Micro/Nano Indentation and Micro-FTIR Spectroscopy Study of Weathering of Coated Engineering Thermoplastics
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Micro/Nano Indentation and Micro-FTIR Spectroscopy Study of Weathering of Coated Engineering Thermoplastics Samik Gupta, Jan Lohmeijer1, Savio Sebastian, Nisha Preschilla and Amit Biswas GE India Technology Center, Hoodi Village, Whitefield Road, Bangalore-560066, India 1 GE Advanced Materials-Europe, Plasticlaan-1, Bergen op Zoom, The Netherlands ABSTRACT A novel combination of depth-sensing nano-indentation, micro-indentation and microFTIR techniques is employed towards understanding the durability of coating layers used on engineering thermoplastics upon exposure to harsh weathering environments. This combination of techniques enables study of changes in surface-to-bulk properties in the clearcoat-substrate system upon weathering; typically observed as a degradation starting from the surface and then proceeding inwards to the bulk of the material. Nano-indentation measurements carried out to understand the mechanical properties of the coating layer provide insights into the changes in hardness and modulus upon prolonged weathering exposure. Depth-sensing micro-indentation and micro-FTIR spectroscopy studies performed to evaluate mechanical performance and chemical changes, respectively, explain the influence of the substrate on the coating layer, especially at the interface upon weathering. This unique combination of depth-sensing indentation and micro-FTIR spectroscopy has led to an understanding of the properties of the coating layer and the substrate individually as well as an integral system as a function of weathering exposure time. Finally, the physico-chemical properties of the coating and substrate are linked to performance prediction, enabling optimization of coating-substrate combinations. INTRODUCTION High performance Engineering Thermoplastics (ETP’s) have widespread outdoor applications and are consequently susceptible to deterioration in physical properties upon exposure to different environmental stresses such as ultraviolet radiation, changes in humidity and ambient temperature etc., though these changes are maximal in the near surface [1,2]. Therefore, a clearcoat layer is usually applied on an ETP, primarily for protecting the bulk of the material from this weathering induced degradation. ETP’s generally possess poor abrasion resistance and inferior gloss, hence clearcoat layers also function as a scratch resistant shielding layer with superior aesthetics thereby offering an overall enhanced visual appeal. However, exposure to ultraviolet radiation may cause photo oxidation of the clearcoat and result in changes in the physical properties affecting the synergistic performance of the coated ETP, typically seen as a catastrophic failure of the coating substrate system or coating delamination [3]. It is thus important to explore and comprehend these weathering induced changes in the mechanical properties and chemical composition of both the coating layer as well as the substrate while developing a coated ETP. Numerous surface analysis and depth sensing techniques have been recently reported
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