JMUUY.
1941
I N D U S T R I A L A N D EN 0 INKERIN 0 C # E M I S T R Y
(€9 Booth. E. W.. I& Rubber W&, aS.M W..1930). (4) Buass. W. F.. IND. Exa. Cnmu.. 28,1194 (1934): Rubber CAm.
TML.8. iai ( 1 % ~ ) . (SI C d d . 8. M.. M d .R. A., Bloman. C. M.,and Yost, F. L.. h. EUQ.b. hfi. . ED.. 12. 18 (1940): Rubber C h T&. 13, 304 (1940). (6) C d d . MedU. Eloman. and Yo& h. Eaa. CRBU.,33,370 (1941): *R Ch-. "4. 14,. 878 (1941). nd N - h . W. J. 8.. Tlotu.I d .
Rv&w Chmm. Tkh.. 4, 184 (1931). Eochea, 8.. Gbn*M.4 , l (Jsn.-Feb.. 1940): R u b k CAm. Tkh.,
13.688 (1940). 0r.y. H.. Kamb. A. 8.. and HUn. R. J.. IND.EUQ.*&mu., AXAL. ED., 6. a86 (1934). Gmta. H. W.,Rubber Aw (N. Y.),38,3%7(1936). Hopkim, 0. E., India R u b b Wmld. 88,34 (8ept.. 1983). Liaks. J. W.. Firestone Tim and Rubbe Co.. u o u b . oommuni-
33
(14) Nul. A. M., .nd Northam, A. J.. h. ma.Carnu.. 23, 1449 (1831): RC A ~ T&., . 5. QO ( 1 9 3 ~ . (16) Nellsn, A. H.. RUMMI A@ (N. Y.),24,873 (1929). (16) Patrikssv. 0. A,. and MslniLov. A. I., C w and RVWV m.8.8.R.). De&. 1940. 1% R&a climr. Tkh.. 14 %S8 (1941). (17) Rehie& E. T..and Qeake. R. H., h. &a. Cnmu..&A&. ED., 7. 388 (1986): RUMMI C h . Ted., 9, 178 (1936). (18) &W.L. B.. and Dimmore, R. P.,Inlh Rubber World,10% 46 ( A i l . 1941). (19) 8omSrville. A. A,. T?-. I d . Rnbber I d . . 6, 130 (1890). (20) 8-t. J. N.. and E M H. L.. Rubber Chm. T d . , 14, 111 (1941). (B) Tomanoe, P. M..and Petamn, L. C.. Indk R u b b 82 (Juls, 1928); Rut
TEMPERATURE COEFFICIENT OF VULCANIZATION OF
*
Tempw tb of vulcanization been dcbnnimd fw tmfve Burr S rtodcr the temperature range 970' to 30% F. average values obtained from phyliaal test
Y
h
c r a ternperi re change of I O o C. a temperature change 01 10' F. n&ng values obtained from cornweIy 9.06 and 1.49, mweativdy.
Y The only ubliaheed mention of the &eot of temperatun on Buna 8 W M in a rshase fmm the office of the Rubber Dir+ar, giving tablea for wnvemion of cum to a atandsrd bemperatn#a (Sf). These tshlea w bMed on a t e m p a h dcisht of 1.Q per 10' F. The l o mof thia informaticm im not available, hqwsMI.. EftXClOFlEMPERA~
This investigation WM devotnd to a atudy of the eE& of bemm t u m on the rata of chemlaal combination M weU M the Mt4 bf change of physical p @ e a of three relate3 Buna 8 reoipS (Table IJ:
INDUSTRIAL A N D ENGINEERING CHEMISTRY
34
Vol. 36, No. 1
/
/ O
/
I
0
Ep
PI
50
100
T I M E OF CURE IN MINUTES
Fi ure 1. Typical Elon ation-Time Curves (toft Black Stock, 3 Per ?ent Added Sulfur)‘
mixer, and all ingredients except the carbon black and @fur were incorporated a t this time. This master batch was divided into three portions. One portion was retained for use as the pure g u m stock; the other two were blended with the requisite portions of pigment and stored before use. The sulfur was incorporated into each stock shortly before use. The proper quantity of sulfur was weighed on an analytical balance and milled into the master batch of stock on a 10-inch mill. The temperature of the platens in the steam-heated presses was checked with a calibrated thermometer and a mercury block. The cured stocks were stored between sheets of heavy parchment gaper before testing. Stress-strain roperties were determined y the A. S. T. M. prqcedure. T o t 3 sulfur was determined by the Cheyney modification of the A. S. T. M. method (9). Free sulfur was determined by a semimicro modification of the method of Oldham, Baker, and Craytor (H).Combined sulfur was calculated as the difference between total sulfur and free sulfur, as determined analytically except for modifications noted below. For comparative purposes, the combined sulfur data for all three stocks were calculated as coefficient of vulcanization (per cent sulfur combined with 100 parts polymer as compounded). The ori ha1 polymer batch contained an appreciable quantit of s d u r (0.25 per cent). The presence of this sdfur and an in4cation of its origin were considered previously (IO). For the calculations re orted here, this “blank ’ sulfur was assumed to be present in tge final vulcanizate entirely in the combined form. the analytical data were corrected accordingly. It was likedise necessary to correct for the sulfur originating in the accelerator, which is 38.57 per cent of the accelerator content, or 0.68 per cent sulfur per 100 parts polymer in the stocks investigated; this type of sulfur was assumed to undergo no chan e during vulcanization. Neither of these assumptions may e! entirely justified. However, a considerably more refined analytical technique, as well as a larger amount of experimental work, would be necessary to evaluate the changes in character of these two types of sulfur during vulcanization.
