Journal of Colloid and Interface Science 349 (2010) 392–401

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Journal of Colloid and Interface Science

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Preparation of uniform monomer droplets using packed column and continuouspolymerization in tube reactor

Masahiro Yasuda a,*, Takashi Goda a, Hiroyasu Ogino a, Wilhelm Robert Glomm b, Hiroaki Takayanagi c

a Department of Chemical Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japanb Department of Chemical Engineering, Norwegian University of Science and Technology, Sem Sælands vei 4, Trondheim N-7491, Norwayc Separation Materials Department, Mitsubishi Chemical Corporation, 14-1 Shiba 4-chome, Minato-ku, Tokyo 108-0014, Japan

a r t i c l e i n f o

Article history:Received 18 February 2010Accepted 17 May 2010Available online 21 May 2010

Keywords:Suspension polymerizationGlass beads packed columnMonomer dropletNarrow droplet size distributionContinuous tube reactor

0021-9797/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.jcis.2010.05.060

* Corresponding author. Tel.: +81 72 254 9299; faxE-mail address: yasuda@chemeng.osakafu-u.ac.jp

a b s t r a c t

A two-step continuous emulsification and polymerization process was developed in which monomerdroplets having narrow size distribution were prepared and polymerized while retaining their monodis-persity. In the emulsification step, a column packed with glass beads, of diameters ranging from 70 lm to1 mm, was used to prepare a monomer O/W emulsion. Monomer droplets were dispersed with an aque-ous solution of poly(vinyl alcohol) (PVA). The droplet size and -distribution was studied with respect tothe effects of diameter of glass beads, concentration of PVA in water phase, degree of polymerization ofPVA, ratio of mass flow of water phase to that of oil phase, linear velocity of water phase and viscosity ofwater phase and oil phase. Droplet size was found to be strongly dependent on the diameter of thepacked glass beads, while the droplet size distribution was affected by the viscosities of the continuousand dispersed phases. Increasing the viscosity of the dispersed phase by addition of poly(styrene) to themonomer mixture resulted in a narrow size distribution of glycidyl methacrylate–ethylene glycoldimethacrylate droplets. Furthermore, these initiator-containing monomer droplets were polymerizedby heating in a tubular reactor, from which polymer particles with a narrow size distribution could besynthesized.

� 2010 Elsevier Inc. All rights reserved.

1. Introduction

Several heterogeneous polymerization processes have beenadopted for the preparation of polymer particles, including emul-sion, soap-free emulsion, dispersion, precipitation and suspensionpolymerization. Suspension polymerization is particularly well-suited for the production of large polymer particles with diametersranging from 5 to 1000 lm [1–3]. The produced particles can be di-rectly utilized in applications such as ion-exchange resin [4,5],immobilization support for enzymes [6,7], copy machine toner[8], carrier for drug delivery system [9], spacer for liquid crystaldisplay [10], and cosmetics [11], or converted various polymermaterials after molding. In this polymerization system, a mono-mer-soluble, water-insoluble initiator was selected, with the vol-ume ratio of monomer mixture to water phase typically variedbetween 0.1 and 0.5. The monomer mixture was suspended inwater by stirring and a suitable suspension agent. By varying theagitation speed or the concentration and species of the suspensionagent, the droplet size could be adjusted to the desired level. Uponincubation at 20–90 �C, the polymerization was carried out. A

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droplet formed by stirring is converted to a particle if aggregationand droplet division do not occur [2,3].

Batch stirred tank reactors are generally used for suspensionpolymerization due to various benefits, such as low cost of reactor,general versatility, well-studied scale-up techniques, and goodoperation performance. However, the shear stress generated fromthe stirring blade or baffle is heterogeneous and has complex dis-tribution in the stirred tank reactor [12,13]. Therefore, dropletbreakup and coalescence occurs randomly, and droplet sizes varyaccording to a normal probability distribution. When the polymercontent in the droplet exceeds a threshold degree of polymeriza-tion, droplet viscosity increases drastically, and droplet breakupis restricted. Upon exceeding this threshold value, the dropletundergoes a phase transition from liquid to gel. The size distribu-tion of the polymer particles is inherited from that of the dropletsat the point of phase transition and is widely spread. To make par-ticle size distribution more narrow, reactor design, shape, andnumber of the stirring blades, as well as chemical species and con-centration of the suspension agent have been studied. However,since these parameters also affect the mean diameter of producedparticles, it is very difficult to adjust particle size while retaining anarrow distribution [14]. This is due to emulsification and poly-merization in particles occurring simultaneously in suspensionpolymerization.

Column packed with glass particles

Monomer mixture (Oil phase reservoir)

PVA aqueous solution (Water phase reservoir)

Plungerpump

Emulsion reservoir

Peristaltic pump

T-junction

A

C

D

Strainer

Peristaltic pump

Air

Product reservoir

Water bath

Poly(ethylene) tube reactor

B

E

F

G

H

Fig. 1. Emulsification and polymerization apparatus.

M. Yasuda et al. / Journal of Colloid and Interface Science 349 (2010) 392–401 393

In order to control the droplet or particle diameter and attain anarrow size distribution, a new process in which emulsificationand polymerization is divided in two steps has been developed.For this purpose, the monomer emulsion is prepared in the pres-ence of a suspension agent as a first step and then, the producedemulsion is polymerized under mild stirring conditions. To emul-sify the monomer, micro device emulsification [15,16], dropletbreakup and coalescence [17,18] using a micro channel and mem-brane or porous glass emulsification [19–22] has been developed.These methods are suitable for preparation of small amounts ofmonodisperse micron size droplets. For continuous industrialemulsification processes, high pressure drops prevent emulsifica-tion of high viscous monomer mixtures and decrease the produc-tivity. Once partial blocking of membrane and channel occurs,the droplet size is changed by a pressure change and the processstability is lost. In the polymerization process of ion-exchange re-sin and toner, high productivity and a comparatively narrow parti-cle size distribution are required. Emulsification using a staticmixer [23] has a high productivity and applicability. However, itis quite difficult for this method to attain narrow size distributionsdue to heterogeneous mixing as described above.

