(449d) Solubility Of Irgacure® 2959 Photoinitiator In Supercritical Carbon Dioxide: Experimental Determination And Correlation

In recent years, supercritical carbon dioxide (CO2) has become an attractive polymerization media solvent, not just because its well known environmental-friendly properties and low-cost, but also because its excellent mass and heat transferring abilities. Furthermore, it does not support chain transfer to solvent during free-radically initiated chain polymerization. Thus, it is an ideal solvent for use in these type of polymerization reactions, despite the fact that it is a poor solvent for most high molecular weight polymers [1,2]. Because most vinyl polymers show poor solubility in supercritical carbon dioxide, heterogeneous polymerizations, and particularly dispersions polymerization and precipitation polymerizations, have been extensively studied in recent years [2]. In these polymerization methods, the initiator must be soluble in the initial homogeneous mixture (also containing the vinyl monomer) and is usually an oil-soluble thermal initiator such as an organic azo-compound (e.g., AIBN) or a peroxide (e.g., BPO) [1,2]. However, and because the use of thermal initiators, high temperatures (usually above 60 ºC) are normally required to initiate these two types of free radical polymerization reactions. Therefore, in some cases and for several reasons, especially when thermo labile substances or devices are also involved in the reaction or process, it would be preferable to use another type of oil-soluble initiator, like a photoinitiator, which will avoid the use of high polymerization and initiation temperatures. Photochemical (ultraviolet or visible light) initiated polymerization reactions are already a well established technology for many industrial applications like offset lithographic and flexographic inks, screen printing, metal decoration, wood coatings, pigmented coatings for textile applications and pigmented primary and secondary optical fiber coatings [3]. Photopolymerization is also commonly used in a broad range of biomedical applications such as dentistry, implants, scaffolds, bioadhesives and drug delivery system [4]. The commercially available CIBA photoinitiator, 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone (Irgacure® 2959), is a highly efficient radical photoinitiator for UV curing systems (comprised by unsaturated monomers and pre-polymers), which is also suitable for use in water-borne systems. Furthermore, and due to its properties and to its low cellular toxicity [4], this photoinitiator can be also employed in the photopolymerization of biocompatible polymers and copolymers intended for biomedical and tissue engineering applications, not only in conventional pharmaceutically-accepted liquid solvents, but also in supercritical CO2. However, for the design of any process based in supercritical fluid technology (like polymerizations in supercritical CO2), it is necessary the precise knowledge of the equilibrium solubility data, in different conditions of temperature and pressure, between the compounds of interest and the supercritical fluid (SCF) solvent. If not completely determined experimentally, the solubility in SCFs must be obtained and extended through correlations based on theoretical or empirical models applied to the existing experimental data. The most common models used for correlating solid-supercritical phase equilibria are cubic equations-of-state (EOSs) from the van der Waals family, like the Peng-Robinson [5] and the Soave-Redlich-Kwong [6] EOSs. As an alternative, several empirical correlations, such as the density-based correlations of Chrastil, Bartle and Méndez-Santiago-Teja [7-9] are also broadly used. In this work, the equilibrium solubility of Irgacure® 2959 in supercritical CO2 was obtained using a static analytical experimental method. Measurements were carried out between 308.15 and 328.15 K and between 10 and 25 MPa. The Soave-Redlich-Kong and the Peng-Robison EOSs, together with the classical van der Waals mixing and combining rules, were applied to correlate the obtained data. The role of estimated critical properties and solid sublimation pressure in the models' capability to successfully correlate solubility data was also investigated, using different sets of properties' values, estimated with different group contribution methods available in the literature. Data were also correlated with the Chrastil, Bartle and Méndez-Santiago-Teja models.

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