Evidence for the Cvclic Structure of Bituminous Coals J
H. C . HOWARD Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pa.
D
ESPITE the large amount of physical and chemical evidence which has accumulated during the past 25 years pointing to the predominantly cyclic structure of bituminous coals, there appear from time to time suggestions that the cyclic compounds which are found t o predominate in all the degradation products of bituminous coals do not represent fragments of the original coal structure, but are formed in secondary cyclizing reactions during the degradation process. As long as the only degradation products from coal which had been studied were those from a carbonizing process operating a t 900' to 1000° C., there was little evidence again& this point of view, but modern investigations of the nature of the products obtained by thermal decomposition in vacuum at temperatures not higher than 525" C., by solvent action a t 250" t o 400" C., by hydrogenation at 300" t o 375" C., and by oxidation reactions at 100" t o 270" C.1 have shown that the degradation products recovered from bituminous coals even in reactions occurring as low as 100" C. are predominantly cyclic. No evidence indicating t h a t the yield of cyclic degradation products is related to the temperature a t which the degradation process is carried out has appeared, thus rendering improbable the hypothesis of the formation of the cyclic compounds by secondary thermal reactions. On the other hand, in low temperature oxidation reactions, for example, a good correlation has been established between the "rank" of the coal reacted and the yield of cyclic compounds. If the cyclic structures were being formed in t h e reaction, one would expect the yield of such compounds to be independent of the nature of the starting material. The early erroneous belief t h a t alkane- (paraffin-) type structures predominate in low temperature tars and that the characteristic difference between high and low temperature tars consists in a high content of cyclic compounds in the former, and acyclic in the latter, seems t o be particularly persistent. For example, it is stated in a recent treatise in organic chemistry (288)t h a t "low temperature tar, obtained by heating soft coal a t about 600" C., contains alkanes, which a t higher temperatures are converted t o aromatic substances." I t is the purpose of the present discussion to review the accumulated physical and chemical evidence on this problem of the source of the cyclic structure found in the degradation products of all bituminous coals. ULTIMATE COMPOSITION
Chemical composition often gives useful clues as to structure. Particularly important as an index of structure in organic compounds is the atomic ratio of carbon t o hydrogen, since it changes from a value of less than 0.5 in purely aliphatic types to five- t o tenfold this value in condensed cyclic types. The atomic carbon-hydrogen ratios for a number of typical American coals ranging in rank from a Pennsylvania anthracite t o a Wyoming subbituminous coal are plotted in Figure 1 as a function of volatile matter. In Figure 2 is shown the atomic carbon-hydrogen ratio as a function of molecular weight of a number of hydrocarbon series; the same ratios for a number of coals are shown on the ordinate for ready comparison. The atomic carbon-hydrogen ratios of the varioub hydrocarbon series increase with molecular weight, but i t will be observed t h a t only in the series containing
