Ind. Eng. Chem. Res. 2008, 47, 6289–6297
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Miniemulsification of Monomer-Resin Hybrid Systems Aitziber Lo´pez,† Abraham Chemtob,† Joseph L. Milton,† Mihaela Manea,† Marı´a Paulis,† Marı´a J. Barandiaran,† Sonja Theisinger,‡ Katharina Landfester,‡ Wolf Dieter Hergeth,§ Ravindra Udagama,| Timothy McKenna,| Franc¸ois Simal,⊥ and Jose´ M. Asua*,† Institute for Polymer Materials (POLYMAT) and Grupo de Ingenierı´a Quı´mica, Departamento de Quı´mica Aplicada, UniVersity of the Basque Country, Joxe Mari Korta zentroa, AVda. Tolosa 72, 20018 Donostia-San Sebastia´n, Spain; Institute for Organic Chemistry III and Polymeric Materials, UniVersity of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany; Wacker Chemie AG, R & D Polymers L-E, Johannes Hess Strasse 24, 84489 Burghausen, Germany; CNRS-LCPP (Laboratory of Chemistry and Processes of Polymerization), 43 BlVd. du 11 NoVembre 1918, 69616 Villeurbanne Cedex, France; and Cytec Surface Specialties S.A./N.V., Anderlechstraat, 33, B-1620 Drogenbos, Belgium
Miniemulsion polymerization is particularly well suited to synthesize hybrid nanocomposites. The first stage in this process is to prepare hybrid monomer-resin nanodroplets by miniemulsification. In this work, the efficiencies of several homogenization equipment (rotor-stator, sonicator, and high-pressure homogenizer) were compared. For the most efficient one, the effect of homogenization conditions, type of resin, emulsifier concentration, and viscosity of the organic phase on the size of the composite droplets was investigated. The results agreed well with a two-step mechanism in series involving droplet breakup and coalescence. Introduction Nanostructured polymer films, which synergistically combine the properties of the different polymers forming the film, are expected to be beneficial for many industrial sectors, providing new and improved properties to coatings, adhesives, cosmetics, and additives for paper and textiles. These films can be produced by using waterborne composite nanoparticles with carefully controlled structure as building blocks. Nanoparticles are highly advantageous for processing films because they offer a means to tailor the size scale of the nanostructure, given that the nanodomains cannot be larger than the size of the particle in which they are contained. A particularly promising type of composite particle is that including a polymer produced by step-growth polymerization and a polymer produced by free-radical polymerization. Miniemulsion polymerization is particularly well suited to synthesize this type of particles.1–4 The preformed step-growth polymer is dissolved in a monomer mixture (polymerizable by free radical polymerization) and a miniemulsion is formed by dispersing this solution in water in the presence of emulsifiers and costabilizers using an adequate dispersion device. The miniemulsion is then polymerized maximizing the extent of droplet nucleation. The minimum size of the composite polymer-polymer particles is that of the composite monomer-polymer droplet. Therefore, in order to produce nanoparticles, it is critical to achieve small droplet sizes. Although the formulation of liquid-liquid dispersions has been thoroughly investigated,5–8 most of these works involved droplets in the supermicron range and when the submicron range was explored, rarely the size of the dispersed phase was below 250 nm, whereas droplets smaller than 100 nm might be required for the present purpose. In addition, the viscosity of the dispersed * To whom correspondence should be addressed. E-mail: jm.asua@ ehu.es. † University of the Basque Country. ‡ University of Ulm. § Wacker Chemie AG. | CNRS-LCPP. ⊥ Cytec Surface Specialties S.A./N.V.
Figure 1. RAYNERI rotor-stator used in this study.
phase was low, while relatively high viscosities of the organic phase (polymer in monomer solution) are common in the present case. Furthermore, the volume fraction of the dispersed phase was in most of the reported cases below 20%, but high solids content (about 50 wt %) is required for most practical applications of the waterborne latexes. Droplet formation under these conditions has not been thoroughly investigated. In this article, the formation of waterborne composite monomer-polymer miniemulsions was investigated. First, the efficiency of several homogenization equipments (rotor-stator, sonicator, and high-pressure homogenizer) was compared and the most efficient one retained for the rest of the study. Then, the effect of the homogenization conditions, type of resin, emulsifier concentration, and viscosity of the organic phase on the size of the composite droplets was investigated aiming to understand the mechanisms ruling the miniemulsification process. Experimental Section In order to obtain small and homogeneously distributed droplet sizes, stirring of a heterophase system containing the monomer/resin mixture and the aqueous surfactant solution is not sufficient. In this work, the homogenization was carried out using different devices: a rotor-stator disperser, a sonicator, and a high-pressure homogenizer. The rotor-stator equipment used was a turbo test homogenizer (Rayneri) consisting of a digital display microprocessor, an overhead drive, and a homogenizing shaft. Figure 1 presents a picture of the mixed-head (5.5 cm) used which was operated at a rotational speed of 3000 rpm. As the blades rotate, a vacuum
10.1021/ie701768z CCC: $40.75 2008 American Chemical Society Published on Web 07/12/2008
6290 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008
Figure 2. (a) Schematics of the cross section of one valve of a high-pressure homogenizer. (b) Loop arrangement.
