Preparation method of aramid nanofiber / polyurethane composite material

Abstract

The invention belongs to the technical field of polyurethane composite materials, and particularly discloses a preparation method of an aramid nanofiber / polyurethane composite material. The method comprises the following steps: (1) preparing an organic silicon modified polyurethane prepolymer; (2) preparing a fluorinated modified aramid nanofiber (FANF); and (3) uniformly dispersing FANF in a solvent to form a dispersion liquid, adding a chain extender into the organosilicon modified polyurethane prepolymer according to a preset chain extension coefficient, then doping the FANF dispersion liquid, and mixing, curing and molding to obtain the high-performance polyurethane elastomer (PUE). Through the synergistic effect of the rigidity enhancement effect of the aramid nanofibers and the interface lubrication characteristic of the polydimethylsiloxane, the wear resistance and mechanical strength of the material are remarkably improved, and the obtained PUE has excellent tensile strength (gt; 35 MPa) and an elongation at break (gt; 400%), and can be widely applied to the fields of industrial sealing, flexible electronics and high-load transmission parts.

Description

Technical Field

[0001] The present invention relates to the technical field of polyurethane composite materials, and in particular to a method for preparing an aramid nanofiber / polyurethane composite material. Background Art

[0002] As a key material in the polymer field, the global market size of polyurethane is expected to exceed US$100 billion in 2025. Due to its unique molecular designability, excellent mechanical strength and flexibility, its application in high-end coatings, high-performance elastomers, lightweight structural foams and other fields continues to increase. However, the performance shortcomings of traditional polyurethane materials under extreme friction conditions are becoming increasingly prominent: unmodified polyurethane is prone to increased surface adhesive wear due to the thermal softening effect under high-load (>5MPa) and high-speed (>2m / s) friction conditions. At the same time, abrasive wear causes an increase in the material volume loss rate, shortening its application life by 30%-50% in key scenarios such as heavy-load transmission components and engineering machinery seals. It is difficult to meet the stringent material durability requirements in the field of high-end equipment.

[0003] In the current technology system, improving the comprehensive performance of polyurethane through fiber reinforcement modification has become the mainstream research direction. Although carbon fiber (CF) and glass fiber (GF) reinforced polyurethane systems have been commercialized, their inherent defects significantly restrict the application boundaries: the high modulus characteristics of carbon fiber lead to increased brittleness of composite materials, and interface delamination failure is prone to occur under dynamic loads; the interface compatibility between glass fiber and polyurethane matrix is ​​poor, and the dynamic heat generated by long-term friction aggravates interface debonding, resulting in a 2-3 times increase in the creep rate of the material. In order to break through the above bottlenecks, in recent years, more and more researchers have focused on the development of new reinforcements such as aramid nanofibers (ANF), whose high strength and high aspect ratio characteristics can significantly improve the load-bearing capacity of composite materials. For example, the preparation technology of powdered modified aramid nanocomposite fillers improves the dispersibility of ANF by grafting polar functional groups on the surface, and composites it with a polymer matrix to improve the tensile strength of the material. However, this method still has certain limitations: first, powdered ANF requires high shear force mechanical dispersion, which can easily lead to fiber breakage and destroy its continuous reinforcement effect; second, there is a lack of chemical bonding between the powder filler and the polyurethane matrix, and the debonding effect is induced due to interfacial stress concentration during dynamic friction, which makes the wear resistance of the composite material lower than the theoretical value; third, the existing technology makes it difficult to achieve the directional arrangement of ANF in the matrix, resulting in significant anisotropy of the material, and the transverse mechanical properties are only 50%-60% of the longitudinal ones, which restricts its application in multi-directional force scenarios.

[0004] In addition, the trend of environmentally friendly manufacturing poses new challenges to the preparation process of polyurethane composite materials: the dispersion of traditional inorganic fillers (such as silica and silicon carbide) relies on organic solvents or high-energy consumption mixing equipment, which is contrary to green manufacturing standards; and although bio-based fillers (such as cellulose nanocrystals) are environmentally friendly, their poor heat resistance and weak interface bonding have not yet been broken through. Therefore, the development of fiber-reinforced polyurethane systems with high interface bonding strength, low process complexity and adaptability to continuous production has become a core breakthrough in meeting the demand for upgraded wear-resistant materials in new energy equipment (such as battery seals) and high-speed rail transportation (shock absorption components). Technological breakthroughs in this direction can not only fill the gap in domestic high-end polyurethane composite materials, but also have significant economic and social benefits. Summary of the Invention

[0005] The present invention aims to overcome the shortcomings of the prior art and provide a method for preparing aramid nanofiber / polyurethane composite materials. Using an in-situ blending method, modified aramid nanofibers are introduced during the polyurethane synthesis process. Through in-situ reaction between ANF surface functional groups (such as -NH2 and -COOH) and polyurethane isocyanate groups (-NCO), a strong covalent bond-dominated interface is established. This improves the dispersibility of the aramid nanofibers and the effective bonding interface with the polyurethane matrix, resulting in excellent mechanical properties and wear resistance.

