WO2006128722A1

WO2006128722A1 - Toughened epoxy adhesive composition

Info

Publication number: WO2006128722A1
Application number: PCT/EP2006/005301
Authority: WO
Inventors: Andreas Lutz; Daniel Schneider
Original assignee: Dow Global Technologies Inc.
Priority date: 2005-06-02
Filing date: 2006-06-02
Publication date: 2006-12-07
Also published as: US8404787B2; JP2013040338A; ATE462762T1; DE602005020260D1; CN101184787A; KR20080013978A; BRPI0613302B1; EP1728825A1; EP1896517B1; US20060276601A1; BRPI0613302A2; KR101299846B1; CN101184787B; JP5647199B2; CA2609774C; EP1896517A1; EP1896517B2; JP5150484B2; EP1728825B2; JP2008542484A

Abstract

The present invention relates to the use of a compound as a toughener in a storage-stable, heat-curable structural epoxy adhesive, said compound comprising an elastomeric prepolymer residue selected from the group of a polyurethane, a polyurea and a polyurea polyurethane having isocyanate end groups, the isocyanate end groups of said prepolymer residue being capped by a capping compound selected from the group consisting of a primary aliphatic, cycloaliphatic, heteroaromatic and araliphatic amine, a secondary aliphatic, cycloaliphatic, aromatic, heteroaromatic and araliphatic amine, a thiol and an alkyl amide, said capping compound being bound to the end of the polymer chain of the elastomeric prepolymer in a manner such that the end to which it is bonded no longer has a reactive group.

Description

TOUGHENED EPOXY ADHESIVE COMPOSITION

FIELD OF INVENTION

[0001] The present invention relates to the use of a compound as a toughener in a storage-stable, heat-curable structural epoxy adhesive, said compound comprising an elastomeric prepolymer residue selected from the group of a polyurethane, a polyurea and a polyurea polyurethane having isocyanate end groups, the isocyanate end groups of said prepolymer residue being capped by a capping compound selected from the group consisting of a primary aliphatic, cycloaliphatic, heteroaromatic and araliphatic amine, a secondary aliphatic, cycloaliphatic, aromatic, heteroaromatic and araliphatic amine, a thiol and an alkyl amide, said capping compound being bound to the end of the polymer chain of the elastomeric prepolymer in a manner such that the end to which it is bonded no longer has a reactive group. The invention further relates to a storage-stable, heat-curable structural epoxy adhesive composition comprising an epoxy resin and the compound defined above and to a process for bonding together two separate surfaces using such a storage-stable, heat- curable structural epoxy adhesive composition.

BACKGROUND OF INVENTION

[0002] Tougheners are flexibilizing agents used in curable compositions for giving an improved flexibility and, thus, a higher dynamic strength to the cured product. [0003] Tougheners are particularly useful in structural adhesives. Structural adhesives are adhesives used to bond structural parts of a structure together, such as for the assembly of the parts of a vehicle such as a car, a truck, a bus or a train. After curing, structural adhesives have to bear both high static and high dynamic loads.

[0004] Conventional structural adhesives are epoxy adhesives. Cured epoxy adhesives per se have a relatively high static strength (i.e. a high tensile and a high lap shear strength), but a rather poor dynamic strength (i.e. a low impact peel strength) . In order to fulfill the requirements of a crash resistant structural adhesive having a high dynamic strength, a toughener is usually added to structural epoxy adhesives.

[0005] Several epoxy adhesive compositions have been described in the state of the art:

US-A-6, 015, 865 relates to an adhesive composition comprising a liquid and a solid epoxy resin and an amino terminated polyalkylene glycol.

JP-A-02150485 describes an adhesive composed of a gelatinized epoxy resin, a latent curing agent and an electrically conductive material. A portion of the epoxy resin comprises an urethane bond and is capped with the polyhydric derivative of bisphenol A diglycidylether .

JP-A-02199116 discloses an epoxy resin composition comprising an epoxy compound having an urethane bond and a curing agent.

JP-A-05156227 discloses a structural adhesive obtained by formulating a urethane-modified epoxy resin, an acrylic rubber-modified epoxy resin and/or a polyalkylene ether- modified epoxy resin with a potential curing agent.

JP-B-02749610 discloses a urethane modified epoxy resin giving flexibility to a cured product.

JP-B-07042449 describes a structural adhesive comprising an epoxy resin, a curing agent, a conductive ingredient and a plasticizer.

WO-A-00/20483 relates to epoxy adhesive compositions comprising a conventional rubber-modified epoxy resin and a condensation product of a carboxylic acid dianhydride with a di- or polyamine and a polyphenol or aminophenol.

WO-A-01/94492 describes the condensation products of cyclic carboxylic acid anhydrides and difunctional polyamines and their use as structural constituents for epoxy adhesive compositions.

WO-A-03/078163 discloses a curable adhesive comprising an epoxy based prepolymer, an acrylate terminated urethane resin and a heat activated curing agent.

EP-A-I 431 325 describes an epoxy adhesive composition comprising an epoxy adduct, a polymer of a defined formula, a thixotropic agent and a hardener.

EP-A-O 457 089 discloses a process for the preparation of urethane- or urea-group containing amines which can be used as hardeners in epoxy resins.

EP-A-O 473 905 refers to epoxy resin mixtures which cure at room temperature.

US-A-3, 636, 133 discloses a curable composition comprising an epoxy resin and a reactive terminated polymeric modifier .

US-A-5, 187 , 253 relates to a method of manufacturing polyesterurethane urea amines and the use of these compounds in the manufacture of epoxy reactive paints, varnishing and coatings.

The mechanical properties of the above epoxy adhesive compositions of the state of the art generally do not fully comply with the requirements of a structural adhesive having both a high static and dynamic strength.

EP-A-O 308 664 discloses an epoxy adhesive composition comprising a butadiene-acrylonitrile copolymer in combination with a polyphenol-terminated polyurethane or polyurea as a toughener.

The tougheners described in EP-A-O 308 664 are known to give a high degree of flexibility to the cured product, making them well suited for the use in structural adhesives. These tougheners are however expensive due to the limited availability of the aminophenol and polyphenol used for preparing the tougheners.

[0006] It is therefore an object of the present invention to provide a toughener for a heat-curable, storage-stable structural epoxy adhesive, said toughener being based on readily available starting materials and having excellent toughening properties.

SUMMARY OF THE INVENTION

[0007] The object is accomplished by the use of a compound as a toughener in a storage-stable, heat-curable structural epoxy adhesive, said compound comprising an elastomeric prepolymer residue selected from the group of a polyurethane, a polyurea and a polyurea polyurethane having isocyanate end groups, the isocyanate end groups of said prepolymer residue being capped by a capping compound selected from the group consisting of a primary aliphatic, cycloaliphatic, heteroaromatic and araliphatic amine, a secondary aliphatic, cycloaliphatic, aromatic, heteroaromatic and araliphatic amine, a thiol and an alkyl amide, said capping compound being bound to the end of the polymer chain of the elastomeric prepolymer in a manner such that the end to which it is bonded no longer has a reactive group, according to Claim 1.