The objective of this work is to develop a new industrial suspen-sion polymerization process in which high productivity and nar-row size distribution can be attained. For this purpose, wepropose an emulsification step by means of a column packed withglass beads and a continuous polymerization step in a tube typereactor. In this study, the droplet size and size distribution in theemulsification step were studied with respect to the effects ofthe diameter of glass beads in the column, concentration of PVAin water phase, degree of polymerization of PVA, ratio of mass flowof the water phase to that of the oil phase, linear velocity of thewater phase and the viscosity of the water phase and the oil phase.For a continuous polymerization step, the monomer emulsion wascontinuously polymerized in the tube reactor.

2. Experimental

2.1. Materials

2,20-Azobis(2,4-dimethylvaleronitrile), ethylene glycol dimeth-acrylate, and glycidyl methacrylate were purchased from WakoPure Chemical Co. (Osaka, Japan). Congo Red, methyl red, poly(vi-nyl alcohol) (PVA) (88% saponified, average molecular mass22,000) and toluene were purchased from Nacalai Tesque (Kyoto,Japan). Poly(vinyl alcohol) with a molecular mass of 88,000 (PVA2000) was provided by Kuraray Co. (Tokyo, Japan). Poly(styrene)(average molecular mass 45,000) was provided by MitsubishiChemical Co. (Tokyo, Japan). All reagents were used withoutpurification.

Glass beads (GB-0.07 (70 lm), GB-0.1 (100 lm), GB-0.2(200 lm), GB-0.3 (300 lm), GB-0.4 (400 lm), and GB-1.0(1000 lm)) were purchased from Kenis Limited (Osaka, Japan)and used for experiments after washing with distilled water.

2.2. Emulsification apparatus and their operation

The emulsification apparatus was composed of a plunger pump(LC-5A; Shimadzu Corporation, Kyoto, Japan) for the water phasefeed, microtubing pump (Mini Puls2; GILSON, Middleton USA) forthe oil phase feed, a glass column (10 mm I.D. � 100 mm;129410; Tokyo Rikakikai Co., Ltd., Tokyo, Japan) packed with beads,and a produced emulsion reservoir as shown in Fig. 1. The glasscolumn size was packed with glass beads of diameters rangingfrom 70 lm to 1000 lm. A Poly(tetrafluoroethrene) tube (1.0 mmI.D. and 1.5 mm O.D.) was used for connection between these

equipments and a pharmed tube (1.60 mm I.D. and 4.80 mmO.D.) was used as the peristaltic part of the microtubing pump.The polymerization apparatus was composed of a microtubingpump for air pressure attenuation in the emulsion reservoir, apoly(ethylene) tube reactor (1.0 mm I.D. and 1.3 mm O.D. �10 m), and a produced polymer particle reservoir.

In the start-up of the emulsification apparatus, 2.2 � 10�2 g s�1

of distilled water was supplied to the column from the water phasereservoir. When the connection line and column was filled up withwater, the aqueous solution reservoir was changed to a PVA aque-ous solution. The flow rate was measured by a gravimetric methodat point E as shown in Fig. 1. After the flow rate of the outlet waschecked in set value and kept in a steady state, the monomer mix-ture was fed from oil reservoir (B) and mixed with the PVA solutionat the T-junction (C) and the resulting mixture was fed to the col-umn from its bottom at point D. The produced emulsion flowed outfrom the top of the column at point E and was guided to emulsionreservoir (F) in which emulsion was mildly suspended with a mag-netic stirrer. Composition of oil phase and water phase is shown inTable 1. PVA concentration (CPVA) of the water phase was variedfrom 3 to 10 wt.%. The flow rate of the water phase was varied from1.4 � 10�4 m s�1 to 4.2 � 10�4 m s�1 (from 1.2 � 10�2 g s�1 to3.5 � 10�2 g s�1), and the ratio of mass flow of water phase to thatof oil phase (Fw/Fo) was varied from 3 to 10.

To attenuate the flow rate of the produced monomer emulsionto the reactor, air was introduced by the microtubing pump, andpressure in emulsion reservoir was attenuated. The monomeremulsion in PE tube reactor was immersed in water bath and poly-merized, and the produced reaction mixture was collected in aproduct reservoir.

To compare emulsification by the glass bead-packed column de-scribed herein with other emulsification methods, 500 ml of waterphase and 100 ml of oil phase were added to 1000 ml in a 4-neckglass flask and stirred at 300 rpm using Teflon shaft stirrers

Table 1Composition of oil phase and water phase.

Phase Component Amount (g)

Water Distilled water 90.0–97.0Poly(vinyl alcohol) 3.0–10.0Glycidyl methacrylate 70.0

Oil (monomerand initiator mixture)

Ethylene glycoldimethacrylate

30.0

2,20-Azobis(2,4-dimethylvaleronitrile)

0.86

Toluene 100.0

10 1 102 103

0

10

20

30

40

50

Droplet diameter [μm]

Fre

quen

cy [

%]

Glass beads packed column Mixing in stirred tank Homogenizer

Emulsion prepared by

Fig. 2. Size distribution of monomer droplets prepared by glass bead-packedcolumn (s), mixing in stirred tank (4), and homogenizer (h).

