POLIMERIZAÇÃO EM MINIEMULSÃO: RECENTES AVANÇOS NA SÍNTESE DE LÁTEX COM

INTERESSE INDUSTRIAL

Marcelo do Amaral

IQT - Indústrias Químicas Taubaté Rua Irmãos Albernaz 300, 12050-190 Taubate, SP Brasil

marcelo.amaral@iqt.com.br

Miniemulsion Polymerization: Novel Industrial Applications The miniemulsion polymerization technique has been explored to synthesize a myriad of products, from hybrid polymer particles to high solids content latexes, from polymer with controlled and tailored macromolecular architecture to superparamagnetic particles, just to name a few. [1-7] The unique feature of changing the polymerization locus to monomer droplets has attracted great attention to explore the possibilities of employing miniemulsion polymerization as a suitable means to synthesize novel materials. This works reviews recent achievements in miniemulsion polymerization. Starting from a new method to analyze droplet size, to the use of high shear devices to prepare miniemulsions, to the synthesis of large particle sized monodisperse particles, the use of polymeric surfactant, and synthesis of polymer hybrids, recent progresses are shown through a perspective of potential industrial application. Introdução The versatility of the miniemulsion polymerization technique is attracting an increased attention in the last few years (1-7). A combination of unsolved scientific aspects and a potential for the synthesis of new products with high value drives the interest of both academia and industry. Miniemulsions are classically defined as finely dispersed oil-in-water dispersions, whose droplet size is in the range of 50 to 500nm (2). Stability against coalescence is provided by the use of conventional surfactants. Ostwald ripening, the diffusional degradation of the monomer droplets, is hindered by the presence of either low molecular weight highly water insoluble compounds, called costabilizers, or high molecular weight water insoluble materials, such as polymers (1), called hydrophobes, which do not have the superswelling capacity of the costabilizers. By using high shear devices, sonifiers, and high pressure homogenizers, monomer droplets in the submicron range can be obtained. The occurrence of predominant droplet nucleation upon addition of initiators to a monomer miniemulsion is the main feature of miniemulsion polymerization. Indeed, the first description of miniemulsion polymerization dates from the early 70’s (8), when the unusual behavior of monomer droplets as nucleation loci was observed. Important aspects concerning the industrial implementation of miniemulsion polymerization are the ability to accurately measure the droplet size of the miniemulsion that is fed into the reactor, the robustness and scale-up of the preparation of the miniemulsion, and the capacity to produce novel products.

This works reviews recent progress on the characterization of miniemulsion droplet size, the preparation of miniemulsion using high pressure homogenizers, the synthesis of large particle size latex, and the use of polymeric surfactants. Experimental (Times New Roman 10, negrito) Materials Technical grade monomers without further purification were used. Hexadecane was used as hydrophobe. Commercial grade surfactants, Disponil A3065 (Cognis –Germany), Dowfax 2A1 (Dow Surfactants – France) and alkali soluble resins Morez 101 and Morez 300 (Rohm and Haas – France) were all used as received. Detailed information about recipes can be found in the references given in the text. Miniemulsification Procedure Miniemulsions were prepared by first dissolving the costabilizer in the organic phase under stirring. Immediately after the pre-mixing time, the coarse dispersion was fed to a Manton-Gaulin homogenizer, model Lab-60TBS (APV – Germany). Ultrasonication was used to prepare miniemulsions using alkali soluble resins as surfactants (Branson 450 W – USA). Droplet and particle size characterization A novel experimental technique using a powerful new STEM (Scanning Transmission Electron Microscopy) imaging system, which allows transmission observations of wet samples in an ESEM (environmental scanning electron microscopy) was used to measure monomer droplet size. Capillary hydrodynamic fractionation (CHDF Matec 2000) was

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employed to measure the latex particle size distribution (PSD). Resultados e Discussão Measurement of monomer droplet size The wet STEM observation is presented in Figure 1. The floating objects are miniemulsion droplets.

