September, 1925
I-VDYSTRIAL AA’D ENGINEERING CHEMISTRY
Discussion of Results Measurements of the active area of fire, as indicated by the diameter of the red hot zone, show that about 93 sq. em. (0.1 square foot) of area was in use; 396.5 liters (14 cubic feet) of gas per minute from this area equals a driving rate of about 68 grams of fuel gasified per square centimeter (140 pounds per square foot) per hour. illlowing for the percentage of voids (previously measured) the total time of contact with fuel was about 0.01 second, and the contact with the “red hot zone” much less. Conclusions 1-Under the conditions of test a particle of carbon undergoing direct oxidation may rise in temperature a t an unknown rate and have an undetermined maximum temperature.
‘931
2-Carbon undergoing oxidation by carbon dioxide to carbon monoxide, or, in other words, the so-called reducing zone, in which heat is being absorbed instead of liberated, could not fairly be assumed to rise in temperature faster than the thermocouple. 3-The temperature of the thermocouple and of the carbon above it is much too low to reduce carbon dioxide to carbon monoxide even with a very long time of contact, much less in the 0.01 second or less allowed in the experiment. 4-Under the conditions of test carbon is either oxidized directly to carbon monoxide or the oxidation and reduction zones are coincident. The substitution of anthracite or coke for the wood charcoal of this experiment does not materially alter the results.
Vulcanization Accelerators-I’ By G. S. Whitby and H. E. Simmons MCGILLUNIVERSITY, MONTREAL. CANADA, AND THE UNIVERSITY OF AKRON,AKRON,OHIO
The accelerator piperidinium pentamethylenedithiocarbamate has been studied with regard to t h e influence of variation in t h e proportions of sulfur and accelerator, and t h e proportion and character of t h e zinc oxide used with t h e rubber. Zinc oxide of extremely fine particle size has a greater effect on t h e activity of t h e accelerator t h a n has t h e ordinary zinc oxide, when the oxide is used in small proportions; b u t when used in large proportions t h e very fine oxide retards vulcanization if t h e proportion of sulfur present is only small. The accelerating effect of piperidinium pentamethylenedithiocarbamate in t h e presence of zinc oxide is not due merely to the formation of t h e zinc salt; the salt-forming molecule of piperidine contributes considerably to the
effect. The addition of a molecule of piperidine greatly enhances the accelerating effect of zinc phenylmethyldithiocarbamate and of di-a-thionaphthoyl disulfide. With all t h e ultra-accelerators studied there is evidence t h a t t h e catalytically active agent is destroyed during vulcanization. One result of such destruction is that, with a given proportion of sulfur, a given proportion of accelerator cannot carry vulcanization beyond a certain point. Therefore with suitable proportions of accelerator and sulfur “flat curing” can always be obtained with accelerators powerful enough t o bring about vulcanization a t temperatures a t which sulfur alone cannot. The accelerator di-a-thionaphthoyl disulfide has been closely studied in this connection.
HE present studies represent an extension of previous investigations2 of the accelerator piperidinium pentamethylenedithiocarbamate (P. P.),3 particularly to ascertain how far, if a t all, the structure R2N.CS.SM (M =
by stating the load a t an elongation of 6; this quantity is represented by Tg. In each case T s is the average of the results for the four test pieces. The tensile strength and the elongation a t break are represented by TB and EB, respectively, and are the average results for those two test pieces showing the highest ultimate tensile strength. Preliminary experiments led to the choice of the following mixture as a standard: rubber 100, sulfur 10, zinc oxide 5, P. P. 0.5 parts. The vulcanization temperature was 115’ C. (10 pounds steam). This mixture can be cured within a reasonably short heating period and, since the mixture can be markedly overcured, the tensile optimum stands out clearly.
