DE2118489A1 - Moldings containing boron and carbon and process for their production - Google Patents

Description

2118489 FARBENFABRIKEN BAYER AG

LEVERKUSEN-Bayerwerk

Central area

Patents, trademarks and licenses

GB / Gg

Moldings containing boron and carbon and process for their production

The invention relates to a method for producing Shaped bodies, in particular fibers, threads, foils, flakes, coatings and foam bodies made from boron carbide and from boron carbide-carbon mixtures. Fibers, threads and leaflets made from this material are resistant to heat and oxidation therefore, due to their physical properties, very good for the reinforcement of plastics, glasses, ceramics Suitable for materials and metals. Boron carbide coatings are suitable for lining apparatus parts that protect against corrosion are to be protected at high temperatures, while foam bodies made of boron carbide or boron carbide-carbon mixtures Advantageously used as a temperature-resistant insulating material or as a filter material for hot, aggressive gases can be.

It is already known to make boron carbide whiskers by that the reaction of boron trichloride and methane, optionally with the addition of hydrogen Lei at a temperature of 1500 ° C - 2000 ° C on suitable substrate surfaces lets (British Patent 974,539). The poor space-time yield, however, does not allow an economical process. It

Le A 13 701

209844/1136

'Is further known to produce boron carbide fibers, characterized in that boron carbide at a temperature of 2000 ⁰ C conventionally prepared powdery - heated 2500 ° C in vacuum and thus vaporous boron carbide on appropriate substrates in the form of whiskers reflected (A. Gatti et al , Sci. Tech. Aerospace Rept. 3 (3) 455, 1965).

Boron carbide filaments can be produced by a known method by using a conventional method produced carbon thread at a temperature of 2000 ° C with a mixture of boron trichloride and hydrogen is made to react (D.J. Beerntsen et al, ^ Polymer Preprints, Atlantic City Meeting Sept. 68, Vol. 9, No. 2, p. 1456). According to another known method Boron carbide continuous fibers can be produced by coating a tungsten thread with boron carbide, which on the tungsten surface, which is kept at 1100 - 1200 ° C, through the reaction of boron trichloride with methane and hydrogen forms (J.B. Higgins et al, J.Electroehem. Soc. 116 (1969) 137).

The present invention relates to a process for the production of shaped articles of boron carbide or boron carbide-carbon mixture, which is characterized in that the solution or melt of a boron-hydrogen \ compound and a polymeric organic, delivered at elevated temperature carbon compound optionally with the addition of auxiliaries, processed into corresponding shaped bodies and these are then heated to temperatures between 800 ° C. and 2350 ° C. under an inert gas.

It is already known that mixtures of a boron component and a carbon component can be converted into boron carbide by suitable heat treatment.

Le A 13 701 - - 2 -

209844/1136

However, the boron component always consisted of compounds with boron-oxygen bonds such as orthoboric acid, Metaboric acid or boron trioxide, while the carbon component is mostly elemental carbon in the form of charcoal, Soot or petroleum coke was used. The temperature required for the reaction was in the range between 2400 ° C and 2600 ° C. The inventive method simplifies the production of boron carbide in such a way that the formation of boron carbide temperatures below 2400 ° C, in particular between 800 ° C and 2350 ° C, are sufficient. The simplified procedure is made possible by the fact that no compound with a boron-oxygen bond is used as the boron component, but rather one that contains at least one boron-hydrogen bond in the molecule.

The Bcr-hydrogen compound to be used can have both salt-like and covalent character. In the case of the salt-like compounds, the ammonium ion, the tetraalkyl, tetraallyl, tetraaryl ammonium ion, the organosubstituted ammonium ions / RNHy> / R ₂ NH ₂ Z ⁺ , / r ₃ NH7 ⁺ , where R = alkyl, allyl, can advantageously be used as the cation Aryl, the hydryzonium ion, the hydroxylammonium ion, and also the ions of the alkali and alkaline earth elements can be used.

