HK1160878B - Heat-shrinkable polyester film - Google Patents

Abstract

A heat-shrinkable polyester film having a heat-shrinkage change per degree Celsius (%/℃) along the main shrinkage direction of 1.5 to 3.0 in the range of 60 ℃ to 70 ℃, 2.5 to 3.5 in the range of 70 ℃ to 80 ℃, 1.0 to 2.0 in the range of 80 ℃ to 90 ℃, and 0.1 to 1.0 in the range of 90 ℃ to 100 ℃, has a good appearance quality after shrinkage and thus suitable for a wrapping material, particularly a label for a bottle.

Description

Heat-shrinkable polyester film

Technical Field

The present invention relates to a heat-shrinkable polyester film suitable for use as a packaging material, more preferably for labeling containers, because it has a good finished appearance after shrinking.

Background

Heat-shrinkable films, which are capable of shrinking back to a pre-stretched form when heated at a predetermined temperature, have been widely used for labeling glass or plastic bottles because of their properties suitable for printing various articles to attract consumers' attention, and suitable for full-wrapping various containers and wrapping a bundle of goods.

Recently, a heat-shrinkable film made of flexible polyvinyl chloride (PVC), which has a limited maximum heat-shrinkage rate and emits toxic pollutants such as dioxin upon combustion, has become undesirable. On the other hand, Oriented Polystyrene (OPS) films, which have uniform shrinkage properties and low specific gravity and are easily removed from PET bottles and recycled, have a problem of poor heat resistance. Therefore, such oriented polystyrene films are not suitable for high shrinkage processing or high temperature filling processing.

Heat-shrinkable polyester films having improved shrink properties and heat resistance compared to those of PVC and OPS films have been developed for packaging glass bottles. However, the shrinkage rate of such polyester films is generally too high, which causes non-uniform shrinkage when a heat-shrinking process using hot air is performed, and thus, a steam-heating type shrinker is required to prevent non-uniform shrinkage of the films. In addition, the shrinkage stress of such polyester films is too high, often resulting in non-uniform shrinkage, with consequent deformation, end-bending or cracking, especially when subjected to secondary heat-shrinking processes, such as sterilization or high-temperature filling processes.

In order to solve the above problems, Korean patent laid-open No.2002-0063158 discloses a copolymer composition comprising terephthalic acid, ethylene glycol, 1, 4-cyclohexanedimethanol and diethylene glycol and a heat-shrinkable polyester film having high flexibility and PVC-like shrinkage properties, which is prepared using the same; and Korean patent laid-open publication No.2002-0062838 discloses a heat-shrinkable polyester film having a polyester elastomer content of 5% or more.

However, such heat-shrinkable polyester films show a skirt phenomenon (curling up) in which a film having fixing points marked on a container such as a square container is drawn up in the center between the fixing points in the direction perpendicular to the shrinking direction, or a shrinking up phenomenon in which a film marked on a round container is drawn up in the direction perpendicular to the shrinking direction, and thus, these conventional films cannot be effectively used for protecting containers or marking goods.

Further, it is also known that the conventional polyester film has disadvantages such as non-uniform thermal shrinkage and shrinkage occurring during the high-temperature shrinkage, deteriorating the appearance and quality of the marking container.

Summary of The Invention

Accordingly, it is an object of the present invention to provide a heat-shrinkable polyester film having good appearance after shrinking, since it can be uniformly heat-shrunk and prevented from the shrink-up or skirt phenomenon, and has improved characteristics in terms of printability, mechanical strength, heat resistance and shrinkage rate.

According to one aspect of the present invention, there is provided a heat-shrinkable polyester film having a heat shrinkage change (%/DEG C) per degree Celsius in a main shrinkage direction of 1.5 to 3.0 at 60 ℃ to 70 ℃, 2.5 to 3.5 at 70 ℃ to 80 ℃, 1.0 to 2.0 at 80 ℃ to 90 ℃, and 0.1 to 1.0 at 90 ℃ to 100 ℃.

The heat-shrinkable polyester film may be prepared from a copolyester composition comprising: (i) a diacid component containing at least 90 mole percent terephthalic acid groups, based on 100 mole percent diacid component; and (ii) a glycol component containing (a)1 to 20 mol% of diethylene glycol, (b)5 to 30 mol% of neopentyl glycol, and (c)50 to 90 mol% of ethylene glycol, based on 100 mol% of the glycol component.

