Vol. 15, KO.7
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
720
acetone and ethylidene as solvents and plastifiers. The author found but a small percentage of the cellulose acetate tested insoluble in acetone, the soluble part being miscible in all proportions. The same observation applies to the various grades of cellulose nitrate tested. RUBBERS-A number of rubbers were extracted with acetone and the percentage of acetone-soluble material found in the vaious samples follow. These figures confirm the statement that acetone is not a rubber solvent, but dissolves only the rubber resins and oils which are found in the better grades of rubbers in amounts less than 5 per cent.
CRUDERUBBERS Balata, Surian s h e e t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Balata, Venezuela block. . . . . . . . . . . . . . . . . . . . . . . . . . Benguellas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Caucha Ball., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Caucha, Central. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crepe, brown. . . . . . . . . . . . . . . . . . . . . . Crepe, first l a t e x . . Gutta-percha.. . . . Guayale ........................................ Pontianac . . . . . . . . . . . . . . . . . Smoked sheet, primed ribbed Upriver Fine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Upper Congo, r e d . . .............................
Percentage Soluble a t Boiling Point of Acetone 41.9
44.5 4.5 2.9 2.8 2.8 2.9 25.0 21.0
.,
95.8 3.15 1.25 5.8
Reactions of Accelerators during Vulcanization’ V-Dithiocarbamates,
Thiurani Disulfides, and the Action of Hydrogen Sulfide By C. W. Bedford and Harold Gray THEB. F. GOODRICH Co., AKRON,OHIO
Vulcanization has been the subject of careful study bg the authors The present discussion is delroted to dithiocarbamates. thiuram disulfides, and hydrogen sulfide, and their reactions during vulcanization. The conclusions reached are: Hydrogen sulfide is formed during vulcanization by the action of sulfur on some constituent of the rubber other than the hydrocarbon, and the reaction i s accelerated by heat. I t decomposes metallic dithiocarbamates, and thus retards or stops curing. The metallic dithiocarbamates m a y be regenerated by the action of metallic oxides on free dithiocarbamic acid at ordinary temperature. Metallic oxides react with hydrogen sulfide to f o r m sulfides and sulfhydrates. thus protecting dithiocarbamates f r o m decomposition. The shifting of dithiocarbamic acid f r o m one metal to another is due to the intervening action of hydrogen sulfide. Hydrogen sulfide, in the presence of metallic oxides, changes thiuram disulfide3 into metallic dithiocarbamates. Metallic sulhydrates react with phenyl mustard oil and tetramethylthiuram disulfide to form metallic dithiocarbamates. of the present and previous papers on the subject.
HYDROGEN SULFIDE IN VULCANIZATIOX
ERRY2 considered the vulcanization of rubber as the
T
substitution of sulfur for hydrogen, but gave no evidence of the formation of hydrogen sulfide, which he postulated. Webers states that not a trace of sulfuretted hydrogen, still less hydrogen, is given off. Schidrowitz4says that “the process of vulcanization is not accompanied by the evolution of any appreciable quantity of sulfuretted hydrogen.” Later, Weber modified his original statement5 as follows: There is scarcely ever a trace of this gas to be discovered in the rubber works atmosphere* * *. I n the vulcanization of hard rubber goods faint but distinct traces of sulfuretted hydrogen are generally, perhaps always, observable, but they could not be ascribed to the vulcanization process proper. Technically pure Para rubber under conditions absolutely precluding the escape of any gaseous product-minute traces of hydrogen sulfide may sometimes be observed * * *with highly purified Para rubber no hydrogen sulfide a t all could be detected. If * * *this “insoluble” part of India rubber is mixed with sulfur, and this mixture subjected to vulcanization temperatures, say about 135’ C., a considerable evolution of hydrogen sulfide takes place * * * certainly 1 Presented before t h e Division of Rubber Chemistry a t t h e 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 t o 8, 1922. 2 J . SOC. Chem. I d . , 11 (1892),970. 3 I b i d . , 18 (1894), 1. 4 Thorpe’s Dictionary of Applied Chemistry, 4 (1913),585. fi “The Chemistry of Rubber,” 1909, p. 87.
