FmLD OF THE INVENTION

The present invention relates to a heat-shrinkable polyester film, which is suitable for wrapping a container such as a glass bottle.

BACKGROUND OF THE INVENTION

A heat-shrinkable film has been mainly used for labeling a glass or plastic bottle because of its properties that are suitable for printing various articles to attract consumers' attention and for packaging a bundle of goods as well as sealing caps.

Heat-shrinkable films made of soft polyvinyl chloride (PVC) have recently become disfavored because they exhibit a limited maximum heat-shrinkage ratio and emit toxic pollutants, e.g., dioxin, on combustion. Oriented polystyrene (OPS) firms, on the other hand, have uniform shrinking properties and low specific gravities, and they can be easily removed from PET bottles for recycling, but they have the problem of poor heat-resistance. Therefore, such oriented polystyrene firms are unsuitable for a high rate-shrinkage process or high temperature-filling process.

Heat-shrinkable polyester films which have improved shrinking properties and heat-resistance as compared with those of PVC and OPS firms have been developed for wrapping a glass bottle. However, the shrinkage rates of such polyester films are generally unacceptably high, which results in non-uniform shrinkage when subjected to a thermal shrinkage process using hot air, which has made it necessary to use a steam-heating type shrinking machine to prevent non-uniform shrinkage thereof, in addition to the problem that the shrinkage stress of such polyester firm is unacceptably high, which often leads to non-uniform shrinkage with consequential distortion, end-bending or rupture, especially when subjected to a secondary thermal shrinkage process, e.g., a sterilization or high temperature-filling process.

In order to solve the above problems, Korean Patent Publication No. 2004-37126 discloses that the shrinkage uniformity of a heat-shrinkable polyester film that does not show whitening, stain, wrinkle, or distortion after shrinkage, can be improved by incorporating therein 1,4-cyclohexanedimethanol and neopentyl glycol in specific amounts each in the range of 12 to 40 % by mole. But, the film prepared by the above method suffers from the above-mentioned problems associated with the secondary thermal shrinkage process, e.g., a sterilization or high temperature-filling process. In addition, Korean Patent Publication No. 2003-84879 discloses a heat-shrinkable polyester film having good cracking-resistance along the oriented direction, which is obtained by controlling the refractive indices of both the longitudinal and transverse directions of the film. Although such heat-shrinkable films show some improvements in terms of cracking-resistance when subjected to a first thermal shrinkage step for packaging, they still suffer from low cracking-resistance and distortion along the undrawn direction when reheated in a subsequent sterilization or high temperature-filling process.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a heat-shrinkable polyester film having improved performance characteristics in terms of uniform heat-shrinkage, printability, mechanical strength, heat-resistance, shrinkage rate, and rupture or distortion resistance after a secondary thermal shrinkage process.

In accordance with one aspect of the present invention, there is provided a heat-shrinkable polyester film comprising a divalent acid component and a diol component, wherein the diol component comprises a diol of formula (I) in an amount ranging from 5 to 95 % by mole, and the film has a residual shrinkage stress of 55 kgf/cnf or less after being dipped in 90 ^° C water for 1 min, and a thermal shrinkage ratio of 40 % or more along the main shrinking direction when treated with 90 °C or 100 ^° C water for 10 seconds:

R ₁

HO-CH ₂ -C-CH ₂ -OH

R2 (I)

wherein, R ₁ and R ₂ are each independently ethyl, propyl, or butyl.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a detailed description of the present invention is given. Examples of the diol component of formula (I) usable in the present invention include 2-butyl-2-ethyl-l,3-propanediol(BEPD), etc.

The polyester film of the present invention comprises the diol component in an amount ranging from 5 to 95 % by mole, preferably from 10 to 25 % by mole. When the amount of the diol component is less than 5 % by mole, an unsatisfactory shrinkage ratio may result and the resultant film wrapped around a container as a label may easily rapture by an external impact due to excessive crystallinity of the film during a heat-treatment process after drawing. Examples of the divalent acid component usable in the present invention are dimethylene terephthalate (DMT), terephthalic acid(TPA), and other acids used in preparing a polyester reaction.

