Food-grade coated ivory board resistant to water seepage and oil seepage and preparation process of food-grade coated ivory board

Abstract

The invention discloses a food-grade coating white cardboard resistant to water seepage and oil seepage and a preparation process thereof, and belongs to the technical field of papermaking. According to the technical scheme, the food-grade coating white cardboard resistant to water seepage and oil seepage comprises body paper and a coating layer, the base paper consists of a surface layer, a core layer and a bottom layer, and the coating layer comprises a bottom coating and a surface coating which are sequentially coated on the front surface of the base paper; the preparation method comprises the following processes: repulping, grinding, proportioning, diluting, forming on a net, squeezing, pre-drying, surface sizing, post-drying, hard calendaring, primary coating, surface coating, front soft calendaring, curling, rewinding, slitting and warehousing. By controlling the ratio and freeness of the slurry and the moisture of the coated ivory board, the water permeation resistance and oil permeation resistance of the coated ivory board are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of papermaking, and in particular relates to a food-grade coated white cardboard resistant to water and oil penetration and a preparation process thereof. Background Art

[0002] In the papermaking industry, food-grade coated white cardboard occupies a pivotal position in the food packaging market with its excellent printing clarity and outstanding physical strength properties. From exquisite candy packaging to the outer boxes of ready-to-eat foods, this paper not only enhances the visual appeal of the product, but also ensures the safety and hygiene of the food. However, despite its wide application, the shortcomings of traditional food-grade white cardboard in the production and manufacturing process cannot be ignored.

[0003] The lack of water and oil resistance is one of the primary challenges. In food packaging, especially for liquid or semi-liquid foods (such as ice cream, milk tea and fast food soup), if the paper cannot effectively resist the penetration of water and grease, it will not only cause the packaging to deform and break, but also may affect the shelf life and flavor of the food. The complex production process involves multiple steps such as slurry proportioning, coating and drying, and each link needs to be strictly controlled to ensure the quality of the final product.

[0004] In view of the diversity of end-customer needs, especially considering that food card coated white cardboard needs to be used in ice cream boxes, milk tea cups, hot and sour noodle cups and other scenarios, these products not only require resistance to low temperature, water penetration and oil penetration, but also need to have good processing and molding performance and adaptability to adapt to the special environmental conditions in different regions, such as pH changes caused by differences in water quality between the north and the south. The existing production process cannot meet the diversified needs. Therefore, it is necessary to develop a preparation process for water- and oil-resistant coated white cardboard for food, aiming to improve the water and oil penetration resistance of food-grade coated white cardboard by optimizing the process, while reducing production costs and improving production efficiency. Summary of the invention

[0005] The invention provides a food-grade coated white cardboard resistant to water and oil penetration and a preparation process thereof, and improves the water and oil penetration resistance of the coated white cardboard by controlling the proportion and freeness of the slurry and the moisture of the coated white cardboard.

[0006] The technical solution of the present invention is: In a first aspect, a food-grade coated white cardboard resistant to water and oil penetration is disclosed, comprising a base paper and a coating layer; The base paper is composed of a surface layer, a core layer and a bottom layer, and the coating layer includes a base coating layer and a top coating layer which are sequentially coated on the front side of the base paper; The pulp in the base paper includes the following raw materials by weight: Surface layer: 25-35% bleached kraft softwood pulp NBKP, 65-75% hardwood pulp LBKP; Bottom layer: 30-40% bleached kraft softwood pulp NBKP, 60-70% hardwood pulp LBKP; Core layer: 0-10% bleached kraft softwood pulp NBKP, 0-10% hardwood pulp LBKP, 70-80% chemical mechanical pulp BCTMP, 10-20% broken paper BROKE; The freeness of pulp in base paper is: bleached kraft softwood pulp: 310-330mL, broadleaf pulp: 340-360mL, chemical mechanical pulp: 390-410mL, and broken paper 190-210mL. Reducing the freeness can increase the degree of fiber fibrillation, increase the strength of the finished paper, and reduce the amount of chemicals added. On the other hand, since the sizing agent can only cover the fiber surface and cannot penetrate into the fiber, water and oil will penetrate along the fiber on the cross section of the finished paper. As the fiber becomes finer, the contact area with the sizing agent in the pulp increases, and the cross-sectional area of ​​a single fiber decreases, which can effectively improve the water and oil penetration of the cross section of the finished paper. The pH of the surface layer, bottom layer and core layer pulp in the base paper is 4-5.5; The base coating comprises the following raw materials in parts by weight: heavy calcium carbonate GCC: 100 parts, NaOH: 0.1-0.15 parts, dispersant: 0.08-0.12 parts, fungicide: 0.008-0.012 parts, defoamer: 0.06-0.1 parts, SA latex: 11-15 parts, potassium zirconium carbonate: 0.2-0.4 parts, lubricant: 0.1-0.3 parts, sodium carboxymethyl cellulose CMC: 0.6-1 parts; The top coating includes the following raw materials by weight: heavy calcium carbonate GCC: 85-90 parts, porcelain clay: 10-15 parts, NaOH: 0.1-0.15 parts, dispersant: 0.1-0.15 parts, fungicide: 0.008-0.012 parts, defoamer: 0.07-0.11 parts, SA latex: 13-18 parts, potassium zirconium carbonate: 0.2-0.4 parts, lubricant: 0.2-0.4 parts, sodium carboxymethyl cellulose CMC: 0.4-0.6 parts. The base coating uses 100% GCC, which is low in cost; the top coating uses 85-90% GCC plus 10-15% porcelain clay to improve the whiteness, glossiness and roughness required for printing.

[0007] Preferably, the dispersant is sodium polyacrylate, the bactericide is isothiazolone, the defoamer is vegetable oil, and the lubricant is modified oxidized polyethylene polymer.

