Silicone rubber compositions and cured products thereof

A silicone rubber composition comprising (A) a vinyl-containing diorganopolysiloxane, (B) an organohydrogenpolysiloxane, and (C) a platinum catalyst is improved in adhesion to metallic, ceramic and plastic substrates by blending therein (D) an alkoxyhydrogensiloxane and (E) a polysiloxane having at least one epoxy group attached to a silicon atom through a carbon atom.

This invention relates to silicone rubber compositions having, improved 
self adhesion which are suitable as protective coatings and adhesive 
,compositions for electric and electronic parts as well as cured products 
thereof. 
BACKGROUND OF THE INVENTION 
Silicone rubber compositions of the type comprising vinylpolysiloxane and 
hydrogen polysiloxane which are heat cured through hydrosilation in the 
presence of a platinum catalyst are known in the art. Because of their 
self adhesion, they are often used as protective coating compositions for 
electric and electronic parts and adhesive as compositions for bonding 
electric and electronic parts to substrates. The prior art silicone rubber 
compositions of this type, however, are not necessarily fully adhesive to 
various substrates, especially to metallic, ceramic, and plastic 
substrates. 
It was proposed in Japanese Patent Publication Nos. 13508/1978 and 
5836/1982 to add various adhesive aids such as hydrogenpoly-siloxanes 
having an epoxy or alkoxy group, alkoxysilanes or hydrolyzates thereof to 
these silicone rubber compositions in order to improve their adhesion to 
various substrates. Despite the addition of these adhesive aids, it is 
necessary to cure these silicone rubber compositions at minimum 
temperatures of 100.degree. to 120.degree. C. in order to obtain improved 
adhesion to the substrate. This is because the adhesion of such 
compositions depends largely on the curing temperature. Thus, curing at 
relatively low temperatures of less than 100.degree. C. often results in 
unsatisfactory adhesion. Therefore, if the silicone rubber compositions 
used as adhesives for electric and electronic parts are cured at 
relatively low temperatures, there can be gaps in the adhesion between the 
composition and the part. These non-adhered gaps can allow moisture and 
contaminants to penetrate which results in part corrosion and deteriorated 
insulation. 
In addition, low-temperature bonding requirements are increasing in recent 
years as a new application or for energy saving. There is a need for a 
silicone rubber composition capable of low-temperature curing to provide 
improved adhesion. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a new and improved 
silicone rubber composition having improved self adhesion so that it may 
be effectively cured to various substrates at relatively low temperatures. 
The inventors have discovered that a silicone rubber composition comprising 
(A) a vinyl-containing diorganopoly-siloxane having at least two CH.sub.2 
.dbd.CH--Si.tbd. linkages in a molecule thereof, (B) an 
organohydrogenpolysiloxane having at least two hydrogen atoms directly 
attached to silicon atoms in a molecule thereof, (C) platinum or a 
platinum compound can be improved in adhesion by blending therein (D) an 
alkoxyhydrogensiloxane of the general formula: 
##STR1## 
wherein R.sup.1 is independently selected from substituted or 
unsubstituted monovalent hydrocarbon groups free of an aliphatic 
unsaturated bond and letter n is equal to 0 or a positive integer, and (E) 
a polysiloxane having at least one epoxy group attached to a silicon atom 
through a carbon atom directly attached to the silicon atom in a molecule. 
More particularly, the co-presence of adhesive modifier compounds (D) and 
(E) in a platinum-catalyzed curing reaction system comprising a 
vinyl-containing diorganopolysiloxane and an organohydrogenpolysiloxane 
allows the system to be cured to various substrates at temperatures of 
100.degree. C. or lower within relatively short times, thereby achieving 
firm bond to the substrates. This silicone rubber composition has improved 
self adhesion to various substrates including metallic, ceramic and 
plastic substrates and is thus suitable for use as protective coating and 
adhesive compositions for electric and electronic parts. 
Briefly stated, the present invention provides a silicone rubber 
composition comprising components (A) to (C) as defined above, (D) an 
alkoxyhydrogensiloxane of formula (1), and (E) an epoxy-containing 
polysiloxane having at least one epoxy group attached to a silicon atom 
through a carbon atom directly attached to the silicon atom in a molecule, 
and cured products thereof. 
