Mechanism of Rubber Aging BERKARD L. JOHNSON AND F R ~ NK. K(:AMEROK, University of North Carolina, Chapel Hill, N. C. tion necessary for a pattern N O K G the present The position of the diffraction lines obtained must be increased, may be intertheories of the aging of on x-ray photographs of rubber does not change preted as meaning that there rubber are two which upon aging the rubber in the oven at 70" C. for had been a depolymerization are closely related to the funda10 days. However, the intensity of the lines deinto the alpha form and the mental structural characteristics creases as thc aging time is increased. The ingreater elongation was required of rubber. This investigation to produce the crystalline units was made to determine to what creased elongation needed to produce jibering in or fibering because the concendegree those structural changes aged rubber is considered necessary to expel the tration of the lower polymers might be responsible for t h e increased concentrat ion of lower polymers, introto be expelled had been inaging of rubber. duced by depolymerization during aging, f r o m the creased by that depolymerizaOne theory is that the drtecrystalline aggregates or jibem allowing the latter tion during aging. Although rioration of rubber is due to its the above explanation is plausunsaturation of carbon, oxidato diffract the x-rays. Oxidation might also ible because xve know that detion taking place a t the double produce the same effect by the formation of obpolymerization takes place durbond. K e b e r ( 5 ) has given stacles to the stretching out of tangled molecules ing oven-aging, i t is not the two facts in support of this or micelles. The rate of depolymerization upon only one which might be adtheory: (1) If vulcanized rubaging vulcanized rubber in the oven at 70" C. is vanced. For instance, if fibering ber is exposed to sunlight in is considered as a stretching out atmospheres of hydrogen, carstudied, and a curve of fhe elongation at which of tangled molecules or micelles bon dioxide, air, oxygen, and in the crystalline modification Jirst appears is ( I ) , then oxidation alone might vacuum, respectively, only the plotted against the time of aging. The same produce obstacles to the prosamples in air and oxygen degeneral curve is obtained for rubber both with duction of fibers. teriorate in a period of several and without phenyl-p-naphthylamine, indicating months. (2) It is known t h a t oxygen carriers, such as copper that a n antioxidant such as phenyl-p-naphthylTYPEOF RUBBER and manganese, have a severe amine does not affect the rate of depolymerizaA specially compounded rubdeteriorating action on rubber. tion of rubber or the elongation necessary to prober was prepared in order that Two inconsistencies in the oxiduce Jibering in aged rubber. i t be as transparent a s possible dation theory listed by the same and not contain fillers which writer are that vulcanized rubber, having a lower degree of unsaturation, is more susceptible would produce diffraction lines on the film which might be to deterioration than crude rubber of a higher degree of unsatu- confused mith those arising from the rubber. Two batches ration, and t h a t over- or undervulcanized rubber deteriorates of stock were mixed which had the following composition b y with increased activity in comparison to properly cured rubber. weight, except that the antioxidant was left out of one batch: A second theory (to be discussed in this paper) i3 that the 1000 Pale crepe 20 Sulfur unstable balance of polymers in the rubber might cause its Zinc oxide 10 deterioration. Evidence of the depolymerization of rubber Stearic acid 25 Accelerator (tetramethylthiuram monosulfide) 2.5 on artificial aging is given in the work of Bary and Fleurent Antioxidant (phenyl-@-naphthylamine) 7.5 ( 2 ) who found that the more polymerized part, &rubber, decreased on oven-aging. The increase in the benzene-soluble This stock was cured into slabs 2 mm. thick. The aging a-rubber with increase of aging temperature is shown in properties of these two stocks were determined by tensile strength tests after aging in the oven a t 70" C. These results the following table given by 13ary and Fleurent: are as follows:
A
AGINGCONDITIONS HOWS c. 75 52 75 63
SOLUBILITY I N BENZLNE
AGINGPERIOD WITH ANTIOXIDANT WITAOCTANTIOXIDANT Days Pounds p e r square znch Unaged 3660 3470 4 3780 3580 7 3680 3130
% 30.1 49 3
The x-ray behavior of rubber may also indicate this depolymerization. Hauser and Mark (4) have shown t h a t with crude rubber the x-ray interferences appear only a t a n elongation of 80 to 100 per cent. They also proved that these interferences do not change position from the moment of their appearance to a n elongation of 1000 per cent, but that their intensity increases proportionally t o t h e elongation. Bary and Hauser (3)have explained the formation of x-ray patterns upon the elongation of rubber on the basis t h a t in the unstretched rubber, crystalline aggregates or fibers exist but are swollen b y the lower polymers so that i t is impossible for them to give a diffraction pattern. When rubber is rapidly stretched, the lower polymers (a-rubber) are expelled, leaying the regularly oriented crystalline aggregates (&rubber) which produce t h e x-ray pattern. From this view the fact (as shown later) that, as the time of oven-aging is increased, the elonga-
Since these data show a distinct difference in behavior of the two stocks after oven-aging, i t was decided to obtain x-ray photographs of the two types of rubber in an attempt to determine whether depolymerization or some change in the rubber molecule or micelle was responsible for the more rapid rate of deterioration of the stock without antioxidant.
X-Ray PROCEDURE The diffraction patterns were obtained on a General Electric diffraction apparatus having a Coolidge tube with a molybdenum target operating a t 30,000 volts potential. The regular set-up for examining a powdered crystal was used, the rubber sample replacing the tube containing the powdered crystal. The radiation from the x-ray tube passed through a slit having a n opening of 0.18 X 12.7 mm. (0.07 X 0.50
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