PREPARATION OF LATEX USING MINIEMULSION TECHNOLOGY

1

PREPARATION OF LATEX USING MINIEMULSION TECHNOLOGY

Eric S. Daniels, E. David Sudol and Mohamed S. El-Aasser

Emulsion Polymers Institute Lehigh University

111 Research Drive Bethlehem, PA 18015

Presented at the International Latex Conference

July 25-26, 2006 Charlotte, NC USA

ABSTRACT

Miniemulsions are typically oil-in-water emulsions most often prepared using a high shear device and a mixed emulsifier system consisting of an ionic surfactant and a costabilizer, the latter being either a fatty alcohol (such as cetyl alcohol) or a long chain alkane (such as hexadecane). These emulsions are characterized by high stability and droplet sizes ranging from 50 to 500 nm. Both “artificial” and “synthetic” latexes can be prepared through the use of miniemulsion technology. The former refers to latexes prepared by emulsifying polymer solutions with subsequent removal of the solvent, allowing the preparation of latexes of polymers that cannot be prepared by emulsion polymerization (e.g., epoxies and polyurethanes). The latter refers to latexes prepared by emulsion polymerization. The focus of this talk will be on the use of miniemulsion technology to prepare polymer colloids as well as a discussion of a number of applications of miniemulsion technology such as the preparation of nanosize particles, the encapsulation of inorganic pigments, and the formation of hybrid composite polymers.

Introduction

Miniemulsion technology is increasingly being utilized industrially to prepare a wide variety of latex particles in the 50 to 500 nm size range [E.D. Sudol and M.S. El-Aasser, “Miniemulsion Polymerization”, Chapter 20, in: Emulsion Polymerization and Emulsion Polymers, P.A. Lovell and M.S. El-Aasser, Eds., John Wiley and Sons, Chichester, 699, 1997; F. J. Schork, Y. Luo, W. Smulders, J.P. Russum, A. Butte, and K. Fontenot, “Miniemulsion

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Polymerization”, Adv. Polym. Sci., 175 (Polymer Particles), 129 (2005) ; J.M. Asua, “Miniemulsion Polymerization”, Prog. Polym. Sci, 27, 1283 (2002); N. Bechthold, F. Tiarks, M. Willert, K. Landfester, M. Antonietti, "Miniemulsion Polymerization: Applications and New Materials”, Macromolecular Symp., 151 (Polymers in Dispersed Media), 549 (2000).] Miniemulsions are usually oil-in-water emulsions that are prepared using high shear and a mixed stabilizer system comprised of an ionic surfactant, such as sodium lauryl sulfate (SLS) used in a concentration range typically employed in emulsion polymerization (i.e., 0.5 to 5 wt%), along with a costabilizer. The costabilizer is typically a long chain alkane or alcohol with a carbon length greater than or equal to 12, with low molecular weight and low water solubility. Typical examples of the costabilizer are hexadecane and cetyl alcohol. The purpose of the costabilizer is to reduce Ostwald ripening or transport of monomer from the miniemulsion droplets. Miniemulsions are opaque in appearance, with droplet sizes in the range of 50 to 500 nm, and are metastable, with shelf-live stabilities on the order of hours to months, compared to conventional macroemulsions that are stable for times ranging from seconds to minutes. Polymerization begins by entry of free radicals into the miniemulsion droplets creating particles (Figure 1a) as compared to the case of conventional emulsion polymerization where particle nucleation typically occurs in micelles or via homogeneous nucleation in the aqueous phase (Figure 1b). The monomer droplets act as reservoirs for monomer to diffuse into the micelles and particles in conventional emulsion polymerizations that are carried out at surfactant concentrations greater than the critical micelle concentration (cmc). Miniemulsion technology may also be employed in instances where it is desired to create a polymer latex out of a polymeric material that cannot be prepared by a free radical polymerization process, such as an epoxy. In this process, termed miniemulsification, the polymer is first dissolved in a good solvent. A costabilizer, such as hexadecane, is then added to the oil phase. A miniemulsion is then prepared by adding the polymer/costabilizer solution (oil phase) into an aqueous phase that contains surfactant and homogenizing the two phases with a high shear device. Solvent is then stripped from this miniemulsion leaving a dispersion of the polymer particles in water. The dispersion formed by this process is often termed an “artificial” latex. This miniemulsification technology can also be used to encapsulate pigments and dyes within a polymer. Miniemulsions were first “discovered” at Lehigh University in 1972 [M.S. El-Aasser and E. D. Sudol, “Miniemulsions: Overview of Research and Applications”, J. Coat. Tech. Research, 1, 21 (2004)] and several early patents were granted to Lehigh for the miniemulsification process [J.W. Vanderhoff, M.S. El-Aasser, and J. Ugelstad; U.S. Patent 4,177,177 (1979); J.W. Vanderhoff, M.S. El-Aasser, and J.D. Hoffman, U.S. Patent 4,070,323 (1979).] Some of the work carried out in the Emulsion Polymers Institute at Lehigh University in the area of miniemulsion technology will be the focus of this paper. A wide variety of monomers have been used in miniemulsion homopolymerizations including styrene [Y.T. Choi, M.S. El-Aasser, E.D. Sudol, and J.W. Vanderhoff, “Polymerization of Styrene Miniemulsions”, J. Polym. Sci.: Polym. Chem. Ed., 23, 2973 (1985); P.J. Blythe, A. Klein, J.A. Phillips, E.D. Sudol, and M.S. El-Aasser, “Miniemulsion Polymerization of Styrene Using the Oil-Soluble Initiator AMBN”, J. Polym. Sci.: Part A: Polym. Chem., 37, 4449 (1999)], and divinylbenzene [S. Mohammed, “Miniemulsion Polymerization of Divinylbenzene-HP: A Facile Route to Highly Crosslinked Particles”, M.S.

