Coated white card and method for its production

Abstract

The application belongs to the technical field of packaging materials, and particularly relates to a coated white cardboard and a preparation method thereof. The coated white cardboard comprises base paper and a composite coating layer arranged on at least one surface of the base paper, and the composite coating layer comprises konjac glucomannan and poly(3-hydroxybutyrate-co-4-hydroxybutyrate). In the application, the konjac glucomannan and the poly(3-hydroxybutyrate-co-4-hydroxybutyrate) are synergistically matched through intermolecular hydrogen bonds, so that the composite coating layer has excellent flexibility and adhesion, the generation of surface cracks in the folding process of the white cardboard is significantly reduced, the coating layer is prepared by using a water-based system, raw materials are biodegradable, the application is environment-friendly, and the application is suitable for the packaging field of food, medicine and high-end consumer goods.

Description

Technical Field

[0001] This invention relates to the technical field of packaging materials, specifically to a coated white cardboard and its preparation method. Background Technology

[0002] During the packaging process, white cardboard typically undergoes post-processing steps such as creasing, bending, and die-cutting. Due to its thickness and high basis weight, white cardboard is prone to surface cracks and bursting defects in the folded areas when subjected to stress and deformation; these defects are commonly referred to in the industry as "color bursting" or "crazing." These defects not only affect the appearance quality of the packaging products but also reduce the performance and processing adaptability of the packaging materials. Therefore, reducing cracking during the creasing and bending processes of white cardboard has always been one of the important technical challenges in the white cardboard processing industry.

[0003] To address the aforementioned problem of bending cracks in white cardboard, various approaches have been explored in the prior art. For example, patent document CN201822209847.7 discloses a high-grade coated white cardboard that increases paper toughness by incorporating tensile-resistant layers (including horizontal and vertical sheets), thereby attempting to improve the paper's crack resistance during processing. Another example is patent document CN201220038472.2, which discloses a novel cardboard that undergoes a lamination process before reverse varnishing printing, applying a biaxially oriented polypropylene (BOPP) protective film to the surface of the white cardboard to increase its strength during die-cutting, folding, embossing, or forming processes, reducing cracking and breakage.

[0004] However, all of the above solutions have some drawbacks in actual production. While constructing a tensile-resistant layer can increase paper toughness, this reinforcement method still cannot fully coordinate the deformation of the crease area subjected to the combined effects of tension, shear, and compression during bending. In particular, the interface bonding and integrity between the tensile-resistant layer and the original coating are insufficient, resulting in limited crack prevention. Although BOPP protective film can improve the surface integrity during bending to some extent, polypropylene film is difficult to degrade, and the paper base material after lamination is difficult to re-pulp and recycle, which does not meet the development requirements of green packaging materials.

[0005] Therefore, the design of white cardboard used for packaging materials still needs to be improved so that it can have green and environmentally friendly characteristics, good interfacial bonding performance and excellent bending and crack resistance. Without affecting the recyclability of the paper base material, it can effectively reduce the surface cracks of white cardboard during the creasing and bending process, thereby meeting the dual requirements of high-end packaging materials for forming quality and environmental friendliness. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a coated white cardboard and its preparation method. The coated white cardboard obtained by the preparation method possesses both excellent flexibility and adhesion, significantly reducing the generation of surface cracks during bending. Furthermore, the coating is prepared using a water-based system, and the raw materials are biodegradable, making it environmentally friendly and suitable for food, pharmaceutical, and high-end consumer product packaging.

[0007] To achieve the above-mentioned objectives, the present invention is implemented through the following technical solution:

[0008] In a first aspect, the present invention provides a coated white cardboard, comprising a base paper and a composite coating disposed on at least one surface of the base paper, the composite coating comprising konjac glucomannan and poly(3-hydroxybutyrate-co-4-hydroxybutyrate).

