Metal Carboxylate/Alcohol Curing Catalyst for Polycyanate Ester of Polyhydric Phenol

The field of art to which this invention pertains is cyanate esters and catalysts for curing such esters.

Cyanate esters, which are described in U.S. Pat. No. 3,553,244, are prepared by reacting polyhydric phenols with cyanogen chloride. Such cyanate esters can be cured by heat alone, but, preferably, are cured by using a catalyst plus heat to form thermoset resins useful as laminating and molding resins. In U.S. Pat. No. 3,962,184, the use of zinc octoate, catechol and triethylenediamine as catalysts for cyanate esters is described. This patent also describes the use of imidazoles alone or combined with organic metal salts, such as zinc octoate, tin octoate, tetrabutyl ester of titanic acid, zinc stearate, tin stearate, or calcium stearate, and phenolic compounds, such as phenol or catechol.

Other references which disclose metal salts and/or aromatic hydroxy compounds as catalysts for cyanate esters are U.S. Pat. Nos. 4,026,913; 4,110,367; 4,195,132; 4,429,112; 4,330,,658 and 4,330,669.

Chelates of iron, cobalt, zinc, copper, manganese, zirconium, titanium, vanadium, aluminum and magnesium with bidentate ligands, e.g., catechol, are described as being useful to catalyze the cure of cyanate esters in U.S. Pat. No. 3,694,410.

Metal salts, when used as catalysts, do not readily dissolve in the cyanate esters. They can form insoluble or gel-encrusted coated droplets which remain as undesirable particulates in the cured compositions. Also such catalysts require high temperatures and/or long cure times in order to complete the cure. Complete cures are obtained when substantially all of the cyanate ester groups are reacted to form triazine ring structures. Compositions which are incompletely cured will whiten, blister, become embrittled, or even soften to a slime consistency when exposed to steam over a period of time.

This invention relates to curable compositions made from polycyanate esters of polyhydric phenols. In one aspect, this invention pertains to curable compositions which are made from polycyanate esters and liquid, readily soluble, catalysts. In another aspect, this invention relates to catalyzed polycyanate ester compositions which cure completely to form heat and moisture resistant thermoset compositions.

The curable compositions of this invention are made from a polycyanate ester of a polyhydric phenol and a catalyst which is a liquid solution of a metal carboxylate and an alcohol. The metal carboxylate is a metal salt of an acid which contains 4 carbons atoms up to about 24 carbon atoms, wherein the metal is classified as a transition metal cation and is characterized as coordinating, i.e., having 9 or more electrons in the next-to-outermost shell. The most useful of these transition metal cations are copper, manganese, tin, lead, zinc, cobalt, and nickel, all in the divalent state; iron and chromium, both in the trivalent state; and tetravalent titanium. The alcohol is a monohydric alcohol which is liquid at room temperature, which has a boiling point greater than 160.degree. C. at 760 mm Hg pressure and which has a solubility of less than 10 percent by weight in water. The metal carboxylate is used in an amount to provide from about 0.001 to about 0.5 part of metal per 100 parts of the polycyanate. The alcohol is used in an amount to provide from about 3 to about 100 milliequivalents of active hydrogen per equivalent of cyanate group. The catalyst blend, that is, the metal carboxylate dissolved in the alcohol, is used in the amount of about 0.5 to about 20 parts by weight per 100 parts by weight of polycyanate.

