Starch-based aqueous polyurethane emulsion and preparation method and application thereof

Abstract

The present invention belongs to the technical field of polymer materials, and specifically relates to a starch-based waterborne polyurethane emulsion and its preparation method and application. The present invention mixes glycerol and polyethylene glycol, stirs and warms to 90-100°C, then adds starch, stirs and warms to 120-140°C, keeps warm and stirs for 1-2h, then adds methanesulfonic acid, stirs for 0.5-1h, cools to room temperature, and obtains starch-based polyols; isophorone diisocyanate, 2,2-dihydroxymethylbutyric acid, and solvent are mixed, stirred and warmed to 60-80°C, then a catalyst is added, stirred and reacted for 2-3h, then starch-based polyols are added, stirred and reacted for 2-3h, cooled to room temperature after the reaction ends, triethylamine is added, stirred and neutralized for 20-40min, then 3-aminopropyltriethoxysilane is added, stirred and reacted for 1-2h, then water is added, high-speed stirring and emulsification is performed for 1-2h, and the solvent is removed by rotary evaporation to obtain. The starch-based waterborne polyurethane emulsion of the present invention has good stability and mechanical properties, and has good degradation ability, safety and environmental protection.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer materials, and specifically relates to a starch-based waterborne polyurethane emulsion and a preparation method and application thereof. Background Art

[0002] Paper is made of fiber pulp. Paper made of fiber pulp has low strength and large pores. After repeated use, the fibers of the pulp will become shorter, and the performance and quality of the finished paper will become worse. Surface sizing agents are chemical substances coated on the surface of paper to improve certain physical properties of paper and reduce its porosity. In the past, in order to improve the performance of paper, vacuum aluminum plating, film coating or polymer coating on the surface of the paper base was generally used. However, the paper produced in this way has poor environmental protection and is difficult to be recycled. In addition, in developed countries such as the European Union, the production of such processes has been banned. At present, the raw materials of surface sizing agents are mainly petroleum-based chemicals, which have poor degradation ability and single function. With the continuous improvement of society's requirements for paper performance, safety and environmental protection, the development of efficient and green surface sizing agents has become an urgent problem to be solved.

[0003] Polyurethane is a block polymer, and the carbamate group (-NHCOO-) in its main chain is obtained by the step-by-step polymerization reaction of the isocyanate group (-NCO-) and the hydroxyl group (-OH) in the polyol. As the industry explores the direction of sustainable development, polyurethane begins to seek a low-pollution transformation. Waterborne polyurethane has gradually replaced solvent polyurethane on the international stage. Waterborne polyurethane emulsion has good low-temperature stability and strong mechanical properties, and has great advantages in environmental protection and transportation. It also has high application value. However, due to the large scale of monomer production in the papermaking industry and low added value, the cost of directly using waterborne polyurethane as a surface sizing agent for packaging paper is high, and as a synthetic polymer, coating it on the paper surface will also result in poor biocompatibility when the paper is recycled. Therefore, it is not feasible to use it directly as a surface sizing agent for packaging paper.

[0004] Starch is one of the most abundant carbohydrates in nature. It has the advantages of high yield, low price, renewable and biodegradable. It can solve the problems of non-renewable, expensive and difficult to degrade petroleum-based raw materials. However, natural starch has the disadvantages of high viscosity, poor fluidity, poor film-forming property, easy coagulation and poor sizing effect, which limits its application. Therefore, it is necessary to integrate the advantages of waterborne polyurethane and natural starch to develop a green and environmentally friendly surface sizing agent that can effectively improve the performance of paper. Summary of the invention

[0005] The first object of the present invention is to provide a method for preparing a starch-based aqueous polyurethane emulsion, the second object of the present invention is to provide a starch-based aqueous polyurethane emulsion prepared by the preparation method, and the third object of the present invention is to provide an application of the starch-based aqueous polyurethane emulsion.

