Dry spinning method for preparing polysilazane fibers and rapid stabilization treatment method initiated by free radicals

Abstract

The present invention discloses a method for preparing polysilazane fibers by dry spinning and a treatment method for rapid infusibilization by radical initiation. Add a polysilazane precursor, xylene, acrylic acid, and an initiator into a beaker, and place it in an ultrasonic cleaner for ultrasonic acceleration of dissolution to form a spinning solution. Then transfer the spinning solution to a spinning cylinder; introduce nitrogen into the spinning cylinder containing the spinning solution, extrude the spinning solution from a spinneret, and wind it on a take-up cylinder to form fiber filaments; heat the fiber filaments, and carry out infusibilization treatment by radical-initiated crosslinking at a temperature below the softening point of the polysilazane to form an insoluble and infusible three-dimensional network crosslinked structure. The present invention realizes the dry spinning of continuous polysilazane fibers. At the same time, rapid crosslinking by radical initiation is used for infusibilization treatment, greatly reducing the time for infusibilization treatment.

Description

Technical Field

[0001] The present invention belongs to the field of precursor converted ceramics fibers, and relates to a method for preparing polysilazane fibers, specifically to a method for preparing polysilazane fibers by dry spinning and a method for rapidly infusibilizing by radical initiation. Background Art

[0002] Silicon nitride fibers have the characteristics of high melting point, good high-temperature resistance, excellent mechanical properties, good corrosion resistance, etc., and have broad prospects in ceramic matrix composites. Silicon nitride ceramic fibers are mainly prepared by the precursor conversion method, which includes four steps: synthesis of polysilazane, spinning, infusibilization treatment, and high-temperature firing. At present, the spinning of polysilazane precursors mainly uses melt spinning, and the infusibilization treatment mostly uses air / oxygen infusibilization.

[0003] The preparation of polysilazane fiber precursor mainly uses melt spinning. Yuan Jia, Han Keqing, Zhao Xi, etc. from Donghua University published "Characterization and Melt Spinning of SiBN(C) Ceramic Fiber Precursors" (Synthetic Fiber Industry, 2011, 34(03): 1-4), and obtained SiBN(C) ceramic precursor fibers by melt spinning. The spinning temperature was 185-205°C, and the fiber diameter was 50-60μm. Li Wenhua from National University of Defense Technology published "Basic Research on the Preparation of Continuous SiBN Ceramic Fibers by Precursor Conversion Method" (National University of Defense Technology, 2012), and prepared SiBN fibers with different compositions by single-hole and multi-hole melt spinning. During the process of preparing polysilazane precursor fibers by melt spinning, due to insufficient degassing treatment, air bubbles are likely to appear inside the fibers. For some high-molecular-weight polysilazanes, due to their high softening point, the melt spinning temperature is too high, and melt spinning is not suitable for use.

[0004] At present, there are few reports on the preparation of polysilazane fibers by dry spinning. There are only some related reports on the dry spinning of polycarbosilane. William Toreki et al. (Composites Science and Technology, 1994, 51: 145) synthesized high-molecular-weight polycarbosilane, dissolved the synthesized high-molecular-weight polycarbosilane in a solvent to form a polycarbosilane concentrated solution, and dry-spun it into PCS fibers, and the high-temperature resistance was increased to 1700°C. The room-temperature mechanical properties of this SiC fiber are similar to those of the fibers obtained by melt spinning in the past, but the high-temperature resistance is more excellent.

