DRYING OILS AND RESINS Mechanism of the “Drying”Phenomenon THEODORE F. BRADLEY American Cyanamid Company, Stamford,
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HE phenomena which occw during the transformation of liquid coating materials into their ultimate, solid form have never ceased to arouse curiosity and interest. Their vital importance to industry and to science has long been recognized and has led to a voluminous literature relating to the theoretical and experimental attempts to complete their elucidation. The extent of this literature and the mass of accumulated, and often seemingly conflicting, data have but served to emphasize the complexity of these phenomena and possibly to exhaust the patience of some investigators. Morre11 and Marks (15) perhaps reflected such an attitude when they wrote in 1929: “lt is extraordinary that in the important paint and varnish industry there is as yet so much uncertainty as to the changes which occur when a drying oil sets t o a solid gel. Every textbook is full of theories and sets forth a formidable array of facts which are inconclusive.” With the growth of the synthetic resin industry and of synthetic polymers in general there has accumulated, especially during the past decade, a fund of experimental and theoretical knowledge which bears an important and significant relation to the chemistry of the natural drying oils and to varnishes, and which, upon proper organization and general application, may be used to coordinate and t o simplify the more fundamental aspects of the “gelation” and “drying” phenomena. Since this has not been generally appreciated, it may be helpful to examine the available data, extending them by additional experiments where necessary, and thereby attempt to reach a more rational and simplified conception of the nature of these phenomena.
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are of requisite hardness and of sufficiently high softening point as to appear neither wet nor sticky upon loss of the solvent, the observed transformation may likewise occur as the result of chemical reactions as in the case of linseed oil, or by both mechanisms as in the case of o i l v a r n i s h e s a n d enamels. Although certain aspects of the physical mechanism of (‘drying” are believed t o require additional study, it is the chemical mechanisms that are of most concern and that require major consideration.
Polymers and Film Formation
The ability to form solid films immediately upon drying is characteristic of most useful coating compositions. The ability to form such films is generally limited to substances which are al: ready polymerized or which are capable Conn. , of undergoing Dolvmerization subsequent towthe& kpplication. Thus the chemistry of coating materials becomes synonymous with that of the polymers, and the mechanism of drying should then be intimately connected with some phase of polymerization. This had been recognized, but the experimental and theoretical attack seemed relatively fruitless until the illuminating work of Kienle (8, 9, IO) and of Carothers (9,3) appeared. And if we may judge from the nature of the subsequent literature (6, 82), the value and significance of their work has thus far been more generally appreciated in England than in this, their own, country. Of particular importance is their observation to the effect that polymerizations are nothing but the ordinary reactions of organic chemistry which are proceeding in multiple fashion because of the multiplicity of reactive or functional groups present in the structure of the polymerizing substance. Likewise the conclusion is important that, a t least in so far as the physical effect is concerned, there is an essential similarity between condensation and addition mechanisms which should permit of their consideration as a whole. The established effect of the degree of ‘(functionality” (effective number of bonds or points of intermolecular attachment) of a system in producing an essentially linear or two-dimensional polymer, on the one hand, or a cross-linked
Knowledge gained from the study of synthetic polymerides has been found applicable to many of the phenomena which have been observed in the case of natural substances of polymerizable nature. The “drying” of certain oils and resins is typical. While primarily physical, the drying of these oils and resins can be definitely related to chemical reactions which, in their more usual form, involve oxidation. The specific nature of these reactions is, however, indicated to be of lesser importance than the functionality equivalents
Drying Phenomena Since coating compositions are invariably of liquid consistency and the word “drying” implies the loss or destruction of liquid, the drying processes are primarily concerned with the conversion of liquid to the solid form, The term “drying,” therefore, relates primarily to a physical transformation of matter. While this physical transformation can in the case of spirit varnishes and lacquers depend solely upon the physical loss of volatile solvent, provided the residual solids 440
or three-dimensional polymer, on the other, is of fundamental importance. Through the combined e x p e r i m e n t a 1 findings of C a r o t h e r s (2, 3), Kienle (8, 9, IO), and Staudinger (I@, this effect may now be considered to apply just as well to addition polymers as to the condensation polymers and, in consequence, to those hybrid systems which, like the natural drying oils, involve both mechanisms. If we accept the postulate, now generally held, that all polymers which are fusible and/or soluble are essentially of the linear form, we must admit that all of our useful film-forming agents, up to the time of their h a 1 application as coatings, are approximately of the linear type. And if, as in the case of spirit varnishes or lacquers, the ultimate coatings or films can be redissolved upon application of a nonreactive liquid or be appreciably softened or fused by heat, then we can say that the filmf o r m i n g agents have remained in linear form. (This form, after aging, may or may not be fully identical with that of the original film, depending upon the extent of additional polymerization or of depolymerisation which may have occurred during its service; yet the solubility and thermoplastic characteristics can still assure us of its physical form.)
