PH response type waterborne polyurethane coating as well as preparation method and application thereof

Abstract

The invention discloses a pH response type waterborne polyurethane coating as well as a preparation method and application thereof. Aiming at the defects of an existing waterborne polyurethane food packaging coating in the aspects of real-time and visual indication of food freshness and active antibacterial performance, the invention provides a polyurethane composition integrating pH response discoloration and contact type antibacterial performance through molecular structure design. Firstly, a tertiary amino group is introduced into a polyurethane polymer chain as a functional reaction site, and then the functional reaction site and a dihalogenated phenolic compound are subjected to quaternization reaction, so that a quaternary ammonium salt antibacterial center and a pH response color development unit are synchronously generated on the chain. The coating is high in adhesive force with a food packaging film base material and stable in performance, and a safe and intelligent active protection solution is provided for food packaging.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a pH-responsive waterborne polyurethane coating, its preparation method, and its application. Background Technology

[0002] Waterborne polyurethane (WPU) has gained widespread attention in the food packaging field due to its advantages such as low volatile organic compound (VOC) emissions and environmental friendliness, gradually replacing traditional oil-based polyurethane. Especially in the field of smart packaging, researchers are dedicated to developing WPU-based smart coatings that combine pH-responsive color changes for real-time monitoring of food freshness with long-lasting antibacterial properties. However, existing technologies still face several key bottlenecks: Firstly, conventional WPU emulsions often use carboxyl-containing hydrophilic chain extenders to achieve water dispersion, resulting in a large number of residual hydrophilic groups after film formation, significantly increasing the coating's water absorption rate. Secondly, in high-humidity environments, swelling or even peeling can easily occur, causing leakage and inactivation of pH-responsive indicators (such as natural anthocyanins) embedded in the coating, severely affecting the accuracy and stability of freshness monitoring. On the other hand, current functionalization strategies generally rely on physical blending to introduce antimicrobial agents (such as quaternary ammonium compounds) and indicators. Due to their poor compatibility with the WPU matrix interface, these agents are prone to migration, aggregation, or phase separation during film formation. This not only weakens the durability of antimicrobial efficacy but may also lead to decreased coating transparency due to uneven distribution of functional components, and even the phenomenon of antimicrobial agents obscuring the color signal of indicators. These systemic defects severely restrict the reliability and practicality of waterborne polyurethane smart coatings in food packaging for the synergistic integration of visual freshness monitoring and antimicrobial protection functions. Innovative solutions from the perspectives of molecular structure design and functional component integration mechanisms are urgently needed. Summary of the Invention

[0003] To address the shortcomings of existing waterborne polyurethane food packaging coatings in terms of food testing and antibacterial properties, this invention provides a pH-responsive waterborne polyurethane coating, its preparation method, and its applications. This invention, through molecular structure design, provides a polyurethane composition that integrates pH-responsive color change and contact antibacterial properties. First, tertiary amino groups are introduced into the polyurethane polymer chain as functionalization reaction sites, followed by a quaternization reaction with dihalophenolic compounds, thereby simultaneously generating quaternary ammonium salt antibacterial centers and pH-responsive colorimetric units on the chain.

[0004] One technical solution of the present invention is to provide a pH-responsive waterborne polyurethane coating, comprising 50-60 parts by weight of a pH-responsive antibacterial waterborne polyurethane emulsion, 2-5 parts by weight of a crosslinking agent, 0.3-0.8 parts by weight of a wetting and dispersing agent, 0.2-0.5 parts by weight of a defoamer, 0.5-1.5 parts by weight of a thickener, and 0.2-0.4 parts by weight of a leveling agent; wherein the pH-responsive antibacterial waterborne polyurethane emulsion is composed of a diol, isocyanate, catalyst, hydrophilic chain extender, and... The preparation of tertiary amine-modified dihydroxy chain extender, neutralizing agent, diluent, dihalophenol and deionized water, wherein the mass ratio of diol, isocyanate, catalyst, hydrophilic chain extender, tertiary amine-modified dihydroxy chain extender, neutralizing agent, diluent, deionized water and dihalophenol is 100: (40~45): (0.1~0.3): (9~15): (6~12): (7~10): (60~95): (200~450): (2~5).

