Analysis of the Combustion Kinetics and Thermal Behavior of an

for fire doors; glazing; ventilation dampers; plastic pipes; building joints; and, ..... consisting of the displacement of the weight-loss curves ...
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Ind. Eng. Chem. Res. 2002, 41, 2107-2114

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Analysis of the Combustion Kinetics and Thermal Behavior of an Intumescent System Carmen Branca and Colomba Di Blasi* Dipartimento di Ingegneria Chimica, Universita´ degli Studi di Napoli “Federico II” Piazzale V. Tecchio, 80125 Napoli, Italy

Heinrich Horacek Intumex Brandschutzprodukte GmbH, Buchnerplatz 1, A4021 Linz, Austria

The combustion kinetics of a laminate with intumescent behavior were determined by thermogravimetry for heating rates between 5 and 20 K/min and a final temperature of 873 K. The devolatilization stage consists of three consecutive first-order reactions that can be associated with the release of (1) water vapor (from aluminum trihydrate), (2) HCl (from polychloroprene), and (3) a gaseous mixture (from expansible graphite), followed by charring. The corresponding activation energies are 114, 140, and 83 kJ/mol. The second stage is the heterogeneous combustion of the active part of the intumescent char. This process is described by a one-step reaction whose rate presents a power-law dependence (n ) 1.98) on the solid conversion and an activation energy of 182 kJ/mol. The thermal response of a composite system that uses the intumescent material as a coating for thin steel slabs was also investigated. For external temperatures of 625-1025 K, the sequential chemico-physical changes undergone by the intumescent coating highly affect the dynamics of steel heating. Introduction Concern about the toxicological impact deriving from the use of halogenated flame retardants in polymeric materials has given rise to extensive research on alternative systems that include intumescent mechanisms.1-3 Two techniques are used for the application of intumescent materials to polymers,4 either as coatings (to the surface of a substrate) or as additives during the preparation of the polymer prior to its fabrication into a product. The main feature in the thermal response of these systems is the formation of a thick char layer (swelling factors usually reported are in the range of 3-30 times the initial thickness).2,3 This layer thermally insulates the substrate or the more internal part of the polymer, reduces the amounts of highly flammable volatile products, and hinders the diffusion of both flammable products toward the flame and oxygen toward the solid. The basic chemistry proposed for numerous intumescent systems always requires1,2 a swelling agent, a dehydrating agent (or a catalyst) and a char forming agent. The specific selection of the components, however, is not straightforward because, as pointed out by Horrocks,1 “ideal physical and chemical compatibility between all components is essential if dehydration to char and release of gas is to occur in a transitional semiliquid state sufficient to enable foaming and expansion to occur followed by full carbonization without char collapse”. Given the complex interaction between chemistry and physics, comprehensive transport models, such as that recently proposed in ref 5 for the intumescent system developed in ref 6, can be profitably applied for the optimization of the fire behavior of new materials. * Corresponding author. Tel.: 39-081-7682232. Fax: 39-0812391800. E-mail:[email protected].

The numerical simulation of transport models for solid combustion requires two sets of data.7 The first set (input parameters) includes physical properties and related submodels, the reaction mechanism, and kinetic constants. The second set of data (output parameters/ variables), needed for model validation, should provide measurements (for instance, the time history of temperature at selected locations) for conditions encountered in practical applications. Unfortunately, apart from the system proposed in ref 6 whose interest is purely academic, measurements concerning the thermal response of commercial systems are not available to provide input and output parameters/variables for transport models. In addition, apart from very few exceptions (for instance, refs 8 and 9), even though the formulation of new intumescent systems often includes thermogravimetry, kinetic constants are generally not estimated. This study discusses the combustion characteristics of an intumescent system of interest in practical applications concerning smoldering10 and ignition. In the first part, thermogravimetric curves are measured and used to formulate a multistep mechanism (devolatilization and char combustion) and to estimate the related kinetic constants. In the second part, the intumescent system is used as a coating for steel, and the thermal response to varying heating conditions is discussed. Materials and Methods The material under study is an intumescent laminate for fire prevention developed by Intumex Brandschutzprodukte GmbH (Linz, Austria). This class of products is commercialized for different applications, such as sealing material for fire doors; glazing; ventilation dampers; plastic pipes; building joints; and, more in general, steel protection. The specimen is a flexible sheet

10.1021/ie010841u CCC: $22.00 © 2002 American Chemical Society Published on Web 03/30/2002

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Figure 1. Schematic representation of the thermogravimetric system.

