I tore off a paper bag of kraft paper, brushed on SOUFA, dried it and ignited it with an alcohol lamp.
SOUFA gives high flame resistance to cellulose, so it is easy to stop burning. However, it does burn.
As for the mechanism of flame retardancy, SOUFA settles on the cellulose of the pulp and reacts to heat when heat is applied, forming a thin glass film on the cotton surface. At the same time, it combines with the carbon of cotton to form an oxygen-blocking film.
Therefore, once the cotton gets burnt, it will not spread further. If it is just a brush coating, the texture will not be affected much.
However, if too much is applied, it is thought to become hard. This is because the boric acid has a property of slightly hardening cellulose.