. Plantation Studies of Crude Rubber Variation GEORGE A. SACKETT The Goodyear Tire & Rubber Company, Akron, Ohio STATION ON
THE
GOODYEAR COMPANY’S SUMATRAPLANTATION TO WHICH COOLIESBRINGLATEX
Here it is weighed and placed in a large tank; at specified times it is taken to the factory in tank trucks.
I
X THE earlier years of the plantation industry it was recog- were made, and a wider knowledge of the causes and extent nized that the product was subject to variation in rate of of variation was gained, Eaton and Grantham (8) compared cure. A number of investigations were made using a the variation of plantation rubber with that of fine Para and rubber-sulfur formula and some were carried out using in- concluded that smoking retarded the rate of cure. More imorganic accelerators. The variation in plantation rubber has portant, however, they decided that the rate of cure was due not received quite so much attention in recent years, probably to the presence of some nonrubber substance in the latex, mainly for two reasons: first, the effect of variation has been possibly a protein or protein degradation product. They also minimized by blending and compounding, and secondly, stated that smoking, or removal of excess serum by washing, the production of a uniform rubber was not economical in the or using preservatives, or any artificial factors which either face of competition by native rubber, since the manufacturers hindered the formation of this material or partially destroyed could easily and cheaply adapt the variable rubber to their i t would lower the rate of cure. They believed that any plantation should be able to make a uniform product by carefully processes and products. It was desirable, however, to study the possibility and controlling conditions, but the product of different plantaeconomy of producing a uniform product from a rather large tions would vary, one from another, because of differences in area under careful control so that, if feasible economically, methods and machines for handling latex and coagulum. the processing could be so arranged that the product of this Whitby (15) found that phenols lowered the rate of cure and area would be of uniform quality, The investigations con- that the purple discoloration of the coagulum, caused by oxiducted in this connection form the basis far this paper. The dation, was without effect. He called attention to the fact work was wholly carried out using an organic accelerator, as that different groups of trees would yield a product of varying rate ofcure, e;en with the same it was felt that the variation of method of production. Comrubber when this material was The effects of a number of factors in the manuparisons of v a r i o u s m e t h o d s used was of far more importance of processing should b e m a d e facture of smoked sheet o n the rate of cure of the than variation in the rubberw i t h a s i n g l e l a r g e l o t of sulfur mixtures previously emfinished product as shown by tests using a purelatex. ployed. I n addition, the prodgum type of stock accelerated with Captax have Further detailed investigauct of the plantation where the been studied on a rubber plantation. tions were m a d e by Eaton, s t u d i e s w e r e m a d e w a s inThe stock used for the tests reported in this Grantham, and Day who intended for use with Captax acpaper shows generally that the rate of cure varies vestigated principally the effect celeration. of various coagulating agents, The term “variation” in this in a manner similar to that reported by various washing, c r e p i n g , and drying p a p e r is t a k e n t o m e a n t h e investigators who used a rubber-sulfur stock in (9). Other investigations were difference in vulcanizing propertheir studies. made by the Rubber Research ties, p a r t i c u l a r l y as reflected The results of a study of the effect of large-scale Scheme of Ceylon (1). De Vries in rate of cure, or in the stiffness blending o n the improvement of uniformity are in Java also made a number of of a s t a n d a r d t e s t stock at a interesting studies (13, 14). given cure. The variation in apgiven. The effects of the length of time the trees Summarizing all this work, i t pearance is not considered, as are in tap, the anticoagulant and coagulant, may be s t a t e d t h a t t h e rate it has been t h o u g h t for some latex concentration, coagulation period, soaking of cure depends largely on the time that n e i t h e r appearance of sheets, and drying temperature are described. n o r h a n d tests of the c r u d e ratio of certain nonrubbers to The conclusion is reached that the consumer rubber give any indication of the the rubber in the finished prodproperties of the finished stock uct. This ratio may be affected probably will hare to blend his rubber in order to not only by the processing of ( 3 )* obtain a uniform raw material because of the It was recognized as e a r l y the latex, but also by natural seasonal variations and the practical and ecoas 1890 that t h e m e t h o d of influences. The natural factors nomic dificulties standing in the way of the coagulation caused the product a f f e c t i n g l a t e x flow a n d , i n production of a n absoluteIy uniform product. to vary (2). F u r t h e r studies turn, the rate of cure are wind, 535
536
INDUSTRIAL AND ENGIYEERING CHEMISTRY
rain, soil, and season. The latter has the most pronounced effect. The artificial factors are the tapping system, the stability of the latex, the kind and amount of coagulant, the period of coagulation, the amount of washing, and the drying system used. It is no simple or cheap matter to overcome the effect of these variables. Thus, even if a plantation were to standard-
OUTSIDEOF FACTORY Truck and trailer waiting t o discharge latex.