IO0
50
TIME O F CURE IN MINUTES Figure 4. Typical Combined Sulfur-Time Curves (Pure Gum Stock, 3 Per Cent Added Sulfur)
more convenient method is to plot time w i n s t temperature on semilog coordinates; the temperature coefficient may then be obtained by the direct ratio of the two time values taken from the straight line obtained. The selection of standard properties for the various stocks required some experimentation. Park, Sandstrom, Morris, and Eliel felt that modulus (load at a fixed elongation) was more reproducible for natural rubber than either ultimate tensile strength or elongation. Attempts to use modulus measurements for these calculations, however, failed to achieve satisfactory results. I n both the pure g u m and soft black stocks, but especially in the former, the modulus was so low that the error introduced by this procedure was rather large. The channel black stocks had moduli of sufficient magnitude, but when plotted, several of the points failed to fall on or near the necessary straight line. The ultimate elongation-time curves finally chosen as standard had the best-defined shape of any of those investigated. I n all stocks the ultimate elongation rapidly reached a maximum value (in some stocks the maximum was already past in the shortest cure) and then decreased almost as rapidly, finally leveling off at a rather constant value. Actual trial showed that the selection of some arbitrary elongation value on the relatively steep portion of the descending part of the curve led to satisfactory results. It is considered likely that the ascending portion of the curve might have proved fully as useful, but this portion was missing in a number of the plots. Furthermore, the relative error in reading a time value in this early portion of the curve is considerably greater. Figure 1 shows a typical set of elongation-time curves; the dotted line illustrates the standard property chosen.
TYPES OF CURVES
The experimental data were plotted against time of cure in the standard way. Inclusion of all forty-eight sets of curves would be impossible, so only the significant data are presented here. The general method of calculating temperature coefficient is based on plotting the log of the time necessary to reach some standard property against temperature; the temperature coefficientis then the antilog of the slope of the resulting curve. A
TABLE I. STOCK RBCIPES Buns 8 Zinc oxide Stearic acid Dibensothiasyl diaulfide Bardol (softener) Channel black Fine thermal blaak Sulfur
100 5 ...
100 5 1 1.5 5 60
V&&ble
Viriable
5
1 1.6
100
5 1
1.5
6 ...
60 Variable
,
’
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1944
I n this connection, the Government memorandum referred to (31) stated that good elongation is the most important charac-
I
1\2-3%S
2 v)
PHYSICAL
OATA
10 -5-
r
2
TEMPERATURE
-
0
OF.
teristic for GR-S tread compounds. Also, elongation was employed in the form of the well-known T-50 test in the investigation of Gerke (16) on the effect of reclaim on temperature coefficient. Here i t showed good agreement with results obtained from modulus and free sulfur data. There was no similar choice of properties for the analytical data. It was necessary to choose some arbitrary percentage of combined sulfur which was within the ascending portion of the curves over the entire temperature range studied. Figure 2 illustrates a family of these combined sulfur-time curves, with the standard property chosen in this case. Table I1 lists the values obtained by this graphical method for the various standard properties, with the property used for each determination. The corresponding log time-temperature plots are given in Figure 3. Table 111lists the temperature coefficients obtained from the slopes of these curves. The values for 10" C. were calculated from those for IO" F., using the relationship: K$.' = K 0. While an effort was made to choose standard properties that would yield satisfactory straight lines, those chosen are not the only ones which would yield the desired results. For instance, Figure 4 gives curves obtained with 600 and 800 per cent elongation, respectively, as the standard properties. The agreement is well within the limits of experimental error. The use of modulus instead of elongation in the cases where the modulus values show
.
