Spontaneous Ignition of Organic
Compounds CHARLES E. FRANK AND ANGUS U. BLACKHAM Applied Science Research Laboratory, University of Cincinnati, Cincinnati, Ohio
T
HE determination and significance of spontaneous ignition
temperature have been reviewed in detail by Helmore (10). More recently, the literature on spontaneous ignition has been summarized by Schull with particular reference to sources of ignition in aircraft (17). As pointed out in these references, the marked lack of agreement in the literature on spontaneous ignition temperature has prevented any but the most general correlation of spontaneous ignition temperature with structure. The present paper reports some of the results obtained in a study on the spontaneous ignition temperature characteristics of organic compounds being conducted for the National Advisory Committee for Aeronautics (9). The broad objective of this work is the development of information which will permit the formulation of lubricants of reduced flammability. Specific objectives have been to determine the effects of structure on ease of spontaneous ignition, and to observe the influence of metal catalysts and various additives on these spontaneous ignition temperatures.
2-minute intervals. The air flow rate generally was kept a t 125 cc. per minute; this is a convenient rate as it sweeps out the reaction chamber within about 2 minutes. A considerable number of determinations also were run a t a 25-cc.-per-minute rate, which gave somewhat lower values because of the richer fuelair ratios thereby possible. Spontaneous ignition temperature values taken as the temperature rose were generally 6" to 10" C. higher than those taken as the block cooled; this apparently was due to a conditioning of the wall surfaces by previous ignitions at the higher temperatures (4,l a ) .
Three different types of oxidation phenomena have been observed and classed as ignitions in determining the spontaneous ignition temperature: a visible flame or flash, blue or yellow in color; a visible glow (cool flame) within the chamber, usually green or blue, which may or may not be followed by a puff of smoke; and a definite puff of smoke after a reasonable induction period. The last two types are usually only observed a t the spontaneous ignition temperature, and temperatures slightly above. I n this range the optimum fuel-air ratio to produce a flame is quite sensiAPPARATUS tive; if it is slightly exceeded, a glow or puff of smoke results instead of the flame. The apparatus used in this work is an adaptation of that deFor the hydrocarbons, alcohols, and olefins the maximum scribed by Sortman, Beatty, and Heron ( I Q ) , and employed b y amount required to effect an ignition was 30 to 40 mg. a t the Zisman and coworkers in their recent flammability studies (3, SO). spontaneous ignition temperature; the induction period was 30 to This apparatus as modified in the present work is illustrated in 60 seconds. The addition of larger than 40-mg. quantities oi Figure 1. Blocks were prepared from both copper and stainless material did not lower the spontaneous ignition temperature more steel; the two gave completely parallel results, although the copthan a degree or two. per block was not as satisfactory at the higher temperatures beFor the ethers, it was necessary to add much larger amounts cause of the rapid formation of a heavy copper oxide layer under of material to produce an ignition in the temperature range just those conditions. above the spontaneous ignition temperature. The induction A convenient modification for observing the effects of various period remained quite short in this range, and even a t the metal surfaces has been the use of metal inserts, each consisting spontaneous ignition temperature, of a base plate, cylinder, and lid, it was only 8 to 12 seconds. Apwhich are accurately machined to parently the large amount of maisolate the ignition chamber from terial was needed so that a high the walls of the block. The preether concentration in the vapor heated air enters the ignition phase could be achieved very soon chamber through four small openafter the addition; otherwise, the ings in the base of the cylinder. I rate of oxidation apparently was The thermocouple is inserted faster than the rate of vaporithrough the base of the block into zation, and accordingly, sufficient a recess in the bottom of the base plate of the ignition chamber. Figconcentration of ether for normal ure 1 shows the details of this igniignition could not be attained. tion cup assembly. The maximum amount of any ether added at the spontaneous PROCEDURE AND PRECISION ignition temperature was 100 mg. -this was in the case of didecyl I n carrying out a determination, the block first was heated to well Figure 1. Cross-Sectional View ~f Metal ether. Because of the very short above the approximate spontaBlack Showing Ignition Cup Assembly induction period with the ethers, neous