Flame-retardant kraft linerboard and surface treatment process thereof

Abstract

The invention relates to the technical field of kraft linerboards, in particular to a flame-retardant kraft linerboard and a surface treatment process thereof. An upper transition layer is arranged below the upper surface layer, a core layer located on a middle base layer of the kraft linerboard is arranged below the upper transition layer, a flame-retardant layer for inhibiting combustion reaction and reducing flame propagation speed through a flame retardant is arranged in the core layer, and a lower transition layer is arranged below the core layer. A lower-layer surface layer which is symmetrical with the upper-layer surface layer and is positioned on the outermost layer of the bottom of the kraft linerboard is arranged below the According to the flame-retardant kraft linerboard, a layered composite structure is adopted, the upper transition layer and the lower transition layer are additionally arranged, the binding force between the layers is enhanced, the layering phenomenon is effectively avoided, the overall strength and durability of the kraft linerboard are greatly improved, the flame-retardant layer adopts a multi-layer composite structure and gradient flame-retardant design, and the flame-retardant efficiency is improved; according to the process, the flatness, glossiness and printing stability of the paperboard are ensured, the quality of a finished product is stable, the defective rate is reduced, and the production efficiency and economic benefits are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of kraft paperboard, in particular to a flame-retardant kraft paperboard and a surface treatment process thereof. Background Art

[0002] Flame-retardant kraft cardboard is an industrial paper with special functions. While retaining the high strength and high toughness of ordinary kraft cardboard, it is given excellent flame-retardant properties through structural design and process treatment. It can be widely used in scenarios with high requirements for fire protection.

[0003] Traditional flame-retardant kraft paperboard is mostly single-layer or simple composite structure, lacking layered optimization design, and no transition layer to enhance the bonding strength of each layer, which is prone to delamination, affecting the overall strength and durability of the paperboard; at the same time, traditional products often use a single flame retardant. Although the flame retardant effect is acceptable, there are environmental issues. When burned, it will release toxic and harmful gases, causing harm to the human body and the environment; and the flame retardant is unevenly distributed, unable to effectively suppress combustion at different stages, and the flame retardant efficiency and stability are poor. The surface layer has not been specifically treated, and it is difficult to meet multiple performance requirements such as flame retardancy, wear resistance, and printing at the same time. The treatment process is relatively simple, and the pre-treatment link is insufficient in cleaning and surface optimization, which affects the coating effect; the coating liquid formula is single and cannot be flexibly adjusted to meet diverse functional requirements; the drying and post-processing processes lack fine control, and it is difficult to ensure the flatness, glossiness and printability of the paperboard, and the quality of the finished product is unstable.

[0004] Therefore, in order to solve the problems of the single structural design, limited flame retardant performance and simple surface treatment process of the above-mentioned flame retardant kraft paper, which lead to unstable quality of the finished product, a flame retardant kraft paper and its surface treatment process can be designed. Summary of the Invention

[0005] In order to overcome the problem that traditional flame-retardant kraft paperboard has a single structural design, low flame-retardant efficiency and simple surface treatment process, it is difficult to ensure the flatness, glossiness and printability of the paperboard, which leads to unstable quality of the finished product.

[0006] The technical solution is as follows: a flame-retardant kraft cardboard, including an upper surface layer; an upper transition layer is provided below the upper surface layer, a core layer located in the middle base layer of the kraft cardboard is provided below the upper transition layer, a flame retardant layer is provided inside the core layer for suppressing the combustion reaction and reducing the flame propagation speed through a flame retardant, a lower transition layer is provided below the core layer, and a lower surface layer is provided below the lower transition layer, which is symmetrical to the upper surface layer and located at the outermost layer at the bottom of the kraft cardboard.

[0007] Furthermore, the flame retardant layer is a multi-layer composite structure, including two flame retardant distribution layers, wherein the first flame retardant layer close to the upper transition layer is mainly composed of phosphorus-based flame retardants, and the second flame retardant layer close to the lower transition layer is mainly composed of nitrogen-based flame retardants. The combustion inhibition effect is optimized through gradient flame retardant design.

[0008] Furthermore, nano-scale flame retardant fillers are added to the flame retardant layer. Specifically, nano-magnesium hydroxide or nano-silicon dioxide is used as the nano-scale flame retardant filler. The filler particle size is 50-200nm. The dispersion and thermal stability of the flame retardant in the substrate are enhanced through the nano effect.

