JPH0456736B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing a biaxially stretched film of a saponified ethylene-vinyl acetate copolymer, and can be particularly used for stabilizing the film during stretching and improving the quality of the product. [Background technology and its problems] Saponified ethylene-vinyl acetate copolymer (hereinafter referred to as EVOH) film has excellent gas barrier properties, oil resistance, antistatic properties, etc., and is used in foods and pharmaceuticals. It is useful as a packaging material such as. However, unstretched EVOH film
Insufficient water resistance, moisture permeability, strength, rigidity, etc.
In order to improve these properties, biaxial stretching has been carried out. Biaxial stretching of EVOH film has strong hydrogen bonds, and the structure is easily fixed.
Therefore, a strong stretching force is required, and the film is likely to break or undergo neck stretching. As a result, if the usual tenter method or tube biaxial stretching method used for stretching polypropylene, polyester, etc. is used as is, an excellent stretched film that is industrially useful cannot be obtained. For this reason, various special methods have been proposed. For example, there is a method in which the film is stretched in close contact with another stretchable film (Japanese Patent Laid-Open No. 51-6276), or a method in which an unstretched film is stretched under specific stretching conditions by increasing its water content (Japanese Patent Laid-Open No. 51-6276). No. 53-88067,
JP-A-52-129776, JP-A-52-129777, JP-A-53-43199), etc. However, the former method requires the use of other films. Also, the latter method
It takes time to increase the moisture content of EVOH film to 4% or more, and it is very difficult to control the moisture content, so it cannot be called an industrial manufacturing method. For this reason, unstretched films with a moisture content of 2% or less obtained from normal EVOH film production are
A method has been proposed in which the film is preheated at 50 to 70°C for 5 seconds or less, then rapidly heated to 70 to 100°C using heated air, and then biaxially stretched (Japanese Patent Application Laid-open No. 57-25920). However, this method requires preheating and also
The film is rapidly heated to a narrow temperature range of 100℃,
Moreover, since this rapid heating is performed by air spray heating, there are disadvantages in that thermal efficiency is low and temperature control is also difficult. [Object of the Invention] The object of the present invention was to solve these drawbacks, and in addition to simplifying production, the stability during stretch forming is improved by stabilizing the stretching start point at a constant position. An object of the present invention is to provide a manufacturing method that allows a high-quality EVOH biaxially stretched film to be obtained. [Means and effects for solving the problem] Therefore, in the present invention, after cooling the molten ethylene-vinyl acetate copolymer saponified resin extruded from a die in a tube shape, the tube-shaped ethylene-vinyl acetate copolymer resin is cooled. saponified polymer film,
When feeding in one direction while heating to a temperature at which stretching is possible in the heating stretching zone, and simultaneously stretching in the feeding direction and the width direction using tension and internal pressure in the feeding direction, the atmospheric temperature in the heating stretching zone is set to 70 - 70°C.
At 200℃, air is directly blown from the upstream side of the heating stretching zone near the stretching start point of the heating stretching zone at an angle of 20 to 60℃ with respect to the film feeding direction, and the stretching ratio in the feeding direction is It is characterized in that it is stretched under conditions that are smaller than the stretching ratio in the width direction. Therefore, the method of the present invention will be explained in more detail.
The EVOH used in the present invention preferably has an ethylene content of 25 to 60 mol% and a saponification degree of 95% or more. If the ethylene content exceeds 60 mol %, the gas barrier properties will deteriorate and the transparency will deteriorate, which is undesirable, and if the ethylene content exceeds 25 mol %, the moisture resistance will become insufficient, which is undesirable. Also,
A degree of saponification of less than 95% is unfavorable in terms of hygroscopicity and gas barrier properties. In addition, in the present invention
As EVOH, other thermoplastic resins may be blended without departing from the gist of the present invention, and additives such as heat stabilizers, plasticizers, lubricants, antistatic agents, colorants, etc. may be added. Moreover, a multilayer film of EVOH and other thermoplastic resins can also be used. EVOH is made into a tube shape using a conventionally known method for manufacturing an unstretched tube film.
