INDUSTRIAL AND ENGINEERING CHEMISTRY
February, 1944
Drying of Liquids. Many organic liquids may be completely dried by contact with activated bauxite. The results in Figure 8 were obtained by percolating portions of a light naphtha containing 70 p.p.m. of water through 14-inch bauxite columns each containing 110 grams of 15-30 mesh adsorbent at the rate of 2 gallons per hour. The water determinations were made with Karl Fischer reagent. Within the limits of experimental error, the effluents produced up to the break points were completely dry. The data show that the bauxite activated at 1100” F. produced about 85% as much dry naphtha as the adsorbent activated a t 700” F. LITERATURE CITED
Achenbach, H., Chem. &de, 6, 307 (1931). AlekseevskiI, E. V., and Belotzerkovskiy, G. M., J . Gen. Chem. (U.S.S.R.), 6 , 370, 382 (1936). Blanc, L., Ann. chim., 6, 182 (1926). Bohm, J., 2. anorg. allgem. Chem., 149, 203 (1925). Bower, J. H., Bur. Standards J . Research, 12, 246 (1934). Bragg, W. L., “Atomic Structure of Minerals”, Ithaca, Cornel1 Univ. Press, 1937. Brunauer, S., Emmett, P. H., and Teller, E., J . Am. Chem. SOC., 60, 309 (1938).
Capell, R. G., et al., Oil Gas J . , 41 (B), 37 (1942); Chem. & Met. (1933).
Foote, H. W., and Dixon, J. K., J . Am. Chem. Soc., 52, 2170 (1930).
Fricke, R., and Severin, H., 2. anorg. allgem. Chem., 205, 287 (1932).
Grim, R. E., and Rowland, R. A,, Am. Mineral., 27, 746 (1942); Haber, F., Naturwissenschaften, 13, 1007 (1925). Hackspill, L., and Stempfel, E., Chimie & industrie, Spec. No., 151 (Feb., 1929).
*
Hansen, W. C., and Brownmiller, L. T., Am. J . Sci., 5, 225 (1928).
Higuti, I., Bull. Inst. Phys. Chem. Research (Tokyo), 18, 15 (1939).
Hubbell, R. H., Jr., and Ferguson, R. P., Oil Gas J., 37, No. 27, 135 (1938).
Hubbeil, R: H., Jr., and Ferguson, R. P., Refiner Natural Gasoline Mfr., 17, 104 (1938). Hiittig, G . F., and Wittgenstein, E. von, 2. anorg. allgem. Chem., 171, 323 (1928).
Inst. pesquisas tech., SLo Paulo, SeccLo chim., Bol. 17 (1937). Jaaitsch, R., 2. phusik. Chem., A174, 49 (1935). Joirdain, A., CGamique, 40, 135 (1937). Klever, E., Trans. Ceram. Soo., 29, 149 (1930). Kurnakov, N. Sa, and Urazov, G. G., Ann. inst. anal. phys. chim. (U.S.S.R.), 2, 495 (1924). La Lande, W. A., Jr., IND.ENQ.C H ~ M33, . , 108 (1941). Lapparent, J. de, Compt. rend., 184, 1661 (1927). McGavack, J., Jr. and Patrick, W. A., J . Am. Chem. SOC.,42, 946 (1920).
Milligan, L. H., J . Phys. Chem., 26, 247 (1922). Munro, L. A., and Johnson, F. M. G., Ibid., 30, 172 (1926). Norton, F. H., J . Am. Ceram. SOL, 22, 54 (1939). Orcel, J., Congr. intern. mines, m6t. g6ol. appl., 7e Session Paris, 1935, 1, 359.
Orosco, E., Ministerio trabalho ind. corn., Inst. nac. tech. (Rio de Janeiro), separate, 1940. Parravano, N., and Malquori, G., Atti. accad. Lincei, 7, 970 (1928).
Eng., 50, 107 (1943).
Edwards, J. D., and Tosterud, M., J . Phys. Chem., 37, 483
+
109
Rao, K. S., Current Soi., 8, 546 (1939). Rao, K. S., and Rao, B. S., h o c . Indian Acad. Sci., 6, 16 (1937). Rooksby, H. P., Trans. Ceram. SOC.,28, 399 (1929). Smith, G. F., “Dehydration Studies Using Anhydrous Magnesium Perchlorate” (1935). (38) Weiser, H. B., and Milligan; W. O., J . Phys. Chem., 36, 3010 (1932). (39) Wohlin, R., Sprechsaal, 46, 719, 733, 749, 767, 780 (1913). (40) Yamaguchi, Y., Takeba, T., and Yazawa, T., Bull. Inst. Phys. Chem. Research (Tokyo), 5, 735 (1926).
?SOkUBlLlTY AND DIFFUSION OF SULFUR IN
*
**
Y
Y
G@d%ehc 0
A. R. KEMP, F. S. MALM, AND B. STlRATELLl
The rate -Rof solution of sulfur in eld5omer.s during milling depends on several factors.+The s o w i l i t y and rate of diffusion of sulfur in various synthetic elastomersare determined b y procedures which were employed in a previous investigation in these laboratories. The results show that sulfur i s more soluble in GR-S than in natural crepe rubber above 55’ C. and less soluble below this temperature. The solubility of sulfur in Perbunan, Hycar OR-I 5, and Butyl B-3 i s less than in rubber or GR-S in the order given. The rate of diffusion of sulfur through elastomers decreases in going from natural crepe rubber to Buna S to Hycar OR-15 to Butyl 8-3. The possibilities of heterogeneous vulcanization with synthetic elastomers are increased.
IBell Telephone Laboratories, Murray Hill, N. J. OR-15), 75 butadiene-25 acrylonitrile copolymer (Stanco Perbunan), isobutylene-isoprene copolymer (Butyl B-3) , polychloroprene (Neoprene GN or GR-M), and natural rubber (Rubber Culture Maatschappij Amsterdam crepe). Three grades of sulfur of different particle size and distribution were employed, designated A, B, and C. B and C are commercial grades of ground sulfur used in the rubber industry, and A is a micronized sulfur with an average particle size of 3 to 4 microns according to the supplier. The particle size distribution curves of the B and C samples are given in Figure 1. R A T E OF SOLUTION O N T H E MILL
N A PREVIOUS investigation (1) the solubility and rate
I
of diffusion of sulfur in natural rubber were determined. The importance of these properties in connection with the vulcanization process was also discussed. The present paper presents data on the solubility and rate of diffusion of sulfur in various synthetic elastomers which are being used to replace natural rubber. A knowledge of sulfur solubility and diffusion will be an aid to a more intelligent approach to the compounding problems arising from the broader use of the synthetics. The elastomers studied were 75 butadiene25 styrene copolymer (Buna S or GR-s), butadiene-nitrile copolymer (Hycar
A study was made of the rate of solution of these sulfurs in various types of synthetic rubberlike polymers. T o standardize the milling procedure, the following conditions were adhered to: A standard batch weight of 300 grams of elastomer was given two hand-tight refinings on the laboratory mill rolls whose initial roll temperature was set a t 43” C. (109’ F.). The rolls were then opened to 0.015 inch, and the milling was started. After 3 minutes the rolls were set at a 0.045-inch separation. During the entire mixing the batch was cut twice per minute. The 6-inch mill roll speed for the front roll was 25 r.p.m. and
Vol. 36, No. 2
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
110
rates of solution for three parts of sulfur in Buna S is approximately 1 : 3 : 5 for the 8, B, and C grades, respectively. These data show the large influence of fineness on rate of solution of sulfur in rubber, as might be expected. The rate of solution of sulfur in natural rubber, B u m S,and Seoprene GK is about the same. The rate is appreciably slolwr for the Hycar OR-18 find Perbunan when the higher milling temperatures are considered. The solution rate OF 1.5 parts of sulfur in Butyl rubber is very slow since the temperature required for solution with thif amount of sulfur as riot reached. Plott,ed on Figure 2 are those values obtained for natural crepe and Goodyear Bunn B in which different, grades of sulfur wc’rc hidied. Figure 2.
Rate cC Solution of Sulfur in-E,lastomarsduring Milling at 78’ C. 1, 2.
3.
PARTICLE
Figure 1.
C1ep2 rubber with sulfur A
GR-5 with sulfur 8
Crepe rubber with sulfur
B
SIZE I N MICRONS
Particle: Size Distribution of Sulfur
for the rear roll 35.5 r.p.m., and the distance between the guides was 10.2 inches. I n intervals of approximately 5 minutes the batch temperature was taken with a thermocouple. Sulfur addition was started after 1.0 minutes of milling and required 2 minutes. Samples were removed and observed under a microscope every 2 minutes after sulfur addition until solution was nearly complete, when the observations were made every minute to determine when solution was complete. The data on time required for the solution of sulfur in elastomers on the mill rolls is given in Table I. The ratio of the
TABLE1. RATEO F
Elastomer
SOI,l.?TIoS
O F SULFUR I N
MILL ROILS
Grade of Sulfur
Parts
S/100 Parts Elastomes
EL~STOJIERS ON
Range of Xillinog C.
