Method for the synthesis of a chlorine-free, pre-ceramic polymer for the production of ceramic molded bodies

Abstract

A method for producing a polysilane includes a disproportionation reaction of a methylchlorodisilane mixture to form chlorine-containing oligosilane, a substitution reaction of the chlorine atoms contained in the oligosilane by the reaction with a primary amine and a cross-linking reaction of the oligosilanes using a chain former to form polysilanes. The obtained polysilanes are infusible and are very suitable for being spun to form green fibers and processed to form silicon carbide fibers and fiber composites. The method is characterized in that it can be carried out cost-effectively and quickly and with very high yields.

Claims

1. A polysilane produced by a process comprising the steps of: performing a disproportionation reaction of a methylchlorodisilane mixture to produce a chlorine-containing oligosilane; performing a substitution reaction of chlorine atoms contained in the chlorine-containing oligosilane by reacting with a primary amine; and performing a cross-linking reaction of the chlorine-containing oligosilane using a chain forming agent to produce polysilanes.

2. The polysilane according to claim 1, wherein: the polysilane has a molecular weight Mw of from 1,000 to 10,000 g/mol; and a bimodal molecular weight distribution and/or a polydispersity of from 1 to 5.

3. The polysilane according to claim 1, wherein the polysilane has a viscosity at 25 C. in a 50 wt. % solution in toluene of from 5 to 50 mPas.

4. The polysilane according to claim 1, wherein the polysilane can be spun during dry spinning with draft factors of from 5 to 10.

5. The polysilane according to claim 4, wherein a fiber filament reduction per hour has a value of from 0 to 75% during the dry spinning.

6. The polysilane according to claim 1, wherein the polysilane is soluble in toluene and/or dioxane in a concentration of from 70 to 90 wt. % and/or has a ceramic yield of at least 60% during pyrolysis up to 800 C.

7. The polysilane according to claim 1, wherein: the polysilane has a molecular weight Mw of from 2,500 to 3,500 g/mol; and a bimodal molecular weight distribution and/or a polydispersity of from 2.5 to 3.5.

8. The polysilane according to claim 4, wherein a fiber filament reduction per hour has a value of from 0 to 20% during the dry spinning.

9. Silicon carbide fibers produced by a process comprising the steps of: producing polysilanes by a process containing the steps of: performing a disproportionation reaction of a methylchlorodisilane mixture to produce a chlorine-containing oligosilane; performing a substitution reaction of chlorine atoms contained in the chlorine-containing oligosilane by reacting with a primary amine; performing a cross-linking reaction of the chlorine-containing oligosilane using a chain forming agent to produce the polysilanes; spinning the polysilanes into polysilane fibers; and converting the polysilane fibers into silicon carbide fibers by pyrolysis.

10. The silicon carbide fibers according to claim 9, wherein a cross-sectional surface of the silicon carbide fibers has an undulating boundary line.

11. A fiber composite material produced by a process comprising the steps of: polysilane fibers produced by a process containing the steps of: performing a disproportionation reaction of a methylchlorodisilane mixture to produce a chlorine-containing oligosilane; performing a substitution reaction of chlorine atoms contained in the chlorine-containing oligosilane by reacting with a primary amine; performing a cross-linking reaction of the chlorine-containing oligosilane using a chain forming agent to produce the polysilane fibers; and transforming the polysilane fibers and a matrix material into a ceramic fiber composite material by pyrolysis.

12. The fiber composite material according to claim 11, which further comprises using the ceramic fiber composite material for lightweight construction, for electrical industry, for space travel, for automobile construction and for aircraft construction.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The single FIGURE of the drawing is an illustration of a silicon carbide fiber according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) Referring now to the single FIGURE of the drawing in detail thereof, there is shown an embodiment of the silicon carbide fiber according to the invention that has a cross-sectional surface 1. Accordingly, a preferred silicon carbide fiber has the cross-sectional surface 1 with an undulating boundary line 10. The degree of undulation can be described as follows. The FIGURE shows the undulating cross section 10, an outer sheathing circle 11 as well as the largest possible circular surface 12 inside the fiber cross-sectional surface 1. The outer circular surface 11 is at least 5%, preferably at least 10%. In this respect, the curvature of the boundary line 10 repeatedly changes sign. At least two radii of curvature with different signs have an amount of at least 1 m in each case. A simple oval fiber cross section is not included by the cross section according to the invention; on the other hand an undulating but basically oval cross-sectional surface is included by the cross-sectional surface according to the invention.

