Primer composition for adhesive bonding and method of use thereof

Abstract

Disclosed herein is a solvent-based bond primer composition containing one or more organic solvents, one or more epoxy resins, one or more curing agents, a silane compound, and a small amount of nanometer-sized, submicron-sized, or micron-sized core-shell rubber particles. Also disclosed is a method of applying the solvent-based bond primer composition onto a metal surface of a first substrate prior to bonding the metal surface to a second substrate with a curable adhesive.

Description

[Technical Field]

[0001] In the manufacture of composite structures, particularly those in the aerospace and automotive industries, it is common to utilize structural adhesives to bond fabricated metal structures to metal or composite adherends or to laminate one or more prepreg plies of resin-impregnated fibrous reinforcement to the fabricated metal structure. Bonding typically requires curing the structural adhesive after the structures are bonded. To ensure maximum bond strength, metal surfaces are generally carefully cleaned of dust, dirt, grease, and metal oxidation products immediately prior to bonding. Unfortunately, this procedure is generally unusable because the cleaning and bonding operations are often separated by long rest periods. During these periods, the metal surface may hydrolyze, reducing the bond strength. A solution to overcome this difficulty is to apply a primer to the cleaned metal surface prior to adhesive bonding. Such primers are often referred to as bonding primers.

[0002] In the manufacture of aerospace structures requiring bonding of a metal structure to another surface, a challenge has been to provide a primer on the metal surface that can maintain an overall balanced bond performance that satisfies both bond strength and long-term corrosion protection requirements. When a primer layer is applied by spraying using a spray nozzle onto the surface of a large metal section on the production floor, e.g., an aluminum alloy section of an aircraft having dimensions (width and / or length) greater than 1.5 m, the target thickness of the primer layer is typically 0.2 mils (i.e., 5.1 μm). Due to limitations in the surface area that can be covered by a single spray nozzle at one time and variations in the distance between the spray nozzle and the metal surface, it has been found that the thickness of the primer layer is not uniform over the large surface area being sprayed. Frequently, there are portions or sections of the primer layer with thicknesses as thin as 0.1 mils (2.5 μm) or as thick as 0.3 mils (7.6 μm) or more. It is well known to those skilled in the art of bonding that bonding primers exhibit unique thickness sensitivity in terms of bond strength, particularly peel strength. It has been found that most bond primer products currently on the market tend to exhibit significant peel strength degradation when the primer thickness exceeds approximately 0.25 mils (6.4 μm). In particular, peel strength at low temperatures, such as −67°F (−55°C), decreases even more significantly for primer layers with thicknesses greater than 0.25 mils (6.4 μm). Peel strength measurements at such low temperatures are typically required for aerospace applications. Therefore, it has been a significant challenge to develop bond primers that exhibit thickness tolerance for low-temperature peel testing without adversely affecting overall performance, such as lap shear strength and handling properties. The term “thickness tolerance,” in this context, refers to the ability to tolerate thickness variations. From the perspective of primer formulation chemistry, improving primer thickness tolerance requires increased reinforcement efficiency in the polymer network of the cured primer layer. Furthermore, any reinforcement material added to the primer composition to enhance such reinforcement efficiency should be compatible with or work synergistically with the primer chemistry without adversely affecting the overall performance of the primer.There remains a need for a method of bonding metal structures using a primer formulation that can provide high toughness in terms of peel strength at low temperatures, particularly below 0°C, e.g., -55°C, and that can be applied at primer thicknesses of 0.3 mils (7.6 μm) or greater. [Brief explanation of the drawings]

[0003] [Figure 1-2] 1 shows the failure mode of a floating roller peel test performed on a test coupon using a 0.2 mil thick primer film. [Figure 3-4] 1 shows the failure mode of a floating roller peel test performed on test coupons using a primer film with a 0.3 mil primer thickness. [Figure 5-6] 1 shows the failure mode of a floating roller peel test performed on a test coupon using a 0.3 mil thick primer film. DETAILED DESCRIPTION OF THE INVENTION

[0004] Disclosed herein is a solvent-based bonding primer composition for application onto a metal surface of a first substrate prior to bonding the metal surface to a second substrate with a curable adhesive.

[0005] The bonding primer composition is a solvent-based dispersion containing a mixture of one or more epoxy resins, a curing agent, a silane compound, a small amount of core-shell rubber (CSR), and an organic solvent, with a solids content of 5% to 30% by weight based on the total weight of the composition (here, solids refers to all components in the primer composition except the liquid solvent). The CSR particles may be nanometer-sized (less than 100 nm), submicron-sized (100 nm to 1000 nm), or micron-sized (1 micron to 100 microns). Preferably, the CSR particles are nano-sized particles with a diameter of about 100 nm or less. The amount of CSR particles is less than 2% by weight (weight percent), preferably in the range of 0.2% to 0.8% by weight, based on the total weight of the composition.

