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Effects of core material, operating temperature and time on microencapsulation by in situ polymerization method
Advanced Powder Technol., Vol. 13, No. 4, pp. 377– 394 (2002) Ó VSP and Society of Powder Technology, Japan 2002. Also available online - www.vsppub.com
Original paper Effects of core material, operating temperature and time on microencapsulation by in situ polymerization method HIROYUKI KAGE 1;¤ , HIDEAKI KAWAHARA 2 , NAOMICHI HAMADA 1 , TSUTOMU KOTAKE 1 , NOBUHIRO OE 1 and HIRONAO OGURA 1 1 Department of
Applied Chemistry, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata, Kitakyushu 804-8550, Japan 2 Kansai R&D Center, Dainippon Ink and Chemicals, Inc., 1-3 Takasago, Takaishi 592-0001, Japan Received 24 August 2001; accepted 12 February 2002 Abstract—Microencapsulation of glass beads was carried out by the in situ polymerization method. Glass beads were chosen as the core material because their size distribution was narrow and their surface was easily changed to be hydrophobic. Melamine and formaldehyde were used as membrane materials of the capsule. The respective effects of amount and size of core material, hydrophobic core surface, addition time of core material, reaction temperature, and time of encapsulation process on microcapsules were investigated systematically. The membrane thickness of microcapsules decreased with larger amounts and larger diameters of core material, while a smoother membrane was obtained with a hydrophobic surface of the core material. It became clear that the membrane growth of capsules was almost nished within 15 min after the addition of initiator; however, it did not progress up to 10 min after the initiator addition at low temperature. Keywords: Microencapsulation; glass bead; melamine; formaldehyde; in situ polymerization.
NOMENCLATURE
A Ch Cf Cm Ct Dp E L N ¤
surface area of core material per unit volume of solution (m2 /m3 ) dosage of dimethyldichlorosilane for surface treatment of core (kg/ kg-core) dosage of formaldehyde (kg/ m3 ) dosage of melamine (kg/ m3 ) total dosage of membrane materials (D Cf C Cm ) (kg/ m3 ) diameter of core material (¹m) efciency (—) membrane thickness (¹m) number of dosed particles (1 / m3 )
To whom correspondence should be addressed. E-mail: [email protected]
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P R T t ta V W
circularity of projected microcapsule (—) stirring rate (r.p.m.) temperature (K) time (min) addition time of core material (min) membrane volume (m3 /m3 ) weight of core material (kg/ m3 )
Greek ½ density of membrane (kg/ m3 )
1. INTRODUCTION
Microcapsules are already widely applied in the chemical, medical, pharmaceutical, food, dyestuff, printing, and fertilizer industries, etc. Many techniques for microencapsulation, such as coacervation, in situ polymerization and interfacial polymerization, have been developed and are commonly known. Many papers on the encapsulation techniques also have been published. However, their purposes were mainly on the development in material design. Therefore, very few papers reported systematically the relation between operational conditions and the characteristics of produced capsules in order to discuss the encapsulation mechanism in detail. The authors have investigated systematically the respective effects of operating factors on the characteristics of microcapsules prepared by the complex coacervation method and reported various experimental data to elucidate its mechanism [1– 4]. In these experiments, glass beads with a narrow size distribution [1, 2] and mono-dispersed droplets prepared by membrane emulsication [3, 4] were used as a solid core and a liquid core, respectively, in order to make a core of uniform size. Following on these reports, the microencapsulation by in situ polymerization, which is more complicated than the coacervation method, is discussed here. In this investigation, melamine and formaldehyde adopted as membrane materials [5, 6] can be supplied from the liquid phase surrounding the outside of glass beads and they microencapsulate glass beads successfully by in situ polymerization. By the adoption of a solid as a core, spherical core materials with a narrow size distribution are easily prepared and the rst emulsication, by which cores are prepared in usual microencapsulation process, can be omitted. Therefore, the microencapsulation mechanism of the in situ polymerization method is successfully investigated by completely excluding the complicated effect of the rst emulsication on microcapsules. The effects of dosages of membrane material, core material, initiator and dispersing agent, and the effects of each operational condition, such as reaction time, temperature, stirring rate, solution volume, addition time of core material, hydrophobic
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core surface, on microcapsules were investigated systematically in this investigation. Among these, respective effects of dosage of core material, core diameter, hydrophobic core surface, addition time of core, reaction time and temperature are mainly discussed here as a follow-up to the previous report [7].