[0008] In the second aspect, a preparation process of the water- and oil-resistant food-grade coated white cardboard is disclosed, including the following processes: pulping → refining → pulping → dilution → screen forming → pressing → front drying → surface sizing → post-drying → hard calendering → bottom coating → top coating → front soft calendering → curling → rewinding → slitting → storage; specifically: 1) Pulping: Use the white water separated during the online forming process as pulping water, and use NaOH to adjust the pH to 6-7, and the pulping concentration is 5%; 2) Refining: The reflux ratio is controlled within 10%, and the freeness of the pulp is controlled to be 310-330mL for bleached kraft softwood pulp, 340-360mL for broadleaf pulp, 390-410mL for chemical mechanical pulp, and 190-210mL for broken paper, so as to reduce the excessive fiber fragmentation caused by repeated pulp refining; 3) Slurry preparation: After slurry preparation according to the proportion, add concentrated sulfuric acid to adjust the pH to 5-6, and add cationic starch with a solid content of 1-1.5% to the core layer to eliminate anionic waste and improve the slurry strength; 4) Dilution: dilute the slurry of the surface layer, bottom layer and core layer; add aluminum sulfate, cationic starch and rosin glue in front of the slurry pump of the surface layer, core layer and bottom layer in turn, fully mix them through the high-speed shear of the slurry pump, add cationic polyacrylamide retention aid CPAM after the slurry pump to improve the retention rate of slurry and chemicals, add silica sol after the pressure screen, adjust the system cationic electron demand PCD, and improve the filtration; after dilution, the pH of the surface layer, core layer and bottom layer is 4-5.5; 5) Forming on the net: The pulp enters the three-layer headbox and then enters the forming net. It is dehydrated by gravity. The core layer is first compounded with the surface layer. The front layer is sprayed with starch to increase the composite strength. After the surface layer and the core layer are compounded, they are compounded with the bottom layer. At the same time, the bottom layer is sprayed with starch to increase the composite strength. After the surface layer, core layer and bottom layer are compounded, they enter the pressing section as base paper; 6) Pressing: The base paper enters the pressing section, where it is first dehydrated by two shoe-type presses, one for one press and the other for two presses, and then goes through a three-press gloss press to eliminate mesh marks and felt marks and reduce the difference between the two sides. After the press, the base paper has a dryness of 45-48%, and then enters the pre-drying section; 7) Pre-drying: The front one-third of the drying cylinder in the pre-drying section is sprayed with Teflon coating to reduce the contact time between wet paper and cast iron drying cylinder, thus preventing the occurrence of yellow spot paper disease. The temperature of the drying cylinder in the entire pre-drying section needs to be steadily increased from 40-50℃ to 130-140℃ to improve the maturity of rosin glue and ensure the water and oil resistance after leaving the machine. The dryness of the base paper is increased from 45-48% before entering to 93-94%, which is conducive to the penetration of the surface sizing liquid; 8) Surface sizing: Enzyme-converted cassava starch is used as the surface sizing agent. The preparation method of enzyme-converted cassava starch is as follows: cassava starch is put into a dissolving tank, water is added to disperse it, and a starch slurry with a concentration of 27-29% is prepared. The starch slurry is transferred to an enzyme conversion tank, and amylase with a mass fraction of 0.017% (to absolute dry cassava starch) is added. Steam is used for heat preservation, and the temperature is kept at 84°C for 15 minutes for enzyme conversion. The cassava starch after enzyme conversion is transferred to a continuous cooker. The temperature of the continuous cooker is 130°C, and the enzyme is inactivated and gelatinized. The cooked starch is sent to a starch storage tank. The viscosity of the prepared glue is 200-220ps, and the solid content is 27-29%. During the glue application process on the machine, the forward and reverse reflux temperatures of the glue are both 72-78°C, the forward and reverse viscosities are 35-43cps and 30-40cps respectively, the forward and reverse solid contents are both 10-14%, and the front glue amount is 2.5-3.5g / m 2 , the amount of glue applied on the back side is 1.5-2.5g / m 2 ; 9) Post-drying: temperature 100-120℃, to ensure that the glue can be fully matured, increase the surface strength of the paper, and reduce the occurrence of hair loss and powder loss during printing; 10) Hard calendering: temperature 170-190℃, pressure 20-30kN / m, the paper surface is modified by high temperature and high pressure to reduce the roughness of the base paper; 11) Primer coating: Use a hard scraper, coating amount 12-14g / m 2 ; 12) Top coating: Use a soft scraper, coating amount 13-15g / m 2 ; 13) Front soft calendering: temperature 140-160℃, pressure 20-30kN / m, finishing the coated surface, improving gloss and smoothness, and reducing roughness; 14) Curling, rewinding, slitting and warehousing.

[0009] Preferably, in step 3), the slurry concentrations of the back layer and the bottom layer are both 3-3.4%, and the slurry concentration of the core layer is 3.5-3.9%.

[0010] Preferably, in step 4), the concentrations of the diluted back layer and bottom layer slurry are both 0.25-0.35%, and the concentration of the core layer slurry is 0.8-1.2%.

[0011] Preferably, in step 3), the amount of cationic starch added to the core layer is 10-15kg / t pulp, in step 4), the amounts of aluminum sulfate added to the surface layer, core layer and bottom layer are 15-30kg / t pulp, 15-30kg / t pulp and 15-30kg / t pulp respectively, and the amount of cationic starch added to the core layer is 10-20kg / t pulp; the amounts of rosin glue added to the surface layer, core layer and bottom layer are 15-30kg / t pulp, 15-30kg / t pulp and 15-30kg / t pulp respectively; the amounts of cationic polyacrylamide CPAM added to the surface layer, core layer and bottom layer are 0.1-0.3kg / t pulp, 0.1-0.3kg / t pulp and 0.1-0.3kg / t pulp respectively; the amount of silica sol added to the core layer is 5-8kg / t pulp.

[0012] Cationic starch increases the bonding strength between fibers and can reduce anionic waste; aluminum sulfate acts as a precipitant and adheres to the fiber surface, which can also reduce pH and anionic waste. The rosin glue added later reacts with aluminum sulfate and adheres to the fiber surface for in-slurry sizing. CPAM, due to its positive charge and long molecular chain, can combine with fibers and fine fibers to improve fiber retention and chemical retention. Silica sol carries a negative charge and neutralizes the charge with the positively charged chemicals attached to the fiber surface, thereby increasing retention and improving water filtration in one step.

[0013] Preferably, in step 5), the amount of starch sprayed on the surface layer is 2-5 kg / t slurry, and the amount of starch sprayed on the bottom layer is 1-2.5 kg / t slurry.

[0014] Preferably, in step 6), the shoe-type pressing pressures in the first and second presses are 775-825 kN / m and 775-825 kN / m respectively, and the three-press gloss press uses a rubber-coated ceramic roller, and the pressure of the three presses is 20-30 kN / m.

