)LUH#6DIHW\

39

Research on Ultra Fire-ResistantMaterials

The objective of the Fire-Resistant Materialsprogram is to eliminate burning cabinmaterials as a cause of death in aircraftaccidents. Long-term activities include thesynthesis of new, thermally stable, low fuelvalue organic and inorganic polymersystems. The synthesis effort is supportedby fundamental research to understandpolymer combustion and fire-resistancemechanisms using numerical and analyticmodeling and the development of newcharacterization techniques. Aircraftmaterials which are targeted for upgradedfire resistance are (1) thermoset resins forinterior decorative panels, secondarycomposites, and adhesives; (2) thermo-plastics for decorative facings, tele-communication equipment, passengerservice units, molded seat parts,transparencies, and electrical wiring;(3) textile fibers for upholstery, carpets,decorative murals, tapestries; and(4) elastomers/rubber for seat cushions,pillows, and sealants. To date, significantprogress has been made in achieving theinterim goal of a 50% reduction in the heatrelease rate of burning cabin materials bythe year 2002 and zero heat release ratecabin materials by 2010, with respect to the1996 baseline for new aircraft.

Thermoset Resins

The flammability of organic polymer matrix,fiber-reinforced composites limits their usein commercial aircraft where fire hazard isan important design consideration becauseof restricted egress. At the present time,affordable, processable resins for fire-resistant aircraft interiors are unavailablesince most organic polymers used for thispurpose ignite and burn readily under fuelfire exposure conditions.

• Polybenzoxazine resins are a new, low-cost phenol-formaldehyde (phenolic)substitute for use in aircraft interiordecorative panels. Polybenzoxazineshave demonstrated an 80% lower heatrelease rate, lower toxicity, and bettersurface finish due to the absence ofvolatile reaction products. A patent hasbeen filed on this technology (CaseWestern Reserve University).

• A zero heat release carbon-silicon resinhas been synthesized which has a 97%char yield when burned. A patent hasbeen filed on this technology (DowCorning).

• Ethynyl polymers give off only 25weight percent of combustible gaseswhen burned (75% char yield). Apatent has been filed on these materials(U. South Carolina).

• A low fuel value polymer is obtainedfrom renewable sources by reactingsilica sand or rice hulls with ethyleneglycol (antifreeze) in the presence of anamine base to yield semi-inorganicmonomers in essentially quantitativeyield. Subsequent curing produces avariety of inexpensive silicon-containing polymers ranging from hardtransparent films to low-density foams.A patent disclosure has been filed onthis technology (U. of Michigan).

• Polycyanurate resins were preparedfrom polystyrene and evaluated forflammability along with commercialcyanate esters (Stockton College, theNational Institute of Standards andTechnology (NIST), and the FAA).

• Inorganic fire retardants based onzirconia and boron oxides are highlyefficient, environmentally benign

40

additives which reduce the peak heatrelease rate by 60%-80% in commodity(nylon, PE, and polypropylene) andengineering (cyanate esters andULTEM polyetherimides) polymers atlow concentrations without increasingsmoke or carbon monoxide. NIST isfiling a patent on this technology(NIST).

Thermoplastics for Molded Parts

Commercial transport aircraft containbetween 1500 and 2500 pounds offlammable plastics as seat trim, windows,window shades, wire insulation, andmiscellaneous parts. At present thesemolded parts are not required to meet theheat release rate regulations imposed onlarge-area interior panels, stowage bins,ceilings, and partitions because of theirsmall size. High-temperature plastics whichpass the heat release rate test do not have therequisite toughness, durability,environmental resistance, and aesthetics tofunction effectively in aircraft interiors.

Nanocomposite technology is an entirelynew generic approach to reducing theflammability of polymeric (plastic) materialsusing environmentally friendly, chemical-free additives. The fire-retardant effect ofnanometer-sized clay particles in plasticswas discovered by the FAA through aresearch grant to Cornell University. NISThas subsequently confirmed the effect in firecalorimeter testing. The approach is todisperse individual, nanometer-sized,layered silicates in a molten polymer tocreate a clay-plastic “nanocomposite.” Theclay particles are about the same size as thepolymer molecules themselves (less thanone millionth of an inch) so they becomeintimately mixed and chemically bonded.This has the overall effect of increasing thethermal stability and viscosity of the plastic

while reducing the transmission of fuelgases generated during burning.

The result is a 60% to 80% reduction in therate of heat released from a burning plasticnanocomposite containing only 5% to 10%clay by weight. This extraordinarily highdegree of fire-retardant efficiency comeswith reduced smoke and toxic gas emissionsand at no sacrifice in mechanical properties.Plastic nanocomposites have twice thestiffness and strength of the original materialand a higher softening temperature (CornellUniversity and NIST).

Additional results on fire-resistantthermoplastics include

• Ultra high-modulus thermoplasticmolecular composites have been testedwhich have low heat release rate andthree times the strength and stiffness ofhigh-temperature engineering plastics(MAXDEM, Inc.).

• New phosphineoxide-polyetherimidethermoplastics have significantly lowerheat release rate when burned thancommercial polyetherimides (ULTEM)currently used in aircraft interiors(Virginia Polytechnic and StateUniversity, Blacksburg).

