Halogen-free layered double hydroxide-cyclotriphosphazene carboxylate flame retardants: effects of cyclotriphosphazene di, tetra and hexacarboxylate intercalation on layered double hydroxides against the combustible epoxy resin coated on wood substrates

Abstract

The development of halogen-free flame retardants as environmentally friendly and renewable materials for heat and fire-resistant applications in the field of electronics is important to ensure safety measures. In this regard, we have proposed a simple and halogen-free strategy for the synthesis of flame retardant LDH-PN materials to decrease the fire hazards of epoxy resin (EP), via a co-precipitation reaction between Mg(NO₃)₂ and Al(NO₃)₃ and the subsequent incorporation of different cyclotriphosphazene (PN) carboxylate anions. The cyclotriphosphazene-based di, tetra and hexacarboxylate-intercalated layered double hydroxides are designated as LDH-PN-DC, LDH-PN-TC and LDH-PN-HC, respectively. Furthermore, the intercalation of cyclotriphosphazene carboxylate anions into the LDH layers was confirmed by PXRD, FT-IR, TGA, solid-state ³¹P NMR, nitrogen adsorption and desorption analysis (BET), HR-SEM and XPS. Evaluation of the flame retardant (vertical burning test and limiting oxygen index) properties was demonstrated by formulating the LDH-PN materials with epoxy resin (EP) in different ratios coated on wood substrates to achieve the desired behaviour of the EP/LDH-PN composites. Structure-property analysis reveals that EP/LDH-PN-TC-20 wt% and EP/LDH-PN-HC-20 wt% achieved a V ₀ rating in the UL-94 V test and achieved higher LOI values (27.7 vol% for EP/LDH-PN-TC-20 wt% and 29 vol% for EP/LDH-PN-HC-20 wt%) compared to the epoxy-coated wood substrate (23.2 vol%), whereas EP/LDH-PN-DC failed in the vertical burning test for various weight percentages of LDH-PN-DC from 5 wt% to 20 wt% in the composites, with a lower LOI value of 22.1 vol%. Excellent flame retardancy was observed for EP/LDH-PN-TC and EP/LDH-PN-HC due to the presence of more binding sites of carboxylate anions in the LDH layers and less or no spiro groups in cyclotriphosphazene compared to that in EP/LDH-PN-DC. In addition, the synergistic flame retardant effect of the combination of LDH and cyclotriphosphazene on the epoxy resin composites remains very effective in creating a non-volatile protective film on the surface of the wood substrate to shelter it from air, absorb the heat and increase the ignition time, which prevents the supply of oxygen during the combustion process. The results of this study show that the proposed strategy for designing flame-retardant properties represents the state-of-the-art, competent coating of inorganic materials for the protection and functionalization of wood substrates.

Scheme 1. Synthesis of cyclotriphosphazene carboxylate anion-intercalated LDH.

Fig. 1. Powder XRD patterns of (a) Mg–Al–NO₃-LDH, L₁ and LDH-PN-DC; (b) Mg–Al–NO₃-LDH, L₂ and LDH-PN-TC; (c) Mg–Al–NO₃-LDH, L₃ and LDH-PN-HC.

Fig. 2. (a) FT-IR spectra of the LDH-PN materials and (b) thermogravimetric analysis of the LDH-PN and Mg–Al–NO₃-LDH materials.

Fig. 3. Solid-state ³¹P NMR spectra of (a) LDH-PN-DC, (b) LDH-PN-TC and (c) LDH-PN-HC; (d) N₂ adsorption and desorption isotherms for LDH-PN-DC, LDH-PN-TC and LDH-PN-HC.

Fig. 4. (a) HR-SEM images of (i) Mg–Al–NO₃-LDH, (ii) LDH-PN-DC, (iii) LDH-PN-TC and (iv) LDH-PN-HC; (b) elemental mapping analysis of C, N, O, P, Mg, and Al in LDH-PN-DC.

Fig. 5. XPS survey spectrum; P 2p, N 1s, O 1s, C 1s, Mg 2p and Al 2p high-resolution spectra of LDH-PN-DC.

Fig. 6. Digital photos before (a, c, e and g) and after (b, d, f and h) the UL-94 vertical burning tests of the epoxy and EP/LDH-PN composite-coated wood substrates.

Fig. 7. Digital photos of the pure epoxy resin, EP/LDH-PN-DC-20 wt%, EP/LDH-PN-TC-20 wt%, and EP/LDH-PN-HC-20 wt% composite-coated wood substrates during the UL-94 V vertical burning process.

Scheme 2. Schematic illustration of the flaming of the EP/LDH-PN composite-coated wood substrate.