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Thesis, Lehigh University, 1992], and a wide variety of copolymerizations including styrene/methyl methacrylate [J.M. Asua, V.S. Rodriguez, C.A. Silebi, and M.S. El-Aasser, “Miniemulsion Copolymerization of Styrene-Methyl Methacrylate: Effect of Transport Phenomena”, Makromol. Chem., Macromol. Symp., 35/36, 59 (1990)], styrene/butyl acrylate [C.D. Anderson, “Miniemulsion Copolymerization of Styrene and n-Butyl Acrylate Using Triton X-405 as Surfactant”, M.S. Thesis, Lehigh University, 1999], vinyl acetate/butyl acrylate [J. Delgado, M.S. El-Aasser, and J.W. Vanderhoff, “Miniemulsion Copolymerization of Vinyl Acetate and Butyl Acrylate. I. Difference between the Miniemulsion Copolymerization and the Emulsion Copolymerization Processes”, J. Polym. Sci.: Part A: Polym. Chem., 24, 861 (1986)], styrene/butadiene [D. Li, E.D. Sudol, and M.S. El-Aasser, “Miniemulsion and Conventional Emulsion Copolymerization of Styrene and Butadiene: A Comparative Kinetic Study”, J. Appl. Polym. Sci., 101, 2304 (2006)], and vinyl acetate/vinyl 2-ethylhexanoate [E. Kitzmiller, “Conventional Emulsion and Miniemulsion Homopolymerization and Copolymerization of Vinyl Acetate and Vinyl 2-Ethylhexanoate”, Ph.D. Dissertation, Lehigh University, 1999],. There are also a wide variety of other (co)monomer systems that have been investigated. (a) (b) Figure 1: Schematic representations of (a) miniemulsion vs. (b) conventional emulsion polymerization.

R.

R.

R.

Precursorparticles

Micelle

Oligomers

Monomer Droplets

Polymerparticles

or

R.

R.

R.R.

R.

R.

Hexadecane

SLS

Cetyl Alcohol

Polymerparticles

4

Some of the advantages and applications for miniemulsion technology include the following. First, monomers or oligomers with very low (or no) water solubility can be polymerized by miniemulsion polymerization since polymerization can occur solely within hydrophobic miniemulsion droplets. Second, pigments and dyes can be encapsulated by either miniemulsion polymerization [B. Erdem, “Encapsulation of Inorganic Particles via Miniemulsion Polymerization”, Ph.D. Dissertation, Lehigh University, 1999; B. Erdem, E. D. Sudol, V.L. Dimonie, and M.S. El-Aasser, "Encapsulation of Inorganic Particles via Miniemulsion Polymerization. I. Dispersion of Titanium Dioxide Particles in Organic Media Using OLOA 370 as Stabilizer." J. Polym. Sci.: Part A: Polymer Chemistry, 38: 4419-4430, 2000)] or miniemulsification [G. H. Al-Ghamdi, “Encapsulation of Inorganic Particles via Miniemulsion and Film Formation of Resulting Composite Latex Particles”, Ph.D. Dissertation, Lehigh University, 2003] since the dye or pigment can be emulsified within the miniemulsion droplet and the monomer polymerized to encapsulate the pigment within the polymer. The use of miniemulsion technology also allows closer control of particle size and particle size distribution by controlling the initial droplet size and size distribution since under ideal cases each droplet may be polymerized to form a polymer particle. Controlling the droplet size and size distribution by the varying the amount and type of shear, and the type and concentrations of the surfactants and costabilizers used in the miniemulsion formulation would control the final particle size and size distribution. Third, miniemulsions may offer an advantage if a pre-emulsion is used to feed monomer into a reactor. In a conventional emulsion polymerization, monomer(s) may be emulsified with water and surfactant and then fed into the reactor and polymerized. However, macroemulsions are usually unstable and the pre-emulsion must be kept stirred as it is fed into a reactor. On the other hand, miniemulsions are stable for much longer time and can be easily fed into a reactor without continual agitation being required. Fourth, hybrid latexes can be prepared by miniemulsion or miniemulsification technology. For example, a seed latex can be prepared by miniemulsification (to form an “artificial latex) of a polymer that cannot be prepared by free radical polymerization such as a thermoplastic elastomer (e.g., Kraton rubber) and then another monomer(s) can be polymerized around the seed polymer particles to form a “core/shell” latex by using a combination of miniemulsion technology with a conventional seeded emulsion polymerization process [M. Merkel, “Morphology of Core/Shell Latexes and their Mechanical Properties”, Ph.D. Dissertation, Lehigh University, 1986]. Finally, miniemulsion polymerization has been utilized to form “living” polymers with controlled molecular weights using stable free radical polymerization (SFRP) [G. Pan, E. D. Sudol, V. L. Dimonie, and M.S. El-Aasser, “Thermal Self-initiation of Styrene in the Presence of TEMPO Radicals: Bulk and Miniemulsion”, J. Polym. Sci., Part A: Polym. Chem., 42, 4921 (2004); M. Cunningham, M. Lin, C. Buragina, S. Milton, D. Ng, C.C. Hsu, and B. Keoshkerian, “Maximizing Polymer Livingness in Nitroxide-mediated Miniemulsion Polymerizations”, Polymer, 46, 1025 (2005); M. Cunningham, M. Lin, J. Smith, J. Ma, K. McAuley, B. Keoshkerian, M. Georges, “Nitroxide-Mediated Living Radical Polymerization in Dispersed Systems”, Prog. Colloid Polym. Sci., 124, 88 (2004)], reversible-addition chain transfer (RAFT) polymerization [X. Huang, “Controlled Radical Miniemulsion Polymerization via the Raft Process”, Ph.D. Dissertation, Lehigh University, 2003]; J.P. Russum, N.D. Barbre, C.W. Jones and F.J. Schork, “Miniemulsion Reversible Addition Fragmentation Chain Transfer Polymerization of Vinyl Acetate”, J. Polym. Sci., Part A: Polym. Chem, 43, 2188 (2005), or atom transfer radical polymerization (ATRP) [K. Matyjaszewski, J. Qiu, N. Tsarevsky, and B. Charleux, “Atom Transfer Radical Polymerization of n-Butyl Methacrylate in an Aqueous Dispersed System: A Miniemulsion Approach”, J. Polym.

