Note: Descriptions are shown in the official language in which they were submitted.
Lo
UAI. CURING Silicone lo Ox PREPARING SUE
END DIELECTRIC SOFT-GFL COMPOSITIONS ll!ERFOF
.
This invention relates to curable polyorganosiloxane
(silicone) compositions particularly adapted for use in
electronic potting applications.
Buckles of their excellent thermal stability, low
temperature flexibility end high dielectric strength,
silicones have been used for petrol and ~ncarslllating
electrical devices such as integrated circuits. They are
lyrically used as elastomeric or gel-like materiels
providing shock, vibrational and turmoil stress
10 protection on fragile electronic components. References
relating generally to silicone gel encapsulant include
U.S. Patents 3,933,712; 4,072,635, 4,087,585; 4,271,425;
4,374,g67; and D. Dickson, Jr., Proceedings
Electric/Electronic into]. Con. 12, 92~1g75). See also
15 prior art description in lJ.S. Patent ~,37~,967. The
present commercially available potting silicones include
one component compositions such as U.S. Patent 4,271,425
which are cured by moisture, requiring from several hours
to days to complete the cure. Easter cures my be
20 obtained from two component systems such as those in U.S.
Patent 4,087,585 if elevated temperatures are provided.
The two-component systems which require a platinum
catalyst, however, are inhibited by organotin compounds,
sulfur, amine, urethanes and Unsaturated hydrocarbon
25 plasticizers Oil the substrate surfaces.
It is known that certain silicones containing
methacrylate or acrylate functional grollps may be cured
by ultraviolet light. U.S. Patents 4,201,808 and
4,348,454 and U.S. Patent Application GO AYE all
30 describe US curing compositions of silicones containing
methacrylates or acrylates for paper release coatings.
Ike compositions do not produce jelly-like or elastomeric
materiels.
I
Accordingly, it is desirable to have US curable
silicone composition which will produce an elastomeric or
soft gel consistency upon cut n no . Furthermore, i t i s
desirable that the composition be capable of clearing by
another mechanism in areas which art not readily
accessible by US irradiation.
Summary of The Invention
The present invention is a silicone composition
10 having the desired characteristics set forth above. Ike
composition cures to a soft gel or elastomeric
consistency in a matter of seconds or minutes noon
irradiation with US light. Furthermore, the composition
will moisture cure in shadow areas. The nester cure
15 capability, however, does not result in loss of the gel
or elastomeric properties.
The compositions of the invention comprise a mixture,
in parts by weight of:
pa) 30-100 parts of a reactive polyorganosiloxane
terminated with acrylic functional dialkoxysilyl
or diaryloxysilyl groups and containing 0.1-5
parts of a silicone moisture curing catalyst;
(b) 0-70 parts ox a trimethylsilyl terminated
silicone oil; and
I an effective amount no a photosensitizes.
The novel reactive polyorganosiloxanes utilized in
the inventive compositions and their method of
preparation are also part of the present invention.
So
~3~5
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Detailed Descrl~tlon of The Inventlorl
The principal ingredient in the formulations of the
invention are polyorganosiloxanes terminated with acrylic
functional dialkoxysilyl or diaxyloxysilyl groups. These
silicones may be represented by formula (1):
R O O R O O R
(1) C112=C-C-O-lR5-SiO~siO-~--Si-R5-0-C-C=C~12
I R R
wherein Al and R2 are organ groups includirlg alkyds, such
as methyl, ethyl etc., halo alkyds such as try-
fluoropropyl, substituted or unsubstituted Aurelius such as
bouncily or furl], and others such as vinyl, methacryloxy-
propel, methoxy, mercaptopropyl, hydrogen or Bunsen groups;
R is the Salle or dirferellt alkyd or aureole; R is If or Clue
alkyd; R5 is alkaline; and n is an integer greater thaJl or
equal to 80.
