JPH0533894B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Industrial Application] The present invention relates to a heat-shrinkable polyester film (including sheets; the same applies hereinafter) that exhibits particularly suitable properties in the field of packaging materials such as covering, binding, and exterior packaging. . [Prior Art] Tube-shaped bodies made of heat-shrinkable plastic film can be used, for example, in containers, bottles (including plastic bottles; the same applies hereinafter), cans, long objects (pipes, rods, wood, and various rod-shaped bodies). etc.) (hereinafter abbreviated as containers), etc. (hereinafter abbreviated as containers), especially for covering a part or the entire surface of the cap, shoulder, body, etc., for marking, protecting, bundling, In addition to being used for the purpose of improving product value, boxes, bottles, etc.
It is widely used in the field of integrated packaging such as plates, sticks, notebooks, etc., or in the field of packaging items in close contact with the packaged items such as skin packs, and is expected to be used in applications that utilize shrinkage properties and shrinkage stress. Conventionally, heat-shrinkable films made of polyvinyl chloride, polystyrene, polyethylene, hydrochloric acid rubber, etc. have been used for the above-mentioned purposes, and the film has been formed into a tube-like body and then covered with the above-mentioned containers or packaged and heat-shrinked. However, these films have the disadvantage that they have poor heat resistance, tend to melt or burst when subjected to boiling or retort processing, and cannot maintain their film state. Furthermore, in applications that require printing, printing pinholes (microscopic irregularities caused by fissures caused by additives or polymer gels in the film) may occur due to poor ink transfer, and even if printing is successful, it may gradually become uneven afterward. There was also the problem that shrinkage of the film (shrinkage at room temperature) progressed, causing dimensional changes in the printing pitch. On the other hand, tubes using polyester heat-shrinkable films have only been made on a trial basis, and it may not be possible to achieve a sufficiently high heat-shrinkage rate in the desired direction, or There is a problem in that it is not possible to reduce heat shrinkage in the orthogonal direction, and furthermore, conventional polyester heat shrinkable films do not adhere tightly to the packaged object in large shrinkage areas and tend to become pocked, especially when used at high speeds and short times. This tendency is especially strong in high-speed packaging that requires shrinkage. In particular, this phenomenon appears in the form of color spots in the case of metallic printing ink, which poses a significant problem in terms of high-quality appearance. Therefore, it has been difficult to apply it to the above-mentioned uses. [Problems to be Solved by the Invention] The above-mentioned conventional techniques that use general-purpose heat-shrinkable films such as polyvinyl chloride, polystyrene, and polyethylene have the following problems. (a) Lack of near-perfect uniaxial shrinkage In applications where it is ideal for the film to exhibit large shrinkage in a certain direction but not to shrink at all in a direction perpendicular to this direction, the above-mentioned conventional film has no uniaxial shrinkage. Not suitable. For example, if we consider attaching a shrink label to the bottle surface by shrinking it in the horizontal direction, shrinking the label in the vertical direction, that is, in the vertical direction of the bolt, means that the label does not come to the specified position and shrinks up. This results in poor appearance. To prevent this, it is necessary to reduce the shrinkage in the vertical direction, but if we simply orient the film only in the horizontal direction for this purpose, it is immediately understood from common sense regarding the properties of polymeric chemicals. It becomes easy to tear and becomes fibrinized, which weakens its strength. Considering that strength in the longitudinal direction is important for preventing bottle breakage, particularly when the bolt is dropped, simple unidirectional stretching is not a good method. In addition, there are many cases where it cannot be used for other purposes unless it has impact resistance. For this reason, it is desired to develop a film that exhibits sufficiently large shrinkage in a specific direction in a specific temperature range, but only extremely small shrinkage in a direction perpendicular to that direction. (b) Insufficient heat resistance None of the conventional films mentioned above can withstand high-temperature boiling or retort processing, making them unsuitable for sterilization. For example, when retort processing is performed, the conventional film breaks or ruptures during processing and loses all its functions as a film. Therefore, it is desired to provide a heat-shrinkable film that can withstand boiling and retorting. (c) Poor printability Pinholes caused by halftone printing,
Each of the above-mentioned conventional films has its own drawbacks in terms of adhesion with a wide variety of inks and the like.
