CN108977045B - Method for chemically modifying water-based wood coating by using nano-cellulose dispersed graphene - Google Patents

Method for chemically modifying water-based wood coating by using nano-cellulose dispersed graphene Download PDF

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CN108977045B
CN108977045B CN201810918168.9A CN201810918168A CN108977045B CN 108977045 B CN108977045 B CN 108977045B CN 201810918168 A CN201810918168 A CN 201810918168A CN 108977045 B CN108977045 B CN 108977045B
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董晓英
李永峰
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Shandong Agricultural University
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Abstract

A method for chemically modifying an aqueous woodenware coating by nano-cellulose dispersed graphene relates to a method for modifying an aqueous woodenware coating. The problem that the existing nano-material modified waterborne wood coating has poor nano-particle dispersibility and poor mechanical property of a waterborne paint film is solved. The method comprises the following steps: firstly, preparing nano-cellulose water solution containing hemicellulose: sequentially carrying out extraction treatment and delignification treatment on a cellulose raw material to obtain holocellulose, and then carrying out mechanical pretreatment or chemical mechanical mixing pretreatment to obtain nano-cellulose water solution containing hemicellulose; secondly, stably dispersing graphene by using nano-cellulose; and thirdly, carrying out in-situ chemical modification on the nano-cellulose dispersed graphene to obtain the waterborne wood lacquer. The method can obviously improve the mechanical properties of the waterborne wood coating, such as adhesive force, wear resistance, hardness, impact resistance and the like, and even endow the paint film with certain functions of electric conduction, heat conduction, ultraviolet resistance, aging resistance and the like. The invention is used in the field of water-based wood coatings.

Description

Method for chemically modifying water-based wood coating by using nano-cellulose dispersed graphene
Technical Field
The invention relates to a method for modifying water-based wood coating.
Background
Wood surface finishing is the primary means of wood protection and decoration. Solvent-based paints and oil-based coatings have been the mainstream paint types in the furniture industry in the past. However, in recent years, with the enhancement of environmental awareness and the emergence of national policy, the application of solvent-based and oil-based coatings with high VOC emission characteristics is greatly limited, and environment-friendly water-based wood coatings with low VOC emission characteristics are more and more favored by the industry and become the mainstream trend of wood coatings development. Unfortunately, the water-based wood coatings on the market have the problems of low solid content, weak acting force between components and between the components and a substrate, and the like, so that the mechanical properties such as film hardness, wear resistance, adhesive force, impact resistance and the like and ultraviolet aging resistance are generally inferior to those of technically mature oil coatings, and thus the modification of the water-based wood coatings is imperative.
In recent years, more methods for modifying the water-based wood coating by utilizing nano materials are provided, ① method for modifying the water-based wood coating by utilizing nano cellulose is provided, but the dispersion of the nano cellulose is regulated by surfactant, the components are complex, the performance of a paint film is greatly influenced by the surfactant of micromolecule, and the adhesive force, the wear resistance and the ultraviolet aging resistance of the coating are improved only by adding the nano cellulose, ② method for preparing the super-hydrophobic wood coating by utilizing the nano cellulose rod-shaped crystal to improve the mechanical properties of the water-based wood coating, such as hardness, the wear resistance and the like, but the mechanical property of the paint film is not obviously improved due to the low length-diameter ratio of the nano cellulose rod-shaped crystal, ③ method for preparing the super-hydrophobic wood coating by utilizing the nano cellulose and nano silica sol to be compounded with water-dispersed resin, but the nano cellulose loaded nano silica is subjected to super-hydrophobic modification firstly, so that the polarity difference between the nano cellulose loaded nano silica and the water dispersed resin is huge, the two-phase affinity is poor, the interface.
In conclusion, the inorganic nanoparticle modified waterborne wood coating disclosed by the disclosed method has the problem of poor coating mechanical property and poor improvement of light transmittance due to poor nanoparticle dispersibility; the organic nano-cellulose is mostly rod-shaped nano-cellulose crystals, the length-diameter ratio is low, and the improvement on the mechanical property of the coating is limited; even if the nano-cellulose and the SiO2、TiO2And (oxidized) graphene are compounded, and the adopted method also has the problems of poor affinity with a polymer matrix and poor dispersibility in the polymer matrix, which leads to poor improvement of the mechanical property of the coating.
Disclosure of Invention
The invention provides a method for chemically modifying an aqueous wood coating by nano-cellulose dispersed graphene, aiming at solving the problem that the existing nano-material modified aqueous wood coating has poor nano-particle dispersibility and poor mechanical property of an aqueous paint film.
The method for chemically modifying the water-based wood coating by using the nano-cellulose dispersed graphene comprises the following steps:
firstly, preparing nano cellulose water dispersion liquid containing hemicellulose:
sequentially carrying out extraction treatment and delignification treatment on a cellulose raw material to obtain holocellulose, and then carrying out mechanical pretreatment or chemical mechanical mixing pretreatment to obtain nano cellulose water dispersion containing hemicellulose;
the extraction treatment in the first step comprises the following specific steps:
the cellulose raw material is crushed into 90-120 mesh powder, and then the benzene alcohol extraction treatment is carried out for 10-12 h. The benzene alcohol is toluene and absolute ethyl alcohol according to a volume ratio of 2: 1, in a mixture of the components.
The delignification treatment in the first step comprises the following specific steps:
① soaking the extracted cellulose powder into sodium chlorite solution with mass concentration of 1-1.2%, adjusting the pH value of the solution to 4-5 with glacial acetic acid, and magnetically heating and stirring for 1-1.5 h in a constant-temperature water bath kettle at 75-80 ℃;
②, soaking the cellulose powder obtained in the step ① in a sodium chlorite solution with the mass concentration of 1-1.2%, adjusting the pH value of the solution to 4-5 by using glacial acetic acid, and magnetically heating and stirring the solution in a constant-temperature water bath kettle at the temperature of 75-80 ℃ for 1-1.5 hours;
③, repeating the step ② 5-6 times to basically remove the lignin, then filtering and washing the obtained liquid by using a Buchner funnel until the filtrate is neutral, and finally obtaining the holocellulose;
or the delignification treatment in the step one comprises the following specific steps:
soaking the extracted cellulose powder into a hydrogen peroxide solution with the mass concentration of 30-35%, adding magnesium silicate (using the magnesium silicate as a stabilizer), and magnetically stirring at room temperature for 48-52 hours to basically remove lignin; then, filtering and washing the obtained liquid by using a Buchner funnel until the filtrate is neutral, and finally obtaining the holocellulose; wherein the mass of the magnesium silicate is 1 to 1.5 percent of the mass of the hydrogen peroxide solution.