cyclic structures do the atomic carbon-hydrogen ratios exceed unity even a t infinite molecular weights. Peripherally condensed systems of the coronene type are characterized by exceptionally high values of carbon-hydrogen ratios even in the range of molecular weights below 1OOO. For example, the fourth member of this series has an atomic carbon-hydrogen ratio of 4.0 and contains almost 98% carbon on a weight basis. On the basis of carbon-hydrogen ratios alone, the series made up of linearly linked phenylene structures falls i n the range of bituminous coals of 45 t o 25% volatile matter, but the condensed type of structure is required t o account for the composition of the higher rank coals. Other facts, however, make such a linear, polyphenylene nuclear structure improbable for even the lower rank coals. All bituminous coals yield methane and other lower paraffin hydrocarbons on pyrolysis, and i t has been shown ( 1 7 ) in a series of 37 coals t h a t the yield of acetic acid on oxidation is related t o the yield of methane. It thus appears probable t h a t there are short aliphatic side chains in coal which are responsible for the appearance of methane on pyrolysis and enter into the formation of acetic acid on oxidation. The introduction of such aliphatic chains into a linear polyphenylene structure results in lowering the carbon-hydrogen ratio; one methyl group per ring decreases the value of the carbon-hydrogen ratio a t infinite molecular weight from 1.5 to 1.16. Further, the evolution of hydrogen halide by coals when treated with halogen (86') and easy loss of hydrogen on heating is evidence for the presence of saturatedt h a t is, alicyclic rings (,%)-and these would also decrease the carbon-hydrogen ratio significantly. The presence of such aliphatic chains or alicyclic rings would necessarily require structures elsewhere in the coal of higher carbon-hydrogen ratio than the polyphenylene type, in order t o maintain the average ratio exhibited by the coal. None of the physical properties of bituminous coals correspond to linear structures, which are characterized by low melting points, thermal plasticity without decomposition, solubility, or swelling a t room temperatures, anomalous viscosities in solution, and fiber-forming properties even a t moderate molecular weights. That the properties which have been mentioned are not connected with any specific type of chemical composition, but rather are characteristic of linear structures, in general, is evident if one considers the properties of compounds of such diverse composition as cellulose, natural rubber, the synthetic vinyl polymers, t h e Bakelite types in which reaction at other than the ortho position has been blocked, and the great variety of condensation polymers where growth is restricted t o a linear form by the exclusive use of bifunctional reactants. Indeed, i t is extremely unfortunate from the standpoint of the use of bituminous coals a~ chemical raw materials, that they show none of the properties of linear molecules. Coals contain atoms other thap carbon and hydrogen and, for purposes of comparison, calculations of average compositions on an atomic basis are presented in Table I for a number of typical coals ranging in rank from a Pennsylvania anthracite to a high oxygen, high volatile matter Wyoming coal (21). These coals, which have a wide range in properties and commercial uses, do not differ greatly in atomic composition. Further, it will be
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1084
Vol. 44, No. 5
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I. ULTIMATE COMPOSITION OF COALS State Pennsylvania Arkansas
Bed Lackawanna Lower Hartshorne
C , 70 71.49 64.25
H, %
Pennsylvania Kentucky Illinois Wyoming
Pitisburgh No. 9 No. 5 Elkol
-
54.20 51.70 50.42 49.77
41.80 42.34 43.11 42.30
25.97 33. I
C
Atomic Composition H, % C/H% 0,% 97.46 2.75 1.62
+
94.04 93.53 92.07
1.22 1.17 1.17
4.22 4.56 7 03
N,%
S,%
0.66
0.26
0.93 0.71 0.71
0.81 1.20 0.19
show the properties of a series of compounds of similar skeletal carbon structure and with the variations in carbon-hydrogen ratios being only what would be expected in members of a condensed cyclic series of varying molecular weight and differing degrees of saturation with hydrogen. THERMAL DECOMPOSITION AT NORMAL’AND REDUCED PRESSURES
.?0
I
JZQ
I .40 1.10 I IO
I
I
20 30 % VOLATILE MATTER ( a . m.f.)