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Sci., Part A: Polym. Chem., 38 (Suppl.), 4724 (2000); M. Li and K. Matyjaszewski, “Further Progress in Atom Transfer Radical Polymerizations Conducted in a Waterborne System”, J. Polym. Sci., Part A: Polym. Chem, 41, 3606 (2003)]. General Preparation of Latexes by Miniemulsion Polymerization Table 1 shows a typical miniemulsion recipe. In the case where cetyl alcohol is used as the costabilizer, it is first added to water along with the surfactant, SLS, and heated and mixed to 65 °C (above the melting point of CA) to form a homogenous solution. The oil phase (monomer(s) is then added to the aqueous phase and initially stirred and emulsified for a given time period. The emulsion is then subjected to higher shear for various times and energy input levels using a high shear device such as a sonifer, Microfluidizer, Omni Mixer (i.e., rotostator homogenizer), or a Manton-Gaulin homogenizer (Figure 2). In the case where HD is used as costabilizer, it is initially mixed in the oil phase before emulsification. Initiator solution can then be added to the miniemulsion at the reaction temperature, and polymerization carried out. Some factors affecting the effectiveness of emulsification include the addition strategy for the surfactant, oil phase and aqueous phase, the type and concentration of emulsifier and costabilizer, the agitation time, intensity and type of shear device, temperature, the density difference between the dispersed and continuous phase, and the viscosity of the continuous phase. As a general rule of thumb, miniemulsions can be prepared if the continuous phase has a viscosity of less than 10,000 cps. Figure 3 shows the effect of the shear device on both the miniemulsion polymerization kinetics as well as the particle size and size distribution. It can be seen that the polymerization rate is fastest in the case when the Microfludizer is used and the lowest when the Omni mixer was used. In addition the particle size was smallest (145 nm) and the size distribution narrowest when the Microfluidizer was used compared to the sonifier (271 nm) or the Omni mixer (300 nm). Under the given processing conditions, the Microfluidizer formed the smallest miniemulsion droplets with higher numbers of particles nucleated compared to the other high shear devices that led to the faster polymerization kinetics and formation of smaller particles.

Table 1: Typical Recipe used for Miniemulsion Polymerizations

Ingredient Amount Water 80 parts Oil Phase (e.g., monomer(s) 20 parts Surfactant (e.g., sodium lauryl sulfate; SLS)† 10 mM Costabilizer†(cetyl alcohol (CA)* or hexadecane (HD)**

* 30 mM; ** 40 mM

† Based on aqueous phase

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Figure 2: Typical high shear devices used to prepare miniemulsions. Figure 3: Influence of shear device on: (a) conversion and (b) particle size; 5 mM SLS/20 mM HD; 20% styrene; From: P.L. Tang, E. D. Sudol, C.A. Silebi, and M.S. El-Aasser, “Miniemulsion Polymerization—A Comparative Study of Preparative Variables”, J. Appl. Polym. Sci., 43, 1059 (1991). One of the challenges in miniemulsion polymerization is to measure and control the miniemulsion droplet size and try to ensure that each miniemulsion droplet finally is converted