The mechanical pretreatment in the step one comprises the following specific operation steps:
① adding the holocellulose into a 250mL beaker, and adding deionized water until the mass fraction of the holocellulose is 0.1-0.3%
②, mechanically treating for 30-40 min by using a high-pressure homogenizing machine at 600bar to obtain nano-cellulose water dispersion containing hemicellulose, wherein the hemicellulose accounts for 20-30% of the mass of the holocellulose;
the chemical mechanical mixing pretreatment in the step one comprises the following specific operation steps:
① adding holocellulose into a 250mL beaker, adding deionized water until the mass fraction of the holocellulose is 0.1-0.3%, then sequentially adding sodium bromide and a TEMPO reagent, then adding sodium hypochlorite at the adding speed of 2.5-3 mL/min under magnetic stirring, adjusting the pH value to 10-10.5 by using 0.5mol/L sodium hydroxide, and continuously reacting until the pH value is unchanged, wherein the mass of the sodium bromide is 12-13% of that of the holocellulose, the mass of the TEMPO reagent is 1-2% of that of the holocellulose, and the mass of the sodium hypochlorite is 25-35% of that of the holocellulose;
②, performing suction filtration and water washing, adding deionized water and sodium chlorite, adjusting the pH value to 4-5 by using glacial acetic acid, magnetically stirring for 1-2 h at 70-80 ℃, and performing suction filtration and water washing to obtain carboxylated cellulose, wherein the mass ratio of the deionized water to the holocellulose is (95-105): 3, and the mass of the sodium chlorite is 25-35% of the mass of the holocellulose;
③, then 0.1-0.3% of carboxylated cellulose aqueous dispersion is prepared again, and mechanical dispersion is carried out for 20-30 min by a high-speed stirrer with 14000-16000 rpm, thus obtaining the nano cellulose aqueous dispersion containing hemicellulose, wherein the hemicellulose accounts for 10-20% of the mass of the holocellulose.
The cellulose raw material in the step one is biomass raw material powder (such as wood powder, crop straw powder, waste paper, leaves, bast fiber raw material and the like) with the granularity of more than 100 meshes;
secondly, stably dispersing the graphene by the nano-cellulose:
①, placing graphene or reduced graphene in the nano-cellulose aqueous dispersion containing hemicellulose obtained in the first step, mechanically dispersing for 1-20 min under the condition of high-speed stirring at 14000-16000 rpm, and then dispersing for 1-20 min under the ultrasonic condition of 900-1100W to obtain uniformly dispersed nano-cellulose/graphene aqueous dispersion, wherein the graphene or reduced graphene accounts for 0.1-0.3% of the mass of the nano-cellulose aqueous dispersion containing hemicellulose;
② heating the nano-cellulose/graphene aqueous dispersion under the stirring condition of 600-1000 rpm, evaporating water to obtain an aqueous dispersion with the mass concentration of 10-20%, adding acetone into the aqueous dispersion to make the acetone account for 30-40% of the volume of the aqueous dispersion, dispersing for 1-20 min under the ultrasonic condition of 400-600W, and centrifuging at 8000-10000 rpm to obtain a precipitate;
③ adding acetone into the precipitate, dispersing for 1-20 min under 500-600W ultrasonic condition, and centrifuging at 8000-9000 rpm to obtain precipitate, wherein the mass ratio of the precipitate to the acetone is 1 (9-11);
④, repeating the step ③ 5-6 times, and then adding acetone into the precipitate to obtain an acetone dispersion liquid of the precipitate, wherein the solid content is 20% -25%;
the preparation method of the reduced graphene in the second step specifically comprises the following steps:
placing the graphene oxide sold in the market in a hydrogen iodide solution sold in the market, standing at room temperature for 20-30 min, and then sequentially and alternately washing with water and ethanol for 3-5 times, wherein the water washing is performed for 1-10 min each time, and the ethanol washing is performed for 1-10 min each time, so as to obtain reduced graphene;
thirdly, in-situ chemical modification of the nano-cellulose dispersed graphene aqueous wood lacquer:
①, mixing isophorone diisocyanate and the acetone dispersion liquid of the precipitate obtained in the second step, stirring uniformly (accounting for 0.5% of the solid content in the water paint), pouring into a dry four-neck flask with a stirrer and a condenser, heating to 65-70 ℃, and reacting for 1-2 hours;
② adding polypropylene glycol-2000 into the reaction system, and reacting for 1.5-2 h at 65-70 ℃;
③ slowly dripping acetone solution of 1, 4-butanediol with mass concentration of 15-25% into the reaction system, controlling the dripping speed of the acetone solution of 1, 4-butanediol to control the temperature of the system at 70-80 ℃ and reacting for 1.5-2 h;
④ dissolving dimethylolpropionic acid with a small amount of N-methyl pyrrolidone, adding into a reaction system, controlling the reaction temperature to be 65-75 ℃, controlling the viscosity of the reaction solution to be lower than 100mPa & S with acetone, and reacting for 3.5-4 h;
⑤ adding triethylamine into the reaction system, and carrying out neutralization reaction for 15-20 min, wherein the mass ratio of isophorone diisocyanate, acetone dispersion of the precipitate obtained in the step two, polypropylene glycol-2000, acetone solution of 1, 4-butanediol, dimethylolpropionic acid and triethylamine is (50-60), (10-20), (45-60), (15-20), (5-8) and (3-5).
⑥ dispersing the reaction solution by a high-speed disperser to obtain the hemicellulose-containing nanocellulose-dispersed graphene in-situ chemically modified waterborne polyurethane emulsion, namely the nanocellulose-dispersed graphene chemically modified waterborne wood coating.
And C, continuously stirring in the modification process of the step III.
The aqueous acrylic ester emulsion can be continuously added into the emulsion obtained from ⑥ to obtain the co-mixed aqueous polyurethane acrylic ester emulsion, and the aqueous acrylic ester emulsion is added to obtain another aqueous wood coating, wherein the performances of the two coatings are different, namely the aqueous polyurethane emulsion is a single-component wood coating, and the blended coating has the advantages of both aqueous polyurethane and acrylic ester.
