Keepin’ it dry: spinning boron oxide fibers
I spent about a year at Kennecott Development working on a project for Kennecott’s Carborundum subsidiary (I think Carborundum is now owned by Saint Gobain). We were developing two processes for making boron nitride ceramic fibers. One process was intended to make tensile BN fibers for use in woven composite materials. The second process made a loose BN fiber mat that looked very much like the pink fiberglass mat used in home construction (the BN wasn’t pink of course ;).
What I want to focus on here is the second fiber mat process. One cannot spin Boron Nitride fibers directly so the process involved spinning boron oxide glass fibers first and then nitriding the BO with ammonia to convert it to BN.
As I mentioned, the fiber mat we wanted to end up with looked a lot like fiberglass wall insulation so the BO glass spinning apparatus was modeled on a commercial fiberglass spinning system. This was comprised of an electrically heated metal tank that stored the molten boron oxide glass, a metering valve, a perforated spinning cup, and a torch system.
The molten boron oxide glass was metered into the spinning cup where it was thrown against the side by the centripetal force. It then extruded through the holes in the cup a formed a cloud of glass fibers surrounding the cup. The torch was positioned so that it melted through the fibers once they reached a certain radius from the center of the cup so that one would get a consistent fiber length in the mat.
In the commercial fiberglass system, the torch would have been supplied with either natural gas or propane. But boron oxide glass is very hygroscopic; in other words it soaks up moisture from the air. When exposed to humidity it ends up looking like sticky cotton candy. And, of course, a major combustion product of both natural gas and propane is H2O.
So what to do? We needed a torch fuel that didn’t produce water vapor as a product of combustion. I don’t think it was me that thought of it but somebody on the team came up with a very elegant solution. We used carbon monoxide as the fuel and that worked quite well. It burned hot enough to cut the fibers as needed and the only combustion product was CO2 so we didn’t end up with cotton candy. Neat, eh?