Microfluidizer

Sonifier

Omni Mixer

Microfluidizer

Sonifier

Omni Mixer

Microfluidizer

Sonifier(145 nm) (271 nm)

(300 nm)Omni Mixer

Microfluidizer


(300 nm)

Microfluidizer


(300 nm)Omni Mixer

SonifierMicrofluidizer

Manton-GaulinHomogenizer

Rotor-stator Homogenizer

SonifierMicrofluidizer

Manton-GaulinHomogenizer

Rotor-stator Homogenizer

7

0 20 40 60 80 100 120 140 160 180 2000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

conventional

0% polymer, mini

0.05% polymer, mini

1% polymer, mini

2% polymer, mini

0.5% polymer, mini

R p, m

ol/d

m3 /s

x10

3

Reaction Time, minutes

into a polymer particle. Miller et al. [C.M. Miller, E.D. Sudol, C.A. Silebi, and M.S. El-Aasser, “Polymerization of Miniemulsions Prepared from Polystyrene in Styrene Solutions 1. Benchmarks and Limits”, Macromolecules, 28, 2754 (1995); ibid: “Polymerization of Miniemulsions Prepared from Polystyrene in Styrene Solutions. 2. Kinetics and Mechanism”, Macromolecules, 28, 2765 (1995); ibid: “Polymerization of Miniemulsions Prepared from Polystyrene in Styrene Solutions. 3. Potential Differences between Miniemulsion Droplets and Polymer Particles”, Macromolecules, 28, 2772 (2005) and Blythe et al. (P.J. Blythe, B.R. Morrison, K.A. Mathauer, E.D. Sudol, and M.S. El-Aasser, “Enhanced Droplet Nucleation in Styrene Miniemulsion Polymerization. 1. Effect of Polymer Type in Sodium Lauryl Sulfate/Cetyl Alcohol Miniemulsions”, Macromolecules, 32, 6944 (1999); P.J. Blythe, A. Klein, E.D. Sudol, and M.S. El-Aasser, “Enhanced Droplet Nucleation in Styrene Miniemulsion Polymerization. 2. Polymerization Kinetics of Homogenized Emulsions Containing Predissolved Polystyrene”, Macromolecules, 32, 3592 (1999); ibid., “Enhanced Droplet Nucleation in Styrene Miniemulsion Polymerization. 3. Effect of Shear in Miniemulsions That Use Cetyl Alcohol as the Cosurfactant”, Macromolecules, 32, 4225 (1999)] found that if they predissolved a certain amount of polymer within the miniemulsion droplets, they would be able to nucleate many more of the droplets to form polymer particles, a phenomenon termed “enhanced droplet nucleation”. This was reflected in faster polymerization kinetics compared to those miniemulsion droplets that did not contain dissolved polymer (see Figure 4). Upon adding a certain polymer concentration (≥ 0.5 wt% in Figure 4) they could obtain faster polymerization kinetics even compared to a parallel conventional emulsion polymerization. Figure 4: Effect of added polystyrene on the rate of polymerization (Rp) in styrene miniemulsion polymerizations showing the phenomenon of enhanced droplet nucleation; 1.33 mM potassium persulfate initiator; 10 mM SLS surfactant ; 0 (conv.) or 30 mM CA); 70 oC.

Conventional: Dv = 106.6 ± 12.4 nm; Np = 3.6 x1017 part./LMiniemulsion w/ NO PS added: Dv = 153.4 ± 21.4 nm; Np = 1.25 x1017 part./LMiniemulsion w/ 1% PS added: Dv = 82.4 ±17.2 nm; Np = 8.08 x1017 part./L

8

Case Study at Lehigh: Preparation of Nanosize Latex Particles by Miniemulsion Polymerization Figure 5 shows the approach taken by Anderson et al. [C.D. Anderson, E.D. Sudol and M.S. El-Aasser, “50 nm Polystyrene Particles via Miniemulsion Polymerization”, Macromolecules, 35, 574 (2002)] to prepare and characterize nanosize latex particles via a miniemulsion polymerization process. The strategy for this process is to carry out the emulsification quickly and then minimize the droplet degradation time by quickly polymerizing the miniemulsion droplets to “lock in” the particle size to a nanosize dimension. In the recipe used by Anderson, styrene monomer was utilized at 20%, sodium lauryl sulfate was varied from 10 to 80 mM, hexadecane was used at 40, 80 or 120 mM, and a redox initiator was used (30% active hydrogen peroxide at 0.46% based on the oil phase (added first) followed by ascorbic acid (0.23 % based on the oil phase)). The miniemulsion was prepared in a typical manner whereby oil and aqueous phases were added together and emulsified with a sonifer (1 min) followed by a Microfluidizer. The sonifer formed a relatively crude emulsion and a finer emulsion was prepared by processing this emulsion through the Microfluidizer unit for a number of passes. The miniemulsion was rapidly polymerized using the redox initiator. The size of the miniemulsion droplets was estimated by a surfactant titration method [B. Erdem, Y. Sulley, E.D. Sudol, V.L. Dimonie and M.S. El-Aasser, “Determination of Miniemulsion Droplet Size via Soap Titration, Langmuir, 16, 4890 (2000)], while the latex particle size was measured by dynamic light scattering (DLS), capillary hydrodynamic fractionation (CHDF) or by transmission electron microscopy (TEM). Figure 5: Schematic diagram of process used to prepare miniemulsion latexes in the nanosize range. Figure 6a shows the TEM micrographs of the miniemulsion latexes prepared by the process described above. It can be seen that particles in the 30 to 90 nm size range can be prepared using this process with the smallest particles formed at the highest SLS concentrations and the largest particles prepared at the lowest SLS concentrations. It can also be observed that the polydispersity index, PDI, increased with increasing SLS concentration; in other words, the