[0006] According to a first aspect of the present invention, there is provided a method for preparing a starch-based aqueous polyurethane emulsion, comprising the following steps:

[0007] S1. Mix glycerol and polyethylene glycol, stir and heat to 90-100° C., then add starch, stir and heat to 120-140° C., keep warm and stir for 1-2 hours, then add methane sulfonic acid, stir at 120-140° C. for 0.5-1 hour, then cool to room temperature, filter out the residue, and obtain starch-based polyol;

[0008] S2. Mix isophorone diisocyanate, 2,2-dihydroxymethylbutyric acid and solvent, stir and heat to 60-80°C, then add catalyst, stir and react at 60-80°C for 2-3h, then add starch-based polyol obtained in step S1, stir and react at 60-80°C for 2-3h, cool to room temperature after the reaction, then add triethylamine, stir and neutralize for 20-40min, then add 3-aminopropyltriethoxysilane, stir and react for 1-2h, then add water, stir and emulsify at high speed for 1-2h, after the reaction is completed, remove the solvent from the reaction solution by rotary evaporation to obtain starch-based waterborne polyurethane emulsion.

[0009] When synthesizing starch-based polyols, the present invention firstly mixes glycerol and polyethylene glycol uniformly to obtain a polyol mixed solution, and then uses the polyol mixed solution as a liquefied liquid and methane sulfonic acid as a catalyst to convert starch into a small molecular polyhydroxy compound at high temperature.

[0010] In the process of synthesizing waterborne polyurethane, the present invention firstly reacts a hydrophilic chain extender 2,2-dimethylolbutyric acid with isocyanate, so as to ensure that the hydrophilic group carboxyl group is kept between molecular chains and that the hydrophilic chain extender is completely grafted, and the hydrophilic group carboxyl group can completely react with the amino group of triethylamine to generate an ionic group, and the generated ionic group can disperse the polyurethane in water to form a stable emulsion; then, a hydroxyl group (-OH) in a starch-based polyol and an isocyanate group (-NCO-) in an isocyanate are polymerized to obtain a carbamate group (-NHCOO-) in the main chain of the waterborne polyurethane molecule; and a silane coupling agent 3-aminopropyltriethoxysilane is introduced in the end-capping stage of the waterborne polyurethane, so as to effectively improve the hydrophobicity of the waterborne polyurethane, so that the obtained waterborne polyurethane has both good hydrophilicity and good hydrophobicity.

[0011] In some embodiments, the starch is corn starch.

[0012] In some embodiments, in step S1, the mass ratio of glycerol to polyethylene glycol is 3:7, the mass ratio of starch to the total amount of glycerol and polyethylene glycol is 1:4, and the amount of methane sulfonic acid used is 0.05% of the total mass of starch, glycerol and polyethylene glycol.

[0013] In some embodiments, in step S2, the molar ratio of the isocyanate group in isophorone diisocyanate to the hydroxyl group in 2,2-dimethylolbutyric acid is 1:(1.3-1.8).

[0014] In some embodiments, in step S2, the solvent is acetone, and the volume ratio of the sum of the masses of isophorone diisocyanate and 2,2-dihydroxymethylbutyric acid to the solvent is (6-7):(5-10).

[0015] In some embodiments, in step S2, the catalyst is dibutyltin dilaurate, and the amount of the catalyst used is 0.01% to 1% of the mass of isophorone diisocyanate.

[0016] In some embodiments, in step S2, the mass ratio of starch-based polyol to isophorone diisocyanate is 1:1.29.

[0017] In some embodiments, in step S2, the molar ratio of triethylamine to 2,2-dimethylolbutyric acid is 1:1, the mass ratio of 3-aminopropyltriethoxysilane to isophorone diisocyanate is 1:3, and the amount of water is 60-70% of the total mass of the reaction raw materials and water. The reaction raw materials here refer to isophorone diisocyanate, 2,2-dimethylolbutyric acid, starch-based polyol, triethylamine and 3-aminopropyltriethoxysilane.

[0018] In some embodiments, the stirring rate is 180-220 rpm, and the high-speed stirring emulsification rate is 1000-1100 rpm.

[0019] According to a second aspect of the present invention, a starch-based aqueous polyurethane emulsion prepared by the above preparation method is provided.

[0020] According to a third aspect of the present invention, there is provided a use of the starch-based aqueous polyurethane emulsion in preparing a paper surface sizing agent.

