Mechanical reinforcement of unsaturated polyester by AL 2 O 3 nanoparticles Mei Zhang, Raman P. Singh * Mechanics of Advanced Materials Laboratory, Department of Mechanical Engineering, State University of New York, Stony Brook, NY 11794, USA Received 1 May 2003; accepted 23 May 2003 Abstract Nanometer-sized Al 2 O 3 particles (15 nm average diameter) were used as reinforcements to enhance the fracture toughness of a highly crosslinked, nominally brittle, thermosetting-unsaturated polyester resin. It was observed that the addition of untreated, as-received Al 2 O 3 particles does not result in enhanced fracture toughness. Instead, the fracture toughness decreases by 15% as the volume fraction of the particles is increased from 0% to 4.5%. Similar degradation in fracture toughness was observed for reinforcement by 1- and 35-Am Al 2 O 3 particles. The lack of reinforcement was attributed to poor particle-matrix bonding, as observed from scanning electron micrographs of the fracture surfaces. However, considerable reinforcement was observed when the nanocomposites were fabricated using an organofunctional silane to enhance particle-matrix interface strength. For the case of a 4.5% volume fraction of well-bonded Al 2 O 3 particles added to the unsaturated polyester, the fracture toughness was increased by almost 100%. D 2003 Published by Elsevier B.V. Keywords: Nanocomposites; Deformation and fracture; Unsaturated polyester; Alumina; Organofunctional silane 1. Introduction Thermosetting polymers, such as polyesters and epoxies, exhibit several useful characteristics due to the high degree of cross-linking between individual polymer chains. Typical qualities include a high glass transition temperature, high modulus and specific strength, creep resistance, and good solvent resistance. Superior properties, coupled with ease of processing, have led to numerous applications of these polymers, perhaps most importantly as matrix materials for fiber-reinforced laminated composites. Unfortunately, this cross-linking also makes these materials inherently brittle with poor resistance to crack initiation and propaga- tion, in comparison to other engineering plastics. Thus, significant improvements in the fracture toughness of ther- mosetting polymers are required to improve the damage tolerance and long-term durability of structures and materi- als based on these polymers, especially fiber-reinforced laminated composites. Improving the fracture toughness of thermosetting res- ins has been the subject of considerable research. Howev- er, the conventional approach of introducing micrometer- sized particles (elastomeric, thermoplastic or ceramic) into the polymer matrix has failed to yield significant improve- ments in the fracture toughness of highly cross-linked thermosetting polymers. Furthermore, other mechanical properties are often compromised, or there are processing problems. Recently, it has been demonstrated that the addition of nanometer-sized fillers can be used to significantly en- hance the mechanical properties of thermosetting polymers [1,2]. Specifically, it has been observed that the enhance- ment in fracture toughness of thermosetting polymer composites is directly related to the size of nanometer- sized reinforcements, with smaller particle sizes resulting in greater increase in fracture toughness [3]. This obser- vation is sharply in contrast with that for conventional reinforcements of thermosetting polymers with micro- meter-sized rubber or glass particles, which lead to moderate or no enhancements of fracture toughness. In light of these observations, this investigation presents an experimental investigation of the fracture behavior of a highly cross-linked unsaturated polyester polymer rein- 0167-577X/$ - see front matter D 2003 Published by Elsevier B.V. doi:10.1016/S0167-577X(03)00512-3 * Corresponding author. Tel.: +1-631-632-4354; fax: +1-631-632- 8544. E-mail address: raman.singh@sunysb.edu (R.P. Singh). www.elsevier.com/locate/matlet Materials Letters 58 (2004) 408 – 412