Waterborne polyurethane emulsion with low water absorption rate as well as preparation method and application of waterborne polyurethane emulsion

Abstract

The invention discloses a low-water-absorption waterborne polyurethane emulsion as well as a preparation method and application thereof, and belongs to the technical field of waterborne polyurethane. According to the invention, diisocyanate, dimer acid polyester diol, a chain extender, a silane coupling agent and the like are used as raw materials, and an aqueous polyurethane emulsion is prepared through addition polymerization. Hydrophobic long-carbon-chain dimer acid polyester glycol and the silane coupling agent are introduced into a waterborne polyurethane structure for composite modification, and the obtained waterborne polyurethane film has low water absorption, high resilience and high strength and can be applied to water-resistant medical elastic film products.

Description

Technical Field

[0001] This invention belongs to the field of waterborne polyurethane technology, and specifically relates to a method for preparing a waterborne polyurethane emulsion. Background Technology

[0002] Waterborne polyurethane, using water as the dispersion medium, boasts significant advantages such as being environmentally friendly and non-toxic, having low VOC content, safe processing, good biocompatibility, and low allergenicity. Furthermore, its flexible molecular structure allows for a wide range of mechanical properties, from high elasticity to high strength, by adjusting the types and proportions of soft and hard segments. It also readily forms dense, continuous films, exhibiting excellent protective performance. Based on these combined advantages, waterborne polyurethane demonstrates enormous application potential in the field of medical elastic films.

[0003] However, in order to obtain and maintain a stable dispersion in the aqueous phase, waterborne polyurethanes must incorporate a certain amount of hydrophilic groups or segments (such as carboxyl groups, sulfonic acid groups, and polyether segments) into the polymer molecular chain. While this necessary structure imparts good stability to the emulsion, it also leads to a significantly high water absorption rate in the coating film. High water absorption significantly reduces the dimensional stability, mechanical properties (such as decreased hardness and strength), water resistance, and chemical resistance of the coating film. Therefore, the high water absorption problem greatly limits the further promotion and application of waterborne polyurethanes in the field of medical elastic film products requiring high elasticity and high water resistance.

[0004] In existing technologies, methods such as organofluorine, organosilicon, and internal / external crosslinking are commonly used to reduce the water absorption rate of waterborne polyurethane. However, these methods still have some problems: too little organosilicon has little water resistance, while too much leads to severe phase separation, affecting mechanical properties; organofluorine modification is costly and environmentally controversial; excessive crosslinking sacrifices the elasticity and toughness of the film. For example, the invention patent with publication number CN109553752A discloses a waterborne polyurethane emulsion for fabric coating with low water absorption rate and its preparation method. By introducing chain extenders containing different active amine groups, the highly active primary amine groups react with isocyanates and are incorporated into the molecular chain, while the less active amine groups remain in the molecular side chain. Combined with an external curing agent of isocyanate, a waterborne polyurethane with a low water absorption rate of 7% is prepared. However, for the requirements of high water resistance, the water absorption rate is still too high, and the tensile strength is not mentioned. For example, the invention patent with publication number CN105601872A discloses a composite modified waterborne polyurethane of hydroxyl silicone oil and silane coupling agent and its preparation method, which makes the water absorption rate of polyurethane film as low as 13%, but its resilience is obviously insufficient and its elongation is only 450%.

[0005] Therefore, developing a waterborne polyurethane emulsion with low water absorption, high elongation at break, high tensile strength, and excellent comprehensive properties has significant industrial application value. Summary of the Invention

[0006] To address the technical problem of high water absorption rate in existing waterborne polyurethanes, this invention proposes a waterborne polyurethane emulsion with low water absorption rate, its preparation method, and its application.

[0007] To achieve the above objectives, the technical solution of the present invention is implemented as follows: This invention provides a waterborne polyurethane emulsion with low water absorption, prepared from raw materials comprising the following parts by weight: 164.2-168.6 parts of polyol, 27-33 parts of diisocyanate, 0.1-0.15 parts of catalyst, 4.1-4.3 parts of carboxylic acid hydrophilic chain extender, 2.1-2.2 parts of small molecule chain extender, 0-1.1 parts of crosslinking agent, 3.1-3.2 parts of neutralizing agent, 0-1.1 parts of sulfonic acid hydrophilic chain extender, 1.1-2.2 parts of silane coupling agent, 0.63-1.5 parts of post-chain extender, 470-475 parts of deionized water, and 140-180 parts of acetone.

