Electrostatic assist for coating: the effect of surface resistivity ☆

Electrostatic forces are used to assist processes for coating liquids onto moving webs of plastic or coated paper. Generation of electrostatic forces at the coating point can be accomplished by: (1) deposition of electrostatic charge on the moving web, (2) application of voltage on the backing roller used in the coating process, or (3) a combination of these two approaches. One specific benefit of electrostatic assist is to enable higher coating speeds before encountering a limitation known as air entrainment; the entrapment of air bubbles between the moving web and the coating liquid. However, the electrical properties of the web play a key role in determining the effectiveness of this electrostatic assist. A simple model is proposed for understanding the impact of these electrical properties, using a parameter called the characteristic electrical length, λ. This parameter provides the length scale over which the web surface transitions from high to low surface potential prior to the wetting line, the point at which the liquid contacts the web. λ depends upon the inverse product of web speed, web capacitance per unit area, and web surface resistivity on the side to be coated. It is desirable to have as short a length λ as possible in order to generate the maximum electrostatic assist. As the web surface resistivity decreases, λ increases and the requisite web charge or backing roller voltage increases in order to provide an electrostatic assist equivalent to that obtained for an insulating web surface. Ultimately, electrical breakdown limits the maximum practical web charge or roller voltage. Experiments were conducted varying web speed, web surface resistivity, and backing roller voltage, and measuring the air entrainment threshold. The experimental results confirmed the essence of this simple model.