FIELD

The present invention relates to a thermoplastic resin that has a high refractive index, and that is able to lower birefringence and to balance heat resistance and moldability.

BACKGROUND

Imaging modules are used in cameras, video cameras, camera-equipped cellular phones, videophones, camera-equipped door phones, and the like. In recent years, downsizing has been a particular requirement for the optical systems used in such imaging modules. However, downsizing of an optical system leads to the major problem of chromatic aberration in the optical system. It is known that chromatic aberration can be compensated for by using a combination of an optical lens material having a high refractive index for the optical lens and a reduced Abbe number for high dispersion, with an optical lens material having a low refractive index and increased Abbe number for low dispersion.

The types of glass conventionally used as materials for optical systems allow the various required optical characteristics to be realized while also having excellent environmental resistance, but their poor workability has been a problem. Therefore, resins that are cheaper than glass materials and have superior workability have come to be used in optical parts. In particular, resins having a fluorene backbone or binaphthalene backbone are used for their high refractive indexes. High refractive index resins with a refractive index of 1.64, using 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene, are described in PTLs 1 and 2, for example. However, their refractive indexes are insufficient, and even higher refractive indexes are in demand. PTL 3 describes a thermoplastic resin comprising 9,9-bis[4-(2-hydroxyethoxy)-3-phenylphenyl]fluorene. The resin described in this patent document, however, is also in need of improvement in terms of the refractive index.

CITATION LIST

Patent Literature

[PTL 1] WO2007/142149

[PTL 2] Japanese Unexamined Patent Publication No. H7-198901

[PTL 3] Japanese Unexamined Patent Publication No. 2015-86265

SUMMARY

Technical Problem

It is an object of the present invention to provide a thermoplastic resin that has a high refractive index, and that is able to lower birefringence and to balance heat resistance and moldability.

Solution to Problem

As a result of much diligent research with the aim of achieving this object, the present inventors have devised this invention upon finding that a thermoplastic resin comprising a specific compound with an aromatic group introduced into a fluorene backbone is able to specifically increase the refractive index. The present inventors have also found that with this thermoplastic resin, it is possible to maintain the high refractive index while reducing birefringence and achieving a high balance between heat resistance and moldability.

Specifically, the present invention provides the following.

1. A thermoplastic resin that includes a repeating unit represented by the following formula (1).


wherein the rings Z represent (the same or different) aromatic hydrocarbon rings, R1and R2each independently represent a hydrogen atom, a halogen atom or a hydrocarbon group of 1 to 12 carbon atoms optionally containing an aromatic group. Ar1and Ar2represent an aromatic group of 6 to 10 carbon atoms optionally containing a substituent, L1and L2each independently represent a divalent linking group, j and k each independently represent an integer of 0 or greater, m and n each independently represent 0 or 1, and W is at least one group selected from groups represented by the following formula (2) or (3).


wherein X represents a divalent linking group.

2. The thermoplastic resin according to 1. above, wherein formula (1) is at least one selected from the group consisting of units represented b the following formulas (1a) to (1d).


wherein R3to R10each independently represent a hydrogen atom, a halogen atom or a hydrocarbon group of 1 to 12 carbon atoms optionally containing an aromatic group. Ar1and Ar2, L1and L2, j and k, m and n and W are the same as in formula (1) above.

3. The thermoplastic resin according to 2. above, wherein formula (1) is at least one selected from the group consisting of units represented by formulas (1b) to (1d) above.

4. The thermoplastic resin according to 2. above, wherein formula (1) is formula (1b).

5. The thermoplastic resin according to any one of 1. to 4. above, wherein Ar1and Ar2in formula (1) are phenyl groups.

6. The thermoplastic resin according to any one of 1. to 4. above, wherein Ar1and Ar2in formula (1) are naphthyl groups.

7. The thermoplastic resin according to any one of 1. to 6. above, wherein L1and L2in formula (1) are ethylene groups, and m and n are 1.

8. The thermoplastic resin according to 4. above, wherein Ar1and Ar2in formula (1b) are phenyl groups, L1and L2are ethylene groups, m and n are 1, and R3to R10are hydrogen atoms.

9. The thermoplastic resin according to any one of 1. to 8. above, wherein X in formula (3) includes at least one selected from the group consisting of phenylene group, naphthalenediyl group, groups represented by the following formula (4) and groups represented by the following formula (5), as a repeating unit.


wherein R11and R12each independently represent a hydrogen atom, a hydrocarbon group of 1 to 12 carbon atoms optionally containing an aromatic group, or a halogen atom.

10. The thermoplastic resin according to any one of 1. to 9. above, which includes at least one selected from the group consisting of units represented by the following formulas (6) to (8), as a repeating unit.


wherein R13and R14each independently represent a hydrogen atom, a hydrocarbon group of 1 to 12 carbon atoms optionally containing an aromatic group, or a halogen atom.


wherein R15and R16each independently represent a hydrogen atom, a hydrocarbon group of 1 to 12 carbon atoms optionally containing an aromatic group, or a halogen atom.


wherein R17and R18each independently represent a hydrogen atom, a hydrocarbon group of 1 to 12 carbon atoms optionally containing an aromatic group, or a halogen atom, and Y is a single bond or a divalent linking group.

11. The thermoplastic resin according to any one of 1. to 10. above, wherein the relative viscosity is 0.12 to 0.40.

12. The thermoplastic resin according to any one of 1. to 11. above, wherein the glass transition temperature is 140 to 185° C.

13. The thermoplastic resin according to any one of 1. to 12. above, wherein the refractive index is 1.660 to 1.730.

14. An optical member comprising a thermoplastic resin according to any one of 1. to 13. above.

15. The optical member according to 14. above, which is an optical lens.

Advantageous Effects of Invention

The thermoplastic resin of the invention has a high refractive index and is able to lower birefringence and to balance heat resistance and moldability, and it therefore exhibits an exceptional industrial effect.