The polycyanate esters of polyhydric phenols useful in this invention are described in U.S. Pat. No. 3,553,244, which is hereby incorporated by reference. These polycyanate esters, which contain more than one cyanate ester group per molecule, are prepared by reacting a cyanogen halide with a polyhydric phenol. Examples of cyanogen halides are cyanogen iodide, cyanogen bromide and cyanogen chloride, with cyanogen chloride being preferred. Polyhydric phenols, from which these polycyanate esters are derived, include resorcinol, p,p'-dihydroxy-diphenyl, o,p'-dihydroxydiphenyl methane, p,p'-dihydroxydiphenyl propane (bisphenol A as it is commonly called), p,p'-dihydroxydiphenyl sulfone, p,p'-dihydroxydiphenyl sulfide, p,p'-dihydroxydiphenyl oxide, 4,4'-methylenebis(2,6-dimethylphenol), 4,4'-(hexafluoroisopropylidene)diphenol, p,p'p'-trihydroxytriphenyl phosphate, dihydroxy naphthalene and novolac resins which contain more than 2 phenol moieties per molecule. A preferred polycyanate ester is the dicyanate ester of bisphenol A, with the most preferred being the dicyanate ester of bisphenol A having a purity equal to, or greater than, 99.0 mole percent. Partially trimerized dicyanates, termed prepolymers, which characteristically have from about 5% to about 60% of the monomeric functionality converted to triazine (or cyanurate) rings by heat processing with attendant increase in molecular weight are also catalyzed by the metal carboxylates and monohydric alcohols of this invention. The monomeric polycyanates and prepolymers obtained therefrom can be catalyzed as neat liquids, hot melts or liquid solutions.

The metal carboxylates useful in this invention are the transition metal soaps of C.sub.4 to about C.sub.24 carboxylic acids. Many of these metal carboxylates are sold as "driers" for oleoresinous coatings and varnishes. The metals useful in this invention are classified as transition metal cations, and are characterized as coordinating, i.e., having 9 or more electrons in the next-to-outermost shell. Examples of these transition metal cations are copper, manganese, nickel, cobalt, zinc, lead and tin, all in the divalent state; iron and chromium, both in the trivalent state; and titanium in the tetravalent state. The carboxylic acid portion of the metal carboxylate contains 4 up to about 24 carbon atoms per molecule. Such acids are butanoic acid, acetoacetic acid, pentanoic acid, hexanoic acid, heptanoic acid, decanoic acid, dodecanoic acid, naphthenic and naphthanoic acids, abietic acid, and the fatty acids derived from vegetable and tall oils. Preferred metal carboxylates are the naphthenates of copper, manganese, nickel, cobalt and zinc, with copper naphthenate being the most preferred.

Alcohols useful in this invention dissolve the metal carboxylates forming stable solutions. Such alcohols are monohydric alcohols which are liquid at room temperature (25.degree. C.), which are high boiling, i.e., having a boiling point above 160.degree. C. at 760 mm Hg pressure and which are hydrophobic, i.e., having a solubility of less than 10 percent by weight in water. Examples of such alcohols are aliphatic alcohols which contain at least 5 carbon atoms, aryl substituted alcohols, cycloaliphatic alcohols and alkyl ether glycols wherein the alkyl group contains at least 5 carbon atoms. Useful aliphatic alcohols include pentanol, hexanol, octanol, 2-ethylhexanol, decanol, dodecanol and the like. Useful aryl substituted alcohols are benzyl alcohol and alkylated benzyl alcohol such as p-butyl benzyl alcohol. Examples of cycloaliphatic alcohols are 5-norbornene-2-methanol, 6,6-dimethylbicyclo-[3.1.1]hept-2-ene-2-ethanol, terpineol, and the like. Examples of alkyl ether glycols are alkyl ethers of ethylene glycol, propylene glycol and butylene glycol wherein the alkyl group contains 5 to about 18 carbon atoms, such as the monoamyl ether of propylene glycol, the monohexyl ether of ethylene glycol and the like. These alcohols are used in the amount of about 3 to about 100 milliequivalents of active hydrogen per cyanate equivalent in the polycyanate of the polyhydric phenol. Minor portions of polyols can be used in conjunction with the monohydric alcohols provided that the blends are liquid at room temperature.

The catalyst solution, i.e, the metal carboxylate dissolved in the alcohol, is used in the amount of about 0.5 to about 20 parts by weight per 100 parts by weight of the polycyanate and, preferably, about 1.0 to about 8 parts by weight per 100 parts by weight of the polycyanate.