Figure 1. Relation between Carbon-Hydrogen Ratio and Typical Coals
For commercisl processing as well as for analytical and research purposes, coals have been subjected to thermal decomposition under a great variety of conditions, a ith respect to both heating rates and maximum temperatures. In all of these decompositions the products recovered have been predominantly cyclic in structure, of varying degrees of saturation and with varying amounts of short aliphatic chains attached (9). The distinction between the tar recovered from by-product oven operation a t 1000” C and that from a pyrolysis a t 500’ C does not lie in any marked difference in the yield of cyclic structures, but rather that in the low temperature tar the caibon cycles may be saturated with hydrogen and may have short, aliphatic, hydrogen-rich side chains (8, 13 15). Any conclusions drawn as to structure when such violent methods of thermal I I 40 50 decomposition are employed are admittedly uncertain, but it appears that certain investigators have overlooked Volatile Matter for alternative explanations in accounting for the condensed cyclic structures found in the degradation moducts from commercial carbonization processes For example, it has been suggested that the cyclic oxygen-containing compound coumarone, found in high temperature tars, is synthesized from an alkylated phenol by a secondary reaction involving ring closure (22). It appears just as probable that the benzofuran skeleton of coumarone represents a part of the original coal network (19) and that the phenolic compounds result from the opening of oxygencontaining rings. The development of the technique of molecular distillation (24) about 20 years ago furnished a new tool for the study of the thermal decomposition of coal under such conditions that secondary reactions are reduced to a minimum (14). In this procedure, there is a short direct path between the surface of the distilling material and the condensing surface, and a t the pressures maintained, the length of this path is much less than the “mean free path” of the molecules leaving the evaporating surface. Thus, secondary cyclizing reactions must have taken place before the distillation products have left the coal surface. The yields and nature of the products recovered from such a thermal decomposition of a Pittsburgh Seam coal, along with comparative figures for a distillation of the same coal a t the same temperature but a t normal pressures, are shown in Table 11. The decompositions in vacuum were carried out on both a 20- to 40-mesh sample and on one ground to about a micron size. Because of the increased ease of escape of the primary products from the small particle-sized material, significant effects of particle size on yield were found in the decomposition in vacuum. However, the outstanding difference between the decompositions in I
noted that carbon and hydrogen constitute over 90% of the atoms in even the low rank western coal and that the sum of these elements is greater than 97% on four of the coals. Even in the highest oxygen coal, the oxygen content amounts to only 7 atomic yo. I n all the coals, nitrogen and sulfur together constitute 1 to 2 atomic %. Conclusions drawn as to the structure of coals by comparison with known hydrocarbon series appear justified not only because the coals are predominantly carbon and hydrogen on an atomic basis, but also because the introduction of oxygen atoms into a hydrocarbon affects carbon-hydrogen ratios relatively little. For example, the introduction of oxygen, by replacing a hydrogen atom by a hydroxyl group, or as a linear ether link between carbon atoms, changes it not at all, and it is only when the oxygen replaces two hydrogen atoms and appears either as a cyclic ether or a carbonyl group, that significant changes are noted. The question is often raised as t o the validity of any conclusions based on the average ultimate composition of material like bituminous coal. Coal is not homogeneous, but if coal were composed of a mixture of chemical compounds differing greatly in chemical structure and consequently in carbon-hydrogen ratios, one would expect to recover from some type of physical or chemical degradation, fractions which differ sufficiently in ultimate composition to indicate the presence of different types of carbon skeletons. With the exception of waxes and resins recovered from materials very low in the scale of coalification, such as the German brown coals, this has not been observed. On the contrary, fractions from R great variety of degradation processes all
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I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
vacuum and at normal pressures lies in the greatly increased yield in the former case of neutral, ether-inpoluble products. These are high molecular weight, solid bituminous substances, which have the high carbon-hydrogen ratio, high density, and color that are characteristic of complex cyclic compounds. Thus, this evidence points strongly to the presence in bituminous coals of cyclic structures, parts of which may be distilled as large units when heated under molecular distillation conditions, but which in heating under normal pressures are converted by secondary reactions to simpler hydrocarbons and to phenols and bases. THERMAL DECOMPOSITION IN PRESENCE OF A LIQUID PHASE
,
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4.20
3.80
3.40
3.00
2.60
I
3 2.20
Heating bituminous coals in the presence of organic solvents results in Q the formation of soluble degradation products of moderate molecular weight gj 1.80 in significant yields. The extent of the a degradation depends both on the temperature and the specific properties of the solvent used. Solvents of low in1.40 ternal pressure, such as aliphatic hydrocarbons, are least effective and those of high internal pressure especially aromatics and those combining I .oo aromatic and alicyclic structures with polar groups, such as hydroxyl and amine, are most effective (16). Typical data for yields from a Pittsburgh Seam 0.6 C coal for two solvents, benzene and Tetralin, are shown in Table 111. Compared with other methods of thermal decomposition, decomposition 0.2c of bituminous coals in solvents is distinguished by the low temperatures a t which significant yields of degradation Figure 2. Relation products can be recovered and by the absence of coking and gas-forming reactions. The products, whether they represent only 18% of the coal, as with benzene, or some SO%, as with Tetralin, are remarkably similar in physical properties and chemical composition. They are viscous, browli bituminous materials, predominantly neutral in character, and all contain some nonreactive oxygen. These products recovered by solvent degradation are similar in many ways to those recovered by distillation in high vacuum. They show even a larger fraction of neutral material and especially of the higher molecular weight, ether-insoluble bitumens, and markedly smaller amounts of the lower molecular weight liquid
TABLE 11. THERMAL DECOMPOSITION AT 525'
c.
