Joining of dissimilar materials using induction hardening

Abstract

A method is provided for joining substrates having different coefficients of thermal expansion using a thermosetting adhesive, which involves a pre-cure step using radio frequency energy followed by a thermal cure step.

Description

[Technical Field]

[0001] The present invention relates to a novel process for obtaining bonded structures using radio frequency energy. [Background technology]

[0002] The use of thermosetting adhesives for interfacial bonding presents various challenges, especially in the automotive industry, where joining plastic to plastic, metal to metal, or plastic to metal is common. When adhesives are heat-cured in an oven to join two dissimilar materials (those with different material properties), warping or distortion can occur, which can compromise the structural integrity of the joined parts. Distortion can occur due in part to a mismatch in the coefficient of thermal expansion (CTE) between the two materials being joined, or due to material degradation in one of the components, or due to the accumulation of thermal stresses in one of the components.

[0003] In the assembly of automotive chassis, a fully assembled or partially assembled chassis is typically subjected to a final oven heating step, such as an electrocoat, where the chassis coating is cured by heating to approximately 180°C. Subassemblies that make up part of the chassis are, of course, simply "in situ" and are therefore subjected to this higher-than-necessary temperature treatment. In many cases, the entire assembly process is designed to cure the adhesives in the subassemblies during this final heating step, thereby reducing overall cycle time and energy usage by replacing two or more heat-curing steps with one. When subassemblies contain substrates with different CTEs and are bonded by heat-curing an adhesive, this process can result in distortion due to differential expansion of the substrates during the heating step. This distortion can compromise the integrity of the adhesive bond and the subassembly.

[0004] WO 2019 / 104216 A1 discloses a method for curing epoxy adhesives using radio frequency energy. The advantage of RF curing is that it allows manufacturers to cure bonded assemblies at a relatively low cost, both in terms of capital investment and in terms of point of use, compared to traditional oven curing. Summary of the Invention [Problem to be solved by the invention]

[0005] What is needed is a process for curing thermosetting adhesives that will bond substrates with different CTEs in a way that avoids distortion of the final assembly. [Means for solving the problem]

[0006] In a first aspect, the present invention provides a method for joining two substrates, comprising: (1) pre-curing a thermosetting adhesive using radio frequency energy, the adhesive including at least one radio frequency susceptor, and the adhesive contacting a first substrate and a second substrate, the first substrate and the second substrate having different thermal expansion coefficients; (2) further curing the thermosetting adhesive using heat; The present invention provides a method comprising:

[0007] In a second aspect, the present invention provides a method for joining two substrates, comprising: (1) providing a first substrate and a second substrate; (2) applying a thermosetting adhesive between a first substrate and a second substrate, the adhesive comprising at least one radio frequency susceptor; (3) pre-curing the adhesive using radio frequency energy, wherein the first substrate and the second substrate have different linear thermal expansion coefficients; (4) further curing the thermosetting adhesive using heat; The present invention provides a method comprising:

[0008] In a third aspect, the present invention provides a bonded assembly comprising a first substrate and a second substrate bonded together, the first substrate and the second substrate having different linear thermal expansion coefficients, and a thermosetting adhesive between the first substrate and the second substrate, the adhesive comprising a radio frequency susceptor.

[0009] In a fourth aspect, the present invention provides a bonded assembly comprising a first substrate and a second substrate bonded together, the first substrate and the second substrate having different linear thermal expansion coefficients, and a thermosetting adhesive between the first substrate and the second substrate, the adhesive comprising a radio frequency susceptor, and the adhesive having been pre-cured to a degree of cure (α) of at least 0.4 using radio frequency energy.

[0010] In a fifth aspect, the present invention provides a method for joining two substrates, comprising: (1) providing an assembly including a first substrate and a second substrate and a thermosetting adhesive in contact with the first and second substrates, the substrates having different thermal expansion coefficients, the adhesive including at least one radio frequency susceptor, and the adhesive having been pre-cured to a degree of cure of at least 0.4 using radio frequency energy; (2) curing the thermosetting adhesive using heat; The present invention provides a method comprising:

[0011] In a sixth aspect, the present invention provides a method for manufacturing an assembly comprising one or more subassemblies, the method comprising: (1) providing a subassembly including a first substrate and a second substrate and a thermosetting adhesive in contact with the first and second substrates, the substrates having different thermal expansion coefficients, the adhesive including at least one radio frequency susceptor, and the adhesive being pre-cured using radio frequency energy; (2) assembling the subassemblies into an assembly; (3) curing the thermosetting adhesive using heat; The present invention provides a method comprising:

[0012] In a seventh aspect, the present invention provides a method for manufacturing an assembly comprising one or more subassemblies, the method comprising: (1) providing a subassembly including a first substrate and a second substrate and a thermosetting adhesive in contact with the first and second substrates, the substrates having different thermal expansion coefficients, the adhesive including at least one radio frequency susceptor, and the adhesive being pre-cured to a degree of cure of at least 0.4 using radio frequency energy; (2) assembling the subassemblies into an assembly; (3) curing the thermosetting adhesive using heat; The present invention provides a method comprising:

[0013] In an eighth aspect, the present invention provides a method for manufacturing an assembly comprising one or more subassemblies, the method comprising: (1) providing a subassembly including a first substrate and a second substrate and a thermosetting adhesive in contact with the first and second substrates, the substrates having different thermal expansion coefficients, the adhesive including at least one radio frequency susceptor, and the adhesive being pre-cured using radio frequency energy; (2) assembling the subassemblies into an assembly; (3) subjecting the assembly including the subassembly to a heat curing step; The present invention provides a method comprising:

[0014] In a ninth aspect, the present invention provides a method for manufacturing an assembly comprising one or more subassemblies, the method comprising: (1) providing a subassembly including a first substrate and a second substrate and a thermosetting adhesive in contact with the first and second substrates, the substrates having different thermal expansion coefficients, the adhesive including at least one radio frequency susceptor, and the adhesive being pre-cured to a degree of cure of at least 0.4 using radio frequency energy; (2) assembling the subassemblies into an assembly; (3) subjecting the assembly including the subassembly to a heat curing step; The present invention provides a method comprising: [Brief explanation of the drawings]

[0015] [Figure 1] 1 shows an example of a configuration for RF curing an adhesive. [Figure 2] 1 shows the setup used to cure adhesives for lap shear testing. [Figure 3] 1 shows the configuration used to cure adhesives according to the method of the present invention. [Figure 4] 1 shows a setup for performing a peel test. [Figure 5] a) Conductivity (S / m) after curing of adhesives containing carbon black (CB) at different carbon black concentrations, b) Heating rate (°C / s) of cured (solid bars) and uncured (hatched bars) adhesives containing carbon black at different carbon black concentrations. [Figure 6] a) Temperature profile for assembling aluminum-adhesive-steel by RF curing. The solid line is the temperature of the adhesive, the dashed line is the temperature of the steel, and the dotted line is the temperature of the aluminum. The stars represent tuning of the RF field during heating; b) Temperature profile for assembling aluminum-adhesive-steel by oven curing. [Figure 7] a) Deflection of the aluminum plate in aluminum-adhesive-steel assemblies using oven-cured carbon black (CB)-free adhesive (triangles), oven-cured adhesive containing 10 wt% CB (circles), RF pre-cured and oven-cured adhesive containing 10 wt% CB (squares), and RF cured adhesive containing 10 wt% CB (diamonds). b) MMB test specimen with corresponding position markers (mm) at which the deflection of the aluminum plate (top bar) is measured. [Figure 8] a) Energy (J) required to propagate a crack in peel tests of MMB specimens (aluminum-steel) cured by various curing methods; b) Average load / width (N / mm) to peel steel-steel specimens bonded with adhesives cured by various curing methods. DETAILED DESCRIPTION OF THE INVENTION

[0016] Disclosed herein is a novel process for obtaining structures comprising the assembly of substrates with significantly different coefficients of thermal expansion (CTE), in which a composite thermosetting adhesive containing a radio frequency (RF) sensitive filler is placed between two substrates and cured by RF electromagnetic energy followed by oven curing.

