A food contactable double-coated coppered paperboard, its preparation method and application

Abstract

The application belongs to the technical field of papermaking, and provides a food contactable double-coated copper-coated card paper, a preparation method and application thereof. The food contactable double-coated copper-coated card paper comprises, from the front side to the back side, a front side surface coating layer, a front side base coating layer, a base paper, a back side base coating layer and a back side surface coating layer which are sequentially stacked, and the base paper comprises, from the front side to the back side, a surface layer, a core layer and a bottom layer which are sequentially stacked, the front side base coating layer and the back side base coating layer are obtained by coating a first coating material, and the front side surface coating layer and the back side surface coating layer are obtained by coating a second coating material, wherein the first coating material comprises the following components: 65-grade ground calcium carbonate, a first latex and a starch-acrylate graft copolymer, and the second coating material comprises the following components: 95-grade ground calcium carbonate, a second latex and a starch-acrylate graft copolymer. The double-coated copper-coated card paper has the advantages of high printing surface strength, high gloss, low surface roughness, low difference between two sides and low heavy metal content.

Description

Technical Field

[0001] This invention relates to the field of papermaking technology, and more specifically, to a food-contact double-coated coated paperboard, its preparation method, and its application. Background Technology

[0002] Coated cardstock, also known as printed coated cardstock, is made by coating the surface of base paper with a coating layer. As a commonly used printing paper material, it is frequently used as the base material for high-end printed products such as books, product nameplates, high-quality cards, and playing cards. It not only features a high basis weight (≥200g / m³), but also... 2 Furthermore, it requires both sides to possess high gloss and high surface smoothness (both sides must meet the following requirements: gloss ≥ 60%, and surface roughness ≤ 1.40 μm). The difference in smoothness, gloss, and other indicators between the two sides of the paper must be low (the difference in gloss and surface roughness between the two sides must both be ≤ 3%), i.e., low two-sided difference. Common coated cardboard products on the market typically use recycled waste paper pulp to produce the base paper, which is then coated on both sides. This often requires the addition of fluorescent whitening agents to the recycled pulp and coating to improve the product's whiteness and the coating quality on both sides of the base paper. Additionally, kaolin is added to the coating formulation to enhance the product's gloss.

[0003] With the emergence of market demand for food-contact double-coated coated paperboard, the coated paperboard currently on the market is difficult to meet the requirements for food contact. This is because the recycled pulp and fluorescent whitening agents used in this paperboard do not meet the requirements for food contact. Moreover, the coating accounts for a large proportion of the paperboard, and the kaolin content in the coating is high, which can easily lead to excessive heavy metals in the finished paper. Obviously, it cannot meet the requirements of "GB 4806.8-2022 Paper and Paperboard Materials and Products for Food Contact" (in the finished paperboard, the requirement is arsenic ≤1.0mg / kg and lead ≤5.0mg / kg), and therefore cannot be used as a material for food contact. Furthermore, during the papermaking process of coated paper, as the paper web gradually moves from low to high vacuum during dewatering and forming, the fine fibers in the pulp are transferred to the side in contact with the forming wire along with the water. This results in differences in flatness, smoothness, and surface roughness between the contact and non-contact surfaces of each pulp layer and the forming wire. The contact surface of the top layer pulp with the forming wire is combined with the core layer, while the non-contact surface of the bottom layer pulp with the forming wire is combined with the core layer. This leads to significant differences in gloss and surface roughness between the front and back sides of the finished paper. During the printing process, this causes differences in ink acceptance and absorption on both sides of the coated paper, resulting in uneven ink depth on both sides of the printed product and low print quality.

[0004] Therefore, there is an urgent need to develop a food-contact double-coated coated paper with high gloss, low surface roughness, and low two-sided difference to meet high printing quality requirements. Summary of the Invention

[0005] This invention aims to solve at least one of the technical problems existing in the prior art. To this end, this invention proposes a food-contact double-coated coated paperboard, its preparation method, and its applications. The double-coated coated paperboard provided by this invention possesses the characteristics of high printing surface strength (printing surface strength ≥1.3m / s), high gloss (front gloss ≥60%, back gloss ≥60%), low surface roughness (front surface roughness ≤1.40μm, back surface roughness ≤1.40μm), low two-sided difference (gloss difference between front and back ≤1%, surface roughness difference between front and back ≤0.03μm), low heavy metal content (lead content as low as 2.1-2.2mg / kg, arsenic content as low as 0.6-0.7mg / kg), and high safety, meeting the requirements for food-contact paper (i.e., complying with the national food safety standard GB 4806.8-2022 "Food Contact Paper and Paperboard Materials and Articles").

[0006] A first aspect of the present invention provides a food-contact double-coated coated paperboard.

[0007] Specifically, a double-coated coated cardboard includes, from front to back, a front top coating layer, a front bottom coating layer, a base paper, a back bottom coating layer, and a back top coating layer stacked sequentially. The base paper includes, from front to back, a top layer, a core layer, and a bottom layer stacked sequentially. The front bottom coating layer and the back bottom coating layer are obtained by coating with a first coating, and the front top coating layer and the back top coating layer are obtained by coating with a second coating.

[0008] The first coating comprises the following components: 65-grade milled calcium carbonate, first latex, and starch acrylate graft copolymer;

[0009] The second coating comprises the following components: 95-grade ground calcium carbonate, second latex, and starch acrylate graft copolymer.

[0010] The two-sided difference mentioned in this invention refers to the difference in properties between the front and back sides of the paper, mainly including differences in smoothness and gloss.

