JP4576886B2 - Laminated polyester film for folding packaging - Google Patents

Description

  The present invention relates to a laminated polyester film for folding packaging. More specifically, the packaging has excellent bending properties and twist fixability while maintaining the practical properties without losing the strength, heat resistance, fragrance retention, water resistance, etc., which are the excellent properties of the stretched polyester film. The present invention relates to a laminated polyester film for folding packaging that is useful as a film for use in bending.

  Conventionally, cellophane has been known as a film excellent in twist fixing property. Cellophane is widely used for various packaging materials and adhesive tapes due to its excellent transparency, easy cutting property, twist fixing property, and the like. However, on the other hand, since cellophane has hygroscopicity, its characteristics fluctuate depending on the season, and it is difficult to always supply a product of a certain quality. In addition, packaging bags using polyethylene terephthalate as a base film have been used to evaluate the excellent properties of stretched polyethylene terephthalate film such as toughness, heat resistance, water resistance, and transparency. On the other hand, it has the disadvantage that it cannot be used for folding packaging or twist packaging because it has no excellent folding property and twist fixing property.

As a method for solving the above problems, the stress-strain curve has a yield point, the average refractive index of the unstretched film of the copolymer is N ₀ , and the average refractive index of the biaxially stretched film is N ₁ . In some cases, an easy-bending polyester film that satisfies 0.003 ≦ N ₁ −N ₀ ≦ 0.021 has been proposed.
Japanese Patent No. 2505474

However, when the stress-strain curve has a yield point and the average refractive index of the unstretched film of the copolymer is N ₀ and the average refractive index of the biaxially stretched film is N ₁ , 0.003 ≦ N _In the method of ₁₋ N ₀ ≦ 0.021, the bending property and the twist fixing property are insufficient, or the film in the width direction is caused by wrinkles due to shrinkage due to heat when processing such as printing or vapor deposition is performed. Dimensional change occurred and could not be used.

Moreover, as a method for improving the shortage of bending property and heat resistance, which are the above-mentioned problems, at least one surface of the polyester resin A layer has a melting point higher by 10 ° C. than the melting point of the polyester resin A layer, And the unstretched laminated film which laminated | stacked the polyester resin B layer of the thickness of 5% or more and 60% or less with respect to the whole thickness at least 10 degreeC lower than melting | fusing point of the polyester resin A layer after uniaxial stretching, and polyester resin There has been proposed a method for producing a polyester film having a good tearability and twist retention, which is heat-treated at a temperature lower than the melting point of the B layer.
Japanese Patent Laid-Open No. 5-104618

  However, at least one surface of the polyester resin layer A has a melting point higher by 10 ° C. or more than the melting point of the polyester resin layer A and has a thickness of 5% or more and 60% or less of the total thickness. In the method of heat-treating an unstretched laminated film laminated with at least uniaxially stretching at a temperature of 10 ° C. lower than the melting point of the polyester resin layer A and lower than the melting point of the polyester resin layer B, the polyester resin layer B having a high melting point Sufficient easy bendability and twist fixability may not be obtained due to the influence. If the thickness of the polyester resin layer B is decreased in order to obtain easy bendability and twist fixability, the film becomes brittle, and when packaging It was not practical because it was torn.

  In light of the above-mentioned problems of the prior art, the present invention pays particular attention to easy bendability and twist fixability, has the property of being bent like paper, and is an excellent property of a polyester film, An object of the present invention is to obtain a laminated polyester film for folding packaging having heat resistance, moisture resistance, transparency, fragrance retention and the like.

In order to achieve the above object, the laminated polyester film for folding packaging of the present invention has at least one kind of crystalline polyester resin having a glass transition temperature of 60 ° C. or more and at least one kind of glass transition temperature of 60 ° C. A biaxially stretched polyester-based film comprising a mixture with the above amorphous copolymerized polyester resin and having at least a three-layer structure, wherein both outer layers are of melting peaks appearing on a differential scanning calorimeter (DSC) curve. Although it shows the maximum temperature, it is a layer made of a resin mixture having a crystallinity of 30 ° C. or higher between the melting peak temperature and the inflection point temperature of the baseline, and an inner layer made of a resin mixture that is not substantially crystalline Yes, and the absolute value of the difference between the refractive index Ny in the longitudinal direction of the refractive index Nx in the width direction of the polyester film (| Nx-Ny |) is 0.005 or less Oh, wherein the Rukoto.

