Back coating glue for preventing high-temperature adhesion of coated card paper and preparation method of back coating glue

Abstract

The invention provides back coating glue for preventing high-temperature adhesion of coated paperboard and a preparation method of the back coating glue. The back coating glue is prepared from the following raw materials in parts by mass: 200 parts of waterborne polyurethane dispersion, 10 to 20 parts of modified filler, 1 to 5 parts of blocked isocyanate cross-linking agent, 0.5 to 2 parts of dispersing agent, 0.2 to 1 part of flatting agent and 0.1 to 0.5 part of defoaming agent. Wherein amino and a polyether-b-polysiloxane chain segment are grafted on the surface of the modified filler, and the polyether chain segment is grafted on one side close to the surface of the filler. According to the invention, the modified filler with a directional grafting structure is constructed and is matched with a thermally-triggered cross-linking system, so that a coating film has good water-based dispersibility and film-forming uniformity at normal temperature, a low-surface-energy film layer can be formed in a hot pressing process, and stable anchoring of the structure is realized; therefore, the bridging and adhesion phenomena generated in high-temperature stacking of the card paper are effectively inhibited.

Description

Technical Field

[0001] The present application relates to the technical field of back-coating adhesives, and in particular to a back-coating adhesive for preventing high-temperature adhesion of coated paperboard and a preparation method thereof. Background Art

[0002] Coated cardboard is widely used in high-end printed packaging applications such as cigarette packaging, food labels, and cosmetic boxes. Its surface typically undergoes multiple layers of coating to achieve excellent printability and visual quality. In practice, to improve production efficiency, coated cardboard is often stacked and stored directly after high-temperature processing such as hot pressing, hot stamping, and lamination. However, during the immediate stacking process after high-temperature processing, the back of the cardboard is prone to bridging, sticking, and even peeling and tearing between the adjacent front sheets, seriously affecting the quality of the finished product and stacking stability.

[0003] To address these issues, the industry typically applies a layer of backing adhesive or anti-stick coating to the back of the cardboard to reduce the surface energy after hot pressing and alleviate interlayer adhesion. Existing backing adhesive systems primarily utilize the following methods: First, paraffin wax emulsions, polyethylene wax emulsions, or silicone oil emulsions are blended into aqueous emulsion systems to form hydrophobic particles or low-surface-energy components during the drying process, which precipitate on the film surface to provide an anti-sticking effect; second, fluorinated surfactants or hydrophobically modified inorganic particles are introduced to improve the film's surface tension and peeling performance during stacking; and third, materials such as EPDM, which have good heat resistance and relatively low surface energy, are used to improve adhesion during stacking.

[0004] However, the above methods still have the following common problems: the coating has poor stability, and the low surface energy components are easy to migrate and lose during storage or use, causing the coating surface to become flowery, the structure to be uneven, or long-term failure; the structure is easily disturbed during the hot pressing process, and the anti-sticking components are redistributed or buried, resulting in adhesion; there is a risk of agglomeration and sedimentation of the mixed particles, which affects the storage stability and application uniformity; at the same time, this type of anti-sticking component usually does not have the ability to synergistically cross-link with the main glue, the film-forming structure is loose, the mechanical adhesion is insufficient, and it is easy to peel, warp or scratch; the low surface energy material needs to be dispersed in an organic solvent, which causes greater pollution to the environment.

[0005] Existing back-coating adhesive systems include water-soluble polyvinyl alcohol and emulsion-based acrylic. For example, related art discloses a back-coating adhesive for high-temperature adhesion-resistant coated cardboard. This adhesive uses water-soluble polyvinyl alcohol as the primary adhesive, supplemented by acrylic emulsion, water-based polyurethane, and wax emulsion. This creates a hydrophobic structure on the film surface, initially enabling post-hot-pressing adhesion control. However, the anti-sticking particles used are still physically dispersed wax emulsions, lacking structural orientation and network intercalation mechanisms. These particles are prone to migration or loss after hot-pressing disturbances. Furthermore, the polyvinyl alcohol primary adhesive itself has poor heat resistance, potentially causing bridging adhesion problems during high-temperature stacking.

[0006] Therefore, there is an urgent need to develop a back-coating adhesive technology solution that has good dispersion stability in the aqueous system, can construct a stable low-surface energy surface layer during the hot pressing process, and can synergistically cross-link with the main adhesive system, so as to effectively alleviate the adhesion problem after hot pressing and improve the structural stability and application reliability of the coated cardboard. Summary of the Invention

[0007] The present application provides a back-coated adhesive for preventing high-temperature adhesion of coated cardboard and a preparation method thereof, aiming to overcome the problems of poor thermal stability of existing back-coated adhesives under high-temperature processing conditions, easy migration and failure of anti-sticking structures, and lack of synergistic curing ability with the main adhesive. The back-coated adhesive has good water-based dispersibility and application stability at room temperature, can construct a stable directional low-surface energy film layer structure during the hot pressing process, and realize spatial anchoring and anti-adhesion performance of the film layer structure through a synergistic cross-linking mechanism, which is suitable for the anti-sticking treatment needs of coated cardboard in high-temperature processing scenarios such as hot pressing, hot stamping, and stacking.

[0008] In the first aspect, the present application provides a back-coating adhesive for preventing high-temperature adhesion of coated cardboard, comprising the following raw materials in parts by mass: 200 parts of aqueous polyurethane dispersion, 10 to 20 parts of modified filler, 1 to 5 parts of blocked isocyanate cross-linking agent, 0.5 to 2 parts of dispersant, 0.2 to 1 part of leveling agent, and 0.1 to 0.5 parts of defoaming agent; wherein, the surface of the modified filler is grafted with amino groups and polyether-b-polysiloxane segments, and the polyether segments are grafted to the side close to the filler surface.

[0009] According to the present application, the back-coating adhesive is constructed with a modified filler having a directional grafted structure and is combined with a heat-triggered cross-linking system to enable the back-coating adhesive to have good water-based dispersibility and film-forming uniformity at room temperature. During the hot pressing process, a low surface energy film layer can be formed and structurally stable anchoring can be achieved, thereby effectively suppressing the bridging and adhesion phenomena of cardboard during high-temperature stacking.

