An aqueous coating composition for improving the transfer rate of electrochemical aluminum and a transfer process thereof

Abstract

This invention discloses a water-based coating composition and its transfer process for improving the transfer rate of electroplated aluminum. The composition comprises a water-based film-forming resin and a latent thermosetting system. The latent thermosetting system consists of a blocked polyisocyanate with a desealing temperature of 110–150°C and a resin containing active hydrogen, which deseales and undergoes irreversible cross-linking under hot stamping heating and pressure conditions. This composition allows the release layer to be permanently cured after the first hot stamping transfer, thus mechanistically avoiding coating softening and misregistration caused by reheating in multi-color hot stamping. The water-based film-forming resin is preferably a blend of low-Tg polyurethane and high-Tg acrylic emulsion, and nano-wax dispersions, surface control agents, thermogenic acid generators, and near-infrared absorbing materials can be added. The transfer process completes drying below the desealing temperature, and curing is triggered during hot stamping. This invention significantly improves the dimensional stability of multi-color printing while ensuring a high transfer rate, and is suitable for multiple hot stamping scenarios such as cigarette packs and wine labels.

Description

Technical Field

[0001] This invention relates to the field of electroplated aluminum hot stamping materials, and more specifically, to a water-based release coating composition coated on a PET base film, and a process for electroplated aluminum transfer using the composition. Background Technology

[0002] Electroplated aluminum hot stamping foil is typically composed of a PET base film, a release layer, a coloring layer, an aluminum plating layer, and a hot melt adhesive layer, layered sequentially. During hot stamping, the hot melt adhesive layer melts upon heating, and the release layer peels off from the PET base film, allowing the coloring and aluminum plating layers to transfer to the substrate surface. At room temperature, the release layer needs to maintain sufficient adhesion to the PET base film to prevent detachment during coating, winding, and transportation; at the hot stamping temperature, it also needs to easily separate from the base film to ensure the integrity and cleanliness of the transfer. To balance this contradiction, existing technologies typically incorporate resin emulsions and wax additives with different glass transition temperatures into water-based release coatings, adjusting the room temperature hardness and high-temperature peelability by controlling the ratio of soft to hard resin segments.

[0003] To further improve transfer precision, existing methods have introduced functional fillers with reversible thermal response. For example, CN121203209A discloses a multi-release electroplated aluminum, whose release layer contains a molybdenum disulfide-cage-locked phase change microcapsule hybrid nanofiller. This method utilizes the endothermic melting of the phase change microcapsules at the hot stamping temperature, which temporarily softens the coating to reduce the release force. At room temperature, the phase change material in the microcapsules returns to a solid state, and the coating hardens again. The entire process is reversible.

[0004] In actual production, especially in fields such as cigarette packs and wine labels where high visual effects of hot stamping are required, the substrate often needs to undergo two or more hot stamping processes to overlay patterns of different colors or effects. The inventors of this case noted that the aforementioned reversible softening mechanism brings a previously unrecognized problem in multi-color hot stamping: the release layer, transferred to the substrate during the first hot stamping, is subjected to heat and pressure again during the second and third hot stamping processes. The phase change microcapsules within the coating, which have not yet lost their function, continue to melt and soften. The edges of the transferred coating are prone to diffusion, and uncontrollable micro-deformation occurs in the pattern details. In severe cases, re-adhesion and re-detachment may even occur, ultimately leading to a decrease in the accuracy of multi-color printing. Moreover, this problem is caused by "heat history accumulation," becoming more pronounced with each additional hot stamping cycle, and is difficult to eradicate simply by adjusting the hot stamping temperature or time.

[0005] Therefore, a new coating design approach is needed to make the coating insensitive to subsequent reheating after the initial hot stamping transfer, thereby eliminating the problem of heat history accumulation in multi-color hot stamping from the source, while maintaining the original environmental friendliness and process adaptability of the water-based system. Summary of the Invention

[0006] In view of this, the present invention proposes an aqueous coating composition to improve the transfer rate of electroplated aluminum, which aims to make the release layer undergo an irreversible curing reaction at the moment of hot stamping transfer, so that the transferred part is permanently fixed and no longer disturbed by subsequent hot stamping heating.