394 M. Yasuda et al. / Journal of Colloid and Interface Science 349 (2010) 392–401

(0253-03, Flon chemical Inc., Osaka, Japan). In the case of homog-enizer, a mixture of water phase and oil phase as shown in Table 1was homogenized at 10,000 rpm for 10 min using a POLYTRON PT-MR3100 homogenizer (KINEMATICA, Luzernerstrasse, Switzer-land) attached with a 20-mm generator shaft.

2.3. Measurement of residence time distribution

A stepwise experiment was performed by switching the aque-ous solution from PVA solution to 0.005 wt.% of Congo Red tracerin PVA solution. Five weight percent of PVA solution fed in the col-umn for 30 min and feed solution was instantly exchanged as tra-cer solution. The eluted mixture from glass bead-packed columnwas collected and centrifuged for 15 min at 15,000 rpm (MX-150,TOMY SEIKO Co. Ltd., Tokyo, Japan). Centrifugation was repeatedthree times. After the centrifugation, the water phase was collectedand absorbance at 519 nm was measured using a UV–VIS spectro-photometer (UV-2100, Shimadzu, Kyoto, Japan). In the case of theoil phase, 0.007 wt.% of methyl red was added to the oil phase andthe eluted mixture from glass bead-packed column was collectedand centrifuged for 15 min at 15,000 rpm. After the centrifugation,oil phase was collected and absorbance at 487.5 nm was measured.

2.4. Measurement of size of monomer droplet and viscosity of waterand oil phase

A sample of the produced emulsion was observed with a micro-scope (IMT-2; OLYMPUS, Tokyo, Japan). Samples of 400 dropletswere measured by a microphotograph and the mean diameterand distribution was calculated by statistical processing. Viscosityof the aqueous solution and monomer mixture were measured at20 �C with an Ostwald viscometer.

2.5. Continuous polymerization apparatus

Produced emulsion was stocked into the reservoir and apoly(tetrafluoroethrene) tube was connected between reservoirand tube reactor immersed in the water bath. The tube reactorwas composed of a poly(ethylene) tube (1.0 mm I.D. and 1.3 mmO.D. � 10 m). The coiled PE tube was wrapped around a glass cyl-inder immersed in the water bath as shown in Fig. 1. Air suppliedfrom a microtubing pump increased the pressure of the reservoir,and the emulsion was continuously fed to the tube reactor. Theemulsion was polymerized by heating in a water bath at 70 �C,and the produced polymer particles were eluted from the reactorto the product reservoir as shown in Fig. 1.

3. Results and discussion

3.1. Emulsion preparation using glass bead-packed column

Bead-packed columns have been used for chromatography andion-exchange, as this method holds the advantage of continuous

operation. Fig. 2 compares size distribution of O/W emulsion pre-pared by stirred tank, homogenizer, and glass bead-packed col-umn. Since acrylic monomer has fast polymerization rate andpolymer particle composed of GMA and EGDMA was commerciallyapplied for HPLC support, we selected the composition shown inTable 1 as model monomer emulsion system. In the glass bead-packed column, 200 lm glass beads were packed, and 5 wt.% ofPVA aqueous solution was selected as a water phase. Mass flowrate of water phase (Fw) and a ratio (Fw/Fo) of Fw divided by volu-metric flow rate of oil phase (Fo) were 2.2 � 10�2 g s�1 and 5,respectively. In the other methods, the composition of the reactionmixture was identical to that in the glass bead-packed column. Asshown in Fig. 2, the emulsion produced using a homogenizer dis-plays a broad size distribution and comparatively small dropletsizes. This was because the oil droplets dispersed in the waterphase were subjected to high shear forces in the small clearanceof the generator shaft in the homogenizer. When the mixture com-posed of PVA aqueous solution and monomer solution was stirredat 300 rpm, droplets ranging from 10 lm to 500 lm were pro-duced. Using the column packed with 200 lm glass beads, the pro-duction of large droplets of diameters ranging from 200 lm to500 lm was suppressed as compared with the stirred tank. Thismay be because the volume limitation of glass bead gaps in thepacked column prevented formation of large droplets. The meandroplet size, standard deviation (SD) and coefficient of variation(CV) are shown in Table 2. The CV value of the glass bead-packedcolumn was the smallest of all three methods tested in this study.We realized that the emulsification technique using a glass bead-packed column was useful for the production of relatively uniformdroplets. Therefore, in order to control the droplet size and retain anarrow size distribution, we studied a pressure drop and the ef-fects of flow rate, composition and properties of oil and waterphase on the size distribution of monomer droplets passingthrough the glass bead-packed column.

3.2. Pressure drop in the glass bead-packed column

Friction shear was used to control the flow in the packed col-umn, as there is a relationship between flow and pressure drop

Table 2Mean droplet size and its distribution of droplets prepared by suspension polymer-ization, homogenizer and GB-0.2 glass bead-packed column.

Emulsion prepared by

Mixing instirred tank

Homogenizer GB-0.2 glass bead-packed column

Mean droplet size,dP (lm)

180.3 30.9 57.0

Standard deviation,SD (lm)

79.2 25.3 13.7

Coefficient of variation,CV (%)

43.9 81.6 24.1

CV was defined as SD/dp � 100.

