structure, and interactions between the polymer and FR. Crosslinking can also
improve FR activity; the functional groups of a FR may be capable of increasing the
degree of crosslinking. Based on their mechanism, FRs are classified into four
categories—gas phase, endothermic, solid phase, and intumescent FRs.
2.1.1 Gas-Phase FRs
These FRs show FR activity by releasing non-flammable gases into the flame zone
area to decrease the heat and oxygen levels, which are responsible for flame
propagation. The quantity of airborne components is low because FRs limit the heat
released into the flame zone area. When polymers containing mineral fillers as FRs
are exposed to heat or flame, the mineral fillers start to decompose and release
non-flammable gases, such as CO 2 , water, SO 2 , and some acids, resulting in a
reduced oxygen content and cooling the flame zone area [1–4].
2.1.2 Endothermic FRs
Endothermic FRs show FR activity by forming endothermic radicals that can
absorb heat energy and reduce flammability by decreasing the amount of heat in the
flame zone area. For example, hydrated fillers when incorporated in polymers
decompose endothermically and decrease the temperature to a value less than that
required for polymer degradation or combustion. Metal hydroxides, halogens,
phosphorus derivatives of low oxidation levels, and antimony-containing polymers
fall under this category. Antimony is normally used as a synergistic agent along
with halogen-based FRs to initiate halogen radical formation and transfer into the
flame zone, which results in flame interruption.
2.1.3 Solid-Phase FRs
Most phosphorus and sulfur derivatives act as solid-phase FRs. When exposed to
flame, they form the corresponding acids, which are capable of fast dehydration,
and form an insulating char layer on the surfaces of polymer materials. The char
layer can protect the substrate from oxygen attack and heat transfer, which is
necessary for flame propagation [5–9].
2.1.4 Intumescent FRs
Intumescent FRs are one of the most important type of FRs because they are
capable of forming highly dense expandable char layers, which can efficiently
2.1 FR Polymers
7
improve FR activity; the functional groups of a FR may be capable of increasing the
degree of crosslinking. Based on their mechanism, FRs are classified into four
categories—gas phase, endothermic, solid phase, and intumescent FRs.
2.1.1 Gas-Phase FRs
These FRs show FR activity by releasing non-flammable gases into the flame zone
area to decrease the heat and oxygen levels, which are responsible for flame
propagation. The quantity of airborne components is low because FRs limit the heat
released into the flame zone area. When polymers containing mineral fillers as FRs
are exposed to heat or flame, the mineral fillers start to decompose and release
non-flammable gases, such as CO 2 , water, SO 2 , and some acids, resulting in a
reduced oxygen content and cooling the flame zone area [1–4].
2.1.2 Endothermic FRs
Endothermic FRs show FR activity by forming endothermic radicals that can
absorb heat energy and reduce flammability by decreasing the amount of heat in the
flame zone area. For example, hydrated fillers when incorporated in polymers
decompose endothermically and decrease the temperature to a value less than that
required for polymer degradation or combustion. Metal hydroxides, halogens,
phosphorus derivatives of low oxidation levels, and antimony-containing polymers
fall under this category. Antimony is normally used as a synergistic agent along
with halogen-based FRs to initiate halogen radical formation and transfer into the
flame zone, which results in flame interruption.
2.1.3 Solid-Phase FRs
Most phosphorus and sulfur derivatives act as solid-phase FRs. When exposed to
flame, they form the corresponding acids, which are capable of fast dehydration,
and form an insulating char layer on the surfaces of polymer materials. The char
layer can protect the substrate from oxygen attack and heat transfer, which is
necessary for flame propagation [5–9].
2.1.4 Intumescent FRs
Intumescent FRs are one of the most important type of FRs because they are
capable of forming highly dense expandable char layers, which can efficiently
2.1 FR Polymers
7