Further, the heat-shrinkable polyester film may be prepared by a method comprising the steps of: (a) melt-extruding the copolyester composition to prepare a sheet, and stretching the sheet in the main shrinkage direction at the temperature of Tg + 5-Tg +20 ℃ to obtain an oriented film; and (b) heat-treating the alignment film at Tg +5 ℃ to Tg +50 ℃.

The heat-shrinkable polyester film of the present invention has improved properties in terms of printability, mechanical strength, heat resistance and shrinkability when used for full packaging. In addition, the film of the present invention can prevent the phenomenon of shrink-up or skirt, and the change in thermal shrinkage is relatively constant with temperature change. Therefore, the film of the present invention shows no shrinkage and deformation after shrinking, and thus can maintain good appearance quality after shrinking.

Drawings

The above and other objects and features of the present invention will become apparent from the following description of the invention when taken in conjunction with the accompanying drawings, which respectively show:

FIG. 1: the polyester film of the present invention changes with the thermal shrinkage of temperature; and

FIG. 2: a schematic diagram showing skirt ratio (skert ratio) of a polyester film, where W is the width of the film before shrinking, d is the width of the film after shrinking, and L is the length of the film.

Detailed Description

The heat-shrinkable polyester film of the present invention has a heat shrinkage change (%/DEG C) per degree centigrade in the main shrinkage direction, which is changed to 1.5 to 3.0 at 60 ℃ to 70 ℃, 2.5 to 3.5 at 70 ℃ to 80 ℃, 1.0 to 2.0 at 80 ℃ to 90 ℃, and 0.1 to 1.0 at 90 ℃ to 100 ℃. For example, the change in thermal shrinkage with temperature in the main shrinkage direction refers to the gradient of the curve shown in fig. 1 at each temperature region.

As shown in fig. 1, the heat-shrinkable polyester film of the present invention has a relatively constant rate of change (i.e., gradient), and thus, the heat-shrinkage rate does not change significantly over the entire temperature range, unlike the conventional film. In particular, as can be seen from FIG. 1, the films of the present invention exhibit a substantially constant rate of change, i.e., an almost linear curve, over the temperature range of 60 ℃ to 80 ℃, similar to the heat shrinkage performance of PVC films.

According to a preferred embodiment of the present invention, the heat-shrinkable polyester film may be prepared from a copolyester composition comprising: (i) a diacid component containing at least 90 mole percent terephthalic acid groups, based on 100 mole percent diacid component; and (ii) a glycol component containing (a)1 to 20 mol% of diethylene glycol, (b)5 to 30 mol% of neopentyl glycol, and (c)50 to 90 mol% of ethylene glycol, based on 100 mol% of the glycol component.

The copolyester composition also contains a conventional diacid component such as dimethyl terephthalate (DMT) or terephthalic acid (TPA), preferably the diacid component contains greater than or equal to 90 mole percent of terephthalic acid (TPA) groups. When the content falls within the above range, the film may have a microcrystalline structure resulting from orientation of the film during stretching and heat treatment, resulting in improved heat resistance.

Further, the copolyester composition further comprises a glycol component containing 1 to 20 mol% of diethylene glycol (DEG), 5 to 30 mol% of neopentyl glycol (NPG) and 50 to 90 mol% of Ethylene Glycol (EG) based on 100 mol% of the glycol component.

When the content of diethylene glycol is 3 mol% or more, the characteristics of the film of the present invention such as low skirt ratio (S/R) and constant shrinkage can be easily produced. When the content of diethylene glycol is 16 mol% or less, the heat-shrinkable film may have improved heat resistance.

When the content of neopentyl glycol is 10-25 mol%, a satisfactory shrinkage rate can be obtained, and the obtained film marked on the container is not easy to crack, because excessive oriented crystals are generated in the heat treatment process after stretching, or secondary oriented crystals are generated in the heat shrinkage process of the container to play a role in protection.

The copolyester composition further contains a reaction catalyst, a polymer stabilizer, a reaction additive, an inorganic substance and the like.

According to a preferred embodiment of the present invention, the copolyester composition further comprises 0.3 to 0.5 wt% of a polymer stabilizer (e.g., trimethyl phosphate), 0.2 to 0.4 wt% of a reaction catalyst (e.g., antimony trioxide) and 0.05 to 0.1 wt% of an inorganic substance (e.g., silica).

The reaction catalyst may use acetates of metals such as magnesium, manganese and zinc.

