Phosphorus-containing epoxy resins are thermally less stable, whereas
phosphorus/silicon-containing epoxy resins exhibit high thermal stability and a
synergetic FR activity; therefore, they are mainly used in advanced electronic and
electrical applications. Epoxy resins containing different percentages of phosphorus
and silica were prepared and they were found to exhibit a high char yield. This is
because phosphorus enriches char formation and silica protects the char from
thermal degradation [144, 145]. The highest LOI value achieved with such epoxy
resins as 42.5%. Neisius et al. [146] prepared a series of FR PU foams by introducing different substituted phosphoramidate FR additives and studied the relationship between the structure and FR activity of these PU forms. They observed
that methyl-substituted phosphoramidates show high FR activity compared to
phenyl-substituted phosphoramidates, while mono-substituted phosphoramidates
showed higher FR activity than di- and tri-substituted phosphoramidates. The
quantity of phosphorus is greater in methyl-substituted phosphoramidates and
hence they showed higher FR activity than other phosphoramidates. Chen et al.
[147] synthesized a novel high UV reactive DGTH and polymer films cured with
DGTH showed excellent FR activity with a high LOI value 48%, which is much
higher than the LOI values of cured films containing phosphorus alone. This is
because there is a synergetic effect between P and N and also P and Si. Further,
Chen et al. prepared DGTH-containing SPUA curable films and studied their FR
properties in terms of their LOI values and using cone calorimetry [148]. They
observed that 2.5 wt% DGTH-containing films displayed a high LOI value of 41%
and a further increase in DGTH content beyond this value decreased the LOI
values. Films with high DGTH contents, during burning, resulted in a poor char
yield owing to the generation of a large amount of nitrogen volatiles. However, the
researchers did not investigate the FR activity mechanism of these resins.
Phosphorus and silica-containing polymers displayed a synergetic FR activity
through a condensed-phase mechanism [149, 150]. Therefore, at increasing DGTH
content, it may be possible that the resins exhibit a condensed-phase mechanism
and the LOI values decreased even though there was a synergetic effect. Various
methods have been developed for the synthesis of phosphorus-containing PUs and
PEA through the grafting of phosphorus groups onto the oligomer backbone [139,
151, 152]. Unsaturated phosphorus was included on skeleton compounds and used
to synthesize UV curable resins with high IFR activity, thermal stability, and char
yield. Phosphorus-containing acrylate DOPO derivatives (DOPO-AC and
DOPO-HEA) were cured by UV; they exhibited both gas-phase and
condensed-phase FR activity with high LOI values compared to pure EA [65, 139].
Hyper-branched UV-cured FR PUs were prepared and they showed high
mechanical and thermal stability owing to a high crosslinking density, which is also
responsible for the improved FR activity [151].
4.4 UV-Curable FRs
37
phosphorus/silicon-containing epoxy resins exhibit high thermal stability and a
synergetic FR activity; therefore, they are mainly used in advanced electronic and
electrical applications. Epoxy resins containing different percentages of phosphorus
and silica were prepared and they were found to exhibit a high char yield. This is
because phosphorus enriches char formation and silica protects the char from
thermal degradation [144, 145]. The highest LOI value achieved with such epoxy
resins as 42.5%. Neisius et al. [146] prepared a series of FR PU foams by introducing different substituted phosphoramidate FR additives and studied the relationship between the structure and FR activity of these PU forms. They observed
that methyl-substituted phosphoramidates show high FR activity compared to
phenyl-substituted phosphoramidates, while mono-substituted phosphoramidates
showed higher FR activity than di- and tri-substituted phosphoramidates. The
quantity of phosphorus is greater in methyl-substituted phosphoramidates and
hence they showed higher FR activity than other phosphoramidates. Chen et al.
[147] synthesized a novel high UV reactive DGTH and polymer films cured with
DGTH showed excellent FR activity with a high LOI value 48%, which is much
higher than the LOI values of cured films containing phosphorus alone. This is
because there is a synergetic effect between P and N and also P and Si. Further,
Chen et al. prepared DGTH-containing SPUA curable films and studied their FR
properties in terms of their LOI values and using cone calorimetry [148]. They
observed that 2.5 wt% DGTH-containing films displayed a high LOI value of 41%
and a further increase in DGTH content beyond this value decreased the LOI
values. Films with high DGTH contents, during burning, resulted in a poor char
yield owing to the generation of a large amount of nitrogen volatiles. However, the
researchers did not investigate the FR activity mechanism of these resins.
Phosphorus and silica-containing polymers displayed a synergetic FR activity
through a condensed-phase mechanism [149, 150]. Therefore, at increasing DGTH
content, it may be possible that the resins exhibit a condensed-phase mechanism
and the LOI values decreased even though there was a synergetic effect. Various
methods have been developed for the synthesis of phosphorus-containing PUs and
PEA through the grafting of phosphorus groups onto the oligomer backbone [139,
151, 152]. Unsaturated phosphorus was included on skeleton compounds and used
to synthesize UV curable resins with high IFR activity, thermal stability, and char
yield. Phosphorus-containing acrylate DOPO derivatives (DOPO-AC and
DOPO-HEA) were cured by UV; they exhibited both gas-phase and
condensed-phase FR activity with high LOI values compared to pure EA [65, 139].
Hyper-branched UV-cured FR PUs were prepared and they showed high
mechanical and thermal stability owing to a high crosslinking density, which is also
responsible for the improved FR activity [151].
4.4 UV-Curable FRs
37