Preparation method of lignin-based aqueous polyurethane emulsion

Abstract

The invention discloses a preparation method of a lignin-based aqueous polyurethane emulsion. The raw materials of the lignin-based aqueous polyurethane emulsion include lignin, oligomeric polyols, polyisocyanates, hydrophilic chain extenders, polyol compounds, organic solvents, etc. In order to solve the disadvantages of traditional aqueous polyurethanes such as poor mechanical properties, poor water resistance, and poor anti-aging ability, the invention compositely blends a certain amount of lignin with polyols, and controls the prepolymerization time through process optimization, so that lignin reacts with isocyanate to form a certain cross-linked structure, greatly improving the mechanical properties and water resistance of the aqueous polyurethane on the premise of ensuring the stability of the emulsion; the invention not only prepares a novel lignin-based aqueous polyurethane emulsion with functional characteristics such as stability, low cost, environmental friendliness, high mechanics, anti-ultraviolet, antibacterial, and hydrophobicity, but also realizes the high-value utilization of lignin as a renewable resource.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of waterborne polyurethane materials, and particularly relates to a method for preparing a stable and multifunctional lignin-based waterborne polyurethane emulsion by compounding lignin and polyol. Background Art

[0002] The full name of polyurethane is polycarbamate (abbreviated as PU), which was developed by German chemist O. Bayer et al. in 1937 and commercially produced by DuPont in 1956. After more than eighty years of research and development, polyurethane can be used to manufacture plastics, rubbers, fibers, rigid and flexible foams, adhesives, coatings, etc. Due to its excellent wear resistance, anti-cutting, impact resistance, solvent resistance and other characteristics, it is widely used in the fields of home, construction, daily necessities, transportation, household appliances, etc. In recent years, with the improvement of people's environmental awareness, the emissions of volatile organic compounds (VOCs) have been restricted globally, and the low-VOC technology has thus been continuously improved. Among them, waterborne polyurethane (WPU) is a kind of polyurethane with water as the dispersion medium. Compared with the traditional solvent-based polyurethane, WPU not only has the advantages of less environmental pollution and reliable production safety, but also avoids the waste of a large amount of organic solvents. With the aggravation of global environmental problems and the continuous improvement of people's awareness of environmental protection, the research and development of green and pollution-free WPU have attracted more and more attention.

[0003] After years of research and practice, a series of WPU products have been put into commercial production and application at home and abroad. For example, Patent CN201310378667.0 provides a method for preparing an aqueous polyurethane emulsion with excellent peel strength and tensile strength by using poly(carbonate-ether) diol and diisocyanate as raw materials, and dimethylolpropionic acid and disulfonate as hydrophilic chain extenders. However, this traditional aqueous polyurethane uses petroleum-based diol as raw material, which is contrary to the national green, low-carbon and circular economy policy. Patent CN201910004160.6 discloses a vegetable oil-based aqueous polyurethane emulsion using vegetable oil as polyol. This patent uses vegetable oil instead of petroleum-based diol to prepare aqueous polyurethane, reducing the use of petrochemical resources. However, as a necessity for human life, vegetable oil will cause the problem of "competing with people for food", and it is difficult to meet the requirements of high performance and functionalization of polyurethane. Patent CN201610640316.6 provides a method for preparing an aqueous polyurethane coating with modified kaolin as filler, enhancing the mechanical properties of the aqueous polyurethane. However, modifying the aqueous polyurethane by adding filler will result in poor compatibility and cannot modify multiple properties simultaneously. Patent CN201510353659.X discloses a method for preparing vegetable oil-based polyurethane modified asphalt. This patent needs to add orthophosphoric acid inhibitor during the prepolymerization process to make the reaction proceed stably to obtain a stable emulsion. Summary of the Invention

[0004] The object of the present invention is to address the above problems in the existing technologies, and propose a novel aqueous polyurethane emulsion based on lignin with high phenolic hydroxyl content and high reactivity. By controlling the lignin content and polymerization reaction conditions, and introducing a biomass resource to complete multiple modifications of the aqueous polyurethane, a novel aqueous polyurethane emulsion with stable, low-cost, green, high mechanical properties, ultraviolet resistance, anti-aging, antibacterial, hydrophobic and other functional characteristics is obtained. At the same time, lignin can also play a certain inhibitory role during the prepolymerization process to make the reaction proceed smoothly without adding additional inhibitors. In addition, the high-value utilization of lignin as a renewable resource is realized, solving the problems of excessive use of petrochemical resources and "competing with people for food" in the preparation of aqueous polyurethane. The invention can effectively broaden the application of aqueous polyurethane emulsion as a coating in harsh conditions in the fields of building protection and the like.

