Moisturizing composite kraft paper

Abstract

The application belongs to the technical field of kraft paper, and particularly relates to a moisturizing composite kraft paper. The moisturizing composite kraft paper comprises an outer layer, a middle layer and an inner layer; the material of the outer layer is kraft paper, the middle layer is an adhesive layer; the inner layer is a modified PBAT film; the preparation method of the modified PBAT film comprises the following steps: 80-85 parts of PBAT, 7-12 parts of PLA, 15-20 parts of polyglycolic acid-hydroxyacetic acid copolymer and 0.7-1 part of a hydrolysis-resistant agent are mixed according to weight fractions, the temperature is increased to 220-230 DEG C, the stirring speed is 80-90 r / min, the stirring reaction time is 2-3 h, 5-8 parts of nano fillers are continuously added, ultrasonic dispersion is carried out for 15-20 min, a mixed material is obtained, the mixed material is added into a casting film machine, a casting film is obtained by casting, and the modified PBAT film is prepared by using a surface temperature 20-25 DEG C water cooling calender roll. The moisturizing composite kraft paper prepared by the application has good waterproof, breathable and oil-proof effects.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of kraft paper, and particularly relates to a moisturizing composite kraft paper. BACKGROUND

[0002] Kraft paper is a common paper material, which is known for its durability, tear resistance and good waterproof performance. As an economical and practical, environmentally friendly paper material, kraft paper is widely used in packaging, printing, writing and other fields.

[0003] Kraft paper is widely used to make food bags, handbags and other packaging materials due to its high strength, tear resistance and environmental characteristics. However, in practical applications, there are still some technical problems: 1. Insufficient waterproof performance, kraft paper has strong natural water absorption, which is easy to be damp, affecting its mechanical properties and appearance. 2. Insufficient oil resistance, kraft paper is easily penetrated by oil, affecting its performance, especially in food packaging. 3. Unstable mechanical properties, the mechanical properties of kraft paper will decrease in humid or high temperature environment. 4. Environmental and food safety issues, the production process and coating materials used in kraft paper may contain harmful substances, affecting environmental protection and food safety. 5. Insufficient weather resistance, kraft paper is easily affected by ultraviolet rays, temperature changes and other factors when used outdoors, resulting in performance degradation. 6. Air permeability problem, the air permeability of kraft paper may cause food spoilage, especially in occasions requiring sealed packaging.

[0004] The prior art makes kraft paper into a composite film to improve the water resistance of kraft paper, such as compounding kraft paper with polyethylene film to form a waterproof barrier. This method can significantly improve the waterproof performance of kraft paper while maintaining its mechanical strength; similarly, using polypropylene film to compound with kraft paper can also provide good waterproof performance. However, these materials have poor degradability, which does not meet the environmental protection concept, and the air permeability of kraft paper is also not ideal, which cannot meet the existing market demand. Therefore, there is an urgent need for a moisturizing composite kraft paper. SUMMARY

[0005] The purpose of the present application is to provide a moisturizing composite kraft paper, which has good air permeability, waterproofness, air permeability and water resistance, and good moisturizing effect.

[0006] In order to achieve the above purpose, the present application provides the following technical solutions:

[0007] A moisturizing composite kraft paper, comprising an outer layer, a middle layer and an inner layer; the material of the outer layer is kraft paper, the middle layer is an adhesive layer; the inner layer is a modified PBAT film; the thickness of the outer layer is 20-25 μm, the thickness of the inner layer is 10-15 μm, and the thickness of the middle layer is 15-20 μm;

[0008] The preparation method of the modified PBAT film comprises the following steps: mixing 80-85 parts of PBAT, 7-12 parts of PLA, 15-20 parts of polylactic acid-hydroxyacetic acid copolymer and 0-7-1 part of an anti-hydrolysis agent, heating to 220-230 DEG C, the stirring speed is 80-90 r / min, the stirring reaction time is 2-3 h, continue to add 5-8 parts of nano filler, ultrasonic dispersion for 15-20 min, to obtain the mixed material, the mixed material is added into the casting film machine, and the casting film is obtained by casting, and the surface temperature is 20-25 DEG C water cooling calender roll, and the modified PBAT film is prepared.

