Paper-based coating material and hot-pressing composite coating preparation method thereof

Abstract

The invention belongs to the field of paper-based material processing, and particularly relates to a paper-based coating material and a preparation method of a hot-pressing composite coating of the paper-based coating material. The paper-based coating material comprises a paper-based material, a cationic starch layer combined on the paper-based material, a PVA and PEI cross-linking layer combined on the cationic starch layer, and a cross-linking layer which is combined on the PVA and PEI cross-linking layer and contains PHA, polysaccharide and mixed wax. According to the paper-based coating material prepared through the hot-pressing composite coating preparation method, the preparation method is simple and efficient, the cost is effectively reduced, and the prepared paper-based coating material has excellent water resistance, oil resistance, gas resistance and heat sealing performance, is environmentally friendly and has wide application prospects.

Description

Technical Field

[0001] The present invention belongs to the field of paper-based material processing, and specifically relates to a paper-based coating material and a method for preparing a hot-pressed composite coating thereof. The invention utilizes hot-pressing crosslinking to crosslink polyhydroxyalkanoate (PHA), glutaraldehyde, and natural polysaccharides (such as chitosan, arabinose, cellulose, etc.), and to crosslink polyvinyl alcohol (PVA) and polyethyleneimine (PEI), so as to improve the barrier properties and heat-sealing properties of the paper-based material. Background Art

[0002] With growing environmental awareness and increasing demand for biodegradable materials, paper-based packaging materials are becoming an ideal alternative to traditional plastic packaging due to their recyclability and biodegradability. However, paper-based materials themselves have problems such as poor water resistance, oil resistance, and gas barrier properties, which limit their widespread application in the packaging field. Traditional methods for modifying paper-based materials mainly include:

[0003] (1) Coating method: Coating emulsion resin on the surface of paper. This method can improve the barrier properties of paper to a certain extent, but the bonding strength between the coating and the paper may be poor, and there are problems with the environmental friendliness of some emulsion resins.

[0004] (2) Lamination method: Lamination with plastic (i.e., coating process). The coating process is to coat the molten plastic (such as PE) on the surface of the paper to form a thin film. This method can give the paper better waterproof, oil-proof and heat-sealing properties, but its adhesion is poor: the bonding strength between the plastic film and the paper is poor and it is easy to peel off; poor environmental protection: the plastic film is difficult to degrade, which is not conducive to environmental protection; and recycling is difficult: the paper-plastic composite material is difficult to recycle.

[0005] A barrier layer or composite layer can be formed on the surface of paper by coating or lamination. For example, a single polymer layer such as polyvinyl alcohol (PVA) or polylactic acid (PLA) can be coated to improve the barrier properties of paper. However, the performance of a single polymer coating is often limited, and it is difficult to simultaneously meet multiple requirements such as high barrier, water resistance, oil resistance, and heat sealing. To solve the problem of single performance, multiple layers of different polymers are coated in sequence to achieve complementary performance, but the bonding force between the layers may be poor, and delamination is prone to occur. To solve the problem of easy delamination, chemical cross-linking is used, and a cross-linking agent is added to the coating. Through chemical reactions, a cross-linked grid is formed between the polymer molecules to improve the strength and stability of the coating. However, chemical cross-linking may use toxic or irritating chemical reagents, and the cross-linking reaction may not be uniform enough.

[0006] The following shortcomings of traditional paper-based material modification methods have hindered their development and application: (1) Insufficient barrier properties: a single coating or simple composite is difficult to achieve high water, oil, and gas resistance at the same time; (2) Poor interlayer adhesion: multi-layer coatings are prone to delamination; (3) Environmental issues: the use of non-degradable plastics or toxic chemical reagents; (4) Poor heat sealing performance: many coating materials do not have good heat sealing properties, which limits their application in the packaging field; (5) High cost: some high-performance coating materials or complex preparation processes are expensive. Summary of the Invention

[0007] In response to the deficiencies in the prior art, the present invention provides a paper-based coating material and a method for preparing a hot-pressed composite coating thereof. The paper-based coating material prepared by the hot-pressed composite coating preparation method of the present invention achieves efficient and uniform cross-linking of the materials in the coating through a hot pressing process, and significantly improves the bonding strength between the coating and the paper base, and between each coating, effectively preventing delamination, and having excellent water-blocking, oil-blocking, gas-blocking, and heat-sealing properties; it also uses environmentally friendly and degradable materials, which is green and environmentally friendly; at the same time, it optimizes the coating formula and preparation process, effectively reducing costs.

[0008] The invention provides a paper-based coating material, comprising a paper-based material, a cationic starch layer bonded to the paper-based material, a PVA and PEI cross-linked layer bonded to the cationic starch layer, and a cross-linked layer containing PHA, polysaccharide and mixed wax bonded to the PVA and PEI cross-linked layer.

[0009] In the PVA and PEI cross-linked layer, the mass of PEI is not less than 0.3 times the mass of PVA, preferably not less than 1 times, more preferably not less than 1.25 times, more preferably 1.25-2 times, more preferably 1.25-1.5 times.