M. Yasuda et al. / Journal of Colloid and Interface Science 349 (2010) 392–401 395

[24]. However, the pressure drop in the packed column was quitehigh by comparison with other processes. Therefore, we checkedthe upper limit of flow rate of the water phase. When 70 lm glassbeads were packed in the 10 mm I.D. � 100 mm column and10 wt.% of PVA solution was fed into it, the tube joint was unableto tolerate the strain. The pressure drop of the packed bed was cal-culated using the Kozeny–Carman equation [25]:

DP ¼ 180lULð1� eÞ2

d2pe3

ð1Þ

where l is the viscosity of the PVA aqueous solution [Pa s], U thevoid velocity of the PVA aqueous solution [m s�1], L the height ofthe packed bed column [m], e the porosity [–], and dg is the diame-ter of the packed glass beads [m]. From Eq. (1), the pressure drop(dg = 70 lm and 10 wt.% PVA solution) was calculated to be1.9 � 106 Pa. The relationship between PVA concentration and vis-cosity is shown in Fig. 3. Viscosity of PVA aqueous solutions wasfound to increase exponentially with linearly increasing PVA con-centration. In order to investigate the oil droplet dividing behaviorin the column, we selected a glass column so as to be able to take apicture. Since the maximum resistant pressure was 1.9 � 106 Pa(19.5 kg cm�2), the linear velocity of the water phase must be keptunder 4.2 � 10�4 m s�1 in the conditions studied here. For a linearwater phase velocity of 4.2 � 10�4 m s�1, the Reynolds number in-

PVA concentration of water phase, CPVA [wt.%]

Vis

cosi

ty o

f w

ater

pha

se,

w [

Pa

s]

0 2 4 6 8 10 0

0.01

0.02

0.03

0.04

0.05

0.06

η

Fig. 3. Viscosity of PVA aqueous solutions at various PVA concentrations. Viscositywas measured using an Ostwald viscometer.

side the column was calculated to be 8.4 � 10�2. Since the Reynoldsnumber is quite small, the flow inside the column was controlled bythe shear stress which acted on the fluid and on the interface be-tween fluid and glass beads.

3.3. Effect of velocity on the emulsification in the glass bead-packedcolumn

To optimize flow rate, mixture composition and properties ofwater and oil phase for producing monomer droplets using theglass bead-packed column, we selected 200 lm glass beads as amodel case. As a first step, the effect of velocities of both waterand oil phase on the size of produced monomer droplets was stud-ied. Fw/Fo was kept in five and the linear velocity of water phase inthe column was varied from 1.4 � 10�4 m s�1 to 4.2 � 10�4 m s�1.Fig. 4 shows the mean droplet sizes and corresponding CV values.When the linear velocity of water phase was high, mean dropletsize was slightly decreased despite a constant CV value. Whenthe mean linear velocity of water phase was doubled, the shearstress acting between the glass bead surfaces and water phasewas quadrupled. However, when the linear velocity was changed,the mean droplet size was almost the same. Therefore, we wouldconclude that the shear stress acting on the interface between glassbeads and water phase had little effect on droplet size, and that thedroplet size was controlled by shear stress acting between the oilphase and the water phase.

3.4. Effect of ratio of mass flow of water phase to that of oil phase onthe emulsification in the glass bead-packed column

We next varied Fw/Fo from 2.9 to 11.0 and kept the mass flowrate of water phase containing 5 wt.% of PVA constant at2.3 � 10�2 g s�1. The column packed with 200 lm glass beadswas used. The linear velocity of water phase in the column was cal-culated to be 2.8 � 10�4 m s�1. The mass flow rates of oil phase atthe column were varied at 2.1 � 10�3, 3.2 � 10�3, 4.4 � 10�3, and7.9 � 10�3 g s�1, respectively. Fig. 5a shows mean droplet sizesand corresponding CV values. The mean droplet sizes of Fw/Fo = 2.9, 5.2, 7.1, and 11.0 were 59.5, 57.0, 57.2, and 56.9 lm,respectively, with corresponding CV values of 25.1%, 24.1%,25.7%, and 25.4%, respectively. Fig. 5b shows the droplet size distri-butions. As can be seen from Fig. 5b, all the droplet size distribu-

0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

20

40

60

80

100

0

20

40

60

80

100

Mea

n dr

ople

t si

ze, d

p [μ

m]

Superficial velocity of water phase, uw [m s–1]

CV

value [%]

Fig. 4. Effect of linear velocity of water phase on the size distribution.

2 4 6 8 10 120

20

40

60

80

100

0

20

40

60

80

100

Mea

n dr

ople

t si

ze, d

p [μ

m]

Ratio of mass flow rate of water phase to that of oil phase, Fw/Fo [–]

CV

value [%]

Fig. 5a. Relationship between mean droplet diameter and linear velocity of waterphase.

Fre

quen

cy [

%%]

Droplet size [μm]

Fw/Fo

2.9 5.2 7.1

11.0

0 30 60 90 120 1500

5

10

15

20

25

30

Fig. 5b. Effect of ratio of mass flow of water phase to that of oil phase on the sizedistribution.

1 10 102 10 3

0

10

20

30

Fre

quen

cy [

%]

Droplet size [μm]

3 wt.%

CPVA

5 wt.%7 wt.%

10 wt.%

Fig. 6. Effect of PVA concentration in water phase on the size distribution.

396 M. Yasuda et al. / Journal of Colloid and Interface Science 349 (2010) 392–401

tions were almost the same. Therefore, there was no effect of themass flow rate within the range tested in this study. When themass flow rate of water phase was set at 1.1 � 10�2 g s�1 (Fw/Fo = 7.1), approximately half of the previous condition, the meandroplet size (dp = 57.0 lm, CV = 21.5%) and profile were almost atthe same level within the experimental error.

Next, we changed the sizes of packed glass beads from 200 lmto 100 lm or 400 lm. The droplet size obtained using 200 lmbeads was different from that (25.2 lm) using 100 lm beads andthat (108.4 lm) using 400 lm beads. However, their droplet sizedistributions were almost the same as for 200 lm within experi-mental error as shown in Fig. 5b.