[0009] The surface treatment process of the flame retardant kraft paperboard uses the flame retardant kraft paperboard as described above, and is characterized by the following steps:

[0010] S1: Base paper pretreatment

[0011] S11: Base paper screening and inspection

[0012] Select flame-retardant kraft paper that meets the specifications, strictly test the physical indicators of basis weight, thickness, stiffness, and tensile strength, and verify the flame retardant performance parameters of the flame retardant layer, including oxygen index and combustion grade, to ensure that the base paper quality meets the standards. Visually inspect the surface of the base paper and eliminate paper with wrinkles, holes, and color spots:

[0013] S12: Cleaning and dust removal

[0014] A multi-stage dust removal method is used. First, a brush roller is used to mechanically remove larger particles of impurities on the surface. Then, a high-voltage electrostatic dust removal device is used to absorb tiny dust and fiber debris to ensure that the coating surface of the upper and lower layers is clean and free of foreign matter, preventing impurities from affecting the coating uniformity and coating adhesion.

[0015] S2 coating solution preparation

[0016] S21: Basic Recipe

[0017] A water-based acrylic resin accounting for 12%-15% is used as a film-forming agent to provide the coating with flexibility and wear resistance; a phosphorus-nitrogen composite flame retardant accounting for 18%-22% is added to further enhance the flame retardancy of the surface layer; a dispersant accounting for 0.8%, a defoamer accounting for 0.3%, and a leveling agent accounting for 0.7% are added as additives to improve the processing performance of the coating liquid;

[0018] S22: Preparation process

[0019] S221: Add a certain amount of water to the stirring tank, turn on the stirrer, and adjust the speed to 300 r / min;

[0020] S222: Add dispersant, defoamer, and leveling agent in sequence and stir for 15 minutes to fully dissolve them;

[0021] S223: Slowly add the phosphorus-nitrogen composite flame retardant, increase the stirring speed to 600 r / min, and stir for 30 minutes to ensure that the flame retardant is evenly dispersed and avoid agglomeration;

[0022] S224: Add film-forming agent and other functional additives, and continue stirring for 20 minutes to mix the components evenly;

[0023] S225: Filter the prepared coating liquid using a 200-mesh filter to remove undispersed particulate impurities to obtain a uniform and fine coating liquid;

[0024] S3: coating process

[0025] S31: To meet the requirements of symmetrical coating of the upper and lower surfaces, a roller coating device with simultaneous double-sided coating is used. The cell depth and line pressure of the concave roller are adjusted according to the viscosity of the coating liquid and the coating amount required.

[0026] S32: coating operation

[0027] S321: Feed the pre-treated base paper smoothly into the coating machine, ensuring that the paper runs with a stable tension, controlled at 15-25N, to avoid wrinkles or deviations;

[0028] S322: During the coating process, the liquid level and temperature of the coating liquid are monitored in real time and maintained at 20-25°C to prevent the coating uniformity from being affected by low liquid level or temperature fluctuations;

[0029] S323: Regularly check the surface cleanliness of the concave roller. If any residual impurities or dried coating liquid are found, stop the machine and clean them in time to avoid coating defects.

[0030] S4: Drying

[0031] S41: Pre-drying

[0032] The coated paperboard first enters the infrared pre-drying channel with a drying temperature of 70-80°C and a drying time of 1.5-2 minutes to quickly remove some of the moisture on the surface of the coating liquid to prevent the coating liquid from sagging or migrating during the subsequent drying process.

[0033] S42: Main Drying

[0034] The pre-dried cardboard enters the hot air drying oven and adopts a three-stage drying process:

[0035] 1) The first stage temperature is 100-110°C, the wind speed is 6-8m / s, and the drying time is 3 minutes to further remove most of the moisture;

[0036] 2) The second stage temperature is 110-120℃, wind speed is 7-9m / s, and drying time is 3 minutes to accelerate water evaporation;

[0037] 3) The third stage is at a temperature of 90-100°C, a wind speed of 5-7m / s, and a drying time of 2 minutes. This stage uses temperature buffering and residual heat drying to stabilize the moisture content of the cardboard at 6%-8%.

[0038] S5: Post-processing

[0039] S51: calendering

[0040] The dried cardboard is calendered on the surface by a soft calender at a calendering temperature of 90-110°C and a pressure of 60-100 kN / m. After 2-3 calendering treatments, the smoothness and glossiness of the upper and lower surfaces are improved, thereby improving printability.

[0041] S52: Slitting and quality inspection

[0042] S521: Use a high-precision slitting machine to cut the cardboard into specified sizes, with a cutting accuracy of ±0.5mm to ensure a neat cut without burrs;

[0043] S522: Conduct comprehensive quality inspection on finished products, including appearance inspection, physical property testing and flame retardant performance retesting. Unqualified products will be handled separately.

