Patent Publication Number: US-2021163789-A1

Title: Method for joining dissimilar materials

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of, and claims priority to, U.S. patent application Ser. No. 16/042,679 filed Jul. 23, 2018, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to joining dissimilar materials, more particularly joining metals and glass with polymer composite materials. 
     BACKGROUND 
     One of the outstanding challenges in the adhesive industry is bonding of dissimilar materials. An adhesive can be optimized for bonding identical parts such as glass-to-glass or carbon fiber reinforced polymer (CFRP), CFRP-to-CFRP but they provide only poor sealing and adhesion between different materials, i.e. glass-to-CFRP. This is because of the different chemical nature and surface free energy properties of these different materials. The problem is particularly relevant for the automotive industry where the introduction of lightweight materials is challenged due to the absence of materials and processes to create mechanically robust and corrosion free load-bearing structures. Examples include joining dissimilar materials such as any combination of the following: carbon fiber reinforced polymer composite (CFRP), fiber glass reinforced polymer (FRP) composite, aluminum, glass, titanium and magnesium. Structural adhesives provide low-cost, corrosion-free bonds and eliminate the need for failure-prone through-holes in CFPC structures, currently used to bond these materials. 
     For example, leading adhesives such as 3M®&#39;s 460 or Dow Chemical®&#39;s Betaforce®, provide highly effective bonding identical aluminum parts, with a bonding strength up to 40 mega Pascals (MPa). Yet, when bonding aluminum to CFRP parts, the performance of these same adhesives is poor (15 MPa). The CFPC side of the bond requires significant improvement. 
     It becomes difficult to identify single regular adhesives that provide strong bonding of dissimilar materials. 
     SUMMARY 
     According to aspects illustrated here, there is provided a deposition system having a multi-material print head, a first reservoir of a first compatible material having particles containing chemical elements similar to a first substrate, a second reservoir of a second compatible material having particles containing chemical elements similar to a second substrate, a third reservoir of an polymer precursor material, and at least one mixer. 
     According to aspects illustrated here, there is provided a method of bonding a joint between dissimilar substrate materials that includes functionalizing a first compatible material having chemical elements similar to a first substrate, mixing the first compatible material with a polymer precursor material, functionalizing a second compatible material having chemical elements similar to a second substrate, mixing the second compatible material with a polymer precursor material, and using the deposition system to deposit the first and second compatible materials and a polymer precursor material on the joint between the first and second substrate materials 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art example of a bond between dissimilar materials. 
         FIG. 2  shows an embodiment of a bond between dissimilar materials. 
         FIG. 3  shows an embodiment of a method of forming a gradient adhesive bond with dissimilar materials. 
         FIG. 4  shows a block diagram of an embodiment of a gradient adhesive print head. 
         FIG. 5  shows an embodiment of a slot die print head suitable for depositing gradient adhesives 
         FIGS. 6-9  shows different bonding techniques. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The ensuing discussion focuses on bonding combinations of different materials such as aluminum, CFRP, glass, fiberglass reinforced polymers (FRP), titanium and magnesium parts, such as commonly occurs in the automotive manufacturing environment. One should understand that this provides only examples and do not limit the invention as claimed to any pair of dissimilar materials. 
     The embodiments here provide a high-performance structural adhesive for bonding dissimilar materials with the following key features. First, the adhesive has direct chemical bonding to each of the dissimilar materials. Second, the adhesive has a graded composition of chemically and mechanically compatible reinforcing particles with a tunable adhesive elasticity that enables modulation of the particle concentration during deposition. Third, chemically linked networks by covalent bonds, of reinforcing particles that prevent particle separation during impact and stress. 
     In the embodiments, functionalized particles are dispersed into a matrix. In embodiment, the matrix may consist of polymer precursor materials. Covalent chemically linked particle networks are formed by reacting the chemical functions present onto the particles surface with the polymer precursors to produce a cured structure. 
     In current bonds of dissimilar materials, as shown in  FIG. 1 , polymer bonds are formed between the two parts. The polymer bonds to the parts with a materials mismatch, and has free dispersed particles. This results in weak bonding. The bonds, such as at  12  and  16  at the interfaces between the adhesive and the materials, are the same as they are in regions such as  14  of the adhesive away from the materials. 
     Direct particle bonding to surfaces results in superior adhesive strength. Current materials achieve their adhesive strength only by the bonding of polymer chains to the surfaces. In the embodiments here, matching particles will directly and covalently bond onto the surface of the joined substrates. Chemical linking forms the bond, or chemical and physical linking simultaneously form the bond in the case of particles. Direct particle bonding to the surface will result from reacting surface functionalized particles with the activated surface using epoxide monomers. 
     When dissimilar materials bond, it is not sufficient to simply having a strong bond for durability. The bond needs to act to absorb differences in material properties between the two parts. Graded interfaces between different materials can reduce the shear stress at an interface and allow two dissimilar surface to become compatible. This approach can serve to grade mechanical properties between the two materials, as well as to bridge the two materials chemically, allowing for increased bonding. 
     The effectiveness of the gradient approach to produce enhanced bounding has been shown recently in plastic materials of different stiffness. Studart has bonded hard structures onto a stretchable substrate, and then stretched them up to 350% before delamination occurred (A. R. Studart, et al.  Nature Communications,  3, 1265 (2102)). Homogenous adhesive bonding failed rapidly at about 200% strain. Using a multilayer graded interface between the dissimilar hard and soft substrates greatly reduced the interlayer shear forces and prevented delamination. This showed an overall increase of the strain level before delamination of approximately 75% when compared to homogenous bonding. 
