A method for preparing aging-resistant cellulose insulating paper

Abstract

The present invention discloses a method for preparing aging-resistant cellulose insulating paper, which belongs to the technical field of oil-paper insulation for electrical equipment. The present invention solves the problems in the prior art that the degree of polymerization of cellulose insulating paper in the "oil-paper" insulation system decreases significantly, resulting in aging, reduced insulation performance, and shortened service life. The invention first pre-treats the natural softwood pulp, performs different degrees of refining, and then mixes it; secondly, the mixed pulp is purified and fine fibers are screened out, and aramid nanofibers are used to replace the screened fine fibers to obtain a mixed pulp containing aramid nanofibers; the mixed pulp containing aramid nanofibers is added to a sheet making machine for dehydration, forming, and drying to obtain base paper, which is then calendered to obtain cellulose insulating paper. The aging resistance and insulation performance of the cellulose insulating paper are significantly improved, and it can meet the performance requirements of insulating paper for ultra-high voltage transformers.

Description

Technical Field

[0001] The invention belongs to the cross-technical field of papermaking industry and material industry, and particularly relates to a method for preparing cellulose insulating paper used in an ultra-high voltage transformer. Background Art

[0002] Transformers in my country are a diverse category. Based on their cooling method, they can be divided into self-cooling and evaporative cooling types. Self-cooling transformers include oil-immersed transformers and dry-type transformers, while evaporative cooling transformers include chlorinated fluorine transformers. Of these three types, oil-immersed transformers are the most traditional, accounting for the highest proportion of transformers currently in operation and having the widest application range. Their insulation system primarily utilizes an "oil-paper" composite system consisting of mineral oil and cellulose insulating paper or cardboard. The insulating oil and cellulose insulating paper in this composite system will age during use. This process, known as aging, degrades under the influence of electrical and thermal stresses, oxygen, and moisture. During aging, the insulating and mechanical properties of the cellulose insulating paper gradually degrade. However, actual operating experience shows that even after long-term use, the insulation breakdown voltage of insulating oil only decreases by approximately 10%, generally not hindering operation. This can be resolved through oil purification, regeneration, or even replacement with new oil, so it is not a major factor affecting transformer life. Cellulose insulation paper is a solid material that acts as an insulator in the windings through a wound process. After aging, it cannot be replaced during transformer operation. Therefore, the life of an oil-immersed transformer depends on the life of the cellulose paper. With the development of the power industry and the continuous increase in grid transmission voltage levels, cellulose paper is susceptible to a decline in insulation performance and mechanical strength over long-term use. This directly leads to a significant decrease in the degree of polymerization (DP) of the cellulose paper, making it difficult for oil-immersed transformers to operate for extended periods. The DP is a characteristic metric of cellulose paper aging, with an average DP of around 1000 to 1300. When the DP drops to 500, the transformer's insulation system is considered to have entered the middle of its lifespan; when it drops to 250, it is considered to have reached the end of its lifespan. In an oil-paper composite system, cellulose paper gradually ages over time due to various aging factors, and this aging process is irreversible. After cellulose insulation paper is impregnated with oil, its fine fibers are the first to degrade. This manifests itself macroscopically as brittleness and microscopically as glycosidic bond breakage. This breakage of fine fibers generates more water, which, combined with other factors, leads to a significant decrease in the paper's degree of polymerization (DOP), ultimately causing a decline in electrical and mechanical properties and resulting in breakdown, significantly shortening the lifespan of the insulation paper. When the DOP drops below 250, the cellulose paper in the oil-paper composite system must be replaced; otherwise, a series of serious insulation failures can occur, leading to accidents.

[0003] Therefore, the development of aging-resistant cellulose insulation paper is imperative, and nanotechnology may be a promising solution. Liao Ruijin et al. improved the DC breakdown strength of nano-modified cellulose insulation paper by adding nano-Al2O3 particles with a low relative dielectric constant to the paper. Tang Chao et al. also improved the AC breakdown strength of cellulose insulation paper by adding nano-TiO2 or nano-montmorillonite to the paper. The results showed that when the nano-TiO2 content was 3wt% or when the nano-Al2O3 particle content was 1wt%, the nanoparticle-modified cellulose insulation paper had the highest AC and DC breakdown strength and the best insulation performance. However, the disadvantages of inorganic nanoparticle-modified cellulose insulation paper are also obvious. First, the density and hydrophilicity of nanoparticles differ from those of natural fibers, making it difficult for nanoparticles to be evenly dispersed in natural fibers, and nanoparticles will agglomerate on the surface and inside the fibers. Second, the small size of nanoparticles makes it very easy for them to flow away with the slurry during the papermaking process, making it difficult to form a paper structure on the filter screen. Retention is a challenge that needs to be overcome. The above two points will directly lead to the decline of the mechanical properties and insulation properties of cellulose insulation paper, which cannot meet the requirements of cellulose insulation paper for ultra-high voltage transformers.