[0021] The beneficial effects of the present invention include:

[0022] (1) The starch-based aqueous polyurethane emulsion of the present invention has good low-temperature stability, and the cured film obtained by curing the starch-based aqueous polyurethane emulsion has strong mechanical properties; moreover, since the main raw material of the starch-based aqueous polyurethane emulsion of the present invention is renewable and cheap biomaterial starch, the utilization of non-renewable petrochemical resources is reduced, the degradation ability is good, and it is safe and environmentally friendly.

[0023] (2) The starch-based waterborne polyurethane emulsion of the present invention has a low relative molecular weight, and its cross-linking groups can generate a triple network structure through curing. When it is applied as a surface sizing agent on the surface of paper and cured to form a uniform and continuous thin film, it can cover the pores and fibers of the paper, leaving no obvious gaps on the surface of the paper, and improving the smoothness of the surface of the paper. At the same time, the thin layer formed on the surface of the paper can also effectively prevent water penetration and improve the water resistance of the paper.

[0024] (3) The preparation method of the present invention is simple, has a fast reaction speed, has no strong corrosiveness of the reaction raw materials, is safe and environmentally friendly, and has simple and easy-to-implement reaction conditions, which is conducive to industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The synthetic route of the starch-based aqueous polyurethane emulsion of Example 1 of the present invention is shown in FIG.

[0026] Figure 2 These are infrared spectra of natural starch, starch-based polyol prepared in Example 1, and starch-based aqueous polyurethane emulsion.

[0027] Figure 3 This is a particle size distribution diagram of the starch-based aqueous polyurethane emulsion prepared in Examples 1 to 6 of the present invention.

[0028] Figure 4 This is a test chart of centrifugal stability of the starch-based aqueous polyurethane emulsions prepared in Examples 1 to 6 of the present invention.

[0029] Figure 5 This is a thermogravimetric analysis diagram of the starch-based aqueous polyurethane emulsions prepared in Examples 1 to 6 of the present invention after film formation.

[0030] Figure 6 The stress-strain curves of paper before and after sizing the paper by diluting the starch-based aqueous polyurethane emulsion prepared in Example 5 of the present invention into emulsions with different solid contents, wherein: Figure 6 (a) is the stress-strain curve of paper before sizing. Figure 6 (b) is the stress-strain curve of the paper after sizing.

[0031] Figure 7 The scanning electron microscope images of paper before and after sizing the paper by diluting the starch-based aqueous polyurethane emulsion prepared in Example 5 of the present invention to an emulsion with a solid content of 20%, wherein: Figure 7 (a) is the surface structure of paper before sizing. Figure 7 (b) is the surface structure of paper after sizing. DETAILED DESCRIPTION

[0032] The present invention is further described in detail below in conjunction with specific examples and drawings, but the embodiments of the present invention are not limited thereto. The materials involved in the following examples can all be obtained from commercial channels.

[0033] In the following examples, the stirring rate is 200 rpm, and the high-speed stirring emulsification rate is 1000 rpm.

[0034] Example 1

[0035] The preparation method of the starch-based aqueous polyurethane emulsion of this embodiment comprises the following steps:

[0036] (1) Add 12 g of glycerol and 28 g of polyethylene glycol to a reaction kettle, stir and heat to 100° C., then add 10 g of corn starch, stir and heat to 130° C., keep stirring for 1 hour, then add two drops of methane sulfonic acid, stir at 130° C. for 0.5 hour, cool to room temperature, filter out the residue, and obtain a light yellow transparent liquid starch-based polyol.

[0037] (2) In another reaction kettle, 4.50 g of isophorone diisocyanate (IPDI), 1.67 g of 2,2-dihydroxymethylbutyric acid, and 8 mL of solvent acetone were added, and the mixture was heated to 70° C. with stirring. Then, 0.03 g of catalyst dibutyltin dilaurate was added, and the mixture was stirred and reacted at 70° C. for 3 h. Then, 3.5 g of the starch-based polyol obtained in step (1) was added dropwise using a constant pressure funnel, and the mixture was stirred and reacted at 70° C. for 3 h. After the reaction was completed, the mixture was cooled to room temperature, and then 1.14 g of triethylamine (TEA) was added dropwise, and the mixture was stirred and neutralized for 0.5 h. Then, 1.5 g of 3-aminopropyltriethoxysilane (APTES) was added, and the mixture was stirred and reacted for 2 h. Then, 22.86 g of deionized water (H 2 O) was added. 2 O), high-speed stirring emulsification for 2h, after the reaction is completed, the reaction solution is evaporated to remove acetone, and a starch-based waterborne polyurethane emulsion with a solid content of 35% is obtained, which is named Si-SWPU1. The synthesis route of the starch-based waterborne polyurethane emulsion of this embodiment is as follows Figure 1 shown.