[0044] Furthermore, after step S12, if the surface of the base paper is too smooth, use sandpaper or a light grinding device to lightly grind the upper and lower surface layers to increase the surface roughness and improve the adhesion effect of the subsequent coating liquid.

[0045] Furthermore, step S21 also includes a silicone waterproofing agent accounting for 5%-8%.

[0046] Furthermore, in step S21, a slip agent accounting for 2%-3% and an anti-blocking agent accounting for 2%-3% are also included.

[0047] Furthermore, in step S31, the roller coating equipment specifically adopts a double-sided gravure roller coater; the viscosity of the coating liquid is controlled to be 15-30s, and the coating amount is required to be 5-12g / m 2 , the cell depth is 40-80μm, and the line pressure is 20-40N / cm.

[0048] Furthermore, after step S51, a surface hardening treatment is added, using a UV curing resin coating with a curing energy of 200-300mJ / cm 2 , so that the surface pencil hardness reaches 3H or above.

[0049] The beneficial effects of the present invention are:

[0050] The flame-retardant kraft paperboard of the present invention adopts a layered composite structure with an additional upper transition layer and a lower transition layer, which enhances the bonding force between the layers, effectively avoids delamination, and significantly improves the overall strength and durability of the paperboard. This structural design enables the paperboard to be subjected to external forces, and the layers can be coordinated to bear the force, making it less likely to suffer structural damage, and can meet the needs of use in more complex scenarios.

[0051] The flame-retardant layer adopts a multi-layer composite structure and gradient flame retardant design, distributing phosphorus-based flame retardants and nitrogen-based flame retardants in different regions. Different flame retardants work at different stages of combustion, synergizing with each other to significantly optimize the combustion suppression effect and improve flame retardant efficiency. At the same time, traditional halogen-containing flame retardants are abandoned and environmentally friendly flame retardants are used. No toxic or harmful gases are released during combustion, which not only ensures the safety of the use environment, but also complies with current environmental protection policies and broadens the application range of the product.

[0052] The flame retardant layer is added with nano-scale flame retardant fillers. By utilizing the nano effect, the flame retardant is dispersed more evenly in the substrate, effectively improving the thermal stability of the flame retardant and further enhancing the flame retardant performance. In addition, the nano fillers can also improve the physical properties of the cardboard, such as increasing its hardness and toughness, thus improving the overall performance of the cardboard.

[0053] In the surface treatment process, by flexibly adjusting the coating liquid formula, functional additives such as silicone waterproofing agents, slip agents, and anti-blocking agents can be added to make the cardboard have multiple functions such as waterproofing, wear resistance, and anti-blocking, meeting the diverse performance requirements of cardboard in different industries and application scenarios;

[0054] From the strict screening and cleaning of base paper pretreatment, to the precise proportioning and dispersion of coating liquid preparation, to the accurate control of the coating process and the refined operations of drying and post-treatment, every link has been optimized. This series of fine processes ensures the flatness, glossiness and printability of the cardboard, stable quality of the finished product, reduced defective rate, improved production efficiency and economic benefits. The surface hardening treatment further enhances the hardness of the cardboard surface, making it more wear-resistant and extending its service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 This is a schematic diagram of the three-dimensional structure of the flame-retardant kraft cardboard of the present invention;

[0056] Figure 2 This is a schematic diagram of the disassembled structure of the flame-retardant kraft paper of the present invention.

[0057] In the figure: 1, upper surface layer; 2, upper transition layer; 3, core layer; 4, flame retardant layer; 5, lower transition layer; 6, lower surface layer. DETAILED DESCRIPTION

[0058] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0059] Example 1

[0060] like Figure 1-Figure 2As shown, the present invention provides an embodiment: a flame retardant kraft paper, comprising an upper surface layer 1; an upper transition layer 2 is provided below the upper surface layer 1, a core layer 3 located at the middle base layer of the kraft paper is provided below the upper transition layer 2, a flame retardant layer 4 is provided inside the core layer 3 for suppressing the combustion reaction and reducing the flame propagation speed through a flame retardant, a lower transition layer 5 is provided below the core layer 3, and a lower surface layer 6 is provided below the lower transition layer 5, which is symmetrical to the upper surface layer 1 and located at the outermost layer of the bottom of the kraft paper.

