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Vol. 18. No. 3
The Absorption of Water by Rubber’ By C. R. Boggs and J. T. Blake SIXPLEXWIRE & CABLECo., BOSTON,MASS.
A vulcanized rubber compound is an absorbent of water, sometimes to the extent of 500 per cent. The water vitally affects its electrical properties. Attempts to reduce the absorption materially by variations i n the compounding have proved unsuccessful. The property of absorption is inherent in the rubber itself. There is no satisfactory mathematical interpretation of the rate of absorption. ’ The absorption of water is somewhat similar to the swelling of rubber in organic solvents. The water is probably first absorbed in the surface layer, then slowly diffuses into the mass, followed by a more or less complete dispersion of the rubber.
The effect of a number of latex coagulants on the rate of absorption of the rubber produced has been determined. Factors in the absorption, such as pressure, temperature, surface tension, and the effect of air have been investigated. Temperature seems to play the most important part. The absorption is largely due to the nonrubber-hydrocarbon constituents of the crude rubber. Purified rubber hydrocarbon absorbs a comparatively small amount of water. Methods of partially purifying the rubber by washing, by coagulating latex so as to lower the protein content, and by chemical means, were studied. It is believed that synthetic rubber has a much lower rate of absorption than natural rubber.
ONTRARY to the general belief, rubber is not waterproof. Raw rubber will absorb water until it is completely dispersed and do this a t room temperature, without the addition of extraneous material. This is in opposition to the old belief that rubber is a nonreversible colloid. Prate’s process of dispersion is only a special case of this principle in which the time is reduced by the addition of more protective colloids. Soft vulcanized rubber acts similarly at higher temperatures, but the disintegration is much slower. Certain samples of vulcanized rubber will absorb as much as 500 per cent water. Since most rubber articles are not immersed in water continuously for long periods, this absorption is not realized by the rubber technologist. Many failures of rubber articles are due to this water absorption. Soft rubber valves which are continuously under water become swollen and flabby and fail to operate. Electric wires insulated with vulcanized rubber lose their high insulation resistance and eventually fail, if allowed to soak continuously in water, either fresh or salt, without proper safeguards. Submarine cables have failed owing to water absorption alone. There is no evidence of deterioration due t o oxidation, but the electrical failure of the rubber is positive and complete. Therefore, transoceanic cables have usually been made of gutta-percha in spite of its many disadvantages. The writers have studied the problem mainly from a physico-chemical viewpoint and have concluded that the absorption is due chiefly to the nonrubber substances in the rubber and not to the hydrocarbon itself. Pure rubber absorbs only a very small amount of water. Prevention of this water absorption therefore lies in the purification of the commercial raw rubber. Vulcanized rubber shows the same relative characteristics as the rubber from which it is made.
brane in osmosis determinations. H e classed rubber as a comparatively inactive absorbent of water. Miller3made several experiments t o show t h a t rubber absorbed water. A piece of unmasticated material increased 17 per cent in weight when soaked in water for 9 months. The same substance in sea water increased 3.6 per cent. A sample of masticated rubber in 9 months absorbed 87 per cent in fresh water and 5.6 per cent in sea water. For comparison, samples of vulcanized rubber absorbed 19 and 1.6 per cent in fresh and sea water, respectively. The Charlottenburg Testing Laboratory of Berlin‘ gives the amount absorbed by rubber as 8 t o 35 per cent. Obachs gives a value of 24 per cent. Payene states t h a t the absorption of water by vulcanized rubber is infinitesimal compared with the raw material. Terry reports that the phenomenon will never come into any importance because Messrs. Hooper say t h a t a vulcanized rubber cable laid by them twenty-five years previous is still in perfect condition. A prominent cable manufacturing company has further claimed t h a t there is no evidence of the absorption of water by vulcanized rubber to t h e extent of interfering with its electrical properties. It is assumed t h a t the 3 or 4 per cent absorbed does not penetrate into the insulation appreciably, but remains wholly on the outer layers, where it does not lead to any trouble.
. . .. ..
C
Historical The literature on the absorption of water by rubber and guttapercha is rather scanty. Hancock, in his classical experiment, sealed water in a bag of raw rubber, and then showed t h a t the water gradually diffused t o the outer surface and evaporated. Somewhat similarly, Flusin2 studied the absorption of fluids by rubber in connection with its use as a semipermeable mem1 Presented by J. T.Blake before the Division of Rubber Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md.. April 6 t o 10, 1925. Revised paper received November 20, 1925. A n n chim. phys., 13, 480 (1908); J. Chem. SOL. (London), 94, 359 (1908).
*
.. .. ..
Preliminary Work
As has been said before, the electrical properties of vulcanized rubber are vitally affected by comparatively small amounts of water. To show the effect of absorbed water on electrical properties, the specific resistance-moisture curves of two typical insulating compounds are shown in Figure 1. The other electrical properties are also affected deleteriously. Today, very few high-tension submarine cables depend on rubber insulation if exposed to the action of water. Lowtension cables prove satisfactory because the insulation wall can conveniently be made thick enough to include a large factor of safety. A study of the rate of absorption of water by rubber is therefore of utmost importance in cable design. An attempt was made to reduce the rate of absorption of water by means of suitable compounding ingredients. Nearly two hundred individual compounds were mixed and cured in this laboratory, and their rates of absorption determined. The compounding ingredients were systematically varied, both in amount and combination. The individual ingredients studied were paraffin, ozocerite, gas black, zinc oxide, barium sulfate, whiting, powdered ebonite, gutta-percha, mineral a 4 6 6
J . Chem. SOC.(London), 18, 273 (1865). Terry, E k f r i c i a n , 48, 916 (1900). “Die Guttapercha,” pp. 66 and 105, Berlin, 1899. Comfit. rend., 54, 2 (1852); 36, 109 (1853).
