Dilatometer Studies of PigmentRubber Systems
H. C. JONES AND H. A. YIENGST The New Jersey Zinc Company, Palmerton, Penna.
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In dilatometer measurements a gum stock decreases in volume, and certain pigmented stocks increase in volume when stretched. Volume decrease is attributable to fibering or orientation of the rubber molecules, and the increase in volume of pigmented stocks, to the formation of vacuoles at the pigmentrubber interfaces. State of cure has a marked effect on dilatometer results. A n undercured gum stock exhibited a greater volume decrease. and
A
NUMBER of years ago Schip
pel (6) observed that, when certain pigmented rubber stocks were stretched, there was a reduction in specific gravity of the compound which he attributed to the separation of the rubber from the pigment, resulting in vacuole formation. Green (3) confirmed Schippel’s explanation with the aid of a microscope. He prep‘ared p h o t o m i c r o g r a p h s of stretched rubber films showing the formation of vacuoles a t coarse pigment particles and undispersed pigment acting as single large particles. These vacuoles were conical in shape and extended from one or both sides of the particle in the direction of e l o n g a t i o n . Depew and Easley (1) reported microscopic results relating particle surface and particle size and shape to the formation of vacuoles in stretched rubber. Several years ago Holt and McPherson ( 4 ) designed a dilatometer to measure the volume change of rubber compounds under tension. I n stretching gum rubber bands, they noted a decrease in volume which they ascribed to the fibering of the rubber and an increase in volume of pigmented rubber bands which they explained by the formation of vacuoles around the pigment particles. Furthermore these investigators pointed out that the decrease in volume of rubber on stretching is influenced by the same considerations a s the x-ray diffraction, since it is
the decrease occurred more readily than with .an optimum cure. In highly pigmented stocks the long cures showed a greater volume increase than the short cures. Zinc oxide was the pigment employed in this study, and particle size and pigment loading influenced the volume increase on stretching; the larger the particle size, the greater the volume increase. For a given zinc oxide, the volume change increased with increments of zinc oxide.
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CAIPILLhRY CRAWATLO TO .OQ M L .
observed only above a certain critical elongation and is greater the higher the elongation, the lower the temperature, and the longer the time the sample is kept stretched. Holt and McPherson dealt almost entirely with gum stocks except that they included measurements on a whiting and a carbon black stock. The object of this investigation was to extend the work of Holt and McPherson to a more complete study of pigmented stocks, with particular reference to those compounded with zinc oxide.
Apparatus
Fxctv~m1. DIAGRAM OF DILATOMETER 1354
The dilatometer was constructed following the design of Holt and McPherson, and a diagram is shown in Figure 1: It consists of a brass tube, about ”8 inch in diameter and 20-inches in length, connected by a side arm to a glass capillary tube. The brass tube is provided with stuffing boxes at each end through which a 1-mm. brass wire can be drawn back and forth. A molded ring-shaped sample, 0.4 cc. in volume, is stretched within the tube by means of a fixed hook sttached at one end of the tube and a movable hook fastened t o the brass wire. I n using the apparatus, the rubber rings are mounted unstretched over the two hooks with the entire system immersed in water. The filling of the tube and capillary is facilitated by a second side tube provided with a stopcock. When the rubber rings are stretched by drawing the brass wire through the tube, any change of the volume of the rubber is indicated by a change in the
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FIGERE 3. EFFECTO F TIMEOF CURE
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ON VOLVME
CRANQE
again recorded. These operations are continued up to a tots1 elongation of 500 per cent, the maximum applied in most instances.
Effect of Time of Cure
FIG-
2. MOR?TING OF DILA-
TOMETER IN
WATERTANK
heightbf the liquid in the capillary. Cuntrolexpmiments in which the brass wire was drawn through the tube without the sample attached showed no appwciable motion of the
tive to changes in temperature, and to o;ercame this objection it n a mounted ~ in a tank of water as shown in Figure 2. The capillary tubes BE immersed in water to within several inches of the tops of the tubes and the height of the capillary made visible by the glass front built in the tank. The volume of water in the tank is sufficient so that the maximum tem erature variation without a thermostatic eontrof is well within 1'C. (1.8" F.) over n period of 8 hours. Two dilatometers are mounted-on a frame which can be withdrawn from the tank by a pulley and counterweieht. The rubber hands are stretched by t h e brass wire, which, in turn, is attached to EI heavy cotton cord running over a pulley on the bottom of the tank and is wound on the windlastss mounted at the top of the panel. The ~hmplesused were molded rings which after vulcrtnization were carefully trimmed before insortion in the dilatometer. In making the actual measurements, the bands are ruhhed with the fingers under water to enawe complete wetting and inserted in the dilatometer, and the zero reading on the eapillary is made. Tho hands &re then stretched 50 per cent and held in this position 2 minutes, and the height of the water in the capillary is again recorded. The hand is then stretched another 50 per cent, and after a >minute i n t e r v a l t h e capillary column height is
