INDUSTRIAL AND ENGINEERING CHEMISTRY
732
provement. This has been found to be true. In compounds containing substantial amounts of alkali process reclaim, a low sulfur and a high accelerator ratio, there is sometimes a rapid reversion causing blowing after the proper cure has been reached. Compounds containing stearic acid show less tendency to reversion under these conditions than compounds containing other softeners. Conclusions
1-Stearic acid does not compare with other softeners in plasticizing efficiency when used in contact with vulcanized rubber scrap during devulcanization.
VOl. 21, KO.8
2-Stearic acid when added as a softener to devulcanized scrap on the mill prior to refining imparts properties which are very desirable. (a) Makes the reclaim batch more plastic, ( b ) improves tubing and calendering properties, (c) reduces nerve without production of excess tack, (d) improves curing properties (higher tensile strength, higher modulus, and improved molding properties) of reclaimed rubber, ( e ) when pigments are added it gives better dispersion with improved physical properties. 3-Stearic acid when used in compounds containing reclaimed rubber improves the curing properties of these compounds.
Stearic Acid in Litharge-Cured Rubber Compounds J. R. Sheppard THEEAGLE-PICHER LEADCOMPANY, JOPLIN, Mo.
I
T HAS been known at least since 1912 that the resins present in crude rubber are essential to vulcanization with litharge. I n that year Weber ( 4 ) reported that deresinated rubber mill not cure and concluded that “resins play an active part in the vulcanization;” but he formulated no theory of the way in which they function. At that time the composition of the rubber resins was not so well known as now. I n 1916 Stevens (3) verified Weber’s conclusions, but believed his findings supported the Esch and Auerbach ( 2 ) theory that litharge accelerated vulcanization through the rise in temperature occasioned by an exothermal reaction. It is now well understood that resins promote vulcanization through their content of organic acid. It was proposed by Bedford and Winkelmann (1) in 1924 that litharge vulcanization proreeds by the following steps: (a) reaction of lead oxide with an organic acid (naturally present in the resin or added during the mixing) to form a rubber-soluble soap; (b) reaction of the lead soap with hydrogen sulfide to form a hydrosulfide salt or a hydrosulfide; ( c ) reaction of the latter with sulfur t o form a disulfide and then a polysulfide; (d) decomposition of the (unstable) polysulfide to yield a very active form of sulfur-the ultimate vulcanizer. The polysulfide theory has been generally accepted as accounting adequately for the known facts. It is clear, then, that the efficiency of litharge as an accelerator is absolutely dependent upon the existence of an organic acid in the rubber as mixed. The mere statement of this general principle, however, by no means gives the compounder all the information he desires in the matter of organic acids as related t o litharge curing. Various questions of practical importance arise. I n the present paper an attempt will be made not comprehensively or systematically to survey all these numerous questions, but only to illustrate a number of scattered but specific cases. It is advisable not to generalize too broadly, but to confine the conclusions to the particular conditions applying to each case. Low-Grade and High-Grade Crude Rubbers-High Oxide Stock
Zinc
Several years ago the writer made a study of various softeners in a high zinc oxide litharge compound, using the formula: Rubber Sulfur Litharge Zinc oxide Softener
100 8 20
100 5
Two series of stocks were made up, one using a typical “highgrade” crude (smoked sheet) and the other a “low-grade”