[0006] In order to achieve the above object, the technical solution of the present invention is: a method for preparing an aramid nanofiber / polyurethane composite material, comprising the following steps:

[0007] (1) Preparation of organosilicon-modified polyurethane prepolymer: using diol, hydroxylated silane and isocyanate as raw materials, reacting at 90-100°C for 2-3h to obtain organosilicon-modified polyurethane prepolymer;

[0008] (2) Preparation of fluorinated aramid nanofibers: Aramid nanofiber (ANF) gel, tetraethyl orthosilicate (TEOS), ethanol solution, and ammonia solution were mixed in a mass volume ratio of (10-40) g: (10-50) g: (100-2000) mL: (5-20) mL, stirred and dispersed, and reacted at 70-80°C for 6-8 hours to obtain an intermediate product (named ANF-SiO2); the intermediate product was mixed with functionalized silane, anhydrous ethanol, and ammonia solution in a mass volume ratio of 10 g: (1-60) g: (100-2000) mL: (10-100) mL, and reacted at 70-80°C for 6-8 hours to obtain fluorinated aramid nanofibers (abbreviated as FANF);

[0009] (3) Preparation of composite materials: The FANF obtained in step (2) is mixed with ethyl acetate solution to obtain a FANF dispersion, a chain extender is added to the prepolymer obtained in step (1), the FANF dispersion is added, and the mixture is injection molded and matured, and demolded to obtain a composite material (abbreviated as PUE).

[0010] Furthermore, in step (1), the molar ratio of the diol, hydroxylated silane and isocyanate is (1-3):0.1:(1.9-3.8).

[0011] Further, the diol in step (1) is selected from at least one of polytetramethylene glycol (PTMG), polypropylene glycol (PPG), polycarbonate diol (PCDL), polyethylene adipate (PEG), and polybutylene succinate (PBS);

[0012] Furthermore, the hydroxylated silane in step (1) is polydimethylsiloxane (PDMS).

[0013] Furthermore, the isocyanate in step (1) is selected from at least one of hydrogenated diphenylmethane diisocyanate (HMDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), xylene diisocyanate (XDI), tetramethyl-m-xylene diisocyanate (TMXDI), and diphenylmethane diisocyanate (MDI).

[0014] Furthermore, the purity of the ethanol solution in step (2) is 75%, and the concentration of the ammonia solution is 1 mol / L.

[0015] Furthermore, the functionalized silane in step (2) is selected from at least one of trimethoxy (3,3,3-trifluoropropyl) silane, 3,3,3-trifluoropropyl trichlorosilane, 3,3,3-trifluoropropyl methyldimethoxysilane, perfluorodecyl trimethoxysilane, and heptafluorodecyl triethoxysilane.

[0016] Furthermore, in step (3), the chain extension coefficient is 1.005, and the chain extender is an aromatic diamine chain extender. Preferably, the chain extender is selected from one of dimethylthiotoluenediamine (DMTDA), ethylenediamine (EDA), isophoronediamine (IPDA), meta-phenylenediamine (MDA), polyoxypropylenediamine (PPDA), diethyltoluenediamine (DETDA), 1,6-hexanediamine (HDA), 4,4'-dicyclohexylmethanediamine (HMDA), meta-phenylenediamine (m-PDA), polytetrahydrofuran diamine (PTHF), and 4,4'-methylenebiscyclohexylamine (H12MDA), and the mass ratio of the silicone modified polyurethane prepolymer to the chain extender is 10:(1 to 5).

[0017] Furthermore, in step (3), the mass ratio of FANF to ethyl acetate solution is (0.1-10):10, the purity of the ethyl acetate solution is 90%-100%, and the mass ratio of the silicone-modified polyurethane prepolymer to FANF is 20:(0.01-10).

[0018] Furthermore, the aging temperature in step (3) is 80 to 120°C, and the aging time is 4 to 48 hours.

[0019] The beneficial effects of the present invention are:

[0020] (1) Through the surface gradient modification technology of aramid nanofiber (ANF), a silicon dioxide (SiO2) inorganic layer is first generated in situ on the fiber surface, and then fluorinated with (3,3,3-trifluoropropyl)trimethoxysilane to construct a fluorinated SiO2 composite interface layer, which significantly improves the dispersion of the fiber in the polyurethane matrix. At the same time, a stable dispersion liquid is formed through the sol-gel method, so that the ANF is evenly distributed in the matrix at the nanoscale, thereby improving the fiber-matrix interface bonding strength;

[0021] (2) Innovatively using hydroxyl-terminated polydimethylsiloxane (PDMS) to blend and modify polyurethane, introducing PDMS segments into the polyurethane network through chemical bonding, giving the material system dynamic lubrication properties, reducing the friction coefficient of the composite material, and improving the hydrophobicity and fatigue resistance of the matrix;