     Conventional adhesives rapidly lose their bonding performance as the environmental temperature increases. For example, 3M&#39;s 460 epoxy lap shear strength decreases from 30 MPa at 23° C. to 5 MPa at 82° C. and further decreases 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 the adhesive at higher temperatures decreases significantly due to the softening of polymer chains as temperatures increase. To improve cohesion at high temperatures, the use of reinforcing particles provides a viable strategy. However, while individual reinforcing particles themselves possess exceptional mechanical strength, translating these properties into macroscopically strong composite structures has not yet been demonstrated. For example, exfoliated graphene particles have outstanding tensile strength (130,000) MPa (Novoselov at al.  Nature,  490, 192-200 (2012)). The tensile strength decreases by orders of magnitude (250 X) in graphene polymer composites (M. A. Rafiee et. al.,  ACS NANO,  3884, (2009). 
     In addition, the free particles are not bonded to the nearby epoxy groups and deformations due to stress can cause permanent dislocation of the particle from the polymer matrix, eventually causing failure. They enable only a limited improvement of the cohesive strength when compared to the polymer material alone. Chemical linking will restrict the movement of the reinforcing particles under mechanical and thermal stresses. As a result, the material should have outstanding thermo-mechanical performance when compared with current adhesives. One expects that the simultaneous enhancement of the cohesive strength, by covalent bonding of the strong particle networks, and of the adhesive strength, by covalent bonding of the strong particle network to each surface, will reduce the slippage between bonded surfaces. Moreover, this will produce adhesive bonds with mechanical thermal cycling and fatigue resistance far exceeding the performance of current adhesives for bonding either identical or dissimilar materials. 
     The selection of the particles that either match or have similar thermo-mechanical and chemical properties to the respective substrates to be bonded depends on the nature of these materials. This may be achieved by choosing particles that contain chemical elements that are similar to the corresponding compatible substrate. The chemical composition of the particles may or may not be identical to the corresponding compatible substrate. For the purpose of this discussion, “similar chemical elements” means that the particle materials contain at least the key chemical element(s) present in the corresponding compatible substrates. The particles materials may contain additional chemical elements that may not be present in the compatible substrate to be bonded. 
     One the embodiment of the compatibilizing particles with compatibility with the substrate consist of metal oxides of the same materials as the metal substrates to be bonded. Another embodiment of compatibilizing particles with compatibility with the substrate consist of carbon particles when bonding carbon fiber reinforced polymers (CFRP) substrates. 
     For example, the particles used for reinforcing the polymer base in the adhesive that is to be placed on the side of a selected substrate are selected based upon the following. For a CFRP substrate, graphene, graphene oxide or carbon nanotube functionalized particles are used. For an aluminum substrate, the compatibilizing particles would consist of aluminum oxide particles (Al 2 O 3 ) particles. For a fiber glass reinforced polymers (FRP) substrate, they would consist of silicon oxide (SiO 2 ) particles. For glass, the process would also use silicon oxide (SiO 2 ) particles. For titanium substrates, the process would use titanium dioxide (TiO 2 ). For magnesium substrates, the process would use magnesium oxide (MgO). 
     The adhesive will have a gradient of these materials to have higher concentrations of the material bonding to one of the dissimilar materials at the interface to that material, and then lower concentrations at the interface to the other material.  FIG. 2  shows a gradient adhesive bond  20 . One should note that in  FIG. 2 , the first material substrate could be a metal substrate, glass, CFRP, FRP, etc., and the second material substrate would be the other material. The corresponding compatible particles for the first substrate will be referred to as the first compatible particles, the corresponding compatible particles for the second substrate will be referred to as the second compatible particles. 
     Gradients can occur in multiple ways. A simple gradient may alter the concentration of reinforcement so that the overall concentration of reinforcement is constant across the entire bond, but the relative concentration of each reinforcement changes across the bond so that concentration at the two ends is entirely the compatible reinforcement. However, in other cases, the gradient may be more complicated. To add compliance or reduce corrosion, it may be desirable to have a less stiff section of the bond between the two components. For example, this may be highly desirable when bonding glass to an aluminum component, in this case the overall reinforcement would vary across the bond, reaching its minimum in the middle of the bond with no or little overlap between the two particles. It also may be of interest to provide a gradient across the width of the bond, decreasing the particle concentration at the edges of the bond will increase resistance to some mechanical loading and may be desirable. 
     As an example, the bonding of a first substrate as CFRP and a second substrate as an aluminum substrate is illustrated. At the CFRP interface  22 , the carbon fibers from the CFRP material chemically bond to the graphene particles as the first compatible particles in the adhesive, with a higher concentration of the graphene at this interface than at the other, and a higher concentration of graphene than the aluminum oxide as the second compatible particles at this interface. The gradient for the aluminum oxide goes in the other direction, with a higher concentration of aluminum oxide at the aluminum interface  26 , and a lower concentration moving towards the CFPC-adhesive interface. The region in the middle, such as  24 , has a mix of the two particles. 
     Multiple adhesive chemistries are suitable for the present invention. Particularly preferred are vinyl, such as acrylic adhesives, and epoxy adhesives. Acrylic adhesives are particularly advantageous in situations when fast curing is required. When acrylic adhesives are being used, they require employing vinyl functionalized particles. They can be obtained for example by reacting X—(R)-Vinyl molecule with —COOH or —OH functionalized particles. An example of a suitable reactive X group is epoxy group. Another approach to vinyl functionalized particles involves coupling of reactive particles with silane coupling agents containing functional groups such as vinyl, including acrylates, and methacrylates. 
     