[0005] The methods of stabilizing silicon nitride fibers are generally divided into air oxidation cross-linking, chemical vapor cross-linking, radiation cross-linking, etc. Yajima et al. from Tohoku University in Japan pioneered the technology of preparing continuous SiC fibers by the precursor conversion method, using air as the oxidation medium for stabilization. Air oxidation cross-linking is to place the precursor filaments in an air atmosphere, where the filaments react with oxygen in the air to undergo cross-linking and branching reactions, and the fibers form an infusible and insoluble three-dimensional network cross-linked structure. Air oxidation cross-linking is relatively simple and easy to operate, but the mechanical properties of the fibers after stabilization are poor. Lipowitz J et al. (Center for Advanced Materials Technology University of Sydney, 1992.5) studied the production of ceramic fibers by pyrolysis at 1600 °C in an Ar atmosphere after stabilization with NO2 and then treatment with BCl3. Hasegawa Y et al. (Composites Science and Technology, 1994, 51: 161) used vapors of unsaturated hydrocarbons such as cyclohexene, heptene or octyne to stabilize the ceramic fiber precursors to improve the high-temperature mechanical properties of the fibers. The process of chemical vapor cross-linking treatment is relatively complex and involves the emission of chemical gases, with poor environmental protection. Radiation cross-linking usually uses electron beam radiation. Free radicals are formed inside the PSZ fibers induced by electron beam radiation, and reactions such as coupling, oxidation, and grafting are initiated by the free radicals to cause cross-linking between the polysilazane fiber molecules, forming a good infusible chemical structure inside the precursor. However, the equipment for electron beam radiation stabilization is expensive, with high costs and complex operation.

[0006] In summary, currently, polysilazane fibers are generally mainly spun by melt spinning. However, bubbles are easily generated in the fibers produced by melt spinning, and the melt spinning temperature of high molecular weight polysilazane is too high, making it difficult to use the melt spinning process. The stabilization method is usually air stabilization, which is simple to operate, but the stabilization time is long, which is not conducive to the heat treatment of the fibers under continuous drawing, and the mechanical properties of the obtained fibers are poor. For chemical vapor cross-linking stabilization, the unsaturated hydrocarbon vapors used are toxic and harmful, with poor environmental protection and a long time. For electron beam radiation stabilization, it is an intermittent operation, the equipment is relatively expensive, and the cost is high. Summary of the Invention

[0007] To solve the problems existing in the above-mentioned background technology, the present invention proposes a method for preparing polysilazane fibers by dry spinning and subjecting them to non-melting treatment through rapid cross-linking initiated by free radicals. In this invention, a spinning solution is prepared by mixing polysilazane with xylene, acrylic acid, and the initiator azodiisobutyronitrile in a certain proportion. Acrylic acid reacts with polysilazane to introduce vinyl active functional groups into the polysilazane. Under the protection and pressure of nitrogen, the spinning solution flows down from the spinning cylinder and is drawn to the winding cylinder to obtain the polysilazane fiber precursor. Then, the obtained precursor is heated to an appropriate temperature, and the initiator forms primary free radicals, which initiate the reaction of the vinyl groups in the polysilazane to carry out rapid non-melting treatment, resulting in polysilazane fibers with a cross-linked structure. Through dry spinning and rapid cross-linking non-melting initiated by free radicals, the present invention obtains continuous polysilazane fibers with uniform diameters, greatly reducing the time for non-melting treatment.

[0008] The technical solution adopted by the present invention is as follows:

[0009] (1) Prepare the spinning solution: Add a certain amount of polysilazane precursor, xylene, acrylic acid, and initiator directly into a beaker to form a preliminary solution. Subsequently, place the preliminary solution in an ultrasonic cleaner and ultrasonicate for 1 h to accelerate dissolution, forming a spinning solution. Then, transfer the spinning solution to the spinning cylinder.

[0010] In the solvent system of the spinning solution of the present invention, acrylic acid is miscible with xylene and the initiator azodiisobutyronitrile. Moreover, polysilazane is a linear molecule. First, it swells in the solvent system, and small solvent molecules first penetrate into the molecular gaps of polysilazane. As polysilazane absorbs the solvent, polysilazane diffuses in the solvent until reaching dissolution equilibrium, and a spinning solution with a certain viscosity can be formed.