2. EXPERIMENTAL
2.1. Materials The same materials as introduced in the previous paper [7] were used in microencapsulation experiment. Glass beads with a narrow size distribution, prepared by 235 and 280 mesh screens, were used as core material. They were adopted because of their spherical shape and easy treatment to obtain a hydrophobic surface. Melamine and formalin (formaldehyde concentration: 37%) were used as membrane materials for the capsule. Sodium polystyrene sulfonate and acetic acid were also used as a dispersing agent and an initiator, respectively. As the surface condition of the core material is very sensitive in microencapsulation, glass beads were used after they were immersed in concentrated sulfuric acid for 4 days and washed well by water. The mean diameter and density of glass beads were 59.3 ¹m and 2520 kg/ m3 , respectively. See the previous report [7] for details of the materials used. 2.2. Experimental procedure Melamine (0.005 kg) and formalin (0.01 kg) (weight ratio D 1 : 2) were weighed and solved by 3:5 £ 10¡5 m3 distilled water in a tall beaker. The solution in the beaker was stirred at 700 r.p.m. by a four-blade stirrer, whose axis was set on the center line of the beaker, and kept at 60 ± C in order to prepare the prepolymer of melamine and formaldehyde. Next, distilled water was added into 0.003 kg glass beads and 0.045 kg dispersing agent aqueous solution (sodium polystyrene sulfonate, 20.8%) to make the total weight of the mixture 0.1 kg. Next, it was added into the prepolymer solution. After the temperature of the system was changed from 60 ± C to the polymerization temperature (65 ± C in usual experiment) with a constant rate of temperature increase, 2 £ 10¡6 m3 initiator (10% acetic acid) was added and the polymerization started. The in situ polymerization was continued for 1 h and the glass beads were microencapsulated. The stirring rate was kept constant throughout the experimental runs. The obtained microcapsules were recovered from the liquid by ltration. Mean diameter, size distribution, standard deviation of the diameter, membrane thickness of microcapsule and circularity of microcapsules projected on a plane were measured and calculated by a image processor (ADS, PIP-4000) after the microcapsules were completely dried. The morphology of microcapsules was observed by a scanning electron microscope (SEM; TOPCON, ABT-32). Details of the measuring method and data treatment are available in the previous report [7].
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The above-mentioned experimental operation was repeated under various experimental conditions where the dosage and the diameter of the core material, the surface of the core, the addition time of the core, and the reaction time and temperature were changed systematically. 2.3. Surface treatment to produce a hydrophobic core In order to examine the effect of a hydrophobic core surface on the microencapsulation, the core surface was treated by the following method. Glass beads (0.05 kg) were immersed in 300 cm3 n-hexane after the usual washing by sulfuric acid and water, and sufcient drying. Next, an adequate amount of dimethyldichlorosilane was added into the hexane with the glass beads and they were stirred for 2 h. After stirring, supernatant liquid was removed, and the remains were mixed with an adequate amount of n-hexane and stirred again for 30 min to stop the reaction. After this washing operation was repeated 3 times, the glass beads were dried well and used as cores in microencapsulation. The hydrophobicity of the glass surface was adjusted by the added amount of dimethyldichlorosilane.
3. RESULTS AND DISCUSSION
3.1. Effect of core material dosage We examined how the added amount of core material inuenced the membrane thickness of microcapsules under constant experimental conditions. In this experiment, two of total dosages of membrane materials, 34.8 and 58.0 kg/m3 were adopted, because favorable microcapsules were obtained with these dosages as reported in the previous paper [7]. The effect of core dosage on the membrane thickness is shown in Fig. 1. The membrane thickness decreased with the core dosage for both total dosages. The thicker membrane was obtained with the higher dosage of membrane material. As many cores were dosed, the membrane thickness might become thin because of the increase of the total surface area of the cores in the system. If the total volume of polymer deposited on the core surface is kept constant independently of the core dosage, the membrane thickness has to decrease due to the expanded surface area. Thus, the relationship between the total volume of the capsule membrane and the total surface area of cores existing in the system was examined. The volume of the membrane was calculated as the product of the total surface area and averaged membrane thickness, where the core was assumed to be a sphere whose diameter and density were 59.3 ¹m and 2520 kg/m3 . The membrane volume shown in Fig. 2 increased gradually with the core dosage and appeared to converge to a maximum volume. In Fig. 2, the efciency, E, is also plotted against W . It was dened by: ¤ £ E D .¼=6/ .Dp C 2L/3 ¡ Dp3 N½=Ct ¼ Dp2 LN½=Ct : (1)
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Figure 1. Effect of core material dosage on membrane thickness.
Figure 2. Effect of core material dosage on membrane volume and efciency.
This indicates what fraction of dosed material was used for the membrane formation of microcapsules. Here, the density of the membrane was assumed to be 1500 kg/ m3 , according to the literature [8]. The efciency also increased gradually with the dosage of the core and it shows that the wider core surface area in the system promotes the utilization of coating materials effectively. In this series of experimental runs, the highest efciency, which was obtained at Ct D 58:0 kg/ m3 and W D 200 kg/ m3 , was 0.17. This relatively low efciency implies that a large amount of membrane material is not
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used for membrane formation but remains in the liquid phase. In practice, after the produced microcapsules were removed by ltration, the ltrate appeared to be cloudy and many small spheres, with a diameter of about 1 ¹m, were observed. Although the circularity, size distribution and standard deviation of capsules were also measured in this experiment, they were almost independent of the core dosage. 3.2. Effect of core diameter Next, the effect of core size on microencapsulation was examined. As microencapsulation takes place on the core surface, the experiment was conducted under conditions where the total surface area of core material was kept constant. The dosed amount of core material was adjusted so as that the core surface area in the liquid phase was kept at 800 m2 /m3 . According to this method, the mass of the dosed core increases with the core size. The measured membrane thickness and circularity are shown in Fig. 3. The efciency, E, is also plotted in Fig. 3. The membrane became thinner and the efciency decreased rapidly with the increase of core size. However, the circularity became worse with the increase core size and shifted from the spherical circularity. The dotted line in Fig. 3 indicates the circularity of the core material. Figure 4 shows microphotographs of microcapsules with the cores whose diameters were 59.3, 80.0 and 160 ¹m. As shown in Fig. 4, polymer drops adhered on the core unevenly and did not cover the whole core surface when large glass beads were used as the core. This uneven adhesion of polymer made the measured membrane thickness and circularity small. When the larger core was used, the number of cores became smaller and then each core had a larger surface area. Then, the core surface existing in the system is dispersed more unevenly in the liquid phase. This uneven spread of surface is thought to prevent the even supply of coating material on the core surface.
