Soft Char intumescent materials produce a lighter char than Hard Char materials, referred to as light char. The light char produced during the heating process has poor heat conduction, which acts as a retardant in the transfer of heat from the heat source (fire) to the material that is coated.
Light char is typically comprised of microporous carbonaceous foam that is a result of the chemical reaction between three main components: melamine, ammonium polyphosphate, and pentaerythritol tetranitrate.
The chemical reaction takes place in a matrix that is formed by a molten binder which is typically based on styrene acrylates or vinyl acetate copolymers.
Soft char is typically used in thin coatings for the fireproofing of structural steel
as well as being present in firestop pillows, which are essentially large pillows used to block holes in which fire is actively trying to pass through. Hard Char
Hard Char is activated in the same manner as soft char but is produced with different chemicals. The primary chemicals used to produce hard char are graphite and sodium silicates.
The matrix used to house these chemicals is like that of Soft Char, but depending on manufacturer the compositions can change per the desired level of charring.