Method for joining dissimilar materials

Abstract

To provide a strong coupling method of dissimilar materials.SOLUTION: A method for coupling a joint between dissimilar substrate materials is configured to functionalize a first compatibility material comprising a chemical element similar to that of the first substrate, mix the first compatibility material with a polymer precursor material, functionalize a second compatibility material having a chemical element similar to that of the second substrate, mix the second compatibility material with the polymer precursor material, use an adhesion system for adhering the first and second compatibility materials and the polymer precursor material to a joint between the first substrate material and the second substrate material.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to joining dissimilar materials, and more particularly to joining metals and glass using polymer composites.

[0002] One of the unsolved problems in the adhesive industry is the joining of dissimilar materials. Adhesives can be optimized for joining identical parts such as glass-to-glass or carbon fiber reinforced polymer (CFRP), CFRP-to-CFRP, etc., but provide only inadequate sealing and adhesion between different materials, i.e., glass-to-CFRP. This is because the chemical properties and surface free energy characteristics of these different materials are different. This problem is particularly relevant to the automotive industry, where the introduction of lightweight materials has been an issue due to the lack of materials and processes for creating mechanically robust and corrosion-resistant load-bearing structures. Examples include joining dissimilar materials such as any combination of carbon fiber reinforced polymer composites (CFRP), fiberglass reinforced polymer (FRP) composites, aluminum, glass, titanium, and magnesium. Structural adhesives provide low-cost and corrosion-resistant joints, eliminating the need for the easily damaged through-holes of the CFPC structures currently used to join these materials.

[0003] For example, major adhesives such as 3M® 460 or Dow Chemical® Betaforce® provide very effective joining of identical aluminum parts with a joint strength of up to 40 megapascals (MPa). Nevertheless, when joining aluminum to CFRP parts, the performance of these same adhesives is poor (15 MPa). Significant improvement is needed on the CFPC side of the joint.

[0004] It becomes difficult to identify a single regular adhesive that provides strong bonding of dissimilar materials.

[0005] According to embodiments described herein, a composition is provided comprising a first compatible material having particles containing the same chemical elements as a first substrate, and a second compatible material having particles containing the same chemical elements as a second substrate, wherein the first and second substrates are chemically distinct. The particles are dispersed in a matrix between the first and second substrates.

[0006] According to embodiments described herein, an adhesion system is provided comprising a multi-material print head, a first storage section for a first compatible material having particles containing the same chemical elements as a first substrate, a second storage section for a second compatible material having particles containing the same chemical elements as a second substrate, a third storage section for a polymer precursor material, and at least one mixer.

[0007] A method for joining seams between dissimilar substrate materials is provided, comprising: functionalizing a first compatible material having the same chemical elements as a first substrate; mixing the first compatible material with a polymer precursor material; functionalizing a second compatible material having the same chemical elements as a second substrate; mixing the second compatible material with a polymer precursor material; and using an adhesion system to adhere the first and second compatible materials and the polymer precursor material to the seam between the first substrate material and the second substrate material. [Brief explanation of the drawing]

[0008] [Figure 1] An example of conventional techniques for joining dissimilar materials is shown. [Figure 2] This shows one embodiment of joining dissimilar materials. [Figure 3] This document describes one embodiment of a method for forming a gradient adhesive bond using dissimilar materials. [Figure 4] A block diagram of one embodiment of a tilted adhesive print head is shown. [Figure 5] This shows one embodiment of a slot die printhead suitable for applying angled adhesive. [Figure 6] This demonstrates different joining techniques. [Figure 7] This demonstrates different joining techniques. [Figure 8] This demonstrates different joining techniques. [Figure 9] This demonstrates different joining techniques. [Modes for carrying out the invention]

[0009] The following discussion will focus on combinations of joining different materials, such as aluminum, CFRP, glass, glass fiber reinforced polymer (FRP), titanium, and magnesium parts, as is commonly done in automotive manufacturing environments. It should be understood that this is merely an example and does not limit the claimed invention to any pair of dissimilar materials.

