STRUCTURAL EPOXY RESIN ADHESIVES CONTAINING EPOXIDE-FUNCTIONAL, POLYPHENOL-EXTENDED ELASTOMERIC TOUGHENERS

This application claims priority from United States Provisional Application No. 61/022,618, filed 22 January 2008.

This invention relates to an epoxy-based structural adhesive containing an epoxide-functional, polyphenol-extended elastomeric toughener.

Epoxy resin based adhesives are used in many applications. In the automotive industry, epoxy resin adhesives are used in many bonding applications, including metal- metal bonding in frame and other structures in automobiles. Some of these adhesives must strongly resist failure during vehicle collision situations. Adhesives of this type are sometimes referred to as "crash durable adhesives", or "CDAs".

In order to obtain the good balance of properties that are needed to meet stringent automotive performance requirements, epoxy adhesives are often formulated with various rubbers and/or "tougheners". A commonly used toughener is a capped polyurethane and/or polyurea as described in U. S. Patent No. 5,278,257. These tougheners have blocked functional groups which, under the conditions of the curing reaction, can become de-blocked and react with an epoxy resin.

An alternative toughening approach uses an epoxide-terminated elastomeric toughener. These epoxide-terminated elastomeric tougheners are described, for example, in US 2007/0066721 and EP 1 741 734. These epoxide-terminated elastomeric tougheners have been reported in these references to be effective when used in conjunction with a thixotropic polyurea dispersion. As reported in EP 1 741 734, these tougheners have not been found to provide good low temperature properties unless the polyurea dispersion and an additional blocked polyurethane prepolymer is also present in the formulation.

The need to include a polyurea dispersion and blocked polyurethane prepolymer into the adhesive formulation, in addition to the elastomeric toughener, increases the cost and complexity of the adhesive formulation. The added complexity can interfere with the development of properties in the cured adhesive, because good property development often depends on the occurrence of phase segregation during curing, and highly complex adhesive formulations may not phase segregate as quickly or as well as less complex compositions.

Applicants have found that, contrary to the disclosure of US 2007/0066721 and EP 1 741 734, an adhesive composition containing certain epoxide-functional elastomeric tougheners can produce a cured adhesive that has good low temperature properties, even in the absence of a thixotropic polyurethane dispersion. Therefore, in one aspect, this invention is a structural adhesive, comprising:

A) at least one epoxy resin;

B) an epoxide-functional, polyphenol-extended elastomeric toughener containing urethane and/or urea groups; and

C) one or more epoxy curing agents, wherein the structural adhesive is substantially devoid of a thixotropic polyurea dispersion.

The invention is also a method comprising applying the foregoing structural adhesive to the surfaces of two metal members, and curing the structural adhesive to form an adhesive bond between the two metal members. The cured adhesive typically has a T _g of at least 8O ⁰ C, sometimes 9O ⁰ C or higher and in some cases at least 95 ⁰ C. The cured adhesive also retains its properties, in particular impact peel strength, well at low temperatures.

The elastomeric toughener used in this invention is an epoxide-functional, polyphenol-extended elastomeric toughener containing urethane and/or urea groups. By "polyphenol-extended", it is meant that an isocyanate-terminated precursor material has been reacted with a compound having two or more phenolic hydroxyl groups to chain- extend the prepolymer, forming a higher molecular weight, isocyanate-terminated intermediate. By "epoxide-functional", it is meant that epoxide groups have been introduced onto the intermediate through reaction with the remaining isocyanate groups.

The elastomeric toughener can be represented by the idealized structure (I)

(D

In structure I, Y ⁴ represents the residue of a hydroxy-functional epoxide, after removal of an epoxide group and a hydroxyl group.

In structure I, Y ³ represents the residue, after removal of phenolic hydroxyl groups, of a polyphenol compound. For purposes of this invention, a polyphenol compound is a compound having at least two hydroxyl groups, each bonded to a carbon atom of an aromatic ring. The hydroxyl groups may be bonded to carbon atoms in the same ring, or may be bonded to carbon atoms of different aromatic rings. The aromatic ring or rings to which the hydroxyl groups are bonded may contain other inert substituent groups, "inert" in this context meaning that those substituent groups are not reactive with an isocyanate group or a hydroxyl group under the conditions at which the elastomeric toughener is prepared.

Y ¹ is the residue of an isocyanate-terminated prepolymer after removal of the terminal isocyanate groups, wherein Y ¹ contains at least one polyether segment or a segment of a butadiene homopolymer or copolymer. The polyether segment or segment of a butadiene homopolymer or copolymer preferably has a weight of from 800 to 5000 daltons, preferably from 1500 to 4000 daltons. The Y ¹ group may contain urethane and/or urea groups, and may in addition contain residues (after removal of hydroxyl or amino groups, as the case may be) of one or more polyol or polyamine compounds having a molecular weight of up to 750, preferably from 50 to 500.

In structure I, y is a number from 1 to 3, m is a number from 2 to 4, and p is a number from 0 to 2. y is preferably 2 or 3 and p is preferably 0 or 1.

