SAG RESISTANT TWO-COMPONENT URETHANE ADHESIVES WITH IMPROVED THERMAL STABILITY

Abstract: An adhesive comprising a mixture of (A) a mixture of an isocyanate prepolymer and a polyepoxide which is essenti­ ally free of hydroxyl groups, and (B) a mixture of a polyol, a urethane catalyst and an epoxy curing catalyst and structures prepared by adhering substrates to one another by means of said adhesive are described.

Description: This invention relates to two component adhesive compositions in which each component has gravity flow character comprising a first component composed of an isocyanate prepolymer and a polyepoxide resin and a second component composed of a curative comprising a mixture of polyols, a di- or tri-primary or secondary amine, a urethane and/or isocyanurate catalyst and an epoxide curing catalyst and to a process for the preparation and cure of said adhesive compositions. Two component urethane adhesives are well known in the art (US-A 3,714,127; 3,812,003; 3,886,122; 3,935,051; and 4,444,976, for instance) and are widely used to adhere substrates such as plastic adherends together. The choice of urethane adhesives over other adhesives is based in part upon their outstanding characteristics with respect to bond strengths, flexibility, chemical inertness, and the like. Such prior art urethane adhesives are also known to have poor resistance to elevated temperatures (approximately 205°C). Two component urethane adhesives having nonsag (do not flow or drip) properties when applied to vertical surfaces plus high heat resistance have not been disclosed previously in the prior art. The present invention is an improvement over EP-A-193 099. The improvements include increased shelf stability of the two components, excellent sag resistance developed immediately upon mixing the two components. These and other improvements are accomplished by including urethane/isocyanurate and epoxy curing catalysts in the curative component. The choice of urethane adhesives over other classes of adhesives is usually based on the outstanding flexibility, bond strength, chemical inertness, etc. of the former. Some of these urethane compositions and their use in two-component adhesives are reported in the above cited U.S. Patents. Generally speaking, conventional polyurethane-based adhesives are gravity flowable, highly viscous two-component systems wherein one component is composed of an isocyanate prepolymer and the second component consists of an admixture of a polyhydroxy compound free of isocyanate groups, small amounts of di- or tri-primary or secondary amines and selected urethane catalysts. The polyurethane adhesives formed by mixing the above two components provide the nonsag properties by the initial immediate reaction of polyisocyanate with polyamine followed by the curing of polyols with polyisocyanate. The conventional adhesives based on the above polymerization reactions are known to have poor high heat resistance and usually suffer loss of adhesion after high temperature exposures near 205°C for 30 minutes or more. Because some of the adhesive operations such as in automotive assembly of fiberglass reinforced plastics, require high heat resistance, prior art urethane adhesives have been found to be unsuitable for such high temperature applications. It is the object of this invention to provide a improved polyurethane based adhesive compositions having significantly improved thermal stability (high heat resistance) in addition to the other desired properties such as (1) gravity flow in each of the two components with good shelf stability in each, (2) an immediate nonsagging behavior particularly on vertical surfaces upon mixing of the two components, (3) noncritical mix ratio (ratio overlap tolerance in mixing of the two components), (4) flexibility to avoid any possible readout in the adhered fiber reinforced plastic parts, (5) reasonable room temperature open time (pot life) and fast cure time at moderately elevated temperatures for the adhesive mixture and (6) high peel and lap shear bond strengths for the adhesive after it is applied to substrates and the samples are cured. This object has been achieved by the two-component, polyurethane-based adhesive compositions of the invention which satisfy all of the above listed properties. The two-component adhesive system of this