Nano boron nitride composite meta-aramid paper-based insulating material and preparation method thereof

Abstract

The invention discloses a nano boron nitride composite meta-aramid paper-based insulating material and a preparation method thereof, and relates to the technical field of insulating materials. The preparation method comprises the following steps: respectively dispersing meta-aramid fibrid and meta-aramid chopped fiber into fibrid slurry and chopped fiber slurry, mixing, adding hexagonal boron nitride nanosheet dispersion liquid and a retention aid for reaction to obtain uniform composite slurry, and shaping to obtain the nano boron nitride composite meta-aramid-based insulating material. The material is well matched with dielectric constant transformer oil, the dielectric loss is still at a lower level, meanwhile, the characteristic breakdown strength and the heat conductivity are synergistically improved, and the multifunctional requirements of high-power electrical equipment for insulating materials are met.

Description

Technical Field

[0001] The present invention belongs to the technical field of insulating materials, and particularly relates to a nano boron nitride composite meta-aramid paper-based insulating material and a preparation method thereof. Background Art

[0002] Modern power systems have gradually developed towards extra-high voltage. As one of the most important devices in the power transmission and transformation system, the safe and stable operation of transformers has a very important impact on the steady development of society. As an important component of the internal insulation of oil-immersed transformers, traditional cellulose insulating paper can no longer meet the increasing voltage levels and capacities.

[0003] In the field of electrical insulating materials, meta-aramid paper-based insulating materials are widely used due to their excellent electrical properties, thermal stability, and mechanical strength. However, there are still some problems to be solved in the existing meta-aramid insulating paper. Although the monopolized Nomex series products abroad perform excellently in terms of performance, their high costs limit their wide application in the domestic market. Although domestic meta-aramid insulating paper has certain cost advantages, there is still room for improvement in key performance indicators such as dielectric strength, tensile strength, and thermal conductivity. In particular, its dielectric strength is usually lower than 24 kV / mm, the tensile strength is lower than 8 kN / m, and the thermal conductivity is only in the range of 0.24 - 0.25 W / (m·K), making it difficult to meet the high requirements for insulating materials in high-temperature environments such as oil-immersed transformers.

[0004] In addition, the problem of the decline in mechanical properties of existing domestic meta-aramid insulating paper at high temperatures also restricts its application range. To improve these properties, the existing technology modifies meta-aramid insulating paper by adding inorganic fillers (such as Al2O3, SiO2). However, this method has problems such as uneven filler dispersion and large interfacial thermal resistance, resulting in unsatisfactory modification effects. Therefore, it is particularly important to develop a meta-aramid paper-based insulating material with high thermal conductivity, high strength, and excellent electrical properties.

[0005] As a new type of inorganic nanomaterial, nano boron nitride (BN) has excellent thermal conductivity and electrical insulation properties. Introducing nano boron nitride into meta-aramid paper-based insulating materials is expected to significantly improve the thermal conductivity, tensile strength, and electrical properties of the materials by constructing a thermal conduction network, regulating free volume, and optimizing interfacial bonding. Based on this idea, the present invention proposes a nano boron nitride composite meta-aramid paper-based insulating material and a preparation method thereof, aiming to solve the problems existing in the existing technology and meet the multi-functional requirements of insulating materials for high-power electrical equipment. Summary of the Invention

[0006] To solve the above technical problems, the present invention proposes a nano boron nitride composite meta-aramid paper-based insulating material and its preparation method, which significantly improves the performance of the insulating material and meets the multifunctional requirements of electrical equipment.

[0007] To achieve the above object, the present invention provides a preparation method of a nano boron nitride composite meta-aramid-based insulating material, comprising the following steps:

[0008] Disperse the meta-aramid precipitated fibers and the meta-aramid short-cut fibers into a precipitated fiber slurry and a short-cut fiber slurry respectively, mix them, and then add a hexagonal boron nitride nanosheet dispersion and a retention aid for reaction to obtain a uniform composite slurry, and then shape it to obtain a nano boron nitride composite meta-aramid-based insulating paper.

[0009] Further, the mass ratio of the meta-aramid precipitated fibers to the meta-aramid short-cut fibers is (5-8):(5-2).

