Lower Flammability Limits
of Methvlchlorosilanes J
E. K . BALIS AND H. -4.L1EBHA4FSKY General Electric Company, Schenectady, J.Y. The lover limit measured for one mixture of these silanes agreed with the ~ a l u calculated e by the reciprocal additivi t y rule of Le Chgtelier. The presence of silicon tetrachloride appears to raise the lower limit of dimethyldichlorosilane; 110 effect of htdrogen chloride was obserFed in two tests. Ier) finely dirided cilica seems to be produced in the combustion of the chlorosilanes.
T h e increasing prominence of the chlorosilanes in the ailicone industry makes it advisable to measure their lower flammability limits in air. The properties of these materials complicate such measurements. The values 3.4 = 0.1% and i . 6 * 0.3% were obtained on a modified Bureau of AIines apparatus for the lower limits of, respectkely, dimethyldichloro- and meth?ltrichlorosilane in air.
CONDESSABILITY. DL)s boils near 'io" C.; XTS, near 66". HE methylchlorodanes are important intermediates in Khen air is admitted t o vertical tubes containing their vapors, the silicone industry. Since these volatile compounds concondensation by compression is likely to occur, especially if the tain oxidizable carbon, hydrogen, and silicon, their vapors might be expected T O form explosive mixtures with air. In order tube is narrow or contains only chlorosilanes near their saturat o eztablish the maximum safe concentration in certain of these tion pressures, and if air is admitted suddenly from the top in a mixtures, l o w r flammability limits (more precisely, loner limits stream directed down the axis of the tube. of self-propagation of flames, 8) have been determined. The corLon- DIFFUBIYITY. rl precise determination of flammability reqponding upper limits probably exceed rhe vapor pressures of limits presupposes thorough mixing; for example, the MTS cont h e chlorosilanes at room temperature, vhich makes these centration in an 8% mixture of that substance must not differ at limits of less immediate practical interest. any point from this over-all ralue by more than 0.1%. The The precise determination of fianimability limits is not simple, molecular neight of MTS is 148.5, and its diffusivity is conseas extensive work done at the E. S. Bureau of Mines (-0) and elsequently l o r . Condensation of 3ITS is therefore particularly where n-ill testify. The only conservative course is to seek the serious; experiments have s h o m that uniform composition is lowest poksihle lower limit, which the apparatus evolved at the not attained for many hours, once condensation has occurred. bureau is deaigned to yield. Because of difficulties peculiar to SOLIDREACTIOX PRODTXTS. The white deposit formed on the the chlorosilnnes, this apparatus had to be modified to give satwalls of a tube in xhich chlorosilanes are burned must be reisfactoi,y re-ults. Determinations on the modified apparatus moved after each experiment. (Scrubbing with a toluene-satuWIT time consuming, and it proved adGable to dr+gi a simpler rated cloth is efYectire.1 Even under the best conditions, a setup for exploratory \vork. irhite film eventually forms in the vacuum system t o which the T h e dimeth~lclicl~loro~ilaiie (DDS) and nietli~lt~ichlorosi~ane explosion tube is connected; satisfactory glass blowing is then (.