Solventless adhesive, paper-plastic composite label and preparation method thereof

Abstract

The application discloses a kind of solventless adhesive, paper-plastic composite label and preparation method thereof, it is related to adhesive technical field, wherein, the raw material of a kind of solventless adhesive includes solventless polyurethane resin, ethylene-acrylic acid copolymer and core-shell microcapsule;The core-shell microcapsule includes gas-generating core layer and protective shell layer sequentially arranged from inside to outside, and the raw material of the gas-generating core layer is inorganic salt that produces gas when encountering alkali, and the raw material of the protective shell layer can form microporous channel when encountering alkali;A kind of solventless adhesive, paper-plastic composite label and preparation method thereof are proposed in the application, to realize the automatic separation and classification recycling of paper-based composite and plastic film under the premise that the label sticking firmness and whole sheet label removal performance of paper-plastic composite label are not influenced.

Description

Technical Field

[0001] This invention relates to the field of adhesive technology, and in particular to a solvent-free adhesive, a paper-plastic composite label, and a method for preparing the same. Background Technology

[0002] With the rapid development of the fast-moving consumer goods industry, glass bottles are widely used in the packaging of products such as beer, beverages, and condiments. To improve display effect and durability, some glass bottles use adhesive labels to adhere paper-plastic composite labels (including a plastic film, an adhesive layer, and a paper-based composite layer stacked from the inside out; the paper-based composite layer includes a varnish layer, a gravure ink layer, and a paper base layer stacked from the inside out) to the bottle body.

[0003] In the glass bottle recycling process, existing technologies typically involve cleaning glass bottles with paper-plastic composite labels in an alkaline sodium hydroxide solution at a temperature of 70–80°C and a concentration of 2–2.5 wt%. This causes the adhesive backing of the label to fail, thereby enabling the paper-plastic composite label to completely detach from the glass bottle surface within 3 minutes (i.e., complete label removal) to meet the recycling requirements of glass bottles.

[0004] However, due to the inherent properties of solvent-free polyurethane adhesives, the paper-based composite label detached from the glass bottle surface remains firmly bonded to the plastic film by the adhesive, forming the paper-based composite of the varnish layer, gravure ink layer, and paper base layer. This prevents automatic separation in alkaline solutions of the same temperature (70–80°C) and concentration (2–2.5 wt%). Because the paper-based composite and the plastic film are made of different materials, they cannot enter the same recycling system, resulting in a large amount of detached paper-based composite labels being incinerated or landfilled as mixed waste, leading to resource waste.

[0005] Therefore, the industry urgently needs a new type of solvent-free adhesive that can achieve automatic separation and recycling of paper-based composites and plastic films without affecting the labeling adhesion and overall label removal performance of paper-plastic composite labels. Summary of the Invention

[0006] The main objective of this invention is to propose a solvent-free adhesive, a paper-plastic composite label, and a method for preparing the same, aiming to achieve automatic separation and classified recycling of the paper-based composite and the plastic film without affecting the labeling adhesion and overall label removal performance of the paper-plastic composite label.

[0007] To achieve the above objectives, the present invention proposes a solvent-free adhesive, wherein the raw materials of the solvent-free adhesive include solvent-free polyurethane resin, ethylene-acrylic acid copolymer and core-shell microcapsules; The core-shell microcapsule comprises a gas-generating core layer and a protective shell layer arranged sequentially from the inside to the outside. The raw material of the gas-generating core layer is an inorganic salt that generates gas when it comes into contact with alkali, and the raw material of the protective shell layer can form microporous channels when it comes into contact with alkali.

[0008] In one embodiment, the inorganic salt includes at least one selected from ammonium bicarbonate, ammonium carbonate, and ammonium chloride; And / or, the raw materials of the protective shell include polymethyl methacrylate and a pore-forming agent; The weight-average molecular weight of the polymethyl methacrylate is 120,000 to 350,000. The pore-forming agent includes at least one of polyethylene glycol-4000, polyethylene glycol-6000, and polyethylene glycol-8000; The protective shell material comprises 95-97% polymethyl methacrylate and 3-5% pore-forming agent by mass percentage.

[0009] In one embodiment, the core-shell microcapsules have a particle size of 2–5 μm; And / or, the thickness ratio of the gas-generating core layer to the protective shell layer is 1:(0.5~2).

[0010] In one embodiment, the acrylic acid content in the ethylene-acrylic acid copolymer is 5-20% by mass percentage.

[0011] In one embodiment, the raw materials for preparing the solvent-free adhesive further include sulfonic acid copolymers; The sulfonic acid copolymer is P(AAm-co-AMPS); the content of AMPS in P(AAm-co-AMPS) is 5-10% by mass percentage.

[0012] In one embodiment, the solvent-free polyurethane resin is an isocyanate-terminated polyurethane prepolymer; The viscosity of the isocyanate-terminated polyurethane prepolymer is 1000–8000 mPa·s; The isocyanate content of the isocyanate-terminated polyurethane prepolymer is 5-15% by mass percentage.

[0013] In one embodiment, the raw materials of the solvent-free adhesive further include at least one of solvent-free defoamer, surfactant, polyethylene glycol regulator and plasticizer / lubricant; The solvent-free defoamer includes at least one of polyether-modified siloxane defoamers, organosilicon defoamers, and non-silicone polymer defoamers; The surfactant includes at least one of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate; The polyethylene glycol modifier includes at least one of polyethylene glycol-4000, polyethylene glycol-6000, and polyethylene glycol-8000; The plasticizing lubricant includes at least one of glycerol, diethylene glycol, propylene glycol, and sorbitol.

