MWCNTs-COOH modified epoxy resin, preparation method and application thereof, and modified epoxy adhesive

Abstract

The invention belongs to the technical field of adhesives, and particularly relates to MWCNTs-COOH modified epoxy resin, a preparation method and application thereof and a modified epoxy adhesive. The preparation method comprises the following steps: firstly, mixing MWCNTs-COOH with acetone, and carrying out ultrasonic treatment to obtain a mixture; and adding epoxy resin into the mixture, carrying out intermittent ultrasonic treatment, and drying, thereby obtaining the product. According to the preparation method disclosed by the invention, an epoxy resin matrix is modified by using MWCNTs-COOH in a manner of combining a specific solvent and intermittent ultrasonic waves, and an experiment shows that the toughness of the epoxy adhesive can be remarkably improved by adding the MWCNTs-COOH. A tensile shear loading experiment on a single lap joint can show that the modification effect is better by only adding the nanofiber MWCNTs-COOH with a specific content, and the shear strength and the fracture energy are remarkably improved respectively.

Description

Technical Field

[0001] The invention belongs to the technical field of adhesives, and particularly relates to a MWCNTs-COOH modified epoxy resin, a preparation method and application thereof, and a modified epoxy adhesive. Background Art

[0002] Epoxy adhesives offer excellent adhesion, chemical stability, high and low temperature resistance, low shrinkage, and low cost. They are widely used for bonding a variety of metals and have become an indispensable foundational material in aerospace, automotive, military, and other fields. However, cured epoxy adhesives have a high crosslink density, which results in poor crack propagation within the cured adhesive layer after interfacial failure, making them susceptible to brittle fracture. Furthermore, epoxy adhesives have poor toughness and are prone to failure under fatigue or impact conditions, which has become a major factor restricting their development.

[0003] Currently, modifiers are commonly added to adhesives to optimize their mechanical properties, thereby improving the toughness of bonded joints. Modification methods include physical and chemical modifications. Physical modification involves adding solid fillers to the adhesive to form a composite structure, leveraging the filler's superior properties to improve the adhesive's shortcomings, such as increasing toughness and temperature resistance. Chemical modification involves adding specific chemical modifiers to the adhesive, which, through chemical reactions, alter specific functional groups within the adhesive to achieve changes in its physical and chemical properties.

[0004] For example, Chinese invention patent publication No. CN105694790A discloses a rapidly disassembled epoxy adhesive and a preparation and disassembly method thereof. The epoxy adhesive comprises the following raw materials in parts by weight: 20-50 parts of epoxy resin; 10-20 parts of epoxy diluent; 50-80 parts of a mixture containing a furan-maleimide adduct; 40-70 parts of a curing agent; 0.5-2 parts of amino-modified carbon nanotubes; and 5-20 parts of thermally expandable microspheres. The mixture containing the furan-maleimide adduct is a mixture of diglycidyl ether of a furan-maleimide adduct and furan glycidyl ether. However, this invention requires the additional preparation of amino-modified carbon nanotubes, which complicates the preparation process of the epoxy adhesive. Furthermore, the addition of thermally expandable microspheres increases production costs, making the epoxy adhesive unsuitable for industrial preparation. Furthermore, the epoxy adhesive has low tensile shear strength and poor mechanical properties.

[0005] Another Chinese invention patent publication number CN117844418A discloses a carbon nanotube-modified epoxy resin adhesive and its preparation method and application. The preparation method of the carbon nanotube-modified epoxy resin adhesive includes the following steps: (1) mixing epoxy resin with an organic solvent to obtain a mixed solution A; mixing and dispersing the mixed solution A and multi-walled carbon nanotubes modified with polyethyleneimine to obtain a dispersion B; heating the dispersion B to obtain a carbon nanotube-modified epoxy resin matrix; (2) mixing a curing agent with the carbon nanotube-modified epoxy resin matrix obtained in step (1), heating and curing, and obtaining the adhesive. However, this invention requires heating the multi-walled carbon nanotubes and polyethyleneimine at 110-130°C for 11-13 hours, and the curing process adopts step-by-step heating. The overall process of preparing the adhesive is complicated and costly, which is not conducive to large-scale production.

[0006] In view of this, there is an urgent need in the art to provide a modified epoxy adhesive with better mechanical properties, simple preparation process and low production cost. Summary of the Invention

[0007] In view of the problems existing in the prior art, the present invention provides a MWCNTs-COOH modified epoxy resin, a preparation method and application thereof, and a modified epoxy adhesive.

