T H E MECHASISM O F T H E ABSORPTIOX OF WATER BY RUBBER BY H. H. LOWRY AND G . T. KOHMAN
Rubber compounds a,s used for purposes of insulation are similar to other substances used for these purposes in that their electrical properties depend to a very great extent upon small amounts of water which are absorbed from the surroundings. In order to make an intelligent attempt to develop an insulating compound from rubber having the best possible insulating properties, it is necessary to have a knowledge of the mechanism by \%-hichwater is taken up by rubber. The data in the literature concerning the absorption of mater by rubber are incomplete, and the experimental work in general was apparently not planned with the object of throwing light upon the mechanism of water absorption. In most capes the exceriments were not carried far enough to enable equilibrium conditions to be predicted and in some cases rates of absorption have been assumed to be proportional to amounts of water absorbed at equilibrium, which, as will be shown, is not always true. Some investigators have studied the absorption of water resulting from contact with pure water only, in which case only information concerning rates can be obtained for a reason which will be made clear later. In order to obtain information concerning the mechanism of water absorption the relat,ions between this process and the following factors were studied: I. Vapor pressure of water external to the rubber. Water-soluble constituents of the rubber. 2. 3 . State of the water external to the rubber (liquid and vapor). 4. Combined sulphur content of the rubber (rigidity). j. Temperature. 6. Hydrostatic pressure upon the rubber compound. 7. Aging of rubber compounds before and during the process of water absorption. A number of investigators have obeerved that rubber compounds absorb appreciable quantities of water and some have investigated the influence of some of the factors dealt with in this paper. I t was observed by Hancock’ in 1876 that a rubber bag filled with 1 2 ounces of water became empty after a period of thirty years. DeVries2and Whitby8 obeerved that the water content of raw rubber varied with the humidity of the atmosphere in which it was stored. Van Rossem4 and his co-workers, Miss van de Leur and Deliker, have shown that the amount of water absorbed by rubber compounds depends upon the humidity, temrerature, and also the amount of water soluble impurities in the rubber. Obach5 has studied the absorption of water by
‘
IVeber: “The Chemistry of India Rubber,” page 13 (1902). “Estate Rubber”, page 602 (1920). J. Soc. Chem. Ind., 37, 278 (1918). Kolloidchem. Beihefte, 10, 43 (1918). “Cantor Lectures on Gutta Percha”, Appendix VIII, p. 100 (1898).
24
H . H . LOWRY AND G. T. KOHMAN
rubber and gutta percha from both fresh wat,er and sea water. He presents data which show that not only do these compounds absorb much less water from sea water than from fresh water, but that the behavior is different in other respects. In fresh water these compounds continue to gain weight, even after very long periods of immersion but in sea water they either reach a constant weight or begin losing weight after comparatively short periods of immerson. TVhit’by’ has shown that the moisture retaining capacity of raw rubber is closely related to the presence of serum solids which are hygroscopic. Andrew and Johnston2 have developed a formula based upon Fick’s law of diffusion which is applicable under certain conditions to the rate of absorption of water by rubber compounds. KirchhoP has shown that the amount of water absorbed by rubber depends upon the acidity of the rubber and also upon the period of vulcanization. Boggs and Blake4 have recently published a paper on the effect of various factors upon the rate of absorption of water by rubber. In a more recent paper Williams and Kemp5 discuss the effect of absorbed water on the electrical properties of submarine insulation. In order definitely to determine the mechanism of water absorption by rubber, a comprehensive study of the process was made, the results of which follow. In making the study it was deemed advisable to eliminate as many complicating factors as possible. The compounds used in the study were therefore in most cases simple rubber sulphur compounds.6 To prevent “blooming” of sulphur and consequently changes in weight and composition of the sheet, the period of vulcanization was so regulated that the free sulphur content in most cases was less than one per cent. Certain of the compounds were therefore somewhat over cured. Experimental Procedure The method of attacking the problem was to make a study of the various factors which influence the amount and rate of absorption of water. The absorption of water was followed by three methods. In most cases test specimens fifty mils in thickness and two inches on I. a side were prepared. These test specimens were dried in vacuum, weighed and immersed in solutions of various aqueous vapor pressures. Sodium chloride solutions were used for the most part, partly because Kah!enberg7 showed that sodium chloride would not diffuse into rubber, and partly because in certain cases it u-as desired to obtain information concerning the absorption from sea water. In order to reduce evaporation of the solutions, the India Rubber World, 59, 141 (1918). J. Am. Chem. Soc., 46, 640 (1924). 