Figura 1. Aspect of the virgin copper grid (SPI supplies holey carbon coated grid): between the copper squares, the carbon layer appears bright, and contains dark holes of diameters ranging from less than 1 µm to 20 µm (dark field STEM conditions). After sample drop deposition, these holes allow to maintain overhanging liquid films on very small areas. Further progress of miniemulsion polymerization has been hampered by the lack of knowledge of some fundamental aspects of this technique (7). Droplet size and droplet size distribution (DSD) are by far the most important characteristics of a monomer miniemulsion, since they affect directly both miniemulsion stability and droplet nucleation. Recent results show the feasibility of using the wet STEM to determine miniemulsion droplet size distribution (9). Current research efforts are being dedicated to further explore this technique, determining of the DSD of miniemulsion prepared under different conditions and using different formulations. Preparation of monomer miniemulsion using high pressure homogenizer The homogenization stage is ubiquitous in miniemulsion polymerization process. During the homogenization process, energy is imparted to the dispersed media in order to subdivide droplets into smaller droplets. High pressure homogenizers are well stablished for many applications including diary and pharmaceutical. Scale-up of these equipments is almost straightfoward, increasing the interest from the industrial point of view. Nevertheless, it is interesting to remark that most of the research on miniemulsion has been mostly focused on ultrasonication (1-7). Figure 2 shows the influence of the pressure set-up on the droplet size of a miniemulsion.

200

220

240

260

280

300

0 2 4 6 8 10

Average Droplet Size - Influence of the Pressure Setup

4000-400 psi5000-500 psi6000-600 psi

Dro

plet

Siz

e (n

m)

number of cycles Figura 2. Effect of the Pressure Setup on the Droplet Size on the corn oil miniemulsion. (●) 4000/400 psi (■) 5000/500 psi (♦) 6000/600 psi Results show that the mean droplet size decreased by both the increasing pressure and increasing number of homogenizing passes. The increasing number of homogenizing passes did also narrow the DSD (10). Stable monomer miniemulsions with different average droplet size were successfully prepared with high pressure homogenizers. The equipment was also observed to be suitable to prepare miniemulsions with rather concentrated organic phase (10). Synthesis of monodisperse high solids latex with latex particle size Using the high pressure homogenizers and different surfactant, but maintaining the basic monomer composition, it was possible to prepare miniemulsions of different droplet size. The anionic surfactant yielded droplets smaller than those obtained with the nonionic surfactant, and droplet size decreased with the surfactant concentration. Droplet size is the result of the interplay between droplet break-up and droplet coalescence during the homogenization process. In a high pressure homogenizer, the maximum droplet size that is not broken by extensional stress depends on the design and operational conditions of the homogenizer, on the viscosities of the continuous and dispersed phases and on the interfacial tensions (7). Experiment Droplet Size

(nm)

Number average

particle size (nm) PDI

Run 1 246 ± 100 232 1.108

Run 2 216 ± 68 273 1.140

Run 3 650 ± 140 711 1.003

Run 4 280 ± 70 372 1.026

Run 5 620 ± 250 632 1.023

Run 6 440 ± 120 622 1.007

Tabela 1. Miniemulsion droplet size and latex particle size. Under the experimental conditions assessed it was possible to obtain a 50% solids content latex with large particle size and narrowly distributed PSD (11). Similar final latex PSD were obtained from miniemulsions of different monomer composition typical of acrylic compositions employed in the

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production of adhesives and paints, where in one case a much more hydrophilic monomer composition was employed. The miniemulsion polymerization process constitutes a direct option for the production of large monodisperse latex, and it is probably more versatile than other techniques availables for obtaining large narrowly distributed aqueous polymer dispersions. Important industrial applications of large particle size include the synthesis of high solids content latexes (12). Alkali soluble resins as polymeric surfactants Amphiphatic macromolecules are widely employed in conventional emulsion polymerization processes in order to obtain aqueous polymer dispersions with improved colloidal properties and enhanced end-use performance (13,14). However, few works have addressed the use of polymeric materials as surfactants in miniemulsion (1-7). ASR was successfully employed in low solids miniemulsion polymerization (10,15). General characteristics of the use of ASR were assessed by varying several experimental conditions, enabling the polymerization of of high solids content latexes. Table 2 shows the formulations used in the high solids experiments.

Run Monomer/ Costabilizer

ASR wt% *

Initiator wt% *

Org. vol. frac. (%)

8 BuA / HD

Morez 101, 2 VA-086, 2

50

9 BuA / Norsocryl

Morez 300, 2 Luperox 256, 2

50

Tabela 2. Formulation used in the batch miniemulsion polymerizations. * Based on the total weight of monomer. The particle size distribution of the latex was also analyzed by means of CHDF, and is shown in Figure 3.