ize the latex handling and control every step of the process with minute care, the natural variations would prevent the production of a uniform material. With the exception of the changes caused by seasonal variations, particularly the effect on the latex of the loss of leaves of the tree during the wintering period, the effect of most of the variables on a given plantation can be overcome to a very great extent. Many plantations use a period system of tapping; that is, each tree in a given area is tapped daily for 2, 4, or 6 weeks, and then allowed to "rest" for a n equal period of time. The latex obtained during the first few days of tapping has a much lower rate of cure than that obtained later. The alternateday system of tapping should not have this disadvantage. Any plantation using the period system should lay its area Q U t in sections, one section changing from rest to tap a t a time. Then by blending latex from areas which have been in tap for varying lengths of time, a latex of uniform characteristics should be obtained. This method was used on the plantation where the experiments detailed below were carried on. The total amount of latex mixed together in any one blend was 16,000 liters of 20 per cent dry rubber content. Besides the advantage of uniformity, it is found that the daily crop is more uniform in size if a plantation arranges to have different areas change from rest to tap at various times. The output of latex on the first few days after changing over is very small. The yield rapidly increases for 4 or 5 days and then begins to drop off gradually. The alternate-day system of tapping, mentioned above, is without doubt the best system from the standpoint of latex uniformity, but it is often not economically as advantageous. There is some question, for instance, about the equality of yield and of cost of supervision. The processing variations have a great deal of influence on the rate of cure of the product. Smoked sheet, because of its method of manufacture, is subject to much wider variation than is pale crepe. Crepe is thoroughly washed, and the coagulum emerges from the final creping machine with a definite and uniform thickness. It is always, or nearly always, made thinner than sheet. The drying of crepe is probably always a t a more uniform temperature than that used for sheet. Sheet always varies slightly in thickness and in the hardness of the coagulum. Since the coagulum is merely rolled out gradually to the desired dimensions, little washing is done. Since the plantation on which the work described
Vol. 26, No. 5
below made sheet as its main product, practically all work was done on that form of rubber. I n brief, any factor which increases latex flow from the tree will produce a latex from which, with a given process, a faster curing rubber will be made; and any factor in processing which causes maturation of the wet coagulum to take place more rapidly, or for a longer time, or any process which does not remove as much of the nonrubber constituents of the latex will be found to give a faster curing product. Although much work had been done on crude rubber variation, all had been carried out with a rubber-sulfur formula. Dinsmore and Zimmerman (6)and Bishop and Fullerton (4) had found that the variations were not the same when using organic accelerators. This investigation was made to study this point more fully, and Captax ( m e r c a p t o benzothiazole) was used as accelerator, not only for the reason already given, that the rubber from this plantation was to be used with Captax in manufacture, but also because this material is more sensitive to slight variations than other accelerators. EXPERIMEXTAL P R O C E D U R E
I n comparing the rate of cure of various samples of rubber, using the same formula and the same time of cure, i t was found sufficient merely to determine the modulus of the stock at 700 per cent elongation. Thus a stiffer stock, having a higher modulus, can be said to be faster curing. This does not allow the setting of a time of best cure, nor can the result be interpreted to mean anything if any of the conditions is changed. However, all the tests made on the plantation were correlated with a system of testing used in the rubber factories. The test formula was as follows: Rubber Zinc oxide Sulfur Captax
100.00 4.00 6.00 0.50
The cure was 40 minutes at 260" F. (126" C.) unless otherwise noted. The cycle of mixing, curing, and testing was 48 hours; that is, the stock was milled one day, cured the next day, and tested on the second day following.
CREPEAND SHEETREADYFOR PACKING
The test was made with a standard dumb-bell strip. It was pulled on a Scott machine with a jaw separation of 20 inches (50.8 cm.) a minute. The usual temperature of test was 90" F. (32' C.) and the relative humidity was about 90 per cent. The results reported are averages of two tests.
