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Browsing by Subject "Photopolymerization"

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    Developing New Strategies Based on Melt Blowing for High Performance Nonwoven Fiber Production
    (2021-07) Banerji, Aditya
    Nonwoven fiber products are used in diverse applications, including medical, hygiene and safety products, filtration, separation, and catalysis modules, and tissue engineering scaffolds. Current synthetic nonwoven fiber production methods typically require transforming pre-formed polymers into a liquid state by heating or adding organic solvents to facilitate fiber spinning. The significant energy demands and the use of volatile organic compounds render these processes suboptimal. Furthermore, conventional synthetic fiber manufacturing processes are limited to thermoplastics since cross-linked thermosets do not flow in a similar manner as thermoplastics. However, the superior thermal and chemical resistance of cross-linked fibers render them attractive targets. In the recent decades, significant research efforts have been focused on producing nanofibers to enhance the specific surface area of nonwoven mats, a critical requirement in most of applications. However, it has proven challenging to obtain defect-free nanofibers using conventional melt blowing. Therefore, there is a necessity to develop alternative strategies to enhance the fiber specific surface area using melt blowing. The first portion of this thesis seeks to address the high energy requirement and VOC emission concerns associated with conventional fiber manufacturing techniques. A new “cure blowing” process is presented that can produce cross-linked nonwoven fibers at room temperature with little or no solvent, using a lab-scale spinning die resembling those used for melt blowing, an approach that currently produces > 10% of global nonwovens. Specifically, a photocurable liquid mixture comprising of multifunctional thiol and acrylate monomers, free-radical photoinitiators, and viscoelasticity modifiers was extruded through a die orifice and drawn by ambient temperature high-velocity air jets into liquid filaments. During flight towards the collector, the liquid filaments undergo rapid photo-cross-linking upon UV irradiation to generate solid, continuous fibers. Preliminary studies revealed that the resulting fiber morphologies were governed by a convoluted and interdependent parameter space arising from the photopolymerization kinetics, material properties, and process operating conditions. The competitive interplay between these categories of parameters were analyzed by quantifying and comparing the corresponding characteristic timescales, namely vitrification time, fluid relaxation time and fiber flight times, respectively. The timescales were chosen such that they can account for the observed fiber morphology transitions irrespective of the photopolymerization mechanism. The universality of this analytical framework was verified by constructing morphology maps in terms of the characteristic timescales for two distinct reactive model systems. Notably, the morphology maps may serve as a predictive tool for implementing different photopolymerization chemistries in cure blowing to produce cross-linked nonwovens of desired morphology. The second portion of this thesis discusses the potential advantages provided by the strategy of leveraging different morphologies available from immiscible polymer blends to expand the scope of melt blowing by introducing new application areas and further improving the existing ones. In this case, the research has focused on improving the specific surface area of the fiber mats to improve their performance in applications that demand a larger surface area. We report a scalable melt blowing method for producing porous nonwoven fibers from model co-continuous polystyrene/high-density polyethylene polymer blends. The co-continuous morphology was utilized as a template to fabricate elongated inter-connected domains upon melt blowing. Consequently, selective solvent extraction of the sacrificial polymer phase generated a network of porous channels within the fibers. Fiber surfaces also exhibited pores that percolate into the fiber interior, indicating the continuous and interconnected nature of the final structure. Pore sizes as small as ~100 nm were obtained, suggesting potential applications as porous nonwovens with improved particulate capture efficiency for various filtration applications.