(3) The polysilane preferably has a ceramic yield of at least 60% during pyrolysis up to 800 C. Here, the ceramic yield is defined as residual mass after pyrolysis under protective gas at 800 C. at a heating rate of 10 K/min. Therefore, after pyrolysis, the polysilane according to the invention has a very good yield.

(4) In principle, pyrolysis or sintering can be carried out in any manner known to a person skilled in the art and with any temperature profile. However, good results are particularly obtained when pyrolysis is carried out with the exclusion of oxygen, i.e. under an inert gas atmosphere such as nitrogen so that the maximum temperature is from 400 to 1200 C., preferably from 600 to 1,000 C. and more preferably from 800 to 900 C. During pyrolysis, the heating rate is set at a value between 0.1 and 200 K/min, preferably between 0.5 and 50 K/min, more preferably between 0.75 and 10 K/min and most preferably at a value of approximately 1.0 K/min. At from 400 to 500 C., the conversion of the polysilane into the silicon carbide is complete. Sintering is preferably carried out at temperatures between 800 and 2,000 C. under an inert gas atmosphere, such as preferably under argon, nitrogen or a nitrogen-hydrogen mixture, and at heating rates of from 1 to 150 K/min. This has the advantage that by increasing the temperature, but still below the melting temperature, the polymer structure changes and the mechanical properties of the fiber are improved. During sintering, the individual fibers do not bind on to each other.

(5) The silicon carbide ceramic according to the invention has an element composition of from 20 to 45 wt. %, preferably from 23 to 40 wt. % of carbon, from 5.0 to 8.0 wt. % of nitrogen, from 0.0 to 4.0 wt. % of oxygen, from 0.0 to 2.0 wt. % of chlorine and from 48 to 72 wt. % of silicon.

(6) According to an embodiment, the silicon carbide ceramic according to the invention in the form of silicon carbide fibers has an element composition of from 38 to 40 wt. % of carbon, from 7.0 to 8.0 wt. % of nitrogen, from 2.0 to 3.0 wt. % of oxygen, from 1.0 to 1.5 wt. % of chlorine and from 48 to 50 wt. % of silicon.

(7) The invention also includes a fiber composite material, characterized in that it contains a silicon carbide fiber according to the invention and a matrix material.

(8) Likewise, the polysilane fibers produced according to the invention can be used in a fiber composite material, characterized in that the fiber composite material contains a polysilane fiber according to the invention and a matrix material. If appropriate, the polysilane fiber is converted into a ceramic fiber by a treatment, preferably by pyrolysis of the fiber composite material.

(9) In this respect, the fibers or matrix can be composed of the silicon carbide according to the invention and of the substance systems SiC, SiCN, SiBNC, Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2 (and mixtures thereof), it being preferred for the fibers and the matrix to be composed of the silicon carbide according to the invention.

(10) Furthermore, the polymer fibers or ceramic fibers can be provided in the form of non-crimp fabrics, fiber mats, woven fabrics, warp-knitted fabrics, weft-knitted fabrics, nonwoven fabrics and/or felts, non-crimp fabrics and/or fiber mats being preferred.

(11) To produce the composite material according to the invention, silicon carbide fibers produced as above or other fiber structures containing other ceramic fibers, preferably SiCN fibers, can be impregnated with the polysilane described according to the invention or with other polymer precursors and then pyrolysed. This means that the fibers and the matrix of the composite material according to the invention can be composed of the silicon carbide according to the invention. Further other possible substance systems for fibers or matrix are SiCN, SiBNC, Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2 and mixtures thereof, it being preferred for the fibers and the matrix to be composed of the silicon carbide according to the invention.

(12) A hardening procedure can optionally be carried out between impregnation and pyrolysis, which hardening can be carried out in a physical or chemical manner, for example using UV light and/or by a temperature treatment. Thereafter, the body produced thus can be impregnated once or several times with polysilane, hardened and pyrolysed.

(13) Furthermore, the described polysilane can be applied as a solution to any fibers or moldings and, after pyrolysis, it forms a protective layer, for example an oxidation protective layer. Due to its outstanding characteristics, in particular its outstanding high temperature resistance and high degree of hardness, the polysilane according to the invention, the silicon carbide fiber according to the invention and silicon carbide moldings which have been produced and silicon carbide-containing fiber composite materials according to the invention, moldings according to the invention and/or fiber composite material are particularly suitable for uses in which the material is exposed to elevated temperatures and oxidative conditions, specifically for example in lightweight construction, in the electrical industry, in space travel, in automobile construction and in aircraft construction.