The copolyester composition used in the present invention preferably has an intrinsic viscosity of 0.6 to 0.9 dl/g. When the intrinsic viscosity is within the above range, the film can achieve satisfactory mechanical properties such as strength and ductility, making the film less likely to break during subsequent processes such as printing and marking.

The heat-shrinkable polyester film of the present invention preferably has a heat shrinkage rate of not less than 50%, preferably not less than 55%, in the main shrinkage direction when treated in hot water at 90 ℃ for 10 seconds. When the heat shrinkage rate is more than or equal to 55 percent, the film can achieve satisfactory shrinkage rate in the full packaging process of containers with various shapes, even container necks.

Further, the heat-shrinkable polyester film of the present invention, which is treated in hot water at 90 ℃ for 10 seconds after fixing both ends, preferably has a skirt ratio (S/R) of 14% or less in the main shrinkage direction as shown by the formula (I):

skirt ratio (%) - (W-d)/Lx100 (I)

Wherein L is the length of the film in the main shrinking direction, W is the width of the film before shrinking, and d is the width of the film after shrinking.

When the skirt ratio is 14% or less, the film exhibits improved skirt phenomenon resistance characteristics, i.e., a film having fixed points marked on a container such as a square container is shrunk over the center between the fixed points in the direction perpendicular to the shrinking direction, or a shrinking phenomenon, i.e., a film marked on a round container is completely shrunk over in the direction perpendicular to the shrinking direction, resulting in good finished appearance and a sign of a printed label.

Further, the heat-shrinkable polyester film of the present invention has a maximum shrinkage stress of 6N or less, preferably 3.0N to 5.5N, when treated in hot water at 90 ℃ for 10 seconds. The maximum shrinkage stress is preferably measured with a film thickness of 40 μm. When the maximum shrinkage stress is 6N or less, the film can exhibit high resistance to the phenomenon of cracking, shrinkages or skirts, preventing deformation of the container during marking.

The heat-shrinkable polyester film of the present invention can be prepared by a method comprising the steps of: (a) melt-extruding the copolyester composition into a sheet, and then stretching the sheet in a main shrinkage direction at Tg +5 ℃ to Tg +20 ℃, preferably by adjusting a roll peripheral speed or using a tenter (stretcher) to obtain an oriented film; and (b) heat-treating the alignment film at Tg +5 ℃ to Tg +50 ℃.

Preferably, the heat treatment process of step (b) is performed by fixing both ends of the oriented film in the main shrinkage direction and then passing the fixed film through a heat treatment zone having a temperature of Tg +5 deg.C to Tg +50 deg.C.

The preferable conditions of the heat treatment process are as follows: constant shrinkage, high heat shrinkage of the resulting film, low shrinkage stress and low skirt ratio. When the heat treatment is in the range of Tg +5 deg.C to Tg +50 deg.C, the film can be sufficiently heat-treated to prevent excessive crystal growth to maintain good mechanical properties.

The following examples are intended to further illustrate the invention but are not intended to limit the scope of the invention.

1. Preparation of copolyester resin and polyester film

The copolyester resin was prepared according to the composition and content in table 1 using a conventionally used and well-known standard preparation method of copolyester in the art (for example, examples 1 to 7 of korean patent No. 10-0987065).

TABLE 1

The polyester films obtained from resins 11 to 14 are shown in Table 2. Each resin was melt-extruded to prepare a sheet, and an oriented film was obtained at 80 ℃ with a draw ratio (rate) of 4.0 in the main shrinkage direction. Fixing both ends of the oriented film along the main shrinkage direction, and then rapidly passing through a heat treatment zone at a temperature ranging from 85 ℃ to 105 ℃ to heat-set the fixed film, thereby obtaining a heat-shrinkable polyester film having a thickness of 40 μm.

TABLE 2

2. Properties of polyester film

The properties of the films obtained in examples 1 to 4 and comparative examples 1 to 5 were evaluated by the following methods, and the results are shown in Table 3.

(1) Skirt ratio (S/R)

The film samples were cut into pieces of 13.0cm (length) by 6.0cm (width) in the direction of shrinkage. The surface of the sample was cleaned of impurities, both ends of the sample were fixed in the main shrinkage direction, and then placed in hot water at 90 ℃ for 10 seconds. As shown in fig. 1, the width of the sample after shrinkage was measured, and the skirt ratio of the sample was calculated by the following equation:

skirt ratio (%) - (width of sample before shrinkage-width of sample after shrinkage)/length of sample ] × 100

(2) Heat resistance

The glass transition temperature (Tg) of the film samples was determined by DSC (differential scanning calorimetry). Tg is more than or equal to 65 ℃ and is rated as O, Tg is less than or equal to 65 ℃ and is rated as X.

