Browsing by Subject "Coating"

Now showing 1 - 8 of 8
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    Cure induced stress generation and viscoelasticity in polymer coatings.
    (2010-01) O’Neal, Daniel Jeffrey
    Coatings solidified by free-radical polymerization and crosslinking (curing) reactions initiated with ultraviolet (UV) light do so quickly and at room temperature. Low viscosity monomer or oligiomer makes the use of volatile solvent unnecessary, decreasing energy use and making the process more environmentally friendly but photoinitiators can be toxic, limiting certain applications. Stress may be generated by a changing specific volume during cure, and stress-induced defects are undesirable. The goal of this research is to understand stress generation in UV irradiated coatings and to model stress generation and viscoelasticity seen during curing. Two new mathematical models were created to accomplish viscoelastic stress modeling. The first, a network model, uses a two-dimensional network of one-dimensional elements to replicate deformation in the coating. The second uses continuum momentum conservation and linear viscoelastic equations. Inertial forces can be neglected and a substitution performed, making the solution more rapid and simple with standard finite element methods. Stress generation in uniformly cured coatings depends on how quickly the specific volume and physical properties change. Reaction kinetics, volume, and stress are calculated simultaneously. Rapid initiation from high initiator concentration or UV light intensity delays volume change, generating more stress because the volume changes with a higher modulus. An optimum curing schedule would insure the actual specific volume and its equilibrium value remain the same. Inhomogeneities in the substrate or the presence of defects change the stress field. Knowing forces on the coating boundaries suggests defect locations and types. Probing the types of geometries and surface roughnesses seen in different types of coatings shows that restricted deformation increases stress concentrations and surface forces seen. Also, avenues for reducing stress via relaxation are discussed. The two-dimensional stress profiles used in these analyses are not possible to measure experimentally, making computational modeling essential. The models developed and methodology presented may be extended to other UV cured coatings or to other methods of coating solidification. Process windows of allowable final conversion-stress-energy-time states suggest what tradeoffs must be made to meet constraints.
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    Dispersible exfoliated zeolite nanosheets and their application in high performance zeolite membrane
    (2013-10) Agrawal, Kumar Varoon
    In the wake of the energy crisis, an efficient separation technology such as membrane is required to replace the energy intensive processes like distillation. High performance zeolite membrane can be fabricated by coating of a thin film of high-aspect-ratio zeolite nanosheets on a porous support. However, the synthesis of highly crystalline and morphologically intact zeolite nanosheets by the direct hydrothermal synthesis has been challenging. Successful reports on the synthesis of zeolite nanosheets by the exfoliation of their layered structure exist, but the synthesis routes provided in these reports often lead to significant damages to the structure and the morphology of nanosheets. This dissertation focuses on the development of a scalable method for the synthesis of zeolite nanosheets, while preserving their structure and the morphology. MWW and MFI nanosheets were prepared by polymer melt compounding of their layered precursors with polystyrene. Zeolite nanosheets were extracted out of the polymer matrix by solution processing of the zeolite-polymer nanocomposite. Exfoliated nanosheets and their coatings were then characterized by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD). A compact, oriented, 300-nm thick zeolite film was fabricated on a symmetric alumina support by a one-step filter coating method. This nanosheet film demonstrated molecular sieving capabilities after a mild hydrothermal treatment. Density gradient centrifugation was used to purify the zeolite nanosheets from the polymer matrix, and the large unexfoliated particles, resulting in a two-fold increase in the yield of nanosheets in the final coating suspension. Sub-100 nm thick films of these nanosheets were made on a symmetric alumina supports. Nanosheet films with thickness ranging from 10 nm to 100 nm were prepared on an asymmetric silica supports. In-plane secondary growth of these films by the impregnation growth method led to b-oriented, 100-150 nm thick zeolite film that separated xylene isomers with separation factors of 100-800, while providing a high permeance of p-xylene (4 x 10-7 moles/m2-s-Pa).</
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    Electrostatic effects in coating and printing processes
    (2015-01) Ramkrishnan, Aruna