[0010] Embodiments of this specification provide high-performance structural adhesives for joining dissimilar materials having the following key features: First, the adhesive directly chemically bonds with each of the dissimilar materials. Second, the adhesive has a gradient composition of chemically and mechanically compatible reinforcing particles having adjustable adhesive elasticity that allows for adjustment of particle concentration during adhesion. Third, a network chemically bonded by covalent bonds of reinforcing particles that prevents particle separation during impact and stress.

[0011] In the embodiment, the functionalized particles are dispersed in a matrix. In the embodiment, the matrix may consist of a polymer precursor material. The covalent chemically bonded particle network is formed by reacting chemically functional groups present on the particle surface with the polymer precursor to generate a cured structure.

[0012] In current dissimilar material bonding, polymer bonds are formed between the two parts, as shown in Figure 1. The polymer has freely dispersed particles that bond to the mismatched parts of the material. This results in a weak bond. Bonds at points 12 and 16, etc., at the interface between the adhesive and the material are the same as those at points 14, etc., in the adhesive away from the material.

[0013] Direct particle bonding to a surface results in superior adhesive strength. Current materials achieve their adhesive strength only through the bonding of polymer chains to the surface. In the embodiments herein, compatible particles are directly and covalently bonded to the surface of the bonding substrate. In the case of particles, chemical bonding forms the bond, or chemical and physical bonding forms the bond simultaneously. Direct particle bonding to a surface results from reacting surface-functionalized particles with an activated surface using epoxide monomers.

[0014] When dissimilar materials are joined, simply having a strong joint is not enough for durability. The joint needs to act in a way that absorbs the differences in material properties between the two parts. A gradient interface between different materials can reduce shear stress at the interface and allow the two dissimilar surfaces to become compatible. This technique not only helps to gradient the mechanical properties between the two materials, but it can also be used to chemically bridge the two materials and increase the bond.

[0015] The effectiveness of gradient techniques for creating reinforced boundaries has recently been demonstrated in plastic materials of varying stiffness. Studart bonded rigid structures to a stretchable substrate and then stretched them to 350% until delamination occurred (AR. Studart, et al. Nature Communications, 3, 1265(2102)). Homogeneous adhesive bonding rapidly failed at approximately 200% strain. By using multilayer gradient interfaces between dissimilar rigid and flexible substrates, interlaminar shear forces were significantly reduced, preventing delamination. This resulted in an overall increase of approximately 75% in pre-delamination strain levels compared to homogeneous bonding.

[0016] Conventional adhesives rapidly lose their bonding performance as ambient temperature rises. For example, the lap shear strength of 3M's 460 epoxy drops from 30 MPa at 23°C to 5 MPa at 82°C, and further to 1.5 MPa at 121°C (http: / / multimedia.3m.com / mws / media / 661220 / 3mtm-scotch-weld-tm-epoxy-adhesive-dp460-ns-and-off-white.pdf). The cohesive strength of adhesives at higher temperatures decreases significantly as the polymer chains soften with increasing temperature. To improve cohesiveness at high temperatures, the use of reinforcing particles offers a viable strategy. However, while individual reinforcing particles themselves possess excellent mechanical strength, translating these properties into macroscopically strong composite structures has not yet been demonstrated. For example, exfoliated graphene particles have excellent tensile strength (130,000) MPa (Novoselov et al. Nature, 490, 192-200 (2012)). Tensile strength decreases by an order of magnitude (250 times) in graphene polymer composite materials (MARafiee et al., ACS NANO, 3884, (2009)).

[0017] Furthermore, the free particles are not bonded to nearby epoxy groups, and stress-induced deformation can cause permanent dislocations of the particles from the polymer matrix, ultimately leading to fracture. They only allow for a limited improvement in cohesive strength compared to the polymer material alone. Chemical bonding restricts the movement of reinforcing particles under mechanical and thermal stress. As a result, the material should have superior thermomechanical performance compared to current adhesives. Simultaneously improving cohesive strength through covalent bonding of a strong particle network and adhesive strength through covalent bonding of a strong particle network to each surface is expected to reduce slip between bonded surfaces. Moreover, this results in an adhesive bond with mechanical and thermal cycling and fatigue resistance far exceeding the performance of current adhesives for joining either identical or dissimilar materials.