ignition temperature, then 1. Metal black, 5 inches high, 5 inches in diameter, the matcrial was added all a t once slowly cooled; near the spontaplaced i n Hoskins electric furnace, Type FD 104 neous ignition temperature the rate rather than dropwise; with com2. Asbestos cover of cooling was 1' C. in 2 to 5 min3. Stainless steel cap pounds showing longer induction 4. Lid; u,top view; b, side view utes. A drop (3 to 4 mg.) of the 5. Cylinder: a, aide view; b , bottom view periods, the few seconds required compound was added from a hy6. Base plate: a,side view; b , bottom view podermic needle and reaction 7. Groove for air passage for dropwise addition had no effect 8. Recess i n base plate for thermocouple noted; if no ignition occurred, on the spontaneous ignition tem9. Thermocouple wires larger quantities of material (up 10. Ignition ehamber, 11/* inches in diameter, l ' / z perature value. to 40 mg.) were added a t about inches high 862
April 1952
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
For some aromatic compounds tested the optimum amount of material for ignition did not exceed 3 to 5 drops (about 9 to 15 mg.) even a t the spontaneous ignition temperature. More than this amount failed t o give ignition in the range just above the spontaneous ignition temperature. This probably results from a rapid rate of vaporization which yields a fuel-air ratio greater than optimum before the rate of oxidation becomes significant. Spontaneous ignition temperature readings below 300' C. have been reproducible generally within &loC.; a t higher temperatures the accuracy is in the range of &3' to 5' C. The temperature actually observed (thermocouple in recess of base plate) differs by no more than 2' C. from the temperature at the surface of the base plate, near which ignition would be expected to occur; in the lower range, this agreement is within 1' C. To obtain a complete picture of the heat distribution throughout the ignition chamber, thermocouples were placed in other parts of the ignition assembly, and observations were made a t various temperatures during both heating and cooling periods. At 200' C. the ten% perature at the center of the chamber was about 3' C. lower than that a t the surface of the base plate; at 600' C. this temperature difference was 9' C. The temperature differentials throughout were considerably less than those reported for the A.S.T.M. apparatus (16). PURIFICATION OF COLMPOUNDS
All compounds for which spontaneous ignition temperature values were determined were carefully purified before use; all constants agreed closely with the best literature data available. The higher n-paraffins were obtained from commercial sources (Eastman, Atlantic Petroleum Co., and Humphrey-Wilkinson Co.) and purified by treatment with chlorosulfonic acid (18)and fractional distillation. The iso-octane and n-heptane were Phillips Pure (99%) Grade. The other branched paraffins were prepared by API Project 45 in greater than 95 to 99% purity. The alpha olefins, five highest alcohols, and two highest ethers were obtained from the Humphrey-Wilkinson Co. and were carefully fractionated before use. The remaining ethers, aromatic hydrocarbons, and alcohols were Eastman Kodak chemicals purified by fractional distillation, usually preceded by refluxing with sodium, through a silica or alumina column, The 1qr by methylnaphthalene was not of the same degree of purity as the other compounds tested; it apparently contained an appreciable amountof the beta compoun~which could not be removed by ordinary means. The compounds tested a8 additives Were Eastman Kod& chemicals purified by a simple distillation or crystallization; the tetraethyllead was supplied by E. I. du Pont de Nemours & Co. SPONTANEOUS IGNITION TEMPERATURES OF PURE COMPOUNDS
The spontaneous ignition temperatures of series of straightchained paraffins, alpha olefins, primary alcohols, and symmetrical ethers containing 10 to 20 carbon atoms are summarized in Figure 2. For the lower n-paraffins, it is known that the spontaneous ignition temperatures decrease (10) and the oxidation rates increase (7) with increasing molecular weight. A similar but more gradual trend in spontaneous ignition temperature is observed here with the paraffins containing from 10 to 14 carbon atoms. The bbserved upward trend in going from 14 t o 20 carbon atoms probably results from the decreasing volatility of these compounds. This is evidenced by the effect of reduced air flow rate (Increased fuel-air ratio) on these values; i.e., a t a 125-cc. perminute air flow rate the higher paraffins simply cannot build up sufficient vapor concentrations to exhibit their minimum spontaneous ignition temperature values. At a 25-cc.air flow ratetheminimum value,225' C.,wasobtained with hexadecane. If volatility were not a factor, it seems likely
863
TABLEI. SPONTANEOUS IGNITION TEMPERATURES OF NORMAL PARAFFINS Compound Heptane Decane Dodecane Tetradecane Hexadecane Octadecane Nonadecane Eicosane
