JP5623848B2 - Manufacturing method of optical film - Google Patents

Description

  The present invention relates to a method for producing an optical film such as a polarizer protective film. Especially this invention relates to the manufacturing method of the optical film which has an acrylic resin as a main component.

  As a typical method for producing an optical film such as a polarizer protective film widely used in a display device such as a liquid crystal display device, one using a solution casting film forming method or one using a melt extrusion method is known. It has been.

  The solution casting film forming method is a method in which a film is obtained by dissolving a resin in a solvent, flowing the solution on a support in a spreading manner, and evaporating the solvent. In order to obtain a polarizer protective film, the film obtained by the solution casting film forming method is further stretched. Although the solution casting film forming method has the advantage of being excellent in film thickness uniformity, equipment for drying the solvent is essential, and there is a problem that the manufacturing equipment must be large-scale. is there.

  On the other hand, the melt extrusion method has a step of extruding a film-like resin from a T die attached to an extruder (for example, Patent Documents 1 to 4). In the melt extrusion method, a thick resin film extruded from an extruder is stretched and thinned to obtain a film. For example, in the case of an acrylic resin or the like, the resin film extruded from the extruder is sandwiched between two pairs of rolls, and the sheet is stretched in the longitudinal direction with the peripheral speed of the front roll in the traveling direction faster than that of the rear roll. That is, longitudinal stretching is performed. In order to obtain a polarizer protective film, the film stretched in the longitudinal direction is further stretched in the lateral direction. That is, the film described above is stretched in the direction perpendicular to the previous stretching direction by sandwiching both ends of the film stretched in the longitudinal direction with clips and widening the gap between the clips at both ends.

  In the melt extrusion method, it is not necessary to dry the solvent, so the construction of the equipment is relatively simple, but there is a problem that the thickness of the film tends to vary. Therefore, in the method disclosed in Patent Document 1, when the molten resin is extruded between the main roll and the touch roll, the thickness of the resin is maximized at the center in the width direction and is minimized at the end corresponding to the pinching region. Adjust the thickness so that That is, when the molten resin is sandwiched between the main roll and the touch roll, it is noted that the roll is bent and the pressing force in the central portion in the width direction is smaller than that of the end portion. When the sheet is pushed out between the touch roll and the touch roll, the sandwiching pressure applied to the resin is made uniform so that the thickness of the resin becomes maximum at the center in the width direction.

  Even the methods disclosed in Patent Documents 2, 3, and 4 are adjusted so that the cross-sectional shape of the film before stretching is maximized at the center in the width direction. That is, when the film is stretched in the transverse direction, the film is stretched from the center in the width direction, and the stretched region gradually spreads toward the end side. Therefore, when a flat film is stretched in the transverse direction, the central portion tends to be thinned, and the cross-sectional shape of the film before stretching is adjusted to be maximum at the center in the width direction.

  FIG. 4 is a diagram schematically showing a method for producing a retardation film disclosed in Patent Documents 2, 3, and 4, together with a configuration of a conventional film production apparatus. A film manufacturing apparatus 101 shown in FIG. 4 includes an extruding unit 102 responsible for an extruding process, a longitudinal stretching part 103 responsible for a longitudinal stretching process, and a transverse stretching part 105 responsible for a transverse stretching process. The resin film F extruded from the T die of the extrusion unit 102 is stretched in the longitudinal direction (longitudinal direction) by the longitudinal stretching unit 103, and then stretched in the lateral direction (width direction) by the lateral stretching unit 105.

4 (c-1) to (c-4) schematically show the cross-sectional shape of the film F in each step, and FIG. 4 (c-1) shows the cross-sectional shape of the resin film immediately after extrusion from the T-die. 4 (c-2) is the cross-sectional shape of the resin immediately after longitudinal stretching, FIG. 4 (c-3) is the cross-sectional shape of the resin during lateral stretching, and FIG. 4 (c-4) is the cross-sectional shape of the resin after lateral stretching. Shape. In the methods disclosed in Patent Documents 2, 3, and 4, as shown in FIG. 4 (c-1), the resin film F whose cross-sectional shape has the maximum thickness at the center in the width direction is stretched in the vertical direction. A longitudinally stretched film having a maximum thickness at the center in the width direction as in 4 (c-2). Thereafter, the longitudinally stretched film is stretched in the transverse direction, and stretching is started from the central portion as shown in FIG. 4 (c-3), and the stretched region is expanded to the end side, as shown in FIG. 4 (c-4). Thus, an optical film F such as a retardation film having a uniform thickness is obtained.
In the prior art, an optical film made of a resin mainly composed of cellulose ester (Patent Document 1) or a norbornene resin (Patent Documents 2, 3, and 4), which is a cyclic olefin resin, is mainly manufactured.

