Production of phase difference film

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Industrial application field]

The present invention relates to a method for producing a retardation film by stretching a polysulfone-based film laterally and uniaxially, and in particular, an improvement of a production method that requires a retardation film having uniform letter determination over the entire surface and excellent viewing angle characteristics. It is about.

【0002】

[Conventional technology]

The retardation film utilizes the birefringence of a uniaxially stretched polymer film (generated because the refractive index in the stretching direction and the direction orthogonal to the stretching direction differ depending on the molecular orientation accompanying the stretching). This is to eliminate the phase difference generated between the polarizations (called phase difference compensation). The phase difference compensation performance is represented by the retardation value Re, that is, the product of the refractive index in the stretching direction and the refractive index difference Δn in the direction orthogonal to the retardation value Δn and the film thickness d.

【0003】

Such a retardation film is described in, for example, Japanese Patent Application Laid-Open No. 2-42406, and is manufactured by uniaxial stretching so that the neck-in rate (shrinkage rate in the direction orthogonal to the stretching direction) is 10% or less. Orthogonal.

【0004】

However, since the retardation value Re of the retardation film produced in this way changes as the incident angle of the incident light increases, color unevenness may occur on the display screen of the liquid crystal display device, or the display screen may be viewed from the front or sideways. A so-called color reversal phenomenon in which black and white are reversed may occur depending on when viewed from the direction.

【0005】

On the other hand, according to JP-A-2-191904, when a retardation film is produced by the "longitudinal uniaxial stretching method", the length in the direction orthogonal to the stretching direction is 1 / a of the ratio of the length before stretching.^1/2~ 1 / a^1/3It is described that the viewing angle characteristic is improved by controlling the angle of view.

【0006】

[Problems to be Solved by the Invention]

By the way, when the polymer film is stretched to produce a retardation film, it is necessary to pay particular attention to the following three points among some important qualities.

【0007】

(1) The above retardation is the same on the entire surface of the film.

【0008】

(2) Good viewing angle characteristics.

【0009】

(3) There should be no defective appearance.

【0010】

On the other hand, as a method for producing a retardation film using a polymer film, there is a "longitudinal uniaxial stretching method" in which stretching is performed in the longitudinal direction of the film by utilizing the difference in rotation speed between rolls sandwiching the film, and mechanically. There are two known "horizontal uniaxial stretching methods" in which the film is stretched in the lateral direction. When the polysulfone-based film is stretched, the "longitudinal uniaxial stretching method" cannot satisfy the quality of (3) above because the film is wound around the roll or the roll and the film are rubbed and scratched.

【0011】

Therefore, the "horizontal uniaxial stretching method" is usually applied as a stretching treatment method for the polysulfone-based film so that there is no poor appearance. Also. The lettering of (1) can be made uniform by optimizing the stretching conditions.

【0012】

However, the viewing angle characteristic of (2) above cannot be improved under stretching conditions. Here, "good viewing angle characteristics" refers to a state in which there is no difference between the characteristics viewed from the film normal direction and the characteristics viewed from the oblique direction, and this viewing angle characteristic is the refractive index anisotropy in the resin. Is closely related to. Here, the "refractive index anisotropy" refers to a state in which the refractive indexes in the three directions of the stretching direction of the film, the direction orthogonal to the stretching direction (the direction perpendicular to the stretching), and the thickness direction of the film are different from each other.

【0013】

Then, in order to improve the viewing angle characteristics, it is easy to make the refractive indexes in the stretching perpendicular direction and the refractive index in the thickness direction equal among the refractive indexes in the three directions. This is due to the following reasons. That is, the in-plane (characteristics seen from the film normal direction) retardation is obtained from the difference in refractive index between the stretching direction and the stretching perpendicular direction. When viewed from an oblique direction, a component in the thickness direction is added to this. When the refractive index in the thickness direction is significantly different from that in the other two directions, the retardation becomes significantly larger or smaller than in-plane. This change in retardation when viewed from an oblique direction is a poor viewing angle. For example, when such a retardation film is mounted on a liquid crystal display, a phenomenon in which colors are inverted between the front and the diagonal occurs. Therefore, in order to suppress the change in the retardation seen from the oblique direction as much as possible, it is preferable to make the refractive index in the stretching perpendicular direction and the refractive index in the thickness direction equal so that the refractive index in the apparent thickness direction does not affect.

