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bles up to this point. Sample A is obtained by removing a strip, weighing out 30 grams, and returning the exce$s to the mill immediately. Five per cent of softener is about the maximum amount used commercially so this percentage was adopted as the standard. Sample B is cut from the finished batch and may be any convenient size. As these batches contain neither sulfur nor accelerator, it is permissible to obtain a plasticity figure as though the batch were crude rubber. The machine used is the one designed by william^,^ and Y value after 5 minutes in the press is obtained on samples A and B. The ratio of difference of the Y values of A and R to the Y value of A is the total softening percentage and is due to two causes-(1) the effect of the additional 4 minutes of milling, and (2) the effect of the ingredient added to the rubber. The total softening effect, minus the percentage drop due to milling as obtained by a control run in which no softener is used, is the effective softening action of the ingredients added. For example, the Y value of A of the control is 296, and of B is 256. Thus the percentage drop due to milling is 40 divided by 296 or 13.5 per cent. The Y value of the A sample before addition of hardwood pitch is 295 and of the B sample is 239, a drop of 56 points, or 19 per cent, which gives an effective softening action due to hardwood pitch of 19 minus 13.5, or 5.5 per cent. Table 11-Softening Stearic acid Pine tar Liquid asphalt Pine oil Degras Rosin oil Stearin pitch
Action Expressed i n Percentage C h a n g e i n Plasticity Per cenf Per rent 22 Perilla oil 11 16.5 Vaseline 9 16.6 Mineral rubber 8 16 Straw paraffin oil S 16 Cottonseed oil S 13 Hardwood pitch 6.6 11 Carnauba wax 5
Softeners
In Table I1 are listed a few of the better known softeners. It may be noted that pine oil and pine tar are similar in
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softening action, which probably explains why various viscosities of pine tar are about alike in their effect on plasticity. Also stearic acid by its superior power justifies a premium in price due to this property alone. Burbridge4 stated that carnauba wax is a stiffener and that it is hard to incorporate, but it has been found to he a softener similar to hardwood pitch. Perhaps the previous investigator did not consider the fact that in raising the initial temperature of the mill he greatly reduced the amount of breakdown. Antisofteners
Khen the total percentage drop of any batch is less than the percentage drop of the control, it is evident that the substance added is a stiffener. It is a well-known fact that some antioxidants exert softening action to such an extent that tubed articles lose their shape due to their weight alone. Several ways are open to overcome this and one of them is to use an antisoftener in the compound. Benzidine, an antioxidant, accelerator-activator, and a fair stiffener of cured stock, is a stiffener of uncured stock of high degree, as evidenced by Table 111. Table 111--Antisoftening Action of Benzidine BENZIDIKE TO ACTUAL TRUE STIFFENING RUBBER STIFFENING Per cent Per cent Per cenl
Another material which behaves in a manner similar to benzidine is p-amidophenol, which gives its optimum stiffening action (43 per cent) when 0.5 per cent on the rubber is used. Other stiffeners which have been noted are 8-phenylenediamine, 8-naphthylamine, a-naphthylamine, tolidine, quinone, and dianisidine.
A Study of Auto-Ignition Temperatures' 11-Pure
Compounds
Henry James Masson and William F. Hamilton? LABORATORY OF CHEMICAL ENGINEERIXG, DEPARTMENT OF CHEMISTRY, KEWYORKUNIVERSITY, NEWYORK,N. Y.