Figure 3. Effect of core size on membrane thickness, efciency and circularity.
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(A)
(B) Figure 4. Microphotographs of microcapsules. (A) Dp D 59:3 ¹m. (B) Dp D 80:0 ¹m. (C) Dp D 160 ¹m.
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(C) Figure 4. (Continued).
Figure 5. Effect of hydrophobic core surface on membrane thickness and circularity.
3.3. Effect of the hydrophobic surface of the core material To investigate the effect of the surface state of the core material on the microencapsulation, the surface of the glass beads was treated by dimethyldichlorosilane and they were then used as cores. Figure 5 shows the relationship between the amount of dimethyldichlorosilane used and the membrane thickness or the circularity of the obtained microcapsules. The membrane thickness decreased suddenly with the
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(A)
(B) Figure 6. Effect of hydrophobic core surface on microcapsule morphology. (A) Ch D 0 kg/ kg-core. (B) Ch D 0:04 kg/ kg-core. (C) Ch D 0:12 kg/ kg-core.
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(C) Figure 6. (Continued).
hydrophobic core surface, but was almost constant independent of the degree of hydrophobicity. However, no change in circularity was observed. Microphotographs of the capsules obtained using normal cores and two kinds of hydrophobic cores are shown in Fig. 6. It is clearly conrmed that the drop-like surface formed by polymers changes to a at coating layer by treating the core surface and that the surface state of the core strongly affects the capsule properties. Comparing the photographs of the two kinds of hydrophobic cores, the membrane surface is found to be smoother at stronger hydrophobicity. The hydrophobic surface did not inuence the standard deviation of the capsule diameters. 3.4. Effect of reaction temperature The effect of operating temperature was examined by the microencapsulation performed at 303, 313, 323, 338 and 348 K. In these experiments, the temperature was changed at a constant rising or cooling rate, 0.5 K/s, from 333 K to each reaction temperature. The initiator was dosed after the system reached each required reaction temperature. Microphotographs of capsules obtained at these reaction temperatures are shown in Fig. 7. The membrane formed at low temperature was thin and an incomplete membrane was observed (Fig. 7A). As the temperature rose, the membrane covering the core surface gradually thickened and became more elaborate (Fig. 7B and C). However, at 348 K, many polymer drops adhered on the
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(A)
(B) Figure 7. Temperature dependency of microcapsule morphology. (A) T D 313 K. (B) T D 323 K. (C) T D 338 K. (D) T D 348 K.
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(C)
(D) Figure 7. (Continued).
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membrane formed directly on the core, as shown in Fig. 7D. At temperatures higher than 348 K the viscosity of the solution increased remarkably and many grown spherical polymers were observed even in the liquid phase. On the other hand, at temperatures lower than 303 K the polymerization did not progress and no coating membrane was formed on the core. Figure 8 indicates the membrane thickness and circularity of microcapsules obtained by these experiments. From Fig. 8 it was conrmed that the polymerization reaction became active and the membrane thickness increased with temperatures higher than 303 K. The circularity shown in Fig. 8 is almost constant and almost the same as that of the core material in the low temperature region; however, it decreases at temperatures higher than 340 K because of the multi-layer adhesion of polymer drops as shown in Fig. 7D. In summary, higher reaction temperatures give us microcapsules with thicker and more elaborate coating membranes, but temperatures higher than 348 K are too high to produce excellent microencapsulation because of high liquid viscosity and the rough membrane surface due to very active reactions. 3.5. Effect of addition time of the core material In the encapsulation procedure introduced up to now, the initiator was dosed after the addition of core material, so the cores existing in the system were encapsulated by the polymerization. In this section, in contrast, the initiator was dosed into the prepolymer solution of melamine and formaldehyde to start the polymerization reaction before the addition of core material. By this procedure, information about the time limitation of core addition for successful microencapsulation can be obtained. The experiment was performed with various times of addition of cores under three reaction temperatures, 328, 338 and 348 K. The addition time of core material, ta , was dened as the time measured from the moment the initiator was dosed. The membrane thickness measured in this experiment is shown in Fig. 9. The membrane thicknesses obtained at 338 and 348 K decreased rapidly in the early stage of ta . However, they were lowered gently with ta values larger than 15 min. From this result, it is judged that the membrane growth on microcapsules almost nished before ta D 15 min and that the core added after this time could not be microencapsulated sufciently. The time dependency of membrane thickness up to 15 min was larger for 348 than 338 K and this result implies that the reaction progressed more actively at the higher temperature. On the other hand, at the lowest temperature, 328 K, the membrane thickness remained constant and then the encapsulation did not progress at all in the early stage of ta . In the following stage, the membrane thickness decreased rapidly and then gently with ta similar to the other temperatures. From the same experiments, the circularity and the standard deviation of the capsule diameter were also conrmed not to be affected by the addition time of the core material.
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Figure 8. Temperature dependency of membrane thickness and circularity.
Figure 9. Relationship between membrane thickness and addition time of core material.
