Two-component curing adhesive

Abstract

A two-part curable adhesive is provided that may be suitable for use in industrial bonding applications, including aerospace and automotive bonding. The adhesive includes a first part containing an epoxy resin and a first oligomeric toughener. The adhesive further includes a second part containing a curing agent reactive with the epoxy resin at ambient conditions and a second oligomeric toughener. The first and second oligomeric tougheners are immiscible with each other at ambient conditions. Advantageously, when cured, the adhesive can simultaneously exhibit both high shear strength and high peel performance at elevated temperatures.

Description

[Technical Field]

[0001] A curable two-part paste adhesive for structural bonding applications is provided. Such adhesives may be useful, for example, in bonding automotive and aerospace structures. [Background technology]

[0002] Over time, structural adhesives have replaced or augmented the use of welds and mechanical fasteners such as screws and rivets. This transition is particularly pronounced in the automotive and aerospace industries, where corrosion resistance is an issue and bonding to softer composite and plastic substrates is becoming increasingly common. Depending on the application, these adhesives may need to exhibit both high peel and shear strength, as well as the ability to withstand extreme environments.

[0003] Epoxy adhesives are known to provide acceptable bond strength in harsh outdoor environments, making them suitable for these critical applications. Epoxy adhesives are typically formulated with a two-part base and hardener component that react with each other when mixed to form a solid polymer material. These parts are stored separately, allowing the adhesive to have a long shelf life even when stored at room temperature.

[0004] Conveniently, the two adhesive parts are provided in a dual-chamber cartridge that can be loaded into a suitable dispenser to mix and apply the reactive mixture. For larger scale operations, these can be provided in bulk and pumped into an in-line mixer and dispenser at the time of use. For convenience and to reduce costs associated with adhesive bonding, ready curing of the adhesive, which occurs at ambient temperatures, is preferred. Summary of the Invention

[0005] The provided two-part adhesive composition addresses the shortcomings of existing high-temperature structural adhesives. Known two-part adhesives, which are mixed and cured at ambient temperatures, can be prepared from a blend of an epoxy resin and a curing agent containing a reactive amine compound. To enhance the adhesive's high-temperature performance, multifunctional epoxy resins can be used to increase crosslink density. Unfortunately, high crosslink density tends to increase brittleness in the cured epoxy resin, which in turn reduces peel performance.

[0006] To compensate for the loss of peel performance, manufacturers incorporate softeners and tougheners into the formulation. However, this has the undesirable effect of impairing the adhesive's shear strength at high temperatures. Because of this dilemma, epoxy adhesives that can be handled at high temperatures tend to exhibit poor peel performance, and those that display acceptable toughness and peel performance tend to exhibit poor shear strength at high temperatures.

[0007] Provided herein are two-part curable adhesives that can simultaneously achieve high-temperature shear strength and high peel performance by incorporating two different toughening oligomers that are incompatible with each other in the separate parts of the adhesive. In the mixed composition, the use of two different oligomers can help form rubbery domains that phase separate on a microscopic scale. The gelation process of the adhesive can prevent macrophase separation of the adhesive as the viscosity rapidly increases during curing.

[0008] When cured, the provided adhesives can surprisingly exhibit overlap shear strengths of greater than 5 MPa at 177°C, along with floating roller peel strengths (measured at ambient temperature) of greater than 75 N / 25 mm.

[0009] In one aspect, a two-part curable adhesive is provided, the two-part curable adhesive comprising a first part comprising an epoxy resin and a first oligomeric toughening agent, and a second part comprising a curing agent reactive with the epoxy resin at ambient conditions and a second oligomeric toughening agent, the first and second oligomeric toughening agents being immiscible with each other at ambient conditions.

[0010] In a second aspect, there is provided a method of bonding comprising mixing a first part and a second part of a two-part curable adhesive to provide a mixture, wherein the first and second oligomeric toughening agents form multiple heterogeneous domains in the mixture; disposing the mixture on at least one substrate to be bonded; and securing the at least one substrate until the two-part curable adhesive is functionally cured.

[0011] definition As used herein, "Ambient conditions" means a temperature of 23°C and a pressure of 1 atmosphere (i.e., 101.3 kPa); "Ambient temperature" refers to a temperature of 23°C, "Average" refers to the numerical average unless otherwise indicated; "Cure" refers to chemically crosslinking (e.g., at room temperature or under heated conditions), such as by exposure to any form of radiation, heating, or undergoing a chemical reaction that results in hardening or an increase in viscosity; "Functionally cured" means that the adhesive has achieved a cured state resulting in an effective adhesive bond; "Macroscopic phase separation" means a state in which at least two distinct compositional domains are visible to the naked eye; "oligomer" means a polymer having a relatively small number of repeating units (e.g., 2 to 20, 2 to 50, 2 to 100, 2 to 200, or 2 to 500, as the case may be); "Polymer" refers to a molecule having multiple repeating units; "Toughener" refers to an additive that increases the toughness of the material to which it is added. DETAILED DESCRIPTION OF THE INVENTION

[0012] As used herein, the terms "preferred" and "preferably" refer to embodiments described herein that may offer certain advantages, under certain circumstances, although other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.