Chemical Drying Mechanism If we accept the p o s t u l a t e t h a t a l l polymers which are infusible and insoluble are of the cross-linked or three-dimensional form, we can then perceive that when, in the drying of a coating, we observe the formation of a film which is infusible and INTRICATE REACTION KETTLESAND CONTROL DEVICES MAKE POSSIBLE THE insoluble, we have witnessed the conversion PRODUCTION OF DRYINQ OILS AND RESINSWHICHARE REQUIRED TO MEET EXACTINQ SPECIFICATIONS of a linear structure to a cross-linked or three-dimensional form. Drying in the chemical sense, then, becomes only a mechamoter. In view of this it appears advisable to stress parnism in which an essentially linear and usually liquid substance ticularly the point that in the case of all of the natural and is converted into a cross-linked or three-dimensional polymer. synthetic drying oils or resins the primary prerequisite for h y means which accomplishes this conversion, whether the drying phenomenon is the possession of a structure which heat, light, or the addition of, or catalytic action of, oxygen, will permit of the formation of a three-dimensional polymer. sulfur, sulfur monochloride, polyvalent metals, etc., may be Hence the emphasis is now placed on the potential converticonsidered as the converting agent or as the converting probility of the system as a whole rather than on oxidation, On this account it is proposed to modify Kienle and Ferguson’s classification of the alkyd resins (8, 9); they recognized heat-convertible, heat-nonconvertible, and oxmen- or eleof the reactants. Drying under the inment-convertible types. I n the proposed revision only confluence of chemical reactions is detervertible and nonconvertible film-forming substances are mined by these functionality equivalents recognized as major types; the first corresponds to such suband requires the transformation of a stances as are capable of forming three-dimensional polymers linear monomer or its polymeride to a and the second to those which cannot be converted beyond the linear f0rm.I The means of conversion, as by oxygen, three-dimensional polymeric form. The heat, light, etc., would then allow of subclassification. This functionality concepts are revised and proposal is intended to cover all film-forming agents, including extended in order to permit of their the alkyds especially. Drying in the chemical sense may then valid application in later work. Active best be considered in the foregoing light and as limited t o the functionality has been differentiated convertible systems or, as Kienle implied, a phenomenon which relates to a special type of sol-gel transition. from potential functionality, and the influence of ring closures, whether by mono- or bimolecular reactions, has for the first time received consideration.
1 Protective ooating films were previously classified as “converted” and “nonconverted” in a paper presented by R. H. Kienle and E. H. Winslow before the Division of Paint and Varnish Chemistry at the 89th Meeting ot the American Chemical Society, New York. N. Y., April 22 to 26, 1936.