3 Kolloid-Z., 35;367 (1924). 4 J. Ind. Eng. Chem., IS, 224 (1926). Williams and Kemp: J. Franklin Inst., 203 (1927). 6 hlany experiments not reported in this paper have shown t h a t in general the amount of water taken up by filled rubbers, if made free from pores, can be calculated from their rubber content unless the filler itself is either water soluble or hygroscopic. 7 J. Phys. Chem., 10, 141 (1906). 1 2
THE MECHASIShl OF THE ABSORPTION OF WATER BY RUBBER
25
vessels containing the test specimens immersed in solutions of equal vapor pressures were placed in a humidifier, and the solutions were changed periodically to correct for changes in concentration due to the absorption of water by the sheets, The absorption of water was followed by removing the sheets from the solution, washing free from sodium chloride, blotting carefully between sheets of hard filter paper in a standard way, transferring t o a weighing bottle and weighing. They were then again immersed in the solution. The blotting and weighing was done as quickly as possible to prevent changes in the moisture content of the sheet. The humidifiers containing the samples at
FJQ.I 2 5 O C . were ordinarily kept in a room the temperature of which was approximately z 5 O C . In certain cases they were immersed in a thermostat kept at a temperature of 25' 0.1. At temperatures other than z 5 O C . they were kept in a constant temperature room the temperature of which was controlled by a thermostat to 1°C. The absorption was followed by observing the changes in pressure of 2. water vapor in an enclosed system of known volume containing the sample. The apparatus used, shown schematically in Figure I , is so designed that stopcocks need not be relied upon t o prevent leakage. The various systems can be closed or opened by regulating the height of the mercury columns above or below the level D E F by controlling the air pressure above the mercury in the reservoirs R,, RZand R1by means of the two way stopcocks SI,SZ and S3 which are connected to both a vacuum line and to atmospheric pressure. The sample in the form of finely cut pieces or very thin sheets is placed in the bulb B and sealed to the apparatus. With the mercury levels at D, H and F, the sample is pumped out by means of mechanical pumps backed by a mercury diffusion pump connected to C. The mercury level of R1is then raised to G and that of R3to I . The mercury level of RPis then slowly lowered
*
H.
26
n. LOWRY
AND G. T. KOHMAN
until water vapor furnished by the evaporation of the water in bulb IT bubbles past the mercury into the system A. Any desired pressure of vapor can be admitted by following the difference of the mercury levels at I which serve as a manometer. These levels are read to =t0.03 mm by a micrometer gage’ not shown. From the volume of the system A and the pressure the amount of water vapor admitted is calculated. The mercury level of R,is then lowered to D which allows the water vapor to come in contact with the sample. When the pressure of water vapor in the system has become constant the
7
5
0
2
4
6
8
IO TIME
-
10 WEEK5
4
pressure is carefully read as before and the amount of mater vapor remaining calculated from this pressure and t,he volume of system A plus that, of bulb B and connecting tubing. By repeating these operations as many measurements as are desired can be obtained at pressures from zero to the vapor pressure of water a t the temperature of the apparatus. The temperature of the samples was controlled in this case by immersing the bulbs containing the samples in a thermostat, the temperature of which was kept at, the specified temperature * 0 . 2 O C . 3. A method similar to that ueed by Miss van de L e d was used in ortier to obtain equilibrium values rapidly. The samples in the form of thin sheets or finely cut pieces in weighing bottles were exposed to water vapor above solutions of known vapor pressure until constant weight had been reached. It was found necessary, in order to prevent oxidation of the samples, to use containers which could be pumped free from air after each weighing of samples. Ferguson: J. Wash. Acad. Sei., 10, 285 (1920). :Kolloidchem. Beihefte, 10, 45 (1918).
T H E 3IECH.4TISJI O F T H E ABSORPTIOS O F WATER BY RUBBER
27
The joints of the containers were then sealed with a beeswax-rosin mixture containing zoc; rosin. h t high relative vapor pressures, (P;'P, where P is the vapor pressure of water over the solution and Pothat of pure ivater), the method was found to be unsatisfactory because of the condensation of moisture on the weighing bottles. The temperature control in this case was the same as that described under method I.