(2.15 mm thick) of initial density 1000 kg/m3. The components include aluminum trihydrate, polychloroprene, expansible graphite (or V-graphite)3 treated with a mixture of nitric and sulfuric acid, and inorganic fillers. Elemental analysis, on a mass basis, gives the following composition: C 54.5%, H 3.15%, N 1.54%, Cl 10% (oxygen and inorganics evaluated as the difference comprise 31%). Thermogravimetric tests were carried out first, and to check data reproducibility, each one was repeated three times. Prior to decomposition, the laminate was milled (particle sizes below 240 µm). The experimental system (Figure 1) has already been presented elsewhere,11-13 so only the main characteristics are discussed here. It consists of a furnace, a quartz reactor, a PID controller, a gas feeding system, an acquisition data set, and a precision balance. The furnace is a radiant chamber that creates a uniformly heated zone where a quartz reactor is located. The sample, a single-particle layer consisting of 6 mg, is exposed to thermal radiation by means of a stainless steel mesh screen, whose sides are wrapped on two stainless steel rods connected to a precision (0.1 mg) balance, which allows the weight of the sample to be continuously recorded. A gas flow (nominal velocity of 0.5 × 10-2 m/s for the tests discussed in this study) establishes the proper reaction environment and reduces the residence time of vapors inside the reactor. The radiative heat flux emitted by the furnace is preregistered imposing a certain heating rate of the sample holder (without sample). A chromel-alumel thermocouple (75-µm bead), positioned in direct physical contact with the sample, is then used to evaluate the temperature deviations with respect to the assigned heating rate. A second set of tests is made to investigate the behavior of the intumescent material under study when used as a coating for steel. The main characteristics of the experimental system and the sample can be seen in Figure 2. The sample (Figure 2A) consists of a substrate (steel of thickness 1 mm and area 7 mm × 20 mm) and intumescent coating (two slabs 17 mm × 25 mm in size). Perfect adherence between the two materials is accomplished through the use of an epoxy resin, and the wider surfaces of the coating avoid direct exposure of any part of the substrate to external heating. Decomposition/combustion of the sample takes place in a steel reactor (Figure 2B) (6.5 × 10-2 m internal diameter and 45 × 10-2 m length), where a forced air flow is distributed through a perforated plate at the bottom. A radiant furnace is used to preheat both the

Figure 2. Schematic representation of the laboratory system for the decomposition of intumescent coating/steel. (A) Sample. (B) Reactor: (1) purging gas, (2) heat carrier gas, (3) sample, (4) isolation valve, (5) gas heating, (6) reactor, (7) furnace, (8) controller, (9) sample holder, (10) acquisition data set, (11) demisters.

reactor and the gas, which is fed through a jacket (internal diameter 8.9 × 10-2 m) at the reactor top (2 × 10-3 m3/min at ambient conditions). Temperature profiles along the reactor axis are measured by seven thermocouples (chromel-alumel type, 500 µm diameter), with their tips exiting from a protective steel tube at chosen distances from the flow distributor. The lower reactor zone (about 15 × 10-2 m) is isothermal at a temperature determined by a proper selection of the furnace temperature (PID controller) that is indicated in the following as Tf. The sample is suspended in the uniform-temperature zone of the reactor (Figure 2B) once the desired thermal conditions are achieved. It is heated by the hot gas flow and, mainly, by reactor wall radiation. A thermocouple is inserted between the coating and the external surface of the steel at the median position. Because of the high thermal conductivity, spatial gradients along the steel thickness are negligible5,14 so that surface measurements are representative of the conditions experienced by the entire slab. The steel temperature is, however, a strong function of time that is highly affected by the temporal and spatial changes experienced by the coating. Results Weight Loss Characteristics. The measured weight loss characteristics are shown in Figure 3A and B by means of the dimensionless volatile mass fraction, Yv ) (Y - Y∞)/(1 - Y∞) (where Y∞ is the final solid mass fraction and Y is the current mass fraction), and the global rate of volatile formation, -dY/dt. The programmed temperature profile is also reported for comparison purposes. The sample holder is first slowly heated to 313 K and then (t ) 0) brought to 873 K at heating rates of 5 and 80 K/min. For both cases, degradation of the sample in air consists of a devolatilization stage of the virgin material and a combustion stage of the intumescent char. Devolatilization starts at temperatures of about 450 K, whereas combustion attains fast rates for temperatures of about 750 K. Both the devolatilization and combustions rate increases with increasing heating rate. Furthermore, the maximun

