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Applications of Polymer, Composite, and Coating Materials
Obtaining Thickness-Limited Electrospray Deposition for 3D Coating Lin Lei, Dylan A. Kovacevich, Michael P. Nitzsche, Jihyun Ryu, Kutaiba AlMarzoki, Gabriela Rodriguez, Lisa Klein, Andrei Jitianu, and Jonathan P. Singer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19812 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018
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ACS Applied Materials & Interfaces
Obtaining Thickness-Limited Electrospray Deposition for 3D Coating
Lin Lei1, Dylan A. Kovacevich1, Michael P. Nitzsche1, Jihyun Ryu1, Kutaiba Al-Marzoki2, Gabriela Rodriguez3, Lisa C. Klein2, Andrei Jitianu3,4, Jonathan P. Singer1*
1
Department of Mechanical and Aerospace Engineering, Rutgers University, New Jersey, 08854, The United States
2
Department of Materials Science and Engineering, Rutgers University, New Jersey, 08854, The United States 3
Department of Chemistry, Lehman College-CUNY, Davis Hall, 250 Bedford Park Boulevard West Bronx, New York 10468, USA New York, The United States
4
Ph.D. Program in Chemistry and Biochemistry, The Graduate Center of the City University of New York, 365 Fifth Avenue, New York, NY 10016, The United States
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KEYWORDS electrospray deposition, functional coatings, polymers, sol-gel, self-assembly ABSTRACT
The electrospray process utilizes the balance of electrostatic forces and surface tension within a charged spray to produce charged microdroplets with a narrow dispersion in size. In electrospray deposition, each droplet carries a small quantity of suspended material to a target substrate. Past electrospray deposition results fall into two major categories: (1) continuous spray of films onto conducting substrates and (2) spray of isolated droplets onto insulating substrates. A cross-over regime, or a self-limited spray, has only been limitedly observed in the spray of insulating materials onto conductive substrates. In such sprays, a limiting thickness emerges where the accumulation of charge repels further spray. In this study, we examined the parametric spray of several glassy polymers to both categorize past electrospray deposition results and uncover the critical parameters for thickness-limited sprays. The key parameters for determining the limiting thickness were (1) field strength and (2) the spray temperature, related to (1) the necessary repulsive field and (2) the ability for the deposited materials to swell in the carrier solvent vapor and redistribute charge. These control mechanisms can be applied to the uniform or controllably varied microscale coating of complex 3D objects.
1. INTRODUCTION In the field of coatings, there is a disconnect between techniques that are used to create macroscale applications and those used to create micro/nanoscale applications. Macroscale coatings, such as paints, are ubiquitous and provide essential protective properties, including UV-
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ACS Applied Materials & Interfaces
protection, dielectric shielding, and resistance to static dissipation, corrosion, and fouling. Micro/nanoscale coatings offer similar performance, but also, if morphology is controlled, provide the added advantage of anomalous beneficial properties, such as anti-reflection, light trapping, or superhydrophobicity. Replacing macroscale coatings with micro/nanoscale coatings also results in more efficient usage of materials. However, widespread adoption of precision micro/nanoscale coatings still faces challenges in scalability. Micro/nanoscale conformal coatings can be applied in either the molecular or condensed matter state. Common molecular deposition techniques, such as electrodeposition, vacuum deposition, atomic layer deposition (ALD), and chemical vapor deposition (CVD), are costly because they generally require a solution bath or high-vacuum, and some also require high-temperature precursor processing. This offsets cost-benefit considerations and limits the size of the component that can be coated. Condensed deposition techniques, such as dip coating, spin coating, and bush or blade coating struggle with 3D surfaces and result in capillary or shadowing effects. It has been one century since Zeleny first reported observations of electrostatically-induced sprays in 1917.1 These results were expanded over the years by Vonnegut and Neubauer2 and, more famously, Taylor,3 leading to the work of Yamashita and Fenn on electrospray ionization mass spectrometry in 1984,4 which resulted in a Nobel prize for Fenn in 2002. In parallel, electrostatic sprays were being developed for the purposes of deposition. ESD with DC fields was first developed by Blumberg et al. of Los Alamos National Laboratory in 1962 as a means to generate thin sources for radioactive particles.5 Simultaneously, electrostatic spray became increasingly common in the 1960s, and Bell-type sprayers are now a standard method for automotive painting, crop spraying, and pharmaceutical coating.6-11 The advantage of electrostatic sprays in these applications is that the spray is directed to a grounded workpiece or leaf, thus
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resulting in highly efficient coverage of even the shadowed components. Due to the build-up of charge and ionization of the surrounding air and parts, self-limiting spray (SLS) effects have been observed in electrostatic powder spray.10, 12-15 The origin of SLS in electrostatic spray has been investigated extensively and has been treated theoretically.12, 16 It arises from three main effects, namely the formation of: (1) a counter field by the arrived powder that cancels the driving field, (2) a field between the powder and ground through a barrier oxide, and (3) a field within the powder itself. The first effect creates a “thicknesslimited” spray at 50~500 µm, while the latter two result in dielectric breakdown due to (2) failure of the barrier oxide or (3) failure of the powder. These latter two effects can create counter-ion “back corona” that further impedes the spray, catastrophic “cratering” that damages the film, or smaller pinholes that reduce performance. Because of these unwanted effects, SLSs have been avoided in most applications, though the work of Barletta and colleagues has taken advantage of SLS in powder spray to smooth their films.14-15 Further, such thickness-limited SLSs have only been reported in powders and have not been reported in 3D coatings, likely due to the importance of the air jet in determining the trajectory of the sprays. SLS effects, if applied to ESD, can be pursued as a means to coat 3D objects with control over the functionality and morphology of the final coatings. ESD, though less widely practiced than electrostatic spray, is also a well-known method for coating. Both methods generate highly monodisperse droplets or powder sprays through the acceleration of particles in a strong electric field (~100 KV/m).17-18 The key difference between ESD and commercial electrostatic spray is the nature of the charge transfer and motion. In electrostatic spray, moving ionized air is used to charge and direct the spray, while in ESD, the electrostatic force is the only driver for transport. In ESD, the droplets are emitted by electrostatic
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breakdown from an electrostatically drawn Taylor cone. As such, ESD tends to use much lower flow rates (on the order of ~1 mL/hr) and exclusively makes use of low solids loadings (generally