• Polycarbodiimides are carbon-nitrogenbackbone polymers, which can beflexible elastomers or rigid resins, havebeen prepared from inexpensivestarting materials. Polymersdepolymerize cleanly to monomerswith extremely low fuel value uponheating to 200°C. Molecular modelingshows that thermal stability can beincreased through stereochemistry (U.of Massachusetts, Amherst).

41

• Novel organic-inorganicinterpenetrating networks (IPN’s) havebeen synthesized by separatelypolymerizing organic and inorganicmonomers in the same reaction mixtureto give a co-continuous morphology.The reaction is solvent free and theinorganic (silica) phase is atomicallydispersed so that the material istransparent, rigid, and has higherthermal stability than the organic phasealone (U. of Massachusetts, Amherst).

• Surface active flame retardantsconcentrate at a burning polymersurface using interfacial free energy asthe driving force. Migration of surfaceactive flame retardants from the bulk tothe burning surface improves flame-retardant efficiency by reducing theloading level required for fireresistance (U. of Massachusetts,Amherst).

Rubber for Seat Cushions

Commercial transport aircraft containbetween 1000 and 2500 pounds offlammable elastomers (rubber) which arefoamed to make seat cushions and pillows orused at full density as sealants and gaskets.Foamed polyurethane rubber seat cushionsare favored for their durability and recovery,but they are the primary fire load in aircraftinteriors. In 1987 the FAA imposedregulations on the flammability of aircraftseat cushions to delay their involvement incabin fires. Manufacturers responded tothese regulations by wrapping thepolyurethane seat cushion in a fire-resistantbarrier fabric. Seat fire blocking allowedmanufacturers to pass the FAA certificationtest, but the cushions burned vigorouslywhen the fire-blocking layer was consumedafter minutes of exposure to a fire.

The flammability of rubber depends on thechemical composition of the polymer fromwhich it is made. Rubbers made fromcarbon-hydrogen-based (organic) polymersare the most flammable because of theirhigh fuel value. Replacing carbon andhydrogen atoms in the polymer withinorganic atoms such as chlorine, silicon,nitrogen, sulfur, or phosphorus results in asemiorganic polymer with reducedflammability because of the lower fuel valueor increased heat resistance. In the Fire-Resistant Materials research program we arefocusing on semiorganic rubbers for seatcushions.

• Silphenylenes are hydrocarbon-silicon-oxygen backbone elastomers, whichare crosslinkable and extremely heatresistant, have been synthesized. Thesilphenylene contains only 30%combustible material and canwithstand temperatures of 600°C(1100°F). A patent is being filed onthis composition of matter (U. ofMassachusetts, Amherst).

• Polyphosphazenes are semiorganicrubbers based on a phosphorus-nitrogen backbone which contains anorganic group that allows the materialto be dissolved or crosslinked.Commercial production ofpolyphosphazene was recentlydiscontinued despite the extremely lowtoxicity and ultra fire resistance ofthese foams because the process formaking them was prohibitivelyexpensive. We are pursuing a newlow-cost, low-temperature, syntheticroute to polyphosphazenes whicheliminates a costly intermediate fromthe process and allows control over themolecular weight of the polymer. Thisnew direct synthetic route has providedthe first phosphazene copolymers

42

including an 80-20 urethane-phosphazene copolymer which doesnot ignite in a flame. A patent hasbeen filed on this process (ThePennsylvania State University).

Fibers for Carpets and Textiles

Approximately 2000 pounds of combustibletextile fibers are used in a moderncommercial aircraft interior as seatupholstery, decorative textiles, wallcoverings, carpeting, tapestries, blankets,curtains, and seat belts. Typical fabricsinclude wool, nylon/wool blends, and fire-retarded polyester, wool, and nylon. Thechemical fire retardants added to these fibersreduce the propensity for small-scaleignition but increase the smoke density andtoxic gas generation once the fibers catchfire and have little or no effect on heatrelease rate. Our research in fibers focuseson materials with unusually high thermalstability which have intrinsic fire resistancewithout the need for chemical additives.Current research includes the following.

• Zero heat release polyimide fibers aremelt and solvent processable forspinning ultrahigh-strength thermallystable fibers and casting films. Fibersof this material exhibit the lowest(microscale) heat release rate of anypolymer tested to date—ten timeslower than aramid (Kevlar™) fibersand 150 times lower than nylon used inseat fabrics (U. of Akron).

• Substituted polyhydroxyamidesgenerate flame-retardant compounds ina fire through a thermally activatedchemical reaction which produces azero heat release polymer(polybenzoxazole) during heating (U.of Massachusetts, Amherst).

Supporting Science and Technology

A microscale combustion calorimeter hasbeen developed to measure flammabilityparameters of milligram polymer (plastic)samples under conditions whichapproximate aircraft cabin fires. The testtakes only a few minutes to run and providesa quantitative measure of the aircraft firehazard of research materials which are onlyavailable in milligram quantities fromuniversity and industrial polymer scientists.Quick feedback on flammability of newmaterials has helped guide the syntheticeffort towards improved fire resistance. Asharp, quantitative, and reproducible heatrelease rate peak is obtained in themicroscale heat release rate test. Afternormalizing the curves for the sample size,the results are independent of the physicalform of the material (e.g., powder, film,fiber, etc.). The microscale heat release ratedata are expressed in kilowatts per gram oforiginal material.