300
Spontaneous Ignition Temperature, C. .4ir flow, 125 cc./min. Air flow, 25 cc./min. 250 236 232 232 232 235 237 240
244 231 229 227 225 227 230 232
that the spontaneous ignition temperature 250 values of the n-paraffins would continue to $ 260 fall with increasing molecular weight, but 5 240 a t a very slow rate. 1-Decene has a spon6 220 taneous ignition temperature about 20' C. 0' higher than that of ndecane. This difference between the alpha Iso 10 12 II 16 I8 20 olefins and correspondNUMBER O F CARHON ATOMS I N COMPOUND . ing n-paraffins decreases with increasing ~ ~ ~ n R~ f ~ ; ~u ~ ,~ h i $ emolecular ~ f weight, as is perature to ~~~~~hof Carbon reasonable on the basis Chain of the increasing preAir flow rate, 125 CO. per minute ponderanceof theparafStainless steel cup finic portion of the molecule. The primarY alcohols possess still higher spontaneous ignition temperature values-in the case of decanol about 45' C. higher than that of decane. Again, with increasing molecular weight, the 5Pontaneous ignition temperature of the alcohol approaches that of the n-paraffin. The reason for these higher Spontaneous i d tion temperature values for compounds such as olefins and alcohols, which are generally more reactive than the paraffins, is perhaps explained by the lower reactivity of the intermediate radicals or peroxides resulting from the initial attack on the molecule. This situation would be analogous to that in addition polymerization, when a reactive monomer such a8 styrene yields a relatively inactive radical (14). Only three of the louer alcohols were tested in the Present work; these comprised ethyl, n-propyl, and isopropyl alcohols with spontaneous ignition temperature values of 425O, 441', and 498" C., respectively, a t a 125-00. air flow rate. The ethers have spontaneous ignition temperature values lower than those of the paraffins, but again approaching those of the paraffins as the paraffinic portion of the molecule increases. By analogy with the olefins, which also are easily peroxidizable, these lower values might seem quite unexpected. However, as Chamberlain and Walsh have suggested ( 6 ) , the radical resulting on abstracting an alpha hydrogen would probably decompose to an aldehyde and an alkyl radical; the alkyl radical would be an active chain initiator and the aldehyde readily susceptible to further attack. It is significant that the odor of acetaldehyde is quite strong when diethyl ether ignites spontaneously. As mentioned earlier, considerably larger quantities of ethers are needed than those required for the paraffins, and the induction period for the ethers is much shorter; Chamberlain and Walsh also have observed this latter phenomenon. Spontaneous ignition temperatures of five ethers a t both 125and 25-cc. air flow rates are contained in Table 11. The low spontaneous ignition temperature value of diethyl ether is in line with the above discussion. The very high spontaneous ignition tem-
E +
t ~
i
864
INDUSTRIAL AND ENGINEERING CHEMISTRY
perature value of diisopropyl ether must result from the relative radical or peroxide, and inertness of the (CH,)z&O-CH(CH,), of the acetone and isopropyl radical fragments. A detailed discussion of the ignition behavior of diisopropyl ether is given by Chamberlain and JTalsh ( 6 ) . .I dose parallel has been observed throughout this work between the spontaneous ignition temperature and the antiknock performIGY ance. This is brought out more strikingly in Table I11 which lists the spontaneous ignition temperature values and critical compression ratios (spark ignition) for a number of aromatic hydrocarbons. As has been previously observed ( I O ) , these values are much higher than those of "'b F C R C E N20T aOLUHE 0 60 8C ' I20 the n-paraffins; this OF 2 2 4 V M THYLPENTAYE presumably is either ~ I S OO c - a ~ ~ E ) M X T J R E S IY i d n YEPTAUE because of difficulty of Figure 3. Spontaneous Ignition Characteristics of Various 2,2,4oxidative attack, as Trimethylpentane n Heptane with benzene, or beMixtures cause of a relative Air flow rate, 125 CC. per minute A . inertness of the resB . Air flow rate, 25 cc. per minute onance stabilized radicals, as with the alkvlsubstituted benzenes. This parallel between the spontaneous ignition temperature and the critical compression ratio is particularly evident with the xylenes. Khile the long-chain alkylated benzenes have not yet been tested in this work, there can be no doubt that the spontaneous ignition temperature will decrease as the length of the unbranched paraffinic chain increases.
Vol. 44, No. 4
TABLE111. RELATIONO F SPONTANEOCS I G N I T I O N TERIPER.4TURE '10 CRITICALCOMPRESSIOS RATIOFOR AROMATIC HYDROCSRBOXS
a
Compound p-Xylene m-Xylene Benzene To1u en e 1,3,5-trimethyIbenxene o-Xylene 1 2 4-Trimethylbenzene 1:2:3-Trimethylbenzene Air Row rate, 125 cc. per minute.