JP 2008-296537 A JP 2008-39807 A JP 2008-39808 A JP 2007-187777 A

By the way, it is desirable that the resin used as the material of the optical film such as the polarizer protective film has high transparency. Therefore, an acrylic resin having high transparency is suitable as a material for the optical film. However, acrylic resins are more fragile than cellulose esters and cyclic olefin-based resins, and special attention is required for stretching in the transverse direction. That is, when performing lateral stretching, the film is heated, and both ends of the film are gripped and stretched in the lateral direction. However, when the temperature of the film is high, the gripped portion is brittlely broken and the yield is poor. Therefore, when the acrylic resin is stretched in the transverse direction, it must be stretched at a relatively low temperature. However, when it is stretched at a relatively low temperature, the techniques disclosed in Patent Documents 2, 3, and 4 are used. When the longitudinally stretched film having the maximum thickness at the center in the width direction is stretched in the transverse direction as shown in FIG. 4 (c-2), the thickness of the film after transverse stretching tends to vary.
Moreover, even if the techniques disclosed in Patent Documents 2, 3, and 4 are adopted and the longitudinally stretched film having the maximum thickness at the center in the width direction is stretched in the lateral direction as shown in FIG. It is difficult to obtain a film having a small thickness variation.
Therefore, when manufacturing optical films, such as a polarizer protective film, using an acrylic resin as a raw material, it is desirable that the cross-sectional shape of the film before lateral stretching is as flat as possible.

On the other hand, according to the knowledge of the present inventors, when a longitudinally stretched film is formed by melt extrusion using an acrylic resin as a raw material, the film after longitudinal stretching has both ends as shown in FIG. 5 (c-2) and FIG. Only the part tends to be thick.
That is, when a resin having a flat cross-sectional shape as shown in FIG. 5 (c-1) is stretched in the longitudinal direction, only both end portions in the width direction are in other parts as shown in FIGS. 5 (c-2) and 6. In comparison, a thick longitudinally stretched film is formed. When a longitudinally stretched film having such a shape is stretched horizontally, it becomes as shown in FIG. 5 (c-3), and finally a film having a small thickness at the center as shown in FIG. 5 (c-4) is obtained. .

  On the other hand, when an acrylic resin having a semicircular cross-sectional shape as shown in FIG. 7A is stretched in the longitudinal direction in the same manner, a part of the semicircular shape remains, contrary to expectation, in the width direction. The thickness of the central part is not uniform. If this longitudinally stretched film is subsequently stretched laterally, a film having a large thickness at the center as shown in FIG.

  Accordingly, the present invention focuses on the above-described problems of the prior art, and provides an optical film made of acrylic resin, which has a high thickness uniformity and a high yield. And

Invention of Claim 1 for solving an above-described subject is an extrusion process which extrudes the resin which has an acrylic resin as a main component continuously in a film form, and extends | stretched the film extruded by the said extrusion process to a longitudinal direction In a method for producing an optical film comprising a longitudinal stretching step, and a transverse stretching step of stretching the film that has undergone the longitudinal stretching step in the transverse direction,
The film subjected to the longitudinal stretching process has a thickness at both ends smaller than the center, a uniform thickness at the center, and a gradual decrease in thickness at both ends. Part,
The width of the uniform thickness portion in the film subjected to the longitudinal stretching step is 50% or more of the total width of the film, and the total width of the gradually decreasing portions at both ends is 20% or more of the total width of the film. The thickness of the portion with the smallest thickness is 95% or less of the thickness of the uniform thickness portion,
The difference between the maximum thickness and the minimum thickness in the uniform part of the film subjected to the longitudinal stretching step is 5 μm or less,
The film thickness is leveled by the longitudinal stretching process, and then the transverse stretching process is performed .
The difference between the maximum value and the minimum value of the thickness in the optical film is within ± 1.5 μm .