[0038] Example 2

[0039] The preparation method of the starch-based aqueous polyurethane emulsion of this embodiment comprises the following steps:

[0040] (1) Add 12 g of glycerol and 28 g of polyethylene glycol to a reaction kettle, stir and heat to 100° C., then add 10 g of corn starch, stir and heat to 130° C., keep stirring for 1 hour, then add two drops of methane sulfonic acid, stir at 130° C. for 0.5 hour, cool to room temperature, filter out the residue, and obtain a light yellow transparent liquid starch-based polyol.

[0041] (2) In another reaction kettle, 4.50 g of isophorone diisocyanate, 1.76 g of 2,2-dihydroxymethylbutyric acid, and 8 mL of solvent acetone were added, and the mixture was stirred and heated to 70° C., and then 0.03 g of catalyst dibutyltin dilaurate was added, and the mixture was stirred and reacted at 70° C. for 3 h. Then, 3.5 g of the starch-based polyol obtained in step (1) was added dropwise using a constant pressure funnel, and the mixture was stirred and reacted at 70° C. for 3 h. After the reaction was completed, the mixture was cooled to room temperature, and then 1.21 g of triethylamine was added dropwise, and the mixture was stirred and neutralized for 0.5 h. Then, 1.5 g of 3-aminopropyltriethoxysilane was added, and the mixture was stirred and reacted for 2 h. Then, 23.15 g of deionized water was added, and the mixture was stirred and emulsified at high speed for 2 h. After the reaction was completed, the reaction solution was rotary evaporated to remove acetone to obtain a starch-based waterborne polyurethane emulsion with a solid content of 35%, which was named Si-SWPU2.

[0042] Example 3

[0043] The preparation method of the starch-based aqueous polyurethane emulsion of this embodiment comprises the following steps:

[0044] (1) Add 12 g of glycerol and 28 g of polyethylene glycol to a reaction kettle, stir and heat to 100° C., then add 10 g of corn starch, stir and heat to 130° C., keep stirring for 1 hour, then add two drops of methane sulfonic acid, stir at 130° C. for 0.5 hour, cool to room temperature, filter out the residue, and obtain a light yellow transparent liquid starch-based polyol.

[0045] (2) In another reaction kettle, 4.50 g of isophorone diisocyanate, 1.87 g of 2,2-dihydroxymethylbutyric acid, and 8 mL of solvent acetone were added, and the mixture was stirred and heated to 70° C. Then, 0.03 g of catalyst dibutyltin dilaurate was added, and the mixture was stirred and reacted at 70° C. for 3 h. Then, 3.5 g of the starch-based polyol obtained in step (1) was added dropwise using a constant pressure funnel, and the mixture was stirred and reacted at 70° C. for 3 h. After the reaction was completed, the mixture was cooled to room temperature, and then 1.28 g of triethylamine was added dropwise, and the mixture was stirred and reacted for 0.5 h. Then, 1.5 g of 3-aminopropyltriethoxysilane was added, and the mixture was stirred and reacted for 2 h. Then, 23.51 g of deionized water was added, and the mixture was stirred and emulsified at high speed for 2 h. After the reaction was completed, the reaction solution was rotary evaporated to remove acetone to obtain a starch-based waterborne polyurethane emulsion with a solid content of 35%, which was named Si-SWPU3.

[0046] Example 4

[0047] The preparation method of the starch-based aqueous polyurethane emulsion of this embodiment comprises the following steps:

[0048] (1) Add 12 g of glycerol and 28 g of polyethylene glycol to a reaction kettle, stir and heat to 100° C., then add 10 g of corn starch, stir and heat to 130° C., keep stirring for 1 hour, then add two drops of methane sulfonic acid, stir at 130° C. for 0.5 hour, cool to room temperature, filter out the residue, and obtain a light yellow transparent liquid starch-based polyol.