[0007] The technical solution of the present invention is achieved as follows: a water-based coating composition for improving the transfer rate of electroplated aluminum is provided, comprising a water-based film-forming resin and a latent thermosetting system. The latent thermosetting system is composed of a blocked polyisocyanate and a resin containing active hydrogen. The unblocking temperature of the blocked polyisocyanate is 110-150°C. The latent thermosetting system does not crosslink with the water-based film-forming resin at room temperature. Under the heating and pressurization conditions of electroplated aluminum hot stamping transfer, the blocked polyisocyanate is unblocked and undergoes irreversible crosslinking with the resin containing active hydrogen.

[0008] This can be understood as follows: after coating and drying but before hot stamping, the latent thermosetting system within the coating is in a "dormant" state. The release layer relies solely on the physical film-forming properties of the water-based film-forming resin to provide mechanical strength and adhesion. At the moment of hot stamping, heat and pressure act simultaneously, causing the end-cap groups of the blocked polyisocyanate to detach, releasing active -NCO groups. These groups rapidly react with the resin containing active hydrogen to form a three-dimensional cross-linked network. Because this cross-linking is an irreversible chemical bonding process, the coating transferred to the substrate acquires permanent dimensional stability, and the heat from subsequent hot stamping no longer softens or flows the cured portion. The untransferred portions remain transferable. Thus, the heat from the initial hot stamping is used to lock in the coating, and the heat from subsequent hot stamping no longer significantly affects the locked portion; the thermal history is decoupled.

[0009] In some embodiments, the resin containing active hydrogen is an aqueous polyester polyol with a hydroxyl value of 50–150 mgKOH / g. This hydroxyl value range provides sufficient crosslinking reaction sites without causing the coating to have excessively high viscosity in the liquid state, affecting coating operability. The blocked polyisocyanate can be selected from aliphatic or alicyclic polyisocyanates using methyl ethyl ketone oxime, caprolactam, or diethyl malonate as blocking agents, preferably hexamethylene diisocyanate trimer or isophorone diisocyanate trimer using methyl ethyl ketone oxime as a blocking agent. The unsealing temperature of 110–150°C matches the commonly used hot stamping temperature (120–160°C) for aluminum foil, ensuring sufficient unsealing during hot stamping without premature curing during the drying stage.

[0010] In some embodiments, the aqueous film-forming resin is a compound of a first aqueous polyurethane emulsion and a second aqueous acrylic emulsion. The glass transition temperature of the first aqueous polyurethane emulsion is -20 to 5°C, and the glass transition temperature of the second aqueous acrylic emulsion is 80 to 110°C. The low-Tg polyurethane component imparts good film-forming flexibility and adhesion to the PET base film at room temperature. Its chain segments exhibit significantly enhanced mobility at hot stamping temperatures, presumably due to a higher probability of collision with the -NCO groups released after unsealing, which is beneficial for accelerating the irreversible curing rate. The high-Tg acrylic component provides the coating with the necessary surface hardness and scratch resistance before hot stamping, preventing damage during intermediate processes such as color layer coating and metallization. With a significant difference in Tg between the two emulsions, the coating itself tends to be "hard at room temperature and soft at high temperature," which works in conjunction with the irreversible curing mechanism: the coating softens moderately in the initial stage of hot stamping to adhere to the substrate surface, followed by a rapid curing reaction to set the coating.

[0011] In some embodiments, the composition further comprises a nano-aqueous wax dispersion and a surface control agent. The wax dispersion has a melting point of 75–95°C, and the surface control agent is a fluorinated surfactant and / or a silicone surfactant. The wax dispersion melts and migrates to the coating surface during the initial hot stamping stage, presumably rapidly constructing a temporary low-shear layer at the interface between the release layer and the PET base film, aiding initial peeling and ensuring transfer continuity even if the crosslinking reaction of the latent curing system is not yet fully complete. The surface control agent reduces the surface energy of the coating during room temperature storage, weakening secondary adhesion to the PET base film and reducing the tendency for gold flyaway.

[0012] In some embodiments, the composition further comprises a thermogenic acid generator. The thermogenic acid generator decomposes at the hot stamping temperature to produce acidic substances, which may catalyze local cross-linking within the coating or promote the condensation reaction of groups such as silanol groups. This helps to create a more pronounced difference in cross-linking density between the pressure-bearing and non-pressure-bearing areas of the graphic, thereby further limiting the transfer range spatially and improving the sharpness of the transfer of fine lines and dots. The dissociation temperature of the thermogenic acid generator can be matched to the hot stamping temperature, for example, an aromatic sulfonate or a triazine derivative. Preferably, the dissociation temperature of the thermogenic acid generator is higher than the baking temperature (100–120°C) in the drying step and not higher than the hot stamping temperature (120–160°C) in the hot stamping transfer step. This temperature window ensures that the thermogenic acid generator does not decompose and produce acid prematurely during the release layer formation stage, effectively releasing the acidic substances at the moment of hot stamping. The thermogenic acid generator can be selected from aromatic sulfonates or triazine derivatives with a dissociation temperature of 130–150°C.