The anion of the salt-like boron component can in the case of exclusive Boron-hydrogen bonds consist, for example, of the following ions:

² "" / bra

, / B ₅ H ₁₀ Z ", / B ₅ H ₁ ^

/ B ₁₀ H ₁₀ Z »Zb ₁₀ H ₁₃ Z, / B ₁₀ H ₁₄ Z, / B ₁₁ H ₁₄ Z

/ B ₂₀ H ₁₈ ² Z ", in which the ion / B ₁₀ H ^_10/2" and

are preferred because the structure of the boron atoms in these ions is very similar to that of boron carbide. Furthermore, the anion of the salt-like boron component can consist of ions in which one or more hydrogen atoms

Le A 13 701 - 3 -

209844/1136

of the boranate ion are substituted, ζ. As demonstrated by halogen atoms, nitro groups or trifluoromethyl groups, such as the following examples: / B ₁₀ H ₃ Br J ₇ ² "/ B ₁₀ H ₉ NO ^_2/2 '/ B ₁₂ H ₁₁ CF ^_3/2".

Higher molecular weight boranes, ie pure boron-hydrogen compounds, can preferably be used as the boron component with a covalent bond. The compounds B ₁₀ H ₁₂ , B ₁₀ H ₁₄ , B ₁₂ H ₁₄ , B ₁₄ H ₂₂ , B ₁₈ H ₂₂ and particularly advantageously their adducts with amines, amides, nitriles and sulfides, such as triethylamine, pyridine, dimethylazetamide, are particularly suitable. Dimethylformamide, acetonitrile and dimethyl sulfide. Examples of such adducts are: B ₁₀ H ₁₂ · 2 (CH ₃ ) ₂ S, B ₁₂ H ₁₄ . 2 (C ₂ H

The carbon component used in the present invention consists from a polymeric organic compound, which upon temperature treatment under inert gas with carbon deposition is pyrolyzable.

A prerequisite for the suitability of an organic material as a carbon supplier is a sufficiently high carbon residue during pyrolysis under inert gas. This should be at least 10 percent by weight of the starting material. From the multitude of as a carbon component for boron carbide possible organic carbon compounds, such compounds can be used particularly well, which in the course the pyrolysis under inert gas does not pass through a fluid phase. This is usually guaranteed when the melting point is significantly above the 'decomposition temperature. For example, you can use: strength, partially degraded or oxidized starch, dextrin, hemicellulose, cellulose derivatives, such as ζ. B. methyl cellulose, polymeric sugars derivatives such as pectin, pectic acid or alginic acid, proteins such as casein, gelatine or isinglass for the internal Organic acid derivatives capable of salt formation, such as glycocolla or betaine, sulfonic acids, their substitution products and

Le A 13 701 - 4 - ■

209844/113 6

Salts, in particular ammonium salts, lignin, ligninsulphonic acid and their salts such as ammonium lignin sulfonate.

According to the method according to the invention, however, it is also possible To use substances as carbon components whose melting or softening point is below their decomposition temperature, provided that the material is used before the actual Makes pyrolysis infusible. From the large number of possible carbon components, the following are exemplary listed: polyvinyl polymers, such as polyvinyl alcohol, in particular Polyacrylonitrile and its copolymers with butadiene and styrene as well as methyl methacrylate, polyolefins, polyester, Polyethers, polyanhydrides, polyurethanes, polyamides, polyureas, Phenol-formaldehyde resins, both as pure polymers and in the form of their graft polymers and their derivatives. In the context of this invention, polymeric organic compounds which do not contain oxygen are particularly preferred Molecule included.

The mixture of the boron component and the carbon component can be in the form of a homogeneous melt or a suspension of the boron component in the molten carbon component be processed into corresponding moldings. On the other hand, the mixture of the two components can be advantageous Dissolve in suitable solvents and process in this form. As a solvent, for example, acetonitrile, Nitromethane, dimethylformamide, N-methy! Pyrrolidone, Tetramethylurea, dimethyl sulfoxide and tetramethylene sulfone be used. For example, the mixture of different parts of diammonium dodecaboranate and Polyacrylonitrile up to high concentrations in dimethylformamide and can in this form he_vorragend to moldings how foils or fibers are processed.