is created that draws the fluid into the assembly. The fluid is then driven toward the periphery of the head by centrifugal forces where it is subjected to milling action in the space between the ends of the blades and the slots of the stator. The fluid is then subjected to high hydraulic shear when it is ejected, at high velocity, through the narrow slots of the stator and circulated in the vessel. The high fluid acceleration at the outlet of the rotor-stator provokes its circulation in the vessel. Therefore, the fluid is continually drawn into the assembly, maintaining the mixing cycle. High energy levels can also be provided by ultrasonication. The sonifier produces ultrasound waves that cause the molecules to oscillate about their main position as the waves propagate. During the compression cycle, the average distance between the molecules decreases, while during rarefaction the distance increases. The rarefaction results in a negative pressure that may cause formation of voids or cavities (cavitation bubbles) that may grow in size. In the succeeding compression cycle of the wave, the bubbles are forced to contract and may even disappear totally. The shock waves produced on the total collapse of the bubbles cause the breakup of the surrounding monomer droplets. With increasing amplitude of the sonifier and increasing time of the sonication process, the droplet size of the emulsion decreases.9 The reason largely is that droplet breakup only occurs in a relatively small region near the sonication tip, and a certain time is needed to pass all the fluid through the sonication region. This strongly depends on the flow patterns in the vessel containing the miniemulsion.10 For every specific composition of an emulsion, a limiting value for ideal application of energy exists, which results in a maximum interface within the emulsion and therefore a minimum size of the droplets. Additional input of energy leads to a reduction of polydispersity of the droplets.11 In this process, the droplets are broken up into smaller droplets, which reaggregate into droplets immediately afterward. The homogenization by sonication is limited to small amounts of emulsion and low viscosities, since the viscous resistance during agitation absorbs most of the energy and creates heat. Figure 2a presents a schematic of the cross section of one valve of the high-pressure homogenizer. The coarse emulsion is pumped through a narrow annular gap. To flow through this gap, the emulsion has to overcome the pressure exerted on the
valve rod. The droplets of the coarse emulsion are broken up as they pass through the valve, yielding a finely dispersed system. Some equipment have a single valve whereas others had two valves in series. For most of the work carried out, a loop arrangement was used (Figure 2b). The miniemulsification was carried out as follows. First, the coarse emulsion was prepared by dispersing an organic phase in an aqueous phase under mechanical agitation. The organic phase was prepared by dissolving a given amount of resin (0-50 wt % based on the total organic phase) in a mixture of monomers. In some formulations a reactive costabilizer (e.g., stearyl acrylate, SA) was also added to the solution. The aqueous phase was prepared by dissolving the emulsifier in water. Prior to being fed to the high-pressure homogenizer, the droplet size of the coarse emulsion may be further reduced (e.g., by using sonication). This emulsion is then homogenized in the high-pressure homogenizer (in most of the work, a two-valve Manton-Gaulin). In the loop arrangement, a cycle is defined by the time needed to pass the volume of the storage tank through the homogenizer. The values of pressure given are the pressures applied by the springs on the valve rods. Droplet z-average diameters were determined by dynamic light scattering using a Coulter N4-Plus. The viscosity of the miniemulsions was measured using a Viscosimeter UK Ltd., Model ELV-8. Results a. Equipment Selection. The performance of a rotor-stator (Rayneri), a sonicator (Branson), and a high-pressure homogenizer (Laboratory 60.10 Manton-Gaulin) to form nanodroplets was assessed. For the experiments carried out for the equipment selection, formulations containing the monomer mixture methyl methacrylate (MMA)/butyl acrylate (BA)/acrylic acid (AA) (49.5/49.5/1 wt/wt), 4 wt % (related to organic phase) of costabilizer (either hexadecane or stearyl acrylate) and varying amounts (0-50 wt % based on the total organic phase) of alkyd resin (Setal 293 XX-99 from Nuplex) were used as dispersed phase. The water phase consisting of 2-6 wt % of the surfactant (Dowfax 2A1; alkyldiphenyloxide disulfonate, Dow Chemical) and distilled water. In some cases, 0.16 wt % sodium hydrogen carbonate was added, in order to increase the ionic strength that
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Figure 3. Droplet size evolution over time generated using the rotor-stator (3000 rpm), for different resin contents at 50% organic phase content.