[0010] The cross-linked layer of PHA, polysaccharide, and mixed wax contains a mass ratio of PHA, mixed wax, and polysaccharide of 85-100:3-15:1, preferably 90-95:5-10:1, and more preferably 90-92:5-8:1. The polysaccharide includes but is not limited to konjac glucomannan, arabinose, chitosan, etc. The mixed wax is a mixture of paraffin wax and palm wax, and the mass ratio of paraffin wax to palm wax is 1-4:1, preferably 1.5-3:1, and more preferably 2-2.6:1. The cross-linked layer of PHA, polysaccharide, and mixed wax also contains a cross-linking agent, and the mass ratio of cross-linking agent to PHA is 1:45-80, preferably 1:50; the cross-linking agent is glutaraldehyde.

[0011] Furthermore, the cross-linked layer of PHA, polysaccharide and mixed wax is formed by hot pressing a coating liquid formed by mixing PHA, polysaccharide solution and mixed wax emulsion, and the mass ratio of PHA, polysaccharide solution and mixed wax emulsion is 3-15:5-30:1, preferably 4.5-7.5:9.5-15.5:1, more preferably 5-7:10.5-15:1; the concentration of the polysaccharide solution is 0.4%-1% (w / w), preferably 0.5% (w / w), and the polysaccharide includes but Not limited to konjac glucomannan, arabinose, chitosan, etc.; the mixed wax emulsion is a mixture of paraffin emulsion and palm wax emulsion, the mass ratio of paraffin emulsion to palm wax emulsion is 1:0.4-1.5, more preferably 1:0.4-1, more preferably 1:0.4-0.5; the concentration of paraffin emulsion is 35%-45% (w / w), preferably 40% (w / w); the concentration of palm wax emulsion is 30%-45% (w / w), preferably 37% (w / w).

[0012] Furthermore, a cross-linking agent is added to the coating solution, and the mass ratio of the cross-linking agent to PHA is 1:45-80, preferably 1:50; the cross-linking agent is glutaraldehyde.

[0013] The present invention provides a method for preparing a hot-pressed composite coating of a paper-based coating material, the steps comprising:

[0014] (1) coating a cationic starch solution on the surface of a paper-based material, and hot pressing to form a cationic starch layer on the surface of the paper-based material;

[0015] (2) coating a mixed solution of PVA and PEI on the cationic starch layer, hot pressing, and bonding the PVA and PEI cross-linked layer on the cationic starch layer;

[0016] (3) coating a coating solution containing PHA, polysaccharide and mixed wax on the cross-linked layer of PVA and PEI, and hot pressing to bond a cross-linked layer containing PHA, polysaccharide and mixed wax on the cross-linked layer of PVA and PEI.

[0017] In step (1), the solid content of the cationic starch solution is 15%-38%, preferably 25%-30%; the coating amount of the cationic starch solution is 8-18g / m 2 , preferably 8-15g / m 2 The concentration and coating amount of cationic starch solution can be adjusted according to the type of starch and coating requirements.

[0018] In step (1), hot pressing is performed at 70°C-90°C for 10-30 seconds at a pressure of 0.6-1.5 MPa; preferably, hot pressing is performed at 80±5°C for 20 seconds at a pressure of 0.6-1.5 MPa. Hot pressing gelatinizes the starch, fills the pores on the paper surface, improves the smoothness, and provides a good adhesion foundation for subsequent coating.

[0019] In step (1), the paper base material is coated paper.

[0020] In step (2), the solid content of the mixed solution of PVA and PEI is 10%-30%, preferably 18%-24%, more preferably 22%; the mass of PEI is not less than 0.3 times the mass of PVA, preferably not less than 1 times, more preferably not less than 1.25 times, more preferably 1.25-2 times, more preferably 1.25-1.5 times. The coating amount of the mixed solution of PVA and PEI is 8-18 g / m 2 , preferably 8g / m 2 , the coating amount is adjusted according to the coating requirements.

[0021] In step (2), hot pressing is performed at 80°C-95°C for 15-25 seconds and a pressure of 0.6-1.5 MPa, preferably at 85±5°C for 25 seconds and a pressure of 0.6-1.5 MPa. This promotes hydrogen bonding and covalent cross-linking between PVA and PEI molecules to form a dense network structure, thereby improving the strength, water resistance and barrier properties of the coating.

[0022] In step (3), the coating liquid containing PHA, polysaccharide and mixed wax is prepared by mixing PHA, polysaccharide solution and mixed wax emulsion. Further, PHA and mixed wax emulsion are first mixed, and then the polysaccharide solution is added and mixed to prepare the coating liquid; the mass ratio of PHA, polysaccharide solution and mixed wax emulsion is 3-15:5-30:1, preferably 4.5-7.5:9.5-15.5:1, more preferably 5-7:10.5-15:1; the concentration of the polysaccharide solution is 0.4%-1% (w / w), preferably 0.5% (w / w), polysaccharides include but are not limited to konjac glucomannan, arabinose, chitosan, etc.; the mixed wax emulsion is a mixture of paraffin emulsion and palm wax emulsion, and the mass ratio of paraffin emulsion to palm wax emulsion is 1:0.4-1.5, more preferably 1:0.4-1, and more preferably 1:0.4-0.5; the concentration of paraffin emulsion is 35%-45% (w / w), preferably 40% (w / w); the concentration of palm wax emulsion is 30%-45% (w / w), preferably 37% (w / w).

