**1. Introduction**

Polyimides (PIs) are one of the most important classes of polymers with high mechanical properties, thermal stability, high temperature resistance, good chemical resistance, and dielectric features [1, 2]. Since their mass production in 1955 [3], they enjoy a superior role in the application of electronics, aerospace, automobile, and military fields for their excellent combined properties under harsh environment [4, 5]. With the rapid development of electronic industry, PI film has even become indispensable in microelectronic and optoelectronic engineering due to their high demand of lightweight, durable, and reliable materials [6]. PI films are used in image display devices, optical films, organic photovoltaics, flexible printing circuit boards, and other optoelectronic devices [7]. However, conventional colored PI films which have lower optical transmittance can't fulfill the need of optical transparency in electronic devices, and a reliable colorless polyimide (CPI) is highly desired.

The lower optical transmittance of conventional PI is caused by the formation of intra- and intermolecular charge transfer complex (CTC) originated from their conjugated structures [8, 9]. Therefore, the basic principle to increase the transparency of CPI is to avoid or reduce the conjugated units and suppress the CTC. To achieve this, two manners of molecular design are adopted in most of the CPI studies. One is to choose diamines with lower electron-donating capability or

dianhydrides with lower electron-accepting capability to decrease the CTC induced by them [9]. For example, 2,20 -bis(trifluoromethyl)-4,40 -diaminobiphenyl (TFMB) [10], which is used widely as the monomer of CPI material, has weaker electrondonating property than many diamines like p-phenylenediamine (PPD), 4,40 oxydianiline (ODA) and will show lighter color if copolymerizing with the same dianhydride [9]. The similar phenomenon will happen to 3,30 ,4,40 -biphenyl tetracarboxylic diandhydride (BPDA) with weaker electron-accepting property compared with pyromellitic dianhydride (PMDA) [9]. The other manner is to break the chemical regularity of the copolymer, like inducing the alicyclic structural unit [11, 12]. But in most cases, these two manners are combined together to improve the transparency of CPI. To be concrete, many CPIs are synthesized by introducing fluorine atoms, cyclic side groups, bulky substituents, flexible linkages, and so on [13–18].

Besides the monomers, the film preparation method can also influence the color of PI films by heat or solvents [15]. There are two conventional ways to prepare CPI films: PAA route and organo-soluble PI route. As heat treatment in air will cause coloration, it is suggested that the film preparation should be under inert gas atmosphere or in vacuum by PAA route, however, the high temperature about 300–350°C will still affect the transparency. If the CPIs are soluble, the solvent method will be a better way to prepare CPI compared to PAA route. But, it needs to be concerned that organo-soluble PIs may have relatively low heat and solvent resistance.

Since the discovery of CPI, quantities of studies have been done. In terms of patent layout, patent applications in the CPI field began in the 1970s and reached a peak in 2011. Among them, the number of patent applications in Japan ranks first, accounting for nearly half of the total number of applications.

After more than 30 years of development, the industrial production of CPI has been realized in recent years. While PI production has high technical barriers, there are only several main suppliers, including DuPont, Kaneka, Ube Industries, South Korea SKC, and Taimide, which occupy more than 90% of the global market. The number of CPI suppliers is even less, such as Kolon Industry, SKC, and Sumitomo Chemical. Sumitomo Chemical took up more than 95% of the global CPI films market in 2018. The main suppliers of CPI and their products are listed in **Table 1** [19].

Although CPI has been used in applications, like OLED and photovoltaics, there are still many aspects to be improved. For designing molecular structure, hightemperature stability, optical transparency, and other properties need to be balanced, as in many cases these properties contradict with each other. For PI film production, the major challenge is the high level of technical capabilities is needed to improve the yield. Other properties like water and gas properties need to be enhanced too.


### **Table 1.**

*The main suppliers of CPI and their products.*

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