Fire retardants silicate

The ceramic approach which appears the best to date because of versatility of composition as well as fire-retardancy is illustrated by the formulation which follows. In future modifications of the recipe, the sodium silicate will be replaced by an insoluble constituent, such as a silicone. 

Recent experiments to determine the dimension stabilizing efficiency of water soluble fire retardent chemicals (41) showed ammonium sulfamate to be superior to phosphate salts, giving anti shrink efficiencies of 51 to 66% compared to polyethylene glycol-1000 values of 63 to 77%. Sodium silicate, because of its alkalinity, caused collapse of the wood that resulted in negative anti shrink efficiencies. Strongly alkaline systems should hence be avoided. 

Talc is a natural magnesium silicate, which when compounded with plastics improves, among other properties, the flame retardancy. For example, talc combined with magnesium hydroxide or ATH improved the fire-retardance behavior of PP and EVA.62-63... 

Calcium metaborate is claimed to be advantageously incorporated into frits as fire retardants in fire doors particularly for those based on sodium silicate. It is believed that calcium borosilicate will be formed upon heating in these compositions.40... 

This chapter develops at first the more frequent combinations of nanoparticles that concern layered silicates associated with phosphorus compounds, as well as metallic hydroxides and halogen compounds. The association of natural layered silicates with intumescent FR (IFR) systems represents one of the main contributions of the combined use of nanoparticles and FRs. Moreover, combinations of layered silicates with other phosphorus compounds have been studied and have led to significant improvements for fire retardancy. 

Several micron-sized layered silicates, such as talcs, can improve the fire retarding behavior of EVA by partial substitution of metal hydroxides. Clerc et al.63 have shown that better fire performance was achieved using higher values of the lamellarity index and specific surface area for four different types of talcs in MH/EVA blends. Expanded mineral and charred layers were formed, similar to intumescent compositions with APP, proving the barrier effect on mass transfer, even at the micron scale for the mineral filler. 

B. Schartel, U. Knoll, A. Hartwig, and D. Ptttz, Phosphonium-modified layered silicate epoxy resins nanocomposites and their combinations with ATH and organo-phosphorus fire retardants, Polym. Adv. Technol., 2006, 17 281-293. 

Bartholmai, M. and Schartel, B. 2004. Layered silicate polymer nanocomposites New approach or illusion for fire retardancy Investigations of the potentials and the tasks using a model system. Polymers for Advanced Technologies 15 355-64. 

New trends involve the use of nanoparticles in synthetic fibers. Polymer-layered silicates, nanotubes, and POSS have been successfully introduced in a number of textile fibers, mainly poly-amide-6, polypropylene, and polyester. Although they reduce the flammability of these fibers, but on their own are not effective enough to confer flame retardancy to a specified level. However, in presence of small amounts of selected conventional FRs (5-10 wt %), synergistic effect can be achieved. With this approach fibers having multifunctional properties can also be obtained, e.g., water repellency or antistatic properties along with fire retardancy. Most of the work in this area at present is on the lab scale and there is a potential to take this forward to a commercial scale. 

Most of the chemicals used in fire-retardant formulations have a long history of use for this purpose, and most formulations are based on empirical investigations for best overall performance. These chemicals include the phosphates, some nitrogen compounds, some borates, silicates, and more recently, amino-resins. These compounds reduce the flame spread of wood but have diverse effects on strength, hygroscopicity, durability, machinability, toxicity, gluability, and paintability (J, 12, 13). 

Leach-Resistant Chemicals. Insoluble Complexes. Leach-resistant fire retardants can be formed by reacting soluble salts with metal salts to form insoluble, metallic salt complexes. Sodium silicate reacted with calcium chloride formed an insoluble, hydrated calcium silicate (95). Application of a 20% diammonium phosphate solution, followed by a 20% magnesium sulfate solution, has been proposed as a ready-to-use treatment for wood roofs (96). This combination forms an insoluble magnesium ammonium phosphate and is recommended for roofs that are 5 years old or older. Test results indicate that this treatment provides increased flame-spread protection. 

