because high percentages of N-PBAAP in PUA coatings induce high crosslinking
density, which are responsible to inhibit the formation of volatile substrates into the
gas phase but they did not discuss about LOI values. Further, Chen et al. prepared
EA/N-PBAAP UV curable resins with varying N-PBAAP contents and observed a
significant IFR activity with high char residue even at high temperature. This result
indicates that N-PBAAP shows greater synergetic and intumescent activity with EA
compared to PUA.
Similarly, Qian et al. [138] prepared acrylate resins containing different percentages of EBAPP and observed that 20 wt% EBAPP-containing resins showed
higher LOI values than 30 wt% EBAPP-containing resins. The 20 wt% EBAPP
resin exhibited a combination of gas-phase and solid-phase FR activity, whereas the
30 wt% EBAPP resin followed only the condensed-phase mechanism. However,
increasing the EBAPP content in the resin drastically reduced the THR and HRR
due to an increase in the quantity of non-flammable gases released into the flammable zone resulting in a dilution of the hot atmospheric zone and cooling it.
NPHE-PU acrylate films were synthesized at different NPHE contents; with
increasing NPHE content, the LOI value increased (the highest achievable LOI
value was 27%) and the resins exhibited excellent thermal, mechanical, and FR
properties [115]. UV-curable IFR resins were prepared at different weight loadings
of N-PBAAP content and it was observed that increasing the N-PBAAP content in
the resin highly decreased the THR and HRR; on the other hand, the char residue
formation increased [139]. Further, these resins exhibit a strong synergetic effect,
which contributed to their high IFR activity. EA/DOPO-HEA epoxy acrylate resins
containing different weight loadings of DOPO-HEA were prepared and it was
found that increasing the DOPO-HEA content in the polymer resin decreased the
THR and PHRR and increased the LOI value and char residue [65]. Most DOPO
derivatives follow the gas-phase FR mechanism and hence display a high FR
activity. Chen et al. [140] synthesized a novel phosphorus skeleton compound
(PEPA), which showed a high photo response and polymerized to form a cured
film. These cured films exhibited high FR activity; their LOI value was 39%, while
the char yield was 53% at 600 °C. Liang et al. [141] prepared UV-curable FR
polymer films from two different phosphorus acrylate monomers containing TAEP
and DAEEP and they systematically demonstrated the correlation between thermal
stability and degradation mechanism and the FR activity. Crosslinking density,
phosphorus content, and thermal degradation mechanism are three important factors
that influence the FR activity of polymer resins. TAEP films show high thermal
stability and higher LOI values than DAEEP even at lesser P contents due to the
higher density of crosslinking and lesser evolution of small volatile products into
the flame zone. Another important observation in the case of DAEEP curable films
is that during thermal degradation, small volatile products (C 2 H 4 ) are generated
from P-O-C 2 H 5 and are highly flammable. Therefore DAEEP showed lesser FR
activity and lower LOI values compared to TAEP curable polymer films [142, 143].
36
4 Types of Flame Retardants Used for the Synthesis of …
density, which are responsible to inhibit the formation of volatile substrates into the
gas phase but they did not discuss about LOI values. Further, Chen et al. prepared
EA/N-PBAAP UV curable resins with varying N-PBAAP contents and observed a
significant IFR activity with high char residue even at high temperature. This result
indicates that N-PBAAP shows greater synergetic and intumescent activity with EA
compared to PUA.
Similarly, Qian et al. [138] prepared acrylate resins containing different percentages of EBAPP and observed that 20 wt% EBAPP-containing resins showed
higher LOI values than 30 wt% EBAPP-containing resins. The 20 wt% EBAPP
resin exhibited a combination of gas-phase and solid-phase FR activity, whereas the
30 wt% EBAPP resin followed only the condensed-phase mechanism. However,
increasing the EBAPP content in the resin drastically reduced the THR and HRR
due to an increase in the quantity of non-flammable gases released into the flammable zone resulting in a dilution of the hot atmospheric zone and cooling it.
NPHE-PU acrylate films were synthesized at different NPHE contents; with
increasing NPHE content, the LOI value increased (the highest achievable LOI
value was 27%) and the resins exhibited excellent thermal, mechanical, and FR
properties [115]. UV-curable IFR resins were prepared at different weight loadings
of N-PBAAP content and it was observed that increasing the N-PBAAP content in
the resin highly decreased the THR and HRR; on the other hand, the char residue
formation increased [139]. Further, these resins exhibit a strong synergetic effect,
which contributed to their high IFR activity. EA/DOPO-HEA epoxy acrylate resins
containing different weight loadings of DOPO-HEA were prepared and it was
found that increasing the DOPO-HEA content in the polymer resin decreased the
THR and PHRR and increased the LOI value and char residue [65]. Most DOPO
derivatives follow the gas-phase FR mechanism and hence display a high FR
activity. Chen et al. [140] synthesized a novel phosphorus skeleton compound
(PEPA), which showed a high photo response and polymerized to form a cured
film. These cured films exhibited high FR activity; their LOI value was 39%, while
the char yield was 53% at 600 °C. Liang et al. [141] prepared UV-curable FR
polymer films from two different phosphorus acrylate monomers containing TAEP
and DAEEP and they systematically demonstrated the correlation between thermal
stability and degradation mechanism and the FR activity. Crosslinking density,
phosphorus content, and thermal degradation mechanism are three important factors
that influence the FR activity of polymer resins. TAEP films show high thermal
stability and higher LOI values than DAEEP even at lesser P contents due to the
higher density of crosslinking and lesser evolution of small volatile products into
the flame zone. Another important observation in the case of DAEEP curable films
is that during thermal degradation, small volatile products (C 2 H 4 ) are generated
from P-O-C 2 H 5 and are highly flammable. Therefore DAEEP showed lesser FR
activity and lower LOI values compared to TAEP curable polymer films [142, 143].
36
4 Types of Flame Retardants Used for the Synthesis of …