Novel low-cost liquid polycarbosilane and preparation method thereof

Abstract

The method comprises the following steps: by taking raffinate RLPS generated during synthesis of polycarbosilane as a raw material, adding a high-activity cracking product (LPS) generated by cracking part of polydimethylsilane (PDMS), firstly converting the raffinate RLPS into viscous polycarbosilane with a certain molecular weight by taking polyboron diphenyl siloxane as a catalyst, and then reacting with triallyl isocyanurate to introduce vinyl, so as to obtain the polycarbosilane. A novel low-cost liquid polycarbosilane precursor is obtained, and the liquid polycarbosilane precursor can be cured and crosslinked at a low temperature. The low-cost liquid polycarbosilane preparation method provided by the invention has the advantages of low cost, adjustable viscosity, high yield, simple preparation process, no need of adding auxiliary solvents and the like, and solves the problems of high cost, low raw material utilization rate, long production period, difficulty in separation of byproducts, recovery and treatment of waste solvents and the like in the prior art.

Description

Technical Field

[0001] The present invention belongs to the technical field of synthesis of high-performance silicon carbide ceramic precursors, and particularly relates to a novel low-cost liquid polycarbosilane and a preparation method thereof. Background Art

[0002] Silicon carbide ceramic matrix composites have extremely high properties such as high temperature resistance, wear resistance, corrosion resistance, radiation resistance, and oxidation resistance, and have wide applications in fields such as aerospace and energy. At present, the processes for preparing silicon carbide ceramic matrix composites mainly include liquid-phase infiltration of silicon, chemical vapor infiltration method, and precursor infiltration-pyrolysis process (PIP), etc. Among them, the PIP method uses an organic polycarbosilane precursor to impregnate a fiber preform, and then obtains an inorganic silicon carbide ceramic matrix through high-temperature pyrolysis conversion. This process has the advantages of good controllability of the microstructure, low cost, and less mechanical and thermal damage to the fibers during the preparation process, and has rapidly developed into the mainstream method for preparing silicon carbide ceramic matrix composites.

[0003] Polycarbosilane precursor is a key raw material for preparing ceramic matrix composites. Currently, the mainstream method is to obtain solid polycarbosilane through high-temperature pyrolysis rearrangement of polydimethylsilane. Since polycarbosilane is in solid form, a large amount of organic solvent is required to dissolve it in the PIP impregnation process before it can be used, which leads to problems such as low ceramic yield, low density of the composite material, long preparation cycle, cumbersome process, and poor production environment. To solve the problems in the application of solid polycarbosilane, another self-crosslinkable liquid polycarbosilane has emerged. This liquid polycarbosilane is a hyperbranched liquid polycarbosilane with hydrogen atoms, alkyl groups, vinyl groups, allyl groups, or alkynyl groups on the side chains. Because of its low viscosity, high ceramic yield, and the silicon-carbon ratio of the pyrolysis product close to the stoichiometric ratio, it has become an ideal precursor for silicon carbide ceramics. The earliest commercialized liquid polycarbosilane is allyl hydrogen-containing polycarbosilane (AHPCS) prepared by Starfire Company in the United States. Currently, the main methods for synthesizing self-crosslinkable liquid polycarbosilane are ring-opening polymerization method and Grignard coupling reduction method, among which the Grignard coupling reduction method is mainly used. The Grignard coupling reduction method uses chloroalkylchlorosilane as the raw material. Metal magnesium selectively recognizes the C-Cl bond and reacts with it to form a Grignard reagent. The Grignard reagent then couples with the Si-Cl in the molecule, and finally, it is reduced with LiAlH4 to obtain self-crosslinkable liquid polycarbosilane. This synthesis route is simple, the raw materials are relatively cheap and easy to obtain, and the synthesis yield is high. In addition, the structure and properties of the product can be achieved by adjusting the types and ratios of monomers, and there have been many studies at home and abroad. Rushkin et al. reacted chlorinated carbosilane with allyl Grignard reagent to finally obtain hyperbranched liquid polycarbosilane containing allyl groups. The allyl group has high activity and can react with the hydrogen on silicon, so it can crosslink at low temperature under the action of a catalyst; Fang et al. dropped bromoethynyl Grignard reagent into chlorinated carbosilane to obtain hyperbranched liquid polycarbosilane containing alkynyl groups. The research shows that the ceramic yield of the alkynyl-containing liquid polycarbosilane after thermal crosslinking is significantly higher than that of the liquid polycarbosilane without alkynyl groups; Li Yongming et al. prepared a photo-curable liquid polycarbosilane ceramic precursor by introducing photosensitive groups such as acrylate. However, there are still many problems with the Grignard coupling reduction method: Although the ceramic yield after crosslinking of the precursor is high, the total yield is still low considering the initial raw materials; the proportion of unsaturated groups introduced into the side chain of liquid polycarbosilane is relatively low, and the waste of raw materials during the synthesis process leads to an increase in cost; this process requires two-step reactions of Grignard coupling and LiAlH4 reduction, with a long reaction time and low production efficiency; in addition, the by-products of LiAlH4 reduction are a mixture of AlCl3 and LiCl, which is not only not conducive to the recovery of lithium salts but also easily hydrolyzes itself, resulting in difficult separation. The use of a large amount of solvent and the large amount of recycling and treatment lead to an increase in cost.