Preferably, Al and R2 are alkyd groups, most preferably
methyl. Phenol groups are also suitable but hill plainly
content silicones may excessively absorb Us irraciiation,
thereby reducing the cure through volume of tile initial US
cure. If R and R include functional groups capable of
cross linking under the US or moisture cure-conditions, such
as vinyl, methacryloxypropyl and methoxy, such groups should
not be present in more than I by repeat unit.
For optimurll moisture curing characteristics, I will !
preferably be alkyds which have 3 or less carbon atoms. For
optimum heat and hydrolytic stability of tile linkage, R
is preferably between 3 and ablate 10 callable atolls.
The nunnery of-repeat units in tile silicone of formula
(i) must be alto 80 or more in order to obtain the desired
gel or elastomeric properties in the cured materials.
though n does not have a theoretical limit, it will
preferably not exceed 1500. Gore than 1500 repeat units
gives a polymer which is difficult to process, apply and
cure.
The moisture curing catalysts in the inventive formula-
lion may be any method sulkily moisture cure catalyst
which does not adversely affect the acrylic (eke. initiating
or inhibiting cure of the acrylic group). referred moisture
catalysts are organotitanate catalysts such as tetraisopropylor-
thotitanate. The higher the amount of tile organotitarlate
catalyst, the faster the curing speed. Consequently, the
moisture cure speed may he tailored as desired. Typically
the moisture cure catalyst will be present at a level of
0.1%-5~ of the amount of reactive silicone of formula (1).
The silicone oils used in the formulations of the
invention are trimethylsilyl terminated polydimethyl-
selections having viscosity between about 100 and 5,000 cups.
these oils are employed as plasticizers to control the
texture and softness of the cured material and as delineates
to adjust the final viscosity of the composition. For I I
electrorlic potting compositions in which gel-like materials I '
are desired, the plasticizing silicone oil should be present ' ,
in tile range offbeat 30-70~ by weight of the composi.tioll.
Lower amounts of silicon oil produce composi.tiorls which
yield soft rubbery materials. Amounts of silicone oil in
excess of about 70~ produce materials which will foe even
after curirlg.
~;23~5
The final ingredient of the inventive compositions is a
photosensitizes. Photosensitizes which will initiate
curing of the acrylic functionalities are jell known in the
art. They include Hanson and substituted enjoins buoyancy-
phenone Michler's kitten, clialkoxybenzophenone diethox~ace-
tophenone, etc. any known photosensitizes can be used as
well as mixtures thereof without departillg from the irlvention
hereof. Further examples of such photosensitizes may be
found in the aforementioned prior patents and in SUP.
Pappas, TV Curing: Science and Technology Technology
Marketing Corp. (1978). The amount of photosensitizes
utilized in the inventive compositions will typically be in
the range of .1~-5% of the composition. Depending on the
characteristics of the particular photosensitizes however,
amounts outside of this range may be employed without
departing from the invention hereof so long as they perform
the function of rapidly and efficiently initiating polymer-
ization of the acrylic groups.
The photosensitizes ingredient may also be polymer
bound, e.g. as a portion of the Al or I groups of the
reactive silicone of formula (1). An example of polymer
bound photo initiators and techniques for producillg same may
be wound in lJ.S. rent 1 271 ~i25. appropriate modifications
of such techniques to produce polyliler bound silicones having
the dialkoxyacrylic-functional termirlal groups are within
the ordinary skill of persons in tulle silicone synthesis art.
The inventive compositions may also contain other
additives so long as they do not interfere with the lo and
moisture curing mechanisms. These include adhesiotl promoters
such as 2,3-epoxypropyltrimetlloxy-silane, trially]-S-triazine-
2,4,6(111,311 truly and others known to Tulsa skilled in
the art; fillers such as silica, microba]loon glass etc.
.
i
qL5
. . 6
Further examples of fillers usable to modify the texture of
the inventive compositions may be found in U.S. Patent
4,072,635 at Cot. 4, line 40 - Cot. 5, line 7. If the
application is for electronic devices, the additives should
not include any ionic species.