invention comprises (A) a first gravity flowable component comprising a mixture of a isocyanate prepolymer obtained by the reaction of a poly (alkylene oxide) diol with an excess of more than two equivalents of NCO per hydroxy group of a polyisocyanate and 2 to 30% by weight based on the weight of the isocyanate prepolymer of essentially hydroxy group free diepoxide resin or their prepolymers and this component optionally may be filled with fillers such as talc, kaolin, aluminum oxide, silica, metals and metal oxides, etc. and (B) a stable gravity flowable curative second component comprising a mixture of a polyol (preferably a polyether polyol or polyester polyol), 0.2 to 5 weight percent of di- or tri-primary or secondary amine group containing compound along with an urethane catalyst and an epoxy-curing catalyst and the curative (B) may optionally be filled with fillers such as talc, silica, kaolin, aluminum oxide, metal oxides and metals. The preferred urethane catalysts include tertiary amines such as triethylene diamine, N, N′, N″-tris-(dimethylaminopropyl) hexahydrotriazine, organotin catalysts such as dibutyltin dicarboxylates, stannous octoate and the like, potassium octoate, iron acetyl acetonate, zinc octoate, nickel acetyl acetonate and the like and mixtures thereof. The epoxy curing catalysts include tertiary amines, imidazoline, onium salts such as tetra alkyl ammonium halides such as methyl trialkyl ammonium chloride titanates boron trifluoride-amine complexes, trifluoride-ethylamine complex, lithium chloride, and phenolics and the like and mixtures thereof. The amounts of each of such urethane and epoxy-curing catalysts used depends on the desired open time and cure time requirements and may be from 0.05 to 8% by weight of the curative component. The polyepoxides useful in this invention can be monomeric or polymeric, saturated or unsaturated, aliphatic, cycloaliphatic, aromatic or heterocyclic, and they may be substituted if desired with other substituents besides the epoxy groups, e.g., ether radicals, halogen atoms, and the like. Typical epoxy compounds suitable in the practice of this invention include those disclosed in, US-A- 2,500,600 and 2,324,483. Preferred in this invention are 1,2-epoxy compounds having an epoxide equivalence greater than 1, that is to say, compounds containing more than one group of the formula: The 1,2-epoxide groups may be either terminal or inner ones. Particularly suitable terminal 1,2-epoxide groups are 1,2-epoxy ethyl or 1,2-epoxy propyl groups. The latter may be linked to an oxygen atom that is to say they are glycidyl ether or glycidyl ester groups such as diglycidyl ether of Bisphenol-A. Compounds with inner epoxide groups usually contain the 1,2-epoxide group in an aliphatic chain or in a cycloaliphatic ring. As epoxy compounds containing an inner 1,2-epoxy group there are suitable epoxidized diolefins, dienes, or cyclic dienes, such as 1,2,5,6-diepoxy hexane, 1,2,4,5,-diepoxy cyclohexane, dicyclopentadiene diepoxide, dipentene diepoxide, vinyl cyclohexane diepoxide, epoxidized diolefinically unsaturated carboxylic acid esters, such as methyl-9,10,12,13-diepoxy stearate or the dimethyl ester of 6,7,10,11-diepoxyhexade-­ cane-1,16-dicarboxylic acid. Furthermore, there may be mentioned epoxidized mono-, di-, or polyesters and mono-, di-, or polyacetals containing at least one cycloaliphatic 5-membered or 6-membered ring, to which at least two 1,2-epoxidized groups are linked. A widely used class of polyepoxides which can be used in the present invention are the epoxy polyethers obtained by reacting a halogen containing epoxide or dihalohydrin, such as epichlorohydrin, epibromohydrin, 3-chloro-1,2-epoxyoctane, and the like with either a polyhydric phenol or a polyhydric alcohol. The polyisocyanates useful in this invention include organic isocyanates having at least two isocyanate groups per molecule. The polyisocyanates can be of low, high or intermediate molecular weight and can be any of a wide variety of organic polyisocyanates including ethylene diisocyanate, trimethylene diisocyanate, dodecamethylene diisocyanate, hexamethylene diisocyanate, hexamethylene diisocyanate trimer, tetraethylene diisocyanate, pentamethylene diisocyanate, propylene-1,2-diisocyanate, 1,4-diisocyanato cyclohexane, cyclopentene-1,3-diisocyanate, p-phenylene