[0008] The polyol is selected from at least one of polycarbonate diol, polytetrahydrofuran ether diol, and dimer polyester diol, wherein the average molecular weight of the polyol is 2000-3000. The carbonate groups in the polycarbonate diol molecular chain have extremely high chemical stability and are not easily hydrolyzed by water molecules. Introducing a small amount of polycarbonate diol can maintain elasticity while imparting hydrolysis resistance to the polymer film. Dimer polyester diol has a unique long-chain alkane branched structure, forming a microscopic hydrophobic barrier within the polymer, effectively blocking the diffusion and adsorption of water molecules. On the other hand, the irregular structure inhibits chain segment crystallization and acts as an internal plasticizer, providing flexibility. Polytetrahydrofuran ether diol, due to its main chain being composed of compliant ether bonds, possesses a structure that endows the polymer chain with excellent high elasticity.

[0009] The diisocyanate is either dicyclohexylmethane diisocyanate or hexamethylene diisocyanate. The combination of dicyclohexylmethane diisocyanate and hexamethylene diisocyanate gives the hard segment micro-region structure both high cohesive strength and moderate flexibility, endowing the film with excellent comprehensive mechanical properties and resistance to yellowing.

[0010] The catalyst is dibutyltin dilaurate.

[0011] The carboxylic acid hydrophilic chain extender is dimethylolpropionic acid, and the sulfonic acid hydrophilic chain extender is sodium ethylenediamine sulfonate. When dimethylolpropionic acid and sodium ethylenediamine sulfonate are combined, the strong sulfonate groups aggregate to form a more compact and stable ion cluster. These physical cross-linking points not only enhance mechanical strength but also greatly inhibit the migration and precipitation of hydrophilic group components, avoiding the formation of a continuous water-rich layer on the surface, thus giving the film excellent whitening resistance.

[0012] The small molecule chain extender is 1,6-hexanediol or diethylene glycol.

[0013] The silane coupling agent is N-(2-aminoethyl)-3-aminopropyltriethoxysilane, which introduces a dual-amino silane coupling agent. Its molecular structure contains both primary and secondary amino groups, which not only provides higher reactivity and crosslinking site density, but also allows the hydrolyzed trimethoxysilane ends to condense inward to form a hydrophobic siloxane network, constructing a uniform and stable hydrophobic crosslinking network. This dual effect not only significantly improves the mechanical properties of the film, but also reduces the water absorption rate of the film by constructing a hydrophobic barrier and increasing the crosslinking density.

[0014] The crosslinking agent is trimethylolpropane. A small amount of trimethylolpropane is introduced as a micro-crosslinking agent to construct a moderate and controllable three-dimensional network structure between molecular chains.

[0015] The neutralizing agent is triethylamine. The chain extender is ethylenediamine or isophorone diamine.

[0016] This invention provides a method for preparing the aforementioned low-water-absorption waterborne polyurethane emulsion, the specific steps of which are as follows: (1) After adding the polyol to the reactor, heat it to 100-120℃ and dehydrate it under vacuum for 3-4 hours. Then, cool it down to 50℃ and add diisocyanate, catalyst and acetone. Heat it up to carry out the reaction. (2) After cooling the raw materials after the reaction in step (1) to 50°C, add the hydrophilic chain extender and heat up to carry out the reaction; (3) After cooling the raw materials after the reaction in step (2) to 50°C, add small molecule chain extender and crosslinking agent, and then heat up to carry out the reaction; (4) After cooling the raw material after the reaction in step (3) to 30-40℃, add neutralizing agent and acetone and stir for 20-30 min. Then add sodium ethylenediamine ethanesulfonate and silane coupling agent and stir for 10-20 min to obtain the prepolymer. (5) Add deionized water to the prepolymer obtained in step (4) and emulsify for 10-20 min. Then add a chain extender and stir for 2-3 h. After removing acetone by rotary evaporation, an aqueous polyurethane emulsion is obtained.

[0017] In step (1), the temperature for the reaction is 70-75℃ and the time is 2-3h; in step (2), the temperature for the reaction is 70-75℃ and the time is 4-5h; in step (3), the temperature for the reaction is 70-75℃ and the time is 2-3h.

[0018] The present invention also provides the application of the aforementioned low-water-absorption waterborne polyurethane emulsion in water-resistant elastic film products.

