Crossing the Traditional Boundaries: Salen-Based Schiff Bases for Thermal Protective Applications

Bio-Based Polyurethane-Urea with Self-Healing and Closed-Loop Recyclability Synthesized from Renewable Carbon Dioxide and Vanillin

Developing recyclable and self-healing non-isocyanate polyurethane (NIPU) from renewable resources to replace traditional petroleum-based polyurethane (PU) is crucial for advancing green chemistry and sustainable development. Herein, a series of innovative cross-linked Poly(hydroxyurethane-urea)s (PHUUs) were prepared using renewable carbon dioxide (CO2) and vanillin, which displayed excellent thermal stability properties and solvent resistance. These PHUUs were constructed through the introduction of reversible hydrogen and imine bonds into cross-linked polymer networks, resulting in the cross-linked PHUUs exhibiting thermoplastic-like reprocessability, self healing, and closed-loop recyclability. Notably, the results indicated that the VL-TTD*-50 with remarkable hot-pressed remolding efficiency (nearly 98.0%) and self-healing efficiency (exceeding 95.0%) of tensile strength at 60 °C. Furthermore, they can be degraded in the 1M HCl and THF (v:v = 2:8) solution at room temperature, followed by regeneration without altering their original chemical structure and mechanical properties. This study presents a novel strategy for preparing cross-linked PHUUs with self-healing and closed-loop recyclability from renewable resources as sustainable alternatives for traditional petroleum-based PUs.

Improving the flame retardancy of waterborne polyurethanes based on the synergistic effect of P-N flame retardants and a Schiff base

A novel reactive intumescent fire retardant hexa-[4-[(2-hydroxy-ethylimino)-methyl]-phenoxyl]-cyclotriphosphazene (HEPCP), containing both cyclotriphosphazene and Schiff base structures, is successfully prepared. The chemical structures of HEPCP and flame-retardant waterborne polyurethane (WPU) (FR-WPU) were characterized via³¹P, ¹H NMR and FT-IR. Thermogravimetric (TG) analysis showed that HEPCP exhibited excellent thermal stability and produced rich char residue under high temperature compared with the control sample. The Schiff base and cyclotriphosphazene had a synergistic effect on the WPU. Limiting oxygen index (LOI) values of up to 26.7% were recorded; the dripping behavior was simultaneously improved and achieved a V-1 rating in the UL-94 test by incorporating 0.5 wt% phosphorus. In contrast to the pure WPU, the peak heat release rate (pHRR) of the FR-WPU/HEPCP5 decreased by 43.8%. The char residues increased from 0.63% to 6.96%, and scanning electron microscopy (SEM) showed a relatively continuous and membranous substance, with few holes. The results of TGA-FIR, Py-GC/MS and SEM indicated that HEPCP displayed a fire-retardant mechanism in the condensed-phase. In addition, the thermomechanical behaviors and the mechanical properties indicated that both mechanical properties and T_gh increased.

A novel reactive intumescent fire retardant hexa-[4-[(2-hydroxy-ethylimino)-methyl]-phenoxyl]-cyclotriphosphazene (HEPCP), containing both cyclotriphosphazene and Schiff base structures, is successfully prepared.

Chain Extension and Synergistic Flame-Retardant Effect of Aromatic Schiff Base Diepoxide on Polyamide 6/Aluminum Diethylphosphinate Composites

A way to suppress the deterioration in mechanical properties of polyamide 6 (PA6) is required, especially with high loading of flame retardants in the matrix. In this study, a novel aromatic Schiff base diepoxide (DES) was synthesized. It exhibited an efficient chain extension effect on PA6 and a synergistic flame-retardant effect with aluminum diethylphosphinate (AlPi) for PA6. The PA6 composite with 16 wt.% AlPi only passed UL-94 V-0 rating at 1.6 mm thickness, while the combination of 1.5 wt.% DES with 13 wt.% AlPi induced PA6 to achieve a UL-94 V-0 rating at 0.8 mm thickness. The tensile, flexural, and Izod notched impact strengths were increased by 16.2%, 16.5%, and 24.9%, respectively, compared with those of V-0 flame-retarded PA6 composites with 16 wt.% AlPi. The flame-retarded mechanism of PA6/AlPi/DES was investigated by cone calorimetry and infrared characterization of the char residues and pyrolysis products. These results showed that DES had a synergistic effect with AlPi in condensed-phase flame retardation by promoting the production of aluminum phosphorus oxides and polyphosphates in the char residues.

Salen Complexes as Fire Protective Agents for Thermoplastic Polyurethane: Deep Electron Paramagnetic Resonance Spectroscopy Investigation

The contribution of copper complexes of salen-based Schiff bases N, N'-bis(salicylidene)ethylenediamine (C1), N, N'-bis(4-hydroxysalicylidene)ethylenediamine (C2), and N, N'-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.

Thermal Stability and Fire Properties of Salen and Metallosalens as Fire Retardants in Thermoplastic Polyurethane (TPU)

This study deals with the synthesis and evaluation of salen based derivatives as fire retardants in thermoplastic polyurethane. Salens, hydroxysalens and their first row transition metal complexes (salen-M) were synthesized (Copper, Manganese, Nickel and Zinc). They were then incorporated in thermoplastic polyurethane (TPU) with a loading as low as 10:1 weight ratio. The thermal stability as well as the fire properties of the formulations were evaluated. Thermogravimetric analysis (TGA) showed that different coordination metals on the salen could induce different decomposition pathways when mixed with TPU. The Pyrolysis Combustion Flow Calorimetry (PCFC) results showed that some M-salen have the ability to significantly decrease the peak heat release rate (−61% compared to neat TPU) and total heat released (−63% compared to neat TPU) when formulated at 10:1 wt % ratio in TPU. Mass Loss Cone Calorimetry (MLC) results have shown that some additives (salen-Cu and salen-Mn) exhibit very promising performance and they are good candidates as flame-retardants for TPU.

Preparation and characterization of chitosan derivatives and their application as flame retardants in thermoplastic polyurethane

Our previous work has demonstrated that chitosan (CS) can be directly used as charring agents. In this paper, efforts were made to further improve the thermal stability and charring ability of CS by reacting with benzaldehyde, salicylaldehyde and hydroxybenzaldehyde respectively to obtain different CS derivatives. CS derivatives in association with ammonium polyphosphate (APP) was then introduced into thermoplastic polyurethane (TPU) to prepare flame retardant TPU composites. The sample containing 18.75% APP and 6.25% salicylaldehyde modified CS (SCS) achieved the maximal limiting oxygen index of 29.5%, passed the V-0 rating and significantly decreased the peak heat release rate from 840.8kW/m² of neat TPU to 206.2kW/m². Thermogravimetric analysis and char morphology observation demonstrated that SCS/APP could promote the char formation to form more intact intumescent char structure. It was proposed that the synergism between CS derivatives and APP might be responsible for the improvement of flame retardancy.

Preparation of Schiff base decorated graphene oxide and its application in TPU with enhanced thermal stability

A facile in situ method of synthesis and immobilization of a copper(salen) complex onto graphene oxide (GO) support has been developed.