U.S. patent application number 10/336259 was filed with the patent office on 2003-06-05 for silicon composition and electrically conductive silicone adhesive formed therefrom.
Invention is credited to Kleyer, Don Lee, Lutz, Michael Andrew.
Application Number | 20030105207 10/336259 |
Document ID | / |
Family ID | 24485758 |
Filed Date | 2003-06-05 |
United States Patent
Application |
20030105207 |
Kind Code |
A1 |
Kleyer, Don Lee ; et
al. |
June 5, 2003 |
Silicon composition and electrically conductive silicone adhesive
formed therefrom
Abstract
A silicone composition for preparing a silicone adhesive, the
composition prepared by mixing (A) an organopolysiloxane containing
an average of at least two silicon-bonded hydroxy groups per
molecule; (B) a crosslinking agent in an amount sufficient to cure
the composition; wherein the agent is selected from (i) at least
one silane having the formula R.sup.2.sub.nSiX.sub.4-n wherein each
R.sup.2 is independently selected from monovalent hydrocarbon and
monovalent halogenated hydrocarbon groups having from 1 to about 8
carbon atoms, n is 0 or 1, and X is --OR.sup.2 or
--OCH.sub.2CH.sub.2OR.sup.2; (ii) a partial hydrolyzate of (i), and
(iii) mixtures comprising (i) and (ii); (C) an electrically
conductive filler in an amount sufficient to impart electrical
conductivity to the silicone adhesive, wherein the filler comprises
particles having at least an outer surface of a metal selected from
the group consisting of silver, gold, platinum, palladium, and
alloys thereof; (D) an effective amount of a hydroxy-functional
organic compound having a molecular weight up to about 1000 and
containing at least one hydroxy group per molecule, provided the
compound does not substantially inhibit cure of the composition;
and (E) a catalytic amount of a condensation catalyst comprising a
metal salt of a carboxylic acid. A silicone adhesive and a
multi-part curable silicone composition.
Inventors: |
Kleyer, Don Lee; (Hemlock,
MI) ; Lutz, Michael Andrew; (Hope, MI) |
Correspondence
Address: |
Dow Corning Corporation
Intellectual Property Department - Mail CO1232
P.O. Box 994
2200 W. Salzburg Road
Midland
MI
48686-0994
US
|
Family ID: |
24485758 |
Appl. No.: |
10/336259 |
Filed: |
January 3, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10336259 |
Jan 3, 2003 |
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09620397 |
Jul 20, 2000 |
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6534581 |
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Current U.S.
Class: |
524/439 ;
524/440 |
Current CPC
Class: |
C08K 5/05 20130101; C08K
5/54 20130101; C08L 2666/44 20130101; C08K 5/053 20130101; C09J
183/04 20130101; C08K 5/04 20130101; C08L 2666/54 20130101; C08K
9/02 20130101; C08K 5/098 20130101; H01B 1/22 20130101; C08L
2666/28 20130101; C08K 5/04 20130101; C08L 83/04 20130101; C08K
5/05 20130101; C08L 83/04 20130101; C08K 5/053 20130101; C08L 83/04
20130101; C08K 5/098 20130101; C08L 83/04 20130101; C08K 5/54
20130101; C08L 83/04 20130101; C08K 9/02 20130101; C08L 83/04
20130101; C09J 183/04 20130101; C08L 2666/44 20130101; C08L 2666/54
20130101; C09J 183/04 20130101; C08L 2666/28 20130101; C08L 2666/54
20130101 |
Class at
Publication: |
524/439 ;
524/440 |
International
Class: |
C08K 003/08 |
Claims
That which is claimed is:
1. A curable silicone composition for preparing a silicone
adhesive, the composition prepared by mixing: (A) an
organopolysiloxane containing an average of at least two
silicon-bonded hydroxy groups per molecule; (B) a crosslinking
agent in an amount sufficient to cure the composition; wherein the
agent is selected from (i) at least one silane having the formula
R.sup.2.sub.nSiX.sub.4-n wherein each R.sup.2 is independently
selected from monovalent hydrocarbon and monovalent halogenated
hydrocarbon groups having from 1 to about 8 carbon atoms, n is 0 or
1, and X is --OR.sup.2 or --OCH.sub.2CH.sub.2OR.sup.2; (ii) a
partial hydrolyzate of (i), and (iii) mixtures comprising (i) and
(ii); (C) an electrically conductive filler in an amount sufficient
to impart electrical conductivity to the silicone adhesive, wherein
the filler comprises particles having at least an outer surface of
a metal selected from the group consisting of silver, gold,
platinum, palladium, and alloys thereof; (D) an effective amount of
a hydroxy-functional organic compound having a molecular weight up
to about 1000 and containing at least one hydroxy group per
molecule, provided the compound does not substantially inhibit cure
of the composition; and (E) a catalytic amount of a condensation
catalyst comprising a metal salt of a carboxylic acid.
2. The silicone composition according to claim 1, wherein the
organopolysiloxane is a hydroxy-terminated polydiorganosiloxane
having the general formula
HOR.sup.1.sub.2SiO(R.sup.1.sub.2SiO).sub.mSiR.sup.1.s- ub.2OH
wherein each R.sup.1 is independently selected from monovalent
hydrocarbon and monovalent halogenated hydrocarbon groups and m has
a value such that the viscosity of the polydiorganosiloxane at
25.degree. C. is from 0.05 to 200 Pa.multidot.s.
3. The silicone composition according to claim 1, wherein R.sup.2
in the crosslinking agent is selected from methyl, ethyl, and
propyl.
4. The silicone composition according to claim 1, wherein the
concentration of the crosslinking agent is sufficient to provide
from 1.0 to 3.0 silicon-bonded --OR.sup.2 or
--OCH.sub.2CH.sub.2OR.sup.2 groups per hydroxy group in component
(A).
5. The silicone composition according to claim 1, wherein the
filler has a concentration from about 20 to about 50 percent by
volume, based on the total volume of the composition.
6. The silicone composition according to claim 1, wherein the
composition cures at a temperature of from about room temperature
to about 150.degree. C.
7. The silicone composition according to claim 6, wherein the
hydroxy-functional organic compound has a normal boiling point
greater than the cure temperature of the composition.
8. The silicone composition according to claim 1, wherein the
hydroxy-functional organic compound is selected from at least one
monohydric alcohol, at least one dihydric alcohol, at least one
polyhydric alcohol, at least one phenol, at least one sugar, at
least one hydroxy aldehyde, at least one hydroxy ketone, at least
one hydroxy acid, at least one hydroxy ester, and a mixture
comprising at least two of the aforementioned compounds.
9. The silicone composition according to claim 8, wherein the
hydroxy-functional organic compound is selected from at least one
monohydric alcohol, at least one dihydric alcohol, at least one
polyhydric alcohol, and a mixture comprising at least two of the
aforementioned compounds.
10. The silicone composition according to claim 1, wherein the
concentration of component (D) is from 0.5 to 1.5 percent by
weight, based on the total weight of the composition.
11. The silicone composition according to claim 1, wherein the
condensation catalyst comprises a metal salt of a carboxylic acid
selected from a tin salt, a lead salt, and a zinc salt.
12. The silicone composition according to claim 1, wherein the
composition further comprises an adhesion promoter.
13. The silicone composition according to claim 12, wherein the
adhesion promoter is selected from at least one amino-functional
alkoxysilane, at least one epoxy-functional alkoxysilane, a
reaction product of at least one amino-functional alkoxysilane and
at least one epoxy-functional alkoxysilane, and at least one vinyl
trialkoxysilane.
