Ih’D USTRIAL -4ND E1VGILI’EERING CHEMISTRY
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and if the temperature varies more than an amount corresponding to a 5 per cent change in the rate of the reaction, the results of the test will be in error by some similar amount. Since it is known in an empirical way that an increase of 10” C. will a t least double the rate of a reaction, deterioration should be a t least twice as rapid a t 70” C. as a t 60” C. Therefore, to maintain an accuracy of 5 per cent, the temperature must vary not over one-half degree above or below 60’ C. Some installations are now equipped with constant-temperature ovens for maintaining the prescribed temperature in the medium surrounding the bomb, others are equipped with a temperature-controlled water bath. Whatever the means employed, a nearly constant temperature must be maintained throughout the test to avoid large errors and unexplained discrepancies in the results. B o m b of Larger Capacity
A sufficiently accurate temperature control and bombs of larger capacity are two of the difficulties which may confront some who are now using the test. As a departure from the present type of bomb, one of larger capacity has been designed. It is shown in Figure 10 in comparison with the present type. Its internal capacity is approximately three and one-half times that of the smaller bomb.
Vol. 17, No. 8.
DISCUSSION
Application to Factory Control of Stocks By John D. Morron and Harold E. Webster UNITEDSTATESRUBBERCo., CLEVELAND, OHIO
ESTS of a large number of factory stocks show that the Bierer-Davis oxygen aging test checks actual life aging and is very valuable as a means of factory control of stocks because of the small amount of time needed to obtain results. Proper Compounding Materials The writers’ first experience with oxygen aging was with two stocks t h a t had been compounded for the same purpose, the actual aging of which was known. Both stocks were “nonblooming” and both gave about the same results in the heat aging test:
Reiore heat aging Aged 1 week at 150’ F. Aged’2 weeks a t 150’ F. Before heat aging Aged 1 week 150’ F. Aged 2 weeks 150” F.
Stretch Per cent Sock 1 280
Permanent elongation Tensile Per cent Lbs./sq. in.
40
4 2 1
Stock 2 320 160 100
12 3 1
60
428 318 321
749 446 385
No great difference would be expected in the natural aging of these two stocks, but actually Stock 1 after less than 2 years was either in a soft, gummy condition or else was so stiff and brittle that it cracked on bending. Stock 2, although actual experience had not run over such a long time, showed perfect aging qualities as far as known. When submitted t o the oxygen aging test these two stocks acted in exact accordance with expectations. Stock 1 came out of the bomb exactly in the condition of the stock which had been aged naturally for about 2 years, being so destroyed t h a t it was impossible to make any tensile tests. Stock 2 showed the following tests:
Before oxygen aging Oxygen aging 16 hours at 60’ C.
Figure 11
Typical Installation for Several Bombs
Figure 11, in turn, represents an installation now used with success with four bombs in operation simultaneously. All four bombs lead to a common header, which serves to charge each from a single tank of oxygen. The header is, of course. the same whatever the number of bombs. Each bomb is equipped with individual piping and a gage so that it can he completely disconnected from the remainder of the system after charging. Likewise, the oxygen tank can be disconnected from the remainder of the system after charging. I n this way one tank of oxygen serves to supply all bombs and when the bombs are under pressure there is no connection between any two. Acknowledgment
The time seems opportune for acknowledging the aid of A. M. Varney, who since the inception of the bomb test has assumed a major part of the burden of the experimental work.
Stretch Per cent 330
300
Permanent elongation Tensile Per cent Lbs./sq. in. 9 792 12
765
This of course indicates an extremely good aging stock. Stock 1 was cured with lime as an accelerator, Stock 2 with an. organic accelerator. Since that time the writers’ experience has led them to believe that it is impossible to make “nonblooming” goods with lime which will age satisfactorily. On the contrary, the presence of even comparatively large amounts of lime in stocks containing large amounts of sulfur does not necessarily cause poor aging, if there is no attempt to make the stock “nonblooming.” For instance, one stock, containing about 25 per cent of lime and 15 per cent sulfur based on the rubber, tested as. follows :
Before oxvgen aging Oxygen aging 16 hours at 60’ C.
