SOUFA was brushed on the cardboard (both sides), and after drying, it was ignited with an alcohol lamp.
SOUFA gives high flame resistance to cellulose, so it is easy to stop burning. However, SOUFA does burn.

As for the flame retardant mechanism, SOUFA settles on the pulp of the cardboard 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 won’t spread further. If it is only brushed on, the texture will not be affected much. In case of corrugated cardboard, there is a possibility of ignition if the fire reaches the inner core. It is because SOUFA does not penetrate to the core of the inside only by painting on the surface. If you let it soak into the core by dipping, it will further stop burning. Or if you treat it at the stage of the base paper of the cardboard, it will be more effective.
However, if too much is applied, it is likely to harden. This is because the boric acid has a property of hardening cellulose a little.