CA1285672C - Method for curing organopolysiloxane compositions and the compositions - Google Patents

Abstract

METHOD FOR CURING ORGANOPOLYSILOXANE
COMPOSITIONS AND THE COMPOSITIONS
ABSTRACT OF THE DISCLOSURE

A method of curing certain organopolysiloxane compositions by a combination of heat and exposure to ultraviolet radiation or electron beam provides the ability to use lower temperatures than heat cures alone and shorter cure times than ultraviolet radiation or electron beam alone. The organopolysiloxane compositions are addition-curable and are made up of alkenyl-containing organopolysiloxane, organohydrogensiloxane, a platinum-type catalyst, an addition-reaction inhibitor selected from alkynol, 3-methyl-3-pentene-1-yne, 3,5-dimethyl-3-hexene-1-yne, methylvinylsiloxane cyclics, and mixtures thereof, and a photosensitizer,

Description

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MEI'HOD FOR CURING ORGANOPOLYSILOXANE COMPOSITIONS
AND THE COMPOSITIONS

The present invention provides a method for curing organopolysiloxane compositions. More specifically, the present invention provides a method for curing organopolysiloxane compositions which consist of a photosensitizer and an addition-curable organopoly-siloxane containing a platinum-type catalyst by heating and by exposure to ultraviolet radiation or to an electron beam.
Background Information It is known in the prior art that organopoly-siloxane compositions can generally be cured by heating;
refer to Japanese Patents 35-13709 (60-13709), 36-1397 (61-1397) and 46-76798 (71-76798). However, all of these methods require elevated temperatures and heating for long periods of time with the resulting drawback of low productivity. -On the other hand, organopolysiloxane compositions which can be cured at room temperature or low temperatures by exposure to ultraviolet radiation or an electron beam; ~refar to Japanese Kokai [Laid-Open]
Patents 50-61386 !75-61386) ~United Kingdom Patent Specification No~ 1,433,461, published April 28, 1976~, 50-61486 (75-61486) (U.S. Patent No. 4,064,027, issued December 20, 1977), and 55-125123 (80-125123) have been developed due to recent demands for increased productivity and the increasing diversity of applications. However, these compositions have certa:Ln drawbacks including one or more af the ~ollowing: a strong mercaptan odor, quite poor pot stability at room temperature, slow curing rates inappropriate for practical applications, and nonuniform curing in interior sections.

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Various methods were examined by the present inventors in order to eliminate the preceding drawbacks with the resulting development of a method for curing organopolysiloxane compositions, including uniform curing in interior sections, by heating and by exposure to ultraviolet radiation or an electron beam for a short period of time at low temperatures.
Summary of the Invention This invention relates to a method for curing organopolysiloxane compositions consisting essentially of heating and exposinq to ultraviolet radiation or an electron beam a photosen~itizer-containing organopoly-siloxane composition which consists essentially of (A) 100 parts by weight of addition-curable organopol~siloxane composition which consists essentially o~ (1) an organo-polysiloxane containing at least two silicon-bonded alkenyl groups per molecule and other organic groups when present are selected from the group consisting of alkyl groups, haiogenated alkyl groups, aryl groups, aralkyl groups and alkaryl groups, (2) an organohydrogenpoly-siloxane containing at least tws organohydrogensiloxane or hydrogensiloxane units per molecule, (3) a platinum-type catalyst, and (4) an addition-reaction retarder selected from the group consisting of alkynols, 3-methyl-3~pentene-1-yne, 3,5-dimethyl-3-hexene-1-yne, methylvinyl-siloxane cyclics, and mixtures thereof, and (B) 0.001 to 30 parts by weight of a photosensitizer.
This invention also relate~ to the compositions , which can be cured by this method More pa~ticularly, the present invention, in one aspect, resides in a method for curing organo-polysiloxane compositions cha~acerized by concurrently heating and exposing to ultraviolet radiation or an eleatron beam a photosensitizer-containing organopoly-siloxane composition which consists of r~