0

0.5

1

1.5

2

2.5

3

3.5

4

100 200 300 400 500 600 700 800

Wei

ght i

nten

sity

Particle Size (nm)

Figura 3. Particle size distribution obtained by CHDF of Run 8. Monomer droplet size and latex particle size were found to be similar, indicating the ocurrance of droplet nucleation. Deviations were credited to the difficulty of measuring the accurate size due to the interference of

the ASR molecule. Miniemulsion polymerization offered the possibility of drastically reducing the amount of ASR needed to generate a stable latex. Conventional emulsion polymerization processes employ up to 30 – 45 wt% (based on the total monomer) or ASR, whereas with 2 wt% the polymerization could be carried out in miniemulsion (15). A great advantage of the miniemulsion process was that different types of ASR were suitable as sole surfactant in aqueous polymer dispersion. Stable latexes were polymerized in the presence of Morez 300. High solids content latexes were produced (16,17). Conclusões Academic research on multiple aspects of miniemulsion polymeriation has been carried out in the past 30 years. More recently, in the quest to lauch new products with higher valued-added, industry is dedicating efforts to bring good ideas from the bench scale to the industrial practice. This work illustrates some recent achiviements is this sense. Accurate measurement of the monomer droplet size is of capital relevance if one aims to understand and control the process of miniemulsification and miniemulsion polymerization. The wet-STEM technique seems to offer great opportunities for future experimentation. The high pressure homonizer is a robust equipment, which can be directly used to prepare miniemulsions in industrial scale. The operational conditions can be tuned to tailor the product particle size, offering a new and straight forward method to prepare latex of varying particle size. The possibility of preparing new product has been illustrated by the use of polymeric surfactants in miniemulsion. Sure, many new alternatives are yet to appear. Fortunately, in the next year many of those will be in the market. Agradecimentos O autor agradece ao Instituto de Materiais Poliméricos (POLYMAT – Espanha), a UCB, ao Groupe d’Etudes de Métallurgie Physique et de Physique des Matériaux (Lyon – França). Referências Bibliográficas

1. M.S. El-Aasser, C.M. Miller,. in Polymeric Dispersions: Principles and Applications; J.M. Asua, (Ed), Kluwer, Dordrecht, 1997.

2. E.D. Sudol, M.S. El-Aasser, in Emulsion Polymerization and Emulsion Polymers; Lovell, P.A.; El-Aasser, M.S. (Eds). Wiley, New York, 1997.

3. I. Capek, C.S. Chern, Adv. Polym. Sci. 2001, 155, 101.

4. J. Qiu, B. Charleux, K.Matyjaszewski, Prog. Polym. Sci. 2001, 26, 2083.

5. M. Antonietti, K. Landfester, Prog. Polym. Sci. 2002, 27, 689.

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6. M.F. Cunningham, Prog. Polym. Sci. 2002, 27, 1039.

7. J.M. Asua, Prog. Polym. Sci. 2002, 27, 1283. 8. Ugelstad, J.; El-Aasser, M.S.; Vanderhoff, J.W. J.

Polym. Sci. Polym. Lett. 1973, 11, 503. 9. M. do Amaral, A. Bogner, C. Gauthier, G.

Thollet, P-H. Jouneau, J-Y. Cavaillé, J.M. Asua, Macromol. Rapid Commun. 2005, 26, 365.

10. M. do Amaral, Tese de Doutorado, Universidad del Pais Vasco, Espanha, 2003.

11. M. do Amaral, J.M. Asua, J. Polym Sci. Polym Chem. 2004, 42, 3936.

12. M. do Amaral, in Leading Edge Polymer Research; R.K. Bregg (Ed), Nova Publishers, New York, 2005.

13. D.H. Napper, “Polymeric Stabilization of Colloidal Dispersions“, Academic Press, New York 1983, pp. 1-15.

14. I. Piirma, “Surfactant Science Series Vol. 42 Polymeric Surfactants“, Marcel Dekker, New York 1992, pp. 127-164.

15. M. do Amaral, H. de Brouwer, S. van Es, J.M. Asua, Macromol. Symp. 2005, 226, 167.

16. M. do Amaral, J.M. Asua, Macromol. Rapid Commun. 2004, 25, 1883.

17. M. do Amaral, J.M. Asua, H. de Brouwer, S. van Es, Patent WO 2004/069879 A1.

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