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Subclassification of the nonconvertible polymers also becomes advisable. Reference is made to the case of certain types of linear polymers which are capable of polymerizing further under the influence of light, oxygen, heat, catalysts, etc., but in which the polymerization is essentially confined to the original direction of growth rather than to cross linkage. There also exist substances which are capable of adding oxygen intramolecularly and without attendant polymerization. Such reactions are capable of hardening the linear polymers and of raising their softening points and of altering the viscosity characteristics of their solutions, but have no real significance in connection with the drying phenomena. We may refer to these as chemical haydening processes.
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suitably activated and in the original reactants may then be considered as only potentially functional. Viewed in this light, the convertible polymers result only in those systems where the functional linkages of the monomer exceeded two-in other words, where the simplest (monomeric) unit is capable of joining with more than two other units a t points on the molecule which differ in each case. But if we consider the geometry of this interpretation, it becomes evident that the main requirement for the production of a convertible polymer is the utilization of a system whose unit molecule possesses a suficient number and type of reactive centers to permit of an unrestricted growth in substantially all directions.
Convertible Polymers
Hybrid Polymers
If drying in the chemical sense is thus restricted to the convertible systems, it is perceived according to the established principles of polymerization that the system should be more than bifunctional in the case of the addition mechanism or more than bi-bifunctional in the case of the usual type of condensation mechanism; otherwise the substance will not dry. We must otherwise then proceed to introduce one or more additional functions into the molecules of the reactants in order to obtain a convertible system. Although the term “functional group” has been applied to all groups which react in a polymerization-for example, the carbon-to-carbon double bond, hydroxyl and carboxyl groups, etc.-perhaps it should be made more clear that, although these groups constitute the active centers, their presence does not necessarily ensure polymerization. (If w e examine this question more closely, we perceive that it is not the mere presence of the active group which determines the functionality of the polymerizing substance but rather the number of points on the unit molecule a t which attachment to other units becomes possible as well as the application of enough energy to activate these centers. Thus we may regard the functional groups as the connecting links or bonds between separate molecular entities.) A plurality of reactive groups does not necessarily confer polyfunctionality since if these should undergo intramolecular or even bimolecular reaction with attendant ring formation they fail to become polyfunctional in the sense of polymerization reactions. This relates to the observation that a plurality of linkages between any two molecules is no more effective in a polymerization mechanism than is one connecting link, and to the fact that the “active groups” become functional only when they are
In his general theory of resin formation Kienle (8) ascribed to the carbon-to-carbon double bond a potential functionality. But in his consideration of the condensation polymers he did not clearly explain the effect of such bonds, when and if they might be present, since he had previously inferred that glycols and dibasic acids always yielded nonconvertible polymers (9). Carothers and Arvin (3) also avoided this contingency and, in their comprehensive studies of the bibifunctional condensations, included the ethylene maleates and fumarates without attempt a t differentiation; while observing the heat convertibility which was unique to those particular systems, they did not attribute this convertibility to the functional activity of the double bonds. Tsuzuki (20) also studied the same systems but did not carry the polymerization far enough to obtain heat conversion. The failure of the original investigators to make themselves clear on this point is evidenced by the lamentable statement of others as follows: “From the considerations already advanced, it should be clear that the reaction between a dihydric alcohol and a dibasic acid can produce only a linear or straight-chain polymeride” (6). It appears, therefore, that in spite of the established knowledge as to the activity of the double bond in purely addition polymerizations and of the corresponding activity of hydroxyl and carboxyl groups in purely condensation polymerizations, there has been insufficient .consideration of the hybrid systems in which both addition and condensation mechanisms become jointly effective. Since the natural drying oils and the oxygen-convertible alkyds belong in this group, these hybrids must be considered of great importance.