The invention has the beneficial effects that:
the invention provides a method for in-situ chemical modification of a water-based wood lacquer by nano-cellulose dispersed graphene with hemicellulose, which is characterized by comprising the following steps:
① hemicellulose has short chain segment, is grafted on the long chain segment of the nano-cellulose to form a brush shape with large specific surface area, ② hemicellulose contains more hydroxyl functional groups than the cellulose, can be stably dispersed in water better and has larger hydroxyl reaction activity, ③ cellulose and hemicellulose both have six-membered ring structures and can generate stronger coupling effect with the six-membered ring of the graphene to form a stable combination body.
The following advantages further result:
① nanometer cellulose with hemicellulose and graphene form stronger coupling force to enable the graphene to be coated by the nanometer cellulose to form stable aqueous dispersion, ② rich hydroxyl functional groups exist to improve the dispersion stability of the nanometer cellulose in water and facilitate the nanometer cellulose to be better dispersed in a polymer matrix so as to modify the performance of a water paint film, ③ rich hydroxyl groups facilitate the nanometer cellulose and an emulsion colloidal polymer of the water paint to form stronger interaction through hydrogen bonds or chemical bonds among polar functional groups so as to better enhance the performance of the water paint film, ④ brush-shaped nanometer cellulose can form dendritic combination with the polymer matrix, the molecular network structure of the polymer matrix is enhanced by using huge specific surface area and chemical/hydrogen bond action so as to theoretically enhance the mechanical strength of the paint film, ⑤ two-dimensional lamellar graphene is uniformly dispersed in the polymer matrix, the mechanical performance of the paint film is significantly improved by using huge specific surface area and mechanical strength, the paint film is endowed with a certain conduction link, a conductive film, an anti-ultraviolet resistance function and the like through lamellar, the nanometer cellulose is easily separated, the energy consumption of the nanometer cellulose dispersed in the nanometer cellulose dispersion process is reduced, the nanometer cellulose dispersion process is simplified, the nanometer cellulose is simplified, the problem of improving the mechanical property of a nanometer cellulose dispersion process of a nanometer cellulose dispersion film, the nanometer cellulose is avoided, the nanometer cellulose dispersion, the nanometer cellulose is reduced, the aging resistance, the aging problem of a nanometer cellulose is reduced, the nanometer cellulose dispersion process of a nanometer cellulose is reduced, the nanometer cellulose dispersion process of a nanometer cellulose dispersion process, and the nanometer cellulose is reduced, the.
The method can obviously improve the mechanical properties of the waterborne wood coating, such as adhesive force, wear resistance, hardness, impact resistance and the like, and even endows the paint film with certain functions of electric conduction, heat conduction, ultraviolet resistance, aging resistance and the like, wherein the adhesive force is improved from the III level before modification to the II level after modification, and the wear resistance is improved from 40 percent of mass loss rate before modification to the modification10 to 20 percent of mass loss rate after modification, the tensile strength is improved from 2.3MPa before modification to 3.5 to 4MPa after modification, the hardness is improved from B grade before modification to HB to H grade after modification, and the impact resistance is improved from 2.1J/m before modification2The impact toughness is improved to 2.8-3.5J/m after modification2The impact toughness and the glossiness are not obviously reduced, the resistance value can reach 950-1200 omega, the heat conductivity coefficient can reach 10-20 w/m.k, and the yellowing index is reduced by 30-45%. The service life of the water-based wood coating is prolonged, the application range of the water-based wood coating is widened, and the added value of wood products is improved.
The method is applicable to the following water-based paint types: waterborne polyurethane, waterborne polyurethane modified acrylate, waterborne epoxy resin modified acrylate and the like.
Drawings
FIG. 1 is a scanning electron microscope image of two-dimensional reduced graphene obtained in step two of example 1;
fig. 2 is a transmission electron microscope image of the hemicellulose-containing nanocellulose composite graphene obtained in step two of example 1;
FIG. 3 is a scanning electron microscope image of the two-dimensional graphene obtained in the first step of example 2;
fig. 4 is a transmission electron microscope image of the nanocellulose composite graphene containing hemicellulose obtained in step two of example 2.
Detailed Description
The technical solution of the present invention is not limited to the following specific embodiments, but includes any combination of the specific embodiments.
The first embodiment is as follows: the method for chemically modifying the water-based wood coating by using the nano-cellulose dispersed graphene comprises the following steps:
firstly, preparing nano cellulose water dispersion liquid containing hemicellulose:
sequentially carrying out extraction treatment and delignification treatment on a cellulose raw material to obtain holocellulose, and then carrying out mechanical pretreatment or chemical mechanical mixing pretreatment to obtain nano cellulose water dispersion containing hemicellulose;
the mechanical pretreatment comprises the following specific operation steps:
① adding deionized water into the holocellulose until the mass fraction of the holocellulose is 0.1-0.3%;
②, mechanically treating for 30-40 min by using a high-pressure homogenizing machine at 600bar to obtain nano-cellulose water dispersion containing hemicellulose, wherein the hemicellulose accounts for 20-30% of the mass of the holocellulose;
the chemical mechanical mixing pretreatment comprises the following specific operation steps:
①, adding deionized water into the holocellulose until the mass fraction of the holocellulose is 0.1-0.3%, then sequentially adding sodium bromide and a TEMPO reagent, then adding sodium hypochlorite at the adding speed of 2.5-3 mL/min under magnetic stirring, adjusting the pH value to 10-10.5, and continuously reacting until the pH value is unchanged, wherein the mass of the sodium bromide is 12-13% of the mass of the holocellulose, the mass of the TEMPO reagent is 1-2% of the mass of the holocellulose, and the mass of the sodium hypochlorite is 25-35% of the mass of the holocellulose;
②, performing suction filtration and water washing, adding deionized water and sodium chlorite, adjusting the pH value to 4-5, performing magnetic stirring at 70-80 ℃ for 1-2 h, and performing suction filtration and water washing to obtain carboxylated cellulose, wherein the mass ratio of the deionized water to the holocellulose is (95-105): 3, and the mass of the sodium chlorite is 25-35% of the mass of the holocellulose;
③, preparing a 0.1-0.3% carboxylated cellulose aqueous dispersion liquid again, and mechanically dispersing for 20-30 min by a high-speed stirrer at 14000-16000 rpm to prepare a nano cellulose aqueous dispersion liquid containing hemicellulose, wherein the hemicellulose accounts for 10-20% of the mass of the holocellulose;
