Charring agents

Liu, W., Chen, D.Q., Wang, Y.Z., Wang, D.Y., and Qu, M.H. 2007. Char-forming mechanism of a novel polymeric flame retardant with char agent. Polym. Deg. Stab. 92 1046-1052. 

Z.L. Ma, W.Y. Zhang, and X.Y. Liu, Using PA6 as a charring agent in intumescent polypropylene formulations based on carboxylated polypropylene compatibilizer and nano-montmorillonite synergistic agent, J. Appl. Polym. Sci., 2006, 101 739-746. 

Modesti, M. Lorenzetti, A. Flame retardancy of polyisocyanurate-polyurethane foams Use of different charring agents. Polym. Degrad. Stab. 2002, 78, 341-347. 

Three different types of halogen-free intumescent charring agents have been identified [17] (Table 7.1). All those compounds lead to the formation of a superficial char layer that prevents further decomposition but they act in three different ways ... 

The catalyst or add source can consist of ammonium phosphate or polyphosphate salts, phosphoric add-derived amides or alkyl or halo-alkyl phosphates. Charring agents are based on molecular structures that can form cross-linked networks such as pentaerythritol, sorbitol, melamine, and phenol-formaldehyde resins. Other polymeric systems capable of intumescence are some polyamides and polyurethanes. Blowing agents help form a porous structure in the char and can fadlitate its formation. Common blowing agents are based on urea and urea-formaldehyde resins, melamines, and polyamides that can liberate moisture. 

Although not strictly a filler, this additive deserves a brief mention. Its mode of action is predominately by the promotion of char formation, resulting from high acidity phosphoric acids produced on pyrolysis. The polyphosphate is used, as it is less water soluble and hygroscopic than the orthophosphate. APP is effective on its own in polymers with suitable chemistry for char formation to occur by this mechanism. In other polymers, it is used in conjunction with a charring agent, such as a melamine derivative [58]. 

The PLA/ramie hybrid alone presents no efficient charring to protect matrix, however, the addition of APP in PLA/ramie hybrid enhances the formation of coherence of carbonaceous charring layer as protective shield and thermal barrier (as shown in Fig. 4.37). The TGA test demonstrates the increase of char. When cooperating with APP, ramie fiber being rich in polyhydric compound acts as a charring agent to form an intumescent flame retardant system. This intumescent mechanism has been discussed in the literature. 

ATH and MH are used primarily in wire and cables in poly( vinyl chloride) (PVC), polyethylene, and various elastomers. There is also some limited application of MH in polyamide-6. To pass flame retardancy tests, 35 to 65 wt% of metal hydroxide is required. Decreasing the loading of metal hydroxides will result in a significant gain in physical properties, especially low-temperature flexibility therefore, combinations with red phosphorus, sUicones, boron compounds, nanoclays (treated montmorillonites), and charring agents have been explored. Surface treatment of metal hydroxides also helps to improve physical properties and sometimes improves flame retardancy, due to better dispersion. 

Dehydration or Chemical Theory. In the dehydration or chemical theory, catalytic dehydration of ceUulose occurs. The decomposition path of ceUulose is altered so that flammable tars and gases are reduced and the amount of char is increased ie, upon combustion, ceUulose produces mainly carbon and water, rather than carbon dioxide and water. Because of catalytic dehydration, most fire-resistant cottons decompose at lower temperatures than do untreated cottons, eg, flame-resistant cottons decompose at 275—325°C compared with about 375°C for untreated cotton. Phosphoric acid and sulfuric acid [8014-95-7] are good examples of dehydrating agents that can act as efficient flame retardants (15—17). 

Oxidizing Properties. Nitric acid is a powerful oxidizing agent (electron acceptor) that reacts violentiy with many organic materials (eg, turpentine, charcoal, and charred sawdust) (19,20). The concentrated acid may react explosively with ethanol (qv). Such oxidizing properties have had military appHcation nitric acid is used with certain organics, eg, furfuryl alcohol and aniline, as rocket propellant (see Explosives AND PROPELLANTS). 

Process development of the use of hydrogen as a radical quenching agent for the primary pyrolysis was conducted (37). This process was carried out in a fluidized-bed reactor at pressures from 3.7 to 6.9 MPa (540—1000 psi), and a temperature of 566°C. The pyrolysis reactor was designed to minimize vapor residence time in order to prevent cracking of coal volatiles, thus maximizing yield of tars. Average residence times for gas and soHds were quoted as 25 seconds and 5—10 rninutes. A typical yield stmcture for hydropyrolysis of a subbiturninous coal at 6.9 MPa (1000 psi) total pressure was char 38.4, oil... 

In chemical activation processes, the precursor is first treated with a chemical activation agent, often phosphoric acid, and then heated to a temperature of 450 -700 °C in an activation kiln. The char is then washed with water to remove the acid from the carbon. The filtrate is passed to a chemical recovery unit for recycling. The carbon is dried, and the product is often screened to obtain a specific particle size range. A diagram of a process for the chemical activation of a wood precursor is shown in Fig. 3. 

Fire Hazards - Flash Point Not pertinent this is a combustible solid Flammable Limits in Air (%) Not pertinent Fire Extinguishing Agents Water Fire Extinguishing Agents Not To Be Used Not pertinent Special Hazards of Combustion Products Irritating fumes may form in fire simations Behavior in Fire The product melts and chars Ignition Temperature Not pertinent Electrical Hazard Not pertinent Burning Rate Not pertinent. 

A somewhat different type of coupling is observed when salts of (i-keto esters, arylacetonitriles (ArCH2CN), and other compounds of the form ZCH2Z are treated with an oxidizing agent such as iodine," " or Cu(II) salts." Arylmethanesulfonyl chlorides (ArCH2S02Cl) couple to give ArCH=CHAr when treated with Et N." ... 

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