[0005] The technical solution adopted by the present invention is as follows:

[0006] It includes the following steps:

[0007] Step 1) Mix lignin, oligomeric polyol PEG-1000 and acetone solvent, put them into a three-necked flask, and place it in an oil bath for stirring to make the lignin and PEG-1000 evenly mixed;

[0008] Step 2) Weigh hexamethylene diisocyanate (HDI) and slowly add it dropwise into a three-necked flask, and carry out the reaction in an oil bath.

[0009] Step 3) After the reaction in Step 2, gradually increase the temperature, add the catalyst dibutyltin dilaurate (DBTDL), and carry out a prepolymerization reaction for a period of time.

[0010] Step 4) Add a certain amount of hydrophilic chain extender dimethylol propionic acid (DMPA) and chain extender 1,4-butanediol (BDO), and carry out a chain extension reaction until the isocyanate groups react completely.

[0011] Step 5) After the reaction in Step 4 is completed, add acetone solvent to reduce the viscosity, gradually lower the temperature, and dropwise add triethylamine (TEA) to carry out a neutralization reaction.

[0012] Step 6) Place the three-necked flask in an ice bath, add deionized water, and carry out emulsification under a high-speed disperser.

[0013] Step 7) Rotate and distill to remove acetone, and a stable lignin-based aqueous polyurethane emulsion is obtained.

[0014] In Steps 1) and 2), the temperature of the oil bath is 50 °C.

[0015] In Step 2), the reaction time in the oil bath is 30 min.

[0016] In Step 1), the hydroxyl value of lignin is greater than 10 mmol / g, and the hydroxyl content of lignin accounts for 10% - 35% of the total hydroxyl content of lignin and PEG-1000.

[0017] In Step 2), calculate the amount of HDI used according to the R value (the molar ratio of the total hydroxyl value to the isocyanate group); the R value is determined to be 1.2 - 1.5, preferably 1.5.

[0018] In Step 3), gradually increase the temperature to 85 °C; in Step 5), gradually lower the temperature to 45 °C.

[0019] In Step 3), the mass percentage of the catalyst dibutyltin dilaurate (DBTDL) is 0.2%.

[0020] In Step 3), the prepolymerization reaction time is 2 - 3 h.

[0021] In Step 4), the amount of hydrophilic chain extender dimethylol propionic acid (DMPA) used is 2 / 3 of the total molar amount of polyol PEG-1000 and lignin; the amount of chain extender 1,4-butanediol (BDO) used is 1 / 3 of the total molar amount of polyol PEG-1000 and lignin.

[0022] The method for preparing the aqueous polyurethane emulsion based on high-phenol lignin of the present invention has the following beneficial effects:

[0023] (1) Based on the three-dimensional network structure of lignin, compounded with polyols and reacted with isocyanates. By controlling the polymerization process, the cross-linking degree of waterborne polyurethane is controllably increased, and the mechanical properties and water resistance of waterborne polyurethane are improved on the premise of emulsion stability;

[0024] (2) Based on the original performance advantages of lignin such as anti-ultraviolet and antibacterial properties, waterborne polyurethane can be simultaneously endowed with multiple functional characteristics such as anti-ultraviolet, anti-aging, antibacterial, and hydrophobic properties;

[0025] (3) The steric hindrance effect of lignin can inhibit the side reactions of isocyanates, promote the reaction of -NCO / -OH, prevent gelation reactions during the prepolymerization process, and solve the problems of easy gelation and unstable storage of waterborne polyurethane in industrial production.

[0026] (4) Using lignin instead of polyols as raw materials has the advantages of wide raw material distribution, high activity, and prominent functionality. It can reduce the utilization of petrochemical resources and does not cause the phenomenon of "competing with people for food", realizing the high-value utilization of lignin as a renewable resource. Description of the Drawings

[0027] Figure 1 It is a graph showing the change in water absorption rate of the waterborne polyurethane film.

[0028] Figure 2 Comparison chart of ultraviolet light transmittance between lignin waterborne polyurethane and waterborne polyurethane. (a) is the ultraviolet light transmittance of the WPU film, and (b) is the ultraviolet light transmittance of the LWPU film.

[0029] Figure 3 Schematic diagram of the molecular structure of waterborne polyurethane.

[0030] Figure 4 It is a schematic diagram of the cross-linked molecular structure of lignin waterborne polyurethane. Specific Embodiments

[0031] The present invention will be further described below in conjunction with the drawings and embodiments.