Figure 4. Viscosity of the organic phase for different alkyd resin contents.
lowered the electrostatic droplet-droplet interaction, and consequently reduced the miniemulsion viscosity. The mixture to be miniemulsified was stirred in a 1 L vessel in an ice bath for 1 h at 2000 rpm, using the rotor-stator setup described above in order to obtain a coarse emulsion. This coarse emulsion was further miniemulsified in the different equipments (high-pressure homogenizer, rotor-stator, or ultrasound). a.1. Miniemulsification Using a Rotor-Stator. The results obtained when the coarse emulsion was further miniemulsified in the rotor-stator at 3000 rpm are presented in Figure 3. As it can be observed, the average droplet size decreased rapidly during the first 20 min of emulsification, and then began to level off asymptotically to a fixed plateau. For low alkyd concentrations, the droplet size achieved a minimum value after a little bit less than 1 h. For higher alkyd concentrations, it can be said that even if we are not at a fixed minimum value, it is clear that prolonging the emulsification time for more time would lead to little (or no) reduction in the average size. We therefore chose to emulsify all mixtures for the same period of 60 min. Furthermore, with increasing alkyd concentration, the final droplet size obtained increased, probably due to the increment of the viscosity of the dispersed phase (Figure 4). Therefore, the rotor-stator system is not effective enough to reduce the droplet size of the dispersed phase to the required size even in the absence of alkyd resin.
Figure 5. Decrease of the droplet size with sonication time, 20 wt % organic phase, 50 wbop % alkyd resin, and 2 wt % surfactant (compared to dispersed phase).
a.2. Miniemulsification Using a Sonifier. In this case, the miniemulsion (30 mL) was prepared by ultrasonicating the preemulsion at a 90% amplitude (Branson sonifier W450 Digital, 1 /2 in. tip) under ice cooling. At first the influence of the sonication time on the droplet size was investigated. Figure 5 illustrates the evolution of the droplet size depending on the sonication time in a system containing 50 wt % of the alkyd resin and 2 wbop % (weight based on the organic phase) of Dowfax 2A1, having an overall organic fraction of about 20 wt %. The droplet size of the miniemulsion decreased until a limiting value was reached at a sonication time of 120 s. Further application of energy led to a marginal decrease of the droplet size, but the polydispersity index decreased until a minimal value of 0.14 which was reached at a sonication time of 150 s. (DLS measurements give the z-average size, which is an intensity mean, and the polydispersity index. The standard cumulant analysis is the fit of a polynomial to the log of the G1 correlation function: ln(G1) ) a + bt + ct2 + dt3 +.... The value of secondorder cumulant b is converted to a size using the dispersant viscosity and some instrumental constants. The coefficient of the squared term c, when scaled as 2c/b2 is known as the polydispersity index.) Therefore, a sonication time of 150 s was commonly used for further investigations. In the next step the influence of the resin content inside the organic phase on the droplet size was investigated. The amount of resin was varied between 0 to 50 wbop % at keeping constant the total organic phase at 20%. Figure 6 shows the increase of the droplet size of the miniemulsion with increasing amount of resin. With increasing amount of alkyd resin, the viscosity of the organic phase exponentially increased (see Figure 4). The increase of viscosity makes more difficult to break up the droplets and resulted in bigger droplets. The viscosity of the resulting miniemulsion against the resin content is also given in Figure 6. In the range of resin contents in which the droplet size remained approximately constant (up top 25%) the viscosity increased with increasing resin content. This might be due to the amphiphilic (or partly hydrophilic) behavior of the resin leading to a more extended hydrodynamic surfactant layer causing higher viscosities. For higher alkyd amounts, the viscosity decreased as the droplet size increased. As increasing the organic phase content leads to a higher probability of coalescence of the droplets, the use of higher
6292 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008
Figure 6. Droplet sizes and viscosities of the miniemulsions in dependence of the alkyd resin content, 2 wt % surfactant, and 20 wt % organic phase.
Figure 8. Viscosities of miniemulsion samples (BA/MMA/AA 49.5/49.5/ 1) at 6 wt % surfactant with different organic phase contents in dependence of the amount of alkyd resin.