[0004] In recent years, aramid nanofibers have gradually become a research hotspot for aging-resistant cellulose insulation paper for electronic and power systems due to their outstanding mechanical properties, thermal stability, and insulation properties. This organic synthetic material is known for its extremely high strength and modulus, low density, and excellent wear resistance. Its chemical structure is stable, similar to that of natural fibers, and can combine more effectively with natural fibers. The large aspect ratio of aramid nanofibers is conducive to the construction of a network structure in natural fibers, effectively solving the problem of uniform distribution and poor retention of nanoparticles in cellulose insulation paper. This study proposes a method for modifying cellulose insulation paper using aramid nanofibers. This method uses unbleached sulfate softwood pulp as raw material, removes the fine fibers, and selects long fibers that degrade more slowly during the aging process. These long fibers are mixed with aramid nanofibers to prepare cellulose insulation paper with good mechanical properties and excellent insulation properties. Summary of the Invention

[0005] During operation, the cellulose insulating paper of the ultra-high voltage transformer is affected by factors such as temperature, electric field, moisture, and oxygen, and will gradually age, resulting in a serious decline in the insulation performance of the ultra-high voltage transformer, which greatly shortens the life of the ultra-high voltage transformer. In response to this shortcoming, the present invention provides a method for preparing aging-resistant cellulose insulating paper, which removes fine fibers from the pulp by screening out the fine fibers, replaces the screened fine fibers with aramid nanofibers, and retains the long fibers, that is, the insulating paper is made by pulping with long fibers that degrade more slowly during the operation of the transformer. After mechanical treatment, the rate of decrease in the degree of polymerization of the cellulose insulating paper can be reduced, and its breakdown strength can be improved. After being soaked in oil, the cellulose insulating paper has great "endurance" and does not pollute the environment, so that it can meet the requirements of long-term safe operation, high-quality, and stable power supply of ultra-high voltage transformers.

[0006] The technical solution adopted in the present invention is:

[0007] A method for preparing aging-resistant cellulose insulating paper, characterized by comprising the following steps:

[0008] (1) Pretreatment of the wood pulp board: soak the wood pulp board in deionized water to loosen it;

[0009] (2) subjecting the pretreated pulp to different degrees of mechanical refining, and mixing them in proportion according to the difference in beating degree to obtain a mixed pulp;

[0010] (3) Purifying the mixed slurry by: hydrochloric acid treatment and magnesium acetate treatment;

[0011] (4) Mechanically screening the treated pulp to remove fine fibers in the pulp and obtain a good pulp without fine fibers;

[0012] (5) replacing the cellulose fine component with an equal amount of aramid nanofibers to obtain a mixed pulp containing aramid nanofibers;

[0013] (6) debonding the obtained mixed pulp and obtaining cellulose base paper through wet forming technology;

[0014] (7) Calendering the obtained cellulose base paper to obtain cellulose insulating paper.

[0015] Preferably, the degree of polymerization of the wood pulp cellulose is 1000-1900.

[0016] Preferably, the refining speed of the mechanical refining is 5000~20000r, the slurry concentration is 5~20%, and the refining gap is 0~4mm.

[0017] More preferably, the refining speed is 8000-15000 r, the refining slurry concentration is 8-10%, and the refining gap is 0-2 mm.

[0018] Preferably, the mixed slurry is compounded according to the beating degree, wherein the low beating degree ranges from 30 to 65°SR, the high beating degree ranges from 70 to 95°SR, and the mixing ratio of the two is 5:5 to 9:1.

[0019] More preferably, the low beating degree ranges from 45 to 65°SR, the high beating degree ranges from 80 to 85°SR, and the mixing ratio of the two is 6:4 to 8:2.

[0020] Preferably, in the purification process, the concentration of the acid treatment slurry is 0.5-5%, and the mass concentration of hydrochloric acid is 0.5-3%; the concentration of the magnesium salt treatment slurry is 0.5-5%, and the concentration of magnesium acetate is 1-5%.

[0021] More preferably, the concentration of the acid-treated slurry is 3-5%, the mass concentration of hydrochloric acid is 1-2%, the concentration of the magnesium salt-treated slurry is 1-3%, and the concentration of magnesium acetate is 2-4%.

[0022] Preferably, the mechanical screening is performed by using a Ball sieving instrument, and the mesh size of the sieve is 20-200 meshes.