Le A 13 701 '- 5 -

209844/1136

If the carbon component consists of a linear high polymer, so the mixture with the boron component can usually be spun directly into fibers in a suitable solvent. This is possible, for example, with high molecular weight polymers such as polyvinyl alcohol, polyacrylonitrile or polystyrene. However, in order to improve their spinning properties and also the large number of low molecular weight carbon-supplying polymers for To be able to use the process according to the invention, it is advantageous to add linear polymeric high molecular weight to the solution of the two components Auxiliaries with degrees of polymerisation over 10,000 to be added in a concentration of 0.01 to 2 percent by weight. Vinyl polymers, Vinyl copolymers, diolefin polymers, polyethers, polythioethers, Polyesters, polyamides and polypeptides can be used. Polyethylene oxide, polyacrylamide, Acrylamide-acrylic acid copolymers or their salts, polyisobutylene, polymethyl methacrylate, polyisoprene and polystyrene proven. Such auxiliary stuffs are not necessary for the production of papers, coatings and foam bodies. but can be added to the mixture without damage.

In the case of the production of fibers, the concentration of the spinning solution can be between 10 and 40 percent by weight based on the Boron and carbon components, it is preferably between 18 and 28 percent by weight. The viscosity of the solutions is between 0.1 and 800 poise.

If the boron component can be dissolved or suspended in a suitable fusible carbon component, so can such mixtures are advantageously spun from the melt. The production of fibers from suitable spinning solutions can be done according to a wet spinning process or a dry spinning process. The first will turn the threads into one spun in a suitable precipitation bath and dried after coagulation. In the dry spinning process go through from the

Le A 13 701 - 6 -

20984A / 1136

Threads emerging from the spinning head enter a heated spinning shaft and are wound onto a bobbin with a delay. Afterward the threads can be drawn under suitable conditions. The spinning solutions can also be made by a jet blowing process or a centrifugal spinning process into staple fibers.

If necessary, the fibers must be subjected to a pretreatment, which results from an oxidation, before the actual pyrolysis can exist and has the purpose of making the carbon-yielding polymer infusible. In a subsequent The remaining solvent residues in the thread are driven out and through a targeted heat treatment Pyrolysis split off all volatile components. For this purpose, the fibers are under a stream of inert gas, such as. B. nitrogen, Argon or hydrogen, heated to a temperature in the range of about 800 - 1200 ° C. In detail, the Temperature treatment according to the used carbon-supplying starting substance and the boron component. The fibers consist of a sintering temperature of around 1200 ° C X-ray amorphous lorocarbide and optionally from X-ray amorphous Carbon. With further heating the formation of crystallized boron carbide begins, which at a temperature a crystallite size of 150 angstroms determined by X-ray analysis at 1600 ° C and a crystallite size of 400 angstroms at 2000 ° C having.

To assess the mechanical properties of the fibers, measurements of the tensile strength and the modulus of elasticity were made carried out. A commercially available micro-tearing machine was used for this (Tecam Tensile Testing Machine from Techne). After heating up de. Fibers at one temperature from around 1200 ° C they already have very good mechanical properties. The tensile strength here is 140 160 kp / mm and the modulus of elasticity 27,000 - 29,000 kp / mm.

Le A 13 701 - 7 -

209844/1136

The modulus of elasticity can be increased if the fibers are heated to higher temperatures under argon for a short time up to 2350 ° C.

The fibers obtained in this way made of boron carbide or of boron carbide-carbon mixtures are insensitive to oxidation or significantly more resistant to oxidation than conventional carbon fibers. They can be used up to very high temperatures and can be based on their mechanical properties Use very well for reinforcing plastics, glasses, ceramics and metals. Furthermore are like that fibers obtained in excellent measure for high-temperature insulation and as filter material for hot, corrosive gases or melting.

For the production of coatings from boron carbide or boron carbide-r Carbon mixtures can be coated with a protective coating. The end of the apparatus is soaked or painted with a suitable solution of boron and carbon components and then be subjected to the temperature treatment. Firmly adhering, gas-tight protective layers of of the desired strength, which are insensitive to corrosion at high temperatures.