T
metal, ammonium, or substituted ammonium) can be varied without destroying the accelerating power. General Procedure The raw rubber used was a large batch of blended smoked sheet. The uncured slabs, 2.5 mm. (0.1 inch) thick, were vulcanized in a press. Four dumbbell-shaped test pieces, 6 mm. (0.25 inch) wide over the parallel portion, from each slab were tested with a Scott tensile testing machine. The load a t each 100 per cent elongation interval was also read. Elongations are stated in fractions of the original length taken as 1. The position of the stress-strain curve is defined From papers presented before t h e Division of Rubber Chemistry of t h e American Chemical Society at its 65th Meeting, New Haven, Conn.. April 2 t o 7. 1923, a n d its 67th Meeting, Washington. D. C., April 21 t o 26, 1924. Received March 5, 1925. 2 Whitby and Walker, THIS JOURNAL, 13,816 (1921); Whitby, I b i d . , 16, 1005 (1923); see also Schidrowitz and Burnand, J. SOC.Chem. I n d . , 40, 2681’ (1921) ; Twiss, Brazier, and Thomas, I b i d . , 41, 8 1 T (19221 8 Other names for this substance are piperidinium piperidine-l-carbothionthiolate, piperidinium dithio-I-piperidinecarboxylate, piperidinium piperidyldithioiormate. It was formerly called, erroneously, piperidine piperidyldithiocarbamate; see Whitby, I n d i a Rubber J . , 67, 1030 (1924).
Variation of Proportions of Sulfur and Accelerator When the proportion of sulfur used was only 3.5 parts, marked over-curing could not be obtained a t 115”C. in any reasonable period of heating with the small proportions of accelerator it was desired to use. With 7.5 parts of sulfur full tensile strength could be developed, and also, if the proportion of accelerator were sufficiently high, marked overcuring occurred within a comparatively short period of heating (Table I). When 0.66 part of the accelerator is used with 7.5 parts of sulfur definite overcuring occurs within an hour. Results given in Table I1 show that when the proportion of sulfur is increased to 10 parts, 0.50 part of the accelerator
INDUSTRIAL A N D ENGINEERING CHEMISTRY
932’
Vol. 17, No. 9
Table I Rubber 100, sulfur 7.5, zinc oxide 5, cured a t 115’ C. with various proportions of piperidinium pentamethylenedithiocarbamate -(a) 0.25 Parf P. P . 7 -(b) 0.33 Purl P. P.4:) 0.50 ~ u 7 P. l P.- ( d ) 0.66 Dart P. P,-
Ts Minutes 10 20 30 40 60 90 120 150
Kg./ sq. cm.
TB Q./
T6 EB
sq. cm.
Kg./ sq. cm.
Ts
TB Kg./
EB
sq. cm.
8.15 7.60 7.30 7.40 6.60 6.35
leads to overcuring in a convenient time, but that 0.33 part does not. I n this table are included the results obtained on curing (at 141’ C.) a nonaccelerated stock corresponding to the accelerated mixture adopted as a basis in the present studies, T a b l e I1 R u b b e r 100, S u l f u r 10, Zinc Oxide 5, Cured w i t h a n d w i t h o u t Piperid i n i u m Pentamethylenedithiocarbamate ----WITH ACCEI,ERATOR-WITHOUTACCSLSRATOR ( a ) 0.33 Part P. P. ( b ) 0.50 Part P. P. (C) 7’8
TK
Minutes Kg./ Kg./ at sq. sq. l l 5 ’ C . cm. cm. 10 58 195 20 118 289 30 40 198 335 60 233 338 120 277 298 160 274 4 Ts = 79. * Ts = 96
Ta EB 8.20 7.50 7.00 7.00 6.10 5.95
TB
Kg.,/ Kg./ sq. sq. cm. cm. 93 275 198 326 2440 289 25lb 256 199 64
Ts
EB 7.50 7.00 6.10 5.95 ,5.00 2.85
TB
Minutes Kg./ Kg./ at sq. sq. 141“ C. cm. cm. 60 16 37 90 22 74 120 28 101 150 32 114 180 41 131 240 61 300 17
EB 8.20 8.65 8.30 8.00 7.90 5.60 3.00