[0023] More preferably, the mesh size is 30 to 100 meshes.

[0024] Preferably, the diameter of the aramid nanofiber is 20-200 nm, and the aspect ratio is 500-2000.

[0025] More preferably, the diameter of the aramid nanofiber is 120-140 nm, and the aspect ratio is 800-1000.

[0026] Preferably, the content of the replaced fine fibers is 1-20% of the mass of the cellulose insulation paper, that is, the content of the aramid nanofibers is 1-20% of the mass of the finished paper.

[0027] More preferably, the content of the replaced fine fibers is 5-15% of the mass of the cellulose insulation paper, that is, the content of the nanofibers is 6-18% of the mass of the finished paper.

[0028] Preferably, the calendering temperature is 50-100° C., the hot pressing roller speed is 0.5-10 m / min, and the calendering line pressure is 5-70 N / mm.

[0029] More preferably, the calendering temperature is 70-80° C., the hot pressing roller speed is 1-3 m / min, and the calendering line pressure is 30-40 N / mm.

[0030] Beneficial effects:

[0031] (1) As the operating time of the ultra-high voltage transformer increases, the long fibers, short fibers and fine fibers in the cellulose insulation paper of the "oil-paper" composite system will degrade first under the influence of various factors, followed by the fine fibers. The cleavage of the glycosidic bonds on the fine fibers will generate more free water, which will directly lead to a decrease in the length of the cellulose chain and a decrease in the polymerization degree of the long fibers, and ultimately lead to a decrease in the mechanical properties of the cellulose insulation paper. The cellulose insulation paper will be broken down during use, and its life will be shortened, which will cause the ultra-high voltage transformer to malfunction or even a serious accident. The purpose of screening pulp in the present invention is to select the pulp fraction with better chemical pulp performance, select the long fibers intercepted by the screen, replace the fine fibers screened out with aramid nanofibers, mix the two, and prepare cellulose insulation paper containing aramid nanofibers. The participation of aramid nanofibers improves the electrical properties of cellulose insulation paper, effectively slows down the degradation rate of the pulp polymerization degree during the aging process, and to a certain extent extends the life of the insulation paper, making the cellulose insulation paper more resistant to aging during long-term operation.

[0032] (2) The present invention uses wet papermaking technology to produce cellulose insulation paper. Screening out fine fibers in the pulp will slightly reduce the mechanical and electrical properties of the cellulose insulation paper. Aramid nanofibers, with their high strength and high modulus properties, have a certain strengthening effect on the strength properties of paper. This strengthening effect is crucial for cellulose insulation paper to resist the performance degradation caused by mechanical stress during aging. Therefore, by replacing the screened fine fibers with aramid nanofibers and further treating the base paper containing aramid nanofibers in the subsequent calendering process, the close connection between the fibers can be promoted. This process ensures the good mechanical properties of the cellulose insulation paper. In addition, due to the presence of aramid nanofibers in the "oil-paper" composite system, the rate of decrease in the degree of polymerization of the cellulose insulation paper during the operation of the ultra-high voltage transformer is slowed down, thereby extending the service life of the transformer. Specific implementation plan

[0033] The present invention is described in detail below with reference to specific embodiments.

[0034] Example 1

[0035] 360g of natural softwood pulp was soaked in deionized water for 4 hours and torn into 25mm x 25mm small pulp sheets. The pulp sheets were poured into a Wall-E pulper and deionized water was added to make the pulp concentration reach 2.5%. The pulp was dispersed until there were no small pulp lumps. The water was squeezed out and then the next beating process was carried out.

[0036] Take 30g of the pretreated pulp and add deionized water to a total weight of 300g. Place it in a PFI beater and beat it at 8000rpm and a beating degree of 45°SR. Take another 30g of the pretreated pulp and add deionized water to 300g. Place it in a PFI beater and beat it at 15000rpm and a beating degree of 85°SR. Mix the pretreated and pretreated pulp in a mass ratio of 6:4. Dehydrate the mixed pulp and set aside.

[0037] Take 30g of the mixed slurry, adjust the slurry concentration to 4%, add 3mL of 3wt% hydrochloric acid, stir evenly, and treat for 60 min; after acid treatment, wash with deionized water three times, each treatment time is 5 min; adjust the slurry concentration after washing to 4%, add 1.5g of magnesium acetate, stir evenly, and treat for 60 min. After magnesium salt treatment, wash with deionized water three times, each treatment time is 5 min. After washing, dehydrate, break up, and balance the moisture to obtain a treated slurry;

[0038] Take 15g of the treated mixed slurry and sieve it using a Bauer sieve. The slurry with a mesh size of 30 is preferably selected. The sieving time is 30min. Repeat this process 24 times and let it stand for 4h to balance the moisture.

