Rubber Vulcanization Formation of Dimethyldithiocarbamic Acid Derivatives in Vulcanization with Tetramethylthiuram Di- and Monosulfides A. F. SHEPARD, Midgley Foundation, Ohio State University, Columbus, Ohio
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HE vulcanizing and accelerating properties of substituted thiuram disulfides have been explained by Bedford and Sebrell(2) and Bedford and Gray ( I ) on the basis of a sequence of reactions involving the intermediate formation of a salt of a substituted dithiocarbamic acid. The data now presented confirm the formation of this type of intermediate product. A mechanism of cure common to both tetramethylthiuram di- and monosulfides, and again probably involving the formation of dimethyldithiocarbamic acid, has been demonstrated by Cummings and Simmons (3) and by Jones and Depew (6). It is now shown that the vulcanization of rubber with a mixture of tetramethylthiuram monosulfide and sulfur does involve the formation of this acid. The following is a summary of the reactions suggested by Bedford and co-workers for thiuram disulfides:
S
L ./ JA
RzNS--S-CNR2 ---f S S + nonrubber constituent +H2S
+ R2N-C-S-S-
NR2
+ ZnO ----f S
// Zn, /S-c-NRz dS S-
+H~O
+s
(3)
+ ZnS + CSz
(4)
-NR,
S
//
pjNR + H2S z ----) S //
Zn\S-&NR,
S / 2R2N--d--S-NHzRz
R2N-C-S-NH2&
+ ZnO + 2RzNH
S //
+ Zn /S-c7NRp
+ H2() (5)
S
//
+ 2CSa + ZnO 4%N-c%s\
/ /Zn R2N--C/-S
+
The following typical procedure proved best for the isolation of dimethylammonium dimethyldithiocarbamate: A 5-gram sample of crepe rubber was milled with 0.25 gram of tetramethylthiuram disulfide. The milled mixture was sealed in vacuum in a Pyrex glass tube and heated 38 minutes at 138' C. The tube was removed while hot, and the empty end was chilled to -78" C. t o collect the volatile dimethylammonium salt. The yield of crude material (melting point, ca. 100' C.) was 0.040 gram. Similarly, 0.035 gram of crude was obtained from 5 grams of pale crepe rubber, 0.2165 gram of tetramethylthiuram monosulfide, and 0.0333 gram of sulfur. From 4.8 grams of pure sol rubber hydrocarbonland 1.68 grams of tetramethylthiuram disulfide, 0.035 gram of crude dimethylammonium salt resulted. In this last case 0.5 gram of unreacted tetramethylthiuram disulfide was recovered by extraction of the rubber.
\S-d-NR2
2%"
EXPERIMENTAL PROCEDURE
(1) (2)
S
H2S
stages of heating rubber, sulfur, zinc oxide, and tetramethylthiuram disulfide. In the present experiments a 15 per cent yield of the volatile dimethylammonium dimethyldithiocarbamate has been obtained by vulcanizing crepe rubber with 5 per cent of:its weight of the disulfide or by a n equivalent amount of monosulfide plus sulfur. When zinc oxide was added to the above cures, the presence of dimethyldithiocarbamic acid, presumably in the form of its zinc salt, was demonstrated in the vulcanizate by color reactions. Pure rubber hydrocarbon also yields dimethylammonium dimethyldithiocarbamate when heated with the thiuram disulfide. I n this case nonrubber constitutents cannot serve as a source of hydrogen sulfide. If the remainder of the foregoing reaction mechanism is accepted, it is probable that there is some substitution of sulfur for hydrogen in this type of vulcanization.
H20 (6)
Crude products obtained in this way were purified by sublimation. The purified material melted at 125' C. (alone and mixed with authentic material), boiled at about 140" C., and was readily soluble in water (6). Oxidation with iodine gave tetramethylthiuram disulfide. Many metal salts of dimethyldithiocarbamic acid (4) are colored and soluble in organic solvents. Solutions of the derivatives from cupric acetate and cobaltous nitrate were prepared in chloroform and were yellow and yellowish green, respectively. T h e absorption spectra of these solutions were directly compared in a spectrophotometer with solutions from dimethylammonium dimethyldithiocarbamate and were found to be identical. These color reactions were applied to the detection of dimethyldithiocarbamic acid in vulcanizates containing zinc oxide. Samples of the following mixes were cured 15 minutes a t 138" C.:
In support of this mechanism they have shown that tetramethylthiuram disulfide and hydrogen sulfide react to form dimethyldithiocarbamic acid and sulfur. Furthermore, a rubber cement containing the thiuram disulfide, zinc oxide, Mix I I1 sulfur, and hydrogen sulfide cures at a low temperature; 100 100 Pale crepe 5 5 Zinc oxide this fact indicates the formation of the extremely active ac5 .... Tetramethylthjuram disulfide Tetramethylthiuram monosulfide ... 4.333 celerator, zinc dimethyldithiocarbamate. Jones and Depew sulfur ... 0.866 have found curing to take place when tetramethylthiuram monosulfide is substituted for disulfide in the above mix. I This material had been prepared by the method of Midgley, Henne, and Finally, Cummings and Simmons observed that alkaline va- Renoll, J . Am. Chem. SOC.,I S , 2733 (1931); analysis by methods to be repors, presumably dimethylamine, were given off in the early ported later gave: nitrogen, 0.01 per cent: carbon, 88.02; hydrogen, 11.84 1200
,
November, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
The cured samples were rubbed with solutions of cupric chloride or cobaltous nitrate until stained, and the stains were dissolved off with chloroform or ether. Direct comparison of the absorption spectra of these solutions with those from authentic material again confirmed the presence of dimethyldithiocarbamic acid.