  The present invention relates to a method for producing an optical film, an extrusion process for continuously extruding a resin mainly composed of an acrylic resin into a film, and a longitudinal stretching process for stretching the film extruded in the extrusion process in the longitudinal direction. And three steps of a transverse stretching step of stretching the film that has undergone the longitudinal stretching step in the transverse direction. And in the manufacturing method of the optical film of this invention, it has the characteristics in the shape of the film provided to a longitudinal stretch process. That is, a film having a thickness-decreasing portion at both ends smaller than the central portion, a uniform thickness portion having a uniform thickness at the central portion, and a gradually decreasing portion at which the thickness gradually decreases at both ends is a longitudinal stretching step. And the film thickness is leveled by the longitudinal stretching step. In the method for producing an optical film of the present invention, since the film subjected to the longitudinal stretching step has the above shape, the thickness of the film is leveled and flattened by the longitudinal stretching step, and only both end portions of the film are thickened. There is no. Therefore, even if it performs a horizontal extending process after that, the thickness of the center part of a film does not become a thin film, but the uniformity of thickness is maintained highly. According to the method for producing an optical film of the present invention, it is possible to produce an optical film made of an acrylic resin with extremely small thickness variation. The present invention is useful for producing a polarizer protective film and the like.

It is preferable that the difference between the maximum thickness and the minimum thickness in the uniform thickness portion of the film subjected to the longitudinal stretching step is within 5% of the maximum thickness (Claim 2).

A configuration in which the optical film is a polarizer protective film is preferred (claim 3 ).

  According to the present invention, it is possible to manufacture an optical film made of an acrylic resin with extremely small thickness variation.

It is a figure explaining the manufacturing method of the optical film of this invention, (a) is a top view which shows the outline of a film manufacturing apparatus, (b) is a side view of (a), (c-1)-(c-4) ) Are cross-sectional views showing the cross-sectional shape of the film at each stage. It is a side view which shows the detail of the longitudinal stretch part of the film manufacturing apparatus of Fig.1 (a), (b). It is an enlarged view of FIG.1 (c-1). It is a figure explaining the manufacturing method of the optical film of a prior art, (a) is a top view which shows the outline of a film manufacturing apparatus, (b) is a side view of (a), (c-1)-(c-4) ) Are cross-sectional views showing the cross-sectional shape of the film at each stage. It is a figure explaining the manufacturing method of the optical film made from an acrylic resin of a prior art, (a) is a top view which shows the outline of a film manufacturing apparatus, (b) is a side view of (a), (c-1)-( c-4) is a cross-sectional view showing the cross-sectional shape of the film at each stage. It is an enlarged view of FIG.5 (c-2). It is sectional drawing which shows the behavior at the time of extending | stretching the acrylic resin film whose cross-sectional shape is semicircle, (a) is cross-sectional shape before longitudinal stretch, (b) shows the cross-sectional shape of a final product.

  Hereinafter, embodiments of the present invention will be described in detail.

  A film manufacturing apparatus 1 shown in FIGS. 1A and 1B includes an extruding unit 2 that performs an extruding process, a longitudinal stretching unit 3 that performs a longitudinal stretching process, and a lateral stretching unit 5 that performs a lateral stretching process. The configuration of the film manufacturing apparatus 1 is basically the same as that of the conventional film manufacturing apparatus 101 shown in FIGS. In addition, in FIG. 1 (a), (b), the structure of the longitudinal stretch part 3 is simplified and shown, and the detailed structure is shown in FIG.

  The extruding unit 2 includes a T-die 7 and can extrude a molten acrylic resin into a sheet shape.

