitive adhesive composition electrically
conductive. Such adhesives are of particular use with gasketing
materials.
Inventors: |
Bessette, Michael D.; (Sun
Lakes, AZ) ; Paul, Sankar K.; (Branford, CT) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
|
Family ID: |
34193169 |
Appl. No.: |
10/903462 |
Filed: |
July 30, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60493181 |
Aug 6, 2003 |
|
|
|
Current U.S.
Class: |
252/500 |
Current CPC
Class: |
C09J 7/38 20180101; H01L
23/552 20130101; B82Y 30/00 20130101; C09J 9/02 20130101; C09J
2483/006 20130101; C09J 11/04 20130101; C09J 2301/314 20200801;
C09J 7/22 20180101; H01L 2924/0002 20130101; H01B 1/24 20130101;
H01L 2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
252/500 |
International
Class: |
D01F 009/12 |
Claims
What is claimed is:
1. An adhesive, comprising a pressure-sensitive adhesive
composition; and carbon nanotubes in an amount effective to render
the pressure-sensitive adhesive composition electrically
conductive.
2. The adhesive of claim 1, having a volume resistivity of about
10.sup.-3 ohm-cm to about 10.sup.8 ohm-cm.
3. The adhesive of claim 1, having a volume resistivity of less
than about 10 ohm-cm.
4. The adhesive of claim 1, having a volume resistivity of less
than about 1 ohm-cm.
5. An article comprising the adhesive composition of claim 1
disposed on a substrate.
6. The article of claim 5, wherein the substrate is electrically
conductive.
7. The article of claim 6, wherein the substrate has a volume
resistivity of about 10.sup.-3 ohm-cm to about 10.sup.8 ohm-cm.
8. The article of claim 7, wherein the substrate has a volume
resistivity of less than about 10 ohm-cm.
9. The article of claim 7, wherein the substrate has a volume
resistivity of less than about 1 ohm-cm.
10. A pressure-sensitive adhesive tape, comprising a backing layer
having a first side and a second side opposite said first side; and
a pressures-sensitive adhesive layer having a first side and a
second side opposite the first side, wherein the first side of the
pressure-sensitive adhesive layer is in contact with at least a
portion of the first side of the backing layer, the second side is
bondable under pressure to a substrate, and the pressure-sensitive
adhesive layer comprises a pressure-sensitive adhesive composition;
and carbon nanotubes in an amount effective to render the
pressure-sensitive adhesive composition electrically
conductive.
11. The tape of claim 10, wherein the backing layer is an
electrically conductive polymer.
12. The tape of claim 10, wherein the backing layer is an
electrically conductive silicone.
13. The tape of claim 10, wherein the backing layer has a volume
resistivity of about 10.sup.-3 ohm-cm to about 10.sup.8 ohm-cm.
14. The tape of claim 12, wherein the backing layer has a volume
resistivity of less than about 10 ohm-cm.
15. The tape of claim 12, wherein the backing layer has a volume
resistivity of less than about 1 ohm-cm.
16. An article, comprising a substrate having a surface; and the
tape of claim 10, wherein the second side of the pressure-sensitive
adhesive layer is bonded to at least a portion of the surface of
the substrate.
17. The article of claim 16, wherein the tape provides an EMI
shielding effectiveness of at least about 60 dB over a frequency
range of about 10 to about 10 GRz.
18. The article of claim 16, wherein the initial peel strength
between the backing and the surface of the substrate is about 1.5
to about 5.0 lb/in (0.26-0.87 N/m).
19. The article of claim 16, wherein the volume resistivity between
the backing and the surface of the substrate is about 10.sup.-3
ohm-cm to about 10.sup.8 ohm-cm.
20. A method of manufacture of an adhesive, comprising mixing a
pressure-sensitive adhesive formulation in a solvent or diluent
with carbon nanotubes; applying the mixture to a substrate or
backing layer; removing the solvent or diluent; and optionally
curing the adhesive.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of the filing of U.S.
Provisional Application No. 60/493,181, filed Aug. 6, 2003, which
is incorporated herein by reference in its entirety.
BACKGROUND
[0002] This invention relates to electrically conductive pressure
sensitive adhesives, methods of manufacture, and uses thereof.