       
         
         
             
             
         
       
     
     Generally speaking, conventional two-part epoxy adhesives consist of a part A, epoxy material, and a part B, hardener. Each of the two adhesives forming the gradient adhesive bond will be deposited 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 parts A or B. For example, in further describing the CFRP and aluminum parts bonding embodiment, appropriately functionalized particles can be present as follows in the formulation: graphene particles can be present into the epoxy part A or into the hardener part B materials. Just prior to deposition, the part A and B are mixed and the mixture represents the CFRP compatible adhesive material. In a similar way, aluminum oxide particles can be placed into the epoxy part A or into the hardener part B materials. Prior to deposition, the part A and B are mixed and the mixture represents the aluminum compatible adhesive material. These premixed formulations are used to produce the gradient adhesive structure when the ratio between the two mixtures is gradually changed. 
     The choice of placing the functionalized particles in the epoxy, part A, or the hardener, part B, is dictated by the type of functional groups present onto particles. The particles must be stable in the base material. For example, functionalized particles in the hardener must not react with the hardener but must react with the epoxy when the hardener/particle dispersion is mixed with the epoxy. Similarly, functionalized particles in epoxy must not react with the epoxy material but must react with the hardener when the epoxy/particle dispersion is mixed with the hardener. Once part A and part B of each of the adhesives are mixed the curing is initiated by the reaction between the amino groups and epoxy groups both those present in the base materials and those present onto the surface of the particles. Depending on the type of hardener, such as amino component, the curing can be performed at the ambient temperature or at higher temperature by heating. Generally, a curing process that may be relatively slow at ambient temperature is significantly accelerated by curing at a higher temperature. 
     One embodiment involves functionalized particles dispersible in the hardener material. The hardener materials contain mostly reactive amino groups. Suitable functionalized particles that are stable and dispersible in hardener material include particles possessing groups such as amino (—NH 2 , —NHR) groups, alcohol groups (—OH) and carboxylic acid groups (—COOH). A hardener formulation containing such functional particles will cure when mixed with the epoxy component, part A, either at room temperature or by heating due to the coupling reaction between these functional groups and the epoxy groups, as it is well known to those skilled in the epoxy adhesives. 
     One exemplary embodiment involves amino functional groups. The process for fabrication of amino functionalized particles depends on the starting particle and more precisely on the coupling functional groups present onto particles. 
     One method for introduction of amino groups onto metal oxide particles such as SiO 2 , Al 2 O 3 , TiO 2  or MgO, involves surface functionalization of metal oxide particles with amino functionalized silane coupling reagents. Amino silane coupling agents have a general structure such as H2N—(X)—Si(OR) 3  where the —Si(OR) 3  groups that react selectively with the —OH groups present onto particles surface to produce particles terminated with amino groups as illustrated in the figure below: 
     
