Metal Oxide/Hydroxides Materials in Inorganic Flame Retardant Coating Applications

Due to the flammability of polymer materials, loss of life and possessions caused by the fire hazards due to the use of polymeric materials has aroused much concern among government regulatory bodies, consumers, and manufacturers. Therefore, to meet multifarious applications, the use of flame retardants to reduce combustibility and suppress the smoke or toxic fume production from the polymers after ignition becomes important; the goal is to explore flame-retardant materials to reduce or to avoid the fire threats. Flame-retardant materials, in particular, flame-retardant polymer materials, have been increasingly required in many fields. The industries dealing with construction, electrical/electronic components, and transportation are the three of greatest importance. Among the metal hydroxides that can be used as flame retardants in polymer materials, the most important are magnesium hydroxide [Mg(OH)2] and aluminum hydroxide [Al(OH)3]. Due to their low toxicity, anti-corrosion properties, low cost, and low emission of smoke during processing and burning, MH has received comprehensive interest for flame retardant systems. Generally, the flame-retardant mechanism of MH is endothermic decomposition into the respective oxides and water when heated, and the released water vapor isolates the flame and dilutes the flammable gases in the gas phase. At the same time, however, it has some serious disadvantages such as relatively lower flame-retardant efficiency and thermal stability, and great deterioration in the physical/mechanical properties of the matrices. Employing Al(OH)3 and Mg(OH)2 powders, which had similar shape and particle size and distribution, as halogen-free flame retardants for ethylene vinyl acetate copolymer (EVA). The effects of the amount of two flame retardants on the mechanical and flame retardant properties of the composites were discussed. The effects of Al(OH)3 and Mg(OH)2 were similar in the improvement of flame-retardant properties as well as tensile strength, accompanied with a decrease in elongation at break. However, as shown by the cone calorimetric test, Al(OH)3 was superior to Mg(OH)2 in the shorter ignition time (88 s of the prior composite versus 120s of the latter one), lower rate of heat release (231.8 versus 271.1kW/m2 in PHRR and 131.4 versus 146.9 kW/m2 in the average HRR), and higher flame performance index (2.63 versus 2.26). At the same weight percentage loading of metal hydroxide, flame retardancy and mechanical properties are related to the particle size and distribution, which promote the developments of micro- or even nano-scaled metal hydroxides as flame retardant used in polymers. Research on Flame retardancy and mechanical properties of nano-scaled and micron-sized Mg(OH)2/ PP has be done and the result indicated that, compared with the micron-sized Mg(OH)2/PP composite, the nano-scaled Mg(OH)2 with proper proportion could enhance the intensity and toughness of the composite with better flame retardancy.However, the effect of particle size on the flame retardancy of micro-Mg(OH)2-filled EVA is not linear as expected. Evaluating the fire behavior by LOI testing, horizontal fire testing, and cone calorimeter, Huang et al. When Mg(OH)2 loading level changes from 35 to 70 wt% in EVA, the composites filled with nano-Mg(OH)2 do not always possess the best flame retardancy, and among the composites filled with micro- Mg(OH)2, the composites filled with 800 mesh Mg(OH)2 show the best flame retardancy; while, the composites filled with 1250 mesh present the worst. The differences were attributed to both particle size effect and distributive dispersion level of Mg(OH)2. The addition of MH improved flame retardancy of MH/polymer composites, but at the same time the mechanical properties of the composites were seriously deteriorated. Therefore, research has been focused on surface modification and encapsulation to improve the dispersibility and miscibility of MH in polymers matrices and improve the mechanical properties as well as flame retardancy. Aluminate, phthalate, titanate, and silane-based coupling agents are usually used for the surface-treatment of MH. Mg(OH)2 nanocrystallines Synthesized with needle- or lamella-like morphologies by a surfactant-mediated solution method, an prepared Mg(OH)2/EVA (1/1 in weight ratio) nanocomposite which has a LOI value as 38.3. Morphological investigation indicated that the Mg(OH)2 nanoparticles dispersed homogeneously in EVA matrix. a PP/loose nano-structured Mg(OH)2 (LN-MH) flame-retardant composite has been prepared. After the LN-MH was encapsulanted, mechanical properties of the encapsulanted specimen were close to those of the pure PP. When LN-MH was further surface-treated with a silane-based coupling agent, the mechanical properties as well as the flame retardancy improved significantly. Additionally, The methyl-blocked novolac (MBN) synthesized ether route and used it as a surfactant with Mg(OH)2 in the application of flame-retardant PA6. The MBN promoted char formation and effectively eliminated the melt drips of PA6 and it was used as an efficient lubricant and compatibilizer between MH and PA6. 50 wt% MH and 5 wt% MBN could give a UL-94 V-0 rating for PA6. An elastomeric polyacrylate latex (EPL) and/or the surfactant TX-10 phosphate (TX) has been used to modify the magnesium hydroxide (MH), giving MH-TX/EP composite particles, which were used in ABS to prepare ABS-based composites with a high loading (60 wt%) of modified MH. The resulting ABS/MH-TX/EP systems had good flame retardancy (LOI 30.0, V-0 rating in UL-94) and much higher impact strength than those filled with unmodified MH due to the good compatibility and boundary adhesion between ABS and MH-EPL/TX. A good flame-retardant effect can usually be obtained by the large addition of a single metal hydroxide; however, a better effect will be achieved by the combination of two or more metal hydroxides. The synergetic effect between flame retardants has also been widely researched. Al(OH)3 composite flame retardant coated with nano-Mg(OH)2 has been prepared. EVA products filled with this composite flame retardant had superior flame retardancy (LOI 1⁄4 39.0) and mechanical properties (tensile strength as 10.2 MPa, while the elongation at break was 180%) to those with uncoated Al(OH)3 and the physically mingled mixture of Al(OH)3/Mg(OH)2. Thermal analysis demonstrated that the composite flame retardant could increase the decomposition temperature and yield of char residues of the EVA products, inhibit the thermal cracking of polyethylene main chain effectively, and promote the charring of the polymers.

کلیدواژه ها:

Inorganic Flame Retardant ، Flammability ، Fire resistance ، Metal oxide/hydroxide