[0006] According to one embodiment, the solvent-based primer composition comprises: (i) an epoxy component comprising 5 to 15 wt. % of one or more epoxy resins; (ii) 10 to 20 parts of a curable component per 100 parts of the epoxy component (the curable component includes one or more curing agents and, optionally, one or more catalysts); (iii) 0.1 to 2 wt. % of an organosilane; (iv) 0.2–0.8 wt % CSR particles; (v) a mixture of two or more organic solvents to provide a solids content of 5% to 30% by weight; It is a dispersion containing

[0007] The weight percentages disclosed above are based on the total weight of the composition.

[0008] Optionally, the primer composition contains up to 3 wt. % of at least one chromate or non-chromate corrosion inhibitor, and / or up to 0.3 wt. % of inorganic fillers in particulate form and / or pigments / dyes, where the term "up to" means greater than zero (0).

[0009] The solvent mixture includes a volatile organic solvent such as methyl ethyl ketone (MEK), diacetone alcohol (DAA), tetrahydrofuran (THF), isopropyl alcohol, acetone, ethylene glycol, xylene, toluene, and ethyl acetate. The solvent does not include water in this context. Water is not added to form the solids disclosed above. In one embodiment, the solvent mixture includes MEK in combination with at least one solvent selected from diacetone alcohol (DAA), acetone, isopropyl alcohol, tetrahydrofuran (THF), ethylene glycol, xylene, toluene, and ethyl acetate. In a preferred embodiment, the solvent mixture includes methyl ethyl ketone (MEK) and diacetone alcohol (DAA). Preferably, the weight ratio of MEK to DAA is 60:40 to 95:05, preferably 70:30 to 90:10. In another embodiment, the solvent mixture consists of MEK, DAA, and THF, wherein the amount of MEK is greater than 50 wt. % based on the total weight of the mixture.

[0010] The solvent is selected to produce a homogeneous solution without phase separation prior to the addition of any corrosion inhibitor. CSR particles are uniformly dispersed throughout this homogeneous solution. A "homogeneous solution" refers to a homogeneous mixture of multiple substances in a single phase, where the solvent or solvent mixture is present in the largest amount. Components (i) through (v) above form a single phase. The homogeneous solution can remain in this single-phase stability for very long periods, e.g., more than one year. The solvent-based primer composition has a long shelf life of at least 180 days at room temperature (23°C to 25°C) and much longer at refrigerated temperatures below 4°C to 8°C, where the CSR particles do not settle or agglomerate to the bottom of the container. Such CSR particle precipitation or agglomeration would adversely affect primer film quality and bonding performance. It has been found that a homogeneous solution with uniformly dispersed CSR particles, in the absence of corrosion inhibitor particles, can remain in this state unaffected at room temperature for more than one year without substantial precipitation or agglomeration of the CSR particles.

[0011] Generally, surface treatments for preparing metal surfaces prior to adhesive bonding include: The method includes applying the solvent-based primer composition disclosed herein onto a metal surface to form a curable primer film having a continuous surface.

[0012] Generally, the primer composition of the present disclosure comprises: (a) applying a solvent-based primer composition onto a metal surface of a first substrate to form a curable primer film; (b) adhesively bonding the first substrate with the primer film thereon to a second substrate, whereby a curable, polymeric adhesive is disposed between the primer film and the second substrate; (c) curing the primer film and adhesive to form a bonded structure; The adhesive is incorporated into a bonding method including:

[0013] The metal surface to be treated may be the surface of a metal selected from aluminium and aluminium alloys, steel, titanium and titanium alloys.

[0014] For adhesive bonding of a metal substrate to another substrate (metal or composite substrate), the solvent-based primer composition of the present disclosure can be applied to the metal surface by spray nozzle or manual brushing to form a curable primer film. The primer composition can be applied to the metal surface in multiple coats, for example, by spraying or brushing, until the desired film thickness is achieved. The primed surface is then allowed to dry in air, for example, for 15 to 40 minutes, before curing and bonding.