In addition to the capping compound defined above, a capping compound selected from the group consisting of a phenol and a polyphenol can be used for capping the isocyanate end groups of the prepolymer residue, according to Claim 2.

Further preferred embodiments are defined in dependent Claims 3 to 11.

In one embodiment, a mixture of the hereinbefore described capping compounds selected from the group consisting of a phenol and a polyphenol may be used. The term "araliphatic amines" means the same as the term aralkylamines . As given above, the term "capped" means bound to the end of the polymer chain of the elastomeric prepolymer in a manner such that the end to which it is bonded no longer has a reactive group, i.e., the capping compound is monofunctional . Thus, the primary aliphatic amine and the secondary amine are bound to the prepolymer through a urea linkage, the thiol is bound to the prepolymer through a thiourea linkage, the alkyl amide is bound to the prepolymer through an acyl urea linkage and the phenol or the polyphenol is bound to the prepolymer through a urethane linkage. The elastomeric prepolymer is capped by a capping compound selected from the group consisting of a primary aliphatic, cycloaliphatic, heteroaromatic and araliphatic amine, a secondary aliphatic, cycloaliphatic, aromatic, heteroaromatic and araliphatic amine, a thiol and an alkyl amide. An additional capping compound selected from the group consisting of a phenol and a polyphenol can be used for capping the isocyanate end groups of the prepolymer residue. Thus, for one and the same compound the capping compound can be a primary aliphatic, cycloaliphatic, heteroaromatic and/or araliphatic amine, a secondary aliphatic, cycloaliphatic, aromatic, heteroaromatic and/or araliphatic amine, a thiol and/or an alkyl amide as well as a mixture of these compounds, optionally in combination with a phenol and/or a polyphenol.

[0008] The addition of the capped elastomeric prepolymer of the present invention to a structural adhesive leads to a cured product having an improved lap shear and impact peel strength. Typically, the cured product of a structural adhesive comprising the toughener of the present invention and an epoxy resin at least partially modified with a copolymer based on a 1,3-diene and a polar, ethylenically unsaturated comonomer has a lap shear strength of more than 25 MPa and an impact peel strength of more than 30 N/mm. Structural adhesives comprising the compound defined above as a toughener have a good storage stability. The structural adhesives also show good bonding to metals with any type of hydrocarbons disposed thereon. Particularly good bonding is achieved on metals having processing oils, such as AP 167/22 oil (available from Pfinder) and ANTICORIT 4107S oil (available from Fuchs) , disposed thereon. Such processing oils are known to a person skilled in the art.

DETAILED DESCRIPTION OF THE INVENTION

[0009] The starting materials for preparing the compounds of the present invention are readily available and the compounds can be prepared at low costs. Moreover, they can be prepared at lower temperatures compared to conventional tougheners .

[0010] Preferably, the capped elastomeric prepolymer has the structure of Formula I

(D

wherein Ri is the elastomeric prepolymer residue, said residue having a valence of p + q = 2 to 6 with p = 1 to 6 and q = 0 to 5,

X is the residue of the primary aliphatic, cycloaliphatic, heteroaromatic and/or araliphatic amine and/or the secondary aliphatic, cycloaliphatic, aromatic, heteroaromatic and/or araliphatic amine, the thiol and/or the alkyl amide and Y is the residue of the phenol and/or the polyphenol,

said compound being soluble or dispersible in an epoxy resin.

[0011] In a preferred embodiment, X is NR₂R₃ and/or SR₄,

[0012] R₂ and R₃ are selected from hydrogen, a straight or branched C₃ to C₂₆ aliphatic or C₅ to C₂₆ cycloaliphatic residue, a C₂ to C₂₆ alkyl carboxy residue and an aromatic or heteroaromatic residue and optionally form together a heterocyclic, aliphatic or aromatic ring, whereby R₂ and R₃ are not both at the same time hydrogen or alkyl carboxy and

R₄ is selected from a straight or branched aliphatic residue, a cycloaliphatic residue, an araliphatic residue and an aromatic residue.

[0013] Thus, each of R₂, R₃ and R₄ may be a straight C₃ to C₂₆ aliphatic residue, such as a butyl, propyl or tridecyl residue, or a branched aliphatic residue, such as an isopropyl residue. R₂, R₃ and R₄ may also be a cycloaliphatic residue, such as a cyclohexyl residue, or an aromatic residue, such as a phenyl or benzyl residue, or a heteroaromatic residue, such as pyrrol. Optionally, R₂ and R₃ may form a ring as in the compounds of Formula I in which X is a heterocyclic secondary amine, such as morpholine or N-alkylpiperidine or imidazol having an active hydrogen atom.

[0014] In a more preferred embodiment, NR₂R₃ is a secondary sterically hindered amine residue, at least one of R₂ and R₃ being of Formula II

-CR₅R₆R? (H) wherein at least R₅ and R₆ are independently a Ci to C₂i aliphatic residue, R₅ and Re may optionally form a ring and R₇ may be hydrogen.

[0015] It is preferred that the elastomeric prepolymer is obtainable by reacting a polyether polyol and/or a polyether polyamine with an excess of polyisocyanate. It is further preferred that during the reaction to obtain the elastomeric prepolymer additionally a polyester diol, a polybutadiene diol and/or a short-chain polyol is co- reacted.

[0016] Preferably, the number of carbon atoms of the capping compound is in the range of 4 to 22. In a particularly preferred embodiment, the secondary amine is dicyclohexylamine and/or diisopropylamine . The thiol is preferably 1-dodecanethiol . The toughening effect of these compounds is generally such that the lap shear strength of the cured product is at least 25 MPa and the impact peel strength is higher than 45 N/mm.

[0017] It is preferred that in the compound of Formula I q is greater than 0. In such an embodiment, the compound of the present invention is thus also capped with a phenol and/or a polyphenol. The term "a phenol and/or a polyphenol" encompasses any compound having a hydroxyl moiety bonded to an aromatic ring structure, wherein the ring structure can have one or more aromatic rings in the structure and may be further substituted with other substituents . Examples of phenols include aminophenols and phenols substituted with an aliphatic residue like allylphenol . [0018] It is preferable that the phenol or the polyphenol is o-allylphenol, bisphenol A and/or o, o^f -diallylbisphenol A.

[0019] The capped elastomeric prepolymer, such as the compound defined above, is generally prepared by a process which comprises

a) reacting a polyether polyol and/or a polyether polyamine with a polyisocyanate in the presence of a catalyst and

b) reacting the product of a) with a primary aliphatic, cycloaliphatic, heteroaromatic and/or araliphatic amine and/or a secondary aliphatic, cycloaliphatic, aromatic, heteroaromatic and/or araliphatic amine, a thiol and/or an alkyl amide and optionally in step c) with a phenol and/or a polyphenol.