INDUSTRIAL AND ENGINEERING
May, 1934
BULKING OF LATEX Four separate series of tests were made in a study of the value of latex bulking. The plantation was operating on a month-period system; that is, any given area was tapped on a half-circumference with a single spiral cut, daily for one month, and then the area was allowed to rest for one month. The whole plantation was divided into twelve sections, and one section was brought into tap every 5 days.
537
CHEMISTRY
tank in successive portions where i t was diluted to 18 per cent dry rubber content. Coagulation, milling, and drying p ~ o ceeded as usual. Representative samples gave the following tests: MODC~LUS AT: TENSILE SAMPLE STRENGTH ELONGATION 500% 700% Ko./cm.' Kg./cm.l 70 32 143 July 2 , 1930 1 254 so0 29 133 2 258 800 30 145 3 so0 257 31 140 4 246 800 5 266 800 31 150 233 800 28 133 1 July 3, 1930 26 128 2 236 800 ' 9 144 3 242 so0 4 266 so0 34 155 30 136 5 238 800 '5 121 July 4, 1930 1 226 800 2 .7 ,> 1 800 28 123 3 29 137 '47 800 4 30 152 229 800 280 800 5 J4 156 31 142 1 273 800 July 5 , 1930 2 32 141 800 279 34 159 3 266 800 4 30 134 '881 800 32 152 254 5 800 1 260 800 32 147 July 6, 1930 2 "33 800 37 158 257 800 3 36 163 4 34 152 26 1 800 5 260 800 '5 163 1 243 800 99 136 July 7, 1930 258 800 31 138 36 165 3 282 80C 4 235 800 30 151 30 150 5 247 800 26 121 1 231 800 July 8, 1930 7 35 152 269 800 237 3 800 25 119 131 29 4 250 800 28 121 5 242 800 800 31 142 Average 252 10.9 14.6 ... 2.5 Average deviation Maximum deviation 31 ... 6 23 DATE
-_
ROLLED4KD
M4RKED
SHEETS BEINGT 4 K E T
IYTO
SYOKE-
HOUSE
The first test mas made before bulking was practiced to any great extent. The latex was brought to the factory by motor truck, in tanks holding about 2000 liters (500 gallons). This quantity mas transferred to a tile-lined tank for dilution to a standard dry rubber content of 18 per cent. Formic acid was used as a coagulant. The latex was coagulated overnight and given a short soaking (about 5 minutes) in running water and dried for 7 or 8 days in a smokehouse at 120" F. (50" C.). Samples were taken from various sheets and tested. The results were as follows: TENSILE MODULUSA T : S.AVPLE STRENGTHELOWXTION5007, 7007, Kg./cm.z % Kg ./cm. 2 M a y 2, 1930 1 243 800 28 127 7 204 108 800 24 3 215 116 800 27 124 800 28 4 238 141 5 236 800 30 M a y 3, 1930 1 168 775 23 102 775 24 2 180 102 800 23 3 216 101 800 27 114 4 207 5 249 130 80C 28 M a y 7, 1930 1 222 800 26 110 800 25 2 187 108 775 32 3 214 141 125 800 28 4 205 800 31 5 220 131 800 22 M a y 10, 1930 1 188 108 800 21 101 2 197 131 3 234 800 28 4 209 800 27 125 103 M a y 12, 1930 1 206 800 21 114 2 211 800 25 775 25 116 3 168 800 23 109 4 176 800 24 112 5 188 800 21 M a y 13, 1930 1 193 101 800 26 188 2 109 800 26 3 213 117 4 216 800 27 114 Average 2 07 796 26 116 Average deviation 17.2 ... 2.3 9.7 hlaximum deviation 42 6 25 DATE
The tensile strength and modulus (average) of this series were higher than of the previous one. I n order to eliminate any seasonal effect, for the first test was made shortly after the wintering period was over, the bulking on a small scale was repeated. This time, however, it mas carried out in units of 6000 to 8000 liters (1500 to 2000 gallons). The method of handling was the same as in the previous tests: DATE
TENSILE SaMPLE
July 9, 1930
July 10, 1930
July 11, 1930
July 12, 1930
July 13, 1930
July 14, 1930
2
...