Vol. 18, No. 3
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is plotted against t / A 2different thicknesses, the points should fall on a single curve. After assuming equilibrium values for the thicker samples, their data do fall on a single curve, which approximates the theoretical one. The authors give no details of the rubber employed. To test the universal application of this equation, two types of compounds were cured in different thicknesses. They were soaked in distilled water a t 70" and 23' C. The increase
l.0
2,0
3,O
4,G
5,G
6,O
7.0
in weight was taken as the water absorbed. The loss of weight caused by solution of soluble constituents of rubber in water was found to be within the experimental error. For any one compound a t a single temperature, the value of X , should be a constant and independent of the thickness of the sample. If we plot values of X against t/A2,the curves for different thicknesses should coincide. COMPOUND 2 240 Smoked sheets 195 Zinc oxide 120 Whiting 15 Sulfur 2 Tuads (tetramethylthiuram
COMPOUND 1 240 Latex-sprayed rubber 180 2inc;oxide 120 Whiting 80 Litharge 15 Sulfur
disulfide) 10 Minutes a t 129.5' C.
60 Minutes a t 135' C.
The curves for Compound 2 are plotted in Figures 3 and 4. Similar curves were obtained for Compound 1. These do not approach a single curve. I n contrast to these, if the gain in weight per unit area of surface, for samples of three different thicknesses, is plotted against the time, the curves very nearly coincide (Figure 5). It has been claimed that the equation does not hold until after water has reached the center of the sheet. I n that event the application is very much restricted, as in one sample over 20 per cent water has been absorbed without the curves coinciding. I n an unpublished report the Research Laboratory of Applied Chemistry a t Massachusetts Institute of Technology, in collaboration with this laboratory, has shown that even after rubber has been soaking for some time the greater part of the moisture is in the outside layers. For raw rubber, where there appears to be no equilibrium, the above equation would be worthless. Fick's law is applicable to water absorption in special cases only. Tompkins12 favors the use of the equation X = K log t. Data have been found to fit it in special cases but usually values of K vary over a wide range. Example: Pale Crepe in distilled water at 24' C. X
Time
Hours 21 43 67 145 169 187 235 310 1130 2712
I n order to have a comparable basis upon which to express absorption rates, an empirical method must be used, since theoretical equations fail to be general. This consists in using a sample of the same thickness in every case and recording absorption in terms of unit area of sample. Such a method is rather unscientific but seems to be the most satisfactory one. Samples of raw rubber used in this investigation were com-
K
1% Trans. Faraday Soc., 16, Discussion Faraday SOC. and Phys. Soc., London, Appendix 11. 162 (1921).
pressed between sheets of aluminium in a frame 0.073 inch thick. A vulcanizing press was used and the temperature maintained for 30 minutes a t 102" C. This gave sheets of reproducible thickness with parallel, smooth surfaces. They were clear and transparent in most cases. Vulcanized samples were cured in the same frame and were equally satisfactory. b',The soaking was done in closed glass containers placed in a thermostatic oven. The temperature was maintained constant within a degree. Air was not excluded from the containers, as preliminary determinations showed that such exclusion did not affect the rate of absorption. Distilled water was used in all cases. Effect of Temperature
Samples were pressed from a lot of first latex pale crepe and the rates of absorption studied a t four temperatures-viz., 24", 60', 70", and 100' C. (Figure 6). The curves in Figure 7 show a steady increase in rate of absorption with a n increase in temperature. If we call the amount absorbed, grams per square centimeter of sample after 20 hours' soaking, a n empirical constant (Km),the values are as follows: Temperature a
c.
24 60 70 100
KIO 0.0110 0.0265 0.0390 0.079
Twenty hours has been found to be the most convenient time interval. At that point absaption has proceeded far enough to be appreciable, and disintegration has not yet begun. For cured samples 100 or 200 hours may be a convenient time as a basis of comparison, owing to the much delayed disintegration. The subscript after the K refers to the time in hours a t which the values were taken. If the logarithm of these empirical constants is plotted against temperature (Figure 7 ) , a straight line is obtained. This implies the same relationship that exists between specific reaction rate and temperature. The equation connecting the two would be:
+
T = 88.9 log K ~ o 198.1
While the specific reaction rate increases from 2 to 3.5 times for a 10 degree rise in temperature, the above increase is only equivalent to 1.30. It is therefore probable that the
INDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1926
rubber, glue, rosin, coumarone resin, phenolic resins, and fatty acids. The individual effect of twenty-nine accelerators was also ascertained. The effect of time of cure was ascertained. The rubber content of the compounds was varied from 10 per cent to 90 per cent. No remarkable improvement was effected. The effect of coating the cured rubber with various waterproof waxes was studied. Rubber was also cured and impregnated in hot paraffin. Although this appreciably lowered the absorption of water, it did not give an entirely waterproof material. The chloride, bromide, and hydrochloride of rubber and benzoyl and phenoxy rubber have been prepared in this laboratory. They proved to be unsatisfactory materials for resisting water when tested alone or mixed with rubber. The rate of absorption of water by hard rubber was shown to be very much lower than that for soft cured rubber. The, phenomenon of absorption seemed to be an inherent property of the rubber rather than a property of the fillers. Phenomenon of Absorption