\ITS) were of good quality. The silicon tetrachloride was difficulr or impossible. ?he middle fraction obtained hy di;?illing the c.p. material xhen needed. EXPLORATORY WORK The .toicliiometry of the reacrtiom betn-eeri the clilurodanes ttnd oxygen was not studied. In all expcrinients the walls of the Csperinients on DDS-air mixtures in liter hulhs with a 0.3-cm, reacrion clianiber ~ v w ecovered Ivith a nhite deposit. The abspark gap between tungsten electrodes sho\ved that flame propaFence of discolorations in these deposit; indicates that combusgation occurred xith 3.27, DDS h u t not n-ith 3.0';. Two violion to silica alKays occurred, The folloiving reactions, though lent explusions dictated a change of app;rratus. not proved, are reasonable: Exploratory work was continued nitii portable explosion ves~ e l s (Figure l) ; they were disconnected from the vac.uum system after filling, n'rapped in a towel, and renioved to a place n.hf,re ignition could be safely attempted Sone of these v~:ssrIs For M T R . CHaSIC'l, i. 2 0 2 = BO? -t CO? 3HC1
T
+
I n all exceot one or t\ro orellminarv experiments, enough oxygen \?-asinitially present for the complete combustion of the chlorosilanes according to these equations. The following properties of the chlorosilanes make it unusually difficult to measure the lower flammability limits of their mixtures with air: REACTIVITY. The chlorosilanes react, readily with rvater; other materials, such as paper and rubber, deteriorate in their presence. Stopcocks, lubricated or not, are liable t o freeze if their bearing surfaces are exposed to chlorosilane vapors. I
_
5"
RUBBER STOPPER
SCALE-I INCW
Figure 1. Portable qpparatus
583
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
584
I
OF TCBE(FIGI-RE 2) WITH Iixows DDSTABLE I. CHARGISG MTS-AIRMIXTURE Operation KO. 1 2*
Open Stopcocks
30
1,7 2,7 3,i
4 5b
1. 2 , 3 , 7 1,2 , 3
Closed Stopcocks 2 , 3 , 4 ,5 , 6 1 , 3 , 4 , 5 ,6 1, 2, 4, 5 , 6 4, 5 , 6 4,5,6,7
6
1,2,3,7
4, 3, A
73
1,3
2, -1. 5. 6 , 7
9
9
1. ?.3,-1, 3 , 1 , 3 , 4 , .i6, .
10
1, 2
3,4,6,6,7
11
I
12 13
1. 5
1 , 2 , 3 ,4 . 5 , 6 1, 2 , 3 , 4 , 6, 7 2,3, 4,6,7
MANOMETER
S
7
EE DETAILOF I T T O M OF TUB1
14
I
Vol. 38, No. 6
Operation I'ump out Pump out Pump out Finish pumping down Determine m,n. leak rate over
2
,
j
Pump down agairi, read manometer (equals b:,,rumerric preasurcj Remove cooling mixture iron, DDY a n d >\ITS reservoirs: wnrm >ITS reservoir to desired pressure: apply leak correction from operation 3 : .\ITS now in tube Puiup d o v n K n r m DDI: reservoir to a p r e s u r e greater than JITS pressure In tube Admit DDS to tube to desired pressure; apply, leak correction from operation 5 Pump down Admit dry air t o system Adjust stopcock 1 so t h a t mercury in manometer rises 1 cm.: readjust to maintain this pressure difference until total pressure in erplolion tuhe approaches atmospheric: open stopcock uide t o obtain atmospheric pressure in tube Close stopcock 1 and admit dry air, to, DDS and NTS reServoIrS in turn
0 Liquids i n reservoirs are permitted to hoil for purging.: they are then cooled to solid COracetone temperatures BS evacuarion is continued. b Highest leak rate observed was 0.04 cm. of Hg per minute (Table 11. KO.i B ) ; in the other tests of Tables I1 a n d I11 the leaks were negiigible.
TGROUND
GLASS PLATE
DETAIL OF BOTTOM OF TUBE
,/I
MIL ALUMINUM FOIL ,GROUND JOINT
Figure 2.
tus as changed gave good results with hydrogen but not with the chlorosilanee. Some of the difficulties iwre: ( u ) condensation invariably occurred when air was admitted, ( b ) t,hc presrnce of hydrolysis products around the mouth of the tube slioived that chlorosilancs began to pour out as the plate was removed prior to attempts a t ignition, and ( c ) chlorosilanes were prohably lost to the mercury in the pump. Further changes in the apparatui: and method of ignition were made t o meet these dificulties.
