Electrodeposition of Rubber Current-Time Relations
W. D. TURNER Columbia University, New York, N. Y. MYRON A. COLER Paragon Paint & Varnish Corporation, Long Island City, N. Y.
The current-time curves for the anodic deposition of rubber have been obtained directly by means of a recording ammeter. Particular attention has been paid to the effects of varying the applied voltage and to the concentration of the slurry.
0
General Characteristics
A
LTHOUGH a great deal of work has been done on the electrodeposition of rubber from latex suspensions, comparatively little attention has been paid to the quantitative current-time relations which must obtain during such deposition. Dogadkin and Sandomirsky ( 2 , 7) showed that for a fixed cell arrangement, voltage, and rubber contept, the form of the current-time dependence may be profoundly altered by the addition of different types and concentrations of electrolytes. The present investigation was based on the assumption that simiar profound variations would be brought about by (a) merely changing the magnitude of the constant voltage applied to the cell or (b) simply diluting a given suspension of latex and electrolytes with water. Moreover, since all of the current-time data so obtained would still be based on a single reference slurry and cell arrangement, it seemed reasonable to expect simple and useful correlations between these data and the composition of a particular slurry.
As is the case with most other electrolytic conductors, it is to be expected that greater applied voltages will tend to yield larger cell currents. However, since the electrodeposited rubber will have a higher specific resistance than the slurry which it replaces, the current will fall off with time. At low voltages, loose and porous deposits may be obtained which offer almost free passage to the solution of electrolytes present so that the resistance of a given weight of deposit will be smaller than that produced a t higher voltages. Thus, the current at low voltages may decrease very slowly with time. On the other hand, the effect may be enhanced a t high voltages as a result of the simultaneous electroosmotic drying of the deposit.
CATHODE
GLASS
Experimental Method The nature of the problem calls for (a) a cell which can be assembled with ease in a reproducible manner, ( b ) means of recording changes in cell current with time even when those changes are taking place very rapidly, and (c) a reference latex suspension which has commercial significance and concerning which a t least some technical data are available. Details of the cell arrangement are indicated in Figure 1. Since the deposit itself was not of primary interest, it proved very convenient to employ a mercury pool as the anode; this ensured a unilateral plating area of fixed dimensions. At the end of a run the cell contents were poured out; after the tube and cathode had been cleaned, a fresh mercury pool and sample of slurry were poured in and the cell was remounted. The cell was placed in series with an Esterline-Angus model A. W. recording ammeter (code designations: case ZOKEM, drive ZEWRO) (3) which plotted automatically and continuously the current-time relations on curvilinear coordinate paper. Power was obtained through a potentiometer operating on a 110-volt direct-current line. Potentials corresponding to approximately 5, 10, 20, 40, and 60 volts were employed. Voltages were read on a Weston model 45 voltmeter. Slurries were prepared from a commercial sample of Revertex ( I , 5, 6),assumed to contain 75 per cent by weight of dry solids and 25 per cent water in accordance with the recommendations of the manufacturer (6). Suspensions containing solids to the extent of 5, 10, 20, 40, and 68 per cent, respectively, were thus prepared. 1282
CLAMP
AND SPACER
TU8€
MARK€R
POOL
FOR JL URRY
ANODE
MARKER FOR
MERCURY
STOPPER
I
5 CM.
I
FIGURE 1. CELLARRANGEMENT
The more dilute slurries will show lower initial conductivities because they possess smaller concentrations of dissolved electrolytes, but since these slurries of lower rubber content also tend to yield more porous deposits, the resistance will again build up more slowly. The representative data of Figure 2 are in accord with this picture. Graphs were obtained directly from the recording ammeter and are here replotted on the more familiar rectangular Cartesian coordinates.
NOVEMBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
1283
FIQURE 3. DEPENDENCE
OF
I N I T I A LC U R RENT ON VOLTAGE
go
/O
at-,-L/ED
30 CO VOLTAGE
50
60
Initial Current Values An analytic treatment which would account in detail for the exact shape of the current-time curves would be rather complicated ; however, the initial current values are obtained under particularly simple conditions-namely, those for which there is a negligible resistance due to deposit formation so that a simple correlation may be expected. In the case of ordinary electrolytes these initial, current values should be directly proportional to the specific conductivities and linearly dependent upon the applied voltage. Figure 3 shows that such a linear relation seems to hold for the latex suspensions studied in the present work; therefore the conductance expressed in milliamperes per volt is, as is the true conductance of most ordinary electrolytes, independent of the field strength in the voltage range being considered here. Figure 4 shows that the slurry conductance based on initial current values varies with the concentration in a manner which is characteristic of many simple electrolytes. It should be noted that the initial current values may depend upon the recording ammeter and in particular upon its time lag and “kick” for the initial reading. Obviously, the time required to obtain an initial current reading is but little longer than the time required to set up the cell.
Summary and Conclusions 1. A cell employing a mercury pool anode and a recording ammeter have been used to study the anodic electrodeposition of rubber. 2. Revertex was selected as an example of a commercially available alkaline latex, and the current-time curves were obtained for a number of voltages and for slurries of several concentrations. 3. Within the concentration and voltage ranges considered, it was found that the higher the voltage and the higher the solids content, the higher will be the initial current and, for sufficiently concentrated slurries, the more rapidly will the current tend to fall off with the time. 4. By considering the porosity and permeability of the deposit being formed, it is possible to explain a t least qualitatively the nature of the current-time curves. Greenspan ( 4 ) has suggested that it may
FIGURE4.
