Polymer compositions for shrink-wrap films

1. A star-shaped block copolymer A having:

two short branches consisting of a single copolymer block (B/S) Ai made from 65 to 95 wt.-% vinylaromatic monomers and 35 to 5 wt.-% dienes and having a glass transition temperature Tg A in the range from 40 to 90° C., and

two long branches of the structure S t -[(B/S) A ] n -(B/S) Ai or [(B/S) A ] n -(B/S) Ai , linked by way of the inner blocks (B/S) Ai , where the block S t is made from 95 to 100 wt.-% of vinylaromatic monomers and 0 to 5 wt.-% of dienes; the block [(B/S) A ] n consists of one or more different or identical copolymer blocks (B/S) A , each made from 65 to 95 wt.-% vinylaromatic monomers and 35 to 5 wt.-% dienes and have a glass transition temperature Tg A in the range from 40 to 90° C.; n is a regular number of at least 1, and the block (B/S) Ai is as defined above, wherein

the block (B/S) Ai has a number average molar mass M n in the range of from 5000 to 15000 g/mol and the entire block [(B/S) A ] n has a number average molar mass M n of 50000 to 150000 g/mol.

2. The star-shaped block copolymer A according to claim 1 , wherein the number-average molar mass Mn of the block S t is in the range from 3000 to 8000 g/mol.

3. The star-shaped block copolymer A according to claim 1 , wherein the copolymer blocks (B/S) A and (B/S) Ai are composed of polymerized vinylaromatic monomers and of dienes with random distribution.

4. The star-shaped block copolymer A according to claim 1 , wherein the block [(B/S) A ] n consists of 2 to 10 different copolymer blocks (B/S) A , where the blocks (B/S) A differ in their molar masses and/or in their vinylaromatic/diene ratio.

5. The star-shaped block copolymer A according to claim 1 , wherein the number-average molar mass Mn of the copolymer block (B/S) Ai is in the range of from 6000 to 12000 g/mol.

6. The star-shaped block copolymer A according to claim 1 , wherein the copolymer blocks (B/S) A , and/or (B/S) Ai are made from 85 to 93 wt.-% of a vinylaromatic monomer and from 7 to 15 wt.-% of a diene.

7. The star-shaped block copolymer A according to claim 1 of the structure:

where S t , and (B/S) Ai are as defined above, (B/S) A1 and (B/S) A2 are two different copolymer blocks (B/S) A as defined above, and X is a coupling center, which is formed by reaction of the living anionic polymer chain with a polyfunctional coupling agent.

8. A process for the preparation of the star-shaped block copolymer A according to claim 1 , characterized by:

a) a double initiation,

b) a coupling step after the last addition and polymerization of a vinylaromatic monomer and a diene, and

c) the second initiation process placed before the last addition and polymerization of a vinylaromatic monomer and a diene.

9. A polymer composition comprising components (a) and optionally (b), (c), and (d):

a) 45 to 100 wt.-% of component a) consisting of:

a1) 20 to 80 wt.-% of at least one star-shaped block copolymer A according to claim 1 , and

a2) 80 to 20 wt.-% of at least one star-shaped block copolymer B, which has

short branches of structure S e -(B/S) B and at least one long branch of structure (B/S) Ae -S i -(B/S) B , linked by way of the blocks (B/S) B , or

short branches of structure S e -(B/S) B -S s and at least one long branch of structure (B/S) Ae -S i -(B/S) B -S s ; linked by way of the blocks S s ;

wherein the polymer blocks S e and S i are identical; the polymer blocks S e , S i , and S s are made from 95 to 100 wt.-% of vinylaromatic monomers and of from 0 to 5 wt.-% of dienes; the copolymer block (B/S) Ae is made from 65 to 95 wt.-% vinylaromatic monomers and 35 to 5 wt.-% dienes and has a glass transition temperature Tg A in the range from 40 to 90° C.; and the copolymer blocks (B/S) B are each made from 20 to 60 wt.-% vinylaromatic monomers and 80 to 40 wt.-% dienes and have a glass transition temperature Tg B in the range from −80° to 0° C.;

b) 0 to 55 wt.-% of at least one further thermoplastic polymer other than block copolymers A and B;

c) 0 to 0.8 wt.-% of at least one plasticizer; and

d) 0 to 3 wt.-% of at least one further additive or processing aid different from c);

where the total amount of components (a) and, if appropriate, (b), (c), and (d) is 100% by weight and the glass transition temperature Tg is determined by DSC based on DIN EN ISO 11357-2:2014-07, with evaluation of the 2nd heating cycle after quick cooling from 180° C. with a heating rate of 20 K/min.

10. The polymer composition according to claim 9 comprising 0.10 to 0.80 wt.-% of component c).

11. The polymer composition according to claim 9 , wherein the number-average molar mass Mn of the copolymer block (B/S) Ae of block copolymer B is in the range from 30000 to 100000 g/mol.

12. The polymer composition according to claim 9 , wherein the blocks Se and Si have a number-average molar mass Mn in the range from 5000 to 30000 g/mol.

13. The polymer composition according to claim 9 , wherein the number-average molar mass Mn of the copolymer blocks (B/S) B is in the range of from 5000 to 40000 g/mol.

14. The polymer composition according to claim 9 , wherein the number average molar mass Mn of the block S s is lower than 2500 g/mol.

15. The polymer composition according to claim 9 , wherein the copolymer blocks (B/S) Ae and (B/S) B are composed of polymerized vinylaromatic monomers and of dienes with random distribution.

16. The polymer composition according to claim 9 , wherein the proportion of the entirety of all of the blocks (B/S) B or (B/S) B -S s of the star-shaped block copolymer B is from 30 to 37 wt.-%, based on the entire star-shaped block copolymer B.

17. The polymer composition according to claim 9 , wherein the star-shaped block copolymers B have the following structures:

where S e , S i , S s , (B/S) Ae and (B/S) B are as defined above and X is a coupling center, which is formed by reaction of the living anionic polymer chain ends with a polyfunctional coupling agent.

18. A process for the preparation of the polymer composition according to claim 9 by melt mixing of component a) and optional components b), c), and d) by aid of a mixing apparatus at a temperature in the range of from 160° C. to 300° C.

19. A method of using the polymer composition according to claim 9 for the production of films.

20. Shrink film produced from the polymer composition according to claim 9 .