Cellulose nanocrystal solvent-free fluid, its preparation method and its application in adhesives

Abstract

The application provides a cellulose nanocrystal solvent-free fluid, a preparation method thereof and application of the cellulose nanocrystal solvent-free fluid in adhesives. The cellulose nanocrystal solvent-free fluid has a core-crown layer structure, cellulose nanocrystals are used as the core, and flexible long-chain oligomers are grafted on the surface as the crown layer, and the cellulose nanocrystals are spherical, rod-shaped or needle-shaped. The cellulose nanocrystal solvent-free fluid prepared by the application realizes the conversion of nanocrystals from a solid phase to a liquid phase at room temperature, and a novel bio-based nanofunctional material with excellent dispersion stability and room-temperature fluidity is obtained. Excellent fluidity makes the material easy to be uniformly dispersed in an epoxy resin matrix, and the rigid core and the flexible crown layer make the material have both rigidity and flexibility, so that the epoxy resin can be reinforced and toughened as a single modifier. Meanwhile, active amine groups in the oligomer crown layer and epoxy groups in the matrix are crosslinked to improve the interfacial interaction between the spherical cellulose nanocrystals and the epoxy matrix, so that the bonding performance of the epoxy glue is effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of functional polymer materials, specifically relating to a solvent-free cellulose nanocrystal fluid, its preparation method, and its application in adhesives. Background Technology

[0002] With the increasing depletion of non-renewable resources such as petroleum, making full use of natural polymer materials has become an inevitable trend. Cellulose nanocrystals are a new type of green and environmentally friendly material derived from cellulose, the most abundant renewable resource in nature. Generally, acid hydrolysis is used to break down the amorphous regions (non-crystalline regions) of cellulose, releasing the crystalline regions and yielding cellulose nanocrystals with smaller size and higher aspect ratio. This results in significant advantages in applications such as biomedicine and nanocomposite materials. Cellulose nanocrystals are composed of dehydrated glucose linked by β-1,4 glycosidic bonds, with each repeating glucose unit containing three hydroxyl groups, giving them extremely strong hydrophilicity. This characteristic weakens the interfacial interaction with the polymer matrix, reducing the stress transfer efficiency from the reinforcing filler to the matrix. Furthermore, strong hydrogen bonds cause cellulose nanocrystals to agglomerate during processing, severely limiting their excellent performance when used as a filler phase.

[0003] Currently, the main method to prevent this is surface modification of cellulose nanocrystals, reducing the number of surface hydroxyl groups through silanization, thereby improving their dispersibility in the polymer matrix. However, surface-modified cellulose nanocrystals are macroscopically solid powders, and still require solvent assistance when used as a filler phase. Solvent-free nanofluids are a novel type of organic-inorganic hybrid material. As the name suggests, they can impart good flowability to solid nanoparticles without solvents. They are generally composed of core particles and a canopy. This core-shell structure remains stable under physical disturbances, so the entire system is considered a single component and macroscopically homogeneous. When mixed with a polymer matrix, they can effectively solve the problem of nanoparticle agglomeration. However, there are currently no reports on solvent-free cellulose nanocrystal fluids and their applications in epoxy adhesives. Summary of the Invention

[0004] To address the aforementioned problems, this invention proposes a solvent-free cellulose nanocrystal fluid, its preparation method, and its application in adhesives.

[0005] This invention first proposes a solvent-free fluid with cellulose nanocrystals, which has a core-canopy structure. The core is cellulose nanocrystals, and flexible long-chain oligomers are grafted onto the surface as a canopy. The cellulose nanocrystals are spherical, rod-shaped, or needle-shaped.

[0006] Furthermore, the flexible long-chain oligomer is composed of an organosilane and an amino-terminated polyether bonded by covalent bonds, wherein the organosilane is γ-glycidoxypropyltrimethoxysilane or β-(3,4-epoxycyclohexane)ethyltrimethoxysilane.

[0007] Furthermore, the terminal amino polyether is one of polyetheramine M-2070, polyetheramine M-1000, and polyetheramine D-2000.

[0008] Furthermore, the cellulose nanocrystals are spherical cellulose nanocrystals with a diameter of 20–100 nm after the amorphous regions in the cellulose fibers have been eliminated.

[0009] The method for preparing the above-mentioned solvent-free fluid containing cellulose nanocrystals includes the following steps:

[0010] Step 1: Preparation of flexible long-chain oligomer canopy

[0011] Equal molar amounts of organosilane and amino-terminated polyether are dissolved in alcohol solvent and stirred at 30-60°C for 12-24 hours to obtain an alcohol solution of oligomer crown.

[0012] Step 2: Preparation of solvent-free fluid from cellulose nanocrystals

[0013] Cellulose nanocrystals were dispersed in deionized water. The cellulose nanocrystal dispersion was added to an alcohol solution of oligomer canopy. After reacting at 35–50 °C for 8–12 h, most of the solvent was removed. Unreacted canopy material was removed by dialyzing in deionized water and then dried to obtain a solvent-free fluid of cellulose nanocrystals.