[0008] To achieve the above object, the technical solution adopted by the present invention is as follows: A method for preparing a MWCNTs-COOH modified epoxy resin comprises the following steps: (1) First, MWCNTs-COOH was mixed with acetone and ultrasonicated to obtain a mixture; (2) Add epoxy resin to the mixture, perform intermittent ultrasonic treatment, and dry to obtain the product.

[0009] Preferably, the mass ratio of the MWCNTs-COOH, acetone and epoxy resin is 0.15-0.3:30:20.

[0010] Preferably, the frequency of the ultrasound in step (1) is 20-30 kHz, the power of the ultrasound is 200-250 W, and the time of the ultrasound is 8-12 min.

[0011] The purpose of this step is to pre-disperse the nanofibers MWCNTs-COOH in the solvent, thereby improving the dispersion stability of the nanofibers in the solvent and reducing the agglomeration and precipitation of the nanofibers in the epoxy resin.

[0012] Preferably, the epoxy resin in step (2) is DP460 epoxy resin.

[0013] Preferably, the intermittent ultrasonic process in step (2) comprises: ultrasonicating for 4-6 min at 4-8°C, 20-30 kHz and 200-250 W, and then stirring at 100-120 rpm for 4-6 min, for a total of 20-40 min.

[0014] The purpose of this step is: ice-water bath and intermittent operation can effectively control the system temperature, avoiding local overheating caused by ultrasound, which may lead to degradation of epoxy resin molecular chains or destruction of cross-linked structure; timely mechanical stirring helps to break up the new agglomerations of nanofibers formed by van der Waals forces, and promotes the uniform dispersion of MWCNTs-COOH in epoxy resin.

[0015] Preferably, the drying process in step (2) comprises: stirring at 100-120 rpm for 4-6 min every 0.5-1.5 h at 40-50° C. for a total of 8-12 h to remove the acetone solvent.

[0016] The present invention also provides MWCNTs-COOH modified epoxy resin prepared by the above preparation method.

[0017] The present invention also provides a modified epoxy adhesive comprising the MWCNTs-COOH modified epoxy resin and a curing agent.

[0018] Preferably, the amount of MWCNTs-COOH added to the modified adhesive is 0.5-1 wt.%.

[0019] The present invention also provides the use of the modified epoxy resin or the modified epoxy adhesive in adhesive joints.

[0020] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses a specific solvent combined with intermittent ultrasound to modify the epoxy resin matrix using MWCNTs-COOH to prepare epoxy adhesive dumbbell-shaped specimens. Static tensile tests on the dumbbell-shaped specimens and combined with scanning electron microscopy analysis revealed that compared with pure epoxy adhesives, the modified epoxy adhesives with the addition of MWCNTs-COOH had improved ultimate tensile strength, elongation at break, and fracture energy. The cross-section of the specimens was rougher and had more obvious wrinkles, indicating that the addition of MWCNTs-COOH significantly improved the toughness of the epoxy adhesive.

[0021] (2) The present invention uses aluminum plate as the substrate and epoxy adhesive as the adhesive to prepare a single lap joint. Tensile shear loading experiments on the single lap joint show that, compared with pure epoxy adhesive, the addition of a specific amount of nanofiber MWCNTs-COOH has a better modification effect, with significantly increased shear strength and fracture energy. When the addition amount of MWCNTs-COOH exceeds a certain level, agglomeration occurs, resulting in a decrease in mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Flow chart for the preparation of MWCNTs-COOH modified epoxy resin.

[0023] Figure 2 The fracture morphology of the standard dumbbell-shaped specimen prepared by the modified epoxy resin in Example 1.

[0024] Figure 3 This is the fracture morphology of the standard dumbbell-shaped specimen prepared from the epoxy resin of Comparative Example 3.

[0025] Figure 4 This is the SEM image of the tensile cross-section of a standard dumbbell-shaped specimen prepared from the modified epoxy resin in Example 1.