wild rubber (Lapori). Each series was master-batched up to the point of adding the various softeners, which were added to portions of the master batches on the laboratory mill. The stocks were vulcanized in the press a t 141’ C. (40 pounds steam) over a series of cures in order adequately to develop the optimum cure and to show the effect of the various softeners on the two grades of rubber. Stressstrains were determined on all the vulcanizates. The results were entirely different for the two crudes, as will be seen from Tables I and 11. I n Table I the physical properties of the stocks of the Lapori series a t their optimum cures (defined by maximum energy) are shown. While the control mix (without softener) came to an optimum in 150 minutes with a tensile of 2115 pounds per square inch, most of the softeners reduced the time to reach optimum and raised the properties a t optimum. This table should not be used to draw fine distinctions between two given softeners, particularly where they are close together in the table. But the outstanding fact is that those softeners which are either acids or salts, or contain an acid, and which therefore can bring the lead into solution in the rubber as a salt, are the ones which significantly aided the cure. On the other hand, in the corresponding data for the smoked sheet series we find no enhancement of cure by any of the softeners. As measured by time to reach optimum and (or) properties a t optimum, most of the softeners inflicted some damage (slight or moderate). It should be noted, however, that this damage in the case of smoked sheet is distinctly less than the benefit conferred by the acid class of softeners on Lapori. A further insight into the results themselves is furnished by Table 11, giving the stress-strains for all the cures on both Lapori and smoked sheet compounded without and with 5 per cent stearic acid. The data for Lapori emphasize how far apart in their properties the control stock and the stearic acid stock are a t a given cure. For example, on this basis stearic acid about doubled the modulus of Lapori. The presence of bloom up to 180 minutes without stearic and its disappearance between 60 and 90 minutes with stearic further confirm the accelerating effect of this acid. I n sharp contrast to these are the results on smoked sheet. Here there is, if anything, a retardation of cure by stearic and a t a given cure the modulus is greatly lowered, although the tensile is not much affected. This serves to emphasize that the observed stiffening of the Lapori vulcanizate is the net result of two opposing tendencies: (a) a stiffening due t o the enhancement of the cure, consequent in turn upon the acid bringing more lead into the available, soluble form;
IiVDCSTRIA L Ah-D ENGIiVEERIAVGCHEMISTRY
August, 1929 T a b l e I-Physical
733
Data for Cures of M a x i m u m Resilient Energy of Litharge-Rubber Stocks Activated w i t h Various Softeners. a t 141' C. (40 P o u n d s S t e a m )
Cured in Press
~
CURE op MAX.ENERGY
SOFTENER
1
Mzn.
TENSILE AT ELONGATIOS OF:
300%
I
L b s . / s q . an.
30 30 60 60 60 60 60 60
455 445 395 475 477 495 585 480
Stearic acid
~
S U~ B P E R M~A N E S T~
SET
Lbs./sq. in.
LITHARGE-LAPORI
Rubberite Benzoic acid Rosin Zinc stearate Pine t a r Rosin oil Zinc benzoate Oleic acid
E
TENSILE
600%
%
R
, P.
ENERGY
T .
Ft-lbs./cu
STOCK
2327 2270 2090 2310 2585 2625 2790 2542
3200 2660 3262 3000 3067 2875 2817 2850
685
640 705 670 645 630 605 630
51 36 54 53 48 46 46 52
219 170 229 200 198 181 170 180
546 433 542 510 499 464 457 446
60
450
2312
2622
635
44
166
419
Phenol Hardwood pitch Lead phenolate C u m a r , grade 310 n-Butyl salicylate Glycerol
90 90 90 90 90 120
642 537 417 305 347 195
2267 2030 1545 1157 1365 730
2535 2232 2142 1760 1837 1150
650 630 675 705 670 705
38 32 42 35 32 38
164 141 144 124 123 81
493 405 382 323 320 208
S i 1 (control)
150
487
1695
2115
655
29
138
387
150 150
397 482
1367 1640
1540 1890
640 640
31 33
95 121
291 357
Piil (control)
15
562
2970
665
36
237
635
Hardwood pitch C u m a r , grade 310 C u m a r , varnish grade Zinc stearate n - B u t y l salicylate Lead phenolate C u m a r , grade 315 Oleic acid Phenol Rubberite Zinc benzoate Kosin oil
15 15 15 20 20 30 30 30 30 30 30 30
627 530 517 420 577 1085 675 753 902 740 655 730
2977 2850 2782 2242 2795
655 660 660 730 670 590 650 610 600 620 630 615
31 37 37 54 35 45 49 52 43 42 42 42
220 226 221 275 226 209 239 223 212 218 218 206
C u m a r , grade 315 C u m a r , varnish grade
>
LITHARGE-SXOKED
in.