Oil & AqueousPhases Miniemulsion

Emulsification

Sonifier & Microfluidizer

DdropletNdroplet

Droplet Sizing Surfactant Titration

LatexPolymerization

Redox InitiatorH2O2 Added First

DparticleNparticle

Particle Sizing

CHDF/NICOMP/TEM/Titration

Nucleation

Mechanism

Oil & AqueousPhases Miniemulsion

Emulsification

Sonifier & Microfluidizer

DdropletNdroplet

Droplet Sizing Surfactant Titration

LatexPolymerization

Redox InitiatorH2O2 Added First

DparticleNparticle

Particle Sizing

CHDF/NICOMP/TEM/Titration

Nucleation

Mechanism

9

particle size distribution became broader as the surfactant concentration increased. This can also be seen more clearly in Figure 6b for particles measured by DLS, CDHF or TEM.

(a)

(b)

Figure 6: (a) TEM micrographs of latex particles prepared by miniemulsion polymerization of styrene monomer as a function of SLS concentration, and (b polydispersity index as a function of reaction time and SLS concentration.

10 mM SLSDv = 91.6 nmPDI = 1.03

20 mM SLSDv = 73.8 nmPDI = 1.06

40 mM SLSDv = 52.3 nmPDI = 1.08

60 mM SLSDv = 40.6 nmPDI = 1.34

80 mM SLSDv = 31.1 nmPDI = 1.38

500 nm

Prepared with 40 mM HD / Redox Initiator @ 50°CNegative Staining / Approx. 1000 particles counted per sample

Smallestparticles -

below CMC

Smallestparticles -

above CMC

Polystyrene Miniemulsion Latex Micrographs

10 mM SLSDv = 91.6 nmPDI = 1.03

20 mM SLSDv = 73.8 nmPDI = 1.06

40 mM SLSDv = 52.3 nmPDI = 1.08

60 mM SLSDv = 40.6 nmPDI = 1.34

80 mM SLSDv = 31.1 nmPDI = 1.38

500 nm

Prepared with 40 mM HD / Redox Initiator @ 50°CNegative Staining / Approx. 1000 particles counted per sample

Smallestparticles -

below CMC

Smallestparticles -

above CMC

Polystyrene Miniemulsion Latex Micrographs

TEM

CHDF

NICOMP

10 mMSLS

80mMSLS 60

mMSLS

40mMSLS

20mMSLS

[HD] = 40 mM

91.6nm

73.8nm

52.3nm

40.6nm

31.1nm

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

0 10 20 30 40 50 60 70 80

Total Reaction Time [min]

PD

I = D

w/D

n

Increase [SLS]tot

Decrease Dv

Decrease Reaction Time

Increase PDI

TEM

CHDF

NICOMP

10 mMSLS

80mMSLS 60

mMSLS

40mMSLS

20mMSLS

[HD] = 40 mM

91.6nm

73.8nm

52.3nm

40.6nm

31.1nm

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

0 10 20 30 40 50 60 70 80

Total Reaction Time [min]