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Figure 3. Dimensionless volatile mass fraction, Yv ) (Y - Y∞)/(1 - Y∞); time derivative of the solid mass fraction, -dY/dt; and programmed temperature, T, as functions of time for nominal heating rates of (A) 5 and (B) 80 K/min and a final temperature of 873 K.

rates are displaced toward successively higher nominal temperatures. The slow heating rate (5 K/min) allows three zones to be seen in the first stage. In accordance with the information already available in the literature, these zones can be attributed mainly to single-component dynamics. This does not mean that component interactions, enhanced by heat- and mass-transfer effects,1,2,4,8 are negligible for kinetic control. As was done in a previous kinetic analysis5 and following component selection during the design of the material under study, the three reaction steps can also be associated with the peculiar processes of intumescence, that is, melting, blowing, and charring, thus providing a qualitative description of the process. However, the formulation of a multistep reaction mechanism and the estimation of the related kinetic constants do not require a knowlege of the composition/properties of the decomposition products. The first devolatilization peak (T ) 541 K) is due to water vapor released from aluminum trihydrate15 via 2Al(OH)3 f Al2O3 + 3H2O. This component is one of the most used flame retardants, and its action is recognized as mainly of a physical nature, reducing the formation of combustion products, diluting the gas phase, and producing Al2O3, which contributes to the insulation of the solid phase. In the presence of chlorine, aluminum trihydrate is also reported16 to reduce the flammability through the formation of AlCl3, which then participates as catalyst in the solid phase and as a source of halogen radicals in the gas phase. Moreover, it is likely that the release of water vapor is responsible for the beginning of the swelling process. Dehydrochlorination is well-documented for PVC, plastic mixtures from domestic waste, and other chlorinecontaining mixtures such as electronic scrap.17 For 1,2polymerized polychloroprene, it leads to the formation of polyacetylene and the release of HCl via (C4H5Cl)n f nHCl + (C4H4)n. Commercial polychloroprene is

usually 1,4-polymerized.18 Following ref 19, the decompositon of these polymers in the low-temperature range (443-673 K) results in the release of about 90% of the theoretical HCl and other compounds such as ethylene, chloroprene (trace), chloroprene dimers, and other modified chain fragments. Consequently, the second devolatilization peak (T ) 587 K) can be associated mainly with the release of HCl. For simplicity, in the following, it is still assumed that the remaining condensed-phase polymer is polyacetylene. Apart from the benefits associated with the release of a noncombustible gas, this step plays a fundamental role in the swelling of the molten material. The third step, corresponding to charring (solidification) of the swollen material, is also preceded by or associated with a further release of gaseous components (mainly water vapor, sulfur dioxide, and nitrogen dioxide) from expansible graphite.3 This additional peak (T ) 629 K) in the devolatilization rate might play a role in the swelling process, provided that the material is still in a semi-liquid phase, a condition closely related to the heating conditions. As the heating rate is increased, the three peaks in the devolatilization rate tend to merge. In particular, for the case of 80 K/min, the three steps of the decomposition occur at comparable times so that only one devolatilization peak appears. The last stage of the process corresponds to the partial combustion of the active part of intumescent char, which includes polyacetylene and graphite. Decomposition of polyacetylene is reported to begin at a temperature of 693 K,20 whereas the combustion rates of graphite become significant only for temperatures above 873 K.21 Hence, given the moderate thermal conditions examined here, it can be assumed that combustion is concerned essentially with the first component. For both cases examined in Figure 3A and B, the final solid yields are about 53% of the initial mass. An examination of the unburned residue reveals that swelling occurs to a greater extent in the case of 80 K/min (factor of about 3 with respect to the case of 5 K/min). The relatively high amount of solid residue might be due, first, to the mineral matter content of the material and to the unburned graphite. However, it can also be postulated that the intumescent behavior hinders oxygen diffusion toward the inner core of the particles, as long as temperatures are not exceedingly high and good structural properties are retained by the intumescent char. Finally, the temperature deviations between the sample and holder are negligible for a heating rate of 5 K/min, whereas, in the other case, the process is globally endothermic in the first stage (maximum deviation of up to 13 K) and exothermic in the second (maximum deviation of about 4 K). Kinetic Mechanism. In the evaluation of intrinsic chemical kinetics, special care is needed to avoid significant temperature deviations between the programmed heating rate and the actual heating conditions experienced by the sample.22 On the basis of thermocouple recording, it was observed that deviations are negligible only for heating rates below 20 K/min, given a final temperature of 873 K. Thus, a set of four thermogravimetric curves obtained for heating rates of 5, 10, 15, and 20 K/min was used (in all cases, for t ) 0, the temperature is 313 K). The final solid residue is always 53% (referred to the initial solid mass), and the

