Aqueous adhesive for bonding, in particular, metallic surfaces.

Abstract

Adhesive for the production of bonded sheet metal stacks made from silicon-containing strip steels, which are bonded under heat and pressure.

Description

The adhesive applied as utility model herein is particularly suitable for bonding metallic surfaces. For this purpose, this adhesive is coated onto metal strips by means of a coil coating process and dried, in order then to be reactivated and bonded by means of heat and pressure at a later point in time.Bonding technology has also proven its most successful in recent years for producing metal composite materials or metal packages.In addition to creating a firm, full-surface and permanent connection, it is of primary importance in the bonding of surfaces, in particular in the bonding of the surfaces of metallic parts, that a connecting layer of adhesive is produced in the bond joint between the surfaces, which layer corresponds as far as possible in its material properties to the material properties of the composite work fabrics over a wide temperature range when used.This objective can only be realized ally with the known adhesive systems.In the electrical industry, the production of bonded laminated cores from magnetizable silicon-containing steel sheets has been known for decades. These are used, above all, in electric motor construction as positively locking composite packs for rotors and stators. It is known to the skilled person that the form-fit bonding offers advantages over mechanical connection methods.The adhesive systems hitherto applied to the metal surfaces by means of roll coating (coil coating) are physically dried above the minimum film-forming temperature of the dispersions present and below the deblocking temperatures of the crosslinking resins or reactive but blocked groups present in the dispersion, in order then to be stored temporarily and further processed. These are then punched out to form sheet metal disks or cut to size by means of a laser and laminated to form laminated cores. These laminated cores are then pressed and heated by means of clamping tools or presses. The bond parameters are typically 1-3 N / mm 2 pressure and 170° C. to 230° C. and a bond time of 30 minutes and a few hours, depending on the size of the component and reactivity of the adhesive. Because of this long adhesion processes and application methods used, baking lacquer systems are also referred to in the case of the adhesive used in this case.This novel adhesive protected here is characterized in that it has the form of a suspension. This consists of a dispersion adhesive as carrier liquid and a cohesive polymer with a higher or preferably high degree of polymerization. The carrier liquid consists of dispersions such as, for example, epoxy resin dispersions, polyvinyl acetate dispersion, polybutadiene dispersion or polyacrylate dispersion, rubber dispersion, EPDM dispersion or copolymer dispersion of the types mentioned. These may be present individually or in mixtures or may be present as a copolymer in a dispersion. These dispersion binders contain reactive end groups on the polymer and can be provided with adhesion-promoting functional comonomers. The reactive groups in the polymer are generally hydroxyl groups which, after the deblocking temperature of the isocyanate has been exceeded, are reacted with blocked polyisocyanates in the bonding process to form a higher-polymeric thermoset or else thermoset polymer. In recent years, silanes have also been used successfully as crosslinkers and adhesion promoters to the substrate as copolymer in dispersions. The crosslinking rate can be accelerated by catalysts.Epoxy dispersions exhibit excellent adhesion strengths to metals and very good chemical resistances and heat resistance. These may be combined in elasticity and reactivity with, for example, other dispersion binders to improve these properties.A disadvantage of dispersion systems hitherto is that, after application and drying to the metal substrate, these have to be reactivated under temperature and pressure and have to be melted and fully reacted, and in the process the melt viscosity and reactivity have to be adjusted such that no adhesive escapes at the adhesive joint. The dispersion adhesives usually have a relatively low glass transition point (Tg) which is degraded in the nature of the monomer composition and the amorphous structure of the polymer in the dispersion. The amorphous structure of the dispersion, however, makes it possible that during the heating process during painting on the metal surface, the dispersion liquifies and forms into a continuous paint film which, in this state, is capable of wetting the metal substrate and at the same time of producing a homogeneous mixture of all paint constituents. After cooling, physically dried polymer film which is adherent to the metal