Temp.,
Time for Soln., Min.
M I L L I N G T I M E IN MINUTES
Figure 3. 1. 2. 3.
Firestone Buna S Hycar OR-I5
C
C
Perbunan
C
Butyl B-3 Neoprene G N
C
a
b
C
3.0
1.5 3.0 1.5 3.0 1.5 1.5 3.0
80 103 104 94 92 80 73 78
*2 *3 6 *4 *6 * 10 d= 2 *4 d=
D a t a from previous publication ( 1 ) . A few sulfur particles remained a t the end of this period.
16
7
21 9
Heat Build-up in Elastomers during Milling Hyear OR-15 Perbunen Crepe rubber
4. 5. 6.
Buna S Neoprene GN
B M W 8-3
Table I shows that the batch temperatures of the various elastomers on the mill varied somewhat. Since this ivas due to differences in internal friction in these materials, a study mTas made of the temperature variation of the batch during milling where all conditions were kept constant, except the type of elastomer used. The data are plotted in Figure 3. The equilibrium batch temperatures of the synthetic elastomers after considerable milling are higher than those obtained for natural crepe rubber.
21
1036
9 18
SOLUBILITY OF SULFUR IN ELASTOMERS
The solubility temperatures for sulfur in the elastomers were obtained, as in previous work ( I ) , by observing the clear-tocloudy transition which takes place in the bank upon cooling
Pebruary, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
g 10 while being masticated on the mill rolls. After I 0 completely dissolving the sulfur, the batch was cooled but not cut on the moving rolls until * e 5 cloudiness became apparentc a t the edges. The W temperatures of the clear and cloudy sections $ 4 in the bank were taken with a thermocouple v) I (Table 11). Several checks were made for each LL 4 elastomer. The slow cooling and constant agita0 2 0 tion in this procedure eliminate the supersatura0 tion effects of the sulfur. p: W P The solubilities a t lower temperatures were v) 1.0 determined by first approximating the solubility $ 0.8 and then making up a series of stocks with small 3 0.6 differences in the dissolved sulfur content on z each side of this approximated value. These 5 0.4 stocks were then placed in constant-temperature rooms at different temperatures for long periods, In and microscopic observations were made. The LL O 0.2 solubility values plot as straight lines on semi* logarithmic paper as shown in Figure 4. c 2 The solubility of sulfur in some elastomers is m 3 0.1 dependent upon the degree of mastication, ino, 3.60 creasing with increased breakdown until the limiting value shown in Table I1 is obtained. This characteristic was observed with natural crepe rubber and with Buna S. Table I11 and Figure 5 record the change in sulfur solubility with mastication of these two elastomers, using 3 parts of sulfur. Discrepancies of sulfur solubility values in rubber recorded in the literature may have been caused by this phenomenon. Solubility values recently given by Williams (9) for Buna S are lower than ours, except at the concentration of 2 parts
111
2
3.50
3.40
3.30
3.20
3.10
3.00
2.90
2.80
2.7a
lo3 Figure 4.
Solubility of Sulfur in Masticated Elastomers
1, Crepe rubber 2. Buna S 3.
4. Perbunan 5. Hvcsr OR-1 5 Buna S (Wllllamr' results)
sulfur. Considering his procedure and the poor miscibility of the polymer and sulfur noted, i t is possible that a slower rate of heating is necessary to establish equilibrium. Therefore. a t high concentrations due to the temperature difference required for solution, higher solubility temperatures are recorded 'than represent the TABLE11. SOLUBILITY OF SULFUR IN MASTICATED ELASTOMERS true saturation temperature. To obtain the true soluParts Parts s/100 s/100 bility temperature by Williams' method, the sample Parts Parts ElastoSoly. ElastoSdY. would have t o remain for a much longer period a t Elastomer mer Temp., C. Elastomer mer Temp., ' C. a constant temperature before equilibrium could be Natural crepe 1.0 14 Firestone 3.0 52.5 1 closely approached. According to Williams' data and rubber" Buna S 2.0 40 Hycar OR-15 0.38 2 4 . 5 1 his plot, the solubility of sulfur in Buna S a t 24.5' C. 3.0 55 1.5 71.5 2 is 1.5%. Our constant-temperature room experiments 4.0 706 3.0 92 A 1 5.0 78.5 on several samples set this value a t 1.0%, established 6.0 88 Perbunan 0.4C 24.5 k 1 GoodyearBunaS 1.OC 24.5 1 1.5 56.5 * 1 after one-month standing. 2.0 43 1.5 3.0 79.5 2 Since Williams master-batched the sulfur and Buna 3.0 54 1 Butyl B-3 1.5 93 3 4.0 64.5 1 NeooreneGN 3.0 54.5 * 1 S, an experiment was conducted to determine the 5.0 70.5 1 effect of this variable on solubility. Goodyear Buns a Data from previous publication (I). S was master-batched with 15 parts of sulfur C b Redetermined, 68 lo C. 6 Constant-temperature room experiments. a t 125' C. for 40 minutes in accordance with
-
f
f
f
f f
f
f
f
f f
TABLE 111. EFFECT OF MASTICATION ON SOLUBILITY OF 3 PARTS SULFURIN CREPERUBBERAND BUNAS
Elastomer
Time of Mastication, Min. Before S After, S addition addition
Temperatures,
Max. batch, during milling
C.
Soly. or transition
8 minutes were required for corn lete solution of 3 parts of sulfur C in this slightly broken-down crepe at &e high temperature. 50 minutes were required for complete solution of 3 arts of sulfur C in this highly broken down crepe; ordinarily 20 minutes (Tatle I) are sufficient.
TIME OF MASTICATION IN MINUTES
Figure 5.
Effect of Milling on Solubility of Sulfur in Crepe Rubber (Curve 1) and in Buna S (Curve 9 )
112
INDUSTRIAL AND ENGINEERING CHEMISTRY
DlFfUSlON
Figure 6.
Vol. 36, No. 2
PERIOD IN HOURS
Rates of Diffusion of Sulfur through Elastomersat 86"C. 1. 0.
Crepe rubber
Buns S
3.
4.
Hvcar OR-1 5
Butyl B-3
Williams' procedure. The mixture was cloudy at the end of this time. Milling with cold water through the rolls was continued for 15 more minutes, a t which time the batch temperature had fallen t o 66" C. The master batch was then removed from the rolls. Additional Buna S was added to a portion of the master batch so that the total mixture contained 3 parts of sulfur in 100 parts of Buna S. This mixture was milled for 30 minutes at 80 * 7" C. ; then cooling water was turned on in order to determine the saturation temperature. No difference from the value in Table I1 was obtained. RATE O F SULFUR DIFFUSION THRQUGH ELASTOMERS
The rates of sulfur diffusion through masticated Goodyear Buna 8, Hycar OR-15, and Butyl B-3 were measured a t 86" C., using the diffusion cell and procedure described previously ( I ) . To obtain smooth sheets, the elastomer was milled 10 minutes and molded between cellophane in a steam-heated press. Small amounts of volatile matter caused the early diffusion values for Buna S and Hycar OR-I5 to be high. Blanks were run and the original values were corrected. The experimental data obtained by the method of the previous investigation ( I ) were corrected to apply to the diffusion rate through a specimen 1 sq. em. in area and 0.030 cm. thick. The
Transformation of Sulfur in Buna S from Dendritic to Rhombic Form (X 45)
Figure 8.
data obtained for the different elastomers are plotted in Figure 6. The diffusion constant D or specific diffusion rate was calculated from Fick's law:
D
where N = z
= =
p
=
A t
Figure 7.
Undissolved Sulfur in Buna
S (X 45)
=
Nx/Atp w i g h t of sulfur lost, grams thickness of rubber sheet, em. area of rubber exposed to diffusion, sq. cm. time of diffusion, hours vapor pressure, mm. =
The calculated diffusion constants are given in Table IV. It is Seen that the rates of sulfur diffusion in the elastomers studied decrease in the order natural crepe rubber, Buna S, Hycar OR-15, Butyl B-3.
INDUSTRIAL AND ENGINEERING CHEMISTRY
February, 1944
C R Y S T A L L I Z A T I O N B E H A V I O R OF SULFUR IN E L A S T O M E R S
Microscopic observations made on a mixture of Goodyear Buna S containing 3 parts of sulfur may be divided into separate categories. If, before complete solution, the sulfur-elastomer mix is allowed to cool and remain at room temperature, the excess dissolved sulfur will recrystallize on the undissolved particles, which results in the production of large crystals in the batch (Figure 7).
