Design and characteristic analysis of a new nozzle for preparing microencapsulated particles by RESS.

Abstract A new nozzle was designed for the technology of making microencapsulated particles by rapid expansion of supercritical solution (RESS). The design is based on the theory of gas dynamics in which the potential energy of high stagnation pressure is converted totally into effective velocity energy. Therefore, a high momentum of the exit jet can be obtained for improving the capability of removing molten debris quickly. Furthermore, the microencapsulated red phosphorus particles were prepared by RESS with the new nozzle, and the structure and property of the microencapsulated red phosphorus particles were characterized by the SEM images, droplet concentration distributions, and moisture absorption ratio. The results show that the process can effectively encapsulate the red phosphorus particles with the paraffin.

Keywords RESS, Microcapsule, Design of nozzle, Technology of coating. Red phosphorus

Introduction

Recently, these literatures have revealed a growing interest in the application of the rapid expansion of supercritical solutions (RESS) for the encapsulation process. (1-4) This interest is due to the possibility of obtaining solid phases with a specific morphology and narrow size distribution in relatively mild processing conditions. In the RESS technique, the solid is first solubilized in a supercritical fluid and the resulting solution highly compressed in the vicinity of the mixture's critical point. The RESS by pressure reduction leads to loss of solvent power and precipitates the solution. (5) The rapid expansion is a very fast process with associated high supersaturating rates that can produce particles of submicroscopic size with narrow size distribution. (3), (6-8) So, it is very important to choose the correct nozzle for the RESS technique. There were some articles written about the microencapsulation of particles by RESS, but all articles used the ordinary capillary nozzle or orifice nozzle, which led to bad encapsulating results and needed the help of other equipment, such as a fluidized bed. So, the disadvantage of complicated preparation processes restricts its application. (9), (10) To overcome the drawbacks of a conventional nozzle, a novel nozzle structure should be established.

In this paper, a novel nozzle suitable for preparing microencapsulated particles by RESS without using a sulfurated bed was designed. The core particles and supercritical [CO.sub.2] (SC-[CO.sub.2]) dissolved with shell materials can be ejected from the nozzle together. During this process, the shell materials dissolved in [CO.sub.2] precipitate and deposit around the core particles. Besides the above advantage, the nozzle has other advantages. First, the aperture can be adjusted, so it can overcome the defects of the traditional capillary nozzle, such as blocking easily, low productivity, and incompact coating. Ejecting and coating are integrative, which is to be expected in making microencapsulated particles by RESS.

Materials and experiments

Materials

Red phosphorus and paraffin were purchased from Shanghai Chemical Reagent Plant. High purity carbon dioxide was supplied by Taiyuan Iron & Steel Company, LTD.

The model of the new nozzle

The model and structure of the new nozzle designed in this paper is shown in Fig. 1. The new nozzle consists of nine parts: a Laval pipe, a rotatable outer shell, a pair of gyrators, a temperature-controlled unit, a carrying pipe of supercritical solution, a carrying pipe of core particles, entrance for the supercritical solution, the entrance for the core particles, and a nozzle connector. After the supercritical solution is obtained in the extraction column, it is sent to the entrance of supercritical solution, and is swirled by the exterior gyrator. Then, the eddy is accelerated to a transonic state by the aperture comprised of the front of the Laval pipe and the front of the rotatable outer shell. At the same time, the core particles are sent into the entrance of core particles by a pump and are swirled after passing the interior gyrator. Then, the eddy is also accelerated to a transonic state by the Laval pipe. The revolving directions of the two gyrators are contrary. At the nozzle's exit, the supercritical fluid collides with the core particles, and the coating is fulfilled in the collection room of microcapsule particle. In this design, the width of the aperture--comprised of the front of the Laval pipe and the front of the rotatable outer shell--can be adjusted, so that the supercritical solution expands rapidly through an exit with a controllable size. This design is also very important for research purposes because the conventional nozzle can easy become stopped up, making the encapsulation process difficult.

[FIGURE 1 OMITTED]

The preparation of microencapsulated red phosphorus particles by rapid expansion of supercritical solution (RESS) with the new nozzle

The microencapsulated red phosphorus particles were prepared according to the literature. (11), (12) The experimental process is shown in the following order: a high-pressure pump charged the liquefied [CO.sub.2] to an extraction column, in which the shell materials (paraffin pieces) were put. The extraction temperature and nozzle temperature were controlled at 120[degrees]C. A magnetic stirrer agitated the melted paraffin in the column to improve the solvating of the paraffin in SC-[CO.sub.2]. The paraffin was further dissolved into a supercritical solution by adjusting the temperature and pressure. After the supercritical solution was obtained, the supercritical solution and the red phosphorus particles with an average size of 45 [micro]m were pumped into a new nozzle at 1.5 g/min core particles to feed mass ratio. At the exit of the nozzle, the supercritical solution and red phosphorus particles collided fiercely. Paraffin precipitated from the expanding rapidly supercritical solution and crystallized around the core particles. The microcapsule particles produced were slipped down from the exit of the collection.