[0022] (3) Construction of a rigid-flexible synergistic reinforcement system: The rigid skeleton of ANF (elastic modulus > 80 GPa) and the flexible chain segments of the polyurethane matrix form a microscopic interpenetrating network. While maintaining the high strength characteristics of ANF, energy dissipation is achieved through the molecular chain slip mechanism, making the composite material have both excellent mechanical properties (tensile strength > 35 MPa, elongation at break > 400%) and crack propagation resistance;

[0023] (4) Through a two-step functional modification process, a fluorosilicone synergistic protective layer is constructed on the ANF surface, which effectively suppresses the interfacial stress concentration phenomenon and enables the composite material to maintain stable friction performance under dynamic load. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 FTIR images of four polyurethanes prepared in Example 1;

[0025] Figure 2 (a) DMA storage modulus curve, (b) DMA loss modulus curve, and (c) Tanδ plot of the four polyurethanes prepared in Example 1 at 1 Hz and a specific temperature range;

[0026] Figure 3 1 is a stress-strain curve diagram of four polyurethanes prepared in Example 1;

[0027] Figure 4 (a) DIN wear pattern, (b) friction coefficient pattern, and (c) SEM image of the worn surface of the four polyurethanes prepared in Example 1;

[0028] Figure 5 The surface morphology SEM images of ANF before and after modification: (a) ANF, (b) ANF-SiO2, (c) FANF, (a'), (b'), (c') are the local enlarged images of (a), (b), (c), respectively;

[0029] Figure 6 The EDS images of the surface elements before and after ANF modification: (a) ANF (b) ANF-SiO2 (c) FANF;

[0030] Figure 7 3 is a stress-strain curve diagram of the SiPU prepared in Example 3 and PUE with different FANF mass contents;

[0031] Figure 8 (a) DMA storage modulus curve and (b) Tanδ graph of SiPU prepared in Example 3 and PUE with different FANF mass contents at 1 Hz and a specific temperature range;

[0032] Figure 9 The following are the SEM images of the SiPU prepared in Example 3 and PUE with different FANF mass contents after brittle fracture: (a) SiPU, (b) SiPU-FANF 0.1％ (c) SiPU-FANF 0.2％ (d) SiPU-FANF 0.4％ , (a'), (b'), (c'), (d') are the partial enlarged views of (a), (b), (c), (d), respectively;

[0033] Figure 10 (a) DIN wear diagram, (b) friction coefficient diagram, and (c) SEM image of the worn surface of SiPU prepared in Example 3 and PUE with different FANF mass contents. DETAILED DESCRIPTION

[0034] The present invention will be further described below with reference to the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it. However, the embodiments are not intended to limit the present invention.

[0035] The reagents used in the present invention are shown in Table 1, and the experimental instruments are shown in Table 2.

[0036] Table 1 Reagents

[0037]

[0038]

[0039] Table 2 Experimental instruments

[0040]

[0041] Example 1:

[0042] Preparation of polyurethanes with different PDMS contents:

[0043] 64.38g PTMG; 62.38g PTMG and 2g PDMS; 60.38g PTMG and 4g PDMS; 58.38g PTMG and 6g PDMS were weighed and added to four three-necked flasks, vacuum dehydrated at 120℃ for 2h, and 35.62g HMDI was added when the temperature dropped to 40℃. After vacuuming and nitrogen flow were repeated three times, the temperature was slowly raised to 90℃ and reacted for 3h to obtain a polyurethane prepolymer. According to the chain extension coefficient of 1.1, 20g of polyurethane prepolymer was taken and added with 2.78g DMTDA, and the mixture was quickly stirred at 500r and evenly poured onto a preheated PTFE plate. After curing at 100℃ in a vacuum drying oven for 24h, polyurethane samples with PDMS mass contents of 0, 2%, 4%, and 6% were obtained after demolding, and were named PU and PU-PDMS respectively. 2％ PU-PDMS 4％ PU-PDMS 6％ After being placed at room temperature for 72 h, the obtained polyurethane samples were used for subsequent testing and characterization.

[0044] Example 2

[0045] Preparation of fluorinated aramid nanofibers:

[0046] 40g of ANF gel, 12.5g of TEOS, 300mL of 75% ethanol solution, and 6.5g of 1mol / L ammonia solution were added to a beaker and mechanically stirred at 500r / min for 10 minutes to achieve uniform dispersion. The mixture was then placed in an oil bath and reacted at 80°C with magnetic stirring at 600r / min for 6 hours. The product was washed three times with anhydrous ethanol and then washed again with anhydrous ethanol to obtain the intermediate product, named ANF-SiO2. 20g of ANF-SiO2 was added to 1.98g of trimethoxy(3,3,3-trifluoropropyl)silane, 200ml of anhydrous ethanol, and 26g of 1mol / L ammonia solution. The mixture was mechanically stirred at 500r / min for 10 minutes to achieve uniform dispersion. The mixture was then placed in an oil bath and magnetically stirred at 70°C with magnetic stirring at 600r / min for 6 hours. The mixture was then filtered and washed three times with anhydrous ethanol to obtain fluorinated aramid nanofibers, named FANF.