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(A) 100 parts by weight of an addition-¢urable organopolysiloxane aompo~ition which aon~ist~ G~
( 1 ) a polydiorganoB$loxane ConBi~ting of diorganosiloxane unit~ ~elected from ths group consisting of dimethylsiloxane units, methyl-phenylqiloxane unit~ methylvinyl~iloxaneunits, and mixtures thereof, ~aid polydiorgano-siloxane being endblocked with dimethylvinyl-5 ~ r i 1~>7 e t ~ 5; I y l ~ ~x~ units, t~ L~ ~y units, or Rilicon-bonded hydroxyl group~, and having a vinyl content of ~rom 0.5 to 3 weight peraent;

(2) a ~ ~ ~ ~ ~ ~ endbloaked polysiloxane containlng methylhydrogen~iloxane units and optionally dimethylsiloxans units, which has a visoo~ity o~ from 0.005 to 0.05 Pa.~ a~ 25C, and which i~ present in an amount of from 2 to 10 parts by weight based on 100 parts by weight of (1);

(3) a platinum aataly~t ~elected from the group con~isting of alcohol-modified chloroplatinic acid, chloroplatinic acid-olefin complexes and chl oroplatinic acid-vinyl-~iloxane coordination ¢ompound~, which i~ pre~ent in an amount of from 50 to 200 parts by weight of elemental platinum per million parts by wsight of (1); and (4) an addltion-reaotion r~tarder, which i~ an alkynol or 3-msthyl-3-pentene-1-yne and whioh i8 pre~ent in an amount of from o.1 to 1.0 part by weight per 100 part~ by weight o~ (1); and .
(B) 0.1 to 5 part~ by weight, per 100 ~arts by weight o~
(A), o~ a photo~en~itizer ~eleated from the group aon~i~ting o~ 4-allylaaetophenone, benzoph~none, benzoin ethyl ether, and benzil dimethyl ketal.

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~ ~ ~r;~7 2 In another aspect, the present invention resides in a composition consisting of (A) 100 parts by weight of an addition-aurable organopolysiloxane composition whiah con~ists of (1) a polydiorganosiloxane consisting of diorganosiloxane units selected from the group aonsisting of dimethylsiloXane units, methylphenylsiloxane units, methylvinylsiloxane units, and mixturas thereof, said polydiorgano~iloxane being endbloaked with et i ~n skh yl V ;/~ / t r ~ ~e~ l~ Y ~ s i~ l ~me~ v~ C~,~3*~ uIllts, ~ ~e-t~ ~ L o,ty units, or siliaon-bonded hydroxyl groups, and having a vinyl content of from 0.5 to-3 weight percent;
(2) a tri~ot~lail~y-endblocked polysiloxane containing methylhydrogensiloxane units and optionally dimethylsiloxane units, which has a viscosity of .from 0.005 to 0.05 Pa. 8 at 25C, and which i6 present in an amount of from 2 to parts by weight based on 100 parts by weight of tl);
; (3) a platinum catalyst selected from the group oonsisting of alcohol-modi~ied chloroplatinic acid, chloroplatinic acid-olefin complexes, and chloroplatinic acid-vinyl-siloxane coordination compounds, which is present in an amou~t of from 50 to 200 parts by weight platinum eleme~t per million parts by weight of ~1); and (4) an addition-reaation retarder, wh~ah i~ a~
alkynol or 3-methyl-3-pentene-1-yne an~ which i~ present in an amount oE rom 0.1 part by weight per 100 parts by weight o~ (1); and (B) 0.1 to 5 part~ by weight, per 100 parts by weight by (A), of a photosensitizer selected from ~he group consisting of 4-allylaaetophenone, benzophenone, benzoin ethyl ether, and banzil dimethyl ketal.