If a sheet of either raw or vulcanized rubber is soaked in water and a t suitable intervals removed and wiped dry, it will be found gradually to increase in weight. The volume of the rubber gradually increases and the process is reminiscent of the swelling of rubber by organic solvents. Flusin and Barry’ investigated the relative swelling powers of organic solvents and water. They made no differentiation between water and the other liquids. It occupied the last place in the table of solvents when they were arranged in the order of their swelling powers. Their constants were obtained from an empirical equation connecting equilibrium and rate and have no theoretical significance. The only conclusion that may be safely drawn from their work is that, since water is absorbed to the least extent of any “solvent,” its rate of absorption is least, and equilibrium conditions are very slowly approached. The swelling of vulcanized rubber by organic solvents takes place rapidly a t first, and then less rapidly, finally coming to a n equilibrium, where the absorption of solvent
226
die-away type, eventually reaching a condition of equilibrium with the water. Pale crepe, when soaked in distilled water a t 70” C., does not show this phenomenon, but steadily absorbs water. At the end of a few days it becomes too sticky to weigh. If allowed t o continue soaking, it disintegrates into a gluey mass and is practically dispersed in the water. Rubber is not an irreversible colloid, but redisperses by itself to give ti latex-like material. It does not need a n added protective colloid. Obviously, the temperature brings about other changes besides the dispersion. At elevated temperatures vulcanized rubber exhibits a n intermediate phenomenon (Figure 2). The sample of latexsprayed rubber is associated with as much water as in the original latex. Although it still retains its general shape, it easily disintegrates in the fingers. Mathematical Relations
Since a comparison of the susceptibility of various rubbers toward water is probably best made by a study of their rates of absorption, a consideration of the mathematics involved is essential. Various investigators have studied the rate of absorption of “solvents” and water by both raw and vulcanized rubber. Their attempts to express the rate mathematically result in several different types of equations. None of these represent a t all accurately the facts in more than a few particular cases. A general equation seems to be impossible. On the data of Kirchhofg and Flusin’O the equation of the first order has been tested
z
dx
=
This equation obviously requires equilibrium values. X is the concentration of solvent in the rubber a t infinite time and X the concentration at time t . A trial on the data of Andrews and Johnstons gives constants that vary up to 1400 per cent. Ostwaldll maintains that the equation is valid and can be used with certainty, but in Kirchhof’s results the values for K vary at least 600 per cent.
/,O
ceases. As the coefficient of vulcanization decreases the swelling equilibrium increases. Eventually, as in the case of raw rubber, the swelling becomes a complete dispersion in the solvent. There is no sharp dividing line between the region of “solution” and swelling ending in equilibrium. This phenomenon is generally true of reversible colloids. Gelatin completely disperses in hot water but in cold water swells to an equilibrium. Andrews and Johnstons found that a sheet of cured rubber soaked in water at room temperature gives a curve of the 8
Compt. rend., 161, 589 (1915). J . A m . Chem. Soc., 46, 041 (1924).
2.0
3,0
4,O
JIO
6,O
Andrews and Johnston have developed Fick’s law of diffusion mathematically so that it might be applied to the absorption of water by sheets of rubber. The final form of the equation, using the same terminology as above is ,
Kt A2
= -0.0851
- 0.933 log
A is the half thickness of the sheet of rubber. This is said, by them, to be a form of the first order equation taking into account the thickness of the sample. Obviously, if X / X , Kolloidchcm. Beiheflc, 6, 1 (1914). Compl. rend., 126, 1497 (1898). 11 “Grundriss der Kolloid Chemie,” 1909, p. 369. @
7
K(X,-X)
10
INDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1926
absorption of water by rubber is not a chemical reaction or a molecular association. The authors have further found the temperature coefficients for cured smoked sheet, para, and pale crepe to be 1.32, 1.44, and 1.43, respectively. A comparison of this coefficient with other temperature coefficients is interesting. The temperature coefficients for the swelling of rubber in benzene9 range from 1.06 to 1.19 a t 40" to 50" C. Lundal'3 gives data on the swelling of pure gum and cured rubber in paraffin oil a t 100' and 16" C . From this a temperature coefficient of 1.33 may be calculated. OstwaldI4 gives the temperature coefficient for the swelling of gelatin in water as 1.5 to 2.0. The coefficient for the adsorption of Congo red by filter paper15is given as 1.36. This does not constitute a proof that the controlling factor in the absorption of water by rubber is an adsorption. It shows, however, that it is analogous to the swelling of rubber in "solvents" and other colloids in water. With organic solvents there is also a certain amount of solvation of the rubber globules. VoLuhm CHANGES--The absorption of %o1vents'' by rubber is accompanied by a slight contraction in total volume. Posnjak'e finds a contraction of about 1 per cent. This is much less than the contraction in total volume when gelatin and starch swell in water. I n six samples that had been soaked in water, measurements were made of the increase in volume of the rubber and compared with the volume of water absorbed. This gives values for change in total volume. Sample 1
2 3
4 5 6
Gain in volume cc.
Gain in weight Grams
7.95 8.30 8.30 8.69 9.90 9.32
8.25 8.26 8.34 8.59 10.01 9.67
Difference
1-0.30 --0.04 +0.04 --0.10 4-0.20
1-0.35
The average difference is +0.12 cc., or approximately 1 per cent, a value within the experimental error. The values obtained by Kirchhof" differ as much as 20 per cent. He nevertheless draws the conclusion that there is no change in total volume when water is absorbed. This would eliminate the hypothesis that the absorption would be due partly to the microporosity of the material, causing entrance of the liquid by capillarity.
