2 research outputs found
Crossing the Traditional Boundaries: Salen-Based Schiff Bases for Thermal Protective Applications
A broad
spectrum of applications of “Salen”-based Schiff bases
tagged them as versatile multifunctional materials. However, their
applicability is often bounded by a temperature threshold and, thus,
they have rarely been used for high temperature applications. Our
investigation of a classical Schiff base, <i>N</i>,<i>N</i>′-bis(4-hydroxysalicylidene)ethylenediamine (L2),
reveals that it displays an intriguingly combative response to an
elevated temperature/fire scenario. L2 resists and regulates thermal
degradation by forming an ablative surface, and acts as a thermal
shield. A polycondensation via covalent cross-linking, which forms
a hyperbranched cross-linked resin is found to constitute the origin
of the ablative surface. This is a unique example of a resin formation
produced with a Schiff base, that mimicks the operational strategy
of a high-heat resistant phenolic resin. Further applicability of
L2, as a flame retardant, was tested in an engineering polymer, polyamide-6.
It was found that it reinforces the polymer against fire risks by
the formation of an intumescent coating. This paves the way for a
new strategic avenue in safeguarding polymeric materials toward fire
risks. Further, this material represents a promising start for thermal
protective applications
Salen Complexes as Fire Protective Agents for Thermoplastic Polyurethane: Deep Electron Paramagnetic Resonance Spectroscopy Investigation
The
contribution of copper complexes of <i>salen</i>-based Schiff
bases <i>N</i>,<i>N</i>′-bis(salicylidene)ethylenediamine
(C1), <i>N</i>,<i>N</i>′-bis(4-hydroxysalicylidene)ethylenediamine
(C2), and <i>N</i>,<i>N</i>′-bis(5-hydroxysalicylidene)ethylenediamine
(C3) to the flame retardancy of thermoplastic polyurethane (TPU) is
investigated in the context of minimizing the inherent flammability
of TPU. Thermal and fire properties of TPU are evaluated. It is observed
that fire performances vary depending upon the substitution of the
salen framework. Cone calorimetry [mass loss calorimetry (MLC)] results
show that, in TPU at 10 wt % loading, C2 and C3 reduce the peak of
heat release rate by 46 and 50%, respectively. At high temperature,
these copper complexes undergo polycondensation leading to resorcinol-type
resin in the condensed phase and thus acting as intumescence reinforcing
agents. C3 in TPU is particularly interesting because it delays significantly
the time to ignition (MLC experiment). In addition, pyrolysis combustion
flow calorimetry shows reduction in the heat release rate curve, suggesting
its involvement in gas-phase action. Structural changes of copper
complexes and radical formation during thermal treatment as well as
their influence on fire retardancy of TPU in the condensed phase are
investigated by spectroscopic studies supported by microscopic and
powder diffraction studies. Electron paramagnetic resonance (EPR)
spectroscopy was fully used to follow the redox changes of Cu(II)
ions as well as radical formation of copper complexes/TPU formulations
in their degradation pathways. Pulsed EPR technique of hyperfine sublevel
correlation spectroscopy reveals evolution of the local surrounding
of copper and radicals with a strong contribution of nitrogen fragments
in the degradation products. Further, the spin state of radicals was
investigated by the two-dimensional technique of phase-inverted echo-amplitude
detected nutation experiment. Two different radicals were detected,
that is, one monocarbon radical and an oxygen biradical. Thus, the
EPR study permits to deeply investigate the mode of action of copper
salen complexes in TPU