Photocyclic initiating systems for free radical photopolymerization under visible light. Application to holographic recording

XAVIER ALLONAS; CHRISTIAN LEY; AHMAD IBRAHIM; OLGA I. TARZI; AURELIE CHANG-YONG; CHRISTIANE CARRE

Cordoba

Conferencia; XI ELAFOT; 2012

Light induced polymerization reaction is employed in quite different technical applications that have become beneficial to humans. These applications include microelectronics, information technologies, optical fibers, dental materials, printing inks,paints, varnishes, ... In other words, various kinds of polymers can be synthesized by lightinduced chemical processes, a technique commonly denoted by the term photopolymerization. A key component of this process is the photoinitiating system, which is responsible of the absorption of light and its conversion into chemical energy. For example, laser direct imaging, graphics arts, holography, and dental materials require irradiation in the visible spectrum to benefit from laser technologies or simply to avoid UV damaging effects on skin. Some dyes absorbing in the visible region have been reported to be photoreducible in the presence of amines. These compounds belong to the families of xanthenes, fluorones, acridines, phenazines, thiazenes, and so on. However, dye/coinitiators systems were not developed significantly in the industry. Very often, dark reactions take place that lead to poor shelf life of the formulation, an effect that was detrimental to their industrial use for a long time. In addition the conversion of the monomer to polymer was generally limited. Indeed, for most of the industrial applications, conversion of more than 60% have to be reached, a goal that is difficult to achieve with conventional dye/coinitiator photoinitiating systems (PIS). Certain additives improve the polymerization efficiency, leading to the development of the so-called three-component PIS or photocyclic initiating systems [1-3]. The mechanism involved is usually rather complex and is based on chemical secondary reactions. It was reported that different radical intermediates generated during the irradiation and in the subsequent polymerization reaction react with the additive to give new reactive radicals. In this paper, a set of photoinitiating systems (PIS) for free radical photopolymerization was studied using time-resolved spectroscopic experiments, real-time FTIR for holographic recording. It is shown that the efficiency of the photoinitiating system can be drastically increased when a redox additive is added to the conventional dye/coinitiator system by virtue of a photocyclic behaviour. Homogeneous photopolymerization process was found to reach a fast vitrification, limiting the conversion at about 55%. By contrast, holographic recording underlines the differences in photoinitiating system reactivity, allowing diffraction efficiencies close to unity for the most reactive PIS [4]. References [1] J.P. Fouassier, X. Allonas, D. Burget, Prog. Org. Coat., 47, (2003), 16 [2] O. Tarzi, X. Allonas, C. Ley, J.P. Fouassier, J. Polym. Sci., Part A : Polym. Chem., 48(12), (2010) 2594-2603. [3] A. Ibrahim, C. Ley, O.I. Tarzi, J.P. Fouassier, X. Allonas, J. Photopolym. Sci. Techn. 23, (2010) 101-108. [4] A. Ibrahim, C. Ley, X. Allonas, O.I.Tarzi, A. Chan Yong, C. Carré, R. Chevallier, Photochem. Photobiol. Sci., 2012, DOI: 10.1039/C2PP25099C.