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weight loss dynamics showed the same qualitative characteristics as discussed for the heating rate of 5 K/min (Figure 3A). Hence, a global mechanism is proposed for the evolution of volatiles from the material as follows K1

A 98 RB + (1 - R)V1 K2

RB 98 βC + (R - β)V2 K3

βC 98 γD + (β - γ)V3 K4

γD 98 γV4

(b1) (b2) (b3) (b4)

24 and 25 to minimize the objective function, F. It is a direct method, belonging to the class of comparison methods, used to find the minimum of a scalar function of n independent variables.26 In contrast to gradient methods, direct methods do not require the derivatives of the scalar function. Different criteria can be used for selecting the orientation of the axis along which the optimum of F should be found. The approach used here combines the Rosenbrock formulas and the golden section method.26 The objective function, for a first-basis estimate, can be expressed in differential form as

F)

[

∑ fj i)1,N ∑ (dY )

j)1,M

It consists of three consecutive reactions, b1-b3, representing the three devolatilization steps, associated with the attainment of a molten phase, swelling, and charring and a fourth reaction, b4, describing combustion (essentially oxidative decomposition of polyacetylene). Moreover, A is the total volatile fraction; B, C, and D are intermediate components; V1-V4 are lumped species representative of gaseous products; and R, β, and γ are stoichiometric coefficients (all expressed as fractions of the dimensionless volatile fraction Yv). The rates of the devolatilization reactions are assumed to present the usual Arrhenius dependence on temperature (A preexponential factor and E activation energy) and to be proportional to the dimensionless volatile fraction Yv

Ri ) -KiYv, Ki ) Ai exp(-Ei/RT), i ) 1, 3 (1) It is worth noting that the mechanism based on successive reactions proposed here takes into account the main features of intumescence, as was done in the set of parallel reactions proposed in ref 6. On the other hand, the two assumptions (successive or parallel reactions) for the description of solid-fuel devolatilization have been shown to give roughly the same kinetic parameters.23 For combustion, the rate of solid disappearance is generally related to the partial pressure of oxygen through an empirical exponent and to the pore surface area available through the reaction volume.12 Given the relatively high air flow rate employed in the tests, it can be assumed that the oxygen mass fraction remains constant during the reaction process. Consequently, its contribution is incorporated in the preexponential factor. Also, a simple power-law (n) expression for the dimensionless volatile mass fraction (available for combustion) is applied to describe the evolution of the pore surface area during the process

R4 ) -K4Yvn, K4 ) A4 exp(-E4/RT)

(2)

The sample temperature, T, is assumed to be coincident with that of the holder, so it is a known function of time, t

T ) T0 + ht

(3)

where T0 (313 K) is the initial temperature and h is the heating rate. Kinetic Constants. The kinetic parameters are estimated through the numerical solution (implicit Euler method) of the mass conservation equations and the application of a method originally proposed in refs

]

(dYij)exp - (dYij)sim ij exp

+ (dYij)sim

2

(4)

where i represents the experimental (exp) or simulated (sim) time derivative (dY) of the solid fraction at time t; j is the heating rate (N is the number of experimental points, and M the number of experiments carried out for each sample by varying the heating rate); and the scale parameter, fj, is expressed27 as

fj )

1 maxj[(dYij)exp]2

(5)

The kinetic parameter values determined in this way are then used as a set of initial values for the final estimates based on the objective function expressed in integral form

F)

∑ ∑

j)1,M i)1,N

[

]

(Yij)exp - (Yij)sim

(Yij)exp + (Yij)sim

2

(6)

The parameters to be estimated are the activation energies, the preexponential factors, the stoichiometric coefficients, and the exponent n. The application of the parameter estimation procedure to multiple curves also allows the compensation effect28 to be avoided, that is, the possibility of different preexponential factor and activation energy combinations to describe reasonably well the same weight-loss curve. Indeed, only one set of data can predict the behavior of the material at several heating rates, consisting of the displacement of the weight-loss curves toward successively shorter times for successively more severe thermal conditions.27 Also, there is always the possibility that only a local minimum is found, instead of the absolute minimum. However, several combinations of kinetic parameters have been tested, and those chosen are always associated with the lowest value of the objective function, F. The minimization procedure based on the differential form of F presents the interesting feature of capturing all of the details of the experiments.23 However, because small experimental errors in the computation of the derivative can result in incorrect kinetic data,27 the integral form is also used for a further improvement in the kinetic constants. The results of the kinetic analysis are summarized in Table 1, and a comparison between predictions and measurements is given in Figure 4 for the solid mass fraction and in Figure 5A and B for the global devolatilization rate. The product distribution, with respect to the total mass, is listed in Table 2. Good agreement is shown between the measurements and the predictions. The preexponential factors, the activation ener-

