Adhesive bonding of substrates with low surface energy

Abstract

Disclosed herein is a method for bonding a first substrate with low surface energy to a second substrate using a thermosetting adhesive composition that has been formulated to enhance the bonding performance of the first substrate. The substrates with low surface energy include thermoplastic substrates and thermoplastic composites. The thermosetting adhesive composition includes: (a) a combination of multifunctional epoxy resins or an epoxy-based pre-react; (b) at least one amine as curing agent; (c) at least one inorganic clay mineral; and (d) a flow control agent that is not inorganic clay mineral.

Description

ADHESIVE BONDING OF SUBSTRATES WITH LOW SURFACE ENERGY

[0001] Generally, the adhesive bonding of substrates with low surface energy, such as thermoplastic substrates, is extremely challenging due to their smooth, hydrophobic surfaces that are difficult to wet out and bond. The low energy surface causes a reduction in adhesion between the substrate surface and the chosen adhesive. Surface energy is a reliable measure of how easy or difficult a surface can be bonded to another surface. A low energy surface is more challenging to bond to than a higher energy surface. As disclosed herein, low surface energy refers to surface free energy of less than 40 dynes / cm as measured by contact angle measurements or dyne testing. Low surface energy materials are difficult to bond because the molecules on their surface have little attraction to other molecules, especially the epoxy and amine chemistries utilized in conventional thermosetting adhesives.

[0002] Generally, a surface with a low surface energy will cause poor wetting. The majority of work investigating the adhesive bonding of substrates with low surface energy in the prior art literature has focused on the use of surface treatments to create a higher energy, more robust and reliable surface for bonding using readily available structural adhesives.

[0003] The various surface pre-treatments investigated in the prior art literature include both abrasive and energetic techniques. The abrasive surface pre-treatments, such as hand sanding, grit blast and peel ply, which are commonly utilized for substrates with high surface energy (such as thermoset composites) do not work as effectively and efficiently for substrates with low surface energy. The ease of adhesion in bonding is primarily measured by evaluating the Gicfracture toughness and resulting failure modes of bonded substrates. The use of these abrasive techniques commonly leads to adhesive failures in the majority of Gic testing pointing to low adhesion properties.

[0004] Energetic surface pre-treatment methods, such as plasma, corona or flame treatment can eliminate surface contaminants and improve the wetting of low energy surfaces more effectively than abrasive methods, which therefore leads to better bonding and adhesion. However, these results are still not comparable to the fracture toughness and cohesive failure modes observed for substrates with high surface energy, pointing to a lower level of adhesion.

[0005] Consequently, while the proper surface treatment is critical for some bonding processes, the highest quality and most robust bonds can only be formed if the properadhesive system is used. Off the shelf structural adhesives commonly lead to a mixture of cohesive, adhesive and laminate failure modes, even when used in conjunction with energetic surface treatments, which is undesirable because of the low bonding robustness and reliability. Consequently, improvements in bonding to low energy substrates are needed. Additionally, most adhesives that have been suitable for low energy surfaces are not ideal for demanding applications such as aerospace applications due to their lower service temperatures and low glass transition temperatures.

[0006] While the use of enhanced surface treatments does improve the adhesive bonding to low energy surfaces, the surface treatment conditions alone are not a comprehensive and all-encompassing solution, as there is still variability observed in the interlaminar fracture toughness values and types of failure modes observed (mixture of cohesive, adhesive and laminate). This leads to questions regarding the repeatability and robustness of relying strictly on surface treatment alone. Consequently, in addition to an optimized surface treatment, there is also the need for the development of an adhesive that can better wet out low energy surfaces and is more compatible with such low energy surfaces.

[0007] Thermoplastic composites, which contain reinforcement fibers embedded in a thermoplastic matrix, are increasingly being utilized in many aerospace, industrial and manufacturing applications. Thermoplastic composites offer many advantages such as unlimited shelf life, short processing times, good impact resistance, high chemical resistance and easier manufacturing conditions as compared to their thermoset composite counterparts. Such thermoplastic composites have very smooth, hydrophobic, low surface energy surfaces that are not easily wet out by conventional thermoset adhesives. As such, it is difficult to bond such thermoplastic composites to each other or to other dissimilar substrates, such as thermoset composites, using conventional thermoset adhesives.

[0008] For an adhesive composition, the term “wetting” refers to the ease with which an adhesive can intimately contact and spread over a given substrate, providing optimum adhesion. An adhesive composition that can provide better compatibility and adhesion to the substrate is ideal to provide the most robust and reliable bonding.

[0009] Disclosed herein is a method for bonding a first substrate with low surface energy to a second substrate using a thermosetting adhesive composition that has been formulated to enhance the bonding performance of the first substrate. The second substrate may be the same or different from the first substrate. Also disclosed is a bonded structure produced from such bonding method.

[0010] Low surface energy refers to surface free energy of less than 40 dynes / cm as measured by contact angle measurements or dyne testing. The substrates with low surface energy include thermoplastic substrates and thermoplastic composites.

[0011] In one embodiment, hereafter referred to as Embodiment A, the thermosetting adhesive composition includes the following components:(a) an epoxy-based pre-react;(b) at least one amine compound as curing agent;(c) at least one inorganic clay mineral selected from phyllosilicates;(d) a flow control agent that is not inorganic clay mineral; and(e) optionally, a trifunctional or tetrafunctional epoxy resin.

[0012] The relative amounts of components (a)-(e) in Embodiment A, in weight percentages, may be as follows: (a) 15 - 25 % epoxy-based pre-react; (b) 5%-16% amine curing agent(s); (c) 1%-20% inorganic clay mineral(s); (d) 1 %-10% flow control agent; and (e) 0%-50% trifunctional or tetrafunctional epoxy resin.

[0013] The above adhesive composition may further include an elastomeric polymer with functional groups as a toughening agent in an amount of up to 30%, based on the total weight of the composition.

[0014] The adhesive composition of Embodiment A is suitable for forming continuous adhesive films.

