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IZT P A R T S B Y JfrEIGHT TABLE I. COMPOUKDIXG FORMULAS, Formula No. A B C Rubber 100 100 100 AleroaptobonzothiazoleQ 1.5 1.5 ... Santocure ... ... 1.5 Sulfur" 2.0 2.0 2.25 5,O 5.0 5.0 Zinc oxidea Carbon blackQ 50.0 50.0 50.0 Stearic acida ... 5.0 5.0 a Standard compounding ingredients obtained from Office of Rubber Reserve.
TABLE11.
PHYSICAL
CHARACTERISTICS
O F THE
45-hIINUTE
\7ULC.4SIZATES
Time of
Sample No.
Run, Hr.
1
22 20 18 16 14 12 20 20
1
1 1 1 1
2 3
Elongation a t hIax. Tensile,
Lb./Sq. I n . Modulus Max. Shore at tensile Hard70 300% strength ness Sodium Ethyl Adduct a8 Emulsifying Agent 7-
_7
Yield,
83 78 72 63 51 38 80 76
Set a t Break,
%
1460 1460 1090 1140
3860 3940 4070" 4010
550 600 700 500
63 63 63 61
b
b h
b
h
b b
b b
450 550
65 68
16 25
b
2370 2200
4430 443OC
22 22
31 16
Sodium Stearate as Emulsifying Agent 4 12 82 1390 2950d 500 64 19 5 12 81 1810 3350d 450 67 16 Tensile strength of the 60-minute cure mas 4320; in all other cases the 45-minute cure was ootimum.
minute cure which had a tensile strength of 740 pounds per square inch and an elongation of 325%. Formula B (Table I) was then uscd for compounding this rubber. By adding 3 parts of Rtearic acid on the mixing rolls, the quality of the vulcanizates was increased to 4110 pounds per square inch and 450% elongation. Homever, a curing time of 165 minutes a t 280' F. was necessary t o reach this state. A third formula (C, Table I) contained a n increased amount of sulfur (2 t o 2.25) and a more powerful accelerator (Santocure) in place of mercaptobenzothiazole. ,4n optimum cure of 4430 pounds and 550% elongation was obtained a t 45 minutes. Since this cure was reasonably satisfactory, this formula was employed for all later work (Table 11). The small batches of rubber were mixed on a special mill with rolls 4 inches in diameter, the distance bet\veen the guides being 1 inch. Speeds of the front and back rolls were 10 and 15.2 revolutions per minute, respectively. The milling was carried out a t atmospheric temperature. During the first 12 minutes all of the compounding ingredients were added in the following order: sulfur, accelerator, zinc oxide, carbon black. When stearic acid
Vol. 41, No. 6
was used, it was added along with the carbon black with an additional &minute milling. The samples were then milled another 5 minutes after all compounding ingredients had been added, and finally sheeted five times with a n opening of 0.011 inch between the rolls. Most of these samples were somewhat nervy, but all broke down readily and could be handled easily by this small scale milling technique. T o make the cures, a speciallv constructed mold was used which consisted of three separate 6 X 6 inch pieces, two polished stainless steel outer plates, and a thinner center plate with three rectangular openings. This mold gave vulcanizates 3 inches long, 0.623 inch wide, and about 0.030 inch thick. Ten vulcanizates could be produced from each compounded 8-gram batch. Stress-strain measurements n-ere made on a standard Scott tensile tester. A special dumbbell with a '/s-inch narron section was used t o cut the small tensile strips. There wab not eriougll raw polymer to make a standard block for determination of hardness, but a n approximate figure was obtained by stacking up the pieces of tensile strips and measuring with a Shore durometer. ,Ipproximately one minutc after the tensile strips had been broken, the distance between the bench marks was measured and the percentage set a t bIeali then calculated. CONCLUSlOUS
No comprehensive compounding study was made of the rubber produced by the small scale technique described here. The substitution of Santocure for mercaptobenzothiasole may not have been necessary, and less stearic acid might have been used just as advantageously. However, the formula as used on samples of rubber prepared with the ethyl ester adduct soap over a n ide range of monomer conversion (63 t o 83%) produced vulcanizateq having tensile strengths around 4000 pounds per square inch with good stretch characteristics. Cures of two sample3 had tensile strengths of 4430 pounds and elongations of 450 and 55OTc,, respectively. Vulcanizates of control samples of Buna 9 prepared with sodium stearate as the emulsifying agent consistently had tensile strengths approximately 1000 pounds lower. This work indicates t h a t rubber of good quality can be produced from butadiene and styrene using the ethyl ester soap of the maleic anhvdride adduct of l-pimaric acid. ACKh-OW LEDGMENT
The authors are indebted to L. -1.Goldblatt for advice and suggestions in carrying out this work, to 11.11.Graff for the p e p aration of the resin acid derivatives, and t o Barbara E. Hillcry for assistance in preparing the butadiene-styrene copolymers LITERATURE CITED
Craig, David, U. S. Patent 2,362,052 (Nov. 7 , 1944). Fleck, E. E., I b i d . , 2,359,980 (Oct. 10, 1944). ( 3 ) Fryling, C . F., IUD. ENG.CHEM.,ANAL.ED.,16, 1-4 (1944). J . Am. Chem Soc., 68, 1937-8 (1946). (4) Graff, M .M., (1) (2)
RECEIT.ED December 10, 1947.
High Temperature Heat Content O f Boron Carbide E. G. KING
P a c s c Experiment Station, u. s.Bureau of illines, Berkeley, Calif.
T
HERlIODYNABIIC data for boron carbide (B&) are of interest in connection with some of its potential industrial applications. I n studies, conductcd elsewhere by the Bureau of Mines, of the possibility of substituting boron carbide, or mixed carbides of which it is a n ingredient, for diamonds in drill bits, the question arose as t o whether the observed brittleness might be connected in some way with transformation points in boron
carbide. Accordingly, high temperature measurements of heat content were undertaken because they offer one of the surelt means of disclosing transitions. S o previous high temperature values of heat content are in the literature; in fact, the only previously available thermodynamic data for boron carbide are the low temperature heat capacities and the entropy a t 298 16" I< reported by Kelley (1).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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40,000
The heat content of boron carbide was determined in the temperature range 298"to 1726"IC. The substance showed normal behavior a t all temperatures. A table of heat content and entropy increments above 298.16' I
