IN D U S T R IA L A N*D E N G IN E E R IN G C H E M IS T R Y
February 1951
475
ELULNT-CHLOROF O R Y + ETHANOL
I
3 N
0
I -
O
100
Figure 5.
.
200
400 500 E L U A N T , ML.
600
300
100
000
Analysis of Sodium Polybutadiene Oxidation Products
Condrsion
*
.
In view of the failure to obtain quantitative yields of the expected carboxylic acids by permanganate oxidation of the several butadiene polymers studied, definite conclusions about their structures cannot be made. The results have a qualitative significance, however, in t h a t the - 10' emulsion polymer was found to produce a larger amount of succinic acid than the 50' emulsion polymer, and less tricarballylic acid, and thus they correlate with general thinking t h a t lowered temperature of polymerization results in a polymer of more regular structure and less branching. The Alfin polymer, although prepared in a system entirely different from the emulsion system, gave results which indicated a microstructure similar to that of 50" emulsion polybutadiene. Sodium polybutadiene was less completely characterized than the others, but the large amounts of carbon dioxide and ,%carboxyadipic acid isolated correlated well with the high percentage of external double bonds known to be present from other methods of analysis.
Literature Cited (1) Boswell, M. C., and Hambleton, A., India Rubber J., 64, 984
(1922). (2) D'Ianni, J. D., Naples, F. J., and Field, J. E., IND. ENO.CHEM., 42, 95-102 (1950). (3) Dunbrook, R. F., India Rubber World, 117, 203-7 (1947). (4) Harries. C.. Ber.. 37. 2708 (1904). (5j Kolthoff, I. M., Lee,'T. S., and Mairs, M. A., J . Polymer Sci., 2, 220-8 (1947).
ELUANT,ML.
Figure 6.
Analysis of Sodium Polybutadiene Oxidation Products Chloroform-butanol eluant
(6) Marvel, C . S., private communication. (7) Marvel, C. S., Bailey, W. J., and Inskeep, G . E., J . PoEymer Sci.. 1, 275-88 (1946). (8) Marvel, C. S., and Light, R. E., Jr., J. Am. Chem. Soc., 72, 3887-91 (1950). (9) Marvel, C. S., and Rands, R. D., Jr., Ibid., 72, 2642-6 (1950). (10) Marvel, C. S., and Shields, D. J., private communication. (11) Morton, A. A., IND. ENG.CHEM.,42, 1488-96 (1950). (12) Rabjohn, N., Bryan, C. E., Inskeep, G. E., Johnson, H. W., and Lawson, J. K., J. Am. Chem. SOC.,69, 314-19 (1947). (13) Robertson, J. M., and Mair, J. A., J . SOC.Chem. Ind., 46, 41T (1927). (14) Rossem, A. van, Kolloid-Beiheft, 10, 9 (1918). (15) Shearon, W. H., Jr., McKenzie, J. P., and Samuela, M. E., IND, ENQ.CHEM.,40, 769-77 (1948). (16) Weidlein, E. R., Jr., Chem. Eng. News, 24, 771 (1946). RECEIVEDOctober 21, 1950. Contribution 182 from the Goodyear Tire & Rubber Co., research laboratory. Investigation oarried out under sponsorship of the Office of Rubber Reserve, Reconstruction Finance Gorp., in connection with the government synthetic rubber program.
Absorption of Oxygen by Ammonia= Preserved Rubber Latex John MoGavaek and E. M. Bevilaoqua United States Rubber Co., PaseaZc, N . d.
d
After i t had been discovered that ammonia latex absorbs oxygen, it was necessary to know the effect of such absorption on the physical and chemical properties of the latex and on the rubber when removed from the latex by coagulation or drying. I t was found t h a t the rate of oxidation of ammonia latex depends on the pH of the latex, increasing as the pH rises to a maximum a t about l0.4 and thereafter decreasing; on the area exposed-greater area, higher rate; on bacterial activity-very rapid in an infected latex; and upon the t e m p e r a t u r e t h e increase being about twofold for a 10' C. rise. The oxygen taken up is absorbed by both the nonrubber constituents and the rubber itself. With a bacterial infected latex the nonrubber constituents absorb almost to the exclusion of the rubber. With a well preserved latex the rate is slow and over 70qo of the oxygen is taken up by the hydrocarbon itself. Mechanical stability is generally increased and the aging life of vulcanized samples is decreased by oxidation.
