I N D U S T R I A L A S D ENGINEERING CHEMISTRY
740
Vol. 22. s o . 7
chiefly of rubber and sulfur explode with violence during vulcanization. The average value for the heat of vulcanization of 10 calories per gram as calculated by Williams and Beaver a t 10 per cent combined sulfur is apparently far below the new value. Their statement that there is an evolution of heat a t the beginning of the reaction followed by a small absorption a t the end seems to be the reverse of the present results. The formation of soft vulcanized rubber and of ebonite seems to be two distinct and separate processes and very different thermochemically. Literature Cited
442 calories per gram of rubber. The significance of this high value may perhaps be best illustrated by the fact that if vulcanization of a hard rubber compound could be started and there were no heat lost in the process, the temperature would rise about 1000” C. or 1800” F. (specific heat = 0.4). It is well known that large masses of hard rubber compounded
Bostrom, Kolloidchem. Beihefle, 24, 467 (1928). Curtis, McPherson, and Scott, Bur. Standards, Sci. Paper 560. Daniels, J . A m . Chem. SOL.,38, 473 (1916). Hock, Kaufschuk, 1927, 207. Kirchhof, Gummi-Zlg., 39, 892 (1925). Kirchhof and Matulke, Ber., 5TB, 1266 (1924). Kirchhof and Wagner, Gummi-Zlg., 39, 357, 372 (1925). Messinger, Trons. Inst. Rubber I n d . , 5, 71 (1929). Perks, J . SOC.Chem. I n d . , 45, 142T (1926). Seidl, Gummi-Ztg., 95, 710, 748 (1911). (11) Weber, “Chemistry of India Rubber,” pp. 106, 114. (12) Williams and Beaver, IND. E N G . CHEM.,15, 255 (1923).
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
11-Vulcanization of Rubber with Nitro Compounds John T. Blake SIMPLEXWIRE & CABLECo., Bosrox MASS.
The Kjeldahl method for nitrogen analysis can be vulcanized with the:-e maadapted to the determination of combined nitrogen terials had approximately the Ostroniuislensky ( 8 ) in rubber vulcanized with nitro compounds. The same degree of unsaturation showed that a number combination of nitro compounds with rubber has been as the unvulcanized rubber. of organic materials possess followed in several cases. Strong evidence is given T h e y t h e r e f o r e concluded the ability to vulcanize rubthat the vulcanization is a chemical reaction. “that ordinary vulcanization ber. The vulcanization does The density of rubber has been shown to change is an unknown or undeternot develop quite such good during vulcanization with dinitrobenzene. The mined type of change in the physical properties in the rubchange approximates the progress of the combination hydrocarbon i n v o l v i n g no ber as sulfur does, but there is of .the vulcanizing agent with rubber. change in the unsaturation, no question that it does take The vulcanization of rubber with dinitrobenzene and and that the chemical union place. Trinitrobenzene, mtrinitrobenzene is monomolecular. A theory of the of sulfur is a secondary redinitrobenzene, and benzoyl mechanism of the vulcanization is advanced. action producing a further peroxide were shown to be the The value of the stoichiometric method in estimatchangewhich, no doubt, gives most satisfactory of the vuling the molecular weight of rubber is discussed. Values properties that are very inicanizing agents. of this constant are suggested by the data. portant in the manufacture We have never been able to Nitro compounds appear to be incapable of producing of rubber goods but which is produce a material resembling a change of degree only, not hard rubber. The amount of combined reagent is only ebonite by using the above of kind.” a small fraction of that required for ebonite formation. materials a s v u l c a n i z i n g Stevens ( 2 1 ) has also inagents. This suggests that the reagents are capable of undergoing only the soft-rubber vestigated the vulcanization of rubber with trinitrobenzene. reaction. They fall, therefore, in the same class with He found t’hat treating the acetone extract of the rubber selenium. This reaction offers a method of studying the with sodium hydroxide gave no red coloration, indicating formation of soft rubber without the complicating effect of the absence of trinitrobenzene. This would imply that the the hard-rubber reaction. The vulcanization of selenium has trinitrobenxene had either been destroyed during vulcanizabeen shown to be a chemical reaction and to follow the mass- tion or had combined with the rubber and therefore could action laws. Vulcanization of rubber with nitro compounds not be extracted with acetone. should follow the same course and throw more light on the Determination of Nitrogen in Vulcanized Rubber mechanism of vulcanization. If a chemical reaction does take place duying vulcanization Fisher and Gray ( 3 ) have investigated the vulcanization of rubber with dinitrobenzene, trinitrobenzene, and benzoyl with nitro compounds, it is probably best followed by analyperoxide. They have determined the unsaturation of the sis. This may be followed, presumably, by determining the vulcanized rubber by the Kemp-Wijs method ( 6 ) . Briefly, combined nitrogen. Since the amount of this nitrogen is this consists of allowing the material t o react for a definite small, the Dumas method for nitrogen determinations is not length of time with an excess of iodine chloride. The excess sensitive enough to be of value. The Kjeldahl method is very is determined and the amount of iodine chloride absorbed is much more sensitive and is universally used for nitrogen a measure of the unsaturation. They found that the rubber determinations on certain types of materials. It is, however, a