[0015] The metal surface is preferably pretreated before applying the primer composition to enhance adhesion of the metal surface to a subsequently applied primer film. The primer film is cured in an oven at elevated temperature, e.g., 250°F to 350°F (121°C to 177°C), for one hour before joining the assembly. The primed surface of the metal substrate is then bonded to a second substrate by providing a curable, polymeric adhesive between the primed surface and the second substrate. The second substrate may be another metal substrate (e.g., aluminum or an aluminum alloy, steel, titanium, or a titanium alloy) or a composite substrate consisting of reinforcing fibers embedded in or impregnated with a matrix resin. The adhesive may be applied onto the surface of the second substrate, or alternatively, the adhesive may be applied onto the primed surface of the first substrate. The resulting assembly is then subjected to an elevated temperature cure to cure the adhesive and thereby produce a bonded structure. Curing may be carried out by applying heat and pressure to the assembly. The primer composition is formulated to be compatible with conventional curing, polymeric adhesives, particularly epoxy adhesives, that are curable at temperatures ranging from 250°F to 350°F (121°C to 177°C).

[0016] The term "substrate" as used herein includes layers and structures of any shape and configuration.

[0017] The terms "cure" and "curing" as used herein refer to the hardening of a polymeric material by cross-linking of polymer chains, induced by chemical additives, ultraviolet light, or heat. A material that is "curable" is one that is capable of being cured, i.e., made hard.

[0018] If the second substrate is a composite substrate consisting of reinforcing fibers and a matrix resin, the matrix resin may be partially or fully cured, or may be uncured. If the matrix resin is uncured or only partially cured prior to adhesive bonding of the two substrates, full curing of the matrix resin occurs simultaneously with curing of the adhesive during the bonding step.

[0019] To enhance adhesion of the metal surface to a subsequently applied polymeric primer film, the metal surface may be pretreated prior to applying the primer composition thereon. Suitable surface treatments include wet etching, anodization such as phosphoric acid anodization (PAA) and phosphoric / sulfuric acid anodization (PSA), and sol-gel processes known to those skilled in the art. A more specific example of a suitable surface treatment is ASTM D2651, which involves cleaning with a soap solution, followed by wet etching, and then anodizing with an acid solution. The solvent-based primer compositions disclosed herein are formulated to be compatible with these various surface treatments.

[0020] PAA typically involves forming a metal oxide surface using phosphoric acid (e.g., ASTM D3933), while PSA typically involves forming a metal oxide surface using phosphoric-sulfuric acid. Anodization creates a porous, rough surface into which the primer composition can penetrate. Adhesion results primarily from the mechanical interlocking between the rough surface and the primer film.

[0021] The sol-gel process typically involves the growth of metal-oxopolymers by hydrolysis and condensation reactions of aqueous solutions of organofunctional silanes and zirconium alkoxide precursors to form an inorganic polymer network on the metal surface. Sol-gel coatings can provide good adhesion between the metal surface and a subsequently applied primer film through covalent chemical bonds.

[0022] Epoxy resin Suitable epoxy resins include multifunctional epoxy resins having a functionality of at least about 1.8, or at least about 2. Epoxy resins are solid, optionally chain-extended, glycidyl ethers of phenols, such as resorcinol and bisphenols, e.g., bisphenol A, bisphenol F, novolacs, and the like. Solid glycidyl derivatives of aromatic amines and aminophenols, such as N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane, are also suitable. Furthermore, the epoxy resins may have an epoxy equivalent weight (EEW) of about 100 to 8000.

[0023] The epoxy resin may be in solid form, dispersion, or solution in a solvent or other continuous phase medium. The epoxy resin in the dispersed phase may be a dispersion of two or more epoxy resins in the form of a mixture of different particles, or may consist of only one type of particle containing two or more epoxy resins. Thus, a flexibilized epoxy, such as a higher molecular weight bisphenol A or bisphenol F epoxy, may be blended with a high-temperature resistant epoxy, such as tetraglycidylmethylenedianiline (TGMDA), and the mixture is then dissolved or dispersed in a solvent or other continuous phase medium. These same epoxy resins may advantageously be dispersed separately without blending.

[0024] Mixtures of different epoxy resins can be used. In one embodiment, the epoxy resin mixture includes a novolac epoxy resin and a diglycidyl ether of bisphenol A ("DGEBA") resin. Examples include novolac epoxy resins such as EPON SU-8, available from Hexion, and bisphenol A epoxy resins such as DER669 and DER664, available from Dow Chemical Co. In another embodiment, the resin mixture contains an epoxy resin having a functionality of about 4 or less and an epoxy resin having a functionality of about 5 or greater. The use of higher functionality epoxy resins, i.e., epoxy resins having a functionality of 5 or greater, is preferred in small amounts, e.g., less than 40 wt. % based on the total weight of all epoxy resins in the composition. The use of such higher functionality epoxy resins in such smaller amounts has been found to increase the solvent resistance of the cured primer composition without substantially reducing adhesive properties.