surface is formed. This polymer film is then melted under heat and pressure in a second downstream pressing process and cured in the process. Only during the second heating process in the pressing tool is the crosslinking mechanism of the adhesive activated by selectively exceeding the blocking point of the crosslinking component.The blocking strength of the polymer layer resulting after the coating process is determined by the resulting filmed polymer mixture, which represents a sum of the glass transition temperature of all individual components. This polymer film is only physically dried in this state and the deblocking temperature of the crosslinking agents present has not yet been exceeded. Usually, the glass transition points in this physically dried polymer layer are just above room temperature. This state of the adhesive layer on the metal rolls (coils) or sheets can then lead to adhesions and incipient physical filming of the touching adhesive films at elevated temperatures in the summer or on tool surfaces. This can lead to such an extent that the adhesive already generates connections in this state by further physical filming, which connections can no longer be released without damage in the further processing process.Also, temperatures arising at room temperature or in the processing process can become so soft and elastic to surfaces and can alter their tribological properties and associated sliding properties of the metal parts coated with the adhesive so negatively that they cannot be punched out and processed to form laminated cores before the actual bonding.Owing to the molar masses and the amorphous structure of the polymers used in the dispersions, these exhibit very low melt viscosities on adhesive bonding under pressure and temperature and cannot transfer adhesive forces in this state. The strength of the polymer only increases with a time delay due to the beginning of the crosslinking of the polymer layer. This crosslinking in the pressing tool leads to a polymer network with a further increase in the molar masses. However, this often results in networks which have an only insignificantly higher melting point and glass transition point than the original dispersion binders. After curing, these are generally from about 90° to 110° C. in softening point and lose their strength when heated above the glass transition temperature. This property corresponds more to the structure of a thermoset and not of a thermoset.However, electronic components such as motors or transformers are used at continuous temperatures of 150° C. to 180° C. Baking lacquer systems customary on the market are therefore only capable of transmitting very low adhesive forces at these use temperatures, which are about 20 to 30% of the adhesive force at room temperature.Since conventional baking lacquers tend to crosslink with respect to a thermoset and, in particular in the case of shorter bonding times and cycle times, cannot form a thermoset network, during the period of bonding and cooling the components are fixed under pressure by means of a tool in order to be secured against relative movements. After the crosslinking has ended, a highly polymeric adhesive has then developed which has thermoset fractions but still has a low glass transition point below the intended continuous use temperature.The adhesive formulation now described herein is based on a principle of combining two polymers that are physically mixed and formulated in the form of an adhesive formulation. In addition to the dispersion, these combination hotmelt adhesives (KLS systems) comprise finely divided hotmelt polymer particles which improve the adhesion properties in such a way that the holding times during the adhesion are significantly reduced and the bond strengths are significantly increased and rapid cycle times in the pressing tool are made possible.These hotmelt polymer particles are known in the adhesive industry as heat-sealing adhesives and are also extruded, inter alia, and are melt-processed from nozzles or gravure rolls. Another form of hot melt adhesives is very fine powders. These can be applied by the powder scattering method or as a paste dot coating. The textile industry has been a wide field of application for decades.The adhesive formulation described herein makes use of this group of hot melt adhesives in finely divided form of the polymer particulate which is incorporated into the lacquer. In addition, use is made of the significantly higher melt viscosities of the hotmelt adhesive in the form of the hotmelt polymer particles compared with the dispersions.This adhesive is a suspension composed of an aqueous dispersion or mixture of a plurality of dispersions and a high molecular weight hotmelt adhesive particle. These hotmelt adhesive particles are generally present as a crystalline or semicrystalline high polymer. The dispersion, on the other