As shown in FIG. 2, the longitudinally extending portion 3 includes a plurality of rolls 8a to 8k, 10a, and 10b. The roll 8a is a touch roll, the rolls 8b, 8c and 8d are cooling rolls, the rolls 8e and 8f are guide rolls, the rolls 8g and 8h are preheating rolls, the rolls 8i and 8j are stretching rolls, and the roll 8k is a cooling roll. Roll 11 a, 11 b are nip rolls, each drawing roll 8i, is provided above the 8j.
The resin extruded from the extrusion unit 2 is formed into a film shape by passing between the touch roll 8a and the cooling roll 8b, and further between the cooling rolls 8b to 8d. Next, the molded film-like resin (film F) passes through the guide rolls 8e and 8f and then passes through the preheating rolls 8g and 8h to be preheated before longitudinal stretching. Preheated film F, draw roll 8i, by passing between 8j and nip rolls 11 a, 11 b, the vertical direction (conveying direction, the longitudinal direction) is stretched (longitudinal stretching step). That is, the film F is stretched in the longitudinal direction by making the rotational speed of the stretching roll 8j larger than the rotational speed of the stretching roll 8i. The longitudinally stretched film F is cooled by a cooling roll 8k and then subjected to a transverse stretching step.
In addition, in this embodiment, the structure which does not have the touch roll 8a is also employable.

  The laterally extending portion 5 includes a pair of guides 10. The guide 10 spreads outward (width direction) with respect to the transport direction of the film F, and a gripping member (not shown) that grips both ends in the width direction of the film F moves along the guide 10, thereby The film F (longitudinal stretched film) supplied from the stretching section 3 is stretched in the transverse direction (lateral stretching step).

  In addition, as a specific example of the above-described longitudinal stretching and lateral stretching methods, for example, the method described in JP-A-2002-221312 can be exemplified.

  In the present embodiment, the cross-sectional shape of the T die 7 provided in the extrusion unit 2 is substantially the same as the shape shown in FIGS. Therefore, a resin sheet having a cross-sectional shape as shown in FIG. 1 (c-1) and FIG. 3 is extruded from the extrusion portion 2 and provided to the longitudinal stretching portion 3.

Details of the cross-sectional shape of the film F provided to the longitudinally stretched portion 3 are shown in FIG. 1 (c-1) and FIG. 3, and the thickness of both end portions 22 a and 22 b is smaller than that of the central portion 21. . The center portion has a uniform thickness portion 23 having a uniform thickness, and both end portions 22a and 22b have gradually decreasing portions 25a and 25b whose thickness gradually decreases.
In a preferred embodiment, the width W2 of the uniform thickness portion 23 is 50% or more of the total width W1 of the film F, and the sum of the widths W3a and W3b of the gradually decreasing portions 25a and 25b is 20% or more of the total width W1 of the film F. The thickness T2 of the portion with the smallest thickness of the gradually decreasing portions 25a and 25b is 95% or less of the thickness T1 of the uniform thickness portion 23. Most preferably, W1 = W2 + W3a + W3b.
In addition, the angle θ (inclination angle θ, 0 °) formed by the sizes of the both end portions 22a and 22b, the uniform thickness portion 23, and the gradually decreasing portions 25a and 25b, and the surface of the film F and the planes forming the gradually decreasing portions 25a and 25b. <Θ <90 °, see FIG. 3) and the like can be freely set using an adjustment bolt for the lip of the T die 7.

  In the film F shown in FIG. 3, W1 is about 1000 mm, W2 is about 600 mm, T1 is about 130 μm, and T2 is about 120 μm, but the present invention is not limited to this.

Regarding the variation in the thickness T1 of the uniform thickness portion 23, in a preferred embodiment, the difference between the maximum thickness and the minimum thickness in the center portion and the region over 50% of the total width W1 of the film F is 5 μm or less. , Smaller than the difference between T1 and T2 (about 10 μm).
In another preferred embodiment, the difference between the maximum thickness and the minimum thickness is within 5% of the maximum thickness.

  In order to facilitate understanding, some sizes are exaggerated in FIG.

  Next, the change in the cross-sectional shape of the film F in the extrusion process, the longitudinal stretching process, and the lateral stretching process will be described sequentially in comparison with the prior art (FIG. 5).