[0013] As used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to an element preceded by "a" or "the" may include one or more of the element and equivalents thereof known to those skilled in the art. Furthermore, the term "and / or" means one or all of the listed elements or a combination of any two or more of the listed elements.

[0014] It should be noted that the terms "comprises" and variations thereof do not have a limiting meaning when these terms appear in the accompanying description. Furthermore, "a," "an," "the," "at least one," and "one or more" are used interchangeably herein.

[0015] Throughout this specification, reference to "one embodiment," "a particular embodiment," "one or more embodiments," or "an embodiment" means that the particular feature, structure, material, or characteristic described with respect to that embodiment is included in at least one embodiment of the invention. Thus, the appearances of phrases such as "in one or more embodiments," "in a particular embodiment," "in one embodiment," or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment of the invention. Where applicable, product names are written in all capital letters.

[0016] Provided herein are two-part adhesives suitable for bonding structural components. Such components, in some embodiments, can include primary and secondary structures in a wide variety of industrial applications. These bonding applications can involve joining similar or dissimilar materials, such as might be encountered in composite bonding, metal bonding, and bonding metal to composites in automotive or aerospace vehicles. In preferred embodiments, these cured adhesives provide structural bonds that can retain their integrity under extreme conditions, including high temperatures and humid environments.

[0017] Advantageously, these two-part adhesives can be mixed and cured at ambient temperature. The curing process is not instantaneous, but occurs gradually so that the operator has sufficient working time to apply and shape the adhesive as needed. This working time can be used to align and prepare the structures to be joined to fit before setting.

[0018] The two-part adhesives provided are epoxy adhesives. Epoxy adhesives are generally formulated with two reactive components, referred to as the base part and the hardener part. Further details about each of these parts are provided in the following sections.

[0019] Main ingredient The base part of a two-part curable adhesive contains at least one epoxy resin, often a miscible blend or two or more epoxy resins.

[0020] Epoxy resins are compounds containing one or more epoxide groups. These compounds may be polymers whose repeating units are derived from monomers with epoxy functionality. Epoxy resins also include silicone-based polymers containing epoxy groups, or organic polymer particles coated or modified with epoxy groups, or particles coated with, dispersed within, or modified by epoxy-containing polymers.

[0021] The epoxide groups allow the resin to undergo crosslinking reactions. Epoxy resins generally have an average epoxide functionality of at least 1, at least 1.5, at least 2, at least 2.5, or at least 3. Epoxy resins with functionalities significantly greater than 2 are capable of inducing crosslinking, and these multifunctional epoxy resins can be used to control crosslink density and critical adhesive properties upon cure.

[0022] Multifunctional epoxy resins having at least three functional groups, such as trifunctional epoxy resins, may be a single polymer or a combination of two or more polymers. Examples of multifunctional epoxy resins include glycidylamine-type epoxy resins and glycidylphenol-type epoxy resins. Examples of glycidylamine-type epoxy resins include triglycidylaminophenol epoxy compounds and triglycidylaminocresol epoxy compounds. Examples of glycidylphenol-type epoxy resins include phenol novolac epoxy compounds and triphenylmethane triglycidyl ether compounds.

[0023] Epoxy resins that can be used as the multifunctional epoxy resins described herein are commercially available from Huntsman Corporation, The Woodlands, TX under the trade names TACTIX and ARALDITE.

[0024] Epoxy resins may be aromatic, aliphatic, cycloaliphatic, or mixtures thereof. Epoxy resins for use herein may be aliphatic or aromatic. Preferred epoxy resins contain glycidyl or polyglycidyl ether moieties. Such moieties can be obtained, for example, by reacting hydroxyl functionality (e.g., but not limited to, dihydric or polyhydric phenols or aliphatic alcohols, including polyols) with epichlorohydrin functionality. Dihydric phenols contain two hydroxy groups attached to the aromatic ring of the phenol, and polyphenols contain at least two hydroxy groups attached to the aromatic ring.

[0025] Examples of useful dihydric phenols include resorcinol, catechol, hydroquinone, and polyphenols such as p,p'-dihydroxydibenzyl, p,p'-dihydroxyphenyl sulfone, p,p'-dihydroxybenzophenone, 2,2'-dihydroxyphenyl sulfone, p,p'-dihydroxybenzophenone, 2,2-dihydroxy-1,1'-dinaphthyl methane, and the 2,2', 2,3', 2,4', 3,3', 3,4', and 4,4' isomers of dihydroxydiphenylmethane, dihydroxydiphenyldimethylmethane, dihydroxydiphenylethylmethylmethane, dihydroxydiphenylmethylpropylmethane, dihydroxydiphenylethylphenylmethane, dihydroxydiphenylpropylenephenylmethane, dihydroxydiphenylbutylphenylmethane, dihydroxydiphenyltolylethane, dihydroxydiphenyltolylmethylmethane, dihydroxydiphenyldicyclohexylmethane, and dihydroxydiphenylcyclohexane.

