Preparation process of environment-friendly degradable thermo-sensitive paper for food

Abstract

The invention discloses a preparation process of environment-friendly degradable thermo-sensitive paper for food, and relates to the technical field of thermo-sensitive paper. Wood pulp and polylactic acid fibers are pulped, lignin is used for backfilling, so that interaction among the fibers is enhanced, pores of prepared paper are reduced along with improvement of the pulping degree, a matrix indirectly achieves the oil-proof effect, then the interior of the matrix is in a porous form through vacuum freeze drying treatment, and under assistance of ultrasonic waves, the oil-proof effect of the matrix is improved. A starch composite zein protein solution is introduced to realize an easy degradation effect, and alkaline protease is utilized to hydrolyze zein protein and react with glucose, so that a dried protein structure becomes compact, a flexible film layer structure is formed on the surface of a matrix, and the transmittance of water molecules and grease molecules is reduced; the easily degradable and oil-proof effects are further improved.

Description

Technical Field

[0001] The invention relates to the technical field of thermal paper, and in particular to a preparation process of environmentally friendly degradable thermal paper for food. Background Art

[0002] Thermal paper is a heat-sensitive material that can be directly printed without other consumables after the surface coating of paper, film and other substrates is processed. The thermal print head releases heat to the thermal coating of the thermal paper, and the dye is colored under the dissolution of the color developer in the coating. Since the invention of the barcode, thermal paper has been widely used in the logistics industry, and has gradually been introduced into various fields by being able to print text, barcodes and other images under the control of printing software. With the continuous improvement of technology, the application field of thermal paper is still expanding.

[0003] In the food industry, thermal paper can not only be used for ordering and issuing bills, but also for recording ingredients, printing recipes, etc. It can also help kitchen chefs record dishes more conveniently and quickly, while reducing the error rate and improving efficiency. In the takeaway industry, because it can print orders and receipts, it is convenient for deliverymen and customers to confirm order information, etc. Therefore, it is necessary to ensure that the printed labels or products will not become blurred due to substances such as grease, thereby reducing work efficiency. In order to effectively prevent the adhesion of oil stains or stains, fluorine-containing compounds and other substances with low surface energy are often used on the market to coat and modify the paper surface to achieve an anti-oil effect. However, scientists have shown that fluoride is carcinogenic to humans; and thermal paper food labels are consumables in daily life. Therefore, it is particularly necessary to invent an environmentally friendly and degradable thermal paper for food. Summary of the invention

[0004] The purpose of the present invention is to provide a preparation process of environmentally friendly degradable thermal paper for food, so as to solve the problems existing in the prior art.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions: a preparation process of environmentally friendly degradable thermal paper for food, the preparation method of environmentally friendly degradable thermal paper for food comprising the following preparation steps: (1) Mix wood pulp, polylactic acid fiber, and deionized water, beat for 20-50 min, add lignin, continue beating for 30 min, and add 6×10 6 Polyacrylamide, polyamide epichlorohydrin resin, alkyl ketene dimer, 500-1000prm, stirring for 3-10min, passing through a 60-mesh sieve, to obtain a slurry with a beating degree of 50-80°SR; (2) The slurry is sequentially sheeted, molded, and dried to obtain a paper base; (3) Immerse the paper base in a starch-composite corn protein solution, add 40 wt % sodium hydroxide aqueous solution to adjust the solution pH to 9, heat to 50 ° C, ultrasonicate at 21 kHz for 20-50 min, heat to 100 ° C, let stand for 5 min, remove, immerse in 10 wt % glucose aqueous solution, heat to 70 ° C, continue ultrasonication for 10-20 min, then wash with deionized water 1-3 times, dry at 40-60 ° C for 4-8 h, and obtain a degradable paper base; (4) The degradable paper base is coated with a thermal coating layer and a protective layer in sequence to obtain an environmentally friendly degradable thermal paper for food.