[0025] In one embodiment, the primer composition comprises the following epoxy resin: 1) 30 to 80% by weight of an epoxy resin having a functionality of about 1.8 to about 4 and an epoxy equivalent weight of about 400 to about 1000; 2) 5 to 35% by weight of an epoxy resin having a functionality of about 1.8 to about 4 and an epoxy equivalent weight of about 2000 to about 8000; and 3) 5 to 25% by weight of an epoxy resin having a functionality of about 5 or more and an epoxy equivalent weight of about 100 to about 400 a mixture of Here, weight percentages total 100% based on the total weight of the epoxy mixture.

[0026] The total amount of epoxy resin may be about 5 to 15 wt % based on the total weight of the primer composition.

[0027] Core-shell rubber particles Individual core-shell rubber (CSR) particles generally have a core made of a polymeric material with elastomeric or rubber-like properties (i.e., a glass transition temperature below about 0° C., e.g., below about −30° C.) surrounded by a shell made of a non-elastomeric polymeric material (i.e., a thermoplastic or thermosetting / crosslinked polymer with a glass transition temperature above ambient temperature, e.g., above about 50° C.). For example, the core may be made of a diene homopolymer or copolymer (e.g., a homopolymer of butadiene or isoprene, a copolymer of butadiene or isoprene with one or more ethylenically unsaturated monomers such as vinyl aromatic monomers, (meth)acrylonitrile, (meth)acrylates, etc.), while the shell may be made of a polymer or copolymer of one or more monomers with a suitable high glass transition temperature, such as (meth)acrylates (e.g., methyl methacrylate), vinyl aromatic monomers (e.g., styrene), vinyl cyanides (e.g., acrylonitrile), unsaturated acids and anhydrides (e.g., acrylic acid), (meth)acrylamide, etc. The polymer or copolymer used for the shell may have acid groups that are ionically crosslinked by metal carboxylate formation (e.g., by forming a salt of a divalent metal cation). The shell polymer or copolymer may also be covalently crosslinked by using a monomer with two or more double bonds per molecule. Other elastomeric polymers, such as polybutyl acrylate or polysiloxane elastomers (e.g., polydimethylsiloxane, especially crosslinked polydimethylsiloxane), may also be suitable for use for the core. The particles may consist of three or more layers (e.g., a central core of one elastomeric material may be surrounded by a second core of a different elastomeric material, or the core may be surrounded by two shells of different compositions, or the particles may have a soft-core-hard-shell-soft-shell-hard-shell structure). Either the core, the shell, or both the core and shell may be crosslinked (e.g., ionically or covalently), as described, for example, in U.S. Pat. No. 5,686,509. The shell may be grafted onto the core.The polymer comprising the shell may have one or more different types of functional groups (e.g., epoxy groups, carboxylic acid groups) that can interact with other components of the primer composition. The particles may have three or more layers (e.g., a central core of one elastomeric material surrounded by a second core of a different elastomeric material, or the core may be surrounded by two shells of different compositions). The core comprises about 50% to about 95% by weight of the particle, while the shell comprises about 5% to about 50% by weight of the particle.

[0028] In a preferred embodiment, the CSR particles have a polybutadiene rubber core, a styrene-butadiene rubber core, or a silicone rubber core and a polyacrylate or butadiene-acrylic copolymer or butadiene-styrene copolymer shell.

[0029] The CSR particles are preferably pre-dispersed in a solvent or resin to maintain their original particle size before and after addition to the primer formulation. For example, they can be pre-dispersed in any of the aforementioned epoxy resins at a concentration of 10% to 60% by weight. Pre-dispersed CSR particles are available under the Kane Ace trademarks, such as MX 120 (a liquid bisphenol A epoxy with approximately 25% by weight CSR), MX 125 (a liquid bisphenol A epoxy with approximately 25% by weight CSR), Kane Ace MX-156 (a liquid bisphenol A epoxy with approximately 25% by weight CSR), MX 215 (an epoxidized phenol novolac with approximately 25% by weight CSR), MX-217 (a phenol novolac epoxy with approximately 25% by weight CSR), and MX-135 (a liquid bisphenol F epoxy with approximately 25% by weight CSR). TMMX is commercially available from Kaneka. Alternatively, CSR in fine powder form can also be used. CSR particles may have nanometer size (less than 100 nm), submicron size (100 nm to 1000 nm), or micron size (1 micron to 100 microns). In certain embodiments, CSR particle size ranges from 10 nm to 3000 nm, preferably 10 nm to 500 nm. Particle size measurements can be performed by scanning electron microscopy (SEM) or scanning transmission electron microscopy (STEM) for sizes less than 1 micron, or by optical microscopy for submicron and larger particle sizes.

