Modeling of Nonwoven Formation Processes

2019-12-01T00:00:00Z (GMT) by Arkaprovo Ghosal
1.1: Meltblowing The aim of this part of the thesis is to describe the development of a robust numerical user-friendly model for prediction of meltblowing in a wide range of the operational parameters, and in particular, to predict the thickness and porosity of laydown formed on a flat collector plate. The experimental work dealt with meltblowing and measurements of the thickness and porosity of the resulting micro- and nanofiber mats. The experimental results were used for validation of the numerical results, and a good agreement was found, even though no adjustable parameters were involved. In addition, such characteristics of meltblown laydowns as their three-dimensional morphology, mass distributions and fiber orientation distributions were predicted under different operational conditions. This work describes the first detailed model of meltblowing process which allows prediction of such integral laydown properties as thickness, porosity and permeability. Also, such laydown properties as the detailed three-dimensional micro-structure, fiber-size distribution and polymer mass distribution are predicted. The effects of the governing meltblowing parameters on the variation of all these laydown properties are accounted for, with the influence of the collector screen velocity being in focus. For this aim numerical solutions of the system of quasi-one-dimensional equations of the dynamics of free liquid polymer jets moving, cooling and solidifying when driven by the surrounding air jet are constructed for a non-isothermal process. Multiple polymer jets are considered simultaneously when they are deposited on a moving screen and forming nonwoven laydown. The results reveal the three-dimensional configuration of the laydown and, in particular, its porosity and permeability, as well as elucidate the dependence of the laydown structure on the forming conditions, in particular, on the velocity of the screen motion. It is shown and explained how an increase in the velocity of the collector screen increases porosity and permeability of the meltblown nonwoven laydown. 1.2: Solution blowing The numerical code developed during the primary work for simulation of meltblowing and the resulting laydown properties was modified and extended also for solution blowing to predict the laydown thickness and porosity, and compare them with the experimental data. The thickness and porosity of the laydown removed from the rotating collector drum was measured using optical profilometry and SEM imaging. Several additional features of solution-blown laydowns were predicted as well, including the three-dimensional morphology in a wide range of operational parameters. In this work the three-dimensional architecture and properties of solution-blown laydown formed on a rotating drum are studied using the system of quasi-one-dimensional equations of the dynamics of free liquid polymer viscoelastic jets moving, evaporating and solidifying, while being driven by the surrounding high-speed air jet. Solution blowing of multiple polymer jets simultaneously issued from a nosepiece and collected on a rotating drum is modelled numerically. The developments shown in this part of the thesis focuse on the computational approach developed to calculate the three-dimensional architecture based on the complex dynamics of fiber formation from a polymer solution under isothermal conditions. Numerical modeling of nonwoven formation processes accounting for detailed dynamics of multiple polymer solution jets moving in air with high relative velocity is undertaken for the first time with the goal to predict the intrinsic parameters of the resulting three-dimensional laydown. In particular, the results predicted the three-dimensional architecture of the laydown, the surface and volumetric porosity and permeability. The effect of the viscoelasticity of polymeric liquids and the governing parameters of the process, for example, of the velocity of the collector screen, are studied in such detail for the first time. The numerical results on the volumetric porosity of nonwoven laydown are compared with the experimental data of the present work. The numerical predictions are in good agreement with the experimental data and elucidate the effect of the angular drum velocity on the mass and angular fiber distribution, as well as the volumetric porosity and permeability of the solution-blown nonwovens. Polymer mass distribution in solution-blown nonwoven laydown, as well as their thicknesses are also elucidated. It was found that instead of doing any upstream modification of the solution blowing process, the easiest way to control the laydown structure (the mass and angular fiber distribution, as well as the volumetric porosity and permeability) is to vary the angular velocity of the collecting drum. The modeling and numerical approaches developed in this work, as well as the numerical code resulting from it are quite unique and can find wide applications in the nonwoven industry and facilitate optimization of fiber-forming processes, such as meltblowing, solution blowing, as well as in describing fiber behavior by modeling post-processing stages such as hydroentanglement using the same framework of the quasi-1-D formulations of free liquid jet dynamics. 1.3: Crystallization kinetics in meltblowing In this part of the work, the theoretical model of crystallization kinetics in meltblowing process is developed. It was also incorporated in the numerical code describing polymer jet formation during meltblowing process and the corresponding laydown structure as mentioned above. This new approach towards accounting for crystallization has been attempted by predicting a distribution pattern and growth behavior of polymer crystal nuclei along the jet pathway. The primary focus of this phase of work has been to determine the effects of the different operational parameters on the properties of the produced fibrous meltblown (nonwoven) laydown and establish a statistical correlation between the final product properties and the corresponding processing conditions. This part of the work also involves experimental validation of the numerically predicted processing parameter effects on the degree of crystallinity in meltblown fibe rmats using the data obtained at the experimental facility of the Nonwovens Institute, NCSU. The numerical simulation tool developed at this phase of the work is capable of predicting the degree of crystallinity along the jet spinline, as well as the distribution of the degree of crystallinity in the laydown collected on a flat moving collector belt. Since meltblowing is a highly non-isothermal process, the thermal profile along the polymer jet path not only affects the rheological parameters, but determines the rate of crystallization, which in turn affects the viscoelastic behavior of the polymer jets, in particular, their stretching and attenuation. The process of formation of polymer crystals in the polymer jets formed under the action of the surrounding high-speed hot air jets is described. The spinline crystallization is studied using numerical solutions of the system of coupled quasi-one-dimensional equations describing the dynamics of free liquid polymer jets moving, cooling and solidifying along the travel path when driven by the surrounding air jets, as well as the nucleation and crystallization kinetics are simultaneously addressed. The developed numerical code predicts the distribution of the degree of polymer crystallinity along the bending jet path in flight, as well as the resulting distribution of the degree of crystallinity in the three-dimensional laydown. Accordingly, the degree of crystallinity maps in the meltblown laydown are predicted. In the numerical solutions multiple polymer jets are considered simultaneously when they are moving toward the collector belt and form an entangled laydown on it. The numerical model is applied for a range of processing parameters, such as the initial velocity and temperature of the surrounding air jets, the die-to-collector distance (DCD), the collector belt velocity, the Deborah number and the activation energy of viscous flow. The results reveal that the crystallization kinetics is sensitive to all the above-mentioned parameters. In addition, the results elucidate the correlation between the onset of the large-amplitude bending perturbations of polymer jets and fluctuations of the degree of crystallinity along the spinline. Acknowledgement: The entire work presented here has been supported by the Nonwovens Cooperative Research Center (NCRC), grant no. 13-149.