For example, in polyvinyl chloride, ink pinholes are likely to occur due to gel-like substances, and in continuous tube processing, pinholes will exist in the middle of a long film. If this is fed to an automatic labeling machine, the product will be manufactured with pinholes left behind, so all products must be inspected at the end, and the labor and reprocessing required by sampling will significantly reduce the actual operating rate. . If this pinhole defect were to be inspected and removed after printing, the film would have to be made into a continuous film again after being cut, and it would be necessary to join it with adhesive tape. As a result, seams form, and the appearance of the seam and the area before and after it becomes poor, resulting in defective products, and these defective packages must be removed during the process. Furthermore, in high-precision printing, the printing pitch decreases (shrinkage over time) due to shrinkage of the film after printing, and furthermore, this shrinkage over time continues to progress under distribution temperature conditions, which is difficult to manage. Therefore, for polyvinyl chloride shrink film, etc., refrigerated trucks and low-temperature warehouses are required. For this reason, it is desired to provide a heat-shrinkable film that can be printed without pinhole defects and that does not change over time after printing. (d) Occurrence of craze Polystyrene film is prone to craze and has poor chemical resistance. Therefore, during use, it is easily damaged by chemicals and the printed surface becomes dirty. Therefore, a film with excellent chemical resistance and durability is desired. (e) Industrial waste issues The amount of plastic bottles used has increased rapidly in recent years. When considering the recovery of this bolt,
In particular, if a different type of film such as polyvinyl chloride or polystyrene is used to cover a polyester bolt, there is a problem in that it cannot be recovered and reused. Furthermore, polyvinyl chloride has the problem of corrosion due to chlorine gas, so a heat-shrinkable film that does not cause waste pollution is desired. (f) Shrinkage area The heat shrinkability of the conventional film mentioned above tends to lack homogeneity, and once areas with sufficient heat shrinkage and areas with insufficient heat shrinkage are formed separately, even if heat is applied again next time. Further re-shrinkage does not occur and the surface becomes uneven. Furthermore, the most important point in terms of application is that in high-speed shrink packaging, labeling, etc., spots tend to appear in areas where the shrinkage rate is large, and when metallic ink is used, although the shrinkage spots are improved in appearance, color spots tend to appear. In the latter case, the difference in local shrinkage rates after finishing appears as is. Therefore, a more uniform shrinkage rate is desired. The present invention has been made in view of these circumstances and aims to provide a polyester film that is free from the defects described in (a) to (f) above. [Means for solving the problem] The heat-shrinkable polyester film of the present invention is a polyester made of a copolyester-containing composition containing terephthalic acid and ethylene glycol as main components and a naphthalene dicarboxylic acid derivative as a copolymerization component. In series films, 100
The film must be stretched more than 4 times so that the thermal shrinkage rate in hot air at ℃ is 30% or more in either the longitudinal direction or the width direction, and the intersection point shrinkage rate is 5% or more. This is a summary. [Function] The constituent material of the heat-shrinkable polyester film of the present invention is a composition containing terephthalic acid and ethylene glycol as main components and a naphthalene dicarboxylic acid derivative as a copolymerization component, and the amount of the naphthalene dicarboxylic acid derivative is equal to the total composition. 1 to 50 mol% in substance
It is preferable that it is in the range of . Particularly preferred is 5 to 40 mol%. If the naphthalene dicarboxylic acid derivative is less than 1 mol%, the retention time of the internal residual stress in the film during heat shrinkage treatment will be shortened, and for example, when coated on a bottle, the shoulders may become loose due to shrinkage and subsequent sterilization treatment. tends to cause undesirable phenomena. On the other hand, if the naphthalene dicarboxylic acid derivative exceeds 50 mol%, the effect of improving the residual stress retention time during heat treatment becomes saturated, and even if the present invention satisfies the requirements described below, amorphousness progresses and the loading resistance decreases. There are some shortcomings like this. Examples of naphthalene dicarboxylic acid derivatives include 1,2-naphthalene dicarboxylic acid, 1,3-