Functionality of Carbon-toCarbon Double Bond While recognizing the potential functionality of the carbon-to-carbon double bond, we should be cautious in attributing a functionality of two to each and every such bond. Would it not be wiser to recognize the wide variations in the degree and mode of activity of these bonds such as are known to be caused by the structure of the molecule as a whole and more particularly by the nature and position of substituent atoms or radicals ( l 7 ) ? As a step in this direction it is proposed first to segregate and recognize the individuality of those double bonds which, as in the methylene and vinyl radicals, occupy a terminal position in an essentially hydrocarbon-like chain. Most of our synthetic linear polymers of high molecular weight are derived from substances which possess this particular type of
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I I
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I I
M S,?POMF/CRjl4rl: w L / n - # r M z
FIGURE 2
peculiarly activated and caused to reverse its normal mode of activity under the influence of even minute traces of oxygen. From the evidence we may conclude that the functionality of this type of bond is generally two. Multiple unsaturation of the benzenoid or aromatic type constitutes a second class, which is ordinarily observed to be nonfunctional. Unsaturation which is confined within an aliphatic hydrocarbon chain may be placed in still another category, and it is this type that is common to the natural drying oils and to the commercial grades of oxygen-convertible alkyd resins.
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have been more inclined to interpret their data by chemistry. Molecular weight determinations of the soluble, polymerized forms of the drying oils and of their related esters have in all cases been of a comparatively low order, rarely exceeding that of a dimer (1). Dimerization through a bimolecular reaction involving ring formation is therefore believed to be a characteristic property of both the conjugated and nonconjugated drying oil acids and of their esters. If some still prefer to attribute this dimerization to secondary valence or association forces rather than to actual primary valence bonds, their opinions may still be acceptable, .provided only that we admit the possibility that the association forces are sufficient to act as a functional linkage in the sense of the Kienle and of the Carothers polymerization mechanisms.
Natural Drying Oils as Convertible Hybrid Polymers Although the natural drying oils are not regarded as polymers in the accepted sense of that word, it should be realized that as glycerol esters of the unsaturated fatty acids they have already undergone one step toward the polymeric state. I n order to extend this polymeriaation, it is necessary to have additional groups which can be caused to function. The only groups which remain are the carbon-to-carbon double bonds. When these are suitably activated as by heat, light, oxygen, etc., polymerization occurs and we finally secure a conversion which, when confined to coating compositions, is ordinarily also termed "drying." That the convertibility of such oils is the resultant of the composite effect of all the functional linkages, regardless of their specific nature, has been shown and in turn provides additional proof of the related effects of the condensation and addition mechanisms. I n spite of extended controversy as to the exact mechanism by which the unsaturated fatty acids and their esters undergo heat or oxygen polymerization, most investigators are now agreed as to the loss of unsaturation which occurs through mutual saturation of the double bonds of the acid radicals by ring closure, and with formation of products which upon analysis are observed to be substantially of dimeric form.
The tendency of nonterminal unsaturation to yield chiefly the more restricted form of polymerization-i. e., dimerization and ring closure-should be more widely appreciated. Dimerization in the absence of extraneous active groups, which results in ring closure, must limit the active functionality from the polymerization viewpoint to only one, since only one molecule can be added under the operative mechanism.
Polymerization Induced by Heat
Polymerization Induced by Oxygen
Ring closure, with respect to heat-polymerized tung oils, was recognized by Ware and Schumann (21) and by Morrell (14) some twenty years ago. Innumerable experimental data and theoretical concepts have been presented during the ensuing years, but the most recent work, such as that of Rossman (16) and of Jordan (6), has again emphasized the original concept. The more recent work indicates that the dimerization of tung oil occurs by a diene reaction, with the formation of a substituted six-membered hydroaromatic ring
Evidence has shown that peroxide formation occurs as the first and most fundamental step in the oxygen conversion of the oils. Beyond that we encounter a mass of controversial data and of opinion. The amount of oxygen required to produce the polymerization varies with the nature of the fatty acid radical, with the amount of heat treatment (addition polymerization) which the compound may have previously experienced and with the nature of the alcoholic radical that is combined with the fatty acid radical of the compound (12).