High-strength, pore-free and flexible foils or flakes made of boron carbide or boron carbide-carbon mixtures can pass through Roll out or spread the solution of the boron and carbon components onto a suitable surface, as follows Drying and peeling and subsequent suitable tempering can be produced.

Foam bodies are obtained when the solution of the boron and carbon component, optionally with the addition of a conventional blowing agent such as ammonium carbonate,

Le A 13 701 - 8 -

209844/1136

21 1 8 A 8 9-

foams by suitable heating in an appropriate mold and then heated to 800 to 2350 ° C. Like that The foam bodies obtained have an extremely low volume weight and are outstandingly suitable for insulation purposes at very high temperatures.

The molded bodies produced, which consist of boron carbide and boron carbide-carbon mixtures, can have a boron content of 78.3 to 5 percent by weight and a carbon content of 21.7 to 95 percent by weight, the sum of the percentages by weight of boron and carbon being at least 90 % target.

Le A 13 701 - 9 -

209844/1 "136

. 40

Example 1.

When 18.5 g of polyacrylonitrile (homopolymer), 31 »5 g of diammonium dodecaboranate and 2.8 g of polyethylene oxide (degree of polymerization 136,400) are dissolved in 225.2 g of dimethylformamide, a spinning solution is obtained which contains 6.7 % polyacrylonitrile, 11.3% diammonium dodecaboranate and contains 1 % polyethylene oxide. At a temperature of 95 ° C, the spinning solution is converted into threads with a diameter of 180 m / min using a conventional dry spinning process, the nozzle cross-section 0.7 mm, the shaft inlet temperature 250 ° C and the shaft outlet temperature 220 ° C, at a take-off speed of 180 m / min spun from 16 / um. They are then tempered under nitrogen for 12 hours at 190 ° C. and brought to a temperature of 1200 ° C. within 7 hours. The resulting fibers consist of X-ray amorphous boron carbide and have a tensile strength of 140 kp / mm and a modulus of elasticity of 29,000 kp / They are heated to 1,800 ° C under argon within 2 hours, their modulus of elasticity increasing further.

Example 2

By dissolving 25 g of polyacrylonitrile (homopolymer), 25 g of diammonium dodecaboranate and 0.75 g of polyethylene oxide (degree of polymerization 136,400) in 199.25 g of dimethylformamide, a spinning viscosity is produced which contains 10 % polyacrylonitrile, 10 % diammonium dodecaboranate and 0.3 % polyethylene oxide. At 80 ° C., the viscose becomes threads with a diameter of 20 μm at a take-off speed of 210 m / min using a conventional dry spinning process, the nozzle cross section being 0.5 mm, the duct inlet temperature 230 ° C. and the duct outlet temperature 180 ° C. spun. They are then at 180 ° C on their

Le A 13 701 '"- 10 -

2098 4 4/1136

stretched eight times its length and heat-treated at 250 ° C. under nitrogen for 25 h. They become infusible and can be heated to 1200 ° C under nitrogen over the course of 7 hours. The result is blue-black, shiny threads which have a tensile strength of 160 kgf / mm and a modulus of elasticity of 28,000 kgf / mm. The fibers obtained in this way consist of 80 % boron carbide, which is in amorphous form, and 20 % carbon.

Example 3

A solution for carbon-rich boron carbide threads is obtained by dissolving 37.5 g of polyacrylonitrile (homopolymer) and 12.5 g of diammonium dodecaboranate in 128.8 g of dimethylformamide. The spinning solution contains 21 % polyacrylonitrile and 7 % diammonium dodecaboranate. Due to its high polyacrylonitrile content, it can be spun without using polyethylene oxide as a spinning aid. The viscose is spun at 110 ° C. using a conventional dry spinning process, the nozzle cross section being 0.2 mm, the duct inlet temperature 260 ° C., the duct outlet temperature 190 ° C. and the take-off speed 270 m / min. The threads are then stretched at a temperature of 110 ° C. to a thread cross section of 13 μm and tempered in air at 170 ° C. for 2 hours. In the course of 8 hours, the threads are heated to 1200 ° C. under nitrogen and then to 2000 ° C. in 1 hour under argon. The threads have a black, shiny appearance and have a tensile strength of 160 kp / mm and a modulus of elasticity of 27,500 kp / mm. They contain 51 % amorphous carbon and 49 % boron carbide with a crystallite size of 50 Sngström-.