[0018] The selection of particles having thermomechanical and chemical properties that match or are similar to those of each substrate to be bonded depends on the properties of these materials. This can be achieved by selecting particles containing chemical elements similar to those of the corresponding compatible substrate. The chemical composition of the particles may or may not be identical to that of the corresponding compatible substrate. For the purposes of this consideration, "similar chemical elements" means that the particle material contains at least one of the important chemical elements present in the corresponding compatible substrate. The particle material may contain additional chemical elements that may not be present in the compatible substrate to be bonded.

[0019] One embodiment of compatible particles having compatibility with a substrate consists of a metal oxide made of the same material as the metal substrate being bonded. Another embodiment of compatible particles having compatibility with a substrate consists of carbon particles when bonding a carbon fiber reinforced polymer (CFRP) substrate.

[0020] For example, the particles used to reinforce the polymer base in the adhesive placed on the sides of a selected substrate are selected based on the following: For CFRP substrates, graphene, graphene oxide, or carbon nanotube-functionalized particles are used. For aluminum substrates, the compatible particles will consist of aluminum oxide particles (Al2O3). For glass fiber reinforced polymer (FRP) substrates, they will consist of silicon oxide (SiO2) particles. For glass, the process will similarly use silicon oxide (SiO2) particles. For titanium substrates, the process will use titanium dioxide (TiO2). For magnesium substrates, the process will use magnesium oxide (MgO).

[0021] The adhesive has an inclination of these materials, has a higher concentration of the material that binds to one of the dissimilar materials at the interface with that material, and then has a lower concentration of the material at the interface with the other material. Figure 2 shows an inclined adhesive joint 20. Note that in Figure 2, the first material substrate can be a metal substrate, glass, CFRP, FRP, etc., and the second material substrate is another material. The compatibility particles corresponding to the first substrate are referred to as the first compatibility particles, and the compatibility particles corresponding to the second substrate are referred to as the second compatibility particles.

[0022] The inclination can occur in multiple modes. A simple inclination can change the concentration of the reinforcing material so that the overall concentration of the reinforcing material is constant across the entire joint, but the relative concentration of each reinforcing material changes across the joint such that the concentrations at both ends are perfectly compatible reinforcing materials. However, in other cases, the inclination can be more complex. It may be desirable to have a less rigid portion of the joint between two components to add compliance or reduce corrosion. For example, this can be highly desirable when joining glass to an aluminum component, in which case the overall reinforcing material changes across the joint and reaches a minimum at the center of the joint where there is no or very little overlap between the two particles. It can also be interesting to provide an inclination across the width of the joint, and reducing the particle concentration at the edges of the joint may be desirable as it increases the resistance to some mechanical loads.

[0023] As an example, the joining of a first substrate as CFRP and a second substrate as an aluminum substrate is shown. At the CFRP interface 22, the carbon fibers from the CFRP material chemically bond to graphene particles as the first compatibility particles in the adhesive, and the concentration of graphene is higher at this interface than at other interfaces, and higher than the concentration of aluminum oxide as the second compatibility particles at this interface. The inclination of aluminum oxide progresses in the other direction, with a higher concentration of aluminum oxide at the aluminum interface 26 and a lower concentration towards the CFPC - adhesive interface. In the central region such as 24, the two particles are mixed.

[0024] Multiple adhesive chemistries are suitable for the present invention. Vinyls such as acrylic adhesives, and epoxy adhesives are particularly preferred. Acrylic adhesives are particularly advantageous in situations where rapid curing is required. When acrylic adhesives are used, they require the use of vinyl-functionalized particles. They can be obtained, for example, by reacting X-(R)-vinyl molecules with -COOH or -OH functionalized particles. An example of a suitable reactive X group is an epoxy group. Another approach to vinyl-functionalized particles involves coupling reactive particles with a silane coupling agent containing functional groups such as vinyls including acrylates and methacrylates.