Permanent Tensile Stretch elongation Lbs./sq. in. Per cent Per cent 340 29 875 280
19
892
This shows a natural, but not excessive, hardening. Innumerable instances confirm the results given for these particular stocks. Another stock, with about 5 per cent of lime. and 20 per cent of sulfur in an equal mixture of shoddy and rubber, showed very good aging:
August, 1925
IhrD USTRIAL A N D ENGINEERING CHEXISTRY
Before oxygen aging Oxygen aging 16 hours at 60‘ C.
Stretch Per cent 210 190
Permanent elongation Tensile Per cent Lbs./sa. in. 8 418 8
Presence of I m p u r i t i e s As is well known, certain ingredients have a harmful effect, such as potassium permanganate which was tested by Bierer and Davis. Prussian blue is also known t o have bad aging properties. It is therefore interesting to see how easily its action is shown by the oxygen aging test, since a great many of the so-called chrome greens on the market contain more or less of this material. Test batches were made up using the following stock: 100 Ceylon 10 Zinc oxide 50 Wbiting
3 Organic accelerator 3 Sulfur
t o which were added 1 per cent and 0.5 per cent of Prussian blue. ‘The resulto were as follows: A Stock
4
Permanent Stretch elongation Tensile Per cent Per cent Lbs./sq. in. Before oxygen aging Base stock 650 28 3175 +0.5% Prussian blue 660 24 3120 1 % Prussian blue 630 23 2915 Oxygen aging at 60’ C. f o r 16 hours Base stock 20 2675 30 570 1638 $0 5 % Prussian blue + l % Prussian blue 450 26 604 Oxygen aging at 75’ C. for 16 hours Base stock 590 22 2662 +0.5% Prussian blue Melted $ 1 % Prussian blue Melted
+
This illustrates a specific case of poor aging and likewise shows that increase in temperature greatly aids in the deterioration. Correct C u r e The greatest value of the oxygen aging test seems to be that it affords an exceedingly valuable means of determining the proper cure of stocks. The following data show no perceptible difference in tensile and physical characteristics at various cures before aging, but very noticeable differences after the aging test, thus making it possible to pick out the correct cure:
B STOCK-This stock contains about 36 per cent rubber and is high in litharge. A range of cures from 20 to 35 minutes a t 45 pounds steam pressure gave before aging very little difference in tensile, but although after oxygen aging the 20-minute cure was hardly affected, the 35-minute cure dropped nearly 800 pounds in strength, a difference of even 5 minutes in the cure having resulted in an appreciable drop of nearly 400 pounds. B Stock Stretch Per cent 490 470 4 70 480 470 460 440 450
Permanent elongation Tensile Per cent Lbs./sq. in. Before oxygen aging 25 1860 25 1860 25 1783 27 1920 Oxygen aging 16 hours at 60’ C . 24 1805 23 1473 23 1486 22 1152
cure dropped only about 100 pounds, whereas the 35-minute cure dropped 800 pounds. C Stock
468
This shows that large amounts of lime as accelerator are not necessarily harmful if the cures are properly adjusted. Actual life aging tests on the last stock confirm this very satisfactorily. This method of testing has been tried up to the present time on a hundred or more different stocks. In most cases poor aging stocks are due not to improper ingredients, but to the wrong cure.