Spontaneous Ignition Temperature, C.G
687 652 645
635 577 551
Critica! Compression Ratio ( 1 1 ) 11.5 11.5 ca. 12 11.3 10.6 8 3
528
8 7
510
7.9
The failure of methane and et'lriane to ignite by a low temperature mechanism also is logica.lly explained on this basis. I n further ajork on spontaneous ignition, an effort Tvill be made to evaluate more thoroughly these two bases of comparison. There is another interesting aspect of these data which illuat,rate one of the reasons for large differences in some of the spontaneous ignition temperature values reported by different investigators. Changing t,he air flow rate from 125 to 25 cc. per minute generally lowers the spontaneous ignit,ion t,emperature by a margin of 5" to 6' C. in t'he low t'emperature region and some 15' to 20" C. a t the high temperatures. An exception to this is apparent in t,he case of 4,5-dimethyloctane where there exists a temperature difference of about 100" C. Thus, while 4,5dimethyloctane and 4-ieopropyllieptane show the expected similar spontaneous ignition temperature behavior a t the 25-cc. air flow rate, they show a surprisingly large difference a t the 125cc. rate. For an explanation of this phenomenon it is necessary to consider briefly the mechanism of hydrocarbon oxidation. As Mulcahy ( I S ) has pointed out, there are two general mechanisms of oxidation: the low temperature mechanism (below 300" C. ) involving peroxide intermediates and depending markedly on structure, and the high temperahre mechanism (above 400' C.) involving simple radical intermediates and depending much less on structure. Compounds or mixtures which can ignite by both the high and IOU- temperature mechanism show a zone of nonignition, where the temperature is too high for peroxide intermediates to exist, and too low for the formation of the highly reTABLE 11. SPONTAXEOKS I G N I T I O X TEMPERSTURES O F ETHERS active simple radicals of the high temperature mechanism. The Spontaneous Ignition Temperature, C. range of the nonignition zone is highly sensitive to such factors as Compound Air flow, 125 cc./min. Air flow, 25 cc./min. the fuel-air ratio, catalytic effects, etc. Propane and the higher nDiethyl ether 193 190 Diisopropyl ether 500 495 paraffins can ignite by either mechanism. The present work Dihexyl ether 200 197 Diootyl ether 210 210 shows that as the degree of branching of the paraffin increases, Didecyl ether 217 217 there is a diminishing tendency to ignite by the low temperature mechanism. This is illustrated by Figure 3 on the ignition behavior of various heptane-iso-octane mixtures. The final series of compounds tested comprised a group of branched paraffins listed in Table IV. With these compounds, it is apparent that the spontaneous ignition temperature, like the TABLEIV. SPONTANEOCS IGSITIOX TEXIPERATURES OF critical compression ratio, increases with increasing branching. BRANCHED PARAFFINS Air Flow/Minute From these and the other data listed it becomes evident that the Compound 125 00. 25 0 0 . most important single factor in determining the spontaneous 2,2,4-Trimethyipentane 513 BO2 ignition temperature of a n organic compound is the length of the 485 2,5,5-Trimethylheptane 463 uninterrupted hydrocarbon chain-the longer the uninterrupted 290 388 4,5-Dimethyloctane 27.3 288 4-Isopropylheptane hydrocarbon chain, the lower the spontaneous ignition tempera235 231 3-Ethyloctane 231 223 2-Methyldecane ture. A similar observation has been made by Cullis and Mulcahy in their study of relative oxidation rates (8). For many comparisons, this generalization is adequate and, in At an air flow rate of 125 cc. per minute, such a niixture ignites the authors' opinion, essentially correct. However, perhaps a by a low temperature mechanism up to about 52% iso-octane. more fundamental basis for comparison of ease of oxidation, or I n the range of 24 to 5270 iso-octane, it has a nonignition zone; spontaneous ignition temperature, is that suggested by Boord above 52Y0 isc-octane and at a 125-cc. airflow rate the mixture ( 1 )-the number of primary, secondary, and tertiary hydrogens ignites only in the high temperature region. The 4,5-dimethylpresent in the molecule. Since the reactivities of these C-H octane has an ignition behavior about like a 1 to 1 mixture of bonds are in the order, tertiary > secondary > primary, the high heptane and iso-octane. A t a high fuel-air ratio, 25 cc. per spontaneous ignition temperature of a highly branched parafin minute, it ignites a t 290" C.; at a leaner fuel-air ratio, 125 cc. per would result from its high percentage of primary hydrogens. BY
- -
~
~~~
April 1952
lTABLE
.'
