hence, it is important to improve the dispersion of nano clays in the polymer matrix.
Inorganic clays are modified with organic compounds by exchanging interlayer
cations with organic cationic surfactants, resulting in a decrease in the surface
energy of the inorganic clay. This improves the compatibility of the nano clay with
the polymer matrix. Organic cationic compounds can increase the interlayer spacing
due to the presence of long alkyl chains. Ray et al. [66] reviewed and studied the
surface modification of different types of clays and polymer nanocomposites.
Typically, the FR activity and other properties are dependent on the dispersion
quality in the polymer matrix and the organic modifier. Polymer-clay nanocomposites show FR activity due to the following reason. During polymer burning, the
viscosity of the molten polymer composite decreases, which increases the migration
of nano clay to the polymer surface and creates FR active sites on the polymer
surface. This accelerates char formation and reduces flammability, similar to other
surface-modified metal oxides and carbon-based nanocomposites. Therefore, clay
surface modification plays an important role in improving the compatibility with the
polymer as well as in increasing the FR activity. Different types of nitrogen and
phosphorus-containing organic compounds are used for the surface modification of
clays to prepare FR polymer nanocomposites and also to meet special requirements
[67, 68]. It is assumed that the synergism between P and N improves the FR activity
of polymer nanocomposites.
Commonly, polymer nanocomposites are prepared by three processes, namely
melt blending, solvent casting, and in situ polymerization. Compared to other
methods, in situ polymerization has certain advantages. The monomer participates in
interfacial interactions with the clays, which helps in enhancing the properties of the
composites. Tai et al. [69] prepared PDEPD/clay composites via in situ polymerization; DDE was intercalated into the clay platelets followed by the addition of
PDCP. The monomer was polymerized inside the clay and during the polymerization
process, HCl was eliminated (Fig. 7.6). Further, PS and PU clay nanocomposites
were prepared at varying nano clay percentages and they were found to exhibit high
FR activity. PU composites showed higher FR activity compared to PS due to the
occurrence of rearrangement reactions involving PDEPD during the combustion of
PUs and increased char density. Zhu et al. [70] prepared PMMA/clay nanocomposites via in situ polymerization with MMT modified with different quaternary
ammonium salts and they observed exfoliated structure formation with unsaturated
quaternary ammonium salts. In the case of saturated ammonium salts, an intercalated
structure was formed. Many studies have indicated that exfoliated polymer
nanocomposites exhibited better properties than intercalated structures. Si et al. [71]
prepared PMMA/clay nanocomposites using a combination of traditional DB and AO
flame retardants. They observed that those nanocomposites containing a combination
of the three components showed excellent FR activity with self-extinguishing ability.
In this case, clay helps in flame quenching, which means that it accelerates char
formation, increases FR dispersion, and catalyzes chain reactions. Similarly, Song
et al. [72] prepared PU/clay nanocomposites with and without MPP as a FR; they
observed that the inclusion of MPP induced excellent FR activity with high
LOI values compared to other nanocomposites owing to a synergetic effect.
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7 Polymer Nanocomposites for Fire Retardant Applications
Inorganic clays are modified with organic compounds by exchanging interlayer
cations with organic cationic surfactants, resulting in a decrease in the surface
energy of the inorganic clay. This improves the compatibility of the nano clay with
the polymer matrix. Organic cationic compounds can increase the interlayer spacing
due to the presence of long alkyl chains. Ray et al. [66] reviewed and studied the
surface modification of different types of clays and polymer nanocomposites.
Typically, the FR activity and other properties are dependent on the dispersion
quality in the polymer matrix and the organic modifier. Polymer-clay nanocomposites show FR activity due to the following reason. During polymer burning, the
viscosity of the molten polymer composite decreases, which increases the migration
of nano clay to the polymer surface and creates FR active sites on the polymer
surface. This accelerates char formation and reduces flammability, similar to other
surface-modified metal oxides and carbon-based nanocomposites. Therefore, clay
surface modification plays an important role in improving the compatibility with the
polymer as well as in increasing the FR activity. Different types of nitrogen and
phosphorus-containing organic compounds are used for the surface modification of
clays to prepare FR polymer nanocomposites and also to meet special requirements
[67, 68]. It is assumed that the synergism between P and N improves the FR activity
of polymer nanocomposites.
Commonly, polymer nanocomposites are prepared by three processes, namely
melt blending, solvent casting, and in situ polymerization. Compared to other
methods, in situ polymerization has certain advantages. The monomer participates in
interfacial interactions with the clays, which helps in enhancing the properties of the
composites. Tai et al. [69] prepared PDEPD/clay composites via in situ polymerization; DDE was intercalated into the clay platelets followed by the addition of
PDCP. The monomer was polymerized inside the clay and during the polymerization
process, HCl was eliminated (Fig. 7.6). Further, PS and PU clay nanocomposites
were prepared at varying nano clay percentages and they were found to exhibit high
FR activity. PU composites showed higher FR activity compared to PS due to the
occurrence of rearrangement reactions involving PDEPD during the combustion of
PUs and increased char density. Zhu et al. [70] prepared PMMA/clay nanocomposites via in situ polymerization with MMT modified with different quaternary
ammonium salts and they observed exfoliated structure formation with unsaturated
quaternary ammonium salts. In the case of saturated ammonium salts, an intercalated
structure was formed. Many studies have indicated that exfoliated polymer
nanocomposites exhibited better properties than intercalated structures. Si et al. [71]
prepared PMMA/clay nanocomposites using a combination of traditional DB and AO
flame retardants. They observed that those nanocomposites containing a combination
of the three components showed excellent FR activity with self-extinguishing ability.
In this case, clay helps in flame quenching, which means that it accelerates char
formation, increases FR dispersion, and catalyzes chain reactions. Similarly, Song
et al. [72] prepared PU/clay nanocomposites with and without MPP as a FR; they
observed that the inclusion of MPP induced excellent FR activity with high
LOI values compared to other nanocomposites owing to a synergetic effect.
94
7 Polymer Nanocomposites for Fire Retardant Applications