[0047] Example 3

[0048] Preparation of aramid nanofiber / polyurethane composites:

[0049] (1) Preparation of silicone-modified polyurethane prepolymer: 60.38 g of PTMG and 4 g of PDMS were weighed and added to a three-necked flask. The mixture was vacuum-dehydrated at 120°C for 2 h. After the temperature dropped to 40°C, 35.62 g of HMDI was added. After vacuuming and nitrogen-filled for three times, the temperature was slowly raised to 90°C and reacted for 3 h to obtain a silicone-modified polyurethane prepolymer, named SiPU.

[0050] (2) Preparation of fluorinated aramid nanofibers: The method is the same as that in Example 2;

[0051] (3) Weigh 0.023 g, 0.046 g, and 0.092 g of FANF prepared in step (2) respectively, add them to a 20 mL sample bottle, add a magnet and 10 g of 98% ethyl acetate solution, set 500 r magnetic stirring and disperse for 24 h, and obtain three FANF dispersions with different FANF contents; according to the chain extension coefficient of 1.005, take 20 g of SiPU prepared in step (1) and add 3.0469 g of DMTDA, and then respectively add the three FANF dispersions with different FANF contents, use a high-speed stirrer to quickly stir and uniformly at 600 r, pour them onto a preheated PTFE plate, mature in a vacuum drying oven at 100 ° C for 24 h, and after demolding, obtain aramid nanofiber / polyurethane composite material samples with FANF mass fractions of 0.1%, 0.2%, and 0.4%, respectively, and name them SiPU-FANF 0.1％ 、SiPU-FANF 0.2％ 、SiPU-FANF 0.4％ After being placed at room temperature for 72 h, the obtained samples were used for subsequent testing and characterization.

[0052] Testing and Characterization

[0053] Fourier transform infrared spectroscopy (FTIR) analysis:

[0054] The chemical structure of the polyurethane molecular chain was analyzed at room temperature using a German Bruker VERTEX-70 Fourier transform infrared spectrometer. The test conditions used the attenuated total reflectance (ATR) mode, and the spectral range was between 600 and 4000 cm -1 , with a resolution of 4cm -1 , the number of scans is 32 times.

[0055] Scanning electron microscopy (SEM) test: The modified ANF was gold-plated, and the surface morphology of different fibers and EDS element analysis were observed using a JSM-6700F scanning electron microscope under the conditions of 5.0 kV accelerating voltage and 50 Pa vacuum chamber.

[0056] Dynamic Mechanical Analysis (DMA): A DMAQ800 DMA analyzer from TA Instruments (USA) was used to analyze and characterize the viscoelastic behavior of the PUE. The test was conducted in tensile mode at a heating rate of 3°C / min.

[0057] Mechanical performance test: The mechanical properties of PUE were evaluated using a Z005 electronic tensile tester produced by Zwick, Germany. The test was conducted in accordance with the GB / T528-2009 standard at a tensile speed of 200 mm / min.

[0058] DIN Abrasion Test: DIN abrasion tests were conducted on the prepared specimens using a DIN abrasion testing machine manufactured by Gaotie Technology Co., Ltd. in accordance with GB / T 9867-2008. The actual weight of the specimens before and after the test was measured using a Precisa-XB220A electronic analytical balance. The density of the specimens was then measured in accordance with GB / T533. The sandpaper used was 60-mesh Al2O3. The roller speed was set at 40 r / min, and the load was 5 N.

[0059] Friction coefficient test: Cut a 50*50mm sample and use the equipment of Gaotie Technology Co., Ltd. for testing, with a load of 1000N, a test speed of 150mm / min, and a distance of 120mm.

[0060] Results and Analysis

[0061] 1. Infrared analysis of silicone modified polyurethane

[0062] The FTIR spectra of the four polyurethane samples prepared in Example 1 are as follows: Figure 1 As shown, at 2260~2270cm -1 Within the range and 1390cm -1 There is no absorption peak at the 3294cm-1 peak, and the characteristic peak of PU is attributed to the stretching vibration peak of -NH -1 ,1533cm -1 ), -C=O stretching vibration peak (1720cm -1 ), the characteristic absorption peak of COC (1070cm -1 ) and -CO- stretching vibration peaks (1230 cm -1 ) [91,92] , indicating that -NHCOO- was generated during the reaction. In addition, at 2260 cm -1 There is no characteristic absorption peak of -NCO at the bottom, indicating that there is no unreacted free -NCO group in the sample and all -NCO groups are involved in the reaction. The characteristic peaks of PDMS modified polyurethane are similar to those of PU, and the main differences are in two points. On the one hand, PU-PDMS 2％ PU-PDMS 4％ PU-PDMS6％ At 1070cm -1 The characteristic absorption peak at 806 cm corresponds to the overlap of COC and Si-O-Si stretching vibration peaks in the modified polyurethane elastomer. Secondly, the stretching absorption peak of -CH3 in Si-(CH3)2 appears at 806 cm -1 And with the increase of PDMS content, the peak intensity increases. Due to the low content of doped PDMS, the peak intensity does not change significantly. In summary, it proves that OH-PDMS is successfully introduced into the main chain of the polyurethane molecule.

