Method of bonding parts for improved adhesion

Abstract

Disclosed is a method for bonding two substrates comprising the sequential steps of - i) applying an expandable one-component thermosetting epoxy resin adhesive on a first substrate S1, and - ii) placing the second substrate S2 so that the distance between the first substrate S1 and the second substrate S2 at the place of application of the expandable one-component thermosetting epoxy resin adhesive is between 2 – 30 mm, and - iii) heating the first substrate S1 and the second substrate S2 to a temperature above the activation temperature of the blowing agent BA to expand the expandable one-component thermosetting epoxy resin adhesive, wherein between step ii) and step iii) the applied expandable one-component thermosetting epoxy resin adhesive is not in contact with the second substrate S2. The expandable one-component thermosetting epoxy resin adhesive comprises epoxy resin, blowing agent BA, latent hardener B for epoxy resins and has a viscosity of 500 to 5000 Pas. The method allows for bonding metallic substrates with good adhesion, especially good lap shear strength, as well as sufficient T-peel values.

Description

[0001] METHOD OF BONDING PARTS FOR IMPROVED ADHESION

[0002] Technical field

[0003] The present invention pertains to the field of expandable one-component thermosetting epoxy resin adhesives and to the use thereof particularly in vehicle construction.

[0004] Prior art

[0005] Thermosetting, one-component epoxy resin adhesives have already been used for some considerable time as adhesives in body construction.

[0006] One important field of use of one-component thermosetting epoxy resin adhesives, accordingly, is in vehicle construction, where typically there are metal substrates such as steel sheets and aluminium present, particularly in the context of adhesive bonding. Following the application of the epoxy resin composition, the bodywork is heated in the CEC (cathodic electrocoating) oven, by means of which the thermosetting epoxy resin composition as well is cured.

[0007] Such structural adhesives must fulfil a variety of requirements including good adhesion on metal substrates such as steel sheets, especially oiled steel sheets, and aluminium, as well as sufficient T-peel values on said substrates. Many assemblies in vehicle construction need to scope with as several millimeter tolerance field. Not connected panels and parts result in bad mechanical performance of a component. In order to compensate these tolerance fields, structural adhesives can be applied in a bead thickness that is larger than the intended gap including the tolerance field. Said bead will then be squeezed accordingly depending on the tolerance reached. It was however found that increasing the thickness of the applied adhesive beads lead to a significant decrease in the adhesion, the lap shear strength in particular. The prior art includes several documents related to expandable adhesive compositions and bonding methods. EP 4303251 discloses a method for bonding substrates. US 2022 / 0396716 describes expandable structural adhesive films for bonding metal parts with specific elongation and expansion properties. US 2009 / 0104448 teaches preformed adhesive bodies. US 2022 / 0228033 discloses shape memory adhesive materials with expandable layers.

[0008] Summary of the invention

[0009] It is an object of the present invention, therefore, to provide a method for bonding substrates, preferably metallic substrates, with good adhesion, especially good lap shear strength, as well as sufficient T-peel values.

[0010] Surprisingly it has been found that this object can be achieved with the method as defined in Claim 1.

[0011] Further aspects of the invention are subjects of further independent claims. Particularly preferred embodiments of the invention are subjects of the dependent claims.

[0012] Certain embodiments of the invention

[0013] The present invention therefore relates to a method for bonding two substrates comprising the sequential steps of

[0014] i. applying an expandable one-component thermosetting epoxy resin adhesive on a first substrate S1, and

[0015] ii. placing the second substrate S2 so that the distance between the first substrate S1 and the second substrate S2 at the place of application of the expandable one-component thermosetting epoxy resin adhesive is between 2 - 30 mm, and

[0016] iii. heating the first substrate S1 and the second substrate S2 to a temperature above the activation temperature of the blowing agent BA to expand the expandable one-component thermosetting epoxy resin adhesive,

[0017] wherein between step ii) and step iii) the applied expandable one-component thermosetting epoxy resin adhesive is not in contact with the second substrate S2.

[0018] In an embodiment, the expandable one-component thermosetting epoxy resin adhesive comprises:

[0019] a) at least one epoxy resin A having on average more than one epoxide group per molecule, preferably a liquid epoxy resin, the fraction of the epoxy resin A being preferably from 20 - 70 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive; and

[0020] b) at least one blowing agent BA; and

[0021] c) at least one latent hardener B for epoxy resins; and

[0022] d) preferably at least one accelerator C for epoxy resins; and

[0023] e) preferably at least one toughness improver D, the fraction of toughness improver D preferably being from 5 - 40 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive.

[0024] The expandable one-component thermosetting epoxy resin adhesive has a viscosity of 500 to 5000 Pas, preferably 500 to 2000 Pas, preferably 500 to 1500 Pas, preferably 500 to 1300 Pas at 25°C, the viscosity being determined oscillographically by means of a rheometer with heatable plate (MCR 301, AntonPaar) (slot 1000 μm, measuring plate diameter: 25 mm (plate / plate), deformation 0.01 at 5 Hz, temperature: 25°C).

[0025] The expandable one-component thermosetting epoxy resin adhesive is one-component, meaning that the constituents of the epoxy resin adhesive, more particularly the epoxy resin and the hardener, are present in one component, without curing taking place at usual ambient temperature or room temperature. The expandable one-component thermosetting epoxy resin adhesive is therefore storage-stable. It can therefore be handled in this form, whereas with two-component systems the components cannot be mixed until immediately prior to use.

[0026] In an embodiment, the curing of the expandable one-component thermosetting epoxy resin adhesive is accomplished by heating, preferably at a temperature of more than 70°C, more preferably 90 °C to 210 °C.

[0027] The prefix “poly” in expressions such as polyol or polyisocyanate denotes that the compound has two or more of the stated groups. A polyisocyanate, for example, is a compound having two or more isocyanate groups. The expression “independently of one another” as used below means that in the same molecule, two or more identically denoted substituents may have identical or different meanings in accordance with the definition.

[0028] The dashed lines in the formulae in this document represent in each case the bond between the substituent in question and the associated remainder of the molecule.

[0029] Room temperature refers here to a temperature of 23°C, unless otherwise indicated.

[0030] The expandable one-component thermosetting epoxy resin adhesive comprises a) at least one epoxy resin A having on average more than one epoxide group per molecule.

[0031] Preferably, the at least one epoxy resin A having on average more than one epoxide group per molecule is a liquid epoxy resin.

[0032] In contrast to the liquid epoxy resins A, solid epoxy resins having on average more than one epoxide group per molecule are solid at room temperature. The glass transition temperature of solid resins is above room temperature, meaning that they can be comminuted to pourable powders at room temperature.

[0033] Preferred liquid epoxy resins A having on average more than one epoxide group per molecule have the formula (II)

[0034]

[0035] In this formula the substituents R’” and R”” independently of one another are either H or CH3. Moreover, the index r is from 0 to 1. Preferably r is less than 0.2.

[0036] The resins in question are therefore preferably diglycidyl ethers of bisphenol-A (DGEBA), of bisphenol-F, and of bisphenol-A / F. Liquid resins of these kinds are available for example as Araldite® GY 250, Araldite® PY 304, Araldite® GY 282 (Huntsman) or D. E. R.™ 331 or D. E. R.™ 330 (Dow) or Epikote 828 (Hexion).

[0037] Additionally, suitable as liquid epoxy resins A are what are called novolaks. These resins have in particular the following formula:

[0038]

[0039] = H or methyl and z = 0 to 7.

[0040] More particularly these are phenol or cresol novolaks (R2 = CH2).

[0041] Epoxy resins of these kinds are available commercially under the trade name EPN or ECN and also Tactix®556 from Huntsman, or among the D. E. N.™ product series from Dow Chemical.

[0042] Preferably, the fraction of the epoxy resin A is from 20 - 70 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive, preferably from 20 - 60 wt.-%, most preferably 25 - 50 wt.-%.

[0043] The expandable one-component thermosetting epoxy resin adhesive comprises b) at least one blowing agent BA.

[0044] A suitable blowing agent BA may be a chemical or physical blowing agent. Preferably, the at least one blowing agent BA is a physical blowing agent, more preferably heat-expandable microspheres HEM.

[0045] The at least one physical or chemical blowing agent BA preferably has an activation temperature from 90 °C to 210 °C.

[0046] Preferably, the at least one blowing agent BA is present in an amount of 0.25 -15 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive.