[0010] Specifically, the waterborne polyurethane dispersion serves as the main adhesive, providing excellent film-forming properties and interfacial adhesion, and possesses active groups that react with isocyanate; the modified filler improves its dispersibility in the aqueous system and forms a stable colloidal structure by grafting polyether segments, and the polyether segments are grafted on the side close to the filler surface, so that the terminal polysiloxane segments have good segment flexibility and spatial mobility; it is understandable that before hot pressing, during the gradual curing process of the coating system, the polyether segments are constrained by the local network structure and cannot undergo effective conformational changes, and remain in a curled or shrunken state; in the subsequent hot pressing process, as the temperature further increases, the film body locally softens, and the polyether segments obtain segment freedom driven by heat, thereby undergoing a conformational transition from a contracted state to an extended state, releasing segment tension, and cooperating with the polysiloxane segments to drive the modified filler to migrate and be exposed on the film surface, forming a uniformly distributed low surface energy structure to reduce the tendency of adhesion between paper sheets after hot pressing.

[0011] At the same time, since the surface of the modified filler retains amino functional groups that do not participate in the grafting reaction, during the hot pressing process, the conformational change of the polyether chain segment can also prompt the originally coated or masked amino groups to be gradually exposed, thereby effectively reducing the influence of steric hindrance on its reaction with isocyanate and improving the reactivity of the amino group; at this time, the isocyanate groups released by the blocked isocyanate cross-linker can efficiently react with the amino groups exposed on the surface of the modified filler and the hydroxyl functional groups in the main glue system to construct an embedded synergistic cross-linking structure between the main glue and the filler; this synergistic cross-linking not only improves the structural stability and heat deformation resistance of the overall coating film, but also can covalently anchor the directionally exposed modified filler to the film surface to prevent it from migrating or backflowing during the cooling and stacking process after hot pressing, thereby ensuring the durability and anti-disturbance of the anti-sticking function.

[0012] Through the synergistic effect of the above-mentioned raw material structures and spatial arrangements, the back-coated adhesive provided in this application is stably dispersed and evenly applied at room temperature. It can achieve the migration construction and covalent anchoring of low surface energy structures during the hot pressing process, and has excellent hot pressing anti-adhesion performance and long-term structural stability. It is particularly suitable for the anti-sticking treatment of coated cardboard in high-temperature stacking processing scenarios such as hot stamping and lamination.

[0013] It should be noted that the aqueous polyurethane dispersion in this application is generally dispersed in water, with a solid content of 35% to 70%. As an example, the solid content of the aqueous polyurethane dispersion in one embodiment of this application is 50%.

[0014] In some embodiments, the modified filler is prepared by the following steps: S1: reacting an inorganic filler with an aminosilane coupling agent to graft active amino groups onto the surface of the inorganic filler to obtain an amino filler; S2: reacting the amino filler with the double-end carboxyl polyether, so that the carboxyl group at one end of the double-end carboxyl polyether undergoes an amidation reaction with part of the amino groups on the surface of the amino filler to obtain a carboxyl-terminated polyether grafted filler; S3: reacting the carboxyl-terminated polyether grafted filler with amino silicone oil to cause an amidation reaction between the carboxyl group of the carboxyl-terminated polyether grafted filler and the amino group on the amino silicone oil to obtain a modified filler.

[0015] In some of the above embodiments, the three-step reaction route realizes the control of the directionality and functionality of the grafted structure on the surface of the modified filler. Specifically, in step S1, a reactive amino group is introduced by an aminosilane coupling agent to provide a basic anchor point for the subsequent amidation reaction; in step S2, when the double-end carboxyl polyether reacts with the amino filler, since the filler surface is a solid interface, the spatial configuration of the molecular chain is limited. Once the polyether segment is grafted to the filler surface through one end carboxyl group, due to steric hindrance, directional arrangement and electrostatic repulsion, the other end carboxyl group is difficult to approach the amino group on the surface of another filler. Therefore, in this process, it is not easy for the fillers to be bridged by the polyether segment, so that one end carboxyl group can be retained for the S3 reaction, and at the same time, some amino groups can be retained on the filler surface; In step S3, the remaining carboxyl groups from the previous step undergo an amidation reaction with an amino-terminated polysiloxane to introduce low-surface-energy segments and construct a polyether-b-polysiloxane block structure. Because the polyether segments are already anchored to the filler surface, the grafted polysiloxane segments are located at the end of the polyether segments away from the filler surface. This provides greater segment freedom and spatial mobility, facilitating the directional exposure of the modified filler to the film surface during hot pressing to create a non-stick structure.

[0016] Furthermore, since some amino groups on the filler surface are not involved in the grafting reaction, they can still participate in the crosslinking reaction with the blocked isocyanate during the subsequent hot pressing process, synergizing with the main adhesive to form a network structure, further enhancing the structural stability of the modified filler in the coating system. Compared with traditional physically adsorbed hydrophobic particles, this type of chimeric modified filler constructed through gradual grafting has higher structural controllability, dispersion stability, and hot pressing anti-sticking durability, making it particularly suitable for back coating of cardboard materials that require long-term stacking or high-temperature treatment.

[0017] In some embodiments, step S1 includes: 50 parts of inorganic filler and 1-5 parts of aminosilane coupling agent are dispersed in 500-1000 parts of ethanol aqueous solution, and reacted at 60-80° C. for 2-4 hours to obtain an amino filler.

[0018] In some of the above embodiments, step S1 is used to introduce an active amino structure on the surface of the inorganic filler to provide an anchoring point for the subsequent directional grafting of polyether segments and polysiloxane segments; the aminosilane coupling agent can be hydrolyzed in an ethanol-water system to form a silanol intermediate, and dehydrate and condense with the hydroxyl groups on the filler surface to form a stable Si-OM structure, thereby covalently anchoring the amino functional group to the filler surface.