3.5. The effect of PVA concentration in water phase and its degree ofpolymerization on the emulsification

Stability of the produced emulsion was found to mainly dependon emulsifier concentration and what type of emulsifier was used.Due to steric hindrance or electrostatic repulsion between emulsi-fier molecules adsorbed at the interface, droplet coalescence is re-duced or prevented, and monomer droplet size is decreased. Thereis an abundance of literature concerning the effect of various typesof emulsifier and their concentration on emulsification – see e.g.Ref. [26] and references therein. PVA is a well known emulsifierused in suspension polymerization, as it does not only possesshydrophilic hydroxyl groups but also a hydrophobic carbon mainchain as the back bone. Upon thorough mixing of an aqueous solu-tion of PVA and monomer mixture, PVA molecules were adsorbedon the interface between monomer droplet and water bulk phase,decreasing the interfacial tension and hence in PVA molecules sta-bilizing the monomer droplets. Emulsification is affected by therate of PVA adsorption speed as well as the interfacial PVA. In thisstudy, the effects of PVA concentration in the water phase and thedegree of polymerization of PVA were studied. Two-hundred mi-cron glass beads were packed in glass columns, and the concentra-tion of PVA in the water phase was varied between 3 wt.% and10 wt.%. The mass flow rate of water phase was set at2.3 � 10�2 g s�1 and Fw/Fo was kept constant at 5.2. Fig. 6 showsthe size distribution of the produced droplets. When excess PVAexisted in the water phase (PVA concentrations were 7 and10 wt.%, respectively), small droplets of sizes ranging from 10 lmto 20 lm were stabilized, and the resulting droplet size distribu-tions were broad. However, when the PVA concentration was 3or 5 wt.%, the formation of small droplets (<20 lm) was completelyprevented. The mean droplet size and its distribution at a PVA con-centration of 3 wt.% were almost the same as what was found for aPVA concentration of 5 wt.% (cf. Table 3). Next, PVA 2000 (degree ofpolymerization = 2000), which is 4-fold larger than that of PVA 500was used as emulsifier. The mass flow rate of water phase was setat 2.3 � 10�2 g s�1 and Fw/Fo was set at 5.2. As shown in Table 3, inthe case of PVA 2000, small droplets of around 10 lm were stabi-lized, and the corresponding droplet size distribution was verybroad. These results indicate that both increasing concentration

Table 3Effect of degree of polymerization of PVA on emulsification.

Degree of polymerization of PVA CPVA (wt.%) dp (lm) CV (%)

500 3 58.1 23.9500 5 57.0 24.1

2000 3 34.6 31.72000 5 21.2 53.3

101 1020

10

20

30

40

50

60

Fre

quen

cy [

%]

Droplet size [μm]

0 wt.% Poly(styrene) conc. in oil phase

10 wt.%

30 wt.%35 wt.%

20 wt.%

Fig. 7. Effect of viscosity of oil phase on the size distribution.

M. Yasuda et al. / Journal of Colloid and Interface Science 349 (2010) 392–401 397

of PVA and increasing the degree of polymerization resulted in thestabilization of small monomer droplet.

3.6. The effect of oil phase viscosity on the emulsification

As described in Section 3.3 above, shear stress acting on theinterface between the glass beads and the water phase had little ef-fect on droplet size. Rather, droplet size may be controlled by theshear stress acting between the water phase and the oil phase. Vis-cosities of the water phase containing 5 wt.% PVA and the oil phaseof monomer mixture were 9.3 � 10�3 Pa s and 1.5 � 10�3 Pa s,respectively. The viscosity of the water phase was six times as highas that of the oil phase, which may result in the formation of smalldroplets or a broad droplet size distribution. Therefore, we addedpoly(styrene) (molecular mass 45,000 g mol�1) to the monomermixture. Table 4 shows the viscosities of oil phase and water phase.When oil phase is flowing in the center of a circular tube and issymmetrically sandwiched by the water phase (assuming thatthe water phase is sheath flow), the shear stress acting on theinterface between the oil and water phase is approximately esti-mated by the theory concerning flow of two adjacent immisciblefluids [27]. The calculated shear stress (see Table 4) is then onlydependent on flow rate and viscosity of the water phase. Here,the viscosity of the oil phase was varied from 1.45 � 10�3 Pa s to5.47 � 10�1 Pa s with addition of poly(styrene) to the monomermixture. Since high poly(styrene) concentrations resulted in smallmean droplet sizes, the shear stress acting between water phaseand oil phase affected the mean droplet size while in a symmetricalsheath flow, mean droplet size did not depended on oil phase vis-cosity. For poly(styrene) concentrations higher than 20 wt.%, theproduced droplet size decreased and CV values were 17%. The addi-tion of poly(styrene) to monomer mixture increased the viscosityof the oil phase, which may prevent droplet deformation and divi-sion by the shear stress acting on the interface. Thus, only theshape and volume of glass bead gaps control the droplet size andits distribution. Fig. 7 shows the droplet size distribution for vari-ous poly(styrene) concentrations. The column packed with200 lm glass beads was used. When the amount of poly(styrene)added to the monomer mixture was <10 wt.%, the mean dropletsize was almost the same as for the system without addedpoly(styrene), but with a narrower droplet size distribution. How-ever, the mean droplet size was slightly decreased with increasingshear stress acting on the interface, and the droplet size distribu-tion became narrow and mono-modal at 20 wt.% of poly(styrene)and above. As Kumar [28] reported, droplet coalescence can be pre-

Table 4Effect of the addition of poly(styrene) to oil phase on shear stress acting on interface and

Poly(styrene) concentrationin oil phase (wt.%)

Viscosity of oilphase (Pa s)

Viscosity of waterphase (Pa s)

Shon

0 1.45 � 10�3

10 4.19 � 10�3

20 3.68 � 10�2 9.33 � 10�3 2.30 2.21 � 10�1

35 5.47 � 10�1

* Shear stress was estimated as Ref. [27].

vented by increasing the viscosity of the oil phase, as is the case inthis system. However, the shear stress acting on the interface wasnot found to be the main factor controlling the droplet size. We ex-pect that the shape and volume of the glass bead gaps affect thevelocity distribution of oil and water phases in the glass bead gaps.

