Pyrolysis Combustion Flow Calorimetry - Lyon 2004

J. Anal. Appl. Pyrolysis 71 (2004) 27–46

Pyrolysis combustion flow calorimetry

Richard E. Lyona,∗, Richard N. Waltersb

a Fire Safety Branch AAR-440, Federal Aviation Administration, William J. Hughes Technical Center,Atlantic City International Airport, Atlantic City, NJ 08405, USAb Galaxy Scientific Corporation, Egg Harbor Township, NJ, USA

Abstract

A method for evaluating the combustibility of milligram samples is described. Pyrolysis-combustionflow calorimetry (PCFC) separately reproduces the solid state and gas phase processes of flamingcombustion in a nonflaming test by controlled pyrolysis of the sample in an inert gas stream followedby high temperature oxidation of the volatile pyrolysis products. Oxygen consumption calorimetryis used to measure the heat of combustion of the pyrolysis products. The maximum amount of heatreleased per unit mass per degree of temperature (J g−1 K−1) is a material property that appears to bea good predictor of flammability.© 2003 Elsevier B.V. All rights reserved.

Keywords: Polymer; Fire; Flammability; Thermal analysis; Combustion; Calorimetry; Fire hazard; Heat releaserate; Oxygen consumption

1. Background

The rate at which heat is released by a burning material is the single most importantparameter determining its hazard in a fire, particularly in an enclosed space such as abuilding, a ship, or an aircraft cabin[1–3]. Several different bench scale methods have beendeveloped for measuring heat release rate during flaming combustion of materials, products,and components[4,5]. These bench scale fire calorimetry methods require replicate sampleson the order of a 100 g each and the results are highly dependent on the ignition source[6],sample thickness[7], sample orientation[8], ventilation[9], and edge conditions[8]. All ofthese factors combine to make the test data configuration dependent and obscure the effectof material properties and composition on burning behavior.

∗ Corresponding author. Tel.:+1-609-485-6076; fax:+1-609-485-6909.E-mail address: [email protected] (R.E. Lyon).

0165-2370/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0165-2370(03)00096-2

28 R.E. Lyon, R.N. Walters / J. Anal. Appl. Pyrolysis 71 (2004) 27–46

The desire for a quantitative analytical laboratory test that correlates fire behavior or flametest performance with material properties has been the motivation to relate thermogravi-metric analyses to flammability[10–13]. To date, most thermogravimetric investigationsof flammability have relied on a single thermal stability parameter (e.g. char yield or ther-mal decomposition temperature) to relate the chemical composition of a material to its fireor flame test performance (e.g. char yield vs. limiting oxygen index). Individually, thesethermal stability parameters have found limited success as material descriptors of flamma-bility and their interrelationship in the context of flaming combustion has remained obscureuntil recently, when it was shown that a particular combination of thermal stability andcombustion parameters could correlate fire behavior[14].

Laboratory methods that directly measure heat release rate of milligram-sized samplesof materials have been developed to study the effect of material properties and chemicalcomposition of materials on combustibility under controlled conditions. Susott[15–17]was the first to measure the heat of combustion of organic materials using transient heatingand oxygen consumption. In Susott’s method milligram-sized samples of forest products(foliage, wood, stems, and bark) were pyrolyzed in an inert gas stream at a constant rate oftemperature rise and the pyrolysis gases were combined with oxygen prior to entering a cat-alytic reactor. Pyrolysis was conducted under inert gas flow to prevent oxidation of the charresidue. The rate of oxygen consumption was determined from the electrolytic oxygen gen-eration rate using a null-balance, closed-loop technique. Only qualitative information wasobtained for the dynamics of the fuel generation process because the oxygen consumptionsignal was distorted by the instrument, and therefore, was not synchronized with the massloss history of the sample. Pyrolysis residue/char fraction was measured by weighing thesample before and after the test and the heat of combustion of the char was determined sepa-rately. Sample heating rates were limited by the dynamic capability of the oxygen generatorto less than 16 K min−1. The rate of heat released by complete oxidation of the pyroly-sis gases during thermal decomposition of the sample was calculated from the measuredoxygen consumption rate using an average gross heat of combustion for forest products or14.0± 0.5 kJ g−1 O2. The water produced in flaming combustion is in the gaseous stateso the relevant heat of combustion is the net value obtained by subtracting the heat of va-porization of water from the gross heat of combustion. Converting Susott’s gross heat ofcombustion to a net value gives 13.3± 0.5 kJ g−1 O2 which is equivalent to the currently ac-cepted value 13.1± 0.7 kJ g−1 O2 used in oxygen consumption fire calorimetry[5,18–20]as determined from data on a wide range of organic solids and polymers[21,22], as well asliquids and gases[23]. Susott did not measure mass loss during the pyrolysis-combustionexperiment but normalized the total heat of combustion of the volatiles to the starting samplemass on an ash-free, dry fuel basis. The gross heat of combustion of the char determined inseparate oxygen bomb calorimetry experiments[24] was found to be relatively independentof the fuel type at 32.0± 0.9 kJ g−1 for the 43 typical forest fuels tested.

Parker[25,26] claims to have improved on Sussot’s non-flaming dynamic combustionmethod by using a step-change in temperature to pyrolyze the sample at a heating rateconsidered to be more typical of the surface conditions in a fire. In Parker’s device samplesof cellulose are inserted into a preheated, nitrogen-purged furnace resulting in a rapid un-controlled temperature change and subsequent pyrolysis. An impinging stream of oxygenis mixed with the inert gas-pyrolyzate stream in a catalytic reactor. Combustion gas con-

R.E. Lyon, R.N. Walters / J. Anal. Appl. Pyrolysis 71 (2004) 27–46 29

centrations (O2, CO2, H2O) are measured and oxygen consumption is used to calculate theheat release rate of volatile fuel combustion. Although mass loss is not measured directlyduring the test, transient oxygen depletion and combustion gas generation were used tocalculate the mass loss rate of cellulose from its known chemical structure. Babrauskas andParker[27] later refined Parker’s device for the measurement of the fuel/oxygen ratio of firegases.