[0019] The amount of aminosilane coupling agent added is preferably controlled between 1 and 5 parts by mass based on 50 parts of inorganic filler. This is to provide sufficient grafting sites while avoiding structural interference caused by excessive surface amino group density. Specifically, if the amino group density is too high, the probability of double-point anchoring or cross-linking reactions occurring during grafting of the polyether segments will increase, affecting subsequent grafting reactions. Furthermore, excess coupling agent may also cause lateral self-condensation, forming a surface coating layer that reduces the filler's aqueous dispersibility and surface reactivity. Therefore, in this application, by controlling the amount of coupling agent added and the reaction time, an amino-containing filler structure with a moderate and uniform amino group density is achieved, providing a stable foundation for subsequent segment construction and cross-linking reactions.

[0020] In some embodiments, the inorganic filler includes at least one of talc, kaolin, and silicon dioxide, and the average particle size of the inorganic filler is 1 to 3 μm.

[0021] In some of the aforementioned embodiments, the average particle size of the inorganic filler is preferably 1-3 μm, which not only improves its dispersibility in the aqueous polyurethane colloid but also facilitates the formation of microscale low-surface-energy structural units within the coating film. Smaller particle sizes provide a larger specific surface area, improving the efficiency of functional group grafting, while also providing a more uniform distribution across the film surface, helping to build a stable and continuous anti-stick structure. During the hot pressing process, the polyether segments grafted onto the surface of the modified filler undergo conformational changes and segment migration, with the physical size of the particles significantly influencing the synergistic nature of this process. Compared with large-particle fillers, small-size fillers with an average particle size in the range of 1~3μm have higher micro-responsiveness and segment cooperative migration ability during the local film softening process. They can undergo subtle arrangement adjustments as the chain segments release tension, making it easier for the modified fillers to be directionally exposed on the film surface. Inorganic particles under this particle size are embedded in the film surface, and a better continuous anti-sticking interface is constructed based on low surface energy segments and rough structures. It can also inhibit the accumulation of surface polysiloxane segments. The two work together to further improve the structural stability and anti-sticking durability of the back-coated film in the post-hot pressing adhesion environment. Too large a particle size is not conducive to migrating to the film surface to form anti-sticking on the film surface. Too small a particle size is not conducive to cooperating with the polysiloxane segments on its surface to form continuous anti-sticking points, which can easily lead to poor anti-sticking effect.

[0022] In some embodiments, in step S2, the double-ended carboxyl polyether is obtained by reacting a double-ended hydroxyl polyether with succinic anhydride, wherein the double-ended hydroxyl polyether comprises a polyethylene glycol segment and a polypropylene glycol segment, the mass percentage of the polyethylene glycol segment in the double-ended hydroxyl polyether is 50% to 80%, and the weight average molecular weight of the double-ended hydroxyl polyether is 1000 to 4000.

[0023] In some of the above embodiments, the design of the polyether structure has a certain influence on the aqueous dispersibility and thermal response behavior of the modified filler. The double-terminated hydroxyl polyether is preferably a polyether block copolymer with certain temperature-sensitive properties, including polyethylene glycol (PEG) segments and polypropylene glycol (PPG) segments. The PEG segments can provide good hydrophilicity and colloidal stability at room temperature. At the same time, during the hot pressing stage, the thermal response characteristics of the PPG segments trigger segment contraction, conformational changes, and segment tension release, providing physical drive for the migration of the modified filler and the exposure of functional groups. By adjusting the weight percentage of polyethylene glycol segments in the double-terminated hydroxyl polyether to 50%–80%, the balance between the hydrophilicity of the polyether segments and their thermally driven responsiveness can be adjusted. A relatively high PEG ratio improves water dispersibility and segment flexibility, facilitating the transition of the polyether segments from a coiled to an extended configuration during hot pressing, thereby synergizing with the terminal polysiloxane segments to promote the migration of the modified filler to the membrane surface. The addition of an appropriate amount of poly(ethylene glycol) (PPG) enhances the thermosensitivity of the polyether segments. During the drying process of the backing adhesive, the PPG segments dehydrate, leading to the aggregation of hydrophobic groups and shrinkage. During the hot pressing process, the thermally driven transition to an extended configuration releases the original conformational tension, providing a thermally responsive driving force for segment migration. However, if the PEG ratio is too high, while the segment flexibility is good, the overall thermal responsiveness decreases, weakening the migration effect. Conversely, if the PPG ratio is too high, the initial dispersion of the system is insufficient, affecting the anchoring and uniformity of the segments. Therefore, the double-terminated hydroxyl polyether with a polyethylene glycol segment mass percentage of 50% to 80% can achieve good synergy between dispersion stability and migration responsiveness.

[0024] The double-terminated hydroxyl polyether is introduced into a double-terminated carboxyl structure by an esterification reaction with succinic anhydride, thereby obtaining the ability to undergo a subsequent amidation reaction with filler amino groups and amino silicone oil. In this reaction, the degree of esterification is easy to control and no secondary functional groups are introduced, which is beneficial to maintaining the structural integrity of the polyether segment and the utilization rate of the functional groups. The weight-average molecular weight of the polyether segment is preferably controlled within the range of 1000 to 4000, so that the segment has appropriate flexibility and ductility: if the molecular weight is too low, the segment's thermal migration ability is insufficient, making it difficult to cross steric hindrance and arrange in a directional manner; if the molecular weight is too high, the utilization rate of the reactive end groups decreases, and the segment is prone to entanglement or migration restriction in the coating film. Therefore, the polyether segment within this molecular weight range can be fully dispersed in an aqueous environment, and at the same time has good segment responsiveness and mobility during the hot pressing stage, further enhancing the anti-adhesion effect of the back coating film after hot pressing.

[0025] In some embodiments, the double-terminated carboxyl polyether is obtained by reacting a double-terminated hydroxyl polyether with succinic anhydride, comprising: dissolving 100 parts of the double-terminated hydroxyl polyether in 300-500 parts of dichloromethane, and stirring to form a uniform solution; adding 1-3 parts of 4-dimethylaminopyridine as a catalyst and 3-5 parts of succinic anhydride to the solution, and reacting at 20-30° C. under a nitrogen atmosphere for 16-24 hours; after the reaction, washing the reaction mixture with a saturated sodium bicarbonate solution and distilled water in sequence to remove unreacted anhydride and catalyst residues, and then drying the organic phase over anhydrous sodium sulfate and removing the solvent by rotary evaporation to obtain a double-terminated carboxyl polyether product.