As commercial latex is imported into this country at a pH where the oxidation rate is the greatest, undue exposure to air should be avoided. A latex having a lower pH would minimize this precaution.
T
HE fact that oxygen is absorbed by latex was forcibly
brought t o the authors' attention many years ago through an incident in shipping of latex t o this country from the Far East. On a certain ship only half of a tank assigned for latex had been filled because of development of a leak. It was suggested t h a t this tank be carefully examined on arrival in this country, and consequently, after the ship docked, someone was sent down into the hold t o examine the latex. Unfortunately, he fell into the latex and had t o be pulled out by a rope. He survived but subsequent examination of the gas above the latex showed it t o be almost completely lacking in oxygen. The latex had undergone a decided change, in t h a t its stability was considerably higher and its viscosity somewhat lower.
Vol. 43, No. 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
476
A s a result of this observation, a laboratory study of absorption of oxygen by Hevea latex was undertaken.
Apparatus In qualitative experiments on the effect of oxygen on ammonia latex, the latex lvas simply treated by bubbling the gas through it or by letting it stand in contact with an atmosphere of oxygen. I n quantitative measurements of the rate a t which oxygen is absorbed by the latex several types of apparatus have been used, all of which consist essentially of a small flat-bottomed flask to which a mercury manometer is attached.
9 16.0
I
x
0
10
EXPT flu3 p H // o./ 8 3 0 /3i Q* 33
m
8
VI
9 I2*O 8.0
8 4.0
20 30 TIME, DAYS
40
with the present known behavior of inadequately preserved latices, although gross signs of spoilage set in much later than the onset of rapid absorption of oxygen. A series of latices was prepared by removing ammonia from normal latex in a slow stream of air, then reammoniating t o various levels. Samples of these were allowed t o stand in contact with oxygen a t 1 atmosphere pressure and the rate of absorption was followed. It was found that about 0.5% ammonia was the minimum concentration of preservative adequate t o prevent the very high oxygen demand observed a t low p H over the period of the authors' experiments. This evidence of the presence of aerobic bacteria in the latex permitted a much more rapid detection of the presence of infection than any of the usual techniques for detecting the presence of bacteria. Figure 1 shows the effect for three latices containing low levels of ammonia. All samples containing 0.1% added ammonia or less (pH 8 or below) absorbed oxygen at very high rates from the start, as indicated by the data of experiment 11. At slightly higher ammonia levels, latices which were initially sterile absorbed oxygen a t low constant rates. Experiment 13 illustrates this for a latex with 0.25% ammonia, p H 9.3. However, inoculation of this latex with 1% of serum froin a putrid latex of lower p H resulted in bacterial growth, shown by the spurt in oxygen demand of sample 13i. When ammonia levels were raised t o a t least 0.5'%, addition of putrid serum did not infect the latex immediately, and no increase in the rate of oxygen absorption was observed over several months.
50
Table I.
Figure 1. Bacterial Attack of Latex
Effect of Hydroxylamineaon Absorption [Normal latex (40.0 T.S.).
In this apparatus the pressure was adjusted manually t o 1 atmosphere a t frequent intervals. The mercury added was used as a measure of oxygen absorbed. This apparatus worked satisfactorily for relatively large samples of latex when a long period of absorption was available, several days t o a month. Recently, more rapid and accurate measurements have been made using a device in which small samples of latex are used and the pressure changes are measured by a Statham laboratory pressure transmitter sensitive to small pressure differences. In this work commercial latices were used. Details of their preparation are not important; the significant differences are indicated in the following outline: Latex Type X-135 X-587 S-18 M(or S)-958 M (or S)-957 M-1135
Concentrat?on, % Total Solids 37 35 63-64 38 62-64 68-70
Preservation Ammonia Ammonia Ammonia Formaldehyde-ammonia Formaldehs de-ammonia rimmonia
Oxygen, Time, PH Millimoles/ Days Start End 100 G. 54 7.51 7.52 2.2 54 7.79 7.79 2.8 54 8.89 8.80 5.6 54 9.21 9.14 9.0 54 14.0 9.94 9.90 54 2.00 16.2 I O . 36 10.25 0 15% added to each aample (Ellis and XcCavaok, unpublished).