An examination of Tables I and I1 brings out the following facts: (a) Cures in the mixtures containing 10 parts of sulfur are, as was expected, distinctly ahead of corresponding cures in similar mixtures containing only 7.5 parts of sulfur (compare columns ( b ) and (c), Table I, with columns (a) and ( b ) , Table 11). ( b ) Excluding the case of the mixture containing the lowest proportion of accelerator (0.25 part), the maximum tensile strengths attained by all the mixtures are similar-that is, they lie between 321 and 338 kg. per sq. cm. (4570 and 4800 pounds per square inch). In the stock containing 0.25 part P. P. and 7.5 parts sulfur the maximum tensile strength is somewhat lower, 308 kg. per sq. cm. In a stock containing 0.25 part P.P. and only 3.5 parts sulfur the maximum tensile strength was still lower, 231 kg. per sq. cm. (see Stock C, Table IV). These results are in accord with the conclusion of Schidrowitz and Burnand‘ that “a certain minimum quantity of accelerator (or of sulfur, or of both) is requisite for the production of a given maximum state of vulcanization.” It would appear that the proportions 7.5 sulfur and 0.25 P. P. are just slightly below those required to produce the maximum tensile strength. The necessity for using a certain minimum proportion of sulfur and an ultra-accelerator in order to obtain maximum tensile strength is considered due to the destruction of the vulcanization catalyst during heating. (c) The course of the figures for TKin the mixtures to which Tables I and I1 refer is worthy of remark. The quantity TO defines the position of the stress-strain curve and makes it-possible to follow the change in stiffness of the vulcanizate. It will be noticed that with a given proportion of accelerator, the curve can be advanced only to a certain, fixed extent. In the mixtures containing 7.5 parts sulfur (Table I) 0.25 part of the accelerator gives a stifhess corresponding only to TO = ca. 148 kg. per sq. cm.; further heating fails to produce greater stiffness With 0.33 part of the accelerator the vulcanizate can be carried to a stiffness represented by Ta = ca. 230 kg. per sq. cm., but no further. In both these cases curing comes to what may be called a “dead end;” the position of the stress-strain curve ceases to alter appreciably and the ultimate tensile strength remains comparatively constant on continued heating. An inspection of the results of experiments with P. P. made by Schidrowitz and Burnand‘ and by Twiss, Brazier, and Thomas6reveals in several cases a similar cessation of movement on the part of the stressstrain curve. Such a state of affairs must indicate destruction of the effective catalytic agent. 4
’
Loc. cit. LOC. c i f .
Tg-
Kg./ sq. cm. 76 159
Kg./ sq. cm. 216 325
7.75 7.15
247 280
336 298
6.50 6.15
206 1R4
5.05 4.45
EB
Ts Kg./
sq. cm. 96 230
TB Kg./ sq. cm. 300 321 267 215 194 88
EB 7.70 6.60 5.90 5.15 3.70 3.10
It will be further noticed that with 0.50 part of the accelerator a greater degree of stiffness can be attained and the rubber begins to develop comparative brittleness-i. e., the ultimate tensile strength falls off noticeably as the heating is prolonged (Column ( c ) , Table I). ( d ) The unaccelerated mixture (Column (c), Table 11) is substantially unvulcanizable a t 115’ C. The influence of the accelerator on the rate of vulcanization of this mixture is so great. that it would be unjustifiable to calculate an “acceleration factor”Kby comparing the time required in the presence and in the absence of the accelerator to bring the stress-strain curve to a given point; for with the accelerator the curve is carried far beyond any point which it can ever reach without the accelerator, no matter how long heating is continued.
