Liquid moisture-curable polyurethane with improved adhesion to aluminum

Abstract

This invention discloses a one-component, liquid, moisture-curable polyurethane adhesive composition comprising a reaction product of a mixture containing at least one polyol and an excess of one or more polyisocyanates and an acid component. This adhesive composition exhibits improved adhesive strength to metals such as aluminum compared to currently available adhesives. This composition is particularly suitable for panel lamination applications.

Description

Technical Field

[0001] This disclosure generally relates to one-component liquid moisture-curable polyurethane adhesive compositions, and more specifically to such compositions having improved adhesion to metal components—particularly aluminum components. This disclosure also relates to methods for improving the adhesive strength of one-component liquid moisture-curable polyurethane compositions to aluminum substrates—particularly mill-grade aluminum substrates, and methods for bonding composite structures including aluminum components using one-component liquid moisture-curable polyurethane adhesive compositions. Background Technology

[0002] The background information provided in this section is not necessarily prior art related to the inventive concept of this disclosure.

[0003] Composite structures are widely used in the manufacture of vehicles for the transportation sector. Examples of such composite structures include commercial trailers, train cars, aircraft components, recreational vehicles, boats, and automobiles. A conventional composite structure includes a welded aluminum frame with a polymer skin bonded to one surface, a wood veneer bonded to the opposite surface, and foam positioned between the polymer skin and the wood veneer. Curable polyurethane adhesives are typically used to bond the polymer skin to the aluminum frame and the wood veneer to the aluminum frame.

[0004] Hot melt adhesives have been proposed as a type of composite adhesive. Hot melt adhesives are solid at room temperature, but melt into a liquid or fluid state upon heating, in which form they are applied to the substrate. Upon cooling, the adhesive reverts to its solid form. One type of hot melt adhesive is the thermoplastic hot melt adhesive. Another type is the reactive polyurethane hot melt adhesive, which is initially a thermoplastic material but crosslinks and cures into an irreversible solid form when exposed to suitable conditions. However, hot melt adhesives require specialized equipment to heat the adhesive and keep it in a molten state, as well as specialized application equipment. Such equipment is not available at all manufacturing sites. Hot melt adhesives solidify rapidly after application, which limits the ability to reposition components after application. Furthermore, once heated, the viscosity of the adhesive increases to unusable levels, thus limiting their usable lifespan.

[0005] Another type of polyurethane-based adhesive is a one-component, moisture-curable polyurethane adhesive that is liquid at room temperature. Their liquid form allows for easy application using conventional equipment without the complex heating equipment required for hot-melt adhesives. They also cure more slowly and allow for repositioning of components some time after application. One-component, moisture-curable polyurethane adhesives are typically based on isocyanate-containing polyurethane prepolymers. The adhesive is stored under moisture-free conditions. When exposed to moisture, the isocyanate portions of the prepolymer irreversibly crosslink to form a cured thermosetting reaction product.

[0006] Good adhesion of polyurethane adhesives to the composite components is desirable for increasing the strength of the composite structure. Ideally, the adhesive bond strength should be greater than that of some or all of the materials it bonds. Additionally, water can penetrate the bonded composite structure. The adhesive should retain as much of its initial bond strength as possible during and after exposure to water.

[0007] Currently, manufacturers use a multi-step process to form composite structures. In the first step, the metal frame is cleaned to remove oil, grease, and dirt. Next, the frame is exposed to conversion coating chemicals applied in a bath or spray, rinsed with water, and dried. The conversion-coated frame is now ready for applying adhesive and assembly into the composite structure. This process requires multiple large chemical tanks, lifting and drying equipment, and considerable space. In a different multi-step process, the metal frame is cleaned to remove oil, grease, and dirt. Next, workers manually wipe the frame and dry it using towels soaked in conversion chemicals (such as Alodine wipes from Henkel Corporation). The conversion-coated frame is now ready for applying adhesive and assembly into the composite structure. This method does not require conversion coating tanks and related equipment. However, manually wiping the entire frame requires significant effort and time, and carries the risk that workers may miss some areas of the frame.

[0008] There is a desire to provide a one-component liquid polyurethane adhesive composition with improved adhesive strength to one or more composite components. There is a desire to provide a one-component liquid polyurethane adhesive composition with improved adhesive strength without requiring conversion coating on metal components. There is a desire to provide a one-component liquid polyurethane adhesive composition that substantially retains the improved strength during and after exposure to water. There is a desire to provide a one-component liquid polyurethane adhesive composition that can be used with existing processes and equipment and can be used without heating to a molten state. Summary of the Invention

[0009] This section provides a general overview of this disclosure and is not a full disclosure of its entire scope or all its features, aspects or subjects.

[0010] In one embodiment, this disclosure is a one-component liquid moisture-curable polyurethane adhesive composition comprising a reaction product of at least one polyol; at least one organic polyisocyanate; and an acid component.

[0011] In one embodiment, this disclosure is a one-component liquid moisture-curable polyurethane adhesive composition comprising the reaction product of at least one polyol; at least one organic polyisocyanate; an acid component; and at least one additive.

[0012] In one embodiment, the polyol in the mixture used to prepare a one-component liquid wet-curable polyurethane adhesive composition comprises one or more polyether polyols, one or more polyester polyols, or one or more polyether polyols and one or more polyester polyols.

[0013] In at least one embodiment, the adhesive composition further comprises a catalyst. In one embodiment, the adhesive composition comprises a metal-free catalyst, such as 2,2'-dimorpholinodiethyl ether.