Reshetnikov et al.[28,29] used an experimental arrangement similar to Parker[25,26]in their study of the gas phase oxidation kinetics of polymer decomposition products. InReshetnikov’s technique milligram polymer samples are decomposed isothermally in aninert gas stream and excess oxygen is added to the volatile fuel stream at various temperaturesto effect oxidation. A gas chromatograph (GC) with flame ionization detector (FID) wasused to sample the fuel stream prior to mixing with oxygen in order to separate and identifythe individual products of decomposition. Gas phase oxidation kinetics of the fuel speciesare computed from the measured oxygen consumption history using isothermal methodsof analysis. Reshetnikov’s device measures the rate of combustion (oxidation) of the fuelgases but not the rate at which these gases are produced by the decomposing solid as it isheated, the latter being the rate limiting process in a fire.

Lasers have been used to pyrolyze polymers for flammability studies by Price[30] andAngel[31] and the pyrolysis gases analyzed by mass spectrometry and laser induced fluores-cence of hydroxyl radicals, respectively. In combination with thermochemical calculationsof the heat of combustion of the fuel species, laser pyrolysis methods can provide an estimateof the total heat released by the polymer.

Commercial thermogravimetric analyzers (TGA) have been used to thermally decom-pose milligram samples under controlled heating and environmental conditions for flamingand nonflaming combustion studies. Coupling the TGA to an evolved gas analysis (EGA)detector which is synchronized with the mass loss signal provides dynamic (rate) capabil-ity during the test and allows for interpretation of the transient evolved gas data in termsof the decomposition kinetics of the solid using established methods of nonisothermalanalysis[32–35]and thermal degradation models for charring and noncharring polymers[36].

Flaming combustion in a TGA was used by Gracik et al.[37,38] in combination withevolved gas (CO, CO2) analysis to study the flammability of fiber reinforced polymercomposites. In Gracik’s test a 50 mg sample is heated at a rate of 20 K min−1 in air ina commercial TGA until ignition occurs. Flaming combustion of the sample in the TGAproduces carbon dioxide (CO2) and some carbon monoxide (CO) which are measured andused to calculate the heat release rate[39]. An advantage of CO2/CO generation calorimetrycompared with oxygen consumption calorimetry as a measure of combustion heat is thehigher sensitivity and lower cost of the CO2/CO detector(s) but the method is more sensitiveto fuel type. Gracik’s method is essentially a scaled down version of the early Factory MutualResearch Corporation fire calorimeter[40] but uses a controlled heating rate instead of acontrolled heat flux to force gasification of the solid.

Nonflaming combustion of volatile fuel products generated in a TGA was used by Kifer[41] who studied fire retarded polymers and wood using a high temperature FID to burn thepyrolysis gases generated in a nitrogen-purged TGA. The FID signal is proportional to thetotal carbon in the pyrolysis gases, so that the integrated value, which was assumed to be


proportional to the total heat release, was related to the flammability of the material. Waltersand Lyon[42,43]and Inguilizian[44] determined the heat release rate of milligram samplesusing controlled heating in a TGA to thermally decompose the polymer and determinedthe heat release rate of the solid from the product of the maximum mass loss rate in theTGA and the net heat of combustion of the volatile pyrolysis products. Walters and Lyonused oxygen consumption calorimetry to measure the heat of combustion of the pyrolysisproducts while Inguilizian calculated the heat of combustion from the oxidation thermo-chemistry of the primary fuel species identified by gas chromatography-mass spectrometry(GC-MS).

The present method of pyrolysis-combustion flow calorimetry (PCFC) seeks to improveupon laboratory pyrolysis-combustion methods by providing dynamic capability for solidswithout the need to measure mass loss rate during the test. Because the PCFC combustibilitytest requires milligram samples rather than the kilogram samples of fire calorimetry, it ismicroscale by comparison to flaming heat release rate tests.

2. Principle of operation

PCFC measures the rate at which the heat of combustion of the fuel gases is released bya solid during controlled pyrolysis in an inert gas stream. The fuel gases are mixed withexcess oxygen and completely oxidized at high temperature and the instantaneous heat ofcombustion of the flowing gas stream is measured by oxygen consumption calorimetry.The rate at which combustion heat Q is liberated per unit time t in the PCFC or in a fire islimited by the rate at which the thermally decomposing solid releases fuel because the gasphase combustion reactions in the flame are rapid by comparison. Thus, the heat releaseratedQ/dt (W) is proportional to the mass generation rate (g s−1) of volatile fuel, i.e. themass loss rate of the solid:

Qc(t) ≡ dQc(t)

dt= h0

c,v(t)dmv(t)

dt= −h0

c,v(t)dms(t)

dt(1)

where a superscripted dot indicates the time derivative; h0c,v (J g−1), the enthalpy (heat) of

complete combustion of the volatile pyrolysis products of mass mv; and ms is the instanta-neous residual mass of solid. In a fire volatiles are generated at the surface of the materialover a range of temperatures and a distribution of molecular weights and atomic composi-tions are produced[45], so that in general h0c,v varies over the mass loss history and cannotbe treated as a constant inEquation 1. In many cases low molecular weight organic and in-organic (e.g. HCl, SOx) species are cleaved from the polymer backbone and released in theinitial stages of fuel generation followed by higher molecular weight organic compoundsin the intermediate and latter stages[45,46]. For materials that form a carbonaceous charduring the fuel generation process the instantaneous atomic composition of the volatiles willnecessarily differ from the atomic composition of the original material[17], with hydrogenbeing evolved in a secondary, high temperature decomposition step[13]. Consequently, aconstant heat of combustion for the volatile fuel that is equal to the heat of combustion ofthe solid cannot be assumed except for the few polymers that thermally decompose by de-polymerization exclusively to monomer (e.g. polymethylmethacrylate, polyoxy-methylene,


poly(�-methylstyrene) or to a single, known species. Thus, h0c,v(t) must be determined

continuously during the course of the fuel generation process to obtaindQc(t)/dt from themass loss rate. Continuous determination of h0

c,v during fuel generation is problematic andtime consuming so average[42–44]values of h0c,v have been used instead.