[0019] The present invention has the following beneficial effects: This invention introduces a combination of hydrophobic long-chain alkyl dimeric acid polyester diol, polycarbonate diol, and polytetrahydrofuran ether diol as soft segments into a waterborne polyurethane structure. The dimeric acid polyester diol possesses a unique long-chain alkane branched structure, forming a microscopic hydrophobic barrier within the polymer, effectively blocking the diffusion and adsorption of water molecules. Furthermore, the irregular structure inhibits chain segment crystallization and acts as an internal plasticizer, providing flexibility. The combination of dimethylolpropionic acid and sodium ethylenediamine sulfonate allows the strong sulfonate groups to aggregate into more compact and stable ionic clusters. These physical crosslinking points not only enhance mechanical strength but also significantly inhibit the migration and precipitation of hydrophilic components, preventing the formation of a continuous water-rich layer on the surface and giving the film excellent whitening resistance. Simultaneously, a diamino silane coupling agent is introduced, whose molecular structure contains both primary and secondary amino groups. This not only provides higher reactivity and crosslinking site density, but also allows the hydrolyzed trimethoxysilane ends to self-condense and form a hydrophobic siloxane network, constructing a uniform and stable hydrophobic crosslinking network. This significantly improves the mechanical properties of the film and reduces its water absorption rate by building a hydrophobic barrier and increasing the crosslinking density. The resulting waterborne polyurethane film has a water absorption rate reduced to 4.3%, an elongation at break greater than 800%, and a tensile strength greater than 20 MPa, making it suitable for use in water-resistant medical elastic film products. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; the materials and reagents used are commercially available unless otherwise specified. In the embodiments of this invention, one part of raw material corresponds to 1g.

[0022] Example 1 A method for preparing a waterborne polyurethane emulsion with low water absorption rate is as follows: (1) 140g of polytetrahydrofuran ether diol with a molecular weight of 3000, 20g of polycarbonate diol with a molecular weight of 2000, and 4.2g of dimer polyester diol with a molecular weight of 2000 were added to a four-necked flask equipped with a stirring and heating device. The mixture was then heated to 120°C and dehydrated under vacuum for 4 hours. After cooling to 50°C, 19.2g of dicyclohexylmethane diisocyanate, 12.2g of hexamethylene diisocyanate, 0.1g of dibutyltin dilaurate catalyst and 40g of acetone were added. The mixture was then heated to 70°C and reacted for 2 hours. (2) After cooling the raw material after the reaction in step (1) to 50°C, add 4.2g of dimethylolpropionic acid and heat to 75°C for 4h. (3) After cooling the raw materials after the reaction in step (2) to 50°C, add 2.1g of diethylene glycol and 1.1g of trimethylolpropane, and heat to 75°C for 3h. (4) After cooling the raw material after the reaction in step (3) to 40°C, add 3.2g of triethylamine and 100g of acetone and stir for 20min. Then add 1.1g of sodium ethylenediaminoethanesulfonate and 1.1g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane and stir for 10min to obtain the prepolymer. (5) Add 460g of ice water to the prepolymer obtained in step (4) and emulsify for 10 min. Then slowly add ethylenediamine aqueous solution (0.65g of ethylenediamine dissolved in 10g of water) and stir for 2 h. Remove acetone by rotary evaporation at 55℃ and -0.1MPa reduced pressure to obtain waterborne polyurethane emulsion.

[0023] Example 2 A method for preparing a waterborne polyurethane emulsion with low water absorption rate is as follows: (1) 140g of polytetrahydrofuran ether diol with a molecular weight of 3000, 20g of polycarbonate diol with a molecular weight of 2000, and 8.6g of dimer polyester diol with a molecular weight of 2000 were added to a four-necked flask equipped with a stirring and heating device. The mixture was then heated to 100°C and dehydrated under vacuum for 3 hours. After cooling to 50°C, 19.6g of dicyclohexylmethane diisocyanate, 12.5g of hexamethylene diisocyanate, 0.15g of dibutyltin dilaurate catalyst and 60g of acetone were added. The mixture was then heated to 75°C and reacted for 3 hours. (2) After cooling the raw material after the reaction in step (1) to 50°C, add 4.3g of dimethylolpropionic acid and heat to 70°C for 5h. (3) After cooling the raw materials after the reaction in step (2) to 50°C, add 2.2g of diethylene glycol and 1.1g of trimethylolpropane, and heat to 70°C for 3h. (4) After cooling the raw material after the reaction in step (3) to 40°C, add 3.2g of triethylamine and 120g of acetone and stir for 30min. Then add 1.1g of sodium ethylenediaminoethanesulfonate and 1.1g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane and stir for 15min to obtain the prepolymer. (5) Add 460g of ice water to the prepolymer obtained in step (4) and emulsify for 10 min. Then slowly add ethylenediamine aqueous solution (0.63g of ethylenediamine dissolved in 10g of water) and stir for 3 h. Remove acetone by rotary evaporation at 55℃ and -0.1MPa reduced pressure to obtain waterborne polyurethane emulsion.