Ind. Eng. Chem. Res., Vol. 41, No. 9, 2002 2111 Table 1. Activation Energies, Preexponential Factors, Reaction Orders, and Stoichiometric Coefficients for the Oxidative Decomposition of the Intumescent Laminate (Reactions b1-b3) and Combustion of Intumescent Char (Reaction b4) reaction

E (kJ/mol)

A (s-1)

n

114.1 140.1 83.0 182.2

5.0 × 1.55 × 1010 1.50 × 104 1.10 × 1010

1 1 1 1.980

b1 b2 b3 b4

108

h (K/min)

R

β

γ

5 10 15 20

0.767 0.745 0.741 0.734

0.662 0.640 0.599 0.575

0.400 0.406 0.417 0.430

Figure 4. Predicted (lines) and measured (symbols) solid mass fractions, Y, as functions of time for heating rates of 5, 10, 15, and 20 K/min and a final temperature of 873 K.

Figure 5. Predicted (lines) and measured (symbols) time derivatives of the mass fraction, -dY/dt, as functions of time for heating rates of (A) 5 and 15 and (B) 10 and 20 K/min and a final temperature of 873 K. Table 2. Yields of Volatiles (V1- V4) and Solid (C∞) Expressed as Percentages of the Initial Solid Mass, from the Thermogravimetric Tests Conducted for Heating Rates between 5 and 20 K/min and a Final Temperature of 873 K h (K/min)

V1

V2

V3

V4

C∞

5 10 15 20

11.0 12.0 12.2 12.5

4.9 5.0 6.7 7.5

12.4 11.0 8.6 6.9

18.9 19.2 19.7 20.3

53.0 53.0 53.0 53.0

gies, and the order of the combustion reaction are the same for all of the curves. In addition, the total volatile fractions evolved from the devolatilization and combustion stages are roughly the same, with values of 2728% and 19-20% (referred to the initial solid mass), respectively, for the range of heating rates considered. The first devolatilization step (b1) is quite rapid and

Figure 6. Time profiles of the time derivative of the steel temperature, dTs/dt (solid line), and the steel temperature, Ts (dashed line), for the composite system (Figure 2A) and a furnace temperature of Tf ) 725 K.

clearly visible, with volatiles released increasing only to about 14% (values from 11 to 12.5%). The other two devolatilization steps are barely visible, and the releases of HCl and other gases are slow and overlapping (even for the slowest heating rate examined). The kinetic model indicates that variations in the volatile yields between these two steps are large. In particular, the amount of volatiles generated in step b2 increases by about 50% (from about 5 to 7.5% of the initial solid mass for h between 5 and 20 K/min) mainly at the expense of the volatiles generated in the charring step b3 (which decrease from 12.4 to 7%). Therefore, the enhancement in swelling observed when the heating rate is increased can be attributed to (1) the higher rate of volatile release with consequent higher blowing velocities and (2) the displacement of the charring step toward higher temperatures, so that gas release in the third step can still contribute. Indeed, the characteristic reaction temperatures, as always observed in thermogravimetric analysis, become successively higher as the heating conditions are made more severe. In particular, for the experimental conditions examined here, the peak temperatures vary from 541 to 569 K (b1), from 587 to 617 K (b2), from 629 to 683 K (b3), and from 782 to 820 K (b4). The activation energies for the devolatilization reactions b1-b3 present the same qualitative features of the mechanism for the intumescent system studied in ref 6. That is, the central swelling stage is characterized by the highest value (140 kJ/mol vs 114 kJ/mol for aluminum trihydrate decomposition and 83 kJ/mol for the charring step), although, given the different nature and composition of the two systems, quantitative differences are high. Also, it is worth noting that the activation energy for HCl evolution estimated in this study is comparable to the values usually reported for the dehydrochlorination of plastics (136-143 kJ/mol17). Finally, the rate of the oxidative decomposition of the active part of the intumescent char, roughly second order in the mass fraction of volatiles generated, is described by a rather high value of the activation energy (182 kJ/mol), close to values reported for carbon combustion.29 Thermal Response under Heat Transfer Control. Experiments with the steel/coating sample were carried out in air for external temperatures, Tf, between 625 and 1025 K. Each test was repeated three times, showing good reproducibility. An example of the heating dynamics for the substrate is shown in Figure 6 for Tf ) 725 K, by means of profiles of the temperature, Ts, and the time derivative of the temperature, dTs/dt. This case is chosen to discuss the main features of the process because, on one side, this furnace temperature is

