JPH0970841A - Cast film manufacturing method - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a method for producing a cast film which prevents whitening of a coating film and is excellent in smoothness, thickness accuracy, transparency and the like. SOLUTION: After a solution of a polymer material is applied on a support in an application part, the solution is conveyed to a drying zone arranged at a constant interval from the application part, and a solvent is evaporated in the drying zone to produce the solution. In the method for producing a cast film to be formed,
Between the coating section and the vicinity of the entrance in the drying zone,
A method for producing a cast film, comprising arranging at least one temperature sensor for a non-dried coating film on a support, and molding while measuring the temperature of the non-dried coating film.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a cast film, and more particularly to a method for producing a cast film having excellent transparency by preventing whitening during coating.

[0002]

2. Description of the Related Art As one of the methods for producing a film from a polymer material, a solution casting method in which a solution prepared by dissolving a polymer material in an organic solvent is coated on a support and the solvent is removed by evaporation (cast method) : Casting method), and the thin film obtained by this method is called a cast film. Conventionally, in the method for producing this cast film, a solution of a polymer material (hereinafter referred to as a resin solution) is applied on a support such as a plastic film or an endless belt, and a drying zone heated by hot air or the like (hereinafter, There is a method in which the solvent is removed by passing it through a furnace drying zone), and then the formed cast film is separated from the support.

The physical properties of the film produced by the casting method are significantly affected by the evaporation rate of the solvent, the temperature and humidity of the environment, the concentration of the resin solution, the coating thickness, the heating temperature in the furnace, the evaporation time and the like. In the casting method, a certain distance is provided between the resin solution coating section and the oven drying zone,
The influence of high heat in the oven drying zone is prevented from reaching the coating part. If the distance between the coating section and the in-furnace drying zone is too close, the heat in the in-furnace drying zone is transferred to the coating section, and as a result, the drying and solidification of the resin solution proceeds in the coating section, causing defects in the cast film. . Further, if such an interval is provided, a leveling effect of flattening the resin solution (undried coating film) applied on the support can be obtained during the period from the application section to the conveyance to the oven drying zone. If the gap between the coating section and the oven drying zone is too short, the leveling time will be reduced, and the smoothness and thickness accuracy of the cast film will be deteriorated. Therefore, the undried coating film applied on the support is usually exposed to the indoor atmosphere for a certain period of time until it is conveyed from the application section to the oven drying zone.
The interval between the coating section and the oven drying zone is usually maintained at room temperature (normal temperature), and here again, the evaporation drying of the undried coating film progresses to some extent. ".

In the casting method, a low boiling point solvent such as acetone or methylene chloride is generally used as an organic solvent for dissolving the polymer material. When a resin solution using such a low boiling point solvent is applied, in the room temperature drying zone between the applying section and the oven drying zone, a part of the solvent is evaporated, and the undried coating film applied by the latent heat of vaporization is applied. Also, the heat of the support is removed and the temperature drops sharply. Due to this rapid temperature drop, moisture in the indoor atmosphere causes dew condensation on the surface of the undried coating film. As a result, the surface of the coating film becomes frosted and the transparency of the cast film is impaired.

In order to prevent the occurrence of dew condensation, it is sufficient to adjust the absolute humidity in the room to 0%, but this is not only difficult to achieve with ordinary air conditioning equipment, and static electricity is generated under such low humidity. It becomes easier and there is a risk of causing a fire. on the other hand,
In the casting method, it is desirable to use a solvent having a boiling point as low as possible from the viewpoint of improving productivity. Moreover, since low boiling point solvents such as methylene chloride are good solvents for polymer materials such as polycarbonate, polyarylate and polysulfone, it is desirable to use these low boiling point solvents also in the casting method. Therefore, it is not practical to avoid the problem of dew condensation by using a high boiling point solvent.

In order to prevent whitening caused by dew condensation, a method of supplying heat from the outside to compensate for latent heat taken by the evaporation of the solvent can be considered. However, as a means for supplying heat, for example, a method using warm air is used. Vibration is applied to the support to deteriorate the thickness accuracy. In the method using an infrared heater, the temperature of the heater becomes high, so there is a problem in disaster prevention when used in a room where an organic solvent is used, and it is difficult to accurately control the temperature. Further, in any heating method, since the heating amount is empirical, it is possible to perform only extremely unstable control, such as overheating to foam the undried coating film or whitening due to insufficient heating. It was not possible and there was no clear standard for control.

