INDUSTRIAL, AND ENGINEERING CHEMISTRY
November. 1929
Table 111 (Continued) TENSILE
MODULUS TENSILE ELONGATION PRODUCT Kg./sq. % Kg./sq. % % % cm. change cm. change $& change change
AGING
(500%) ~-
Table IV-Coefficients of Correlation between Oven Aging and Bomb Aging OVEN AGING-96 COEF.
147 147 156 133 139
Original 41 Natural aeine. 1 vear 46 'S,7-00 c. Oven. 96 6011; Bomd,24hours,6OoC. Bomb,48hours,6O0C 35
0 f 6 -10 6
-
+ 10
% 2;; -15
(500%)
Original Naturalaging, l y e a r Oven,96hours,7O0C. Bomb,24hours.60°C. Bomb,48hours,60°C.
72 67 75 51 42
- 8 +-30 4 -42
-11 -22 -17
65 43 57 45 45
34 -12 -30 -30
95 83 95 63 43
570 -13 556 - 1 565 -34 550 -55 507
-
4
-
(300%) Orieinal 63 105 NzF&ila&ine. -1 vear 71 1 1 4 94 -10 Oven 96hours 70'C. 90 -13 71 + I 3 1 Bom6,24hour&60°C. 64 1 104 8 80 -23 Bomb.48hours,6O0C. 58
_.-_.
I
+-
(300%) (7) Original 86 Naturalaging, l y e a r 87 Oven,96hours.7OoC. 90 Bomb,24hours,6O0C. 89 Bomb,48hours,60°C. 86
+2 + + 053
562 575 520 535 540
190 183 169 149 158
-
150 136 127 150 142
-
-E!
--
29
!I
24
25
-37 -34
-
2 I.
78 66 76 49 31
-15 3 -37 -60
63 50 45 65 46
-21 -29 3 -27
-
410 362 375 390
425 375 350 440 400
440 9 425 -15 400 0 437 5 443
-
- 4k
152 150 125 113 122
4- :!
if
4:
-11
.-1:I -1s
+- 4 €0
+
3 $1
I 1
-
1
-18 -26 -20
.- -24.,-
94 82 72 93 90
HOCRSBOMBAGING-% COEF. OF
OF
,I,
Original Naturalaging, 1 year Oven,96hours,70°C. Bomb,24hours.60°C. Bomb,48hours,60°C.
1015
-
+
-13 -23 1
-- 4
HOIJRS BEST APPROXIMATION
TO
CORRE- PROBABLE CORRE- PROBABLE NATURAL PROPERTY LATION ERROR LATION ERROR AGING Lightly Compounded Stocks Modulus 0.610 t0.24000 0.940 +0.04600 Bomb Tensile 0.990 *0.00074 0.900 *0.07400 Oven Elongation 0.470 *0.34000 0.390 *0.33000 .,, Tensile product 0.950 =+0.03500 0.890 *0.07700 Oven Mediam Compounded Stocks Modulus 0.410 t0.12500 0.500 t0.11600 0.420 0.770 * O 09200 Bo'mb Tensile *0.18400 0.760 0.550 =tO 10800 Oven Elongation *0.06300 0.740 *0.06100 0.845 t0.03800 Bomb Tensile product Heavily Compounded Stocks Modulus -0.830 *0.07900 -0,570 tO.17300 Oven Tensile -0,720 *0.12200 -0.147 t0.25000 Oven Elongation 0 440 *0.22000 0.380 t0.22000 ... Tensile product 0.072 t0.25000 0.200 *0.25000 ...
Acknowledgment
Grateful acknowledgment is made of the assistance of
H. A. Braendle, of Binney & Smith, for outlining the method of working out the coefficient of correlation, and to J. M. Ball, of R. T. Vanderbilt Company, for help in drawing up the graphs.