[0017] An automobile chassis is composed of many bonded subassemblies. These subassemblies are assembled into a chassis, and in the final stage, the chassis is subjected to an electrocoating process and oven curing, typically at approximately 180°C. It is common to simultaneously cure the adhesives of the various subassemblies using the electrocoating heat cure step. Subassemblies in which substrates with different CTEs are bonded together may be prone to distortion due to differential expansion of the substrates during exposure to the high temperatures used in the electrocoating heat cure step. Traditionally, this has been addressed by fastening the subassemblies using fastening means other than adhesive. Adding such a fastening step adds time to the overall cycle, as well as material and extra weight in the form of the fastening means. The inventors have discovered that distortion can be significantly reduced by subjecting such subassemblies to a radio frequency pre-cure before assembling them into a chassis and subjecting them to a heat cure step, such as the electrocoating heat cure step. RF pre-cure secures the substrates in a stress-free configuration, thereby reducing distortion during the heat cure step, and upon cooling, the subassemblies are more likely to return to their starting state with minimal stress.

[0018] glue The method of the present invention uses a thermosetting adhesive. A thermosetting adhesive is a polymeric resin that can be cured using heat and / or heat and pressure. The adhesive undergoes a chemical reaction upon curing, resulting in the formation of a structure with superior strength and environmental resistance. The present invention can be used with any thermosetting or heat-accelerated adhesive system, including, but not limited to, both one-part and two-part adhesive systems. Exemplary thermosetting adhesives used herein include, but are not limited to, epoxy-based thermosetting adhesives, urethane-based thermosetting adhesives, (meth)acrylic-based thermosetting adhesives, various thermoplastic hot melt adhesives, or mixtures thereof. One-part adhesives are particularly suitable for the method of the present invention.

[0019] Epoxy-based adhesives are preferred. Epoxy resins useful in the adhesive composition of the present invention include various curable epoxy compounds and combinations thereof. Useful epoxy resins include liquids, solids, and mixtures thereof. Typically, the epoxy compound is an epoxy resin, also known as a polyepoxide. The polyepoxides useful herein can be monomers (e.g., diglycidyl ether of bisphenol A, diglycidyl ether of bisphenol F, diglycidyl ether of tetrabromobisphenol A, novolac-based epoxy resins, and trifunctional epoxy resins), higher molecular weight resins (e.g., diglycidyl ether of bisphenol A chain-extended with bisphenol A), or unsaturated monoepoxides (e.g., glycidyl acrylate, glycidyl methacrylate, allyl glycidyl ether, etc.) polymerized into homopolymers or copolymers. Most desirably, the epoxy compound contains an average of at least one pendant or terminal 1,2-epoxy group (i.e., vicinal epoxy groups) per molecule. Solid epoxy resins that can be used in the present invention can preferably include, or preferably be based on, bisphenol A. Some preferred epoxy resins include, for example, DER330, DER331, and DER671, all of which are commercially available from Dow Chemical Company.

[0020] Suitable epoxy resins include polyhydric phenolic compounds such as resorcinol, catechol, hydroquinone, bisphenol, bisphenol A, bisphenol AP (1,1-bis(4-hydroxyphenyl)-1-phenylethane), bisphenol F, bisphenol K, bisphenol M, diglycidyl ethers of tetramethylbisphenol, diglycidyl ethers of aliphatic glycols and polyether glycols, e.g., C 2~24Examples of suitable adhesives include diglycidyl ethers of alkylene glycols and poly(ethylene oxide) or poly(propylene oxide) glycols; phenol-formaldehyde novolac resins, alkyl-substituted phenol-formaldehyde resins (epoxy novolac resins), phenol-hydroxybenzaldehyde resins, cresol-hydroxybenzaldehyde resins, dicyclopentadiene-phenolic resins, and polyglycidyl ethers of dicyclopentadiene-substituted phenolic resins, as well as any combination thereof. Bisphenol A-based epoxy adhesives are more preferred. In a particularly preferred embodiment, the adhesive comprises bisphenol A-based epoxy resin, diglycidyl ether of polypropylene oxide, and glycidylpropyltrimethoxysilane.

[0021] Other suitable additional epoxy resins are cycloaliphatic epoxides, which have the following structure I: [ka] (wherein R is an aliphatic group, a cycloaliphatic group, and / or an aromatic group, and n is a number from 1 to 10, preferably from 2 to 4), the cycloaliphatic epoxide comprises a saturated carbocyclic ring with epoxy oxygens bonded to two adjacent atoms within the carbocyclic ring, as exemplified by the formula: (wherein R is an aliphatic group, a cycloaliphatic group, and / or an aromatic group, and n is a number from 1 to 10, preferably from 2 to 4). When n is 1, the cycloaliphatic epoxide is a monoepoxide. When n is 2 or greater, a diepoxy or epoxy resin is formed. Mixtures of monoepoxy, diepoxy, and / or epoxy resins can be used. The cycloaliphatic epoxy resins described in U.S. Pat. No. 3,686,359 can be used in the present invention. Particularly noteworthy cycloaliphatic epoxy resins are (3,4-epoxycyclohexyl-methyl)-3,4-epoxycyclohexanecarboxylate, bis-(3,4-epoxycyclohexyl)adipate, vinylcyclohexene monoxide, and mixtures thereof.

[0022] The epoxy resin is preferably a bisphenol-type epoxy resin or a mixture thereof, with the amount of other types of epoxy resin being 10 weight percent or less. Preferably, the bisphenol-type epoxy resin is a liquid epoxy resin or a mixture in which a solid epoxy resin is dispersed in a liquid epoxy resin. The most preferred epoxy resins are bisphenol-A and bisphenol-F epoxy resins. A particularly preferred epoxy resin is a mixture of a diglycidyl ether of at least one polyhydric phenol, preferably bisphenol A or bisphenol F, having an epoxy equivalent weight of 170 to 299, especially 170 to 225, and at least one diglycidyl ether of a second polyhydric phenol, also preferably bisphenol A or bisphenol F, having an epoxy equivalent weight of at least 300, preferably 310 to 600. The ratio of these two resins is such that the mixture of the two resins preferably has an average epoxy equivalent weight of 225 to 400. This mixture can optionally contain up to 20%, preferably up to 10%, of one or more other epoxy resins.

[0023] Examples of suitable epoxy adhesives include: Bisphenol A liquid epoxy resin (DER331) or bisphenol F liquid epoxy resin (DER 354), Bisphenol A solid epoxy resins (DER 661, DER 663, DER 667, etc.), Diglycidyl ether of polypropylene oxide (DER 732), · Glycidylpropyltrimethoxysilane (Silquest A187).

[0024] The one-component adhesive will contain a latent curing agent. The curing agent, along with an optional catalyst, is selected so that the adhesive cures when heated to temperatures above 80°C, preferably at least 100°C, but cures very slowly, if at all, at room temperature (about 22°C) and temperatures up to at least 50°C. Suitable curing agents of this type include boron trichloride / amine complexes and boron trifluoride / amine complexes, dicyandiamide, melamine, diallylmelamine, guanamines such as acetoguanamine and benzoguanamine, aminotriazoles such as 3-amino-1,2,4-triazole, hydrazides such as adipic dihydrazide, stearic dihydrazide, isophthalic dihydrazide, and semicarbazide, and aromatic polyamines such as cyanoacetamide and diaminodiphenylsulfone. It is particularly preferred to use a curing agent selected from dicyandiamide, isophthalic dihydrazide, adipic dihydrazide, and 4,4'-diaminodiphenylsulfone. Dicyandiamide is particularly preferred.