[0010] Further, the mass ratio of the meta-aramid precipitated fibers to the meta-aramid short-cut fibers is 7:3.

[0011] Further, the slurry concentration of the precipitated fiber slurry is 0.3 wt%, and the slurry concentration of the short-cut fiber slurry is 3 wt%.

[0012] Further, the hexagonal boron nitride nanosheet dispersion is a micellar solution of sodium dodecylbenzenesulfonate and water.

[0013] Further, the mass ratio of the hexagonal boron nitride nanosheets to sodium dodecylbenzenesulfonate in the hexagonal boron nitride nanosheet dispersion is (5-15):5.

[0014] Further, the mass ratio of the hexagonal boron nitride nanosheets in the hexagonal boron nitride nanosheet dispersion to the total mass of the meta-aramid precipitated fibers and the meta-aramid short-cut fibers is 5-15 wt%.

[0015] Further, the mass ratio of the hexagonal boron nitride nanosheets in the hexagonal boron nitride nanosheet dispersion to the total mass of the meta-aramid precipitated fibers and the meta-aramid short-cut fibers is 10 wt%.

[0016] Further, the retention aid is a polyacrylamide retention aid.

[0017] The present invention also provides a nano boron nitride composite meta-aramid-based insulating material prepared by the above preparation method.

[0018] Compared with the prior art, the present invention has the following advantages and technical effects:

[0019] In the present invention, by adding 10 wt% hexagonal boron nitride nanosheets (BN), the thermal conductivity of the nano boron nitride composite meta-aramid-based insulating material is increased from 0.244 W / (m·K) of pure meta-aramid to 0.857 W / (m·K), with an increase of 3.51 times; the addition of nano boron nitride increases the tensile strength of the composite material from 7.53 kN / m to 9.06 kN / m, an increase of about 20%, and the elongation at break is increased to 4.48%. The decreasing trend of the shear modulus with the increase of temperature slows down, indicating that the material can still maintain excellent structural stability at high temperatures.

[0020] For the nano boron nitride composite meta-aramid-based insulating material of the present invention, the dry paper dielectric strength is increased from 23.69 kV / mm to 31.52 kV / mm (an increase of 33%), the partial discharge inception voltage is increased from 15.99 kV to 17.76 kV (an increase of 11%), and the volume resistivity is increased from 1.038×1016 Ω·m to 3.158×1016 Ω·m (an increase of 3.04 times), significantly enhancing the electric breakdown resistance and charge storage performance of the insulating paper.

[0021] The doping of hexagonal boron nitride nanosheets increases the glass transition temperature (Tg) from 538 K of pure meta-aramid to 586 K (an increase of 8.84%). The free volume fraction of the composite material decreases significantly at high temperatures (350 K), inhibiting the molecular chain movement and delaying the transition of the material from the glassy state to the rubbery state. Detailed Embodiments

[0022] The various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and implementation embodiments of the present invention.

[0023] There are no special restrictions on the sources of all raw materials of the present invention, and they can be purchased on the market or prepared by conventional methods well-known to those skilled in the art.

[0024] There are no special restrictions on the purity of all raw materials of the present invention. The present invention preferably uses analytically pure raw materials or the raw material purity commonly used in the field of chemical synthesis.

[0025] As an embodiment of the present invention, the meta-aramid precipitated fibers are purchased from Liaoyang Huaqi Chemical Fiber Products Co., Ltd., and the main parameters are as follows:

[0026] Test Items Unit Standard Average Value Dryness ％ 18±2 18.6 Beating Degree °SR 70～80 79.0 Whiteness ％ ≥84 87.3 DMAC Content mg / kg ≤500 203.0 Logarithmic Viscosity dL / g 1.7±0.1 1.78 Appearance White Fibers, without obvious color difference, impurities, etc.

