crystallinity is destroyed due to the formation of a viscous melt, which can be
processed by conventional thermoplastic techniques, such as extrusion and injection
molding, for conventional material preparation [127].
5.6 Interactions Between P,N-Based FRs and PUs for FR
Activity
Commonly, phosphorus compounds are considered to accelerate char formation in
hetero atom (oxygen and nitrogen)-containing polymers. This is because at high
temperatures during polymer burning, they can form thermally stable 3D char
networks with P–O and P–N compounds. This char network acts like a thermoset
material and protects the polymer substrate from the flame. The formation of a
stable char structure mainly depends on the interactions between the polymer and
P-based FR derivatives at high temperatures. In the case of polyurethanes
(PUs) containing –PO 4 functionalities in their structure, the formation of phosphoric
acid is accelerated, which further interacts with PU to yield isocyanate and alcohol,
as shown in Scheme 5.3. These isocyanates recombine with phosphoric acid to
form complex crosslinked structures, which can easily convert into azophosphonates at high temperatures. This chain propagation reaction continues to complete
degradation of PUs, which finally lead to the formation of crosslinked char, CO 2 ,
and NO 2 . Overall, the inclusion of phosphorus-nitrogen functionalities in the
polymer structure improves its FR activity to a great extent.
The introduction of FRs into thermoplastic PU (TPU) polymers through covalent
or non-covalent bonds can delay the degradation of the polymers and improve the
FR activity with increasing char residue and release of non-flammable gases.
Literature indicates that polymer nanocomposites (PNCs) exhibit high thermal
stability with low HRR values; they accelerate the formation of char residue but do
not self-extinguish. Further, they do not pass the UL-94 test. Therefore, for a
polymer to exhibit these properties; it is necessary to prepare PNCs with a combination of 2 or 3 FRs, either additive FRs or reactive FRs. Toldy et al. [129]
prepared a series of TPU composite polymers using a combination of various FRs
and comprehensively studied their FR properties (Table 5.1). The advantage of
using a combination of FRs in polymer composites is that they might exhibit a
synergetic effect in improving the properties of the polymer. TPU composites
containing different additive and reactive FRs displayed strong FR activity due to
the synergetic effect of the incorporated additives. An increase in the phosphorus
content in the TPU increases the LOI value, but the polymer specimens could not
pass the UL-94 test because most P-based FRs cannot prevent dripping by themselves. However, the combination of P-containing FRs in TPUs with other FRs
such as clays, boron derivative compounds, and hindered amines increased the LOI
values and the specimens could pass the UL-94 test without melt dripping.
58
5 Flame-Retardant Polyurethanes
processed by conventional thermoplastic techniques, such as extrusion and injection
molding, for conventional material preparation [127].
5.6 Interactions Between P,N-Based FRs and PUs for FR
Activity
Commonly, phosphorus compounds are considered to accelerate char formation in
hetero atom (oxygen and nitrogen)-containing polymers. This is because at high
temperatures during polymer burning, they can form thermally stable 3D char
networks with P–O and P–N compounds. This char network acts like a thermoset
material and protects the polymer substrate from the flame. The formation of a
stable char structure mainly depends on the interactions between the polymer and
P-based FR derivatives at high temperatures. In the case of polyurethanes
(PUs) containing –PO 4 functionalities in their structure, the formation of phosphoric
acid is accelerated, which further interacts with PU to yield isocyanate and alcohol,
as shown in Scheme 5.3. These isocyanates recombine with phosphoric acid to
form complex crosslinked structures, which can easily convert into azophosphonates at high temperatures. This chain propagation reaction continues to complete
degradation of PUs, which finally lead to the formation of crosslinked char, CO 2 ,
and NO 2 . Overall, the inclusion of phosphorus-nitrogen functionalities in the
polymer structure improves its FR activity to a great extent.
The introduction of FRs into thermoplastic PU (TPU) polymers through covalent
or non-covalent bonds can delay the degradation of the polymers and improve the
FR activity with increasing char residue and release of non-flammable gases.
Literature indicates that polymer nanocomposites (PNCs) exhibit high thermal
stability with low HRR values; they accelerate the formation of char residue but do
not self-extinguish. Further, they do not pass the UL-94 test. Therefore, for a
polymer to exhibit these properties; it is necessary to prepare PNCs with a combination of 2 or 3 FRs, either additive FRs or reactive FRs. Toldy et al. [129]
prepared a series of TPU composite polymers using a combination of various FRs
and comprehensively studied their FR properties (Table 5.1). The advantage of
using a combination of FRs in polymer composites is that they might exhibit a
synergetic effect in improving the properties of the polymer. TPU composites
containing different additive and reactive FRs displayed strong FR activity due to
the synergetic effect of the incorporated additives. An increase in the phosphorus
content in the TPU increases the LOI value, but the polymer specimens could not
pass the UL-94 test because most P-based FRs cannot prevent dripping by themselves. However, the combination of P-containing FRs in TPUs with other FRs
such as clays, boron derivative compounds, and hindered amines increased the LOI
values and the specimens could pass the UL-94 test without melt dripping.
58
5 Flame-Retardant Polyurethanes