Processed into EVOH film. The cooling method in this case is not particularly limited, but it is preferable to use a method with a high rapid cooling effect, such as a water cooling method, to reduce the crystallinity of the EVOH film in order to improve the stretchability. First, this tubular EVOH film is held by, for example, a pair of upper and lower nip rolls, heated between them and fed in one direction, and simultaneously stretched in the feeding direction and the width direction by the tension and internal pressure in the feeding direction. At this time, regarding the stretching ratio during stretching, the condition is that the stretching ratio in the feed direction is smaller than the stretching ratio in the width direction. At the same time, air is directly blown onto the stretching start point of the heated stretching zone from the upstream side at a predetermined angle, preferably within an angle range of 20 to 60 degrees, with respect to the film feeding direction. If the air blowing angle is within the above range, the air blown to the stretching start point of the tubular film will flow along the outer surface of the tubular film, creating a flowing air film on the film surface, resulting in the stretching. The temperature distribution at the starting point can be made uniform. Therefore, the stretching start point can be stabilized at a certain position, and the film (bubble) after stretching can be stabilized.
can prevent shaking. In this case, the blown air is usually room temperature, but heated air may also be used. By such a simple method, an EVOH film with high stability in stretch forming and high quality can be produced. [Example] The figure shows an example of a manufacturing apparatus used in the manufacturing method of the present invention. In the figure, compressed air is supplied to the inside of molten EVOH which is extruded into a tube shape downward from an annular die 2 by an extruder 1. The EVOH supplied with compressed air is cooled through an air ring 3 and a cooling tank 4, and then taken up by a nip roll 5 and sent to the next process. The air ring 3 is disposed directly below the die 2 and cools the outer surface of the molten EVOH extruded from the die 2 into a tube shape with air.
The cooling tank 4 is disposed below the air ring 3, and has a hole in the center through which the molten EVOH extruded from the die 2 passes, and is filled with water. As a result, the molten EVOH extruded from the die 2 is first air-cooled by the air ring 3, and then rapidly cooled and solidified by the water-cooling effect of the cooling tank 4. Tube-shaped tube sent out from Nitzprol 5
EVOH film has two guide rolls 6 and 7.
After passing through, it passes through an upper nip roll 8 and a lower nip roll 9, and is then wound up, for example, by a winder (not shown). At this time, the circumferential speed of the lower nip roll 9 is higher than the circumferential speed of the upper nip roll 8, and compressed air is injected into the tube-shaped film between the upper nip roll 8 and the lower nip roll 9, so the tube-shaped film is When heated to a temperature that allows it to be stretched,
The tubular film is simultaneously stretched in the feeding direction and the width direction by the action of both, that is, the expansion force by the compressed air and the tension in the feeding direction. In this case, the stretching ratio during stretching may be appropriately determined depending on the type of EVOH, but the condition is that at least the stretching ratio in the feeding direction is smaller than the stretching ratio in the width direction. This difference in magnification, that is, the stretching ratio in the width direction - the stretching ratio in the feeding direction, is particularly preferably from 0.6 to 2.0. Note that the stretching ratio in the feed direction is usually 1.1~
5.0, preferably about 1.2 to 4.0 times. A stretching heating section 10 is provided between the upper nip roll 8 and the lower nip roll 9 to heat the tubular film to a temperature at which it can be stretched.