[0047] Suitable chemical blowing agents include, but are not limited to, azo compounds, hydrazides, nitroso compounds, carbamates, carbazides, bicarbonates, polycarboxylic acids, and salts of polycarboxylic acids. According to one or more embodiments, the at least one blowing agent BA is selected from the group consisting of azodicarbonamide, azoisobutytronitrile, azocyclohexyl nitrile, dinitrosopentamethylene tetramine, azodiamino benzene, benzene-1,3-sulfonyl hydrazide, calcium azide, 4,4'-diphenyldisulphonyl azide, p-toluenesulphonyl hydrazide, p-toluenesulphonyl semicarbazide, 4,4’-oxybis(benzenesulphonylhydrazide), trihydrazino triazine, and N, N’-dimethyl-N, N’-dinitrosoterephthalamide, and combinations thereof, preferably azodicarbonamide or 4,4’-oxybis(benzenesulphonylhydrazide).

[0048] Suitable physical blowing agents further include heat-expandable microspheres, consisting of a thermoplastic shell filled with thermally expandable fluids or gases. Suitable expandable microspheres are commercially available, for example, under the trademark of Expancel® microspheres (from AkzoNobel).

[0049] Preferably, the at least one blowing agent BA is a physical blowing agent, more preferably a preferably heat-expandable microspheres HEM.

[0050] Preferably, the expandable thermosetting one-component epoxy resin adhesive contains 1.25 - 2.5 wt.-% of heat-expandable microspheres HEM, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive.

[0051] More preferably, the amount of the heat-expandable microspheres HEM is 1.0 - 2.5 wt.-%, most preferably 1.25 - 2.25 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive.

[0052] In an embodiment, when the heat-expandable microspheres having such properties are heated, the contained substance expands in volume and the resin material constituting the outer shell softens, so that capsules (expanded capsules) expanded by internal pressure can be obtained.

[0053] Preferably, the heat-expandable microspheres HEM contain a liquid compound inside an outer shell made of a resin material (resin composition).

[0054] The liquid compound which is the inclusion substance constituting the heatexpandable microsphere preferably has a boiling point (atmospheric pressure condition) equal to or lower than the softening temperature of the resin material constituting the outer shell of the heat-expandable microsphere. Preferably, the liquid compound is selected from the list consisting of n-butane, isopentane, cyclobutane, n-pentane, isopentane, cyclopentane, n-hexane, 2-methylpentane, 2,2-dimethylbutane, cyclohexane, n-heptane, cycloheptane, n-octane, cyclooctane and hydrocarbons, preferably hydrocarbons, more preferably hydrocarbons having 1 to 5 carbon atoms, most preferably hydrocarbons having 4 to 5 carbon atoms.

[0055] The liquid compound may be only one kind or two or more kinds. The liquid compound is preferably a hydrocarbon, and particularly preferably contains a low boiling point hydrocarbon having 4 to 5 carbon atoms.

[0056] The resin material constituting the outer shell preferably has thermoplasticity. Preferably, the resin material contains a structural unit derived from the list consisting of acrylonitrile, methacrylonitrile, acrylic acid alkyl ester, methacrylic acid alkyl ester, vinyl chloride, vinylidene chloride, vinyl acetate and aromatic vinyl compound, preferably acrylonitrile. More preferably, the resin material is a acrylonitrile-based copolymer.

[0057] Preferably, the shape of the heat-expandable microspheres HEM is spherical or elliptical.

[0058] In an embodiment, the heat-expandable microspheres HEM preferably have a mean particle size D (0.5) between 30 - 75 pm, preferably between 30 - 60 μm, most preferably between 35 - 50 μm.

[0059] The term “mean particle size” here preferably relates to the D (0.5) value of the cumulative volume distribution curve, in which 50% by volume of the particles have a particle size that is smaller than the value. The mean particle size or the D (0.5) value is preferably determined by laser diffractometry.

[0060] The activation temperature, in this case called expansion-initiation temperature (Ts), of the heat-expandable microspheres HEM is preferably between 90 °C -115 °C, preferably between 95 °C - 115 °C, most preferably between 100 °C -115 °C.

[0061] Preferably, the maximum expansion temperature (Tmax) of the heat-expandable microspheres HEM is between 130 °C - 170 °C, preferably between 140 °C -165 °C, most preferably between 150 °C - 165 °C.

[0062] Preferably, the expansion-initiation temperature (Ts) and the maximum expansion temperature (Tmax) are determined with a dynamic mechanical analysis (DMA Q800, manufactured by TA Instruments), more preferably in the following way:

[0063] In an aluminum cup 4.8 mm deep and 6.0 mm in diameter (5.65 mm in inside diameter), 0.5 mg of heat-expandable microspheres are placed, and the cup is covered with an aluminum cap 0.1 mm thick and 5.6 mm in diameter to prepare a sample. The sample is subjected to the pressure of 0.01 N with the compression unit of the device, and the height of the sample is measured. The sample is then heated at temperatures elevating at a rate of 10 °C / min in the temperature range from 20 to 300 °C, being subjected to the pressure of 0.01 N with the compression unit, and the vertical change of the position of the compression unit is measured. The temperature at which the compression unit starts to change its position to the positive direction is determined as the expansion-initiation temperature (Ts), and the temperature

[0064] at which the compression unit indicats the greatest change is determined as the maximum expansion temperature (Tmax).

[0065] Preferred thermally expandable microsphere HEM are commercially available as "FN-78D": microspheres containing hydrogen carbide (acrylonitrile-based copolymer shell type), product name: Matsumoto Microsphere FN-78D (manufactured by Matsumoto Yushi-Seivaku Co., Ltd.), average particle diameter: 35 - 50 mm, expansion initiation temperature (Ts): 100 - 115 °C, maximum expansion temperature (Tmax): 150 - 165 °C.

[0066] Preferably, the expandable one-component thermosetting epoxy resin adhesive further comprises at least one epoxy-bearing reactive diluent G. Such reactive diluents are known to those skilled in the art. Preferred examples of epoxy-bearing reactive diluents are:

[0067] - glycidyl ethers of monofunctional saturated or unsaturated, branched or unbranched, cyclic or open-chain C4-C30 alcohols, e.g. butanol glycidyl ether, hexanol glycidyl ether, 2-ethylhexanol glycidyl ether, allyl glycidyl ether, tetrahydrofurfuryl and furfuryl glycidyl ether, trimethoxysilyl glycidyl ether etc.;

[0068] - glycidyl ethers of difunctional saturated or unsaturated, branched or unbranched, cyclic or open-chain C2-C30 alcohols, for example ethylene glycol glycidyl ether, butanediol glycidyl ether, hexanediol glycidyl ether, octanediol glycidyl ether, cyclohexanedimethanol diglycidyl ether, neopentyl glycol diglycidyl ether etc.;

[0069] - glycidyl ethers of tri- or polyfunctional, saturated or unsaturated, branched or unbranched, cyclic or open-chain alcohols, such as epoxidized castor oil, epoxidized trimethylolpropane, epoxidized pentaerythritol or polyglycidyl ethers of aliphatic polyols such as sorbitol, glycerol or trimethylolpropane etc.;

[0070] - glycidyl ethers of phenol compounds and aniline compounds, such as phenyl glycidyl ether, cresyl glycidyl ether, p-tert-butylphenyl glycidyl ether, nonylphenol glycidyl ether, 3-n-pentadecenyl glycidyl ether (from cashewnutshell oil), N, N-diglycidylaniline, etc.;

[0071] - epoxidized amines such as N, N-diglycidylcyclohexylamine etc.;

[0072] - epoxidized mono- or dicarboxylic acids, such as glycidyl neodecanoate, glycidyl methacrylate, glycidyl benzoate, diglycidyl phthalate, tetrahydrophthalate and hexahydrophthalate, diglycidyl esters of dimeric fatty acids etc.;

[0073] - epoxidized di- or trifunctional, low to high molecular weight polyether polyols, such as polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether etc.

[0074] Particular preference is given to hexanediol diglycidyl ether, cresyl glycidyl ether, p-te / t-butylphenyl glycidyl ether, polypropylene glycol diglycidyl ether and polyethylene glycol diglycidyl ether.

[0075] Preferably, the fraction of the epoxy-bearing reactive diluent G is from 0.5 - 10 wt.-%, 0.5 - 7.5 wt.-%, 0.5 - 5 wt.-%, more preferably 0.5 - 3 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive.