[0026] Figure 5 This is the SEM image of the tensile cross-section of a standard dumbbell-shaped specimen prepared from the epoxy resin of Comparative Example 3. DETAILED DESCRIPTION

[0027] It is worth noting that the raw materials used in this invention are all commercially available products. MWCNTs-COOH, carboxylated multi-walled carbon nanotubes (with performance parameters shown in Table 1), were purchased from Shenzhen Suiheng Technology Co., Ltd.; the epoxy resin and curing agent were both derived from 3M's DP460 two-component epoxy adhesive.

[0028] Table 1 Performance parameters of MWCNTs-COOH

[0029] Example 1 A preparation method of MWCNTs-COOH modified epoxy resin (flow chart as shown in FIG Figure 1 As shown), the steps are as follows: (1) Weigh 0.15 g of MWCNTs-COOH and place it in a clean 50 mL container. Then, add 30 g of acetone and stir at 120 rpm for 5 min. Then, place it in an ultrasonic cell disruptor and sonicate continuously at 25 kHz frequency and 225 W power for 10 min to obtain a mixture.

[0030] (2) After adding 20 g of epoxy resin to the mixture, intermittent ultrasonication was performed. The entire ultrasonication process was as follows: ultrasonication for 5 min at 4°C, 25 kHz, and 225 W, followed by stirring at 120 rpm for 5 min, for a total of 30 min. The mixture was then placed in a constant temperature drying oven at 47°C and stirred at 120 rpm for 5 min every 1.5 h for a total of 12 h to remove the acetone solvent, thereby obtaining MWCNTs-COOH modified epoxy resin.

[0031] Example 2 A method for preparing a MWCNTs-COOH modified epoxy resin comprises the following steps: (1) First, weigh 0.3 g of MWCNTs-COOH and place it in a clean 50 mL container. Then, add 30 g of acetone and stir it at 120 rpm for 5 min. Then, place it in an ultrasonic cell disruptor and sonicate it continuously at 30 kHz frequency and 200 W power for 10 min to obtain a mixture.

[0032] (2) After adding 20 g of epoxy resin to the mixture, intermittent ultrasonication was performed. The entire ultrasonication process was as follows: ultrasonication for 4 min at 8°C, 30 kHz, and 200 W, followed by stirring at 100 rpm for 6 min, for a total of 30 min. The mixture was then placed in a constant temperature drying oven at 40°C and stirred at 100 rpm for 4 min every 1.5 h for a total of 12 h to remove the acetone solvent, thereby obtaining MWCNTs-COOH modified epoxy resin.

[0033] Comparative Example 1 Compared with Example 1, the only difference is that acetone is replaced by butyl acetate.

[0034] Comparative Example 2 Compared with Example 1, the only difference is the ultrasonication method in step (2), specifically: ultrasonication at 4°C, 25 kHz and 225 W for 30 min.

[0035] Comparative Example 3 Epoxy resin is derived from commercially available DP460 two-component epoxy adhesive.

[0036] Comparative Example 4 Compared with Example 1, the only difference is that the added amount of MWCNTs-COOH is 0.6 g.

[0037] Test Example 1 To determine the mechanical properties of the modified adhesive, this test example referenced EN ISO 527-2012. Static tensile tests were performed on standard dumbbell-shaped specimens prepared using the epoxy resins of Example 1 and Comparative Examples 1-3. Scanning electron microscopy (SEM) was used to analyze the fracture surfaces of typical fractured specimens. Three replicate specimens were used in each experiment.

[0038] 1. The preparation process of dumbbell-shaped specimens is as follows: (1) Mixing. Add an accurately measured amount of curing agent to the epoxy resin (the mass ratio of epoxy resin to curing agent is 2:1) and stir slowly in a clockwise direction using a glass rod.

[0039] (2) Degassing. Due to factors such as ultrasonic and stirring treatment, bubbles may exist in the mixed epoxy adhesive, which needs to be removed. Place the mixed adhesive in a vacuum pump and evacuate it. To prevent the adhesive from overflowing during the degassing process, pour an appropriate amount of adhesive each time. After the degassing is completed, pour the next layer and repeat the process until there are no obvious bubbles in the adhesive.

[0040] (3) Mold cleaning. Use alcohol cotton pads to clean the polytetrafluoroethylene mold to avoid residual dust, oil or residues from previous experiments on the mold surface that may affect the surface quality and internal structure of the specimen and lead to inaccurate mechanical properties test results.