[0009] Konjac glucomannan (KGM) is a water-soluble nonionic polysaccharide whose main chain is composed of D-mannose and D-glucose linked by β-1,4 glycosidic bonds. Its molecular chain contains abundant hydroxyl (-OH) and acetyl groups, giving it excellent film-forming properties. However, as a natural polysaccharide polymer, KGM has a relatively rigid molecular chain, resulting in insufficient flexibility when used alone to form a film. Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) has a significantly flexible molecular chain with a glass transition temperature (Tg) as low as -9°C, and is in a highly elastic state at room temperature. This structural feature endows P34HB with excellent flexibility and extensibility. However, as a hydrophobic polyester, its interfacial bonding ability with the hydrophilic white cardboard glossy surface (containing inorganic pigments and latex) is weak, making it difficult to form a stable and complete adhesion layer when coated alone. The inventors discovered that when KGM and P34HB are mixed in an aqueous system, intermolecular hydrogen bonds can form between the abundant hydroxyl groups (-OH) on the KGM molecular chain and the ester groups (-COO-) on the P34HB molecular chain. This hydrogen bond interaction acts as a "molecular bridge" between the two polymer chains, enabling KGM and P34HB, which originally had significant differences in hydrophilicity and hydrophobicity, to achieve good compatibility at the molecular level, effectively preventing macroscopic phase separation. Simultaneously, the interconnection of hydrogen bonds forms a more compact three-dimensional network structure than a single component. In this network structure, the flexible segments of P34HB impart excellent ductility to the material, absorbing and dissipating external energy, while the rigid segments of KGM provide structural support to maintain the material's load-bearing capacity. Furthermore, as a reversible non-covalent interaction, hydrogen bonds can dissipate energy through breakage and recombination under external forces, further delaying crack initiation and propagation. As a result, the tensile strength and elongation at break of the KGM / P34HB composite coating are significantly improved, far superior to any single-component coating.

[0010] Preferably, the mass ratio of konjac glucomannan to poly(3-hydroxybutyrate-co-4-hydroxybutyrate) in the composite coating is 3:7 to 7:3.

[0011] Preferably, the thickness of the composite coating is 1–5 μm.

[0012] Preferably, the composite coating further includes a plasticizer.

[0013] Preferably, the plasticizer is glycerin.

[0014] Preferably, the content of the plasticizer is 10% to 20% of the total mass of konjac glucomannan and poly(3-hydroxybutyrate-co-4-hydroxybutyrate).

[0015] Secondly, the present invention also provides a method for preparing coated white cardboard, comprising the following steps:

[0016] S1. Add konjac glucomannan to water and hydrate and dissolve the konjac glucomannan under stirring to obtain an aqueous phase of konjac glucomannan.

[0017] S2. Add poly(3-hydroxybutyrate-co-4-hydroxybutyrate) aqueous dispersion to the aqueous phase of konjac glucomannan, mix evenly, and obtain a composite aqueous coating liquid;

[0018] S3. Apply the composite aqueous coating liquid to the surface of white cardboard and dry it to prepare coated white cardboard.

[0019] Preferably, in step S1, the mass concentration of konjac glucomannan in the aqueous phase of the konjac glucomannan is 0.5% to 2.0%.

[0020] Preferably, in step S2, the aqueous dispersion of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) is prepared by the following method: poly(3-hydroxybutyrate-co-4-hydroxybutyrate) is added to water to form an aqueous suspension, a natural emulsifying stabilizer is added, and the mixture is dispersed and homogenized to obtain the aqueous dispersion of poly(3-hydroxybutyrate-co-4-hydroxybutyrate).

[0021] Preferably, the natural emulsifying stabilizer is selected from at least one of low-methoxyl pectin and gum arabic.

[0022] Preferably, in step S1 or step S2, a plasticizer is also added, wherein the amount of plasticizer added is 10% to 20% of the total solids of konjac glucomannan and poly(3-hydroxybutyrate-co-4-hydroxybutyrate).

[0023] Preferably, in step S3, the coating amount of the composite aqueous coating liquid is 4.5 to 10.0 g / m².

[0024] Preferably, in step S3, the drying temperature is 50–70°C.

[0025] Preferably, after step S3, the method further includes a step of hot-pressing the coated white cardboard.

[0026] Preferably, the hot pressing conditions are: temperature 100-140℃, pressure 0.05-0.30 MPa, and time 1-10 s.

[0027] Preferably, after step S3, the method further includes a step of placing the coated white cardboard in a constant temperature and humidity environment for equilibration treatment.

[0028] Preferably, the temperature of the constant temperature and humidity environment is 23-27°C and the relative humidity is 50%-55%.