[0011] This invention incorporates starch-acrylate graft copolymer into both the first and second coatings, meaning both the front and back coatings contain starch-acrylate graft copolymer. This copolymer exhibits excellent water absorption, swelling to form a gel upon absorbing water and resisting dehydration under external force. During coating dispersion, the hydrophilic carboxyl and hydroxyl groups in the starch-acrylate graft copolymer form hydrogen bonds with water molecules. The starch chain structure within the copolymer contains numerous spaces capable of accommodating water molecules, and the presence of osmotic pressure further enhances its ability to absorb large amounts of water. This prevents water migration during application, reducing the evaporation rate and effectively improving the coating's water retention. Furthermore, the hydrogen bonding between the starch-acrylate graft copolymer and other components in the coating further reduces the evaporation rate, contributing to improved water retention. This higher water retention allows the coating to remain moist for an extended period after application, with slow water release, improving the coating's leveling and permeability, thereby enhancing its gloss and smoothness. Simultaneously, the repulsive effect of ions leads to the expansion and structural extension of the polymer. The swollen and hydrated polymer occupies space in the solution, and the hydrophobic groups on the polymer associate with each other, increasing the viscosity. Furthermore, the starch-acrylate graft copolymer has a high affinity for calcium carbonate particles. Its polar functional groups are attracted to the polar surfaces of pigments (the pigments in this invention refer to inorganic mineral pigment particles such as kaolin and calcium carbonate in the coating, which can typically enhance paper whiteness and cover the fiber base color), using a bridging mechanism to bind them together. This results in a high degree of cross-linking in the entire system, forming a strong network structure that facilitates the adhesion of pigment particles. As the coating wets, penetrates, adheres, and cures on the surface of the base paper fibers, the bonding between particles gradually tightens, enhancing the strength of the coating. The typical paper coating weight is approximately 30 g / m². 2 The coating weight of conventionally produced coated cardstock (i.e., the sum of the total coating weight on the front and the total coating weight on the back) is as high as approximately 60 g / m². 2This invention effectively improves the printing surface strength and reduces coating powdering under the high coating weight requirements of coated cardstock. Therefore, the use of starch-acrylate graft copolymer solves the problem of low coating strength caused by a large coating weight. Furthermore, the acrylic polymer component in the starch-acrylate graft copolymer has high thermoplasticity, allowing it to adapt well to calendering. After calendering, the roughness and micro-smoothness of the coating are significantly improved, which is beneficial for enhancing the gloss and smoothness of the paper. Therefore, this invention utilizes starch-acrylate graft copolymer to improve the water retention, gloss, and printing surface strength of the coating. The second coating of this invention also contains 95-grade ground calcium carbonate. When the second coating comes into contact with the base paper, the largest pigment particles and particles near the boundary layer are first fixed at the boundary layer between the coating and the base paper. Liquid and smaller particles may still move within the porous network formed by the fixed particles. As the coating dries, the smaller particles fill the porous network structure and simultaneously accumulate on the coating surface to form a dense particle layer, improving gloss. The front and back undercoat layers of this invention are both coated with a first coating material, and the front and back topcoat layers are both coated with a second coating material, thereby further reducing the difference between the two sides of the paper and making the gloss and roughness of the two sides more similar.

[0012] Preferably, the base paper comprises, from front to back, a surface sizing layer, a surface layer, a core layer, a bottom layer, and a bottom surface sizing layer, which are stacked sequentially.

[0013] Preferably, the basis weight of the food-contact double-coated coated cardboard is ≥200g / m³. 2 .

[0014] More preferably, the basis weight of the food-contact double-coated coated cardboard is 300-400 g / m³. 2 .

[0015] More preferably, the basis weight of the food-contact double-coated coated cardboard is 345-355 g / m³. 2 .

[0016] Preferably, the first coating comprises the following components by weight: 100 parts of 65-grade ground calcium carbonate, 10-20 parts of first latex, and 0.1-2 parts of starch-acrylate graft copolymer.

[0017] More preferably, the first coating comprises, by weight, the following components: 100 parts of 65-grade ground calcium carbonate, 10-15 parts of first latex, and 0.1-1 parts of starch-acrylate graft copolymer.

[0018] More preferably, the first coating comprises, by weight, the following components: 100 parts of 65-grade milled calcium carbonate, 12-14 parts of the first latex, and 0.1-0.6 parts of starch-acrylate graft copolymer.

[0019] Preferably, the second coating comprises, by weight, the following components: 50-80 parts of 95-grade ground calcium carbonate, 10-20 parts of second latex, and 0.1-2 parts of starch-acrylate graft copolymer.

[0020] More preferably, the second coating comprises, by weight, the following components: 60-80 parts of 95-grade ground calcium carbonate, 15-20 parts of the second latex, and 0.5-2 parts of starch-acrylate graft copolymer.

[0021] More preferably, the second coating comprises, by weight, the following components: 70-75 parts of 95-grade milled calcium carbonate, 17-20 parts of second latex, and 0.5-1.5 parts of starch-acrylate graft copolymer.

[0022] Preferably, the second coating also includes grade 99 calcium carbonate.

[0023] Preferably, the second coating further comprises 20-50 parts of 99-grade ground calcium carbonate by weight.

[0024] More preferably, the second coating also includes 20-40 parts of 99-grade ground calcium carbonate by weight.

[0025] More preferably, the second coating also includes 30-40 parts of 99-grade ground calcium carbonate by weight.

[0026] The second coating of this invention also contains grade 99 calcium carbonate. The heavy metal content of grade 99 calcium carbonate is much lower than that of kaolin (the lead content in grade 99 calcium carbonate is ≤4.0 mg / kg, while the lead content in kaolin is ≤70.0 mg / kg), thus solving the problem of excessive heavy metal content in the coating. Moreover, grade 99 calcium carbonate has the characteristics of small particle size and narrow particle size distribution (concentrated 2μm and 1μm particle size distribution), which can provide a good stacking effect, better fill the pits on the paper surface, improve the microscopic smoothness of the coating, reduce the roughness of the paper surface, reduce the diffusion ratio of reflected light in the coating, and thus improve gloss, which can compensate for the gloss loss due to the lack of kaolin.

[0027] Preferably, the ISO whiteness of the 65-grade milled calcium carbonate is 93%-95%, and / or, the proportion of particles with a particle size of less than 2μm in the 65-grade milled calcium carbonate is 60-65%.

[0028] Preferably, the ISO whiteness of the 95-grade ground calcium carbonate is 92%-94%, and / or, the proportion of particles with a diameter less than 2μm in the 95-grade ground calcium carbonate is 93-96%, and / or, the proportion of particles with a diameter less than 1μm in the 95-grade ground calcium carbonate is 72-76%.

[0029] Preferably, the ISO whiteness of the 99-grade ground calcium carbonate is 91%-93%, and / or, the proportion of particles with a diameter less than 2μm in the 99-grade ground calcium carbonate is 97%-99.5%, and / or, the proportion of particles with a diameter less than 1μm in the 99-grade ground calcium carbonate is 82%-84%.

[0030] Preferably, the first coating further includes additive I, and / or the second coating further includes additive II, and / or additive I and additive II are each independently selected from at least one of dispersant, water-resistant agent, lubricant, and rheology modifier.