The peak microscale heat release rate (HRR)measured on milligram samples of pureplastics in the microcalorimeter correlatesextremely well (correlation coefficient =0.95) with the heat release rate measured for100-gram samples of the same material in aconventional fire calorimeter.

Full-scale fire tests at the FAA William J.Hughes Technical Center have shown thatthe heat release rate of interior materialsmeasured in a fire calorimeter correlateswith passenger escape time in a simulatedpostcrash fuel fire. The good correlationbetween fire- and microcalorimeter resultsshows that the microcalorimeter is also agood predictor of passenger escape timeand, therefore, of full-scale fire hazard. ADOT/FAA patent has been filed on thisinvention (FAA and Galaxy ScientificCorporation).

43

The figure below shows microcalorimeterresults for some of the new materials beingdeveloped in the program and how theycompare materials currently being used inaircraft. It is clear that most of the newmaterials tested show significantly lowerheat release rate when combusted than dothe current (baseline) aircraft materials.

In addition to creating a metric for materialsdevelopment (i.e., the microscalecalorimeter), supporting science andtechnology provided the followingtechnology base.

• Thermal degradation of polymers wasmodeled by computer to study theprocess which generates combustiblefuel in a fire. The computer code (MDREACT) uses a commercial program(Discover 95, Molecular Simulations)to create the polymer chains for themodel and then simulates the thermaldegradation process at firetemperatures. This code will provideinsight into the process and products of

thermal degradation to allow the designof thermally stable, less toxic, fire-resistant polymers (NIST).

• A mechanistic fuel generation modelwas developed for polymers in fireswhich includes the competingprocesses of gasification and charformation. The simple analytic resultprovides the fuel generation rate of aburning polymer from conventionallaboratory thermal analysis data (FAAWilliam J. Hughes Technical Center).

• Molecular modeling of flammabilityutilizes computational quantumchemistry to predict thethermochemistry and kinetics of solid-and gas-phase combustion processes ofburning polymers. Initial results forthe effects of chain size andstereoregularity on the bonddissociation energy of novelpolycarbodimides were obtained (U. ofMassachusetts, Amherst).

0 500 1000 1500 2000 2500 3000

MICROSCALE HEAT RELEASE RATE, Watts/gram

Plastic APlastic B

ULTEM PEI PLASTIC

Fiber AFiber B

NYLON FIBER

Rubber ARubber B

URETHANE RUBBER

Resin AResin B

PHENOLIC RESIN

CABIN MATERIAL

Current

Developmental

Heat Release Rate of Current and Developmental Aircraft Cabin Materials

44

• Atomistic models of polybenzoxazinehave been developed and solved on acomputer which provide insight intothe high thermal stability and unusualvolumetric expansion onpolymerization (U. of Akron).

• Computational modeling ofintumescence has shown theimportance of bubble nucleationkinetics and thermal properties on theinsulation value of this importantcommercial fireproofing technology(NIST).

• Thermomechanical stability of fireexposed resins and composites arestudied using a new noncontacttechnique—ultrasonic spectroscopy.Results indicate surface embrittlementof the resin in a fire initiates surfacecracks which mechanically destabilizethe composite (U. of Massachusetts,Amherst).

• An integral method of nonisothermalkinetic analysis provides a new, semi-exact solution of the mass loss integralwhich is useful for extractingflammability parameters from

laboratory thermal analysis data (FAAWilliam J. Hughes Technical Center).

• In situ flame chemistry by remote laserspectroscopy has been successful atidentifying flame species in burningpolymers to study the chemicalreactions which lead to combustioninhibition and toxic byproducts inflame-retarded polymers. A patent isbeing filed on this technology (U.South Carolina).

• Fire calorimetry of flame-retardantpolymers showed that fire retarded(FR) polymers ignited and burnedreadily under aircraft cabin fireexposure conditions with increasedsmoke and toxic gas productioncompared to the original (non-FR)material—validating the FAA’s focuson inherently fire-resistant polymersand composites (Omega Point Labs).

POC: Dr. Richard Lyon, AAR-422, (609)485-6076.

45

A Simple Fuel Generation Modelfor Bur ning Plastics

A molecular basis for polymer burning wasdeveloped from thermal degradation kineticsand compared to experimental data forflaming heat release rate in fires. The solid-state fuel generation rate in fires has beenderived [1] from the polymer thermaldegradation scheme

assuming that the majority of pyrolysisgases, G, and char, C, are producedanaerobically from the reactive intermediateI* in a single step via parallel reactions. Thesystem of rate equations for the species attime, t, is

dPdt

= – kpP + k–pI* (1)

dI*dt

= kpP – k–p+ kg + kc I* (2)

dGdt

= kgI* (3)

dCdt

= kcI* (4)

The stationary-state assumption dI*/dt • 0eliminates I* from Equations 1-4. Definingan initial mass, mo = P + G + C + I* ≈ P + G+ C, and a sensible mass, m = P + C + I* ≈ P+ C, the maximum fractional mass loss rateat a constant heating rate dT/dt = β is