The reactive toughener can be prepared in the manner described in US 2007/0066721 and EP 1 741 734. In a first step, an isocyanate-terminated prepolymer

is reacted with a polyphenols compound. The isocyanate-terminated prepolymer preferably corresponds to the structure: wherein Y ¹ and p are as defined before. This prepolymer can be prepared by reaction of one or more polyol or polyamine compounds with a stoichiometric excess of a diisocyanate compound, provided that at least one of the polyol or polyamine compounds includes a polyether segment or a segment of a butadiene homopolymer or copolymer. The segment preferably has a weight of from 800 to 500 daltons, preferably from 1500 to 4000 daltons. This segment is preferably linear or at most slightly branched. The diisocyanate may be an aromatic diisocyanate, but it is preferably an aliphatic diisocyanate, such as isophorone diisocyanate, hexamethylene diisocyanate, hydrogenated toluene diisocyanate, hydrogenated methylene diphenylisocyanate (Hi ₂ MDI), and the like.

In the simplest case, only one polyol or polyamine is used to make the prepolymer. However, it is also possible to use a mixture of polyols or polyamines to make the prepolymer. It is preferred that at least 50%, more preferably at least 80% and even more preferably at least 90% by weight of the polyol or polyamine materials include a polyether segment or a segment of a butadiene homopolymer or copolymer. Other polyols and polyamines that can be used in combination therewith include those having a molecular weight of up to about 750. More preferred are aliphatic polyols and polyamines having an equivalent weight of up to 150 and from 2 to 4, especially from 2 to 3, hydroxyl and/or primary or secondary amino groups. Examples of these materials include polyols such as trimethylolpropane, glycerine, trimethyolethane, ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, sucrose, sorbitol, pentaerythritol, ethylene diamine, triethanolamine, monoethanolamine, diethanolamine, piperazine, aminoethylpiperazine and the like.

The prepolymer may contain urethane or urea groups.

The prepolymer is extended by reaction with a polyphenol compound, as described before. The polyphenol compound may take the form Y ³ -(OH) _m , where Y ³ and m are as defined before. The OH groups are bound to aromatic rings on the Y ³ group. Preferred polyphenols have from 2 to 4, especially from 2 to 3 and more preferably 2 phenolic groups per molecule. Examples of suitable polyphenol compounds include, for

example, resorcinol, catechol, hydroquinone, bisphenol, bisphenol A, bisphenol AP (1,1- bis(4-hydroxylphenyl)-l-phenyl ethane), bisphenol F, bisphenol K, bisphenol M, tetramethylbiphenol, O,O'-diallyl-bisphenol A and the like.

The extension reaction is conducted using a stoichiometric excess of the prepolymer, so that, upon reaction, a chain-extended product having free isocyanate groups is formed. The chain extended product can in some embodiments be represented by structure (III):

wherein Y ¹ , Y ³ , m and p are as defined before. It is noted that the foregoing structure is somewhat idealized, as are the preceding structures. In particular, it is possible that not all of the phenolic groups on the polyphenol groups will react in forming the chain extended prepolymer.

The proportions of starting materials is preferably selected so that the polyphenol-extended prepolymer has an isocyanate content of from 0.5 to 5% by weight, more preferably from 1 to 4% by weight and even more preferably from 1.5 to 3% by weight. In terms of isocyanate equivalent weight, a preferred range is from 840 to 8400, a more preferred range is from 1050 to 4200, and an even more preferred range is from 1400 to 2800.

The polyphenol-extended prepolymer is then reacted with an epoxy compound that has one or more hydroxyl groups. The epoxy compound may have one or more epoxy groups. Suitable hydroxyl-containing epoxy compounds can be prepared by reacting an excess of a polyol compound with epichlorohydrin to produce an ether having at least one hydroxyl group and at least one epoxide group. Suitable polyols for producing such ethers include, for example, trimethylolpropane, glycerine, trimethyolethane, ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, sucrose, sorbitol, pentaerythritol and the like. This reaction of the polyphenol- extended prepolymer and the epoxy compound is conducted until the isocyanate content

of the product is reduced to no greater than 0.1 weight percent, and preferably until the isocyanate content is essentially zero.

The resulting toughener has terminal epoxide groups and essentially no isocyanate functionality. Its number average molecular weight is preferably from about 3000 to about 30,000, more preferably from about 5000 to about 25,000 and even more preferably from about 7,500 to about 20,000. Tougheners having a number average molecular weight of from 10,000 to 30,000 are particularly useful in some formulations. Toughener molecular weights for purposes of this invention are determined by GPC.

The structural adhesive contains at least one epoxy resin which is in addition to and different than the toughener. It is preferred that at least a portion of the epoxy resin is not rubber-modified. A non-rubber-modified epoxy resin may be added to the structural adhesive as a separate component. In some embodiments of the invention, a core-shell rubber product is used, which may be dispersed in some quantity of epoxy resin. Some amount of non-rubber-modified epoxy resin may be brought into the structural adhesive in that manner. In other embodiments, a rubber-modified epoxy resin product used as a component of the structural adhesive may also contain a certain amount of epoxy resin which is not reacted with the rubber (and thus is not rubber- modified). Some non-rubber-modified epoxy resin may be brought into the adhesive in that manner as well. A wide range of epoxy resins can be used as a non-rubber-modified epoxy resin, including those described at column 2 line 66 to column 4 line 24 of U.S. Patent 4,734,332, incorporated herein by reference.