[0029] The beneficial effects of this invention are as follows:

[0030] This invention employs a composite water-based coating formed by blending KGM and P34HB and applying it to the surface of white cardboard. The two components complement each other in terms of mechanical properties: KGM imparts good film-forming properties and adhesion to the paper substrate, while P34HB provides excellent flexibility. The tensile strength of the composite coating can reach 26.2. With a strength of MPa and an elongation at break of up to 41.9%, this composite coating is far superior to any single-component coating. It effectively disperses localized strain concentration in the crease area during bending, reducing the strain concentration coefficient from 5.83 in the uncoated base paper to 1.26. After 20 cycles of 150° bending, the percentage of crack area decreased from 0.91% to 0.10% (a reduction of approximately 89%), significantly inhibiting the generation and propagation of bending cracks. Furthermore, the coating is prepared using a water-based system, with raw materials consisting entirely of natural or biodegradable polymers, avoiding the adverse effects of traditional plastic coating methods on paper repulping. It combines excellent bending crack resistance, interfacial bonding performance, and environmentally friendly characteristics, making it particularly suitable for food, pharmaceutical, and high-end consumer product packaging applications where high molding quality and environmental friendliness are required. Attached Figure Description

[0031] Figure 1 This is a cross-sectional SEM image of the coating in Example 1;

[0032] Figure 2 This is a schematic diagram of the DIC test during the bending process in Example 1. In the diagram, the white dashed box indicated by the white arrow is the strain analysis area. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0034] Unless otherwise specified, all raw materials, reagents and instruments used in the following examples and comparative examples are commercially available.

[0035] Raw material: Konjac glucomannan (KGM, molecular weight 1.0 × 10⁻⁶) 6 The g / mol of glycerol was purchased from Hubei Qiangsen Konjac Technology Co., Ltd.; the analytical grade of glycerol was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; and the poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) powder was purchased from Beijing Microstructure Workshop Biotechnology Co., Ltd. The basic performance parameters are as follows: particle size 1.8 μm, glass transition temperature (Tg) -9 ℃, melting point 170 ℃, and nominal strain at tensile fracture >10%.

[0036] The white cardboard substrate is commercially available coated white cardboard with a basis weight of 250 g / m². 2 The thickness is 0.32 mm.

[0037] Example 1

[0038] (1) Preparation of KGM solution

[0039] Weigh 1 g of KGM powder and add it to a beaker containing 100 mL of deionized water. Place the beaker in a 60 ℃ water bath and stir at 500 rpm for 30 min to fully hydrate and dissolve the KGM. Then remove the beaker from the water bath and allow it to cool naturally to room temperature to obtain a KGM solution with a mass concentration of 1%.

[0040] (2) Preparation of P34HB aqueous dispersion

[0041] P34HB powder was added to deionized water to form an aqueous suspension with a P34HB mass fraction of 20%. A natural emulsifying stabilizer, low-methoxyl pectin, was added at 5% of the P34HB mass. After pre-dispersing under mechanical stirring (800 rpm) for 30 min, the mixture was sequentially ground in a colloid mill (3000 rpm, 20 μm gap, 20 min) and subjected to high-pressure homogenization (first-stage homogenization pressure 50 MPa, second-stage homogenization pressure 10 MPa, repeated 3 times) to obtain a homogeneous and stable aqueous dispersion of P34HB. The dispersion was found to have a solid content of 19.5%, an average particle size of 1.2 μm, and a viscosity of 18 mPa·s.

[0042] (3) Preparation of composite coating liquid

[0043] Take the above 1% KGM solution and P34HB aqueous dispersion, make the solid mass ratio of KGM to P34HB 7:3, and add 15% of glycerol as plasticizer as the total solid mass. Stir at 500 rpm for 10 min at 25 ℃ to obtain KGM / P34HB (7:3) composite coating liquid.

[0044] (4) Coating the surface of white cardboard

[0045] Cut the white cardboard into sheets of 190 mm × 150 mm. Take 1.5 mL of the above-mentioned composite coating liquid and apply it evenly to the glossy surface of the white cardboard using a wire bar coater, controlling the coating amount to be 8.0 g / m². After coating, place the samples in a 60 ℃ hot air conditioner to dry for 5 min.

[0046] (5) Hot pressing treatment

[0047] The dried sample was hot-pressed at 120 °C and 0.15 MPa for 5 s.

[0048] (6) Balancing

[0049] The hot-pressed sample was placed in an environment with a temperature of 25 ℃ and a relative humidity of 55% for 72 h to ensure the stability of the sample's moisture content and internal structure.