  In this case, the tensile elongation at break in the longitudinal direction after being left in a 40 ° C. atmosphere for one week can be 50% or more.

  Further, the thermal shrinkage in the longitudinal direction of the film when left at 150 ° C. for 30 minutes can be 5.0% or less.

  Further, the heat treatment after biaxial stretching can be performed at the melting peak temperature of −5 ° C. or lower and a temperature of the inflection point temperature of the baseline or higher.

  Moreover, 90 mol% or more of the acid component which comprises an amorphous polyester resin can consist of terephthalic acid.

  Moreover, the ratio of the total thickness of both outer layers and the thickness of the inner layers can be set to both outer layers / inner layers = 1/4 to 4/1.

  According to the polyester-based resin film of the present invention, the laminated film has excellent properties such as strength, heat resistance, moisture proofness, transparency, and fragrance retention properties, but also has excellent bendability and twist fixability. And has an advantage that it can be suitably used for folding packaging and twist packaging of chocolate, candy, gum and the like.

Hereinafter, the present invention will be described in detail.
The laminated polyester film for folding wrapping of the present invention is a mixture in which both outer layer and inner layer are a mixture of a specific crystalline resin and a specific amorphous resin, and both outer layers are a resin mixture having crystallinity. There is a layer made of a resin mixture that is substantially non-crystalline in the inner layer.

  The crystalline polyester resin used for producing the laminated polyester film for folding packaging of the present invention has a glass transition temperature of 60 ° C. or higher, more preferably 67 ° C. or higher. When the glass transition temperature is less than 60 ° C., embrittlement progresses due to processing after film formation or heat during storage, making practical use difficult. Moreover, since film characteristics change immediately after film formation and when post-processing or packaging is used, it is difficult to guarantee quality, which is not preferable.

  Examples of the crystalline polyester resin used for producing the laminated polyester film for folding packaging of the present invention include polyethylene terephthalate, polyethylene naphthalate, or a copolymer mainly composed of these constituents. Preferably, the polyester resin comprises 95% by weight or more of terephthalic acid and 95% by weight or more of ethylene glycol, more preferably 98% by mole or more of terephthalic acid and 97% by mole or more of ethylene glycol. The polyester resin consisting of 90% by weight or more based on the whole crystalline polyester resin.

  When the content of terephthalic acid and / or ethylene glycol is less than 95 mol%, the crystallinity is lowered and it is difficult to ensure the rigidity of the film, and the ester is mixed and extruded with the amorphous resin. This is not preferable because it tends to be a uniform resin by an exchange reaction.

  The amorphous polyester resin used for producing the laminated polyester film for folding packaging of the present invention has a glass transition temperature of 60 ° C. or higher, more preferably 67 ° C. or higher. When the glass transition temperature is less than 60 ° C., embrittlement progresses due to processing after film formation or heat during storage, making it impossible to use practically. In addition, since film properties at the time of film formation and processing or packaging change, it is difficult to guarantee quality, which is not preferable.

  As an amorphous polyester resin used for producing the laminated polyester film for folding packaging of the present invention, terephthalic acid is mainly used for the acid component, ethylene glycol is mainly used for the glycol component, and other acid components and Polyesters containing / or other glycol components as copolymerization components are preferred. Other acid components include aliphatic dibasic acids (for example, adipic acid, sebacic acid, azelaic acid) and aromatic dibasic acids (for example, isophthalic acid, diphenyldicarboxylic acid, 5-tert-butylisophthalic acid, 2,2,6,6-tetramethylbiphenyl-4,4-dicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,1,3-trimethyl-3-phenylindene-4,5-dicarboxylic acid) . Examples of the glycol component include aliphatic diols (for example, neopentyl glycol, diethylene glycol, propylene glycol, butanediol, hexanediol), alicyclic diols (for example, 1,4-cyclohexanedimethanol), or aromatic diols (for example, xylylene Renglycol, bis (4-β-hydroxyphenyl) sulfone, 2,2- (4-hydroxyphenyl) propane derivative) are used.

