CA2002876A1 - Uv curable compositions - Google Patents

Abstract

IMPROVED UV CURABLE COMPOSITIONS
ABSTRACT
UV curable compositions suitable for use as potting and encapsulating compounds for electrical and electronic devices are provided. The compositions comprise: (a) a UV
curable compound polymerizable by a free radical process and containing reactive unsaturated groups, (b) a polysilane photoinitiator, and (c) a peroxide or azo compound photo-initiator or mixture of peroxide and azo compound photo-initiators. The use of the two photoinitiators enables one to get a more complete cure, both at the surface and deep into the coating.

Description

20~28~

IMPROVED UV CURABLE COMPOSITIONS

UV curable compositions suitable for use as potting and encapsulating compounds for electrical and electronic devices are provided. The compositions provide more effective cure at both the surface and in deep section and comprise (a) a W curable compound which polymerizes by a free radical process and which contains reactive unsaturated groups, (b) a polysilane photoinitiator and (c) a peroxide or azo compound photoinitiator or mixture of peroxide and azo compound photoinitiators.
Japa~ese patent (5~)49717 describes a UV curable composition comprising (a) a polyorganosiloxane, (b) a polysilane initiator and (c) a sensitizer.
It has been found, however, that compositions with a polysilane as the sole initiator only induce a surface cure (e.g. up to a thickness of about 50 mil.). Moreover, to applicants k~owledge, there is no known single initiator that is effective in inducing W cure for coatings of the type described herein; coatings up to 180 mil or more.
It has been found that a complete cure can be obtained by the use of two photoinitiators, one a polysilane and the other a peroxide or azo compound. The compositions also comprise a W curable compound polymerizable by a free radical process and having reactive unsaturated groups.
Additional improvements can be obtained by incorporation of W chromophores onto the peroxide molecule. In addition to improved photocure, the peroxide or azo compound provides the capability to thermally cure shadow regions not exposed to ultraviolet radiation. The scope and a better understanding ' -~ Z0028~;

of the invention, however, will be apparent from the following detailed description of the invention.
The W curable compound includes any compound which polymerizes by a free radical process and which contains reactive unsaturated groups. The preferred compounds have carbon-carbon double bonds. A still more preferred composition contains at least a part and preferably consists essentially all of an alkenyl functional polysiloxane as the W curable compound. Exemplary of suitable reacti~e unsaturated groups are: acrylics, methacrylics, styrenics, acrylamides, acrylonitriles, vinyl acetates, alkynes and alkenes such as vinyl and allyl.
Exemplary of suitable UV curable compounds, which are well known to those skilled in the art, are: siloxanes having alkene reactive groups as illustrated by U.S. Patent Nos. 4,064,027, 4,596,720 and 4,608,270; trior~anosiloxane-endblocked polydiorganosiloxane fluids having mercaptoalkyl radicals; mercapto-olefins and a methylvinyl polysiloxane as illustrated by U.S. Patent No. 4,052,529; organopolysiloxanes having acrylic functionality such as exemplified by GB 2,151,243 A, U.S. Patent Nos. 4,697,026 and 4,563,539, methacrylate functional polymers; acetylenic functional polymers; acrylated polymers such as urethanes, epoxies and acrylic esters; unsaturated polyesters such as polyvinyl-esters and polyvinyl acetals; vinyl endblocked butadiene and polyisoprene polymers and oligomers; and organopolysiloxanes containing acrylamidoalkyl functionality.
Typical polysilanes for use in the invention are disclosed in U.S. Patent Nos., 4,2~0,780, 4,276,424, 4,314,956, 4,324,901 and JP 58: 049,717. They include linear and branched peralkyl polysilanes, such as Me(Me2Si)xMe and Me~PhMeSi)xMe, cyclic peralkyl polysilanes such as (Me2Si)x 2~02876 and (PhMeSi)x and polysilacycloalkanes but is preferably a cyclosilane of the formula:
(RR'Si)X
wherein R and R' are independently selected from aliphatic and aromatic radicals and x is an integer of from 4 through 7. In the above formula and elsewhere in the specification and claims, Me and Ph represent the methyl and phenyl radicals respectively.
Suitable aromatic radicals include aryl such as phenyl, napthyl and benzyl and may be substituted with alkyl and alkoxy groups of from 1 to 10 carbon atoms and other substituents such as halogen, carboxylate and nitrogen containing groups. Suitable aliphatic radicals are alkyl groups which have from 1 to 10 carbon atoms, which can be substituted with halogen, such as methyl, ethyl, propyl, isopropyl, cyclohexyl, 3,3,3-trifluoropropyl and tertiary butyl. The most preferred silane has the formula:
(Me2Si)6 The peroxide and azo compound should be compatible in the composition and be activated by W light to form free radicals. Only a minor but effective amount of peroxide or azo compound and silane is required. Generally, from about 0.1~b to about 10% of each or preferably from 2% to 5% is employed of each.
Exemplary of suitable peroxides (to include peroxy compounds) are: dibenzoyl peroxide, cumene hydroperoxide, di-t-butyl peroxide, diacetyl peroxide, hydrogen peroxide, peroxydisulphates and 2,2-bis(terbutylperoxy)-2,5-dimethyl-hexane.
Exemplary of suitable azo compounds are:
Azo-bis(i~obutyronitrile), diazirine, 3-acetoxy-3,5,5-trimethyl-l-pyrazoline, tetramethyltetrazene and alpha-bis-l-cyclohexanecarbonitrile.