3.6. Effect of reaction time In the previous section, it was found that 15 min after the start of polymerization was very important for the microencapsulation and that the growth of capsule membranes at 328 K was much different from that at higher temperatures. Thus, the effect of reaction time on the encapsulation was examined here. In this experiment, the core was added in advance of initiator and various experimental times shorter than 1 h were adopted under the three reaction temperatures, 328, 338 and 348 K. The growth of membrane in this experiment is shown in Fig. 10. At 338 and 348 K the membrane grew quickly for a few minutes immediately after the start of polymerization and its growth became mild gradually. The change of membrane thickness almost nished within about 10 min for 338 K and 15 min for 348 K. However, at 328 K the membrane was not formed for nearly 10 min. After
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Figure 10. Growth of capsule membrane with time.
Figure 11. Effect of stirring rate on membrane thickness at 348 K.
that, the membrane thickness began to increase rapidly and the membrane growth became gentle gradually. These results agreed very well with those introduced in Section 3.5. To investigate the relationship between the reaction temperature and membrane thickness in detail, the experiments were repeated with different stirring rates. Experimental temperatures of 328 and 348 K were chosen because the membrane thickness at these temperatures had different time dependencies, as introduced in Fig. 10. The results for 348 and for 328 K are shown in Figs 11 and 12, respectively. At 348 K, the growth of membrane was promoted with a faster stirring rate and it appeared to converge with a stirring rate faster than 700 r.p.m. In Fig. 12, it is clearly veried that the stirring rate at 328 K remarkably inuenced the delay of microencapsulation and it shortened the delay time. However, the growth of membranes after the delay was not improved so much by the stirring rate.
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Figure 12. Effect of stirring rate on membrane thickness at 328 K.
Figure 13. Illustration of microencapsulation mechanism.
According to the experimental results introduced up to now, microencapsulation is considered to progress by a two-stage mechanism as shown in Fig. 13. By the addition of initiator, some prepolymers composed of melamine and formaldehyde gather each other to make a small cluster (rst stage). Next, the clusters adhere on the core surface and grow up to polymer drops by the activated polymerization reaction. Therefore, the encapsulation is delayed at the lower temperature, because the rst stage reaction for making clusters is sensitively decelerated. However, as shown in Fig. 12, the delay of encapsulation can be improved when this rst stage
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reaction is accelerated by stronger stirring. The delay of encapsulation disappears at higher temperatures, because the reaction in the rst stage becomes very rapid. When the cores treated by dimethyldichlorosilane are dosed in the system, the molecules of membrane materials are easily adsorbed directly on the hydrophobic surface of cores. Then, the polymerization reaction may progress actively not only on the clusters, but also on the hydrophobic surface of the cores. This hypothesis agrees well with the observed result that hydrophobic cores made smooth capsule membranes and they changed from drop-like surfaces to at coating layers. 4. CONCLUSION
Microencapsulation of solid cores with a narrow size distribution was performed by the in situ polymerization method with melamine and formaldehyde. Spherical glass beads were microencapsulated systematically under various experimental conditions, and the respective effects of dosage, diameter, surface and addition time of core material, and reaction time and temperature on microencapsulation were investigated. The following conclusions were drawn from the experimental results: (1) As the core dosage increased, the membrane thickness of the capsules became thinner. The total volume of the membrane and the utilized efciency of membrane materials increased gradually with the core dosage and the maximum efciency was 0.17 in these experiments. (2) The membrane thickness and the efciency of membrane materials decreased as the larger core material was dosed under conditions where the total surface area of the core material in the system was kept constant. (3) Using hydrophobic core surfaces, the membrane formed on the cores became smooth and changed from a drop-like surface to a at coating layer. The membrane thickness decreased with the hydrophobic surface, but the circularity was almost constant. (4) Microcapsules with thicker and more elaborate coating membranes were obtained as the reaction temperature rose. However, temperatures higher than 348 K were too high for good microencapsulation because of the high liquid viscosity and rough membrane surface. (5) The growth of capsule membranes nished within 15 min after the addition of initiator. Core material added later than 15 min was impossible to be microencapsulated sufciently. At 328 K the encapsulation did not progress at all until 10 min. These experimental results imply that the microencapsulation progresses by a two-stage mechanism. REFERENCES 1. H. Kage, N. Yada, M. Kunimasa, H. Ogura and Y. Matsuno, Membrane thickness of microcapsules generated by complex coacervation method, Kagaku Kogaku Ronbunshu 22, 342– 349 (1996).
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2. H. Kage, M. Kunimasa, H. Kawahara, H. Ogura and Y. Matsuno, Operating condition and membrane thickness of microcapsules generated by complex coacervation method, Kagaku Kogaku Ronbunshu 22, 365– 371 (1996). 3. H. Kage, H. Kawahara, H. Ogura and Y. Matsuno, Complex coacervation microencapsulation of mono-dispersed droplets prepared by membrane emulsication, Kagaku Kogaku Ronbunshu 23, 652– 658 (1997). 4. H. Kage, H. Kawahara, H. Ogura and Y. Matsuno, Microencapsulationof mono-disperseddroplets by complex coacervation method and membrane thickness of generated capsules, Kagaku Kogaku Ronbunshu 23, 659– 665 (1997). 5. Y. Hoshi and T. Hayashi, Bisho Capsule no Seizou Houhou, Japanese Patent Disclosure S5651238 (1981). 6. S. Washisu, Y. Saeki and S. Tatsuta, Microcapsule no Seizou Houhou, Japanese Patent Disclosure S62-129141 (1987). 7. H. Kage, H. Kawahara, N. Hamada, T. Kotake and H. Ogura, Operating condition and microcapsules generated by in situ polymerization method, Advanced Powder Technol. 13, 265– 285 (2002). 8. T. Mitsuwa, in: Kiso Gosei-jushi no Kagaku, pp. 263– 265. Gihodo, Tokyo (1975).
