INDUSTRIAL A N D ENGINEERING CHEMISTRY
November, 1926
and attention must be shown to a criterion such as free sulfur. ARTIFICIAL AGING-The data we have shown as to the position of the time-aged tensile product maximum are not meant to serve as a practical guide in the selection of the best cure, but merely to indicate the general relation between the aging maximum and the vulcanizing skeleton. I n any given case the quick determination of the aging maximum can be made only on the basis of accelerated aging tests. Unfortunately, neither the heat aging test, nor the oxygen bomb, nor any other artificial aging test, can claim entire, or even approximately entire, correspondence with time aging. Thus, for instance, the mixing designed for dry heat curing will show up astonishingly well on a heat aging test, whereas another mixing, built for press curing, may fall down badly; and yet both may be equally proof against the ravages of time. Here the complexity of the technical cure reaches its climax. Some knowledge of the actual time aging behavior must be a t hand before the accelerated aging results for any given mixing can be safely applied. With such general knowledge the aging maximum can be determined quite closely by an artificially aged series. One of our correspondents6 has been kind eriough’to state a convenient short-cut to this determination as follows: The tensile figures obtained a t cures 20 per cent below and 20 per cent above the factory cure are expected to show equal values on aging. This, it will be observed, is merely a convenient way of determining the apex of the aged curve. It is of vital importance in each case to work out the time of cure corresponding to the aged maximum, since this may be regarded as the lower limit of technical curing. 6
J. W. Schade, The B.
F.Goodrich Co.,Akron, Ohio.
1163
From this limit the technologist moves his cure toward the right to the minimum extent necessary to attain freedom from bloom, or exceptionally high modulus (soling and heels), immediate abrasive resistance, low hysteresis, etc. Until the technic of artificial aging comes closer to the effects of time, the practical compounder must continue to be ultra-conservative regarding the change of curing “skeleton.” Where he lacks all time aging data he must wait till these can be obtained before attempting a drastically new compounding set-up, based solely on artificial aging. TEAR AND TECHNICAL CURE-Tear has been briefly mentioned in connection with the standard or optimum cure. Tear is, however, already playing a useful role in certain types of technical cure determinations. An excellent beginning in the technic of tear testing is that of E. C. Zimmermann, which provides means of separating the true resistance to tear from the observed data. Tear is obviously a highly complex phenomenon. It is hoped that there will be an increasing volume of published data relating tear to other well-known physical properties both in the green and aged condition. Tear determinations become less sensitive to cure the higher the pigmentation, and also grain effects become increasingly disturbing. Tear deserves careful attention with a view to its refinement as a guide to a state of cure which lies within the limits we have prescribed as suitable for technical cure. The chief difficulty, a t present, seems to be ready and quantitative means for its determination and calculation. Acknowledgment The writer wishes to express his deep appreciation of the important part played throughout the preparation of this paper by his colleague, D. F. Cranor.
The Present and Future of Reclaimed Rubber By H. A. Winkelmann PHILADELPHIA RUBBERWORKSCo.,AKRON,OHIO
HE consumption of reclaimed rubber is greater today than ever before in the history of the industry. The volume consumed in rubber products is larger than that of any other compounding ingredient. The fact that products made with reclaimed rubber are capable of rendering service equivalent to those without it is resulting in its greater use. A survey of the literature on reclaimed rubber shows that, despite its importance, there has been very little work published on this subject, except in the patent literature. Any further increase in the consumption of reclaimed rubber above that which is due to the normal growth of the rubber industry will depend both upon the reclaimer and rubber manufacturer. In recent years we have learned a great deal about the use of reclaimed rubber through proper compounding, with the result that it is now being used with satisfactory results where formerly it was not used a t all, or only in small amounts. Improvements in its quality can only be attained through research and development. Utilization of scrap rubber is an economic necessity. The position of the rubber industry is rather unusual in that it produces a by-product for which it is the only outlet. At present the reclaiming industry is wholly dependent upon the rubber industry. The development of other outlets for scrap or reclaimed rubber, or products made from them, would tend to stabilize the industry.
