Char formation

An additive molar function for the char-forming tendency Cft 

The results of pyrolysis experiments with nearly 100 polymers enabled Van Krevelen (1975) to quantify the char-forming tendency, defined as the amount of char per structural unit divided by 12 (the atomic weight of carbon), i.e. the amount of C equivalents in the char per structural units of polymer. 

The use of the additive molar Cpx function is restricted to polymers exempt from halogen. Halogen atoms are in-built soot formers (and consequently in-built flame retardants in the presence of oxygen) engaged in secondary reactions, thus influencing the char formation markedly. 

We want to emphasise that the char-forming tendency is a statistical concept. The fact that the phenyl group has a Cpx value of 1 C equivalent means that on the average only one out of six phenyl groups in the polymer goes into the residue (five going into the tar and gas). 

The standard deviation from the experimental values for the polymers investigated is 3.5%. Fig. 21.4 shows the CR values experimentally found and calculated by means of Table 21.4. The agreement is satisfactory. 

Applications. The method is used to determine the effect of formulation variables on char formation and the stability of char to further pyrolysis. 

Testing procedure. A tube furnace is 1 m long and has a 7.2 cm diameter. At one end of the tube, a forced air flow is supplied. The furnace temperature is measured by a thermocouple located 30 cm Ifom the end of air supply. The temperature is constant within t2°C and is kept at 600 C. The sample is placed in a holder made Ifom a ceramic material with a tungsten foil lining. In the oxygen-Ifee mode, no air flows in the furnace, but a dry air is supplied at the rate of 1 nT/rnin in the oxygen- 

In a pioneering study, Shafizadeh and Sikiguchi [28] employed the permanganate oxidation method (common procedure for analysis of coal, lignin and other carbonaceous material) for the separation and analysis of the aromatic compounds as benzene carboxylic acids. Cellulosic chars were prepared over the temperature range of 

Treatment (°C) Char yield (wt.%) C(%) H(%) o(%) Empirical formula H/C ratio 

300-500 °C and subsequently analysed the extent of aromatisation was investigated as a function of HTT. This technique has been also applied to chars prepared in the presence of inorganic additives, thus assessing the effect of these latter on the development of aromaticity and the relationship between the structure and combustibility of char. The additives chosen were flaming inhibitors, which include both an inhibitor and an enhancer of smouldering combustion. The influence of these additives on cellulose degradation will be discussed more in detail in Section 14.3.5. 

By using IR, Shafizadeh and co-workers have identified phthalic (1,2-benzene dicar-boxylic acid), isophthalic (1, 3), hemimellitic (1,2, 3), trimellitic (1, 2,4), prehnitic (1, 2,3,4), mellophanic (1,2,3,5), pyromellitic (1,2,4,5), benzene penta and hexa carboxylic acids [32]. Hemimellitic and trimellitic acids were formed approximately at 2 3 ratio and prehnitic, mellophanic and pyromellitic acid approximately at 2 1 1 ratio. At the lower HTTs, in addition to the above compounds, lower amounts of monohydroxy benzene dicarboxylic acid, monohydroxy benzene tricarboxylic acid, and monohydroxy benzene tetracarboxylic acid have been detected, as well. 

An analogous study on Whatman filter paper has been carried out by Soares and co-workers by using FTIR and solid state NMR [33]. Once again, the use of the latter technique turned out to be very important for better clarifying the phenomena already 

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The industrial value of furfuryl alcohol is a consequence of its low viscosity, high reactivity, and the outstanding chemical, mechanical, and thermal properties of its polymers, corrosion resistance, nonburning, low smoke emission, and exceUent char formation. The reactivity profile of furfuryl alcohol and resins is such that final curing can take place at ambient temperature with strong acids or at elevated temperature with latent acids. Major markets for furfuryl alcohol resins include the production of cores and molds for casting metals, corrosion-resistant fiber-reinforced plastics (FRPs), binders for refractories and corrosion-resistant cements and mortars. 

Step 4 of the thermal treatment process (see Fig. 2) involves desorption, pyrolysis, and char formation. Much Hterature exists on the pyrolysis of coal (qv) and on different pyrolysis models for coal. These models are useful starting points for describing pyrolysis in kilns. For example, the devolatilization of coal is frequently modeled as competing chemical reactions (24). Another approach for modeling devolatilization uses a set of independent, first-order parallel reactions represented by a Gaussian distribution of activation energies (25). 

Phosphoms compounds are effective flame retardants for oxygenated synthetic polymers such as polyurethanes and polyesters. Aryl phosphates and chloroalkyl phosphates are commonly used, although other compounds such as phosphonates are also effective. The phosphoms compounds can promote char formation, thereby inhibiting further ignition and providing an efficient thermal insulation to the underlying polymer. 

One of the interesting properties of PBPCP [187] was its fast heat dissipation characteristics and so it was tested by the well-known oxy-acetylene panel test (ASTM 285-70) for ablative materials. Figure 13 shows the survival of a flower for 100 s. kept on the 6.35-mm asbestos fiber-reinforced hexamine-cured panel. The ablation rate value of this material was 3.2 x 10 in/s in comparison with 3.6 x 10 in/s for asbestos-phenolic. As the char content of PBPCP was only 27% compared with 60% for conventional phenolics, mechanisms involving transpiration processes rather than heat blocking by char formation might be playing a greater role in this case [188]. 

Cements, polyester, 30 CFCs. See Chlorofluorocarbons (CFCs) Chain conformation, 54 Chain extenders, 213-214 structure of, 219 Chain extension, 216 Chain-growth polymerizations, 4 Char formation, 421, 423 Chelated phosphine ligands, 488 Chemical recycling, 208 Chemical structure... 

