T H E RATE OF ABSORPTION O F WATER BY BAKELITE BY H. GENEVA LEOPOLD AND JOHN JOHNSTON
I n a paper dealing with the rate of absorption of water by rubber Andrews and Johnston' showed that the observations on a series of sheets of different thickness of the same rubber compound follow practically a single curve2 when the fractional saturation of the sheet as a whole-as determined by the increase in weight on immersion-is plotted against t/a2, t being the period of immersion in water, and a the half-thickness of the sheet. It was recognized that rubber is not the ideal substance for such a test of the general theory which leads to this conclusion, because of the oxidative and other changes which it might well undergo during the long period of immersion necessary; so we looked round for other materials, readily obtainable in uniform sheets of different thickness, which would be more stable under the conditions of experiment, and concluded to try cellulose acetate, celluloid and clear Bakelite (yellow and brown).$ We found however that the sheets of cellulose acetate reach substantial saturation in a few minutes, so rapidly indeed that a very thin sheet would promise to be useful as the absorptive member in a hygrometer; further that the celluloid sheets yielded in time to the water in which they were immersed so much of the original solvent or of camphor that conclusions based upon change of weight of the immersed sheets would be quite meaningless. The sheets of yellow Bakelite (presumably therefore those which in the course of manufacture were cured a t a lower temperature or for a shorter time) upon immersion increased in weight for about two days, but thereafter lost steadily for upwards of two years, the final ~ than the original dry weight; corresponding to weight being about 1 7less this, phenol diffused out into the water and was easily recognisable. There remained therefore as satisfactory material for our purpose only the brown Bakelite, sheets of which showed a regular increase of weight throughout the period of immersion; but even with this material we found, when at the conclusion of the experiments the sheets were dried for three days at I I O O (by which time the weight had become practically constant), that the final dry weight was less than the original weight by an amount averaging about 1 7 ~ . This difference may be due, in part or wholly, to the fact that the samples were not so thoroughly dried before the initial weighing; to this loss of material of the sheets may be attributed the fact that the points for the thicker sheets corresponding to the very longest periods of immersion lie slightly below the 1 Andrews and Johnston: J. Am. Chem. SOC.,46,640 (1924);see also Boggs and Blake: Ind. Eng. Chem., 18,224 (1926);Lowry and Kohman: J. Phys. Chem., 31,23 (1927). 2 Immediately after immersion, and until the water, entering from either surface of the sheet, reaches its central plane, the rate of absorption is, of course, dependent on the area but not upon the thickness of the sheet. 8 For sheets of these materials we are indebted to the Eastman Kodak Company, the DuPont Company, and the Bakelite Company respectively.
RATE OF ABSORPTION OF WATER BY BAKELITE
877
curve. I n any case, this difference was not taken into account in defining the increase of weight as a measure of the amount of water absorbed; and consequently the amount recorded as absorbed is rather below than above the true value. The sheets used were 3 X I inch in three thicknesses, about 0.04,0.07 and 0.14 inch respectively; they were dried ordinarily, weighed and immersed separately in distilled water in a stoppered bottle. At intervals the samples were removed, wiped with a piece of old linen in a standard way, transferred to a stoppered weighing bottle and weighed; they were then immediately reimmersed in distilled water. The increase of weight of three or four sheets of each thickness was observed in this way, and the results for each thickness agreed very closely; it suffices therefore to tabulate the data for one sheet of each thickness, as is done in Table I, Q being the percentage increase over the original dry weight of the sample. It may be mentioned that the thickness of the samples after soaking checked with the original within I or 2 mils (I mil = 0.001inch).
TABLE I The percentage increase in weight (Q) of sheets of brown Bakelite 2u mils thick immersed in distilled water for t days B h C 2u = 7 1 mils 2a = 42 mils 2u = 139mils orig. wt. = 4.623 j g. orig wt. = 8.7850 g. orig. wt. = 2.4904 g. t
Q
Q'
t
Q
days
obs.
curve
days
0.7 1.7 5.7 7.7 I3
0.25
0.18 .33
2
0.27
7
.52
.72
IO
.57
.83 1.03
I4 18 32 35 42 56
.75
I5
I9 26 33 40 54 69 98 200
223 354 382 464 552 876
.39 .61
.75 .92 1.16 1.29 1.48 1.66 1.87 2.23 2.54 2.99
4.18 4.34 5.16 5.18 5.40 5.56 6.02
1.10
1.28 1.48 1.68 1.85 2.18
85 187
2.50
210
3.00 4.12 4.30 5.08 5.17 5.39 5.60 6.00
246 341 451 539 568 845
obs.
.83 1.02
1.06 1.09 1.29 1.52
2.36 2.46 2.66 3.24 3.68 3.96 4.06 4.94
Q'
curve 0.18
.48 . j8 .67 .74 .99
days obs.
t
Q
curve
I
0.09 .14 .zz
0.06
.24
.19 .26
2
6 8 IO
.27
1.02
13 16
1.09
20
1.27
24
1.56 2.42
27
.31 .33 ,37 .42 .45 .48 .58 .66 .79
2.57
2.79 3.27
3.73 4.02
4.10 4.78
34 48 62
Q'
.IO
.17
.29
.31 .37 .42
'45 .54 .62 .72
.86
91 193 216
1.14
1.20
1.20
252
1.31
458 545 574 851
1.27 1.37 1.87
1.80 1.98 2.07 2.04 2.13 2.86 2.66
H. GENEVA LEOPOLD AND JOHN JOHNSTON
878
After
months immersion each of the thinnest samples had increased in weight; it may be that they were still gaining weight very slowly, but we have assumed that 6.0% represents substantially the saturation value Q. of this type of Bakelite. When the observed value Q, or the relative saturation Q/Qs, is plotted against t / a 2 (t is number of days immersion, a is the half-thickness of the sheet), the points for all the sheets lie, with very few exceptions, very close,to a single curve, so close that, on a diagram of the size which can be printed here, they would be indistinguishable from the curve. Consequently we show the degree of agreement of the results in Table I with this curve by presenting in the column Q’ values interpolated a t the designated values of t , or rather of t / a 2 , from the curve drawn on a large scale. This agreement is all that could be expected under the circumstances. The advantage of this way of expressing the results is that one may from the curve predict for thick sheets the relative state of saturation, and thus avoid the necessity of continuing the experiments over long periods of years. This may be done from the following rounded values of t / a 2 (in days per square mil) corresponding to even arguments of the fractional saturation 6.00
*
28
0.02%
&I/&.:
Q’/Qs
0.1
0.2
t/a2
0.01
0.04 0 . 0 9
0.3
0.4
0.5
0.15
0.22
0.6 0.33
0.7 0.8 0.48 0 . 6 8
0.9 1.07
1.0
1.98
From this one may predict that the 140-mil sheets would not be essentially saturated until they had been immersed some twenty-six years! From these results i t is clear that the rate of penetration of water into this type of Bakelite is so very slow that under all ordinary atmospheric conditions the amount of water absorbed would be negligible; this is doubtless a factor in the quality of this material as an electrical insulator. From these experiments alone little can be said as to the mechanism of this water absorption; but there is no reason to believe that it differs essentially from the absorption of water by rubber. I n this case, as the result of very careful and extensive investigation, Loary and Kohman conclude that the process is essentially one of solution of water in the rubber, the end result being influenced by the presence of water-soluble impurities in the rubber, and by its rigidity whkh determines the ease with which the rubber sheet may swell and so provide space for the entering water. Yale University, New Haoen, Conn.