    Coating and printing are interfacial processes that are highly relevant in industry. Precision coatings impart functionalities and boost the performance of products. On the other hand, high-resolution roll-to-roll printing is being increasingly explored for creating dense and flexible printed electronics at high speeds. Electrostatic effects often significantly influence both these processes. However, in industry, much of the current understanding of these effects is empirical and has not received a rigorous treatment. This thesis discusses how electrostatics and hydrodynamics couple in coating and printing applications, and presents different modes of investigation: simplified thin-film models and flow visualization experiments, to understand the underlying physics of these processes. Throughout this work, the electric response of liquids has been described by the perfect (non-conducting) and leaky dielectric (partially conducting) models, which are representative of many liquids used in industry. In coating processes, electrostatic charges are known to accumulate on the substrate due to various upstream operations (e.g. corona treatment, friction in roll-to-roll equipment). This leads to the buildup of an electric field in the subsequently coated film, which in turn causes the formation of defects due to electrostatically driven flows. Thus, in order to obtain high quality coatings, it is desirable to keep them resistant to electrostatic destabilization. We have carried out a systematic study via the construction of electrohydrodynamic lubrication models to understand the influence of charged substrates and charged interfaces on the leveling of liquid coatings. Based on our findings, we develop simple heuristics that can be used to design coatings that are stable to substrate charging and charge contamination. Electric fields are also present in some printing processes. Developed in the late1960s, electrostatic assist (ESA) has been long used to remove printing defects and enhance image quality in gravure printing, a high-resolution roll to-roll process. ESA involves the application of an electric field to pull ink out of cavities and transfer it onto the desired substrate. However, there is limited understanding of how this process works, which hinders its development as a tool for printed electronics. In order to address this issue, we develop a model for electrostatically assisted meniscus deformation near a cavity (this describes the first stage of electrostatic assist). Our calculations show that electric fields pull up the ink meniscus either at the edges or at the center of the cavity, depending on the ink conductivity. This suggests that ink contact with the substrate will be improved during ESA but air entrapment occurs for a certain range of conductivities, which would be detrimental to print quality. Our model also enables us to investigate the effect of cavity shape and spacing on the mode of deformation of the ink surface. In order to validate the findings from our electrohydrodynamic model, we have carried out flow visualization experiments to track the deformation of liquids contained in cavities, and these corroborate the qualitative trends of meniscus deformation predicted by the model.
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    In situ characterization of dynamic structures of coatings
    (2012-04) Song, Jin-Oh
    We have developed new methods and apparatus to characterize the structure changes of coatings in situ. The techniques enabled the study of critical factors to control during drying or curing process to avoid excess materials use and coating defects. The instruments have been used to investigate the effect of process conditions on the structure development and final coating properties. A magnetic microrheometer for in situ measurement of local viscosity of coatings was designed since conventional bulk rheometry cannot be used to follow the temporal and spatial gradients of viscosity in drying or curing coatings. Micron-sized magnetic probe particles under a magnetic field gradient act as probes for local rheological responses in coatings. Viscosity-time profiles were measured in drying aqueous PVA coatings, and the results revealed the correlation between sagging defects and viscosity build-up. The development of viscosity gradient through the thickness in UV curing epoxy coatings was also characterized to study wrinkling defects, skin formation, and structure or composition gradients through the coating thickness. The microstructure of drug-polymer coatings was also characterized using confocal Raman microscopy. A drying apparatus was built to control the coating method, drying temperature, and air flow during the characterization since the size and distribution of drug phase and polymer structure in the coating strongly depend on the process conditions. The dependence of environmental inputs to the drug and polymer coating morphology during drying was investigated in order to elucidate and optimize either the processing conditions or coating formulation.