I n general, the use of a metallic dithiocarbamate with the oxide of another metal gives results comparable with the use of the dithiocarbamate of the metal of the oxide. Lead dithiocarbamates are exceptions. Lead dithiocarbamates, if “protected” by zinc oxide, will accelerate vulcanization without decomposition to lead sulfide. Lead dithiocarbamates act only at relatively high temperatures. Zinc dithiocarbamates function at both high and low temperatures. They are activated by amines but retarded by hydrogen sulfide. Magnesium and calcium dithiocarbamates air-cure more rapidly than the zinc salts, but are less actioe at higher temperatures. Ammonia gas and other amines f o r m addition products with zinc dithiocarbamates and accelerate curing. Ammonia gas changes thiuram disulfides to dithiocarbamates and thioureas, accelerating air-curing in the presence of zinc oxide. No evidence has yet been presented contrary to the theory that metallic dithiocarbamates are true accelerators, activating sulfur by the formation of polysulfides whose “super sulfur” i s capable of vulcanizing at a low temperature.
it proceeds much slower than the vulcanization process. The insoluble constitutent of India rubber combines with sulfur under vulcanizing conditions a t a very slow rate, with evolution of hydrogen sulfide.
The admirable work of these investigators was directed foward the question of substitution or addition of sulfur with rubber. There is no cause for questioning their conclusions in this respect, Their statements, however, have been the cause of a general impression t o the effect that hydrogen sulfide is not formed in rubber stocks during hot vulcanization, and most certainly not a t ordinary temperatures. This viewpoint should be corrected since the question of hydrogen sulfide is of vital importance in the study of the mechanism of the action of accelerators. A few observations are therefore recorded which are easily reproducible and which throw new light on the question. The acetone extract of pale crepe was placed in a test tube with sulfur, under a piece of lead acetate paper held by the stopper. The paper blackened in from 5 to 8 days at room temperature or in 3 0 t o 45 min. in a benzene bath a t 80” C. A mixture of 100 parts rubber and. 10 parts sulfur was milled and portions placed in a closed desiccator under lead acetate paper. The paper darkened in from 2 to 4 wks. at room temperature and became black in 2 t o 3 mo. The same mix placed under test paper in the bottom of a beaker showed the black color in 2 hrs. on the steam bath or in less than 1 hr. in an oven a t 115” C. Test papers scattered around the
*
IND UXTRIAL A N D ENGINEERING CHEMISTRY
July, 1923
room proved the atmosphere to be free from hydrogen sulfide. These tests were checked several times with both pale crepe and smoked sheet. A cement containing only rubber, sulfur, litharge, and solvent will blacken on standing a t room temperature. A mixture of rubber and sulfur slowly evolves hydrogen sulfide a t ordinary temperatures, the rate of formation of the gas increasing with rise of temperature.
DITHIOCARBAMATES Twiss, Brazier, and Thomas6 describe zinc dithiocarbamates as being almost inactive except in the presence of zinc oxide. The foregoing observations on hydrogen sulfide furnish the explanation of this phenomenon. Hydrogen sulfide precipitates zinc sulfide from solutions of zinc dithiocarbamates, and thereby decomposes the accelerator. This action is the same as that of hydrogen sulfide on zinc acetate or zinc: salts of inorganic acids. R2N- C = S I
2RzX - C - SH
I/
s
\
/ S I
Zn
S
+ HzS+
f ZnS
RzN - C =S
--+
RzK-C-S-NHiRzf
I1
CSZ f ZnS
5
1 2 3
4
5
TABLE I sulfur-5 0, oxide-5.0, Zn[(S CS N ( C H z l a l ~ 03 Press cures 15 min. a t 286' F. OXIDS PRESSCURE AIR CURE AIR CUREAFTER S H a OK, 11 days None None, 26 days None OK None, 26 days OK, 11 days ZnO OK None, 26 days None, 26 days PbO Under OK, 13 days OK, 11 days MgO OK, 1 3 d a y s OK, 11 days Ca(OH)I Under
TABLE I1 Same as Table I using Zn(S. CS. N . C a H l o ) a O .39 -PRESS, 286' F.OXIDE Min. Cure AIR CIJRE None 15 Under None, 38 days ZnO 5 OK OK, 38 days 10 OK &Tone, 38 days PbO MgO 15 OK OK, 15 days Ca(OHh 15 OK OK, 15 days
In high-temperature cures without zinc oxide an increment of curing power is obtained from zinc dithiocarbamates prior t o their decomposition by hydrogen sulfide. Later, the acceleration is produced by the combined action of the decomposition products which consist of amine and carbon disulfide. The cure therefore proceeds the same as if the amine salt of the dithiocarbamic acid had originally been compounded as such, with the exception that more carbon disulfide is now present. 0
J . SOC. Chem. I n d . , 4 1 (1922), 811.
