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

Abstract

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, N,N'-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.

Phosphorus‐containing Salen‐Ni metal complexes enhancing the flame retardancy and smoke suppression of epoxy resin composites

“…Meanwhile, the strong absorption peak around 3400 cm −1 is attributed to OH stretching vibration, which is assigned to the presence of phenol. It is the key role to form graphitized carbon residue structure . Moreover, the bands at 1609 cm −1 are attributed to the vibration of CC/CN, and the characteristic absorption peaks of PO (1205 cm −1 ) and POC (1045 cm −1 ) could also be observed.…”

“…Further experiments showed that salen‐based Schiff bases displayed a polycondensation by covalent crosslinking. The reactions and curing processes of N , N ‐bis (4‐hydroxysalicylidene) ethylenediamine had close resemblance to that of phenolic/resorcinol resins, which are frequently used as starting materials for the production of chars . Naik et al .…”

“…Although the metal‐doped Salen structure exhibits good flame retardancy to the polymer material, it is not difficult to find that its flame‐retardant effect mainly plays a role in the condensed phase . As is well‐known that phosphorus‐based flame retardants have the significant flame‐retardant role both in gas and condensed phase .…”

“…The reactions and curing processes of N,Nbis (4-hydroxysalicylidene) ethylenediamine had close resemblance to that of phenolic/resorcinol resins, which are frequently used as starting materials for the production of chars. 8 Naik et al illustrated the degradation pathway of Salen-Cu/TPU composite material and found that the complexation of Salen-Cu increased the thermal stability and avoided unnecessary reactions of potential active sites to affect the flame-retardant effect. 9 Ramgobin et al investigated a serious of metal-based Salen bases (Salen-M, M = Ni, Cu, Zn.Mn) as FRs in TPU polymer.…”

“…Although the metal-doped Salen structure exhibits good flame retardancy to the polymer material, it is not difficult to find that its flame-retardant effect mainly plays a role in the condensed phase. 8 As is well-known that phosphorus-based flame retardants have the significant flame-retardant role both in gas and condensed phase. [12][13][14] For gas-phase flame-retardant, it is believed that the phosphorus-containing components and their derivatives can easily evolve into phosphorus-based radicals (e.g., PO 2 • and PO•), which will further quench the other free-radicals to realize the disruption of free-radical chain reactions in gas-phase.…”