100
TABLE 11. 1%
2%
3% PHISICAL
35
OATA
l d % S
5%
'rIMES TO
REACH STANDARD
270 280 290 300
PROPBRTIES Channel Black Stacks, Min. 600% E. 0.50% S 100 80
72 88 30
50 40 23
270 280 290 300
600% E. 1 00% S 81 128 85 53 66 41 26 38
700% E. 1.00% S 07 60 34 40 20 30 20 12
4 5 0 p E. 1.00% S 1 0 82 9s 50 80 37 57 22
270 280 290 300
500% E. 2 00% S 80 134 .55 93 37 64 26 40
500% E. 1 50% S 69 40 47 25 36 18 20 12
450% E. 2.00% 87 94 65 60 43 47 27 28
270 280 290 300
500% E. 3.00% S 58 82 39 39 26 39 20 24
350% E. 4 5070 S 70 160 47 102 35 76 25 50
250% E. 3.50% S 110 80 74 50 64 40 36 22
Q;E. = elongation. 0
TFMPERATURE
TABLE 111.
-OF.
Added Sulfur, %
TEMPERATURE COEFFICIENTS 7-Chemical10' F. 10°C. ----Physical~
loo F.
loo C.
Pure Gum Stocks
1 2 3 5
cn
Av.
w 2 I- 20 3
2.09 1.95 1.95 1.93 1.98
1.45 1.49 1.49 1.52 1.49
1.97 2.04 2.04 2.13 2.05
1.52 1.53 1.47 1.47 1.50
2.07 2.17 2.00 2.00 2.07
Soft Black Stocks
z
=
1.51 1.45 1.45 1.44 1.46
2 3 5
IO
Av.
1.42 1.43 1.44 1.44 1.43
1.88 1.90 1.92 1.92 1.91
Channel Black Stocks
1 2 3 5
2
TEMPERATURE Figure 3.
-
0 O F .
Log Time-Temperature Curves ( A ) Pure gum stocks, ( B ) soft black stocks. (C)channel black rtocks
Av. Av. for d l stocks Govt. figure (81) Natural rubber with same accelerator (18)
1.47 1.45 1.47 1.44 1.46
2.00 1.94 2.00 1.93 1.97
1.48 1.48 1.51 1.48 1.49
2.03 2.03 2.09 2.03 2.05
1.45 1.43
1.95
..
1.49
..
2.06
1.50
2.00
1.52
2.08
..
s
INDUSTRIAL A N D E N GINEERING CHEMISTRY
36
I
I
TEMPERATURE
-
TEMPERATURE
- OF.
3 0
F.
the mechanism of reinforcement bears no direct relation to that of vulcanization. Unlike much of the reported data for natural rubber vulcanizates, there is no significant difference in the values obtained from physical and chemical data. However, Morris (I??), using the same accelerator in natural rubber stocks, obtained only a slight difference in the values from the two sources. It is worth noting, however, that here, as in all the reported work with natural rubber, the values obtained from chemical data were somewhat larger than those obtained from physical data, even though the differences were slight. The constancy of the values obtained over a fivefold variation in sulfur content and with a corresponding variation in sulfuraccelerator ratio is also a striking result. It is remarkable that the values obtained were in such close agreement with the arbitrary value obtained from the Government memorandum (51).
