Shear orientation of nematic phases of clay nanosheets: processing of barrier coatings

Maximilian Röhrl,Christoph Habel,Franz Durst,Holger Schmalz,Judith H Mettke,Josef Breu,Ulrich Mansfeld,Sabine Rosenfeldt,Renee L Timmins

doi:10.1007/s11998-021-00535-4

Abstract

When suspensions are exposed to shear forces, the particles may form ordered structures depending on their shapes, concentrations, and the material. For some processes, e.g., for wet-film coating, it is important to know how fast these structures form in shear fields and for how long the structures persist when the shear is relaxed. To obtain information on the particle structure formation and the decay time, the effective viscosity of nematic suspensions of Na-hectorite nanosheets was investigated by rheology employing a cone-plate measurement geometry. The necessary time for the formation textured nematic films could be deduced by carrying out effective viscosity measurements at constant time steps. Information could also be obtained on the lifetime of the platelet textures when shear is relaxed. All this information was employed to identify geometrical requirements for slot dies to produce barrier liners with nanosheet layers oriented parallel to PET substrates. Thereby, we obtained green and simple coatings that are in line with state-of-the-art high-performance materials such as metalized plastic foils in terms of oxygen barrier properties.

Highlights

When small particles are added to a fluid and shear is applied, as in a Couette flow, the moving suspensions show effective viscosities of different complexities
Laun et al showed that the measured effective viscosities as a function of the applied shear stress of polystyrene–ethyl acrylate latex particles in water depend on the particle concentration and the surface charge characteristics of the latex particles.[2,3]
This means that effective viscosity measurements can be employed to reveal structural changes of suspensions induced by increasing shear rates

Summary

Introduction

When small particles are added to a fluid and shear is applied, as in a Couette flow, the moving suspensions show effective viscosities of different complexities The latter depend on the suspension medium, the particle concentration, its size and shape, and on the hydrophobic or hydrophilic surface properties of the particle material.[1] For instance, Laun et al showed that the measured effective viscosities as a function of the applied shear stress of polystyrene–ethyl acrylate latex particles in water depend on the particle concentration and the surface charge characteristics of the latex particles.[2,3] In his seminal work, Giesekus[4,5] showed that spherical particles, when exposed to shear flows, arrange themselves in such a way that they form ‘‘strings of particles’’. Such suspensions still comprise of a fluid with its original viscosity, but the interactions of the fluid with dispersed particles and particle–particle interactions yield forces that are recorded by viscometers as a decrease in the measured momentum of a rheometer.[5,6] The increase in the regularity of the

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Conclusion