[0025]

Chemical formula

[0026] Generally speaking, conventional two-component epoxy adhesives consist of an epoxy material in part A and a curing agent in part B. Each of the two adhesives forming a gradient adhesive joint is applied as a mixture of its own part A and part B. For each of these two adhesives, the reinforcing particles disclosed in the present invention can be present in either of the two corresponding part A or part B. For example, when further explaining an embodiment of joining a CFRP and an aluminum component, appropriately functionalized particles may be present in the formulation as follows: Graphene particles can be present in the epoxy part A or the curing agent part B material. Immediately before application, part A and part B are mixed, and the mixture represents a CFRP-compatible adhesive material. Similarly, aluminum oxide particles can be incorporated into the epoxy part A or the curing agent part B material. Before application, part A and part B are mixed, and the mixture represents an aluminum-compatible adhesive material. These premixed formulations are used to produce a gradient adhesive structure when the ratio between the two mixtures is gradually varied.

[0027] The choice of incorporating functionalized particles into epoxy (part A) or curing agent (part B) is determined by the type of functional groups present on the particles. The particles must be stable within the substrate. For example, functionalized particles in a curing agent must not react with the curing agent, but must react with the epoxy when the curing agent / particle dispersion is mixed with the epoxy. Similarly, functionalized particles in epoxy must not react with the epoxy material, but must react with the curing agent when the epoxy / particle dispersion is mixed with the curing agent. Once parts A and B of each adhesive are mixed, curing is initiated by a reaction between amino groups and epoxy groups (both those present in the substrate and those on the particle surface). Depending on the type of curing agent, such as amino components, curing can occur at ambient temperature or at higher temperatures through heating. Generally, a curing process that may be relatively slow at ambient temperature is significantly accelerated by curing at higher temperatures.

[0028] One embodiment includes functionalized particles that can be dispersed in a curing agent material. The curing agent material mainly contains reactive amino groups. Suitable functionalized particles that are stable and dispersible in the curing agent material include particles having groups such as amino (-NH2, -NHR) groups, alcohol groups (-OH), and carboxylic acid groups (-COOH). As is well known to those skilled in the art of epoxy adhesives, a curing agent formulation containing such functionalized particles hardens when mixed with an epoxy component (partial A) at room temperature or by heating resulting from a coupling reaction between these functional groups and epoxy groups.

[0029] One exemplary embodiment includes an amino functional group. The process for producing amino-functionalized particles depends on the starting particles, or more precisely, the coupling functional groups present on the particles.

[0030] One method for introducing amino groups onto metal oxide particles such as SiO2, Al2O3, TiO2, or MgO involves surface functionalization of the metal oxide particles using an amino-functionalized silane coupling agent. The aminosilane coupling agent has a general structure such as H2N-(X)-Si(OR)3, and the -Si(OR)3 group selectively reacts with -OH groups present on the particle surface to produce particles terminated with amino groups, as shown in the figure below.

[0031] [ka]

[0032] Amino-functionalized exfoliated graphene sheets can be manufactured in several ways. This process can use any available graphene material as a starting material. This includes graphene, graphite, and graphene containing any substituted or doped chemical elements, such as graphene containing oxygen functional groups, as in the case of graphene oxide. Graphene containing other chemical elements is also suitable for the manufacture of amino-functionalized exfoliated graphene sheets. One process is disclosed by Choi et al. (Chem.Commun., (2010), 46, 6320-2). Exfoliation is a process that transforms a three-dimensional molecule into a two-dimensional sheet. In this process, the amino-functionalized sheet is manufactured by exfoliating the amino-functionalized graphene sheet simultaneously with a mild Friedel-Crafts substitution reaction onto graphite particles. Another method for producing amino-functionalized graphene sheets starts from a graphene oxide (GO) sheet. Graphene oxide contains a large number of -COOH and -OH functional groups that can be used to introduce amino functional groups, for example, by reacting GO with a molecule of the general structure X-(R)-NH2 (wherein X has selective reaction with -OH or -COOH groups in the presence of a catalyst). Suitable X groups include -NH2 and silane coupling agent (-Si(OR)3) groups. Other methods for producing amino-functionalized graphene particles may be available and are all suitable for the present invention.