[0061] Furthermore, the flame retardant layer 4 is a multi-layer composite structure, including two flame retardant distribution layers, wherein the first flame retardant layer close to the upper transition layer 2 is mainly composed of phosphorus-based flame retardants, and the second flame retardant layer close to the lower transition layer 5 is mainly composed of nitrogen-based flame retardants. The combustion inhibition effect is optimized through gradient flame retardant design.

[0062] Furthermore, nano-scale flame retardant fillers are added to the flame retardant layer 4. Specifically, the nano-scale flame retardant fillers are nano-magnesium hydroxide or nano-silicon dioxide. The filler particle size is 50-200 nm. The dispersion and thermal stability of the flame retardant in the substrate are enhanced through the nano effect.

[0063] The surface treatment process of the flame retardant kraft paperboard uses the flame retardant kraft paperboard as described above, and is characterized by the following steps:

[0064] S1: Base paper pretreatment

[0065] S11: Base paper screening and inspection

[0066] Select flame-retardant kraft paper that meets the specifications, strictly test the physical indicators of basis weight, thickness, stiffness, and tensile strength, and verify the flame retardant performance parameters of the flame retardant layer, including oxygen index and combustion grade, to ensure that the base paper quality meets the standards. Visually inspect the surface of the base paper and eliminate paper with wrinkles, holes, and color spots:

[0067] S12: Cleaning and dust removal

[0068] A multi-stage dust removal method is used. First, large surface impurities are removed mechanically with a brush roller. Then, high-voltage electrostatic dust removal equipment is used to absorb tiny dust and fiber debris to ensure that the coating surface of the upper surface layer 1 and the lower surface layer 6 are clean and free of foreign matter. This prevents impurities from affecting the coating uniformity and coating adhesion. If the surface of the base paper is too smooth, use sandpaper or a light grinding device to lightly grind the surface of the upper surface layer 1 and the lower surface layer 6 to increase the surface roughness and improve the adhesion of the subsequent coating liquid:

[0069] S2 coating solution preparation

[0070] S21: Basic Recipe

[0071] A water-based acrylic resin, accounting for 12%-15%, is used as a film-forming agent to provide the coating with flexibility and wear resistance; a phosphorus-nitrogen composite flame retardant, accounting for 18%-22%, is added to further enhance the flame retardancy of the surface layer; a dispersant, accounting for 0.8%, a defoamer, accounting for 0.3%, and a leveling agent, accounting for 0.7%, are added as additives to improve the processing performance of the coating liquid; and a silicone water repellent, accounting for 5%-8%.

[0072] S22: Preparation process

[0073] S221: Add a certain amount of water to the stirring tank, turn on the stirrer, and adjust the speed to 300 r / min;

[0074] S222: Add dispersant, defoamer, and leveling agent in sequence and stir for 15 minutes to fully dissolve them;

[0075] S223: Slowly add the phosphorus-nitrogen composite flame retardant, increase the stirring speed to 600 r / min, and stir for 30 minutes to ensure that the flame retardant is evenly dispersed and avoid agglomeration;

[0076] S224: Add film-forming agent and silicone waterproofing agent, and continue stirring for 20 minutes to mix the components evenly;

[0077] S225: Filter the prepared coating liquid using a 200-mesh filter to remove undispersed particulate impurities to obtain a uniform and fine coating liquid;

[0078] S3: coating process

[0079] S31: To meet the requirements of symmetrical coating on the upper and lower surfaces, a roller coating device with double-sided coating is used. The cell depth and line pressure of the gravure roller are adjusted according to the viscosity of the coating liquid and the coating amount requirements. The roller coating device specifically uses a double-sided gravure roller coater; the coating liquid viscosity is controlled to 15-30s, and the coating amount requirement is 5-12g / m 2 , the cell depth is 40-80μm, and the line pressure is 20-40N / cm;

[0080] S32: coating operation

[0081] S321: Feed the pre-treated base paper smoothly into the coating machine, ensuring that the paper runs with a stable tension, controlled at 15-25N, to avoid wrinkles or deviations;

[0082] S322: During the coating process, the liquid level and temperature of the coating liquid are monitored in real time and maintained at 20-25°C to prevent the coating uniformity from being affected by low liquid level or temperature fluctuations;

[0083] S323: Regularly check the surface cleanliness of the concave roller. If any residual impurities or dried coating liquid are found, stop the machine and clean them in time to avoid coating defects.

[0084] S4: Drying

[0085] S41: Pre-drying

[0086] The coated paperboard first enters the infrared pre-drying channel with a drying temperature of 70-80°C and a drying time of 1.5-2 minutes to quickly remove some of the moisture on the surface of the coating liquid to prevent the coating liquid from sagging or migrating during the subsequent drying process.