naphthalene dicarboxylic acid, 1,4-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid,
1,6-naphthalene dicarboxylic acid, 1,7-naphthalene dicarboxylic acid, 1,8-naphthalene dicarboxylic acid, 2,3-naphthalene dicarboxylic acid, 2,
4-naphthalene dicarboxylic acid, 2,5-naphthalene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid, 2,7-naphthalene dicarboxylic acid, 2,8-
Examples include naphthalene dicarboxylic acid and lower alkyl esters thereof, among which 1,5-naphthalene dicarboxylic acid and 2,6-naphthalene dicarboxylic acid are particularly preferred. The copolyester itself in the present invention can be produced by a conventional polyester production method. Examples include a method in which terephthalic acid and naphthalene dicarboxylic acid derivatives are directly esterified with ethylene glycol, and a transesterification method in which dimethyl naphthalate and dimethyl ester of naphthalene dicarboxylic acid are reacted with ethylene glycol. Furthermore, the copolyester composition of the present invention can also be produced by blending with copolyesters, homopolyesters, or other copolyesters within and outside the above-mentioned condition ranges of the present invention, and by controlling these, a more preferable film can be obtained. can be obtained. That is, we have discovered that the copolymer of the present invention can not only improve the retention characteristics of the residual shrinkage stress, but also reduce the shrinkage spots that normally occur during shrinkage by adjusting the temperature sensitivity during shrinkage and controlling the shrinkage speed. . In the present invention, it is recommended that the naphthalene dicarboxylic acid derivative occupies a range of 1 to 50 mol% in the composition. It is necessary to satisfy the intersection shrinkage rate described below.
The heat-shrinkable polyester of the present invention has terephthalic acid as its main acid component, but other acid components can be copolymerized as long as these properties do not change significantly. Aliphatic dibasic acids such as adipic acid, sebacic acid, azelaic acid: isophthalic acid, diphenyldicarboxylic acid, 5-tert-butyl isophthalic acid, 2,2,6,6-tetramethylbiphenyl-4, Aromatic dibasic acids such as 4-dicarboxylic acid: 2,6-naphthalene dicarboxylic acid,
Examples include aromatic dibasic acids such as 1,1,3-trimethyl-3-phenylindene-4,5-dicarboxylic acid. Similarly, the glycol component is mainly composed of ethylene glycol, but other components may be copolymerized as long as their properties are not significantly changed. For example, aliphatic diols such as diethylene glycol, propylene glycol, butanediol, hexanediol, or 1,4-cyclohexanedimethanol, xylylene glycol, bis(4-β-hydroxyphenyl)sulfone,
Examples include alicyclic or aromatic diols such as 2,2-(4-hydroxyphenyl)propane derivatives. Furthermore, lubricants such as titanium dioxide, particulate silica, kaolin, and calcium carbonate may be added as necessary, and further antistatic agents, antiaging agents, ultraviolet ray inhibitors, and colorants (dyes, etc.) may be added. . The preferable intrinsic viscosity of the film base material is 0.55 to 1.3 dl/g, preferably 0.58 to 1.2 dl/g.
dl/g, particularly preferably 0.63 to 1.2 dl/g. The film of the present invention is a film suitable for high-speed shrinking work, and while satisfying the above requirements, it needs to have an intersection shrinkage rate of 5% or more and a stretching ratio of more than 4 times. The intersection point shrinkage rate is determined as shown in Figure 1, which shows the internal residual stress curve when a given shrinkage rate is given to the film, and when the film is shrunk to a value greater than that shrinkage rate. It is defined as the shrinkage rate that intersects with the tensile force curve required to return the shrinkage amount to the shrinkage amount corresponding to the arbitrary shrinkage rate. Therefore, below this intersection point shrinkage rate, even if color spots or shrinkage spots occur on the film due to local shrinkage, the internal shrinkage stress is greater than the force that stretches the locally contracted part, and the locally contracted part will temporarily shrink. This is based on the discovery of an extremely new fact that even if the spots are removed, there is always a force that tries to bring them back together and eliminate the spots. If the intersection shrinkage rate is less than 5%, internal residual stress will be released even by a slight shrinkage, and the above-mentioned force for correcting other shrinkage parts will not be generated.