between Atomic Carbon-Hydrogen Ratio and Molecular Weight for Hydrocarbons hydrocarbona, phenols, and bases. Data on the ultimate composition of these materials are shown in Table IV. Compared with the original coal, both the products recovered with benzene and Tetralin show lower, and the residues higher, carbon-hydrogen ratios. The markedly lower ratios in the Tetralin extracts point to hydrogen addition by transfer from the Tetralin. That the simpler, neutral, ether-soluble materials, as well a s the phenols and bases, are largely decomposition products of the ether-insoluble solid bitumens is indicated by the data of Figure 3 which Ehow that preheating the coal in the 350" to 450' C. range
TABLE 111. DECOMPOSITION BY SOLVENT EXTRACTION 3'% of Coal
% of Coal Composition of , Bise of Sample Distillates 20- to 40-mesha Microns Neutral Ether-insoluble 7.11 17.10 Ether-soluble 8.61 8.20 Phenols and acids 2.56 1.37 RSWA. n 3n __""" ".-0.16 Water and loss 2.32 1.50 0 Decomposition in vacuum. 6 Decomposition at atmospheric pressure.
20- to 40-meshb 2.78 7.51 2.58 0.40 3.98
Extra06 Neutral Ether-ineoluble Ether-soluble Phenols and aoids
-SolventBenzene0
-
Tetralinb
61.4 64.39 36.3 32.92 1.19 2.17 Bases 0.38 0.51 Temperature, 160' to 26da C.; 15 stages; total yield of dry, ash-free ooal, 18.0%. b Temperatura, 250' to 360' C.; 79 stages; total yield of dry, ash-free cod, 81.6%.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1086 TABLE IV. Original coal Benzeneextract Benzene residue Tetralinextract Tetralinresidue
0.60
1.11
dation method used in obtaining this extract material from the original coal (as has been already pointed out these decompositions in liquid phase result neither in gas nor coke), it seems improbable that the cyclic structures isolated could have been formed by secondary reactions.
0.80 0.14 2.22
1.40 1.02 1.61
EIY DROG ENATIOX
ULTIMATECOMPOSITION O F EXTRACTS
c, % 85.51 85.62 86.51 83.80 84.99
If, % 5 33 6 39 5 15 6.81
4.38
0, 70
N,
6 07 5.77 5.94 8.81 7.18
1.78 1.60 1.57 0.44 1.23
9%
8 %70 1.08
-4tomic
C/H 1.28
ec-Figure 3.