S H E E T STOCK
3567
3062 3565 3527 3280 3102 3192
Stearic acid
30
580
3067
3430
635
52
217
564
Benzoic acid Glycerol Pine t a r Rosin
30 30 45 45
620 560 718 715
3240 2480 3075 3220
3540 3015 3725 3485
620 650 650 625
42 49 46 44
219 19s 242 217
562 510 654 597
Conversion factor for tensiles: 1 Ib. per sq. in. = 0.0703 kg. per sq. cm. Conversion factor for energies: 1 ft-lb. per cu. in. = 0.84 kg-cm. per cc.
Table 11-Effect of 5 Per Cent Stearic Acid in Litharge Stock Loaded w i t h Zinc Oxide
TENSILE AT ELOSGATION OF:
CUREAT 1410
c.
100%
200%
300%
4OOC4
500%
600%
700%
SUBPERTENSILE ELOSG.M A N E X T T. P. SET
800%
Lbs./sq. in.
I
~
ENERGYBLOOM"
l ;
%
1593 1625 1872 2115 2000
715 690 670 653 650
32 24 25 29 27
114 112 125 138 130
~.~ 2R4
2702 2622 2050
675 635 565
45 44 40
182 166 116
410 419 320
3180 3367 3573
675 663 655
36 36 39
214 237 234
521 635 633
825 710 700 635
51 45 48 52 .53 ..
259 222 235 217
547 517 554 564
Lbs./sq. in.
Fl-lbs./cu. in.
LAPORI TVITH N O STEARIC ACID
60
1
85 !34 137 145
180
150 165 222 300 280
235 282 372 487 465
350 420 555 740 692
595 io0 920 1090 1062
213 300 367
320 450 520
310
1022 134i 1477
1000 1062 1402 1695 1652
1
1520
~~
~~~
283 335 387 365
+
3 +
4-
L h P O R l TVITH 6 P E R C E N T STEARIC ACID
137 90
232
717 837
1935 2312
+ + -
S V O K E D S H E E T TVVITH K O STEARIC ACID
10
20 15
I
105 167 19i
257 332 ,400
425 562 652
767 1065 1107
1407 1880 1940
2395 2970 2992
,
~~~
SMOKED S H E E T W I T H 5 P E R C E N T STEARIC ACID
10
S5
30 45
157 165 238 277
a
130 210 270 385 ,430
202 335 397 580 665
320 570 645 1025 1113
575 1117 1272 1868 1970
1110 1982 2222 3067 3040
1922 3055 3366
2922
I
3140 3135 3365 3430 3040
600
1 8.2_ .
4Q8 ._ -
A plus sign (+) means sulfur bloom present, a minus sign ( - ) means bloom absent.
( b ) a softening quite analogous to that of any non-acidic softener such as mineral oil or mineral rubber. What the net result will be in a given case depends upon the magnitude of the two component effects. Where, as in many low-grade crudes, the natural resin content is low or deficient in acid, an addition of stearic acid, oleic acid, pine tar, or similar
material greatly accelerates otherwise slow curing and enhances otherwise low properties; on the other hand, where the natural acid content is relatively high, a further addition shortens the time of cure little or none (it may retard) and the resulting observed effect is likely to be a distinct softening at a given cure.
734
INDUSTRIAL AND ENGINEERING CHEMISTRY
The effect of the softeners on set should be noted. This was measured on specimens broken in the course of determining stress-strains and was taken from 5 to 10 seconds after the break. With few exceptions set was increased by the softeners, this being true for both grades of rubber but more marked with smoked sheet. This phenomenon is doubtless closely allied to softening action, both being expressions of the reduction of elastic properties. Hardwood pitch-which however only moderately enhanced the curing of Lapori-in the case of both grades of crude had about the least effect on set.