The stretching heating section 10 includes a heating source such as an infrared heater arranged in an annular shape along the inner peripheral surface of a cylindrical body whose upper and lower ends are open. The ambient temperature in the heated stretching zone varies depending on the melting point of the EVOH to be stretched, but is usually maintained at 70 to 200°C, preferably 80 to 170°C. As a result, the tubular film is heated to at least the temperature at which it can be stretched, mainly by radiant heat from the heating source. On the upstream side of the stretching/heating section 10, that is, on the nip roll 8 side, there is an air ring device 11 that directly blows air at a predetermined angle with respect to the film feeding direction near the stretching start point of the heating stretching zone of the tubular film. It is provided. Air ring device 11
In this case, an air outlet such as a slit or a plurality of pores is provided along the inner peripheral surface of the annular ring body. Therefore, air is blown uniformly in the circumferential direction of the tubular film. In addition, the blowing angle α from these air blowing ports is the angular range in which the air blown from there can smoothly flow from the film stretching start point along the outer surface of the film in the feeding direction, that is, in the film feeding direction. 20-60° against, preferably
It is set within the angle range of 30~50°. Also,
Room temperature air is usually used as the blown air, but heated air may also be used. Further, a preheating section 12 is provided upstream of the air ring device 11, that is, on the nip roll 8 side, if necessary. The preheating section 12 has the same configuration as the stretching heating section 10, and includes an infrared heater and the like arranged along the inner peripheral surface of the cylindrical body. On the other hand, on the downstream side of the stretching/heating section 10, that is, on the lower nip roll 9 side, guide rollers 13 are provided for sequentially flattening the tube-shaped film after stretching and introducing it into the lower nip roll 9. The guide rollers 13 are arranged in a V-shape such that the width of the plurality of guide rollers becomes gradually narrower as they go downward. As a result, the tube-like film after stretching is moved to the guide roller 13.
After being guided to the lower nip roll 9 while being successively folded into a flat shape, the sheet is heat-fixed if necessary, and then wound onto a winder (not shown), for example. Next, the operation of this embodiment will be explained. First, the molten EVOH extruded into a tube shape from the die 2 by the extruder 1 is expanded into a tube shape by compressed air injected inside, and is expanded into the tube shape by the air ring 3.
Then, it is sequentially cooled in the cooling tank 4 and solidified into a tube shape. At this time, the tube-shaped film is rapidly cooled in the cooling bath 4 to suppress crystallization as much as possible, and is formed to have a moisture content of 2% or less. Subsequently, it is sent from the nip roll 5 to the upper nip roll 8 through guide rolls 6 and 7. The tube-shaped film sent to the upper nip roll 8 is heated in a stretching heating section 10 to a temperature at which it can be stretched. Then, due to the compressed air injected into the EVOH film between the upper nip roll 8 and the lower nip roll 9 and the difference in circumferential speed between the nip rolls 8 and 9, the tube-shaped film has a stretching ratio in the feeding direction that is smaller than the stretching ratio in the width direction. Under these conditions, it is simultaneously stretched in the feed direction and the width direction. Incidentally, the difference in stretching ratio is within the above-mentioned range. At this time, since air is blown from the air ring device 11 near the stretching start point of the stretching zone of the tube-shaped film, the air ring device 11
The air blown from the tubular film is blown to the stretching start point of the tubular film and then flows along the outer surface of the tubular film in the film feeding direction, so that the film surface is surrounded by a fluidized air film. As a result, the temperature distribution at the stretching start point becomes uniform, so that the stretching start point is stabilized at a constant position and the bubbles are prevented from shaking after stretching. After that, the tube-shaped film after stretching is
After being folded flat by the guide roller 13,
It is taken up by the lower nip roll 9. The tube-shaped film taken up by the lower nip roller 9 is heat-set if necessary and then wound up. Therefore, according to this embodiment, the tube-shaped
Near the stretching start point of the heated stretching zone of the EVOH film, 20 to 60° with respect to the film feeding direction.