[0005] Further, the preparation method of the tertiary amine modified dihydroxy chain extender is as follows: 15-20 parts by weight of a substance containing dihydroxyl and primary amine are placed in 100-300 parts by weight of isopropanol and stirred thoroughly to disperse. Under continuous stirring at 50-75°C, 5-20 parts by weight of sodium hydroxide are added to the mixture and stirred for 1-2 hours. Subsequently, 10-20 parts by weight of a tertiary amine haloalkane substance dissolved in isopropanol are added dropwise and reacted for 3-5 hours. After the reaction is completed, the product is fractionated and dried to obtain the tertiary amine modified dihydroxy chain extender. The substance containing dihydroxyl and primary amine includes one or more of 2-amino-1,3-propanediol, 2-amino-2-methyl-1,3-propanediol, and tromethamine. The tertiary amine haloalkane substance includes one or more of N,N-dimethylaminoethane bromide hydrobromide and N,N-dimethylaminopropyl chloride.

[0006] Further, the diol includes one or more of polyether diols, polyester diols, special functional diols, or copolymers thereof. Molecular weight M n The above-mentioned diol polymers with a concentration of 500~4000 are all applicable to this invention, wherein the water content of the diol is ≤0.05%; The isocyanate is one or more of isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), dicyclohexylmethane diisocyanate (HMDI), lysine diisocyanate (LDI), and diphenylmethane diisocyanate. The catalyst includes one or more of organozinc compounds, organobismuth compounds, and tertiary amine compounds; Hydrophilic chain extenders include one or more of 2,2-dimethylolbutyric acid (DMBA) and 2,2-dimethylolpropionic acid (DMPA); The neutralizing agent includes one or more of triethylamine, monoethanolamine, and ammonia. The diluent includes one or more of acetone, N-methylpyrrolidone, ethyl acetate, and butyl acetate; Dihalophenols include one or more of 2,3-dichlorophenol, 2,4-dichlorophenol, 2,5-dichlorophenol, 2,3-dibromophenol, 2,4-dibromophenol, and 2,5-dibromophenol.

[0007] Furthermore, the crosslinking agent includes one or more of the following: organic peroxide crosslinking agents, isocyanate crosslinking agents, and carbodiimide crosslinking agents; Wetting and dispersing agents include one or more of sodium alkylnaphthalene sulfonate, alkylphenol polyoxyethylene ether, and polyacrylate; Defoamers include one or more of polydimethylsiloxane, ethylene oxide, and isooctanol; Thickeners include one or more of hydroxyethyl cellulose, sodium carboxymethyl cellulose, hydrophobically modified nonionic polyurethane, xanthan gum, and fumed silica. Leveling agents include one or more of polyether or polyester-modified polydimethylsiloxane, acrylate copolymer solutions, and associative polyurethanes.

[0008] Furthermore, the preparation method of the pH-responsive antibacterial waterborne polyurethane emulsion is as follows: (1) Mix diol, isocyanate and catalyst at 60-80℃ and react for 3-5 hours; add hydrophilic chain extender and tertiary amine modified dihydroxy chain extender to extend the product; after the reaction is complete, cool to 35-50℃, add neutralizer and diluent, add deionized water for emulsification, and react for 5-15 minutes; obtain polyurethane emulsion with tertiary amine groups. The chain extension reaction temperature is 60-75℃ and the reaction time is 2-4 hours.

[0009] (2) Add dihalophenolic substances to polyurethane emulsions with tertiary amine groups at 20~50℃ and react for 3~8h; (3) The emulsion is subjected to reduced pressure to remove the diluent and halogen ions, resulting in a pH-responsive antibacterial waterborne polyurethane emulsion.

[0010] In addition, as a common technique in this field, chain extension reaction and emulsification reaction are carried out under stirring conditions. Generally, the stirring speed for emulsification reaction is 400~1000 r / min, and the stirring speed for other reactions such as chain extension and reaction of dihalophenols with the main chain is 150~350 r / min.