secondly, stably dispersing the graphene by the nano-cellulose:
①, placing graphene or reduced graphene in the nano-cellulose aqueous dispersion containing hemicellulose obtained in the first step, mechanically dispersing for 1-20 min under the condition of high-speed stirring at 14000-16000 rpm, and then dispersing for 1-20 min under the ultrasonic condition of 900-1100W to obtain uniformly dispersed nano-cellulose/graphene aqueous dispersion, wherein the graphene or reduced graphene accounts for 0.1-0.3% of the mass of the nano-cellulose aqueous dispersion containing hemicellulose;
② heating the nano-cellulose/graphene aqueous dispersion under the stirring condition of 600-1000 rpm, evaporating water to obtain an aqueous dispersion with the mass concentration of 10-20%, adding acetone into the aqueous dispersion to make the acetone account for 30-40% of the volume of the aqueous dispersion, dispersing for 1-20 min under the ultrasonic condition of 400-600W, and centrifuging at 8000-10000 rpm to obtain a precipitate;
③ adding acetone into the precipitate, dispersing for 1-20 min under 500-600W ultrasonic condition, and centrifuging at 8000-9000 rpm to obtain precipitate, wherein the mass ratio of the precipitate to the acetone is 1 (9-11);
④, repeating the step ③ 5-6 times, and then adding acetone into the precipitate to obtain an acetone dispersion liquid of the precipitate, wherein the solid content is 20% -25%;
thirdly, in-situ chemical modification of the nano-cellulose dispersed graphene aqueous wood lacquer:
①, mixing isophorone diisocyanate and the acetone dispersion liquid of the precipitate obtained in the second step, uniformly stirring, and heating to 65-70 ℃ for reaction for 1-2 hours;
② adding polypropylene glycol-2000 into the reaction system, and reacting for 1.5-2 h at 65-70 ℃;
③, dropwise adding the acetone solution of 1, 4-butanediol with the mass concentration of 15-25% into the reaction system, and controlling the dropwise adding speed of the acetone solution of 1, 4-butanediol to control the temperature of the system at 70-80 ℃ for reaction for 1.5-2 h;
④ dissolving dimethylolpropionic acid by using N-methyl pyrrolidone, adding the solution into a reaction system, controlling the reaction temperature to be 65-75 ℃, controlling the viscosity of the reaction solution to be lower than 100mPa & S by using acetone, and reacting for 3.5-4 h;
⑤ adding triethylamine into the reaction system, and carrying out neutralization reaction for 15-20 min;
⑥ dispersing the reaction solution by a high-speed disperser to obtain the hemicellulose-containing nanocellulose-dispersed graphene in-situ chemically modified waterborne polyurethane emulsion, namely the nanocellulose-dispersed graphene chemically modified waterborne wood coating.
The second embodiment is as follows: the first difference between the present embodiment and the specific embodiment is: the extraction treatment in the first step comprises the following specific steps:
crushing a cellulose raw material into powder of 90-120 meshes, and then performing benzyl alcohol extraction treatment for 10-12 hours; the benzene alcohol is toluene and absolute ethyl alcohol according to a volume ratio of 2: 1, in a mixture of the components. The rest is the same as the first embodiment.
The third concrete implementation mode: the first difference between the present embodiment and the specific embodiment is: the delignification treatment in the first step comprises the following specific steps:
① soaking the extracted cellulose powder into sodium chlorite solution with mass concentration of 1-1.2%, adjusting the pH value of the solution to 4-5 with glacial acetic acid, and magnetically heating and stirring for 1-1.5 h in a constant-temperature water bath kettle at 75-80 ℃;
②, soaking the cellulose powder obtained in the step ① in a sodium chlorite solution with the mass concentration of 1-1.2%, adjusting the pH value of the solution to 4-5 by using glacial acetic acid, and magnetically heating and stirring the solution in a constant-temperature water bath kettle at the temperature of 75-80 ℃ for 1-1.5 hours;
③ repeating the step ② 5-6 times to remove lignin basically, then filtering and washing the obtained liquid by a Buchner funnel until the filtrate is neutral, and finally obtaining the holocellulose.
The fourth concrete implementation mode: the first difference between the present embodiment and the specific embodiment is: the delignification treatment in the first step comprises the following specific steps:
soaking the extracted cellulose powder into a hydrogen peroxide solution with the mass concentration of 30-35%, adding magnesium silicate, and magnetically stirring at room temperature for 48-52 hours to basically remove lignin; then, filtering and washing the obtained liquid by using a Buchner funnel until the filtrate is neutral, and finally obtaining the holocellulose; wherein the mass of the magnesium silicate is 1 to 1.5 percent of the mass of the hydrogen peroxide solution. The rest is the same as the first embodiment.
The fifth concrete implementation mode: the first difference between the present embodiment and the specific embodiment is: the cellulose raw material in the step one is biomass raw material powder with the granularity of more than 100 meshes. The rest is the same as the first embodiment.
The sixth specific implementation mode: the fifth embodiment is different from the fifth embodiment in that: the biomass raw material powder is wood powder, crop straw powder, waste paper, leaves or bast fiber raw materials. The rest is the same as the fifth embodiment.
The seventh embodiment: the first difference between the present embodiment and the specific embodiment is: the preparation method of the reduced graphene in the second step specifically comprises the following steps:
and placing the graphene oxide in a hydrogen iodide solution, standing at room temperature for 20-30 min, and then alternately washing with water and ethanol for 3-5 times in sequence, wherein the water washing is performed for 1-10 min each time, and the ethanol washing is performed for 1-10 min each time, so as to obtain the reduced graphene. The rest is the same as the first embodiment.
The specific implementation mode is eight: the first difference between the present embodiment and the specific embodiment is: and C, continuously stirring in the modification process of the step III. The rest is the same as the first embodiment.
The specific implementation method nine: the first difference between the present embodiment and the specific embodiment is: and C, continuously adding the water-based acrylate emulsion into the emulsion obtained in the step three to obtain the co-mixing water-based polyurethane acrylate emulsion. The rest is the same as the first embodiment.
The detailed implementation mode is ten: the first difference between the present embodiment and the specific embodiment is: in the third step, the mass ratio of isophorone diisocyanate, acetone dispersion liquid of the precipitate obtained in the second step, acetone solution of polypropylene glycol-2000, 1, 4-butanediol, dimethylolpropionic acid and triethylamine is (50-60): (10-20): (45-60): (15-20): (5-8): (3-5). The rest is the same as the first embodiment.
