May, 1934
I N D U S T R I A L .4.N D E N G I N E E R I N G C H E M I S T R Y
cause failure. The stocks did'not deteriorate to any great extent during the service test, as judged by tear and adhesion tests; while the extent of checking of the tread and side-wall stock was somewhat greater than normal, it was not serious. Since the stocks containing guayule rubber had a rate of cure different from that of stocks containing Hevea rubber, a direct comparison of the wearing qualities of guayule stocks with the corresponding Hevea rubber stocks could not be made in the same tire. This eliminated the possibility of testing so-called "half-and-half" treads, and there were no facilities, where the tests were carried out, for making a fair comparison of whole tires, which must be rotated from wheel to wheel if the test is to be reliable. However, it is known that a t that time standard tires under similar service conditions were averaging approximately 16,000 miles before failure. I n other words, the best of the guayule tires gave approximately 60 per cent of the mileage obtained with Hevea tires. It would be difficult to estimate the expected mileage for these tires in present-day service, but it would be poor in
543
comparison with the mileages that are now obtained. However, it is believed that cord tires made today with guayule rubber compounds would compare favorably in mileage rendered with the fabric tires made during the days of the World War. There is no doubt that, with further study of the compounding of guayule stocks, this mileage could be increased several thousand miles, but it is felt that a n outstanding improvement can be made only with more completely deresinified rubber than was used here. LITER.4TURE CITED (1) Carnahan, C. H., Chem. &. M e t . Eng., 38, 128 (1931). (2) Carnahan, C. H., IND. Ewo. CHEM.,18, 1125 (1926). (3) McCallum, W. B., Ibid., 18, 1121 (1926). (4) Spence, D., Ibid.,18, 1126 (1926). ( 5 ) Ibid., 22, 384 (1930). (6) Spence, D., and Roone, E., Bur. Standards, Tech. P a p e r 353 (Sept. 33, 1927). RECEIVEDSeptember 20, 1933. Presented before the Division of Rubber Chemistry a t the 86th Meeting of the 4merican Chemical Society. Chicago. Ill., September 10 to 15, 1933.
Fibering of Rubber Time Lag and Its Relation to Rubber Structure JOHN D. LONG,WILLIAM E. SINGER, AND WHEELER P. DAVEY The Pennsylvania State College, State College, Pa. By means of experiments on the time lag of within *0.2" c. A molybdeOR several years it ha;; num target x-ray tube, operated been known that in the fibering, the most probable pictures as to the a t 30 kilovolts, furnished the unstretched c o n d i t i o n macroscopic nature of rubber have been narrowed x-rays. A z i r c o n i u m o x i d e o r d i n a r y rubber acts toward x-rays like an amorphous madown SO that one type appears much more filter g a v e a s u b s t a n t i a l l y probable i'han the others. This most probable monochromatic beam. Various terial, but that, when it is sui%type of picture considers rubber to be a tangle of samples of rubber sheet were ciently stretched, it acts toward spiral (or zigzag)molecules or molecular furnished by the Bell Telephone the r a y s l i k e a f i b r o u s maL a b o r a t o r i e s , Inc., a n d b y terial* In lg3' plexes. So f a r , this type seems to be consistent the B, F. Goodrich Company. and Davey (1) reported that a t room temperature a time innot only with experiments Of the Present authors These sheets had been milled o n the time lag of jibering, but also with all other as l i t t l e a s possible, and were terval was required to build up experiments, conducted along entirely different vulcanized only to the e x t e n t the fibrous structure in cyclically necessary to make them withstretched rubber. lines, which have been found in the literature. stand the c y c l i c s t r e t c h i n g gation showed that the time-lag effect could not be a c c o u n t e d required by the investigation. for in terms of a temperature change during the act of All the results reported here were obtained from samples cut stretching. Even if all the mechanical energy of stretching from the same sheet. The composition was: were instantaneously changed into sensible heat, the temperaPale crepe 90 0 Accelerator 0 5 Mineral rubber 3 0 Zinc oxide 2 5 ture of the rubber sample could not have been increased moSulfur 1.5 Antioxidant 1 0 mentarily by more than 5.2" C., whereas the fibering of rubber continuously stretched at 420 per cent elongation could be Time of milling was 20 minutes on warm rolls. The tliickiiers demonstrated up to a temperature of 47" C. Experimentally of the unstretched rubber was 0.2 cm. Several preliminary diffraction patterns were taken from no temperature rise greater than 1.0" C. could be found in the samples used. Since the time-lag effect