[0015] In another embodiment, hereafter referred to as Embodiment B, the thermosetting adhesive composition includes:(a) an epoxy component which includes at least two different multifunctional epoxy resins;(b) a toughening component comprising at least one toughener selected from: acrylic block copolymers, elastomeric polymers with functional groups, epoxyelastomer adduct, polyethersulfone-polyetherethersulfone (PES-PEES) copolymer;(c) at least one amine compound as curing agent;(d) at least one inorganic clay mineral selected from phyllosilicates; and(e) a flow control agent that is not inorganic clay mineral.

[0016] The relative amounts of components (a)-(e) in Embodiment B may be as follows: a) 45 - 60 % epoxy component; (b) 15% - 30% toughening component; (c) 5%-16 % amine curing agent(s); (d) 1%-20% inorganic clay mineral(s); and (e) 1 %-10% flow control agent.

[0017] The epoxy component may have the following combination of resins:(i) diglycidyl ether of bisphenol A or bisphenol F + triglycidyl ether of aminophenol or tetraglycidyl ether of methylene dianiline + a multifunctional epoxy novolac resin; or(ii) liquid difunctional epoxy resin with CSR particles dispersed therein + triglycidyl ether of aminophenol and / or tetraglycidyl ether of methylene dianiline + a multifunctional epoxy novolac resin; or(iii) liquid difunctional epoxy resin with CSR particles dispersed therein + triglycidyl ether of aminophenol and / or tetraglycidyl ether of methylene dianiline + difunctional cycloaliphatic epoxy.

[0018] The adhesive composition of Embodiment B is suitable for forming an adhesive paste, which has a lower viscosity as compared to that of the adhesive film.

[0019] The adhesive paste may be supplied as a two-part system composed of a resinous part A and a curative part B. Such two-part system can be stored separately at room temperature until the adhesive is used. Mixing of part A and part B is required before prior to applying the adhesive to a surface. The resinous part A includes the epoxy component (a) and the toughening component (b) as mentioned above. The curative part B includes one or more amine compounds, preferably, aliphatic or cyclic amine compounds. Both part A and part B further include inorganic clay mineral and flow control agent.

[0020] For the resinous part A, the relative amounts of the components, in weight percentages based on the total weight of part A may be as follows: 50% - 65% epoxy component; 10% - 30% toughening component; 10% - 20% inorganic clay mineral; and 5% - 10% flow control agent. For the curative part B, the relative amounts of the components, in weight percentages based on the total weight of part B may be as follows: 75% - 90% amine compound(s); 5% - 15% inorganic clay mineral; and 5% - 10% flow control agent.

[0021] For the two-part system, the weight ratio of the resinous part A to the curative part B is within the range of 3:2 to 10:2. In a preferred embodiment, the weight ratio of part A to part B is 2:1.Epoxy-based Pre-react

[0022] In Embodiment A, the epoxy-based pre-react may be a reaction product of a liquid difunctional epoxy resin with core-shell rubber (CSR) particles dispersed therein and a polyethersulfone-polyetherethersulfone (PES-PEES) copolymer with terminal amine functional groups. The PES-PEES copolymer with terminal amine functional groups is alsoreferred herein as “amine-terminated PES-PEES copolymer”. As another option, the epoxybased pre-react is a reaction product of one or more difunctional epoxy resin(s), CSR particles, a multifunctional epoxy novolac resin, and amine-terminated PES-PEES copolymer. As another option, the epoxy-based pre-react is a reaction product of one or more difunctional epoxy resin(s), CSR particles, a multifunctional epoxy novolac resin, and amine-terminated PES-PEES copolymer Bisphenol A or F, tetrabromobisphenol A (TBBA) and a catalyst such as triphenylphosphine, Other possible catalysts include additional organophosphorous compounds phosphonium halides, imidazole based compounds (e.g. 2- methylimidzole); any nucleophilic or lewis bases catalysts capable of facilitating the phenolepoxide ring-opening reaction.

[0023] In the preparation of the epoxy-based pre-react, the components are mixed and the resulting mixture is heated at an elevated temperature for a duration sufficient to form a pre-react. For example, the reacting step may be carried out at a temperature in the range of 90° to 150° C for a duration of one-half to 3 hours. In preparing the composition according to Embodiment A, the epoxy-based pre-react preferably is initially prepared in a first stage.

[0024] When reacting the epoxy resin(s) with the amine-terminated PES-PEES copolymer, the ratio of epoxy resin (s) to PES-PEES copolymer is adjusted such that there is an excess of epoxy groups relative to the amine groups so that the amine groups reacted completely. As such, the resulting pre-react contains active epoxy groups, but is essentially free of amine groups. Preferably, there is a 1.5 to 10 fold excess, for example a 3.5 fold excess of epoxy groups over the active hydrogen equivalents (AHEW) of the amine- terminated PES-PEES copolymer.Epoxy Resins

[0025] The multifunctional epoxy resins to be used in the thermosetting adhesive compositions are those polyepoxides containing an average of two to four epoxy groups (oxirane rings) per molecule with the epoxy groups being the terminal groups. A difunctional epoxy resin is an epoxy resin that contains an average of two epoxy groups per molecule, a trifunctional epoxy resin is an epoxy resin that contains an average of three epoxy groups per molecule, and a tetrafunctional epoxy resin contains an average of four epoxy groups per molecule.

[0026] Suitable difunctional epoxy resins include digylcidyl ethers of bisphenol A (e.g. Epon™ 828 (liquid epoxy resin) from Hexion, DER 331 , D.E.R. 661 (solid epoxy resin)supplied by Dow Chemical Co., Tactix 123, and Araldite® 184 supplied by Huntsman Advanced Materials. Additional difuctional epoxy resins include diglycidylethers of Bisphenol F, diglycidylethers of Bisphenol S, diglycidylethers of Bisphenol Z, diglycidylethers of tetrabromo bisphenol A, and diepoxide of hydrogenated Bisphenol A.

[0027] The multifunctional epoxy novolac resins used in the adhesive compositions are different from the difunctional epoxy resins. Suitable epoxy novolac resins include phenol- based or cresol-based novolac resins, and hydrocarbon epoxy novolac resins.

