Preparation of Porous Silicone Resin Sheet with Phase Inversion in Parallel with Non Solvent Induced Phase Separation and Application to Hollow Particle Formation ()
1. Introduction
Polymeric porous membranes [1] -[3] and particles [4] [5] have attracted much attention for various applications such as membrane separation and purification, solid supports for catalysts and dyes, scaffolds for biological cells and so on [6] .
The polymeric porous membranes and particles have been prepared by the several methods such as the multiple emulsion method, the phase separation method, the removal method of inorganic salts, carbon dioxide forming method and so on [5] .
The phase separation method among these preparation methods has been of great interest because of the simple and low cost preparation process [7] [8] . Non solvent induced phase separation is on the basis of the removal of the good solvent from the uniform solution composed of polymer, good and poor solvents for polymer. With increasing the removed volume of good solvent, polymer and poor solvent may be separated and the droplets of poor solvent come to be formed in polymer swollen by the good solvent. The polymeric porous membrane can be prepared by removing water from the droplets of poor solvent. Accordingly, in order to prepare the higher porous membrane, it is necessary to stably disperse the smaller droplets of poor solvent in the matrix polymer.
In the case of the system composed of silicone resin, ethyl acetate, water and ethyl alcohol, after complete removing of the good solvent, the silicone resin sheet containing the liquid droplets of poor solvent is prepared. Then, the porous silicone resin sheet can be prepared by removing these liquid droplets due to drying.
It is well known that the diameter and number of dispersed droplets are determined by the dispersing behavior of droplets such as coalescence and breakup which are strongly affected by the turbulent energy level in the continuous phase and the physical properties of liquids concerned [9] -[11] .
Also, the finer liquid droplets are able to be easily prepared by utilizing the phase inversion under mass transfer of amphiphilic solvent [12] .
As the pore size and pore number density in the porous sheet prepared by the phase separation method are inevitably dependent on the diameter and number of liquid droplets of poor solvent formed by removing the good solvent, it is necessary to investigate how the dispersing behavior of dispersed liquid droplets affects the pore size and porosity of sheet in detail.
In this study, it was tried to prepare the porous silicon resin sheet with the phase inversion method in parallel with the non solvent induced phase separation method.
In the experiment, ethyl acetate and water were adopted as the good and the poor solvent for silicone resin, respectively and ethyl alcohol as an amphiphilic solvent was used to increase the solubility of ethyl acetate in water and decrease the interfacial tension between the oil phase and the aqueous solution due to mass transfer.
The purposes of this study are to investigate how the porosity and pore number density of silicone resin sheet are affected by the experimental conditions such as silicone resin concentration and the oil soluble surfactant species, to discuss the effect of dispersing behavior of dispersed liquid droplets of poor solvent on the porosity and pore number density and to apply to the preparation of hollow silicone resin particles.
2. Experimental
2.1. Materials
Silicone resin and hardening agent for silicone resin were silicone KE-106 (Shinetsu Kagaku Kogyo, Tokyo, Japan) and CAT-RG (Shinetsu Kagaku Kogyo, Tokyo, Japan), respectively.
Ethyl acetate (Wako Pure Chemical Industries, Tokyo, Japan) and distilled water were used as a good solvent and a poor solvent for silicone resin, respectively. Ethyl alcohol as an amphiphilic solvent (Wako Pure Chemical Industries, Tokyo, Japan) was used to decrease the interfacial tension between the oil solution and the aqueous solution by rapid mass transfer and to increase the solubility of ethyl acetate in water.
2.2. Preparation of Porous Silicone Resin Sheet
Figure 1 shows the schematic diagram of experimental apparatus together with the flow chart for preparing the porous silicone resin sheet. The reactor was the separable flask with the diameter of 8.5 cm and the effective volume of 300 ml. The aqueous solution dissolving ethyl alcohol was poured into this reactor. Then, the oil solution composed of silicone resin, ethyl acetate, hardening agent and oil soluble surfactant was dropped into the
Figure 1. Shematic diagram of experimental apparatus and flow chart for preparing porous silicone resin sheet.
aqueous solution with the syringe pump under stirring with the six-bladed disc turbine impeller (revolution speed: 5 s−1).