Original paper Effects of core material, operating temperature and time on microencapsulation by in situ polymerization method HIROYUKI KAGE 1;¤ , HIDEAKI KAWAHARA 2 , NAOMICHI HAMADA 1 , TSUTOMU KOTAKE 1 , NOBUHIRO OE 1 and HIRONAO OGURA 1 1 Department of
Applied Chemistry, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata, Kitakyushu 804-8550, Japan 2 Kansai R&D Center, Dainippon Ink and Chemicals, Inc., 1-3 Takasago, Takaishi 592-0001, Japan Received 24 August 2001; accepted 12 February 2002 Abstract—Microencapsulation of glass beads was carried out by the in situ polymerization method. Glass beads were chosen as the core material because their size distribution was narrow and their surface was easily changed to be hydrophobic. Melamine and formaldehyde were used as membrane materials of the capsule. The respective effects of amount and size of core material, hydrophobic core surface, addition time of core material, reaction temperature, and time of encapsulation process on microcapsules were investigated systematically. The membrane thickness of microcapsules decreased with larger amounts and larger diameters of core material, while a smoother membrane was obtained with a hydrophobic surface of the core material. It became clear that the membrane growth of capsules was almost nished within 15 min after the addition of initiator; however, it did not progress up to 10 min after the initiator addition at low temperature. Keywords: Microencapsulation; glass bead; melamine; formaldehyde; in situ polymerization.
NOMENCLATURE
A Ch Cf Cm Ct Dp E L N ¤
surface area of core material per unit volume of solution (m2 /m3 ) dosage of dimethyldichlorosilane for surface treatment of core (kg/ kg-core) dosage of formaldehyde (kg/ m3 ) dosage of melamine (kg/ m3 ) total dosage of membrane materials (D Cf C Cm ) (kg/ m3 ) diameter of core material (¹m) efciency (—) membrane thickness (¹m) number of dosed particles (1 / m3 )
To whom correspondence should be addressed. E-mail: [email protected]
H. Kage et al.
378
P R T t ta V W
circularity of projected microcapsule (—) stirring rate (r.p.m.) temperature (K) time (min) addition time of core material (min) membrane volume (m3 /m3 ) weight of core material (kg/ m3 )
Greek ½ density of membrane (kg/ m3 )
1. INTRODUCTION
Microcapsules are already widely applied in the chemical, medical, pharmaceutical, food, dyestuff, printing, and fertilizer industries, etc. Many techniques for microencapsulation, such as coacervation, in situ polymerization and interfacial polymerization, have been developed and are commonly known. Many papers on the encapsulation techniques also have been published. However, their purposes were mainly on the development in material design. Therefore, very few papers reported systematically the relation between operational conditions and the characteristics of produced capsules in order to discuss the encapsulation mechanism in detail. The authors have investigated systematically the respective effects of operating factors on the characteristics of microcapsules prepared by the complex coacervation method and reported various experimental data to elucidate its mechanism [1– 4]. In these experiments, glass beads with a narrow size distribution [1, 2] and mono-dispersed droplets prepared by membrane emulsication [3, 4] were used as a solid core and a liquid core, respectively, in order to make a core of uniform size. Following on these reports, the microencapsulation by in situ polymerization, which is more complicated than the coacervation method, is discussed here. In this investigation, melamine and formaldehyde adopted as membrane materials [5, 6] can be supplied from the liquid phase surrounding the outside of glass beads and they microencapsulate glass beads successfully by in situ polymerization. By the adoption of a solid as a core, spherical core materials with a narrow size distribution are easily prepared and the rst emulsication, by which cores are prepared in usual microencapsulation process, can be omitted. Therefore, the microencapsulation mechanism of the in situ polymerization method is successfully investigated by completely excluding the complicated effect of the rst emulsication on microcapsules. The effects of dosages of membrane material, core material, initiator and dispersing agent, and the effects of each operational condition, such as reaction time, temperature, stirring rate, solution volume, addition time of core material, hydrophobic
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core surface, on microcapsules were investigated systematically in this investigation. Among these, respective effects of dosage of core material, core diameter, hydrophobic core surface, addition time of core, reaction time and temperature are mainly discussed here as a follow-up to the previous report [7].
2. EXPERIMENTAL
2.1. Materials The same materials as introduced in the previous paper [7] were used in microencapsulation experiment. Glass beads with a narrow size distribution, prepared by 235 and 280 mesh screens, were used as core material. They were adopted because of their spherical shape and easy treatment to obtain a hydrophobic surface. Melamine and formalin (formaldehyde concentration: 37%) were used as membrane materials for the capsule. Sodium polystyrene sulfonate and acetic acid were also used as a dispersing agent and an initiator, respectively. As the surface condition of the core material is very sensitive in microencapsulation, glass beads were used after they were immersed in concentrated sulfuric acid for 4 days and washed well by water. The mean diameter and density of glass beads were 59.3 ¹m and 2520 kg/ m3 , respectively. See the previous report [7] for details of the materials used. 2.2. Experimental procedure Melamine (0.005 kg) and formalin (0.01 kg) (weight ratio D 1 : 2) were weighed and solved by 3:5 £ 10¡5 m3 distilled water in a tall beaker. The solution in the beaker was stirred at 700 r.p.m. by a four-blade stirrer, whose axis was set on the center line of the beaker, and kept at 60 ± C in order to prepare the prepolymer of melamine and formaldehyde. Next, distilled water was added into 0.003 kg glass beads and 0.045 kg dispersing agent aqueous solution (sodium polystyrene sulfonate, 20.8%) to make the total weight of the mixture 0.1 kg. Next, it was added into the prepolymer solution. After the temperature of the system was changed from 60 ± C to the polymerization temperature (65 ± C in usual experiment) with a constant rate of temperature increase, 2 £ 10¡6 m3 initiator (10% acetic acid) was added and the polymerization started. The in situ polymerization was continued for 1 h and the glass beads were microencapsulated. The stirring rate was kept constant throughout the experimental runs. The obtained microcapsules were recovered from the liquid by ltration. Mean diameter, size distribution, standard deviation of the diameter, membrane thickness of microcapsule and circularity of microcapsules projected on a plane were measured and calculated by a image processor (ADS, PIP-4000) after the microcapsules were completely dried. The morphology of microcapsules was observed by a scanning electron microscope (SEM; TOPCON, ABT-32). Details of the measuring method and data treatment are available in the previous report [7].