[0013] In some embodiments, the composition further comprises a near-infrared absorbing material, which is indium tin oxide nanoparticles and / or tungsten oxide cesium nanoparticles. When the hot stamping equipment uses infrared heating or a heating method with a high proportion of thermal radiation, the near-infrared absorbing material can more efficiently convert radiant energy into heat energy, allowing the release layer to quickly rise to the unsealing temperature, shortening the time required for the latent curing system to complete cross-linking, and making it more suitable for high-speed hot stamping. Simultaneously, these inorganic nanoparticles have high transparency in the visible light region, having minimal impact on the appearance of the colored layer.

[0014] In some embodiments, based on the weight parts of solid content of each component, the low-Tg waterborne polyurethane emulsion accounts for 40-60 parts, the high-Tg waterborne acrylic emulsion accounts for 10-25 parts, the nano-waterborne wax dispersion accounts for 5-15 parts, the surface control agent accounts for 0.1-2 parts, the blocked polyisocyanate and the resin containing active hydrogen are added at an NCO / OH equivalent ratio of 0.8-1.2, and the two together account for 10-30 parts of the total solid content of the coating, the thermogenic acid generator accounts for 0.5-5 parts, the near-infrared absorbing material accounts for 0.1-3 parts, and the balance is film-forming aids, wetting and leveling agents, defoamers, and other additives. Within the specified proportion range, after the composition is sealed and stored at 50°C for 7 days, the viscosity change does not exceed 15% of the initial value, there is no visible layering or gel particles, and the release layer obtained after coating and drying is transparent and free of pinholes, indicating that the latent thermosetting system did not undergo significant premature reaction during wet storage and drying film-forming stages.

[0015] The present invention also provides a transfer process for improving the transfer rate of electroplated aluminum, using an aqueous coating composition of any of the foregoing embodiments, comprising the following steps: (1) The aqueous coating composition is coated onto a PET base film and dried to obtain a release layer; (2) A coloring layer, an aluminum plating layer and a hot melt adhesive layer are sequentially formed on the release layer to produce an electroplated aluminum hot stamping foil; (3) The hot stamping is transferred to the substrate at a hot stamping temperature of 120-160℃ and a hot stamping pressure of 3-8 kgf / cm². During the hot stamping process, the latent thermosetting system undergoes irreversible cross-linking.

[0016] In some embodiments, the coating in step (1) is performed using micro-gravure coating, with a wet film thickness of 3–8 μm; the drying is a two-stage process, with the first stage at 60–80°C to remove surface water, and the second stage at 100–120°C for baking, resulting in a dry release layer film thickness of 0.5–1.5 μm. The second stage baking temperature is intentionally controlled below the unsealing temperature of the blocked polyisocyanate. After baking, the release layer shows no significant dissolution or swelling when immersed in acetone for 30 minutes, indicating that the coating has only completed physical film formation and moisture evaporation at this stage, and the crosslinking reaction of the latent thermosetting system has not been triggered. The final curing of the coating is completely delayed until the hot stamping station, which is an important difference between this process and conventional thermosetting coating processes.

[0017] In some embodiments, the hot stamping process involves multiple stampings. After the first stamping, the transferred coating undergoes irreversible cross-linking and permanently cures, remaining in a cured state during subsequent hot stamping processes and reheating, without softening or flowing. When the sample after the first stamping is placed on a hot plate at 150°C for 30 seconds, the coating surface shows no stickiness, no indentations, and no transferability. Under the same conditions, the release layer on the PET base film, which has not been stamped, shows significant softening. This comparison indicates that the irreversible cross-linking reaction is essentially completed during the first stamping process, and the cross-linked coating possesses sufficient thermal stability to resist subsequent thermal cycling. Subsequent stampings continue to repeat the irreversible curing process on the untransferred coating portions, ensuring that the different colors do not interfere with each other, and that the multi-color overprinting accuracy does not deteriorate with the number of stampings.