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    Liquid-film coating on rotating discrete objects
    (2018-01) Li, Weihua
    The flow of liquid films on discrete objects is encountered in coating processes for a wide range of products such as biomedical devices, automobiles, and food. Describing the shape of liquid films as they flow over discrete objects is a challenging task due to the large number of forces at play. These include gravitational, inertial, viscous, surface-tension, and centrifugal forces, and the complex interplay among them may lead to the growth of instabilities that degrade the quality of the final product. Motivated by the need to improve fundamental understanding of coating flows on discrete objects, we pick cylinders that rotate about their horizontal axes as model discrete objects and investigate four model problems highly relevant to industrial coating processes for rotating discrete objects. In each model problem, the interplay among all the forces is systematically examined to reveal the critical conditions for which a smooth coating can be obtained. For coating of surfactant-laden liquids on rotating cylinders, we applied lubrication theory to derive coupled nonlinear evolution equations to describe the variation of the film thickness and surfactant concentration as a function of time, the angular coordinate, and the axial coordinate. In the absence of gravitational effects, linear stability analysis reveals that surfactant-induced Marangoni stresses suppress the growth rate of instabilities driven by centrifugal effects and hinder the leveling of perturbations to the film thickness in both the angular and axial directions. When gravitational effects are present, Marangoni stresses lower the critical rotation rate needed to cause a liquid lobe to form and rotate in the angular direction. These stresses also lead to faster damping of this lobe, giving rise to a more axisymmetric coating. With the growth of axial instabilities at long times, Marangoni stresses significantly weaken the stabilizing effect of surface-tension forces, which are found to be responsible for keeping the coating axially uniform in a stable speed window. In addition, Marangoni stresses tend to reduce the spacing between droplets that form at low rotation rates, and suppress the growth rate of rings that form at high rotation grates. Flow visualization experiments yield observations that are qualitatively consistent with our simulation results. For cylinders with complex surface geometries (i.e., topographically patterned cylinders and elliptical cylinders), the Galerkin finite-element method is used to solve the Stokes equations, augmented with a term accounting for centrifugal forces, in a rotating frame of reference. For rapidly rotating cylinders where gravitational forces are negligible, surface-tension forces tend to drive liquid to the low-surface-curvature areas (e.g., pattern troughs) leading to the formation of liquid pools, while centrifugal forces tend to drive liquid in the opposite direction, giving rise to liquid droplets. The number of droplets or pools at steady state depends on the rotation rate, strength of surface tension, pattern frequency, and cylinder aspect ratio. When gravitational forces become significant, it is possible to obtain a coating that closely conforms to the cylinder surface in the patterned-cylinder case. With an increase in the pattern amplitude, recirculation regions start to form inside the troughs, which may strongly influence mixing, mass transport, and heat transport. These reciprocation regions can appear and vanish as the cylinder rotates due to the variation of gravitational forces around the cylinder surface. In the elliptical-cylinder case, simulation results show that smaller aspect ratio corresponds to less liquid that can be supported on the cylinder and also larger gradients in film thickness. A suitably chosen time-dependent rotation rate can greatly improve coating smoothness relative to the constant-rotation-rate case. For cylinders with sufficiently small aspect ratio, film rupture and liquid shedding may occur over the cylinder tips, so simultaneous drying and rotation along with the introduction of Marangoni stresses will likely be especially important for obtaining a smooth coating.
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    Physics of coating tensioned-web over slot die.