[0030] It is desirable that the CSR particles form a highly stable dispersion, i.e., do not aggregate and maintain their original particle size after being added to the primer composition for an extended period of time, preferably more than one month.

[0031] Curing Agents and Catalysts The solvent-based primer composition contains one or more amine-containing curing agents, preferably latent amines that are not highly reactive at room temperature (23°C to 25°C). Suitable curing agents include aromatic diamines, dicyandiamide (DICY), and hydrazides. Specific examples of aromatic diamines include 2,2-bis-4-(4-aminophenoxy)phenylpropane (BAPP) (available from BASF), 4,4'-diaminodiphenylsulfone (4,4'DDS), and 3,3'-diaminodiphenylsulfone (3,3'DDS). Hydrazides include dihydrazides, trihydrazides, and tetrahydrazides. Dihydrazides are represented, for example, by the active group [H2NHNC(=O)-RC(=O)NHNH2] (where R is any polyvalent organic group), such as carbodihydrazide (R=CH2). Specific examples include adipic dihydrazide (ADH), sebacic dihydrazide (SDH), valine dihydrazide (VDH), isophthalic dihydrazide (IDH), phthalic dihydrazide, terephthalic dihydrazide, and naphthalenedicarboxylic dihydrazide. Other hydrazide curing agents include 1,2,3-benzenetricarboxylic trihydrazide, trimellitic trihydrazide, trimesic trihydrazide, aromatic monohydrazides, aliphatic monohydrazides, aliphatic monohydrazides, aliphatic dihydrazides, aliphatic trihydrazides, aliphatic tetrahydrazides, aromatic monohydrazides, aromatic dihydrazides, aromatic trihydrazides, aromatic tetrahydrazides, and naphthoic hydrazide.

[0032] A catalyst or accelerator may be added as an optional component to accelerate the curing / crosslinking of the thermosetting resin or to enable curing at lower temperatures. Such a catalyst / accelerator may be added if a particular curing agent is not sufficiently active to achieve cure of the primer composition at the heating temperature of the primer composition. For example, if the curing agent is active at 350°F, a catalyst is added to enable curing at about 250°F. The catalyst / accelerator may be in particulate form, with a particle size such that essentially 100% of the particles have an average diameter of less than about 30 μm. Preferred catalysts / accelerators include, but are not limited to, bis-urea and imidazoles. A preferred bis-urea is toluene-2,4-bis(N,N'-dimethylurea). In some embodiments, a combination of BAPP and bis-urea is used.

[0033] The curing agent, alone or in combination with one or more catalysts / accelerators, may be present in a total amount (ie, total amount of epoxy or epoxies) of about 2 to 30 parts per 100 parts of epoxy resin.

[0034] Silane Compounds The silane compound in the solvent-based primer composition has a silane functional group that can react or bond with the material to be bonded to the metal surface. Suitable silane compounds include organosilanes. Organosilanes with hydrolyzable groups are preferred. In certain embodiments, the organosilane has the general formula: [ka] wherein n is 0 or greater; each X is OH, OCH3, and OCH2H5; R1 is CH=CH2; [ka] or CH2-CH2-CH2-Y (wherein Y is NH2, SH, OH, NCO, NH-CO-NH2, NH-(CH2)3NH2, NH-aryl, [ka] wherein each R2 is alkyl, alkoxy, aryl, substituted aryl, or R1. It has.

[0035] Examples of suitable commercially available organosilane compounds are Silquest TM A-186, beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; Silquest TM A-187, gamma-glycidoxypropyl-trimethoxysilane; Silquest TM A-189, gamma-mercaptopropyltrimethoxysilane; Silquest TM A-1100, gamma-aminopropyltriethoxysilane; Silquest TM A-1170, bis-(gamma-trimethoxy-silylpropyl)amine; and Y-9669, N-phenyl-gamma-aminopropyl-trimethoxysilane, available from Momentive Performance Materials Inc. Other suitable commercially available organosilanes include DOWSIL 1000 series manufactured by The Dow Chemical Company. TM Z-6040 Silane, gamma-glycidoxypropyl-trimethoxysilane.

[0036] Generally, the organosilane is present in the solvent-based primer composition in an amount ranging from about 0.1 to 2 weight percent, based on the total weight of the composition.

[0037] Corrosion inhibitors The solvent-based primer compositions disclosed herein may also include corrosion inhibitors to further improve long-term, rust-preventing performance.

