JPH1158648A - Heat-shrinkable polyester film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat shrinkable film having sufficient shrinkability and not generating whitening and the lowering of its strength after a heat sterilizing process. SOLUTION: This heat shrinkable polyester film is composed of a multilayered film consisting of a surface layer comprising polyester (A) wherein 1,4-butane diol is contained in an amt. of 50 mol.% or more in glycol components and an inner layer comprising polyester (B) wherein ethylene glycol is contained in an amt. of 60 mol.% or more in the glycol components and characterized by that the shrinkage factor thereof in an ethylene glycol bath at 100 deg.C is 30% or more and the haze thereof after one-hr heat treatment in saturated steam at 120 deg.C is 0.22%/μm or less and the tensile elongation at break thereof in the direction right-angled to the main shrinkage direction of the heat-treated film is 2.0% or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-shrinkable polyester film having excellent whitening resistance, strength and heat-shrinkability against heat sterilization for food applications.

[0002]

2. Description of the Related Art Plastic shrink films are used for labels and cap seals of containers, coating of electronic parts, and the like. Conventionally, polyvinyl chloride has been used as a resin for shrink film, but PVC has been developed due to generation of harmful gas during incineration.

[0003] Polystyrene as an alternative to polyvinyl chloride,
Although there is polyethylene or the like, it has a drawback that it cannot be maintained in a film state due to melting and rupture in a heat sterilization step such as boil treatment or retort treatment due to low heat resistance.

On the other hand, there is a polyester-based heat-shrinkable film which is superior in heat resistance and mechanical strength.

[0005]

Since polyester has high heat resistance and high mechanical strength, it does not fuse in the heat sterilization process and is suitable for this use. However, heat treatment at high temperature causes hydrolysis to decompose oligomers and the like. There is a disadvantage that the substance is deposited on the film surface and whitened. In addition, there is a disadvantage that crystallization proceeds in the heat sterilization step and fibrillates in the stretching direction (shrinkage direction) to cause breakage or cracking due to external force such as impact or friction during transportation.

A polyester label for retort food has been proposed in Japanese Patent Application Laid-Open No. 9-52965. Here, the use of PET copolymerized with sulfoisophthalic acid suppresses fusion in the heat sterilization step, but is insufficient for suppressing whitening and reduction in strength after the heat sterilization step.

An object of the present invention is to provide a heat-shrinkable polyester film which has sufficient shrinkage and does not cause whitening and strength reduction after a heat sterilization step.

[0008]

The gist of the present invention is as follows.
A multilayer film having a polyester (A) containing 50 mol% or more of butanediol in the glycol component as a surface layer and a polyester (B) containing 60 mol% or more of ethylene glycol in the glycol component as an inner layer;
The shrinkage in a triethylene glycol bath at 30 ° C. is 30% or more, the haze of the film after heat treatment in saturated steam at 120 ° C. for 1 hour is 0.22% / μm or less, and the main shrinkage direction of the heat-treated film Is a heat-shrinkable polyester film having a tensile elongation at break of 20% or more in a direction perpendicular to the polyester film.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The polyester (A) constituting the surface layer of the heat-shrinkable polyester film of the present invention contains 1,4-butanediol in an amount of 50 mol% or more in a glycol component. When the total amount of the components and the total glycol components is 100 mol%, polyesters in which the total of the acid components other than terephthalic acid and the glycol components other than 1,4-butanediol are in the range of 5 to 55 mol% are preferred, and especially 10 to 50 mol. % Polyester is preferred.

If the total of the acid component other than terephthalic acid and the glycol component other than 1,4-butanediol (hereinafter referred to as the third component) is less than 5 mol%, polyethylene terephthalate having a high degree of crystallinity upon stretching and a high shrinkage ratio. (Hereinafter PE
It is called T. ) -Based polymer as an inner layer does not provide a sufficient shrinkage, and if it exceeds 55 mol%, the crystallinity of polybutylene terephthalate (hereinafter referred to as PBT) is significantly reduced, and is fused during the heat sterilization step (retort treatment). Tend to be.

As an acid component usable as the third component,
Isophthalic acid, 1,4- or 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid, azelaic acid,
Conventionally known monomers such as glutaric acid, oxalic acid and succinic acid are exemplified.

Examples of the glycol component include conventionally known monomers such as ethylene glycol, diethylene glycol, triethylene glycol, 1,4-cyclohexanedimethanol, neopentyl glycol, and an ethylene oxide adduct of bisphenol A.

In order to shorten the polymerization time or to suppress shrinkage unevenness and wrinkles, trihydric or higher polyhydric alcohols such as trimethylolpropane, pentaerythritol, trimellitic acid and pyromellitic acid, and polybasic acids may be used. .

In order to impart flexibility, a polyalkylene glycol such as polyethylene glycol or polyoxytetramethylene glycol can be added in the polyester polymerization step.

