ROLE OF SURFACE ACTIVE AGENTS IN PREVENTING CRACK-FORMING IN T H E DRYING OF SILICA-ALUMINA HYDROGEL BEADS H l R O S H l U K I H A S H I AND RYUZABURO F U R U I C H I Research Laboratory, Asahi Glass Co., Ltd., Bentencho, Tsurumi-ku, Yokohama, Japan
Silica or silica-alumina hydrogel beads usually crack or break into fragments during drying, unless temperature and humidity are controlled. Cracking or breakage i s markedly decreased by immersing the hydrogel in an aqueous solution of a surface active agent prior to drying. If the yield of perfect gel beads after drying i s about 2070 without this pretreatment, it increases to 70 to 80% after pretreatment for 20 to 30 minutes with a 0.3% solution of a surface active agent. To understand the mechanism of this protective effect, the permeation into the gels, the amount of surface active agents adsorbed on them, and the drying rate of the hydrogel beads were measured.
manufacture of silica or silica-alumina gel beads, beads are usually dried in a n air stream with controlled humidity. Simple drying with hot air accelerates the rate of drying, but increases breakage or crack forming. The factors that affect the yield of hydrogels during drying are the size of primary micelles (ultimate silica units), Si02 content, pH, the kind of ions in the silica sol from which the hydrogel is produced; and the conditions of the preceding processes (aging, ion exchange? and washing). The mechanism of this phenomenon is considered to be as follows: While hydrogel is dried, its volume decreases gradually and a t the final stage is about 1/15 of that a t the initial stage. Since the water in the hydrogel does not vaporize uniformly from the entire bead, the localized stress grows, because the contraction force among the micelles differs a t different parts in the hydrogel. Thus, cracking may arise a t the point where the bonding force between the micelles is smaller than the stress. The effect of treating hydrogels with surface active agent solutions prior to drying has been studied. The yields of uncracked gels after drying and the rate of drying were measured for treated and untreated hydrogels. T h e permeation and adsorption of the surfactants into and on the hydrogels were also examined under various conditions. Along with the anionic, cationic, and nonionic surface active agents, higher alcohols and a dye solution were used as reference substances. N THE
I hydrogel
porcelain dish 15 cm. in diameter. After a definite time, the solution was decanted. and the hydrogel beads were dried a t 80" C. in an electric oven. The yields were determined by selecting and counting beads that had no cracks visible to the eye. The yield value was defined as the weight percentage of uncracked hydrogel beads in the total dried beads. The rate of drying was determined from the decrease in weight of the hydrogel during drying. Permeability of the surface active agent into the gel was measured by the use of a disk-like hydrogel 5 mm. thick, which was gelated in a rubber mold, and fixed between two glass tubes. one filled with water and another with a 0.5% solution of the surface active agent. A sample of the water in the tube was removed by a pipet and qualitatively analyzed every 30 minutes to check permeation of the surfactants. The amount of surface active agent adsorbed was determined by means of disk-like hydrogel specimens 5.3 cm. in diameter and 0.9 cm. thick, which after the usual processing were immersed in a beaker containing 300 cc. of surfactant solution per 100 grams of gel for a fixed time a t room temperature. The amount of adsorption was determined by the difference in solution concentration. The Epton method was used for quantitative analysis of the anionic and cationic surfactants. and the Schonfeldt method for the nonionic surfac tan ts.
Santomerse S
Surface Active Agents Used Chemical Structure Maker Monsanto Chemical Sodium salt of dodecyl
Aerosol OT
American Cyanamid
Table 1. Commercial .Vame
Experimental
Perex NB
Materials. The silica-alumina hydrogel beads were made by mixing sodium silicate solution and sulfuric acid containing aluminum sulfate, followed by gelation and shaping of the sol in the machine oil tower, aging, ion exchange, and washing. During the process the temperature was kept constant a t 25 C. The hydrogel beads thus prepared were 8 to 10 mm. in diameter. After washing. the beads were immersed in the surface active agent solutions listed in Table I. and then dried. T o prepare solutions of definite concentration, the original agents were dissolved in or diluted by pure water on the basis of the purity or concentration specified by the manufacturer. Experimental Procedures and Measurements. After washing and draining, 100 grams of the hydrogel beads were dipped into 200 ml. of the surface active agent solution in a
Arquad 12
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I&EC PRODUCT RESEARCH A N D DEVELOPMENT
Penetronyx D-30 Soygen EX-170
Noygen EA-83
benzene sulfonate Sodium dioctyl sufosuccinate Sodium butyl naphKao Soap (Japan) thalene sulfonate Dodecyl trimethyl amArmour & Co monium chloride Onxy Oil & Chemi- Alkyl dimethyl hydroxyethyl ammocal nium chloride Daiichi Pure Chemi- Polyethylene glycol nonyl phenyl ether cals (Japan) (degree of polymerization of ethylene oxide, 25) Daiichi Pure Chemi- Polyethylene glycol nonyl phenyl ether cals (D.P., 8.5)
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I
T
80 -
20--0
20
'
ao
40
120
1
TIEATMENT TIME, MlH.