Figure 7. Droplet sizes and viscosities of the miniemulsions in dependence of organic phase content, 4 wt % surfactant, and 50 wt % alkyd resin related to organic phase.
amounts of surfactant and longer sonication times were necessary to obtain stable droplets and to reach the possible minimal value of the droplet size. Therefore, the amount of surfactant was raised up to 4 wt % related to the dispersed phase and the sonication time was increased to 3 min. The resulting viscosities and droplet sizes for the investigated miniemulsions containing 50 wt % alkyd resin in the dispersed phase are illustrated in Figure 7. The droplet size was approximately constant whereas the viscosity showed an increase with increasing organic phase content, so that insufficient mixing during sonication occurred. In fact, the sonicator acts only in the small volume near the sonication tip. Mixing is provided by a magnetic stirrer. Therefore, when the viscosity increased, the magnetic stirrer was not able to mix well the flask. Stable miniemulsion with organic phase content of 50 wt % by using 4 wt % of the surfactant in consequence of the strong droplet interactions could not be obtained. An increase of the surfactant level to 6% and the addition of small amounts of sodium hydrogen carbonate were required to reduce the strong droplet interactions. The miniemulsion was stirred during the sonication step, and the sonication time was raised up to 4 min and separated into sonication steps of 15 s followed by pause of 30 s to cool down the miniemulsion in the ice bath. Thereby it was possible to produce a miniemulsion having a organic phase content of 50 wt % containing 50 wbop % alkyd
Figure 9. Droplet sizes of the miniemulsions obtained after being subjected to different amounts of cycles in the Manton-Gaulin homogenizer, for presonified and not presonified coarse emulsions (2 wt % Dowfax 2A1, 15 wbop % alkyd resin, and 50% organic phase content).
resin. The droplet size of this miniemulsion was 101 nm, and the miniemulsion viscosity was 1006 mPa · s. An overview of the viscosities at different organic phase contents using 6% of surfactant is shown in Figure 8. It is worth pointing out that the droplet size that can be achieved by sonication strongly depends on the dispersion volume. Thus, sonication of 500 mL of 2 wt % Dowfax2A1, 15 wbop % alkyd resin, and 50% orgnic phase content yielded a droplet size of 152 nm. Therefore, sonication seems appropriate to prepare small volumes of monomer miniemulsions with droplets small enough, but sonicating larger volumes appears to be a problem; namely, it is not well adapted to scale-up. Therefore, work with the highpressure homogenizer was started, which was expected not to offer problems in the scaling up, as this equipment is used in the high tonnage dairy industry. a.3. Miniemulsification Using a High-Pressure Homogenizer. Figure 9 presents the miniemulsion droplet sizes obtained when using the Manton-Gaulin high-pressure homogenizer, for miniemulsification of 1 L of sample containing 2% Dowfax 2A1, 15 wbop % alkyd resin, and 50% organic phase content.
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Figure 10. Effect of organic phase content, resin content, and emulsifier concentration on droplet size and miniemulsion viscosity.
First of all, it has to be pointed out that the droplet size obtained after several cycles in the Manton-Gaulin was 119 nm, much smaller than the one obtained after sonication of 500 mL(15 min, power 9, duty cycle 80%) 152 nm. On the other hand, it can be seen that the number of cycles needed to reach the minimum droplet size was less when the size of the emulsion fed to the homogenizer was reduced. Nevertheless, the same final droplet size was obtained in both cases. The droplet size distribution as estimated by light scattering became narrower with the number of cycles. Therefore, the high-pressure homogenizer was retained for the rest of the work. b. High-Pressure Homogenization of Different Polymer/ Monomer Systems. As has been seen in Figure 9, the droplet size rapidly decreased in the Manton-Gaulin homogenizer until a certain minimum value was reached. Longer homogenization processes did not yield smaller sizes. This minimum value is characteristic of each equipment, homogenization conditions and formulation, and it will be used in all the plots presented below.
b.1. Acrylic-Alkyd System. Figure 10 presents the effect of the organic phase content, resin content and emulsifier concentration on the droplet size and viscosity of the miniemulsion for a formulation containing acrylic monomers (MMA/ BA/AA ) 49.5/49.5/1), alkyd resin (SETAL 293 XX-99), costabilizer (stearyl acrylate, 4% wbm (weight based on monomer)) and Dowfax 2A1 as surfactant. A buffer concentration of 0.024 M NaHCO3 was used for the formulations having 2 and 4 wbop % Dowfax 2A1 and 0.039 M NaHCO3 when working at 6 wbop % Dowfax 2A1, to lower the viscosity. Miniemulsification was conducted in a Manton-Gaulin highpressure homogenizer using a pressure of 41.4 × 106 Pa in the first valve and 41.4 × 105 Pa in the second valve. Since the pressures are the pressures applied by the string, they closely correspond to the pressure drop in the valve. Figure 10 shows that droplet size increased as organic phase content and resin contents increased and as emulsifier concentration decreased. Manea et al.12 have proposed that the droplet size is the result of two consecutive processes: droplet break up and coalescence.