Conventionally, as a method for solving this problem, in a room temperature drying zone from the coating section to the in-furnace drying zone, a planar heating element is arranged below the support, and the planar heating element is arranged. Has proposed a method of supplying heat (Japanese Patent Laid-Open No. 3-316380). However, even with this method, it was difficult to sufficiently prevent unevenness in the thickness of the coating film due to uneven heating and foaming due to forced heating from the lower surface. As described above, according to the conventional method for producing a cast film, it is extremely difficult to obtain a plastic film having excellent smoothness, thickness accuracy, transparency, etc., which can be used for optical applications.

[0008]

DISCLOSURE OF THE INVENTION The object of the present invention is to produce a cast film excellent in smoothness, thickness accuracy, transparency, etc. by preventing whitening of a coating film in a method of producing a cast film by a solution casting method. To provide a method.
The present inventors, in the method for producing a cast film by the solution casting method, when observing the mechanism of whitening in more detail, crystallization of water mixed in the undried coating film due to dew condensation,
It was found that whitening occurs due to the generation of bubbles due to the evaporation of crystallized water. Further, such a whitening phenomenon does not always occur only in the room temperature drying zone,
It was found that it also occurs near the entrance of the furnace drying zone. That is, whitening often occurs after about 1 to 2 minutes from the time of coating, and the undried coating film applied on the support may already be conveyed into the oven drying zone. When the whitened undried coating film is heated and dried in the oven drying zone, the surface of the obtained dried coating film becomes frosted glass, and smoothness and transparency are impaired.

Therefore, as a result of further research, the present inventor has (1) forcedly applied between the coating section and the in-furnace drying zone (room temperature drying zone) in order to avoid thickness unevenness and foaming of the coating film. Room temperature drying is performed without performing specific heating, and (2)
It was found that a method of measuring the temperature of the undried coating film from the room temperature drying zone to the in-furnace drying zone and controlling the molding conditions so as to keep the temperature at which dew condensation does not occur is extremely effective. More specifically, one or a plurality of temperature sensors for the undried coating film on the support are arranged, and depending on the measurement temperature, the temperature below 0 ° C. at which the water mixed by dew condensation crystallizes, preferably Can prevent whitening caused by dew condensation by controlling the molding conditions such as the concentration of the resin solution to be applied, the coating thickness, and the amount of hot air in the oven drying zone so that the temperature does not fall below 0 ° C. it can. The present invention
It has been completed based on these findings.

[0010]

According to the present invention, a solution of a polymer material is coated on a support in the coating section and then conveyed to a drying zone arranged at a constant interval from the coating section. In a method for producing a cast film in which a solvent is evaporated in a drying zone to form a film, at least one temperature sensor for the undried coating film on the support is arranged between the coating section and the vicinity of the inlet in the drying zone. Then, the method for producing a cast film is provided, which comprises molding while measuring the temperature of the undried coating film. Further, according to the present invention, there is provided the method for producing a cast film, wherein the molding conditions are controlled so that the temperature of the undried coating film does not drop below 0 ° C. according to the temperature measured by the temperature sensor.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a coating section, a belt (support for coating liquid), a drying oven (in-furnace drying zone), and a resin solution are provided in a room, and a polymer is provided in the coating section. After coating the resin solution of the material on the support, it is conveyed to the drying zone in the furnace, in the method for producing a cast film by evaporating the solvent, in both the room temperature drying zone or the drying zone in the furnace or each zone The necessary number of temperature sensors are installed, and the temperature of the undried coating film is measured. The undried coating film is less than 0 ° C, preferably 0 ° C or less, more preferably 2 ° C.
By controlling the molding conditions so as not to reach a low temperature of ℃ or less, it is possible to provide a highly transparent cast film with suppressed whitening.

