1648
ANALYTICAL CHEMISTRY
Remove the flask and if the solution is a greenish-yellow color instead of the normal pink color then heat on the steam bath until the normal pale pink calor of the ferric acetate complex is obtained. Cool the solution to room temperature and dilute to the 50-ml. mark. Mix thoroughly and measure the optical density a t a wave length of 337.5 mp against a blank solution containing 0.2 ml. of hydrochloric acid in 50 ml. of 50% acetic acid by volume. The optical density reading multiplied by 2.321, the reciprocal of the slope of the standard Beer's law curve (Figure 5), gives the value in milligrams of iron per 50.0 ml.
Three samples of iron ore of the sefiquioxide type, designated A, B, C,were analyzed. The weighed samples were brought into solution by heating with concentrated hydrochloric acid. T h e respective solutions were filtercd quantitatively into 100.0-ml. volumetric flasks. Suitable aliquot portions of these solutions were analyzed according to t,he mocedure dexribed above. The results are given in Tahle 11. LlTERhTURl
As large as 3.00 mg. of iron can be determined directly by thifi procedure. Larger samples can be analyzed hy making an appropriate dilution with 50.0% acetic acid from the original solution oontained in the 50.0-ml. volumetric flask, Increased volumes of hot 1 N hydrochloric acid me needed to dissolve larger amounts of ferric hydroxide. Citlculations are made with the aid of the following exprossion: milligrams of iron in 50.0 ml. = optical density a t 337.5 mp X dilution factor X 2.321. Typical results are given in Table I.
(1) Broda, Z., Chem. Lisly. 37,2R9 (2) Cooke, W. D.. Hazel, J. P., and McNahh. W. At., 21, 643, 1011 (1949). (3) Riban, J., Bull. am. chim.. 6 , 3, 916-20 (1891).
ANAL.CHEM..
R E C E ~ V Ef oDr review J~nuary26, 1952. Aooepted June 20, 1952. Presented before the Meeting-in-Miniature of the Philadelphia Section. AYERICAN CHEMICAL SOCIETT, January 1911. Abstracted from s portion of B diasertation presented by Wilhelm Reisa to the Faculty of the Graduate School of the University of Pennsylvania in partial fulfillment of the requirements f o r the degree of doctor of philosophy
Preparing Crude Rubber Test Specimens for Oxygen-Absorption Measurements WILLIAM J. GOWANS
U. S. N a t u r a l Rubber Reswrc :hStation, Salinas, Cnlif. ECENT investigations (a, 5, 8) have shown the advantages of using a volumetric oxygen-absorption apparatus for accelerated aging tests on crude rubber. Current work in this laboratory has confirmed the advantages of this apparatus as a rapid means for determining the storage stability of a given crude rubber stock. Unfortunately, crude rubber test specimens cannot be placed in the oxygen-absorption apparatus in the same manner as can test specimens of vuloaniaates. The tendency of crude rubber t o flow, particularly during the more advanced stages of oxidation, m y cause errors in oxygen-absorption mea& urements by limiting the diffusion of oxygen into the test specimen. For this reason it was necessary t o devise a method of preparing rubber test specimens for oxygen-absorption measurements which is applicable t o crude rubber. Test specimens of crude ruhber suitable far oxygen-abmrption measurements can be readily prepared with the aid of an alumi num mold and a vulcanizing press. The method of pressing tbe crude ruhher into a sheet from which the specimens are taken is a slight modification of that used by McPherson and Cummings (3) in preparing samples far refractive-index measurements. Oxygen-absorption, data. were obtained in an apparatus similar to the one described by Shelton and Winn (6). The results indicate several important advantages of the present hoepress method of sample preparation as compared with the test tuhe-film method used by Glazer and coworkers ( 8 ) in preparing crude GR-S samples for oxy,. scuoies. ,,. gen-aosorpuan
.
EXPERIMENTAL
The mold used for pressing the crude rubber eonsisted of three pieces, each 12 inches square, cut from 20-gage sheet aluminum. One piece had a n %inch square opening which constituted the mold cavity when placed between the other two pieces. Approui-
necessary to ensure complete flow of the guayule and Hevea rubber samples used thus far. Antioxidants or other additives, which are to he investigated for their effects upon oxygen ahsorption, can be milled into the rubber on compounding rolls immediately before the pressing step. Milling guayule rubber for as long as 10 minutes had no apparent effect on the rate of oxygen absorption. Duplicate test specimens were cut from the pressed rubber sheet with the aid of a 1.25 X 3.25 inch template and weighed. Test specimens prepared by the ahove method w&gh between 2 and 3 grams and have a thickness of approximately 0.040 inch. Each specimen was enclosed in B 30-mesh stainless steel envelope, tied securely in place with stainless steel wire a6 shown in Figure 1, and placed in the oxygen-absorption unit. This means of supporting the sample prevents the rubber from flowing until late in the autocatalytic stage of oxidation. The screen is easily cleaned by boiling in nitric acid. It has been found that test specimens having a thickness in the range of 0.010 to 0.020 inoh may he more readily prepared without use of the press. The desired thickness in this range may be obtained by Eimply passing about 5 grams of the rubber sample twice through closely set rolls of a 4 X 9 inch laboratory mill
i \
A Figure 1. Method of S u p p o r t i n g Test Specimen in a Stainless Steel Screen Envelope
V O L U M E 2 4 , NO. 10, O C T O B E R 1 9 5 2
." o TEST T U B E M E T H O D DISSOLVED R U B B E R . HOT PRESS S H O T PRESS (1 C O L D PRESS
i
x MILLED SHEETS
,
0
IO
2 . 0
30
1649
11 1
40
50
60
TIME-HOURS
Figure 2.
