Some specific compounds, such as zinc borate, can improve the melt viscosity of
polymers in the condensed phase during the initial stages of polymer composite
burning. Wang et al. [27] designed efficient halogen-free composite flame retardants
(CFR) using a nano engineering pathway; the CFRs consisted of a brucite core and
fine zinc borate Zn 6 (OH)(BO 3 ) 3 . They found out that Zn 6 (OH)(BO 3 ) 3 accelerates
the formation of a specific char structure, which can prevent contact between stored
combustible gases and oxygen. Several studies reported that zinc borate may
enhance the rigidity of the char layer and form a glassy structure, which can
enhance the FR activity and mechanical properties of the composite [28, 29]. Guo
et al. [30] coated Mg–Al LDH on wood surfaces and observed improved
mechanical and FR properties compared to uncoated wood. Shao et al. [31] prepared PP nanocomposites using APP modified with ethylene diamine via an ion
exchange reaction (MAPP); they observed that 40 wt% MAPP-containing PP
showed higher LOI values and V-0 rating in the UL-94 test compared to the neat
polymer. MAPP showed better FR activity than APP, especially in inhibiting the
fire rate growth (FRG) and SPR. The rate of heat release is an important factor
affecting the flammability of a polymer. When the HRR increases, the degradation
of a polymer is accelerated, resulting in the formation of combustible components.
Therefore, an analysis of HRR and THR is necessary to evaluate FR activity. Some
specific compounds highly inhibit HRR and THR. Different types of metal phenyl
phosphonates were synthesized and used to prepare 2 wt% PS nanocomposites. The
nanocomposites exhibited excellent thermal stability and low HRR due to high
graphitization and char formation, which can prevent heat release and act as a
thermal insulating agent [32]. Polyamides and polyimines are mostly used as FRs
and they can efficiently decrease dripping. Imine compounds in combination with
APP exhibited excellent intumescent characteristics. At lower temperatures, APP
can decompose into NH 3 , which is helpful for the formation of intumescent char
[33–35]. Similarly, Jin et al. [36] synthesized AM-APP and used it to prepare
polyamide 11 with TiO 2 nanoparticles at different loadings. The results revealed
that 22% AM-APP and 3% TiO 2 containing polymer nanocomposites exhibited a
high LOI value, V-0 rating in the UL-94 test, and no dripping. In this case, TiO 2
migrates at high temperatures to the surface of the polymer and aids the formation
of a strong char structure. Further, it accelerates the decomposition of APP,
resulting in the release of non-flammable gases. Tan et al. [37] used hyper branched
polyimide (PI)-modified APP to prepare EP nanocomposites and observed an
enhancement in the FR activity with excellent smoke suppression, high T g values,
and V-0 rating in UL-94 test. Liu et al. [38] synthesized epoxy resins with and
without thermal cross-linkers and observed that in the presence of azobenzene and
phenyl acetylene, the epoxy resins showed efficient FR activity. This is because azo
functionality-containing FRs led to the release of N 2 gas. In addition, the formation
of thermally crosslinked intermediate products improves the intumescent properties
of the composite and char compactness. The same research group synthesized
different azo benzene and acetylene derivatives for EP resins and confirmed that
they effectively enhanced the FR activity by the formation of crosslinked graphitic
86
7 Polymer Nanocomposites for Fire Retardant Applications
polymers in the condensed phase during the initial stages of polymer composite
burning. Wang et al. [27] designed efficient halogen-free composite flame retardants
(CFR) using a nano engineering pathway; the CFRs consisted of a brucite core and
fine zinc borate Zn 6 (OH)(BO 3 ) 3 . They found out that Zn 6 (OH)(BO 3 ) 3 accelerates
the formation of a specific char structure, which can prevent contact between stored
combustible gases and oxygen. Several studies reported that zinc borate may
enhance the rigidity of the char layer and form a glassy structure, which can
enhance the FR activity and mechanical properties of the composite [28, 29]. Guo
et al. [30] coated Mg–Al LDH on wood surfaces and observed improved
mechanical and FR properties compared to uncoated wood. Shao et al. [31] prepared PP nanocomposites using APP modified with ethylene diamine via an ion
exchange reaction (MAPP); they observed that 40 wt% MAPP-containing PP
showed higher LOI values and V-0 rating in the UL-94 test compared to the neat
polymer. MAPP showed better FR activity than APP, especially in inhibiting the
fire rate growth (FRG) and SPR. The rate of heat release is an important factor
affecting the flammability of a polymer. When the HRR increases, the degradation
of a polymer is accelerated, resulting in the formation of combustible components.
Therefore, an analysis of HRR and THR is necessary to evaluate FR activity. Some
specific compounds highly inhibit HRR and THR. Different types of metal phenyl
phosphonates were synthesized and used to prepare 2 wt% PS nanocomposites. The
nanocomposites exhibited excellent thermal stability and low HRR due to high
graphitization and char formation, which can prevent heat release and act as a
thermal insulating agent [32]. Polyamides and polyimines are mostly used as FRs
and they can efficiently decrease dripping. Imine compounds in combination with
APP exhibited excellent intumescent characteristics. At lower temperatures, APP
can decompose into NH 3 , which is helpful for the formation of intumescent char
[33–35]. Similarly, Jin et al. [36] synthesized AM-APP and used it to prepare
polyamide 11 with TiO 2 nanoparticles at different loadings. The results revealed
that 22% AM-APP and 3% TiO 2 containing polymer nanocomposites exhibited a
high LOI value, V-0 rating in the UL-94 test, and no dripping. In this case, TiO 2
migrates at high temperatures to the surface of the polymer and aids the formation
of a strong char structure. Further, it accelerates the decomposition of APP,
resulting in the release of non-flammable gases. Tan et al. [37] used hyper branched
polyimide (PI)-modified APP to prepare EP nanocomposites and observed an
enhancement in the FR activity with excellent smoke suppression, high T g values,
and V-0 rating in UL-94 test. Liu et al. [38] synthesized epoxy resins with and
without thermal cross-linkers and observed that in the presence of azobenzene and
phenyl acetylene, the epoxy resins showed efficient FR activity. This is because azo
functionality-containing FRs led to the release of N 2 gas. In addition, the formation
of thermally crosslinked intermediate products improves the intumescent properties
of the composite and char compactness. The same research group synthesized
different azo benzene and acetylene derivatives for EP resins and confirmed that
they effectively enhanced the FR activity by the formation of crosslinked graphitic
86
7 Polymer Nanocomposites for Fire Retardant Applications