GB2159826A - Stabilization of polyfunctional acrylate compositions against premature anaerobic polymerization - Google Patents

Abstract

Polyfunctional acrylate monomer compositions, of the type polymerized by radiation-initiated polymerization under vacuum conditions to form dielectrics for capacitors, are stabilized against premature polymerization by incorporating therein a minor amount of a substantially stable free radical of formula <IMAGE> wherein R<3> is a tertiary alkyl radical and X is CH or N.

Description

SPECIFICATION Stabilization of polyfunctional acrylate compositions against premature anaerobic polymerization This invention relates to polymerizable compositions stabilized against premature polymerization, a method for so stabilizing such compositions and the use of compositions so stabilized.

Ethylenically unsaturated compounds such as esters of acrylic and methacrylic acid undergo facile polymerization to form polymers useful for many purposes. It is well known that such polymerization ordinarily takes place by a free radical mechanism, which is typically initiated by such means as the use of chemical initiators or contact of the unsaturated compound with ultraviolet or electron beam radiation or the like.

It is frequently desired to initiate polymerization under anaerobic conditions; that is, in the absence of air. However, spontaneous polymerization frequently occurs under conditions since a principal inhibitor, oxygen, is present only in extremely small concentrations if at all. When controlled anaerobic polymerization is desired, the monomeric composition may be stored in contact with air (i.e., under aerobic conditions), optionally in the additional presence of a coinhibitor such as p-methoxyphenol, and thereby stabilized against premature polymerization.

Under some circumstances, however, it is important for polymerization to be inhibited at least temporarily when the monomer is in an anaerobic environment. An example of such a circumstance is the formation of capacitor dielectrics by vacuum deposition of a monomer composition followed by radiation (i.e., electron beam)-initiated polymerization. Reference is made to copending, commonly assigned applications Serial Nos. 562,779; 562,871; 562,872; 562,873; 562,893 and 562,894, all filed December 19, 1983, and Serial No. [36CA-3594].

These applications describe in detail various polyfunctional acrylate monomer systems useful for the preparation of polymeric dielectrics in such capacitors, the polymers thereof useful as dielectrics, and a method for vacuum deposition and electron beam polymerization for dielectric formation. The disclosures of all of the above-listed applications are incorporated herein by reference.

In the manufacturing method used for such capacitors, it is necessary that monomer deposition be complete before polymerization takes place. Since such deposition is effected in an anaerobic system, and since the presence of coinhibitor under such conditions is generally not a sufficient condition for inhibition, it is extremely important to insure against premature polymerization in the vacuum system employed, while providing conditions which permit radiation-initiated polymerization when desired.

A principal object of the present invention, therefore, is to provide a method for stabilization of monomeric polyfunctional acrylate systems against premature anaerobic polymerization.

A further method is to provide polyfunctional acrylate compositions so stabilized.

A further method is to provide a method by which monomeric polyfunctional acrylates may be stabilized against anaerobic polymerization until they are deposited on an electrode surface, and then may be readily polymerized to form dielectric layers.

Other objects will in part be obvious and will in part appear hereinafter.

In its broadest aspect, the present invention is directed to compositions stabilized against anaerobic polymerization and methods for so stabilizing them, said compositions comprising: (A) a major proportion of at least one polyfunctional acrylate having the formula in Fig. 1, wherein R' is an aliphatic or alicyclic radical containing at least 10 carbon atoms, R2 is hydrogen or a hydrocarbon radical containing 1-5 carbon atoms and n is from 2 to 6; and (B) a minor anaerobic polymerization-inhibiting proportion of at least one substantially stable free radical having the formula in Fig. II, wherein each R3 is independently a tertiary lower alkyl radical and X is CH or N.

Component A in the compositions of this invention is, as previously noted, at least one polyfunctional acrylate having formula I. (For the sake of brevity, references hereinafter to formulas in the drawings will frequently be in the form "formula I", etc., rather than "the formula in Fig. I".) In that formula, the R' value is an aliphatic or alicyclic radical containing at least 10 carbon atoms. It may be a hydrocarbon or substituted hydrocarbon radical; the types of substituents permissible in the latter will be apparent to those skilled in the art and generally include substituents having only minor effects on the electrical properties of the molecule.

Illustrative substituents of this type are hydroxy, alkoxy, carbalkoxy and nitrile substituents. Most often, however, the R' value is an unsubstituted hydrocarbon radical. There is no real upper limit to the number of carbon atoms therein; it is within the scope of the invention to use compounds of formula I in which R' is a polymeric radical containing, for example, as many as 500 and preferably up to about 300 carbon atoms.