[0003] Electrically conductive materials, particularly elastomers
and foams, are of utility in a wide variety of applications such as
electrical contacting devices, in sensors, and in applications
requiring EMI/RFI shielding and/or electrostatic dissipation.
Electrically conductive addition cure silicone compositions have
been described, for example, in U.S. Pat. No. 5,932,145 to Mitani
et al., U.S. Pat. No. 6,017,587 to Kleyer et al., European Patent
No. 0 839 870, and European Patent No. 0 971 367. Other
electrically conductive elastomers and foams are also known, for
example polyurethanes and polyolefins.
[0004] It is often desirable in the above applications to use an
electrically conductive adhesive, particularly a pressure-sensitive
adhesive (PSA), to adhere the elastomer or foam to a substrate.
Electrical conductivity is most often achieved in PSAs by adding
electrically conductive fillers such as particulate polyaniline
(see U.S. Pat. No. 5,645,764); particulate metals such as silver or
copper (see U.S. Pat. No. 3,475,213 and U.S. Pat. No. 4,258,100);
and carbonyl nickel powder (U.S. Pat. No. 3,762,946). Incorporation
of enough filler to produce the desired conductivity can be
difficult, however, especially without significantly adversely
affecting the properties of the adhesive such as tack, peel
strength, shear, and the like.
[0005] There accordingly remains a need in the art for compositions
and methods whereby pressure sensitive adhesives can be provided
with electrical conductivity without significant adverse effect on
one or more physical properties desired for a particular
application.
BRIEF SUMMARY
[0006] The above-described drawbacks and disadvantages are
alleviated by a pressure sensitive adhesive comprising a pressure
sensitive adhesive composition and carbon nanotubes in an amount
effective to render the pressure sensitive adhesive electrically
conductive. Use of carbon nanotubes allows the pressure sensitive
adhesive compositions to attain a volume resistivity of about
10.sup.-3 ohm-cm to about 10.sup.8 ohm-cm without significantly
adversely affecting the properties of the compositions such as peel
strength, tack, and shear.
[0007] Further disclosed is the above-described composition in the
form of a tape, and an article comprising the electrically
conductive pressure sensitive adhesive disposed on a substrate. The
adhesives, tapes, and articles are of particular utility in
applications such as EMI/RFI shielding, sensors, electrical
contacts, and the like. The above discussed and other features and
advantages of the present invention will be appreciated and
understood by those skilled in the art from the following detailed
description.
BRIEF DESCRIPTION OF THE FIGURE
[0008] The invention is illustrated by the following Figure, which
is a cross-sectional view of on embodiment of an adhesive tape of
the present invention.
DETAILED DESCRIPTION
[0009] The inventors hereof have unexpectedly found that carbon
nanotubes can be used to confer high electrical conductivity to
pressure sensitive adhesives without significantly affecting the
physical properties of the adhesives. The adhesives are
particularly suitable for use in adhering elastomers and foams that
can provide electromagnetic shielding and/or electrostatic
dissipation to articles. Uses include applications involving
complicated geometries and forms, such as in computers, personal
digital assistants, cell phones, medical diagnostics, and other
wireless digital devices, electronic goods such as cassette and
digital versatile disk players, as well as in automobiles, ships
and aircraft, and the like, where high strength to weight ratios
are desirable.
[0010] As used herein, the term "carbon nanotube" is inclusive of a
variety of very small carbon fibers having average diameters of
less than or equal to about 2000 nanometers (nm) and having
graphitic or partially graphitic structures. Suitable carbon
nanotubes include those wherein the outer surface of the graphitic
or carbon layers is derivatized, for example bonded to a plurality
of oxygen-containing groups such as carbonyl, carboxylic acid,
carboxylic acid ester, epoxy, vinyl ester, hydroxy, alkoxy,
isocyanate, or amide group, or derivatives thereof, for example,
sulfhydryl, amino, or imino groups.