  In order to set the glass transition temperature of the polyester resin used in the present invention to 60 ° C. or higher, for example, an acid component having a rigid molecular structure based on a benzene ring, or a monomer component having a ring structure and difficult to rotate. A glycol component with a large amount is preferred. For example, introduction of a copolymer component having a methylene chain is not preferable because the glass transition temperature is lowered.

  In the polyester resin used in the present invention, among the copolymer polyesters, terephthalic acid-isophthalic acid-ethylene glycol copolymer, terephthalic acid-ethylene glycol-neopentyl glycol copolymer, terephthalic acid-ethylene glycol are particularly preferable. -Cyclohexanedimethanol copolymer, etc., more preferably terephthalic acid-ethylene glycol-neopentyl glycol copolymer, terephthalic acid-ethylene glycol-cyclohexanedimethanol copolymer. These copolyesters are produced industrially and can be suitably used from the viewpoint of cost, quality, supply stability, and the like.

  Furthermore, it is preferable that 90 mol% or more of the acid component of the amorphous polyester resin used in the present invention is terephthalic acid. Terephthalic acid is very rigid because of its molecular structure, and is preferable for increasing the glass transition temperature. Also, when used as an amorphous resin component, terephthalic acid helps maintain film rigidity when formed into a film.

  In the present invention, both outer layers must have crystallinity in a state where a crystalline polyester resin having a glass transition temperature of 60 ° C. or higher and an amorphous polyester resin having a glass transition temperature of 60 ° C. or higher are formed on a film. The melting peak temperature (hereinafter sometimes referred to as the melting point) is preferably 220 ° C. or higher and 250 ° C. or lower, more preferably 225 ° C. or higher and 240 ° C. or lower.

  In the present invention, both outer layers are preferably mixed in a ratio of crystalline polyester resin / amorphous polyester resin = 40/60 to 80/20 wt%, and more preferably 55/45 to 75/35 wt%. Is preferred. By mixing at these ratios, the crystalline polyester resin and the amorphous polyester resin are combined and the melting point thereof is set to 220 ° C. or higher and 250 ° C. or lower, so that even better bendability can be exhibited.

  In the present invention, when the melting point of the polyester resin forming both outer layers is less than 220 ° C., the heat shrinkage rate is deteriorated due to insufficient heat resistance, and dimensional stability due to heat during processing such as printing and vapor deposition is reduced. It is not preferable because it deteriorates. Moreover, when the melting point exceeds 250 ° C., the crystallinity of both outer layers becomes too high, so that the bending property may not be exhibited, which is not preferable.

  The melting point of the polyester resin forming both outer layers may change depending not only on the type and mixing ratio of the polyester resin to be mixed, but also on the kneading state during film formation, the extrusion temperature, the residence time, etc. It is preferable to appropriately adjust these conditions so that the temperature is not lower than 250 ° C. and not higher than, for example, kneading is preferably performed using a screw having a low shear rate, lowering the extrusion temperature, and shortening the residence time. Since it often depends on equipment limitations, it is preferable to adjust the mixing ratio between the crystalline polyester resin and the amorphous polyester resin.

  Furthermore, the polyester resin that forms both outer layers is melted although it exhibits the highest temperature among melting peaks appearing on the differential scanning calorimeter (DSC) curve of the polyester resin that constitutes both outer layers after biaxially stretching the unstretched laminated sheet. The difference between the peak temperature and the inflection point temperature of the baseline is preferably 30 ° C. or higher, and more preferably 35 ° C. or higher.

  In the present invention, the base inflection point temperature (Tm1) of the melting peak appearing at the maximum temperature in the differential scanning calorimeter (DSC) curve of the polyester resin constituting both outer layers after biaxially stretching the unstretched laminated sheet. ) To the melting peak temperature (TmP) −5 ° C., the desired easy bendability and twist fixability are exhibited by heat treatment after biaxial stretching. Further, in order to better express the bendability and the twist fixing property, it is preferable to perform heat treatment at a temperature of the inflection point temperature (Tm1) + 5 ° C. or higher and the melting peak temperature (TmP) −10 ° C. or lower.

