INDUSTRIAL A X D ENGINEERING CHEMISTRY
September, 1927
(containing 1 per cent alcohol) and the solution filtered; 35 cc. of this solution were added to 100 cc. of a 2 per cent solution of purified two-phase rubber in carbon tetrachloride or chloroform and the mixture was shaken. The solution gradually became darker and in about 5 minutes deposited a brownish jelly. This mixture when placed in 150 cc. of 95 per cent alcohol turned white as the FeCls split off, and deposited a fibrous, crumbly material. The product was broken up and washed with alcohol until the washings were no longer yellow and then extracted in a Soxhlet, first with alcohol, and then ether The final product was a white, crumbly powder insoluble in all reagents. The yield was quantitative.
1037
from solution in ether. The product was dried in vacuo (1 mm.) a t 85" C. During the drying the rubbery mass swelled to a spongy solid which, when completely dried, could be powdered. It formed a snow-white powder which oxidized very rapidly in the air with formation of a yellow color. Analysis: 0.1008 gram substance: 0.1048 gram H10 and 0 . 3 2 1 8 gram CO 7uC % H
(CsHd,
Calcd.: Found:
88.15 87.06
11.85 11.65
The polymer is very soluble in ether and in the usual rubber solvents, in all of which it gives non-viscous solutions. It is insoluble in alcohol or acetone. The compound softens a t Analyszs. 0 1050 grdm substance 0 1124 gram HzO and 0 3365 gram COz %C "oH c> Ash 110" C., becomes pasty at 120" C., and melts with decompo(CsHs), sition above 135" C. I t therefore differs from the soluble Calcd 88 15 11 s5 Found 87 42 12 00 0 26 polymer obtained from natural rubber a t room temperature. POLYMERIZED IsoPREm-Isoprene was polymerized by Diffusion rubber behaved in the same way. SbC16 gave a heating in a sealed tube for 7 months at 50" C. The rubbersimilar product. like mass was treated in benzene solution at room temperature Reactions with Isoprene, Polymerized Isoprene, and with SnCI4 and alcohol as described under (A). It gave a Balata red addition product which, after splitting with alcohol, IsoPREruE-Twenty-five cubic centimeters of pure isoprene formed a fine, white powder that was completely qoluble in (b. p. 33-34" C.) prepared by cracking dipentene in V U C U O ~ ~ benzene a t 0" C. This polymer softened at 175-180" C. were mixed with 25 cc. of pure dry benzene under nitrogen in The product was not entirely oxygen-free. a flask fitted with reflux. A solution of 4.6 cc. of anhydrous .4nal%sis: 0 1060 qram substance 0 1052 gram €I20 and 0 3287 pram COa SnCld in 10 cc. of benzene was then slowly added through a 0 ' c "IC H dropping funnel. I n about 5 minutes the solution turned (CsHd, Calcd.: 88 15 11 85 Found: 11.12 84 57 yellow and began to evolve heat. The reaction was allowed to continue for about 1 hour, during which time the solution BALATA-B~~B~R which had been purified by freezing out reached 50" C. and became deep brown. It was filtered from gasoline solution followed by acetone extraction reacted from a small amount of insoluble matter through a cotton similarly at room temperature with SnC14, giving a white plug and precipitated with 300 cc. of 95 per cent alcohol. powder completely soluble in cold benzene. There was no A flocculent white precipitate first separated, which imme- indication of a second hydrocarbon phase. The polymer diately coagulated to a glistening rubber-like mass. The softens at 200" C. and melts with decomposition at 250" C. supernatant liquid was poured off, and the mass washed with alcohol. It was reprecipitated three times from benzene A n a l y s i s : 0 1004 gram substance 0 1040 gram HzO and 0 3209 gram COz %'oc % H R Ash solution with alcohol, followed by precipitation with alcoho! (CsHd ~
11
Staudinger and Klener, Ber , 44, 2215 (1911)
Calcd. : Found:
88.15 87.17
11.85 11.61
0.12
Some Accelerator Characteristics as Revealed by Coefficients of Vulcanization' By A. F. Hardman and Frank L. White KELLY-SPRINGFIELD TIRE COMPANY, CUXBERLAND, hID.