[0027] As an embodiment of the present invention, the meta-aramid short cut fibers are purchased from Yantai Taihexing Materials Technology Co., Ltd., and the main parameters are as follows:

[0028] Test Items Unit Average Value Length Mm 6 Linear Density DTEX 2.07 Breaking Strength CN / DTEX 3.82 Elongation at Break ％ 26.97 Young's Modulus CN / DTEX 75.20 Whiteness Value WICIE(D65 / 10) 24.38 Dry Heat Shrinkage at 300℃×15min ％ 2.75 Moisture Content ％ 37.64

[0029] As an embodiment of the present invention, the hexagonal boron nitride nanosheets are purchased from Shanghai Kemi Chemical Technology Co., Ltd., and the main parameters are as follows:

[0030] Density: 0.9 - 1.1 g / mL (25 °C), bulk density: 120 kg / m 3 , specific gravity 3.48, particle size 300 - 500 mm, resistivity: 10 19 ρ / μΩ·cm; crystal structure: hexagonal.

[0031] The mechanism of action involved in the present invention:

[0032] Thermal conduction network construction mechanism: The hexagonal lattice structure of nano-boron nitride forms an efficient phonon transmission channel through strong in-plane covalent bonds. Molecular dynamics simulations show that boron nitride sheets form a three-dimensional interconnected network in the meta-aramid matrix, and the average free path of phonons propagating along the layer is significantly increased, reducing the phonon scattering probability caused by the disordered arrangement of molecular chains in the amorphous region; the hydroxyl groups on the surface of boron nitride form hydrogen bonds with the amide groups of meta-aramid, reducing interface defects. The interfacial thermal conductivity is much higher than that of the pure meta-aramid interface, verifying the contribution of the decrease in interfacial thermal resistance to the overall thermal conductivity.

[0033] Free volume regulation mechanism: Nano-boron nitride particles fill the free volume between the meta-aramid molecular chains, and the free volume fraction is significantly reduced. The compression of the molecular chain movement space increases the glass transition temperature, and at the same time, the relaxation time of the molecular chain segments is prolonged; the reduction of free volume shortens the average free path of electrons. According to the impact ionization theory, the breakdown field strength is inversely proportional to the free path, and the experimental data is consistent with the theoretical prediction.

[0034] Interface bonding strengthening mechanism: The hydroxyl groups on the surface of boron nitride are combined with the amide groups of the meta-aramid molecular chains through hydrogen bonds and van der Waals forces, significantly enhancing the interface binding energy and increasing the fracture energy density; boron nitride particles act as pinning points, increasing the deflection angle of the crack propagation path, and synchronously improving the tensile strength and elongation at break.

[0035] Thermal stability mechanism of action: Boron nitride particles act as nano-level crosslinking points, restricting the slippage and rotation of meta-aramid molecular chains. Dynamic mechanical analysis shows that the rate of decrease in the storage modulus in the high-temperature region slows down, and the thermal decomposition temperature is significantly increased; Free volume-temperature correlation: Boron nitride doping inhibits the thermal expansion movement of molecular chains at high temperatures, and the coefficient of thermal expansion of free volume is significantly reduced.

[0036] Electrical property optimization mechanism: The wide bandgap of boron nitride forms a potential barrier, inhibiting electron transition. The depth of electron traps at the interface increases, and the volume resistivity is significantly improved; the uniform dispersion of boron nitride reduces the distortion of the local electric field. Finite element simulations show that the maximum electric field strength decreases, and the partial discharge inception voltage increases, which is consistent with the experimental results.

[0037] Example 1

[0038] A preparation method of a nano boron nitride composite meta - aramid - based insulating material, comprising the following steps:

[0039] (1) Weigh 70 g of meta - aramid precipitated fibers, add them to deionized water according to a slurry concentration of 0.3 wt%, and control the water temperature at 25 ± 2 °C. Using a standard fiber beater, first dissociate the fibers at an initial speed of 1500 revolutions per minute for 10 minutes, and then increase the speed to 3600 revolutions per minute for fine beater treatment for 30 minutes to obtain a precipitated fiber slurry. After the beater treatment, immediately seal the tank opening to prevent fiber oxidation.

[0040] (2) Weigh 30 g of meta - aramid short - cut fibers, add them to deionized water according to a slurry concentration of 3 wt%. Use an electric stirrer to continuously stir at a speed of 300 revolutions per minute for 66 minutes until the fibers are evenly dispersed without agglomeration, obtaining a short - cut fiber slurry.