[0038] Chromate or non-chromate corrosion inhibitors can be used in solvent-based primer compositions, but non-chromate compounds are preferred to comply with environmental, health, and safety regulations. Examples of suitable chromate corrosion inhibitors include strontium chromate, barium chromate, zinc chromate, and calcium chromate. Non-chromate corrosion inhibitors include NaVO3, VO4, VO2O 7、 Examples of inorganic, non-chromate corrosion inhibitors include inorganic compounds containing one or more ions selected from the group consisting of phosphate, phosphonate, molybdate, cerium, and borate. Examples of inorganic, non-chromate corrosion inhibitors include, but are not limited to, metavanadate anions, such as sodium metavanadate, combinations of molybdate and metavanadate, or any combination of molybdate, metavanadate, phosphate, phosphonate, cerium, or borate. Carbon-based materials, such as graphene, are also suitable. Organic corrosion inhibitors, such as those chemically fixed or encapsulated on the surface of particles and releasable in the event of corrosion, are also suitable. Examples of such releasable organic corrosion inhibitors are described in U.S. Patent Application Publication No. 2010 / 0247922, published September 30, 2010. Combinations of different corrosion inhibitors may be used.

[0039] If added, the total amount of corrosion inhibitors may be up to 3 wt % based on the total weight of the primer composition.

[0040] Optional Additives The solvent-based primer composition may optionally contain conventional dyes, pigments, and inorganic fillers. The total amount of such optional additives is less than 3% by weight, for example, 0.1% to 2% by weight. The advantage of a composition containing a dye or pigment is that surface coverage can be more easily assessed by visual methods. Inorganic fillers in particulate form may also be added to control rheology for application process and stability. Suitable inorganic fillers include fumed silica, clay particles, and the like. [Example]

[0041] The following examples demonstrate the performance results obtained using solvent-based bond primer formulations with low amounts of CSR particles compared to other primer formulations that do not contain such CSR particles.

[0042] Example 1 Primer Formulations 1 and 2 were prepared according to the formulations disclosed in Table 1.

[0043] [Table 1]

[0044] Each primer formulation was sprayed onto a surface-treated aluminum alloy sheet using a spray gun to form a primer film with a thickness of 0.2 mil. This priming process was repeated on another aluminum alloy sheet to form a primer film with a thickness of 0.3 mil. The surface treatment followed ASTM D 2651, including cleaning, FPL etching, and PAA anodizing. The resulting uncured film was air-dried at ambient temperature. A floating roller peel test (ASTM D3167) was performed to measure the bonding performance of the primer film. These tests were performed after bonding the primed surface to another aluminum alloy sheet using a curable, epoxy-based adhesive (FM 73M and FM 94M from Cytec Industries Inc.) and subsequent curing.

[0045] The floating roller peel test results for primer formulation 1 without CSR nanoparticles are reported in Table 2.

[0046] [Table 2]

[0047] The floating roller peel test results for primer formulation 2 with core-shell rubber nanoparticles at 3.5% loading based on total solids are reported in Table 3.

[0048] [Table 3]

[0049] Figures 1 and 2 show the failure modes of floating roller peel test coupons using Formulations 1 and 2, respectively, with a 0.2 mil primer thickness and FM 73M adhesive.

[0050] Figures 3 and 4 show the failure modes of floating roller peel test coupons using Formulations 1 and 2, respectively, with a 0.3 mil primer thickness and FM 73M adhesive.

[0051] Figures 5 and 6 show the failure modes of floating roller peel test coupons using Formulations 1 and 2, respectively, with a 0.3 mil primer thickness and FM 94M adhesive.

[0052] Formulations 1 and 2 in Table 1 are primer compositions without and with CSR particles, respectively. Peel test results at 0.2 mil and 0.3 mil thicknesses based on both formulations and two adhesives (FM 73 and FM 94 bond adhesives) are compared in Tables 2 and 3 and Figures 1-6. The low-temperature peel strength at -55°C (-67°F) based on Formulation 2 with a low CSR particle loading is substantially higher than that for Formulation 1, especially for the 0.3 mil thickness. The failure mode of the coupons tested with Formulation 2 is also much more cohesive than that of Formulation 1, as shown in Figures 2-4. In fact, Formulation 2 exhibits nearly identical strengths for both the 0.2 mil and 0.3 mil thicknesses at room temperature and the low test temperature. These remarkable results demonstrate the effectiveness of utilizing CSR particles to improve the toughness of the primer layer. Furthermore, Formulation 2 with a low CSR loading exhibits identical spray and film-forming performance to those of Formulation 1. The 3000 hour corrosion performance for both Formulations 1 and 2 easily passed all performance requirements.

[0053] Example 2 Primer Formulations 3 and 4 were prepared according to the formulations disclosed in Table 4.