EVERAL years ago
S
INDUSTRIAL, A S D E.VGISEEIZIYG C H E J f l S T R Y
July, 1930
generally accepted belief that the method is unreliable in the case of compounds containing nitrogen linked to oxygen. Brinton, Schertz, Crockett, and Merkel ( 1 ) have attempted to adapt the Kjeldahl method to the determination of nitrogen in nitronaphthalenes. This was done by applying a correction factor, but obviously this method is useless for the present work. Investigation showed that the determinations of nitrogen in dinitrobenzene by the Kjeldahl method were low. The loss of nitrogen is due to the fact that the nitro compound slowly distilled out of the sulfuric acid during the heating and was lost before it could be attacked and absorbed. If the nitro compound was mixed into rubber and a nitrogen determination made on the mixture, excellent values were obtained. Apparently the presence of the rubber is sufficient to hold the nitro compound in the acid until it has been converted into a nonvolatile nitrogen-containing material. X batch of smoked sheets was set aside for the work and the nitrogen content determined as follows: 0.498, 0.489, 0.468, 0.508, 0.477; average, 0.488 per cent. Preliminary tests showed that dinitrobenzene and trinitrobenzene were soluble enough in acetone so that this solvent could be used to remove the free nitro compound from the rubber. A qeries of cures were made of the following compounds: COMPD A
36
COMPD B Parls 600 60 60
Per cenl 86 2 80 1
Per cenl 83 3 77 5
Parts 600
Smoked sheets Litharge Dinitrobenzene Trinitrobenzene
60
Rubber Rubber hjdrocarbon
COMPDC Parts 600
60
36 Per cent 86 2 so 1
T'ulcanization was carried out between sheets of aluminum. Accurate timing was obtained by having the plates hot when the sample wa5 inserted in the press. The samples were chilled in cold water on renioval froni the press and dried in a vacuum desiccator. The vulcanized samples were rolled thin on the mill, wrapped in extracted filter paper, and acetone-extracted for 16 hours. Preliminary results -bowed that the extraction was complete in 8 hours. The samples mere freed of solvent in the vaculim desiccator and analyzed for nitrogen. The values for caombined nitrogen have been corrected for the crude-rubber content of the compound, the nitrogen content of ihe raw rubber, and then calculated t o the rubber hydrocarbon basis, assuming that the raw rubber contained 93 per cent hydrocarbon. TIMEO F CURE Jlin 10 20 30 45 60 75 105 240 15 30 45 60 75 90 240 a 10 15
20 30 40 J
10 15 20 60
80
COMBINED XITROGEN 1 1 Av. P e i cent Per cent Per cent Compound A-145' 0 512 0 492 0.502 0.814 0 785 0.800 LOT2 1058 1.065 1 211 1.264 1 238 1 230 1.266 1 248 1 282 1 278 1.280 1.243 1.236 1.240 1 190 0 1.190 Compound B-132' 0 512 0 493 0 '474 0 670 0 670 0 670 0 811 0 809 0 812 0 939 0 935 0 943 1 024 1 012 1 1136 1 060 1 038 1 082 1 229 1 240 1 118 0 1 1 1 1 1
946 090 196 236 235 249
0.729 1.008 1.140 1.192 1.212 1.240
0 963 1 158 1 198 1 213 1 268 1 310 Compound 0 720 1,032 1. 140 1 '228 1.230 1.290
0 1 1 1 1
954 124 197 225 261 1 280 C-124' 0 724 1.020 1.140 1.210 1.221 1.265
01 R U B B E R ON HYDRORUBBERCORRECTEDC A R B O X Per cenl Per cent Per cent C. (293' F . ) 0.103 0.582 0.094 0.473 0.928 0 440 0.816 1.246 0 758 1.437 0 949 1.020 1.033 1.448 0.980 1.072 1 485 0.997 1.023 1.440 0 952 0 062 1.382 0.894 C (270' F ) 0 592 0 104 0.112 0 804 0 316 0,340 0 973 0 465 0.522 1 128 0 640 0.688 1 230 0 742 0 797 1 272 0 764 0.843 1 475 0 967 1.061
1 105 1 303 1 389
1 422 1 463 1 485 C . (255' F 0.840 1 182 1 322 1.403 1.418 1.468
517 815
0 0 0 0 0 0
901 934 975 997
0 664 0.877 0 970 1 004 1 048 1.071
0 0 0 0 0 0
3j2 694 834 915 930 960
0 379 0.747 0.897 0.985 1 000 1.052
74 1
The above figures indicate that there is a definite end point to the reaction in each case. It is interesting to compare the maximum values for combined nitrogen due t o the nitro compound in each of the above cases. The amount available for combination is also tabulated. NITROGES TEMP.OF CURE Available E n d value c. F. Per cent Per renl CURED WITH DINITROBGNZESE 145 293 1 075 1.072 132 270 1,790 1 061 C U R E D WITH TRINITROBENZENE 129 265 1 271 1Oil 1 052 255 1 271 124
COMPOUND
.4 B
C
C
It is significant that the end values check each other so closely, when in three caies there was an appreciable excess of vulcanizing agent. I n addition to this, the curing temperatures covered a wide range and two different nitro compounds were used. Changes in Density during Vulcanization
While the chemical analyses leave no doubt that the vulcanization with nitro compounds is a chemical reaction, it i. desirable to confirm the progress of vulcanization independently. Many chemical reactions involve a change in density of the mixture of reactants. Curtis, McPherson, and Scott ( 2 ) have shown that hard rubber has a density several per cent different from that of raw rubber. Soft vulcanized rubber has only about one-thirtieth the combined sulfur of hard rubber. I n spite of this it was hoped that there would be enough change in density of rubber during vulcanization with dinitrobenzene 50 that the course of the reaction could be followed.
Samples of B cured for various lengths of time were freed of air by immersing in water and evacuating for 30 minute< in a vacuum desiccator. Densities were determined in a pycnometer in a thermostat maintained a t 25' C. 132' C (270" F ) Mrnules Unvulcanized 15 30 45
CLRE AT
60 240
DE\SITY 4T 25' C . 1 0289 1 0267 1 0292 1 0337 1 0463 1.0419
The deiisitv determination