[0076] In an embodiment, the expandable one-component thermosetting epoxy resin adhesive further comprises c) at least one latent hardener B for epoxy resins. Latent hardeners are substantially inert at room temperature and are activated by elevated temperature, typically at temperatures of 70°C or more, thereby initiating the curing reaction. The customary latent hardeners for epoxy resins can be used. Preference is given to a latent epoxy resin hardener B containing nitrogen.

[0077] The latent hardener B is preferably selected from the list consisting of dicyandiamide, guanamines, guanidines, aminoguanidines and derivatives thereof, substituted ureas, imidazoles and amine complexes, preferably dicyandiamide.

[0078] The latent hardener B is preferably used in a stoichiometric amount based on the epoxy groups in the composition. The molar ratio of the epoxy groups to the active hydrogen of the latent hardener B is preferably 0.8 to 1.2, in particular 0.9 to 1.1, preferably 0.95 to 1.05.

[0079] The fraction of the latent hardener B is preferably 0.5 to 12 wt%, more preferably 1 to 8 wt%, more particularly 2-6 wt%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive.

[0080] The expandable one-component thermosetting epoxy resin adhesive preferably further comprises d) at least one accelerator C for epoxy resins. Such accelerating curing agents are preferably substituted ureas, for example 3-(3-chloro-4-methylphenyl)-1,1 -dimethylurea (chlortoluron) or phenyldimethylureas, especially p-chlorophenyl-N, N-dimethylurea (monuron), 3-phenyl-1,1-dimethylurea (fenuron) or 3,4-dichlorophenyl-N, N-dimethylurea (diuron). In addition, it is possible to use compounds from the class of the imidazoles, such as 2-isopropylimidazole or 2-hydroxy-N-(2-(2-(2-hydroxyphenyl)-4,5-dihydro-imidazol-1-yl)ethyl)benzamide, imidazolines, trihalide complexes, preferably BF3complexes, blocked amines and encapsulated amines.

[0081] Preferably, the accelerator C for epoxy resins is selected from the list consisting of substituted ureas, imidazoles, imidazolines and blocked amines, preferably substituted ureas.

[0082] More preferably, the accelerator C for epoxy resins is selected from the list consisting of substituted ureas and blocked amines, especially when the latent hardener B is a guanidine, especially dicyandiamide.

[0083] Most preferably, the latent hardener B is a guanidine, especially dicyandiamide, and the expandable one-component thermosetting epoxy resin adhesive additionally includes an accelerator C for epoxy resins, selected from the list consisting of substituted ureas and blocked amines, especially substituted ureas.

[0084] Preferably, the fraction of the accelerator C for epoxy resins is from 0.05 - 2 wt.-%, 0.1 - 1 wt.-%, 0.15 - 0.5 wt.-%, more preferably 0.2 - 0.3 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive.

[0085] The expandable one-component thermosetting epoxy resin adhesive preferably comprises e) at least one toughness improver D. The toughness improvers D may be solid or liquid.

[0086] The fraction of toughness improver D is preferably from 5 -40 wt.-%, preferably from 7.5 - 35 wt.-%, 10 - 30 wt.-%, more preferably 15 - 25 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive.

[0087] More particularly the toughness improver D is selected from the group consisting of terminally blocked polyurethane polymers D1, liquid rubbers D2 and core-shell polymers D3. With preference the toughness improver D is selected from the group consisting of terminally blocked polyurethane polymers D1 and liquid rubbers D2, most preferably a terminally blocked polyurethane polymer D1.

[0088] Where the toughness improver D is a terminally blocked polyurethane polymer D1, it is preferably a terminally blocked polyurethane prepolymer of the formula (I).

[0089]

[0090] In this formula, R1is a p-valent radical of a linear or branched polyurethane prepolymer terminated by isocyanate groups, following the removal of the terminal isocyanate groups, and p has a value of 2 to 8.

[0091] Moreover, R2independently at each occurrence is a substituent which is selected from the group consisting of

[0092]

[0093] In these formulae, R5, R6, R7and R8each independently of one another are an alkyl or cycloalkyl or aralkyl or arylalkyl group, or R5together with R6, or R7together with R8, forms part of a 4- to 7-membered ring which is optionally substituted.

[0094] Moreover, R9’ and R10each independently of one another are an alkyl or aralkyl or arylalkyl group or are an alkyloxy or aryloxy or aralkyloxy group, and R11is an alkyl group.

[0095] R12, R13and R14each independently of one another are an alkylene group having 2 to 5 carbon atoms, which optionally has double bonds or is substituted, or are a phenylene group or are a hydrogenated phenylene group. R15, R16and R17each independently of one another are H or are an alkyl group or are an aryl group or an aralkyl group, and R18is an aralkyl group or is a mono- or polycyclic, substituted or unsubstituted aromatic group which optionally has aromatic hydroxyl groups. Lastly, R4is a radical of an aliphatic, cycloaliphatic, aromatic or araliphatic epoxide containing a primary or secondary hydroxyl group, after the removal of the hydroxyl and epoxide groups, and m has a value of 1, 2 or 3.

[0096] R18should be considered to comprise, in particular, on the one hand, phenols or polyphenols, more particularly bisphenols, after removal of a hydroxyl group. Preferred examples of such phenols and bisphenols are, in particular, phenol, cresol, resorcinol, pyrocatechol, cardanol (3-pentadecenylphenol (from cashew nut shell oil)), nonylphenol, phenols reacted with styrene or with dicyclopentadiene, bisphenol A, bisphenol F and 2,2’-diallylbisphenol A. R18should be considered on the other hand to comprise, in particular, hydroxybenzyl alcohol and benzyl alcohol after removal of a hydroxyl group. If R5, R6, R7, R8, R9, R9’, R10, R11, R15, R16or R17is an alkyl group, this group more particularly is a linear or branched C1-C20 alkyl group.

[0097] If R5, R6, R7, R8, R9, R9, R10, R15, R16, R17or R18is an aralkyl group, this moiety is more particularly an aromatic group bonded via methylene, more particularly a benzyl group.

[0098] If R5, R6, R7, R8, R9, R9’ or R10is an alkylaryl group, this group is more particularly Ci to C20 alkyl group bonded via phenylene, such as tolyl or xylyl, for example.

[0099] The radicals R2are preferably the substituents of the formulae

[0100]

[0101]

[0102] A preferred substituent of the formula is s-caprolactam after removal of the NH proton.

[0103] _ r\ _lo

[0104] Preferred substituents of the formula are monophenols or polyphenols, more particularly bisphenols, after removal of a phenolic hydrogen atom. Particularly preferred examples of such radicals R2are radicals which are selected from the group consisting of

[0105]

[0106] The radical Y in these formulae is a saturated, aromatic or olefinically unsaturated hydrocarbyl radical having 1 to 20 carbon atoms, more particularly having 1 to 15 carbon atoms. Preferred as Y are, in particular, allyl, methyl, nonyl, dodecyl, phenyl, alkyl ether, carboxylic ester or an unsaturated C15 alkyl radical having 1 to 3 double bonds.

[0107] Most preferably

[0108]

[0109] The terminally blocked polyurethane prepolymer of the formula (I) is prepared from the linear or branched polyurethane prepolymer, terminated by isocyanate groups, with one or more isocyanate-reactive compounds R2H. If two or more such isocyanate-reactive compounds are used, the reaction may take place sequentially or with a mixture of these compounds.

[0110] The reaction preferably takes place such that the one or more isocyanatereactive compounds R2H are used stoichiometrically or in a stoichiometric excess, in order to ensure that all of the NCO groups have undergone reaction. The polyurethane prepolymer having isocyanate end groups on which R1is based may be prepared from at least one diisocyanate or triisocyanate and also from a polymer QPM having terminal amino, thiol or hydroxyl groups and / or from an optionally substituted polyphenol Qpp.

[0111] Suitable diisocyanates are aliphatic, cycloaliphatic, aromatic or araliphatic diisocyanates, especially commercial products such as methylenediphenyl diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), tolidine diisocyanate (TODI), isophorone diisocyanate (IPDI), trimethylhexamethylene diisocyanate (TMDI), 2,5- or 2,6-bis(isocyanatomethyl)bicyclo[2.2.1]heptane, naphthalene 1,5-di isocyanate (NDI), dicyclohexylmethyl diisocyanate (H12MDI), p-phenylene diisocyanate (PPDI), m-tetramethylxylylene diisocyanate (TMXDI), etc., and also their dimers. Preferred are HDI, IPDI, MDI or TDI.