       
         
         
             
             
         
       
     
     Amino functionalized exfoliated graphene sheets can be fabricated in several ways. The process can use any available graphene material as a starting material. This includes graphene, graphite, graphene with any substituting or doping chemical elements such as for example graphene containing oxygen functions, such as in the case of graphene oxide. Graphene with other chemical elements is also suitable for fabrication of amino functionalized exfoliated graphene sheets. One process as disclosed by Choi, et al. ( Chem. Commun ., (2010), 46, 6320-2). Exfoliation is a process of turning three-dimensional molecules into two-dimensional sheets. In this process, amino functionalized sheets are fabricated through a mild Friedel-Crafts substitution reaction onto graphite particles with simultaneous exfoliation of amino functionalized graphene sheets. Another approach to producing amino functionalized graphene sheets starts from graphene oxide (GO) sheets. Graphene oxide contains a large content of —COOH and —OH functional groups that can be used for introduction of amino functional groups, for example by reacting GO with molecules of the general structure X—(R)—NH2 where X has a selective reaction with —OH or —COOH groups in the presence of a catalyst. Suitable X groups include —NH 2  and silane coupling agents (—Si(OR) 3 ) groups. Other methods for producing amino-functionalized graphene particles may be available and they are all suitable for this invention. 
     In a further embodiment, the functionalized particles are dispersed in the hardener, part B, the reactive functional groups of are either —COOH or —OH groups. Most often, particles such as SiO 2 , Al 2 O 3  or TiO 2  have hydroxyl groups present onto their surface. In the case of graphene sheets, a suitable particle is graphene oxide that contains a large content of —COOH and —OH functional groups. 
     A second embodiment involves functionalized particles dispersed in the epoxy material (part A). Suitable examples include epoxy functionalized particles because they are stable and compatible with epoxy base materials. Epoxy functionalized particles can be fabricated by reacting —OH or —COOH functional groups present onto the surface of the particles with epoxy containing reagents of a general structure X—(R)-Epoxy where X reacts selectively with —OH and —COOH groups. The process is applicable to any type of —OH and —COOH functionalized particles, including Al 2 O 3 , SiO 2 , TiO 2 , MgO and similar as well as graphene oxide. A preferred reactive X function is an epoxy group. 
     
       
         
         
             
             
         
       
     