[0018] The present invention has the following advantages over the prior art: The coating composition of this invention initiates irreversible crosslinking of the latent thermosetting system only under hot stamping heating and pressure conditions, ensuring that the transferred release layer is permanently locked in the first hot stamping, fundamentally avoiding the problems of coating softening, diffusion, and misregistration caused by reheating during multi-color hot stamping. Compared with schemes that rely on repeated softening by reversible phase change materials, the thermal stability of the transferred coating is significantly improved, maintaining dimensional and morphological stability even after reheating at the same temperature as the hot stamping. Through the compounding of low-Tg and high-Tg components in the aqueous film-forming resin, and the assistance of wax dispersions and surface control additives, the coating's adhesion to the PET base film before hot stamping meets the processing requirements of subsequent color layer coating and metallization processes, and the surface hardness meets the requirements of winding and slitting processes. The hot stamping process can quickly complete the peeling and curing synergistic process, resulting in high transfer rate and clear pattern edges. Thermoacid generators and near-infrared absorbing materials, as optional reinforcing components, can further optimize the regional selectivity and process window from the perspectives of chemical catalysis and photothermal conversion, respectively. The entire composition is an all-water-based system, which meets environmental protection requirements. The preparation and coating processes are compatible with existing electroplated aluminum production lines, and the industrial conversion is easy. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1 This embodiment provides a basic aqueous coating composition, the components of which are as follows by weight based on solid content: 50 parts of low-Tg waterborne polyurethane emulsion (Tg: -10℃, Covestro Bayhydrol UH series), 20 parts of high-Tg waterborne acrylic emulsion (Tg: 95℃, BASF Joncryl series), 10 parts of nano-waterborne polyethylene wax dispersion (wax melting point 85℃, particle size approximately 120nm, nonionic emulsion system, BYK Aquacer series), 0.5 parts of silicone surfactant (Tego Wet series), 10 parts of blocked hexamethylene diisocyanate trimer (using butanone oxime as the blocking agent, unblocking temperature approximately 130℃), 9 parts of waterborne polyester polyol (hydroxyl value approximately 90 mgKOH / g, NCO / OH equivalent ratio 1.0:1.0), 0.2 parts of organic bismuth catalyst (Borchi Kat series), 3 parts of film-forming aid (dipropylene glycol butyl ether), 0.3 parts of wetting and leveling agent, and 0.2 parts of defoamer.

[0021] The coating is prepared as follows: Low-Tg waterborne polyurethane emulsion and high-Tg waterborne acrylic emulsion are added to a dispersion vessel and stirred at 300 rpm for 5 min to obtain a resin premix. In another container, a closed-cell HDI trimer, waterborne polyester polyol, and organic bismuth catalyst are premixed uniformly. Under stirring at 400 rpm, the premixed latent thermosetting system is slowly added to the resin premix, followed by the sequential addition of a waterborne wax dispersion, a silicone surfactant, a film-forming aid, a wetting and leveling agent, and a defoamer. The stirring speed is increased to 600 rpm for 20 min, and the mixture is filtered through a 300-mesh screen to obtain the waterborne coating composition. After being stored sealed at 50°C for 7 days, the viscosity of this composition changes by less than 10%, and no stratification or gel particles appear.

[0022] Preparation of electroplated aluminum hot stamping foil: A 15μm thick PET base film was subjected to online corona treatment until the surface dyne value was ≥42mN / m; the above-mentioned aqueous coating composition was coated onto the base film using a micro-gravure coating method, with a wet film thickness of approximately 5μm; after two-stage drying—pre-baking with hot air at 70℃ for 4s in the first stage and baking with hot air at 110℃ for 8s in the second stage—a release layer with a dry film thickness of approximately 0.8μm was obtained. A small amount of the release layer sample was immersed in acetone for 30 minutes, and the coating showed no obvious dissolution or swelling, indicating that the cross-linking reaction was not triggered during the baking stage. An aqueous coloring layer, vacuum aluminum plating (aluminum layer thickness of approximately 380Å), and an aqueous hot melt adhesive layer (EVA type, coating amount of approximately 3g / m²) were sequentially coated onto the release layer, and the electroplated aluminum hot stamping foil was obtained after slitting.