Effect of Character of Zinc Oxide The preliminary experiments included some tests on the effect of varying the proportion and the character of the zinc oxide in the mixture. Results recorded in Table I11 indicate that in the absence of an accelerator there is little difference between the ordinary (XX) form of zinc oxide and a very fine form (“1-A” or “Black Label Kadox”) which has been described by Breyer,’ a t least when only 5 parts of the oxide are used; but that in the presence of the accelerator the fine oxide leads to more rapid vulcanization than does the ordinary oxide. The figures for T Sshow that the mixture containing the fine oxide vulcanizes somewhat more quickly than that containing the ordinary oxide. Further experiments (Table IV) showed that, although, in a mixture containing the accelerator, replacement of the ordinary by the very fine oxide increased the accelerating effect, when the proportion of zinc oxide was comparatively small, 5 parts or 28.75 parts (5 volumes), the accelerating effect was reduced when the proportion of zinc oxide was high -115 parts (20 volumes). I n these experiments the proportion of sulfur in the mixtures was low-3.5 parts. Hence the retarding effect of the fine oxide when used in large amount is probably due to the greater ease with which this oxide reacts with sulfur, thus reducing the active mass of sulfur available for vulcanization. Comparison of Piperidinium a n d Zinc Salts of Pentamethylenedithiocarbamic Acid On account of the strong influence of zinc oxide on the accelerating activity of P. P. and other disubstituted ammonium disubstituted dithiocarbamates, it is not surprising that the zinc salts of dithiocarbamic acids have been found to be powerful accelerators.8 However, no quantitative comparisons have hitherto been made of the degree of activity of corresponding zinc and disubstituted ammonium salts of these acids. If the activity of the latter salts is due entirely to the formation from them of zinc salts by reaction with zinc oxide, or with the salts that zinc oxide may form with the fatty acids naturally present in rubberlg then equi0 Twiss and Brazier, J . SOC.Chcm. I n d . , 39, 125 (1920). 7 I n d i a Rubber Would, 67, 19 (1922); India Rubber J . , 64, 557 ( l 9 2 2 ) , U. S. Patent 1,522,098 (1925). 8 Rruni and Romani, Rend. accad. Lincei, 30 (5). 337 (1921); India Rubber J., 62, 63 (1921). 9 Bedford and Winkelmann, THIS JOURNAL, 16, 32 (1924); Whitby, J. Soc. Chem. I n d . , 42, 369R (1923).
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1925
molecular proportions of the zinc and of the disubstituted ammonium dithiocarbamates should show substantially the same degree of activity. A comparison of the piperidinium and zinc saltslo of pentamethylenedithiocarbamic acid (Table V) shows that the zinc salt is markedly less active than an equimolecular proportion of the piperidinium salt: it fails to carry the stress-strain curve so far (T6)and to produce so high a tensile strength. Hence it appears that the saltforming molecule of piperidine in P. P. contributes to the accelerating action. The effect on the activity of the zinc salt of adding one molecular proportion of piperidine was therefore examined. The results (Section (c) Table V) show that the addition increases the accelerating activity and makes it substantially equal to that of the piperidinium salt. Influence of Piperidine on Ultra-Accelerators