T I M E (WFEKS)
Experimental Results In Table I A-E arc given the average percent Iveight gains of sheets of n r i o u s caombined sulphur contents after the specified time intervals of immersion in salt solutions of various vapor pressures. The vapor pressures given in the tables are those of the solutions at zs0C. X more complete description of the compounds used is given in Table I F. The data \\-ere obtained by method I. The humidifiers in which the sheets were kept lyere not kept free from air. I n Figs. 2 and 3 the results for sheets S o . j and I O of Table I A-E are plotted. The results for the other sheets fall on similar curves. If the water content of each sheet at the end of 2x0 days be plotted against vapor pressure of the solution in which the sheet was inmereed curves of the type shown in Figure 4 are obtained. There are no experimental determinations on the portions of the curves of Fig. 4 below 18 m.m. because vapor pressures of sodium chloride solutions at 2 5 O C have a lower limit of 1 7 . 9 m.m., thnt being the vapor pressure of a saturated sodium chloride solution. In order to investigate this portion of the curve and also to determine whether or not the vapor pressure of water external to the rubber is the factor which causes the amount of water absorbed
H. H. LOWRY A S D G . T. KOHMAS
28
o5
6
' Io '
I I 5
PER
CENT
'
2'5
3b
WATER
TABLE
'
I 3'5
1
Mean Percentage Gain in Veight of Rubber Sheets Immersed in Water and in Salt Solutions
A Distilled Water, Vapor Pressure--23.6 m.m. Time idad 1
7
Rewashed Smoked Sheet Sheet S o . 2 3 4
I , 14
0 .jI
0.IO 0.20
I4
1.39
21
I , j2
28
I .60
35 49 56 70
1,75 I .86
0.69 0.84 1.07 1.37
2.00
1.51
2.21
I .61
84
2.33
98
2.51
1.74 1.79
2 60
2.66 3.45 3.14 7.38
440
I2.4j
I12
I60 200
__
2.33 2.49 2.82 3.75
0.24
0.29 0.37
0.23 0.28 0.26 -
0.40
0.47 0.44 0.46 0.48 0.49
-
0.54 0.59 0.76
0.36 0.40
Commercial SmokedSheet Sheet S o . 5
2.86 3.91 4.44 4.88 j.40 5.90 6.47 6.78 7.18 7.49 7.80 8.81 9.62
6
7
1.53
0.49 0.69 2.05 2.40 0.86 2.59 2,89 0.95 3'23 I ,015 3.49 I .09 3.71
I.Ij
3.92
1.17
4.11
I . 19
4,32 j,44
6.jj 10.68 7.73 14.70 1j.30
1.26 1.27
I .40 2.08
THE MECHANISM O F THE ABSORPTION O F WATER BY RUBBER
29
B I . O ~Sodium ~
Time (days)
7 IO
I4 21
35 52
77 105
150 2 IO
290
Chloride Solution, Vapor Pressure--23.5
Rewashed Smoked Sheet Sheet No. 9
IO
I1
0.51 - - 0.68 0.19 1.00 0.70 1.28 1 . 1 2 0.77 1 . 2 8 1.18 0 . 8 6 1.39 1.37 1.11 1 . 5 0 I . ~ I1.32 __ - 1 . 7 6 1.59 2 . 2 8 2.11 1 . j 4 2 . 8 0 2.86 _ _ 3.58 0.91
0.82
m.m.
Commerical Smoked Sheet Sheet S o . Ij 16 17 I8 1.18 0.99 0.34 0.29
I2
I3
14
0.29 0.32 0.38
0.23 0.26
1.52
- - 1.19
0.28
1.95
0.41 0.48
0.28
0.29 0.36 0.33 0.74 0 . 3 j 0.82 0.3; 1.04 0 . 3 ; 1.45 0 . 3 j
0.60 0.63
0.41 0 . 3 1 1.43 0.47 0.35 2 . 2 4 1.78 1 . 6 5 0 . 5 1 0 . 3 5 2 . j ~2 . 0 3 1,94 0.61 0.39 2.96 2.46 2.34 0 . 7 2 0.43 3.2; 2 . 8 7 2 . 7 0 0 . 7 8 0.42 3 . 0 9 0 . 9 0 0.47 4.04 4.43 3.43 0.95 0.46 4.61 5.53 4 . 1 7 1.15 0.47 - - 5 . 0 5 1.48 0 . 4 7 ~
1.52
~
C
3. jyc Sodium Chloride Solution, Vapor Pressure--z3.1 Rewashed Smoked Sheet Sheet S o .