EXAMPLES

(14) In the following, the present invention is described on the basis of practical examples according to the invention in comparison with a comparative example which has been carried out without a cross-linking reaction but with a thermal cross-linking, the practical examples describing the present invention without restricting it. The comparative example and the practical example are compared with one another in respect of the necessary reaction temperature, time and yield. In addition, in Table 1, the concentration of polysilane which can be used for dry spinning, i.e. the concentration of polysilane which is still dissolving, the viscosity, spinnability and the pyrolysing characteristic of the polysilane for the polysilane of the invention according to Example 1, for the comparative example and for the oligosilane are compared with one another.

(15) The viscosity of the polysilane or of the spinning masses was determined using a rotation rheometer Physica MCR 301 manufactured by Anton Parr. The measurements were made using a plate/plate geometry with approximately from 300-500 mg of the respective sample. The viscosity of the diluted polymer solutions was measured on a falling-ball viscosimeter manufactured by HAAKE at 25 C. in 50% solution with a 16.25 g steel ball.

(16) Comparative example from the prior art according to international patent disclosure WO 2010072739:

(17) The comparative example was produced in three steps, namely first the preparation of an oligosilane, second the modification of the oligosilane with gaseous dimethylamine and third the thermal cross-linking of a dimethylamine-modified oligosilane.

Step 1: Preparation of an Oligosilane by Disproportionation

(18) 600 g of a methylchlorodisilane mixture (disilane fraction from a Mller-Rochow process, consisting of respectively 45 mol. % Cl.sub.2MeSiSiMeCl.sub.2 and Cl.sub.2MeSiSiMe.sub.2Cl as well as 10 mol. % ClMe.sub.2SiSiMe.sub.2Cl; by 150-155 C.) are mixed with 14 g N-methylimidazole and 69 g phenyltrichlorosilane and heated to 180 C. at 0.5 K/min. Approximately 450 ml of a distillate are obtained consisting of MeSiCl.sub.3, Me.sub.2SiCl.sub.3 and Me.sub.2ClSiSiMe.sub.2Cl, as well as 153 g of a dark brown hydrolysis-susceptible oligosilane which is solid at room temperature and has a chlorine content of approximately 25% by mass. This is dissolved in toluene or xylene to produce a 60% by mass solution containing oligosilane.

Step 2: Modification of an Oligosilane Using Gaseous Dimethylamine

(19) Introduced into a double-wall 2-L reaction vessel with a bottom valve, reflux cooler, KPG stirrer, internal thermometer and gas inlet tube are 1,500 ml of a 60% solution of an oligosilane obtained by disproportionation of the disilane fraction in toluene or xylene, which solution is then cooled to 0 C. Thereafter, approximately 700 g of gaseous dimethylamine are introduced under the liquid level with vigorous stirring within 3 hours. In so doing, the temperature of the mixture rises to 30-35 C. and falls again towards the end of the reaction. The product is removed via the bottom valve under pressurized argon and the separated dimethylammonium chloride is filtered off via a pressure nutsche filter. The solvent is distilled off from the filtrate. The modified oligosilane still contains approximately 1.5-2% by mass of chlorine.

Step 3: Thermal Cross-Linking of a Dimethylamine-Modified Oligosilane

(20) 600 g of the modified oligosilane are slowly heated in a distillation apparatus to an end temperature of approximately 400 C. During the heating procedure, approximately 200 ml of a yellowish distillate are obtained; the solidification of the mass indicates the end point of cross-linking. After cooling, the copolymer which is obtained, the chlorine content of which is now only approximately 0.5% by mass, is dissolved in toluene and can be used in a dry spinning process for the production of green fibers.

(21) The yield of polysilane is rounded off at 60%. During thermal cross-linking, temperatures of above 300 C. are required, the reaction lasts 4-6 hours. The viscosity of the polysilane is 100 Pas at 30 C. The concentration of polysilane which can be used for dry spinning is 70%. The polysilane is spinnable and immediately pyrolysable.

