To completely analyze fire hazard properties of the PU composites, we calculated the fire growth index (FGI) and fire performance index (FPI) from HRR plots,
and the obtained values are also reported in Table 7.1. The FGI value allows the
estimation of flame spread rate, and a lower FGI value indicates delayed flashover.
The FGI value is significantly decreased from 4.4 kW/m
2 s for PU to 2.4 kW/m
2 s
for PU/M-MoS 2 -5%. However, for PU/MoS 2 -5% the FGI value increased to
6.3 kW/m
2 s. The reason is that the combustion of PU/MoS 2 forms a porous and
unstable char, as reported below. Similarly, the FPI values of PU, PU/MoS 2 -5%,
and PU-M-MoS 2 -5% are 0.09, 0.08, and 0.12 m
2 s/kW, respectively. These values
clearly show that PU composites containing M-MoS 2 exhibit strong FR activity
with decreased fire risks. Hence, we conclude that surface modification of M-MoS 2
nanosheets improves the FR activity of PU/MoS 2 composites, since the abundant –
NH 2 groups in melamine mean strong interactions with silicate and urethane
functionalities of PU chains, and the extra hydrogen bonds cause more extensive
interfacial mixing of organic and inorganic phases. Therefore, during combustion,
the PU/M-MoS 2 composites form a strong and highly crosslinked char layer, which
is responsible for the better FR properties. Figure 7.5c reports the total smoke
release (TSR) plots of all the PU composites, and the corresponding data is included
in Table 7.1. Clearly, the TSR values decreased from neat PU to PU/M-MoS 2
composites, and it also decreased with increasing M-MoS 2 content in the PU
composite, owing to the formation of highly crosslinked char. Generally, smoke can
develop from the release of incompletely burned volatile components during
combustion. If the formed char is porous and loose, then more such volatile
components will be released and condense to form a dense smoke. Figure 7.5d and
Table 7.1 report the percentage of char residues after cone calorimetry test. From
neat PU to PU/MoS 2 -5% and PU/M-MoS 2 -5%, the ratio of char residues significantly increased, with the respective percentages of 15.2%, 18.3%, and 28.3%.
Thus, the PU/M-MoS 2 composite produces a significantly increased amount of
residual char.
7.2 Clay-Based Flame-Retardant Polymer
Nanocomposites
Nano clays are layered mineral silicates and based on the arrangement of the silicate
structure and chemical components, nano clays are classified into several types,
such as MMT, kaolinite, bentonite, and halloysite. Generally, polymer nanocomposites can be prepared by the introduction of silicate layers into the polymer
matrix. Many studies have been conducted on the polymer layered silicate
nanocomposites for different applications. The presence of silicate layers in the
polymer matrix can enhance its thermal, mechanical, barrier, and flammability
properties owing to their high aspect ratio, large surface area, and nanoscale dispersion. Most of the aforementioned properties depend on the quality of dispersion;
7.1 FR Polymer Nanocomposites Based on Various Nanoparticles
93
and the obtained values are also reported in Table 7.1. The FGI value allows the
estimation of flame spread rate, and a lower FGI value indicates delayed flashover.
The FGI value is significantly decreased from 4.4 kW/m
2 s for PU to 2.4 kW/m
2 s
for PU/M-MoS 2 -5%. However, for PU/MoS 2 -5% the FGI value increased to
6.3 kW/m
2 s. The reason is that the combustion of PU/MoS 2 forms a porous and
unstable char, as reported below. Similarly, the FPI values of PU, PU/MoS 2 -5%,
and PU-M-MoS 2 -5% are 0.09, 0.08, and 0.12 m
2 s/kW, respectively. These values
clearly show that PU composites containing M-MoS 2 exhibit strong FR activity
with decreased fire risks. Hence, we conclude that surface modification of M-MoS 2
nanosheets improves the FR activity of PU/MoS 2 composites, since the abundant –
NH 2 groups in melamine mean strong interactions with silicate and urethane
functionalities of PU chains, and the extra hydrogen bonds cause more extensive
interfacial mixing of organic and inorganic phases. Therefore, during combustion,
the PU/M-MoS 2 composites form a strong and highly crosslinked char layer, which
is responsible for the better FR properties. Figure 7.5c reports the total smoke
release (TSR) plots of all the PU composites, and the corresponding data is included
in Table 7.1. Clearly, the TSR values decreased from neat PU to PU/M-MoS 2
composites, and it also decreased with increasing M-MoS 2 content in the PU
composite, owing to the formation of highly crosslinked char. Generally, smoke can
develop from the release of incompletely burned volatile components during
combustion. If the formed char is porous and loose, then more such volatile
components will be released and condense to form a dense smoke. Figure 7.5d and
Table 7.1 report the percentage of char residues after cone calorimetry test. From
neat PU to PU/MoS 2 -5% and PU/M-MoS 2 -5%, the ratio of char residues significantly increased, with the respective percentages of 15.2%, 18.3%, and 28.3%.
Thus, the PU/M-MoS 2 composite produces a significantly increased amount of
residual char.
7.2 Clay-Based Flame-Retardant Polymer
Nanocomposites
Nano clays are layered mineral silicates and based on the arrangement of the silicate
structure and chemical components, nano clays are classified into several types,
such as MMT, kaolinite, bentonite, and halloysite. Generally, polymer nanocomposites can be prepared by the introduction of silicate layers into the polymer
matrix. Many studies have been conducted on the polymer layered silicate
nanocomposites for different applications. The presence of silicate layers in the
polymer matrix can enhance its thermal, mechanical, barrier, and flammability
properties owing to their high aspect ratio, large surface area, and nanoscale dispersion. Most of the aforementioned properties depend on the quality of dispersion;
7.1 FR Polymer Nanocomposites Based on Various Nanoparticles
93