Probing Boundary Lubrication Phenomena On Textile Relevant Surfaces

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Boundary lubrication is a fundamental phenomenon in textile processing. While the study on the boundary can provide a unique insight of the chemical interactions between lubricants, finishes, and fibers, boundary lubrication has not received as much attention by the scientific community as hydrodynamic lubrication. The disparity in the scientific efforts to develop a comprehensive understanding of lubrication phenomena can be explained in part by experimental limitations when the characteristic dimensions of the phenomena become submicron. The work reported in this dissertation introduces lateral force microscopy as a feasible tool to probe boundary lubrication phenomena at the nanoscale and establishes correlations between this new technique and more established macroscopic analytical methods. The findings and research contributions of this work are expected to provide a systematic guide to design intelligent and efficient textile finishes and lubricants. We studied boundary lubrication on model surfaces of common textile materials such as polypropylene (PP), polyethylene (PE), and cellulose. Thin films of these polymeric materials were made using the spin-coating method. The friction coefficient of the films was measured via lateral force microscopy (LFM) under three environmental conditions, in air, in water, and in lubricant solutions. The lubricant solution consisted of aqueous solutions of a PEO-PPO-PEO triblock copolymer commercially known as Pluronic (R) surfactants. LFM results indicated that when the lubricant was applied, PP and PE surfaces exhibited lower values of the friction coefficient than those on the cellulose specimen. This behavior was explained on the basis of the lubricant molecular configuration. It was determined that the PEO segment of the triblock copolymers had higher affinity towards the cellulose surface while the PPO segment exhibited a higher tendency to deposit on the PP and PE substrates. These differences in affinity were responsible for the diverse configurations of the triblock copolymer hence their lubrication performance. Unique molecular structures of the triblock copolymer were proposed including a buoy (PEO)-anchor (PPO)-buoy (PEO) structure on both PP and PE surfaces and an anchor (PEO)-buoy (PPO)-anchor (PEO) structure on cellulose. The improved lubrication performance of the triblock copolymer on the PP and PE surfaces was attributed to the flexibility of the PEO segments extending from the surface while the poor lubrication on the cellulose surface was attributed to the configuration of the PEO segments being anchored to the surface. Two important parameters, critical normal force and adhesion hysteresis, were found to correlate and to predict lubrication performace. High critical normal force and low values of adhesion hysteresis were good predictors of low friction coefficients. The reported findings are valuable to explain the behavior of finishing additives and lubricants commonly used in textile and fiber processing operations, as well as to relate the morphology of the adsorbed layers to friction and wear phenomena.
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