Suitable epoxy resins include the diglycidyl ethers of polyhydric phenol compounds such as resorcinol, catechol, hydroquinone, bisphenol, bisphenol A, bisphenol AP (l,l-bis(4-hydroxylphenyl)-l-phenyl ethane), bisphenol F, bisphenol K, tetramethylbiphenol, diglycidyl ethers of aliphatic glycols and polyether glycols such as the diglycidyl ethers of C2-24 alkylene glycols and poly(ethylene oxide) or poly(propylene oxide) glycols; polyglycidyl ethers of phenol-formaldehyde novolac resins, alkyl substituted phenol-formaldehyde resins (epoxy novolac resins), phenol- hydroxybenzaldehyde resins, cresol-hydroxybenzaldehyde resins, dicyclopentadiene- phenol resins and dicyclopentadiene-substituted phenol resins, and any combination thereof.

Suitable diglycidyl ethers include diglycidyl ethers of bisphenol A resins such as are sold by Dow Chemical under the designations D.E.R.® 330, D.E.R.® 331, D.E.R.® 332, D.E.R.® 383, D.E.R. 661 and D.E.R.® 662 resins.

Commercially available diglycidyl ethers of polyglycols that are useful include those sold as D.E.R.® 732 and D.E.R.® 736 by Dow Chemical.

Epoxy novolac resins can be used. Such resins are available commercially as D.E.N.® 354, D.E.N.® 431, D.E.N.® 438 and D.E.N.® 439 from Dow Chemical.

Other suitable additional epoxy resins are cycloaliphatic epoxides. A cycloaliphatic epoxide includes a saturated carbon ring having an epoxy oxygen bonded to two vicinal atoms in the carbon ring, as illustrated by the following structure IV:

wherein R is an aliphatic, cycloaliphatic and/or aromatic group and n is a number from 1 to 10, preferably from 2 to 4. When n is 1, the cycloaliphatic epoxide is a monoepoxide. Di- or polyepoxides are formed when n is 2 or more. Mixtures of mono-, di- and/or polyepoxides can be used. Cycloaliphatic epoxy resins as described in U.S. Patent No. 3,686,359, incorporated herein by reference, may be used in the present invention. Cycloaliphatic epoxy resins of particular interest are (3,4-epoxycyclohexyl-methyl)-3,4- epoxy-cyclohexane carboxylate, bis-(3,4-epoxycyclohexyl) adipate, vinylcyclohexene monoxide and mixtures thereof.

Other suitable epoxy resins include oxazolidone-containing compounds as described in U. S. Patent No. 5,112,932. In addition, an advanced epoxy-isocyanate copolymer such as those sold commercially as D.E.R. 592 and D.E.R. 6508 (Dow Chemical) can be used.

The non-rubber-modified epoxy resin preferably is a bisphenol-type epoxy resin or mixture thereof with up to 10 percent by weight of another type of non-rubber- modified epoxy resin. The most preferred epoxy resins are bisphenol-A based epoxy resins and bisphenol-F based epoxy resins. These can have average epoxy equivalent weights of from about 170 to 600 or more, preferably from 225 to 400.

An especially preferred non-rubber-modified epoxy resin is a mixture of a diglycidyl ether of a polyhydric phenol, preferably bisphenol-A or bisphenol-F, having an

epoxy equivalent weight of from 170 to 299, especially from 170 to 225, and a second diglycidyl ether of a polyhydric phenol, again preferably bisphenol-A or bisphenol-F, this one having an epoxy equivalent weight of at least 300, preferably from 310 to 600. The proportions of the two resins are preferably such that the mixture of the two resins has an average epoxy equivalent weight of from 225 to 400. The mixture optionally may also contain up to 20%, preferably up to 10%, of one or more other non-rubber-modified epoxy resins.

The structural adhesive further contains a curing agent. For the preferred one- component adhesive products, the curing agent preferably is selected together with any catalysts such that the adhesive cures when heated to a temperature of 8O ⁰ C or greater, preferably 100 ⁰ C or greater, but cures very slowly if at all at room temperature (~22°C) and temperatures up to at least 5O ⁰ C. Suitable such curing agents include materials such as boron trichloride/amine and boron trifluoride/amine complexes, dicyandiamide, melamine, diallylmelamine, guanamines such as acetoguanamine and benzoguanamine, aminotriazoles such as 3-amino-l,2,4-triazole, hydrazides such as adipic dihydrazide, stearic dihydrazide, isophthalic dihydrazide, semicarbazide, cyanoacetamide, and aromatic polyamines such as diaminodiphenylsulphones. The use of dicyandiamide, isophthalic acid dihydrazide, adipic acid dihydrazide and/or 4,4'- diaminodiphenylsulphone is particularly preferred. The adhesive of the invention is substantially devoid of a thixotropic polyurea dispersion. Thixotropic polyurea dispersions of this type are described in EP 1 152 019, US 2007/0066721 and EP 1 741 734. They are generally prepared by forming a polyurethane prepolymer having blocked isocyanate groups, and then reacting a polyisocyanate and an amine compound in the presence of the prepolymer, to form a dispersion of polyurea particles in the prepolymer.

In addition, the adhesive of the invention is substantially devoid of a polyurethane prepolymer which has blocked isocyanate groups but does not contain epoxide groups.

