RU2767238C1 - Method for producing pre-ceramic fiber-forming oligoorganosilazanes - Google Patents

Abstract

FIELD: ceramic materials.

SUBSTANCE: invention relates to a method for producing pre-ceramic fiber-forming oligoorganosilazanes for producing ceramic fibers of the SiCN composition. The reaction mixture of tri- and difunctional organochlorosilanes with their total molar ratio of more than 0.66, but less than 0.85 is subjected to ammonolysis. Ammonolysis is carried out stepwise by sequential introduction of the first, second and third compositions, each of which consists of a mixture of tri- and difunctional organochlorosilanes in toluene. The ammonolysis product is treated with a catalyst and polymerized from 3 to 6 hours at atmospheric and reduced pressure at temperatures of more than 149°C and less than 171°C to obtain polysilazanes with a non-volatile substance content of more than 97, wt. %, with an inorganic residue yield of about 75, wt. %, after pyrolysis up to 1200°C both in an inert atmosphere and in air. This method can be effectively implemented in the industrial production of pre-ceramic fiber-forming oligoorganosilazanes, obtained with high yield and stability in the form of granules for up to 6 months.

EFFECT: fiber-forming polysilazanes provide continuous molding from the melt and ceramization of SiCN fibers with specified characteristics with high physical and mechanical properties.

1 cl, 4 tbl, 10 ex., 2 dwg

Description

The claimed invention relates to the field of chemical technology of nitrogen-containing silicon compounds, in particular, to methods for producing preceramic fiber-forming oligoorganosilanes by the interaction of organohalogensilanes with ammonia, disilazanes, followed by treatment of the ammonolysis product with a basic catalyst that deprotonates NH groups. Ceramic fibers, due to their performance characteristics: light weight, fire resistance, high strength, wear resistance, stability at high temperatures in aggressive and oxidizing environments, low coefficient of thermal expansion (CTE), provide ceramic matrix composite materials (CMC) when used as reinforcing components performance improvement. Along with fibers, oligoorganosilazanes make it possible to implement various methods for obtaining ceramic materials that cannot be obtained by traditional methods, namely: ceramic coatings on various substrates or CMC with a ceramic matrix based on carbonitride-, silicon-nitride.

The authors of the patent (US No. 4,543,344, IPC SW 35/58, 1985) report on a method for obtaining preceramic fiber-forming polyorganosilazane containing in its structure from one to several cyclosilazane units. Polysilazanes are obtained by reacting trichlorosilane and organodisilazane with organic substituents of various natures at silicon to form a copolymer, which is further treated with a basic catalyst that deprotonates the NH and SiH groups located nearby. The rearrangements that occur in the reaction mixture are accompanied by an increase in the molecular weight of polyorganosilazanes.

The disadvantages of the method include the fact that the resulting polymer is not considered as a commercial product.

The patent (US No. 4,395,460, IPC C04B 35/52, 1983) describes a method for preparing preceramic polysilazanes by ammonolysis of a mixture of organochlorodisilanes with various substituents at silicon in an inert solvent, followed by treatment of the ammonolysis product with a basic catalyst capable of deprotonating NH groups. The advantage of this process is the ability to stop the reaction at any time by cooling the reaction mass, which makes it possible to control the viscosity of the polymers, and, consequently, their molecular weight. In appearance, polysilazanes can be: liquids with different viscosities or solid glassy materials. The invention describes the production of polysilazanes with a high content of silicon, hydrolytically stable and easy to use. After pyrolysis of polysilazanes at 1200°C in an inert atmosphere, the yield of inorganic residue was 46.3 wt. % and had the following composition: carbon - 29 wt. %; hydrogen - 7-8 wt. %; silicon - 45 wt. % and nitrogen - 8.1 wt. %.

The disadvantages of the method include the fact that in the process of polymerization it is not possible to achieve high molecular weights, therefore, to ensure the formation of fibers, it is necessary to use finely dispersed fillers. Also, the disadvantage of this method is to obtain a low yield of inorganic residue after pyrolysis. The patent does not report on the stability of fiber-forming polysilazanes in the form of granules, especially for compositions with a content of non-volatile substances of more than 97 wt. %, which is a necessary condition for the formation of fibers from the melt.