[0015] Preferably, in step 7), the drying area is divided into nine groups of drying cylinders, and the first group of drying cylinders, the second group of drying cylinders and the third group of drying cylinders are sprayed with Teflon coating, the drying temperature of the first group of drying cylinders is 40-50°C, and the ventilation humidity of the bag area is ≤120g / kgDA; the drying temperature of the second group of drying cylinders is 50-70°C, and the ventilation humidity of the bag area is ≤120g / kgDA; the drying temperature of the third group of drying cylinders is 70-90°C, and the ventilation humidity of the bag area is ≤120g / kgDA; the drying temperature of the fourth group of drying cylinders is 90-100°C, and the ventilation humidity of the bag area is ≤150g / kgDA; the fifth group The drying temperature of the drying cylinder is 90-100℃, and the ventilation humidity in the bag area is ≤150g / kgDA; the drying temperature of the sixth group of drying cylinders is 100-115℃, and the ventilation humidity in the bag area is ≤150g / kgDA; the drying temperature of the seventh group of drying cylinders is 110-120℃; the drying temperature of the eighth group of drying cylinders is 120-130℃; the drying temperature of the ninth group of drying cylinders is 130-140℃. It should be noted that the base paper is basically in a dry state when dried by the seventh, eighth and ninth groups of drying cylinders, and there is no need to control the ventilation humidity in the bag areas of these three groups.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention improves the water and oil resistance of the finished coated white cardboard by controlling the pulp ratio and freeness in the base paper, the pulp pH, adding cationic starch and rosin glue during the dilution process, and controlling the moisture content of the discharged white cardboard in combination with other preparation processes.

[0017] 2. The present invention optimizes the pulp ratio of the surface layer, bottom layer and core layer, and combines the rational use of chemical mechanical pulp (BCTMP) and broken paper (BROKE), which not only improves the utilization rate of raw materials, but also reduces production costs. At the same time, precise pulp pH adjustment and the use of chemicals ensure the stability of pulp performance and further reduce losses in the production process.

[0018] 3. The present invention uses enzyme-converted cassava starch as a surface sizing agent, which reduces the amount of chemicals used and reduces the impact on the environment. At the same time, by optimizing the drying process, the energy utilization efficiency is improved and the energy consumption is reduced.

[0019] 4. The present invention significantly enhances the water and oil resistance of paper through a carefully designed coating formula, especially a reasonable ratio of heavy calcium carbonate GCC and porcelain clay used in the primer and topcoat, and high-efficiency latex as an adhesive, meeting the strict requirements of food packaging for high water and oil resistance.

[0020] 5. The present invention significantly improves the gloss, smoothness and strength of paper and reduces roughness through precise coating amount and coating process, as well as high temperature and high pressure hard calendering and soft calendering treatment, making the paper more suitable for high-precision printing and packaging needs.

[0021] 6. The present invention does not require a lot of changes to the paper machine hardware. It can achieve the production of high-quality products by optimizing the process technology, such as fine control of pulping, refining, pulping, dilution, screen forming, pressing, drying, coating and calendering. This not only simplifies the production process, but also improves production efficiency and shortens the production cycle. DETAILED DESCRIPTION

[0022] In order to enable persons skilled in the art to better understand the technical solution in the present invention, the technical solution will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0023] The dispersant used in the examples and comparative examples is sodium polyacrylate (Changshu Juhe Chemical Co., Ltd.), the fungicide is isothiazolone (Shanghai Emerson Chemical Co., Ltd.), the defoamer is vegetable oil (Shanghai Emerson Chemical Co., Ltd.), and the lubricant is modified oxidized polyethylene polymer (Jiuzhou Biotechnology (Suzhou) Co., Ltd.).

[0024] The preparation method of enzyme-converted cassava starch is as follows: cassava starch is put into a dissolution tank, water is added to disperse it, and a starch slurry with a concentration of 28% is prepared. The starch slurry is transferred to an enzyme conversion tank, and amylase with a mass fraction of 0.017% (to absolute dry cassava starch) is added, and steam is used for insulation. The temperature is kept at 84°C for 15 minutes for enzyme conversion. The cassava starch after enzyme conversion is transferred to a continuous cooker. The reaction temperature of the continuous cooker is 130°C. The enzyme is inactivated and gelatinized. The starch after cooking is sent to a starch storage tank for standby use.

[0025] Example 1 In this embodiment, the basis weight is 260gsm, and the basis weights of the surface layer, core layer and bottom layer are 32gsm, 139gsm and 36gsm respectively.

[0026] Water- and oil-resistant food-grade coated white cardboard, including base paper and coating layer; The base paper is composed of a surface layer, a core layer and a bottom layer, and the coating layer includes a base coating layer and a top coating layer which are sequentially coated on the front side of the base paper; The pulp in the base paper includes the following raw materials by weight: Surface layer: 30% bleached kraft softwood pulp, 70% LBKP hardwood pulp; Bottom layer: 35% bleached kraft softwood pulp NBKP, 65% hardwood pulp LBKP; Core layer: 5% bleached kraft softwood pulp NBKP, 10% hardwood pulp LBKP, 70% chemical mechanical pulp BCTMP, 15% broken paper BROKE; The freeness of the pulp in the base paper after grinding is: bleached kraft softwood pulp NBKP: 320mL, hardwood pulp LBKP: 350mL, chemical mechanical pulp BCTMP: 400mL, broken paper 200mL; The base coating includes the following raw materials in parts by weight: heavy calcium carbonate GCC60 (60% of particle size ≤2μm): 100 parts, NaOH: 0.12 parts, dispersant: 0.1 parts, bactericide: 0.01 parts, defoamer: 0.08 parts, SA latex: 13 parts, potassium zirconium carbonate: 0.3 parts, lubricant: 0.2 parts, sodium carboxymethyl cellulose CMC: 0.7 parts; the base coating has a solid content of 67.5% and a viscosity of 850cps; The top coating coating includes the following raw materials in parts by weight: heavy calcium carbonate GCC95 (95% of particle size ≤2μm): 87 parts, kaolin: 13 parts, NaOH: 0.12 parts, dispersant: 0.12 parts, bactericide: 0.01 parts, defoamer: 0.09 parts, SA latex: 15 parts, potassium zirconium carbonate: 0.3 parts, lubricant: 0.25 parts, sodium carboxymethyl cellulose CMC: 0.5 parts; the solid content of the top coating coating is 66% and the viscosity is 1000cps.