[0009] Further, the anti-hydrolysis agent is bis (2, 6-diisopropylphenyl) carbodiimide.

[0010] Further, the nano filler comprises nano calcium carbonate, nano zinc oxide and nano lanthanum oxide in a weight ratio of 1: (1.2-1.5) : (0.6-0.8).

[0011] Further, the particle size of the nano calcium carbonate is 15-25 nm, and the specific surface area is 30-60 m 2 / g, the particle size of the nano lanthanum oxide is 30-50 nm, and the specific surface area is 15-30 m 2 / g. The particle size of the nano lanthanum oxide is 30-50 nm, and the specific surface area is 30-50 m 2 / g.

[0012] PBAT has good degradability, which is beneficial to environmental protection, and the present application attempts to compound it with kraft paper to improve the waterproofness of the composite kraft paper, but the waterproof effect under high temperature is not ideal. The present application can improve the waterproof performance of kraft paper under high temperature by compounding PEAT with PLA and polylactic acid-hydroxyacetic acid copolymer. Analysis is that PLA and PLGA have high surface hydrophobicity, which can improve the waterproof performance of the composite kraft paper. At the same time, the flexibility and ductility of PBAT can be combined with the rigidity and strength of PLA, so as to improve the mechanical properties of the composite material.

[0013] At the same time, the addition of nano filler can improve the air permeability of kraft paper. The nano filler can form a microporous structure in the composite material, providing a channel for gas molecules to pass through, and the nano filler can improve the interfacial compatibility between PBAT, PLA and PLGA, forming a more uniform composite material, and good interfacial compatibility can reduce the defects in the material, improving the air permeability.

[0014] Further, the preparation method of the adhesive layer comprises the following steps: mixing 80-85 parts of PBAT, 7-12 parts of PLA, 15-20 parts of polylactic acid-hydroxyacetic acid copolymer and 0-7-1 part of an anti-hydrolysis agent, heating to 220-230 DEG C, the stirring speed is 80-90 r / min, the stirring reaction time is 2-3 h, continue to add 5-8 parts of nano filler, ultrasonic dispersion for 15-20 min, to obtain the mixed material, the mixed material is added into the casting film machine, and the casting film is obtained by casting, and the surface temperature is 20-25 DEG C water cooling calender roll, and the modified PBAT film is prepared.

[0015] (1) 15-20 parts of brominated butyl rubber is plasticized at 170-180℃ for 5-10 min, 0.3-0.5 parts of silane coupling agent KH550, 2-4 parts of magnesium stearate, 100 parts of thermoplastic polyurethane A are continuously mixed for 15-20 min, and then crushed at room temperature to obtain modified thermoplastic polyurethane;

[0016] (2) 10-14 parts of thermoplastic polyurethane B, 20-25 parts of modified thermoplastic polyurethane, 0.04-0.07 parts of ultraviolet absorber and 0.02-0.05 parts of antioxidant are mixed, a single screw extruder is used to obtain a cast film, and a surface temperature of 20-25℃ water-cooled calender roll is used to calender to obtain an adhesive layer.

[0017] The tensile strength of the composite kraft paper is not ideal when using commercially available adhesive materials, which is because the compatibility of the commercially available adhesive materials with the kraft paper or the PBAT layer is poor. The tensile strength of the composite kraft paper can be improved by modifying the thermoplastic polyurethane A with brominated butyl rubber and then compounding with the thermoplastic polyurethane B. At the same time, the texture of the composite kraft paper is improved. The main reason is that the brominated butyl rubber has excellent elasticity and toughness, which can significantly improve the impact strength and tear resistance of the thermoplastic polyurethane A, form an energy dissipation zone in the stress concentration area, reduce the crack propagation, and improve the tensile strength; the modified thermoplastic polyurethane and the thermoplastic polyurethane B have different physical and chemical properties, and the performance complementation can be achieved by compounding, so that the tensile strength and elasticity of the composite material can be improved at the same time. The brominated butyl rubber has good softness and elasticity, which can improve the touch of the composite material; the surface smoothness of the adhesive layer is higher through the compounding of the materials, the roughness of the surface is reduced, and the surface of the composite kraft paper is more delicate.