Sample
a
Temp. 25O C.]
% NHa Start 0.08 0.11 0.20 0.30 1.00
Further evidence supporting the view that this behavior is due t o the growth of bacteria n-as obtained by measuring the oxygen absorbed by latices preserved with small amounts of hydroxylamine (4) or with silver cyanide (Table I). I n these, even a t low ammonia levels, no rise in oxygen demand was observed, and the latices show no signs of spoilage after long standing in contact with oxygen. Inoculation of these preserved latices with putrid serum did not produce infection. Titration of the serum from spoiled latices with potassium hydroxide showed that the oxygen absorbed could be accounted
Only the last two of these are now being produced. Quantitative measurements were made a t constant temperature, usually 30" C., and a t 1 atmosphere of oxygen in most experiments. Some difficulty was experienced in obtaining reproducible rates with undiluted concentrates. Because normal latices did not give this difficulty, all experiments were made a t 35 t o 40% total solids concentrations, adjusted with dilute ammonia solutions where necessary. I n this work, oxygen absorbed is always recorded in millimoles per hundred grams of latex solids, and rates are per hour.
Results Bacterial Invasion of Latex. I n the first work it was found t h a t much more rapid absorption of oxygen occurred a t low pHt h a t is, in the range 7 to 8.5-than a t high pH, around 10. Investigation of this phenomenon showed that the latices of low p H appeared to spoil more rapidly than those of higher pH, in line
2
Figure 2.
4
6 8 TIME, D A Y S
10
12
14
Representative Absorption Curves
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1951
for nearly quantitatively as carboxyl, assuming 1mole of -COOH per mole of oxygen. I n oxidized sterile latices a much smaller fraction of the oxygen can be detected this way. The data are given in Table 11.
Table 11. Absorption Titration Data
4
c
Time, PH Sample Days Start 11 107 8.3 13i 111 9.3 13 65 9.3 a Me. KOH per gram of latex Total millimoles of oxyge?
*
Oxygen KOHa Millimole) End Me./G: G. KOH/O 0.604 1.05 7.4 0.637 0.809 0.79 7.6 0.640 0.45 0.069 0.152 9.2 solids gained during oxidation. absorbed per gram of latex solids.
This observation was a forerunner of work recorded elsewhere, which showed t h a t it is essential t o observe strict cleanliness in the handling of latex and t o preserve the latex adequately from the start, as latex is susceptible t o bacteria from the air as well as to bacteria of anaerobic type. $16 X
8
I
4
1
I
7
I
8
I
1
I
I
9
IO
11
I¶
J
PH
Figure 3.
I
G. 100 200 300
the intercept, is much larger than t h a t required t o saturate the latex with oxygen in solution, but it may not be larger than the amount t h a t can be trapped as small bubbles in the latex. The observed rates are reproducible within a few per cent; the intercepts vary widely. Effect of pH. When the original marked effect of p H on the rate of oxygen absorbed was traced t o the effect of aerobic bacteria, new experiments were made, using care t o prevent infection of the latex used. Under these conditions it is found t h a t the rate of absorption is still a strong function of the p H but that the rate increases with increasing pH, instead of decreasing. A number of commercial latices have been examined and show similar behavior in this respect. Figure 3 shows the rate as a function of p H for a normal latex of about 40% solids (958). The relative rates are calculated from the total absorbed after 50 days. The abscissa is initial pH; a slow, small, drop in p H occurs during the oxidation. The most rapid rate occurs a t about the p H at which most commercial latex is brought into this country. This means that any effect of oxygen on the latex or the rubber will be a t its highest in present commercial latices because this p H represents the minimum a t which long preservation from bacterial infection can be obtained with ammonia. I n a sterile latex the rate is independent of the base used t o adjust the pH. Effect of Latex Age. Although the rate of absorption of oxygen appears t o be constant during the time of the experiments, there is a considerable variation in the observed rate with some of the latices, depending on the shipment studied. Figure 4 shows a set of experiments with different shipments of latex (Type 957) arranged according t o age. The earliest point is a t about 3 months, because this length of time usually elapses between tapping and receipt in this country. The observed rate decreases with latex age up t o about 12 months, when it remains fairly constant at about 10-8 millimole per 100 grams of solids per hour