Heterogeneous vulcanization involves the incomplete solution or local high concentration of sulfur or accelerators in rubber during vulcanization as previously shown. With a micromold (Figure 10) i t was possible t o determine comparative rates of resolutionof different size sulfur crystals in natural rubber. The micromold was brought to a temperature of 141.5' C. and loaded with the sample of crepe rubber containing the fine sulfur crystals previously described. Observations were then made under a microscope at a magnification of 40 diameters. These small crystals required only 1.5 minutes for complete solution. The larger crystals previously described, which separated upon standing a t room temperature, required 13 minutes to dissolve at the same temperature, a difference of about ten times. It is this difference in rate of re-solution, which is dependent upon the solubility, and the rate of diffusion of the sulfur in the elastomer that will determine whether a heterogeneous or a homogeneous vulcanizate will result.
TABLE IV.
CALCULATED SULFURDIFFUSION CONSTANTS FOR ELASTOMERS AT 86" C.
Elastomer Nstural crepeo
Figure 9. Fine Sulfur Crystals in Buna S (Oblique Illumination, X 45)
Bum €3
Hycar OR-15 Butyl B-3
Sheet Thickness, Cm.
Av. Wt. Loss, G./Hr.
0.0305 0.0312 0.0429 0.0305
0.000160 0.000111
PYREX GLASS WINDOWS?
FIBERGLASS4 NSULATED
Diffusion Constant Db G./Hr./Cm./Mm. Hp'
01000070
0,000064
Data fromprevious ublication (I). h Area of diffusion celrwas 5.06 sq. om. is 2.2 X 10-8 mm. H g (1).
If, after complete solution, the mix is slowly cooled to and held at room temperature, the sulfur precipitates in the dendritic and rhombic states. Transformation from dendritic form into large rhombic crystals takes place upon standing as shown in Figure 8. If, after complete solution, the mix is milled below its solubility temperature, the sulfur precipitates as numerous, very fine, uniformly dispersed rhombic crystals. This pattern (Figure 9), which causes the cloudiness visible to the naked eye, is retained upon standing. Blooming is completely eliminated. Heating during vulcanization causes these very small crystals to redissolve quickly, which is favorable for the formation of a homogeneous vulcanizate. If, after complete solution, the mix is chilled in ice water, the sulfur precipitates in masses of dendrites. Short storage periods at room temperature will not alter tk;s sulfur pattern, which dissolves quickly upon heating and yields a homogeneous vulcanizate.
113
4.4x 3.1 x 2.7 x 1.7 x
10-4 10-4 10-4 10-4
Vapor pressure of sulfur a t 86' C.
CONCLUSIONS
1. The rate of solution of sulfur in various elastomers during milling depends upon the temperature of the elastomer, solubility, and rate of diffusion of the sulfur in the elastomer, particle size and type of sulfur used, and degree of breakdown of the elastomer. 2. The relative heat build-up on the mill rolls for each elastomer after 30-minute milling increases in the order: natural crepe rubber, Butyl, Buna S, Neoprene GN, Perbunan. Hycar OR-15. Conclusions as to extent of cross linkage, mutual attraction of the chain molecules, and plasticity are indicated from data of this type. 3. The solubilitx of sulfur for the several elastomers increases with the degree of mastication, as shown in the cases of natural crepe and Buna S, until a limiting value is obtained. 4. The specific diffusion rate of sulfur through various elastomers has been determined. These rates of diffusion decrease in the following order: natural crepe rubber, Buna S (GR-S), Hycar OR-15, Butyl B-3. 5. I n synthetic elastomers the change in sulfur solubility with increased temperature is greater than in natural rubber, the solubility is less a t room temperature, and the rates of diffusion are slower; therefore the possibilities of heterogeneous vulcanization with these elastomers are increased. ACKNOWLEDGMENT
The authors are indebted to R. H. Walcott of the Stauffer Chemical Company for the particle size analysis of the sulfur samples. LITERATURE CITED
(1) Kemp, A. R., Malm, F. S., Winspear, G. G., and Stiratelli, B., IND.
[email protected].,32,1075 (1940). (2) Williams, Ira, India Rubber World, 108,35 (1943).
TRAN~SITE Figure 10.
THERMOCOUPLE
Micromold for Observation of Sulfur in Elastomers during Heating
PRBY~I#NTSD before the fall meeting of the Division of Rubber Chemistry of the AMERICAN CH~~MJC SOCIETY, AL in New York, N. Y . . 1943.