16. The silicone composition according to claim 1, further
comprising a solvent having a normal boiling point greater than the
cure temperature of the composition.
17. The silicone composition according to claim 1, further
comprising a nonfunctional silicone fluid.
18. A silicone adhesive comprising a reaction product of the
composition of claim 1.
19. A multi-part curable silicone composition for preparing a
silicone adhesive, the composition comprising: (A) an
organopolysiloxane containing an average of at least two
silicon-bonded hydroxy groups per molecule; (B) a crosslinking
agent in an amount sufficient to cure the composition; wherein the
agent is selected from (i) at least one silane having the formula
R.sup.2.sub.nSiX.sub.4-n wherein each R.sup.2 is independently
selected from monovalent hydrocarbon and monovalent halogenated
hydrocarbon groups having from 1 to about 8 carbon atoms, n is 0 or
1, and X is --OR.sup.2 or --OCH.sub.2CH.sub.2OR.sup.2; (ii) a
partial hydrolyzate of (i), and (iii) mixtures comprising (i) and
(ii); (C) an electrically conductive filler in an amount sufficient
to impart electrical conductivity to the silicone adhesive, wherein
the filler comprises particles having at least an outer surface of
a metal selected from the group consisting of silver, gold,
platinum, palladium, and alloys thereof; (D) an effective amount of
a hydroxy-functional organic compound having a molecular weight up
to about 1000 and containing at least one hydroxy group per
molecule, provided the compound does not substantially inhibit cure
of the composition; and (E) a catalytic amount of a condensation
catalyst comprising a metal salt of a carboxylic acid, provided
neither component (A) nor component (D) are present with components
(B) and (E) in the same part.
Description
CROSS REFERENCE
[0001] This application is a divisional of U.S. patent application
Ser. No. 09/620,397, filed on 20 July, 2000, now allowed. This
application claims priority to U.S. patent application Ser. No.
09/620,397 under 35 U.S.C. .sctn.120.
FIELD OF THE INVENTION
[0002] The present invention relates to a curable silicone
composition for preparing a silicone adhesive and more particularly
to a condensation-curable silicone composition containing an
electrically conductive filler and a hydroxy-functional organic
compound. The present invention also relates to an electrically
conductive silicone adhesive produced from such composition.
BACKGROUND OF THE INVENTION
[0003] Silicone adhesives are useful in a variety of applications
by virtue of their unique combination of properties, including high
thermal stability, good moisture resistance, excellent flexibility,
high ionic purity, low alpha particle emissions, and good adhesion
to various substrates. For example, silicone adhesives are widely
used in the automotive, electronic, construction, appliance, and
aerospace industries.
[0004] Condensation-curable silicone compositions comprising an
organopolysiloxane containing silicon-bonded hydroxy groups, a
crosslinking agent, and a curing catalyst are known in the art.
Illustrative of such compositions are U.S. Pat. Nos. 3,769,064;
2,902,467; 2,843,555; 2,927,907; 3,065,194; 3,070,559; 3,127,363;
3,070,566; 3,305,502; 3,575,917; 3,696,090; 3,702,835; 3,888,815;
3,933,729; 4,388,433; 4,490,500; 4,547,529; 4,962,152; and European
Patent Application No. 0816437A2. However, the aforementioned
references do not teach the electrically conductive filler and the
hydroxy-functional organic compound of the present invention.
[0005] Furthermore, Japanese Laid-Open Patent Application (Kokai)
No. 8-302196 to Fujiki et al. discloses a silicone composition
comprising a silanol group-containing organopolysiloxane, a
hydrolysable organosilicon compound, a conductive filler composed
of silver particles or silver-covered particles surface-treated
with a silicone compound, and a curing catalyst. However, the
preceding patent application does not teach the hydroxy-functional
organic compound of the present invention.
SUMMARY OF THE INVENTION
[0006] The present inventors have discovered that a
condensation-curable silicone composition containing an
electrically conductive filler and a hydroxy-functional organic
compound cures to form an adhesive having unexpectedly superior
electrical conductivity. Specifically, the present invention is
directed to a curable silicone composition for preparing a silicone
adhesive, the composition prepared by mixing:
[0007] (A) an organopolysiloxane containing an average of at least
two silicon-bonded hydroxy groups per molecule;
[0008] (B) a crosslinking agent in an amount sufficient to cure the
composition; wherein the agent is selected from (i) at least one
silane having the formula R.sup.2.sub.nSiX.sub.4-n wherein each
R.sup.2 is independently selected from monovalent hydrocarbon and
monovalent halogenated hydrocarbon groups having from 1 to about 8
carbon atoms, n is 0 or 1, and X is --OR.sup.2 or
--OCH.sub.2CH.sub.2OR.sup.2; (ii) a partial hydrolyzate of (i), and
(iii) mixtures comprising (i) and (ii);
[0009] (C) an electrically conductive filler in an amount
sufficient to impart electrical conductivity to the silicone
adhesive, wherein the filler comprises particles having at least an
outer surface of a metal selected from the group consisting of
silver, gold, platinum, palladium, and alloys thereof;
[0010] (D) an effective amount of a hydroxy-functional organic
compound having a molecular weight up to about 1000 and containing
at least one hydroxy group per molecule, provided the compound does
not substantially inhibit cure of the composition; and
[0011] (E) a catalytic amount of a condensation catalyst comprising
a metal salt of a carboxylic acid.
[0012] The present invention is also directed to a silicone
adhesive comprising a reaction product of the above-described
composition.
[0013] The present invention is further directed to a multi-part
curable silicone composition comprising components (A) through (E)
in two or more parts, provided neither component (A) nor component
(D), are present with components (B) and (E) in the same part.
[0014] The silicone composition of the present invention has
numerous advantages, including good flow, low VOC (volatile organic
compound) content, and adjustable cure. Moreover, the present
silicone composition cures to form a silicone adhesive having good
adhesion and unexpectedly superior electrical conductivity as
evidenced by low contact resistance and/or volume resistivity.
[0015] The silicone composition of the present invention is useful
for preparing an electrically conductive silicone adhesive. The
silicone adhesive of the present invention has numerous uses,
including die attach adhesives, solder replacements, and
electrically conductive coatings and gaskets. In particular, the
silicone adhesive is useful for bonding electronic components to
flexible or rigid substrates.
[0016] These and other features, aspects, and advantages of the
present invention will become better understood with reference to
the following description and appended claims.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The present invention is directed to a curable silicone
composition for preparing a silicone adhesive, the composition
prepared by mixing:
[0018] (A) an organopolysiloxane containing an average of at least
two silicon-bonded hydroxy groups per molecule;
[0019] (B) a crosslinking agent in an amount sufficient to cure the
composition; wherein the agent is selected from (i) at least one
silane having the formula R.sup.2.sub.nSiX.sub.4-n wherein each
R.sup.2 is independently selected from monovalent hydrocarbon and
monovalent halogenated hydrocarbon groups having from 1 to about 8
carbon atoms, n is 0 or 1, and X is --OR.sup.2 or
--OCH.sub.2CH.sub.2OR.sup.2; (ii) a partial hydrolyzate of (i), and
(iii) mixtures comprising (i) and (ii);
[0020] (C) an electrically conductive filler in an amount
sufficient to impart electrical conductivity to the silicone
adhesive, wherein the filler comprises particles having at least an
outer surface of a metal selected from the group consisting of
silver, gold, platinum, palladium, and alloys thereof;
[0021] (D) an effective amount of a hydroxy-functional organic
compound having a molecular weight up to about 1000 and containing
at least one hydroxy group per molecule, provided the compound does
not substantially inhibit cure of the composition; and
[0022] (E) a catalytic amount of a condensation catalyst comprising
a metal salt of a carboxylic acid.