Effect of pH on Oxygen Absorption
Effect of Agitation on Latex Oxidation. I n the first semiquantitative experiments, rates of absorption of oxygen were measured in a static system-that is, the latex and oxygen were introduced into the sample flasks and let stand undisturbed. Rates were found of the order of millimole of oxygen to per 100 grams of solids per hour. As shown in the following experiment, under these conditions the apparent rate of oxygen absorption is a function of exposed surface area rather than of sample size. Varying amounts of latex 5-958 were placed in three 500-ml. Erlenmeyer flasks under oxygen and the amount absorbed after 10 days was measured. The rates are proportional t o the exposed area and not related t o the amount of latex present: Latex,
477
Area Exposed sq. Cm.' 69.6 55.7 43.0
%%end ~~illimoleL 1.26 1.14 0.85
Oxygen Millimoles'/ S q . Cm. Surface 0.18 0.20 0.20
As a result of this observation, all later work was carried out in apparatus mounted on a shaker t o make sure t h a t diffusion was not the rate-controlling step in the reaction. More recent experiments indicate t h a t the limiting factor may be the rate of solution of oxygen at the surface rather than entirely slow diffusion in the aqueous phase. This surface effect is similar t o that which has been observed in work with solution of gases in organic liquids. When adequate agitation is used, the curves of oxygen absorbed versus time are essentially linear for the latices examined in this work, over periods of 2 t o 3 weeks, and are five t o ten times as fast as those found in static systems. This is illustrated in Figure 2, which shows data for two commercial latices. The curves intersect the zero time axis a t a slight positive value of oxygen absorbed. The amount absorbed, as indicated by the height of
I
4
a
I2
I
AGE, MONTHS
Figure 4.
Effect of Latex Age on Absorption Rate
(at p H 10.4). It was suspected t h a t the limiting rate is the rate a t which the hydrocarbon suspension reacts with oxygen, and the higher early rate is caused by water-soluble materials in the serum, which are slowly destroyed on storage. This was confirmed by measuring the absorption of a triply centrifuged latex, in which the concentration of nonadsorbed water solubles is 2.5% t h a t in normal latex. It absorbs at the same rate as aged 957. This rate, a t p H 10.4, is about ten times as fast as t h a t for dry rubber from the same latex. Effect of Temperature and Pressure. A few experiments have shown that minor variations in oxygen pressure do not have any effect on the observed rate of absorption of oxygen by the latex a t high pressures (about 1 atmosphere). When air is substituted for oxygen, the rate falls to about 0.2 that observed at 1 atmosphere. Increased temperatures cause a n increase in the observed rate, the increase being about twofold for a 10" C. rise of temperatures.
INDUSTRIAL AND ENGINEERING CHEMISTRY
478
data of this curve were obtained by treating the latex with oxygen until a specified amount had been absorbed, drying the latex in a slow stream of air, and measuring the Mooney viscosity of the dried films. Five millimoles of oxygen per 100 grams correspond t o about 0.15%oxygen, so t h a t very little is required t o produce a marked change in the viscosity of the rubber. These results may be roughly compared with those of Farmer and Sundralingham ( I ) , using an empirical correlation between Mooney viscosity and intrinsic viscosity. The assumptions involved permit only qualitative comparison. The data are compared in Table V, using the authors' figures for rubber from normal laticrs.
105 r*
2 100
8
K 5
Vol. 43, No. 2
95
e;
z
c
4 90
-1
e Y
85
~~
Table V.
PO
10
30
50
40
70
60
STABILITY, M I N U T E S
Figure 5.