  In the present invention, when the temperature difference (TmP−Tm1) (hereinafter sometimes referred to as “ΔTm”) from the inflection point temperature (Tm1) to the melting peak temperature (TmP) is less than 30 ° C., easy bending This is not preferable because the range from the expression of the folding property to the melt fracture is narrowed and the adjustment becomes difficult and the productivity of the bending process is deteriorated.

  In the present invention, it is assumed that a mixture of a crystalline polyester resin and an amorphous polyester resin used for both outer layers may be partially copolymerized by an ester exchange reaction in an extrusion process. In this case, the DSC curve of the mixture after this step is a DSC curve of a mixture of a crystalline polyester resin and an amorphous polyester resin that is simply a chip or powder mixed, or a copolymer that is copolymerized so as to have a composition of the mixture. The melting peak value is different from the DSC curve of the polymer, and the shoulder portion is broad.

  That is, the resin composition used to form both outer layers of the biaxially stretched polyester film of the present invention has a composition ratio of the crystalline polyester resin and the amorphous polyester resin that is higher than the portion where the crystalline polyester resin concentration is high. There is a portion where the crystalline polyester resin concentration is high, the shoulder portion of the peak, which is the melting peak exhibited by the portion where the amorphous polyester resin concentration is high, becomes gentle, and excellent melting is achieved by melting that portion. Flexibility is obtained.

  At this time, the shape of the DSC curve changes depending on various conditions such as the resin composition ratio, extrusion temperature, residence time, etc., but the difference between the inflection point temperature (Tm1) and the melting peak temperature (TmP) is 30 ° C. or more. By doing so, stable bendability is obtained.

  In the present invention, the inner layer is a mixture of a crystalline polyester resin having a glass transition temperature of 60 ° C. or higher and an amorphous polyester resin having a glass transition temperature of 60 ° C. or higher in a ratio that makes a resin mixture that is not substantially crystalline. It is composed of a resin mixture, preferably mixed in a ratio of crystalline polyester resin / amorphous polyester resin = 3/97 to 30/70% by weight, more preferably in a ratio of 5/95 to 20/80% by weight. Have been mixed. In addition, when there are two or more inner layers, if at least one layer is a mixture of crystalline polyester resin / amorphous polyester resin = 3/97 to 30/70 wt%, the other inner layers are The combination is not limited as long as the characteristics of the present invention are not impaired.

  When the ratio of the crystalline polyester resin in the inner layer is less than 3% by weight, the rigidity of the inner layer is remarkably lowered, and the rigidity of the entire film becomes insufficient. On the other hand, if it exceeds 30% by weight, it may show crystallinity after being mixed with an amorphous polyester resin, and the excellent easy bendability characteristic of the present invention cannot be obtained, which is not preferable.

  The laminated polyester film for folding packaging according to the present invention preferably has a heat shrinkage of 5.0% or less, more preferably 3% or less when left at 150 ° C. for 30 minutes. When the thermal shrinkage rate exceeds 5%, the dimensions change in a process having a drying process such as a printing process or a vapor deposition process, or a process having a heating process, which causes a pitch shift or causes wrinkles or talmi.

  In the folded polyester laminated film of the present invention, in order to reduce the heat shrinkage rate to 5% or less, crystallizing the crystalline resin component of both outer layers of the film in the heat treatment after biaxial stretching, A relaxation treatment that relaxes stretching during the heat treatment is preferred.

  In the laminated polyester film for folding packaging according to the present invention, the inner layer is a layer made of a resin mixture that is not substantially crystalline, so that it is in a molten state in the heat treatment process, and neither crystallization nor stress relaxation occurs. Since it has crystallinity, crystallization and stress relaxation occur.

  In the laminated polyester film for folding packaging according to the present invention, the melting point of both outer layers should be 220 ° C. or higher, which is a necessary condition for improving the heat shrinkage rate. Is about 200 ° C. or less for preventing melt fracture of the film, which causes a deterioration of the heat shrinkage rate.

