Aromatic, char formation

Table IV. Effect of Inorganic Additives on Aromatic Char Formation...

Boronic acids (69 and 70) (Fig. 45) with more than one boronic acid functionality are known to form a polymer system on thermolysis through the elimination of water.93 Specifically, they form a boroxine (a boron ring system) glass that could lead to high char formation on burning. Tour and co-workers have reported the synthesis of several aromatic boronic acids and the preparation of their blends with acrylonitrile-butadiene-styrene (ABS) and polycarbonate (PC) resins. When the materials were tested for bum resistance using the UL-94 flame test, the bum times for the ABS samples were found to exceed 5 minutes, thereby showing unusual resistance to consumption by fire.94... 

SCT-SRC could be further processed at temperatures above that of dissolution to produce a clean solid fuel of reduced sulfur content. Char formation tendency would be lowered by prior removal of mineral matter and undissolved coal. At higher temperatures, desulfurization would proceed rapidly light gas formation might be minimized by keeping the time very short. Hydrogen consumption would be minimized because aromatic-hydroaromatic equilibria favor aromatics as temperatures increase. 

The principles needed to design a polymer of low flammability are reasonably well understood and have been systematized by Van Krevelen (5). A number of methods have been found for modifying the structure of an inherently flammable polymer to make it respond better to conventional flame retardant systems. For example, extensive work by Pearce et al. at Polytechnic (38, 39) has demonstrated that incorporation of certain ring systems such as phthalide or fluorenone structures into a polymer can greatly increase char and thus flame resistance. Pearce, et al. also showed that increased char formation from polystyrene could be achieved by the introduction of chloromethyl groups on the aromatic rings, along with the addition of antimony oxide or zinc oxide to provide a latent Friedel-Crafts catalyst. 

FACTOR Char Formation in Aromatic Engineering Polymers... 

Fuel, oxygen, and high temperature are essential for the combustion process. Thus, polyfluorocarbons, phosphazenes, and some composites are flame-resistant because they are not good fuels. Fillers such as alumina trihydrate (ATH) release water when heated and hence reduce the temperature of the combustion process. Compounds such as sodium carbonate, which releases carbon dioxide when heated, shield the reactants from oxygen. Char, formed in some combustion processes, also shields the reactants from a ready source of oxygen and retards the outward diffusion of volatile combustible products. Aromatic polymers, such as PS, tend to char and some phosphorus and boron compounds catalyze char formation aiding in controlling the combustion process. 

Char also shields the reactants from oxygen and in addition retards the outward diffusion of volatile combustible products. Aromatic polymers tend to char, and some phosphorus and boron compounds tend to catalyze char formation. 

The aromatic structure of polyimides, in particular, ensures that they are thermally resistant, and hence, characterized by high char formation on pyrolysis, low flammability (LOI > 30 vol%), and low smoke production. 

Phenolic resins have a low flammability by themselves due to the high aromatic content which leads to a high char formation on thermal degradation. However, end-capped brominated epoxy resins are used when necessary. Decabromodiphenyl ether in combination with antimony oxide is also used. 

Factor A. Char formation in aromatic engineering polymers. In Fire and Polymers, ACS Symposium Series 425. American Chemical Society Washington, 1990 chap. 19, pp. 274—287. 

Flame retardants may not only catalyze dehydration of the cellulose to more char and fewer volatiles but also enhance the condensation of the char to form cross-linked and thermally stable polycyclic aromatic structures (60). Cellulose was treated with various additives and then charred at 400 °C. The chars were then oxidized with permanganate see Chapter 13) and the results are in Table IV. The char yield was slightly higher for the sodium chloride-treated sample (17.5%) and substantially more for the sample containing diammonium phosphate (28.9%), as compared to the yield from the untreated sample (15.3%). Furthermore, the increased char formation was accompanied by increased aromaticity, as measured by the amount of the aromatic carbon obtained from the char and the amount obtained from the original cellulose molecules (60). 

Flame Retardance. The most important reason for phenolic foam being an excellent flame retarder is that the phenolic polymer is easily carbonized and the char part formed as a result is highly stabilized. This mechanism of char-formation is considered that of a multi-aromatic ring with chemically stabilized strong bond formed through a dehydrogenation reaction by heating and oxidation. 

Char formation is a common result of side chain reactions that maintain the C-C bonds and eliminate small molecules (such as H2O, CO2. etc.) or hydrogen. The formation of polyphenylene structures prior to char formation is common. Certain temperature resistant polymers that typically contain aromatic rings are more prone to the formation of char than other polymers. Pure graphite can be represented by an ideal formula as shown below ... 

A sequence of H-abstractions due to the very reactive chlorine radical and / -decomposition gives rise to the formation of polyene molecules and HC1 which is released from the melting phase. Cross-linking reactions between polyenes lead to alkyl aromatic intermediates which can further decompose, releasing tar compounds and/or can polymerize with char formation. These transformations are experimentally observed as a second weight loss in PYC thermo gravimetric analysis. 

Fig. 39. Predicted dynamic TGA of PVC with an heating rate of 10°C/min, experimental data (Marks) (Montaudo and Puglisi, 1991). Panel (a) Residue (%wt) behaviour and identification of the main thermal decomposition phases. Panel (b) Benzene, PAH and char formation profiles. The TAR fraction represents the total amount of volatile aromatics.

The polymers were stable up to 300°C in air and nitrogen but started losing weight at higher temperatures (Fig. 5). The anaerobic char yields of the polymers at 800°C were in the range of 55%-61% (Table V). Presence of fused aromatic rings in these polymers apparently did not influence the char formation tendency. The temperature of the maximum rate of decomposition was reduced in the presence of air, and an almost complete loss in weight occurred at about 600°C (Table V). 

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