In simple terms, casting a membrane is the controlled precipitation of nitrocellulose from the lacquer. Technologically, however, the equipment required to achieve uniform precipitation is complex (2), in part because the process must be continuous to achieve a high level of production efficiency. The first step in the casting process is the spreading of a thin, uniform layer of lacquer on a moving belt. The belt carries the lacquer into a series of chambers where air flow, temperature, and humidity are adjusted to control the evaporation rate of the solvents from the lacquer. As the solvents evaporate, the nitrocellulose strands migrate within the liquid phase, eventually reaching a concentration where they precipitate. The size of the pores within the structure is dictated by the evaporation rate, with more open structures achieved by reducing the evaporation rate. Overall, the casting process is slow, running at a rate of <2 linear ft/min. Because a single casting run may be hundreds of linear meters long, the casting equipment must be capable of maintaining constant conditions for several days.
Nitrocellulose is inherently hydrophobic (2,10), and membranes made solely from it would be incompatible with the aqueous systems used to apply reagents and run samples. For this reason, the casting process must include a technique for introducing a wetting agent into the membrane. This can be done through inclusion of the wetting agent in the lacquer or by application of the wetting agent to the membrane in a separate step. Regardless of the technique used, the final product must be uniformly wettable so that it allows even adsorption of reagent solutions and uniform lateral flow of samples.
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