[0026] Suitable blends of epoxy resins can include epoxy resins containing or consisting of glycidyl ethers or polyglycidyl ethers of dihydric or polyhydric phenols, such as bisphenol A, bisphenol F, and combinations thereof. These epoxy resins can include one or more repeat units derived from bisphenol A and / or F. Such ethers, or such repeat units, can be obtained, for example, by polymerization of glycidyl ethers of bisphenol A and / or F with epichlorohydrin.

[0027] Instead of or in addition to the above-mentioned aromatic epoxy resins, their corresponding alicyclic compounds may be used. For example, cyclic or acyclic, linear or branched epoxy resins may be used instead of or in addition to the aforementioned aromatic epoxy resins. Blends of two or more epoxy resins are also possible, and those skilled in the art may attempt to optimize adhesive properties.

[0028] Commercially available examples of useful epoxy resins include diglycidyl ethers of bisphenol A (available from Hexion Specialty Chemicals GmbH, Rosbach, Germany, under the trade names EPON 828, EPON 830, EPON 1001, or EPIKOTE 828, or from Dow Chemical Co. under the trade names DER-331 or DER-332), diglycidyl ethers of bisphenol F (available, for example, from Dainippon Ink and Chemicals, Inc., as EPICLON 830, or from Dow Chemical Co., Schwalbach / Ts., Germany, as DER-354), and diglycidyl ethers of a blend of bisphenol A and bisphenol F (available, for example, from Hexion Specialty Chemicals GmbH, Rosbach, Germany, as EPIKOTE 232).

[0029] In addition to the epoxy resin or epoxy resin blend, the base portion of the two-part curable adhesive further comprises a first oligomeric toughener. In preferred embodiments, the oligomeric toughener is compatible (i.e., miscible) with the epoxy resin or epoxy resin blend such that the base portion is macroscopically homogeneous.

[0030] The first oligomeric toughening agent can be reactive with at least one other component in the two-part curable composition. In some embodiments, the first oligomeric toughening agent in the base part can react directly with the curing agent in the curing agent part when the base and curing agent parts are mixed at ambient temperature.

[0031] Alternatively, the first oligomeric toughening agent can be reactive with at least one epoxy resin in the base portion to form a reactive epoxy adduct, which can then react with the curing agent in the hardener portion when the base and hardener portions are mixed at ambient temperature.

[0032] The first oligomeric toughening agent can have any suitable molecular weight. The first oligomeric toughening agent can have a number average molecular weight of less than, equal to, or greater than 500 g / mol to 15,000 g / mol, 1,000 g / mol to 10,000 g / mol, 1,500 g / mol to 10,000 g / mol, or in some embodiments, 500 g / mol, 600, 700, 800, 900, 1000, 1200, 1500, 1700, 2000, 2500, 3000, 3500, 4000, 4500, 5000, 6000, 7000, 8000, 9000, 10,000, 11,000, 12,000, 13,000, 14,000, or 15,000 g / mol.

[0033] In an exemplary embodiment, the first oligomeric toughening agent is an adduct of a polyetheramine and at least one epoxy resin present in the epoxy portion of the two-part curable composition. Useful polyetheramines include difunctional, trifunctional, and other multifunctional polytetrahydrofuranamines, which may have number average molecular weights within the ranges listed above. The polyetheramine may be linear or branched. Branched polyetheramines may have an amine functionality of 3, 4, 5, or even more. The choice of polyetheramine may be influenced by the desired crosslink density, hardness, flexibility, and degree of water uptake.

[0034] Polytetrahydrofuranamine grades can have different degrees of functionality and molecular weight and are commercially available as curing agents from a variety of manufacturers, including BASF SE, Ludwigshafen, Germany.

[0035] Another exemplary embodiment uses an epoxy-terminated butadiene-acrylonitrile oligomer as the first oligomeric toughening agent. The epoxy-terminated butadiene-acrylonitrile oligomer may be an adduct of the diglycidyl ether of bisphenol A and a carboxylic acid-terminated butadiene-acrylonitrile (CTBN) oligomer. Various carboxylic acid-terminated butadiene-acrylonitrile reactive liquid oligomers can be offered under the trade name HYPRO by Emerald Kalama Chemical, LLC, Vancouver, WA.

[0036] Proper selection of the acrylonitrile content in butadiene-acrylonitrile oligomers allows the oligomers to be fully miscible in epoxy resin blends. As the resin system cures, the epoxy functional groups react with the matrix resin, causing the synthetic butadiene-acrylonitrile rubber phase to separate and form distinct rubber domains. These domains, on the nanometer to micrometer size scale, are capable of absorbing strain energy and toughening the cured adhesive.

[0037] The butadiene-acrylonitrile oligomers may have an acrylonitrile content of from 5 percent to 60 percent, from 10 percent to 50 percent, from 15 percent to 45 percent, or in some embodiments less than, equal to, or greater than 5 percent, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, or 60 weight percent based on the total weight of the butadiene-acrylonitrile oligomers.

[0038] The adduct of the diglycidyl ether of bisphenol A and butadiene-acrylonitrile oligomer can have a viscosity of 50,000 cps to 1,000,000 cps at ambient temperature, which can be measured, for example, by a Brookfield LV series viscometer provided by Brookfield AMETEK, Middleboro, MA.

