J . MICROENCAPSULATION, 1991, VOL. 8, NO, 4, 525-536

Interfacial polymerization encapsulation of a viscous pigment mix: emulsification conditions and particle size distribution

HOCK SENG TAN, T I E HWEE NG and HAD1 KHAN MAHABADI Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1

(Received 14 December 1990; accepted 24 February 1991)

A viscous organic phase, containing up to 65 per cent solid pigment, was dispersed into water with an emulsifier by a rotor-stator homogenizer and the droplets formed were encapsulated by interfacial polymerization. Microcapsules with volume median diameters d,, ranging from 10 to 25pm and geometric standard deviation (GSD), from 1.25 to 1.65, were obtained depending on emulsification conditions. Larger impellers gave smaller d,, and slightly narrowed GSD; d,, decreased and GSD increased as volume fraction of dispersed phase is decreased. Higher homogenizer speed and emulsifier con- centration decreased d,, but slightly increased GSD. Increasing pigment content in dispersed phase decreased d,, but had little effect on GSD. These effects were assessed quantitatively by fitting an empirical model to the data.

Introduction Many applications of microencapsulation require the microcapsules to have

particle sizes in the range 1-50pm. For example, for application in carbonless copying paper it is desirable to have the microcapsule size in the 1-10pm range (Baxter 1977), microcapsules containing pesticides have size ranges of 30-50 pm (Ivy 1972) and those containing perfumes 10-50pm (Curt 1987). One method of preparing microcapsules of these sizes is interfacial polymerization, where droplets are first formed by dispersing an organic phase consisting of the core ingredients and an oil-soluble reactive shell monomer into an aqueous phase containing a small fraction of emulsifier. Following emulsification a water-soluble reactive monomer is added and polymer shells form rapidly at the interface of the droplets. T h e encapsulated particles must then be separated from the continuous phase by washing, filtering and drying. In many applications it is often desirable to have a narrow particle size distribution. For example, in carbonless copying application the marking fluid capsules with narrow size distribution give sharp and uniform images on the transfer sheet. The emulsification step is the primary determining step in establishing the size and size distribution of microcapsules. This step may be influenced by physical parameters such as mixer/vessel configuration, speed of mixing and volume ratio of the two phases, and physicochemical properties such as interfacial tension, respective viscosities, densities and chemical composition of the two phases in contact.

Chang et al. (1966) used a magnetic stirrer and Span 85 as emulsifier to produce droplets for collodion and nylon microcapsules in the range 10-80 pm. Though they did not present the size distribution data, they reported that the mean diameters

0265-2048/91 S3.00 Q 1991 Taylor & Francis Ltd.

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5 26 Hock Seng Tan et al.

decreased with increase in stirring speed and emulsifier concentration, but levelled off beyond emulsifier concentration of 5 per cent. Mori et al. (1 972) used an Omuni mixer at 600-2000rpm and prepared nylon microcapsules ranging from 10 to 500 pm. They found the capsule sizes tended to be smaller at higher rates of stirring and concentration of Span 85. Koishi et al. (1969) studied the emulsification conditions on the size distribution of polyamide microcapsules having a mean diameter ranging from approximately 2 to 10pm. They found that increasing mechanical agitation in the absence of an emulsifier makes the size distribution narrower and as the emulsifier concentration increases the distribution becomes narrower first and then remains unchanged. The data reported in all the studies noted above are limited in number and range, and quantitative assessment has been used only to a very limited extent. Recently, some quantitative evaluation of the effects of experimental conditions on size and size distribution of droplets ranging from about 5 to 30pm has been reported for emulsion (Chaffey et al. 1989) and suspension polymerization (Tanaka et al. 1989, Almog and Levy 1982).

Data on dispersion of liquid droplets containing solid powders such as pigment and other additives in a suspension medium are scarce. Chaffey et al. (1989) dispersed an organic phase containing about 7 per cent by weight of a pigment in water with emulsifier, but did not report the effects of pigment content on particle size and size distribution. Tanaka et al. (1989) suspended an organic phase with solid loading of about 10-50 per cent in aqueous solution of a stabilizer, but did not measure the initial droplet size. They measured the sizes of the particles after suspension polymerization, which in some cases have grown significantly during the course of the polymerization.