6294 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 Table 1. Relative Viscosities of the Monomer Solutions of Alkyd Resins relative viscosity (ηd/ηr)a resin content 30% 50%
Setal 293-XX-99
Setal X-11 768
5 8
6 10
a ηd, ηr: viscosity of the disperse phase and continuous phase, respectively.
The droplets of the coarse emulsion suffer extensional deformation at the entrance of the valve. Although elongational flow may break up highly viscous droplets, it is believed that the time that droplets spend at the entrance of the gap is too short to break them.13 Nevertheless, the deformation is important to facilitate the breakup by turbulence at the exit of the valve. The newly formed droplets present a part of their surface not well covered by the surfactant, and hence they may coagulate. The final droplet size was determined by the mechanism giving the larger droplet size. Increasing resin content resulted in a higher viscosity of the organic phase and hence in less intensive breakup. Therefore, droplet size increased with resin content. Increasing organic phase content increased the probability of coalescence leading to larger droplet sizes. The use of higher concentrations of emulsifier improved the stability of the newly formed droplets yielding smaller droplet sizes. It was found that both the viscosity and the droplet size of the miniemulsions increased with the resin content. This is surprising because dispersion viscosity usually decreases when the size of the dispersed phase increases.14 Therefore, the viscosity increase suggests that droplet-droplet interaction was stronger when a higher amount of resin was used. The effect of the hydrophobicity of the resin on the adsorption of the emulsifier may be the reason for the stronger droplet-droplet interaction. A higher resin content resulted in a stronger droplet emulsifier adsorption (smaller parking area) which led to a higher surface coverage (the contribution of the smaller total surface area was also substantial) and a lower concentration of emulsifier in the aqueous phase. Therefore, both the surface charge density and the surface potential increased, and hence in a low ionic strength medium the droplet-droplet interaction was very strong, leading to a viscosity increment. The effect of using a more hydrophobic alkyd resin (Setal X-11768) was investigated maintaining constant the other components of the formulation. The monomer solution of this resin was slightly more viscous than that of Setal 293 XX-99 (Table 1). Figures 11 and 12 present the effect of the homogenization pressure on the droplet size for two contents of the resins Setal 293 XX-99 and Setal X-11768 (30 and 50 wbop %) and several emulsifier concentrations (2, 4, and 6 wbop %). The surface tensions of these miniemulsions are presented in Table 2. For an aqueous solution of NaHCO3 (0.024 M) the cmc was 0.0035 g/L and the surface tension at the cmc was 30 mN/m. Therefore, in the runs presented in Table 2 there were no micelles and most of the emulsifier was located on the droplets. Figures 11 and 12 support the two-step mechanism for droplet formation. For the less viscous organic phase (30% resin) at low emulsifier concentrations, the droplet size was not affected by the valve pressure (i.e., by the energy applied) because the minimum size that could be stabilized by the emulsifier available was larger than that resulting from the breakup process. At high emulsifier concentrations, the droplet size initially decreased with pressure and remained constant at higher values of the homogenization pressure. At low pressures, the droplet size
Figure 11. Comparison between Setal X-11768 (filled symbols) and Setal 293 X-99 (empty symbols) for a 30 wbop % of resin (O, 2%Dowfax; 0, 4% Dowfax; ], 6% Dowfax).