[0039] Other epoxide-compatible toughening agents include acrylate core-shell graft copolymers, where the core or backbone has a glass transition temperature (T) below about 0° C. g ), such as polyacrylate polymers having a T above about 25°C, such as polymethyl methacrylate. g and polybutadiene, polybutyl acrylate, or polyisooctyl acrylate to which has been grafted a polymethacrylate polymer having the formula: Optionally, the core-shell graft copolymer can be pre-dispersed in a diglycidyl ether of bisphenol A. These are other useful toughening agents described in EP 0 623 151 (Tarbutton et al.).

[0040] The epoxy resin or resins can represent any suitable proportion of the two-part curable adhesive. The weight of said epoxy resin, excluding the first oligomeric toughener, can be 30 to 80 percent, 40 to 80 percent, 50 to 70 percent, or in some embodiments less than, equal to, or greater than 30, 40, 50, 60, 70, or 80 percent of the total weight of the base and hardener portions.

[0041] The first oligomeric toughener is preferably present in an amount that improves the peel performance of the cured adhesive over a wide range of temperatures while maintaining solubility of the toughener in the epoxy resin. The weight of the first oligomeric toughener can be less than, equal to, or greater than 3 percent to 30 percent, 5 percent to 30 percent, 5 percent to 20 percent, or in some embodiments, 3 percent, 5, 10, 20, 30, or 40 percent of the total weight of the base resin portion and the hardener portion.

[0042] Hardener part As previously mentioned, the hardener portion of the two-part curable adhesive is mixed manually or automatically with the base portion to obtain the cured adhesive.

[0043] In the provided adhesive, the hardener portion includes a curing agent that is reactive with the epoxy resin. The hardener portion further includes a second oligomeric toughening agent different from the first oligomeric toughening agent. Preferably, the hardener is reactive with the epoxy resin at ambient temperatures to provide a cured adhesive within a reasonable working time for hand-bonding applications.

[0044] Known curing agents that can be used in the adhesive include any of a variety of aliphatic and aromatic amines. The amines preferably contain primary and / or secondary amine groups. The curing agents can have an amine equivalent weight of up to 350 g / mol, up to 300 g / mol, up to 250 g / mol, or up to 200 g / mol.

[0045] Exemplary amine curing agents include 4,7,10-trioxytridecane-1,13-diamine, 4-7-dioxydecane-1,10-diamine, diethylenetriamine, triethylenetetraamine, tetraethylenepentamine, higher polyalkylenepolyamines, aminoethylpiperazine, TCD diamine, bis(aminopropyl)piperazine, norbornanediamine, methylenedicyclohexylamine, methyl or dimethylmethylenedicyclohexylamine, isophorone amine or diamine, and polyamidoamine. Blends of one or more of the above are also possible.

[0046] The curing agent, when mixed, can be from 20 percent to 70 percent, from 25 percent to 60 percent, from 40 percent to 60 percent, or in some embodiments, less than, equal to, or greater than 20 percent, 25, 30, 40, 50, 60, or 70 percent of the total weight of the two-part curable composition.

[0047] Additional curing agents, curing catalysts, or accelerators may also be present in the curing agent portion of the two-part curable adhesive. These can include, for example, tertiary amine catalysts and calcium catalysts to accelerate cure time. Tertiary amine catalysts include, for example, tris-2,4,6-dimethylaminomethylphenol, as described in U.S. Patent Nos. 4,518,749 (Waddili), 4,800,222 (Waddili), and 6,773,754 (Whiter), available from Evonik Industries AG, Essen, Germany, under the trade name ANCAMINE. Useful calcium catalysts, including calcium nitrate tetrahydrate, are available from suppliers such as MilliporeSigma (St. Louis, MO). Curing accelerators can be incorporated into either the base or curing agent portion to shorten cure time.

[0048] The second oligomeric toughener can be directly reactive with the epoxy resin in the base part of the two-part curable adhesive. For example, the second oligomeric toughener can be composed of a polyetheramine. The polyetheramine can be difunctional, trifunctional, or even higher functional.

[0049] Suitable polyether amines can include polytetrahydrofuran amines, particularly any of the polytetrahydrofuran amines useful in making the adducts described in the previous subsection.

[0050] Alternatively, the second oligomeric toughening agent can be reactive with at least one curing agent to provide a precursor such as a curing agent adduct reactive with an epoxy resin. An exemplary curing agent adduct is an amine-terminated butadiene-acrylonitrile oligomer derived from any of the aforementioned butadiene-acrylonitrile oligomers.

[0051] Like the first oligomeric toughening agent, the second oligomeric toughening agent can have any suitable molecular weight. The second oligomeric toughening agent can have a number average molecular weight of less than, equal to, or greater than 500 g / mol to 15,000 g / mol, 1,000 g / mol to 10,000 g / mol, 1,500 g / mol to 10,000 g / mol, or in some embodiments, 500 g / mol, 600, 700, 800, 900, 1000, 1200, 1500, 1700, 2000, 2500, 3000, 3500, 4000, 4500, 5000, 6000, 7000, 8000, 9000, 10,000, 11,000, 12,000, 13,000, 14,000, or 15,000 g / mol.