The polyester film of the present invention has a residual shrinkage stress of 55 kgf/cnf or less, preferably 40 kgf/cnf or less, attained with a film sample (110 mm long in the main shrinking direction and 15 mm wide in the direction perpendicular to the main shrinking direction) which is fixed an a holder equipped with chucks 95 mm apart and dipped in 90 ^° C water for 1 min. When the residual shrinkage stress of the film exceeds 55 kgf/cnf, cracking-resistance of the film becomes poor, causing a riding up or skirt phenomenon of a film label, or distortion of the substrate container.

In addition, the inventive firm has a thermal shrinkage ratio of 40 % or more, preferably from 50 % to 80 % along the main shrinking direction when treated with 90 ^° C or 100 ^° C water for 10 seconds. When the thermal shrinkage ratio is less than 40 %, satisfactory shrinkage in concave parts such as the neck of a container may be not achieved.

The inventive film may further comprise various divalent acid and diol components besides the above-mentioned major components to the extents which do not adversely affect the film properties. For instance, for the purpose of enhancing the glass transition temperature (Tg), i.e., the heat-resistance of a film, the inventive film may further comprise a divalent acid component selected from the group consisting of naphthalene dicarboxylic acid (NDA), isophthalic acid(IPA), succinic acid, glutaric acid, adipic acid, suberic acid, axealaic acid, sebacic acid, ester derivatives thereof, and a mixture thereof, as well as a diol component selected from the group consisting of 1,4-cyclohexanedimethanol (1,4-CHDM) ₅ cyclobutanediol (CBD), l,2-propanediol(l,2-PDO), l,3-propanediol(l,3-PDO), and a mixture thereof. Further, the inventive film may comprise a straight chain alcohol having a carbon number of 4 or more; at least one diol component selected from the group consisting of diethylene glycol, and a polytetramethylene ether glycol; or a mixture thereof.

In accordance with the present invention, in order to enhance the processibility after the drawing and heat-set processes, the inventive film may further comprises a runnability enhancing agent, i.e., a slipping agent, an organic or inorganic inert powder, in an amount ranging from 0.01 to 0.5 % by weight based on the total weight of the film. A preferable runnability enhancing agent that may be used in the present invention is silica gel, calcium carbonate, alumina, or a mixture thereof, having an average particle diameter of 0.01 to 10 μm. For example, for the purpose of obtaining a transparent film having a high gloss index, it is prefer to use an organic or inorganic inert powder having an average particle diameter of less than 4 μm in an amount of less than 0.1 % by weight, and for the purpose of obtaining an opaque and mat film, it is prefer to use an organic or inorganic inert powder having an average particle diameter of 3 μm or more in an amount of 0.15 % by weight or more.

The inventive film may further comprise titanium dioxide(TiO ₂ ) and silica gel which act to enhance the whitening degree of the film in amounts of 0.1 to 1 % by weight and 0.6 to 1 % by weight based on the total weight of the divalent acid component, respectively.

The inventive film may be prepared by conventional methods including a blowing technique or tenter method. The use of the tenter method results in enhanced slipping property as well as the dimensional stability in the non-shrinking direction of the resultant film.

Especially, in case of a tenter method, it is preferable that the drawing process is performed at a temperature which is 3 to 15 ^° C higher than a glass transition temperature (Tg) of a resin composition at a total drawing ratio of 3 to 6.

For the purpose of enhancing the shrink finishing quality of the firm when the film is passed through the steam heating type shrinking tunnel after the drawing process, it is preferable that a heat treating process is conducted at a temperature which is 2 to 15 ^° C higher than the drawing temperature. The afore-mentioned drawing condition contributes to low shrinkage of the resultant firm when treated with 90 ⁰ C water for 10 sec.

The inventive firm preferably has a thickness of 5 to 70 μm. For the purpose of enhancing the antistatic property to the film and also lowering the thermal sticking property of the firm within a high-temperature shrinking machine, an antistatic agent or a water-soluble and thermal sticking-resistant polymer may be coated on one or both surfaces of the film. For the purpose of enhancing a slipping property the film, if necessary, a slipping agent may be coated on one or both surfaces of the firm.

The coating with an antistatic agent suppresses the surface resistance of the film at 10 ¹⁴ Ω or less, which enables accurate capping of the upper part of a container with a firm label in a sleeve process. And the coating with an thermal sticking-resistant polymer give a thermal sticking-resistant to the film at 100 °C .