[0032] The stable and multifunctional waterborne polyurethane emulsion prepared by the present invention based on lignin takes emulsion stability, mechanical properties, water absorption rate, anti-ultraviolet performance, and antibacterial performance as the main evaluation indicators, and focuses on investigating the influence of process conditions such as lignin content and reaction time in the system on the comprehensive performance of the prepared lignin polyurethane emulsion.

[0033] Example 1:

[0034] 1) Weigh 30 g of PEG-1000, 18 g of acetone, and 0.0225 g of phosphoric acid into a three-necked flask, and place it in an oil bath at 50 °C;

[0035] 2) Weigh 15.14 g of HDI and slowly add it dropwise into the three-necked flask, and react in an oil bath at 50 °C for 30 min;

[0036] 3) Gradually raise the temperature to 85 °C, add 0.045 g of DBTDL and 2.68 g of DMPA, and react for 2 h;

[0037] 4) Add 0.9012 g of BDO and continue to react for 3 h until the isocyanate groups react completely;

[0038] 5) After the reaction is completed, add acetone to reduce the viscosity, gradually cool down to 45 °C, add 2.0238 g of TEA dropwise and then react for 20 min;

[0039] 6) Place the three-necked flask in an ice bath, add 53.4 g of deionized water and emulsify it under a high-speed disperser;

[0040] 7) Rotate and evaporate to remove acetone to obtain the aqueous polyurethane emulsion WPU-1.

[0041] Example 2:

[0042] 1) Weigh 30 g of PEG-1000 and 18 g of acetone into the three-necked flask and place it in an oil bath at 50 °C;

[0043] 2) Weigh 15.14 g of HDI and slowly add it dropwise into the three-necked flask, and react in an oil bath at 50 °C for 30 min;

[0044] 3) Gradually raise the temperature to 85 °C, add 0.045 g of DBTDL and 2.68 g of DMPA, and react for 2 h;

[0045] 4) Add 0.9012 g of BDO and continue to react for 3 h until the isocyanate groups react completely;

[0046] 5) After the reaction is completed, add acetone to reduce the viscosity, gradually cool down to 45 °C, add 2.0238 g of TEA dropwise and then react for 20 min;

[0047] 6) Place the three-necked flask in an ice bath, add 53.4 g of deionized water and emulsify it under a high-speed disperser;

[0048] 7) Rotate and evaporate to remove acetone to obtain the aqueous polyurethane emulsion WPU-2.

[0049] Example 3:

[0050] 1) Weigh 0.5 g of lignin (hydroxyl content is 12 mmol / g), 27 g of PEG-1000 and 18 g of acetone into the three-necked flask, and place it in an oil bath at 50 °C and stir for 15 - 20 min to mix lignin and PEG-1000 evenly;

[0051] 2) Weigh 15.14 g of HDI and slowly add it dropwise into a three-necked flask, and react in an oil bath at 50 °C for 30 min;

[0052] 3) Gradually raise the temperature to 85 °C, add 0.045 g of DBTDL and 2.68 g of DMPA, and react for 2 h;

[0053] 4) Add 0.9012 g of BDO and continue to react for 3 h until the isocyanate groups react completely;

[0054] 5) After the reaction is completed, add acetone to reduce the viscosity, gradually cool down to 45 °C, add 2.0238 g of TEA dropwise and react for 20 min;

[0055] 6) Place the three-necked flask in an ice bath, add 53.4 g of deionized water and emulsify it under a high-speed disperser;

[0056] 7) Rotate and evaporate to remove acetone, and then the lignin-based aqueous polyurethane emulsion LWPU-0-10% is obtained.

[0057] The preparation method of lignin in step 1) is as follows: Place the waste lignocellulose in a deep eutectic solvent for dissolution. After the reaction is completed, add ethanol, shake it well, and then filter to obtain the filtrate as the lignin dissolution solution; Wash and filter the lignin dissolution solution with pure water, and dry it to obtain powdered lignin.

[0058] Example 4:

[0059] 1) Weigh 0.5 g of lignin (hydroxyl content is 12 mmol / g), 27 g of PEG-1000 and 18 g of acetone and put them into a three-necked flask, and place it in an oil bath at 50 °C and stir for 15 - 20 min to make the lignin and PEG-1000 evenly mixed;

[0060] 2) Weigh 15.14 g of HDI and slowly add it dropwise into a three-necked flask, and react in an oil bath at 50 °C for 30 min;

[0061] 3) Gradually raise the temperature to 85 °C, add 0.045 g of DBTDL, and carry out a prepolymerization reaction for 2 h;

[0062] 4) Add 0.9012 g of BDO and 2.68 g of DMPA, and continue to react for 3 h until the isocyanate groups react completely;

[0063] 5) After the reaction is completed, add acetone to reduce the viscosity, gradually cool down to 45 °C, add 2.0238 g of TEA dropwise and react for 20 min;

[0064] 6) Place the three-necked flask in an ice bath, add 53.4 g of deionized water, and emulsify it under a high-speed disperser;

[0065] 7) Rotate and evaporate to remove acetone to obtain a stable lignin aqueous polyurethane emulsion LWPU-2-10%.

