Polyester composition, polyester film, and magnetic recording medium

1. A copolyester comprising:

(A) an aromatic dicarboxylic acid component;

(B) an alkylene glycol component; and

(C1) a dimer acid component and/or (C2) a dimer diol component,

wherein the copolyester contains, with reference to a molar number of a total dicarboxylic acid component, 0.5 to 3.5 mol % of the dimer acid component (C1) and/or 0.3 to 4.0 mol % of the dimer diol component (C2), and wherein the dimer acid component (C1) and/or the dimer diol component (C2) has an average carbon number of at least 20.

2. The copolyester according to claim 1 , wherein the dimer acid component (C1) and/or the dimer diol component (C2) are/is derived from a mixture of an aliphatic dicarboxylic acid component and/or aliphatic diol component having an average carbon number of 20 to 55.

3. The copolyester according to claim 1 , comprising repeating units (I) to (III):

—C(O)—R A —C(O)—  (I)

—C(O)—R B —C(O)—  (II)

—O—R C —O—  (III)

wherein the repeating unit (I) is the dimer acid component (C1), and R A is an alkylene group which has 31 to 51 carbon atoms, and may contain a cyclo ring and/or a branched chain,

the repeating unit (II) is the aromatic dicarboxylic acid component (A), and R B is a phenylene group or a naphthalenediyl group, and

the repeating unit (III) is the alkylene glycol component (B), and R C is an alkylene group having 2 to 6 carbon atoms.

4. The copolyester according to claim 1 , wherein

the copolyester contains, with reference to a molar number of the aromatic dicarboxylic acid component, 0.3 to 4.0 mol % of the dimer diol component (C2),

the aromatic dicarboxylic acid component (A) is aromatic dicarboxylic acid having 6 or more carbon atoms,

the alkylene glycol component (B) is alkylene glycol having 2 to 4 carbon atoms, and

the dimer diol component (C2) is aliphatic dimer diol having 31 to 50 carbon atoms.

5. The copolyester according to claim 1 , wherein

the copolyester comprises the dimer diol component (C2), and

the dimer diol component (C2) comprises a branched chain and/or a cyclohexane ring structure.

6. A polyester film comprising the copolyester according to claim 1 .

7. The polyester film according to claim 6 , wherein the polyester film has a temperature expansion coefficient of 14 ppm/° C. or less in at least one direction in the plane direction of the polyester film.

8. The polyester film according to claim 6 , wherein the polyester film has a humidity expansion coefficient of 1 to 8.5 ppm/% RH in at least one direction in the plane direction of the polyester film.

9. A laminated polyester film comprising:

a film layer A which comprises a polyester film; and

a film layer B which comprises a polyester film and is laminated over the film layer A,

wherein at least one of the film layer A and the film layer B is the polyester film according to claim 6 .

10. The laminated polyester film according to claim 9 , wherein a content (W AB ) of the dimer acid component (C1) is within a range of 0.5 to 3.5 mol % or a content (W AB ) of the dimer diol component (C2) is within a range of 0.5 to 5.0 mol %, with reference to a molar number of a total dicarboxylic acid component in a whole of the laminated polyester film.

11. The laminated polyester film according to claim 9 , wherein a difference between a content (W A ) of the dimer acid with reference to a molar number of a total dicarboxylic acid component in the film layer A and a content (W B ) of the dimer acid with reference to a molar number of a total dicarboxylic acid component in the film layer B is 0.3 mol % or more.

12. The laminated polyester film according to claim 9 , wherein a difference between a content (W A ) of the dimer diol with reference to a molar number of a total dicarboxylic acid component in the film layer A and a content (W B ) of the dimer diol with reference to a molar number of a total dicarboxylic acid component in the film layer B is 0.3 mol % or more.

13. The laminated polyester film according to claim 9 , wherein a difference between surface roughness (Ra A ) of the film layer A and surface roughness (Ra B ) of the film layer B is 1.0 nm or more.

14. The laminated polyester film according to claim 9 , wherein at least one film layer of the film layers A and B contains, with reference to a mass of the film layer, 0.5 to 25% by mass of at least one selected from the group consisting of polyimide, polyether imide, polyether ketone, polyether ether ketone, and a mixture thereof in addition to the copolyester.

15. The laminated polyester film according to claim 9 , wherein the laminated polyester film is used for a base film of a magnetic recording medium.

16. A magnetic recording medium comprising:

the laminated polyester film according to claim 9 ; and

a magnetic layer formed on one surface of the laminated polyester film.

17. The laminated polyester film according to claim 9 , wherein a ratio (dA/dB) of a thickness (dA) of the film layer A and a thickness (dB) of the film layer B is 0.03 to 33.

18. The laminated polyester film according to claim 9 , wherein the laminated polyester film has a temperature expansion coefficient of 14 ppm/° C. or less in at least one direction in the plane direction of the laminated polyester film.

19. The laminated polyester film according to claim 9 , wherein the laminated polyester film has a humidity expansion coefficient of 1 to 8.5 ppm/% RH in at least one direction in the plane direction of the laminated polyester film.