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~ 72 - - Detailed Description of the Invention The addition-curable organopolysiloxane compo-sition which contains a platinum-type catalyst and which comprises component (A) is a composition whose principal - 2c -. . ,' . :
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components are organopolysiloxane containing silicon-bonded alkenyl groups, organohydrogensiloxane, a platinum-type catalyst, and an addition-reaction retarder and which may contain organic solvents, extender fillers, thermal stabilizers, flame retardants, etc., as optional com-ponents.
The organopolysiloxane containing silicon-bonded alkenyl groups, (A), preferably contains at least 2 silicon-bonded alkenyl groups in each moLecule from the standpoint of curability. The molecular structure of this organopolysiloxane can be straight chain, branched chain, cyclic, or network and the degree of polymerization is arbitrary. Examples of the alkenyl group are vinyl, propenyl, and butenyl. The organic ~roups present in this organopolysiloxane other than the alkenyl groups are alkyl groups such as methyl, ethyl, propyl, hutyl, and octyl; halogenated alkyl groups such as 3-chloropropyl, and 3,3,3-trifluoropropyl; aryl such as phenyl; aral~yl groups such as 2-ph0nylethyl and 2-phenylpropyl; and alkaryl groups such as tolyl.
Examples of the preceding organopolysiloxane are polydiorganosiloxanes such as dimethylvinylsilyl-terminated polydimethylsiloxane, dimethylallylsilyl~
terminated polydimethylsiloxane, phenylmethylvinylsilyl-terminated diphenylsiloxa~e-dimethylsiloxane copolymer, trimethylsilyl terminated methylvinylsiloxane-dimethyl-siloxane copolymer, silanol-terminated dimethylsiloxane-methylvinylsiloxane copolymer, and dimethylvinylsilyl-terminated polymethyl~3,3,3-trifluoropropyl)siloxane; and examples of other organopolysiloxanes are polyvinylsil-sesquioxane and copolymers of dimethylvinylsiloxane units and SiO2 units.

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The organohydrogenpolysiloxane (2) must contain orgarlohydrogensiloxane or hydrogensiloxane units and can be composed of only organohydrogensiloxane units or hydrogensiloxane units or can contain other organosiloxane units. At least 2 organohydrogensiloxane units must be present in each molecule from the standpoint of curability.
The molecular structure of the organohydrogenpolysiloxane can be straight chain, branched chain, cyclic, or network and the degree of poymerization ls arhitrary. The organohydrogenpolysiloxane (2) has a viscosity range of ~rom 0.005 to 0.05 Pa's at 25C., and is present in an amount of from 2 to 10 parts by weight based on 100 parts by weight of organopolysiloxane ~1).
Examples of the preceding organohydrogenpoly-siloxane are dimethylhydrogensilyl-terminatea dimethyl-siloxane-methylhydrogensiloxane copolymer, trimethylsilyl-terminated dime~hyl~iloxane-methylhydrogensiloxane copolymer, dimethylphenylsilyl- erminated dimethylsiloxane-methylhydrogensiloxane copolymer, trimethylsilyl-terminated methylhydrogenpolysiloxane, and cyclic methylhydrogenpoly-siloxane.
The mixing ratio of organopolysiloxane which contains silicon-honded alkenyl to organohydrogenpoly-siloxane is generally 0.1 to 10 moles silicon-bonded hydrogen in ~2) per 1 mole silicon-bonded alkenyl group in (1). Curing will be inadequate when this value is less than 0.1 mole. When the above value exceeds 10 moles, hydrogen gas bubbles will be generated whlch adversely affects the stability. However, the above value may exceed 10 moles when a cured foamed product is to be produced.
The platinum-type catalysts ~3) include finely divided platinum particles, finely divided platinum , ~ n~ 72 par~icles adsoxbed on a carbon powder ~upport, chloro-platinic acid, alcohoL-modified chloroplatinic acid, chloroplatinic acid-olefin complexes, chloroplatlnic acid-vinylsiloxane coordination compounds, platinum 4a B