[0023] Component (A) of the present invention also referred to
herein as the "polymer," is at least one organopolysiloxane
containing an average of at least two silicon-bonded hydroxy groups
(silanol groups) per molecule. The structure of the
organopolysiloxane can be linear, branched, or resinous. The
organopolysiloxane can be a homopolymer or a copolymer. The
silicon-bonded organic groups in the organopolysiloxane are
independently selected from monovalent hydrocarbon and monovalent
halogenated hydrocarbon groups. These monovalent groups typically
have from 1 to about 20 carbon atoms, preferably from 1 to 10
carbon atoms, and are exemplified by, but not limited to alkyl such
as methyl, ethyl, propyl, pentyl, octyl, undecyl, and octadecyl;
cycloalkyl such as cyclohexyl; alkenyl such as vinyl, allyl,
butenyl, and hexenyl; aryl such as phenyl, tolyl, xylyl, benzyl,
and 2-phenylethyl; and halogenated hydrocarbon groups such as
3,3,3-trifluoropropyl, 3-chloropropyl, and dichlorophenyl.
Preferably, at least 50 percent, and more preferably at least 80%,
of the organic groups in the organopolysiloxane are methyl.
[0024] The viscosity of the organopolysiloxane at 25.degree. C.,
which varies with molecular weight and structure, is typically from
0.05 to 200 Pa.multidot.s, preferably from 2 to 100 Pa.multidot.s,
and more preferably from 5 to 50 Pa.multidot.s.
[0025] A preferred organopolysiloxane according to the present
invention is a hydroxy-terminated polydiorganosiloxane having the
general formula
HOR.sup.1.sub.2SiO(R.sup.1.sub.2SiO).sub.mSiR.sup.1.sub.2OH wherein
each R.sup.1 is independently selected from monovalent hydrocarbon
and monovalent halogenated hydrocarbon groups, as defined above;
and subscript m has a value such that the viscosity of the
polydiorganosiloxane at 25.degree. C. is from 0.05 to 200
Pa.multidot.s. Preferably, R.sup.1 is methyl.
[0026] Examples of organopolysiloxanes useful in the silicone
composition of the present invention include, but are not limited
to polydiorganosiloxanes having the following formulae:
HOMe.sub.2SiO(Me.sub.2SiO).sub.mSiMe.sub.2OH,
HOMe.sub.2SiO(Me.sub.2SiO).-
sub.0.25m(MePhSiO).sub.0.75mSiMe.sub.2OH,
HOMe.sub.2SiO(Me.sub.2SiO).sub.0-
.95m(Ph.sub.2SiO).sub.0.05mSiMe.sub.2OH,
HOMe.sub.2SiO(Me.sub.2SiO).sub.0.-
98m(MeViSiO).sub.0.02mSiMe.sub.2OH, and
HOPhMeSiO(Me.sub.2SiO).sub.mSiPhMe- OH, where Me and Ph denote
methyl and phenyl respectively and the subscript m is as defined
above.
[0027] Component (A) can be a single organopolysiloxane or a
mixture comprising two or more organopolysiloxanes that differ in
at least one of the following properties: structure, viscosity,
average molecular weight, siloxane units, and sequence.
[0028] Methods of preparing organopolysiloxanes suitable for use in
the composition of the present invention, such as hydrolysis and
condensation of the corresponding organohalosilanes or
equilibration of cyclic polydiorganosiloxanes with a chain stopper
such as water or a low molecular weight silanol-terminated
diorganosiloxane, are well known in the art.
[0029] Component (B) of the present invention is a crosslinking
agent selected from (i) at least one silane having the formula
R.sup.2.sub.nSiX.sub.4-n wherein each R.sup.2 is independently
selected from monovalent hydrocarbon and monovalent halogenated
hydrocarbon groups having from 1 to about 8 carbon atoms, n is 0 or
1, and X is --OR.sup.2 or --OCH.sub.2CH.sub.2OR.sup.2; (ii) a
partial hydrolyzate of (i), and (iii) mixtures comprising (i) and
(ii). Examples of monovalent hydrocarbon and monovalent halogenated
hydrocarbon groups include, but are not limited to, alkyl such as
methyl, ethyl, propyl, pentyl, octyl, undecyl, and octadecyl;
cycloalkyl such as cyclohexyl; alkenyl such as vinyl and allyl;
aryl such as phenyl, tolyl, xylyl, benzyl, and 2-phenylethyl; and
halogenated hydrocarbon groups such as 3,3,3-trifluoropropyl,
3-chloropropyl, and dichlorophenyl. Preferably, R.sup.2 is alkyl
and more preferably, R.sup.2 is methyl, ethyl, or propyl.
[0030] Examples of silanes having the formula
R.sup.2.sub.nSiX.sub.4-n wherein R.sup.2 and n are defined above
include, but are not limited to, silanes having the formulae:
Si(OCH.sub.3).sub.4, Si(OCH.sub.2CH.sub.3).s- ub.4,
Si(OCH.sub.2CH.sub.2CH.sub.3).sub.4,
Si[O(CH.sub.2).sub.3CH.sub.3].s- ub.4,
Si[O(CH.sub.2).sub.4CH.sub.3].sub.4, CH.sub.3Si(OCH.sub.3).sub.3,
CH.sub.3Si(OCH.sub.2CH.sub.3).sub.3,
CH.sub.3Si(OCH.sub.2CH.sub.2CH.sub.3- ).sub.3,
CH.sub.3Si[O(CH.sub.2).sub.3CH.sub.3].sub.3,
CH.sub.3CH.sub.2Si(OCH.sub.3).sub.3,
CH.sub.3CH.sub.2Si(OCH.sub.2CH.sub.3- ).sub.3,
CH.sub.3CH.sub.2Si(OCH.sub.2CH.sub.2CH.sub.3).sub.3,
CH.sub.3CH.sub.2CH.sub.2Si(OCH.sub.2CH.sub.3).sub.3,
CH.sub.3CH.sub.2CH.sub.2Si(OCH.sub.2CH.sub.2CH.sub.3).sub.3,
CH.sub.3(CH.sub.2).sub.3Si(OCH.sub.3).sub.3,
CH.sub.3(CH.sub.2).sub.3Si(O- CH.sub.2CH.sub.3).sub.3,
(CH.sub.3).sub.2CHCH.sub.2Si(OCH.sub.3).sub.3,
CH.sub.3(CH.sub.2).sub.5Si(OCH.sub.3).sub.3,
C.sub.6H.sub.11Si(OCH.sub.3)- .sub.3,
C.sub.6H.sub.5Si(OCH.sub.3).sub.3, C.sub.6H.sub.5CH.sub.2Si(OCH.su-
b.3).sub.3, C.sub.6H.sub.5Si(OCH.sub.2CH.sub.3).sub.3
CH.sub.2.dbd.CHSi(OCH.sub.3).sub.3,
CH.sub.2.dbd.CHCH.sub.2Si(OCH.sub.3).- sub.3,
CH.sub.2.dbd.C(CH.sub.3)CH.sub.2Si(OCH.sub.3).sub.3,
CF.sub.3CH.sub.2CH.sub.2Si(OCH.sub.3).sub.3,
Si(OCH.sub.2CH.sub.2OCH.sub.- 3).sub.4,
Si(OCH.sub.2CH.sub.2OC.sub.2H.sub.5).sub.4,
CH.sub.3Si(OCH.sub.2CH.sub.2OCH.sub.3).sub.3,
CF.sub.3CH.sub.2CH.sub.2Si(- OCH.sub.2CH.sub.2OCH.sub.3).sub.3,
CH.sub.2.dbd.CHSi(OCH.sub.2CH.sub.2OCH.- sub.3).sub.3,
CH.sub.2.dbd.CHCH.sub.2Si(OCH.sub.2CH.sub.2OCH.sub.3).sub.3,
C.sub.6H.sub.5Si(OCH.sub.2CH.sub.2OCH.sub.3).sub.3, and
CH.sub.2.dbd.C(CH.sub.3)CH.sub.2Si(OCH.sub.2CH.sub.2OCH.sub.3).sub.3.