–mmaxmo

=β(1 – µ)Ea

eRTp2 (5)

where Ea is the global molar activationenergy for the rate limiting decompositionstep, µ = C(∞)/mo is the equilibrium charfraction at the peak mass loss ratetemperature, Tp, R is the gas constant, and eis the natural number. Multiplying Equation5 by the heat of complete combustion of thepyrolysis gases hc

o gives the kinetic heatrelease rate

Qc(W/kg) = hc

o – mmaxmo

=hc

oβ(1 – µ)Ea

eRTp2

(6)

The pyrolysis zone thickness for steadyburning of a polymer of thermalconductivity κ at net surface heat flux qnet is

δ =

eκqnet

RTp2

Ea

(7)

The areal density of pyrolyzing polymer ofbulk density ρ and surface area S is mo/S =ρδ so that the macroscopic heat release rateper unit area of burning surface is

qc(W/m2) = –χ hc

o mS = χρδ hc

o–mmo

(8)

or with Equation 6

qc = χρδ Qc (9)

Equation 9 states that the ratio of the steady-state macroscopic heat release rate to thepeak kinetic heat release rate for comparabletemperature histories is in the range

qc

Qc

= χρδ ≈ 0.4 ± 0.2 kg/m2 (10)

using typical values for the gas phasecombustion efficiency in a fire, χ = 0.7 ±0.2, the polymer density, ρ = 1000 ± 100kg/m3, and the pyrolysis zone thickness, δ =

→→Polymer, P

ReactiveIntermediate, I*

Gas, G

Char, C

∆

↓

↑

46

0.5 ± 0.2 mm at qnet = 50 kW/m2 fromEquation 7.

The heating rate at the surface of a steadilyburning polymer is

β = dT

dt x = 0=

qnet2

κρcp Tp – To

(11)

with cp = c(Tp) the polymer heat capacity atthe peak pyrolysis temperature. SubstitutingEquation 11 along with Equations 6 and 7into Equation 9 recovers the continuumresult for thermal diffusion limited steadyburning

qc(W/m2) = χ hc

o mS = χ

hco

hgqnet (12)

with the heat of gasification defined hg ≡cp(Tp– To)/(1–µ) = qnet /(mY/S).

In principle the macroscopic heat releaserate at any net surface heat flux qnet is nowcalculable from the pyrolysis kineticparameters and heat of combustion of thevolatiles using Equations 6, 7, 9, and 11.However, these kinetic and combustionparameters are rarely known with anycertainty, particularly at the surface heatingrates in a fire.

Consequently, our approach [2] was tomeasure the kinetic heat release rate directlyusing pyrolysis-combustion flowcalorimetry (PCFC). In the PCFC test, a 1-to 3-milligram polymer sample is pyrolyzedby heating in an inert environment to amaximum temperature Tmax = 923 K at alinear heating rate β ≈ 8 K/s (calculatedfrom Equation 11) to approximate the rapid,anaerobic heating of a polymer surface in afire or fire calorimeter at qnet = 50 kW/m2.

The pyrolysis gases are swept from thesmall-volume pyrolysis chamber of thePCFC by flowing nitrogen and mixed withexcess oxygen prior to entering a 900°Cfurnace to effect complete combustion.Combustion products are scrubbed from thegas stream prior to entering an oxygenanalyzer. Heat release rate is calculatedfrom oxygen consumption and the measuredflow rate of the gas stream.

Results of these PCFC tests are compared toliterature values for average macroscopicheat release rate at qnet = 50 kW/m2 in acone calorimeter in the figure below.

0

200

400

600

800

1000

1200

1400

Con

e A

vera

ge H

RR

(kW

/m2 )

Kinetic Heat Release Rate (kW/g)

ABS

phenolic triazine POM

PET

PEEK

polyetherimide

substituted polyphenylene Nylon 66

PMMA PBT

polyethylene

polypropylene

polysulfone

0.6 3.60 1.2 1.8 2.4 3.0 4.2

polystyrene

Macroscopic Versus Kinetic Heat ReleaseRate For Some Polymers

Proportionality is observed between themacroscopic and kinetic heat release rateswith slope qc / Qc = 0.33 kg/m2, which iswithin the range of predicted values usingEquation 10.

We conclude that steady polymer burning isa thermal diffusion controlled kineticprocess in which the rates of the solid-statereactions (pyrolysis) and gas phase reactions(combustion) are rapid in comparison to therate of heat transfer at the flaming surface.The pyrolysis zone depth is determined bythe coupling between thermal diffusion andpyrolysis kinetics and effectively correlates

47

the molecular and macroscopic heat releaserates.

REFERENCES

1. R. E. Lyon, Polymeric Degeneration andStabilization, 61 (2), 201-210 (1998).

2. R. N. Walters and R. E. Lyon,Proceedings of the 42nd InternationalSAMPE Symposium, 42 (2), 1335-1344(1997)

POC: Dr. Richard Lyon, AAR-422, (609)485-6076.