When a simple unaccelerated mixture of rubber and N THE earlier days of proportional to the time of sulfur is vulcanized at constant temperature, the comrubber c h e m i s t r y t h e heating up to a point where bination of sulfur proceeds at a uniform rate to a point coefficient of vulcanizathe free sulfur approaches exwhere the sulfur approaches exhaustion. Where ortion-that is, the percentage haustion. ganic accelerators are used, the rate of combination of chemically combined sulfur With the advent of organic usually decreases as the cure proceeds, in some cases calculated to 100 parts rubber accelerators it was early realbecoming almost zero. Various accelerators were hydrocarbon-was considered ized that the old idea that examined in respect to this property of persistence duran index of considerable imcorrectly vulcanized rubber ing cure and some significant differences are pointed Dortance in determining the should have a coefficient of state of cure. Furthermore, out. 3 to 5 was no longer true. various investigators* estabIt was further discovered that lished the fact that in the ordinary types of soft rubber different accelerators, even when used in proportions that mixtures vulcanized at constant temperature for different would give equivalent physical cures under the same conperiods of time the coefficients of vulcanization are directly dition of vulcanization, did not produce the same coefficients of sulfur combination.3 I n fact, variations were so wide that 1 Presented before the Division of Rubber Chemistry a t the 73rd Meetthe coefficient of vulcanization as an index of cure has been ing of the American Chemical Society, Richmond, Va., April 11 t o 16, 1927. * Whitby, "Plantation Rubber and Testing of Rubber," p. 313, Long- generally discarded.
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mans, Green & C o . , London.
* Shepherd and Krall. THISJOURNAL,14,951 (1922).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1038
A few observers4 have mentioned the fact that when certain accelerators are used the vulcanization coefficients of a series of cures plotted against the time of cure fall, not on straight line, but on a curve concave to the time axis. An especially notable example of this change of direction of the combined-sulfur curve, as affected by the metallic xanthates, is shown by Twiss and tho ma^.^ They point out
Yol. 19, x o . 9
and observe that it is their experience that all ultra-accelerators in the classes of disubstituted dithiocarbamates or corresponding thiuram sulfides, or salts of the dithiocarboxylic acids and the corresponding dithioacyl disulfides, are active but for a limited time during the cure. While the ultra type affords the most striking example of what have been termed “fugitive” or “transient” accelerators, nearly all organic accelerators exhibit this property to a greater or less extent and since the number of accelerators now available is rather large and so little has been published in regard to their “persistent”or “transient”qua1ities during the cure, it was thought that a series of comparisons of these properties of several of the commonly used accelerators would be of interest and practical value to the rubber compounder. Experimental Method
The simple formula-rubber 100, sulfur 5, and zinc oxide used as the base, to which was added such an amount of each accelerator as would give fairly equivalent cures and would show a tensile maximum within the curing range selected. Blended smoked sheet was the rubber used. Slabs about 1/8 inch (3 mm.) thick were cured in the steamheated platen press, usually at intervals of 15 minutes to 2 hours. The physical properties were determined on a Scott testing machine, two or more test pieces being broken from each slab and the best value taken as the tensile strength. The combined sulfurs were determined by difference. Samples 5 parts-was
M ~ E I U ~Cure P S@ Figure 3.
I
< $
4-
-5
T \
;s:
3-
ZS8OR.
A-Merca tobenzothiazole, 0.5 Part: B-“808,” 0 . 8 P a r t per 100 Rubber
were extracted overnight and the free sulfur was oxidized with bromine and determined as barium sulfate in the usual way. By deducting the values obtained from the total sulfur added to the compound, the figures for combined sulfur were obtained. The method is subject to certain errors, in that no allowance is made for the small amounts of sulfur that may combine with the zinc or materials other than the rubber hydrocarbon, nor for the sulfur which is a constituent part of certain of the accelerators themselves; but since the chief interest in this investigation is the shape of the sulfur curve relative to the time axis, and not the degree of sulfur combination at any particular cure, the method selected is quite accurate enough to give consistent comparative values. Results
Twelve different accelerators, most of them well known, were examined in the manner described. The results are graphically represented in Figures 1to 5, where both the combined sulfur (continuous lines) and the tensile at break (dotted lines) are plotted against the time of cure. I n the case of the tensile curves each unit of the scale on the left represents
INDUSTRIAL AND EXGINEERING CHEMISTRY
September, 1927
1000 pounds per square inch, while the corresponding values in kilograms per square millimeter are shown on the right. All other physical tests, such as elongation a t break and load a t certain fixed elongation, have been omitted in order t o avoid tedious and space-consuming tables. It should be mentioned that, when mixing the compounds containing as accelerators the lead and zinc salts of dithiofuroic acid,
ization. The falling off with diphenylguanidine (Figure 2) is relatively uniform, the sulfur curve being a fairly regular arc; but with the other semipersistent accelerators it is more rapid during the earlier than the later part of the curing period, the sulfur curves showing a deflection in the early part. The sulfur curve for tetramethylthiurammonosulfide (Figure 4) is less concave than that for the corresponding disulfide.
1
MieufPL
1039
4
C 4 r e @ 239%
Diethyl Dithiocarbamate 0.2 Part. BTetramethylthiuramrnonosulfide 0.125 Part: C2Tetram;thylthiuramdisulfide, 0.125 Part per 1bO Rubber
Figure 5. A-Zinc Dithiofuroate, 0.2 Part; B-Lead Dithiofuroate, 0.3 Part; C-Zinc Ethylxanthate, 2.5 Parts per 100 Rubber
0.5 per cent cottonseed oil was added to each batch to promote accelerator dispersions. The accelerator and oil were mixed together to form a paste before being added to the rubber batch. Without such treatment these accelerators have a tendency t o stick on the rolls of the mill and form lumps which cannot subsequently be broken up and therefore cause spotting when the mix is cured.
The corresponding zinc dithiocarbamate shows about the same initial rate of cure as the disulfide, but does not fall off a t the same rate as the latter. The lead salt of dithiofuroic acid (Figure 5 ) is R little slower a t the beginning of the cure than the zinc salt. The extreme case of transient acceleration is found in zinc sthylxanthate. The instability of this accelerator is so much greater even than that of the salts of dithiofuroic acid that more than ten times as much xanthate is required as of the latter t o produce equivalent cures.
Figure 4.
A-Zinc
Discussion
The twelve accelerators examined fall broadly into three classes. One class, represented by hexamethylenetetramine, Conclusion triphenylguanidine, and “808,” consists of persistent accelerX o attempt has been made to evolve a theory to explain ators, the sulfur curve for which is a straight line from bethe different behavior of these various accelerators, since ginning to end. Another class, represented by lead and zinc dithiofuroate and zinc ethylxanthate, consists of highly tran- each one seems to be a case of its own. But it is evident sient accelerators. the sulfur curve for which rapidly becomes that both the stability of the accelerator itself towards heat parallel to the time axis. The accelerators in this class exert and towards the other vulcanizing agents and the nature of their full effect during the first 15 or 30 minutes of vulcan- the decomposition products formed are the factors which ization, and thereafter become inert. Intermediate between determine the shape of the sulfur curve. I n other words, these two classes is a class, represented by the remaining an accelerator that is quickly decomposed, whether by heat accelerators, which may be called semipersistent, and which or by reaction with sulfur, etc., into inert decomposition products will give sulfur curves which are typified by the give a sulfur curve concave t o the time axis. The tensile curve for the stock containing hexa (Figure 1) xanthates or salts of the dithiocarboxylic acids. On the is represented only as far as the 105 minutes’ cure, since with other hand, with an accelerator which is extremely resistant this particular mixture the rubber became brittle during the to the disintegrating influences during vulcanization or one last 15 minutes of cure and its tensile strength fell off almost in which the various decomposition products continue to exert to zero. Other iiivestigators7 have reported somewhat similar a curing action equivalent to that of the original substance, results with this accelerator, so this tendency to an abrupt there would be produced the persistent, straight-line cure orercure may be considered characteristic of hexa and prob- as shown by hexa and triphenylguanidine, and between these ably of any fairly powerful persistent accelerator. Tri- two extremes the majority of the now commonly used accelphenylguanidine, although also a persistent accelerator, is erators may be placed, all of which indicate by their sulfur much slower than hem, and in consequence no sudden break curves progressive changes in curing effect during the course was reached in the tensile curve during the curing period. of vulcanization. It is believed that a study of these changes as revealed The sulfur curre for the accelerator known commercially as “808” (Figure 3), although not quite a straight line, is by sulfur coefficients will prove an aid t o the compounder sufficiently close to one to justify the classification of the in choosing the accelerator best suited to any particular artiaccelerator as persistent. It appears that the dability of cle of rubber manufacture. this accelerator a t 20 pounds steam pressure approaches that General Discussion of hexa a t 40 pounds. Q. What happens to the accelerator in these straight-line The curing power of the semipersistent accelerators falls I assume the reason the sulfur curves become straight off, but does not disappear with increasing period of vulcan- cures? is that the accelerator changes in such a way that it will no longer
’ Trickey and Leuck, THIS
JOURNAL,
18, 812 (1926).
function when a proper cure is reached.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
A. It is assumed that the accelerator decomposes into inactive products. What they are I am unable t o say. Q. Was any work done on combinations of accelerators along that line? A. Not in this particular investigation. Q. For a stock t o be non-blooming with one of these accelerators, would the accelerator disappear during the cure? A. To make non-blooming stocks the sulfur would have t o be kept very low so that the free-sulfur ratio would not be more than about 1 per cent on the rubber. The accelerator would disappear regardless of the amount of sulfur present. Q. Have you been able t o apply this method to determine the best amount of sulfur to use in a stock with any given accelerator? It seems, since i t was discovered that the combined sulfur is not an index of cure, t h a t i t has been neglected more than it should have been. Although it (the sulfur coefficient) certainly cannot tell anything about the state of cure, i t is highly important t o determine the actual amount of sulfur combined by any given
Vol. 19, No. 9
accelerator. This paper seems t o be valuable in its suggestiveness of what may be developed along these lines. Q. Is i t not possible that the flattening of the curves is due to the mass action of the sulfur rather than t o the decompositions of the accelerator? A. No, i t has been established by a number of investigators t h a t mass action does not seem t o enter into i t a t all until the free sulfur is almost gone. It would make little difference whether the sulfur content were 5 or 10 per cent, the accelerator of this fugitive type would combine just about the same amount of sulfur. Q. What would be the effect of a change of temperature on these curves? A. I cannot exactly say, b u t I think a change of curing temperature would merely change the position of the curve but not its general shape, except possibly in the case of the xanthates, which are so unstable that if you attempted t o cure with them at high temperatures, there would be almost no cure a t all.
Heat of Solution of Paraffin Wax' By F. W. Sullivan, Jr., W. J. McGill, a n d A u g u s t F r e n c h STANDARD OIL COMPANY (INDIANA), WHITING, IND.