The objective of this paper was to provide new data on size and size distribution of microcapsules loaded with a viscous mixture of liquid and solid pigment. Thus, microcapsules of 1 &25 pm were prepared using an interfacial polycondensation method. Dispersion of the viscous non-Newtonian mixture was done using a high- shear rotor-stator homogenizer. The effects of several variables of practical importance on the size and size distribution were examined, and quantitatively correlated.

Experimental Materials

The systems studied were based on a proprietary formulation, in which the organic dispersed phase consisted of 21-36 per cent polyisobutylene (Polysciences) or a mixture of polyisobutylene and a methacrylate monomer such as n- laurylmethacrylate (Polysciences), 14 per cent polyfunctional isocyanates such as mixture of toluene diisocyanate (Bayer) and Desmodur RF (Bayer) as the oil-soluble shell formation monomers, and 50-65 per cent magnetite iron oxide (Pfizer) as a pigment. The insoluble pigment in the form of solid particles small compared to the eventual capsule size, was included to simulate a highly viscous organic phase, in the order of 100000-300000cp, and to make the droplets visible. The density of the organic phase was about 1.85 g/cm3.

The aqueous phase was distilled water containing a small fraction by mass of 88 per cent hydrolysed polyvinylalcohol (averaged molecular weight 77 000-79 000, Aldrich) as emulsifier. The water-soluble shell formation monomer was a difunc- tional amine such as diethylenetriamine (Aldrich).

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Interfacial polymerization encapsulation 527

Method of microencapsulation Microcapsules were prepared using the interfacial polycondensation technique.

The organic phase, containing the oil-soluble shell-forming monomer, was first dispersed into the aqueous phase, with the aid of the emulsifier, to form an oil-in- water emulsion and subsequently encapsulation was achieved by addition of shell- forming water-soluble reactant. Dispersion was done using a Brinkmann rotor- stator homogenizer. Three sizes of Brinkmann generators, or heads, were used: 35, 45 and 50 mm. The 35 mm generator has four rotating and stationary rings, and those of 45 and 50 mm have six rings. For example, the 45 mm homogenizer has six concentric toothed rings of which alternate ones rotate at a high speed; the outermost ring, which is stationary, has an outer diameter of 45 mm, while the next ring, which rotates, is separated from it by about 0.5 mm and is about 40 mm in outer diameter. The speed of rotation was controlled by a Reco speed controller. According to the manufacturer, homogenization occurs from both mechanical shearing and cavitation.

In an experiment the pigment, core polymer and monomer, and the oil-soluble shell monomers were mixed to form a homogeneous organic phase, which was weighed into a 2-litre glass vessel and the aqueous phase was added. The internal diameter of the vessel was 12.5 cm and the amounts of materials were chosen to give a desired volume fraction 0 and a total volume of about 1200ml. Then the homogenizer was lowered into the mixture and run at a constant speed N for 2 min. Tests showed that steady state was established in less than 1 min. T h e emulsion was then transferred to another vessel equipped with a mechanical stirrer, and solution of the water-soluble reactant was added. The dispersion was kept agitated at 500 rpm for 30 min for the completion of the interfacial polycondensation reaction.

Particle size measurement The size of microcapsules was measured by a Coulter Multisizer interfaced with

a Hewlett-Packard personal computer. Samples were diluted with Isoton I I electrolyte solution containing two drops of Triton X-100 dispersant. The aperture tube ordinarily used had an aperture of diameter 100pm. The instrument has 128 channels and 250000 droplets were counted in one measurement. After a run, the numbers in the 128 channels, together with data for calibration, were transmitted to the computer and the 16th, 50th and 84th percentiles of the volume distribution of droplet diameters, d , , , d , , and d,, were calculated. A plot of the distribution was also drawn.

