May, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
volumes are necessary. In the case of zinc oxide, blanc fixe, barytes, and the like, lower volumes will be necessary to obtain satisfactory resistance to tear. 3. The stocks should not be markedly overcured, especially if the cure is carried out at 50 pounds steam pressure (298" F.) or above. This is the opposite of former practice with stocks of higher sulfur ratio. 4. The accelerator chosen must be strong enough to combine practically all of the sulfur present, and yet should not be too scorchy to handle safely in the plant. 5 . Tests showed that the use of 2 per cent of a good antioxidant was justified. One per cent gave good results, but especially %-henagings longer than 18 hours were carried out the stocks containing 2 per cent were markedly superior. 6. Zinc oxide, blanc fixe, barytes, or precipitated whiting can be successfully employed in stocks of this type. The modulus and tensile figures are lower than xhen P33 Black is used, but the losses in tensile strength due t o air-bomb aging are comparable t o P33 Black, and these fillers would permit the manufacture of colored tubes such as red, green, and yellonr. 7 . Heat-resisting inner tube stocks of this type compared to the ordinary tread, carcass, and tube stocks will necessarily be
,559
more scorchy, and care must be taken to cool the stocks properly in processing through the factory. The possibility of the migration of sulfur from the carcass stocks to the inner tube, or the migration of accelerator from the inner tube stock to the carcass stock must be taken into account. Today tire manufacturers are using as low as 2 per cent sulfur in the carcass stocks of bus and truck tires. These low sulfur ratios in the carcass stock tend to prevent the migration of sulfur. The usual Geer oven migration tests were carried out on stocks containing Iireka C as accelerator. The Geer oven was maintained at 93.3" C. (200" F.) for 14-day periods, and in no case did these tests show appreciable migration of sulfur or accelerator. Controls were run during the migration tests and alone. RECEIVED March 3, 1932. Presented before the Meeting of the Division of Rubber Chemistry of the American Chemical Society, Detroit, Mich., February 25 and 26, 1932.
Grit in Carbon Black Effect on Flexing Resistance of Vulcanized Rubber JOHN N. STREET,The Firestone Tire and Rubber Company, Akron, Ohio
T
Rubber channel black has been found to contain and 1.92 kg. per sq. mm., respecHAT the g r i t f o u n d in amounts of grit retainable on a 32j-mesh Screen tively. Abrasion tests showed c a r b o n b l a c k used in the a b r a s i o n resistance to be rubber products is detrivarying from 0.05 to 1.7 per cent. This residue 1.06, and l,oo, respectively. mental to the quality of such products has been pointed out is shown to be objectionable f r o m the standpoint Thus the variations in abrasion by a number of investigators. of the flexing resistance of the resulting rubber resistance were w i t h i n expericompounds, higher concentrations being coprem e n t a l e r r o r , although lower Gallie (5) states that the presence of this i m p u r i t y i n g a s spondinglr harmful than lower concentratensile strengths and r e s i l i e n t energies resulted in the cases of black used for rubber manufactions. The Coarser fractions appear to give rise the blacks containing grit. ture may be expected to give rise to trouble in the followto larger cracks and earlier appearance of crackThe difficulties in s a m p l i n g and in methods of testing carbon ing than the Jiner material. Even that which ing directions, a m o n g others: passes a 200-mesh screen but is retained on a blacks for grit are d i s c u s s e d (1) degradation of tensile and by M u r p h y (8),G a l l i e a n d other mechanical p r o p e r t i e s ; 325-mesh is objectionable. ( 2 ) production of discontinuity Porritt ( 6 ) ,Esch (S), and Evers i n r u b b e r films r e q u i r e d t o (4). T h e s e w r i t e r s have not retain or exclude gases or liquids; (3) unsatisfactory surface discussed, however, the effect of grit on the quality of the finish on goods prepared by extruding, calendering, and resulting rubber product. The evidence points to the conclusion that the coarser spreading; and (4) points of weakness in rubber articles subjected t o severe mechanical usage or repeated compression grit lowers tensile strength, the fine grit is not detrimental and stressing. Gallie records the finding that the addition of to tensile strength, and that neither is objectionable from the 5 per cent of grit, collected on a n 80-mesh screen, to a rubber- standpoint of abrasion resistance. Experimental data on its sulfur mix (95:s) resulted in reducing the tensile strength effect on flexing resistance, which is of distinct importance from 1.0 kg. per sq. mm. to 0.75 kg. per sq. mm. Reduc- in the compounding of tire treads and sidewalls, where the tion of the grit to finer particle size by grinding caused i t major amount of rubber channel black is used, do not appear to function "merely as a filler." Kearsley and Park (7) to have been recorded in the literature. The flexing resistance of rubber compounds is becoming concur in this opinion. 