The compositions of this invention can be cured by heating at elevated temperature for a time sufficient to obtain a complete cure. The curing reaction can be conducted at one temperature or can be conducted by heating in steps. If conducted at one temperature, the curing temperatures will vary from about 250.degree. to about 450.degree. F. When conducted by stepwise heating, the first step, or gelation step, is performed at a temperature of about 150.degree. to about 350.degree. F. The curing step is conducted at a temperature of about 300.degree. to about 450.degree. F., and the optional post-curing step is conducted at a temperature of about 400.degree. to about 500.degree. F. Generally the overall curing reaction will take from about 5 minutes to about 8 hours.

When the polycyanate and the curing catalyst of this invention are heated, the aromatic cyanate functionality trimerizes more extensively leaving less than 20%, preferably, less than 5%, of the original cyanate groups untrimerized. Unreacted cyanate groups in the crosslinked resin lower thermal performance and are especially deleterious to performance measured after conditioning in moist environments. With exposure to water or water vapor, cyanates hydrolyze to carbamate esters, which are both hydrophilic and function as carbon dioxide blowing agents at elevated temperatures. Residual cyanate groups in polycyanurate networks result in higher moisture absorption, which advesely affects: (a) dimensional stability (swelling and higher coefficients of thermal expansion); (b) mechanical strength and stiffness at elevated temperatures (lower heat distortion temperature); (c) electrical insulating properties; (d) stability in contact with molten solders; and (e) cyanurate hydrolysis, blistering and outgassing.

The catalysts of this invention are also useful in curing blends of polycyanate esters and polyepoxide resins wherein up to about 70 weight percent of the blend is the polyepoxide resin. Such polyepoxide resins are the well-known glycidyl ethers of polyhydric phenols which are made by reacting an epihalohydrin, preferably, epichlorohydrin, with a polyhydric phenol. Such polyhydric phenols are described hereinbefore in the description of polycyanate esters. Preferred polyepoxide resins are those derived from bisphenol A and tetrabromobisphenol A having an epoxide equivalent weight of about 180 to about 750.

When formulating for particular end uses, additional components can be incorporated in the polycyanate compositions. Such components include minor amounts of thermoplastic resin tougheners, reinforcing fibers, colloidal silica flow modifiers, mineral fillers and pigments.

The cured composition of this invention can be used in vacuum bagged structural composites, transfer molded encapsulants, filmed structural adhesives, printed wiring boards and composites for aircraft primary structures. Manufacturers of printed wiring boards, structural composities and plastic encapsulated semiconductors prefer that curing be accomplished at temperatures of 350.degree. F. or below.

The invention is described in more detail in the following examples. Parts and percentages, unless otherwise indicated, are parts and percentages by weight.

A catalyst package was prepared by blending 1.3 parts of benzyl alcohol with 0.25 part of copper naphthenate (8% Cu grade from Mooney Chemical Company). A clear liquid was obtained. Bisphenol a dicyanate monomer (100 parts), analyzing 99.7 mole percent pure by Differential Scanning Calorimeter (DSC) melt analysis, was melted in a 250 ml. 3-neck flask in an oil bath. At a temperature of 200.degree. F., the above described catalyst package was added and stirred to dissolve the catalyst into the molten dicyanate. Catalyst concentrations were 0.02 part per hundred resin (phr) copper (as metal) and 16 milliequivalents (meq) active hydrogen per cyanate equivalent. Afer vacuum deairing, the catalyzed dicyanate melt was poured into an aluminum sheet mold preheated to 220.degree. F. The molten material gelled in 120 minutes at 220.degree. F., after which the oven temperature was increased to 350.degree. F. and the casting was cured for a period of 3 hours. An optically clear, yellow casting of 1/8 inch thickness was obtained, 1/2 of which was sawed and milled without chipping into a series of test bars. The following properties were obtained:

No visual change was apparent in the test bars immersed in solvent and 200.degree. F. moisture vapor.