[0011] The second technical solution of the present invention provides a method for preparing the above-mentioned pH-responsive waterborne polyurethane coating, wherein the pH-responsive waterborne polyurethane coating is prepared into a film by a pH-responsive waterborne polyurethane resin mixture, and the preparation of the pH-responsive waterborne polyurethane resin mixture includes the following steps: Add 50-60 parts by weight of pH-responsive antibacterial waterborne polyurethane emulsion to a reaction vessel and preheat to 25±3℃ while stirring at 200-400 rpm. Then, add 2-5 parts by weight of crosslinking agent while stirring for 10-15 minutes. Next, add 0.3-0.8 parts by weight of wetting and dispersing agent and 0.2-0.5 parts by weight of defoamer in sequence, increase the speed to 800-1000 rpm, then add 0.2-0.4 parts by weight of leveling agent and stir at 300-500 rpm for 10-15 minutes. Add 0.5-1.5 parts by weight of thickener in batches, and make up the total volume to 100 parts by weight with the remaining water. Finally, continue stirring at 200-400 rpm for 15-20 minutes to obtain a pH-responsive waterborne polyurethane resin mixture.

[0012] pH-responsive waterborne polyurethane resin mixtures can be used to prepare films on polyurethane films, polyethylene films, and polyethylene terephthalate films.

[0013] Specifically, a pH-responsive waterborne polyurethane resin mixture can be placed in a tetrafluoroethylene mold and placed at 50°C for 24 hours to form a film.

[0014] The third technical solution of the present invention is to provide an application of the above-mentioned pH-responsive waterborne polyurethane coating.

[0015] The present invention also provides a food packaging protective film, including the pH-responsive waterborne polyurethane coating.

[0016] The beneficial effects of this invention are as follows: Tertiary amino groups are introduced into the polyurethane polymer chain, which then react with dihalophenolic substances to generate quaternary ammonium groups. Simultaneously, a phenolic structure is introduced into the polymer chain. After the resin mixture is coated onto the surface of a food protective film, it is used to wrap and preserve food. When food (especially high-protein meats and fish) begins to spoil, microorganisms decompose proteins, producing alkaline volatile basic nitrogen compounds. These substances diffuse into the protective film coating, causing a local increase in pH. When the environment becomes alkaline, the phenolic hydroxyl group loses a proton, forming a phenolic anion, thereby forming a quinone structure. This changes the color of the coating, achieving the purpose of monitoring food freshness. At the same time, the quaternary ammonium groups endow the protective film coating with good antibacterial properties, inhibiting bacterial growth. Attached Figure Description

[0017] Figure 1 This refers to the antibacterial properties of pH-responsive waterborne polyurethane coatings.

[0018] Figure 2 This refers to the pH-responsive color change of a pH-responsive waterborne polyurethane coating. Detailed Implementation

[0019] The following examples are provided to further illustrate the present invention and are intended to explain the invention, not to limit its scope. Unless otherwise specified, all figures are expressed in parts by weight and weight percentages.

[0020] Unless otherwise specified, the raw materials used in this invention are all conventional commercially available products; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0021] The embodiments of the present invention will be further described below with reference to several examples.

[0022] It should be understood that the described embodiments are merely some, not all, of the embodiments in this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0023] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0024] Example 1 15 parts of 2-amino-1,3-propanediol were added to 100 parts of isopropanol and stirred thoroughly to disperse the mixture. While stirring continuously, 20 parts of sodium hydroxide were added to the mixture at 50°C, and the reaction was allowed to proceed for 2 hours. 15 parts of N,N-dimethylaminobromoethane hydrobromide were dissolved in isopropanol and slowly added dropwise to the system, allowing the reaction to proceed for 3 hours. After the reaction was complete, the product was fractionated and dried to obtain a tertiary amine-modified dihydroxy chain extender.

[0025] 100 parts of polyester diol (Mn=2000, pre-dehydrated), 45 parts of isophorone diisocyanate, and 0.3 parts of organozinc catalyst were added to a container and reacted at 70°C and 150 r / min for 5 h. 10 parts of DMPA and 12 parts of tertiary amine-modified dihydroxy chain extender were added to the system and reacted at 65°C and 150 r / min for 3 h. The reaction temperature was lowered to 35°C, and 10 parts of triethylamine were added to neutralize the system, while 60 parts of acetone were added for dilution. 250 parts of deionized water were added and emulsified at 800 r / min for 15 min. 5 parts of 2,5-dichlorophenol were added to the system and reacted at 50°C and 200 r / min for 5 h. The product was heated under reduced pressure to remove the diluent, halogen ions were removed by adsorption, and the product was filtered to obtain the polyurethane emulsion.