Otherwise, due to significant shrinkage spots, there is no internal residual stress sufficient to alleviate the shrinkage spots, so in any case, the spots once formed cannot be removed. As a result, the finished product loses its appearance significantly. On the other hand, if only the intersection shrinkage rate is high, then the object of the present invention cannot be achieved by that alone.
This is because a film with high rigidity that does not shrink by heat has a high intersection point shrinkage rate, and the shrinkage rate conditions described above must be satisfied. On the other hand, unoriented films and films with a low degree of orientation that generally have low rigidity have a high intersection point shrinkage rate because the residual stress is less reduced and the tensile force increases less with respect to the recovery rate. Therefore, the film must satisfy the above-mentioned shrinkage ratio. A film obtained by any method such as an extrusion method or a calendar method using such a polymer has a film that is more than 4 times larger in one direction than 10
Stretched by 1 to 2 times, preferably 1.1 times in the direction perpendicular to the above direction, preferably from 4.6 times to 7.0 times.
Stretched by 1.8 times. There is no problem in the order of stretching, whichever comes first. Stretching in a direction perpendicular to the main stretching direction is effective in further improving the impact resistance and tear resistance of the film of the present invention. However, if it is stretched more than twice as much, the thermal shrinkage in the direction orthogonal to the main shrinkage direction will also become too large, resulting in a wavy finish. To suppress this waving, increase the heat shrinkage rate in the direction perpendicular to the main shrinkage direction to 15.
%, preferably 8-9% or less, most preferably 5% or less shrinkage, or 5% or less elongation. As the stretching method, a conventional device is used, and methods such as roll stretching, long gap stretching, tenter stretching, tubular stretching, etc. are applied, and the shape does not matter whether it is flat, tubed, etc. Further, the stretching may be carried out by successive biaxial stretching, simultaneous secondary stretching, uniaxial stretching, or a combination thereof. Furthermore, the film of the present invention is stretched in one axis in the longitudinal direction, one in the transverse direction, and two axes in the longitudinal and lateral directions. Particularly in biaxial stretching, the stretching in the longitudinal and lateral directions is carried out sequentially in one direction or the other.
Axial stretching is effective, and either order may be used first. When simultaneous biaxial stretching is performed, the stretching order may be either simultaneous in longitudinal and lateral directions, first in longitudinal direction, or first in transverse direction. Heat setting in these stretching processes is carried out depending on the purpose, but in order to prevent dimensional changes under high temperatures in summer, it is recommended that the film be passed through a heating zone of 30°C to 150°C for about 1 second to 60 seconds. Further, expansion up to a maximum of 70% may be applied either before or after such processing, or both. In particular, it is preferable to stretch in the main direction and relax in the non-shrinkage direction (direction perpendicular to the main shrinkage direction), and it is better not to stretch in the perpendicular direction. In order to exhibit the preferable characteristics of the present invention, in addition to the above-mentioned stretching ratio, preheating must be performed at a temperature that is higher than the average glass transition temperature (Tg) of the polymer composition and lower than Tg + 80°C. /10 or less is Tg
Stretch at +75℃ or less, remaining 1/10 or more is Tg +60℃
Hereinafter, it is desirable to stretch preferably at Tg+50°C or lower. This kind of requirement is suitable for obtaining the unique shrinkage behavior of the present invention. In addition, the above-mentioned processing temperature in the main direction stretching (main shrinkage direction) suppresses the heat shrinkage rate in the direction orthogonal to the direction, and is 80±25
It is extremely important to have its minimum value in the temperature range of °C. Further, after stretching, the shrinkage characteristics can be made better and more stable by cooling the film while applying stress by keeping it in a stretched or tensioned state, or by cooling it further successively. The planar orientation coefficient of the film thus obtained was 40