T
The relation between crude and reclaimed rubber consumption for the United States from 1919 to 1926 is shown in Table I. At prese.nt 1 pound of reclaimed rubber is consumed for every 2 pounds of crude rubber. In 1921 and 1922, when the price differential between these two products amounted to only a few cents, 1 pound of reclaimed rubber was consumed for every 4.5 pounds of crude. The reason for the present use of reclaim in such volume is that there are certain advantages to be obtained thereby. There is a saving of labor and power due to its plasticity. Compounds containing it may be mixed in a shorter time and pigments can be incorporated into them more quickly than when no reclaimed rubber is present. For tubing machine work the presence of reclaim often gives a smoother product at an increased rate of speed. Table I-Relation
between Crude a n d Reclaimed Rubber Consumption in United States (From figures by Rubber Association of America) Consumotion -~
Year 1919 1920
1921
1922 1923 1924 1926 1926
Crude rubber consumed Pounds 406,231,000 373 507 000 383:OOO:OOO 631 680 000 683:200:000 750,400,000 863 600 000 (6 months) 406:300:000
~~
-RECLAIMEDRUBBERratio of reConsumed Produced claimed rubber Pounds Pounds to crude 169,504,000 0.42 179 980 000 0.48 85:OOO:OOO 0.22 133,870,000 0.21 173,000,000 0.25 189,300,000 0.25 307,600,000 0.35 206,400,000 0.50
1164
INDUSTRIAL 4ND ENGINEERING CHEMISTRY
Vol. 18, No. 11
Reclaiming Process
Vulcanization consists in the addition of sulfur to rubber while the latter is being disaggregated by heat. The lower the temperature of cure the less the disaggregation and the stronger the product. The faster the addition of the sulfur to the rubber the less the disaggregation. The purposo of
Figure 1-Effect
of Steam a t Various Temperatures on the Tensile Strength of a Tread Compound
devulcanization is to impart to vulcanized rubber its original plasticity. The problem is to impart the desired plasticity without obtaining sufficient disaggregation to produce a materially weaker product. Reclaiming processes depend for their effectiveness upon the application of heat in the presence of devulcanizing agents, softeners, etc. The lowest temperature used in reclaiming is around 300' F. s o that the disaggregating influence of heat is probably the most important factor in every process. Dubosc,l using hexa, and Spence,2 using metallic sodium dissolved in aniline, showed that it is possible to remove a considerable portion of the combined sulfur, which the commercial reclaiming processes are ineffective in removing. If they were effective in removing the combined sulfur, there is some question as to whether the character of the reclaimed rubber would be changed, because during the manufacture of reclaimed rubber changes take place in the rubber which may have as great an effect on its quality as combined sulfur. The principal sources of supply for reclaimed rubber are pneumatic tires, solid tires, and inner tubes. Boots and shoes, hose, and miscellaneous mechanical products comprise the other sources. Scrap rubber, therefore, divides itself into two classes, fiber-free scrap and scrap containing fiber. By far the greatest proportion of scrap rubber contains fiber which must be removed prior to or during the devulcaniaation process before the scrap can be used to give a satisfactory reclaimed rubber. The separation of fabric from rubber may be accomplished in the following ways: (1) mechanical separation, ( 2 ) acid process, (3) alkali process, and (4)solution. When the scrap is fiber-free it may be devulcaniaed in pans which are placed in a heater and subjected to live steam for the desired length of time. The time and temperature vary
I
.