[0023] Furthermore, a cross-linking agent is finally added to the coating solution and mixed evenly, with the mass ratio of the cross-linking agent to PHA being 1:45-80, preferably 1:50; the cross-linking agent is glutaraldehyde.

[0024] Furthermore, the coating amount of the coating liquid of PHA, polysaccharide and mixed wax is 8-18g / m 2 , preferably 10-12 / m 2 , the coating amount is adjusted according to the coating requirements.

[0025] In step (3), hot pressing is performed at 140°C-160°C for 10-20 seconds at a pressure of 0.6-1.5 MPa; preferably, hot pressing is performed at 150±5°C-160°C for 15 seconds at a pressure of 0.6-1.5 MPa. Hot pressing promotes cross-linking of the PHA, polysaccharide, and glutaraldehyde: the aldehyde group (-CHO) of glutaraldehyde reacts with the hydroxyl or carboxyl groups of the PHA and the hydroxyl, amino, or aldehyde groups of the polysaccharide to form acetal bonds, imine bonds (Schiff bases), or ester bonds, thereby connecting the PHA and polysaccharide molecules to form a three-dimensional network structure, further improving the strength, water resistance, oil resistance, and barrier properties of the coating.

[0026] During the coating in steps (1), (2) and (3), a wet film preparation device is used to perform multiple coatings, and the coating thickness each time is 80 microns.

[0027] Compared with the prior art, the advantages of the present invention are:

[0028] 1. Excellent barrier properties: Through multi-layer coating and hot-pressing cross-linking, the paper-based composite coating material prepared has excellent water-proof, oil-proof and gas-proof properties, which can effectively extend the shelf life of packaged items.

[0029] 2. The water contact angle can reach about 100°.

[0030] 3. Cobb value can be controlled at 15-20g / m 2 .

[0031] 4. Good heat sealing performance: The prepared paper-based material can achieve double-sided heat sealing at 210°C, meeting the requirements of packaging sealing

[0032] 5. High strength and stability: The dense network structure formed by hot pressing cross-linking significantly improves the mechanical strength, water resistance and oil resistance of the coating, and prevents the coating from delamination.

[0033] 6. Environmental protection and biodegradability: It uses environmentally friendly and biodegradable materials such as starch, PVA, PEI, PHA and natural polysaccharides, which is in line with the development trend of green packaging.

[0034] 7. Simple and efficient process: The hot pressing process is simple to operate, has high production efficiency, and is easy to realize industrial production.

[0035] 8. Cost controllable: By optimizing material selection and process parameters, production costs can be effectively controlled. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a performance test result diagram of the paper-based coating material prepared in Example 1.

[0037] Figure 2The metallographic microscope image of the coating surface of the paper-based coating material prepared in Example 1 (left figure), the metallographic microscope image of the coating cross-section (middle figure), and the scanning electron microscope image of the coating surface of the paper-based coating material (right figure) are shown.

[0038] Figure 3 This is a performance test result diagram of the paper-based coating material prepared at different mass ratios of PVA and PEI (4:5, 5:6, 7:3) in Example 2.

[0039] Figure 4 These are metallographic microscopy images of the coating surface (left figure), metallographic microscopy images of the coating cross-section (middle figure), and scanning electron microscopy images of the coating surface (right figure) of the paper-based coating material prepared at different mass ratios of PVA and PEI (4:5, 5:6, and 7:3) in Example 2.

[0040] Figure 5 This is a performance test result diagram of paper-based coating materials prepared with mixed wax emulsions of different solid contents (10%, 20%, 30%) in Example 3.

[0041] Figure 6 Metallographic microscopy images of the coating surface (left figure), metallographic microscopy images of the coating cross-section (middle figure), and scanning electron microscopy images of the coating surface (right figure) of the paper-based coating material prepared from mixed wax emulsions with different solid contents (10%, 20%, 30%) in Example 3.

[0042] Figure 7 This is a performance test result diagram of the paper-based coating material prepared in Example 4.

[0043] Figure 8 The metallographic microscope image of the coating surface of the paper-based coating material prepared in Example 4 (left figure), the metallographic microscope image of the coating cross-section (middle figure), and the scanning electron microscope image of the coating surface of the paper-based coating material (right figure) are shown. DETAILED DESCRIPTION

[0044] Example 1

[0045] Objective: To promote the cross-linking of PHA, glutaraldehyde and polysaccharide through hot pressing process, and simultaneously realize the cross-linking of PVA and PEI, so as to improve the barrier properties, heat sealing properties and mechanical properties of paper-based coating.