Figure 12. Comparison between Setal X-11768 (filled symbols) and Setal 293 X-99 (empty symbols) for a 50 wbop % of resin (O, 2% Dowfax; 0, 4% Dowfax; ], 6% Dowfax). Table 2. Surface Tensions (mN/m) of the Miniemulsions Prepared with Setal 293 XX-99 at Different Pressures first valve pressure (41.4 MPa)
30% resin
50% resin
first valve pressure (6.9 MPa)
30% resin
50% resin
2% emulsifier 4% emulsifier 6% emulsifier
32.4 33.0 33.1
33.2 33.2 33.4
2% emulsifier 4% emulsifier 6% emulsifer
31.5 31.7 34.2
31 33.8 33.9
determined by the breakup was higher than the one that could be stabilized by the emulsifier amount; therefore, droplet breakup determined the droplet size, which was lower for higher pressures. However, as pressure increased, the amount of emulsifier available to stabilize the smaller droplets resulting from breakup was not enough, and the droplet size was determined by the emulsifier amount (droplet coagulation). On the other hand, for a given homogenization pressure and emulsifier concentration, droplet size increased with the resin content, which can be seen as an interplay between the higher viscosity of the organic phase, leading to a more difficult breakup and higher hydrophobicity of the organic phase, which led to a higher demand of emulsifier for its stabilization. A detailed comparison of the results obtained with the two resins shows that the droplet sizes obtained with Setal X-11768 were larger and the plateau showing that the droplet size was
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Figure 13. In-line arrangement of the high-pressure homogenizer. Table 3. Homogenization of Resin Euroresin ER51110 Containing Formulations monomers
PU content (wbop %)
droplet size (nm)
MMA/BA/AA MMA/BA/AA BA/AA BA/AA 2EHA/AA 2EHA/AA
0 10 0 10 0 10
122 97 134 98 345 191
controlled by the availability of surfactant was less evident. This indicates that droplet breakup was more difficult for resin Setal X-11768, which was in agreement with a higher viscosity of the organic phase (Table 1). In addition, the size of the droplets at the plateau was larger for Setal X-11768, which was the consequence of its higher hydrophobicity. b.2. Acrylic-Polyurethane System. Several monomer-polyurethane (PU) formulations were homogenized. Two PUs were used. Euroresin ER51110 was a rather hydrophilic nonreactive polyurethane and Incorez 701 was a less hydrophilic reactive PU. Euroresin ER51110 containing formulations were homogenized passing four times through a Manton-Gaulin in an inline arrangement (Figure 13), whereas Incorez 701 was homogenized using a Manton-Gaulin in a loop arrangement. Table 3 summarizes the results obtained with resin Euroresin ER51110, for formulations with 4 wt % of stearyl acrylate and 2 wt % Dowfax 2A1 at 50% organic phase content. It can be seen that the addition of the hydrophilic PU led to a reduction of the droplet size, showing that for this system the hydrophilicity (which is related to the coalescence of newly formed droplets) counteracted the increase in viscosity due to the presence of the resin. The reason is that the driving force for droplet-droplet coalescence is van der Waals attraction, which is proportional to the effective Hamaker constant. This constant is proportional to the droplet-water interfacial tension;15 i.e., it decreases with the hydrophilicity of the organic phase. Table 3 also shows that the use of more hydrophobic monomers (e.g., BA and 2-ethylhexyl acrylate (2EHA)) led to higher droplet sizes. Figures 14–16 show the effect of the homogenization pressure on the droplet size for several Incorez 701 contents at two emulsifier concentrations. The organic phase content was 52 wt % and the acrylic monomer mixture was BA/MMA/ methacrylic acid (MAA) (90/9/1 wt/wt); 4 wbm % SA was used as costabilizer and Dowfax 2A1 as emulsifier. It can be seen that droplet size was almost not affected by valve pressure, which indicates that the size was controlled by the availability of emulsifier (coalescence). Droplet size showed a relatively weak dependence with respect the resin content, suggesting that Incorez 701 had a hydrophobicity similar to that of the monomer mixture.
Figure 14. Effect of valve pressure and emulsifier concentration on droplet size for 5 wbop % of Incorez 701 (O, 2% Dowfax; 9, 4% Dowfax).
Figure 15. Effect of valve pressure and emulsifier concentration on droplet size for 10 wbop% of Incorez 701 (O, 2% Dowfax; 9, 4% Dowfax).
Figure 16. Effect of valve pressure and emulsifier concentration on droplet size for 20 wbop % of Incorez 701 (b, 1% Dowfax; O, 2% Dowfax; 9, 4% Dowfax).
b.3. Acrylic-Biocompatible Polymers System. For some applications, the use of biocompatible materials is required. Therefore, a formulation containing biocompatible monomers (MMA/BA/AA 49.5/49.5/1), polymers (cellulose acetate butyrate, CAB and poly(ε-caprolactone), CAPA) surfactant (di-
6296 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 Table 5. Physical Chemical Parameters of the Silicone Oils silicone oil
AK1
AK10
AK500 AK5000
shear viscosity (mPa · s) spindle No. shear rate (rpm) apparent extensional viscosity (mPa · s) surface tension (mN/m)
4 1 10 not detectable
12 1 10 not detectable
500 2 10 3000
5000 7 10 30 000
16.7
19.3
20.7
19.5
Table 6. Experimental Conditions for Emulsification of Silicone Oils: (I) Formulations without Ethyl Acetate. (II) Formulations with Ethyl Acetate variable
values
Formulations without Ethyl Acetate
Figure 17. Effect of valve pressure and emulsifier concentration on droplet size for cosmetic formulations containing 30 wbm % of CAB (O, 2% DSG; b, 4% DSG; 0, 6% DSG).