Practical Example

(22) The first and second steps of the polysilane preparation, namely the disproportionation and the modification using gaseous dimethylamine according to international patent disclosure WO 2010072739 (see comparative example) can be carried out for the preparation of a polysilane according to the invention. The third step of the preparation process, thermal cross-linking according to WO 2010072739, is not carried out according to the invention. Instead, polymerisation takes place by cross-linking with a chain forming agent. According to the invention, the cross-linking reaction can take place in accordance with the now described protocol.

(23) 1400 g of oligosilane solution (chlorine-free, 57%) are introduced into a 2 L flat flange vessel, fitted with a reflux cooler, an argon connection and an anchor stirrer. The reactor was previously rendered inert by applying a vacuum for 30 minutes and was flooded with argon. 70 g of 1,6-diaminohexane (hexamethylenediamine, HMDA) as chain forming agent are dissolved in 233.3 g toluene while being heated (40 C.).

(24) This solution is added to the oligosilane over a period of 2 minutes at room temperature in an argon counter flow while being stirred (108 rpm). To remove the resulting dimethylamine (DMA), a light stream of argon (5 cm3/min) is passed into the vessel and through the cooler.

(25) The reaction is started by heating the solution to reflux within a period of from 20-30 minutes. After reaching the reflux temperature (approximately 111 C.), the solution is stirred for a further 2 hours (108 rpm).

(26) At the end of the reaction time, the solution is cooled and degassed at 100 mbar for 10 minutes at 30 C. The solution is then filtered over a 1 m depth-filter with 3 bars nitrogen pressure.

(27) The polysilane according to the invention is obtained in a yield of 95%. In this example, the maximum reaction temperature in the cross-linking step is 111 C. The reaction duration is 2 hours. Compared with the prior art, the yield is significantly higher, the reaction duration is shorter and the reaction temperature in the last step is lower. Therefore the method is more economical and faster. The viscosity is 100 Pas at 30 C. The concentration of polysilane which can be used for dry spinning is 85% and thus is higher than in the comparative example. Furthermore, the polysilane is spinnable and immediately pyrolysable. The oligosilane, however, is neither spinnable nor pyrolysable.

(28) TABLE-US-00001 TABLE 1 Oligosilane acc. to step 2 of comparative Comparative example example Practical example Concentration 70% 70% 85% (wt. %) of polysilane which can be used for dry spinning Viscosity at 30 C. <1 Pas 100 Pas 100 Pas Spinnable No Yes Yes Immediately No Yes Yes pyrolysable

Practical Example 2

(29) The reaction of practical example 2 is carried out analogously to that of practical example 1. Instead of HMDA, 1,2-ethylendiamine is used as chain forming agent.

Practical Example 3

(30) The reaction of practical example 3 is carried out analogously to that of practical example 1. Instead of HMDA, melamine is used as chain forming agent.

Practical Example 4

(31) The reaction of practical example 4 is carried out analogously to that of practical example 1. Instead of HMDA, triethylenediamine (TREN) is used as chain forming agent.

Practical Example 5

(32) The reaction of practical example 5 is carried out analogously to that of practical example 1. Instead of HMDA, glycerine is used as chain forming agent.

Practical Example 6

(33) The reaction of practical example 6 is carried out analogously to that of practical example 1. Instead of HMDA, triethanolamine is used as chain forming agent.

Practical Example 7

(34) The reaction of practical example 7 is carried out analogously to that of practical example 1. Instead of the stated concentration of HMDA, it is also possible to use from 1-20 mol. % of HMDA.

Practical Example 8

Processing in the Dry Spinning Process

(35) The polysilane prepared according to the invention is processed into green fibers by dry spinning. For this purpose, the polysilane is dissolved in toluene, THF or dioxane and conveyed by a pump at 30 C. through the spinneret having a diameter of 75 m. The resulting pressures are from 5-250 bars. The spinning-column contains a nitrogen atmosphere at 30 C. At a take-off rate of from 50-200 m/min, draw factors of at least 6-8 can be realized. It is possible to obtain suitable green fibers for the subsequent steps.

Practical Example 9

Pyrolysis

(36) The hardened fibers are pyrolysed under a protective gas atmosphere up to 1,200 C. The heating rate is 10 K/min. At 400 to 500 C., the conversion of the polysilane into silicon carbide is complete. Ceramic SiCN fibers are produced which have, for example a diameter of from 19 to 25 m and are composed of 50 wt. % of silicon, 39 wt. % of carbon, 7 wt. % of nitrogen, 3 wt. % of oxygen and 1 wt. % of chlorine by elemental analysis.