[0014] Furthermore, the alcohol solvent mentioned in step 1 is methanol or ethanol.

[0015] Furthermore, the dialysis operation described in step 2 uses dialysis bags with a molecular weight of 3500–5000 Da.

[0016] Furthermore, the mass ratio of cellulose nanocrystals to oligomer canopy in step 2 is 1:10 to 30.

[0017] Furthermore, the core of the solvent-free cellulose nanocrystal fluid is spherical cellulose nanocrystals after the amorphous regions in cellulose have been eliminated. The method for eliminating the amorphous regions in cellulose is ultrasonic-assisted acid hydrolysis, specifically: microcrystalline cellulose is treated with an alkaline solution, filtered, washed, and dried to obtain a white powder; the alkaline-treated white powder is treated with dimethyl sulfoxide, filtered, and washed to obtain a slurry; the slurry is added to an acid hydrolysis solution, and acid hydrolyzed at 55–75°C with ultrasonic stirring for 8–10 hours; deionized water is added to terminate the reaction, the supernatant is removed, and the process of adding deionized water and removing the supernatant is repeated until the supernatant becomes turbid. Dialysis is then performed in deionized water until the pH is constant to obtain an aqueous dispersion of spherical cellulose nanocrystals. Further, the dialysis operation uses a dialysis bag with a molecular weight of 8000–14000 Da.

[0018] Furthermore, the alkaline solution is an aqueous sodium hydroxide solution with a concentration of 5-6 mol / L, and the ratio of microcrystalline cellulose to alkaline solution is 100-120 g: 1 L.

[0019] Furthermore, the ratio of microcrystalline cellulose to dimethyl sulfoxide is 100–120 g: 1 L.

[0020] Furthermore, the acid hydrolysate is a mixture of concentrated hydrochloric acid, concentrated sulfuric acid, and deionized water in a ratio of 1-2:2-3:6-7; the ratio of the acid hydrolysate to microcrystalline cellulose is 20-30 mL / g.

[0021] This invention further proposes the application of cellulose nanocrystalline solvent-free fluid in adhesives. Using the above-mentioned cellulose nanocrystalline solvent-free fluid as a raw material, a cellulose nanocrystalline solvent-free fluid-modified epoxy adhesive is obtained by mixing the cellulose nanocrystalline solvent-free fluid, epoxy resin and amine curing agent. The content of cellulose nanocrystalline solvent-free fluid in the adhesive is 0-1 wt.%.

[0022] Furthermore, the epoxy resin is bisphenol A type E51 epoxy resin.

[0023] Furthermore, the amine curing agent is one of triethylenetetramine, isophorone diamine, or polyamide.

[0024] The principle of this invention is as follows:

[0025] The nanoscale effect and excellent mechanical properties of cellulose nanocrystals make them potential bio-based nanomaterials for polymer reinforcement. However, the hydrophilicity leading to filler agglomeration and weakened interfacial interactions greatly limits their application in epoxy adhesives. Therefore, this study designs a solvent-free cellulose nanocrystal fluid by grafting flexible long-chain oligomers onto the surface of cellulose nanocrystals, which can then be used as a single modifier to comprehensively improve the performance of epoxy adhesives.

[0026] Pretreatment of microcrystalline cellulose with NaOH aqueous solution and DMSO roughens the fiber surface, even causing cracks, exposing the internal fibrous strands. This facilitates the entry of the hydrolysate into the amorphous region of cellulose during subsequent acid hydrolysis. Then, ultrasonic-assisted acid hydrolysis removes the amorphous region (amorphous region) with poor acid resistance, releasing the crystalline region and preparing small-sized, large-specific-surface-area spherical cellulose nanocrystals. A covalent canopy is then prepared through the ring-opening reaction of organosilanes and terminal amino polyethers. The canopy is grafted onto the nanocrystal surface via the condensation reaction of hydroxyl and silanol groups on the cellulose nanocrystal surface, creating a solvent-free fluid from cellulose nanocrystals. Using the flexible oligomer canopy as a flow medium imparts excellent flow properties to the cellulose nanocrystals. The removal of the amorphous region (amorphous region) with poor acid resistance and the release of the crystalline region, along with the preparation of small-sized, large-specific-surface-area spherical cellulose nanocrystals, further increases the grafting density of the oligomer canopy, giving the solvent-free cellulose nanocrystal fluid even better flow properties.

