Preparation of flame retardant and conductive epoxy resin composites by incorporating functionalized multi‐walled carbon nanotubes and graphite sheets

Construction of carbon-based flame retardant composite with reinforced and toughened property and its application in polylactic acid

To simultaneously improve the flame retardancy, strength and toughness of polylactic acid (PLA) fibers, a composite flame retardant CNTs-H-C was prepared with carbon nanotubes (CNTs) as the core, hexachlorocyclotriphosphazene as linker, and chitosan grafted on the surface. The prepared CNTs-H-C was introduced into a PLA matrix to obtain CNTs-H-C/PLA composites and fibers via a melt-blending method. The morphology, structure, flame retardant properties and mechanical properties were thoroughly characterized, and the flame retardant mechanism was studied. Results showed that the prepared CNTs-H-C displayed a nanotube-like morphology with good compatibility and dispersion in the PLA matrix. After blending with PLA, CNTs-H-C/PLA composites exhibited outstanding flame retardancy with limiting oxygen index (LOI) increasing from 20.0% to 27.3%, UL94 rating reaching V-0. More importantly, the introduction of CNTs-H-C did not affect the spinnability of PLA. Compared with pure PLA fibers, the LOI of CNTs-H-C/PLA fibers with a CNTs-H-C content of 1.0 wt% increased by 32.5%, and meanwhile the breaking strength and elongation increased by 28.2% and 30.4%, respectively. Mechanism study revealed that CNTs-H-C/PLA possessed a typical condensed phase flame retardancy mechanism. In short, we have developed a CNT-based composite flame retardant with reinforced and toughened properties for the PLA matrix. The prepared CNTs-H-C showed great potential in polymer flame retardancy and mechanical enhancement.

A CNT-based flame retardant was synthesized and introduced into PLA to simultaneously improve the flame retardancy, strength and toughness of PLA.

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

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 V₀ 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.

The flame retardant properties of the different types of cyclotriphosphazene carboxylate-intercalated LDH materials are emphasized by increasing the number of binding sites and decreasing the number of spiro groups in the cyclotriphosphazene core.