Nov., 1921
THE JOURN.4L OF INDUSTRIAL AND ENGINEERING CHEMISTRY
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Volume Increase of Compounded Rubber under Strain’ COXWENT8 ON AN ABTlClE BY E. F. SCEIPPEL’
By Henry Green RSSBARCAL ~ ~ o n m o a NBW u, J a n s ~ vZ r ~ cCo.. PALXEPITON, PA.
This discussion is not intended as a criticism on Mr. Schippel’s work. It was written expressly for t,he purpose of offering a suggestion, to those interested in the subject of “volume increase.” as to a method of studying this phenomenon not specifically mentioned in ilk.Scbippel‘s paper. The followinc excernt will be sufficient to uresent t.lie idea involved : I
While studying the nature of the stress-strain curves for rubber
t h i t poss;bly when th; rubber body was elongated sufficie& the rubber might pull away from the particles of pigment in their axes of stress and cause vacua to be formed on both sides of each particle. the net result of which would be a considerable increase in the volume of the rubber body as a whole A preliminary test was made by preparing a transparent vulcanized compound containing a fair proportion of medium-sired lead shot. When this compound was stretched, the formation
reference to a microscopic examination, since otherwise t.hese minute vacua could not have been shown t,o ex& AB no such reference appeared in his paper, the author believed that it would he both interesting and novel to eliminate the shot, introducing in its place a material like ground barytes, a.nd to observe microscopically the conditions arising during subsequent stretching of the rubher compound. EXPERIMENTAL DETAILS Rubber sheets approximately 0.01 in. in thickness containing the barytes which it TVSS desired to study were prepared. These sheets were cut into strips 3.5 in. x 0.25 in. in dimension. One end of a strip was fastened t u a microscope slide by means of sealing wax. hft,er the solidification of the wax the rubber was stretched to the deaired percentage elongation and the strip fastened at the other end of the slide. 4 suit,able mounting medium such as glycerol or cedar oil
I !The large partide in the center is the only one pxreaent which io in dose contact with the rubher; the others azc surrounded by B Elm of gas and will, therefore. produce ‘ ‘ Y ~ C U ~ : ’ The particle In the lower right hnod eorncr ~ p p ~ taonhc a bubble. There is actually a particle present, however, sc1 was revealed by closer focusing.)
integral phenomenon was actially found to take place
&om the last sentence it is somewhat difEcult to decide whether shot was introduced in the rubher compounds and the vacua observed in connection with that matwial, or whether the vacua appeared at the ends of the miscellaneous pigments which presumably were also present in these rubber compounds. It seemed to the author that, if this last condition was the one implied, Mr. Schippel would naturally have made some I Presentedbefaethe Divi.iaooiRubberChrmirtr~atthe8lEtMeetiog .,i the ~ ~ eChemical r i satiety. ~ ~ Rahesier, ~ N. Y.,April 2g, 1g21. * ~ a r JOURNAL, s ia (1820).33.
was then allowed to flow under the strip and some also placed on the upper side. After placing a cover glass upon this, the specimen was ready for examination. The result is shown in Fig. 1. The barytes particles are clearly shown with conical vacua at the ends, extending in the direction of elongation. The clear white field is the pure rubber containing a large number of small barytes particles which evidently have produced no vacua. The experiment was modified as follows: The rubber strip mas fastened to the glass slide at one end only, and the specimen was placed on the microscope (which was equipped with a mechanical stage), and slowly stretched. As the elongation
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THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY
Val. 13, No. 11
RUBSSESxowlr m Pxo.3. r~ A STBBTUIBD COIDITIMI. NOTICS T.HAI TPB A ~ G P B D ~ T Aae B S Sop.? A N D E ~ s a u
Fro. 4-Snars
Pro. %UNSTR&ICBBDR ~ a a mC o l i r r i ~ r ~“AWLO~(BBIIBD o Mnssso” OF PICUBNI. Encs BLIW SPOII9 Coar~osHo01 PB~xd*s TBO~SANDS 011 P A R ~ C L S S
D B P O ~ Y BTO D LONOS T B B A ~
increased the stage was moved in the opposite direction so fact that they are in turn composed of perhaps many thouthat any individual particle of barytes could be made to re- sands of individual particles. A pellet then is an agglomerated main in the field during observation. I n this way the actual mass. If such a pigment is not thoroughly irtcorporated in formation of the vacua could be studied. It was noticed a compounded rubber these masses will exist intact. (Fig. 3.) that certain particles invariably formed vacua, while with The immediate problem now is to determine whether or not others it was almost impossible to produce them. The reason there is any change in the volume occupied by these pellets for this fact is probably explained in Fig. 2. Of the eleven whcn the rubber is stretched. (Fig. 4.) It is apparently an accepted fact that there is no measurable largest particles present tho white one in the center is the only one in close contact with the rubber. (As in the pre- change in the volume of pure unpigmented rubber upon exceding figure the clear field is the rubber in which the barytes tension. Granting this to be so, it then follows that if a solid is imbedded.) The other particles are surrounded by a rubber cylinder (Fig. 5) is stretched to 1z times its length, its circular cross-section will be reduced t,o l/n tirnesitsori&ml gaseous film, which on account of its low index of refraction causes the particles to appear dark. It is hardly necessary area. In other words, the radius has been decreased from R to state that the so-called “vacua” will be formed in this R to r where r = 4; ’ latter case, white with the large particle in the center t.he Let us now suppose that in the center of this cylinder, result cannot safely be predicted. Finer pigments than barytes have also been investigated tangent to its walls, there should be a vacuous spherical by this method. For materials as fine as zinc oxide, carbon cavity and that the cylinder itself is placed in a vacuum. black, etc., a 2-mm. oil immersion objective and a magnifi- The volume of such a cavity will be cation preferably of 15M) diameters are required. The cover V = 4/31 a X a X a glass is dispensed with and the objective immerscd directly in the mounting fluid. So far, the results of experiment have where a is its radius. Let the cylinder now be stretched to n times its length; then the volume of the deformed cavity indicated that no vacua are formed with fine pigments when compounded in small percentages. This naturally leads to the point discussed hy Mr. Schippel in regard to the effect ( T LA,::.; T @ produced by agglomerated mawes on volume increase. +’ L , 4 , Where the percentage of pigment is high, aggregation is more /‘ likely to be present, and it was suggested that if aggregated y =$re.a.‘a. A ,A masses tended to augment the volume on extension, it would rii be easily understood why highly pigmented rubber possessed this property to such a marked degree. > ;-:
I ?
THEORETICAL DISCUSSION A t first glance this conclusion seems so obvious that further discussion would be unnecessary. On closer analysis, however, it is not. quite so self-evident why agglomerated masses tend to cause volume increase when subjected to elongation. If any fine-grained powder he examined microscopically it will he found to be composed of small soft pellets. Microscopic examination of these pellets will further reveal the
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Nov., 1921
T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
or
FIG.&MODEL OF AGGLOMERATSD MASSESO F PIGMENT W H E N STRETCHED. ILLUSTRATING PROBABLE CONDITION WHICHPRODUCES “VOLUME INCREASE”
will be the volume of an ellipsoid of revolution about the axis of elongation and will be equal to a a Ve = 4/3 B (na) - & 4;i or Vs = Ve That is, there is no change in the volume of the cavity when the rubber is stretched. If this cavity were filled with a gas a t a given pressure, and the rubber stretched adiabatically, it necessarily follows that there can be no change in volume as a result thereof, though of course we are now dealing with a larger volume (due to gas pressure) than was the case when the cavity was empty. If, instead of a gas-filled cavity, we should have one completely occupied by a pellet of pigment (an agglomerated mass), it is not so evident just why there should be volume increase upon stretching. In the empty cavity, as already stated, there can be no increase in volume. Any form of material placed in the cavity that would cause such a condition to take place upon stretching, would have exerted upon it, by the walls of the cavity: a compressive force acting in the opposite direction.