Le A 13 701 - 11 -

2098U / 1 136

Example 4

To produce a boron carbide film, the solution of 18.5 g of polyacrylonitrile (homopolymer) and 31.5 g of diammonium dodecaboranate in 122.5 g of dimethylformamide, which has a concentration of 10.7% polyacrylonitrile and 18.3 % diammonium dodecaboranate, is rolled onto a tefionized metal surface and dried at 170 ° C. under nitrogen. The film is peeled off the base and heated at 220 ° C. under nitrogen for 8 hours. The film is then heated to 1200 ° C. within 5 hours. A flexible, blue-black film made of amorphous boron-P carbide and having a thickness of 20 μm is obtained.

Example 5

A foam body made of boron carbide can be produced by applying the solution in a layer of approx.

1 cm thickness is placed in a porcelain boat and under nitrogen at a temperature of 180 ° C for 5 hours is dried. The resulting glassy, transparent, brittle mass is then inside nitrogen

Heated to 1200 ° C for 2 hours. The resulting foam foam Pyrolysis gases increase the body to 10 times its original volume. The fine-pored

P foam body made of boron carbide is excellent for high-temperature insulation.

Le A 13 701 - 12 -

209844/1 136

Claims (14)

Patent claims: 1. Process for the production of shaped bodies from boron carbide or boron carbide-carbon mixtures, characterized in that the solution or the melt of a boron-hydrogen compound and a polymeric organic compound thermally decomposable to carbon, optionally under Addition of auxiliaries to the corresponding porous bodies processed and then heated under inert gas to temperatures between 800 ° C and 2350 ° C. 2. The method according to claim 1, characterized in that a dry spinning process is used for the production of fibers will. 3. The method according to claim 1 and 2, characterized in that dissolved linear polymeric organic substances as auxiliaries with degrees of polymerisation above 10,000 can be used. 4. · The method according to claim 3, characterized in that the linear polymeric high molecular weight substances in one Concentration of 0.01 to 2 wt .-? 6 can be used. 5. The method according to claim 3 and 4, characterized in that the linear polymeric high molecular weight substances are vinyl polymers and copolymers, diolefin polymers, polyethers, Polythioethers, polyesters, polyamides and polypeptides are used will. 6. The method according to claim 5, characterized in that the linear polymeric high molecular weight substances polyethylene oxide, Polypropylene oxide, polyacrylamide, acrylamide-acrylic acid copolymers or their salts, polyisobutylene, polymethyl methacrylate, polyisoprene and polystyrene can be used. Le A 13 701 - 13 - 209844/1136 7. The method according to claim 1 to 6, characterized in that boranates are used as boron compounds, in particular Diammonium decaboranate, diammonium dodecarboranate or their alkyl, allyl and ary! Ammonium compounds. 8. The method according to any one of claims 1 to 7, characterized in that the carbon supplying organic Compounds used are those that do not contain any oxygen bound in the molecule. 9. The method according to claim 8, characterized in that ^ ■ polyacrylonitrile and its copolymers with at least 85 % polyacrylonitrile or copolymers of polyacrylonitrile with butadiene or with butadiene and styrene are used as carbon-supplying organic compounds. 10. The method according to claim 7 and 9, characterized. that the mixture of the boron compound and the carbon-supplying organic compound dissolved in dimethylformamide is spun. 11. Paving, produced according to any one of claims 1 to 10. 12. Fibers containing ™ Boron from 78.3 to 5% by weight and carbon from 21.7 to 95% by weight, the sum of the% by weight boron and carbon being at least 90 % . 13. Use of the fibers according to claim 11 and 12 to . Reinforcement and insulation purposes. 14. Shaped body produced according to one of claims 1 to Le A 13 701 ■ * - 14 - 209844/1136