[0026] Fifty parts of a pH-responsive antibacterial aqueous polyurethane emulsion were added to a reaction vessel and preheated to 25°C with low-speed stirring at 200 rpm. Then, 5 parts of an isocyanate crosslinking agent were slowly added with low-speed stirring for 10 minutes. Next, 0.3 parts of sodium alkylnaphthalene sulfonate and 0.5 parts of polydimethylsiloxane were added sequentially, and the stirring speed was increased to 800 rpm. Then, 0.2 parts of an acrylate copolymer solution were slowly added, and the mixture was stirred at 300 rpm for 10 minutes. 1.5 parts of hydroxyethyl cellulose were slowly added in batches, and the remaining water was used to bring the total to 100 parts. Finally, the mixture was stirred continuously at 400 rpm for 20 minutes to obtain the resin mixture.

[0027] pH-responsive antibacterial waterborne polyurethane composition: ; R1 is derived from a tertiary amine-modified dihydroxy chain extender, and R2 is a polyurethane repeating unit including R1.

[0028] Example 2 20 parts of 2-amino-1,3-propanediol were added to 300 parts of isopropanol and stirred thoroughly to disperse the mixture. While stirring continuously, 20 parts of sodium hydroxide were added to the mixture at 50°C, and the reaction was allowed to proceed for 1 hour. 20 parts of N,N-dimethylaminobromoethane hydrobromide were dissolved in isopropanol and slowly added dropwise to the system, allowing the reaction to proceed for 3 hours. After the reaction was complete, the product was fractionated and dried to obtain a tertiary amine-modified dihydroxy chain extender.

[0029] 100 parts of polyether diol (Mn=2000, pre-dehydrated), 40 parts of isophorone diisocyanate, and 0.2 parts of organozinc catalyst were added to a container and reacted at 60°C and 150 r / min for 5 h. 13 parts of DMPA and 10 parts of tertiary amine-modified dihydroxy chain extender were added to the system and reacted at 75°C and 150 r / min for 3 h. The reaction temperature was lowered to 50°C, and 9 parts of triethylamine were added to neutralize the system, while 80 parts of acetone were added for dilution. 450 parts of deionized water were added and emulsified at 800 r / min for 5 min. 3 parts of 2,4-dichlorophenol were added to the system and reacted at 20°C and 200 r / min for 8 h. The product was heated under reduced pressure to remove the diluent, halogen ions were removed by adsorption, and the product was filtered to obtain the polyurethane emulsion.

[0030] Sixty parts of a pH-responsive antibacterial aqueous polyurethane emulsion were added to a reaction vessel and preheated to 25°C with low-speed stirring at 200 rpm. Then, five parts of an isocyanate crosslinking agent were slowly added with low-speed stirring for 10 minutes. Next, 0.3 parts of sodium alkylnaphthalene sulfonate and 0.5 parts of polydimethylsiloxane were added sequentially, and the stirring speed was increased to 800 rpm. Then, 0.2 parts of an acrylate copolymer solution were slowly added, and the mixture was stirred at 300 rpm for 10 minutes. One and a half parts of hydroxyethyl cellulose were then slowly added in batches, and the mixture was brought to a final volume of 100 parts with the remaining water. Finally, the mixture was stirred continuously at 400 rpm for 20 minutes to obtain the resin mixture.

[0031] Example 3 17 parts of 2-amino-1,3-propanediol were added to 100 parts of isopropanol and stirred thoroughly to disperse the mixture. While stirring continuously, 20 parts of sodium hydroxide were added to the mixture at 75°C, and the reaction was allowed to proceed for 2 hours. 12 parts of N,N-dimethylaminobromoethane hydrobromide were dissolved in isopropanol and slowly added dropwise to the system, allowing the reaction to proceed for 5 hours. After the reaction was complete, the product was fractionated and dried to obtain a tertiary amine-modified dihydroxy chain extender.

