Boron nitride thin fibres obtained from a new copolymer borazine–tri(methylamino)borazine precursor

Boron nitride thin fibres have been obtained using the melt drawn technique from a new molecular precursor prepared by reacting borazine (HBNH)3 with trimethylamino-borazine (CH3NHBNH)3, (MAB). Borazine reacted very slowly with MAB at room temperature but this reaction was very enhanced when heated. Polymers have been prepared with different borazine/MAB ratio and several reaction temperatures and times. According to the precursor properties required for the melt drawing technique, the best results were obtained with copolymer prepared with a borazine/MAB molar ratio of 90/10 heated at 75 °C for 36 h. Its glass transition was 22.5 °C and no decomposition occurred up to 100 °C. The ceramic yield was 78%, however the ceramisation, even under an ammonia atmosphere, was very slow up to 300 °C. Using a spinning machine, a mother fibre was easily extruded at 85 °C under a 40 daN pressure with a diameter of about 200 μm but its mechanical properties were too low to allow stretching of this crude fibre to lower its diameter down to 20 μm without a lot of breaks. After a chemical and thermal treatment up to 1800 °C, samples of BN fibres have been obtained. If the diameter from the crude fibres was thin enough, short samples of a thin ceramic BN fibre have been obtained, but if the diameter of the crude fibre was to important, the ceramisation yield to catastrophic defects on the surface of the fibre. Theses results were related to the low glass transition of the copolymer and to the poor reactivity of the crude fibre during the preceramisation treatment as showed by TGA.