[0066] Examples 5 - 7:

[0067] Change the prepolymerization reaction time in Example 4 to 1 h, 3 h, and 4 h, and keep the other conditions the same as in Example 1 to obtain lignin aqueous polyurethane emulsions LWPU-1-10%, LWPU-3-10%, and LWPU-4-10%.

[0068] Examples 8 - 11:

[0069] Increase the lignin substitution rate (the ratio of the hydroxyl value of lignin to the total hydroxyl value) in Example 4 to 15%, 25%, 35%, and 45% respectively, and keep the other conditions the same as in Example 1 to obtain stable lignin aqueous polyurethane emulsions LWPU-2-15%, LWPU-2-25%, LWPU-2-35%, and LWPU-2-45%.

[0070] Experimental results:

[0071] Effect 1: Emulsion stability test

[0072] To verify the storage stability of the emulsion, centrifuge all samples at 3000 r / min for 15 min to simulate the storage performance in the actual environment. If no precipitation occurs after centrifugation, it is confirmed that the emulsion has a storage time of at least 6 months. The test results are shown in Table 1:

[0073] Table 1 Storage stability of aqueous polyurethane emulsions

[0074]

[0075]

[0076] Comparing Examples 1 - 3, when lignin is not added, due to the high activity of isocyanate, a phosphoric acid inhibitor needs to be added to ensure the stable progress of the reaction. After adding lignin, the reaction can proceed smoothly without adding an inhibitor to obtain a stable emulsion; comparing Examples 3 - 7, by changing the prepolymerization reaction time, the reaction degree between lignin and isocyanate is changed, thereby controlling the crosslinking degree of the aqueous polyurethane. When the prepolymerization time is too long and the crosslinking degree is too large, the stability of the emulsion becomes poor and it is easy to gel. Therefore, the prepolymerization time needs to be controlled within 4 h; comparing Example 4 with Examples 8 - 11, too high a lignin substitution rate will lead to unstable emulsion because the three-dimensional amorphous structure of lignin will damage the ordered molecular structure of the aqueous polyurethane and increase the particle size of the aqueous polyurethane.

[0077] Effect 2: Mechanical property test

[0078] The aqueous polyurethane emulsions of Examples 1, 3 - 6, 8 - 10 were made into films for mechanical property test. The films were cut into dumbbell shapes with a cutter and subjected to tensile test on a universal tensile testing machine. The test results are shown in Table 2:

[0079] Table 2 Mechanical properties of aqueous polyurethane films

[0080]

[0081]

[0082] Comparing Examples 1, 3 - 6, since the activity of lignin is lower than that of DMPA, if the chain extender is directly added without pre - polymerization reaction, lignin cannot fully react with isocyanate, which will instead cause certain defects and reduce the mechanical properties. Therefore, the pre - polymerization time needs to be controlled. It is necessary to ensure that a certain degree of cross - linking is formed between lignin and isocyanate and the cross - linking degree should not be too large to cause gelation. From the mechanical properties, the optimal pre - polymerization time is 2 - 3 h; comparing Examples 1, 4 with Examples 8 - 10, because lignin has a rigid structure with benzene rings and acts as a hard segment in the aqueous polyurethane molecular chain, and a certain cross - linked structure and hydrogen bond network will be formed after adding a small amount of lignin, which can enhance and toughen the polyurethane simultaneously. However, if too much lignin is added, the hard segment will be too large and certain toughness will be lost;

[0083] Effect 3: Antibacterial effect

[0084] Some common pathogenic bacteria mainly include Escherichia coli, Staphylococcus aureus, Bacillus subtilis, and Neisseria gonorrhoeae. In this experiment, the antibacterial effects of the aqueous polyurethane emulsions of Examples 1, 4, 8 - 10 were tested with the above - mentioned pathogenic bacteria. The test results are shown in Table 3:

[0085] Table 3 Antibacterial properties of aqueous polyurethane emulsions

[0086]

[0087] As can be seen from the above table, the LWPU emulsion prepared by the present invention has a certain antibacterial effect on most pathogenic bacteria and can meet the requirements of general antibacterial coatings. This is because lignin contains a large number of phenolic hydroxyl groups, and the reaction activity of phenolic hydroxyl groups is relatively low and cannot react completely. Therefore, there will be some free phenolic hydroxyl groups in the LWPU emulsion.