[0020] Preferably, reaction step a) , b) and c) are carried out at a temperature between about 40⁰C and about 120⁰C, more preferably at a temperature of about 6O⁰C to 100⁰C and most preferably about 85°C. The polyetherpolyol can for example be a polytetrahydrofuran (PTHF) or a polypropylene oxide (PPO) .

Any aliphatic isocyanate may be used in the invention which reacts with the polyols described herein and which gives the final product the properties defined herein. Examples of the preferred aliphatic polyisocyanate compound used in the process of the present invention are

1, 6-diisocyanatohexane (HDI), 5-isocyanato-l-

(isocyanatomethyl) -1, 3, 3-trimethylcyclohexane (IPDI) and

2, 4, 4-trimethyl-hexamethylen-l, 6-diisocyanate (TMDI). The polyisocyanate is preferably hexamethylene-1, 6- diisocyanate (1, 6-diisocyanatohexane) (HDI).

[0021] The catalyst used for the reaction of the polyether polyol and the polyisocyanate can be any catalyst known to a person skilled in the art which catalyses the reaction of isocyanate groups with active hydrogen containing compounds. Among preferred catalysts are organotin compounds, metal alkanoates, and tertiary amines. Preferred organotin compounds useful as catalysts include alkyl tin oxides, stannous alkanoates, dialkyl tin carboxylates and tin mercaptides. Stannous alkanoates include stannous octoate. Alkyl tin oxides include dialkyl tin oxides, such as dibutyl tin oxide and its derivatives. The organotin catalyst is preferably a dialkyltin dicarboxylate or a dialkyltin dimercaptide . The dialkyltin dicarboxylate preferably corresponds to the formula (R⁹OC (O) ) ₂~Sn- (R⁹) ₂ wherein R⁹ is independently in each occurrence a Ci_io alkyl, preferably a C₁-₃ alkyl and most preferably a methyl. Dialkyl tin dicarboxylates with lower total carbon atoms are preferred as they are more active catalysts in the compositions of the invention. Another preferred class of organotin catalysts are the dialkyl tin Cβ-Cia carboxylates such as the dibutyl tin Cs- C₁8 carboxylates. The preferred dialkyl dicarboxylates include 1, 1-dimethyltin dilaurate, 1, 1-dibutyltin diacetate and 1,1-dimethyl dimaleate. Metal carboxylate catalysts include bismuth, zinc, and zirconium carboxylates, including bismuth octoate, bismuth neodecanoate, cobalt-neodecanoate, zinc-neodecanoate and zirconium-neodecanoate. The organo tin or metal carboxylate catalyst is present in an amount of about 60 parts per million or greater based on the weight of the composition, more preferably 120 parts by million or greater. The organo tin or metal carboxylate catalyst is present in an amount of about 1.0 percent or less based on the weight of the composition, more preferably 0.5 percent by weight or less and most preferably 0.1 percent by weight or less. Preferred tertiary amine catalysts include dimorpholinodialkyl ether, a di ( (dialkylmorpholino) alkyl) ether, bis- (2- dimethylaminoethyl) ether, triethylene diamine, pentamethyldiethylene triamine, N, N- dimethylcyclohexylamine, N, N-dimethyl piperazine 4- methoxyethyl morpholine, N-methylmorpholine, N-ethyl morpholine and mixtures thereof and metal alkanoates, such as bismuth octoate or bismuth neodecanoate . A preferred dimorpholinodialkyl ether is dimorpholinodiethyl ether. A preferred di ( (dialkylmorpholino) alkyl) ether is (di-(2-

(3, 5-dimethylmorpholino) ethyl) ether . Tertiary amine catalysts are preferably employed in an amount, based on the weight of the composition, of about 0.01 percent by weight or greater, more preferably about 0.05 percent by weight or greater, even more preferably about 0.1 percent by weight or greater and most preferably about 0.2 percent by weight or greater and about 2.0 percent by weight or less, more preferably about 1.75 percent by weight or less, even more preferably about 1.0 percent by weight or less and most preferably about 0.4 percent by weight or less. Preferably the catalyst is a bismuth catalyst, such as bismuth carboxylates (e.g. NEOBI 200), a dialkyl tin dicarboxylate, such as a dibutyltin dilaureate catalyst (e.g. METATIN™ 712), a dialkyl tin mercaptide, such as a dibutyltin mercaptide catalyst (e.g. METATIN™ 713 (8-oxa- 3, 5-dithia-4-stannatetradecanoic acid, 4, 4-dibutyl-10) ) .

[0022] In a preferred process, the secondary amine reacted in step b) is dicyclohexylamine and/or diisopropylamine and the thiol reacted in step b) is preferably 1- dodecanthiol, in accordance with the preferred compounds given above.

[0023] If the product of step a) or b) is also reacted with a phenol or polyphenol, the phenol or polyphenol is preferably allylphenol, bisphenol A and/or o,o'- diallylbisphenol A.

[0024] In a further preferred embodiment, a polybutadiene diol and/or a polyester diol is additionally added and reacted in step a) .

[0025] In order to increase the functionality of the prepolymer, a short-chain polyol, such as 1,1,1- trimethylolpropane (TMP) or pentaerythrit, can additionally be added and reacted in step a) .

[0026] Preferably, the amounts of the components are such that based on the total weight of the composition

a) 10 wt percent to 90 wt percent of the polyether polyol, more preferably about 25 wt percent to about 80 wt percent, up to 35 wt percent of the polybutadiene diol, more preferably about 10 to about 35 weight percent, up to 40 wt percent of the polyester polyol more preferably about 5 to about 40 weight percent, up to 5 wt percent of the short-chain polyol, more preferably about 0 to about 3 weight percent, and 5 wt percent to 35 wt percent of the polyisocyanate, more preferably about 10 to about 20 weight percent are reacted in the presence of 0.1 wt percent to 1 wt percent of a catalyst, and

b) 2 wt percent to 20 wt percent, more preferably about 3 to about 15 weight percent of the primary amine, the secondary amine, the thiol and/or the amide are added to the mixture obtained in a) , corresponding to an amount equimolar to the isocyanate of the prepolymer if there is no reaction step c) .

[0027] In an optional subsequent step c) 0.5 wt percent to 50 wt percent, more preferably about 1.5 to about 30 weight percent of the phenol and/or the polyphenol are added to the mixture obtained in b) , corresponding to a slight molar excess with respect to the remaining isocyanate groups after step b) .

