Browsing by Subject "Wetting"

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    Development of Lactide-based Macromonomers for Copolymerization with Acrylates to produce Adhesives and Coatings of High Renewable Contents
    (2019-01) Gu, Cheng
    A new approach was introduced for incorporating renewable biomass into existing commercial pressure-sensitive adhesive (PSA) polymers in the form of acryloyl macromonomers (MM). MMs were prepared with L-lactide and ε caprolactone via a bulk ring-opening polymerization initiated by N-hydroxyethyl acrylamide (HEAA). Acrylic adhesive copolymers were synthesized by free-radical solution polymerization in presence of 2-ethylhexyl acrylate (EHA), acrylamide and MMs. A series of MMs, synthesized using catalyzed ring-opening polymerizations, were produced containing a broad range of lactic acid and caprolactone repeat units. Results indicate that the properties and performance of adhesive polymers are strongly dependent on lactide composition. In general, increasing lactide content increases polymer hardness enhancing cohesive strength, while reducing it (i.e., increasing caprolactone content) softens the polymer. Optimal adhesion is found to require a balance between these tendencies as indicated by the existence of a clear maximum in both tack and peel data. The results demonstrate that a broad range of properties is achievable through relatively minor modifications to MM composition. It is expected that these hybrid materials can be optimized for a variety of self-adhesive applications. With the new MM approach, the relation between the dynamic wetting behavior on a soft viscoelastic surface and the rheological properties of materials can be studied. The mechanical properties of polymers are tailored through changing MM composition to provide a broad range of viscoelastic responses. It was found the wetting of these polymers supports the existence of two distinct wetting regions as opposed to the several, one in which the wetting line and ridge propagate smoothly together, and a second in which the ridge slows propagation and is eventually dropped leaving behind a residual deformation ridge. The focus is on ridge formation and properties controlling its propagation prior in the neglected former region. Although most past experimental studies emphasize the rate dependency of this process, results presented here indicate that ridge propagation is governed to a similar extent by film thickness and the vertical surface tension force. The data is used to develop a semi-empirical model consistent with the contribution of both viscous and elastic responses to the process. The ideas presented provide a new and more comprehensive view of the wetting of soft substrates.
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    Onset of dynamic wetting failure: the mechanics of high-speed fluid displacement
    (2013-07) Vandre, Eric Allen
    Dynamic wetting is crucial to processes where a liquid displaces another fluid along a solid surface, such as the deposition of a coating liquid onto a moving substrate. Numerous studies report the failure of dynamic wetting when process speed exceeds some critical value. Typically, wetting failure is a precursor to air entrainment, which produces catastrophic defects in coatings. However, the hydrodynamic factors that influence the transition to wetting failure remain poorly understood from empirical and theoretical perspectives. This work investigates the fundamentals of wetting failure in a variety of systems that are relevant to industrial coating flows. A hydrodynamic model is developed for planar and axisymmetric geometries where an advancing fluid displaces a receding fluid along a smooth, moving substrate. Numerical solutions predict the onset of wetting failure at a critical substrate speed, which coincides with a turning point in the steady-state solution path for a given set of system parameters. Flow-field analysis reveals a physical mechanism where wetting failure results when capillary forces can no longer support the pressure gradients necessary to steadily displace the receding fluid.Novel experimental systems are used to measure the substrate speeds and meniscus shapes associated with the onset of air entrainment during wetting failure. Using high-speed visualization techniques, air entrainment is identified by the elongation of triangular air films with system-dependent size. Air films become unstable to thickness perturbations and ultimately rupture, leading to the entrainment of air bubbles. Meniscus confinement in a narrow gap between the substrate and a stationary plate is shown to delay air entrainment to higher speeds for a variety of water/glycerol solutions. In addition, liquid pressurization (relative to ambient air) further postpones air entrainment when the meniscus is located near a sharp corner along the plate. Recorded critical speeds compare well to predictions from the model, supporting the hydrodynamic mechanism for the onset of wetting failure. Lastly, the common practice of curtain coating is investigated using the hydrodynamic model. Due to the complexity of this system, a new hybrid method is developed to reduce computational cost associated with the numerical analysis. Results show that the onset of wetting failure varies strongly with the operating conditions of this system. In addition, stresses from the air flow dramatically affect the steady wetting behavior of curtain coating. Ultimately, these findings emphasize the important role of two-fluid displacement mechanics during high-speed wetting. Although this work was motivated by coating flows, it is also relevant to a number of other applications such as microfluidic devices, oil-recovery systems, and splashing droplets.