[0046] First coat:

[0047] Preparation of coating solution: Use a cationic starch solution with a solid content of 25%. Cationic starch has excellent cross-linking properties and good hydrophilicity.

[0048] Coating: evenly coat the cationic starch solution on the surface of the paper base (coated paper) with a coating amount of 8g / m 2 .

[0049] Hot pressing: Hot pressing at 80°C for 20 seconds (no vacuum mode, pressure 0.6-1.5MPa). During the hot pressing process, the cationic starch is gelatinized to fill the gaps on the surface of the paper, making the paper smoother.

[0050] Second coating:

[0051] Preparation of coating solution: Polyvinyl alcohol (PVA) and polyethyleneimine (PEI) were mixed in a ratio of 5:6 to prepare a mixed solution with a solid content of 22%.

[0052] Coating: Apply the above PVA and PEI mixed solution on the first starch coating layer with a coating amount of 10g / m 2 .

[0053] Hot pressing: Hot pressing at 85°C for 20 seconds (no vacuum mode, pressure 0.6-1.5 MPa). The hot pressing process allows hydrogen bonds and covalent bonds between PVA and PEI to crosslink to form a dense network structure, further enhancing the mechanical strength, water resistance and gas barrier properties of the coating.

[0054] The third coating:

[0055] Preparation of the coating solution: Dissolve 0.5g of polysaccharide (such as konjac glucomannan, arabinose, or chitosan) in 99.5g of water to prepare a 0.5% (w / w) polysaccharide solution. Mix 3.15g of a 40% solids paraffin wax emulsion, 1.35g of a 37% solids palm wax emulsion (the mixed wax emulsion accounts for 15% of the PHA solids content, a weight ratio of 7:3), and 30g of PHA. Add 65.5g of the 0.5% polysaccharide solution, and finally, add 0.6g of glutaraldehyde (2% of the PHA solids weight) to obtain 100g of the third coating solution. PHA reacts with the polysaccharide during cross-linking, improving the coating's water resistance, oil resistance, and gas barrier properties.

[0056] Coating: Apply the above PHA emulsion on the second coating layer with a coating amount of 15g / m 2 .

[0057] Hot Pressing: Heat press at 150°C for 15 seconds (no vacuum mode, pressure 0.6-1.5 MPa). During the hot pressing process, PHA reacts with glutaraldehyde through its hydroxyl or carboxyl groups. Glutaraldehyde simultaneously reacts with hydroxyl, amino, or aldehyde groups in the polysaccharide molecules to form acetal bonds, imine bonds, or acetal networks, thereby constructing a three-dimensional cross-linked structure. The polysaccharide serves as the backbone component of the cross-linked network, and glutaraldehyde acts as a bifunctional cross-linking agent. Through the condensation reaction under hot pressing conditions, the PHA and polysaccharide are covalently cross-linked, enhancing the material's mechanical properties and biocompatibility. Coating Tool: An 80-micron wet film applicator is used.

[0058] Cross-linking mechanism

[0059] 1. PVA and PEI cross-linking mechanism:

[0060] The crosslinking of PVA and PEI primarily forms a network structure through hydrogen bonding and covalent bonding. PVA molecules contain multiple -OH groups, while PEI molecules contain multiple -NH2 groups. These functional groups form a crosslinked network through hydrogen bonding. At high temperatures, hydrogen bonding between PEI's amine groups and PVA's hydroxyl groups creates a tightly bound crosslinked network between PVA and PEI on the paper surface.

[0061] 2. PHA and natural polysaccharide cross-linking mechanism:

[0062] Glutaraldehyde as a bridge: The glutaraldehyde molecule contains two active aldehyde groups (-CHO), which can serve as a cross-linking bridge to connect different molecules. In the cross-linking reaction, the aldehyde groups of glutaraldehyde react chemically with the active functional groups (such as hydroxyl, amino or aldehyde groups) in PHA and polysaccharides to form chemical bonds. Participation of PHA: The hydroxyl (-OH) or terminal carboxyl (-COOH) groups in PHA react with glutaraldehyde under hot pressing or catalytic conditions to form stable acetal bonds or ester bonds. PHA serves as a matrix material, providing good biodegradability and mechanical properties. Participation of polysaccharides: Hydroxyl (-OH): reacts with glutaraldehyde to form acetal bonds, enhancing the strength and flexibility of the material. Amino (-NH2) (such as chitosan): reacts with glutaraldehyde to form imine bonds, increasing the cross-linking density and stability. Aldehyde (-CHO) (such as oxidized polysaccharides): reacts with glutaraldehyde or other aldehyde groups to further form a three-dimensional network. Hot pressing promotes reaction: Hot pressing conditions (high temperature and pressure) can activate the activity of reactants, accelerate the formation of chemical bonds, and ensure uniform molding of materials.