[0032] 100 parts of polyether diol (Mn=4000, pre-dehydrated), 40 parts of diphenylmethane diisocyanate, and 0.1 parts of organozinc catalyst were added to a container and reacted at 80°C and 150 r / min for 3 h. 9 parts of DMPA and 6 parts of tertiary amine-modified dihydroxy chain extender were added to the system and reacted at 75°C and 150 r / min for 3 h. The reaction temperature was lowered to 35°C, and 7 parts of triethylamine were added to neutralize the system, while 95 parts of acetone were added for dilution. 450 parts of deionized water were added and emulsified at 800 r / min for 15 min. 3 parts of 2,4-dibromophenol were added to the system and reacted at 50°C and 200 r / min for 5 h. The product was heated under reduced pressure to remove the diluent, halogen ions were removed by adsorption, and the product was filtered to obtain the polyurethane emulsion.

[0033] Fifty parts of pH-responsive antibacterial waterborne polyurethane emulsion were added to a reaction vessel and preheated to 25°C with low-speed stirring at 200 rpm. Then, two parts of a carbodiimide crosslinking agent were slowly added with low-speed stirring for 10 minutes. Next, 0.8 parts of alkylphenol polyoxyethylene ether and 0.2 parts of ethylene oxide were added sequentially, and the stirring speed was increased to 800 rpm. Then, 0.4 parts of polyester-modified polydimethylsiloxane were slowly added, and the mixture was stirred at 300 rpm for 10 minutes. Finally, 0.5 parts of sodium carboxymethyl cellulose were slowly added in batches, and the mixture was brought to a final volume of 100 parts with the remaining water. The mixture was then stirred continuously at 400 rpm for 20 minutes to obtain the resin mixture.

[0034] Example 4 15 parts of 2-amino-1,3-propanediol were added to 100 parts of isopropanol and stirred thoroughly to disperse the mixture. While stirring continuously, 20 parts of sodium hydroxide were added to the mixture at 50°C, and the reaction was allowed to proceed for 2 hours. 15 parts of N,N-dimethylaminobromoethane hydrobromide were dissolved in isopropanol and slowly added dropwise to the system, allowing the reaction to proceed for 3 hours. After the reaction was complete, the product was fractionated and dried to obtain a tertiary amine-modified dihydroxy chain extender.

[0035] 100 parts of polyester diol (Mn=2000, pre-dehydrated), 45 parts of isophorone diisocyanate, and 0.3 parts of organozinc catalyst were added to a container and reacted at 70°C and 150 r / min for 3 h. 15 parts of DMPA and 12 parts of tertiary amine-modified dihydroxy chain extender were added to the system and reacted at 65°C and 150 r / min for 3 h. The reaction temperature was lowered to 35°C, and 10 parts of triethylamine were added to neutralize the system, while 60 parts of acetone were added for dilution. 200 parts of deionized water were added and emulsified at 800 r / min for 15 min. 2 parts of 2,5-dichlorophenol were added to the system and reacted at 50°C and 200 r / min for 5 h. The product was heated under reduced pressure to remove the diluent, halogen ions were removed by adsorption, and the product was filtered to obtain the polyurethane emulsion.

[0036] 52 parts of pH-responsive antibacterial waterborne polyurethane emulsion were added to a reaction vessel and preheated to 25°C with low-speed stirring at 200 rpm. Then, 5 parts of isocyanate crosslinking agent were slowly added with low-speed stirring for 10 minutes. Next, 0.3 parts of sodium alkylnaphthalene sulfonate and 0.5 parts of polydimethylsiloxane were added sequentially, and the stirring speed was increased to 800 rpm. Then, 0.2 parts of acrylate copolymer solution were slowly added, and the mixture was stirred at 300 rpm for 10 minutes. 1.5 parts of hydroxyethyl cellulose were slowly added in batches, and the mixture was brought to a final volume of 100 parts with the remaining water. Finally, the mixture was stirred continuously at 400 rpm for 20 minutes to obtain the resin mixture.