The closest in technical essence and adopted by us as a prototype is the method proposed by the authors in the patent (US No. 4,482,669, IPC S08K 3/34, 1984) for the preparation of polyorganosilanes in a solvent in an inert atmosphere by the interaction of organodihalosilane with ammonia to form an ammonolysis product, for which, after treatment with a basic catalyst capable of deprotonating nearby NH and SiH groups, an increase in the molecular weight of polysilazane is observed. The preceramic polymer is characterized by a cyclolinear structure, since it contains many cyclic and/or linear units interconnected by bridges of the composition Si ₂ N ₂ . Such polymers form new ladder or flat lattice structures and can be in various physical states: in the form of powders soluble in organic solvents; in the form of waxy solid or viscous materials. Pyrolysis of polymers leads to a high yield of ceramics up to 83 wt. % containing silicon nitride, silicon carbide and carbon in a molar ratio of 0.88:1.27:0.75, respectively. Preceramic oligosilazanes are used to obtain ceramic coatings, fibers and ceramic products based on them, for various applications.

The disadvantage of this method is the lack of studies of polysilazane with a content of non-volatile substances of more than 97 wt. %, and the content of non-volatile substances is more than 97 wt. % is a necessary condition for the formation of fibers from the melt. In addition, there are no data on the possibility of obtaining polysilazanes in the form of granules with high storage stability. The stability of polysilazanes in the form of granules significantly reduces the time of contact with air during dosing, which prevents the appearance of oxygen in the polymer.

The objective of the present invention is to develop a universal method for obtaining preceramic fiber-forming oligoorganosilazanes with a high yield from readily available and relatively cheap domestic starting materials with a content of non-volatile substances of more than 97 wt. % with the yield of inorganic residue not less than 75 wt. % after pyrolysis up to 1200°C both in an inert atmosphere and in air, with stability in the form of granules up to 6 months, ensuring the formation of fibers from the melt in a continuous mode with high physical and mechanical characteristics. Moreover, the method can be effectively implemented in industrial production.

The task is achieved by the proposed method for obtaining preceramic fiber-forming oligoorganosilazanes in an inert atmosphere in a solvent by direct ammonolysis of organodichlorosilanes, followed by polymerization of the ammonolysis product at an elevated temperature in the presence of a catalyst, which consists in the fact that ammonolysis is subjected to a reaction mixture consisting of a mixture of organotri- and organodichlorosilanes in various combinations with their total molar ratio in the range of more than 0.66, but less than 0.85, and ammonolysis is carried out stepwise by sequentially introducing the first equimolar composition consisting of dimethyldichlorosilane and methyltrichlorosilane with a concentration of 3.42 mol in toluene; upon completion of the process - the second composition, consisting of methyldichlorosilane, vinyltrichlorosilane, dimethyldichlorosilane with concentrations of 1.39 mol, 0.138 mol, 0.138 mol, respectively; upon completion of the process - the third equimolar composition with a concentration of 0.294 mol in toluene, consisting of methyldichlorosilane, phenyltrichlorosilane with separation of the resulting ammonium chloride precipitate and removal of the solvent by known methods after the introduction of the third composition and completion of the process, with subsequent polymerization of the ammonolysis product from 3 to 6 hours at atmospheric and reduced pressure at temperatures above 149°C, but less than 171°C in the presence of polyalkoxyphosphorus titanoxane as a catalyst with a concentration of at least 0.2 wt. % and not more than 2 wt. %, leading to the formation of polysilazane with a content of non-volatile substances of more than 97 wt. %, with the yield of inorganic residue not less than 75 wt. % after pyrolysis up to 1200°C both in air and inert atmosphere, with the stability of polysilazane in the form of granules up to 6 months.

Organic substituents on silicon in organochlorosilanes can be selected from the proposed list both of the same type and in various combinations: hydrogen, lower alkyl groups: methyl, ethyl, n-propyl, isopropyl, etc.; vinyl, allyl, benzyl; dimethyl-, methylethyl-, diisopropylamino, etc.; trimethyl-, dimethyl-, triethylsilyl and other groups; lower aryl groups: phenyl, tolyl, xylyl, etc., preferably: hydrogen, methyl, phenyl, vinyl.

The solvents used in the ammonolysis, deprotonation/polymerization process may be selected from: ethers; cyclic ethers; aliphatic hydrocarbons; aromatic hydrocarbons, preferably toluene.

The presence of toluene in the reaction mixture does not significantly affect the properties of the final product. The calculated amount of toluene should ensure the achievement of a homogeneous mixture at all stages of ammonolysis, while maintaining the preferred ratio of toluene to the sum of organochlorosilanes equal to 6:1.

The content in the reaction mixture of organotri- and orgnoadichlorosilanes in various combinations is at a total molar ratio of more than 0.66, but less than 0.85.

