JPS637573B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a heat-shrinkable polyester film with excellent suitability for shrink packaging. Traditionally, polyester films, typified by polyethylene terephthalate film, are mechanically
Due to its excellent optical properties and high heat resistance, it is used in a variety of ways as a packaging material for Shrink packaging. A number of problems have been recognized above, and it is difficult to say that it is necessarily suitable. That is, crystalline resins such as polyethylene terephthalate undergo rapid crystallization at the same time as molecular orientation during the film forming process, for example, at the initial stage of film stretching. Not only is shrinkage inhibited, but heat sealability is also significantly reduced. However, when shrink packaging is performed using a polyester film that has undergone oriented crystallization during stretching, the shrinkage is insufficient, resulting in uneven shrinkage, leaving wrinkles in some areas, or In addition to the appearance problem of large protrusions (hereinafter referred to as dog ears), polyethylene terephthalate originally has poor sealing strength, as if contradicting its characteristic that its shrinkage stress is extremely strong compared to other resins. Because of its weakness, it had the fatal problem of easily tearing the seal, and was never satisfactory as a packaging material for shrinkage. The present inventors aimed to improve the suitability of shrink packaging for such polyester films.
As a result of extensive testing, we found that in order to provide excellent shrinkability, the high crystallinity of polyethylene terephthalate should be suppressed by making it a copolymer component with isophthalic acid, and the heat shrinkage rate of the polyester film should be reduced. We have completed the present invention by discovering that excellent shrink packaging without uneven shrinkage or tearing of the sealed portion can be achieved after shrink packaging only when the seal strength and heat shrinkage stress are limited to a certain range. That is, the present invention provides a polyester film containing isophthalic acid as a copolymerization component, which has a heat shrinkage rate of 30% in both the length and width directions in a 120°C glycerin bath. or more, and the product of the shrinkage rates in both directions is 1200% or more, and the ratio of fusing seal strength and heat shrinkage stress at a thickness of 12 microns is as follows (1)
The present invention relates to a heat-shrinkable polyester film that satisfies the following formula. α/β≧12 …(1) However, α: Fused seal strength [g/15mm] β: Heat shrinkage stress in 120°C glycerin bath [Kg/
cm ² ] The present invention will be explained in detail below. The heat-shrinkable polyester film of the present invention contains isophthalic acid as a copolymerization component.
It consists of 50 mol% copolymerized poly(ethylene terephthalate/ethylene isophthalate) (hereinafter referred to as copolyester) or a mixture of the copolyester and polyethylene terephthalate. The reason why isophthalic acid is included as a copolymerization component is that in the polyethylene terephthalate molecular chain,
By partially introducing isophthalate molecules, oriented crystallization of the polyester film is suppressed, giving it high shrinkage rate and fusing seal strength, as well as reducing heat shrinkage stress as a resin property. It also has an effect. Here, if the amount of isophthalic acid as a copolymer component forming the copolyester is 3 mol% or less, the effect of suppressing orientational crystallization during stretching will be small, and therefore, this paper on shrinkage rate, fusion seal strength, heat shrinkage stress, etc. It is not possible to satisfy the scope of the invention, and on the other hand,
If the isophthalic acid content is 50 mol% or more, the crystallinity of the copolyester will be significantly reduced, and the copolyester will become extremely sticky when heated during the film manufacturing process, such as during drying or transport of raw materials. Many problems arise, such as raw materials adhering to each other, making uniform dispersion and transportation impossible, which makes stable production difficult, and the heat resistance of the resulting heat-shrinkable polyester film decreases significantly, making it difficult to put into practical use. This results in a lack of sexuality. Note that the heat-shrinkable polyester film containing isophthalic acid as a copolymerization component of the present invention may contain third and fourth acids, such as phthalic acid, adipic acid, and sebacic acid, within a range that does not significantly change their properties. or alcohol components such as butanediol, hexanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, etc. may be copolymerized, and if necessary, various additives such as lubricants, colorants, etc. may be copolymerized. Needless to say, antioxidants, antistatic agents, etc. can be added to the heat-shrinkable polyester film of the present invention. This includes those coated with various paints for the purpose of prevention. The heat-shrinkable polyester film of the present invention, which is highly suitable for shrink packaging, has a heat shrinkage rate of 30% or more in either the length or width direction in a 120°C glycerin bath, and a shrinkage rate in both directions. The product of is 1200% or more, and the ratio of the fusing seal strength and heat shrinkage stress when the thickness is 12 microns is expressed by the following equation (1) α/β≧12 ... (1) where α: fusing seal strength [ g/15mm] β: It is necessary to satisfy the heat shrinkage stress [Kg/cm ² ] in a 120°C glycerin bath, and the relationship between the heat shrinkage rate, fusion seal strength, heat shrinkage stress, etc. is determined according to the present invention. It is essential to achieving the purpose. In the present invention, the