[0050] Example 2

[0051] It is basically the same as Example 1, except that:

[0052] (3) Preparation of composite coating liquid:

[0053] Take the above 1% KGM solution and P34HB aqueous dispersion, make the solid mass ratio of KGM to P34HB 3:7, and add 15% of glycerol as plasticizer as the total solid mass. Stir at 500 rpm for 10 min at 25 ℃ to obtain KGM / P34HB (3:7) composite coating liquid.

[0054] Comparative Example 1

[0055] The same creasing and bending processes as in the examples were performed directly on the uncoated white cardboard substrate (same specifications as in the examples).

[0056] Comparative Example 2

[0057] Take the P34HB aqueous dispersion prepared in step (2) of Example 1, add 15% of glycerol as a plasticizer, and stir at 500 rpm for 5 min at 25 °C to obtain pure P34HB coating liquid.

[0058] Take 1.5 mL of the above-mentioned pure P34HB coating liquid and apply it evenly to the glossy surface of white cardboard using a bar coater, controlling the coating amount to be 8.0 g / m². The remaining coating, drying, hot pressing, and equilibration conditions are the same as in Example 1.

[0059] Comparative Example 3

[0060] Prepare a 1% KGM solution according to step (1) of Example 1, add 15% glycerol as a plasticizer, and stir at 500 rpm for 10 min at 25 °C to obtain a pure KGM coating solution.

[0061] Take 1.5 mL of the above-mentioned pure KGM coating liquid and apply it evenly to the glossy surface of white cardboard using a bar coater, controlling the coating amount to be 8.0 g / m². The remaining coating, drying, hot pressing, and equilibration conditions are the same as in Example 1.

[0062] Composite membrane preparation (for mechanical property testing)

[0063] Eighty g each of the pure KGM coating liquid from Comparative Example 3, the KGM / P34HB (7:3) composite coating liquid from Example 1, and the KGM / P34HB (3:7) composite coating liquid from Example 2 were poured into plastic molds with a diameter of 14 cm and dried in an oven at 60°C for 6 h to prepare pure KGM membranes, KGM / P34HB (7:3) composite membranes, and KGM / P34HB (3:7) composite membranes, respectively. The prepared membranes were equilibrated at 25°C and 55% relative humidity for 3 days for subsequent mechanical property testing. A separate attempt was made to prepare a membrane using the pure P34HB coating liquid from Comparative Example 2 using the same method. However, due to its weak film-forming properties, a complete self-supporting membrane could not be obtained, and therefore its mechanical properties were not measured.

[0064] Indentation and bending treatment

[0065] Using an indentation tester, the coated and uncoated glossy surfaces of Examples 1-2 and Comparative Examples 1-3 were indented. The indentation length was 50 mm, the width was 1.7 mm, and the depth was 0.85 mm. The indentation line was parallel to the CD direction (transverse). The indented samples underwent two bending treatments:

[0066] (a) Angle gradient bending

[0067] The sample was manually bent at a constant speed along the center line of the indentation to a preset angle. The bending angles were 0°, 30°, 60°, 90°, 120°, and 150°, with the bending speed controlled at approximately 30° / s. The bent sample was then used for subsequent observation of the evolution of surface cracks with the bending angle using a polarizing microscope.

[0068] (ii) Cyclic bending

[0069] Manually bend the sample along the center line of the indentation at a constant speed, from 0° to 150°, then return it to 0°, and then bend it to 150° again. Repeat this process 20 times, with a bending speed of approximately 30° / s. The sample after cyclic bending is used for subsequent observation of surface crack morphology under a polarizing microscope.

[0070] 1. Performance Testing

[0071] The performance of each sample prepared in the above embodiments and comparative examples was tested according to the following methods.

[0072] 1.1. Layer thickness measurement

[0073] Following the method specified in GB / T 451.3-2002 "Determination of Thickness of Paper and Paperboard", the sample was cut into 1 cm × 1 cm specimens, totaling 20 pieces. The thickness of each specimen was measured using a digital thickness gauge, with one measurement point for each specimen. The coating thickness was calculated according to formula (1):

[0074]

[0075] In the formula:

[0076] : Coating thickness, in micrometers (μm);

[0077] : The thickness of the paper base after coating, in micrometers (μm);

[0078] : Thickness of the uncoated paper base, in micrometers (μm).