A Flammability Test Method forThermal Acoustical Insulation

An aircraft thermal acoustical insulationblanket is typically comprised of a fiberglasscore encapsulated by a film cover or bag.Blankets of various thicknesses are placedagainst the aircraft skin throughout theaircraft interior circumference. Theirfunction is to reduce noise and act as athermal barrier, depending on the location ofthe blankets. The two principle reasons thatfilms are used as coverings on the aircraftblankets are for containment of thefiberglass (keeping the batting in place) andresistance to moisture permeation. Selectionof films is based on durability, fireresistance, weight, and installationconsiderations.

Between 1993 and 1995, a number of cabin

fires involving flame propagation on thermal

acoustical insulation blankets were reported.As a result of these incidents and a requestfrom the aircraft industry, the InternationalAircraft Materials Fire Test Working Group,chaired and administered by the FAAWilliam J. Hughes Technical Center’s FireSafety Section, formed an ad hoc task groupfor the purpose of conducting and evaluatingflammability tests performed on insulationblankets. Members of the group includedBoeing, Douglas, Facile, Jehier, Airbus,Orcon, Govmark, and the FAA. Verticalflammability testing of all insulation blanketcomponents (film cover, core material, andcomplete assembly) is mandated in theFederal Aviation Regulations (FAR) 25.853.In tests performed by the task groupmembers on insulation blankets, it wasfound that vertical testing does not produce

48

consistent test results and therefore may notbe a suitable indicator of the flammability ofthermal acoustical insulation assemblies.

Previously, a test employing a flamingcotton swab at two locations on a foldedblanket, developed by a commercial aircraftmanufacturer, promised to be effective inconsistently identifying thermal acousticalinsulation films that propagate flame (seefigure below). As part of the test program,the task group evaluated the cotton swab testin a series of round-robin testing performedon the most widely used film covers andassemblies. The test method was slightlymodified in terms of sample size and swabplacement. The round-robin test results

showed that the cotton swab test was a morereproducible test and slightly more severethan the vertical flammability test. This wasespecially apparent for a particularmetallized film/fiberglass assembly. Thefront face of this particular sample wasconsumed when subjected to the cottonswab test, although the same sample passedthe vertical tests of most laboratories.Because of these findings, the metallizedfilm/fiberglass blanket is no longer beinginstalled in aircraft.

A final technical report entitled “Evaluationof Fire Test Methods for Thermal AcousticalInsulation,” DOT/FAA/AR-97/58, waspublished and distributed. The reportcontains the round-robin test findings andthe version of the standardized cotton swabtest developed by the test group.Additionally, the standardized test methodwill be included in the Materials Fire TestHandbook that is scheduled for release inlate 1998. Although not mandated, thecotton swab test is now performed by theworld’s largest commercial aircraftmanufacturers as part of their own internalspecifications.

POC: Ms. Patricia Cahill, AAR-422, (609)485-6571.

49

Oxygen Cylinder Overpack Testing

A fatal in-flight fire occurred onboard aValuJet DC-9 on May 11, 1996. During thisaccident, an extremely intense fire fueled bysolid oxygen generators erupted in the classD compartment, burned out of control intothe passenger cabin, and eventually causedthe aircraft to crash, resulting in 110fatalities. In the wake of this accident, theshipment of oxidizers was banned in allcommercial aircraft cargo compartments. InFebruary 1998, industry, pilot, and usergroups requested an exemption to allow forthe shipment of bottled oxygen in class Ccargo compartments, which have firedetection and suppression systems thatshould detect and suppress a fire. In theevent that the fire is not fully extinguished,which would be the case if the origin were adeep-seated fire, the temperatures in thecompartment could reach 400oF. The majorconcern with the shipment of oxygencylinders under this scenario is that theelevated temperatures would cause thecylinder pressure to increase, causing thepressure relief mechanism to open. If thisoccurs, the oxygen could cause significantintensification of the fire.

In order to determine the level of hazardassociated with gaseous oxygen releaseduring a Halon 1301-suppressed cargocompartment fire, four full-scale tests wereconducted inside a 169-cubic-foot LD-3cargo container (see photograph at top ofpage). Previously, tests were conductedinside a large industrial convection oven todetermine the temperature and time requiredto cause pressure relief activation of threedifferent-sized oxygen cylinders commonlyused in commercial transport aircraft. Thecylinders were placed inside the oven and anexternally mounted pressure gauge wasconnected to the cylinder valve head tomonitor the pressure rise. As the test began,the oven temperature was ramped to 400oF,

which represented the temperature leveltypically reached during a Halon 1301-suppressed cargo compartment fire.Pressure relief activation typically occurredafter the surface temperature of the cylinderhad reached only 300oF, within 15 minutesfrom the start of the test. This informationwas then used to set up the full-scale tests inwhich gaseous oxygen was bled into theactual deep-seated fire.

During the full-scale tests, the cargo fireswere allowed to burn for a short durationbefore Halon 1301 was discharged into thecontainer. After the suppressantconcentration stabilized to about 3%, theminimum design concentration for inerting,a quantity of oxygen from a remotelylocated cylinder was discharged to simulatethe relief of oxygen from an overpressurizedcylinder. When the surface temperature ofan empty “dummy” cylinder inside the LD-3test container reached 300oF, the flow ofoxygen was initiated. Although the amountsof oxygen were very modest in comparisonto the amount typically stored in one crewbreathing cylinder, which range from 76 ft3

to 115 ft3, the tests demonstrated that ahalon system could not always contain anoxygen-enriched fire (see photograph onnext page).