[0112] Suitable triisocyanates are trimers or biurets of aliphatic, cycloaliphatic, aromatic or araliphatic diisocyanates, more particularly the isocyanurates and biurets of the diisocyanates described in the preceding paragraph. It is of course also possible to use suitable mixtures of di- or triisocyanates.

[0113] Particularly suitable as polymers QPM having terminal amino, thiol or hydroxyl groups are polymers QPM having two or three terminal amino, thiol or hydroxyl groups.

[0114] The polymers QPM advantageously have an equivalent weight of 300-6000, more particularly of 600-4000, preferably of 700-2200 g / equivalent of NCO-reactive groups.

[0115] Preferred polymers QPM are polyols having average molecular weights of between 600 and 6000 daltons, selected from the group consisting of polyethylene glycols, polypropylene glycols, polyethylene glycol-polypropylene glycol block polymers, polybutylene glycols, hydroxyl-term inated polybutadienes, hydroxyl-term inated butadiene-acrylonitrile copolymers, and mixtures thereof.

[0116] Especially preferred as polymers QPM are dihydroxypolyalkylene glycols having C2-C6 alkylene groups or having mixed C2-C6 alkylene groups, which are terminated with amino, thiol or, preferably, hydroxyl groups. Particularly preferred are polypropylene glycols or polybutylene glycols. Further particularly preferred are hydroxyl-group-term inated polyoxybutylenes.

[0117] Especially suitable as polyphenol Qpp are bis-, tris- and tetraphenols. The term refers not only to pure phenols, but instead also, where appropriate, to substituted phenols. The nature of the substitution can be very diverse. Understood more particularly by this is substitution directly on the aromatic ring system to which the phenolic OH group is bonded. Phenols, moreover, are not only monocyclic aromatics, but also polycyclic or fused aromatic or heteroaromatics, having the phenolic OH group directly on the aromatic or heteroaromatic moiety.

[0118] In one preferred embodiment, the polyurethane prepolymer is prepared from at least one diisocyanate or triisocyanate and also from one polymer QPM having terminal amino, thiol or hydroxyl groups. The polyurethane prepolymer is prepared in a manner known to the person skilled in the polyurethane art, more particularly by using the diisocyanate or triisocyanate in a stoichiometric excess in relation to the amino, thiol or hydroxyl groups of the polymer QPM.

[0119] The polyurethane prepolymer having isocyanate end groups is preferably elastic in nature. It preferably exhibits a glass transition temperature Tg of less than 0°C.

[0120] The toughness improver D may be a liquid rubber D2. This may be, for example, a carboxyl-term inated or epoxide-term inated polymer.

[0121] In a first embodiment, this liquid rubber may be a carboxyl- or epoxide-term inated acrylonitrile / butadiene copolymer or a derivative thereof. Liquid rubbers of this kind are available commercially, for example, under the name Hypro / Hypox® CTBN and CTBNX and ETBN from Emerald Performance Materials. Suitable derivatives are, in particular, elastomer-modified prepolymers containing epoxide groups, of the kind marketed commercially under the product line Polydis®, especially from the product line Polydis® 36.., by the company Struktol® (Schill+Seilacher Group, Germany), or under the product line Albipox (Evonik, Germany).

[0122] In a second embodiment, this liquid rubber may be a polyacrylate liquid rubber, which is fully miscible with liquid epoxy resins and which separates only when the epoxy resin matrix is cured, to form microdroplets. Liquid polyacrylate rubbers of this kind are available, for example, under the designation 20208-XPA from Dow.

[0123] It is of course also possible to use mixtures of liquid rubbers, more particularly mixtures of carboxyl- or epoxide-term inated acrylonitrile / butadiene copolymers or of derivatives thereof.

[0124] The toughness improver D in a third embodiment may be a core-shell polymer D3. Core-shell polymers consist of an elastic core polymer and a rigid shell polymer. Particularly suitable core-shell polymers are composed of a core of elastic acrylate polymer or butadiene polymer, surrounded by a rigid shell of a rigid thermoplastic polymer. This core-shell structure either forms spontaneously by separation of a block copolymer, or is dictated by the polymerization regime as latex or suspension polymerization with subsequent grafting. Preferred core-shell polymers are those known as MBS polymers, which are available commercially under the trade name Clearstrength™ from Arkema, Paraloid™ from Dow or F-351 ™ from Zeon.

[0125] With particular preference the expandable one-component thermosetting epoxy resin adhesive comprises terminally blocked polyurethane polymers D1, most preferably only terminally blocked polyurethane polymers D1.

[0126] In one preferred embodiment, the expandable one-component thermosetting epoxy resin adhesive further comprises at least one filler F. Preference here is given to mica, talc, kaolin, wollastonite, feldspar, syenite, chlorite, bentonite, montmorillonite, calcium carbonate (precipitated or ground), dolomite, quartz, silicas (fumed or precipitated), cristobalite, calcium oxide, aluminium hydroxide, magnesium oxide, hollow ceramic beads, hollow glass beads, hollow organic beads, glass beads, colour pigments. Particularly preferred are fillers selected from the group consisting of calcium carbonate, calcium oxide and fumed silicas.

[0127] The total fraction of the overall filler F is advantageously 10 - 70 weight-%, preferably 15 - 65 weight-%, 30 - 60 weight-%, most preferably 40 - 50 weight-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive.

[0128] The expandable one-component thermosetting epoxy resin adhesive may comprise further constituents, especially stabilizers, particularly heat and / or light stabilizers, plasticizers, solvents, dyes and pigments, corrosion inhibitors, surfactants, defoamers and adhesion promoters.

[0129] The expandable one-component thermosetting epoxy resin adhesive has a viscosity of 500 to 5000 Pas at 25°C, preferably the viscosity is from 500 to 2000 Pas, more preferably 500 to 1500 Pas at 25°C. Most preferably, the viscosity is from 500 to 950 Pas at 25°C or from 1000 to 1500 Pas at 25°C. The viscosity is determined oscillographically by means of a rheometer with heatable plate (MCR 301, AntonPaar) (slot 1000 μm, measuring plate diameter: 25 mm (plate / plate), deformation 0.01 at 5 Hz, temperature: 25°C). This is advantageous with respect to higher values in lap shear strength and T-peel values. A range of 500 to 950 Pas at 25°C is especially advantageous with respect to high values for lap shear strength, whereas a range of 1000 to 1500 Pas at 25°C is especially advantageous with respect to T-peel strength. This can be seen, for example in the comparison of E1 with E2 in table 3.

[0130] The expandable one-component thermosetting epoxy resin adhesive is preferably tacky at 23 ° C. The term "tacky" is understood here to mean immediate tack or "tack", which is sufficiently high at 23 ° C that when pressed with a thumb with a pressure of about 5 kg for 1 second on the surface of the expandable one-component thermosetting epoxy resin adhesive, the thumb does stick to the surface of the expandable one-component thermosetting epoxy resin adhesive and the adhesive can be raised.

[0131] A particularly preferred expandable one-component thermosetting epoxy resin adhesive comprises:

[0132] -20 - 70 wt.-%, preferably 25 - 50 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive, of at least one epoxy resin A having on average more than one epoxide group per molecule, preferably a liquid epoxy resin;

[0133] -1.0 - 2.5 wt.-%, preferably 1.25 - 2.25 wt.-%, of heat-expandable microspheres HEM, based on the total weight of the expandable one- component thermosetting epoxy resin adhesive, with an expansioninitiation temperature (Ts) between 95 °C - 115 °C, preferably between 100 °C - 115 °C, and a mean particle size D (0.5) between 30 - 60 μm, preferably between 35 - 50 μm;

[0134] - preferably 0.5 - 7.5 wt.-%, more preferably 0.5 - 3 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive, of at least one epoxy-bearing reactive diluent G;

[0135] -1-8 wt%, preferably 2-6 wt%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive, of at least one latent hardener B for epoxy resins, more particularly dicyandiamide;

[0136] - preferably 0.05 - 2 wt.-%, more preferably 0.2 - 0.3 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive, of at least one accelerator C for epoxy resins;

[0137] -7.5 - 35 wt.-%, preferably 15 - 25 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive, of at least one toughness improver D, preferably a terminally blocked polyurethane polymers D1;

[0138] -preferably 15 -65 weight-%, more preferably 40 - 50 weight-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive, of a filler F selected from the group consisting of calcium carbonate, calcium oxide and fumed silicas.