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sufficiently high to cause rapid devolatilization and combustion of the intumescent char. On the other hand, it is still low enough to permit a good resolution between the different reaction stages. Indeed, even though the spatial gradients along the coating are nonnegligible, the differences in the characteristic temperatures for the three devolatilization steps and for combustion are significant (see Figures 3 and 4), so that the moderate thermal conditions established in the reactor cause the sequential occurrence of the different processes. In accordance with previous experimental observations14 and detailed numerical simulations,5 different zones in the heating rate curve can be seen, associated with the occurrence of specific chemical/physical processes across the degrading coating. The first three zones of local minima in the heating rate can be associated with the three sequential steps of devolatilization. For very short times, when the coating has not yet attained temperatures high enough to initiate the decomposition process, the thickness of the sample is relatively small, and the thermal conductivity high, so that the substrate heating rate attains a maximum (about 4 K/s). However, within about 30 s, for Ts ) 387 K, a decrease and the attainment of a constant value are shown. It can be postulated that this is a consequence of the endothermicity of the decomposition reaction of aluminum trihydrate,15 with convective cooling caused by the release of hot vapors (reaction b1) and the beginning of the swelling process. The subsequent remarkable decrease in the heating rate and the attainment of a local minimum of about 1.8 K/s (for a time of about 55 s and Ts ) 470 K) are due to the second devolatilization step (b2), with both HCl vapor release, which causes convective cooling, and intense swelling of the coating, which hinders inward heat transfer owing to a reduction in the effective thermal conductivity. In addition, this process is endothermic. As these processes approach completion, the substrate temperature starts to increase again, and a new maximum in the heating rate (slightly above 3.5 K/s for a time of about 90 s) is attained. The third minimum in the heating rate might be associated with possible endothermicity of gas evolution (and further swelling) preceding or jointly occurring with the charring process (time of about 100 s and Ts of about 610 K) and, again, to convective cooling. Finally, because of the partial combustion of the intumescent char, a third maximum in the temperature rise is observed. As a consequence of reaction exothermicity, the temperature first attains values higher than those of the furnace and then slowly approach the furnace value. Finally, it should be noted that, as a consequence of the insulating effects of the coating, the steel temperatures corresponding to the three delays in the heating rate are lower than the characteristic reaction temperatures observed in thermogravimetric analysis. Although the process retains roughly the same basic characteristics as the external temperature is varied, important changes also take place. Heating dynamics for different Tf values are shown in Figures 7 and 8. As expected, the initial maximum in the substrate heating rate increases with the external temperature (values between about 2.5 and 9 K/s for the range of temperatures investigated). The effects of the melting step (associated with aluminum trihydrate decomposition) are clearly visible on the heating-rate curve only for Tf ) 625 and 725 K (corresponding to a Ts value of about

Figure 7. Time profiles of the time derivative of the steel temperature, dTs/dt, and the steel temperature, Ts, for the composite system (Figure 2A) and furnace temperatures of Tf ) 625 K (dashed lines) and Tf ) 825 K (solid lines).

Figure 8. Time profiles of the time derivative of the steel temperature, dTs/dt, and the steel temperature, Ts, for the composite system (Figure 2A) and furnace temperatures of Tf ) 925 K (dashed lines) and Tf ) 1025 K (solid lines).