0
Figure 4. Curves Obtsined with 600 and 800 Per Cent Elongation (IS Standard (Pure Gum Stock, 1 Per Cent Added Sulfur) Figure
Vol. 36, No. 1
5. Comparison of Modulus and Elongation Values (Channel Black Stock, 3 Per Cent Added Sulfur)
good agreement with one another yields values of the same order of magnitude; this is illustrated for a typical stock in Figure 5. ACCURACY OF VALUES REPORTEID. The final value in each case is obtained by a combination of two graphs. I n some of these the points fall almost exactly on the curve and define the latter with considerable accuracy. I n others there is some variation, and the investigator is forced to exercise some judgment in selecting the proper path for the curve. I n the log time-temperature curves, which tend to average out some of the variations of the original experimental ones, a difference of 0.01 in the slope of the curve is relatively easy to obtain. This leads to a difference of 0.04-0.05 in the temperature coefficient calculated from it, or about 2.5 per cent variation in the absolute values. I n a semilog plot such as those reported here, the error may be slightly less but varies considerably, depending upon which portion of the line is used for the determination. The higher values give coefficients with greater accuracy, but these values are more difficult to read from the semilog plot with accuracy. As a result the error tends to average out over the entire range. The over-all error is probably within 5 per cent and the values are considered accurate to this extent. C O M P A R I S O N WITH O T H E R D A T A
Comparison of the data with previously reported values for natural rubber stocks emphasizes the remarkable agreement in the magnitudes of the values obtained. There is evidently a close similarity between the mechanisms of vulcanization of the two elastomers, a t least in so far as the effect of temperature is concerned. Agreement with the values of Morris (I@, who used the same accelerator, is unusually good. Although similar temperature coefficients were obtained from both physical and chemical data, at any given temperature the rate of chemical combination of sulfur and polymer is not directly related to the rate of change of physical properties. The combined sulfur approaches a limiting value with no further change, whereas a property such as ultimate elongation goes through a t least three types of change: (1) rapid increase to a maximum, (2) relatively rapid decrease from this value, and (3)leveling off to a fairly constant value. Another interesting feature of these results is the fact that, within the limits of experimental error, the same values were obtained in the presence or absence of reinforcing pigment. This may be regarded as evidence, although not conclusive proof, that
ACKNOWLEDGMENT
The major portion of the work upon which this paper is based was performed at the University of Akron. It was made possible by a fellowship grant to the junior author by The Firestone Tire and Rubber Company. This investigation was suggested by 0. D. Cole. Several other members of the Firestone organization contributed valuable suggestions and criticisms, especially J. R. Rafter, R.F. Dunbrook, and R.Wands. LITERATURE CITED
(1) Am.SOC. for Testing Materials, Standards on Rubber Products Designation D15-41 (1941). (2) Ibid., Designation D412-41 (1941). (3) Anonymous, India Rubber World, 106, 458, 575 (1941); 107, 37 (1942). (4) Bourne, India Rubber J.,45, 120 (1913). (5) Bourne, India Rubber World, 33, 93 (1905). (6) Brackley, Rubber Age (N. Y . ) ,54, 331 (1943). (7) Carlton and Reinbold, India Rubber World, 108, 141 (1943). (8) Carlton and Reinbold, Rubber Age (N.Y.), 52, 29 (1942). (9) Cheyney, IND.ENQ.CHEM.,ANAL.ED., 15, 164 (1943). (10) Cheyney and Robinson, IND. ENQ. CHEM.,35, 976 (1943). (11) Cranor, India Rubber World, 61, 137 (1919). 51, 483 (1942). (12) Drogin, Rubber Age (N. Y.), (13) Eliel, Trans. Inst. Rubber Ind., 12, 161 (1936). Fisher, IND. ENQ.CHEM.,31, 1381 (1939). Garvey and Freeze, Ibid., 34, 1277 (1942). Gerke, Ibid., 31, 1478 (1939). Jones and Snyder, India Rubber World, 108, 137 (1943). Morris, IND. ENQ.CHEM.,25, 1400 (1933). Morris, James, and Werkenthin, Ibid., 35, 864 (1943). Morris, Mitton, Montermoso, and Werkenthin, Ibid., 35, 646 (1943). Neal, Rubber Age (N. Y . ) ,53, 31 (1943). Oldham, Baker, and Craytor, IND.ENQ.CHEM.,ANAL.ED., 8, 41 (1936). Park, IND.ENO.CEIEM.,22, 1004 (1930). Park and Maxwell, Ibid.. 24, 148 (1932). Rossem, A. van, C m m u n . Netherlanda Gout. Inst. Advisiw Rubber Trade and I d . , V I , 179 (1917). Sandstrom, IND.ENQ. CHEM.,25, 684 (1933). Sebrell, Ibid., 35, 736 (1943). Sebrell and Dinsmore, India Rubber World, 103, No. 6 , 37; 104, No. 1, 45 (1941). Sheppard, Ibid., 80, 56 (1929). Shegpard and Krdl, IND.ENQ.CHEM.,14, 951 (1922). Simmons, I n d k Rubber World, 107, 497 (1943); Rubber Age (N. YJ,52, 305 (1943). Spence and Young, Z . Chem. Ind. Kolloide, 11, 28 (1912). Street, Vanderbilt Rubber Handbook, p. 65 (1942). Street and Dillon, J . Applied Phya., 12, 45 (1941). Street and Ebert, Rubber Chem. Tech., 14,211 (1941). Twiss and Branier, J . SOC.Chem. Ind., 39, 125T (1920). Vila, IND.ENQ.CHBX.,34, 1269 (1942). Williams, in Davis and Blake’s “Chemistry and Technology of Rubber”, A. C. 8. Monograph 74, Chap. VI. New York, Reinhold Pub. Corp., 1937. PRESENTED before the fall meeting of the Division of Rubber Chemistry, AMZRWANCHEMICAL Soarmy, In New York, N. Y., 1943.