【0014】

By the way, this refractive index is primarily proportional to the internal stress of the resin. The polysulfone film originally has an intrinsic index of refraction of 1.633, but when this resin is pulled from the inside to the outside, the refractive index increases, and when it is compressed from the outside to the inside, it decreases.

【0015】

When the change in the refractive index is applied to stretching, the isotropic refractive index is pulled in the stretching direction as shown in FIG. 5 (A), so that it becomes large. On the other hand, it becomes smaller in the thickness direction because it is compressed as shown in FIG. 5 (B). In addition, there is little change because it is regulated without changing in the direction perpendicular to the stretching.

【0016】

On the other hand, in the "horizontal uniaxial stretching method", it is difficult for a force to act in the direction orthogonal to the stretching direction (the direction perpendicular to the stretching) (because the roll interval for transporting the film is fixed and regulated). The change in the perpendicular direction is small (actually, the compressive force remains, so it becomes slightly smaller).

【0017】

Considering the unit volume of the film during the stretching process, as shown in FIG. 6 (A), when the film is pulled a times in the stretching direction (in this case, the x-axis direction), it is pulled in the other two directions (y-axis direction and z). (Axial direction) is usually 1 / a^1/2It doubles (that is, the volume of the film before and after stretching is constant). In this case, since the same stress is applied to the refractive index in both the stretching perpendicular direction and the thickness direction, the refractive index (n) in the stretching perpendicular direction is as shown in FIG. 6 (B)._y) And the refractive index in the thickness direction (n)_z) Are equal.

【0018】

In this state, the viewing angle characteristics are good.

【0019】

However, in the "horizontal uniaxial stretching method", the longitudinal direction of the film is regulated as described above, so that the film does not shrink in the longitudinal direction (y-axis direction) as shown in FIG. 7 (A). Therefore, the thickness direction contracts 1 / a times, and the refractive index (n) in the stretching perpendicular direction as shown in FIG. 7 (B)._y) And the refractive index in the thickness direction (n)_z) Is different, and there is a problem that the viewing angle characteristic is deteriorated.

【0020】

The present invention has been made by paying attention to such a problem, and the problem is that in the lateral uniaxial stretching method using a polysulfone film, the retardation is uniform over the entire surface and the viewing angle characteristics are excellent. It is an object of the present invention to provide a method for producing a retardation film.

【0021】

[Means for solving problems]

That is, the invention according to claim 1 is premised on a method of producing a retardation film by thermally shrinking the polysulfone-based film in the longitudinal direction after the polysulfone-based film is stretched horizontally. The polysulfone-based film is shaped so that its longitudinal cross-sectional shape has a corrugated shape, and while maintaining this corrugated shape, both ends of the polysulfone-based film in the lateral direction are gripped, and then heat shrinkage treatment is performed to perform polysulfone. The longitudinal length of the polysulfone is 1 / a'of the length before heat shrinkage.^1/2It is characterized in that it is controlled to be twice or more (where a'is the theoretical stretching ratio of the lateral uniaxial stretching treatment), and the invention according to claim 2 is the retardation film according to claim 1. Assuming the manufacturing method, re-lettering the manufactured retardation film when a sodium D line with a wavelength of 589.8 nm is incident on the manufactured retardation film from a direction parallel to the normal line.₀And the lettering when it is incident from the direction of 40 degrees with respect to the normal is Re₄₀When, it is characterized in that the following equation (1) is satisfied.

【0022】

0.90 Re₄₀/ Re₀ 1.10 (1) According to the inventions according to claims 1 and 2, even when the lateral uniaxial stretching method is applied, both ends of the lateral uniaxially stretched polysulfone film can be gripped and the film can be shrunk in the direction perpendicular to the stretching. , It is possible to make the refractive indexes in the direction perpendicular to the stretching direction and the refractive index in the thickness direction uniform. That is, the length in the direction perpendicular to the stretching of the film is 1 / a'of the length in the direction perpendicular to the stretching before heat shrinkage.^1/2It is doubled or more, and the refractive index in the direction perpendicular to the stretching approaches the refractive index in the thickness direction, and the difference in retardation Re due to the difference in the incident angle is made small.

