Method of manufacturing optical compensator

Abstract: A method of manufacturing an optical compensator plate comprising: autoclaving step of applying heat and pressure to an ionomer resin to form a resin film having birefringence; and annealing step of annealing the resin film after autoclaving step, at an anneal temperature in a temperature range not less than the softening temperature and not higher than the melting point of said resin, e.g. 6⊘-1⊘⊘°C for at least about 3⊘ minutes.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a method of manufacturing an optical compensator plate, and more particularly to a method of manufacturing an optical compensator plate having a negative optical anisotropy or activity. Description of the Related Art The optical compensator is an optical component having an optical anisotropy, used for compensating the optical anisotropy of another optical component of interest. Liquid crystal molecules used in such devices as liquid crystal display device ordinarily have an elongated shape, and has a high refractive index in the direction parallel to the molecular axis and a low refractive index in directions perpendicular to the molecular axis. When such liquid crystal molecules are oriented perpendicular to the substrate between a pair of transparent substrates, the liquid crystal cell will show a positive optical anisotropy. Performance of the liquid crystal cell in this state has a strong dependency on the angle measured from the normal of the substrates. For compensating this positive optical anisotropy, there is needed a material having a negative optical anisotropy having a low refractive index in the direction of thickness, and a high refractive index in the directions in the plane. Further, it is necessary to adjust the degree of optical anisotropy Δn.d of the optical compensator plate, where Δn=n ⊘ -n e , n is the refractive index, n ⊘ is the refractive index for the ordinary ray, and n e is the refractive index for the extraordinary ray, and d is the thickness of the compensator plate, in accordance with the degree of the optical anisotropy to be compensated. However, it is not easy to manufacture an optical medium having desired optical properties. SUMMARY OF THE INVENTION An object of this invention is to provide a method of manufacturing an optical compensator plate which exhibits little variation of the optical property in long term use. According to an aspect of this invention, there is provided a method of manufacturing an optical compensation plate comprising: a first step of applying heat and pressure to a thermo-plastic resin film to form a resin film having birefringence; and a second step of annealing said resin film after said first step, at an anneal temperature in a temperature range not less than the softening temperature and not higher than the melting point of said resin, for about 3⊘ minutes or more. The optical properties of the optical compensator can be stabilized by performing annealing after manufacture of the optical compensator plate. After manufacture of the optical compensator plate, the variation of the thickness d is of a negligible amount, and hence the variation of the optical anisotropy Δnd, is considered to be ascribed mainly to the variation of the refractive index n. Just after the manufacture of the optical compensator plate, thermal hysteresis occurred in the manufacture is considered to remain in the film as internal stresses. Also, it can be considered that the crystalinity of the plate is not yet stabilized. By performing annealing at a temperature above the softening temperature, the internal stress can be relieved, thereby to decrease variation thereafter, and the variation of optical anisotropy Δnd can be enhanced to reach a stable value. Therefore, variations in the properties of the optical compensator plate thereafter can be decreased. In case of using the optical compensator plate in combination with a liquid crystal device, the manufacturing steps of the optical compensator plate and the annealing step to be done thereafter can be adjusted to give desired optical anisotropy Δnd, to provide an appropriate optical compensator plate for compensating the optical anisotropy of the liquid crystal cell. Thus, an optical compensator plate having stabilized optical properties can be provided. BRIEF DESCRIPTION OF THE DRAWING Figs. 1A and 1B are graphs showing variation of the optical anisotropy of optical compensator plates which are subjected to the treatment according to the embodiment of this invention. Fig. 2 is a schematic diagram of a liquid crystal device. