Progress in Organic Coatings 72 (2011) 522–527 Contents lists available at ScienceDirect Progress in Organic Coatings j ourna l ho me p ag e: www.elsevier.com/locate/porgcoat UV-cured siloxane-modified acrylic coatings containing birifrangent calcarenitic stone particles: Photo-calorimetric analysis and surface properties C. Esposito Corcione ∗ , M. Frigione Department of Engineering for Innovation, University of Salento, via per Monteroni, Lecce, Italy a r t i c l e i n f o Article history: Received 15 October 2010 Received in revised form 17 May 2011 Accepted 21 June 2011 Keywords: Coatings Photo-polymerizable acrylic resins UV radiation Surface properties a b s t r a c t An experimental study was carried out for the development and characterization of innovative pho- topolymerizable siloxane-modified acrylic formulations for possible use as protective coatings for stone substrates. Two amounts of calcarenitic stone particles (25 wt.% and 35 wt.%) were added to a siloxane-modified acrylic mixture. The effect of stone particles inclusion on the reactivity and surface properties of the pho- topolymerizable systems was investigated. To this aim, the kinetics of the radical photo-polymerization reaction, induced by UV radiations in the presence of a suitable photoinitiator, was studied by calori- metric analysis, both in air and nitrogen atmosphere. The formulations, coated on a glass substrate, were photo-cured by using a medium pressure Hg UV lamp in air. On the coatings photo-polymerized in air were measured: gel content, transparency, scratch and surface hardness. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Polymers based on acrylic and methacrylic monomers are widely used for the protection and conservation of stone build- ings, due to their ability to form water repellent and optically clear coatings [1–17]. There are, however, durability issues on the out- door use of acrylic systems for the long term protection of historical buildings [18–20]. Acrylic coatings exert a poor adhesion to porous substrates and provide a sufficient drainage of the water from the coated surface. In order to overcome these deficiencies, water-borne silane modified acrylic resins and partially fluorinated acrylic copolymers have recently been proposed [3–17]. The free radicals for the initi- ation of the polymerization reactions of such systems are generally produced by the decomposition of a peroxide initiated by the heat generated through IR lamps. However, this technique is not suitable for the coating of large area substrates. In a first stage of research, methacrylate resin based mixtures were proposed for the protection of stone using as curing method the exposure to ultraviolet (UV) and visible radiations [21]. This procedure was proposed, in fact, to overcome the intrinsic defi- ciencies of acrylic coatings and to explore an alternative high energy radiation curing. UV sources are widely used for the cur- ing of coatings, inks and even adhesives; little use has been made so far of this method in the applications of coatings for the pro- tection of stones [22,23]. The selected methacrylate monomer ∗ Corresponding author. E-mail address: carola.corcione@unile.it (C.E. Corcione). (trimethylolpropane trimethacrylate, TMPTMA) was modified with a methacrylate silane coupling agent (trimethoxypropyl silane methacrylate, MEMO) to enhance the adhesion to the inorganic substrate [3]. In addition, a high molecular weight polysiloxane unsaturated oligomer was employed to increase the hydrophobic- ity and the viscosity of the acrylic–silane mixture. The increase of the hydrophobicity is required to enhance the water repellency of the coatings, while the increase of the viscosity is useful to control the penetration of the monomer characterized by a low viscosity [3]. A further increase in viscosity of the acrylic based formulations was obtained by adding an inorganic filler, consisting of a finely ground calcarenitic stone, known as pietra gentile. The results of the rheological measurements of the mixtures as function of shear rate and solid volume fraction of stone particles are reported in a previous work [24]. The primary reason for choosing this filler was to provide a color matched pigmentation for similar calcarenitic stone substrates. In relation to the specific application, the curing reactions of a thermosetting resin should achieve a minimum degree of con- version, often very high, and the reaction must proceed at a suitable rate. Cure kinetics strongly depend on parameters, such as temperature, time, and composition. The possibility to pre- dict how each parameter can affect kinetics of the curing process would make it possible to design coating systems for any specific application. In a previous paper [21], the kinetic of photo-polymerization process of the siloxane modified methacrylate resin, initiated by the addition of an appropriate photoinitiator for UV activated free radical reactions, was analyzed by photo-calorimetric analysis. 0300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2011.06.012