In accordance with another aspect of the present invention, there is provided a sleeve prepared by using heat-shrinkable polyester film.

As described above, the inventive heat-shrinkable polyester film has high performance characteristics in terms of uniform heat-shrinkage, minimized rupture or distortion even after secondary thermal shrinkage e.g., a sterilizing or high temperature-filling process, printing processibility, mechanical strength, heat-resistance, and shrinkage rate sufficient for full wrapping, and thus, it can be advantageously used for full wrapping a container, particularly a glass bottle.

The following Preparation Examples and Examples are intended to further illustrate the present invention without limiting its scope.

Preparation Example 1 (Polymer A) 25 parts by mole of 2-butyl-2-ethyl- 1 ,3 -propanediol (BEPD) and 170 parts by mole of ethyleneglycol base on 100 parts by mole of dimethylene terephthalate (DMT) were placed in a stainless steel monomer-preparation reactor equipped with a stirrer, a distillation column, and a condenser, and the reactor temperature was raised to 150 ⁰ C . Manganese acetate (a catalyst) diluted in ethylene glycol was added thereto in an amount of 0.03 % by weight based on the weight of DMT. While the temperature was raised to 220 ^° C over a period of 120 min, methanol formed during the reaction was continuously removed. The reaction was allowed to proceed until no methanol was formed. After completion of the reaction, a phosphoric acid (a heat stabilizer) dissolved in ethylene glycol was added to the reaction mixture in an amount of 0.04 % by weight based on the weight of DMT and stirred for 10 min. The temperature of the mixture thus obtained was raised to 250 ⁰ C . Antimony trioxide dissolved in ethylene glycol was added thereto in an amount of 0.04 % by weight based on the weight of DMT and stirred for 5 min to obtain a monomer. The monomer thus obtained was transferred to a polymerization reactor equipped with a vacuum unit, and the polymerization was allowed to proceed at 280 ^° C for 80 min under vacuum to _' obtain a polyester copolymer. The resulting polyester copolymer was analyzed by H-NMR ₅ and the result showed that it contained 21 % by mole of butylethylpropanediol (BEPD) moiethy based on the 100 moles of DMT.

Preparation Example 2 (Polymer B)

The procedure of Preparation Example 1 was repeated except for using 22 parts by mole of neopentylglycol (NPG) and 170 parts by mole of ethylene glycol based on the 100 moles of DMT ₅ to obtain a polyester copolymer. The resulting polyester copolymer was analyzed by H-NMR ₅ and the result showed that it contained about 18 % by mole of NPG moiety based on 100 moles of DMT.

Preparation Example 3 (Polymer C)

LUPOX HV-1010 grade (available from LG Chemicals Inc.) was employed as polybuthylene terephthalate.

Preparation Example 4 (Polymer D)

The procedure of Preparation Example 1 was repeated except for using 22 parts by mole of 2-methyl- 1,3 -propanediol (MPDO) and 170 parts by mole of ethylene glycol based on the 100 moles of DMT ₅ to obtain a polyester copolymer. The resulting polyester copolymer was analyzed by H-NMR ₅ and the result showed that it contained about 20 % by mole of MPDO moiety based on 100 moles of DMT.

Preparation Example 5 (Polymer E) The procedure of Preparation Example 1 was repeated except for using

DMT ₅ ethylene glycol and inorganic powder master chips (available from SKC Co., Ltd.) which comprise 18,000 ppm of a silica gel having an average particle diameter of 2.7 μm (a slipping agent) to obtain a polyethylene terephthalate (PET).

Preparation Example 6 (Polymer F) The procedure of Preparation Example 1 was repeated except for using 17 parts by mole of diethylene glycol (DEG) and 170 parts by mole of ethylene glycol based on the 100 moles of DMT, to obtain a polyester copolymer. The resulting polyester copolymer was analyzed by H-NMR, and the result showed that it contained about 16 % by mole of DEG moiety based on 100 moles of DMT.

The compositions, glass transition temperatures, and intrinsic viscosities (IV) of the polymers prepared above are shown in Table 1.

