Rapid Curing Epoxide Composition Employing Stable Carbenium Salt Catalyst

This invention relates to adhesive compositions which fixture in approximately one minute or less and reach their full properties in a short time thereafter in the manner of cyanoacrylates. It relates more particularly to epoxy formulations where rapid curing is achieved by introducing stable carbenium ion initiators into the epoxy.

A major disadvantage of prior art fast room temperature curing epoxies, i.e., mercaptan cured, is that the rapid gelation retards the development of full properties through the restriction of molecular motion and the cure mechanism involved. Extremely fast cures result in overly plasticized product due to the need to use a high level of curative. In addition, for epoxies cured by conventional amine or mercaptan curing agents, there is a critical mix ratio or stoichiometry for the resin/hardener mixture with thorough mixing necessary to achieve full cure.

In U.S. Pat. No. 4,092,296 a catalyst system for room temperature curing of epoxy resins is described which comprises a metallic fluroborate salt such as the tin, copper, zinc, nickel, lead or silver salt.

In U.S Pat. No. 4,396,754 there are described compositions of epoxy compounds and catalysts selected from lithium or group II metal salts of non-nucleophilic acids such as LiBF.sub.4, Ca(BF4).sub.2, Mg(BF4).sub.2, LiPF.sub.6, LiSbF.sub.6, LiAsF.sub.6, etc. Some of these compositions are reported to give very rapid cure at room temperature.

High temperature rapid curing epoxy resin systems are available utilizing high temperature or radiation curing. In these cases cationic ring opening polymerization of the epoxy rather than addition polymerization is achieved.

Suitable photoinitiators are diaryliodonium, diarylsulfonium, dialkylphenarylsulfonium, dialkylphenarylsulfonium, dialkyl4-hydroxyphenylsulfonium, triarylselenonium salts and related materials described in such patents as U.S. Pat. Nos. 4,069,054; 4,069,055; 4,138,255; 4,175,972; 4,175,973; 4,367,261; 4,374,066; 4,398,014; 4,399,071; 4,417,061, etc.

High temperature catalysts commonly employed are Lewis acid amine complexes such as Boron trifluoride monoethylamine which dissociate at elevated temperatures releasing the Lewis acid catalyst BF.sub.3. BF.sub.3 amine complex initiation of epoxies is reported to be described in U.S. Pat. No. 2,717,285.

While BF.sub.3 amine complexes are available which dissociate at room temperature, e.g., BF.sub.3 N-diphenylamine, room temperature curing using stable carbenium ions has not been commercialized.

Kinetic studies of trityl salt initiated polymerization of 1,2-butylene oxide in solution are reported in J. Polymer Sci: Polymer Chem Ed, 11, 1825-1839 (1973) and 14, 2853-2866 (1976). The polymers produced were low molecular weight oligomers which would not be useful as cured adhesives. Storage stable dispersions of trityl salts as initiators for curing epoxy resins to high molecular weight solids are not suggested.

Mechanistic studies of trityl and tropylium salt initiated polymerization of alkyl vinyl ethers, tetrahydrofuran and N-vinylcarbazole are reported by A. Ledwith in ACS Advanced Chem. Ser., 91 317-334 (1969).

It is an aspect of the present invention to produce epoxy compositions which cure rapidly and reach full properties in a short time after gelation.

It is a further aspect of the present invention to produce an epoxy resin system cured by stable carbenium ion catalysts.

It is still a further aspect of the present invention to produce catalyst dispersions which can cure epoxy resins to high molecular weight rapidly and at room temperature.

It is a yet further aspect of the present invention to produce catalyst dispersions which can be mixed with epoxy resins in a convenient mix ratio, where the said catalysts are extended in an inert filler vehicle mixture.

In accordance with the invention there is provided a two part adhesive composition of the type which exhibits rapid room temperature curing, which composition comprises:

At least one oxirane functional resin selected from di- and polyepoxide compounds.

Part one may additionally comprise polymeric additive(s) or filler(s).

(a) A stable carbenium ion salt, i.e., a triphenylmethyl or tropylium salt as hereinafter defined.

(b) A solvent for the said salt.

(c) Optionally a liquid vehicle in which the salt solution may be stored or extended.

(d) Optionally fillers and additive(s).

The diepoxide and polyepoxide compounds of Part I include any material containing a plurality of oxirane functional groups. Preferred resins are those which result from bisphenol-A (4,4'-isopropylidene diphenol) and epichlorohydrin, i.e., bisphenol-A diglycidyl ether (DGEBA), and extended prepolymers thereof. Other particularly suitable resins include cycloaliphatic epoxies, and epoxidized novolacs.

Stable carbenium ion salts or carbocation salts include certain triphenyl methyl (Ph.sub.3 C) or tropylium salts. These salts are defined by the formulas Ph.sub.3 C.sup.+ X.sup.- or C.sub.7 H.sub.7.sup.+ X.sup.-, wherein X.sup.- is a weak nucleophile such as SbCl.sub.6.sup.-, SbCl.sub.3.sup.-, SbF.sub.6.sup.-, AsF.sub.6.sup.-, SnCl.sub.5.sup.-, PF.sub.6.sup.-, BF.sub.4.sup.-, CF.sub.3 SO.sub.3.sup.-, ClO.sub.4.sup.-.