[0018] Further, the thermoplastic polyurethane A is a thermoplastic polyurethane with a model number of 9670A. Brand: Bayer, Germany.

[0019] Further, the thermoplastic polyurethane B is a thermoplastic polyurethane with a model number of 9370AU. Brand: Bayer, Germany.

[0020] Brominated butyl rubber, model: EXXPRO TM 3433, brand: Exxon, USA.

[0021] Further, the barrel temperature of the single screw extruder is 100-105℃ for the first section, 130-135℃ for the second section, 145-150℃ for the third section, 160-165℃ for the fourth section, and 150-155℃ for the fifth section.

[0022] Further, the preparation method of the moisture-retention composite kraft paper comprises the following steps: sequentially placing the outer layer, the middle layer and the inner layer on a hot roller press machine from top to bottom, and sequentially laminating and hot-rolling to obtain the moisture-retention composite kraft paper.

[0023] Further, the rolling temperature is 130-135 DEG C, and the pressure is 4.5-5 kg / cm 2 .

[0024] Compared with the prior art, the application has the following advantages and beneficial effects:

[0025] 1. The PEAT is compounded with PLA and poly(lactic-co-glycolic acid) to improve the waterproof performance of kraft paper at high temperature.

[0026] 2. The addition of nano fillers in the modified PBAT film can improve the air permeability of kraft paper.

[0027] 3. The brominated butyl rubber is used to modify the thermoplastic polyurethane A, and then the modified thermoplastic polyurethane A is compounded with the thermoplastic polyurethane B to improve the tensile strength of the composite kraft paper, and meanwhile, the texture of the composite kraft paper is improved. DETAILED DESCRIPTION

[0028] The technical solutions in the embodiments of the application will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0029] The raw materials used in the following embodiments of the application are all commercially available goods:

[0030] Kraft paper, maple leaf refined kraft paper, number: 205B-50, gram weight 40-80 grams.

[0031] PBAT, Shanghai Yiding Plastic Co., Ltd., brand: TH801T.

[0032] PLA, NatureWorks, USA, model: 4060D.

[0033] Poly(lactic-co-glycolic acid), Suzhou Renfu Plastic Co., Ltd., brand JJ901.

[0034] Ultraviolet absorber, BASF, ultraviolet absorber 234.

[0035] Antioxidant, BASF, antioxidant 1010.

[0036] Example 1

[0037] The embodiment provides a kind of moisturizing composite kraft paper, including outer layer, middle layer and inner layer;The material of the outer layer is kraft paper, the middle layer is adhesive layer;The inner layer is modified PBAT film;The thickness of the outer layer is 22 μm, the thickness of inner layer is 12 μm, the thickness of middle layer is 18 μm.

[0038] The preparation method of the modified PBAT film includes the following steps: 82 parts of PBAT, 10 parts of PLA, 17 parts of polyglycolic acid-hydroxyacetic acid copolymer and 0.8 parts of hydrolysis-resistant agent are mixed, the temperature is raised to 225 DEG C, the stirring speed is 85 r / min, the stirring reaction time is 2.5 h, 7 parts of nano filler is continuously added, ultrasonic dispersion is carried out for 18 min, the mixture is obtained, the mixture is added into a casting film machine, a casting film is obtained by casting, and the modified PBAT film is prepared by using a surface temperature of 22 DEG C water-cooled calender roll.

[0039] The hydrolysis-resistant agent is bis (2, 6-diisopropylphenyl) carbodiimide.