The polyester (B) constituting the inner layer of the heat-shrinkable polyester film of the present invention contains ethylene glycol in an amount of 60 mol% or more in a glycol component, and further contains a total acid component and a total glycol component of the polyester. When each is 100 mol%, a polyester having a total of the third component in the range of 15 to 45 mol% is preferable, and a polyester having a total of 20 to 40 mol% is particularly preferable. If the total amount of the third component is less than 15 mol%, the present invention in which the surface layer is a PBT polymer cannot provide a sufficient shrinkage, and if the total exceeds 40 mol%, the shrinkage reaches saturation, resulting in an unnecessary modification rate. Become.

As the acid component usable as the third component,
Isophthalic acid, 1,4- or 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid, azelaic acid,
Conventionally known monomers such as glutaric acid, oxalic acid and succinic acid are exemplified.

Examples of the glycol component include conventionally known monomers such as diethylene glycol, triethylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, and ethylene oxide adduct of bisphenol A.

A trihydric or higher polyhydric alcohol such as trimethylolpropane, pentaerythritol, trimellitic acid, pyromellitic acid, or a polybasic acid may be used for shortening the polymerization time or suppressing shrinkage unevenness and wrinkles. .

For imparting flexibility, a polyalkylene glycol such as polyethylene glycol or polyoxytetramethylene glycol can be added in the polyester polymerization step.

The heat shrinkable polyester film of the present invention has a haze of 0.22% / μm or less after heat treatment in saturated steam at 120 ° C. for 1 hour.

The heat-shrinkable polyester film of the present invention has a shrinkage ratio in a 100 ° C. triethylene glycol bath of 30% or more, preferably 40% or more. If the shrinkage is less than 30%, it is insufficient as a shrink film.

After heat-treating the heat-shrinkable polyester film of the present invention in saturated steam at 120 ° C. for 1 hour, the film has a tensile elongation at break of 20 in the direction perpendicular to the main shrinkage direction of the film.
% Or more, more preferably 30% or more.

The polyester film has a drawback that the crystallization proceeds in the stretching direction in the heat sterilization step (retort treatment) and fibrillation occurs, so that the polyester film is broken or broken by external force such as friction or impact. Although there is a method of increasing the degree of polymerization to prevent cracking of PET due to external force, it is insufficient to suppress cracking in the heat sterilization step (retort treatment).

In the present invention, a heat-shrinkable polyester film which is hardly torn even after retort treatment is obtained by forming a multilayer structure of a PBT polymer having excellent flexibility and a PET polymer having excellent shrinkability.

In the structure of the multilayer film, the ratio of the PBT-based polymer to the film thickness is preferably from 20% to less than 80%, more preferably from 30% to less than 70%. When the ratio of the PBT polymer to the film thickness is less than 20%, the film is easily damaged after the retort treatment because the PBT polymer layer is thin, and when the ratio of the PBT polymer to the film thickness is 80% or more, the shrinkage ratio is small. In some cases, a sufficient percentage of shrinkage cannot be obtained because the proportion of a PET polymer having a high

The polyester resin of the present invention can be produced by a known direct polymerization method or transesterification method, and the degree of polymerization is not particularly limited. / Measured at 25 ° C. using a mixed solution of equal weight in tetrachloroethane as a solvent).
Those having 5 to 1.5 are preferred.

The obtained polyester resin is formed into a heat-shrinkable polyester film by the following method, for example. First, the polyester resin is dried or a decrease in the degree of polymerization due to hydrolysis is suppressed by a vent-type extruder, and a multilayer film raw material is formed by a multilayer extruder or an extrusion laminating method.

Next, this raw film is stretched 1.5 to 5.0 times, preferably 1.0 to 4.8 times in the machine direction or the transverse direction at a temperature higher than the Tg of the PET polymer by 3 ° C. or more, and has a high shrinkage. The rate is given to the film. Further, if necessary, the film is stretched 1.0 to 1.8 times, preferably 1.0 to 1.5 times in the direction perpendicular to the stretching direction. This is effective for increasing the strength in the direction perpendicular to the main shrinkage direction of the film and preventing the shrinkage in the stretching direction from unnecessarily shrinking.

The film is stretched by simultaneous biaxial stretching, sequential biaxial stretching, or uniaxial stretching. Either the transverse stretching or the longitudinal stretching may be performed first. The stretched heat-shrinkable film can be used as a product as it is, but may be subjected to a heat treatment at a temperature of 50 to 150 ° C. for several seconds to several tens of seconds from the viewpoint of dimensional stability. By performing such a heat treatment, preferable properties such as adjustment of the shrinkage ratio in the shrinkage direction of the polyester film of the present invention, reduction of shrinkage with time of storage of the unshrinkable film, and reduction of shrinkage unevenness can be exhibited.

The thickness of the heat-shrinkable polyester film of the present invention is not particularly limited, but a thickness in the range of 1 to 600 μm is practically used.

In the present invention, conventionally known various processings and appropriate additives can be blended in order to further impart specific performance.