【0023】

By the way, in general, the polymer chains constituting the stretched film are entangled with the adjacent polymer chains, and the orientation state is maintained by the shearing force generated by the entanglement. Then, in the once stretched film, the stretched state is maintained by this shearing force even when the film is heated to a temperature equal to or lower than the glass transition point.

【0024】

However, depending on the type of polymer, the stretched state cannot be maintained due to the low shearing force. This slippage between the resins is called "slip" for convenience, but the amount of slippage of the polysulfone-based resin is larger than that of other optical resins.

【0025】

In the contracted state after the above-mentioned lateral uniaxial stretching, the stretching perpendicular direction remains 1 and the thickness direction is 1 / a times (see Fig. 7A), and 1 / a for simple elastic recovery.^1/2It becomes stable, but due to the above "slip", 1 / a after the actual recovery^1/2Greater. It is an important point in controlling the refractive index of the polysulfone film to perform recovery control in consideration of this stress relaxation.

【0026】

This 1 / a^1/2Larger expected recovery is 1 / a'^1/2Can be set. That is, the film stretched a times is treated as a film stretched a'fold (a'<a) apparently by offsetting the "slip" amount.

【0027】

Then, the stretching perpendicular direction and the thickness direction are 1 / a'.^1/2A good viewing angle can be obtained by controlling to.

【0028】

This a'(a': theoretical stretch ratio in consideration of slippage) can be obtained from actual measurement.

【0029】

Hereinafter, this point will be described more specifically. For example, when a polysulfone film having an intrinsic refractive index of 1.633 is stretched 1.5 times in a lateral uniaxial manner, the refractive index in the stretching direction increases to 1.6357.

【0030】

Then, when the film is heated while maintaining this stretched state (that is, while the four sides of the stretched film are fixed), the refractive index drops to 1.6347.

【0031】

Here, assuming that the increase in length due to stretching is proportional to the increase in refractive index, the following equation (2) holds, where a is the lateral uniaxial stretching ratio, n is the refractive index, and k is the constant of proportionality.

【0032】

n = k × (a-1) +1.633 (2) However, (a-1) in the formula means the increase in length due to stretching divided by the length before stretching.

【0033】

Then, in the case of a = 1.5, since n = 1.6357, k = 0.0054, and the above (2) can be rewritten as the following equation (3).

【0034】

n = 0.0045 (a-1) +1.633 (3) Since the refractive index of the film after heating while maintaining the stretched state is 1.6347, if the stretching ratio (theoretical stretching ratio) of the stretched film having the same refractive index is a', the following equation ( 4) holds.

【0035】

1.6347 = 0.0054 (a'-1) + 1.633 (4) When this is calculated, a'= 1.315 is obtained, and although the film after heating is apparently stretched 1.5 times, it has substantially the same orientation as the film stretched 1.315 times. , Can be treated equivalently to a laterally uniaxially stretched polysulfone film stretched at this theoretical stretch ratio a'.

【0036】

Therefore, the length in the stretching perpendicular direction (longitudinal direction of the horizontally uniaxially stretched polysulfone film) is 1 / a'of the length before heat shrinkage.^1/2When controlled more than twice, the refractive index in the direction perpendicular to the stretching becomes equal to the refractive index in the thickness direction, the retardation value Re is constant regardless of the incident angle, and the viewing angle characteristic is excellent.

【0037】

The inventions according to claims 1 and 2 have been made based on such technical reasons.

【0038】

In these inventions, the theoretical stretching ratio a'specified by the refractive index after heating while maintaining the stretched state is the refractive index n before heating, the stretching ratio a, and the refractive index after heating according to the preliminary experiment described above. n'can be measured and calculated from the following equations (5) and (6).

【0039】

That is, n = k × (a-1) + n₀ (Five) n'= k × (a'-1) + n₀ (6) However, n₀Is the intrinsic refractive index before stretching, and k is the constant of proportionality.

【0040】

And from equations (5) and (6) a'= 1 + (a-1) × (n'-n₀) / (Nn₀) (7) According to the result of repeating the experiment to obtain this theoretical stretching ratio, Generally 1 / a^1/2 <1.05 / a^1/2 <1 / a'^1/2The relationship holds.

