Zero-solvent aqueous polyurethane and method of making

Abstract

The application discloses a kind of zero solvent water-based polyurethane and preparation method, sequentially including the steps of synthesizing prepolymer, neutralization, high-temperature dispersion, post-chain extension, etc., the method does not add solvent in the process of prepolymer synthesis, utilizes the exponential nonlinear influence law of temperature to polymer viscosity in dispersion stage, adopts high-temperature fast shear dispersion strategy, completely clears the fluid mechanics barrier of solvent stirring shear, and simultaneously, the method strictly controls high-temperature dispersion time in 5 seconds to 15 minutes, so that system NCO retention rate is greater than 85%, which guarantees efficient emulsification in water phase, and effectively inhibits the excessive side reaction of NCO group and water.The application completely solves the existing technical problems, such as emulsion failure, NCO loss and serious deterioration of coating film mechanical properties, caused by the necessity of relying on organic solvent to reduce viscosity or forcibly dispersing without solvent in the preparation process of current commercial water-based polyurethane.

Description

Technical Field

[0001] This invention relates to the field of polymer material preparation technology, and in particular to a solvent-free waterborne polyurethane and its preparation method. Background Technology

[0002] Waterborne polyurethane (WPU) is environmentally friendly due to its lack of or minimal presence of volatile organic compounds (VOCs), making it a mainstream trend in coatings, adhesives, and other fields. However, most current commercial WPU manufacturing processes (such as the acetone method or prepolymer mixing method) still require the use of a certain amount of organic solvents to reduce the system viscosity during the chain extension or dispersion stage. Subsequently, the solvent must be removed through energy-intensive operations, making it not a truly solvent-free and environmentally friendly process.

[0003] In the prior art, the prepolymer must be dispersed in water at a relatively low temperature (such as 30-40℃ or room temperature). The core purpose is to avoid the free NCO groups at higher temperatures from undergoing violent side reactions with water, which would generate ineffective urea bridge structures.

[0004] However, this "low-temperature dispersion" approach has a fatal flaw that is difficult to overcome: in a solvent-free state, the viscosity of polyurethane prepolymers is extremely high at low or room temperature, making them highly prone to gelation and agglomeration. When water is added to this extremely high-viscosity system, the stirring paddle struggles to shear and disperse it, often resulting in extremely large emulsion particle sizes, poor storage stability, and ultimately, severe degradation of the mechanical properties of the coating product. Therefore, how to balance reducing prepolymer viscosity with suppressing NCO loss is a critical technical bottleneck that the entire solvent-free waterborne polyurethane process urgently needs to overcome. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a solvent-free waterborne polyurethane and its preparation method, thereby solving the problems of existing technologies, such as the need to add solvents or high NCO loss during the preparation of waterborne polyurethane.

[0006] To achieve the above objectives, the technical solution adopted by this invention is: a method for preparing solvent-free waterborne polyurethane, comprising the following steps: Step S1, Synthesis of prepolymer: 100 parts by weight of polyol, 1-5 parts by weight of small molecule chain extender, and 2-8 parts by weight of hydrophilic chain extender are placed in a reactor, protected by nitrogen and heated, and 20-60 parts by weight of diisocyanate are added. The reaction is carried out under mechanical stirring at 50-500 r / min to obtain polyurethane prepolymer. Step S2, neutralization: Add 2-8 parts by weight of neutralizing agent to the polyurethane prepolymer to neutralize the acidic groups on the hydrophilic chain extender; Step S3, high temperature dispersion: Keep the system temperature at 60℃~95℃, add 100~300 parts by weight of deionized water to the polyurethane prepolymer, and perform high-speed shear dispersion at a speed of 500~5000 r / min for 5 seconds to 15 minutes. After dispersion, a dispersion emulsion is obtained, and the dispersion emulsion is cooled to room temperature. Step S4, post-chain extension: Add 10-40 parts by weight of 10% ethylenediamine solution to the dispersion emulsion, and react to obtain all-waterborne polyurethane.