[0039] Weigh 0.4 g of aramid nanofibers (accounting for 16.85% of the mass of the base paper) to replace fine fibers, and mix the aramid nanofibers with the preferred slurry;

[0040] The treated slurry was decomposed at a decomposition speed of 10000r, and the obtained suspension was added to a sheet making machine for dehydration and molding to prepare a sheet with a weight of 75g / m 2 The wet paper sheet was dried at 95°C for 10 min to obtain cellulose base paper;

[0041] The obtained cellulose base paper was calendered at a calendering temperature of 70° C., a hot pressing roller speed of 1 m / min, and a pressure of 30 N / mm to obtain cellulose insulating paper.

[0042] The performance parameters of the cellulose insulating paper prepared in Example 1 are as follows: the ash content of the paper is 0.23% (≤0.25%), the sodium ion content in the ash is 23.3 mg·kg -1 (≤30mg·kg -1), thickness of 78μm, tensile strength of 6.39kN / m, tear strength of 1307mN, folding endurance of 1017 times, breakdown voltage of 1.29kV, breakdown strength of 13.43kV / mm; the degree of polymerization of cellulose insulation paper impregnated with oil for 0d (day) is 1089, and the degree of polymerization of cellulose insulation paper impregnated with oil for 25d is 263; the breakdown strength of cellulose insulation paper impregnated with oil for 0d is 18.0kV / mm, and the breakdown strength of cellulose insulation paper impregnated with oil for 25d is 10.42 kV / mm.

[0043] Example 2

[0044] 360g of natural softwood pulp was soaked in deionized water for 4 hours and torn into 25mm x 25mm small pulp sheets. The pulp sheets were poured into a Wall-E pulper and deionized water was added to make the pulp concentration reach 2.5%. The pulp was dispersed until there were no small pulp lumps. The water was squeezed out and then the next beating process was carried out.

[0045] Take 30g of the pretreated pulp and add deionized water to a total weight of 300g. Place it in a PFI beater and beat it at 8500rpm and a beating degree of 55°SR. Take another 30g of the pretreated pulp and add deionized water to 300g. Place it in a disc mill and beat it at 15000rpm and a beating degree of 85°SR. Mix the two in a mass ratio of 6:4. Dehydrate the mixed pulp and set aside.

[0046] Take 30g of the mixed slurry, adjust the slurry concentration to 4%, add 3mL of 3wt% hydrochloric acid, stir evenly, and treat for 60 min; after acid treatment, wash with deionized water three times, each treatment time is 5 min; adjust the slurry concentration after washing to 4%, add 1.5g of magnesium acetate, stir evenly, and treat for 60 min. After magnesium salt treatment, wash with deionized water three times, each treatment time is 5 min. After washing, dehydrate, break up, and balance the moisture to obtain a treated slurry;

[0047] Take 15g of the treated mixed slurry and use a Bauer sieving instrument to sieve it. The slurry retained by both 30 mesh and 100 mesh is selected. The sieving time is 30min. Repeat this process 24 times and let it stand for 4h to balance the moisture.

[0048] Weigh 0.3 g of aramid nanofibers (accounting for 12.62% of the mass of the base paper) to replace fine fibers, and mix the aramid nanofibers with the preferred slurry;

[0049] The treated slurry was decomposed at a decomposition speed of 10000r, and the obtained suspension was added to a sheet making machine for dehydration and molding to prepare a sheet with a weight of 75g / m 2 The wet paper sheet was dried at 95°C for 10 min to obtain cellulose base paper;

[0050] The obtained cellulose base paper was calendered at a calendering temperature of 80° C., a hot pressing roller speed of 1 m / min, and a pressure of 3 N / mm to obtain cellulose insulating paper.

[0051] The performance parameters of the cellulose insulation paper prepared in Example 2 are as follows: the ash content of the paper is 0.24% (≤0.25%), the sodium ion content in the ash is 23.4 mg·kg -1 (≤30mg·kg -1 ), thickness is 78μm, tensile strength is 6.38KN / m, tear strength is 1309mN, folding endurance is 1019 times, breakdown voltage is 1.28kV, breakdown strength is 13.38kV / mm; the degree of polymerization of cellulose insulation paper impregnated with oil for 0d is 1091, and the degree of polymerization of cellulose insulation paper impregnated with oil for 25d is 261; the breakdown strength of cellulose insulation paper impregnated with oil for 0d is 17.87kV / mm, and the breakdown strength of cellulose insulation paper impregnated with oil for 25d is 10.12kV / mm.