RzN
2R2N- C - S - N H ~ R f z ZnO+ I
S
-C =S I s \
Zn
/
+ 2R&H + HzO( 2 )
S
I
RAY- C = S
If one mol of carbon disulfide is added to one mol of the amine salt, with a further excess of carbon disulfide to moderate the violent reaction, the addition of zinc oxide gives a quantitative yield of the zinc salt with no other reaction product except water.
4-CS2+ZnO--Zn(S-CS-"Rz)~
S
(1)
Rubber-100,
4
After hydrogen sulfide has decomposed the zinc dithiocarbamate and liberated the amine and carbon disulfide (free dithiocarbamic acid), the action of zinc oxide comes into play and reforms the zinc dithiocarbamate, as shown by the following experiments: If the amine salt of a dithiocarbamic acid is mixed with zinc oxide and sufficient solvent to moderate the otherwise violent reaction, the zinc salt is a t once formed.
I1
If zinc dithiocarbamates activate sulfur for reaction with rubber, it may be assumed that they also activate sulfur for reaction with rubber resins or proteins whereby the formalion of hydrogen sulfide would also be accelerated. This hydrogen sulfide then decomposes the zinc dithiocarbamate and further air-curing of the rubber mix will cease. As a check on this assumption i t has been found that aircuring is prevented and heat cures are greatly retarded if stocks containing zinc dithiocarbamates are placed in an atmosphere of hydrogen sulfide for a few hours. The use of from 5 to 10 per cent of zinc dithiocarbamate was then investigated for the purpose of over-riding the effect of hydrogen sulfide. Such a stock, containing no zinc oxide, proceeds to air-cure the same as if zinc oxide and a lower amount of accelerator were used.
1 2 3
FUKCTION OF ZINCOXIDE
R2N--C-S--T\"zR2
S
72 1
+ HzO (3)
A literature search has failed to locate any published description of these two reactions. Primary aromatic amines such as aniline, toluidine, and many others have been found to give violent reactions on mixing with zinc oxide and then adding carbon disulfide. If the temperature is controlled the reaction product will air-cure rubber cements, while if the reaction mixture is allowed to heat up, the odor of mustard oils is very strong and the low-temperature curing power is lost. Neither the cold nor the hot reaction product shows high curing power in heat cures. By the use of secondary amines such as piperidine-or dimethylamine, there is no possibility of a decomposition to mustard oils, and quantitative yields of the zinc dithiocarbamates may be obtained when working with pure materials. Dimethylamine dimethyldithiocarbamate when pure is water-soluble. On standing i t slowly develops an increasing water-insoluble content with loss of dimethylamine. One sample went to 40 per cent water-insoluble in 4 mo. This insoluble portion seems to be tetramethylthiuram disulfide. For a study of the reactions shown in Equations 2 and 3, it is therefore necessary to use freshly prepared amine salts. When this precaution is taken, one mol of dimethylamine dimethyldithiocarbamate will react with zinc oxide and carbon disulfide to give a product whose air-dry weight shows the fixing of an additional mol of carbon disulfide, and corresponds to one mol of the zinc salt. An ash analysis of the product gives a zinc oxide residue which corresponds very closely to the formula for the zinc salt. The need of zinc oxide to bring out the full curing power of zinc dithiocarbamates is for the purpose of maintaining a given concentration of the accelerator in the rubber mix. Hydrogen sulfide is continually destroying the accelerator and zinc oxide is continually reforming the same.
SUBSTITUTES FOR ZINCOXIDE The reaction of tetramethylthiuram disulfide with hydrogen sulfide to form dithiocarbamic acid has been previously described.' If this reaction proceeds as easily as is claimed, the thiuram disulfide should tend to remove hydrogen sulfide 7
Bedford and Sebrell, THISJOURNAL, 14 (1922), 25.