viscosity (mPa · s) no. of cycles organic phase content (%) pressure (MPa) surfactant concentration (%)
1, 10, 500, 5000 1, 2, 3, 4, 5 20, 30, 40 80, 100, 120 2, 3, 5
Formulations with Ethyl Acetate organic phase content (%) pressure (MPa) surfactant concentration (%)
20, 30, 40 30, 50, 80 2, 3, 5
Table 7. Influence of Homogenization Pressure on Droplet Size (20% Organic Phase, 2% Surfactant)
Figure 18. Effect of valve pressure and emulsifier concentration on droplet size for cosmetic formulations containing 50 wbm % of CAPA (O, 2% DSG; b: 4% DSG; 0, 6% DSG). Table 4. Relative Viscosities of the Miniemulsions for Cosmetics Formulations relative viscosity ηd/ηc 30 wbm % CAB 50 wbm % CAPA
300 42
sodium stearoyl glutamate, DSG), and costabilizers (cetyl alcohol) was used. 40 wt % organic phase content miniemulsions containing either 30 wbm % of CAB or 50 wbm % of CAPA were prepared. Figures 17 and 18 present the effect of the valve pressure and emulsifier concentration on droplet size. These plots are different from those shown above for alkyd and PU resins as neither effect of emulsifier concentration on droplets size nor plateau at high valve pressures were observed. This indicates that droplet size was mainly controlled by droplet breakup, namely, that in all cases there was enough surfactant to stabilize the miniemulsions. The reason was the high viscosities of the organic phase (Table 4 vs Table 1). Comparison between Figures 17 and 18 shows that under similar miniemulsification conditions the droplet size was larger for the formulations containing CAB. This was due to the fact that the viscosity of the organic phase was higher for the CAB containing system (Table 4). b.4. Silicone-Containing Systems. Miniemulsification of silicone oils was also investigated. An IKA high-pressure homogenizer 2000/4-SH5 was used for the homogenization
sample ID
press. (MPa)
no. of cycles
droplet size (nm)
sp surf. area (m2/g)
AK1, 80 MPa AK1, 100 MPa AK1, 120 MPa AK10, 80 MPa AK10, 100 MPa AK10, 120 MPa AK500, 80 MPa AK500, 100 MPa AK500, 120 MPa AK5000, 80 MPa AK5000, 100 MPa AK5000, 120 MPa
80 100 120 80 100 120 80 100 120 80 100 120
2 2 2 2 2 2 3 3 3 3 3 3
155 170 141 128 170 117 2539 1748 393 4877 4877 3687
46.77 44.52 46.19 50.25 38.82 56.18 9.63 13.27 17.55 2.00 2.87 7.32
experiments. The pressure built up in the homogenizer was up to 200 MPa, the working quantity was 200 mL/min, and the flow rate was up to 3 L/h, adjustable via speed adjustment. AK 1, AK 10, AK 500, and AK 5000 (Wacker) silicone oils were used. Their shear viscosity, extensional viscosity, and surface tension are listed in Table 5. The extensional viscosity for Newtonian fluids is typically 3 times the shear viscosity (Trouton ratio, Tr ) 3). However, according to Table 5, the extensional viscosities of AK 500 and AK 5000 are 6 times their shear viscosity, indicating that these two silicone oils were non-Newtonian fluids. Table 6 summarizes the experiments. Table 7 presents the effect of the homogenization pressure on the droplet size for silicone oils of different viscosity. It is important pointing out that all the resins had the same chemical composition and therefore a similar interfacial tension. Table 7 shows that for low viscous silicone oils, droplet size did not vary with homogenization pressure, whereas it decreased for high-viscosity oils. This is coherent with the two-step process discussed above. For low-viscosity oils, the energy supplied by the homogenizer was enough to break up the droplets to a size smaller than the one that could be stabilized by the emulsifier. Therefore, the size was controlled by coagulation and similar droplet sizes were obtained for oils AK1 and AK10. For highviscosity oils, droplet size was controlled by droplet break up and droplet size decreased with homogenization pressure. Table 8 shows that the droplet size was strongly reduced when the
Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 6297 Table 8. Miniemulsions Prepared in the Presence of Ethyl Acetate (20% Organic Phase, 2% Surfactant) sample ID AK500, 80 MPa AK500 + 15% EA, 80 MPa
viscosity no. of droplet sp surf. cycles size (µm) area (m2/g) (MPa · s) 3 2
2.539 0.088
9.632 67.99
500 6