appeared, therefore, stretched samples cut from this sheet to find whether or not to be a real effect, it seemed worth while to study it in detail. the sheet had been so affected during the process of manuI t is the purpose of this paper to report: (1) typical data on facture as to make i t easier for the rubber micelles to become the effects on the time lag, of temperature, previous tempera- oriented in some one direction rather than in any other. It ture history, mechanical working and aging, time of relaxa- was found that the direction in which the sample was cut had tion, time of stretch, and rate of stretch; and ( 2 ) the relation no effect on the diffraction patterns obtained under constant stretch. I n spite of these results there was the possibility of these results to the Dossible structure of the rubber fiber. that some small orientation due to the process of manufacture -APPARATUS AND EXPERIMENTAL PROCEDURE might introduce a directional effect in the time lag of fibering. The main part of the apparatus was identical with that For this reason, in the final work all samples (length 2.5 cmy, used by Acken, Singer, and Davey (1). For the present work width 0.8 cm.) were cut with their lengths in the same direca n air thermostat was built to enclose the specimen holder and tion with respect to the sheet. Cutting was done with a the photographic film. This held the temperature constant sharp razor blade. Because of the small size of the sample it
F
INDUSTRIAL AND ENGINEERING CHEMISTRY
544
was necessary for the dimensions to be quite accurate, and the edges had to be smooth. After each sample was cut, its ends were punched for the brass clamps which were later to fasten it to the supports of the cyclic stretching mechanism (citation 1, M I and M 2 of Figure 1). One end of the sample was then clamped to a table, and the sample was stretched to a proximately the extent desired in the experiment. Two parafel ink lines were then ruled on it perpendicular t o the direction of stretch-i. e., parallel to the short dimension of the sample. These lines were 3 or 4 cm. apart.
Vol. 26, No. 5
of intensity of spots. The re$uIts are given in Table I and are shown diagrammatically in curves A , C, D, E , and F of Figure 1. It is evident that, with cyclic stretching under the conditions of the experiment, an increase in temperature from 15" to 30" C. causes very little time lag. The changes from 15" to 27" C. are just perceptible; the change caused by an increase from 30" to 35" C. is very marked.
B. EFFECTOF PREVIOUS TEMPERATURE HISTORY.A
series of films was taken to show the growth of the fiber structure in a certain s p e c i m e n ( c y c l i c s t r e t c h , elongation 510 per cent). The specimen was then given the following treatment: (1) heated to 55" C., (2) stretched to 510 per cent, and (3) quickly cooled to 25" C. T h e s p e c i m e n was kept a t 55" C. for only a few minutes to minimize the effect of any possible chemical changes. A new series of films was then taken to show the growth of the fiber structure during stretch a t 25" C. (elongation 510 per cent), using the same stretching cycle as before heat treatment. The two sets of t i m e - l a g d a t a were 5.0 found to be identical. Evidently, then, within 1.0 2.0 3.0 9.0 the limits' of the experiment, the time lag of TIHE IN S E C O N D S AFTER STRETCHJNG fibering is independent of the previous temperature history of the sample. FIGURE 1. EFFECTOF TEMPER.4TURE UPON TIMELAGOFFIBERING
*
CURVE TEMP. TIME OF R E L A X A T I O N c. Sec. A 15 0.6 B 15 2.0 C 25 0.6
D E F
c.
See. 0.6 0.6
27 30 30
The part of the rubber through which the x-ray beam was to pass later was midway between the lines. After the ink had dried, the sample was relaxed and the distance between bhe lines (0.6 t o 0.8 cm.) was measured with vernier calipers. Rubber facing strips were then placed on the ends of the sample t o Prevent dxasion and wear, and the sample was clamped to the supports of the cyclic stretching mechanism* The stretching apparatus was set for the position of maximum stretch and the exact height of the support was adjusted by a set screw t o give the desired percentage stretch. In every case the stretch was calculated by caliper measurements between the ink lines on the sample. Any sample which showed distorted lines after stretching was taken off and reclamped. In most cases this made the lines straight and parallel in both the stretched and unstretched condition. Samples which for some reason could not be adjusted properly were discarded as being under suspicion of not being capable of uniform stretching. Between successive diffraction patterns, the cyclic stretching was continued in order to keep the rubber in a standard reproducible condition. New specimens were used every two weeks t o avoid any cumulative tendency toward "racking."