[0041] (4) Pouring and curing. The degassed adhesive is slowly injected into the mold using a syringe in batches. Each time a single layer of adhesive is injected, carefully check for visible bubbles. If there are bubbles, use a wire to break and remove them. After pouring each layer, use a scraper to gently scrape along the mold surface to remove excess adhesive, so that the adhesive surface is flat and flush with the upper surface of the mold. The specimen is placed at room temperature for 7 days to cure.

[0042] (5) Demolding and post-processing. After curing is completed, demolding is performed and the dumbbell-shaped specimen is taken out. To ensure the accuracy of the tensile test data, the specimen needs to be pre-processed. Use a knife to carefully remove burrs, flash, and protrusions at the clamping end to avoid interference of eccentric bending moment on the test results.

[0043] 2. The static stretching test is as follows: Tensile tests were conducted on a 20 kN universal testing machine. The specimen was placed between upper and lower clamps, ensuring that its central axis remained perpendicular to the ground. During this process, a DIC system was used to measure the specimen's three-dimensional strain and displacement contactlessly. The tensile tests employed displacement-controlled loading at a rate of 2 mm / min. Load-displacement data and DIC measurements were collected simultaneously throughout the tensile process until specimen failure occurred, at which point the DIC measurement and the tensile machine were discontinued.

[0044] 3. The experimental results are as follows: The fracture morphologies of the standard dumbbell-shaped specimens prepared from the modified epoxy resin of Example 1 and the epoxy resin of Comparative Example 3 are as follows: Figure 2 and Figure 3 As shown, the SEM images of the tensile cross-section are Figure 4 and Figure 5 It can be seen that the dumbbell-shaped specimen with the addition of nanofiber MWCNTs-COOH (ie, Example 1) has a rougher cross section and more obvious wrinkles, which can effectively improve the toughness of the adhesive.

[0045] The mechanical properties data of the dumbbell-shaped specimens are shown in Table 2. It can be seen from Table 2 that the addition of MWCNTs-COOH to the epoxy adhesive can significantly improve its mechanical properties. Compared with the unmodified epoxy adhesive dumbbell-shaped specimens, the elastic modulus, ultimate tensile strength, failure strength, elongation at break and fracture energy of the dumbbell-shaped specimens prepared in Example 1 (i.e., incorporating 0.5wt.% of WCNTs-COOH) increased by 2.49%, 9.71%, 8.55%, 39.09% and 61.97% respectively; compared with other reaction solvents, such as butyl acetate, the elastic modulus, ultimate tensile strength, failure strength, elongation at break and fracture energy of the dumbbell-shaped specimens prepared in Example 1 increased by 2.49%, 9.71%, 8.55%, 39.09% and 61.97% respectively. The elastic modulus, ultimate tensile strength, failure strength, elongation at break and fracture energy of the dumbbell-shaped specimen prepared in Example 1 increased by 8.31%, 35.78%, 32.88%, 15.72% and 59.72%, respectively. Although the effect of MWCNTs-COOH on improving the stiffness of the material is limited, it shows a significant effect in enhancing the toughness of the material.

[0046] Table 2 Mechanical properties test data of dumbbell specimens

[0047] Test Example 2 To determine the effect of modified epoxy resins on joint bonding performance, single-lap bonded joints were prepared using the epoxy resins of Examples 1-2 and Comparative Examples 1-4, respectively, and AA6061-T6 aluminum alloy. Quasi-static tensile-shear properties of these joints were tested using a tensile testing machine according to ASTM D5868. Three replicates were used for each experiment.

[0048] 1. The preparation process of single lap joint is as follows: (1) Surface treatment of aluminum plate. The surface of the aluminum plate (aluminum plate thickness 1.5 mm) was carefully cross-grinded (±45° to the loading direction) using a grinder to ensure that the entire bonding surface was uniformly rough. After grinding, the surface of the sample was first wiped with an alcohol cotton pad to remove metal chips and dust. After the ethanol evaporated, it was further cleaned with acetone to ensure that there was no residue on the surface. The aluminum plate was then placed at room temperature to dry naturally. After the solvent completely evaporated, bonding was performed.

[0049] (2) Preparation of adhesive. Add an accurately measured amount of curing agent to the epoxy resin (the mass ratio of epoxy resin to curing agent is 2:1) and stir slowly in a clockwise direction using a glass rod. Place the mixed adhesive in a vacuum drying oven and evacuate to remove air bubbles.

