INDUSTRIAL AND ENGINEERING CHEMISTRY
378
-
TABLE
v.
TENSILE STRENGTHS
OF
Vol, 34, No. 3
ACETICACID-AND ALUXI-COAGULATED STOCKS
.lcetic Acid-Coagulated Sheet . Press Cu?,e a t -AM-173*-2AX-173br-AhI-174a60°F.,I\lin. 600% 700y0 A t break BOO% 700% At break 000% 70076 2230 130 3310 (775%) 90 1160 2230 (8207 ) 1100 570 30 300 2540 3290 (7607 173 1290 1500 2700 (800%) 775 45 420 2580 3400 ( 7 7 0 4 j 230 1310 1780 2900 (800%) 840 BO 2620 010 2940 ( 7 3 0 h ) 1520 280 1780 3000 (780%) 75 890 610 2575 3200 (750%) 310 1310 3100 ( 8 l O 7 ) 1950 960 90 GOO 350 2620 3290 (750%) 1375 3160 ( 8 0 0 4 ) 1890 940 120 a Rubber 100 zinc oxide 6, sulfur 3.5 Captax 0.5 stearic acid 0.5. b Rubber 100: zinc oxide 6, sulfur 3.5: Captax 0 5 : stearic acid 0.5, NaH2POcFI?O 0.37, IGHl‘04 0.70
--
- -
TABLE VI. TEKSILE STRENQTHS OF
cIIIIIi?iEL
Slum-Coagulated Sheet c -
At break 300 (830%) 600 (810%) 1090 840%) 975 1800%) 1420 (850%) 1600 (860%)
BLACK STOCKS
GOO%
740 850 980 1020 1030 1010
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0.5 Captax/100 Rubber 1.0 Captax/100 RubberPress Cure at -.----3AC-l43=-3AD-143b.--3AE-1430-260°F., Min. 400% 500% At break 400% 000% At break 400% 500% A t break 400% 1710 1320 1900 2700(625%) 1030 1590 2680(6707) 2300(71097,) 890 1275 30 2300 3375(630 ) 2220 3275 (640%) 1930 1490 2660 (640%) 1625 1900 1220 45 60 1610 2260 3010(600%) 1840 2630 3575 625%) 1760 2530 3700(650%) 2200 90 1990 2760 3475 (590%) 2090 2920 3450 [660%) 2050 2875 3900 (610%) 2420 a Rubber (first latex crepe) 100, zinc oxide 6, sulfur 3.5, Captax 3.5, stearic acid 2 . 5 , channel black 50 b Same as 3AC-143 plus 0.37 NaHzPOI.Hz0 and 0.70 KzHP04. c Rubber (first latex crepe) 100, zinc oxide 6, sulfur 3.5, Captax 1.0, stearic acid 2.5, channel black 50. d Same as 3AE-143 plus 0.37 h-aHzP01.Hz and 0.70 KzHPO4. 7
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additional 0.5 gram of Captax per 100 grams of rubber (3AE-143).
Discussion The hypothesis is suggested that a factor, perhaps a major one, in determining rate of cilre of rubber is the acid-base equilibrium developed during cure; it may be said for brevity (somemay think inaccurately) that the pH of the system and its buffer capacity enable it t o maintain a given reaction and thus condition the rate of cure of a given rubber. This assumed equilibrium could result in rubber-sulfur stocks, for example, from the components (a) nonrubber constituents naturally present in the crude rubber mass and (h) acidic materials, such as hydrogen sulfide (4), generated during vulcanization. The extent to which hydrogen sulfide would influence the equilibrium would depend on its rate of formation and rate of removal-that is, its concentration a t any given time. Even with the same rubber this concentration (and resulting acid-base equilibrium) would be expected to be different in the case of rubber-sulfur stocks (assumed high concentration of hydrogen sulfide) as compared with accelerated stocks (assumed low concentration of hydrogen sulfide). Moreover, it might be expected to vary with the type of accelerator used. Addition of various reinforcing agents or fillers, particularly in large volume, could be expected to affect the acid-base equilibrium; but any such effect, on the whole due to addition of standard pigments, would probably be of less magnitude if the batch were compounded with
2AX-174b700% A t break 2430 ( 7 9 0 7 ) 1460 2650 ( 7 8 0 d ) 1740 2900 7 7 5 7 ) 1970 1990 2990 1775%) 2040 2940 (775%) 1040 3040 (780%)
3AF-143d500% At break 2440 3920(660’7) 4000 (660%) 2770 3090 4340(640%) 3375 4100 (575%)
a whole rubber, such as L. S. made from ammoniated latex, than if it were compounded with well-washed (but unfermented) rubbers such as first latex crepe.