Vol. 21, No. 8
by Table IV is significant a t all), while in Table I1 for smoked sheet 5 per cent stearic cut the modulus in half; and why with the black have we evidence of (slight) activation of cure but with zinc oxide appreciable retardation by the stearic added? Two possible explanations suggest themselves: (1) With gas black an appreciable part of the stearic may be adsorbed, leaving only the free part to soften the rubber phase. If this is the case, then conceivably there was a moderate deficiency of free organic acid in 4984 which was made up in 49A5 by the added acid. A much smaller adsorptive effect is t o be expected from the zinc oxide. (2) Probably the dispersion of the black was improved by the stearic, introducing thus a stiffening factor which would operate against the softening action of the stearic on the rubber phase. If so, with zinc oxide there would be no corresponding stiffening factor consequent upon improved dispersion and higher reenforcement, since zinc oxide disperses readily without added stearic. Stearic Acid and Litharge in Conjunction with Mercaptobenzothiazole
The observations made above are on stearic acid in stocks accelerated by litharge. I t is of interest to note what obtains when litharge is used in conjunction with an organic accelerator as an “activator,” analogously to zinc oxide. For this purpose mercaptobenzothiazole (Captax) was employed, since (a) litharge strongly activates this accelerator and ( b ) it is well known that when zinc oxide is used as the activator a full curing propensity is highly dependent on stearic acid, especially in the presence of gas black. There is little doubt in the writer’s mind that a smaller quantity of softener than used throughout the present series mould, in the case of the more active ones a t least, have furnished maximum activation. It is clear that it is useless to go beyond a certain limit which, in so far as curing considerations are concerned, is perhaps nil for standard smoked 5heet and for the present type of formula. In the case of Lapori, perhaps from 2 to 3 per cent of stearic acid would have yielded maximum activation. To exceed the limit for full activation means to increase the softening action on the vulcanizate.
no
Stearic Acid in a Pure Gum and a Gas Black Stock
In the remaining cases we are dealing with smoked sheet, which normally does not require an addition of acid in so far as enhancement of cure with litharge is concerned. Tables I11 and IV illustrate this, Table I11 showing results for a pure gum mix and Table IV for a gas black stock. The pure gum stock (4982) with 1 per cent of stearic acid follows closely in its properties that with no added acid (49A1), and has an identical rate of cure. There is a very slight softening effect o n the part of the stearic. With gas black stearic improved the tensile significantly. As the object mas t o determine not the dispersing action of the stearic on the black, but rather the effect on curing, the softener was so added as t o minimize any benefit it might confer on dispersion; i. e., the rubber, sulfur, black, and litharge were master-batched and 4 per cent stearic was subsequently milled into half of this batch to yield 49A5. There is meager, and only meager, evidence of enhancement of cure by the stearic. The increase in tensile a t all cures, on the other hand, is distinct; it is due to increased elongation (not stiffness), and probably represents improved dispersion, notwithstanding the precaution to the contrary. Why have we only a slight softening of the gas black stock b y 4 per cent stearic (assuming that the softening exhibited
1
1 (0
20
I
f i f l P Fffcct o f Jfcorrc n o d m i , f h o r g 4 o p f r r r ond Zinc Ox/de.fopfgx Pure Gum S t o c k s Cured of 130’G ‘Cure ,n Mmufsg o f / 3 G % (2r/& SPeoml 40
80
/bo
110
10
40
1
’
80
/bd
320
Two base stocks were master-batched: PUREG L MBASE Smoked sheet 100 Sulfur 4 Captax 0 7.5
G4S
BLACKBASE
Smoked sheet Sulfur Captax Gas black
100 4 0 75
45
Each stock was then subdivided into four parts each and to these portions the following additions were made: Litharge Litharge Stearic acid Zinc oxide Zinc oxide Stearic acid
S S 4
10 10 4
Thus there mere studied all the combinations which arise from varying the following items: Reenforcement (and concomitant adsorptive effects).. , , , . , , . . , . . . , . , , . . , Pure gum v s . high gas black Activation.. . . . . , . . . . . . , . , . . . . . , . . . . , . Litharge u s . zinc oxide Acidic softener (added), . . . , . . , , . . . . . . Stearic absent u s . stearic present
735
I N D U S T R I A L A N D ENGINEERING CHE.VIISTRY
August, 1929
of 1 Per C e n t Stearic Acid i n a Pure G u m Litharge Stock
Table 111-Effect
FORMULAS 49A1 100 5 15
Smoked sheet Sulfur Litharge Stearic acid
49A2 109 J
15
...