[0007] In a further technical solution, the polyol mentioned in step S1 is at least one of polycarbonate diol, carbon dioxide-based polyol, polyether polyol, or polyester polyol; the small molecule chain extender is at least one of 1,4-butanediol, ethylene glycol, or diethylene glycol; and the hydrophilic chain extender is one of 2,2-dimethylolpropionic acid or 2,2-dimethylolbutyric acid.

[0008] In a further technical solution, the diisocyanate in step S1 is at least one of isoflurane diisocyanate, hexamethylene diisocyanate, dicyclohexylmethane diisocyanate, diphenylmethane diisocyanate, or toluene diisocyanate.

[0009] In a further technical solution, the neutralizing agent in step S2 is triethylamine.

[0010] In a further technical solution, the deionized water in step S3 is added in two stages. The first stage involves adding deionized water for pre-dispersion, which takes 30-150 seconds. Then, heating is stopped and deionized water is added a second time for stirring and dispersion, followed by cooling to room temperature.

[0011] In a further technical solution, step S3 involves adding deionized water to cool the water to room temperature while simultaneously dispersing it through stirring.

[0012] In a further technical solution, the reaction temperature in step S3 is 60℃ ~ 85℃.

[0013] A solvent-free waterborne polyurethane is prepared using the above-described method for preparing a solvent-free waterborne polyurethane.

[0014] The advantages of this invention compared to existing technologies are: it solves the problem of solvent addition or significant NCO loss during the preparation of waterborne polyurethane in existing technologies. This technical solution does not add solvent during the prepolymer synthesis stage; instead, it generates NCO groups through reaction. During dispersion, a high-temperature, rapid dispersion and emulsification method is used. The reaction mechanism utilizes the exponential nonlinear effect of temperature on polymer viscosity. By performing short-time water dispersion at a relatively high temperature of 50℃ to 95℃, the viscosity of the polymer prepolymer can be significantly reduced to below 20,000 mPa·s, overcoming the hydrodynamic barrier of solvent-free stirring and shearing. Simultaneously, this method strictly controls the high-temperature dispersion time within 5 seconds to 15 minutes, ensuring an NCO retention rate of greater than 80% to 90%, guaranteeing efficient emulsification in water while avoiding excessive NCO side reactions and failure. The resulting waterborne polyurethane emulsion after cooling and chain extension has fine particle size and extremely uniform dispersion, and the prepared coating film exhibits excellent tensile strength and structural toughness. Attached Figure Description

[0015] Figure 1 This is a diagram illustrating the synthesis mechanism of solvent-free waterborne polyurethane. Figure 2 The graph shows the viscosity curves of the prepolymer at different temperatures. Figure 3 is a thermogravimetric (TG) comparison diagram of polyurethane films prepared using the waterborne polyurethane in Example 1; Figure 4 is a comparison of the micro-quotient thermogravimetric (DTG) curves of polyurethane films prepared using Example 1. Figure 5 is a comparison of the differential scanning calorimetry (DSC) curves of the polyurethane films prepared using the waterborne polyurethane in Example 1. Figure 6 is a thermogravimetric (TG) comparison diagram of polyurethane films prepared using the waterborne polyurethane in Example 3; Figure 7 is a comparison of the micro-quotient thermogravimetric (DTG) curves of the polyurethane films prepared using Example 3. Figure 8 is a comparison of the differential scanning calorimetry (DSC) curves of the polyurethane films prepared using the waterborne polyurethane in Example 3. Detailed Implementation

[0016] Example 1 (1) Synthesis of prepolymer: 100 parts by weight of polycarbonate diol (PCD-2000), 8 parts by weight of hydrophilic chain extender 2,2-dimethylolpropionic acid (DMPA), and 1 part by weight of small molecule chain extender 1,4-butanediol (BDO) were added to a three-necked flask equipped with a mechanical stirrer. Under nitrogen protection and heating conditions, 36 parts by weight of isoflurane diisocyanate (IPDI) were added, and the mixture was stirred at 200 r / min for 2 hours. (2) Neutralization: Cool down to a slightly lower temperature, add 4 parts by weight of triethylamine (TEA), and maintain stirring for 15 minutes to neutralize the carboxyl groups in the system; (3) High temperature dispersion: Adjust and maintain the reaction temperature at 80℃, add 215 parts by weight of deionized water to the system at one time, and immediately start high-speed shear dispersion at 5000 r / min. The dispersion time is strictly controlled to 30 seconds to obtain a micron-sized dispersion emulsion with a bluish appearance. Then immediately transfer and cool to room temperature. (4) Post-chain extension: Slowly add 28 parts by weight of 10% ethylenediamine (EDA) aqueous solution, stir and react for 1 hour to complete the polymer chain extension, and obtain a uniform and stable all-water WPU emulsion with a solid content of 40%.