The fact that there is no change in total volume would perhaps imply that the rate of absorption would not be changed by a change in pressure on the system. .4n attempt was made to check this observation. Samples of a pure gum latex-sprayed stock were soaked a t 24" C. under a pressure of 80 pounds per square inch, in an autoclave so arranged that Ann. phys., 66, 741 (1898). Loc. ciL, p. 370. 16 Ibid. p. 426. 16 Kolloidchem. Beiheftc. 3, 431 (1912). 17 K o l b i d - Z . . 36, 373 (1924). I* 14
227
air could be forced into it under pressure. Blank experiments were run a t atmospheric pressure at the same temperature. LeChatelier's principle would imply that the equilibrium values are independent of the pressure on the system. Time Hours
27 54 144 216
-GAIN, GRAMS/SQ. IN.SAM+Atmospheric pressure 80 Ibs./sq. in. pressure 0.0253 0.0272 0.0442 0.0435 0.0541 0.0541 0.0690 0.0686
The values check within the experimental error and show that the rate .of absorption is independent of the pressure on the system within the range of the experiment. It may therefore be seen that the rate of absorption follows no general mathematical law. It probably consists of an adsorption of the water in the surface layer, followed by a slow diffusion into the mass. This in turn may be followed by a dispersion, either partial or complete. Effect of Change in Environment
WATERVAPOR-Rubber swells in the vapors of an organic solvent a t a slower rate than in the solvent itself a t a given temperature. If the vapor pressure of the liquid were the determining factor, the rate of swelling should be dependent on the temperature alone. The rubber immersed in the liquid should have the same rate as that in the vapor phase. Samples of a 9O:lO mix of pale crepe (cured 120 minutes at 141' C.) were suspended over water in a vacuum desiccator from which the air had been removed. They were thus completely surrounded by water vapor and a determination of rate was made a t 70" C. A determination was made on samples of latex-sprayed rubber a t 24" C. Latexsprayed rubber was used on account of its high rate and consequent quick results. The constants a t 50 hours are: KSQ 0.0470 0.0476 0.0374 0.0430
70' C.
Samples in vapor Blank in liquid 24' C. Samples in vapor Blank i n liquid
At 70" C. practically no difference in rate was observable over the period covered. At room temperature there is a difference of about 15 per cent.
DILUTE SULFURIC ACIDAND DILUTESoDIuhf HYDROXIDEA sheet of pressed pale crepe was soaked in 5 per cent sulfuric acid a t 70" C. Similar determinations were made in 2 per cent and 5 per cent aqueous sodium hydroxide a t 70" C. Distilled water
5% HzSOi
2 7 NaOH
58 NaOH
Kno 0.078 0.0170 0.060 0.001
The decrease in rate due to the sulfuric acid is considerable, but the effect of dissolved sodium hydroxide is tremendous-
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so that absorption practically ceases beyond 5 per cent concentration. SODIUM CmoRIDE-Samples of a 9O:lo mix of latex-sprayed rubber were soaked in 3, 7, 15, and 35 per cent sodium chloride solutions a t 70" C. Kao Distilled water NaCl solution: 3% 7% 15% 35 %
0.250
0.148 0.105 0.0672 0.0368
The 3 per cent salt solution corresponds to sea water. The values of the constant, if plotted against the concentration of the sodium chloride, give a curve of the hyperbolic type (Figure 8). OSMOTICPRESSURE-The Western Electric Company in a recent patent1* have advanced the theory that osmotic pressure is the cause of water absorption by rubber. The Bureau of Standardslg has subscribed to the same theory, maintaining that the rubber acts as a semipermeable membrane. The crystalloids (probably both organic and inorganic) in the rubber, by osmotic pressure, attract the water. This causes its diffusion into the body of the rubber. According to this principle, since there is a finite amount of crystalloids in the rubber, a piece soaked in distilled water should absorb a n infinite amount of water and never reach an equilibrium. Osmotic pressure may play some part in the absorption. A sample of vulcanized rubber that had gained 58 per cent water by soaking 4200 hours in distilled water a t 70" C. showed no signs of approaching equilibrium. Sample 1 of this soaked rubber was placed in a 35 per cent sodium chloride solution a t 70" C. Sample 2 was placed in a similar solution a t 24' C. The water content fell rapidly in both cases. Time Hours
-PBR
Sample 1
n
.5R. 0
26 44
16.8 13.4
68
11.7
C&NT WATER-
Sample 2 58.0 29.6 21.6 19.4
Osmotic pressure may be the force drawing the water out of the rubber. SURFACETENSION-In the preliminary investigation the effect of surface tension was considered. Rates of absorption
Vol. 18, No. 3
a straight-line function of the concentration expressed as mols of solute per mol of solvent. If the rates of absorption of rubber in such solutions are plotted against their surface tensions, the rate falls off with a n increase in surface tension, as predicted. The remarkable difference between the effects of 5 per cent sulfuric acid and 5 per cent sodium hydroxide on the rate is qualitatively explicable on this basis. Sodium hydroxide raises the surface tension of water a t a much greater rate than sulfuric acid. KIP 0.078
Distilled water sulfuric acid sodium hydroxide
$ W
0.0170 0.001
Increase of surface tension
...
0.20 1.8
Most substances when dissolved in water raise its surface tension. There are several, however, that lower it. Among these are aniline, phenol, acetic acid, sodium oleate, alcohol, glycerol, sugar, and ammonia. A 40 per cent latex-sprayed compound was soaked in solutions of these substances a t 70' C. SOLUTE Sodium chloride Glycerol Sugar Distilled water Ethyl alcohol Sodium oleate Phenol Aniline Ammonia Acetic acid
Per cent 35 30 5.3 Blank 10.0 2.5
5.0
3.5 5.0
20.0
Kso 0.0368 0.110 0.173 0.280 0.346 0.336 0.539 0.622 1.002 2.006
Surface tension 86.3 71.5 63.6 72.8 51.2 26
... ...