[0006] Furthermore, the wood pulp in step (1) is prepared by mixing and beating a bleached coniferous wood pulp board and a bleached hardwood wood pulp board in a mass ratio of 7:3 to a SR of 37°.

[0007] Furthermore, the beating pressure in step (1) is 3.5-8 kg.

[0008] Furthermore, in step (1), the wood pulp, polylactic acid fiber, deionized water, lignin, and a molecular weight of 6×10 6 The mass ratio of polyacrylamide, polyamide epichlorohydrin resin and alkyl ketene dimer is 1~5:0.1~0.6:80~150:0.05:0.05:0.2~0.5:0.3.

[0009] Furthermore, the basis weight of the sheet in step (2) is 200-250 g / m 2 .

[0010] Furthermore, the molding process parameters of step (2) are as follows: temperature of 168-175° C., pressure of 6 MPa, and time of 2-5 min.

[0011] Furthermore, the drying conditions in step (2) are: vacuum degree of 10-50 Pa, temperature of -56°C, and time of 24-48 h.

[0012] Furthermore, the starch-composite corn protein solution in step (3) is prepared by mixing corn protein, alkaline protease, cationic starch and deionized water in a mass ratio of 5-10:0.1:5-10:100-200.

[0013] Furthermore, the coating of the heat-sensitive coating in step (4) is prepared by mixing silica, 3-(3-methylureido)phenyl-4-methylbenzenesulfonate, 1,2-diphenoxyethane, polyvinyl alcohol with a molecular weight of 20,000, 2-(2-4-xylamino)-3-methyl-6-diethylaminofluoran, and calcium stearate in a mass ratio of 7:15:10:3:6:2.5; the coating of the protective layer is prepared by mixing silica, polyvinyl alcohol with a molecular weight of 20,000, polyamide polyurea, and calcium stearate in a mass ratio of 8:6:3:3.

[0014] Furthermore, the coating amount of the thermal-sensitive coating in step (4) is 5-12 g / m 2 ; The coating amount of the protective layer is 1~5g / m 2 .

[0015] Compared with the prior art, the beneficial effects achieved by the present invention are: The thermal paper of the invention is prepared from wood pulp and polylactic acid fiber through pulping and freezing treatment to obtain a paper base, and then immersed in a starch composite corn protein solution to achieve the effects of easy degradation and oil resistance.

[0016] Firstly, the present invention uses wood pulp as raw material and polylactic acid fiber as auxiliary material. During the beating process, the polylactic acid fiber can fill the gaps between fibers in the paper forming process, thereby enhancing the interaction between the fibers, which is beneficial to increasing the bonding area and using lignin for backfilling. As the beating degree increases, the fiber fibrillation will reduce the pore radius, while increasing the hydrogen bond content and increasing the bonding force between the fibers, thereby reducing the pores in the prepared paper and reducing the passage of grease, thereby indirectly making the matrix achieve an oil-proof effect. Subsequently, freeze-drying treatment is performed to directly freeze the suspension into ice to form a fine pore structure to give the material surface sufficient roughness, and then the solid solvent is sublimated by low-pressure vacuuming to make the inside of the matrix porous. At the same time, the fibers are kept in a fibrillated state after being fibrillated to avoid keratinization, and the surface microfibrillation morphology is maintained.

[0017] Secondly, the paper is immersed in a starch-composite corn protein solution, and with the assistance of ultrasound, alkaline protease is used to hydrolyze corn protein, and the composite starch enters the porous structure of the matrix, and the starch decay is used to cause the material to split into fine particles, so that the paper base can achieve an easily degradable effect. Then, the temperature is increased to gelatinize the starch solution, which is thoroughly adhered to and filled inside the paper. At the same time, the active protease is eliminated, so that the hydrolyzed amino acids react with glucose to undergo Maillard reaction, promoting the intermolecular force and making the dried protein structure compact, thereby forming a flexible film structure on the surface of the matrix, reducing the permeability of water molecules and oil molecules, and further improving the easy degradation and oil-proof effects. DETAILED DESCRIPTION

[0018] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0019] In order to more clearly illustrate the method provided by the present invention, the following examples are used to describe in detail the various index testing methods of the environmentally friendly degradable thermal paper for food prepared in the following examples are as follows: Oil resistance: The same size of the embodiment and the comparative example were tested for oil resistance according to Oil Kit Test-TAPPI 557, and the results were expressed as oil resistance grades.

