flame, as shown clearly in Fig. 6.1. FR epoxy resins were synthesized using DOPO
either in combination with OPS or alone. When 5 wt% of OPS or DOPO was used
alone for EP composites, the obtained specimens did not achieve any rating in the
UL-94 test, but a 5 wt% mixture (2.5 wt% OPS and 2.5 wt% DOPO) led to a V-1
rating in the UL-94 test due to a strong “blowing-out effect” and also possible
synergism between OPS and DOPO [12]. The same research group studied the FR
properties of epoxy resins using two different amide curing agents, including aliphatic oligomeric poly amide (PA 650) and aromatic 4,4′-diaminodiphenylsulphone
(DDS). The UL-94 test results revealed that DGEBA/DDS with DOPO-POSS
exhibited a stronger “blowing-out effect” than DGEBA/PA650, which means that
emission of pyrolytic gases is very less in the case of aromatic amides compared to
aliphatic amides even at low percentages. It is assumed that aromatic DDS leads to
the formation of a strong crosslinked structure and dense char [13]. Further, EP
resins were synthesized using a combination of various FRs such as PEPA, APP, and
DOPO with or without OPS and flammability studies were conducted. A clear
difference could be observed in the combustion behavior of the resultant resins due
to differences in the chemical structures of the FRs. Organic phosphorus compounds,
PEPA and DOPO, showed better FR activity than inorganic phosphorus compounds,
such as APP, AP, and MPP [14]. Qian et al. [15] synthesized TGIC-DOPO, which
contains phosphaphenanthrene and triazine-trione as the FRs. They observed that the
12% TGIC-DOPO/DGEBA/DDS system showed the highest LOI value of 33.3%
and achieved a UL-94 V-0 rating. Further, they also observed that TGIC-DOPO
exhibited a dual-phase FR mechanism (gas phase and solid phase) during combustion. The phosphaphenanthrene groups are responsible for the condensed-phase
FR activity and the triazine-trione group is responsible for the gas-phase FR activity.
Similarly, Trif-DOPO was also synthesized and used as a FR for epoxy resins; it was
observed that the systems containing very little phosphorus such, as 1.2%
Trif-DOPO/DGEBA/DDS exhibited a V-0 rating in the UL-94 test and showed a
high LOI value of 36%. However, the researchers did not analyze why Trif-DOPO
showed high efficient FR activity even at low weight percentages compared to
TGIC-DOPO. Based on literature, it can be assumed that Trif-DOPO with more
aromatic groups in its structure induces higher crosslinking density and might participate in a synergistic reaction [16].
Li et al. [17] synthesized PET/PN6/PPPMS composite polymers and studied their
flammability properties. They observed that PET/10%PN6/10%PPPMS showed the
highest LOI value of 30.9%, V-0 rating in the UL-94 test, and no dripping. The melt
dripping of polymer composites mainly depends on the melt viscosity during
polymer combustion; upon the addition of PPPMS to PET/PN6, its viscoelastic
properties are drastically changed. PET/10%PN6/10%PPPMS behaved like a
completely elastic solid at all test frequencies and hence it exhibited excellent anti
dripping properties. Wang and Cai [18] synthesized triazole-containing DOPO
derivatives (DTA), as shown in Scheme 6.1, and used it as a co-curing FR agent for
epoxy resins. In the presence of DTA, melt dripping was completely inhibited and a
V-0 rating could be achieved in the UL-94 test. It was observed that 4 wt%
DTA-containing EP showed the highest LOI value, low average combustion time,
6.1 Relationship Between the Melting Dripping Characteristics …
71
either in combination with OPS or alone. When 5 wt% of OPS or DOPO was used
alone for EP composites, the obtained specimens did not achieve any rating in the
UL-94 test, but a 5 wt% mixture (2.5 wt% OPS and 2.5 wt% DOPO) led to a V-1
rating in the UL-94 test due to a strong “blowing-out effect” and also possible
synergism between OPS and DOPO [12]. The same research group studied the FR
properties of epoxy resins using two different amide curing agents, including aliphatic oligomeric poly amide (PA 650) and aromatic 4,4′-diaminodiphenylsulphone
(DDS). The UL-94 test results revealed that DGEBA/DDS with DOPO-POSS
exhibited a stronger “blowing-out effect” than DGEBA/PA650, which means that
emission of pyrolytic gases is very less in the case of aromatic amides compared to
aliphatic amides even at low percentages. It is assumed that aromatic DDS leads to
the formation of a strong crosslinked structure and dense char [13]. Further, EP
resins were synthesized using a combination of various FRs such as PEPA, APP, and
DOPO with or without OPS and flammability studies were conducted. A clear
difference could be observed in the combustion behavior of the resultant resins due
to differences in the chemical structures of the FRs. Organic phosphorus compounds,
PEPA and DOPO, showed better FR activity than inorganic phosphorus compounds,
such as APP, AP, and MPP [14]. Qian et al. [15] synthesized TGIC-DOPO, which
contains phosphaphenanthrene and triazine-trione as the FRs. They observed that the
12% TGIC-DOPO/DGEBA/DDS system showed the highest LOI value of 33.3%
and achieved a UL-94 V-0 rating. Further, they also observed that TGIC-DOPO
exhibited a dual-phase FR mechanism (gas phase and solid phase) during combustion. The phosphaphenanthrene groups are responsible for the condensed-phase
FR activity and the triazine-trione group is responsible for the gas-phase FR activity.
Similarly, Trif-DOPO was also synthesized and used as a FR for epoxy resins; it was
observed that the systems containing very little phosphorus such, as 1.2%
Trif-DOPO/DGEBA/DDS exhibited a V-0 rating in the UL-94 test and showed a
high LOI value of 36%. However, the researchers did not analyze why Trif-DOPO
showed high efficient FR activity even at low weight percentages compared to
TGIC-DOPO. Based on literature, it can be assumed that Trif-DOPO with more
aromatic groups in its structure induces higher crosslinking density and might participate in a synergistic reaction [16].
Li et al. [17] synthesized PET/PN6/PPPMS composite polymers and studied their
flammability properties. They observed that PET/10%PN6/10%PPPMS showed the
highest LOI value of 30.9%, V-0 rating in the UL-94 test, and no dripping. The melt
dripping of polymer composites mainly depends on the melt viscosity during
polymer combustion; upon the addition of PPPMS to PET/PN6, its viscoelastic
properties are drastically changed. PET/10%PN6/10%PPPMS behaved like a
completely elastic solid at all test frequencies and hence it exhibited excellent anti
dripping properties. Wang and Cai [18] synthesized triazole-containing DOPO
derivatives (DTA), as shown in Scheme 6.1, and used it as a co-curing FR agent for
epoxy resins. In the presence of DTA, melt dripping was completely inhibited and a
V-0 rating could be achieved in the UL-94 test. It was observed that 4 wt%
DTA-containing EP showed the highest LOI value, low average combustion time,
6.1 Relationship Between the Melting Dripping Characteristics …
71