  In the laminated polyester film for folding packaging according to the present invention, both outer layers are layers having easy bendability while maintaining the strength and rigidity of the film, and the inner layer is a layer having excellent bendability. Is playing a role. Therefore, the ratio of the total thickness of both outer layers to the thickness of the inner layer is preferably both outer layers / inner layers = 1/4 to 4/1. If this range is exceeded, the bendability is excellent, but the film strength Or, on the contrary, a film having insufficient bending property is not preferable.

  The intrinsic viscosity of the polyester resin that can be used for the laminated polyester film for folding packaging of the present invention is preferably 0.55 to 1.3 dL / g, more preferably 0.62 to 0.78 dL / g. g.

  Moreover, it is preferable to make the difference in intrinsic viscosity between the outer layer and the inner layer after mixing within ± 0.3. When the intrinsic viscosities of the resins of the respective layers are remarkably different, in the present invention, layers of two or more kinds of polyester resins selected from the intrinsic viscosities within these ranges are laminated. When the block method is used, if the intrinsic viscosities are remarkably different, the resin flow becomes non-uniform, and it is not preferable because uniform characteristics cannot be obtained in the width direction.

  The thickness of the laminated polyester film for folding packaging according to the present invention is 12 μ to 30 μm when used for folding packaging, twist packaging, etc., which are the intended purposes of the present invention, but is particularly limited. It can be arbitrarily selected depending on the size, weight, shape or packaging form of the contents.

  The laminated polyester film for folding packaging according to the present invention has a molecular orientation that the absolute value | Nx−Ny | of the difference between the refractive index Nx in the longitudinal direction and the refractive index Ny in the width direction is 0.005 or less. This is preferable because the degree of disappearance of the non-existing layer is constant. Here, it is more preferably 0.002 or less.

  When the absolute value of the difference between the refractive index Nx in the longitudinal direction and the refractive index Ny in the width direction exceeds 0.005, the polyester resin layer having no crystallinity used for the inner layer is oriented. Therefore, it is difficult to obtain excellent bendability and twist fixability, which are the intent of the present invention.

  The laminated polyester film for folding packaging of the present invention may be added with various known additives such as lubricants, pigments, antioxidants, antistatic agents, etc., as long as the effects of the present invention are not impaired. .

  Next, an example of a method for producing a laminated polyester film for folding packaging according to the present invention will be described.

  Using vacuum-dried crystalline polyester resin and amorphous polyester resin, mix the mixed raw material for outer layer and mixed raw material for inner layer, respectively, and supply them to two different extruders, respectively, exceeding the melting point of crystalline polyester resin Are melted at the same temperature, laminated by a multi-manifold die system, extruded as two types and three layers, cooled and solidified to form an unstretched laminated film.

  The unstretched laminated film thus obtained is stretched 2 to 4 times in the machine direction at a temperature not lower than the glass transition temperature of the crystalline polyester resin and not higher than the glass transition temperature + 40 ° C., and immediately cooled to 20-40 ° C. .

  Next, the film is stretched 3 to 4.5 times in the transverse direction at a temperature equal to or higher than the longitudinal stretching temperature + 10 ° C. and lower than the crystallization temperature of the crystalline polyester resin.

  The biaxially stretched film is heat-treated at a temperature not lower than the inflection point temperature from the baseline when the film melting point is measured by DSC and lower than the melting peak temperature of −5 ° C. In this heat treatment, a relaxation treatment may be performed as necessary.

  Through the above steps, both outer layers are easily foldable while maintaining the strength and rigidity of the film, and the inner layer is a layer having rigidity while having excellent foldability. A biaxially stretched polyester film excellent in bendability is obtained.

  Next, the present invention will be specifically described with reference to examples and comparative examples. About the evaluation method in this specification, it performed by the following method.

(A) Glass transition temperature, inflection point temperature, melting peak temperature (melting point)
Using a DSC-60 type differential scanning calorimeter (DSC) manufactured by Shimadzu Corporation, about 5.0 mg of the film was measured in the range of 30 ° C. to 280 ° C. at a temperature rising rate of 20 ° C./min. The tangential intersection of the obtained DSC curve displacement was defined as the glass transition temperature (Tg). Further, for the melting peak of the obtained DSC curve, the inflection point temperature (Tm1) and the melting peak temperature (TmP) from the baseline were determined. Further, a temperature difference (TmP−Tm1) from Tm1 to TmP was set to ΔTm.