[0054] [Table 4]

[0055] Each of the primer formulations was sprayed onto a surface-treated aluminum alloy sheet using a spray gun to form a primer film with a thickness of 0.3 mil. Floating roller peel tests were performed as described in Example 1 using FM 94 M adhesive. The results for primer formulations 3 and 4 using powdered CSR at a thickness of 0.3 mil are reported in Table 5.

[0056] [Table 5]

[0057] Formulations 3 and 4 in Table 4 are primer compositions containing two powdered CSR products (Paraloid EXL-2691A from DOW and ZEFIAC F351 from AICA Kogyo Co., Ltd.) at the same CSR loading of approximately 3.5% on a dry solids basis. Both formulations can demonstrate very good ambient and cold peel strength at 0.3 mil thickness for FM 94 adhesive (Table 5). However, the cured primer film using Paraloid EXL-2691A is too rough and does not meet the film appearance requirements. In comparison, the ZEFIAC F351 film exhibits a smooth cured film, achieving balanced overall performance. At first glance, Paraloid EXL-2691A exhibits more cohesion in the primer formulation than ZEFIAC F351. These results clearly demonstrate the importance of powdered CSR particles being well dispersed in the primer network matrix to achieve significantly improved low-temperature peel strength without any sacrifice in other performance.

Claims

1. 1. A method for treating a metal surface prior to adhesive bonding, comprising: applying a primer composition onto a metal surface to form a curable primer film having a continuous surface, wherein the primer composition comprises: (i) one or more epoxy resins; (ii) at least one amine-containing curing agent; (iii) a silane compound having at least one hydrolyzable group; (iv) core-shell rubber (CSR) particles; and (v) Mixtures of organic solvents Including, the primer composition has a solids content of 5% to 30% and the amount of the CSR particles is less than 2 wt. % (weight percent), preferably in the range of 0.2 wt. % to 0.8 wt. %, based on the total weight of the primer composition; No water is added to the primer composition; Components (i) to (v) in the primer composition form a homogeneous solution, the mixture of organic solvents comprises methyl ethyl ketone (MEK) in combination with at least one solvent selected from diacetone alcohol (DAA), acetone, isopropyl alcohol, tetrahydrofuran (THF), ethylene glycol, xylene, toluene, and ethyl acetate; The at least one amine-containing curing agent is selected from aromatic diamines, dicyandiamide (DICY), and hydrazides. method.

2. The method of claim 1, wherein the at least one amine-containing curing agent is 2,2-bis-4-(4-aminophenoxy)phenylpropane (BAPP) and the primer composition further comprises a bis-urea as a catalyst.

3. A method for treating a metal surface prior to adhesive bonding, comprising: applying a primer composition onto a metal surface to form a curable primer film having a continuous surface, wherein the primer composition comprises: (i) one or more epoxy resins; (ii) at least one amine-containing curing agent; (iii) a silane compound having at least one hydrolyzable group; (iv) core-shell rubber (CSR) particles; and (v) Mixtures of organic solvents Including, the primer composition has a solids content of 5% to 30% and the amount of the CSR particles is less than 2 wt. % (weight percent), preferably in the range of 0.2 wt. % to 0.8 wt. %, based on the total weight of the primer composition; No water is added to the primer composition; Components (i) to (v) in the primer composition form a homogeneous solution, the mixture of organic solvents comprises methyl ethyl ketone (MEK) in combination with at least one solvent selected from diacetone alcohol (DAA), acetone, isopropyl alcohol, tetrahydrofuran (THF), ethylene glycol, xylene, toluene, and ethyl acetate; The method wherein the mixture of organic solvents is a mixture of methyl ethyl ketone (MEK) and diacetone alcohol (DAA).

4. 4. The method of claim 3, wherein the weight ratio of MEK to DAA is from 60:40 to 95:05, preferably from 70:30 to 90:

10.

5. The method according to any one of claims 1 to 4, wherein the CSR particles have a particle size in the range of 10 nm to 3000 nm, preferably 10 nm to 500 nm.

6. The method of any one of claims 1 to 5, wherein the primer composition is applied by spraying or brushing.

7. The method of any one of claims 1 to 6, wherein the primer composition further comprises a corrosion inhibitor compound.

8. The method according to any one of claims 1 to 7, wherein the metal surface is the surface of a metal substrate selected from aluminium and aluminium alloys, steel, titanium and titanium alloys.