hand, is amorphous and serves as a carrier liquid for the hot melt particle in the suspension.The polymers of the dispersion and of the hotmelt adhesive particle are present in the liquid adhesive in very different particle sizes. The particle size of the dispersion is between 15 nm and 5 μm, preferably from 50 nm to 200 nm. These dispersion particles are capable of liquifying and filming during the painting and baking process, owing to relatively low minimum film-forming temperatures (MFT), in order to wet both the metal surface and the hotmelt polymer present therein and to bond it into the adhesive film.The minimum film forming temperature (MFT) of a dispersion describes the minimum temperature to be achieved, which is necessary after evaporation of the water and co-solvent for the dispersion particles to physically fuse and thus physically film and form a polymer film. After the water is evaporated in the drying process, the dispersion particles inevitably contact each other. This is referred to as pasture point, since the dispersion particles are still present in fine particles and, depending on the particle size, produce a white or opaque color shade of the dispersion film by refraction of light. Owing to the cosolvents present in this hole, the dispersion particles are very soft and amorphous in their structure and begin to flow physically by further temperature increase. The cosolvents are largely evaporated at the end of the drying process, but a small portion usually remains in the film and acts as an external plasticizer in the adhesive film. These remaining cosolvents are also referred to as volatile organic components (VOCs) and describe that these organic molecules often escape from the polymer layer and evaporate only with a time delay. The remaining portion of VOC lowers the glass transition point for this period of time and causes, among other things, additional tackiness at the surface of the adhesive film.The hotmelt polymer particles are present in the suspension having a particle size of from 5 μm to at most 63 μm, preferably from 5 μm to 20 μm, and have a melting point of from 110° C. to 260° C., preferably 170° C. The melting point of the hotmelt polymer particle is thus far above the minimum film-forming temperature (MFR) of the dispersions and is therefore not completely melted during the drying of the adhesive film and is therefore present in a separately distributed manner therein. These hot-melt adhesive particles are only softened completely when their glass transition point is exceeded in the second heating process in the pressing tool during bonding and are then only completely melted and liquefied when their melting point is reached. Its viscosity in the molten state is determined by the melt viscosity of the hotmelt adhesive polymer and can be varied by the type of hotmelt adhesive of the type and adapted for use.The properties present in the hotmelt polymer, such as glass transition point, melting point, crystallinity, and internal tensile strength and flexural strengths, can be used to control the mechanical properties of the bond with respect to adhesive strength and elasticity of the bond bond. This can also have a positive effect in particular on the oscillation behavior and the generation of noise at different temperatures.In addition to the material properties of the hotmelt adhesive polymer used, thermosetting and thermoplastic properties in the cured adhesive layer can be varied by the mixing ratio of dispersion and hotmelt polymer. The formulation additionally contains the stoichiometrically necessary proportion of crosslinking groups corresponding to the reactive groups contained in the dispersion and the hotmelt polymer. The crosslinking density of the cured adhesive can be varied by means of this ratio.The specific melting point of the hotmelt polymer particle can be chosen such that it is far above the deblocking point of the curing agent, but its recrystallization point is chosen so high that further reaction of the dispersion curing agent mixture after partial cooling has taken place can continue until solidification takes place thereafter. In the case of hotmelt adhesives, the skilled worker denotes the temperature range between melting point and recrystallization point of the polymer used as open time of the hotmelt adhesive, which can be used in a technically targeted manner. Thus, it is possible, after the recrystallization point has been undershot and the hot melt adhesive polymer contained in the adhesive has solidified, for the pressing or holding tool to be able to be opened already and then to be crosslinked further without pressure and with further temperature control below this temperature.The use of admixtures of hotmelt polymer particles produces a specific surface roughness which improves the blocking resistance and further processing in the punching tool. Conventional