[0052] The second oligomeric toughener is preferably present in an amount that improves release performance over a wide range of temperatures while maintaining solubility of the toughener in the curing agent component. The weight of the second oligomeric toughener, when combined, can be from 0.5 percent to 25 percent, from 2 percent to 25 percent, from 5 percent to 20 percent, or in some embodiments, less than, equal to, or greater than 0.5 percent, 2, 5, 10, 20, or 25 percent of the total weight of the base and curing agent portions.

[0053] The first and second oligomeric toughening agents, relative to one another when mixed, can be present in a weight ratio of 0.2:1 to 5:1, 0.4:1 to 4:1, 0.6:1 to 3:1, or in some embodiments less than, equal to, or greater than 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.7:1, 1:1, 3:2, 2:1, 3:1, 4:1, or 5:1 by weight of the total base and curative portions.

[0054] Either the base or curative part may further contain any number of other optional additives, including, for example, adhesion promoters, fillers, rheology modifiers, pigments, additional rubbers, diluents (which may or may not be reactive), plasticizers, extenders, flame retardants, thixotropic agents, flow control agents, thickeners, gelling agents, fillers, dyes, and antioxidants.

[0055] In addition to the filler, the provided two-part curable composition can contain any of various inorganic submicrometer particles known in the art. Inorganic submicrometer particles can include, for example, inorganic nanoparticles. Such particles can be present in the base part, the curing agent part, or both.

[0056] Exemplary additives may include one or more of silica gel, calcium silicate, phosphate, molybdate, fumed silica, carbon black, clays such as bentonite, organoclay, aluminum trihydrate, hollow glass microspheres, hollow polymeric microspheres, and calcium carbonate. In some embodiments, these fillers may be used to adjust the modulus, promote adhesion, improve corrosion resistance, control the rheological properties of the adhesive, and / or reduce shrinkage during cure.

[0057] Exemplary commercially available fillers include SHIELDEX AC5 (artificial amorphous silica, calcium hydroxide mixture available from W.R. Grace, Columbia, MD), CAB-O-SIL TS 720 (hydrophobic fumed silica treated with polydimethylsiloxane polymer available from Cabot Corporation, Boston, MA), AEROSIL VP-R-2935 (hydrophobic fumed silica available from Evonik Industries AG, Essen, Germany), glass beads Class IV (250 micrometers to 300 micrometers), Micro-billes de verre 180 / 300 (available from CVP SA, France), Glass Bubbles K37, amorphous silica (available from 3M Deutschland GmbH, Neuss, Germany), MINSIL SF 20 (available from Minco Inc., 510 available from Midway, Tennessee, USA), amorphous, fused silica, and APYRAL 24 ESF (epoxysilane-functionalized (2 wt%) aluminum trihydrate available from Nabaltec GmbH, Schwandorf, Germany).

[0058] The inclusion of small amounts of inorganic sub-micrometer particles can provide a significant increase in modulus in the cured composition. Advantageously, this increase in modulus can partially or completely offset the decrease in modulus (if any) due to the presence of core-shell particles in the curable composition, while maintaining the high fracture toughness imparted by the core-shell particles. Useful sub-micrometer particles can include surface-bound organic groups that act to improve compatibility between the inorganic sub-micrometer particles and the epoxy resin.

[0059] Useful sub-micrometer particles include those derived from silicon dioxide (ie, silica) and calcium carbonate.

[0060] How to use The provided adhesives are cured by mixing the first and second parts of a two-part curable adhesive to provide a mixture, placing the mixture on one or more substrates to be joined, and then fixing the substrates until the mixture has functionally cured. The mixture is typically dispensed onto the joining surface of a bead or layer.

[0061] If desired, these two-part adhesives can be packaged in dual-chamber cartridges and conveniently metered, mixed, and dispensed using any of several suitable applicator systems known to those skilled in the art, such as the 3M EPX applicator system offered by 3M Company, St. Paul, Minn. In larger-scale operations, the reactive portion is provided in bulk and pumped from a drum or other storage container into an in-line mixer and dispenser, which is placed at the point of use. By mixing the adhesive at the point of use, such an applicator can help reduce waste.

[0062] In some cases, the cured adhesive may only be bonded to a single structure or substrate, such as may be the case when used as an isolator, void filler, or potting compound.

[0063] When mixed, the first and second oligomeric toughening agents form numerous heterogeneous domains within the mixture. The individual heterogeneous domains are so small that they are invisible to the naked eye, thereby preserving uniform adhesive properties on a macroscopic scale. Furthermore, the relatively rapid increase in viscosity during the curing process prevents macroscopic phase separation of the first and second oligomeric toughening agents.

[0064] Generally, the two-part curable adhesive is functionally cured within 10 hours of mixing at ambient temperature. In some embodiments, the two-part curable adhesive reaches functional cure within 9 hours, 8 hours, 7 hours, 6 hours, 5 hours, 4 hours, 3 hours, or 2 hours of mixing.

