We have tried to prepare the porous silicone 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 a good solvent and a 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 by mass transfer from water to ethyl acetate. The concentration of silicone resin in ethyl acetate and the oil soluble surfactant species were changed. Increasing the concentration of silicone resin could depress coalescence between the water droplets in the (W/O) dispersion and increase the porosity and pore number density of silicone resin sheet. Span 80 among the oil soluble surfactant species made the porosity and pore number density larger. The effect of physical proparties of liquids concerned on the porosity and pore number density was discussed on the basis of dispersing behavior of liquid droplets in the liquid-liquid dispersion. The hollow silicone resin particles could be prepared by applying the preparation method presented here.
Polymeric porous membranes [
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 [
The phase separation method among these preparation methods has been of great interest because of the simple and low cost preparation process [
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 [
Also, the finer liquid droplets are able to be easily prepared by utilizing the phase inversion under mass transfer of amphiphilic solvent [
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.
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.
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 [
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
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
. Experimental conditions
Oil solution | 18 ml |
---|---|
concent of silicone resin | Cs = 50, 67, 75, 80 [wt%] |
ethyl acetate | 1.6 g |
hardening agent | 1.0 g |
surfactant | Span 20 (HLB = 8.6), |
Span 40 (HLB = 6.7), | |
Span 80 (HLB = 4.3) | |
Aqueous solution | 15 ml |
distilled water | 12 ml |
ethanol | 3 ml |
Phase separation condition | |
impeller speed | 5 s−1 |
syringe pump flow rate | 10 ml/h |
Reaction condition | |
temperature | 35˚C |
reaction time | 12 h |
Drying-in-liquid condition | |
temperature | 44˚C |
drying time | 24 h |
. Experimental conditions
Oil solution | |
---|---|
silicone resin | 16.0 g |
ethyl acetate | 4.0 g |
hardening agent | 1.6 g |
surfactant (Span 80) | 1.0 g |
Inner aqueous solution | |
distilled water | 3 ml |
ethyl alcohol | 2 ml |
Continuous water phase | |
distilled water | 300 ml |
surfactant | 12.0 g |
revolution speed | 6.7 s−1 |
Reaction condition | |
temperature | 70˚C |
reaction time | 3 h |
revolution speed | 5 s−1 |
Drying-in-liquid condition | |
temperature | room temperature |
revolution speed | 11.7 s−1 |
Schematic diagram of experimental apparatus and flow chart for preparing hollow silicone resin particles
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.
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.
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.
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 [
. Measured values of viscosity of oil solution and interfacial tension (T = 35˚C)
Cs [wt%] | 50 | 67 | 75 | 80 |
---|---|---|---|---|
μs [mPa∙s] | 5.0 | 18.2 | 38.5 | 58.2 |
γ [N/m] | 5.9 | 6.0 | 6.0 | 6.2 |
SEM photographs of silicone resin sheet Cs = (a) 50 (b) 67 (c) 75 (d) 80 wt%
sin, the interfacial tension slightly increase and the viscosity extremely increase.
From
In general, the water droplet diameters may inevitably change due to coalescence or breakup of liquid droplets. Also, coalescence between the liquid droplets is depressed by increasing the viscosity of continuous phase. As an example, the dependences of the coalescence velocity (Rc) and the mean liquid droplet diameter (dp) on the physical properties of liquids such as the viscosity of continuous phase (μc) and the interfacial tension (γ) are expressed as follows [
where n and t are the number of liquid droplets per unit volume of the dispersion and time, respectively. Accordingly,
From Equation (1), it may be quantitatively understood that the coalescence velocity is able to be decreased with increasing the viscosity of continuous phase and the interfacial tension. On the other hand, the liquid droplet diameters become smaller with increasing the viscosity of continuous phase and decreasing the interfacial tension.
Taking these results into consideration, the following things may be discussed. In the case of the lower concentration of silicone resin, the water droplets frequently coalesce with each other to form the larger water drop-
Dependences of mean water droplet diameter, mean pore sizes and degree of dispersion on viscosity of oil solution
lets during the removal process of good solvent. In the case of the higher concentration of silicone resin, the water droplets have to be stabilized in the oil solution. Accordingly, in the range of the lower concentration of silicone resin, the pore sizes have to become larger due to coalescence of the water droplets. As a result, the degree of dispersion of pore sizes should become larger. In the range of the higher concentration of silicone resin, the pore sizes have to become close to the water droplet diameters due to depression of coalescence. As a result, the degree of dispersion of pore sizes should become smaller, namely the pore sizes become more uniform. The degrees of coalescence
As the degree of coalescence at the concentration of 50 wt% is 35.4, the stability of water droplets in the oil solution is very low in comparison with 1.6 at the concentration of 80 wt%.