[0031] Preferably, by weight, the additive I comprises the following components: 0.05-0.2 parts of dispersant, 0.5-1.0 parts of water-resistant agent, 0.5-1.5 parts of lubricant, and 0.15-0.5 parts of rheology modifier.

[0032] Preferably, according to parts by weight, the additive II comprises the following components: 0.1-0.25 parts of dispersant, 0.5-1.0 parts of water-resistant agent, 1.0-2.0 parts of lubricant, and 0.15-0.5 parts of rheology modifier.

[0033] Preferably, the first latex is styrene-butadiene latex and / or styrene-acrylate latex, and / or the second latex is styrene-butadiene latex and / or styrene-acrylate latex.

[0034] Preferably, the dispersant is at least one of sodium polycarboxylate dispersant, polyacrylate, and polyphosphate.

[0035] Preferably, the water-resistant agent is a zirconium carbonate water-resistant agent and / or a polyamide-polyurea water-resistant agent.

[0036] Preferably, the lubricant is calcium stearate lubricant and / or polyoxyethylene wax.

[0037] Preferably, the rheology modifier is a modified carboxylic acid terpolymer rheology modifier and / or polyacrylate.

[0038] Preferably, the viscosity of the first coating at 35°C is 1500-1800 cps, and / or the solid content of the first coating is 67-70%, and / or the viscosity of the second coating at 35°C is 1500-1800 cps, and / or the solid content of the second coating is 67-70%. Solid content refers to the weight percentage of solid components contained in a substance.

[0039] Preferably, the raw materials for preparing the surface layer include: bleached softwood sulfate pulp and bleached hardwood sulfate pulp.

[0040] More preferably, the raw materials for preparing the surface layer, by weight percentage, include: 10%-30% bleached softwood sulfate pulp and 70%-90% bleached hardwood sulfate pulp.

[0041] Preferably, the raw materials for preparing the core layer include: bleached chemithermomechanical pulp, bleached hardwood sulfate pulp, and bleached softwood sulfate pulp.

[0042] More preferably, the raw materials for preparing the core layer, by weight percentage, include: 30%-50% bleached chemithermomechanical pulp, 20%-40% bleached hardwood sulfate pulp, and 10%-30% bleached softwood sulfate pulp.

[0043] Preferably, the raw materials for preparing the core layer also include process recycled paper (process recycled paper refers to waste paper generated during the production process, including paper webs with cut edges, waste paper obtained from the reuse of unqualified products, etc.).

[0044] More preferably, the raw materials for preparing the core layer also include 10%-30% recycled paper by weight.

[0045] Preferably, the raw materials for preparing the bottom layer include: bleached softwood sulfate pulp and bleached hardwood sulfate pulp.

[0046] More preferably, the raw materials for preparing the bottom layer, by weight percentage, include: 10%-30% bleached softwood sulfate pulp and 70%-90% bleached hardwood sulfate pulp.

[0047] Preferably, the bleached softwood sulfate pulp has an ISO brightness of 88-90%, and / or the bleached hardwood sulfate pulp has an ISO brightness of 90-92%, and / or the bleached chemithermomechanical pulp has an ISO brightness of 78-80%, and / or the process recycled paper has an ISO brightness of 84-86%.

[0048] Preferably, the wood pulp raw materials (i.e., white softwood sulfate pulp, bleached hardwood sulfate pulp, and bleached chemithermomechanical pulp) used in the top layer, core layer, and bottom layer are all 100% water-washed pulp. This invention uses 100% water-washed pulp as raw material, and no alkali is added during the pulping process to avoid a decrease in the whiteness of the raw materials during production. No fluorescent whitening agents are needed during pulp preparation, and the use of recycled pulp with high heavy metal content and uncontrollable microorganisms is avoided, reducing food safety risks and improving the safety of the paper products of this invention.

[0049] Preferably, the starch acrylate graft copolymer is prepared by a method comprising the following steps: preparing a starch solution using starch, then mixing the starch solution, emulsifier, initiator, crosslinking agent and acrylate monomers, and obtaining the starch acrylate graft copolymer by free radical emulsion polymerization.

[0050] Preferably, the method for preparing the starch solution includes the following steps: mixing starch and water, and cooking at 90-100°C to obtain the starch solution.

[0051] Preferably, the starch is at least one of tapioca starch, corn starch, and wheat starch.

[0052] Preferably, the temperature of the free radical emulsion polymerization reaction is 70-90°C, and / or the time of the free radical emulsion polymerization reaction is 3-8 hours.

[0053] Preferably, the raw materials of the starch acrylate graft copolymer, by weight, include: 100 parts starch, 5-10 parts emulsifier, 2-3 parts initiator, 1-3 parts crosslinking agent and 35-40 parts acrylate monomer.

[0054] Preferably, the acrylate monomer is at least one selected from acrylic acid, methacrylic acid, and methyl methacrylate.

[0055] More preferably, the acrylate monomer is acrylic acid, methacrylic acid, and methyl methacrylate.

[0056] More preferably, the weight ratio of acrylic acid, methacrylic acid, and methyl methacrylate is 0.5-1.5:0.5-1.5:0.5-1.5.

[0057] More preferably, the weight ratio of acrylic acid, methacrylic acid, and methyl methacrylate is 1-1.5:1-1.5:1-1.5.

[0058] More preferably, the weight ratio of acrylic acid, methacrylic acid, and methyl methacrylate is 1:1:1.

[0059] Preferably, the emulsifier is sodium dodecyl sulfate and / or polyvinyl alcohol.

[0060] Preferably, the initiator is at least one selected from ammonium persulfate, potassium persulfate, and sodium thiosulfate.

[0061] Preferably, the crosslinking agent is N,N-methylenebisacrylamide and / or N-hydroxymethylacrylamide.

[0062] A second aspect of the present invention provides a method for preparing food-contact double-coated coated paperboard.

[0063] A method for preparing food-contact double-coated coated paperboard includes the following steps:

[0064] (1) Mix the raw materials for the preparation of the surface layer, core layer and bottom layer respectively to prepare the surface layer slurry, core layer slurry and bottom layer slurry respectively. Then mold the surface layer slurry, core layer slurry and bottom layer slurry respectively to obtain the surface layer, core layer and bottom layer respectively.