Caoutchouc & nutla-bncha. 16. 9440 (1918). . . ZU. S. Patent i,235,~850(1917). I
Figure 2-Effect of Steam a t Various Temperatures on :the Stress-Strain Curve of a.Tread Compound
I n 1899 Marks9 obtained a patent on a process which has come to be known as the alkali process. About 13 per cent of caustic soda is used, based on-the weight of the dry scrap,
* British
Patent 2933 (1853). British Patent 1461 (1853). U. S. Patent 20,242 (1858). 8 U. S. Patent 249,970 (1881). 7 U. S. Patent 19,172 (1858). 8 U. S. Patent 40,407(1863). e U. S. Patent 646,230 (1900). 4
*
INDUSTRIA I, A N D ENGINEERING CHEMISTRY
November, 1926
and in addition to this a certain percentage of softeners such as those mentioned above may be added. The temperature varies from 125 t o 200 pounds of steam. There has been a tendency in recent years toward a higher temperature and a shorter time. Depending on the scrap the time may vary from 10 to 20 hours. The caustic soda performs a number of functions-it removes free sulfur, hydrolyzes the cotton, and plasticizes the rubber.
3000
2500
2000
1500
1000
500
lo
190
Figure 3-Effect
290
390
4QO
500
690
of Steam a t Various Temperatures on the StressStrain Curves of a Tread Compound
Physical Properties of Reclaimed Rubber
Reclaimed rubber is being used as a compounding ingredient in many instances to replace a portion of the crude rubber without decreasing the service which the product will render. Reclaimed rubber, however, is very different from crude rubber in its physical properties. That this should be the case will be evident when we consider the cycle from crude rubber to reclaimed rubber. The crude is weakened by the milling and calendering processes. Vulcanization produces profound changes in its structure and properties. During the life and service of the product it is subjected to the deteriorating effects of light, oxidation, and aging. Mechanical deterioration also takes place. After three to five years this product is devulcanized a t a high temperature for a long period of time. The reclaim is then dried, refined, and strained a t fairly high temperatures. The problem is-can we with existing methods expect to obtain a better reclaim from scrap rubber? One of the greatest problems of the reclaimer is the variability of the scrap, not only in chemical composition, but also in physical properties. The treads of tires, for example, vary in carbon black from 15 to 25 per cent, which materially changes their toughness and the ease with which they respond to reclaiming processes. The amount of rubber, state of cure, and the age of the tire also affect the reclaiming process. Standardized reclaimed rubbers can be, and are being, made by sorting of scrap and close chemical and physical supervision through the various steps of manufacture. Table I1 gives the results of tests of tensile strength and elongation and resistance to tear of treads from thirteen tires prior to reclaiming. Present whole tire or tread reclaims will give 500 to 1000 pounds tensile strength when cured with 5 per cent of sulfur. As a result of service the tensile strength of the tread has dropped from 3500 pounds to 1649 pounds.
Table 11-Physical
Average of 13 tires Maximum Minimum
1165
Properties of Treads from Old Tires RESISTANCE TO TEAR Elongation Trans. Long. Per cent Pounds Pounds 415 21.7 22.6. 537 51.4 45.9 283 7.1 6.8
Tensile Pounds 1649 3127 637
The problem is to attain a product which will not only give the tensile strength of the scrap, but which will approach the physical properties of the original compound. This involves a more intimate knowledge of the structure of rubber than we have a t the present time. As stated above, the lowest temperature used in the devulcanization of vulcanized rubber is around 300" F. The effect of steam a t various temperatures on a tread compound was determined by exposing strips for varying lengths of time up to 24 hours a t 250", 275O, 300°, and 350" F. After exposure the samples were allowed to stand for several days to allow full time for recovery. The decrease in tensile strength is shown in Figure 1. With the exception of 250" F., the tensile drop has almost reached a maximum in 4 hours or less. The tensile strength after 4 hours a t 350' F. is no higher than for a good grade of tread reclaim. The deteriorating action of heat is so great that it probably tends to eliminate one of the variables in the original scrap-namely, tensile strength. The stress-strain curves (Figure 2) after one hour's heating show additional cure and lower tensile strength a t 275" F. At 300" and 350" F. the stress-strain curve is weaker with a correspondingly lower tensile strength. The stress-strain curves (Figure 3) after 4 hours' heating show additional cure at 250" F. and n-eakening a t 275" F. The stress-strain curves for 300" and 350" F. coincide. The action of heat and steam at the temperatures usually employed in devulcanization has a deteriorating influence on the physical properties of the rubber. If a satisfactory reclaimed rubber can only be made by application of high temperatures, some means must be found for the prevention of the disaggregation a t higher temperatures.