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The above-mentioned experimental operation was repeated under various experimental conditions where the dosage and the diameter of the core material, the surface of the core, the addition time of the core, and the reaction time and temperature were changed systematically. 2.3. Surface treatment to produce a hydrophobic core In order to examine the effect of a hydrophobic core surface on the microencapsulation, the core surface was treated by the following method. Glass beads (0.05 kg) were immersed in 300 cm3 n-hexane after the usual washing by sulfuric acid and water, and sufcient drying. Next, an adequate amount of dimethyldichlorosilane was added into the hexane with the glass beads and they were stirred for 2 h. After stirring, supernatant liquid was removed, and the remains were mixed with an adequate amount of n-hexane and stirred again for 30 min to stop the reaction. After this washing operation was repeated 3 times, the glass beads were dried well and used as cores in microencapsulation. The hydrophobicity of the glass surface was adjusted by the added amount of dimethyldichlorosilane.
3. RESULTS AND DISCUSSION
3.1. Effect of core material dosage We examined how the added amount of core material inuenced the membrane thickness of microcapsules under constant experimental conditions. In this experiment, two of total dosages of membrane materials, 34.8 and 58.0 kg/m3 were adopted, because favorable microcapsules were obtained with these dosages as reported in the previous paper [7]. The effect of core dosage on the membrane thickness is shown in Fig. 1. The membrane thickness decreased with the core dosage for both total dosages. The thicker membrane was obtained with the higher dosage of membrane material. As many cores were dosed, the membrane thickness might become thin because of the increase of the total surface area of the cores in the system. If the total volume of polymer deposited on the core surface is kept constant independently of the core dosage, the membrane thickness has to decrease due to the expanded surface area. Thus, the relationship between the total volume of the capsule membrane and the total surface area of cores existing in the system was examined. The volume of the membrane was calculated as the product of the total surface area and averaged membrane thickness, where the core was assumed to be a sphere whose diameter and density were 59.3 ¹m and 2520 kg/m3 . The membrane volume shown in Fig. 2 increased gradually with the core dosage and appeared to converge to a maximum volume. In Fig. 2, the efciency, E, is also plotted against W . It was dened by: ¤ £ E D .¼=6/ .Dp C 2L/3 ¡ Dp3 N½=Ct ¼ Dp2 LN½=Ct : (1)
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Figure 1. Effect of core material dosage on membrane thickness.
Figure 2. Effect of core material dosage on membrane volume and efciency.
This indicates what fraction of dosed material was used for the membrane formation of microcapsules. Here, the density of the membrane was assumed to be 1500 kg/ m3 , according to the literature [8]. The efciency also increased gradually with the dosage of the core and it shows that the wider core surface area in the system promotes the utilization of coating materials effectively. In this series of experimental runs, the highest efciency, which was obtained at Ct D 58:0 kg/ m3 and W D 200 kg/ m3 , was 0.17. This relatively low efciency implies that a large amount of membrane material is not
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used for membrane formation but remains in the liquid phase. In practice, after the produced microcapsules were removed by ltration, the ltrate appeared to be cloudy and many small spheres, with a diameter of about 1 ¹m, were observed. Although the circularity, size distribution and standard deviation of capsules were also measured in this experiment, they were almost independent of the core dosage. 3.2. Effect of core diameter Next, the effect of core size on microencapsulation was examined. As microencapsulation takes place on the core surface, the experiment was conducted under conditions where the total surface area of core material was kept constant. The dosed amount of core material was adjusted so as that the core surface area in the liquid phase was kept at 800 m2 /m3 . According to this method, the mass of the dosed core increases with the core size. The measured membrane thickness and circularity are shown in Fig. 3. The efciency, E, is also plotted in Fig. 3. The membrane became thinner and the efciency decreased rapidly with the increase of core size. However, the circularity became worse with the increase core size and shifted from the spherical circularity. The dotted line in Fig. 3 indicates the circularity of the core material. Figure 4 shows microphotographs of microcapsules with the cores whose diameters were 59.3, 80.0 and 160 ¹m. As shown in Fig. 4, polymer drops adhered on the core unevenly and did not cover the whole core surface when large glass beads were used as the core. This uneven adhesion of polymer made the measured membrane thickness and circularity small. When the larger core was used, the number of cores became smaller and then each core had a larger surface area. Then, the core surface existing in the system is dispersed more unevenly in the liquid phase. This uneven spread of surface is thought to prevent the even supply of coating material on the core surface.