[0014] In one embodiment, the solvent-free polyurethane resin in the raw materials of the solvent-free adhesive is 90 to 100 parts by weight. And / or, calculated by mass parts, the ethylene-acrylic acid copolymer in the raw materials of the solvent-free adhesive is 50 to 90 parts; And / or, calculated by mass parts, the core-shell microcapsules in the raw materials of the solvent-free adhesive are 1 to 5 parts; And / or, by weight, the raw materials of the solvent-free adhesive further include 1 to 3 parts of sulfonic acid copolymer; And / or, calculated by weight, the raw materials of the solvent-free adhesive further include 0.05 to 0.3 parts of solvent-free defoamer; And / or, calculated by mass parts, the raw materials of the solvent-free adhesive further include 0.05 to 0.3 parts of surfactant; And / or, calculated by mass parts, the raw materials of the solvent-free adhesive further include 0.1 to 0.8 parts of polyethylene glycol modifier; And / or, by weight, the raw materials of the solvent-free adhesive further include 0.4 to 1.5 parts of plasticizing lubricant.

[0015] The present invention also proposes a paper-plastic composite label, comprising a plastic film, an adhesive layer and a paper base composite layer stacked sequentially from the inside out; the paper base composite layer comprises a varnish layer, a gravure ink layer and a paper base layer stacked sequentially from the inside out, and the adhesive layer is made using a solvent-free adhesive as described above; And / or, the density of the plastic film is 0.91–0.95 g / cm³. 3 ; And / or, the density of the paper base layer is 1.05–1.1 g / cm³. 3 .

[0016] This invention also proposes a method for preparing a paper-plastic composite label, comprising the following steps: A gravure ink layer and a varnish layer are sequentially applied to the surface of a paper base layer to obtain a paper base composite; The raw materials for solvent-free adhesives are mixed evenly to obtain solvent-free adhesives; A solvent-free adhesive is applied to the surface of a paper-based composite and cured to obtain an adhesive layer. A plastic film is laminated onto the surface of the adhesive layer to obtain a paper-plastic composite label; And / or, the density of the plastic film is 0.91–0.95 g / cm³. 3 ; And / or, the density of the paper base layer is 1.05–1.1 g / cm³. 3 .

[0017] In the technical solution of this invention, when the adhesive is applied to a paper-plastic composite label (including a plastic film, an adhesive layer, and a paper-based composite layer stacked sequentially from the inside out; the paper-based composite layer includes a varnish layer, a gravure ink layer, and a paper base layer stacked sequentially from the inside out), in the early stage of bottle washing (0-3 minutes), due to the dense nature of the paper base layer's heavy fibers and the multiple barriers of the varnish layer and gravure ink layer, the alkaline solution mainly acts on the label's back adhesive. The alkaline solution has not yet fully penetrated into the adhesive layer formed by the solvent-free adhesive. Furthermore, because the saponification reaction and gas generation process require a certain amount of time, the adhesive layer maintains its structural integrity during this stage and does not collapse. This ensures that the paper-plastic composite label maintains its intact paper-plastic composite state in the early stage of bottle washing. The paper-plastic composite label can be detached entirely. After the label is removed from the washing bottle (i.e., after label removal), as the washing time increases (after 3 minutes), the alkaline solution penetrates through the paper-based composite to the interface of the adhesive layer. The ethylene-acrylic acid copolymer begins to undergo a saponification reaction, and the adhesive layer gradually disintegrates. At the same time, the gas generated by the rupture of the core-shell microcapsules creates internal pressure inside the adhesive layer, further accelerating the disintegration of the adhesive layer. The two work synergistically, so that after an additional 5-8 minutes of immersion in an alkaline solution of 2-2.5 wt% sodium hydroxide at a temperature of 70-80°C, the paper-based composite and the plastic film can be automatically separated (i.e., paper-plastic separation). The paper-based composite and the plastic film are then sorted and recycled based on their density difference. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0019] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly.

[0020] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the use of "and / or," or "and / or" throughout the text implies three parallel solutions. For example, "A and / or, B" includes solution A, solution B, or a solution where both A and B are satisfied. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0021] In the glass bottle recycling process, existing technologies typically involve cleaning glass bottles with paper-plastic composite labels in an alkaline sodium hydroxide solution at a temperature of 70–80°C and a concentration of 2–2.5 wt%. This causes the adhesive backing of the label to fail, thereby enabling the paper-plastic composite label to completely detach from the glass bottle surface within 3 minutes (i.e., complete label removal) to meet the recycling requirements of glass bottles.

[0022] However, due to the inherent properties of solvent-free polyurethane adhesives, the paper-based composite label detached from the glass bottle surface remains firmly bonded to the plastic film by the adhesive, forming the paper-based composite of the varnish layer, gravure ink layer, and paper base layer. This prevents automatic separation in alkaline solutions of the same temperature (70–80°C) and concentration (2–2.5 wt%). Because the paper-based composite and the plastic film are made of different materials, they cannot enter the same recycling system, resulting in a large amount of detached paper-based composite labels being incinerated or landfilled as mixed waste, leading to resource waste.

[0023] This invention proposes a solvent-free adhesive, wherein the raw materials of the solvent-free adhesive include solvent-free polyurethane resin, ethylene-acrylic acid copolymer and core-shell microcapsules; The core-shell microcapsule comprises a gas-generating core layer and a protective shell layer arranged sequentially from the inside to the outside. The raw material of the gas-generating core layer is an inorganic salt that generates gas when it comes into contact with alkali, and the raw material of the protective shell layer can form microporous channels when it comes into contact with alkali.