[0025] Epoxy adhesive compositions will almost always contain a catalyst for curing the adhesive. Preferred epoxy catalysts include ureas such as p-chlorophenyl-N,N-dimethylurea (monuron), 3-phenyl-1,1-dimethylurea (fenuron), 3,4-dichlorophenyl-N,N-dimethylurea (diuron), N-(3-chloro-4-methylphenyl)-N',N'-dimethylurea (chlortoluron), tert-acryl- or alkyleneamines such as benzyldimethylamine and 2,4,6-tris(dimethylaminomethyl)phenol, piperidine or its derivatives, imidazole derivatives, and generally C1-C 12 Alkylene imidazoles or N-arylimidazoles, such as 2-ethyl-2-methyl-imidazole or N-butylimidazole, 6-caprolactam, and a preferred catalyst is 2,4,6-tris(dimethylaminomethyl)phenol (as described in EP 0 197 892) incorporated into a poly(p-vinylphenol) matrix.

[0026] The adhesive may further comprise one or more toughening agents. Preferred toughening agents are core-shell rubber toughening agents and copolymers having at least one block segment miscible or partially miscible with epoxy resins and at least one block segment immiscible with epoxy resins. Examples of block segments that are miscible in epoxy resins include polyethylene oxide blocks, polypropylene oxide blocks, poly(ethylene oxide-co-propylene oxide) blocks, and poly(ethylene oxide-ran-propylene oxide) blocks, as well as mixtures thereof. Examples of block segments that are immiscible in epoxy resins include polyether blocks prepared from alkylene oxides containing at least four carbon atoms, preferably butylene oxide, hexylene oxide, and / or dodecylene oxide. Examples of block segments that exhibit immiscibility in epoxy resins include, inter alia, polyethylene oxide, polyethylene-propylene, polybutadiene, polyisoprene, polydimethylsiloxane, and polyalkylmethacrylate blocks, as well as mixtures thereof.

[0027] The toughening agent can be a phenolic end-capped polyurethane toughening agent. In one embodiment, the polyurethane-based toughening agent comprises a polyurethane polymer that is a reaction mixture of a polyol and an aliphatic diisocyanate, such as 1,6-hexane diisocyanate or isophorone diisocyanate. Preferably, the polyurethane-based toughening agent according to the present invention comprises either end groups reactive with an epoxy curing agent or end groups that are removed to make the isocyanate group available for reaction with the epoxy curing agent. Examples of diisocyanates that can be used in the preparation of polyurethane polymers include aromatic diisocyanates, toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI), aliphatic and cycloaliphatic isocyanates, such as 1,6-hexamethylene diisocyanate (HDI), 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane (isophorone diisocyanate, IPDI), and 4,4'-diisocyanatodicyclohexylmethane (HDI). 12 The polyol component can include polyether polyols produced by the reaction of an epoxide with a starting compound containing active hydrogen or polyester polyols produced by the polycondensation of a polyfunctional carboxylic acid with a hydroxyl compound. In one embodiment, the isocyanate groups of the polyurethane-based toughening agent can be capped or blocked with end groups such as phenolic compounds, aminophenolic compounds, carboxylic acid groups, or hydroxyl groups. Preferred end-capping groups include phenolic compounds such as bisphenol-A, diallyl bisphenol-A, cardanol, and diisopropylamine.

[0028] Some examples of toughening agents are: Polyurethane prepolymer derived from PTMEG and HDI and end-capped with bisphenol A, Polyurethane prepolymer derived from PTMEG and HDI and end-capped with diisopropylamine, Epoxy end-capped carboxyl-terminated butyronitrile rubber (CTBN), Polyurethane prepolymer derived from PTMEG, HDI, and polybutadiene and end-capped with cardanol is.

[0029] A particularly preferred thermosetting adhesive is an epoxy adhesive derived from PTMEG, HDI, and toughened with a bisphenol A end-capped polyurethane prepolymer.

[0030] In addition to at least one susceptor, fillers, rheology modifiers, and / or pigments may be present in epoxy adhesive compositions. These may serve several functions, including (1) modifying the rheology of the epoxy adhesive to a desired extent, (2) reducing overall cost, (3) absorbing moisture or oil from the epoxy adhesive or the substrate to which it is applied, and / or (4) promoting cohesive rather than adhesive failure. Examples of such materials include calcium carbonate, calcium oxide, talc, coal tar, carbon black, textile fibers, glass particles or fibers, aramid pulp, boron fiber, carbon fiber, inorganic silicates, mica, quartz powder, hydrated aluminum oxide, bentonite, wollastonite, kaolin, fumed silica, silica aerogel, or metal powders such as aluminum powder or iron powder. Among these, calcium carbonate, talc, calcium oxide, fumed silica, and wollastonite, used alone or in some combination, are often preferred because they promote the desired cohesive failure mode. The epoxy adhesive composition may further include other additives such as diluents, plasticizers, extenders, pigments and dyes, flame retardants, thixotropic agents, flow control agents, thickeners such as thermoplastic polyesters, gelling agents such as polyvinyl butyral, adhesion promoters, antioxidants, etc.

[0031] High Frequency Susceptor The method of the present invention involves using an adhesive that includes at least one radio frequency (RF) susceptor. An RF susceptor is any substance that can absorb radio frequency energy and convert it into heat. In principle, any material that exhibits this property can be used, provided that it can be incorporated into the adhesive without compromising the final adhesive strength. Examples include: 1. Carbon materials such as carbon black, carbon fiber, graphene, carbon nanofiber, carbon nanotube, and any mixture thereof; 2. Metals such as metal flakes, fibers, filaments, powders; 3. Polymer dielectric materials such as polycaprolactone (PCL) Examples include:

[0032] Particularly preferred are carbon materials selected from carbon black, carbon fibers and carbon nanotubes, and mixtures thereof.

[0033] The shape and size of the RF-sensitive filler used herein are not limited. For example, the RF-sensitive filler may be spherical, plate-like, tubular, or irregular. Alternatively, the RF-sensitive filler may be spherical with an average diameter ranging from about 5 nm to about 500 nm, plate-like with an average thickness ranging from about 0.5 nm to about 2 nm and an average diameter ranging from about 2 nm to about 1 μm, or tubular with a length ranging from about 1 nm to about 1 mm.

[0034] The susceptor is preferably present in the adhesive at 0.1 to 35 wt%, more preferably 1 to 30 wt%, 2 to 25 wt%, and particularly preferably 7.5 to 12.5 wt%. In a preferred embodiment, the susceptor is present at 10 wt%.

[0035] In a preferred embodiment, the susceptor is carbon black. Preferably, the carbon black is present in an amount of 5 to 20 wt %, more preferably 5 to 15 wt %, and especially preferably 7.5 to 12.5 wt %.

[0036] In another preferred embodiment, the susceptor is carbon nanotubes, which are preferably present in an amount of 5 to 20 wt %, more preferably 5 to 15 wt %, and particularly preferably 7.5 to 12.5 wt %.

[0037] The susceptor is incorporated into the thermosetting adhesive by pre-curing or mixing prior to curing.

[0038] Base material: The present invention involves bonding two substrates together, where the two substrates have different thermal masses or the substrates have different coefficients of thermal expansion (CTE). Different CTE means that the CTE of the materials is greater than or equal to 5×10 -6 m / (m-℃) or more, more preferably 8×10 -6 This means that the difference (ΔCTE) is more than m / (m-℃).

[0039] Some examples of pairs of materials that can be bonded include, but are not limited to:

[0040] [Table 1]

[0041] Particularly common substrates in automotive applications are metal-metal, plastic-metal, and plastic-plastic. More specific examples include aluminum-steel [ΔCTE=11×10 -6 m / (m-℃)], plate glass-aluminum [ΔCTE=13×10 -6 m / (m-℃)], nylon glass fiber reinforced steel [ΔCTE=12×10 -6 m / (m-℃)], magnesium-steel [ΔCTE=15×10 -6 m / (m-℃)], magnesium-plate glass [ΔCTE = 17 × 10 -6 m / (m-°C)], with aluminum-steel being particularly preferred.

[0042] The substrates may be surface treated prior to adhesive bonding. For example, suitable surface treatments for plastic materials include, but are not limited to, chemical, mechanical, or high-energy surface treatments. Suitable surface treatments for metals used herein include, but are not limited to, galvanization, passivation or conversion coatings, powder coating, etc.