50

Additional oven tests were conducted usinga 76.5 ft3 oxygen cylinder placed insideseveral types of cylinder cases, commonlyreferred to as overpacks. The overpackswere available in a variety of constructions,all for the purpose of protecting the cylinderfrom impact damage that may occur duringshipment (see photograph below). The testswere run to determine the level of thermalprotection, if any, that might result when thecylinders are subjected to elevated

temperatures. Two custom-made overpackswere tested containing insulating materialsaimed specifically at providing thermal

protection. Tests showed that somecommon overpacks have the ability toprotect the cylinder from pressure reliefactivation for nearly 60 minutes in a 400oFenvironment, while other types designedspecifically for thermal protection canprevent rupture disc activation for extendedperiods of time.

The FAA is considering a thermal-protection performance standard recentlydeveloped by the FAA William J. HughesTechnical Center’s Fire Safety Section foroxygen cylinder overpacks based on thesetests. The standard will consist of two tests,an open-flame burnthrough test and an oventest, which address thermal protectionrequirements before fire detection (andagent discharge) and during fire suppression,respectively. During the oven test, anitrogen-charged cylinder placed in the testoverpack will be exposed to a 400oFenvironment. The overpack must preventthe cylinder rupture disc from activating for90 minutes. During the burnthrough test, a16- by 24-inch sample representing theoverpack construction will be exposed to anintense, 1800oF open flame for 5 minutes.The test sample must prevent flamepenetration, and the backface temperaturemust not exceed 400oF at any time duringthe test. By meeting this combinedstandard, the cylinder overpacks willeffectively prevent the release of oxygenduring a typical deep-seated cargocompartment fire.

POC: Mr. Timothy Marker, AAR-422,(609) 485-6469.

51

Ini tial Development of anExploding Aerosol Can Simulator

The Montreal Protocol Treaty, signed bynearly all industrialized nations around theworld, banned the manufacture of ozonedepleting halons. Halons are effectivegaseous extinguishing agents that are used infour major applications in commercialaircraft: cargo compartments, enginenacelles, hand-held extinguishers, andlavatory trash receptacles. Because of thediminishing availability of halons, the FAAhas been developing minimum performancestandards (MPS) for replacement agents andsystems in each of these areas. Incommercial transport aircraft, the largestquantity of halon is used in the extinguishersin the cargo compartment. The MPS beingdeveloped for cargo compartmentsencompasses four fire test burningscenarios: surface, simulated bulk loadedluggage, simulated containerized luggage,and exploding aerosol cans.

Previous research and testing has shown thatcargo compartment fires involving aerosolcans are particularly dangerous. For morethan a decade the chlorofluorocarbonpropellant used in aerosol cans has beenreplaced with hydrocarbon blends thatinclude fractions of propane,butane, and isobutane. Theseflammable gases wouldnormally be prohibited onpassenger carrying airplanes,but there is an exception for upto 75 ounces per person formedicinal and toilet articleswhen carried in checkedbaggage only. During a fire,heated aerosol cans willoverpressurize and rupture,releasing the flammablehydrocarbon-based propellantwith explosive force. The

resultant overpressure in the compartment isof particular concern. The compartmentlining system may become dislodgedallowing the protective fire suppressionagent to escape, and the fire and smoke mayspread to other parts of the airplane. Halon1301 has proven to be extremely effective atmitigating an explosion caused by heatedaerosol cans and therefore the halonreplacement agents and systems must alsoperform at an equivalent level.

Because aerosol can explosions areextremely variable and unpredictable, asimulator was constructed which canreplicate an exploding aerosol can. Theinitial simulator design used a steel pipepressure vessel mated to a high-ratedischarge (HRD), electrically actuatedsolenoid valve. The vessel contained portsand valves to allow for the transfer of thebase product (initially isopropyl alcohol)and hydrocarbon propellant (typicallypropane). The contents were heated byblowing a hot-air gun against the surface ofthe steel vessel, effectively raising thepressure. When the pressure was sufficientto burst a standard can, approximately 210psig, the contents were released over a set ofdirect current (DC) spark igniters (seedrawing below).

52

Initial tests were performed using thesimulator in an open area to observe theflame propagation pattern and to test thegeneral operation of the device. The mix ofconstituents used were representative of alarge hairspray can (16 ounces), consistingof 3.52 ounces (weight) of liquid propaneand 2.50 ounces (weight) of isopropylalcohol. The mix produced a large fireballof about 12 feet in diameter, slightly largerthan a typical exploding hairspray can,which produces about an 8-foot-diameterfireball (see photograph below). Thesimulator tests were then conducted in theconfined space of a B727 cargocompartment. The simulator was mountedto the forward bulkhead, and the mix wasreleased into the compartment over the sparkigniters, which were situated approximately3 feet from the nozzle exit. The ensuingexplosion caused severe damage to theentire compartment. The forward and aftbulkheads collapsed, and the cabin floorabove the compartment ceiling liner wasblown out of place. There was alsoevidence of severe overpressure inside thestructure, as several rivet heads on theexterior skin surface failed under tension. Inorder to measure the effectiveness of Halon1301 at mitigating this event, a repeat test

was performed with the entire compartmentinerted with Halon 1301 at a concentrationof 6.5%. Upon release of the heatedconstituent mix into the inertedcompartment, no explosion event took placeand no perceived pressure rise inside thecompartment was observed.