66.5 49.8
Six of the solutions give greater rates of absorption, while sugar and glycerol solutions have lower rates than water. In some cases there is undoubtedly some absorption of solute. A solution of aniline in water is exceptional in that its surface tension increases with a rise in temperature.20 It might, therefore, be predicted that the rate of absorption in such a solution should have a lower temperature coefficient than the same rubber in distilled water. The rates of absorption will be higher. A temperature coefficient undoubtedly is the combined effect of temperature and change in surface tension. The temperature coefficient for a 40 per cent latexsprayed compound in a 3.5 per cent aniline is 1.36, while in distilled water it is 1.52.
405
404 403 402
were determined for a sample of rubber in a sodium chloride and in a cane sugar solution. The rate of absorption was decreased in both cases, although the surface tension was changed in opposite directions. If the absorption is a dispersion process, rather than an osmosis, a lowering of the surface tension of the soaking medium should increase the rate of absorption, since the interfacial tension is usually lowered. The surface tension of aqueous sodium chloride solutions is 18
19
British Patent 223,644 (1924). Rubber A g e , 16, 287 (1925).
The two exceptions in the above determinations are glycerol and cane sugar. A number of years ago, during an investigation of the absorption of oils by rubber, it was noticed by the writers that castor oil was not appreciably absorbed. Many other oils were investigated a t that time and it was found in general that substances with high dielectric constants did not swell rubber as much as those with low constants. The high dielectric constants of glycerol and water are attributed t o the presence of a relatively high percentage 10
Bancroft, "Colloidal Chemistry." p. 130.
IA-D USTRIAL A S D ENGINEERING CHEJIISTRY
hIarch, 1926
of hydroxyl groups in the molecule. The general relationship has recently been brought out by Whitby.21 Ostwald has formulated mathematically a relationship. His equationzzconnects the swelling powers of a solvent with its dielectric constant. 2.16
d m X D = K K = a constant JV = swelling power of solvent D = dielectric consi:ant
It may be, in the case of sugar and glycerol, that the effect of the groups causing a high dielectric constant has overbalanced the effect due to the lowering of the surfitce tension. LangmuirZ3has shown that in the case of t'he higher fatty acids there is a selective solution of part of the aolecule in water. The carboxyl group actually enters the water, while the hydrocarbon residue is completely insoluble. Similarly, it is possible that there is a selective absorption !of the nonhydroxyl portions of the molecule by rubber. As the ratio of hydrocarbon residue to hydroxyl group increases, we should expect the swelling power of the liquid to increase. Amyl alcohol swells rubber to a greater extent than ethyl alcohol. This in turn swells it to a greater extent than methyl alcohol. Effect of Latex Coagulants
The phenomenon of the absorption having been studied, a tentative hypothesis as to its cause was formulated. By previous work the absorption was shown to be an inherent property of the rubber itself. Rubber, however, consists of two portions-the nonrubber constituents (resins, carbohydrates, proteins, et(!.) and the rubber hydrocarbon. It is possible that these nonrubber const'ituents act as dispersing agents for the hydrocarbon, thus causing the absorption. The rubber globule in latex probably consists of the hydrocarbon portion surrounded by an envelope of protein. The remainder of the nonrubber constituents are adsprbed on the surface. If the envelope could be rendered inactive as a dispersing agent, rubber would become practically waterproof. There are four general methods of bringing this about: (1) Latex may be coagulated with a substance which mill destroy the characteristics of the envelope. ( 2 ) The latex may be coagulated under condit'ions which vould leave less nonrubber const'ituents in the rubber, the remainder being discarded in the serum. (3) Some of the material may be removed by washing the rubber after coagulation with warm water. (4) The nonrubber constituents may be removed by solution of the rubber, mechanical separation of t.he protein, and reprecipitation of the hydrocarbon. A number of methods of producing rubber from ammoniapreserved latex by the use of various coagulants were tried. The effect of these coagulants on the rate of wat'er absorption was ascertained. The following results are only preliminary as the field to be covered is so large that much more time will be required for complete data. One hundred grams of preserved latex were treated with the coagulant dissolved in enough water to give 100 grams of solution. The coagulum was allowed t o stand several hours in the serum, then washed and rolled on the corrugated roll mill. It was air-dried and rolled into crepe on the mill. A rate of absorption was determined a t 70" C. as described. ACETICACID-cOmnlerCially, most of t'lie coagulation of latex is brought about by the use of about 1 per cent acetic 21 "Relations between t:he Chemical Character of Liquid:; a n d Their Ability t o Swell o r Disperse Rubber." Presented before t h e Division of Rubber Chemistry a t t h e 65th Sleeting of t h e American Chemical Society, A-ew H a v e n , Conn., April 2 t o 7, 1923. 2 2 Kolioid-Z., 19, 100 (1'321). * 3 J . A m . Chsm. SOL.,39, 1848 (1917).
229
acid. The effect of the strength of the acid on the rate of absorption was determined a t 70" C. Strength of acid Per cent 2 4
riz0 0.069 0,040 0.027 0,020 0.040 0.040
7
10
30 40
The rate of absorption varies appreciably with the strength of the acid, a minimum occurring a t about 14 per cent (Figure 9). It is believed that this point represents a maximum solubility of the protein in the serum, the rubber having a minimum rate of absorption. OTHERhcrDb--deveral other acids were tried as coagulants and their effect on the rate of water absorption determined. Five per cent acid was used. The constants a t 70" C. mere Kzo 0 0 0 0
Chloroacetic Trichloroacetic H j drochloric bulfuric Oxalic Tannic H) droferrocyanic
043 035 065 06.2-
0 041s
0 036 0 034
One of these coagulants proved t o be better than 5 per cent acetic acid. S o radical improvement was observable, however. OTHER PROTEIN PREcrPITAruTs-Several protein precipitants, in addition to those included above, were tried as coagulants. Formaldehyde is reported to act mliolly as a preservative for latex. It occupies an anomalous position for a protein precipitant. Per cent Ammonium sulfate Acetic acid Formaldehyde Sodium sulfate Formaldehyde Acetic acid Alum Stannous chloride Mercuric chloride
3 2: 1 1 10 l!