[0043] High frequency pre-hardening The method of the present invention includes a radio frequency pre-curing step. A preferred RF frequency is typically about 30 kHz to about 300 GHz, more preferably 100 to 250 MHz, and particularly preferably 140 MHz. The optimal frequency for conducting the experiments was determined for each configuration. This optimal frequency depends on the shape of the test specimen and the properties of the adhesive.

[0044] The power level of the RF energy is typically in the range of 50-300W, particularly 100 or 200W.

[0045] The method of applying RF energy is not particularly limited. A typical configuration is shown in Figure 3. The RF electromagnetic field can be generated by any suitable applicator design, such as direct contact, non-contact parallel plate, or non-contact fringing field. When using a non-contact parallel plate applicator, an assembly is placed between the two parallel plates of the applicator and connected to an RF source, generating an electromagnetic wave between the parallel plates of the applicator. This type of non-contact parallel plate applicator is suitable for joining structures in which one or both of the parts to be joined are made of a non-conductive plastic material. In a non-contact fringing field applicator, the two strips of the applicator are placed horizontally on the same plane on a non-conductive support block (e.g., Teflon sheet) and connected to an RF source. This generates an electromagnetic field between the strips, and a weaker fringing electromagnetic field in the out-of-plane space directly above the strips. When using this type of non-contact fringing field applicator, the assembly is placed above the two strips of the applicator within the range where a fringing electromagnetic field would be generated when the RF source is connected. When the two substrates to be joined are conductive (e.g., metal substrates), a direct contact applicator is most suitable. In this configuration, the two conductive joining components themselves serve as electrodes of a capacitor connected to an RF source. If one of the joining components is made of a conductive material and the other is made of a non-conductive material, the application device is designed so that only the non-conductive component is positioned within the fringing electromagnetic field.

[0046] A typical RF setup consists of a power supply to generate RF energy, a control unit to manipulate the RF power, an autotuner to minimize reflected power, and an assembly containing the RF-sensitive adhesive. A typical setup using two conductive substrates is shown in Figure 1. The optimal frequency for conducting the experiments was determined for each setup. This optimal frequency depends on the specimen geometry and adhesive properties. Reflected power can be reduced by using low RF power (5 W to 20 W) at the optimal frequency and using an autotuner. This is accomplished by an automatic matching circuit (autotuner) that uses lumped elements (capacitors and impedances) to match the connected load. The power can then be increased to reach the desired adhesive temperature.

[0047] Pre-curing is preferably carried out until the adhesive has a degree of cure, α, of at least 0.4. The degree of cure can be assessed by differential scanning calorimetry (DSC) of the adhesive. The absorbed heat versus temperature data is used to determine the enthalpy change, ΔH, of the curing reaction. The degree of cure is then calculated using the following equation:

number

[0048] The pre-cure allows the adhesive between the two substrates to be fixed in a relatively strain-free configuration (due to the strain reduction that can be achieved by RF curing), so that during the subsequent thermal cure, the partially cured adhesive acts to prevent the substrates from fully distorting upon thermal curing, which also means that after the substrates cool, internal forces in the adhesive will substantially restore the strain-free configuration.

[0049] During the RF pre-cure step, an autotuner can be used to determine the optimal frequency depending on the geometry and adhesive properties, and the power intensity can be adjusted to achieve the desired pre-cure temperature. Pre-cure is preferably carried out until the adhesive reaches a cure degree of at least 0.4, more preferably 0.5, 0.6, 0.7, 0.9, or 0.9. Pre-cure reduces the time required for the final heat-cure step and also stabilizes the substrate by gelling the adhesive, reducing distortion during the final heat-cure step.

[0050] The pre-cured assembly may be heat cured immediately after RF pre-curing or may be stored for later heat curing. The present invention extends to such pre-cured assemblies and to methods involving a single step of heat curing a previously RF pre-cured assembly.

[0051] heat curing The RF pre-cure step is followed by a heat cure step, which can be carried out by any heating method, including but not limited to convection heating, forced air heating, and infrared heating, and can be carried out, for example, in an oven.

[0052] The heat curing step involves exposing the pre-cured assembly to, for example, a forced air oven.

[0053] Heat curing may also occur in an assembly that is part of a larger assembly as a result of the larger assembly being exposed to heat to cause other changes, such as in an electrocoat coating. For example, an RF pre-cured assembly according to the present invention may be attached to a larger assembly (such as an automobile chassis), and heat curing may occur when the larger assembly is exposed to heat to cure or paint other components within the larger assembly.

[0054] Heating is typically performed at about 130°C to 220°C, more preferably 140°C to 180°C. The heat-curing time is selected to ensure complete cure. Heat-curing is usually performed for about 5 to 60 minutes, more preferably 10 to 30 minutes. A shorter heat-curing time is preferred, as longer heat-curing times increase the tendency for the assembly to warp.

[0055] Heat curing is typically carried out until the adhesive has a degree of cure of 1.

[0056] Examples of Preferred Embodiments Below are some examples of how the method or assembly of the present invention can be used.

[0057] 1. A method for joining two substrates to form a bonded assembly, comprising: (1) pre-curing a thermosetting adhesive using radio frequency energy, wherein the adhesive includes at least one radio frequency susceptor, and the adhesive is in contact with a first substrate and a second substrate, and the first substrate and the second substrate have different linear thermal expansion coefficients; (2) incorporating the bonded assembly into a larger assembly, such as an automobile chassis, door, hood, or hatch door closure, and simultaneously curing other components of the larger assembly, such as electrocoating, using heat to cure the thermosetting adhesive and to cure adhesives in other portions of the larger assembly; A method comprising:

[0058] 2. A method for joining two substrates, comprising: (1) pre-curing a thermosetting adhesive to a degree of cure of at least 0.4 using radio frequency energy, the adhesive including at least one radio frequency susceptor, and the adhesive contacting a first substrate and a second substrate, the first substrate and the second substrate having different linear thermal expansion coefficients; (2) further curing the thermosetting adhesive using heat; A method comprising:

[0059] 3. A method for joining two substrates, comprising: (1) pre-curing a thermosetting adhesive to a degree of cure of at least 0.4 using radio frequency energy, the adhesive including at least one radio frequency susceptor, and the adhesive contacting a first substrate and a second substrate, the first substrate and the second substrate having different linear thermal expansion coefficients; (2) heating the assembly to 130 to 220°C, more preferably 150 to 190°C, to further harden the thermosetting adhesive; A method comprising:

[0060] 4. A method for joining two substrates, comprising: (1) pre-curing a thermosetting adhesive to a degree of cure of at least 0.4 using radio frequency energy, the adhesive including at least one radio frequency susceptor, and the adhesive contacting a first substrate and a second substrate, the first substrate and the second substrate having different linear thermal expansion coefficients; (2) further curing the thermosetting adhesive by heating the assembly to 130 to 220°C, more preferably 150 to 190°C, for 5 to 60 minutes, more preferably 10 to 30 minutes; A method comprising:

[0061] 5. A method for joining two substrates, comprising: (1) pre-curing a thermosetting adhesive to a degree of cure of at least 0.4 using radio frequency energy, the adhesive including at least one radio frequency susceptor, and the adhesive contacting a first substrate and a second substrate, the first substrate and the second substrate having different linear thermal expansion coefficients; (2) further curing the thermosetting adhesive using heat in a convection oven, infrared oven, or forced air oven; A method comprising:

[0062] 6. A method for joining two substrates to form a bonded assembly, comprising: (1) pre-curing a thermosetting adhesive using high frequency energy, wherein the adhesive includes at least one high frequency susceptor, and the adhesive contacts a first substrate and a second substrate, and the first substrate and the second substrate are 5×10 -6 and a step having a thermal expansion coefficient different by m / (m-°C) or more; (2) incorporating the bonded assembly into a larger assembly such as an automobile chassis, body structure, closure, etc., and curing the thermosetting adhesive while simultaneously curing other components of the larger assembly, such as electrocoating, using heat to cure the adhesive in other portions of the larger assembly; A method comprising:

[0063] 7. A method for joining two substrates, comprising: (1) pre-curing a thermosetting adhesive to a degree of cure of at least 0.4 using radio frequency energy, wherein the adhesive comprises at least one radio frequency susceptor, and the adhesive is in contact with a first substrate and a second substrate, and the first substrate and the second substrate are in contact with at least one radio frequency susceptor. -6 and a step having a thermal expansion coefficient different by m / (m-°C) or more; (2) further curing the thermosetting adhesive using heat; A method comprising:

[0064] 8. A method for joining two substrates, comprising: (1) pre-curing a thermosetting adhesive to a degree of cure of at least 0.4 using radio frequency energy, wherein the adhesive comprises at least one radio frequency susceptor, and the adhesive is in contact with a first substrate and a second substrate, and the first substrate and the second substrate are in contact with at least one radio frequency susceptor. -6 and a step having a thermal expansion coefficient different by m / (m-°C) or more; (2) heating the assembly to 130 to 220°C, more preferably 150 to 190°C, to further harden the thermosetting adhesive; A method comprising:

[0065] 9. A method for joining two substrates, comprising: (1) pre-curing a thermosetting adhesive to a degree of cure of at least 0.4 using radio frequency energy, wherein the adhesive comprises at least one radio frequency susceptor, and the adhesive is in contact with a first substrate and a second substrate, and the first substrate and the second substrate are in contact with at least one radio frequency susceptor. -6 and a step having a thermal expansion coefficient different by m / (m-°C) or more; (2) further curing the thermosetting adhesive by heating the assembly to 130 to 220°C, more preferably 150 to 190°C, for 5 to 60 minutes, more preferably 10 to 30 minutes; A method comprising:

[0066] 10. A method for joining two substrates, comprising: (1) pre-curing a thermosetting adhesive to a degree of cure of at least 0.4 using radio frequency energy, wherein the adhesive comprises at least one radio frequency susceptor, and the adhesive is in contact with a first substrate and a second substrate, and the first substrate and the second substrate are in contact with at least one radio frequency susceptor. -6 and a step having a thermal expansion coefficient different by m / (m-°C) or more; (2) further curing the thermosetting adhesive using heat in a convection oven, infrared oven, or forced air oven; A method comprising:

[0067] 11. A method for joining two substrates, comprising: (1) pre-curing a thermosetting adhesive using high frequency energy, wherein the adhesive includes at least one high frequency susceptor, and the adhesive contacts a first substrate and a second substrate, and the first substrate and the second substrate are 5×10 -6 and a step having a thermal expansion coefficient different by m / (m-°C) or more; (2) further curing the thermosetting adhesive using heat; A method comprising:

[0068] 12. A method for joining two substrates, comprising: (1) providing a first substrate and a second substrate; (2) applying a thermosetting adhesive between the first substrate and the second substrate; (3) Pre-curing the adhesive using high frequency energy, wherein the first substrate and the second substrate are -6 m / (m-°C) or more different thermal expansion coefficients, and the adhesive includes a high frequency susceptor; (4) further curing the thermosetting adhesive using heat; A method comprising:

[0069] 13. A first substrate and a second substrate joined together, each of which is 5×10 -6 A bonded assembly comprising a first substrate and a second substrate having thermal expansion coefficients that differ by at least m / (m-°C), and a thermosetting adhesive between the first substrate and the second substrate, wherein the adhesive comprises a radio frequency susceptor.

[0070] 14. A first substrate and a second substrate joined together, each of which is 5×10 -6 A bonded assembly comprising a first substrate and a second substrate having thermal expansion coefficients that differ by m / (m-°C) or more, and a thermosetting adhesive between the first substrate and the second substrate, wherein the adhesive comprises a radio frequency susceptor, and the adhesive has been pre-cured to a degree of cure of at least 0.4 using radio frequency energy.

[0071] 15. A method for joining two substrates, comprising: (1) An assembly including a first substrate and a second substrate, and a thermosetting adhesive in contact with the first and second substrates, wherein the substrates are 5×10 -6 providing an assembly having thermal expansion coefficients that differ by more than m / (m-°C), the adhesive including at least one radio frequency susceptor, and the adhesive having been pre-cured to a degree of cure of at least 0.4 using radio frequency energy; (2) curing the thermosetting adhesive using heat; A method comprising:

[0072] 16. A method for manufacturing an assembly (automotive chassis) comprising one or more subassemblies, (1) providing a subassembly including a first substrate and a second substrate and a thermosetting adhesive in contact with the first and second substrates, the substrates having different thermal expansion coefficients, the adhesive including at least one radio frequency susceptor, and the adhesive being pre-cured to a degree of cure of at least 0.4 using radio frequency energy; (2) assembling the subassemblies into an assembly; (3) curing the thermosetting adhesive using heat, for example during an electrocoating process that includes a heat curing step; A method comprising:

[0073] 17. The embodiment of any one of embodiments 1-16, wherein the first substrate is aluminum and the second substrate is steel.

[0074] 18. The embodiment of any one of embodiments 1-16, wherein the first substrate and the second substrate are selected from the following pairs: aluminum-steel, aluminum-magnesium, aluminum-reinforced plastic (carbon fiber reinforced epoxy, carbon fiber reinforced polyamide, etc.). [Example]

[0075] material glue The adhesive used was an epoxy adhesive having the following components: Epoxy resin: Bisphenol A liquid epoxy resin (DER 331) Bisphenol A solid epoxy resin (DER 66X, etc.) Diglycidyl ether of polypropylene oxide (DER 732) Glycidylpropyltrimethoxysilane (Silquest A187) toughening agent RAM F: Polyurethane prepolymer derived from PTMEG and HDI, end-capped with bisphenol A RAM DIPA: A polyurethane prepolymer derived from PTMEG and HDI and end-capped with diisopropylamine. Epoxy end-capped carboxy-terminated butyronitrile rubber (CTBN) Dicyandiamide (Dicy) Epoxy Catalyst calcium oxide talc: Al(OH)3 PDMS-treated fumed silica

[0076] Carbon susceptors were added to the above adhesives, and the mixing procedures were different for carbon nanotubes (CNTs) and carbon black.

[0077] The compositions of the comparative and experimental compositions are shown in Table 2.

[0078] [Table 2]

[0079] Five different concentrations of carbon black (5.0, 7.5, 10.0, 12.5, and 15.0 wt%) were dispersed at the indicated weight concentrations in the adhesive by high speed stirring at 1000 rpm under vacuum (<27 mmHg).

[0080] CNTs were mixed into the adhesive using a solution mixing process. The desired weight of multi-walled CNTs (Cheaptubes, USA) was mixed with 5 g of acetone to achieve a CNT content of 0.1–15 wt% in 50 g of adhesive. The CNT-acetone solution was treated in an ultrasonic bath for 5 minutes and then added to 50 g of adhesive. This composition was first mixed using a Thinky mixer for 2 hours, and then further mixed using a magnetic stirrer at 100 rpm at 40–50 °C until the acetone evaporated (approximately 24 hours).

[0081] Three curing methods were investigated for curing adhesives used to bond metal substrates. These were: (a) a 30-minute oven cure, (b) a 5-minute RF precure followed by a 25-minute oven postcure, and (c) a 30-minute RF cure. The oven cure and RF field cure are described in detail below.

[0082] Oven-Cure Specimens: The oven-cure step involves placing either the uncured or partially cured assembly in a forced-air oven and preheating to 160°C for up to 30 minutes.

[0083] RF curing: The RF setup consists of a power supply that generates RF energy, a control device for manipulating the RF power, an autotuner to minimize reflected power, and an assembly containing the RF-sensitive adhesive. A typical setup is shown in Figure 1. The optimal frequency for conducting the experiments was determined for each setup. This optimal frequency depends on the geometry of the test specimen and the properties of the adhesive. Then, low RF power (5 W to 20 W) is used at the optimal frequency, and the autotuner is used to reduce reflected power. This is achieved by an automatic matching circuit (autotuner) that uses lumped elements (capacitors and impedances) to match the connected load. The power is then increased to reach the desired adhesive temperature.