First, the (O/W) dispersion was formed at room temperature and then, with increasing the volume of oil solution poured, the (W/O)/W dispersion and the (W/O) dispersion due to phase inversion were formed in turn [12] . The (W/O) dispersion was kept in the thermostatted water bath at temperature of 35˚C. From this point, the hardening reaction and the removal of ethyl acetate and ethyl alcohol were performed for 12 h under reduced pressure. With removing ethyl acetate and ethyl alcohol by the drying-in-liquid operation, the water droplets were formed in the silicone resin sheet swollen by ethyl acetate.
Then, the silicone resin sheet containing the water droplets was dried at temperature of 44˚C for 24 h to remove water perfectly. After this operation, the porous silicone resin sheet has been prepared.
In the fundamental preparation process stated just above, the concentration of silicone resin and the oil soluble surfactant species were changed.
The experimental conditions adopted here are shown in Table1
2.3. Preparation of Hollow Silicone Resin Particle
Figure 2 shows the schematic diagram of experimental apparatus and the flow chart for preparing the hollow silicone resin particles. The reactor was the separable flask with the diameter of 8.5 cm and the effective volume of 400 ml, in which four baffles with 0.85 cm width made from aluminum plate were settled on the reactor wall. This reactor was set in the water bath to keep temperature of the reactor constant. A six bladed disc type impeller with the diameter of 3.5 cm was used to form the (W/O) dispersion and the (W/O)/W dispersion. The inner aqueous solution was prepared by dissolving ethyl alcohol in the water phase. The oil solution was prepared by dissolving ethyl acetate, hardening agent and oil soluble surfactant in silicone resin.
First, the (W/O) dispersion was formed at room temperature by dropping the inner aqueous solution into the oil solution with the syringe pump under stirring with the six bladed disc turbine impeller (revolution speed: 11.7 s−1).
Second, the (W/O)/W dispersion was formed at room temperature by dropping the (W/O) dispersion into the continuous water phase in which sodium dodecyl sulphate was dissolved as a water soluble surfactant.
After formation of the (W/O)/W dispersion, the gelated reaction was performed at temperature of 70˚C with removing ethyl acetate and ethyl alcohol by the drying-in-liquid method. The experimental conditions are shown in Table2
Table 1. Experimental conditions.
Table 2. Experimental conditions.
Figure 2. Schematic diagram of experimental apparatus and flow chart for preparing hollow silicone resin particles.
3. Characterization
3.1. Observation of Cross Section of Porous Silicone Resin Sheet
The cross section of porous silicone resin sheet was observed by scanning electron microscopy (JSM-S800; JEOL Ltd., Tokyo, Japan).
From these SEM photographs, the pore size and pore number density were measured directly.
3.2. Measurement of Water Droplet Diameters in the (W/O) Dispersion
The water droplet diameters were measured from the optical microscopic photographs of the water droplets taken by the optical microscope (BH-2; OLYMPUS Grp., Tokyo, Japan) just after formation of the (W/O) dispersion. From these photographs, the mean diameters were measured directly.
3.3. Measurement of Physical Properties of Liquids Concerned
Interfacial tension and viscosity were measured by the Auto Tensiometer (CBVP-A3; Kyowa Kaimen Kogaku Ltd., Tokyo, Japan) and the plate vibrating viscometer (VM-1A-L; Yamaichi Denki Ltd., Tokyo, Japan), respectively.
4. Results and Discussion
4.1. Effect of Concentration of Silicone resin
Figure 3 shows the SEM photographs of the cross section of porous silicone resin sheet prepared at the various concentrations of silicone resin without any oil soluble surfactants.
It is found that the pore sizes drastically decrease with the concentration (Cs) of silicone resin. As the pore sizes are strongly dependent on the diameters of water droplets in the (W/O) dispersion, it is necessary to investigate the dispersion stability of water droplets in the oil solution.
In general, the stability of liquid droplets in the liquid-liquid dispersion is affected by the physical properties of liquids such as the viscosity of continuous phase and dispersed phase, the interfacial tension and the difference in density between the dispersed and the continuous phase under the given conditions of the revolution speed and the concentration of stabilizer [10] . As the viscosity of oil solution and the interfacial tension specially strongly affect the dispersing behavior of droplets, the measured values of the viscosity (μs) of oil solution and the interfacial tension (γ) are shown in Table3 It is found that, with increasing the concentration of silicone re