A. EFFECT OF TEMPERATURE ON TIMELAG. A base line was established by taking diffraction patterns of continuously stretched rubber at various temperatures from 15" to 70" C. At a continuous elongation of 420 per cent, excellent diffraction spots were present a t 47" but not at 50" C. At a continuous elongation of 515 per cent there was very little diminution in intensity from 15" to 50" C. There was more loss in intensity in going from 50" to 55" C. than from 15" to 50" C. At 65" C. the spots were quite weak, and a t 70" they were absent. The time lag of fibering was then investigated at 15", 25", 2 7 O , 30", and 35" C., using the following stretching cycle, called "cycle 1":
r) (1)
3) 4)
C. EFFECTOF M E C H A N I C AWORK L AND AGING. Three specimens, 60, 74, and 87, which had been used in 1932 were retested in 1933 to 0.6 determine whether they still showed a time lag of fibering. It was found that a higher percentage stretch was necessary to bring out the diffraction spots. H ~the samples ~ did~show a ~ time lag~at this higher ~ per, centage stretch, a,nd the growth of a fiber structure took Place just as it did in fresh samples. The effect of aging alone would not esplain this change because there was no change in the t h e - l a g effect in the original unstptched rubber, The amount of mechanical working to which samples were subjected was not sufficient to make any change in the time-lag effect a t the time they were used in 1932, Therefore it Seems probable that the change found in 1933 in tile specimens used in 1932must have been caused by the combinationofmechanical working and subsequent aging.
C U R Y E TEMP. TIME OF RELAXATION
See.
Maintaining stretched condition (515%) Unstretching Msintajning a substantially relaxed condition Stretching Total time of cycle
4.8 0.4
TABLE I. EFFECT OF TEMPER.4TURE UPON TIMELAGOF FIBERING TIMEBEFORE STARTOF EXPOSURE AFTER
7 -
STEP STRETCHINQ15'C.
-
INTEN3ITY OF DIFFRACTION SPOTSa 25'C. 27'C. 30° C. 35O C.
Sec.
; 5
Continuous 4.2 0.0 3.2 2.2 1.2
Curves
'
D l
Strong Strong Absent Absent Strong Strong Strong Strong Strong Strong E' F' Figure
''
Strong Absent Strong Strong Strong
Strong Weak Absent Absent Strong Weak Strong Very weak Strong Barely present
TABLE 11. ELONGATION OF SAMPLES OF RUBBER SUBJECTED TO CYCLIC STRETCHING OF 515 PERCEST ELONGATION AFTER TEMP.
c. 15 15 25 27 35
T I M EO F ELONQATION AT RELAXATION ENDOF R U N See. 470
2-4 WEEK0 O F
CONTINUOU3 RELAXATION
% 3.3 5.3
0.6
16
2.0 0.6
16 16.9
0.6 0.6
4.6
16.6 17.5
10.3
7.7
0.6
0.4 6.2
A fresh sample was used for each temperature. Each photographic film was compared visually with every other film to see which had stronger diffraction spots, Tabulations of the data obtained in this way made i t easy to pick out the order
If a given specimen is stret'ched cyclically for a long time (between 2 and 4 weeks), it tends to build up a fiber structure which persisbs beyond the period of relaxation (0.6 second) of the writers' standard cycle. It is as though the rubber had gradually become "racked." This is the reason for taking a fresh sample every 2 weeks, and also in studying t'he effect of
May, 1934
I N D U S T R I A 1, A N D E N G I N E E R I N G C H E M I S T R Y
temperature (section A) for taking a fresh sample for each temperature studied. It is also the reason for the time schedule listed in Table I, which caused the evidence for the time lag to appear in spite of any possible racking rather than because of it. The racking effect on prolonged cyclic stretching is consistent with the observation of Katz ( 3 ) that a fiber structure can be detected for 20 to 30 minutes after the relaxation of rubber which has been subjected to long continuous stretching. For purposes of record, Table I1 gives, for the samples used in preparing the data for Table I and Figure 1, the elongation found when the samples were removed from the apparatus. D. EFFECT OF TIMEOF RELAXATION. I n order to make sure that, under the conditions of the experiment, the degree of fibering and the time-lag effect were not functions of the time of relaxation of the rubber, the work recorded under section A was repeated a t 15" C. using a different cycle (called "cycle 2") as follows: Sec. (1) (2) (3) (4)