  In the present invention, since the inner layer is amorphous, the inner layer does not have a melting peak. Therefore, in the present invention, the melting peak of both outer layers can be observed by measuring the film. Alternatively, the melting peaks of both outer layers can be observed by measuring only the surface layer of the film or unstretched film.

(B) Bending property After dividing the entire width of the sample into three equal parts, n = 2 from the central portion of each position, and a sample for measurement having a length of 60 mm and a width of 10 mm is taken with respect to the longitudinal direction of the film, and glass A sheet of silicon was laid on the plate, bent 180 degrees at 20 mm from the edge, applied to the bent part for 10 seconds to remove 0.5 kg / cm ² , removed, and recovered 30 seconds later I read the angle. The smaller this angle, the better the bendability, and the measured value of the paper was 20 degrees.
◎: Recovery angle less than 30 degrees ○: Recovery angle 30 degrees or more and less than 50 degrees △: Recovery angle 50 degrees or more and less than 70 degrees ×: Recovery angle 70 degrees or more

(C) Elongation at break after aging The film was allowed to stand in an atmosphere of 40 ° C. for 7 days, and then the tensile elongation at break in the longitudinal direction was measured. For the measurement, an autograph made by Shimadzu Corporation was used, and after dividing the entire sample width into 5 equal parts, two samples each having a longitudinal direction of 200 mm and a width direction of 15 mm were cut out from the central portion of each position and held at a tensile interval of 100 mm. Then, 10 pieces were measured at a pulling speed of 200 mm / min to obtain the minimum value.
◎: Breaking elongation 70% or more ○: Breaking elongation 50% or more △: Breaking elongation 30% or more ×: Breaking elongation less than 30%

(D) Thermal shrinkage rate After dividing the entire sample width into three equal parts, samples with a width of 10 mm and a length of 250 mm were cut out from the central part of each position in the longitudinal direction of the film, marked at intervals of 200 mm, and a constant tension of 5 g. Interval A was measured. Subsequently, it was left in an oven in an atmosphere at 150 ° C. for 30 minutes with no load. After taking out from the oven and cooling to room temperature, the interval B was obtained with a constant tension of 5 g, the heat shrinkage rate was calculated by the following formula, and the average value was obtained.
Thermal contraction rate = (A−B) / A × 100 (%)
◎: Thermal shrinkage rate 3% or less ○: Thermal shrinkage rate 3% to 5% or less △: Thermal shrinkage rate 5% to 7% or less ×: Thermal shrinkage rate 7% or more

(E) Crystallinity Using a DSC-60 type differential scanning calorimeter (DSC) manufactured by Shimadzu Corporation, about 5.0 mg of the film was measured in the range of 30 ° C. to 280 ° C. at a temperature rising rate of 20 ° C./min. After adjusting 10 mg of the sample to an aluminum pan, it is set in a DSC apparatus (reference: aluminum pan of the same type without a sample), the sample is heated at a rate of 10 ° C./min, and the temperature is 285 ° C. for 15 minutes. After heating, it was quenched with liquid nitrogen. The sample was heated at a rate of 10 ° C./min, and the crystallization exothermic peak and the melt endothermic peak were measured from the DSC chart. The resin having no crystallization exothermic peak and no melt endothermic peak was defined as a substantially non-crystalline (amorphous) polyester resin.

(F) Refractive index The refractive index was measured with NaD light by an Abbe refractometer. The mount liquid was methylene iodide, and the temperature was 25 ° C. The refractive index (Nx) in the longitudinal direction, the refractive index (Ny) in the width direction, and the refractive index (Nz) in the thickness direction were measured.

(G) Twist Fixability Double twist packaging was performed at a rate of 200 pieces per minute using a twist packaging machine TA200 type manufactured by Tenchi. The film is twisted 1.5 turns (540 degrees) into individual wrapping, but after twisting, the angle returns to some extent, so the angle that is held in the twisted state is adjusted for each of the 30 samples. The average value of n = 60 (referred to as holding angle) was determined.
The larger the angle is, the better the twist fixing property is. The average value when measuring currently used cellophane is 250 degrees, and the minimum value is 200 degrees.
○: Twist holding angle 240 degrees or more Δ: Twist holding angle 180 degrees or more and less than 240 degrees ×: Twist holding angle less than 180 degrees

  The polyester resins used in Examples and Comparative Examples are as follows.