diisocyanate, 1-methyl phenylene-2,4-diisocyanate, naphthalene-1,4-diisocyanate, toluene diisocyanate, diphenyl-4,4′-diisocyanate, benzene-1,2,4-triisocyanate, xylene-1,4-diisocyanate, xylylene-1,3-diisocyanate, 4,4′-diphenylene methane diisocyanate, 4,4′-diphenylene propane diisocyanate, 1,2,3,4-tetraisocyanato butane, butane-1,2,3-triisocyanate, polymethylene polyphenyl isocyanate, and other polyisocyanates having an isocyanate functionality of at least two more fully disclosed in US-A- 3,350,362 and 3,382,215. Polyisocyanates which are polymeric in nature including isocyanate prepolymers of all types are included in this invention such as a prepolymer which is prepared by the reaction of methylene bis (phenyl isocyanate) with a poly (propylene oxide) diol. The polyols useful in this invention include those having at least two hydroxyl groups per molecule and having equivalent weights falling in the range of from 30 to 5000. Specific polyols include butane diol, cyclohexane dimethanol, tripropylene glycol, amide diols, urethane diols, polyether polyols such as poly (tetramethylene ether) diols, poly (propylene ether) polyols, polyester polyols, and the like. Polyhydroxy polyethers are suitable and preferably those having at least 2 hydroxyl groups per molecule. Polyhydroxy polyethers can be prepared by polymerization of epoxides such as ethylene oxide, propylene oxide, butylene oxide, tetrahydrofuran, styrene oxide, or epichlorohydrin either on their own or by chemical addition to other materials such as ethylene glycol, propylene glycol, trimethylol propanes and 4,4′-dihydroxy diphenyl propane. Sucrose polyethers also may be used. Polybutadienes having hydroxyl groups as well as other known hydroxyl containing vinyl addition polymerized polymers can be used. According to the present invention, hydroxyl containing polyesters, polythioethers, polyacetals, polycarbonates or polyesteramides of the types known for the formation of polyurethanes such as the reaction product of an exess of poly (propylene oxide) tetraol with toluene diisocyanate may also be used. The primary or secondary di- or tri-amines used for the rapid thixotrophy build up to give the non-sag behavior in the mixed adhesive include those conforming to the formula H₂NRNH₂ (where R is a hydrocarbon or hydrocarbon ether group having from 2 to 50 carbon atoms) such as ethylene diamine and the like and piperazine and its derivatives such as amino ethyl piperazine and the like. Additives such as plasticizers, antioxidants, colorants, pigments and the like may be included in either or both of the components. The adhesive compositions of this invention involving the two gravity flow components exhibit excellent adhesion to a wide variety of substrates including fiberglass reinforced plastic, metal, wood and the like and combinations thereof, and the adhesive bonds withstand high heat bake cycles of around 205°C for a period of one hour or more. The two components, (A) and (B) may be mixed to form an adhesive in such a manner that the total isocyanate functionality to the combined hydroxy and epoxy functionality of the ingredients are in the equivalent ratio of 0.8 : 1 to 3 : 1, preferably 0.9 : 1 to 0.5 : 1. This invention is further illustrated in the following representative examples. EXAMPLE 1 An isocyanate prepolymer having long shelf life of about six months or more was prepared by mixing 118 g of liquid methylene bis(phenyl isocyanate) (NCO equivalent weight of 144) at 8O°C with 132.1 g of a mixture obtained by blending 59 g of ethylene oxide capped poly(propylene oxide) diol (hydroxy equivalent weight of 500), 28.3 g of liquid diglycidyl ether of Bisphenol-A (epoxy equivalent weight 180-190) and 62.3 g of Kaophile (alumina treated kaolin) filler. The prepolymer was found to have about 11.7% by weight of NCO groups. EXAMPLE 2 An isocyanate prepolymer was prepared by reacting 43.7 parts by weight of 2000 molecular weight poly(propylene oxide) diol capped with ethylene oxide with 132.5 parts by weight of oligomeric methylene bis(phenyl isocyanate) (NCO functionality of 2.7) followed by mixing with 23.7 parts by weight of liquid methylene bis(phenyl isocyanate) (NCO functionality of 2.1) and 23.8 parts by weight of dry talc filler. This isocyanate prepolymer was mixed with 20% by weight of liquid glycidyl