[0024] The solvent-free adhesive of this invention contains solvent-free polyurethane resin, ethylene-acrylic acid copolymer, and core-shell microcapsules. Specifically, in a 2-2.5 wt% alkaline sodium hydroxide solution at 70-80°C, the ethylene-acrylic acid copolymer undergoes a saponification reaction, while the protective shell of the core-shell microcapsules forms microporous channels, allowing the gas-generating core layer to produce gas upon contact with the alkali. The above reaction is not instantaneous but requires a certain reaction time and alkaline solution penetration time.

[0025] When adhesives are applied to paper-plastic composite labels (comprising a plastic film, adhesive layer, and paper base composite layered sequentially from the inside out; the paper base composite layer comprising a varnish layer, gravure ink layer, and paper base layer layered sequentially from the inside out), in the early stage of bottle washing (0–3 min), due to the dense structure of the paper base fiber and the multiple barriers of the varnish layer and gravure ink layer, the alkaline solution mainly acts on the label adhesive. The alkaline solution has not yet fully penetrated into the adhesive layer formed by the solvent-free adhesive. Furthermore, because the saponification reaction and gas generation process require a certain amount of time, the adhesive layer maintains its structural integrity during this stage and does not disintegrate. This ensures that the paper-plastic composite label maintains its intact paper-plastic composite state in the early stage of bottle washing, thus ensuring the paper-plastic composite... The label can be detached entirely. After the paper-plastic composite label is removed from the washing bottle (i.e., after label removal), as the washing time increases (after 3 minutes), the alkaline solution penetrates through the paper-based composite to the interface of the adhesive layer. The ethylene-acrylic acid copolymer begins to undergo a saponification reaction, and the adhesive layer gradually disintegrates. At the same time, the gas generated by the rupture of the core-shell microcapsules creates internal pressure inside the adhesive layer, further accelerating the disintegration of the adhesive layer. The two work together to achieve automatic separation of the paper-based composite from the plastic film (i.e., paper-plastic separation) after an additional 5-8 minutes of immersion in an alkaline solution of 2-2.5 wt% sodium hydroxide at a temperature of 70-80°C. The paper-based composite and the plastic film are then sorted and recycled based on their density difference.

[0026] Furthermore, solvent-free polyurethane resin provides the main bonding strength at room temperature, while ethylene-acrylic acid copolymer maintains stable bonding performance at room temperature, meeting the labeling adhesion requirements of paper-plastic composite labels.

[0027] In embodiments of the present invention, the inorganic salt includes at least one of ammonium bicarbonate, ammonium carbonate, and ammonium chloride; And / or, the raw materials of the protective shell include polymethyl methacrylate and a pore-forming agent; The weight-average molecular weight of the polymethyl methacrylate is 120,000 to 350,000. The pore-forming agent includes at least one of polyethylene glycol-4000, polyethylene glycol-6000, and polyethylene glycol-8000; The protective shell material comprises 95-97% polymethyl methacrylate and 3-5% pore-forming agent by mass percentage.

[0028] This type of inorganic salt triggers the production of ammonia gas upon contact with alkali; at the same time, this type of inorganic salt is widely available, inexpensive, and has good thermal stability (able to withstand the curing temperature of adhesive composites). When combined with a protective shell layer that can form microporous channels upon contact with alkali, it can achieve a response mechanism that triggers alkali and continuously produces gas, effectively assisting in the disintegration and separation of the adhesive layer.

[0029] It's important to note that alkali-triggered operation does not mean instantaneous triggering or destruction. The alkaline solution's penetration into the protective shell and the formation of microporous channels upon contact with the alkali require time. Gas production necessitates sufficient contact area and time with the alkaline solution. Within the 0-3 minute window, limited by the small contact area, short contact time, and restricted alkaline solution penetration path, the gas production of the microcapsules is controlled at an extremely low level, ensuring no negative impact on the label's normal label removal performance.

[0030] High molecular weight polymethyl methacrylate (PMMA) (weight average molecular weight of 120,000–350,000) imparts excellent mechanical strength and room temperature stability to the protective shell, ensuring structural integrity and non-swelling under normal environmental conditions (neutral, room temperature), thus preventing premature gas generation or leakage during storage, transportation, and use. Simultaneously, 3–5 wt% polyethylene glycol is added to the raw material of the protective shell as a pore-forming agent. In a 2–2.5 wt% alkaline sodium hydroxide solution at 70–80°C, the polyethylene glycol dissolves to form microporous channels, accelerating the penetration of the alkaline solution into the gas-generating core layer, triggering the slow release of gas from the core layer, ensuring that the gas-generating reaction occurs as needed, thereby assisting in the disintegration and separation of the adhesive layer.

[0031] In an embodiment of the present invention, the particle size of the core-shell microcapsules is 2–5 μm; And / or, the thickness ratio of the gas-generating core layer to the protective shell layer is 1:(0.5~2).

[0032] Solvent-free lamination involves extremely small coating gaps, typically only a few micrometers to tens of micrometers. This technical solution, by limiting the particle size of the core-shell microcapsules to 2–5 μm, ensures that the microcapsules are uniformly dispersed in the solvent-free adhesive without clogging the coating equipment. This prevents the core-shell microcapsules from clogging the coating head, doctor blade, or anilox roller, causing coating lines or streaks, thus ensuring product yield. Simultaneously, this particle size range matches the adhesive layer thickness (typically 1–5 μm), facilitating uniform distribution of the core-shell microcapsules within the adhesive layer and efficient gas generation after penetration into the alkaline solution. The particle size of the core-shell microcapsules includes, but is not limited to, 2 μm, 3 μm, 4 μm, 5 μm, or any two of these values ​​as endpoints.