Thornton[23] made the experimental observation that the net heat of complete com-bustion of typical organic molecules per mol of oxygen consumed is relatively constant atC= 419± 19 kJ mol−1 O2 = 13.1± 0.6 kJ g−1 O2, and is essentially independent of thechemical composition of the combusted material. Sussot[15], Huggett[21], Babrauskas[22], and Walters[47] later confirmed this result for a wide range forest products, chemicalcompounds, and organic polymers, thereby establishing oxygen consumption as the pre-ferred method for determining the heat released in flaming combustion. Thus, to a goodapproximation (± 5%) the net heat of complete combustion of the volatile degradationproducts, regardless of chemical composition, is:

mv(t)h0c,v(t) = C�mO2(t) (2)

where�mO2(t) is the mass of oxygen consumed. Substituting the time derivative ofEquation2 into Equation 1:

dQ(t)

dt≡ Qc(t) = −h0

c,v(t)dms(t)

dt= C

d�mO2(t)

dt= C�mO2(t) (3)

shows that the instantaneous heat release rate of the solidQc(t) (W) resulting from completeand instantaneous combustion of the volatile decomposition products can be determinedfrom the product of the mass loss rate and heat of combustion of the fuel, or more simplyfrom the mass consumption rate of oxygen�mO2(t) = m0

O2− mO2. Equation 3shows that

the rate at which heat is released by combustion of the fuel gases during polymer thermaldegradation can be obtained by measuring the mass of oxygen consumed—and this resultis independent of the composition of the fuel. The total heat of combustion of the volatiles(J) after the pyrolysis process is complete and the rate of oxygen consumption returns tozero is simply the time integral ofEquation 3:

Qc =∫ ∞

0Qc(t)dt = C

∫ ∞

0�mO2(t)dt (4)

WhileEquation 4provides a means for relating the total oxygen consumed to the total heatreleased by combustion of the pyrolysis products, the heat release history of the polymer(Equation 3) requires that the oxygen consumption measured downstream be synchronizedwith the fuel generation (mass loss) history of the sample in the pyrolyzer. Unfortunately,the flow of combustion gases from the pyrolyzer (or burning surface) to the oxygen analyzerthrough PCFC cold traps and/or scrubbers distorts or smears the oxygen signal. Distortion ofthe oxygen consumption signal does not effect the total (time integrated) heat of combustion[15,25,28], but the heat release rate is no longer related to the fuel generation rate of thesample[37,42]. Heat release rates computed from the product of mass loss rate and heatof combustion (Equation 3) did not compare well with those obtained from the oxygenconsumption history in the PCFC unless the oxygen signal was deconvoluted to correctfor instrumental broadening. A simple mixing model for the pyrolysis-combustion flow


calorimeter (seeAppendix A) was confirmed by experiment and gives the heat release rateof the solid (W) in terms of the change in oxygen concentration� measured at the detector:

Qc(t) = C�F

(� + �

d�

dt

)(5)

where� and F are the gas stream density and volumetric flowrate, respectively, and� is theresponse time of the instrument. DividingEquation 5by the initial mass of the sample m0(g) gives the specific heat release rate in units of W g−1.

qc (W g−1) = Qc(t)

m0= C�F

m0

(� + �

d�

dt

)(6)

If the sample is pyrolyzed at a constant rate of temperature rise� (K s−1) and thermaldecomposition to inert or char fraction,� and/or volatile fuel occurs in a single step thenthe maximum specific heat release rate of the sample is related to the pyrolysis kineticparameters[48,49]:

�c (J g−1 K−1) ≡ qmaxc

�= C�F

�m0

(� + �

d�

dt

)∣∣∣∣max

= hoc,sEa

eRT2p

(7)

where Ea is the global activation energy for pyrolysis; Tp, the temperature of maximum massloss rate; e, R are the natural number and gas constant, respectively, and h0

c,s=(1−�)hoc,v is

the heat of combustion of the fuel gases per unit initial mass of solid. The derived expressionfor the maximum specific heat release rate normalized for heating rate on the right handside of Equation 7is a collection of thermochemical properties that has the units (andsignificance) of a heat release capacity�c and depends only on the composition of thematerial[14]. The following sections describe the construction, calibration, and operationof a pyrolysis combustion flow calorimeter that measures the heat release rate, total heat,and heat release capacity of polymers using only milligram samples.