[0027] Good dispersibility and strong filler / matrix interfacial interactions are key to improving the performance of nanoparticle-modified epoxy adhesives. Nanofluids exhibit good flowability at room temperature, promoting uniform dispersion in the epoxy matrix and preventing agglomeration. The active amine groups in the oligomer canopy also undergo cross-linking reactions with the epoxy groups in the epoxy matrix, significantly enhancing the interfacial interaction between cellulose nanocrystals and the epoxy matrix. Furthermore, the solvent-free cellulose nanocrystal fluid possesses a unique core-canopy structure. Cellulose nanocrystals, acting as rigid organic nanoparticles, provide reinforcement, while the long-chain flexible oligomer canopy not only increases the dispersibility of cellulose nanocrystals in the epoxy matrix but also provides toughening. When cellulose nanocrystals are introduced into the epoxy system as a single-component functional nanofiller, they simultaneously enhance and toughen the epoxy adhesive, thereby further improving its performance.

[0028] Beneficial effects:

[0029] 1. The flexible long-chain oligomers in solvent-free cellulose nanocrystal fluid prepared by the present invention are composed of organosilanes and terminal amino polyethers bonded by covalent bonds. Compared with ionic bonds, the amine groups exist in a protonated form. The secondary amine groups in covalent bonds can react during polymer modification. Furthermore, the covalent bonds have relatively higher bond energies than ionic bonds. The relatively stable structure has a positive effect on improving the thermal properties of polymer composites.

[0030] 2. The solvent-free fluid of cellulose nanocrystals prepared by this invention exhibits excellent flowability without the need for solvent assistance, solving the problem of easy agglomeration of cellulose nanocrystals. It maintains a flowing state at room temperature, thus improving the processing performance of cellulose nanocrystals.

[0031] 3. The present invention uses solvent-free cellulose nanocrystal fluid as a functional filler for epoxy adhesives, which shows significant modification effects. Its liquid-like behavior allows it to be uniformly dispersed in the epoxy matrix. The active amine groups in the canopy can improve the interfacial interaction between cellulose nanocrystals and epoxy matrix, and can effectively improve the adhesive performance of the adhesive under low loading.

[0032] 4. The solvent-free cellulose nanocrystal fluid provided by this invention has a special core-canopy structure. When introduced into the epoxy system as a single-component functional nanofiller, it can simultaneously enhance and toughen the epoxy system, thus broadening the application range of cellulose nanocrystals. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the synthesis route of solvent-free fluid for cellulose nanocrystals in Examples 1 and 2 of the present invention;

[0034] Figure 2 Here is a SEM image of the spherical cellulose nanocrystals from Example 1;

[0035] Figure 3 These are macroscopic images of solvent-free fluids containing cellulose nanocrystals in Examples 2, 3, and 4.

[0036] Figure 4 Infrared comparative analysis of solvent-free fluids of spherical cellulose nanocrystals from Example 1 and Example 2;

[0037] Figure 5 Example 2: Solvent-free fluid XPS structure analysis of spherical cellulose nanocrystals;

[0038] Figure 6 The DSC curve of the solvent-free fluid containing cellulose nanocrystals is shown in Example 2.

[0039] Figure 7 Thermogravimetric curve of solvent-free fluid containing cellulose nanocrystals in Example 2;

[0040] Figure 8 This is a comparison chart of the bonding strength of the epoxy adhesives in Examples 13, 14 and Comparative Example 1;

[0041] Figure 9 This is a comparison chart of the bonding strength of the epoxy adhesives in Examples 15, 16 and Comparative Example 2;

[0042] Figure 10 This is a comparison chart of the bonding strength of the epoxy adhesives in Examples 17, 18 and Comparative Example 3;

[0043] Figure 11 Here is a SEM image of the cross-section of the epoxy adhesive in Comparative Example 3;

[0044] Figure 12This is a SEM image of the cross-section of the solvent-free fluid-modified epoxy adhesive with cellulose nanocrystals from Example 18. Detailed Implementation

[0045] This invention proposes a solvent-free cellulose nanocrystal fluid, its preparation method, and its application in adhesives.

[0046] This invention first proposes a solvent-free fluid containing cellulose nanocrystals, featuring a core-canopy structure. The core consists of cellulose nanocrystals grafted onto the surface as a canopy, with the cellulose nanocrystals being spherical, rod-shaped, or needle-shaped. The flexible long-chain oligomers are covalently bonded to organosilanes and terminal amino polyethers. The organosilane is either γ-glycidyl etheroxypropyltrimethoxysilane or β-(3,4-epoxycyclohexane)ethyltrimethoxysilane. The terminal amino polyether is one of polyetheramine M-2070, polyetheramine M-1000, and polyetheramine D-2000. Optionally, the cellulose nanocrystals are spherical cellulose nanocrystals with a diameter of 20–100 nm after eliminating amorphous regions from cellulose fibers.

[0047] The method for preparing the above-mentioned solvent-free fluid containing cellulose nanocrystals includes the following steps:

[0048] Step 1: Preparation of flexible long-chain oligomer canopy

[0049] Equal molar amounts of organosilane and amino-terminated polyether are dissolved in an alcohol solvent, and the mixture is stirred at 30–60°C for 12–24 h to obtain an alcohol solution of the oligomer crown; the alcohol solvent is methanol or ethanol.