Time 9
7 10
I4 21
35
-52 i l
105
IjO 2 IO
290
IO
0.74
0.81
m.m.
11
12
13
0.j0
0.27
0.22
Commerical Smoked Sheet Sheet S o . 14 15 16 17 I8 1 . 2 9 1.01 0.92 0 . 3 1 0 . 2 7
0.31 0.38
0.25
-
0.26
1.56
0.40
0.26
1.77 1 . 4 0 1 . 4 4 0.48
0.45 0.54
0.31 0.31
1.86
1.51
2.02
1.71
1.6j 0.61 1.99 0.70
0.57
0.31
2.17
1.97
2.17
0.6; 0.76 0.9j 1.19
0.31 0.3j 0.35
-
- _ _ 0.68 0.87 0.86 O . ~ I 0.92 0 . 9 1 0 . 7 4 0.86 0.92 0.86 0.93 1 . 0 2 1.00 0.93 1.19 I . I j - - 1.30 0.93 1.46 1.64 0.79 1.58 2.08 - _ _ 2.84
0.41
~
1.24
1.12
0.38
1.27
0.44
0.70
0.81 2 . 3 0 2.31 2 . 7 1 0 . 9 0 2 . 2 6 2 . 5 6 3 . 1 3 1.09 - __ 3 . 6 1 1 . 3 4 ~
2.55
0.31 0.34 0.3j 0.38 0.41
0.40 0.41 0,45 0.45 0.j3
D
10.07~ Sodium Chloride Time (days) i
Rewashed Smoked Sheet Sheet S o . 9 0.j1
IO
11
0.j4 o 45
IO
~0.52
I4
0.59
21
35
-52 i l
‘05 ‘50 2 IO
290
Solution, T’apor Pressure-21.9
12 0.24
0.30 0.54 0 . 3 j 0 . 5 7 0.87 0.55 0.52 0.j8 0 . 6 6 0 . 3 8 0 . 5 4 0 . 6 4 0 . 8 0 0.49 0 . 6 4 0 . 7 5 0.8; 0 . 5 1 0.98 0 . j 7 0.64 0.94 1.12 0.65 0.64 1.09 1.57 0.74 - - 1.82 0.88 0.62 0.61
m.m.
0.22
Commerical Smoked Sheet Sheet S o . 14 15 16 17 18 0.95 0.80 0 . 7 8 0.30 0.23
0.27
- - 0 . 9 0 0.37
0.32
0.27
0.99
0.41
0.32
1.12
0.44
0.32
1 . 2 6 0.52 1.37 0.62 1.43 0.62 1.51 0 . 6 8 1.64 0.72
0.37
0.35
1.03 0.91 1.08 0 . 9 6 1 . 0 j 0.96 1 . 0 9 1.04 1.09 1.15 __ 1.22 1.42 1.25 1.67
0.38
- -
2.01
13
0.27
0.30 0.36 0.30 0.33 0.35
1,7;
0.82 0.94
0.41
0.38 0.40
0.41 0.43 0.46
H. H. LOWRY AND G. T. KOHMAN
30
E Saturated Sodium Chloride Solution,.~Vapor PressureTime (days)
7 IO
9
IO
0.21
0.25
0.22
0.25
0.22
0.34
0.27
0.22
0.39
0.27
0.42
0.33 0.31
0.48 0,43 0 . 4 8
0.34 0.34
o.zj 0 . 2 j
0 . 3 3 0.4j 0.48 0 . 3 4 -~ 0.60 0.37
0.25
77 105 150
0.32
__ 0.32
0.33 0,34 0.34 0.34 0.35 __
17
18
0.22
0.21
0.23
0.24
0.43
0.26
0.24
0.43 0 . 5 0 0 . 2 6 0.42 0.46 0 . 5 3 0.33 0 . 4 2 0 . 4 6 0.54 0.36 0.45 0.49 0.57 0.36 __ -_ 0 . 6 0 0 . 3 8 0 . 4 8 0 . 5 7 0 . 6 1 0.41 0.48 0.59 0.61 0.41 - __ 0 . 6 8 0 . 4 3
0.24
0.25
F Description of Sheets Sumbers Sheet h-o . I 2
3 4
5 6 7 8 9 IO
I1 I2
13 14 15
16 If 18
Rubber
Rewashed* S. 8. ),
lf
,,
1, 1, a,
,)
I,
,,
Commercial S. S.