A second portion of the casting was post-cured for 1 hour at 450.degree. F. and tested in an identical manner. The results, shown below, indicate that aside from an expected increase in dry HDT, little improvement in performance is achieved with higher temperature curing.

To demonstrate the effectiveness of the use of the combination of metal and active hydrogen compound as catalyst versus the metal withough active hydrogen compound and the active hydrogen compound without metal, the molten dicyanate ester of bisphenol A having a purity of 99.7% was mixed with the following reactants and was heated to form a thermoset plastic and evaluated: (A) benzyl alcohol and copper naphthenate solution; (B) dibutylphthalate and copper naphthenate solution; (C) benzyl alcohol; (D) copper naphthenate.

In (B) the copper naphthenate was dissolved in dibutylphthalate, which contains no active hydrogen. The amounts of each component and the cured properties are listed in Table I.

Using the same procedure described in Example 1, the dicyanate ester of bisphenol A was cured with copper naphthenate predissolved in the monohexyl ether of ethylene glycol (Hexyl Cellosolve). The amounts of each component and the cure properties are listed in Table II.

A catalyst blend was prepared by heating and stirring 13 parts of liquid diglycidyl ether of bisphenol A having an epoxide equivalent weight of 185 with 1.0 part of zinc octoate (18% Zn grade) until a clear liquid was obtained. Upon cooling the blend was a viscous hazy, but uniform, liquid. The high purity grade of bisphenol A dicyanate used in the previous examples was melted and 140 parts were mixed at 200.degree. F. with 1.4 parts of the zinc octoate/liquid epoxy resin catalyst to form a clear, uniform catalyzed melt containing 0.013 phr zinc (as metal) and <5 meq. of active hydrogen per cyanate group. The melt was poured into an aluminum mold preheated to 300.degree. F. and gelled within 5 minutes. After curing a total of 3 hours at 450.degree. F. plus 2 hours at 482.degree. F., the casting was sawed and milled into test bars. A dry Heat Distortion Temperature (HDT) value of 226.degree. C. was obtained. After conditioning 64 hours at 200.degree. F. and >95% Relative Humidity in a humidity chamber, the bar had absorbed 3.08% moisture and had partially hydrolyzed as noted by a slimy outer layer and warped appearance.

This example shows that even long, high temperature curing cycles are incapable of complete cyanate trimerization when the dicyanate is of high purity and no active hydrogen compounds are incorporated.

Using the same procedure described in Example 1, catalyst packages were prepared from a number of transition metal carboxylates and benzyl alcohol. Again using the procedure described in Example 1, bisphenol A dicyanate monomer was cured with the catalyst blends. Details of the composition and the cured properties are listed in Table III.

Using the same procedure described in Example 1, the dicyanate ester of 4,4'-methylenebis(2,6-dimethylphenol) was cured with a catalyst blend of copper naphthenate in benzyl alcohol and zinc naphthenate in benzyl alcohol. Details of the compositions and cured properties are listed in Table IV.

The dicyanate ester of bisphenol A, 64 parts, was blended with 46 parts of a glycidyl polyether of tetrabromobisphenol A having an epoxide equivalent weight of 675 and 50 parts of a diglycidyl ether of bisphenol A having an epoxide equivalent weight of 190 at a temperature of 210.degree. F. A blend of 0.30 part of zinc naphthenate (8% zinc) and 1.7 parts of benzyl alcohol was added to the resin blend at 200.degree. F. and was well mixed and deaired. The molten mixture was poured into an aluminum sheet mold preheated to 220.degree. F. The resinous mixture gelled in 25 minutes at 220.degree. F. The gelled casting was then heated at 350.degree. F. for one hour. One-half of the casting was sawed and milled without chipping into a series of test bars. Infrared spectra of the casting showed no residual cyanate or epoxy groups. The following properties were obtained from the test bars:

A second portion of the casting was post cured for 1 hour at 420.degree. F. and was tested as described above.

The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention.