JPH0416060B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a method for producing a coextruded stretched laminated polyester film using the Cheebler process, which is heat-sealable and has excellent thickness uniformity. [Prior art] Conventionally, biaxially oriented polyester film
Because it has excellent heat resistance, mechanical strength, and optical properties, it is widely used in various packaging materials, including food packaging materials, and as industrial films. However, polyester film has a high melting point and therefore has poor heat-sealability. Therefore, when it is used for general packaging purposes, it is usually dry laminated or extruded laminated with a film such as polyolefin that has heat-sealability on the surface of the polyester film. Furthermore, a method of imparting heat sealability by applying a coating agent having good heat sealability, such as vinyl chloride-vinyl acetate copolymer, has been adopted. However, in these methods, heat-sealability is imparted in a secondary process, which makes the process extremely complicated and costly, and there is also a risk of ignition and explosion due to the organic solvents used in the laminating or coating process, as well as hygiene issues. There are many troublesome problems, including the need to adapt to the work environment. On the other hand, in order to solve this problem, a so-called coextrusion stretching method has recently been proposed in which homopolyester resin and amorphous polyester resin are coextruded and then stretched continuously, and this method has attracted much attention. Since the method attempts to stretch resins with different thermal properties at the same time, it is extremely difficult to set the conditions, especially when stretching with the tubular method. The thickness was not uniform in the circumferential direction and was not satisfactory. [Problems to be Solved by the Invention] The present invention solves all at once the problems of stability and thickness uniformity during stretching that were observed in the conventional coextrusion stretching method of homopolyester resin and amorphous polyester resin. The method for manufacturing laminated polyester film that we have solved aims to stably produce a laminated polyester film of uniform thickness that has heat sealability and has excellent heat resistance, mechanical strength, optical properties, etc. The point is that it provides a possible method. [Means for Solving the Problems] In order to stably produce a laminated polyester film that has excellent heat sealability, heat resistance, mechanical strength, and optical properties and has a uniform thickness, the present inventors have As a result of intensive studies on a method of coextruding and stretching a polyester resin and an amorphous polyester resin by the tubular method, we found that in the tubular stretching method of a coextruded unstretched film, conditions are such that stretching starts almost simultaneously in the circumferential direction. It is not possible to stably produce a laminated film with satisfactory thickness uniformity unless it is stretched using We came to the conclusion that there is a limit to this, and based on this knowledge, we conducted further studies and found that if the crystalline polyester resin used as the support was composed of a copolymerized polyester resin with a specific crystallization induction time, it would be possible to achieve a uniform thickness. The present invention was completed by discovering that the problems of stability and stretching stability could be solved at once. That is, the present invention supplies crystalline polyester resin and amorphous polyester resin to separate extruders,
A tube-shaped coextruded unstretched film whose total thickness is made up of crystalline polyester resin for at least 50% of its total thickness by melt extrusion and rapid cooling while being joined together in a die is heated between upper and lower nip rolls while blowing compressed air. In a method for producing a laminated film in which simultaneous biaxial stretching is carried out by blowing and further heat setting as necessary, when the crystalline polyester resin is crystallized at a temperature of 150°C in an amorphous non-oriented state, The present invention relates to a method for producing a laminated polyester film with excellent thickness uniformity, characterized by using a copolyester resin having a crystallization induction time of 400 seconds to 400 seconds. [Function] Hereinafter, the function based on the manufacturing method of the present invention will be explained in detail with reference to the drawings. The method for producing a laminated polyester film with excellent thickness uniformity according to the present invention involves stretching an unstretched film coextruded with a crystalline polyester resin and an amorphous polyester resin, which account for 50% or more of the total thickness, by a tubular method. At this time, the aim is to stably produce a laminated polyester film with excellent thickness uniformity by stretching so that the stretching starts almost simultaneously along the circumferential direction as shown in FIG. In other words, as is well known, crystalline polyester resins such as polyethylene terephthalate have a temperature of 70 to 80°C.
Although it has a glass transition temperature in the vicinity, the free kinetic energy of the molecules changes extremely within an extremely narrow temperature range beyond this glass transition temperature, so when drawing this using the tubular method, as shown in Figure 2 The stretching start points 2 tend to be concentrated at several points or more in the circumferential direction and become irregular, which makes the stretching unstable and the thickness of the obtained film also becomes non-uniform. The problem of film thickness uniformity caused by the unstable stretching characteristic of crystalline polyester resins can be solved by coextruded films in which 50% or more of the total thickness is made of crystalline polyester resins, as in the present invention. is a common phenomenon, and in order to solve this type of problem, many proposals have been made, mainly from the perspective of the stretching operation, such as making the temperature distribution uniform or selecting optimal conditions for the temperature gradient. However, these methods have limitations in that they cannot cope with subtle changes in the film during stretching, and therefore cannot be said to be a satisfactory solution to the problem of thickness uniformity. The method for producing a laminated polyester film with excellent thickness uniformity according to the present invention involves stretching a coextruded unstretched film made of a crystalline polyester resin and an amorphous polyester resin by a tubular method, using a crystalline polyester film as a support. resin
By being composed of a copolyester resin that has a crystallization induction time of 30 to 400 seconds when crystallized at a temperature of 150°C, it prevents uneven stretching start points and at the same time improves thickness uniformity and stretching. This also solves the stability problem all at once. That is, the present inventors have solved the problem of thickness uniformity and stretching stability that occur during tubular stretching of a coextruded unstretched film using a crystalline polyester resin as a support. To prevent this, the crystalline polyester resin used as the support should be composed of a copolymerized polyester resin in which the free kinetic energy of the molecules that occurs at the glass transition temperature changes over a certain temperature range. is an essential requirement, and as a copolyester resin that meets these requirements, when crystallized at a temperature of 150°C,
The present invention was achieved by discovering that a polyester resin having a crystallization induction time of 400 seconds to 400 seconds is basically applicable. Although the correlation between the crystallization induction time and the changes in molecular behavior that occur at the glass transition temperature is not completely clear, the effect of crystallization nucleation, which is probably the direct factor determining the crystallization induction time, is not completely clear. There is some relationship between the difficulty level and the changes in molecular behavior that occur at the glass transition temperature.