DETAILED DESCRIPTION OF THE INVENTION 
Component (A) forming the silicone rubber composition of the present 
invention is a vinyl-containing diorganopolysiloxane having at least two 
CH.sub.2 .dbd.CH--Si.tbd. linkages in a molecule thereof, preferably at 
either molecular chain end. In the vinyl-containing diorganopolysiloxane, 
vinyl groups may be present solely at both ends, or at both ends and an 
intermediate position or positions of a molecule thereof. Preferred are 
vinyl-containing diorganopolysiloxanes of the general formula (2): 
##STR2## 
wherein R.sup.2, which may be identical or different, is independently 
selected from substituted or unsubstituted monovalent hydrocarbon groups 
free of an aliphatic unsaturated bond, letter 1 is equal to 0 or a 
positive integer, and m is equal to 0 or a positive integer. 
In formula (2), substituent R.sup.2 is a substituted or unsubstituted 
monovalent hydrocarbon group free of an aliphatic unsaturated bond, 
preferably having 1 to 10 carbon atoms, more preferably 1 to 7 carbon 
atoms. Examples of substituent R.sup.2 include lower alkyl groups such as 
methyl, ethyl, propyl, and butyl groups; aryl groups such as phenyl, 
tolyl, and xylyl groups; aralkyl groups such as benzyl and phenylethyl 
groups; cycloalkyl groups such as cyclohexyl; and substituted ones thereof 
in which some or all of the hydrogen atoms are replaced by halogen atoms 
or cyano groups, for example, chloromethyl, cyanoethyl, and 
3,3,3-trifluoropropyl groups. Letter 1 is equal to 0 or a positive 
integer, and m is equal to 0 or a positive integer, preferably 
0&lt;1+m.ltoreq.10,000, more preferably 0&lt;1+m.ltoreq.2,000 and 
0.ltoreq.m/(1+m).ltoreq.0.2. 
The diorganopolysiloxanes of formula (2) preferably have a viscosity of 10 
to 1,000,000 centistokes at 25.degree. C. 
Component (B) is an organohydrogenpolysiloxane having at least two hydrogen 
atoms directly attached to silicon atoms in a molecule thereof. Preferred 
are organohydrogen-polysiloxanes having at least two hydrogen atoms 
directly attached to silicon atoms in a molecule thereof as represented by 
the general formula (3): 
EQU H.sub.a R.sup.3.sub.b SiO(4-a-b)/2 (3) 
wherein R.sup.3, which may be identical or different, is independently 
selected from substituted or unsubstituted monovalent hydrocarbon groups 
free of an aliphatic unsaturated bond, and letters a and b are: 0&lt;a&lt;2, 
1.ltoreq.b .ltoreq.2, and 1.ltoreq.a+b.ltoreq.3. 
In formula (3), substituent R.sup.3 is a substituted or unsubstituted 
monovalent hydrocarbon group free of an aliphatic unsaturated bond, 
preferably having 1 to 10 carbon atoms, more preferably 1 to 7 carbon 
atoms. Examples of substituent R.sup.3 include lower alkyl groups such as 
methyl, ethyl, propyl and butyl, aryl groups such as phenyl and tolyl, 
cycloalkyl groups, and aralkyl groups as previously described for R.sup.2. 
Letters a and b are: 0&lt;a&lt;2, 1.ltoreq.b.ltoreq.2, and 
2.ltoreq.a+b.ltoreq.3, preferably 0.3.ltoreq.a.ltoreq.1 and 
2.ltoreq.a+b.ltoreq.2.7. 
These organohydrogenpolysiloxanes are generally prepared by hydrolysis of 
chlorosilanes such as R.sup.3 SiHCl.sub.2, R.sup.3.sub.3 SiCl, 
R.sup.3.sub.2 SiCl.sub.2, and R.sup.3.sub.2 SiHCl or by further 
equilibrating siloxanes resulting from such hydrolysis. Some illustrative, 
non-limiting examples of the organohydrogen-polysiloxane are given below. 