While the average deviation was not great, the maximum was too large. I n an effort to improve the uniformity, a run was made in which bulking was carried out on a much larger scale. The procedure was as follows: All latex received was stabilized with 4 cc. of 18.5 per cent ammonia solution to each kilogram of latex. The entire day's crop, or about 80,000 pounds (36,000 kg.) of latex with 35 per cent dry rubber content, was mixed in a single large tank. It was run into a smaller
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1
July 15, 1930
3 4 5 1 2 3 4 5
Average Average deviation Maximum deviation
i
STRENGTH E L O N G I T I O N
Kg./cm.Z 262 248 232 234 238 235 256 250 235 238 235 237 240 240 231 2 50 258 235 229 264 265 252 246 238 235 233 225 226 226 230 270 254 247 234 239 242 10.1 28
% 800 800 so0 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800
...
M
AT'
~
500% 700% Kg./cm.= 28 139 29 139 26 12 1 27 142 30 141 25 122 29 136 30 142 26 121 26 117 26 116 29 139 31 138 28 12 1 26 124 28 139 34 147 34 139 30 142 32 151 30 148 30 143 31 144 27 128 31 152 28 124 30 143 31 140 31 150 28 129 34 159 35 157 32 156 32 157 29 153 30 139 2.0 9.6 5 23
The average deviation in this last series is less than in the one before, but it is of about the same magnitude. We may
~
INDUSTRIAL AND ENGINEERING CHEMISTRY
538
safely say, therefore, that nothing is gained in uniformity by larger scale bulking. The samples used for test were selected at random from the regular production, so that the variations are easily accounted for by the unavoidable slight differences in processing. Each sample represents in every test, however, a single diluted mixture of latex.
EFFECT OF TIMETHE TREES AREIN TAP As mentioned before, the trees when &st tapped after a rest period give a slower curing rubber than they do later. This point was checked to see if it still held true in the accelerated stock used for test. The area when the test was made was on a period system of alternate 6 weeks. A large number of tests was made, and only the averages of the 700 per cent modulus are reported here as these are sufficient for rate-of-cure comparison : TIMEIN TAPAFTER REST
MODULUS AT 700%
Days
Kg./cm.* 125 134 141 149
11 24 33 40
Kg./cm.g
ELONQATION
%
1 Av.
244 199 242 229
800 773 800 792
MODULUS AT: 500% 700% Kg./cm.z 32 28 30 30
144 129 137 137
27 28 36 30
130 134 159 141
23 27 28 26
114 128 131 114
AMMONIA
1 2 3 Av.
246 201 214 220
800 800 800 800
20 30 40 60 80
148 148 164 168 184
790 760 755 745 735
27 30 32 37 39
AMMONIA-STABILIZED
20 30 40 60 80
168 166 176 158 182
810 775 775 735 760
27 30 31 37 39
169 188 182 189 181
89 105 116 129 135
705 690 660 660 660
61
48 53
735 685 675 660 650
43 52 59 64 65
730 695 680 675 685
38 47 56 58 55
66 65
LATEX^ 186 175 185 181 173
92 108 115 128 135
FORMALDEHYDE-STABILIZED
UNSTABILIZED
2 3
%
OXYQEN
BOMB
500% 700% Kg./cm.a
TION
12 DAYSIN
Tensile Elonga- Modulua strength tion at 500% Kg./cm.* % &./em.*
UNSTABILIZED LATEX
EFFECT OF ANTICOAQULANT
TENSILE
AQED
CURE” STRENQTH Min. Kg./cm.l
Tests were made on rubber from latices which had been stabilized with ammonia, with sodium sulfite, and with formaldehyde, and these were compared with rubber made from latex which had not been stabilized. The results of these comparisons are as follows: STRENQTH
The ammonia, as above, was added as an 18.5 per cent solution, using 3 cc. per liter of latex. The sodium sulfite was added in the form of a 10 per cent solution, using 9 cc. per liter. These two series of tests show that both formaldehyde and sodium sulfite give a rubber with a lower rate of cure. Formaldehyde is a very strong bactericide, and sodium sulfite a mild one. While it is true that free formaldehyde would react with the Captax in the test formula, and this would slow down the cure, it is doubtful if this is the case here. Rather, it is a prevention of maturation, which aIways produces a rubber with a slower rate of cure. The rubber from the first series was tested after aging the vulcanized sheet in the oxygen bomb in order to determine whether the anticoagulant had any effect on aging. The results are as follows: TENSILEELONGA-MODULUS AT:
Unfortunately no area was using the alternate-day system, so it was not possible to make a direct comparison. However, it is not unreasonable to suppose that the product would be very uniform, except for seasonal variations.