3.7. The effect of the diameter of glass beads on the emulsification

As described in Section 3.6 above, we hypothesize that the glassbead gaps control droplet size and droplet size distribution. There-fore, we varied the diameter of glass beads in the range from70 lm to 1000 lm in order to change the glass bead gaps. In orderto investigate the effect of the glass bead diameter, a water phasecontaining 5 wt.% of PVA was used, with the mass flow rate set to2.3 � 10�2 g s�1, and Fw/Fo was kept constant at 5.2. Fig. 8 showsthe droplet size distribution for various sizes of glass beads. Inthe case of an open rhombus in which 1000 lm glass beads werepacked in the column, the droplet size distribution was very broad.When the ratio of the diameter (D) of the column to the diameter(dg) of packed beads exceeds 10, both wall effects and void distri-bution – both of which result in heterogeneous flow distribution inthe packed column – can be neglected [29]. In the column packedwith 1000 lm glass beads, D/dg was 10. Therefore, the broad drop-let size distribution was attributed to the channeling flow in thecolumn or the wall flow of the column. When the sizes of glassbeads were 70, 100, 200, and 400, so that D/dg was higher than25, the mean droplet size was related to the diameter of the glassbeads. Table 5 summarizes the effect of the size of glass beads on

emulsification.

ear stress actedinterface* (kg m�1 s�2)

Mean droplet size (lm) CV of droplet sizedistribution (%)

57.0 24.160.9 21.8

40 � 10�4 56.4 17.546.2 17.543.7 17.1

Droplet size [μμm]

Fre

quen

cy [

%]

70 μm100 μm200 μm400 μm

1000 μm

Size of packed beads

1 10 100 10000

10

20

30

Fig. 8. Effect of diameter of glass beads on the size distribution.

Table 5Effect of diameter of glass beads on droplet size.

Size of glass beads, dg (lm) Mean size of droplet, dp (lm) CV (%) dg/dp

70 19.2 27.8 3.65100 25.2 25.1 3.97200 57.0 24.1 3.51400 108.4 24.3 3.69

1000 236.6 41.6 4.23

Laminar flow

Plug flow

uL=2.8 ×× 10–4 m s–1

uO=5.2 × 10–5 m s–1

Outlet of water phase

Inlet of water phase

Outlet of oil phase

Fθ

[–]

θ [–]

Inlet of oil phase

0 1 2 30

0.2

0.4

0.6

0.8

1

Fig. 9a. Time course of dimensionless cumulative distribution, F(h) of water phaseand oil phase at inlet and outlet of the column. Dimensionless cumulativedistribution function was defined in Ref. [27].

398 M. Yasuda et al. / Journal of Colloid and Interface Science 349 (2010) 392–401

the produced droplet size and its distribution. The ratio (dg/dp) ofthe size of glass beads (dg) to the mean droplet size (dp) was nearlyconstant as 3.7 except in the case of the 1000 lm glass beads.There is a linear relationship between the size of glass beads andthe gap volume of packed beads. Therefore, the droplet size ismainly controlled by the gap volume of the packed beads.

3.8. The mechanism of droplet formation

In an O/W emulsion preparation using an SPG membrane or amicro device, oil droplets were pushed out against a continuouswater phase in which the velocity distribution was parabolic in acircular tube or closed channel. Since the oil phase was discontin-uously pushed out at a left angle and the shear stress required fordroplet division is only proportional to the flow rate of water andoil phase, respectively, the produced droplet size was monodis-perse. However, these methods hold the disadvantage of low pro-ductivity. Furthermore, since channel closing increases pressureacting on the channel increasing and degradation of surface mod-ification varies the flow profile of water and oil phase, these meth-ods lack stability over long periods of production. In the case of thestirred tank and homogenizer, large amounts of emulsion can beeasily and reproducibly prepared. However, quite strong shearstress between water and oil phase force the oil phase into micronsize droplets. When the mixture composed of water and oil phasewas emulsified by laminar or turbulent flow, size distribution ofproduced droplet became broad because of heterogeneous shearstress distribution [2]. Therefore, in order to diminish broadeningof the droplet size distribution, ideal flow, plug flow, in which ver-

tical the flow rate distribution against flow direction is propor-tional, is recommended. Some studies [24,30–32] have reportedthat the flow velocity distribution in a packed bed column is homo-geneous and similar to plug flow. Therefore, we selected the glassbead-packed column as a continuous emulsification apparatus.

As a first step, to study the mechanism of droplet formation,residence time distributions of the water phase and oil phase inglass bead-packed column were measured by a tracer method.We used two tracer reagents, the water-soluble Congo red, andthe oil-soluble methyl red. For measurement of residence time dis-tribution of the water phase in the packed column, the PVA solu-tion was exchanged with a PVA solution containing Congo red.The PVA solution containing Congo red was added to the oil phaseand fed to the glass bead-packed column. In the case of residencetime distribution of the oil phase, methyl red was used. From thestep tracer experiment, a cumulative distribution curve, F(t) wasobtained from the time course of visible absorption at point E asshown in Fig. 1. To obtain the normalized function F(h), a parame-ter h was defined as

h ¼ t=s ð2Þ

where t is time [s] and s is a space time [s]. Space time s was de-fined as

s ¼ Vv f

ð3Þ

where V is the volume of flow channel and the void volume of col-umn [m3] and vf is the volumetric flow rate [m3 s�1].