The duration of heat treatment is from 3 to 6 hours, depending on the total molar ratio of organotri- and organodichlorosilanes during the process at atmospheric and reduced pressure at temperatures above 149°C, but less than 171°C. Polyalkoxyphosphortitanoxane of the following formula is used as a polymerization catalyst: [ ¹ -R _n TiO ² R _(4-n)- ], where ¹ R=P; n=1-3; ² R= lower alkyls (C ₁ -C ₅ ), with a concentration of more than 0.2 wt. %, but not less than 2 wt. %. Since the nature of the radical ( ² R) does not significantly affect the properties of the obtained polysilazanes, butoxy, a derivative of titanoxane, was used in this process, since butyl alcohol is the cheapest.

The quality of the obtained fiber-forming polymers at the stage of primary testing was evaluated by their ability to provide the formation of fibers, which should retain their shape after oxidation in air for 30 minutes at 60°C.

The essence of the invention is illustrated by the following examples, which should be considered for illustration only and not as a limitation. Synthesis conditions and properties of fiber-forming polysilazanes are given in the tables:

table 1 - "Compositions of compositions of organochlorosilanes in toluene for the production of fiber-forming polysilazanes";

table 2 - "Concentration of the original organochlorosilane composition No. 2";

table 3 - "Modes for obtaining fiber-forming polysilazanes and their properties."

Example 1

In a four-necked jacketed reactor equipped with a stirrer, a reflux condenser and an addition funnel, in nitrogen at a temperature of 0°C, equimolar composition No. 1 was charged with a concentration of 3.58 mol in 4 kg of toluene (Tables 1, 2). Further, after completion of the ammonolysis process, composition No. 2 is introduced in an amount of 200 g, consisting of methyldichlorosilane, vinyltrichlorosilane, dimethyldichlorosilane with concentrations of 1.39 mol, 0.138 mol, 0.138 mol, respectively. Then, after completion of the ammonolysis process, an equimolar composition No. 3 is introduced into 3.5 kg of toluene with a concentration of 0.294 mol, consisting of phenyltrichlorosilane, methyldichlorosilane. The total molar ratio of organotri- and organodichlorosilanes during ammonolysis is 0.744 (Tables 1, 2, 3). After removing the precipitate of ammonium chloride and distillation of the solvent by known methods, a polymerization catalyst is added to the ammonolysis product, which was used as polyalkoxyphosphorus titanoxane with a concentration of 1.5 wt. %. At a temperature of 160°C with stirring, polymerization is carried out for 6 hours. For fiber-forming polysilazane obtained the following characteristics: dry residue 97 wt. %; stability in the form of granules reaches 6 months (Tables 1, 2, 3); the yield of inorganic residue is about 75 wt. % both in air and in argon; molecular weight distribution characteristics are equal to Mw=4275; Mn=988; D=4.32.

Example 2. In example 2, a different sequence of introducing compositions of organochlorosilanes was used: composition No. 2 is first introduced, then No. 1, and then composition No. 3 (Tables 1, 2, 3).

Example 3. In example 3, the following sequence of input compositions of organochlorosilanes was used: composition No. 3 was introduced first, then composition No. 2, and then composition No. 1 (Tables 1, 2, 3).

Example 4. In example 4, composition No. 2 is administered in an amount of 100 g (Tables 1, 2, 3).

Example 5. In example 5, compositions No. 2 are administered in an amount of 300 g (Tables 1, 2, 3).

While maintaining all other parameters of example No. 1 in examples 2-10, the selection of optimal parameters was carried out: the sequence of entering compositions (examples 1-3); the amount of composition No. 2 introduced into the reaction mixture (examples 1, 4, 5), polymerization temperature (examples 1, 6, 7) and polymerization duration (examples 1, 8), catalyst concentration (example 9-10).

The sequence of input of organochlorosilane compositions determines the properties of the obtained polysilazanes, and, consequently, the characteristics and modes of carrying out the process of their production. The initial testing of the fiber-forming ability of polysilazanes made it possible to select samples of polysilazanes obtained in examples 1,4-9 for further testing. Used in example 1, the sequence of input compositions of chlorosilanes provides polysilazane with increased thermal-oxidative stability, since the yield of inorganic precipitate is about 75 wt. % both in air and in argon. The sequence of injection of the compositions used in examples 2 and 3 does not provide polyorganosilazane corresponding to the invention, since the fibers formed from them did not pass the initial test, because they melted during heat treatment at 60°C for 30 minutes.