Several additional proof-of-concept testsemploying the aerosol simulator wereconducted using LD-3 Unit Loading Device(ULD) containers as the confining space.The hydrocarbon propellant mix explodedwith violent force, blowing the bifold dooroff its hinges. The container sustainedheavy damage in the form of long cracksdue to overpressure. During the simulatorevent, a pressure rise of 8 psig wasmeasured in the container. This test resultwas more severe than an actual aerosol canexplosion, which typically generates enoughforce to disengage and partially swing openthe bifold door. Several additional testscorroborated these results. As with theB727 tests, the effectiveness of Halon 1301at mitigating these types of events was alsoinvestigated. Several additional tests wereconducted in the LD-3 container which wasinerted to various concentrations prior tosimulator activation. During the tests, there

was no explosion in the LD-3container even when theconcentration of Halon 1301 was aslow as 1%. These results illustratedthe effectiveness of halon againstthis type of threat, even atsubstantially reducedconcentrations. Although the testsexhibited the ability of Halon 1301to prevent explosions at 1%concentration, the results differfrom published literature, whichindicates higher concentrations(e.g., 6.7% for propane) arerequired to suppress this type ofexplosion. Further testing will be

53

conducted to examine this discrepancy. Theresults of this work are published inTechnical Note DOT/FAA/AR-TN97/103,“Initial Development of an ExplodingAerosol Can Simulator.”

Although the simulator used a mix ofconstituents representative of a largehairspray can, the explosion event is muchmore severe. A major reason for theconsistent potency of the simulator lies in itsability to form a large, combustible vaporcloud, promoting complete combustion.

Further refinement of the simulator iscurrently underway. At present, largeaerosol cans are being overpressurized under

a variety of conditions in an attempt to causethe most violent explosion. These tests arebeing conducted in a large pressure vesseloutfitted with high-speed pressuretransducers to measure the pressure rise.This data will be used to adjust the output ofthe simulator so that a severe but realisticMPS test condition can be achieved. Halonreplacement agents, most notably waterspray, will be tested to determine theireffectiveness against the standard explodingaerosol can fire threat.

POC: Mr. Timothy Marker, AAR-422,(609) 485-6469.

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Effect of Acid Spills on AircraftStructure

The FAA conducted tests to evaluate theeffects of a spill of a strong corrosive acidsuch as hydrochloric acid (HCl) on aircraftinterior skin and to determine the timerequired for a spill of Department ofTransportation (DOT) allowable volumesand concentrations to cause catastrophicfailures.

This work was conducted in response to arequest by the agency that regulatestransport of hazardous materials, theResearch and Special ProgramsAdministration, Department ofTransportation (RSPA DOT).

Four spill tests were conducted withconcentrated HCl onto various sizedsections of a Boeing 747 aircraft skin.Epoxy paint was used to coat all the cut orscraped surfaces that were not beingevaluated by the test.

Test specimens for the first three testsranged in size from a 6" x 6" section to a 3' x3' section. Each specimen included asection of the ribs and the rivets connectingthe skin to the ribs. One liter ofconcentrated HCl, the maximum allowableDOT volume for passenger aircraft, waspoured onto the interior surface of the 3' x 3'skin or placed into a Pyrex glass pancontaining the smaller samples.

The last test used a 2" by 7" strip of a B-747skin with two scratches placed scratchedsurface up in a Pyrex container holding 1liter of concentrated HCl.

Test data indicated that the epoxy coatedinterior aluminum skin is resistant to acidattack. The acid reacted vigorously withscratched skin surfaces, creating a wide hole

in the skin along the scratch line. Test dataalso indicated that a spill of concentratedHCl can eat completely through the rivetsand ribs and may result in a significant lossof structural rib strength.

A detailed report describing this study,“Effects of Concentrated Hydrochloric AcidSpills on Aircraft Aluminum Skin,”DOT/FAA/AR-TN97/108, was published inJuly 1998.

A 6" x 6" Section of B-747 Skin After an85-Minute Exposure to 1 Liter of

Concentrated HCl Solution

Scratched 2" x 7" Strip of B-747 Skin Aftera 90-Minute Immersion in 1 Liter of

Concentrated HCl Solution

POC: Ms. Louise Speitel, AAR-422, (609)485-4528.

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A Computer Program forPredicting the Probabili ty of a FuelTank Explosion

A software model was developed to assessthe hazard of a fuel tank explosion atdiscrete points in time. The model employsexperimental data of ignition energy (Eign)required to cause fuel tank vapors to igniteas a function of fuel flash point temperature,fuel temperature, and altitude (pressure). Inorder to assess the probability of a fuel tankexplosion during an entire flight andevaluate the effect of changes to the fueltemperature and/or fuel flash point, it isnecessary to sum the total of Eign < Espark

from time equal to zero till the timecorresponding to the end of fl ight, whereEspark is the energy of the ignition source.Dividing by the total flight time in minuteswould determine the norm of Eign < Espark.

