Mcdonald, Glen E

Schalla, Rose L

English

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FORAERONAUTICS
TECHNICALNOTE3405
TEMPERATURE -COMPOSITION LIMITS OF SPONTANEOUS
EXPLOSION FOR NINE ALKYLSILANES WITH AIR
AT ATMOSPHERIC PRESSURE
By Rose L. SchaIla and Glen E. McDonald
Lewis Flight Propulsion Laboratory
Cleveland, Ohio
Washington
February1955
https://ntrs.nasa.gov/search.jsp?R=19930084114 2020-06-17T17:51:44+00:00Z
TECHLIBRARYKAFB,NM
TEMPERA’IURE-COMPOHTIONLIMITSOFSPONTANEOUSEXJ?IQSIONFORNINE
AIJWL811ANESWITHAIRATATMOSPHERIC!PRESSURE
ByRoseL. SchallasndGlenE.McDonald
SUMMARY
Toprovidesafe-handling information, the spontaneousexplosion
limits of nine alkylsihmesweredeterminedas a functionoftemperature
andfuel-airccznpositionata pressureof 1 atznosphere.Overa fuel
concentrationrangeofabout2 to 9 percentby volumein dryair,the
fuelsstudiedwerenonexplosiveb lowthefollowingtemperatures: tet -
rsmethylsilane(CH3)Asi,450°C;trimethylsilane(CH3)3SiH,300°C; di-
ethylsiwe (C%) #iH2, 2550C; dimethylsihne(CH3)2SiH2j2N0 C;
ethylsilane(C#5)SiE3,21-50 Cj isobutylsilane(~-C4~)SiE3,200°Cj
~-butylsilane(~-C4~)SiH3,1850 C;methylsilane(CH3)SiH3,125°C; and
vinylsilaneH2C=CH-SiE3>90°C. Althoughtheexplosiontemperatures
applyonlytotheapparatususedherein,it seemsveryprobablethat
l thesesilanescanbe stielyhandledin thegasphaseatrocmtemperature
withoutdangerof a spontaneousreaction.Sincetheexplosiontempera-
turesofthealkylsilxmesarelowerthanthuseofanalogoushydrocarbons
. (500°to 600°C) anddecreaseashydrogenatomsaresubstitutedformethyl
groups,it appearedthattheSi-Hbondismorereadilysusceptibleto oxi-
dationthantheC-Hbond.
INTRODUCTION
CoWustionpropertiesofhydrombonfuelssuchasflsmespeed,
fl.amabilitylindt,ignitionenergy,andquenchingdistancehavebeen
extensivelyinvestigated.Forcertaincombustionprocesses,it isde-
sirableto employfuelsthathavennrefs,vorablecombustioncharacter-
isticsthsnthehydrocarbons,butthatpossessimilarphysicalproper-
ties. A classof compoundswhichmightmeettheserequirementsi the
alkylsilanes.Consequently,severalalkylsilanesweresynthesizedat
theWA Lewislaboratory;
point,meltingpoint,heat
ence1. At thetimethese
.
whichtheycouldbe safely
l
-h physicalproperties(suchasboiling
of mmbustion,etc.) s.rereportedinrefer-
silanesweresynthesized,theconditionsunder
hsndledandstored,particulsxlyb contact
2 NACATN 3425
withair>wereunkm.own.Thepresentreportdescribesa studydirected
towerdeatablisbingtheconditionsoftemperatureendconcentrationin
dryairthatwillpermitsafehandlingofthisclassoffuels.
Methyl-,dimethyl-,trimethyl-} tetremethyl-,ethyl-,diethyl-,
~-butyl-,isobutyl-j andvi@lsilaneswereinvestigatedinthisstudy.
Premixedfuel-airmixtures,withthesilsmeconcentrationrangingfrom
about2 to 10percentby voWine,weresdmittedto a heated,evacuated
reactionvesselto a pressureof1 atmosphere;andthelowestaupera-
tureatwhichexplosionwouldoccurwasthendetermined.Theexplosion +
regionof theninealkylsilmesasa functionoftemperatureendfuel- 3
aircompositionwasthereforestablished.A comparisonoftheexplosion
regiono% theallcylsilaneswiththehydrocarboncompoundthatwouldresult
by replacingthesiliconatomtitha ce&bonatomwasmadewherever
possible.
APPARATUSANDPROCECXJRE
Fuels.- Allthefuelsusedinthisstudywerepreparedby reduc-
tionoftheproperalkylchlorosilsnewithlithiumaluminumhydridein
anhydTousethyletherordi.oxaneasdescribedinref=ence1. Original
puritywasestimatedtobe 95percentorgreater.
Procedure.- A diagramofthea~eratususedfordeterminingthe
spontaneousexplosioulimitsof thealkylsilanesi showninfigure1.