[0063] The water contact angle was measured using a contact angle meter. 5 μL of water was added and the contact angle was recorded after standing for 5 minutes. The effect of the waterproof coating on the water absorption of the paperboard was measured using a Cobb value tester according to GB / T 1540-2012 standard. The Cobb value (unit: g / m 2 ); Use air permeability and moisture permeability testers to measure the effect of the coating on air permeability and water vapor permeability, and record the air permeability and moisture permeability data (unit: g / m 2 24h, cm 3 / m 2 ·s), the performance test results of Example 1 are shown in Figure 1 .

[0064] The metallographic microscope image of the coating surface of the paper-based coating material prepared in Example 1, the metallographic microscope image of the coating cross section (as shown in the figure, the left side is the coating side) and the scanning electron microscope image of the coating surface are as follows: Figure 2 As shown, Figure 2The microstructure of the paper-based coating material prepared in Example 1 is shown. Metallographic microscopy images of the surface show that the coating surface is relatively flat and smooth, without noticeable roughness, particles, or holes. Scanning electron microscopy images further confirm the coating's compactness and uniformity. Metallographic microscopy cross-sectional images show that the coating is located on the left side of the paper-based material, forming a thin layer. The coating is tightly bonded to the paper-based material, with no delamination or shedding, and the coating thickness is relatively uniform. These results demonstrate that the coating prepared in Example 1 has good surface quality, a dense structure, and uniform thickness, which is beneficial for improving the barrier properties of the coating.

[0065] Example 2

[0066] Objective: To study the effects of different cross-linking ratios on the mechanical properties, water resistance and gas barrier properties of the coatings by adjusting the mass ratio of PVA to PEI, with an emphasis on the role of hot pressing in the cross-linking reaction.

[0067] First coat:

[0068] Preparation of coating liquid: A cationic starch solution with a solid content of 25% is used. Cationic starch has good hydrophilicity and excellent cross-linking performance in the cross-linking reaction.

[0069] Coating: evenly coat the cationic starch solution on the surface of the paper base (coated paper) with a coating amount of 15g / m 2 .

[0070] Hot pressing: Hot pressing at 80°C for 20 seconds (no vacuum mode, pressure 0.6-1.5 MPa) to gelatinize the cationic starch, fill the gaps on the paper surface, and make the paper smoother.

[0071] Second coating:

[0072] Preparation of coating solution: PVA and PEI were mixed at a mass ratio of 4:5, 5:6, and 7:3, respectively, to prepare a solution with a solid content of 22%.

[0073] Coating: The mixed solutions of PVA and PEI in different proportions were coated on the first starch coating layer, with a coating amount of 8g / m 2 .

[0074] Hot pressing: Hot pressing at 90°C for 25 seconds (no vacuum mode, pressure 0.6-1.5MPa). During the hot pressing process, PVA and PEI are cross-linked through hydrogen bonds and covalent bonds to form a dense cross-linked network, further enhancing the mechanical strength, water resistance and gas barrier properties of the coating.

[0075] The third coating:

[0076] Preparation of the coating solution: Dissolve 0.5g of polysaccharide (such as konjac glucomannan, arabinose, or chitosan) in 99.5g of water to prepare a 0.5% (w / w) polysaccharide solution. Mix 2.25g of a 40% solids paraffin wax emulsion, 2.25g of a 37% solids palm wax emulsion (the mixed wax emulsion accounts for 15% of the PHA solids content, a 1:1 weight ratio), and 30g of PHA. Add 65.5g of the 0.5% polysaccharide solution, and finally, add 0.6g of glutaraldehyde (2% of the PHA solids weight) to obtain 100g of the third coating solution. PHA reacts with the polysaccharide during cross-linking, improving the coating's water resistance, oil resistance, and gas barrier properties.

[0077] Coating: Apply the above PHA emulsion on the second coating layer with a coating amount of 12g / m 2 .

[0078] Hot pressing: Hot pressing is performed at 150°C for 15 seconds (no vacuum mode, pressure 0.6-1.5 MPa). During the hot pressing process, PHA reacts with glutaraldehyde through its hydroxyl or carboxyl groups. At the same time, glutaraldehyde reacts with the hydroxyl, amino or aldehyde groups in the polysaccharide molecules to form acetal bonds, imine bonds or acetal networks, thereby constructing a three-dimensional cross-linked structure. Polysaccharide serves as the skeleton component of the cross-linked network, and glutaraldehyde serves as a bifunctional cross-linking agent. The covalent cross-linking of PHA and polysaccharide is achieved through the condensation reaction under hot pressing conditions, thereby enhancing the mechanical properties and biocompatibility of the material.

[0079] Coating tool: Use 80 micron wet film preparer.

[0080] Performance testing: The composite coatings prepared with different ratios of PVA to PEI in Example 2 were subjected to relevant performance tests. A contact angle meter was used to measure the water contact angle. 5 μL of water was added and the contact angle was recorded after standing for 5 minutes. A Cobb value tester was used to measure the effect of the waterproof coating on the water absorption of the paperboard according to the GB / T 1540-2012 standard. The Cobb value (unit: g / m 2 ); Use air permeability and moisture permeability testers to measure the effect of the coating on air permeability and water vapor permeability, and record the air permeability and moisture permeability data (unit: g / m 2 24h, cm 3 / m 2 ·s), the obtained data are used to evaluate the effect of different cross-linking ratios on coating performance. The parameter table is shown in Figure 3 .