3000
2500
D.P.G.
I500
SOFTENER
M R.
i
Z I N C OXIDE
1000
4.5
6
-seoo
IO0
Strength of Various Reclaims in a Friction Compound reclaim; 4-carcass reclaim; 5, 6, 7-whole tire reclaim
Figure 4-Tensile 1, 2, 3-tube
The physical properties of a reclaimed rubber in the uncured state are fully as important, and very often more so, than physical tests on the cured reclaim. The ideal reclaim should be clean and smooth as well as have the maximum physical properties possible. There is a tendency to evaluate too much on tensile strength. The real significance of the tensile strength of a reclaim and its relation to the physical properties of the compound in which it is used have yet to be determined.
1166
INDUSTRIAL A N D ENGINEERING CHEMISTRY
In Table I11 are listed the tensile strength and elongation of a number of inner tube and tire reclaims cured with 5 per cent sulfur. The tensile strength varies from 178 to 1392 pounds. Twenty-four per cent of each of these reclaims was added to a friction compound. The tensile strengths and elongations obtained are given in Figure 4. Note that the tensile strength of the friction compound does not follow the tensile strength of the reclaim a t all. The stress-strain curves (Figure 5 ) are very close together, but when 0.1 per cent accelerator was added to numbers 2 and 3 (Figure 6) all the compounds checked within experimental error. The reclaims which give a low tensile strength when cured with sulfur may, however, have the correct degree of disaggregation for yielding good results with rubber and other ingredients. When the amount of reclaimed rubber in the friction compound is increased substantially above 24 per cent, results such as the above cannot be obtained owing to variation in rubber value and manufacturing processes of the reclaims. Table 111-Physical Properties of Reclaimed Rubbers Reclaim 100; sulfur 5 All cures at 40 lbs. steam pressure (287OF.or 141.6' C.) 15 MINUTES 20 MINUTES 25 MINUTES 30 MINUTES Elong. Elong. Elong. Elong. Ty eof Tens. Per Tens. Per Tens. Per Tens. Per No. recraim Pounds cent Pounds cent Pounds cent Pounds cent 1 Tube 1272 597 1254 574 1392 558 1314 517 2 Tube 257 350 286 310 272 337 287 328 3 Tube 178 353 193 347 197 327 198 307 4 Carcass 379 404 399 406 419 431 369 407 5 Whole tire 518 341 571 290 555 321 567 303 6 Whole tire 546 365 548 321 567 324 564 349 7 Whole tire 453 309 463 296 475 278 479 263
Behavior toward Accelerators
Reclaimed rubber differs materially from crude rubber in its behavior toward accelerators. Accelerators increase the tensile strength of reclaimed rubber but slightly, or not a t all. In some cases the rate of cure is increased with but little change in the tensile strength. I n Table IV eight commercial grades of reclaimed inner tubes were compared with and without accelerators. Hexamethylenetetramine, diphenylguanidine, thiocarbanilide, and B. B. were used as accelerators. Reclaim 2 shows an increase in tensile strength of 30 to 50 per cent. It also shows an increase in the rate of cure as evidenced by the decreased elongation. The addition of the accelerator has practically no effect on the other reclaims except in some cases an increase in the rate of cure.