10
10
K?o 0.135 0,082 0,026 0.110
0.046 0.026
LIercuric chloride and formaldehyde, the strongest protein precipitants, give the lovest rates of absorption. They are, however, no better than the same amount of acetic acid. ORGAXICLroums-Organic liquids, if stirred with latex, coagulate it. The coagulation is probably due t o the insolubility of the protein in organic liquid. Likewise, some of the other constituents tend to remain in the rubber instead of being discarded in the serum. On the whole, these rubbers give higher rates of water absorption. E t h y l acetate Chloroform I3en ze n e Acetone Ether E t h y l alcohol Amyl alcohol
cc. 110 120 120 80 110 110 SO
KlO
0.067 0.040 0.036 0.062 0.117 0,058 0.027
I n each case approximately the minimum amount necessary t o coagulate 100 grams latex TTas used. DILUTIOX OF Lamx-Proteins are dissolved by water t o a greater or lesser degree. Dilution of the latex before coagulation should incredse the amount of water-soluble material discarded in the serum and thus lower the rate of absorption. One hundred grains latex were diluted with 300 cc. water and coagulated with 100 cc. 7 per cent acetic acid. The 20hour constant was 0.021, and that for 7 per cent acetic acid without dilution preceding coagulation, 0.027. Dilution of the latex lowers the constant about 25 per cent. HE.u--Fresh latex is coagulated by warming, which should increase the amount of solids discarded in the serum. With preserved latex coagulation could not be brought about
405
ai 7 4 ° C
by the action of heat, except by actual evaporation. This applied also to latex acidified just short of coagulation. Coagulation of hot latex with the usual reagents gave a product apparently the same as that produced by a cold treatment. CEPiTRIFUGIKG-Fresh latex is coagulated when centrifuged. Experiments with preserved material produced only a cream. This cream was no different from that formed when the latex is allowed to stand for a moderate length of time. T 7 a ~ ~ o REAGEKTS us AND ~ ~ C E T I C hcrn-Proteins may be acted on by certain reagents and not be precipitated by them. Certain colloidal substances may be adsorbed by them. I n either case the additional material makes no appreciable difference in the ability of the latex to be coagulated by acid. Five grams of various reagents dissohed in 100 cc. of water were added to 100 grams latex and the mixture allo\Ted to stand 30 minutes. Coagulation was produced by 100 grams 5 per cent acetic acid. The rubbers were treated as usual and their rates of absorption determined. R~AGWIT Aniline Soap Egg albumin G u m arabic Sodium oleate Phenol Glue Gelatin
KZQ 0.018 0.019
0,025 0.027
0.019 0.0155 0.0195 0.042
The rubber produced by means of the phenol has the least rate of absorption, although some of the other reagents produce a substance better than that produced by acetic acid. Various combinations of these beneficial reagents were tried without any substantial betterment of the rubber. Modification of Rubber .~PPRECIATION OF PALECREPE-At the beginning of the investigation a lot of pale crepe was set aside. A short time after its rate of absorption had first been determined,
a futile attempt was made t o check it. Several times later further attempts were made, with poorer results than before. The curves (Figure 10) show a steady decrease with time. A plot of the constants a t 20 hours gives a smooth curve (Figure 11). Pale crepe appreciates with age in regard to its rate of water absorption. Gu!rTA-PERCHA-Gutta-percha was first suggested for use as an insulator in 1848 by Faraday. Since then it has been constantly applied in the manufacture of submarine cables. Gutta-percha contains finely powdered bark and other impurities. A sample of Macassar gutta was washed 10 minutes on a corrugated roll mill with warm water. The warm water softened the gutta and made it thoroughly plastic. The removal of the extraneous impurities was complete, as was shown by dissolving the gutta in a chloroform-benzene mixture when only the small amount of protein remained insoluble. Rates of absorption were determined in distilled water a t 24" C. (Figure 12). Attempts a t 70" C. were only partially satisfactory because the sheets lose their shape a t that temperature. The constant a t 200 hours is 0.0122. The corresponding constant for raw pale crepe a t the same temperature is 0.0356. It can thus be seen that, although a perfectly waterproof rubber is desirable, the improvement of present materials need be of only a moderate magnitude to be satisfactory. STATEOF AGGREGATION-It was thought that the degree of polymerization, or the state of aggregation, of the rubber might have an appreciable effect on the rate of absorption. One method of changing it is by changing the time necessary to obtain the correct technical cure. This may be brought about by means of accelerators. Three different compounds were cured to a maximum tensile strength and their rates of absorption determined (Figure 13). v77 240 Rubber 195 Zinc oxide 120 1Vhiting 15 Sulfur 2 Tuads 10 Minutes a t 129' C.
v5 240 Rubber 180 Zinc oxide
120 Whiting 30 Litharge 1 i Sulfur 60 Minutes a t 135' C.
Vi6
240 Rubber 195 Zinc oxide 120 Whiting 15 Sulfur
I50 M i n u t e s a t 141OC.