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I n general those substances such as are known to possess a which must inevitably result in a three-dimensional polymer conjugate system of double bonds are found to polymerize whether the ultimate conversion is effected by heat Or by oxygen mechanisms* and by addition Or most readily and with less oxygen requirement than comFigure 2 is a similar representation of the course of a polypounds Of nonconjugate structure* The mass Of merization of the glycol esters of the drying oil acids, which result data has not yet been resolved to provide a clear understandonly in the linear or two-dimensional form. Figure 3 similarly represents the more restricted polymerizaine: of the mechanism by which oxygen acts in each case.2 It tion (dimerization) of the saturated may act differently monohydric alcohol esters of the for various cases, just drying oil acids. as the mechanism of Although the molecular coma n a d d i t i o n polyplexity iS increased by the nummerization may differ ber of active groups on the alcoaccording to the parholic radical, regardless of the ticular structure of ultimate s i z e of t h e polymer, the compound and to saponification and hydrolysis will t h e environmental in each case result in the recovery conditions. I n the of the acids in dimeric form. mass of accumulated Further, of these three only the data we should be triesters of glycerol are of the coni m p r e s s e d by the vertible or drying type. This is comparatively 1o w in accordance with the observaorder of the molecutions of many investigators who lar weights such as reported the nonconvertibility of have been observed the mono- and dihydric alcohol in all of the oxidation esters of the drying oil acids and s t u d i e s and which the convertibility of those esters prevail up to the solwhich are derived from alcohols gel transition or conpossessed of three or more hyversion point of the droxyl groups (IS). oils, and likewise by It is also to be expected that, the low m o l e c u l a r as we further increase the number weights of the acids of f u n c t i o n a l linkages in the which have been reinitial condensation polymer or covered. This indiby a preliminary heat treatment cates that, even if the LARGE-SIZE BATCHSTILLSUSEDTO PRODUCE DRYING OILS extend the initial degree of polyso-called chain reacAND RESINS merization, we should in either tions (19) b e c o m e case increase the rate of convertiinvolved in any such bility and require less oxygen or less energy to effect the ulticase, these chains are fairly well limited to a bimolecular mate conversion, This theory is in accord with the observaring closure, a t least in so far as the fatty acid radicals tions of Long and his co-workers (1.2)and also suggests the themselves are concerned; thus from a physical standpoint reason for the increased degree of convertibility which is so they differ but little from the polymerizations induced by characteristic of the modern synthetic finishes such as those heat. which are derived from the phenol-formaldehyde and from the alkyd resins. Functionality of the Drying Oil Acids We would likewise expect that anything which may serve Such considerations suggest that the fatty acids of the to prevent an active group from functioning will thereby renatural drying oils possess a normal functionality of but two, strict the polymerization and thus inhibit or prevent the conone function resulting from the carboxyl group and the version of the normally convertible systems. Thus we may second from the entire system of unsaturated carbon-to-carbon inhibit the conversion of a drying oil by a substance which, double bonds. (Under somewhat unusual conditions an like an excess of glycerol, can effect the condensation mechaadditional function may be derived from the unsaturated nism by mono- or diglyceride formation, or on the other hand system, particularly where the acids are possessed of three by an antioxidant such as hydroquinone which can prevent double bonds.) the activation of the unsaturated system by oxygen. If the foregoing conceptions are essentially correct, one I n conclusion it may be stated that further progress in the would then expect the drying oil acids to form convertelucidation of the polymerization and drying mechanisms of ible systems when combined with alcohols having three or the drying oils will be largely influenced by the further study more active groups and nonconvertible systems when comand proper classification of the various carbon-to-carbon bined with alcohols having a lesser number of active groups. double bond systems and by researches which will more defiThe application of these principles may be illustrated as nitely ascertain the shape and size of the molecular aggregafollows : tions at the sol-gel transition point. Experimental evidence bearing upon these fundamental Figure 1 represents, from the functionality viewpoint, the problems and upon the validity of the concepts of this inessential steps in the polymerization of the natural drying oils, troductory paper has been accumulated and it is to be pre2 During the period which has intervened since the original presentation sented in forthcoming publications.s of this paper, Morrell and Davis have supplied experimental data relating to the oxidation mechanism of eleostearic acid and its maleic anhydride derivatives which greatly clarify this situation. These data appear to support the contention that the double bonds of the fatty acid radicals are not equivalent but react in such a manner as to restrict the active functionality of the molecules [ J . Soe. Chem. I n d . , 66, 237-46T (193611.