In other words, the resin in which crystallization nuclei are less likely to form, in other words, the longer the crystallization induction time is, the more gradual the molecular behavior change occurs after reaching the glass transition temperature, and this is thought to be largely responsible for preventing irregularities in the stretching start point. It is inferred. Therefore, in the present invention, when a copolyester resin with a crystallization induction time of less than 30 seconds at 150°C is used, the free kinetic energy of the molecules around the glass transition temperature changes within a relatively narrow temperature range. Therefore, it is not possible to prevent irregularities in the stretching start point during tubular stretching, and as a result, it is thought that the thickness uniformity and stretching stability deteriorate. On the other hand, if a polyester resin whose crystallization induction time at 150°C is longer than 400 seconds is used, it is possible to prevent unevenness of the stretching start point during tubular stretching, but the oriented crystallization due to stretching is slow. The stability of the tube deteriorates significantly and satisfactory stretching becomes impossible. The crystallization induction time in the present invention can be determined by the change in density when an amorphous non-oriented polymer is subjected to isothermal crystallization treatment in a glycerin bath at 150°C. As illustrated in FIG. 3, the processing time is expressed by the processing time at the intersection point C between the extended line of part A, where the density at the initial stage of processing hardly changes, and the drawn line of part B, where the density rapidly increases. In the present invention, the copolyester resin characterized by such crystallization induction time is
Among polyester resins that use terephthalic acid and ethylene glycol as starting materials, some of the terephthalic acid is replaced with dicarboxylic acids such as isophthalic acid, adipic acid, and sebacic acid, or some of the ethylene glycol is replaced with propylene glycol or tetramethylene. Copolymerized polyester resins substituted with glycol, neopentyl glycol, cyclohexanedimethanol, etc. can be used alone, or a mixture of these and polyethylene terephthalate can be used, but it is important that the crystallization induction time at 150°C is 30 to 30 minutes.
It is important that the resin is a copolymerized polyester resin with a molecular weight of 400 seconds, and the most suitable example is a copolymerized polyester resin synthesized from a dicarboxylic acid component consisting of 90 mol% of terephthalic acid and 10 mol% of isophthalic acid and ethylene glycol. Examples include polyester resins. The amorphous polyester resin to be coextruded and laminated on one or both sides of the copolymerized polyester resin is preferably a copolymerized amorphous polyester consisting of three components: terephthalic acid, ethylene glycol, and cyclohexanedimethanol. Other copolymers consisting of three components of terephthalic acid, isophthalic acid, and cyclohexanedimethanol, or amorphous polyester resins similar to these may also be used, and even if they are not completely amorphous, they have similar properties in terms of heat sealability, etc. It is also possible to use a copolymerized polyester resin having the following. In addition, thermoplastic resins such as polyethylene and polypropylene may be added to these polyester resins within a range that does not significantly change their properties, and if necessary, lubricants, ultraviolet absorbers, antistatic agents, etc. may be added as appropriate. It goes without saying that there is no harm in doing so. In the laminated polyester film of the present invention, the crystalline polyester resin must account for 50% or more of the total thickness, and the reason is that if the crystalline polyester resin accounts for less than 50% of the total thickness This is because orientation crystallization during stretching is insufficient, which not only makes stretching extremely unstable, but also deteriorates the heat resistance of the resulting laminated film. By the way, the laminated polyester film of the present invention having excellent thickness uniformity is manufactured through the following steps.
That is, the crystalline polyester resin and the amorphous polyester resin are introduced into an annular die having the same nozzle using different extruders, coextruded into a tube shape, and then immediately quenched. The tubular coextruded unstretched film thus obtained is stretched by blowing compressed air while heating it between upper and lower nip rolls at a temperature of approximately 60 to 120°C after undergoing an appropriate preheating process. ,that time,
As shown in FIG. 1, the stretching of the film occurs almost simultaneously in the circumferential direction of the unstretched film 1, so there is almost no deviation in the stretching start point 2, and therefore the stretchability is extremely stable and can be obtained. The thickness uniformity of the laminated film is also extremely excellent.
The stretching operation is carried out in two directions, the longitudinal direction and the radial direction, and if necessary, heat setting is performed to obtain the laminated polyester film which is the object of the present invention. [Examples] Hereinafter, the present invention will be described in more detail based on Examples, but the present invention is of course not limited to these Examples. The evaluation method is as follows. (1) Condition of the stretching start point This evaluates the netting condition of the stretching start point. ○ indicates that the stretching starts almost simultaneously in the circumferential direction, △ indicates that the stretching starts at the same time in the circumferential direction; The case was marked as (x). (2) Stretching stability This evaluates the fluctuation of bubbles in the stretching zone and the frequency of punctures.
Those that could be stably stretched were classified as (good), and those that showed frequent valve fluctuation or punctures were classified as (bad). (3) Thickness uniformity The range of thickness variation with respect to the nominal thickness of 12.4 microns was ranked and evaluated as A, B, and C. Those with a variation range of less than 1.0 micron were evaluated as (A), and those with a variation range of 1.0 to 1.5 micron were evaluated. (B), 1.5
Those larger than microns were designated as (C). Examples 1 to 4, Comparative Examples 1 to 4 The crystalline polyester resins shown in Table 1 were put into a first extruder, and the dicarboxylic acid component was terephthalic acid, the diol component was 60 mol% ethylene glycol, and 40 mol% ethylene glycol. Amorphous polyester resin made of cyclohexanedimethanol is supplied to a second extruder, and extruded together with the amorphous polyester resin from the same die so that the ratio of the crystalline polyester resin to the total thickness becomes 80%. A two-layer unstretched tube with a total thickness of 170 microns was obtained.
Subsequently, this unstretched tube was simultaneously biaxially stretched by 3.7 times in the longitudinal and radial directions by blowing compressed air under heating at 60 to 110°C, and then further stretched to 180 to 110°C.
A laminated polyester film with a nominal thickness of 12.4 microns was produced by tension heat setting at a temperature of 230°C. When producing these laminated polyester films, the state of the stretching start point, stretching stability, and thickness uniformity of the resulting laminated polyester films were evaluated for cases where crystalline polyester resins with various crystallization induction times were used. The results are summarized in Table 1, and the table shows that in a laminated polyester film using a crystallized polyester resin whose crystallization induction time falls within the range of the present invention, the start of stretching occurs almost simultaneously along the circumferential direction. Moreover, it was confirmed that there were no problems such as bubble fluctuation or puncture during stretching over a long period of time, and the thickness uniformity of the obtained laminated polyester film was also extremely excellent.