[0081] The metallographic microscope image of the coating surface of the paper-based coating material prepared in Example 2, the metallographic microscope image of the coating cross section (as shown in the figure, the left side is the coating side) and the scanning electron microscope image of the coating surface are as follows: Figure 4As shown, the microstructure of the paper-based coating material prepared in Example 2 at different PVA:PEI ratios. The surface image of the metallographic microscope shows that when the PVA:PEI ratio is 4:5, the coating surface is highly flat and smooth; the scanning electron microscope image further confirms that it has a dense and uniform cross-linked network structure without obvious pores or defects; the cross-sectional image of the metallographic microscope shows that the cross-section between the coating and the paper base is clear and tightly bonded, the coating thickness is uniform (about 8-10μm), and there is no delamination. When the ratio is adjusted to 5:6 and 7:3, the coating surface shows a significant increase in roughness, with local protrusions or depressions, and microcracks or loose structures can be seen in some areas under the scanning electron microscope; the cross-sectional image shows that the coating thickness fluctuates greatly (12-15μm), there is a tendency for interlayer peeling in local areas, and the porosity increases significantly, indicating that the incomplete cross-linking reaction leads to a decrease in structural stability. The above results show that the coating with a PVA:PEI ratio of 4:5 has better surface quality, cross-sectional uniformity and potential barrier properties.

[0082] Example 3

[0083] Objective: To study the effect of the ratio of mixed paraffin wax emulsion and palm wax emulsion in the third layer of PHA coating on the cross-linking of PHA and natural polysaccharides, and to further optimize the waterproofness, oil resistance and gas barrier properties of the coating.

[0084] First coat:

[0085] Coating liquid preparation: Use 30% solid content cationic starch solution, coating amount is 15g / m 2 .

[0086] Hot pressing: Hot pressing at 80°C for 25 seconds (no vacuum mode, pressure 0.6-1.5 MPa) to gelatinize the cationic starch, fill the gaps on the paper surface, and make the paper smoother.

[0087] Second coating:

[0088] Preparation of coating solution: PVA and PEI were mixed in a mass ratio of 5:6 to prepare a mixed solution with a solid content of 22%. The coating amount was 8 g / m 2 .

[0089] Hot pressing: Hot pressing at 90°C for 30 seconds (no vacuum mode, pressure 0.6-1.5 MPa), PVA and PEI are cross-linked through hydrogen bonds and covalent bonds, which improves the mechanical properties and oxygen barrier properties of the coating.

[0090] The third coating:

[0091] Case 1: Mixed wax emulsion accounts for 10% of PHA solid content

[0092] Preparation of coating liquid: 0.5g of polysaccharide (konjac glucomannan, arabinose, chitosan, etc.) was dissolved in 99.5g of water to prepare a 0.5% (w / w) polysaccharide solution for use; 2.1g of paraffin emulsion with a solid content of 40%, 0.9g of palm wax emulsion with a solid content of 37% (the total mass of the mixed wax emulsion is 3g, accounting for 10% of the PHA solid content, and the mass ratio of paraffin to palm wax is 7:3) and 30g of PHA (solid) were mixed, and then 66.4g of 0.5% polysaccharide solution was added, and finally 0.6g of glutaraldehyde (2% of the PHA solid mass) was added and stirred evenly to obtain 100g of the third layer of coating liquid.

[0093] Case 2: Mixed wax emulsion accounts for 20% of PHA solid content

[0094] 0.5 g of polysaccharide (konjac glucomannan, arabinose, chitosan, etc.) was dissolved in 99.5 g of water to prepare a 0.5% (w / w) polysaccharide solution for use; 4.2 g of paraffin emulsion with a solid content of 40%, 1.8 g of palm wax emulsion with a solid content of 37% (the total mass of the mixed wax emulsion was 6 g, accounting for 20% of the PHA solid content, and the mass ratio of paraffin to palm wax was 7:3) and 30 g of PHA (solid) were mixed, and then 63.4 g of 0.5% polysaccharide solution was added, and finally 0.6 g of glutaraldehyde (2% of the PHA solid mass) was added and stirred evenly to obtain 100 g of the third layer coating liquid.

[0095] Case 3: Mixed wax emulsion accounts for 30% of PHA solid content

[0096] 0.5 g of polysaccharide (konjac glucomannan, arabinose, chitosan, etc.) was dissolved in 99.5 g of water to prepare a 0.5% (w / w) polysaccharide solution for use; 6.3 g of paraffin emulsion with a solid content of 40%, 2.7 g of palm wax emulsion with a solid content of 37% (the total mass of the mixed wax emulsion was 9 g, accounting for 30% of the PHA solid content, and the mass ratio of paraffin to palm wax was 7:3) and 30 g of PHA (solid) were mixed, and then 60.4 g of 0.5% polysaccharide solution was added, and finally 0.6 g of glutaraldehyde (2% of the PHA solid mass) was added and stirred evenly to obtain 100 g of the third layer coating liquid.

