HEAT-SHRINKABLE POLYESTER FILM, AND RETORT FOOD INCLUDING SAME
A polyester film according to an embodiment is characterized in that the shrinkage properties in a direction perpendicular to a main shrinkage direction are adjusted under certain heat treatment conditions, and thus further deformation of the polyester film can be suppressed even when the polyester film goes through a harsh retort process after being tightly adhered to a container by means of heat shrinkage. Thus, the polyester film according to the embodiment can be effectively applied, as a heat-shrinkable label or packing material, to containers of various products including retort food.
1 . A polyester film, which comprises a copolymerized polyester resin in which two or more dials and an aromatic dicarboxylic acid are polymerized, wherein when the main shrinkage direction is a first direction, and when a direction perpendicular to the main shrinkage direction is a second direction, ΔL % defined by the following equation is 2% or less in the second direction:
Δ L %=[( L 1− L 2)/ L 0]×100
in the above equation, L 0 is the initial dimension, L 1 is the dimension upon first thermal treatment for 0.1 minute under the hot air condition of 250° C., and L 2 is the dimension upon second thermal treatment for 30 minutes under the conditions of 125° C. and 1.4 atm after the first thermal treatment.
2 . The polyester film of claim 1 , wherein when the shrinkage rate (%) of the polyester film upon thermal treatment at a temperature of VC for 10 seconds is defined as T x , the following Relationship (1) in the second direction is satisfied:
5%≤ T 100 −T 95 −T 90 ≤15% (1)
3 . The polyester film of claim 1 , wherein when the shrinkage rate (%) of the polyester film upon thermal treatment at a temperature of VC for 10 seconds is defined as T x , the following Relationship (2) in the second direction is satisfied:
T 100 <0%< T 125 (2)
4 . The polyester film of claim 1 , wherein the polyester film has a maximum expansion rate of 4% to 13% at a temperature of 90° C. to 1100° C. in the second direction.
5 . The polyester film of claim 1 , wherein when the polyester film is fixed in the first direction and thermally treated at 90° C. for 10 seconds, SR % according to the following equation is 10% or less:
SR %=[( x 1− x 2)/ y]× 100
in the above equation, x 1 is the dimension (mm) before thermal treatment in the second direction, x 2 is the dimension (mm) after thermal treatment in the second direction, and y is the dimension (mm) in the first direction.
6 . The polyester film of claim 1 , wherein once two sheets of the polyester film have been adhered by tetrahydrofuran (THE), the peel strength is 300 gf/in or more.
7 . The polyester film of claim 1 , wherein the diol comprises ethylene glycol in an amount of 65% by mole to 83% by mole and diethylene glycol in an amount of 1% by mole to 15% by mole.
8 . The polyester film of claim 1 , wherein the diol comprises at least one selected from neopentyl glycol and cyclohexanedimethanol in an amount of 20% by mole to 30% by mole, and the content of isophthalic acid in the aromatic dicarboxylic acid may be less than 1% by mole.
9 . The polyester film of claim 1 , wherein the polyester film is heat-set at 85′C to 100° C.
10 . The polyester film of claim 1 , wherein the polyester film is for use as a heat-shrinkable label or packaging material.
11 . Retort food, which comprises the polyester film of claim 1 as a heat-shrinkable label or packaging material.