Borazine fibers

The area of organoboron polymers containing borazine and its derivatives is covered in Chapter 5 of this book by Miele and co-workers. Miele and Bernard also describe the utilization of these polymers in ceramics, fibers, and so on, in Chapter 3 of this book. In this section, the utilization of polymers containing borazine or in some cases the bicyclic boron ligand, 9-BBN, for the production of SiC or Si/C/B fibers is briefly described. Recent advances in polypyrazolylborate or pyrazabole-containing polymers and other boron ring system-derived polymers also have been briefly described. 

L Organoboron Polymers that Contain Borazine or 9-BBN with Utility in the Production of High-Performance Fibers... 

Polymer 1 is decomposed by reaction with ammonia, yielding methylamine and the intermediate A-trimethyl-.8-tri(amino)borazine 2, which was partially sublimed resulting in a major loss of fiber integrity (scheme 2). 

Poly[5-(alkylamino)borazines] 7-10 exhibited suitable viscoelastic and thermal stabilities to be extruded in the molten state through the monohole spinneret of a lab-scale melt-spinning apparatus. Thus, an extruded filament (diameter, 200 gm) was drawn with a windup unit, that is, a graphite spool. This provided green fibers with a wide range of diameters (16 =s (f> =s 50 gm Table 2), depending on polymer architecture. 

Borazines, particularly polymeric compounds, have been extensively investigated as preceramic materials from which coatings and fibers of boron nitride can be produced upon thermolysis. B-aryl and halogeno-amino borazines are reported to have use as fire retardants in cotton and nylon textiles. Other reported uses for borazines are as epoxy resin catalysts, polymerization inhibitors of unsaturated alcohols and esters, and catalysts for polymerization of alkenes (95). 

Another method is to extrude borazine network polymers ([-B(NH2)-N(C6H5)-]3) with addition of B203 at T > 300 °C into fibers and subsequently firing in nitrogen stream at 1800 °C [150]. 

The synthesis of boron nitride (BN) fiber has been attempted. The thermoplastic precursor for BN fiber was obtained by the reaction between B-tris(methy-lamino)borazine and lauric amine [28,29],... 

Boron nitride derived from the borazine oligomer has been successfully used to replace the carbonaceous matrix in carbon fiber/carbon matrix composites (C/C) [15,18]. The carbon fiber/boron nitride matrix composites (C/BN) exhibit improved oxidation resistance over the C/C. Excellent mechanical strength and toughness have also been observed in the C/BN. As shown in Figure 2.16, both the flexural strength and modulus of pitch carbon fiber/BN specimens actually increase with extended heat treatment [18]. 

Transmission electron microscopy (TEM) of ceramic fiber/BN matrix composite has verified the preferential orientation of BN parallel to the interface (Figure 2.17). In this figure, the diffraction pattern in the BN matrix reveals a very well aligned crystalline structure. This evidence suggests that the liquid crystallinity of the borazine oligomer provides... 

It has been confirmed that the presence of bridge-type bonds in such poly (B-alkylaminoborazines) confers flexibility and an inaeased melt-spirmability, thus leading to the conclusions that melt-spinnability inaeases from 1- to 4-daived polymers. In addition, the ceramic yield is reduced with the increased proportion of -NlCHj) in the B-(alkylamino)borazine leading to important shrinkages during the furlha- polymer-to-ceramic conversion of green fibers derived therefrom. Hence, it is relevant to assume... 

A boron nitride fiber derived from a borazine precursor fiber has recently been reported [32]. As with boron nitride fibers from B2O3, the final heat treatment of boron nitride fibers from borazine must be carried out in a reactive atmosphere (also ammonia) to chemically complete the conversion of the precursor fiber. This nitridation step renders the reaction sequence a chemical vapor infiltration process. 

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