[0139] Preferably, the expandable one-component thermosetting epoxy resin adhesive has a viscosity of 500 to 2000 Pas, 500 to 1500 Pas, preferably 500 to 1300 Pas at 25°C.

[0140] It may further be advantageous if the preferred expandable one-component thermosetting epoxy resin adhesive consists to an extent of more than 80 weight%, preferably more than 90 weight%, more particularly more than 95 weight%, especially preferably more than 98 weight%, most preferably more than 99 weight%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive, of the aforementioned constituents.

[0141] It is advantageous when, after step iii), the cured and expanded expandable one-component thermosetting epoxy resin adhesive bonding the two substrates has the following properties:

[0142] -Lap shear strength determined according to SAE J1523, preferably as described in the experimental section: > 1.50 MPa, preferably > 2.50 MPa, most preferably > 2.75 MPa, after curing for 20 min at 180 °C; and / or

[0143] -T-peel strength determined according to ASTM D1876, preferably as described in the experimental section: > 1.2 N / mm, preferably > 1.5 N / mm, most preferably > 2.0 N / mm, after curing for 20 min at 180 °C; and / or -Flexural strength determined according to ASTM D 7264 M, Procedure B, preferably as described in the experimental section: 2 mm deflection: > 500 N, preferably > 750 N, most preferably > 1000 N, after curing for 10 min at 160 °C; and / or

[0144] Peak load: > 800 N, preferably > 1000 N, most preferably > 1250 N, after curing for 10 min at 160 °C; and / or

[0145] -expansion, determined according to DIN EN ISO 1183 using the water immersion method (Archimedes principle), preferably determined as described in the experimental section: 100 - 250 %, preferably 110 - 225 %, more preferably 125 - 175 %.

[0146] The method for bonding two substrates comprises the sequential steps of

[0147] - i) applying an expandable one-component thermosetting epoxy resin adhesive on a first substrate S1, and

[0148] - ii) placing the second substrate S2 so that the distance between the first substrate S1 and the second substrate S2 at the place of application of the expandable one-component thermosetting epoxy resin adhesive is between 2 - 30 mm, and

[0149] iii) heating the first substrate S1 and the second substrate S2 to a temperature above the activation temperature of the blowing agent BA to expand the expandable one-component thermosetting epoxy resin adhesive,

[0150] wherein between step ii) and step iii) the applied expandable one-component thermosetting epoxy resin adhesive is not in contact with the second substrate S2.

[0151] As shown in the comparison of E1 with Ref1-Ref5 in table 3, the applied expandable one-component thermosetting epoxy resin adhesive in E1 that is not in contact with the second substrate S2 after the positioning of S2 leads to improved values in lap shear strength and T-peel strength. Surprisingly is was found that, as shown in the comparison of the examples Ref1-Ref5 in table 3, that increasing bead thickness lead to a reduction in lap shear strength in case the beads were in contact with both substrates before the curing / expansion step.

[0152] Many assemblies in vehicle construction need to cope with tolerance fields of greater than + / -2 mm. Normal adhesives cannot compensate for such larger tolerance fields, and not connected panels result in bad mechanical performance of a component. In contrast to conventional approaches, the inventive method is able to achieve good mechanical performance even with these challenging tolerance requirements.

[0153] The technical effect of the present invention stems from the unique combination of the expandable adhesive composition and the specific application method. The expandable one-component thermosetting epoxy resin adhesive may be applied in a bead thickness that is smaller than the intended gap, including the tolerance field. The bead may then be positioned with the second substrate at a controlled distance, for example with a distance of 2-15 mm, ensuring that the adhesive is not in contact with the second substrate prior to the heating step.

[0154] During the heating process, the blowing agent activates and causes controlled expansion of the adhesive composition. This expansion allows the adhesive to bridge the gap between the substrates and establish contact with both surfaces, effectively compensating for dimensional variations and tolerance fields that would otherwise result in poor or no bonding with conventional adhesives.

[0155] The critical technical advantage is that this expansion occurs under controlled conditions where the adhesive can flow and conform to both substrate surfaces optimally, rather than being constrained by premature contact. This results in superior lap shear strength and T-peel values compared to conventional thick adhesive applications, where increased bead thickness typically leads to decreased adhesion performance. The inventive method thereby enables reliable bonding of components with significant dimensional variations while maintaining consistent mechanical performance across the assembly.

[0156] The synergistic effect of the controlled distance, non-contact positioning, and specific viscosity requirements provides unexpected advantages in bonding performance. The viscosity range of 500 to 5000 Pas at 25°C ensures that the adhesive maintains sufficient flow properties to enable controlled expansion while preventing premature spreading or sagging that could lead to unintended contact with the second substrate. When combined with the non-contact positioning where the adhesive, in particular the adhesive bead, is not touching the second substrate S2 prior to heating, this viscosity range allows the adhesive to maintain its applied geometry and position during the placement step. The controlled distance of e.g. 2-30 mm between substrates at the position of the applied adhesive works in conjunction with these viscosity properties to create an optimal expansion environment where the adhesive can undergo uniform volumetric expansion without being constrained by surface contact forces. This three-way interaction between non-contact positioning, and viscosity optimization results in enhanced mechanical properties that exceed what would be achieved by any single parameter alone, as the adhesive can expand freely in all directions before making contact with both substrates, leading to improved wetting and adhesion characteristics upon curing.

[0157] In an embodiment, the expandable one-component thermosetting epoxy resin adhesive is a structural adhesive.

[0158] As used herein, the term "structural adhesive" may refer to an adhesive composition that is capable of transferring loads between adherends and maintaining structural integrity under various stress conditions. A structural adhesive may exhibit mechanical properties that enable it to bear significant structural loads in bonded assemblies. In some aspects, a structural adhesive may demonstrate lap shear strength values of at least 1.0 MPa, in some cases at least 2.0 MPa, and in certain embodiments at least 2.5 MPa when tested according to standard test methods such as SAE J1523. The structural adhesive may also exhibit T-peel strength values of at least 0.8 N / mm, in some cases least 1.0 N / mm, in some cases at least 1.5 N / mm, and in certain embodiments at least 2.0 N / mm when tested according to standard methods such as ASTM D1876. Additionally, a structural adhesive may demonstrate flexural strength properties, for example showing load values of at least 400 N, in particular at least 450 N, in particular at least 500 N, at 2 mm deflection and peak loads of at least 700 N, in particular at least 750 N, in particular at least 800 N when tested according to methods such as ASTM D 7264 M, Procedure B. In some aspects, structural adhesives may be characterized by their elongation at break properties when tested according to methods such as ASTM D638, typically exhibiting less than 5% elongation, which distinguishes them from flexible sealants that may show greater than 20% elongation, thereby affecting their suitability for different automotive bonding applications. In some aspects, structural adhesives may be distinguished from non-structural adhesives by their ability to maintain bond integrity under dynamic loading conditions and environmental stresses encountered in applications such as automotive, aerospace, and construction industries.

[0159] In the context of the present invention, the term "not in contact" means that the applied expandable one-component thermosetting epoxy resin adhesive is free of contact with the second substrate S2, or alternatively has no substantial contact with the second substrate S2. In some aspects, "not in contact" may refer to a gap or air space being maintained between the applied adhesive and the second substrate, allowing for unimpeded expansion of the adhesive during the heating step. The absence of substantial contact ensures that the adhesive can expand freely without being constrained by premature adherence to both substrates, which may otherwise limit the expansion process and reduce the mechanical properties of the final bond.

[0160] In an embodiment, it is advantageous that in step i), the expandable one-component thermosetting epoxy resin adhesive is applied in the form of a bead with a width of 1.5 - 25.0 mm, preferably 2.5 - 15.0 mm, more preferably 3.0 -8.0 mm, most preferably 4.0 - 7.0 mm.

[0161] In an embodiment, it is further advantageous that in that in step i), the expandable one-component thermosetting epoxy resin adhesive is applied in the form of a bead with a height of 1.5 - 12.0 mm, preferably 2.5 - 10.0 mm, more preferably 3.0 - 8.0 mm, most preferably 4.0 - 7.0 mm. This is advantageous with respect to higher values in flexural strength. This can be seen, for example in the comparison of E7 - E10 in table 4.

[0162] In an embodiment, it is also advantageous if in step i) the expandable one-component thermosetting epoxy resin adhesive is applied in the form of a bead with a length of 10 mm to 2 m, preferably with a length of 2 cm to 10 cm or with a length of 10 cm to 1 m. The first is advantageous if the composition is applied as spotted bead, for example to bond a vehicle bonnet. The later is beneficial if the composition is continuously applied over a long distance, for example to bond a vehicle roof or a vehicle pillar.