The R' radical may contain aliphatic, alicyclic or aromatic moieties or a mixture thereof.

Preferably, however, all moieties present therein are aliphatic, alicyclic or both.

The R2 value, as previously noted, may be hydrogen or an alkyl radical containing 1-5 carbon atoms. Most often, R2 is hydrogen or methyl and especially hydrogen. Thus, the term "acrylate" as used herein embraces both acrylates and a-substituted acrylates including methacrylates and the like, although the acrylates (i.e. the compounds in which R2 is hydrogen) are usually preferred. The value of n may be from 2 to 6, and is most often 2 or 3.

From the foregoing, it will be apparent that component A may typically be prepared by esterification of acrylic acid, methacrylic acid or the like, or functional derivatives thereof such as the acid chlorides, anhydrides and lower alkyl esters, with a wide variety of compounds containing the R1 value. Illustrative compounds of this type are polyhydroxy compounds and polyepoxides. Particularly useful polyfunctional acrylates of formula I are the following: Acrylates of aliphatic polyhydroxy compounds containing about 10-20 carbon atoms, illustrated by a,-diol mixtures having an average chain length of 12-20 carbon atoms.

Acrylates of polymers having groups capable of reacting with acrylic acid or derivatives thereof. Illustrative are various hydroxy-terminated diene polymers such as polybutadiene, typically having a number average molecular weight within the range of about 1000-5000.

Acrylates of aromatic diepoxides and polyepoxides, illustrated by diepoxy derivatives of 2,2bis-(4-hydroxyphenyl)propane or "bisphenol A".

A number of commercially available polyfunctional acrylates are useful for the preparation of dielectrics in the capacitors of this invention. Among them are the following, many of which are identified by trademarks as indicated: Bisphenol A diacrylate.

Celanese "Celrad 3700", a diacrylate of a diepoxide derived from bisphenol A.

"Sartomer SR-349", a diacrylate of ethoxylated bisphenol A.

Sartomer "Chemlink 2000", a diacrylate of an cu,w-alkanediol containing an average of 1 4-1 5 carbon atoms.

A diacrylate of Arco "Poly bd", which is a hydroxy-terminated butadiene resin having a number average molecular weight of about 3000.

While it is within the scope of the invention to use a single polynfunctional compound as component A, it is often preferred to employ copolymers prepared from blends of two or more such compounds. This may result in lower dissipation factor than is the case for most polyfunctional acrylates used alone. It may also optimize the physical properties of the material, e.g., lower the viscosity of the monomer composition to facilitate its handling and deposition.

It is also within the scope of the invention to use copolymers prepared from mixtures of at least one compound of formula I with at least one monoacrylate having formula III, wherein R2 is as previously defined and R4 is a hydrocarbon or substituted hydrocarbon radical, typically containing about 4-25 carbon atoms. Illustrative monoacrylates of formula III are cyclohexyl methacrylate and various acrylated derivatives of fatty acids containing such substituents as carbomerthoxy or niltrile. Typical copolymers of compounds of formulas I and III contain about 10-50% by weight, most often about 13-35%, of compounds of formula Ill with the balance being compounds of formula I.

With respect to these commercially available acrylates and the like, it is frequently preferred to remove residual acrylic acid and other ionic compounds and impurities which may be present in the materials as received from the supplier. Such impurities are readily removed by known methods such as the use of absorption columns or ion exchange resins.

Illustrative polyfunctional acrylate compositions whose polymers are suitable as dielectrics in the capacitors of this invention are illustrated in Tables I and II. All percentages in these tables are by volume.

TABLE I Ingredient Example 1 2 3 "Poly bd" diacrylate 100 50 "Chemlink 2000" --- 50 100 TABLE II Ingredient Example 4 5 6 7 8 9 10 11 Trimethylol- 50 100 -- -- -- -- -- 25 propane tri acrylate Neopentyl -- -- 50 60 40 40 50 25 glycol diacrylate Bisphenol A -- -- -- -- -- 20 -- - acrylate "Sartomer -- -- 50 20 -- -- -- - SR-349" "Celrad 3700" 50 -- -- 20 50 40 50 50 Cyclohexyl .- -- -- -- 10 -- -- -methacrylate A preferred subgenus of polyfunctional acrylates consists of those in which R2 is hydrogen or methyl, n is from 2 to 4, and R' is an aliphatic, alicyclic or mixed aliphatic-alicyclic radical having about 10-40 carbon atoms which optionally contains up to about three olefinic linkages, said olefinic linkages being non-conjugated.