Maintaining stretched condition (515%) Unstretching Maintaining a subatantially relaxed condition Stretching
4.8
Total time of cycle
7.6
0.4
2.0 0.4
-
The results shown in curve B of Figure 1 indicate that the original cycle was not sensitive to reasonable increases in the time of relaxation. This conclusion was later confirmed a t 25" C., 510 per cent stretch, using a slightly different pair of cycles (called "cycle 3 and 4") in which the tiines of relaxation were 0.5 and 3.2 seconds, respectively (toi,al times of the cycles, 6.5 and 9.2 seconds). Cycle 3, described under section E, is essentially the same as cycle 1. E. EFFECT OF TIMEOF STRETCH.Two stretching cycles were arranged to determine the effect on the time lag of fibering of holding the rubber in the stretched condition for different lengths of time during the cycle. The two following cycles were used:
543
knowledge of the chemical nature of rubber. Of the types of pictures considered, one seems to be consistent with the known facts. Data on rubber in the literature have a t various times been interpreted in terms of three types of pictures a$ to the nature of rubber: I. Pictures expressed in colloid terminology (i. e., in terms of a disperse phase, a dispersion medium, and possibly a stabilizing agent). 11. Long molecular complexes (so-called beta rubber) coiled like helical springs, with shorter complexes (alpha rubber) filling in the vacant spaces (2). Obviously zigzags can be substituted for coils if desired (4). 111. A random distribution of tangled molecules or molecular complexes. In order to account for the ability of stretched rubber to return to approximately its original length, we must assume that each of the tangled molecules or complexes has a spiral or zigzag structure.
Picture I11 is obviously a more complicated version of picture 11, with the tangle fulfilling the role of the alpha rubber of picture 11. It will be shown below that the simple pictures I and I1 must be ruled out, but that all the data are consistent with pictures of type 111. A. EFFECTOF TEMPERATURE. It is easy t o show that picture I is hardly consistent with the present data. For instance, in a typical version of this picture due to Hauser (2) it is assumed in effect: (1) that the more polymerized portion (beta rubber) is "swollen" by the less polymerized portion (alpha rubber); ( 2 ) that the fibering cannot be shown by x-ray methods unless the more liquid phase is expelled, thus allowing the highly polymerized "swollen" molecules to
CYCLE3 CYCLE5 Unstretching Maintajning a substantially relaxed condition Stretching
Sec. 5.0 0.5 0.5 0.5
Sec. 2.0 0.5 0.5 0.5
T o t a l time of cycle
6.5
3.5
(1) Maintaining stretched condition (510y0) (2) (3) (4)
-
-
No evidence was found of any difference in the rate of growth of the fibrous structure for the two cycles considered. F. EFFECT OF RATEOF STRETCH. I n order to determine the influence of the rate of stretching on the time lag of fibering, the following two cycles were used in which the rate of stretching in cycle 6 is one-fourth that of cycle 3: CYCLE3 CYCLE6 (1) Maintaining stretched condition (510$%o) ( 2 ) Unstretching (3) Maintaining a substantially relaxed condition ( 4 ) Stretching
Sec. 5.0 0.5 0.5
0.5
See. 5.0 2.0 0.5 2.0
The results are shown diagrammatically in Figure 2. Cycle 3 gives the same sort of time lag previously found (curve A ) , but the slow rate of stretching gives a much shorter apparent time-lag (curve B ) . The diffraction spots were very distinct immediately after stretching, and their intensity increased only slightly even as long as 3.2 seconds after stretching. This could be explained easily by assuming that the fibering had a chance nearly to complete itself during the long period of stretching. This explanation was later confirmed by direct experiment.