[0163] In an embodiment, preferably, in step i), the expandable one-component thermosetting epoxy resin adhesive is applied in the form of a bead. Preferably the ratio of the width to the height of the bead is 1 - 5, preferably 1 - 3, more preferably 1 - 2.

[0164] In an embodiment, it was also found to be advantageous if in step i), the expandable one-component thermosetting epoxy resin adhesive is applied in the form of a bead and in step ii)

[0165] the ratio between the distance DI between the first substrate S1 and the second substrate S2 and the height HT of the bead at the place of application of the expandable one-component thermosetting epoxy resin adhesive (DI / HT) is from 1.10 - 3.00, preferably 1.20 - 2.00, most preferably 1.25 - 1.75. This is advantageous with respect to higher values in lap shear strength and T-peel values. This can be seen, for example in the comparison of E1 with Ref5 in table 3.

[0166] The height HT of the bead is measured before expansion of the expandable one-component thermosetting epoxy resin adhesive.

[0167] In an embodiment, in a preferred method, in step i), the expandable one-component thermosetting epoxy resin adhesive is applied in the form of a bead, the height HT of the bead is from 1.5 - 12.0 mm, preferably 2.5 - 10.0 mm, more preferably 3.0 - 8.0 mm, most preferably 4.0 - 7.0 mm, and the distance DI between the first substrate S1 and the second substrate S2 at the place of application of the expandable one-component thermosetting epoxy resin adhesive is from 2.0 - 15 mm, preferably 3.0 - 12.5 mm, more preferably 4.0 - 10.0 mm, most preferably 5.0 - 9.0 mm. This is advantageous with respect to higher values in flexural strength. This can be seen, for example in the comparison of E7 - E10.

[0168] In an embodiment, it is further advantageous, if after step iii) the expanded one-component thermosetting epoxy resin adhesive has an expansion of 100 - 250 %, preferably 110 - 225 %, more preferably 125 - 175 %, determined according to DIN EN ISO 1183 using the water immersion method (Archimedes principle). This is advantageous with respect to higher values in lap shear strength and T-peel values. This can be seen, for example in the comparison of E1 with E3-E6 in table 3. In an embodiment, it can be further advantageous if in step i) the expandable one-component thermosetting epoxy resin adhesive is applied in the form of several beads, preferably 2 - 4 beads, that are aligned parallel to each other, preferably the distance between the neighboring beads is 0 - 30 mm, preferably 0 -20 mm, more preferably 5 - 15 mm.

[0169] In an embodiment, the first substrate S1 and / or second substrate S2 preferably are metal substrates, more preferably oiled metal substrates. If appropriate, however, heat-resistant plastics, are also conceivable. The first substrate S1 and second substrate S2 may be made from the same or different materials. In an embodiment, preferred metal substrates are as utilized, for example, in the construction of modes of transport, for example in the automobile industry, or in the production of white goods. The method of the invention is therefore especially suitable for bonding of metal substrates, for the manufacture of modes of transport, especially automobiles, buses, trucks, rail vehicles, ships or aircraft, or white goods, especially washing machines, tumble dryers or dishwashers, or parts thereof, preferably motor vehicles or installable components thereof.

[0170] In an embodiment, most preferably, the first substrate S1 is a supporting frame of a vehicle engine hood, trunk or door or a crossbeam of a vehicle roof and the second substrate S2 is a vehicle engine hood, trunk or door panel or a vehicle roof panel or the first substrate S1 and the second substrate S2 are parts of a vehicle pillar, preferably an A-pillar, a B-pillar or a C-pillar.

[0171] In an embodiment, preferably said metal substrates are oiled substrates meaning they are covered with corrosion protection oils known to the person skilled in the art. An example of such a corrosion protection oil is Anticorit PL 3802-39S.

[0172] Preferred examples of said metal substrates are metal substrates selected from the list consisting of steel, aluminium and magnesium, especially selected from the list consisting of electrolytically galvanized steel, hot-dip galvanized steel, bonazinc-coated steel, subsequently phosphated steel, aluminium, magnesium and magnesium alloys. Preferably the substrates are oiled substrates.

[0173] In an embodiment, the expandable one-component thermosetting epoxy resin adhesive is applied to the first substrate S1 in step (a) of the method of the invention. This is effected, for example, at an application temperature of the expandable one-component thermosetting epoxy resin adhesive of 10°C to 80°C, preferably of 25°C to 50°C, more preferably of 30 to 40°C. The application is preferably effected in the form of a bead. Automatic application is preferred.

[0174] In an embodiment, the expandable one-component thermosetting epoxy resin adhesive can be applied over the entire surface or over part of the surface of the first substrate S1 facing the second substrate S2. In a preferred application, the expandable one-component thermosetting epoxy resin adhesive is applied only on a part, preferably less than 20 %, less than 10 %, less than 5 %, preferably less than 2 %, of the surface of the substrate of the first substrate S1 facing the second substrate S2, especially if the first substrate S1 is a supporting frame of a vehicle engine hood, trunk or door or parts of a vehicle pillar.

[0175] In an embodiment, in a later step ii), the second substrate S2 is placed so that the distance between the first substrate S1 and the second substrate S2 at the place of application of the expandable one-component thermosetting epoxy resin adhesive is between 2 - 30 mm. This refers to the gap at the adhesive location. Preferably, the first substrate S1 and the second substrate S2 are fixed in position after the placement, preferably with mechanical fixation means, preferably mechanical clamping, more preferably mechanical clamping followed by spot welding or laser welding, so that the distance between the first substrate S1 and the second substrate S2, preferably at the location where the expandable one-component thermosetting epoxy resin adhesive is applied on a first substrate S1, does not change until step iii) is completed.

[0176] In an embodiment, after step ii), the first substrate S1 and the second substrate S2 are heated to a temperature above the activation temperature of the blowing agent BA to expand the expandable one-component thermosetting epoxy resin adhesive, wherein between step ii and step iii) the applied expandable one-component thermosetting epoxy resin adhesive is not in contact with the second substrate S2.

[0177] In an embodiment, to heat and preferably cure the expandable one-component thermosetting epoxy resin adhesive in step iii), the expandable one-component thermosetting epoxy resin adhesive is preferably heated to a temperature in the range from 150 to 220°C, preferably 160 to 200°C, preferably 170 to 190°C, most preferably 180°C. The heating can be effected, for example, by means of infrared radiation or induction heating or in an oven, for example a cathodic electrocoating oven. In this way, the adhesive bond with the pumpable thermally foaming filler composition is obtained.

[0178] In an embodiment, preferably the duration of said heating step is from 10 - 60 min, preferably 10 -40 min, 10 -30 min, most preferably 10 -20 min.

[0179] In an embodiment, preferably, after step iii), the expandable one-component thermosetting epoxy resin adhesive is expanded and cured and forms an adhesive bond between the first substrate S1 and the second substrate S2 by being in direct contact with the two substrates.

[0180] In an embodiment, the expandable one-component thermosetting epoxy resin adhesive in the adhesive bond can be cured in one step, but curing in two or more steps is also possible, in which case intermediate operating steps between or during the curing steps are possible, for example a wash and / or a dip-coating operation, for example a cathodic electrocoating operation, of the substrates with a subsequent wash.

[0181] Another aspect of the present invention is an article obtained from said method, especially a construction of modes of transport, especially in the automobile industry, or an article of white goods, most preferably selected from the list consisting of vehicle roof, engine hood, trunk hood, vehicle door and vehicle pillar, preferably an A-pillar, a B-pillar or a C-pillar.

[0182] The article may be characterized not only by its structural components but also by the unique method of formation and resulting properties that cannot be adequately defined by structure alone. The article exhibits properties that arise from the specific non-contact positioning method during formation, where the expandable one-component thermosetting epoxy resin adhesive undergoes controlled expansion without being constrained by premature substrate contact. This process-dependent characteristic results in a bonded assembly having a distinctive microstructure and performance profile that may differ significantly from articles formed by conventional adhesive application methods where the adhesive contacts both substrates prior to curing. The resulting article may demonstrate superior mechanical properties, including enhanced lap shear strength and T-peel values, that are directly attributable to the method of formation rather than merely the chemical composition or physical arrangement of the components.