The effect of methylaniline on the activity of‘ the dithiocarbamic acid derived from methylaniline was then examined to ascertain whether the enhancing effect of a base on the activity of the zinc salt of the dithiocarbamic acid derived from that base is general. The results (Table VI) show that one molecular equivalent of methylaniline has only a slight effect on the activity of zinc phenylmethyldithiocarbamate. However, piperidine again greatly increases the rate of vulcanization and the maximal tensile strength. The mode of action of the added piperidine will be discussed in a later paper. Any attempt to express in quantitative terms the effect of piperidine on the activity of the two zinc dithiocarbamates meets a difficulty similar to that noticed in connection with the “acceleration factor” of P. P.; for the piperidine leads to a degree of stiffness in the vulcanizate which, with the given proportion of accelerator and a t the given temperature, is unattainable in its absence. Hence no valid comparison of the activity of the accelerator with and without piperidine can be made on the basis of the time required to bring the stress-strain curve to a given position. For example, if a stiffness represented by Tg = 100 were taken as a basis of comparison, it might perhaps be stated that one molecular
** A recrystallized sample of the zinc salt was used. For its preparation and physical properties, see Whitby and Matheson, Proc. Roy. SOC.Canada, 18, 111 (1924). T a b l e 111-Comparison
933
proportion of piperidine doubles the rate of vulcanization of the two zinc dithiocarbamates in question. If a stiffness represented by TB= 230 were taken as a basis of comparison, it would have to be stated that piperidine increases the rate of vulcanization indefinitely, since the stocks containing no piperidine are incapable of reaching such a degree of stiffness no matter how long they are heated. Transient Catalysis
Reference has already been made to the phenomenon of curing coming to a “dead end” in the presence of an ultraaccelerator. The same phenomenon may be seen in the stocks to which Table VI refers. After a certain period of heating the state of cure (best indicated by the stiffness, Ts) ceases to change substantially. Thus the state of cure of Stocks (a) and (b) is changed only very slightly by doubling 6
I
MlrlUfCS
Progress of C o m b i n a t i o n of S u l f u r in S t o c k s A, B , C, T a b l e VI1
the period of cure-from 60 to 120 minutes. This phenomenon was studied more closely in accelerated stocks containing as the accelerator di-0-thionaphthoyl disulfide, with the results shown in Table VII. The results for Stock B show incidentally that one molecular proportion of piperidine markedly enhances the accelerating effect of the disulfide. The vulcanization coefficients of the various vul-
of Ordinary a n d Very F i n e Zinc Oxide
-
A-Unaccelerated stock: rubber 100, sulfur 10, zinc oxide 5 B-Accelerated stock: rubber 100, sulfur 7.5, zinc oxide 5, P.P.0.33 A B-Ordinary ZnO-Fine ZnO-Ordinary ZnO-Fine Minutes Ts TB Ts TB Minutes T6 TB To at Kg./ Kg./ Kg./ Kg./ at Kg./ Kg./ Kg./ 141OC. sq. cm. sq. cm. EB sq. cm. sq. cm. EB 115°C. sq. cm. sq. cm. EB sa. cm. 155 8.20 36 147 8.40 120 41 20 66 233 8.16 166 7.30 67 201 40 134 7.60 IS0 75 324 7.60 1%
32 32
240 300
3.60 3.05
31 26
60 90 120 150
3:60 2.70
164 195 229 230
335 330 316 306
7.30 7.40 6.60 6.35
188 219 230 245
-
--ZnO--
TB Kg./ s o . cm. 189 344 328 319 295 300
En 8.10 7.50 7.05 6.75 6.25 6.15
T a b l e IV-Comparison of Ordinary a n d Very F i n e Zinc Oxide (All tests except D were carried out on a batch of smoked sheet. D was carried out on latex crepe,) Basal stock: rubber 100 sulfur 3.5, accelerator (P. P.) 0.25parts -Ordinary ZnO-Fine ZAO-Ordinary Zn+-Fine Zn0Minutes at 115’C.
30 60 90 120 150 180 30 60 90 120 150 195
Ts Kg./ sq. cm.
12 33 40 43 5s 63 52 105 149 167 193 203
TB
7.6
Kg./ sq. cm.
44 124 169 224 231 222 160 244 271 313 317 339
Kg./ EB sq. cm. C-5 $arts ZnO 20 43
81
TB Kg./ sq. cm.
Ts
EB
79 190 241 272
66 71 280 78 254 E--28.75 parts (5 sols.) ZnO 8.25 64 182 8.00 123 249 7.10 160 309 7.30 172 330 7.10 176 335 7.05 193 332
Kg./ sq. cm.
TB Kg./ sq. cm.
19 42 51 65
73 174 219 256
86
260
EB D-5
139 166 186 190 214
I98 259 262 267 281
TB Kg./ sq. cm.
EB
purls ZnO
21 55 73 82 F-115
7.85 7.50 7.25 7.20 7.40 7.05
TS Kg./ sq. cm.