[0027] The preparation process of the water- and oil-resistant food-grade coated white cardboard includes the following processes: pulping → refining → pulping → dilution → screen forming → pressing → front drying → surface sizing → post drying → hard calendering → bottom coating → top coating → front soft calendering → curling → rewinding → slitting → storage; specifically: 1) Pulping: Use the white water separated during the screen forming as pulping water, and use NaOH to adjust the pH to 6.5, and the pulping concentration is 5%; 2) Refining: The reflux ratio is controlled at 8%, and the freeness of the pulp is controlled to be 320 mL for bleached kraft softwood pulp NBKP, 350 mL for broadleaf pulp LBKP, 400 mL for chemical mechanical pulp BCTMP, and 200 mL for broken paper, so as to reduce the excessive fiber fragmentation caused by repeated pulp refining; 3) Slurry preparation: After slurry preparation according to the proportion, add concentrated sulfuric acid to adjust the pH to 5.5, and add cationic starch with a solid content of 1.2% and an addition amount of 15kg / t slurry to eliminate anionic waste and improve slurry strength. After slurry preparation, the slurry concentration of the upper and lower layers is 3.2%, and the slurry concentration of the core layer is 3.7%; 4) Dilution: dilute the slurry of the surface layer, bottom layer and core layer. After dilution, the slurry concentration of the surface layer and bottom layer is 0.3%, and the slurry concentration of the core layer is 1%. Before the slurry pump, add 18kg / t slurry, 30kg / t slurry and 18kg / t slurry aluminum sulfate in turn before the slurry pump of the surface layer, core layer and bottom layer, add 20kg / t slurry cationic starch in the core layer, add 17kg / t slurry, 17kg / t slurry and 17kg / t slurry rosin glue in turn to the surface layer, core layer and bottom layer, and fully mix them after the high-speed shear of the slurry pump. After the slurry pump, add 0.3kg / t slurry, 0.3kg / t slurry and 0.15kg / t slurry of CPAM in turn to the layer, core layer and bottom layer to improve the retention rate of slurry and chemicals. After the pressure screen, add 8kg / t slurry of silica sol to the core layer, adjust the system PCD, and improve water filtration; After dilution, the pH of the rear layer, core layer and bottom layer were 4.9, 5.2 and 4.8 respectively. It should be noted that the pH here was obtained by measuring the white water under the net, and the PCD was -10ueq / L, -25ueq / L and -8ueq / L respectively; 5) Forming on the net: The pulp enters the three-layer headbox and then enters the forming net. It is dehydrated by gravity. The core layer is first compounded with the surface layer. The front layer uses 3.64kg / t of spray starch to increase the composite strength. After the surface layer and the core layer are compounded, they are compounded with the bottom layer. At the same time, the bottom layer uses 1.67kg / t of spray starch to increase the composite strength. After the surface layer, core layer and bottom layer are compounded, they enter the pressing section as base paper; 6) Pressing: The base paper enters the pressing section, and first passes through two shoe presses for dehydration, and then passes through a three-press gloss press. The shoe press pressures in the first and second presses are 810kN / m and 800kN / m respectively, and the three-press gloss press uses a rubber-coated ceramic roller, and the pressure of the three-press is 25kN / m. Eliminate the net marks and felt marks, reduce the difference between the two sides, and the base paper dryness is 47% after passing through the pressing section, and then enter the front drying; 7) Pre-drying: The drying area is divided into nine groups of drying cylinders. The temperature of the drying cylinders in the entire pre-drying section needs to be steadily increased from 40-50°C to 130-140°C. The dryness of the base paper is increased from 47% before entering to 93%. The first, second and third groups of drying cylinders are sprayed with Teflon coating. The process parameters of the drying area are shown in Table 1. 8) Surface sizing: Enzyme-converted cassava starch was used as the surface sizing agent. The viscosity of the prepared glue solution was 208cps, and the solid content was 28.1%. During the glue application process, the forward and reverse reflux temperatures of the glue solution were 78°C and 75°C, respectively. The forward and reverse viscosities were 43cps and 38cps, respectively. The forward and reverse solid contents were 13.5% and 11.5%, respectively. The front sizing amount was 3g / m 2 , the amount of glue applied on the back side is 2g / m 2 ; 9) Post-drying: The temperature is 110℃, to ensure that the glue can be fully matured, increase the surface strength of the paper, and reduce the occurrence of hair loss and powder loss during printing; 10) Hard calendering: temperature 180℃, pressure 22kN / m, to modify the paper surface through high temperature and high pressure to reduce the roughness of the base paper; 11) Primer coating: Use a hard scraper, coating amount 13g / m 2 ; 12) Top coating: Use a soft scraper, coating amount 14g / m 2 ; 13) Front soft calendering: temperature 150℃, pressure 25kN / m, finishing the coated surface, improving gloss and smoothness, and reducing roughness; 14) Curling, rewinding, slitting and warehousing.

[0028] Example 2 In this embodiment, the basis weight is 260gsm, and the basis weights of the surface layer, core layer and bottom layer are 32gsm, 139gsm and 36gsm respectively.

[0029] Water- and oil-resistant food-grade coated white cardboard, including base paper and coating layer; The base paper is composed of a surface layer, a core layer and a bottom layer, and the coating layer includes a base coating layer and a top coating layer which are sequentially coated on the front side of the base paper; The pulp in the base paper includes the following raw materials by weight: Surface layer: 25% bleached kraft softwood pulp, 75% LBKP hardwood pulp; Bottom layer: 30% bleached kraft softwood pulp NBKP, 70% hardwood pulp LBKP; Core layer: 80% chemical mechanical pulp BCTMP, 20% broken paper BROKE; The freeness of the pulp after grinding in the base paper is: bleached kraft softwood pulp NBKP: 310mL, hardwood pulp LBKP: 340mL, chemical mechanical pulp BCTMP: 390mL, and broken paper 190mL; The base coating includes the following raw materials in parts by weight: heavy calcium carbonate GCC60 (60% of particle size ≤2μm): 100 parts, NaOH: 0.1 parts, dispersant: 0.08 parts, bactericide: 0.008 parts, defoamer: 0.06 parts, SA latex: 11 parts, potassium zirconium carbonate: 0.2 parts, lubricant: 0.1 parts, sodium carboxymethyl cellulose CMC: 0.6 parts; the base coating has a solid content of 67.5% and a viscosity of 850cps; The top coating coating includes the following raw materials in parts by weight: heavy calcium carbonate GCC95 (95% of particle size ≤2μm): 90 parts, porcelain clay: 10 parts, NaOH: 0.1 parts, dispersant: 0.1 parts, bactericide: 0.008 parts, defoamer: 0.07 parts, SA latex: 13 parts, potassium zirconium carbonate: 0.2 parts, lubricant: 0.2 parts, sodium carboxymethyl cellulose CMC: 0.4 parts; the solid content of the top coating coating is 66% and the viscosity is 1000cps.