[0183] In an embodiment, the article exhibits at least one of the following properties: -lap shear strength of at least 1 MPa and

[0184] - T-peel strength of at least 0.8 N / mm; and

[0185] - flexural strength at 2 mm deflection of at least 450; and

[0186] - expansion of 100-250%. In an embodiment, the article exhibits the following properties:

[0187] - lap shear strength at least 1 MPa and

[0188] - T-peel strength of at least 0.8 N / mm; and

[0189] - flexural strength at 2 mm deflection at least 450 N; and

[0190] - expansion of 100-250%

[0191] The combination of properties exhibited by the article demonstrates a unique technical effect resulting from the inventive non-contact expansion process. The lap shear strength of > 1 MPa indicates strong adhesive bonding capability suitable for structural applications, while the T-peel strength of at least 0.8 N / mm demonstrates resistance to peeling forces that commonly occur in automotive assemblies under dynamic loading conditions. The flexural strength at 2 mm deflection of > 750 N reveals the article's ability to withstand bending forces without failure, which is critical for maintaining structural integrity in vehicle body applications where panels may experience flexural stresses during operation or impact events. This high flexural strength is particularly significant given the expanded nature of the adhesive, as conventional expanded adhesives typically exhibit reduced mechanical properties due to increased porosity. The expansion capability of 100-250% enables the adhesive to effectively bridge gaps of 2-30 mm between substrates while maintaining its structural properties. This controlled expansion, achieved through the noncontact positioning method, allows the adhesive to fill tolerance variations without being constrained by premature substrate contact, resulting in optimal density distribution and mechanical performance. The synergistic combination of these properties - high adhesive strength, peel resistance, flexural strength, and controlled expansion - creates a technical effect that addresses the fundamental challenge in automotive assembly: achieving reliable structural bonding across variable gap dimensions while maintaining consistent mechanical performance. This combination is not achievable with conventional adhesive application methods where premature contact limits expansion and compromises final properties.

[0192] In an embodiment, the article provides adhesive bonding between metal substrates separated by 2-30 mm with cohesive failure mode at lap shear strengths of at least 0.8 MPa when tested according to SAE J1523.

[0193] The article's ability to provide adhesive bonding between metal substrates separated by 2-30 mm with cohesive failure mode at lap shear strengths above 0.8 MPa demonstrates a critical technical advancement over conventional adhesive systems. The cohesive failure mode indicates that the bond strength exceeds the internal strength of the adhesive material itself, representing optimal adhesive performance where failure occurs within the adhesive rather than at the substrate interface. This technical effect is particularly significant because conventional adhesives typically exhibit adhesive failure (interfacial debonding) when applied across large gaps of 2-30 mm, especially when substrate separation varies due to manufacturing tolerances. The inventive non-contact positioning method enables the expandable adhesive to achieve uniform contact with both substrate surfaces during controlled expansion, eliminating air entrapment and ensuring complete wetting that would otherwise be compromised by premature substrate contact. The maintenance of cohesive failure mode across the entire 2-30 mm gap range, combined with lap shear strengths above 0.8 MPa, indicates that the adhesive bond strength remains consistently high regardless of the actual gap dimension within the specified range. This consistency is achieved because the non-contact expansion process allows the adhesive to adapt its expansion ratio to completely fill the available space while maintaining optimal density and mechanical properties. This technical effect enables reliable structural bonding in automotive applications where panel-to-panel gaps vary significantly due to manufacturing tolerances, providing consistent mechanical performance that would be unattainable with conventional adhesive application methods that suffer from incomplete gap filling or compromised bond strength due to constrained expansion.

[0194] In an embodiment, the article exhibits lap shear strength values that are at least 15% higher than comparable articles produced by methods where the adhesive contacts both substrates prior to heating. This is for example demonstrated in Examples E1 vs. Ref1-Ref5.

[0195] In an embodiment, the features and properties described for the article obtained from the method may be equally applicable to the method itself, and vice versa. The disclosure encompasses all possible combinations of the features described for both the method and the resulting article. In some aspects, any feature described in relation to the method may be combined with any feature described in relation to the article, and any technical effect or advantage described for one aspect may be realized in the other aspect. The synergistic effects achieved through the non-contact positioning method may be reflected in the properties of the resulting article, while the superior performance characteristics of the article may validate the effectiveness of the method parameters and process conditions.

[0196] The invention is further explained in the following experimental part which, however, shall not be construed as limiting the scope of the invention.

[0197] Examples

[0198] Raw materials used for preparing the impact modifier D1 and the compositions C1 -C5 were as follows:

[0199] Table 1

[0200]

[0201]

[0202] Preparation of the toughness improver (D1)

[0203] 300.0 g of PolyTHF®2000 (BASF) and 56.7 g of hexamethylene diisocyanate (HDI) were mixed together with 0.04 g of dibutyltin dilaurate (DBTL) in a vessel. After mixing had taken place under reduced pressure at 60°C for 2 hours, an NCO content of 4.1% was measured. The NCO-terminated polyurethane polymer thus formed was admixed with 39.3 g of phenol (Aldrich), stirred further under reduced pressure at 100°C for 3 hours and stored at 70°C for 12 hours. Thereafter the NCO content measured was 0%. Preparation of the compositions

[0204] Table 2

[0205]

[0206] The composition C1 - C5 as indicated in Table 2 was used in the experiments as shown in table 3 and table 4.

[0207] Viscosity

[0208] The viscosity of the compositions C1 - C5 were measured and found to be between 850 to 900 Pas at 25°C, the viscosity being determined oscillographically by means of a rheometer with heatable plate (MCR 301, AntonPaar) (slot 1000 μm, measuring plate diameter: 25 mm (plate / plate), deformation 0.01 at 5 Hz, temperature: 25°C). For example, the viscosity of the composition C1 was determined to be 850 Pas at 25°C. A sample of the composition C1 was stored in a sealed container for 6 months at room temperature and its viscosity was then determined to be 1060 Pas at 25°C. This higher viscosity sample (after storage) was used as C1* in table 3.

[0209] Adhesion Strength (Lap shear strength “LSS”)

[0210] The lap shear strength was determined according to SAE J1523 with 1.5 mm- thick and 25.4 mm wide high strength hot dip galvanized steel (HDG) coupons, 25.4 mm length overlap, 13 mm / min pull rate. The samples were cured for 20 min at 180 °C. The thickness of the samples is indicated in table 3. For all examples in table 3, spacers between the coupons were used with the indicated thickness (“DI”). In the examples Ref1 - Ref5, the thickness of the applied bead (“HT”) was identical to the indicated thickness (DI / HT = 1), hence the applied compositions were in direct contact with both coupons before curing / expansion. For the examples E1 - E6, the thickness of the applied bead (“HT”) was only 80 % of the spacer thickness (DI / HT = 1.25), so the applied compositions were in contact with only one of the coupons before curing / expansion.

[0211] T-Peel test (“T-peel”)

[0212] The T-peel strength was determined according to ASTM D1876 with 90-degree bend with 0.8 mm-thick and 25.4 mm wide mild hot dip galvanized steel (HDG) coupons, substrate dimensions: 100 x 25.4 x 0.8 mm bent at 75 mm, 75 mm length overlap, 127 mm / min pull rate. The samples were cured for 20 min at 180 °C. The thickness of the samples is indicated in table 3. For all examples in table 3, spacers between the coupons were used with the indicated thickness (“DI”). In the examples Ref1 - Ref5, the thickness of the applied bead (“HT”) was identical to the indicated thickness (DI / HT = 1), hence the applied compositions were in direct contact with both coupons before curing / expansion. For the examples E1 - E6, the thickness of the applied bead (“HT”) was only 80 % of the spacer thickness (DI / HT = 1.25), so the applied compositions were in contact with only one of the coupons before curing / expansion.

[0213] Flexural Strength, 3-point bending test (“FS”)

[0214] The flexural strength was determined according to ASTM D 7264 M, Procedure B, with the exception of utilizing 25 mm x 203 mm x spacer height indicated in table 4 sandwich samples instead of material bar samples. 100 mm constant support span, 6.4 mm radius loading nose, 5 mm / min constant crosshead rate, compression, load applied to substrate surface. The samples were conditioned at ambient conditions (23°C / 50%RH) for a minimum of 4 hours prior to testing. The composition C1 was applied to result in an overlap area of the sandwich of 150 mm length. The load at 2 mm deflection (“2mmD”) as well as the peak load (“PL”) was determined (in N) at 23°C. The samples were cured for 10 min at 160 °C. The thickness of the samples (spacer height) is indicated in table 4. For all examples in table 4, spacers between the coupons were used with the indicated thickness (“DI”). For the examples E7 - E10, the thickness of the applied bead (“HT”) was only 80 % of the spacer thickness (DI / HT = 1.25), so the applied compositions were in contact with only one of the coupons before curing / expansion.