[0011] Suitable carbon nanotubes for imparting electrical
conductivity to the pressure sensitive adhesive compositions have
diameters of about 0.5 to about 2000 nm, with aspect ratios greater
than or equal to about 5. Preferably, the carbon nanotubes have an
aspect ratio greater than or equal to about 10, more preferably
greater than or equal to about 100, and even more preferably
greater than or equal to about 1000. Carbon nanotubes as defined
herein include vapor grown carbon nanofibers (VGCF) and multi-wall
and single carbon nanotubes obtained from processes such as laser
ablation, carbon arc, chemical vapor deposition and other
processes.
[0012] The VGCF have diameters of about 3.5 to about 2000 nm and
are generally produced by chemical vapor deposition. Within this
range, the VGCF generally have diameters of greater than or equal
to about 3, preferably greater than or equal to about 4.5, and more
preferably greater than or equal to about 5 nm. Also desirable
within this range are diameters of less than or equal to about
1000, preferably less than or equal to about 500, and more
preferably less than or equal to about 100, and even more
preferably less than or equal to about 50 nm. The VGCF may be
hollow or solid and may have outer surfaces comprising amorphous or
graphitic carbon. Solid VGCF are often referred to as carbon
nanofibers. VGCF typically exist in the form of clusters, often
referred to as aggregates or agglomerates, which may or may not
contain embedded catalyst particles utilized in their
production.
[0013] Other carbon nanotubes are presently produced by
laser-evaporation of graphite or by carbon arc synthesis, yielding
fullerene-related structures that comprise graphene cylinders that
may be open or closed at either end with caps containing pentagonal
and/or hexagonal rings. These nanotubes may have a single wall of
carbon, and are therefore generally called single wall carbon
nanotubes. Preferred single wall carbon nanotubes have a diameter
of about 0.5 to about 3 nm. Within this range it is desirable to
use single wall carbon nanotubes having diameters of greater than
or equal to about 0.6, preferably greater than or equal to about
0.7 nm. Also desirable within this range are single wall carbon
nanotubes having diameters less than or equal to about 2.8,
preferably less than or equal to about 2.7, and more preferably
less than or equal to about 2.5 nm.
[0014] Carbon nanotubes having multiple concentrically arranged
walls produced by laser-evaporation of graphite or by carbon arc
synthesis are generally called multiwall carbon nanotubes.
Multiwall nanotubes used in the polymeric foams and elastomers
generally have diameters of about 2 nm to about 50 nm. Within this
range it is generally desirable to have diameters greater than or
equal to about 3, preferably greater than or equal to about 4, and
more preferably greater than or equal to about 5 nm. Also desirable
within this range are diameters of less than or equal to about 45,
preferably less than or equal to about 40, more preferably less
than or equal to about 35, even more preferably less than or equal
to about 25, and most preferably less than or equal to about 20 nm.
Single wall or multiwall carbon nanotubes generally exist in the
form of clusters, (also often referred to as agglomerates and
aggregates) and may or may not contain embedded catalyst particles
utilized in their production. Single wall carbon nanotubes tend to
exist in the form of ropes due to Van der Waal forces, and clusters
formed by these ropes may also be used. Single wall nanotubes may
be metallic or semi-conducting. It is preferable to use
compositions having as high a weight percentage of metallic carbon
nanotubes as possible for purposes of electromagnetic
shielding.
[0015] Carbon nanotubes containing impurities such as amorphous
carbon or soot, as well as catalytic materials such as iron,
nickel, copper, aluminum, yttrium, cobalt, sulfur, platinum, gold,
silver, or the like, or combinations comprising at least one of the
foregoing catalytic materials, may also be used. In one embodiment,
the carbon nanotubes may contain impurities in an amount less than
or equal to about 80 weight percent (wt %), preferably less than or
equal to about 60 wt %, more preferably less than or equal to about
40 wt %, and most preferably less than or equal to about 20 wt %,
based upon the total weight of the carbon nanotubes and the
impurities.
[0016] Effective amounts of VGCF, single wall, and/or multiwall
carbon nanotubes, alone or in combination, will vary depending on
the nature of the adhesive, the nature of the nanotubes, the
intended use, the desired electrical conductivity, and similar
factors, and can be readily determined by one of ordinary skill in
the art. In general, the nanotubes will comprise about 0.0001 to
about 50 wt % of the total weight of the pressures sensitive
adhesive composition. Within this range, it is generally desirable
to have the carbon nanotubes present in an amount of greater than
or equal to about 0.05, preferably greater than or equal to about
0.1 of the total weight of the adhesive composition. Also desirable
are carbon nanotubes present in an amount less than or equal to
about 40, preferably less than or equal to about 20, and more
preferably less than or equal to about 5 wt % of the total weight
of the composition.