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~2 !35~72 black, palladium, and rhodium catalysts. The quantity of platinum-type catalys~ used is determined by the require-ment that it can semi-cure the above organopolysiloxane composition by heating at 50 to 100C and is generally 1 to 1,000 ppm~as platinum metal relative to the total weight of the organopolysiloxane (1).
The addition-reaction retarder (4) is an essential component which has the effect of inhibiting the reaction at room temperature in order to impart pot stability to the composition. Examples thereof are alkynols such as 3-methyl-1-but~ne-3-ol, 3,5-di-methyl-l-hexyne-3-ol, 3-methyl-1-pentyne-3-ol, and phenylbutynol; 3-methyl-3-pentene-1-yne; 3,5-dimethyl-3-hexene-l-yne; methylvinylsiloxane cyclics, and ben-zotriazole. These compounds can be used as reaction inhibitors singly or as mixtures of two or more com-pounds. The quantity of addition-reaction retarder is determined by the condition that a practical pot life be imparted to the present composition. Although the reaction-inhibiting ef~ect will depend on molecular structure, this quantity is preferably 0.05 to 10 parts by weight per 100 parts by weight organopolysiloxane (1) and may be adjusted up or down depending on the purpose.
The photosensitizer comprising component (B) accelerates the curing reaction of the present com-position upon exposure to ultra~iolet radiation or an electron beam and is a known compound usually used ~or ultraviolet-curable resins. Examples are axomatic ketones such as acatophenone, benzophenone, trimethyl-silylated benzophenone, propiophenone, 3~methylacetophenone, 4-methylacetophenone, benzoin ethyl ether, dimethylpoly-siloxane in which both ends are bonded through ether linkages with benzoin, ' B

(4-isopropyl)phenyl-1-hydroxyisopropyl ketone, 4-allyl~
acetophenone, 3-benzylacetophenone, and ~-methoxyben-zophenone; amine compounds such as triphenylamine; and azo compounds such as azobisisobutyroni~rile.
The preceding photosensitizer must be used at 0.001 to 30 parts by weight and preferably 0.05 to 20 parts by weight per 100 parts by weight of the platinum catalyst-containing, addition-curable organopolysiloxane composition, component (A).
In the method of the present invention, the photosensitizer-containing organopolysiloxane composition consisting of the preceding components (A) and (B) is converted into a rubbery elastomer or cured resin by heating and by exposure to ultraviolet radiation or an electron beam.
The heating conditions depend on the type of preceding organopolysiloxane, the quantity of addition of platinum-type catalyst and the thickness of the coated film; however, the heating conditions are determined by the requirement that the present composition be semi-cured by the heat treatment alone. For example, the heating temperature is 50 to 100C and the heating time is several tens of seconds or less.
The ultraviolet-generating source is any such source known in the prior art and examples thereof are hydrogen discharge tubes, xenon discharge tubes, and low-pressure, medium-pressure, and high-pressure mercury lamps.
The electron beam source is any such source known in the prior art and examples thereof are scanning electron beam generators and curtain electron beam generators.

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The conditions of exposure to the ultraviolet radiation or electron beam are arbitrary as long as exposure alone can semi-cure the present composition.
With regard to the order of treatment, ultra-violet or electron beam exposure can be executed after the heat treatment, the heat treatment can be executed after ultraviolet or electron beam exposure, or the heat treatment and ultraviolet or electron beam exposure can be executed simultaneously. However, when the composition o~ the present invention is dissolved or dispersed in organic solvent, it is preerably exposed to ultraviolet radiation or an electron beam after the heat treatment.
According to the method of the present invention, the composition is completely cured by a combination of heating at low temperatures for a brief period with ultraviolet or electron beam exposure for a brief period.
The composition is cured at temperatures much lower than in a prior-art heat treatment alone and is cured in times much shorter than in prior-art ultraviolet or electron beam exposure alone. Process productivity is thus increased and the mild heating conditions are appropriate for applications such as in the electronic materials industry, in coating synthetic resins and in the use of release agents.
The present invention will be explained using demonstrational examples. Parts in the examples are parts by weight, % denotes wt~, and the viscosity was measured at 25~C.