[0014] In at least one embodiment, one or more organic polyisocyanates comprise polymeric MDI, or a mixture of polymeric MDI and MDI isomers.

[0015] In at least one embodiment, this disclosure is a method of bonding a skin or panel to a metal frame to form a reinforced composite structure, the method comprising: providing a one-component liquid moisture-curable polyurethane adhesive composition as described in any of the embodiments; disposing the one-component liquid moisture-curable polyurethane adhesive composition on a surface of at least one of the panel or the metal frame; arranging the surface of the panel in contact with the disposed adhesive and adjacent to a surface of the metal frame; and exposing the disposed adhesive to conditions that would induce curing. In a preferred embodiment, the metal frame is aluminum and is not treated with a conversion coating. In a preferred embodiment, the metal frame is rolled aluminum.

[0016] In one embodiment, this disclosure includes articles comprising a disclosed one-component liquid wet-curable polyurethane adhesive composition in cured or uncured form.

[0017] In one embodiment, this disclosure includes the cured reaction product of the disclosed one-component liquid wettable curable polyurethane adhesive composition.

[0018] These and other features and advantages of this disclosure will become more apparent to those skilled in the art from the detailed description of preferred embodiments. Generally, unless otherwise expressly stated, the disclosed materials and processes can be alternatively formulated to include, consist of, or substantially consist of any suitable components, portions, or steps disclosed herein. The disclosed materials and processes can also be alternatively formulated to be free of or substantially free of any components, materials, ingredients, additives, portions, substances, and steps used in prior art compositions or otherwise unnecessary for achieving the function and / or purpose of this disclosure. Attached Figure Description

[0019] Referring now to the attached diagrams, similar elements are numbered similarly in several of the figures:

[0020] Figure 1 Examples illustrate the adhesion % (%) of some one-component liquid wet-curable polyurethane adhesive compositions to first-rolled aluminum substrate samples.

[0021] Figure 2 Examples illustrate the adhesion of some one-component liquid wet-curable polyurethane adhesive compositions to second-rolled aluminum substrate samples.

[0022] Figure 3 Examples illustrate the adhesion of some one-component liquid, moisture-curable polyurethane adhesive compositions to first-rolled stainless steel substrate samples. Detailed Implementation

[0023] The singular forms “a,” “one,” “a kind,” “the,” and “the” include plural referents unless the context clearly specifies otherwise.

[0024] Unless otherwise specified, “about” or “approximately” used with numerical values ​​means ±10%, preferably ±5%, and more preferably ±1% or less of that value.

[0025] Unless otherwise specified, “%” refers to weight percentage.

[0026] The term “substantially free” is intended herein to mean that, based on the weight of the defined composition, the applicable groups, compounds, mixtures or components constitute less than 10% by weight, typically less than 1% by weight, preferably less than 0.5% by weight, more preferably less than 0.1% by weight, and ideally no more than trace amounts.

[0027] Unless otherwise specified, "at least one" means one or more, i.e., 1, 2, 3, 4, 5, 6, 7, 8, 9 or more. Regarding components, this designation refers to the type of component rather than the absolute number of molecules. "At least one polymer" therefore means, for example, at least one type of polymer, i.e., a polymer of one type or a mixture of several different polymers may be used.

[0028] Unless otherwise specified, the terms “comprising,” “containing,” and “including” as used herein are synonymous with “including,” are inclusive or open-ended, and do not exclude additional unlisted members, elements, or method steps.

[0029] When quantities, concentrations, dimensions, and other parameters are expressed as ranges, preferred ranges, upper limits, lower limits, or preferred upper and lower limits, it should be understood that any range obtainable by combining any upper limit or preferred upper limit with any lower limit or preferred lower limit is also specifically disclosed, regardless of whether the obtained range has been explicitly mentioned in the context.

[0030] Unless otherwise defined, "liquid" means liquid or flowable under standard room temperature conditions. Typically, liquid materials have a viscosity of 50,000 cP or less, more typically 20,000 cP or less, or preferably 10,000 cP or less, all values ​​at room temperature. As used herein, room temperature is 23°C ± about 2°C.

[0031] As used herein, preferred and preferred means that embodiments of the present disclosure may provide particular benefits in certain circumstances. However, the enumeration of one or more preferred or preferred embodiments does not imply that other embodiments are unavailable and is not intended to exclude those other embodiments from the scope of the present disclosure.

[0032] Unless otherwise specified, throughout this specification and claims, the term molecular weight when referring to a polymer means the number-average molecular weight (Mn). Number-average molecular weight M n Molecules can be calculated based on end-group analysis (OH number according to DIN EN ISO 4629, free NCO content according to EN ISO 11909), or determined by gel permeation chromatography using THF as the eluent according to DIN 55672. Unless otherwise specified, all given molecular weights are those determined by gel permeation chromatography.

[0033] The liquid, moisture-curable polyurethane adhesive composition comprises an isocyanate (NCO)-functional polyurethane prepolymer reaction product formed by the reaction of a mixture of one or more polyols and one or more polyisocyanates. An excess of isocyanate equivalents relative to OH molar amounts is present, thereby making the prepolymer is isocyanate (NCO) functional. The adhesive composition may optionally contain a catalyst to control the curing rate of these adhesive compositions, and may optionally contain other additives to control rheological properties and other processing characteristics. The disclosed liquid, moisture-curable polyurethane adhesive composition cures in the presence of moisture, said moisture being either from the atmosphere, present on the substrate, or added by the operator during use. The cured reaction product forms a polyurethane crosslinked network, wherein CO2 is released as a byproduct of the curing process. The thermosetting reaction product forms a structural bond with good heat resistance, good chemical resistance, and good adhesion to metals with conversion coatings; said thermosetting reaction product is solvent-free and can often be applied at approximately room temperature.