CoMPAR1soN OF SPoNTANEoUs TBMPERATURE AND CETANBKUMBERS OF HYDROCARBONS
Compound n-Hexadecane n-Dodecsne 7i-Heptane 1-Hexadecene 1-Tetradecene 2,2,4-Trimethylpentane I-Methylnaphthalene Toluene a
INDUSTRIAL AND ENGINEERING CHEMISTRY
Spontaneous Ignition Temperature,
c.a
232 232 250 2 53 255 515 553 635
Cetane Number (16) 100 80 57 88 79 12 0 -5
Air flow, 125 eo. per minute
865
done by completely enclosing the ignition chamber in the metal t o be observed as is illustrated in Figure 1. Results are Bummarized in Tables VI and VII. Surprisingly, a change in the metal surface had substantially no effect on the spontaneous ignition temperature value of those compounds which undergo spontaneous ignition below 290" C. This is apparent from an examination of Table VI, where the compounds listed include three normal paraffins, P two straight-chained $ alpha olefins, trvo straight-chained normal $ f alcohols, and one ? straight-chained ether. The condition of the metal surface also appeared to have little or no effect on these spontaneous ignition temperature values (Table PERCENT BY VOLUME OF HEXAOECANE VII). (CETANE) IN MIXTURES WITH With compounds posd- HETHYLNAPH THALENE sessing high spontaneous ~!'?!~~' ignition temperature decane -1-Methylnaphthalene values, the catalytic efMixtures fect of the metal surface flow rate, 12s cC. per minute becomes pronounced. A. B. Air flow rate, 25 CC. per minute While it has not been possible in the present investigation to study a large number of compounds with all these metal surfaces, the information in Table VI1 demonstrates quitclearly the importance of that surface at the higher temperatures. This table lists results obtained on five compounds employing a
minute, conditions are shifted sufficiently to miss this small zone .of low temperature ignition, and a value of 388" C. is obtained. The 4-isopropylheptane has a n ignition behaTior similar to that .of a mixture a little richer in n-heptane, as is evidenced by the value of 275' C. for the spontaneous ignition temperature a t a '25-cc. air flow rate, compared with 290' C. for 4,Bdimethyl.octane. Accordingly, even at a 125-cc. air flow rate the isopropylheptane will ignite by the low temperature mechanism. I n considering Figure 3, i t is important to emphasize that the ,curve was determined by testing a range of charge sizes a t each experimental point in order t o secure the maximum extension of the ignition boundary at the 125-cc. air flow rate; the curve therefore was obtained a t the optimum fuel-air ratios possible at this rate. The dotted (partial) curve obtained at a 25-cc. airflow rate represents results obtained at the somewhat richer fuel-air ratio .obtained under those conditions. It also is possible to prepare a family of curves at a fixed air flow rate where each curve repre.sents the spontaneous ignition temperature change with composition a t a given charge size, roughly fuel-air ratio. Such a family of curves may be SwerimPosed upon one another to yield three-dimensional plot, in which the third ordinate is the fuel-air ratio. I n continuing work in this field, a number of such three*dimensionalignition-boundary surfaces have been prepared and found exceedingly instructive in an understanding *of the complexities of spontaneous ignition phenomena. OF METALS ON IGNIT~ON OF COMPOUNDS OF LOW TABLE VI. EFFECT A similar but inverse relationship exists between SPONTANEOUS IGNITION TEMPERATURE VALUES the spontaneous ignition temperature and cetane Spontaneous Ignition Temperature, a C.a number; thus, a bomb ignition apparatus has been
F$z2;s
Dode-
Tetra- Octa-
Dode-
Octa-
Decrt- Dode- Dioctyl