[0020] Easy to degrade: Take the same size of the embodiment and the comparative example for soil degradation, and observe the degradation degree of the samples at 50d and 150d.

[0021] Example 1; (1) bleached coniferous wood pulp board and bleached broadleaf wood pulp board were mixed and beaten to 37°SR in a mass ratio of 7:3 to obtain wood pulp; the wood pulp, polylactic acid fiber and deionized water were mixed and beaten for 20 minutes at a beating pressure of 3.5 kg, lignin was added, and the beating was continued for 30 minutes, and a molecular weight of 6×10 6 polyacrylamide, polyamide epichlorohydrin resin, alkyl ketene dimer, stirred at 500prm for 3min, passed through a 60-mesh sieve to obtain a pulp with a beating degree of 50°SR; the wood pulp, polylactic acid fiber, deionized water, lignin, molecular weight 6×10 6 The mass ratio of polyacrylamide, polyamide epichlorohydrin resin, and alkyl ketene dimer is 1:0.1:80:0.05:0.05:0.2:0.3; (2) The slurry is made into sheets with a basis weight of 200 g / m 2 Then, the mold was pressed at 168°C and 6MPa for 2 minutes, and finally, it was dried at -56°C and 10Pa in vacuum for 24 hours to obtain a paper base. (3) Mix corn protein, alkaline protease, cationic starch and deionized water in a mass ratio of 5:0.1:5:100 to obtain a starch-composite corn protein solution; immerse a paper base in the starch-composite corn protein solution, add a 40 wt % sodium hydroxide aqueous solution to adjust the solution pH to 9, heat to 50 ° C, ultrasonicate at 21 kHz for 20 min, heat to 100 ° C, stand for 5 min, remove, immerse in a 10 wt % glucose aqueous solution, heat to 70 ° C, continue ultrasonication for 10 min, then wash once with deionized water, and dry at 40 ° C for 4 h to obtain a degradable paper base; (4) Silicon dioxide, 3-(3-methylureido)phenyl-4-methylbenzenesulfonate, 1,2-diphenoxyethane, polyvinyl alcohol with a molecular weight of 20,000, 2-(2-4-xylamino)-3-methyl-6-diethylaminofluoran, and calcium stearate were mixed in a mass ratio of 7:15:10:3:6:2.5 to obtain a heat-sensitive coating; silicon dioxide, polyvinyl alcohol with a molecular weight of 20,000, polyamide polyurea, and calcium stearate were mixed in a mass ratio of 8:6:3:3 to obtain a protective layer coating; the coating amount was 5 g / m 2 and 1g / m 2 A thermal coating material and a protective layer coating are coated on a degradable paper base to obtain environmentally friendly degradable thermal paper for food.