[0028] The present invention relates also to a storage- stable, heat-curable structural epoxy adhesive composition comprising an epoxy resin and the capped elastomeric prepolymer defined above, according to Claim 12. Epoxy resins which may be employed in the compositions of the invention are those which contain groups illustrated in the following formula

wherein R⁸ is hydrogen or Ci_₄ alkyl, preferably hydrogen or methyl and most preferably hydrogen. Preferred epoxy resins are epoxy resins having bisphenol moieties in the backbone of the epoxy resin. Representative of preferred bisphenol resins useful in this invention are those disclosed in U.S. 5,308,895 at column 8, line 6 and represented by Formula 6. Relevant portions of such patent are incorporated herein by reference. Preferably, the epoxy resin is a liquid epoxy resin or a mixture of a solid epoxy resin dispersed in a liquid epoxy resin. The most preferred epoxy resins are bisphenol A and bisphenol- F based resins. The first epoxy resin preferably can be a liquid epoxy resin, such as D.E.R™ 330 and D.E.R.™ 331 Bisphenol A based epoxy resins (available from The Dow Chemical Company) , or a solid epoxy resin, such as Bisphenol A based epoxy resin D.E.R.™ 671 (available from The Dow Chemical Company) , or a mixture thereof. Typical liquid and solid epoxy resins are used such as D.E.R.™ 330, D.E.R.™ 331 and D.E.R.™ 671 (available from The Dow Chemical Company) .

[0029] Preferably, the epoxy resin is used in an amount of 30 to 80 parts, more preferably 40 to 70 parts and most preferably 45 to 60 parts per hundred parts of the adhesive composition.

[0030] Likewise, the curable epoxy composition can also be a two-component epoxy adhesive composition.

[0031] Further preferred is a storage-stable, heat-curable structural epoxy adhesive composition wherein at least a part of the epoxy resin is modified with a copolymer based on a 1,3-diene and a polar, ethylenically unsaturated comonomer and/or comprises a core-shell rubber. The term "modified" means herein that the copolymer is blended with, grafted to or reacted with the epoxy resin, i.e., an adduct. Preferably, the copolymer is an adduct to the epoxy resin. Such copolymers are described in detail in U.S.-B-5,278,257 at column 2, line 11, to column 4, line 5, the disclosure of which is incorporated herein by reference. Examples of 1,3-dienes are butadiene, isoprene and chloroprene. Copolymers based on butadiene are preferred. Examples of polar, ethylenically unsaturated comonomers used in the copolymer are acrylic acid, methacrylic acid, esters of acrylic or methacrylic acid, for example, the methyl or ethyl esters, amides of acrylic or methacrylic acid, fumaric acid, itaconic acid, maleic acid or esters or half-esters thereof, for example, the monomethyl or dimethyl esters, or maleic anhydride or itaconic anhydride, vinyl esters, for example, vinyl acetate, polar styrenes, such as styrenes chlorinated or brominated in the nucleus, or, in particular, acrylonitrile or methacrylonitrile . Besides, polar, ethylenically unsatured comonomers, the copolymer can also contain other non-polar, ethylenically unsaturated comonomers. Examples of these are ethylene, propylene or, in particular, styrene or substituted styrenes, such as vinyltoluene . Copolymers can be a statistical copolymer, a block copolymer or a graft copolymer. This component can be solid, in particular pulverulent, or, preferably, can be liquid. It can also be a thermoplastic, a thermoplastic elastomer or an elastomer. The proportion of the comonomers in the copolymer can vary within wide ranges. The monomers are chosen that an elastomer phase is formed in combination with an epoxide resin. These can be a homogeneous or heterogeneous system.

[0032] The composition comprises an epoxy resin modified with an acrylonitrile-butadiene rubber. Preferably, the epoxy resin modified with a copolymer comprises at least one of the acrylonitrile-butadiene rubbers selected from the group of X13, X8, X31 or any mixture of X8, X31 and X13 (wherein X stands for an acrylonitrile-butadiene rubber of the CTBN (carboxy- terminated butadiene-rubber) type and the term "mixture" means a "mixture of two or three of the components") .

X8 is an acrylonitrile-butadiene-rubber comprising 17 percent acrylonitrile. X13 is an acrylonitrile-butadiene-rubber comprising 26 percent acrylonitrile .

X31 is an acrylonitrile-butadiene-rubber comprising 10 percent acrylonitrile.

Preferable copolymers are carboxy terminated butadiene acrylonitrile rubbers, such as HYCAR™ CTBN 1300 X8, 1300 X13 and 1300 X31 (available from Noveon) . More preferably, the storage-stable, heat-curable structural epoxy adhesive composition comprises 5 to 30 wt percent, more preferably about 10 to about 20 percent by weight of the copolymer modified epoxy resin and/or of the core-shell rubber. Core-shell rubbers are known to a person skilled in the art and are for example described in US-A-5 290 857 and US-A-5 686 509, incorporated herein by reference, as well as in EP-A-I 359 202 (paragraph [0037], incorporated herein by reference, the disclosure of which is enclosed herewith) .

[0033] A preferred storage-stable, heat-curable structural epoxy adhesive composition comprises about 5 weight percent to about 40 weight percent of the compound of the present invention, more preferably about 8 to about 30 weight percent and most preferably about 10 to about 25 weight percent.

[0034] In a preferred embodiment, the storage-stable, heat-curable structural epoxy adhesive composition further comprises one or more additives selected from the group of hardeners, accelerators, adhesion promoters, epoxy silane, fumed silica, wetting agents and inorganic fillers.

It is particularly preferred that the curable epoxy composition of the present invention comprises one or more additives selected from the group of hardeners, such as dicyandiamide and imidazoles, accelerators, such as EP 796, adhesion promoters, epoxy silane, fumed silica, wetting agents and inorganic fillers, the epoxy compositions being curable by heat.

[0035] The hardener present is a sufficient amount to fully cure the epoxy resin component. Preferably, the hardener is present in an amount of about 1 to aboutlO weight percent and more preferably about 2 to about 7 weight percent based on the weight of the epoxy composition. The accelerator may be used in a sufficient amount to result in the cure of the epoxy resin component within the desired time period. Preferably, the amount of accelerator is about 0 to about 5 weight percent and more preferably about 0.2 to about 2 weight percent based on the weight of the epoxy composition. Suitable curing agents are familiar to persons skilled in the art. Several suitable curing agents are taught in "Handbook of Epoxy Resins" (Lee, H & Neville, K., McGraw-Hill Book Company, 1967, page 20-11) and in "Powder Coatings" (Tess, Epoxy Resins-Chemistry and Technology, 2^nd Ed., 1988, pages 112-11Q) . Examples of suitable curing agents include dicyandiamide and other amines and amides, polyhydric phenols, and polyanhydrides . The optimum ratio of curing agent to epoxy resin varies depending upon the curing agent selected and the intended use of the resin. Usually, the equivalent ratio of curing agent to epoxy resin is preferably 0.1:1 to 10:1, and more preferably 0.2:1 to 2:1. [0036] For wash-off resistant adhesive compositions, the storage-stable, heat-curable structural epoxy adhesive composition further comprises a thermoplastic polymer comprising a polyester segment, said polymer being at least partially crystalline at room temperature and having a softening temperature in the range of 40° to 125°C, more preferably from about 40 to about 90 ⁰C. Preferably, the amount of the polymer is from 2 to 20 weight-percent, more preferably from 5 to 15 weight-percent based on the total weight of the adhesive composition. Such a storage-stable, heat-curable structural epoxy composition has a rather low basic viscosity and, without being pre-cured, a high wash-off resistance. Softening temperature is used herein to mean the temperature where segments of polyester polyols start to melt in the adhesive formulation.