Examples of the processing include ultraviolet rays, α rays, β
Irradiation with rays, γ rays, or electron beams, treatments such as corona treatment, flame treatment, etc., application and lamination of resins such as vinylidene chloride, polyvinyl alcohol, polyamide, and polyolefin, and vapor deposition of metal.

Examples of additives include resins such as polyamide, polyolefin, polymethyl methacrylate, and polycarbonate; inorganic particles such as silica, talc, kaolin, and calcium carbonate; pigments such as titanium oxide and carbon black; ultraviolet absorbers; Agents, flame retardants and the like.

The thus obtained heat-shrinkable polyester film of the present invention can be used for food labels, cap labels, and the like that involve a retort treatment step.

[0035]

【Example】

(Resin adjustment: Resins 1 to 6)
Put into a reaction vessel equipped with a stirrer 500 p
pm of tetrabutylentitantanate and the reaction temperature was 14
The reaction was carried out at 0 ° C. to 240 ° C. while distilling off by-produced methanol until the methanol distillation amount reached 90% of the theoretical amount to obtain an esterified product.

Then, while distilling off the glycol component by-produced from the esterified product at 250 ° C. under a high vacuum of 5 torr or less, PE having a melt viscosity at 250 ° C. of an intrinsic viscosity of 0.7.
Melt polymerization was performed until the melt viscosity of T became equal to the melt viscosity at 285 ° C. to obtain a polyester resin.

(Preparation of Resin: Resins 7 to 11) The raw materials shown in Table 1 were placed in a reaction vessel equipped with a reflux tower and a stirrer, and then 200
The reaction was carried out at a temperature of from 240 ° C. to 240 ° C. while distilling off by-produced water until the amount of distilled water became 90% of the theoretical amount to obtain an esterified product.

Then, 500 ppm of antimony trioxide with respect to the weight of the raw material was added to the esterified product, and 285 ° C.
While distilling off the glycol component by-produced at a high vacuum of 5 torr or less, melt polymerization was performed until the melt viscosity at 285 ° C. became equal to the melt viscosity at 285 ° C. of PET with an intrinsic viscosity of 0.7 to obtain a polyester resin. .

The obtained polyester resin had the composition shown in Table 1.

[0040]

[Table 1]

(Formation of Film) Examples 1 to 5, Comparative Examples 1 to 7 These resins were dried by vacuum drying until the water content became 100 ppm or less, and the discharge rate was adjusted by a multilayer extruder to obtain a predetermined composition. A three-layer film raw material having a surface layer, an inner layer and a surface layer was obtained. The extrusion temperature was 250 ° C. for Resins 1 to 6, and 260 ° C. for Resins 7 to 11.

The raw film is stretched 3.8 times in the TD direction and 1.0 times in the MD direction at a temperature higher than the glass transition temperature of the PET polymer by 3 ° C. or more by a batch-type biaxial stretching machine. A heat-shrinkable polyester film having a thickness of 40 μm was obtained.

Table 2 shows the shrinkage ratio, transparency after retort treatment, tensile elongation at break in a direction perpendicular to the shrinkage direction, and fusion characteristics.

[0044]

[Table 2]

The method for measuring physical properties will be described below.

Heat shrinkage: A polyester film cut out to a size of 150 mm in the stretching direction and 20 mm in a direction perpendicular to the stretching direction is marked with a mark at a distance of 100 mm.
And immersed in a 100 ° C. triethylene glycol bath, and determined from the length between the marked lines after one minute.

Shrinkage (%) = {(LL ′) / L} × 100 L: Length before shrinkage L ′: Length after shrinkage ・ Retort treatment method The stretching direction of the film is the circumferential direction of the glass bottle. And heat-treated in saturated steam at 120 ° C for 1 hour. The following evaluation was performed on the film after the retort treatment.

Haze The haze of the film after the retort treatment was measured by a haze meter (MODELTC-HIII) manufactured by Tokyo Denshoku Co., Ltd., and the haze was divided by the thickness of the film.

Tensile rupture elongation The film after retort treatment is cut into a width of 5 mm in a direction perpendicular to the shrinkage direction, a chuck interval of 30 mm, and a speed of 50 mm.
Measure the tensile elongation at break in mm / min.

Fused: Determined by the degree of fusion when peeled from the glass bottle. ○: Peeled clean without leaving any trace.

[0051]

Industrial Applicability The polyester film of the present invention is a film which has sufficient shrinkage and has sufficient transparency and strength even after a heat sterilization step (retort treatment) and can be used for food labels.

Claims (1)

[Claims] 1. A polyester (A) containing at least 50 mol% of 1,4-butanediol in a glycol component in a surface layer, and 60 mol% of ethylene glycol in a glycol component.
It is composed of a multilayer film having the polyester (B) as an inner layer, and has a shrinkage ratio of 30% or more in a triethylene glycol bath at 100 ° C. and 1% in saturated steam at 120 ° C.
Haze of film after heat treatment for 0.22% / μ
m, the heat-shrinkable polyester film having a tensile elongation at break of 20% or more in a direction perpendicular to the main shrinkage direction of the heat-treated film.