[0022] Example 2: (1) bleached coniferous wood pulp board and bleached broadleaf wood pulp board were mixed and beaten to 37°SR in a mass ratio of 7:3 to obtain wood pulp; wood pulp, polylactic acid fiber and deionized water were mixed and beaten for 35 min at a beating pressure of 5.5 kg, lignin was added, and beating was continued for 30 min, and a molecular weight of 6×10 6 polyacrylamide, polyamide epichlorohydrin resin, alkyl ketene dimer, stirred at 750prm for 6min, passed through a 60-mesh sieve to obtain a pulp with a beating degree of 65°SR; the wood pulp, polylactic acid fiber, deionized water, lignin, molecular weight 6×10 6 The mass ratio of polyacrylamide, polyamide epichlorohydrin resin, and alkyl ketene dimer is 3:0.4:120:0.05:0.05:0.35:0.3; (2) The slurry is made into sheets with a basis weight of 225 g / m 2 Then, the mold was pressed at 170°C and 6MPa for 3.5min, and finally, it was dried at -56°C and 20Pa in vacuum for 36h to obtain the paper base; (3) Mix corn protein, alkaline protease, cationic starch and deionized water in a mass ratio of 8:0.1:8:170 to obtain a starch-composite corn protein solution; immerse a paper base in the starch-composite corn protein solution, add a 40 wt % sodium hydroxide aqueous solution to adjust the solution pH to 9, heat the solution to 50°C, ultrasonicate at 21 kHz for 35 min, heat the solution to 100°C, let stand for 5 min, remove the paper base, immerse the paper base in a 10 wt % glucose aqueous solution, heat the solution to 70°C, continue ultrasonicating for 15 min, then wash the paper base twice with deionized water, and dry the paper base at 50°C for 6 h to obtain a degradable paper base; (4) Silicon dioxide, 3-(3-methylureido)phenyl-4-methylbenzenesulfonate, 1,2-diphenoxyethane, polyvinyl alcohol with a molecular weight of 20,000, 2-(2-4-xylamino)-3-methyl-6-diethylaminofluoran, and calcium stearate were mixed in a mass ratio of 7:15:10:3:6:2.5 to obtain a heat-sensitive coating; silicon dioxide, polyvinyl alcohol with a molecular weight of 20,000, polyamide polyurea, and calcium stearate were mixed in a mass ratio of 8:6:3:3 to obtain a protective layer coating; the coating amount was 8 g / m 2 and 3g / m 2 A thermal coating material and a protective layer coating are coated on a degradable paper base to obtain environmentally friendly degradable thermal paper for food.

[0023] Example 3; (1) bleached coniferous wood pulp board and bleached broadleaf wood pulp board were mixed and beaten to 37°SR in a mass ratio of 7:3 to obtain wood pulp; the wood pulp, polylactic acid fiber and deionized water were mixed and beaten for 50 min at a beating pressure of 8 kg, lignin was added, and the beating was continued for 30 min, and a molecular weight of 6×10 6 polyacrylamide, polyamide epichlorohydrin resin, alkyl ketene dimer, stirred at 1000prm for 10min, passed through a 60-mesh sieve to obtain a pulp with a beating degree of 80°SR; the wood pulp, polylactic acid fiber, deionized water, lignin, molecular weight 6×10 6 The mass ratio of polyacrylamide, polyamide epichlorohydrin resin, and alkyl ketene dimer is 5:0.6:150:0.05:0.05:~0.5:0.3; (2) The slurry is made into sheets with a basis weight of 250 g / m 2 Then, the mold was pressed at 175°C and 6MPa for 5 minutes, and finally, it was dried at -56°C and 50Pa in vacuum for 48 hours to obtain a paper base. (3) Mix corn protein, alkaline protease, cationic starch and deionized water in a mass ratio of 10:0.1:10:200 to obtain a starch-composite corn protein solution; immerse the paper base in the starch-composite corn protein solution, add 40 wt % sodium hydroxide aqueous solution to adjust the solution pH to 9, heat to 50 ° C, ultrasonicate at 21 kHz for 50 min, heat to 100 ° C, stand for 5 min, remove, immerse in 10 wt % glucose aqueous solution, heat to 70 ° C, continue ultrasonication for 20 min, then wash with deionized water 3 times, and dry at 60 ° C for 8 h to obtain a degradable paper base; (4) Silicon dioxide, 3-(3-methylureido)phenyl-4-methylbenzenesulfonate, 1,2-diphenoxyethane, polyvinyl alcohol with a molecular weight of 20,000, 2-(2-4-xylamino)-3-methyl-6-diethylaminofluoran, and calcium stearate were mixed in a mass ratio of 7:15:10:3:6:2.5 to obtain a heat-sensitive coating; silicon dioxide, polyvinyl alcohol with a molecular weight of 20,000, polyamide polyurea, and calcium stearate were mixed in a mass ratio of 8:6:3:3 to obtain a protective layer coating; the coating amount was 12 g / m 2 and 5g / m 2 A thermal coating material and a protective layer coating are coated on a degradable paper base to obtain environmentally friendly degradable thermal paper for food.