1
CUREAT
I
TENSILE AT ELONGATION OF:
49A2--1
200%
40070
300%
J
15
45 4
TENSILE 600%
500%
STEARIC
ELONG.
200%
3007,
400%
1
P E R C E N T S T E A R I C ACID
500%
600y0
700%
TESSILE
SOOc7,
Lbs./sq. i n . 49BS--8
64
40 80
96 79
81
70 106
BUI~KCY
ACID
TENSILE AT ELONGATIOX OF:
10
T. P.
iOO(7,
of Stearic Acid o n Litharge-Captax a n d Zinc Oxide-Captax Pure G u m Stocks, Cured a t 141O C. (40 Pounds S t e a m )
1
80
100
...
49A6-4
40
49A.5
100
5 15 45
49A4-XO
,win.
49.44
TENSILE AT ELONGATIOX OF:
100%
/ L U .i n .
P E R C E X T S T E A R I C ACID
Smoked shei:t Sulfur Litharge Gas black Stearic acid
,
Fl-lbs.
STEARIC ACID
FORXULAS
Table V-Effect
%
ENERGY
of 4 Per C e n t Stearic Acid i n a Gas Black Litharge Stock
Table IV-Effect
100%
T. P.
~
49A1-NO
CUREAT 141O c.
ELONG.
1 Lbs./sq. in.
Lbs./sq. in. ~
TENSILE
115 142 144 135 128
93 161 185 139 “12
158 244 248 2 50 227
285 406 433 425 345
49 5 670 742 750 582
947 1500 1600 1540 1045
49B2--10
P E R C E N T Z I N C O X I D E , 4 P E R C E X T S T E 4 K I C ACID
432
682 681
566
465 7 S8 I040
1270 1300
860
1605 2080
2460
!
1960 2800 2900 2850 1960
PER C E N T L I T H A R G E , 4 P E R C E N T S T E A R I C A C I D
279 402
T. P.
ESERGY
Lbs./sg. i n
%$2800 3300 3150 3150 2320
765 725 71.5 720 735
214 239 225 227 171
381 433 427 435 344
810 750 700 660 590
253 215
220 151
426 531 508 473 309
Fl-lbs./cu. i n .
N O SETARIC ACID
49B6--8
163 253 324 405
~
PER CENT LITHARGE,
ELONG.
1619 2870 3610
‘2600
~
i:
3750 32.50
2380
140
INDUSTRIAL A N D ENGINEERING CHEMISTRY
736
I
It will be noted that here, as in an earlier experiment, the stearic was added after the milling of the gas black into the gum, in order to influence dispersion in a minimal way. These eight stocks were then press-cured over a range a t 141O C. (40 pounds steam) and a t 130' C. (25 pounds steam) and stress-strains were determined on all vulcanizates. The complete physical data including tensile product (T. P.) and energy are displayed in Tables V to VIII, and certain of these data are, for readier assimilation, graphed in Figures 1 t o 4. I n the graphs will be found breaking tensiles, elongations, and moduli, the latter being represented by tensiles a t 600 per cent elongation ( T w ) for pure gum stocks and a t 400 per cent elongation ( T m ) for gas black stocks. Each graph is based directly on one of the tablesfor example, Figure 1 on Table V, 2 on VI, etc. The curves obtained with and without stearic acid (other conditions being constant) are superimposed in the same space (stearic present represented by solid lines, stearic absent by broken lines). I n this way the effect of stearic acid in a given set of conditions is observable a t a glance. In reading the graphs due account of the logarithmic time scale should be taken. The outstanding result is that the behavior of the lithargeCaptax combination toward stearic acid is in sharp contrast Table VI-Effect
Min.