Reclaim 1
2
3
Table V-Whole Tire Reclaim with Various Accelerators Reclaim 100; sulfur 5. All cures at BO lbs. steam pressure (141' C. or 287' F.) ACCELERATOR0.75 ACC&LERATOR0.75 ZINC OXIDE 3 Cure Tens. Elong. Tens. Elong. Min. Pounds Per cent Set Pounds Per cent Set N o accelerator 15 500 320 11 20 519 287 8 25 555 247 7 30 660 242 6 Hexa 15 549 260 7 540 342 11 20 445 175 2 604 297 11 245 7 25 429 140 2 629 223 6 30 506 135 2 609 Vulcone 15 644 320 11 579 288 8 20 283 8 522 173 4 628 25 263 573 195 4 619 7 531 165 3 240 6 30 574 Diphenylguanidine 15 657 328 12 657 251 7 20 645 215 8 590 235 5 25 725 240 6 629 203 5 30 701 208 4 661 168 3 A-19 15 549 265 6 591 273 8 534 253 5 620 230 4 20 532 4 577 155 3 25 200 6 614 168 4 30 534 185 9 697 298 7 662 290 B. B. 15 6 707 270 6 20 618 240 25 617 5 642 175 4 232 30 606 225 5 640 162 2
The effect of accelerators on a whole tire reclaim with and without the addition of zinc oxide is shown in Table V. The action of the accelerators in three instances gives an increase of about 20 to 25 per cent in tensile strength, but the addition of zinc oxide to these same reclaims gives a lower elongation than without the zinc oxide present, except in one case. The use of ultra-accelerators from data which have been obtained to date gives the same results, but at a lower temperature, as the accelerators mentioned above. These data will be published later. The action of organic accelerators on reclaimed rubber is further evidence that devulcanization has resulted in such changes that ordinary accelerators are not so effective as they are on crude rubber. I n other words, we need a new type of accelerator which will increase the degree of aggregation of the rubber in the reclaim. Mechanism of Devulcanization
Compared with the amount of work that has been done on the theory of vulcanilration, but very little attention has been devoted to the theory and mechanism of devulcanization. Very little actual evidence has been put forth in the support
Table IV-Inner Tube Reclaims with Various Accelerators Reclaim 100: sulfur 5. All cures at 40 lbs. steam pressure (141' C. or 287" F.) N o ACCELERATOR -HEXA 0.75--D. P.G.0 . 7 6 7 -B. B. 0 . 7 5 7 Tens. Elong. Tens. Elong. Tens. Tens Elong. Lbs. % Set % Set Lbs. (m Set Lbs. % Set Time Lbs. Elong. 1 204 272 2i3 1 230 1 227 15 183 312 1 204 1 212 263 232 1 202 1 259 20 197 315 2 213 1 214 242 2m 1 222 1 267 25 210 327 2 193 235 232 1 203 1 282 1 260 302 2 236 30 221 495 405 9 310 5 282 5 437 15 300 445 9 433 434 353 6 290 3 238 2 388 458 20 293 432 8 474 350 6 282 4 258 3 407 445 25 337 418 8 467 327 5 290 4 288 5 430 403 458 8 30 349 1201 550 27 523 25 577 34 1159 1222 15 1093 580 30 1064 500 21 518 24 563 31 1209 1344 20 1184 578 31 1129 482 20 493 21 550 27 1106 1185 25 1209 595 33 1060 480 18 400 14 562 23 880 931 . . ~ 30 1163 528 28 262 287 1 288 1 263 1 294 1 181 277 15 201 274 288 227 0 278 1 257 1 221 283 20 212 1 257 275 208 0 238 1 288 220 25 256 295 1 278 298 1 157 0 208 1 219 223 1 30 254 290 1224 468 25 500 27 612 42 1020 1382 15 1233 658 42 1116 380 17 340 21 538 36 752 1304 20 1350 613 41 885 305 8 300 16 530 37 714 1417 25 1378 570 40 880 308 9 180 3 428 23 510 505 32 1168 30 1310 179 278 1 o -2ns __ 233 0 153 2 169 15 133 295 171 227 1 202 0 228 1 163 186 20 150 282 1 242 285 1 230 0 232 1 195 210 1 25 152 253 221 228 0 225 0 232 1 220 242 2 30 173 295 191 243 2 145 0 207 2 198 4 188 15 220 323 218 250 1 145 0 238 3 225 275 20 229 380 3 216 255 2 143 1 230 2 221 302 360 3 25 280 241 263 2 -.125 2 160 2 226 2 283 325 30 327 326 378 5 220 2 308 4 233 5 332 1'5 216 39s 308 330 4 212 2 265 3 247 354 4 20 283 440 324 323 3 225 2 228 3 282 25 224 340 275 4 307 282 3 253 3 293 3 335 332 30 293 373 5 ~
4
5
6
7
___
8
Vol. 18, No. 11
-Thio Tens. Lbs. 191 211 201 199 435 434 428 441 1198 1122 1088 1015 250 305 259 253 1283 1285 874 928 157 178 192 197 240 293 216 268 264 244 268 277
-2 Elong.