[0034] The disclosed polyurethane adhesive compositions may include one-component compositions. Two-component polyurethane adhesive compositions (in which each component is stored separately and the curing reaction is initiated by mixing the two components) are formulated in a different manner than one-component polyurethane adhesives. Two-component polyurethane adhesives require special handling and equipment and are not interchangeable with one-component polyurethane adhesives for every application. The manufacture of reinforced composite panels requires a relatively slow-curing adhesive, allowing multiple panels time to be fitted onto the structural metal frame with the adhesive, and also allowing for later adjustment of the panels' position on the frame. Two-component adhesives cure more quickly and do not allow for the necessary positional adjustments during manufacturing.

[0035] In a less preferred embodiment of the application in the manufacture of unreinforced composite panels, the polyurethane adhesive composition according to this disclosure may comprise a two-component composition. One component comprises an isocyanate-functionalized polyurethane prepolymer reaction product and other typical ingredients, and a separate second component comprises an isocyanate-reactive material such as a polyol and other typical ingredients.

[0036] Isocyanate-functionalized polyurethane prepolymer reaction products can be formed by reacting at least one polyol with an excess of at least one polyisocyanate to form an isocyanate-functionalized prepolymer. Alternatively, a small amount of acid component can be added to the prepolymer reaction mixture or subsequently to the formed prepolymer. Optional additives can be added to the adhesive composition to contribute to its stability, rheology, and curing properties. The disclosed adhesive compositions exhibit long curing times and surprisingly high adhesive strength to metals such as aluminum. In some embodiments, this increased strength is maintained during and after exposure to water. The adhesive compositions can be particularly used in reinforced composite panel lamination processes—such as recreational vehicle assembly processes.

[0037] The polyols that can be used include those used to prepare polyurethanes, including but not limited to polyether polyols, polyester polyols, polycarbonate polyols, polybutadiene polyols, polyacetal polyols, polyamide polyols, polyesteramide polyols, polyalkylene polyether polyols, polysulfide polyols, and mixtures thereof, preferably polyether polyols, polyester polyols, polycarbonate polyols, and mixtures thereof. In one embodiment, polyether polyols are preferred.

[0038] Available polyester polyols include those obtainable by reacting dicarboxylic acids with polyols in a polycondensation reaction. The dicarboxylic acids can be aliphatic, alicyclic, or aromatic, and / or their derivatives such as anhydrides, esters, or acyl chlorides. Specific examples of these substances are succinic acid, glutaric acid, adipic acid, pimelic acid, octanoic acid, azelaic acid, sebacic acid, dodecanoic acid, phthalic acid, terephthalic acid, isophthalic acid, trimellitic acid, phthalic anhydride, tetrahydrophthalic anhydride, glutaric anhydride, maleic acid, maleic anhydride, fumaric acid, dimer fatty acids, dodecanoic acid, and dimethyl terephthalate. Examples of suitable polyols are monoethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 3-methylpentane-1,5-diol, neopentanediol (2,2-dimethyl-1,3-propanediol), 1,6-hexanediol, 1,8-otaneglycol cyclohexanedimethanol, 2-methylpropane-1,3-diol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, polypropylene glycol, dibutylene glycol, tributylene glycol, tetrabutylene glycol, and polybutanediol. Alternatively, they can be obtained by ring-opening polymerization of cyclic esters—preferably caprolactone. Polyester polyols are commercially available, such as Piotane polyol from Panolam Industries International and Dynacoll polyol from Evonik. Other suppliers include Stepan, COIM, and Lanxess. In some embodiments, polyhexane adipate polyol is preferred.

[0039] Useful polyether polyols that can be used include linear and branched polyethers having hydroxyl groups. Examples of polyether polyols may include polyoxyethylene polyols, such as polyethylene glycol, polypropylene glycol, polybutane glycol, etc. Additionally, homopolymers and copolymers of polyoxyethylene polyols may also be used. Particularly preferred copolymers of polyoxyethylene polyols may include at least one adduct selected from the following compounds: ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, 2-ethyl-1,3-hexanediol, glycerol, 1,2,6-hexanetriol, trimethylolpropane, trimethylolethane, tris(hydroxyphenyl)propane, triethanolamine, triisopropanolamine, ethylenediamine, and ethanolamine. Most preferably, the polyether polyol comprises polypropylene glycol. Preferably, the polyether polyol has a number average molecular weight of 500 to 6000 Daltons, more preferably in the range of 1000 to 3000 Daltons. Polyether polyols may include mixtures of polyether polyols.

[0040] Available polycarbonate polyols can be obtained by reacting carbonate derivatives (e.g., diphenyl carbonate, dimethyl carbonate, or phosgene) with diols. Suitable examples of such diols include ethylene glycol, 1,2- and 1,3-propanediol, 1,3- and 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, neopentyl glycol, 1,4-dimethylolcyclohexane, 2-methyl-1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, dipropylene glycol, polypropylene glycol, dibutylene glycol, polybutanediol, bisphenol A, bisphenol F, tetrabromobisphenol A, and lactone-modified diols. In some embodiments, the diol component preferably comprises 40 to 100% by weight of hexanediol (preferably 1,6-hexanediol) and / or hexanediol derivatives. More preferably, the diol component includes examples exhibiting ether or ester groups in addition to the terminal OH groups. The polycarbonate polyol should be substantially linear. However, they can be optionally slightly branched by introducing multifunctional components—particularly low molecular weight polyols. Suitable examples include glycerol, trimethylolpropane, 1,2,6-hexanetriol, 1,2,4-butanetriol, pentaerythritol, p-cyclohexanediol, mannitol and sorbitol, methyl glycosides, and 1,3,4,6-dianhydrohexites.

