Surfactant-free emulsion polymerization of styrene using crosslinked seed particles Eshuis, A.; Leendertse, H.J.; Thoenes, D.

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1 Surfactant-free emulsion polymerization of styrene using crosslinked seed particles Eshuis, A.; Leendertse, H.J.; Thoenes, D. Published in: Colloid and Polymer Science DOI: /BF Published: 01/01/1991 Document Version Publisher s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. The final author version and the galley proof are versions of the publication after peer review. The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication Citation for published version (APA): Eshuis, A., Leendertse, H. J., & Thoenes, D. (1991). Surfactant-free emulsion polymerization of styrene using crosslinked seed particles. Colloid and Polymer Science, 269(11), DOI: /BF General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Download date: 26. Apr. 2018

2 Colloid & Polymer Science Colloid Polym Sci 269: (1991) Surfactant-free emulsion polymerization of styrene using crosslinked seed particles A. Eshuis, H. J. Leendertse, and D. Thoenes Department of Chemical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands Abstract: The aim of this research was to prepare a monodisperse polystyrene latex without surfactants adsorbed at the particle surface. Conventional polymerization formulations usually lead to large amounts of oligomers. Furthermore, they are characterized by a low reproducibility with respect to particle size. This was overcome by using a seed latex that was crosslinked in order to overcome dissolution in the monomer phase. By adjusting the seed concentration, any desired particle size in the range #m could be obtained. The monodispersity was very good. Key words: Polystyrene; latex; monodispersity; seeded emulsion polymerization; surfactant-fr~e emulsion-polymerization Introduction Emulsion polymerization is used extensively in industry to prepare a wide range of polymers. Compared to bulk processes, it has the advantage that more conventional equipment can be used and much higher concentrations can be handled than in solution polymerization. One disadvantage is the relatively large amount of surfactants required [1, 2]. This adds to the cost of raw materials, may result in additional waste-water treatment, and, in some instances, it gives rise to undesired contaminants in the end product. The latter aspect is important when the latex is used as a model colloid in coagulation research, especially because the surfactants may subsequently be partly desorbed to an extent that is difficult to control [3, 4]. In the literature, some methods that seem to overcome this problem are described: a) In the method described first by Kotera et al., the "surfactant is left out completely and an excess of initiator is used, most often potassium persulfate. Oligomers containing sulfonic acid groups and radical functions will then act effectively as surfactants [5-7]. They are b) c) subsequently incorporated in the polymer molecules. The polymer particles formed are sufficiently stable and will not contain any "contaminants" that ~can be desorbed. However, considerable amounts of oligomers may be formed, which results in a low overall yield to solid polymer. There is also a serious risk that even low amounts of solid contaminants ("dust"), which are difficult to avoid completely, will act as seed particles. This may result in a particle size distribution that is almost impossible to control. The dust problem is a direct consequence of the absence of any surface-active agent during the initial stage of the polymerization. Polymeric surfactants that show a very low tendency to desorb can be used. The (kinetic) stability of particles prepared by this method is based on both electrostatic and steric effects [8]. This makes them less suitable as model systems in our coagulation experiments [3, 4]. The method developed by Ugelstad makes use of diluents and/or chain-transfer agents to improve swelling of seed particles [9]. Usually, these additives are insoluble in water, but during prolonged periods of time their migration K931

3 Eshuis et al., Surfactant-free emulsion polymerization of styrene using crosslink.ed seed particles 1087 out of these particles cannot be dismissed. The use of a surfactant (polymeric or not) constitutes a further problem. d) If polymerizable surfactants are used, it is absolutely vital that the incorporation of these molecules in the polymeric chains is 100%. Otherwise, desorption of these surface-active agents is still possible. This is very hard to accomplish. Obviously, all these methods have serious disadvantages. In the next sections we will describe a method by which all of these problems are largely overcome. Table 1. Typical formulations to prepare seeds and final products Seeds Final product Water 1800 ml 1800 ml Styrene 180 ml 200 ml Divinylbenzene 20 ml - Potassium persulfate g g Oleic acid g - Sodium hydroxide g - Seeds g Polymerization temperature 60 ~ 60 ~ Polymerization time 24 h 48 h Seeded surfactant-free emulsion polymerization Polystyrene can be polymerized with high yields without any surfactant by using suitable seeds. The relative amount of seeds may be on the order of a few percent of the total final product. Seeds can be made by emulsion polymerization using a surfactant, because this will be included completely inside the final particles. However, the use of seed particles of polystyrene would not be practical, because these would dissolve in the monomer phase. Therefore, we prepared seed particles by copolymerizing styrene and divinylbenzene, which resulted in a crosslinked polymer that did not dissolve in styrene. It appeared that 1% (by volume) of divinylbenzene was sufficient to render the polymer insoluble. Experiments Polymerizations were carried out in a stirred cylindrical vessel with 88 mm in diameter and 330 mm high. All products were prepared by batch processes and typical polymerization formulations are shown in Table 1. Water was distilled twice. The monomers were purified by distillation under reduced pressure. All chemicals were obtained from Merck Darmstadt, FRG. Oleic acid was used as a surfactant, because it was expected to copolymerize with the monomers so that it could not be desorbed. We were not able to prove experimentally if this took place. The seed latex was diluted with an equal volume of ethanol to prevent any coagulation during storage. Seeds preserved in this way were used in subsequent polymerization steps for over one year after their preparation. The particle-size distribution of these latex samples was measured by photon correlation spectroscopy (Malvern Autosizer 2C). The final polystyrene particles were washed and filtered over a nylon membrane with a pore size of 0.2/lm (NYLAFLO, Gelman Sciences, Ann Arbor, Mich., USA). Photographs of these spheres were made (after covering them with a thin gold film) using a scanning electron microscope (JEOL JSM 80A). Results and discussion The relative amount of divinylbenzene in the monomer mixture was varied between 1 and 10%. In all cases, a crosslinked polymer was formed that did not dissolve in styrene. In subsequent experiments, 1% of comonomer was used. By varying the amount of oleic acid between 0.4 and 6.0 g (per 2 1 of reaction mixture), the diameter of the seeds varied between 100 and 300 nm (Fig. 1). Based on the experiments of Goodwin et al., a particle size of 500 nm was to be expected with an emulsion-free formulation [6]. This point is also shown in Fig. 1. A scanning-electron-microscope picture of the seeds is shown in Fig. 2. The diameter of these particles is about 270 nm with a standard deviation of 43 nm (as determined by photon correlation spectroscopy). In the second step of the process, these seeds were used in various concentrations. A typical result is shown in Fig. 3. The particles in the end product appear to be quite uniform in size. In this example, the particle size is about 900 nm with a standard deviation of 100 nm. Of course the diameter of the final product follows from a mass balance, given the amounts of seeds and monomer in the second step (~s and ~, respectively), as long as the particles are uniform in size. From Fig. 4 it follows that deviations from this