[0030] The preparation process of the water- and oil-resistant food-grade coated white cardboard includes the following processes: pulping → refining → pulping → dilution → screen forming → pressing → front drying → surface sizing → post drying → hard calendering → bottom coating → top coating → front soft calendering → curling → rewinding → slitting → storage; specifically: 1) Pulping: Use the white water separated during the screen forming as pulping water, and use NaOH to adjust the pH to 6, and the pulping concentration is 5%; 2) Refining: The reflux ratio is controlled at 8%, and the freeness of the pulp is controlled to be 310 mL for bleached kraft softwood pulp NBKP, 340 mL for broadleaf pulp LBKP, 390 mL for chemical mechanical pulp BCTMP, and 190 mL for broken paper, so as to reduce the excessive fiber fragmentation caused by repeated pulp refining; 3) Slurry preparation: After slurry preparation according to the proportion, add concentrated sulfuric acid to adjust the pH to 5, and add cationic starch with a solid content of 1% and an addition amount of 10kg / t slurry to eliminate anionic waste and improve slurry strength. After slurry preparation, the slurry concentration of the upper and lower layers is 3%, and the slurry concentration of the core layer is 3.5%; 4) Dilution: dilute the slurry of the surface layer, bottom layer and core layer. After dilution, the slurry concentration of the upper and lower layers is 0.25%, and the slurry concentration of the core layer is 0.8%. Before the slurry pump, add 15kg / t slurry, 15kg / t slurry and 15kg / t slurry aluminum sulfate to the surface layer, core layer and bottom layer in turn, add 10kg / t slurry cationic starch to the core layer, add 15kg / t slurry, 15kg / t slurry and 15kg / t slurry rosin glue to the surface layer, core layer and bottom layer in turn, and fully mix them after the high-speed shear of the slurry pump. After the slurry pump, add 0.1kg / t slurry, 0.1kg / t slurry and 0.1kg / t slurry of CPAM to the upper layer, core layer and bottom layer in turn to improve the retention rate of slurry and chemicals. After the pressure screen, add 5kg / t slurry of silica sol to the core layer to adjust the PCD system and improve water filtration. After dilution, the pH of the rear layer, core layer and bottom layer were 5.3, 5.5 and 5.2 respectively. It should be noted that the pH here was obtained by measuring the white water under the net, and the PCD was -240ueq / L, -530ueq / L and -265ueq / L respectively; 5) Forming on the net: The pulp enters the three-layer headbox and then enters the forming net. It is dehydrated by gravity. The core layer is first compounded with the surface layer. The front layer uses 2kg / t of spray starch to increase the composite strength. After the surface layer and the core layer are compounded, they are compounded with the bottom layer. At the same time, the bottom layer uses 1kg / t of spray starch to increase the composite strength. After the surface layer, core layer and bottom layer are compounded, they enter the pressing section as base paper; 6) Pressing: The base paper enters the pressing section, and first passes through the first and second shoe presses for dehydration, and then passes through the third press for gloss pressing. The shoe press pressures in the first and second presses are 775kN / m and 775kN / m respectively, and the third press uses a rubber-coated ceramic roller, and the pressure of the third press is 20kN / m. Eliminate the net marks and felt marks, reduce the difference between the two sides, and the base paper dryness is 45% after passing through the pressing section, and then enter the front drying; 7) Pre-drying: The drying area is divided into nine groups of drying cylinders. The temperature of the drying cylinders in the entire pre-drying section needs to be steadily increased from 40-50°C to 130-140°C, and the dryness of the base paper is increased from 45% before entering to 93%. The process parameters of the drying area are shown in Table 1; 8) Surface sizing: Enzyme-converted cassava starch was used as the surface sizing agent. The viscosity of the prepared glue solution was 200cps, and the solid content was 22%. During the glue application process, the forward and reverse reflux temperatures of the glue solution were 72°C and 72°C, respectively. The forward and reverse viscosities were 35cps and 30cps, respectively. The forward and reverse solid contents were 10% and 10%, respectively. The front glue application amount was 2.5g / m 2 , the amount of glue applied on the back side is 1.5g / m 2 ; 9) Post-drying: temperature 100℃, to ensure that the glue can be fully matured, increase the surface strength of the paper, and reduce the occurrence of hair loss and powder loss during printing; 10) Hard calendering: temperature 170℃, pressure 20kN / m, to modify the paper surface through high temperature and high pressure to reduce the roughness of the base paper; 11) Primer coating: Use a hard scraper, coating amount 12g / m 2 ; 12) Top coating: Use a soft scraper, coating amount 13g / m 2 ; 13) Front soft calendering: temperature 140℃, pressure 20kN / m, finishing the coated surface, improving gloss and smoothness, and reducing roughness; 14) Curling, rewinding, slitting and warehousing.

[0031] Example 3 In this embodiment, the basis weight is 260gsm, and the basis weights of the surface layer, core layer and bottom layer are 32gsm, 139gsm and 36gsm respectively.

[0032] Water- and oil-resistant food-grade coated white cardboard, including base paper and coating layer; The base paper is composed of a surface layer, a core layer and a bottom layer, and the coating layer includes a base coating layer and a top coating layer which are sequentially coated on the front side of the base paper; The pulp in the base paper includes the following raw materials by weight: Surface layer: 35% bleached kraft softwood pulp, 25% LBKP hardwood pulp; Bottom layer: 40% bleached kraft softwood pulp NBKP, 60% hardwood pulp LBKP; Core layer: 10% bleached kraft softwood pulp NBKP, 5% broadleaf pulp LBKP, 75% chemical mechanical pulp BCTMP, 10% broken paper BROKE; The freeness of the pulp after grinding in the base paper is: bleached kraft softwood pulp NBKP: 330mL, hardwood pulp LBKP: 360mL, chemical mechanical pulp BCTMP: 410mL, and broken paper 210mL; The base coating includes the following raw materials in parts by weight: heavy calcium carbonate GCC60 (60% of particle size ≤2μm): 100 parts, NaOH: 0.15 parts, dispersant: 0.12 parts, bactericide: 0.012 parts, defoamer: 0.1 parts, SA latex: 15 parts, potassium zirconium carbonate: 0.4 parts, lubricant: 0.3 parts, sodium carboxymethyl cellulose CMC: 1 part; the base coating has a solid content of 67.5% and a viscosity of 850cps; The top coating coating includes the following raw materials in parts by weight: heavy calcium carbonate GCC95 (95% of particle size ≤2μm): 85 parts, kaolin: 15 parts, NaOH: 0.15 parts, dispersant: 0.15 parts, bactericide: 0.012 parts, defoamer: 0.11 parts, SA latex: 18 parts, potassium zirconium carbonate: 0.4 parts, lubricant: 0.4 parts, sodium carboxymethyl cellulose CMC: 0.6 parts; the solid content of the top coating coating is 66% and the viscosity is 1000cps.