  PES-A: A crystalline polyethylene terephthalate resin composed of terephthalic acid as an acid component and ethylene glycol as a glycol component. Glass transition temperature 75 ° C., melting point 255 ° C., intrinsic viscosity 0.75 dL / g.

  PES-B: A crystalline polybutylene terephthalate resin comprising terephthalic acid as an acid component and 1,4-butanediol as a glycol component. Glass transition temperature 30 ° C., melting point 224 ° C., intrinsic viscosity 0.84 dL / g.

  PES-C: a copolymerized polyester resin obtained by copolymerizing terephthalic acid as an acid component, 70 mol% of ethylene glycol and 30 mol% of neopentyl glycol as a glycol component. The glass transition temperature was 72 ° C., the intrinsic viscosity was 0.74 dL / g, and the crystallization temperature and melting point were not shown.

  PES-D: a copolyester resin comprising terephthalic acid as an acid component, 70 mol% of ethylene glycol as a glycol component, and 30 mol% of cyclohexanedimethanol. The glass transition temperature was 77 ° C., the intrinsic viscosity was 0.80 dL / g, and the crystallization temperature and melting point were not shown.

Example 1
Both outer layers were mixed with 55% by weight of crystalline PES-A and 45% by weight of amorphous PES-C. As the inner layer, 5% by weight of crystalline PES-A and 95% by weight of amorphous PES-C were mixed and used. Each melt was melted by a separate extruder at a temperature of 285 ° C., the melt was joined by a multi-manifold die with a residence time of about 3 minutes, extruded from a T die, and rapidly cooled by a cooling drum adjusted to 30 ° C. Thus, a three-layer unstretched laminated film having an outer layer / inner layer / outer layer structure was obtained.

  The unstretched laminated film was stretched 3.7 times in the machine direction at 105 ° C. and then 4.0 times in the transverse direction at 110 ° C., and then heat treated at a temperature of 228 ° C. while relaxing 3%. An 18 μm film having a ratio of 1/2/1 was obtained. The properties of the obtained film are shown in Table 2.

(Example 2)
Both the outer layer was mixed with 45% by weight of crystalline PES-A and 55% by weight of amorphous PES-C, and the inner layer was mixed with 25% by weight of crystalline PES-A and 75% by weight of amorphous PES-C. A film having a thickness of 18 μm was obtained in the same manner as in Example 1. The properties of the obtained film are shown in Table 2.

(Example 3)
A film having a thickness of 18 μm was obtained in the same manner as in Example 1 except that 75% by weight of crystalline PES-A and 25% by weight of amorphous PES-C were mixed as both outer layers. The properties of the obtained film are shown in Table 2.

Example 4
A film having a thickness of 18 μm was obtained in the same manner as in Example 1 except that amorphous PES-D was used instead of amorphous PES-C. The properties of the obtained film are shown in Table 2.

(Comparative Example 1)
A film having a thickness of 18 μm was obtained in the same manner as in Example 1 except that 90% by weight of crystalline PES-A and 5% by weight of amorphous PES-C were mixed as both outer layers. The properties of the obtained film are shown in Table 3.

(Comparative Example 2)
Both the outer layer and the inner layer were formed by mixing 5% by weight of crystalline PES-A and 95% by weight of amorphous PES-C, and the resulting film had no melting point. During film formation, when the heat treatment temperature was lowered to a temperature just before the film melted and fractured, the heat shrinkage rate was remarkably inferior and the bendability was not good. The properties of the obtained film are shown in Table 3.

(Comparative Example 3)
A film having a thickness of 18 μm was obtained in the same manner as in Example 1 except that 45% by weight of PES-A and 55% by weight of PES-B were mixed and used for the inner layer. The DSC curve of the obtained film had two melting peaks. When only the inner layer was measured from the unstretched sheet for confirmation, it had a melting peak. The properties of the obtained film are shown in Table 3.