[0041] The polyisocyanates that can be used include single polyisocyanates and mixtures of polyisocyanates. There are no restrictions on the one or more polyisocyanates used, as long as the weight-average functionality of the polyisocyanate or mixture of polyisocyanates is 2.0 or higher and less than 2.7, preferably less than 2.6 and more preferably less than 2.5 to improve the adhesive strength of the adhesive composition. The weight-average functionality (fNCO) of the polyisocyanate mixture is calculated as follows: fNCO = (wt%NCO1*fNCO1) + (wt%NCOi*fNCOi) + ... In other words, the weight-average functionality is the sum of the products of the weight percentage of each individual polyisocyanate multiplied by its functionality, based on the total weight percentage of the polyisocyanates. For example, an adhesive composition containing 30% by weight of a polyisocyanate with a functionality of 2.7 and 15% by weight of a polyisocyanate with a functionality of 2.0 based on the total weight of the composition produces the following weight-average functionality: (30 / (30+15))*2.7+(15 / (30+15))*2=2.47. Available polyisocyanates include: diisocyanates, such as 4,4'-diphenylmethane diisocyanate (4,4'MDI); toluene diisocyanate; 1,4-diisocyanate phenylene (PPDI); 2,4'-diphenylmethane diisocyanate; 1,5-naphthalene diisocyanate; polymeric MDI; bitolylene diisocyanate; 1,3-xylene diisocyanate; p-TMXDI; 1,6-diisocyanate-2,4,4-trimethylhexane; CHDI; BDI; H6XDI; IPDI; H 12 MDI, any of the above substances in polymeric form (e.g., polymeric MDI), their modified forms (e.g., urethane, carbodiimide, and biuret), and mixtures thereof.

[0042] The disclosed adhesive composition comprises at least one acid component. Suitable acids for this component include inorganic acids and / or organic acids. Inorganic acids are those derived from one or more inorganic compounds. Organic acids are acids containing a carbon atom. Some exemplary inorganic acids include sulfuric acid, sulfurous acid, disulfuric acid, persulfuric acid, thiosulfate, nitric acid, dinitric acid, phosphoric acid, hydrochloric acid, chloric acid, perchloric acid, hydrofluoric acid, hypophosphitic acid, phosphonic acid, hypodiphosphoric acid, diphosphoric acid, triphosphoric acid, boric acid, hydrogen sulfide, chromite, perchromic acid, selenic acid, permanganic acid, silicic acid, antimonyic acid, molybdic acid, and fluorosilicic acid. Some exemplary organic acids include p-toluenesulfonic acid, methanesulfonic acid, ethanesulfonic acid, trifluoromethanesulfonic acid, acetic acid, propionic acid, citric acid, fumaric acid, maleic acid, ethanedioic acid, adipic acid, succinic acid, lactic acid, tartaric acid, benzoic acid, salicylic acid, gallic acid, trimellitic acid, pyromellitic acid, tannic acid, glutaric acid, stearic acid, acetylsalicylic acid, phthalic acid, and isophthalic acid. Inorganic acids (e.g., phosphoric acid, sulfuric acid, and nitric acid) are preferred. Combinations of different acids may also be used. Some or all of the acid components may also be in the form of weak salts, such as products of the reaction of acids with tertiary amines.

[0043] The disclosed adhesive composition may optionally comprise liquid oils, such as mineral oils, paraffin oils, and aromatic oils. Many liquid paraffin oils and aromatic oils can be used, such as n-paraffin oil, isoparaffin oil, and other branched paraffins, cycloalkanes (cycloanes), condensed cycloalkanes (including steranes and hopanes), and other substances having alkyl side chains in the ring system. The paraffin oil may be 100% n-alkane-based paraffin oil with the molecular formula CH3[CH2]. n CH3. This paraffin oil is also known as liquid paraffin, white mineral oil, or liquid petrolatum. Commercially available examples of paraffin oil include Citation from Avatar Corporation. TM Those with the NF designation. Suitable aromatic oils for use herein include oils containing at least one ring having a conjugated π-electron system with (4n+2) π electrons, where n is an integer, such as 0, 1, or 2. Such aromatic oils include those aromatic hydrocarbons, including benzene systems, fused aromatic systems, fused aromatic cycloalkyl systems, and other systems having alkyl side chains on the ring. An example of an aromatic oil suitable for use herein is a complex mixture of 100% aromatic hydrocarbons, available from Crowley Chemical Company under the trade name... and And from Shell Company by product name Commercially available. More general information about liquid paraffin oils and aromatic oils can be found in The Chemistry and Technology of Petroleum, 4th edition, James Speight, CRC Press, the contents of which are explicitly incorporated herein by reference.

[0044] The composition may optionally include a catalyst. The catalyst can be any moisture-curing catalyst for isocyanates, such as 2,2'-dimorpholinodiethyl ether, triethylenediamine, dibutyltin dilaurate, and stannous octoate. While metal-based catalysts can work, they are preferably not used. Organic catalysts are preferred, such as the tertiary amine catalyst 2,2'-dimorpholinodiethyl ether (DMDEE).