[0097] Hot pressing: Hot pressing at 150°C for 20 seconds (no vacuum mode, pressure 0.6-1.5MPa). During the hot pressing process, PHA reacts with glutaraldehyde through its hydroxyl or carboxyl groups. At the same time, glutaraldehyde reacts with the hydroxyl, amino or aldehyde groups in the polysaccharide molecules to form acetal bonds, imine bonds or acetal networks, thereby constructing a three-dimensional cross-linked structure. Polysaccharide serves as the skeleton component of the cross-linked network, and glutaraldehyde serves as a bifunctional cross-linking agent. The covalent cross-linking of PHA and polysaccharide is achieved through the condensation reaction under hot pressing conditions, thereby enhancing the mechanical properties and biocompatibility of the material.

[0098] Coating tool: Use 80 micron wet film preparer.

[0099] Performance testing: The composite coatings prepared at different mixing ratios in Example 3 were subjected to relevant performance tests. A contact angle meter was used to measure the water contact angle. 5 μL of water was added and the contact angle was recorded after standing for 5 minutes. A Cobb value tester was used to measure the effect of the waterproof coating on the water absorption of the paperboard according to the GB / T 1540-2012 standard. The Cobb value (unit: g / m 2 ); Use air permeability and moisture permeability testers to measure the effect of the coating on air permeability and water vapor permeability, and record the air permeability and moisture permeability data (unit: g / m 2 24h, cm 3 / m 2 ·s), the data obtained are used to evaluate the effect of different mixing ratios on coating performance. The parameter table is shown in Figure 5 .

[0100] The metallographic microscope image of the coating surface of the paper-based coating material prepared in Example 3, the metallographic microscope image of the coating cross section (as shown in the figure, the left side is the coating side) and the scanning electron microscope image of the coating surface are shown in FIG. Figure 6 As shown, Figure 6 The microstructures of paper-based coatings prepared in Example 3 with varying PHA solids contents are shown. Surface images obtained through metallographic microscopy reveal that the coating with a 20% PHA solids content is the flattest and smoothest, with virtually no visible defects or holes. Coatings with 10% and 30% PHA solids contents, on the other hand, exhibit relatively rough surfaces, with numerous lines, bumps, and particles. Scanning electron microscopy images further confirm that the coating with 20% PHA solids content is the most dense and uniform. Cross-sectional metallographic microscopy images reveal that all three coatings adhere well to the paper substrate. However, the coating with 20% PHA solids content exhibits moderate and uniform thickness, while the coating with 10% PHA solids content is thinner, and the coating with 30% PHA solids content is thicker and may exhibit uneven thickness. These results indicate that the coating with a 20% PHA solids content exhibits superior surface quality, cross-sectional uniformity, and density, which may be related to its superior barrier properties.

[0101] Example 4

[0102] This example differs from Example 1 in that the ratio of PVA to PEI is 4:5, and the mass ratio of paraffin wax emulsion to palm wax emulsion is 1:1. Objective: To verify, through comparison, the key role of hot pressing in cross-linking each coating and improving the overall performance of the paper-based coating.

[0103] First coat:

[0104] Preparation of coating solution: Use a cationic starch solution with a solid content of 25%. Cationic starch has excellent cross-linking properties and good hydrophilicity.

[0105] Coating: evenly coat the cationic starch solution on the surface of the paper base (coated paper) with a coating amount of 15g / m 2 .

[0106] Hot pressing: Hot pressing at 80°C for 20 seconds (no vacuum mode, pressure 0.6-1.5 MPa) to gelatinize the cationic starch, fill the gaps on the paper surface, and make the paper smoother.

[0107] Second coating:

[0108] Preparation of coating solution: Polyvinyl alcohol (PVA) and polyethyleneimine (PEI) were mixed in a ratio of 4:5 to prepare a mixed solution with a solid content of 22%.

[0109] Coating: Apply the above PVA and PEI mixed solution on the first starch coating layer with a coating amount of 8g / m 2 .

[0110] Hot pressing: Hot pressing at 85°C for 20 seconds (no vacuum mode, pressure 0.6-1.5 MPa). The hot pressing process allows hydrogen bonds and covalent bonds between PVA and PEI to crosslink to form a dense network structure, further enhancing the mechanical strength, water resistance and gas barrier properties of the coating.