[0033] Limiting the thickness ratio of the gas-generating core layer to the protective shell layer to 1:(0.5-2) not only ensures sufficient mechanical strength and sealing of the protective shell layer to prevent premature leakage or gas generation during storage and use, but also avoids an excessively thick protective shell layer, which would lead to slow penetration of alkaline solutions and delayed response. This achieves a balanced effect of stable storage at room temperature and alkali-triggered activation. Furthermore, limiting the thickness ratio of the gas-generating core layer to the protective shell layer allows for adjustment as needed. For example, if faster paper-plastic separation is required, the thickness ratio can be increased; conversely, it can be decreased, improving flexibility. The thickness ratio of the gas-generating core layer to the protective shell layer includes, but is not limited to, 1:0.5, 1:0.8, 1:1, 1:1.2, 1:1.5, 1:0.8, 1:2, or any two of the above values ​​as endpoints.

[0034] In an embodiment of the present invention, the acrylic acid content in the ethylene-acrylic acid copolymer is 5-20% by mass percentage.

[0035] If the acrylic acid content in the ethylene-acrylic acid copolymer is too low, there will be insufficient carboxyl groups, resulting in substandard composite strength and slow saponification reaction. This leads to excessively long separation time between the paper-based composite and the plastic film, affecting production efficiency. Conversely, if the acrylic acid content in the ethylene-acrylic acid copolymer is too high, although it improves bonding strength and label adhesion, the saponification reaction becomes overly sensitive, easily causing premature swelling in the early stages of bottle washing (0-3 minutes) due to contact with a small amount of alkaline solution at the edges, affecting the complete label removal. Therefore, this technical solution limits the acrylic acid content in the ethylene-acrylic acid copolymer to 5-20%, ensuring strong adhesion of the paper-plastic composite label during normal use, and enabling rapid and controllable effective separation between the paper-based composite and the plastic film after label removal. The acrylic acid content in the ethylene-acrylic acid copolymer includes, but is not limited to, 5%, 8%, 10%, 13%, 15%, 17%, 20%, or any two of the above values ​​as endpoints.

[0036] In embodiments of the present invention, the raw materials for preparing the solvent-free adhesive further include sulfonic acid copolymers; The sulfonic acid copolymer is P(AAm-co-AMPS); the content of AMPS in P(AAm-co-AMPS) is 5-10% by mass percentage.

[0037] Sulfonic acid copolymers such as P(AAm-co-AMPS) added to the formulation rapidly ionize their sulfonic acid groups in alkaline solutions, generating osmotic pressure. During the alkaline washing process after label removal, this osmotic pressure accelerates the swelling and rupture of the solvent-free adhesive, thereby shortening the overall alkaline washing separation time.

[0038] If the AMPS content in P(AAm-co-AMPS) is too low, the content of sulfonic acid groups will be insufficient, resulting in low ionization and weak osmotic pressure in alkaline solutions, and the swelling and cracking effect will be insignificant. If the AMPS content in P(AAm-co-AMPS) is too high, the copolymer will absorb water too strongly, which will not only make it easy to absorb water at room temperature, affecting the bonding strength, but also make it easy to swell prematurely or cause a decrease in the cohesive strength of the adhesive layer, affecting the process window and stability.

[0039] In an embodiment of the present invention, the solvent-free polyurethane resin is an isocyanate-terminated polyurethane prepolymer; The viscosity of the isocyanate-terminated polyurethane prepolymer is 1000–8000 mPa·s; The isocyanate content of the isocyanate-terminated polyurethane prepolymer is 5-15% by mass percentage.

[0040] Solvent-free polyurethane resin is an isocyanate-terminated polyurethane prepolymer, which allows the isocyanate group (-NCO) at the end to react with moisture in the air or active hydrogen such as hydroxyl and amino groups on the substrate surface, achieving room temperature or medium temperature curing without the need for external curing agents. Meanwhile, limiting the viscosity of the isocyanate-terminated polyurethane prepolymer to 1000–8000 mPa·s ensures the uniformity of solvent-free adhesive coating, preventing uneven coating due to excessively high viscosity or sagging due to excessively low viscosity, thus ensuring product quality. The viscosity of the isocyanate-terminated polyurethane prepolymer includes, but is not limited to, 1000 mPa·s, 2000 mPa·s, 3000 mPa·s, 4000 mPa·s, 5000 mPa·s, 6000 mPa·s, 7000 mPa·s, 8000 mPa·s, or any two of the above values ​​as endpoints.

[0041] Furthermore, the isocyanate content of 5-15% provides a moderate crosslinking density and a controllable curing speed. After curing, it forms a strong and tough polyurethane network, providing high peel strength and anti-delamination ability. This improves adhesion while allowing for curing within 18-24 hours, ensuring a smooth operating window and high production efficiency. The isocyanate content of the isocyanate-terminated polyurethane prepolymer includes, but is not limited to, 5%, 8%, 10%, 13%, 15%, or any two of these values ​​as endpoints. It should be noted that the isocyanate-terminated polyurethane prepolymer is model Wanhua SF6100.

