Interdependencies Between Fluid Design and Coating Methods

By Rick Daniels, General Manager, Carestream Advanced Materials and Tollcoating

richard.daniels@carestream.com

Choosing a specific precision coating method depends on a variety of factors, including the desired final product characteristics, anticipated processing steps, and the nature of the material being coated. There is a highly interdependent relationship between coating techniques and fluids, making consideration of both at the same time very important.

“Fluid design” is influenced by the desired optimal final coating thickness, the number of layers, the expected curing and handling of the material post coating, the degree of accuracy required, and many other physical, chemical and optical properties. The list of interrelated considerations in the design of fluid rheology includes viscosity, dispersability and stability of solids, Newtonian/non-Newtonian/thixotropic/shear thinning characteristics, surface tension of fluid relative to base and adjoining layers, adhesion requirements, ability to wet a surface, and many others.

The creation of an optical quality antireflective film with a precise “quarter wavelength” thick coating is an example where fluid and coating choices are interrelated. To avoid optical distortion, coating requires extreme thickness control both across and down the web. Two commonly used methods that can deliver this level of quality are high-precision gravure and slot-die coating. Both work well with very low solids concentration (typically a polymer) at low viscosity (due to the solvent), allowing self-leveling and uniform shrinking in the “z” direction during drying. If the rest of the system is optimized with chatter-free drives, high-efficiency filtration, tension control and zoned-temperature drying, virtually defect-free coatings can be achieved.

Finding an experienced high-precision, quality coating partner with diverse past process expertise saves a great amount of time in product scale up. Evaluating alternative combinations of both fluid design and coating methods at the early stages of product definition will allow optimal selection for successful manufacturing at large scale with good economics and high quality.

Precision Casting: Membranes and Other Advanced Materials

By Jason Payne, PhD | Director

jason.a.payne@carestream.com

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 (using methylene 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 Tollcoating 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.

The Movement Toward Thinner Films

By Todd Arndorfer

todd.arndorfer@carestream.com

As consumer and industrial products continue to feature smaller, thinner and higher performing attributes, there has been a resulting shift in the coatings industry toward using thinner films and substrates for these products. For example, where a 4-mil film used to be standard, an application may now require a 2-mil film. There are two primary drivers for this: reduced material cost, resulting in a lower price for customers or increased margins for manufacturers, and a reduction in weight/size to enable both aesthetic and functional attributes, as well as easier handling and lower shipping costs.

A common type of application that requires the use of thinner materials is LCD, OLED and bistable displays for handheld devices or computer displays. This category of products incorporates multiple layers of different types of films, up to 20, which are ultimately layered or assembled together. Each layer is important as it provides specific performance attributes. In LCD displays, for example, there are protective, prism, diffusing, liquid crystal and polarizing layers, among others, that work together to create the image. The thickness of those layers adds up in terms of both light transmission efficiency and weight. To alleviate this, thinner film layers can help improve brightness and substantially reduce weight and thickness.

The same principle applies to a variety of industries; as products become more sophisticated, multiple layers are a highly effective way to improve product performance. For example enhanced packaging films, where several layers with individual attributes are combined to make a higher-performing film: one layer to provide a barrier to oxygen and moisture to protect the product, another to print high quality graphics for branding, and a third layer on top to protect during handling and distribution. Again, thinner layers are required to reduce cost, bulk and weight. (Full Article)