Fuelanddryairwereedmittedatthepositionsindicatedandmixedin
l
a l-literstoragebulbbymeansof a magneticstirrer.Thepressureof
—
themixtureinthestoragebulbwassuchthat,whenopenedtotheheated - A
reactionvessel,theresultingpressurewas1 atmosphere.H an explosive
reactionoccurredbeforethepressureinthereactionvesselreached1
atmosphere,or ifno exp10siontookplaceat 1 atmosphere,thereaction
vesselwasevacuatedandthetemperatured creasedor increasedaccord-
inglyl Thetemperatureofthepyrexreactionvessel,~~inchesindiam-
eterand1($inchesin length,wasmeasuredusinga thermocoupleattached
to theoutside”ofthevessel.
Admissionof themixturetothereactionvesselwasusuallymadeas
rapidlyaspossi.blealthough,asdescribedlater,themixturewassome-
timesadmittedveryslowlytodeterminethepresenceof coolflemes.
Occurrenceof snexplosionwasindicatdby a surgeofthemsrcury
inthemanometer,endoccasionallya flemewasseentotravelupthetube
leadi@fromthereactionvessel.AlthoughSUChflamesusuallydidnot
travelveryfar,theydidon severaloccasionsflashbackintothemixing
bulbandcausea seriousexplosion.Consequently,a rollofwiregauze
wasplacedbetweenthemixingbulbandthereactionvesselto serveasa
flametrap.Afterthisflamearresterwasinstalled,nofurthertrouble
wasencounteredfromflashback.
NACATN 3405 3
.
.
Theminimumtemperatureforexplosionwasdeterminedwithin15°C for
variousfuel-airmixturesofeachgivenfuel. Theseminimumtemperatures
delineatetheregionofexplosionandnonexplosionwithrespectotemper-
atureandcomposition.
RESULTSANDDISCUSSION
TheexplosionMmit curvesfortheninefuelsstudiedarepresented
infigure2, andsresummerizedinfigure3. In thisinvestigation,two
distinctlydifferenttypesofexplosionlimitwereencountered.The
firsttype,typicalofthosereportedforhydrocarbonsata pressureof
1 atmosphere,wasfoundfortetramethyl-,trimethyl-,enddiethylsilane.
In thecaseofthesethreefuel.s,thefuel-airmixturewasadmitted
rapidlyb theevacuatedreactionvesselto a pressureof 1 atmosphere,
@ ata sufficientlyhightemperatureem explosionoccurred.The-
explosionusuallyoccurredwithin30 secondstiteradmission.If no
rapidreactiontookplaceinthislengthof time,onlya veryslowin-
creaseinthepressurewasobservedandno explosionwaslaterdetected
eventitertimeintervalsof 1 hour.
Thetemp@?ature-compositioncurvesdelineatingthespontaneousex-
plosiveregionfortetrsmetbyl-,trimethyl-~smddtethylsi.laneov ra
fuel-concentrationrengefromabout2 to 10percentareshowninfigures
2(a)and(c). Determinationsweremileofboththetemp=atureatwhich
explosionoccurredin 15 secondsor lessandthetemperatureb lowwhich
no explosivereactionoccurred.As showninfigures2(a)and(c),these
twotemperaturesdifferby lessthan15°C; snd,consequently,themin-
imumexplosiontemperatureundertheconditionsof thetestlieswithin“
this15°C temperatureange.
A secondtypeofexplosionlimitwasencounteredfordimethyl-,
methyl-,ethyl-,n-butyl-jisobutyl-,amlvinylsilane.Thetemperature-
compositioncurve=forthesecompcundscouldnotbe obtainedby the
techniquejustdescribed.Thete~eraturesbelowwhichno ~losion
occurred,showninfigures2(b)and(c),couldbe determinedunambiguously.
Abovethesetemperatures,however,theobservedphenomenadependedupon
therateof admisstonof thefuel-airmlxtweto thereactionvessel.
Inthosecaseswherethemixfxrewasintroducedrapidly,snexplosion
tookplacebeforethepressureinthereactiontubereachedatmospheric
pressure.Mixtureswerethereforeadmittedveryslowlyto observethe
pressureatwhichreactionwasfirstindicated.By thisprocedureit
wasobservedthata seriesofminorexplosions,probablycoolflames$
occurredovera pressurerangeof50 to 100millimetersofmercury.
Thesewererecordedas explosions.Afterthispressurerangewasex-
ceeded,nofurthereactiontookplacewhenthemixturewasslowly
admittedto a pressureof 1 atmosphereattemperaturesjustabovethe
none~losiontemperature.