PD

I = D

w/D

n

Increase [SLS]tot

Decrease Dv

Decrease Reaction Time

Increase PDI

10

Case Study at Lehigh: Encapsulation of Inorganic Pigments by Miniemulsion Technology Figure 7 shows two different approaches that were utilized at Lehigh University to encapsulate titanium dioxide (TiO2) pigment particles by miniemulsion technology. The first approach, depicted in Figure 7a, shows the use of miniemulsion polymerization to encapsulate TiO2 [Erdem et al., ibid]. The second approach, given in Figure 7b, demonstrates the use of miniemulsification technology to encapsulate the TiO2 in a polymer shell [Al-Ghamdi et al. ibid.]. The miniemulsion polymerization approach utilized by Erdem was carried out as follows. First, TiO2 pigment was dispersed in an oil phase comprised of styrene, hexadecane costabilizer, and a polymeric stabilizer for the TiO2, such as OLOA 370, or Solsperse 3000 or 24000. Erdem determined that these particular stabilizers were effective in dispersing the titanium dioxide in the styrene oil phase. This dispersion was then sonified with a sonifer (output power of 10, 70% duty cycle) for up to 30 minutes. An aqueous phase comprised of SLS and sodium bicarbonate buffer in water was added to the oil phase and emulsification was carried out first by sonifying the oil and aqueous phases for 30 seconds and then passing the crude miniemulsion though a Microfluidizer unit for 10 passes. Water-soluble persulfate initiator was then added to the miniemulsion and polymerization was carried out at 70 °C. The particles were imaged in a transmission electron microscope. They were also placed in a density gradient column (DGC) and separated via ultracentrifugation to determine the encapsulation efficiency (percentage of TiO3 encapsulated by percentage of PS) and the size of the encapsulated particles and their portion of TiO2 (i.e., estimated number of TiO2 particles encapsulated in each PS particle). The amount of polystyrene particles with no TiO2 present within them was also estimated. Table 2 summarizes the results obtained with this approach; i.e., where pigment particles are initially present within miniemulsion droplets that are then subsequently polymerized to encapsulate the TiO2. Based on the type of polymeric dispersant and the type of TiO2 to be encapsulated, encapsulation efficiencies of 30 to 80% could be achieved. The major disadvantage of this process was that only low loading levels of TiO2 (i.e., ~ 3 wt%) could be incorporated into the polymer particles. Higher encapsulation efficiencies and higher loading levels of TiO2 could be achieved by using a miniemulsification approach (Figure 7b). In this approach, the TiO2 particles are still dispersed in an oil phase with a polymeric stabilizer present (e.g., Solsperse 24,000) by sonification. However, the oil phase also contains a copolymer (in this case a film-forming poly(styrene-co-butyl acrylate) copolymer) dissolved in solvent (toluene); hexadecane costabilizer is also present in the oil phase. The oil phase is then redispersed with sonification. An aqueous phase (containing conventional surfactant, SLS, is then added to the mixed oil phase and miniemulsification is carried out by homogenizing the oil and aqueous phases together. The solvent is then stripped out of the emulsion leaving TiO2 pigments particles dispersed or encapsulated within a copolymer shell. This approach allows one to obtain much higher PVC (pigment volume concentration) values (11 to 70%) and higher TiO2 loading levels. In addition, films prepared with this material exhibited superior optical properties (i.e. hiding power) compared to those films prepared by simply blending a film-forming PBA-PS latex with TiO2 pigment.

11

(a)

(b)

Figure 7: (a) Encapsulation of TiO2 particles by: (a) miniemulsion polymerization, and (b) by miniemulsification.

1. Dispersion1. Dispersion

TiO2

Styrene(Hexadecane)(Polystyrene)

StabilizerSonifier

@10 OP, 70DC

0-30 min

3. Polymerization3. PolymerizationInitiator70 oC

DGC & TEM

H2O

2. Emulsification2. EmulsificationSLS & NaHCO3

in H2OSonifier & MF

(30 sec & 10 passes)

1. Dispersion1. Dispersion

TiO2


StabilizerSonifier

@10 OP, 70DC1. Dispersion1. Dispersion

TiO2


StabilizerSonifier

@10 OP, 70DC

0-30 min



DGC & TEMDGC & TEM

H2O


in H2OSonifier & MF


H2O


in H2OSonifier & MF



in H2OSonifier & MF


: Copolymer chains in Oil-phase

: Hexadecane : Aggregate of TiO2 Particles: Stabilizer in oil phase

1. Dispersion

Sonification @ 10 O.P. and 70 % D.C. for 0-20 min.

Encapsulated Particles

2- RedispersionStirring andSonification

@ 10 O.P/70% D.C for 0-5 min.

: Stabilized TiO2 particles in oil phase: Copolymer Chains

: Hexadecane

4- Stripping

of the Solvent

3- Miniemulsification

H2O/SLS, Homogenizer, e.g., Sonfier for 0-7min.


: Hexadecane


: Hexadecane


: Hexadecane : Aggregate of TiO2 Particles: Stabilizer in oil phase: Aggregate of TiO2 Particles: Stabilizer in oil phase: Aggregate of TiO2 Particles: Stabilizer in oil phase

1. Dispersion


1. Dispersion


1. Dispersion


Encapsulated ParticlesEncapsulated Particles


@ 10 O.P/70% D.C for 0-5 min.


: Hexadecane


@ 10 O.P/70% D.C for 0-5 min.


: Hexadecane


@ 10 O.P/70% D.C for 0-5 min.


@ 10 O.P/70% D.C for 0-5 min.