[0028] Suitable epoxy novolac resins include polyglycidyl derivatives of phenolformaldehyde novolac resin or cresol-formaldehyde novolac resin having the following chemical structure (Structure I):wherein n is 0 to 5, and R is H or CH3. When R is H, the resin is a phenol-based novolac resin. When R=CH3, the resin is a cresol-based novolac resin. The phenol-based novolac resin is commercially available as DEN 428, DEN 431, DEN 438, DEN 439, or DEN 485 from Dow Chemical Co. The cresol-based novolac resin is commercially available as ECN 1235, ECN 1273, or ECN 1299 from Ciba-Geigy Corp.

[0029] The multifunctional hydrocarbon epoxy novolac resin may be a dicyclopentadiene based epoxy resin (DCPD) having the following chemical structure (Structure II):whereOR = CH— C-CH2, n = 0.2 to 3.H

[0030] Commercially available hydrocarbon epoxy novolac resins include Tactix® 71756, Tactix® 556, and Tactix® 756.

[0031] Examples of tri-functional epoxy resin (having three epoxy functional groups per molecule) include triglycidyl ether of aminophenol. Specific examples of commercially available tri-functional epoxy resins are Araldite® MY 0510, MY 0500, MY 0600, MY 0610 supplied by Huntsman Advanced Materials.

[0032] Examples of tetra-functional aromatic epoxy resin (having four epoxy functional groups per molecule) include tetraglycidyl ether of methylene dianiline. Examples of commercially available tetra-functional epoxy resins are Araldite® MY 9663, MY 9634, MY 9655, MY-721, MY-720, MY-725 supplied by Huntsman Advanced Materials.

[0033] Suitable cycloaliphatic epoxies comprise compounds that contain at least one cycloaliphatic group and at least two oxirane rings per molecule. Specific examples include diepoxide of cycloaliphatic alcohol, hydrogenated Bisphenol A (EPALLOY™ 5000, 5001 supplied by Huntsman Advanced Materials) represented by the following structure:Curing Agents and Accelerators

[0034] The curing agent for the thermosetting adhesive composition may be selected from primary and secondary amines, aliphatic and aromatic amines. Suitable curing agents include a variety of latent amine-based curing agents, which are activated at elevated temperatures, e.g. temperature above 150°F (65°C). The term “amine-based” means containing an amine compound or group. Examples of suitable curing agents include dicyandiamide (DICY), 4, 4'-diamino-diphenylsulfone (4,4’DDS), and 3,3’- diaminodiphenylsulfone (3,3’DDS), guanamine, guanidine, aminoguanidine, piperidine, combinations and derivatives thereof. The amine curing agent may be a single amine compound or a combination of different amines. In certain embodiments, the curing agent is a combination of DICY and 3,3’ or 4,4’- DDS. In other embodiments, the curing agent is a mixture of dicyandiamide (DICY) and bisurea.

[0035] The amine curing agent may be present in an amount within the range of about 5% to about 16% by weight based on the total weight of the thermosetting adhesive composition.

[0036] A curing accelerator may be used in conjunction with the amine-based curing agent to promote the curing reaction between the epoxy resins and the amine-based curing agent. Suitable curing accelerators may include alkyl and aryl substituted ureas (including aromatic or alicyclic dimethyl urea), and bisureas based on toluenediamine or methylene dianiline. One example of bisurea is 4,4’-methylene bis(phenyl dimethyl urea), commercially available as Omicure U-52 or CA 152 from CVC Chemicals, which is a suitable accelerator for dicyandiamide. Another example is 2,4-toluene bis(dimethyl urea), commercially available as Omicure U-24 or CA 150 from CVC Chemicals. The curing accelerator may be present in an amount within the range of about 0.5% to about 5% by weight based on the total weight of the adhesive composition.

[0037] For the two-part paste adhesive, the amine curing agents for the curative part B are preferably aliphatic or cyclic amine compounds capable of reacting with the multifunctional epoxy resins in part A to form highly cross-linked resin matrix. In a preferred embodiment, the amine compounds for part B are selected from the group consisting of aliphatic and cycloaliphatic amines, aromatic tertiary amines, polyethylene polyamines, amine-terminated piperazines, imidazoles, and combinations thereof.

[0038] Suitable cycloaliphatic amines include dicyclohexylamines such as bis- (paminocyclohexyl)methane (PACM) having the following structure:PACM and dimethyl PACM having the following structure:

[0039] Aromatic tertiary amine is a tertiary amine with an aromatic ring structure. An example of a suitable tertiary amineis tris-(dimethylaminomethyl) phenol (available as Ancamine® K54 from Air Products), having the following chemical structure:Ancamine K54 structure

[0040] An example of a suitable aliphatic amine is diethylene glycol, di(3-aminopropyl) ether(available as Ancamine® 1922A), having the following chemical structure:Ancamine 1922A structure

[0041] An example of a suitable polyethylene polyamine is tetraethylene pentamine (linear C-8 pentamine) having the following chemical structure:

[0042] Other examples of polyethylene polyamines include diethylenetriamine (linear C- 4 diamine), triethylenetetramine (linear C-6 triamine), and pentaethylenehexamine (linear C- 10 hexamine).

[0043] Suitable amine-terminated piperazines include 1,4-bisaminopropyl piperazine and aminoethyl piperazine. Suitable imidazoles include 2-ethyl-4-methyl imidazole.CSR Particles

[0044] The CSR particles in the adhesive compositions are relatively small in size, preferably, 300 nm or less in particle size. For example, the CSR particle size may be from about 30 nm to about 300 nm, including particle size in the range of 50-100 nm or 100-300 nm. Particle size can be measured by a laser diffraction technique, for example, using a Malvern Mastersizer 2000 instrument.

[0045] The CSR particles may be any of the core-shell particles where a soft core is surrounded by a hard shell. Preferred CSR particles are those having a polybutadiene rubber core or butadiene-acrylonitrile rubber core and a polyacrylate shell. CSR particles having a hard core surrounded by a soft shell may also be used, however.