I S D C-STRIAL LI*D E S G t S E E R I S G CHE-IIIS'TR I'
March, 1926
The equivalent times of cure at 129" C., assuming a temperature coefficient of 2.5, are 10, 90, and 430 minutes, respectively. The lolyer the state of aggregation the greater i? the rate of absorption. Its effect is small, however. when the fact that the tensile strength of VT7 is 2900 pounds per square inch and F'76 is 900 pounds per square inch is considered. Sodium polymerizes butadiene, isoprene, and dimethyl butadiene to give rubber. This principle applied Lo the further polymerization of rubber has been made the basis of a patent by S p e n c P for use on the cheaper grades: Smoked sheet was broken down on the mill and 1 per cent finely sliced metallic sodium mixed in during 5 minutes. An appreciable change took place in the rubber. It stiffened somewhat, became lighter in color and opaque. It mas rolled and washed with cold water on the corrugated roll mill and dried. Another sample was milled similarly for 15 minutes with sodium, washed, and dried. Rates of absorption were determined on both samples and a blank a t 70" C. (Figure 14). The constants are very nearly proportional to the time of treatment with sodium. These results are directly contrary, on the aggregation basis, to the previous results by the vulcanization method. This anomalous result will be mentioned later. TIMEOF T'uLCaxIza'rIox-It was thought that in an individual mix the time of vulcanization might be an appreciable factor in the rate of absorption. A 9O:lO mix of pale crepe was cured 2, 4, and 8 hours a t 141" C. and rates of absorption were determined in distilled water a t 70" C. (Figure 15). The latter two samples were obviously overcured. It is evident that an increase in the time of vulranization decreaqes the rate of absorption. This decrease in the constant is approximately a logarithmic function, since the 4-hour curve is very nearly intermediate between the other two. This same principle has been checked by Kirchhof.9 H A R D RUBBER-The foregoing results would iniply, and previous work indicates, that hard rubber or ebonite would hare very low constants. Two samples were prepared. I.30 300 Rubber 175 Sulfur 3 p-Nitrosodimethylanillne 120 hfinutes a t 148' C.
V29 600 Rubber 350 Sulfur 240 Minutes a t 14s' C
Their rates of absorption were determined in distilled water a t 70" C. The constants a t 100 hours are: RIQO
1'30 T729
0.0137 0 0110
They are considerably less than the 9O:lO smoked sheet mix soft cured (100-hour constant = 0.068). Two samples of commercial hard rubber gave very similar constants. The chief cause of a decreased rate with an increase in the degree of rulcanization is probably the fact that the degree of vulcanization is a measure of the extent to which the nonrubber constituents have been changed or destroyed. RECLAIMED RuBBER--Reclaimed rubber finds a large use in compounding. There are two general types---material reclaimed by either the alkali or acid process. The rates of absorption a t 70" C. were determined for a sample of each kind. K50 Pale crepe Acid reclaimed rubber Alkali reclaimed rubber
0.12 0 . 15 0.93
The acid reclaimed is not radically different from pale crepe, while the alkali reclaimed has nearly eight times the rate of ** British P a t e n t 17,667 ( 1 Q 1 2 ) , U. S. Patent 1,112,938(1911).
23 1
ahorption. The remarkable rate of absorption of alkali reclaim i- in line with other phenomena. The smoked sheet that had been milled with sodium, although uiidoubtedly of a higher state of aggregation as judged by texture and stiffness, had a very much higher rate of absorption. The rubber, although very thoroughly washed, undoubtedly contained a small amount of alkali adsorbed by it. The reclaimed rubber also probably retains a finite amount in an adsorbed condition. Acid-reclaimed rubber is practically neutral owing to the action of the residual fillers, such as whiting, etc. This is in accord with the fact that pale crepe, when treated with alcoholic sodium hydroxide to destroy the protein, had a higher rate of absorption although it had been very thoroughly washed with water. .dCCELERATORS-The effect of the addition of nitrogenous accelerators on rates of absorption is still being investigated. Preliminary results indicate that they are m-ithout effect. Causes of Absorption
Sheets of latex-sprayed, smoked sheets, Para rubber, and first latex crepe were prepared by pressing a t 102" C., and their rates of absorption determined in distilled water. Since the rate increases materially with a rise in temperature and quick results were desired, determinations were made a t 70" C.-the temperature of the Geer oven. To compare these with cured rubber a t room temperature. samples of the raw rubbers were made into a standard 40 per cent compound. 240 Rubber 180 Zinc oxide 120 Whiting