The following examples are given to illustrate the present invention, and the following examples are carried out on the premise of the technical solution of the present invention, and give detailed embodiments and specific procedures, but the scope of the present invention is not limited to the following examples.
Example 1:
the method for chemically modifying the water-based wood coating by using the nano-cellulose dispersed graphene comprises the following steps:
firstly, preparing nano cellulose water dispersion liquid containing hemicellulose: sequentially carrying out extraction treatment and lignin removal treatment on a cellulose raw material to obtain holocellulose fibers, and then carrying out mechanical pretreatment to obtain nano cellulose aqueous dispersion containing hemicellulose;
the extraction treatment in the first step comprises the following specific steps:
the cellulose raw material is crushed into 100-mesh powder and then is subjected to extraction treatment for 10 hours by using the benzene alcohol. The benzene alcohol is prepared from toluene and absolute ethyl alcohol according to a volume ratio of 2: 1 is prepared from (1).
The delignification treatment in the first step comprises the following specific steps:
① soaking the extracted cellulose powder in 1% sodium chlorite solution, adjusting pH to 4.5 with glacial acetic acid, and magnetically heating and stirring in 75 deg.C water bath for 1 h;
② soaking the cellulose powder obtained in step ① in 1% sodium chlorite solution, adjusting pH to 4.5 with glacial acetic acid, and magnetically heating and stirring in a 75 deg.C constant temperature water bath for 1 h;
③ repeating step ② 5 times to remove lignin basically, filtering the obtained liquid with Buchner funnel, washing until the filtrate is neutral to obtain holocellulose;
the mechanical pretreatment in the step one comprises the following specific operation steps:
① adding the holocellulose into a 250mL beaker, and adding deionized water until the mass fraction of the holocellulose is 0.3%;
② homogenizing with high pressure of 600bar for 30min to obtain nanometer cellulose water dispersion containing hemicellulose 25 wt% of the holocellulose;
the cellulose raw material in the step one is wood powder with the granularity of more than 100 meshes.
Secondly, stably dispersing the graphene by the nano-cellulose:
① placing graphene oxide sold in the market in hydrogen iodide solution sold in the market, standing for 20min at room temperature, then alternately washing for 3 times with water and ethanol, washing for 10min each time, and washing for 10min each time with ethanol to obtain reduced graphene, placing the reduced graphene in the nano-cellulose aqueous dispersion containing hemicellulose obtained in the step one, mechanically dispersing for 20min under the condition of high-speed stirring at 15000rpm, and dispersing for 20min under the ultrasonic condition of 1000W to obtain uniformly dispersed nano-cellulose/graphene aqueous dispersion, wherein the mass of the graphene or the reduced graphene is 0.1% of that of the nano-cellulose aqueous dispersion containing hemicellulose;
② heating the nano-cellulose/graphene aqueous dispersion under the stirring condition of 600rpm, evaporating water to obtain an aqueous dispersion with the mass concentration of 15%, adding acetone into the aqueous dispersion to enable the acetone to account for 30% of the volume of the aqueous dispersion, dispersing for 20min under the ultrasonic condition of 500W, and centrifuging at 8000rpm to obtain a precipitate;
③ adding acetone into the precipitate, dispersing for 20min under 500W ultrasonic condition, and centrifuging at 8000rpm to obtain precipitate, wherein the mass ratio of the precipitate to acetone is 1: 10;
④ repeating step ③ 5 times, and adding acetone to the precipitate to obtain acetone dispersion of precipitate with solid content of 25%;
thirdly, in-situ chemical modification of the nano-cellulose dispersed graphene aqueous wood lacquer:
①, weighing 56g of isophorone diisocyanate and 10ml of acetone dispersion liquid of the precipitate obtained in the second step, uniformly stirring, pouring into a dry four-neck flask with a stirrer and a condenser tube, and heating to 65 ℃ for reaction for 1 h;
② weighing 50g of polypropylene glycol-2000, adding into the reaction system, and reacting for 1.5h at 65 ℃;
③ weighing 17g of acetone solution of 1, 4-butanediol, wherein the weight of the acetone solution contains 10.5g of 1, 4-butanediol, slowly dripping the acetone solution into the reaction solution, controlling the dripping speed of the acetone to control the temperature of the system to be 75 ℃, and reacting for 1.5 h;
④ weighing 5.8g dimethylolpropionic acid, dissolving with a small amount of N-methyl pyrrolidone, adding into the reaction solution, controlling the reaction temperature at 70 deg.C, controlling the viscosity of the reaction solution to be lower than 100 mPa.S with acetone, and reacting for 3.5 h;
⑤ weighing 4.3g triethylamine, adding into the reaction system, neutralizing for 15 min;
⑥ dispersing the liquid by a high-speed dispersion machine to obtain the hemicellulose-containing nano-cellulose dispersed graphene in-situ chemical modified aqueous polyurethane emulsion.
Fig. 1 is an SEM image of reduced graphene obtained in the second step of example 1, and fig. 2 is an SEM image of nanocellulose composite graphene obtained in the second step of example 1, it can be seen that the side length of the obtained two-dimensional graphene sheet is greater than 50um, the diameter of nanocellulose is less than 100nm, the length is greater than 50um, and nanocellulose with high aspect ratio is uniformly distributed on the graphene sheet, so that the surface property of graphene is well coated and regulated, and a foundation is laid for a subsequent reinforced modified paint film.