[0041] (3) Dissolve 5 g of sodium dodecylbenzenesulfonate (SDBS) in 250 mL of deionized water, place it in a 50 °C constant - temperature water - bath magnetic stirrer (rotation speed 1200 revolutions per minute) and stir for 5 minutes until completely dissolved. Add 10 g of hexagonal boron nitride nanosheets, and then transfer it to an ultrasonic cell disruptor. Under the condition of an ice - water bath, ultrasonically treat it at a power of 400 W for 30 minutes to obtain a stable BN suspension.

[0042] (4) Preparation of the mixed slurry

[0043] Mix the precipitated fiber slurry in step (1) with the short - cut fiber slurry in step (2), and add the BN suspension in step (3). Add 0.04 g of polyacrylamide (PAM) as a retention aid to the mixed system, maintain the 50 °C water - bath condition and stir at 1200 revolutions per minute for 10 minutes to form a uniform composite slurry.

[0044] (5) Inject the composite slurry into a standard paper sheet former, and supplement the deionized water volume to 9 L. Start the rapid stirring mode (2000 revolutions per minute) and disperse for 5 minutes, then turn on the vacuum pump to dehydrate for 70 seconds until there is no obvious free water. Transfer the wet paper sheet to a vacuum drying oven, set the air pressure in the oven to be 0.05 MPa lower than the standard atmospheric pressure, and conduct constant - temperature drying: dry at 110 °C for 1 minute → turn over → dry at 110 °C for 1 minute → turn over → dry at 110 °C for 2 minutes → turn over → dry at 110 °C for 2 minutes → in the final stage, dry at 110 °C for 5 minutes until the moisture content ≤ 0.5%.

[0045] (6) Attach the dried paper sheet to the surface of the rotating wheel of a double - roll hot press, set the roller surface temperature at 120 °C, and pre - heat for 60 seconds. Apply a pressure of 0.6 MPa, and roll - press the four sides along the longitudinal direction of the paper sheet in turn (the rolling - press contact time for each side is 7.5 seconds). After a total rolling - press time of 30 seconds, take out the finished product to obtain the nano boron nitride composite meta - aramid - based insulating material.

[0046] Example 2

[0047] It is only different from Example 1 in that the addition amount of hexagonal boron nitride nanosheets in step (3) is 5 g.

[0048] Example 3

[0049] It is only different from Example 1 in that the addition amount of hexagonal boron nitride nanosheets in step (3) is 15 g.

[0050] Example 4

[0051] It is only different from Example 1 in that the addition amount of polyacrylamide (PAM) in step (4) is 0.02 g.

[0052] Example 5

[0053] It is only different from Example 1 in that the addition amount of polyacrylamide (PAM) in step (4) is 0.06 g.

[0054] Example 6

[0055] It is only different from Example 1 in that the addition amount of meta-aramid precipitated fibers in step (1) is 50 g, and the addition amount of meta-aramid short fibers in step (2) is 50 g.

[0056] Example 7

[0057] It is only different from Example 1 in that the addition amount of meta-aramid precipitated fibers in step (1) is 80 g, and the addition amount of meta-aramid short fibers in step (2) is 20 g.

[0058] Example 7

[0059] It is only different from Example 1 in that the slurry concentration of meta-aramid precipitated fibers in step (1) is 0.6 wt%.

[0060] Example 8

[0061] (6) It is only different from Example 1 in that in step (5), the wet paper sheet is transferred to a vacuum drying oven and dried according to a stepwise temperature rising program: dried at 70 °C for 1 minute → turn over → dried at 70 °C for 1 minute → turn over → dried at 90 °C for 2 minutes → turn over → dried at 90 °C for 2 minutes → final stage dried at 110 °C for 5 minutes until the moisture content ≤ 0.5%.

[0062] Control Example

[0063] Taking pure poly-m-phenylene isophthalamide (meta-aramid, PMIA) as the control.

