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How thin films deposited using solution casting method?

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October 29, 2012 12:43AM

In a nutshell, the process of producing thin films or membranes involves the following steps:

a polymer solution or melt is forced through a die/coating machine onto a backing or inert support, such as a sheet of nylon or even cellulosic paper. The newly deposited film on its backing is then fed, in the form of a continuously driven sheet, through a treatment process specific to the application. Finally, the sheet is packaged for later use, often by rolling it back up.

Here is a longer step by step description of the process:

1) polymer system is prepared. This involves the melting or dissolution of one or more polymers into a solvent system. This step may involve more complicated procedures than simply adding solids to a solvent or heating a solid until it melts. The specifics are ultimately determined by the application, the capacity and equipment available, and the

2) the polymer melt or solution is then fed through a die coating machine onto a backing of some kind. This is by far the most common process, and even when applications do not use a backing/support the film is still cast this way (later processing will peel it away).

other processes exist, such a spin coating, spray coating, etc. but this method is the most widely used in industry for the preparation of thin films or membranes.

This step is typically considered to be the most precise part of the entire process. The polymer must be fed at a constant, consistent, and reliable/accurate pressure, temperature, and flow rate. This must be achieved while depositing approximately 100-200 micrograms of polymer solution (ultimately about 20-40 micrograms of solids after the solvent has been removed) per square milimeter over a width of several feet and a length of many thousands of feet. A small scale operation should produce around 7,000 feet of membrane per day per casting machine, though it can be much much higher.

3) The newly coated polymer sheet is now treated by drawing the sheet through some collection of processes along a series of cylindrical rollers. Drying, heating or cooling, photo irradiation, doping (example: exposure to a dye, a particular chemical, a salt solution, etc.), pH balancing, and rinsing are common examples of treatment stages.

4) the sheet, now treated, is typically rolled back up for later processing.

It should be noted that not all films require the use of backing or support in their application

there are many applications which require that the polymer film be carefully peeled away from the backing at some later processing stage.

As a side note, I will mention the following about this particular method of thin film casting:

Its main historical application, and that which produced the most R&D and understanding of the theory underpinning this field of materials science, was the production of large quantities of Cellulosic (nitrate or acetate) film backing for use in chemical film. Unsurprisingly, Eastman Kodak held many of the patents and trade secrets related to this technology for many years, and they (Eastman Chemical Company) still operate as one of the main suppliers of cellulose acetate used for the production of membranes or films.