In addition to coating single layer product structures, the web coating process can also produce multi-layer products. This multilayer capability can be used to produce a variety of novel and cost effective products. The initial methods to produce these structures were not cost effective and restricted the general development of multilayer products. Three new multilayer pre-metered coating methods have become available and are cost and quality effective.
Initial Methods
• Coat & dry one layer at a time and the recoat in second pass to coat a new formulation to give a second layer. This is not economically competitive because each pass through the coater reduces yields and adds an additional conversion cost to the product
• Place several coating and drying section in series in the coating line. In this configuration, the first layer would be applied and dried and the go into a second coater dryer for the second layer. Several units could be placed in the line for the total number of layers required. The disadvantage of this approach is that coating line could be expensive. In addition, it may not be possible to optimize the all of the separate zones and operating conditions would be a compromise that could affect cost and quality. The unit could also not be flexible enough to accommodate.
• Coat wet layer on wet layer by building a coater with two coating stations in series. The concern with this approach is that the layers may not be compatible in this sequence and produce poor quality
Current Pre-metered Methods
The current multilayer pre-metered method, slot die, curtain and slide, can apply several layers simultaneously with excellent coating weight uniformity, good quality and high speeds. These methods are commercially available at competitive process. There is a strong technology base that permits custom design and excellent operability and quality. Also, improvement in automated metal fabrication and metrology has resulted in low cost flexible designs.
Currently a slot die coater is one of the preferred methods for multilayer coating. It can coat up to three layers simultaneously up to three layers by stacking the distribution plates, see attached image, The quality, coating weight uniformity, maximum speed and viscosity range make it very versatile and economically competitive process.
In Slide Coating Multiple layers are created by stacking multiple plates) together. Each layer flows out onto the slide and does not mix with the other layers as they all flow together down the slide, across the gap, and onto the web. It is possible to coat up to 20 layers simultaneously. This method is not in general usage because it requires low viscosity, limited to aqueous and has speed limitations.
The Multilayer curtain coating is also an attractive process. The layers are formed on a slide and then flow of the slide and form a curtain which then transfers the fluid to the substrate. See attached image. The advantages of this process are high speed and a wide gap which does not trap particles, thereby reducing streaks.