[0033] In further embodiments, the functionalized particles are dispersed in a curing agent (partial B), and the reactive functional group is either a -COOH or -OH group. In most cases, particles such as SiO2, Al2O3, or TiO2 have hydroxyl groups present on their surface. In the case of graphene sheets, suitable particles are graphene oxide containing a large amount of -COOH and -OH functional groups.

[0034] The second embodiment includes functionalized particles dispersed in an epoxy material (part A). Since epoxy functionalized particles are stable and compatible with epoxy substrates, suitable examples include epoxy functionalized particles. Epoxy functionalized particles can be produced by reacting the -OH or -COOH functional groups present on the particle surface with an epoxy-containing reagent of the general structure X-(R)-epoxy (wherein X selectively reacts with -OH and -COOH groups). This process is applicable to all kinds of -OH and -COOH functionalized particles, including Al2O3, SiO2, TiO2, MgO, and similar materials, as well as graphene oxide. The preferred reactive X functional group is an epoxy group.

[0035] [ka]

[0036] In an embodiment of aluminum being bonded to CFPC, the resulting bond has a higher concentration of aluminum oxide than graphene at the aluminum interface and a higher concentration of graphene than aluminum oxide at the CFPC interface. In a more general sense, the compound has two materials as curing agents in the epoxy resin, each material having a gradient composition in which the concentration is higher on one side of the substrate and decreases toward the other side of the substrate, and the two materials have gradient compositions in opposite directions.

[0037] In different embodiments using epoxy-functionalized particles, a type of one-component epoxy formulation is included in which the epoxy material is mixed with epoxy-functionalized particles and a curing initiator that is activated only when heated at a specific temperature. This type of formulation is advantageous because it can be stable in that the formulation does not harden and can be shipped and handled for a long period of time before application. This formulation is also advantageous because it reduces the number of mixing steps required for each part A and part B of each adhesive that uses an amino curing agent. After adhesion, curing is carried out by heating at a temperature above ambient temperature, for example 80°C to 100°C or higher, for various periods such as several minutes to several hours. Optionally, the second heating step can be carried out at a higher temperature such as 160°C, 200°C, or 220°C. This process ensures completion of curing and generally results in a stiffer bond compared to curing at lower temperatures only.

[0038] The method of the present invention is generally applicable to joining any two different materials. For example, when joining CFRP to glass, the CFRP side of the adhesive layer contains functionalized graphene particles, while the glass side contains SiO2-functionalized particles. When joining aluminum to glass, the aluminum side of the adhesive layer contains functionalized Al2O3 particles, while the glass side contains SiO2-functionalized particles.

[0039] Figure 3 shows one embodiment of a method for bonding a gradient adhesive for dissimilar materials using an epoxy embodiment. The example used here shows the use of amino-functionalized particles dispersed in a curing agent when bonding CFRP and aluminum. This process uses exfoliated aminographene 40 and alumino-alumininated aluminum 42 particles. In 44, aminographene is mixed with the curing agent material to obtain a graphene curing agent, which is then mixed with the epoxy material 48 in the next step. Similarly, aminoAl2O3 42 is first mixed with the curing agent to obtain an Al2O3 curing agent in 46, which is then mixed with the epoxy material in the next step 50. The curing agent and epoxy material may be the same for each of the two adhesive particles.

[0040] These materials are supplied to the printhead. In the context of the present invention, the printhead may include any extrusion and co-extrusion deposition device, such as a slot die or conventional extrusion via a needle. The following examples illustrate deposition via a slot die.

[0041] A series of layers, from the first layer 52 of a pure aminographene / epoxy mixture to the last layer 56 of a pure aminoAl2O3 / epoxy formulation (with the concentration of aminographene / epoxy gradually decreasing and the concentration of aminoAl2O3 / epoxy increasing in between), are bonded via a slot die. An aluminum substrate is placed on top of the bonded gradient adhesive (58), and the bonded structure is cured (59).

