Explosive Properties of Methyl FormateAir Mixtures Effect of Molecular Constitution on Behavior G. W. JONES,W. E. MILLER,AND HENRYSEAMAN, Bureau of Mines Experiment Station, Pittsburgh, Pa.
T
HE Bureau of Mines since its organization has sought
to become more intimately acquainted with the phenomena of explosions and the behavior of explosive substances. It was early recognized that reactions in the gaseous phase were of greater simplicity and more under observational control than reactions in the liquid or solid phase, since gases have a much greater simplicity than solid or liquid explosives, especially compounded explosives such as dynamites. Attention has therefore first been given t o the development of apparatus for the determination of the limits of inflammability of combustible gases and vapors, the pressures developed when these mixtures are ignited, and their ignition and flame temperatures, ,Om"lali"g
pYmp
This consisted of a metal bomb 4 inches in internal diameter and 38 inches long, and having a capacity of 8.05 liters (8.5 quarts). The bomb was mounted upright on a heavy metal base. The inflammable mixtures were ignited at the spark gap, k , and the flame was propagated vertically upward. Observation of the progress of the flame was made through the three glass windows f, g, and h. Records of the pressures develo ed were obtained on the Bureau of Mines manometer, j. T i e bomb was designed and used in an investigation to determine the explosive properties of acetone-air mixtures (8),to which reference should be made for detail of o eration and methods used for the preparation and analysis o r the vapor-air mixtures. This apparatus gives slightly wider limits than the 2-inch glass tube apparatus previously used, in which ignitions were made at the open end. Table I gives the results obtained on the limits of inflammability, pressures developed, and average speed of flames of methyl formate-air mixtures. The mixtures were roughly dried by passing through Dehydrite before introduction into the bomb. The lower inflammable limit was 5.08 per cent by volume, the upper limit, 22.7 per cent.
PRESSURES DEVELOPED
A
i
s
,
FIGURE 1. DIAGRAM OF APPARATUS
The determination of the explosive properties of methyl formate-air mixtures by these methods was undertaken because this material is now commercially available and may be rather extensively used industrially in the future, and also because its molecular formula, CZH402,may be written in the form CH4C02,thus having a composition the same as methane plus one molecule of carbon dioxide. If the internal structure of a molecule has no effect on the limits of inflammability, methyl formate should have approximately the same limits as a methanecarbon dioxide mixture containing each gas in equivalent amounts. LIMITSOF INFLAMMABILITY OF METHYLFORMATE-AIR MIXTURES The limits of inflammability of methyl formate-air mixtures were determined in the apparatus shown in Figure 1.
The pressures developed by methyl formate-air mixtures are given in Table I and shown graphically in Figure 2. The results show that, starting with the lower inflammable limit, the pressures developed increase quite uniformly as the concentration of methyl formate is increased until a maximum is reached a t about 10.5 per cent. As the concentration of methyl formate is further increased, the pressures become lower, gradually a t first and then with a rather abrupt drop a t about 16 per cent. From this point on, the pressures again decrease more slowly until the upper limit is reached. The maximum pressures developed in the bomb equaled 106 pounds per square inch above atmospheric, and all mixtures which propagated flame gave pressures of 35 pounds or higher. The rather abrupt drop in the pressures developed when the concentration of methyl formate increased from 16 to 17 per cent appears to be due to a distinct change in the mechanism of combustion within this range. An abrupt drop in pressure does not occur for combustibles such as methane. The results of similar tests made on methane-air mixtures are also shown in Figure 2 for comparison. Both methane and methyl formate require the same amount of air for complete combustion, and this ratio is shown by the vertical line in Figure 2. The maximum pressures produced by both combustibles occur when the oxygen present is insufficient for complete combustion. I n the case of methane the maximum is produced when there is about 9.8 per cent methane present, whereas for methyl formate the percentage is about 10.5. The pressures produced by methyl formate-air mixtures are much higher than those produced by methane-air mixtures.
