WO2005012403A1

WO2005012403A1 - Packaging polyester film with easy opening property

Info

Publication number: WO2005012403A1
Application number: PCT/KR2004/001830
Authority: WO
Inventors: Sang-Il Kim; Chang-Ju Lee; Yong-Won Kim
Original assignee: SKC Co Ltd
Current assignee: SKC Co Ltd
Priority date: 2003-08-05
Filing date: 2004-07-22
Publication date: 2005-02-10
Anticipated expiration: 2006-02-05
Also published as: KR100560214B1; KR20050015304A

Abstract

The present invention relates to a packaging polyester film prepared by biaxially drawing a polyester sheet having a glass transition temperature (Tg) of 60 °C or higher, followed by heat-treating, the packaging film having a refractive index of 1.64 or less, a strength of 6 kgf/mm² or more and an energy at break of 90 kgf ·mm or less in the longitudinal direction, and a birefringence of 0.03 or more. A package made of the inventive polyester film having no notch can be easily opened.

Description

PACKAGING POLYESTER FILM WITH EASY OPENING PROPERTY

FIELD OF THE INVENTION The present invention is directed to a packaging polyester film having an easy opening property.

BACKGROUND OF THE INVENTION Packaging films having easy opening properties have been used for packaging beverages, foods or various daily products. Conventional packaging films such as cellophane films and biaxially oriented polypropylene films are associated with various environmental and hygienic problems, and accordingly, there have been attempts to use polyester films having good mechanical and chemical properties, which suffer from no environmental or food hygienic problems. However, a conventional biaxially drawn polyester film still needs a notching process to confer an easy opening property thereto.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a polyester film having improved easy opening property, printability and processibility in addition to good mechanical and chemical properties, environmental acceptability and food sanitary safety. In accordance with the present invention, there is provided a packaging polyester film prepared by biaxially drawing a polyester sheet having a glass transition temperature (Tg) of 60 °C or higher, followed by heat-treating, the packaging film having a refractive index of 1.64 or less, a strength of 6 kgf/mnf or more and an energy at break of 90 kgf mm or less in the longitudinal direction, and a birefringence of 0.03 or more. DETAILED DESCRIPTION OF THE INVENTION

The packaging polyester film according to the present invention is characterized by having a refractive index of 1.64 or less, a strength of 6 kgf/mn or more together with an energy at break of 90 kg mm or less in the longitudinal direction, and a birefringence of 0.03 or more. The polyester resin suitable for use in preparing the packaging polyester film of the present invention may be made from a polyester prepared by polycondensing an acid component comprising an aromatic dicarboxyhc acid with a glycol component comprising an alkylene glycol, melting and extruding as an amorphous sheet. In a preferred embodiment, the aromatic dicarboxyhc acid is dimethyl terephthalate or terephthalic acid. It is also preferable that the acid component further comprises other aromatic dicarboxyhc acid such as isophthalic acid, naphthalene dicarboxyhc acid, dimethyl isophthalate, dimethyl-2,6-naphthalene dicarboxylate, etc. In a preferred embodiment, the alkylene glycol is ethylene glycol. It is also preferable that the alkylene glycol further comprises other alkylene glycol component such as dimethylene glycol, trimethylene glycol, teteramethylene glycol, 1,4-cyclohexane dimethanol or 2,2-dimethyl(- 1,3 -propane) diol. Tg of the polyester resin used in the present invention should be 60°C or higher. If Tg of the polyester resin is less than 60°C, mechanical and thermal properties of the polyester become unsatisfactory. The polyester resin sheet used in the present invention may be prepared from a blend of polyester resins obtained by polymerizing the aforementioned aromatic dicarboxyhc acid and glycol components. According to the present invention, the polyester resin sheet is preferably biaxially drawn in the longitudinal direction at a draw ratio of 1.2 to 2.5 and in the transverse direction at a draw ratio of 3.0 to 5.0 during the film making process to provide a film having satisfactory mechanical strengths in both directions. Drawing conditions such as drawing ratio or drawing temperature influence the refractive index of the resulting polyester film. The refractive index represents the degree of orientation of the molecular structure, which, in turn, determines the mechanical properties such as strength or elongation of the polyester film. The inventive polyester film has a refractive index in the longitudinal direction of 1.64 or less and an energy at break in the longitudinal direction of 90 kgf mm or less and can be easily cut in the longitudinal direction. Also, the inventive polyester film has a birefringence of 0.03 or more.

The birefringence of a polyester film is the value obtained by subtracting the refractive index in the longitudinal direction from that in the transverse direction, and it preferably is 0.03 or more to obtain sufficient strength in the transverse direction. The inventive polyester film has a strength in the longitudinal direction of

6 kgf/mnf or more, preferably 7 to 15 kgf/muf to prevent breakage of the film during the film making or packaging step. Further, it is preferable that the specific gravity of the polyester film is 1.372 or more to provide a proper strength and elongation in the longitudinal direction. The polyester film according to the present invention is further heat-treated at a temperature which is higher than Tg of the polyester sheet by 55°C or more to obtain a good cutting property in the longitudinal direction and sufficient stiffness in the transverse direction. Meanwhile, the inventive polyester film has a thickness of 8 to 25 and a longitudinal elongation of 100 % or less, preferably 80 % or less. If the longitudinal elongation is more than 100 % in such a thickness range, the film cannot be easily cut. The polyester film according to the present invention may further comprise other components such as a polycondensation catalyst, dispersant, electrostatic - A -

generator, antistatic agent, sun-bloking agent, antiblocking agent and inorganic lubricant. Also, the inventive polyester film may be corona-treated by a conventional method to improve the printability thereof. Thus, the inventive polyester films for packaging has good easy opening property which can be easily cut without notching, in addition to good printability and processibility.