"Substantially devoid" means, for purposes of this invention, that the adhesive composition contains no more than 1 weight percent of the mentioned material. More preferably, the adhesive composition contains no more than 0.25 weight percent of such material, and more preferably contains no more than 0.05 weight percent thereof. The adhesive composition most preferably will contain none of the mentioned material.

Applicants have found, surprisingly and contrary to US 2007/0066721 and EP 1

741 734, that good properties, in particular good low temperature properties, can be achieved in an adhesive that contains an epoxide-functional, polyphenol-extended elastomeric toughener containing urethane and/or urea groups, as described above, even in the absence of the polyurea dispersion or even the blocked polyurethane prepolymer.

The toughener should constitute at least 15 weight percent of the composition. Better results are typically seen when the amount of toughener is at least 20 weight percent or at least 23 weight percent. The toughener may constitute up to 45 weight percent thereof. The amount that is needed to provide good properties, particularly good low temperature properties, in any particular adhesive composition may depend somewhat on the other components of the composition, and may depend somewhat on the molecular weight of the toughener.

One preferred adhesive of the invention includes one or more diglycidyl ethers of a polyhydric phenol, especially bisphenol A or bisphenol F, having an equivalent weight of from 170 to 299, a liquid rubber-modified epoxy resin, a core-shell rubber or both a liquid rubber-modified epoxy resin and a core-shell rubber, and from about 23 to 45 weight percent of the toughener.

Another preferred adhesive of the invention includes one or more diglycidyl ethers of a polyhydric phenol, especially bisphenol A or bisphenol F, having an equivalent weight of from 170 to 299, a liquid rubber-modified epoxy resin, a core-shell rubber or both a liquid rubber-modified epoxy resin and a core-shell rubber, and from about 15 to 45 weight percent of toughener as described herein, having a number average molecular weight of from 10,000 to 30,000. Another preferred adhesive of the invention includes one or more diglycidyl ethers of a polyhydric phenol, especially bisphenol A or bisphenol F, having an average equivalent weight of from 225 to 400, no liquid rubber-modified epoxy resin or core-shell rubber, and from 25 to 45 weight percent of the toughener. In this case, the diglycidyl ether of the polyhydric phenol is most preferably a mixture of a diglycidyl ether of a polyhydric phenol having an epoxy equivalent weight from 170 to 299, especially from 170 to 225, and another diglycidyl ether of a polyhydric phenol having an epoxy equivalent weight of at least 300.

Yet another preferred adhesive of the invention includes one or more diglycidyl ethers of a polyhydric phenol, especially bisphenol A or bisphenol F, having an average

equivalent weight of from 225 to 400, a liquid rubber-modified epoxy resin or a core-shell rubber or both, and from 15 to 45 weight percent of the toughener. In this case, the diglycidyl ether of the polyhydric phenol is most preferably a mixture of a diglycidyl ether of a polyhydric phenol having an epoxy equivalent weight from 170 to 299, especially from 170 to 225, and another diglycidyl ether of a polyhydric phenol having an epoxy equivalent weight of at least 300.

The non-rubber-modified epoxy resin is used in sufficient amount to impart desirable adhesive and strength properties. Preferably, the non-rubber-modified epoxy resin will constitute at least about 25 weight percent of the structural adhesive, more preferably at least about 30 weight percent, and still more preferably at least about 35 part weight percent. The non-rubber-modified epoxy resin may constitute up to about 60 weight percent of the structural adhesive, more preferably up to about 50 weight percent. These amounts include amounts of non-rubber-modified epoxy resin (if any) that may be brought into the composition with any core-shell rubber and/or any liquid rubber-modified epoxy resin(s) as may be used.

The curing agent is used in sufficient amount to cure the composition. Typically, enough of the curing agent is provided to consume at least 80% of the epoxide groups present in the composition. A large excess of that amount needed to consume the epoxide groups is generally not needed. Preferably, the curing agent constitutes at least about 1.5 weight percent of the structural adhesive, more preferably at least about 2.5 weight percent and even more preferably at least 3.0 weight percent. The curing agent preferably constitutes up to about 15 weight percent of the structural adhesive composition, more preferably up to about 10 weight percent, and most preferably up to about 8 weight percent. The structural adhesive will in most cases contain a catalyst to promote the cure of the adhesive. Among preferred epoxy catalysts are ureas such as p-chlorophenyl-N,N- dimethylurea (Monuron), 3 -phenyl- 1,1 -dimethylurea (Phenuron), 3,4-dichlorophenyl- N,N-dimethylurea (Diuron), N-(3-chloro-4-methylphenyl)-N',N'-dimethylurea

(Chlortoluron), tert-acryl- or alkylene amines like benzyldimethylamine, 2,4,6- tris(dimethylaminomethyl)phenol, piperidine or derivates thereof, imidazole derivates, in general C1-C12 alkylene imidazole or N-arylimidazols, such as 2-ethyl-2- methylimidazol, or N-butylimidazol, 6-caprolactam, a preferred catalyst is 2,4,6- tris(dimethylaminomethyl)phenol integrated into a poly(p-vinylphenol) matrix (as described in European patent EP 0 197 892). The catalyst may be encapsulated or

otherwise be a latent type which becomes active only upon exposure to elevated temperatures. Preferably, the catalyst is present in an amount of at least about 0.1 weight percent of the structural adhesive, and more preferably at least about 0.5 weight percent. Preferably, the epoxy curing catalyst constitutes up to about 2 weight percent of the structural adhesive, more preferably up to about 1.0 weight percent, and most preferably up to about 0.7 weight percent.