ether of Bisphenol-A and 5% by weight of talc. EXAMPLE 3 A curative component was prepared by reacting 53 parts by weight of poly(propylene oxide) tetraol (hydroxy equivalent weight 450) and 28.6 parts by weight of ethylene oxide capped poly(propylene oxide) tetraol (hydroxy number 453) with 1.96 parts by weight of toluene diisocyanate followed by mixing with 5.3 parts by weight of Aerosil 200 (fumed silica) 1.51 parts by weight of piperazine, 0.4 parts by weight of triethylene diamine and 2 parts by weight of boron trifluoride-ethylamine complex. EXAMPLE 4 A curative component was prepared by reacting 53 parts by weight of poly(propylene oxide) triol capped with ethylene oxide (hydroxy equivalent weight of 1167) with 2.95 parts by weight of toluene diisocyanate followed by mixing with 34.76 parts by weight of ethylene oxide capped poly(propylene oxide) tetraol (hydroxy equivalent weight 450). The resulting mixture was filled with 41.8 parts by weight of dry kaolin filler and mixed with 0.12 parts by weight of iron (III) acetonyl acetonate, 0.064 parts by weight of stannous octoate and 1.54 parts by weight of piperazine. EXAMPLE 5 A curative component was prepared by mixing 0.6% by weight of methyl trialkyl (C₈-C 10 ) ammonium chloride catalyst (Adogen 464 from Sherex Chemical Company, Inc.) with the curative of Example 4. EXAMPLE 6 The following is the general procedure followed for preparing the adhesives and the adhesive bonds used throughout the following Examples. The two components were mixed in appropriate weight amounts under an inert atmosphere (nitrogen) at room temperature. The resulting adhesive mixture was applied to the substrate in the form of a 3/8 inch bead across the substrate SMC (sheet molding compound) laminates which measured 30.5 × 10.2 × 0.25 cm (12 × 4 × 0.1 inch). The substrate was first wiped clean with a dry rag or with a rag containing methylene chloride. After sprinkling a few 0.75 mm (30 mils) diameter glass beads on the top of the adhesive to get a final glue line thickness of 0.75 mm (30 mils), the other laminate sheet was placed on top of the first with a one-inch overlap. The samples were kept at room temperature under contact pressure until the adhesive had gelled, (usually in from 10 to 60 minutes) and then were placed in an oven at 140°C for postcuring for 30 minutes. Test specimens were cut from these cured samples into 2.54 cm (1 inch) wide strips. The following tests were carried out by using a set of at least three samples for each type of test: (A) Shear strength test at room temperature (B) Postbaked at 205°C for an additional hour and tested shear strength at room temperature (C) Postbaked at 205°C for one hour and tested shear strength at 82°C (D) Shear strength at room temperature after immersion for 24 hours in 97°C water (E) Shear strength at 54°C after 7 days immersed in water at 54°C (F) Shear strength at 54°C after 205°C postbaked (1 hour), samples immersed in 54°C water for seven days. EXAMPLE 7 The prepolymer of Example 1 (23.5 g) and the curative of Example 5 (25.2 g) were mixed for about one minute to give a nonsagging adhesive mixture. This mixture was applied on a fiberglass reinforced plastic panel (fiberglass reinforced polyester, Phase Alpha SMC from Ashland Chemical Company) by following the general bonding procedure of Example 6. The lap shear bond test results, which show good adhesion even after the 205°C for 1 hour postbake cycle, are given in Table 1. EXAMPLE 8 The prepolymer of Example 1 (25 g) was mixed with the curative of Example 4 (25 g) to give a nonsagging adhesive paste. The paste was applied to unprimed SMC from Rockwell (RW 9468) and the cured adhesive bonds were tested. The results, which clearly show good adhesion even after the most severe tests, are summarized in Table 2. EXAMPLE 9 The prepolymer component of Example 2 (40 g) and the curative component of Example 3 (10 g) were mixed to give a nonsagging adhesive paste [weight ratio of first (prepolymer) component to second (curative) component of 4 : 1] which was applied on Diversitech General SMC (GT 7113) and bonded according to the procedure of Example 6. Similarly 60 g of the prepolymer of Example 2 were mixed with 10 g of the curative of Example 3 (ratio of 6 : 1) and tested as an adhesive. The results of these tests are given in Table 3.