[0037] In a further preferred embodiment, the storage- stable, heat-curable structural epoxy adhesive composition comprises as an accelerator a tertiary polyamine embedded in a polymer matrix. A preferred example is EP 796, i.e., 2, 4 , 6-tris (dimethylaminomethyl) phenol integrated into a poly (p-vinylphenol) matrix as described in EP-A-O 197 892.

[0038] When bonding together separate surfaces, the epoxy adhesive composition is applied by common dispensing equipment to at least one surface, the surfaces are brought together and the epoxy adhesive composition is cured at a temperature of 120⁰C to 210⁰C. Such heat curable epoxy composition are usually cured during 15 to 60 minutes, and more preferably for 20 to 30 minutes at a temperature of 120⁰C to 210°C, preferably 140°C to 200⁰C, and more preferably at 175°C to about 185°C for about 25 to 35 minutes.

[0039] Thus, the present invention also relates to a process for preparing a cured epoxy adhesive wherein the storage-stable, heat-curable structural epoxy adhesive composition is heated to a temperature as described above. The epoxy adhesive composition is preferably used for the assembly of parts of a vehicle, such as a car, a van, a lorry, a bus and a train, i.e., as structural adhesive. It can also be used for assembling parts of boats and aircrafts.

[0040] The epoxy adhesive composition of the present invention can be applied manually or automatically by a robot as normal beads, by swirling or by jet-streaming.

The curing is starting at temperatures above 120⁰C and preferably above 140⁰C.

[0041] In one embodiment the invention is a method of bonding two substrates together comprising applying a composition described herein to at least one surface of one substrate, the surface of a second substrate is brought together with the surface of the first substrate such that the composition is located between the surfaces of the substrates and the composition is cured.

[0042] In a preferred process for bonding together two separate surfaces, the adhesive composition is applied to at least one surface, the surfaces are brought together and the epoxy adhesive composition located between the surface is cured at a temperature above about 120⁰C, preferably above about 140⁰C for at least about 15 minutes and preferably at least about 20 minutes, preferably, no more than about 60 minutes and more preferably no more than about 50 minutes. Preferably, the curing is performed at about 210⁰C and more preferably about 200⁰C or less. In this embodiment the adhesive composition cures after a short period of time. In such a process, the epoxy adhesive composition can be cured up to about 4 weeks after bringing together the surfaces with the adhesive located between them. In the embodiment where the composition is a two-part composition, the components are mixed just prior to applying the mixture to a substrate.

[0043] The present invention also relates to the use of the two-component epoxy adhesive composition for bonding together parts of a vehicle into a crash-stable assembly. Accordingly, the present invention also relates to parts bonded together by the epoxy adhesive composition into a crash-stable assembly.

In general, the number average molecular weight of the compounds of the present invention ranges from 10000 to 45000 Da. Preferably, the number average molecular weight of the compound ranges from 15000 to 40000 Da, most preferably from 20000 to 35000 Da.

The number average molecular weight is determined by a standard gel permeation chromatography (GPC) method using a GPC apparatus comprising a pre-column, a first column (PL gel 3 μm MIXED E™ available from Polymer Laboratories) and a second column (PL gel 5 μm MIXED D™ available from Polymer Laboratories) .

The eluent of the GPC method is tetrahydrofuran (absolute puriss over molecular sieve >99.5 percent (GC)), the flow rate being 0.9 ml/minute (min.). The detector used is an RI-detector (refractive index-detector) . For calibrating the GPC apparatus, a polystyrene standard is used, the range of calibration being 160 Da to 100000 Da.

For determining the number average molecular weight of the compound defined in Claim 1, only peaks over 2000 Da are taken into account. In most cases, only one peak over 2000 Da is detected. In the rare cases where two peaks over 2000 Da are detected, the number average molecular weight of the toughener is determined by calculating the mean of these peaks.

EXAMPLES

1. Preparation of a dicyclohexylamine-capped compound according to Formula I

Reaction step a) :

72.6 wt percent (based on the total weight of the resulting composition) of polytetrahydrofuran (PTHF 2000,

BASF), 0.5 wt percent of 1, 1, 1-trimethylolpropane (TMP;

Fluka) and 0.6 wt percent of a bismuth carboxylate catalyst (NEOBI 200 (bismuth neodecanoate) available from

Shepherd) are mixed at 85°C to homogeneity. Then, 13.1 wt percent of hexamethylene-1, 6-diisocyanate (HDI;

Bayer/Merck) is added and the mixture is allowed to react at 85°C for 1 hour.

Reaction step b) :

13.2 wt percent dicyclohexylamine (Merck) is added and the mixture is stirred for additional 20 minutes at 85°C. Thereby, a dicyclohexylamine-capped compound according to Formula I is obtained (Example 1) . 2. Preparation of a diisopropylamine-capped toughener according to Formula I

Reaction step a) :

77.1 wt percent (based on the total weight of the resulting composition) of polytetrahydrofuran (PTHF 2000,

BASF), 0.5 wt percent of 1, 1, 1-trimethylolpropane (TMP;

Shepherd) are mixed at 85°C to homogeneity. Then, 14 wt percent of hexamethylene-1, 6-diisocyanate (HDI;

Bayer/Merck) is added and the mixture is allowed to react at 85°C for 1 hour.

Reaction step b) :

7.8 wt percent of diisopropylamine (Merck) is added and the mixture is stirred for additional 20 minutes at 85°C. Thereby, a diisopropylamine-capped compound according to Formula I is obtained (Example 2) .

3. Preparation of a dodecanthiol-capped toughener according to Formula I

Reaction step a) :

71.5 wt percent (based on the total weight of the resulting composition) of polytetrahydrofuran (PTHF 2000, BASF), 0.5 wt percent of 1, 1, 1-trimethylolpropane (TMP; Fluka) and 0.6 wt percent of a bismuth carboxylate catalyst (NEOBI 200 (bismuth neodecanoate) available from Shepherd) are mixed at 85°C to homogeneity. Then, 12.9 wt percent of hexamethylene-1, 6-diisocyanate (HDI; Bayer/Merck) is added and the mixture is allowed to react at 85°C for 1 hour.

Reaction step b) :

14.5 wt percent of dodecanthiol (Merck) is added and the mixture is stirred for additional 20 minutes at 85°C. Thereby, a dodecanthiol-capped compound according to Formula I is obtained (Example 3) .

4. Preparation of a compound according to Formula I capped with both dicyclohexylamine (80 val percent (val percent = equivalent percent) ) and allylphenol (20 val percent)

Reaction step a) :

73.1 wt percent (based on the total weight of the resulting composition) of polytetrahydrofuran (PTHF 2000,

BASF), 0.5 wt percent of 1, 1, 1-trimethylolpropane (TMP;

Shepherd) are mixed at 85°C to homogeneity. Then, 13.2 wt percent of hexamethylene-1, 6-diisocyanate (HDI;

Bayer/Merck) is added and the mixture is allowed to react at 85°C for 1 hour.