[0017] Other electrically conductive fillers may be used in
addition to the carbon nanotubes, for example carbon black, carbon
fibers such as PAN fibers, metal-coated fibers or spheres such as
metal-coated glass fibers, metal-coated carbon fibers, metal-coated
organic fibers, metal-coated ceramic spheres, metal-coated glass
beads and the like, inherently conductive polymers such as
polyaniline, polypyrrole, polythiophene in particulate or fibril
form, conductive metal oxides such as tin oxide or indium tin
oxide, and combinations comprising at least one of the foregoing
conductive fillers may also be used. The amount of these fillers is
preferably selected so as to not adversely affect the final
properties of the pressure sensitive adhesive compositions. Typical
amounts, when present, are about 0.1 to about 50 wt % based on the
total weight of the composition. Within this range it is generally
desirable to have an amount of greater than or equal to about 1.0,
preferably greater than or equal to about 5 wt % of the total
weight of the composition. Also desirable is an amount of less than
or equal to about 40, more preferably less than or equal to about
25 wt %, of the total weight of the composition.
[0018] The term "pressure sensitive adhesive" or "PSA" is used
herein in its conventional sense to mean that the composition is
formulated to have a glass transition temperature, surface energy,
and/or other properties such that it exhibits some degree of tack
at normal room temperature. Thus, the constituent polymers and/or
copolymers of the composition generally will have a glass
transition temperature of less than about 0.degree. C. such that
the mass of the composition is tacky at ambient temperatures and is
thereby bondable under an applied pressure to a surface or other
substrate. In general, the formulation of the adhesive composition
specifically may be selected to exhibit an affinity, as may be
measured by lap shear, die shear, static or dynamic shear, peel, or
other adhesion, to the material forming the substrate or substrates
involved in the particular application, but which affinity is less
than to the material forming a backing layer as described below.
Such adhesion affinities may depend particularly on the surface
energy of the materials involved, and may be developed from surface
tension, valence, polar, electrostatic, van der Waals, or other
attractive forces, mechanical interlocking action, or a combination
thereof.
[0019] In use, pressure sensitive adhesives are generally provided
in the form of a tape. An exemplary embodiment of a pressure
sensitive adhesive tape is shown generally at FIG. 1, and comprises
tape 10, which may have an overall thickness of between about 0.5
and about 10 mils, may be provided in the form of, or as formed
from, a sheet, roll, tape, die-cut part, or the like. Tape 10,
which may be of an indefinite length and/or width, includes a
backing strip, sheet, or other generally flat layer 20, an adhesive
layer 22 on at least one side or portion of backing layer 20, and,
optionally, a release liner 24 for covering adhesive layer 22
during shipping and handling. Although adhesive layer 22 is shown
as being coated on substantially the entirety of backing layer 20,
adhesive layer 22 may alternatively be applied in a pattern or
otherwise to cover only a portion of backing layer 20. For most
applications, backing layer 20 can have a thickness of about 0.5-8
mils (0.013-0.203 mm), with adhesive layer 22 having a thickness of
about 0.5-2.5 mils (0.013-0.064 mm).
[0020] Backing layer 20 has a first side or surface 24 and an
opposing second side or surface 26. Adhesive layer 22 may be coated
or laminated on, or otherwise bonded to or in intimate contact with
second side 26 of the backing layer 20 to provide the laminar
structure of tape 10. Depending on the intended application, a
second adhesive layer 22 may be coated on backing layer first side
24 (not shown). Adhesive layer 22 has an inner face 30 adhesively
or otherwise bonded to second side 26, and an opposite outer face
32 that is adhesively bondable under an applied pressure to a
surface of a substrate.