Vol. 15, No. 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
722
If dry hydrogen sulfide is passed over zinc oxide a t room temperature, the oxide becomes hot, water is liberated, and zinc sulfide is formed. A quantitative yield of zinc sulfide is not obtained by this reaction of powder with gas which seems to show only a superficial action of hydrogen sulfide A B C Rubber 100 100 100 on the zinc oxide crystals. The action of hydrogen sulfide Sulfur 5 0 5 on litharge and calcium hydroxide a t ordinary temperatures Zinc salt 0.25 .... 0.12 Thiuram .... 0.5 0.25 is well known. With zinc oxide it 'was believed that further The third mix, C, was made by mixing the proper pro- laboratory tests were necessary to show its rapid reaction portions of Stocks A and B. These three mixes were cured with hydrogen sulfide. Hydrogen sulfide also reacts rapidly for 45 min. a t 186' F. in an electric oven (open heat). -4 with zinc oxalate and zinc soaps. was barely set or badly undercured; B gave no cure; C gave a snappy overcure. ' Tetramethylthiuram disulfide not only protects zinc dimethyldithiocarbamate from decomposition by hydrogen sulfide, thereby acting as a substitute for zinc oxide, but gives the additional effect of free amine on the zinc salt, as will be described subsequently. from a rubber stock and thereby prevent the decomposition of zinc dithiocarbamates, whereby it might be considered as a substitute for zinc oxide. The following stocks were prepared :
TABLE I11 Same a s Table I using Pb(S.CS.NCaHio)r-O.53 -PRESS, 286' F.OXIDE Min. Cure AIR CURE 15 Under None 3 8 d a y s None ZnO 10 OK None: 38 days 10 OK &Tone, 38 days PbO 15 OK None, 38 days MgO 1.5 OK None. 38 days Ca(0H)a
1 2 3 4 5
TABLE IIIa Same as Table I using Pb(S.CS.NCsHia)a-O.5 .--PRESSCURES- OVENCURES,186' F. OXIDE Min. O F . Cure Min. Cure AIR 110 None None, None 15 286 Under 50 Over ZnO 15 260 None OK 50 Over 15 260 OK None: PbO 50 Under None, MgO 15 286 OK Under None, Ca(OH)a 15 286 OK 50
1 2 3 4
5
1 2 3 4
5
CURE 38 days 38 days 38 days' 38 days 38 days
TABLEI V Same as Table I using P b [ S . CS.N(CHdz]a-0.44 PRESSCURB OXIDE 1 5 min. 286' F. AIR CURE None ZnO OK None, 75 days PbO Fair MgO Under Under, 13 days Ca(OH)a Under
It has also been found that several zinc salts of weak organic acids may be substituted for zinc oxide. To a standard mix consisting of 100 parts rubber, 4 parts sulfur, and 0.5 parts zinc dimethyldithiocarbamate, the following zinc salts were added: ( a ) none (control); (b) zinc oxide, 3 parts; (c) zinc stearate, 20 parts; (d) zinc resinate, 20 parts; (e) zinc oxalate, 10 parts; and (f) zinc dust (contains ZnO), 20 parts. These six mixes were cured simultaneously in a n electric oven for 30 min. a t 215" F., with results as follows: (a) no cure, ( b ) and (c) snappy overcure, ( d ) and ( e ) fair cure, and (f) snappy overcure. Further experiments show that zinc soaps will react with dimethylamine dimethyldithiocarbamate in carbon disulfide t o form the zinc dithiocarbamates and free fatty acid. Zinc salts of weak acids, therefore, act in the same manner as zinc oxide and remove hydrogen sulfide or reform zinc dithiocarbamates after they have been decomposed by hydrogen sulfide. The oxygen of zinc oxide is nonessential; it is the zinc radical which is effective. TRUEACCELERATORS Further evidence that the metallic dithiocarbamates are true accelerators is found in the action of lead dithiocarbamates. Zinc oxide, when used with this accelerator, apparently reacts with hydrogen sulfide direct, thereby protecting the lead salt from decomposition. A mix consisting of: rubber, 100; sulfur, 5; zinc oxide, 10; and lead dimethyldithiocarbamate 0.5 will cure in 45 min. a t 186O F. without turning black. I n the absence of zinc oxide, lead sulfide is formed and no cure results.
B
b
4 \1 FIG.1
Zinc compounds, therefore, may act as secondary accelerators to metallic dithiocarbamates in two different ways. (1) They may react with hydrogen sulfide and thereby protect the primary accelerator from decomposition. ( 2 ) They may reform the accelerator after i t has once been decomposed. The ultimate effect by either method is the same.