[0011] (2) Prepare polysilazane fibers by dry spinning: Introduce nitrogen into the spinning cylinder containing the spinning solution. Under the action of nitrogen pressure, the spinning solution flows down from the spinning cylinder and is then drawn and wound onto a winding cylinder rotating at a certain speed to obtain the fiber precursor.

[0012] (3) Rapid cross-linking initiated by free radicals: Heat the fiber precursor to an appropriate temperature, and at a temperature below the softening point of polysilazane, carry out non-melting treatment by rapid cross-linking initiated by free radicals, so that the fiber precursor forms an insoluble and infusible three-dimensional network cross-linked structure as the final product of polysilazane fibers.

[0013] The polysilazane precursor is prepared by ammonolysis and is a colorless, semi-transparent, thermoplastic brittle solid. The softening point of the polysilazane precursor is 200 °C to 220 °C.

[0014] In the present invention, a solvent system is formed by blending xylene, acrylic acid, and the initiator. Among them, xylene is used as the main solvent, which has a relatively low boiling point, is easy to volatilize, and has relatively low toxicity.

[0015] The acrylic acid can not only dissolve in xylene as a solvent, but also react with the polysilazane, so that vinyl active functional groups are introduced into the polysilazane.

[0016] In the spinning solution, acrylic acid reacts with polysilazane to introduce vinyl into the polysilazane, and then free radical initiated cross-linking polymerization is carried out by an azodiisobutyronitrile initiator.

[0017] Specifically, the azodiisobutyronitrile initiator can dissolve in xylene and acrylic acid, and at the same time act as a thermal initiator. It can decompose into primary free radicals by heat to initiate primary polymerization. Azodiisobutyronitrile initiates the re-polymerization of the vinyl functional groups introduced after the reaction of polysilazane and acrylic acid, so that the polysilazane raw filaments form a cross-linked structure.

[0018] The mass ratio between the polysilazane precursor and the solvent system composed of xylene, acrylic acid and initiator is controlled such that the mass content of the polysilazane precursor is 70 wt% - 93 wt%.

[0019] In the solvent system, the volume ratio of acrylic acid to xylene is 1:5. In a specific implementation, the addition amount of the initiator azodiisobutyronitrile is 0.01 g.

[0020] The initiator used is azodiisobutyronitrile.

[0021] In the dry spinning to prepare polysilazane fibers in step (2), the nitrogen gas flow rate is 0.1 - 0.2 L / min.

[0022] In step (2), the rotation speed of the take-up bobbin for drawing and winding is 200 - 300 r / min.

[0023] The diameter of the outlet of the spinning cylinder is 0.5 mm.

[0024] In step (2), the diameter of the fiber raw filaments is 10 μm - 30 μm.

[0025] In step (3), for free radical initiated rapid cross-linking, the fiber raw filaments are heated to 140 - 160 °C and kept warm for 10 - 20 min, and the fiber raw filaments can form an infusible and insoluble cross-linked structure.

[0026] The beneficial effects of the present invention are:

[0027] (1) The present invention proposes a method for preparing polysilazane fibers by dry spinning and performing non-melting treatment by free radical initiated rapid cross-linking.

[0028] In the traditional melt spinning process for preparing polysilazane fibers, bubbles are likely to form inside the fibers. Due to the high softening point of high-molecular-weight polysilazane, it is difficult to use the melt spinning method for spinning. In the present invention, dry spinning can effectively solve the problems of too high spinning temperature in melt spinning due to the high molecular weight of polysilazane and the easy formation of bubbles in melt-spun fibers.

[0029] The traditional dry spinning process is to extrude the spinning stream through the spinneret holes and then enter the duct to contact with the hot air flow, so that the solvent volatilizes and the fiber solidifies. In the present invention, by preparing a polysilazane spinning solution with an appropriate concentration, the polysilazane fibers can be directly formed without entering the duct.