INTERPRETATION OF RESULTS The following discussion shows that it is reasonable, in the light of the authors' data, to discard certain types of pictures of rubber structure without having to draw upon our limited
TIMEIN SECONDS A F T E RSTRLTCHING
FIGURE 2. EFFECTOF RATEOF STREL'CH ou TIMELAGOF FIBERING A. B.
Normal cycle (cycle 3) Slow stretch (cycle 5 )
align themselves fiber fashion; (3) that the act of stretching is sufficient to cause the necessary expulsion of the alpha material. The colloid picture is hard to reconcile with the facts. This type of theory would seem to require that we explain the results of Figure 1 a t 35" C. by assuming additional swelling and therefore the production of additional alpha rubber. The additional alpha material can be obtained only at the expense of the beta material. Such a decrease in beta content is extremely improbable since the writers' diffraction spots of continuously stretched rubber (515 per cent elongation) were practically as strong a t 50" as a t 15" C., and their diffraction spots of continuously stretched rubber (420 per cent elongntion) were practically as strong a t 45" as a t 15' C. There can, therefore, be a t 30" to 35" C. only a small decrease in the number of molecular complexes capable of forming fibers. We shall, therefore, consider picture I as being untenable. It is a consequence of pictures I1 and I11 that, if the amplitude of the transverse thermal vibration of molecules (or molecular complexes) becomes too great, adjacent spirals or zigzags will become somewhat tangled together. The time of
I X D U S T R I A 1,
546
A N D E N G I N E E R I N G C H E MI S T R Y
stretch necessary t80 align the spirals or zigzags into true fibers will therefore be greatly increased. This great increase in the time lag of fibering should, then, occur at temperatures above the one which corresponds to some limiting amplitude of transverse vibration (35" C. in these experiments).
t
P FIGURE3.
IDEALIZEDSTRUCTURE OF RUBBER
(a) The relaxed state
(a) Immediately after atretching (c)
At the end of the time lag
B. EFFECTOF PREVIOUS TEMPERATURE HISTORY. When the rubber was given its heat treatment, it was heated to 55" C. (20" above the critical temperature of the time lag). It was hoped that part of the change taking place might be retained by rapid cooling. However, although the tendency of the rubber to form fibers is sharply decreased or even removed a t 55" C., the ability to recover, a t room temperature, from subsequent fibering is unaffected. This must mean that the physical nature of the rubber has not been fundamentally altered by heating, but that the heat vibrations a t 55" C. have merely prevented that extreme degree of alignment necessary for x-ray diffraction. Because of the different degrees of loading and tension of the vibrating molecules or complexes, no two fibers would have the same natural period of vibration. -4s soon as the tendency for these fibers to vibrate out of phase with each other becomes greater than the tendency of the cohesive forces to produce substantially perfect alignment of parallel fibers, the x-ray fiber pattern should disappear. As soon as the rubber is cooled to the point where this tendency becomes less than the sideways forces associated with alignment, the fibering should again predominate. This must mean that the elastic properties of the spirals or zigzags are not destroyed by heating. I n terms of picture I11 it means, in addition, that the ends of the tangled molecules (or molecular complexes) are not loosened from their anchorages by a temperature of 55" C.; otherwise the tendency to re-form the tangles would disappear with heating. C . EFFECTOF MECHANICALWORK AND AGING. The fact that more stretch was required to bring out the diffraction spots in specimens which had been cyclically stretched a year before, means that the combination of mechanical work and age caused the rubber molecules (or complexes) to resist the lining-up process more effectively. Although the rubber contained antioxidant, it might be that the cyclic stretching followed by aging would cause chemical changes such as oxidation. A mechanism is suggested later by which this might occur. The effect of additional atoms along the rubber molecules (or complexes) would be to make it more difficult for the molecules (or complexes) to slide over each other. D. EFFECT OF TIMEOF RELAXATION.Since the relaxation of rubber required less than 0.5 second, the departure from the fiber state must take place very rapidly. I n terms of picture 111,the portions of the molecules (or complexes) pulled
Vol. 26, N o . 3