S U M B E R of o b The auto-ignition temperatures of an additional range of auto-ignition terns e r v e r s 1 t o 5,* have number of pure organic compounds have been deterperatures was far wider than p o i n t e d out the immined in air at ordinary pressures. The compounds was anticipated. The range portance of auto-ignition ternselected cover a wide range of properties and structures of the pyrometer was therep e r a t u r e s in arriving a t a in order to provide data for use in studying various fore extended to make possicombustion reactions. A study has also been made better understanding of the ble the study of a larger nummolecular structure of organic of the catalytic effect of various surfaces on autober of compounds. The wellignition temperatures. known catalytic activity of compounds, and the mechanism of their c o m b u s t i o n , p l a t i n u m has indicated the and in determining the suitability of certain mixtures of them desirability of determining auto-ignition temperatures on as fuels for use in the internal-combustion engine. Although various surfaces other than this metal. the auto-ignition temperatures of several pure substances The previous paper6 describes the apparatus used, the have been reported by the authorsI6it was considered desir- experimental method followed, and the results obtained for a able to determine these temperatures for additional pure number of pure organic compounds. The results given in substances of widely different properties and structure before Table I include for comparison determinations previously studying the various mixtures as represented by typical com- reported as well as those subsequently made. The commercial fuels. Previous determinations showed that the pounds have been arranged according to the conventional organic groupings in order to facilitate comparisons. I Received April 25, 1928. A comparison of these results with those of other investiResearch fellow, Atlantic Refining Company Fellowship. * Numbers refer to bibliography a t the end of the article. gators shows a fair general agreement. Owing, however, to
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the different forms of apparatus, technic, and catalytic activity of the surface, it was not expected that agreement would be close. Temperatures of Pure C o m p o u n d s at Normal Pressure on P l a t i n u m SUBSTANCE P. c. SUBSTANCE F. c. Hydrocarbons : Ethyl esters: n-Pentane (tech.) Formate 1074 579 1070 577 Acetate n-Hexane 968 520 1130 610 n-Heptane 844 451 Propionate 1116 602 %Butyrate n-Octane 856 468 1134 612 n-Decane n-Valeriate 797 425 1095 590 IsoGctane n-Caproate 1041 56 1 1079 582 Isododecane 993 534 n-Caprylate 1059 57 1 Benzene 1212 656 Pelargonate 975 524 To1uen e 1172 633 Caprate 919 493 1145 618 Palmitate 9-Xylene 731 388 Malonate 1027 553 Ethyl benzene 1005 54 1 1149 621 Benzoate Mesitylene 1192 644 $-Cymene Oleate 87 1 466 667 353 u-n-Heptylene 6300 332a Acetates: Alcohols: Methyl 1210 654 Methyl 1065 574 1130 610 Ethyl Ethyl: n-Propyl 1223 662 1054 568 Amyl (tech.) 1045 563 (95%) 1034 557 Absolute Benzyl 1091 588 1004 540 n-Propyl Ethylene glycol di938 503 n-Butyl acetate 1175 635 1148 620 Isopropyl Ethers: 1007 542 Isobutyl Diethyl: 965 518 Isoamyl (10% alcohol) 908 487 935 502 Benzyl AbsoI ute 915 491 971 522 Ethylene glycol Benzylethyl 924 496 Glycerol 974 523 Isoamyl 803 428 Aldehydes : Miscellaneous: Paraldehyde 1005 54 1 1148 620 Aniline Propionaldehyde 787 419 Ethylaniline 895 479 n-Butaldehyde 767 408 o-Toluidine 999 537 Ketones : m-Toluidine 1075 580 1340a 7270 Acetone Nitrobenzene 1033 556 Diethylketone 1127 608 o-Cresol 1110 599 1067 575 Ethylpropyl m-Cresol 1158 626 1062 572 Methylhexyl Chloroform Above 1800 . , . Methylcyclohexanlone1109 598 Carbon tetrachloride 983 534 Methylheptenone Above 1800 1034 557 Cyclohexanone Carbon disulfide 300a 149. Menthone 942 50R Acids: 940 504 Pilegon 798 426 Formic Acetic (glacial) 1110 699 Ethyl bromide 1109 588 n-Propionic 1105 596 n-Butyl bromide 901 483 Benzyl chloride 1161 627 n-Butyric 1026 552 n-Heutanoic 974 523 Bromobenzene 1270a 688. a Temperature above or below millivoltmeter scale. Error is within 100 F . , or 5.5O C. Table I-Auto-Ignition
...