[0046] The CSR particles may be pre-dispersed in a liquid difunctional epoxy resin. Such dispersion may contain 20% to 40% CSR particles based on the total weight of the dispersion. The dispersion of CSR particles in liquid epoxy resin is commercially available from Kaneka Texas Corporation, for example, Kane Ace™ MX 120, MX 125, and MX 156 (which contain 25%-37% by weight of CSR particles in D.E.R.™ 331 epoxy resin).Toughening Component

[0047] With reference to Embodiment B, suitable acrylic block copolymers include copolymer of poly(methyl methacrylate) and polybutyl acrylate (PMMA-pBuA), copolymer of polymethyl methacrylate and polybutadiene, and also triblock copolymers of poly(methyl methacrylate) - polybutyl acrylate - poly(methyl methacrylate) and polystyrene - polybutadiene - poly(methyl methacrylate).

[0048] Commercially available acrylic block copolymers include NANOSTRENGTH® triblock copolymers from Arkema, MAM grades M51, M52, M53, M52M. Each triblock copolymer constitutes of a center block of poly(butyl acrylate) and two side blocks of poly(methyl methacrylate).

[0049] Suitable elastomeric polymers with functional groups include carboxyl-term inated butadiene homopolymer (CTB), carboxyl terminated butadiene nitrile polymer (CTBN) and amine- terminated butadiene acrylonitrile (ATBN) elastomers. Examples of commercially available elastomeric polymers include HYPRO® 2000X162 CTB and HYPRO® ATBN 1300x16 from Huntsman.

[0050] The epoxy-elastomer adduct is a reaction product formed by reacting epoxy resin with carboxyl-terminated butyl nitrile elastomer or amine-terminated butadiene acrylonitrile (ATBN) elastomer. A specific example is EPON™ 58005 (available from Westlake Epoxy) which is an elastomer-modified epoxy-functional adduct formed from the reaction of the diglycidyl ether of bisphenol A and a carboxyl-terminated butadiene-acrylonitrile elastomer. Another example of epoxy-elastomer adduct is EPON 58006 available from Westlake Epoxy.

[0051] The polyethersulfone-polyetherethersulfone (PES-PEES) copolymer comprises a combination of polyethersulfone (PES) and polyetherethersulfone (PEES) ether-linked repeating units and contains amine terminal groups. Such copolymer is described in U.S. Patent No. 7084213.Inorganic Clay Minerals

[0052] As mentioned above, the inorganic clay mineral is selected from phyllosilicates. In general, phyllosilicates form parallel sheets of silicate tetrahedral (Si2Os) that are hydrated with water or hydroxyl groups and can also include elements such as magnesium, iron or aluminum. The phyllosilicates that may be used in the adhesive composition include talc, kaolin, pyrophyllite, muscovite, and combination thereof.

[0053] Talc refers to hydrated magnesium silicate with chemical formula of Mg3Si4O (OH)2. Kaolin refers to hydrated aluminum silicate with chemical formula of Al2Si2Os(OH)4). Pyrophyllite has the chemical formula of Al2Si4O (OH)2, and muscovite has the chemical formula KAl2(AISi3)O (OH2).

[0054] The amount of inorganic clay minerals in the adhesive composition may be in the range of 1% to 20% by weight based on the total weight of the adhesive composition.

[0055] It has been discovered that the presence of the clay minerals in the adhesive compositions improves the wetting out of thermoplastic substrates that are difficult to bond such as PAEK substrates, thereby, resulting in better adhesion of the thermoplastic substrates.Flow Control Agents

[0056] Flow control agent may be added to the adhesive composition to modify the rheological properties of the adhesive composition. The presence of flow control agents helps maintain the desired viscosity for the adhesive and also improves the sagging resistance of the adhesive during application and curing. Sagging or slump resistance is desirable when the adhesive is applied on vertical or high-angle surfaces. The flow control agents may be provided in particulate form such as flakes, powders, fibers, spheres, and pellets. It should be pointed out that the flow control agent in the adhesive composition is made of a material that is different from the inorganic clay mineral.

[0057] Flow control agents can include: various group or precipitated chalks, quartz powder, alumina, metallic aluminum powder, aluminum oxide, zinc oxide, calcium oxide, silver flakes, graphite particles or flakes, granite particles, carbon fibers, glass fibers, polymeric fibers, titanium dioxide, fused silica, fumed silica (e.g. CAB-O-SIL® TS 720), ceramic microsphers, aluminum trioxide, calcium carbonate, calcium sulfate, sand, carbon black, calcium oxide, etc.

[0058] Ceramic microspheres are small, spherical, hollow bodies. Each microsphere consists of an outer shell enclosing a hollow core. As an example, the ceramicmicrospheres are made of an inert silica-alumina ceramic material. The ceramic microspheres may have a crush strength of over 60,000 psi, a dielectric constant of about 3.7-4.6, a softening point in the range of 1000-1100°C (or 1832-2012T), and particle diameters ranging from 0.1 micron to 50 microns, or 1 to 50 microns. The high softening point of the ceramic microspheres enables them to be nonabsorbent to solvents, nonflammable, and highly resistant to chemicals. An example of commercially available ceramic microspheres which are particularly suitable for use in the adhesive composition are sold by Zeelan Industries, Inc. under the trade name Zeeospheres ®, for example, G-200, G210 and W-200. These are hollow, silica-alumina spheres with thick walls, odorless, and light gray in color.

[0059] When present, the flow control agent may be present in an amount ranging from 1% to 10% by weight based on the total weight of the adhesive composition.Bonding Method

[0060] The thermosetting adhesive composition disclosed herein is capable of bonding substrates with low surface energy, such as thermoplastic substrates or thermoplastic composites, which are difficult to bond using conventional thermosetting adhesives due to the low surface energy. The applicable bonding method includes joining a first substrate with low surface energy to a second substrate with a curable adhesive layer between the substrate, followed by curing at an elevated temperature above 25°C. The curable adhesive layer is formed from the thermosetting adhesive compositions described above. The adhesive composition may be applied as a coated film or a paste. Upon curing, the thermosetting adhesive compositions will form a substantially continuous rigid phase. The product of such bonding method is a bonded structure having a cured adhesive layer interposed between two substrates.

[0061] Curing of some adhesive compositions, particularly, adhesive compositions of Embodiment A, can be performed at a temperature in the range of 325°F- 400°F (or 162°C - 204°C), for a duration of 60 to 120 minutes. For other embodiments, the adhesive compositions can be cured at a temperature in the range of 225°F - 300°F (or 107°C - 149°C) for a duration of 60-120 minutes.