[0023] Hot stamping transfer: The hot stamping foil was transferred on a flatbed hot stamping machine at a temperature of 150℃, a pressure of 5 kgf / cm², and a time of 0.5 s. The substrate was coated white cardboard. After hot stamping, the pattern was complete, the edges were clear, and there was no gold fly. The hot-stamped sample was wiped 10 times in acetone, and the coating did not dissolve or peel off. Another hot-stamped sample was heated on a 150℃ hot plate for 30 seconds. The coating surface was dry and hard, and there were no indentations or stickiness when pressed with a finger. In contrast, the unstamped release layer softened significantly under the same conditions, indicating that the irreversible cross-linking was completed to a degree sufficient to resist subsequent heat cycling during the 0.5 s hot stamping process.

[0024] Example 2 The component ratios were adjusted based on Example 1: the low-Tg waterborne polyurethane emulsion was increased to 55 parts, the high-Tg waterborne acrylic emulsion was reduced to 15 parts, the waterborne wax dispersion was reduced to 6 parts, and the total amount of the latent thermosetting system was increased to 22 parts. The sources, preparation methods, and hot stamping foil manufacturing processes of the remaining components were the same as in Example 1. The hot stamping temperature was adjusted to 140℃, and the pressure was 4 kgf / cm². After hot stamping, the coating did not dissolve or peel off after wiping with acetone 10 times, and the coating did not become sticky or soften after heating with a hot plate at 150℃ for 30 seconds, consistent with the results of Example 1.

[0025] Example 3 Based on the component ratio of Example 1, an additional thermogenic acid generator (thionium salt type thermogenic acid generator, San-Apro TA-100, which, when combined with epoxy resin, has a DSC curing onset temperature of approximately 137°C and a peak temperature of approximately 150°C) was added. Two parts of the mixture were prepared, along with 1.5 parts of indium tin oxide (ITO) nanoparticles (approximately 40 nm in diameter, 30% solid content, aqueous dispersion) as a near-infrared absorbing material. The thermogenic acid generator and the near-infrared absorbing material were added to the resin premix before the latent thermosetting system was added. After preparation, the composition showed a viscosity change of less than 12% after 7 days of sealed storage at 50°C. Visually, there was no layering, sedimentation, or gel particles, and the color was a uniform light blue-gray, indicating that the ITO nanoparticles were stably dispersed within the system, and the introduction of the thermogenic acid generator did not affect the storage stability of the composition. The coating preparation and hot stamping conditions were the same as in Example 1, except that the hot stamping equipment was equipped with an infrared heating auxiliary section. The hot stamping speed was approximately 20% higher than in Example 1. After hot stamping, the coating did not dissolve or peel off after 10 acetone wipes, and the coating did not become sticky or soften after 30 seconds of heating on a hot plate at 150°C. The transfer integrity rate of fine lines (0.05 mm in width) was over 95%.

[0026] The aforementioned TA-100 is a thionium salt-type hot acid generator produced by San-Apro Ltd., belonging to the aromatic sulfonium salt class, CAS number [1365091-47-9]. When combined with epoxy resin, its curing initiation temperature, as determined by DSC, is approximately 137°C. In this embodiment, it is used in a waterborne polyurethane-acrylic system, where it decomposes to produce acid at hot stamping temperatures (145-155°C), thus acting as an auxiliary catalyst for crosslinking. Example 4 – Multicolor Hot Stamping The electroplated aluminum hot stamping foil prepared in Example 3 was subjected to three consecutive hot stamping processes (gold → red → blue). The hot stamping plates of each color had an overprinting alignment relationship. The hot stamping process parameters were as follows: the hot stamping temperature of the first color (gold) was 155°C, the hot stamping temperature of the second color (red) was 150°C, and the hot stamping temperature of the third color (blue) was 145°C. The pressure was 5 kgf / cm² and the time was 0.5 s.

[0027] After the first color hot stamping was completed, a sample was tested: the sample was placed on a 150℃ hot plate and heated for 30 seconds. The coating surface was dry and hard, without stickiness, indentation, or transferability. After the second and third colors were hot stamped, the transferred coatings maintained dimensional stability, with no edge diffusion or pattern softening or deformation, and the multi-color printing deviation was not perceptible to the naked eye.