[0021] Backing layer 20 may be a formed of a synthetic, natural, or
glass fiber fabric, paper, or foamed or unfoamed plastic, resin,
elastomer, or other polymeric or other material conventionally used
in tape construction. In one embodiment, backing layer 20 is
removable after bonding of adhesive layer 22 to the surface of a
substrate. A second substrate may then be applied to the exposed
face 30. In another embodiment, for example EMI shielding
applications, backing layer 20 can be formed of an
electrically-conductive material such as a conductive polymer, a
conductive metal foil, or a cloth plated with a conductive metal
such as copper, aluminum, nickel, silver, or alloys or mixtures
comprising at least one of the foregoing conductive metals. Where
similar materials are used for the substrate and backing layer 20,
the backing layer second side 26 may be coated, prior to the
application of the adhesive layer, with a higher surface energy
"tie" layer so as to increase the affinity of the adhesive layer 22
thereto relative to the substrate surface. Such tie layer may be
formed as a chemical bond coat, such as a thermoplastic dissolved
in a solvent, which is applied to the side 26 and dried or
otherwise cured thereon to form an intermediate tie layer between
the side and the adhesive layer 22. Alternatively, other known
surface treatments may employed such as cleaning or roughening the
side 26 with one or more of compressed gas, chemical or solvent
etching/cleaning, grit blasting, such as with aluminum oxide or
other abrasive, or plasma, such as may be generated from the
ionization of oxygen, argon, or another gas or mixture of
gases.
[0022] Exemplary release liners 24 include face stocks or other
films of polyolefins, plasticized polyvinyl chloride, polyesters,
cellulosics, metal foils, composites, and waxed, siliconized, or
other coated paper or plastic having a relatively low surface
energy to be removable without appreciable lifting of the adhesive
layer 22 from the backing layer 20.
[0023] Manufacture of the pressure sensitive adhesives is by
processes recognized in the art. In general, the compositions for
formation of the adhesive, additives, e.g., catalyst, crosslinking
agent, additional fillers, and the like (which are described in
further detail below), and the carbon nanotubes are mixed, frothed
and/or blown if desired, shaped (e.g., cast), then cured, if
applicable. Stepwise addition of the various components may also be
used, e.g., the carbon nanotubes may be provided in the form of a
masterbatch, and added after the other components are mixed.
[0024] During manufacture, it is generally desirable to disentangle
any clusters, aggregates or agglomerates of carbon nanotubes with
minimal damage to the aspect ratio, in order to provide enhanced
electrical conductivity, in particular enhanced electromagnetic
shielding or electrostatic dissipative properties at lower weight
percentages of nanotubes. It may therefore be desirable that any
mixing during manufacture be carried out at as low a viscosity as
possible, as mixing at lower viscosities substantially reduces the
shear forces acting on the nanotubes. Accordingly, it may be
desirable to use a removable solvent or diluent (.g., an organic
solvent and/or water) during mixing, to substantially reduce the
melt viscosity of the composition. The diluent may be removed after
some or all of the dispersion of the nanotubes in the elastomer is
completed. Alternatively, the nanotubes may be pre-disbursed at a
higher concentration in at least one of the adhesive components,
i.e., masterbatch. In the preparation of foamed adhesives, it may
be desirable to introduce desired blowing agents into the
composition prior to the introduction of the nanotubes to
facilitate dispersion while minimizing damage to the nanotubes. The
blowing of the foam may produce a similar effect, in that it
disentangles nanotubes with-low or minimal damage to the aspect
ratio, because the expansion of any polymer trapped in a nanotube
cluster or agglomerate or aggregate may cause the disentangling of
the individual nanotubes with minimal damage. Thus, mixing
nanotubes with the polymer at a reduced viscosities and
subsequently foaming the polymer may achieve excellent conductivity
at low loading levels, because of the preservation of nanotube
aspect ratio. Low carbon nanotube loading aids in preserving the
desirable physical properties of the adhesives.