[0084] Lap shear test: The adhesive bond strength was evaluated using a lap shear test.

[0085] Test specimens were prepared using two 1.5 mm thick steel substrates measuring 25.4 mm x 101.6 mm. After cleaning the steel substrates with acetone, the adhesive was spread over an area measuring 12.7 mm (lap length) x 25.4 mm (width). 0.5 mm diameter glass beads were sprinkled on the adhesive to ensure uniform spacing between the metal substrates. Test specimens were prepared using three different heating methods: (1) oven curing, (2) RF curing, and (3) RF pre-curing followed by oven post-curing. The configuration of the RF-cured lap shear specimens is shown in Figure 2. Lap shear tests were performed on an MTS tensile testing machine at a loading rate of 12.7 mm / min and a hydraulic chuck clamping pressure of 10 MPa.

[0086] RF curing and pre-curing for the lap shear test were carried out using the configuration shown in Figure 2.

[0087] Distortion and Peel Test The effects of three different heating methods were evaluated by conducting dissimilar material bonding (MMB) and strain tests to assess the impact of coefficient of thermal expansion (CTE) mismatch. A hollow rectangular steel channel, measuring 25.4 mm x 25.4 mm and 3 mm thick, was bonded to a 1 mm thick 6061 aluminum plate (see Figure 3). The steel channel was 250 mm long, and the aluminum plate had the same width and length as the steel channel. Adhesive was applied to one side of the steel channel, and 0.5 mm diameter glass beads were evenly sprinkled on top. The aluminum plate was then pressed against the adhesive, squeezing out excess adhesive and ensuring there were no visible gaps between the two metals. Three specimens were prepared for each curing method described in the previous section. A typical RF field-based setup for curing these specimens is shown in Figure 3. The gap between the steel and aluminum of the cured specimens was measured to evaluate the strain generated during the curing process.

[0088] To demonstrate the benefits of minimizing distortion with the method of the present invention, peel tests were conducted on test specimens. The setup for conducting the peel tests is shown in Figure 4. As shown in Figure 4, a fastener was inserted into one end of a steel channel and the aluminum was peeled off at a constant displacement rate of 127 mm / min. In these tests, failure began at a 90° peel and ended at a 180° peel, with the adhesive stress state transitioning from uniaxial to biaxial, where it is subjected to shear stress. When calculating the fracture energy, only the range of 25–90% of the total displacement was considered to eliminate edge effects from the calculation.

[0089] Microscopic observation The surface morphology of the fracture surface in the lap shear test was observed using a scanning electron microscope (SEM). The specimens were coated with 10 nm of iridium, and images were taken using an FEI SEM, Quanta 600.

[0090] result Metal-metal assemblies were fabricated by bonding metal substrates using an RF field to locally heat and cure the adhesive. The effect of different concentrations of carbon nanofillers in the adhesive on the heating rate upon exposure to the RF field was evaluated.

[0091] Lap shear specimens were used to evaluate the strength of the adhesive, while MMB specimens were used to evaluate the strain of composite assemblies bonded with different curing methods.

[0092] Adhesive Characterization The effect of carbon black concentration on heating rate was evaluated by measuring electrical properties and by directly assessing the RF field-induced heating response of the adhesive. The AC conductivity of cured adhesive films containing 5 to 15 wt% carbon black was measured, and then the heating response of uncured and cured adhesives containing various concentrations of carbon black was measured using a non-contact fringing field applicator.

[0093] The AC conductivity of cured adhesive films containing five different carbon black concentrations was measured, and the percolation threshold of carbon black in the adhesive was found to be between 12.5 and 15 wt% (Figure 5a).

[0094] The heating response of uncured and cured carbon black-containing adhesive films was measured using a fringing field applicator at 138 MHz and 10 W of power. As shown in Figure 5b, the heating rate was highest at 10 to 12.5 wt% carbon black. For the uncured adhesive, the heating rate plateaued at 10 wt%. For the cured adhesive, the highest heating rates were observed at 10 and 12.5 wt% carbon black. The highest heating rate was achieved with a 10 wt% carbon black loading. This optimal range was used for RF field-based bonding in subsequent experiments.

[0095] Lap shear test: Assemblies were made using three different cure methods: 1) a 30 minute oven cure, 2) a 5 minute RF partial cure followed by a 25 minute oven cure, and 3) a 30 minute RF cure. As a control, assemblies were made using the same adhesive without carbon black and cured in an oven for 30 minutes.

[0096] The RF power transfer is maximized when the impedance between the RF source and the system (applicator, cable, and test specimen) is matched. Impedance is a combination of resistance and reactance (capacitance and inductance) and is a function of frequency. The frequency at which the impedance is best matched, resulting in the fastest heating rate, is selected. In this case, as the heat generated by the RF field cures the epoxy, the impedance of the specimen also changes during cure. To address this issue, an automatic matching circuit, or autotuner, was added to the RF circuit. The autotuner contained lumped elements (capacitors and inductors) that automatically matched to minimize the energy reflected from the circuit, allowing maximum heating of the adhesive. The selected frequency was used to heat the assembly, and the autotuner was used to minimize the reflected energy during the curing process.

[0097] For mechanical testing of the lap shear specimens, additional tabs were attached to each end of the specimen to ensure pure shear on the adhesive during testing. The shear strength of the adhesive is shown in Table 3.

[0098] [Table 3]

[0099] The shear strength of the adhesive without carbon black was 32.6 MPa. Adhesives containing carbon black pre-cured at RF and then cured in an oven exhibit similar strengths, ranging from 33.2 to 36.5 MPa. The data in Table 3 show that the curing method has very little effect on adhesive strength.

[0100] Joining different materials Dissimilar material bonded (MMB) assemblies were fabricated by bonding aluminum plates to steel channels using a base adhesive containing 10 wt% carbon black (as shown in Figure 3). Three curing methods were evaluated: 1) a 30-minute oven cure, 2) a 5-minute RF partial cure followed by a 25-minute oven cure, and 3) a 30-minute RF cure. Additionally, a control assembly was fabricated using only the adhesive without carbon black. This control assembly was oven cured for 30 minutes.

[0101] A 5 minute RF pre-cure results in a cure degree of at least 0.4.

[0102] The specimens cured using an RF field used a similar setup to that described above for curing the lap shear specimens. The MMB experiments were conducted at 20 MHz, with 10 W power at t = 0 and an autotuner used to minimize the reflected power (Figure 6a). The input RF power was then increased to 100 W. Multiple tuning passes (marked with stars in Figure 6a) were performed, during which the power was reduced to 10 W and then increased back to 100 W after tuning. At t = 7 min, the adhesive reached 120 °C, while the aluminum and steel remained below 70 °C. In contrast, for the oven-cured specimens, the metal heated up before the adhesive, and the adhesive reached a temperature sufficient for curing only after the aluminum and steel also reached such high temperatures (approximately >120 °C). RF curing allows for rapid energy input to the adhesive without significantly heating the metal substrate, compared to oven-curing processes, which require longer times to heat the adhesive to the required temperature and also heat the substrate to high temperatures.

[0103] The distortion of the aluminum plate (due to CTE mismatch) was measured in the dissimilar material bonding experiment. Figure 7a shows the deflection of the aluminum plate under four different test conditions. Figure 7b shows the "0" position on the aluminum plate where the deflection measurements were performed. The results are listed in Table 4.

[0104] [Table 4]

[0105] The deflection observed for oven-cured specimens was the highest. With RF curing, either pre-cured or fully cured, the deflection observed was much less.

[0106] degree of hardening The relationship between degree of cure and strain in bonded assemblies was investigated as follows: Dissimilar material bonded (MMB) assemblies were prepared as described above using the base adhesive with 10 wt% carbon black added. After RF pre-curing, each assembly was cut perpendicular to the aluminum surface into two pieces. One piece was evaluated by differential scanning calorimetry (DSC) to determine the degree of cure (α), and the other piece was oven-cured at 160°C for 30 minutes and then evaluated for strain.