A probability input screen provides the userthe ability to specify flash point(s) of thefuel (TFP) in °F and assign a distribution toTFP (if desired). The spark probability(Fspark) distribution can be selected using acurve that is resident in the program orinputted by the user.

The flight scenario screen allows the user todevelop the flight profile. Here, an altitude(Z) versus time profile is entered. Altitudeis entered in thousands of feet and time inminutes.

Also entered in the flight scenario screen isthe temperature of the fuel (Tfuel) versustime profile. Fuel temperature is input as °Fand time is entered in minutes. It should benoted that the temperature being entered(Tfuel) should be the hottest measured orcalculated fuel temperature in the tank atthat time with the following exception.When the fuel temperature starts to decreasein flight and the hottest measured or

calculated ullage temperature at a specifictime is hotter than the fuel temperature atthat same time, one should use themaximum fuel temperature recorded up tothat time or the ullage temperature at thatspecific time, whichever is less.

From the above input, the model willextrapolate Tfuel and altitude (Z) andcalculate the probability of an explosion,given that a spark occurs, for each minute ofthe flight profile (P). These probabilitieswill then be weighted (based on the flashpoint distribution), totaled, and divided bythe total flight time of the flight profile.This yields the average probability of anexplosion over the entire flight, given aspark occurs (Pf).

The primary output of the model (as shownin the graphic on the following page) is asfollows:

• Graphical representation of the flightprofile

• Altitude (1,000 feet) versus time(minutes)

• Fuel temperature (°F) versus time(minutes)

• Color-coded potential hazard rangebased on the altitude and flash pointthat show when Tfuel versus timecrosses into the range of a potentialhazard.

• Average probability of an explosionover the entire flight, given a sparkoccurs (Pf).

• Maximum probability of anexplosion over the entire flight, attime (t) into flight, given that a sparkoccurs (Pmax).

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The results are estimates based on a limitedamount of available data. Although it was

felt that for most cases to be analyzed thatboth off gassing of O2 from the fuel andmisting or sprays of fuel can be ignored, thatmay not always be the case. For largequantities of fuel and small ullage spaces,the off gassing of O2 from the fuel may raisethe O2 level in the ullage significantly (3%to 5%) thus reducing the spark energyneeded for ignition. Should fuel be sprayedinto a tank during flight, the lowerflammability limit would be greatlydecreased, making the above calculationinvalid.

POC: Mr. Lawrence Fitzgerald, AAR-422,(609) 485-5582.

Flight ScenarioExponential Distribution

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Flammabili ty Hazard of Jet A FuelVapor in Civil Transport FuelTanks

A study was undertaken by a FuelFlammability Task Group composed ofrecognized fuel and combustion specialiststo investigate the flammability andexplosiveness of fuel within an aircraft fueltank. The task group reviewed all availablereports on the subject and discussed the datawith technical experts from BoeingCommercial Airplane Company, CaliforniaInstitute of Technology, and the NationalTransportation Safety Board. The scope ofthe review included jet fuel definitions andspecifications, jet fuel flammability data,influences of various factors on fuelflammability, and predictive analyses andmodels for flammability. The groupreported their findings in DOT/FAA/AR-98/26, “A Review of the FlammabilityHazard of Jet A Fuel Vapor in CivilTransport Aircraft Fuel Tanks.” The reportdiscusses the impact of this knowledge onthe needs for in-flight fuel fire prevention.The report did not offer a single, definitiveanswer to the question of when fuel tankscontain a flammable vapor, but it doesidentify the research necessary for a betterunderstanding of fuel flammability inaircraft fuel tanks. It also presents amethodology that, within limits, can be usedto compare the probability of a flammablemixture in a tank when the fuel temperature,flash point, and/or aircraft altitude arevaried. While adequate for a fuel tanksafety analysis to be carried out in the nearterm, it should be recognized that thecorrelation formulas are based on severalassumptions and, therefore, have seriouslimitations. The formulas represent only aninterim solution to a complex problem.Accordingly, it is strongly recommendedthat it be replaced with an improvedapproach when new data are developed. A

comparison between the formula andexperimental data is shown below. Theimplications of analytical approximation interms of ignition energy contours in thetemperature-altitude plane are shown in thegraph at the bottom of the page.

Comparison of Correlation With MinimumIgnition Energy (MIE) Data for Jet A Fuel at

14,000-ft Altitude

The task group found present methods forpredicting in-fli ght fuel temperatures to beinadequate. The development of reliableheat transfer models and the ability tocalculate the flammability of the ullagespace in an aircraft fuel tank are in the earlystages. Therefore, the ability to reliablyevaluate different strategies to reduce theflammability of fuel in aircraft fuel tanks hasnot been proven.

Predicted Constant Ignition EnergyContours for a Fuel with a Flash Point of

120°F (49°C)

POC: Mr. Richard Hill, AAR-422, (609)485-5997.