~ e 1 100 parts dimethylvinylsilyl-terminated dimethyl-siloxane methylvinylsiloxane copolymer with a 1.0 Pa s viscosity and a 2.0% vinyl group content were mixed with ~ 7~

8 parts trimethylsilyl-terminated methylhydrogenpoly-siloxane with a 0.006 Pa-s viscosity, 0.3 part 3-methyl-1-butyne-3-ol as the addition-reaction retarder, and 2 parts benzophenone as photosensitizer and this was then combined with a vinylsiloxane-chloroplatinic acid complex ~100 ppm as platinum based on the total weight) and mixed to homogeneity. The resulting composition, denoted as Sample 1, had a viscosity of 0.81 Pa s.
To provide Comparison Example 1, a composition was produced by the same method as for Sample 1 with the exception that the benzophenone was omitted. ~he resulting composition had a viscosity of 0.816 Pa s. In addition, compositions for Comparison Examples 2 and 3 were prepared by the same method as for the preceding Sample 1 by omitting the 3-methyl-1-butyne-3-ol or the platinum catalyst, respectively. The composition of Comparison Example 3 had a viscosity of 0.808 Pa s while the composition of Comparison Example 2 gels in mixing and its viscosity could not be measured.
When the preceding compositions was allowed to stand at 25C, Sample 1 and the compositions of Comparison Examples 1 and 3 did not exhibit an increase in viscosity over 1 day and thus were quite stable.
Sample 1 and the compositions of Comparison Examples 1 and 3 were each coated at 1.0 g/m2 on poly-propylene film, heated in a hot air-circulation oven at 70C for 5 minutes, and then immediately irradiated with a 160 W/cm high-pressure mercury lamp at a distance of 5 cm for 0~1 second. Sample 1 was completely cured while the compositions of Comparison Examples 1 and 3 were not cured. Sample 1 was not cured only by heating at 70C
for 120 seconds or only by exposure to ultraviolet radiation under the same conditions as above.

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Example 2 100 parts dimethylvinylsilyl-terminated dimethyl-siloxane-methylphenylsiloxane-methylvinylsiloxane copolymer with a 3.0 Pa-s viscosity`(containing 1.8% vinyl groups and 3.0% phenyl groups) were mixed with 8 parts trimethyl-silyl-terminated dimethylsiloxane-methylhydrogensiloxane copolymer with a viscosity of 0.02 Pa s (Si-bonded H:methyl molar ratio=1:3~, 0.6 part 3-methyl-3-pentene-1-yne addition-reaction retarder and 1.5 parts benzil dimethyl ketal as photosensitizer, and this was then combined with a vinylsiloxane-chloroplatinic acid complex (180 ppm as platinum based on the total weigh-t) followed by mixing to homogeneity. The resulting composition, denoted as Sample 2, had a viscosity of 2.36 Pa~s.
- Compositions for Comparison Examples 4, 5, and 6 werP prepared by the same method as for the preceding Sample 2 with the respective omission of the preceding dimethylsiloxane-methylhydrogensiloxane copolymer, the benzil dimethyl ketal photosensitizer, or the vinyl-siloxane-chloroplatinic acid complex. The viscosities of the samples were 2.92 Pa s for Comparison Example 4, 2.48 Pa s for Comparison Example 5, and 2.39 Pa-s for Comparison Example 6.
When the preceding compositions were allowed to stand at 25C for 1 day, they did not present an increase in viscosity and were thus stable. Sample 2 and the compositions of Comparison Examples 4, 5, ànd 6 were each coated at 1.5 g/m2 on a polyethylene-laminated kraft paper, irradiated with a 160 W/cm high-pressure mercury lamp from a distance of 5 cm for 0.1 second and then immediately heated in a hot air-circulation oven at 80C
for S seconds. Sample 2 was completely cured while the compositions for Comparison Examples 4, 5, and 6 were not , , ~ .

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cured. Sample 2 was not cured only by heating at 80C
for 120 seconds or only by ultraviolet exposure under the above-mentioned conditions.