Fig. 9a shows the cumulative distribution F(h) of tracer at vari-ous points for the column packed with 200 lm glass beads. Here,the water phase contains 5 wt.% of PVA and the mass flow ratewas set at 2.3 � 10�2 g s�1. Fw/Fo was kept constant at 5.2. The lin-ear velocities of water phase (uL) and oil phase (uO) were2.8 � 10�4 m s�1 and 5.2 � 10�5 m s�1, respectively. The closed tri-angle represents F(h) of the water phase at point D (inlet of the col-umn). As shown in Fig. 1, the PVA aqueous solution was fed fromthe water phase reservoir (A) using a plunger pump, and the oilphase was added to the water phase at point C. Since the waterphase flowed in a PTFE circular tube from A to D, the velocity pro-

0 1 2 30

0.2

0.4

0.6

0.8

1

Fθ

[–]

θ [–]

Laminar flow Plug flow

uL=1.4 × 10–4 m s–1

uL=2.8 × 10–4 m s–1

uL=4.2 × 10–4 m s–1

Fig. 9b. Time course of dimensionless cumulative distribution, F(h) of water phaseat uL = 1.4 � 10�4 m s�1, 2.8 � 10�4 m s�1, and 4.2 � 10�4 m s�1. The dimensionlesscumulative distribution function was defined in Ref. [27].

(a) T junction (Point C) (b) Column inlet (Point D)

(c) Produced emulsion (Point E)

Fig. 10. Pictures of oil phase containing methyl red and water phase at the T-junction (a), inlet of the column (b), and outlet of the column (c). (For interpretationof the references to colour in this figure legend, the reader is referred to the webversion of this article.)

M. Yasuda et al. / Journal of Colloid and Interface Science 349 (2010) 392–401 399

file of the PVA aqueous solution became orderly laminar flowthrough wall friction [27]. The velocity profile at the inlet of thecolumn was not the same as that of a laminar flow (solid line).The PVA solution represents a typical non-Newtonian dilatant fluid[33], and the velocity profile of the water phase in the circular tubeapproached plug flow owing to high viscosity of the PVA aqueoussolution. In the case of the effluent from the column at point E,both the h F h profile of water phase and that of the oil phase be-came closer to the h F h profile of plug flow (dashed line) than thatof laminar flow. At low velocities, the dilatant fluid flows easily. Athigher velocities, the friction greatly increases, resulting in a solid-like flow of the dilatant. In the packed column, the strong shearstress controlled velocity profiles of the water phase and madethem homogeneous. Closed circles (outlet of oil phase) representthe h F h profile of the oil phase. The oil phase also flows like plugflow. The residence times of both phases were almost the identical.Due to the hydrophilic properties of the glass beads, their surfacewas coated with the water phase. Viscosities of the water phasecontaining 5 wt.% PVA and the oil phase were 9.3 � 10�3 Pa s�1

and 1.5 � 10�3 Pa s�1, respectively. The viscous water layer pre-vented approach of oil phase to the surface of the glass beads,and so the oil phase could not push aside or pass the water phase.Since the oil phase was forced by the water phase, both oil, andwater phases have similar h F h profiles.

When the linear velocity of the water phase was varied from1.4 � 10�4 m s�1 to 4.2 � 10�4 m s�1, the h F h profiles as shownin Fig. 9b were almost similar (0.8 < h < 1.0) within the experimen-tal error. The slope of the h F h profile monotonically increased(0.8 < h < 1.0) and monotonically decreased (1.0 < h < 1.2) and theslope was symmetrical at h = 1. This result indicates that there isno channeling, and that the bypassing in the glass packed columnand water phase flow were homogeneous [34]. For h > 1.0, the pro-file of the water phase (uL = 1.4 � 10�4 m s�1) was the same as thatof the outlet of the oil phase as shown in Fig. 9a. At high linearvelocities, F h was far from 1.0. We consider that there was deadspace at a corner of the column and thus that there was no ex-change between the water phase placed in the dead space andthe bulk water phase. Only diffusion of tracer molecules took place.At a high linear velocity, the ratio of the diffusion rate to the mass

transfer rate of the tracer was quite high. The existence of the deadphase in which tracer concentration was low resulted in the slowslope of F h. All of the residence time distribution functions wereplaced between a plug flow function and a laminar flow function.Since the viscosity of the aqueous PVA aqueous was high, the flowprofile in the glass bead-packed column was controlled via fric-tional shear stress on the interface between the glass beads andthe emulsion mixture. In the case of the 400 lm and 1000 lm glassbeads, all of the residence time distribution functions were similarto that of 200 lm.

As shown in Fig. 9a, the h F h profiles were almost identicalwithin the experimental error, and the oil phase was moved withwater phase as a unit because their h F h profiles were almost iden-tical. Fig. 10 shows a picture of the oil phase containing methyl or-ange and the water phase at the T-junction (a), inlet of column (b),and outlet of column (c). At the T-junction, oil phase was pushed atleft angles with respect to the water phase. Water phase and oilphase were alternately supplied to the column as shown in (b).The pulse supplied oil phase could not push aside or pass the waterphase, and was forced to glass bead gaps of the first layer by nextsupplied water phase. In this situation, oil phase would be dividedin proportion to the glass bead gap volume and the effect of shearstress acting on glass beads would be weakened. The variation ofvelocity of water and oil phases only controlled flow rate and theflow of both phases approximated plug flow.

