High speed very thin films with reverse roll coatings. An experimental investigation of reverse roll coating of fluids using rigid and deformable rolls at high speeds.

Abstract

The objective of a coating operation is to transfer a defect free liquid film onto a continuous substrate in order to meet the requirements of the final products. Mainly two concerns govern the process. The first concern is the economics of the process and the second concern is the quality of the coated film. The economics of the process are dictated by the speed of coating and the film thickness. Clearly, higher speeds mean better productivity hence less cost of operation and thinner films are desirable because less material is being used. Quality is governed by film uniformity and integrity, indicating that the film will perform as designed. Film defects such as streaks or tiny air bubbles are indication that the film properties are not uniform rendering it unacceptable to customers. One of the most versatile coating systems to achieve thin films at high speeds is reverse roll coating which has been used for a long time all over the world. At low speed, typically 1m/s, this coating operation is inherently stable and with small gaps of order 100 microns can ii lead to film thickness of order 30-50 microns. Much research, theoretical and experimental, has been devoted to this coating flow but only at low speeds and for large gaps (>100 microns). There are no comprehensive data how very thin films, 20 microns and less (particularly lower limits in the region of 5 microns) can be achieved at high speeds, of 2 or more metres per second. This study is concerned precisely with this aim, that of investigating the effect of large speeds and small roller gaps (rollers nearly touching or in elastohydrodynamic contact) to achieve the very thin films desired by modern applications (electronics, medical and others). In order to achieve this aim, a rig was designed and built to enable to understand the effect of various coating conditions and liquid properties on the metered film thickness and coating instability. To achieve thin films at high speeds, small roll gap and low viscosity are needed, however flow instabilities will develop under these conditions. To achieve stable coating window at high speeds high surface tension is needed. It was found that the roll gap and the viscosity have complicated effect on the coating window. In the case of low viscosity liquid (7mPa.s), small roll gaps are needed, whereas in the case of high viscosity liquid (more than 30mPa.s), large gaps are needed. It was found that Weber number is better describer for ribbing instability in rigid reverse roll coating unlike in rigid forward roll coating in which capillary number is the one. In addition the potential of reverse deformable roll coating (rolls in elastohydrodynamic contact) was investigated in order to achieve much thinner films at higher speeds. As a result of the investigation of reverse deformable roll coating, it was found that there is a possibility to get much thinner stable films at much higher speeds compared to reverse rigid roll coating. The liquid transfer from an applicator roller to a PET film was investigated in this study. It was found that air stagnation at downstream meniscus and air entrainment at upstream meniscus depend on the liquid properties such as viscosity and surface tension and coating conditions such as web tension and wrap angle of web. As a result, wet film instability also depends on liquid properties and coating conditions. It was found that air stagnation causes streaks on the wet film and air entrainment caused bubbles on the wet film. To get a stable wet film, it was found that suitable viscosity and high surface tension were needed.