[0031] Methods of preparing silanes suitable for use in the
silicone composition of the present invention, such as the reaction
of chlorosilanes with alcohols, are well known in the art.
[0032] A partial hydrolyzate of at least one silane having the
formula R.sup.2.sub.nSiX.sub.4-n wherein R.sup.2 and n are defined
above is commonly referred to as a "polysilicate."Methods of
preparing polysilicates are well known in the art. For example, the
partial hydrolyzate can be prepared by partially hydrolyzing the
aforementioned silane in the presence of a small amount of an acid
to a point where the hydrolyzate is a water-insoluble liquid that
can be isolated from the reaction mixture. In particular, the
controlled partial hydrolysis of ethyl orthosilicate can be carried
out by treating a mixture of the orthosilicate in water with an
acid or an acid-forming metal salt to obtain a two-phase mixture
and then separating the water-insoluble, partially hydrolyzed
orthosilicate from the aqueous phase. Examples of acid-forming
metal salts include, but are not limited to, ferric chloride,
cupric chloride, aluminum chloride, and stannic chloride.
[0033] Component (B)(i) can be a single silane or a mixture
comprising two or more different silanes. Similarly, component
(B)(ii) can be a partial hydrolyzate of one silane or a mixture
comprising two or more different silanes. Finally, component (B)
can be a crosslinking agent as defined by (B)(i) or (B)(ii) or a
mixture comprising (B)(i) and (B)(ii).
[0034] The concentration of component (B) in the silicone
composition of the present invention is sufficient to cure
(crosslink) the composition. The exact amount of component (B)
depends on the desired extent of cure, which generally increases as
the ratio of the number of moles of silicon-bonded --OR.sup.2 or
--OCH.sub.2CH.sub.2OR.sup.2 groups in component (B) to the number
of moles of hydroxy groups in component (A) increases. Typically,
the concentration of component (B) is sufficient to provide from
0.8 to 10 silicon-bonded --OR.sup.2 or --OCH.sub.2CH.sub.2OR.sup.2
groups per hydroxy group in component (A). Preferably, the
concentration of component (B) is sufficient to provide from 1.0 to
3.0 silicon-bonded --OR.sup.2 or --OCH.sub.2CH.sub.2OR.sup.2 groups
per hydroxy group in component (A).
[0035] Component (C) of the present invention is at least one
electrically conductive filler comprising particles having at least
an outer surface of a metal selected from silver, gold, platinum,
palladium, and alloys thereof. Fillers comprising particles
consisting of silver, gold, platinum, palladium, and alloys thereof
typically have the form of a powder or flakes with an average
particle size of from 0.5 to 20 .mu.m. Fillers comprising particles
having only an outer surface consisting of silver, gold, platinum,
palladium, and alloys thereof typically have an average particle
size of from 15 to 100 .mu.m. The core of such particles can be any
material, electrical conductor or insulator, which supports a
surface consisting of the aforementioned metal and does not
adversely affect the electrical properties of the silicone
adhesive. Examples of such materials include, but are not limited
to, copper, solid glass, hollow glass, mica, nickel, and ceramic
fiber.
[0036] In the case of electrically conductive fillers comprising
metal particles having the form of flakes, the surface of the
particles may be coated with a lubricant, such as a fatty acid or
fatty acid ester. Such lubricants are typically introduced during
the milling process used to produce flakes from a metal powder to
prevent the powder from cold welding or forming large aggregates.
Even when the flakes are washed with a solvent after milling, some
lubricant may remain chemisorbed on the surface of the metal.
[0037] The electrically conductive filler of the present invention
also includes fillers prepared by treating the surfaces of the
aforementioned particles with at least one organosilicon compound.
Suitable organosilicon compounds include those typically used to
treat silica fillers, such as organochlorosilanes, organosiloxane,
organodisilazanes, and organoalkoxysilanes.
[0038] Component (C) can be a single electrically conductive filler
as described above or a mixture of two or more such fillers that
differ in at least one of the following properties: composition,
surface area, surface treatment, particle size, and particle
shape.
[0039] Preferably, the electrically conductive filler of the
present invention comprises particles consisting of silver and more
preferably particles consisting of silver having the form of
flakes.
[0040] The concentration of component (C) in the silicone
composition of the present invention is sufficient to impart
electrical conductivity to the adhesive formed by curing the
composition. Typically, the concentration of component (C) is such
that the silicone adhesive has a contact resistance less than about
1 .OMEGA. and a volume resistivity less than about 0.01
.OMEGA..multidot.cm, as determined using the methods in the
Examples below. The exact concentration of component (C) depends on
the desired electrical properties, surface area of the filler,
density of the filler, shape of the filler particles, surface
treatment of the filler, and nature of the other components in the
silicone composition. The concentration of component (C) is
typically from about 15 to about 80 percent by volume and
preferably from about 20 to about 50 percent by volume, based on
the total volume of the silicone composition. When the
concentration of component (C) is less than about 15 percent by
volume, the silicone adhesive typically does not have significant
electrical conductivity. When the concentration of component (C) is
greater than about 80 percent by volume, the silicone adhesive
typically does not exhibit further substantial improvement in
electrical conductivity.
[0041] Methods of preparing electrically conductive fillers
suitable for use in the silicone composition of the present
invention are well known in the art; many of these fillers are
commercially available. For example powders of silver, gold,
platinum, or palladium, or alloys thereof are typically produced by
chemical precipitation, electrolytic deposition, or cementation.
Also, flakes of the aforementioned metals are typically produced by
grinding or milling the metal powder in the presence of a
lubricant, such as a fatty acid or fatty acid ester. Particles
having only an outer surface of at least one of the aforementioned
metals are typically produced by metallizing an appropriate core
material using a method such as electrolytic deposition,
electroless deposition, or vacuum deposition.
[0042] As stated above, the electrically conductive filler of the
present invention can be a filler prepared by treating the surfaces
of the aforementioned particles with at least one organosilicon
compound. In this case, the particles can be treated prior to
admixture with the other ingredients of the silicone composition or
the particles can be treated in situ during the preparation of the
silicone composition.
[0043] Component (D) of the present invention is at least one
hydroxy-functional organic compound having a molecular weight up to
about 1000 and containing at least one hydroxy group per molecule,
provided the compound does not substantially inhibit cure of the
composition. When the molecular weight of the hydroxy-functional
organic compound is greater than about 1000, the silicone adhesive
formed by curing the composition does not have substantially
improved electrical conductivity relative to an adhesive formed by
curing a similar silicone composition lacking only the
hydroxy-functional organic compound. As used herein, the term
"substantially inhibit cure" means to prevent cure or retard cure
to the point where the cure rate is impracticably slow, for
example, several days, at all temperatures from room temperature to
about 150.degree. C. Preferably, the silicone composition of the
present invention cures in less than about 16 hours at 70.degree.