[0028] Comparative Example 1 (Conventional scheme without latent thermosetting system) The composition ratio is as follows: 50 parts of low-Tg waterborne polyurethane emulsion (same as Example 1), 20 parts of high-Tg waterborne acrylic emulsion (same as Example 1), 10 parts of nano-waterborne polyethylene wax dispersion (same as Example 1), 0.5 parts of silicone surfactant, 3 parts of film-forming aid, 0.3 parts of wetting and leveling agent, and 0.2 parts of defoamer. It does not contain blocked polyisocyanates, resins containing active hydrogen, or catalysts. The preparation method, coating process, and hot stamping foil preparation steps are the same as in Example 1.

[0029] Single-color hot stamping can complete the transfer normally. Under three-color continuous hot stamping conditions (according to the conditions of Example 4), the first color coating shows perceptible edge diffusion after being reheated by the second color hot stamping; after the third color hot stamping is completed, the edges of the first and second color patterns show varying degrees of softening and deformation, and the registration accuracy is significantly reduced. When the sample after the first color hot stamping is placed on a hot plate at 150°C for 30 seconds, the coating surface shows stickiness and indentation transferability.

[0030] Comparative Example 2 (Reversible Phase Change Microcapsule Solution) The main formulation of the release layer is the same as that of Comparative Example 1, except that 5 parts of paraffin core-silica shell phase change microcapsules (D50 particle size approximately 2 μm, phase change temperature approximately 130℃, prepared according to CN121203209A using the Stöber method) are added to replace an equal amount of the film-forming resin solids. In the preparation method, the phase change microcapsules are introduced by low-speed stirring after the wax dispersion is added and before the additives are added. The hot stamping temperature is 155℃, and the pressure is 5 kgf / cm².

[0031] The integrity of the single-color hot stamping transfer was comparable to that of Example 1. Under three-color continuous hot stamping conditions, the first color coating showed visible pattern diffusion under the reheating effect of the second color hot stamping; after the third hot stamping, the first two color coatings showed varying degrees of smearing and misregistration. When the sample after the first color hot stamping was placed on a hot plate at 150°C for 30 seconds, the coating surface became sticky, indicating that the phase change microcapsules reversibly melted upon reheating, causing the transferred coating to soften.

[0032] Comparative Example 3 (single-component blocked isocyanate, resin without active hydrogen) Based on Example 1, only 10 parts of blocked HDI trimer were added, and no aqueous polyester polyol or catalyst was added. The remaining components and preparation methods were the same as in Example 1.

[0033] After hot stamping transfer, the sample was wiped five times in acetone, resulting in partial dissolution and peeling of the coating. Heating on a 150°C hot plate for 30 seconds caused the coating surface to become sticky. Under three-color hot stamping conditions, the first color coating softened and diffused during subsequent stampings. It is speculated that the lack of a co-reacting resin containing active hydrogen means that the blocked isocyanate, after unblocking, mainly reacts with moisture in the air or trace amounts of hydroxyl groups in the coating, resulting in low crosslinking density and insufficient formation of an effective irreversible curing network.

[0034] Performance Testing and Results Summary The coatings of Examples 1-4 and Comparative Examples 1-3 were subjected to the following performance tests: (1) Single-color hot stamping transfer effect: Visually observe the integrity of the pattern after hot stamping, the clarity of the edges, and whether there is any gold flying phenomenon.

[0035] (2) Stability after reheating at 150℃: Place the hot stamped sample on a 150℃ hot plate for 30 seconds and observe whether the coating surface becomes sticky, softened or the indentation can be transferred.

[0036] (3) Multicolor hot stamping registration accuracy: Perform three consecutive hot stampings (gold → red → blue), observe whether there is diffusion at the edge of each color coating, and measure the final registration deviation.

[0037] (4) Fine line transfer integrity rate: The transfer was carried out using a hot stamping plate with a line width of 0.05mm, and the proportion of lines that were completely transferred was statistically analyzed.

[0038] The test results are summarized in the table below.

[0039]

[0040] Note: "—" indicates that this item was not specifically tested in this embodiment; the 0.05mm line integrity rate of Comparative Example 3 is close to that of Example 1 under single-color hot stamping conditions, but it decreases significantly after multi-color hot stamping due to coating softening.

[0041] As can be seen from the table above: The common feature of Examples 1-4 is that the coatings do not become sticky after hot stamping and are reheated at 150°C, the three-color printing misalignment is controlled within 0.1 mm, and the coating edges remain clear. This indicates that the latent thermosetting system completes irreversible cross-linking during the hot stamping process, and the transferred coating acquires dimensional stability to resist subsequent thermal cycling.