The microcapsule size was assumed to follow a log-normal distribution, and thus the geometric mean diameter was given by d , , and the geometric standard deviation (GSD), a representation of size distribution, was given by ( ~ & ~ / d , ~ ) ' / ~ (Chaffey et al . 1989).

Variables to be studied In a turbulent dispersion to produce droplets, the size has been found to depend

on several variables. Important physicochemical properties include the rheological parameters of the two phases and the interfacial tension between the two phases. In our experiments the viscosity of the dispersed phase was varied by changing the solid (pigment) weight fraction in the organic phase (S), from 0.50 to 065. T h e interfacial tension was varied by changing the concentration of emulsifier in the aqueous phase, from 0.05 to 0.10 per cent by weight. Surface tension, measured by a Fisher

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528 Hock Seng T a n et al.

Autotensionat surface tension analyser, of the aqueous phase over this range was about 4548mN/m (Moffat and Jhamb 1987, unpublished). The dispersion of the droplets also depends on the mechanical energy input, which is characterized by the length L of dispersing impeller, the agitation speed N, and the volume fraction CD of dispersed phase. Three impeller lengths (35, 45 and 50 mm), characterized by the sizes of the homogenizer, were used. The speed was varied from 7000 to 14 000 rpm and volume fraction of dispersed phase, from 005 to 0.22.

Results Forty-six experiments were done, and figure 1 shows a typical particle size

distribution curve obtained from the Coulter counter. I t has been shown that for log- normal distribution (Allen 1981), d,,d,,/d:,= 1. In our experiments the average of dI6da4/d:, = 0.962, with standard deviation of 0.039, indicating the distribution did not deviate significantly from being log-normal.

Effects of variables In figures 2 to 6 the volume median diameter d,, and estimated geometric

standard deviation (GSD) are plotted against one of the experimental variables. The scales are logarithmic with the same base used for both ordinate and abscissa. Figure 2 shows that increasing the homogenizer size L decreased both d,, and GSD, though the effect on GSD was not as pronounced as that on dsO. Specifically increasing L from 35 to 50mm decreased d, , by about 24 per cent, but reduced GSD by only about 2 per cent.

I 1.0 -

I- z 0.8 3 0 0

w 0.6

-I q I 0 z

-

n

N -

0.4 -

0.2 -

I - Volume

- - - Volume

Dif ferentlal

Cumulative

I - Volume

- - - Volume

Dif ferentlal

Cumulative

Figure 1. A typical plot from a Coulter counter, showing size distribution of microcapsules.

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Znterfacial polymerization encapsulation

1.

U u) m

1.1 31

2( - E v

0 In U

10

\.

- -o- - 12000 RPM

529

Figure 2. Plots on logarithmic scales of geometric standard deviation (GSD), above, and volume median diameter d,,, below, against homogenizer size L. E=O10 per cent, @=0.22, S=0.62.

From figures 3 and 4, decreases in dsO and increases in G S D are observed with increasing homogenizer speed N and emulsifier concentration E, but the effect of emulsifier concentration is smaller than that of homogenizer speed. According to figure 3, doubling N from 7000 to 14 000 rpm decreased dSo by about 32 per cent, but increased G S D by about 3.3 per cent. Figure 4 shows that d,, was reduced by about 12 per cent while G S D widened by about 3 per cent as E increased from 0.05 to 0.1 per cent.

Figure 5 shows that dsO may decrease as volume fraction CP of the dispersed phase decreases. The geometric standard deviation, however, increased significantly as CP is decreased. For example, figure 5 shows that increasing @ from 0.05 to 0.22 increased dSo by about 42 per cent but decreased GSD by about 19 per cent.

Figure 6 shows the dependence of dso and GSD on pigment content S in the organic phase. Increasing the pigment content from 050 to 0.65 reduced d,, by about 21 per cent but had little effect on GSD.

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1.

