I
T. H. LING',
E. G.
BOBALEK, and G. W. BLUM2
Case Institute of Technology, Cleveland 6, Ohio
Reinforcement of Synthetic Elastomers
Effect of Carbon Fillers in Bonding Butyl Rubber to 70/30 Brass S T U D I E S of the practical problems of obtaining a good bond between vulcanited butyl inner-tube stock and brass were reported by Kaercher and Blum ( 3 ) in 1951. Only one compounded rubber stock was used and the test method proposed involved tensile rupture of a brass-rubber sandwich, where different brass alloys and different surface treatments were employed in the preparation of tensile test specimens. These studies established an optimum choice of alloy composition, and a procedure for surface treatment of the metal in the preparation of tensile samples. I t was then possible to investigate more precisely variables introduced by modifying the formula of the rubber adhesive. The force required to rupture a metalrubber laminate at the adhesive bond depends on a composite of properties, involving at least the physical properties of rubber in thin films and the chemical or physical attractive forces prevailing at the rubber-metal interfaces. Which effect is the more important is of fundamental concern in any research on adhesion. Especially in practical problems, the experimenter would follow different directions of rubber compounding if it were known that the cohesive strength of the rubber is a more important determinant of bond strength than are chemical interfacial reactions between rubber and metal. The experiments reported here were designed to seek information regarding this qualitative question. Varying the type or quantity of carbon black filler changes markedly the bulk properties of rubber. A systematic way of evaluating the reinforcing effect of filler has been proposed by Wiegand ( 5 ) . If the strength of rubber-brass bonds showed a direct correlation with the reinforcing effect of carbon in the rubber, this would suggest that surface reactions are less important within certain limits than the bulk physical properties of the adhesive, l Present address, Anaconda Wire & Cable Go., Marion, Ind. * Present address, Goodyear Tire and Rubber Go., Akron, Ohio.
Carbon Black Fillers Several types of carbons of varying physical and chemical properties were used in this study (Table I). The diphenylguanidine (DPG) value should be noted. As the variously compounded adhesives were cured according to a fixed time schedule, the retardation of cure might be expected to be greater for the carbons that have more volatiles and a greater diphenylguanidine value. The apparent reinforcing effects observed represent a composite of the filler and vulcanizing effect on physical properties.
inch and blend in pine tar and Paraflux (3 minutes). 3. Add carbon and mix (8 minutes), then cut four times each way (5 minutes). 4. Add zinc oxide and sulfur and mix (3 minutes). 5. Add Captax and thiuram and mix (3 minutes). 6. Cut four times each way and band six times (5 minutes). 7. Sheet out at 0.055 f 0.005-inch thickness, and age sheet for 24 to 48 hours at 70 O F. and 40y0 relative humidity. 8. Masticate the aged sheet again (5 minutes) and sheet out again at a thickness of 0.005 inch.
Formula and Compounding All rubber compounds were made up within less than 6 months from one batch stock of butyl rubber polymer (Firestone). The following schedule of compounding the formula and production of ASTM tensile test bars was adhered to rigidly:
From this flattened sheet, slabs were cut to fit the curing press, and small squares were cut for use in preparation of the brass-rubber tensile test sandwiches. The large slabs were cured for 30 minutes in a mold 0.050 inch deep at a pressure of 1750 pounds per square inch and temperature of 325' F. After the cured slabs had cooled, tensile test dumbbells were punched out with a die having a minimum neck width of 0.250 inch. Each test specimen was marked on the neck with a 1-inch stamp and the thick-
1. O n a two-roll mill at a clearance of 0.150 inch, break down the rubber (3 minutes). 2. Tighten the mill to 0.55 f 0.005
Table 1.
Carbon Blacks Used in Compounding Butyl Rubber
Type' Monarch 71 Monarch 74 Monarch 81 Carbolac 2 Supercarbovar Vulcan R Sterling 95R Sterling R
(Data from Godfrey L. Cabot, Inc.) Sq. Meters per Cham Electron NB Volatiles, microscopy adsorption % 145 13 1
95 178 168
... 54 26
419 328 142 714 347 115 50 22
5 5 5 12 5 1 1 1
Grams 212 6 6 16 4 4
Material Butyl polymer
a
DPG Value 65 60 40 85 75 10 8 3
Pine tar Paraflux (C. P. Hall) Zinc oxide (New Jersey Zinc, Kadox 15) Captax (Vanderbilt) Thiuram M (Du Pont) Sulfur (Dow Mike) 6 Carbon, variable type (each black tried at weight loadings of 42.4, 84.8, and 127.2 grams, corresponding to 20,40,and 60 parts of black per 100 parts of butyl) First four carbons are channel blacks. other three furnace types.
VOL. 48, NO. 1 1
0
NOVEMBER 1956
2083
Table 11.
after the adhesive bond had been formed. The variation of film thickness within a test piece was on the average f0.1mil. In several duplicate test pieces it might vary between 0.1 and 0.4 mil. Within this range, the bond strength data showed no definite dependence on film thickness. The occasional sample where a very thin film of less than 0.1 mil or a very thick film of more than 0.4 mil was obtained was rejected from the average, as here film thickness effects were indicated and the number of samples was insufficient to clarify the film thickness relationship more precisely.