It is preferably at least ×10 ⁻³ and at most 120×10 ⁻³ . If the plane orientation coefficient is less than 40×10 ⁻³ , the above-mentioned shrinkage characteristics cannot be expressed, and various defects such as insufficient shrinkage finish, wrinkles, color spots, and sagging due to secondary heating are observed. Furthermore, not only this, but when subjected to heat treatment such as bolting, retort treatment, and boiling water sterilization when or after heat shrinking, it becomes cloudy and the appearance deteriorates significantly. On the other hand, if it exceeds 120 x 10 ^-3 , the bottle breakage prevention effect will be reduced and the bottle will be more likely to break, even with the slightest external injury. On the other hand, the birefringence is 20×10 ^-3 ~175×
10 ^-3 is preferred; if the birefringence is less than 20 x 10 ^-3 , it is difficult to obtain the above-mentioned intersection point shrinkage, and wrinkles, color spots, etc. seen in metallic ink are undesirable. Also 175
If it exceeds ×10 ^-3 , the shrinkage speed will be too fast, resulting in temperature unevenness during heating, such as temperature differences between areas near and far from the heat source, temperature differences between areas that are easily exposed to hot air and areas that are not. As a result, shrinkage spots are extremely likely to occur. The film of the present invention will be explained below in terms of its uses. Boiling and retort processing are used for packaging purposes, particularly for food and beverage packaging. None of the existing heat-shrinkable films can sufficiently withstand these treatments. The film of the present invention can withstand heat sterilization by boiling and retort processing, and is a novel technology in that the crater-shaped shrinkage areas caused by shrinkage spots and color spots of metallic ink are corrected during heating. It has a higher thermal shrinkage stress than polyvinyl chloride film and has excellent cohesiveness. Therefore, even containers with a large diameter ratio can be coated with uniform adhesion, and the same applies to deformed containers. Furthermore, it is also possible to shrink plastic bottles such as polyester bottles and other polyethylene, polypropylene, polyvinyl chloride, and styrene containers at low temperatures to prevent them from being deformed by heating, and the film shrinks until the temperature at which the bottle deforms is reached. has been completed. In addition, it is possible to cover or bundle heavy items or deformed molded items with a strong covering or bundling that does not dislodge, and at the heat shrinkage level of 50 to 70% required for packaging, it can be perpendicular to the main shrinkage direction. Since it has broad heat shrinkability with the lowest heat shrinkage rate in the direction, the process from the beginning of heat shrinkage to the completion of shrink wrapping should be performed in the temperature range (80±25℃) that shows the minimum shrinkage amount. become. As a result, a feature was obtained in which the error in finished dimensions was reduced. For packaging that utilizes heat shrinkability, heat shrinkage is complete (a state in which the package is in close contact with the packaged item and cannot be shrunk any further, even if it has the ability to shrink further).
Subsequent heating has become a common procedure and plays an important role in achieving complete shrinkage while accounting for numerous product variations.
The same applies to boiling and retort processing. At this time,
If the shrinkage of the film has reached saturation, and not only this but also the tensile force of adjacent high shrinkage parts is higher than this residual shrinkage stress, the shrinkage spots and color spots that have already occurred will be removed by continuous heating. There is a problem in that the film is not corrected even when it is fixed, and the film instead undergoes linear expansion, and even after taking the pains to shrink it tightly, it ends up becoming loose. In order to prevent such a situation, the present invention not only increases the shrinkage stress, but also increases the residual retention level of the shrinkage stress and stretches the adjacent high shrinkage parts from this shrinkage stress. By achieving such a retention level, a completed heat-shrinkable film could be obtained. This will be described in more detail below. (a) Unidirectional shrinkability: One of the roles of a shrink film is to prevent the destruction of the packaged items and the collapse of the package, but for this purpose, it is necessary to have high impact resistance and large shrinkage in the main direction. It is necessary to obtain a rate. In this respect, the film of the present invention has a high shrinkage rate and high impact resistance, so that beautiful packaging can be obtained, and moreover, it exhibits excellent durability in terms of protecting the packaged items. This trend is evidenced by container drop tests. Further, near-perfect unidirectional shrinkability is extremely ensured by stretching more than 4 times in the main stretching direction, and even containers with large dimensional ratios have good finished dimensional stability after shrink-wrapping. (b) Heat resistance: All