RUN
Vol. 26, No. 5
LATEXC
167 23 75 20 145 825 26 89 175 30 150 790 40 141 183 30 108 745 188 60 162 760 31 112 35 120 183 80 176 765 A t 126O C. b 2 cc. of 18.5 per cent NHiOH per liter of latex. C 1 cc. of 40 per cent HCHO per liter of latex.
There is no difference in aging properties of the samples; however, the vulcanized product made from the formaldehyde-stabilized latex is not quite as desirable as the other two. The cure curve has too little “plateau.” It is possible, however, that this might be corrected by compounding. It is evident that the use of anticoagulant is important. The use of formaldehyde should be permitted only if great care is exercised to see that an excess is not used.
FORMALDEHYDE
1 2 3 Av.
221 211 214 215
775 775 800 783
The ammonia was added as an 18.5 per cent solution, using 2 cc. per liter of latex. The formaldehyde was added in the form of a 40 per cent solution, using 1cc. per liter of latex. It was neutralized before use. The same latex was used for a single run, dividing it into three parts. The figures are averages of several tests. A second series was run to observe the effect of sodium sulfite on the rubber, as compared to the effect of ammonia. The results are as follows: RUN
TENSILE STRENQTH
Kg./cm.P
ELONQATION
%
MODULUS AT: 500% 700% Kg./cm.*
UNSTABILIZED
1 2 3 Av.
227 174 194 198
775 775 805 785
35 24 16 25
162 114 95 124
35 29 23 29
172 139 119 143
27 20 20 22
128 98 101 109
AMMONIA
775 775 805 785 SODIUM S U L F I T E
1 2 3 Av.
190 188 185 191
775 800 805 795
EFFECT OF AMOUNT OF COAQULANT The effect of various coagulants was not studied, but a series of sheets was made using varying amounts of formic acid. The latex was diluted to 22 per cent dry rubber content in each case. No stabilizer was used. The results of the tests of the rubbers were as follows: 5% FORMIC ACIDPER LITER LATEX
.
CC 26.6 31.1 35.6 40.0 44.3 48.3 53.3
r
Tensile strength Kg./cm.l 200 238 232 234 244 257 246
RUBBERTE8T Elongation
Modulus at: 500%Kg./cm.* 700%
% 26 28 26 26 24 32 26
104 125 122 120 117 144 146
The rate of cure is fairly uniform over a considerable range of acid proportions, but, when the amount is very much in excess, the rate of cure is increased. This agrees with the findings of the Ceyl.on Research Scheme (1).
EFFECTOF LATEXCONCENTRATION De Vries (11) found that the dilution of the latex had a marked effect on rate of cure. This was investigated using Captax as an accelerat.or. The results follow:
20 15 10
276 203 217
173 123 119
39 27 27
SM)
800
800
The 20 per cent latex is faster in cure than the 15 or 10 per cent latex. This is a reasonable result, for the effect of soaking (as given later) shows that the natural accelerators are removable by water, and the greater dilution should thus remove more nonruhhers in the serum.
EFFECT OF COAOULATION PERIOD The longer a given coagulum stands, between acid addition and milling, the more maturation takes place and the harder and tougher i t becomes. Thus not only is there a greater amount of natural accelerator present in a sheet which has stood for a longer period before milling, but because of the increased hardness and tougliness it is harder to squeeze out the serum substances, and they are not as easily removed in the soaking process. The result is that the sheet is mu& faster in wre. In addition to this, the hardness of the coagulum requires much more power for milling. A series of tests was made to determine the relative curing proporties of rubbers made from coagula milled as soon as possible and from coagula milled the following morning. The results follow: OvmnsIonT C O * ~ r m A ' " O X Tensile Elonga- Modulua st: tion 500% 700% Ko./cm.* % Kdcrn.3 239 800 27 123 261 800 30 136 249 800 28 124 800 25 150 263 250 800 27 118 28 127 251 son 262 xoo 28 130 stieiigth
Kp./om.Z
%
1 2
247 246
800
3
242
ROO
4 5
237 225 248 241
6
Av
xnn
800 800 800 SW
Kv./crn.l 29 153 2s 120 26 113 17 122 22 98 27 104 27 115
EOPECT OF SO.4KING SHEETS
It is customary on most plantations to give all sheet rubber a soaking, or at least a rinsing, after the sheets are rolled and marked. This is done in order to prevent the development of "rust," and to improve the appearance of the product. The effect of soaking is to remove the soluble and the waterdispersed nonrubber substances. Since this N O U Ihave ~ an effect on cure as shown by Eaton and Granthani (a), a series of tests was made to determine whether the results were similar in a Captax-accelerated stock. The following arc averages of at least eight different runs:
The rate of cure decreases very gradually with increased time of soaking. I n order to produce a uniform sheet, a standard time of soaking should be maintained. It would he better to have this soaking period fairly long. For example, some sheets which had high initial ditrerences in rate of cure were especially watched through a soaking period. The results obtained slrowed that a sheet with a higher initial rate of cure decreased in rate of cure more rapidly than one which had a lower initial rate of cure. The results were as follows: Pmnroo VP Sonmlro
Nolle 1 mi".