The nano-cellulose dispersed graphene chemically modified water-borne wood coating and the unmodified wood coating obtained in the example 1 are mixed at a ratio of 120g/m2The spraying amount is respectively applied to the surfaces of maples in a spraying mode, the maples are dried for 72 hours at the temperature of 30 ℃ to obtain the nano-cellulose dispersed graphene chemically modified waterborne wood paint film obtained in the first embodiment and an unmodified wood paint film, and the mechanical properties and the glossiness of the two paint films are tested according to the national standard GBT-17657 plus 2013 'test method for the physical and chemical properties of artificial boards and decorative artificial boards'. The result shows that the hardness of the nano-cellulose dispersed graphene chemically modified waterborne wood coating paint film obtained in the example 1 is improved from the B level before modification to the HB level, the wear resistance is improved from 40% of mass loss rate before modification to 15% of mass loss rate after modification, the adhesive force is improved from the III level before modification to the II level after modification, and the impact toughness is improved from 2.1J/m before modification2Increased to 3.5J/m after modification2The tensile strength is improved from 2.3MPa before modification to 3.7MPa after modification, the glossiness is only reduced from 86.2 to 78.5, and the paint film has certain functions of electric conduction (the resistance value is 1100 omega), heat conduction (the heat conductivity coefficient reaches 15w/m.k) and ultraviolet resistance and aging resistance (the yellowing index is reduced by 35%); in contrast, the pure nanocellulose dispersed graphene modified waterborne wood coating paint filmThe hardness of the modified polyurethane is improved from B grade before modification to B grade to HB grade, the wear resistance is improved from 40 percent of mass loss rate before modification to 35 percent of mass loss rate after modification, the adhesive force is improved from III grade before modification to III grade to II grade after modification, and the impact toughness is improved from 2.1J/m before modification2Increased to 2.6J/m after modification2The tensile strength is improved from 2.3MPa before modification to 2.9MPa after modification, the glossiness is only reduced from 86.2% to 82%, and the paint film has certain functions of electric conduction (the resistance value is 1820 omega), heat conduction (the heat conductivity coefficient reaches 14w/m.k) and ultraviolet resistance and aging resistance (the yellowing index is reduced by 46%), which shows that the comprehensive performance of the waterborne wood paint can be effectively improved by using the method for dispersing the graphene by using the nano-cellulose containing hemicellulose, and the comprehensive effect is better than that of the waterborne wood paint modified by the pure nano-cellulose dispersed graphene.
Example 2:
the method for chemically modifying the water-based wood coating by using the nano-cellulose dispersed graphene comprises the following steps:
firstly, preparing nano cellulose water dispersion liquid containing hemicellulose: sequentially carrying out extraction treatment and lignin removal treatment on a cellulose raw material to obtain holocellulose fibers, and then carrying out chemical-mechanical mixing pretreatment to obtain nano cellulose water dispersion containing hemicellulose;
the extraction treatment in the first step comprises the following specific steps:
the cellulose raw material is crushed into 100-mesh powder and then is subjected to extraction treatment for 10 hours by using the benzene alcohol. The benzene alcohol is prepared from toluene and absolute ethyl alcohol according to a volume ratio of 2: 1 is prepared from (1).
The delignification treatment in the first step comprises the following specific steps:
soaking the extracted cellulose powder into 30% hydrogen peroxide solution, adding magnesium silicate (using magnesium silicate as stabilizer), and magnetically stirring at room temperature for 48 hr to remove lignin; then, filtering and washing the obtained liquid by using a Buchner funnel until the filtrate is neutral, and finally obtaining the holocellulose; wherein the mass of the magnesium silicate is 1 percent of the mass of the hydrogen peroxide solution.
The chemical mechanical mixing pretreatment in the step one comprises the following specific operation steps:
① adding holocellulose into a 250mL beaker, adding deionized water until the mass fraction of the holocellulose is 0.3%, then sequentially adding sodium bromide and a TEMPO reagent, then adding sodium hypochlorite at the adding speed of 2.5mL/min under magnetic stirring, adjusting the pH value to 10 by using sodium hydroxide with the concentration of 0.5mol/L, and continuously reacting until the pH value is unchanged, wherein the mass of the sodium bromide is 12% to 5% of that of the holocellulose, the mass of the TEMPO reagent is 1.25% of that of the holocellulose, and the mass of the sodium hypochlorite is 30% of that of the holocellulose;
②, performing suction filtration and water washing, adding deionized water and sodium chlorite, adjusting the pH value to 4.5 by using glacial acetic acid, magnetically stirring for 1h at 70 ℃, and performing suction filtration and water washing to obtain carboxylated cellulose, wherein the mass ratio of the deionized water to the holocellulose is 100:3, and the mass of the sodium chlorite is 30% of the mass of the holocellulose;
③ and preparing 0.3% carboxylated cellulose aqueous dispersion, and mechanically dispersing for 20min with a high speed mixer at 15000rpm to obtain nanometer cellulose aqueous dispersion containing hemicellulose, wherein the hemicellulose accounts for 18% of the weight of the holocellulose.
The cellulose raw material in the step one is wheat straw powder with the granularity of more than 100 meshes.
Secondly, stably dispersing the graphene by the nano-cellulose:
①, placing commercially available graphene in the nano-cellulose aqueous dispersion containing hemicellulose obtained in the first step, mechanically dispersing for 20min under the condition of high-speed stirring at 15000rpm, and then dispersing for 20min under the ultrasonic condition of 1000W to obtain uniformly dispersed nano-cellulose/graphene aqueous dispersion, wherein the mass of graphene or reduced graphene is 0.1% of that of the nano-cellulose aqueous dispersion containing hemicellulose;
② heating the nano-cellulose/graphene aqueous dispersion under the stirring condition of 600rpm, evaporating water to obtain an aqueous dispersion with the mass concentration of 15%, adding acetone into the aqueous dispersion to enable the acetone to account for 30% of the volume of the aqueous dispersion, dispersing for 20min under the ultrasonic condition of 500W, and centrifuging at 8000rpm to obtain a precipitate;
③ adding acetone into the precipitate, dispersing for 20min under 500W ultrasonic condition, and centrifuging at 8000rpm to obtain precipitate, wherein the mass ratio of the precipitate to acetone is 1: 10;
④ repeating step ③ 5 times, and adding acetone to the precipitate to obtain acetone dispersion of precipitate with solid content of 25%;
thirdly, in-situ chemical modification of the nano-cellulose dispersed graphene aqueous wood lacquer:
①, weighing 56g of isophorone diisocyanate and 10ml of acetone dispersion liquid of the precipitate obtained in the second step, uniformly stirring, pouring into a dry four-neck flask with a stirrer and a condenser tube, and heating to 65 ℃ for reaction for 1 h;
② weighing 50g of polypropylene glycol-2000, adding into the reaction system, and reacting for 1.5h at 65 ℃;
③ weighing 17g of acetone solution of 1, 4-butanediol, wherein the weight of the acetone solution contains 10.5g of 1, 4-butanediol, slowly dripping the acetone solution into the reaction solution, controlling the dripping speed of the acetone to control the temperature of the system to be 75 ℃, and reacting for 1.5 h;
④ weighing 5.8g dimethylolpropionic acid, dissolving with a small amount of N-methyl pyrrolidone, adding into the reaction solution, controlling the reaction temperature at 70 deg.C, controlling the viscosity of the reaction solution to be lower than 100 mPa.S with acetone, and reacting for 3.5 h;
⑤ weighing 4.3g triethylamine, adding into the reaction system, neutralizing for 15 min;
⑥ dispersing the liquid by a high-speed dispersion machine to obtain the hemicellulose-containing nano-cellulose dispersed graphene in-situ chemical modified aqueous polyurethane emulsion.