Some Notes on Artificial Aging Tests for Rubber w. w. vogt GOODYEAR 'CIRE & RUBBERCOMPANY, AKRON, OHIO
ROM a strictly yuautitative point of view we should expect an artificial age test to duplicate in all respects the changes that take place on natural aging. In other words, all of the measurable physical and chemical properties should be changed in the same direction and to the same degree for all types of compounds. If we are looking for such performance from an artificial aging test, then in the writer's opinion none of the present tests fulfil these conditions to a sufficient degree f,o warrant any extensive use of correlation factors. It must, moreover, be realized that the present methods of artificial aging seek only to duplicate the natural aging of rubber on shelf storage in the dark. If we wish information that will predict aging of rubber products under their normal service condifions, then we must get into the field of specialized testing wherein the conditions with respect to light, temperature, humidity, etc., may be made to approach more nearly those of the actual service.
F
Limitations of Present Aging Methods
With these points in mind we may now consider a few of the more serious limitations of present methods. LIMITATION1-All of the measurable properties should change in the same direction and to the same degree as in natural aging. I n this connection the writer wishes to present a few data by C. R. Park. The formula is-rubber 100, ZnO 12.5, PbO 6.25, and sulfur 6.25. This stock was used for several years as a tube stock. General experience and records over a long period have shown that it stiffens on aging and that the tensile qualities hold up fairly well. From the data given in Table I we find: (1) On natural aging the tensile product decreases and the stiffness index increases. (2) In the Geer oven the tensile product decreases and the stiffness index decreases very strongly. (3) I n the
Bierer-Davis bomb the tensile product increases slightly, but the stiffness index decreases quite markedly. Table I
TENSILESTIFFNESS PRODUCT^ INDEX^ Original 150 85 76 76 3 days Geer oven a t 70' C. 6 days Geer oven a t 70' C. 65 70 9 days Geer oven a t 70' C. 60 64 16 hours Bierer-Davis bomb (at 50' C. and 400 Ibs. per sq. in. or 28.2 kg. per dq. cm.) oxygen pressure I55 77 6 months natural 140 95 12 months natural 120 102 a Tensile in kg. per sq. cm. X elongation + 1000. b Difference in kg. per sq. cm. of the moduli a t 650 and 450 per cent. AGINQ
This behavior is typical of several similar types of litharge stocks and one is forced to conclude that it is rather hopeless to attempt their evaluation by present methods of accelerated aging. LIMITATION 2-Another question that is constantly being asked is-how many hours in the oven or the bomb does it take to equal a given period of natural aging? In other words, what is the equivalence factor? Park (1) and Vogt (6)have shown that to produce changes equivalent to those produced by a given period of natural aging the times required for both the oven and the bomb test vary as much as tenfold depending upon the type of stock. This extreme variation can only force us to the conclusion that in the present state of the art accelerated aging cannot be said to be a quantitative method. Even under the most ideal conditions one can get very diversified results as to the equivalence factor, depending upon what property one chooses upon which to make the comparisons. The data given in Table I1 also bear directly upon Limitation 2. They represent the average of the results obtained on nineteen stocks which differed from one another only in the percentage and kind of softener, all other conditions being fixed.
I N D U S T R I A L A N D Eh'GliVEERlNG CHEMISTRY
1016
The base stock was a high-grade black tread. The data are from the average of three cures on each stock, a correct, slight under, and slight over-cure. Table I1 16Ilouns BOMB
SO 118
16 HOURS 6 DAYS 6 MONTHS 9 MONTHS BOMB OVEN NATURAL NATURAL EQUALS EQUALS Months Months Per cent of original tensile product 7s 83 77 7 0 so Per cent of original modulus a t 300 per cent 137 137 140 2s 6 0
Table 111 gives the results of variations in tensile product and tear resistance for several widely different stocks which were subjected to a slow oxidation test, the conditions being 150 pounds per square inch (10.5 kg. per sq. cm.) of oxygen at 40" C.