to that of the zinc oxide-Captax combination. The former is seen to be relatively independent of added stearic for its activity, coming to approximately full properties without the acid; the latter is highly dependent on, and produces its maximum results only with, added acid. These general conclusions hold for both the pure gum and the gas black stocks, though more pronounced for the black. Examining the graphs in detail we find as follows: I n Figure 1 (pure gum mixes cured a t 141' C.), in the case of litharge the tensile and modulus are appreciably raised by stearic acid but the properties as a whole are not changed so radically as in the case of zinc oxide. With the latter we have an otherwise low modulus greatly raised and an otherwise high elongation considerably lessened, especially in the longer cures. These two effects on modulus and stretch influence the tensile oppositely so that i t has not undergone any more change than it did with litharge. Sotwithstanding the near-identity in the influence of stearic acid on breaking tensile in the case of the pure gum litharge and pure gum zinc oxide stocks, the dependence of modulus and elongation in the case of zinc oxide constitutes a difference in vulcanizing behavior which is significant from the practical standpoint certainly, and from the theoretical perhaps. Figure 2, displaying proper-
of Stearic Acid on Litharge-Captax a n d Zinc Oxide-Captax Pure G u m Stocks, Cured a t 130° C. (25 P o u n d s S t e a m ) TEBSILE AT ELOXGATKOX OF:
CURE
100%:
Vol. 21. No. 8
200%
300%
500%
400%
I
600%
700%
1 Lbs./sa. in.
Lbs./sq. in. 49B6--8
TENSILE
800%
ELOBG.
T. P.
W
ENERGY Ft-lbs./cu. in.
P E R CENT LITHARGE, N O STEARIC ACID
~~
90 120
48
72 119 125
120 167 205 230 214
169 286 319 394 363
88 74
137 142
66
88 124 167 174 146
131 187 256 298 267
219 280 443 432 473
313 452 557 689 599
49BB--8
90 120
82 97
90 120
76 97 131 134 148
101 170 219 223 245
339 457 822 893 837
164 291 339 400 393 49B2--10
4s 90 120
116 129
72 123 193 232 269
96 202 338 419 456
169 290 554 709 773
1095 1980 2265 2690 2375
2075
2555 3050 3390
840 785 750 740 735
215 240 254 249 204
362 427 415 457 380
2600
s25 820 705 715 695
214 255 247 279 222
365 474 457 513 425
3075
830 805 780 755 775
179 236 254 235 238
317 449 466 450 483
2290 3175 3740 3535 3155
775 700 635 620
sso
202 246 262 225 196
368 439 520 453 439
P E R C E N T LITHARGE, 4 P E R C E N T STEARIC ACID
49BI--10
51
566 906 1145 1380 1095
612 861 1735 1905 1805
1245 1765 3515 3610
2176
3200
P E R C E N T ZIBC O X I D E , S O STEARIC ACID
304 485 558 656 664
557 896 1085 1220 1140
1035 1780 2100 2355 2260
1845 2905
P E R C E N T ZINC O X I D E , 4 P E R C E N T S T E A R I C ACID
277 548 1020 1425 1590
506 1085 2170 2840 2900
933 2010 3740
1555
2155 2930
1
I N D U S T R I A L A N D ENGINEERING CHEMIXTRY
August, 1929 Table VII-Effect
of Stearic Acid on Litharge-Captax a n d Zinc Oxide-Captax G a s Black Stocks, Cured at 141° C. (40 P o u n d s S t e a m )
TENSILE AT ELONGATION OF: CCRE
100%
200%
300%
400%
500%
TENSILE
600%
I
Lbs./sg. in. 49Bi-8
4988-8
10 20 30 40 60 80 e
,
236 305 345 352 350 350
549 876 934 957 941 914
123 234 318 400 408 457
so
Table VIII-Effect
2060 2970 3120 3075 3025 2995
586 1085 1355 1505 1735 1980
3100 4150 4305 4200 3980
4160
I
1095 1640 2260 2405 2695 2$180
1740 2800 3310 3505 3775
2460 3765 4385
200%
300%
400%
500%
376 352
4750 4530 4670 4755 4020 3900
655 530 545 530 510 480
311 240 255 252 205 187
971 777 878 811 713 614
2975 4270 4540 4150 4020 3700
665 655 625 565 510 450
198 280 284 234 205 167
569 885 939 765 670 558
TENSILE 600%
375 576 791 959 868
827 1200 1670 1875 1745
1590 2120 2720 2935 2825
Lbs./sq. in.
