PROCESS FOR PRODUCING A SHAPED BORON NITRIDE PRODUCT

Abstract: A process for the production of boron nitride is carried out by condensing a mixture of B-tris­ (loweralkylamino)borazine with an alkylamine in an inert gas atmosphere at a temperature of 150°C or above to prepare a precursor, forming the precursor into a desired shape and then slowly heating the shaped precursor in an ammonia gas atmosphere until the precursor changes to a white boron nitride product. The shaped boron nitride in the form of a fiber is particularly useful as a reinforcement.

Description: This invention relates to a process for producing a shaped boron nitride product. Further it relates to a process for producing a shaped boron nitride product from its precursor. There is known a polymer compound, which is generally called boron nitride, having a recurring structure of boron/nitrogen bond. This boron nitride has a high melting point and excellent electrical, mechanical and chemical properties. Thus it is expected to be available for various uses including a reinforcement for composite materials, the diaphragm of a molten salt battery, an IC base plate and a crucible. A known process for producing boron nitride fiber comprises heating B,B,B-triamino-N,N,N-triphenylborazine (H₂NBN-C₆H₅)₃ to 250°C in a nitrogen atmosphere to give a precursor, a condensed polymer of borazine, which would begin to soften at 150°C to 200°C; melting the obtained precursor by heating; spinning the molten precursor; and then slowly heating the precursor fiber thus obtained to a temperature higher than 1000°C in a nitrogen or an ammonia atmosphere (JP-78-37837, examined Japanese Patent Publication). Other shaped boron nitride products, for example thin film, may be produced in a similar manner from the precursor. However the shaped boron nitride product obtained by the above process is not purely white but slightly gray or pale gray in colour. This coloration seems to be caused by the incomplete removal of phenyl groups bound to nitrogen atoms. It is an object of the present invention to provide a process for producing a shaped boron nitride product which comprises heating a mixture of B-tris(loweralkylamino)-borazine and a higher alkylamine to yield a precursor, forming the resulted precursor into a desired shape and then slowly heating the shaped precursor to obtain a white boron nitride product, wherein the heating takes place in an ammonia gas atmosphere until the temperature reaches at least 600°C. Figs. 1 to 3 are scanning electron microscopic photographs each showing the form of a boron nitride fiber produced in Example 1. Fig. 4 is a scanning electron microscopic photograph which shows the form of a precursor fiber produced in Example 1. According to the present invention, the shaped boron nitride product is produced in three steps; preparation of the precursor, forming the precursor into a desired shape such as a fiber or sheet, and calcination of the formed precursor. In the first step, the precursor is produced by heating a mixture of B-tris(loweralkylamino)borazine and higher alkylamine. The starting B-tris(loweralkylamino)borazine is represented by general formula (I) below; wherein R represents a lower alkyl group, preferably a methyl or ethyl group. B-tris(methylamino)borazine is the most preferred. The borazine used in this invention easily gives a precursor with good properties in comparison with other borazines having organic substituents bound to the nitrogen atom in the borazine ring. The borazine used in this invention is believed to be easily condensed to form the precursor without cleaving the borazine ring during the condensation reaction. On the other hand, borazines having organic substituents bound to the nitrogen atom of the ring are likely to split between the nitrogen atom and boron atom of the ring during the condensation reaction. The difference in the reaction mechanisms seems to cause the difference in the precursors. The starting higher alkylamine is mono-alkylamine having an alkyl group which is higher than the alkyl group bonding to the starting borazine. Usually the starting higher alkylamine having four to twenty carbon atoms, preferably eight to eighteen carbon atoms, is used. The mixture usually contains five to thirty parts by weight of the higher alkylamine with respect to one hundred parts by weight of the borazine. The precursor obtained from the mixture has a lower molecular weight and better properties in comparison with that obtained from the borazines without the higher alkylamine when both are prepared by heating at the same temperature. The precursor resulting from the mixture is especially suitable to produce a fiber by means of the melt-spinning process. The ratio of the higher alkylamine in the mixture may be lowered, but the precursor obtained may prove too viscous and have poor spinning properties. On the other hand, a mixture having a higher ratio of the higher alkylamine is likely to produce a precursor having significantly too low a viscosity to be spun into fibers. Thus, the mixture preferably has five to fifteen parts by weight of the higher alkylamine with respect to one hundred parts by weight of the borazine. The mixture is heated over the melting point of the borazine in an inert gas atmosphere, usually at 140° to 300°C, preferably 150° to 250°C, for over 30 minutes. The heating time has less effect on the properties of the resulting precursor than the heating temperature. The precursor thus obtained is solid at room temperature, but melts into a viscous liquid at about 90°C. It seems to be formed a mixture of condensed borazines having higher alkylamine bonded to the boron atom. Then the precursor thus obtained is formed into any desired shape. Particularly, the precursor of the present invention may be spun into fibers, which are one of the most important and expective utilities thereof to be used as reinforcements. Of course, the precursor may be formed into a film, an IC base plate or a crucible, if desired. The shaped product is then calcined by heating it slowly in an ammonia gas atmosphere until the precursor changes into boron nitride. In the calcining process, the shaped product is to be heated in ammonia gas atmosphere until the temperature is at least at 600°C. The ammonia gas used herein may be diluted with inert gas, such as nitrogen gas, to a certain extent. The heating is preferably continued until the temperature reaches 1500°C to 1800°C since a higher temperature yields a boron nitride product with better properties. The heating may be carried out in two steps; in the first step the precursor is heated in an ammonia atmosphere until it reaches at least 600°C, and in the second step is further heated in an inert gas such as nitrogen atmosphere up to a temperature of 1500°C to 1800°C. The heating rate is usually 2 to 20°C/min., preferably 5 to 15°C/min. A lower heating rate may yield a boron nitride product with better properties, but in most cases heating at lower rates, such as lower than 2°C/min., is not desirable from an economical standpoint. The process of the present invention may be shown, with reference to the following scheme, as a model. The formula (III) represents the model of the chemical structure of the precursor. It illustrates that some of the borazine rings have a lauryl amino group connected to the boron atom and the rest do not. The formula (IV) represents the model of the chemical structure of the boron nitride fibers thus obtained. According to the present invention, a boron nitride product of white color and good properties is easily produced. The present invention is especially suitable for the production of boron nitride fiber since the precursor obtained according to the present invention is very easily available for producing precursor fibers via the melt-spinning process. To further illustrate the present invention, but without any limitation, the following Examples will be given. Example 1 50 g of laurylamine was added to 500 g of B-tris(methylamino)borazine represented by the formula (I) and thoroughly mixed therewith. The obtained mixtures were heated to 150°, 200° and 250°C respectively in a nitrogen gas atmosphere for two hours. Thus 455 g, 430 g and 375 g of a precursor were obtained respectively. The precursor obtained at 200°C was subjected to melt-spinning through a nozzle of approximately 70µm in diameter at 100°C. The obtained precursor fibers were calcined in an ammonia gas atmosphere until the temperature reached 1000°C, 1400°C and 1800°C respectively, each at a heating rate of 10°C/min. to yield boron nitride fibers in each case. In the calcining process that was carried out above 1000°C, the ammonia was replaced with nitrogen gas when the temperature reached 1000°C. The product obtained by calcining at 1800°C has a white color. Figs. 1 to 3 are scanning electron microscopic photographs of the boron nitride fiber calcined at 1000°C, 1400°C and 1800°C respectively, while Fig. 4 is that of the melt-spinning fiber precursor at 100°C. These boron nitride fibers give scanning electron microscopic photographs which show a smooth surface without any defects in the cross section. These figures indicate that each boron nitride fiber has a dense structure. The boron nitride fiber calcined at 1000°C had a tensile strength of 0.5 GPa and a elastic modulus of approximately 50 Gpa. The boron nitride fibers calcined at 1400°C and 1800°C each showed improved tensile strength and an elevated elastic modulus, compared with the one calcined at 1000°C. On the other hand, boron nitride fibers obtained by calcining the precursor fiber under nitrogen gas atmosphere throughout the calcining process at a temperature of as high as 1800°C showed a gray color which might be caused by the residual carbonized organic groups.