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As a gas is a fluid it must obey the viscous law, that is, aa infinitely small force will cause it to flow. If then, at any time during stretching, the gas should exert an extensive action it will be met by an equal and opposite compressive force which will be completely used in molding the gas into its new form. On the other hand, a pellet of powder, being similar to a plastic material, will require the application of a finite force of definite magnitude, before it undergoes a change in shape. As such is the case, all of the compressive force is not utilized, and there will exist a residual quantity of it equal to the previously mentioned finite force necessary to caiise the aggregate to flow. The presence, therefore, of this small quantity of force implies that there has been a volume increase. Fig. 6 is an attempt to visualize this condition. The white sections in this figure represent air spaces; the shaded ones, a mixture of pigment and occluded air. It should be understood that the figure is only a theoretical model never attained in practice. The air spaces would be extremely minute and overlap in such a manner that to detect them with a microscope would be impossible. CONCLUSION Theoretically, a t least, Mr. Schippel’s contention that agglomerated masses of pigment are responsible for a part of the volume increase, is justifiable. Whether or not this evidently small volume increase is sufficient to account ‘for the quantity found experimentally, will be difficult to decide. No doubt there are other forms of aggregation, such as very small ones like doublets, which upon extension will produce sufficient volume increase to explain fully Mr. Schippel’s interesting discovery.
Discussion of the Tetrabromide Method for Estimating Rubber Hydrocarbon1 By Harry L. Fisher, Harold Gray and Ruth Merling F.GOODRICH Co., AKRON,OHIO
RESEARCH LABORATORIES, THEB.
The tetrabromide method is the oldest method for estimating rubber hydrocarbon. As originally described2 it consisted of the preparation of the rubber tetratromide in solution, precipitation with alcohol, washing and drying, and determination of the bromine in the product. Several modifications followed, mostly in the methods of preparation and of determination of the bromine in the precipitated sample. The latest modification of the method has been made by W. K. Lewis and W. H. McAdams13 who have applied the method of McIlhiney4 for the esfimation of unsaturated oils by means of bromine in an organic solvent. Briefly, the rubber is dissolved in carbon tetrachloride and treated with a solution of bromine in the same solvent. After standing from 2 to 4 hrs. the excess of bromine is titrated with sodium thiosulfate after the addition of a solution of potassium iodide, using starch as the indicator. Since substitution always takes place, as shown by the presence of hydrogen bromide, the amount of bromine used, as determined by titration, does not give the correct figure for the amount actually added to the double bonds. This is remedied by immediately adding some potassium iodate solution which converts the hydrogen bromide into bromine. This bromine, as in the previous case, reacts with the potassium iodide to give iodine, which is titrated with additional sodium thiosulfate. By subtracting twice this
*
The greater part of this paper was given in the Symposium on Analytical Chemistry hefore the Rubber Division at the 60th Meeting of the American Chemical Society, Chicago, I l l , September 6 to 10, 1920. The work on vulcanized samples has been completed since that time. * Th. Budde, “The Valuation of Cold Vulcanized Rubber Ware by the Tetrabromide Method,” Apotb. Z t g . , 24,529;C.A . , 3 (I909), 3013. 8 TEISJOURNAL, 12 (1920), 673. 4 1. Am. Chcm. Soc., 21 (1599), 1084.
latter amount from the total bromine determined by the first titration, the true value of bromine addition is obtained. There is no doubt but that this titration method is theoretically correct, so far as the chemistry of rubber is understood, The present authors have tried out this method in a sympathetic manner, with the full expectation that excellent results would be obtained. They have been somewhat disappointed in its workings although they believe that it will eventually be of good service.
DISCUSSION OF ERRORS Substitution apparently always takes place. Hinrichsen’ states that when the temperature is around 0’ C., only the“” tetrabromide is formed, even though an excess of bromine is present. Our experience does not corroborate this. I n a number of preparations of the tetrabromide from ordinary first latex pale crepe, as well as from specially purified rubber, we have always noticed that hydrogen bromide begins coming off after the first addition of the bromine solution. I n these preparations the mixtures were well stirred and cooled approximately to 0” C. The amount of moisture in the solution or in the room m7as not enough to account for all the hydrogen bromide which came out of the solution. All the preparations, however, were made in diffused daylight, and the light, no doubt, had considerable effect on the reaction. One theoretical possibility should be discussed here. Some of the hydrogen bromide formed in the substitution may add to a double bond. Such a reaction would prevent the regular addition of bromine. Since the amount of substitution is measured from the amount of hydrogen bromide 1
Z.anorg. Chem., 81 (1913), 70; C.A., 7 (19131,2861.