Yo
342 340 315 295 405 393 380 383 558 540 520 513 248 250 203 178 483 420 335 340 242 248 240 228 303 300 317 310 347 302 287 315
Set 2 2 2 1 9 9 8 8
34 26 25 22 0 0 0 0
29 23 12 12 0 1 0 0
3 2 2 4 4 4 3 4
I N DUSTRIAL A N D ENGINEERING CHEMISTRY
November, 1926
Table VI-Chloroform
Type of reclaim Inner tube Whole tire Solid tire
Acetone extract (8 hrs.) Per cent 7.5 8.73 7.86 9.49 11.89
CHCla extract (48 hrs.) Per cent 30.42 27.08 21.77 25.47 12.1
Total Per cent 3.182 2.978 2.63 2.93 2,207
Extract of Reclaimed R u b b e r Combined sulfur CHCh in CHCla Sulfur Ratio extract after Ratio in insoluble pigments S. soluble in CHCla CHCla extract extract Per cent Per cent S. insol. in CHCla Rubber inreclaim sulfur 0.15 2.26 0.68 0.0663 0.435 2.33 2.34 0.29 0.284 0.123 0.47 2.17 0.187 0.237 0.0861 0.362 1.50 0.199 2.3 0.336 0.0865 1.88 0.385 2.75 0.151 1.74 0.299 0.0867 0.29
Sulfur in CHCIa soluble extract Per cent
Sulfur in acetone extract Per cent 0.092 0.064 0.036 0.095 0.017
Table VII-Effect Time heating Hours 2.5 5
Acetone extract Per cent 7.63 7.45
CHCla extract Per cent 29.15 36.98
Total sulfur Per cent 3.02 2.926
1167
Cyg&eh
of T i m e on Devulcanization Combined Sulfur sulfur Sulfur in CHCla in CHCla in extract insoluble extract pigments Per cent Per cent Per cent 0.29 2.52 0.131 0.335 2.25 0.267
Sulfur in acetone extract Per cent 0.079 0.068
of any theory. Acetone-extracted reclaimed rubber is partly extractable with chloroform. L. E. WeberlO called attention to the fact that the chloroform extract might be intimately connected with the mechanism of devulcanization. The devulcanization of a highly cured scrap with caustic soda produces a more plastic material than devulcanization with live steam. Dr. Weber obtained the following chloroform extracts by devulcanizing a tire tread with caustic soda and live steam for varying lengths of time: PERCENT CHLOROFORM EXTRACT BASEDON RUBBER PRESENT 7 hours 20 hours 23.4 23.7 NaOH, 3 per cent 19.6 22.0 Water
A jacketed devulcanizer was used, and in each case steam a t 125 pounds pressure was admitted into the jacket. The products obtained from the caustic soda devulcanization are more plastic on the mixing mill than those prepared under water. There is very little difference in plasticity between the products resulting from the 7- and 20-hour devulcanization with caustic soda, whereas there is considerable difference
Ratio Ratio in CHC1r CHC1a extract S . insol. in. CHCla Rubber in reclaim 0.115 0.506 0.144 0.636
s.
acetone extract, chloroform extract, and chloroform-insoluble portion was then determined. The amount of chloroform extract varies with the reclaiming process, the type of reclaim, and rubber value of the reclaim. The ratio of the chloroform extract to the rubber value of the reclaim, however, is of the same order for the two tube and the two tire reclaims. The sulfur in the chloroform extract is very low. This fact is very significant inasmuch as we have from 43 to 47 per cent of the rubber for tube reclaims, and 36 to 38 per cent of the rubber for tire reclaims containing less than 10 per cent of the combined sulfur. The amount of sulfur in the chloroform extract as given here is less than that reported by Stafford." The combined sulfur of the reclaimed rubber has not decreased. To date it has not been possible to fractionate vulcanized rubber by means of solvents. It has been assumed that during vulcanization all the rubber combines with some of t!:e sulfur because the rubber becomes progressively more insoluble. If this is actually the case we must account for the low sulfur content of the chloroform extract. There are two explanations for the mechanism 3000 C U R E 45'~287"F.( 141 6°C)
!