[0079] 1.2. Moisture Content Determination

[0080] Following the method specified in GB / T 462-2008 "Determination of Moisture Content in Analytical Samples of Paper, Paperboard and Pulp", the sample was dried to constant weight at (105 ± 2) °C. The moisture content was calculated according to formula (2):

[0081]

[0082] In the formula:

[0083] Moisture content, expressed as a percentage (%).

[0084] Mass of the sample before drying, in grams (g);

[0085] Mass of the dried sample, in grams (g).

[0086] 1.3. Surface morphology observation

[0087] The sample was cut into 10 mm × 30 mm pieces and placed flat on a glass slide for observation using a polarizing microscope. First, the crack area was precisely located using a 5× objective lens, and then the objective lens was switched to 10× to photograph the crack morphology within the crease area. The image was then imported into Image-Pro Plus 6.0 software to statistically analyze the crack length and width in the image, and then the average crack length, average crack width, and crack area were calculated. Finally, the percentage of crack area per unit area was calculated according to formula (3):

[0088]

[0089] In the formula:

[0090] : Percentage of crack area per unit area, in percentage (%);

[0091] Total crack area within the observation area, in square micrometers (μm²).

[0092] Total area of ​​the observation region, expressed in square micrometers (μm²).

[0093] The average crack length and average crack width are calculated according to equations (4) and (5), respectively:

[0094]

[0095] In the formula:

[0096] : Average crack length, in micrometers (μm);

[0097] : Average crack width, in micrometers (μm);

[0098] The total number of cracks within the observation area;

[0099] : No. The length of the crack, in micrometers (μm);

[0100] : No. The width of the crack, measured in micrometers (μm).

[0101] 1.4. Observation of cross-sectional morphology

[0102] The sample was cut into 15 mm × 5 mm pieces and fixed with the cross-section facing upwards on the stage of a scanning electron microscope using conductive adhesive. The sample was then sputtered with gold under a vacuum of 13.3 Pa using a particle sputtering instrument. The sample was then observed using a scanning electron microscope (SEM). The main parameters were set as follows: accelerating voltage of 30 kV and magnification of 200×.

[0103] 1.5. Non-contact full-field strain measurement

[0104] The indented paper samples were cut into 40 mm × 40 mm square specimens, with the indentation located at the center of the specimen. A thin layer of matte black-gray paint was evenly sprayed onto the specimen surface and allowed to dry naturally before use. Strain measurement was performed using the VIC-3D non-contact full-field strain measurement system. The centerline of the specimen indentation was aligned with the edge of a fixed rectangular iron sheet (approximately 1 mm thick, roughly the same as the width of the indentation), which served as the support point for bending rotation. The specimen was manually bent at a constant speed, continuously bending it from 0° to 180° around the edge of the iron sheet, with the bending speed controlled at approximately 18° / s (0°~180° takes approximately 10 s). During the bending process, two industrial cameras continuously acquired speckle images of the specimen surface at a sampling frequency of 5 Hz. A sequence of images of the entire bending process was recorded.

[0105] The acquired image sequence was imported into VIC-3D software for strain field analysis. The analysis area was a rectangular region (50 mm × 2 mm) centered on the indentation centerline, with 1 mm on each side. The bending angle was extracted frame by frame from the image sequence using the software algorithm, and the principal strain field at the corresponding angle was calculated simultaneously. Strain data were extracted for bending angles of 30°, 60°, 90°, 120°, and 150°, respectively, and the average strain and maximum strain were calculated. Then, the strain concentration factor was calculated according to equation (6):

[0106]

[0107] In the formula:

[0108] — Strain concentration factor, dimensionless;

[0109] — The maximum principal strain within the analysis area, expressed as a percentage (%);

[0110] — The average strain within the analysis area, expressed as a percentage (%).

[0111] 1.6. Coating Adhesion Test

[0112] According to the method specified in GB / T 9286-2021 "Cross-cut test for paints and varnishes", cross-cut treatment is performed on the coating surface. After peeling off with adhesive tape, the coating peeling is observed and rated from 0 to 5 (0 is the best and 5 is the worst).

[0113] 1.7. Nanoscale Scratch Test

[0114] Furthermore, the glossy coating of white cardboard was characterized using a progressive load nano-scratching method. The testing equipment was a KLA iMicro nanoindenter with a diamond conical indenter, a tip radius of approximately 5 μm, and a cone angle of 90°. The testing conditions were: scratch length 200 μm, scratch speed 10 μm / s, normal load gradually increased from 0.01 mN to 7 mN, and the testing environment was 25 ℃ and 50% relative humidity. The test included three stages: pre-scan, main scratch, and post-scan. The load for both the pre-scan and post-scan was 0.02 mN. The critical load Lc was determined based on the characteristic changes in the scratch response curve. Each sample was tested three times, and the average value was taken.