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---` 200~:876 In addition to the conventional chromaphores, chromophoric groups can be incorporated into the peroxide to increase photosensitivity such as nitro, azo, quinoid or -C:CHCO- radicals.
While the thickness of the coating will depend upon the particular use, typical coatings will have a thickness of between about 2 mils and about 500 mils. Coatings can be applied to glass, metals, electrical and electronic devices and other con~entional substrates by conventional techniques such as draw bar coating and dip coating. The compositions can be cured by exposure to a dose of ultraviolet (W ) radiation by conventional equipment such as a broad spectrum wavelength ultraviolet source. The compositions may require high doses of W radiation to cure in air but readily cure under an inert atmosphere such as nitrogen in the amount of as little as 200 millijoules per square centimeter and still heat cure in the shadow areas or dark areas, i.e., where the composition is not exposed to the ultraviolet radiation. As is well known in the art, shadow cure is beneficial in that it allows one to cure areas that are not easily cured by W
to obtain optimum cured physical properties. The temperature necessary to heat cure the compositions will depend upon the particular composition but will generally be between about 80C. and about 150C. Conventional heating equipment can be employed such as convection ovens and infrared cure reactors.
It does not matter whether the composition is first subjected to W radiation or first heated but it is preferred to subject it to W radiation for ease of handling.
If desired, other compatible ingredients can be incorporated in the composition and are well known to those skilled in the art. Typical ingredients include reinforcin~
agents, thickeners, flow control additives, adhesion ;~002~76 additives and other conventional processing and performance aids.
The following examples will serve to illustrate the invention. All parts and percentages in said examples, and elsewhere in the specification and claims, are by weight unless otherwise indicated.
Example 1 Employing a UV curable polysiloxane of the average structural formula ~ (MeSiO3/2)3(Me2SiO)g5(Me3SiO~)1 3(ViMe2SiO~)0 7]
wherein Me is methyl and Vi is vinyl, various compositions were prepared as shown in the following table. The compositions were prepared by making a physical blend of the desired ingredients in a vial. The compositions were prepared using 20g portions of the W curable polysiloxane.
To the 20g portions was added the desired amount of photo-sensitization system. Compositions containing (~e2Si)6 were prepared by first adding it to the W curable polysiloxane, heating for several minutes at 80C. to aid disolution, cooling and then adding the remainder of the photo-sensitization system.
W cure was obtained by placing a 10 g portion of the composition in an aluminum weighing dish (approx. 2" in diameter) and irridating with an ultraviolet cure unit (Ashdee W -12H/2 Horizontal Cure Reactor manufactured by Ashdee Products, Koch Technical Division) which contained two median pressure Hg vapor arc lamps. The conveyor speed was 33 ft/min and both lamps were on the high setting. This resulted in an irradiation time of 3.65 sec. and a dose of 112 mJ/cm for each pass. The cure dose was obtained using a W Powermeter equipped with a 365 nm filter manufactured by Optical Associates, Inc. Time was allotted for samples to cool between passes. The appearance at the surface of the 200;~

composition, whether fluid or cured, was then noted. Next, the cured thickness was measured with a hand held caliper manufactured by B. C. Ames Company, Waitham, Mass. The cured thickness was first measured without releasing the pressure foot. The pressure foot was then released and a second thickness measured. In this manner, a measure of the W cure depth (no pressure reading) and also the cure effectiveness (pressure reading) was obtained. The larger either reading, the better the W cure.
~ hermal cure response was measured by placing 5 g of the composition into an aluminum weighing dish or placing 1 g of the composition into a small capped vial, followed by thermal aging in a forced air oven for l hour at 128C.
Table I shows the results of both heat curing and sub~ecting several compositions to UV radiation. Of the seven compositions, only the last is a composition of the invention (B+C), and it can be seen that it was the only composition to cure both on the surface and to the bottom.

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~ 2~02a~6 ExamPle 2 The above procedure was repeated employing a vinyl functional polymer of the average structural formula ViMe2SiO(Me2SiO)145(ViMeSiO)3SiMe2Vi. The other ingredients were the same and the results are shown in Table II. Of the six compositions, only the last (B+C) is a composition of this invention.

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2C~0287~, From the data reported in the tables, it can be seen that only the compositions with the dual photoinitiators of this invention provide both a surface cure and a deep cure. Similar results are found with the other materials of the invention described, and they can be both shadow cured and heat cured.
The above disclosure contains the Best Mode and describes a number of materials which can be employed.
Nevertheless, obvious modifications may appear to one of ordinary skill and thus the inv~ntion is intended to be limited only by the appended claims.

Claims (5)

1. An improved composition comprising (a) a UV
curable compound polymerizable by a free radical process and containing reactive unsaturated groups, (b) a polysilane photoinitiator, and (c) a peroxide or azo compound photoinitiator or mixture of peroxide and azo compound photoinitiators.

2. The composition of claim 1 wherein the UV
curable compound contains carbon-carbon multiple bonds.

3. A method of coating a substrate which comprises applying to said substrate, a composition comprising (a) a UV
curable compound polymerizable by a free radical process and containing reactive unsaturated groups, (b) a polysilane initiator, and (c) a peroxide or azo compound photoinitiator or mixture of peroxide and azo compound photoinitiators, and curing said composition with UV radiation.

4. The method of claim 3 wherein the substrate coated is an electrical or electronic device.

5. The method of claim 3 wherein the composition is also cured with heat.