4 NACATN 3405
Sincea veryvigorousreactiontookylacewhenthemixturewas
rapidly-admitted,itisbelievedthatthecool-flsmer actionoccurring
at a pressureof50to 3.00millimeterssetoffan explosivereaction
whenadditionalfuel-airmixturewassuppliedat a sufficientlyrapid
rate. Thetaperature-compositionexplosioncurveformethyl-,dimethyl-,
ethyl-,~-butyl-,andisobutyl-,endvinylsilane(figs.2(b)and(c))
actuallyrepresenttheappe=anceof coolflames.Thecoolflamesare
capableof causinganexplosionifadditionalfuel-airmixtureisimme-
diatelyavailabletoreact.If coolflmesdidno$occur,theexplosion
te~ratureforthemixturesataninitialpressureof1 atmospherewould
be expectedtobehigherthanthetemperaturesrecordedinfigures2(b)
and(c)fortheoccurrenceofcoolflames.
d
Forrichmtitures,theoexplo~iontemp=aturesof ethyl-,~-butyl-,
andisobutylsilanewere215, 185, and200°C,respectively.Sincethe
rangeM experimentalerrormaybe asgreatas*20°C,theexplosion
limitsof thesethreefuelscanyrohablybe consideredthessme.This
representsanincreaseinexplosiontemp~atureofabout80°overmethyl-
silane(seefig.3). Sincemonosilme.(EdH4)is spontaneouslyflsmmable
atroomtaperature(ref.2),theadditionof onemethylgroqpinmethyl-
silaneappreciablyreducesthereactivity.Itappearsthattheaddition
of a sidechainof twocarbonatoms(C##iH3)reducesthereactivityof
themoleculeas comparedwitha sidechainofonecarbonatom(CH3SiH2).
Howeverfurtherincreasesinthelengthofthesidechaintofox carbon
tatoms,butylsilanes)producesonlya verysmallchangeintiereactivity.
Ifthesidechainisunsaturatedasinvinylsilsme(~C=CH-SiH3),the *
reactivityappesrsgreaterthsn foreith= ethyl-orraethylsilane.
“Vinylsileneis theonlyco~oundwitha positiveheatofformationamong -
thesilanestudied(ref.1). .
In figure 4 are presentedtietemperature-composition~losion
curvesforseveralhydrocarboncompoundsasgiveninreferences3 to 6.
Thespontaneousexplosionlimitcurvesforthehydrocarbonsfellbetween
500°slld6~0 C. Thetemperature-compositionexplosioncurvesforthe
allcylsilsnessxesimilsrtothoseofthehydrocarbons,butoccurat
appreciablyowertemperatures.As showninfigure4, theeqlosion
curvesof thehydrocarbonsshowlittlevariationwithchangesinfuel
structure.A msrkedseparationi explosiontemperatureswasobserved
amongmenibersofthealkylsil~eseries.As showninfigure3,thefive
fuelspossessingthreeSi-Hbondshavethelowestexplosiontemperature.
As thenumberofSi-Hbondsisreduced,thetemperaturequiredforan
explosivereactionis increased.FortetramethylsilanewherenoSi-H
bondssrepresent,theexplosiontemperatureapproachesthatofthe
hytcrocsrbonsl
Sincetheexplosiontemperatureislowerfortheslkylsilanesthan ‘
forthehydrocarbonssndsinceit decreasesashydrogenatomsaresub-
stitutedformethylgroups,it a~eem veryprobablethattheSi-Hbond .
NACATN 3405 5
ismorereadilysubJectoo~dationthantheC-Ebond. Thisconclusion
d mightalsohavebeenreachedby consideringthetheoreticalbond-strength
calculationsofreference7,whichshowthestrengthoftheSi-Hbondto
be ~reciabl.yweakerthanthatof theC-Hbond.
Inreference8, itisproposedthatoxidationreactionsofthe
hydrocarbonsproceedby freeradicals.Topermittheformationofthese
radicals,thefiratstepintheoxidationmustinvolvethevulnerability
ofthemuleculetobond-breakingreactions.ItwasTroposedinreference
8 thatrdicalssrefirstformedbyrenmvalof a hydrogenatimframthe
holecule,whichisthensubjecto attackby additionof02 tothefree
carbonvalence.A generalov=-allschemestartingwitha freeradical
R canthereforebepresentedasfollows(ref.9):
o~ RH
R* ROO+ROOH+R (1)
Thefactthat thealkylsilmesoxidizemorereadilythanthehydrocarbons,
andthathydrogenatomscanbe removedmorereadily fromthe silsnesj
wouldlendsupportotheinitialstepproposedfcmthehydrocarbonx-
idation,nsmely,theremovalof a hydrogenatom.Ifthesil-ane-~tion
thenfollowsthemchanismofthehydrocarbonxidation,theschemepre-
sentedin equation(1}mightalsobe adopted.