[0063] 2. Dynamic mechanical properties of silicone modified polyurethane

[0064] Dynamic mechanical analysis is often used to analyze the viscoelastic motion of polymers and the subtle motion of molecules. The storage modulus, loss modulus and loss factor curves of polyurethanes prepared with different PDMS contents prepared in Example 1 are shown in FIG. Figure 2 The mass percentages of hard segments in these four groups of polyurethane samples are the same, and the differences are mainly due to the different PDMS contents.

[0065] Figure 2 (a) shows the storage modulus curves of the four groups of PU. At a certain temperature, the storage modulus is positively correlated with the PDMS content. With the increase of PDMS content, it shows excellent low temperature resistance. Below -100℃, the storage modulus increases with the increase of PDMS content. PU-PDMS 6％ Shows excellent low temperature resistance and high storage modulus. Figure 2 (b) shows the loss modulus curves of four groups of polyurethane elastomers. It can be seen from the figure that below -100°C, the loss modulus is higher when the PDMS content is higher. As the temperature increases, the loss modulus is negatively correlated with the PDMS content. The higher the PDMS content, the smaller the loss modulus. At the same time, two temperature peaks appear, representing the T of the PTMG soft segment and the hard segment respectively. g It can be seen that with the increase of PDMS content, the two peaks are gradually moving away, indicating that appropriate microphase separation has occurred. Figure 2 (c) is the Tanδ diagram of four groups of polyurethane elastomers. It can be seen that the four groups of polyurethane elastomers show a wider damping temperature range and better damping effect. This is due to the specific structure of the urea group. Polyurethane elastomers usually have a higher degree of cross-linking. This cross-linking structure enables the material to disperse and absorb energy more effectively when impacted or vibrated, reducing the propagation of vibration. This means that it can more effectively convert kinetic energy into heat energy during the energy absorption process, thereby reducing vibration and noise and showing a lower loss factor. The two peaks in the figure are consistent with the loss modulus curve, representing the T values ​​of the PDMS soft segment and hard segment in the polyurethane elastomer. g, showing appropriate microphase separation.

[0066] 3. Mechanical properties of silicone modified polyurethane

[0067] The mechanical properties of silicon-containing polyurethane are closely related to its microphase separation structure. In order to study the effect of different PDMS contents on its mechanical properties, the tensile properties of polyurethane modified with different PDMS contents prepared in Example 1 were tested. The measured stress-strain curves are shown in Figure 2. Figure 3 As shown. In order not to reduce the mechanical properties of polyurethane itself, the added PDMS content is controlled within 6%. With the increase of PDMS content, the tensile strength shows a gradual upward trend, reaching the maximum at 6%. This is because a lower content of PDMS can effectively promote the free movement of molecules and produce appropriate microphase separation between soft and hard segments, which is conducive to the improvement of the degree of hydrogen bonding. Consistent with the infrared test fitting results, its elongation at break shows a trend of first increasing and then decreasing with the increase of PDMS content. This is because PDMS itself has low surface energy, is highly helically twisted, and has weak intermolecular forces. A large amount of PDMS will reduce the cohesive strength of the polyurethane itself. At the same time, when the degree of microphase separation is too large, the hard segments will be tightly stacked. The intermolecular force will dominate the weakening of the cohesive strength, resulting in a decrease in the force between the soft and hard segments and a decrease in the elongation at break. Therefore, the introduction of PDMS content should not be too much.

[0068] 4. Wear resistance of silicone modified polyurethane

[0069] The polyurethane modified with different PDMS contents prepared in Example 1 was subjected to DIN abrasion, friction coefficient and wear surface tests. The results are as follows: Figure 4 The detailed data are shown in Table 3. Figure 4 As can be seen in (a), the addition of PDMS can significantly improve the wear resistance of polyurethane and reduce the wear loss. As the PDMS content increases, the wear loss gradually decreases. Adding 6% PDMS can achieve a wear loss of 110 mm 3 Reduced to 56.9mm 3 , wear resistance is improved by about 48%. Figure 4 (b) is a friction coefficient diagram tested under a load of 1000N, a measuring stroke of 120mm, and a sliding speed of 150mm / min. It can be seen that with the addition of PDMS, its friction coefficient and the friction coefficient of polyurethane without PDMS are both reduced, and the higher the content, the more obvious the decrease. Figure 4(c) is a SEM test image of the worn surface after the DIN abrasion test. It can be seen from the figure that the polyurethane surface without PDMS is severely worn, and fatigue wear and adhesive wear dominate the wear behavior of the polyurethane. The addition of PDMS effectively improves the degree of surface wear and reduces adhesive wear. The addition of 6% PDMS shows milder surface wear and improved wear resistance. This is because the addition of PDMS will migrate to the polyurethane surface to form a lubricating layer, which effectively reduces the friction coefficient and surface wear of the polyurethane.