[0017] Example 2 (1) Synthesis of prepolymer: 100 parts by weight of polycarbonate diol (PCD-2000), 2 parts by weight of hydrophilic chain extender 2,2-dimethylolpropionic acid (DMPA), and 2 parts by weight of small molecule chain extender 1,4-butanediol (BDO) were added to a three-necked flask equipped with a mechanical stirrer. Under nitrogen protection and heating conditions, 20 parts by weight of hexamethylene diisocyanate (HDI) were added, and the mixture was stirred at 200 r / min for 2 hours. (2) Neutralization: Cool down to a slightly lower temperature, add 2 parts by weight of triethylamine (TEA), and maintain stirring for 15 minutes to neutralize the carboxyl groups in the system; (3) High-temperature dispersion: Adjust and maintain the reaction temperature at 95℃, add 100 parts by weight of deionized water to the system for pre-dispersion for only 5 seconds; then immediately stop heating and add the remaining 200 parts by weight of deionized water for high-speed dispersion for 15 seconds and then cool down instantly. When adding deionized water for the first time, the shear dispersion speed is 500 r / min. When adding deionized water for the second time, start high-speed shear dispersion at 4000 r / min to obtain a micron-sized dispersion emulsion with a bluish appearance. Then immediately transfer and cool down to room temperature. (4) Post-chain extension: Slowly add 10 parts by weight of 10% ethylenediamine (EDA) aqueous solution, stir and react for 1 hour to complete the polymer chain extension, and obtain a uniform and stable all-water WPU emulsion with a solid content of 40%.

[0018] Example 3 (1) Synthesis of prepolymer: 100 parts by weight of polytetrahydrofuran ether diol (PTMEG-2000), 5 parts by weight of hydrophilic chain extender 2,2-dimethylolpropionic acid (DMPA), and 3 parts by weight of small molecule chain extender diethylene glycol (DEG) were added to a three-necked flask equipped with a mechanical stirrer. Under nitrogen protection and heating conditions, 40 parts by weight of dicyclohexylmethane diisocyanate (HMDI) were added, and the mixture was stirred at 200 r / min for 2 hours. (2) Neutralization: Cool down to a slightly lower temperature, add 6 parts by weight of triethylamine (TEA), and maintain stirring for 15 minutes to neutralize the carboxyl groups in the system; (3) High temperature dispersion: Adjust and maintain the reaction temperature at 60℃, add 180 parts by weight of deionized water to the system at one time, and immediately start high-speed shear dispersion at 3000 r / min. The dispersion time is strictly controlled to 3 minutes to obtain a micron-sized dispersion emulsion with a bluish appearance. Then immediately transfer and cool to room temperature. (4) Post-chain extension: Slowly add 30 parts by weight of 10% ethylenediamine (EDA) aqueous solution, stir and react for 1 hour to complete the polymer chain extension, and obtain a uniform and stable all-water WPU emulsion with a solid content of 40%.

[0019] Example 4 (1) Synthesis of prepolymer: 100 parts by weight of polybutylene adipate polyol (PBA), 7 parts by weight of hydrophilic chain extender 2,2-dimethylolpropionic acid (DMPA), and 5 parts by weight of small molecule chain extender diethylene glycol (DEG) were added to a three-necked flask equipped with a mechanical stirrer. Under nitrogen protection and heating conditions, 50 parts by weight of a mixture of diphenylmethane diisocyanate (MDI) and toluene diisocyanate (TDI) were added, and the mixture was stirred at 200 r / min for 2 hours. (2) Neutralization: Cool down to a slightly lower temperature, add 8 parts by weight of triethylamine (TEA), and maintain stirring for 15 minutes to neutralize the carboxyl groups in the system; (3) High temperature dispersion: Adjust and maintain the reaction temperature at 85℃, add 100 parts by weight of deionized water to the system at once, and immediately start high-speed shear dispersion at 1000 r / min. The dispersion time is strictly controlled to 45 seconds to obtain a micron-sized dispersion emulsion with a bluish appearance. Then immediately transfer and cool to room temperature. (4) Post-chain extension: Slowly add 40 parts by weight of 10% ethylenediamine (EDA) aqueous solution, stir and react for 1 hour to complete the polymer chain extension, and obtain a uniform and stable all-water WPU emulsion with a solid content of 40%.