A specific example of the installation position of the temperature sensor is shown in FIG. In FIG. 1, an infrared radiation thermometer is installed as a temperature sensor. The temperature may be measured by measuring the unused portion such as the end of the coating film with a contact type thermometer. In the coating unit 1, the resin solution 5 is coated on the support 4 and then conveyed into the box-shaped in-furnace drying zone 2. The coating unit 1 is a lip coater, a comma coater, or the like that is generally used for producing cast films. The infrared radiation thermometer changes the measurement wavelength depending on the type of resin to be coated. For example, in a polycarbonate resin, the absorption wavelength of infrared rays is in the range of about 7.9 to 8.3 μm (see FIG. 2), so it is preferable to measure within this range. Outside this range, the transmission area will be measured, and the temperature outside the measurement target, for example, the temperature of the surrounding equipment or the temperature of the surrounding workers, will be sensed, and accurate temperature cannot be measured. . The infrared absorption spectrum of the polysulfone resin film is shown in FIG.

The polymer material to be applied is not particularly limited as long as it is a resin which can be formed into a film by a casting method, and examples thereof include polycarbonate, polystyrene, polyarylate, polyetherketone, polysulfone and the like. These resins have a viscosity of 200 to 20000c which is dissolved in a solvent and has a viscosity that can be used for film formation by a usual casting method.
Adjust to ps range. Viscosity of resin solution is 200cps
If it is less than 2000, the thickness accuracy is deteriorated due to liquid sag, and 2000
If it exceeds 0 cps, defects are likely to occur due to the inclusion of bubbles, and both are not preferable. Examples of the support include films and sheets of polyethylene terephthalate (PET), polyimide, etc., metal belts of stainless steel, etc. The solvent is not particularly limited as long as it can be dissolved without decomposing the resin, but acetone, methylene chloride and other low boiling point organic solvents used in ordinary casting methods are preferable.

As a heating means in the drying zone in the furnace, hot air, infrared rays and the like can be mentioned. High frequency heating may be used as long as the surface of the dry coating film is not rough. The drying temperature is preferably at or below the boiling point of the solvent, but may be at or above the boiling point for rapid drying. The indoor humidity of the coating section and the room temperature drying zone is preferably low, but is preferably about 40% from the viewpoint of safety such as suppressing generation of static electricity. If there are structural restrictions, it may be about 60 to 70%. The solution concentration of the resin is preferably not less than 80% of the saturated concentration. For example, if the saturation concentration of the resin is 30% by weight, the solution concentration is preferably 24% by weight or more. The coating thickness is preferably thin.
The coating thickness is preferably 300 μm or less, but 500 μm
It is possible to some extent. When the coating thickness is 1 mm or more, not only whitening cannot be suppressed but also foaming cannot be suppressed, and it cannot be formed as a smooth film.

The environment is preferably no wind. The presence of wind accelerates the drying of the solvent in the room temperature drying zone, and causes whitening even under conditions that are not normally whitened. However, there is also a method of avoiding bleaching by stopping the wind even if the situation changes, if the condition that does not bleach even if the wind is applied is found in advance. When whitening is observed during coating, whitening can be improved by applying any of the above measures such as controlling the concentration of the resin solution or the coating thickness, but the temperature of the resin solution during coating can be improved. If the temperature gradually decreases, if the start of the decrease can be confirmed by the temperature sensor, the above measures can prevent whitening. Further, in the in-furnace drying zone, there is also a method of suppressing whitening by increasing the hot air amount within a range of the hot air amount that does not roughen the surface.

The whitening of the undried coating does not occur immediately after coating, but usually occurs after about 1 to 2 minutes. Therefore, a temperature sensor such as an infrared radiation thermometer can be used 1-2 times after coating a resin solution. It is preferable to install it at a position where the undried coating film reaches after a minute. In this case, the undried coating film is often already in the oven drying zone, and the time to reach the minimum temperature may change. Therefore, the number of temperature measurement points is not limited to one, but it is preferable to install the temperature measurement at a plurality of points with an interval, and it is necessary to pay close attention to the temperature sensor showing the lowest temperature among them. If the temperature sensor is installed inside the drying zone in the furnace, the temperature of the temperature sensor may rise due to heating, and the temperature of the undried coating may not be measured accurately.In that case, use the temperature sensor. It may be air-cooled or water-cooled.