4988-8
2460 3105 3905 4070 4000
3350 4200
I
15
I
180
515
%
Ft-lbs / c u . in.
700 665 59 5 545 540
306 320 291 246 240
915 1021 967 843 790
655 520 505 465
148 116 123 113
482 355 399 386
2425
650 635 550
229 299 235
678 948 826
P E R C E N T ZINC O X I D E , 4 P E R C E N T S T E A R I C ACID
I
Pornns
159
ENERGY
P E R C E N T ZINC O X I D E , N O S T E A R I C ACID
2265
30
T. P.
P E R C E N T L I T H A R G E , 4 P E R C E N T S T E A R I C ACID
4983-10
4981-10
ELOXG.
700%
L b s . / s q . in.
120 180
Ft-lbs./cu. in.
of Stearic Acid on Litharge-Captax a n d Zinc Oxide-Captax Gas Black Stocks, Cured a t 130' C. (25 Pounds S t e a m )
Mtn.
159
%
ENERGY
P E R CEHT ZINC O X I D E , K O S T E A R I C ACID
TENSILE AT ELONGATION OF: 100%
T. P.
PER CENT LITHARGE, 4 PER CENT STEARIC ACID
1175 1790 1970 1900 1850 1850
288 566 723 830 929 1105
Lbs./sq. in.
ELONG.
P E R CENT LITHARGE. N O S T E A R I C ACID
4983-10
10 20 30 40 60
737
39s 690 979 1130
829 1320 1765 2085
1455 2180 2755 3216
2170 3180 3905
ties on the same four stocks as Figure 1, but cured a t 131" C., leads to the same conclusions. The conclusion, discernible on analysis of the pure gum results, that there is a significant difference in the behavior toward stearic acid of litharge-Captax and of zinc oxideCaptax stocks, is confirmed by the data for the gas black mixes. Both Figure 3 (141" C. cures on gas black stocks) and Figure 4 (130" C. cures) show an almost complete lack of influence by added stearic when litharge is used for activation but a marked dependence on the acid when zinc oxide is used. For example, a t 141" C. either with or without stearic, the litharge-activated stocks reached a tensile of 4400 to 4700 early in the cure and maintained this value approximately unchanged to the 40-minute cure. On the
2995 4310
3525 4140
46.5
193
fi44
other hand, while the zinc oxide stock with stearic showed a maximum tensile of 4540 reached in the 30-minute cure, without stearic only 2750 was reached (also in 30 minutes). Similarly, a t 130" C. the litharge stocks, with or without stearic, maintained a tensile of approximately 4400 to 4900 over the curing range of 15 to 180 minutes. With zinc oxide and stearic a maximum of 4710 was reached in 60 minutes, but without stearic a maximum of only 2440 was reached (in 120 minutes). There is obviously some, a t least quantitative, difference in the mechanism of mercaptobenzothiazole acceleration in conjunction with the two oxide activators, a difference which is accentuated when t o the gum there is added an agent which powerfully adsorbs the organic acid and (or) the
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accelerator. Of what this difference in mechanism consists we shall not attempt to say. Possibly it has to do with the extent to which lead and zinc, respectively, are made available to the mercaptan so as to permit of mercaptide formation, this in turn being influenced by the solubilities or mobilities of lead stearate and of zinc stearate; or, on the other hand, it might have to do with a difference in the chemistry of the lead and of the zinc mercaptides after their formation. From the practical standpoint, a compound formulated along the line of 49B7 offers desirable qualities as a tread stock including high physical properties maintained over a wide range of cures. This sort of compound would appear also to be adapted to the present-day tendency toward lowtemperature cures, with the advantage which that practice has of minimizing the damage t o the rubber by heat. Because the litharge-Captax tread stock cures fully without the addition of stearic acid, it is not to be inferred that none should be added. That depends on requirements other than curing which may demand this ingredient, including obtaining a proper dispersion of the black and a desirable plasticity for tubing or calendering. But the point is that the stearic can be regulated in whatever way satisfies these other considerations with the assurance that curing requirements will have been met incidentally. Possibly in this way the amount of stearic used in general practice in a tread could be reduced, resulting in a lessened tendency toward stearic acid blooming. Summary
Although an organic acid is essential to vulcanization with litharge, smoked sheet usually has enough natural acid for