Effect of Composition of Liquid
An extended study of compounds covering a wide range of properties and structures discloses large differences in auto-ignition temperatures. The auto-ignition temperature is affected more by the composition of the substance than any other factor. It is this variation of auto-ignition temperature with composition that constitutes the primary purpose of this investigation. A study of the compounds listed in Table I brings out several interesting facts. For instance, substances of widely different structures and compositions will have almost identical auto-ignition temperatures, while substances having closely related structures and compositions will have widely different temperatures. Several observers7n8,9have shown that as molecular weight or complexity increases the auto-ignition temperature decreases. The results presented in Table I show that this conclusion is not entirely warranted. If for a given series of compounds auto-ignition temperatures are plotted against molecular weight or the number of carbon atoms in the molecule, it will be found that auto-ignition temperatures do not decrease progressively. Curves showing these relationships for %-hydrocarbons, alcohols, acids, and esters are shown in Figure 1. Effect of Surface Temperature, Sensitivity, and Heat Capacity
By making that part of the surface upon which the drop falls essentially the hot junction of a thermocouple, an accurate measurement of the surface temperature just prior to contact with the drop is obtained. Since the drop is a t room temperature when introduced into the furnace, and
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probably undergoes only a comparatively small increase in temperature through absorption of heat from the hot gases inside the furnace, the plate must supply the bulk of the heat required to raise the liquid to the boiling point, supply the latent heat of vaporisation and, together with heat from the coils, raise the vapor-air mixture to the auto-ignition temperature. If the plate is of low heat capacity, or a poor conductor, a considerable drop in temperature will take place when the liquid comes in contact with the surface. The loss of heat from the plate to the drop may be so great as not to give any flash or only after a long ( 5 or 6 seconds) time interval. If the heat capacity for a given material is increased by making the plate very thick, a loss of sensitivity will take place, although this will depend somewhat on the location of the thermocouple. It has been found, however, that this may be overcome with apparently no loss of accuracy by making the plate quite heavy and placing several thermocouples in series around the point of contact of the drop with the surface and just above the plate. A number of determinations made in this manner gave consistent results as well as greater sensitivity due to the use of multiple couples. When the surface is below the auto-ignition temperature, an immediate lowering of the temperature of the plate is observed due to the cooling effect of contact with the liquid. This is followed by a rise in temperature due to combustion. The drop in temperature is probably of no importance, but the rise in temperature a t or near the auto-ignition temperature due to the very rapid combustion taking place seems to be of some significance. This rise has been studied for a number of pure substances and gasolines of different "knock" ratings. Very interesting results have been obtained, but as yet the relationship of this rise to other characteristics of the substance have not been sufficiently investigated to make a report. It is found that results are reproducible only when the plate is a t equilibrium with the air inside the furnace, and that if the temperature of the furnace is raised or lowered too rapidly, deviations occur due to the time required for heat transfer to and from the plate. If the rate of change of plate temperature does not exceed 1" C. per minute, the results are found to be duplicable. I n the procedure previously described,6 the auto-ignition temperature was approximated by varying the furnace temperature. After lowering the furnace temperature about 5" C., the plate was heated by alternating current by means of its own resistance to the auto-ignition temperature of the substance. This procedure was found not to be necessary, as the auto-ignition temperature was satisfactorily reached by means of the furnace heating wires. The time required for the flash to take place after the drop strikes the plate will be a complex function of several factors, such as vapor pressure, heat capacity, and transfer of the plate, rate of diffusion, etc. With substances like ether the flash occurs almost instantly after the liquid makes contact with the plate. The auto-ignition temperature will be very sharp; as, for instance, in the caSe of ether there will be no flash a t 490" C., but a sharp flash will occur a t 491" C., as soon as the drop makes contact with the surface. So far as can be observed, in no case does the flash occur before the drop makes contact with the plate. On the other hand, glycerol does not flash a t 522" C. At 523" C. there will be a flash within 6 seconds. If the rate of change of temperature of the system does not exceed 1" C. per minute, the direction of approach to the auto-ignition temperature is immaterial. Effect of Composition of Surface