Results and discussions

Design theory of the new nozzle

Laval pipe section

The function of Laval pipe is to provide the core particles with enough momentum from an existing jet, resulting in the core particles colliding with shell material with enough speed so the core particles could be effectively encapsulated by the shell materials. The structure of the Laval pipe is shown in Fig. 2.

[FIGURE 2 OMITTED]

The Laval pipe should consist of three sections: convergent, throat, and divergent. According to the theory of throttle and expand, the goal is for the uniform flow to obtain supersonic speed after getting across the Laval pipe. When the design of the three sections is logical, the uniform flow obtains subsonic speed, velocity of sound, and supersonic speed through the convergent, throat, and divergent, respectively. So, the design of the throat section is relatively important because it is a transitional cross-sectional area that transfers subsonic speed into supersonic speed. Just as mentioned above, the cross-sectional area closer to the throat section cannot be easily varied. The dimensions of the throat in the Laval pipe need to be designed correctly and calculated precisely on the basis of the following equation, according to the theor (13)

[d/d*] = [square root of ([1/m] [[([[2]/[[gamma] + 1]]) * (1 + [[[gamma] - 1]/2] [M.sup.2])].sup.[[gamma] + 1/2[gamma] - 1] (1)

where the d and d* are the arbitrary Laval pipe diameter and the throat diameter, respectively: the [gamma] is the ratio of specific heats of the flow uniform; and M is the Mach number.

The function of the convergent section is to accelerate gas How. but also to keep the flow uniform and parallel as shown in Fig. 3. The characteristics of the convergent section are mainly determined by the factors, one being the converging ratio--i.e., [d/d*] (d is the convergent inlet diameter and d* is the throat diameter)--which accelerates the gas flow and ensures the speed of flow to reach sonic speed, and the second being the convergent curve, which maintains the velocity of flow uniform.

[FIGURE 3 OMITTED]

There are numerous theories about the design of a converging curvature. However, most of them are quite complicated. In this paper, a simpler but more practical equation is applied in the design of the convergent section, which is derived from the conclusion of an ideal axial incompressible symmetry flow (14):

r = [[r.sub.1]/[{1-[[1-[([[[r.sub.1]]/[[r.sub.2]]]).sup.2]][([1-[[[x.sup.2]]/[[l.sup.2]]]).sup.2]]/[[([1 + [x.sup.2]]/3[[l.sup.2]]]).sup.3]}.sup.1/2]] (2)

where r is the radius of an arbitrary convergent cross-sectional urea: [r.sub.1] is the radius of the throat sectional: [r.sub.2] is the radius of the stable sectional area: l is the length of the convergent section; and x is the length of an arbitrary convergent section.

Rotatable outer shell and narrow orbicular channel section

The function of the rotatable outer shell is used to form narrow orbicular channels, as shown in Fig. 4. The uniform flow containing the shell materials is ejected and collides with the core particles in the section. So. the design and structure of the channels are also very important to the formation of the core-shell structure of particles.

[FIGURE 4 OMITTED]

First, the width of the channels determines the exit size of nozzle and the size of the shell materials. It not only avoids the blockage of the nozzle by core and shell materials, but also improves the encapsulated quality of core particles. So. the width of the channels needs to be controlled correctly and calculated precisely on the basis of the following equation

m = h cos 25[degrees] (3)

where m is the width of the channel and h is the rotatable distance of the outer shell. Second, the angle. [[alpha].sub.3] of the channels also needs to be controlled correctly, and the uniform flow containing the shell materials can be ejected to collide with the core particles at supersonic speed. Then, the shell materials can be well encapsulated on the surface of core particles.

Gyrators section

There are two gyrators in the new nozzle, as shown in Fig. 1, which are inside and outside the Laval pipe. They are shown in Fig. 5. The function of the gyrators is to make the uniform flows rotate at supersonic speed. Furthermore, the rotatable directions of two gyrators are contrary, resulting in a good encapsulating quality of core particles. Design details of the first gyrator geometry include four 3 mm thick vanes, as shown in Fig. 5a. The diameter of the gyrator and hole are 25 mm and 9 mm. respectively. The angle ([theta]) between vanes indicated in the top view is 90[degrees]. As shown in Fig. 5b, the annular 2 mm think swirl vanes are fixed on a hub with a diameter of 2 mm. and the swirler is fixed in the inlet rotatable outer shell. The angle subtended by a vane at the axis is 45[degrees], giving an overlap of 90[degrees] between adjacent vanes.