[0024] Example 3 A method for preparing a waterborne polyurethane emulsion with low water absorption rate is as follows: (1) 140g of polytetrahydrofuran ether diol with a molecular weight of 3000, 20g of polycarbonate diol with a molecular weight of 2000, and 8.4g of dimer polyester diol with a molecular weight of 2000 were added to a four-necked flask equipped with a stirring and heating device. The mixture was then heated to 110°C and vacuum dehydrated for 3 hours. After cooling to 50°C, 16.6g of dicyclohexylmethane diisocyanate, 10.4g of hexamethylene diisocyanate, 0.1g of dibutyltin dilaurate catalyst and 60g of acetone were added. The mixture was then heated to 70°C and reacted for 2.5 hours. (2) After cooling the raw material after the reaction in step (1) to 50°C, add 4.2g of dimethylolpropionic acid and heat to 73°C for 4h. (3) After cooling the raw material after the reaction in step (2) to 50°C, add 2.1g of 1,6-hexanediol and heat to 75°C for 2h. (4) After cooling the raw material after the reaction in step (3) to 40°C, add 3.2g of triethylamine and 120g of acetone and stir for 30min. Then add 1.1g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane and stir for 20min to obtain the prepolymer. (5) Add 460g of ice water to the prepolymer obtained in step (4) and emulsify for 20 min. Then slowly add isophorone diamine aqueous solution (1.5g isophorone diamine dissolved in 15g water) and stir for 2 h. Remove acetone by rotary evaporation at 55℃ and -0.1MPa reduced pressure to obtain waterborne polyurethane emulsion.

[0025] Example 4 A method for preparing a waterborne polyurethane emulsion with low water absorption rate is as follows: (1) 140g of polytetrahydrofuran ether diol with a molecular weight of 3000, 20g of polycarbonate diol with a molecular weight of 2000, and 8.6g of dimer polyester diol with a molecular weight of 2000 were added to a four-necked flask equipped with a stirring and heating device. The mixture was then heated to 120°C and vacuum dehydrated for 3.5h. After cooling to 50°C, 20g of dicyclohexylmethane diisocyanate, 13g of hexamethylene diisocyanate, 0.15g of dibutyltin dilaurate catalyst and 60g of acetone were added. The mixture was then heated to 73°C and reacted for 2h. (2) After cooling the raw material after the reaction in step (1) to 50°C, add 4.3g of dimethylolpropionic acid and heat to 75°C for 4.5h. (3) After cooling the raw materials after the reaction in step (2) to 50°C, add 2.2g of diethylene glycol and 1.1g of trimethylolpropane, and heat to 73°C to react for 2.5h; (4) After cooling the raw material after the reaction in step (3) to 40°C, add 3.2g of triethylamine and 120g of acetone and stir for 25min. Then add 1.1g of sodium ethylenediamine ethanesulfonate and 2.2g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane and stir for 10min to obtain the prepolymer. (5) Add 460g of ice water to the prepolymer obtained in step (4) and emulsify for 15min. Then slowly add ethylenediamine aqueous solution (0.63g of ethylenediamine dissolved in 10g of water) and stir for 2.5h. Remove acetone by rotary evaporation at 55℃ and -0.1MPa reduced pressure to obtain waterborne polyurethane emulsion.