<Table 1>

Example 1

As shown in Table 2, 96 % by weight of polymer A pellets obtained in Preparation Example 1 and 4 % by weight of polymer E pellets obtained in Preparation Example 5 were mixed and dried for about 6 hours at 55 ⁰ C using a dehumidifying dryer(dew point temperature: -40 ^" C). Then, the dried mixture was melted at 245 ⁰ C and extruded through a T-die using gear pump. The extrudate thus obtained was passed over a casting roller maintained at about 20 °C , to obtain an amorphous sheet. The amorphous sheet was transferred to a tenter and passed through a heated zone thereof maintained at 90 ⁰ C , and the preheated sheet thus obtained was drawn in a total draw ratio of 4 by performing a first drawing process at about 85 ^° C and a second drawing process at about 80 ^" C , and the resulting drawn film was heat-set at 75 ^° C within the tenter in order to minimize the shrinkage of the film after drawing. The heat-set sheet was cooled at 50 ⁰ C just before exiting the tender, to obtain a polyester film having a thickness of 50 μm.

Example 2

As shown in Table 2, the procedure of Example 1 was repeated except that 76 % by weight of polymer A pellets obtained in Preparation Example 1, 20 % by weight of polymer C pellets obtained in Preparation Example 3, and 4 % by weight of polymer E pellets obtained in Preparation Example 5 were mixed, dried and melted at 260 ^° C , to obtain an amorphous sheet. The amorphous sheet was transferred to a tenter in which the temperature was controlled by hot air, and passed through a heated zone thereof maintained at 85 ^° C . The preheated sheet thus obtained was drawn in a total draw ratio of 4 by performing a first drawing process at about 80 ⁰ C and a second drawing process at about 75 ⁰ C, and the resulting drawn film was heat-set at 75 ^° C within the tenter. The heat-set sheet was cooled at 50 °C just before exiting the tender, to obtain a polyester film having a thickness of 50 μm.

Example 3 The amorphous sheet obtained in Example lwas transferred to a tenter and passed through a heated zone thereof maintained at 85 ⁰ C , and the preheated sheet thus obtained was drawn in a total draw ratio of 3 by performing a first drawing process at about 80 ^° C and a second drawing process at about 75 ^" C, and the resulting drawn film was heat-set at 75 ^° C within the tenter. The heat-set sheet was further drawn in a draw ratio of 1.5 by performing a drawing process at 95 ⁰ C and the resulting drawn film in a total draw ratio of 4.5 was cooled at 50 ^° C just before exiting the tender, to obtain a polyester film having a thickness of 50 μm.

Comparative Example 1

As shown in Table 2, the procedure of Example 1 was repeated except that 96 % by weight of polymer B pellets obtained in Preparation Example 2 and 4 % by weight of polymer E pellets obtained in Preparation Example 5 were mixed, dried, and melted at 260 ^° C , to obtain an amorphous sheet. The amorphous sheet was transferred to a tenter and passed through a heated zone thereof maintained at

95 "C , and the preheated sheet thus obtained was drawn in a total draw ratio of 4 by performing a first drawing process at about 85 ^° C and a second drawing process at about 80 ^° C , and the resulting drawn film was heat-set at 75 ^° C within the tenter. The heat-set sheet was cooled at 50 ⁰ C just before exiting the tender, to obtain a polyester film having a thickness of 50 μm.

Comparative Example 2

As shown in Table 2, the procedure of Example 3 was repeated except that

96 % by weight of polymer B pellets obtained in Preparation Example 2 and 4 % by weight of polymer E pellets obtained in Preparation Example 5 were mixed, to obtain a polyester film having a thickness of 50 μm.

Comparative Example 3

As shown in Table 2, the procedure of Example 2 was repeated except that

76 % by weight of polymer B pellets obtained in Preparation Example 2, 20 % by weight of polymer C pellets obtained in Preparation Example 3, and 4 % by weight of polymer E pellets obtained in Preparation Example 5 were mixed, to obtain a polyester film having a thickness of 50 μm.

Comparative Example 4

As shown in Table 2, the procedure of Example 2 was repeated except for using 96 % by weight of polymer D pellets obtained in Preparation Example 4 and 4 % by weight of polymer E pellets obtained in Preparation Example 5, controlling a pre-heating temperature to 90 ^° C , and controlling respective drawing temperatures to 85 °C and 80 "C ₅ to obtain a polyester film having a thickness of 50 μm.

