Browsing by Author "Yoon, Tae-Ho"
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- Adhesion study of thermoplastic polymides with Ti-6Al-4V alloy and PEEK-graphite compositesYoon, Tae-Ho (Virginia Tech, 1991)High glass transition (eg. 360 °C) melt processable thermoplastic polyimide homopolymers and poly(imide-siloxane) segmented copolymers were prepared from a number of diamines and dianhydrides via solution imidization, polydimethylsiloxane segment incorporation and molecular weight control with non-reactive phthalimide end-groups. The adhesive bond performance of these polyimides was investigated as a function of molecular weight, siloxane incorporation, residual solvent, test temperature, and polyimide structure via single lap shear samples prepared from treated Ti-6AI-4V alloy adherends and compression molded film adhesives or scrim cloth adhesives. The adhesive bond strengths increased greatly with siloxane segment incorporation at 10, 20 and 30 weight percent, and decreased slightly with total polymer molecular weight. As the test temperature was increased, adhesive bond strength increased, decreased or showed a maximum at some temperatures depending on the polyimide structure and siloxane content. The presence of residual solvent increased adhesive bond strength at ambient temperature but decreased the strength at the elevated temperatures. The variation of adhesive bond strength with residual solvent, siloxane and test temperature was attributed to the influence of these parameters on the brittle-ductile transition behavior of the polyimide system. This conclusion was supported by stress-strain measurements which indicated that tensile strength and modulus decreased with siloxane concentration and test temperature, demonstrating that there was an optimum combination of strength and strain for maximum adhesive bond strength. A model was developed to describe this behavior. The poly(imide-30%siloxane) segmented copolymer and a miscible poly(ether-imide) also demonstrated excellent adhesive bond strength with poly(arylene ether ketone) PEEK®-graphite composites. Oxygen or ammonia gas plasma treatment was very effective in further improving adhesive bond strength of melt laminated PEEK®-graphite composites.
- Phosphate stabilization by non-chromate post-rinse treatmentYoon, Tae-Ho (Virginia Polytechnic Institute and State University, 1987)Zinc phosphate conversion coating has been applied to improve the corrosion resistance and paint adhesion. However, zinc phosphate crystals dissolve in a highly alkaline environment, which reduces the corrosion resistance of base metal. To improve the phosphate stability in a highly alkaline environment, a post-rinse treatment has been applied to phosphate coating by rinsing with an aqueous solution which contains certain anions or cations. Chromatepost-rinse treatment is the most widely used method and has shown a great improvement in corrosion resistance. But, due to the environmental problems caused by chromate ions, non-chromate post-rinse treatment should be developed, which has equal or better corrosion resistance than does chromate post-rinse treatment. In this research, inorganic silicate with addition of Ca²⁺, Ba²⁺, Ni²⁺, Mg²⁺, has been extensively evaluated together with silane solution, γ -aminopropyltriethoxysilane ( γ -A.P.S., NH₂(CH₂)₃Si(OC₂H₅)₃), which was applied after the post-rinse treatment. The evaluation was carried out by the highly advanced surface analysis techniques such as SEM/EDX, AES, SIMS, and XRD and polarization measurements.