The structural adhesive of the invention may include at least one liquid rubber- modified epoxy resin. The rubber-modified epoxy resin is a reaction product of an epoxy resin and at least one liquid (at 25 ⁰ C) rubber that has epoxide-reactive groups, such as amino or preferably carboxyl groups. The resulting adduct has reactive epoxide groups which can be cured further when the structural adhesive is cured. It is preferred that at least a portion of the liquid rubber has a glass transition temperature (T _g ) of -4O ⁰ C or lower, especially -5O ⁰ C or lower. Preferably, each of the rubbers (when more than one is used) has a glass transition temperature of -25 ⁰ C or lower. The rubber T _g may be as low as -100 ⁰ C or even lower.

The liquid rubber is preferably a homopolymer of a conjugated diene or copolymer of a conjugated diene, especially a diene/nitrile copolymer. The conjugated diene rubber is preferably butadiene or isoprene, with butadiene being especially preferred. The preferred nitrile monomer is acrylonitrile. Preferred copolymers are butadiene-acrylonitrile copolymers. The rubbers preferably contain, in the aggregate, no more than 30 weight percent polymerized unsaturated nitrile monomer, and preferably no more than about 26 weight percent polymerized nitrile monomer.

The rubber preferably contains from about 1.5, more preferably from about 1.8, to about 2.5, more preferably to about 2.2, epoxide-reactive terminal groups per molecule, on average. Carboxyl-terminated rubbers are preferred. The molecular weight (M _n ) of the rubber is suitably from about 2000 to about 6000, more preferably from about 3000 to about 5000.

Suitable carboxyl-functional butadiene and butadiene/acrylonitrile rubbers are commercially available from Noveon under the tradenames Hycar ^® 2000X162 carboxyl- terminated butadiene homopolymer, Hycar ^® 1300X31, Hycar ^® 1300X8, Hycar ^® 1300X13, Hycar ^® 1300X9 and Hycar ^® 1300X18 carboxyl-terminated butadiene/acrylonitrile copolymers. A suitable amine-terminated butadiene/acrylonitrile copolymer is sold under the tradename Hycar ^® 1300X21.

The rubber is formed into an epoxy-terminated adduct by reaction with an excess of an epoxy resin. Enough of the epoxy resin is provided to react with substantially all of the epoxide-reactive groups on the rubber and to provide free epoxide groups on the resulting adduct, but without significantly advancing the adduct to form high molecular weight species. A ratio of at least two equivalents of epoxy resin per equivalent of epoxy-reactive groups on the rubber is preferred. More preferably, enough of the epoxy resin is used that the resulting product is a mixture of the adduct and some free epoxy resin. Typically, the rubber and an excess of the polyepoxide are mixed together with a polymerization catalyst and heated to a temperature of about 100 to about 25O ⁰ C in order to form the adduct. Suitable catalysts include those described before. Preferred catalysts for forming the rubber-modified epoxy resin include phenyl dimethyl urea and triphenyl phosphine.

A wide variety of epoxy resins can be used to make the rubber-modified epoxy resin, including any of those described above. Preferred polyepoxides are liquid or solid glycidyl ethers of a bisphenol such as bisphenol A or bisphenol F. Halogenated, particularly brominated, resins can be used to impart flame retardant properties if desired. Liquid epoxy resins (such as DER™ 330 and DER™ 331 resins, which are diglycidyl ethers of bisphenol A available from The Dow Chemical Company) are especially preferred for ease of handling. The rubber-modified epoxy resin(s), if present at all, may constitute about 1 weight percent of the structural adhesive or more, preferably at least about 2 weight percent. The rubber-modified epoxy resin may constitute up to about 25 weight percent of the structural adhesive, more preferably up to about 20 weight percent, and even more preferably up to about 15 weight percent. The structural adhesive of the invention may contain one or more core-shell rubbers. The core-shell rubber is a particulate material having a rubbery core. The rubbery core preferably has a T _g of less than -2O ⁰ C, more preferably less than -5O ⁰ C and even more preferably less than -7O ⁰ C. The T _g of the rubbery core may be well below -100 ⁰ C. The core-shell rubber also has at least one shell portion that preferably has a T _g of at least 50°C. By "core", it is meant an internal portion of the core-shell rubber. The core may form the center of the core-shell particle, or an internal shell or domain of the core-shell rubber. A shell is a portion of the core-shell rubber that is exterior to the rubbery core. The shell portion (or portions) typically forms the outermost portion of the core-shell rubber particle. The shell material is preferably grafted onto the core or is

crosslinked. The rubbery core may constitute from 50 to 95%, especially from 60 to 90%, of the weight of the core-shell rubber particle.

The core of the core-shell rubber may be a polymer or copolymer of a conjugated diene such as butadiene, or a lower alkyl acrylate such as n-butyl-, ethyl-, isobutyl- or 2- ethylhexylacrylate. The core polymer may in addition contain up to 20% by weight of other copolymerized monomers having a single polymerizable ethylenically unsaturated group, such as styrene, vinyl acetate, vinyl chloride, methyl methacrylate, and the like.