conventional general-purpose films cannot withstand high-temperature boiling or retort processing, and are not compatible with high-temperature sterilization. For example, the film may tear, tear, become cloudy, etc. Furthermore, it has the property that shrinkage spots are corrected during the above treatment. In contrast, the film of the present invention exhibits excellent utility as a heat-shrinkable film that can be subjected to boiling or retort processing. (c) Printability: Conventional films have inherent drawbacks such as the generation of pinholes due to halftone printing and adhesion with ink, but the polyester film of the present invention has chemical resistance and the advantages of copolymerization. Printability was improved due to improved adhesion. (d) Industrial waste issues: The use of plastic bottles has been rapidly expanding in recent years. When considering the recovery of such bottles, homogeneity is preferable in terms of quality stability of recycled products, and application of the film of the present invention to the packaging of polyester bottles is advantageous in this respect. (e) Shrinkage spots: The film of the present invention has a large shrinkage rate and high shrinkage stress, and if it is continued to be heated during secondary heating, the excessive shrinkage area is tensilely corrected by the shrinkage stress of the lower shrinkage area. Therefore, shrinkage spots are corrected and eliminated, and color spots such as metallic ink are also eliminated. [Example] Examples will be described below, and the measurement methods used in the examples are as follows. (1) Haze Measured based on JIS-K6714. (2) Heat shrinkage rate The film was cut into a width of 15 mm with a standard sample spacing of 200 mm and measured at each temperature. For heating
Heating was performed using hot air at 80°C and 100°C for 1 minute each. (3) Intersection shrinkage rate (%) To determine the thermal shrinkage stress, use a tensioner to take a sample piece with a width of 20 mm and a length of 150 mm, mark a 100 mm mark on the film, and mark a 100 mm mark on the film. Attach the sample piece to the upper and lower chucks set to a larger arbitrary dimension (L ₁ ), and
The film was shrunk by treatment in hot air at ℃.
The residual shrinkage force at this time was determined, and the stress was determined using the following formula. Shrinkage force/cross-sectional area = residual shrinkage stress The shrinkage rate at that time was calculated from the following formula. Shrinkage rate = 100 - L ₁ /100 x 100 (%) On the other hand, the tensile stress is determined by the distance between the chucks after 50% heat shrinkage in the measurement of the residual shrinkage stress.
The tensile force required to return to an arbitrary chuck distance L ₂ greater than 50 mm and less than 100 mm was determined, and the tensile stress was determined using the following formula. Tensile force/cross-sectional area = tensile stress Re-stretch rate = L ₂ -50/50 x 100 (%) Graph showing the relationship between the above residual shrinkage stress and shrinkage rate, as well as tensile stress and re-stretch rate (Figure 1)
The contraction rate corresponding to the intersection point determined by the above is defined as the intersection contraction rate. (4) Heat shrinkage residual stress holding time (at 50% relaxation) Using Tensilon, prepare a sample piece in the same manner as for heat shrinkage stress, mark a 100mm gauge line on the sample piece's film, and set the upper and lower chucks at 50mm. 100mm mark line accurately, process it in hot air at 170℃, and check the time until the shrinkage stress becomes 0 or
Determine the residual stress after 10 minutes. If the stress is to be maintained after 10 minutes, calculate it in the same way as the heat shrinkage stress. Example 1 Using a stainless steel autoclave, 60 mol% of terephthalic acid and 2,6-
Uses 40 mol% naphthalene dicarboxylic acid and 100 mol% ethylene glycol as the glycol component.
Polycondensation was carried out by a direct esterification method using 0.025 mol of antimony trioxide (based on the acid component) as a catalyst. This copolymer has an intrinsic viscosity of 0.71 dl/
It was hot at g. This polyester was melt-extruded at 290°C to obtain an unstretched film with a thickness of 180 μm. In order to stretch the film in the transverse direction, it was heated at 130°C for 8
Preheat for seconds, then set 1/2 area of the entire stretching section to 88℃,
The remaining 1/2 area was heated to 80°C and stretched 5.2 times. After stretching, the film was cooled to 40° C. while being stretched by about 3% in the lateral direction. The obtained film is a heat-shrinkable film with a thickness of 40 μm, and the birefringence and plane orientation coefficient are as follows.
They were 102×10 ^-3 and 65×10 ^-3 . The physical property values are shown in Table 1. As is clear from Table 1, the quality is high, and good results were obtained in practical tests.