SUMMARY Dimethyldithiocarbamic acid derivatives are formed in the vulcanization of rubber with tetramethylthiuram disul-
1201
fide or with tetramethylthiuram monosulfide and sulfur in the presence or absence of zinc oxide.
LITERATURE CITED (1) Bedford and Gray, IND.ENQ.C H ~ M15, . , 720 (1923). (2) Bedford and Sebrell, Ibid., 14, 25 (1922). (3) Cummings and Simmons, Ibid., 20, 1173 (1928). (4) Delepine, Bull. SOC. chim., [4] 3, 651 (1908); Cambi and nasso, Atti. acad. Lincei, [6] 13, 254, 809 (1931). ( 5 ) Hutin, A , , Moni. sci., 7, 193-6 (1917). (6) Jones and Depew, IND. ENQ.CHEM.,23, 1467 (1931).
Cag-
RECEIVED 4ugust 14, 1934
Supersaturation and Crystal Formation in Seeded Solutions Hsw HUAIT I N GAND ~ WARRENL. MCCABE,University of Michigan, Ann Arbor, Mich.
T
however, the solution is capable HE formation of crystals I n continuously cooled, stirred, seeded solufrom solutions is closely of crystallizing spontaneously tions of magnesium sulfate, two reproducible suand nucleus formation will occur related to the phenomepersaturation curves are obtained: one where new non of supersaturation. Much once the labile region is reached. nuclei are first observed, and the other, lying f a r Ostwald called the line of detheoretical and experimental atther in the supersaturation field, where a sudden m a r c a t ion b e t ween the two tention has been given to the regions the “metastable limit.” degree of supersaturation supincrease in the rate of formation of new crystals portable by a s o l u t i o n under Miers and his followers (3-1 O), and a pronounced heat effect are observed. given conditions, to the mechaamong whom may be mentioned T h e supersaturation curves are dependent o n nism of precipitation from such Isaac, Jones, Hartley, and rate of cooling, speed of stirring, and number and a solution, and to the effect of the Fouquet, have done work supimportant variables on such a p o r t i n g Ostwald’s metastable size of seed crystals. process, but very little of this l i m i t c o n c e p t . Their experiThe data indicate that both mechanical stimuments covered a large variety of work was done with artificially lus and inoculation effects are of importance in seeded solutions. This paper substances in aqueous solutions, the process of nucleus formation in seeded solureports results of experiments on in organic solvents, and in binary tions. the formation of new crystals in and ternary m i x t u r e s . The a cooling. suDersaturated. seeded Drouess of the crvstallization solution-and on.thermal effects accompanying crystallization was noted by following changes Tn the refractive index of the from such a solution. cooling and crystallizing solution and comparing the index with Seeded solutions were chosen for several reasons. It is a known calibration. Any change in the concentration of a well known that the initial formation of crystal nuclei is solution became immediately known. It was found that a t profoundly influenced by the chance presence of very small definite temperatures sudden changes in concentration took seed crystals of the solute, of a substance isomorphous with it, place, and that such changes corresponded to the formation of or even of foreign dust particles, and is also sensitive to shock. showers of crystals. To check the refractive index experiments It was found that, if the solution was artifically seeded, the an independent method was used: A sample of solution which effects of fortuitous seeding and of minor mechanical effects had been sealed into an ampoule containing fragments of were so far overshadowed by the action of the seeds that the glass, platinum tetrahedra, or other hard bodies to promote results were more reproducible and the experiments under mechanical friction was cooled slowly and the ampoule shaken better control than if unseeded solutions were used. Also, a t regular intervals until crystals were discernible to the naked once crystals have formed in an originally unseeded solution, eye. I n general, these investigators used unseeded soluthe process from that point on is that of crystallization in a tions. Their conclusions can be summarized as follows: seeded solution. Finally, the use of seeded solutions comIn each case the dissolved substance in the solution has a defimercially is a promising method of controlling the size distri- nite supersolubility, which can be represented by a definite tembution of the product of the crystallization, even in cases where perature-concentration curve. This is true for stirred or agithe number of nuclei introduced initially as seeds is small in tated solutions. This curve, called by Miers the “supersolubility curve,” is traceable in each case and is found t o run a proxicomparison with new nuclei formed during the process. mately parallel to the solubility curve. For a solution o?a substance possessing a normal solubility curve and cooled past its PREVIOUS WORK saturation temperature, no crystal formation takes place at temperatures higher than that on the supersolubility curve. Ostwald (11) suggested that the unstable field lying on the When, however, the supersolubility curve is reached, the cooling supersaturation side of the solubility curve may be divided solution will usually crystallize spontaneously. A solution careinto two regions, “metastable” and “labile.” In the meta- fully rotected against mechanical shock, however, can be cooled stable region, inoculation is necessary to start the process of into txe labile region without crystal formation. crystallization; if there are already seed crystals present, Such a supersolubility curve, as first conceived by Miers, crystallization will take place by depositing material on these corresponds to Ostwald’s metastable limit and marks the seeds and new crystals will not form. I n the labile region, sudden change of a supersaturated solution from the metaPresent addreas, Peiyang Engineering College, Tientsin, China. stable state to the labile state. This conception has been