[0026] Example 5 A method for preparing a waterborne polyurethane emulsion with low water absorption rate is as follows: (1) 140g of polytetrahydrofuran ether diol with a molecular weight of 3000, 20g of polycarbonate diol with a molecular weight of 2000, and 4.2g of dimer polyester diol with a molecular weight of 2000 were added to a four-necked flask equipped with a stirring and heating device. The mixture was then heated to 120°C and dehydrated under vacuum for 3 hours. After cooling to 50°C, 19.2g of dicyclohexylmethane diisocyanate, 12.2g of hexamethylene diisocyanate, 0.12g of dibutyltin dilaurate catalyst and 40g of acetone were added. The mixture was then heated to 70°C and reacted for 2 hours. (2) After cooling the raw material after the reaction in step (1) to 50°C, add 4.1g of dimethylolpropionic acid and heat to 75°C for 4h. (3) After cooling the raw materials after the reaction in step (2) to 50°C, add 2.15g of diethylene glycol and 0.5g of trimethylolpropane, and heat to 75°C for 3h. (4) After cooling the raw material after the reaction in step (3) to 40°C, add 3.1g of triethylamine and 110g of acetone and stir for 20min. Then add 0.5g of sodium ethylenediaminoethanesulfonate and 1.5g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane and stir for 10min to obtain the prepolymer. (5) Add 460g of ice water to the prepolymer obtained in step (4) and emulsify for 10 min. Then slowly add ethylenediamine aqueous solution (1g of ethylenediamine dissolved in 12g of water) and stir for 2 h. Remove acetone by rotary evaporation at 55℃ and -0.1MPa reduced pressure to obtain waterborne polyurethane emulsion.

[0027] Comparative Example 1 A method for preparing a waterborne polyurethane emulsion with low water absorption rate, which differs from Example 1, is as follows: (1) 140g of polytetrahydrofuran ether diol with a molecular weight of 3000 and 20g of polycarbonate diol with a molecular weight of 2000 were added to a four-necked flask equipped with a stirring and heating device. The mixture was then heated to 120°C and dehydrated under vacuum for 4 hours. After cooling to 50°C, 19.2g of dicyclohexylmethane diisocyanate, 12.2g of hexamethylene diisocyanate, 0.1g of dibutyltin dilaurate catalyst and 40g of acetone were added. The mixture was then heated to 70°C and reacted for 2 hours. (2) After cooling the raw material after the reaction in step (1) to 50°C, add 4.2g of dimethylolpropionic acid and heat to 75°C for 4h. (3) After cooling the raw materials after the reaction in step (2) to 50°C, add 2.1g of diethylene glycol and 1.1g of trimethylolpropane, and heat to 75°C for 3h. (4) After cooling the raw material after the reaction in step (3) to 40°C, add 3.2g of triethylamine and 100g of acetone and stir for 20min. Then add 1.1g of sodium ethylenediamine ethanesulfonate and stir for 10min to obtain the prepolymer. (5) Add 460g of ice water to the prepolymer obtained in step (4) and emulsify for 10 min. Then slowly add ethylenediamine aqueous solution (0.65g of ethylenediamine dissolved in 10g of water) and stir for 2 h. Remove acetone by rotary evaporation at 55℃ and -0.1MPa reduced pressure to obtain waterborne polyurethane emulsion.

[0028] Implementation Results Example Aqueous polyurethane emulsion was deposited on a tetrafluoroethylene plate at room temperature. After the water evaporated, the film was dried in an oven at 60°C for 12 hours, resulting in a film thickness of 0.2 mm. The performance of the films prepared from the aqueous polyurethane emulsions of Examples 1-4 and Comparative Example 1 was tested.

[0029] 1. Water absorption performance test The water absorption properties of the films prepared from the waterborne polyurethane emulsions of Examples 1-4 and Comparative Example 1 were tested as follows: The dried waterborne polyurethane film was cut into 2cm×2cm samples and weighed M1. Then, it was soaked in deionized water at room temperature for 24 hours and then removed. The surface moisture was absorbed with filter paper and weighed M2. The water absorption rate of the film was calculated according to the following formula: Water absorption rate W / % = (M2-M1) / M1×100%.

[0030] 2. Mechanical property testing The mechanical properties of the films prepared from the aqueous polyurethane emulsions of Examples 1-4 and Comparative Example 1 were tested, as follows: The tensile strength and elongation at break of the specimens were tested using a universal tensile testing machine at a tensile speed of 500 mm / min at a test temperature of 25℃. The results are shown in Table 1. Table 1. Test results of film performance The performance test results of the aqueous polyurethane emulsions prepared in Examples 1-4 and Comparative Example 1 are shown in Table 1. According to the data in Table 1, the water absorption rate of the aqueous polyurethane film modified with dimer acid polyester diol and silane coupling agent in this invention is significantly lower than that before modification, while maintaining a high elongation at break. Specifically, the aqueous polyurethane films obtained in Examples 1-4 have high tensile strength (20.2-26.1 MPa), high elongation at break (717.1-890.7%), and low water absorption rate (2.7-4.8%), and can be used in water-resistant medical elastic film products.

