CA1234445A - Dual curing silicone, method of preparing same and dielectric soft-gel compositions thereof - Google Patents

Abstract

ABSTRACT

Novel dual curing silicones are terminated with acrylic functional dialkoxy or diaryloxy silyl groups.
They are prepared by condensation of a silanol terminated silicone having a viscosity of bettween 500 and 50,000cps, (mPas) with a silane represented by the formula where R3 is alkyl or aryl, R4 is H or C1-5 alkyl and R5 is alkylene, the ratio of silane to silanol silicone being between about 2.1 and 6.1.
Compositions of the novel silicones with moisture cure catalysts, photoinitiators and optionally up to 70% trimethyl silyl terminated silicone oils, are both moisture and UV curable and produce soft gel or rubbery cured materials suitable for electronic potting appli-cations.

Description

Lo UAI. CURING Silicone lo Ox PREPARING SUE
END DIELECTRIC SOFT-GFL COMPOSITIONS ll!ERFOF
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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.

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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 .. ;- 3 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.

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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.

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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.

~23~
.. 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 ~23~
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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Claims (14)

The embodiments of the invention in which an exclusive property of privilege is claimed are defined as follows:

1. A composition capable of curing by both moisture cure and UV cure mechanisms, the composition comprising a mixture in parts by weight of:
(a) 30-100 parts of a reactive polyorganosiloxane terminated with acrylic functional dialkoxy-or diaryloxy-silyl groups and containing about 0.1-5% of a silicone moisture curing catalyst;
(b) 0-70 parts of a trimethylsilyl terminated silicone oil; and (c) an effective amount of a photosensitizer, wherein the reactive polyorganosiloxane comprises the reaction product of a silanol terminated silicone represented by the formula:

wherein R1 and R2 are organo groups, no more than 2% of which are cross-linkable under UV or moisture cure conditions, and n is an integer, said silanol having a viscosity of between about 500 and 50,000cps, with an acrylic functional trialkoxy- or triaryloxy-silane of the formula:

wherein R3 are the same or different alkyl or aryl groups; R4 is H or C1-5 alkyl and R5 is alkylene, in the presence of a condensation catalyst, the mole ratio of said silane to said silanol being between 2:1 and 6:1.

2. A composition as in claim 1, wherein substantially all R3OH produced by the reaction of said silanol and silane is removed.

3. A composition as in claim 1, wherein said photosensitizer is selected from substituted or unsubstituted benzoin, benzophenone, dialkoxybenzophenone, Michlers' ketone and diethoxyacetophenone.

4. A composition as in claim 1, wherein said R1 or R2 organo groups on said silanol terminated silicone include photosensitizing groups, said groups comprising said photosensitizer ingredient c.

5. A composition as in claim 4, wherein said R1 or R2 photosensitizing groups comprise benzoin or benzoin ether groups.

6. A composition as in any one of claims 1 to 3, wherein said silanol terminated silicone has a viscosity of between about 600cst and 20,000cst.

7. A composition as in any one of claims 1 to 3, wherein said silanol terminated silicone has a viscosity of between about 600cst and 4000cst.

8. A composition as in any one of claims 1 to 3, containing a said trimethysilyl terminated silicone oil, the silicone oil having a viscosity of between 100 and 5,000cps.

9. A composition as in any one of claim 1 to 3, comprising between 30 and 70% of said silicone oil.

10. A composition as in any one of claims 1 to 3 wherein said moisture cure catalyst comprises an orthotitanate catalyst.

11. A composition as in any one of claims 1 to 3, wherein said reactive polyorganosiloxane has a molecular weight of between about 12,000 and 50,000.

12. A composition as in any one of claims 1 to 3, further comprising an adhesion promoter.

13. A composition as in any one of claims 1 to 3, further comprising an ion trapping compound.

14. A composition as in any one of claims 1 to 3, wherein said silane to silanol mole ratio is between 2:1 and 2.5:1.