[0042] To achieve sufficiently fast adhesion speeds for automotive applications, printheads capable of generating angled adhesive bonds can apply adhesive in a single pass. Conventional automated automotive adhesive dispensers consist of a single needle tip for dispensing, mounted on the front of a robot with a single material supply system. The robot is pre-programmed or uses a vision system to locate the appropriate dispensing position and dispense the required amount of adhesive. Figures 4 and 5 show one embodiment of a slot-die printhead.

[0043] In Figure 4, the slot die printhead 60 has a material supply unit 62 for epoxy, graphene and curing agent, as well as aluminum oxide and curing agent. A pump 64 typically supplies pressure to move the material. One or more mixers 66 receive the material from the material supply unit and provide the desired concentration to the adhesive joint to be formed. A manifold 68 then combines the supplied materials, and the die 70 itself adheres the adhesive across the interface between the dissimilar materials to form the joint. Figure 5 shows an image of such a printhead.

[0044] A printhead can bond a small number of layers by pre-mixing the formulation and bonding all three layers simultaneously. An ideal gradient bond may require more layers. To achieve multi-layer bonding simultaneously, the printhead may mix the epoxy formulation, i.e., the combination of curing agent and resin, within the printhead itself. This also allows for customization of the gradient for specific bonds by changing the gradient composition as needed. Doing so reduces equipment costs and maximizes the flexibility of the automotive plant.

[0045] Figures 6-9 show variations of different bonding methods. Figure 6 shows one embodiment of bonding using traditional bonding techniques. A monolithic layer 62 of adhesive bonds a substrate of a first material, such as carbon fiber 60, to a substrate of a second material, such as aluminum 64. There are abrupt changes in properties between various layers.

[0046] In contrast, the embodiments discussed herein have a gradient of compatible material particles within the substrate. In Figure 7, the high-rigidity bond maintains a high particle concentration in the adhesive layer 66, allowing for the transfer of mechanical properties from graphene to aluminum oxide.

[0047] Figure 8 shows a flexible gradient joint for compensating for dimensional or temperature differences. The adhesive layer 68 has a flexible gradient joint with a lower concentration of particle reinforcement, allowing for flexibility while maintaining gradual changes in properties such as thermal or dimensional differences.

[0048] Figure 9 shows an adhesive layer 70 with a particle gradient on the bonding plane. This allows for an increase in overall bonding strength.

[0049] In this way, the embodiments achieve a structural adhesive having enhanced cohesive strength enabled by a chemically bonded network of reinforcing particles that prevent particle movement between impacts and stresses. The adhesive has maximum strength due to the direct chemical bonding of particles that conform to the surface. The embodiments create a reinforced interface between dissimilar materials having a graded composition of chemically and mechanically compatible reinforcing particles. These embodiments can be readily integrated with current robotic adhesion systems by replacing the current nozzle with a multi-material slot die.

Claims

1. A composition that forms a multilayer structure together with a substrate, A first compatible material having particles containing the same chemical elements as the first substrate, A composition comprising a second compatible material having particles containing the same chemical elements as the second substrate, wherein the first substrate and the second substrate are chemically different, and the particles of the first compatible material and the particles of the second compatible material are dispersed in a matrix between the first substrate and the second substrate. The first material has a gradient of the first compatible material, wherein the concentration is higher toward the first substrate and lower toward the second substrate. The second compatible material has a gradient such that its concentration is higher toward the second substrate and lower toward the first substrate. A composition in which compatible particles from the first and second compatible materials form a network of chemically bonded particles when cured.

2. The composition according to claim 1, wherein the first substrate and the second substrate are selected from the group consisting of carbon fiber reinforced polymer, glass fiber reinforced polymer, glass, aluminum, magnesium, and titanium.

3. The composition according to claim 1, wherein the matrix is ​​an acrylic adhesive, or a two-component epoxy adhesive comprising an epoxy material and a curing agent material.

4. The composition according to claim 1, wherein the first substrate is aluminum and the second substrate is a carbon fiber reinforced polymer composite (CFPC).

5. The composition according to claim 4, wherein the first particle comprises aluminum oxide and the second particle comprises either graphene or graphene oxide.

Images