Inregsrdto safehandling,vaporphasemixturesofthemostre-
activefuelstudi&dsrenonexplosiveb lowabout90°C; ccnsequently~
eventhoughtheseresultstri”btlya plyonlyto theconditionsand
apparatusgeometryused,whenhandledatroomtemperature(60°or 70°C
lowerth= thelowestexplosiontemperature)theprobabilityof spon-
taneousexplosionfromtiesefuelsisextremelysmall.
SUMMARYOFREmLTs
Inen investigationoftheexplosionlimitsofninesilanesasa
functionof temperatureandconcentrationat a pressureof 1 atmo~here,
thefollowingresultswereobtained:
1.Overa fuelmncen’trationrsngeof2 to9 percentby volumein
dryair,thefuelsstudiedwerenonewlosivebelowthefollowingtempera-
tures:tetrszAhylsi.lane(CH3)@, 45@ j trimethylsilane(CH3)3SiH,~0°;
diethylsilane(C!2H5)#iJ12,255°;dimethylsilme<CH~&H , 220°;ethyl-8Silane(C2H5)SiH3,215°j isobutylsilane(i-C4H9)SiH3,200j n-btiylsilane
(~-C4~) SiH3, 185°; methylsil=e (cH3)Sfi~, 125°;andvinyls~lane
H2WH-SIH3,90°C. Thelargevariationinexplosiontemperaturesamong
.
thesilanesindicatedthat,substitutinghydrogenatmm forthemethyl
groupsattachedtothesiliconatomdecidedlytncreasedtheirreactivity.
*
6 NACATN 3405
2.Althoughthereactivityofthe“silanesstudieda~earedgreat=
thanforanalogousQdracarbonsjwhichhadspontaneousexplosiontemper-
aturesbetween500°aud6000C,itappearsveryprobablethateventhe
vaporsofthemostreactivefuelstudiedcanbe stielyhandledin drysir
atroomtemperature.
3.Sincetheexplosiontemperaturewaslowerforthealkylsilanes
thanforthehydrocarbonsandsinceitdecreasedashydrogenatomswere
substitutedformethylgroups,theSi-Hbondisprobablymoresusceptible
tooxidationthantheC-Hbond.
LewisFlightPropulsionLaboratory
NationalAdvisoryCommitteeforAeronautics
Cleveland,Ohio,December17,1954
REFERENCES
1. Tannaibaum,Stsnley,Kaye,Ssmuel,andLeweti,GeorgeF.: Synthesis
audPropertiesofSomeAlkylsilanes.Jour.Am.Chem.Sot.,vol.
75,no.15,Aug.5, 1953,pp.3753-3757.
—.
2.Rockow,EugeneG.: ChemistryoftheSilicones.Seconded.,John
Wiley& SonsInc.,1951,p. 5.
<
3.lbwnend,DonaldT.A.,@ M@leker, M. R.: TheInfluenceofPressure
ontheSpont=eousIgnitionofInflammableGas-AirI&-xtures.11 -
Pentane-AirMixtures.I%oc.Roy.Soc.(London),ser.A, vol.143, .
no.A848,Dec.4, 1.934,pp.168-176.
4.To~m, D. T.A.,Cohen,L.L..,- M=dlti} M.R.: TheWluence
ofPressureontheSpontaneousIgnitionof I@lsmmableGas-Air
Mixtures.111- Hexane-andIsobutame-AirMixtures.Proc.ROY.
Sot.(London),ser.A, vol.146,no.A856~Aug.1~1934,pp.11-3-~9c
5. Townend,D. T.A.,andChamberlain%El A. Cl: The~~uence ofpres-
sureontheSpontaneousIgnitionofInflammableGas-AirMixtures.
rv - Methane-,Ethane-,andPropsne-AirMixtures.-c. Roy.Soc.
(London),ser.A, vO1.154,no.A881,~. 3, 1936,PP.95-112l
6.Kane,G.P.,amdTownend,D. ‘1.A.: TheInfluence of pressureon the
SpontaneousI@tion ofInfMnmuibleGas-AirMixtures- TheSimpler
Olefins. Proc.ROY*Soc.(Lotion),ser.A, vol.160$no.A901,
May18,1937,pp.174-187.
7.Pauling,Linus:TheNatureoftheChemicalBond. Seconded.,
CornellUniv.Press,1940,p. 53.
.
u
NACATN 3405 7
8.Walsh,A.D.: ProcessesintheOxidationofHydrocarbonFuels.I -
d ThePointofOxygenAttack.~aTiS.Fsmdw SOC.,vol.42,no.286,
Msr.+pr. 1946,pp.269-283.(Seealsopt.11,vol.43,2.947,
pp.297-33.3.}
9. Lewis,Bernard,ardvonIXbe.Guenther:Ccmibustion,Flamesand
E@losionofGases.AcademicPress,Inc.(NewYork),1951.
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Temperature-composition limits of spontaneous explosion for nine alkylsilanes with air at atmospheric pressure

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