Toray Industries Inc developed a coating material that can be used to readily form a retarder inside an LCD panel.
Although the new coating material is optimized for vertical alignment (VA) LCD panels, it can also be utilized for twisted nematic (TN) panels. Toray plans to use the material in color filters for its small panels, etc with a view to commercializing it by 2010.
A retarder is a component to increase the viewing angle of LCD panels, and it is usually a film attached to a glass substrate. In order to reduce the stress variation caused by film expansion and contraction in large panels or to reduce thickness of small panels, there has been an increasing demand to form a retarder inside an LCD panel.
However, the formation process using the existing coating material for retarder has not been so widespread because it requires more than six preliminary steps including the coating of alignment film, orientation processing and exposure.
The retarder formation process using the new material requires only two steps: The material is first applied to color filters, etc, and then the coating is subjected to thermal treatment.
Toray reduced the number of steps to less than 1/3 that of the existing process by optimizing the design of polyimide coating material. Specifically, the company eliminated the need for orientation processing and other steps by utilizing the characteristic of a straight-chain polymer with a structure elongated in the planar direction, which aligns by itself in the direction parallel to the substrate.
The company did not unveil the details of the material, but it stressed the improvements added to the polymer's three-dimensional structure and electron structure. The planar orientation can be increased by designing the molecular structure in such a way that it conforms to a "rod-like" shape. But this also reduces the transparency, posing a trade-off problem, the company said.
The new material was designed to optimize the electron migration between two constituents of the polyimide, thereby ensuring the transparency.
This time, Toray used an alignment film with a thickness of several microns and has viewing angle properties equal to or higher than those of the existing retarders used in VA LCD panels. If the surface irregularity of the underlying color filter is within 0.3μm, a retarder can be formed on an as-is basis without performing the planarization such as over coating.
The retardation properties do not change after the thermal treatment at about 230°C, which is required in LCD panel production. In addition, the retarder has a sufficient resistance to a high polarity solvent, which is used to form an alignment film on the retarder, according to the company.
Toray announces the retarder made of the new material at International Display Workshop '08 (IDW '08), which runs from Dec 3 to 5, 2008, in Niigata Prefecture, Japan.
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