Reaction step b) :

10.6 wt percent of dicyclohexylamine (Merck) is added and the mixture is stirred for additional 20 minutes at 85°C.

Reaction step c) : 2 wt percent of o-allylphenol (Fluka) is added and the mixture is stirred for additional 20 minutes at 85°C. Thereby, a compound of Formula I capped with both dicyclohexylamine (80 val percent percent) and allylphenol (20 val percent percent) is obtained (Example 4) .

5. Preparation of a compound according to Formula I capped with both diisopropylamine (80 val percent) and allylphenol (20 val percent)

Reaction step a) :

76.7 wt percent (based on the total weight of the resulting composition) of polytetrahydrofuran (PTHF 2000, BASF), 0.5 wt percent 1, 1, 1-trimethylolpropane (TMP; Fluka) and 0.6 wt percent of a bismuth carboxylate catalyst (NEOBI 200 (bismuth neodecanoate) available from Shepherd) are mixed at 85°C to homogeneity. Then, 13.9 wt percent hexamethylene-1, 6-diisocyanate (HDI; Bayer/Merck) is added and the mixture is allowed to react at 85°C for 1 hour.

Reaction step b) :

6.2 wt percent diisopropylamine (Merck) is added and the mixture is stirred for additional 20 minutes at 85°C.

Reaction step c) :

2.1 wt percent o-allylphenol (Fluka) is added and the mixture is stirred for additional 20 minutes at 85°C. Thereby, a compound of Formula I capped with both diisopropylamine (80 val percent) and allylphenol (20 val percent) is obtained (Example 5) . 6. Preparation of a compound according to Formula I capped with both dodecanthiol (80 val percent) and allylphenol (20 val percent)

Reaction step a) :

72.2 wt percent (based on the total weight of the resulting composition) of polytetrahydrofuran (PTHF 2000, BASF), 0.5 wt percent of 1, 1, 1-trimethylolpropane (TMP; Fluka) and 0.6 wt percent of a bismuth carboxylate catalyst (NEOBI 200 (bismuth neodecanoate) available from Shepherd) are mixed at 85°C to homogeneity. Then, 13.1 wt percent of hexamethylene-1, 6-diisocyanate (HDI; Bayer/Merck) is added and the mixture is allowed to react at 85°C for 1 hour.

Reaction step b) :

11.7 wt percent of dodecanthiol (Merck) is added and the mixture is stirred for additional 20 minutes at 85⁰C.

Reaction step c) :

2 wt percent of o-allylphenol (Fluka) is added and the mixture is stirred for additional 20 minutes at 85°C. Thereby, a compound of Formula I capped with both dodecanthiol (80 val percent) and allylphenol (20 val percent) is obtained (Example 6) . 7. Preparation of a diisopropylamine-capped compound according to Formula I based on a polybutadiene diol- containing prepolymer

Reaction step a) :

52.8 wt percent (based on the total weight of the resulting composition) of polytetrahydrofuran (PTHF 2000, BASF), 24.5 wt percent of polybutadiene diol (PBD, Elf Atochem) , 0.5 wt percent of 1, 1, 1-trimethylolpropane (TMP; Fluka) and 0.6 wt percent of a dibutyltin mercaptide catalyst (METATIN™ 713 available from Acima which is 8- oxa-3, 5-dithia-4-stannatetradecanoic acid, 4, 4-dibutyl-10) are mixed at 85°C to homogeneity. Then, 13.7 wt percent of hexamethylene-1, 6-diisocyanate (HDI; Bayer/Merck) is added and the mixture is allowed to react at 85°C for 1 hour.

Reaction step b) :

7.9 wt percent of diisopropylamine (Merck) is added and the mixture is stirred for additional 20 minutes at 85°C. A diisopropylamine-capped compound according to Formula I based on a polybutadiene diol-containing prepolymer is obtained (Example 7).

8. Preparation of a compound according to Formula I capped with both diisopropylamine (80 val percent) and o,o'- diallylbisphenol A (20 val percent)

Reaction step a) :

74.7 wt percent (based on the total weight of the resulting composition) of polytetrahydro (PTHF 2000, BASF), 0.5 wt percent of 1, 1, 1-trimethylolpropane (TMP; Fluka) and 0.6 wt percent of a dibutyltin mercaptide catalyst (METATIN™ 713 available from Acima) are mixed at 85°C to homogeneity. Then, 13.5 wt percent of hexamethylene-1, 6-diisocyanate (HDI; Bayer/Merck) is added and the mixture is allowed to react at 85°C for 1 hour.

Reaction step b) :

6.1 wt percent of diisopropylamine (Merck) is added and the mixture is stirred for additional 20 minutes at 85°C.

Reaction step c) :

4.7 wt percent of o, o' -diallylbisphenol A (Huntsman Corporation) is added and the mixture is stirred for additional 20 minutes. Thereby, a compound of Formula I capped with both diisopropylamine (80 val percent) and o, o' -diallylbisphenol A (20 val percent) is obtained (Example 8) .

9. Preparation of a cyclohexylamine-capped compound according to Formula I

Reaction step a) :

77.2 wt percent (based on the total weight of the resulting composition) of polytetrahydrofuran (PTHF 2000, BASF), 0.5 wt percent of 1, 1, 1-trimethylolpropane (TMP; Fluka) and 0.6 wt percent of a dibutyltin mercaptide catalyst (METATIN™ 713 available from Acima) are mixed at 85°C to homogeneity. Then, 14.0 wt percent of hexamethylene-1, 6-diisocyanate (HDI; Bayer/Merck) is added and the mixture is allowed to react at 85°C for 1 hour.

Reaction step b) :

7.7 wt percent cyclohexylamine (Merck) is added and the mixture is stirred for additional 20 minutes at 85⁰C. Thereby, a cyclohexylamine-capped compound according to Formula I is obtained (Example 9) .

10. Preparation of an n-octylamine-capped compound according to Formula I

Reaction step a) :

75.4 wt percent (based on the total weight of the resulting composition) of polytetrahydrofuran (PTHF 2000, BASF), 0.5 wt percent of 1, 1, 1-trimethylolpropane (TMP; Fluka) and 0.6 wt percent of a dibutyltin mercaptide catalyst (METATIN™ 713 available from Acima) are mixed at 85⁰C to homogeneity. Then, 13.7 wt percent of hexamethylene-1, 6-diisocyanate (HDI; Bayer/Merck) is added and the mixture is allowed to react at 85°C for 1 hour.

Reaction step b) :

9.8 wt percent n-octylamine (Fluka) is added and the mixture is stirred for additional 20 minutes at 85⁰C.

Thereby, an n-octylamine-capped compound according to Formula I is obtained (Example 10) .