Metal cane decane decane cene decene no1 csnol ether used as an indication of the approximate cetane number where only small quantities of compounds were Stainless steel 232 232 235 257 251 291 283 207 available (16). The bomb employed approaches actCopper 230 233 240 261 255 288 278 206 Aluminum 235 230 235 256 251 289 280 ual conditions of Diesel operation, however, and it Blackiron 235 233 237 257 252 294 284 208 234 234 241 255 253 289 278 208 was of interest to determine how wall the spontane235 230 236 256 251 292 281 207 cous ignition temperature values obtained in this Aluminumalloy (24-S) 232 231 237 256 251 289 280 208 .. . 233 .. . 257 252 ... . .. . . . apparatus would correlate with cetane ratings. Magnesium Table V shows a relationship between the spontaneDifference and low values, betweena high C. 5 4 6 6 4 6 6 2 a u s ignition temperature and cetane rating similar t o that observed between the spontaneous ignition OF CONDITION OF METALSURFACE ON TABLE VII. EFFECT temperature and octane rating. SPONTANEOUS IGNITION TEMPERATURE OF HYDROCARBOKS A further comparison of cetane number and spontaneous igniSpontaneous Ignition Condition tion temperature is contained in Figure 4, where the spontaneous Temperature, C . of Stainless ignition temperature values of various cetane-l-methylnaph125 cc./min. 25 OG /min. Steel Compound Surface air flow air flow thalene mixtures are plotted. As with the heptane-iso-octane mixtures, an S-shaped curve was obtained showing the marked a-Methylnaphthalene Oxjdlzed 579 Bright metal 553 547 sensitivity of the spontaneous ignition temperature to comp-Xylene Highly oxidized position; thus, at about 20y0 hexadecane, a 5% change in comFirst run 697 Second run 708 704 position may change the observed spontaneous ignition temperaThird run 710 7 08 ture by more than 200' C. Bright metal 657 650 EFFECT OF METAL SURFACES ON THE SPONTANEOUS IGNITION TEMPERATURE
One of the objectives of this work has been to determine the effect of the various metals used in aircraft construction on the spontaneous ignition temperature of hydrocarbons, This has been
m-Xylene Toluene Cetane
Highly oxidized Bright metal Slightly oxidized Bright metal Highly oxidized Bright metal
652 644 635 234 235
700 645 640 630 232 232
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
866
Vol. 44, No. 4
may amount to well over 50' C. This fact proves extremely EFFECT OF METALSURFACE O N THE SPONTANEOUS troublesome in determining reproducible spontaneous ignition I G N I T I O N TEMPERATURE O F 4-ISOPROPYLHEPTANE temperature values for materials which ignite at these high tem-
TABLE VIII.
Spontaneous Ignition Temperature, O C.O Metal 257 Inconel 256 Copper 288 Stainless steel 281 Aluminum alloy Air flow, 125 cc. per minute.
peratures; under these conditions the metal surface is rapidly oxidized, and must be cleaned frequently; otherwise, the spontaneous ignition temperature value observed will rise progressively, as is apparent from the data listed for p-xylene. 0 The only additional data on the effect of metal surfaces on compounds of relatively high spontaneous ignition temperature values stainless steel cup pretreated in several ways; also contained is a comprise some observations of the spontaneous ignition temperacomparison of results at two air flow rates. With those comture of 4-isopropylheptane in four different clean metal cups. Repounds of high spontaneous ignition temperature values, the difsults are summarized in Table VIII. The differences observed ference between a heavily oxidized and a bright metal surface here demonstrate appreciably different catalytic activity of the various metals at higher temperatures. While the information thus TABLEIX. SPOKTANEOUS IGNITIOX TEMPERATURE OF DODECANE CONTAININQ VARIOUS far is meager, the following . ADDITIVES ( 5 MOLEyo) generalizations may be justiSpontaneous Ignition Temperature, C.a fied. Referring to Figures 3 Additive 230 238 240 245 250 255 260 266 270 h-one and 4,it appears that the spon232 Amines taneous ignition temperatures Aniline 243 +Toluidine 255 of compounds igniting by the p-Toluidine 2 59 low temperature mechanism 2-Amino-1 ,a-dimethyibenzene 265 2-Amino-l,4-dimethylbenzene 266 will be affected little by metal 5-Amino-l,3-dimethylbenzene 265 4-Amino-I ,3-dimethylbenzene 267 catalysts. On the other hand, N-Met hylaniline 268 compounds which ordinarily E t h 1-o-toluidine 271 Di Kenylamine 272 fail to ignite at these low temo-&oroaniline 237 m-Chloroaniline 239 peratures or which, like 4isop-Chloroaniline 245 propylheptane, show a borderN,N-Dimethylaniline 232 Triethanolamine b 234 line behavior, may undergo a Diethanolamineb 233 Mono-ethanolamine6 234 considerable change in sponPyridine 233 taneous ignition temperature o-Nitroaniline 235 m-PU itroaniline 234 value with change in the metal p-Nitroaniline 23 1 Diethylamine 248 surface.