[0065] Although not required, curing can be accelerated by heating the mixed composition to an elevated temperature. The gel time, or the time for the adhesive to reach a functionally cured state, can be less than, equal to, or greater than 0.25 hours to 8 hours, 0.5 hours to 6 hours, 1 hour to 4 hours, or in some embodiments, 0.25 hours, 0.5, 0.75, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.6, 5, 6.5, 7, 7.5, or 8 hours.

[0066] The first and second oligomeric toughening agents are preferably completely soluble in their respective parts of the two-part curable adhesive, but the first and second oligomeric toughening agents are immiscible with each other at ambient temperature.

[0067] In some embodiments, the first oligomeric toughener comprises an epoxy-terminated butadiene-acrylonitrile oligomer and the second oligomeric toughener comprises a polyetheramine, wherein the epoxy-terminated butadiene-acrylonitrile oligomer is miscible with the epoxy resin at ambient conditions and the polyetheramine is miscible with the curing agent at ambient conditions.

[0068] In another embodiment, the first oligomeric toughener comprises a polyetheramine-epoxy adduct and the second oligomeric toughener comprises an amine-terminated butadiene-acrylonitrile oligomer, wherein the epoxy-terminated butadiene-acrylonitrile oligomer is miscible with the epoxy resin at ambient conditions and the amine-terminated butadiene-acrylonitrile oligomer is miscible with the curing agent at ambient conditions.

[0069] Upon curing, the adhesive mixture can provide a degree of bond strength and reliability sufficient for structural adhesive applications at the target temperature, as evidenced by an average floating roller peel performance of 10 N / 25 mm to 450 N / 25 mm at ambient temperature and an average overlap shear strength of 3 MPa to 25 MPa at 150°C, an average floating roller peel performance of 50 N / 25 mm to 350 N / 25 mm at ambient temperature and an average overlap shear strength of 5 MPa to 20 MPa at 150°C, or an average floating roller peel performance of 75 N / 25 mm to 300 N / 25 mm at ambient temperature and an average overlap shear strength of 5 MPa to 15 MPa at 150°C.

[0070] The high peel performance, along with the unexpectedly higher overlap shear strength at elevated temperatures, can be attributed to the immiscibility of the toughening oligomers in the two adhesive parts. After the base and curing agent parts are mixed and the curing process is initiated, this miscibility can advantageously induce (or accelerate) phase domain separation of the first and second oligomeric tougheners from the surrounding curable matrix resin. An additional advantage of using two mutually immiscible tougheners is that it allows for the use of tougheners with lower molecular weights than would otherwise be possible, which in turn improves processability. This effect stems from the inverse relationship between the molecular weight of the toughener and its compatibility with the matrix.

[0071] The adhesive compositions and methods provided can be exemplified by the following non-exhaustive list of embodiments:

[0072] 1. A two-part curable adhesive comprising a first part comprising an epoxy resin and a first oligomeric toughening agent, and a second part comprising a curing agent that is reactive with the epoxy resin at ambient conditions and a second oligomeric toughening agent, wherein the first and second oligomeric toughening agents are immiscible with each other at ambient conditions.

[0073] 2. The two-part curable adhesive of embodiment 1, wherein the first oligomeric toughener is reactive with the epoxy resin, resulting in an epoxy adduct.

[0074] 3. The two-part curable adhesive of embodiment 1, wherein the first oligomeric toughening agent is reactive with the curing agent.

[0075] 4. The two-part curable adhesive of any one of embodiments 1-3, wherein the second oligomeric toughener is reactive with epoxy resins.

[0076] 5. The two-part curable adhesive of any one of embodiments 1-3, wherein the second oligomeric toughening agent is reactive with the curing agent to form a curing agent adduct.

[0077] 6. The two-part curable adhesive of any one of embodiments 1 to 5, wherein the first oligomeric toughener has a number average molecular weight of 500 g / mol to 15,000 g / mol and the second oligomeric toughener has a number average molecular weight of 500 g / mol to 15,000 g / mol.

[0078] 7. The two-part curable adhesive of any one of embodiments 1-6, wherein either the first or second oligomeric toughening agent comprises a polyetheramine.

[0079] 8. The two-part curable adhesive of embodiment 7, wherein the first oligomeric toughener comprises a polyetheramine-epoxy adduct.

[0080] 9. The two-part curable adhesive of embodiment 7, wherein the second oligomeric toughening agent comprises polytetrahydrofuranamine.

[0081] 10. The two-part curable adhesive of any one of embodiments 1-9, wherein either the first or second oligomeric toughening agent comprises a butadiene-acrylonitrile oligomer.

[0082] 11. The two-part curable adhesive of embodiment 10, wherein the first oligomeric toughener comprises an epoxy-terminated butadiene-acrylonitrile oligomer comprising an adduct of a diglycidyl ether of bisphenol A and a carboxylic acid-terminated butadiene-acrylonitrile (CTBN) oligomer.

[0083] 12. The two-part curable adhesive of embodiment 10, wherein the second oligomeric toughener comprises an amine-terminated butadiene-acrylonitrile oligomer.