[0050] Step 2: Preparation of solvent-free fluid from cellulose nanocrystals

[0051] Cellulose nanocrystals were dispersed in deionized water. The cellulose nanocrystal dispersion was added to an alcoholic solution of the oligomer canopy. After reacting at 35–50°C for 8–12 hours, most of the solvent was removed. Unreacted canopy material was removed by dialysis in deionized water, and the mixture was dried to obtain a solvent-free fluid of cellulose nanocrystals. The dialysis operation used dialysis bags with a molecular weight of 3500–5000 Da. The mass ratio of cellulose nanocrystals to oligomer canopy was 1:10–30.

[0052] When the core of the solventless cellulose nanocrystal fluid is spherical cellulose nanocrystals after the amorphous regions in cellulose have been eliminated, the method for eliminating the amorphous regions in cellulose is ultrasonic-assisted acid hydrolysis. Specifically, the microcrystalline cellulose is treated with an alkaline solution, filtered, washed, and dried to obtain a white powder; the alkaline-treated white powder is treated with dimethyl sulfoxide, filtered, and washed to obtain a slurry; the slurry is added to an acid hydrolysis solution, and acid hydrolyzed at 55–75°C under ultrasonic stirring for 8–10 hours; deionized water is added to terminate the reaction, the supernatant is removed, and the process of adding deionized water and removing the supernatant is repeated until the supernatant becomes turbid. Dialysis is performed in deionized water until the pH is constant to obtain an aqueous dispersion of spherical cellulose nanocrystals. Further, the dialysis operation uses a dialysis bag with a molecular weight of 8000–14000 Da. The alkaline solution is an aqueous sodium hydroxide solution with a concentration of 5–6 mol / L, and the ratio of microcrystalline cellulose to alkaline solution is 100–120 g:1 L. The ratio of microcrystalline cellulose to dimethyl sulfoxide is also 100–120 g:1 L.

[0053] Furthermore, the acid hydrolysate is a mixture of concentrated hydrochloric acid, concentrated sulfuric acid, and deionized water in a ratio of 1-2:2-3:6-7; the ratio of the acid hydrolysate to microcrystalline cellulose is 20-30 mL / g.

[0054] Example 1 is a specific embodiment of the method for preparing spherical cellulose nanocrystals; Examples 2-9 are specific embodiments of preparing solvent-free fluids using pretreated spherical cellulose nanocrystals; Examples 10-12 are specific embodiments of preparing solvent-free fluids using untreated cellulose nanocrystals.

[0055] Example 1:

[0056] Step 1: Treat 3g of microcrystalline cellulose with 25mL of 5mol / L sodium hydroxide solution at 80℃ with mechanical stirring for 3h. Filter and wash with water until pH=7. Dry at 40℃ for 24h to obtain a white powder. Treat the alkali-treated powder with 25mL of dimethyl sulfoxide at 80℃ with mechanical stirring for 3h. Filter and wash with water three times to obtain a slurry. Add the slurry to 78mL of acid hydrolysis solution (prepared with concentrated hydrochloric acid, concentrated sulfuric acid, and deionized water in a ratio of 1:3:6). Acid hydrolyze at 65℃ by mechanical stirring and ultrasonic treatment for 8h. Add deionized water to terminate the reaction. Centrifuge at 9000r / min for 10min, discard the supernatant, add water, shake, and centrifuge again. Repeat the operation until the upper layer is turbid. Dialyze in deionized water using a dialysis bag with a molecular weight of 10000Da until the pH is constant to obtain an aqueous dispersion of spherical cellulose nanocrystals.

[0057] Example 2:

[0058] Step 1: Add 7.5 mmol of γ-glycidoxypropyltrimethoxysilane to 35.45 g of methanol, and then add it dropwise to a methanol solution of 7.5 mmol of polyetheramine M-2070 (150 g of methanol). Stir the mixture magnetically at 50 °C for 12 h to obtain an alcohol solution of the oligomer crown.

[0059] Step 2: Add the aqueous dispersion of 0.2g of spherical cellulose nanocrystals obtained in Example 1 to the alcohol solution of the oligomer canopy. After reacting at 35°C for 12h, remove most of the solvent. Remove the unreacted canopy material in deionized water using a dialysis bag with a molecular weight of 3500Da. Dry to obtain a solvent-free fluid of spherical cellulose nanocrystals.

[0060] Example 3:

[0061] Step 1: Add 10 mmol of γ-glycidoxypropyltrimethoxysilane to 47.27 g of methanol, and then add it dropwise to a methanol solution of 10 mmol of polyetheramine M-2070 (200 g of methanol). Stir the mixture magnetically at 50 °C for 12 h to obtain an alcohol solution of the oligomer crown.

