Molding material for plastic lens

Details Molding material for plastic lens Molding material for plastic lens

1999-12-06

2002-11-26

Szekely, Peter (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C264S001320, C351S159000, C351S16000R, C351S166000, C523S106000, C525S333300, C525S338000, C525S339000

Reexamination Certificate

active

06486262

ABSTRACT:

TECHNICAL FIELD
The present invention relates to molding materials for plastic lenses, and more particularly to molding materials for plastic lenses, which have excellent strength properties, heat resistance, and molding and processing ability and are capable of forming plastic lenses sufficiently small in birefringence. The present invention also relates to plastic lenses molded from such a molding material. The present invention further relates to hydrogenated products of aromatic vinyl polymers, which are suitable for use in such optical materials, and a production process thereof.
BACKGROUND ART
Optical lenses molded from a plastic material have such features that they are excellent in impact resistance and formability, light-weight, capable of being colored and low in cost. As properties of polymers used in molding materials for plastic lenses, it is particularly important to be excellent in transparency, moldability, mechanical properties, low water absorption property, heat resistance, chemical resistance and the like and small in birefringence. Among plastic materials, polymethyl methacrylate (PMMA) has far excellent transparency and weather resistance and is a typical isotropic substance hardly causing birefringence. However, it has drawbacks that the water absorption property is relatively high, and its surface is easy to be damaged and it tends to cause solvent cracking by solvents. On the other hand, polystyrene is somewhat poor in transparency compared with PMMA, but has features that the light transmittance is high, the moldability and mechanical properties are good and the water absorption property is low. However, polystyrene has a drawback that it tends to cause birefringence.
The birefringence means a phenomenon that two refracted rays appear when light is incident on an anisotropic medium. More specifically, the birefringence is a phenomenon that light incident on an anisotropic material is divided into two light waves respectively having vibration directions perpendicular to each other. Accordingly, plastic materials used in application fields such as optical lenses in particular are required to be optically isotropic and hardly cause birefringence. The optical anisotropy is mainly controlled by a difference in the principal polarizability of a repeating unit. Benzene is great in the difference between polarizability in a plane direction and polarizability in a vertical direction. Accordingly, polymers having a benzene ring in their molecules, such as polystyrene, tend to cause birefringence.
Therefore, it has heretofore been attempted to reduce the birefringence of an aromatic vinyl polymer such as polystyrene by hydrogenating carbon-carbon double bonds in the aromatic rings (typically, benzene rings) of the aromatic vinyl polymer to saturate the double bonds, thereby producing a hydrogenated product of the aromatic vinyl polymer, which is excellent in transparency, moldability, mechanical properties, low water absorption property, and moreover low in birefringence and hence suitable for use in a molding material for plastic lenses.
For example, Japanese Patent Application Laid-Open No. 132603/1989 discloses a process for producing a hydrogenated product of polystyrene, in which azobisisobutyronitrile is used as a polymerization initiator to radical polymerize styrene, thereby synthesizing polystyrene, and the benzene rings in the polystyrene are then hydrogenated at a hydrogenation rate (degree of hydrogenation) of at least 30%. With respect to the hydrogenated products of polystyrene specifically disclosed in this publication, however, the hydrogenation rate of the benzene rings is as low as 60 to 80% in the case of a molecular weight as comparatively high as 100,000 to 120,000 in terms of a weight average molecular weight (Mw), and so the birefringence is not sufficiently reduced, whereas the hydrogenation rate of the benzene rings is high in the case of a molecular weight as comparatively low as 60,000 to 80,000 in terms of a weight average molecular weight (Mw), and so the birefringence is sufficiently reduced. However, such a low-molecular weight polymer is poor in strength properties. In addition, the hydrogenated products of polystyrene disclosed in the publication involve a problem that since the molecular weight distribution of polystyrene itself is comparatively wide, and the molecular weight distribution becomes wider due to the hydrogenation reaction, the strength properties and heat resistance of the hydrogenated products are lowered. Accordingly, the hydrogenated products described in the publication are not polymers having high strength properties and sufficiently small birefringence, and are hence poor in the balance between them.
Japanese Patent Application Laid-Open No. 75001/1992 discloses a process for producing a hydrogenated product of polystyrene, in which benzoyl peroxide is used as a polymerization initiator to radical polymerizing styrene, thereby synthesizing polystyrene having a weight average molecular weight (Mw) of 320,000, and the benzene rings in the polystyrene are then hydrogenated at a hydrogenation rate of 100%. In this hydrogenated product of polystyrene, the birefringence is sufficiently reduced since the hydrogenation rate is high. However, this hydrogenated product is wide in molecular weight distribution and contains a low-molecular weight component in a high proportion and hence involves a problem that when a plastic lens is molded therefrom, the lens cannot be provided as a plastic lens having sufficient strength properties and heat resistance.
Further, the plastic materials used in optical lenses tend to cause birefringence based on molecular orientation and mechanical strain upon molding. In general, the plastic materials are excellent in moldability and can be molded into optical lenses by injection molding, extrusion, press molding or the like. However, the plastic materials tend to cause molecular orientation and also to cause mechanical strain resulting from residual stress upon molding. Accordingly, the plastic materials are required to reduce birefringence based on molecular orientation and mechanical strain so as to permit the molding of optical lenses little in optical strain. Further, the plastic materials are required not to cause such problems that an optical lens cracks upon molding. As described above, the molding materials for plastic lenses are required to have excellent molding and processing ability that neither birefringence nor cracking is caused.
However, there has heretofore not been known any hydrogenated product of an aromatic vinyl polymer, which is far excellent in strength properties and heat resistance and low in birefringence, and has excellent molding and processing ability that neither birefringence nor cracking is caused upon molding.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a molding material for plastic lenses, which is far excellent in strength properties and heat resistance and low in birefringence, and has excellent molding and processing ability that neither birefringence nor cracking is caused upon molding.
Another object of the present invention is to provide a plastic lens molded from the molding material for plastic lenses having such excellent various properties.
A further object of the present invention is to provide a hydrogenated product of an aromatic vinyl polymer, which has such excellent various properties in combination and is suitable for use as optical materials including a molding material for plastic lenses, and a production process thereof.
The present inventors have carried out an extensive investigation with a view toward solving overcoming the above-described problems involved in the prior art. As a result, it has been found that hydrogenated products obtained by hydrogenating an aromatic vinyl polymer and having a hydrogenation rate (degree of hydrogenation) of aromatic rings as high as at least 97%, a weight average molecular weight within a comparatively high range and a narrow molecular weight distribution exh

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