##STR3## 
The amount of component (B) or organohydrogenpoly. siloxane blended in the 
composition is not particularly limited although it is preferably blended 
in such proportion that the organohydrogenpolysiloxane provides 0.5 to 4 
hydrogen atoms, more preferably 2 to 4 hydrogen atoms per vinyl group in 
diorganopolysiloxane (A). 
Component (C) is a platinum catalyst effective for promoting addition 
reaction between silicon-attached vinyl groups of the vinyl-containing 
organopolysiloxane as component (A) and silicon-attached hydrogen atoms of 
the organohydrogenpolysiloxane as component (B). The platinum catalyst may 
be platinum or platinum compounds which are commonly used in conventional 
silicone rubber compositions of the type contemplated herein. 
Examples of the platinum catalyst include elemental platinum, H.sub.2 
PtCl.sub.6.nH.sub.2 O, NaHPtCl.sub.6.nH.sub.2 O, KHPtCl.sub.6.nH.sub.2 O, 
Na.sub.2 PtCl.sub.6.nH.sub.2 O, K.sub.2 PtCl.sub.6.nH.sub.2 O, 
PtCl.sub.4.nH.sub.2 O, PtCl.sub.2, Na.sub.2 PtCl.sub.4.nH.sub.2 O, and 
H.sub.2 PtCl.sub.4.nH.sub.2 O. Also useful are complexes of these platinum 
compounds with hydrocarbons, alcohols, and vinyl-containing cyclic 
siloxanes. 
The platinum catalyst is used in a catalytic amount, for example, in 
concentrations of 0.1 to 1000 ppm, preferably 0.1 to 100 ppm of platinum 
based on the total organopolysiloxanes of components (A) and (B). 
According to the present invention, a combination of (D) an 
alkoxyhydrogensiloxane of formula (1) and (E) an epoxy containing 
polysiloxane is blended as an adhesion modifier to an addition reaction 
type silicone rubber composition comprising components (A), (B) and (C) 
defined above. 
More particularly, component (D) is an alkoxyhydrogen-siloxane formula (1) 
which is free of a silicon-to-carbon bond. 
##STR4## 
In formula (1), R.sup.1, which may be identical or different, is 
independently selected from substituted or unsubstituted monovalent 
hydrocarbon groups free of an aliphatic unsaturated bond, preferably 
having 1 to 10 carbon atoms, more preferably 1 to 7 carbon atoms. Examples 
of substituent R.sup.1 include lower alkyl groups such as methyl, ethyl, 
propyl, and butyl groups; aryl groups such as phenyl, tolyl, and xylyl 
groups; aralkyl groups such as benzyl and phenylethyl groups; cycloalkyl 
groups such as cyclohexyl; and substituted ones thereof in which some or 
all of the hydrogen atoms are replaced by halogen atoms, for example, 
chloromethyl and 3,3,3-trifluoropropyl groups, with the lower alkyl groups 
being preferred. Letter n is equal to 0 or a positive integer, preferably 
from 0 to 20, more preferably from 0 to 6. The alkoxyhydrogensiloxane of 
formula (1) is sufficiently effective to attain the objects of the 
invention insofar as n is at least 0, but preferably in view of 
compatibility with the base fluid, n ranges from 0 to 6, more preferably n 
is equal to 0 or 1. 
The alkoxyhydrogensiloxanes may be synthesized, for example, by mixing an 
alkoxysilane of the general formula (4): 
##STR5## 
wherein R.sup.1 is as defined above with an alcohol of the general formula 
(5): 
EQU R.sup.1 OH (5) 
wherein R.sup.1 is as defined above, and adding dropwise pure water, 
preferably pure water in alcohol to the mixture with cooling whereby 
reaction readily proceeds without a catalyst. By varying the amount of 
water added, the molecular weight of the alkoxyhydrogensiloxane of formula 
(1), that is, the value of n can be controlled. The reaction mixture as 
such is ready for use as component (D). It is also possible to isolate 
respective components from the reaction mixture by distillation. 
Examples of the trialkoxysilane represented by formula (4) include 
trimethoxysilane, triethoxysilane, tri-n-propoxysilane, and 
tri-n-butoxysilane. Examples of the alcohol represented by formula (5) 
include methanol, ethanol, n-propanol, and n-butanol. 
