One of the many technologies that will drive the growth of the web coating process (also called Roll-to-roll) is the development of Simultaneous Multilayer Coating process. Several technology and manufacturing advances have resulted in a process that has the ability to coat a wide range of properties at high productivity, with excellent quality and is cost effective. This capability will give product developers the ability to develop new multilayer products without an economic penalty.
The technology advances have been in the development of the basic understanding of the fluid and solution rheology in these devices. It has resulted in computer programs to design these devices for all uses and to help troubleshoot problems. It has also provided a process understanding to optimize the process for quality, yield lowest possible cost and productivity. Improvements in automated machining tools and metrology have resulted in applicators with reasonable cost and high quality.
A significant obstacle to the development of multilayer commercial products has been the inability to efficiently coat simultaneous layers in the laboratory; so that the potential of new product concepts can be evaluated This drawback ahs been eliminated with the recent availability of simultaneous multilayer laboratory coaters that can produce excellent quality coatings that can be realistically evaluated.
There are three Multilayer Coating Applicators slot die, curtain and slide coaters. They give excellent coating weight uniformity, good quality, high speed, the necessary hardware is commercially available and they have the flexibility to design versatile product structures. There is a strong technology base that permits custom design and excellent operability and quality. Often they can be inserted into existing coaters and do not require custom machine as in the previous methods. The disadvantage of these methods are high cost initial cost and they require a strong technology base efficiently operate them. Table 1 summarizes their the operating range
Currently a slot die coater is the preferred methods. It can coat up to three layers simultaneously by stacking the distribution plates, The excellent 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 distribution plates.. 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 10 + layers simultaneously. This method is not in general usage because it requires low viscosity, is limited to aqueous and few product structures require this many layers
In multilayer curtain coating, the layers are formed on a slide and then flow down the slide and form a curtain, which then transfers the fluid to the substrate, The advantages of this process are high speed and a wide gap, between coater and substrate, which does not trap particles, thereby reducing streaks. The disadvantage is that a high minimum coating speed, ~ 400 fpm. Is required to operate the process.