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Effect of Heat Treatment of Coal on Composition of Extracts
results in almost complete disappearance of the high molecular weight, ether-insoluble components and significant increases in yields of the lower molecular weight neutral ether-soluble products and phenols and bases. In addition to the evidence from ultimate composition and physical properties, structural studies of these primary decomposition products by further degradation, through oxidation and hydrogenation, have led to the isolation and identification of coxiipounds of simple cyclic and of condensed cgclic structure. English workers ( 5 ) isolated from the alkaline permanganate oxidation products of the petroleum ether-insoluble fraction of extract from Busty coal, a mixture of oiganic acids, of which the benzenoid fraction constituted 62% of the weight of extract used. Hydrogenation (9)with a copper-chromium oxide catalyst of the petroleum ether-insoluble fraction of the benzene extract of a Pittsburgh Seam coal, resulted in degradation to a miyture of hvdrocarbons covering a boiling range of 70” to 400” C. The determination of such properties as density, refractive index, specific refraction, molecular weight, and normal boiling points demonstrated that except for the low boiling materials which contained small amounts of aliphatics, these oils were made up of wholly or partly hydrogenated aromatics On the basis of this Tvork Biggs (3) presented the following picture of the structure of these materials: “The general type of structure characteristic coal extracts is represented by a network of carbon rings, with occasionally an oxygen atom, or, much more rarely, a nitrogen or sulfur atom, occupying a position either in the ring, or linking two nuclei. The atomic carbon-hydrogen ratio of the original extract indicates that some of the rings may be hydroaromatic. The distribution of oxygen, nitrogen, and sulfur is probably a random one, in some places leaving large sections of continuous hydrocarbon structure, and in others possibly separating a mono- or bicyclic unit from the remaining structure. Variations can be, and probably are, almost unlimited.” I n view of the mild degra-
The presence of single- and condensed-ring struct.ures in the degradation products obtained by hydrogenation of biturninow coals has been demonstrated by man?; workers ( 4 , 18,27), and the view that the carbon skelet,ons of these compounds represent fragments of the original coal network has been generally accepted. However, the degree of saturation of the rings is regarded as without significance, since it is known that certain double bonds in cyclic structures are so reactive that they are readily saturable even with such catalysts as copper-chromium oxide, and that loss of hydrogen from alicyclic rings may also occur. This view as to the significance of the products from hydrogenation is furt’her supported by the general observation that the recovery of condensed cyclic structures from the hydrogenation products of coal is not dependent on the use of special temperatures, hydrogen pressures, or cat’alysts, but that the yield and the distribution of molecular sizes in the products depends on the rank of t,he coal used (18). Studies on the hydrogenolysis of lignin are also of interest in this connection. It has been shown that if a sample of freshly prepared lignin from young aspen trees: isolated by the action of metha,nol and hydrochloric acid is subjected to hydrogenation a t 260 C. in the presence of a copper-chromium oxide catalyst the characteristic and distinctive compounds isolated have carbon skeletons consisting of a six-membered ring with a three-carbon aliphatic chain (12). If, on the ot’herhand, a commercial sample of lignin, isolat.ed by the action of alkaline reagents on wood a t elevakd temperatures, is employed as the starting material and subjected to hydrogenolysis under comparable conditions, the product,s recovered show properties simihr t o the mixtures of condensed cyclic compounds obtained from coal, and the simple phenylpropane skeleton is not found in significant amounts (1). Thus, with lignin also, it seems evident that the structure of the hydrogenolysis products reflects the structure of the starting material and that t,he condensed cyclic structures do not result from cvclization during the hydrogenation procew. OXIDATION