[0004] Precursor polycarbosilane is an important raw material for the preparation of high-performance SiC ceramics and SiC fibers. The current annual domestic production scale is about 100 tons, and nearly 100 tons of synthetic residual liquid RLPS are produced during the production process. RLPS is a mixture of linear and cyclic silane oligomers, among which those with smaller molecular weight are generally linear structures; those with larger molecular weight are generally cyclic structures, showing inactivity or low activity. Although some Si-CH2-Si bonds are formed on the main chain of the molecule, Si-Si bonds still dominate. In addition, some oxygen is introduced into RLPS to form siloxanes such as hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane, or inserted into Si-Si bonds to form Si-O-Si chain segments. Due to its low activity, low flash point and flammability, RLPS is difficult to directly react and utilize, and most of them are converted into solids and treated as solid waste.

[0005] The present invention utilizes residual liquid RLPS generated during the synthesis of polycarbosilane as a raw material, adds a high-activity cracking product (LPS) generated by the cracking of a portion of polydimethylsilane (PDMS), uses polyboron diphenylsiloxane as a catalyst to first convert the residual liquid RLPS into a viscous polycarbosilane with a certain molecular weight, and then reacts with triallyl isocyanurate to introduce vinyl, thereby obtaining a novel low-cost liquid polycarbosilane precursor, which can achieve curing and cross-linking at a relatively low temperature. Summary of the invention

[0006] The object of the present invention is to provide a new type of low-cost liquid polycarbosilane and a preparation method thereof. The liquid polycarbosilane has the advantages of low cost, adjustable viscosity, high yield, simple preparation process, and no need to add auxiliary solvents, etc., which solves the problems of high cost, low raw material utilization, long production cycle, difficult separation of by-products, and recovery and treatment of waste solvents in the prior art.

[0007] The present invention provides a method for preparing a novel liquid polycarbosilane, comprising the following steps:

[0008] 1. Add polydimethylsilane powder into a flask, raise the temperature to 360-420°C under a nitrogen atmosphere, and keep it warm for 2-6 hours to decompose the polydimethylsilane powder to obtain liquid LPS;

[0009] 2. Fully mix the cleavage product LPS and the residual liquid RLPS to obtain a mixed solution of the two, wherein the mixing ratio of LPS to the residual liquid RLPS is between 1:10 and 1:20;

[0010] 3. Add polydiphenylborane to the mixed solution, react for 4-8 hours at a certain temperature under a nitrogen atmosphere, and then cool down. The amount of polydiphenylborane added is 0.5%-5.0% of the total mass of the mixed solution. The temperature of the polycondensation reaction is controlled between 280-380°C.

[0011] 4. Add triallyl isocyanurate to the cooled viscous liquid in a certain proportion, and stir and react at 70 - 118 °C for 2 - 8 h to finally obtain liquid polycarbosilane. The addition amount of triallyl isocyanurate is 20 - 60% of the product mass. Description of the Drawings

[0012] Figure 1 TGA diagram of the product in Example 1 Detailed Description of the Invention

[0013] Example 1

[0014] Add 1600 g of polydimethylsilane powder to a 5000 mL flask equipped with a condenser and a receiver flask. Under a nitrogen atmosphere, heat up to 360 - 420 °C and keep warm for 2 - 6 h to decompose the polydimethylsilane powder by heating to obtain 1400 g of liquid LPS for use. Weigh 1094 g of residual liquid RLPS, 54.7 g of liquid LPS, and 45.9 g of polyborodiphenylsiloxane respectively and add them to a dried 5000 mL three-necked flask and mix evenly. Then connect a thistle-type fractionating column, a condenser, a vacuum adapter, and a 250 mL ground-glass spherical dropping funnel. Start heating under a nitrogen flow atmosphere. When the temperature rises to 180 °C, low-molecular liquid distills out. After condensation, the liquid is collected in the spherical dropping funnel, and the liquid in the funnel is refluxed into the flask to continue participating in the reaction by adjusting the flow rate. As the reaction temperature increases, small-molecule substances also polymerize into larger-molecule substances, and the boiling point of the reactants gradually increases. After reacting at 320 °C for 5 h, terminate the reaction. After cooling, a light yellow viscous liquid is obtained. Take 68.4 g of the light yellow viscous liquid, add 41.3 g of triallyl isocyanurate, and heat up to 110 °C in a nitrogen atmosphere and stir and react for 4 h to obtain vinyl-containing liquid polycarbosilane. The viscosity of the liquid polycarbosilane is 300 mPa·s, and it can be cured at low temperature at 150 °C. The TGA diagram of the cured product is as Figure 1 shown, and the ceramic yield is 75.4%.

