Fluid Mechanics of High Speed Deformable Roll coating. An experimental and theoretical study of film thickness and stability in high speed deformable roll coating flow with Newtonian and non-Newtonian liquids

Abstract

High operation speeds and thin coating films are desirable in many industrial applications. But the quality of the product, which is primarily determined by an optimum process window, is affected by non-uniformities and instabilities originating at higher operation speeds.

Unlike other academic works, because of associated industrial relevance, particular attention was given towards the use of industrially relevant coating systems or generation of model fluids, which replicate the real coating solutions. One of the novelties of the research proposed lie in an integrated approach, utilising a range of comparative rheometrical techniques, with a focus on measurement of: (i) high strain rate shear viscosity [η], (ii) high strain rate uniaxial extensional viscosity [ηE] (iii) high strain rate elasticity (N1).

Deformable roll coating, in terms of classification, is the ultimate metered coating flow. The flow is controlled by the combination of hydrodynamic force and elastic deformation effect, which could be complicated by the presence of non-Newtonian fluid. This study necessitated the design and construction of a sophisticated deformable coating rig with the ability to operate over a wide range of conditions. Although the irregularities and instabilities associated with the roller coating process has been studied previously, the concerned speeds of operation in this study is around three times higher.

The main objectives of this study was to carry out a comprehensive experimental programme establishing relationship exist between film thickness, film quality and operating parameters. Main identified operating parameters are roll speeds, roll gaps or applied load between the rolls, the elasticity with thickness of elastomeric layer and different rheological properties of the coating fluids. Surface instabilities and air entrainment are identified as the major limitations to being able to coat at higher speeds.