110 238 258 266
103 266 parts (20 vols.) ZnO 65 7.00 49 7.00 96 170 7.00 121 226 6.60 144 241 6.50 150 281
7.15 7.10 7.10 7.50 7.50
INDUSTRIAL AND ENGIA-EERING CHEJlISTRY
934
Vol. 17, No. 9
Table V Basal stock: rubber 100, sulfur 10, zinc oxide 5 WITHPIPERIDIWIUM PENTAMETHYLENE-DITHIOCARBAMATE (0.50 PART)-YWITH ZINC PENTAMETHYLEWDITHIOCARBAMATE ( 0 . 3 9 2 PART)-(a) ( b ) Alone ( 6 ) Plus piperidine (0.173 p a r l ) Minutes -Teat Kg./ 1 1 5 0 ~ .sq. cm. 93 10 198 20 244 30 262 40 60 90
-TB-
--TBLb:/ sq. in. 1330 2810 3470
3T3:J
Kg./ Lb,/ sq. cm. sq. in. 275 3910 326 4640 280 4100 256 3650 199 2530 64 910
EB 7.50 7.00 6.10 5.95 5.00 2.85
Kg./ sq. cm. 47 96 144
168 150
-TBLb,/ sq. in. 25.50 3620
EB 8.2;
2050 2400 2560
3900 3600 2910
7.15 6.i0 6.15
274 253 20.5
canizates, recorded in the table, are shown graphically in the accompanying figure. From Table VI1 it is clear that in all three stocks the accelerator has practically exhausted itself within 60 minutes of the commencement of heating. After 60 minutes the state of cure, as judged by the stiffness of the vulcanizate (re)progresses no further-indeed, in Stock A some retrogression apparently occurs. The rate of combination of sulfur with the rubber also falls off greatly after 60 minutes, although not so sharply as does the advance in stiffness. Stocks A and B show what has been called “flat curing” or a broad “curing plateau;’’ that is, the tensile properties of the vulcanizate change comparatively little over a wide range of curing times. This plateau effect is obviously due to the circumstance that after the catalyst has largely or entirely disappeared the temperature a t which the rubber is being heated (115’ C.) is not high enough to bring about any ordinary unaccelerated vulcanization. It cannot, of course, be ascribed to lack of sulfur, as more than 7 parts of free sulfur are stlll present when the rubber attains a static condition. I n Stock C the plateau is not so broad as in the other two stocks, presumably because the amount of accelerator originally added was higher than that in the other stocks, and consequently there is a slight residual accelerating effectsufficient to carry the vulcanizate into a state of overcure. At the temperature in question vulcanization by sulfur alone proceeds so slowly that the entire effect observed must be ascribed to the accelerator. It is therefore of interest to note that the effect of the accelerator (a) on the stiffness, ( b ) on the ultimate tensile strength, and (c) on the combination of sulfur does not entirely correspond. After a certain period of heating the stiffness ceases to change, but the tensile strength still changes somewhat; the stress-strain curve no longer changes its position, but breaks off short. Combination of sulfur continues, although after the maximum stiffness has been reached the rate of combination falls off greatly. I n Stock C the rate of combination of sulfur is considerably higher than in Stock B, although wlcanization, as judged by the tensile properties, progresses a t approximately the same rate. Since the present paper was read, Twiss and Thomas” have remarked that dithiocarbamates and thiuram disulfides when used in the presence of zinc oxide are active for only 11
J. SOC.Chem. I n d , 44, lOOT (1925)
-TB-
-T4-
Lh,/ Kg./ sq. in. sq. cm. 676 180 1345 254
i.aa
Lb./
Kg./ sq. cm. 92 IS1
sq. in. 1310 2570
242
3445
Kg./
Lb,/
sq. cm. sq. in.