a Following the presentation of this paper on April 14, 1936, R. H. K i e d e [ J . SOC.Chem. I n d . , 55, 229-37T (l936)l adopted much of the theory oontained therein and revised his original postulates to conform with it. R . Houwink [Ibid., 55, 247-48T (1936)l subsequently recogniied the necessity of revising the functionality ooncepts in accordance with these ideas.
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Literature Cited (1) Caldwell, B. P., and Mattiello, J., IND.ENQ.CHEM.,24, 158-62 (1932); Long, J. S.,and Wentz, G., Ibid., 18,1245-8 (1926); Long, J. S.,and Amer, W. J., Ibid., 18, 1252-3 (1926); Long, J. S., Egge, W. S., and Wetterau, P. C., Ibid., 19, 903-7 (1927); Long, J. S.,Reineck, A. E., and Ball, G. L., Ibid., 25, 1086-91 (1933); Rhodes, F. H., and Welz, C., Jr., Ibid.,
19, 68-73 (1927). (2) Carothers, W. H., J. Am. Chem. Soc., 51, 2548-59 (1929); Chem. Rev., 8, 353-426. (3) Carothers, W. H., and Arvin, J. A., J . Am. Chem. SOC., 51, 2560-70 (1929). (4) Elm, A. C., IND.ENQ.CHEM.,23, 881-6 (1931). (5) Jordan, L. A.,J. Oil Colour Chem. Assoc., 17, 47-66 (1934). (6) Jordan, L. A., and Cutter, J. O., J . SOC.Chem. I n d . , 54, 90T (1935). (7) Kharasch, M. S., and Mayo, F. R., J. Am. Chem. SOC.,55, 2468-96 (1933); Kharasch, M. S.,and McNab, M. C., J. SOC.Chem. Ind., 54, 989-90 (1935). (8) Kienle, R. H., IND.EIQ. CHEM.,22, 590-4 (1930). (9) Kienle, R. H., and Ferguson, C. S., Ibid., 21, 349-52 (1929) (10) Kienle, R.H., and Hovey, A. G., J . Am. Chem. SOC.,51,509-19 (1929);52, 3636-45 (1930);Kienle, R. H., IND.ENQ.CHEM., 23. 1260-1 (1931): 25. 971-5 (1933). (11) Kino’, K., Sei. Papers ‘Inst. P i y s . Chem. Research (Tokyo), 26, 61-7 (1935). (12) Long, J. S., and McCarter, W. S. M., IND.ENQ.CHEM.,23, 786-91 (1931); Long, J. S.,and Chataway, H. D., Ibid., 23, 53-7 (1931).