I
N CALCULATIONS concerning refrigeration requirements for the chilling of wax bearing distillates, 63.27 B. t. u. per pound is commonly used as the heat of solution of the wax. This figure seems to have been taken from the 1894 edition of Landolt-Bornstein's "Tabellen."* Graefe3 gives a value of 70.2 B. t. u. per pound; Batelli,4 63.0 B. t. u. per pound; Kozicki and Pilat,' 70 B. t. u. per pound for 126" F. (52.2' C.) melting point paraffin wax and 78.9 B. t. u. per pound for 149.6" F. (65.3' C.) melting point wax. I n making heat balances for refrigerating operations on wax distillate, discrepancies have been found which it was thought might be due to the use of this commonly accepted figure of 63.27 B. t. u. per pound. The heat of crystallization of the wax found in the wax distillate cut from midcontinent crude was consequently determined and found to be 72.5 B. t. u. per pound.
This stirrer mixed the contents of the calorimeter very thoroughly. The amount of heat developed thereby was negligible in these experiments. I n 45 minutes of stirring at room temperature in a vacuum bottle the temperature of the oil was raised only 0.1"C. The water bath was kept a t a constant level by means of tube, e , attached to a suction line. The bath was adjusted to any desired temperature by running in water from the hot-water line, h , or the cold-water line, f. The heating coil was operated by current from a 24-volt storage battery. The current consumed was measured by means of a voltmeter and ammeter which were read at 50second intervals. Materials
The oils were pressed distillate from midcontinent crude and a wax-free gas oil. With the latter it was possible to Apparatus work close to room temperature because of its greater solvent An adiabatic calorimeter was used in this work (Figure 1). effect for the wax. The wax was obtained by recrystallization of a slack wax The inner vessel, a, consisted of an old 1-quart, wide-mouth vacuum bottle, the vacuum of which had been broken. It until it was practically oil-free. This wax was ground in was left mounted in its original jacket. It was fitted with a food chopper into particles less than l/8 inch (3 mm.) in a thermometer, t, graduated in 0.2" C., a heating coil, c, diameter. These were formed into the shape of an easily of No, 24 nichrome resistance wire, a '/*inch (13-mm.) crumbled rod by plunging a cork-borer into 8 pile of the opening, 0, fitted with a glass tube for introducing the wax. wax particles several times and then ejecting the wax from and a stirrer, s. The stirrer was of the reciprocating type the cork-borer with one of the next smaller size. These consisting of two flat rings, r, mounted on two upright wires rods disintegrated readily upon being introduced into the and spaced above each other 2 inches apart. These rings oil so that the wax dissolved completely in a short time. were of sheet copper of about 2 inches outside diameter and Procedure 1 inch inside diameter and had l/d-inch (6-mm.) holes drilled Four hundred grams of the oil were weighed out into the about inch (13 mm.) apart midway between the two edges. The ends of the upright wires were sealed into narrow calorimeter. The motor driving the stirrer was started and glass tubing with cement. The glass tubing passed through the speed was adjusted by means of the regulator until the brass sleeves in the cork covering the inner vessel. The stirrer made 60 cycles per minute. The oil was brought t o stirrer was driven by a string which passed over a pulley, the desired temperature by means of the electric heating coil. p , actuated by a crank attached to a reducing gear driven Some of the first experiments were done a t higher temperatures, but the later ones were done close to room temby an electric motor. perature to reduce any possible errors due to radiation 1 Presented before the Division of Petroleum Chemistry at the 73rd losses. When the desired temperature was reached the Meeting of the American Chemical Society, Richmond, Va , April 11 to 16, current was shut off and the stirring continued. Mean1927 2 Day, Handbook of the Petroleum Industry, Vol 11, p 823, John while the temperature of the water bath was adjusted by Wiley & Sons, I n c , 1922 adding either hot or cold water until it was exactly the same "Laboratoriumsbuch fur Braunkohlenteerindustrie as the temperature of the calorimeter proper. When no 4 Physik Z , 9, 671 (1908) increase or decrease in the temperature of the calorimeter 6 Pelvoleurn, 14, 12 (1918) J
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