Suspension polymerization of styrene with pickering emulsifiers

The suspension polymerization of styrene in water with hydroxyapatite and c alcium carbonate as Pickering emulsifiers and sodium dodecylbenzenesulfonat e (NaDDBS) and poly(vinylpyrrolidone) (PVP) as surfactants was investigated . The adsorption of NaDDBS and PVP at the surface of the solids, the surfac e tensions of the aqueous solutions, the interfacial tensions between the a queous solution and styrene and the contact angles versus water and styrene , respectively, were measured in dependence of the surfactant and the solid s concentrations. In contrast to assumptions in literature, the adsorption equilibrium was found to be far on the side of desorption. The decrease of surface and interfacial tensions with increasing surfactant concentrations especially in the case of NaDDBS were stronger than expected due to an enha nced dissolution of the solids caused by the surfactant. The contact angles of the binary systems were measured by the Washburn method. Surprisingly t heir absolute values depended on the packing densities of the solids. The t heory of Good and Girifalco was used to calculate the interfacial tensions and the contact angle between solids and water in the ternary system solid/ water/styrene. For NaDDBS, the contact angle was higher than 90 degrees eve n at very low surfactant concentrations, causing unstable suspensions. Ther efore, the modulation effect of NaDDBS seems to be very low and developing very slowly due to the slow adsorption process. Also for PVP the contact an gle was higher than 90 degrees, however, stable suspensions were obtained. This may be due to the fact that PVP stabilizes polymer suspensions also wi thout any solids and that in such a system the stabilizing effect of solids is of minor importance. In the case of PVP the particle size of polystyren e could be controlled by the PVP concentration and the length of the contac t time between PVP and solid.