activity and self-extinguishing ability. Lai et al. [126] synthesized PETBP and used
it alone as an IFR to prepare FR-PP polymers with varying contents of
PETBP. They observed that 25 wt% PETPB-containing PP showed a high LOI
value and a V-0 rating in the UL-94 test; further, this polymer exhibited low THR
and PHRR. A novel phosphorus and sulfur-containing caged bicyclic FR compound, BCPPS, was synthesized and used with PP polymers; a high FR activity
with a high LOI value and V-0 rating in the UL-94 test was observed [127] as the
presence of sulfur led to an increased FR activity. Fullerenes (C 60 ) are reported to
be good flame retardants acting via a gas-phase mechanism; they delay the thermal
oxidative degradation of PP polymers [128]. Song et al. [129] synthesized a novel
P–N containing IFR, where PDBPP was introduced along with fullerenes into the
polymer. They observed that C 60 -d-PDBPP-containing PP exhibited reduced thermal degradation, high initial ignition temperature, and a significantly low PHRR
compared to neat PP. The researchers also commented on the dual-phase activity of
the FR additive; fullerenes (C 60 ) exhibited a gas-phase mechanism at the same time
as PDBPP displayed a condensed-phase mechanism, which induced excellent FR
activity.
4.4 UV-Curable FRs
UV curing technology is an emerging technology in polymer science; in this process, the unsaturated parts of the monomers are polymerized in the presence of
incident UV radiation. Compared to traditional solvent-cured films, radiation-cured
polymer resins exhibit certain advantages, such as requiring less energy, rapid and
efficient curing, selective part curing, and no environmental pollution, which means
that no solvent is eluted during curing. Hence, this technology is being used
increasingly in many fields, such as the coating industry, microelectronics, and for
the protection of different material surfaces. However, there are several disadvantages including the need for a UV source and oxygen inhibition; molecular oxygen
is highly active in terminating polymerization. However, a number of methods have
been developed to utilize oxygen scavengers and high radiation intensity to overcome these drawbacks [132]. During UV curing, only the unsaturated parts of
acrylic oligomers are involved in polymerization. The acrylic derivative compounds
treated using this method are polyester acrylates, epoxy acrylates, polyether acrylates, silicon acrylates, and urethane acrylates [133–136]. Urethane acrylate oligomers exhibit excellent properties, such as high impact and tensile strength,
abrasion resistance, toughness, and durability. Therefore, UV curable PUs are used
in many applications, including composite wood, plastics, and ceramics. Some
acrylate oligomer structures are shown in Scheme 4.12; these structures can be used
to synthesize FR PUs. Chen et al. [137] synthesized UV curable FR-PUA coatings
by introducing different percentages of N-PBAAP (Scheme 4.13) and studied their
thermal and FR properties. It was observed that increasing the N-PBAAP content in
PUA coatings significantly decreased the THR and HRR values but increased the
34
4 Types of Flame Retardants Used for the Synthesis of …
it alone as an IFR to prepare FR-PP polymers with varying contents of
PETBP. They observed that 25 wt% PETPB-containing PP showed a high LOI
value and a V-0 rating in the UL-94 test; further, this polymer exhibited low THR
and PHRR. A novel phosphorus and sulfur-containing caged bicyclic FR compound, BCPPS, was synthesized and used with PP polymers; a high FR activity
with a high LOI value and V-0 rating in the UL-94 test was observed [127] as the
presence of sulfur led to an increased FR activity. Fullerenes (C 60 ) are reported to
be good flame retardants acting via a gas-phase mechanism; they delay the thermal
oxidative degradation of PP polymers [128]. Song et al. [129] synthesized a novel
P–N containing IFR, where PDBPP was introduced along with fullerenes into the
polymer. They observed that C 60 -d-PDBPP-containing PP exhibited reduced thermal degradation, high initial ignition temperature, and a significantly low PHRR
compared to neat PP. The researchers also commented on the dual-phase activity of
the FR additive; fullerenes (C 60 ) exhibited a gas-phase mechanism at the same time
as PDBPP displayed a condensed-phase mechanism, which induced excellent FR
activity.
4.4 UV-Curable FRs
UV curing technology is an emerging technology in polymer science; in this process, the unsaturated parts of the monomers are polymerized in the presence of
incident UV radiation. Compared to traditional solvent-cured films, radiation-cured
polymer resins exhibit certain advantages, such as requiring less energy, rapid and
efficient curing, selective part curing, and no environmental pollution, which means
that no solvent is eluted during curing. Hence, this technology is being used
increasingly in many fields, such as the coating industry, microelectronics, and for
the protection of different material surfaces. However, there are several disadvantages including the need for a UV source and oxygen inhibition; molecular oxygen
is highly active in terminating polymerization. However, a number of methods have
been developed to utilize oxygen scavengers and high radiation intensity to overcome these drawbacks [132]. During UV curing, only the unsaturated parts of
acrylic oligomers are involved in polymerization. The acrylic derivative compounds
treated using this method are polyester acrylates, epoxy acrylates, polyether acrylates, silicon acrylates, and urethane acrylates [133–136]. Urethane acrylate oligomers exhibit excellent properties, such as high impact and tensile strength,
abrasion resistance, toughness, and durability. Therefore, UV curable PUs are used
in many applications, including composite wood, plastics, and ceramics. Some
acrylate oligomer structures are shown in Scheme 4.12; these structures can be used
to synthesize FR PUs. Chen et al. [137] synthesized UV curable FR-PUA coatings
by introducing different percentages of N-PBAAP (Scheme 4.13) and studied their
thermal and FR properties. It was observed that increasing the N-PBAAP content in
PUA coatings significantly decreased the THR and HRR values but increased the
34
4 Types of Flame Retardants Used for the Synthesis of …