[0033] The preparation process of the water- and oil-resistant food-grade coated white cardboard includes the following processes: pulping → refining → pulping → dilution → screen forming → pressing → front drying → surface sizing → post drying → hard calendering → bottom coating → top coating → front soft calendering → curling → rewinding → slitting → storage; specifically: 1) Pulping: Use the white water separated during the screen forming as pulping water, and use NaOH to adjust the pH to 7, and the pulping concentration is 5%; 2) Refining: The reflux ratio is controlled at 8%, and the freeness of the pulp is controlled to be 330 mL for bleached kraft softwood pulp NBKP, 360 mL for broadleaf pulp LBKP, 410 mL for chemical mechanical pulp BCTMP, and 210 mL for broken paper, so as to reduce the excessive fiber fragmentation caused by repeated pulp refining; 3) Slurry preparation: After slurry preparation according to the proportion, add concentrated sulfuric acid to adjust the pH to 6, and add cationic starch with a solid content of 1.5% and an addition amount of 14kg / t slurry to eliminate anionic waste and improve slurry strength. After slurry preparation, the slurry concentration of the upper and lower layers is 3.4%, and the slurry concentration of the core layer is 3.9%; 4) Dilution: dilute the slurry of the surface layer, bottom layer and core layer. After dilution, the slurry concentration of the surface layer and bottom layer is 0.35%, and the slurry concentration of the core layer is 1.2%. Before the slurry pump, add 30kg / t slurry, 30kg / t slurry and 30kg / t slurry aluminum sulfate to the surface layer, core layer and bottom layer in turn, add 18kg / t slurry cationic starch to the core layer, add 30kg / t slurry, 30kg / t slurry and 30kg / t slurry rosin glue to the surface layer, core layer and bottom layer in turn, and fully mix them after the high-speed shear of the slurry pump. After the slurry pump, add 0.15kg / t slurry, 0.15kg / t slurry and 0.3kg / t slurry of CPAM to the layer, core layer and bottom layer in turn to improve the retention rate of slurry and chemicals. After the pressure screen, add 7kg / t slurry of silica sol to the core layer, adjust the system PCD, and improve water filtration; After dilution, the pH of the rear layer, core layer and bottom layer were 4, 4.4 and 4 respectively. It should be noted that the pH here was obtained by measuring the white water under the net, and the PCDs were -301ueq / L, -285ueq / L and -312ueq / L respectively; 5) Forming on the net: The pulp enters the three-layer headbox and then enters the forming net. It is dehydrated by gravity. The core layer is first compounded with the surface layer. The front layer uses 5kg / t of spray starch to increase the composite strength. After the surface layer and the core layer are compounded, they are compounded with the bottom layer. At the same time, the bottom layer uses 2.5kg / t of spray starch to increase the composite strength. After the surface layer, core layer and bottom layer are compounded, they enter the pressing section as base paper; 6) Pressing: The base paper enters the pressing section, and first passes through the first and second shoe presses for dehydration, and then passes through the third press for gloss pressing. The shoe press pressures in the first and second presses are 825kN / m and 825kN / m respectively, and the third press uses rubber-coated ceramic rollers, and the pressure of the third press is 30kN / m. Eliminate the net marks and felt marks, reduce the difference between the two sides, and the base paper dryness is 48% after passing through the pressing section, and then enter the front drying; 7) Pre-drying: The drying area is divided into nine groups of drying cylinders. The temperature of the drying cylinders in the entire pre-drying section needs to be steadily increased from 40-50°C to 130-140°C, and the dryness of the base paper is increased from 48% before entering to 94%. The process parameters of the drying area are shown in Table 1; 8) Surface sizing: Enzyme-converted cassava starch was used as the surface sizing agent. The viscosity of the prepared glue solution was 220cps, and the solid content was 29%. During the glue application process, the forward and reverse reflux temperatures of the glue solution were 78°C and 75°C, respectively. The forward and reverse viscosities were 40cps and 40cps, respectively. The solid contents of the forward and reverse were 14% and 14%, respectively. The front sizing amount was 3.5g / m 2 , the amount of glue applied on the back side is 2.5g / m 2 ; 9) Post-drying: The temperature is 120℃, to ensure that the glue can be fully matured, increase the surface strength of the paper, and reduce the occurrence of hair loss and powder loss during printing; 10) Hard calendering: temperature 190℃, pressure 30kN / m, to modify the paper surface through high temperature and high pressure to reduce the roughness of the base paper; 11) Primer coating: Use a hard scraper, coating amount 14g / m 2 ; 12) Top coating: Use a soft scraper, coating amount 15g / m 2 ; 13) Front soft calendering: temperature 160℃, pressure 30kN / m, finishing the coated surface, improving gloss and smoothness, and reducing roughness; 14) Curling, rewinding, slitting and warehousing.

[0034] Comparative Example 1 The difference from Example 1 is that the surface layer of the base paper pulp is: 25% bleached sulfate coniferous pulp, 75% broadleaf pulp LBKP; the bottom layer is: 30% bleached sulfate coniferous pulp NBKP, 70% broadleaf pulp LBKP, and the rest of the preparation process is the same as Example 1.

[0035] Comparative Example 2 The difference from Example 1 is that in step 3), during the slurry preparation process, concentrated sulfuric acid is not added to adjust the pH value; in step 4), during the dilution process, the amounts of rosin glue added to the surface layer, core layer and bottom layer are 28kg / t slurry, 29kg / t slurry and 28kg / t slurry, respectively; 30kg / t slurry, 55kg / t slurry and 30kg / t slurry of aluminum sulfate are added to the surface layer, core layer and bottom layer before the slurry pump, and the pH value of the white water under the core layer net after dilution is 6.9. The rest of the preparation process is the same as that of Example 1.

[0036] Comparative Example 3 Different from Example 1, in step 7), the temperature of the drying cylinder in the front drying section is increased from 40-50°C to 90-100°C, and the dryness is increased to 88%. The process parameters of the drying zone are shown in Table 1.

[0037] The process parameter settings of the baking zone in Examples 1-3 and Comparative Example 3 are shown in Table 1.

[0038] Table 1 Process parameters of the baking zone in Examples 1-3 and Comparative Example 3

[0039] Comparative Example 4 Different from Example 1, in this comparative example, after refining in step 2), the freeness of the pulps is 370 mL for bleached kraft conifer pulp NBKP, 380 mL for broadleaf pulp LBKP, 450 mL for chemical mechanical pulp BCTMP, and 300 mL for broken paper.

[0040] The performance of the paper prepared in the above examples and comparative examples was tested. The test methods and results are shown in Table 2.

[0041] Table 2 Test results and test methods of paper properties prepared in Examples and Comparative Examples

[0042] Compared with Example 1, the values ​​of Example 2 are all smaller than those of Example 1. As can be seen from Table 2, when the thickness increases by 10 μm, the CD folding endurance decreases by 43 times, the MD folding endurance decreases by 34 times, and the interlayer bonding strength decreases by 24 J / m. 2 , the front IGT is reduced by 0.6m / s, the back IGT is reduced by 0.9m / s, the front TV 16 is reduced by 2 times, the back TV 16 is reduced by 2.5 times, the CD elongation is reduced by 0.86, the 95℃ water edge penetration is increased by 3.1mm, and the 95℃ palm oil edge penetration is increased by 2.0mm. Compared with Example 1, the folding resistance, interlayer, IGT and TV 16 of Example 2 are reduced, and the water and palm oil edge penetration are increased. The product can meet the general daily use scenarios.

