Microcapsules can contain as the core materials the various substances such as solid, liquid and gas and release optionally them [1] [2] .

Also, microcapsules containing water among these core materials have been applied in a lot of fields such as pharmacy, adhesives, agriculture, cosmetics, food, textile, information recording materials and so on [1] [2] , because water is the solvent being able to dissolve various inorganic and organic materials.

Until now, many works with respect to microencapsulation of water have been reported, in which water was microencapsulated using multiple emulsion like the (W/O)/W emulsion [3] [4] [5] [6] .

In this preparation method, how to keep the (W/O) emulsion stably is extremely important in order to prepare the satisfactory microcapsules containing water.

Tanaka et al. have reported the results tried to form the stable (W/O) emulsion by increasing the viscosity of oil phase, changing the oil soluble surfactant species and their concentrations, conducting the interfacial polycondensation reaction between the inner water droplets and the oil phase and using the inverse Pickering emulsion [7] [8] .

These works were aimed at decreasing the inner droplet diameter and preventing coalescence between the inner water droplets in an oil droplet.

Shiomori et al. have prepared the porous polystyrene beads applied to preparation of the microcapsules containing water with suspension polymerization by using the (W/O)/W emulsion [9] [10] . They have formed the stable (W/O)/W emulsion by selecting the oil and the water soluble surfactants.

Sekine et al. have formed the stable (O/W)/O emulsion using liquid paraffin as the inner oil phase and the outer oil phase, where hydrophobic nonionic surfactant and organophilic montmorillonite were added into the outer oil phase [4] .

They have reported that the stable (O/W)/O emulsion could be formed mainly by increasing the viscosity of the outer oil phase in the (O/W)/O emulsion with lipophilic nonionic surfactant and organophilic montmorillonite.

The other works with regard to the formation of the stable (W/O)/W emulsion have investigated the effects of the optimum hydrophilic and lipophilic surfactant species and the physical properties of multiple emulsion [11] [12] .

Also, Tanaka et al. have prepared the microcapsules containing water, in which the (W/O) emulsion was stabilized by adding TiO₂ powder as a particulate stabilizer into the oil phase [13] and investigated the effect of contained water on expansion behavior.

In this study, they modified TiO₂ powder with silane coupling agents so as to adher on the interface between the inner water droplets and the oil phase.

In this study, taking the formation of the stable (W/O)/W multiple emulsion and obtaining the higher water content into consideration, the microcapsules containing water were prepared with two kinds of microencapsulation methods, namely, the first was the microencapsulation of water with suspension polymerization and the second was the microencapsulation of water with suspension polymerization in parallel with interfacial polycondensation reaction. These preparation methods were easy operation compared with the stabilization of the (W/O)/W emulsion by the modified TiO₂ powder or SiC powder [14] .

The purposes of this study are to prepare the microcapsules containing water by forming the stable (W/O)/W multiple emulsion and to investigate the effect of the microencapsulated water on the expansion behaviour of microcapsules.

Materials used to prepare the microcapsules containing water were as follows.

Styrene monomer (St: Wako Pure Chemical Co., Ltd., Tokyo, Japan) and Trimethyl propane triacrylate (TMPTA: Wako Pure Chemical Co., Ltd., Tokyo, Japan) were used as a polymerizable monomer and a crosslinking agent in the suspension polymerization process, respectively.

Toluene diisocyanate (TDI: Wako Pure Chemical Co., Ltd., Tokyo, Japan) and polyethylene glycol (PEG: Wako Pure Chemical Co., Ltd., Tokyo, Japan) were used as the oil soluble reactant and the water soluble reactant in the interfacial polycondensation process, respectively.

2,2’-Azobis 2,4-dimethylvaleronitrile (V-65: Wako Pure Chemical Co., Ltd., Tokyo, Japan) was used as the oil soluble initiator in the suspension polymerization process.

As the water soluble stabilizers, polyvinyl alcohol (PVA: Polym.degree 500 Wako Pure Chemical Co., Ltd., Tokyo, Japan) and hydroxyl propylmethyl cellulose (HPMC (60 SH-50): Shinetsu Chemical, Co., Ltd., Tokyo, Japan) were dissolved in the continuous water phase. As an oil soluble surfactant, polyglycerol polyricin oleate (POEM PR-100, Riken Vitamin Co., Ltd., Tokyo, Japan) was used.

As the oil soluble additives added to increase the water content, wax (Kanto Chemical Ltd., Tokyo, Japan) was added in the monomer phase in the suspension polymerization process. Also, as the hydrophilic additives, agarose and silicagel (Kanto Chemical Ltd., Tokyo, Japan) were added into the inner water phase.

The experimental apparatus was the same as one used in the previous work [15] [16] .

Briefly, the reactor was the separable flask with the effective volume of 500 cc and the four aluminum baffle plates, and set in the thermostatted water both. The impeller was the six bladed turbine with the diameter of 5.0 cm.

Figure 1 shows the flow sheet for preparing the microcapsules. The suspension polymerization method was as follows, where all the abbreviations such as

PVA, HPMC, TDI, et al., are explained in the section of 2.1 materials.

St and TMPTA were mixed to form the dispersed oil phase (O), where the oil soluble additives and the oil soluble surfactant were dissolved according to the experimental conditions.

The inner water (W) was added into the dispersed oil phase (O) and homogenized with the homogenizer to form the (W/O) emulsion. Here, the water soluble additives were added into the inner water according to the experimental conditions beforehand. After this operation, the initiator of a given weight was added into the (W/O) emulsion.

Then, the (W/O) emulsion was added into the continuous water phase (W) and stirred with the impeller to form the (W/O)/W emulsion. The water soluble stabilizer (PVA, HPMC) was dissolved in the continuous water phase beforehand. Also, sodium nitrite (NaNO₂) of a given weight was dissolved as an inhibitor in the continuous water phase.

After stirring the (W/O)/W emulsion during the constant time (10 min), suspension polymerization was performed for 7 h keeping reaction temperature at 70˚C.

Hereafter, the suspension polymerization method and the microcapsules prepared thus are called Process A and MCS, respectively.

Next, the suspension polymerization in parallel with interfacial polycondensaiton method was as follows.

TDI and PEG were added into the dispersed oil phase and the inner water phase, respectively. Then, (W/O) emulsion and the (W/O)/W emulsion were formed with the same method as one described above. Suspension polymerization in parallel with interfacial polymerization was performed for 7 h at 70˚C. Interfacial polycondensation was conducted to microencapsulate the inner water droplets and to increase the stability of the (W/O) emulsion and the water content.

Hereafter, the suspension polymerization in parallel with interfacial polycondensation method and the microcapsules prepared thus are called Process B and MCSI, respectively.

The experimental conditions were shown in Table 1.

The microcapsules prepared with each preparation method were observed by optical microscope (OM: BH-2, OLYNPUS Co., Ltd., Tokyo, Japan) and scanning electron microscope (SEM, JST-5800, Nihon Denshi Co., Ltd., Tokyo, Japan).

The shape, section area and inner structure of microcapsules were evaluated from these results.

Also, the mean diameters of microcapsules were evaluated directly from the photographs taken by OM or SEM, and measured by particle size analyzer (SALD-3000, Shimazu Seisakusho Ind., Co., Ltd., Kyoto, Japan).

The water content (R) was defined and calculated according to Equation (1).

R [ % ] = ( M w − M o ) / M w × 100 (1)