Locia-
RITHMIC
PLOT FOR
INITIAL
CURRENT-
DEPENDENCE OF
VOLTAGERATIOON CONCENTRATION OF SLURRY
FIGURE 2. VARIATION OF CURREKT WITH TIME Curves: A , 5 volts;. B ,10 volts; C, 20 volts; D, 40 volts; E, 60 volts
I. 5 per cent solids 11. 20 per cent solids 111. 68 per cent solids
I 10
P€R
I
I
20 CO C€NT SOIIDS CONTENT
I
80
1284
INDUSTRIAL AND ENGINEERING CHEMISTRY
prove feasible and convenient to prepare reproducible membranes by such electrodeposition since the recording ammeter plots should tell a great deal about the history of the formation of the membrane, thus providing a check on the assumed similarity of procedures. 5. The initial current varies with the applied voltage in accordance with Ohm’s law and varies with the dilution in the simple manner characteristic of many electrolytes. Since, in addition, the initial current readings may be obtained with ease and rapidity, the taking of such data recommends itself as a control method. Thanks are due to the Revertex Corporation of America for the samples of Revertex used in this work.
VOL. 30, NO. 11
Literature Cited Budiloff, Kautschuk, 9, 1, 20 (1933); Rubber Chem. Tech., 6 , 422 (1933). Dogadkin and Sandomirsky, Rev. g i n . caoutchouc, 10, No. 97, 17 (1933); Rubber Chem. Tech., 7, 314 (1934). Esterline-Angus Go., Catalog 336, esp. p. 42 (1936). Greenspan, J., private communication, 1937. Hauser, “History of the ‘Revertex’ Process,” Revertex Corp. of America, 1929. Revertex Corp. of Am., “Revertex Concentrated LatexSuggestions for Its Treatment and Use,” 1929. Sandomirsky and Dogadkin, Russian Patent 43,733 (1936). RECEIVED February 11, 1938.
Oxidizability of Roasted Coffee
S
EVERAL attempts have been made to find an objective
method of measuring flavor deterioration during the air storage of roasted coffee. Balart (1) reported that the oxidation-reduction potential of coffee beverage has been studied with the hope of correlating this measurement with changes in coffee flavgr. He also mentions studies on coffee oil made with a similar end in view. Bengis (2) attempted to correlate staling with changes in the fat fraction of roasted coffee, but the work of Elder (4) indicated that the fat of roasted coffee is stable and that Bengis’ measurements were probably concerned with impurities in the fat. No objective method of measuring flavor deterioration has yet been reported. It has been suggested that staling of coffee, or flavor deterioration, which occurs on exposure to air is due to absorption of oxygen by the coffee and reaction of the absorbed oxygen with flavor constituents. Regardless of the complexity of t h e mixture of flavor constituents, it is reasonable that the rate of absorption of oxygen by these substances is a factor in a possible measurement of their concentration and consequently of the freshness of a coffee sample. Since the amount of flavor substances in coffee is small, it has been necessary to resort to a inicromethod in order to study the oxidation of coffee by molecular oxygen. The BarcroftWarburg assembly has been found to be ideally applicable in measuring oxygen absorptions. By means of this assembly it has been possible to measure the rate of absorption of oxygen by a given weight of roasted coffee and to show that the rate of oxygen absorption decreases as the coffee stales. The studies showed that fresh coffee contains an amount of oxidiaable material, dependent on the degree of roast, which decreases slowly during air storage because of reaction with atmospheric oxygen. Consequently, the measured oxidizability, or rate of oxygen absorption, also decreases and may be correlated with the loss in fresh coffee flavor a t any given time.
Apparatus and Procedure The Barcroft-Warburg equipment and technic are described by Dixon ( 3 ) . The equipment was constructed by the American Instrument Company. The flask volumes were approximately 35 cc., and the manometric fluid used was Brodie solution. Preliminary work on the oxygen absorption of coffee was concerned with the measurement of absorption by dry roasted coffee having the usual moisture content of 1 to 3 per cent.
WILLIAM R. JOHNSTON The Fleischmann Laboratories, Standard Brands Incorporated, New York, N. Y.
The Barcroft-Warburg assembly has been applied to the measurement of the rate of oxidation of roasted coffee infusions. Measurements on the rate of absorption of oxygen by roasted coffee have shown that only a small amount of oxygen is necessary to produce staleness. Moisture, p H , temperature, and other factors affecting the oxidation of coffee are discussed.
It was soon found, however, that daiIy variations in humidity caused such large fluctuations in measured absorptions that results were too variable. Measurements of oxygen absorptions of water infusions of coffee were next tried with much more satisfactory results. The infusions were simply suspensions of ground coffee in water and were not filtered beverages. The pH of the infusion influenced the oxidation rate, so that a buffered infusion was later employed. A number of experiments were made to determine a satisfactory rate of shaking, a suitable ratio of coffee to buffer, a proper amount of 30 per cent potassium hydroxide to be used in the inner cup of the Barcroft flask, and a convenient temperature of measurement. The following procedure was finally devised : The temperature is set at 40’ C., the shaking rate at one hundred and ten oscillations per minute, and the amplitude at 2 cm. The shaking rate and amplitude are sufficient to permit a maximum rate of absorption at a given temperature. The measurement of oxidizability is made by weighing a 0.500-gram sample of ground dry roasted coffee into a Barcroft flask, placing 0.2 cc. of 30 per cent potassium hydroxide in the inner cup, adding 5 cc. of Walpole acetate buffer (pH 5.20) at 25’ C., attaching the flask to the ground joint of the manometer, and then equilibrating the system at 40’ C. for 20 minutes. Ten minutes are a1lowed for the preparative period. After equilibration the system is sealed by turning a stopcock, and manometric measurements are started.