: Hexadecane


: Hexadecane

4- Stripping

of the Solvent



4- Stripping

of the Solvent



12

Table 2: Summary of Encapsulation Efficiency of TiO2 particle using the Miniemulsion Polymerization Approach

Experiment Type of Stabilizer

Type of TiO2 Encapsulated TiO2 (%)

Encapsulating PS (%)

ME62-0.5 OLOA 370 Hydrophilic 34.5 29.0 ME62-1 " " 82.8 72.6 ME62-2 " " 80.8 71.5

ME63-0.5 Solsperse 3000 Hydrophilic 53.2 36.8 ME63-1 " " 70.32 71.6 ME63-2 " " 66.17 54.4

ME64-0.5 Solsperse 24,000 Hydrophilic 37.3 31.3 ME64-1 “ " 77.01 68.5 ME64-2 “ " 83.9 64.6

ME65-0.5 Solsperse 3000 Hydrophobic 38.05 22.6 ME65-1 " " 50.3 46.2 ME65-2 " " 40.4 23.6

ME66-0.5 Solsperse 24,000 Hydrophobic 38.4 24.8 ME66-1 " " 44.1 38.9 ME66-2 " " 64.6 60.1

Case Study at Lehigh: Preparation of Polyurethane/Acrylic Hybrid Nanoparticles using Miniemulsion Polymerization

Li et al. prepared a series of nanosized polyurethane/acrylic hybrid latexes utilizing miniemulsion technology [M. Li, E.S. Daniels, V.L. Dimonie, E.D. Sudol, and M.S. El-Aasser, “Preparation of Polyurethane/Acrylic Hybrid Nanoparticles via a Miniemulsion Polymerization Process”, Macromolecules, 38, 4183 (2005)]. The approach used by Li et al. is depicted in Figure 8 and a master recipe is shown in Table 3. The objective of this program was to prepare a hybrid material that combined the desirable properties of polyurethanes (e.g., toughness, chemical resistance) with those of acrylates (e.g., good weatherability, non-yellowing, good adhesion to substrates, etc.). The first part of the synthesis entailed the preparation of polyurethane prepolymers. These were prepared by condensation polymerization of polypropylene glycol (PPG) with a molecular weight of 2000 with either MDI or IPDI isocyanates. A small amount of 2-hydroxyethyl methacrylate (HEMA) was also incorporated into the prepolymer to introduce some double bonds into the urethane prepolymer that could act as grafting site for the subsequent free radical polymerization with methacrylate monomer. This prepolymer was then added to an oil phase comprised of butyl methacrylate monomer and hexadecane. The oil phase was then

13

added to an aqueous phase that contained a given concentration of SLS surfactant and mixed with magnetic stirring. A miniemulsion was then formed by homogenizing the aqueous and oil phases with a sonifer for 10 minutes (duty cycle = 70%, power output of 8). An ice bath was used during sonification to reduce any temperature rise resulting from the sonification process. A hydrogen peroxide/ascorbic acid redox initiator (as used by Anderson et al., ibid) was used to polymerize the miniemulsions at 30 °C. In addition, a chain extender (bisphenol A) was utilized that can have a dramatic effect on the film properties of the hybrid polymer (see Figure 9). Table 4 summarizes the miniemulsion droplet size as well as the resulting particle sizes of the urethane/acrylate particles. The results shown in this table indicate that there is a very good correspondence between the miniemulsion droplet size and the resulting latex particle size. In addition, the resulting latex particles are in the nanosize range. Finally Figure 10 shows that the mechanical properties of the hybrid urethane/acrylate films are superior to films prepared by physically blending polyurethane latexes with polyacrylate latexes. Figure 8: Schematic representation of process used to prepare polyurethane/acrylic hybrid latexes using miniemulsion technology.

1.Prepolymer Synthesis

PU Prepolymer (NCO end-groups

and some double bonds)

Pure MDI or IPDIPPG-Diol (MW = 2000)HEMA: 0 to 1.5%

PU Prepolymer (NCO end-groups

and some double bonds)

Pure MDI or IPDIPPG-Diol (MW = 2000)HEMA: 0 to 1.5%

% NCO as f(t) (Titration)MW as f(t) (GPC)[MDI] or [IPDI] (GPC)

% NCO as f(t) (Titration)MW as f(t) (GPC)[MDI] or [IPDI] (GPC)

U-NCO/BMAMiniemulsions

BMAHDSLS

U-NCO/BMAMiniemulsions

BMAHDSLS

Sonification: Size as f(t) (Nicomp)Droplet stability% Decrease NCO (Titration)



2.Miniemulsification2.Miniemulsification

PU/P(BMA) NanosizedHybrid Latexes

Redox initiation at 30 oC

PU/P(BMA) NanosizedHybrid Latexes

Redox initiation at 30 oC

Particle size & stability(CHDF)Polym’n kinetics (RC1)Particle morphology(TEM)

Particle size & stability(CHDF)Polym’n kinetics (RC1)Particle morphology(TEM)

Films from Hybrid LatexesFilm Formation Films from Hybrid LatexesFilm Formation

Film morphology (microtoming; TEM)Mechanical Properties (σ-ε)

Film morphology (microtoming; TEM)Mechanical Properties (σ-ε)

4. Film Formation/Properties4. Film Formation/Properties

3. Miniemulsion Polymerization3. Miniemulsion Polymerization

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Table 3: Recipe used to Prepare Urethane/Acrylic Hybrid Latexes by Miniemulsion Polymerization and Schematic Representation of Final Polymer Structure

INGREDIENTS WEIGHT (g) Urethane prepolymer 2.0 to 8.0 n-Butyl Methacrylate 18.0 to 12.0 Sodium lauryl sulfate 0.17 (7.5 mM) Hexadecane 0.54 Chain extender Varies Catalyst Trace Deionzed water 78.9 10% Hydrogen peroxide 0.69 (25.2 mM) 10% Ascorbic acid 0.39 (2.77 mM) Figure 9: Effect of chain extender on mechanical behavior of urethane/acrylic hybrid polymer.