[0024] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that step (1) is different, and step (1) is changed to: mixing and beating the bleached coniferous wood pulp board and the bleached hardwood wood pulp board at a mass ratio of 7:3 to 37° SR to obtain wood pulp; mixing the wood pulp and deionized water, beating for 35 minutes, the beating pressure is 5.5 kg, adding lignin, and continuing beating for 30 minutes, adding a molecular weight of 6×10 6 polyacrylamide, polyamide epichlorohydrin resin, alkyl ketene dimer, stirred at 750prm for 6min, passed through a 60-mesh sieve to obtain a pulp with a beating degree of 65°SR; the wood pulp, deionized water, lignin, molecular weight 6×10 6 The mass ratio of polyacrylamide, polyamide epichlorohydrin resin and alkyl ketene dimer is 3:120:0.05:0.05:0.35:0.3; the remaining steps are the same as those in Example 2.

[0025] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that step (1) is different, and step (1) is changed to: mixing and beating the bleached coniferous wood pulp board and the bleached hardwood wood pulp board in a mass ratio of 7:3 to 37° SR to obtain wood pulp; mixing the wood pulp, polylactic acid fiber, and deionized water, beating for 65 minutes, the beating pressure is 5.5 kg, and adding a molecular weight of 6×10 6 polyacrylamide, polyamide epichlorohydrin resin, alkyl ketene dimer, 750prm stirring for 6min, through a 60-mesh sieve, to obtain a pulp with a beating degree of 65°SR; the wood pulp, polylactic acid fiber, deionized water, molecular weight 6×10 6 The mass ratio of polyacrylamide, polyamide epichlorohydrin resin and alkyl ketene dimer is 3:0.4:120:0.05:0.35:0.3; the remaining steps are the same as those in Example 2.

[0026] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that step (1) is different, and step (1) is changed to: bleached coniferous wood pulp board and bleached hardwood wood pulp board are mixed and beaten to 37°SR in a mass ratio of 7:3 to obtain wood pulp; wood pulp, polylactic acid fiber and deionized water are mixed and beaten for 10 minutes at a beating pressure of 2.5 kg, lignin is added, and beating is continued for 10 minutes, and a molecular weight of 6×10 6 polyacrylamide, polyamide epichlorohydrin resin, alkyl ketene dimer, stirred at 750prm for 6min, passed through a 60-mesh sieve to obtain a pulp with a beating degree of 40°SR; the wood pulp, polylactic acid fiber, deionized water, lignin, molecular weight 6×10 6 The mass ratio of polyacrylamide, polyamide epichlorohydrin resin and alkyl ketene dimer is 3:0.4:120:0.05:0.05:0.35:0.3; the remaining steps are the same as those in Example 2.

[0027] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that step (2) is different, and step (2) is changed to: the slurry is sheeted, and the quantitative amount is 225g / m 2 Then, the mold is pressed at 170°C and 6 MPa for 3.5 min, and finally, it is dried at 50°C for 5 h to obtain a paper base. The remaining steps are the same as those in Example 2.