[0015] Example 2

[0016] After drying, 1471.2 g of residual liquid RLPS, 146.3 g of liquid LPS, and 54.7 g of polyborodiphenylsiloxane were successively added to a 5000 mL three-necked flask. After mixing evenly, a fractionating column with thorns, a condenser, a vacuum adapter, and a 250 mL ground glass spherical dropping funnel were connected. Heating was started under a flowing nitrogen atmosphere. After the temperature rose to 180 °C, low-molecular liquids were distilled out. After condensation, the liquid was collected in the spherical dropping funnel, and the liquid in the funnel was refluxed into the flask by adjusting the flow rate to continue participating in the reaction. As the reaction temperature increased, small-molecule substances also polymerized into larger-molecule substances, and the boiling points of the reactants gradually increased. The reaction was terminated after reacting for 2 h at 360 °C. After cooling, a light yellow honey-like viscous liquid was obtained. 95.6 g of the light yellow viscous liquid was taken, and 57.4 g of triallyl isocyanurate was added. After heating to 80 °C and stirring for 8 h in a nitrogen atmosphere, a vinyl-containing liquid polycarbosilane was obtained. The viscosity of the liquid polycarbosilane was 1000 mPa·s, and the ceramic yield after curing was 75.5%.

[0017] Example 3

[0018] After drying, 1324.7 g of residual liquid RLPS, 88.6 g of liquid LPS, and 42.4 g of polyborodiphenylsiloxane were successively added to a 5000 mL three-necked flask. After mixing evenly, a fractionating column with thorns, a condenser, a vacuum adapter, and a 250 mL ground glass spherical dropping funnel were connected. Heating was started under a nitrogen flowing atmosphere. After the temperature rose to 180 °C, low-molecular liquids were distilled out. After condensation, the liquid was collected in the spherical dropping funnel, and the liquid in the funnel was refluxed into the flask by adjusting the flow rate to continue participating in the reaction. As the reaction temperature increased, small-molecule substances also polymerized into larger-molecule substances, and the boiling points of the reactants gradually increased. The reaction was terminated after reacting for 8 h at 280 °C. After cooling, a light yellow viscous liquid was obtained. 79.6 g of the yellow viscous liquid was taken, and 27.9 g of triallyl isocyanurate was added. After heating to 115 °C and stirring for 2 h in a nitrogen atmosphere, it was obtained. The viscosity of the liquid polycarbosilane was 250 mPa·s, and the ceramic yield after curing was 71.5%.

[0019] Example 4

[0020] After drying a 5000 mL three-necked flask, 1254.8 g of residual liquid RLPS, 253 g of liquid LPS, and 60.4 g of polyborodiphenylsiloxane were successively added. After mixing evenly, a thistle-type fractionating column, a condenser, a vacuum adapter, and a 250 mL ground-glass spherical dropping funnel were successively connected. Heating was started under a nitrogen flow atmosphere. After the temperature rose to 180 °C, low-molecular liquids were distilled out. After condensation, the liquid was collected in the spherical dropping funnel, and the liquid in the funnel was refluxed into the flask by adjusting the flow rate to continue participating in the reaction. As the reaction temperature increased, small-molecule substances also polymerized into larger-molecule substances, and the boiling point of the reactants gradually increased. The reaction was terminated after reacting for 6 h at 300 °C. After cooling, a light yellow viscous liquid was obtained. 83.4 g of the yellow viscous liquid was taken, 40 g of triallyl isocyanurate was added, and the mixture was heated to 105 °C and stirred for 5 h in a nitrogen atmosphere to obtain liquid polycarbosilane. The viscosity of the liquid polycarbosilane was 150 mPa·s, and the ceramic yield after curing was 69.9%.

Claims

1. A novel low-cost liquid polycarbosilane and a preparation method thereof, comprising the following steps: (1) Add polydimethylsilane powder into a flask, raise the temperature to 360-420°C under a nitrogen atmosphere, and keep the temperature for 2-6 hours to thermally decompose the polydimethylsilane powder to obtain liquid LPS; (2) fully mixing the cleavage product LPS and the residual liquid RLPS to obtain a mixed solution of the two, wherein the mixing ratio of LPS to the residual liquid RLPS is between 1:10 and 1:20; (3) adding polydiphenylboronosiloxane to the mixed solution, reacting at a certain temperature for 4-8 hours under a nitrogen atmosphere and then cooling down, wherein the amount of polydiphenylboronosiloxane added is 0.5%-5.0% of the total mass of the mixed solution, and the temperature of the polycondensation reaction is controlled between 280-380° C.; (4) adding triallyl isocyanurate in a certain proportion to the cooled viscous liquid, and stirring the reaction at 70-118° C. for 2-8 hours to finally obtain liquid polycarbosilane, wherein the amount of triallyl isocyanurate added is 20-60% of the mass of the product.

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