[0052] Example 3

[0053] 360g of natural softwood pulp was soaked in deionized water for 4 hours and torn into 25mm x 25mm small pulp sheets. The pulp sheets were poured into a Wall-E pulper and deionized water was added to make the pulp concentration reach 2.5%. The pulp was dispersed until there were no small pulp lumps. The water was squeezed out and then the next beating process was carried out.

[0054] Take 30g of the pretreated pulp and add deionized water to a total weight of 300g. Place it in a PFI beater and beat it at 9000rpm and a beating degree of 65°SR. Take another 30g of the pretreated pulp and add deionized water to 300g. Place it in a PFI beater and beat it at 15000rpm and a beating degree of 85°SR. Mix the two in a mass ratio of 8:2. Dehydrate the mixed pulp and set aside.

[0055] Take 30g of the mixed slurry, adjust the slurry concentration to 4%, add 3mL of 3wt% hydrochloric acid, stir evenly, and treat for 60 min; after acid treatment, wash with deionized water three times, each treatment time is 5 min; adjust the slurry concentration after washing to 4%, add 1.5g of magnesium acetate, stir evenly, and treat for 60 min. After magnesium salt treatment, wash with deionized water three times, each treatment time is 5 min. After washing, dehydrate, break up, and balance the moisture to obtain a treated slurry;

[0056] Take 15g of the treated mixed slurry and use a Bauer sieving instrument to sieve it. The slurry with a mesh size of 30 mesh and 50 mesh is preferably taken. The sieving time is 30min. Repeat this process 24 times and let it stand for 4h to balance the moisture.

[0057] Weigh 0.16g of aramid nanofibers (accounting for 6.88% of the mass of the base paper) to replace fine fibers, and mix the aramid nanofibers with the preferred slurry;

[0058] The treated slurry was decomposed at a decomposition speed of 10000r, and the obtained suspension was added to a sheet making machine for dehydration and molding to prepare a sheet with a weight of 75g / m 2 The wet paper sheet was dried at 95°C for 10 min to obtain cellulose base paper;

[0059] The obtained cellulose base paper was calendered at a calendering temperature of 75° C., a hot pressing roller speed of 1 m / min, and a pressure of 35 N / mm to obtain cellulose insulating paper.

[0060] The performance parameters of the cellulose insulation paper prepared in Example 3 are as follows: the ash content of the paper is 0.22% (≤0.25%), the sodium ion content in the ash is 22.6 mg·kg -1 (≤30mg·kg -1 ), thickness of 78μm, tensile strength of 6.56kN / m, tear strength of 1217mN, folding endurance of 1085 times, breakdown voltage of 1.25kV, breakdown strength of 13.37kV / mm; the degree of polymerization of cellulose insulation paper impregnated with oil for 0d is 1109, and the degree of polymerization of cellulose insulation paper impregnated with oil for 25d is 268; the breakdown strength of cellulose insulation paper impregnated with oil for 0d is 17.56kV / mm, and the breakdown strength of cellulose insulation paper impregnated with oil for 25d is 10.13kV / mm.

[0061] Example 4

[0062] 360g of natural softwood pulp was soaked in deionized water for 4 hours and torn into 25mm x 25mm small pulp sheets. The pulp sheets were poured into a Wall-E pulper and deionized water was added to make the pulp concentration reach 2.5%. The pulp was dispersed until there were no small pulp lumps. The water was squeezed out and then the next beating process was carried out.

[0063] Take 30g of the pretreated pulp and add deionized water to a total weight of 300g. Place it in a PFI beater and beat it at 8500rpm and a beating degree of 55°SR. Take another 30g of the pretreated pulp and add deionized water to 300g. Place it in a disc beater and beat it at 15000rpm and a beating degree of 85°SR. Mix the two in a mass ratio of 8:2. Dehydrate the mixed pulp and set aside.

[0064] Take 30g of the mixed slurry, adjust the slurry concentration to 4%, add 3mL of 3wt% hydrochloric acid, stir evenly, and treat for 60 min; after acid treatment, wash with deionized water three times, each treatment time is 5 min; adjust the slurry concentration after washing to 4%, add 1.5g of magnesium acetate, stir evenly, and treat for 60 min. After magnesium salt treatment, wash with deionized water three times, each treatment time is 5 min. After washing, dehydrate, break up, and balance the moisture to obtain a treated slurry;

[0065] Take 15g of the treated mixed slurry and use a Bauer sieving instrument to sieve it. The slurry retained by the sieve mesh of 30 mesh and 50 mesh is preferably selected. The sieving time is 30min. Repeat this process 24 times and let it stand for 4h to balance the moisture.