[0028] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that step (3) is different. Step (3) is changed to: corn protein, alkaline protease, cationic starch and deionized water are mixed in a mass ratio of 8:0.1:8:170 to obtain a starch-composite corn protein solution; a paper base is immersed in the starch-composite corn protein solution, a 40wt% sodium hydroxide aqueous solution is added to adjust the solution pH to 9, the temperature is raised to 50°C, the reaction is carried out for 35 minutes, the temperature is raised to 100°C, the solution is allowed to stand for 5 minutes, the paper base is removed, the solution is immersed in a 10wt% glucose aqueous solution, the temperature is raised to 70°C, the reaction is carried out for 15 minutes, and then the paper base is washed twice with deionized water and dried at 50°C for 6 hours to obtain a degradable paper base; the remaining steps are the same as those in Example 2.

[0029] Comparative Example 6: The difference between Comparative Example 6 and Example 2 is that step (3) is different. Step (3) is changed to: mix corn protein, alkaline protease and deionized water in a mass ratio of 8:0.1:8:170 to obtain a corn protein solution; immerse the paper base in the corn protein solution, add 40wt% sodium hydroxide aqueous solution to adjust the solution pH to 9, heat to 50°C, ultrasonicate at 21kHz for 35min, heat to 100°C, let stand for 5min, remove, immerse in 10wt% glucose aqueous solution, heat to 70°C, continue ultrasonication for 15min, then wash twice with deionized water, and dry at 50°C for 6h to obtain a degradable paper base; the remaining steps are the same as Example 2.

[0030] Comparative Example 7: The difference between Comparative Example 7 and Example 2 is that step (3) is different. Step (3) is changed to: cationic starch and deionized water are mixed in a mass ratio of 8:170 to obtain a starch solution; the paper base is immersed in the starch solution, ultrasonicated at 21kHz for 35min, heated to 100°C, allowed to stand for 5min, taken out, washed twice with deionized water, and dried at 50°C for 6h to obtain a degradable paper base; the remaining steps are the same as those in Example 2.

[0031] Comparative Example 8: The difference between Comparative Example 8 and Example 2 is that step (3) is different. Step (3) is changed to: mix corn protein, cationic starch and deionized water in a mass ratio of 8:0.1:8:170 to obtain a starch-composite corn protein solution; immerse the paper base in the starch-composite corn protein solution, ultrasonicate at 21kHz for 35min, heat to 100°C, let stand for 5min, remove, wash twice with deionized water, and dry at 50°C for 6h to obtain a degradable paper base; the remaining steps are the same as Example 2.

[0032] Effect example Table 1 below shows the performance analysis results of the environmentally friendly degradable thermal paper for food using Examples 1 to 3 of the present invention and Comparative Examples 1 to 8.

[0033] Table 1

[0034] From the comparison of the experimental results of the embodiment and the comparative example in Table 1, it can be found that the present invention uses wood pulp as a raw material, uses polylactic acid fibers to fill the gaps between fibers during pulping, enhances the interaction between fibers, and uses lignin for backfilling. As the degree of beating increases, the fiber fibrillation will reduce the pore radius, while also increasing the hydrogen bond content and increasing the binding force between fibers, thereby reducing the passage of grease and indirectly enabling the matrix to achieve an oil-proof effect. Next, a vacuum freeze-drying treatment is performed, and a fine pore structure is formed by directly freezing the suspension into ice to give the material surface sufficient roughness, and the interior of the matrix presents a porous morphology. With the assistance of ultrasound, a starch-composite corn protein solution is introduced, and at the same time, alkaline protease is used to hydrolyze corn protein, so that the paper base achieves an easily degradable effect. Next, the hydrolyzed amino acids react with glucose to undergo a Maillard reaction, so that the dried protein structure becomes compact, thereby forming a flexible film structure on the surface of the matrix, reducing the permeability of water molecules and oil molecules, and further enhancing the oil-proof effect.

[0035] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations within the meaning and scope of the equivalent elements of the claims be included in the invention. Any marking in a claim should not be considered as limiting the claim to which it relates.