     In the embodiment of aluminum bonding aluminum 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 hardeners in an epoxy resin, where each material has a graded composition of a higher concentration at one of the substrates moving towards a lower concentration at the other substrate, and the two materials have graded compositions in the opposite direction. 
     In a different embodiment with epoxy functionalized particles involves a type of one part epoxy formulations wherein an epoxy material is mixed with epoxy functionalized particles and with a curing initiator that is activated only when heated at a certain temperature. This type of formulation is advantaged by the fact that the formulation can be stable in that is does not cure and can be shipped and handled for long periods of time before application. This formulation is also advantageous because it reduces the number of mixing steps require for each of the part A and B of each of the adhesives using amino hardeners. After deposition the curing takes place by heating at a temperature above the ambient temperature such as at 80 deg. C or 100 deg. C or higher for various time durations such as from minutes to hours. Optionally, a second heating step can be performed at a higher temperature, such as 160° C. or 200° C. or 220° C. This process ensures curing completion and generally produces stiffer bond when compared with curing at only lower temperatures. 
     The present approach is generally applicable to bonding any two different materials. For example, when bonding CFRP to glass, the CFRP side of the adhesive layer contains functionalized graphene particles while on the glass side the adhesive contains SiO 2  functionalized particles. When bonding aluminum to glass, the aluminum side of the adhesive layer contains functionalized Al 2 O 3  particles while on the glass side the adhesive contains SiO 2  functionalized particles. 
       FIG. 3  shows an embodiment of a method of depositing a gradient adhesive for dissimilar materials using epoxy embodiment. The examples used here illustrate the use of amino functionalized particles dispersed in a hardener when bonding CFRP and aluminum. The process uses exfoliated amino graphene  40  and amino aluminum oxide  42  particles. At 44 Amino graphene is mixed with hardener material to provide a graphene hardener, which is then mixed in the next step with epoxy material  48 . Similarly, amino Al 2 O 3  at  42  is first mixed with the hardener to provide an Al 2 O 3  hardener at  46  which is then mixed in the next step with epoxy material at  50 . The hardener and epoxy materials may be the same for each of the two adhesive particles. 
     These materials are provided to the print head. A print head, in the context of the present embodiments may include any extrusion and coextrusion deposition device, such as slot die or conventional extrusion through a needle. The following examples illustrate the deposition through a slot die. 
     Successive layers ranging from a first layer of pure amino graphene/epoxy mixture  52 , up to a last layer consisting of pure amino Al 2 O 3 /epoxy formulation  56  with intermediate gradual decreasing concentration of the amino graphene/epoxy and increasing concentration of amino Al 2 O 3 /epoxy  54  are deposited through a slot die. The aluminum substrate is placed on top of the deposited gradient adhesive  58  and the bonded structure is cured  59 . 
     To achieve deposition rates fast enough for automotive applications, a print head capable of producing gradient adhesive bonds can deposit the adhesive in a single pass. Traditional automated automotive adhesive dispensers consist of a single needle point application mounted on the front of a robot with a single material feed system. The robot has either undergone pre-programming or uses a vision system to locate the appropriate application point and dispense the needed amount of adhesive.  FIG. 4   5  show an embodiment of a slot die print head. 
     In  FIG. 4 , the slot die print head  60  has material feeds  62  of the epoxy, the graphene and hardener, and the aluminum oxide and hardener. A pump  64  typically provides the pressure to move the materials. The mixer or mixers  66  receive the materials from the material feeds and provide the desired concentrations for the adhesive bond being formed. The manifolds  68  then merge the feeds, and the die itself  70  deposits the adhesive across the interface between the dissimilar materials to form the bond.  FIG. 5  shows an image of such a print head. 
     The print head can deposit small numbers of layers by premixing formulas and depositing all three layers simultaneously. The ideal graded bond may require larger numbers of layers. To accomplish multiple layers simultaneously, the print head may mix the epoxy formulations, meaning the combination of hardeners and resin, within the print head. This also allows for changes in gradient composition if necessary, customizing the gradient for a specific bond. This may reduce equipment costs and maximize flexibility in automotive factories. 
       FIGS. 6-9  show variations of the different bonds.  FIG. 6  shows an embodiment of a bond in accordance with the traditional bonding techniques. A monolithic layer of adhesive  62  bonds a substrate of a first material, such as carbon fiber,  60  and a substrate of a second material, such as aluminum  64 . There are sharp changes in the properties between the various layers. 
     In contrast, the embodiments discussed here have gradients within a base material of particles of compatible materials. In  FIG. 7 , a high rigidity bond has a particle concentration kept high in the adhesive layer  66  allowing the mechanical properties to transition from graphene to aluminum oxide. 
       FIG. 8  shows a flexible gradient bond to compensate for dimensional or thermal differences. The adhesive layer  68  has a flexible gradient bond with a lower concentration of particle reinforcement, allowing for flexibility while maintaining gradual changes in properties such as thermal or dimensional differences. 
       FIG. 9  shows an adhesive layer  70  that has a particle gradient in the plane of the bond. This allows for an increase in the overall bond strength. 
     In this manner, the embodiments achieve a structural adhesive that had enhanced cohesive strength enabled by chemically linked networks of reinforcing particles that prevent particle motion during impact and stress. The adhesive has maximized strength due to direct chemical bonding of matched particles to the surfaces. The embodiments create an enhanced interface between dissimilar materials with the graded composition of chemically and mechanically compatible reinforcing particles. These embodiments easily integrate with current robotic deposition systems with the replacement of the current nozzles with a multi-material slot die. 
     It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.