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... 

The significance of the above-described work is that in all of the presently developing coal liquefaction processes, the initial step in the conversion is thermal fragmentation of the coal structure to produce very fragile molecules which are highly functional, low in solubility, and extremely reactive toward dehydrogenation and char formation. A more detailed discussion of the chemical nature of these initial products has been presented elsewhere (4). ... 

As discussed above, the composition of the solvent used in short contact time conversions can be important. The concentration of H-donors is one factor to be considered. It is known that in long contact time conversions, solvents having high H-donor contents have a better ability to prevent char formation as sulfur is removed from the SRC. Thus, higher yields of upgraded liquids are observed when solvents containing high concentrations of H-donors are used. 

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. 

Since char formation results from dehydrogenation and condensation, a reduction in conversion temperature (which is accessible only with liquid catalysts) will allow lower hydrogen pressures to be utilized without threat of char formation. 

We have discovered that ZnC, in combination with methanol, constitutes an active liquid-phase catalytic medium for conversion of coal to pyridine-soluble material. There are several possible explanations for this effect improved contact between coal and melt higher activity of the ZnCl2 in the methanol medium methylation of cleaved bonds resulting in reduced char formation and extraction of the reaction products leaving the coal more accessible. 

Melamine and melamine salts (e.g., borates, phosphates) act in a similar manner promoting char formation and intumescence. 

The symposium was planned as a state-of-the-art meeting, focusing on the basic science. Program areas included high heat polymers, fire performance of polymers, hazard modeling, mechanism of flammability and fire retardation, char formation, effects of surfaces on flammability, smoke assessment and formation mechanisms, and combustion product toxicity. 

An efficient flame retardant effect was demonstrated with 2-mil zinc coatings on polyphenylene oxide-polystyrene blends (Notyl) by Nelson (21). The action may relate to enhanced char formation by chemistry specific to this blend. However, other metal coatings on some other polymers also appeared to contribute a measurable flame retardant effect. 

One very efficient system which may work this way was reported by Chien and Kiang (32) who found that 1.5% chromium, introduced by the tard reaction, raised the oxygen index of polypropylene to 27 (Fig. 4) and char formation was promoted. The hypotheses as to mode of action included the idea that dehydrogenation catalysis might be involved. 

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. 

The initial step is an oxidative addition of RhCI(PPh3)3 to a C-0 bond of the ester moiety and produces rhodium-carbon and rhodium-oxygen bonds. Adjacent rhodium species can undergo further reaction with the formation of anhydride linkages. This anhydride formation may occur between adjacent pairs of reactants, between pairs in the same chain, or between pairs that are present in different chains. All of these reactions are observed, and in however the last reaction is the one of interest here since this leads to cross-linking and char formation. Rhodium is present in both the chary material and in the soluble fractions. From the reaction pathway in order for rhodium elimination to occur, two rhodium-inserted... 

It is unlikely that CIRh(PPh3)3 will ever be useful as a flame retardant due to its red color, expense, and the potential toxicity associated with a heavy metal. An additional disadvantage of the rhodium system is the fact that char formation occurs at a temperature of 250°C, since this is near the processing temperature of PMMA char formation may occur during processing rather than under fire conditions. This discovery is nonetheless... 

Char formation and reduced monomer production are observed for all of these additives upon reaction with PMMA. Char formation increases as a function of temperature, for the hydrido cobalt compound, there is 5% char at 262°, 8.5% at 322°, 15% at 338°, and 19% at 375°C the cobalt(lll) cyanide produces 3% char at 338° and 11% at 375°C the cobalt(ll) cyanide yields 11% char at 375°C. At the highest temperature, 375°C, the amount of monomer formation is 22% for K3Co(CN)5, 11% for K3Co(CN)6, and 10% for HCo[P(OPh)3]4. Ideally one would hope to observe no monomer formation and complete char production. Such is not the case here, these materials probably have no utility as flame retardant additives for PMMA since monomer formation, even at a reduced level, will still permit a propagation of the burning process. While somewhat positive results for these three additives do not prove the validity of the hypothesis, we take this to be a starting point in our search for suitable additives, further work is underway to refine the hypothesis and to identify other potential hydrogenation catalysts and other additives that may prove useful as flame retardants for PMMA... 

It is well known that char formation during polymer burning is an important mechanism by which polymers resist burning. For example Van Krevelen (1-2) has shown that a correlation exists between the oxygen index (OI) of a polymer containing no heteroelements and the amount of char it forms when pyrolyzed in the absence of air. As shown in Table I, a similar correlation is seen to hold for a number of common engineering thermoplastics. 

FACTOR Char Formation in Aromatic Engineering Polymers... 

C Solid State NMR. Earlier workers have used 13 C solid state NMR to monitor char formation during the pyrolysis of cellulose (18). They found the 13C NMR of cellulose heated at progressively higher temperatures shows the loss of aliphatic carbons and the appearance of small broad new resonance peaks at... 

An initial experiment involved determination of Arapahoe Smoke Chamber results for samples with and without the zinc coating present. Data are presented in Table II. Depending upon orientation of the sample, an increase in char occurred for some samples with zinc present, while no change in smoke formation was seen. Initial pyrolysis GC/mass spectroscopy results at 90CPC in helium showed no difference in volatiles formed with or without zinc. These results suggested enhanced char formation as the origin of the Radiant Panel results for zinc on modified-polyphenylene oxide (m-PPO). Zinc oxide is a known, effective thermal stabilizer in the alloy. The next work then focused on DSC/TGA studies. 

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