Minutes of cure at 45 lbs. steam pressure
20 25 30 35 20 25 30 35
C SmcK-This is a white stock containing about 30 per cent rubber with a n excess of sulfur and magnesium oxide as curative. Cures from 15 to 35 minutes a t 50 pounds steam pressure gave little difference in tensile. On oxygen aging, the ?&minute
867
Stretch Per cent
480 540 520 520 550 470 480 480 360 200
Permanent elongation Tensile Per cent Lbs./sq. in. Bejore oxygen aging 31 1085 37 1100 40 1420 42 1240 48 1335 Oxygen aging 16 hours at 60‘ C. 35 1055 40 985 41 1321 30 723 ‘ 10 508
Minutes of cure at 50 lbs. steam pressure 15 20 25 30 35 15 20 25 30 35
These are but two out of a great many examples of aging in oxygen to determine cures which have given satisfactory results. Naturally, some stocks are much more sensitive to changes in cure than others, high-grade stocks resisting comparatively large changes, but it provides a very quick means of determining whether the stocks have a wide range of cure and of modifying them to obtain such results. Shows w h e n Stocks Are Overworked Oxygen aging also makes it possible t o determine the ability of a stock to resist excessive working or to show when the stock has been overworked. Most rubber manufacturers have certain products in which the percentage of returned raw scrap is high, and any method of rapidly determining when the stock has been overworked is of value. To carry on these experiments three stocks were selected. A large batch of each was mixed on the mill and after mixing in the proper manner was milled for 2.5 hours, a sample being taken off the mill every half hour. The proper cure was then determined by means of a series of experiments on the properly milled stock, and finally a sample of stock from each milling was given the proper cure. The tests before milling showed no very alarming differences in quality of the stock, but after oxygen aging deterioration was very noticeable in some of these stocks. This undoubtedly explains a great many stock troubles which arise from time to time and gives a means of checking the difficulty. The value of this method of testing is illustrated by the following tests:
1-D stock is a red stock containing about 30 per cent of rubber, crimson antimony for coloring, low sulfur, and an organic accelerator, the main filler being whiting. In the process of manufacture this stock is subjected t o considerable manipulation and the percentage of raw waste is very high. Tests before and after aging the variously milled batches were as follows, the cure being 25 minutes a t 40 pounds steam pressure: D Stock Permanent Stretch elongation Tensile Per cent Per cent Lbs./sq. in. Before oxygen aging Before working 380 22 907 0.5 400 27 863 1 410 30 875 1.5 420 32 850 2 400 33 902 2.5 420 36 842 Oxygen aging at 60’ C. for 16 hours Before working 350 18 806 0.5 420 27 815 1 390 27 771 1.5 400 27 726 2 400 31 778 2.5 410 34 744 Hours worked
This stock stands up fairly well with excessive working. %The next stock is a red “nonblooming” stock, E, containing iron oxide for coloring and likewise a low percentage of sulfur and a n organic accelerator. This stock also has a rubber content around 30 per cent and the main filler is whiting. The proper cure having been previously determined, all samples were cured 25 minutes a t 40 pounds steam pressure.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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E Stock Permanent Stretch elongation Tensile Lbs./sq. in. Per cent Per cent Before oxygen aging Before working 370 16 1266 0.5 360 21 1147 340 20 1049 1 390 24 1072 1.5 2 330 23 952 370 27 925 2.5 Oxygen aging a1 60’ c. for 16 hours 350 15 1103 Before working 360 20 981 0.5 370 22 953 1 . ~ . 370 i.5 23 889 340 23 823 2 25 703 360 2.5 Hours worked
~~
The data indicate a marked deterioration as the working is increased. 3-The last stock is the C stock which was previously referred t o as showing decided changes in quality with the cure. Samples were prepared in the same manner and the following tests were obtained before and after oxygen aging, the cure being 25 minutes at 50 pounds steam pressure: C Stock Permanent elongation Tensile Stretch Per cent Per cent Lbs./sq. in. Before oxygen aging Before working 540 39 1479 0.5 540 43 1404 1 540 43 1334 1.5 530 47 1315 2 530 48 1265 540 50 1176 2.5 Oxygen aging a t 60’ C . for 16 hours 520 Befor’e working 0.5 530 1 380 1.5 240 2 60 2 329 2.5 70 0 335 Hours worked
This stock in particular shows the very decided effect which may be obtained with oxygen aging and which does not show up in the original cure. It will be noted that the tensile has only dropped from 1479 pounds on the original milled stock to 1176 pounds on the stock which was milled an extra 2.5 hours, but in the oxygen aging test it has dropped from 1308 pounds t o 335 pounds.