【table】

[Table] [Effects of the Invention] In the laminated polyester film obtained by the production method of the present invention as described above, the so-called netting portion during tubular stretching is extremely stable, so that the thickness of the obtained film is uniform. In addition to having good properties, it also has excellent heat sealability, heat resistance, mechanical strength, optical properties, etc. In addition, the production method of the present invention can stably produce a laminated polyester film having such excellent properties by coextrusion and stretching, resulting in a high yield and without the need for complicated processes. , its industrial utility value is extremely high.

[Brief explanation of drawings]

FIG. 1 is an explanatory view showing an example of a stretching start portion in the production method of the present invention, and FIG. 2 is an explanatory view showing a stretching start portion observed during tubular stretching of a conventional laminated polyester film. Moreover, FIG. 3 is a diagram showing the temporal change in density during isothermal crystallization treatment of an amorphous non-oriented polymer. 1...Tube-shaped coextruded unstretched film, 2
...Stretching start point, A...A part where the density at the initial stage of crystallization hardly changes, B...A part where the density rapidly increases, C...Crystallization induction time (time to the intersection of the extension lines of A and B) ).

Claims (1)

[Claims] 1. A crystalline polyester resin and an amorphous polyester resin are fed into separate extruders, joined together in a die, melted and extruded at the same time, and rapidly cooled to obtain a full-thickness resin.
A lamination process in which a tube-shaped coextruded unstretched film composed of 50% or more of crystalline polyester resin is simultaneously biaxially stretched by blowing compressed air while heating between upper and lower nip rolls, and further heat-set as necessary. In the film manufacturing method, the crystalline polyester resin is a copolymerized polyester resin that has a crystallization induction time of 30 to 400 seconds when crystallized at a temperature of 150°C in an amorphous, non-oriented state. A method for producing a laminated polyester film with excellent thickness uniformity.