Literature Cited (1) Bruce, A., Trop. Aar. (Ceylon), 59, 267 (1922). (2) Bruni and Levi, in Memmler’s “Science of R u b b e r ” , p. 308 (1934). (3) E a t o n , G r a n t h a m , a n d D a y , Agr. Bull. Federated .?laZaz/ States, 27 (1918). (4) Fisher, H. L., IPTD. E N G .CHEM., 31, 1381 (1939). (5) Glidden, A. A , , personal communication; Weber, L. E . , “Chemistry of Rubber Manufacture”, p. 241 (1926). (6) Hastings and Rhodes, IND. ENG.CHEM.,31, 1455 (1939). (7) M o r t o n , H. A., U. S. P a t e n t 1,893,868 (Jan. 10, 1933). (8) Oenslager, G., IND.E N G . CHEM.,25, 299 (1933). (9) Sackett, G. A., I b i d . , 26, 535 (1934). (10) Vries, 0. de, Arch. RubbeTcuZtuur, 2, 67 (1918); “Estate R u b ber”, p. 101 (1920). (11) Vnes, 0. de, “ E s t a t e R u b b e r ” , 1920. (12) Ibid., pp. 463 et seq. (13) Weber, C. O., “Chemistry of India R u b b e r ” , 1902; quoted in Whitby’s “Plantation R u b b e r ” , p. 306 (1920). (14) Weber, C. O., Gummi-Ztg., 17, 898 (1903); quoted in Bedford and Winklemann’s, “Systematic Survey of Rubber Chemistry”, p. 217 (1923). (15) Whitby, G. S., “Plantation Rubber and t h e Testing of R u b b e r ” , pp.219etsep. (1920). (16) Wiegand, W. B., IND. ENG.C H E M . 29, , 953 (1937).
PRESENTED before the Division of Rubber Chemistry a t the 102nd Meeting of the AMIERICINCEEMICAL SOCIETY, Atlantic City, N. J.
EVALUATION OF THE BUFFER CAPACITY OF CRUDERUBBERS E. B. NEWTON’ AND E. A. WILLSOE’ Malayan Research Laboratories, T h e
B. F. Goodrich
T
HE hypothesis was advanced in the preceding paper that an important factor in determining rate of cure of rubber is the acid-base equilibrium developed during vulcanization, such an equilibrium being determined to a large extent by the nat’ure and quantity of latex serum material remaining in the crude rubber mass. In effect, this serum material-was regarded as a buffer substance. 1
Present address. The B. F. Goodrich Company, Akron, Ohio.
Company, Kuala Lumpur, Federated Malay States
If this were true, it follows that differences in buffer capacities of various crude rubbers should be demonstrable, and in the following discussion two sets of experiments designed with this object in view are described. Experimental In the first series two rubbers were chosen-first latex crepe and a commercial latex-sprayed (made by spray drying am-
March, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
Two sets of data are presented to show that distinctions in the buffer capacities of different rubbers can be measured relatively. In the first set, gradually increasing quantities of phosphoric acid added to a rubbersulfur stock compounded in one case with crepe and in the other with a whole rubber showed that the latter, which contains more serum matter, is less influenced in rate of cure by the acid addition; it behaves as though the whole rubber had greater buffer capacity than the crepe. In the second set, within the limitations of the method described, the serum materials in the series of crude rubbers examined were titrated electrometrically with hydrochloric acid, the results showing that differences exist. Moreover, the rubbers containing serum material which possessed both greater alkalinity and buffer capacity also showed more rapid cure in the A. C. S . Captax test mix and in a 100 rubber-10 sulfur stock. The possibility of using this technique as a means of controlling uniformity in rate of cure of plantation rubber is suggested.