viscosity of the organic phase was reduced by the addition of ethyl acetate thus simulating a silicone-vinyl monomer mixture. Conclusions In the foregoing, the formation of waterborne monomer-resin miniemulsions was investigated. The goal was to prepare small size (about 100 nm) high organic phase content (50 wt %) miniemulsions containing a high concentration of resin (50 wt % based on the organic phase). The efficiency of three homogenization devices (rotor-stator, sonicator, and highpressure homogenizer) was compared in terms of the minimum droplet size that could be obtained at high organic phase and high resin contents. It was found that the rotor-stator was not effective enough to reduce the droplet size of the dispersed phase to the required size. Sonication was effective to prepared small volumes of miniemulsions with small droplet sizes, but it is not well adapted for scale-up. High-pressure homogenization allowed preparing small size, high organic phase content miniemulsions containing high concentrations of resin. Miniemulsion droplet size was determined by the interplay between two processes occurring mainly in series: droplet breakup and droplet coalescence. The size of the droplets was determined by the process giving the larger droplet size. The droplet size resulting from droplet breakup increased with increasing resin concentration (increasing viscosity of the organic phase) and by decreasing the energy applied (decreasing valve pressure). The droplet size resulting from coalescence increased as surfactant concentration decreased and the hydrophobicity of the organic phase increased. The droplet size of miniemulsions with low-viscosity organic phase was mainly controlled by the emulsifier used. On the other hand, equipment was the factor determining the droplet size in systems with a high-viscosity organic phase. Acknowledgment A.L. acknowledges the financial support by the Basque Government. The financial support from the European Union
(NAPOLEON NMP3-CT-2005-011844), the Diputacio´n Foral de Gipuzkoa, and the Ministerio de Ciencia y Tecnologı´a (CTQ2006-03412) projects is gratefully acknowledged. Literature Cited (1) Asua, J. M. Miniemulsion Polymerization. Prog. Polym. Sci. 2002, 27, 1283–1346. (2) Schork, J. F.; Luo, Y.; Smulders, W.; Russum, J. P.; Butte´, A.; Fontenot, K. Miniemulsion Polymerization. AdV. Polym. Sci. 2005, 175, 129–255. (3) Landfester, K. Miniemulsions for Nanoparticle Synthesis. Top. Curr. Chem. 2003, 227, 75–123. (4) Landfester, K. Synthesis of Colloidal Particles in Miniemulsions. Annu. ReV. Mater. Res. 2006, 36, 231–279. (5) Shinnar, R.; Church, J. M. Predicting Particle Size in Agitated Dispersions. J. Ind. Eng. Chem. 1960, 52, 253–256. (6) Calabrese, R. V.; Changm, T. P. K.; Dang, P. T. Drop Breakup in Turbulent Stirred-Tank Contactors. Part I: Effect of Dispersed-Phase Viscosity. AIChE J. 1986, 32, 657–666. (7) Chatzi, E. G.; Kiparissides, C. Steady-State Drop-Size Distributions in High Holdup Fraction Dispersion Systems. AIChE J. 1995, 41, 1640– 1652. (8) Schultz, S.; Wagner, G.; Urban, K.; Ulrich, J. High-Pressure Homogenization as a Process for Emulsion Formation. Chem. Eng. Technol. 2004, 27, 361–368. (9) Behrend, O.; Ax, K.; Schubert, H. Influence of Continuous Phase Viscosity on Emulsification by Ultrasound. Ultrasonics Sonochem. 2000, 7, 77–85. (10) do Amaral, M.; Arevalillo, A.; Santos, J. L.; Asua, J. M. Novel Insight into the Miniemulsification Process: CFD Applied to Ultrasonication. Prog. Colloid Polym. Sci. 2004, 124, 103–106. (11) Li, M. K.; Fogler, H. S. Acoustic Emulsification. Part 1. The Instability of the Oil-Water Interface to Form the Initial Droplets. J. Fluid. Mech. 1978, 88, 499. (12) Manea, M.; Chemtob, A.; Paulis, M.; de la Cal, J. C.; Barandiaran, M. J.; Asua, J. M. Miniemulsification in High Pressure Homogenizers. AIChE J. 2008, 54, 289–297. (13) Floury, J.; Belletre, J.; Legrand, J.; Desrumaux, A. Analysis of a New Type of High Pressure Homogenizer. A Study of the Flow Pattern. Chem. Eng. Sci. 2004, 59, 843–853. (14) Arevalillo, A.; do Amaral, M.; Asua, J. M. Rheology of Concentrated Polymeric Dispersions. Ind. Eng. Chem. Res. 2006, 45, 3280–3286. (15) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: New York, 1987; Vol. I., pp 332-333.
ReceiVed for reView December 26, 2007 ReVised manuscript receiVed May 21, 2008 Accepted May 29, 2008 IE701768Z