[0062] Step 2: Add the aqueous dispersion of 0.2g of spherical cellulose nanocrystals obtained in Example 1 to the alcohol solution of the oligomer canopy. After reacting at 35°C for 12h, remove most of the solvent. Remove the unreacted canopy material in deionized water using a dialysis bag with a molecular weight of 3500Da. Dry to obtain a solvent-free fluid of spherical cellulose nanocrystals.

[0063] Example 4:

[0064] Step 1: Add 12.5 mmol of γ-glycidoxypropyltrimethoxysilane to 59.09 g of methanol, and then add it dropwise to a methanol solution of 12.5 mmol of polyetheramine M-2070 (250 g of methanol). Stir the mixture magnetically at 50 °C for 12 h to obtain an alcohol solution of the oligomer crown.

[0065] Step 2: Add the aqueous dispersion of 0.2g of spherical cellulose nanocrystals obtained in Example 1 to the alcohol solution of the oligomer canopy. After reacting at 35°C for 12h, remove most of the solvent. Remove the unreacted canopy material in deionized water using a dialysis bag with a molecular weight of 3500Da. Dry to obtain a solvent-free fluid of spherical cellulose nanocrystals.

[0066] Example 5:

[0067] Step 1: Add 12.5 mmol of γ-glycidoxypropyltrimethoxysilane to 59.09 g of methanol, and then add it dropwise to a methanol solution of 12.5 mmol of polyetheramine M-2070 (250 g of methanol). Stir the mixture magnetically at 30 °C for 12 h to obtain an alcohol solution of the oligomer crown.

[0068] Step 2: Add the aqueous dispersion of 0.2g of spherical cellulose nanocrystals obtained in Example 1 to the alcohol solution of the oligomer canopy. After reacting at 35°C for 12h, remove most of the solvent. Remove the unreacted canopy material in deionized water using a dialysis bag with a molecular weight of 3500Da. Dry to obtain a solvent-free fluid of spherical cellulose nanocrystals.

[0069] Example 6:

[0070] Step 1: Add 12.5 mmol of β-(3,4-epoxycyclohexane)ethyltrimethoxysilane to 59.09 g of methanol, and then add it dropwise to a methanol solution of 12.5 mmol of polyetheramine M-2070 (250 g of methanol). Stir the mixture magnetically at 40 °C for 12 h to obtain an alcohol solution of the oligomer crown.

[0071] Step 2: Add the aqueous dispersion of 0.2g of spherical cellulose nanocrystals obtained in Example 1 to the alcohol solution of the oligomer canopy. After reacting at 35°C for 12h, remove most of the solvent. Remove the unreacted canopy material in deionized water using a dialysis bag with a molecular weight of 3500Da. Dry to obtain a solvent-free fluid of spherical cellulose nanocrystals.

[0072] Example 7:

[0073] Step 1: Add 12.5 mmol of γ-glycidoxypropyltrimethoxysilane to 59.09 g of methanol, and then add it dropwise to a methanol solution of 12.5 mmol of polyetheramine M-2070 (250 g of methanol). Stir the mixture magnetically at 50 °C for 24 h to obtain an alcohol solution of the oligomer crown.

[0074] Step 2: Add the aqueous dispersion of 0.2g of spherical cellulose nanocrystals obtained in Example 1 to the alcohol solution of the oligomer canopy. After reacting at 35°C for 12h, remove most of the solvent. Remove the unreacted canopy material in deionized water using a dialysis bag with a molecular weight of 3500Da. Dry to obtain a solvent-free fluid of spherical cellulose nanocrystals.

[0075] Example 8:

[0076] Step 1: Add 12.5 mmol of γ-glycidoxypropyltrimethoxysilane to 59.09 g of methanol, and then add it dropwise to a methanol solution of 12.5 mmol of polyetheramine M-1000 (250 g of methanol). Stir the mixture magnetically at 60 °C for 24 h to obtain an alcohol solution of the oligomer crown.

[0077] Step 2: Add the aqueous dispersion of 0.2g of spherical cellulose nanocrystals obtained in Example 1 to the alcohol solution of the oligomer canopy. After reacting at 35°C for 12h, remove most of the solvent. Remove the unreacted canopy material in deionized water using a dialysis bag with a molecular weight of 3500Da. Dry to obtain a solvent-free fluid of spherical cellulose nanocrystals.

[0078] Example 9:

[0079] Step 1: Add 12.5 mmol of γ-glycidoxypropyltrimethoxysilane to 59.09 g of methanol, and then add it dropwise to a methanol solution of 12.5 mmol of polyetheramine D-2000 (250 g of methanol). Stir the mixture magnetically at 60 °C for 24 h to obtain an alcohol solution of the oligomer crown.