[0030] Therefore, compared with melt spinning, the dry spinning in the present invention can also obtain continuous fine fibers, with uniform diameter of the fiber precursor and smooth surface, and no obvious defects.

[0031] (2) The present invention uses free radical-induced rapid crosslinking to achieve infusibilization treatment. Acrylic acid in the spinning solution reacts with polysilazane to introduce vinyl groups into polysilazane. The fiber precursor is heated to 140-160 °C and kept warm for 10-20 min. Azodiisobutyronitrile decomposes into primary free radicals when heated, initiating the crosslinking of vinyl group-containing polysilazane, greatly shortening the infusibilization treatment time.

[0032] A crosslinked structure is formed inside the polysilazane fiber precursor of the present invention, providing necessary conditions for the high-temperature pyrolysis ceramization under the subsequent fiber drawing. Description of the Drawings

[0033] Figure 1 It is a graph showing the relationship between the apparent viscosity and shear rate of a spinning solution containing 70 wt% - 93 wt% of a polysilazane precursor prepared by the method of the present invention. It can be seen from the figure that the spinning solution containing 82 wt% - 87 wt% of the polysilazane precursor is the most suitable spinning concentration;

[0034] Figure 2 It is a SEM image of the polysilazane fiber precursor prepared by the method of the present invention. Detailed Embodiments

[0035] The present invention will be further described below with reference to the drawings and embodiments. These embodiments are only used to illustrate the present invention and not to limit the scope of the present invention.

[0036] Embodiments of the present invention are as follows:

[0037] Example 1

[0038] a. Weigh 2.47 g of the polysilazane precursor, add it to a beaker, use a pipette to measure 1 ml of xylene, 0.2 ml of acrylic acid, and 0.01 g of 2,2'-azobis(2-methylbutyronitrile) into the beaker, place it in an ultrasonic machine and ultrasonicate for 1 h to accelerate the dissolution of the polysilazane precursor to form a spinning solution, and then transfer the spinning solution to a spinning cylinder. At this time, the mass fraction of the polysilazane precursor is 70 wt%, the viscosity of the spinning solution is low, and the fluidity is strong.

[0039] b. Place the spinning cylinder on an iron stand higher than the winding cylinder, introduce nitrogen into the spinning cylinder, and the nitrogen flow rate is 0.1 - 0.2 L / min. Under nitrogen protection and pressure, the spinning solution flows down from the spinning orifice, is drawn and wound onto the winding cylinder, and the winding speed of the winding cylinder is 200 - 300 r / min. The SEM image of the obtained as-spun fiber is as Figure 2 shown, the fiber diameter is 10 μm - 30 μm, the surface is smooth, and there are no obvious defects.

[0040] c. Heat the wound polysilazane as-spun fiber. 2,2'-azobis(2-methylbutyronitrile) decomposes by heating to form primary free radicals, which initiate the crosslinking of the vinyl-containing polysilazane. Raise the temperature to 140 - 160 °C and hold for 10 - 20 min, and the fiber forms an infusible and insoluble crosslinked structure.

[0041] Example 2

[0042] a. Weigh 5.0 g of the polysilazane precursor, add it to a beaker, use a pipette to measure 1 ml of xylene, 0.2 ml of acrylic acid, and 0.01 g of 2,2'-azobis(2-methylbutyronitrile) into the beaker, place it in an ultrasonic machine and ultrasonicate for 1 h to accelerate the dissolution of the polysilazane precursor to form a spinning solution, and then transfer the spinning solution to a spinning cylinder. At this time, the mass fraction of the polysilazane precursor is 82 wt%.

[0043] Steps b and c are the same as those shown in Example 1.

[0044] Example 3

[0045] a. Weigh 6.0 g of the polysilazane precursor, add it to a beaker, use a pipette to measure 1 ml of xylene, 0.2 ml of acrylic acid, and 0.01 g of 2,2'-azobis(2-methylbutyronitrile) into the beaker, place it in an ultrasonic machine and ultrasonicate for 1 h to accelerate the dissolution of the polysilazane precursor to form a spinning solution, and then transfer the spinning solution to a spinning cylinder. At this time, the mass fraction of the polysilazane precursor is 85 wt%.