Figure 3. Effect of core size on membrane thickness, efciency and circularity.
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(B) Figure 4. Microphotographs of microcapsules. (A) Dp D 59:3 ¹m. (B) Dp D 80:0 ¹m. (C) Dp D 160 ¹m.
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(C) Figure 4. (Continued).
Figure 5. Effect of hydrophobic core surface on membrane thickness and circularity.
3.3. Effect of the hydrophobic surface of the core material To investigate the effect of the surface state of the core material on the microencapsulation, the surface of the glass beads was treated by dimethyldichlorosilane and they were then used as cores. Figure 5 shows the relationship between the amount of dimethyldichlorosilane used and the membrane thickness or the circularity of the obtained microcapsules. The membrane thickness decreased suddenly with the
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(B) Figure 6. Effect of hydrophobic core surface on microcapsule morphology. (A) Ch D 0 kg/ kg-core. (B) Ch D 0:04 kg/ kg-core. (C) Ch D 0:12 kg/ kg-core.
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(C) Figure 6. (Continued).
hydrophobic core surface, but was almost constant independent of the degree of hydrophobicity. However, no change in circularity was observed. Microphotographs of the capsules obtained using normal cores and two kinds of hydrophobic cores are shown in Fig. 6. It is clearly conrmed that the drop-like surface formed by polymers changes to a at coating layer by treating the core surface and that the surface state of the core strongly affects the capsule properties. Comparing the photographs of the two kinds of hydrophobic cores, the membrane surface is found to be smoother at stronger hydrophobicity. The hydrophobic surface did not inuence the standard deviation of the capsule diameters. 3.4. Effect of reaction temperature The effect of operating temperature was examined by the microencapsulation performed at 303, 313, 323, 338 and 348 K. In these experiments, the temperature was changed at a constant rising or cooling rate, 0.5 K/s, from 333 K to each reaction temperature. The initiator was dosed after the system reached each required reaction temperature. Microphotographs of capsules obtained at these reaction temperatures are shown in Fig. 7. The membrane formed at low temperature was thin and an incomplete membrane was observed (Fig. 7A). As the temperature rose, the membrane covering the core surface gradually thickened and became more elaborate (Fig. 7B and C). However, at 348 K, many polymer drops adhered on the
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(B) Figure 7. Temperature dependency of microcapsule morphology. (A) T D 313 K. (B) T D 323 K. (C) T D 338 K. (D) T D 348 K.
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(C)
(D) Figure 7. (Continued).
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membrane formed directly on the core, as shown in Fig. 7D. At temperatures higher than 348 K the viscosity of the solution increased remarkably and many grown spherical polymers were observed even in the liquid phase. On the other hand, at temperatures lower than 303 K the polymerization did not progress and no coating membrane was formed on the core. Figure 8 indicates the membrane thickness and circularity of microcapsules obtained by these experiments. From Fig. 8 it was conrmed that the polymerization reaction became active and the membrane thickness increased with temperatures higher than 303 K. The circularity shown in Fig. 8 is almost constant and almost the same as that of the core material in the low temperature region; however, it decreases at temperatures higher than 340 K because of the multi-layer adhesion of polymer drops as shown in Fig. 7D. In summary, higher reaction temperatures give us microcapsules with thicker and more elaborate coating membranes, but temperatures higher than 348 K are too high to produce excellent microencapsulation because of high liquid viscosity and the rough membrane surface due to very active reactions. 3.5. Effect of addition time of the core material In the encapsulation procedure introduced up to now, the initiator was dosed after the addition of core material, so the cores existing in the system were encapsulated by the polymerization. In this section, in contrast, the initiator was dosed into the prepolymer solution of melamine and formaldehyde to start the polymerization reaction before the addition of core material. By this procedure, information about the time limitation of core addition for successful microencapsulation can be obtained. The experiment was performed with various times of addition of cores under three reaction temperatures, 328, 338 and 348 K. The addition time of core material, ta , was dened as the time measured from the moment the initiator was dosed. The membrane thickness measured in this experiment is shown in Fig. 9. The membrane thicknesses obtained at 338 and 348 K decreased rapidly in the early stage of ta . However, they were lowered gently with ta values larger than 15 min. From this result, it is judged that the membrane growth on microcapsules almost nished before ta D 15 min and that the core added after this time could not be microencapsulated sufciently. The time dependency of membrane thickness up to 15 min was larger for 348 than 338 K and this result implies that the reaction progressed more actively at the higher temperature. On the other hand, at the lowest temperature, 328 K, the membrane thickness remained constant and then the encapsulation did not progress at all in the early stage of ta . In the following stage, the membrane thickness decreased rapidly and then gently with ta similar to the other temperatures. From the same experiments, the circularity and the standard deviation of the capsule diameter were also conrmed not to be affected by the addition time of the core material.
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Figure 8. Temperature dependency of membrane thickness and circularity.
Figure 9. Relationship between membrane thickness and addition time of core material.
3.6. Effect of reaction time In the previous section, it was found that 15 min after the start of polymerization was very important for the microencapsulation and that the growth of capsule membranes at 328 K was much different from that at higher temperatures. Thus, the effect of reaction time on the encapsulation was examined here. In this experiment, the core was added in advance of initiator and various experimental times shorter than 1 h were adopted under the three reaction temperatures, 328, 338 and 348 K. The growth of membrane in this experiment is shown in Fig. 10. At 338 and 348 K the membrane grew quickly for a few minutes immediately after the start of polymerization and its growth became mild gradually. The change of membrane thickness almost nished within about 10 min for 338 K and 15 min for 348 K. However, at 328 K the membrane was not formed for nearly 10 min. After
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Figure 10. Growth of capsule membrane with time.