[0080] Step 2: Add the aqueous dispersion of 0.2g of spherical cellulose nanocrystals obtained in Example 1 to the alcohol solution of the oligomer canopy. After reacting at 35°C for 8 hours, remove most of the solvent. Remove unreacted canopy material in deionized water using a dialysis bag with a molecular weight of 4000 Da. Dry to obtain solvent-free fluid of spherical cellulose nanocrystals.

[0081] Example 10:

[0082] Step 1: Add 12.5 mmol of γ-glycidoxypropyltrimethoxysilane to 59.09 g of methanol, and then add it dropwise to a methanol solution of 12.5 mmol of polyetheramine M-2070 (250 g of methanol). Stir the mixture magnetically at 60 °C for 24 h to obtain an alcohol solution of the oligomer crown.

[0083] Step 2: Add an aqueous dispersion of 0.2g of rod-shaped cellulose nanocrystals to an alcoholic solution of oligomer canopy. After reacting at 45°C for 9 hours, remove most of the solvent. Remove unreacted canopy material in deionized water using a dialysis bag with a molecular weight of 5000 Da. Dry to obtain a solvent-free fluid of rod-shaped cellulose nanocrystals.

[0084] Example 11:

[0085] Step 1: Add 12.5 mmol of γ-glycidoxypropyltrimethoxysilane to 59.09 g of methanol, and then add it dropwise to a methanol solution of 12.5 mmol of polyetheramine M-1000 (250 g of methanol). Stir the mixture magnetically at 60 °C for 24 h to obtain an alcohol solution of the oligomer crown.

[0086] Step 2: Add an aqueous dispersion of 0.2g of rod-shaped cellulose nanocrystals to an alcoholic solution of oligomer canopy. After reacting at 50°C for 10 hours, remove most of the solvent. Remove unreacted canopy material in deionized water using a dialysis bag with a molecular weight of 3500 Da. Dry to obtain a solvent-free fluid of rod-shaped cellulose nanocrystals.

[0087] Example 12:

[0088] Step 1: Add 12.5 mmol of γ-glycidoxypropyltrimethoxysilane to 59.09 g of methanol, and then add it dropwise to a methanol solution of 12.5 mmol of polyetheramine D-2000 (250 g of methanol). Stir the mixture magnetically at 60 °C for 24 h to obtain an alcohol solution of the oligomer crown.

[0089] Step 2: Add an aqueous dispersion of 0.2g of needle-shaped cellulose nanocrystals to an alcoholic solution of oligomer canopy. After reacting at 50°C for 11 hours, remove most of the solvent. Remove unreacted canopy material in deionized water using a dialysis bag with a molecular weight of 3500 Da. Dry to obtain a solvent-free fluid of needle-shaped cellulose nanocrystals.

[0090] This invention also proposes the application of the solvent-free cellulose nanocrystal fluid prepared by the above method in the field of epoxy adhesives, including the following steps:

[0091] Using the above-mentioned solvent-free cellulose nanocrystal fluid as a raw material, a cellulose nanocrystal solvent-free fluid-modified epoxy adhesive is obtained by mixing the solvent-free cellulose nanocrystal fluid, epoxy resin, and amine curing agent. The content of the solvent-free cellulose nanocrystal fluid in the adhesive is 0-1 wt.%. The epoxy resin is bisphenol A type E51 epoxy resin. The amine curing agent is one of triethylenetetramine, isophorone diamine, and polyamide.

[0092] Examples 13-18 are application examples of solvent-free cellulose nanocrystal fluids in epoxy adhesives.

[0093] Example 13:

[0094] The solvent-free cellulose nanocrystal fluid obtained in Example 2 was added to 10g of epoxy resin E51 and 1.24g of triethylenetetramine, with the addition amount controlled at 0.5wt.%. After stirring and mixing evenly, the air bubbles were removed by vacuuming to obtain epoxy adhesive.

[0095] Example 14:

[0096] The solvent-free cellulose nanocrystal fluid obtained in Example 2 was added to 10g of epoxy resin E51 and 1.24g of triethylenetetramine, with the addition amount controlled at 1wt.%. After stirring and mixing evenly, the air bubbles were removed by vacuuming to obtain epoxy adhesive.

[0097] Example 15:

[0098] The solvent-free cellulose nanocrystal fluid obtained in Example 2 was added to 10g of epoxy resin E51 and 2.17g of isophorone diamine, with the addition amount controlled at 0.5wt.%. After stirring and mixing evenly, the air bubbles were removed by vacuuming to obtain epoxy adhesive.

[0099] Example 16:

[0100] The solvent-free cellulose nanocrystal fluid obtained in Example 2 was added to 10g of epoxy resin E51 and 2.17g of isophorone diamine, with the addition amount controlled at 1wt.%. After stirring and mixing evenly, the air bubbles were removed by vacuuming to obtain epoxy adhesive.