Such catalysts are effective at levels of about 0.1-10% by weight (%W) and are especially preferred at levels of 2-5%W of the total formulation. The catalysts are suitably present in the part 2 composition at levels of 2-25%W.

Suitable solvents for the carbenium ion salts include some ketones, ethers and esters. Particularly effective solvents are 3,5,5-trimethyl-2-cyclohexen-1-one(isophorone), 2,6-dimethyl-4-heptanone(isovalerone), diethyleneglycol monoethyl ether, tetrahydrofurfuryl alcohol, 2-ethoxyethylacetate (Cellsolve.sup.R Acetate), and 2-(2-ethoxyethoxy)ethyl acetate (Carbitol.sup.R Acetate). Preferably the solvent is nonvolatile, non-nucleophillic and moisture free. Solvents commonly comprise about 15%W of the part 2 formulation.

Likewise liquid vehicles for extending the salt solutions must be non-nucleophillic and moisture free. Suitable vehicles include polyepichlorohydrin, polybutenes, hydroxy and terminated polybutadienes, carboxy terminated polybutadienes, polyglycols and polyglycol esters. Such vehicles commonly comprise 40%W of the catalyst dispersions.

Since alkaline materials retard or inhibit cationic curing, fillers and pigments should be acidic. the acidity of fillers is determined by ASTM test D 1208-7.8. Since the trityl and tropylium salts are moisture sensitive fillers are also chosen on the basis of moisture content which is preferably less than 0.5% and still more preferably less than 0.1%.

Examples of acceptable fillers are Anhydrous aluminum silicates, Anhydrous calcium silicates, Barytes, Amorphous silica (Imsil 54), Kaolin and powdered aluminum. Fillers commonly comprise 40%W of the catalyst dispersions.

The invention will be better understood by reference to the following examples.

A two-part epoxy formulation described below was prepared. Mixing both parts 1:1 generates cure in 30 seconds and gives the following properties after 16 hours of room temperature aging. It should be noted a large exotherm accompanies cure. Gelation and vitrification are effectively simultaneous and follow an induction period.

Tensile Shear Strength Solvent Wiped Mild Steel

ASTM-D-1002-64: 1360 psi

Shore D Hardness: 65 D

Hence the catalyst may be extended to give a convenient mix ratio, two-part epoxy formulation which exhibits rapid curing and high strengths.

Table 1 illustrates the difference in the rate of strength development for cationically and Loctite E-POX-E.sup.R QM-50, a typical bisphenol-A resin based two-part mercaptan cured fast epoxy system. The resin cured by carbocation catalysis reaches effectively full strength after 30 minutes and has greater than 50% ultimate strength after ten (10) minutes. The mercaptan cured epoxy gels in five (5) minutes but requires up to two hours for 50% strength development.

A two-part epoxy formulation described below was prepared. A 30 second cure time was recorded on mixing two grams of each component. The following properties were obtained after 16 hours of room temperature aging.

Tensile Shear Strength Solvent Wiped Mild Steel

ASTM-D-1002-64: 1480 psi

Shore D Hardness: 65 D

A two-part epoxy formulation described below was prepared. The formulation was cured in a no-mix fashion, i.e., with Part A applied to one substrate and Part B to a second with a bond being formed when the two substrates were brought together. The substrates were solvent wiped mild steel lapshears and a 30 second fixture time was recorded.

Tensile shear strength of 310 psi were recorded after 16 hours room temperature aging.

Hence it can be seen that with these catalysts stoichiometry is non-critical. Thorough mixing is required in examples one and two to ensure homogeneity with respect to the fillers.

Examples 1 to 3 illustrate preferred embodiments of the development. The remaining examples describe some of the catalyst, solvent, carrier, resin, and filler variations utilized within the scope of the invention.

Example 4 illustrates the use of these catalysts with epoxidized novalak resins. Additionally Example 4 describes the use of a new solvent. A two-part epoxy formulation described below was prepared. On mixing the two parts in equal weights (2 gms of each) a 50 second cure time was recorded. The following properties were obtained after 16 hours room temperature aging.

Tensile Shear Strength Solvent Wiped Mild Steel

ASTM-D-1002-64: 1000 psi

Shore D Hardness: 70 D

Example 5 illustrates the use of non-mineral fillers, i.e., powdered metal filler, synthetic polymer thickener. A two-part epoxy formulation described below was prepared. Cure time on mixing equal parts by weight was 35 seconds. The following properties were obtained after 16 hours of room temperature aging.

Tensile Shear Strength Solvent Wiped Mild Steel

ASTM-D-1002-64: 1487 psi

Shore D Hardness: 65 D

Example 6 illustrates the use of other fillers. The formulations and results are described below.

Example 7 illustrates the use of other carbocation catalysts.

Example 8 illustrates the use of alternative liquid vehicles as described previously Part 2 (III).