The alkoxyhydrogensiloxanes of formula (1) may be used alone or in 
admixture of two or more as component (D) of the present composition. 
Component (E) is an epoxy-containing polysiloxane having at least one epoxy 
group attached to a silicon atom through a carbon atom directly attached 
to the silicon atom in a molecule. Preferred are polysiloxanes of the 
general formula (6): 
EQU R.sup.4.sub.c R.sup.5.sub.d SiO(.sub.4-c-d) /2 (6) 
wherein R.sup.4 is an epoxy-containing monovalent organic group selected 
from 
##STR6## 
wherein 
X is a divalent organic group having 1 to 6 carbon atoms, such as 
--(CH.sub.2).sub.1-6 --, --(CH.sub.2).sub.1-3 --O--(CH.sub.2).sub.1-3 --, 
R.sup.5 is a hydrogen atom or a monovalent hydrocarbon group as defined for 
R.sup.2 and R.sup.3, preferably having 1 to 10 carbon atoms, and 
letters c and d are 0&lt;c.ltoreq.1, 1.ltoreq.d&lt;3, 1.5.ltoreq.c+d.ltoreq.3, 
preferably 1.8.ltoreq.c+d.ltoreq.2.2. 
Exemplary polysiloxanes are cyclic or straight chain polysiloxanes having 
one or more cyclic or acyclic epoxy groups added thereto, and their 
typical examples are given below. 
##STR7## 
The amounts of components (D) and (E) blended in the composition are not 
particularly limited. Preferably, the alkoxyhydrogensiloxane (D) is used 
in an amount of 0.01 to 5%, more preferably 0.1 to 1% by weight and the 
epoxy-containing organopolysiloxane (E) is used in an amount of 0.1 to 5%, 
more preferably 0.2 to 2% by weight, based on the total weight of the 
vinyl-containing organopolysiloxane (A) and the organohydrogenpolysiloxane 
(B). 
In addition to components (A) to (E), the silicone rubber compositions of 
the invention may contain other additives such as reinforcing or extending 
inorganic fillers if desired. Examples of the reinforcing inorganic filler 
include fumed silica and fumed titanium dioxide, and examples of the 
extending inorganic filler include fillers commonly used in conventional 
silicone rubber compositions, such as ground quartz, calcium carbonate, 
calcium silicate, titanium dioxide, ferric oxide, and carbon black. The 
fillers need not be blended in the compositions of the invention. When 
blended, the fillers may be used in amounts of 0 to 200 parts by weight 
per 100 parts by weight in total of the remaining components of the 
composition. 
The silicone rubber composition of the present invention may be divided 
into two parts like conventional silicone rubber compositions and cured by 
combining the two parts although the composition can also be used as a 
single part if a minor amount of a curing retarder such as acetylene 
alcohol is added. 
In one preferred embodiment where the composition is divided into two 
parts, a first part is a blend of the vinyl-containing organopolysiloxane 
(A) with the platinum catalyst (C) and a second part is comprised of the 
organo-hydrogenpolysiloxane (B). The alkoxyhydrogensiloxane (D) and 
epoxy-containing organopolysiloxane (E) may be blended in either the first 
or second part and either together or separately. 
The silicone rubber compositions of the present invention cure under 
conditions as commonly used for conventional silicone rubber compositions. 
Advantageously, the compositions of the invention can be cured at low 
temperatures of up to about 100.degree. C., especially about 80.degree. to 
about 90.degree. C. for short times of about 1/2 to about 2 hours while 
achieving improved adhesion. Therefore, the compositions can be cured at 
relatively low temperatures to various substrates of electric or 
electronic parts to form protective coatings thereon or used as adhesives 
for bonding electric or electronic parts to various supports. 
There have been described silicone rubber compositions which can be cured 
to substrates or parts at relatively low temperatures within relatively 
short time to accomplish a firm bond while preventing the underlying parts 
from corrosion or insulation loss. Therefore, the compositions of the 
invention may find a wide variety of applications as protective coating 
and adhesive compositions for electric and electronic parts. The 
compositions are also applicable to less heat resistant parts and in new 
uses while providing energy and labor savings.