111 their work the grit was ground to pass a 100-mesh screen, and was then added t o the standard of increasing importance. I n the past, the major cause of tire black sample, in amounts varying from 0 t p 10 per cent. The failures under normal operating conditions has been tread results indicated that in amounts up t o 3 per cent this ma- wear. The increase in power and in braking facilities on the terial had no measurable effect on the tensile strength or newer cars is causing still faster wear. To overcome this modulus of the stock. difficulty, the rubber compounder is naturally turning to the Twiss (9) concluded that grit has no effect on abrasion use of higher black loadings to make the tires more resistant resistance although it may lower tensile strength. His to abrasion. The increased loading in itself, through the procedure was as follows: Mixing A contained no grit (90 increase in volume of pigment and through the stiffening of mesh), B contained a different black having 0.09 per cent the compound, will lead to a greater tendency toward flexgrit, and C contained the same black as A, except that enough cracking. Even under present conditions, tires occasionally of the grit separated from another sample of the black used in crack. This leads to an unsightly appearance and raises B was added t o give a sample containing 0.1 per cent. Tests the question in the mind of the user as to the length of service on these samples showed the tensile strengths to be 2.03, 1.84, that will be rendered. Consequently it appears timely t o
560
I N D 11 S T I{ I A L A N D
I3 N G I N 1s E I{ I N G C It 12 hl I S 'I I< Y
Vol. 24, No. 5
merits of rubber channel black. The combined residues showed lit.tlo or no tinctorial effect as judged by the appearance of a sample of pale crepe into which 0.1 per cent of the residue was milled. This indicat.ed t.bat the grit contained little or no aggregated black-i. e., soft aggregates which would disperse easily. This method has proved of considcrrthle interest and value in examining residues from screening tests and their dispersability in rubber. A sample of pale crepe amounting to approximately 500 grams is broken down in the usual manner, allowing 4 to 5 minntes for breakdown. The residue is then added and the milling continued for 10 more minutes. The batch is allowed to stand for 24 hours and is then remilled for 10 minutes, after which it is cut off in a sheet approximately 0.05 inch (0.127 em.) in thickness. A batch containing grit will be light in color and still translucent, the g r i t p a r t i c l e s b e i n g easily observable and evenly distributed throughout the batch. A hatch containing FIounE 1. CARBON BLACK GRIT soft aggregates will be black and completely opaque. present data on the effect of carbon black grit 0x1 tlie flexing The general nature of the resistance of rubber. grit a s t o size a n d s h a p e may he obtained from FigTsSrINo PItOCEDUIiE ure 1. The grit retained on The grit used in this work was separated from tlie black by a 200-mesh s c r e e n , a f t e r wet-screening on a 325-mesh sieve (44-micron openings). passing 100 mesh, is illusThe operation was facilitated by hrushing the black on the trated as being representascreen to assist in breaking up the aggregates and by directing tive of the other samples. a fine spray of water (under 20 pound9 pressure) onto the The coarser and finer mascreen from time to time. Endres (2) lias reported that a terial appeared todiffer only black that has been wet can thereafter he dried and satis- in size. It is probable that A E factorily dispersed in rubber by the usual milling processes. in the screening operation G~~~ i3xperiment.s in this laboratory have substantiated this con- s o m e , if n o t all, of t h e F~~~~~ 3. E~~~~~ ADDED TO BUCK BEPORE clusion. Consequently, the wet-screening method should not sharper corners have been MILL~NG be objectionable from the sdandpoint of subsequent use of worn off, making tho materirtl appear more nearly the gritty material in compounding experiments. It is of interest, in passing, to note that the various samples round than it actually was in the original sample of black. of black tested showed percentages of residue varying It was also noted in making this examination that the amount from 0.05 to over 1.00 per cent. One sample slrowed 1.74 of sand was very low and the amount of iron oxide, with per cent of grit. All samples were t.aken from regular ship- adhering black, represented a relatively small percentage of the total. I n testing the effect of grit on flexing, a masterhatch of tread stock, containing an easily dispersible black of low grit content (0.10 per cent), wasused. Varyingamountsofgritweremilied into port,ions of this bat,ch. All batches, including tlie control, were remilled after standing for 24 hours. I n this way the grit received two millings, giving it every opportunity for satisfactory dispersion. The formula of the masterhatch was as follows: Smoked sheets Ssliur %ne oxide Pine tar Rteario arid Carbon blsok ~~
Meresptobe~roLliia~ula
100
....