3. Instrument construction

Fig. 1 is a schematic diagram showing how the component processes of flaming com-bustion are reproduced in pyrolysis-combustion flow calorimetry. The apparatus is basedon Susott’s original concept[15–17]of linear programmed heating of milligram samples inan inert (non-oxidizing) atmosphere to separate the solid state and gas phase processes offlaming combustion as normally occur in a fire. In particular, char formation in a fire occursat or below the burning solid surface in contact with a fuel rich/oxygen deficient gas layer[46]. In the present device the sample is heated using a linear temperature program and thevolatile thermal degradation products are swept from the pyrolysis chamber by an inert gasand combined with excess oxygen in a tubular furnace at flame temperatures to force com-plete non-flaming combustion (oxidation) of the fuel. Combustion products CO2, H2O, andacid gases are scrubbed from the gas stream and the transient heat release rate is calculatedfrom the measured flow rate and oxygen concentration after correcting for flow dispersionas perEquation 6. Time-integration of the PCFC heat release rate gives the heat of complete


Fig. 1. Schematic of flaming combustion and pyrolysis combustion flow calorimetry.

combustion/oxidation of the pyrolysis gases and the char yield is measured by weighingthe sample before and after the test. If the pyrolysis is conducted in air so that the organicchar is completely oxidized during the high temperature hold period, time-integration ofthe oxygen consumption signal gives the net heat of complete combustion of the solid aswould be determined in a high pressure oxygen bomb calorimeter[50].

3.1. Pyrolyzer

Initial studies using a commercial TGA to pyrolyze the polymer samples[42,43] wereunsuccessful due to condensation of the thermal decomposition products in the TGA celland in the heated transfer line. Smearing of the output signal (heat release rate) was alsoobserved because of dilution of the pyrolysis gases with nitrogen in the large mixing volumeof the TGA cell. Moreover, the maximum heating rate capability of the TGA (100–200 Kmin−1) is at the lower range of the surface heating rates in fires[48]. For these reasons atemperature-controlled pyrolysis chamber was designed that could be continuously purgedwith gas, coupled directly to the combustion furnace, and accept a commercial probe py-rolyzer (Pyroprobe 1000/2000, CDS Analytical) to thermally decompose the sample. Thisarrangement provided consistent temperature and minimum dead-volume with the probein place for the experiment as shown inFig. 2. The probe pyrolyzer body is 6.4 mm indiameter and contains a 3 mmdiameter, 25 mm long platinum resistance coil that heatsthe sample at a constant rate in the reported range� = 20×10− 3–20×103 K s−1. At thehighest heating rate the temperature history of the sample approximates a step change to apreset temperature and in this mode can be used to study the isothermal pyrolysis kineticsof liquids and solids by pulsed heating[51,52].

Mass transfer efficiency from the heated pyrolysis chamber to the combustor was studiedfor a few polymers (polyethylene, polyetheretherketone (PEEK), and KEVLAR) to deter-mine the minimum temperature necessary to maintain all of the pyrolysis products in thegaseous state entering the combustor. Mass transfer efficiency was calculated as the ratioof the time-integrated heat release rate (total heat of combustion) of the pyrolysis gases atcell temperature T, i.e. h0c,s(T) to the maximum value obtained in the experiments, i.e. masstransfer efficiency= h0

c,s(T)/h0c,s(max). The results of these studies are plotted inFig. 3as the


Fig. 2. PCFC pyrolyzer and pyrolysis chamber.

mass transfer efficiency versus pyrolysis chamber temperature. The data inFig. 3 indicatethat high molecular weight thermal degradation products are generated during pyrolysis thatvaporize at temperatures approaching the decomposition temperature of the polymer. Lossof these low volatility fuel products by condensation between the pyrolyzer and combustorreduces the peak heat release rate and total heat release unless the pyrolysis chamber temper-ature is held to within a few degrees of the onset (1%) weight loss temperature of the sample.

Mass flow controllers measure and regulate the flow of nitrogen, oxygen, or air, depend-ing on the experiment. To prevent condensation of high molecular weight decompositionproducts on the walls of the pyrolysis chamber it is held several degrees below the on-set decomposition temperature of the sample determined in a separate TGA experiment atmoderate (10–20 K min−1) heating rate.

3.2. Combustor

The combustor is a coiled, 5 m length of 6.35 mm OD Inconel tubing having a wallthickness of 0.89 mm, a coiled length of 24 cm, and an outer coil diameter of 5 cm asshown in cross-section inFig. 4. The gas stream from the pyrolyzer passes through the

Fig. 3. Effect of pyrolysis cell wall temperature on mass transfer efficiency of pyrolysis products for PEEK,KEVLAR, and polyethylene.


Fig. 4. Combustor cross-section and temperature distribution for 900◦C setpoint.

coiled combustion tube contained in a cylindrical ceramic furnace capable of maintaining amaximum temperature of 1200◦C. The ceramic furnace is surrounded with 5 cm of mineralwool insulation and the entire assembly is enclosed in a 3 mmthick cylindrical aluminumshell. The combustion tube length was selected to provide a residence time of approximately60 s at a volumetric flow rate of 100 cm3 min−1 in order to completely oxidize the pyrolyzatestream. Published studies of the oxidation of the products of flaming combustion indicatedthat a residence time of 60 s at 1000◦C was required to completely oxidize the largest sizesoot particles observed real fires[27]. However, gaseous pyrolysis products and fire gasesshould be completely (> 99%) oxidized in less than a few seconds at the nominal 900◦Ccombustor temperature, as deduced from high temperature oxidation kinetics of methane[53] and volatile polymer pyrolysis products[28].

The temperature distribution along the length of the Inconel tubing coil was measuredusing a shielded thermocouple probe positioned at several locations along the inside surfaceof the coil with nitrogen flowing through the coil at 100 cm3 min−1. These experimentswere repeated for various set point temperatures. A nearly symmetric parabolic temperaturedistribution about the coil midpoint location, x= 0, as shown inFig. 5for the nominal 900◦Cset point. Temperature distributions for set point temperatures from 500 to 1000◦C weresimilar.

