ABSTRACT
The phenomenon of high-temperature oxidation in magnesium alloys constitutes a significant obstacle to their application in the aerospace field. However, the incorporation of active elements such as alloys and rare earth elements into magnesium alloys alters the organization and properties of the oxide film, resulting in an enhancement of their antioxidation capabilities. This paper comprehensively reviews the impact of alloying elements, solubility, intermetallic compounds (second phase), and multiple rare earth elements on the antioxidation and flame-retardant effects of magnesium alloys. The research progress of flame-retardant magnesium alloys containing multiple rare earth elements is summarized from two aspects: the oxide film and the matrix structure. Additionally, the existing flame-retardancy models for magnesium alloys and the flame-retardant mechanisms of various flame-retardant elements are discussed. The results indicate that the oxidation of rare earth magnesium alloys is a complex process determined by internal properties such as the structure and properties of the oxide film, the type and amount of rare earth elements added, the proportion of multiple rare earth elements, synergistic element effects, as well as external properties like heat treatment, oxygen concentration, and partial pressure. Finally, some issues in the development of multi-rare earth magnesium alloys are raised and the potential directions for the future development of rare earth flame-retardant magnesium alloys are discussed. This paper aims to promote an understanding of the oxidation behavior of flame-retardant magnesium alloys and provide references for the development of rare earth flame-retardant magnesium alloys with excellent comprehensive performance.
ABSTRACT
A novel MoS2-DOPO hybrid has been successfully synthesized through the grafting of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) on the surface of MoS2 nanosheets using allyl mercaptan as an intermediate. MoS2-DOPO was used as a flame retardant additive to prepare flame-retardant flexible polyurethane foam (FPUF). The influence of MoS2-DOPO on the mechanical, thermal stability, and flame retardancy properties of FPUF composites were systematically investigated. The incorporation of MoS2-DOPO could not deteriorate greatly the tensile strength and 50% compression set of FPUF composites, but effectively improves the char residue. The cone calorimeter and smoke density tests results revealed that the peak heat release rate, total heat release, and the maximum smoke density of the MoS2-DOPO/FPUF composite were reduced by 41.3, 27.7, and 40.5%, respectively, compared with those of pure FPUF. Furthermore, the char residue after cone calorimeter tests and pyrolysis gaseous products of the MoS2-DOPO/FPUF composite were analyzed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and thermogravimetric analysis/infrared spectrometry. The results suggested that the MoS2-DOPO hybrid played a synergistic flame retardant effect of gas and condensed bi-phase action. In addition, a possible flame retardancy and smoke suppression mechanism of the MoS2-DOPO/FPUF composite were proposed. This study provides a facile and promising strategy for the fabrication of polymer materials with excellent flame retardancy and smoke suppression properties.
ABSTRACT
Phosphorus- and silicon-modified graphene oxide was prepared to improve the thermal stability and flame retardancy properties of epoxy resin. 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and vinyltriethoxysilane (VTES) were successfully grafted onto the surface of graphene oxide (GO) through solvothermal synthesis and hydrolysis-condensation reaction, respectively. Subsequently, the functionalized graphene oxide grafted by DOPO and VTES (DOPO-VTES-GO) was incorporated into the epoxy resin by the solution blending method. The effect of DOPO-VTES-GO on the thermal stability and flame-retardant properties of epoxy resin was systematically studied. Thermogravimetric analysis showed that the thermal stability and char residue yield of DOPO-VTES-GO/epoxy were increased obviously compared with those of pure epoxy resin and DOPO-GO/epoxy. Cone calorimeter test results showed that DOPO-VTES-GO/epoxy had better flame retardancy than pure epoxy resin and DOPO-GO/epoxy on reducing the peak of heat release rate, total heat release, and total smoke production. Furthermore, the char residue after the cone calorimeter tests was investigated by scanning electron microscopy-energy-dispersive X-ray spectrometry, Raman spectroscopy, and Fourier transform infrared measurements. These results demonstrated that the DOPO-VTES-GO can enhance the graphitization degree of char residues and promote the formation of the thermally stable char. In addition, the mechanism of flame retardancy was proposed, and DOPO-VTES-GO exerts the synergistic effect mainly by means of catalytic charring in the condensed phase and capturing hydroxyl or hydrogen radicals from thermal decomposition of epoxy resin in the gas phase. This work provides novel insights into the preparation of phosphorus-silicon-graphene oxide ternary synergistic flame retardants for thermosetting polymer materials.