UV generation of a multifunctional hyperbranched thermal crosslinker to cure epoxy resins D. Foix a , X. Ramis b , A. Serra a , M. Sangermano c, * a Department of Analytical and Organic Chemistry, University Rovira i Virgili, C/Marcel$lí Domingo s/n, 43007 Tarragona, Spain b Thermodynamics Laboratory, ETSEIB University Politècnica de Catalunya, C/Av. Diagonal 647, 08028 Barcelona, Spain c Department of Material Science and Chemical Engineering, Politecnico di Torino, C.so Duca degli Abruzzi 24,10129 Torino, Italy article info Article history: Received 8 March 2011 Received in revised form 21 April 2011 Accepted 18 May 2011 Available online 2 June 2011 Keywords: Hyperbranched polymers UV curing Epoxy resin abstract A new hyperbranched polymer (HBP) was obtained via an iterative synthetic procedure that consists of esterification and thiol-ene click reaction. This polymer was used as a latent multifunctional macro- initiator for the dual curing of a commercially available cycloaliphatic epoxy resin and this process was studied by photo-DSC and FTIR. The presence of thioether groups in the HBP structure leads, by pho- toirradiation, to the formation of sulfonium salts, which are thermal cationic initiating species. The materials obtained were characterized by DMTA, TGA, gel content and FESEM. By means of the last technique domains of HBP as a second phase within the epoxy matrix were observed. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Epoxy resins are widely used as thermosets in technological applications such as coatings, adhesives, structural applications or electronics but they lack of good toughness characteristics for some specific applications. This is due to their high crosslinking character and rigid structure, which leads to a poor resistance to crack formation and its growth [1]. Toughening of epoxy thermosets can be done by the reduction of the crosslinking density or by the addition of polymeric rubber particles that leads to phase separated morphologies, but the deterioration of other characteristics, such as the Tg and stiffness are common side effects accompanying these solutions [2]. Recently, Le et al. [3] reported that the addition of rubber particles of nanometer size doubled the fracture toughness of the modified epoxy without observing loss of strength, modulus and glass transition temperature. Also H-J. Sue et al. [4] reported the use of block copolymers in epoxy thermosets to improve the fracture toughness in a 300% without compromising its modulus by a nanocavitation mechanism of fracture. Another approach to increase toughness in epoxy resin has been the addition of hyper- branched polymers (HBPs). The possibility to tailor their structure and the presence of a great number of reactive or non-reactive groups as chain ends can lead to homogeneous or phase sepa- rated epoxy thermosets with improved characteristics [5,6]. The use of HBPs as reactive modifiers does not affect substantially the Tg and stiffness if their structure is efficiently tailored [7,8] and even can present some other advantages, such as the reduction of the shrinkage on curing [9] and increased reworkability [7,10]. In addition, the curing process can be accelerated in reference to the curing of the neat resin and the conversion at the gelation can be increased [9]. A multitude of high-tech and electronic applications, such as coatings in electronic fibers, microelectronics and printed circuit boards use UV curable formulations. This is because the formation of the film is a very quick process with low energy consumption and absence of VOC emissions. Cationic photopolymerization is a very advisable technique because the lack of inhibition by oxygen and the good characteristics of the thermosets obtained. The photo- crosslinking of HBP-epoxy formulations has been extensively re- ported in the literature [11,12]. Thiol-ene photocurable coatings present advantages such as inherently rapid reaction rate, low shrinkage, reduced oxygen inhibition, photoinitiator-free polymerization and formation of homogeneous polymer structures. However, they present odor problems, low surface hardness and poor mechanical properties [13]. To increase mechanical characteristics it seemed interesting to incorporate epoxy resins into these systems to be polymerized by * Corresponding author. Tel.: þ390115644651; fax: þ390115644699. E-mail address: marco.sangermano@polito.it (M. Sangermano). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.05.029 Polymer 52 (2011) 3269e3276