Diagram of nlodified Bureau of >lines 4pparatus
broke, the rubber stopper being ejected when ignition t i i l l occur: only mistures near the flammability limit, rvere tried. The first portable vessels were short-necked glass bulbs. Ignition occurred in these bulbs for 4.0% DDS at diameters of 2, 3. and 5 inches, which indicates that the lower limit for DDS can~iot be very sensitive to the diameter above 2 inches. Bfter tests n5th longer tubes, the apparatus of Figure 1 was finally adopted. The longer tubes gave erratic result>. Ii ignition was attempted immediately upon removal from the vacuum systcm, mising was incomplcte: during a Rait for completc mixing, the chloroiilane reacted with the stopper. (I'rotccting t l u . lat,ter with metal foil was incffective.) Thc tube was accordingly shortened to expedite misiug: at a length of 5 iiiches, tests oil DDS shoved thc mixing with air to be complete during the charging of the tube. The laxer limits obtained on the final apparatw were: DDS 3.4% and l l T 8 10%. The former value ir sati-factory, but the latter is too high, which is not surprising since MTS makes the greater demands upon tlic npparatus. MODIFIED BUREAU OF 1 I I I E S APPARATUS
Attempts were made to use a setup devised by the Bureau of Mines ( 8 ) ; since the method of ignition proved unsuitable for the chlorosilanes, electrical ignition was provided. The appara-
Figure 3.
Electron Micrograph of Siliceous Combustion Product
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1946
TABLE 11. LOWERFLAMMABILITY LIMITSFOR CHLOROSILAKES IX A I R
7 by Chlorosilane
XIethyltrichloro(.\ITP)b
so.
1B 2B 3B 4B 5B
6B
7BC 8B
9Bd 10B 11B
l l i x t . of DDS-
.\ITS6
IC
2C
3c
1C
5C
1-01,
Hr. of F l a m e P r o p a Standing gation
7.3
16 G 16
5.0
3
9.3 8.6 7.6
i.9 7.8 i.8 7.: 8 , i.8 4.15 3 88 3.99 3 97 3.84
Pes Tes ?io Y es s o
Pea 40
s o NO
IC)
NO
0
Yes Yes
6
16 6 16 6 22
Yea NO
Yes Yes NO
Lower flammability limit, 3.4 =t0.1% DDS. t h e mean of 3.46y0 ( S o . 5.4) a n d 3.31% (No. 4A). b Lower flammability l i m i t , 7.6 * 0 . 3 % .\ITS ( a r b i t r a r y choice). C Loss of >ITS probable: mercury a d d e d t o give 5 cm. head n e a r m o u t h of tube. d Leak observed; p l a t e reground a f t e r experiment. .\Iole fractions, DDS 0.719 a n d .\ITS 0.281; 6 Chlorine, 60,y0by weight. lower f l a m m a b i l t y l i m i t , 3 . 9 % mixture.
585
The value in Table I1 (3.47,) for the lower flammability limit of D D S is identical with t h a t previously chosen from tests in the portable apparatui, and slightly higher than that (3.lYc)obtained n-ith the 1-liter bulb. I t appears t o be reliable. T h e lower limit for ;\ITS is somexhnt less certain, partly bccause leaks occurred even after precautions had been taken. I n this v o r k a white ring formed a t the periphery of t h e contact area bctn-een tube and glass plate (Figure 2 1 , and this ring hecame progres.sively heavier until the plate had t o be reground after run 9B (Table 11) because there l i d been a leak. The ring contained a chlorosilane hydrolysis product ; calomel waz absent. ( K i t h DDS, presumably because the concentration3 n-ere loFer, the ring rras very faint and leakage negligib1e.i Severtheless, the value 7.6 * 0.3% seems t o be sufficiently reliable for practical purposes; the higher result (10%) from the portable apparatus is untrustworthy. According to the reciprocal additivity relationship first assumed for flammability limits by Le Chhtelier ( I ) , the value for the methylchloroiilane mixture of Table I1 should be
0
T h e modified apparatus finally developed (Figure 2) differs a s follows from t h e original: h i r is admitted from the side a t the bottom, instead of from the center a t the t o p ; this change eliminated condensation. Complete mixing is obtained by waiting a minimum of 5 hours instead of with a mercury pump. A 1-mil aluminum foil held between phosphor bronze split rings is added to eliminate the loss of chlorosilanes from the tube prior t o ignition (Figure 2). The ignition source is a fine oxyhydrogen flame t,hat cuts instantly through the foil, a wooden support, the vessel of mercury, and the glass plate having been removed. T h e steps required t o fill the tube for a run are given in Table I. Pure materials were used in all tests, and concentrations were calculated directly from the pressures measured after the admission of each component to the tube. After 5 hours or longer, ignition of t h e mixture was attempted in a darkened room t o see whether flame propagation occurs. T h e tube was then dismounted, swabbed out with toluene, and set u p for t h e next run. The substances added t o D D S (Table 111) were introduced as follom: Silicon tetrachloride from a reservoir, as though a DDS)ITS mixture were being made, and hydrogen chloride from a gas cylinder through stopcock 6.
