Optimisation of Fluid Properties and Process Parameters for Batch Slot Die Coating of Dilute Polymer Systems

Abstract

The effects of viscosity and surface tension are well documented in the case of roll-to-roll slot die systems. However their influence in combination with process parameters for batch scale coaters are less often discussed. Lack of coating uniformity attained using a bespoke slot die coater at the CPI National Printable Electronics Centre led to the requirement for an investigation to identify the sources of error.

This project outlines a method used to determine the effects of material and process parameters on coating quality without the requirement for modification of equipment. It also describes the development of a series of versatile formulations containing polyethylene oxide that can be used to mimic similar polymer formulations for cost-effective initial process development of new materials.

Various dilute polymer systems were developed whereby solids loading, viscosity and surface tension were all fully controlled. Design of Experiments techniques were employed to determine relationships between parameters and fluid properties. Chosen parameters included coating velocity, coating acceleration, coating gap (gap between slot die lips and substrate), volume infuse (volume of material initially infused) and flow infuse (flow rate of material initially infused). Their effects on coating thickness and thickness uniformity were quantified for the bulk region of the coating, resulting in the development of a statistical model to aid optimisation of parameters for new materials.

Surface tension had a minimal effect on coating thickness and uniformity during the screening study so this was not included in the final model. Coating acceleration and flow infuse exhibited a proportional relationship relating to coating thickness uniformity. Volume infuse did not have a significant effect on coating thickness or uniformity. Particular focus was placed on viscosity and coating velocity; in the case of this bespoke tool they were found to be inversely proportional in relation to coating thickness and uniformity. They strongly influenced the flow rate and the resultant coating thickness due to the effects of fluid resistance, though the wet coat thickness parameter was held at a constant value. This suggested that the system does not work by a feedback loop to correct the flow rate for fluid resistance and internal pressure effects. Regardless of this, an operating window was found in which coating thickness and uniformity can be controlled for polymer solutions with a range of viscosities.