[0026] In some embodiments, step S2 includes: 10 parts of double-terminated carboxyl polyether, 20-40 parts of amino filler, 1-2 parts of 1-ethyl-(3-dimethylaminopropyl)carbodiimide, and 0.5-1 part of N-hydroxysuccinimide are dispersed in 100-200 parts of water, and the mixture is reacted at a pH of 5-6 and 20-40°C for 6-8 hours to obtain a carboxyl-terminated polyether grafted filler.

[0027] In some of the above embodiments, the reaction is an amide bond construction step, the main purpose of which is to selectively cause one end of the double-ended carboxyl polyether to undergo an amidation reaction with part of the amino groups on the surface of the amino filler to form a single-end anchoring structure of the polyether segment, while retaining the other end carboxyl group for subsequent grafting of the polysiloxane segment, and retaining some unreacted amino functional groups on the filler surface for subsequent cross-linking with isocyanate.

[0028] Among them, the mass ratio between the double-ended carboxyl polyether and the amino filler is preferably 1:2-4, that is, 10 parts of polyether corresponds to 20-40 parts of amino filler. By controlling the mass ratio between the two, the grafting density of the carboxyl polyether chain segment on the surface of the amino filler can be controlled. When the mass ratio of the two is controlled within the above range, the filler surface has an appropriate grafting density, which can reduce the influence of excessive grafting density on the migration of the chain segments and reduce the influence of excessive grafting density on the integrity of the low surface energy film, thereby achieving better high-temperature adhesion effect.

[0029] The EDC / NHS (1-ethyl-(3-dimethylaminopropyl)carbodiimide / N-hydroxysuccinimide) carboxyl activation system efficiently activates polyether terminal carboxyl groups at pH 5-6 and room temperature, promoting their directional reaction with amino groups. This mild reaction environment not only maintains the stability of the polyether backbone structure, but also promotes the physical stability of the colloidal dispersion system during the reaction.

[0030] In summary, this implementation method realizes a carboxyl-terminated polyether grafted filler with good structural directionality and cross-linking capability through reasonable control of the ratio between polyether and filler, providing a structural and functional basis for the subsequent low surface energy segment construction and main adhesive network synergy, further improving the anti-adhesion performance and long-term structural stability of the back coating film during the hot pressing process.

[0031] In some embodiments, in step S3, the weight average molecular weight of the amino silicone oil is 500-2000.

[0032] In some of the above embodiments, the weight-average molecular weight of the amino silicone oil is preferably controlled within the range of 500 to 2000, so that the modified filler can take into account both low surface energy construction ability and chain segment flexibility; if the molecular weight is too low, the polysiloxane chain segment is too short, and the ability to promote the migration of the modified filler to the surface energy regulation is insufficient; if the molecular weight is too high, the chain segment is too long and easily entangled, the migration drive is insufficient, and even physical phase separation may occur in the reaction system, affecting the grafting efficiency and subsequent dispersion stability.

[0033] As an example, in one embodiment of the present application, amino polydimethylsiloxane with a weight average molecular weight of 1000 is used.

[0034] In some embodiments, step S3 includes: 10 parts of carboxyl-terminated polyether grafted filler, 1-2 parts of amino silicone oil, 0.1-0.3 parts of 1-ethyl-(3-dimethylaminopropyl)carbodiimide, and 0.05-0.15 parts of N-hydroxysuccinimide are dispersed in 100-200 parts of N,N-dimethylformamide, and reacted at 20-40°C for 6-8 hours to obtain a modified filler.

[0035] In some of the above embodiments, this step is intended to utilize the amidation reaction between the carboxyl group and the amino group to cause the carboxyl-terminated polyether segment retained in the previous step to undergo a directional grafting reaction with the amino-terminated polysiloxane to construct a polyether-b-polysiloxane block structure, and ultimately form a low surface energy modified filler with spatial arrangement characteristics.

[0036] The amino silicone oil is added in an amount of 1-2 parts by mass per 10 parts of carboxyl-terminated polyether grafted filler to ensure that the carboxyl groups at the ends of the polyether segments participate fully in the grafting process while preventing excess unreacted silicone oil residue. This ratio not only improves grafting efficiency but also creates a quantitatively controllable distribution of polysiloxane segments during the hot pressing process, preventing excessive hydrophobization or segment agglomeration that could affect structural uniformity.

[0037] Because the polyether segments are anchored to the filler surface, amino silicone oil grafting occurs at the distal ends of the segments, creating a well-defined block chain structure: a unilaterally extended filler-polyether-polysiloxane configuration. During hot pressing and softening, the polysiloxane segments migrate and quickly become exposed to the film surface, collaborating with the filler to create low-surface-energy regions, further enhancing the anti-blocking properties of the back-coated film after hot pressing.

[0038] In some embodiments, the waterborne polyurethane dispersion includes Wanhua Adwel 1630B. Based on the above embodiments, Wanhua Adwel 1630B has a moderate hydroxyl content and film-forming strength, and combines flexibility and heat resistance. This facilitates the reaction of the main adhesive with the blocked isocyanate during hot pressing to form a dense cross-linked structure, while maintaining the compatibility transition of the film between drying and hot pressing.

[0039] In some embodiments, the blocked isocyanate crosslinker includes Covestro Imprafix 2794. Based on the above embodiment, Covestro Imprafix 27944 is a pyrolytic NCO with a pyrolysis temperature of approximately 110°C. It is stable during the thermal drying process and releases isocyanate groups during the hot pressing stage. These react with the hydroxyl groups in the aqueous polyurethane dispersion and the residual amino groups on the surface of the modified filler to form a synergistic crosslinking structure between the main glue and the filler, further enhancing the overall stability of the film and its resistance to deformation during hot pressing.

[0040] In some embodiments, the dispersant includes BASF Disponil A 1080. Based on the above embodiment, BASF Disponil A 1080 is a non-ionic surfactant that can improve the dispersion uniformity and component stability of the system in an emulsion state.

[0041] In some embodiments, the leveling agent includes Digo 410. Based on the above embodiment, Digo 410 helps to improve the tension balance and flow uniformity of the coating surface and prevent local film surface blooming after hot pressing.