5 min. 30 min.
60 min.
2 hours 8 houra
6 hours 18 hours
--Sasa;~ No 1 -Sxem No 2Tensile Elqnga- Modulus st: Tensile J3lonaa- Modulivs at: strenatli tioo 500% 700% strength t i o n 500% 700% Ka.1cm.Z y/n K .d.c m . 9 Ko.1cm.S 9G Ka.lcm.3 .. .. .. .. so0 21 id0 29 206 96 244 121 son 23 21 217 196 83 800 93 85 189 800 20 21 218 800 91 810 17 21 79 182 207 800 86 xin 17 21 196 184 70 810 90 174 826 16 76 15 170 810 67 825 12 131 55 157 826 16 72 123 825 13 52 69 180 825 17 825 12 49 825 12 127 110 50
Tbe sheet which had been soaked overniglit was nincli lighter in color after smoking than was tlie unsoaked sheet. It also vas very resistant to molding, as no mold developed on i t for months while hanging in the open air in the tropics. The unsoaked sheet developed mold in a few days. The soaked sheet when subjected to higher temperatures than usual for drying became tacky quickly, while the unsoaked sheet required a mucli longer time. The study of the effect of soaking also seenis to show that pale crepe rubber, mhictr is washed thoroughly, would have a lower rate of cure than sheet; it is well known that such is actually the case. Similarly, pale crepe should be less susceptible to mold and should also become tacky at relatively low temperatures. Those points have been well established, and it is also true that, generally speaking, pale crepe is more uniform in rate of cure than is sheet.
EFFECT OF SHEET THICKNESS Eaton (6) has pointed out that the effect of soaking varies with the t,hickness of the sheet. Not only is tlie surface per
MODERN SHEETING MAL~NE
540
INDUSTRIAL AND ENGINEERING CHEMISTRY
unit volume greater, but the nonrubber substances may diffuse more from the center of a thin sheet than from a thick one. Van Harpen (IO)found that the rate of drying varies with the square of the thickness of the sheet. It is not unreasonable to assume that the same rule :nay hold for the removal of the nonrubber substances from the coagulum by soaking. There are several causes of variation in the thickness of sheet, assuming the rolls for milling the coagulum are set with a constant opening. The majority of the machines used are not too well constructed, with a result that the rolls may be forced wider apart with tougher coagulum, owing to play in the bearings or a springing of the rolls. There is also apt to be a difference in the slip between roll and coagulum, depending upon the condition and material of the roll, and on the toughness or hardness of coagulum. The hardness or toughness of coagulum may be varied by changing the dry rubber content of the latex and by changing the time interval between addition of coagulant and rolling the coagulum. The increase of dry rubber content makes the coagulum more dense. More rolling is required to bring the coagulum to the same thickness, while the length and width of the sheet are greater than that from a more dilute latex. The longer a slab of coagulum stands before rolling, the harder and t,ougher it becomes. This is caused by syneresis. Consequently, there is an opt,iinumlength of time between addition of coagulant and rolling; the coagulum should be firm enougil to be rolled without splitting or tearing and yet soft enough that it can be easily rolled out to the proper thickness. The harder coagulum will require much more power to roll out as well as more machining to obtain the proper thickn-ss.