SPEEDOF FLAME Numerous investigations have been made by m e e l e r , Coward, Payman, Bone, and others on the speed of uniform flame movement of various gases and vapors. If an inflammable mixture is enclosed in t-i tube of uniform bore, and the mixture ignited a t the open end, flame will initially propagate down the tube towards the closed end at a uniform rate 694
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INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLEI. PRESSURES DEVELOPED AND AVERAGESPEEDOF FLAME FOR METHYLFORMATE-AIR MIXTURES [Initial temperature, 23' t o 25.7' C. (73.4' t o 77.13' F.); pressure, 738 t o 748 mm. mercury] P R E S ~ U R E A v . FLAME TEST CHaCOOH PROPAQATED DEVELOPED SPEED % by v o l . L b . / s q . in. Ft /wc. 4 4.76 NO 5 4.92 NO 47 4.94 No 53 5.00 NO 50 5.07 No ... 52 5.08 Yes 35.0 2.26 51 5.12 Yes 35.0 2.00 49 5.23 Yes 37.0 2.00 6 5.43 Yes 43.0 2.76 3 5.65 Yes 45.0 2.79 7 6.25 Yes 48.0 3.11 7.14 62.0 8 Yes 5.70 8.12 73.0 10.25 9 Yes 10 88.0 20.15 9.08 Yes 11 Yes 21.70 9.44 102.0 12 9.82 Yes 103.0 20.80 10.51 13 Yes 106.0 23.60 11.18 14 Yes 21.70 106.0 15 11.85 Yes 22.45 101.0 13.32 16 Yes 15.60 97.0 17 14.09 Yes 16.00 97.0 18 14.68 Yes 12.75 89.0 19 15.78 82.0 Yes 8.55 20 16.43 Yes 64.0 4.05 21 16.64 Yes 61.0 3.80 22 17.29 Yes 56.0 2.85 54 17.29 Yes 57.0 2.65 55 17.66 52.0 Yes 2.25 23 17.86 Yes 52.0 2.43 24 18.46 Yes 53.0 2.45 25 18.83 Yes 53.0 2.36 30 20.12 Yes 52.0 2.32 27 20.14 Yes 51.0 2.31 43 20.88 Yes 46.0 2.47 28 21.00 Yes 49.0 2.36 29 21.33 Yes 46.0 2.37 44 21.58 Yes 42.0 2.42 34 21.65 Yes 45.0 2.48 45 21.70 Yes 43.0 2.52 38 21.73 Yes 42.0 2.23 32 22 00 Yes 42 0 2.38 41 22.46 Yes 42 0 2.45 36 22 72 Y e8 40.0 2.07 42 22.72 No 39 23.29 NO ...
... ... ... ...
.. ...
...
of speed. After a time, depending on a number of factors, vibrations may be set up, and the flame speed may increase rapidly and then slow down as it nears the closed end of the tube. If the uniform flame speeds obtained by this method are plotted with reference to concentration of a given combustible present, the values obtained increase quite uniformly from both the lower and upper inflammable limits, and reach a maximum when the combustible is present in proportions slightly on the rich side for complete combustion. I n a closed apparatus the flame speeds may accelerate from the moment of ignition, and a t definite concentrations there may be an abrupt change in the speed. A knowledge of the concentrations which show these abrupt changes is very important in the safe handling of explosive mixtures. It has been shown that in a cylinder of sufficient length (b), when a mixture i s ignited a t one end of the cylinder, flame will reach the other 1
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end of the cylinder a t approximately the same time that the maximum pressure is developed; therefore, knowing the distance the flame travels from the point of ignition to the end of the bomb, and having the time-pressure records obtained on the manometer, the average rate of flame speed through the bomb can be determined. The values thus obtained are not exact constants but are useful in giving the relative flame speeds of different inflammable mixtures when tested under the same conditions. The speeds as obtained by this method
1
1
6
l
FIGURE 3.