EB
275 325
3960 ,4625
7.80 7.00
293
4170 1040 560
6.20 4.05 2 45
a limited period of heating, and have applied to accelerators showing such an effect the term “fugitive.” The present authors have always found that, if conditions are suitably chosen, all ultra-accelerators in the classes of disubstituted dithiocarbamates, the corresponding thiuram mono- and disulfides, salts of dithiocarboxylic acids, and the corresponding dithioacyl disulfides appear to be active for only a limited time. I t ’ i s reasonable to suppose that this limitation is due to disappearance of the catalytically active agent. Table VI Basal stock: rubber 100, sulfur 10, zinc oxide 5, zinc carbamate 0.437 parto ( b ) Methylaniline (a) No addition 10.218 part) added TB To T6 TB Minutes Kg./ Kg./ Kg./ Kg./ at sq. sq. 39. sq. 115’ C. cm. cm. EB cm. cm. EB 10 44 172 8.10 53 191 8.25 7.50 20 71 226 7.65 07 251 40 121 276 7.35 138 279 7.26 60 131 260 7.20 145 279 7.15 90 129 252 7.10 148 252 7.00 120 134 247 7.00 142 250 7.05 a Equivalent t o 0.50 part P. P.
phenylmethyldithio(6)
(0.173 TI Kg./ sq. cm. 102 190 230 231
Piperidine part) added
TB Kg./ sq.
crn. 275 330 319 292 251 237
Ultra-Accelerators and Tensile Strength
-4 comparison of the result of vulcanizing the mixture rubber 100, sulfur 10, zinc oxide 5 in the absence of an accelerator with the results of vulcanizing the mixture in the presence of powerful accelerators shows that accelerators are capable of producing a degree of tensile strength that is unattainable in their absence. Thus, whereas the unaccelerated mixture is capable of attaining only a stiffness represented by Te = ca. 45 kg. per sq. cm. and ultimate tensile strength of a t most 140 kg. per sq. cm. (2000 pounds per sq. in.), when powerfully accelerated the mixture may attain a stiffness represented by Te = > 200 kg. per sq. cm. and an ultimate tensile strength of nearly 350 kg. per sq. cm. (5000 pounds per sq. in.). Some writers have considered that the greater strength produced in rubber by accelerated vulcanization is due merely t o the shorter time for which the rubber is heated or the lower temperature to which it is subjected. They would apparently attribute the extra strength due to accelerators solely to the fact that accelerators save the rubber largely from the weakening effect of heating. The present authors believe that the merely negative or “sparing” effect of ultra-accelerators is insufficient to explain the remarkable tensile re-
Table VI1 di-a-thionaphthoyl diw!fide. Basal stock, rubber 100, sulfur 10. zinc oxide 5 -C-0.816 B-0.413 01. -r 0.173 P l p i p e r i d i n e pariaKg./ TB Td TB 1-ulcn. Vulcn. Kg / Kg./ s q . cm. sq. cm. coeff. EB coeff. sq. cm. s q . em. EB 82 19 1.08 158 9.10 9.50 0.65 42 TO 5.iB 1.18 ,a 258 8.10 1 83 243 261 120 2.15 S.35 1.65 124 ?so 7.50 23s 135 19s 150 223 160 6 . 4 0 2 . 6 5 ‘706 2 74 163 7.80 176 162 232 8.50 3.00 3.26 163 7.70 62 3.44 6.20 181 3.s2 16S i.45 1.14 7.45 4.26 7.40 4.39 7.25
Accelerator: p.4-0.413
TS TB Minutes Kg./ Kg./ at sa. cm. sa. cni. 115’ C. 11 40 10 40 166 20 56 204 30 40 50 240 60 s9 236 90 207 150 SS i6 17s 210 1i4 270 i4 159 73 330 a Equivalent t o 0.50 part P. P.
EB 7.60 7.00 6.60 6.3O 6.10 6.00
pariVulcn.
EB 9.25
!.!?