C.
[0044] The structure of the hydroxy-functional organic compound can
be linear, branched, or cyclic. The hydroxy group(s) in the
hydroxy-functional organic compound may be attached to a primary,
secondary or tertiary aliphatic carbon atom; an aromatic carbon
atom; or a doubly bonded carbon atom in the molecule. Furthermore,
there are no restrictions on the stereochemistry of the
hydroxy-bearing carbon atom(s) or the molecule.
[0045] The hydroxy-functional organic compound can contain one or
more functional groups other than hydroxy, provided the compound
does not substantially inhibit cure of the composition. Examples of
suitable functional groups include, but are not limited to, --O--,
>C.dbd.O, --CHO, --CO.sub.2--, --C.ident.N, --NO.sub.2,
>C.dbd.C<, --C.ident.C--, --F, --Cl, --Br, and --I. However,
hydroxy-functional organic compounds containing functional groups
that strongly complex the metal in the condensation catalyst,
component (E), may substantially inhibit cure of the composition.
For example, when a tin catalyst is used, hydroxy-functional
organic compounds containing thiol (--SH) groups are generally
avoided. The degree of inhibition depends on the particular
functional group and metal and the mole ratio thereof. The
suitability of a particular hydroxy-functional organic compound for
use in the silicone composition of the present invention can be
readily determined by routine experimentation using the methods in
the Examples below.
[0046] The hydroxy-functional organic compound can be a naturally
occurring or synthetic compound having a liquid or solid state at
room temperature. Also, the hydroxy-functional organic compound can
be soluble, partially soluble, or insoluble in the silicone
composition. The normal boiling point of the hydroxy-functional
organic compound, which depends on the molecular weight, structure,
and number and nature of functional groups in the compound, can
vary over a wide range. Preferably, the hydroxy-functional organic
compound has a normal boiling point greater than the cure
temperature of the composition. Otherwise, appreciable amounts of
the hydroxy-functional organic compound may be removed by
volatilization during cure, resulting in little or no enhancement
in the conductivity of the silicone adhesive. Also, excessive
volatilization of the hydroxy-functional organic compound during
curing may cause formation of voids in the silicone adhesive.
[0047] Examples of hydroxy-functional compounds suitable for use in
the composition of the present invention include, but are not
limited to, monohydric alcohols such as methanol, ethanol,
propanol, butanol, pentanol, hexanol, cyclohexanol, hepatanol,
nonanol, decanol, undecanol, 1-phenylethanol, benzyl alcohol, allyl
alcohol, 3-nitrobenzyl alcohol, 3-chlorobenzyl alcohol,
3-bromobenzyl alcohol, 3-iodobenzyl alcohol, and diethylene glycol
butyl ether; dihydric alcohols such as ethylene glycol, propylene
glycol (1,2-propanediol), polyethylene glycol, polypropylene
glycol, polytetrahydrofuran, benzopinacole, 1,2-butanediol,
1,3-butanediol, 1,4-butanediol, trimethylene glycol
(1,3-propanediol), 1,5-pentanediol, 1,6-hexanediol, and
bis(2-hydroxyethyl) ether; polyhydric alcohols such as glycerol,
pentaerythritol, dipentaerythritiol, tripentaerythritol,
trimethylolethane, trimethylolpropane, ditrimethylolpropane,
1,3-dihydroxyacetone dimer, sorbitol, and mannitol; phenols such as
phenol, 1-hydroxynaphthalene, 1,2-dihydroxynaphthalene,
hydroquinone, catechol, resorcinol, phloroglucinol
(1,3,5-trihydroxybenzene), p-cresol, vitamin E, 2-nitrophenol,
2,4-dinitrophenol, picric acid (2,4,6-trinitrophenol),
4-chlorophenol, 2-bromophenol, 2-iodophenol,
2,4,5-tricholorophenol, chlorohydroquinone, pentachlorophenol;
sugars such as glucose, mannose, allose, altrose, idose, gulose,
galactose, talose, ribose, arabinose, xylose, lyxose, erythrose,
threose, glyceraldehyde, fructose, ribulose, lactose, maltose, and
sucrose; hydroxy aldehydes such as 2-hydroxybutyraldehyde,
4-hydroxybenzaldehyde, and 2,4-dihydroxybenzaldehyde; hydroxy
ketones such as hydroxyacetone, 1-hydroxy-2-butanone,
2',4'-dihydroxyacetophenone, benzoin, and 3-hydroxy-2-butanone;
hydroxy acids such as citric acid, gluconic acid, 3-hydroxybutyric
acid, 2-hydroxycinnamic acid, and salicylic acid (2-hydroxybenzoic
acid); and hydroxy esters such as ascorbic acid, TWEEN 20
(polyoxyethylene (20) sorbitan monolaurate), methyl salicylate,
methyl 3-hydroxybenzoate, and methyl 2-hydroxyisobutyrate.
[0048] Component (D) is present in an effective amount in the
silicone composition of the present invention. As used herein, the
term "effective amount" means that the concentration of component
(D) is such that the silicone composition cures to form a silicone
adhesive having improved electrical conductivity, initial contact
resistance and/or volume resistivity, compared with a similar
silicone composition lacking only the hydroxy-functional organic
compound. Typically, the concentration of component (D) is such
that the adhesive exhibits at least about a ten-fold improvement in
either contact resistance or volume resistivity, as determined
using the methods in the Examples below. The concentration of
component (D) is typically from about 0.1 to about 3 percent by
weight and preferably from about 0.5 to about 1.5 percent by
weight, based on the total weight of the composition. When the
concentration of component (D) is less than about 0.1 percent by
weight, the silicone adhesive typically does not exhibit improved
electrical conductivity. When the concentration of component (D) is
greater than about 3 percent by weight, the silicone adhesive
typically does not exhibit further substantial improvement in
electrical conductivity. The effective amount of component (D) can
be determined by routine experimentation using the methods in the
Examples below.
[0049] Methods of preparing hydroxy-functional organic compounds
suitable for use in the silicone composition of the present
invention are well known in the art; many of these compounds are
commercially available.
[0050] Component (E) of the present invention is at least one
condensation catalyst comprising a metal salt of a carboxylic acid,
which promotes the condensation reaction of component (A) with
component (B). As used herein, the term "metal salt of a carboxylic
acid" includes salts containing hydrocarbon groups attached to the
metal. The carboxylate component of the metal salt can be derived
from a monocarboxylic acid or a dicarboxylic acid having from 2 to
20 carbon atoms. The metal ion component of the metal salt is
typically derived from lead, tin, nickel, cobalt, antimony, iron,
cadmium, chromium, zirconium, zinc, manganese, aluminum, and
titanium. Preferably, the metal ion in the metal salt is derived
from tin, lead, or zinc. Examples of metal salts of carboxylic
acids include, but are not limited to, lead naphthenate, cobalt
naphthenate, zinc naphthenate, tin naphthenate, lead octoate, tin
octoate, zinc octoate, iron stearate, tin oleate, chromium octoate,
antimony octoate, tin butyrate, dibutyltin diacetate, dibutyltin
dioctoate, dibutyltin dilaurate, dibutyltin adipate, dibutyltin
dibenzoate, dibutyltin dilactate, lead sebacate, and zirconium
acetylacetonate. The tin and lead salts are particularly preferred
because they are generally soluble in the organopolysiloxane,
Component (A), and have high catalytic activity.
