Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2001-06-05
2003-07-01
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
active
06586515
ABSTRACT:
This application is a 371 application of PCT/JP00/06880 filed Oct. 3, 2000.
TECHNICAL FIELD
This invention relates to optical resin materials, more particularly to non-birefringent optical resin materials that show substantially no birefringence or which show small enough birefringence to cause no adverse effects in practice, as well as applications of such non-birefringent optical resin materials.
BACKGROUND ART
Recently, high molecular weight resins are increasingly supplanting the conventional glass-based materials for use not only in general optical parts such as eyeglass lenses and transparent sheets but also in optoelectronic optical parts such as those to be used in laser-related equipment as exemplified by optical disk apparatus for recording acoustic, video, character and other information. This is because optical materials made of high molecular weight resins, namely, optical resin materials, are generally better suited to efficient processing and high-volume production than glass-based optical materials since they are lighter in weight, have higher impact resistance and allow for easier application of molding techniques such as injection molding and extrusion molding.
These characteristics are of course useful for the various kinds of optical parts mentioned above; in addition, they are even more useful when optical resin materials are used in various members that compose the liquid-crystal device as the principal component of a liquid-crystal display. Liquid-crystal displays have come to be used extensively as the display element of various electronic equipment. As their use has expanded, liquid-crystal displays are increasingly required to be lighter and thinner, with the added need for improvements in strength performance such as higher impact resistance. These requirements can be met by effective utilization of those characteristics possessed by high molecular weight resin materials.
Thus, optical resin materials have the potential to show excellent characteristics as optical parts and they are expected to find extensive use in optical parts. In fact, however, they have not been used as much as expected. This is primarily because the products manufactured by applying the aforementioned molding techniques to optical resin materials show by no means small birefringence which sometimes impairs the functions of optical parts in which they are used.
The occurrence of birefringence in high molecular weight resin materials is widely known per se, inclusive of its cause. To be more specific, for almost all high molecular weight resin materials that are commonly used as optical materials, the monomers of which they are formed have optical anisotropy in refractive index and if the polymer is oriented or shows alignment in a given direction, this optical anisotropy of the monomers is expressed to develop birefringence in the high molecular weight resin material. More specifically, the following phenomena take place.
The polymer as produced by polymerization reaction has the linked chains intertwined randomly, i.e., the linked chains of the polymer are not oriented. In this state, the optical anisotropy of one monomer unit cancels that of another monomer unit and the polymer shows no birefringence. However, upon molding such as injection molding or extrusion molding, an external force is applied and the random linked chains in the polymer become oriented, causing the polymer to show birefringence. This state is shown schematically in FIG.
1
. As shown, the high molecular weight resin material that has undergone the molding process accompanied by an external force is in such a state that a number of units (monomers)
1
which form the linked chains in the polymer are linked spatially in a specified direction of orientation. And as already mentioned, for almost all high molecular weight resin materials that are commonly used as optical materials, every unit
1
has optical anisotropy in refractive index. In other words, the refractive index n
pr
for a polarized wave component travelling in a direction parallel to that of orientation is different from the refractive index n
vt
for a polarized wave component in a direction vertical to that of orientation.
As is well known, this optical anisotropy can be expressed by the index ellipsoid. Referring to
FIG. 1
, each linked unit
1
has an elliptic mark
2
which represents the index ellipsoid. Take, for example, polymethyl methacrylate (PMMA); the refractive index of each unit (methyl methacrylate)
1
is relatively small in the direction of orientation and relatively large in a direction vertical to it. Therefore, the index ellipsoid as viewed at a macroscopic scale is oblong in the vertical direction as indicated by
3
in FIG.
1
. In other words, n
pr
is smaller than n
vt
with polymethyl methacrylate. The difference obtained by subtracting n
vt
from n
pr
(&Dgr;n=n
pr
−n
vt
) is called the value of birefringence in orientation. The following Table 1 lists the values of intrinsic birefringence for typical optical resin materials.
TABLE 1
Intrinsic Birefringence Values of Typical
High Molecular Weight Resins
Birefringence value;
Resin name
&Dgr;n = n
pr
− n
vt
Polystyrene
−0.100
Polphenylene oxide
+0.210
Polycarbonate
+0.106
Polyvinyl chloride
+0.027
Polymethyl methacrylate
−0.0043
Polyethylene terephthalate
+0.105
Polyethylene
+0.044
The value of intrinsic birefringence is the value of birefringence exhibited by any one of these polymers when they are fully oriented in a certain direction. In fact, depending on the degree of its orientation, the polymer assumes a value between zero and the intrinsic birefringence.
For example, the polymethyl methacrylate shown in
FIG. 1
has &Dgr;n between −0.043 and 0, and polystyrene has &Dgr;n between −0.100 and 0. With polyethylene, &Dgr;n has a positive value between 0 and +0.044. Hereinafter, if the sign of &Dgr;n is positive (&Dgr;n>0), the expression “the sign of birefringence is positive” is to be used and if the sign of &Dgr;n is negative (&Dgr;n<0), the expression “the sign of birefringence is negative” is to be used.
The birefringence in orientation is particularly problematic in applications where polarization characteristics are important. A typical example of such applications is a group of optical parts in the recently developed write/erasable magnetooptical disk. The write/erasable magnetooptical disk uses polarized beams as reading or writing beams, so if birefringent optical elements (e.g. the disk per se or lenses) are within the optical path, the precision in reading or writing is adversely affected.
An application where the birefringence in the members used plays a more important role is a liquid-crystal device. As is well known, the liquid-crystal device consists of a liquid-crystal layer sandwiched between a polarizer and an analyzer that form crossed or parallel Nicols and the liquid-crystal layer switches between the transmission and non-transmission of light by rotating the plane of polarization of polarized light. Hence, the birefringence of the members which compose the liquid-crystal device poses a great problem which prevents extensive application of optical resin materials to the liquid-crystal device.
With a view to eliminating this problem of birefringence in orientation, various proposals have heretofore been made. A typical example is disclosed in commonly assigned PCT/JP95/01635 (International Publication WO96/06370). According to this technique, a matrix made of a transparent high molecular weight resin is mixed with a low molecular weight organic substance that can be oriented in the same direction as the linked chains in the matrix forming high molecular weight resin are oriented under an external force and the birefringence in orientation of the high molecular weight resin is cancelled out by the birefringence of the low molecular weight organic substance to produce a non-birefringent optical resin material.
To be more specific, the matrix forming high molecul
Cain Edward J.
Wenderoth , Lind & Ponack, L.L.P.
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