in APP/PER-containing polymer systems using IR and solid-state nuclear magnetic
resonance (NMR) studies. It was found that instead of water formation, phosphoric
acids were formed. The polyethylene links between poly aromatic species were
rearranged, which enhances the mechanical strength of the intumescent coatings and
also provides strong barrier properties. Liu et al. [38] synthesized FR PP composites
with different percentages of PP/MP/PER and PP/MP/PER/TPU-containing polymers and they observed that the presence of TPU improved the FR activity and led to
high LOI values and V-0 rating in the UL-94 test. These observations can be
attributed to the following causes. (i) The charring agent PER reacts TPU during
melt-blending instead MP (PER) reacts with MP and (ii) prevention of the leaching
of PER from the nonpolar PP polymer matrix. Additionally, it has been noticed that
MPP works more effectively in the reactive mode rather than in the additive mode;
reactive-mode MPP-containing epoxy resins showed high LOI values and more char
yield [39]. Tang et al. [40] synthesized TPUs using different percentages of APP and
APP/DPER (dipentaerythritol). In the case of APP, they observed that with
increasing APP content the FR activity of the TPU improved but there was a marked
decrease in its thermal and mechanical properties owing to the catalytic action of
APP and the accelerated decomposition of TPU. However, when APP was combined
with DPER in the TPU, with increasing APP/DPER content, there was a significant
improvement in the FR properties and no adverse impact on the mechanical and
thermal properties. The important observation here is that at high temperatures,
esterification takes place between APP and DPER, resulting in an improvement in
the thermal, mechanical, and FR properties. Similarly, Lv et al. [41] synthesized a
series of PP polymers containing MP and MP/PER and MP/DPER and MP/TPER
and characterized them at the same FR loading. The FR activity of the polymers
increased with high LOI values and all of them, except PP/MP, achieved a V-0 rating
in the UL-94 test. These results clearly indicate that in the presence of PER, FR
activity increases, while THR, PHRR, and smoke production decrease. To improve
FR activity, APP modified with different chemical components was used to prepare
polymer composites; the results showed improved FR activity at high loadings, but
the mechanical properties were disturbed [42]. Therefore, it is necessary to modify
and develop efficient APP derivatives. The modified chemical functionalities should
be able to improve FR activity either by increasing the crosslinking density or
releasing non-flammable gases. Shao et al. [43] prepared a series of PP polymers
with piperazine-modified APP (PA-APP) and carried out flammability analysis; they
observed that PP/PA-APP (piperazine-modified ammonium polyphosphate) led to
superior FR activity compared to PP/APP composites. In the presence of PA-APP,
flame propagation in PP and the SPR were reduced by 86.2% and 78.2%, respectively. The same research group synthesized epoxy polymers with multifunctional
groups using organic and inorganic hybrid DETA (diethylenetriamine)-APP at different loading levels and evaluated the FR properties of the resultant polymers.
A large number of functional groups led to a greater crosslinking density, which
conferred strong barrier properties helpful in preventing the evolution of combustible
substrates and heat transfer into the flammable zone. The researchers also noticed
that DETA-APP is an efficient curing agent for epoxy polymers. With increasing
4.2 Nitrogen and Phosphorus-Based Salts as FRs
19
resonance (NMR) studies. It was found that instead of water formation, phosphoric
acids were formed. The polyethylene links between poly aromatic species were
rearranged, which enhances the mechanical strength of the intumescent coatings and
also provides strong barrier properties. Liu et al. [38] synthesized FR PP composites
with different percentages of PP/MP/PER and PP/MP/PER/TPU-containing polymers and they observed that the presence of TPU improved the FR activity and led to
high LOI values and V-0 rating in the UL-94 test. These observations can be
attributed to the following causes. (i) The charring agent PER reacts TPU during
melt-blending instead MP (PER) reacts with MP and (ii) prevention of the leaching
of PER from the nonpolar PP polymer matrix. Additionally, it has been noticed that
MPP works more effectively in the reactive mode rather than in the additive mode;
reactive-mode MPP-containing epoxy resins showed high LOI values and more char
yield [39]. Tang et al. [40] synthesized TPUs using different percentages of APP and
APP/DPER (dipentaerythritol). In the case of APP, they observed that with
increasing APP content the FR activity of the TPU improved but there was a marked
decrease in its thermal and mechanical properties owing to the catalytic action of
APP and the accelerated decomposition of TPU. However, when APP was combined
with DPER in the TPU, with increasing APP/DPER content, there was a significant
improvement in the FR properties and no adverse impact on the mechanical and
thermal properties. The important observation here is that at high temperatures,
esterification takes place between APP and DPER, resulting in an improvement in
the thermal, mechanical, and FR properties. Similarly, Lv et al. [41] synthesized a
series of PP polymers containing MP and MP/PER and MP/DPER and MP/TPER
and characterized them at the same FR loading. The FR activity of the polymers
increased with high LOI values and all of them, except PP/MP, achieved a V-0 rating
in the UL-94 test. These results clearly indicate that in the presence of PER, FR
activity increases, while THR, PHRR, and smoke production decrease. To improve
FR activity, APP modified with different chemical components was used to prepare
polymer composites; the results showed improved FR activity at high loadings, but
the mechanical properties were disturbed [42]. Therefore, it is necessary to modify
and develop efficient APP derivatives. The modified chemical functionalities should
be able to improve FR activity either by increasing the crosslinking density or
releasing non-flammable gases. Shao et al. [43] prepared a series of PP polymers
with piperazine-modified APP (PA-APP) and carried out flammability analysis; they
observed that PP/PA-APP (piperazine-modified ammonium polyphosphate) led to
superior FR activity compared to PP/APP composites. In the presence of PA-APP,
flame propagation in PP and the SPR were reduced by 86.2% and 78.2%, respectively. The same research group synthesized epoxy polymers with multifunctional
groups using organic and inorganic hybrid DETA (diethylenetriamine)-APP at different loading levels and evaluated the FR properties of the resultant polymers.
A large number of functional groups led to a greater crosslinking density, which
conferred strong barrier properties helpful in preventing the evolution of combustible
substrates and heat transfer into the flammable zone. The researchers also noticed
that DETA-APP is an efficient curing agent for epoxy polymers. With increasing
4.2 Nitrogen and Phosphorus-Based Salts as FRs
19