    (2008-11) Park, Eungsik
    Coating tensioned-web running over slot die is an efficient way of depositing thin and uniform layers on flexible web at high speed with a relatively simple set-up, a combination of slot die pre-metering and carefully controlled web transport. It is different from conventional slot die coating, where the coating gap is maintained by the clearance between backup roll and slot die, in that coating gap is maintained by elastohydrodynamic balance between the tension-resultant force of the curved and tensioned web and the pressure in the coating bead; with this setup much thinner coating gap is made than is possible in conventional slot die coating. For successful operation in tensioned-web coating system, properly designed die is necessary. In conventional slot die coating, pressure profile in the coating bead is well understood and efficiently predicted by considerations based on the one-dimensional viscocapillary model. The model to describe the pressure and gap profiles in tensioned-web slot die coating needs to incorporate the interaction between the liquid flow and the web deformation, leading to a nonlinear system of equations. In this thesis, coating bead flows and gap profiles in both single-layer and two-layer tensioned-web slot die coating were studied by theoretical model based on shell theory and lubrication approximation, hybridized with two-dimensional flow model when two-dimensional flow model is necessary, and visualization using bench-top web transport system with slot die mounted. The results from visualization and theoretical model study show that a tensioned-web slot die coating die can be viewed as a combination of component foil bearings. Interpreting the results in this manner, it is easier to predict how the change in die shape and process condition will affect the pressure and gap profiles and therefore easier to design the die shapes required to achieve desired process states, as thin and fast coating, stable two layer coating, or to predict and remove the appearance of microvortexes. By the two-dimensional model on the flow of the two-layer coating, positioning of separation line and appearance of the microvortexes inside the feed slot were studied with the changes in operation condition.
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    Stress development in particulate, nano-composite and polymeric coatings.
    (2009-09) Jindal, Karan
    The main goal of this research is to study the stress, structural and mechanical property development during the drying of particulate coatings, nano-composite coatings and VOC compliant refinish clearcoats. The results obtained during this research establish the mechanism for the stress development during drying in various coating systems. Coating stress was measured using a controlled environment stress apparatus based on cantilever deflection principle. The stress evolution in alumina coatings made of 0.4 micron size alumina particles was studied and the effect of a lateral drying was investigated. The stress does not develop until the later stages of drying. A peak stress was observed during drying and the peak stress originates due to the formation of pendular rings between the particles. Silica nanocomposite coatings were fabricated from suspension of nano sized silicon dioxide particles (20 nm) and polyvinyl alcohol (PVA) polymer. The stress in silica nano-composite goes through maximum as the amount of polymer in the coating increases. The highest final stress was found to be ~ 110MPa at a PVA content of 60 wt%. Observations from SEM, nitrogen gas adsorption, camera imaging, and nano-indentation were also studied to correlate the coatings properties during drying to measured stress. A model VOC compliant two component (2K) acrylic-polyol refinish clearcoat was prepared to study the effects of a new additive on drying, curing, rheology and stress development at room temperature. Most of the drying of the low VOC coatings occurred before appreciable (20%) crosslinking. Tensile stress developed in the same timeframe as drying and then relaxed over a longer time scale. Model low VOC coatings prepared with the additive had higher peak stresses than those without the additive. In addition, rheological data showed that the additive resulted in greater viscosity buildup during drying.
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    Understanding particulate coating microstructure development.
    (2010-09) Roberts, Christine Cardinal
    How a dispersion of particulates suspended in a solvent dries into a solid coating often is more important to the final coating quality than even its composition. Essential properties like porosity, strength, gloss, particulate order, and concentration gradients are all determined by the way the particles come together as the coating dries. Cryogenic scanning electron microscopy (cryoSEM) is one of the most effective methods to directly visualize a drying coating during film formation. Using this method, the coating is frozen, arresting particulate motion and solidifying the sample so that it be imaged in an SEM. In this thesis, the microstructure development of particulate coatings was explored with several case studies. First, the effect of drying conditions was determined on the collapse of hollow latex particles, which are inexpensive whiteners for paint. Using cryoSEM, it was found that collapse occurs during the last stages of drying and is most likely to occur at high drying temperatures, humidity, and with low binder concentration. From these results, a theoretical model was proposed for the collapse of a hollow latex particle. CryoSEM was also used to verify a theoretical model for the particulate concentration gradients that may develop in a coating during drying for various evaporation, sedimentation and particulate diffusion rates. This work created a simple drying map that will allow others to predict the character of a drying coating based on easily calculable parameters. Finally, the effect of temperature on the coalescence and cracking of latex coatings was explored. A new drying regime for latex coatings was identified, where partial coalescence of particles does not prevent cracking. Silica was shown to be an environmentally friendly additive for preventing crack formation in this regime.