A simple test was conducted to make the paper tube flame-retardant. Sodium polyborate (SOUFA) was able to provide flame retardancy.
I used a paper tube sold on Amazon.

It was cut to a length of about 10 cm. After that, apply SOUFA aqueous solution. It seems that the surface is coated, and once applied, it did not penetrate much. So I paint over and over again.

Apply several times.

After that, it was naturally dried and the conditions were as follows. I think the weight will still increase if you apply more than once.

Before impregnation

・ 15g

After impregnation

・ 16g

• Because the scale can only be measured in 1g units, the exact weight is not measured, but I think it is about 110% by weight after drying.
  It was possible to give flame retardancy. Please check the video below.

SOUFA’s flame-retardant mechanism is oxygen blocking by carbonized layer formation.