[0065] (2) The top layer, core layer and bottom layer are laminated in the order of top layer, core layer and bottom layer from front to back to obtain a wet paper sheet;

[0066] (3) The wet paper is sizing and drying to obtain the base paper. Then the base paper is subjected to hard calendering, coating and soft calendering in sequence to obtain the double-coated coated cardboard.

[0067] In step (3), the coating is performed by applying the first coating and the second coating to the front side of the base paper in sequence, and then applying the first coating and the second coating to the front side in sequence, to obtain the front undercoat and the front topcoat, respectively. In addition, the first coating and the second coating are applied to the back side of the base paper in sequence, and then applying the first coating and the second coating to the back side in sequence, to obtain the back undercoat and the back topcoat, respectively.

[0068] Preferably, in step (1), before molding, the surface slurry, core slurry, and bottom slurry are crushed, ground, mixed, screened and slag removed, and then enter the triple mesh molding process to obtain the surface layer, core layer, and bottom layer, respectively.

[0069] Preferably, in step (3), the drying includes pre-drying and post-drying.

[0070] More preferably, in step (3), the wet paper sheet is sequentially subjected to pressing, pre-drying, surface sizing and post-drying to obtain the base paper.

[0071] Preferably, in step (3), the coating amount of the front undercoat is 14-15 g / m². 2 And / or, the coating amount of the front surface coating is 14-15 g / m². 2 And / or, the coating amount of the back undercoat is 16-17 g / m². 2 And / or, the coating amount of the back surface coating is 16-17 g / m². 2 And / or, the total coating weight on the front side of the double-coated coated cardstock is 28-30 g / m². 2 And / or, the total coating weight on the back of the double-coated coated cardstock is 32-34 g / m². 2 The total coating amount on the front side is the sum of the coating amounts of the front undercoat and the front topcoat, and the total coating amount on the back side is the sum of the coating amounts of the back undercoat and the back topcoat.

[0072] Preferably, in step (3), the temperature of the hot roller for hard calendering is 150-180°C, and / or the linear pressure of hard calendering is 30-50 kN / m.

[0073] This invention applies intense pressure to the surface of the base paper by controlling the pressure and temperature of hard calendering. This adjusts the thickness between the fiber layers of the base paper, increases the density and strength of the paper, and at the same time smooths out the protrusions and irregularities on the surface, improves the smoothness of the paper surface, reduces the difference in thickness across widths, optimizes the flatness of the paper surface, and helps to improve the uniformity of the paper coating in subsequent processes. This invention can improve the flatness and thickness uniformity of the base paper surface.

[0074] Preferably, in step (3), the soft calendering is performed twice, including a first soft calendering and a second soft calendering in sequence. The upper roller of the first soft calendering is a hot roller, and / or the lower roller of the first soft calendering is a soft roller, and / or the temperature of the hot roller of the first soft calendering is 150-180℃, and / or the pressure of the first soft calendering is 30-50kN / m, and / or the lower roller of the second soft calendering is a hot roller, and / or the upper roller of the second soft calendering is a soft roller, and / or the temperature of the hot roller of the second soft calendering is 150-180℃, and / or the pressure of the second soft calendering is 50-70kN / m.

[0075] The first soft calendering is mainly to improve the gloss and smoothness of the front side; the second soft calendering is mainly to improve the gloss and smoothness of the back side. Both soft calendering processes help reduce the difference between the two sides of the paper.

[0076] This invention cleverly solves the problem of large differences in coating effects between the front and back sides of coated cardstock by setting different total coating amounts for the front and back sides, and adjusting the temperature and pressure of the first and second soft calendering, thus reducing the difference between the two sides.

[0077] Preferably, in step (3), after the soft calendering is completed, the temperature is lowered to 40-50°C and then the roll is wound up.

[0078] A third aspect of the invention provides an application of food-contact double-coated coated paperboard.

[0079] Application of a food-contact double-coated coated paperboard in the preparation of packaging materials, decorative materials or printing materials.

[0080] A fourth aspect of the present invention provides a food packaging material.

[0081] A food packaging material made from the aforementioned double-coated coated paperboard.

[0082] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0083] The food-contact double-coated coated paperboard provided by the present invention comprises, from front to back, a front top coating layer, a front bottom coating layer, a base paper, a back bottom coating layer, and a back top coating layer stacked sequentially. The base paper comprises, from front to back, a top layer, a core layer, and a bottom layer stacked sequentially. The front bottom coating layer and the back bottom coating layer are obtained by coating with a first coating, and the front top coating layer and the back top coating layer are obtained by coating with a second coating. The first coating layer comprises the following components: 65-grade ground calcium carbonate, a first latex, and a starch-acrylate graft copolymer. The second coating layer comprises the following components: 95-grade ground calcium carbonate, a second latex, and a starch-acrylate graft copolymer. This invention adds starch-acrylate graft copolymer to the first and second coatings, forming a strong network structure in the coating system. This reduces the migration of the starch-acrylate graft copolymer in the wet coating. The starch-acrylate graft copolymer dries to form a film, reducing the air permeability of the paper surface. At the same time, the film formed is not sensitive to moisture and is not prone to swelling due to water absorption. It has little impact on the smoothness and gloss of the coating. The double-coated coated cardboard provided by this invention has the advantages of high printing surface strength (printing surface strength up to 1.4-1.7m / s), high gloss (front gloss reaches 60-65%, back gloss reaches 60-64%), low surface roughness (front surface roughness 0.98-1.22μm, back surface roughness 0.99-1.21μm), and low two-sided difference (gloss difference between front and back ≤1%, surface roughness difference between front and back ≤0.03μm). Moreover, the raw materials selected in this invention are safe and have low heavy metal content. The whiteness of the product is improved by optimizing the production process. No fluorescent whitening agents are added, which meets the requirements of the national food safety standard GB 4806.8-2022 "Food Contact Paper and Paperboard Materials and Products" (lead content as low as 2.1-2.2 mg / kg, arsenic content as low as 0.6-0.7 mg / kg). It can be used to prepare food contact paper. Attached Figure Description

[0084] Figure 1 This is a schematic diagram of the structure of the double-coated copperplate cardboard obtained in Embodiment 1 of the present invention;

[0085] Figure 2 This is a flowchart illustrating the production process of double-coated coated paperboard according to Embodiment 1 of the present invention. Detailed Implementation

[0086] To enable those skilled in the art to more clearly understand the technical solutions described in this invention, the following embodiments are provided for illustration. It should be noted that the following embodiments do not constitute a limitation on the scope of protection claimed by this invention.