Other compounds such as aluminum trihydrate and silicate compounds have also been tried as fire retardants for wood. These compounds work best in combination with other chemicals, especially those in which the behavior is synergistic. 

The variety of substances used as additives in polymers is considerable. For example, the fillers may include china clay, various forms of calcium carbonate, talc, silicas (diatomaceous silica), silicates, carbon black, etc. The impact modifiers typically include other polymers. Plasticizers include certain polymers with low (oligomers), dialkyl phthalates, dialkyl sebacates, chlorinated paraffin waxes, liquid paraffinic fractions, oil extracts, etc. Heat stabilizers include heavy metals salts such as basic lead carbonate, basic lead sulfate, dibasic lead phosphite (also acting as a light stabilizer), dibasic lead phthalate, stearates, ricinoleates, palmitates and octanoates of cadmium and barium, epoxide resins and oils, amines, diphenylurea, 2-phenylindole, aminocrotonates. The antioxidants include tris-nonyl phenyl phosphite, 2,6-di-ferf-butyl-p-cresol (BHT), octadecyl-3,5-di-terf-butyl-4-hydroxyhydrocinnamate, etc. The UV stabilizers include modified benzophenones and benzotriazoles. Processing lubricants include calcium stearate, stearic acid, lead stearate, various wax derivatives, glyceryl esters and long-chain acids. Fire retardants include antimony oxide, some pyrophosphates, etc. 

Zanetti, M. Camino, G. Canavese, D. Morgan, A.B. Lamelas, F.J. Wilkie, C.A. Fire retardant halogen-antimony-clay synergism in polypropylene layered silicate nanocomposites. Chem. Mater. 2002, 14, 189-193. 

The concept of fire-retardancy is remarkably old. The Greek historian, Herodotus, in 484-431 BC recorded that the Egyptians imparted fire-resistance to wood by soaking it in a solution of alum (potassium aluminum sulfate) [Browne, 1958]. The Romans added vinegar to the alum for the same purpose. Vitruvius in the first century BC described the natural fire-retardant properties of the larch tree and some military applications of fire retardant materials such as plaster of clay reinforced with hair [Vitruvius, I960]. In 1638, Circa recorded that Italian theaters were painted with a mixture of clay and gypsum (potassium aluminum silicate and hydrated calcium sulfate) to protect them from fire. Wild was issued a British patent in 1735 for his process of treating wood with a mixture of alum, ferrous sulfate and borax (sodium tetraborate decahydrate). And Gay-Lussac in 1821 showed that a solution of ammonium phosphate, ammonium chloride and borax acts as a fire-retardant for wood. 

Cables are covered with a pasty composition. When exposed to fire, the composition forms a glass-like coating and does not ignite or firme. When heated by fire, the latex component of the composition chars, but the silicate component converts to a non-combustible, vitreous coating. The resultant fire-retardant jacket over the cable inhibits further burning and prevents re-ignition. The rigid non-per-meable siliceous jacket formed by the composition serves to prevent flow of molten cable material underneath it. 

Nanocomposites refer to the combination of nanosized fillers (10 m diameter) with polymers, rather than the combination of polymer matrix (filled with nanoparticles) and fiber reinforcement The most popular fillers used as fire retardants are layered silicates. Loading of 10% or less (by weight) of such fillers significantly reduces peak heat release rates and facilitates greater char production [7]. The char layer provides a shielding effect for the composites below and the creation of char also reduces the toxicity of the combustion products, as less carbon is available to form the CO and CO2. 

Connell and co-workers [87] investigated silicate siloxane fire retardant composites derived from vermiculite by reaction with hydroxyl-terminated polydimethylsiloxanes. Cone calorimetry was used to obtain HRR measurements. The results show that even at the highest irradiance levels the samples have very long ignition times. Ignition resistance deceases as the hydrocarbon content of the composite increases. The results from the cone calorimeter were obtained by Py-GC-MS, which show that small, volatile, silicone-containing molecules are formed during pyrolysis. 

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