[0042] Although both Comparative Examples 1 and 2 could complete normal transfer in single-color hot stamping, they both became sticky after reheating at 150℃, with a three-color registration deviation exceeding 0.3mm and obvious diffusion at the coating edges. Comparative Example 2 (phase change microcapsule solution) and Comparative Example 1 (conventional solution) showed similar thermal stability, further confirming that the reversible softening mechanism cannot avoid the accuracy degradation caused by heat history accumulation in multi-color hot stamping.

[0043] Although Comparative Example 3 was comparable to Example 1 in terms of transfer integrity in single-color hot stamping, it became sticky in the reheat test, and the coating spread significantly after three-color hot stamping. This indicates that without a co-reactive resin containing active hydrogen, the reaction of the blocked isocyanate alone is insufficient to construct an effective irreversible curing network.

[0044] Industrial Application Notes All raw materials used in this water-based coating composition are commercially available, and the coating process is compatible with existing electroplated aluminum gravure coating production lines. The coating speed can be performed at conventional parameters of 80-150 m / min. During the hot stamping process, the baking temperature in the drying section should be controlled below the unsealing temperature of the closed polyisocyanate to ensure that the "dormant" state of the latent thermosetting system is maintained until the moment of hot stamping.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A water-based coating composition for improving the transfer rate of electroplated aluminum, characterized in that, It includes waterborne film-forming resins and latent thermosetting systems. The latent thermosetting system consists of a blocked polyisocyanate and a resin containing active hydrogen, wherein the unblocking temperature of the blocked polyisocyanate is 110–150°C. The latent thermosetting system does not cross-link with water-based film-forming resin at room temperature, but is desealed under heating and pressurization conditions during electroplated aluminum hot stamping transfer, and undergoes irreversible cross-linking with resin containing active hydrogen.

2. The aqueous coating composition according to claim 1, characterized in that, The resin containing active hydrogen is an aqueous polyester polyol with a hydroxyl value of 50-150 mgKOH / g.

3. The aqueous coating composition according to claim 1, characterized in that, The aqueous film-forming resin is compounded from a first aqueous polyurethane emulsion and a second aqueous acrylic emulsion. The glass transition temperature of the first aqueous polyurethane emulsion is -20 to 5°C, and the glass transition temperature of the second aqueous acrylic emulsion is 80 to 110°C.

4. The aqueous coating composition according to claim 3, characterized in that, The composition further comprises a nano-aqueous wax dispersion and a surface control aid, wherein the wax melting point of the nano-aqueous wax dispersion is 75–95°C, and the surface control aid is a fluorinated surfactant and / or a silicone surfactant.

5. The aqueous coating composition according to claim 1, characterized in that, The composition further comprises a thermogenic acid generator, the dissociation temperature of which is higher than the baking temperature in the drying step but not higher than the hot stamping temperature in the hot stamping transfer step.

6. The aqueous coating composition according to claim 5, characterized in that, The composition further comprises a near-infrared absorbing material, wherein the near-infrared absorbing material is indium tin oxide nanoparticles and / or tungsten oxide cesium nanoparticles.

7. A transfer process for improving the transfer rate of electroplated aluminum, characterized in that, The water-based coating composition used according to any one of claims 1-6 comprises the following steps: (1) The aqueous coating composition is coated onto a PET base film and dried to obtain a release layer; (2) A coloring layer, an aluminum plating layer and a hot melt adhesive layer are sequentially formed on the release layer to produce an electroplated aluminum hot stamping foil; (3) The latent thermosetting system is transferred to the substrate under the conditions of hot stamping temperature of 120-160℃ and hot stamping pressure of 3-8 kgf / cm². During the hot stamping process, the latent thermosetting system undergoes irreversible cross-linking.

8. The transfer process according to claim 7, characterized in that, In step (1), the coating is done using micro-gravure coating, with a wet film thickness of 3-8 μm; the drying is done in two stages, with the first stage temperature at 60-80℃ and the second stage temperature at 100-120℃, resulting in a release layer dry film thickness of 0.5-1.5 μm.

9. The transfer process according to claim 8, characterized in that, The temperature of the second segment is lower than the desealing temperature of the blocked polyisocyanate.

10. The transfer process according to claim 7, characterized in that, The hot stamping process involves multiple hot stampings. After the first hot stamping, the coating has been transferred and irreversibly cross-linked, and it remains in a cured state during the reheating of subsequent hot stampings.

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