The R' radical in this preferred subgenus may be aliphatic, alicyclic or mixed aliphaticalicyclic; it may optionally contain up to about three olefinic linkages which are non-conjugated, and contains about 10-40 carbon atoms. Suitable polyhydroxy compounds include straight chain compounds such as hexadecanediol and octadecanediol, with the hydroxy groups being located anywhere on the chain, and branched chain isomers thereof. By "branched chain" is meant that at least one carbon atom is present in a branch. Thus, configurations such as those of formulas IV and V are unbranched, while those of formulas VI and VII are branched.

A first preferred class of polyhydroxy compounds within this subgenus consists of those characterized by being branched and also by having at least 1 8 carbon atoms in a single chain; that is, at least 1 8 carbon atoms are successively bonded without branching. Particularly suitable polyfunctional acrylates derived therefrom are those having formulas VIII and IX, wherein r and s are each 7 or 8 and the sum of r and s is 1 5. They may be obtained, for example, by acrylic acid esterification of the hydroformylation products of oleic acid, as disclosed in U.S. Patent 4,243,818. Another suitable compound is 1,12-octadecanediol diacrylate, formed by hydrogenolysis of ricinoleic acid followed by esterification.

Also within this first preferred class of polyhydroxy compounds are single compounds and mixtures, usually mixtures, in which R' is at least one aliphatic or alicyclic radical containing about 20-40 carbon atoms and optionally up to about three non-conjugated olefinic linkages.

At least about 40%, and preferably at least about 50%, of the total number of R' radicals therein are alicyclic. Thus, the polyhydroxy compounds may be entirely alicyclic or may be mixtures of acyclic and alicyclic compounds satisfying these percentage limitations.

It is frequently convenient to prepare such polyhydroxy compounds by reduction of at least one corresponding polycarboxylic acid or ester thereof,which may be saturated or may contain olefinic linkages. A typical suitable polycarboxylic acid is linoleic acid dimer (hereinafter "dimer acid"), a mixture consisting essentially of acyclic, monocyclic and bicyclic dicarboxylic acids which typically contain up to two olefinic bonds per molecule. A particularly suitable dimer acid is sold by Emery Industries under the trade designation "Empol 1 010". According to Kirk Othmer, Encyclopedia of Chemical Technology, Third Edition, Volume 7, pp. 768-770, formulas X, XI and XII are structures of typical molecular species present in the methyl ester of dimer acid. Thus, free dimer acid obviously comprises free dicarboxylic acids having corresponding structures.

The esters of formulas X, Xl and XII, their corresponding free acids, and similar polycarboxylic acids and esters may be reduced by known methods, such as by hydrogen in the presence of a hydrogenation catalyst or by lithium aluminum hydride, to produce diols useful for preparation of the polyfunctional acrylates. Depending on the method of reduction of these or similar acids or esters, the reduction product may be saturated or may contain olefinic linkages. For example, lithium aluminum hydride reduction normally will not affect olefinic linkages while some hydrogenation methods (e.g., in the presence of a palladium catalyst) will reduce them to saturated linkages.Thus, reduction of the esters of formulas X, XI and XII may produce diols of formulas XIII, XIV and XV, respectively, wherein the broken lines and hydrogen atoms in brackets indicate that the corresponding carbon-carbon bonds may be single or double bonds depending on the method of reduction. It is frequently found that the compounds which contain only single bonds have properties somewhat more favorable than those of the analogous doublebonded compounds. Suitable diol mixtures of this type are commercially available from Henkel Corporation under the trade name "Dimerol".

Other suitable polyhydroxy compounds within this first preferred class may be prepared by reduction of various acrylic acid-unsaturated fatty acid condensation products. These polyhydroxy compounds may be illustrated by formula XVI, wherein m may be, for example, from 3 to 5, p may be from 7 to 9 and the sum of m and p is 1 2. A typical commercially available dicarboxylic acid which may be reduced to a diol of formula XVI is sold under the trade designation "Westvaco 1 550 Diacid"; it has formula XVI I, and is an adduct of linoleic and acrylic acids. It is also described in Kirk-Othmer, op. cit., at p. 779.