[Table] * Examples 2 and 3 and Comparative Examples 1 and 2 using polyvinyl chloride In the same manner as in Example 1, polyethylene terephthalate (intrinsic viscosity 0.8 dl/g) was added to polyester having the composition listed in Table 1. A mixed polyester composition was prepared by adding 60% by weight of the former and 40% by weight of the latter. Comparative Example 1 is polyester terephthalate with an intrinsic viscosity of 0.6, and Comparative Example 2 is polyvinyl chloride. The stretching conditions were the same as in Examples 2 and 3. The planar orientation coefficient of the film of the present invention was determined in Example 2.
The birefringence was 103×10 ^-3 for the ^former and 102×10 ^-3 for the latter, and 58×10 ^-3 for Example 3. Comparative Example 1 was stretched 4.2 times in the transverse direction at 95°C and cooled to 40°C. Planar orientation coefficient is 42×10 ^-3 , birefringence is 70×
It was 10 ^-3 . The physical property values are shown in Table 1. From the same table, products copolymerized with naphthalene dicarboxylic acid derivatives have a high intersection point shrinkage rate, good residual stress retention properties, a low longitudinal heat shrinkage rate, no wrinkles in high shrinkage areas, and metallic inks. No color spots were observed at all. In addition, the jagged waves seen in the comparative example were not observed at all on the top and bottom of the label in the example, and there was no vertical tilt or large undulation of the label, which was consistent with the shape of the polyester bottle used. I was able to cover it without any problems.
Polyethylene terephthalate and a copolymer consisting of terephthalic acid and isophthalic acid shown as comparative examples have a high thermal shrinkage rate, and in practical tests, wrinkles, color spots, and vertical waving were observed in the high shrinkage area in the small diameter area at the top of the bottle. The product had an extremely poor appearance and could no longer be used as a commercial product. Using the same raw material compositions as Examples 4 to 6, Comparative Example 3, and Example 3, films having intersection point shrinkage rates as shown in Table 1 were formed, and the physical property values and practical test results were evaluated. Shown in Table 1. The results show that when the intersection point shrinkage rate is low, the finish of the high shrinkage part is poor and it is not possible to eliminate color spots in the metallic twin ink part. Planar orientation coefficients are 76×
10 ^-3 , 78×10 ^-3 , 84×10 ^-3 , birefringence is 110×
^10-3 , 115× ^10-3 , and 117× ^10-3 . For comparison, physical property and practical tests were conducted on films that did not satisfy the intersection shrinkage rate condition using the same resin (Comparative Examples 3 and 4). Planar orientation coefficients are 35×10 ^-3 , 30×10 ^-3 ,
The birefringence was 35×10 ⁻³ and 30×10 ⁻³ . Comparative Example 5 20 mol% of terephthalic acid and 80 mol% of naphthalene dicarboxylic acid were used as the dibasic acid component, and ethylene glycol (100 mol%) was used as the glycol component.
A copolymerized polyester film was formed using the same method as in Example 1, but the heat resistance of the polyester was low, and clip breakage occurred in the tenter, making it impossible to form a stable film. Example 7 Using a composition in which 40% by weight of polyethylene terephthalate was mixed with the copolymer of Comparative Example 5, Example 1
The film stretched under the same conditions as above showed practically effective properties. The plane orientation coefficient is 65×10 ^-3 and the birefringence is
It was 113×10 ^-3 . [Effects of the Invention] Since the film of the present invention is constructed as described above, it exhibits stable heat shrinkability in a specific direction, and can provide a beautiful and strong packaging state in covering packaging or bundling packaging. It has various effects such as improving printing pitch stability and heat resistance, and can exhibit excellent utility value in a wide range of fields.

[Brief explanation of drawings]

FIG. 1 is a graph for explaining the intersection shrinkage rate.

Claims (1)

[Scope of Claims] 1. A polyester film comprising a copolyester-containing composition containing terephthalic acid and ethylene glycol as main components and a naphthalene dicarboxylic acid derivative as a copolymerization component,
The film must be stretched more than 4 times so that the thermal shrinkage rate in hot air at ℃ is 30% or more in either the longitudinal direction or the width direction, and the intersection point shrinkage rate is 5% or more. A characteristic heat-shrinkable polyester film. 2 Naphthalene dicarboxylic acid derivative component from 1 to 50
The heat-shrinkable polyester film according to claim 1, comprising a copolyester-containing composition containing mol %.