⑦ and continuously adding the water-based acrylic ester emulsion into the emulsion obtained from ⑥ according to the solid content ratio of 1:1 to obtain the co-mixed water-based polyurethane acrylic ester emulsion.
Fig. 3 is an SEM image of reduced graphene obtained in step two of example 2, and fig. 4 is an SEM image of nanocellulose composite graphene obtained in step two of example 2. Therefore, the side length of the obtained two-dimensional graphene sheet layer is more than 500um, the diameter of the nano-cellulose is less than 100nm, the length of the nano-cellulose is more than 10um, and the nano-cellulose with high length-diameter ratio is uniformly distributed on the graphene sheet layer, so that the surface property of the graphene is well coated and regulated, and a foundation is laid for a subsequent reinforced modified paint film.
The nano-cellulose dispersed graphene chemically modified water-borne wood coating and the unmodified wood coating obtained in the embodiment 2 are mixed at a ratio of 120g/m2The spraying amount of the nano-crystalline cellulose dispersed graphene chemically-modified waterborne wood paint film and the unmodified wood paint film obtained in the second embodiment are obtained after the nano-crystalline cellulose dispersed graphene chemically-modified waterborne wood paint film is respectively applied to the surfaces of maples in a spraying mode and dried at the temperature of 30 ℃ for 72 hours; the mechanical properties and the glossiness of the two paint films are tested according to the national standard GBT-17657 plus 2013 physicochemical property test method for artificial boards and decorative artificial boards. The result shows that the hardness of the nano-cellulose dispersed graphene chemically modified waterborne wood coating paint film obtained in the example 2 is improved from the B level before modification to the HB level, the wear resistance is improved from 40% of mass loss rate before modification to 18% of mass loss rate after modification, the adhesive force is improved from the III level before modification to the II level after modification, and the impact toughness is improved from 2.1J/m before modification2Increased to 3.3J/m after modification2The tensile strength is improved from 2.3MPa before modification to 3.5MPa after modification, the glossiness is only reduced from 86.2 to 77, and the paint film has certain functions of electric conduction (the resistance value is 1200 omega), heat conduction (the heat conduction coefficient reaches 14w/m.k) and ultraviolet resistance and aging resistance (the yellowing index is reduced by 37%); as a contrast, the hardness of a paint film of the pure nano-cellulose dispersed graphene modified waterborne wood coating is improved from B level before modification to B level to HB level, the wear resistance is improved from 40% of mass loss rate before modification to 35% of mass loss rate after modification, the adhesive force is improved from III level before modification to III level after modification, and the impact toughness is improved from 2.1J/m before modification to 2.1J/m2Increased to 2.6J/m after modification2The impact toughness and tensile strength are improved from 2.3MPa before modification to 2.9MPa after modification, the glossiness is only reduced from 86.2% to 82%, and a paint film has certain functions of electric conduction (the resistance value is 1820 omega), heat conduction (the heat conductivity coefficient reaches 14w/m.k) and ultraviolet resistance and aging resistance (the yellowing index is reduced by 46%), which shows that the method for dispersing graphene by using nano-cellulose containing hemicellulose can effectively improve the comprehensive performance of the waterborne wood coating, and the waterborne wood coating is modified by using the pure nano-cellulose dispersed grapheneThe wood lacquer has better comprehensive effect.

Claims (10)

1. The method for chemically modifying the water-based wood coating by using the nano-cellulose dispersed graphene is characterized by comprising the following steps of:
nanometer cellulose water containing hemicelluloseDispersingPreparation of the solution:
sequentially carrying out extraction treatment and delignification treatment on a cellulose raw material to obtain holocellulose, and then carrying out mechanical pretreatment or chemical mechanical mixing pretreatment to obtain nano-cellulose water containing hemicelluloseDispersingLiquid;
the mechanical pretreatment comprises the following specific operation steps:
① adding deionized water into the holocellulose until the mass fraction of the holocellulose is 0.1-0.3%;
②, mechanically treating for 30-40 min by using a high-pressure homogenizing machine at 600bar to obtain nano-cellulose water dispersion containing hemicellulose, wherein the hemicellulose accounts for 20-30% of the mass of the holocellulose;
the chemical mechanical mixing pretreatment comprises the following specific operation steps:
①, adding deionized water into the holocellulose until the mass fraction of the holocellulose is 0.1-0.3%, then sequentially adding sodium bromide and a TEMPO reagent, then adding sodium hypochlorite at the adding speed of 2.5-3 mL/min under magnetic stirring, adjusting the pH value to 10-10.5, and continuously reacting until the pH value is unchanged, wherein the mass of the sodium bromide is 12-13% of the mass of the holocellulose, the mass of the TEMPO reagent is 1-2% of the mass of the holocellulose, and the mass of the sodium hypochlorite is 25-35% of the mass of the holocellulose;
②, performing suction filtration and water washing, adding deionized water and sodium chlorite, adjusting the pH value to 4-5, performing magnetic stirring at 70-80 ℃ for 1-2 h, and performing suction filtration and water washing to obtain carboxylated cellulose, wherein the mass ratio of the deionized water to the holocellulose is (95-105): 3, and the mass of the sodium chlorite is 25-35% of the mass of the holocellulose;
③, preparing a 0.1-0.3% carboxylated cellulose aqueous dispersion liquid again, and mechanically dispersing for 20-30 min by a high-speed stirrer at 14000-16000 rpm to prepare a nano cellulose aqueous dispersion liquid containing hemicellulose, wherein the hemicellulose accounts for 10-20% of the mass of the holocellulose;
secondly, stably dispersing the graphene by the nano-cellulose:
①, placing graphene or reduced graphene into the nano-cellulose aqueous dispersion containing hemicellulose obtained in the step one, mechanically dispersing for 1-20 min under the condition of high-speed stirring at 14000-16000 rpm, and then dispersing for 1-20 min under the ultrasonic condition of 900-1100W to obtain uniformly dispersed nano-cellulose/graphene aqueous dispersion, wherein the graphene or reduced graphene is the nano-cellulose aqueous dispersion containing hemicelluloseDispersing0.1-0.3% of the liquid;