On increasing the viscosity of oil solution, the pore number density increases from 28 at 5.0 mPa∙s to 62 at 58.2 mPa∙s.
In order to increase the pore number density, it is necessary to decrease the water droplet diameters and to increase the stability of water droplets in the (W/O) dispersion.
According to Equation (2), the water droplet diameters can be decreased by lowering the interfacial tension due to addition of the oil soluble surfactant into the oil solution under the given revolution speed to form the (W/O) dispersion.
In general, it is well known that the effect of surfactant on the stability of the (W/O) dispersion is strongly dependent on the HLB value and molecular structure of oil soluble surfactant, because the amount and configuration of surfactant absorbed on the interface are affected by them. In order to investigate the optimum surfactant species, three kinds of oil soluble surfactants were added.
The pore sizes decrease and the pore number density increases due to addition of the oil soluble surfactants.
In order to quantitatively know the degree of effects of surfactant species,
As these results are considered to be due to the fact that the interfacial tension and the viscosity of oil solution are changed by addition of the surfactants, these physical properties are measured and shown in
Dependences of mean water droplet diameter, mean pore sizes and degree of dispersion on viscosity of oil solution
SEM photographs of silicone resin sheet (a) no (b) Span 20 (c) Span 40 (d) Span 80 (effect of surfactant species)
Effect of oil soluble surfactant species on mean pore size and pore number density
Effect of oil soluble surfactant species on interfacial tension and viscosity of oil solution
In the case of Span 80, as the interfacial tension considerably decreases and the viscosity slightly increases, the water droplets have to become smaller and been stabilized with both the absorption layer of surfactant and the increase in viscosity of oil solution as stated just above.
Taking the results obtained above into consideration, the formation mechanism of the porous sheet may be discussed as follows.
First, the (O/W) dispersion has to be formed in the early stage by pouring the oil solution into the aqueous solution. Ethyl alcohol of amphiphilic solvent may transfer from the aqueous solution to the oil solution and lower the interfacial tension [
Then, the (W/O)/W dispersion has to be formed because of increase in the oil droplet diameter with the volume of oil solution poured and the dispersion of the fine inner water droplets into a larger oil droplet.
Finally, the (W/O) dispersion has to be formed by the phase inversion occurred by addition of the volume of oil solution larger than that of aqueous solution.
The water droplets in the (W/O) dispersion have to be stabilized by the higher viscosity of oil solution and the adsorption layer of surfactant during the removal of solvents such as ethyl acetate and ethyl alcohol.
In the consecutive processes stated above, the fine inner droplets dispersed in the oil droplets can be made smaller by lowering the interfacial tension due to mass transfer of ethyl alcohol [
As a result, it is found that the non solvent induced phase separation method in parallel with phase inversion is extremely effective for preparing the porous silicone resin sheet.
Also, it was tried to apply the method for preparing the porous silicone resin sheet to the formation of the hollow silicone resin particles as follows.
Namely, the (W/O) dispersion before gelation as shown in
From the formation of the (W/O)/W dispersion, the hardening reaction and the removal of solvents such as ethyl acetate and ethyl alcohol were started under temperature of 70˚C and reduced pressure.
In this preparation method, any oil soluble surfactants were not added to promote coalescence between the fine inner water droplets in the oil droplets and to make the hollow structure.
Formation mechanism of porous silicone resin sheet
Photographs of hollow particle. (a) optical microscopic photograph, (b) SEM photograph
by the preparation method presented in this study. The formation of the hollow structure is inevitably determined by the coalescence velocity between the fine inner water droplet and the formation velocity of microcapsules shell. Also, these velocities are affected by the concentrations of surfactants and the reaction conditions. Accordingly, it may be necessary to establish the detailed preparation conditions and to develop the simple method for preparing the hollow particles.
It was tried to prepare the porous silicone resin sheet with the phase inversion in parallel with the non solvent induced phase separation method. Ethyl acetate and water were adopted as the good solvent and the poor solvent for silicone resin, respectively.
The preparation method presented here was applied to prepare the hollow silicone resin particles. The following results were obtained.
1) With increasing the concentration of silicone resin, coalescence between the water droplets in the (W/O) dispersion could be depressed and the water droplets diameters could be decreased. As a result, the pore sizes could be decreased and the pore number density could be increased.
2) By adding Span 80 (HLB 4.3), the pore sizes could be drastically decreased and the pore number density could be increased.
3) The effects of physical properties of liquids changed by the concentration of silicone resin and Span 80 on the porosity and pore number density was discussed on the basis of dispersing behavior of dispersed droplets in the liquid-liquid dispersion.
4) The hollow silicone resin particles could be prepared by the preparation method prepared in this study.