【0041】

Also, 1 / a^1/3And 1 / a'^1/2The magnitude relationship differs depending on the stretching conditions. As described in JP-A-2-191904, 1 / a^1/3Control within the following range can only be achieved under limited stretching conditions as long as the polysulfone film stretched laterally uniaxially is used.

【0042】

Next, the amount of shrinkage of the polysulfone film to be heat-shrinked is (1-1 / a'^1/2) Is appropriate, but the exact amount of shrinkage cannot be given due to a defect. For example, 1-1 / a as described in JP-A-2-191904.^1/2~ 1 / a^1/3If the heat shrinkage treatment is performed by loosening in the direction perpendicular to the stretching in the range of 1-1.05 / a^1/2(To be exact, 1-1 / a'^1/2), The film cannot be completely shrunk due to slippage, and wavy wrinkles remain on the surface, making it unusable as an optical film.

【0043】

On the contrary, (1-1 / a'^1/2) When held with a significantly smaller amount of slack (for example, when the amount of neck-in is close to 0), the refractive index does not change because it does not shrink in the direction perpendicular to the stretching even if heat shrinkage treatment is applied. Therefore, since it is difficult to bring the refractive index in the stretching perpendicular direction close to the refractive index in the thickness direction, improvement in viewing angle characteristics cannot be expected. In addition, since the film is inadvertently displaced, the difference in refractive index in the film surface becomes small, and it becomes difficult to obtain a predetermined retardation.

【0044】

Therefore, 1.05 / a^1/2Larger, 1 / a'^1/2That's it, and 1 / a'^1/2It is essential to control it so that it does not grow significantly larger.

【0045】

Further, in the above-mentioned Japanese Patent Application Laid-Open No. 2-42406, it is described that the neck-in rate is suppressed to 10% or less, preferably 0%, but when the neck-in is bound and stretched, the neck-in is stretched as described above. The direction orthogonal to the direction is 1 / a'^1/2Since it cannot be doubled, the refractive indexes in the direction perpendicular to the stretching direction and the refractive index in the thickness direction are not equal. Therefore, improvement in viewing angle characteristics cannot be expected. In particular, in the case of a'> 1.24 film, when the neck-in ratio is 10% or less, the refractive indexes in the stretching perpendicular direction and the thickness direction are absolutely not equal, and a retardation film having good viewing angle characteristics cannot be obtained.

【0046】

In the inventions according to claims 1 and 2, the above 1 / a'^1/2A polysulfon-based film that has been subjected to a lateral uniaxial stretching treatment (a treatment in which the vertical length perpendicular to the transverse stretching direction is equal to the length before stretching) as described above in order to realize a range of more than double. Is shaped so that its vertical cross-sectional shape becomes a corrugated shape, and after grasping both ends of the polysulfon-based film in the lateral direction while maintaining this corrugated shape, the lateral direction of the film is held as it is. However, it causes heat shrinkage in the vertical direction.

【0047】

The horizontal uniaxial tenter stretching method is preferable as the method for performing the horizontal uniaxial stretching treatment.

【0048】

Then, in the inventions according to claims 1 and 2, the reduction ratio in the vertical direction is the length of the film after shrinkage (straight line distance along the axis of the waveform) in the case where the waveform is triangular as shown in FIG. When (appeared) is α and the length of the film along the waveform is β, it is given as (β-α) / β × 100 (%), and the desired values for the wave pitch, height, and shape. Can be controlled. Further, there are no particular restrictions on the waveform shape. As a method of appropriately shaping the polysulfone film that has been subjected to the laterally uniaxially stretched treatment, the following method can also be applied as long as the film is not torn and damaged. Further, as for the method of holding the waveform shape, the entire waveform may be gripped or a partial grip may be performed.

【0049】

Further, as an apparatus for performing heat treatment in the inventions according to claims 1 and 2, an apparatus obtained by modifying a horizontal uniaxial stretching machine or the like can be applied.

【0050】

Further, in the stretching treatment, various conditions such as stretching temperature, magnification, stretching speed, heat setting (heat treatment after stretching) temperature, heat setting time and the like are appropriately set so as to have desired retardation values.

【0051】

On the other hand, the heat treatment in the inventions according to claims 1 and 2 includes a step of shaping a laterally uniaxially stretched polysulfone film into a waveform, a step of gripping the film to hold the waveforms of both ends in the lateral direction, and heat shrinkage. It consists of steps, and conditions such as heating temperature and heating time are appropriately set.