[0066] Weigh 0.16g of aramid nanofibers (accounting for 6.88% of the dry weight of the base paper) to replace fine fibers, and mix the aramid nanofibers with the preferred slurry;

[0067] The treated slurry was decomposed at a decomposition speed of 10000r, and the obtained suspension was added to a sheet making machine for dehydration and molding to prepare a sheet with a weight of 75g / m 2 The wet paper sheet was dried at 95°C for 10 min to obtain cellulose base paper;

[0068] The obtained cellulose base paper was calendered at a calendering temperature of 75° C., a hot pressing roller speed of 1 m / min, and a pressure of 40 N / mm to obtain cellulose insulating paper.

[0069] The performance parameters of the cellulose insulating paper prepared in Example 4 are as follows: the ash content of the paper is 0.25% (≤0.25%), the sodium ion content in the ash is 23.8 mg·kg -1 (≤30mg·kg -1 ), thickness is 78μm, tensile strength is 6.32kN / m, tear strength is 1295mN, folding endurance is 1094 times, breakdown voltage is 1.36kV, breakdown strength is 13.91kV / mm; the degree of polymerization of cellulose insulation paper impregnated with oil for 0d is 1017, and the degree of polymerization of cellulose insulation paper impregnated with oil for 25d is 269; the breakdown strength of cellulose insulation paper impregnated with oil for 0d is 18.07kV / mm, and the breakdown strength of cellulose insulation paper impregnated with oil for 25d is 10.06kV / mm.

[0070] Comparative Example 1

[0071] The difference from Example 1 is that 30°SR and 90°SR slurries were mixed and sieved in a ratio of 7:3, and then the slurry retained by both the 30-mesh and 50-mesh meshes was mixed with 0.16 g (6.88% of the base paper mass) of aramid nanofibers for papermaking. The remaining processes and procedures were the same as those in Example 1. The performance parameters of the cellulose insulating paper produced in Comparative Example 1 were: thickness 78 μm, tensile strength 5.53 kN / m, tear strength 967 mN, folding endurance 321 times, breakdown voltage 0.77 kV, and breakdown strength 9.17 kV / mm; the degree of polymerization of the cellulose insulating paper impregnated with oil after 0 days was 1002, and the degree of polymerization of the cellulose insulating paper impregnated with oil after 25 days was 206; the breakdown strength of the cellulose insulating paper impregnated with oil after 0 days was 15.33 kV / mm, and the breakdown strength of the cellulose insulating paper impregnated with oil after 25 days was 8.42 kV / mm.

[0072] Comparative Example 2

[0073] The difference from Example 1 is that 30°SR and 90°SR were mixed and sieved in a ratio of 8:2 to obtain 30-mesh and 100-mesh cutoff slurries, and 0.3 g (accounting for 12.62% of the base paper mass) of aramid nanofiber was weighed for mixed papermaking. The remaining processes and procedures were the same as those in Example 1. The performance parameters of the cellulose insulating paper produced in Comparative Example 2 were: thickness 78 μm, tensile strength 5.35 kN / m, tear strength 932 mN, folding endurance 328 times, breakdown voltage 0.69 kV, and breakdown strength 9.87 kV / mm; the degree of polymerization of the cellulose insulating paper impregnated with oil after 0 days was 1001, and the degree of polymerization of the cellulose insulating paper impregnated with oil after 25 days was 201; the breakdown strength of the cellulose insulating paper impregnated with oil after 0 days was 16.05 kV / mm, and the breakdown strength of the cellulose insulating paper impregnated with oil after 25 days was 8.78 kV / mm.

[0074] Comparative Example 3

[0075] The difference from Example 1 is that 40° SR and 80° SR were mixed and sieved in a ratio of 7:3, and a 30-mesh, 50-mesh, and 100-mesh slurry was retained. 0.04 g (2.66% of the base paper mass) of aramid nanofiber was weighed for mixed papermaking. The remaining processes and procedures were the same as those in Example 1. The performance parameters of the cellulose insulating paper produced in Comparative Example 3 were: thickness 78 μm, tensile strength 5.63 kN / m, tear strength 985 mN, folding endurance 457 times, breakdown voltage 0.91 kV, and breakdown strength 10.01 kV / mm; the degree of polymerization of the cellulose insulating paper after 0 days of oil impregnation was 1003, and the degree of polymerization of the cellulose insulating paper after 25 days of oil impregnation was 203; the breakdown strength of the cellulose insulating paper after 0 days of oil impregnation was 15.67 kV / mm, and the breakdown strength of the cellulose insulating paper after 25 days of oil impregnation was 8.56 kV / mm.