In order to prevent whitening, a method of forcibly heating the undried coating film from the outside can be considered, but it is not desirable in view of the uniformity of the film thickness and the risk of foaming. It was found that the cause of the whitening was the crystallization of water in the resin due to the latent heat of radiation and the residual of bubbles in the film due to the evaporation of the crystallized water after the film formation. Therefore, according to the production method of the present invention, it becomes possible to prevent whitening by keeping the temperature of the undried coating film at a temperature of 0 ° C. or higher, preferably higher than 0 ° C. We were able to obtain a guideline for control during filming.

[0018]

The present invention will be described more specifically below with reference to examples and comparative examples.

Example 1 A commercially available polycarbonate resin (K-1285, Teijin Chemicals Ltd.) was dissolved in methylene chloride (CH ₂ Cl ₂ special grade, Junsei Chemical Co., Ltd.). A resin solution was prepared by changing the concentration of the resin solution to some extent and coated on a support by a coating machine (coating section) as shown in FIG. The base film used as the support is PT manufactured by Teijin Chemicals Ltd.
E film (grade OX) was used. At the time of coating, the amount of gap between the base film and the roll bar 7 in FIG. 1 was determined. The resin solution was poured into the coating part and coated on the base film. The coating thickness set with the roll bar 7 is 3
It was 20 μm. The base film proceeded at a speed of 10 cm / min. At the room temperature drying zone with a length of 50 cm,
The temperature inside the drying furnace was 100 ° C.

The coated base film was run in a windless room. On the other hand, at a position 200 mm above the undried coating film, an infrared radiation thermometer 9 having a wavelength of about 8 μm as the center of the measurement wavelength was installed to measure the temperature of the undried coating film (coating resin solution) 5. . The infrared radiation thermometer 9 is installed 200 mm above the undried coating film, 6 places in the room temperature drying zone and 4 places in the oven drying zone, with a space of 5 cm between them, and the temperature of the undried coating film after coating. It was possible to measure at any time until the end of the drop. The infrared radiation thermometer in the furnace was water cooled. When the room temperature was measured by the room temperature measuring thermometer 10, it was 22 ° C. and the humidity was 55%.

The results of the experiment are shown in Table 1 and the relationship between the minimum temperature of the undried coating film and whitening is shown. All the undried coating films whose temperature became lower than 0 ° C were whitened. That is, it can be seen that the whitening phenomenon is caused by crystallization of water mixed in due to dew condensation or the like, or water previously contained in the resin. From the results in Table 1, it can be seen that the higher the concentration (weight concentration) of the resin solution, the lower the temperature of the undried coating film is. In addition, in this case, the dissolved concentration is approximately 30% by weight in this case, so if the concentration is about 80% (25% by weight in this case, more precisely, 83% of the upper limit), whitening is less likely to occur. I found out. Therefore, if the measurement temperature of the undried coating film becomes less than 0 ° C. and whitening is likely to occur during coating, the concentration of the resin solution may be increased.

[0022]

[Table 1] (Footnote) (1) Whitening: The presence or absence of whitening of the undried coating film was visually observed and judged according to the following criteria. ○: transparent, △: slightly transparent, ×: whitening. (2) Haze value: The total light transmittance and parallel light transmittance of the undried coating film were measured, and the haze value was calculated by the following formula.
The haze value indicates that the larger the numerical value, the greater the degree of haze. Haze value = [(total light transmittance)-(parallel light transmittance)]
/ (Total light transmittance)