[0087] S42: Main Drying

[0088] The pre-dried cardboard enters the hot air drying oven and adopts a three-stage drying process:

[0089] 1) The first stage temperature is 100-110°C, the wind speed is 6-8m / s, and the drying time is 3 minutes to further remove most of the moisture;

[0090] 2) The second stage temperature is 110-120℃, wind speed is 7-9m / s, and drying time is 3 minutes to accelerate water evaporation;

[0091] 3) The third stage is at a temperature of 90-100°C, a wind speed of 5-7m / s, and a drying time of 2 minutes. This stage uses temperature buffering and residual heat drying to stabilize the moisture content of the cardboard at 6%-8%.

[0092] S5: Post-processing

[0093] S51: calendering

[0094] The dried cardboard is calendered on the surface by a soft calender at a calendering temperature of 90-110°C and a pressure of 60-100 kN / m. After 2-3 calendering treatments, the smoothness and glossiness of the upper surface layer 1 and the lower surface layer 6 are improved, the printability is improved, and the surface is hardened. UV curing resin coating is used with a curing energy of 200-300 mJ / cm 2 , so that the surface pencil hardness reaches 3H or above;

[0095] S52: Slitting and quality inspection

[0096] S521: Use a high-precision slitting machine to cut the cardboard into specified sizes, with a cutting accuracy of ±0.5mm to ensure a neat cut without burrs;

[0097] S522: Conduct comprehensive quality inspection on finished products, including appearance inspection, physical property testing and flame retardant performance retesting. Unqualified products will be handled separately.

[0098] Example 2

[0099] like Figure 1-Figure 2As shown, the present invention provides an embodiment: a flame retardant kraft paper, comprising an upper surface layer 1; an upper transition layer 2 is provided below the upper surface layer 1, a core layer 3 located at the middle base layer of the kraft paper is provided below the upper transition layer 2, a flame retardant layer 4 is provided inside the core layer 3 for suppressing the combustion reaction and reducing the flame propagation speed through a flame retardant, a lower transition layer 5 is provided below the core layer 3, and a lower surface layer 6 is provided below the lower transition layer 5, which is symmetrical to the upper surface layer 1 and located at the outermost layer of the bottom of the kraft paper.

[0100] Furthermore, the flame retardant layer 4 is a multi-layer composite structure, including two flame retardant distribution layers, wherein the first flame retardant layer close to the upper transition layer 2 is mainly composed of phosphorus-based flame retardants, and the second flame retardant layer close to the lower transition layer 5 is mainly composed of nitrogen-based flame retardants. The combustion inhibition effect is optimized through gradient flame retardant design.

[0101] Furthermore, nano-scale flame retardant fillers are added to the flame retardant layer 4. Specifically, the nano-scale flame retardant fillers are nano-magnesium hydroxide or nano-silicon dioxide. The filler particle size is 50-200 nm. The dispersion and thermal stability of the flame retardant in the substrate are enhanced through the nano effect.

[0102] The surface treatment process of the flame retardant kraft paperboard uses the flame retardant kraft paperboard as described above, and is characterized by the following steps:

[0103] S1: Base paper pretreatment

[0104] S11: Base paper screening and inspection

[0105] Select flame-retardant kraft paper that meets the specifications, strictly test the physical indicators of basis weight, thickness, stiffness, and tensile strength, and verify the flame retardant performance parameters of the flame retardant layer, including oxygen index and combustion grade, to ensure that the base paper quality meets the standards. Visually inspect the surface of the base paper and eliminate paper with wrinkles, holes, and color spots:

[0106] S12: Cleaning and dust removal

[0107] A multi-stage dust removal method is used. First, large surface impurities are removed mechanically with a brush roller. Then, high-voltage electrostatic dust removal equipment is used to absorb tiny dust and fiber debris to ensure that the coating surface of the upper surface layer 1 and the lower surface layer 6 are clean and free of foreign matter. This prevents impurities from affecting the coating uniformity and coating adhesion. If the surface of the base paper is too smooth, use sandpaper or a light grinding device to lightly grind the surface of the upper surface layer 1 and the lower surface layer 6 to increase the surface roughness and improve the adhesion of the subsequent coating liquid:

[0108] S2 coating solution preparation

[0109] S21: Basic Recipe

[0110] A water-based acrylic resin, accounting for 12%-15%, is used as a film-forming agent to provide the coating with flexibility and wear resistance; a phosphorus-nitrogen composite flame retardant, accounting for 18%-22%, is added to further enhance the flame retardancy of the surface layer; a dispersant, accounting for 0.8%, a defoamer, accounting for 0.3%, and a leveling agent, accounting for 0.7%, are added as additives to improve the processing performance of the coating liquid; a slip agent, accounting for 2%-3%, and an anti-blocking agent, accounting for 2%-3%, are added;

[0111] S22: Preparation process

[0112] S221: Add a certain amount of water to the stirring tank, turn on the stirrer, and adjust the speed to 300 r / min;

[0113] S222: Add dispersant, defoamer, and leveling agent in sequence and stir for 15 minutes to fully dissolve them;

[0114] S223: Slowly add the phosphorus-nitrogen composite flame retardant, increase the stirring speed to 600 r / min, and stir for 30 minutes to ensure that the flame retardant is evenly dispersed and avoid agglomeration;

[0115] S224: Add film-forming agent, slip agent and anti-blocking agent, and continue stirring for 20 minutes to mix all the components evenly;

[0116] S225: Filter the prepared coating liquid using a 200-mesh filter to remove undispersed particulate impurities to obtain a uniform and fine coating liquid;

[0117] S3: coating process

[0118] S31: To meet the requirements of symmetrical coating on the upper and lower surfaces, a roller coating device with double-sided coating is used. The cell depth and line pressure of the gravure roller are adjusted according to the viscosity of the coating liquid and the coating amount requirements. The roller coating device specifically uses a double-sided gravure roller coater; the coating liquid viscosity is controlled to 15-30s, and the coating amount requirement is 5-12g / m 2 , the cell depth is 40-80μm, and the line pressure is 20-40N / cm;

[0119] S32: coating operation

[0120] S321: Feed the pre-treated base paper smoothly into the coating machine, ensuring that the paper runs with a stable tension, controlled at 15-25N, to avoid wrinkles or deviations;

[0121] S322: During the coating process, the liquid level and temperature of the coating liquid are monitored in real time and maintained at 20-25°C to prevent the coating uniformity from being affected by low liquid level or temperature fluctuations;

[0122] S323: Regularly check the surface cleanliness of the concave roller. If any residual impurities or dried coating liquid are found, stop the machine and clean them in time to avoid coating defects.

[0123] S4: Drying

[0124] S41: Pre-drying

[0125] The coated paperboard first enters the infrared pre-drying channel with a drying temperature of 70-80°C and a drying time of 1.5-2 minutes to quickly remove some of the moisture on the surface of the coating liquid to prevent the coating liquid from sagging or migrating during the subsequent drying process.

[0126] S42: Main Drying

[0127] The pre-dried cardboard enters the hot air drying oven and adopts a three-stage drying process:

[0128] 1) The first stage temperature is 100-110°C, the wind speed is 6-8m / s, and the drying time is 3 minutes to further remove most of the moisture;

[0129] 2) The second stage temperature is 110-120℃, wind speed is 7-9m / s, and drying time is 3 minutes to accelerate water evaporation;

[0130] 3) The third stage is at a temperature of 90-100°C, a wind speed of 5-7m / s, and a drying time of 2 minutes. This stage uses temperature buffering and residual heat drying to stabilize the moisture content of the cardboard at 6%-8%.

[0131] S5: Post-processing

[0132] S51: calendering

[0133] The dried cardboard is calendered on the surface by a soft calender at a calendering temperature of 90-110°C and a pressure of 60-100 kN / m. After 2-3 calendering treatments, the smoothness and glossiness of the upper surface layer 1 and the lower surface layer 6 are improved, the printability is improved, and the surface is hardened. UV curing resin coating is used with a curing energy of 200-300 mJ / cm 2 , so that the surface pencil hardness reaches 3H or above;

[0134] S52: Slitting and quality inspection

[0135] S521: Use a high-precision slitting machine to cut the cardboard into specified sizes, with a cutting accuracy of ±0.5mm to ensure a neat cut without burrs;

[0136] S522: Conduct comprehensive quality inspection on finished products, including appearance inspection, physical property testing and flame retardant performance retesting. Unqualified products will be handled separately.

[0137] Experimental example

[0138] The examples 1 and 2, as well as the traditional flame-retardant kraft paper as a control group, were divided into three groups. Comparative tests were conducted on the three aspects of physical properties, flame retardancy, and functional properties. Each indicator was tested using industry standard methods. Each group of experiments was repeated five times and the average value was taken to ensure data reliability.