As shown in Figs. 9a and 9b, the h F(h) profile at the outlet issimilar to that of plug flow while the h F(h) profile at inlet is similarto that of laminar flow. A decrease of the width of the distributionmeans that a flow rate fraction was subjected to a shear stress withthe friction at an interface between mobile phase and glass beads,and was subsequently decelerated. Therefore, the friction at aninterface makes the flow rate distribution in a packed columnhomogeneous. This condition constitutes an ideal environmentfor producing uniform droplets. However, we have neglected theemulsion formation caused by channel spreading in the inlet ofthe column and channel reducing in the outlet of the column ondroplet dividing behavior. We have also neglected monomer drop-

0 100 2000

10

20

Size [μm]

Fre

quen

cy [

%]

Monomer dropletPolymer particle

Fig. 11. Size distribution of monomer droplets prepared using GB-0.3 glass bead-packed column (s) and polymer particles prepared using tubular continuousreactor (d).

400 M. Yasuda et al. / Journal of Colloid and Interface Science 349 (2010) 392–401

let dividing caused by shear stress at point C (T-junction). Further-more, we have adopted the condition in which oil mass flow ratewas smaller than that of water phase. For such conditions, thespace gap occupied by water phase was bigger than that of theoil phase. When the flow rate of water phase was increased withconstant Fw/Fo, there was no effect on the mean droplet size andits distribution as described in Section 3.3. This was because thefriction between glass beads wall and the mobile phase (mainlywater phase) had little effect on the formation of monomer drop-lets. The droplet size was proportional to the geometrical shapein the packed column gap.

3.9. Continuous polymerization of monomer droplets in tubularreactor

Prior to the continuous polymerization of monomer droplets intubular reactor, monomer emulsion stability was studied. Mono-mer emulsion was prepared by GB-0.3 (300 lm) glass beadspacked column. The mass flow rate of water phase was set at2.3 � 10�2 g s�1 and a water phase containing 5 wt.% of PVA wasused and Fw/Fo was kept constant at 5.2. When monomer emulsionwas mixed with magnetic stirrer, mean droplet size and CV valuewas almost constant for 180 min. Furthermore, when monomeremulsion not containing initiator was introduced into the tubularreactor (the mass flow rate of monomer emulsion was 6.88 g s�1)and heated at 70 �C, the residence time was about 32 min andmean droplet size and CV value was almost the same as fed mono-mer emulsion.

In order to synthesize polymer particles, the monomer emul-sion prepared with the 300 lm of glass bead-packed column wascontinuously polymerized in a tube reactor. Poly(styrene) did notadded to monomer emulsion. When monomer emulsion was fedto the reactor using a pump, such as a plunger pump or peristalticpump, significant droplet coalescence and the formation of poly-mer plugs took place. Table 6 shows time course of conversion ofmonomer emulsion stirred with 100 rpm at 70 �C. Since the aver-age residence time of monomer emulsion was 32 min, the poly-merization in monomer droplet proceeded quickly and pulsationof pump resulted in the accumulation and aggregation of polymerparticle.

To prevent droplet coalescence, emulsion was fed to the reactorby charging pressure to the emulsion reservoir using a micro tub-ing pump as shown in Fig. 1. Then, emulsion containing an initiatorwas heated in the water bath and monomer droplets were subse-quently polymerized (the mass flow rate of monomer emulsionwas 6.88 g s�1). When the flow rate was low and the droplet con-centration was high, particles aggregated, which resulted in aggre-gates holding back the tube in the water bath. However, we couldobtain polymer particle having narrow size distribution at highPVA concentrations (=5 wt.%). Fig. 11 compares the size distribu-tion of monomer droplets with that of polymerized particles, and

Table 6Time course of conversion of batch reactor.

Reaction time (min) Conversion (%)

0 010 5.420 52.430 71.740 73.850 83.660 89.390 90.2

120 88.6

Monomer emulsion was mixed by magnetic stirrer (100 pm) and was immersed inwater bath at 70 �C.

shows that the size distribution of polymer particles was almostthe same as that of monomer droplet. This result indicates thatwe have succeeded in polymerizing monomer droplets whileretaining monodispersity.

4. Conclusion

A two step continuous emulsification and polymerization pro-cess capable of producing monomer droplets and polymer particleswith narrow size distributions has been developed. In the emulsi-fication step, when the monomer mixture and aqueous solutioncontaining 3–10 wt.% of PVA were introduced into the glassbead-packed column, relatively monodisperse monomer dropletemulsion can easily and continuously be prepared. Velocity ofwater phase and ratio of mass flow of water phase to that of oilphase had little effect on size and its distribution of the monomerdroplets. Droplet size was reduced with increasing concentrationand degree of polymerization of PVA in the water phase. Size dis-tribution of monomer droplet became broad with increasing vis-cosity of the water phase, and narrow with increasing oil phaseviscosity. This result indicates that viscosities of water phase andoil phase were the most important parameters to improve the sizedistribution of monomer droplets produced using the glass bead-packed column. In order to study the mechanism of droplet forma-tion in the glass bead-packed column, residence time distributionof the water phase and oil phase were investigated. From the res-idence time distribution of the water phase and oil phase in theglass bead-packed column, flow profiles of water phase and oilphase were almost identical. When diameter of the glass beadswas varied from 70 lm to 400 lm, uniform droplets with meandiameters ranging from 19.2 lm to 108.4 lm were obtained. Theratio (dg/dp) of the diameter of the glass beads, dg to the meandiameter, dp was about 3.7 and there was a linear relationship be-tween the diameter of glass beads and the gap volume of packedbeads. Thus it appears that the gap volume of packed beads wasthe main factor controlling the droplet size. Furthermore, themonomer droplet was continuously polymerized using a tubularreactor and the size distribution of produced polymer particle

M. Yasuda et al. / Journal of Colloid and Interface Science 349 (2010) 392–401 401

was almost the same as that of the monomer droplets. This indi-cates that the combination of emulsification by a glass bead-packed column and polymerization using tubular reactor is veryuseful to produce droplets and particles with narrow sizedistributions.

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