[0042] In embodiments of the present invention, the raw materials of the solvent-free adhesive further include at least one of solvent-free defoamer, surfactant, polyethylene glycol regulator and plasticizer / lubricant; The solvent-free defoamer includes at least one of polyether-modified siloxane defoamers, organosilicon defoamers, and non-silicone polymer defoamers; The surfactant includes at least one of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate; The polyethylene glycol modifier includes at least one of polyethylene glycol-4000, polyethylene glycol-6000, and polyethylene glycol-8000; The plasticizing lubricant includes at least one of glycerol, diethylene glycol, propylene glycol, and sorbitol.

[0043] Solvent-free defoamers eliminate air bubbles generated during formulation and coating, preventing defects such as pinholes and craters in the adhesive layer and ensuring coating uniformity. Surfactants reduce the surface tension of the system, improve wettability to the substrate, promote uniform dispersion of components, and enhance adhesive adhesion. Polyethylene glycol regulators not only help adjust the viscosity and flowability of the system and improve coating suitability, but also enhance hydrophilicity and assist in the rapid establishment of osmotic pressure. Plasticizers and lubricants help reduce the coefficient of friction and improve the slippage and anti-blocking properties of the film surface. It should be noted that the model of polyether-modified siloxane defoamer can be Airex900; the model of organosilicon defoamer can be Dow Corning DC163; and the model of non-silicone polymer defoamer can be Clariant Foamaster NS1.

[0044] In an embodiment of the present invention, the solvent-free polyurethane resin in the raw materials of the solvent-free adhesive is 90 to 100 parts by weight. And / or, calculated by mass parts, the ethylene-acrylic acid copolymer in the raw materials of the solvent-free adhesive is 50 to 90 parts; And / or, calculated by mass parts, the core-shell microcapsules in the raw materials of the solvent-free adhesive are 1 to 5 parts; And / or, by weight, the raw materials of the solvent-free adhesive further include 1 to 3 parts of sulfonic acid copolymer; And / or, calculated by weight, the raw materials of the solvent-free adhesive further include 0.05 to 0.3 parts of solvent-free defoamer; And / or, calculated by mass parts, the raw materials of the solvent-free adhesive further include 0.05 to 0.3 parts of surfactant; And / or, calculated by mass parts, the raw materials of the solvent-free adhesive further include 0.1 to 0.8 parts of polyethylene glycol modifier; And / or, by weight, the raw materials of the solvent-free adhesive further include 0.4 to 1.5 parts of plasticizing lubricant.

[0045] This technical solution limits the amount of each raw material added to the solvent-free adhesive, ensuring that the amount of each raw material is within an optimal range. This helps to balance the adhesive strength of the product and the automatic separation effect between the paper-based composite and the plastic film after label removal. Simultaneously, limiting the amount of each raw material added to the solvent-free adhesive also facilitates its compatibility with high-speed solvent-free lamination production lines of 200–250 m / min, improving production efficiency and meeting the green and efficient requirements of modern label production.

[0046] The mass fractions of solvent-free polyurethane resin include, but are not limited to, 90 parts, 95 parts, 100 parts, or any two of the above points as endpoints.

[0047] The mass fractions of the ethylene-acrylic acid copolymer include, but are not limited to, 50 parts, 55 parts, 60 parts, 65 parts, 70 parts, 75 parts, 80 parts, 85 parts, 90 parts, or any two of the above points as endpoints.

[0048] The mass fractions of the core-shell microcapsules include, but are not limited to, 1 part, 2 parts, 3 parts, 4 parts, 5 parts, or any two of the above points as endpoints.

[0049] The mass fractions of the sulfonic acid copolymer include, but are not limited to, 1 part, 1.5 parts, 2 parts, 2.5 parts, 3 parts, or any two of the above points as endpoints.

[0050] The mass fractions of the solvent-free defoamer include, but are not limited to, 0.05 parts, 0.1 parts, 0.2 parts, 0.3 parts, or any two of the above points as endpoints.

[0051] The mass fractions of the surfactant include, but are not limited to, 0.05 parts, 0.1 parts, 0.2 parts, 0.3 parts, or any two of the above points as endpoints.

[0052] The mass fractions of polyethylene glycol modifiers include, but are not limited to, 0.1 parts, 0.2 parts, 0.3 parts, 0.4 parts, 0.5 parts, 0.6 parts, 0.7 parts, 0.8 parts, or any two of the above points as endpoints.

[0053] The mass fractions of the plasticized lubricant include, but are not limited to, 0.4 parts, 0.5 parts, 1 part, 1.2 parts, 1.5 parts, or any two of the above points as endpoints.

[0054] The present invention also proposes a paper-plastic composite label, comprising a plastic film, an adhesive layer and a paper base composite layer stacked sequentially from the inside out; the paper base composite layer comprises a varnish layer, a gravure ink layer and a paper base layer stacked sequentially from the inside out, and the adhesive layer is made using a solvent-free adhesive as described above; And / or, the density of the plastic film is 0.91–0.95 g / cm³. 3 ; And / or, the density of the paper base layer is 1.05–1.1 g / cm³. 3 .

[0055] Solvent-free adhesives are used to firmly bond plastic film and paper-based composites, which not only leverages the excellent moisture resistance, gas barrier properties, and mechanical strength of plastic film, but also retains the printability, stiffness, and environmentally friendly biodegradability of the paper base. At the same time, a varnish layer inside the paper-based composite protects the gravure ink layer, which can improve the abrasion resistance, scratch resistance, and gloss of the printed graphics. Thus, paper-plastic composite labels combine strong adhesion, aesthetics, and environmental friendliness.