Oxidative degradations have contributed important evidence on the structure of coals. A significant fraction, over X%, of the carbon of bituminous coals can be recovered in the form of a mixture of organic acids by the action of such reagents as nitric acid or alkaline permanganate a t temperatures riot higher than 100” C. With alkaline permanganate or other alkaline oxidants, complete carbon balances can be readily obtained and it has been found that the carbon is distributed between three types of products-carbonic acid, the simple aliphatic acids, acetic and oxalic, and a mixture of “aromatic” acids. With any given coal, the fraction of the carbon appearing as these aromatic acids depends on the extent of the oxidation, and a useful memure of this is the fraction of the carbon appearing as carbon dioxide. When this fraction is about 20%, the aromatic acids consist largely, but not exclusively, of the dark colored, alkali-soluble, acid-precipitable “humic” type, n-ith equivalent weights in the range 200 to 300 With higher conversion of carbon to carbon dioxide, the aromatic acids become pale yellow, are soluble in both acid and alkaline media, and have equivalent weights ranging down to 80. The properties of these acids, whether of the complex, high equivalent weight, humic type or of the simpler, low equivalent weight, soluble type, indicate that they are mixtures of polycarboxylic acids with cyclic nuclei. The atomic carbon-hydrogen ratios for humic acids prepared from various coals and by several different
INDUSTRIAL AND ENGINEERING CHEMISTRY
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1087
procedures are shown in 0.310 Table V ; for comparison there are a l s o p r e s e n t e d values for the simpler soluble acids from coal oxidations as well as those for 0.300 certain acid series of known s t r u c t u r e . The observed 2 carbon-hydrogen ratios for 0 Ithe acids from oxidation of V a coal can be accounted for a 0.290 only on the basis of cyclic a nuclear structures or linear I! types which are completely LL carboxylated, and no eviV W dence for the presence of the a v) latter has so far appeared. 0.280 English workers (5, 6) have determined the fraction of the carbon of carb o n a c e o u s m a t e r i a l s of various ranks appearing as 0.270 aromatic acids on oxidation. The data are shown in Table VI. If the aromatic 0.262 acids were formed during 200 300 400 500 600 700 the oxidation process, one MOLECULAR WEIGHT would not expect the correFigure 4. Specific Refraction-Molecular Weight Relations for Butyl Esters lation observed between the fraction of the carbon apoearing- in these aromatic acids and the rank of the material oxidized. While it is true that TABLE VI. CARBON DISTRIBUTION IIi OXIDATION PRODUCTS O F only small amounts of aromatic acids of authentic purity have CARBONACEOUS MATERIALSOF VARIOUSRANKS been isolated from the mixtures, the high yields of the mixed % ' of Carbon as acids and their average composition are significant,. Oxalic Aromatic Carbpnic Acetic acid acid acid acids During the past few years, studies a t the Coal Research 11.8-15 8 2.7-6.0 21-22 Lignin 57-60 Laboratory on the action of oxygen gas a t elevated temperatures 10-25 3.1-5.6 15-28 Feats 49-61 22-34 3 0-7.5 9-23 Brown coals and lignites 45-57 and pressures on an aqueous alkaline suspension of bituminous 3946 1.74.6 13-14 Bituminous coals 36-42 coal have made available water-soluble, polycarboxylic acids in 7 50 43 2 Anthracite larger amounts (IO). The acids were prepared a t a temperature of 270" C. and at total pressures of about 900 pounds per square inch gage. This oxidation furnishes the same types of acids as in the number of acid groups per molecule but also in the type of obtained in the alkaline permanganate reaction at 100' C.nuclear structure. Samples of mixed butyl asters were subjected carbonic, acetic, oxalic, and a mixture of yellowish aromatic acids. to fractionation in a small centrifugal molecular still over a Fractionation of both the free acids and their ester8 (2) has temperature range of 100' to approximately 200 O C. and a number shown that the mixture consists of components varying not only of properties of the distillable fractions and of the residue were