[0115] 1.8. Mechanical property testing of composite membranes

[0116] Tensile tests were conducted using a texture analyzer. The test parameters were set as follows: tensile speed 30 mm / min, initial clamp spacing 20 mm, ambient temperature 25 ℃, and relative humidity 55%. The stress-strain curves of the specimens were recorded during the test. The tensile strength was calculated according to equation (7), and the elongation at break was calculated according to equation (8).

[0117]

[0118] In the formula:

[0119] Tensile strength, measured in megapascals (MPa).

[0120] : The maximum load that the specimen can withstand before it breaks, in Newtons (N).

[0121] Width of the narrow portion of the sample, in millimeters (mm);

[0122] : The thickness of the sample, in millimeters (mm).

[0123]

[0124] In the formula:

[0125] Elongation at break, in percentage (%).

[0126] : The distance between the fixtures when the specimen breaks, in millimeters (mm);

[0127] : The initial distance between the sample clamps, in millimeters (mm).

[0128] 2. Performance Test Results

[0129] 2.1. Basic Properties of Coatings

[0130] The basic properties of the coating on the coated samples were characterized. The results showed that at a coating amount of 8.0 g / m², the coating properties were optimal. 2 Under these conditions, the thickness of each coating is approximately 5 μm, and the coating exhibits a uniform and dense micron-scale film structure (as shown in the cross-sectional SEM image of Example 1). Figure 1 (As shown). The equilibrium moisture content of each coated sample was approximately 6.2% to 6.5%, indicating that the combination of KGM and P34HB had little effect on the moisture content of the paper base.

[0131] 2.2. Quantitative Results and Analysis of Surface Cracks in Paper Substrate under Different Bending Angles

[0132] The damage to the glossy surface of white cardboard during indentation and bending can be divided into two stages: initial damage from indentation and the evolution of cracks from a single bend. In the stage after indentation but before bending (0°), surface cracks appeared on the uncoated glossy surface of the white cardboard, while no obvious cracks were observed after coating with KGM or KGM / P34HB, indicating that the polymer coating had an impact on the surface damage behavior during the indentation stage.

[0133] During a single bending stage, the cracks exhibit a clear staged evolution characteristic as the bending angle increases. Taking uncoated paper as an example, discrete microcracks begin to appear in the crease area at 30°; between 60° and 120°, the number of cracks gradually increases and extends along the crease direction; when the bending angle increases to 150°, the crack distribution becomes denser and the opening size increases significantly, indicating that the crease area has undergone a typical "crack initiation, propagation, and localization" damage path.

[0134] The percentage of crack area for each sample under different bending angles is shown in Table 1.

[0135] Table 1. Percentage of crack area (%) for each sample under different bending angles

[0136] Comparative Example 1 (Uncoated base paper) 0.15 0.28 0.36 0.44 0.51 Comparative Example 2 (Pure P34HB coating) 0.08 0.14 0.20 0.26 0.32 Comparative Example 3 (Pure KGM Coating) 0.10 0.16 0.22 0.28 0.34 Example 1 - KGM / P34HB (3:7) 0.06 0.10 0.14 0.18 0.22 Example 1 - KGM / P34HB (7:3) 0.05 0.08 0.11 0.14 0.18

[0137] As shown in Table 1, the percentage of crack area in the uncoated base paper increases continuously with the bending angle, reaching 0.51% at 150°. The percentages of crack area for the pure P34HB coating and the pure KGM coating at 150° are 0.32% and 0.34%, respectively, both significantly lower than those for the uncoated base paper. The KGM / P34HB composite coating sample of this invention further reduces the percentage of crack area, with the KGM / P34HB (7:3) sample showing only 0.18% at 150°, exhibiting the best crack resistance.

[0138] 2.3. Quantitative Results and Analysis of Surface Cracks in Paper Base under Different Bending Numbers

[0139] Under 150° cyclic bending conditions, the influence of different coating ratios on the stable propagation behavior of cracks became more apparent. When the number of bends increased from 0 to 20, the uncoated paper substrate surface gradually evolved from a crack-free state to the initiation, propagation, and connection of microcracks and the formation of a main crack, indicating that cyclic bending can lead to continuous accumulation of damage.