Average Values of Bonding Strength
Monarch 74
20 40 60
Monarch 71
20
No. OJ' Measurements 4 4 5 4 5 6 5 6 5 6 14 11 5 4 7 4 6
40
10
60
5
m7t. 7 0
No black
0
Sterling R
20 40 60
Sterling 95R
20 40 60 20 40 60 20 40 60
Vulcan R
Monarch 81
Supercarbovar
20 40 60
Carbolac 2
dv. Bonding Strength, Lb./Sq. Inch 63 220 310 360 238 358 446 259 490 657 2 94 537 746 359 5 96 765 273 551 715
9 .
20 40 60
6 6
864 716
6 7 4
369 22 1 59
An. Deviation,
70 3.2 1.3 i 6.8 i 3.8 5.5 5.5 i 5.6
* *
i: 5.6
i 1.9 i 3.4 i 7.3 i 4.1 i 2.6
Bond Strengfh Data The average values of bonding strength are shown in Table 11. The ASThl tensile test data for the dumbbell test bars can be summarized and compared in terms of the calculated Wiegand (5)function.
i 4.0 i 3.8 i 4.0 =k 3.7
rf 6 . 6 i 7.2
.... * 9.8
A.4 =
i 3.3
4.6 i 7.4 *14.5
Table 111. Correlation between Average Values Tensile Data of ASTM Test Bars and Tensile Strength of Adhering Brass Buttons Bonded with Same Rubber (Cure a t 325' i So F.after 29 minutes a t pressure of 1750 lb./sq. inch)
miCarbon,
wt. 70
Sterling R
Sterling 95R
VulcanIR
Monarch 81
Monarch-74
MonarchZ71 Carbolac 2
Black, Vol. % 8.5 15.7 21.8 8.3 15.4 21.4 8.2 15.2 20.9 8.2 15.1 20.9 8.1 15.0 20.8 8.1 15.0 8.1
15.0 20.8
No carbon
Load, Lb./Sq. Inch, at Elongation o f Final 200% SOO% 397 512 610 993 60 1 74 1 1102 912 754 440 582 745 1263 688 859 1400 82 1 1060 465 662 1140 72 1 1060 1940 1128 1635 1943 600 1487 452 2860 550 758 746 1085 3083 42 1 571 2300 2753 540 753 3720 954 666 2347 340 550 523 699 3077 2480 370 462 409 512 2270 502 2900 687 309 448 388
ness of each was measured separately. The test bars were pulled a t a constant rate of 20 inches per minute, using a Tinius-Olsen-Schopper tester. The test method conformed to specification D 412-51T of ASTM Committee D-11 (7)The preparation of the brass-rubber adhesive sandwich has been described 13). The time, temperature, and pressure used in curing the slabs were used also to cure the adhesive layer between the brass buttons. The bonded brass
2084
~
mate tion,
70 367 450 433 400 470 467 457 515 423 553 670 653 63 7 640 717 660 712 763 843 790 353
l
~ Actual Strength 2,850 5,460 5,980 3,725 7,210 7,940 6,350 11,950 10,200 9,700 22,000 23,200 17,000 20,400 30,400 17,000 25,100 21,400 21,400 25,800 2,030
~
~
AA 109 670 1842 137 1125 2558 274 2100 3960 840 3600 7455 1710 4620 8305 1220 4220 2520 5945 9430
test piece was pulled on a Tate-Emery No. 508-A machine a t a constant cross head speed of 0.025 to 0.030 inch per minute. In the preparation of the bonded brass sandwiches, some variation of film thickness of the adhesive could not be avoided. The film thickness was estimated by difference after micrometer measurements of the assembled pair of brass plates had been taken at various points near the circumference before and
INDUSTRIAL A N D ENGINEERING CHEMISTRY
SI:'
Ey dV
where Ey is the area under the stressstrain curve (proof resilience). The AA function represents in energy units the reinforcing effect of the black between volume fractions of V Oand VI in the rubber mix. The manufactuier's values for specific gravity were used to compute volume fractions of caibon in the total ~ - formula-i.e., it was assumed all volumes were additive to give total volume of compound. AA values are shown in Table 111. The average values of ultimate tensile strength of the 200 and 300y0 moduli and actual tensile strength calculated from the cross-sectional area at break (2, 4 ) are also given in Table 111. The bond strength of the brass-rubber adhesive sandwiches using the same rubber shows an apparent qualitative dependence on tensile properties as represented by the data listed, and various correlations might be deduced. The most systematic relationship exists if the bond strength ( 3 ) of the adhesive sandwiches is considered a function of effect of volume loading of the rubber with carbon-that is, if 2 is plotted as some function of AA, irrespective of what is the rubber stock that provides these values (Figure 1). An analytic curve fitting the data yields the following equation, where all data are given equal weight in determining the average curve. Z = 7.963 ( A A ) ' / 2-/- 60
This empirical equation shows only that there is a correlation between adhesive strength as measured by this particular test method and the physical properties of the rubber adhesive as measured by ASTM tensile tests. Other methods of measurement of either property might produce comparable correlation equations that differ only in the
Z-60 = Z'= 7.963 ( A A ) ' "
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