TIMEFACTOR AND MASSACTION The theory of decomposition of a zinc dithiocarbamate by hydrogen sulfide and its re-formation by reaction with zinc oxide requires the migration of the free amine and carbon disulfide (dithiocarbamic acid) from the place in the rubber mix where the zinc salt was decomposed to the location of a particle of zinc oxide. Zinc oxide, as such, is not in solution in rubber, and therefore cannot migrate by diffasion. The free dithiocarbamic acid must therefore diffuse to the zinc oxide particle. With large amounts of zinc oxide distributed through a rubber stock, its action may be chiefly the removal of hydrogen sulfide. With high amount& of accelerator and high zinc oxide, the decomposed accelerator will find plenty of fresh zinc oxide in its immediate vicinity and the zinc salt will reform at once. Low accelerator and low zinc oxide, however, present an entirely different set of conditions. I n such a stock the condition might easily arise where a large portion of or all the zinc dithiocarbamate had been decomposed and had not yet migrated to the location of unreacted zinc oxide. Such a stock would tend to show a flat curing curve, and yet after removal from the press
might proceed to air-cure further on standing where time would allow migration and re-formation of the zinc salt. Many instances of this phenomenon have been found, as illustrated by cures on a stock containing: rubber, 100; zinc oxide, 5 ; sulfur, 2 ; and zinc dimethyldithiocarbamate, 0.2. The original tensiles are shown by the lower curve in Fig. 1. The same sheets from which the tensile pieces were cut were placed in a dark cabinet and stored for 75 days at room temperature. They were then retested, with the results shown in the upper curve. The question of equilibrium is, therefore, an important one in the study of the action of accelerators, and where mass action is low the time factor seems to dominate.
THIURAM DISULFIDE The action of hydrogen sulfide on tetramethylthiuram disulfide7 forms free dithiocarbamic acid, which then reacts with zinc compounds to form zinc dithiocarbamates. R2N- C S
I I S I
S
+ HzS
+ 2RzN-'2-SH
R~N-CZS
I
I
S
I
I/
+S
(4)
S
RzN-C=S
S
+ HzS +ZnO+
R~N-CES
RzN-C=S
I
"Zn S/
+HzO
+S
I
RzN-C=S
TABLEV Same as Table I using Mg(S.CS,hTCsHlo)a0.5 -PRESS CURE- OVENCURES, 185' F. OXIDE Min. F. Cure Min. Cure AIR CURE None 15 286 Under 110 Under None, 38days ZnO 8 240 OK 20 OK Under 5 days PbO 15 260 Fair 50 Fair hTone,'38 days 15 286 OK 50 Under None, 38 days OK 50 Under Under, 12 days EA%H)I 15 260
1
3 4
5
1 2 3 4 5
1
2 3 4 5
of such accelerators. A laboratory study of these reactions shows that this is not the case. Hydrogen sulfide, when passed into a rubber-sulfur cement containing litharge, produces a t once black lead sulfide. If the cement is chilled, the red color of the lead sulfhydrate is first developed, changing shortly to black. On the addition of tetramethylthiuram disulfide to the cement, the action of hydrogen sulfide first shows the red sulfhydrate color which, instead of turning black, changes to white or gray, and no lead sulfide is formed until either the litharge or the thiuram is nearly all reacted upon. A rapid stream of gas will give black sulfides at once, but a slow stream with stirring gives the results described. Lead dimethyldithiocarbamate is formed. The same action is found with phenyl mustard oil and litharge on passing in hydrogen sulfide. The lead salt of phenyl dithiocarbamic acid is formed. The low curing power of phenyl mustard oil as found by Twisss is therefore probably due to the formation of a small amount of the zinc salt of phenyl dithiocarbamic acid. Bruni's reactionB of phenyl mustard oil with sulfur to form mercapto benzothiazol does not take place as easily a t the temperature of 138' C., used by Twiss, as the reaction of sulfur on rubber resins (or proteins) to produce hydrogen sulfide. Tlie presence of metallic oxides aids, rather than prevents, the formation of dithiocarbamates by the action of hydrogen sulfide on tetramethylthiuram disulfide. ACTIONO F AninioxIA AND AMINES
(5)
This reaction in rubber has been studied in the light of recent developments. The liberation of sulfur according to Equation 4 has not vulcanized rubber cements. I n a zinc oxiderubber-sulfur cement or dry stock a small amount of hydrogen sulfide will cause air-c-uring with tetramethylthiuram disulfide. A large excess of hydrogen sulfide decomposes the zinc dithiocarbamates which are formed, reacts with the available zinc oxide, and prevents air cures. Organic softeners, such as pitches, oils, resins, etc., often show a retarding action. This is attributed to the increase of hydrogen sulfide resulting from their reaction with sulfur.