[0056] Furthermore, based on the characteristics of the adhesive layer, the paper-plastic composite label can be completely detached in the early stage of bottle washing (0-3 min). After the label is detached, as the bottle washing time is extended (after 3 min), the alkaline solution penetrates through the paper-based composite to the interface of the adhesive layer. By additionally soaking in a 2-2.5 wt% sodium hydroxide alkaline solution at 70-80℃ for 5-8 min, the paper-based composite and the plastic film can be automatically separated.

[0057] Furthermore, the paper-based composite material mainly depends on the density of the paper base layer, which is typically between 1.05 and 1.1 g / cm³. 3 At that time, the density of the paper-based composite was 1.05–1.1 g / cm³. 3 The density of the paper-based composite is greater than that of the plastic film, so that after the paper-based composite and the plastic film are automatically separated, the plastic film floats and the paper-based composite sinks due to the density difference between the two, thus achieving classified recycling and reuse.

[0058] This invention also proposes a method for preparing a paper-plastic composite label, comprising the following steps: A gravure ink layer and a varnish layer are sequentially applied to the surface of a paper base layer to obtain a paper-based composite. The raw materials of the above solvent-free adhesive are mixed evenly to obtain the solvent-free adhesive; A solvent-free adhesive is applied to the surface of a paper-based composite and cured to obtain an adhesive layer. A plastic film is laminated onto the surface of the adhesive layer to obtain a paper-plastic composite label; And / or, the density of the plastic film is 0.91–0.95 g / cm³. 3 ; And / or, the density of the paper base layer is 1.05–1.1 g / cm³. 3 .

[0059] This technical solution first forms a gravure ink layer and a varnish layer on the paper base layer during the preparation of paper-plastic composite labels. This ensures exquisite printed graphics, saturated colors, and scratch resistance, improving the aesthetics of the paper-plastic composite labels. Then, a solvent-free adhesive is used to bond the paper-based composite to the plastic film. Utilizing its environmentally friendly, residue-free, strong initial tack, and good resistance to media after curing, a strong bond is achieved between the paper-based composite and the plastic film. The entire process requires no solvent discharge and is suitable for high-speed coating production. The resulting paper-plastic composite labels not only possess the moisture-proof, tear-resistant, and aesthetic properties of plastic, but also allow for automatic separation of the paper-based composite from the plastic film after label removal in a 70–80℃, 2–2.5wt% sodium hydroxide alkaline solution. No manual tearing or additional equipment is required; the separation process is fast, thorough, and non-adhesive, facilitating classified recycling and improving resource utilization.

[0060] In an embodiment of the present invention, in the step of coating a solvent-free adhesive onto the surface of a paper-based composite and curing it to obtain an adhesive layer, the coating temperature is 85–95°C, and the adhesive application amount is 2.0–3.0 g / m³. 2 The coating speed is 200-250 m / min; And / or, the curing temperature is 40-50°C and the time is 18-24 hours.

[0061] It should be noted that the gravure ink layer in this solution is printed using conventional inks in existing technologies, and the varnish layer is coated using conventional varnishes in existing technologies. Further description of the inks and varnishes is not provided here. The paper base can be coated paper, matte paper, or kraft paper; the specific type is not limited here.

[0062] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following specific embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0063] Example Examples 1-3 Prepare the ingredients: Solvent-free adhesives, by weight parts, comprise the following raw materials: (1) Solvent-free polyurethane resin: Isocyanate-terminated polyurethane prepolymer (Wanhua SF6100), 90 parts; the viscosity of the isocyanate-terminated polyurethane prepolymer is 4000 mPa·s, and the isocyanate content is 10 wt%; (2) Ethylene-acrylic acid copolymer: 85 parts; the acrylic acid content in the ethylene-acrylic acid copolymer is 10 wt%; (3) Core-shell microcapsules: 1 part; the core-shell microcapsules include a gas-generating core layer and a protective shell layer arranged sequentially from the inside to the outside; the particle size of the core-shell microcapsules is 4 μm; the thickness ratio of the gas-generating core layer to the protective shell layer is 1:1; the raw material of the gas-generating core layer is ammonium bicarbonate; the raw material of the protective shell layer, calculated by mass percentage, includes 97% polymethyl methacrylate and 3% polyethylene glycol-4000; the weight average molecular weight of polymethyl methacrylate is 200,000; (4) Solvent-free defoamer: 0.1 parts; the solvent-free defoamer is a polyether-modified siloxane defoamer (Dega Airex900). (5) Surfactant: 0.2 parts; the surfactant is sodium dodecyl sulfate; (6) Polyethylene glycol modifier: 0.2 parts; the polyethylene glycol modifier is polyethylene glycol-4000; (7) Plasticizing lubricant: 0.5 parts; the plasticizing lubricant is glycerol.

[0064] The preparation method of paper-plastic composite labels includes the following steps: At a density of 1.05 g / cm³ 3 A gravure ink layer and a varnish layer are sequentially applied to the surface of coated paper to obtain a paper-based composite. The raw materials for solvent-free adhesives are mixed evenly to obtain solvent-free adhesives; Solvent-free adhesive was applied at 90°C at a speed of 230 m / min and a coating density of 2.5 g / m. 2 The adhesive is applied to the surface of the paper-based composite and then cured at 45°C for 24 hours to obtain the adhesive layer. The surface composite density of the adhesive layer is 0.91 g / cm³. 3 Plastic film is used to obtain paper-plastic composite labels.

[0065] As shown in Table 1, based on Example 1, the formula was changed (as shown in Table 1 below) to obtain Examples 2 to 3.

