58
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 23. No. 1
Some Experiments in the Soaking of Silk’ Harvey A. Neville and Theodore H. Marshall2 W M . H. C H A X D L E R C H E M I C A L
LABORATORY, LEHICHU X I V E R S I T Y ,
BETHLEHEM, P A
The initial process in the treatment of raw silk involves soaking in an oil emulsion to soften and lubricate the fibers. By means of dilatometers the relative hydration, or adsorption of water by the silk, was measured and the effects of variation in pH, temperature, mass-volume ratio, and other factors were evaluated. Hydration of silk is greatest in acid solution and decreases with increasing pH. Hydration increases with decrease in the temperature of the soaking bath, but the time required to reach equilibrium is much greater at the lower temperatures. The conditions which favor maxim u m hydration do not, in general, result in a corresponding maximum adsorption of oil by the silk. The gain in weight of the silk, as determined by the boil-off method, increases in a series under conditions which cause a decrease in the hydration. I n emulsions made with soap as the emulsifying agent the proportion of soap used has an important bearing
upon the degree of dispersion, stability, and action of the soaking emulsion. Photomicrographs of such emulsions are shown to explain these effects. The hydration of silk decreases regularly as the concentration of soap in the emulsion is increased, but the gain in weight increases with the soap concentration. The maximum dispersion of oil in the emulsion is obtained when the concentration of the soap is 0.25 per cent. Additional quantities of soap are objectionable because of too rapid creaming, decreased dispersion, and increased alkalinity. The use of triethanolamine, both as the oleate and in combination with sulfonated oil, in emulsions for silk soaking results in a higher degree of dispersion and improves the penetrating quality of the emulsion. As a result the desired gain in weight of the silk may be obtained by soaking in these emulsions for a shorter time and with a lower concentration of oil.
ATURAL silk is a thin continuous filament spun by the silkworm in the preparation of its cocoon. The worm has two glands, situated on each side of its body, connected by a duct with a capillary opening or spinneret in the head. These glands secrete two transparent liquids. The one called “fibroin” forms the interior portion of the filaments and constitutes approximately 75 per cent of the whole; the othersecretion, “sericin,” som‘et imes called “silk glue,” completely covers the two strands of fibroin and cements them together. I n order to reel the silk into skeins the cocoons are placed in warm water to soften the sericin and several filaments are combined to form a single thread of silk. These f i l a m e n t s are twisted together in the process of reeling and are firmly cemented into a uniform thread as the sericin again hardens. A single skein of this silk weighs about 70 grams and contains approximately 40,000 yards of silk thread. These threads of raw silk, although they may consist of six or eight original iilaments, are still too fine and deliFigure 1-Dilatometer cate for ordinary use. They undergo a series of operations called throwing; the object of which is to twist a number of such threads into a more substantial yarn and to prepare it for the manufacture of textiles. Raw silk is quite harsh and stiff owing to the nature and amount of sericin present. The initial treatment in the throwing process hence involves soaking the skeins in an oil-in-water emulsion to soften and lubricate the fibers. The
present investigation was undertaken to measure some of the factors involved in this soaking process.
N
1 Received September 19, 1930. Presented before the Division of Leather and Gelatin Chemistry at the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September 8 to 12, 1930. 1 R. K. Laros Silk Company Fellow, 1928-30.
Apparatus and Methods The method employed was similar to that previously described in connection with the study of the hydration and swelling of gelatin and animal skins (2, 4). It has been shown that when such materials are soaked in water or solutions the decrease in the volume of the system, resulting from the compression of the adsorbed water, may be taken as a relative measure of hydration. I n the experiments with silk two sizes of dilatometers were used, having capacities of 700 and 2200 cc., respectively. The type is illustrated in Figure 1; its use has been described in the previous papers (2, I+). s i;
60
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ZOO
$000
600
6-0
/DO0
,200
/%o
/600
7ims w? M,nufes
Figure 2-Hydration
of Silk in Pure Water
All silk used in this investigation was from the same lot of high-grade, uniform Japan silk, Single skeins were used in each experiment and, owing to the hygroscopic nature of silk, they were conditioned before all weighing in an insulated cabinet a t about 25” C. over a solution saturated with potassium nitrate and ammonium chloride. This solution maintains a relative humidity of 71.2 per cent a t 2.5’ C.