EXAMPLE 
Examples of the invention are given below by way of illustration and not by 
way of limitation. All parts are by weight. 
First, synthesis of the alkoxyhydrogensiloxane used in Examples is 
illustrated. 
Synthesis of alkoxyhydrogensiloxane 
A reactor was charged with 1 mol of trimethoxysilane and 0.5 mol of 
methanol. With ice cooling, 0.5 mol of pure water was added dropwise to 
the reactor, allowing reaction to take place. 
The reaction mixture was stripped at 120.degree. C. under atmospheric 
pressure. By gel chromatography (GC), it was found to be a mixture of 
alkoxyhydrogensiloxanes of the following formula: 
##STR8## 
wherein n is an integer of 0 to 6. 
EXAMPLE 1 and COMATIVE EXAMPLES 1-4 
A composition of Example 1 was prepared by blending and thoroughly 
agitating 100 parts of dimethylpolysiloxane having two methylvinylsiloxane 
units in a molecule thereof (viscosity 400 centistokes), 5.0 parts of 
methylhydrogen-polysiloxane containing 1.0 ml/100 gram of .tbd.SiH bond, 
0.05 parts of an octyl alcohol modified solution of chloroplatinic acid 
(platinum content 2% by weight), 30 parts of ground quartz, 0.15 parts of 
the alkoxyhydrogensiloxane mixture obtained in Synthesis, and 1.5 parts of 
an epoxy-containing siloxane of formula (7) obtained by partially adding 1 
mol of allyl glycidyl ether to 1 mol of 
1,3,5,7-tetramethylcyclotetrasiloxane. The composition was coated to five 
substrates shown in Table 1 to form a coating of 5 cm.times.2 cm.times.2 
mm (thick) thereon and then heated at 80.degree. C. for 2 hours for 
curing, obtaining specimens of Example 1. 
The specimens were examined by the following qualitative adhesion test. 
Adhesion test 
Using a micro-spatula, the cured coating was damaged and stripped from the 
substrate. Areas of cohesive failure and stripping were determined to 
evaluate the degree of adhesion according to the following criterion. 
O: good adhesion (cohesive failure&gt;80%) 
.DELTA.: partial adhesion (cohesive failure 20-80%) 
X: no adhesion (cohesive failure&lt;20%) 
Comparative compositions were prepared using approximately the same 
formulation and procedure as above. Comparative Example 1 was a 
composition which omitted the epoxy-containing siloxane, Comparative 
Example 2 was a composition which omitted the alkoxyhydrogensiloxane 
mixture, Comparative Example 3 was a composition which omitted both the 
epoxy-containing siloxane and trimethoxysilane hydrolyzate, and 
Comparative Example 4 was a composition which used 0.15 parts of 
trimethoxysilane instead of the alkoxyhydrogensiloxane mixture. Coated 
specimens were prepared from these compositions and subjected to the 
adhesion test. 
The results are shown in Table 1. 
TABLE 1 
______________________________________ 
E1 CE1 CE2 CE3 CE4 
______________________________________ 
Alkoxyhydrogen- 
yes yes -- -- trimethoxy- 
siloxane mixture silane 
Epoxy-containing 
yes -- yes -- yes 
siloxane 
Substrate 
Aluminum .largecircle. 
X X X .DELTA. 
Stainless steel 
.largecircle. 
X X X .DELTA. 
Nickel .largecircle. 
X X X X 
Silicon wafer 
.largecircle. 
X X X .DELTA. 
Glass .largecircle. 
.DELTA. X X .DELTA. 
______________________________________ 
As seen from Table 1, the organopolysiloxane composition of the invention 
(Example 1) adhered firmly to all the aluminum, stainless steel, nickel, 
silicon wafer and glass substrates. In contrast, the compositions which 
did not contain the alkoxyhydrogensiloxane mixture and/or epoxy-containing 
siloxane (Comparative Examples 1-3) did not adhere well to the metal 
substrates, and the composition which contained the epoxy-containing 
siloxane, but trimethoxysilane instead of the alkoxyhydrogensiloxane 
mixture (Comparative Example 4) did not adhere firmly to the substrates. 