[0070] Table 3 Wear resistance parameters of silicone modified polyurethane prepared in Example 1

[0071]

[0072] 5. Micromorphology analysis of aramid nanofibers

[0073] The surface morphology SEM images of ANF before and after modification are as follows: Figure 5 As shown. ANF without surface modification tends to aggregate together and show clusters, as shown Figure 5 As shown in (a), (a') is a partial enlarged view of (a). After TEOS surface modification to form SiO2 on the surface of ANF (the intermediate product ANF-SiO2 prepared in Example 2), its fiber agglomeration is improved compared with ANF, and filamentous fibers are dispersed on the surface, and white nanoparticles such as Figure 5 As shown in (b), (b') is a partial enlargement of (b), which proves that white mesh-like nanoparticles of silica are formed in situ on the ANF surface. After TFP-TMS treatment (FANF prepared in Example 2), the agglomeration of the ANF surface is further improved compared with ANF-SiO2, and each fiber shows a clear separation phenomenon. At the same time, the fiber is wrapped by the silane coupling agent, the particle size increases, and the white nanoparticles on the surface become smaller and wrapped on the ANF surface. At the same time, the particle distribution is more uniform, as shown in the figure. Figure 5 As shown in (c), (c') is a local enlarged view of (c).

[0074] 6. Surface element analysis of aramid nanofibers

[0075] EDS was used to analyze the changes in surface elements before and after ANF modification. The obtained element scan and surface element composition were as follows: Figure 6As shown in Table 4, ANF is primarily composed of three elements: C, N, and O. After TEOS surface modification, the Si content increased from 0.84% ​​to 2.32%, a 1.48% increase in Si content. This demonstrates the formation of SiO2 on the ANF surface after TEOS modification, confirming successful modification and the successful preparation of ANF-SiO2. After TFP-TMS treatment, the Si content of ANF-SiO2 increased from 2.32% to 6.07%, and the F element appeared, reaching a content of 6.68%, indicating the successful grafting of TFP-TMS onto the ANF-SiO2 surface and confirming the successful preparation of FANF.

[0076] Table 4 Changes of surface elements before and after ANF modification

[0077]

[0078] 7. Mechanical properties of aramid nanofiber / polyurethane composites

[0079] The stress-strain curves of SiPU prepared in Example 3 and PUE with different FANF mass contents are shown in Figure 3. Figure 7 The results show the effects of varying FANF content on polyurethane composites. Table 5 shows that the incorporation of ANF significantly improves the mechanical properties of polyurethane (PU), with a significant increase in tensile strength and limited improvement in elongation at break. With the addition of 0.1% FANF, the tensile strength increases from 28.8 to 35.53, and the elongation at break rises from 436% to 444%. With increasing FANF content, the tensile strength initially increases and then decreases. Generally speaking, the mechanical properties of polyurethane composites strongly depend not only on the dispersion of the nanofiller and the filler-PU interfacial adhesion, but also on intermolecular interactions or hydrogen bonds within the polyurethane matrix. The introduction of FANF significantly improves its dispersion and interfacial adhesion in PU, thereby significantly enhancing the tensile strength and toughness of the polyurethane composite. When the FANF content is further increased to 0.4%, the tensile strength and elongation at break show a downward trend. This is because when the filler content exceeds a critical value, the filler aggregates in the matrix, leading to performance degradation.

[0080] Table 5 Stress-strain parameters of aramid nanofiber / polyurethane composites

[0081]

[0082] 8. Dynamic mechanical properties of aramid nanofiber / polyurethane composites

[0083] The dynamic mechanical loss test was carried out on the SiPU prepared in Example 3 and PUE with different FANF mass contents. Figure 8 As shown, Figure 8 (a) is the storage modulus curve, Figure 8 (b) is the loss factor curve. Figure 8 As can be seen in (a), in the range of -40 to -120°C, after adding FANF to polyurethane, the storage modulus of the polyurethane composite material can be improved, and as the FANF content increases, the storage modulus gradually increases. The reason is that after the introduction of FANF, the hydrogen bonding between the molecular chain and the FANF produces a molecular chain reinforcement effect, which improves the storage modulus of the material. The high storage modulus (especially in the high temperature zone) indicates that the material maintains rigidity under stress, and its ability to resist deformation is enhanced, which can effectively reduce adhesive wear. Figure 8 As can be seen in (b), as the storage modulus increases, the Tanδ peak shows a downward trend. Tanδ is the ratio of the loss modulus to the storage modulus, which is used to evaluate the energy consumed by the molecular chain movement and interface friction during the tensile vibration process. It reflects the internal friction of the material and, to a certain extent, represents the filling network of the composite material. At the glass transition temperature (T g ) region, since the filler network is difficult to be destroyed and the energy required to destroy the filler network is very high, the dissipation in the composite material is mainly caused by the friction between the molecular chains.