30 Litharge 15 Sulfur 60 Minutes a t 135' C.
The rates of absorption of these samples were determined in distilled water a t 70" and 23" C. (Figures 16 to 18). The 50-hour constants are: Temp. LatexC. sprayed
Raw rubbers 407,compound 40% compound
70 70 23
0.38 0.236 0.0312
Smoked sheets
0.103
0.045 0.0097
Para 0.087 0.040 0.0079
Pale crepe 0.072 0.035 0.0070
There is but one anomaly in the table, that of the 40 per cent latex-sprayed compound at 70" C. The raw rubbers a t 70" C. and the same samples cured in a 40 per cent compound a t 23" C. maintain a constant ratio. The constants show that quick results may be obtained at higher temperatures, either with raw or vulcanized rubber. They will allow a prediction of relative rates of absorption at room temperature. Omitting the latex-sprayed rubber, the vulcanized samples give an average temperature coefficient of 1.40. This value is in fair agreement with 1.30, the value for raw pale crepe. The extraordinary absorptive power of latex-sprayed rubber is especially significant. This rubber is produced by a quick evaporation of the latex in a current of warm air. It therefore contains, nearly intact, all the nonrubber constituents present in the latex. -1rubber produced by coagulation obviously loses many of the soluble crystalloids, protein, and other materials which are discarded in the serum. I n Para rubber the nonrubber constituents are subjected to the destroying action of heat and the phenolic constituents of the drying smoke. Whitby25 and Van Rossemz j have shown that the moisture content of crude rubber varies with the atmospheric humidity. Whitby has separated serum solids which he describes as being exceedingly hygroscopic. This, with the remarkable properties of latex-sprayed rubber, implies that the nonJ . SOL.Chem. I n d . , 37, 27ST (iQl8i. :e Iiolloidchem. Beiheflir. 10, 44 (19131.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
232
VOl. 18, Yo. 3
rubber constituents, the impurities, are the cause of water absorption. THERESIN-The effect of the presence of the natural resins occurring in rubber on water absorption was ascertained. A sample of pale crepe was rolled thin, wrapped in tinfoil, and acetone-extracted in the usual manner for 24 hours. It was placed in vacuum for 6 hours to free it of acetone. A rate of absorption a t 70" C. was determined (Figure 19). Obviously, the rate is materially reduced by the elimination of the acetone-soluble material. This is very similar to the data of K i r ~ h h o f . ~It~ is contrary, however, to the statement of Weber?* "It (the absorption of water) increases
separated and the solvent evaporated in vacuum. The rubber was maintained in the absence of light and air until used. The insoluble material was repeatedly washed with benzene to free it of adhering rubber and dried in vacuum. The protein thus separated was a brownish powder. On exposure to the air it proved to be nonhygroscopic. The benzene-acetone mixture on evaporation yielded a small amount of gummy material. This also proved to be nonhygroscopic under the same conditions of humidity. PURIFIED RUBBER-The absorption curve of the purified rubber mas determined and the amount absorbed was found to be very low, almost within the experimental error. The
materially by freeing the rubber of its resinous impurities by extraction with acetone." The extracted resins were exposed to the air (average relative humidity 66 per cent) but were found not to gain in weight by the absorption of moisture. They were separated into two fractions, the water-soluble and the water-insoluble portions. Neither fraction was hygroscopic under these same conditions. The resins, although in themselves apparently nonhygroscopic a t atmospheric humidities, assist the absorption of water by rubber. WATER-SOLUBLE MATERIAL-van Rossem has shown that washing rubber makes its final moisture content lower. A sample of smoked sheet was washed 45 minutes with boiling water on a corrugated roll mill and the absorbed water removed by vacuum drying. A sheet of the washed material was pressed and its rate of water absorption determined a t 70" C. Since in this manner of washing there is considerable mechanical working-approaching breakdown to some extent-a piece of the original material was broken down on the mill, allowed to stand for 2 days and then pressed into a sheet. Its rate of absorption was determined as was that of the original material (Figure 20). Water-soluble constituents that are removed by washing increase the rate of absorption. Breaking down the rubber has no real effect on the rate, showing that it is independent of the mechanical structure. Kirchhofg has also shown that washing decreases the rate of water absorption. Western Electric Company has recently obtained a patent involving the principle.29 OTHER CONSTITUENTS-Twenty-five grams smoked sheet were acetone-extracted 24 hours and freed of solvent in a vacuum desiccator. This was allowed to stand under C. P. benzene until disintegration was complete and the mixture was then centrifuged. The insoluble material was thus thrown out of suspension and the clear benzene solution decanted. The rubber was precipitated by the addition of C. P. acetone. It was separated, redissolved in benzene, centrifuged again, and reprecipitatedlwith acetohe. It was
rate is a t least less than one-seventieth that of pale crepe (Figure 21). The pure rubber obviously is oxidized rather easily by the air dissolved in the water. This soon renders the samples difficult to handle. The absorption of this very small amount of water may be due to its actual solubility in the rubber hydrocarbon. It has been shown that the nonrubber constituents are the cause of the absorption of water. This suggests that they may play the part of protective colloids in the dispersion of the rubber in water. This is entirely in line with the recent work of Pohle.30 He has attempted by various methods to produce dispersions of synthetic methyl rubber. The material, of course, consisted of the pure hydrocarbon and contained no resins, proteins, etc. All direct attempts to disperse it were failures. The only successful method was to atomize a solution of the hydrocarbon into a dilute soap solution. The concentrations obtainable did not approach that of latex and the dispersions were not very stable. It is believed that synthetic rubber absorbs water to a very small degree compared with natural rubber.
-
m ~ ~ i i ~ i d35, - z .371 , (1924). 28 "Chemistry of India Rubber," p. 14. 29 British P a t e n t 223,644 (1924).
Note-After this work had been completed, a n article b y Kirchhof [Kollozdchem. Beihefte, 35, 367 (1924)l appeared, covering a few phases of t h e work. 80
Kolloidchem. Beihefte, 13, 1 (1920).
Detailed Foreign-Trade Figures for 1925 Available Complete and detailed preliminary figures covering imports and exports of the United States in the calendar year 1925 are now available. The Department of Commerce publishes summaries of this trade by principal articles in its monthly bulletins, but the December summary also includes figures for the whole year, copies of which may be obtained from the Superintendent of Documents for 15 cents each. Except in a very few cases the monthly summary does not give import or export figures of a given article by countries of origin or destination. Such detailed figures are published only in the annual volume, "Commerce and Navigation," which will not appear until the latter part of the year. But these detailed figures are all tabulated in the Department of Commerce, and available t o any one who wants to copy them. Members who live away from Washington can arrange through THISJOURNAL t o have desired figures copied at comparatively small expense.