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(13) Morrell, R. S., J . Oil Colour Chem. Assoc., 7, 159 (1924);Fonrobert, E., and Pallauf, F., Chem. Umschau, 33, 44 (1926); Miller, A. B.,and Claxton, E., IND.ENQ.CHEM.,20, 46-7 (1928); Miller, A. B., and Rohrbach, R., Ibid., 21, 338-42 (1929); Thurman, B. H., and Crandall, W. R., Ibid., 20, 1390-2 (1928); Kino, K., J. SOC. Chem. I n d . J a p a n , 33, Suppl., 153 (1930); Drinberg, A. Y.,and Blagonravova, A. A., J. Gen. C h m . (U. S. 9. R.), 5, 1226-32 (1935). (14) Morrell, R. S., J. SOC.Chem. I n d . , 34, 105-9 (1915). (15) Morrell, R. S.,and Marks, S., J. Oil. Colour Chem. Assoc., 12, 184-202 (1929). (16) Rossman, E., Fettehem. Umschau, 40, 96-123 (1933). (17) Staudinger, H.,Trans. Faraday SOC.,32 (I), 97 (1936). (18) Staudinger, H., and Huseman, E., Ber., 68, 1618 (1935); Staudinaer. H.. and Heuer. W.. Ibid.. 67. 1164-6 (1934): Staudinger; H.,’Heuer,W., and Huseman, E:, Trans. F k d a y SOC.,32 (I), 323-32 (1936). (19) Stephens, H.N., IND.ENQ.CHIM., 24, 918-20 (1932). (20) Tsuzuki, Y.,Bull. Chem. SOC.(Japan);lO, 17-26 (1935). (21) Ware, E. E., and Schumann, C. L., J. IND.ENQ.CHEM.,7, 571-3 (1915); Schumann, C. L., Ibid., 8,7-15 (1916). (22) Wornum, W. E.,J . Oil Coolour Chem. Assoc., 16, 231-42 (1933); 17, 119-45 (1934); Jordan, L. A.,Ibid., 17, 47-66 (1934); Cutter, J. 0..and Jordan, L. A.,Ibid., 18, 5-11 (1935). RECEIVED April 21,1936. Presented before the Division of Paint and Varnish Chemistry a t the 91st Meeting of the American Chemiaal Sooiety, Kansas City, Mo., April 13 to 17, 1936.
Flammability of Propane-Air Mixtures R a n g e at Low Pressures H.w.VAN DER H O m N N. V. De Bataafsche Petroleum FJIaatschappij, Amsterdam, Holland
LTHOUGH t h e explosive properties of mixtures of combustible gases and air have been repeatedly examined, most investigators have experimented with mixtures under atmospheric or increased pressure (1, I, 6). Only methane-air, hydrogen-oxygen, and carbon monoxide-oxygen mixtures have been studied a t low pressures (3, 4, ?‘), so that it seemed worth while t o publish some measurements on propane-air mixtures at reduced pressure. The limits of flammability of propane-air mixtures under reduced pressure were determined as follows (Figure 1): Measured quantities of propane and air were carefully mixed in a buret, E , over mercury. The air had previously been dried over calcium chloride and phosphorus pentoxide; the propane, which was freshly distilled, was already dry. Five to seven milliliters of the mixture were forced under pressure into an explosion buret, A , which contained two fused-in platinum electrodes, C (diameter about 0.3 mm., length in the gas 4.5 mm.), 1mm. apart. After the mixture had been introduced into the explosion buret ond the pressure had been adjusted to one atmos here, the buret was closed at the to , B, and the ressure reducefto the re uired extent by means o r a mercury-fiEed leveling bottle, D. 8ubsequently the gas was ignited by a s ark discharge between the electrodes, which was produced by a gord induction coil.
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After each determ i n a t i o n the remaining gas was expelled and the buret d r i e d b y moving the mercury level up and down several times; as a result the water formed evaporated in the fresh supply of dry air. T h e l i m i t s of FIGURE1. APPARATUSFOR THE DEflammability were TERMINATION OF EXPLOSION LIMITS approximated from AT PRESSURES BELOW ONE ATMOBPHERE b o t h sides. The same gas mixture was examined at different pressures until finally no further explosion occurred at low pressures. As was to be expected, the range of flammability was found to decrease a t lower pressures. With the apparatus used, the lowest pressure at which explosion still occurred was 210 mm. of mercury a t a concentration of 4 to 5 per cent propane by volume.
The results of these measurements are shown graphically by curve 111, Figure 2. The pressure in millimeters is plotted
