.August, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
869
Use in Control of Rubber Stocks
windows in the door. As a result, the total loss from the explosion consisted of the rubber samples, the safety valve, and t h e thermometer in the oven.
By W. E. Glancy HOODRUBBERCo., WATERTOWN. MASS.
Aging of Rubber Goods in Oxygen at Atmospheric Pressure By A. A. Somerville R. T. VANDERBIL’I CO., N E W
YORg,
N. Y.
N L’IEW of the growing interest in aging tests using oxygen and the importance which tests of this character are rapidly assuming, it would seem an opportune moment to describe a somewhat different oxygen aging test now in use a t the Pirelli Laboratories. It was the writer’s privilege recently t o visit the main plant and laboratories of the Societd Italiana Pirelli, at Milano, Italy, and to examine in detail their methods of testing rubber. The test of most interest in connection with this symposium is that developed by Marzetti. This has already been described in some detail in two journals,’ and has since that time been further developed and utilized. Instead of using oxygen under pressure in a bomb, according t o the method of Bierer and Davis, which is now being used by so many companies, Marzetti has obtained similar results by using pure oxygen at atmospheric pressure. This necessitates a longer time to obtain the same deterioration, and for this reason a little higher temperature is used than that commonly used in the Bierer-Davis test, where the oxygen is under high pressure. The work of Marzetti and Bruni with this method has led to certain general and very interesting conclusions. The most important of these are that aging is essentially an ordinary oxidation due to a reaction of the rubber with atmospheric oxygen, and that rubber compounds deteriorate a t a rate proportional to the rate at which they absorb oxygen. To reach an advanced physical state of deterioration only about 1 per cent of oxygen based on the rubber is absorbed and the varying resistance to oxidation of different rubber compounds is manifest, not in the amount of oxygen absorbed, but in %e rate a t which it is absorbed. Bruni and Marzetti have found also that the degree of cure has a great influence on this rate of absorption of oxygen and that rubber compounds which deteriorate rapidly in natural aging because of overcure absorb oxygen rapidly in their test and deteriorate correspondingly rapidly. This is of great practical utility to them, for it has given them a means of foreseeing the aging of their compounds and of adjusting their cures. Besides the extent to which the stock is cured, the quality of the crude rubber influences the aging. Thus, a slow, poorly curing rubber takes up oxygen faster than a first-grade plantation rubber and deteriorates correspondingly faster. When organic accelerators are used to obtain good cures in shorter times, the rate of absorption of oxygen is greatly decreased and the aging is much improved. In this connection it is the opinion of Bruni that the proper use of organic accelerators improves the aging qualities of rubber compounds so enormously that such accessory agents as antioxidants are usually unnecessary. Furthermore, antioxidants are for the most part retarders of vulcanization and accordingly tend to defeat their own purpose in that the rubber must be cured longer. Their work in general confirms in a very striking manner the fact that aging is simply oxidation by atmospheric oxygen and that a n artificial aging test involves nothing more than some convenient means of increasing the rate of absorption of this oxygen. 1
Giorn. chim. i n d . applicato, 5, 122 (1923); Rubber Age, 13, 433 (1923).
HIS laboratory has been using an oxygen aging bomb for the past five months. During this period data have been secured which lead to the belief that this new method of artificial aging of rubber goods is a valuable addition to existing methods of test for the control of compounding and degree of vulcanization of rubber stocks. The advantage of obtaining results quickly is especially desirable for compounding development work. It should be possible to speed the development of new compounds considerably because of the new test. Not all the results obtained in using the bomb are consistent with known facts of natural aging and with Geer oven tests which have previously been checked with life aging. However, generally speaking, stocks of high quality, such as are used for tire treads, inner tubes, reducing corsets, etc., which give good aging qualities as shown in the Geer oven test, will also show good aging qualities in the oxygen bomb. Inner tube stocks which would not age properly in the Geer test have also been prepared and these stocks also aged poorly in the oxygen bomb, as is shown by the following examples: TRKSILE STRENGTHS, POUNDS
TYPEOF STOCK
Original 3350 2200 1850
Carbon black tread Good aging tube Poor aging tube
After Geer test 3350 2200 950
INCH After oxygen test
P E R SlUUARE
3150
2100 850
The stocks in the Geer test were held 6 days a t 71’ C. and in the oxygen test 16 hours a t 60” C. with a pressure of 300 pounds per square inch. A comparison of a large number of stocks aged artificially by these two methods showed an average drop in tensile strength by the Geer test of approximately 10 per cent and by the oxygen test of approximately 20 per cent. Mr. Morronl has pointed out that the oxygen test can be used to determine the correct cure very readily and with greater accuracy than has hitherto been possible. Experience in this laboratory confirms this fact, but it is believed that the results obtained tend to exaggerate the conditions found in natural aging. For instance, an inner tube stock-which, however, has been cured in a flat sheet mold-shows a very large reduction in tensile strength by the oxygen method after it has been overvulcanized. TEXSII.E STRENGTHS, POIINDS PER SQUARE INCH Cure Minutes
30
60 90 120
OriFinal 3100 3280 3100 3030
6-day Geer test 2250 2900 2550 1860
Oxygen test 2450 2400 400 350
Another inconsistency has been found in the changes in direction of the stress-strain curves of various stocks. Very often stocks which have been properly vulcanized tighten with age, as is shown by a movement of the stress-strain curve to the left. In numerous cases it has been found that the stress-strain curve of a stock aged naturally moves to the left somewhat farther than the stress-strain curve of the same stock aged for an equivalent time in the Geer oven. After aging in oxygen, under the conditions noted above, the stocks give stress-strain curves which are in some cases identical with the original stress-strain curves and in practically all cases the oxygen aging seems to cause less tightening than does the heat treatment. The present feeling in this laboratory is that some stocks age more rapidly because of their easier susceptibility t o oxidation; other stocks are not so easily oxidized but tend t o get “drier” and harder for other reasons. It is believed that !he oxygen aging test is going to be most valuable when used for stocks of 1
Page 866, thisissue.