[0049] (2) In another reaction kettle, 4.50 g of isophorone diisocyanate, 2.00 g of 2,2-dihydroxymethylbutyric acid, and 8 mL of solvent acetone were added, and the mixture was stirred and heated to 70° C., and then 0.03 g of catalyst dibutyltin dilaurate was added, and the mixture was stirred and reacted at 70° C. for 3 h. Then, 3.5 g of the starch-based polyol obtained in step (1) was added dropwise using a constant pressure funnel, and the mixture was stirred and reacted at 70° C. for 3 h. After the reaction was completed, the mixture was cooled to room temperature, and then 1.37 g of triethylamine was added dropwise, and the mixture was stirred and neutralized for 0.5 h. Then, 1.5 g of 3-aminopropyltriethoxysilane was added, and the mixture was stirred and reacted for 2 h. Then, 23.90 g of deionized water was added, and the mixture was stirred and emulsified at high speed for 2 h. After the reaction was completed, the reaction solution was rotary evaporated to remove acetone to obtain a starch-based waterborne polyurethane emulsion with a solid content of 35%, which was named Si-SWPU4.

[0050] Example 5

[0051] The preparation method of the starch-based aqueous polyurethane emulsion of this embodiment comprises the following steps:

[0052] (1) Add 12 g of glycerol and 28 g of polyethylene glycol to a reaction kettle, stir and heat to 100° C., then add 10 g of corn starch, stir and heat to 130° C., keep stirring for 1 hour, then add two drops of methane sulfonic acid, stir at 130° C. for 0.5 hour, cool to room temperature, filter out the residue, and obtain a light yellow transparent liquid starch-based polyol.

[0053] (2) In another reaction kettle, 4.50 g of isophorone diisocyanate, 2.14 g of 2,2-dihydroxymethylbutyric acid, and 8 mL of solvent acetone were added, and the mixture was stirred and heated to 70° C. Then, 0.03 g of catalyst dibutyltin dilaurate was added, and the mixture was stirred and reacted at 70° C. for 3 h. Then, 3.5 g of the starch-based polyol obtained in step (1) was added dropwise using a constant pressure funnel, and the mixture was stirred and reacted at 70° C. for 3 h. After the reaction was completed, the mixture was cooled to room temperature, and then 1.46 g of triethylamine was added dropwise, and the mixture was stirred and reacted for 0.5 h. Then, 1.5 g of 3-aminopropyltriethoxysilane was added, and the mixture was stirred and reacted for 2 h. Then, 24.33 g of deionized water was added, and the mixture was stirred and emulsified at high speed for 2 h. After the reaction was completed, the reaction solution was rotary evaporated to remove acetone to obtain a starch-based waterborne polyurethane emulsion with a solid content of 35%, which was named Si-SWPU5.

[0054] Example 6

[0055] The preparation method of the starch-based aqueous polyurethane emulsion of this embodiment comprises the following steps:

[0056] (1) Add 12 g of glycerol and 28 g of polyethylene glycol to a reaction kettle, stir and heat to 100° C., then add 10 g of corn starch, stir and heat to 130° C., keep stirring for 1 hour, then add two drops of methane sulfonic acid, stir at 130° C. for 0.5 hour, cool to room temperature, filter out the residue, and obtain a light yellow transparent liquid starch-based polyol.

[0057] (2) In another reaction kettle, 4.50 g of isophorone diisocyanate, 2.25 g of 2,2-dihydroxymethylbutyric acid, and 8 mL of solvent acetone were added, and the mixture was heated to 70° C. with stirring. Then, 0.03 g of catalyst dibutyltin dilaurate was added, and the mixture was stirred and reacted at 70° C. for 3 h. Then, 3.5 g of the starch-based polyol obtained in step (1) was added dropwise using a constant pressure funnel, and the mixture was stirred and reacted at 70° C. for 3 h. After the reaction was completed, the mixture was cooled to room temperature, and then 1.71 g of triethylamine was added dropwise, and the mixture was stirred and neutralized for 0.5 h. Then, 1.5 g of 3-aminopropyltriethoxysilane was added, and the mixture was stirred and reacted for 2 h. Then, 24.68 g of deionized water was added, and the mixture was emulsified with high-speed stirring for 2 h. After the reaction was completed, the reaction solution was rotary evaporated to remove acetone, and a starch-based waterborne polyurethane emulsion with a solid content of 35% was obtained, which was named Si-SWPU6.