Figure 2. Effect of treatment time on yield of uncracked gel 0 Arquad 12, 0.1% A Santomerse S, 0.3% 0
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0
0.a CONCENTRATION OF SURFACTANTS, % 0.2
0.4
0.6
1.0
0
Arquad 12, 0.3% Santomerse S, 0.1 %
A
Naygen EA 170,0.3% Noygen EA 170, 0.7%
Figure 1 . Effect of surfactant concentration on yield of uncracked gel 0
A J
0
Arquad 12 Santomerse S Perex NB Noygen EA 1 7 0 Penetronyx D 3 0
0
Noygen EA 83 Methylene blue Octyl alcohol Dodecyl alcohol
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Results and Discussion
Figure 1 shows the effect on yields of uncracked gel beads of varying the concentrations of surfactants in which the hydrogels Lvere pretreated for 30 minutes prior to drying. The yield a t Oyc corresponds to that of untreated hydrogel. T h e optim u m surfactant concentration in most cases appears to be about 0.3Y0. Represented in the figure are also the yields obtained with the gels pretreated with octyl and dodecyl alcohols and a n aqueous solution of methylene blue. T h e alcohols, having a lower surface tension than the surfactant solutions, are considered to replace water in the hydrogel. T h e methylene blue solution has almost the same surface tension as water, and hardly permeates the hydrogel. T h e changes of yield with treatment time are plotted in Figure 2 a t definite surfactant concentrations. The yield increases until the treatment time reaches 20 to 30 minutes. Longer treating times d o not appear to be harmful a t concentrations below about 0.3%, although treatment with more concentrated solutions decreases yields with longer times. T h e best protective effect is gained with Arquad 12 or Santomerse S solution of 0.3% concentration with a treatment time of 20 to 30 minutes. Effectiveness of various surfactants falls in the series: Arquad 12 > Perex NB > Santomerse S > Koygen EA 170 > Penetronyx D 30 > Noygen EA 83. With methylene blue solution the maximum yield (70%) was obtained at 0.17, concentration; use of octyl or dodecyl alcohol increased yields slightly, reaching values of 50 and 30%, I cspectively. The hydrogels began to crack when their weight was reduced to 2 5 to 357, of the original. \$/eight of completely dried beads \vas only about 10% of the original.
'
-~
Figure 3. 1. 2. 3.
a
4
0
12 TIME, HOUIS
16
20
Drying rate of hydrogel beads
None 0.1 % methylene blue 0.3% Noygen EA 1 7 0
4. 0.3% octyl alcohol 5. 6.
0.3% Arquad 12 0.3% Santomerse S
The rate of drying was measured under the same conditions for hydrogel beads untreated and treated with various additives (Figure 3). The drying rate of the hydrogel treated with a surfactant solution is slightly faster than that of the untreated gel. and also depends on the kind of surfactant. The penetration times through the 5-mm. hydrogel disk were 30 minutes for Santomerse S and Noygen EA 170 and 4 hours for Arquad 12. Since the bead hydrogel has a diameter of 8 to 10 mm., the anionic and nonionic surfactants are estimated from the above data to penetrate to the center of the beads \tithin 30 minutes, while the cationic surfactant penetrates only about 1 mm. from the surface of gel in the same time. LIethylene blue did not permeate through the disk even after 24 hours. Figures 4 and 5 give the results of measurement of the amount of surfactant adsorbed. The VOL.
2
NO. 3
SEPTEMBER 1 9 6 3
233
V
IO
20
30
ao
50
120
TREATMENT TIME, MIN.
Figure 5. Effect of treatment time on amount of nonionic surfactant adsorbed on hydrogel Noygen EA 170
TREATMENT TIME, MIN.
Figure 4. Effect of treatment time on amount of cationic and anionic surfactant adsorbed on hydrogel Arquad 12
0
1.0%
0
0.5%
A
0.1%
Sontomerse
S A 0
1.0% 0.5% 0.7%
amount adsorbed onto the hydrogel depends markedly on concentration of the solution, and while it increases with treatment time, the curves bend a t a definite time. I n Figure 2, this bending point seems to correspond approximately to the time for the optimum yield of uncracked beads. Thus the crack forming or breakage occurring during drying of the hydrogel beads was decreased by pretreating them with a solution of surface active agent, although the rate of drying was accelerated. Treatment with a long-chain alcohol of low surface tension also increased the yield, but less than with methylene blue solution, whose surface tension is not much different from that of water. T h e results suggest that the protective action of the surfactants may consist of two factors.
234
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
C
l.O?&
0
0.5% 0.1%
Lowering of the surface tension of the water in the hydrogel may lower the contractive force between micelles, easing the stress arising a t the intermicellar bond during shrinkage of hydrogels in the drying process. O r formation of a n adsorption layer of a surface active agent on the outer surface of the hydrogel bead may prevent a n initial excessively high rate of evaporation of water from the surface, thus equalizing the rate of drying of both the inner and outer parts of the hydrogel beads (7, 2). The latter factor seems to be the more likely. Assuming that the adsorption is entirely on the outer surface of the gel bead, the thickness of the adsorbed layer is calculated as 4.5 to 6.4 X lC14A. for the three surface active agents, where the gel bead was immersed in a 370 surfactant solution for 30 minutes, to attain maximum yield. The number of molecular layers is estimated to be the order of IO3. Considering the relationship between the yield of uncracked gel bead and the amount of surfactant molecule adsorbed, excessive adsorption appears detrimental, possibly because of the recurrence of irregular shrinking. literature Cited (1) Archer, R. J., La Mer, V. K.: J . Phys. Chem. 5 9 , 200 (1955). ( 2 ) Rosano, H. L., La Mer, V. K., Ibid., 60, 348 (1956).
RECEIVED for review July 9, 1962 RESUBMITTED April 23, 1963 ACCEPTED May 20, 1 9 6 3