[0140] The quantitative statistical results of cracks in each sample after 20 cyclic bending cycles are shown in Table 2.

[0141] Table 2 Crack parameters of each sample after 20 cyclic bending cycles

[0142] Comparative Example 1 (Uncoated base paper) 394 10.2 0.91 Comparative Example 2 (Pure P34HB coating) 168 6.8 0.26 Comparative Example 3 (Pure KGM Coating) 175 7.2 0.28 Example 1 - KGM / P34HB (3:7) 125 5.0 0.14 Example 1 - KGM / P34HB (7:3) 110 4.2 0.10

[0143] Table 2 shows that the percentage of crack area in the uncoated sample increases continuously with the number of bends, reaching 0.91% after 20 bends. The percentages of crack area in the pure KGM and pure P34HB coated samples are 0.28% and 0.26%, respectively, indicating that a single coating can suppress crack propagation, but the effect is limited. The percentage of crack area in the KGM / P34HB composite coated samples of this invention is significantly reduced to 0.10%–0.14%, with the KGM / P34HB (7:3) sample showing only 0.10%, exhibiting the best resistance to cyclic bending.

[0144] 2.4. Non-contact full-field strain analysis

[0145] Strain field analysis was performed on the sample bent to 150°. Since the out-of-plane deformation of the bending area increases significantly when the bending angle exceeds 150°, the DIC system cannot effectively match it. Therefore, the strain data at 150° was selected for analysis. The average strain, maximum strain and strain concentration factor (calculated according to formula (6)) of different samples are shown in Table 3.

[0146] Table 3. Strain parameters of each sample when bent at 150°

[0147] Comparative Example 1 (Uncoated base paper) 1.2 7.0 5.83 Comparative Example 2 (Pure P34HB coating) 1.5 4.2 2.80 Comparative Example 3 (Pure KGM Coating) 1.4 4.5 3.21 Example 1 - KGM / P34HB (3:7) 1.8 2.8 1.56 Example 1 - KGM / P34HB (7:3) 1.9 2.4 1.26

[0148] As shown in Table 3:

[0149] The maximum strain of the uncoated base paper was 7.0%, and the strain concentration factor was as high as 5.83, indicating that the strain was highly concentrated in the crease area.

[0150] The maximum strain of the pure KGM and pure P34HB coated samples decreased to 4.5% and 4.2%, respectively, and the strain concentration factors decreased to 3.21 and 2.80, respectively, indicating that a single coating can alleviate strain concentration to a certain extent.

[0151] The maximum strain of the KGM / P34HB composite coating samples of this invention was further reduced to 2.4%–2.8%, and the strain concentration factor was reduced to 1.26–1.56, while the average strain increased (1.8%–1.9%). This indicates that the composite coating can disperse local strain over a larger area, effectively alleviating stress concentration at the crease tip. Among them, the KGM / P34HB (7:3) sample had the lowest strain concentration factor (1.26) and the best crack resistance.

[0152] 2.5. Coating Adhesion

[0153] The coating adhesion of each sample was tested, and the results are shown in Table 4.

[0154] Table 4 Adhesion ratings for each coated sample

[0155] Comparative Example 2 (Pure P34HB coating) Level 2 Comparative Example 3 (Pure KGM Coating) Level 0 Example 1 - KGM / P34HB (3:7) Level 1 Example 1 - KGM / P34HB (7:3) Level 0

[0156] Table 4 shows that the pure KGM coating exhibits the best adhesion, reaching grade 0, indicating good adhesion stability on the glossy surface of white cardboard. This may be related to the presence of numerous hydrophilic hydroxyl groups in the KGM molecule, resulting in better film-forming properties and surface adhesion, which facilitates better contact and coverage with the original pigment / latex coating on the glossy surface of the white cardboard. In contrast, the pure P34HB coating shows poor adhesion, at grade 2, indicating that P34HB, when used alone as a coating, has relatively weak interfacial bonding ability with the glossy surface of white cardboard. For the composite coating, the KGM / P34HB (7:3) coating has an adhesion grade of 0, comparable to the pure KGM coating; the KGM / P34HB (3:7) coating has an adhesion grade of 1, still at a good level, indicating that the introduction of KGM helps improve the adhesion performance and interfacial bonding state of P34HB on the glossy surface of white cardboard. Furthermore, the nano-scratch test results show that the average critical load Lc of the coating system is 8.9 mN, indicating that the coating has good interfacial stability on the glossy surface of white cardboard.