EFFECT OF DRYIXG TENPERATURE The temperature of the drying chamber has an effect on the rate of cure. This was first point'ed out by Eaton ('7) in his experiments with slab rubber. His tests mere conducted at 160" and 130" F. (71.1"and 54.3" C.). The following comparison shows the effect of raising the drying bemperature from 40" C. (104" F.) to 50" C. (122' F.);the rubber used was pale crepe: DRIED24 HOURSIT 50' c. DRIED30 HOURSAT 40' c. CUREA T Tensile Elonga- Modulus a t : 500% 700% 128O C. strength tion Kg./crn.? Min. Kg.lcrn.2 % 47 16 850 20 105 18 56 840 30 126 66 820 21 40 134 70 23 136 810 60 24 73 810 80 143
Tensile Elonga- Modulus a t : 500% 700% strength tion Kg./cm.' % Ks./~rn.~ 915 14 38 110 17 50 106 840 17 51 855 120 820 20 60 117 815 21 64 126
I n sheet nianufacture the temperature must be held at such a point that blisters will not form from the excessively rapid evaporation of moisture. This temperature is dependent, among other things, on the thickness of the sheet. Thinner sheet may be dried a t higher temperatures. The higher temperature decreases the mat'uration of the coagulum, and, beepuse the sheet is thinner, the center becomes warmer in a shorter time. COKCLUSIOSS 1. A comparison of the results of the present work with that of previous investigators shows that certain natural and
processing variations have the same effect on a Captaxaccelerated stock as they have on a rubber-sulfur stock. 2. I n attempting to produce a uniform sheet on a plantation the following points should be remembered: Latex should be blended so that the effect of the tapping system is minimized. Latex should be brought to a standard dry rubber content before coagulation. The anticoagulant and coagulant must be carefully controlled as to quality and quantity. The length of time between acid addition and rolling should be carefully standardized.
Vol. 26, No. 3
The rolling equipment should be sturdily constructed and the
rolls so fixed that they cannot be easily changed. The temperature of drying should be so fixed that it is always the same and at such a point that fuel consumption is a t a
minimum.
3. The causes of variation are so many that unless el'erything is held absolutely constant, the product may vary in rat'e of cure as much as if nothing is done in the way of standardization. 4. While it is conceivable that a plantation may produce a uniform product for a short space of time through careful supervision, no amount of care in processing can entirely overcome the seasonal variation, or the variations due to rainfall and wind. It is not probable, however, that all plantations will be able to produce a material with the same rate of cure as all the others, for they must overcome variations caused by climate and geographical location as well as those caused by different types of all the manufacturing equipment of all kinds. The consumer of crude rubber must bear the burden of eliminating most of these variables by blending the crude rubber just prior to use, for only at that time does the rubber accumulate in such a way that blending is econornical.
LITERATURE CITED (1) A n o n y m o u s , Bull. I m p . I m t . , 18, 1 (1920), 20, 431 (1923). ( 2 ) Anonymous, Gu?nmi-Ztg., 4, No. 14, 4 (1590). (31 A n o n y m o u s , India Rubber J . , 47, 125 (1914). (4) Bishop a n d F u l l e r t o n , J . Ruhber Researchlnst. -lIaLaya, 3, 129-49 (1932). ( 5 ) D i n s m o r e a n d Z i m m e r m a n , IND.ENG.CHEM., 18, 144 (1926). (6) E a t o n . B. J.. Am. Bull. Federatrd .bfalilu States. 5,372 (1917).
t7j r i i , 6, i47'(1917).
(8) E a t o n , B. J., a n d G r a n t h a m , J., J . Soc. Chern. I n d . , 34, 989
(1915). (9) Eaton, B. J., Grantham, J., a n d Day, F. W. F., 4 g r . Bull. Federated X a l a y Stales, 17, 1-395 (1918). 110) H a r u e n . van. Arch. RrdJbeicuituur, 14. 432 (1930). ( l l j Vriei, 0. d e , ' I b i d . , 1, 35 (1917). (12) Ibid., 1, 169 (1917). (13) Ibicl., 4, 313, 331 (1920). (14) \-rim, 0. d e , "Estate R u b b e r , " Ruygrok & Co., Batavia, 1920. (l5j Thitby, G. S., J . SUC.C'hem. I d , 3 5 , 493 (1916). (16) K h i t b y , G. S., "Plantation R u b b e r a n d T e s t i n g of Rubber," L o n g m a n s , 1920. RECEIVEDNovember 23, 1933. Presented before the Ohio-Michigan Regional Meeting of the American Chemical Society, Akron, Ohio, October 13 t o 14, 1933.