[0037] Example 5 15 parts of 2-amino-1,3-propanediol were added to 100 parts of isopropanol and stirred thoroughly to disperse the mixture. While stirring continuously, 5 parts of sodium hydroxide were added to the mixture at 50°C, and the reaction was allowed to proceed for 2 hours. 10 parts of N,N-dimethylaminobromoethane hydrobromide were dissolved in isopropanol and slowly added dropwise to the system, allowing the reaction to proceed for 3 hours. After the reaction was complete, the product was fractionated and dried to obtain a tertiary amine-modified dihydroxy chain extender.

[0038] 100 parts of polyester diol (Mn=2000, pre-dehydrated), 45 parts of isophorone diisocyanate, and 0.3 parts of organozinc catalyst were added to a container and reacted at 70°C and 150 r / min for 3 h. 10 parts of DMPA and 12 parts of tertiary amine-modified dihydroxy chain extender were added to the system and reacted at 65°C and 150 r / min for 3 h. The reaction temperature was lowered to 35°C, and 10 parts of triethylamine were added to neutralize the system, while 60 parts of acetone were added for dilution. 250 parts of deionized water were added and emulsified at 800 r / min for 15 min. 5 parts of 2,5-dichlorophenol were added to the system and reacted at 50°C and 200 r / min for 5 h. The product was heated under reduced pressure to remove the diluent, halogen ions were removed by adsorption, and the product was filtered to obtain the polyurethane emulsion.

[0039] Fifty parts of a pH-responsive antibacterial aqueous polyurethane emulsion were added to a reaction vessel and preheated to 25°C with low-speed stirring at 200 rpm. Then, 5 parts of an isocyanate crosslinking agent were slowly added with low-speed stirring for 10 minutes. Next, 0.3 parts of sodium alkylnaphthalene sulfonate and 0.5 parts of polydimethylsiloxane were added sequentially, and the stirring speed was increased to 800 rpm. Then, 0.2 parts of an acrylate copolymer solution were slowly added, and the mixture was stirred at 300 rpm for 10 minutes. 1.5 parts of hydroxyethyl cellulose were slowly added in batches, and the remaining water was used to bring the total to 100 parts. Finally, the mixture was stirred continuously at 400 rpm for 20 minutes to obtain the resin mixture.

[0040] Comparative Example 1 The difference from the example is that no dihalophenols were added, specifically: 15 parts of 2-amino-1,3-propanediol were added to 100 parts of isopropanol and stirred thoroughly to disperse the mixture. While stirring continuously, 20 parts of sodium hydroxide were added to the mixture at 50°C, and the reaction was allowed to proceed for 2 hours. 15 parts of N,N-dimethylaminobromoethane hydrobromide were dissolved in isopropanol and slowly added dropwise to the system, allowing the reaction to proceed for 3 hours. After the reaction was complete, the product was fractionated and dried to obtain a tertiary amine-modified dihydroxy chain extender.

[0041] 100 parts of polyester diol (Mn=2000, pre-dehydrated), 45 parts of isophorone diisocyanate, and 0.3 parts of organozinc catalyst were added to a container and reacted at a reactant temperature of 70°C and a stirring speed of 150 r / min for 3 h. 10 parts of DMPA and 12 parts of tertiary amine-modified dihydroxy chain extender were added to the system and reacted at a temperature of 65°C and a stirring speed of 150 r / min for 3 h. The reaction temperature was lowered to 35°C, and 10 parts of triethylamine were added to neutralize the system, while 60 parts of acetone were added for dilution. 250 parts of deionized water were added, and emulsification was carried out at 800 r / min for 15 min. The product was heated under reduced pressure to remove the diluent, yielding the polyurethane emulsion.

[0042] Fifty parts of polyurethane emulsion were added to a reaction vessel and preheated to 25°C with low-speed stirring at 200 rpm. Then, 5 parts of isocyanate crosslinking agent were slowly added with low-speed stirring for 10 minutes. Next, 0.3 parts of sodium alkylnaphthalene sulfonate and 0.5 parts of polydimethylsiloxane were added sequentially, and the stirring speed was increased to 800 rpm. Then, 0.2 parts of acrylate copolymer solution were slowly added, and the mixture was stirred at 300 rpm for 10 minutes. 1.5 parts of hydroxyethyl cellulose were slowly added in batches, and the remaining water was used to bring the total to 100 parts. Finally, the mixture was stirred continuously at 400 rpm for 20 minutes to obtain the resin mixture.