[0157] 2.6. Mechanical properties of composite membranes

[0158] Table 5 Mechanical properties of films prepared with different coatings

[0159]

[0160] Table 5 shows that the tensile strength of the pure KGM coating is 14.3 MPa, and the elongation at break is 26.2%. The tensile strength and elongation at break of the KGM / P34HB composite coating of this invention are significantly higher than those of the pure KGM coating. Specifically, the KGM / P34HB (7:3) composite coating has a tensile strength of 26.2 MPa and an elongation at break of 41.9%, exhibiting the best comprehensive mechanical properties; the KGM / P34HB (3:7) composite coating has a tensile strength of 21.2 MPa and an elongation at break of 30.9%. These results indicate that the introduction of P34HB can significantly improve the flexibility and tensile strength of the composite coating, which is beneficial for the coating to absorb and disperse strain during the bending process of white cardboard, thereby inhibiting the generation and propagation of cracks.

[0161] 3. Conclusion

[0162] Based on the above test results, the KGM / P34HB composite coating provided by this invention can improve the bending and crack resistance of white cardboard while maintaining good interfacial bonding performance. Among them, the KGM / P34HB (7:3) composite coating exhibits superior overall performance, showing a lower crack area percentage and a lower strain concentration coefficient under both single bending and cyclic bending conditions. Furthermore, the raw materials used are natural polymers or biodegradable polymers, and the composite coating is prepared using an aqueous system, meeting the requirements for the development of green packaging materials.

Claims

1. A coated white cardboard, characterized in that, It includes a base paper and a composite coating disposed on at least one surface of the base paper, the composite coating comprising konjac glucomannan and poly(3-hydroxybutyrate-co-4-hydroxybutyrate).

2. The coated white cardboard according to claim 1, characterized in that, The mass ratio of konjac glucomannan to poly(3-hydroxybutyrate-co-4-hydroxybutyrate) in the composite coating is 3:7 to 7:

3.

3. The coated white cardboard according to claim 1, characterized in that, The thickness of the composite coating is 1–5 μm.

4. The coated white cardboard according to claim 1, characterized in that, The composite coating also includes a plasticizer.

5. The coated white cardboard according to claim 4, characterized in that, The plasticizer content is 10% to 20% of the total mass of konjac glucomannan and poly(3-hydroxybutyrate-co-4-hydroxybutyrate).

6. A method for preparing coated white cardboard as described in any one of claims 1-5, characterized in that, The steps include the following: S1. Add konjac glucomannan to water and hydrate and dissolve the konjac glucomannan under stirring to obtain an aqueous phase of konjac glucomannan. S2. Add poly(3-hydroxybutyrate-co-4-hydroxybutyrate) aqueous dispersion to the aqueous phase of konjac glucomannan, mix evenly, and obtain a composite aqueous coating liquid; S3. Apply the composite aqueous coating liquid to the surface of white cardboard and dry it to prepare coated white cardboard.

7. The method for preparing coated white cardboard according to claim 6, characterized in that, In step S1, the mass concentration of konjac glucomannan in the aqueous phase of the konjac glucomannan is 0.5% to 2.0%.

8. The method for preparing coated white cardboard according to claim 6, characterized in that, In step S2, the poly(3-hydroxybutyrate-co-4-hydroxybutyrate) aqueous dispersion is prepared by the following method: poly(3-hydroxybutyrate-co-4-hydroxybutyrate) is added to water to form an aqueous suspension, a natural emulsifying stabilizer is added, and after dispersion and homogenization treatment, the poly(3-hydroxybutyrate-co-4-hydroxybutyrate) aqueous dispersion is obtained.

9. The method for preparing coated white cardboard according to claim 8, characterized in that, The natural emulsifying stabilizer is selected from at least one of low-methoxyl pectin and gum arabic.

10. The method for preparing coated white cardboard according to claim 6, characterized in that, In step S1 or step S2, a plasticizer is also added, wherein the amount of plasticizer added is 10% to 20% of the total solids of konjac glucomannan and poly(3-hydroxybutyrate-co-4-hydroxybutyrate).

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