[0020] Comparative Example 1 The process and formulation were completely consistent with Example 1, except that after neutralization, the system was directly cooled to below 40°C. At this point, due to the lack of solvent assistance, the viscosity of the prepolymer increased sharply, exhibiting solid rheological properties similar to chewing gum. Adding deionized water at this point resulted in a large-area gelation and clumping of the prepolymer due to its extremely high viscosity barrier; even a 2000 r / min stirring paddle could not break it up. The reaction system failed to form an emulsion and the experiment failed.

[0021] Comparative Example 2 The difference from Example 1 is that the dispersion time with water at 80°C was extended to 20 minutes. Testing showed that after dispersion, almost all NCO in the system reacted with water to form polyurea precipitate, and the NCO retention rate decreased to less than 15%. After cooling and adding EDA, the chain extension completely failed due to the lack of NCO crosslinking sites. The resulting emulsion film formation was extremely difficult.

[0022] Test method: Determination of NCO group content and calculation of NCO retention rate According to the chemical industry standard HG / T 2409-1992, the di-n-butylamine back titration method was used. The specific procedure was as follows: A quantitative amount of the polyurethane prepolymer obtained from the reaction (before and after dispersion) was dissolved in anhydrous toluene. An excess of di-n-butylamine-toluene standard solution was added, and the mixture was reacted at room temperature for 15 minutes to allow it to fully react with free NCO. Then, isopropanol was added for dilution, and bromocresol green indicator was added dropwise. Titration was performed with 0.1 mol / L hydrochloric acid standard solution until the blue color changed to yellow. A blank experiment was conducted. NCO content (%) = [(V0 - V1) × C × 4.202] / m. NCO retention rate = (residual NCO content after dispersion / theoretical NCO content before dispersion) × 100%.

[0023] The NCO group is the most reactive functional group in polyurethane synthesis. By measuring the change in the NCO value in the reaction system, the progress of the prepolymerization reaction can be accurately determined. The performance of waterborne polyurethane largely depends on the ratio of "hard segments" to "soft segments" in its microstructure. The amount of NCO groups consumed has a significant impact on the mechanical properties of waterborne polyurethane (such as tensile strength, elongation at break, and energy absorption capacity).

[0024] The NCO retention rates of Examples 1-4 and Comparative Examples 1-2 were measured, and the results are shown in the table below: Table 1 NCO Retention Rate

[0025] As shown in the table above, the NCO group retention rates in Examples 1-4 all exceeded 85%, resulting in good emulsion film formation. Comparative Example 1 could not form an emulsion due to the high viscosity of the prepolymer; Comparative Example 2 had a very low NCO group retention rate, making film formation difficult.

[0026] Figure 2 The graph shows the viscosity curves of the prepolymer at different temperatures. As can be seen, the viscosity is low at 60°C, making it easy to disperse by stirring. In Comparative Example 1, the system temperature is too low to disperse and form an emulsion. This technical solution does not add any other solvents during the preparation process. It mainly utilizes the exponential nonlinear effect of temperature on polymer viscosity. By performing a short-term water dispersion operation at a relatively high temperature of 50°C to 95°C, the viscosity of the polymer prepolymer can be significantly reduced to below 20,000 mPa·s, facilitating subsequent emulsion dispersion.

