Normally composed of textile fibres, mineral and/or wood fibres can be added to the pre-polymerized felt. It is called pre-polymerized felt because the resin is not cured.
Whatever the origin of the fibres may be they can be piled with thermosetting resin of thermoplastic origin. 

The resins may be phenolic or non-phenolic and may also contain neutral or expandable fireproofing agents.

Among the main uses of pre-polymerized felt is that it can be subsequently moulded or shaped in a hot mould.