3.3. Combustion gas scrubbing

The gas stream exiting the combustor contains nitrogen, combustion products, and resid-ual oxygen. This combustion gas stream enters two 6 mm diameter Teflon tubes, 20-cmlong, connected in series and tightly packed with anhydrous calcium sulfate (DrieriteTM)and sodium hydroxide coated silica (AscariteTM), respectively, to remove any H2O, acidgases, and CO2 from the sample stream. After cooling and scrubbing, the combustion gasstream contains only nitrogen and the residual oxygen which was not consumed in thecombustion reactions. Removing the combustion products from the gas stream ensures thatthey do not dilute the oxygen concentration and prolongs the life of the oxygen analyzer.The flowrate of the scrubbed combustion gas stream (≈100 cm3 min−1) is measured by amass flowmeter and continuously recorded by the data acquisition system.


Fig. 5. Heat release rate signal for square wave methane calibration pulses before and after signal deconvolution.

3.4. Data acquisition

The mass flow rate of pure oxygen entering the nitrogen-pyrolyzate stream is set bythe mass flow controller assuming ideal gas behavior. The mass flowrate of oxygen in thescrubbed sample stream after combustion of the volatile polymer degradation products isdetermined from the measured mass flow rate of the gas stream and the concentration ofoxygen measured by a high speed commercial oxygen analyzer. For the present work azirconia oxygen analyzer (Panametrics Series 350) was used having a 90% response timeof less than 1 s and an accuracy± 1% of full scale (0–20% O2, v/v). Temperatures ofthe pyrolysis probe, pyrolysis chamber, combustor, and gases are monitored continuouslyduring the test as well as gas and sample stream flow rates and oxygen concentration. Dataare acquired at 5 Hz on a personal computer during the experiment using a multichanneldata acquisition board and software (National Instruments). To determine the heat releaserate from oxygen consumption the mass flow rate, temperature, and oxygen concentrationof the gas stream before and after combustion of the pyrolysis products are measured.

4. Experimental

4.1. Materials

Polymer samples were unfilled, natural, or virgin-grade commercial resins obtained fromAldrich Chemical Company, Scientific Polymer Products, or directly from manufacturers.Methane, oxygen, and nitrogen gases used for calibration and testing were dry, >99.99%purity grades obtained from Matheson Gas Products, Welco, and Praxair.

4.2. Instrument calibration

A heating rate calibration of the pyrolysis probe was performed using a fine thermocouplein the quartz sample tube to measure the actual rate of temperature rise of the sample at


the nominal programmed rate. Heat release rate calibrations were performed by meteringmethane gas directly into the pyrolyzer in a continuous stream and measuring the oxygendepletion of the scrubbed gas stream after complete combustion. Steady-state oxygen de-pletion was in quantitative agreement with expected values using the net heat of combustionand flow rate of methane and the stoichiometric fuel/oxygen ratio. Square-wave fuel pulseswere generated using a syringe pump containing methane to test the dynamic response ofthe flow calorimeter to an instantaneously applied (� = ∞) heat release rate. A character-istic time� = 6 for the instrument was obtained from the dynamic response of the oxygendepletion signal to the step change in methane flow (heat release rate) as per Equation A6in theAppendix A. Equation 6with � = 6 s was then used to deconvolute the oxygen con-sumption history for the square wave methane heat release rate pulse with the results shownin Fig. 5. It is seen that the square wave pulse is reasonably reproduced by the deconvolutionprocedure with only a slight overshoot at the step changes because of signal noise.

4.3. Test procedure

Polymer samples were dried for at least 8 h at 80◦C in a convection oven and held ina dessicant chamber until testing. A dry sample weighing 1.0± 0.2 mg is placed into apre-weighed, thin-walled, quartz capillary tube which is 2.5 mm in diameter and 10–12mm long. The sample and tube are weighed on a microbalance to an accuracy of± 2 �gto determine the initial, mo, and final, m, sample mass after subtracting the weight of thequartz tube. The quartz tube containing the sample is inserted into the heating coil of thepyrolysis probe and the probe is inserted into the pyrolysis chamber and sealed. The py-rolysis chamber is equilibrated at a temperature a few degrees below the onset degradationtemperature of the sample as determined in separate TGA experiments at a heating rate inthe range of 10 K min−1.

A constant rate of temperature rise (ramp) is used to heat the sample from the initialpyrolyzer temperature to a maximum temperature Tmax where it is held for a period of time.The parameters relevant to polymer flammability are measured using anaerobic pyrolysisand a soak temperature that is well above the thermal decomposition range of the solid toforce complete thermal decomposition. Under these conditionsEquation 7applies and themaximum heat release rate normalized for heating rate has physical significance[48,49]as the capacity of a material to release its heat of combustion in a fire. Thus, the standardflammability test in the pyrolysis-combustion flow calorimeter involves heating the sampleat a constant rate of temperature rise (258 K min−1, typically) in nitrogen flowing at 82cm3 min−1 to a maximum temperature Tmax = 930◦C and holding the sample at Tmax forbetween 10 and 120 s to effect complete pyrolysis. The volatile pyrolysis products generatedduring the temperature ramp are swept from the pyrolyzer by the nitrogen purge gas and18 cm3 min−1 of pure oxygen is added to the pyrolyzate/nitrogen gas stream at the inlet tothe combustor. Combustion products carbon dioxide, water, and acid gases (if any) exitingthe combustor are removed by the scrubbers and the dry nitrogen and residual oxygen passthrough a flowmeter and oxygen analyzer. Deconvolution of the oxygen consumption signalis performed numerically during the test and the heat release rate, heat release capacity, andtotal heat of combustion are calculated and displayed. The quartz tube is weighed after thetest to determine the residual mass of the sample.