In electrorlic applications, ion trapping compoullds such
as crown ethers and cr~ptates may be useful for reducing
ionic conductivity. examples are 18-crown-6, 12-crown-4 and
karma. See also U.S. Patent 4,271,425 Weller the use of
crown ethers in conventional Rev silicone encapsulant is
described.
The reactive silicones of formula (1) are easily
prepared from silallol terminated silicones of formula to):
Al
(2) ilO-~-SiO~
R2 n
where Al and R2 are defined as in fornl-31a (1). Silal-ol
terminated dimethylsiloxanes of viscosity between about
500cps and 50,000cps will produce reactive silicones within
the desired molecular weight rheology previously specified.
For producing soft gel potting compounds sullenly terminated
dimethylsiloxanes of between about 600cst and 20,000cst are
preferred, more preferably between 600cst and 4000cst.
Silanols of formula (2) are reacted with Solon
compositions of formula (3)
O R
(3) (R Oozier OC-C=C~l2
Jo .
~4~5
where R3 I and R5 are also as defined as in formula 11) in
the presence of a condensation catalyst such- as an organ-
titan ate to produce the reactive silicones of formula (1).
Because of commercial availability and good reactivity of
both the alkoxy and the acrylic groups methacryloxytri-
methoxysilane is the preferred compound of formula (3)
The mole ratio of Solon to sullenly terminate; selection
is between 2 and 6 (1-3 moles of trialkoxysilane per equivalent
of sullenly hydroxyl). Ratios of moles Solon (3) to moles
sullenly (2) which are less than 2:1 produce golfed or
taffy-like materials which cannot be usefully employed in
the inventive compositions. As the molecular weights of the
sullenly increases the minimum level of Solon has boon
observed to increase slightly. Thus, a ratio of at least
15 2.2:1 is recorrmended when a 28 WOW sullenly terminated
dimethylsilc~;ane is used whereas a level of 2:1 is satisfactory
for a 12 MOE sullenly terminated dimethylsiloxane.
The titan ate catalyst is added in an amount of between
0.1 and I by weight. The preferred catalyst is tetraisopropanol
titan ate. The catalyst and reactallts are reacted between
10C and 200C preferably 60C to 120C. A vacuum is
applied during or after the reaction to remove substantially
all of the theoretical amount of R301~ which is liberated
from the reaction of the sullenly groups of silicone I with
Solon (3).
The silicone oil and photoil~iticltcr ingredients of the
inventive compositions may be added to the composition
before or after the reaction of silicone (2) and Solon (3).
After the reaction the titan ate catalyst level may be
adjusted to alter the moisture cure characteristics of the
composition.
... .
I
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The compositions of the inventiorl will typically
produce slightly softer materials if moisture cured compared
-to W cured samples of the same compositions. The moisture
cure is usually complete within 2~-72 hours. after Us cure
or completion of moisture cure the samples do not become
noticeably harder on prolonged agirlg.
Tile actual physical characteristics of the cured
products obtained from a given compositioll will depend on
the molecular weight of the reactive silicone of formula (1)
as well as on tile Matilda of cure employed and the amount of
oil. In genera] the higher the molecular weight of the
reacted silicone the softer tile cured product end the slower
the cure. it'll silicones of formula (1) which have a
molecular weight above about 35 000 the level of oil above
which plowable cured products are produced may be somewhat
less than 70~.
The following examples are illustrative of the involution.
Example 1
In a round-bottomed flask 50.0g of a hydroxyl terminated j
polydinlethylsiloxane of 680cst viscosity (0.3~ hydroxyl by
weight) 2.62g methacryloxypropyltrimethoxysilalle an 0.21g
tetraisopropylorthotitanate were reacted on a rotary evaporator
and loom lug pressure end fake for two hours. Tile viscosity
of the mixture increased for lie first flour arid then
decreased during the second hour. full vacuum
approximately Nina fig) was applied for gin additional Jo
hours at 80C. The flask was removed from the evaporate]
and flushed with nitrogen. Tile reactive silicone product
was a yellow liquid.