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the embodiments of this invention, prior work of manufacturing an optical compensator plate utilizing autoclaving furnace will be described. For example, a thermo-plastic resin film is sandwiched between a pair of glass plates to form a laminated structure. This laminated structure is contained in an evacuatable bag. The inside of the bag is evacuated and then the open end of the bag is sealed. The sealed bag is loaded in an autoclaving furnace. When pressure is applied in the autoclaving furnace, the pressure is applied to the thermo-plastic resin film through the pair of glass plates along the direction of thickness. Also, the autoclaving furnace is heated to a predetermined temperature. Thus, the thermo-plastic resin film is subjected to a pressure and a heat treatment. When the thermo-plastic resin film is formed of ionomer resin which is formed by bridging ethylene-acryl acid or ethylene-metacryl acid copolymer molecules with metal ions, a resin film having a negative optical anisotropy can be provided by the above pressure and heat treatment. According to such a method of manufacturing an optical compensator plate as described above, however, the optical properties of the optical compensator plate tends to vary with the lapse of time. The variation is not significant by the use at the ordinary temperatures, but when the compensator plate is kept at a raised temperature above about 6⊘ °C, the product Δn.d of the difference of the refractive indice Δn for the ordinary ray and the extraordinary ray and the film thickness d does increase. In a liquid crystal panel, even if an optical compensator plate initially matched to the optical anisotropy of the liquid crystal is combined with the liquid crystal cell, appropriate compensation cannot be done if the optical properties of the optical compensator plate varies with the lapse of time in use. Now, embodiments of this invention will be described. First, an optical compensator plate is made in an autoclaving furnace as described above. Between a pair of transparent substrates such as glass plates, ionomer resin film is sandwiched, which is formed by bridging ethylene-acryl acid or ethylene-metacryl acid copolymer molecules with metal ions. The sandwiched structure is put in an evacuatable hermetic bag and the bag is evacuated. The the evacuated bag is sealed and loaded in an autoclaving furnace. The temperature and the pressure of the autoclaving furnace are raised to desired values and then are kept constant for a predetermined time period. For example, the autoclaving furnace is kept at a temperature of 1⊘⊘-15⊘°C at a pressure of 1-5 kg/cm² for about 3⊘ minutes. Then, the temperature and the pressure of the autoclaving furnace are lowered to retrieve the laminated structure. When the laminated structure is used as it is, as an optical compensator plate, the optical property will change with the lapse of time as described above. As an annealing step after manufacture, the laminated structure is loaded in an annealing furnace and annealing at about 6⊘-1⊘⊘ °C is done for about 1-3 hours. Here, the annealing temperature is preferably not less than the softening temperature and not higher than the melting point. If the temperature is below the softening temperature, the annealing effect is low. If the temperature is above the melting point, the once formed optical anisotropy will decrease. In case of HI-MILAN (trade name) available from Mitsui Du-pont Polychemical, Japan, the annealing temperature is preferably not higher then 9⊘ °C. The annealing time may depend on the annealing temperature, but is preferably not shorter than about 3⊘ minutes. When the annealing time is below 3⊘ minutes, the annealing effect may not be fully exhibited. After predetermined annealing treatment is done, the temperature of the annealing furnace is lowered and the laminated structure is taken out to be used as an optical compensator plate. By such annealing treatment, the variation of the refractive indice thereafter is decreased, and also distribution of the optical properties in the plane of optical compensator plate is decreased. Further, although the maximum use temperature was about 4⊘ °C in the conventional case, the maximum use temperature has increased to, for example about 6⊘ °C by performing the annealing treatment at about 6⊘ °C. Variation of the amount of optical anisotropy (Δnd) when the annealing treatment as described above is performed are shown in Fig. 1A and 1B, taking examples of the ionomer resin bridging ethylene-metacryl acid copolymer molecules with sodium ions, and the ionomer resin bridging