[0084] The incorporation of FANF forms a strong filling network with PU, restricting the movement of molecular chains. The molecular migration of FANF fibers in the polyurethane system is restricted, and the low surface energy of fluoride reduces energy dissipation, thereby reducing the energy consumed to overcome the mutual friction between molecular chains, resulting in a decrease in the peak value of Tanδ. With the increase of FANF doping content, the peak value of Tanδ shows a gradual downward trend. At the same time, with the doping of FANF, its glass transition temperature gradually shifts to the left. This is due to the formation of flexible chain segments Si-O-Si on the surface of FANF, indicating that the addition of FANF can effectively reduce the glass transition temperature of the substrate and improve the low-temperature resistance.

[0085] 9. Microscopic morphology of the cross section of aramid nanofiber / polyurethane composite material

[0086] The dispersion of aramid nanofibers in the substrate is the key to determining whether its performance is improved. To this end, the SiPU prepared in Example 3 and PUE tensile specimens with different FANF mass contents were subjected to liquid nitrogen extreme cold fracture, and the dispersion of FANF in the SiPU matrix was characterized by field emission scanning electron microscopy. Figure 9 Middle (a) shows the cross-sectional morphology of the SiPU matrix after brittle fracture. The cross-sectional surface of the SiPU shows obvious black and white phases, with a large number of white wrinkles, showing obvious microphase separation morphology. Figure 9 (b) is the cross-sectional morphology of FANF added to SiPU substrate at 0.1%. It can be seen that the fibers are doped in the polyurethane matrix and covered by polyurethane, with obvious extraction phenomenon. Figure 9 (c) shows the addition of 0.2% FANF. It can be seen that the extracted fibers are covered with polyurethane and a circular interface effect is presented with the fiber as the center. Figure 9 Figure (d) shows the addition of 0.4% FANF. Higher FANF levels can lead to slight agglomeration, resulting in decreased mechanical properties. This is consistent with the mechanical properties test, which also exhibits a circular interfacial effect centered on the fiber. Therefore, excessive FANF content is not recommended. Furthermore, increasing FANF content increases the degree of microphase mixing. This is due to the reduced electron density of the C=O groups caused by the introduction of fluorocarbon chains, which hinders hydrogen bonding between urethanes. This reduces the number of hydrogen bonds in the hard segments, leading to increased phase mixing in the corresponding composites.

[0087] 10. Wear resistance of aramid nanofiber / polyurethane composite materials

[0088] The SiPU prepared in Example 3 and PUE with different FANF mass contents were subjected to DIN wear, friction coefficient and wear surface tests. The measured results are as follows: Figure 10 The detailed data are shown in Table 6. Figure 10 As can be seen in (a), the addition of FANF can significantly improve the wear resistance of polyurethane and reduce the wear loss. As the FANF content increases, the wear loss gradually decreases. Adding 0.4% FANF can achieve a wear loss of 68.8 mm. 3 Reduced to 36.3mm 3 , wear resistance is improved by about 47%. Figure 10 (b) is a friction coefficient graph tested under a load of 1000N, a measuring stroke of 120mm, and a sliding speed of 150mm / min. It can be seen that with the addition of FANF, its friction coefficient decreases compared with the friction coefficient of polyurethane without adding FANF, and shows a gradually decreasing trend with the increase of FANF content. This is because fluorine has lower surface energy and lubricating effect.

[0089] Figure 10 (c) is a SEM test image of the wear surface of each sample after the DIN wear test. It can be seen from the figure that the polyurethane surface without FANF addition is severely worn, and fatigue wear and adhesive wear dominate the wear behavior of polyurethane. The addition of FANF effectively improves the degree of surface wear and reduces adhesive wear. Adding 0.4% FANF shows milder surface wear and improved wear resistance. This is because the PDMS lubricating phase and the fluorinated ANF interface synergistically reduce the surface friction coefficient, thereby reducing friction loss.