[0215] Expansion

[0216] The expansion is quantified for each sample by measuring the density before and after expansion (curing). The densities were determined according to DIN EN ISO 1183 using the water immersion method (Archimedes principle) in deionized water and a precision balance to measure the mass.

[0217] Table 3,

[0218]

[0219] C1* = composition C1 after storage for 6 months at 23°C in a sealed container, n.d. = not determined Table 4

[0220]

Claims

Claims1. A method for bonding two substrates comprising the sequential steps of- i) applying an expandable one-component thermosetting epoxy resin adhesive on a first substrate S1, and- ii) placing the second substrate S2 so that the distance between the first substrate S1 and the second substrate S2 at the place of application of the expandable one-component thermosetting epoxy resin adhesive is between 2 - 30 mm, and- iii) heating the first substrate S1 and the second substrate S2 to a temperature above the activation temperature of the blowing agent BA to expand the expandable one-component thermosetting epoxy resin adhesive,wherein between step ii) and step iii) the applied expandable one- component thermosetting epoxy resin adhesive is not in contact with the second substrate S2,and the expandable one-component thermosetting epoxy resin adhesive has a viscosity of 500 to 5000 Pas, preferably 500 to 2000 Pas, preferably 500 to 1500 Pas, preferably 500 to 1300 Pas at 25°C, the viscosity being determined oscillographically by means of a rheometer with heatable plate (MCR 301, AntonPaar) (slot 1000 μm, measuring plate diameter: 25 mm (plate / plate), deformation 0.01 at 5 Hz, temperature: 25°C).

2. Method according to claim 1, wherein the expandable one-component thermosetting epoxy resin adhesive is a structural adhesive3. Method according to any of the proceeding claims, wherein the expandable one-component thermosetting epoxy resin adhesive comprises:a) at least one epoxy resin A having on average more than one epoxide group per molecule, preferably a liquid epoxy resin, the fraction of the epoxy resin A preferably being from 20 - 70wt.-%, based on the total weight of the expandable one- component thermosetting epoxy resin adhesive; andb) at least one blowing agent BA; andc) at least one latent hardener B for epoxy resins; andd) preferably at least one accelerator C for epoxy resins; and e) preferably at least one toughness improver D, the fraction of toughness improver D being preferably from 5 - 40 wt.-%, based on the total weight of the expandable one-component thermosetting epoxy resin adhesive.

4. The method according to any of the proceeding claims, characterized in that in step i) the expandable one-component thermosetting epoxy resin adhesive is applied in the form of a bead with a width of 1.5 - 25.0 mm, preferably 2.5 - 15.0 mm, more preferably 3.0 - 8.0 mm, most preferably 4.0 - 7.0 mm.

5. The method according to any of the proceeding claims, characterized in that in step i) the expandable one-component thermosetting epoxy resin adhesive is applied in the form of a bead with a height of 1.5 - 12.0 mm, preferably 2.5 - 10.0 mm, more preferably 3.0 - 8.0 mm, most preferably 4.0 - 7.0 mm.

6. The method according to any of the proceeding claims, characterized in that in step i) the expandable one-component thermosetting epoxy resin adhesive is applied in the form of a bead with a length of 10 mm to 2 m, preferably with a length of 2 cm to 10 cm or with a length of 10 cm to 1 m.

7. The method according to any of the proceeding claims, characterized in that in step i) the expandable one-component thermosetting epoxy resin adhesive is applied in the form of a bead, preferably the ratio of the width to the height of the bead is 1 - 5, preferably 1 - 3, more preferably 1 – 2.

8. The method according to any of the proceeding claims, characterized in that in step i) the expandable one-component thermosetting epoxy resin adhesive is applied in the form of a bead and in step ii) the ratio between the distance DI between the first substrate S1 and the second substrate S2 and the height HT of the bead at the place of application of the expandable one-component thermosetting epoxy resin adhesive (DI / HT) is from 1.10 - 3.00, preferably 1.20 - 2.00, most preferably 1.25 - 1.75.

9. The method according to any of the proceeding claims, characterized in that in step i) the expandable one-component thermosetting epoxy resin adhesive is applied in the form of a bead, the height HT of the bead is from 1.5 - 12.0 mm, preferably 2.5 - 10.0 mm, more preferably 3.0 - 8.0 mm, most preferably 4.0 - 7.0 mm, and the distance DI between the first substrate S1 and the second substrate S2 at the place of application of the expandable one-component thermosetting epoxy resin adhesive is from 2.0 - 15 mm, preferably 3.0 - 12.5 mm, more preferably 4.0 - 10.0 mm, most preferably 5.0 - 9.0 mm.

10. The method according to any of the proceeding claims, characterized in that in step i) the expandable one-component thermosetting epoxy resin adhesive is applied in the form of several beads, preferably 2 -4 beads, that are aligned parallel to each other, preferably the distance between the neighboring beads is 0 - 30 mm, preferably 0 - 20 mm, more preferably 5 - 15 mm.

11. The method according to any of the proceeding claims, characterized in that the expandable one-component thermosetting epoxy resin adhesive has a viscosity of 500 to 1500 Pas, preferably 500 to 950 Pas at 25°C or preferably 1000 to 1500 Pas at 25°C, the viscosity being determined oscillographically by means of a rheometer with heatable plate (MCR 301, AntonPaar) (slot 1000 μm, measuring plate diameter: 25 mm (plate / plate), deformation 0.01 at 5 Hz, temperature: 25°C).

12. The method according to any of the proceeding claims, characterized in that after step iii) the expanded one-component thermosetting epoxy resin adhesive has an expansion of 100 - 250 %, preferably 110 - 225 %, more preferably 125 - 175 %, determined according to DIN EN ISO 1183 using the water immersion method (Archimedes principle).

13. The method according to any of the proceeding claims, characterized in that the at least one blowing agent BA is a physical blowing agent, more preferably heat-expandable microspheres HEM and / or, preferably and, the blowing agent BA has an activation temperature from 90°C to 210°C.

14. The method according to any of the proceeding claims, characterized in that the latent hardener B is selected from the list consisting of dicyandiamide, guanamines, guanidines, aminoguanidines and derivatives thereof, substituted ureas, imidazoles and amine complexes, preferably dicyandiamide.

15.

13. The method according to any of the proceeding claims, characterized in that the expandable one-component thermosetting epoxy resin adhesive is tacky at 23 ° C, wherein the term "tacky" is understood here mean immediate tack, which is sufficiently high at 23 ° C that when pressed with a thumb with a pressure of about 5 kg for 1 second on the surface of the expandable one-component thermosetting epoxy resin adhesive, the thumb does stick to the surface of the expandable one-component thermosetting epoxy resin adhesive and the adhesive can be raised.

16. The method according to any of the proceeding claims, characterized in that the first substrate S1 is a supporting frame of a vehicle engine hood, trunk or door or a crossbeam of a vehicle roof and the second substrate S2 is a vehicle engine hood, trunk or door panel or a vehicleroof panel or the first substrate S1 and the second substrate S2 are parts of a vehicle pillar, preferably an A-pillar, a B-pillar or a C-pillar.

17. An article obtained by the method of claim 1 to claim 16, especially a construction of modes of transport, especially in the automobile industry, or an article of white goods, most preferably selected from the list consisting of vehicle roof, engine hood, trunk hood, vehicle door and vehicle pillar, preferably an A-pillar, a B-pillar or a C-pillar.

18. An article according to claim 17, wherein the article fulfills at least one of the following conditions:- lap shear strength of at least 1 MPa and- T-peel strength of at least 0.8 N / mm; and- flexural strength at 2 mm deflection of at least 450; and- expansion of 100-250%; and19. An article according to one of claims 17-18, wherein the article fulfills at least one of the following conditions:- provides adhesive bonding between metal substrates separated by 2-30 mm with cohesive failure mode at lap shear strengths of at least 0.8 MPa when tested according to SAE J1523; and20. An article according to one of claims 17-19, wherein the article fulfills at least one of the following conditions:- exhibits lap shear strength values that are at least 15% higher than comparable articles produced by methods where the adhesive contacts both substrates prior to heating.

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