6 DAYS OVEN
It is seen that the bomb test in particular shows a quite wide spread, 16 hours being equivalent to 7 or 3 months' natural aging, depending upon whether we take tensile product or modulus as the basis of comparison. LIMITATION 3-Ali types of stock, whether using different pigments, different degrees of pigment loading, or different kinds of accelerators, should change in the same manner and degree as in natural aging. No new data are presented on this point as Tables I and I1 are sufficient t o bring out the fact that the litharge tube stock reacts very differently from the black tread stocks. Furthermore, papers by Park (1) and Vogt @) supply additional data. It seems to the writer that the fact that different accelerators, for example, give widely varying results when compared with natural aging results is one of the most serious criticisms of present methods of artificial aging. If one interprets the results of the unknown on the basis of experience built up on the known and then finds later by sad experience that the new did not behave like the old a very healthy distrust is liable t o be engendered in the mind of the experimenter. Value of Oxygen Bomb Test
Now in spite of the limitations of present methods the writer believes that artificial aging tests have been of great value to the rubber industry and that in general better aging rubber products are being made now than ever before. In this connection the writer considers the high-pressure oxygen bomb test as proposed by Bierer and Davis to be a sensitive and reliable method for determining the oxidizability of rubber stocks. In a series of tests involving the determination of the antioxidant properties of approximately three hundred compounds, it has been found that, without exception, those compounds ccjuld be classified as good, fair, or worthless by either a high-pressure oxygen test or by the results of natural aging. The method of compounding, previously mentioned in various papers from the Goodyear laboratories, involves the use of acetone-extracted rubber together with zinc oxide, stearic acid, sulfur, and an accelerator with no antioxidant properties such as hexa or D.P.G., to which basic mix is added the compound to be tested for antioxidant properties. Properly cured sheets of the basic mix will suffer practically complete deterioration in tensile properties in 6 t o 12 months in the dark and in even less time in diffuse daylight. A few hours (3 to 5) in the high-pressure oxygen bomb a t 70" C. will also accomplish complete loss of tensile properties. Furthermore, the deterioration can be checked equally well by either loss in tensile or decrease in tear resistance. If one uses the criterion of weight increase as the basis of comparison, the results of artificial and natural aging tests show good correlation, but the lack of complete correlation is probably due to the difficulties of the experimental method (volatility of ingredients, changes due to humidity variations, etc.) rather than to any real significant differences in behavior.
Vol. 21, N o . 11
STOCK
Average
Table I11 (Per cent of original properties) 7 DAYS 14 DAYS T P Tear T . P. Tear
90
03
85
83
28 DAYS
T . P.
Tear
76
76
On the whole the agreement between the two properties is good. These data are typical and lead to the conclusion that, under test conditions wherein the oxidation effect is enhanced and the heat effect suppressed, both tensile properties and tear resistance are effected in a similar manner. For those rubber stocks whose life is limited by their oxidizability some form of the high-pressure oxygen test furnishes a reliable qualitative estimate of their worth, but for those stocks whose service is limited by other factors this test is not satisfactory. In dealing with stocks that are relatively resistant to oxidation, through the use of age-resisting accelerators, proper cure, or antioxidants, the primary effect of the accelerated test is brought about through the elevated temperature employedi. e., through the heat effect. And in ronnection with the study of the heat effect widely different results are obtained depending on the criterion used for judging the effect of the heat treatment. For example, a simple rubber 100-sulfur 6.5 stock gives the following tests when heated in nitrogen and air at TO" C. Table IV (Per cent of original properties) 6 DAYS 12 D A Y S Air Nitrogen Air h'itrogen Tensile 112 110 84 10s Tear resistance 57 63 48 45
The results in Table IV show that the tear resistance suffers much more than tensile properties and that there is little difference between heating in air or nitrogen. Importance of Specific Tests
In view of the data presented and of other experiences, it is probably asking too much of any single test to demand that it reproduce quantitatively all of the phenomena associated with natural aging in the same degree on all physical properties, and it is far better to determine by independent tests designed to accentuate one phase and suppress the others the propensities of a stock with regard to oxidizability and after vulcanization. In the Goodyear laboratories a rather slow oxidation test has been adopted which consists in heating the sheets of rubber in oxygen n t a pressure of 150 pounds per square inch (10.5 kg. per sq. cm.) a t 50" C. for 6 to 12 days. If they come out of the bomb with fair tensile properties it is fairly certain that the stock is not going to fail by oxidation. To determine after-vulcanization or stiffening or autopolymerization, or whatever one wishes t o call it, the rubber sheets are heated for 6 and 12 days in nitrogen a t atmospheric pressure and a t a temperature of 70" C. From this test one can judge whether or not a stock is going to become unduly short and brittle on natural aging. Literature Cited (1) Park, Rubber Age (Loxdon), 7, 64 (1926). (2) Vogt, IND. ENO.CHEM.,17, 870 (1925).