Comparative Example 5

As shown in Table 2, the procedure of Example 2 was repeated except for using 71 % by weight of polymer B pellets obtained in Preparation Example 2, 25 % by weight of polymer F pellets obtained in Preparation Example 6, and 4 % by weight of polymer E pellets obtained in Preparation Example 5, controlling a pre-heating temperature to 90 ^° C , and controlling respective drawing temperatures to 85 ^° C and 80 ^° C , to obtain a polyester film having a thickness of 50 μm.

Experiments

The properties of the films manufactured by using polymers of Preparation Examples were measured by the following methods.

(1) Thermal (90 °C/ 100 ⁰ C water) shrinkage ratio (%)

A film sample was cut into a 300 mm (length) x 15 mm (width) piece and put in a water bath maintained at 90 ^° C or 100 ^° C for 10 seconds, and the change in the film length after the heat-treatment was measured. Using the following equation, the degree of shrinkage was calculated.

Thermal shrinkage ratio (%)=[(300-Length of the piece after the heat-treatment) / 300] x 100

(2) Maximum shrinkage stress and Residual shrinkage stress A film sample was cut into a 120 mm (length) x 15 mm (width) piece and indicated at the points of 5 mm far from both sides to the length direction. The 110 mm-long film sample thus obtained was applied to an apparatus having the distance between chucks of 95 mm and equipped with a load cell for sensing a shrinkage stress attached to one of grips thereof (see FIG. 1). Thereafter, the apparatus equipped with the film sample was put in a water bath maintained at 90 °C , followed by heat-treatment for 1 min when the degree of shrinkage of 13.6 % was observed. The shrinkage stress value after the heat-treatment was represented as the unit of kgf/cnf.

(3) Finishing quality after shrinkage

A film sample prepared in Examples was subjected to a solvent adhesion to obtain sleeve of which a lay flat was 105 mm. The sleeve was cut into a 200 mm-long piece. A "Byul" glass bottle (available from Kuksundang, KR) was wrapped with the cut sleeve and passed through the steam-heating hype shrinking tunnel. The temperature in each zone of the tunnel was set at 110 ^° C, 140 ^" C and 160 ^° C and the passing rate was varied to 5.4 m/min, 9 m/min, and 13.5 m/min. Thereafter, the shrink finishing quality of the firm depending on the passing rate was observed. The shrunk film was evaluated based on the following criteria. © : The film strain and the shrinkage-insufficiency were not found o : The firm strain and the shrinkage-insufficiency were rarely found Δ : The film strain and the shrinkage-insufficiency were partly found x : The firm strain and shrinkage-insufficiency were seriously found

(4) Cracking-resistance

A film sample was subjected to a solvent adhesion to obtain a sleeve of which a lay flat was 105 mm. The sleeve was cut into a 200 mm-long piece. A

"Byul" glass bottle (available from Kuksundang, KR) was wrapped with the cut sleeve. The glass bottle was put in a water bath maintained at 90 ^° C for about

30 sec, and took out thereof to be cooled to an atmosphere temperature.

Thereafter, the shrunk sleeve was separated from the glass bottle, and cut into a piece of 70 mm (length direction of the bottle corresponding to the main shrinking direction of sleeve before heat-treatment) x 15 mm (width). The sleeve was elongated at a rate of 200 mm/min using Universal Tester (UTM) having the distance between chucks of 50 mm and the degree of elongation at rupture was measured. An average elongation value derived from 3 tests was taken for each sample, as graded according to the following standards. o: The average elongation value was 100 % or more

Δ: The average elongation value was 50 % or more and less than 100 % x : The average elongation value was less than 50 %

The measured properties of the polyester films in Examples 1 to 3 and

Comparative Examples 1 to 5 are shown in Table 3. <Table 3>

As shown in Table 3, the films of Examples had the residual shrinkage stress of 25 to 27 kgf/cπf after being dipped in 90 ^° C water for 1 min, and the thermal shrinkage ratio of 40% or more along the main shrinking direction when treated with 90 ^° C or 100 ^° C water for 10 seconds, thereby showed improved properties in terms of finishing quality after shrinkage and cracking-resistance as compared with those of Comparative Examples.

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.