[0043] Compared with Example 3, Example 1 has values ​​greater than those of Example 1. As can be seen from Table 2, CD folding resistance increases by 43 times, MD folding resistance increases by 46 times, interlayer bonding strength increases by 104 J / ㎡, gloss decreases by 4.5, front whiteness increases by 0.75, front IGT increases by 0.5 m / s, front TV 16 increases by 1 time, back TV 16 increases by 1 time, CD elongation increases by 0.89, 95°C water edge penetration decreases by 1.1 mm, and 95°C palm oil edge penetration decreases by 1 mm. The folding resistance, interlayer, and CD elongation of Example 3 increase, which can meet more complex molding processes. IGT and TV 16 increase, which can adapt to faster printing speeds. Water and palm oil edge penetration decreases, which can meet the needs of longer liquid storage. The product prepared according to Example 3 can meet the use requirements in various extreme situations.

[0044] Example 1 is compared with Comparative Example 1: the NBKP ratio of the surface layer and the bottom layer of Example 1 is 5 points higher, and the other processes remain the same. It can be seen from Table 2 that the CD folding endurance of Example 1 is 31 times higher than that of Comparative Example 1, the MD folding endurance is 30 times higher, and the CD elongation is 0.98 higher. The other indicators are basically the same, which shows that the water- and oil-resistant food-grade coated white cardboard prepared in Example 1 has a higher yield rate and a lower burst rate in the back-end cupping and bowl-forming process.

[0045] Compared with Comparative Example 2, Example 1 has three layers of rosin glue, namely, the surface layer, the core layer, and the bottom layer, which are 11 / 12 / 11 kg / t pulp lower, respectively, and aluminum sulfate lower than 12 / 25 / 12 kg / t pulp. The pH of the white water under the core layer is 1.7 lower. Other processes remain the same. It can be seen from Table 2 that the edge penetration data of 95°C water and 95°C palm oil in Example 1 are lower than those in Comparative Example 2. This is because in Example 1, concentrated sulfuric acid is added to adjust the pH of the slurry to 4-5.5 after the core layer is prepared, and aluminum sulfate is added before the slurry pump to match the rosin glue to achieve the optimal pH range for rosin sizing and obtain the best edge penetration data. Comparative Example 2 failed to add concentrated sulfuric acid in advance to adjust the pH, and a large amount of aluminum sulfate and rosin glue were added before the slurry pump, but the edge penetration data obtained was not very ideal. This is because aluminum sulfate decomposes and dissolves in water when pH>6, and the amount of aluminum sulfate that reacts and fixes with the fiber is reduced, resulting in poor sizing effect, increased cutting costs, and greatly increased chances of water penetration and oil penetration during use.

[0046] Compared with Example 3, Example 1 has a lower drying cylinder temperature, a lower maximum temperature of about 40°C, a lower dryness of 5 points, and a lower interlayer bonding strength of 11 J / m 2 , the front IGT decreased by 0.5m / s, the back IGT decreased by 0.9m / s, the front TV16 decreased by 2 times, the back TV 16 decreased by 2 times, this is due to the low paper dryness, high water content, less glue penetration, resulting in reduced surface strength and interlayer bonding strength; 95℃ water edge penetration increased by 4.1mm, 95℃ palm oil edge penetration increased by 7mm, this is due to the low drying cylinder temperature, the rosin glue cannot be fully matured, resulting in poor sizing effect in the pulp. These will increase the probability of printing bubbles and water and oil penetration during the use of the product.

[0047] Compared with Example 4, Example 1 shows that the freeness of bleached kraft softwood pulp in Example 4 is 50 mL higher, the freeness of broadleaf pulp is 30 mL higher, the freeness of chemical mechanical pulp is 50 mL higher, and the freeness of broken paper is 100 mL higher. The front PPS of Example 4 is 0.44 μm higher, the smoothness is 180S lower, the CD folding endurance is 43 times lower, the MD folding endurance is 35 times lower, and the interlayer bonding strength is 39 J / m lower. 2, TV16 on the front side is 0.5 times higher, TV16 on the back side is 0.5 times higher, CD elongation is 0.93 lower, 95℃ water edge penetration is 3.9mm higher, and 95℃ palm oil edge penetration is 6mm higher. As the freeness increases, the fiber fibrillation degree is low, resulting in low fiber strength, reduced folding resistance, interlayer bonding strength, and elongation; as the freeness increases, the fiber coarseness is high, the paper surface has many pits, and the flatness is poor, which affects the coating quality, increases PPS, and reduces smoothness; as the freeness increases, the fiber coarseness is high, the cross-sectional area is large, liquid penetration increases, and water and oil resistance deteriorates.

Claims

1. Food grade coated white cardboard resistant to water and oil penetration, characterized by: Including base paper and coating layer; The base paper is composed of a surface layer, a core layer and a bottom layer, and the coating layer includes a base coating layer and a top coating layer which are sequentially coated on the front side of the base paper; The pulp in the base paper includes the following raw materials by weight: Surface layer: 25-35% bleached kraft softwood pulp, 65-75% hardwood pulp; Bottom layer: 30-40% bleached kraft softwood pulp, 60-70% hardwood pulp; Core layer: 0-10% bleached kraft softwood pulp, 0-10% hardwood pulp, 70-80% chemical mechanical pulp, 10-20% broke paper; The freeness of pulp in base paper is: bleached kraft softwood pulp: 310-330mL, broadleaf pulp: 340-360mL, chemical mechanical pulp: 390-410mL, and broken paper 190-210mL; The pH of the surface layer, bottom layer and core layer pulp in the base paper is 4-5.5; The primer coating comprises the following raw materials in parts by weight: heavy calcium carbonate: 100 parts, NaOH: 0.1-0.15 parts, dispersant: 0.08-0.12 parts, fungicide: 0.008-0.012 parts, defoamer: 0.06-0.1 parts, SA latex: 11-15 parts, potassium zirconium carbonate: 0.2-0.4 parts, lubricant: 0.1-0.3 parts, sodium carboxymethyl cellulose: 0.6-1 parts; The top coating paint includes the following raw materials in parts by weight: heavy calcium carbonate: 85-90 parts, china clay: 10-15 parts, NaOH: 0.1-0.15 parts, dispersant: 0.1-0.15 parts, bactericide: 0.008-0.012 parts, defoamer: 0.07-0.11 parts, SA latex: 13-18 parts, potassium zirconium carbonate: 0.2-0.4 parts, lubricant: 0.2-0.4 parts, sodium carboxymethyl cellulose: 0.4-0.6 parts.