First, as shown in FIG. 1 (c-1) and FIG. 3, the film F extruded from the extrusion part 2 has both end portions 22a and 22b smaller in thickness than the center portion 21 and has a thickness in the center portion. Has uniform thickness-equalizing portions 23, and both end portions 22a and 22b have gradually decreasing portions 25a and 25b whose thickness gradually decreases.
Here, in the case of a film made of an acrylic resin, only the both end portions tend to be thick in the film after longitudinal stretching. Therefore, if the film before longitudinal stretching has a flat shape as in the prior art (FIG. 5 (c-1)), the film after longitudinal stretching has both ends as shown in FIG. 5 (c-2) and FIG. Only the part becomes thick.
However, in this embodiment, since the film before longitudinal stretching has a shape as shown in FIG. 1 (c-1) and FIG. 3, the “thickness portion 23 is formed in the central portion after longitudinal stretching. The thicknesses of both end portions 22a and 22b coincide with the thickness of the central portion, and the thickness is leveled. As a result, the film F after longitudinal stretching has a flat shape as shown in FIG.

The film F (longitudinal stretched film) that has finished the longitudinal stretching process is sent to the lateral stretching section 5 and used in the lateral stretching process. At this time, since the film F (longitudinal stretched film) subjected to the transverse stretching step is a flat one having a cross-sectional shape as shown in FIG. 1 (c-2), as shown in FIG. 1 (c-3). A flat shape is maintained even during transverse stretching, and a flat shape as shown in FIG. 1 (c-4) is retained even at the end of the transverse stretching step. As a result, it is possible to obtain an optical film F in which the uniformity of the thickness is maintained at a high level and the thickness variation is extremely small.
According to this embodiment, a film having a small thickness at the central portion as shown in FIG. In addition, a film having a large thickness at the center as shown in FIG.

  Any acrylic resin used in the present invention can be used as long as it is generally used as a material for optical films. For example, a glutarimide resin obtained by reacting methylamine with a polymethyl methacrylate or a methyl methacrylate-styrene copolymer can be employed. More specifically, a glutarimide resin containing a repeating unit represented by the following general formulas (1) and (2) or (1)-(3) can be employed. The details of the glutarimide resin are described in, for example, International Publication No. 2005/54311 pamphlet and International Publication No. 2005/108438 pamphlet.

（ここで、Ｒ¹およびＲ²は、それぞれ独立に、水素原子または炭素数１〜８のアルキル基を示し、Ｒ³は、水素原子、炭素数１〜１８のアルキル基、炭素数３〜１２のシクロアルキル基、または炭素数６〜１０のアリール基を示す。）

(Here, R ¹ and R ² each independently represent a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, and R ³ represents a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, or 3 to 12 carbon atoms. Cycloalkyl group or an aryl group having 6 to 10 carbon atoms.)

（ここで、Ｒ⁴およびＲ⁵は、それぞれ独立に、水素原子また炭素数１〜８のアルキルを示し、Ｒ⁶は、炭素数１〜１８のアルキル基、炭素数３〜１２のシクロアルキル基、または炭素数６〜１０のアリール基を示す。）

(Wherein R ⁴ and R ⁵ each independently represent a hydrogen atom or alkyl having 1 to 8 carbon atoms, R ⁶ represents an alkyl group having 1 to 18 carbon atoms or a cycloalkyl group having 3 to 12 carbon atoms. Or an aryl group having 6 to 10 carbon atoms.)

（ここで、Ｒ⁷は、水素原子または炭素数１〜８のアルキル基を示し、Ｒ⁸は、炭素数６〜１０のアリール基を示す。）

(Here, R ⁷ represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, and R ⁸ represents an aryl group having 6 to 10 carbon atoms.)

  Other examples of the acrylic resin include those having a lactone structure and those having a glutaric anhydride unit.

  In the above-described embodiment, an example in which an optical film is manufactured using the film manufacturing apparatus 1 having the configuration shown in FIGS. 1A, 1B, and 2 is shown. However, the film manufacturing apparatus used in the present invention is this. Of course, it is not limited to. For example, the number of cooling rolls in the longitudinal stretching portion can be freely set according to the purpose.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

  First, a method for measuring physical properties and evaluation values shown as results of each experiment will be described.

(1) Imidation ratio 1H-NMR 1H-NMR measurement of resin was performed using BRUKER Avance III (400 MHz). The area A of the peak derived from the O-CH3 proton of methyl methacrylate near 3.5 to 3.8 ppm and the area B of the peak derived from the N-CH3 proton of glutarimide near 3.0 to 3.3 ppm were determined, The imidation ratio Im (%) was calculated by the following formula.
Im = {B / (A + B)} × 100
Here, the “imidization rate” refers to the ratio of the imide carbonyl group in the total carbonyl group.