(Comparative Example 4)
A film having a thickness of 18 μm was obtained in the same manner as in Example 1 except that only crystalline PES-A was used for the inner layer. The obtained film showed melting | fusing point of each outer layer and inner layer. The properties of the obtained film are shown in Table 3.

(Comparative Example 5)
A film having a thickness of 18 μm was obtained in the same manner as in Example 1 except that both outer layers were mixed with 45% by weight of crystalline PES-A, 10% by weight of crystalline PES-B, and 45% by weight of amorphous PES-C. The obtained film had characteristic changes with time. The characteristics are shown in Table 3.

(Comparative Example 6)
A film having a thickness of 18 μm was obtained in the same manner as in Example 1 except that only PES-A was used as both outer layers and PES-D was used as the inner layer. Table 4 shows the properties of the obtained film.

(Comparative Example 7)
A film having a thickness of 18 μm was obtained in the same manner except that the layer ratio of Comparative Example 6 was set to 1/8/1. The obtained film had excellent bendability, but the film strength was low, and the film was torn off during the twist fixing evaluation. The characteristics are shown in Table 4.

  Tables 1 to 4 show the evaluation results of the polyester resins used in Examples and Comparative Examples, and the films obtained in Examples 1 to 4 and Comparative Examples 1 to 7, respectively.

  As is clear from Examples 1 to 4 and Comparative Examples 1 to 7, in the biaxially stretched polyester film having at least three layers, each layer is a crystal having at least one kind of glass transition temperature of 60 ° C. or more. The outer layer is a layer made of a resin mixture having crystallinity, and the inner layer is substantially crystalline. It is understood that the laminated polyester film for folding packaging, which is a layer composed of a resin mixture that is not curable, has excellent easy bendability, excellent heat resistance, and no change in properties over time.

  As described above, the laminated polyester film for folding packaging according to the present invention has been described based on a plurality of examples, but the present invention is not limited to the configuration described in the above examples, but is described in each example. The configuration can be changed as appropriate within a range not departing from the gist, such as appropriately combining the configurations.

  The biaxially stretched polyester film of the present invention has excellent bendability, twist fixability, excellent heat resistance, and properties that do not change even when processed or stored. It can be used in a wide range of applications as a film for solid packaging, folding packaging, or twist packaging.

Regarding the melting peak of the DSC curve, the inflection point temperature (Tm1) and the melting peak temperature (TmP) from the baseline are shown.

Claims (6)

At least a three-layer structure comprising a mixture of at least one kind of crystalline polyester resin having a glass transition temperature of 60 ° C. or more and at least one kind of amorphous copolymer polyester resin having a glass transition temperature of 60 ° C. or more. The biaxially stretched polyester-based film having both outer layers exhibit the highest temperature among melting peaks appearing in a differential scanning calorimeter (DSC) curve, but the difference between the melting peak temperature and the inflection point temperature of the baseline is a layer made of a resin mixture having a 30 ° C. or more crystalline, have a layer of the resin mixture is substantially non-crystalline to the inner layer, the refractive index of the refractive index in the longitudinal direction Nx and the width direction of the polyester film Ny absolute value of the difference between (| Nx-Ny |) is laminated packaging polyester film bending Ori characterized der Rukoto 0.005.   2. The laminated polyester film for folding packaging according to claim 1, wherein the tensile elongation at break in the longitudinal direction after being left in a 40 ° C. atmosphere for 1 week is 50% or more.   The laminated polyester film for folding packaging according to claim 1 or 2, wherein the thermal shrinkage in the longitudinal direction of the film when left at 150 ° C for 30 minutes is 5.0% or less.   The bending according to claim 1, 2, or 3, wherein the melting peak temperature is -5 ° C or lower and a heat treatment after biaxial stretching is performed at a temperature equal to or higher than a baseline inflection point temperature. Laminated polyester film for packaging.   The laminated polyester film for folding packaging according to claim 1, 2, 3 or 4, wherein 90 mol% or more of the acid component of the amorphous polyester resin comprises terephthalic acid.   The laminate for folding packaging according to claim 1, 2, 3, 4, or 5, wherein the ratio of the total thickness of both outer layers to the thickness of the inner layer is both outer layers / inner layers = 1/4 to 4/1. Polyester film.

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