[0027] Figure 3-5 The TG / DTG / DSC curves obtained from testing the waterborne polyurethane film prepared in Example 1 show that, apart from the small low-temperature peak caused by trace amounts of moisture / monomers, the main peak of the prepared waterborne polyurethane is very single and sharp. TG shows very little residue, and the DSC melt peak is narrow, indicating high material purity, good control of the synthesis process, and a relatively concentrated molecular weight distribution. The initial decomposition temperature is above 300°C, and the maximum decomposition rate temperature is close to 380°C. As the soft segment of polyurethane, this means that the final synthesized polyurethane material will have excellent heat resistance, far superior to ordinary polyester or polyether polyols.

[0028] Figure 6-8 The TG / DTG / DSC curves obtained from testing the waterborne polyurethane film prepared in Example 3 show that the initial decomposition temperature of the prepared waterborne polyurethane is 350°C, and the peak weight loss rate occurs between 410°C and 420°C. This is a very high temperature, indicating that the bonding force between the COC ether bond and the CC bond is very strong, requiring high energy to break, thus demonstrating good tensile and abrasion resistance. The DSC curves show that the main endothermic peak is located between 145°C and 150°C, and the formed polyurethane elastomer exhibits significant micro-region separation at room temperature, providing excellent physical and mechanical strength.

[0029] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and variations without departing from the concept of the present invention, and these modifications all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing a solvent-free waterborne polyurethane, characterized in that: Includes the following steps, Step S1, Synthesis of prepolymer: 100 parts by weight of polyol, 1-5 parts by weight of small molecule chain extender, and 2-8 parts by weight of hydrophilic chain extender are placed in a reactor, protected by nitrogen and heated, and 20-60 parts by weight of diisocyanate are added. The reaction is carried out for 2 hours under mechanical stirring at 50-500 r / min to obtain polyurethane prepolymer. Step S2, neutralization: Add 2-8 parts by weight of neutralizing agent to the polyurethane prepolymer to neutralize the acidic groups on the hydrophilic chain extender. The neutralization reaction time is 15 minutes. Step S3, high temperature dispersion: Keep the system temperature at 60℃~95℃, add 100~300 parts by weight of deionized water to the polyurethane prepolymer, and perform high-speed shear dispersion at a speed of 500~5000 r / min for 5 seconds to 15 minutes. After dispersion, a dispersion emulsion is obtained, and the dispersion emulsion is cooled to room temperature. Step S4, post-chain extension: Add 10-40 parts by weight of 10% ethylenediamine solution to the dispersion emulsion and react for 1 hour to obtain solvent-free waterborne polyurethane.

2. The method for preparing a solvent-free waterborne polyurethane according to claim 1, characterized in that: The polyol mentioned in step S1 is at least one of polycarbonate diol, carbon dioxide-based polyol, polyether polyol, or polyester polyol; the small molecule chain extender is at least one of 1,4-butanediol, ethylene glycol, or diethylene glycol; and the hydrophilic chain extender is one of 2,2-dimethylolpropionic acid or 2,2-dimethylolbutyric acid.

3. The method for preparing a solvent-free waterborne polyurethane according to claim 1, characterized in that: In step S1, the diisocyanate is at least one of isoflurone diisocyanate, hexamethylene diisocyanate, dicyclohexylmethane diisocyanate, diphenylmethane diisocyanate, or toluene diisocyanate.

4. The method for preparing a solvent-free waterborne polyurethane according to claim 1, characterized in that: The neutralizing agent mentioned in step S2 is triethylamine.

5. The method for preparing a solvent-free waterborne polyurethane according to claim 1, characterized in that: The deionized water in step S3 is added in two stages: first, 10% to 60% of the total amount of deionized water is added for pre-dispersion, and the pre-dispersion time is 30 to 150 seconds; then heating is stopped and the remaining deionized water is added for high-speed shear dispersion and cooling to room temperature.

6. The method for preparing a solvent-free waterborne polyurethane according to claim 1, characterized in that: In step S3, 100-300 parts by weight of deionized water are added at once, while the mixture is dispersed by stirring and cooled to room temperature.

7. The method for preparing a solvent-free waterborne polyurethane according to claim 1, characterized in that: The system temperature in step S3 is 60℃ ~ 85℃.

8. A solvent-free waterborne polyurethane, characterized in that: It is prepared using the method for preparing a solvent-free waterborne polyurethane according to any one of claims 1 to 7.

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