Conclusion It can readily be seen from all these data that the oxygen aging test affords an excellent method of determining: (1) the proper compounding materials; (2) the presence of impurities in compounding materials; (3) the correct cure; and (4)whether the stock has been overworked. Although it may be possible t o determine some of these factors successfully on high-grade stocks by means of the heat aging test, as far as the writers’ experience goes and in accordance with the general opinion, i t is not possible to rely on the heat test for low-grade stocks. This is now perfectly possible with the oxygen aging test. Moreover, the heat aging test, which takes two weeks t o complete, is eliminated in many cases on account of the time required. The oxygen aging test provides a ready means of determining every night whether the work is progressing satisfactorily.
Application to Tire and Tube Stocks By H. B. Pushee THE GENERALTIRE & RUBBERCo., AKRON,OHIO
’
H E oxygen bomb aging test has been applied in the writer’s laboratory t o a number of tire and tube stocks that have been under observation for several years, so t h a t their age-resisting qualities are well known. The results of the bomb test check the results of natural aging very satisfactorily. Furthermore, the effect of variation of the state of cure on the ageresisting properties of these stocks has been correctly indicated by the results of the bomb test.
Vol. 17, No. 8
Value in Testing Rubber Tubing and Rubberized Fabrics By Hugh L. Thomeon AEOLIANCo., MERIDEN, CONN.
H E following remarks about the Bierer-Davis aging test are solely from the standpoint of a user of rubber products. For about twenty-five years the Aeolian Company has been a large-scale consumer of rubber tubing and rubberized fabrics. The properties of the rubber products necessary in the manufacture of player pianos make the requirements difficult t o meet, and these requirements are further complicated by the numerous sizes, shapes, and weights of material necessary for each instrument. It has been possible, however, t o obtain rubber products entirely suitable except in one respect-the assurance of satisfactory aging. Experience has shown that certain rubber compounds and types of material fail consistently after a comparatively short time, and furthermore there has been no certainty that other and supposedly better materials would stand up for a longer period. When it is considered that the ability of a few pounds of rubber to age well is vital t o the successful performance of a n instrument which costs several thousand dollars, the desirability of advance assurance for every lot of material is self-evident. Many methods have been tried in the effort t o foresee in a reliable manner the aging of tubing and rubberized fabrics, varying all the way from simple exposure to light and air-in which case the deterioration has been accelerated by subjecting the rubber t o stress-to the forced air oven. This latter method, though doubtless valuable for some compounds, d o 9 not seem applicable to the class of rubber materials used by this company. On the other hand, results with the Bierer-Davis aging test have been most encouraging. The company has accurate data on the “expectation of life” of certain grades of rubber compounds, and the oxygen aging test apparently checks these results in all cases. It is surely of immense value in comparing one material with another. At the present time 12 hours’ exposure in the bomb under 300 pounds per square inch pressure of oxygen a t 60” C. is considered t o be equivalent to one year in darkness under the conditions prevailing in the piano. I n the tests by this company compounded rubber materials always last longer than so-called “pure-gum” products. Furthermore, the equivalent time of 12 hours in the bomb and one year of aging under ordinary exposure is not correct when the material is exposed to light of any intensity. For this reason experiments were started with a quartz bomb in the hope of checking more accurately the effect of light in addition t o t h a t of oxygen. The danger of ignition of rubber samples in the oxygen bomb has been emphasized and illustrated by Bierer and Davis in their original paper. A similar but much less serious explosion has recently occurred in this company’s laboratory. Heating is carried on in a conditioning oven having an intermittent heater of 300 watts and two continuous heaters of 400 and 500 watts, respectively. The 300-watt intermittent heater alone is used for the oxygen bomb aging test. On a recent occasion the 500watt heater was inadvertently turned on, resulting in a rise of temperature sufficient t o ignite the rubber samples in the bomb. These samples had a total weight of about 48 grams. The standard safety valve, with which the Emerson bomb is now equipped, gave way, relieving the pressure without damage to the bomb itself. The flame was of sufficient intensity, however, t o burn away a large portion of the brass safety device. The pressure inside the oven forced open the door but did not break the mica