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full activation. For example, when 5 per cent stearic acid (or similar softeners) was added to a standard smoked sheet in a high zinc oxide stock the properties were lowered. On the other hand, a “low-grade” rubber, Lapori, was greatly improved in a high zinc oxide formula by stearic and by other acids. Acidic softeners in a high zinc oxide stock increased set, even when they promoted vulcanization. I n a pure gum litharge formula with smoked sheet, 1 per cent stearic had but little effect, while in a high gas black formula, 4 per cent stearic raised the tensile (probably due t o improved dispersion) but had no marked effect on rate of cure. When litharge was used as the activator for mercaptobenzothiazole, stearic acid had but little effect.either in pure gum or high gas black stocks. On the contrary, when zinc oxide was used proper curing was highly dependent on stearic, especially with gas black. The litharge-mercaptobenzothiazole compounds with gas black in tread stock proportions, either with or without stearic, yielded a high tensile over a wide range of cures. Acknowledgment
The writer wishes to acknowledge the valuable assistance of J. 8. Blakeney in the experimental work. Literature Cited (1) Bedford and Winkelmann, IND. ENG.CHEM.,16, 32 (1924). (2) Esch and Auerbach, Gummi-Mark!, 1911, 123. (3) Stevens, J . SOC. Chem. I n d . , 36, 874 (1916). (4) Weber, Orig. Comm. 8th Intern. Congr. A p P l . Chem., 9, 95 (1912)
[ENDO F SYMPOSIUM]
Economic Factors in Chemical Plant Location’ Chas. W. Cuno INDUSTRIAL BUREAUOF
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INDUSTRIAL CLUBO F ST.
N A STUDY of the development of chemical and metallurgical industry in the United States, the first generalization is not so much the tendency for the centralization of industry, although that is the most obvious, but the fact that industry is continually migrating and that this migration is due to the influence of certain definite factors of plant location. The remarkable growth of American industries in a period of scarcely more than one hundred years is marked by the constant migration of industry from points of centralization to other favorable points. Profound changes are especially notable in the essentially technical industries, such as iron and steel, fertilizer, glass, coke, alkali, paper and wood pulp, coal-tar products, paint and varnish, sulfuric acid, and soap. These are usually classed as basic industries in that they demand sources of especially cheap power, proximity t o raw materials, or an advantageous distribution, because the products are t o be sold on a highly competitive market.
I
Migration of Different Classes of Industries
Before discussing the factors of plant location, I wish to correct some commonly accepted ideas which are prevalent in this connection. There are, for instance, arbitrary state1 Received April 2, 1929. Presented under the titie “Sound Industrial Development” before the Division of Industrial and Engineering Chemistry at the 77th Meeting of the American Chemical Society, Columbus, Ohio, April 29 to May 3, 1929.
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ments made which seem to apply t o certain industries. It has been said that in the iron industry the ore goes to the fuel; that in carbide, abrasive, and aluminum manufacture the industry goes to cheap hydroelectric power; that the measure of sulfuric acid manufacture is local consumption; and that soda ash and caustic industry must sit on top of a salt mine. But these views are often superficial. The editor of the Executives Magazine (Vol. 12, No. 5, 1928) divides industries into three classes-the raw material industries, the competitive industries, and the population industries. Of the raw material industries he remarks, “industries which come t o a region because the existence of heavy raw materials there makes it necessary to build plants on the ground.” It is hardly ever true that industries come to a region because of the existence of raw materials. An examination of industrial location will show other factors are usually more important. BASICINDUSTRIES-hSiC industries whose raw material is, for the most part, not processed and whose products in the main are the raw materials for other industries must, to succeed, locate near cheap fuel or cheap power. It is true that major raw materials must be obtainable (1) advantageously as to quality and price, or a t least (2) on a competitive basis with the same industries in other localities; but these two factors alone will not serve to establish a basic industry, for basic industries demand cheap fuel and advantageous raw materials in order to compete in the national market. It is