[0139] Quantitative test: According to GB / T451.2-2002 "Paper and board - Determination of quantitative weight", use a quantitative sampler to cut a standard sample, weigh it on an analytical balance, and calculate the mass per unit area;

[0140] Thickness test: According to GB / T451.3-2002 "Paper and board - Determination of thickness", use a thickness gauge to measure 10 times at different positions of the sample and take the average value;

[0141] Tensile strength test: According to GB / T12914-2018 "Paper and board - Determination of tensile strength", use a tensile strength testing machine to stretch the sample at a constant speed and record the maximum force at break;

[0142] Refer to GB / T454-2002 "Determination of bursting strength of paper" and use a bursting strength tester to apply uniform pressure to the sample until it breaks, and read the bursting strength value;

[0143] Oxygen index test: According to GB / T2406.2-2009 "Determination of combustion behavior of plastics by oxygen index method Part 2: Room temperature test", use an oxygen index meter to adjust the oxygen and nitrogen mixing ratio and measure the minimum oxygen concentration required for the sample to just maintain combustion;

[0144] Vertical burning test: According to the UL94 "Test for Flammability of Plastic Materials for Parts of Equipment and Appliances" standard, fix the sample vertically, burn it with a standard flame for 10 seconds, then remove it, and record the afterburning time, smoldering time and damaged length;

[0145] Waterproof performance test: Using the surface absorption weight method (Cobb value), according to GB / T1540-2002 "Determination of water absorption of paper and paperboard (Cobb method)", the sample is exposed to water for 60 seconds and the water absorption per unit area is measured;

[0146] Wear resistance test: Use a wear tester to set a certain number of friction times and pressure, observe the degree of wear on the sample surface, and express the wear resistance as the percentage of gloss reduction;

[0147] Printability test: Through solid density and dot gain evaluation, a standard pattern is printed on the sample using a printability meter, and relevant parameters are measured using a densitometer and a magnifying glass:

[0148]

[0149]

[0150] Therefore, the descriptions limiting the shapes, quantities, etc. explicitly stated above are illustrative descriptions for the purpose of facilitating understanding of the present invention, and are not descriptions limiting the present invention. Therefore, descriptions using the names of components other than some or all of the limitations on shapes, quantities, etc. should also be included in the present invention.

Claims

1. A flame retardant kraft paperboard, characterized in that: It includes an upper surface layer; an upper transition layer is provided below the upper surface layer; a core layer located at the middle base layer of kraft paper is provided below the upper transition layer; a flame retardant layer is provided inside the core layer for suppressing combustion reaction and reducing flame propagation speed through flame retardant; a lower transition layer is provided below the core layer; a lower surface layer is provided below the lower transition layer and is located symmetrically with the upper surface layer and is located at the outermost layer at the bottom of the kraft paper.

2. The flame retardant kraft paper according to claim 1, characterized in that: The flame retardant layer is a multi-layer composite structure, including two flame retardant distribution layers. The first flame retardant layer close to the upper transition layer is mainly composed of phosphorus-based flame retardants, and the second flame retardant layer close to the lower transition layer is mainly composed of nitrogen-based flame retardants. The combustion inhibition effect is optimized through gradient flame retardant design.

3. The flame retardant kraft paper according to claim 1, characterized in that: Nano-scale flame retardant fillers are added to the flame retardant layer. Specifically, nano-magnesium hydroxide or nano-silicon dioxide is used. The filler particle size is 50-200nm. The dispersion and thermal stability of the flame retardant in the substrate are enhanced through the nano effect.

