Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified... – With bonding or intermediate layer of specified composition
Reexamination Certificate
2001-12-13
2004-06-08
Pyon, Harold (Department: 1772)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
With bonding or intermediate layer of specified composition
C428S001310, C536S069000, C536S080000, C558S211000, C156S239000, C349S096000
Reexamination Certificate
active
06746732
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of optical components and, in particular, to protective triacetyl cellulose polymeric films that contain a plasticizer combination including a triphenyl monophosphate compound and a lower alkyl-substituted triphenyl phosphate ester compound.
BACKGROUND OF THE INVENTION
Plasticizing agents have long been known to enhance the physical properties of solvent cast cellulose triacetate (also called triacetyl cellulose or TAC) polymeric films. Plasticizers also aid in the release of solvents during the cast sheet formation and drying process. TAC films have found a large market in polarizing plate manufacturing. In this process, dyed and oriented polyvinyl alcohol (PVA) sheets are glued between protective layers of TAC sheet. The acetate cover layers (both sides) protect the active PVA layer from physical damage and environmental agents that could cause chemical degradation to the PVA, with resulting polarization efficiency loss. Water, particularly with heat, can dissolve PVA, potentially altering the active, oriented PVA sheet. A hot, high-humidity environment can result is a dissolution/relaxation of the oriented PVA, and/or degradation of the oriented dye that provides the polarizing effect, with a subsequent loss in polarization efficiency.
Small reductions in moisture or water vapor transmission of the protective layer will improve the life of polarizing plates under normal and high humidity ambient conditions. This attribute is important for liquid crystal displays, which incorporate two polarizing plates that may function in high heat and humidity situations, such as in automobiles.
Many different polymers have been proposed as protective cover layers for the optically active PVA sheet. To date, polymer extrusion has not been able to produce coversheets with the desired low birefringence properties. In general, all current polarizer coversheets are made from solvent cast TAC. Current industry specifications require LCD polarizer coversheets to posses a birefringence of less than 5 nanometers. The advantage of solvent casting is the extremely low stresses exerted on the polymer film as solvent leaves the forming web. Solvent sheet casting contains 20 to 40 percent polymer solids in a suitable solvent. The polymer chains are mobile in the solvent system and the stresses of hopper coating are easily relaxed, on the casting surface prior to solvent removal. Low stresses result in low orientation and the accompanying optical anisotropy.
Extrusion is a melt/cast process. The extrusion die induces stress into the rapidly cooling polymer web. There is little opportunity for polymer relaxation in the quick thermal quenching of the formed sheet. Tentoring can be employed to produce a more stress-balanced sheet. However, results to date have not met the stringent requirements of low birefringence for polarizer coversheets.
TAC polymers have a cellulose chain backbone with varying degrees of acetylation. TAC can range in substitution from approximately 2.4 to 3 acetyl substitution points on the cellulose backbone. Other substitutions on the cellulose backbone could be hydroxyl, propyl or butyl groups. LCD coversheet is made with TAC substitution in the 2.8 to 2.9 range. This degree of acetyl substitution results in optimum polymer properties (such as clarity, physical strength, and polymer solubility.) TAC tends to be high in polymer molecular weight. This is due to the extremely large cellulose chains it is formed from. In the conversion process long cellulose chains are broken down in molecular weight and acetylated. Cellulose chains are helical in nature. The acetyl groups added in conversion to cellulose triacetate add bulky side groups to the polymer chain. This results in a polymer system that has long helical chains with bulky side groups. TAC cannot be thermally extruded, as it does not posses a melting point (it will oxidize first). Hydrogen bonding also plays a role in cellulose sheet formation. Long helical polymer chains, with bulky side groups, combined with rapid hydrogen bonding, results in a polymer system with very low order. TAC is thus a very good polymer for forming amorphous polymer sheet. The fundamental lack of TAC polymer orientation combined with the low stresses of solvent casting, forms a unique polymer system for extremely isotropic LCD coversheets. These fundamental advantages have allowed solvent cast cellulose triacetate to capture the vast majority of LCD coversheet applications.
Low birefringence of protective layers is critical for clarity in liquid crystal displays. Orientation in the coversheet counteracts the specific orientation in the polyvinyl alcohol active layer, damaging the polarization efficiency of the complete package. LCD screens contain two polarizers with four coversheets. Improper orientation in the coversheets will also reduce the focus and clarity of the resulting display. The overriding need for low retardation coversheets has driven the industry to a universal acceptance of TAC sheet but water vapor transmission is a problem.
Solutions have been proposed. U.S. Pat. No. 5,516,456 suggests the use of extruded and solvent cast polynorbornene. Example 2b shows LCD coversheet (80 micron) variations in retardation orientation of +/−5% (40 nanometers). This is far greater than the 5 nanometers typically required for 80 micron LCD coversheet. Polynorbornene has very low moisture absorption, but the optical retardation is too high to meet the requirements of high-grade LCD displays. The patent also states that polyethylene terephthalate and polycarbonate sheets display poor retardation quality, which distorts LCD display images.
An alternative solution is presented in U.S. Pat. No. 4,416,946. In this case a double layer is used to sandwich the polarizing element. In this case, fluorinated polymers are laminated outside of a polyester or cellulose acetate butyrate inner layer. This procedure adds additional expense for manufacturing. An additional layer also causes light path interface, which adds to the diffraction of light, which increases light path retardation, and results in poor LCD screen clarity.
Another proposed solution is covered in U.S. Pat. No. 4,564,545. In this case, the inherently high birefringence of extruded polypropylene and polyethylene is intentionally oriented. The cover sheets are oriented in the same direction as the active polarizer they are laminated with. This approach attempts to intentionally control the effect of optical anisotropy in the coversheets. With a non-orthogonal beam of light, the coversheet will impart orientation to the light before it reaches the polarizing layers. The defect of this approach is that the orientation of the coversheet will make the undistorted viewing angle of the resulting LCD screen very narrow. Another problem with this method is the requirement for exact orientation of the two anisotropic coversheets. If the coversheets are not exactly oriented with each other and the active polarizing layer, there will be a degradation of the polarizer efficiency.
Another way to reduce water vapor transmission is to raise the levels of triphenyl phosphate (TPP), a component often present as a plasticizer in the TAC coversheet. This results in a modest decrease in water vapor transmission for the TAC sheet, but it is not nearly as effective as is desired. More importantly, there are other difficulties with simply increasing the TPP loading in the coversheet. TPP is mobile and migrates to the sheet surface as the sheet surface collapses down during sheet curing. This causes TPP generated dirt problems in the sheet forming machinery. This leads to surface defects and additional machine cleaning expense. A sheet rich in surface TPP may also cause polarizer lamination difficulties and the enriched surface will be more porous and easier to scratch. TAC polarizer coversheets commonly contain 10 to 12 weight percent TPP in the TAC sheet. If this level is raised to 15 weight percent, the mechanical properties of the TAC sheet will fal
Gamble William J.
Lippert Joseph L.
Eastman Kodak Company
Hon Sow-Fun
Kluegel Arthur E.
Pyon Harold
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