ethylene-metacryl acid copolymer molecules with magnesium ions. In the figures, absicissa represents the annealing time and the ordinate represent Δnd. Here, the zero point on the time axis represents a time when an optical anisotropic plate manufactured by the autoclaving technique is set in an annealing furnace and annealing treatment is started. Annealing is first done at 6⊘ °C for 6 hours. Then the temperature is raised to 7⊘ °C, and the annealing is continued for 6 hours. Then, the temperature is raised to 8⊘ °C and annealing is continued for 6 hours. Then the temperature is raised to 9⊘ °C and annealing is continued for 6 hours. And so on. In this way, the annealing treatment is done by raising the temperature stepwise. As shown Fig. 1A, in the case of sodium ion type ionomer resin, the value of Δnd rapidly rises at the initial stage, and then varies very gradually thereafter. Namely, it can be considered that the variation of Δnd in a first period can be fully exhibited by performing heat treatment at about 6⊘ °C for about 3⊘ minutes or more. The stabilized optical anisotropy Δn was about -1.3x1⊘⁻³. In the case of magnesium ion type ionomer resin as shown in Fig. 1B, the initial variation appears more intensively. Namely, by the annealing at 6⊘°C, the value of Δnd increases from about 19⊘ to about 28⊘ in an arbitrary scale by annealing of about one hour. Thereafter, the increase becomes gradual but the value of Δnd continues to be varied. When the temperature is raised to 7⊘ °C, the value Δnd varies rapidly again. when the temperature is raised 8⊘ °C, the value of Δnd now shows decrease. But along with the lapse of annealing time, the value of Δnd shows slight increase. When the temperature is raised 9⊘°C, the value of Δnd decreases further intensively. Namely, in the case of magnesium of ion type, the value of Δnd becomes maximum around 7⊘ °C. In the case of magnesium ion type ionomer, also, the variation of Δnd in the initial stage can be forcedly exhibited by performing the heat treatment at a temperature not less than about 6⊘ °C for about 3⊘ minutes or more, to decrease the variation thereafter. Fig. 2 shows a liquid crystal display device. A homeotropic liquid crystal layer 13 is sandwiched between a pair of glass substrates 12 and 14 provided with electrodes. A pair of crossed polarizers 11 and 16 having crossed polarization axes P1 and P2 are positioned outside the liquid crystal cell. An optical compensator film 15 manufactured according to the above described embodiment is inserted between the liquid crystal cell 12, 13, 14 and one of the polarizers 16. When the liquid crystal molecules in the liquid crystal layer 13 is aligned perpendicular to the substrates. A positive optical anisotropy is established The compensator plate 15 has a negative optical anisotropy to compensate the positive optical anisotropy of the liquid crystal layer 13. Although description has been made embodiments of this invention, the present invention is not limited thereto. It will be apparent for those skilled in the art that various changes, substitutions, improvements, combinations, etc. are possible within the scope of the present invention, as defined in the dependent claims.

Claims: A method of manufacturing an optical compensator plate comprising: a first step of applying heat and pressure to a thermo-plastic resin film to form a resin film having birefringence; and a second step of annealing said resin film after said first step, at an anneal temperature in a temperature range not less than the softening temperature and not higher than the melting point of said resin, for at least about 3⊘ minutes. A method of manufacturing an optical compensator plate according to claim 1, wherein said anneal temperature is a temperature in a range of 6⊘-1⊘⊘°C. A method of manufacturing an optical compensator plate according to claim 1, wherein said resin is ionomer resin. A method of manufacturing an optical compensator plate according to claim 3, wherein said ionomer resin is formed by bridging ethylene-metacryl acid copolymer molecules with metal ions. A method of manufacturing an optical compensator plate according to claim 4, wherein said annealing temperature is a temperature in a range of 6⊘-9⊘°C. A method of manufacturing an optical compensator plate according to claim 1, wherein said first step includes heating the thermoplastic resin to a temperature in a range of 1⊘⊘-15⊘°C.

Inventors: HIROSE SHINICHI, HIROSE, SHINICHI
Applicants: STANLEY ELECTRIC CO LTD, STANLEY ELECTRIC CO., LTD.
Application year: 1991
Main classification: B29D11
IPC codes: B29C55, G02B5, G02F1, B29C71, B29D11