l 8
l
l
10
l
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12
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1 14
l
l
16
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1.8
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METHYL FORMATE IN AIR, PER CENT BY VOLUME
S P E E D OF
FLAME PROPAGATION
IN
l
20
l
l
2 3 '
BOMB
are shown in Figure 3. When the concentration of methyl formate exceeds about 7 per cent, the speed increases rapidly and reaches a maximum a t about 10.5 per cent. It then drops rapidly as 16.5 per cent is approached. Also, all mixtures containing methyl formate varying from S.5 to 16.0 per cent gave sharp "pings" when exploded in the bomb. Tests which have been made on different gases and vapors in the bomb have shown that, for rating the hazardous nature of different combustibles, the flame speeds obtained by the method outlined give a better comparison of the conditions that take place in a closed system than values given by the speeds of uniform flame movement. The data obtained on methyl formate-air mixtures and a few other combustible gases and vapors showing the limits of inflammability, pressures developed, speed of flame, and ignition temperatures are given in Table 11. The ignition temperatures of the different combustibles in air were determined by the authors in an apparatus similar to that used by Mason and Wheeler (3). It consisted of a transparent quartz bulb of SS cc. capacity, mounted in an electric furnace in which the temperature could be accurately controlled. Coward, Jones, Dunkle, and Hess (1) give a detailed description of the apparatus used and method of conducting the tests. Since the ignition temperature of combustibles varies with the concentration of the combustible present, the time the mixture is in contact with the heated surface (or the lag), the initial pressure of the mixture, the concentration of the oxygen present, the material of which the apparatus is constructed, and other variables, it is apparent that data given for the ignition temperature are of little value for comparing different combustibles unless all tests are made under the same relative conditions. The data given in Table I1 were made with the same apparatus under the same test conditions. The values given in the table are the lowest values found for all concentrations tested and having the longest lag.
EXPLOSIVE PROPERTIES OF METHYL FORMATE-AIR MIXTURES COMRUSTIBLE. PER CENT BY V O L W
FIGURE 2. PRESSURES PRODUCED IN CYLINDRICAL BOMB
Methyl formate has a wider range of inflammability than acetone or methane, but a narrower one than manufactured gas. The maximum pressures developed are greater than
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TABLE11. DATAON COMBUSTIBLE GASES LINITSOF
MAX.PRESEURE COMBUSTIBLE A T MAX.-4V. Upper DEVELOPED Max. PRESSURE FLAME SPEED IQXITION TEXP Per cent by uolume Lb./sq. in. % Ft./sec. c. ( 0 F . ) Methyl formate 5.05 22.7 106 10.5 23.0 498 ( 928.4) Manufactured gas" 6.50 36.0 93 20.5 29.4 568 (1054.4) Acetone 2.55 12.8 78 5.5 18.8 561 1041 8 ) Methane 4.90 15.0 66 9.8 12.7 . 645 i i m j Composition: Carbon dioxide 2.2 per cent by volume; illuminants 4 . 1 , oxygen 2.0,hydrogen 45.0,carbon monoxide 10.7, methane 23.3, ethane 1.4, and nitrogen 11.3. COMBUSTIBLE
INFLAMMhBILITY
Lower
any of the combustibles listed and much higher than methane or acetone. The maximum average flame speed is higher than that for methane or acetone but less than that for manufactured gas. The ignition temperature of methyl formate and the other combustibles is within a range of 150' C., and each offers approximately the same hazard from the standpoint of ignitions. Compounds, such as carbon disulfide or ethyl ether which have ignition temperatures much below the figures given above, would offerconsiderably greater ignition hazards than the combustibles given above. When methyl formate, whose empirical formula may be written CHaC02, is compared with the inflammable limits of the equimolecular mixture CH4 COZ,the lower inflammable limit of methyl formate in air was found to be 5.08 per cent; previous experiments have shown that a 50-50 mixture by volume of methane and carbon dioxide has a lower limit (methane content) of 5.5 per cent. The lower limit values show that there is an approximate agreement between the
+
lower limits as determined and the limits which would be predicted from an inspection of the composition of the molecule. The upper limit for methyl formate-air mixtures is 22.7 per cent; the upper limit for a 50-50 methane and carbon dioxide mixture is (methane content) 11.0 per cent, indicating that there is no agreement whatever between the two. It is therefore apparent that the internal structure of the molecule is of decided importance in determining the limits of inflammability, especially so in the case of the upper limits of inflammability.