I------
Beneylamine Triethylamine Phenols Phenolb Resorcinolb Hydro uinoneb Guaico? Hydrocarbons Benzene Toluene &Xylene m-Xylene p-Xylene Mesitylene 1 2 3-Trimethylbenzene 1 :2:4-Trimethylbeneene Triethylbenzene Hexameth lbenaene Hexaethylgenzene h-aphthalene I-Methylnaphthalene Biphenyl Alcohols Isopropyl alcohol Benzyl alcohol D ec anol Carbonyl Compounds Aceto henone Benzaydehyde Cyclohexanone Halogen Compounds Rromobenzene Chlorobenzene Carbon tetrachloride 1,l,Z-Trichloroethane Blisoellaneous E t h y l benzoate Nitrobenzene or-Methyl-o-trifluoromethylstyrene Isopropyl ether Cumene hydroperoxide tert-Butyl peroxide
238 234
239 235 245 244
E F F E C T O F ADDITIVES ON T H E SPONTANEOUS IGNITION TEMPERATURE OF n-PARAFFINS
232 232 233 234 234 234 234 238 236 246 239 236 238 23 1
Some 60 compounds have been tested at a concentration of 5 mole % €or their effects on the spontaneous ignition tomperature of n-dodecane; these results are summarized in Table IX. Itappears that only the most powerful antiknocksLe., compounds capable of inhibiting chain reactions in the cool flame region-can haveany major effect on the spontaneous ignition temperature value. As knock is simply the result of a premature spontaneous ignition, it is entirely reasonable that tetraethyllead should prove an outstanding additive for raisingthe spontaneous ignition temperature. I n this work, 1% tetraethyllead raised the spontaneous ignition temperature of n-dodecane by 200' to 300" C., depending on the fuel-air ratio; a similar marked effect of tetraethyllead also has been observed by Sortman, Beatty, and Heron (19). The largest effect of any of
237 235 240 232 231 233 232 232 230 232 232 231
-
__-
237 234 230 199
10s
-
t-
Tetraethyllead 5 mole Yo 1 mole yo * .Air flow rate, 125 00. per minute. b S o t completely miscible with dodecane.
1 2 8 cc. /niin. 580 a? 5
Air Flow Rate 25 co./min. 434 436
None 420 420
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
April 1952
867
rise in spontaneous ignition temperature value with the addition B small amount of an effective inhibitor.
TABLE X. COMPARISON OF ANTIKNOCK AND IGNITION TEMPER- of ATURE
Amine Diphenylamine N-Methylaniline Toluidines (average) Aniline Diethylamine Dimethylaniline
RATINGS
ACKNOWLEDGMENT
Reciprocal of Moles Required t o Give Antiknock Effect Equivalent t o 1 Mole of Aniline ( 8 ) 1.5 1.4 1.2 1.0 0.5 0.2
40 36 25 11 16 2
the more usual inhibitors of free-radical reaction was an increase of about 40’ C. obtained with the three amines, N-ethyl-o-toluidine, diphenylamine, and N-methylaniline, These results also correlate with the effects of additives on octane ratings. Thus, in discussing antiknocks for spark-ignition engines, Boyd (3)has reported the marked effectiveness of the primary and secondary aromatic amines. Table X shows an interesting comparison of corresponding antiknock and spontaneous ignition temperature ratings. While not perfect, the agreement appears as good as that commonly observed among the various methods of evaluating antiknock performance. Aside from tetraethyllead and the amines, additives tested included phenols, aromatic hydrocarbons, carbonyl compounds, halogen compounds, alcohols, and a number of miscellaneous materials. The phenols had relatively small effects; the aromatic hydrocarbons were substantially ineffective, as were also the carbonyl compounds, halogenated hydrocarbons, alcohols, and ethers. These results again are generally in accord with those on the antiknock ratings in spark-ignition engines (22). They further confirm the remarkable effectiveness of metallic atoms as chain-reaction inhibitors; as Walsh has pointed out @ I ) , there can be no doubt that it is the lead, rather than the ethyl radicals, which contributes the antiknock properties to tetraethyllead. The effect of additives on the spontaneous ignition temperature values of compounds igniting in the high temperature range has not yet been studied in this work. A converse generalization of the action of metal catalysts on spontaneous ignition temperature values, however, is t o be expected. Again referring to Figures 3 and 4, it appears that the spontaneous ignition temperature values of compounds igniting by the high temperature mechanism will be raised little by additives. On the other hand, compounds which ordinarily ignite a t low temperatures-e.g., dodecane-or which show a borderline behavior, may undergo a considerable
This work was done under National Advisory Committee for Aeronautics Contract NAw-5838; the authors are grateful t o the NACA for the sponsorship of this research and for permission t o publish these results. They are indebted also t o C. E. Boord of Ohio State University and API Project 45 for generously providing samples of the branched decanes and undecane listed in Table IV. LITERATURE C I T E D
Boord, C. E., “Third Symposium on Combustion, Flame, and Explosion Phenomena,” p. 416, Baltimore, Williams & Wilkins Co., 1949. Bovd. T. A.. IND. ENQ.CHEM..16. 893 (1924). Bri”ed; E. M.’, Kidder, H. F., Murphy, C.‘M., and Zisman, W. A. Ibid., 39, 484 (1947).