[0042] In some embodiments, the defoaming agent includes BYK 024. Based on the above embodiments, BYK 024 can effectively control bubble generation during the sizing, mixing, and drying stages to ensure film density.

[0043] In some embodiments, the back-coating adhesive further includes water, and the solid content of the back-coating adhesive is 20 wt % to 35 wt %. Based on the above embodiment, within this range, the system has a moderate coating viscosity and construction fluidity, which not only facilitates uniform application, avoids scratches and bubbles, but also forms a continuous and dense film structure after drying.

[0044] In a second aspect, the present application provides a method for preparing a back-coated adhesive for preventing high-temperature adhesion of coated paperboard, comprising: Providing a raw material for the back-coating adhesive according to any embodiment of the first aspect; The raw materials are mixed and water is added to make the solid content of the mixture be 20 wt % to 35 wt %, and the mixture is mixed evenly to obtain a back coating adhesive.

[0045] According to the present application, by dispersing and mixing the raw materials in the formulation according to predetermined proportions and controlling the solids content of the system within a range of 20 wt% to 35 wt%, a back-coating adhesive product with stable colloid and suitable for coating and sizing processes can be obtained. The method does not rely on special equipment or reaction conditions, has excellent operational simplicity and industrial adaptability, and because it includes the raw materials for the back-coating adhesive of the first aspect, the prepared back-coating adhesive exhibits the beneficial effects of the first aspect.

[0046] In a third aspect, the present application provides a coated cardboard, comprising: a base paper, and a back coating layer arranged on one side of the base paper, wherein the back coating layer is obtained by drying the back coating glue described in any embodiment of the first aspect or the back coating glue prepared by the method described in any embodiment of the second aspect.

[0047] According to the present application, the coated cardboard, while maintaining the original graphic printing suitability and packaging function, relies on the structural arrangement of the modified filler in the back coating layer and the synergistic cross-linking effect of the main glue, so that the back of the paper can form a stable low surface energy structure after hot pressing or hot stamping, and has good dimensional stability and interlayer anti-adhesion ability; the back coating layer realizes the directional migration of low surface energy chain segments during the hot pressing process to expose the modified filler on the surface, and at the same time, the blocked isocyanate in the main glue and the residual amino group of the modified filler jointly participate in the cross-linking reaction to form an embedded network structure inside the coating film, thereby realizing the migration of the film surface structure after hot pressing to construct a low surface energy film and covalent anchoring dual effects.

[0048] Through this structural design, the coated cardboard provided in this application is suitable for high-temperature processing scenarios such as hot stamping, embossing, and laminating. It is particularly suitable for process paths that require rapid stacking, transportation, glazing, or printing after processing is completed. It can effectively suppress the risk of overprint deviation, surface tearing, or finished product scrapping caused by problems such as thermal bridging of cardboard and back adhesion, and significantly improve the thermal stability and yield of paper in the post-processing process chain.

[0049] It should also be noted that the back-coated adhesive provided in this application needs to be heat-treated and the blocked isocyanate will only have a cross-linking effect after thermal decomposition. At the same time, the temperature in the water-based polyurethane coating system generally needs to be above 110°C so that the chain segments are more likely to migrate and form a low surface energy film. Therefore, the corresponding blocked isocyanate can be selected according to the temperature of the hot-pressing treatment of the coated cardboard. If the hot-pressing treatment stability is relatively low, the back-coated adhesive can be heat-treated above the thermal decomposition temperature of the blocked isocyanate. At this time, the structure formed by the back-coated adhesive has been cross-linked and fixed, and has a good high-temperature anti-adhesion effect. It is not easy to cause high-temperature adhesion when hot-pressing treatment is performed again. That is, the coated cardboard coated with the back-coated adhesive can be directly used for hot-pressing processing (such as hot stamping, hot lamination, etc.), and can be subjected to appropriate heat treatment according to the raw materials of the system before hot-pressing processing.

[0050] Compared with the prior art, the present invention has the following advantages: The provided back-coating adhesive constructs a modified filler with a directional grafted structure and is combined with a heat-triggered main adhesive cross-linking system. The coating film is synergistically driven by low-surface energy segments and polyether segments during the hot pressing process, which can achieve effective migration of the modified filler and stable structural anchoring. Therefore, it has good dispersibility and coating performance at room temperature, and has a long-lasting anti-adhesion effect after hot pressing. It is particularly suitable for the anti-adhesion treatment of coated cardboard in post-processing scenarios such as high-temperature stacking. DETAILED DESCRIPTION

[0051] The various embodiments or implementation schemes in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments.

[0052] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples" means that a specific feature, structure, material, or characteristic described in conjunction with an embodiment or example is included in at least one embodiment or example of the present application. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0053] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of such features. Throughout the description of this application, "plurality" means at least two, for example, two, three, etc., unless otherwise specifically defined.

[0054] In the description of this specification, unless otherwise specified, "parts" refer to "parts by mass".

[0055] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0056] Pluronic L35 is a commercially available polyether block polymer with a HO-PEG-b-PPG-b-PEG-OH configuration, a weight-average molecular weight of approximately 1900, and a PEG segment content of approximately 60%.

[0057] Pluronic L62 is a commercially available polyether block polymer with a HO-PEG-b-PPG-b-PEG-OH configuration, a weight-average molecular weight of approximately 2500, and a PEG segment content of approximately 20%.

[0058] Preparation Example 1-1 Preparation of double-terminated carboxyl polyether: 100 parts of Pluronic L35 were dissolved in 400 parts of dichloromethane and stirred to form a uniform solution; 2 parts of 4-dimethylaminopyridine as a catalyst and 4.4 parts of succinic anhydride were added to the solution, and the mixture was reacted at 25°C under a nitrogen atmosphere for 20 hours; after the reaction, the reaction mixture was washed three times with a saturated sodium bicarbonate solution and distilled water, and the organic phase was dried over anhydrous sodium sulfate and the solvent was removed by rotary evaporation to obtain a double-end carboxyl polyether A.

[0059] Preparation Example 1-2 Preparation of double-terminated carboxyl polyether: The preparation method is similar to that of Preparation Example 1-1, except that PEG2000 is used instead of Pluronic L35 to obtain double-end carboxyl polyether B.