[0101] Example 17:

[0102] The solvent-free cellulose nanocrystal fluid obtained in Example 2 was added to 10g of epoxy resin E51 and 5g of polyamide, with the addition amount controlled at 0.5wt.%. After stirring and mixing evenly, the air bubbles were removed by vacuuming to obtain epoxy adhesive.

[0103] Example 18:

[0104] The solvent-free cellulose nanocrystal fluid obtained in Example 2 was added to 10g of epoxy resin E51 and 5g of polyamide, with the addition amount controlled at 1wt.%. After stirring and mixing evenly, the air bubbles were removed by vacuuming to obtain epoxy adhesive.

[0105] Comparative Example 1: 10g of epoxy resin E51 and 1.24g of triethylenetetramine were compounded to obtain an epoxy adhesive.

[0106] Comparative Example 2: 10g of epoxy resin E51 and 2.17g of isophorone diamine were compounded to obtain an epoxy adhesive.

[0107] Comparative Example 3: 10g of epoxy resin E51 and 5g of polyamide were compounded to obtain an epoxy adhesive.

[0108] Analysis and Testing

[0109] Figure 1 These are schematic diagrams illustrating the synthesis routes of solvent-free fluids for cellulose nanocrystals in Examples 1 and 2 of this invention. Figure 1 As can be seen, microcrystalline cellulose undergoes swelling treatment and ultrasonic-assisted acid hydrolysis in step 1 to remove amorphous regions on its surface, yielding spherical cellulose nanocrystals. In step 2, organosilanes and amino-terminated polyethers generate flexible long-chain oligomer canopies through ring-opening reactions of amine and epoxy groups. In step 3, the oligomer canopies undergo dehydration condensation reactions with the hydroxyl groups on the surface of the cellulose nanocrystals, and are covalently grafted onto the surface of the cellulose nanocrystals, endowing them with excellent flowability.

[0110] Figures 2-7 The results are the test results for Examples 1 to 4.

[0111] Figure 2 The image shows the SEM image of the spherical cellulose nanocrystals in Example 1. It can be seen that all the cellulose nanocrystals are approximately spherical in appearance and their size distribution is concentrated below 100 nm, indicating that the target product, spherical cellulose nanocrystals, has been successfully prepared.

[0112] Figure 3 The images show macroscopic images of the solvent-free cellulose nanocrystal fluids in Examples 2, 3, and 4. It can be seen that after being placed vertically on a glass plate, all three products exhibit excellent flow properties. Furthermore, the flow properties of the products are even better with the increase of the content of the flexible oligomer canopy, which initially indicates the successful synthesis of the solvent-free cellulose nanocrystal fluids.

[0113] Figure 4 Infrared comparative analysis of solvent-free fluids of spherical cellulose nanocrystals from Example 1 and Example 2 shows that at 1100 cm⁻¹... -1 and 845cm -1 The absorption peaks at the point are caused by Si-O-Si and O-Si-O, indicating that the flexible canopy was successfully grafted onto the surface of cellulose nanocrystals.

[0114] Figure 5 XPS structural analysis of the solvent-free fluid containing spherical cellulose nanocrystals in Example 2 was conducted to further analyze the chemical composition of the product. The solvent-free fluid containing cellulose nanocrystals contained four elements: oxygen, nitrogen, carbon, and silicon. The C1s fine spectrum showed significant CO, CC, C-Si, and CN bonds, representing cellulose nanocrystals and flexible oligomer canopies, indicating the successful synthesis of the solvent-free fluid containing cellulose nanocrystals.

[0115] Figure 6 The DSC curve of the solvent-free fluid of cellulose nanocrystals in Example 2 clearly shows a melting peak at 3°C, indicating that the product exhibits a liquid-like state at room temperature.

[0116] Figure 7 The thermogravimetric curve of the solvent-free fluid of cellulose nanocrystals in Example 2 shows that there was no significant weight loss of the product at 200°C, and there was no solvent in the system, proving that the fluidity of the solvent-free fluid of cellulose nanocrystals is inherent.

[0117] Figures 8-12 This study analyzes and tests the adhesive properties and toughening mechanism of the solvent-free fluid-modified epoxy adhesives based on cellulose nanocrystals from Examples 13-18.

[0118] Figure 8 The image shows a comparison of the bonding strengths of the epoxy adhesives in Examples 13, 14, and Comparative Example 1, which are 5.30 MPa, 4.94 MPa, and 4.29 MPa, respectively.

[0119] Figure 9 The image shows a comparison of the bonding strength of the epoxy adhesives in Examples 15, 16, and Comparative Example 2, which are 12.52 MPa, 11.36 MPa, and 9.88 MPa, respectively.

[0120] Figure 10 The chart shows a comparison of the adhesive strengths of the epoxy adhesives in Examples 17, 18, and Comparative Example 3, which are 12.62 MPa, 14.70 MPa, and 10.86 MPa, respectively. This demonstrates that the solvent-free cellulose nanocrystal fluid has a significant reinforcing effect on the epoxy adhesive, greatly improving its adhesive strength.