[0062] In one embodiment, the paste adhesive composition based on a two-part system is curable at or below 200°F (93°C), including room temperature (20-25 °C or 68-77 °F). For example, this paste adhesive may be cured at an elevated temperature within the range of 60-80 °C for a period of one hour.

[0063] In the present disclosure, the terms “cure” and “curing” as used herein encompass polymerizing and / or cross-linking of monomers or oligomers brought about by heating at elevated temperatures or exposure to ultraviolet light and radiation.

[0064] The adhesive film used for bonding may be prepared by combining the components of the thermosetting adhesive composition and forming a resinous film from the composition on a release paper using a conventional film coating technique. The coating step can be performed at temperatures in the range of 65°C - 94°C (or 150°F - 200°F). The adhesive film may have a film weight of from 0.02 psf to 0.15 psf (or 100 gsm to 700 gsm). The adhesive film may further include a carrier embedded therein to control bondline thickness between the bonded surfaces. The carrier may be in the form of a woven fabric, a knitted mat, or a nonwoven veil, composed of fibers derived from glass, polyester, nylon, or other polymeric materials. The carrier may be embedded by pressing it into the resinous film while the film is in a soften or molten state. Alternatively, the carrier is pressed between two resinous films.

[0065] For the paste adhesive, the components of the adhesive compositions are combined to form a paste. In the case of the two-part system, the resinous Part A and the curative Part B are mixed to form a paste. The paste adhesive can be applied by conventional dispensing means such as bead or film application onto one or more surfaces to be bonded.

[0066] The first substrate with low surface energy may be a thermoplastic substrate or a thermoplastic composite substrate. The second substrate may be a second thermoplastic substrate, a second thermoplastic composite substrate, a thermoset substrate, a thermoset composite substrate, or a metal substrate.

[0067] With respect to the thermoplastic substrate with low surface energy, the substrate contains one or more thermoplastic polymers, including high-performance thermoplastic polymers. The thermoplastic polymer(s), in total, constitutes more than 50%, e.g., 80% - 100%, by weight of the thermoplastic substrate.

[0068] As used herein, the term “high-performance thermoplastic polymer” refers to any thermoplastic polymer that has a high melting temperature (Tm) of greater than or equal to 170°C. High performance thermoplastics differ from standard plastics and engineering plastics based on their temperature stability, chemical resistance and mechanical properties. They have a continuous service temperature of greater than 170°C. Tmis defined as the position in degrees Celsius of the endothermic melting peak present in the DifferentialScanning Calorimetry (DSC) thermogram of the polymer. Such Tmcan be determined using the standard test method ASTM D 3418. High-performance polymers include polyaryletherketones (PAEK), such as polyether ketone ketone (PEKK), polyether ether ketone (PEEK), and polyether ketone (PEK), polyphenylenesulfides (PPS), polyphenylene oxide (PPO), polyetherimide (PEI), polyvinylidene difluoride (PVDF), and polyarylamide.

[0069] The composite substrates in this context refer to fiber-reinforced polymer composites composed of reinforcement fibers impregnated or infused with a polymer or a resin, or reinforcement fibers embedded in a matrix material. As used herein, the term “matrix material” refers to a mass of resin or polymer, and the expression “embedded in a matrix material” means firmly fixed or positioned within a surrounding mass of resin or polymer.

[0070] For thermoplastic composite substrates, the matrix material contains a thermoplastic polymer or a blend of thermoplastic polymers, wherein the thermoplastic polymer or blend of thermoplastic polymers constitutes more than 50%, e.g., 80% - 100%, by weight of the matrix material.

[0071] The thermoset substrate refers a curable thermoset substrate, which can be cocured with the thermosetting adhesive composition, or a cured thermoset substrate that has been fully cured or partially cured prior to bonding. The curable thermoset substrate contains one or more thermoset resins, which will harden upon thermal curing, and optional additives such as curing agents, catalysts, co-monomers, rheology control agents, tackifiers, rheology modifiers, inorganic or organic fillers, thermoplastic or elastomeric toughening agents, stabilizers, inhibitors, pigments / dyes, flame retardants, reactive diluents, and other additives well known to those skilled in the art for modifying the properties of the resin matrix before or after curing. “Fully cured” as used herein refers to 100% degree of cure. “Partially cured” as used herein refers to less than 100% degree of cure.

[0072] The thermoset resins in the thermoset substrate may include, but are not limited to, epoxy resin, unsaturated polyester resin, bismaleimide, polyimide, cyanate ester, phenolic, etc. In one embodiment, the curable thermoset substrate contains one or more multifunctional epoxy resins (i.e. polyepoxides) and at least one amine curing agent. The epoxy resin(s) and curing agent, combined, constitute more than 50 wt.%, e.g., 80 wt%-100 wt.%, of the curable thermoset substrate.

[0073] For thermoset composite substrates, the matrix material contains one or more thermoset resin(s) and the optional additives as described above in reference to the curablethermoset substrate. In one embodiment, the thermoset composite substrate contains reinforcement fibers impregnated with or embedded in a matrix resin containing one or more multifunctional epoxy resins (i.e. polyepoxides) and at least one amine curing agent. Multifunctional epoxy resins refer to polyepoxides having a plurality of epoxy groups per molecule

[0074] Suitable epoxy resins include polyglycidyl derivatives of aromatic diamine, aromatic mono primary amines, aminophenols, polyhydric phenols, polyhydric alcohols, polycarboxylic acids. Examples of epoxy resins include polyglycidyl ethers, which are prepared by reaction of epichlorohydrin or epibromohydrin with a polyphenol in the presence of alkali. Additional examples include polyglycidyl ethers of bisphenol A or bisphenol F, and polyglycidyl ethers of cresol and phenol based novolacs.

[0075] The addition of curing agent(s) and / or catalyst(s) may increase the cure rate and / or reduce the cure temperatures of the matrix resin. The curing agent for thermoset resins is suitably selected from known amine curing agents, for example, primary and secondary amines, aliphatic and aromatic amines, and mixtures thereof.