[0060] Preparation Examples 1-3 Preparation of double-terminated carboxyl polyether: The preparation method is similar to that of Preparation Example 1-1, except that Pluronic L62 is used instead of Pluronic L35 to obtain double-end carboxyl polyether C.

[0061] Preparation Example 2-1 Preparation of modified filler: S1: 50 parts of talc (average particle size 2 μm) were ultrasonically dispersed in 800 parts of a mixture of ethanol and water in a volume ratio of 8:2, and then 2.5 parts of 3-aminopropyltriethoxysilane were added. The mixture was refluxed at 70°C for 3 hours. After the reaction, the mixture was centrifuged, washed three times with ethanol, and vacuum dried to obtain amino talc. S2: Disperse 30 parts of the above-mentioned amination talc powder and 10 parts of double-terminated carboxyl polyether A in 150 parts of water, adjust the pH to 5.5, add 1.5 parts of EDC and 0.8 parts of NHS, and react at 25°C for 6 hours. After the reaction, centrifuge and wash three times to obtain a carboxyl-terminated polyether grafted filler; S3: Subsequently, 10 parts of the above-mentioned carboxyl-terminated polyether grafted filler were dispersed in 150 parts of N,N-dimethylformamide, and 1.5 parts of amino-terminated polydimethylsiloxane (weight-average molecular weight of about 1000), 0.2 parts of EDC and 0.1 parts of NHS were added. The reaction was continued at 30°C for 6 hours. After the reaction was completed, it was washed with anhydrous ethanol three times, the solvent was removed by centrifugation, and vacuum dried to obtain modified filler A.

[0062] Preparation Example 2-2 Preparation of modified filler: The method is substantially the same as Preparation Example 2-1, except that in step S2, the amount of amination talc is 10 parts, that is, the ratio of amination talc to double-end carboxyl polyether A is 1:1, and modified filler B is obtained.

[0063] Preparation Example 2-3 Preparation of modified filler: The method is substantially the same as Preparation Example 2-1, with the only difference being that in step S2, the amount of amination talc is 20 parts, that is, the ratio of amination talc to double-end carboxyl polyether A is 2:1, to obtain modified filler C.

[0064] Preparation Example 2-4 Preparation of modified filler: The method is substantially the same as Preparation Example 2-1, with the only difference being that in step S2, the amount of amination talc is 40 parts, that is, the ratio of amination talc to double-end carboxyl polyether A is 4:1, to obtain modified filler D.

[0065] Preparation Example 2-5 Preparation of modified filler: The method is substantially the same as Preparation Example 2-1, with the only difference being that in step S2, the amount of amination talc is 50 parts, that is, the ratio of amination talc to double-end carboxyl polyether A is 5:1, to obtain modified filler E.

[0066] Preparation Example 2-6 Preparation of modified filler: The modified filler F is substantially the same as Preparation Example 2-1, except that double-terminated carboxyl polyether B is used instead of double-terminated carboxyl polyether A.

[0067] Preparation Example 2-7 Preparation of modified filler: The modified filler G is substantially the same as Preparation Example 2-1, except that double-terminated carboxyl polyether C is used instead of double-terminated carboxyl polyether A.

[0068] Preparation Example 2-8 Preparation of modified filler: The modified filler H was obtained by using a method similar to Preparation Example 2-1, except that talc powder with an average particle size of 0.5 μm was used instead of talc powder with an average particle size of 2 μm.

[0069] Preparation Example 2-9 Preparation of modified filler: The modified filler I was obtained by preparing the filler in a manner similar to that in Preparation Example 2-1, except that talc powder with an average particle size of 5 μm was used instead of talc powder with an average particle size of 2 μm.

[0070] Comparative Preparation Example 2-1 Preparation of modified filler: 50 parts of talc powder (average particle size of 2 μm) were ultrasonically dispersed in 800 parts of a mixture of ethanol and water in a volume ratio of 8:2. Then, 2.5 parts of 3-aminopropyltriethoxysilane were added and refluxed at 70°C for 3 hours. After the reaction, the mixture was centrifuged, washed three times with ethanol, and vacuum dried to obtain amino talc powder as modified filler J.

[0071] Comparative Preparation Example 2-2 Preparation of modified filler: 50 parts of talc (average particle size of 2 μm) were ultrasonically dispersed in 800 parts of a mixture of ethanol and water in a volume ratio of 8:2, and then 2.5 parts of 3-aminopropyltriethoxysilane were added. The mixture was refluxed at 70°C for 3 hours. After the reaction, the mixture was centrifuged, washed three times with ethanol, and dried in vacuo to obtain amino talc. 30 parts of the above-mentioned amination talc powder and 10 parts of double-terminated carboxyl polyether A were dispersed in 150 parts of water, the pH was adjusted to 5.5, 1.5 parts of EDC and 0.8 parts of NHS were added, and the mixture was reacted at 25°C for 6 hours. After the reaction, the mixture was centrifuged and washed three times to obtain a carboxyl-terminated polyether grafted filler as modified filler K.

[0072] Example 1

[0073] Preparation of back coating: Disperse 15 parts of modified filler A in an appropriate amount of water and pre-disperse it with a high-speed disperser for 10 minutes. Then add 1 part of BASF Disponil A 1080, 0.8 parts of Digo 410 and 0.3 parts of BYK024 and stir evenly. Then slowly add 200 parts of Wanhua Adwel 1630B waterborne polyurethane dispersion with a solid content of approximately 50% and stir and disperse it for 30 minutes. After uniform dispersion, add 3 parts of Covestro Imprafix 2794 and add an appropriate amount of water with low-speed stirring to control the solid content of the system to 28%. After stirring, let it stand for 1 hour to degas. This will give the back coating adhesive.

[0074] Example 2

[0075] Preparation of back coating: The process is substantially the same as Example 1, with the only difference being that modified filler B is used instead of modified filler A.

[0076] Example 3

[0077] Preparation of back coating: The process is substantially the same as Example 1, with the only difference being that modified filler C is used instead of modified filler A.

[0078] Example 4

[0079] Preparation of back coating: The process is substantially the same as Example 1, with the only difference being that modified filler D is used instead of modified filler A.