slip, hardness and abrasion measurements, such as pendulum hardness or buchholz hardness and Taber-Abraser measurements, show, by the addition of hot melt polymer particles which protrude inside the lacquer layer and from the latter, a significantly higher glass transition temperature and thus higher hardnesses and better abrasion resistances and sliding properties than the surrounding adhesive layer. This clearly improves the storage stability and further processing before final bonding.By varying the ratio of the adhesive layer thickness to the particle size of the hot melt polymer particle used, it is possible to adjust the extent to which the particle looks out of the surface and an increase is produced in the adhesive film. See FIGS. 1 and 2, which produces a visually visible structure and significantly reduces the degree of gloss of the adhesive paint film and can then be determined, for example, by means of a gloss meter or wave scan and an old mobility tester.The embedded hotmelt polymer particles are capable, during the subsequent renewed heating and pressing of the adhesive lacquer layer against the next same layer or another suitable adhesion promoter layer, of spontaneously bonding in the melt after the melting point has been exceeded as a result of the liquefaction occurring and the formation of a viscous melt, and of producing an adhesive bond.The hotmelt adhesive particles assume the primary function of producing cohesive forces in the adhesive layer and have tough elastic properties. The crosslinking adhesive dispersion embeds the hotmelt polymer particles and can be incorporated into the polymer film via terminal reactive groups in the hotmelt polymer. The adhesive layer consisting of a reactive dispersion is cured with a time delay to form the thermosetting plastic and takes over the primary task of transmitting adhesive forces to the metal substrate and to the embedded hotmelt polymer particles.Since hot melt adhesives can have different melting points and different melt viscosities by corresponding copolymerization, by suitable selection of these in combination with the reactivity, the adhesive can be adjusted such that it does not escape during the bonding of the laminated cores to the edge of the respective layer of the latter.In addition, mechanically acting spacers in the form of thermosetting spheres made of polyacrylate or glass microspheres can be contained in the adhesive. These then have the effect mechanically that pressing of the adhesive layer to a minimum gap dimension is reduced and thus prevents an edge leakage of the adhesive and the relative parallelism of the layers in the sheet metal packs is additionally established. This is of particular interest in very thin steel sheets below 0.3 mm, since the ratio of steel sheet to adhesive layers shifts significantly such that a planar position in the laminated core is strongly influenced by the adhesive.Melting point, melt viscosity and reactivity of the adhesive suspension and pressing pressure, pressing temperature and pressing time can be chosen such that no adhesive escapes at the cut edge, but the adhesive layer between the sheets is closed over the full surface. Thus, media-tight packages can be produced in which no liquids or gases can any longer penetrate into the laminated cores. This is of interest in particular for electric motors which can be cooled in the future by gases or liquids through internal cooling ducts in the laminated core.The melting point and glass transition point of the hot-melt adhesives used can be chosen such that they are above the desired creep temperature strengths of, for example, 150° to 180° C. Hotmelt polymers based on polyamide 12 and their co-polyamide polymers are about 180° C. glass transition point or melting point. Hot-melt adhesives based on polyamide 6 and their co-polyamide polymers can have glass transition points or melting points of about 210° C. and are referred to as high-temperature hot-melt adhesives. Thus, with this adhesive described here, a distinct increase in the softening point of the adhesive and thus improved strengths of the components under prevailing temperature loads in continuous operation are achieved.The combination of low molecular weight and more reactive dispersion adhesives with incorporated high molecular weight hotmelt polymer particles enables faster heating and cooling speeds, since a fast cohesive connection takes place via the viscous fraction which is introduced into the adhesive by the latter. Cycle times during the production of the laminated cores can thereby be shortened.The final thermosetting curing of the component can alternatively also take place after falling below the recrystallization point of the hotmelt polymer, utilizing the operating temperatures of 180° C., for example, since the thermoplastic bonding already present in part in the adhesive