Effects of Pigment and its Dispersion on UV Degradation of Epoxy and Acrylic Urethane Polymeric Systems
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Effects of Pigment and its Dispersion on UV Degradation of Epoxy and Acrylic Urethane Polymeric Systems Stephanie Watson1, Amanda Forster1, I-Hsiang Tseng2, Li-Piin Sung1, Justin Lucas1, and Aaron Forster1 1 NIST, Gaithersburg, MD, 20899 2 NIST, Gaitherburg, MD, 20899 ABSTRACT The effects of surface treatment, particle size, and concentration of titanium dioxide (TiO2) and its dispersion on the photodegradation of two resin systems, epoxy and acrylic urethane, were examined. A suite of techniques, including laser scanning confocal microscopy (LSCM) and attenuated total reflectance –Fourier transform infrared spectroscopy (ATR-FTIR), was used to follow the degradation process of TiO2/polymer films as a function UV exposure. LSCM was used to show that both pigment dispersion and durability of the polymer matrix influenced the generation of pits/holes in pigmented polymer film. The type of pigment had a greater influence on the more durable polymer matrix, the acrylic urethane system. The epoxy system showed the greatest extent of degradation regardless of the TiO2 choice. The LCSM results were supported by the ATR-FTIR data. INTRODUCTION Titanium dioxide (TiO2) and other pigments/fillers are heavily utilized in building and construction applications. For most applications these components are added to increase the opacity and improve the appearance of a coating system. However, the addition of pigments has been found to affect the properties of the coating, especially its durability [1-3]. Basically, pigments can interact with their polymer binder in three different (but not mutually exclusive) ways: (a) Protecting the resin from direct photochemical degradation: Some polymers are particularly susceptible to direct ultraviolet (UV) degradation, which has sufficient energy to break chemical bonds within the resin. In this case, absorption of UV by TiO2 mitigates direct photochemical attack. Absorption of UV by TiO2 continues and will lead to photocatalytic attack as well, but is an initially less favored process. (b) Degrading the resin by photocatalytic degradation: Radicals generated by TiO2 oxidize the polymeric binder and is the process TiO2 manufacturers attempt to minimize via surface treatments of TiO2. (c) Introducing physical interactions with the resin: Degradation of a coating can lead to chalking, which is the physical separation/exposure of a pigment. Another factor, which is particularly important, especially in binders susceptible to photochemical attack, is the relative degree of dispersion of TiO2 particles. Well-dispersed TiO2 absorb more UV and give a smoother surface resulting in enhanced gloss retention [4]. The goal of this study is to examine the effects of type (i.e. surface treatment), particle size, and concentration of TiO2 on the photodegradation of resin systems exposed on the Simulated Photodegradation by High Energy Radiant Exposure (SPHERE) at the National Institute of Standards and Technology (NIST). The influence of the photoreactivity of TiO2 located below the polymer su
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