[0066] Examples 4-5 Based on Example 1, the raw materials of the solvent-free adhesives in Examples 4 and 5 also include sulfonic acid copolymers, namely P(AAm-co-AMPS); the content of AMPS in P(AAm-co-AMPS) is 8% by mass percentage, and the specific formulation is shown in Table 1.

[0067] Comparative Example 1 The raw materials and preparation methods of Comparative Example 1 and Example 1 are the same, the only difference being that ethylene-acrylic acid copolymer was not added in Comparative Example 1.

[0068] Comparative Example 2 The raw materials and preparation methods of Comparative Example 2 are the same as those of Example 1, the only difference being that no core-shell microcapsules were added in Comparative Example 1.

[0069] Comparative Example 3 Comparative Example 3 uses a solvent-free adhesive formulation from the prior art. That is, based on parts by weight, the solvent-free adhesive comprises the following raw materials: (1) Solvent-free polyurethane resin: Isocyanate-terminated polyurethane prepolymer (Wanhua SF6100), 100 parts; the viscosity of the isocyanate-terminated polyurethane prepolymer is 4000 mPa·s, and the isocyanate content is 10 wt%; (2) Solvent-free defoamer: 0.15 parts; the solvent-free defoamer is a polyether-modified siloxane defoamer (Dega Airex900); (3) Surfactant: 0.15 parts; the surfactant is sodium dodecyl sulfate; (4) Polyethylene glycol modifier: 0.3 parts; the polyethylene glycol modifier is polyethylene glycol-4000; (5) Plasticizing lubricant: 0.7 parts; the plasticizing lubricant is glycerol.

[0070] Table 1 Recipe List

[0071] The paper-plastic composite labels prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to performance tests according to the following methods: 1. Appearance quality inspection The visual inspection method is adopted. Under the standard light source color matching light box (D65 light source, illuminance 800-1200Lux), the paper-plastic composite label sample is laid flat on the observation table. The sample surface and edges are observed at a 45° angle from 30-50cm away from the paper-plastic composite label sample. Check for defects such as bubbles, interlayer separation, wrinkles or printing indentations. At the same time, gently touch the surface of the paper-plastic composite label sample to confirm that there is no obvious unevenness.

[0072] 2. Flatness Detection After cutting the paper-plastic composite label sample to A4 size, place it flat on a horizontal testing table. Use a feeler gauge or flatness tester to measure the maximum gap between the four corners and edges of the sample and the table surface, and record the measurements (negative values ​​indicate downward warping, such as -3mm indicating a 3mm edge sagging). Test five paper-plastic composite labels in parallel and calculate their average maximum gap, which is the flatness of the paper-plastic composite label.

[0073] 3. Peel strength test Refer to GB 8808-1988 "Test Method for Peel Strength of Soft Composite Plastic Materials". Cut 5 specimens with a width of 15 mm and a length of 200 mm along the longitudinal and transverse directions of the paper-plastic composite label sample respectively. Manually pre-peel 50 mm along the length direction at one end of the specimen; use an electronic tensile testing machine, clamp both ends of the peeled part of the specimen on the upper and lower fixtures respectively, set the peel speed to 300 mm / min, and the equipment automatically records the change of force value during the peeling process after starting the test. Calculate the average peel force, which is the peel strength.

[0074] 4. Detection method for label removal performance: Use a commercially available alkali-soluble labeling adhesive (manufactured by Henkel AG & Co. KGaA, model Optal LG 32-35 series) to paste the paper-plastic composite label on the surface of the glass bottle to obtain a test sample. Immerse the test sample completely in a sodium hydroxide alkaline solution with a temperature of 70-80 °C and a concentration of 2-2.5 wt% and gently stir, and start timing at the same time. The soaking time is 3 min. Observe the detachment situation of the test sample during the soaking process: First, observe whether the paper-plastic composite label completely falls off the surface of the glass bottle within 3 min; then, observe whether the paper-plastic composite label after detachment can achieve automatic separation of the paper-based composite body and the plastic film after continuing to soak in the hot alkaline solution for 5-8 min. If the paper-plastic composite label completely falls off the surface of the glass bottle within 3 min, and the paper-plastic composite label after detachment can achieve automatic separation of the paper-based composite body and the plastic film within 5-8 min (i.e., paper-plastic separation), it is determined that the performance is qualified.

[0075] Table 2 List of performance parameters [[ID=im11]]

[0076] It can be seen from Table 2 that the paper-plastic composite labels prepared with the solvent-free adhesive in Examples 1-5 have no apparent bubbles, no delamination, no wrinkles, no printing indentation, no obvious unevenness, the flatness is controlled within 0 to -6 mm, the peel strength is ≥ 2.0 N / 15 mm, and it completely falls off the surface of the glass bottle within 3 min, and the paper and plastic are separated and collected within 5-8 min after detachment. It is proved that the solvent-free adhesive obtained in this technical solution can achieve automatic separation and classified recycling of the paper-based composite body and the plastic film without affecting the labeling firmness and the whole label removal performance of the paper-plastic composite label.

[0077] In Comparative Example 1, due to the lack of ethylene-acrylic copolymer, the compatibility and dispersibility of the system are significantly reduced, and defects such as orange peel, shrinkage holes or micro-particle-like may appear on the surface of the adhesive layer, and the flatness becomes worse; at the same time, the peel fastness is significantly reduced, and the adhesive layer is not fully dispersed in the alkaline solution, resulting in difficulty in paper-plastic separation after label removal and inability to achieve automatic separation.