reason why the shrinkability of polyester film is expressed as the heat shrinkage rate in a 120°C glycerin bath is that when a crystalline polymer film such as polyester is heated in a shrink tunnel device, The shrinkage rate reaches a certain level instantaneously, but due to the subsequent heating, the shrinkage rate cannot be expected to increase much due to the crystallization that progresses at the same time as the shrinkage due to the unique shrinkage behavior. Based on the knowledge that the size of the shrinkage rate at the initial stage of heating is the biggest factor determining the appearance after Shrink packaging, we have developed a method that can most quantitatively measure the size of the shrinkage rate at the early stage of heating. The evaluation was based on the heat shrinkage rate in a 120°C glycerin bath. Therefore, when shrink-wrapping an article using a heat-shrinkable polyester film, if the shrinkage rate deviates from the range of the present invention, no matter how you manipulate the heating conditions of the shrink tunnel device, it will not be possible to completely eliminate shrinkage unevenness. Not only is it impossible to erase, but the dog ears are large, resulting in a significant loss of product value in terms of appearance. Furthermore, in the heat-shrinkable polyester film of the present invention, the relationship between the fusing seal strength and the heat-shrinkage stress is an extremely important requirement in determining the finished state of the sealed portion after shrink packaging. In other words, as it is well known, polyethylene terephthalate has high crystallinity, which makes it difficult to heat seal, but on the other hand, it has contradictory properties such as being unparalleledly strong in terms of heat shrinkage stress. Even if shrink packaging is attempted using such polyethylene terephthalate, the heat shrinkage stress is too strong compared to the seal strength, and the sealed portion often tears, making it impossible to achieve satisfactory shrink packaging. In order to avoid such inconvenient situations during Shrink packaging, the present inventors investigated the relationship between the fusing seal strength and heat shrinkage stress from various angles, and as a result, the specific relationship shown in equation (1) above was established. It has been concluded that a complete solution to the problem cannot be achieved unless the above conditions are satisfied.On the other hand, if the relationship between the fusing seal strength and heat shrinkage stress is outside the scope of the present invention, This poses a fatal drawback in shrink packaging, in that the seal part is easily torn. In the heat-shrinkable polyester film of the present invention, there is a difference between the fusing seal strength and the heat-shrinkage stress.
In order to satisfy the relationship of formula (1), for example, the copolymerization ratio of isophthalic acid in the copolyester must be 3 to 3.
It may be selected as appropriate within the range of 50 mol%, or the mixing ratio of copolyester and polyethylene terephthalate may be changed as appropriate.In addition, in this case, it is necessary to maintain the cohesive strength after shrink packaging, which is unique to polyester. for,
It is preferable to select the content of isophthalic acid so that the thermal shrinkage stress is at least 50 [Kg/cm ² ] or more. Hereinafter, the present invention will be explained in more detail based on Examples. The physical properties of the heat-shrinkable polyester film of the present invention were measured and evaluated as follows. Heat shrinkage rate [%]: parallel to the longitudinal direction of the film,
A sample film cut into a square of 100 mm both vertically and horizontally was immersed in a 120°C glycerin bath for 10 seconds.
Thermal shrinkage rate in the length and width directions [%] is determined by the following formula:
was calculated. 100-A/100×100% However, A indicates the length in the vertical or horizontal direction (unit: mm) after immersion. Fusing seal strength [g/15mm]: With two 12 micron thick heat-shrinkable polyester films stacked on top of each other, the fusing sealing machine (Fuji Manufacturing PSC-
310E model), and then the peel strength [g/15 mm] of the sealed portion was measured. Fusing sealing conditions: Voltage = 100V, current 3.8A, energizing time = approximately 0.8 seconds, heater wire thickness = approximately 0.55φ In addition, if the film thickness is other than 12 microns,
From the test data conducted by the present inventors, a converted value was calculated using the following formula. However, T: Melting seal strength at thickness t (unit: microns) [g/15mm] Heat shrinkage stress [Kg/cm ² ]: Sample films taken at a width of 15mm from a heat-shrinkable polyester film are set at intervals of 30mm. After attaching it to the upper and lower chucks, it was immersed in a glycerin bath at 120°C for 30 seconds, and the maximum shrinkage stress during that time was defined as heat shrinkage stress [Kg/cm ² ]. Examples 1 to 3, Comparative Examples 1 to 2 Copolyester raw materials with various copolymerization ratios of isophthalic acid were melt-extruded at a temperature of 150 to 285°C to obtain amorphous tubes with a thickness of 180 microns.
The amorphous tube was simultaneously stretched 3.5 times in the longitudinal direction and 4.2 times in the width direction using compressed air under heating, and then heat-set at a temperature of 80 to 100 degrees Celsius and heat-shrinked to a thickness of 12 microns. A polyester film was obtained. The shrink suitability of these films is shown in Table 1, and it can be seen from the table that if the relationship between heat shrinkage rate, fusing seal strength, and heat shrinkage stress is within the range of the present invention, there will be no shrinkage unevenness after shrink packaging. It can be confirmed that the finish of the seal part is extremely excellent.