Claims: A process for producing a shaped boron nitride product which comprises heating a mixture of a B-tris(loweralkylamino)borazine and a higher alkylamine to yield a precursor, forming the resulted precursor into a desired shape and then slowly heating the shaped precursor to obtain a white boron nitride product, wherein the heating takes place in ammonia gas atmosphere until the temperature reaches at least 600°C. A process for producing a shaped boron nitride product as claimed in Claim 1, wherein the B-tris(loweralkylamino)-borazine is B-tris(methylamino)-borazine. A process for producing a shaped boron nitride product as claimed in Claim 1 or 2, wherein the alkyl group in the higher alkylamine has from four to twenty carbon atoms. A process for producing a shaped boron nitride product as claimed in Claim 3, wherein the alkyl group in the higher alkylamine has from eight to eighteen carbon atoms. A process for producing a shaped boron nitride product as claimed in any of Claims 1 to 4, wherein the higher alkylamine is mixed at a ratio of 5 to 30 parts with respect to one hundred parts by weight of B-tris(loweralkylamino)borazine. A process for producing a shaped boron nitride product as claimed in any of Claims 1 to 5, wherein the heating of the mixture is conducted at a temperature between 140°C to 300°C to form a precursor. A process for producing a shaped boron nitride product as claimed in any of Claims 1 to 6, wherein the heating of the shaped precursor is conducted at a rate of 5 to 15°C/min. until the temperature reaches at least 600°C. A process for producing a shaped boron nitride product as claimed in any of Claims 1 to 7, wherein the heating of the shaped precursor is conducted until the temperature reaches 1500°C or more. A process for producing a shaped boron nitride product as claimed in any of Claims 1 to 8, wherein the shaped boron nitride product is in the form of a fiber.

Inventors: KIMURA, YOSHIHARU
Applicants: MITSUBISHI KASEI CORPORATION
Application year: 1989
Main classification: C04B35
IPC codes: D01F9, C08G79, C04B35, C01B21