[0040] The nano filler includes nano calcium carbonate, nano zinc oxide and nano lanthanum oxide in a weight ratio of 1:1.4:0.7. 2 The nano calcium carbonate has a particle size of 15 nm-25 nm and a specific surface area of 30-60 m 2 / g. The nano lanthanum oxide has a particle size of 30 nm-50 nm and a specific surface area of 15-30 m 2 / g.

[0041] The preparation method of the adhesive layer includes the following steps: 17 parts of brominated butyl rubber are plasticated at 175 DEG C for 8 min, 0.4 parts of silane coupling agent KH550, 3 parts of magnesium stearate and 100 parts of thermoplastic polyurethane A are continuously added, and mixing is carried out for 18 min, and then the modified thermoplastic polyurethane is obtained by crushing at room temperature.

[0042] (1) 17 parts of brominated butyl rubber are plasticated at 175 DEG C for 8 min, 0.4 parts of silane coupling agent KH550, 3 parts of magnesium stearate and 100 parts of thermoplastic polyurethane A are continuously added, and mixing is carried out for 18 min, and then the modified thermoplastic polyurethane is obtained by crushing at room temperature.

[0043] (2) 12 parts of thermoplastic polyurethane B, 22 parts of modified thermoplastic polyurethane, 0.06 parts of ultraviolet absorber and 0.04 parts of antioxidant are mixed, a single-screw extruder is used, the barrel temperature of the single-screw extruder is 102 DEG C, 133 DEG C, 147 DEG C, 162 DEG C and 152 DEG C in turn, and then a casting film is obtained by casting, and the adhesive layer is obtained by using a surface temperature of 22 DEG C water-cooled calender roll.

[0044] The thermoplastic polyurethane A is a thermoplastic polyurethane with a model number of 9670A, and the brand is Bayer, Germany. The thermoplastic polyurethane B is a thermoplastic polyurethane with a model number of 9370AU, and the brand is Bayer, Germany. The brominated butyl rubber has a model number of EXXPROTM 3433, brand: Exxon, USA.

[0045] The preparation method of the moisture-retention composite kraft paper comprises the following steps: sequentially placing the outer layer, the middle layer and the inner layer on a hot roller press machine from top to bottom, sequentially stacking them, hot roller pressing and bonding, the roller pressing temperature is 132 DEG C, the pressure is 4.7 kg / cm 2 , and the moisture-retention composite kraft paper is obtained.

[0046] Embodiment 2

[0047] The embodiment provides a moisture-retention composite kraft paper, which comprises an outer layer, a middle layer and an inner layer; the material of the outer layer is kraft paper, the middle layer is an adhesive layer, and the inner layer is a modified PBAT film; the thickness of the outer layer is 22 microns, the thickness of the inner layer is 12 microns, and the thickness of the middle layer is 18 microns.

[0048] The preparation method of the modified PBAT film comprises the following steps: mixing 80 parts of PBAT, 12 parts of PLA, 15 parts of polylactic acid-glycolic acid copolymer and 1 part of an anti-hydrolysis agent according to weight proportions, heating to 220 DEG C, stirring at a speed of 90 r / min, stirring for 2 hours, continuously adding 8 parts of nano-filler, ultrasonic dispersion for 15 minutes, obtaining a mixed material, and adding the mixed material into a casting film machine to obtain a casting film through casting, and the casting film is rolled by using a water-cooling rolling mill with a surface temperature of 25 DEG C.

[0049] The anti-hydrolysis agent is bis(2,6-diisopropylphenyl)carbodiimide.

[0050] The nano-filler comprises nano-calcium carbonate, nano-zinc oxide and nano-lanthanum oxide at a weight ratio of 1:1.2:0.8; the particle size of the nano-calcium carbonate is 15 nm-25 nm, the specific surface area is 30-60 m 2 / g, the particle size of the nano-lanthanum oxide is 30 nm-50 nm, the specific surface area is 15-30 m 2 / g, and the particle size of the nano-zinc oxide is 30 nm-50 nm, the specific surface area is 30-50 m 2 / g.