Xext to composition of the liquid, composition of the surface is the most important factor affecting the deter-
August, 1928
I S D C S T R I A L A.VD ESGISEERIAVG CHEIITISTRY
mination of aut,o-ignition temperatures. Re,wIts obtained on one type of surface may diffcr considrraldy from IliO.-e obtained on another, owing to difference? in catalytic arlivity of the surfaces. Uiiless this factor is taken into consideration deductions are liable t,o be misleading. The peculiar behavior of, and the inability to obtain duplicable resiilts on. a pure, clean platjnuin surface suggwted a study of the catalytic activity of 1-arious surfacw, eqpecially as t o their effect upon auto-ignition temperatures and deductions relating thereto. Also. jn view of the inany investigations which are being carried out in an eff:)rt to explain the mechanism of combustion and "knock" in the internal-combustion engine, it seemed desirable to indicate the importance of catalysis and its effect on results and deductions. KO attempt has been made to study coniprehensively the complex conditions which may result through catalysis, but t o add emphasis to what has been PO well expressed by others as to the importance of catalysis in the study of combustion. I t will be apparent that in the study of auto-ignition temperatures we are dealing with combustions of two types: (1) combustion in the gaseous phase (indicated by the flash), and (2) surface cornbustion (flameless combustion on the surface of the plate). When a drop of liquid is allowed to fall upon a surface of low catalyt'ic activity, a t or above the ignition temperature, practically all the combustion will take place in the gaseous phase, a minimum amount taking place on the surface. If the surface is entirely devoid of catalytic activity, it may be assumed that t'he combustion will take place entirely in t)he gaseous phase. On the other hand, if the surface is very active catalytically, or large in size, it would be impossible to obtain a flash because little or no combustion mould take place in t'he gaseous phase, all the reactions t'aking place on the surface. Therefore, assuming extreme cases, no flash would be possible in the case of a catalyst of infinite activity, the combustion being confined wholly to the surface, and in the case of a surface of zero catalytic activity the combustion would be confined wholly to the gaseous phase. However, since all known surfaces lie between t'hese extremes, it is impossible to confine the combustion exclusively bo the surface in the case of an active catalyst, or to the gaseous phase in the case of :t surface of low activity. It is obvious, therefore, that in general two surfaces of different catalytic activity will not give the same autoignition temperature. If, a t a given temperature, the surface of low catalytic activity gives a flash, no flash will he obtained on the surface of higher catalytic activity, it being necessary to raise the temperature of the latter surface in order to cause sufficient combustion to take place in the gaseous phase to produce a flash. Therefore, all other factors being constant, the auto-ignition temperature will increase with the increase in catalytic activity of the surface. Howe m - , the catalytic activity of a given surface and material is a function of the temperature, in general approaching a maxinium for each case. At high (incandescent) temperatures all distinction disappears and one surface is as good a catalyst as another. As the range of auto-ignition temperatures is below the tempera,ture of marinium catalytic activity for most substances, differences in auto-ignition temperatures are to be expected. In addition, since the action of many catalysts is specific, differing somewhat n-ith vtrious substances, the auto-ignition temperature will be affected accordingly. These factors may be practically eliniinated by choosing a surface of very small catalytic artivity and of small temperature coefficient of activity. A surface of nil catalytic activity and zero temperature coefficient of activity should give a value of auto-ignition temperatures which
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approaches the ignition temperature of a gaseous mixture. In accordance with the above it is apparent that in deterriiining the ignition temperature of a gaseous mixture the 1 and the igniting source should have a niiiiiiiium catalytic effect , Catalytic Effect of Various Surfaces
In order to determine nhich surface n-odd be best to choose For prcvnt purposei the results may be cwwidered under the following divisions: (1) Change in auto-ignition temperature using same surface and liquid; ( 2 ) c h n g e in auto-ignition teniperature using snine surface and different liquids, and using different surface. and the same liquid. C'HASGES USTSGSAMESVRFACE ASD LIqvm-That there may be a change in the catalytic activity of a given material when used in a given system is well known. The study considering this point mas limited to benzene on platinum. Because of the long time interval required and the desire not to interfere with the use of the platinum surface for other purposes, the change in auto-ignition temperat'ure with time was not carried to completion. Benzene was chosen as a standard or reference substance. ai; n standard, sevcr:il different inaterials were studied.