② heating the nano-cellulose/graphene aqueous dispersion under the stirring condition of 600-1000 rpm, evaporating water to obtain an aqueous dispersion with the mass concentration of 10-20%, adding acetone into the aqueous dispersion to make the acetone account for 30-40% of the volume of the aqueous dispersion, dispersing for 1-20 min under the ultrasonic condition of 400-600W, and centrifuging at 8000-10000 rpm to obtain a precipitate;
③ adding acetone into the precipitate, dispersing for 1-20 min under 500-600W ultrasonic condition, and centrifuging at 8000-9000 rpm to obtain precipitate, wherein the mass ratio of the precipitate to the acetone is 1 (9-11);
④, repeating the step ③ 5-6 times, and then adding acetone into the precipitate to obtain an acetone dispersion liquid of the precipitate, wherein the solid content is 20% -25%;
thirdly, in-situ chemical modification of the nano-cellulose dispersed graphene aqueous wood lacquer:
①, mixing isophorone diisocyanate and the acetone dispersion liquid of the precipitate obtained in the second step, uniformly stirring, and heating to 65-70 ℃ for reaction for 1-2 hours;
② adding polypropylene glycol-2000 into the reaction system, and reacting for 1.5-2 h at 65-70 ℃;
③, dropwise adding the acetone solution of 1, 4-butanediol with the mass concentration of 15-25% into the reaction system, and controlling the dropwise adding speed of the acetone solution of 1, 4-butanediol to control the temperature of the system at 70-80 ℃ for reaction for 1.5-2 h;
④ dissolving dimethylolpropionic acid by using N-methyl pyrrolidone, adding the solution into a reaction system, controlling the reaction temperature to be 65-75 ℃, controlling the viscosity of the reaction solution to be lower than 100mPa & s by using acetone, and reacting for 3.5-4 h;
⑤ adding triethylamine into the reaction system, and carrying out neutralization reaction for 15-20 min;
⑥ dispersing the reaction solution by a high-speed disperser to obtain the hemicellulose-containing nanocellulose-dispersed graphene in-situ chemically modified waterborne polyurethane emulsion, namely the nanocellulose-dispersed graphene chemically modified waterborne wood coating.
2. The method for chemically modifying the aqueous wood coating by using the nano-cellulose dispersed graphene according to claim 1, which is characterized by comprising the following steps: the extraction treatment in the first step comprises the following specific steps:
crushing a cellulose raw material into powder of 90-120 meshes, and then performing benzyl alcohol extraction treatment for 10-12 hours; the benzene alcohol is toluene and absolute ethyl alcohol according to a volume ratio of 2: 1, in a mixture of the components.
3. The method for chemically modifying the aqueous wood coating by using the nano-cellulose dispersed graphene according to claim 1, which is characterized by comprising the following steps: the delignification treatment in the first step comprises the following specific steps:
① soaking the extracted cellulose powder into sodium chlorite solution with mass concentration of 1-1.2%, adjusting the pH value of the solution to 4-5 with glacial acetic acid, and magnetically heating and stirring for 1-1.5 h in a constant-temperature water bath kettle at 75-80 ℃;
②, soaking the cellulose powder obtained in the step ① in a sodium chlorite solution with the mass concentration of 1-1.2%, adjusting the pH value of the solution to 4-5 by using glacial acetic acid, and magnetically heating and stirring the solution in a constant-temperature water bath kettle at the temperature of 75-80 ℃ for 1-1.5 hours;
③, repeating the step ② 5-6 times to basically remove the lignin, then filtering and washing the obtained liquid by using a Buchner funnel until the filtrate is neutral, and finally obtaining the holocellulose.
4. The method for chemically modifying the aqueous wood coating by using the nano-cellulose dispersed graphene according to claim 1, which is characterized by comprising the following steps: the delignification treatment in the first step comprises the following specific steps:
soaking the extracted cellulose powder into a hydrogen peroxide solution with the mass concentration of 30-35%, adding magnesium silicate, and magnetically stirring at room temperature for 48-52 hours to basically remove lignin; then, filtering and washing the obtained liquid by using a Buchner funnel until the filtrate is neutral, and finally obtaining the holocellulose; wherein the mass of the magnesium silicate is 1 to 1.5 percent of the mass of the hydrogen peroxide solution.
5. The method for chemically modifying the aqueous wood coating by using the nano-cellulose dispersed graphene according to claim 1, which is characterized by comprising the following steps: the cellulose raw material in the step one is biomass raw material powder with the granularity of more than 100 meshes.
6. The method for chemically modifying the aqueous wood coating by using the nano-cellulose dispersed graphene according to claim 5, which is characterized by comprising the following steps: the biomass raw material powder is wood powder, crop straw powder, waste paper, leaves or bast fiber raw materials.
7. The method for chemically modifying the aqueous wood coating by using the nano-cellulose dispersed graphene according to claim 1, which is characterized by comprising the following steps: the preparation method of the reduced graphene in the second step specifically comprises the following steps:
and placing the graphene oxide in a hydrogen iodide solution, standing at room temperature for 20-30 min, and then alternately washing with water and ethanol for 3-5 times in sequence, wherein the water washing is performed for 1-10 min each time, and the ethanol washing is performed for 1-10 min each time, so as to obtain the reduced graphene.
8. The method for chemically modifying the aqueous wood coating by using the nano-cellulose dispersed graphene according to claim 1, which is characterized by comprising the following steps: and C, continuously stirring in the modification process of the step III.
9. The method for chemically modifying the aqueous wood coating by using the nano-cellulose dispersed graphene according to claim 1, which is characterized by comprising the following steps: and C, continuously adding the water-based acrylate emulsion into the emulsion obtained in the step three to obtain the co-mixing water-based polyurethane acrylate emulsion.
10. The method for chemically modifying the aqueous wood coating by using the nano-cellulose dispersed graphene according to claim 1, which is characterized by comprising the following steps: in the third step, the mass ratio of isophorone diisocyanate, acetone dispersion liquid of the precipitate obtained in the second step, acetone solution of polypropylene glycol-2000, 1, 4-butanediol, dimethylolpropionic acid and triethylamine is (50-60): (10-20): (45-60): (15-20): (5-8): (3-5).
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