[0051] The concentration of component (E) is sufficient to catalyze
the condensation reaction of components (A) and (B). The exact
amount of component (E) depends on the desired extent and rate of
cure, the activity of the catalyst, the type of crosslinking agent,
and the nature of the other components in the silicone composition.
The concentration of component (E) is typically from 0.1 to 10
percent by weight and preferably from 0.5 to 5 percent by weight,
based on the weight of the organopolysiloxane, component (A).
[0052] The silicone composition of the present invention can also
comprise additional ingredients, provided the ingredient does not
prevent the composition from curing to form an adhesive having
improved contact resistance and/or volume resistivity compared with
a similar silicone composition lacking only the hydroxy-functional
organic compound. Examples of additional ingredients include, but
are not limited to, adhesion promoters, solvents,
organopolysiloxane resins, precrosslinked silicone elastomer
particulates, and non-functional silicone fluids.
[0053] The silicone composition of the present invention can
further comprise at least one adhesion promoter that effects strong
unprimed adhesion of the silicone composition to substrates
commonly employed in the construction of electronic devices; for
example, silicon; passivation coatings, such as silicon dioxide and
silicon nitride; glass; metals, such as copper and gold; ceramics;
and organic resins, such as polyimide. The adhesion promoter can be
any adhesion promoter typically employed in condensation-curable
silicone compositions, provided it does not adversely affect cure
of the composition or the physical properties of the silicone
adhesive, particularly contact resistance and volume
resistivity.
[0054] Examples of adhesion promoters suitable for use in the
silicone composition of the present invention include, but are not
limited to, amino-functional alkoxysilanes such as
H.sub.2N(CH.sub.2).sub.3Si(OCH.sub- .3).sub.3,
H.sub.2N(CH.sub.2).sub.3Si(OCH.sub.2CH.sub.3).sub.3,
H.sub.2N(CH.sub.2).sub.3Si(OCH.sub.2CH.sub.2OCH.sub.3).sub.3,
H.sub.2N(CH.sub.2).sub.4Si(OCH.sub.3).sub.3,
H.sub.2NCH.sub.2CH(CH.sub.3)-
CH.sub.2CH.sub.2SiCH.sub.3(OCH.sub.3).sub.2,
H.sub.2N(CH.sub.2).sub.2NH(CH- .sub.2).sub.3Si(OCH.sub.3).sub.3,
H.sub.2N(CH.sub.2).sub.2NH(CH.sub.2).sub-
.3Si(OCH.sub.2CH.sub.2OCH.sub.3).sub.3,
CH.sub.3NH(CH.sub.2).sub.2NH(CH.su- b.2).sub.3 Si(OCH.sub.3).sub.3,
H.sub.2N(CH.sub.2).sub.2NH(CH.sub.2).sub.3-
Si(CH.dbd.CH.sub.2)(OCH.sub.3).sub.2; epoxy-functional
alkoxysilanes such as 3-glycidoxypropyltrimethoxysilane,
1,2-epoxy-4-(2-trimethoxysilylethyl- )cyclohexane, and
1,2-epoxy-2-methyl-4-(1-methyl-2-trimethoxysilylethyl)cy-
clohexane; reaction products of at least one amino-functional
alkoxysilane and at least one epoxy-functional alkoxysilane such as
reaction products of 3-glycidoxypropyltrimethoxysilane and
3-aminopropyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane
and [3-(2-aminoethyl)aminopropyl]trimet- hoxysilane,
1,2-epoxy-4-(2-trimethoxysilylethyl)cyclohexane and
3-aminopropyltrimethoxysilane,
1,2-epoxy-4-(2-trimethoxysilylethyl)cycloh- exane and
[3-(2-aminoethyl)aminopropyl]trimethoxysilane,
1,2-epoxy-2-methyl-4-(1-methyl-2-trimethoxysilylethyl)cyclohexane
and 3-aminopropyltrimethoxysilane, and
1,2-epoxy-2-methyl-4-(1-methyl-2-trime- thoxysilylethyl)cyclohexane
and [3-(2-aminoethyl)aminopropyl]trimethoxysil- ane; and vinyl
trialkoxysilanes such as (CH.sub.3O).sub.3SiCH.dbd.CH.sub.2- ,
(CH.sub.3CH.sub.2O).sub.3SiCH.dbd.CH.sub.2, (CH.sub.3
CH.sub.2CH.sub.2O).sub.3SiCH.dbd.CH.sub.2, (CH.sub.3CH.sub.2
CH.sub.2 CH.sub.2O).sub.3SiCH.dbd.CH.sub.2, and
(CH.sub.3OCH.sub.2CH.sub.2O).sub.3- SiCH.dbd.CH.sub.2.
[0055] The concentration of the adhesion promoter in the
composition of the present invention is sufficient to effect
adhesion of the composition to a substrate, such as those cited
above. The concentration can vary over a wide range depending on
the nature of the adhesion promoter, the type of substrate, and the
desired adhesive bond strength. The concentration of the adhesion
promoter is generally from 0.01 to about 10 percent by weight,
based on the total weight composition. However, the optimum
concentration of the adhesion promoter can be readily determined by
routine experimentation.
[0056] Methods of preparing amino-functional alkoxysilanes are well
known in the art as exemplified in U.S. Pat. No. 3,888,815 to
Bessmer et al. Methods of preparing epoxy-functional alkoxysilanes,
such as the hydrosilylation addition reaction of alkenyl-containing
epoxysilanes with trialkoxysilanes, and methods of preparing vinyl
trialkoxysilanes, such as the reaction of vinyltrichlorosilane with
alcohols, are also well known in the art. Reaction products of
amino-functional alkoxysilanes and epoxy-functional alkoxysilanes
can be prepared using well known methods of reacting
epoxy-containing compounds with amines. The reaction is typically
carried out using about a 1:1 mole ratio of epoxy groups in the
epoxy-functional alkoxysilane to nitrogen-bonded hydrogen atoms in
the amino-functional alkoxysilane. The two compounds can be reacted
either in the presence of an inert organic solvent, such as
toluene, or in the absence of a diluent. The reaction can be
carried out at room temperature or an elevated temperature, for
example, from about 50 to about 100.degree. C.
[0057] The silicone composition of the present invention can
further comprise an appropriate quantity of a solvent to lower the
viscosity of the composition and facilitate the preparation,
handling, and application of the composition. Preferably, the
solvent has a normal boiling point greater than the cure
temperature of the composition. Otherwise excessive volatilization
of the solvent during curing may cause formation of voids in the
silicone adhesive. Examples of suitable solvents include, but are
not limited to, saturated hydrocarbons having from 1 to about 20
carbon atoms; aromatic hydrocarbons such as xylenes; mineral
spirits; halohydrocarbons; esters; ketones; silicone fluids such as
linear, branched, and cyclic polydimethylsiloxanes; and mixtures of
such solvents. The optimum concentration of a particular solvent in
the present silicone composition can be readily determined by
routine experimentation.
[0058] The silicone composition of the present invention can also
comprise at least one organopolysiloxane resin consisting
essentially of R.sup.3.sub.3SiO.sub.1/2 siloxane units and
SiO.sub.4/2 siloxane units wherein each R.sup.3 is independently
selected from monovalent hydrocarbon and monovalent halogenated
hydrocarbon groups having from 1 to 20 carbon atoms and the mole
ratio of R.sup.3.sub.3SiO.sub.1/2 units to SiO.sub.4/2 units in the
organopolysiloxane resin is from 0.65 to 1.9.
