In recent years, in the field of display devices such as liquid crystal displays and organic electroluminescent displays, due to the demands for reduction in weight and thickness, improvement in flexibility, resistance to breakage and the like, glass substrate and cover glasses have been replaced to plastic substrates. Particularly, in personal digital assistants such as mobile phones, smart phones and tablet PCs, there is a strong demand for plastic substrates.
Moreover, LEDs (Light Emitting Diodes) which is taking advantage of the characteristic features such as long service life and low power consumption, have been expanding their range of use to various applications including general household lighting, automotive lighting and backlights of liquid crystal displays. In response to the demand for miniaturization of information terminal devices such as mobile phones, smart phones and tablet PCs, LED elements are increasingly surface-mounted on circuit boards. In those cases where LED elements are surface-mounted on a circuit board, for the efficiently extracting the emitted light from the LEDs to the front, a reflective layer is commonly formed on the circuit board surface. The material of the reflective layer used in such cases is obtained by filling a white pigment into a transparent resin at a high concentration. However, many kinds of the transparent resins that are conventionally used have problems in that they induce yellowing when exposed to a high-temperature condition for a prolonged period and their reflectance is thus decreased with time. In association with increase in the brightness of LEDs, There is an ever increasing trend for the calorific value of LEDs; therefore, it is in urgent need to improve such thermal yellowing.
It is strongly demanded that developing a resin material which has not only heat resistance and mechanical properties that conform to these market requirements but also, excellent transparency from the viewpoint of design properties as well as to realize the high visibility and is possible to highly inhibit the yellowing caused by exposure to heat and light.
Among organic materials, polyimides are known as polymeric materials that have the highest levels of heat resistance, chemical resistance and electrical insulation properties. In the electrical and electronic industries, for example, “KAPTON (registered trademark)” manufactured by DuPont, which is polymerized from pyromellitic dianhydride (PMDA) and 4,4′-diaminodiphenyl ether (pDADE), and “UPILEX (registered trademark)” manufactured by Ube Industries, Ltd., which is polymerized from biphenyltetracarboxylic dianhydride (BPDA) and p-phenylene diamine (pPD), have been widely used as heat-resistant insulating materials. However, these polyimides have a disadvantage in that they are colored in yellowish brown under a steady state due to intramolecular conjugation and formation of charge transfer complexes.
In order to solve this disadvantage, a number of methods have been proposed so far. Specifically, there have been proposed, for example, a method where a fully aliphatic polyimide entirely constituted by an aliphatic and/or alicyclic compound without any aromatic compound being used as a polyimide constituent is utilized for inhibiting coloration caused by intramolecular conjugation (see, for example, Patent Document 1); a method of obtaining a semi-aliphatic polyimide satisfying both heat resistance and transparency by using an aromatic compound as either an acid anhydride or diamine component and an aliphatic and/or alicyclic compound as the other component (see, for example, Patent Document 2); a method of utilizing a flexible polyimide in which the formation of intermolecular charge transfer complexes is inhibited by incorporating, as constituents, an acid dianhydride and/or a diamine that contain a sterically bulky substituent(s) and/or have a flexible structure (see, for example, Patent Documents 3 and 4); and a method of providing transparency by utilizing the structural distortion of a 7-membered cyclic acid dianhydride to force node formation in π-conjugated system and to thereby localize π-electrons (see, for example, Patent Document 5).
In the method where an aliphatic and/or alicyclic compound is used as a constituent of a polyimide, there are problems that the aliphatic material causes deterioration in the heat resistance and mechanical strength and that yellowing occurs due to oxidation in a heat-treatment process. The problem of heat resistance can be improved by incorporating a rigid aromatic material into the resin structure; however, such a polyimide usually has a decreased transparency and is often hardly soluble to solvents. Therefore, a coating film is formed using a polyamic acid solution which is a precursor, and the resulting film is subsequently thermally or chemically imidized. However, the polyamic acid solution is easily affected by the humidity, handling and storage thereof are difficult.
Meanwhile, in the method of utilizing a flexible polyimide, although excellent solubility to organic solvents and excellent transparency are attained, the polyimide is not possible to ensure sufficient heat resistance by a reduction of the glass transition temperation (Tg) caused by the flexible structure.
Further, in the method of providing transparency by utilizing the structural distortion, it is required to use a special material which is an acid dianhydride having a 7-membered ring structure and the 7-membered ring is an unstable as a constituent; therefore, the resultant is likely to have insufficient mechanical strength, long-term heat resistance and thermal yellowing resistance.