59
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1931
with only slight variations for a temperature change of * s o C. It was also found necessary to use weighing bottles for the silk because of its rapid change in moisture content on exposure to the atmosphere. The soaking liquid, the dilatometer, and the silk were always brought to uniform temperature in a thermostat before each experiment, and during the soaking period that temperature was kept constant. Except as otherwise noted, the temperature of soaking was that used in plant p r a c t i c e
chart in Figure 3 illustrate the effect of varying this factor. Small quantities of sodium hydroxide or hydrochloric acid were added to distilled water to prepare these soaking solutions. It would appear from these results that hydration is greatest in an acid medium and decreases with increasing pH. The differences between the initial and final pH values of the soaking solutions indicate that silk adsorbs or combines with both acid and alkali, but that it adsorbs acid preferentially, Hydration in Dye Solutions
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Some skeins of silk were hydrated in solutions of watersoluble tints such as are commonly used in soaking baths for temporary dyeing. The results obtained with two of these dyes are represented in Figure 4. The concentration of dye used in each case was 0.0566 gram per liter of water. The rise in the hydration curves at the beginning of the experiments is caused by the almost complete adsorption of the dye from the solution during this period. This temporary increase in the volume of the systems may be attributed to the heat of adsorption of the dye. An analogous effect is observed when the hydration of plaster of Paris or Portland cement is measured by the volume-change method (1). The amount of hydration obtained in the dye solutions is somewhat greater than in pure water, probably because of the slight acidity of the dye solutions. The curves also show that the red dye acts as an antiseptic, and this was confirmed by the absence of odor in this preparation at the end of the experiment.
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Figure 4-Hydration
of Silk in Dye Solutions
Hydration in Soap Solutions
The next experiments were performed to determine the effect of different concentrations of soap upon the hydration of raw silk. h neutral soap (sodium oleate) was used to prepare the soap solutions. The results (Table I) show Table I-Effect CONCN.
of Concentration of Soap Solution on Hydration of hfAX.
Raw Silk BOIL OFF
Soap HYDRATIOA Initial Grams per liter Cc. % 2.57 7,303 17.81 5.15 4.416 17.81 10.30 1 860 17.81 1 760 17.81 15.40 1.628 17.81 20.60 OF
Final
G A I X IX WEIGnT
% 19.44 20.31 21.08 22.57 23.68
1.63 2.56 3.27 4.76 5.87
5%
that as the concentration of soap in the soaking solution is increased the amount of hydration becomes less. However, the gain in weight of the silk due to the amount of soap adsorbed increases regularly with increasing concentration of the soap solution. The data in Table I represent the conditions at the end of 600 minutes, at which time equilibrium is practically established, and were obtained by means of the 700-cc. dilatometers. Other similar experiments were
Vol. 23, KO.1
INDUSTRIAL A N D ENGINEERISG CHEMISTRY
60
permitted to continue 36 hours and the absence of any odor in the silk or upward trend in the hydration curves demonstrates the antiseptic action of the soap.