[0090] Table 6 Wear resistance parameters of aramid nanofiber / polyurethane composites

[0091]

[0092] The present invention uses SiPU prepared with 4wt% PDMS as the matrix and prepares aramid nanofiber / polyurethane composite material by fluorinated SiO2 grafted with ANF, achieving a double breakthrough in the dispersion of nano-reinforcement and interface bonding:

[0093] FTIR, TG, SEM and EDS analysis showed that fluorinated SiO2 was successfully grafted onto the ANF surface (the F element content reached 6.68%), the dispersion of the fiber was improved, and the grafting rate was about 23%. When the FANF content increased from 0.1wt% to 0.4wt%, the tensile strength of the composite material showed a trend of first increasing and then decreasing, reaching a peak of 36.24MPa at 0.2wt% (an increase of 25.8% compared to the matrix). At the same time, the elongation at break was maintained at 460%, indicating that the fluorination modification effectively avoided the problem of increased brittleness caused by traditional nanofillers. In the DMA test, the addition of FANF increased the storage modulus of the composite material and reduced the loss factor, indicating that the interfacial friction dominated the energy dissipation and effectively improved the wear resistance. The DIN test showed that the volume wear rate was further reduced to 36.3mm after doping with 0.4wt% FANF. 3 , a 47.2% decrease compared to the base material. SEM analysis revealed that the wear surface transitioned from adhesive wear to fatigue wear, and the fluorinated SiO2 grafted onto the ANF surface effectively inhibited the initiation of microcracks and slowed down friction loss.

[0094] This invention offers a new paradigm for the design of highly wear-resistant polymer composites: through dual-path optimization, matrix microphase separation and filler interface modification, it overcomes the traditional material "struggle triangle" of strength, toughness, and wear resistance. The developed aramid nanofiber / polyurethane composite has significant application potential in areas such as mining machinery liners and conveyor belts.

[0095] The above-described embodiments are only preferred solutions of the present invention and are not intended to limit the present invention in any form. Other variations and modifications are possible without exceeding the technical solutions described in the claims.

Claims

1. A method for preparing an aramid nanofiber / polyurethane composite material, characterized in that: The following steps are involved: (1) Preparation of silicone-modified polyurethane prepolymer: Using diol, hydroxylated silane and isocyanate as raw materials, the silicone-modified polyurethane prepolymer is prepared by reacting at 90-100°C for 2-3 hours; (2) Preparation of fluorinated aramid nanofibers: Aramid nanofiber gel, tetraethyl orthosilicate, ethanol solution, and ammonia solution were mixed in a mass volume ratio of (10-40) g: (10-50) g: (100-2000) mL: (5-20) mL, stirred and dispersed, and reacted at 70-80° C. for 6-8 hours to obtain an intermediate product; the intermediate product was mixed with functionalized silane, anhydrous ethanol, and ammonia solution in a mass volume ratio of 10 g: (1-60) g: (100-2000) mL: (10-100) mL, and reacted at 70-80° C. for 6-8 hours to obtain fluorinated aramid nanofibers, referred to as FANF; (3) Preparation of composite materials: The FANF obtained in step (2) is mixed with ethyl acetate solution to obtain a FANF dispersion, a chain extender is added to the prepolymer obtained in step (1), the FANF dispersion is added, and the mixture is injection molded and matured, and demolded to obtain a composite material.

2. The preparation method according to claim 1, wherein: The molar ratio of the diol, hydroxylated silane and isocyanate in step (1) is (1-3):0.1:(1.9-3.8).

3. The preparation method according to claim 1, wherein: The diol in step (1) is selected from at least one of polytetramethylene glycol, polypropylene glycol, polycarbonate glycol, polyethylene glycol, and polybutylene succinate glycol.

4. The preparation method according to claim 1, wherein: The hydroxylated silane in step (1) is hydroxyl-terminated polydimethylsiloxane.

5. The preparation method according to claim 1, wherein: The isocyanate in step (1) is selected from at least one of dicyclohexylmethane diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, tetramethylxylylene diisocyanate and diphenylmethane diisocyanate.

6. The preparation method according to claim 1, wherein: The functionalized silane in step (2) is selected from at least one of trimethoxy(3,3,3-trifluoropropyl)silane, 3,3,3-trifluoropropyltrichlorosilane, 3,3,3-trifluoropropylmethyldimethoxysilane, perfluorodecyltrimethoxysilane, and heptadecafluorodecyltriethoxysilane.

7. The preparation method according to claim 1, wherein: The purity of the ethanol solution in step (2) is 75%, and the concentration of the ammonia solution is 1 mol / L.

8. The preparation method according to claim 1, wherein: The chain extension coefficient in step (3) is 1.005, the chain extender is an aromatic diamine chain extender, and the mass ratio of the organosilicon-modified polyurethane prepolymer to the chain extender is 10:(1-5).

9. The preparation method according to claim 1, wherein: In step (3), the mass ratio of FANF to ethyl acetate solution is (0.1-10):10; the purity of the ethyl acetate solution is 90%-100%, and the mass ratio of the organosilicon-modified polyurethane prepolymer to FANF is 20:(0.01-10).

10. The preparation method according to claim 1, characterized in that: The aging temperature in step (3) is 80-120° C., and the aging time is 4-48 hours.

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