The size of the separated phase determines the pore size of SPG. The pore size is closely related to the temperature (T) and time (t) of the heat treatment. Nakashima and Shimizu (11) revealed that the average pore radius (r) can be expressed by Eqn. (1) when T is changed under a constant t

Also r depends on t as is expressed by Eqn. (2) when t is varied under a fixed temperature T

where E is the apparent activation energy, R is the gas constant, and A, B and C are the constants determined by the composition of primary glass.

2.2. Physical Properties of SPG

One of the features of SPG is the unique porous microstructure, which is related to mechanical properties such as strength. Figure 2 displays scanning electron micrographs of SPG membranes, and pore size distribution histogram measured by a mercury penetration porosimetry. It is clearly seen in Figure 2 that the outlets of Membrane 1 are uniformly controlled in shape and size. However, the irregularities can be seen for the surfaces of Membranes 2 and 3, while the pore diameter in all three membranes is narrowly distributed. This surface roughness of Membranes 2 and 3 could be due to the dissolution of SPG in a highly acidic solution resulting in enlargement of the pore size. The pore size of SPG can be varied over a wide range from several nanometers to 10 micrometers. This is the distinguishing feature of SPG. Also, the many hydroxyl groups which cover the surface of SPG membrane make it an excellent material for surface modification by reaction with organic silanes such as octadecyl trichlorosilane. Table 3 shows physical properties of a typical SPG membrane. Detailed information on SPG is given elsewhere (11).

2.3. Availability and price of SPG

The SPG membarne can be formed into tubes, flat sheets, particles or beads of various sizes. Tubular membranes are typically made in 10 mm outer diameter, and are generally available incorporated in filtration module. Flat sheets are cut into disc or square configurations. Several beads and particles are also made for chromatography separations. The standard products and their prices are listed in

Table 3

Physical Properties of SPG

Pore diameter Porosity Pore volume Specific surface area True density

Thermal expansion coefficient Heat resisting temperature Compressive strength Bending strength

20 nm to 10 |im 50 to 60 % 0.4 to 0.6 cm3/g 0.2 to 8.0 m2/g 2.5 g/cm3 60x10-7 K-l 650 °C

2000-3000 kgf/cm2 710-840 kgfcm2

Table 4.

Ax Membrane 1

Membrane 2

Membrane 3

0 0

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