[0058] In order to verify the comprehensive performance of the starch-based waterborne polyurethane emulsion of the present invention, the starch-based waterborne polyurethane emulsions prepared in Examples 1 to 6 were subjected to the following performance tests.

[0059] Test Example 1

[0060] The starch-based polyol and starch-based waterborne polyurethane emulsion prepared in Example 1 were structurally characterized: natural starch, starch-based polyol prepared in Example 1, and starch-based waterborne polyurethane emulsion Si-SWPU1 were subjected to infrared spectroscopy detection. The detection results are as follows: Figure 2 As shown in the figure, Natural starch is natural starch, STP is starch-based polyol, and Si-SWPU is starch-based waterborne polyurethane emulsion. It can be seen from the figure that at 1100cm -1 The strong absorption peaks on the left and right are the characteristic absorption peaks of the antisymmetric stretching vibration of the ether bond (COC) in the polyether polyol. The absorption peak intensity of the starch-based polyol is greater than that of the natural starch here, which means that new ether bonds are generated, indicating that the product is a polyether polyol. The infrared spectrum of Si-SWPU is at 2250-2270cm -1 The absence of the band at 3300-3600 cm-1 confirms the absence of free NCO in the polymer structure, indicating that the reaction has been completed. -1 The bands observed between 2800 and 3000 cm-1 belong to the NH stretching vibration of polyurethane.-1 The energy band in the range is connected to -CH 2 and -CH 3 Aliphatic CH stretching vibration, 1240 cm -1 (Si-CH 2 CH 2 CH 2 )、771cm -1 Absorption occurs at the (Si-OC) frequency. The detection of Si-OC confirms that the hydroxyl group (Si-OH) of silane reacts chemically with the isocyanate (N=C=O) of WPU, indicating that silane reacts with the isocyanate on the other side of IPDI and the end-capping is successful, and the starch-based waterborne polyurethane emulsion is successfully synthesized.

[0061] Test Example 2

[0062] (1) The stability of Si-SWPU1-6 prepared in Examples 1-6 was characterized: the particle size distribution of the emulsion was tested. The test results were as follows: Figure 3 As shown in the figure, it can be seen that the particle size distribution of the starch-based aqueous polyurethane emulsions prepared in Examples 1 to 6 is all between 10 and 100 nm, the particle size of the emulsion is small, and the smaller the particle size, the better the stability, indicating that the emulsion has good stability.

[0063] (2) Si-SWPU1-6 prepared in Examples 1-6 were centrifuged at 3000 r / min for 15 min. The treatment results were as follows: Figure 4 As shown in the figure, it can be seen that there is no obvious precipitation in the six samples after centrifugation, so it is considered that the emulsion has a storage stability period of 6 months or more.

[0064] Test Example 3

[0065] The starch-based aqueous polyurethane emulsions prepared in Examples 1 to 6 were placed in a polytetrafluoroethylene mold, and then demolded after being placed at room temperature and pressure for 48 hours to obtain a cured film.

[0066] The cured films obtained by curing Si-SWPU1 to 6 of Examples 1 to 6 were subjected to thermal stability analysis and determination. The results are as follows: Figure 5 As shown. Figure 5 The thermal degradation of the cured film was analyzed and found to be roughly divided into three stages. The first stage was at around 250°C, the second stage was at around 320°C, and the third stage was at around 400°C. The maximum carbon residue rate of the cured film at 600°C was 6.26% to 8.81%.

[0067] Test Example 4

[0068] The starch-based aqueous polyurethane emulsion prepared in Example 5 was diluted to emulsions with solid contents of 10%, 15%, 20%, 25%, 30% and 35%, respectively, and then the emulsions of different concentrations were coated on the surface of base paper to obtain sized paper.