Effect of Method of Sample Preparation on Rate of 0 x 3 gen Absorption 70' C., 760 nim.
With the aid of the template thr te-t specimen is easily obtained from the sheeted sample. DISCUSSIOA
Ere of the stainless steel envelope proved to be very effective in preventing sample flo~vduring all but the late stage of autocataii did not appear to affect the lytic, oxidation. The ~ ~ r e r itself rate of oxygen absorption, hioe practically identical oxygenabsorption curves n-ere obtained on a specimen tested with and without the screen envelope. In making the above comparison, a sample was selected which was sufficirntly stable against flow when tested over that, portion of the oxygen-absorption range normally used for antioxitlarit Ptudies. Figure 2 shows ho\v the method of sample preparation affects the rate of oxygen absorption of a typical t,est specimen of guayule rubber. Each oxygen-absorpt)ion curve represents a different method of sample preparation. F1.oni Figure 2 it appears that the test, specimen prepared by the test tube-film method of Glazer et al. ( 2 ) shows the most rapid rate of osygen absorption. The method used to prepare the test tube-film specimen appears t o be the primary cause for the rapid rate of oxygen absorption. .Ilthough the test tube film has a thickness of 0.004 inch as compared to an equivalent t,hickiiess of 0.009 inch for the other test specimens of Figure 2, thP nuthor does not believe that this difference in thickness accounts for the differences shown in the rate of oxygen absorption. Test specimens having a thickness in the range of 0.004 to 0.008 inrh are believed to be free from diffusion limitations. Blum, Shrlton, and Winn ( 1 ) reported t,hnt hevea gum vulcanimtrs \\-it11L: thickness of 0.028 inch were
free from diffusion limitations when tested at' 70' C. and 760 mm. Dissolving a rubber sample in benzene and recasting it as a film according to Glazer et al. ( 2 ) appears to increase the rate of oxygen absorption of test specimens prepared in this laboratory. Precautions \\-ere taken in the preparation of these specimens to guard against light and oxygen. However, it is possible other types of crude rubber specimens can be prepared by this method without influencing the rate of oxygen absorption. For the test specimen which was dissolved in benzene, dried, and pressed on the hot press, there exists again the possibility that the rate of oxygen absorption is increased by dissolving the sample in benzene. The preparation of a test specimen by the hot-press met,hod (142' C.) does not appear to affect the rate of oxygen absorption significantly as compared to the rates of oxygen absorption of test specimens prepared by the cold-press method (50' C . ) and the milled-sheet method. However, Morgan and Kaunton ( 4 ) and later Van Amerongtn ( 7 ) observed that when lightindurcd peroxides present in vulcanizates were destroyed by heat a sigiiificant decrease in the rat>eof oxygen absorption was noted. The heat (142' C.) of the hot-press method of preparing test specimens could possibly produce a recurrence of this phenomenon, if there Fere peroxides present in sufficient quantity in the original rubber sample. This phenomenon has not been observed in any of the crude-rubber test specimens tested thus far. The hot-press method of sample preparation appears to have the following advant,ages over the test tube-film method: 1. In evaluating the effects of antioxidants, the content of an added antioxidant is believed to be more precisely known, since the antioxidant is milled directly into the sample before pressing. Also, the procedure appears to present a minimum of opportunity for osidation to take place before the sample is placed under test. 2. KO special techniques of sample preparation are required. I t is not necessary to use a solvent with the accompanying dificulties of dissolving the rubber, followed by casting and drying a film. X sample of any desired thickness can readily be obtained simply by varying the mold cavity thickness.
-illthough Figure 2 shom no significant difference in ratrs of oxygrn absorption in test specimens prepared by the cold-press method (50' C.) and specimens prepared by the hot-press method (142" C.)! the hot-press method appears to be preferable. The rubhcr sample used t o obtain the data of Figure 2 had a Mooney viscosity of 70, R-hich is relatively low, but enabled it to be pressed adequately a t 50" C. It requires a much higher temperature, however, to press, adequately, crude rubber samples posses,qing a llooney viscosity of 90 t o 100. This fact, plus its convenience, recommends the hot vulcanizing press (142' C.) for the pressing of crude rubber samples. If prior mastication is not objectionable. high viscosity rubbers can br maptirated to viscosities adaptable for cold pressing (50" C.). ACKNOW~LEDGXIEST
The aut,hor wishes to expreps his appreciation for valuable suggestions given by R. H. Taylor of this lahoratory. LITERATURE CITED
(1) Blum, G. IT., Shelton, J. It., and Winn, Hugh, I n d . Eng. Chem., 43.464 (1951). (2) Glazer, E. J., Parks, C. R., Cole. .J. 0..and D'Innni, J. D., Ibzd., 41,2270 (1949). (3) McPherson, A. T.. and Cummings, A. I),, J . Research S a t l . B U T . Standards, 14,553 (1935). (4) hforgan, L. R., and Naunton. W.J. S., Rubber Chem. and Techno2.. 12,235 (1939). ( 5 ) Parks. C . R., Cole, J. O., and D'Ianni, J. D., Ind. Eng. Chem., 42,2553 (1950). (6) Shelton, J. R., and Winn, Hugh, Ibid., 38, 71 (1946). (7) T'an Amerongen, G. J., Rubber Chem. und Technol., 19, 174 (1946). ( 8 ) Winn, Hugh, a n d Shelton, J. R., I n d . Eng. Chrm., 40, 2051 (1945).
RECEIVED for review March
12. 1952.
Accepted July 1. 1952