,>
,,
ff
,%
,I
f,
11
,I
,,
Renashed
,, ,, ,,
>,
8. S. f,
j.
,)
)>
TJ
9,
,,
,9
Commercial S. S.
,,
>>
I,
f ,
,, ,,
,) .,
)9
J )
19
)I
Ij
0.45 0.4j
0.25
16
0.39 0.38 0.44 --- 0 . 5 0
14
0 . 2 2
0.28
_ _ _ _
0.30
290
13
0.19
52
2 10
I2
0.21
35
21
I1
0.31 0.33 0.33 0.33
0 . 29
0,30 0.30 0.30
I4
I 7.0 . _ m.m. Commerical Smoked Sheet Sheet S o .
Rewashed Smoked Sheet Sheet No.
I
0.50
7
hr. hr.
I 2
I j’
30’ I 2
hr. hr.
I Of
ISf
30’ I 2
hr. hr.
IO
S.**
Free
”c .I
.31
0.I 8
20
0.18
30
0.44
3 7
IO
5
0.jj 0.50
IO
20
0.12
14 3?4 5 7
30
0.46
IO
0.18
IO
20
0.12
14
30 3
0.48 0,39 0.14
3
1.13
5
0.56
Ij’
5
5
3 0‘
I
IO
2
0.31 0.31 0.31 0.31
14
315
hr. hr.
0.28
IC
IO’
I
0.31
to 18 inc.
Cure at 50 lbs Steam Pressure Total S. Rise Cure (hrs.) 2 IS’ 3 5
3 0’
0.27
0.11
IO
20
0.13
14
30
0.95
* The rewashed rubber used in these experiments was washed by a special process developed to remove the water soluble constituents from rubber with ‘a minimum amount of physical deterioration. Prior to washing the rubber 1s passed through, a creping mill for 18 minutes during which time it is subjected to a spray of water ranging in temperature between 43 and 5 2 T . The creped rubber is then washed on a Werner and Pfleiderer washing machine and subjected to a spray of water the temperature of which is lowered from 6 j to 45‘C during the process. After wahsing the rubber is again creped to uniform thicknesP and dried in vacuum. ** I n order to reduce the free sulphur content t o these values the compounds in some cases were badly over-cured. I t was necessary. however, t o keep it below approximately one per cent, (the solubility of sulphur in rubber a t room temperature) to prevent blooming which would make it impossible to follow the absorption of water by weighing the sample.
THE MECHANISM OF THE ABSORPTION OF WATER BY RUBBER
31
by sheets immersed in sodium chloride solutions to decrease as the salt concentration increases, method 2, which enables the absorption to be studied over the entire vapor pressure range, was developed. In Table I1 the amounts of water absorbed a t equilibrium by various samples of rubber as determined by this method are given. The results fall also on curves of the type of that in Fig. 4 as is shown by Fig. j . Wilson and Fuwa' have discussed briefly various tyFes of curves showing the relation between vapor pressure and
0
04
08
12
I C
20
2 4
28
PER CENT WATER
amount of water absorbed. The character of the curves such as are shown in Fig. 5 together with other results which will be discussed later have led to a theory of the mechanipm of the absorption of water by rubber which has been found to be in harmony with all of the observations concerning this process that have come to the attention of the writers. This theoiy will be discussed now in order that it may be of help in explaining the results of further experimentation. I t will be obserred from Fig. 5 that the solubility of water in rubber a t vapor pressures below approsimately 16 m.m. at 25' is directly proportional to the first power of the pressure. At these pressures then the solubility obeys Henry's Law and therefore it is probable that the water and rubber form a true solution. Other facts as well point to this conclusion. For example, as is shown by the (lata plotted in Figs. z and 3 , the soluble impurities in the rubber do not affect the amount of water absorbed a t low vapor pressures appreciably. The sheets from which the soluble impurities have been rei