[0045] The composition may optionally contain one or more additives. Common adhesive additives include, for example, adhesion promoters, colorants, UV pigments, fillers, oils, plasticizers, rheology modifiers, and combinations thereof. Alternatively, the composition may be substantially free of any or all of these additives.

[0046] The adhesive compositions according to this disclosure may contain solvents. Preferably, the adhesive compositions according to this disclosure may be substantially free of any solvents or water at any stage of formulation.

[0047] In one embodiment, the liquid wet-curable polyurethane adhesive composition is a reaction product comprising a mixture of the following substances:

[0048] Polyisocyanates 10-90 20-75 30-60 polyols 10-90 20-80 30-70 acid 0.001-5.0 0.05-1.5 0.05-0.5 Oil 0-15 0-8 0-5 catalyst 0-3.5 0.001-3.0 0.01-2.5 additive 0-50 0.1-25 0.1-5

[0049] The preparation of the composition does not require a special method and can be performed using standard practices. In one embodiment, the adhesive composition can be prepared by adding a polyol and an acid component to a reaction vessel. The reaction vessel is heated and placed under a vacuum or inert gas atmosphere to remove trace amounts of moisture. The catalyst can be added before or during heating. Once the reaction vessel is substantially free of moisture, the polyisocyanate is added under mixing and allowed to react with the polyol. If used, additives can be added before the polyisocyanate without interfering with the polyisocyanate-polyol reaction, or they can be added after the reaction is complete. The final adhesive composition is transferred to a moisture-proof container and sealed to remove moisture. In other embodiments, the acid component is added after the polyisocyanate and polyol have completely reacted.

[0050] The disclosed liquid, moisture-curable polyurethane adhesive compositions are particularly suitable for use as adhesives in reinforced composite structures. One example is the manufacture of large reinforced composite panels used in recreational vehicles. Such reinforced composite panels typically comprise one or two panels, or “skins,” laminated to opposite sides of a reinforced metal frame. Skins can include, for example, wood or wood products, plastics, fiber-reinforced plastics (FRP), metal or metal foil, high-pressure laminate (HPL) skins, or other materials. Typically, the outer skin is plastic or a plastic composite to resist weathering. If an inner skin is required, it is typically wood or laminated wood (e.g., Lauan laminate). The frame typically comprises multiple tubular metal sections welded together to form a structural frame. Typically, the tubular metal sections have a quadrilateral cross-sectional shape, with bonding surfaces defined on opposite sides of this shape. Structural aluminum components are used in recreational vehicles almost exclusively to reduce the weight of the frame and the vehicle. Materials such as expanded polystyrene (EPS) foam boards can be arranged between the skins in spaces not occupied by the frame. The panel lamination method includes: applying a one-component liquid adhesive to several surfaces to be laminated; optionally atomizing the adhesive with water to accelerate curing; placing one or more skins in contact with the adhesive applied to the frame surfaces; placing the assembled components in a press to apply pressure and heat to the assembled components and hold the components in place until substantially cured; and removing or storing the components from the press after the adhesive has initially cured. For multilayers, this process is repeated until a final lamination stack is assembled. The final lamination stack is then moved to a pressing station, where the press applies pressure to the lamination stack, and the adhesive can develop its initial strength (i.e., green strength) through initial curing, as is known in the industry. Once sufficient initial or green strength has been developed, the lamination stack is removed from the press and conveyed to the next station. The disclosed liquid, wet-curable polyurethane adhesive composition provides improved bond strength to aluminum frames compared to conventional adhesives.

[0051] For reinforced composite panels used in vehicles, it is desirable to have an adhesive strength of at least about 30%, preferably at least about 50%, more preferably at least about 70%, and most preferably 90%. 100% adhesive strength is ideal because it indicates that the substrate breaks down before the adhesive bonds. While some of these adhesive strengths can be achieved using conventional adhesives in combination with anodized or conversion-coated aluminum frame components, it is not possible to consistently achieve even 30% adhesive strength using conventional adhesives with rolled aluminum frame components (e.g., uncleaned and unconversion-coated or unanodized aluminum frame components received from a rolling mill).

[0052] Experimental data

[0053] The viscosity of the product, expressed in centipoises (cP), was measured at 25°C using a Brookfield viscometer, model DV-IPrime, equipped with a spindle of size 27.

[0054] NCO% was monitored using a Brinkman Metrohm automatic titrator.

[0055] Adhesion strength % is tested by applying the test composition at an effective coating weight of 10 to 12 grams per square foot (gsf) to an untreated rolled-grade hollow rectangular aluminum tube specimen. The rolled-grade aluminum is used as is, without cleaning, anodizing, or conversion coating prior to testing. The applied composition is atomized with water. A sheet of Lauan laminate (approximately 3 mm thick) is placed on the applied adhesive and vacuum-pressed onto the adhesive and tube for 1 hour. The laminate is allowed to cure for two days at room temperature and ambient humidity. The adhesion strength of the Lauan laminate to the aluminum is tested by attempting to pry the laminate off the tube with a trowel. The percentage of Lauan laminate failure is visually assessed based on the amount of wood still bonded to the aluminum; for example, 90% adhesion means 90% of the wood remains bonded (good result), while 10% adhesion means 10% of the wood remains bonded (failure result). Results are recorded.