(3) Appearance quality after shrinkage

A cube container weighing 22g and measuring 6cm on a side was wrapped with a film and passed through a steam heated shrinking oven. The appearance quality of the film after shrinkage was then observed. Each film was measured 20 times. If a shrinkage defect such as shrinkage or shrink-up of the film or deformation of the container is found to be 2 times or less, the film is rated as "O", and if a shrinkage defect such as shrinkage or shrink-up of the film or deformation of the container is found to be 3 times or more, the film is rated as "X".

(4) Thermal shrinkage rate

The film sample was cut into small pieces of 300mm (length) × 15mm (width) in the main shrinkage direction, placed in a water bath at 60 ℃, 70 ℃, 80 ℃, 90 ℃ or 100 ℃ for 10 seconds, and the change in length of the film after heat treatment was measured. The shrinkage was calculated using the following equation:

heat shrinkage (%) [ (300-sample length after heat treatment)/300 ] × 100.

(5) Maximum shrinkage stress

The film sample was cut into pieces of 120mm (length) × 15mm (width), and marked at positions 5mm from each side in the longitudinal direction. The resulting 110mm long film samples were applied to a device having chucks (chucks) spaced 95mm apart and fitted with load cells attached to one of its shanks for detecting shrinkage stress. The device with the film sample was then placed in a water bath at 90 ℃. When a degree of shrinkage of 13.6% was observed, the film sample was heat-treated for 1min, and the maximum shrinkage stress was obtained.

TABLE 3

3. Analysis of Heat shrinkage Pattern of polyester film

The films obtained in examples 1 to 3, comparative examples 1 and 2, and the PVC and OPS films were measured for heat shrinkage mode, and the results are shown in table 4 and fig. 1.

TABLE 4

The heat shrinkage rate in table 4 was used to calculate the change in heat shrinkage (%/deg.c) per degree celsius in the main shrinkage direction in each temperature range, which is shown in table 5.

TABLE 5

As shown in tables 3 to 5 and FIG. 1, the films of examples 1 to 3 of the present invention have improved characteristics in mechanical strength, heat resistance and heat shrinkage compared to conventional films, and the films of the present invention have uniform heat shrinkability required for full packaging.

Although the invention has been described above with reference to specific embodiments, it will be appreciated that various modifications or improvements of the invention may be made by those skilled in the art, which also fall within the scope of the invention as defined in the appended claims.

Claims (3)

1. A heat-shrinkable polyester film having a heat-shrinkage change per degree centigrade in the main shrinkage direction in%/° C, which is changed from 1.5 to 3.0 at 60 ℃ to 70 ℃, from 2.5 to 3.5 at 70 ℃ to 80 ℃, from 1.0 to 2.0 at 80 ℃ to 90 ℃, and from 0.1 to 1.0 at 90 ℃ to 100 ℃,

wherein the heat-shrinkable polyester film is prepared from a copolyester composition prepared by copolymerizing:

(i) a diacid component containing at least 90 mole percent terephthalic acid groups, based on 100 mole percent diacid component; and

(ii) a glycol component containing (a)3 to 16 mol% of diethylene glycol, (b)5 to 30 mol% of neopentyl glycol, and (c)50 to 90 mol% of ethylene glycol, based on 100 mol% of the glycol component,

wherein the heat-shrinkable polyester film has a heat shrinkage rate of 58% or more in a main shrinkage direction when treated in hot water at 90 ℃ for 10 seconds, a skirt ratio (S/R) represented by formula (I) of 11% or less, and a maximum shrinkage stress of 6N or less:

skirt ratio (%) - (W-d)/L x100 (I)

Wherein L is the length of the film along the main shrinkage direction, W is the width of the film before shrinkage, and d is the width of the film after shrinkage;

wherein the intrinsic viscosity of the copolyester composition is 0.6 to 0.9 dl/g.

2. The heat-shrinkable polyester film of claim 1, wherein the copolyester composition further comprises at least one selected from the group consisting of: reaction catalysts, polymer stabilizers, reaction additives and inorganic substances.

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

(a) melt-extruding the copolyester composition to obtain a sheet, and then stretching the sheet in a main shrinkage direction at a temperature of Tg +5 ℃ to Tg +20 ℃ to obtain an oriented film; and

(b) heat-treating the oriented film at Tg +5 ℃ to Tg +50 ℃.