[0043] Conduct antibacterial performance testing: The coating mixture was placed in a tetrafluoroethylene mold and incubated at 50°C for 24 hours to form a film. The activated test strain was inoculated into liquid culture medium and cultured with shaking at 37°C until the logarithmic growth phase. The bacterial cells were collected by centrifugation, resuspended in PBS, and their concentration was adjusted to approximately 1.5 × 10⁻⁶ using UV spectrophotometry. 8 A CFU / mL suspension was prepared. 100 μL of the bacterial suspension was carefully and evenly added dropwise to the surface of a polyurethane film sample in a sterile petri dish. The dish was then transferred to a 37°C incubator and co-cultured for 24 hours under suitable humidity. After co-culture, the sample was eluted with PBS to obtain the initial eluent. The initial eluent was diluted with physiological saline to obtain the diluted bacterial solution. 100 μL of the diluted solution was added dropwise to the surface of a solid agar plate and evenly spread using a sterile spreader. The plate was then inverted and incubated at 37°C for 24 hours. After incubation, the number of colonies on the plate was observed. The results are as follows: Figure 1 As shown, Comparative Example 1 has poor antibacterial properties, while the Example 1 has better antibacterial properties.

[0044] pH response test: The coating mixture was placed in a PTFE mold and incubated at 50°C for 24 hours to form a film. A pH 8 buffer solution was placed in a glass petri dish. The sample was then immersed in the pH buffer solution, ensuring complete coverage, and the timer was started.

[0045] Remove the sample after 5 minutes of soaking and blot the surface liquid with filter paper. Observe and record the color change. Figure 2 As shown, the coating combination liquids of Examples 1 and 2 have good pH-responsive color-changing properties, while Comparative Example 1 did not change color and did not have pH-responsive color-changing properties.

[0046] The above embodiments describe in detail the structure, features, and effects of the present invention. The above description is only a preferred embodiment of the present invention. Any changes made in accordance with the concept of the present invention, or equivalent embodiments modified to have equivalent changes, shall still fall within the scope of protection of the present invention if they do not exceed the scope covered by the specification.

Claims

1. A pH-responsive aqueous polyurethane coating, characterized in that, The pH-responsive antibacterial waterborne polyurethane emulsion 50-60 parts by weight, crosslinking agent 2-5 parts by weight, wetting dispersant 0.3-0.8 parts by weight, antifoaming agent 0.2-0.5 parts by weight, thickening agent 0.5-1.5 parts by weight, leveling agent 0.2-0.4 parts by weight; the pH-responsive antibacterial waterborne polyurethane emulsion is prepared from dihydric alcohol, isocyanate, catalyst, hydrophilic chain extender, tertiary amine modified double hydroxyl chain extender, neutralizing agent, diluent, dihalogenated phenol and deionized water, the mass ratio of dihydric alcohol, isocyanate, catalyst, hydrophilic chain extender, tertiary amine modified double hydroxyl chain extender, neutralizing agent, diluent, deionized water, dihalogenated phenol is 100: (40-45): (0.1-0.3): (9-15): (6-12): (7-10): (60-95): (200-450): (2-5).

2. The pH-responsive aqueous polyurethane coating of claim 1, wherein, The preparation method of the tertiary amine modified double hydroxyl chain extender is: 15-20 parts by weight of a substance with double hydroxyl and primary amine is placed in 100-300 parts by weight of isopropyl alcohol and stirred to disperse, 5-20 parts by weight of sodium hydroxide is added to the mixture under continuous stirring at 50-75℃, and stirred for 1-2h; then 10-20 parts by weight of a tertiary amine halogenated alkyl substance dissolved in isopropyl alcohol is added dropwise and reacted for 3-5h, after the reaction is completed, the product is fractionated and dried to obtain the tertiary amine modified double hydroxyl chain extender; the substance with double hydroxyl and primary amine includes one or more of 2-amino-1,3-propanediol, 2-amino-2-methyl-1,3-propanediol, tromethamine; the tertiary amine halogenated alkyl substance includes one or more of N,N-dimethylamino bromoethane hydrobromide, N,N-dimethyl aminopropyl chloride.