[0056] The compounds used in the experimental formulations are as follows: PPG 1000 is polypropylene glycol with a hydroxyl functionality of 2.0 and a molecular weight of about 1000, and is available from Covestro. PPG 2000 is polypropylene glycol with a hydroxyl functionality of 2.0 and a molecular weight of about 2000, and is available from Covestro. Mondur MR is a mixture containing a 4,4'-methylene diphenyl diisocyanate (4,4'-MDI) isomer and polymeric MDI (p-MDI) with a functionality of about 2.7, and is available from Covestro. MondurMRS2 is a mixture containing a 4,4'-methylene diphenyl diisocyanate (4,4'-MDI) isomer and polymeric MDI (p-MDI) with a functionality of about 2.2, and is available from Covestro. 2,2'-Dimorpholinodiethyl ether (which is available from Huntsman Corp. as...) DMDEE (obtained) is a tertiary amine catalyst. Silquest A-Link35 is an isocyanate-functionalized silane with a trimethoxysilyl functional group and is available from Momentive Performance Materials. Silquest A 1110 is a primary amino silane and is available from Momentive Performance Materials. Sylvalite 10 is a liquid rosin ester tackifier available from Kraton. The acids used are well-known laboratory materials and are commercially available.

[0057] These compositions have the same universal formulation as shown below.

[0058] PPG 1000 27 PPG 2000 27 Polyisocyanates 46 additive Less than approximately 1.5

[0059] Comparative Example A

[0060] 270 parts PPG 1000, 270 parts PPG 2000, and 4.7 parts DMDEE were added to the reactor and heated to 80°C under a nitrogen atmosphere. 460 parts Mondur MRS2 were added to the reactor with stirring. The mixture was stirred at 80°C under a nitrogen atmosphere for 1.5 hours. The reaction product was transferred to a moisture-proof container and sealed.

[0061] Example 1

[0062] 270 parts PPG 1000, 270 parts PPG 2000, 1.5 parts sulfuric acid (laboratory grade 96%), and 4.7 parts DMDEE were added to a reactor and heated to 80°C under a nitrogen atmosphere. 460 parts Mondur MRS2 were added to the reactor with stirring. The mixture was stirred at 80°C under a nitrogen atmosphere for 1.5 hours. The reaction product was transferred to a moisture-proof container and sealed.

[0063] Example 2

[0064] 270 parts PPG 1000, 270 parts PPG 2000, 1.5 parts phosphoric acid (commercial grade 85%), and 4.7 parts DMDEE were added to a reactor and heated to 80°C under a nitrogen atmosphere. 460 parts Mondur MRS2 were added to the reactor with stirring. The mixture was stirred at 80°C under a nitrogen atmosphere for 1.5 hours. The reaction product was transferred to a moisture-proof container and sealed.

[0065] Example 3

[0066] 270 parts PPG 1000, 270 parts PPG 2000, 1.5 parts methanesulfonic acid (laboratory grade), and 4.7 parts DMDEE were added to a reactor and heated to 80°C under a nitrogen atmosphere. 460 parts Mondur MRS2 were then added to the reactor with stirring. The mixture was stirred at 80°C under a nitrogen atmosphere for 1.5 hours. The reaction product was transferred to a moisture-proof container and sealed.

[0067] Example 4

[0068] 270 parts PPG 1000, 270 parts PPG 2000, 1.5 parts propionic acid (laboratory grade), and 4.7 parts DMDEE were added to a reactor and heated to 80°C under a nitrogen atmosphere. 460 parts Mondur MRS2 were then added to the reactor with stirring. The mixture was stirred at 80°C under a nitrogen atmosphere for 1.5 hours. The reaction product was transferred to a moisture-proof container and sealed.

[0069] Comparative Example B

[0070] 270 parts PPG 1000, 270 parts PPG 2000, 10 parts Sylvalite 10, and 4.7 parts DMDEE were added to the reactor and heated to 80°C under a nitrogen atmosphere. 460 parts Mondur MRS2 were then added to the reactor with stirring. The mixture was stirred at 80°C under a nitrogen atmosphere for 1.5 hours. The reaction product was transferred to a moisture-proof container and sealed.

[0071] Comparative Example C

[0072] 270 parts PPG 1000, 270 parts PPG 2000, and 4.7 parts DMDEE were added to the reactor and heated to 80°C under a nitrogen atmosphere. 460 parts Mondur MRS2 were added to the reactor with stirring. The mixture was stirred at 80°C under a nitrogen atmosphere for 1.5 hours. The temperature was lowered to approximately 60°C, and 4 parts Silquest A 1110 were added with stirring. The reaction product was transferred to a moisture-proof container and sealed.

[0073] Comparative Example D

[0074] 270 parts PPG 1000, 270 parts PPG 2000, and 4.7 parts DMDEE were added to the reactor and heated to 80°C under a nitrogen atmosphere. 460 parts Mondur MRS2 were added to the reactor with stirring. The mixture was stirred at 80°C under a nitrogen atmosphere for 1.5 hours. The temperature was lowered to approximately 60°C, and 4 parts Silquest A-Link 35 were added with stirring. The reaction product was transferred to a moisture-proof container and sealed.

[0075] Example 5

[0076] 270 parts PPG 1000, 270 parts PPG 2000, 1.5 parts sulfuric acid (laboratory grade 96%), and 4.7 parts DMDEE were added to the reactor and heated to 80°C under a nitrogen atmosphere. 230 parts Mondur MRS2 and 230 parts Mondur MR were added to the reactor with stirring. The mixture was stirred at 80°C under a nitrogen atmosphere for 1.5 hours. The reaction product was transferred to a moisture-proof container and sealed.

[0077] Comparative Example E

[0078] 270 parts PPG 1000, 270 parts PPG 2000, 1.5 parts sulfuric acid (laboratory grade 96%), and 4.7 parts DMDEE were added to a reactor and heated to 80°C under a nitrogen atmosphere. 460 parts Mondur MR were then added to the reactor with stirring. The mixture was stirred for 1.5 hours under a nitrogen atmosphere at 80°C. The reaction product was transferred to a moisture-proof container and sealed.