J. Ind. En&.Chem., 14, 913 (1922>.
H. H. LOWRY AND G. T. KOHMAN
32
moved by washing absorb very nearly as much water as the sheets of commercial rubber at a vapor pressure of 17.9 m.m. while a t higher pressures the
TABLEI1
yG Water absorbed at Equilibrium by Rubber Sheets as determined by Method Temp. =
P
=
P(m.m.)
3
Pressure of water vapor in equilibrium with the samples. Compound Numbers VI1 VI11 IX X 0.07 0.05 0.06 0.06
5
0 .I O
0.11
IO
0.19 0.28 0.47 0.78 1.58
0.24
15
18 20 22
23 23.2
0.36 0.52
2.80
0.78 1.30 2.16 2.50
2.50
0.08 0.16 0.24 0.28 2.46 3.30
0.10
4.50
I
8.0
I . 16
XI 0.03 0.07 0.16 0.23 0.29 0.33 0.45 0.60 0.75
Xv 0.15
XS’I 0.03
0.23
0.04
0.47 0.68 0.88
0.09 0.16
1.21
0.21
53 2.16 __
0.30 0.41
XI1
XI11
XIV
3 5
0.Oj
0.04 0.06
0.04
IO
0.23 0.38 0.60
P(m.m.)
0 . IO
I5
18 20
22
23 23.2
2
2jOC.
0.88 1.68 2.95 4.75
0.11
0.06 0.13
0.17
0.19
0.27
0.28 0.41 0.63 0.94 I .06
0.45 0.76 I.2 0 I
.40
0.21
0.31 0.41 0.52
0.77 ,oi
1’
0.14
0 . j2
Description of Compounds of Table I1 Corn ound
rs,.
Description of Compound
VI1 Commercial smoked sheet, 4.25% combined sulphur, 0 . 7 5 7 ~ free sulphur. VI11 Commerciallatex crepe, 2 . 6 1 7 ~combined sulphur, 0.397; free sulphur. I X Rewashed smoked sheet, 4.3670 combined sulphur, 0 . 6 4 7 ~ free sulphur, 0.4% sodium chloride. X Gutta Percha Cable Compound. X I Hard rubber dust, commercial smoked sheet, 23y0 combined sulphur. XI1 Commercial raw smoked sheet. XI11 Washed balata. XIV Deresinated balata. xv Gutta Percha. XVI Filled Rubber Compound (Silica)
T H E J I E C H A S I S M O F T H E ABSORPTION O F WATER BY R U B B E R
33
soluble impurities have a very noticeable effect. That pores in the rubber are not a factor is shown by the charact'er of the vapor pressure curves. If pores were present in the sheets the water would enter the sheet at constant pressure until they were filled, and therefore the lower parts of the curves of Fig. 5 would deviate from unbroken straight lines. Further evidence t h a t pores do not play a part in water absorption is the fact that rubber sheets, as will be shown later, expand in volume an amount very nearly equal t o the volume of water absorbed. Above a certain vapor pressnre (approximately 16 m.m. at 2 j"C) the soluliility no longer obeys Henry's Law hut is much greater than the law noultl predict, and Figs. z and 3 show that this deviation is much greater in the case of commercial rubber than of rewashed ruther. It x a s . therefore, concludeti that when the vapor pressure of water esternal to the ruliber is raised slightly above that of a saturated solution of the water-soluble impurities in the r u h h r , water diffuses in t o form such a solution and then t o dilute it until the va1:or pressnrcs of the internal and external solutions are equal, the driving force tieing the vapor pressure difference. If these soluble impurities be washetl out. the aniount of water ahsorbed should lie reduced t o that actually tlissolvetl in the ruhhrr as a loivrr limit, The fact that thr soluldity docs not Iiegin to tlrviate from Henry's Law until the vn1)or prcswrc is r a i d ahove a h i t 16n i . i t i . indicates that the gross soluhility of the ini~iiiritiesin rubber is such that the vapor prrssnre of a satur:itctl