[0059] When the silicone composition further comprises an
organopolysiloxane resin, the concentration of the crosslinking
agent is typically sufficient to provide from 0.8 to 10
silicon-bonded --OR.sup.2 or --OCH.sub.2CH.sub.2OR.sup.2 groups and
preferably from 1 to 3 silicon-bonded --OR.sup.2 or
--OCH.sub.2CH.sub.2OR.sup.2 groups, per silicon-bonded hydroxy
group in component (A) and the organopolysiloxane resin
combined.
[0060] The concentration of the organopolysiloxane resin in the
silicone composition of the present invention is typically from 5
to 90 parts by weight per 100 parts by weight of component (A) and
the resin combined.
[0061] The organopolysiloxane resin of the present invention can be
prepared by well known methods in the art as exemplified in U.S.
Pat. No. 2,676,182 to Daudt et al.
[0062] The silicone composition of the present invention can
further comprise at least one precrosslinked silicone elastomer
particulate. The silicone particulate provides the cured
composition with good elasticity, low hardness, and a low permanent
compression set. The particulate is a powder of a precrosslinked
silicone elastomer consisting of spherical or irregularly shaped
particles. The particles typically have an average particle size of
from 0.1 to 500 .mu.m and preferably have an average particle size
of from 0.5 to 300 .mu.m.
[0063] The concentration of the precrosslinked silicone elastomer
particulate in the silicone composition of the present invention is
typically from 10 to 150 parts by weight and is preferably from 15
to 80 parts by weight, per 100 parts by weight of components (A)
and (B) combined.
[0064] Several methods of preparing precrosslinked silicone
elastomer particulates are known in the art, including spray drying
and curing a curable organopolysiloxane composition as disclosed in
Japanese Patent Application No. 59-96122; spray drying an aqueous
emulsion containing a curable organopolysiloxane composition as
disclosed in U.S. Pat. No. 4,761,454; curing an emulsion of a
liquid silicone rubber microsuspension as disclosed in U.S. Pat.
No. 5,371,139; and pulverizing a precrosslinked silicone
elastomer.
[0065] The composition of the present invention can also comprise
at least one non-functional silicone fluid. As used herein, the
term "non-functional" means that the silicone fluid is free of
functional groups that react with components (A), (B), or (D) under
normal cure conditions. The silicone fluid can be used to alter the
viscosity of the present silicone composition, depending on the
requirements for a particular application. Moreover, the silicone
fluid reduces the cost of the silicone composition. The structure
of the silicone fluid is typically linear, branched, or cyclic,
preferably linear or branched, and more preferably linear. Also,
the silicone fluid typically has a viscosity at 25.degree. C. of
from about 0.05 to about 200 Pa.multidot.s and preferably has a
viscosity of from about 2 to about 100 Pa.multidot.s.
[0066] Examples of silicone fluids suitable for use in the
composition of the present invention include, but are not limited
to linear silicone fluids such as the trimethylsiloxy-terminated
dimethylsiloxane fluids sold by Dow Corning Corporation under the
trade name "Dow Corning.RTM. 200 Fluids" and branched silicone
fluids such as Me.sub.3SiO[(OSiMe.sub.3- ).sub.2SiO]SiMe.sub.3 and
Me.sub.3SiO[(OSiMe.sub.3)MeSiO]SiMe.sub.3.
[0067] The concentration of the non-functional silicone fluid in
the composition of the present invention is typically up to about
20 parts by weight, preferably from about 5 to 15 parts by weight,
per 100 parts by weight of component (A). The optimum concentration
of the non-functional silicone fluid for a particular application
can be readily determined by routine experimentation.
[0068] The silicone composition of the present invention can be a
one-part composition comprising components (A) through (E) in a
single part or, alternatively, a multi-part composition comprising
components (A) through (E) in two or more parts, provided neither
component (A) nor component (D), are present with components (B)
and (E) in the same part. For example, a multi-part silicone
composition can comprise a first part containing all of component
(A), a portion of component (C), all of component (D), and a second
part containing the remaining portion of component (C), and all of
components (B) and (E).
[0069] The one-part silicone composition of the instant invention
is typically prepared by combining components (A) through (E) and
any optional ingredients in the stated proportions at ambient
temperature with or without the aid of a solvent, which is
described above. Although the order of addition of the various
components is not critical if the silicone composition is to be
used immediately, the condensation catalyst is preferably added
last at a temperature below about 30.degree. C. to prevent
premature curing of the composition. Also, the multi-part silicone
composition of the present invention can be prepared by combining
the particular components designated for each part. Mixing can be
accomplished by any of the techniques known in the art such as
milling, blending, and stirring, either in a batch or continuous
process. The particular device is determined by the viscosity of
the components and the viscosity of the final silicone
composition.
[0070] The silicone composition of the present invention can be
applied to a wide variety of solid substrates including, but not
limited to, metals such as aluminum, gold, silver, tin-lead,
nickel, copper, and iron, and their alloys; silicon; fluorocarbon
polymers such as polytetrafluoroethylene and polyvinylfluoride;
polyamides such as Nylon; polyimides; polyesters; ceramics; and
glass. Furthermore, the silicone composition of the instant
invention can be applied to a substrate by any suitable means such
as spraying, syringe dispensing, screen or stencil printing, or ink
jet printing.
[0071] A silicone adhesive according to the present invention
comprises a reaction product of the silicone composition containing
components (A) through (E), described above. The silicone
composition of this invention can be cured at a temperature from
about room temperature to about 150.degree. C., preferably from
about 40 to about 100.degree. C., and more preferably from about 50
to about 80.degree. C., for a suitable length of time. For example,
the present silicone composition can be cured in about 10 to 20
hours at room temperature and in less than about 16 hours at
70.degree. C.
[0072] The silicone composition of the present invention has
numerous advantages, including good flow, low VOC (volatile organic
compound) content, and adjustable cure. Moreover, the present
silicone composition cures to form a silicone adhesive having good
adhesion and unexpectedly superior electrical properties.
[0073] With regard to flow, the present silicone composition
possesses the rheological properties required for a number of
applications and is easily dispensed and applied using standard
equipment.
[0074] Furthermore, absent optional solvent, the silicone
composition of the present invention has a very low VOC content.
Consequently, the composition avoids the health, safety, and
environmental hazards associated with solvent-borne silicone
compositions. In addition, the solventless composition of the
present invention typically undergoes less shrinkage during curing
than solvent-borne silicone compositions.
[0075] The silicone composition of the present invention cures
rapidly at moderately elevated temperatures. Moreover, the cure
rate of the silicone composition can be conveniently adjusted by
regulating the type and amount of both the catalyst and
crosslinking agent and the cure temperature.
[0076] Further, the silicone composition of the present invention
cures to form a silicone adhesive having good adhesion to a wide
variety of materials, including metals, glass, silicon, silicon
dioxide, ceramics, polyesters, and polyimides.
[0077] Importantly, the silicone composition of the present
invention cures to form a silicone adhesive having unexpectedly
improved electrical conductivity, as evidenced by a low initial
contact resistance and/or volume resistivity, compared with a
similar silicone composition lacking only the hydroxy-functional
organic compound.
