The heat- and flame-resistant properties are engineered into the molecular structure of the fiber, i.e., they are inherent and permanent.
Nomex® is a synthetic aromatic polyamide polymer, or aramid for short. It’s a man-made material with ring-like molecules that are bound together in long, very strong fibers.
Thanks to its molecular structure, it can’t melt, drip or combust. When exposed to heat, a special reaction takes place to absorb the energy of the heat in the fabric.