[0121] Figure 11 The SEM images of the cross-section of the epoxy adhesive in Comparative Example 3 show that the cross-section of the pure epoxy adhesive exhibits typical brittle fracture, with a very smooth surface and river-like cracks.

[0122] Figure 12 The SEM image of the cross-section of the solvent-free cellulose nanocrystal-modified epoxy adhesive in Example 18 shows that the cross-section of the modified epoxy adhesive is very rough. Furthermore, due to the crack deflection effect of the solvent-free cellulose nanocrystal-modified fluid, the adhesive absorbs more energy, resulting in crisscrossing cracks. This demonstrates that the uniform dispersion of the spherical cellulose nanocrystal-modified fluid and its enhanced interfacial interaction with the epoxy matrix effectively improve the transmission of the failure load when the adhesive is under stress.

Claims

1. A method for preparing a solvent-free fluid containing cellulose nanocrystals, characterized in that, Includes the following steps: Step 1: Preparation of flexible long-chain oligomer canopy Equal molar amounts of organosilane and amino-terminated polyether were dissolved in alcohol solvent and stirred at 30-60 °C for 12-24 h to obtain an alcohol solution of oligomer crown. Step 2: Preparation of solvent-free fluid from cellulose nanocrystals Cellulose nanocrystals were dispersed in deionized water. The cellulose nanocrystal dispersion was added to an alcohol solution of oligomer canopy. After reacting at 35-50 °C for 8-12 h, most of the solvent was removed. Unreacted canopy material was removed by dialyzing in deionized water and then dried to obtain a solvent-free fluid of cellulose nanocrystals. The solvent-free fluid of the cellulose nanocrystal has a core-canopy structure, with cellulose nanocrystals as the core and flexible long-chain oligomers grafted onto the surface as the canopy. The cellulose nanocrystals are spherical, rod-shaped or needle-shaped. The flexible long-chain oligomer is composed of an organosilane and an amino-terminated polyether bonded together by covalent bonds. The organosilane is γ-glycidoxypropyltrimethoxysilane or β-(3,4-epoxycyclohexane)ethyltrimethoxysilane.

2. The method for preparing solvent-free cellulose nanocrystal fluid according to claim 1, characterized in that, The core is spherical cellulose nanocrystals after the amorphous regions in cellulose have been eliminated. The method for eliminating the amorphous regions in cellulose is ultrasonic-assisted acid hydrolysis, specifically: microcrystalline cellulose is treated with an alkaline solution, filtered, washed, and dried to obtain a white powder; the alkaline-treated white powder is treated with dimethyl sulfoxide, filtered, and washed to obtain a slurry; the slurry is added to the acid hydrolysis solution and acid hydrolyzed at 55-75 °C under ultrasonic stirring for 8-10 h; deionized water is added to terminate the reaction, the supernatant is removed, and the process of adding deionized water and removing the supernatant is repeated until the supernatant becomes turbid. The supernatant is then dialyzed in deionized water until the pH is constant to obtain an aqueous dispersion of spherical cellulose nanocrystals.

3. The method for preparing solvent-free cellulose nanocrystal fluid according to claim 1, characterized in that, The mass ratio of cellulose nanocrystals to oligomer canopy in step 2 is 1:10~30.

4. The method for preparing solvent-free cellulose nanocrystal fluid according to claim 2, characterized in that, The acid hydrolysate is a mixture of concentrated hydrochloric acid, concentrated sulfuric acid and deionized water in a ratio of 1~2:2~3:6~7; the ratio of the acid hydrolysate to microcrystalline cellulose is 20~30 mL / g.

5. The method for preparing solvent-free cellulose nanocrystal fluid according to claim 1, characterized in that, The alcohol solvent mentioned in step 1 is methanol or ethanol.

6. The solvent-free cellulose nanocrystal fluid prepared according to any one of claims 1 to 5, characterized in that, The terminal amino polyether is one of polyetheramine M-2070, polyetheramine M-1000 and polyetheramine D-2000.

7. The solvent-free cellulose nanocrystal fluid prepared by the method according to any one of claims 1 to 5, characterized in that, The cellulose nanocrystals are spherical cellulose nanocrystals with a diameter of 20~100nm after the amorphous regions in cellulose fibers have been eliminated.

8. The application of the solvent-free cellulose nanocrystalline fluid as described in any one of claims 6 to 7 in adhesives, characterized in that, A cellulose nanocrystal solvent-free fluid modified epoxy adhesive is obtained by mixing the cellulose nanocrystal solvent-free fluid, epoxy resin and amine curing agent, using cellulose nanocrystal solvent-free fluid as raw material. The content of cellulose nanocrystal solvent-free fluid in the adhesive is 0~1 wt.%.

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