Viscosity and Stability of Oxygen-Treated Latex
Effect of Oxygen Absorbed on Latex. It was noted previously that the stability and viscosity of the large shipment inadvertently treated with oxygen were altered by this treatment. Examination of other normal latices shipped subsequently showed this to be a general effect. Figure 5 illustrates one experiment in which a latex (X-135) was treated with oxygen at 50" C. and 5 atmospheres. The relative viscosity, measured with a capillary viscometer, fell slightly. The stability ($), measured with a high speed stirrer, rose severalfold. Successive points on the curve represent successive 24-hour intervals. Similar changes in stability were observed in other shipments of normal latex, when treated with oxygen or mild oxidizing agents. Representative data are shown in Tables 111 and IV for latex X-135. All stabilities are in minutes.
Table 111.
Effect of Oxygen and Oxidants on Latex Stability
Latex Shipment .4
Original StabilitJ10 8
B
c
9 12 10 6 15
D
E
F
0 O-
Perborate Treated 17
Ouygeq"
Treated 15 17
9 13 22 18 11
3a
..
1%
56
Chain Scission by Molecular Oxygen
Oxygen Absorbed, Millimoles/100 G
Molecular Weight. 34 x 10-b TBrV
WORK
6 8 (est.) 5 1
0 2 3
41 24 17 12
3 7 2 6 1 8
4 5
FARMER AND 0 23 31 48 90 134
Moles Oxygen Absorbed per Bond Broken
Sl-\UR4LI\CIH&.U
..
3.24 0.93
30 32 44 60
0.75
0.67 0 .55 0.48
78
Io Table 111,the third column is obtained by dividing the oxygen absorbed per mole of (original) rubber by the number of bonds broken per mole of (original) rubber, calculated from the relation Bonds broken =
Mo
-
Ad
- 1
The efficiency of the absorbed oxygen in breaking bonds is of the same order of magnitude, but there appears t o be greater efficiency of breakdown in the authors' experiments. Possibly the rubber oxidized in the latex, where peroxides are rapidly destroyed, is broken down more efficiently than in the photo-oxidation, where the peroxide is relatively stable.
29
1 atmosphere of oxygen, 3 weeks at room tempernture. Sodium perborate, 0.2%. 4 weeks at room temperature.
Table IV.
Effect of Perborate Concentration on Latex Stability SnBOs,
wt.
70
0.0
0 .I 0.2 0.3 0.4 0.5
Stability (after 2 Weeli-9) 11 18 20 22 28 25
Normal latex is no longer shipped in quantity, and the improvements in preservation associated with the development of concentrated latices have resulted in latices of such high stability and low viscosity t h a t no marked change is produced in the early stages by this mild oxidation of serum constituents in concentrates. Effect of Oxygen Absorbed on Rubber. Although some of t h e oxygen attacks the water-soluble materials in latex, m indicated above, at least part of it also attacks the rubber hydrocarbon. Figure 6 shows the most immediately observable effect -the reduction in the Mooney viscosity of the rubber. The
4 4 6 2 4 M I L L I M O L E S O X Y G E N / 1 0 0 Grams
Figure 6.
6
Breakdown of Latex Rubber with Oxygen
More surprising is the effect of mild oxidation of the crude on the aging of the rubber after cure. TabIe VI shows the average properties of several samples of latex rubber, all treated in the same fashion. One half of the latex was stirred under nitrogen, the other half treated with oxygen until an average of 2.5 milli-
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
February 1951
moles per 100 grams latex of solids had been absorbed. were mill-mixed and cured in the following formula:
These
100 6 0.5 0.5 3.5
*
*
Time of Cure, Days 20 40
lined by one of the authors (3). The simplest of these is that containers be kept as full as possible to minimize the effects of oxygen.
Acknowledgment
Green tensile and T-50s are not significantly different, but on aging in the oxygen bomb the rubber from the latex treated with oxygen fell in tensile much more rapidly. Presumably this effect is due t o the destruction of the natuial antioxidant by the oxygen absorbed in the alkaline latex.
Table VI.
The authors wish t o extend credit t o Ralph H. Tefft, S. B. Ellis, and W. J. Hart, formerly with the General Laboratories, United States Rubber Go., and working with the senior author, for helping t o develop some of the ideas and experiments reported in this paper. They also wish t o acknowledge helpful advice from C. E. Rhines and aid in preparation of the data from H . S. Witt.