[FIGURE 5 OMITTED]

Effect of the new nozzle structure on preparation of microencapsulated red phosphorus particles

Effect of the gyrators section

Figures 6a and 6b show a typical spray photo of red phosphorus (RP) particles, which are ejected by the new nozzle without and with gyrators, respectively. From Fig. 6, it can be seen that the diameter of spray using the new nozzle without gyrators (a) is far smaller than that using the new nozzle with gyrators (b). The result indicates that the atomization of the RP particles is better for the nozzle with gyrators. which is very important to improving the encapsulating quality of the particles. (15)

[FIGURE 6 OMITTED]

The encapsulating quality of the RP particles using the new nozzle with and without gyrators is further compared by the SEM images shown in Fig. 7. As per the above discussion, the diameter of RP particles is about 45 [micro]m. Figure 7a shows almost the same size, suggesting that the RP particles are not encapsulated by the paraffin with the new nozzle without gyrators. Contrarily, the increase in size (65 [micro]m) is observed as shown in Fig. 7b. The result is attributed to the fact that the RP particles are encapsulated by the shell materials of paraffin and the size of the shell is about 10 [micro]m. Moreover, Fig. 7b also shows that the outer surface of the particles, when magnified, is perfectly smooth, indicating that the RP particles are completely and effectively encapsulated by the paraffin. At the same time, the break on the surface of microencapsulated RP particles further indicates the formation of a core-shell structure of microencapsulated RP particles, according to the magnified SEM image (Fig. 7b).

[FIGURE 7 OMITTED]

Effect of the rotatable outer shell

The effect of angle [[alpha].sub.3] of the rotatable outer shell section on the moisture absorption ratio of paraffin-microencapsulated RP particles is further studied from 10[degrees] to 50[degrees], as shown in Fig. 8. It can be noticed that the moisture absorption ratio decreases with an increase in angle [[alpha].sub.3] from 10[degrees] to 20[degrees]. However, when angle [[alpha].sub.3] is further increased, the moisture absorption ratio drastically increases. According to previous works, (16), (17) the lower moisture absorption ratio indicates a better encapsulating quality because the shell materials of paraffin effectively prevent the core materials from absorbing water from the atmosphere. So, these results suggest that the RP particles can be well-encapsulated by the paraffin at an angle of 20[degrees]. Compared with the conventional nozzle. (18) the moisture absorption ratio here is relatively lower, suggesting that the new nozzle is well-suited to be used in the encapsulated techniques of particles by RESS. The increase in angle [[alpha].sub.3] actually corresponds to two opposite effects: an increase of flow mass of paraffin particles (and thus of paraffin solubility), and an increase in the particle size of the paraffin. For the range of 10[degrees] to 20[degrees] used in this investigation, the former effect is dominating, so the flow mass of particles increases with an increase in angle [[alpha].sub.3], which leads to a thicker coaling. On the other hand, when angle [[alpha].sub.3] is further increased, the latter effect is dominating, so the size of shell materials increases with an increase in angle [[alpha].sub.3]. The result is that it is difficult to encapsulate the RP particles with big paraffin particles, which leads to an increase in the moisture absorption ratio.

[FIGURE 8 OMITTED]

Conclusions

A new structure nozzle consisting of nine parts was designed that has good gas dynamic characteristics and controlled channels for two kinds of materials. So. when the design of four parts--such as Laval pipe section, rotatable outer shell, narrow orbicular channel section, and gyrators section--is logical, it is suitable for the preparation of microencapsulated particles by RESS. Microencapsulated RP particles were prepared with the new nozzle. which was further characterized by the SEM images, droplet size distributions, and moisture absorption ratio. The result shows that the design of the new nozzle is very important to the formation of microencapsulated RP particles with encapsulated quality, which is expected in the preparation of microencapsulated particles by RESS.

Acknowledgments This work was supported by National Natural Science Foundation of China (N 50303005), Natural Science Foundation of Shanxi Province (N 20041029), and Project of Science and Technology of Shanxi Province (N 012078). The authors are grateful for the financial support.

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Y.-q. Liu ([??]). F.-x. Zhang. Y.-y. Sun ([??]),

G.-z. Zhao

Research Center for Engineering Technology of Polymeric Composites of Shanxi Province, North University of China.

Taiyuan 030051, China

e-mail: [e-mail]

Y.-y. Sun

e-mail: [e-mail]

Y.-q. Liu, X.-y. Li

Shandong Universities, Jinan 250100, China

Y.-q. Liu, X.-y. Li

Shandong Non-Metallic Materials Research Institute,

Jinan 250031, China

X.-y, Li

China North Industries Group Corporation, Beijing 100089. China