[0069] The stress-strain curve of the paper before and after sizing was tested. The results are as follows: Figure 6 As shown, Figure 6 (a) is the stress-strain curve of paper before sizing. Figure 6 (b) is the stress-strain curve of the paper after sizing. Figure 6 It can be seen that the tensile strength of the paper after sizing is greatly improved compared with that before sizing, indicating that using starch-based water-based polyurethane emulsion as a paper surface sizing agent can improve the mechanical properties of paper.

[0070] Test Example 5

[0071] The starch-based aqueous polyurethane emulsion prepared in Example 5 was diluted to an emulsion with a solid content of 20%, and then the emulsion was coated on the surface of base paper to obtain sized paper.

[0072] The surface structure of paper before and after sizing was characterized by scanning electron microscopy. Figure 7 As shown, Figure 7 (a) is the surface structure of paper before sizing. Figure 7 (b) is the surface structure of paper after sizing. Figure 7 It can be seen that there are large gaps between the paper fibers on the surface of the base paper, and there are small fibers arranged loosely on the surface; after sizing, there are no fine fibers on the surface of the paper, and the pores between the fibers become smaller. It can be seen that after the starch-based water-based polyurethane emulsion is sizing on the surface of the paper, the ions of the starch-based water-based polyurethane emulsion will fill the gaps between some of the paper fibers, so that the smoothness of the paper surface is significantly improved.

[0073] The above are only some embodiments of the present invention. For those skilled in the art, several modifications and improvements can be made without departing from the creative concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for preparing a starch-based aqueous polyurethane emulsion, characterized in that: The steps include: S1. Mix glycerol and polyethylene glycol, stir and heat to 90-100°C, then add starch, stir and heat to 120-140°C, keep warm and stir for 1-2h, then add methane sulfonic acid, stir at 120-140°C for 0.5-1h, then cool to room temperature, filter out the residue, and obtain starch-based polyol; S2, isophorone diisocyanate, 2,2-dihydroxymethylbutyric acid, and a solvent are mixed, stirred and heated to 60-80° C., then a catalyst is added, stirred and reacted at 60-80° C. for 2-3 hours, then the starch-based polyol obtained in step S1 is added, stirred and reacted at 60-80° C. for 2-3 hours, and then the reaction is cooled to room temperature after the reaction is completed, and then triethylamine is added, stirred and neutralized for 20-40 minutes, and then 3-aminopropyltriethoxysilane is added, stirred and reacted for 1-2 hours, and then water is added, and emulsified with high-speed stirring for 1-2 hours. After the reaction is completed, the reaction solution is rotary evaporated to remove the solvent to obtain a starch-based waterborne polyurethane emulsion; In step S1, the mass ratio of glycerol to polyethylene glycol is 3:7, the mass ratio of starch to the total mass of glycerol and polyethylene glycol is 1:4, and the amount of methanesulfonic acid used is 0.05% of the total mass of starch, glycerol and polyethylene glycol; In step S2, the molar ratio of the isocyanate contained in the isophorone diisocyanate to the hydroxyl contained in the 2,2-dimethylolbutyric acid is 1:(1.3-1.8); In step S2, the solvent is acetone, and the volume ratio of the sum of the mass of isophorone diisocyanate and 2,2-dihydroxymethylbutyric acid to the solvent is (6-7): (5-10); In step S2, the catalyst is dibutyltin dilaurate, and the amount of the catalyst is 0.01% to 1% of the mass of isophorone diisocyanate; In step S2, the mass ratio of the starch-based polyol to isophorone diisocyanate is 1:1.29; In step S2, the molar ratio of triethylamine to 2,2-dihydroxymethylbutyric acid is 1:1, the mass ratio of 3-aminopropyltriethoxysilane to isophorone diisocyanate is 1:3, and the amount of water used is 60-70% of the total mass of the reaction raw materials and water.

2. The method for preparing the starch-based aqueous polyurethane emulsion according to claim 1, characterized in that: The stirring rate is 180-220 rpm, and the high-speed stirring emulsification rate is 1000-1100 rpm.

3. A starch-based aqueous polyurethane emulsion prepared by the preparation method according to claim 1 or 2.

4. Use of the starch-based aqueous polyurethane emulsion according to claim 3 in preparing a paper surface sizing agent.

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