Figure 11. Effect of stirring rate on membrane thickness at 348 K.
that, the membrane thickness began to increase rapidly and the membrane growth became gentle gradually. These results agreed very well with those introduced in Section 3.5. To investigate the relationship between the reaction temperature and membrane thickness in detail, the experiments were repeated with different stirring rates. Experimental temperatures of 328 and 348 K were chosen because the membrane thickness at these temperatures had different time dependencies, as introduced in Fig. 10. The results for 348 and for 328 K are shown in Figs 11 and 12, respectively. At 348 K, the growth of membrane was promoted with a faster stirring rate and it appeared to converge with a stirring rate faster than 700 r.p.m. In Fig. 12, it is clearly veried that the stirring rate at 328 K remarkably inuenced the delay of microencapsulation and it shortened the delay time. However, the growth of membranes after the delay was not improved so much by the stirring rate.
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Figure 12. Effect of stirring rate on membrane thickness at 328 K.
Figure 13. Illustration of microencapsulation mechanism.
According to the experimental results introduced up to now, microencapsulation is considered to progress by a two-stage mechanism as shown in Fig. 13. By the addition of initiator, some prepolymers composed of melamine and formaldehyde gather each other to make a small cluster (rst stage). Next, the clusters adhere on the core surface and grow up to polymer drops by the activated polymerization reaction. Therefore, the encapsulation is delayed at the lower temperature, because the rst stage reaction for making clusters is sensitively decelerated. However, as shown in Fig. 12, the delay of encapsulation can be improved when this rst stage
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reaction is accelerated by stronger stirring. The delay of encapsulation disappears at higher temperatures, because the reaction in the rst stage becomes very rapid. When the cores treated by dimethyldichlorosilane are dosed in the system, the molecules of membrane materials are easily adsorbed directly on the hydrophobic surface of cores. Then, the polymerization reaction may progress actively not only on the clusters, but also on the hydrophobic surface of the cores. This hypothesis agrees well with the observed result that hydrophobic cores made smooth capsule membranes and they changed from drop-like surfaces to at coating layers. 4. CONCLUSION
Microencapsulation of solid cores with a narrow size distribution was performed by the in situ polymerization method with melamine and formaldehyde. Spherical glass beads were microencapsulated systematically under various experimental conditions, and the respective effects of dosage, diameter, surface and addition time of core material, and reaction time and temperature on microencapsulation were investigated. The following conclusions were drawn from the experimental results: (1) As the core dosage increased, the membrane thickness of the capsules became thinner. The total volume of the membrane and the utilized efciency of membrane materials increased gradually with the core dosage and the maximum efciency was 0.17 in these experiments. (2) The membrane thickness and the efciency of membrane materials decreased as the larger core material was dosed under conditions where the total surface area of the core material in the system was kept constant. (3) Using hydrophobic core surfaces, the membrane formed on the cores became smooth and changed from a drop-like surface to a at coating layer. The membrane thickness decreased with the hydrophobic surface, but the circularity was almost constant. (4) Microcapsules with thicker and more elaborate coating membranes were obtained as the reaction temperature rose. However, temperatures higher than 348 K were too high for good microencapsulation because of the high liquid viscosity and rough membrane surface. (5) The growth of capsule membranes nished within 15 min after the addition of initiator. Core material added later than 15 min was impossible to be microencapsulated sufciently. At 328 K the encapsulation did not progress at all until 10 min. These experimental results imply that the microencapsulation progresses by a two-stage mechanism. REFERENCES 1. H. Kage, N. Yada, M. Kunimasa, H. Ogura and Y. Matsuno, Membrane thickness of microcapsules generated by complex coacervation method, Kagaku Kogaku Ronbunshu 22, 342– 349 (1996).
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2. H. Kage, M. Kunimasa, H. Kawahara, H. Ogura and Y. Matsuno, Operating condition and membrane thickness of microcapsules generated by complex coacervation method, Kagaku Kogaku Ronbunshu 22, 365– 371 (1996). 3. H. Kage, H. Kawahara, H. Ogura and Y. Matsuno, Complex coacervation microencapsulation of mono-dispersed droplets prepared by membrane emulsication, Kagaku Kogaku Ronbunshu 23, 652– 658 (1997). 4. H. Kage, H. Kawahara, H. Ogura and Y. Matsuno, Microencapsulationof mono-disperseddroplets by complex coacervation method and membrane thickness of generated capsules, Kagaku Kogaku Ronbunshu 23, 659– 665 (1997). 5. Y. Hoshi and T. Hayashi, Bisho Capsule no Seizou Houhou, Japanese Patent Disclosure S5651238 (1981). 6. S. Washisu, Y. Saeki and S. Tatsuta, Microcapsule no Seizou Houhou, Japanese Patent Disclosure S62-129141 (1987). 7. H. Kage, H. Kawahara, N. Hamada, T. Kotake and H. Ogura, Operating condition and microcapsules generated by in situ polymerization method, Advanced Powder Technol. 13, 265– 285 (2002). 8. T. Mitsuwa, in: Kiso Gosei-jushi no Kagaku, pp. 263– 265. Gihodo, Tokyo (1975).