Heat-shrinkable polyester film and preparation method thereof

1. A heat-shrinkable polyester film having a thermal shrinkage ratio of 15% or more in the longitudinal direction when treated at 100° C. for 10 seconds, a tensile strength at rupture in the longitudinal direction of 7 kgf/mm 2 or more, and a difference between elongations (%) at the hardening and yield points in the longitudinal direction of 15% or more.

2. The film of claim 1 , which is a uni-directionally shrinkable film.

3. The film of claim 1 , which is drawn in the longitudinal direction at a total drawing ratio of 1.2 to 7.

4. The film of claim 1 , which is composed of a polyester resin which comprises

60 to 97% by mole of ethylene terephthalate repeating unit; and

3 to 40% by mole of a repeating unit obtained by copolymerization of a component selected from the group consisting of terephthalic acid, dimethyl terephthalate, naphthalenedicarboxylic acid and a mixture thereof with a branched alkylene glycol component having a carbon number of 3 or more.

5. The film of claim 4 , wherein the polyester resin further comprises 20% or less by mole of a repeating unit obtained by copolymerization of a component selected from the group consisting of terephthalic acid, dimethyl terephthalate, naphthalenedicarboxylic acid and a mixture thereof with a linear alkylene glycol component having a carbon number of 3 or more.

6. The film of claim 5 , wherein the linear alkylene glycol component is selected from the group consisting of 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol and a mixture thereof.

7. The film of claim 4 , wherein the branched alkylene glycol component is selected from the group consisting of 1,2-octanediol, 1,3-octanediol, 2,3-butanediol, 1,3-butanediol, 2,2-dimethyl-1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol, 2,2-diethyl-1,5-petanediol, 2,4-diethyl-1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,1-dimethyl-1,5-pentanediol and a mixture thereof.

8. A method for preparing the heat-shrinkable polyester film according to claim 1 , comprising the steps of:

i) melt-extruding a polyester resin at 200 to 300° C.;

ii) casting the extruded resin with cooling, to produce an undrawn sheet; and

iii) drawing the undrawn sheet in the longitudinal direction at a total drawing ratio of 1.2 to 7.

9. The method of claim 8 , wherein the drawing is conducted using at least two rolls having different peripheral velocities, the peripheral velocity ratio of the rear drawing roll to the front drawing roll being 1.1 to 6.5.

10. A wrapping material comprising the heat-shrinkable polyester film according to claim 1 .