[0084] 13. The two-part curable adhesive of embodiment 1, wherein the first oligomeric toughener comprises an epoxy-terminated butadiene-acrylonitrile oligomer and the second oligomeric toughener comprises a polyetheramine, the epoxy-terminated butadiene-acrylonitrile oligomer being miscible with the epoxy resin at ambient conditions, and the polyetheramine being miscible with the curing agent at ambient conditions.

[0085] 14. The two-part curable adhesive of embodiment 1, wherein the first oligomeric toughener comprises a polyetheramine-epoxy adduct and the second oligomeric toughener comprises an amine-terminated butadiene-acrylonitrile oligomer, the epoxy-terminated butadiene-acrylonitrile oligomer being miscible with the epoxy resin at ambient conditions, and the amine-terminated butadiene-acrylonitrile oligomer being miscible with the curing agent at ambient conditions.

[0086] 15. A method of bonding, comprising: mixing a first part and a second part of the two-part curable adhesive of any one of embodiments 1-14 to provide a mixture, wherein the first and second oligomeric toughening agents form multiple heterogeneous domains in the mixture; disposing the mixture on at least one substrate to be bonded; and securing the at least one substrate until the two-part curable adhesive is functionally cured. [Example]

[0087] Objects and advantages of the present disclosure are further illustrated by the following non-limiting examples, but the specific materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit the disclosure. Unless otherwise stated, all parts, percentages, ratios, etc. in the examples and elsewhere in the specification are by weight.

[0088] [Table 1]

[0089] Test Method: Grade 2024-T3 bare aluminum panels were obtained from Erickson Metals of Minnesota, Inc., Coon Rapids, Minnesota. Prior to bonding with the structural adhesive, the panels were subjected to the following panel preparation process.

[0090] Panel preparation Bare aluminum panels were immersed in OAKITE 165 (BASF Corporation) caustic cleaning solution at 85°C (185°F) for 10 minutes. The panels were then immersed in tap water at 21°C (69.8°F) for 10 minutes, followed by a continuous tap water spray rinse for approximately 3 minutes. The panels were then immersed in Forest Products Laboratory (FPL) etching solution at 66°C (151°F) for 10 minutes, after which the panels were spray rinsed with water at 21°C (69.8°F) for approximately 3 minutes, allowed to drip dry for an additional 10 minutes, and then dried in an oven at 54°C for 30 minutes. The etched panels are ready for adhesive bonding and should be used within 8 to 12 hours.

[0091] Overlap Shear (OLS) Testing of Adhesive Films FPL etched panels of 2024-T3 bare aluminum measuring 10.16 cm x 17.78 cm x 0.16 cm (4 in x 7 in x 0.063 in) were prepared for testing as described above under "Panel Preparation." The etched panels were bonded together in an overlapping relationship along their longitudinal dimension. An example adhesive was applied to the edges of the etched aluminum panels measuring 4 in x 7 in x 0.063 in (10.16 cm x 17.78 cm x 0.16 cm). A second, identically sized etched aluminum panel was then applied over the adhesive, overlapping 0.5 in (12.7 mm), and the assembly was pressed between metal blocks at a pressure of approximately 2 psi to 5 psi (13.8 KPa to 34.5 KPa). The panel assemblies were cured at 70°F (21.1°C) for 72 hours and then cut into 1 inch x 7 inch (2.54 cm x 17.78 cm) strips. Overlap shear strength was measured in accordance with ASTM D-1002 using a Model "SINTECH-30" tensile tester obtained from MTS Corporation, Eden Prairie, Minnesota, at a grip separation rate of 0.05 in / min (1.3 mm / min). Six test panels were prepared and evaluated for each example.

[0092] Floating Roller Peel (FRP) strength test for adhesive film 20.3 cm x 7.6 cm x 0.16 cm (8.0 in x 3.0 in x 0.063 in) and 25.4 cm x 7.6 cm x 0.064 cm (10 in x 3 in x 0.025 in) 2024-T3 bare aluminum etched panels were prepared for testing as described above under "Panel Preparation." Adhesives corresponding to the Examples or Comparative Examples were applied onto the 20.3 cm x 7.6 cm x 0.16 cm 2024-T3 bare aluminum etched panels. Primed 25.4 cm x 7.6 cm x 0.064 cm aluminum panels were then applied onto the 20.3 cm x 7.6 cm x 0.16 cm panels to which adhesive had already been applied. The assemblies were then pressed between metal blocks at a pressure of approximately 2 psi to 5 psi (13.8 KPa to 34.5 KPa). The panel assemblies were allowed to cure for 72 hours at ambient temperature and then evaluated for floating roller peel strength according to ASTM D-3167-76 with the following modifications: Five samples were tested for each example or comparative example, and the average value (in N / 25 mm) was reported. 1.27 cm (0.5 inch) wide specimens were cut along the length of the bonded aluminum panel. Tests were conducted at ambient temperature at a speed of 30.5 cm / min (6 in / min). For each test, a thinner substrate was peeled from a thicker substrate, and the results were normalized to a width of 25 mm (approximately 1 inch).