From the data published to date, i t can he postulated that physical linkages of some kind are formed within the rubber during vulcanization, and that as cure progresses a greater number of these linkages or bridges are developed. These bonds tie the long-chain rubber molecules together in a more o r less rigid body. Therefore, when an undercured compound is stretched, the rubber molecules orient readily, whereas an overcure will require a greater amount of tension to orient or fiber the rubber. This mechanism is consistent with the results plotted in Figure 3 for a stock elongated in the dilatometer. The rubher compounds employed in these expenments had the following base composition and were cured a t 126" C. (258.8" F.):
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loading of zinc oxide (16.4 volumes) the volume of vacuoles formed is directly proportional to the particle size of the oxide. Volume of vacuoles formed is directly related to the pigment content of the stock for an oxide of 0.40 micron as illustrated in Figure 9. With the lower loading (2 volumes) the volume increase is of the same order of magnitude as the decrease in volume due to fibering, which results in a zero net volume change. The dilatometer measurements were made on bands cured to an equal state of cure. Pigments other than zinc oxide were examined in the dilatometer, and the results are shown in Figure 10. Although precise determination of the particle size of the various pigments and inerts has not been made, the indications from these curves are that the volume increase on stretching is largely influenced by the particle size rather than the surface characteristics of the material. Comparable cures were selected for the dilatometer tests. The data discussed up to this point have dealt with the initial elongation of the rubber bands from 0 to 500 per cent elongation in 50 per cent stages with 2-minute intervals between each stage. Volume change measurements were also determined on the rubber bands during the retraction cycle and then during the second extension cycle to 500 per cent and the second retraction cycle. Figure 11 shows the results on the 45-minute cure for the compound discussed in Figure 6. On the first retraction cycle the volume of voids or vacuoles around the pigment particle decreases slightly as the stress on the rubber band is released. Below a load of 5 pounds the change is more rapid. When this rubber band is elongated the second time, there is a slight increase in volume, but on continued stretching there is an inflection point in the curve and a decrease in volume is noted. At the end of the second retraction cycle, the original volume of the rubber stock is not quite restored, perhaps owing to a permanent set or deformation. The volume change-load relations for this cure are also charted in Figure 11. The proposed mechanism also serves to explain these data. During the first extension cycle the rubber is stiff, and vacuoles are formed around the pigment particles. A load of 27 pounds is required to elongate the test piece to 500 per
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1 100 200 300 400 500 E L O N G A T l O N P E R CE N T
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FIGURE 7. EFFECT OF 0.40 MICHOX PARTICLE SIZEZINC OXIDE (16.4 V o ~ u l r l ~ON s ) VOLUMECHANGE
A similar set of curves for the same loading of a zinc oxide of 0.40 micron is plotted in Figure 7. These data show a
somewhat greater volume increase on stretching than the finer particle size oxide (0.22 micron). I n the case of the larger oxide particles greater shearing stresses are developed around the pigment particles when the rubber is stretched, and vacuoles begin to form as soon as the stock is subjected to the slightest load. Fibering of the rubber takes place simultaneouqly but is not detectable in the dilatometer measurements. The effect of particle size of zinc oxide on the volume change on stretching of ruhher i i shown in Figure 8. For the same
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E L O N G A T 1 0 N PE R CE N T
FIGURE8. EFFECTO F PARTICLE SIZE ON VOLUJlE CH.4SGE .A CPON STRETCHIXG
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cycle. A parallel example is the data in Figure 6 showing the effect of cure. The first extension cycle is similar to the stiff overcures which are accompanied by vacuole formation; the second extension cycle corresponds to the soft undercured bands which exhibit volume decrease on stretching. I04
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OF REPEATED STRETCHING ON THE COMFIGURE11. EFFECT POUND CONTAINIKG 0 . 2 2 - M I C R o N ZINC OXIDE
ELONGATION- P E R C E N T
FIGURE9. EFFECTO F VOLUME LOADINGO F 0.40-MICRON ZINC OXIDEON STRETCHING
cent the first cycle and 8.5 pounds the second cycle; this indicates that a considerable breakdown or softening of the rubber occurred during the first cycle. The rubber is fibered a t the beginning of the first retraction cycle, and presumably while the rubber is in this condition the volume of vacuoles FIGURE 10. EFFECT OF 16.4 VOLUME LOADIKG OF VARIOUS PIGMENTS
Since all the preceding stocks were accelerated with mercaptobenzothiazole, several compounds accelerated with diphenylguanidine were examined in the dilatometer. This accelerator requires neither zinc oxide nor fatty acids for activation and offers a n opportunity of studying the effect of these compounding ingredients. A gum stock containing 4 per cent sulfur and 2 per cent accelerator showed essentially no change in volume when stretched from 0 to 400 per cent, but when 5 per cent of zinc oxide was added to this compound there was a definite decrease in the volume of the rubber bands during stretching. The data for the 60-minute cure a t 141.5' C. (286.7' F.) are plotted in Figure 12. The compound containing zinc oxide was considerably stiffer than the zincoxide-free stocks; 12.5 pounds were required to stretch the zinc oxide stock to 400 per cent, and only 2.5 pounds for the zinc-oxide-free stock through the same range. The explanation offered for the molecular orientation in the previous cases does
ON
VOLUME INCREASE AT 500 PER CENT ELONGATION 1. 2. 3.