(2) Acid value 0.3 g of resin was dissolved in 37.5 mL of methylene chloride, and 37.5 mL of methanol was further added. Next, 5 mL of 0.1 mmol% sodium hydroxide aqueous solution and several drops of ethanol solution of phenolphthalein were added. Next, back titration was performed using 0.1 mmol% hydrochloric acid, and the acid value was determined from the amount of hydrochloric acid required for neutralization.

(3) Glass transition temperature Using a differential scanning calorimeter (DSC, model DSC-50 manufactured by Shimadzu Corporation) using 10 mg of resin, the glass transition temperature was measured at a heating rate of 20 ° C./min in a nitrogen atmosphere. Determined by point method.

(4) Thickness and thickness variation Measurement was performed using a stylus type continuous film thickness meter (film thickness tester KG601B and electronic micrometer K3001A) manufactured by Anritsu Corporation. Specifically, in the case of an original fabric, a sample of 30 mm (flow direction of the film) × 1,000 mm (full width in the width direction of the film) was cut out, and the thickness in the width direction of 1,000 mm was continuously measured at 1 mm intervals, and the average value Was the thickness. In the case of a uniaxially stretched film (film after longitudinal stretching), a thickness of 30 mm in width and 900 mm in length was continuously measured in the film width direction. Furthermore, in the case of a sequential biaxially stretched film, a sample of 30 mm (film flow direction) × 1,400 mm (film width direction full width) is cut out, and the thickness excluding 75 mm from both ends in the width direction is 1 mm apart in the width direction. The average value was taken as the thickness. The thickness variation was defined as the difference between the maximum value and the minimum value of the thickness. However, regarding the variation of the raw film in Examples 1 and 2, the difference between the maximum value and the minimum value of the thickness corresponding to W2 was taken.

[Example 1]
1. Preparation of Resin A resin was prepared by the following procedure using a tandem reaction extruder in which two extrusion reactors were arranged in series. As for the tandem type reactive extruder, the first extruder (1) and the second extruder (2) are both in the same direction meshing type with a diameter of 75 mm and L / D (ratio of the length L to the diameter D of the extruder) of 74. Using a twin-screw extruder, the raw material resin was supplied to the raw material supply port of the first extruder using a constant weight feeder (manufactured by Kubota Corporation). The degree of vacuum of each vent in the first extruder and the second extruder was set to -0.095 MPa. Moreover, the 1st extruder and the 2nd extruder were connected by piping of diameter 38mm and length 2m. The resin (strand) discharged from the second extruder was cooled by a cooling conveyor and then cut by a pelletizer to form pellets.

  Regarding the first extruder, a polymethyl methacrylate resin (Mw: 105,000) was used as a raw material resin, and monomethylamine was used as an imidizing agent to produce an imide resin intermediate 1. Regarding the second extruder, the imidization reaction reagent and by-products remaining in the vent were devolatilized, and then a mixed solution of dimethyl carbonate and triethylamine was added as an esterifying agent to produce an imide resin intermediate 2. Furthermore, after removing the esterifying agent with a vent, the resin composition was obtained by extruding from a strand die, cooling in a water tank, and pelletizing with a pelletizer. The imidation ratio of this resin composition was 3.7%, the acid value was 0.29 mmol / g, and the glass transition temperature was 130 ° C.