4. A surface treatment process for flame-retardant kraft paper, using the flame-retardant kraft paper according to any one of claims 1 to 3, characterized in that: The steps are as follows: S1: Base paper pretreatment S11: Base paper screening and inspection Select flame-retardant kraft paper that meets the specifications, strictly test the physical indicators of basis weight, thickness, stiffness, and tensile strength, and verify the flame retardant performance parameters of the flame retardant layer, including oxygen index and combustion grade, to ensure that the base paper quality meets the standards. Visually inspect the surface of the base paper and eliminate paper with wrinkles, holes, and color spots: S12: Cleaning and dust removal A multi-stage dust removal method is used. First, a brush roller is used to mechanically remove larger particles of impurities on the surface. Then, a high-voltage electrostatic dust removal device is used to absorb tiny dust and fiber debris to ensure that the coating surface of the upper and lower layers is clean and free of foreign matter, preventing impurities from affecting the coating uniformity and coating adhesion. S2 coating solution preparation S21: Basic Recipe A water-based acrylic resin accounting for 12%-15% is used as a film-forming agent to provide the coating with flexibility and wear resistance; a phosphorus-nitrogen composite flame retardant accounting for 18%-22% is added to further enhance the flame retardancy of the surface layer; a dispersant accounting for 0.8%, a defoamer accounting for 0.3%, and a leveling agent accounting for 0.7% are added as additives to improve the processing performance of the coating liquid; S22: Preparation process S221: Add a certain amount of water to the stirring tank, turn on the stirrer, and adjust the speed to 300 r / min; S222: Add dispersant, defoamer, and leveling agent in sequence and stir for 15 minutes to fully dissolve them; S223: Slowly add the phosphorus-nitrogen composite flame retardant, increase the stirring speed to 600 r / min, and stir for 30 minutes to ensure that the flame retardant is evenly dispersed and avoid agglomeration; S224: Add film-forming agent and other functional additives, and continue stirring for 20 minutes to mix the components evenly; S225: Filter the prepared coating liquid using a 200-mesh filter to remove undispersed particulate impurities to obtain a uniform and fine coating liquid; S3: coating process S31: To meet the requirements of symmetrical coating of the upper and lower surfaces, a roller coating device with simultaneous double-sided coating is used. The cell depth and line pressure of the concave roller are adjusted according to the viscosity of the coating liquid and the coating amount required. S32: coating operation S321: Feed the pre-treated base paper smoothly into the coating machine, ensuring that the paper runs with a stable tension, controlled at 15-25N, to avoid wrinkles or deviations; S322: During the coating process, the liquid level and temperature of the coating liquid are monitored in real time and maintained at 20-25°C to prevent the coating uniformity from being affected by low liquid level or temperature fluctuations; S323: Regularly check the surface cleanliness of the concave roller. If any residual impurities or dried coating liquid are found, stop the machine and clean them in time to avoid coating defects. S4: Drying S41: Pre-drying The coated paperboard first enters the infrared pre-drying channel with a drying temperature of 70-80°C and a drying time of 1.5-2 minutes to quickly remove some of the moisture on the surface of the coating liquid to prevent the coating liquid from sagging or migrating during the subsequent drying process. S42: Main Drying The pre-dried cardboard enters the hot air drying oven and adopts a three-stage drying process: 1) The first stage temperature is 100-110°C, the wind speed is 6-8m / s, and the drying time is 3 minutes to further remove most of the moisture; 2) The second stage temperature is 110-120℃, wind speed is 7-9m / s, and drying time is 3 minutes to accelerate water evaporation; 3) The third stage is at a temperature of 90-100°C, a wind speed of 5-7m / s, and a drying time of 2 minutes. This stage uses temperature buffering and residual heat drying to stabilize the moisture content of the cardboard at 6%-8%. S5: Post-processing S51: calendering The dried cardboard is calendered on the surface by a soft calender at a calendering temperature of 90-110°C and a pressure of 60-100 kN / m. After 2-3 calendering treatments, the smoothness and glossiness of the upper and lower surfaces are improved, thereby improving printability. S52: Slitting and quality inspection S521: Use a high-precision slitting machine to cut the cardboard into specified sizes, with a cutting accuracy of ±0.5mm to ensure a neat cut without burrs; S522: Conduct comprehensive quality inspection on finished products, including appearance inspection, physical property testing and flame retardant performance retesting. Unqualified products will be handled separately.

5. The surface treatment process of flame retardant kraft paper according to claim 4, characterized in that: After step S12, if the surface of the base paper is too smooth, use sandpaper or a light grinding device to lightly grind the upper and lower surface layers to increase the surface roughness and improve the adhesion effect of the subsequent coating liquid.

6. The surface treatment process of flame retardant kraft paper according to claim 4, characterized in that: In step S21, a silicone waterproofing agent accounting for 5%-8% is also included.

7. The surface treatment process of flame retardant kraft paper according to claim 4, characterized in that: In step S21, a slip agent accounting for 2%-3% and an anti-blocking agent accounting for 2%-3% are also included.

8. The surface treatment process of flame retardant kraft paper according to claim 4, characterized in that: In step S31, the roller coating equipment is specifically a double-sided gravure roller coater; the coating liquid viscosity is controlled to 15-30s, and the coating amount is required to be 5-12g / m 2 , the cell depth is 40-80μm, and the line pressure is 20-40N / cm.

9. The surface treatment process of flame retardant kraft paper according to claim 4, characterized in that: After step S51, a surface hardening treatment is added, using UV curing resin coating with a curing energy of 200-300mJ / cm 2 , so that the surface pencil hardness reaches 3H or above.

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