[0064] Performance Test

[0065]

[0066] Principle of thermal conductivity change:

[0067] Under room temperature conditions, the PMIA composite insulating paper with 10wt% BN added has the best improvement in thermal conductivity, reaching 0.857 W / (m·K), which is 3.51 times that of the unmodified meta-aramid insulating paper. This is because as the content of the thermal conductive filler increases, the nano boron nitride with good thermal conductivity fills the volume occupied by air in the pure PMIA insulating paper, forming a thermal conductive network and improving the thermal conductivity of the insulating paper. However, as the addition amount of the filler continues to increase to more than 10wt%, the distribution of the thermal conductive filler nano boron nitride in the PMIA insulating paper becomes wider with the continuous increase of the addition amount, and the probability of the boron nitride filler distributed on the surface of the insulating paper gradually increases, resulting in an increase in the surface roughness of the paper. This change directly leads to an increase in the interfacial thermal resistance of the PMIA insulating material, causing a more serious phonon scattering phenomenon, hindering the phonon propagation inside the insulating paper, and gradually reducing the thermal conductivity of the PMIA paper.

[0068] Principle of mechanical property change: The best effect is achieved when 10wt% of nano boron nitride is doped, which increases the tensile strength of the composite material from 7.53 kN / m to 9.06 kN / m, an increase of about 20%. This is because the nano boron nitride has a small particle size, a large specific surface area, and a high surface energy. As a filler, it can play a bonding role, making the precipitated fibers and short-cut fibers in the PMIA matrix combine more tightly, and improving the tensile strength to a certain extent. However, as the amount of nano filler further increases, due to the high surface energy of the nano filler itself and the insufficient tight combination between the inorganic nano filler and the organic fiber, the addition of the thermal conductive filler hinders the hydrogen bond combination between the PMIA fibers, increasing the voids in the formed paper structure and reducing the tensile strength of the PMIA paper.

[0069] Principle of dielectric strength change: The dielectric strength of the composite aramid paper first increases and then decreases with the increase of the addition amount of nano filler. This is because the addition of nano boron nitride changes the space charge and electric field distribution of the original PMIA insulating material, improving the electric field uniformity of the formed paper, and thus improving the dielectric strength of the PMIA paper-based insulating material. However, with the continuous addition of nano particles, due to the high surface energy of nano boron nitride, particle agglomeration occurs, resulting in many pores in the formed paper. At this time, when an electric field is applied, breakdown will occur first at defects such as pores, leading to a decrease in the overall dielectric strength of the PMIA insulating paper.

[0070] Principle of glass transition temperature change:

[0071] The boron nitride nanosheets form a physical cross - linked network with the PMIA aromatic rings through π - π interactions, significantly restricting the thermal motion ability of molecular segments. When the BN addition reaches 10 wt%, molecular dynamics simulations show that the free volume fraction of PMIA decreases from 24.3% in the pure matrix to 18.1%, resulting in the activation energy of chain segment motion increasing from 42.1 kJ / mol in the pure state to 54.8 kJ / mol. This compression effect of free volume directly drives the Tg to rise from 538 K of pure PMIA to 586 K (an 8.9% increase). However, when the BN content exceeds 10 wt%, the filler agglomerates to form local stress concentration areas (aggregates larger than 200 nm are observed by SEM), leading to a rebound of the free volume fraction to 19.8% and the Tg increase falling back to 579 K; BN realizes temperature - related structural stability by compressing free volume and suppressing its thermal expansion behavior: the BN nanosheets occupy the gaps between molecular chains, and positron annihilation lifetime spectroscopy (PALS) shows that the average radius of free volume holes decreases; during the heating process, the coefficient of thermal expansion of the free volume in the BN - doped system decreases, indicating that the thermal expansion motion of molecular chains is effectively constrained at high temperatures.

[0072] Relationship between PAM addition and properties:

[0073] With the increase in the addition amount of the dispersing aid PAM, the hydrophilic groups in it gradually increase the surface tension of the fibers in water, and the dispersion effect becomes better. However, with the further increase in the dispersant content, the slurry viscosity increases sharply, resulting in a poor pumping effect during papermaking, a decrease in the paper formation uniformity, and further a decrease in the mechanical and dielectric properties of the formed paper.