Since air is blown at an angle of , and stretching is performed under conditions where the stretching ratio in the feed direction is smaller than the stretching ratio in the width direction, the stability of stretch forming can be improved by a simple method. In other words, the air blown at the starting point of stretching of the tubular film flows in the film feeding direction along the outer surface of the tubular film, creating a fluidized air film on the film surface, which changes the temperature distribution at the starting point of stretching. It can be made uniform. Therefore, under conditions where the stretching ratio in the feed direction is smaller than the stretching ratio in the width direction, the stretching start point can be stabilized at a constant position, and the bubbles can be prevented from shaking after stretching. Incidentally, if the blowing angle α from the air blowing port is outside the above range, the air blown at the stretching start point will not flow smoothly along the outer surface of the tubular film, and will not flow uniformly over the film surface. It is difficult to create an air film. As a result, in this example, it is possible to obtain a high-quality biaxially stretched film with good thickness unevenness accuracy, excellent water resistance, strength, etc. In addition, since the manufacturing equipment uses a heating source such as an infrared heater to heat the film to a temperature at which it can be stretched, it has high thermal efficiency and is easy to control. Since it is sufficient to install one air ring device 11 that blows air at a predetermined angle with respect to the film feeding direction near the stretching start point, equipment costs and manufacturing costs are low, and operation control is extremely easy. Can be done. Moreover, since film formation and stretching can be carried out in an integrated line, this also has the advantage of low manufacturing costs. Therefore, the above points will be clarified based on an example in which a biaxially stretched film was manufactured under the following conditions.
A saponified ethylene-vinyl acetate copolymer (manufactured by Nippon Gosei Kagaku Co., Ltd., trade name: Soarnol ET) with an ethylene content of 38 mol% and a melting point of 173°C is first extruded from a bottom-blown water-cooled extruder and solidified by water-cooling. Diameter 60
A substantially non-oriented tube-shaped film having a diameter of mmφ and a thickness of 100 μm was obtained. Next, using the stretching device shown in the figure, the peripheral speed of the nip roll 8 for delivery was set to 6 m/
min, the transverse (width direction) stretching ratio by internal pressure was 2.8, the ambient temperature around the stretching start point was 130°C (infrared heater temperature 350°C), and the air ring device 11 ( Inner diameter
270mmφ, slit gap 2mm), blow air at 20℃ at a rate of ^10m3 /min at a 45 degree angle toward the starting point of stretching.
Stretch molding was performed while spraying. Films with longitudinal (feeding direction) stretching ratios of 1.2 (Example 1), 1.6 (Example 2), and 2.0 (Example 3) were obtained by changing the peripheral speed of the Nippro roll 9 for taking off. The stability of the bubbles and the state of uneven thickness in Examples 1, 2, and 3 are shown in the following table along with Comparative Examples 1 and 2.

[Table] In the table, the conditions of Comparative Example 1 were as follows: Example 1 except that the conditions were 2.8 in lateral stretch and 3.0 in longitudinal stretch.
Stretching was carried out according to. In addition, as for the conditions of Comparative Example 2, stretching was performed according to Example 1 except that no air ring was used.
In both cases, net stretching occurred and a good film could not be obtained. [Effects of the Invention] As described above, according to the present invention, the production is simple, and since the stretching start point can be stabilized at a constant position, the stability during stretch forming is high, and the quality of the obtained film is improved. It is also possible to provide a method for producing a good EVOH biaxially stretched film.

[Brief explanation of the drawing]

The figure is an explanatory diagram showing an EVOH biaxially stretched film manufacturing apparatus in which the present invention is implemented. 8... Upper nip roll, 9... Lower nip roll, 10... Stretching heating section, 11... Air ring device.

Claims (1)

[Claims] 1. After cooling the molten saponified ethylene-vinyl acetate copolymer resin extruded into a tubular shape from a die, the tubular saponified ethylene-vinyl acetate copolymer film is heated to a temperature at which it can be stretched in a heated stretching zone. While simultaneously feeding in one direction and stretching simultaneously in the feeding direction and the width direction by tension and internal pressure in the feeding direction, the atmospheric temperature of the heated stretching zone is set to 70 to 200 ° C., and
From the upstream side of the heated stretching zone to the vicinity of the stretching start point of the heated stretching zone, the
A biaxially stretched film of a saponified ethylene-vinyl acetate copolymer, which is characterized by blowing air directly at an angle of ~60° and stretching under conditions where the stretching ratio in the feed direction is smaller than the stretching ratio in the width direction. manufacturing method.