[0087] Unless otherwise specified, the raw materials, reagents or devices used in the following examples are available from conventional commercial sources or can be obtained by existing known methods.

[0088] The main raw materials used in the embodiments and comparative examples of this invention are as follows:

[0089] Starch-acrylate graft copolymer: A starch solution with a solute mass percentage of 20% was prepared using 100 parts of cassava starch and water. The solution was then cooked and gelatinized at 95°C to obtain a starch slurry. Then, using a free radical emulsion polymerization method, 5 parts of sodium dodecyl sulfate (emulsifier), 3 parts of ammonium persulfate (initiator), and 3 parts of N,N-methylenebisacrylamide (crosslinking agent) were added dropwise under constant pressure. 35 parts of acrylate monomers (acrylic acid, methacrylic acid, and methyl methacrylate in a weight ratio of 1:1:1) were added to the starch slurry to carry out the polymerization reaction. The reaction was carried out at 80°C and stirred at 300 r / min for 5 hours to obtain the product, denoted as starch-acrylate graft copolymer A.

[0090] Grade 65 ground calcium carbonate: ISO whiteness is 93.9%, and particles with a diameter of less than 2μm account for 63.5%.

[0091] Grade 95 ground calcium carbonate: ISO whiteness is 93.7%, and the proportion of particles with a diameter of less than 2μm is 94.2%.

[0092] Grade 99 ground calcium carbonate: ISO whiteness is 92%, particle size is less than 2μm, and particle size is less than 1μm.

[0093] Kaolin: ISO whiteness is 89.2%, particles smaller than 2μm account for 98.6%, and particles smaller than 1μm account for 94.7%.

[0094] Bleached coniferous sulfate pulp: ISO brightness is 89.8%.

[0095] Bleached hardwood sulfate pulp: ISO brightness is 91.6%.

[0096] Bleached chemithermomechanical pulp: ISO brightness 79.5%.

[0097] Process recycled paper (waste paper): ISO whiteness is 85.2%.

[0098] Example 1

[0099] A double-coated coated cardboard includes, from front to back, a front top coating layer, a front bottom coating layer, a top surface sizing layer, a base paper, a bottom surface sizing layer, a back bottom coating layer, and a back top coating layer, which are stacked sequentially. The base paper includes, from front to back, a top layer, a core layer, and a bottom layer, which are stacked sequentially. The front bottom coating layer and the back bottom coating layer are coated with a first coating, and the front top coating layer and the back top coating layer are coated with a second coating.

[0100] The above-mentioned method for preparing food-contact double-coated coated paperboard (e.g.) Figure 2 The process includes the following steps:

[0101] 1. Preparation of the surface layer slurry: The raw materials include 30wt% bleached softwood sulfate pulp and 70wt% bleached hardwood sulfate pulp. After being broken down with clean water and pulped in a disc mill, the pulp is mixed, screened to remove slag in the flow section, and then evenly sprayed onto the surface screen through a headbox. The surface layer basis weight is 36g / m². 2 .

[0102] 2. Preparation of core layer pulp: The raw materials include 50wt% bleached chemithermomechanical pulp, 20wt% bleached hardwood sulfate pulp, 10wt% bleached softwood sulfate pulp, and 20wt% recycled process paper. After pulping and beating in a disc mill, the pulps are mixed, screened and descaled in the headbox, and then evenly sprayed onto the core web for forming. The core layer basis weight is 198 g / m³. 2 .

[0103] 3. Preparation of the base layer slurry: The raw materials include 30wt% bleached softwood sulfate pulp and 70wt% bleached hardwood sulfate pulp. After being pulped with clean water and beaten in a disc mill, the pulp is mixed, screened to remove slag in the headbox, and then evenly sprayed onto the bottom screen for molding. The basis weight of the base layer is 44g / m³. 2 .

[0104] 4. Preparation of Base Paper: The face, core, and bottom layers of paper are laminated to produce a wet paper sheet with face, core, and bottom layers. This wet paper sheet undergoes pressing, pre-drying, surface sizing, and post-drying to obtain the base paper. Surface sizing uses a two-roll film transfer sizing machine to evenly spread the sizing solution on the paper surface. The solid content of the surface layer feed tank is 15%, and the surface sizing amount is 3.0 g / m². 2 The solid content of the bottom feeding trough is 15%, and the amount of adhesive applied to the bottom surface is 3.0 g / m². 2 .

[0105] The pressing process utilizes a combination of double-boot pressing and a light pressing, which increases fiber bonding, improves paper density and strength, eliminates wire marks, enhances paper smoothness, and reduces surface-to-surface differences. Surface sizing uses cassava surface-sizing starch, which, compared to native cassava starch, has better film-forming properties, thus improving the smoothness of the base paper.

[0106] 5. Hard calendering

[0107] The linear pressure of the hard calender is 40kN / m. The upper roller is a hot roller, and the lower roller is a normal temperature roller. The surface temperature of the hot roller is 180℃.

[0108] 6. Coating

[0109] After hard calendering, the paper undergoes two coating processes on both sides. First, a first coating is applied to both the front and back sides of the base paper, resulting in a front base coat and a back base coat, respectively. Then, a second coating is applied to both the front and back sides of the base paper, resulting in a front top coat and a back top coat, respectively. The total coating weight of the front base coat and the front top coat is 30 g / m². 2 The total coating weight of the back primer and back topcoat is 32 g / m². 2 The total basis weight of the finished paper is 350 g / m³. 2 ,in:

[0110] Top coat application rate: 15 g / m² 2 The coating amount on the back side is 16g / m². 2 The first coating, by weight percentage, comprises: 100 parts of 65-grade ground calcium carbonate, 14 parts of styrene-butadiene latex, 0.6 parts of starch-acrylate graft copolymer A, 0.2 parts of sodium polycarboxylate dispersant, 1.0 part of potassium zirconium carbonate water-resistant agent, 1.0 part of calcium stearate lubricant, and 0.5 parts of modified carboxylic acid terpolymer rheology modifier. The first coating has a solid content of 68%, and the pH is adjusted to 9.5-10.5 with a 10% sodium hydroxide solution.