Schematic of Structure :C C

OC=ONH

UrethaneNCO

C C

OC=ONH

UrethaneNCO

Chain ExtenderChain Extender

PBMA chain

PBMA chain

HEMA

HEMA

Schematic of Structure :C C

OC=ONH

UrethaneNCO

C C

OC=ONH

UrethaneNCO


PBMA chain

PBMA chain

HEMA

HEMA

C C

OC=ONH

UrethaneNCO

C C

OC=ONH

UrethaneNCO


PBMA chain

PBMA chain

HEMA

HEMA

0.0

1.0

2.0

3.0

4.0

5.0

0 100 200 300 400

Strain (%)

Stre

ss (M

Pa)

No chain extension (25/75 PU/PBMA)

Using bisphenol A chain extender (25/75 PU/PBMA)

0.0

1.0

2.0

3.0

4.0

5.0

0 100 200 300 400

Strain (%)

Stre

ss (M

Pa)



Using bisphenol A chain extender (25/75 PU/PBMA)Using bisphenol A chain extender (25/75 PU/PBMA)

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Table 4: Miniemulsion Droplet Sizes and Particle Sizes of Urethane/Acrylate Latexes

Droplet Size

Final Particle Size*** Sample Type of U-NCO

SLS mM)

Dv (nm) Dn (nm) Dv (nm) Dw (nm) PDI PBMA — 10.0 72.2 ± 4.4 76.8 ± 1.2 78.7 ± 1.5 82.0 ± 1.3 1.07

IPDI 7.5 55.1 ± 5.8 43.9 ± 1.9 47.1 ± 2.4 51.4 ± 2.9 1.17 PU/PBMA Hybrids MDI 7.5 58.7 ± 7.5 47.7 ± 2.4 52.4 ± 1.7 63.9 ± 1.2 1.33

Figure 10: Comparison of the mechanical properties of films prepared from hybrid urethane/acrylate latexes compared to films prepared from simple blends. Summary Miniemulsion technology has been shown to be a versatile technology which offers a number of advantages when compared to conventional emulsion polymerization processes such as the ability to polymerize monomers with very low to no water solubility (such as macromonomers), the ability to encapsulate a variety of materials within the miniemulsion polymer particles (e.g., TiO2 pigment), and a possible advantage in terms of preemulsion stability in feeding monomer into a reactor during a polymerization among many others. Artificial latexes can also be prepared starting from polymers that cannot be prepared by a free radical polymerization process using a miniemulsification approach. Some drawbacks in the use of miniemulsion technology include the need for a capital intensive high shear device and the requirement for a volatile costabilizer such as hexadecane among other factors (although there are ways to work around these limitations). An increasing use of miniemulsion technology is

Hybrid system

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 100 200 300 400

Strain (%)

Stre

ss (M

Pa)

PBMA

PU/PBMA: 10/90

25/75

40/60

Instron test: crosshead speed = 25 mm/min

Blend system

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 100 200 300 400

Strain (%)

Stre

ss (M

Pa)

PBMA

PU/PBMA:10/90

25/7540/60

Hybrid system

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 100 200 300 400

Strain (%)

Stre

ss (M

Pa)

PBMA

PU/PBMA: 10/90

25/75

40/60

Hybrid system

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 100 200 300 400

Strain (%)

Stre

ss (M

Pa)

PBMA

PU/PBMA: 10/90

25/75

40/600.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 100 200 300 400

Strain (%)

Stre

ss (M

Pa)

PBMA

PU/PBMA: 10/90

25/75

40/60


Blend system

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 100 200 300 400

Strain (%)

Stre

ss (M

Pa)

PBMA

PU/PBMA:10/90

25/7540/60


Blend system

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 100 200 300 400

Strain (%)

Stre

ss (M

Pa)

PBMA

PU/PBMA:10/90

25/7540/60

Blend system

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 100 200 300 400

Strain (%)

Stre

ss (M

Pa)

PBMA

PU/PBMA:10/90

25/7540/60

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 100 200 300 400

Strain (%)

Stre

ss (M

Pa)

PBMA

PU/PBMA:10/90

25/7540/60

16

being reported in the literature exploring the many ways in which miniemulsions may be applied industrially. The intent of this paper is to offer a brief introduction to miniemulsion technology and highlight some of the studies into this technology that have been carried out in the Emulsion Polymers Institute at Lehigh University Acknowledgments

The authors gratefully acknowledge the many graduate students who have studied in the Emulsion Polymers Institute over a 30+ year period and have made many valuable contributions to the understanding of miniemulsions. Some of these previous students whose work are cited in this paper include Phan Tang, Chris Miller, John Blythe, Chris Anderson, Bedri Erdem, Ghurmallah Al-Ghamdi, and Mei Li.

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