9. A method for treating a metal surface prior to adhesive bonding, comprising: applying a primer composition onto a metal surface to form a curable primer film having a continuous surface, wherein the primer composition comprises: (i) one or more epoxy resins; (ii) at least one amine-containing curing agent; (iii) a silane compound having at least one hydrolyzable group; (iv) core-shell rubber (CSR) particles; and (v) Mixtures of organic solvents Including, the primer composition has a solids content of 5% to 30% and the amount of the CSR particles is less than 2 wt. % (weight percent), preferably in the range of 0.2 wt. % to 0.8 wt. %, based on the total weight of the primer composition; No water is added to the primer composition; Components (i) to (v) in the primer composition form a homogeneous solution, the mixture of organic solvents comprises methyl ethyl ketone (MEK) in combination with at least one solvent selected from diacetone alcohol (DAA), acetone, isopropyl alcohol, tetrahydrofuran (THF), ethylene glycol, xylene, toluene, and ethyl acetate; The method wherein the metal surface is subjected to an anodization or sol-gel process to form a metal oxide coating prior to applying the primer composition.

10. forming a curable primer film on the metal surface of a first substrate according to the method of any one of claims 1 to 9; adhesively bonding the first substrate having the curable primer film thereon to a second substrate, whereby a curable, polymeric adhesive is disposed between the curable primer film and the second substrate; curing the polymer adhesive to form a bonded structure; A bonding method comprising:

11. (i) 5 to 15 weight percent of an epoxy component comprising one or more epoxy resins; (ii) 10 to 20 parts per 100 parts of the epoxy component of a curable component, the curable component comprising at least one amine-containing curing agent, and optionally one or more catalysts or accelerators; (iii) 0.1 to 2 wt. % of an organosilane; (iv) 0.2 to 0.8 wt. % CSR particles; (v) a mixture of organic solvents to provide 5% to 30% solids; A solvent-based primer composition comprising: the weight percentages are based on the total weight of the solvent-based primer composition; the organic solvent mixture is water-free, the mixture of organic solvents comprises methyl ethyl ketone (MEK) in combination with at least one solvent selected from diacetone alcohol (DAA), acetone, isopropyl alcohol, tetrahydrofuran (THF), ethylene glycol, xylene, toluene, and ethyl acetate; Solvent-based primer compositions.

12. 12. The solvent-based primer composition of claim 11, wherein the at least one amine-containing curing agent is selected from aromatic diamines, dicyandiamide (DICY), and hydrazides.

13. The solvent-based primer composition of claim 12, wherein the curable component comprises a combination of 2,2-bis-4-(4-aminophenoxy)phenylpropane (BAPP) and a bis-urea as a catalyst.

14. 12. The solvent-based primer composition of claim 11, wherein the mixture of organic solvents is a mixture of methyl ethyl ketone (MEK) and diacetone alcohol (DAA).

15. 15. The solvent-based primer composition of claim 14, wherein the weight ratio of MEK to DAA is from 60:40 to 95:05, preferably from 70:30 to 90:

10.

16. The solvent-based primer composition according to any one of claims 11 to 15, wherein the CSR particles have a particle size in the range of 10 nm to 3000 nm, preferably 10 nm to 500 nm.

17. The solvent-based primer composition of any one of claims 11 to 16, further comprising a corrosion inhibitor compound, preferably in an amount of 3 wt% or less, based on the total weight of the solvent-based primer composition.

18. 18. The solvent-based primer composition according to any one of claims 11 to 17, further comprising inorganic fillers in particulate form and / or pigments / dyes, preferably in an amount of 0.3 wt. % or less, based on the total weight of the solvent-based primer composition.

19. 1. A surface treatment for preparing a metal surface prior to adhesive bonding, comprising: Applying the solvent-based primer composition of any one of claims 11 to 18 onto a metal surface to form a curable primer film having a continuous surface. A method comprising:

20. (a) applying the solvent-based primer composition of any one of claims 11 to 18 onto a metal surface of a first substrate to form a curable primer film; (b) adhesively bonding the first substrate having the curable primer film thereon to a second substrate, whereby a curable, polymeric adhesive is disposed between the curable primer film and the second substrate; (c) curing the curable primer film and the polymer adhesive to form a bonded structure; A bonding method comprising:

21. 21. The bonding method of claim 20, wherein the first substrate is formed of a metal selected from aluminum, aluminum alloys, steel, titanium, and titanium alloys.

22. 22. The bonding method according to claim 20 or 21, wherein the second substrate is a composite substrate comprising (i) an uncured resin matrix or a cured polymer matrix and (ii) reinforcing fibers.

23. 22. The bonding method according to claim 20 or 21, wherein the second substrate is a metal substrate.

24. 24. The bonding method of claim 23, wherein the second substrate is formed of a metal selected from aluminum, aluminum alloys, steel, titanium, and titanium alloys.

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