The use of precision coating assets and techniques to solvent cast advanced materials has recently experienced resurgence due to new uses in filtration, batteries, electronics and optical markets. While creating plastic films via extrusion of molten polymer through a die with or without biaxial stretching is common practice, its disadvantages include thermal degradation and lower thickness uniformity. In contrast, film casting or coating a polymer dissolved in a solvent onto a temporary substrate features many advantages, from uniform thickness distribution to high optical purity to excellent transparency. It also results in virtually isotropic optical orientation, as well as excellent flatness and dimensional stability. In some cases, the cast film or membrane can be coated with functional coatings in highly economic in line processes.
There are many examples of cast membranes, including microporous filtration media for gases and liquids, a variety of advanced battery and fuel cell applications, and even high performance clothing. Microporous membranes can be made from many polymers, including polyethylene, polypropylene, polyethersulfone, polyimide or polyvinylidene fluoride (PVDF). While extrusion or melt casting is possible, solvent or wet casting is prevalent for creating membranes with either uniform pore size throughout the membrane or graduated pore size across the membrane where larger pore sizes act as pre-filters and reduce membrane plugging. The main pore creation methods include chemical porogens (pore generators), stretching and coagulation – all achieved through a series of specific and highly proprietary steps.
In addition to cast membranes, there are also cast films for display, barrier and high temperature applications where special properties such as electrical resistivity/conductivity, optical clarity, temperature resistance, scratch resistance and/or uniform thickness are essential. In these situations, the cast film may be used as a substrate or as a barrier layer to prevent damage to a less resistant material where chemical migration is an issue or where adhesion can be enhanced. Common materials for cast films include a.) cellulose triacetate (usingmethylene chloride) for photographic and polarizing films b.) polycarbonate (using methylene chloride and other solvents) for BlueRay and other disc media c.) polyimide (using DMF) for flexible electronics d.) polyvinyl chloride (using THF or MEK) for tamper proof labels, mid- to low-value packaging and shrink films e.) polyvinyl alcohol (using water and methanol) for polarizing layers in liquid crystal displays.
Regardless of the final application (membrane or film), casting quality is paramount and driven by the manufacturer’s expertise in precision coating methods and drying control. Carestream has a depth of experience in helping customers develop products with the distinct advantages of cast film and membranes. The company’s professional staff of engineers has developed a proven record of assisting customers throughout all stages of the product development cycle, from initial inception to standardized production.