Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-07-02
2003-10-07
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C359S483010
Reexamination Certificate
active
06630970
ABSTRACT:
TECHNICAL FIELD
This invention relates to liquid crystal displays, and more particularly to polarizers for use with liquid crystal displays.
BACKGROUND
Liquid crystal displays are optical displays used in devices such as laptop computers, hand-held calculators and digital watches. A typical liquid crystal display includes a liquid crystal display cell and an electrode matrix disposed between a pair of absorbing polarizers. The liquid crystal display cell contains, e.g., twisted nematic or super twisted nematic molecules. In the liquid crystal display, the optical state of portions of the liquid crystal display cell is altered by the application of an electric field using the electrode matrix. This creates an optical contrast for light passing through the liquid crystal display cell that results in the appearance of pixels of polarized light on the liquid crystal display.
A typical liquid crystal display includes a front polarizer and a rear polarizer. These polarizers may be plane polarizers that absorb light of one polarization orientation more strongly than they absorb light of the orthogonal polarization orientation. The transmission axis of the front polarizer is usually crossed with the transmission axis of the rear polarizer in a liquid crystal display. The angle by which these transmission axes are crossed can vary from zero degrees to ninety degrees.
In general, unpolarized ambient light waves vibrate in a large number of directions without having a single characterizing electromagnetic radiation vector. By contrast, plane polarized light consists of light waves having a direction of vibration along a single electromagnetic radiation vector. Also, circularly polarized light has a direction of vibration along an electromagnetic radiation vector that rotates as the light propagates through space. Polarized light has many applications in electro-optical devices, such as the use of plane and circular polarizing filters to reduce glare in displays.
Further, much commercial attention has been directed to the development and improvement of flat panel displays, particularly thin, compact flat panel displays. A problem encountered in the construction of plastic flat panel displays is the development of “black spots,” which arise from the formation of bubbles in the liquid crystal material from gas that has permeated through the plastic display materials. Another problem associated with plastic flat panel displays is moisture contamination of the liquid crystal display cell. These problems are avoided in conventional liquid crystal displays by using low permeability glass substrates instead of plastic. With respect to plastic flat panel displays, these problems are addressed by adding additional gas and moisture barrier layers to the liquid crystal display structure and/or the plastic substrates. However, adding such gas and moisture barrier layers increases the thickness, weight and cost of the displays.
Polarizers in the form of synthetic polarizing films exhibit comparative ease of manufacture and handling and comparative ease with which they may be incorporated into electro-optical devices such as flat panel displays. In general, plane polarizing films have the property of selectively passing radiation vibrating along a given electromagnetic radiation vector and absorbing electromagnetic radiation vibrating along a second electromagnetic radiation vector based on the anisotropic character of the transmitting film medium. Plane polarizing films include dichroic polarizers, which are absorbing plane polarizers utilizing the vectorial anisotropy of their absorption of incident light waves. The term “dichroism” refers to the property of differential absorption of the components of incident light, depending on the vibration directions of the component light waves. Light entering a dichroic plane polarizing film encounters two different absorption coefficients along transverse planes, one coefficient being high and the other coefficient being low. Light emerging from a dichroic film vibrates predominantly in the plane characterized by the low absorption coefficient.
Dichroic plane polarizing films include H-type (iodine) polarizers and dyestuff polarizers. For example, an H-type polarizer is a synthetic dichroic sheet polarizer including a polyvinyl alcohol-iodine complex. Such a chemical complex is referred to as a chromophore. The base material of an H-type polarizer is a water-soluble high molecular weight substance, and the resulting film has relatively low moisture and heat resistance and tends to curl, peel or otherwise warp when exposed to ambient atmospheric conditions. Further, H-type polarizers are inherently unstable, and require protective cladding, e.g., layers of cellulose triacetate, on both sides of the polarizer to prevent degradation of the polarizer in a normal working environment such as in a liquid crystal display.
In contrast to H-type polarizers and other similar synthetic dichroic plane polarizers are intrinsic polarizers and thinly cladded or encapsulated polarizers. Intrinsic polarizers polarize light due to the inherent chemical structure of the base material used to form the polarizer. Such intrinsic polarizers are also typically thin and durable. Examples of intrinsic polarizers are K-type polarizers. A thinly cladded or encapsulated polarizer may be, e.g., an iodine polarizer coated on both surface with polymer coatings each having a thickness of only about 5 microns, and is also thin and durable.
A K-type polarizer is a synthetic dichroic plane polarizer based on molecularly oriented polyvinyl alcohol (PVA) sheets or films with a balanced concentration of light-absorbing chromophores. A K-type polarizer derives its dichroism from the light absorbing properties of its matrix, not from the light-absorbing properties of dye additives, stains, or suspended crystalline materials. Thus, a K-type polarizer may have both good polarizing efficiency and good heat and moisture resistance. A K-type polarizer may also be very neutral with respect to color.
An improved K-type polarizer, referred to as a KE polarizer, is manufactured by 3M Company, Norwood, Mass. The KE polarizer has improved polarizer stability under severe environmental conditions, such as high temperatures and high humidity. In contrast to H-type polarizers, in which the light absorption properties are due to the formation of a chromophore between PVA and tri-iodide ion, KE polarizers are made by chemically reacting the PVA by an acid catalyzed, thermal dehydration reaction. The resulting chromophore, referred to as polyvinylene, and the resulting polymer may be referred to as a block copolymer of vinylalcohol and vinylene.
For H-type polarizers, stability is achieved by sandwiching the polarizer between two plastic substrates, such as two layers of cellulose triacetate, one on each side of the polarizer. However, even in these structures the application of heat, humidity and/or vacuum can adversely affect the properties of the polarizer. By contrast, K-type polarizers such as KE polarizers do not need to be sandwiched between sheets of cellulose triacetate. The polyvinylene chromophore of the KE polarizer is an extremely stable chemical entity, since the chromophore is intrinsic to the polymer molecule. This chromophore is thermally stable as well as resistant to attack from a wide range of solvents and chemicals.
A K-type polarizer such as a KE polarizer has several advantages over other types of polarizers, e.g., iodine and dyestuff polarizers. K-type polarizers have more durable chromophores, are thinner, and may be designed with variable transmission levels. Most notably, K-type polarizers such as KE polarizers may be used in applications that require high performance under severe environmental conditions, including high temperatures and high humidity, such as 85° C. and 85% relative humidity, for extended periods of time. Under such extreme environmental conditions, the stability of iodine polarizers is greatly reduced, thus limiting their usefulness in applications such as flat panel dis
Branca John C.
Cael John
Foresyth David M.
Gordon James
Pugh William
3M Innovative Properties Company
Dudek James
Kirkpatrick & Lockhart LLP
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