U 0 m

1. 3

h g 21 v

0 n -0

1(

-0-

--o-- E = 0.10 %

I 1 1 1 1 . 1 ,

6 8 10 12 14 16

N x ~ o - ~ (RPM)

Figure 3 . Plots as figure 1 against homogenizer speed N ; L=45 mm, @=0.22, S=0.62

1.

'0 ln m

1.( 3(

2( - E v

0 n U

i a

0

I 1

0.05 0.1

E (%) Figure 4. Plots as figure 1 against emulsifier concentration E, L=45mm, N=9000rpm,

@=0.22, S = 0 6 2 .

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Interfacial polymerization encapsulation 531

Fitting of empirical model A power law model in the form of

d, , = KLaNbECcPdS‘, (1)

where K and K are constants and where a to e and a’ to e’ are exponents, was used to quantitatively characterize the effects of each variable. The data of 46 experiments were used to fit the model by linear regression of logarithms of equations (1 ) and (2)

(3)

(4)

Results of analysis indicate that for the fitted GSD model the constant, log (K), was not statistically significant at 5 per cent significance level, and the model was refitted without the constant. The fitted parameters, and the associated 95 per cent confidence intervals after +, are given in table 1 . The model predictions from equations (3) and (4) are shown as straight lines in figures 2 to 6. The adequacy of the model was assessed by examination of plots of the residuals obtained from the fitted model. The residuals were plotted against the respective experimental variables and predicted response variables. No evidence of inadequacy was found.

log d,, = log K + a log L + b log N + c log E + dlog@+ e log S ,

log GSD = log K’+ a’ log L +b’ log N + c’ log E + d log @+ e’ log S .

Discussion Particle formation in the turbulent dispersion depends on a stress balance

between the pressure fluctuations due to turbulent eddies tending to break up the droplets and the stresses from interfacial tension and internal viscosity holding a droplet together (Chaffey et al. 1989). The stress tending to break up a droplet directly depends on the power input per unit mass or power dissipated per unit mass. Larger impeller size L and higher mixing speed N increase the power input per unit mass, and thus increase the turbulent stress. This results in smaller droplet size, which was generally observed in figures 2 and 3. Chaffey et al. (1989) showed that increasing volume fraction @of the dispersed phase reduces the power dissipated per unit mass and thus produces larger droplets (figure 5). The stress due to interfacial tension could be reduced by increasing the emulsifier or stabilizer concentration E and this led to formation of smaller particles (figure 4).

The size distribution of the droplets depends on the distribution of the turbulent force throughout the suspension mixture; the more uniform the mixing force distribution, the narrower the size distribution of the droplets, and hence that of the final capsules. Larger L provides more uniform distribution of energy throughout the mixing vessel, and hence results in narrower GSD (figure 2). Higher N , however, increases the gradient of energy distribution from the vessel wall to the centre of impeller, and thus gives wider GSD (figure 3) .

Increasing volume fraction @ of dispersed phase apparently resulted in greater coalescence of smaller droplets in the break-up/coalescence equilibrium and thus narrowed the GSD (figure 5). This is supported by the plots of d16 and d,, as a function of @ on figure 5 , which shows that the change of d16 was much faster than that of d,, as @ is increased.

The results of fitting equations (3 ) and (4) can be compared to those in the literature. In the preparation of composite particles containing a mixture of monomers, a copolymer of the monomers and a pigment, Chaffey et al . (1989)

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Tab

le 1

. F

itte

d p

aram

eter

s in

eq

uat

ion

s (3

) and

(4)

.

Var

iabl

e C

onst

ant

L

N

E Q

, S

Par

amet

er

log

K

a b

C d

e

40

10-4 f 072

- 0.77 f 01 3

-056

f 0.06

-0.1 9

f 0.04

023f0.04

-072f021

Par

amet

er

log Ic

a' b'

C' Cr

e'

GSD

-006

0+00

57

0047f0.023

0-04OfO.022

-0142f0.021

0088+012

4 c

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Slop. I

0-0-0*23

*/ 0.40 /------

4. -.A 0: I I I

0 05 0.10 0.15 0.20 0.30

0-0-0*23

*/ 0.40 /------

4. y o /

0 05 0.10 0.15 0.20 0.30

6 Figure 5 . Plots on logarithmic scales of GSD (above), and d,, , dSo and da4 (below) against

volume fraction (D of organic phase; L=45mm, N=9000rpm, E=007 per cent, S = 0 6 2 .

t

h

E v

0 u) TI

20 '

151 I I

0.4 0.5 0.6 0.7

S Figure 6. Plots as figure 1 against solid content S in organic phase; L=45mm,

N=9000rpm, E=0.05 per cent, @ = 0 2 2 .