1.22
7.00 6.50
6.60 6.20 4.40
coeff. 1.15 1.96 2.99 :3.66 4.17 4.54 4.97
5,ST
September, 1925
I N D C S T R I A L ,4SD EAVGINEERISGCHEMISTRY
sults which they are capable of producing. They believe that ultra-accelerators have a positive effect-that’ they improve the tensile properties of rubber chiefly because of the vigor with which they bring about the colloidal change or the polymerization on which dcanization depends. (The combination of sulfur with caoutchouc is probably only incidental to this main colloidal or polymerization
935
effect.) The rate a t which and the extent to which the stiffness of rubber is advanced in progressive curing is probably a better measure of the vigor of the action in question, and thus of the potency of a given accelerator, than is the tensile strength or the x-ulcanization coefficient. It is noten-orthy that the most vigorous accelerators display most clearly the phenomenon of transient catalysis.
Adjustment of pH of Culture Media under Sterile Conditions’ By Leo M. Christensen and Ellis I. Fulmer IOWASTATECOLLEGE, AMES,1.4.
T
H E accompanying apparatus for adjusting the hydrogenion concentration of culture media can be readily sterilized and allows sterile conditions to be maintained during the adjustment. Its height may be adjusted to the size of the autoclave by using a smaller buret or by lowering the buret. The shield should be of Pyrex glass to allow repeated flaming. The rest of the apparatus may be of ordinary soft glass, although Pyrex is preferable. As there is no stopcock in the line between the flask and the buret there is little chance for contamination. Air entering the system first passes through the U-tubes B, containing glass beads and just enough concentrated sulfuric acid t o fill the bottom of the tube. The beads become covered with a film of acid, so that a large surface of acid is presented and the air is effectively sterilized. Care must be taken to prevent splashing the acid out into the side arms. The soda lime tube absorbs the carbon dioxide from the air and should be used on the acid and water as well as the alkali apparatus. It is connected tc the lower U-tube by rubber tubing. The buret is filled by applying suction to the upper U-tube a t H . The stopcock I prevents too rapid filling of the buret and the consequent too rapid passage of air through the U-tubes, which might cause splashing of the acid. When the buret is filled, suction is removed and the stopcock I is opened and the buret is ready for use. The shield keeps out any organisms 1% hile the reagent is being added. Three such pieces of apparatus must be set up for pH adjustment-one for acid, one for alkali, and one for water. The strength of alkali or acid depends upon the range of hydrogen-ion concentration to be used and the amount of buffer present; 0.1 N is generally a convenient strength. The flasks will hold 1 liter without boiling over in the autoclave if care is taken. After the flask is filled it is closed with a cotton plug. The stopcocks should be closed and caps placed over the openings of the U-tubes to prevent entrance of an undue amount of water during autoclaving. After sterilization the cotton plug is trimmed level on top, pushed down into the neck of the flask, and covered with sealing wax t o make an air-tight stopper. The soda lime tubes are then connected. The condensate that collects on the sides of the flask must be rinsed down by shaking each time before use. The approximate amount of acid or alkali required to give the extremes of the series should be previously determined. The medium should then be made up so‘that the addition of this amount of reagent or water -rill give the desired concentration, with allowance for loss by evaporation during sterili1
Received June 15, 1926.
zation. Equal amounts of this medium are measured out carefully and sterilized. Enough flasks must be made up to provide for the determination of the titration curve. These flasks are sterilized exactly like those to be inoculated. When they have cooled one flask is diluted with water t o the desired concentration, the next with 1 cc. of acid and the required amount of water, the next with 2 cc. of acid and the required amount of water, and so on to cover the range of hydrogen-ion concentration desired. The pH of each solution thus obtained is determined and the titration curve is plotted from this data. From this curve the amount of acid or alkali and water required to produce a given pH a t the desired concentration of the medium can be read. The adjustment can then be carried out rapidly and with little chance for contamination if care is taken to flame the flask and the inside of the shield thoroughly.
A--_”-liter Pyrex flask B-U-tubes with glass beads and sulfuric acid C-Calcium chloride tube filled with soda lime D-2.3.~~. buret T h e shield is fitted with a stopper consisting of two aluminium disks, F . and cotton, G, fastened toaether and held t o the stoncock by means of copper wire, E
