4.2 Nitrogen and Phosphorus-Based Salts as FRs
Generally, nitrogen-based salts and nitrogen and phosphorus-based salts, such as
ammonium melamine phosphates, show IFR activity. A number of research articles
report that melamine salt-containing polymers exhibited excellent IFR, such as
melamine phosphate (MP), melamine polyphosphate (MPP), melamine salts of
pentaerytropolyol phosphoric acid, dicyclic phosphorous-melamine derivative
compounds, and bis(pentaerythritol phosphate) phosphoric acid compounds [13–15].
At high temperatures, the presence of phosphorus induces the formation of char and at
the same time, the presence of nitrogen induces the generation of non-flammable
gases. This results in an expandable char layer on the surface of the polymer, which
can protect it from heat and oxygen transfer. Recently, IFRs have drawn much
attention due to low toxic gas and smoke evolution and high efficiency [16–21].
To exhibit IFR activity, the system must contain an acid source (the dehydration
catalyst for char formation), carbon source (the carbonization agent), and gas source
(the blowing agent) [22–24]. In the presence of acid catalysts, the carbonizing agent
forms a char layer, which further expended by releasing blowing gases [24]. The FR
mechanism mainly depends on the type of additives and their interactions with the
polymer. Intumescent PU coatings may be used to protect various substrates, especially plastic substrates. APPs are considered to be highly effective IFRs for different
types of polymers [25, 26] and in particular, for PU polymers [27]. Duquesne et al.
[28] prepared FR-PU coatings using different percentages of APP as FR additives
and they investigated the IFR mechanism. They observed that with increasing
APP content, the LOI values increased. At 40 wt% APP, the PUs exhibited the
highest LOI value of 44%. However, high APP loadings can adversely the
mechanical properties; hence, to decrease the loading amount required for high IFR
activity, nanocomposites were prepared. Yuan et al. synthesized rigid PU foams/
expandable graphite (RPUF/EG) using a combination of phosphorus and nitrogenbased polyols (BHPP and MADP) and demonstrated the synergetic effect of these
compounds towards FR activity [29]. A phosphorus tungstic acid (PTA)-catalyzed
MPP system showed remarkably high FR activity without disturbing the mechanical
properties when compared to non-catalyzed MPP [30]. This is because PTA acts as an
effective synergist and decreases the reaction temperature as well.
Lubczak et al. [31] prepared MPP-modified PUF and reported high thermal stability, LOI values, and self-extinguishing character. Semi-rigid PUFs were prepared
using soluble ammonium polyphosphate (SAPP) and water as the blowing agent;
these systems exhibited excellent mechanical, thermal, and FR properties [32].
Further, in these systems, a strong dripping phenomenon was observed before char
formation due to a strong synergetic effect. Combinations of APP/PER act as efficient
FRs for polyolefin-based formulations [33–35] and especially for ethylene-butyl
acrylate maleic anhydride formulations [36]; all these FR formulations achieved high
LOI values and V-0 rating in the UL-94 test [37]. Bourbigot et al. [34] systematically
studied intumescent char formation and protection mechanism at high temperatures
18
4 Types of Flame Retardants Used for the Synthesis of …
Generally, nitrogen-based salts and nitrogen and phosphorus-based salts, such as
ammonium melamine phosphates, show IFR activity. A number of research articles
report that melamine salt-containing polymers exhibited excellent IFR, such as
melamine phosphate (MP), melamine polyphosphate (MPP), melamine salts of
pentaerytropolyol phosphoric acid, dicyclic phosphorous-melamine derivative
compounds, and bis(pentaerythritol phosphate) phosphoric acid compounds [13–15].
At high temperatures, the presence of phosphorus induces the formation of char and at
the same time, the presence of nitrogen induces the generation of non-flammable
gases. This results in an expandable char layer on the surface of the polymer, which
can protect it from heat and oxygen transfer. Recently, IFRs have drawn much
attention due to low toxic gas and smoke evolution and high efficiency [16–21].
To exhibit IFR activity, the system must contain an acid source (the dehydration
catalyst for char formation), carbon source (the carbonization agent), and gas source
(the blowing agent) [22–24]. In the presence of acid catalysts, the carbonizing agent
forms a char layer, which further expended by releasing blowing gases [24]. The FR
mechanism mainly depends on the type of additives and their interactions with the
polymer. Intumescent PU coatings may be used to protect various substrates, especially plastic substrates. APPs are considered to be highly effective IFRs for different
types of polymers [25, 26] and in particular, for PU polymers [27]. Duquesne et al.
[28] prepared FR-PU coatings using different percentages of APP as FR additives
and they investigated the IFR mechanism. They observed that with increasing
APP content, the LOI values increased. At 40 wt% APP, the PUs exhibited the
highest LOI value of 44%. However, high APP loadings can adversely the
mechanical properties; hence, to decrease the loading amount required for high IFR
activity, nanocomposites were prepared. Yuan et al. synthesized rigid PU foams/
expandable graphite (RPUF/EG) using a combination of phosphorus and nitrogenbased polyols (BHPP and MADP) and demonstrated the synergetic effect of these
compounds towards FR activity [29]. A phosphorus tungstic acid (PTA)-catalyzed
MPP system showed remarkably high FR activity without disturbing the mechanical
properties when compared to non-catalyzed MPP [30]. This is because PTA acts as an
effective synergist and decreases the reaction temperature as well.
Lubczak et al. [31] prepared MPP-modified PUF and reported high thermal stability, LOI values, and self-extinguishing character. Semi-rigid PUFs were prepared
using soluble ammonium polyphosphate (SAPP) and water as the blowing agent;
these systems exhibited excellent mechanical, thermal, and FR properties [32].
Further, in these systems, a strong dripping phenomenon was observed before char
formation due to a strong synergetic effect. Combinations of APP/PER act as efficient
FRs for polyolefin-based formulations [33–35] and especially for ethylene-butyl
acrylate maleic anhydride formulations [36]; all these FR formulations achieved high
LOI values and V-0 rating in the UL-94 test [37]. Bourbigot et al. [34] systematically
studied intumescent char formation and protection mechanism at high temperatures
18
4 Types of Flame Retardants Used for the Synthesis of …