[0050] (3) Joint bonding. First, place the surface-treated aluminum plate on a dust-free workbench. Then, use a syringe to evenly apply the pre-mixed adhesive to the bonding area of ​​the aluminum plate (area size is 25.4 mm × 25.4 mm), and use a copper wire to control the adhesive layer thickness to 0.3 mm. After the adhesive is evenly applied, gently connect another aluminum plate to it and fine-tune its position to ensure that the two aluminum plates are accurately aligned in the bonding area. Use a butterfly clip to fix the bonding area and apply appropriate pressure to promote the flow and penetration of the adhesive. Use an alcohol cotton pad or paper towel to gently wipe away the overflowed adhesive to ensure the cleanliness and integrity of the bonding surface.

[0051] (4) Joint curing: Place the joint horizontally indoors and cure at room temperature for 7 days.

[0052] (5) Joint post-processing. After curing is complete, use a knife to scrape off the adhesive that overflows from the bonding area to make the sides of the bonded joint smooth. To avoid the tension at both ends of the joint being non-collinear and causing external torque, a calibration gasket with the same thickness as the substrate is attached to each of the two substrates, and the gasket is fixed again with a butterfly clip and left at room temperature for 24 h.

[0053] 2. The experimental results are as follows: The experimental results of the single lap bonded joint specimens are shown in Table 3. It can be seen from Table 3 that: (1) Compared with Comparative Example 3, the addition of MWCNTs-COOH in Example 1 can significantly enhance the failure load, failure displacement, shear strength and fracture energy of the single lap joint specimens; compared with Comparative Examples 1 and 2, only the specific reaction solvent acetone and intermittent ultrasonic method can improve the physical properties of the single lap joint specimens.

[0054] (2) Compared with the pure epoxy adhesive of Comparative Example 3, when the MWCNTs-COOH content in the modified adhesive was 0.5 wt.% (Example 1) and 1 wt.% (Example 2), the shear strength of the prepared single lap joint specimens increased by 31.62% and 20.18%, respectively, and the fracture energy increased by 74.62% and 34.01%, respectively; however, when the MWCNTs-COOH content was increased to 2 wt.% (Comparative Example 4), the shear strength decreased by 14.01% and the fracture energy decreased by 47.21%.

[0055] Table 3 Experimental results of single lap bonded joint specimens

[0056] Note: C indicates cohesive failure of the adhesive layer, and D indicates debonding at the adhesive layer / aluminum plate interface.

[0057] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions of the technical solution of the present invention by ordinary technicians in this field do not deviate from the essence and scope of the technical solution of the present invention.

Claims

1. A method for preparing MWCNTs-COOH modified epoxy resin, characterized in that: The steps include: (1) First, MWCNTs-COOH was mixed with acetone and ultrasonicated to obtain a mixture; (2) Add epoxy resin to the mixture, perform intermittent ultrasonic treatment, and dry to obtain the product.

2. The preparation method according to claim 1, characterized in that The mass ratio of the MWCNTs-COOH, acetone and epoxy resin is 0.15-0.3:30:

20.

3. The preparation method according to claim 1, characterized in that The ultrasonic frequency in step (1) is 20-30 kHz, the ultrasonic power is 200-250 W, and the ultrasonic time is 8-12 min.

4. The preparation method according to claim 1, characterized in that The epoxy resin in step (2) is DP460 epoxy resin.

5. The preparation method according to claim 1, characterized in that The intermittent ultrasonic process in step (2) includes: ultrasonicating for 4-6 min at 4-8°C, 20-30 kHz and 200-250 W, and then stirring at 100-120 rpm for 4-6 min, for a total of 20-40 min.

6. The preparation method according to claim 1, characterized in that The drying process in step (2) includes: stirring at 100-120 rpm for 4-6 minutes every 0.5-1.5 hours at 40-50°C for a total of 8-12 hours.

7. A MWCNTs-COOH modified epoxy resin prepared by the preparation method according to any one of claims 1 to 6.

8. A modified epoxy adhesive, characterized in that: The invention comprises the MWCNTs-COOH modified epoxy resin according to claim 7 and a curing agent.

9. The modified epoxy adhesive according to claim 8, characterized in that The amount of MWCNTs-COOH added to the modified adhesive is 0.5-1 wt.%.

10. Use of the modified epoxy resin according to claim 7 or the modified epoxy adhesive according to claim 8 or 9 in adhesive joints.

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