379
The results of the curing tests are shown in Figure 1; time of cure to reach a modulus of 2100 pounds per square inch a t 800 per oent elongation is plotted against quantity of HsP04 per 650 grams of rubber. The addition of a given quantity of acid has far less effect on slowing cure in L. S. (latexsprayed) than it has in crepe; moreover, for any given range of acid additions, the slope of the curve is less for L. S. than for crepe. The L. S. acts as though it were more strongly buffered than does the crepe rubber ( 1 ) . The other series of experiments amounted to an attempt to determine the acid-neutralizing power of the serum material in different crude rubbers. With due regard to whatever limitations may have been imposed by diffusion or by reaction of acid with rubber hydrocarbon, the method employed gave an electrometric titration of the serum material. The procedure employed was as follows: The dry crude rubber was passed four or five times through close-set mill rolls to form a thin crepe sheet about 1/82 inch thick. A 2gram sample in rectangular form was cut out and rolled spirally but loosely into a cylinder without any dusting or parting layer. Generally eight samples (2 grams each) were prepared from each crude rubber tested. They were placed in Pyrex glass test tubes (soft glass must be avoided) and covered with 10 cc. of hydrochloric acid solution, as listed in Table I1 and plotted in Figure 2. A short length of Pyrex rod bent as a V or L was dropped onto the rubber to keep it submerged. A control series of solutions without rubber but with the Pyrex weights was always run. The tubes were closed with corks wrapped in tin foil (glass stoppers would have been preferable). The solutions were not in contact with the metal. The tubes were heated overnight (16 hours) in a
moniated latex)-showing widely different rates of cure in a rubber-sulfur stock. The rubbers were compounded in basic recipe D (100 rubber-10 sulfur), and graded quantities of phosphoric acid were introduced into these batches (Table I). The acid was mixed with the sulfur powder which was then worked into the rubber running with a rolling bank on the mixing mill. The lower concentrations of acid (up to 8 grams HaPO4per 650 grams of rubber) were added a 8 aqueous solutions, with the total liquid volume kept a t 10 cc. or less. Above 8 grams HaPO4 per 650 grams of rubber, the acid was added as 85 per cent sirupy phosphoric acid.
GRAMS H3P04 / 6 5 0 GRAMS OF RUBBER
FIGURE1. INFLUENCE OF PHOSPHORIC ACIDON RATEOF CUREOF FIRSTLATEXCREPEAND LATEX-SPRAYED RUBBERS IN BASICSTOCK D
TABLEI. EFFECTOF PHOSPHORIC ACIDADDITIONS ON CUREOF BATCHES OF RUBBER D" &%/%ch Time of oureb
49
..
%%%%oh Time of oureb
5DA 0.297 49
5DB 0.396 49
5DC 0.495 49
5DD 0.594 4s
0.693 52
5DF 0.792 52
Reci e No. HaP&/batch Time of cureb
5DL 2.376 65
5DM 2.772 67
5DN 3.168 67
5DO 3.960 74
5DR 4.752 77
5DS 6.544 85
D
5DV 0.033 153
5DW 0.066 153
5DX 0.132 155
0
i38
4D9
0.048
49
4DT 0.072 48
SPRAYBD LAT~X RUBBER 4DU 4DV 4DW 0.096 0.125 0.144 47 46 46
4DR 0.024 48
@%:/F$Cb Time of cureb
D
4DX 0.168 47
4DY 0.192 45
4DZ 0.198 49
5DG .0990 52
5DEI 1.188 55
5DI 1.386 52
5DJ 1.584 55
5DK 1.98 58
5DT 6.336 93
5DU 7.920 107
6DF 11.10 127
6DG 12.95 139
6DH 14.80 149
6DI 18.50 167
C R B I PRUBBBR ~ 5DY 5DB 6DA 0.398 0.462 0.264 164 166 165
6DB 0.528 170
6DC 0.660 176
6DE 0.693 186
5DF 0.792 184
6DJ 0.891 192
5DE
Press-cured at 298' F. in aluminum foil. a modulus of 2100 pounds per square inch at 800 per cent elongation.
h To reaoh
5D0 0.990 195
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
380
Vol. 34, No. 3
was by far the fastest in the rubber-sulfur mix, but which proved slower than skim a. rubber 974 and L. 8. rubber 537 in the A. C. S. Captax stock. This crossing over 7. is indicated by the titration curves if the assumption is made that the concentration of acidic materials (for example, hydrogen 6 sulfide, 3) is greater in the rubber-sulfur I stock during cure than in the A. C. S. 5 Captax stock. Inasmuch as the titration data indicate that the buffer capacity of 4 rubber 936 is low relative t o that of rubbers 974 and 537, the higher concentration (as3 sumed) of hydrogen 'sulfide in the rubbersulfur stock during cure would be expected * . o l ~1 1 ~ I ~ 1 ~1 1 ~ 1 ~ 1 1 1 1 1 1 1 1 1 1 1 1 ' to shift the acid-base equilibrium to some I 2 3 5 IOCC. IOCC cc. of .001 1 HCI .002"Cl .005 HCI lower value, possibly lower than the equilibrium in the other two rubbers. (The FIGURE 2. TITRATION OF SERUM MATERIALIN RUBBERS PREPARED BY presence of zinc oxide in the Captax stock VARIOUSMEANS Time of Cureb of Tensile of Recipe may possibly assist, but the action of Recipe D, Min. A M @ , Lb./Sq. In. Rubber N o Treatment zinc oxide in a stock is regarded as someSkim