[0079] The adhesive strength of the composition was tested. The formulation and results are provided in the table below. All quantities are parts by weight.

[0080]

[0081] 1. Sylvalite 10 tackifier

[0082] 2Silquest A1110 Silane Adhesion Accelerator

[0083] 3Silquest A-Link 35 Silane Adhesion Accelerator

[0084] 4 see Figure 1

[0085] The composition of Example A illustrates a conventional polyurethane adhesive for bonding composite structures containing aluminum frames (e.g., uncleaned and unconverted or unanodized aluminum received from a rolling mill) under worst-case commercial conditions. This adhesive exhibits low bond strength, which is undesirable in many applications.

[0086] A common way to improve the adhesive strength of an adhesive is to add a tackifier to the adhesive composition. The composition example in Example B illustrates an attempt to improve adhesive strength by using a tackifier additive. The addition of the tackifier did not improve the adhesive strength of that composition.

[0087] A common way to improve the adhesive strength of an adhesive is to add a silane adhesive accelerator to the composition. The composition example in Example C illustrates an attempt to improve adhesive strength by using an aminosilane adhesive accelerator. However, the addition of this aminosilane adhesive accelerator did not improve the adhesive strength.

[0088] The composition example in Example D illustrates an attempt to improve adhesive strength by using an isocyanate-functionalized silane adhesive accelerator having a trimethoxysilyl functional group. The addition of this isocyanate-functionalized silane adhesive accelerator did not improve the adhesive strength of that composition.

[0089] Another conventional way to improve adhesive bond strength is through conversion coating of aluminum substrates. Tests have shown that bonding Lauan laminates to properly anodized aluminum substrates using acid-free conventional adhesives provides an increased bond strength compared to bonding on rolled-grade aluminum substrates. However, this improvement requires additional labor, time, and cost for the additional transportation and processing of the aluminum. The disclosed adhesive provides stronger adhesion to rolled-grade aluminum that has not undergone any cleaning or conversion coating. Using the disclosed adhesive with anodized aluminum substrates consistently shows improved bond strength compared to conventional adhesives.

[0090] Example 1 illustrates the effect of adding a small amount of acid to the conventional composition of Example A. The addition of 1.5 parts by weight (approximately 0.15% by weight) of sulfuric acid provided an unexpected and surprising increase in bond strength, from 10% of the bonded wood residue to 90%. Even more surprisingly, this increase was achieved without any conversion coating or other pretreatment of the aluminum, and without washing to remove dirt, grease, and oil.

[0091] Example 2 illustrates the effect of adding a small amount of acid to the conventional composition of Example A. The addition of 1.5 parts by weight (approximately 0.15% by weight) of phosphoric acid provided an unexpected and surprising increase in bond strength, from 10% of the bonded wood residue to 90%. Even more surprisingly, this increase was achieved without any conversion coating or other pretreatment of the aluminum, and without washing to remove dirt, grease, and oil.

[0092] Example 3 illustrates the effect of adding a small amount of acid to the conventional composition of Example A. The addition of 1.5 parts by weight (approximately 0.15% by weight) of methanesulfonic acid provided an unexpected and surprising increase in bond strength, from 10% of the bonded wood residue to 80% of the bonded wood residue. Even more surprisingly, this increase was achieved without any conversion coating or other pretreatment of the aluminum, and without washing to remove dirt, grease, and oil.

[0093] Example 4 illustrates the effect of adding a small amount of acid to the conventional composition of Example A. The addition of 1.5 parts by weight (approximately 0.15% by weight) of propionic acid provided an unexpected and surprising increase in bond strength, from 10% of the bonded wood residue to 80% of the bonded wood residue. Even more surprisingly, this increase was obtained without any conversion coating or other pretreatment of the aluminum, and without washing to remove dirt, grease, and oil.

[0094] Example 5 illustrates the effect of isocyanate functionality on the adhesive strength of the composition. Example 1 used 460 parts of Mondur MRS2 (a mixture containing 4,4'-methylene diphenyl diisocyanate (4,4'-MDI) isomer and polymeric MDI (p-MDI) with a functionality (f) of about 2.2) and 1.5 parts by weight (about 0.15% by weight) of sulfuric acid, which provided an unexpected and surprising increase in adhesive strength, from 10% to 90% of the bonded wood residue without conversion coating. Example 5 used the same composition as Example 1, but replaced 230 parts of Mondur MRS2 with 230 parts of Mondur MR (a mixture containing 4,4'-methylene diphenyl diisocyanate (4,4'-MDI) isomer and polymeric MDI (p-MDI) with a functionality of about 2.7).

[0095] Example 5 has higher functionality than Example 1.

[0096] Although the increased polyisocyanate functionality reduced the adhesive strength to 30%, it was still a surprising increase of 10% over the adhesive strength of Example A.

[0097] Example E used the same composition as Example 1, but replaced all of Mondur MRS2 (f = 2.2) with the same amount of Mondur MR (f = 2.7). The average functionality of Example E (f = 2.7) was higher than that of Example 1 (f = 2.2). Example E had a higher isocyanate functionality than Example 1, but had a much lower adhesive strength. The adhesive strength of Example E did not show an improvement over Example A. Therefore, the increased isocyanate functionality offset any adhesive strength advantage provided by the addition of acid.

