Mechanical and thermal characterization of a structural polyurethane adhesive modified with thermally expandable particles M.D. Banea a,b,n , L.F.M. da Silva b , R.J.C Carbas b , R.D.S.G. Campilho c a IDMEC, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal b Departamento de Engenharia Mecânica, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal c Departamento de Engenharia Mecânica, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Rua Dr. António Bernardino de Almeida, 431, 4200-072 Porto, Portugal article info Article history: Accepted 3 June 2014 Available online 18 June 2014 Keywords: Thermally expandable particles (TEPs) Adhesive joints Fracture toughness abstract Thermally expandable particles (TEPs) are used in a wide variety of applications by industry mainly for weight reduction and appearance improvement for thermoplastics, inks, and coatings. In adhesive bonding, TEPs have been used for recycling purposes. However, TEPs might be used to modify structural adhesives for other new purposes, such as: to increase the joint strength by creating an adhesive functionally modified along the overlap of the joint by gradual heating and/or to heal the adhesive in case of damage. In this study, the behaviour of a structural polyurethane adhesive modified with TEPs was investigated as a preliminary study for further investigations on the potential of TEPs in adhesive joints. Tensile bulk tests were performed to get the tensile properties of the unmodified and TEPs- modified adhesive, while Double Cantilever Beam (DCB) test was performed in order to evaluate the resistance to mode I crack propagation of unmodified and TEPs-modified adhesive. In addition, in order to investigate the behaviour of the particles while encapsulated in adhesives, a thermal analysis was done. Scanning electron microscopy (SEM) was used to examine the fracture surface morphology of the specimens. The fracture toughness of the TEPs-modified adhesive was found to increase by addition of TEPs, while the adhesive tensile strength at yield decreased. The temperature where the particles show the maximum expansion varied with TEPs concentration, decreasing with increasing the TEPs content. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Adhesive bonding is a viable technique for joining a wide range of materials [1]. However, increasing the lifetime, reducing the costs and improving the safety of structures are highly demanded, nowadays. Hence, the development of new technologies and processes for easy recycle, heal or self-heal of bonded structures are becoming of great interest for the industry [2]. If bonds can be broken without damage of the components, recycling is easier. Also, for an environmental friendly disassembly of bonded struc- tures, it is necessary to separate the joint between the bonded components so that the different materials can be reused on a qualitatively high level. One example is the case of the adhesives modified with thermally expandable particles (TEPs), which can be dismounted by heating the joint. However, TEPs might be used to modify structural adhesives for other new purposes, such as: to increase the joint strength by creating an adhesive functionally modified along the overlap of the joint by gradual heating and/or to heal the adhesive in case of damage. Thermally expandable particles, microcapsules or microsphere, were developed by Dow Chemical Co in the early 1970s [3] and are particles made up of a thermoplastic shell filled with liquid hydrocarbon. On heating them, two transformations occur (see Fig. 1). One is the softening of shell material and the other is the gasification of the hydrocarbon liquid inside it. As a consequence, the shell expands as the gas inside it pushes the softened shell from inside out causing it to grow in size. When fully expanded, the growth in volume of the particle can be from 50 to 100 times. When heat is removed, the shell stiffens and the particle remains in its expanded form. Expansion temperatures range from 70 1C to 285 1C depending on particle and grade [4]. They are commercia- lized worldwide by the companies Expancel Nobel Industries (Sweden), under the trademark of Expancel [5], and Matsumoto Yushi Seiyaku (Japan), under the trademark of Micropearl [4]. Owing to this unique behaviour, TEPs are used by the industry in a wide variety of applications [6–9]. This innovative idea has been extended to structural adhesives for recycling purposes by Nishiyama et al. [10]. The simple heating of the joint over 100 1C leads to an easy separation of the bonded materials. The adhesive Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ijadhadh International Journal of Adhesion & Adhesives http://dx.doi.org/10.1016/j.ijadhadh.2014.06.008 0143-7496/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author at: IDMEC, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal. Tel.: þ351 225081750; fax: þ351 225081445. E-mail address: mbanea@fe.up.pt (M.D. Banea). International Journal of Adhesion & Adhesives 54 (2014) 191–199