[0093] Examples 1 to 6 (EX1 to EX6): Preparation of Agent A Step 1: A predefined amount (in grams) of EPON828, MX257, MY721, and / or RA840 was added to a predetermined amount (in grams) of TEPA, BAPP, and / or N-AEP, as shown in Table 2. The ingredients were then thoroughly mixed using a SPEEDMIXER DAC 400 FVZ high-shear mixer (Flack Tek, Inc., Landrum, SC, United States) at 2200 rpm for 2 to 5 minutes. The mixture was heated to 80°C with stirring and held for 60 minutes. The mixture was then cooled to ambient temperature.

[0094] Step 2: Predefined amounts (in grams) of Ca(NO3)2·4H2O and K54 were added to the mixture and mixed at 2200 rpm for 3-5 minutes, as shown in Table 2. Oligomeric tougheners were added only if they contained amine groups, such as THF1700.

[0095] Step 3: A predefined amount of TS720 (in grams) was added as shown in Table 2, and the mixture was mixed at 2200 rpm for 4 minutes. The walls of the mixing bowl were scraped to ensure uniform mixing.

[0096] Step 4: A predefined amount of any remaining ingredients (in grams) was added and the mixture was mixed again for 2 to 5 minutes.

[0097] [Table 2]

[0098] Preparation of Agent B Step 1: Predefined amounts (in grams) of EPON828, MX257, MY721, and other epoxy resins, as listed in Table 3, were mixed in a SPEEDMIXER DAC 400 FVZ high-shear mixer at 2200 rpm for 2 to 5 minutes until the components were thoroughly mixed. An oligomeric toughener was added to the mixture. If it contained epoxy groups, the toughener was added directly to the mixture. If it contained amine groups, such as THF1700, the mixture was heated to 80°C and held at that temperature for 60 minutes with continuous mixing. The mixture was then cooled to ambient temperature.

[0099] Step 2: Predefined amounts of TS720 and the remaining ingredients (in grams) were added as shown in Table 3, and the mixture was mixed at 2200 rpm for 4 minutes. The walls of the mixing bowl were scraped to ensure uniform mixing.

[0100] [Table 3]

[0101] Mixing Agent A and Agent B The components of Part A and Part B were mixed together in the weight ratios shown in Table 4 using a SPEEDMIXER DAC 400 FVZ high shear mixer at 2200 rpm for 30 seconds to 1 minute until the components were thoroughly mixed.

[0102] [Table 4]

[0103] test The OLS and FRP tests were carried out under specific conditions, and the test conditions and results are shown in Table 5.

[0104] Comparative example 1 (CE1) Samples of LOCTITE EA 9394 AERO adhesive obtained from Henkel Corporation of Dusseldorf, Germany were subjected to OLS and FRP testing. The results are presented in Table 5.

[0105] [Table 5]

[0106] All references, patent documents, and patent applications cited in the above patent application are incorporated herein by reference in their entirety for consistency. In the event of any inconsistency or contradiction between the incorporated reference portions and this application, the information in the foregoing description shall prevail. The foregoing description is intended to enable one skilled in the art to practice the disclosure as set forth in the claims, and should not be construed as limiting the scope of the present disclosure, which is defined by the claims and all equivalents thereof.

Claims

1. A two-component curable adhesive, a first part comprising an epoxy resin and a first oligomeric toughening agent; a second part comprising a curing agent that is reactive with the epoxy resin at ambient conditions, and a second oligomeric toughening agent; the first and second oligomeric toughening agents are immiscible with each other at ambient conditions; A two-part curable adhesive wherein the first oligomeric toughener is reactive with the epoxy resin to provide an epoxy adduct.

2. The two-part curable adhesive of claim 1 , wherein the first oligomeric toughening agent is reactive with the curing agent.

3. The two-part curable adhesive of claim 1 , wherein the second oligomeric toughener is reactive with the epoxy resin.

4. A two-component curable adhesive, a first part comprising an epoxy resin and a first oligomeric toughening agent; a second part comprising a curing agent that is reactive with the epoxy resin at ambient conditions, and a second oligomeric toughening agent; the first and second oligomeric toughening agents are immiscible with each other at ambient conditions; A two-part curable adhesive wherein the second oligomeric toughener is reactive with the curing agent to form a curing agent adduct.

5. 10. The two-part curable adhesive of claim 1, wherein either the first or second oligomeric toughener comprises a polyetheramine, and optionally a branched polyetheramine.

6. 6. The two-part curable adhesive of claim 5, wherein the first oligomeric toughener comprises a polyetheramine-epoxy adduct, and optionally a branched polyetheramine-epoxy adduct.

7. 6. The two-part curable adhesive of claim 5, wherein the second oligomeric toughener comprises polytetrahydrofuranamine, and optionally branched polytetrahydrofuranamine.

8. 10. The two-part curable adhesive of claim 1, wherein either the first or second oligomeric toughener comprises a butadiene-acrylonitrile oligomer.

9. A method of bonding, comprising: mixing a first part and a second part of the two-part curable adhesive of claim 1 to provide a mixture, wherein the first and second oligomeric toughening agents form multiple non-uniform domains in the mixture; disposing the mixture on at least one substrate to be bonded; and clamping the at least one substrate until the two-part curable adhesive is functionally cured.

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