2 76
8
3
3.5 40 0.75
102 00
Test slabs were cured for 60 minutes a t 280" F. (137.8' C.) which was judged to be the optimum commercial cure. Dumbbell test strips were died out in the usual manner. The flexing tests were carried out on the Firestone flexingmachine ( I ) , the test strips being flexed to 100 per cent elongation a t the rate of 400 flexes per minute. Experience over tlie past six A n C D FIGURE 2. EFFECT OF TOTAL RESIDUEON 325-M~swSCREEN years has shown that this test parallels drum and road tests unusually well. Grit added to tread masterbatah
The flexing results on stocks containing varying amounts of the total residue on a 325-mesh screen are shown in Figure 2. The control stock is marked A. The amounts of grit added were 0.5 per cent (U), 1.0 per cent (C), and 2.0 percent (D). The concentrations of grit are given as percentages of the total black content of the stock. The strips with the high grit content. (C and D) were removed from the flexing machine after 3.5 hours, as the cracking was severe a t the end of that time. The remaining two strips were flexed 4.5 hours. As the test does not lend itself to numerical evaluation, thi: comparison is made by visual observation of the test strips after a given time of flexing, judging by the numbor and intensity of the cracks. Tlie photograph shows that the stock containing 0.5 per cent of added grit (U) has cracked more than the control, and that those with higher percentages (C and D) are correspondiiigly worse. This and subsequent tests have been duplicated. I n order to determine whether the method of mixing affected the results materially, a comparison was made in whicli 1.0 per cent of grit wm added directly to the black itself before mixing (E), rather than to the masterbatch as above. Results of this test are shown in Fipsre 3. I t is apparent that the strip containing the black to whicli grit has been added has cracked decidedly more than the control. Comparison of the two stocks containing 1.0 per cent of added grit---E in Figure 3, and C in Figure %shows that thr method of adding the grit makes no appreciable diffcrence in the results. For the purpose of obtaining additional evidence as t u whether the grit was uniformly dispersed in the batch, the test slabs were examined microscopically. A typical case is illustrated in Figure 4. In order to obtaisi a field sufficiently large to be representative of the entire surface, very low magnification was used. A grit particle at or near tho cut surface of the sample causes a "hump" or unevenness. These are readily observable using reflected light. Thus, in Figure 4 it is noted that the number of such Irumps hecomes increasingly greater in comparing the stocks in the order A, 13, C, and U. In all stocks these were uniformly distributed over the field, showing uniform distribution of the grit. Tlie high magnifications normally used in examining treads, with the methods now employed, are not applicable to stocks
containing grit. Tlie grit retained 011 a 325-iziesh screen must be approximately 44 microns in one direction, whereas the thickness of a slide from a troad must be less than o n e tenth of this thickness in order to bo examined by transmitted light. It then appeared desirable to determine how coarse the grit must be to affect flexing adversely. A 10-gram portion of the 325-mesh residue was screened successively through 40-, loo-, 200-, and 325 mesh sieves. It is of interest to note the relative amounts collected on the various screens: ON
T.lRO"liH
Asar
40'&h
7" 4.4 24.0 41.2 24.2
40 medi
ma
ZW mc8h 325 mesh
too mesh zoo
._
T i l e fact that the percentage reiriainiiig on tlle 325-mesh sieve
after passing the 200 mesh is srnaller than that collected on the 200-mesh screen, is again indicative that it is grit or
A
K
3
I
Pir;i!aii 6. I h w m OF R ~ s t n uOY~ 200-M~siiScniir:~AFTER PASSING 100 ~ l ; s i i
foreign iiiaterial rather tlirtri agglomerated black. Were this material agglomerated black, normal distribution of tlie sniric slabs were used in the tensile and flexing comparison. range of particle size would require increasing anioilnts of Tlrese values are given in Table 1. Pour strips \yere broken residues with decreasing size of mesh openings OS the various and averaged iii arriving at tlie recorded values. A11 slabs screens. were cured ti0 minutes at 280" F. (137.8' C.). The flexing results sliorni in E'igure 5 arc those obtained in A11 tensile-strengtli values are probably within the limits the comparison of the control, A , with stocks to which liad US experimental error. In those tests, the st.ocks containing been added 0.5 per cent (P), 1.0 per ceiit (G), and 1.5 per cent the coarser grit slioiv irrdications of a very slight lowering of (If) of the material passing 40 mesh, but reinaining on a 100- teirsile strength (stocks B, C, U, F, G , and I1 compared to tnesli sicve. It was a.gaiii observed that stock E', t o wliich 0.5 stock A). However, tho general conclusion is reached that per ceni, had been added, cracked rriore than tlie control A. the fine grit does not lower tensile strength in the concentraThe higher grit loadings appeared worse. tions used in tlicse tests. It has been noted, in tests not In t.lie same way, Figure 6 shows tile test, strips [:oi~taitiii~~ rsjiorted, that in many cases tlie presence of relatively high 0.5 per cent (I), 1.0 per cent (J), and 1.5 pix cent (I
