T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C R E M I S T R Y
1rj6
erally carry g j to 58 per cent of zinc oxide instead of only 46 per cent, as in this case. There is no appreciable difference in aging, however, between compounds containing 46 and 58 per cent zinc oxide, as shown by evidence not included in this report. T A ~ LI-Co~wsmon B os C o r r ~ o u x ~ s ,-By Weight-
C
A
92
92
LOO
..___....... .. 20. . ........._____. ..
25
First later.
...,. .....__92
n black Lithopone
Volurnh
-By
B
A
.. ..
..
8
C
1W
1W
5:o
14.5
..
The compounds were aged in an air bath at 7 0 ° C., as was suggested by Dr. Geer as an accelerated aging test. Tires, however, heat up from internal friction while running, and this temperature is often exceeded. In solid tires, in fact, the temperature at times exceeds I000
c.
The physical properties of the compounds after aging are tabulated in Table 11.
Vol.
12,
No.
IZ
The test specimens were about 0.17 in. thick, and accordingly the tensile strength and elongation did not record an interesting hardening on the surface as fully as thinner specimens would have done. This hardening penetrated about o 03 in. in 16 days. The effect is shown in Fig. z , where the surface of t h e rubber compounded with carbon black is cracked deeply by bending the rubber. There is no evidence of this surface hardening in the case of Compounds A and B. This particular hardening is probably responsible to a considerable degree for the accelerated wear of a black tread tire with time, the hard surface being continually cracked and torn away by the road abrasion. The cause is uncertain, but appears to be oxidation. The adsorptive power of carbon for oxygen has been recognized for some time, and it may be reasoned t h a t t h e carbon black acts as a catalyst and oxidizes t h e soft rubber t o a hard rubber. S O M E MICROSECTIONS CUT FROM VULCANIZED RUBBER ARTICLES' By Harlan A. Depew and I. R. Ruby R S B B A L*so**To%r ~~~ OB TRH NSW JBpsSp Zrrc Co., PALXBLITON, PI.
R-ved
45 3410 626 60 3594 645 90 2764 495 120 3154 564
2650 1251
SomMc A 639 2832 591 563 2849 544 485 785 190 266 915 184
3144 629 60 3203 605 90 2212 485 120 1321 339
3050 2708 1396 734
589 538 359 I89
385 577 652 566
3243 3233
45
45 60 90 120
3306 3408 3808 3217
3702 3605
1645 iW8
2435 544 1746 429 844 197 653 132 SomQIr C 540 1763 386 525 1087 263 298 773 I41 191 607 106
2761 1322 786 992
576 330 185 147
2831 2403 855 766
577 512 202
2403 526 1525 394 740 169 591 99
1445 5% 504
129 i70
519
87
1342 308 1470 314 684 115 543 79
1W
100
588 182 502 I40 496 71 550 124
The aging for the forty-five minute cure is shown graphically in Fig. I . Elongation is the typical property of rubber, and is a fairly satisfactory gage of the aging of its compounds.
Pia. 2
I t appears that the compound containing lithopone ages somewhat less satisfactorily than a straight XX Red zinc oxide compound. The carbon black compound is much inferior t o the others on the basis of tensile strength and elongation on aging. This is especially serious when it is realized that, as the percentage of carbon black in a tread compound increases, the heat developed by iliternal friction increases faster than a linear function.