silk but more soap and oil are taken up. The increase in alkalinity of the solution as the concentration of soap is increased is probably an important factor in decreasing the hydration, The adsorption of the soap, particularly in the higher concentrations, is indicated in the upward trend of the hydration curves in the first stage of the soaking period. It is also of interest to note that the gain in weight of the silk, due to the soap and oil taken up, is greater in the small dilatometers where the silk is closely packed than it is in the larger volume of emulsion. This is probably due to a more complete breaking of the emulsion in the smaller volume. On the other hand, the hydration is greater in the larger volume of emulsion under otherwise comparable conditions. of Soap Concentration on Stability of Emulsion CONCN.OF SOAP BREAKINGTIME
Table 111-Effect
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Figure 5-Hydration
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4 days, slight cream 3 hours thick cream 2.5 hodrs, thick cream 2 hours, thick cream
0.257 1.03 1.54 2.06
of Silk in Soap Emulsions
In order to prepare the most stable and uniform emulsions, it was found that the best procedure was to dissolve the soap in the water and, after this was thoroughly dispersed, to add the oil slowly with vigorous stirring to insure a fine particle size. Samples of each emulsion used in these experiments were set aside in test tubes and allowed to stand in order to determine their relative stabilities. As shown in Table I11 the emulsion containing approximately 0.25 per cent of soap was found to be the most stable. It is of interest to recall that this concentration of soap was found to be most efficient in detergent action ( 3 ) . I
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rme in
Figure 6-Hydration
I
roo $00 N/t/nufes
I 660
of Silk in Various Soaking Liquids
Soaking in Soap-Oil Emulsions
A mixture of mineral oil and neat's-foot oil, such as is used in plant practice, was emulsified in soap solutions and these emulsions were used as soaking media. The concentration of oil was uniformly 14.3 cc. per liter. The results are shown in Table 11. The hydration curves for the last five experiments are given in Figure 5. Table 11-Effect of Soaking in Oil-Soap Emulsions Time, 800 minutes; temperature, 37.8' C. CONCN. INITIAL FINAL MAX. GAININ OP S O A P PH PH HYDRATION WEIGHT Crams per liter cc. % 7WCC. DILATOMETERS
0.50 1.00 1.50 2.00 2.57 5.15 10.30 15.40 20.60
8.72 8.82 8.98 9.15 9.26 9.32 9.37 9.40 9.57
7.03 7.08 7.20 7.28 7.37 8.21 8.73 8.81 8.89
6.279 5.871 5.797 4.917 4.680 2.418 1.784 1.585 0.986
22WCC. DILATOMETERS
0.50 1.00
1.50 2.00 2.57 5.15 10.30 15.40 20.60
8.72 8.82 8.98 9.15 9.26 9.32 9.37 9.40 9.57
7.12 7.37 7.87 8.64 8.68 8.72 8.81 8.89 8.98
8.346 6.897
...
5:iis 3.214 2.866 2.029 1.814
.. 3:;s 4.41 4.66 5.67 6.62 7:i2
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I 200
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Figure 7-Influence
rn
I
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C#O
600
Mmufer
of Temperature upon t h e Hydration of Silk
Emulsions containing less than 0.25 per cent of soap creamed fairly rapidly and also showed a separation of oil. I n plant practice emulsions of this type commonly contain 1 per cent of soap. The influence of the concentration of soap upon the character of the emulsion as observed microscopically will be discussed later in this paper.
..
Soaking in Sulfonated Oil Emulsions
4:i5
All the emulsions made with sulfonated oil were fine and partially translucent. They did not appear to be so rich as soap-oil emulsions which contained the same quantity of oil, and this difference in appearance may be attributed to the higher degree of dispersion which obtains in the sulfonated oil emulsions as revealed by microscopic examination. The larger droplets in the soapoil emulsion scatter light more completely and hence this emulsion appears more opaque. The data in Table I V are for soakings made with three emulsions of the following compositions:
e.
4:s7 5.30 5.77 5.84
It is again apparent that as the concentration of soap in the emulsion becomes greater the amount of hydration decreases, but the percentage gain in weight as determined by the boil-off method increases steadily. That is, with increasing concentration of soap less water is adsorbed by the
INDUSTRIAL A N D ENGINEERING CHEMISTBY
January, 1931
Emulsion E-1
B-I-Partially sulfonated neat's-foot oil. B-2-Equal parts of sulfonated neat's-foot oil and raw neat'sfoot oil.
B-3-Mixture
of 50 per cent sulfonated neat's-foot oil, 25
per cent raw neat's-foot oil, 25 p a cent mineral oil.
The total quantity of oil used in each emulsion was 15 cc. per liter. Table IV-Effect of Soaklng In Suffunared Oil Emulsions Time, 600 minutes; temperature. 37.8O C. INlTlAL I3hl"LSLON
FIX*,.
PI