A second preferred class of polyhydroxy compounds consists of 1,2-alkanediols in which R' has formula XVI II, wherein R5 is an alkyl radical containing about 8-28 carbon atoms. Examples of suitable R5 radicals are 1-octyl, 2-methylheptyl, 1-nonyl, 2,3-dimethylheptyl, 1-decyl, 2dodecyl, 1-tetradecyl, 1-octadecyl, 1-eicosyl and 1-docosyl. Radicals having the formula R6CH2, wherein R6 is an alkyl and especially a straight chain alkyl radical having about 7-27 and most often about 9-1 7 carbon atoms, are preferred as R5.

In another embodiment of the invention, the polyfunctional acrylates of formula I are used in admixture with at least one ester of formula XIX, wherein R7 is an aliphatic hydrocarbon radical containing about 1-20 carbon atoms and free from acetylenic and polymerizable ethylenic unsaturation; n is from 2 to 4, and m is less than n and is from 1 to 3; at least about 50% of the carboxylate moieties in said admixture having formula XX. This embodiment is of particular value when capacitors having a low dissipation factor over a vary wide temperature range are desired, and typically involves the use of polyfunctional acrylates from the previously described preferred subgenus.

As will be apparent, the esters of formula XIX are mixed esters containing both acrylate moieties and carboxylate moieties derived from a carboxylic acid of the formula R7COOH in which R7 contains about 2-1 9 carbon atoms and is free from acetylenic and polymerizable ethylenic unsaturation. The R7 groups are preferably alkyl radicals, especially those containing 1-7 and most often 2-4 carbon atoms. Illustrative carboxylic acids of this type are acetic, propionic, butyric, 2-ethylhexanoic, lauric, palmitic, stearic and oleic acids.

Since the capacitors of the aforementioned copending applications are frequently produced by a method which includes vapor deposition of monomers followed by polymerization to form the dielectric member, vapor deposition conditions for the ester mixtures are optimized when the carboxylate moieties are substantially similar in their contributions to the volatility of the ester mixture. Therefore, it is most preferred that R7 be of a size and molecular weight similar to that of the corresponding moiety in the acrylate radical, and specifically that R7 be the ethyl radical; that is, that R7COOH be propionic acid.

Of the total number of carboxylate moieties in the ester mixtures of this embodiment of the invention, at least about 50%, usually about 65-99% and preferably about 70-90%, have formula XX. Thus, a major proportion of the carboxylate moieties are polymerizable.

Component B in the compositions of this invention is at least one substantially stable free radical having formula II. In that formula, each R3 is independently a tertiary lower alkyl radical, the term "lower" denoting the presence of up to seven carbon atoms. Illustrative tertiary lower alkyl radicals are t-butyl and 3-ethyl-3-pentyl. The X value may be either CH or N.

In formula II as written, the unpaired electron is shown as present on one of the oxygen atoms. However, those skilled in the art are aware that the molecular structure of the radical is readily a hybrid and that the unpaired electron is delocalized over the entire molecule. It should be understood, therefore, that formula II is merely a convenient shorthand way to represent the molecular structure of component B.

The preferred free radical of formula II for use as component B is the one in which each R3 is t-butyl and X is CH. This compound is commercially available under the designation "Galvinoxyl". The corresponding compound in which X is N is also known and may be designated "azagalvinoxyl".

Component A constitutes the major ingredient in the compositions of this invention.

Component B is present in a minor anaerobic polymerization-inhibiting amount, typically about 75-1500 ppm, and most often about 100-1000 ppm. The compositions may also contain other materials in minor amounts, exemplified by the coinhibitors previously mentioned.

Another embodiment of the present invention is a method for inhibiting premature anaerobic polymerization of a composition comprising component A as defined hereinabove which comprises adding thereto a minor anaerobic polymerization-inhibiting proportion of component B. This method is particularly valuable as an improvement in a method for manufacturing a capacitor which includes the step of depositing on an electrode, under vacuum conditions, a monomer composition comprising component A and subsequently subjecting said monomer composition to radiation-initiated polymerization to form a dielectric layer.

The invention is illustrated by an example in which a monomer system consisting of epoxylated trimethylolpropane triacrylate containing 100 ppm of p-methoxyphenol was heated rapidly under nitrogen from 25"C to 1 76"C and then maintained at 1 76or. The time to peak polymerization exotherm, as detected by differential colorimetry, was determined. For a composition containing no further inhibitor, the peak exotherm occurred at 4.2 minutes. For a composition containing 1000 ppm. of "Galvinoxyl", the peak exotherm occurred at 32 minutes.