layer provides sufficient strengths for the startup of the component.Example Formulation:Epoxy resin dispersion 53% in waterEPIREZ 3546WH5376,6%Polyamide 12, 10 μm, melting point 177° COrgasol 2001 EXD NAT16,6%Polyisocyanate, water-dilutableBAYHYDUR BL XP 27063,4%Defoamer, silicone-freeMoussex SF5610,2%Deionized Water for Viscosity Adjustment13,2%Sum Sum100%Application data: Coating:Viscosity: about 50 seconds DIN 4 mm Outflow viscosity at 20° CSubstrate: Si steel N270, 0.27 mmDrying:Dwell time 40sMetal temperature 155° C. PMT (Peak Metal Temperature)average layer thickness of adhesive layer in the painted state about 7 μmmaximum layer thickness in the region of the incorporated PA 12 particles of about 10 μmList of reference characters1 Steel sheet 0.27 mm 2 Dispersion adhesive layer 7 mμ (standard) 3 KLS adhesive system consisting of dispersion and hot melt particles as a mixture 4 Hot melt polymer particles 10 μm embedded in 7 μm adhesive layer and protruding from the adhesive layer 3 μm 5 Adhesive layer 7 μm of the KLS adhesive 6 Differential layer thickness adhesive layer to diameter hot melt polymer particles of 3 μm

Claims

Adhesive for the production of bonded laminated cores made of silicon-containing steel strips which are bonded under heat and pressure.An adhesive comprising a suspension of at least one aqueous polymer dispersion and finely divided high-polymer hotmelt polymer particles.Adhesive with an organic co-solvent and VOC content of 0% to 20%.The dispersion used has a particle size of 15 nm to 5 μm, preferably 50 nm to 300 nm.The hotmelt polymer particles have a particle size of from 1 μm to 40 μm, preferably from 5 μm to 20 μm.The glass transition temperature of the high-polymer hotmelt polymer particles is above the minimum film-forming temperature (MFR) and the glass transition point (TG) of the dispersion used.The mixing ratio in percent by weight of the aqueous dispersion to the proportion of hotmelt polymer particles is between 99% dispersion to 1% hotmelt polymer particles to 5% dispersion to 95% hotmelt polymer particles, preferably 90%:10% to 40%:60% dispersion to hotmelt polymer particles.The high-polymer hotmelt polymer particles consist of: a. polyester b. copolyester c. polyamide 6 d. polyamide 6.6 e. polyamide 6.11 f. polyamide 11 g. polyamide 12 h. co-polyamide i. ethylene-vinyl acetate, EVA j. EVA copolymerThe hotmelt polymer particles are prepared by the following processes: a. cryoscopic grinding with subsequent sieving and sieving b. precipitation from solvents c. Preparation as bead polymerThe adhesive may additionally contain colored or functional pigments and also fillers in the adhesive, which lead to improvement in corrosion protection, heat absorption and electrical breakdown resistance.The hotmelt polymer particles may contain reactive end groups in the hotmelt polymer, which can react with the reactive groups present in the dispersion and are thus crosslinked from the thermoplastic to the thermoset or thermosetting plastic.The adhesive can additionally contain thermosetting microspheres made of polyacrylate or mineral glass, which lead to the setting of the minimum film thickness and additional parallel alignment of the laminated cores during pressing.Adhesion promoters of from 0.1% to 5% can additionally be formulated in the formulation in order to improve the adhesion of the adhesive to metal oxides or pretreatments on the metal surface. a. phenoxy resins b. carboxy resins c. epoxy resins d. phosphoric acid ester resins e. silanes f. thiolsThe adhesion groups mentioned under point 10 can also be present as copolymers in the dispersions or hotmelt polymer particles used.The adhesive is applied to the metal substrate by means of a coil coating process.The layer thickness in the painting process is 1 μm to 50 μm, preferably 5 μm to 15 μm, for metallic materials.The drying temperature of the metal substrates in the painting process is 40° C. to 300° C., preferably 110° C. to 160° C.The drying time in the painting process is from 1 second to 10 minutes, preferably from 10 seconds to 60 seconds.After drying, the adhesive layer can be wound up into coils or stacked as sheets or stamped parts and is then block-proof and stable in storage.The metal substrates coated with adhesive can be stamped or cut and laminated into laminated cores and then bonded together by means of temperature and pressure.The pressure during bonding in the pressing tool is 1-50 N / mm 2.The time for bonding in a pressing tool is from 0.1 second to 120 minutes.The temperature during bonding in the pressing tool is 120° C. to 350° C.The substrates coated with this adhesive can be pressed and bonded with the adhesive layer against the adhesive layer and can be pressed and bonded on the substrate with a coating of a different nature suitable for the adhesive.

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