from the tangle in the direction of the stretching force must snap back into the tangle when the stretching force is removed. This permits of a restoring force in the tangle itself in addition to that in the stretched molecules or complexes. Such an additional force is implied in the picture of the tangle itself. Each molecule (or complex) must have other molecules (or complexes) looped about it in random fashion. Some of these loops will have a direction roughly perpendicular to the axis of the stretched molecule (or complex) under consideration. The stretching force would tend to straighten out the molecules (or complexes) which are parallel to the stretching force, and this very process would produce a tension in the loops which are roughly perpendicular to the stretched molecules (or complexes). When the stretching force is removed, this sideways pull would snap the lined-up molecules (or complexes) back into the tangle. Obviously this process could take place very rapidly. E. EFFECTOF TIMEOF STRETCH. It was found experimentally that no difference could be found in the time lag of fibering whether the rubber was held stretched for 2 seconds or for 5 seconds during each cycle. I n both cases the fibering was apparently reduced to zero a t the end of the relaxation portion of the cycle (0.5 second). These results emphasize the temporary effect of cyclic stretching on the state of the rubber and indicate that stretching does not affect the anchorage of the ends of the molecules (or complexes) which compose the stretched fiber. F. EFFECTOF R ~ T EOF STRETCH.The rapid rate of stretching gave a much longer time lag of fibering than the slow rate. I n terms of picture 11, fibering would consist of two processes: (1) the stretching out of the beta molecules (or complexes) and (2) the expulsion of the alpha molecules (or complexes). For a slow rate of stretching, these two processes could occur simultaneously. As a result t8hefiber pattern would show up fairly strongly just a t the end of the stretching process. The rapid rate of stretching would cause the beta rubber to be stretched out before the alpha rubber had time to be expelled. Such a picture explains the time lag and its dependence upon the rate of stretching. The chief difficulty with this theory is that it does not explain what sort of process the alpha rubber must go through which requires it to interfere with the stringent requirements for x-ray fiber patterns before but not after expulsion. It does not explain why, if the alpha rubber is once squeezed out between fibers, it finds i t necessary to return so quickly when the rubber is relaxed. We therefore have some justification, independent of any other type of data, for definitely discarding a simple picture of type 11. I n terms of picture I11 we seem to have no such difficulties. This picture assumes that the act of stretching pulls molecules (or complexes) a t least part way out of the tangle and aligns them in the direction of the stretch. Expressed in terms of the mechanism detailed under section D, in the case of the slow stretch, the stretching out of the loose ends of the molecules (or complexes) and the straightening out of the tangle (in the direction of the stretch) have both taken place when the maximum stretch is reached. For the rapid rate of stretching, the untangling process is assumed not to be complete until some time after the maximum stretch has been attained. We may assume that, in rapid stretching, the ends of the molecular complexes are first overstretched, and that this excess strain is relieved by the untangling process after the completion of the macroscopic stretching. After the untangling occurs, the fiber structure is perfect enough to show the x-ray fiber pattern. This is illustrated in Figure 3 in which, of course, zigzags may be substituted for spirals if desired. We may assume that, when the rubber is stretched, the tangle is not completely unraveled; the other mole-
May, 1934
INDUSTRIAL
AND ENGINEERING CHEMISTRY
cules would still be looped around those molecules which take part in the fibering. These other molecules also would have their spirals (or zigzags) stretched out but would have no opportunity to align themselves accurately so that they would not contribute to the fiber structure. They would, however, make possible the forces necessary to reestablish the tangle after the external forces of stretching are removed. It is assumed, of course, that the ends of the molecular complexes would stretch out rapidly but that the straightening out of the tangle would require more time, so that reasonably perfect alignment will occur only after a time lag. Such a picture is apparenty consistent with a mass of other data in the literature (see literature references a t end of citation 1). The picture may appear to be open to the objection that the size of the loops in the tangle may be too small to allow the overstretched ends just to release the strain of overstretching. This difficulty may be met if we assume that, as a rule, there is more length available in the tangle than is required to release the strain of overstretching. The relaxing of the ends would then take place until the secondary valence forces have a chance to tie adjacent molecular complexes together into a fiber structure which would persist during the rest of the stretching period. I n the case of certain fibers it is likely that there may not be enough untangling to relieve the overstretched ends. This would result in torn fibers resulting in turn, in the failure of Hooke’s lam, racking, and
547
finally in tearing. The chemical reactivity of torn molecules (or complexes) might conceivably account, too, for the combined effect of cyclic and aging mentioned under section C.