September 4. 1920
It has long been recognized t h a t pigments for use in rubber work should consist of very fine particles. Microscopic work done in this laboratory on pigments in linseed oil and other paint vehicles has shown t h a t fine particles tend t o exist as flocculates. In certain vehicles this flocculation is largely or entirely overcome. I n the case of water suspensions, as well as of paints, the physical properties of a defloccnlated and a flocculated suspension differ greatly. Carrying the suspension analogy still further t o t h e very much more Viscous (properly, plastic) medium, rubber, it seems reasonable t o assume t h a t dispersion and flocculation of pigments play important parts in determining the physical properties of compounded rubber. Attempts to determine the dispersion of pigments in cornpounded rubber by reflected light have failed, because with the high magnifying powers necessary to see the individual particles of, for example, zinc oxide or carbon black, the illumination of the surface is entirely too weak, particularly where the reflecting properties of the medium and the snspenaed,@gment are close. Our success with transmitted light l n water and oil suspensions pointed to this method of illumination for t h e examination of cornpounded rubber. There is considerable difficulty in cutting sufficiently thin sections of vulcanized rubber, owing t o its elasticity and toughness, especially in the case ,of rubber highly compounded with zinc oxide or carbon black, where the thickness mnst not exceed I #. In less highly compounded matter the section can be 2 0 # thick, or even thicker. The elasticity can be destroyed by immersing in liquid air, but this makes the sample too hard t o cnt. Accordingly, the rubber must b e frozen t o an extent just sufficient t o destroy its elasticity and yet not make it too bard. 1 Presented before the Rubber Divisiion s t the 60th Meeting of the American Chemical Soeiety. Chicago, nt., September 6 to 10, 1920.
Dec., r g a o
T E E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
Fro. 2
9". 4
FIG. 1
Pro. 3
pro. 6
Fro. 8
Fio. 9
'157
1158
T E E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
FIG. 10
c
The method finally developed consists in freezing the rubber by expansion of carbon dioxide on the stage of an o r d i n a r y Spencer microtome. The sample is held tight on the stage by means of awaterglycerol solution (90 : IO) which, on cooling,solidifies t o a white mass. This solution is used because it does FZG.13 not become very brittle a t the low temperatures. The carbon dioxide does not cool the sample t o a sufficiently low temperature, and a surface flash cooling is given by liquid air. The liquid air can be obtained at a convenient liquid air manufacturing plant and transported without difficulty in an “Icy-Hot” bottle, which will keep liquid air for several days. The liquid air may be ejected very satisfactorily from the bottle by introducing carefully a one-hole rubber stopper through which a glass tube about 3 t o 4 mm. in diameter is passed into the bottle. The pressure developed by the continual evaporation will force a stream of the liquid out, which can be directed on the sample. A very small amount will freeze the sample sufficiently. The section is then cut with the knife. Raising the stage is best controlled by hand. The thin section is mounted in Canada balsam on a microscope slide and preferably examined visually a t I 5 0 0 diameters, using a Zeiss 2 mm. apochromatic oil-immersion lens.
Vol.
12,
No.
12
Fro. 12
For photographing, sharper negatives are obtained at 800 diameters, DESCRIPTION OF CUTS
In Fig. I zinc oxide is shown uniformly dispersed in rubber. A typical triplet crystal can he noted and also the grain in the rubber in the direction of working is shown by the definite alignmeut of the needle-shaped crystals. In Fig. 2 a good lithopone is shown very well dispersed. Fig. 3 contains alnminium flake. The grain is shown very decidedly, and it is not difficultto believe tbat the tensile strength and elongation will vary according to the direction of testing. Fig. 4 shows whiting. Fig. 5 is iron oxide. It is well dispersed hut contains many large particles. Fig. 6 was litharge when introduced and should have shown clear centered particles when in focus, but the particles have become black, due to B surface coating of sulfide. Fig. 7 is a well-known white tread with magnesia as an a3elerator. Chunks of magnesia stand out in sharp contrast to the uniform distribution of the zinc oxide. The individual particles of zinc oxide do not show up well except near the edge, owing to the thickness of the section which was necessary to show the large particles of magnesia. Fig. 8 is a section from a widely advertised composition sole bought from e shw repairman. It shows the iron oxide used in coloring, the zinc oxide used in reinforcing. and the fiher used for stiffening. The photograph is pwr, but the section can he seen more distinctly by direct observation with the microscope. Fig. q shows a zinc oxide tread with a very small amount of lampblack to give a gray color. The lampblack is badly flocculated. ’ Fig. IO shows e poorly dispersed carbon black. Fig. I I shows a well-dispersed carbon black together with zinc oxide. Fig. 12 is a section from the sidewall containing zinc oxide and whiting. Fig. 13 is a section of a cushion from a tire made by a large mabufacturer, showing reclaimed rubber and a small amount of zinc oxide. Microscopic examination opens u p the following possibilities: (E) A knowledge of the dispersion of t h e pigment in rubber which will tell a good deal about the quality
.