[0076] The reinforcement fibers in the thermoplastic or thermoset composite substrate may in the form of chopped or continuous fibers, tows composed of multiple filaments, continuous unidirectional fibers, nonwoven mat / veil of randomly oriented fibers, and woven or nonwoven fabrics. Nonwoven fabrics include non-crimped fabric that contains unidirectional fibers held in place by stitching. The term “unidirectional” as used herein means aligning in parallel in the same direction. Reinforcement fibers may be selected from carbon or graphite fibers, glass fibers and fibers formed of silicon carbide, alumina, boron, quartz, and ceramics, as well as fibers formed of polymers such as for example polyolefins, poly(benzothiazole), poly(benzimidazole), polyarylates, poly(benzoxazole), aromatic polyamides, polyaryl ethers and the like, and may include mixtures having two or more such fibers. In some embodiments, the fibers are selected from glass fibers, carbon fibers and aromatic polyamide fibers, such as fibers sold under the trade name KEVL R.

[0077] The thermoplastic or thermoset composite substrate may take the form of prepregs or prepreg layups such as those used for making aerospace composite structures. The term “prepreg” as used herein refers to a layer of fibrous material (e.g., in the form of unidirectional tows, nonwoven mat, or fabric ply) that has been impregnated with a matrix material. The term “prepreg layup” as used herein refers to a plurality of prepreg plies that have been laid up in a stacking arrangement.

[0078] The reinforcement fiber content in the thermoplastic composite substrate may in the range of 50% to 70% by weight based on the total weight of the composite substrate. The reinforcement fiber content in the thermoset composite substrate may in the range of 50% to 70% by weight based on the total weight of the composite substrate.

[0079] The use of the thermosetting adhesive compositions disclosed herein leads to a higher fracture toughness (Gic) at adhesive joint, more specifically, Gicof greater than 1200 J / m2, and higher percentage of cohesive failure. In general, a higher fracture toughness value and a higher percentage of cohesive failure modes indicates better bond strength and better adhesion to the substrate.

[0080] Fracture toughness is a quantitative way of expressing a material's resistance to brittle fracture when a crack is present. Gicfracture toughness of an adhesive can be determined by testing the adhesive joint between bonded substrates according to ASTM D5528, a double cantilever beam peel test.

[0081] Cohesive failure means the adhesive itself at the adhesive joint between the bonded substrates has fractured. The bond between the adhesive and the substrates is stronger than the adhesive. In analyzing an adhesive joint that has been tested to destruction, the mode of failure is often expressed as a percentage cohesive or adhesive failure. The ideal failure is a 100% cohesive failure in the adhesive layer itself.

[0082] The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context, (e.g., includes the degree of error associated with measurement of the particular quantity). For example, a number following “about” can mean the recited number plus or minus 0.1% to 1% of that recited number. The suffix “(s)” as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., the metal(s) includes one or more metals). Ranges disclosed herein are inclusive of the endpoints and all intermediate values of the ranges, for example, “1 % to 10%” includes 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, etc.

[0083] EXAMPLES

[0084] The following examples are intended for illustration purposes and are not to be construed as limiting the scope of the various embodiments of the thermosetting adhesive compositions described above.1

[0085] An adhesive composition was prepared based on the components and weight percentages (wt%) shown in Table 1.TABLE 1

[0086] MX 156 refers to Kane Ace™ MX 156 consisting of 25 wt% CSR particles in liquid difunctional epoxy.

[0087] Components no. 1-4 were used to form an epoxy-based pre-react by mixing the components and reacting the mixture at 250°F (121°C) for 60 minutes. To this pre-react, components 5-9 were added. The resulting mixture was stirred under vacuum for 30 minutes to form an adhesive composition. The adhesive composition was then coated onto a release paper at a coating temperature of 150°F (65.5°C) to form film can be coated at a weight of 0.05 psf (244 gsm).Example 2

[0088] An adhesive composition was prepared based on the components and weight percentages (wt%) shown in Table 2.TABLE 2

[0089] Components no. 1-3 were used to form an epoxy-based pre-react by mixing the components and reacting the mixture at 250°F (121°C) for 60 minutes. To this pre-react, components 4-8 were added. The resulting mixture was stirred under vacuum for 30 minutes to form an adhesive composition. The adhesive composition was then coated onto a release paper at a coating temperature of 150°F (65.5°C) to form film can be coated at a weight of 0.05 psf (244 gsm).Example 3

[0090] An adhesive composition was prepared based on the components and weight percentages (wt%) shown in Table 3.TABLE 3

[0091] Components no. 1-4 were used to form an epoxy-based pre-react by mixing the components and reacting the mixture at 250°F (121°C) for 60 minutes. To this pre-react, components 5-9 were added. The resulting mixture was stirred under vacuum for 60 minutes to form an adhesive composition. The adhesive composition was then coated ontoa release paper at a coating temperature of 150°F (65.5°C) to form film can be coated at a weight 0.05 psf (244 gsm).Example 4

[0092] An adhesive composition was prepared based on the components and weight percentages (wt%) shown in Table 4.TABLE 4

[0093] All components (minus talc, 4,4’DDS, DICY and fumed silica) were added stepwise at 200°F and mixed under vacuum for 60 minutes. The mixture was cooled to 155°F (68°C) and the remaining ingredients were added stepwise and mixed under vacuum for 60 minutes to form an adhesive composition. The adhesive composition was then coated onto a release paper at a coating temperature of 150°F (65.5°C) to form film can be coated at a weight of 0.05 psf (244 gsm).Example 5

[0094] An adhesive composition was prepared based on the components and weight percentages (wt%) shown in Table 5.TABLE 5

[0095] MX 156 was reacted with PES-PEES copolymer at 250°F (121°C) for 60 minutes to form an epoxy-based pre-react. To this pre-react, components no. 3-8 were added. The resulting mixture was stirred under vacuum for 60 minutes to form an adhesive composition. The adhesive composition was then coated onto a release paper at a coating temperature of 150°F (65.5°C) to form film can be coated at a weight of 0.05 psf (244 gsm).Example 6

[0096] An adhesive composition (Formulation 6) was prepared based on the components and weight percentages (wt%) shown in Table 6. For comparison, a similar adhesive composition was prepared based on Formulation C shown in Table 6. Note that Formulation C does not contain any talc.TABLE 6

[0097] For each adhesive composition, components no. 1-6 were used to form an epoxy-based pre-react by mixing the components and reacting the mixture at 250°F (121°C) for 60 minutes. To this pre-react, the remaining components were added. The resulting mixture was stirred under vacuum for 60 minutes to form an adhesive composition. The adhesive composition was then coated onto a release paper at a coating temperature of 150°F (65.5°C) to form film can be coated at a weight of 0.05 psf (244 gsm).Adhesive Bondina Performance

[0098] The bonding performance of the adhesive compositions based on Formulations 1-6 and C were measured. Fracture toughness Gicwas determined based on ASTM D5528 (Standard Test method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber- Reinforced Polymer Matrix Composites).