480 K). For higher Tf values, the effects of melting and swelling are combined to give a clearly visible minimum in the heating rate, which occurs for Ts roughly between 460 and 478 K. Also, the duration of the delay in the temperature rise becomes progressively shorter as the external heating conditions are made more severe. The minimum in the heating rate associated with water vapor evolution from the charring process, occurring for Ts between 597 K (Tf ) 625 K) and 610 K (Tf ) 1025 K), becomes successively less important as the external temperature is increased. This can be explained by the successively more important role played by char combustion (completely absent for Tf ) 625 K). In particular, for Tf ) 1025 K, the combustion of char is responsible for the maximum heating rate attained by the substrate, which, for lower Tf, is observed during the initial transients when the decomposition rates are negligible. For all of the external temperatures studied, complete combustion of the intumescent char never occurs. Elemental analysis of the residue (Tf ) 725 K) gives the following composition: C ) 61%, H ) 1.7%, and N ) 1.7% (total oxygen and inorganics, by difference, comprises 35.6%). However, because of the configuration of the experimental system and the frangibility of char, it was not possible to carry out accurate measurements of the dependence of the conversion and swelling factors (roughly between 10 and 20) on the furnace temperature. Conclusions In this study, the combustion characteristics of a laminate with intumescent behavior were examined with the aim of producing a kinetic mechanism and basic information about its effectiveness when used as a coating for steel. The data provided are useful for the development and validation of transport models. De-

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tailed process simulations can be helpful for understanding the complex interactions between physical and chemical processes of intumesecent material combustion, thus leading to improvements in the design of fireresistant materials. Even though the chemico-physical details might be material-specific, the system investigated here exhibits the main features of intumescence and can be useful for such a purpose. Combustion under kinetic control occurs according two sequential stages: the first is the devolatilization of the virgin material, and the second is the partial combustion of the intumescent char. Oxidative degradation can be described by three consecutive reactions, corresponding to melting (with the beginning of the swelling process), swelling, and charring, processes that are always associated with intumescent behavior. Kinetic parameters were estimated numerically, through the use of thermogravimetric curves obtained for several heating rates, and led to good agreement between measurements and predictions in terms of both solid mass fraction and global devolatilization rate. The activation energies and preexponential factors are invariant with respect to the heating conditions (values are particular to the system under study and are affected by the dynamics of the main components, aluminum trihydrate, polychloroprene, and expansible graphite), whereas the amounts of volatiles evolved in each step are affected by the actual reaction temperature. As successively more severe thermal conditions are established, swelling is enhanced through two concomitant effects, that is, the increase in blowing velocity and the displacement of the charring process toward higher temperatures. Combustion of char is roughly a secondorder process in the mass fraction of evolved volatiles and occurs with an activation energy (182 kJ/mol) comparable to those reported for the combustion of carbon. The thermal history experienced by steel when coated with the intumescent material indicates that a delay in the heating rate is the most important feature. More specifically, for the moderate thermal conditions examined here (heating temperatures below 1025 K), each step in the decomposition process appears as a more or less visible minimum in the steel heating rate as a consequence of reaction endothermicity, convective cooling, and swelling. Only for the highest temperature, when intumescence plays the most effective role, are the effects of these processes simultaneous. The importance of the delay in substrate heating is well-known,5,14 especially when intumescent coatings are used to protect highly reactive systems. Indeed, it also causes a delay in unwanted reactions, thus increasing the escape time. Future activities, before a transport model can be applied for quantitative predictions of the fire performance of the intumescent material considered here, concern both the development of input parameters/ submodels and the collection of further data for experimental validation of the model. As for the first point, adequate submodels for the physical properties should be formulated, in particular, for the effective thermal conductivity to be applied as devolatilization (with swelling) and combustion take place. Also, the thermal response of the system steel/coating requires further investigation for (a) the influences of more severe heating conditions (fire level heat fluxes) and (b) the influence of gravity in view of aerospace applications.