3. The pH-responsive aqueous polyurethane coating of claim 1, wherein, The molecular weight Mn of the dihydric alcohol is 500-4000; the isocyanate is one or more of isophorone diisocyanate, hexamethylene diisocyanate, dicyclohexyl methane diisocyanate, lysine diisocyanate, diphenyl methane diisocyanate; the catalyst includes one or more of organic zinc compound, organic bismuth compound and tertiary amine compound; the hydrophilic chain extender includes one or more of 2,2-dimethylol butyric acid, 2,2-dimethylol propionic acid; the neutralizing agent includes one or more of triethylamine, monoethanolamine, ammonia; the diluent includes one or more of acetone, N-methyl pyrrolidone, ethyl acetate, butyl acetate; the dihalogenated phenol includes one or more of 2,3-dichlorophenol, 2,4-dichlorophenol, 2,5-dichlorophenol, 2,3-dibromophenol, 2,4-dibromophenol, 2,5-dibromophenol.

4. The pH-responsive aqueous polyurethane coating of claim 1, wherein, The cross-linking agent includes one or more of organic peroxide cross-linking agent, isocyanate cross-linking agent, and carbodiimide cross-linking agent; the wetting dispersant includes one or more of sodium alkyl naphthalene sulfonate, alkyl phenol polyoxyethylene ether, and polyacrylate; the defoaming agent includes one or more of polydimethylsiloxane, oxirane, and isooctanol; the thickening agent includes one or more of hydroxyethyl cellulose, sodium carboxymethyl cellulose, hydrophobically modified nonionic polyurethane, xanthan gum, and fumed silica; and the leveling agent includes one or more of polyether or polyester modified polydimethylsiloxane, acrylate copolymer solution, and associative polyurethane.

5. The pH-responsive aqueous polyurethane coating of claim 1, wherein, The preparation method of the pH-responsive antibacterial waterborne polyurethane emulsion is as follows: (1) mixing dihydric alcohol, isocyanate, and catalyst at 60-80°C, and reacting for 3-5 h; adding hydrophilic chain extender and tertiary amine modified double hydroxyl chain extender to the product for chain extension reaction; after sufficient reaction, cooling to 35-50°C, adding neutralizing agent and diluent, adding deionized water for emulsification, and reacting for 5-15 min; obtaining a polyurethane emulsion with tertiary amine groups; (2) adding a bis-halogenated phenol substance to the polyurethane emulsion with tertiary amine groups at 20-50°C, and reacting for 3-8 h; (3) removing the diluent from the emulsion under reduced pressure to remove halogen ions, and obtaining a pH-responsive antibacterial waterborne polyurethane emulsion.

6. A method of preparing a pH-responsive aqueous polyurethane coating as claimed in claim 1, characterized in that, The pH-responsive waterborne polyurethane coating is prepared into a film by using a pH-responsive waterborne polyurethane resin combination liquid, and the preparation of the pH-responsive waterborne polyurethane resin combination liquid has the following steps: 50-60 parts by weight of the pH-responsive antibacterial waterborne polyurethane emulsion is put into a reaction container, preheated to 25±3°C under stirring at a speed of 200-400 r / min, then 2-5 parts by weight of a cross-linking agent is added under stirring, and stirring is performed for 10-15 min; then 0.3-0.8 parts by weight of a wetting dispersant and 0.2-0.5 parts by weight of a defoaming agent are added in sequence, the speed is increased to 800-1000 r / min, then 0.2-0.4 parts by weight of a leveling agent is added, stirring is performed at a speed of 300-500 r / min for 10-15 min, 0.5-1.5 parts by weight of a thickening agent is added in batches, and the remaining water is added to make up to 100 parts by weight; finally, stirring is continuously performed at a speed of 200-400 r / min for 15-20 min to obtain a pH-responsive waterborne polyurethane resin combination liquid.

7. The method of claim 6, wherein, The pH-responsive waterborne polyurethane resin combination liquid is placed into a tetrafluoroethylene mold, and a film is formed after being placed at 50°C for 24 h.

8. Use of the pH-responsive waterborne polyurethane coating according to claim 1.

9. A food packaging protective film, characterized by, The food packaging protective film comprises the pH-responsive waterborne polyurethane coating according to claim 1.

Images