[0078] In Comparative Example 2, due to the lack of internal stress release and toughening effect of core-shell microcapsules, the adhesive layer is prone to shrinkage stress during the drying and curing process, resulting in deterioration of flatness (possibly exhibiting edge lifting or wavy deformation); at the same time, the flexibility of the adhesive layer decreases, brittleness increases, peel strength decreases, the adhesive layer is difficult to disintegrate in alkaline solutions, and the paper-plastic parts are stuck together and cannot be automatically separated.

[0079] The above description is merely an exemplary embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent transformations made using the present invention's specification under the technical concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A solvent-free adhesive, characterized in that, The raw materials for the solvent-free adhesive include solvent-free polyurethane resin, ethylene-acrylic acid copolymer, and core-shell microcapsules; The core-shell microcapsule comprises a gas-generating core layer and a protective shell layer arranged sequentially from the inside to the outside. The raw material of the gas-generating core layer is an inorganic salt that generates gas when it comes into contact with alkali, and the raw material of the protective shell layer can form microporous channels when it comes into contact with alkali. The inorganic salt includes at least one of ammonium bicarbonate, ammonium carbonate, and ammonium chloride; The protective shell is made of polymethyl methacrylate and a porogen, wherein the porogen is at least one of polyethylene glycol-4000, polyethylene glycol-6000, and polyethylene glycol-8000.

2. The solvent-free adhesive as described in claim 1, characterized in that, The weight-average molecular weight of the polymethyl methacrylate is 120,000 to 350,000. The protective shell material comprises 95-97% polymethyl methacrylate and 3-5% pore-forming agent by mass percentage.

3. The solvent-free adhesive as described in claim 1, characterized in that, The core-shell microcapsules have a particle size of 2–5 μm; And / or, the thickness ratio of the gas-generating core layer to the protective shell layer is 1:(0.5~2).

4. The solvent-free adhesive as described in claim 1, characterized in that, The acrylic acid content in the ethylene-acrylic acid copolymer is 5-20% by mass percentage.

5. The solvent-free adhesive as described in claim 1, characterized in that, The raw materials for preparing the solvent-free adhesive also include sulfonic acid copolymers; The sulfonic acid copolymer is P(AAm-co-AMPS); The content of AMPS in the P(AAm-co-AMPS) is 5-10% by mass percentage.

6. The solvent-free adhesive as described in claim 1, characterized in that, The solvent-free polyurethane resin is an isocyanate-terminated polyurethane prepolymer; The viscosity of the isocyanate-terminated polyurethane prepolymer is 1000–8000 mPa·s; The isocyanate content of the isocyanate-terminated polyurethane prepolymer is 5-15% by mass percentage.

7. The solvent-free adhesive as described in claim 1, characterized in that, The raw materials of the solvent-free adhesive also include at least one of solvent-free defoamers, surfactants, polyethylene glycol modifiers, and plasticizers and lubricants; The solvent-free defoamer includes at least one of polyether-modified siloxane defoamers, organosilicon defoamers, and non-silicone polymer defoamers; The surfactant includes at least one of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate; The polyethylene glycol modifier includes at least one of polyethylene glycol-4000, polyethylene glycol-6000, and polyethylene glycol-8000; The plasticizing lubricant includes at least one of glycerol, diethylene glycol, propylene glycol, and sorbitol.

8. The solvent-free adhesive according to any one of claims 1 to 7, characterized in that, According to the mass fraction, the solvent-free polyurethane resin in the raw materials of the solvent-free adhesive is 90 to 100 parts; And / or, calculated by mass parts, the ethylene-acrylic acid copolymer in the raw materials of the solvent-free adhesive is 50 to 90 parts; And / or, calculated by mass parts, the core-shell microcapsules in the raw materials of the solvent-free adhesive are 1 to 5 parts; And / or, by weight, the raw materials of the solvent-free adhesive further include 1 to 3 parts of sulfonic acid copolymer; And / or, calculated by mass parts, the raw materials of the solvent-free adhesive further include 0.05 to 0.3 parts of solvent-free defoamer; And / or, calculated by mass parts, the raw materials of the solvent-free adhesive further include 0.05 to 0.3 parts of surfactant; And / or, calculated by mass parts, the raw materials of the solvent-free adhesive further include 0.1 to 0.8 parts of polyethylene glycol modifier; And / or, by weight, the raw materials of the solvent-free adhesive further include 0.4 to 1.5 parts of plasticizing lubricant.

9. A paper-plastic composite label, characterized in that, The invention comprises a plastic film, an adhesive layer, and a paper-based composite layer stacked sequentially from the inside out; the paper-based composite layer comprises a varnish layer, a gravure ink layer, and a paper base layer stacked sequentially from the inside out, and the adhesive layer is prepared using a solvent-free adhesive as described in any one of claims 1 to 8. And / or, the density of the plastic film is 0.91–0.95 g / cm³. 3 ; And / or, the density of the paper base layer is 1.05–1.1 g / cm³. 3 .

10. A method for preparing a paper-plastic composite label, characterized in that, Includes the following steps: A gravure ink layer and a varnish layer are sequentially applied to the surface of a paper base layer to obtain a paper-based composite. The raw materials of the solvent-free adhesive according to any one of claims 1 to 8 are mixed evenly to obtain the solvent-free adhesive; A solvent-free adhesive is applied to the surface of a paper-based composite and cured to obtain an adhesive layer. A plastic film is laminated onto the surface of the adhesive layer to obtain a paper-plastic composite label; And / or, the density of the plastic film is 0.91–0.95 g / cm³. 3 ; And / or, the density of the paper base layer is 1.05–1.1 g / cm³. 3 .

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