Acknowledgment The research carried out at the University of Napoli “Federico II” was funded in part by the Italian Space Agency (ASI) under Contract I/R/080/01 (Flammability and Smolder of Insulating Materials in Microgravity). Thanks are also due to the anonymous reviewers for their useful comments on the manuscript. Literature Cited (1) Horrocks, A. R. Developments in flame retardants for heat and fire resistant textilessThe role of char formation and intumescence. Polym. Degrad. Stab. 1996, 54, 143. (2) Lewin, M. Physical and chemical mechanisms of flame retarding of polymers. In Fire Retardancy of PolymerssThe Use of Intumescence; Le Bras, M., Camino, G., Bourbigot, S., Delobel, R., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1998; pp 3-32. (3) Horacek, H.; Pieh, S. The importance of intumescent systems for fire protection of plastic materials. Polym. Int. 2000, 49, 1106. (4) Kandola, B. J.; Horrocks, S.; Horrocks, A. R. Evidence of interaction in flame-retardant fiber-intumescent combinations by thermal analytical techniques. Thermochim. Acta 1997, 294, 113. (5) Di Blasi, C.; Branca, C. A mathematical model for the nonsteady decomposition of intumescent coatings. AIChE J. 2001, 47, 2359. (6) Cagliostro, D. E.; Riccitello, S. R.; Clark, K. L.; Shimizu, A. B. Intumescent coating modelling. J. Fire Flammability 1975, 6, 205. (7) Di Blasi, C. The state of the art of transport models for charring solid degradation. Polym. Int. 2000, 49, 1133. (8) Le Bras, M.; Bourbigot, S.; Siat, C.; Delobel, R. Comprehensive study of protection of polymers by intumescences Application to ethylene vinyl acetate copolymer formulations. In Fire Retardancy of PolymerssThe Use of Intumescence; Le Bras, M., Camino, G., Bourbigot, S., Delobel, R., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1998; pp 266-279. (9) Neininger, S. M.; Kandola, B.; Hill, N. J.; Horrocks, R.; Staggs, J. E. J. Modelling the charring of an intumescent flame retardant, polymeric system. Presented at the 8th European Conference on Fire Retardant Polymers, Alessandria, Italy, June 2001. (10) Ohlemiller, T. J. Modeling smoldering combustion propagation. Prog. Energy Combust. Sci. 1985, 11, 277. (11) Lanzetta, M.; Di Blasi, C.; Buonanno, F. An experimental investigation of heat transfer limitations in the flash pyrolysis of cellulose. Ind. Eng. Chem. Res. 1997, 36, 542-552. (12) Di Blasi, C.; Buonanno, F.; Branca, C. Reactivities of some biomass chars in air. Carbon 1999, 37, 1227. (13) Di Blasi, C.; Branca, C. Kinetics of primary product formation from wood pyrolysis. Ind. Eng. Chem. Res. 2001, 40, 5547. (14) Anderson, C. E.; Wauters, D. K. A thermodynamic heat transfer model for intumescent systems. Int. J. Eng. Sci. 1984, 22, 881. (15) Troitzsch, J. International Plastics Flammability Handbook, 2nd ed.; Hanser Publishers: Munich, Germany, 1990. (16) Lyons, J. W. The Chemistry and Uses of Fire Retardants; Wiley-Interscience: New York, 1970. (17) Bockorn, H.; Hornung, A.; Hornung, U.; Jakobstroer, P.; Kraus, M. Dehydrochlorination of plastic mixtures. J. Anal. Appl. Pyrolysis 1999, 49, 97. (18) Ullmann’s Encyclopedia of Industrial Chemistry A 23; VCH: Weinheim, Germany, 1993; p 252. (19) Liggat, J. Products of Thermal Degradation of Polymers. In Polymer Handbook, 4th ed.; John Wiley & Sons: New York, 1999; Vol. II, pp 451-480. (20) Takeo, I.; Hideki, S.; Sakuji, I. Thermal cis-/trans-isomerization and decomposition of polyacetylene. J. Polym. Sci. 1975, 13, 1943. (21) Shafizadeh, F.; Sekiguchi, Y. Oxidation of chars during smoldering combustion of cellulosic materials. Combust. Flame 1984, 55, 171. (22) Antal, M. J.; Varhegyi, G. Cellulose pyrolysis kinetics: The current state of knowledge. Ind. Eng. Chem. Res. 1995, 34, 703.

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(23) Varhegyi, G.; Antal, M. J.; Szekely, T.; Szabo, P. Kinetics of the thermal decomposition of cellulose, hemicellulose, and sugar cane bagasse. Energy Fuels 1989, 3, 329. (24) Buzzi Ferraris, G. Metodo automatico per trovare l’ottimo di una funzionesNota I. Ing. Chim. Ital. 1968, 4, 171. (25) Buzzi Ferraris, G. Metodo automatico per trovare l’ottimo di una funzionesNota II. Ing. Chim. Ital. 1968, 4, 180. (26) Edgar, T. F.; Himmelblau, D. M. Optimization of Chemical Processes; McGraw-Hill: New York, 1988. (27) Conesa, J. A.; Marcilla, A.; Caballero, J. A.; Font, R.

Comments on the validity and utility of the different methods for kinetic analysis of thermogravimetric data. J. Anal. Appl. Pyrolysis 2001, 58-59, 617. (28) Chornet, E.; Roy, C. Compensation effect in the thermal decompostion of cellulosic materials. Thermochim. Acta 1980, 35, 389. (29) Hurt, R. H.; Calo, J. M. Semi-global intrinsic kinetics for char combustion modeling. Combust. Flame 2001, 125, 1138.

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