The total heat of combustion of the solid, as opposed to just the pyrolysis gases, can bemeasured by switching the purge gas from nitrogen to air after the temperature ramp, or byusing air as the purge gas during the ramp and hold cycles. Thus, selective thermoxidativedegradation of the solid or char can be accomplished and the oxygen consumption can bemeasured to determine the heat of complete combustion since any residual organic materialincluding carbonaceous char, will be completely oxidized in air at 930◦C. Experimentswere conducted in which dry air was the purge gas in the pyrolyzer during the heating cycleso that the sample was thermally degraded in the presence of oxygen and the remainingcarbonaceous char was oxidized during a 2 min hold period at 930◦C. The quartz tube isweighed after the test to determine the residual mass of the sample, if any.

5. Results and discussion

All results are the average of three to five replicated determinations for each polymersample. Data on polymers from different sources was averaged to obtain generic values ofthe combustion parameters. The repeatability of heat release rate measurements for a singleoperator is estimated to be± 3% and accuracy is± 7%, with the majority of the errorassociated with moisture pickup during the weighing and handling of small (1 mg) samplesand the noise in the deconvoluted oxygen consumption signal. Repeatability and accuracyof total heats of combustion obtained by time integration of the heat release rate is± 5%.

5.1. Heat release rate

Experiments were conducted in which the heat release rate of 1 mg samples of severalcommon polymers was measured over a range of nominal heating rates from 0.1 to 10 Ks−1. Fig. 6 shows the results of these experiments plotted as the maximum specific heatrelease rate versus heating rate. Below a nominal heating rate of about 5 K s−1 the maximumspecific heat release rate is proportional to the heating rate as perEquation 7with slope equal

Fig. 6. Specific heat release rate versus heating rate for 1 mg samples of several common polymers.


Fig. 7. Specific heat release rate data for several common polymers at a heating rate of 258 K min−1 (horizontallyshifted for clarity).

to the heat release capacity of the sample. Above a nominal heating rate of about 5 K s−1

the specific heat release rate is no longer proportional to the heating rate due to thermal lagof the sample and quartz tube. In practice a nominal heating rate of 5 K s−1 is used for theexperiment and the maximum heat release rate (c.f. peak height inFig. 3) is divided by theactual heating rate in the test (� = 4.3 K s−1) to obtain the heat release capacity.

Fig. 7 shows experimental heat release rate data for several polymers heated in nitro-gen at 258 K min−1 to a maximum temperature of 930◦C and held at the maximumtemperature for 10 s. Included in this data are polyethylene (PE), polypropylene (PP),polystryene (PS), acrylonitrile–buatdiene–styrene terpolymer (ABS), polymethylmethacry-late (PMMA), polyethyleneterephthalate (PET), polyetheretherketone (PEEK), and poly-benzimidazole (PBI). The maximum specific heat release rates of these commercialpolymers varies by more than a factor of 10 and the magnitude is in qualitative agree-ment with their ignition resistance[54].

To calibrate the PCFC and test the derived flammability relationship (Equation 7), heat re-lease capacities were measured in the PCFC at� = 4.3 K s−1 (258 K min−1) and comparedwith values obtained for the same samples by TGA-gas chromatography/mass spectrometry(TGA/GC-MS)[44,55]at a nominal heating rate of 10 K min−1. In the TGA/GC-MS theinstantaneous heat of combustion ho

c,v(t) at peak mass loss rate was computed from the ther-mochemistry of the major species identified by mass spectroscopy. The heats of combustionso calculated were multiplied by the instantaneous maximum fractional mass loss rate in theTGA to obtain the maximum specific heat release rates as perEquation 3. Dividing the maxi-mum heat release rate by the initial sample mass and heating rate (� = 10 K min−1) gives theheat release capacity by TGA-GC/MS for comparison to PCFC values at� = 258 K min−1.Results for the same samples tested by the two different test methods are listed inTable 1.The average weighted difference between the heat release capacities obtained by the twodifferent techniques (i.e. PCFC and TGA/GC-MS) is± 9% for the 14 polymers inTable 1.

Table 2is a partial listing of heat release capacities (�c), total heat release (h0c,s), and

char yield (�) of the hundreds of polymers tested in our laboratory by PCFC for which theChemical Abstract Service (CAS) registry numbers were available[56]. Data for generic


Table 1Comparison of heat release capacities by PCFC and TGA-GC/MS

Polymer Heat release capacity (J g−1 K−1)

PCFC (� = 258 K min−1) TGA/GC-MS (� = 10 K min−1)

Polyethyene (PE) 1600 1422Polyproplylene (PP) 1391 1338Polystyrene (PS) 1198 1302Poly(�-methyl)styrene (PMS) 730 695Polyoxyphenylene (POP) 553 635Polyhexamethyleneadipamide (PA66) 494 509Polyethyleneterephthalate (PET) 393 407Polycarbonate (PC) 390 470Polymethylmethacrylate (PMMA) 376 360Poly(p-aramide) (KEVLAR) 292 207Polyoxymethylene (POM) 261 233Polyetheretherketone (PEEK) 180 222Polyphenylenesulfide (PPS) 156 118Polyimide (PI) 29 31

polymers from different sources were averaged to obtain the values inTable 2with theexception of PMMA for which the range of values is given. Polymers are listed in descendingorder of heat release capacity�c which span the entire range measured to date for polymericsolids. Heat release capacities can vary by± 20% between polymers from different sources,e.g. compareTables 1 and 2and “polyimides” inTable 2. Heat release capacity variationsprobably reflect real differences in thermal stability, backbone chemical structure, molecularweight, chain defects, end groups, thermal processing history, and additive package betweenpolymers from different sources. Total heat release h0

c,s in Table 2is the heat of completecombustion of the pyrolysis gases per unit initial mass of polymer. Low total heat releaserelative to the heat of complete combustion measured by oxygen bomb calorimetry (seeTable 3) indicates that a fraction of the pyrolysis products were not oxidized in the PCFCtest (e.g. acid gases or char). The theoretical relationship between heat release capacity andflammability is an active area of research[48,49] that is being driven by empirical datashowing that low heat release capacity is a good predictor of ignition resistance and lowheat release rate in flaming combustion[56].