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534 Hock Seng Tan et al.

dispersed the core material mixture, containing about 7 per cent by weight of pigment, into water with emulsifier by a 35 mm Brinkmann homogenizer. Their emulsifications conditions, in the present notation, were N=4000-11000 rpm, E=0.1-3 per cent, @=0.05-0*5, and fraction of monomer in the organic phase M=0*1-1*0. Volume median diameters of the droplets were typically 4-20pm, proportional to N-0'8'E-0'28Cp0'1'M-0'99 but the effect of @ was not statistically significant, possibly because a relatively high concentration of emulsifier was used. A high GSD of about 1.7 was generally independent of experimental conditions.

Tanaka et al. (1 988) prepared microcapsules containing temperature-indicating pigments by dispersing the pigmented solution of polystyrene and dichloromethane as a solvent into water containing polyvinyl alcohol. The capsules were formed by evaporation of the solvent inside the particles. Their conditions were N = 250- 800rpm, E=0*05-5 per cent, @ = 0 * 0 1 4 3 , S=0*02-0.2, and amount of polymer in the organic phase P = 0 .0541 5. Sauter (volumearea) mean diameters ranged from 100 to 700 pm, proportional to N-0'74E-0'4@-0'4~''Z. The solid (pigment) content S did not significantly affect the mean particle size and size distribution. The reported values of particle size dispersity (ratio of standard deviation to Sauter mean size) imply GSD values of 1.3-1.4 generally independent of experimental conditions.

In the preparation of composite pigmented polymeric particles, Tanaka et al. (1989) studied the emulsification conditions affecting the particle size and size distribution. They used tricalcium phosphate as a stabilizer, in the range 02-0-6 per cent by weight; the solid powders present in the dispersed phase were magnetic and carbon black, and its weight fraction ranged from about 0.08 to 0 5 ; and N= 54& 3300 rpm. Sauter mean diameters, in the range 7-25 pm, were proportional to N- 1'3 up to 9000 rpm and did not change significantly afterwards; they decreased with S (proportional to S-O") up to 0 3 but increased as Sincreased from 0.3 to 0.5. The size dispersity increased slightly with S and decreased with N up to 9000rpm and remained constant afterwards. Effect of @ on mean size and dispersity was not reported.

Almog and Levy (1982) dispersed styrene with or without polystyrene into aqueous solutions of polyvinyl alcohol using a Waring blender with a Polytron head. The mixing time to attain a steady-state particle size, about 360 s, was longer than that of our experiments. Their conditions were N=4500-13 000 rpm, E=O5-5 per cent, Cp=04-0*17, and M = 0.88-1.0; d,, was in the range 4-25 pm, and GSD about 1.38-3.7. Chaffey et al. (1989) fitted their data and reported d,, was proportional to N-"37E-0"s@0"4M-3'14t-0'23, where t is time of blending. The size distribution, characterized by the ratio of mass median to number median diameter, generally increased with increase of N, E and amount of polymer in the dispersed phase, and was apparently independent of @.