[0051] The preparation method of the adhesive layer comprises the following steps: mixing 15 parts of brominated butyl rubber at 180 DEG C for plasticizing for 10 minutes, continuously adding 0.5 parts of silane coupling agent KH550, 2 parts of magnesium stearate and 100 parts of thermoplastic polyurethane A, continuously mixing for 20 minutes, and crushing at room temperature to obtain modified thermoplastic polyurethane.

[0052] (1) mixing 15 parts of brominated butyl rubber at 180 DEG C for plasticizing for 10 minutes, continuously adding 0.5 parts of silane coupling agent KH550, 2 parts of magnesium stearate and 100 parts of thermoplastic polyurethane A, continuously mixing for 20 minutes, and crushing at room temperature to obtain modified thermoplastic polyurethane.

[0053] (2) 10 parts of the thermoplastic polyurethane B, 25 parts of the modified thermoplastic polyurethane, 0.04 parts of the ultraviolet absorber and 0.05 parts of the antioxidant are mixed, a single screw extruder is used, the barrel temperature of the single screw extruder is 105℃ for the first section, 130℃ for the second section, 150℃ for the third section, 160℃ for the fourth section and 155℃ for the fifth section, a cast film is obtained by casting, and the cast film is calendered by using a water-cooled calender roll with a surface temperature of 20℃ to obtain the adhesive layer.

[0054] The thermoplastic polyurethane A is a thermoplastic polyurethane with a model number of 9670A. Brand: Bayer, Germany. The thermoplastic polyurethane B is a thermoplastic polyurethane with a model number of 9370AU. Brand: Bayer, Germany. The brominated butyl rubber has a model number of EXXPRO TM 3433. Brand: Exxon, USA.

[0055] The preparation method of the moisture-retention composite kraft paper comprises the following steps: sequentially placing the outer layer, the middle layer and the inner layer on a hot roller press from top to bottom, sequentially stacking them, and hot roller pressing and bonding, with a roller pressing temperature of 135℃ and a pressure of 4.5kg / cm 2 , to obtain the moisture-retention composite kraft paper.

[0056] Comparative Example 1

[0057] The difference between the present comparative example and Example 1 is that the preparation method of the modified PBAT film comprises the following steps: mixing 78 parts of PBAT, 20 parts of PLA, 11 parts of poly(lactic acid-co-glycolic acid) and 0.8 parts of a hydrolysis-resistant agent by weight, heating to 225℃, stirring at a speed of 85r / min, stirring for 2.5h, continuously adding 7 parts of nano fillers, ultrasonic dispersion for 18min, obtaining a mixed material, and adding the mixed material into a cast film machine to obtain a cast film, which is calendered by using a water-cooled calender roll with a surface temperature of 22℃ to obtain the modified PBAT film.

[0058] Comparative Example 2

[0059] The difference between the present comparative example and Example 1 is that the nano fillers comprise nano calcium carbonate, nano zinc oxide and nano lanthanum oxide in a weight ratio of 1:1:1.

[0060] Comparative Example 3

[0061] The difference between the present comparative example and Example 1 is that the nano calcium carbonate has a particle size of 40nm-55nm and a specific surface area of 10-20m 2 / g, the nano lanthanum oxide has a particle size of 60nm-80nm and a specific surface area of 5-10m 2 / g, and the nano zinc oxide has a particle size of 70nm-90nm and a specific surface area of 5-10m 2 / g.

[0062] Comparative Example 4

[0063] The difference between the present comparative example and Example 1 is that the preparation method of the adhesive layer comprises the following steps: mixing 12 parts of the thermoplastic polyurethane B, 22 parts of the thermoplastic polyurethane A, 0.06 parts of the ultraviolet absorber and 0.04 parts of the antioxidant by weight, using a single screw extruder, the temperature of the barrel of the single screw extruder is 102℃ at the first section, 133℃ at the second section, 147℃ at the third section, 162℃ at the fourth section and 152℃ at the fifth section, and a cast film is obtained by casting, and the cast film is calendered using a water-cooled calender roll with a surface temperature of 22℃ to obtain the adhesive layer.