[0088] Effect 4: Water absorption test

[0089] The water absorption rates of the aqueous polyurethanes prepared in Examples 1, 4, and 8-10 were tested by preparing them into films. The aqueous polyurethane films were cut into small squares of 2 cm * 2 cm (recording their initial weight m0), immersed in water for 24 h, and then the weight m after water absorption was recorded. The water absorption rate was calculated by the formula (m - m0) / m0. The test results are as Figure 1 shown: Since lignin itself has a certain hydrophobicity and the LWPU molecules form a cross-linked structure, the hydrophobicity of LWPU has been improved.

[0090] Effect 5: Anti-ultraviolet performance test

[0091] The aqueous polyurethanes of Examples 1 and 4 were prepared into films with a thickness between 1.35 and 1.50 mm, and the transmittance of the WPU films was measured in the range of 200 - 800 nm using an ultraviolet / visible spectrophotometer. The test results are as Figure 2 and Figure 3 shown: Due to the presence of the benzene ring in lignin, lignin has good ultraviolet absorption ability. It can be seen from the curve of 280 nm - 400 nm that the anti-ultraviolet ability of LWPU has been significantly improved. It is expected to be an ultraviolet blocking coating.

[0092] Figure 3 and Figure 4 As shown in the molecular structure schematic diagram, lignin reacts with isocyanate due to its three-dimensional network structure, and the prepared LWPU molecules form a cross-linked structure, thereby improving the mechanical properties and water resistance of the emulsion; and some phenolic hydroxyl groups of lignin do not react completely, making LWPU have a certain antibacterial effect.

Claims

1. A preparation method of a lignin-based aqueous polyurethane emulsion, characterized in that, It includes the following steps: Step 1) Mix lignin, oligomeric polyol PEG-1000 and acetone solvent, then put them into a three-necked flask and place it in an oil bath pot for stirring to make the lignin and PEG-1000 evenly mixed; Step 2) Weigh hexamethylene diisocyanate HDI and slowly add it dropwise into the three-necked flask, and carry out the reaction in an oil bath pot; Step 3) After the reaction in Step 2, gradually increase the temperature, add the catalyst DBTDL, and carry out a pre-polymerization reaction for a period of time; Step 4) Add a certain amount of hydrophilic chain extender DMPA and chain extender BDO to carry out a chain extension reaction until the isocyanate groups completely react; Step 5) After the reaction in Step 4 ends, add acetone solvent to reduce the viscosity, gradually lower the temperature, and dropwise add triethylamine TEA to carry out a neutralization reaction; Step 6) Place the three-necked flask in an ice bath, add deionized water, and carry out emulsification under a high-speed disperser; Step 7) Rotate and distill to remove acetone to obtain a stable lignin aqueous polyurethane emulsion; In the said Step 1), the hydroxyl value of lignin is greater than 10 mmol / g, and the lignin hydroxyl content accounts for 10% - 35% of the total hydroxyl content of lignin and PEG-1000; In the said Step 3), the pre-polymerization reaction time is 2 - 3 h.

2. The preparation method of a lignin-based aqueous polyurethane emulsion according to claim 1, characterized in that: In the said Step 1) and Step 2), the temperature of the oil bath pot is 50 °C; In the said Step 2), the reaction time in the oil bath pot is 30 min.

3. The preparation method of a lignin-based aqueous polyurethane emulsion according to claim 1, characterized in that: In the said Step 2), calculate the dosage of HDI according to the R value; the R value is determined to be 1.2 - 1.

5.

4. The preparation method of a lignin-based aqueous polyurethane emulsion according to claim 1, characterized in that: In the said Step 3), gradually increase the temperature to 85 °C; in the said Step 5), gradually lower the temperature to 45 °C.

5. The preparation method of a lignin-based aqueous polyurethane emulsion according to claim 1, characterized in that: In the said Step 3), the mass percentage of the catalyst DBTDL is 0.2%.

6. The preparation method of a lignin-based aqueous polyurethane emulsion according to claim 1, characterized in that: In the said Step 4), the dosage of the hydrophilic chain extender DMPA is 2 / 3 of the total molar amount of polyol PEG-1000 and lignin; the dosage of the chain extender BDO is 1 / 3 of the total molar amount of polyol PEG-1000 and lignin.

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