The core polymer is optionally crosslinked. The core polymer optionally contains up to

5% of a copolymerized graft-linking monomer having two or more sites of unsaturation of unequal reactivity, such as diallyl maleate, monoallyl fumarate, allyl methacrylate, and the like, at least one of the reactive sites being non-conjugated.

The core polymer may also be a silicone rubber. These materials often have glass transition temperatures below -100 ⁰ C. Core-shell rubbers having a silicone rubber core include those commercially available from Wacker Chemie, Munich, Germany, under the trade name Genioperl™.

The shell polymer, which is optionally chemically grafted or crosslinked to the rubber core, is preferably polymerized from at least one lower alkyl methacrylate such as methyl-, ethyl- or t-butyl methacrylate. Homopolymers of such methacrylate monomers can be used. Further, up to 40% by weight of the shell polymer can be formed from other monovinylidene monomers such as styrene, vinyl acetate, vinyl chloride, methyl acrylate, ethyl acrylate, butyl acrylate, and the like. The molecular weight of the grafted shell polymer is generally between 20,000 and 500,000.

A preferred type of core-shell rubber has reactive groups in the shell polymer which can react with an epoxy resin or an epoxy resin hardener. Glycidyl groups such as are provided by monomers such as glycidyl methacrylate are suitable.

A particularly preferred type of core- shell rubber is of the type described in EP 1 632 533 Al. Core- shell rubber particles as described in EP 1 632 533 Al include a crosslinked rubber core, in most cases being a crosslinked copolymer of butadiene, and a shell which is preferably a copolymer of styrene, methyl methacrylate, glycidyl methacrylate and optionally acrylonitrile. The core-shell rubber is preferably dispersed in a polymer or an epoxy resin, also as described in EP 1 632 533 Al.

Preferred core- shell rubbers include those sold by Kaneka Corporation under the designation Kaneka Kane Ace, including Kaneka Kane Ace MX 156 and Kaneka Kane Ace MX 120 core-shell rubber dispersions. The products contain the core-shell rubber

particles pre-dispersed in an epoxy resin, at a concentration of approximately 25%. When such a dispersion is used, the epoxy resin contained in the dispersion will form all or part of the non-rubber-modified epoxy resin component of the structural adhesive of the invention. The core-shell rubber particles can constitute from 0 to 15 weight percent of the structural adhesive.

The total rubber content of the structural adhesive of the invention can range from as little as 0 weight percent to as high as 30 weight percent. A preferred rubber content for a crash durable adhesive is from 1 weight percent to as much as 20 weight percent, preferably from 2 to 15 weight percent and more preferably from 2 to 15 weight percent.

Total rubber content is calculated for purposes of this invention by determining the weight of core-shell rubber, plus the weight contributed by the liquid rubber portion of any rubber-modified epoxy resin as may be used. No portion of the elastomeric toughener is considered in calculating total rubber content. In each case, the weight of unreacted (non-rubber-modified) epoxy resins and/or other carriers, diluents, dispersants or other ingredients that may be contained in the core-shell rubber product or rubber-modified epoxy resin is not included. The weight of the shell portion of the core-shell rubber is counted as part of the total rubber content for purposes of this invention.

The structural adhesive of the invention may contain various other optional components.

At least one filler, rheology modifier and/or pigment is preferably present in the structural adhesive. These can perform several functions, such as (1) modifying the rheology of the adhesive in a desirable way, (2) reducing overall cost per unit weight, (3) absorbing moisture or oils from the adhesive or from a substrate to which it is applied, and/or (4) promoting a cohesive failure mode, rather than an adhesive failure mode. Examples of these materials include calcium carbonate, calcium oxide, talc, carbon black, textile fibers, glass particles or fibers, aramid pulp, boron fibers, carbon fibers, mineral silicates, mica, powdered quartz, hydrated aluminum oxide, bentonite, wollastonite, kaolin, fumed silica, silica aerogel or metal powders such as aluminum powder or iron powder. Another filler of particular interest is a microballon having an average particle size of up to 200 microns and a density of up to 0.2 g/cc. The particle size is preferably about 25 to 150 microns and the density is preferably from about 0.05

to about 0.15 g/cc. Heat expandable microballoons which are suitable for reducing density include those commercially available from Dualite Corporation under the trade designation Dualite™, and those sold by Akzo Nobel under the trade designation Expancel™. Fillers, pigment and rheology modifiers preferably are used in an aggregate amount of about 2 parts per hundred parts of adhesive composition or greater, more preferably about 5 parts per hundred parts of adhesive composition or greater. They preferably are present in an amount of up to about 25 weight percent of the structural adhesive, more preferably up to about 20 weight percent, and most preferably up to about 15 weight percent.

The structural adhesive can further contain other additives such as diluents, plasticizers, extenders, pigments and dyes, fire -retarding agents, thixotropic agents, expanding agents, flow control agents, adhesion promoters and antioxidants. Suitable expanding agents include both physical and chemical type agents. The adhesive may also contain a thermoplastic powder such as polyvinylbutyral or a polyester polyol, as described in WO 2005/118734.

The adhesive composition can be applied by any convenient technique. It can be applied cold or be applied warm, as desired. It can be applied by extruding it from a robot into bead form on the substrate, it can be applied using mechanical application methods such as a caulking gun, or any other manual application means, or it can also be applied using a swirl technique. The swirl technique is applied using an apparatus well known to one skilled in the art such as pumps, control systems, dosing gun assemblies, remote dosing devices and application guns. Preferably, the adhesive is applied to the substrate using a jet spraying or streaming process. Generally, the adhesive is applied to one or both substrates. The substrates are contacted such that the adhesive is located between the substrates to be bonded together.