4 1088 Colloid and Polymer Science, Vol No. 11 (1991) Oleic acid (g) Fig. 1. Diameter of P.S. spheres as a function of the amount of oleic acid. 9 = our experiments; 9 = calculated from literature data [5] Fig. 3. Particles obtained by seeded surfactant-free emulsion polymerization * i 0.4 O.G % d s. ( i + r~/~s)lls Fig. 2. Particles obtained by emulsion polymerization with 0.4 g oleic acid Fig. 4. Diameter of particles, as determined by SEM, prepared with crosslinked seeds as a function of the diameter resulting from a mass balance assumption occur at relatively low seed concentrations (i.e., large ~/~s ratios). Products made under these conditions are no longer monodisperse (Fig. 5). Apparently, other nucleating mechanisms have a change at low seed concentrations. All experiments represented in Fig. 4 have been performed to obtain a final latex with a total solid content of 10% by volume. Hayashi et al. have recently prepared particles of polyvinylacetate by a similar process. During the preparation of their seeds they used very large amounts of potassium persulfate. The resulting high surface charge on these particles prevented their dissolution into the monomer [10]. When seeds were used with a higher degree of crosslinking (prepared with up to 10% divinylbenzene), a somewhat lower monodispersity of second-generation particles was obtained. This was reproduced several times, but we were not able to explain this convincingly.

5 Eshuis et al., Surfactant-free emulsion polymerization of styrene using crosslinked seed particles 1089 samples to be used in coagulation studies. It might also be promising for the industrial manufacture of emulsion polymers, because much less surfactant is needed compared to conventional processes. Fig. 5. Particles formed by seeded surfactant-free emulsion polymerization and by self association of ionized radicals Conclusion It was shown that a two-step polymerization process could be used to prepare a monodisperse polystyrene latex without any surfactant adsorbed on the particle surface. The first step was a copolymerization with divinylbenzene using oleic acid as a surfactant. The resulting particles of about 0.3 #m were used as seeds in the second step. The amount of seeds determined the final particle size and this could be controlled between 0.5 and 1.2 #m. The method is practical for preparing latex References 1. Smith WH, Ewart JJ (1948) Chem Phys 16: Napper DH, Gilbert RG (1990) In: Candau F, Ottewill RH (eds) An introduction to polymer colloids. Kluwer Academic Publishers, Dordrecht 3. De Boer GBJ, Hoedemakers GFM, Thoenes D (1989) Chem Eng Res Des 67: De Boer GBJ, De Weerd C, Thoenes D (1989) Chem Eng Res Des 67: Kotera A, Furusawa K, Takeda Y (1970) Lolloid Z u Z Polymere 239: Goodwin JW, Hearn J, Ho CC, Ottewill RH (1974) Colloid Polym Sci 252: Vanderhoff JW (1981) In: Basset DR, Hamielec AE (eds) Emulsion polymers and emulsion polymerization. ACS Sym Series, 165, Washington 8. Zecha H (1990) Makromol Chem Macromol Symp 31: Ugelstad J, Mork PC, Kaggerud KH, Ellinger T, Berge A (1980) Adv Colloid Interface Sci 13: Hayashi S, Komatsu A, Hirai TJ (1989) Polym Sci Polym Chem Ed 27:157 Authors' address: A. Eshuis Eindhoven University of Technology P.O. Box MB Eindhoven, The Netherlands Received October 29, 1990; accepted January 15, 1991