[Table] Examples 4-5, Comparative Examples 3-4 Melt extrusion of copolyester raw materials with various copolymerization ratios of isophthalic acid at temperatures of 200 to 285°C
It was rapidly cooled on a casting drum kept at 40°C to obtain an amorphous sheet with a thickness of 170 microns. This amorphous sheet was heated at 80 to 100℃ for 3.2 degrees in the width direction.
After successively stretching 3.8 times in the longitudinal direction at 85 to 110°C, heat-setting at 100°C increases the thickness.
A heat-shrinkable polyester film of 14 microns was obtained. The shrink suitability of these films is shown in Table 2. From the table, it can be seen that films whose relationship between heat shrinkage rate, fusing seal strength and heat shrinkage stress are within the scope of the present invention have uneven shrinkage after shrink packaging. Not at all,
It was confirmed that the finish of the seal part was also extremely excellent.

[Table] (2) Syring suitability was evaluated using the same method as in Table 1.
Examples 6 to 8, Comparative Examples 5 to 6 A raw material prepared by mixing a copolyester copolymerized with 10 mol% of isophthalic acid and polyethylene terephthalate in various proportions was melt-extruded at a temperature of about 285°C to a thickness of 90 microns. Obtained an amorphous tube. This amorphous tube was simultaneously stretched 3.8 times in the longitudinal direction and 3.9 times in the width direction at 86°C using compressed air, and then heat-set at a temperature of 130°C to a thickness of 6.1 times.
A micron heat-shrinkable polyester film was obtained. The shrink suitability of these films is shown in Table 3. From the table, it can be seen that films whose heat shrinkage rate, fusing seal strength, and heat shrinkage stress are within the range of the present invention have uneven shrinkage after shrink packaging. Not at all,
It was confirmed that the finish of the seal part was also extremely excellent.

[Table] (2) Syring suitability was evaluated using the same method as in Table 1.
Examples 9 to 11, Comparative Examples 7 to 8 Raw materials prepared by mixing copolyester copolymerized with 30 mol% isophthalic acid and polyethylene terephthalate in various proportions were melt-extruded at a temperature of about 280°C,
It was rapidly cooled on a casting drum maintained at 50°C to obtain an amorphous sheet with a thickness of 200 microns. This amorphous sheet was made 3.4 times thicker in the longitudinal direction at 90℃.
After successively stretching 3.9 times in the width direction at 95℃,
A heat-shrinkable polyester film having a thickness of 15.1 microns was obtained by heat setting at a temperature of 105°C. The shrink suitability of these films is shown in Table 4. From the table, it can be seen that films whose relationship between heat shrinkage rate, fusing seal strength and heat shrinkage stress are within the scope of the present invention have uneven shrinkage after shrink packaging. Not at all,
It was confirmed that the finish of the seal part was also extremely excellent.

[Table] (2) Syring suitability was evaluated using the same method as in Table 1.
As described above, the heat-shrinkable polyester film of the present invention not only has excellent suitability for shrink packaging, but also has the advanced mechanical and optical properties unique to polyester, making it ideal as a packaging material for shrink packaging. , and its application fields are extremely wide-ranging.

Claims (1)

[Scope of Claims] 1. A polyester film containing isophthalic acid as a copolymerization component, which has a heat shrinkage rate of 30% in both the length and width directions in a 120°C glycerin bath. or more, and the product of the shrinkage rates in both directions is 1200% or more, and the ratio of the fusing seal strength to the heat shrinkage stress at a thickness of 12 microns satisfies the following formula (1). Heat-shrinkable polyester film. α/β≧12 …(1) However, α: Fused seal strength [g/15mm] β: Heat shrinkage stress in 120°C glycerin bath [Kg/
cm ² ] 2 The polyester film absorbs 3 isophthalic acids.
The heat-shrinkable polyester film according to claim 1, comprising poly(ethylene terephthalate/ethylene isophthalate) copolymerized with 50 mol% to 50 mol%. 3 The polyester film absorbs 3 isophthalic acids.
The heat-shrinkable polyester film according to claim 1, comprising a mixture of poly(ethylene terephthalate/ethylene isophthalate) and polyethylene terephthalate copolymerized at 50 mol% to 50 mol%.