Carbonaceous char

Activated carbons are made by first preparing a carbonaceous char with low surface area followed by controlled oxidation in air, carbon dioxide, or steam. The pore-size distributions of the resulting products are highly dependent on both the raw materials and the conditions used in their manufacture, as maybe seen in Figure 7 (42). 

Carbonaceous char barriers may be formed by the normal mode of polymer burning, and besides representing a reduction in the amount of material burned, the char may act as a fire barrier. The relationship of char yield, structure, and flame resistance was quantified by Van Krevelen (5) some years ago. For polymers with low char-forming tendencies, such as polyolefins, one approach to obtain adequate char is to add a char-forming additive. Such additives generally bear a resemblance to intumescent coating ingredients (6, 7). 

However, other types of barriers besides carbonaceous char have been shown to function in flame retardancy. In brief, these include the following ... 

Char Analysis. Analyses of char samples were performed on specimens prepared at 2CPC/minute and held at temperature for 30 minutes. Below 55CPC carbonaceous char is present. Above 55CPC in air and above 60CPC in nitrogen the residue consists of zinc, zinc oxide, glass and other inorganic species as shown in Table III. 

A wide range of intumescent epoxy coatings are available. These can be described as a mix of thermally reactive chemicals in a specific epoxy matrix formulated for fireproofing applications. Under fire conditions, they react to emit gases, which cool the surface while a low density carbonaceous char is formed. The char then serves as a thermal barrier. 

Figures 15.8 and 15.9 illustrate examples of how cone calorimeter data can be used in the development of flame-retarded materials. PA 66-GF without Pred showed typical fire behavior for noncharring polymers containing inorganic glass fiber as inert filler,69 when high external heat flux is applied. The shape of the HRR curve is divided in two different parts. In the beginning, the surface layer pyrolysis shows a sharp peak, followed by a reduced pyrolysis rate when the pyrolysis zone is covered by the glass fiber network residue layer. When Pred was added, the PA 66-GF samples were transformed into carbonaceous char-forming materials, which led to a...

Recently, new approaches on flame retardancy deal often with nanofillers and in this section some examples of improvements of fire behavior of polymeric foams obtained by use of nanoclays or nanofibers will be shown. Much more details on flame retardancy of polymeric nanocomposite may be found elsewhere as for example in the book edited by A. B. Morgan and C. A. Wilkie105 or in scientific review.106 Polymer nanocomposites have enhanced char formation and showed significant decrease of PHRR and peak of mass loss rate (PMLR). In most cases the carbonaceous char yield was limited to few weight %, due to the low level of clays addition, and consequently the total HRR was not affected significantly. Hence, for polymer nanocomposites alone, where no additional flame-retardant is used, once the nanocomposite ignites, it burns slowly but does not self-extinguish... 

The carbonaceous coke formed during plastics pyrolysis is automatically scraped off and accumulates in the bottom of the pyrolysis chamber where it is reduced by attrition to a free-flowing black powder. The internal agitator/scraper constantly removes the carbonaceous char by-product before it acts as a thermal insulator and lowers the heat transfer to the plastic. The char residue produced is about 2-3% of the output for relatively clean polyolefln feedstocks and up to 8-10% for PET-rich feedstocks. 

Nickel silicate and ferrous silicate are the preferred catalysts in the Smuda process. The Smuda catalyst is a layered silicate clay framework with ordered nickel (or iron) atoms inside. The catalyst is charged at 10 wt% ratio of the plastic feedstock. The catalysts are based on layered silicates with Lewis acid activity [24]. Catalytic cracking results in very little noncondensable gas (<1%) and minimal carbonaceous char. The hfe of the Smuda catalyst is approximately 1 month [24]. 

Phenol-formaldehyde resins are relatively resistant to heat. They start decomposing at about 250° C still maintaining some mechanical resistance, the decomposition rate increasing significantly around 300° C. In an inert atmosphere at 750° C, phenol-formaldehyde resins form more than 50% char [2, 3]. The volatile materials consist of xylene (76%). traces of phenol, cresol, and benzene [4]. The heating in air above 300° C leads to the oxidation of the carbonaceous char and complete volatilization of the polymer [5], More information regarding pyrolysis products of phenol-formaldehyde... 

Thermal analysis experiments have clearly shown that tin-based fire retardants markedly alter both the initial pyrolysis and the oxidative burn off stages that occur during polymer breakdown These changes have been interpreted as being indicative of an extensive condensed phase action for the tin additive, in which the thermal breakdown of the polymer is altered to give increased formation of a thermally stable carbonaceous char at the expense of volatile, flammable products. The consequent reduction in the amount of fuel supplied to the flame largely accounts for the beneficial smoke-suppressant properties associated with zinc stannates and other tin-based fire retardants. 

Glass-fiber incorporation increases the fire resistance of polyisocyanurate foam laminates. The glass fibers embedded in the foam prevent the development of deep fissures in the protective carbonaceous char that is formed when polyisocyanurate foam is exposed to high temperature flames. ... 

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