[0111] Secondary coating amount on the front side: 15g / m 2 The amount of the second coating on the back side is 16g / m². 2 The second coating, by weight percentage, comprises: 70 parts of 95-grade ground calcium carbonate, 30 parts of 99-grade ground calcium carbonate, 17 parts of styrene-butadiene latex, 0.6 parts of starch-acrylate graft copolymer A, 0.2 parts of sodium polycarboxylate dispersant, 1.0 part of potassium zirconium carbonate water-resistant agent, 1.5 parts of calcium stearate lubricant, and 0.5 parts of modified carboxylic acid terpolymer rheology modifier. The second coating has a solid content of 67%, and the pH is adjusted to 9.5-10.5 with a 10% sodium hydroxide solution.

[0112] 7. Soft calendering

[0113] The pressure of the first soft calendering is 40 kN / m, and the surface temperature of the hot roller is 180℃. The pressure of the second soft calendering is 60 kN / m. In the first soft calendering, the upper roller is a hot roller, and the lower roller is a soft roller. In the second soft calendering, the lower roller is a hot roller, and the upper roller is a soft roller.

[0114] 8. Winding

[0115] After soft calendering, the paper is cooled in a cold air box to a temperature of 48°C. It is then rolled into large rolls for easier handling and subsequent slitting.

[0116] The structure of each layer of the food-contact double-coated coated paperboard prepared in Example 1 is as follows: Figure 1As shown, from front to back, the layers are as follows: front topcoat (main components of the second coating: grade 95 calcium carbonate, grade 99 calcium carbonate, and second latex), front base coat (main components of the first coating: grade 65 calcium carbonate and first latex), topcoat surface sizing layer (main component is starch), topcoat (main component is fiber), core layer (main component is fiber), bottom layer (main component is fiber), bottom layer surface sizing layer (main component is starch), back base coat (main components of the first coating: grade 65 calcium carbonate and first latex), and back topcoat (main components of the second coating: grade 95 calcium carbonate, grade 99 calcium carbonate, and second latex).

[0117] Example 2

[0118] A food-contact double-coated coated cardboard differs from Example 1 in that the starch-acrylate graft copolymer A in the first and second coating formulations is increased from 0.6 parts to 0.8 parts.

[0119] Example 3

[0120] A food-contact double-coated coated cardboard differs from Example 1 in that the starch-acrylate graft copolymer A in the first and second coating formulations is increased from 0.6 parts to 1.0 parts.

[0121] Example 4

[0122] A food-contact double-coated coated cardboard differs from Example 1 in that the starch-acrylate graft copolymer A in the first and second coating formulations is increased from 0.6 parts to 1.5 parts.

[0123] Examples 5-7

[0124] The food-contact double-coated coated cardboard provided in Examples 5-7 differs from that in Example 1 in that the coating amounts on the front and back sides, as well as the linear pressure of the first and second soft calendering processes, are different in the preparation methods of Examples 4-6, as detailed in the table below:

[0125] Table 1. Front and back coating amounts and pressure during two soft pressing operations in Examples 5-7.

[0126]

[0127] Examples 8-13

[0128] Examples 8-13 provide food-contact double-coated coated cardboard, which differs from Example 3 in that the starch-acrylate graft copolymer A in the first and second coating formulations is different. Specifically, the weight ratio of acrylate monomers in preparing starch-acrylate graft copolymer A is different, as shown in the table below:

[0129] Table 2. Acrylate monomers and their weight ratios in starch-acrylate graft copolymers A of Examples 8-13.

[0130]

[0131] Comparative Example 1

[0132] A double-coated coated cardboard differs from Example 1 in that 0.6 parts of starch acrylate graft copolymer in the first coating are replaced with an equal part by weight of sodium carboxymethyl cellulose.

[0133] Comparative Example 2

[0134] A double-coated coated cardboard differs from Example 1 in that 0.6 parts of starch acrylate graft copolymer in the second coating are replaced with an equal part by weight of sodium carboxymethyl cellulose.

[0135] Comparative Example 3

[0136] A double-coated coated cardboard differs from Example 1 in that 0.6 parts of starch acrylate graft copolymer in both the first and second coatings are replaced with an equal part by weight of sodium carboxymethyl cellulose.

[0137] Comparative Example 4

[0138] A double-coated coated cardboard differs from Example 1 in that 30 parts of 99-grade ground calcium carbonate in the second coating are replaced with an equal part by weight of kaolin.

[0139] Product effectiveness test

[0140] 1. Testing Method

[0141] Basis weight: GB / T 451.2-2023 Determination of basis weight of paper and paperboard.

[0142] Thickness: GB / T 451.3-2023 Determination of thickness of paper and paperboard.

[0143] Printing surface strength: 《GB / T 22365-2008 Determination of printing surface strength of paper and paperboard》. Printing surface strength evaluates the bonding strength between fibers, fillers, and sizing agents on the paper surface, or between coating particles on the paper surface and between the coating and the paper base. It indicates the paper's ability to resist ink splitting during the printing process. The greater the printing surface strength, the greater the coating's resistance to ink splitting.

[0144] Gloss: GB-T8941-2013 Determination of specular gloss of paper and paperboard.

[0145] Surface roughness: 《GB / T 22363-2008 Determination of roughness of paper and paperboard (air leakage method) Bentsen method and printing surface method》. Surface roughness is an indicator that characterizes the degree of unevenness of the surface of paper and paperboard. The greater the surface roughness, the rougher the paper surface, and the less smooth the paper surface.

[0146] Heavy metal content: GB 31604.49-2023 National Food Safety Standard - Determination of multiple elements and determination of multiple element migration in food contact materials and articles.

[0147] Smoothness: GB / T 456-2002 Determination of smoothness of paper and paperboard (Beck method)

[0148] 2. Test Results

[0149] The performance test results of the double-coated coated paperboards prepared in Examples 5-7 are as follows:

[0150] Table 3 shows the two sides of the double-coated coated paperboard in Examples 5-7.

[0151]

[0152] As shown in the table above, compared with Example 6, Example 5 has the same coating amount on both the front and back sides, while Example 6 has different coating amounts on both sides, resulting in a significant reduction in the two-sided difference in Example 6. Compared with Example 7, Example 6 uses the same pressure during the two soft-pressing operations, while Example 7 uses different pressure during the two soft-pressing operations, resulting in a further reduction in the two-sided difference in Example 7. Therefore, this invention significantly reduces the difference in coating effect between the front and back sides of coated cardstock by setting different total coating amounts on the front and back sides, as well as different pressure during the two soft-pressing operations.