Claims: 1. Two-component adhesive composition comprising (A) a first gravity flowable component comprising a mix­ ture of a isocyanate-prepolymer obtained by the reac­ tion of a poly(alkyneoxide)diol with an excess of more than two equivalents of NCO per hydroxy group of a polyisocyanate and 2 to 30% by weight based on the weight of the isocyanate-prepolymer of essentially hydroxy group free diepoxide resin or their prepo­ lymers and optionally fillers, and (B) a stable gravity flowable second component comprising a mixture of a polyol, 0.2 to 5% by weight of di- or tri-primary or secondary amine group containing mole­ cule along with an urethane catalyst and an epoxy-­ curing catalyst and optionally fillers, in which the two components (A) and (B) are mixed in such a manner that the total isocyanate functionality to the combined hydroxy and epoxy functionality of the ingre­ dients are in the equivalent ratio of 0.8 : 1 to 3 : 1. 2. The adhesive compositions of claim 1 wherein the poly epoxide of (A) is a compound containing more than one group of the formula : 3. The adhesive compositions of anyone of claims 1 and 2 wherein the urethane catalyst in (B) is triethylene diamine, N,N′,N″-tris (dimethylamino­ propyl ) hexahydrotriazine, a organotin catalyst, potassium octoate, iron acetyl acetonate, zinc octoate, nickel acetyl acetonate or a mixture thereof. 4. The adhesive compositions of anyone of claims 1 to 3 wherein the epoxy curing catalyst is a tertiary amine, imidazoline, an onium salt, titanate, boron trifluoride-amine complex, lithium chloride, phenolic compound or a mixture thereof. 5. The adhesive compositions of anyone of claims 1 to 4 wherein the polyol of (B) is a compound having at least two hydroxyl groups per molecule and having an equivalent weight in the range of from 30 to 5000. 6. The adhesive compositions of anyone of claims 1 to 5 wherein the isocyanate prepolymer of (A) is one prepared by the reaction of methylene bis (phenyl isocyanate) with a poly (propylene oxide) diol and the polyepoxide is a diglycidyl ether of Bisphenol-A. 7. The adhesive compositions of anyone of claims 1 to 6 wherein the polyol of (B) is obtained by reacting an excess of poly(propylene oxide) tetraol with toluene diisocyanate, the urethane catalyst is triethylene diamine and the epoxy curing agent is a boron trifluoride-ethylamine complex. 8. The adhesive compositions of anyone of claims 1 to 7 wherein (B) also contains methyl trialkyl ammonium chloride as epoxy curing catalyst. 9. The adhesive compositions of anyone of claims 1 to 8 wherein the fillers are talc, kaolin, aluminum oxide, silica, metals or metaloxides. 10. The process for preparing a laminated structure comprising applying to at least one surface of a first substrate the adhesive of anyone of claims 1 to 9 and contacting said surface with the surface of a second substrate and allowing said adhesive to cure. 11. The process of Claim 10 wherein the substrates are fiber reinforced plastic, metal, wood and combinations thereof. 12. The process of Claim 11 wherein the substrates are fiberglass reinforced polyester panels.

Inventors: GOEL, ANIL B., RICHARDS, HARVEY J.
Applicants: ASHLAND OIL, INC.
Application year: 1987
Main classification: C09J201
IPC codes: C08G18, C09J175, C08J3