determined. Values of density, dq6, ranged from 1.04 to 1.14, and of refractive index, ng, from 1.48 t o 1.55. These high RATIOSOF VARIOUSACIDS T ~ B Lv. E CARBON-IIYDROOEN values are characteristic of cyclic structures. Ultimate composiCoal Oxidation Method Atomic C / H tions of a number of the fractions of these butyl esters were also HUMICACID determined. From these and the number of functional groups British East Kirby Hydrogen peroxide 1.86 per molecule, it is possible to calculate the ultimate compositions Canadian Estavan Alkaline permanganate 1.41 of the hypothetical, parent nuclear structure, since the composiU.S.A., P h s b u r g h 1 N nitric acid 1.57-1.63 British, Parkgate Air a t 150' C. followed tion of the ester group itself is known. Such calculations lead to by alk. permanganate 1.65 U.S.A., Upper Freeport 16 N nitric acid 1.56 values of atomic carbon-hydrogen ratio of from 1.04 to 1.15. Aliphatic compounds have values not greater than 0.5. SOLUBLE ACIDS .4ustralian, Morwpll Alkaline permanganate 1.31 The relation between specific refraction and molecular weight British, Busty Alkaline permanganate 1.42 which has been shown by several workers (18,65) t o be a powerful ACIDSO F KNOWNSTRUCTURE tool in determining the structure of mixtures, demonstrates Acid Type (Figure 4) that the butyl esters have cyclic nuclei and t h a t these 1.16-2.0 nuclei in the higher fractions must be more complex than the benzene ring (EO). It is not possible on the basis of specific refraction-molecular weight relations alone t o distinguish beMonocarboxy aliphatic, H(CHz)zCOOH 0.5 tween condensed and polyphenylene types of cyclic nuclei; howDicarboxy diphatic, HOOC-(CHZ)ZCOOH 1--0, 5 ComDletelv 1-1.5 . - carboxvlated linear structure. ever, the intense color shown by the higher fractions of these COOH esters points toward the condensed structure, since the largest HOO.(-C\COOH polyphenylene compounds synthesized, up t o 15 rings, are c o l o ~ less ( I I ) , while color appears in condenPed cyclic types containing \ dOOH/z only four rings (7).
-
v
-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1088
LITERATURE CITED
Adkins, H., Frank, R. L., and Bloom, E. S., J . Am. Chem. SOC., 63, 549 (1941). Berman, N., and Howard, H. C., Fuel, 29, 109-11 (1950). Biggs, B. S.,J . Am. Chem. Sor., 58, 484 (1936). Biggs, B. S.,and Weiler, J. F., Ibid., 59, 369 (1937). Bone, W. A., Horton, L., and Ward, S. G., Proc. Rov. SOC. (London), 1278, 508 (1930). Ibid., 148A, 521 (1935). Clar, E., “Aromatisohe Kohlenwasserstoffe,” p. 170, Berlin, Springer, 1941. Fieldner, A. C., and Davis, J. D., U. S.Bur. Mines, Monograph 5 11934). - -, Fisher, C. H., U. S. Bur. Mines, Bull. 412 (1938). Franke, N. W., and Kiebler, M. W., Chem. Inds., 58, 580 (1946). Gillam, A. E., and Hey, D. H., J . Chem. Son., 1939, 1170. Harris, E. E., D’Ianni, J., and Adkins, H., J . Am. Chem. Soc., 60, 1467 (1938). Hicks, D., and Xing, J. G., (Brit.), Dept. Sei. Ind. Research, Fuel Research Tech. Paoer 34 11931). --
Vol. 44, No. 5
ed.. PP. 737, 745, chap. 19, New York. John Wiles & Sons, I ~ c . ;i945. Kinney, J. R., J. Am. Chem. SOC.,69, 284 (1947). Le Claire, C. D., Ibid., 63, 343 (1941). Orlov, N. A., and Belopolsky, M. A., Ber., 62, 17524 (1929). Ruof, C. H., and Howard, H. C., Division of Gas and Fuel Chemistry, AM. CHEM.Soc., Pittsburgh Divisional Meeting, May 9 and 10, 1949. (21) Selvig, W. A., Pittsburgh Station, U. S. Bur. Mines, private communication. (22) Spielmann, P. E., “Constituents of Coal Tar,” p. 137, London, Longmans, Green and Co., 1924. (23) Travers, M. W.,J . Inst. Fuel, 6, 253 (1932-33). (24) Washburn, E. W., Bur. Standards J . Research, 2, 476 (1929). (25) Waterman, H. I., J . Inst. Petroleum Technol., 21, 661, 707 (1935). (26) Weiler, J. F., “Chemistry of Coal Utilization,” H. H. Lowry, ed., chap. 8, New York, John Wiley & Sons, Inc., 1945. (27) Ibid., chap. 10, p. 382. (28) Whitmore, Frank C., “Organic Chemistry,” p. 6, New York, D. Van Nostrand Co., Inc., 1937. (17) (18) (19) (20)
RECEIVED for review August 1, 1951. ACCEPTED December 17, 1951. Presented as part of the Symposium on the Nature of Bituminous Materials before t h e Division of Colloid Chemistry and the Division of Gas, Fuel, and Petroleum Chemistry at the 118th Meeting of the AMERICANCHEMICAL SOCIETY, Chicago, Ill., September 1950.