[0153] The performance test results of the double-coated coated paperboards prepared in each embodiment and comparative example are as follows:

[0154] Table 4. Performance test results of double-coated coated paperboard in each embodiment and comparative example.

[0155]

[0156]

[0157] As shown in the table above, the double-coated coated cardstock provided by this invention has a high printing surface strength of 1.4-1.7 m / s, a front gloss of 60-65%, a back gloss of 60-64%, a gloss difference between the front and back sides ≤1%, a front surface roughness of 0.98-1.22 μm, a back surface roughness of 0.98-1.21 μm, a roughness difference between the front and back sides ≤0.02 μm, and a lead content as low as 2.1-2.2 mg / kg and an arsenic content as low as 0.6-0.7 mg / kg. This indicates that the double-coated coated cardstock provided by this invention combines high printing surface strength, high gloss, low surface roughness, low two-sided gloss difference, low heavy metal content, and high safety. The second coating of this invention incorporates a starch-acrylate graft copolymer, which utilizes a bridging mechanism to form a high degree of cross-linking throughout the coating system, creating a robust network structure that facilitates the adhesion between pigment particles. As the coating wets, penetrates, adheres, and cures on the surface of the base paper fibers, the bonding between particles gradually tightens, enhancing the coating strength and forming a dense coating that improves the gloss and surface roughness of both sides of the paper. Simultaneously, the acrylic polymer component has high thermoplasticity, making it more suitable for calendering than thermosetting films formed from natural products such as sodium carboxymethyl cellulose. This results in a more significant improvement in the roughness and micro-smoothness of the coating after calendering, further enhancing gloss and reducing surface roughness. Furthermore, both the front and back coatings are double-layered and use the same coating (i.e., both the front and back undercoats are coated with the first coating, and both the front and back topcoats are coated with the second coating), reducing surface differences.

[0158] Compared with Example 1, Example 2 increased the starch acrylate graft copolymer from 0.6 parts to 0.8 parts, which further enhanced the coating strength, improved the gloss and surface roughness of both sides of the paper, increased the surface strength to 1.5 m / s, increased the gloss to 63 / 63% on the front and back sides, and decreased the surface roughness to 1.10 / 1.12 μm on the front and back sides.

[0159] Compared with Example 2, Example 3 further increased the amount of starch acrylate graft copolymer to 1.0 part, which further enhanced the strength of the coating, improved the gloss and surface roughness of both sides of the paper, increased the surface strength to 1.7 m / s, increased the gloss to 65 / 64% on the front / back side, and decreased the surface roughness to 0.98 / 0.99 μm on the front / back side.

[0160] Compared to Example 3, Example 4 further increased the amount of starch-acrylate graft copolymer to 1.5 parts, but the overall paper quality performance decreased slightly. This indicates that the improvement of paper quality performance by starch-acrylate graft copolymer is not necessarily better the more it is used. When the amount is too high, it can easily cause the coating components to be too tightly crosslinked, the coating viscosity to be too high, the fluidity to be reduced, and affect the overall leveling effect of the coating, thereby affecting the paper quality performance.

[0161] Compared to Example 3, Examples 8-10 adjusted the types of acrylate monomers in the starch-acrylate graft copolymers of the first and second coating formulations. Compared to Example 3, the printing surface strength, gloss, and surface roughness of Examples 8-13 decreased. Acrylic monomers can improve the water solubility and dispersibility of starch, facilitating the crosslinking of the starch-acrylate graft copolymer with pigment particles; methacrylic monomers can improve the rigidity of the copolymer; methyl methacrylate monomers can reduce the hygroscopicity of starch, improving the moisture resistance of the copolymer. Examples 11-13 adjusted the ratio of acrylate monomers in the starch-acrylate graft copolymers of the first and second coating formulations. In Example 11, the amount of acrylic monomer increased, resulting in overly tight crosslinking with pigment particles, excessively high coating viscosity, reduced fluidity, and affecting the overall leveling effect of the coating, thus impacting the paper's properties. In Example 12, the increased amount of methacrylic monomer improved the overall rigidity of the coating, resulting in better calendering and finishing effects, but slightly negatively impacting the printing surface strength of the coating. In Example 13, the increased amount of methyl methacrylate monomer improved the flexibility of the starch and reduced its moisture absorption, but slightly weakened the overall calendering and finishing effect of the coating. Example 3, by using three acrylate monomers and their specific ratio combination to achieve a synergistic effect, showed the best improvement in paper properties.

[0162] Compared to Example 1, Comparative Example 1 replaced 0.6 parts by weight of starch-acrylate graft copolymer in the first coating with an equal part by weight of sodium carboxymethyl cellulose. This resulted in a decrease in both the gloss and surface roughness of the paper, as well as a reduction in the printed surface strength. The surface strength decreased to 1.3 m / s, the gloss decreased to 58 / 57% on both sides, and the surface roughness increased to 1.42 / 1.45 μm on both sides.

[0163] Compared to Example 1, in Comparative Example 2, the starch-acrylate graft copolymer was replaced with an equal part by weight of sodium carboxymethyl cellulose in the second coating, resulting in a decrease in the gloss and surface roughness of the paper, as well as a reduction in the printed surface strength. The surface strength decreased to 1.2 m / s, the gloss decreased to 56 / 56% on both sides, and the surface roughness increased to 1.57 / 1.56 μm on both sides.

[0164] Compared to Example 2, in Comparative Example 3, the starch-acrylate graft copolymer was replaced with an equal part by weight of sodium carboxymethyl cellulose in both the first and second coatings, resulting in a decrease in the gloss and surface roughness of the paper, as well as a reduction in the printed surface strength. The surface strength decreased to 1.0 m / s, the gloss decreased to 54 / 53% on both sides, and the surface roughness increased to 1.65 / 1.68 μm on both sides.

[0165] Compared with Example 1, Comparative Example 4 replaced 30 parts of 99-grade ground calcium carbonate in the second coating with an equal part of kaolin. Although the product's gloss, roughness, and surface strength met the requirements, the lead content increased to 3.6 mg / kg and the arsenic content increased to 1.5 mg / kg (approximately 71% and 150% higher than in Example 1, respectively). The heavy metal content seriously exceeded the standard, did not meet the food safety requirements, and could not be used as a food contact material.

