Heat-shrinkable polyester film
The present invention relates to a heat-shrinkable polyester film, which has a heat-shrinkage of 20% or more in both the longitudinal and transverse directions when treated with 80° C. water for 30 seconds and a bidirectional balance parameter in the range of 0.25 to 2.5 and exhibits an excellent properties in die-cutting and removing from a container such as a glass bottle for recycling by washing with hot water.
1. A heat-shrinkable polyester film having a heat-shrinkage of 20% or more in both the longitudinal and transverse directions when treated with 80° C. water for 30 seconds, and a bidirectional balance parameter A of formula (I) in the range 1.16 to 1.19:
A =( M D −M S )/( T D −T S ) (1)
in which, A is the bidirectional balance parameter; M D and T D are elongations (%) at break in the longitudinal and transverse directions, respectively; M S and T S are strengths (kg/mm 2 ) at break in the longitudinal and transverse directions, respectively, A being calculated with the numerical values of M D , M S , T D , and T S as defined above, disregarding the units thereof,
wherein the heat-shrinkable polyester film is drawn in both the longitudinal and transverse directions at a drawing ratio of 2.5 to 4.5,
the difference of the drawing ratios in the longitudinal and transverse directions is 1.0 or less, and
the heat-shrinkable polyester film is (i) drawn in the longitudinal direction, (ii) drawn in the transverse direction, and (iii) further drawn in the transverse direction after being passed through a neutral section.
2. The heat-shrinkable polyester film of claim 1 , wherein the feedstock polyester resin is prepared by reacting an acid component selected from the group consisting of terephthalic acid, isophthalic acid, and naphthalene dicarboxylic acid with a diol component selected from the group consisting of ethylenediol, propanediol, butanediol, hexanediol, ethylene glycol, neopentyl glycol, and 1,4-cyclohexanedimethanol.
3. The heat-shrinkable polyester film of claim 1 , which comprises a polyethylene terephthalate repeating unit in an amount of 60 to 90 wt % based on the total weight of the film.
4. The heat-shrinkable polyester film of claim 1 , which is drawn at a temperature in the range from 80 to 120° C.
5. The heat-shrinkable polyester film of claim 1 , which is prepared by drawing and then cooling at a temperature which is lower than the glass transition temperature (Tg) of the feedstock polyester resin.