Prevention of Mold Growth in Optical Instruments PANAMA CANAL ZONE EXPOSURE LEONARD TEITELL AND SIGMUND BERK Pitman-Dunn Laboratories, Frankford Arsenal, Philadelphia, Pa.
I
T I S well known that cotton and leather goods can be dam-
aged by molds (mildew). Deterioration caused by molds is not limited only to cotton and leather, for even optical instmments, such as microscopes, binoculars, and telescopes, are affected. Molds are found growing on the polished glass that is used for line optical equipment. Molds require dampness for growth and thrive best in warm (20’ to 30” C.) places. Tropical climates are warm and humid, and optiral instruments that are stored or used in the tropics tend to become moldy. The air inside the instruments becomes as humid as the surrounding tropical air and the instrument acts as an incubation chamber for the molds. Molds also grow inside optical instruments stored in temperate or even arctic climates, if the storage conditions are warm and humid. However, during the war moldy binoculars and telescopes n-ere encountered mostly in tropical areas, such as New Guinea, islands of the Southwest Pacific, the Philippines Burma, Malaya, and the Panama area. The damage caused by molds growing on the internal parts of binoculars and other optical equipment was known to people who lived in the tropics and used these instruments before the war. Prevention a t that time was mostly a matter of special storage under dry conditions If a n instrument became moldy, it was returned t o the manufacturer for cleaning. Before the war several companies had started investigations on mold growth in binoculars. However, with the advent of a global war large numbers of binoculars, telescopes, cameras, etc., were sent to tropical areas. Storage conditions were poor and binoculars became moldy so fast that there were insufficient personnel to repair and clean them. I n one of the earlier surveys on tropical deterioration, Magee, Hansen, and Grant (9) reported that the presence of fungal colonies in optical equipment in New Guinea was the rule rather
than the exception, and that of hundreds of items observed, the few instruments that were free from molds were either recently received in New Guinea or recently cleaned in workshops. Hutchinson (14) estimates that 70 t o SOYo of the repairshop work in the Panama Canal Zone was primarily a result of mold growth. During the course of the investigation reported here, visits were made t o repair shops a t Fort Gulick and Coroaal in the Canal Zone, and more than half of the binoculars t h a t were in for repair had mold growth on the lenses. The problem was considered so serious that it was attacked almost simultaneously by investigators from three countries-by the British (b), who used Nigeria, West Africa, as a test eite; by the University of Melbourne staff ( 1 , ,$?I),who used New Guinea as a test area; and in the United States by the National Defense Research Committee of the Office of Scientific Research and Development (10-18), who used Barro Colorado Island in the Canal Zone as a testing area. Optical instruments present a unique problem in mold growth. Small amounts of mold, which on materials like cotton or leather would probably go unnoticed and certainly would do little if any damage, create a serious problem in optical instruments since the mold is in the optical path close to the viewer’s eye. Apparently the optical glass does not act as a food for the molds but provides a surface on which the molds can grow and water vapor can condense. Some mold spores seem t o require liquid water for germination. The mold growth that takes place is initiated by sorption of water from a condensed droplet or from vapor in the air. The reserve food present in the mold spore, along with the sorbed water, allows sufficient mold growth to interfere with function of the instrument. More luxuriant mold growth occurs on a lens surface if accessory food, such as dust, debris, or small dead arthropods, is present on the glass.