5.2. Heats of complete combustion

Fig. 8 shows heat release rate history for oxidative pyrolysis of the polycarbonate ofbisphenol-A (PC), a polymer that has a single heat release rate peak and forms 20–25% charwhen pyrolyzed under anaerobic conditions. The total oxygen consumed in this experimentis proportional to the net heat of complete combustion of the polymer as determined in anoxygen bomb calorimeter[24,50]. The net heat of complete combustion of polycarbonatecalculated from oxygen consumption is the time integrated heat release rate inFig. 8.Table 3shows that the net heats of complete combustion measured by PCFC are within 2%of literature values[22,47]on average for the ten polymers listed inTable 3.


Table 2Heat release capacity, total heat released, and char yield of selected polymers

Polymer (ISO abbreviation) CAS number HR capacity(J g−1 K−1)

Total HR(kJ g−1)

Char(%)

Polyethylene (LDPE) 9002-88-4 1676 41.6 0Polypropylene (PP) 25085-53-4 1571 41.4 0Polyisobutylene (PIB) 9003-27-1 1002 44.4 0Polystyrene (PS) 9003-53-6 927 38.8 0Isotactic polystyrene (PS-iso) 25086-18-4 880 39.9 0Polyhexamethylene sebacamide (PA610) 9008-66-6 878 35.7 0Poly-2-vinylnaphthalene (PVN) 28406-56-6 834 39 0Polyvinylbutyral (PVB) 63148-65-2 806 26.9 0.1Polylaurolactam (PA12) 25030-74-8 743 33.2 0Poly �-methylstyrene (PMS) 52014-31-7 730 35.5 0Polyhexamethylene dodecanediamide (PA612) 26098-55-5 707 30.8 0Poly(acrylonitrile-butadiene-styrene) (ABS) 9003-56-9 669 36.6 0Bisphenol-A epoxy/phenoxy-A (EP) 001675-54-3 657 26.0 3.9Polyethyleneoxide (PEOX) 25322-68-3 652 21.6 1.7Polyhexamethylene adipamide (PA66) 32131-17-2 615 27.4 0Polyvinylalcohol≥99% (PVOH) 9002-89-5 533 21.6 3.3Polycaprolactone (PCL) 24980-41-4 526 24.4 0Dicyclopentadienyl bisphenol cyanate ester 1355-71-0 493 20.1 27.1Polycaprolactam (PA6) 25038-54-4 487 28.7 0Polybutyleneterephthalate (PBT) 26062-94-2 474 20.3 1.5Polyethylmethacrylate (PEMA) 9003-42-3 470 26.4 0Polymethylmethacrylate (PMMA) 9011-14-7 376-514 23.2 0Polyepichlorohydrin 24969-06-0 443 13.4 4.8Poly-n-butylmethacrylate 9003-63-8 412 31.5 0Poly-2,6-dimethyl-1,4-phenyleneoxide (PPO) 25134-01-4 409 20.0 25.5Polyisobutylmethacrylate 9011-15-8 406 31.3 0Polyethylmethacrylate (PEMA) 9003-42-3 380 26.8 0Polycarbonate of bisphenol-A (PC) 24936-68-3 359 16.3 21.7Polysulfone of bisphenol-A (PSU) 25135-57-7 345 19.4 28.1Polyethyleneterephthalate (PET) 25038-59-9 332 15.3 5.1Bisphenol E cyanate ester 47073-92-7 316 14.7 41.9Polyvinylacetate (PVAC) 9003-20-7 313 19.2 1.2Polyvinylidenefluoride (PVDF) 24937-79-9 311 9.7 7Polyethylenenaphthylate (PEN) 24968-11-4 309 16.8 18.2Poly(p-phenyleneterephthalamide) (KEVLARTM) 308069-56-9 302 14.8 36.1Bisphenol A cyanate ester 1156-51-0 283 17.6 36.3Tetramethylbisphenol F cyanate ester 101657-77-6 280 17.4 35.4Poly(styrene-maleicanhydride) (SMAH) 9011-13-6 279 23.3 2.2Epoxy novolac/phenoxy-N (EP) 028064-14-4 246 18.9 15.9Polynorbornene 25038-76-0 240 21.3 6Bisphenol-M cyanate ester 127667-44-1 239 22.5 26.4Polychloroprene 9010-98-4 188 16.1 12.9Polyoxymethylene (POM) 9002-81-7 169 14 0Polyacrylic acid (PAA) 9003-01-4 165 12.5 6.1Poly-1,4-phenylenesulfide (PPS) 9016-75-5 165 17.1 41.6Liquid crystalline polyarylate (LCP) 70679-92-4 164 11.1 40.6Polyetheretherketone (PEEK) 29658-26-2 155 12.4 46.5Polyphenylsulphone (PPSU) 25839-81-0 153 11.3 38.4Polyvinylchloride (PVC) 9002-86-2 138 11.3 15.3


Table 2 (Continued )

Polymer (ISO abbreviation) CAS number HR capacity(J g−1 K−1)

Total HR(kJ g−1)

Char(%)