1
(0.719/3.470)
RESULTS AXD DISCUSSION
a,,
= 4.0%
where the denominator contains t h e mole fraction of each silane divided by the corresponding lower limit; these mole fractions characterize t h e mixture containing 607, chlorine by weight. T h e actual result (3.9%) from Table I1 proves t h a t this relationship holds for the present case. Also, the last four runs in the table show t h a t it was possible t o attain considerable precision in norking with these reactive materials. It seemed plausible t h a t silicon tetrachloride would react with the reactive intermediates involved in t h e propagation of chlorosilane flames, which suggested t h a t a small amount of this substance might significantly raise t h e loiver flammability limit. Accordingly, five runs were carried out with silicon tetrachloride a s added substance (Table 111). T h e results d o indicate a deactivating effect of t h e kind sought, but a t silicon tetrachloride concentrations too high t o be of much practical interest. T h e two experiments with hydrogen chloride serve as a blank, for this substance is less likely than silicon tetrachloride t o react with the intermediates in question
TABLE 111. EFFECTOF ADDED LIMITS O F
SUBSTANCES ON FL.4lIlI.%BILITY
DI~IETHYLDICHLOROSILANE % h? volume
Added
Substance
D a t a for various chlorosilane-air mixtures are given in Table 11. When flame propagation occurs in a darkened room, a clazd i n g white hemisphere, equator d o ~ n n - a r d ,moves up the tube and reaches the top in several seconds. Incandescent silicalike particles appear t o be responsible for the dazzling nhiteneq.s. -1s the flame progreases, the walls in the burned zone become covered with a white deposit. Large white floc;. of silicalike material float dornn-ard for about a minute after the flame has traveled the full length of the tube. T h e flocculent material produced by the combustion of D D S m-as examined in the electron microscope (Figure 3). The material appears t o be a loose three-dimensional netivork of very fine, adhering particles. T h e average diameter of these particles, less than 100 is many times smaller than t h a t attained by grinding silica. I n mixtures just below the flammability limit, combustion without flame propagation may occur. K h e n this happens, only t h e lower portion of the tube is ivhitened, a reliable indication t h a t the lower limit is near.
+ (0.281/7.67,)
Sic14
HC1
Run So.
DDS
1 2 3
3 61
3.61 3,59
i
3 59 3 en
1
3.39 3.58
9
Added Substance
4.01 1.29 0 71 0 19 0 39 0 68 3.90
Time of Sranding, Hours lfi 16 16
piame
Propagation NO N O X O
1b
Pee
1I;
6
Pes Yes
1R
Yes
ACKNOTLEDGJIENT
T h e authors \$-ish t o thank ST. J. Scheiber and C. A. Burkhartl for supplying the pure compounds, and David Harker for tilo clectron microscope work on the siliceous conihwtion proilucr. LITERATURE CITED
(1) Cowaid, H.F., and Jones. G. W., U.
S. Bur. Mines, Bull.
279, 5-10 (1938). (2) Jones, G. W., Kennedy, R. E., and Thomas, G. J., U. S. Bur. ;\lines, R e p t . Investigation 3589, esp. Fig. 1 (1941). (3) Lewis, B., and Elbe, G. von, “Combustions, Flames and Explosions of Gases”, p. 141, Cambridge Cniv. Press, 1938.
