New hyperbranched polyester modified DGEBA thermosets with improved chemical reworkability David Foix a , Michael Erber b , Brigitte Voit b , Albena Lederer b , Xavier Ramis c , Ana Mantecón a , Angels Serra a, * a Department of Analytical and Organic Chemistry, University Rovira i Virgili, C/Marcel$lí Domingo s/n, 43007 Tarragona, Spain b Leibniz-Institute of Polymer Research Dresden e.V., Hohe Str. 6, 01069 Dresden, Germany c Thermodynamics Laboratory, ETSEIB University Politècnica de Catalunya, C/Av. Diagonal 647, 08028 Barcelona, Spain article info Article history: Received 22 October 2009 Received in revised form 7 January 2010 Accepted 13 January 2010 Available online 21 January 2010 Keywords: Epoxy resins Hyperbranched polymers Cationic polymerization Reworkability Shrinkage abstracts The chemical reworkability of epoxy thermosets in alkaline solutions has been increased by adding a hyperbranched polyester (HBPE) as a reactive modifier to the diglycidyl ether of bisphenol A (DGEBA) cured by lanthanide triflates as cationic initiators. The presence of hydroxyl chain-ends in the HBPE allows the modifier to be linked covalently to the epoxy matrix through the monomer activated prop- agation mechanism, which occurs in cationic polymerizations. Yb(OTf) 3 leads to quicker curing than the lanthanum salt and to materials with the highest glass transition temperature (T g ). The addition of HBPE does not affect adversely the thermal stability and leads to a slight reduction of the global shrinkage. The T g of the materials, the relaxed modulus and the linear thermal expansion coefficients are practically maintained by the addition of HBPE. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Epoxy resins are among the most widely used materials in the coating of electronic devices. This is because they present good properties in terms of electrical insulation, adhesion to various components and thermal stability. However, they are currently lacking in terms of reworkability, meaning the encapsulated or protected material can not be recovered to be repaired or recycled. The concept of reworkability is related to the ability of the material to break-down under controlled conditions in order to remove the coatings from the substrate, but it does not mean that the polymeric material can be reused or recycled. The introduction of labile linkages, thermally cleavable over 200  C, has been proposed to facilitate the reworking of the thermosets in which they are chemically incorporated [1e3]. The first studies were mainly based on new cycloaliphatic epoxy resins which contained ester groups [4], but they usually lead to high crosslinking densities, which worsen the mechanical characteristics. Our research has been focused on increasing the reworkability of epoxy thermosets. In previous papers [5e7] we reported a new route of introducing ester groups in thermosetting materials by the copolymerization of commercial epoxy resins with lactones via ring-opening mechanism. This strategy presents some advantages such as the possible modulation of the properties of the final materials on changing the characteristics of the epoxy resin and the co-monomer, their feed ratio and the initiator used, but it leads to a decrease of the T g , caused by the aliphatic structure of the co- monomer. As a new strategy, the reworkability of modified epoxy thermosets has also been increased on adding hyperbranched polyesters as modifiers [8,9]. Usually, hyperbranched polymers have been added to epoxy thermosets to improve toughness [10,11]. Moreover, the addition of these modifiers can reduce the shrinkage during curing by reducing the internal stresses, which in turn affects the durability of the coating and its protection capability [8]. Taking all these improve- ments into account, the development of these modifiers can be of a great technological importance [12]. HBPs have some structural advantages such as a large number of functional groups at the end of the branches and a versatile structure depending on the monomeric units selected in their synthesis. Their branched structure leads to a lack of entangle- ments, which reduce the melt viscosity. In this way, HBPs can be considered as versatile reactive modifiers to improve epoxy coat- ings [13]. * Corresponding author. Fax: þ34 977558446. E-mail address: angels.serra@urv.cat (A. Serra). Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab 0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.01.011 Polymer Degradation and Stability 95 (2010) 445e452