[0078] The silicone composition of the present invention is useful
for preparing an electrically conductive silicone adhesive. The
silicone adhesive of the present invention has numerous uses,
including die attach adhesives, solder replacements, and
electrically conductive coatings and gaskets. In particular, the
silicone adhesive is useful for bonding electronic components to
flexible or rigid substrates.
EXAMPLES
[0079] The following examples are presented to further illustrate
the silicone composition of this invention, but are not to be
considered as limiting the invention, which is delineated in the
appended claims. Unless otherwise noted, all parts and percentages
reported in the examples are by weight. The following methods and
materials were employed in the examples:
[0080] The Contact resistance of a silicone adhesive on copper was
determined using a Keithley Instruments Model 580 Micro-ohm Meter
equipped with a 4-pole probe having spring-loaded, gold-plated,
chisel point tips. A contact resistance joint was prepared by
bonding two rectangular copper bars (0.254 cm.times.0.254
cm.times.2.032 cm) with the silicone composition according to the
following procedure: One face (rectangular) of each copper bar was
cleaned by sanding with 400 grit silicon carbide waterproof
sandpaper, scrubbing with a Kimwipe dampened with heptane followed
by a Kimwipe dampened with isopropanol, and air drying at room
temperature for at least one hour. A small aliquot of the silicone
composition was applied at approximately the center (lengthwise) of
one bar. A second bar was then oriented perpendicular to the first
bar with the center (lengthwise) of each bar facing the other and
the silicone composition forming a bondline of 0.025 cm. Finally,
the cross-shaped (+) fixture was cured in a forced air oven at
70.degree. C. for sixteen hours. After allowing the sample to cool
to room temperature, the initial contact resistance of the joint
was measured. The reported values for contact resistance, expressed
in units of ohms, represent the average of three measurements, each
performed on identically prepared test specimens.
[0081] The volume resistivity of a silicone adhesive was determined
using a Keithley Instruments Model 580 Micro-ohm Meter equipped
with a four-point probe having spring-loaded, gold-plated,
spherical tips. A test specimen was prepared by first placing two
strips of 3M Scotch brand tape 0.25 cm apart on a glass microscope
slide to form a channel extending the length of the slide. An
aliquot of the silicone composition was deposited at one end of the
slide and over the channel. The composition was then spread over
the entire channel by drawing a razor blade through the composition
and across the surface at an angle of 45.degree.. The tape strips
were removed and the specimen was cured in a forced air oven at
70.degree. C. for sixteen hours. After allowing the sample to cool
to room temperature, the voltage drop between the two inner probe
tips was measured at an appropriate current to give a resistance
value in ohms. The initial volume resistivity of the adhesive was
then calculated using the following equation:
V=R(W.times.T/L)
[0082] where V is volume resistivity in ohm-centimeters, R is
resistance (ohms) of the adhesive measured between two inner probe
tips spaced 2.54 cm apart, W is the width of the adhesive layer in
centimeters, T is the thickness of the adhesive layer in
centimeters, and L is the length of the adhesive layer between the
inner probes in centimeters (2.54 cm). The thickness of the
adhesive layer, typically about 0.004 cm, was determined using an
Ames Model LG3500-0-04 thickness gauge. The reported values for
volume resistivity, expressed in units of ohm-centimeters,
represent the average of three measurements, each performed on an
identically prepared test specimen.
[0083] Silicone Base: a mixture consisting of 12% of a
hydroxy-terminated polydimethylsiloxane having a viscosity of about
50,000 mm.sup.2/s; 22% of a hydroxy-terminated polydimethylsiloxane
having a viscosity of about 10,000 mm.sup.2/s; 0.8% of a
trimethylsiloxy-terminated polydimethylsiloxane having a viscosity
of about 350 mm.sup.2/s; 35% of a trimethylsiloxy-terminated
polydimethylsiloxane having a viscosity of about 325 mm.sup.2/s;
23% of a hexamethyldisilazane-treated precipitated silica; 6% of
zirconium silicate (ZrSiO.sub.4) having a particle size of 5 .mu.m;
and 0.6% of water.
[0084] Curing Agent: a mixture consisting of 80% of a
trimethylsiloxy-terminated polydimethylsiloxane having a viscosity
of about 50 mm.sup.2/s, 11% of ethyl polysilicate, 6% of
phenyltrimethoxysilane, 2% of tetraethyl orthosilicate, and 1% of
dibutyltin dilaurate.
[0085] Adhesion Promoter: a mixture consisting of 98% of a reaction
product of 3-glycidoxypropyltrimethoxysilane and
3-aminopropyltrimethoxys- ilane, and 2% methanol. The reaction
product was prepared by reacting 3 parts of
3-glycidoxypropyltrimethoxysilane with 1 part of
3-aminopropyltrimethoxysilane at 50.degree. C. for 10 hours.
[0086] Filler: a silver flake sold under the name SF-22 by Degussa
Corporation. The filler has a tap density of 3.5 g/cm.sup.3; a
surface area of 1.07 m.sup.2/g; weight loss of 0.05% at 110.degree.
C.; weight loss of 0.45% at 538.degree. C.; and a particle size
distribution of 9.7 .mu.m (95%), 7.5 .mu.m (90%), 3.0 .mu.m (50%),
and 1.0 .mu.m (10%).
[0087] TERGITOL TMN-6: a nonionic surfactant commercially available
from Union Carbide Corporation. The surfactant consists of 87% of
2,6,8-trimethyl-4-nonyloxypoly-ethyleneoxyethanol (reaction product
of 2,6,8-trimethyl-4-nonanol and ethylene oxide), 10% of water, 2%
of polyethylene glycol, and 1% of 2,6,8-trimethyl-4-nonanol.
Comparative Example 1
[0088] A silicone composition was prepared by first mixing 18.00
parts of Silicone Base, 1.82 parts of Curing Agent, and 0.23 part
of Adhesion Promoter in a 1 oz. plastic cup. The components were
then mixed for 26 seconds using an AM 501 Hauschild dental mixer.
Then 79.95 parts of Filler was added to the mixture and the
components were blended for 26 seconds. The mixture was cooled to
room temperature by immersing the cup in a water bath and the
aforementioned mixing and cooling procedure was repeated. The
contact resistance (copper) and volume resistivity of the cured
silicone adhesive are shown in Table 1.
Examples 1-3
[0089] In each of Examples 1-4, a silicone composition was prepared
by combining the hydroxy-functional organic compound specified in
Table 1 with a sample of the silicone composition prepared in
Comparative Example 1, according to the following procedure: In a
1/4 oz. plastic cup, 0.04 part (1%) of the hydroxy-functional
organic compound was added to 4.00 parts of the silicone
composition in Comparative Example 1. The components were mixed for
26 seconds using an AM 501 Hauschild dental mixer. The mixture was
then cooled to room temperature and the aforementioned mixing and
cooling procedure was repeated. The contact resistance (copper) and
volume resistivity values of the cured silicone adhesives are shown
in Table 1.
1TABLE 1 Contact Volume Hydroxy-Functional Resistance Resistivity
Example Organic Compound (.OMEGA.) (.OMEGA. .multidot. cm) Comp. 1
-- 4.03 2.03 .times. 10.sup.-2 1 glycerol 7.83 .times. 10.sup.-4
4.35 .times. 10.sup.-4 2 Tergitol TMN-6 4.01 2.18 .times. 10.sup.-3
3 vitamin E 2.03 .times. 10.sup.2 8.52 .times. 10.sup.-4 4
1,3-dihydroxyacetone dimer .sup. 2.90 .times. 10.sup.-2 1.75
.times. 10.sup.-1
* * * * *