[0099] Twelve (12) plies of thermoplastic composite tapes composed of fibers impregnated with PEKK polymer (APC (PEKK-FC) / AS4D from Syensqo) were laid up and consolidated in an autoclave to form a panel with a thickness of 0.063 inch. Consolidation was carried out by heating with a 5°F / min ramp to 707°F and holding for 15 min at 707°F while under full vacuum and pressure. For each bonding test, two of such consolidated panels were prepared.

[0100] The thermoplastic composite panels were then surface treated. Following surface treatment of the thermoplastic composite panels, the panels were immediately laid up and bonded with the adhesive. The adhesive was applied to the surface of one of the treated thermoplastic substrates immediately after plasma treated. The second substrate was then placed on top of the adhesive. The bonded panel was then cured in an autoclave using the recommended adhesive curing cycle (350°F for 120 min or 250°F for 120 min).

[0101] The ease of bonding was determined by the Gicvalues (J / m2) and failure mode of the resulting tested panels. Table 7 below indicates the results of tested Gic panels and the resulting average fracture toughness values in J / m2. The average fracture toughness value was determined based on the average of four test measurements.

[0102] Failure mode was determined by both visual inspection and inspection with the use of a microscope.

[0103] The glass transition temperature (Tg) was determined by the use TMA (thermomechanical analysis) using a ramp rate of 10°C / min from room temperature to 250°C on cured specimens.TABLE 7

[0104] The control (Formulation C) which has no talc incorporation has a very low fracture toughness value as compared to and exhibits adhesive failure.

[0105] Note for Table 7: Surface Treatment #1 includes solvent wipe, then surface abrasion (e.g. grit blasting), and atmospheric pressure plasma treatment; Surface Treatment #2 includes solvent wipe, then atmospheric pressure plasma treatment.Example 7

[0106] Two paste adhesives were prepared based on formulations shown in Table 8. Amounts are shown in weight percentages (wt%).TABLE 8

[0107] MX 257 refers to Kane Ace™ MX 257 containing CSR particles dispersed in difunctional bisphenol A epoxy.Example 8

[0108] Two paste adhesives were prepared based on the two-part formulations shown in Table 9. Amounts are shown in weight percentages (wt%).TABLE 9

[0109] The resinous Part A and the curative Part B based on the above formulations were prepared separately. To form a paste adhesive, Part A and Part B were mixed.Adhesive Bonding Performance

[0110] The bonding performance of the paste adhesives based on Formulations 10-13 were carried out using the same testing procedures described above for Formulations 1-6.

[0111] Table 10 below indicates the results of tested Gicpanels, the resulting average fracture toughness values in J / m2and the failure mode for the bonded panels using the adhesives based on Formulations 10-13.TABLE 10

Claims

CLAIMSWhat is claimed is:

1. An adhesive bonding method comprising: joining a first substrate to a second substrate with a curable adhesive layer between the substrates; and curing the adhesive layer at an elevated temperature above 25°C to form a bonded structure, wherein the first substrate is a thermoplastic substrate or a thermoplastic composite substrate, and wherein the curable adhesive layer is formed from a thermosetting resin composition comprising:(a) an epoxy-based pre-react formed by reacting a mixture comprising one or more multifunctional epoxy resin(s), core-shell rubber (CSR) particles, and an amine-terminated polyethersulfone-polyetherethersulfone (PES-PEES) copolymer;(b) at least one amine compound as curing agent;(c) at least one inorganic clay mineral selected from phyllosilicates;(d) a flow control agent that is not inorganic clay mineral; and(e) optionally, a trifunctional or tetrafunctional epoxy resin.

2. The adhesive bonding method according to claim 1 , wherein the epoxy-based prereact is a reaction product of a liquid difunctional epoxy resin with core-shell rubber (CSR) particles dispersed therein and amine-terminated PES-PEES copolymer.

3. The adhesive bonding method according to claim 1 , wherein the epoxy-based prereact is formed by reacting a mixture comprising one or more difunctional epoxy resin(s), CSR particles, a multifunctional epoxy novolac resin, and amine-terminated PES-PEES copolymer.

4. The adhesive bonding method according to claim 1 , wherein the epoxy-based prereact is formed by reacting a mixture comprising one or more difunctional epoxy resin(s), CSR particles, a multifunctional epoxy novolac resin, amine-terminated PES-PEES copolymer, Bisphenol A or F, and a catalyst.

5. The adhesive bonding method according to any of the preceding claims, wherein the curable adhesive layer comprises a carrier embedded therein, and said carrier is in the form of a woven fabric, a knitted mat, or a nonwoven veil.

6. The adhesive bonding method according to claim 5, wherein the carrier comprises fibers selected from glass fibers, polymeric fibers, and combination thereof.

7. An adhesive bonding method comprising: joining a first substrate to a second substrate with a curable paste adhesive between the substrates; and curing the paste adhesive at an elevated temperature above 25°C to form a bonded structure, wherein the first substrate is a thermoplastic substrate or a thermoplastic composite substrate, and wherein the curable paste adhesive is formed from a thermosetting resin composition comprising:(a) an epoxy component comprising at least two different multifunctional epoxy resins;(b) a toughening component comprising at least one toughener selected from: acrylic block copolymers, elastomeric polymers with functional groups, epoxyelastomer adduct, polyethersulfone-polyetherethersulfone (PES-PEES) copolymer;(c) at least one amine compound as curing agent;(d) at least one inorganic clay mineral selected from phyllosilicates; and(e) a flow control agent that is not inorganic clay mineral.

