Role of Nanoparticles in Life Cycle of ZnO/Polyurethane Nanocomposites Xiaohong Gu, Guodong Chen, Minhua Zhao, Stephanie. S. Watson, Paul E. Stutzman, Tinh Nguyen, Joannie W. Chin and Jonathan W. Martin Materials and Construction Research Division National Institute of Standards and Technology, Gaithersburg, MD 20899, USA xiaohong.gu@nist.gov ABSTRACT This study investigated the role of ZnO nanoparticles in the photodegradation of a waterborne polyurethane (PU) nanocomposite during exposure to ultraviolet (UV) radiation. The effects of loading and size of ZnO nanoparticles on the photodegradation of ZnO/PU films were evaluated. Chemical and surface morphological changes of exposed specimens were examined by Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM), respectively. The results clearly showed that ZnO nanoparticles can act as a photo-catalyst or a photo-stabilizer, depending on the UV exposure conditions. Both size and loading of the ZnO nanoparticles had a strong effect on photodegradation of the nanocomposites.The polymer in the vicinity of the nanoparticles preferentially degrades when the nanoparticles act as a photo-catalyst, and is shielded when they behave as a photo-stabilizer. Keywords: nanoparticles, nanocomposites, UV exposure, life cycle, AFM 1 INTRODUCTION Recent developments in nanoparticle technology have initiated the use of inorganic UV absorbers such as zinc oxide (ZnO) for UV protection of polymers [1]. This approach is based on the unique UV absorbing ability of these metal oxides. Compared to the traditional organic UV absorbers, the inorganic UV absorbers offer transparency in the visible range and are non-migratory in the polymer matrix. They also offer enhancements in electrical, mechanical, and antibacterial properties for the polymers [1]. However, due to the photoreactivity of some metal oxide nanoparticles [2], their contributions to UV protection of the polymeric matrix can be complicated. In this paper, we have investigated the role of ZnO nanoparticles in the life-cycle of a waterborne PU nanocomposite exposed to UV radiation. Nano-filled polyurethane thin films were prepared with ZnO nanoparticles having various sizes and loadings. The UV exposure of the nanocomposites was conducted on both NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) and an UV/Ozone (UVO) chamber. Chemical changes of the nanocomposite films were examined by FTIR. Changes in film morphology were characterized by AFM. The photoreactivity of ZnO nanoparticles was measured by electron paramagnetic resonance (EPR) spectroscopy. The results clearly showed that the ZnO nanoparticles played a critical role in the life cycle of the ZnO/PU nanocomposites. They acted either as a photo-catalyst or a photo-stabilizer, depending on the UV exposure conditions. The degradation behavior of the polymers in the vicinity of the nanoparticles exposed to different conditions was discussed. 2 EXPERIEMNTALS* 2.1 Materials and Specimen Preparation A commercial waterborne dispersion of polyurethane (PU, Bayer Material Science) and a series of waterborne dispersions of ZnO nanoparticles (BYK Additives & Instruments) were selected for preparing ZnO/PU nanocomposites. The PU dispersion was a one-component, anionic dispersion of an aliphatic polyester urethane resin in water/n-methyl-2-pyrrolidone solvent. The nominal average diameters of the ZnO nanoparticles were 20 nm, 40 nm, and 60 nm, and their respective specific surface areas were 54 m 2 /g, 33 m 2 /g and 18 m 2 /g (as provided by the manufacturer). ZnO/PU nanocomposite films (hereafter referred to as ZnO/PU films) were prepared by mixing the PU dispersion with different loadings and/or different sizes of ZnO nanoparticles using a mechanical stirrer (Dispermat, VMA) at 315 rad/s (3000 rpm) for 20 minutes. Three ZnO nanoparticle loadings, 1 %, 2 %, and 5 % (based on mass of the solid PU), were used for 20 nm ZnO nanoparticles. Similarly, three sizes of nanopartilces, 20 nm, 40 nm and 60 nm were used at 5 % loading. After degassing for 1 h in vacuum, the mixture was then applied to the substrates. Thin films having a thickness of approximately 5 m were prepared by spin coating onto calcium fluoride (CaF 2 ) substrates. Thick films having a nominal thickness of approximately 100 μm were prepared by drawdown technique on a glass substrate that was pretreated with a release agent. All films were dried overnight under ambient conditions, followed by an oven post-curing at 150 C for 10 min. Free-standing ZnO/PU films were obtained by removing the thick films from glass substrates. In addition, NSTI-Nanotech 2010, www.nsti.org, ISBN 978-1-4398-3401-5 Vol. 1, 2010 709