Modeling and experimental investigation of coating of electrostatically charged polymeric microspheres to a carbon fiber surface
Date
2021
Authors
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Publisher
University of Delaware
Abstract
In recent years, there is renewed interest in the manufacturing of thermoplastic prepregs. A wide range of impregnation processes has been developed to investigate the feasibility of producing easy to process and cost-effective thermoplastic prepreg. Dry powder impregnated thermoplastic prepregs that use ozone-generated fluidized bed are an attractive approach due to the high recyclability of the polymeric particles, as well as no solvents/surfactants is needed. Moreover, the availability of small particle sizes offers the potential for much higher microstructural quality through the control of particle packing efficiency and coverage of fibers. This can lead to higher process throughput than other methods. The goal of this thesis is to investigate and model the mechanisms of powder coating in Carbon Fiber (CF) monofilaments for smaller Polystyrene (PS) particle sizes and identifying the best set of design parameters such as resin particle diameter, carbon fiber diameter, resin charge density, resin charge decay rates as a function of relative humidity, electrostatic forces, and the particle-to-particle equilibrium spacing in both along the fiber axis as well as around the fiber circumference that will enhance our understating in maximizing throughput in obtaining the desired microstructure during prepreg manufacturing. ☐ The influence of relative humidity (RH) on charge decay on PS particles and PS film is investigated at room temperature by using Faraday cup, Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy, and Kelvin Probe Force Microscopy (KPFM). The PS (2.8±0.4 µm in diameter) particles were charged by using an ozone setup, while the PS film was charged by applying a voltage pulse. The charge decayed exponentially, and the decay time constants were quantified using Faraday cup for various relative humidity conditions at room temperature. The charge decay time constant was measured to be 1.04×105 (s) and 4.55×103 (s) under 5% and 98% RH conditions, respectively, which are long (hours) when compared to the real-world impregnation process (which can take minutes). A generalized master charge decay curve was established for any PS surfaces (particles/films). ☐ Electrostatic dry powder impregnation processing of composites requires a uniform coverage of polymeric microspheres on the surface of reinforcement fibers. To gain a fundamental understanding of the electrostatic forces acting on the charged particles and their effect on particle packing, a series of analytical and numerical models were developed to predict the attractive and repulsive electrostatic forces between two charged particles in space and two charged particles in contact with a grounded carbon fiber, where both particles are located along the fiber axis and around the circumference of the fiber. From the parametric study performed on a 3µm polystyrene particle size and a 7µm carbon fiber diameter, the required particle-to-particle spacing (surface-to-surface) in a hexagonal packing of particles to produce a 60% fiber volume fraction (FVF) composite was found to be approximately 1.1µm. ☐ To demonstrate the feasibility of coating the fiber surface uniformly with PS particles, a customized fluidized bed setup was fabricated to charge, transport, and coat the 7 µm in diameter grounded CF with the PS particles. The effect of the PS mass, nitrogen flow rate, and sample location on the final particle arrangements were investigated. As a result, the particle-to-particle equilibrium spacing along the fiber axis and around the fiber circumference were experimentally measured to back-calculate the associated sliding friction coefficient that exists between the PS particles and the grounded CF based on the equilibrium spacing predicted by the model.
Description
Keywords
Coating, Electrostatically charged polymeric microspheres, Carbon fiber surface