After application, the structural adhesive is cured by heating to a temperature at which the curing agent initiates cure of the epoxy resin composition. Generally, this temperature is about 8O ⁰ C or above, preferably about 100 ⁰ C or above. Preferably, the temperature is about 22O ⁰ C or less, and more preferably about 18O ⁰ C or less.

The adhesive of the invention can be used to bond a variety of substrates, including wood, metal, coated metal, aluminum, a variety of plastic and filled plastic substrates, fiberglass and the like. In one preferred embodiment, the adhesive is used to bond parts of automobiles together or parts to automobiles. Such parts can be steel,

coated steel, galvanized steel, aluminum, coated aluminum, plastic and filled plastic substrates.

An application of particular interest is bonding of automotive frame components to each other or to other components. The frame components are often metals such as cold rolled steel, galvanized metals, or aluminum. The components that are to be bonded to the frame components can also be metals as just described, or can be other metals, plastics, composite materials, and the like.

Assembled automotive frame members are usually coated with a coating material that requires a bake cure. The coating is typically baked at temperatures that may range from 14O ⁰ C to over 200 ⁰ C. In such cases, it is often convenient to apply the structural adhesive to the frame components, then apply the coating, and cure the adhesive at the same time the coating is baked and cured.

The adhesive composition once cured preferably has a Young's modulus of about

1200 MPa as measured according to DIN EN ISO 527-1. Preferably the Young's modulus is about 1500 MPa or greater, more preferably at least 1800 MPa and even more preferably at least 2200 MPa. Preferably, the cured adhesive demonstrates a tensile strength of about 30 MPa or greater, more preferably about 38 MPa or greater, and most preferably about 45 MPa or greater. Preferably, the lap shear strength of a 1.5 mm thick cured adhesive layer on cold rolled steel (CRS) and galvaneal is about 15 MPa or greater, more preferably about 20 MPa or greater, and most preferably about 25 MPa or greater measured according to DIN EN 1465.

The cured adhesive of the invention demonstrates excellent adhesive properties

(such as lap shear strength and impact peel strength) over a range of temperatures down to -4O ⁰ C or lower. The low temperature performance is quite surprising due to the lack of the thixotropic polyurea dispersion that has heretofore been considered necessary in order to achieve good results when the toughener is epoxide-terminated.

The following examples are provided to illustrate the invention but are not intended to limit the scope thereof. All parts and percentages are by weight unless otherwise indicated. Products used in the following examples are identified as follows:

DER™ 671 is a solution of a solid diglycidyl ether of bisphenol A, available from The Dow Chemical Company. It has an epoxy equivalent weight of approximately 425- 550 (not including solvent).

DER™ 330 is a liquid diglycidyl ether of bisphenol A, available from The Dow Chemical Company. It has an epoxy equivalent weight of approximately 180.

DER™ 331 is a liquid diglycidyl ether of bisphenol A, available from The Dow Chemical Company. It has an epoxy equivalent weight of approximately 187. Epoxy Resin Blend A is an experimental blend of liquid diglycidyl ethers of bisphenol A having an epoxy equivalent weight of 180-187 and a solid diglycidyl ether of bisphenol A having an epoxy equivalent weight of about 425-550. Its epoxy equivalent weight is from 275-300.

Struktol™ 3614 is a reaction product of approximately 60% a liquid diglycidyl ether of bisphenol F, and 40% of Hycar 1300X13 rubber (a carboxy-terminated butadiene-acrylonitrile copolymer having a T _g greater than -4O ⁰ C, available from Noveon). It is commercially available from Schill & Seilacher.

Cardolite NC700 is an alkylated phenol wetting agent, available from Cardura.

Cardura™ ElO is versatic acid monoepoxy ester, available from Shell Chemicals. Silquest Silane A187 is an epoxy silane available from GE Silicones.

Al 87 is a commercially available glycidyl silyl ether.

EP796 is tris (2,4,6-dimethylaminomethyl)phenol in a poly(vinylphenol) matrix.

PVB is polyvinylbutyral.

Polypox R 18 is 1,6-hexanediol diglycidyl ether. Elastomeric Toughener A is an epoxide-functional, polyphenol-extended elastomeric toughener containing urethane groups. It is prepared by first reacting 57.9 parts of a 2000 molecular weight polytetrahydrofuran with 13.8 parts isophorone diisocyanate in the presence of a tin catalyst to form a prepolymer having an isocyanate content of 3.9% by weight. 5.1 parts bisphenol M are then reacted with the prepolymer until the isocyanate content is reduced to 2.0%. 23.2 parts of a trimethylolpropane di/tri-glycidyl ether then added and the reaction continued until the isocyanate content is reduced to 0.0%. The toughener has a M _n of 9100 and a M _w of 49,800 by GPC.