[0166] As can be seen from the comparative examples 1-3 above, the double-coated coated cardboard provided by this invention uses starch-acrylate graft copolymer in the coating to replace the traditionally used sodium carboxymethyl cellulose. This copolymer can associate and form a strong network structure in the coating system, reducing its migration in the wet coating. Simultaneously, the starch-acrylate graft copolymer dries to form a thin film, reducing the air permeability of the paper surface. This film is also insensitive to moisture, preventing water absorption and swelling, thus improving the water retention of the coating and having minimal impact on the smoothness and gloss of the coating. Therefore, it can ensure high gloss and low surface roughness of the finished paper, thereby guaranteeing coating quality. Furthermore, the second coating of this invention also adds 99-grade ground calcium carbonate to replace kaolin, significantly reducing the heavy metal content (the lead content in the finished paper is only 2.1-2.2 mg / kg, and the arsenic content is only 0.6-0.7 mg / kg). In addition, the base paper uses 100% wood pulp, making it more environmentally friendly and ensuring the application of double-coated coated cardboard in food contact paper.

Claims

1. A type of double-coated coated cardstock, characterized in that, The paper comprises, from front to back, a front surface coating, a front base coating, a base paper, a back base coating, and a back surface coating, which are stacked sequentially. The base paper comprises, from front to back, a surface layer, a core layer, and a bottom layer, which are stacked sequentially. The front base coating and the back base coating are coated with a first coating, and the front surface coating and the back surface coating are coated with a second coating. The first coating comprises the following components by weight: 100 parts of 65-grade milled calcium carbonate, 10-20 parts of first latex, and 0.1-2 parts of starch-acrylate graft copolymer. The second coating comprises, by weight, the following components: 50-80 parts of 95-grade milled calcium carbonate, 10-20 parts of the second latex, 0.1-2 parts of starch-acrylate graft copolymer, and 20-50 parts of 99-grade milled calcium carbonate. Grade 65 ground calcium carbonate: 63.5% of the particles are smaller than 2μm; Grade 95 ground calcium carbonate: 94.2% of particles are smaller than 2μm; Grade 99 ground calcium carbonate: 99% of the particles are smaller than 2μm, and 83% of the particles are smaller than 1μm. The raw materials of the starch acrylate graft copolymer, by weight, consist of the following components: 100 parts starch, 5-10 parts emulsifier, 2-3 parts initiator, 1-3 parts crosslinking agent, and 35-40 parts acrylate monomers. The starch acrylate graft copolymer is prepared by a method including the following steps: starch is used to prepare starch solution, and then starch solution, emulsifier, initiator, crosslinking agent and acrylate monomer are mixed and the starch acrylate graft copolymer is obtained by free radical emulsion polymerization reaction. The acrylate monomers are acrylic acid, methacrylic acid, and methyl methacrylate; the weight ratio of acrylic acid, methacrylic acid, and methyl methacrylate is 0.

5. 1.5:0.5 1.5:0.5 1.

5.

2. The double-coated coated paperboard according to claim 1, characterized in that, The first coating further includes additive I, and the second coating further includes additive II, wherein additive I and additive II are each independently selected from at least one of dispersant, water-resistant agent, lubricant, and rheology modifier.

3. The double-coated coated paperboard according to claim 1, characterized in that, The first latex is a styrene-butadiene latex, and / or the second latex is a styrene-butadiene latex.

4. The double-coated coated paperboard according to claim 1, characterized in that, The first coating has a viscosity of 1500-1800 cps at 35°C, and / or the first coating has a solid content of 67-70%, and / or the second coating has a viscosity of 1500-1800 cps at 35°C, and / or the second coating has a solid content of 67-70%.

5. The method for preparing double-coated coated cardboard according to any one of claims 1-4, characterized in that, Includes the following steps: (1) Mix the raw materials for the preparation of the surface layer, core layer and bottom layer respectively to prepare the surface layer slurry, core layer slurry and bottom layer slurry respectively. Shape the surface layer slurry, core layer slurry and bottom layer slurry respectively to obtain the surface layer, core layer and bottom layer respectively. (2) The top layer, core layer and bottom layer are laminated in the order of top layer, core layer and bottom layer from front to back to obtain a wet paper sheet; (3) The wet paper is sizing and drying to obtain the base paper, and then the base paper is subjected to hard calendering, coating and soft calendering in sequence to obtain the double-coated coated cardboard. In step (3), the coating is performed by applying the first coating and the second coating to the front side of the base paper in sequence, and then applying the first coating and the second coating to the front side in sequence, to obtain the front undercoat and the front topcoat, respectively. In addition, the first coating and the second coating are applied to the back side of the base paper in sequence, and then applying the back undercoat and the back topcoat, respectively, to obtain the back undercoat and the back topcoat, respectively.

6. The preparation method according to claim 5, characterized in that, The coating amount of the front undercoat is 14-15 g / m². 2 The coating amount of the front surface coating is 14-15 g / m². 2 The coating amount of the back undercoat is 16-17 g / m². 2 The coating amount of the back surface coating is 16-17 g / m². 2 The total coating weight on the front side of the double-coated coated cardstock is 28-30 g / m². 2 The total coating weight on the back of the double-coated coated cardstock is 32-34 g / m². 2 The total coating amount on the front side is the sum of the coating amounts of the front undercoat and the front topcoat, and the total coating amount on the back side is the sum of the coating amounts of the back undercoat and the back topcoat.

7. The preparation method according to claim 5, characterized in that, The temperature of the hot roller for hard calendering is 150-180℃, and / or the linear pressure of hard calendering is 30-50kN / m.

8. The preparation method according to claim 5, characterized in that, The soft calendering is performed twice, including a first soft calendering and a second soft calendering. The upper roller of the first soft calendering is a hot roller, and the lower roller of the first soft calendering is a soft roller. The temperature of the hot roller in the first soft calendering is 150-180℃, and the pressure of the first soft calendering is 30-50kN / m. The lower roller of the second soft calendering is a hot roller, and the upper roller of the second soft calendering is a soft roller. The temperature of the hot roller in the second soft calendering is 150-180℃, and the pressure of the second soft calendering is 50-70kN / m.

9. The use of the double-coated coated paperboard according to any one of claims 1-4 in the preparation of packaging materials, decorative materials or printing materials.

10. A food packaging material, characterized in that, It is made using the double-coated coated paperboard as described in any one of claims 1-4.

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