2. Production of Optical Film Basically, an optical film was produced by the method shown in FIGS. That is, after drying the obtained resin composition at 100 ° C. for 5 hours, it was extruded into a sheet using a 50 mm single screw extruder and a 1,100 mm wide T-die (extrusion process), and the sheet was discharged with a metal cooling roll. Was cooled to obtain an original fabric having a width of 1,000 mm. The cross-sectional shape of this film is as shown in FIG. 1 (c-1) and FIG. 3, and the sizes in FIG. 3 are W1 = 1000 mm, W2 = 600 mm, W3a = W3b = 200 mm, T1 = 130 μm, T2 = 120 μm, average thickness was 128 μm, and thickness variation was ± 1.0 μm.
Next, the raw material was subjected to a longitudinal stretching process using a roll longitudinal stretching machine to obtain a longitudinal uniaxially stretched film. Specifically, the raw fabric was preheated to 100 ° C. with a preheating roll of a longitudinal stretching machine, and then stretched 2.0 times with a stretching roll of 130 ° C. to obtain a longitudinally uniaxially stretched film having a width of 900 mm. The average thickness of this film was 80 μm, and the thickness variation was ± 1.0 μm.
Furthermore, the longitudinally uniaxially stretched film was subjected to a transverse stretching process using a transverse stretching machine to obtain a sequential biaxially stretched film (optical film). Specifically, a longitudinally uniaxially stretched film is preheated to 130 ° C. in a preheating zone of a transverse stretching machine, and then stretched 2.0 times in a stretching zone of 130 ° C. to obtain a sequential biaxially stretched film having a width of 1,700 mm. It was. The cross-sectional shape of this film was a flat shape as shown in FIG. 1 (c-4), the average thickness was 40 μm, and the thickness variation was ± 1.0 μm.

[Example 2]
A biaxially stretched film was obtained in the same manner as in Example 1 except that the cross-sectional shape of the original fabric (FIGS. 1 (c-1) and 3) was changed to “W2 = 700 mm, W3a = W3b = 150 mm”. The cross-sectional shape of this film was a flat shape as shown in FIG. 1 (c-4), the average thickness was 40 μm, and the thickness variation was ± 1.5 μm.

[Comparative Example 1]
A film having a cross-sectional shape as shown in FIG. 5 (c-1) was subjected to a longitudinal stretching step and a lateral stretching step similar to those in Examples, to obtain a sequentially biaxially stretched film. The cross-sectional shape of this film was as shown in FIG. 5 (c-4), the average thickness was 40 μm, and the thickness variation was ± 4.0 μm. That is, the sequential biaxially stretched film obtained in this comparative example did not have a flat shape as in the examples, but had a large thickness variation.

[Comparative Example 2]
A film having a cross-sectional shape as shown in FIG. 7 (a) was subjected to the same longitudinal stretching step and lateral stretching step as those in Examples, to obtain sequentially biaxially stretched films. The cross-sectional shape of this film was as shown in FIG. 7B, the average thickness was 40 μm, and the thickness variation was ± 3.0 μm. That is, the sequential biaxially stretched film obtained in this comparative example did not have a flat shape as in the examples, but had a large thickness variation.

The results of Examples 1 and 2 and Comparative Examples 1 and 2 are summarized in Table 1.

  As described above, according to this example, it was possible to manufacture an optical film made of an acrylic resin that has high uniformity of thickness and extremely small variation in thickness.

F film (optical film)
21 Center part 22a, 22b Both end part 23 Thickness part 25a, 25b Decrease part

Claims (3)

An extrusion process for continuously extruding a resin mainly composed of an acrylic resin into a film, a longitudinal stretching process for stretching the film extruded in the extrusion process in the longitudinal direction, and a film subjected to the longitudinal stretching process in the lateral direction In a method for producing an optical film comprising a transverse stretching step of stretching,
The film subjected to the longitudinal stretching process has a thickness at both ends smaller than the center, a uniform thickness at the center, and a gradual decrease in thickness at both ends. Part,
The width of the uniform thickness portion in the film subjected to the longitudinal stretching step is 50% or more of the total width of the film, and the total width of the gradually decreasing portions at both ends is 20% or more of the total width of the film. The thickness of the portion with the smallest thickness is 95% or less of the thickness of the uniform thickness portion,
The difference between the maximum thickness and the minimum thickness in the uniform part of the film subjected to the longitudinal stretching step is 5 μm or less,
The film thickness is leveled by the longitudinal stretching process, and then the transverse stretching process is performed .
The difference between the maximum value and the minimum value of the thickness in the optical film is within ± 1.5 μm . 2. The optical film according to claim 1, wherein the difference between the maximum thickness and the minimum thickness in the uniform thickness portion of the film subjected to the longitudinal stretching step is within 5% of the maximum thickness. Production method. The method for producing an optical film according to claim 1 or 2, characterized in that the optical film is a polarizer protective film.

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