[0046] Steps b and c are the same as those shown in Example 1.

[0047] Example 4

[0048] a. Weigh 7.1 g of polysilazane precursor and add it to a beaker. Use a pipette to measure 1 ml of xylene, 0.2 ml of acrylic acid, and 0.01 g of 2,2'-azobis(2-methylbutyronitrile) into the beaker. Place it in an ultrasonic machine and ultrasonicate for 1 h to accelerate the dissolution of the polysilazane precursor, forming a spinning solution. Then transfer the spinning solution to a spinning cylinder. At this time, the mass fraction of the polysilazane precursor is 87 wt%.

[0049] Steps b and c are the same as those shown in Example 1.

[0050] Example 5

[0051] a. Weigh 14.08 g of polysilazane precursor and add it to a beaker. Use a pipette to measure 1 ml of xylene, 0.2 ml of acrylic acid, and 0.01 g of 2,2'-azobis(2-methylbutyronitrile) into the beaker. Place it in an ultrasonic machine and ultrasonicate for 1 h to accelerate the dissolution of the polysilazane precursor, forming a spinning solution. Then transfer the spinning solution to a spinning cylinder. At this time, the mass fraction of the polysilazane precursor is 93 wt%, and the fluidity is very small at room temperature.

[0052] Steps b and c are the same as those shown in Example 1.

[0053] The experimental conditions of each example are compared as Figure 1 shown. It can be seen from the figure that the spinning solution containing 82 wt% - 87 wt% of the polysilazane precursor is the most suitable spinning concentration.

Claims

1. A method for preparing polysilazane fibers by dry spinning, characterized in that: (1)Prepare the spinning solution: Add a polysilazane precursor, xylene, acrylic acid, and an initiator into a beaker to prepare a preliminary solution. Then, place the preliminary solution in an ultrasonic cleaner for ultrasonic acceleration of dissolution to form a spinning solution. Subsequently, transfer the spinning solution to a spinning cylinder. (2)Prepare polysilazane fibers by dry spinning: Pass nitrogen gas into the spinning cylinder containing the spinning solution. Under the action of nitrogen pressure, the spinning solution flows down from the spinning cylinder and is then drawn and wound onto a take-up cylinder rotating at a certain speed to obtain fiber rovings. (3)Rapid cross-linking by free radical initiation: Heat the fiber rovings and conduct infusibilization treatment by free radical-initiated cross-linking at a temperature below the softening point of the polysilazane to form an infusible and insoluble three-dimensional network cross-linked structure as the product of the final polysilazane fibers. The mass ratio between the polysilazane precursor and the solvent system composed of xylene, acrylic acid, and an initiator is controlled such that the mass content of the polysilazane precursor is 82 wt% - 87 wt%. The initiator used is 2,2'-azobis(2-methylbutyronitrile).

2. The method for preparing polysilazane fibers by dry spinning according to claim 1, characterized in that: The polysilazane precursor is prepared by ammonolysis and is a colorless, semi-transparent, thermoplastic brittle solid.

3. The method for preparing polysilazane fibers by dry spinning according to claim 1, characterized in that: In the solvent system, the volume ratio of acrylic acid to xylene is 1:

5.

4. The method for preparing polysilazane fibers by dry spinning according to claim 1, characterized in that: The nitrogen gas flow rate is 0.1 - 0.2 L / min.

5. The method for preparing polysilazane fibers by dry spinning according to claim 1, characterized in that: The rotation speed of the take-up cylinder for drawing and winding is 200 - 300 r / min.

6. The method for preparing polysilazane fibers by dry spinning according to claim 1, characterized in that: Heat the fiber rovings to 140 - 160 °C and keep them warm for 10 - 20 min.

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