ACKNOTVLEDGMEKT The writers are greatly indebted to C. W. Scliarf of the Bell Telephone Laboratories, Inc., and to W. F. Busse of the B. F. Goodrich Company for samples of rubber, and to the General Electric Company, Schenectady, N.Y., for making available equipment without which the work would have been extremely difficult, if not impossible. The authors are also grateful to W. F. Busse, A. T. hlcPherson, and L. F. Curtiss for their kindness in examining the data and conclusions during the preparation of the manuscript.
LITERATURE CITED (1) Acken, M. F., Singer, W. E., and Davey, \V. P., IND ENQ. CHEM., 24, 54 (1932). (2) H a u s e r , E. A., Gummi-Ztg., 40, 2090 (1926); IND.ENG.CREM, 19, 169 (1927). (3) K a t e , J. R., C h e w - Z t g . , 49, 353 (1925). (4) S a u t e r , E., 2. physik. Chem., 21, 161 (1933). RECEIVED November 22, 1933.
Abstracted from the theses submitted by
J. D. Long and W. E. Singer t o the Graduate School of The Pennsylvania State College in partial fulfillment of the requirementa for the degree of doctor of philosophy in physical chemistry and chemical physics
Nitrification in Sewage Mixtures EMERY J. THERIAULT AND
PAUL
D.
hlCNA4MEE
U. S. Public Health Service, Stream Pollution Investigations, Cincinnati, Ohio PREVIOUS publication ( 8 )has shown that the adjustment of the various factors which may affect the rate and extent of a biological oxidation in a sewage mixture is not critical when observations are restricted to the first or carbonaceous stage of this purification process. Ample indications were obtained, however, that the specifications should be far more rigid in studies extending to the second or nitrification stage of a bacterial oxidation. In the first instance the work of oxidation is carried out by mixed cultures which, taken collectively, are adaptable to wide variations in pH adjustments and other factors affecting bacterial growth. I n the subsequent stage of nitrification the oxidation process depends on the activity of highly specialized groups of organisms whose cultural characteristics must be carefully considered. Essentially, therefore, any study of nitrification even in such a heterogeneous mixture as sewage, becomes a “pure culture” problem with all of the difficulties attending such work. Among the variables which may affect a nitrification process are the nature and concentration of the mineral salts. the carbon dioxide tension, and the dissolved oxygen content. Except for conditions either of extreme deficiency or of abnormal concentration, these variables may be regarded as secondary in influence to the more important factor of pH. The discussion which follows will accordingly be limited largely to the effects of pH adjustment and control on the nitrification process.
A
EXPERIMENTAL PROCEDURE The general plan of experimentation mas to inoculate sewage mixtures buffered a t various p H values, generally pH 6.0, 7.2, and 8.2, with nitrifying organisms acclimated to one
or the other of these pH values. The sewage mixtures were prepared by diluting ordinary domestic sewage with the Clark-Lubs phosphate buffer solutions, suitably diluted, as described in previous work ( 7 ) . The cultures of nitrifying organisms were obtained by the repeated transfer of actively growing organisms to appropriate media buffered a t selected pH values. While this procedure is not expected to give a pure culture in the bacteriological sense, it does assure a preponderance of nitrifying organisms acclimated to a particular pH value. Cultures of nitrifying organisms which were satisfactory at pH 6.0 were also obtained from the activated sludge plant a t Rockville Center, where the pH value of the raw sewage is around 6.0.
LABORATORY EXAMINATIONS The pH value of the experimental solutions was determined colorimetrically with frequent comparisons against the quinhydrone electrode. The course of nitrification was followed by determinations of nitrogen present as ammonia, nitrites, and nitrates, using more or less standardized procedures. The methods used in the determinations of dissolved oxygen and oxygen demand have been fully presented elsewhere ( 7 ) . The Rideal-Stewart or permanganate modification of the Winkler method was used throughout. Of the numerous analytical precautions to be observed, mention should be made of the necessity for using ammoniafree water for dilution purposes. Ordinary distilled water, especially where the chloramine process of water sterilization is used, may readily contain 0.2 or 0.3 p. p. m. of oxidizable nitrogen. Applying a factor of 4.6, this minute trace of nitrogenous impurity will lead to a huge error in the oxygen demand determination. The difficulty may not be aroided