[0111] The third coating:

[0112] Coating solution preparation: Dissolve 0.5g of polysaccharide (such as konjac glucomannan, arabinose, or chitosan) in 99.5g of water to prepare a 0.5% (w / w) polysaccharide solution. Mix 2.25g of a 40% solids paraffin wax emulsion, 2.25g of a 37% solids palm wax emulsion (the mixed wax emulsion accounts for 15% of the PHA solids content, a 1:1 weight ratio), and 30g of PHA. Add 65.5g of the 0.5% polysaccharide solution, and finally, add 0.6g of glutaraldehyde (2% of the PHA solids weight) to obtain 100g of the third coating solution. PHA reacts with the polysaccharide during cross-linking, improving the coating's water resistance, oil resistance, and gas barrier properties.

[0113] Coating: Apply the above PHA emulsion on the second coating layer with a coating amount of 12g / m 2 .

[0114] Hot pressing: Hot pressing at 150°C for 15 seconds (no vacuum mode, pressure 0.6-1.5 MPa). During the hot pressing process, PHA reacts with glutaraldehyde through its hydroxyl or carboxyl groups. At the same time, glutaraldehyde reacts with the hydroxyl, amino or aldehyde groups in the polysaccharide molecules to form acetal bonds, imine bonds or acetal networks, thereby constructing a three-dimensional cross-linked structure and enhancing the mechanical properties and biocompatibility of the material.

[0115] Coating tool: Use 80 micron wet film preparer.

[0116] The metallographic micrograph of the coating surface of the paper-based coating material prepared in Example 4, the metallographic micrograph of the coating cross section (as shown in the figure, the left side is the coating side) and the scanning electron micrograph of the coating surface are shown in FIG. Figure 8 shown.

[0117] Performance test: The composite coating prepared in Example 4 was subjected to relevant performance tests. A contact angle meter was used to measure the water contact angle. 5 μL of water was added and the contact angle was recorded after standing for 5 minutes. A Cobb value tester was used to measure the effect of the waterproof coating on the water absorption of the paperboard according to the GB / T 1540-2012 standard. The Cobb value (unit: g / m 2 ); Use air permeability and moisture permeability testers to measure the effect of the coating on air permeability and water vapor permeability, and record the air permeability and moisture permeability data (unit: g / m 2 24h, cm 3 / m 2 ·s). Parameter table see Figure 7 , and the performance comparison results with the embodiment are shown in Table 1:

[0118] Table 1 Performance comparison results of this embodiment and embodiment 1

[0119]

Claims

1. A paper-based coating material, characterized in that: The invention comprises a paper-based material, a cationic starch layer bonded to the paper-based material, a PVA and PEI cross-linked layer bonded to the cationic starch layer, and a cross-linked layer containing PHA, polysaccharide and mixed wax bonded to the PVA and PEI cross-linked layer.

2. A paper-based coating material according to claim 1, characterized in that: In the PVA and PEI cross-linked layer, the mass of PEI is not less than 0.3 times the mass of PVA; The cross-linked layer of PHA, polysaccharide and mixed wax contains PHA, mixed wax and polysaccharide in a mass ratio of 85-100:3-15:

1.

3. The method for preparing a hot-pressed composite coating of a paper-based coating material according to claim 1 or 2, characterized in that the steps include: (1) coating a cationic starch solution on the surface of a paper-based material, and hot pressing to form a cationic starch layer on the surface of the paper-based material; (2) coating a mixed solution of PVA and PEI on the cationic starch layer, hot pressing, and bonding the PVA and PEI cross-linked layer on the cationic starch layer; (3) coating a coating solution containing PHA, polysaccharide and mixed wax on the cross-linked layer of PVA and PEI, and hot pressing to bond a cross-linked layer of PHA, polysaccharide and mixed wax on the cross-linked layer of PVA and PEI.

4. The method for preparing a hot pressing composite coating according to claim 3, characterized in that: In step (1), the solid content of the cationic starch solution is 15%-38%.

5. The method for preparing a hot pressing composite coating according to claim 3, characterized in that: In step (1), hot pressing is performed at 70°C-90°C for 10-30s and a pressure of 0.6-1.5 MPa.

6. The method for preparing a hot pressing composite coating according to claim 3, characterized in that: In step (2), the solid content of the mixed solution of PVA and PEI is 10%-30%, and the mass of PEI is not less than 0.3 times the mass of PVA.

7. The method for preparing a hot pressing composite coating according to claim 3, characterized in that: In step (2), hot pressing is performed at 80°C-95°C for 15-25s and a pressure of 0.6-1.5 MPa.

8. The method for preparing a hot pressing composite coating according to claim 3, characterized in that: In step (3), the coating liquid containing PHA, polysaccharide and mixed wax is prepared by mixing PHA, polysaccharide solution and mixed wax emulsion, and the mass ratio of PHA, polysaccharide solution and mixed wax emulsion is 3-15:5-30:

1.

9. The method for preparing a hot-pressed composite coating according to claim 8, characterized in that: In step (3), a cross-linking agent is finally added to the coating solution and mixed, and the mass ratio of the cross-linking agent to PHA is 1:45-80.

10. The method for preparing a hot pressing composite coating according to claim 3, characterized in that: In step (3), hot pressing is performed at 140°C-160°C for 10-20s and a pressure of 0.6-1.5 MPa.

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