[0076] Comparative Example 4

[0077] The difference from Example 1 is that 55° SR slurry was used for papermaking; the remaining processes and procedures were the same as in Example 1. The performance parameters of the cellulose insulating paper produced in Comparative Example 4 were: thickness 78 μm, tensile strength 4.43 kN / m, tear strength 865 mN, folding endurance 121 times, breakdown voltage 0.67 kV, and breakdown strength 8.67 kV / mm. The degree of polymerization of the cellulose insulating paper impregnated with oil after 0 days was 1089, and the degree of polymerization of the cellulose insulating paper impregnated with oil after 25 days was 108. The breakdown strength of the cellulose insulating paper impregnated with oil after 0 days was 8.45 kV / mm, and the breakdown strength of the cellulose insulating paper impregnated with oil after 25 days was 4.78 kV / mm.

[0078] Comparative Example 5

[0079] The difference from Example 1 is that 45° SR slurry is used for papermaking, and the remaining processes and procedures are the same as those in Example 1. The performance parameters of the cellulose insulating paper produced in Comparative Example 5 are: thickness 78 μm, tensile strength 4.32 kN / m, tear strength 876 mN, folding endurance 131 times, breakdown voltage 0.71 kV, breakdown strength 8.97 kV / mm; the degree of polymerization of the cellulose insulating paper impregnated with oil after 0 days is 1089, and the degree of polymerization of the cellulose insulating paper impregnated with oil after 25 days is 103; the breakdown strength of the cellulose insulating paper impregnated with oil after 0 days is 8.97 kV / mm, and the breakdown strength of the cellulose insulating paper impregnated with oil after 25 days is 5.12 kV / mm.

[0080] Comparative Example 6

[0081] The difference from Example 1 is that 65° SR slurry is used instead of papermaking, and the rest of the processes and procedures are the same as Example 1. The performance parameters of the cellulose insulating paper obtained in Comparative Example 6 are: thickness 78 μm, tensile strength 4.09 kN / m, tear strength 765 mN, folding endurance 118 times, breakdown voltage 0.68 kV, breakdown strength 8.77 kV / mm; the degree of polymerization of the cellulose insulating paper impregnated with oil after 0 days is 1089, and the degree of polymerization of the cellulose insulating paper impregnated with oil after 25 days is 101; the breakdown strength of the cellulose insulating paper impregnated with oil after 0 days is 9.12 kV / mm, and the breakdown strength of the cellulose insulating paper impregnated with oil after 25 days is 5.84 kV / mm.

[0082] Comparative Example 7

[0083] The difference from Example 1 is that the calendering process is omitted from the process, and the remaining processes and processes are the same as Example 1. The performance parameters of the cellulose insulating paper prepared in Comparative Example 7 are: thickness 110 μm, tensile strength 4.67 kN / m, tear strength 634 mN, folding endurance 571 times, breakdown voltage 0.65 kV, breakdown strength 8.56 kV / mm; the degree of polymerization of the cellulose insulating paper impregnated with oil after 0 days is 1019, and the degree of polymerization of the cellulose insulating paper impregnated with oil after 25 days is 175; the breakdown strength of the cellulose insulating paper impregnated with oil after 0 days is 14.68 kV / mm, and the breakdown strength of the cellulose insulating paper impregnated with oil after 25 days is 7.42 kV / mm.

[0084] Comparative Example 8

[0085] The difference from Example 1 is that no pulp purification treatment was performed. The ash content of the pulp raw material was 0.43% (≤0.25%) and the sodium ion content in the ash was 53.1 mg·kg -1 (≤30mg·kg -1 ) have exceeded the standard and have no paper value.