[0080] Example 5

[0081] Preparation of back coating: The process is substantially the same as Example 1, with the only difference being that modified filler E is used instead of modified filler A.

[0082] Example 6

[0083] Preparation of back coating: The process is substantially the same as Example 1, with the only difference being that modified filler F is used instead of modified filler A.

[0084] Example 7

[0085] Preparation of back coating: The process is substantially the same as Example 1, with the only difference being that modified filler G is used instead of modified filler A.

[0086] Example 8

[0087] Preparation of back coating: The process is substantially the same as Example 1, with the only difference being that modified filler H is used instead of modified filler A.

[0088] Example 9

[0089] Preparation of back coating: The process is substantially the same as Example 1, except that modified filler I is used instead of modified filler A.

[0090] Comparative Example 1 The process is substantially the same as Example 1, with the only difference being that modified filler J is used instead of modified filler A.

[0091] Comparative Example 2 The process is substantially the same as Example 1, with the only difference being that modified filler K is used instead of modified filler A.

[0092] Comparative Example 3 Disperse 10 parts of talc powder (average particle size of 2 μm) in an appropriate amount of water and pre-disperse it using a high-speed disperser for 10 minutes. Then add 3 parts of Pluronic L35, 2 parts of polydimethylsiloxane (weight-average molecular weight of approximately 1000), 1 part of BASF Disponil A1080, 0.8 parts of Digo 410, and 0.3 parts of BYK024, and stir evenly. Then slowly add 200 parts of Wanhua Adwel 1630B waterborne polyurethane dispersion with a solid content of approximately 50% and stir and disperse it for 30 minutes. After uniform dispersion, add 3 parts of Covestro Imprafix 2794 and add an appropriate amount of water while stirring at low speed to control the solid content of the system to 28%. After stirring, let it stand for 1 hour to degas. This will give the back coating adhesive.

[0093] Comparative Example 4 Disperse 10 parts of talc (average particle size of 2 μm) in an appropriate amount of water and pre-disperse for 10 minutes using a high-speed disperser. Then add 3 parts of Pluronic L35, 2 parts of amino-terminated polydimethylsiloxane (weight-average molecular weight of approximately 1000), 1 part of BASF Disponil A 1080, 0.8 parts of Digo 410, and 0.3 parts of BYK024, and stir evenly. Then slowly add 200 parts of Wanhua Adwel 1630B waterborne polyurethane dispersion with a solid content of approximately 50% and stir and disperse for 30 minutes. After uniform dispersion, add 3 parts of Covestro Imprafix 2794 and add an appropriate amount of water while stirring at low speed to control the solid content of the system to 28%. After stirring, let it stand for 1 hour to degas. This will give the back coating adhesive.

[0094] Test section Connectivity test: Cardboard substrates from the same batch were coated, and the backing adhesive prepared in each example and comparative example (dry film thickness of approximately 8 μm) was applied to one side of the cardboard. The adhesive was dried at 70°C for 10 minutes. The cardboard was cut into 100 mm × 100 mm square pieces, with 500 sheets of cardboard per test sample. The cardboard samples coated with back glue were hot-pressed at 150°C and 0.2 MPa for 20 seconds. After hot-pressing, they were directly stacked and cooled to room temperature. The adhesion rate was calculated by manual separation method. Adhesion rate = number of adhesion-prone sheets / total number of sheets × 100%. The results are shown in Table 1.

[0095] Table 1

[0096] According to Table 1, the back-coating adhesives prepared in each embodiment showed a low adhesion rate after high-temperature hot pressing and stacking, which was significantly better than the comparative examples. This shows that the back-coating adhesive provided by the present application has good anti-adhesion performance under high-temperature hot pressing conditions, can effectively inhibit the occurrence of bridging and tearing of cardboard during the stacking process, and reflects the ability to construct low-surface energy structures and the thermal stability of the film layer. This may be due to the modified filler having a directional polyether-polysiloxane segment structure, combined with the blocked isocyanate cross-linker to release reactive activity during the hot pressing process, realizing a synergistic mechanism of migration exposure of the modified filler on the film surface and covalent structure locking. In Comparative Example 1, the modified filler is not grafted with polyether segments and polysiloxane segments, lacks migration segments and low surface energy exposed structures, and the molecular arrangement of the film surface is disordered after hot pressing, and it is impossible to construct an effective anti-sticking area, resulting in a adhesion rate as high as 30.6%; in Comparative Example 2, although the modified filler contains polyether segments, it lacks the low surface energy segments at the end of polysiloxane and cannot form a hydrophobic migration structure during the hot pressing process. The anti-sticking ability of the coating surface is insufficient, and the adhesion rate is still as high as 32.0%, indicating that the low surface energy segments at the end are one of the keys to constructing the hot pressing anti-sticking structure; in Comparative Example 3, the filler is grafted with polyether segments and lacks the low surface energy segments at the end of polysiloxane. , polyether compounds and silicone oil are directly added to the back coating adhesive, among which the silicone oil has poor compatibility with the system, is easily distributed on the surface of the film layer during the drying process, and is easily lost during storage and hot pressing. At the same time, during the hot pressing process, the driving effect of each chain segment is lacking, the filler is not easy to migrate, and a complete anti-adhesion film layer cannot be formed, resulting in reduced anti-adhesion performance; in Comparative Example 4, amino silicone oil can improve the compatibility with the system to a certain extent, but it is also easy to lose during storage and hot pressing, and it is not easy to form a complete film anti-adhesion layer, resulting in its anti-adhesion effect is still poor.

[0097] Examples 1-5 show that, assuming all other conditions remain constant, as the amount of amino filler increases from 10 to 50 parts (corresponding to a grafting ratio from 1:1 to 1:5), blocking initially decreases and then increases. A grafting ratio of 1:3 achieves the best results, with a blocking rate of only 1.4%. This suggests that excessive polyether usage can lead to excessive graft density on the filler surface, impacting segment migration during hot pressing. Conversely, excessive amino filler usage can result in insufficient grafting, a decrease in the density of low-surface-energy segments on the film surface, discontinuous coverage of the anti-sticking zone, and a rebound in blocking. Therefore, when preparing the modified filler, maintaining a mass ratio of double-end carboxyl polyether to amino filler between 1:2 and 4 can further improve the high-temperature anti-blocking properties of coated paperboard.