Example 3 100 parts hydroxyl-terminated dimethylsiloxane-methylvinylsiloxane copolymer gum with a 1.0~ vinyl group content were dissolved in 1,000 parts toluene and this was combined with 4 parts trimethylsilyl~terminated methylhydrogenpolysiloxane with a 0.02 Pa-s viscosity, 0.2 part phenylbutynol as addition-reaction retarder, and 4 parts benzoin ethyl ether as photosensitizer ancl this was then stirred to give a homogeneous solution.
An alcohol solution of chloroplatinic acid (60 ppm based on the total polysiloxane weight) was homo-geneously dissolved in the preceding polysiloxane solution.
The resulting composition, denoted as Sample 3, had a viscosity of 0.48 Pa s.
Compositions for Comparison Examples 7, 8, and 9 were prepared by the same method as for the preceding Sample 3 with the respective omission of benzoin ethyl ether, the alcohol solution of chloroplatinic acid, or -phenylbutynol. The preceding compositions were allowed to stand at 25C for 1 day. Sample 3 and the com-positions of Comparison Examples 7 and 8 did not present an increase in viscosity. The composition of Comparison Example 9 underwent an increase in viscosity and com-pletely gelled in 2 minutes after combination with the alcoholic chloroplatinic acid and the attempt to stir to homogeneity.
Sample 3 and the compositions of Comparison Examples 7 and 8 were each coated at 1.2 g/m2 as solids on aluminum plates. The toluene was evaporated at room . .

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temperature and the compositions were then heated in a hot air-circulation oven at 70C for 5 seconds and then immediately irradiated with a 160 W/cm high-pressure mercury lamp from a distance of 5 cm for 0.07 seconds.
Sample 3 was completely cured while the compositi~ns of Comparison Examples 7 and 8 were not cured. The com-positions of Comparison Examples 7 and 8 were not cured by irradiation with the mercury lamp for 5 seconds under the above conditions.
Sample 3 was not cured only by heating at 70C
for 120 seconds or only by irradiation with ultraviolet radiation under the above conditions.

Example 4 100 parts trimethylsilyl-terminated dimethyl~
siloxane-methylvinylsiloxane copolymer 10 Pa s viscosity, 1.7~ vinyl group content) were combined with 10 parts fumed silica and heated at 160C for 2 hours. This mixture was then dissolved in 200 parts toluene and then combined and mixed with 5 parts trimethylsilyl-terminated methylhydrogenpolysiloxane with a 0.04 Pa-s viscosity, 0.4 part 3-methyl-1-butyne-3-ol, and 4 parts ~enzophenone as photosensitizer to give a homogeneous solution. The resulting mixture was then combined with vinylsiloxane-chloroplatinic acid complex (180 ppm as platinum based on the entire quantity of polysiloxane~ and ~ixed to homo-geneity. The resulting composition, denoted as Sample 4, had a viscosity of 0.29 Pa s.
For Comparison Example 10l a composition was prepared by the same method as above with the use of a trimethylsilyl-terminated polydimethylsiloxane with a lO
Pa s viscosity instead of the dimethylsiloxane-methyl-vinylsiloxane copolymer used for Sample 4.

, Sample 4 and the composition of Comparison Example 10 were allowed to stand at 25C for 2 days;
however, they did not present an increase in viscosity.
These compositions were each coated at 1.1 g/m2 as solids on polyester film, heated in a hot air-circulation oven at 70C for 5 seconds and then immediately irradiated with a 160 W/cm high-pressure mercury lamp from a distance of 5 cm for 0.1 second. Sample 4 was completely cured while the composition of Comparison Example 10 was not cured. In addition, Sample 4 was not cured only by heating at 70C for 120 seconds or only by ultraviolet irradiation under the above conditions.

Example 5 100 parts dimethylvinylsilyl-terminated climethyl-siloxane-methylvinylsiloxane copolymer with a viscosity of 1.5 Pa s and containing 2.0% vinyl groups were combined with 5 parts trimethylsilyl-terminated methylhydrogenpoly-siloxane with a viscosity of 0.01 Pa s, 0.3 part, 3,5-dimethyl-1-hexyne-3-ol, and 0.5 part 4-allylaceto-phenone as photosensitizer and mixed to give a solution.
The resulting solution was combined with vinyl-siloxane-chloroplatinic acid complex (120 ppm as platinum based on the total weight) and this was mixed to homogen-eity. The resulting composition, denoted as Sample 5, had a viscosity of 1.31 Pa s. For Comparison Example 11, a composition was prepared by the same method as used for Sample 5 with the exception that the vinylsiloxane-chloro-platinic acid complex was omitted. ~ample 5 and the composition of Comparison Example 11 were both allowed to stand at 25C for 2 days; however/ they did not exhibit an increase in viscosity. These compositions were each coated at 0.8 g/m on polyester film, ~eated in a hot . . .. .. .
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air-circulation oven at 70C for 4 seconds and then immediately irradiated with a 1 Mrad electron beam does from a curtain eleetron beam generator. Sample 5 was completely cured while the eomposition of Comparison Example 11 was not completely eured. In addition, the cured product o~ Sample 5 did not have an undesirable odor.