[0025] Accordingly, in the production of commercial quantities of
tape 10, the formulation for the adhesive layer 22 may be
compounded in a conventional mixing apparatus as an admixture of a
PSA composition, the carbon nanotubes, any additional fillers
and/or additives, and a solvent or diluent. The formulation may be
coated or otherwise applied to side 26 of the backing layer 20 in a
conventional manner such as, for example, by a direct process such
as spraying, knife coating, roller coating, casting, drum coating,
dipping, dispensing, extrusion, screen printing, or like, or an
indirect transfer process. After coating, the resultant film may be
dried to remove the solvent or otherwise cured or cooled to develop
an adherent film on backing layer 20. Curing, where used, is
inclusive of polymerization, crosslinking, vulcanization, or
otherwise chemical or physical changes that result in formation of
a generally solid film from the applied composition. As a result of
the inherent tack of the PSA film, an adhesive and/or mechanical
bond may be developed between the layers 20 and 22 to form the
integral, laminate structure of tape 10. Alternatively, the
adhesive layer 22 may be separately formed and laminated under
conditions of elevated temperature and/or pressure to the backing
layer 20 in a separate operation.
[0026] A variety of pressure sensitive adhesive formulations may be
suitable for use with the carbon nanotubes, and include
film-forming materials such as a natural or synthetic rubber or
elastomer, or other resin, plastic, or polymer exhibiting
rubber-like properties of compliancy, resiliency or compression
deflection, low compression set, flexibility, and an ability to
recover after deformation. Examples of such materials include
styrene-butadienes, styrene-isoprenes, polybutadienes,
polyisobutylenes, polyurethanes, silicones, fluorosilicones and
other fluoropolymers, chlorosulfonates, butyls, neoprenes,
nitriles, polyisoprenes, plasticized nylons, polyesters, polyvinyl
ethers, polyvinyl acetates, polyisobutylenes, ethylene vinyl
acetates, polyolefins, and polyvinyl chlorides, copolymer rubbers
such as ethylene-propylene (EPR), ethylene-propylene-diene monomer
(EPDM), styrene-isoprene-styrene (SIS), styrene-butadiene-styrene
(SBS), nitrile-butadienes (NBR) and styrene-butadienes (SBR),
blends such as ethylene or propylene-EPDM, EPR, or NBR, and
mixtures, blends, and copolymers thereof.
[0027] These materials may be compounded with a tackifier, which
may be a resin such as glyceryl esters of hydrogenated resins,
thermoplastic terpene resins, petroleum hydrocarbon resins,
coumarone-indene resins, synthetic phenol resins, low molecular
weight polybutenes, or a tackifying silicone. Generally, the
tackifying resin may be compounded into the resin material at
between about 40-150 parts per hundred parts of the resin.
[0028] Additional fillers and additives may be included in the PSA
composition depending upon the requirements of the particular
application, for example conventional wetting agents or
surfactants, pigments, dyes, and other colorants, opacifying
agents, anti-foaming agents, anti-static agents, coupling agents
such as titanates, chain extending oils, lubricants, stabilizers,
emulsifiers, antioxidants, thickeners, and/or flame retardants such
as aluminum trihydrate, antimony trioxide, metal oxides and salts,
intercalated graphite particles, phosphate esters, brominated
diphenyl compounds such as decabromodiphenyl oxide, borates,
phosphates, halogenated compounds, glass, silica, silicates, and
mica. Typically, these fillers and additives are blended or
otherwise admixed with the formulation, and may comprise between
about 0.05-80% or more by total volume thereof.
[0029] Aqueous pressure sensitive adhesive compositions are useful
with carbon nanotubes, for example those comprising a mechanically
stable aqueous emulsion of polyethylene particles having an average
molecular weight ranging from about 7,000 to 40,000 as described in
U.S. Pat. No. 3,734,686; ethylene polymer latexes containing
ethylene polymer particles of submicron size prepared by dispersing
in water an ethylene polymer and a water-soluble block copolymer of
ethylene oxide and propylene oxide as described in U.S. Pat. No.
3,418,26; latexes prepared from copolymers of ethylene and
C.sub.3-C.sub.20 .alpha.-olefins as in U.S. Pat. No. 5,574,091; or
compositions comprising homogenous ethylene/alpha-olefin
interpolymers and substantially random interpolymers as disclosed
in U.S. Pat. No. 6,521,696.