Generally, our results show the same trends for the dependence of particle size on N, E, and 0 as those previously reported, although the quantitative results differ. Our exponent for N of 0.56 is slightly lower than those reported by Chaffey et al. (1 989) and Tanaka et al. (1 988), and considerably lower than that of Almog and Levy (1982). This is probably due to the larger ratio of mixing vessel to mixer used in our experiments. Sprow (1 967) has given equations for droplet break-up and coalescence in agitated liquid-liquid systems, and the equations indicated that when droplet break-up predominates the mixing speed exponent a is - 1-2 for break-up by inertial effects or - 1.5 for break-up due to viscous shear, and when coalescence predomi- nates, a is -0.75. In this study our value of a is closer to -0.75, indicating

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Interfacial polymerization encapsulation 535

coalescence of droplets can be controlling. The increase of GSD with higher Nand E observed in our experiments is also in agreement with the results of Almog and Levy (1982).

Our results for the effect of 0 on d50 and GSD are consistent with Almog and Levy (1 982) and Chaffey et al. (1 989), but not Tanaka et al. (1 988). I t is well known that, in the dispersion of two immiscible liquids, increase in volume fraction of dispersed phase usually results in increase in mean particle size (for example, Sculles 1976, Lee and Soong 1985, Arshady 1989). It was not clear why Tanaka et al . (1 988) obtained the opposite results.

Our results of effect of S on d5o were also observed by Tanaka et al. (1989) but only in the range S= 0.08-0.30. However, the primary droplet size before polymeriz- ation reported by Tanaka et al. (1989) indeed increased with increase of S up to 0.4, and possibly up to 0.5. Our exponent for S of - 0.72 was close to - 0.9 (for S < 0.30) reported by Tanaka et al. (1 989).

Data on dispersion of liquid droplets containing solid powders are limited, but it is generally believed that these droplets will be larger because the viscosity of the dispersed phase increased; thus there is a greater resistance to deformation and breakage of the droplets (Tanaka et al. 1989). However, our results and those of Tanaka et al. (1989) indicate otherwise. We believe that the pigment we used in dispersed phase functioned as a stabilizer for the droplets in the aqueous phase, and prevented the coalescence of the droplets. Therefore, higher pigment content reduced coalescence, and produced smaller droplets. Tanaka et al. (1 989) attributed their results to a phenomenon in that, as the viscosity of the dispersed phase increases, many small droplets are chipped off from the surfaces of the much larger droplets, resulting in a decrease in mean droplet size.

Reports on effects of impeller length L o n size of droplets formed in emulsion are scarce. Nevertheless, our results can be compared to those related to dispersion of organic liquid in water in the absence of emulsifier. Vermeulen et al . (1955) dispersed a wide range of organic. liquids in water and correlated the experimental variables with mean drop size. Their equation indicated that the mean size is proportional to L - o ’ 8 . Our results of d , , o ~ L - ” ~ ~ agree closely with their equation.

Conclusions We have prepared microcapsules containing high pigment loading by dispersing

a viscous mixture of pigment, an organic compound and oil-soluble shell monomer into an aqueous solution containing polyvinyl alcohol, and subsequently en- capsulated the droplets by interfacial polymerization. The effects of several variables of practical importance on the microcapsule size and size distribution were studied, and correlated by an empirical power-law model. The volume median diameters d50

were typically 1&25pm, and can be reduced by increasing the size and speed of homogenizer, and the solid content in the dispersed phase. T o a lesser extent, d, , becomes smaller when the interfacial tension is lowered b y increasing the emulsifier concentration, or when the volume fraction of dispersed phase is reduced. These results are consistent with those reported in the literature.

The breadth of distribution of microcapsule sizes was characterized by geometric standard, standard deviation (GSD), estimated from (ds4/dI6)l’’. The effects of the emulsification conditions on GSD were not as pronounced compared with those of

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536 Interfacial polymerizat ion encapsulation

&,. T h e GSD typically ranged from 1.25 to 1.65, increasing with higher homogenizer speed and emulsifier concentration, and decreasing with higher volume fraction of dispersed phase. Larger homogenizer size slightly reduced GSD, and change in pigment content in the dispersed phase did not affect GSD.

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MORI, T., SATA, T., MATUO, Y., TOSA, T., and CHIBATA, I., 1972, Preparation and characteristics of microcapsules containing asparaginase. Biotechnology and Bioengineering, 14, 663-573.

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