Elastomeric Toughener B is an epoxide-functional, polyphenol-extended elastomeric toughener containing urethane groups. It is prepared by first reacting 60.3 parts of a 2000 molecular weight polytetrahydrofuran with 10.8 parts hexamethylene diisocyanate in the presence of a tin catalyst to form a prepolymer having an isocyanate content of 4.0% by weight. 4.7 parts O,O'-diallyl bisphenol A are then reacted with the prepolymer until the isocyanate content is reduced to 2.1%. 24.2 parts of a trimethylolpropane di/tri-glycidyl ether then added and the reaction continued until the

isocyanate content is reduced to 0.0%. The toughener has a M _n of 12,800 and a M _w of 86,000.

The Polyurea Dispersion is a 21.5% solids dispersion of butylamine-MDI polyurea particles in a caprolactam blocked polyurethane prepolymer.

Examples 1 and 2 and Reference Sample 1

Adhesive Examples 1 and 2 are prepared without a thixotropic polyurea dispersion and with a zero rubber content, from the following formulations. Reference Sample 1 is based on the formulation described in EP 1 741 734 example Z3, using the same reactive toughener as is used in Examples 1 and 2.

Table 1

Impact peel testing is performed in accordance with ISO 11343 wedge impact method. Testing is performed at an operating speed of 2 m/sec. The substrate is 0.8-mm electrogalvanized steel (DC04-B+ZE). Impact peel testing is performed at 23 ⁰ C, -3O ⁰ C and -4O ⁰ C, with strength in N/mm and energy in Joules being measured.

Test coupons for the impact peel testing are 90 mm X 20 mm with a bonded area of 30 X 20 mm. The samples are prepared by wiping them with acetone. A 0.15 mm X 10 mm wide Teflon tape is applied to the coupons to define the bond area. The structural adhesive is then applied to the bond area of latter coupon and squeezed onto the first coupon to prepare each test specimen. The adhesive layer is 0.2 mm thick. Duplicate

samples are cured for 30 minutes at 18O ⁰ C. Results of the impact peel testing are as indicated in Table 3.

Duplicate test coupons are prepared and are evaluated for lap shear strength in accordance with DIN EN 1465. Testing is performed at a test speed of 10 mm/minute. The substrate is the same as used in the impact peel testing. Testing is performed at 23 ⁰ C.

Test samples are prepared using each adhesive. The bonded area in each case is 25 X 10 mm. The adhesive layer is 0.2 mm thick. Duplicate test specimens are cured at for 30 minutes at 18O ⁰ C.

The glass transition temperature (T _g ) of the cured adhesive is also measured.

Results are as indicated in Table 2.

Table 2

The results in Table 2 indicate that Examples 1 and 2 have impact peel properties that are very comparable to that of the reference example. This data indicates that the polyurea dispersion of Reference Example 1 is not necessary to obtain these properties, in particular the low temperature impact peel properties, when the amount of toughener is increased slightly from about 23% (Ref. Sample 1) to about 25- 30%. Examples 1 and 2 also exhibit a smaller loss in properties at -4O ⁰ C than does Reference Sample 1.

Examples 3-7 and Reference Sample 2

Adhesive Examples 3-7 are prepared with a rubber-modified epoxy resin, usin^ the following formulations.

Table 3

These adhesives are tested in the manner described with regard to Examples 1 and 2, with results as indicated in Table 4.

Table 4

These results again show that adhesive compositions having good properties, and good low temperature properties in particular, are obtained then the adhesive contains from about 21 to 40 weight percent of the epoxide-terminated, phenol-extended toughener, even in the absence of a thixotropic polyurea dispersion. In these Examples, a cured adhesive having at least three phases is believed to exist, the phases being the cured epoxy resin phase, the rubber phase (contributed by the rubber-modified epoxy resin), and at least one toughener phase.

Examples 8 and 9

Adhesive Examples 8 and 9 are prepared with a rubber-modified epoxy resin, using the following formulations. In these cases, the higher equivalent weight, solid epoxy resin is omitted from the formulation.

Table 5

These adhesives are tested in the manner described with regard to Examples 1 and 2, with results as indicated in Table 6.

Table 6

Example 9 exhibits excellent impact peel at all temperatures tested, and has the further benefit of a particularly high glass transition temperature. The results for Example 8 show that the low temperature properties decrease somewhat at the lower level of toughener (compared to Ex. 9). These results, taken together with the results from Examples 3-7, indicate that the presence of the higher equivalent weight epoxy resin provides a benefit in low temperature properties in these adhesive systems, at a

comparable level of toughener. As demonstrated by Example 9, the lack of a higher equivalent weight epoxy resin can be compensated for by increasing the amount of the toughener. In these Examples, a cured adhesive having at least three phases is believed to exist, similar to Examples 3-7.

Example 10

Adhesive Example 10 is prepared with a rubber-modified epoxy resin, using the following formulation.

Table 7

This adhesive is tested in the manner described with regard to Examples 1 and 2, with results as indicated in Table 8.

Table 8

In this Example, a cured adhesive having at least three phases is believed to exist, similar to Examples 3-7.

The results for Example 10 indicate that Toughener B is more effective than Toughener A in imparting good low temperature impact properties to a structural

adhesive that contains a mixture of high and low equivalent weight epoxy resins and a phase, but no thixotropic polyurea dispersion. Here, about 17.4% of the reactive toughener is sufficient to provide the desired low temperature impact peel strength, whereas, Toughener A provides similar results at somewhat higher levels (see Examples 3-7). The difference in performance may relate to the higher molecular weight of Toughener B compared to Toughener A.