2500
SMOKED SHEET
2500
RECLAIM
I 58 24
3 0.5
2000
MAGNESIUM OXIDE 2 2 M. R. A.5
I500
IO00 IO00
500
500
lo 10
Io0
ZQO
340
4QO
590
690
700
1
Figure 5-Substitution of Various Reclaims in a Friction Compound 1, 2, 3-tube reclaim;4--carcass reclaim; 5, 6, 7-whole tire reclaim
between the products devulcanized under water for similar periods. By a determination of the sulfur present in the acetone and chloroform extracts and in the chloroform-insoluble portion of reclaimed rubber, we have found that considerably more information can be obtained relative to the mechanism of devulcanization. (Table VI) Each sample of reclaimed rubber was extracted for 8 hours with acetone followed by a 48-hour extraction with chloroform. The sulfur in the '0
"The Chemistry of Rubber Manufacture." p. 270.
10'0
200
300
400
500
600
700
I
Figure 6-Substitution of Various Reclaims in a Friction Compound, 1-tube reclaim; 2,3--tube reclaim -0.1 S.S., +0.1 D.P.G.; 4-carcass reclaim; 5 , 6, 7-whole tire reclaim
in this case, both of which satisfy existing theories of vulcanization: 1-The aggregation of the rubber which takes place during vulcanization is just as important as the rubber-sulfur combination. A part of the rubber becomes more highly aggregated in the presence of sulfur without combining with it. It may be this portion of the rubber which is more easily disaggregated on devulcanization. 2-If we assume t h a t the rubber molecule consists of a chain of C6Hs groups and t h a t the sulfur is added a t one end of the mole11
India Rubber J . , 71, 59 (1925).
INDUSTRIAL 4 N D ENGINEERING CHEMISTRY
1168
cule, it would then be possible to break off CjHs groups by disaggregation during devulcanization.
.
This would account for the low sulfur content of the chloroform extract. Fisher and Gray12 have shown that heat causes a change in both the chemical unsaturation and viscosity of crude rubber. The unsaturation of the chloroform-soluble and -insoluble portions was determined on the first tube reclaim given in Table VI.
Chloroform-soluble Chloroform-insoluble
Unsaturation Per cent 85.25 65.75
The unsaturation of a vulcanized sample of inner tube is around 92 to 96 per cent. Thus it is shown that the reclaiming process has resulted in deep-seated changes in the rubber. The effect of time devulcanization was determined by devulcanizing a tube scrap for different periods. (Table VII) The steam introduced into the heater was a t 60 pounds pressure. The acetone extract does not increase with increasing time of devulcanization, although the chloroform extract is appreciably increased. The sulfur in the chloroform extract remains practically unchanged. The plasticity of the two stocks was very different, the 5-hour 11
THISJOURNAL, 18, 414 (1926).
Vol. 18, No. 11
heating period giving a product which was much softer and easier to smooth out on a mixing mill. The plasticity of reclaimed rubber is no doubt dependent on the chloroform extract. It is suggested that a method based on the chloroform extract might be worked out to insure a uniform consistency of reclaimed rubber. The chloroform extract is transparent and looks like rubber obtained by evaporation of the solvent from a solution of rubber. The ash varies from 0.5 to 1.0 per cent. The viscosity of the chloroform extract is very much less t,han that of crude rubber. The viscosity is similar to the viscosity of a solution of rubber made from rubber which has been milled for a very long time, such as 6 to 8 hours. The chloroform extract can be vulcanized with sulfur, a comparison of the physical properties of the chloroform extract and chloroform-insoluble portion is now in progress. This is highly desirable in view of the difference in unsaturation pointed out above. It is interesting to note that on vulcanization of the reclaimed rubber with sulfur (last column Table VI) the solubility in chloroform decreases again to a very low figure. We sometimes hear the question as to what kind of reclaimed rubber will be obtained from vulcanized rubber which already contains substantial proportions of reclaim. If vulcanized rubber is disaggregated under proper conditions it may be possible to aggregate it on revulcanization so as to lose but little of its original value.
