compared to epoxy/DOPO and epoxy/PEPA, owing to the synergism between
DOPO and PEPA. These FR compounds can be incorporated in the polymer
structure as chain extenders or as co-monomers. DOPO was introduced into
polymeric diols and reacted with diisocyanates under suitable conditions to prepare
PUs for FR applications [72]. Phosphorylation of poly(epichlorohydrin) (PECH)
was conducted by reacting the P–H bond of DOPO with the chloromethyl group of
PECH; it resulted in the formation of hydroxyl-containing PECH as the product.
These hydroxyl groups are capable of reacting with diisocyanates to form
phosphorus-containing PUs, as shown in Scheme 4.6. FR-PU polymers were
synthesized using DOPO-phosponamidates and their FR activity was compared
with that of various commercial available FRs, such as TCCP (tris (chloroisopropyl) phosphate) and DOPO. They observed that the thermal and FR properties of
DOPO-phosponamidates are superior compared to those of commercial FRs [73].
Wang et al. [74] synthesized triazole-containing DOPO derivatives and used them
as curing agents for epoxy resins; the resultant epoxy resins exhibited high thermal
and FR properties with high LOI values and a V-0 rating in the UL-94 test.
Similarly, different curing agents based on DOPO derivatives containing –OH and
–NH 2 functionalities were synthesized and used either as chain extenders or curing
agents to prepare FR epoxy resins. The resulting epoxy resins exhibited a high
T g value, thermal stability, low thermal expansion, and excellent FR properties
[66, 75–78]. Liu et al. [79] prepared a series of PU foams with different DOPO
derivatives along with various P-oxides as FR additives; the DOPO-derivativecontaining PU foams displayed high thermal stability and FR activity with low
smoke production and high char residue formation. DOPO in combination with
hyper-branched polyamide (HBPA) exhibited excellent IFR activity as shown in
Fig. 4.1; the ·PO radicals released by DOPO can interrupt coating degradation,
while HBPA induces the formation of a strong crosslinked char layer and APP
(APP incorporated along with DOPO and HBPA) releases non-flammable gases.
HBPA exhibits good expandability and plays an important role as a carbonization
and blowing agent as it contains large amounts of C and N atoms, which can
increase the IFR activity by improving the quality of the char layer.
4.3.2 Phosphorus-Based FRs with Different Oxidation
States
Most phosphorus derivatives and phosphorus-containing polymers show FR
activity by virtue of the presence of phosphorus. During burning, they can form
phosphorus and phosphoric acid; phosphoric acid further esterifies and dehydrates
the polymer, resulting in the formation of a protective carbonaceous char layer on
surface of the polymer substrate. This char layer acts as a heat-resistant layer at high
temperatures and it also protects the substrate from oxygen and heat transfer [81].
Previous studies have shown that phosphorus-based FRs show efficient FR activity
4.3 Phosphorus-Based FRs
25
DOPO and PEPA. These FR compounds can be incorporated in the polymer
structure as chain extenders or as co-monomers. DOPO was introduced into
polymeric diols and reacted with diisocyanates under suitable conditions to prepare
PUs for FR applications [72]. Phosphorylation of poly(epichlorohydrin) (PECH)
was conducted by reacting the P–H bond of DOPO with the chloromethyl group of
PECH; it resulted in the formation of hydroxyl-containing PECH as the product.
These hydroxyl groups are capable of reacting with diisocyanates to form
phosphorus-containing PUs, as shown in Scheme 4.6. FR-PU polymers were
synthesized using DOPO-phosponamidates and their FR activity was compared
with that of various commercial available FRs, such as TCCP (tris (chloroisopropyl) phosphate) and DOPO. They observed that the thermal and FR properties of
DOPO-phosponamidates are superior compared to those of commercial FRs [73].
Wang et al. [74] synthesized triazole-containing DOPO derivatives and used them
as curing agents for epoxy resins; the resultant epoxy resins exhibited high thermal
and FR properties with high LOI values and a V-0 rating in the UL-94 test.
Similarly, different curing agents based on DOPO derivatives containing –OH and
–NH 2 functionalities were synthesized and used either as chain extenders or curing
agents to prepare FR epoxy resins. The resulting epoxy resins exhibited a high
T g value, thermal stability, low thermal expansion, and excellent FR properties
[66, 75–78]. Liu et al. [79] prepared a series of PU foams with different DOPO
derivatives along with various P-oxides as FR additives; the DOPO-derivativecontaining PU foams displayed high thermal stability and FR activity with low
smoke production and high char residue formation. DOPO in combination with
hyper-branched polyamide (HBPA) exhibited excellent IFR activity as shown in
Fig. 4.1; the ·PO radicals released by DOPO can interrupt coating degradation,
while HBPA induces the formation of a strong crosslinked char layer and APP
(APP incorporated along with DOPO and HBPA) releases non-flammable gases.
HBPA exhibits good expandability and plays an important role as a carbonization
and blowing agent as it contains large amounts of C and N atoms, which can
increase the IFR activity by improving the quality of the char layer.
4.3.2 Phosphorus-Based FRs with Different Oxidation
States
Most phosphorus derivatives and phosphorus-containing polymers show FR
activity by virtue of the presence of phosphorus. During burning, they can form
phosphorus and phosphoric acid; phosphoric acid further esterifies and dehydrates
the polymer, resulting in the formation of a protective carbonaceous char layer on
surface of the polymer substrate. This char layer acts as a heat-resistant layer at high
temperatures and it also protects the substrate from oxygen and heat transfer [81].
Previous studies have shown that phosphorus-based FRs show efficient FR activity
4.3 Phosphorus-Based FRs
25