LITERATURE CITED (1) Coward, Jones, Dunkle, and Hess, Bur. Mines and Carnegie Inst. Tech., Codperalive Bull. 30, 34 (1926). (2) Jones, Harris, and Miller, Bur. Mines, Tech. P a p e r 544, 3 (1932). (3) Mason and Wheeler, J. Chem. Soc., 121, 2079 (1922). RECEIVED November 25, 1932. Published by permission of the Director, U. S. Bureau of >fines. (Not subject t o copyright.)
Nature and Constitution of Shellac VI. Preparation of Heavy Metal Soaps of Refined Bleached Shellac' WM. HOWLETTGARDNER, WILLETF. WHITMORE, AND HARRYJ. HARRIS Polytechnic Institute of Brooklyn, Brooklyn, N. Y.
T
tain only phenolic groups. On H E preparation of true Many factors enter into the preparation of the other hand, there is excellent heavy m e t a l s o a p s of heavy metal soaps, and unquestionably have an evidence from the investigations substances s u c h as important influence upon the properties of the into the constitution of shellac shellac, which exhibits certain ProdUcts obtained. This paper describes seaeral to show that the resin molecules colloidal properties in solution, is methods of preparation and a study of the effect contain uncombined c a r b o x y l not always as simple as it appears. I n many cases pseudogroups (6,8). of various influences on the yield and composiThe potentiometric titration compositions are obtained when tion of the soaps. H~~~~metal soaps of shellac c u r v e s in 95 per c e n t e t h y l for all general purposes it may are of general commercial interest because they alcohol given by and seem that a direct replacement Play an important Part in the spirit narnish Whitmore (4) present m a r k e d of the h y d r o g e n of the acid Jield in the study of such technical problems as evidence of the close similarity groUDS bv the metal has taken of s h e l l a c to the f a t t y acids. place. clay and fuller's earth in livering and corrosion. C o m p a r i s o n of these c u r v e s water f u r n i s h well-known exwith that given for phenol leaves amples of this type of behavior, and yield products which are actually adsorption compounds, little doubt as to the lack of phenolic properties in such soluI n a number of cases this can be discerned only upon close tions, Furthermore, it has been shown in the same laboratory examination or through a careful study of the nature of the that clear aqueous alkali soap solutions of both orange and acidity of the substance of which the salt is desired. bleached shellac can be prepared by carefully neutralizing CHARACTER OF ACIDITYOF SHELLAC alcoholic solutions t o a pH of 10, adding water, and then cauShellac shows a marked anomaly in its acidic behavior tiously evaporating all of the spirit solvent. One of the toward aqueous solutions of the alkali carbonates (8, 14). authors has demonstrated that these solutions, as well as It is practically insoluble in the bicarbonate solutions and dilute alcoholic solutions of wax-free shellac, contain no dissolves with little or no evolution of carbon dioxide in the colloidally suspended material since they pass completely carbonates. As solubility in the alkali carbonate solutions through a membrane filter delicate enough to retain gold is used as a means of differentiating between carboxyl and sol. This result is in accord with some preliminary work phenolic types of compounds (8) shellac would seem to con- which has been carried out on the molecular weight of shellac. These experiments eliminate the possibility that the acidity 1 For previous parts of this article, see footnote1 t o P a r t V, IND.EXQ. of shellac solutions results from the colloidal properties maniCHEM., 25, 550 (1933). -
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