2
723
INDUSTRIAL A N D ENGINEERING CHEMISTRY
July, 1923
TABLEVI Same as Table I using C ~ ~ S . C S . N C ~ H I O ) ~ - O . ~ --PRESS CURE-OVENCURES,1 8 5 O F. OXIDE Min. F. Cure Min. Cure None 15 286 Under 110 Under ZnO 8 240 OK 20 OK PbO 15 260 OK 50 OK MgO 15 286 OK 110 Under Ca(0H)z 15 260 OK 110 Under TABLE VI1 Same as Table I using ( C H Q ) ~ W H CS.N(CH3)-0.33 ~.S. --PRESS CUREOXIDE Min. O F . Cure AIR CURE None 15 286 Under None, 75days ZnO 15 230 OK OK, 13 days PbO 15 286 OK None, 13 days 15 286 Under OK, 13 days %%HI2 15 286 Under OK, 13days
Litharge reacts so readily with hydrogen sulfide to form lead sulfide that i t might be expected to prevent the action of this gas on thiuram disulfides and thereby retard the action
FrommlO shows that ammonia, aniline, and other amines easily react with disulfides which have a double bond adjacent to the =C-S-S-C= group, such as tetramethylthiuram disulfide, according to the following equation: RzN-C=S
I
RzN-C=S
R2N - C- S- NHd
/I
S
Cadwellll uses aniline with thiuram disulfides, dithiobenzoyl disulfide, metallic xanthogenates, etc., to induce rapid air cures in the presence of zinc oxide. In Tables IX and X it is shown that ammonia has the same effect on the cure, by reaction with thiuram disulfides to produce dithiocarbamic acids, which in turn form zinc salts by reacting with zinc oxide. The action of ammonia on thiuram disulfides does not stop with the formation of the zinc dithiocarbamat,es. Bedford and Sebrell7 noted the accelerating action of aniline on zinc dithiocarbamates, but were somewhat in doubt as to whether the rapid air-curing was due to other than the solvent action of aniline for the zinc salt. Ammonia has the same effect as aniline. A zinc dithiocarbamate of a secondary amine air-cures far more rapidly after the dry stock has been kept over night in an atmosphere of dry ammonia gas. The stock is then exposed t o air and kept at room temperature. The effect of ammonia cannot be due to solvent action, It is evidently connected with the formation of an additive reaction product with the zinc dithiocarbamate. Dry ammonia gas was passed upward through a cylinder containing one mol (303 g.) of zinc dimethyl dithiocarbamate. An exothermic reaction took place. The cylinder was allowed to cool before the flow of gas was interrupted, and the contents were then spread out in air to remove all odor of ammonia. J . SOC.Chem. Ind., 40 (1921), 2421. Giorn. chim. ind. applicata, 8 (1921), 351; C. A , , 15 (1921), 3915. lo Ann., 348 (1906), 144. 11 Brit. Patent Application 174,915 (1922). 6
0
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
724
The weight increase was 17 g. or the equivalent of 1 mol of ammonia. The formula of the new compound probably is
]
(CHs)2N - C - S
[
!I
s
Zn.NH3
2
Lead dithiocarbamates do not give addition products with ammonia, nor does ammonia cause them to air-cure.
1 2 3 4
5
TABLEVI11 Same as Table I using CsHloNHa S . C S . NCrHlo-0.49 --PRESS CURE---OXIDE Min. O F . Cure AIR CURE h-one 15 286 Under None, 75 days ZnO 15 230 OK OK, 1 3 d a y s 15 286 PbO OK None, 13 days MgO 15 286 Under OK, 13 days 15 286 Under OK, 13days Ca(0H)z
Rubber-100;
sulfur-4.0; PRESS CURE 15 min., OXIDE 280° F.
; zll0” 3 PbO
4 5
.....
MgO Ca(0H)n
TABLEI X oxide-0.5; tetramethylthiuram disulfide-0.25 -OVEN CURE-50 min., 176’F. 7--AIR CURE---As is AfterNH3 As is After N H I
.,... . . . . .