[0064] (1) 17 parts of brominated butyl rubber is plasticized at 175℃ for 8 min, 0.4 parts of silane coupling agent KH550, 3 parts of magnesium stearate, 100 parts of thermoplastic polyurethane A are continuously mixed for 18 min, and the mixture is crushed at room temperature to obtain modified thermoplastic polyurethane.

[0065] (2) 17 parts of thermoplastic polyurethane B, 17 parts of modified thermoplastic polyurethane, 0.06 parts of ultraviolet absorber and 0.04 parts of antioxidant are mixed, a single screw extruder is used, the temperature of the barrel of the single screw extruder is 102℃ at the first section, 133℃ at the second section, 147℃ at the third section, 162℃ at the fourth section and 152℃ at the fifth section, a cast film is obtained by casting, and the cast film is calendered using a water-cooled calender roll with a surface temperature of 22℃ to obtain the adhesive layer.

[0066] Comparative Example 5

[0067] The difference between the present comparative example and Example 1 is that the preparation method of the adhesive layer comprises the following steps: mixing 12 parts of the thermoplastic polyurethane B, 22 parts of the thermoplastic polyurethane A, 0.06 parts of the ultraviolet absorber and 0.04 parts of the antioxidant by weight, using a single screw extruder, the temperature of the barrel of the single screw extruder is 102℃ at the first section, 133℃ at the second section, 147℃ at the third section, 162℃ at the fourth section and 152℃ at the fifth section, and a cast film is obtained by casting, and the cast film is calendered using a water-cooled calender roll with a surface temperature of 22℃ to obtain the adhesive layer.

[0068] Comparative Example 6

[0069] The difference between the present comparative example and Example 1 is that the thermoplastic polyurethane A is a thermoplastic polyurethane with model number 9370AU, and the brand is Bayer, Germany. The thermoplastic polyurethane B is a thermoplastic polyurethane with model number 9670A, and the brand is Bayer, Germany.

[0070] Performance test

[0071] The composite kraft paper prepared in Examples 1-2 and Comparative Example 6 is subjected to performance test.

[0072] Mechanical property test: the tensile strength is tested according to the national standard GB / T1040.3-2006.

[0073] Barrier property: the oxygen transmission rate [cm3 / (m2.d.atm0)] is tested according to the national standard GB / T1038-2000.

[0074] Waterproof performance: the composite kraft paper is quickly heat sealed by air cushion machine, then placed in water for 30 min, then the water tank is vacuumed to observe the air pillow air leakage, the vacuum degree is-30 kpa, and the specific test conditions are: bag making specification 200*100 mm, air volume of air inflator 28 L / min, heat sealing temperature 200℃, speed 15 m / min, heat sealing number 100, 50℃ or 25℃ water standing for 30 min, vacuum degree-30 kpa, the vacuum air leakage rate is detected = the number of air leakage / total number*100%.

[0075] Sensory test: according to the smoothness and softness of the surface of the composite kraft paper, it is graded as 1-3, wherein 1 is the best and 3 is the worst.

[0076] The results are shown in Table 1 below.

[0077] Table 1 performance test results

[0078]

[0079]

[0080] From the above performance test results, it can be seen that the composite kraft paper of examples 1-2 has good waterproof and air permeability, especially the comprehensive performance of example 1 is the most outstanding, which is mainly because the outer layer, middle layer and inner layer play a synergistic effect.

[0081] The comparative examples are significantly inferior to the examples in the corresponding performance tests because they do not use the necessary technical solutions. In comparative example 1, the ratio of PBAT, PLA and poly(lactic acid-co-glycolic acid) is changed, which can be seen to reduce the waterproof effect at high temperature. In comparative example 2, the types of fillers are different, in comparative example 3, the parameters of fillers are different, which can be seen to affect the air permeability effect. In comparative example 4, the ratio of thermoplastic polyurethane B and modified thermoplastic polyurethane is different, in comparative example 5, the thermoplastic polyurethane A is not modified, which can be seen to reduce the tensile effect. In comparative example 6, the types of thermoplastic polyurethane A and thermoplastic polyurethane B are different, which can be seen to affect the tensile strength and sensory quality of the composite kraft paper. The above experimental results further prove the importance of the technical solutions defined in the present application to its technical effects.