[0107] RF curing was performed at 200 W and 13 MHz with autotuning, and the target adhesive temperature was set to 160°C.

[0108] After dividing the assembly into two parts, one part was disassembled by peeling off the aluminum coupon and evaluated by DSC to determine the degree of cure of the adhesive. To perform the DSC experiment, the adhesive was equilibrated at 50°C and then heated at 10°C / min to 230°C. Heat flow versus temperature was measured and plotted on a graph with temperature on the x-axis and heat flow on the y-axis. The area under the curve can be used to determine ΔH (i.e., the energy released by the exothermic reaction of cure). The degree of cure, α, can then be calculated using the following equation:

number

[0109] The other part of the assembly was oven-cured at 160°C for 30 minutes and the distortion was evaluated as described above. The distortion of the aluminum plate of the assembly (due to the CTE mismatch) was measured. Figure 7b shows the location on the aluminum plate where the deflection was measured.

[0110] Table 5 shows the degree of cure for various lengths of RF pre-curing, as well as the deflection of the aluminum plate at position “0” shown in Figure 7b.

[0111] [Table 5]

[0112] As expected, the results in Table 5 show that a longer RF pre-cure increases the degree of cure. Additionally, when RF pre-cured samples are subsequently subjected to an oven cure step, the degree of cure affects the amount of distortion observed; the higher the degree of cure in the pre-cure, the less distortion there is in the final assembly.

[0113] Peel test Peel tests were performed on MMB specimens cured using the different methods described above. In all tests, the bonded aluminum plate was peeled from the channel steel, and the load versus displacement (elongation) was recorded and plotted on a graph with load on the Y-axis and elongation on the X-axis. The area under the curve represents the energy required to propagate the fracture. The results are shown in Table 6 and graphically in Figure 8a.

[0114] [Table 6]

[0115] The peel tests show that more energy is required to propagate failure in composite specimens cured using an RF field due to the lack of deflection and strain observed in the RF-cured specimens. The fracture energy was also calculated from the load vs. elongation graph, and showed an approximately 590% increase in energy for the RF-cured specimens compared to the oven-cured tests. This improvement is due to the relaxation of strain caused by the CTE mismatch during adhesive curing.

[0116] The peel resistance of adhesives cured using different curing methods was evaluated to determine whether differences in the force required to gradually separate two bonded flexible steel substrates (i.e., substrates with no difference in CTE) were observed. Note that these experiments did not alter the peel angle compared to previous peel tests performed on MMB specimens. Because two similar substrates (steel) were bonded in this experiment, there was no strain due to CTE mismatch compared to the previous MMB experiment. Therefore, this peel resistance test only measured the impact of the adhesive on peel resistance. Specimens containing 10 wt% CB were cured in an oven, an RF-oven, and RF-only, along with a base case where the adhesive alone was cured in an oven for 30 minutes.

[0117] Mechanical testing of the cured specimens was performed at a constant displacement rate, and load versus displacement data was recorded. The average load recorded from 25 mm to the end of the experiment was divided by the width of the specimen. The results are shown in Table 7 and graphically in Figure 8b.

[0118] [Table 7]

[0119] Although a slight increase in load per unit width was observed with the addition of CB to the adhesive, no significant difference was evident compared to the adhesive containing CB. The observed differences in peel resistance between CB adhesives cured by different methods were very small. This indicates that when materials with similar CTEs (i.e., steel-steel) are used, different curing methods do not significantly affect the peel strength of the adhesive. Therefore, the differences observed for materials with mismatched CTEs (i.e., steel-aluminum) are due to strain-induced degradation of the adhesive bond strength, rather than inherent differences in adhesive strength. The present invention includes the following aspects. [Aspect 1] 1. A method for joining two substrates, comprising: (1) pre-curing a thermosetting adhesive using radio frequency energy, the adhesive including at least one radio frequency susceptor, and the adhesive in adhesive contact with a first substrate and a second substrate, the first substrate and the second substrate having different thermal expansion coefficients; (2) subjecting the thermosetting adhesive to a heat treatment; A method comprising: [Aspect 2] 1. A method for joining two substrates, comprising: (1) providing a first substrate and a second substrate; (2) applying a thermosetting adhesive between the first substrate and the second substrate, the adhesive including at least one radio frequency susceptor; (3) pre-curing the adhesive using high frequency energy, wherein the first substrate and the second substrate have different linear thermal expansion coefficients; (4) subjecting the thermosetting adhesive to a heat treatment; A method comprising: [Aspect 3] 1. A method for joining two substrates, comprising: (1) providing an assembly including first and second substrates and a thermosetting adhesive in contact with the first and second substrates, the substrates having different thermal expansion coefficients, the adhesive including at least one radio frequency susceptor, and the adhesive being pre-cured to a degree of cure of at least 0.4 using radio frequency energy; (2) subjecting the thermosetting adhesive to a heat treatment; A method comprising: [Aspect 4] 1. A method for manufacturing an assembly including one or more subassemblies, comprising: (1) providing at least one subassembly including first and second substrates and a thermosetting adhesive in contact with the first and second substrates, the substrates having different thermal expansion coefficients, the adhesive including at least one radio frequency susceptor, and the adhesive being pre-cured to a degree of cure of at least 0.4 using radio frequency energy; (2) assembling the subassemblies into the assembly; (3) subjecting the thermosetting adhesive to a heat treatment; A method comprising: [Aspect 5] 5. The method of any one of aspects 1 to 4, wherein the pre-curing is carried out to a degree of cure of at least 0.4. [Aspect 6] The first substrate and the second substrate are 5×10 -6The method of any one of aspects 1 to 5, wherein the thermal expansion coefficients differ by at least m / (m-°C). [Aspect 7] The first substrate and the second substrate are 8×10 -6 7. The method of any one of aspects 1 to 6, wherein the thermal expansion coefficients differ by at least m / (m-°C). [Aspect 8] Aspect 8. The method of any one of aspects 1 to 7, wherein the adhesive is selected from an epoxy-based thermosetting adhesive, a urethane-based thermosetting adhesive, a (meth)acrylic-based thermosetting adhesive, a thermoplastic hot melt adhesive, or a mixture thereof. [Aspect 9] Aspect 9. The method of any one of aspects 1 to 8, wherein the adhesive is an epoxy adhesive. [Aspect 10] Aspect 10. The method of any one of aspects 1 to 9, wherein the adhesive is an epoxy adhesive based on a bisphenol epoxy resin. [Aspect 11] 11. The method of any one of aspects 1-10, wherein the adhesive cures when heated to a temperature of 80°C, preferably at least 100°C or higher, but cures very slowly, if at all, at room temperature (about 22°C) and temperatures up to at least 50°C. [Aspect 12] 12. The method of any one of aspects 1-11, wherein the adhesive comprises a curing agent selected from boron trichloride / amine complexes and boron trifluoride / amine complexes, dicyandiamide, melamine, diallylmelamine, guanamines such as acetoguanamine and benzoguanamine, aminotriazoles such as 3-amino-1,2,4-triazole, hydrazides such as adipic dihydrazide, stearic dihydrazide, isophthalic dihydrazide, semicarbazide, cyanoacetamide, and aromatic polyamines such as diaminodiphenylsulfone. [Aspect 13] 13. The method of any one of aspects 1 to 12, wherein the adhesive comprises dicyandiamide. [Aspect 14] 14. The method of any one of aspects 1 to 13, wherein the adhesive comprises a catalyst for curing the adhesive selected from ureas such as p-chlorophenyl-N,N-dimethylurea (monuron), 3-phenyl-1,1-dimethylurea (fenuron), 3,4-dichlorophenyl-N,N-dimethylurea (diuron), N-(3-chloro-4-methylphenyl)-N',N'-dimethylurea (chlortoluron), tert-acryl- or alkyleneamines such as benzyldimethylamine, 2,4,6-tris(dimethylaminomethyl)phenol, piperidine or a derivative thereof, an imidazole derivative, typically a C1 to C12 alkylene imidazole or an N-arylimidazole, for example, 2-ethyl-2-methyl-imidazole or N-butylimidazole, or 6-caprolactam. [Aspect 15] 15. The method of any one of aspects 1-14, wherein the adhesive comprises 2,4,6-tris(dimethylaminomethyl)phenol incorporated into a polyvinylphenol matrix. [Aspect 16] 16. The method of any one of aspects 1 to 15, wherein the at least one radio frequency susceptor is selected from carbon materials such as carbon black, carbon fiber, graphene, carbon nanofibers, and carbon nanotubes; metals such as metal flakes, metal fibers, metal filaments, and metal powders; polymeric dielectric materials such as polycaprolactone (PCL); and mixtures thereof. [Aspect 17] 17. The method according to any one of aspects 1 to 16, wherein the at least one radio frequency susceptor is present in the adhesive in an amount of 0.1 to 35 wt %, more preferably 1 to 30 wt %, 2 to 25 wt %, and particularly preferably 7.5 to 12.5 wt %. [Aspect 18] 18. The method according to any one of aspects 1 to 17, wherein the at least one radio frequency susceptor is carbon black, and the at least one radio frequency susceptor is present in an amount of 5 to 20 wt %, more preferably 5 to 15 wt %, and particularly preferably 7.5 to 12.5 wt %. [Aspect 19] 19. The method according to any one of aspects 1 to 18, wherein the at least one radio frequency susceptor is carbon black and is preferably present in an amount of 5 to 20 wt %, more preferably 5 to 15 wt %, and especially preferably 7.5 to 12.5 wt %. [Aspect 20] 20. The method of any one of aspects 1 to 19, wherein the at least one radio frequency susceptor is carbon nanotubes, preferably present in an amount of 5 to 20 wt %, more preferably 5 to 15 wt %, and particularly preferably 7.5 to 12.5 wt %. [Aspect 21] 21. The method according to any one of aspects 1 to 20, wherein the RF pre-curing is carried out using an RF frequency of about 30 kHz to about 300 GHz, more preferably 100 to 250 MHz, and particularly preferably 140 MHz. [Aspect 22] Aspect 22. The method of any one of aspects 1-21, wherein the heat curing step is carried out by heating to a temperature of 120° C. or greater. [Aspect 23] Aspect 23. The method of any one of aspects 1 to 22, wherein the heat curing step is the heat curing of an electrodeposition coating process. [Aspect 24] 1. A bonded assembly comprising: a first substrate and a second substrate bonded together, the first substrate and the second substrate having different linear thermal expansion coefficients; and a thermosetting adhesive between the first substrate and the second substrate, the adhesive including a radio frequency susceptor, and the adhesive being cured to a degree of cure of at least 0.4.