In examples 1, 3, 4, the amount of input composition No. 2 affects the mole ratio of organotri- and organodifunctional chlorosilanes in the reaction mixture in the range of 0.662-0.850. So, in example 4, composition No. 2 is introduced in an amount of 100 g (10 wt.%). In this case, the mole ratio of tri- and difunctional chlorosilanes is 0.850. The polysilazanes obtained in this way do not provide a continuous mode of spinning fibers from the melt, and the ceramics obtained after pyrolysis is characterized by increased brittleness (Tables 1,2,3). In example 5, composition No. 2 is introduced in an amount of 300 g (30 wt.%) with a molar ratio of tri- and difunctional chlorosilanes equal to 0.662 (Table 1, 2, 3). This reduces the viability of the fiber-forming polysilazane in the form of granules during storage. There is a decrease in the yield of inorganic residue (URR) during pyrolysis and a decrease in their thermal-oxidative stability, since there are significant differences in the URH after pyrolysis in argon and in air.

In the proposed method, obtaining fiber-forming oligoorganosilanes with molecular weights in a wide range of values is achieved by controlling the duration of polymerization of the reaction mass from 3 to 6 hours when the temperature changes from 149°C to 171°C. The choice of polymerization temperature while maintaining the remaining synthesis conditions of example 1 was carried out in examples 6 and 7. Thus, in example 6, when polymerization was carried out at 149°C, a polysilazane with a content of non-volatile substances of less than 97 wt. %, and when polymerized at 171°C in example 7, polysilazane did not provide melt spinning.

It has been found that when polymerization is carried out for 2.8 hours, the fiber shape is not retained when held at 60°C for 30 minutes, so a polymerization time of less than 3 is insufficient. (Table 3, example 8). Heat treatment for 6 hours (Table 3, examples 1-7.9) ensures that the shape of the fiber is retained when held for 30 minutes. at 60°C, so a further increase in the duration of the heat treatment process is not required.

Carrying out the process at different concentrations of the catalyst showed that the concentration of the catalyst is more than 0.2 wt. %, but not less than 2 wt. % is optimal. With the introduction of the catalyst in an amount of more than 2.1 wt. % (Table 3, example 9), the obtained polysilazanes do not provide a continuous mode of fiber spinning from the melt, and the ceramics obtained after pyrolysis is characterized by increased brittleness. The catalyst concentration is less than 0.2 wt. % (Table 3, example 10) does not provide a polyorganosilazane corresponding to the invention, since the fibers formed on their basis did not pass the initial test, as they melted at 60°C for 30 minutes.

The use of each mixture separately does not lead to the production of a polysilazane that allows the object of the invention to be accomplished. Apparently, when using three mixtures in accordance with the proposed invention, a synergistic effect is observed, leading to the production of polysilazane, according to the present invention.

Thus, when the process is carried out in a controlled mode by sequentially introducing three compositions of organochlorosilazanes in the sequence shown in example 1, with a mole ratio of organotri- and organodichlorosilanes of more than 0.66, but less than 0.85, after separating the resulting precipitate of ammonium chloride and removing the solvent known methods after the introduction of the third composition and completion of the process, followed by polymerization of the ammonolysis product from 3 to 6 hours at atmospheric and reduced pressure at temperatures above 149°C, but less than 171°C in the presence of polyalkoxyphosphorus titanoxane with a concentration of more than 0.2 wt. %, but not less than 2 wt. % taken as a catalyst are necessary to obtain polysilazane corresponding to the proposed invention.

Outside the above ranges of process parameters, the obtained polymers do not provide the desired properties.

In example 1, the optimal conditions for the synthesis of polyorganosilazane with properties in accordance with the invention are established: the sequence of input of compositions of organochlorosilanes, their composition; the content of composition No. 2 in the reaction mixture in the amount of 200 g (20 wt.%) at a molar ratio of tri - and difunctional chlorosilanes equal to 0.744; polymerization temperature of 160°C; with a polymerization time of 6 hours.

The technical result of the invention is the production of oligoorganosilanes with a content of non-volatile substances of more than 97 wt. % with a molecular weight that ensures the production of highly heat-resistant, oxidation-resistant oligoorganosilazanes with a high ceramic yield of about 75 wt. % after pyrolysis up to 1200°C both in air and in an inert atmosphere, consisting mainly of silicon, nitrogen, carbon, which is confirmed by the data presented in Fig. 1 "Thermogravimetric curves of fiber-forming polysilazane in air and in argon (optimal sample)".

The stability of the obtained polysilazanes in the form of granules reaches six months. Fiber-forming polysilazane, according to the present invention, has a structure different from the structure of the polysilazane polymer according to the prototype, due to the presence of different types of repeating sesquisilazane units connected by different bridges, which provides improved performance and the emergence of new properties.