11. Preparation of a diisopropylamine-capped compound according to Formula I based on a polyester diol- containing prepolymer

Reaction step a) :

27.9 wt percent (based on the total weight of the resulting composition) of polytetrahydrofuran (PTHF 2000, BASF), 51.4 wt percent of a polyester diol (DYNACOLL™ 7380 available from Degussa) , 0.4 wt percent of 1,1,1- trimethylolpropane (TMP; Fluka) and 0.4 wt percent of a dibutyltin mercaptide catalyst (METATIN™ 713 available from Acima) are mixed at 85⁰C to homogeneity. Then, 12.0 wt percent of hexamethylene-1, 6-diisocyanate (HDI; Bayer/Merck) is added and the mixture is allowed to react at 85°C for 1 hour.

Reaction step b) :

7.9 wt percent of diisopropylamine (Merck) is added and the mixture is stirred for additional 20 minutes at 85°C. A diisopropylamine-capped compound according to Formula I based on a polyester diol-containing prepolymer is obtained (Example 11).

12. Preparation of a diisopropylamine-capped compound according to Formula I based on a polyester diol- containing prepolymer

Reaction step a) :

64.6 wt percent (based on the total weight of the resulting composition) of polytetrahydrofuran (PTHF 2000, BASF), 13.2 wtpercent of a polyester diol (DYNACOLL™ 7380 available from Degussa), 0.5 wt percent of 1,1,1- trimethylolpropane (TMP; Fluka) and 0.5 wt percent of a dibutyltin mercaptide catalyst (METATIN™ 713 available from Acima) are mixed at 85°C to homogeneity. Then, 13.4 wt percent of hexamethylene-1, 6-diisocyanate (HDI; Bayer/Merck) is added and the mixture is allowed to react at 85°C for 1 hour.

Reaction step b) :

7.8 wt percent of diisopropylamine (Merck) is added and the mixture is stirred for additional 20 minutes at 85 °C. A diisopropylamine-capped compound according to Formula I based on a polyester diol-containing prepolymer is obtained (Example 12) . This prepolymer on which the compound is based contains less polyester diol than the one of Example 11 (Example 12) .

13. Preparation of a diisopropylamine-capped toughener according to Formula I based on a polyurea prepolymer

Reaction step a) :

76.7 wt percent (based on the total weight of the resulting composition) of a polyoxypropylenediamine (JEFFAMINE™ D 2000 available from Huntsman Corporation) and 1.9 wt percent of a polyoxypropylenetriamine

(JEFFAMINE™ T 403 available from Huntsman Corporation) are mixed under cooling to homogeneity. Then, 13.7 wt percent of hexamethylene-1, 6-diisocyanate (HDI; Bayer/Merck) is added and the mixture is allowed to react under cooling for 10 minutes.

Reaction step b) : 7.8 wt percent of diisopropylamine (Merck) is added and the mixture is stirred for additional 10 minutes at 85°C. Thereby, a diisopropylamine-capped compound according to Formula I based on a polyurea prepolymer is obtained (Example 13) .

In analogy to Examples 1 to 3, Examples 14 to 22 are prepared by adding in step b) the corresponding compounds given below.

Example 14: dibutylamine

Example 15: dipropylamine

Example 16: ditridecylamine

Example 17: dibenzylamine

Example 18: diallylamine

Example 19: ethylcyclohexylamine

Example 20: N-methylacetamide

Example 21: morpholine

Example 22: 1-butanethiol

[0044] All mixing steps in the above preparation processes are performed under nitrogen.

[0045] After reaction step a) , the NCO content is measured by back-titration with HCl solution after reacting the polyisocyanate with an excess of dibutylamine.

[0046] The number average molecular weight (Mn) of the obtained compounds is determined by gel permeation chromatography as described above (first column: PL Gel 3μm Mixed E; second column: PL Gel 5μm Mixed D; both available from Polymer Laboratories; flow: 0.9 ml/min THF; standard: polystyrene) .

[0047] The results are given in Table 1. As a reference example, the result for FLEXIBILIZER™ DY 965, i.e., a toughener corresponding to Example 13 of EP-B-O 308 664, is also given.

Table 1

Toughening effect of compounds according to Formula I

[0048] For each of Examples 1 to 22, a heat curable composition comprising 14 wt percent of the respective toughening compound of the present invention, 55 wt percent of a mixture of epoxy resins D. E. R. TM 330 and

D. E. R. TM 671 (both available from The Dow Chemical

Company) , 15 wt percent of an acrylonitrile-butadiene rubber, 4.5 wt percent of dicyandiamide, 1 wt percent of accelerator EP 796, about 5 wt percent of fumed silica and about 5 wt percent of polyvinylbutyral has been prepared. As a comparative example, an analog composition comprising FLEXIBILIZER™ DY 965 has been prepared.

A sample of each of the thus prepared heat curable compositions is cured at a temperature of 18O⁰C for 30 minutes .

[0049] The lap shear strength and the impact peel strength of the cured product is determined by the following methods :

[0050] The lap shear strength was determined at 23°C according to DIN EN 1465 using degreased cold-rolled steel (CRS 1403, 1.5 mm thick) as substrate.

[0051] The bonding area was 25 mm x 10 mm, the adhesive layer thickness was 0.2 mm. The test speed was 10 mm/min.

[0052] The impact peel strength was determined at 23°C according to ISO 11343 using a degreased cold-rolled steel (CRS 1403, 1 mm thick) as substrate. The bonding area was 30 mm x 20 mm, the adhesive layer thickness was 0.2 mm. The test speed was 2 m/s.

[0053] The results are shown in Table 2. The corresponding values of the heat curable composition comprising

FLEXIBILIZER TM DY 965 (RAM 965) is given as a reference 0 example .

Table 2

[0054] Particularly good results in respect of both the lap shear strength and the impact peel strength are obtained when the capping compound (i.e. residue X in Formula I) is a sterically hindered secondary amine, such as dicyclohexylamine and diisopropylamine, or a combination of such an amine with o-allylphenol or o, o' diallylbisphenol A. Also, good results are obtained when the capping compound is a thiol such as 1- 0 dodecanethiol . Compounds in which the capping compound is a mixture of dicyclohexylamine and o-allylphenol or o,o'- diallyl bisphenol A, a mixture of diisopropylamine and o- allylphenol or 0,0' -diallyl bisphenol A or a mixture of a 1-dodecanethiol and o-allylphenol or 0,0' -diallyl bisphenol A are also preferred.

Lap shear strength on different metals treated with different oils

[0055] For Examples 1 to 3, the lap shear strengths at 23°C according to DIN EN 1465 (bonding area: 25 mm x 10 mm; adhesive layer thickness: 0.2 mm) of the corresponding formulations were determined on different metals treated with different oils. As substrates, H340LAD+Z (hot-dipped zinc coated, 0.7 mm thick), DC04-B+ZE (electrogalvanized zinc coated, 0.8 mm thick) and AA6016 (Aluminum, 1.2 mm thick, pretreated using ALODINE™ 2040) were used. The oils used in the tests were AP 167/22 (available from Pfinder) and ANTICORIT 4107S (available from Fuchs) . The test speed was 10 mm/min. The results are shown in Table 3. The values are given in MPa.