8. The adhesive bonding method according to claim 7, wherein the epoxy component comprises diglycidyl ether of bisphenol A or bisphenol F, triglycidyl ether of aminophenol and / or tetraglycidyl ether of methylene dianiline, and a multifunctional epoxy novolac resin.

9. The adhesive bonding method according to claim 7, wherein the epoxy component comprises a liquid difunctional epoxy resin with CSR particles dispersed therein, triglycidyl ether of aminophenol and / or tetraglycidyl ether of methylene dianiline, and a multifunctional epoxy novolac resin.

10. The adhesive bonding method according to claim 7, wherein the epoxy component comprises a liquid difunctional epoxy resin with CSR particles dispersed therein, triglycidyl ether of aminophenol and / or tetraglycidyl ether of methylene dianiline, and a difunctional cycloaliphatic epoxy.

11. An adhesive bonding method comprising: forming a two-part adhesive system comprising a resinous Part A and a curative PartB;mixing Part A with Part B to form a curable paste adhesive; joining a first substrate to a second substrate with the curable paste adhesive between the substrates; and curing the paste adhesive at an elevated temperature above 25°C to form a bonded structure, wherein the first substrate is a thermoplastic substrate or a thermoplastic composite substrate, and wherein the resinous Part A comprises:(a) an epoxy component comprising at least two different multifunctional epoxy resins;(b) a toughening component comprising at least one toughener selected from: acrylic block copolymers, elastomeric polymers with functional groups, epoxyelastomer adduct, polyethersulfone-polyetherethersulfone (PES-PEES) copolymer;(c) at least one inorganic clay mineral selected from phyllosilicates; and(d) a flow control agent that is not inorganic clay mineral; and the curative Part B comprises:(e) at least one amine compound selected from: cycloaliphatic amines, polyethylene polyamines, amine-terminated piperazines, imidazoles, and combinations thereof(f) at least one inorganic clay mineral selected from phyllosilicates; and(g) a flow control agent that is not inorganic clay mineral.

12. The adhesive bonding method according to any one of the preceding claims, wherein the at least one inorganic clay mineral selected from: talc, kaolin, pyrophyllite, muscovite, and combinations thereof.

13. The adhesive bonding method according to any one of the preceding claims, wherein the flow control agent is selected from: precipitated chalks, quartz powder, alumina, metallic aluminum powder, aluminum oxide, zinc oxide, calcium oxide, granite particles, titanium dioxide, fused or fumed silica, ceramic microspheres, aluminum trioxide, calcium carbonate, calcium sulfate, sand, calcium oxide, silver flakes, graphite flakes or particles, carbon black particles, carbon fibers, glass fibers, polymeric fibers.

14. The adhesive bonding method according to any one of the preceding claims, wherein the thermoplastic substrate or thermoplastic composite substrate comprises one or more thermoplastic polymers selected from: polyaryletherketones (PAEK), such as polyetherketone ketone (PEKK), polyether ether ketone (PEEK), and polyether ketone (PEK), polyphenylenesulfides (PPS), polyphenylene oxide (PPO), polyetherimide (PEI), polyvinylidene difluoride (PVDF), and polyarylamide.

15. The adhesive bonding method according to any one of the preceding claims, wherein the thermoplastic composite substrate comprises reinforcement fibers embedded in a thermoplastic polymer matrix.

16. The adhesive bonding method according to any one of the preceding claims, wherein the second substrate is the same as or different from the first substrate in composition.

17. The adhesive bonding method according to any one of the preceding claims, wherein the second substrate is selected from a thermoplastic substrate comprising one or more thermoplastic polymers, a thermoplastic composite substrate comprising one or more thermoplastic polymers and reinforcement fibers, a thermoset substrate comprising one or more thermoset resins, a thermoset composite substrate comprising one or more thermoset resins and reinforcement fibers, or a metallic substrate.

18. A bonded structure produced by the adhesive bonding method according to any one of the preceding claims.

19. A laminate structure comprising a first substrate joined to a second substrate by a curable adhesive layer between the substrates, wherein the first substrate is a thermoplastic substrate or a thermoplastic composite substrate, and wherein the curable adhesive layer is formed from a thermosetting resin composition comprising:(a) an epoxy-based pre-react formed by reacting a mixture comprising one or more multifunctional epoxy resin(s), core-shell rubber (CSR) particles, and an amine-terminated polyethersulfone-polyetherethersulfone (PES-PEES) copolymer;(b) at least one amine compound as curing agent;(c) at least one inorganic clay mineral selected from phyllosilicates;(d) a flow control agent that is not inorganic clay mineral; and(e) optionally, a trifunctional or tetrafunctional epoxy resin.

20. A laminate structure comprising a first substrate joined to a second substrate by a curable adhesive between the substrates, wherein the first substrate is a thermoplastic substrate or a thermoplastic composite substrate, and wherein the curable adhesive is formed from a thermosetting resin composition comprising:(a) an epoxy component comprising at least two different multifunctional epoxy resins;(a) a toughening component comprising at least one toughener selected from: acrylic block copolymers, elastomeric polymers with functional groups, epoxyelastomer adduct, polyethersulfone-polyetherethersulfone (PES-PEES) copolymer;(b) at least one amine compound as curing agent;(c) at least one inorganic clay mineral selected from phyllosilicates; and(d) a flow control agent that is not inorganic clay mineral.

21. A laminate structure comprising a first substrate joined to a second substrate by a curable adhesive between the substrates, wherein the first substrate is a thermoplastic substrate or a thermoplastic composite substrate, and wherein the curable adhesive is formed from a thermosetting resin composition comprising:(a) an epoxy component comprising a diglycidyl ether of bisphenol A or bisphenol F and at least one other multifunctional epoxy resin selected from: (i) trifunctional or tetra-functional epoxy resins, and (ii) multifunctional epoxy novolac resins;(b) a toughening agent selected from: acrylic block copolymers, elastomeric polymers with functional groups, epoxy-elastomer adduct, polyethersulfonepolyetherethersulfone (PES-PEES) copolymer;(c) at least one amine compound as curing agent;(d) at least one inorganic clay mineral selected from phyllosilicates; and(e) optionally, a flow control agent that is not inorganic clay mineral.

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