[0030] Another useful type of pressure sensitive adhesive
composition is based on (meth) acrylates (i.e., acrylates and
methacrylates). Such compositions include, for example, copolymers
derived from compositions containing, based on the total weight of
the monomer components, about 50 to about 99 weight percent of
C.sub.4-C.sub.18 alkyl esters of (meth)acrylic acids, about 1 to
about 50 weight percent of polar ethylenically unsaturated
comonomers such as itaconic acid, certain substituted acrylamides
such a N,N-dimethyl acrylamide, N-vinyl-2-pyrrolidone, or n-vinyl
caprolactam, acrylonitrile, acrylic acid, glycidyl acrylate, and
the like, and optionally, up to about 25 weight percent of a
non-polar ethylenically unsaturated comonomer such as cyclohexyl
acrylate, n-octyl acrylamide, t-butyl acrylate, methyl
methacrylate, and the like, and/or a tackifier.
[0031] Other additives such as crosslinking agents may also be
present, for example include di- and triacrylates, for instance
1,6-hexanediol diacrylate; and photoinitiators such as
1-hydroxycyclohexyl phenyl ketone or
2,2-dimethoxy-2-phenylacetophenone, which are commercially
available from Ciba-Geigy under the trade names respectively of
IRGACURE 184 and IRGACURE 651, or other photoinitiators for
ethylenically-unsaturated monomers which are well known in the art.
Cross-linking agents and photoinitiators, are each generally used
in amounts of about 0.005 to about 0.5 weight percent, based on
total weight of monomer composition. Suitable (meth)acrylate
pressure sensitive adhesives are disclosed, for example, in U.S.
Pat. No. 4,223,067; U.S. Pat. No. 4,181,752 U.S. Pat. No.
5,183,833; U.S. Pat. No. 5,645,764, and U.S. Pat. No. Re. 24,906.
Suitable compositions are also commercially available, for example
from Ashland Chemicals under the trade name AEROSET.
[0032] The (meth)acrylate containing monomer mixture may be
polymerized by various techniques, preferably photoinitiated bulk
polymerization as described, for example, in U.S. Pat. No.
5,620,795, wherein the polymerizable comonomers and a
photoinitiator are mixed together in the absence of solvent and
partially polymerized to a viscosity of about 500 to about 50,000
cps to achieve a coatable syrup. Alternatively, the monomers may be
mixed with a thixotropic agent such as fumed hydrophilic silica to
achieve a coatable thickness. A crosslinking agent, the carbon
nanotubes, and any other components (including any tackifiers) are
then added to the prepolymerized syrup. Alternatively, these
components (including any tackifiers but with the exception of the
crosslinking agent) can be added directly to the monomer mixture
prior to pre-polymerization.
[0033] The resulting composition is coated onto a substrate (which
may be transparent to ultraviolet radiation) and polymerized in an
inert (i. e., oxygen free) atmosphere, e.g., a nitrogen atmosphere
by exposure to ultraviolet radiation. Examples of suitable
substrates include release liners (e.g., silicone release liners)
and tape backings (which may be primed or unprimed paper or
plastic). A sufficiently inert atmosphere can also be achieved by
covering a layer of the polymerizable coating with a plastic film
which is substantially transparent to ultraviolet radiation, and
irradiating through that film in air as described in the
aforementioned Martens et al. patent using ultraviolet lamps. The
ultraviolet light source preferably has 90% of the emissions
between 280 and 400 nm (more preferably between 300 and 400 nm),
with a maximum at 351 nm. Where multi-layer tape constructions are
desirable, a variety of conventional techniques may be used. For
example, the coatings may be applied concurrently (e.g., through a
die coater), after which the entire multi-layer structure is cured
at the same time. The coatings may also be applied sequentially
whereby each individual layer is partially or completely cured
prior to application of the next layer.
[0034] Use of carbon nanotubes enables the production of
electrically conductive adhesives having a volume resistivity of
about 10.sup.-3 ohm-cm to about 10.sup.8 ohm-cm. Within this range,
the volume resistivity can be less than or equal to about 10.sup.6,
less than or equal to about 10.sup.4, or less than or equal to
about 10.sup.3, and is preferably less than or equal to about
10.sup.2, more preferably less than or equal to about 10, and most
preferably less than or equal to about 1 ohm-cm.