[0098] Examples 6 to 10

[0099] A second test was conducted using a second rolled aluminum sample, different from the first rolled grade aluminum sample, and the previous binder composition. The aluminum was used as is and was not cleaned or converted prior to testing. Formulations and results are provided in the table below. All quantities are parts by weight. Results are shown in... Figure 2middle.

[0100] PPG 1000 270 270 270 270 270 PPG 2000 270 270 270 270 270 Mondur MRS2 460 460 460 460 460 Average NCO functionality (f) f＝2.2 f＝2.2 f＝2.2 f＝2.2 f＝2.2 sulfuric acid 1.5 0 0 0 0 Phosphoric acid 0 1.5 0 0 0 mesylate 0 0 1.5 0 0 propionic acid 0 0 0 1.5 0 DMDEE 4.7 4.7 4.7 4.7 4.7 additive 0 0 0 0 0 total 1006.2 1006.2 1006.2 1006.2 1004.7 Adhesion strength % 80 70 10 20 0

[0101] Compared to previous rolled aluminum samples, this second rolled aluminum sample formed a weaker bond. Therefore, the ability of rolled aluminum to form adhesive bonds varies between different batches and from different suppliers. Despite this variability, compositions 6 and 7 containing inorganic acids maintained surprisingly improved bond strength. Compositions 8 and 9 containing organic acids had improved bond strength compared to comparative adhesive F, but were not preferred because their strength was significantly lower than the disclosed adhesive compositions.

[0102] Examples 10 to 11

[0103] The adhesive composition was tested for adhesion to untreated stainless steel using the same procedure as previously used for aluminum. The stainless steel samples were used as is and were not cleaned or converted prior to testing. The formulation and results are provided in the table below. All quantities are parts by weight. Results are shown in... Figure 3 middle.

[0104] PPG 1000 270 270 270 PPG 2000 270 270 270 Mondur MRS2 460 460 460 Average NCO functionality (f) f＝2.2 f＝2.2 f＝2.2 sulfuric acid 1.5 0 0 Phosphoric acid 0 1.5 0 DMDEE 4.7 4.7 4.7 additive 0 0 0 total 1006.2 1006.2 1004.7 Adhesion strength % 0 0 0

[0105] When using comparative adhesive composition G or the disclosed adhesive compositions 10 or 11, the stainless steel substrate exhibits virtually no adhesion to the Lauan substrate. While the use of the disclosed adhesive compositions did not improve adhesion to the stainless steel substrate, the same adhesive compositions significantly improved adhesion to aluminum. This selectivity confirms the surprising effectiveness of the disclosed adhesive compositions on aluminum.

[0106] The terminology used herein is for the purpose of describing specific illustrative implementations only and is not intended to be restrictive. The methods, procedures, and operations described herein should not be construed as requiring their addition in the specific order discussed or illustrated, unless specifically identified as such. It should also be understood that additional or alternative steps may be employed.

Claims

1. A method for manufacturing an adhesive reinforced composite structure, the method comprising: An aluminum frame is provided having a first bonding surface, wherein at least one aluminum bonding surface is an uncleaned rolled grade; Provide a first panel with an adhesive surface; A one-component, liquid, moisture-curable polyurethane adhesive composition is provided, wherein the adhesive composition comprises: a reaction product of a mixture containing at least one polyol, an excess of at least one organic polyisocyanate, and an acid component in the form of an acid. The organic polyisocyanate described herein has an average functionality in the range of 2.0 to less than 2.

5. Based on the total weight of the mixture, the acid component is present in an amount of 0.001 to 5.0% by weight, and The acid component is selected from nitric acid, sulfuric acid, phosphonic acid, phosphoric acid, pyrophosphate, methanesulfonic acid, propionic acid, and combinations thereof; The adhesive composition is applied to at least one bonding surface; The bonding surface of the first panel is arranged to contact the bonding surfaces of the arranged adhesive and aluminum frame to form a composite structure; and The adhesive is cured to bond the first panel to the aluminum frame.

2. The method according to claim 1, further comprising: The step of exposing the applied adhesive to water prior to the step of arranging the first panel bonding surface in contact with the arranged adhesive and aluminum frame bonding surfaces.

3. The method according to claim 1 or claim 2, wherein the method comprises: The composite structure is placed in a press, wherein the curing step is completed at least in part while the composite structure is in the press.

4. The method according to claim 1 or 2, wherein: The aluminum frame has a quadrilateral cross-section and a second adhesive surface opposite to the first adhesive surface; and the method includes: Provide a second panel with an adhesive surface; The adhesive composition is applied to at least one of the second bonding surface of the aluminum frame or the second panel bonding surface; The second panel bonding surface is arranged to contact the arranged adhesive and the second bonding surface of the aluminum frame; and The adhesive is cured to bond the second panel to the aluminum frame.

5. The method according to claim 1 or 2, wherein at least one aluminum bonding surface does not have a surface conversion coating and / or anodizing.

6. The method according to claim 1 or 2, wherein at least one aluminum bonding surface is converted and coated.

7. The method according to claim 1 or 2, wherein the first panel comprises a cured polymer and / or a laminate.

8. The method of claim 1 or 2, wherein the composite structure has no mechanical fasteners to hold the first panel on the aluminum frame.

9. The method according to claim 1 or 2, wherein the organic polyisocyanate has an average functionality in the range of 2.0 to 2.

4.

10. The method according to claim 1 or 2, wherein the acid component comprises at least one inorganic acid.

11. The method according to claim 1 or 2, wherein the acid component is selected from nitric acid, sulfuric acid, phosphonic acid, phosphoric acid, pyrophosphate, and combinations thereof.

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