Claims (6)

1. A method for curing organopolysiloxane compositions characterized by concurrently heating and exposing to ultraviolet radiation or an electron beam a photosensitizer-containing organopolysiloxane composition which consists of (A) 100 parts by weight of an addition-curable organopolysiloxane composition which consists of (1) a polydiorganosiloxane consisting of diorganosiloxane units selected from the group consisting of dimethylsiloxane units, methyl-phenylsiloxane units, methylvinylsiloxane units, and mixtures thereof, said polydiorgano-siloxane being endblocked with dimethylvinyl-silyl units, trimethylsilyl units, or silicon-bonded hydroxyl groups, and having a vinyl content of from 0.5 to 3 weight percent;
(2) a trimethylsilyl endblocked polysiloxane containing methylhydrogensiloxane units and optionally dimethylsiloxane units, which has a viscosity of from 0.005 to 0.05 Pa.s at 25°C, and which is present in an amount of from 2 to 10 parts by weight based on 100 parts by weight of (1);
(3) a platinum catalyst selected from the group consisting of alcohol-modified chloroplatinic acid, chloroplatinic acid-olefin complexes and chloroplatinic acid-vinyl-siloxane coordination compounds, which is present in an amount of from 50 to 200 parts by weight of elemental platinum per million parts by weight of (1); and (4) an addition-reaction retarder, which is an alkynol or 3-methyl-3-pentene-1-yne and which is present in an amount of from 0.1 to 1.0 part by weight per 100 parts by weight of (1); and (B) 0.1 to 5 parts by weight, per 100 parts by weight of (A), of a photosensitizer selected from the group consisting of 4-allylacetophenone, benzophenone, benzoin ethyl ether, and benzil dimethyl ketal.

2. The method according to claim 1 in which the heating is at a temperature of from 50 to 100°C.

3. A composition consisting of (A) 100 parts by weight of an addition-curable organopolysiloxane composition which consists of (1) a polydiorganosiloxane consisting of diorganosiloxane units selected from the group consisting of dimethylsiloxane units, methylphenylsiloxane units, methylvinylsiloxane units, and mixtures thereof, said polydiorganosiloxane being endblocked with dimethylvinylsilyl units, trimethylsilyl units, or silicon-bonded hydroxyl groups, and having a vinyl content of from 0.5 to 3 weight (2) a trimethylsilyl-endblocked polysiloxane containing methylhydrogensiloxane units and optionally dimethylsiloxane units, which has a viscosity of from 0.005 to 0.05 Pa.s at 25°C, and which is present in an amount of from 2 to parts by weight based on 100 parts by weight of (1);
(3) a platinum catalyst selected from the group consisting of alcohol-modified chloroplatinic acid, chloroplatinic acid-olefin complexes, and chloroplatinic acid-vinyl-siloxane coordination compounds, which is present in an amount of from 50 to 200 parts by weight platinum element per million parts by weight of (1), and (4) an addition-reaction retarder, which is an alkynol or 3-methyl-3-pentene-1-yne and which is present in an amount of from 0.1 part by weight per 100 parts by weight of (1); and (B) 0.1 to 5 parts by weight, per 100 parts by weight by (A), of a photosensitizer selected from the group consisting of 4-allylacetophenone, benzophenone, benzoin ethyl ether, and benzil dimethyl ketal.

4. The composition according to claim 3 in which (4) is an alkynol.

5. The method in accordance with claim 1 in which the composition is cured by concurrently heating and exposing to ultraviolet radiation.

6. The method in accordance with claim 1 in which the composition is cured by concurrently heating and exposing to an electron beam.