2. The water- and oil-resistant food-grade coated white cardboard according to claim 1, characterized in that: The dispersant is sodium polyacrylate, the bactericide is isothiazolone, the defoamer is vegetable oil, and the lubricant is modified oxidized polyethylene polymer.

3. A process for preparing the water- and oil-resistant food-grade coated white cardboard as claimed in claim 1, characterized in that: It includes the following processes: pulping → refining → pulp preparation → dilution → screen forming → pressing → front drying → surface sizing → post drying → hard calendering → bottom coating → top coating → front soft calendering → curling → rewinding → slitting → storage; specifically: 1) Pulping: Use the white water separated during the online forming process as pulping water, and use NaOH to adjust the pH to 6-7, and the pulping concentration is 5%; 2) Refining: The reflux ratio is controlled within 10%, and the freeness of the pulp is controlled to be 310-330mL for bleached kraft softwood pulp, 340-360mL for broadleaf pulp, 390-410mL for chemical mechanical pulp, and 190-210mL for broken paper; 3) Slurry preparation: After slurry preparation, add concentrated sulfuric acid to adjust the pH to 5-6, and add cationic starch with a solid content of 1-1.5% to the core layer; 4) Dilution: dilute the slurry of the surface layer, the bottom layer and the core layer; add aluminum sulfate, cationic starch and rosin glue in front of the slurry pumps of the surface layer, the core layer and the bottom layer in turn, add cationic polyacrylamide retention aid after the slurry pump, add silica sol after the pressure screen, adjust the system PCD, and improve the filtration; after dilution, the pH of the surface layer, the core layer and the bottom layer is 4-5.5; 5) Forming on the net: The pulp enters the three-layer headbox and then enters the forming net. It is dehydrated by gravity. The core layer is first compounded with the surface layer. The front layer is sprayed with starch. After the surface layer and the core layer are compounded, they are compounded with the bottom layer. At the same time, the bottom layer is sprayed with starch. After the surface layer, the core layer and the bottom layer are compounded, they enter the pressing section as base paper. 6) Pressing: The base paper enters the pressing section, firstly, it passes through two shoe-type pressings for dehydration, and then passes through the three-press gloss pressing. After passing through the pressing section, the base paper dryness is 45-48%, and then enters the pre-drying; 7) Pre-drying: The front one-third of the drying cylinder in the pre-drying section is sprayed with Teflon coating. The temperature of the drying cylinder in the entire pre-drying section needs to be steadily increased from 40-50℃ to 130-140℃, and the dryness of the base paper is increased from 45-48% before entering to 93-94%; 8) Surface sizing: Enzyme-converted cassava starch is used as the surface sizing agent. The viscosity of the prepared glue solution is 200-220cps, and the solid content is 27-29%. During the glue application process, the reflux temperature of the glue solution is 72-78℃, the viscosity of the forward and reverse sides is 35-43cps and 30-40cps respectively, the solid content of the forward and reverse sides is 10-14%, and the front glue amount is 2.5-3.5g / m 2 , the amount of glue applied on the back side is 1.5-2.5g / m 2 ; 9) Post-drying: temperature 100-120℃, to ensure the glue is fully matured; 10) Hard calendering: temperature 170-190°C, pressure 20-30kN / m; 11) Primer coating: Use a hard scraper, coating amount 12-14g / m 2 ; 12) Top coating: Use a soft scraper, coating amount 13-15g / m 2 ; 13) Front soft calendering: temperature 140-160℃, pressure 20-30kN / m; 14) Curling, rewinding, slitting and warehousing.

4. The process for preparing the water- and oil-resistant food-grade coated white cardboard according to claim 3, characterized in that: In step 3), the slurry concentration of the back layer and the bottom layer is 3-3.4%, and the slurry concentration of the core layer is 3.5-3.9%.

5. The process for preparing the water- and oil-resistant food-grade coated white cardboard according to claim 3, characterized in that: In step 4), the concentration of the diluted back layer and bottom layer slurry is 0.25-0.35%, and the concentration of the core layer slurry is 0.8-1.2%.

6. The process for preparing the water- and oil-resistant food-grade coated white cardboard according to claim 3, characterized in that: In step 3), the amount of cationic starch added to the core layer is 10-15kg / t pulp. In step 4), the amounts of aluminum sulfate added to the surface layer, core layer and bottom layer are 15-30kg / t pulp, 15-30kg / t pulp and 15-30kg / t pulp respectively, and the amount of cationic starch added to the core layer is 10-20kg / t pulp; the amounts of rosin glue added to the surface layer, core layer and bottom layer are 15-30kg / t pulp, 15-30kg / t pulp and 15-30kg / t pulp respectively; the amounts of cationic polyacrylamide added to the surface layer, core layer and bottom layer are 0.1-0.3kg / t pulp, 0.1-0.3kg / t pulp and 0.1-0.3kg / t pulp respectively; the amount of silica sol added to the core layer is 5-8kg / t pulp.

7. The process for preparing the water- and oil-resistant food-grade coated white cardboard according to claim 3, characterized in that: Step 5) The amount of starch sprayed on the surface layer is 2-5kg / t slurry, and the amount of starch sprayed on the bottom layer is 1-2.5kg / t slurry.

8. The process for preparing the water- and oil-resistant food-grade coated white cardboard according to claim 3, characterized in that: Step 6) The shoe-type pressing pressures in the first and second presses are 775-825 kN / m and 775-825 kN / m respectively, and the three-press gloss press uses rubber-coated ceramic rollers, and the pressure of the three presses is 20-30 kN / m.

9. The process for preparing the water- and oil-resistant food-grade coated white cardboard according to claim 3, characterized in that: Step 7) The drying area is divided into nine groups of drying cylinders. The first, second and third groups of drying cylinders are sprayed with Teflon coating. The drying temperature of the first group of drying cylinders is 40-50°C, and the ventilation humidity of the bag area is ≤120g / kgDA; the drying temperature of the second group of drying cylinders is 50-70°C, and the ventilation humidity of the bag area is ≤120g / kgDA; the drying temperature of the third group of drying cylinders is 70-90°C, and the ventilation humidity of the bag area is ≤120g / kgDA; the drying temperature of the fourth group of drying cylinders is 9 0-100℃, ventilation humidity in bag area ≤150g / kgDA; the drying temperature of the fifth group of drying cylinders is 90-100℃, ventilation humidity in bag area ≤150g / kgDA; the drying temperature of the sixth group of drying cylinders is 100-115℃, ventilation humidity in bag area ≤150g / kgDA; the drying temperature of the seventh group of drying cylinders is 110-120℃; the drying temperature of the eighth group of drying cylinders is 120-130℃; the drying temperature of the ninth group of drying cylinders is 130-140℃.

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