The present invention is further described and illustrated in Examples, which are, however, not intended to limit the scope of the present invention.

The polyester films manufactured in examples were examined for the following properties.

(1) Specific Gravity: Density gradient tube method (ASTM D1505, JIS K7112)

(2) Refractive Index and Birefringence: Refractive indexes in the longitudinal and transverse directions of film were measured with Abbe Refractor

Birefringence = R.I. (TD) - R.I. (LD) wherein R.I. (TD) is the refractive index in the transverse direction and R.I. (LD) is the refractive index in the longitudinal direction

(3) Glass transition temperature : Differential Scanning Calorimeter (Perkin-Elmer DSC-7) Temperature rising 20 °C/min (4) Film Strength, Elongation and Energy at break (ASTM D882): Strength and Elongation at break were measured using 50mm x 15mm specimen at a rate of 200rnm/min and at room temperature (23 ±2 °C). An Energy at break is a value integrating the area of load-displacement curve in a tensile test.

(7) Film thickness difference: The film thickness was measured at 50 points which were arranged at 5 cm intervals in both the longitudinal and transverse directions. Thickness difference (%) = (T^ -T^) /T_ave X 100 wherein T,^ is maximum thickness of film; T_π^ is minimum thickness of film; and T_ave is average thickness of film.

Example 1

100 parts by mole of dimethyl terephthalate and 180 parts by mole of ethylene glycol were placed in an autoclave equipped with a distillation column, heated to 150°C, and manganese acetate (an interesterification catalyst) was added thereto in an amount of 0.05 % by weight based on the weight of dimethyl terephthalate. While removing methanol formed, the temperature was raised to 220°C over a period of 120 min. After the interesterification was complete, trimethylphosphate (a stabilizer) was added in an amount of 0.045 % by weight based on the weight of dimethyl terephtalalte, and silicon dioxide having an average particle diameter of 2 micron (an antiblocking agent), in an amount of 0.2 % by weight based on the amount of dimethyl terephthalate. After 10 minutes, antimonytrioxide (polymerization catalyst) was added in an amount of 0.03 % by weight based on the weight of dimethyl terephthalte, and the resulting mixture was transferred to a second reactor equipped with a vacuum unit, and reacted at 280°C for about 140 minutes, to obtain a polyethylene terephthalate (Tg 78°C) having an intrinsic viscosity of 0.65 dl/g.

The polyester thus obtained was melted at 280°C, extruded through a conventional T-die and cooled by a casting roller maintained at 30°C, to obtain an amorphous sheet of 92 μm thickness. The amorphous sheet was drawn 1.5 times in the longitudinal direction at 95 °C and drawn 4.1 times in the transverse direction at 110°C, and then conducted heat-treatment at 170°C, followed by relaxation to a degree of 2 %, to obtain a biaxially drawn polyester film of 15 μm thickness.

The drawing conditions for the polyester sheet obtained above are shown in Table 1, and the properties of the resulting biaxially drawn films are shown in Table 2.

Examples 2 to 6 and Comparative Examples 1 to 6

The procedure of Example 1 was repeated except that various drawing conditions shown in Table 1 were used. The resulted properties of the biaxially drawn polyester films are shown in Table 2.

Table 1

Table 2

As shown in Table 2, the film of Examples 1 through 6 show improved properties suitable for easy opening package, while the films of Comparative Examples 1 through 6 were evaluated to be poor.

While the invention has been described with respect to the above specific embodiments, it should be recognized that various modifications and changes may be made to the invention by those skilled in the art which also fall within the scope of the invention as defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A packaging polyester film prepared by biaxially drawing a polyester sheet having a glass transition temperature (Tg) of 60 °C or higher, followed by heat-treating, the packaging film having a refractive index of 1.64 or less, a strength of 6 kgf/mnf or more and an energy at break of 90 kgf mm or less in the longitudinal direction, and a birefringence of 0.03 or more.

2. The packaging polyester film of claim 1, wherein the biaxially drawing process is conducted in the longitudinal direction at a draw ratio of 1.2 ~ 2.5 and in the transverse direction at a draw ratio of 3.0 ~ 5.0

3. The packaging polyester film of claim 1, wherein the heat-treating process is conducted at a temperature which is higher than Tg of the polyester sheet by 55°C or more.

4. The packaging polyester film of claim 1, wherein the strength in the longitudinal direction is 7 to 15 kgf/mnf.

5. The packaging polyester film of claim 1, which has a specific gravity of 1.372 or more.

6. The packaging polyester film of claim 1, which has a thickness of 8 to 25 μm and a longitudinal elongation of 100 % or less.

7. The packaging polyester film of claim 6, wherein the longitudinal elongation is 80 % or less.