2 Silica Nucleation, Polymerization, and Growth Preparation
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of Monodispersed Sols Akitoshi Yoshida Central Research Institute, Nissan Chemical Industries, Ltd., 722-1, Tsuboicho, Funabashi-shi, Chiba 274, Japan
The industrial development of silica sol manufacturing methods is reviewed. Primary attention is focused on the preparation of monodispersedsolsfrom water glass by the ion-exchange method. Details are given for variations of manufacturing process and for the characteristics of both the processes and sols obtained. Furthermore, the following surface modifications of particles are demonstrated: silica sols stabilized with ammonia, amine, and quaternary ammo nium hydroxide; aluminum-modified or cation-coated silica sol; and lithium silicate. Finally, future trends in silica sol manufacturing are discussed from the viewpoint of not only raw materials and improve ment of the procedures but also the function of the silica sols and their particle shape.
M
ONODISPERSED SOL IS COMMONLY REFERRED TO as Silica sol ΟΓ colloidal
silica. The term "colloidal silica" here refers to a stable dispersion of discrete, amorphous silica particles. The industrial development of silica sols first began with the initial research by Graham (1) in 1861 involving the addition of hydrochloric acid to an aqueous solution of sodium silicate followed by dialysis to obtain dilute silica sol. In 1933, silica sol containing 10% S 1 O 2 was first marketed. This step was followed in 1941 by the announcement of an ion-exchange process (2), including procedures for stabilization and for concentration of the sol by heating, that used an ion-exchange resin to remove sodium ions 0065-2393/94/0234-0051$08.00/0 © 1994 American Chemical Society
In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
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52
T H E C O L L O I D CHEMISTRY O F
SILICA
from the dilute aqueous solution of sodium silicate. This sodium removal process is the most common today. In 1951, a process (3) for creating colloidal silica particles of uniform and controlled size was announced for the first time. In 1956, a method (4) was established for making stable sols consisting of microscopic particles having a diameter of only 8 nm yet containing more than 3 0 % of silica. The history of the development of silica sols and a description of the fundamental and application research up to and throughout the 1950s was summarized in detail by Her (5). Various raw materials can be used in the manufacturing of monodispersed sol. Examples of these materials include silicon metals (6), silicon tetrachloride (7), ethyl silicate (8), water glass (2), and silica powder (9). In this chapter, I focus attention on the preparation of monodispersed sols from water glass, a raw material that is presently used in large amounts industrially for the inexpensive production of silica sols. The manufacturing processes of silica sols can be broadly divided into the following three steps: 1. formation of active silicic acid by removal of alkali ions from a dilute aqueous solution of water glass 2. formation of a dilute silica sol by nucleation from the active silicic acid and growth of discrete silica particles by polymerization 3. concentration of the dilute silica sol In addition, I will discuss modification of the surfaces of silica particles.
Manufacturing Methods of Silica Sols and Their Characteristics History of Silica Sol Manufacturing Methods. Although numerous techniques have been proposed for manufacturing of silica sols, including dialysis (10), electrodialysis (II), peptization (12), acid neutralization (13), and ion exchange (3,14), the last three methods have come to be used most commonly. At present, the ion-exchange method is considered the most prominent technique. Although the dialysis method, which involves the reaction of dilute water glass with an acid followed by dialytic removal of the sodium salt as the formed electrolyte, was proposed in 1861, it was never applied on an industrial basis. Furthermore, the electrodialysis method was attempted in various ways as a method for electrically removing sodium salt from water glass. But, unlike the last three methods mentioned, it did not reach the level of practical application.
In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
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2.
YOSHIDA
Silica Nucleation, Polymerization, and Growth Preparation
53
Peptization Method. As shown in Figure 1, an acid such as sulfuric or hydrochloric is added to a dilute aqueous solution of water glass while stirring, or while heating as necessary, to neutralize and obtain a silica gel containing salt. Next, the crude silica gel is washed with water to remove the salt and obtain a silica wet gel. Then, water and an aqueous sodium hydroxide solution are added to the gel so that the p H of the resulting solution is 8.5-10 and a silica gel slurry is formed. The slurry is then heated for several hours in an autoclave at 120-150 °C to allow the gel to peptize and form a sol. Thus, a silica sol is prepared. For example, the resulting silica sol contains 30 wt% S 1 O 2 , has a p H of 10, has a molar ratio of Si02:Na20 of about 100, and consists of irregular particles having a diameter of 10-20 nm. However, it is difficult to obtain silica sols having a desired particle size or high purity with this method. Neutralization
WATER GLASS aq. soin.
PRECIPITATED SILICA GEL
H 2 SO 4 aq. soin. etc. + Na2S0 and other salts
Washing
4
Water and NaOH aq. soin.
PEPTIZATION
—>
WET GEL
with Water ^
s
m
G
SLURRY
E
Heating
L
j n
a u t o c
j
a v e
SILICA SOL
Figure 1. Flow chart of peptization method for manufacturing silica soh. Acid-Neutralization Method. As is indicated i n Figure 2, an acid, such as sulfuric or hydrochloric, is added to a dilute aqueous solution of water glass while heating and stirring. This addition results i n silica sol nucleation and particle growth to obtain a dilute silica sol containing salt. The salt is removed by either dialysis or electrodialysis, and the solution is concentrated to obtain a silica sol. In recent years, as the removal of salt has been made easier through the use of ultrafiltration membranes, this method has come to be considered as having potential for the future. Ion-Exchange Method. Sophisticated ion-exchange resins have been developed to efficiently remove sodium ions at the industrial application level. This method is currently the primary means of manufacturing silica sols. The following section provides a detailed description of this method.
In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
54
T H E C O L L O I D CHEMISTRY O F SRJCA
WATER GLASS aq. soin.
ι
Heating, Partial Neutralization z> Particle Growth I
SILICA SOL + Na2S04 and other salts
Dilute H 2SO4, HCI aq. soin. Removal of Salts
Dilute SILICA SOL
Dialysis, Electrodialysis
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Concentration SILICA SOL Evaporation, Ultrafiltration
Figure 2. Flow chart of acid-neutralization method for manufacturing silica soh.
Silica Sol Manufacturing Using the Ion-Exchange Method. The silica sol manufacturing method using ion-exchange is as shown in the basic flow chart of Figure 3. Ion Exchange. A n aqueous water-glass solution as the raw material in this process can have a S1O2 content of 30%, a Na2