Table 3

Impact peel strength on different metals and oils

[0056] For Examples 1 to 3, the impact peel strengths of the corresponding formulations were determined at 23⁰C according to ISO 11343 on different metals and oils

(bonding area: 30 mm x 20 mm, adhesive layer thickness: 0.2 mm, test speed: 2 m/s) . As substrates, H340LAD+Z (0.7 mm thick), DC04-B+ZE (0.8 mm thick) and AA6016 (Aluminum, 1.2 mm thick, pretreated using ALODINE™ 2040) were used. The oils used in the tests were AP 167/22 (available from Pfender) and ANTICORIT 4107S (available from Fuchs) . The results are shown in Table 4. The values are given in N/mm. Table 4

[0057] As can be seen from Table 4, the formulations comprising a compound of the present invention have generally higher impact peel strengths than the formulation comprising the reference FLEXIBILIZER™ DY 965.

Impact peel strength at low temperature

[0058] In a further test, the impact peel strength of the structural adhesive composition comprising the compound of

Example 7 has been determined according to ISO 11343 at room temperature and at -40⁰C (bonding area: 30 x 20 mm, adhesive layer thickness: 0.2 mm, test speed: 2 m/s, substrate: cold-rolled steel, thickness lmm) . As a reference example, the values of the adhesive composition comprising reference FLEXIBILIZER™ DY 965 are also given.

The results are shown in Table 5. The values are given in N/mm. Table 5

[0059] Table 5 shows a superior impact peel strength of the structural adhesive composition of the present invention over the structural adhesive composition comprising the reference toughener.

Claims

Use of a compound as a toughener in a storage-stable, heat-curable structural epoxy adhesive, said compound comprising an elastomeric prepolymer residue selected from the group of a polyurethane, a polyurea and a polyurea polyurethane having isocyanate end groups, the isocyanate end groups of said prepolymer residue being capped by a capping compound selected from the group consisting of a primary aliphatic, cycloaliphatic, heteroaromatic and araliphatic amine, a secondary aliphatic, cycloaliphatic, aromatic, heteroaromatic and araliphatic amine, a thiol and an alkyl amide, said capping compound being bound to the end of the polymer chain of the elastomeric prepolymer in a manner such that the end to which it is bonded no longer has a reactive group.

The use according to claim 1 wherein an additional capping compound selected from the group consisting of a phenol and a polyphenol is used for capping the isocyanate end groups of the prepolymer residue.

The use according to claim 1 or 2 wherein the compound used as a toughener has the structure of Formula I

(D wherein Ri is the elastomeric prepolymer residue, said residue having a valence of p + q = 2 to 6 with p = 1 to 6 and q = 0 to 5,

X is the residue of the primary aliphatic, cycloaliphatic, heteroaroitiatic and/or araliphatic amine and/or the secondary aliphatic, cycloaliphatic, aromatic, heteroaromatic and/or araliphatic amine and/or the thiol and/or the alkyl amide and

Y is the residue of the phenol and/or the polyphenol,

said compound being soluble or dispersible in an epoxy resin.

4. The use according to claim 3 wherein

X is NR2R₃ and/or SR₄,

R₂ and R₃ are selected from hydrogen, a straight or branched C₃ to C₂₆ aliphatic or C₅ to C₂₆ cycloaliphatic residue, a C2 to C₂₆ alkyl carboxy residue and an aromatic or heteroaromatic residue and optionally form together a heterocyclic, aliphatic or aromatic ring, whereby R₂ and R₃ are not both at the same time hydrogen or alkyl carboxy and

5. The use according to claim 4 wherein NR₂R₃ is a secondary sterically hindered amine residue, at least one of R₂ and R₃ being of Formula II

-CR₅R₆Rv (H) - A A -

wherein at least R₅ and Re are independently a Ci to C₂₁ aliphatic residue, R₅ and RQ may optionally form a ring and R₇ may be hydrogen.

6. The use according to any of the preceding claims wherein the elastomeric prepolymer is obtainable by reacting a polyether polyol and/or a polyether polyamine with an excess of a polyisocyanate.

7. The use according to claim 6 wherein during the reaction to obtain the elastomeric prepolymer additionally a polyester diol, a polybutadiene diol and/or a short-chain polyol is co-reacted.

8. The use according to any of the preceding claims wherein the secondary amine is dicyclohexylamine and/or diisopropylamine.

9. The use according to any of the preceding claims wherein the thiol is 1-dodecanethiol .

10. The use according to claims 3 to 9 wherein q is greater than 0.

11. The use according to any of claims 2 to 10 wherein the phenol or the polyphenol is o-allylphenol, bisphenol A and/or o, o' -diallylbisphenol A.

12. A storage-stable, heat-curable structural epoxy adhesive composition comprising an epoxy resin and the compound defined in any of claims 1 to 11.

13. A storage-stable, heat-curable structural epoxy adhesive composition according to claim 12 wherein at least a part of the epoxy resin is modified with a copolymer based on a 1,3-diene and a polar, ethylenically unsaturated comonomer and/or comprises a core-shell rubber.

14. A storage-stable, heat-curable structural epoxy adhesive composition according to claim 13 comprising 5 to 30 wt percent of the copolymer modified epoxy resin and/or of the core-shell rubber.

15. A storage-stable, heat-curable structural epoxy adhesive composition according to any of claims 12 to

14 comprising 5 wt percent to 40 wt percent of the compound defined in any of claims 1 to 11.

16. A storage-stable, heat-curable structural epoxy adhesive composition according to any of claims 12 to

15 comprising 8 wt percent to 30 wt percent of the compound defined in any of claims 1 to 11.

17. A storage-stable, heat-curable structural epoxy adhesive composition according to any of claims 12 to

16 comprising 10 wt percent to 25 wt percent of the compound defined in any of claims 1 to 11.

18. A storage-stable, heat-curable structural epoxy adhesive composition according to any of claims 12 to

17 further comprising one or more additives selected from the group of hardeners, accelerators, adhesion promoters, epoxy silane, fumed silica, wetting agents and inorganic fillers.

19. A storage-stable, heat-curable structural epoxy adhesive composition according to any of claims 12 to

18 further comprising a thermoplastic polymer comprising a polyester segment, said polymer being at least partially crystalline at room temperature and having a softening temperature in the range of 40° to 125°C.

20. A storage-stable, heat-curable structural epoxy adhesive composition according to any of claims 12 to 19 further comprising as an accelerator 2,4,6- tris (dimethylaminomethyl) phenol integrated into a poly (p-vinylphenol) matrix.

21. A process of bonding together separate surfaces wherein the storage-stable, heat-curable structural epoxy adhesive composition according to any of claims 12 to 20 is applied to at least one surface, the surfaces are brought together and the structural epoxy adhesive composition is cured at a temperature of 120⁰C to 210⁰C.