Crystallization of J
Vulcanized Rubber NORMAN BEKKEDAHL AND LAWRENCE A. WOOD
Crystallization has not hitherto been recognized as a factor of practical importance in commercial vulcanized rubber products. In cold climates some rubber products have been observed to undergo slowly a great increase in rigidity and permanent set. The present investigation, reporting the crystallization of unstretched specimens of vulcanized rubber of low sulfur content, offers a basis for the explanation of such phenomena. The work has been concerned with a fundamental study of some of the factors affecting crystallization rather than with the development of any practical recommendations. A few of these factors have been varied systematically, but the study is still largely exploratory in nature.
National Bureau of Standards, Washington, D. C .
Preparation of Specimens Two types of compounds were selected for study, the vulcanizing agent consisting of sulfur in the one type and of tetramethylthiuram disulfide (Tuads) in the other. The rubber-sulfur compounds contained no other ingredients; they are not "practical" compounds but were chosen for study because of their simplicity. They were prepared as follows:
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Smoked sheet = 100 X parts by weight Sulfur = X Total = I n n where X = 0, 0.66-0.10, 0.12,'0.15, 0.20, 0.30, 0.35, 0.40, 0.43, 0.46, or 0.50 Vulcanization = 10 hours a t 150' C. Several of the Tuads compounds represented a type of lowsulfur vulcanizates of considerable practical application. The general formula for the compounds which contained Tuads was as follows:
-HE
formation of crvstals at room temperature stretching rubber, vulcanized or unvulcanizid, has been the subject of considerable study. The crystallization of unstretched rubber at low temperatures is also well known, but with a single exception (18) to be discussed later, the effect has commonly been considered to be limited to the unvulcanized material. In the present investigation, however, the crystallization of unstretched specimens of vulcanized rubber of low sulfur content has been accomplished. In commercial vulcanized rubber products, crystallization has not hitherto been recognized as a factor of practical importance. It is probably significant in cold climates where some rubber products slowly undergo a great increase in rigidity and permanent set. Automobile traffic counters, for example, have been rendered inoperative by the hardening of the rubber tubing used with them. Laboratory tubing and other products made of a number of different commercial rubber compounds have become rigid after storage for some weeks in a refrigerator a t about 0' C. Previous work on unvulcanized rubber (3) showed that it can be crystallized a t temperatures between +lo' and -40" C., the crystals melting in a range from about 6' to 16" C. Crystallization and fusion are accompanied by changes in volume (9), heat capacity (4), light absorption ( I @ , birefringence (14, 16, 16), x-ray diffraction (Q), and mechanical properties such as hardness (1.2). X-ray diffraction and birefringence, of course, give the most direct evidence of crystalline structure, but in the present work change of volume, as measured in a mercury-filled dilatometer, was chosen as the criterion of crystallization or fusion. Quantitative results are more easily obtained in this manner, and the experimental observations are simple. Furthermore, the method is well adapted to continuous observations over long periods of time, such as were found necessary in the present work.
1
Blended smoked sheet = 98 - X parts by weight Zinc oxide (Kndox) = 2.00 X Tuads = Total = 100 where X = 0,0.375,0.750, 1.125,1.500,2.500,3.000, or 4.000 Vulcanization = 30 minutes at 126" C. Vulcanization in all cases was performed in a j/s-inch (15.9-mm.) rod mold. The specimens, which were cylindrical and about 15 mm. in diameter, were cut t o the desired length of about 80-100 mm. and then weighed.
Construction and Calibration of Dilatometers The dilatometers used to measure the volume changes were made of Pyrex glass, and each consisted merely of a bulb with a capillary tube sealed to it. Changes in volume of a specimen were determined from the changes in the height of a confining liquid in the capillary tube. Mercury was selected as the confining liquid because it seems to have no effect on rubber, even when the two are in contact for periods of a year or more. Some absorption, swelling, and softening usually occur with other liquids. The capillary tubes, 2 to 3 mm. in inside diameter and about 45 cm. in length, were calibrated by observing the lengths of weighed amounts of mercury at three or more different points along each tube, Each capillary was then sealed to a glass tube about 15 mm. in inside diameter, the other end of the tube being left open. Into the glass tube were inserted the rubber specimen and a hollow glass bulb of about the same diameter as the specimen and about 40 or 50 mm. long (Figure 1). Finally the open end of the glass tube was sealed off t o form the bulb of the dilatometer. This construction made it possible to avoid any heating of the specimen during the sealing operation, without unduly increasing the net volume of the dilatometer bulb. I n most cases the volume of the specimen was between two and three times the volume of the 381
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mercurv in the bulb of the dilatometer. The effectofthe insertion of the h o ~ o wbulb is equivalent to a mere reduction of the net volume of the dilatometer bulb. Since the inner hollow bulb was made of the same type of glass as the rest of the dilatometer, neither its volume nor its expansivity entered directly into any of the calculations. The dilatometer was next evacuated for a day or two in order to remove gas. With the dilatometer again at atmospheric pressure, the mercury was added through the capillary, into which a small wire had been temporarily inserted to facilitate the mercury flow. The dilatometer was weighed before and after the addition of the mercury. Further evacuation was carried out, until the height of the mercury in the capillary did not change by more than a few millimeters as the pressure was again raised t o that of the atmomhere.
Frcum 1. DILATOMETER
The volumes of specimen and mercury at 25' c. were determined from their weights and densities. The densities of the rubber specimens at this temperature were measured bg the method of hydrostatic weighings. The volume of the dilatometer up t o the level of the mercury in the capillary at this temperature was obtained as the sum of the volumes of the mercury and the specimen, A t any other temperature, the volume up to the same point on the capillary was found from the known expansivity of Pyrex glass. The volume of the dilatometer up t o any other ooint on the cauillarv could then be calculated from the calibrahon of the cap1ilary.- The volume Of the specimen corresponding to each observation was obtained by subtracting from this volume the volume of the mercury at the temperature of the nhqoriratinn -I"--
Experimental Procedure After the suecimens had been ureuared and Dlaced in the dilatometers, {hey were put in a stipred water bath^at a precisely
the upper incomplete curve of Figure 3, are of the same sigmoid type; the volume decreases very slowly at first, then more and more rapidly until the change is about half complete, and finally less and less rapidly until the volume has reached an approximate equilibrium value. Since the volume is changing very sloTvly in the final stages, a determination from the graphs Of the time required for completion is somewhat uncertain. The time required for half the total volume change is much more precisely determinable since the curves are steepestin this region, This quantity has therefore been measured and is plotted in Figure 4, which shows the time required for 50 per cent of the total volume change as a function of the amount of combined sulfur in the compound. The assumption is made that all the )sulfur in the rubber-sulfur compounds became combined during the 10 hours of vulcanization. For the Tuads compounds i t is assumed that one atom of sulfur per molecule of the disulfide, or 13.3 per cent of its weight, is capable of combination with the rubber during the vulcanization (2, 7 ) . The further assumption that au the available sulfur became combined during the 30 minwas based on the work of utes Of vulcanization a t 126" Bruni (7) and Bedford and Sebrell (2'). If the amount of combined sulfur was actually less than this maximum, the curve labeled "Tuads" in Figure 4 would then be shifted further t o the left and would indicate a still greater difference between this curve and that labeled "sulfur". Figures 2 and 3 that the magnitude of the total velume change on crystallization is somewhat less, the higher the amount of combined sulfur. I n several instances (Figures 2 and 3) the experiments were discontinued before the changes were complete. Consequently in Figure 4 the data for the highest Dercentazes of Tuads are not included. The be\avi
