*Design, Fabrication, and Application of Colorless Polyimide Film for Transparent and Flexible… DOI: http://dx.doi.org/10.5772/intechopen.93428*

Because the curing process of the pre-iminated PI solution is almost a pure physical process of solvent evaporation, the curing process can be realized under a lower temperature (≤250°C) which will not cause coloring. CPI films prepared this way will have a good surface smoothness and better transparency.

The excellent performance of PI film is related to its heteroaryl molecular structure as well as their unique production technology. Compared with laboratory preparation, industrial production also use the solvent casting process, however, the biggest difference between them is the stretching process. There are two stretching methods, uniaxial stretching (machine direction, MD) and biaxial stretching (transverse direction, TD, and mechanical direction, MD) technologies, as shown in **Figure 2** [56]. Biaxial stretching is always used to ensure the evenness of the film. The gelation of the PAA membrane used for stretching can be achieved by partial evaporation of the solvent, or by chemical treatment of the dehydrating agent (acetic anhydride, dicyclohexylcarbodiimide, etc.) and its catalyst (pyridine). During the stretching process, whether it is uniaxial stretching or biaxial stretching, the gel-like PAA film will lead to a complete orientation and stretching of the PI molecular chain. From the perspective of polymer physics, stretching will greatly improve the mechanical properties of the resulting PI film. CPI films produced by biaxial stretching have good optical transparency, heat resistance, and reduced dimensional change.

In production, the monomer of polyimide is firstly sent to a polymerization reactor for polycondensation to obtain PAA solution. The solution is degassed and casted to a continuous film on a heated, rotating steel drum forming a selfsupporting PAA film. The gel-like PAA film will be peeled off from the metal drum and stretched in the machine direction (MD) and lateral direction, which is performed at a temperature of about 350°C to promote the imidization of PAA. This method above has been widely used in the preparation of polyimide films industrially. Whereas, due to the high temperature (350°C) of full imidization of PAA, the above-mentioned manufacturing process may be difficult for producing colorless polyimide film. Therefore, a novel manufacturing technology has been developed in recent years. It uses soluble PI resin as the starting material instead of PAA. The key elements of this procedure include the following: (1) the PI resin must be soluble in volatile solvents; (2) the formed PI solution should be stable with a reasonable solids

**Figure 2.** *Production process of polyimide films, adopted from [56].*

content and viscosity; and (3) it must be able to form a uniform film and be releasable from the casting support. Mitsubishi Gas Chemical Company introduced a method [57, 58] for producing colorless and transparent PI films using this technology. The starting soluble PI resin was prepared from 1,2,4,5-cyclohexanetetracarboxylic dianhydride and aromatic diamine by one-step high-temperature polycondensation. The PI film was then stretched 1.01 times in the machine direction and 1.03 times in the transverse direction under the condition of nitrogen at a temperature of 250°C and dried. The obtained PI film showed good properties, with a thickness of is 200 mm, a light transmittance 89.8%, and a yellow index 1.9.

The major challenge of polyimide film market is the high level of technical capabilities involved in the film processing. Besides, the strong heat and chemical resistance of polyimide lead to processing difficulties, such as lack of solubility. This makes it difficult to integrate other materials into the polymer matrix. At the same time, there are some other key issues in processing of CPI film, high unevenness, the thickness decline caused by high stress (50–60% of the deposited value), and poor adhesion.