Burgoyne, J. H., and Silk, J. A., J . Chem. SOC.,1951,572. Chamberlain, G.H. N., and Walsh, A. D., “Third Symposium on Combustion, Flame, and Explosion Phenomena.” D. 368 Baltimore, Williams & Wilkins Co., 1949. Ibid., p. 375. Cullis, C. F., and Hinshelwood, C. N., Discussions Faraday Soc., 2, 117 (1947). Cullis, C. F., and Mulcahy, M. F. R., Rev. inst. franc pBlrole et Ann. combustibles Eiquides, 4, 283 (1949). Frank, C . E., and Blackham, A. U., Natl. Advisory Comm. Aeronaut., Tech. Note 2549 (January 1952). Helmore, W., “Science of Petroleum,” Vol. IV, p. 2970,London, Oxford University Press, 1938. Lovell, W. G., IND. ENQ.CHEM.,40,2388 (1948). Mulcahy, M. F. R., Discussions Faraday Soc., 2, 128 (1947). Mulcahy, M. F. R., Trans. Faraday Soc., 45, 537 (1949). Price, C. C., J . Polymer Sci., 1, 83 ( 1946). Puckett, A. D., and Caudle, B. H., U. 8. Bur. Mines, Inform. Circ. 7474 (July 1948). Scott, G. S., Jones, G. W., and Scott, F. E., Anal. Chem., 20,238 (1948). Scull, W. E., Natl. Advisory Comm. Aeronaut., Tech, Note 2227 (Deoember 1960). Shepard, A. F.,Henne, A. L.,and Midgley, T., Jr., J. Am. Chem. Soc., 53, 1948 (1931). Sortman, C., Beatty, H., and Heron, S.,IND.ENQ.CHEM.,33, 357 (1941). Sullivan, M. V., Wolfe, J. K., and Zisman, W. A., Ibid., 39, 1607 (1947).
Walsh, A. D.,“Third Symposium on Combustion, Flame and Explosion Phenomena,” p. 389, Baltimore, Williams & Wilkins Co., 1949. Walsh, A. D., Trans. Faraday Soc., 45, 1043 (1949). RBCEIVEDfor review June 21, 1951. ACCEPTEDNovember 21, 1951. Abstracted from a thesis submitted in partial fdflllment of the requirements for the Ph.D. degree, University of Cincinnati. Presented before the Pivision of Organic Chemistry a t the 119th Meeting of the AMERICAN CHEMICAL SOCIETY, Cleveland, Ohio.
Light Stability of Polvstvrene and Polvvinvlidene Chloride J
J
J
J L. A. MATHESON’ AND R. F. BOYER
Physical Research Laboratory, The Dow Chemical Co., Midland, Mich,
T
HIS paper attempts to compare and contrast the light stability of two typical polymers, polystyrene and polyvinylidene chloride (or its copolymers), which react quite differently t o light. With polystyrene, aging represents the photochemically catalyzed addition of oxygen to unsaturated groups. With polyvinylidene chloride, aging seems to be the photochemically catalyzed loss of hydrogen chloride with subsequent formation of conjugated 1 Present address, Rocky Flats Plant, The Dow Chemical Co., Denver, Colo. ri
double bond systems, which may then absorb oxygen. In both polymers the colored reaction products are formed preferentially a t the surface, thus protecting the interior of the specimen from further degradation by light. Polystyrene is stabilized by strong organic amines, whereas polyvinylidene chloride requires an extremely mild acid acceptor. These and other points of difference will be discussed in more detail. In general, studies on the stability of organic high polymers to light and heat offer problems which combine technological impor-