[0074] Relationship between the proportion of precipitated fibers and property changes:

[0075] Since the formed paper has more pores and poor paper uniformity when the amount of precipitated fibers is small, breakdown is likely to occur at the non - uniform parts. As the proportion of precipitated fibers in the meta - aramid paper continuously increases, the fine film - like precipitated fibers gradually fill the pores in the aramid paper matrix, improving the paper uniformity, so the dielectric strength gradually increases. However, when the proportion of precipitated fibers is higher than 70%, due to the high proportion, the precipitated fibers can already fill the vast majority of pores in the formed paper, so further increasing the proportion of precipitated fibers has little effect on improving the uniformity of the aramid formed paper, and the dielectric strength tends to a stable value.

[0076] Relationship between the concentration of precipitated fiber slurry and property changes:

[0077] The physical packing between the fibers in the high-concentration precipitation is more compact, and the porosity decreases to form a continuous heat conduction path, which increases the thermal conductivity from 0.244 W / (m·K) in the control example to 0.841 W / (m·K). However, excessive fibers lead to a sharp increase in the viscosity of the slurry system, and fiber entanglement and agglomeration are likely to occur during the shear dispersion process, reducing the effective contact area and hydrogen bond binding sites, resulting in a decrease in the tensile strength from 9.06 kN / m in the optimal Example 1 to 8.24 kN / m. At the same time, the locally over-dense fiber packing causes stress concentration points, weakening the interfacial bonding force, resulting in the dielectric strength (27.56 kV / mm) and volume resistivity (2.635×10 16 Ω·m) being lower than 31.52 kV / mm and 3.158×10 16 Ω·m in Example 1.

[0078] Relationship between drying method and property change:

[0079] In the stepped drying method, since the initial temperature is relatively low, the water evaporation is slow during the drying process. After turning over, the water accumulates more seriously under the paper, resulting in an increase in the water content of the dried paper. During the rolling process, the water vaporizes due to the high temperature, leaving gaps between the fibers, reducing the uniformity of the fiber spatial arrangement and the rolling forming effect.

[0080] The embodiments described above are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A preparation method of a nano boron nitride composite meta-aramid-based insulating material, characterized in that, The method includes the following steps: Disperse the meta-aramid precipitated fiber and the meta-aramid short fiber into a precipitated fiber slurry and a short fiber slurry respectively, mix them, then add a hexagonal boron nitride nanosheet dispersion and a retention aid to react to obtain a uniform composite slurry, and then shape it to obtain a nano-boron nitride composite meta-aramid-based insulating material.

2. The preparation method according to claim 1, wherein The mass ratio of the meta-aramid precipitated fiber to the meta-aramid short fiber is (5-8):(5-2).

3. The preparation method according to claim 2, characterized in that, The mass ratio of the meta-aramid precipitated fiber to the meta-aramid short fiber is 7:

3.

4. The preparation method according to claim 1, characterized in that, The slurry concentration of the precipitated fiber slurry is 0.3 wt%, and the slurry concentration of the short fiber slurry is 3 wt%.

5. The preparation method according to claim 1, characterized in that, The hexagonal boron nitride nanosheet dispersion is a micellar solution of sodium dodecylbenzenesulfonate and water.

6. The preparation method according to claim 5, wherein The mass ratio of the hexagonal boron nitride nanosheets to sodium dodecylbenzenesulfonate in the hexagonal boron nitride nanosheet dispersion is (5-15):

5.

7. The preparation method according to claim 1, characterized in that, The mass ratio of the hexagonal boron nitride nanosheets in the hexagonal boron nitride nanosheet dispersion to the total mass of the meta-aramid precipitated fiber and the meta-aramid short fiber is 5-15 wt%.

8. The preparation method according to claim 7, characterized in that, The mass ratio of the hexagonal boron nitride nanosheets in the hexagonal boron nitride nanosheet dispersion to the total mass of the meta-aramid precipitated fiber and the meta-aramid short fiber is 10 wt%.

9. The preparation method according to claim 1, characterized in that, The retention aid is a polyacrylamide retention aid.

10. A nano boron nitride composite meta-aramid-based insulating material, characterized in that, It is prepared by using the preparation method described in any one of claims 1-9.

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