The sequence of input compositions of organochlorosilanes, selected in example 1, provides the formation of a polymer structure, in which after spinning from the melt and carrying out oxidative curing of the fibers in air for 1.5 hours, the composition and shape of the fiber are observed during subsequent pyrolysis in nitrogen up to 1200°C . The surface morphology of ceramic fibers and their composition were studied by SEM and X-ray microanalysis. In FIG. 2 shows "Micrograph of a fiber made on the basis of the obtained fiber-forming polysilazane and the results of elemental analysis."

Despite the fact that the development of methods for obtaining preceramic fiber-forming polymers for SiCN fibers has been carried out by foreign and domestic researchers since 1970 (“Ceramic Fibers Based on SiC and SiCN Systems: Current Research, Development, and Commercial Status”, Flores O., Bordia R ., Advanced engineering materials, 2014, 16, No. 6, pp. 621-635 "Processing and characterization of large diameter ceramic SiCN monofilaments from commercial oligosilazanes", Flores O., Bordia R., et al., RSC Adv. 2015. Vol 5. P. 107001-107011 "Selective cross-linking of oligosilazanes to tailored meltable polysilazanes for the processing of ceramic SiCN fibres", Flores O., Schmalz T., Krenkel W. et al., J. Mater Chem. A. 2013) at this stage there are no commercially available fibers due to their low viability. The lack of commercially available fiber-forming pre-ceramic polymers on the market is one of the main reasons preventing the emergence of commercially available fibers. It should be noted that the characteristics of SiCN fibers (tensile strength (1.1-1.3) GPa) based on oligoorganosilazane, achieved by us, using oxidative curing, are in the range of maximum values obtained by foreign researchers (“Ceramic Fibers Based on SiC and SiCN Systems: Current Research, Development, and Commercial Status)), Flores O., Bordia R., Advanced engineering materials, 2014, 16, no. 6, p. 621-635; "Processing and characterization of large diameter ceramic SiCN monofilaments from commercial oligosilazanes", Flores O., Bordia R., et al., RSC Adv. 2015. Vol. 5. P. 107001-107011). Moreover, the maximum strength characteristics (1.3 GPa) are observed in fibers cured only by electron beam irradiation.

Fiber-forming oligoorganosilazanes obtained on the basis of domestic raw materials are commercial products for SiCN ceramic fibers.

The developed technology based on available domestic raw materials can be implemented on an industrial scale with the release of volumes that meet all the needs of the industry. Technological characteristics of fiber-forming polysilazanes for fibers of SiCN composition correspond to TU 20.16.57-250-00209013-2018 (Table 4 - “Characteristics of pre-ceramic fiber-forming polysilazane”).

Claims (2)

1. A method for obtaining pre-ceramic fiber-forming oligoorganosilanes in a solvent in an inert atmosphere by direct ammonolysis of organodichlorosilanes followed by polymerization of the ammonolysis product in the presence of a catalyst, characterized in that a reaction mixture consisting of a mixture of organotri- and organodichlorosilanes in various combinations is subjected to ammonolysis with their total molar ratio of the range of more than 0.66, but less than 0.85, and ammonolysis is carried out stepwise by sequentially introducing the first equimolar composition consisting of dimethyldichlorosilane and methyltrichlorosilane with a concentration of 3.42 mol in toluene; upon completion of the process - the second composition, consisting of methyldichlorosilane, vinyltrichlorosilane, dimethyldichlorosilane with concentrations of 1.39 mol, 0.138 mol, 0.138 mol, respectively; upon completion of the process - the third equimolar composition in toluene with a concentration of 0.294 mol, consisting of dimethyldichlorosilane, phenyltrichlorosilane, with the separation of the resulting precipitate of ammonium chloride and removal of the solvent by known methods after the introduction of the third composition and completion of the process, followed by polymerization of the ammonolysis product from 3 to 6 hours at atmospheric and reduced pressure at temperatures above 149°C, but less than 171°C in the presence of polyalkoxyphosphorus titanoxane with a concentration of more than 0.2 wt.%, but less than 2 wt.%, taken as a catalyst, leading to the formation of polysilazanes with a content of non-volatile substances more than 97 wt.%, with an inorganic residue yield of at least 75 wt.% after pyrolysis both in air and in an inert atmosphere up to 1200 ° C with the stability of polysilazane in the form of granules up to 6 months. 2. The method according to p. 1, characterized in that as a polymerization catalyst used polyalkoxyphosphorus titanoxane of the following formula: [ ¹ R _n TiO ² R _(4-n)- ], where ¹ R=P; n=1-3; ² R= lower alkyls (C ₁ -C ₅ ).

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