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.. g
The product was then used to formulate mixtures with
laxity or laxity trimethylsilyl terminated silicone oils.
The mixtures ranged from 0-90~ silicone oil. To eat}- of
these mixtures was added I by weight diethoxyacetophenone
as photo initiator. Samples of these compositions, about
1/4" thick, were then cured by 1-2 minute exposure to US
irradiatioJl Or about 70,000 microwatts/cm or by I hour
moisture cure at ambient temperature and humidity. Above
about 70~ oil content, curing was very difficult and non-
plowable products were not achieved by the moisture cure mechanism. Samples containing between 0 and 70% oil ranged,
respectively, from a soft rubbery material shore A 10-15)
to a very soft sticky gel (Shore A unmeasurable) which
showed some tendency to creep but would not flow.
Example 2
The synthetic procedure of Example 1 was repeated using
50.0g of a 3500cst hydroxyl terminated polydimethylsiloxane
(WOW), 1.05g methacryloxypropyl trimethoxysilane and
OWE tetraisopropylorthotitanate. The product was a
viscous liquid. A sample of the product was mixed With I
diethoxyacetophenorle and cured with W irradiation of 70,000
microwatts/cm2 to a stretchable soft rubber. Another sample
of the sane composition cured lo a slightly sol ton rubber
after 2 1/2 days exposure to ambient temperature and humidity.
Example 3
A methacryloxypropyldimethoxysilyl -terminated dimly-
selection was prepared from 101.7g of a 20,000cst sullenly
terminated dimethylsiloxane, 2.6g methacryloxypropyltri-
methoxysllane an 0.3g tetraisopropylorthotitanate in a
I
Jo
manner similar to that of Examples 1 and 2 except that
100. 9g of a laxity trimethylsilyl terminated silicone oil
was added to the mixture prior to heating. The resulting
product had a viscosity of 21, 200cps.
3.059 of the 21,200cps proc1~1ct were then mixes White
0.6C1 of the same laxity silicone oil and 0.079 dimetl1cxyace-
topheno1-le. This composition, containing 58~ oil, cured to a
soft gel l/16 deep upon exposure to US of about 70,000
microwatts/cm for 60 seconds.
lo 193. a of the 21, 200cps product were mixed with 459 of
. the laxity silicone oil and 59 ~imethoxyacetop11eno1le. The
mixture was cast into 1/8 thick open top newlyweds and cured by
TV light of the same intensity for six intervals of 20
seconds each, with a cooling period between each interval.
lo The samples were then allowed to moisture cure overnight
after which they were removed from the newlyweds in 4 x
square pieces, turtled over, and the bGttorn surfaces subjected
to three 20 second intervals of TV exposure to reduce
tackiness. x 4 pieces were they'll subjected to electrical
testing, the results of which are given in Table I below
Table I
Test Performed Individual values voyeur Valve
_ _ _ _ _
ASSET D-l49, Dip
25 electric S~renath,
volts/mil 340 230 210 250 260 260
ASSAY D-256, Volume
Resistivity,
I ohm-cn1 3.8 3.9 4.B 3.5 3.5 3.9
30 ASSET D-lS0, Dip
electric Constar1t,
lo 2.87 ~.28 3.5
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it
Test ~erEormed Individual V2 us revalue
ASSUME b-150, by-
electric Constant,
lo Liz 2. 5 2. ~33 2. 71
S ASTM D-150, bus-
sipation Factor,
lo Liz o. 0018 0. 0017 0. 0(~18
ASSUME D-150, Disk
slpatlon Factor,
lo 105 Isles 0.0012 0.(!~12 0.0012
.
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