R2R coating of sheet materials
Another use of release coatings is as an interim process where individual, small-dimensional substrates may be laminated to a roll of carrier substrate so that the advantages of R2R coating may be obtained but without changing from the individual substrates. Once the individual substrates have been coated, they may be released back to being discrete items. In the display industry, some manufacturers rather than transition from glass sheets directly to rolls of flexible substrate would bond sheets of thin glass onto a carrier web ito try to get the benefits of a R2R-coating process but with sheet substrates. The glass sheets aew removed from the carrier at the end of the process usually by a rapid heating process that heats only the release layer, letting the carrier to be peeled away.
Often a key part of using a carrier web is the release coating. Vacuum deposition heats up the web, and most roll coaters use a cooled deposition drum to manage the heat. To maintain the effectiveness of this way of cooling the substrate during deposition, it is essential for the release coating to be a good thermal bridge between the substrate and carrier webs. This layer needs to survive the thermal load during deposition but still provide release when required. In some applications, it is important that none of the release layer is left on the product; in other applications it is required that none of the release layer is left on either the product or the carrier, whereas in some applications it does not matter if some of the release layer is left behind.
The intricacies of release layers
There are so many disparate applications where release layers have been developed as part of the process that it is no surprise that there are many different types of release coatings. They range from inorganic to organic layers and even in one unusual case – water. The water was frozen onto the substrate surface and the coating deposited onto the ice, which was then melted, allowing the free-standing coating to be peeled from the surface.
The inorganic coatings, such as NaCl or NaF, may be dissolved in a suitable solvent to free the coating or substrate. The waxes, such as beeswax or paraffin wax, may be heated to transform the solid, rigid wax to a softer state or even to a liquid, enabling quick release of the coating or substrate. Similarly, the oligomer, monomer or copolymer release layer may be dissolved away. The alternative may be to have a material that can absorb the solvent, which gives a dimensional change that promotes a delamination at the interface. Some use the dimensional change produced by swelling due to the absorbed solvent or heating to create crack at the interface that will rapidly propagate and so very quickly separate the layer.
One system uses bubbles deliberately produced during the release layer as it was coated to form a weak layer. When the gas in the bubbles is heated, the expansion ruptures the bubbles and so produces the fracture plane, allowing the coating to separate from the substrate or two substrates to be separated.