Polyetherketone (PEK) 27380-27-4 124 10.8 52.9Novolac cyanate ester 173452-35-2 122 9.9 51.9Polyetherimide (PEI) 61128-46-9 121 11.8 49.2Poly-1,4-phenyleneethersulfone (PESU) 25667-42-9 115 11.2 29.3Polyacrylamide 9003-05-8 104 13.3 8.3Polyetherketoneketone (PEKK) 74970-25-5 96 8.7 60.7Poly(m-phenylene isophthalamide) (NOMEX) 24938-60-1 52 11.7 48.4Poly-p-phenylenebenzobisoxazole (PBO) 852-36-8 42 5.4 69.5Polyimide (PI) 105030-42-0 38 6.7 57Polybenzimidazole (PBI) 25928-81-8 36 8.6 67.5Polytetrafluoroethylene (PTFE) 9002-84-0 35 3.7 0Polyamideimide (PAI) 42955-03-3 33 7.1 53.6Hexafluorobisphenol-A cyanate ester 32728-27-1 32 2.3 55.2Polyimide (PI) 26023-21-2 29 6.6 51.9Polyimide (PI) 79062-55-8 14 3.4 57Polyimide (PI) 87186-94-5 13 2.9 52

Table 3Net heats of combustion of selected polymers by PCFC and oxygen bomb calorimetry

Polymer Net heat of combustion (kJ g−1)

PCFC Oxygen bomb

Polyethylene 44.1 43.3Polystyrene 40.1 39.8Polyetheretherketone 30.9 30.2Phenolic triazine 29.5 29.8Polycarbonate 29.1 29.8Poly(p-aramide) 28.1 27.8Polybutyleneterephthalate 26.3 26.7Polymethylmethacrylate 25.0 24.9Polyethyleneterephthalate 23.2 21.8Polyoxymethlene 15.0 15.9

Fig. 8. Heat release rate for oxidative pyrolysis-combustion of polycarbonate.


6. Conclusions

Pyrolysis-combustion flow calorimetry is a viable method for determining combustionparameters of materials when only milligram samples are available for testing. Measuredquantities include the specific heat release rate under simulated fire conditions, the heat ofcombustion of the fuel gases, the heat of combustion of the solid, and a derived quantitycalled the heat release capacity. While PCFC is a quantitative technique, the generic polymerheat release capacities inTable 2are qualitative because of real differences in polymersamples from different sources. Polymer heat release capacities (Table 2) span two ordersof magnitude and ongoing work in our laboratory suggests that the heat release capacity isa reliable indicator of fire hazard.

Acknowledgements

The authors are indebted to Taline Inguilizian, Huiqing Zhang, and Stanisalv Stoliarov ofthe University of Massachusetts for valuable contributions in testing and analysis. Certaincommercial equipment, instruments, materials and companies are identified in this paper inorder to adequately specify the experimental procedure. This in no way implies endorsementor recommendation by the Federal Aviation Administration.

Appendix A

In the pyrolysis-combustion flow calorimeter (Fig. 1) the oxygen concentration is mea-sured several meters downstream from the point at which the pyrolysis gases are generated,mixed with excess oxygen in the heated manifold, and enter the combustor. As the oxygendepletion profile moves through the instrument mixing and diffusion of the flow stream inthe combustion tube and scrubbers tends to broaden and attenuate the peak. These effectsmust be corrected to synchronize the fuel generation history of the sample with the oxygenconsumption history measured at the downstream detector. If the oxygen signal distortionis assumed to be the result of perfect mixing in an idealized dead volume V0, which maybe distributed over the flow path, the mass of oxygen in the mixing volume at any timet is:

mO2 = �V0[O2] = �V0[O2]out (A1)

where� is the gas density and [O2] is the instantaneous volume fraction (concentration) ofoxygen in the gas stream which, if V0 is well mixed, is equal to the oxygen concentrationleaving V0 and entering the detector, so [O2] = [O2]out. For a constant volumetric flowF the change in the mass of oxygen in the mixing volume over any time interval, dt,is:

dmO2 = (moutO2

− moutO2

)dt = (�F[O2]in − �F[O2]out)dt (A2)

whereminO2

and minO2

are the mass flow rates of oxygen into and out of the well mixedvolume V0. Differentiating Equation A1 with respect to time and equating the result to the


differential form of Equation A2:

�mO2 ≡ m0O2

− minO2

= �F[O2]0 − �F[O2]in

= �F([O2]0 − [O2]out) − �V0d[O2]out

dt(A3)

wherem0O2

is the baseline mass flowrate of oxygen and�mO2 is the mass consumption

rate of oxygen relative to the baseline. Making the substitutions,� = [O2]0−[O2]out and� = V0/F, the specific heat release rate in terms of the oxygen concentration at the detector is:

Qc(t) ≡ C�mO2(t) = K

(� + �

d�

dt

)(A4)

where K= C�F (a constant with units of W). Equation A4 synchronizes the oxygen signalwith the heat release history of the solid. Inverting Equation A4 with� time variable ofintegration:

�(t) = 1

K�

∫ t

−∞exp(−(t − �)/�)Qc(�)d� (A5)

shows that the oxygen depletion measured at the detector�(t) is distorted (convoluted) byan exponential apparatus function. The convolution integral (Equation A5) can be solvedexactly for a heat release rate history having an analytic form. An experimentally convenientheat release rate history is a step change, i.e.Qc(t) = 0 for −∞ < t < 0, and,Qc(t) = Q0

c(a constant) for t≥ 0, which when substituted into Equation A5 and solved:

�(t) = Q0c

K�

∫ t

−∞exp(−(t − �)/�)u(�)d� = Q0

c

K(1 − exp(−(t/�))) (A6)

shows that� approaches an equilibrium valueQ0c/K as t→ ∞, and that t= � when� equals

63% of this steady-state value for a constant heat flowQ0c. This result and the step change

heating history is the basis of the calibration procedure for the PCFC (seeFig. 5).

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