[0031] Compared with Comparative Example 1, Examples 1 and 2 show that the introduction of dimer acid polyester diol increases the elongation at break and decreases the water absorption rate. However, the content should not be too high, otherwise it will affect the mechanical properties. The comparison between Example 2 and Example 4 shows that excessive introduction of silane coupling agent leads to excessive crosslinking and reduces the elongation at break.

[0032] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A waterborne polyurethane emulsion with low water absorption rate, characterized in that, It is prepared from raw materials containing the following parts by weight: 164.2-168.6 parts of polyol, 27-33 parts of diisocyanate, 0.1-0.15 parts of catalyst, 4.1-4.3 parts of carboxylic acid hydrophilic chain extender, 2.1-2.2 parts of small molecule chain extender, 0-1.1 parts of crosslinking agent, 3.1-3.2 parts of neutralizing agent, 0-1.1 parts of sulfonic acid hydrophilic chain extender, 1.1-2.2 parts of silane coupling agent, 0.63-1.5 parts of post-chain extender, 470-475 parts of deionized water, and 140-180 parts of acetone.

2. The waterborne polyurethane emulsion with low water absorption rate according to claim 1, characterized in that: The polyol is selected from at least one of polycarbonate diol, polytetrahydrofuran ether diol, and dimer acid polyester diol, and the average molecular weight of the polyol is 2000-3000.

3. The waterborne polyurethane emulsion with low water absorption rate according to claim 1, characterized in that: The diisocyanate is dicyclohexylmethane diisocyanate or hexamethylene diisocyanate.

4. The waterborne polyurethane emulsion with low water absorption rate according to claim 1, characterized in that: The catalyst is dibutyltin dilaurate; the carboxylic acid hydrophilic chain extender is dimethylolpropionic acid; and the sulfonic acid hydrophilic chain extender is sodium ethylenediamine ethanesulfonate.

5. The waterborne polyurethane emulsion with low water absorption rate according to claim 1, characterized in that: The small molecule chain extender is 1,6-hexanediol or diethylene glycol.

6. The waterborne polyurethane emulsion with low water absorption rate according to claim 1, characterized in that: The silane coupling agent is N-(2-aminoethyl)-3-aminopropyltriethoxysilane.

7. The waterborne polyurethane emulsion with low water absorption rate according to claim 1, characterized in that: The crosslinking agent is trimethylolpropane, the neutralizing agent is triethylamine, and the chain extender is ethylenediamine or isophorone diamine.

8. A method for preparing a low-water-absorption waterborne polyurethane emulsion according to any one of claims 1-7, characterized in that, The specific steps are as follows: (1) After adding the polyol to the reactor, heat it to 100-120℃ to dehydrate it, then cool it down to 50℃ and add diisocyanate, catalyst and acetone, and then heat it up to carry out the reaction. (2) After cooling the raw materials after the reaction in step (1) to 50°C, add the hydrophilic chain extender and heat up to carry out the reaction; (3) After cooling the raw materials after the reaction in step (2) to 50°C, add small molecule chain extender and crosslinking agent, and then heat up to carry out the reaction; (4) After cooling the raw materials after the reaction in step (3) to 30-40℃, add neutralizing agent and acetone and stir. Then add sodium ethylenediamine ethanesulfonate and silane coupling agent, stir and react to obtain prepolymer; (5) Add deionized water to the prepolymer obtained in step (4) to carry out emulsification reaction, then add chain extender and stir, and obtain waterborne polyurethane emulsion after rotary evaporation.

9. The preparation method according to claim 8, characterized in that: In step (1), the temperature for the reaction is 70-75℃ and the time is 2-3h; in step (2), the temperature for the reaction is 70-75℃ and the time is 4-5h; in step (3), the temperature for the reaction is 70-75℃ and the time is 2-3h; in step (4), the stirring time is 20-30min and the stirring reaction time is 10-20min; in step (5), the emulsification time is 10-20min and the stirring time is 2-3h.

10. The use of the low water absorption waterborne polyurethane emulsion according to any one of claims 1-7 in water-resistant medical elastic film products.

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