Claims

1. 1. A method for joining two substrates, comprising: (1) pre-curing a thermosetting adhesive using radio frequency energy, the adhesive including at least one radio frequency susceptor, and the adhesive in adhesive contact with a first substrate and a second substrate, the first substrate and the second substrate having different thermal expansion coefficients; (2) subjecting the thermosetting adhesive to a heat treatment; A method comprising:

2. 1. A method for joining two substrates, comprising: (1) providing a first substrate and a second substrate; (2) applying a thermosetting adhesive between the first substrate and the second substrate, the adhesive including at least one radio frequency susceptor; (3) pre-curing the adhesive using high frequency energy, wherein the first substrate and the second substrate have different linear thermal expansion coefficients; (4) subjecting the thermosetting adhesive to a heat treatment; A method comprising:

3. 1. A method for joining two substrates, comprising: (1) providing an assembly including a first substrate and a second substrate and a thermosetting adhesive in contact with the first and second substrates, the substrates having different thermal expansion coefficients, the adhesive including at least one radio frequency susceptor, and the adhesive having been pre-cured to a degree of cure of at least 0.4 using radio frequency energy; (2) subjecting the thermosetting adhesive to a heat treatment; A method comprising:

4. 1. A method for manufacturing an assembly including one or more subassemblies, comprising: (1) providing at least one subassembly including first and second substrates and a thermosetting adhesive in contact with the first and second substrates, the substrates having different coefficients of thermal expansion, the adhesive including at least one radio frequency susceptor, and the adhesive being pre-cured to a degree of cure of at least 0.4 using radio frequency energy; (2) assembling the subassemblies into the assembly; (3) subjecting the thermosetting adhesive to a heat treatment; A method comprising:

5. The method according to any one of claims 1 to 4, wherein the pre-curing is carried out to a degree of cure of at least 0.

4.

6. The first substrate and the second substrate are 5×10 －６ The method according to any one of claims 1 to 5, wherein the thermal expansion coefficients differ by more than m / (m-°C).

7. The first substrate and the second substrate are 8×10 －６ The method according to any one of claims 1 to 6, wherein the thermal expansion coefficients differ by at least m / (m-°C).

8. The method according to any one of claims 1 to 7, wherein the adhesive is selected from an epoxy-based thermosetting adhesive, a urethane-based thermosetting adhesive, a (meth)acrylic-based thermosetting adhesive, a thermoplastic hot melt adhesive, or a mixture thereof.

9. The method according to any one of claims 1 to 8, wherein the adhesive is an epoxy adhesive.

10. The method according to any one of claims 1 to 9, wherein the adhesive is an epoxy adhesive based on a bisphenol epoxy resin.

11. The method of any one of claims 1 to 10, wherein the adhesive cures when heated to a temperature of at least 100°C or higher.

12. 12. The method of any one of claims 1 to 11, wherein the adhesive comprises a curing agent selected from boron trichloride / amine complexes and boron trifluoride / amine complexes, dicyandiamide, melamine, diallylmelamine, acetoguanamine, benzoguanamine, 3-amino-1,2,4-triazole, adipic acid dihydrazide, stearic acid dihydrazide, isophthalic acid dihydrazide, semicarbazide, cyanoacetamide, and diaminodiphenylsulfone.

13. The method of any one of claims 1 to 12, wherein the adhesive comprises dicyandiamide.

14. 14. The method of any one of claims 1 to 13, wherein the adhesive comprises a catalyst for curing the adhesive selected from p-chlorophenyl-N,N-dimethylurea (monuron), 3-phenyl-1,1-dimethylurea (fenuron), 3,4-dichlorophenyl-N,N-dimethylurea (diuron), N-(3-chloro-4-methylphenyl)-N',N'-dimethylurea (chlortoluron), benzyldimethylamine, 2,4,6-tris(dimethylaminomethyl)phenol, piperidine, 2-ethyl-2-methyl-imidazole, N-butylimidazole, and 6-caprolactam.

15. The method of any one of claims 1 to 14, wherein the adhesive comprises 2,4,6-tris(dimethylaminomethyl)phenol incorporated into a polyvinylphenol matrix.

16. 16. The method according to any one of claims 1 to 15, wherein the at least one radio frequency susceptor is selected from carbon black, carbon fibers, graphene, carbon nanofibers, carbon nanotubes, metal flakes, metal fibers, metal filaments, metal powders, polycaprolactone (PCL), and mixtures thereof.

17. The method of any one of claims 1 to 16, wherein the at least one radio frequency susceptor is present in the adhesive at 0.1 to 35 wt%.

18. The method of any one of claims 1 to 17, wherein said at least one radio frequency susceptor is carbon black, and said at least one radio frequency susceptor is present at 5 to 20 wt%.

19. The method of any one of claims 1 to 18, wherein said at least one radio frequency susceptor is carbon nanotubes and is present in an amount of 5 to 20 wt%.

20. The method of any one of claims 1 to 19, wherein the RF pre-curing is carried out using an RF frequency between 30 kHz and 300 GHz.

21. The method according to any one of claims 1 to 20, wherein the heat curing step is carried out by heating to a temperature of 120°C or higher.

22. The method according to any one of claims 1 to 21, wherein the heat curing step is the heat curing of an electrocoating process.

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