Significance of the Resin of Hevea Rubber in Vulcanization and in the Aging of Raw Rubber‘ By G. S. Whitby and H. Greenberg MCGILLUNIVBRSITY, MONTREAL,CANADA
A
N EXAMINATION2 of the resin which constitutes on an average 2.8 per cent of the weight of raw Hevea rubber in the form of latex crepe or sheet has shown it to be of the approximate composition shown in Table I. Dekker3 and Bruni4 have confirmed the presence of some of these constituents. I n addition, Dekker3 has indicated the presence of formic acid and probably other lower fatty acids. In this paper the influence of the individual components of the resin on vulcaniaation and the secular change which the resin undergoes when raw rubber is stored are considered. Table I-Constituents of Hevea Resin Per cent Per cent ~. A phytosterol ester 0.075 Quebrachitol Very small amount Sitosterolin 0,175 Stearic acid 0.15 A phytosterol 0.225 Oleic acid 1.25 d-Valine 0.015 Linoleic acid
1
Influence of Resin Acids on Vulcanization
Experiments to date indicate that the free fatty acids of the resin have an importance in the vulcanization of rubber with many accelerators which overwhelms the importance of the other resin components mentioned above. Observations by Bedford and Winkelmann6 show that the acids play an essential part in vulcanization with the aid of inorganic oxide accelerators, especially litharge. Experiments by 1
Sebrell and Vogt6 and by Whitby and Simmons7 have indicated that they are also of great importance in vulcanization with the aid of many organic accelerators. The following new experiments show clearly the importance of fatty acids in vulcanization by the aid of powerful organic accelerators. In these and all other experiments recorded in this paper four ring test pieces with an approximate crosssectional area of 0.25 sq. cm. were tested a t each cure; and the numerical data given refer in each case to the mean of those two of the four test pieces showing the highest ultimate tensile strength. The ultimate tensile strength in kilograms per square centimeter is represented by T B ; the load a t an elongation of ZOO per cent by T,, and the ultimate percentage elongation by EB. I n comparing different cures little importance should be attached to differences in ultimate tensile strength unless very marked, because of the comparatively small number of test pieces examined and the limited number of cures made. Differences in rate of cure are best seen from the load-strain curves and from the figures of T,. ( a ) From two-thirds of a sample of Hevea latex crepe rubbers the resin was removed by extraction in the cold for a week with a mixture of acetone and light petroleum (7: 3 by volume), the liquid being poured off and replaced by fresh liquid five times at intervals of 24 or 48 hours. After the solvent had evaporated from the extracted rubber, stocks made up as shown below gave on vulcanization the results shown in Table I1 and Figure 1 .
Presented by G. S. Whitby under the title “The Testing of Raw Rub-
ber.”
THISJOURNAL, 16, 792 (1924). Whitby, Trans. Inst. Rubber I n d . , 1, 12 (1925). In the term “Hevea latex rubber” the qualification “latex” is used to indicate rubber prepared by artificial coagulation (usually by acetic acid) from the latex of Hevea brasiliensis in contradistinction to the various grades of scrap rubber derived from such latex by spontaneous coagulation. 6
Whitby, B r i f . Assoc. Aduancemenl Sci., Repts., 69, 432 (1923); Whitby, Dolid, and Yorston, J . Chem. Soc. (London), SO, 1448 (1926). a I n d i a Rubber J . , 7 0 , 815 (1925). 4 Giorn. chim. i n d . applicafa, 7 , 447 (1925). 5 THIS JOURNAL, 16, 32 (1924). 2
7
8