[0086] In comparative examples 1, 2, and 3, the tensile strength, folding endurance, and tear strength of the cellulose insulating paper before oil immersion all decreased with changes in the beating degree, fiber mixing ratio, and aramid nanofiber content, and were all lower than the strength of the cellulose insulating paper in the examples. After oil immersion, the breakdown strength of the cellulose insulating paper at 0d decreased from 15.68kV / mm to 8.58kV / mm at 25d. The results show that under high-temperature, aerobic working conditions, the fine fibers of cellulose insulating paper will degrade first, and the water produced by them will aggravate the degradation of long fibers, reduce the length of the cellulose macromolecular chain, and make the cellulose insulating paper more susceptible to breakdown after the mechanical properties decrease. After the cellulose insulating paper is oil-immersed, its average degree of polymerization at 0d is around 1100, and its degree of polymerization at 25d is around 200. The results show that under long-term high-temperature, aerobic working conditions, the cellulose insulating paper ages severely, and its service life enters the late stage. Adjustments need to be made according to actual conditions to ensure the normal use of the transformer, otherwise it will be dangerous. In Comparative Examples 4, 5, and 6, cellulose insulation paper was made from a pulp with a single beating degree. Compared with the cellulose insulation paper made by mixing pulps with two different beating degrees, its mechanical properties decreased significantly. After oil immersion, the degree of polymerization of the cellulose insulation paper decreased from 1100 at 0d to around 100 at 25d. The results show that the aging degree of cellulose insulation paper becomes more serious with the increase of operating time. The service life of cellulose insulation paper enters the end and it can no longer be used. New cellulose insulation paper needs to be replaced to ensure the safe operation of the transformer. In Comparative Example 7, the cellulose insulation paper that was not calendered increased in thickness and decreased in tightness compared to the cellulose insulation paper that was calendered. Calendering helps to improve the tightness and flatness of the paper, thereby improving its overall performance. After 25d of oil immersion, the degree of polymerization of the cellulose insulation paper was lower than 180, the degree of aging was very serious, the breakdown strength decreased significantly, and the service life entered the late stage. It needs to be decided according to the actual situation, otherwise it will be dangerous. In Comparative Example 8, the pulp for making cellulose insulating paper was not purified, and the chemical composition indexes detected failed to meet the indexes of cellulose insulating paper used in ultra-high voltage transformers, making it difficult to proceed with subsequent steps.

[0087] In summary, the long fibers retained by a small-mesh screen are used, and the fine fibers screened out from the pulp are replaced with aramid nanofibers. The two are mixed in a certain proportion to make paper. The cellulose insulating paper obtained after calendering has better insulation performance and breakdown strength than the cellulose insulating paper in the comparative example. After aging, the polymerization degree of the former is significantly higher than that of the latter. This method not only ensures the good mechanical properties of the cellulose insulating paper, but also slows down the aging rate of the polymerization degree of the paper-made cellulose insulating paper, effectively extending the service life of the cellulose insulating paper and making the cellulose insulating paper more resistant to aging. By comparing the results of the examples and comparative examples, we can clearly understand the effects of different conditions on the performance of cellulose insulating paper, as well as the important role of aramid nanofibers in improving the performance of insulating paper.

[0088] The present invention uses specific examples to illustrate the principles and implementation methods of the present invention. The above-mentioned embodiments are only preferred implementation methods of this patent and are only used to help understand the method and core ideas of the present invention. At the same time, for those skilled in the art, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for preparing aging-resistant cellulose insulating paper, characterized in that: The following steps are involved: (1) Pre-treating the wood pulp board: soaking the wood pulp board in deionized water to disintegrate the wood pulp board; the cellulose polymerization degree of the wood pulp board is 1000-1900; (2) subjecting the pretreated pulp to mechanical refining to different degrees, and mixing them in proportion according to the difference in beating degree to obtain a mixed pulp; the mixed pulp refers to the pulp compounded according to the size of the beating degree, wherein the low beating degree ranges from 30 to 65°SR, the high beating degree ranges from 70 to 95°SR, and the mixing ratio of the two is 5:5 to 9:1; (3) Purifying the mixed slurry by: hydrochloric acid treatment and magnesium acetate treatment; (4) mechanically screening the treated pulp to remove fine fibers in the pulp and obtain a good pulp without fine fibers; the mesh size of the mechanical screening is 20 to 200 meshes; (5) replacing the cellulose fine component with an equal amount of aramid nanofibers to obtain a mixed pulp containing aramid nanofibers; the nanofibers are aramid nanofibers with a diameter of 20 to 200 nm and an aspect ratio of 500 to 2000; the content of the replaced fine fibers is 1 to 20% of the mass of the cellulose insulation paper, that is, the content of the aramid nanofibers is 1 to 20% of the mass of the finished paper; (6) debonding the obtained mixed pulp and obtaining cellulose base paper through wet forming technology; (7) Calendering the obtained cellulose base paper to obtain cellulose insulating paper.

2. The preparation method according to claim 1, characterized in that The mechanical refining has a refining speed of 5000-20000 r, a pulp concentration of 5-20%, and a refining gap of 0-4 mm.

3. The preparation method according to claim 1, characterized in that The acid treatment slurry concentration is 0.5-5%, and the hydrochloric acid mass concentration is 0.5-3%; the magnesium salt treatment slurry concentration is 0.5-5%, and the magnesium acetate concentration is 1-5%.

4. The preparation method according to claim 1, characterized in that The calendering temperature is 50-100° C., the hot pressing roller speed is 0.5-10 m / min, and the calendering line pressure is 5-70 N / mm.