None None None None
Over Under Under Fair
Over None Under Under
..
...
None None None Good, 8 days
Faii,’8 ‘days None hTone Good, 8 days
Vol. 15, No. 7
show a t ordinary temperatures. When used with zinc oxide or litharge they assume the properties of the zinc or lead salts. Ammonia causes zinc dithiocarbamates of secondary amines to air-cure in absence of metallic oxides so that they function as rapidly as by the aid of calcium hydroxide or magnesia. The ammonia, is believed to be held as an addition product to the zinc salts, and probably increases the basicity of the accelerator, thereby giving greater curing power. With thiuram disulfides the ammonia first forms the corresponding dithiocarbamate. The data check the previous assumptions and show the shifting from one metallic salt to another during vulcanization. This is believed to be due to the interfering action of hydrogen sulfide. Metallic oxides apparently do not fix hydrogen sulfide sufficiently t o prevent the interchange of metallic radicals. Litharge stops the zinc salt from aircuring and magnesia speeds its action. Lead and magnesium sulfhydrates evidently react with zinc dithiocarbamates to form lead or magnesium salts and precipitate the zinc as the sulfide. It will be recalled that litharge does not prevent the action of hydrogen sulfide on thiuram disulfides.
TABLE X Rubber--100;
Mg0-0.5;
PRESS CURE
SIILPUR 5 min.. 240 -_ 4 0 Under 6 0 Fair 8.0 OK
F.
tetramethylthiuram disulfide-0.5 ----AIR CURE------^ As is After N H I
OK, 5 0 d a y s O K 34 days Ovkr, 27 days
OK, 13 days OK, 13 days OK, 13 days
RELATIVE ACTIONOF VARIOUSOXIDES The idea of continual decomposition and re-formation of metallic dithiocarbamates during vulcanization a t once suggests a possibility of the shifting from one metallic salt to that of another metal. Upon studying the action of various oxides on a certain metallic dithiocarbamate, their action was found to be so widely different that the results could be recorded qualitatively by the feel of the cured vulcanizate. Considering acceleration purely as a time factor, the relative action of the common oxides has been recorded through the time necessary to give an under, fair, OK, or over cure. This was partly necessary in order t o correlate air cures properly with press or oven cures, since, air cures do not lend themselves readily to machine tests. The treatment with ammonia gas consisted merely of exposing the stocks, in thicknesses not over 0.25 in., to the dry gas over night a t atmospheric pressure, or for shorter periods when using higher pressure. All stocks were then exposed to air for a t least one day before being cured in press or oven. The entire data are given in Tables I to X.
DISCUSSIOK OF DATA From the foregoing data it was found that lead dithiocarbamates do not air-cure except by the aid of calcium hydroxide or magnesia, and then but slowly. The lead salts are high-temperature accelerators and require the presence of litharge or zinc oxide. Litharge used as an aid to the dithiocarbamates of zinc, calcium, or magnesium gives practically the same results as if the lead salt had been used. Zinc dithiocarbamates air-cure most rapidly with calcium hydroxide and magnesia, more slowly with zinc oxide, and not a t all in the presence of litharge. For high-temperature cures calcium hydroxide and magnesia lose their value and become secondary to zinc oxide and litharge. I n each case the resultant cure corresponds closely to that of the metallic salt of the oxide used. Calcium and magnesium dithiocarbamates of secondary amines are rather unstable, decomposing with rising temperature and thereby losing the high curing power which they
FIG.2
I n conclusion, there is shown in Fig. 2 a graphical outline of the many reactions which take place with dithiocarbamates and thiuram disulfides derived from secondary amines. All these reactions take place a t room temperature. Nitrostarch a s a Constituent of Explosives The use of nitrostarch in the manufacture of safety explosives is increasing, and this substance is now important in the explosives industry, state C. A. Taylor and W. H. Rinkenbach, assistant explosives chemists of the Interior Department, who have conducted a series of studies of the materials, constitution, and analysis of numerous types of explosives a t the Pittsburgh Experiment Station of the Bureau of Mines. Nitrostarch is made by nitrating starches with a mixture of sulfuric acid and nitric acid, details of the method varying considerably among different manufacturers. Commercial nitrostarch is in reality a mixture of compounds of various degrees of nitration, being comparable in this respect with nitrocellulose. Like the nitrocellulose, nitrostarch is not a true nitro compound, being an orgstnic nitrate.