[0035] Use of carbon nanotubes unexpectedly allows the manufacture
of pressure sensitive adhesives that have excellent electrical
conductivity and physical properties, particularly tack and peel
strength. These characteristics permit the adhesives to be used
with a variety of articles, particularly where electromagnetic
and/or electrostatic dissipative properties are desired. The
articles are suitable for use in a variety of commercial
applications such as cell phones, global positioning systems, disk
drives, personal digital assistants, personal or laptop computers,
airplanes, radio receiver or transmitter, network server, cellular
communication base station or other telecommunications equipment,
or other articles of commerce.
[0036] The following examples, which are meant to be exemplary, not
limiting, illustrate compositions and methods of manufacturing of
some of the various embodiments of the pressure sensitive adhesives
described herein.
EXAMPLES
[0037] As is known, particular values for volume resistivity and
electrostatic shielding will depend on the particular test methods
and conditions. For example, it is known that volume resistivity
and shielding effectiveness may vary with the pressure placed on
the sample during the test. Useful electrical equipment and test
fixtures to measure volume resistivity in the sample below are as
follows. The fixture is a custom fabricated press with gold plated,
2.5 cm.times.2.5 cm (1 inch.times.1 inch) square, and electrical
contacts. The fixture is equipped with a digital force gauge that
allows the operator to control and make adjustments to the force
that is applied to the surface of the sample. The Power supply is
capable of supplying 0 to 2 amps to the sample surface. The Voltage
drop and ohms across the sample are measured using a HP 34420A Nano
Volt/Micro Ohmmeter. The electronic components of the fixture are
allowed to warm up and, in the case of the HP 34420 A, the internal
calibration checks are done. The samples are allowed to
equilibrate, for a period of 24 hours, to the conditions of the
test environment. Typical test environment is 50% Relative Humidity
(% RH) with a room temp of 23.degree. C. (70.degree. F.).
[0038] The sample to be tested is placed between the platens of the
test fixture and a load is applied to the surface. The applied load
is dependent on the type of sample to be tested, soft elastomers
are tested using small loads while solids are tested using a load
range from about 10 to 100 pounds per square inch). Once the load
has been applied, the current is applied to the sample and the
voltage drop through the sample thickness is measured. A typical
test would include measurements at 4 different amp settings, 0.5,
1.0, 1.6, and 2.0 amps. For a conductive composite the resulting
calculated volume resistivity for all four of the amp settings will
be similar. The calculation for the volume resistivity is as
follows:
Volume resistivity (ohm-cm)=(E/I)*(A/T)
[0039] wherein E=voltage drop (V), I=current (amps), A=area
(cm.sup.2), and T=thickness (cm).
[0040] To make volume resistivity measurements, a conductive
silicone elastomer is coated with a commercially available pressure
sensitive adhesive composition (e.g., Aeroset 1450 from Ashland
Chemicals) in an organic solvent and comprising 0.001 to 50 wt % of
carbon nanotubes. After as described by the manufacturer, the
elastomer is adhered to conductive silicone elastomer from BISCO,
and a voltmeter is used to make resistance measurements as
described above. The volume resistivity of such samples is lower
than for comparable samples made using a commercially available
conductive PSA available from 3M under the trade designation
"9713"). The samples can have a volume resistivity of about
10.sup.-3 ohm-cm to about 10.sup.8 ohm-cm. Within this range, the
volume resistivity can be less than or equal to about 10.sup.6,
less than or equal to about 10.sup.4, or less than or equal to
about 10.sup.3, and is preferably less than or equal to about
10.sup.2, more preferably less than or equal to about 10, and most
preferably less than or equal to about 1 ohm-cm.
[0041] Typically, the inventive pressure sensitive adhesive will be
bondable to the substrate surface under firm hand pressure, and
will exhibit thereon a 180.degree. peel adhesion, such as may be
determined in accordance with PSTC-1 (Pressure Sensitive Tape
Council Test Methods for Pressure Sensitive Adhesive Tapes,
Pressure Sensitive Tape Council, Northbrook, Ill. 60062), of
between 0.5-5.0 lb/in initial. Preferably, such adhesion will be
observed to increase or "build" on aging.
[0042] While preferred embodiments have been shown and described,
various modifications and substitutions may be made thereto without
departing from the spirit and scope of the invention. Accordingly,
it is to be understood that the present invention has been
described by way of illustration and not limitations.
* * * * *