[0082] The above is a preferred embodiment of the present application, it should be noted that for ordinary skilled in the art, without departing from the principles of the present application, can make a number of improvements and refinements, these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A moisture-retaining composite kraft paper, characterized in that, It includes an outer layer, a middle layer, and an inner layer; the outer layer is made of kraft paper, the middle layer is an adhesive layer, and the inner layer is a modified PBAT film; The preparation method of the modified PBAT film includes the following steps: by weight, 80-85 parts of PBAT, 7-12 parts of PLA, 15-20 parts of polylactic acid-glycolic acid copolymer and 0.7-1 parts of anti-hydrolysis agent are mixed, heated to 220-230℃, stirred at a speed of 80-90 r / min, and stirred for 2-3 hours. Then, 5-8 parts of nanofiller are added and ultrasonically dispersed for 15-20 minutes to obtain a mixture. The mixture is added to a casting film machine and cast to obtain a cast film. The film is then calendered using a water-cooled calendering roller with a surface temperature of 20-25℃ to obtain the modified PBAT film. The nanofiller comprises nano-calcium carbonate, nano-zinc oxide, and nano-lanthanum oxide in a weight ratio of 1:(1.2-1.5):(0.6-0.8). The method for preparing the adhesive layer includes the following steps: by weight, (1) Plasticize 15-20 parts of brominated butyl rubber at 170-180℃ for 5-10 min, then add 0.3-0.5 parts of silane coupling agent KH550, 2-4 parts of magnesium stearate, and 100 parts of thermoplastic polyurethane A, and continue to mix for 15-20 min. Crush at room temperature to obtain modified thermoplastic polyurethane. (2) Mix 10-14 parts of thermoplastic polyurethane B, 20-25 parts of modified thermoplastic polyurethane, 0.04-0.07 parts of UV absorber and 0.02-0.05 parts of antioxidant, and use a single screw extruder to cast the film. Then, use a water-cooled calendering roll with a surface temperature of 20-25℃ to calender the film to obtain an adhesive layer. The nano-calcium carbonate has a particle size of 15nm-25nm and a specific surface area of ​​30-60m². 2 / g, the particle size of nano zinc oxide is 30nm-50nm, and the specific surface area is 15-30m². 2 / g; the particle size of nano-lanthanum oxide is 30nm-50nm, and the specific surface area is 30-50m². 2 / g; The anti-hydrolysis agent is bis(2,6-diisopropylphenyl)carbodiimide.

2. The moisture-retaining composite kraft paper according to claim 1, characterized in that, Thermoplastic polyurethane A is thermoplastic polyurethane of type 9670A; thermoplastic polyurethane B is thermoplastic polyurethane of type 9370AU.

3. The moisture-retaining composite kraft paper according to claim 1, characterized in that, The barrel temperature of a single screw extruder is 100-105℃ in section 1, 130-135℃ in section 2, 145-150℃ in section 3, 160-165℃ in section 4, and 150-155℃ in section 5.

4. The moisture-retaining composite kraft paper according to claim 1, characterized in that, The preparation method of the moisture-retaining composite kraft paper includes the following steps: placing the outer layer, middle layer and inner layer on a hot roller press in sequence, stacking them from top to bottom, and hot roller pressing them together to obtain the moisture-retaining composite kraft paper.

5. The moisture-retaining composite kraft paper according to claim 4, characterized in that, The rolling temperature is 130-135℃, and the pressure is 4.5-5 kg / cm². 2 .

6. The moisture-retaining composite kraft paper according to claim 1, characterized in that, The outer layer has a thickness of 20-25 μm, the inner layer has a thickness of 10-15 μm, and the middle layer has a thickness of 15-20 μm.

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