Conolnsfons
Aging Properties of Oxidized Latex Rubber
*
_Tensile_Green_ ~Tensile Aged b a b a b -2.0 3500 3500 2400 1670 -1.0 3500 3600 -9.0 -9.1 1870 1000 3500 3700 60 -14.2 -14.7 1100 650 -17.3 3300 3500 800 300 80 -17.3 a a. Nitrogen-treated control, MIA-4(212' F.) 110. h . Oxygen-treated (2.5 millirnoles/100 g . ) . ML-4 (212' F.)ca. 90. Aged 96 hours a t 70° C. and 300 Ib. oxygen. T-50a
419
a
The development and production of a latex which can be handled, shipped, and stored at a lower pH would be desirable. This would not only get away from the changes due t o oxygen absorption inherent in the usual commercial ammonia latex, but also would require smaller amounts of base for its preservation.
Literature Cited (1) Farmer, E. H., and Sundralingham, A., J. Chern. SOC.,1943,
125.
Precautions in Handling Latex. The marked effects of rather small amounts of oxygen on latex rubber described above indicate that care must be used in exposing the latex t o oxygen wherever these effects might be deleterious. This observation formed a part of the basis for the rules for handling latex in quantity out-
(2) Jordan, H. F., Brass, P. D., and Roe, C. P., IND.ENG.CHEW.
ANAL.ED.,9, 182 (1937). (3) McGavaok, John, India Rubber Work?, 113, 808 (1946): 115. 362 (1946). (4) MoGavack, John, U. S. Patent 2,126,268 (1938). RECEIVED September 27, 1950.
Rheometrie Tests and Extrusion. Pirelli
Silvio Eeoher per Arioni, Milan, Italg
1% cylindrical rheometer of the Couette type, suitable for
F
I
the experimental determination of the rheological properties of extruded materials was designed to provide data which could not be obtained with existing plastometers. The purpose of this study was strictly practical as work was performed in connection with a study of extruders. The results obtained on twenty-five different materialsnatural and synthetic rubbers and compounds of both with various fillers-are reported; measurements fall In this within shear rate limits from 1to 100 seconds-'. interval the relationship between log D (rate of shear) and log T (shear stress) is nearly a straight line. I t may therefore be analytically interpreted by the power law D =
D
URING a study of extruders, the need was felt to know, more completely than was permitted with the use of existing plastometers, the rheological properties of extruded materials. A cylindrical rheometer of the Couette type suitable for,rubber a n d rubber compounds was therefore designed to obtain the rheometric curve D - 7,where D = rate of shear in seconds-' and T = shear strem in kg. per sq. rm. The purpose of this study was strictly practical, and therefore precise equipment was not employed. Mooney ( 4 ) and Hamm (1) have already used cylindrical rheometers for testa on rubber. Mooney applies a pressure to the material from the outside by means of two retaining rings. This p r e m r e is selected with reference to I, and ranges from
- ( I / c ) ~where , n and c are parameters characteristic of the material. A s the power law is known to be of limited validity, attempts were made to ascertain the limits of its application in laminar flow through a cylindrical hole. The results of measurements carried out on a 2-inch extruder employing the same materials as were tested by the rheometer are repqrted. Measurements of pressure and flow were made using discharge holes of various diameters and operating the screw a t various speeds. Reasonable agreement was found between values of flow and pressure determined with an extruder and those calculated from parameters n and c determined with the cylindrical rheometer.
0 to 5.6 kg. per sq. cm. According to Mooney, the maximum T values are 2 kg. per sq. cm. and shear rates, 100 seconds-'. Hamm uses a rheometer derived from that of Mooney, but without movable rings. He therefore relies on the pressure generated when the instrument closes, which pressure is not determined. The T values reached do not exceed 1 kg. per 99. cm. and the highest rates of shear are about 1 second-'; these are not high enough to be of practical interest. Hamm states that the instrument does not allow for the use of shearing stresses of the same magnitude as those that occur during the technical procedures t o which rubber and rubber mixtures are exposed in manufacturing practice-for example, the forcing process. This is the chief limitation of his accurate work. In the rheom-