4. 5. 6.
7. 8. 9.
10. g
9
Precipitated whiting Zinc sulfide Lithopone Titanium dioxide Blanc fixe Clay Soft carbon black Fine particle size whiting Channel carbon black Gum stock
1
changes only a slight amount. When the tension on the band is released so that the rubber is amorphous, the vacuoles disappear in a short range between 5 pounds and zero load. I n the second extension cycle there is a decrease in volume because the rubber is softer and fibers more readily, and the shearing stresses around the pigment particles have been considerably lessened as the result of the first extension
0
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FIGURE 12. EFFECTOF ZINC OXIDE ON A DIPHENYLGUANIDINE STOCK
not apply here. I n those instances fibering took place in the soft cures, but in this case fibering occurs with the stiffer stock. Perhaps without zinc oxide in a rubber stock a type of linkage is developed during vulcanization which does not permit a ready fibering of the rubber when it is stretched. The diphenylguanidine stocks were examined by x-ray; there was
OCTOBER. 1940
INDIJSTRIAL AND ENGINEERING CHEMISTRY
no evidence of molecular iirieritation in tho zinc-oxide-free compound elongated to 300 per cent, but the characteristic fiber pattern was noted for the stock containing 5 per cent zinc oxide. The zinc-oxide-free stock exhibited the amorphous ring diagram of unstretched rubber.
Effect of Dispersing Agent The value of stearic acid 86 a dispersing agent for zinc oxide is demonstrated by the data charted in Figure 13 in which a diphenylguanidine stock containing 16.4 volumes of zinc oxide is compounded with various amounts of stearic acid. Without stearic acid in the compound there is a definite volume increase on stretching, and the addition of 0.5
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b2-h
PARAFFIN
13.59
from crystalline to amorphous. The experiments were conducted on a DeMattia flexometer, and when a gum stock was flexed from 0 to 250 per cent the flexing life was one fortieth that of the same compound flexed from 100 to 350 per cent. In the 0 to 250 per cent cycle the rubber changes from an amorphous to a fibered state, and it is believed that the frictional heat associated with this transition (that is, the Joule effect) is responsible for the premature fatigue of the ruhher. The rubber molecules are probably oriented somewhat throughout the entire flexing cycle of 100 to 350 per cent, so that the Joule effect which would induce prematnre faiiwe has been minimized.
Acknowledgment The helpful criticism snd assistance of G. S. Haslam in this investigation are appreciated by the writers. Thanks are also due to M. D. Rrewster for assistance in constructing the apparatus and making some of the preliminary measurements, and to M. I,. Fuller for the preparation of the x-ray diapams.
Literature Cited (1) Depew. H. A., and Easlcy. M . K.. ISD. &e. CHEM.,26, 1187 (1934). (2) Gehmsn, S. D., and Field, J. E.. Xubhor C h . Tech., 12, 706
(1939).
W., IND. EN-. CHEX.,13, 1029 (1921). (4) Holt, W. 1%. and MoPhorson, A. T..J . Research Nall. BUT. Standards. 17,657 (1936). ( 5 ) Sohippel, H . F.,IND.Ewo. CHBM..12.33 (1920). (3) Green,
PR~~ENT befoie E D tho Division of Rubbei Chemistry at the 99th Xeetinz of the American Chemical Society, Cincinnati. Ohio.
FIGURE 13. EFFECTOF STELRIC ACID O N ~DIPUENYLGOANIDINE-ZINC OXIDESTOCK
per cent stearic acid did not influence the dilatometer results to any extent. However, with 1.0 per cent stearic acid in the compound there is a substantial reduction, and with 2 per cent stearic acid an additional slight reduction in the volume of vacuoles formed in the dilatometer. The measurements on a 2 per cent paraffin compound are essentially the same as those for a stock without added stearic acid. The significance of these observations is that, with an insufficient amount of fatty acid in the stock, incomplete dispersion of the zinc oxide results, and the undispersed pigment acts as single large particles when the rubber is stretched and a large volume of vacuoles is formed. I n this case (16.4 volumes of zinc oxide of 0.22-micron particle size) the most striking improvement in the dispersion of the zinc oxide is obtained with 1 per cent stearic acid. Stearic acid is an effective dispersing agent for zinc oxide in rubber, whereas paraffn, having a nonpolar molecule, does not aid in the dispersion of zinc oxide in rubber.
Relation to Dynamic Fatigue
It is the opinion that the fibering of rubher and the flexing properties of rubber compounds are closely related, and that a consideration of this approach to the flexing problem should lead to a better understanding of the dynamic fatigue of rubber. Preliminary experimental evidence with elongated tensile specimens has indicated that, when the complete flexing cycle is conducted with the rnbber in a fibered state, the flexing life of the rnhber compound is far greater than if the flexing cycle includes a range in which the rubber changes
CRUD~O BUTYL RUBBER (See text, pagea 1283 t o 1282.)