[Example 2] When the resin solution prepared in Example 1 was used for coating in the same manner as in Example 1, the gap of the roll bar 7 was changed to change the coating film thickness. It was The weight concentration of the resin solution was fixed at 25% by weight. Table 2 shows the results
It was shown to. The molding conditions such as the installation position of the infrared radiation thermometer, the room temperature drying zone, and the temperature inside the furnace were the same as in Example 1. Table 2 shows the results. Similar to Example 1, when the minimum temperature of the undried coating film was lower than 0 ° C., all were whitened. The coating thickness in this case was greater than 642 μm. Even if the coating thickness is 642 μm, it is slightly whitened,
In order to obtain a cast film having excellent transparency, approximately 500 μm can be considered as the upper limit of the coating thickness. When the coating thickness is 140 μm, the temperature of the undried coating film is sufficiently high, and it seems that it can be used even if the concentration of the resin solution is reduced, but the film thickness after drying becomes 140 μm × 25% = 35 μm, Difficult to handle. It is easier to handle after drying if it is applied as thick as possible. based on the above results,
It has been found that it is effective to reduce the coating thickness (as much as possible) when whitening easily occurs.

[0024]

[Table 2] ○: transparent, △: slightly transparent, ×: whitening.

[Comparative Example 1] Coating was carried out using the resin solution of Example. Resin solution concentration 25% by weight, coating thickness 320μ
I went in m. The line speed was 10 cm / min, the room temperature drying zone length was 50 cm, and the furnace temperature was 100 ° C. Immediately after coating, when a wind speed of 150 mm / sec was applied and dried,
The temperature of the undried coating film rapidly decreased and turned white.
Therefore, from this result, it is understood that it is better not to perform the action of increasing the evaporation rate of the solvent in the room temperature drying zone. That is, it is preferable not to apply wind. The results are shown in Table 3.

[0026]

[Table 3] ○: transparent, △: slightly transparent, ×: whitening.

[Comparative Example 2] The resin solution of Example 1 was applied onto a glass surface. The resin solution concentration was 25% by weight, and the coating thickness was 320 μm. Other settings were made in the same manner as in Example 1. A heating heater (manufactured by Corning-stirrer / Hotplat) is provided on the lower side of the glass surface to be coated.
e) was installed and heated after coating until the end of drying. Room temperature 2
There was no change when heated to 2 ° C to 30 ° C. However, when heated to 50 ° C., the undried coating film foamed, and a smooth film was not obtained. In the case of forced heating, the heating surface must be as uniform as possible, which is practically difficult. It is desirable not to perform forced heating as much as possible. <Summary> The results of Examples and Comparative Examples are summarized in Table 4.

[0028]

[Table 4] ○: transparent, △: slightly transparent, ×: whitening. From Table 4, it can be seen that the undried coating film is completely whitened at a temperature lower than 0 ° C. To prevent whitening,
It is necessary to mold at 0 ° C or higher, and it is desirable to mold at a temperature higher than 0 ° C.

[0029]

According to the present invention, there is provided a method for producing a cast film, which is excellent in smoothness, thickness accuracy, transparency, etc. in the method for producing a cast film, which prevents whitening of the coating film. Therefore, according to the manufacturing method of the present invention, for example, smoothness, thickness accuracy, which is provided for optical use,
It is possible to obtain a plastic film having excellent transparency.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a specific example of installation of an apparatus and an infrared radiation thermometer used in the method for producing a cast film of the present invention.

FIG. 2 is an infrared absorption spectrum of a polycarbonate resin film (thickness 10 μm).

FIG. 3 is a polysulfone resin film (thickness 10
μm) infrared absorption spectrum.

[Explanation of symbols]

 1: coating part 2: drying zone (drying zone in furnace) 3: drying zone (drying zone in furnace) 4: support 5: coated resin solution (undried coating film) 6: roll 7: roll bar 8: Roll 9: Temperature sensor (infrared radiation thermometer) 10: Room temperature measuring thermometer

Claims (2)

[Claims] 1. A solution of a polymer material is applied on a support in the application section, and then the solution is conveyed to a drying zone arranged at a constant interval from the application section, and the solvent is evaporated in the drying zone. In the method of manufacturing a cast film to be molded,
Between the coating section and the vicinity of the entrance in the drying zone,
A method for producing a cast film, comprising arranging at least one temperature sensor for a non-dried coating film on a support, and molding while measuring the temperature of the non-dried coating film. 2. The method for producing a cast film according to claim 1, wherein the molding conditions are controlled so that the temperature of the undried coating film does not drop below 0 ° C. according to the temperature measured by the temperature sensor.