Figure 1
I n determining the temperatures listed in Table I, the apparatus was checked from time to time against benzene. During the early life of the plate checks were readily obtained, but in time it was noticed that the auto-ignition temperature increased steadily. This rise was studied until the autoignition temperature of the benzene had &en from 656" C. to 732" C., a t which point the teat waq discontinued. The rise of approximately 76" C. in the auto-ignition temperature after an inten a1 of about 6 weeks indicated a change in the catalytic acth ity of the surface. Since the auto-ignition temperature had increased, it indicated, in accordance with the ideas previously outlined, that the surface had become activated. Such changes as the abole are not unubual, similar changes in the catalytic activity of surfaces having been reported by many observers. Photomicrographic inT eatigations of platinum and silrer gauze indicate that there is a gradual increase in catalytic activity associated with the formation of minute craters or pits, the fringe of each crater being tinged with "black" metal. S o photomicrographic studies were made of the platinum plate, but it has probably been affected in the same manner. Upon heating the plate to a higher temperature by means of an oxy-acetylene torch, the
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original condition was not restored, making it impossible to use the plate. I n determining auto-ignition temperatures therefore-for example, using platinum-the temperature must be checked from time to time against some standard reference material. Also, it is necessary that the liquid under test be free from any material which might affect the surface adversely. I n studying gasolines containing lead tetraethyl this became a serious matter, making necessary frequent replacements of the platinum surface. This procedure was followed in determining the values in Table I whenever checks could not be obtained.
mating catalytic effect, but the change in catalytic activity wit’h temperature previously mentioned. Because of the very nature of the measurement it was impossible to keep the temperature of the surface constant and so eliminate this factor. A comparison of auto-ignition temperatures on platinum, a cat’alytically active metal, and on glass, a substance of much less catalytic activity, is interesting. I n accordance wit’h the theory previously evolved, it would be expected that a subst’ance of. marked catalyt’ic activity would raise the auto-ignition temperature. With the exception of nCHLVGES WITH DIFFERENTSURFACESOR DIFFEREKT heptane, a-n-heptylene, and p-cymene, all the substances LIQuIDs-The change of auto-ignition temperature with com- have higher auto-ignition temperatures on platinum than on position of the surface and liquid was, however, somewhat glass. The recent work of Coward and Guestlo on natural more fully investigated. The surfaces selected were Dlati- gas-air mixtures is of interest in this connection. They num, gold, silver, and glass (Pyrex). The results are given found that “in parallel experiments the metals of greater in Table 11. catalytic effect must be hotter to cause ignition than metals or other substances of smaller catalytic effect.” As one set T a b l e 11-Change of A u t o - I g n i t i o n T e m p e r a t u r e w i t h C o m p o s i t i o n of experiments was carried on initially in the gaseous state of S u r f a c e a n d L i q u i d , SUB5TANCE PLATIKUMGLASS GOLD SILVER and the other in the liquid, the results are not entirely comc. oc. oc. c. parable. The formation of a fiim of silver oxide on the silver a-n-Heptylene 33% 33% ... ... caused difficulties. The presence of such a film affects the n-Heptane 451 474 477 476 n-Octane 458 435 482 476 auto-ignition temperatures in an anomalous manner. The +-Cymene 466 ... 508 Diethyl etherI(l0 %:alcohol) 487 ... 479 474 difficuky of obtaining oxidized surfaces which would produce Benzyl alcohol 502 519 443 510 constant and duplicable results has limited observations to 518 449 Isoamyl alcohol 513 492 520 n-Hexane 515 536 538 this or other oxidized surfaces. It is appreciated that this 522 Ethylene glycol 535 457 ... 523 Glycerol 429 ... phase of the subject has been studied only in a preliminary ... 534 Isododecane 499 ... ... fashion, but sufficient has been presented to show the im540 n-Propyl alcohol 526 537 511 542 Isobutyl alcohol 522 540 512 portance of catalysis in the determination of auto-ignition 553 Ethyl benzene 557 ... ... 556 Nitrobenzene temperatures. ... 526 513 Ethyl alcohol, a b s o l ~ t e ~ 9 5 7 ~ 557 523 522 544 Experimental work is now in progress on the auto-ignition 568 532 528 552 561 Isooctane 583 584 537 temperatures of binary mixtures of pure substances, gasolines 548 Methanol 574 558 536 579 557 543 n-Pentane (tech.) of known “knock” rating, and other mixtures of interest. 564 .
Benzyl acetate o-Cresol Acetic acid Ethyl acetate 4-Xvlene iso