EXAMPLE 2 and COMATIVE EXAMPLES 5-7 
A composition of Example 2 was prepared by blending and thoroughly 
agitating 50 parts of dimethylpolysiloxane having two methylvinylsiloxane 
units in a molecule thereof (viscosity 5000 centistokes), 50 parts of a 
copolymer consisting of SiO.sub.2 units, trimethylsiloxy units and 
dimethylvinylsiloxy units in a molar ratio of 1:1:0.15, 6.0 parts of 
methylhydrogenpolysiloxane containing 1.2 mol/100 gram of .tbd.SiH bond, 
0.05 parts of an octyl alcohol solution of chloroplatinic acid (platinum 
content 2% by weight), 0.05 parts of siloxane-modified acetylene alcohol, 
0.35 parts per 100 parts of the dimethylpolysiloxane of the 
alkoxyhydrogen. siloxane mixture obtained in Synthesis, and 2.0 parts of 
an epoxy-containing siloxane of formula (8). The composition was coated to 
nine substrates shown in Table 2 to form a coating of 5 cm.times.2 
cm.times.2 mm (thick) thereon and then heated at 100.degree. C. for 1 hour 
for curing, obtaining specimens of Example 2. The specimens were examined 
for adhesion as in Example 1. 
Comparative compositions were prepared using approximately the same 
formulation and procedure as above. Comparative Example 5 was a 
composition which omitted the epoxy-containing siloxane, Comparative 
Example 6 was a composition which omitted the alkoxyhydrogensiloxane 
mixture, and Comparative Example 7 was a composition which omitted both 
the epoxy-containing siloxane and alkoxy. hydrogensiloxane mixture. Coated 
specimens were prepared from these compositions and subjected to the 
adhesion test. 
The results are shown in Table 2. 
TABLE 2 
______________________________________ 
E2 CE5 CE6 CE7 
______________________________________ 
Alkoxyhydrogensiloxane mixture 
yes yes -- -- 
Epoxy-containing siloxane 
yes -- yes -- 
Substrate 
Aluminum .largecircle. 
X X X 
Stainless steel .largecircle. 
X X X 
Nickel .largecircle. 
X X X 
Silicon wafer .largecircle. 
.DELTA. .DELTA. 
X 
Glass .largecircle. 
.DELTA. .DELTA. 
X 
Polyester .largecircle. 
.DELTA. .DELTA. 
X 
Polyimide .largecircle. 
X X X 
Glass-reinforced epoxy 
.largecircle. 
.DELTA. .DELTA. 
X 
Phenolic resin .largecircle. 
.DELTA. .DELTA. 
X 
______________________________________ 
As seen from Table 2, the organopolysiloxane composition of the invention 
(Example 2) adhered firmly to all metallic substrates of aluminum, 
stainless steel and nickel, silicon wafer and glass substrates, and 
plastic substrates of polyester, polyimide, glass-reinforced epoxy resin 
and phenolic resin, as compared with the compositions which did not 
contain the alkoxyhydrogensiloxane mixture and/or epoxy-containing 
siloxane (Comparative Examples 5.7). 
EXAMPLE 3 
The alkoxyhydrogensiloxane mixture obtained in Synthesis example was 
fractionated by distillation, isolating the following compounds. 
##STR9## 
Two compositions were prepared using the same formulation and procedure as 
in Example 1 except that each of these isolated compounds was blended 
instead of the alkoxyhydrogensiloxane mixture. They were tested for 
adhesion to find equivalent results to the results of the mixture. 
EXAMPLE 4 
The procedure of Example 3 was repeated according to Example 2, finding 
equivalent results to the results of Example 2. 
The results of Examples 3 and 4 reveal that the alkoxy. hydrogensiloxane 
compounds, when used alone, are as effective for improving adhesion as the 
alkoxyhydrogen-siloxane mixture. 
Although some preferred embodiments have been described, many modifications 
and variations may be made thereto in the light of the above teachings. It 
is therefore to be understood that within the scope of the appended 
claims, the invention may be practiced otherwise than as specifically 
described.