Claims

1. A process for preparing environmentally friendly degradable thermal paper for food, characterized in that: The method for preparing the environmentally friendly degradable thermal paper for food comprises the following preparation steps: (1) Mix wood pulp, polylactic acid fiber, and deionized water, beat for 20-50 min, add lignin, continue beating for 30 min, and add 6×10 6 Polyacrylamide, polyamide epichlorohydrin resin, alkyl ketene dimer, 500-1000prm, stirring for 3-10min, passing through a 60-mesh sieve, to obtain a slurry with a beating degree of 50-80°SR; (2) The slurry is sequentially sheeted, molded, and dried to obtain a paper base; (3) Immerse the paper base in a starch-composite corn protein solution, add 40 wt % sodium hydroxide aqueous solution to adjust the solution pH to 9, heat to 50 ° C, ultrasonicate at 21 kHz for 20-50 min, heat to 100 ° C, let stand for 5 min, remove, immerse in 10 wt % glucose aqueous solution, heat to 70 ° C, continue ultrasonication for 10-20 min, then wash with deionized water 1-3 times, dry at 40-60 ° C for 4-8 h, and obtain a degradable paper base; (4) The degradable paper base is coated with a thermal coating layer and a protective layer in sequence to obtain an environmentally friendly degradable thermal paper for food.

2. The method for preparing an environmentally friendly degradable thermal paper for food according to claim 1, characterized in that: The wood pulp in step (1) is prepared by mixing and beating a bleached coniferous wood pulp board and a bleached broadleaf wood pulp board in a mass ratio of 7:3 to a SR of 37°.

3. The method for preparing an environmentally friendly degradable thermal paper for food according to claim 1, characterized in that: The beating pressure in step (1) is 3.5~8kg.

4. The method for preparing an environmentally friendly degradable thermal paper for food according to claim 1, characterized in that: The wood pulp, polylactic acid fiber, deionized water, lignin, and molecular weight of 6×10 6 The mass ratio of polyacrylamide, polyamide epichlorohydrin resin and alkyl ketene dimer is 1~5:0.1~0.6:80~150:0.05:0.05:0.2~0.5:0.

3.

5. The method for preparing an environmentally friendly degradable thermal paper for food according to claim 1, characterized in that: The basis weight of the sheet in step (2) is 200-250 g / m 2 .

6. The method for preparing an environmentally friendly degradable thermal paper for food according to claim 1, characterized in that: The molding process parameters of step (2) are as follows: temperature of 168-175° C., pressure of 6 MPa, and time of 2-5 min.

7. The method for preparing an environmentally friendly degradable thermal paper for food according to claim 1, characterized in that: The drying conditions in step (2) are as follows: vacuum degree of 10-50 Pa, temperature of -56°C, and time of 24-48 h.

8. The method for preparing an environmentally friendly degradable thermal paper for food according to claim 1, characterized in that: The starch-composite corn protein solution in step (3) is prepared by mixing corn protein, alkaline protease, cationic starch and deionized water in a mass ratio of 5-10:0.1:5-10:100-200.

9. The method for preparing an environmentally friendly degradable thermal paper for food according to claim 1, characterized in that: The coating of the heat-sensitive coating in step (4) is prepared by mixing silicon dioxide, 3-(3-methylureido)phenyl-4-methylbenzenesulfonate, 1,2-diphenoxyethane, polyvinyl alcohol with a molecular weight of 20,000, 2-(2-4-xylamino)-3-methyl-6-diethylaminofluoran, and calcium stearate in a mass ratio of 7:15:10:3:6:2.5; the coating of the protective layer is prepared by mixing silicon dioxide, polyvinyl alcohol with a molecular weight of 20,000, polyamide polyurea, and calcium stearate in a mass ratio of 8:6:3:

3.

10. The method for preparing an environmentally friendly degradable thermal paper for food according to claim 1, characterized in that: The coating amount of the thermal-sensitive coating in step (4) is 5-12 g / m 2 ; The coating amount of the protective layer is 1~5g / m 2 .

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