In a series of tests similar to the above except that heating to 1 50 C was effected at 1 0 per minute, no peak exotherm had occurred in the composition containing "Galvinoxyl" after 32 minutes. Substantially shorter times to peak exotherm were observed with known inhibitors such as 1,2- and 1,4-naphthoquinone and the 2,2.6.6-tetramethylpiperidnoxy free radical.

Similar results are obtained for a composition comprising a major proportion of the difunctional acrylate of formula VIII and 1 pO ppm. each of p-methoxyphenol and "Galvinoxyl" or azagalvinoxyl.

Claims (21)

1. A composition comprising: (A) a major proportion of at least one polyfunctional acrylate having the formula in Fig. 1, wherein R' is an aliphatic or alicyclic radical containing at least 10 carbon atoms, R2 is hydrogen or a hyrocarbon radical containing 1-5 carbon atoms and n is from 2 to 6; and (B) a minor anaerobic polymerization-inhibiting proportion of at least one substantially stable free radical having the formula in Fig. II, wherein each R3 is independently a tertiary lower alkyl radical and X is CH or N.

2. A composition according to claim 1 wherein each R3 is t-butyl.

3. A composition according to claim 2 wherein R2 is hydrogen or methyl and n is 2 or 3.

4. A composition according to claim 3 wherein X is CH.

5. A composition according to claim 4 wherein component B is present in the amount of about 75-1500 ppm.

6. A composition according to clzaim 5 wherein R1 is an aliphatic, alicyclic or mixed aliphatic-alicyclic radical having about 10-40 carbon atoms which optionally contains up to about 3 olefinic linkages, said olefinic linkages being non-conjugated.

7. A composition according to claim 6 wherein component A has the formula in Fig. VEIL, wherein rand s are each 7 or 8 and the sum of r and s is 15.

8. A method for inhibiting premature anaerobic polymerization of a composition comprising at least one polyfunctional acrylate having the formula in Fig. I, wherein R1 is an aliphatic or alicyclic radical containing at least 10 carbon atoms, R2 is hydrogen or a hydrocarbon radical containing 1-5 carbon atoms and n is from 2 to 6; which comprises adding thereto a minor anaerobic polymerization-inhibiting proportion of at least one substantially stable free radical having the formula in Fig. II, wherein each R3 is independently a tertiary lower alkyl radical and X is CH or N.

9. A method according to claim 8 wherein each R3 is t-butyl.

10. A method according to claim 9 wherein R2 is hydrogen or methyl and n is 2 or 3.

11. A method according to claim 10 wherein Xis CH.

1 2. A method according to claim 11 wherein the amount of said free radical is about 75-1500 ppm.

1 3. A method according to claim 12 wherein R' is an aliphatic, alicyclic or mixed aliphaticalicyclic radical having about 10-40 carbon atoms which optionally contains up to about 3 olefinic linkages, said olefinic linkages being non-conjugated.

14. A method according to claim 1 3 wherein the polyfunctional acrylate has the formula in Fig. VEIL, wherein r and s are each 7 or 8 and the sum of r and s is

1 5.

1 5. In a method for manufacturing a capacitor which includes the step of depositing on an electrode, under vacuum conditions, a monomer composition comprising at least one polyfunctional acrylate having the formula in Fig. I, wherein R' is an aliphatic or alicyclic radical containing at least 10 carbon atoms, R2 is hydrogen or a hydrocarbon radical containing 1-5 carbon atoms and n is from 2 to 6, and subsequently subjecting said monomer composition to radiation-initiated polymerization to form a dielectric layer; the improvement which comprises inhibiting premature anaerobic polymerization of said monomer composition by incorporating therein a minor anaerobic polymerization-inhibiting proportion of at least one substantially stable free radical having the formula in Fig. II, wherein each R3 is independently a tertiary lower alkyl radical and X is CH or N.

1 6. A method acording to claim 1 5 wherein each R3 is t-butyl.

1 7. A method according to claim 1 6 wherein R2 is hydrogen or methyl and n is 2 or 3.

18. A method according to claim 17 wherein X is CH.

1 9. A method according to claim 1 8 wherein the amount of said free radical is about 75-1500 ppm.

20. A method according to claim 1 9 wherein R' is an aliphatic, alicyclic or mixed aliphatic or alicyclic radical having about 10-40 carbon atoms which optionally contains up to about 3 olefinic linkages, said olefinic linkages being non-conjugated.

21. A method according to claim 20 wherein the polyfunctional acrylate has the formula in Fig. VEIL, wherein r and s are each 7 or 8 and the sum of r and s is 1 5.