[0098] According to Examples 1, 6, and 7, the polyether segment structure has a significant impact on the migration effect of the modified filler. Example 6 uses a pure PEG segment, which lacks thermoresponsiveness and has poor segment conformational change ability, which will affect the migration efficiency of the modified filler during hot pressing, and the adhesion rate increases to 4.8%; Example 7 uses a polyether segment with a PEG ratio of 20%. Although it has certain thermoresponsiveness, the system has poor dispersibility due to enhanced hydrophobicity, which may lead to a decrease in the grafting efficiency of the polysiloxane segment, and the adhesion rate increases to 6.2%. In contrast, the block polyether segment with a PEG ratio of 60% in Example 1 has good flexibility and moderate thermoresponsiveness. The segment conformation changes fully during hot pressing, the migration efficiency is high, and the anti-sticking effect is the best, indicating that a moderate hydrophilic-thermosensitive structure can achieve controlled migration of the modified filler, thereby further improving the high-temperature anti-sticking performance of the coated cardboard.

[0099] Examples 1, 8, and 9 show that modified fillers with different particle sizes significantly impact the anti-adhesion properties of the back-coating adhesive. In Example 8, the too-small particle size hinders the formation of continuous anti-adhesion points with the polysiloxane segments on its surface, leading to a poor anti-adhesion effect. In Example 9, the too-large particle size hinders migration to the film surface to form anti-adhesion properties. In contrast, in Example 1, the modified filler with the appropriate particle size, driven by the synergistic effects of the polyether and polysiloxane segments, can migrate smoothly to the film surface during hot pressing and achieve directional anchoring, resulting in superior anti-adhesion properties.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A back-coating adhesive for preventing high-temperature adhesion of coated paperboard, characterized in that: Including the following raw materials by weight: 200 parts of waterborne polyurethane dispersion, 10-20 parts of modified filler, 1-5 parts of blocked isocyanate crosslinker, 0.5-2 parts of dispersant, 0.2-1 parts of leveling agent, 0.1-0.5 parts of defoamer; The surface of the modified filler is grafted with amino groups and polyether-b-polysiloxane segments, and the polyether segments are grafted on a side close to the surface of the filler.

2. The back coating adhesive according to claim 1, characterized in that: The modified filler is prepared by the following steps: S1: reacting an inorganic filler with an aminosilane coupling agent to graft active amino groups onto the surface of the inorganic filler to obtain an amino filler; S2: reacting the amino filler with the double-end carboxyl polyether, so that the carboxyl group at one end of the double-end carboxyl polyether undergoes an amidation reaction with part of the amino groups on the surface of the amino filler to obtain a carboxyl-terminated polyether grafted filler; S3: reacting the carboxyl-terminated polyether grafted filler with amino silicone oil to cause an amidation reaction between the carboxyl group of the carboxyl-terminated polyether grafted filler and the amino group on the amino silicone oil to obtain a modified filler.

3. The back coating adhesive according to claim 2, characterized in that: The step S1 comprises: 50 parts of inorganic filler and 1-5 parts of aminosilane coupling agent are dispersed in 500-1000 parts of ethanol aqueous solution, and reacted at 60-80° C. for 2-4 hours to obtain an amino filler.

4. The back coating adhesive according to claim 3, characterized in that: The inorganic filler includes at least one of talc, kaolin, and silicon dioxide, and the average particle size of the inorganic filler is 1-3 μm.

5. The back coating adhesive according to claim 2, characterized in that: In step S2, the double-ended carboxyl polyether is obtained by reacting a double-ended hydroxyl polyether with succinic anhydride, wherein the double-ended hydroxyl polyether includes a polyethylene glycol segment and a polypropylene glycol segment, the mass percentage of the polyethylene glycol segment in the double-ended hydroxyl polyether is 50% to 80%, and the weight-average molecular weight of the double-ended hydroxyl polyether is 1000 to 4000.

6. The back coating adhesive according to claim 5, characterized in that: The step S2 comprises: 10 parts of double-terminated carboxyl polyether, 20-40 parts of amino filler, 1-2 parts of 1-ethyl-(3-dimethylaminopropyl)carbodiimide, and 0.5-1 part of N-hydroxysuccinimide are dispersed in 100-200 parts of water, and the mixture is reacted at a pH of 5-6 and 20-40°C for 6-8 hours to obtain a carboxyl-terminated polyether grafted filler.

7. The back coating adhesive according to claim 6, characterized in that: The step S3 comprises: 10 parts of carboxyl-terminated polyether grafted filler, 1-2 parts of amino silicone oil, 0.1-0.3 parts of 1-ethyl-(3-dimethylaminopropyl)carbodiimide, and 0.05-0.15 parts of N-hydroxysuccinimide are dispersed in 100-200 parts of N,N-dimethylformamide, and reacted at 20-40°C for 6-8 hours to obtain a modified filler.

8. The back coating adhesive according to any one of claims 1 to 7, characterized in that: The back coating glue satisfies at least one of the following conditions: 1) The aqueous polyurethane dispersion includes Wanhua Adwel 1630B; 2) The blocked isocyanate crosslinker includes Covestro Imprafix 2794; 3) The dispersant includes BASF Disponil A 1080; 4) The leveling agent includes Digo 410; 5) The defoaming agent includes BYK024; 6) The back coating adhesive further includes water, and the solid content of the back coating adhesive is 20 wt% to 35 wt%.

9. A method for preparing a back-coating adhesive for preventing high-temperature adhesion of coated paperboard, characterized in that: include: Providing a raw material for the back coating adhesive according to any one of claims 1 to 8; The raw materials are mixed and water is added to make the solid content of the mixture be 20 wt % to 35 wt %, and the mixture is mixed evenly to obtain a back coating adhesive.

10. A coated cardboard, characterized in that: include: A base paper, and a back coating layer provided on one side of the base paper, wherein the back coating layer is obtained by drying the back coating adhesive according to any one of claims 1 to 8 or the back coating adhesive prepared by the method according to claim 9.

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