【0052】

[Action]

According to the invention of claim 1, the polysulfon-based film that has been subjected to the laterally uniaxially stretched treatment is shaped so that its vertical cross-sectional shape has a corrugated shape, and the polysulfon-based film is maintained in this corrugated shape. After grasping both ends in the horizontal direction, heat shrinkage treatment is performed to reduce the length of the polysulfon film in the vertical direction to 1 / a'of the length before heat shrinkage.^1/2A retardation film having a good appearance and having substantially the same refractive index in the stretching perpendicular direction and the thickness direction because it is controlled to be twice or more (however, a'is the theoretical stretching magnification of the above-mentioned lateral uniaxial stretching treatment). Can be manufactured.

【0053】

Further, according to the invention of claim 2, the retardation when a sodium D line having a wavelength of 589.8 nm is incident on the manufactured retardation film from a direction parallel to the normal is re-resolved.₀And the lettering when it is incident from the direction of 40 degrees with respect to the normal is Re₄₀When, this manufactured retardation film 0.90 Re₄₀/ Re₀ 1.10 (1) Since Eq. (1) is satisfied, the refractive index in the direction perpendicular to the stretching and the refractive index in the thickness direction are approximated, and the change in the retardation value due to the incident angle is small, so that the viewing angle characteristic can be improved.

【0054】

[Example]

Hereinafter, examples of the present invention will be described in detail with reference to the drawings.

【0055】

[Example 1] A polysulfone film (Tg = 190 ° C) having a width of 430 mm and 100 μm is horizontally stretched at a stretching temperature of 190 ° C, a stretching ratio of 1.5 times, a heat setting temperature of 170 ° C, and a heat setting time of 30 sec. It was uniaxially stretched. The theoretical draw ratio a'was calculated to be 1.386 times.

【0056】

And the theoretical reduction rate = (1-1 / a'^1/2) × 100 (%) Then, the theoretical reduction rate in this case was 15.05%.

【0057】

Next, the obtained laterally uniaxially stretched film p was shaped into the wavy shape shown in FIG. 1 (setting reduction ratio 14%), and while maintaining this corrugated shape, both ends of the film p in the lateral direction were gripped, and then 190. Heat shrinkage treatment was performed at ° C for 2 minutes to reduce the vertical dimensions.

【0058】

The evaluation of the obtained retardation film is based on the R value and (Re).₄₀/ Re₀).

【0059】

The R value is a phase difference value when the measurement wavelength and the phase difference value are equal.

【0060】

Also, (Re₄₀/ Re₀) Is the retardation value Re when the film is rotated 40 degrees with the stretching axis and the axis orthogonal to the stretching axis (in the film plane) as the axis.₄₀Lettering value Re at (590nm) and 0 degrees₀Is measured and the ratio is taken.

【0061】

And as a result of the evaluation, the R value is 580.9nm, (Re₄₀/ Re₀) = 1.09, 0.92, 0.90 Re₄₀/ Re₀ 1.10 (1) It had the characteristics of.

【0062】

[Example 2] A polysulfone film (Tg = 190 ° C) having a width of 430 mm and 100 μm is horizontally stretched at a stretching temperature of 190 ° C, a stretching ratio of 1.5 times, a heat setting temperature of 170 ° C, and a heat setting time of 30 sec. It was uniaxially stretched. The theoretical draw ratio a'was calculated to be 1.386 times.

【0063】

And the theoretical reduction rate = (1-1 / a'^1/2) × 100 (%) Then, the theoretical reduction rate in this case was 15.05%.

【0064】

Next, the obtained laterally uniaxially stretched film was shaped into the wavy shape shown in FIG. 2 (setting reduction ratio 14%), and while maintaining this corrugated shape, both ends of the film in the lateral direction were gripped, and then 190 ° C. , Heat shrinkage treatment was performed for 2 minutes to reduce the vertical dimension.

【0065】

Then, with respect to the retardation film obtained in the same manner as in Example 1, the R value and (Re)₄₀/ Re₀) Was evaluated, and the R value was 580.2 nm, (Re₄₀/ Re₀) = 1.09, 0.92, 0.90 Re₄₀/ Re₀ 1.10 (1) It had the characteristics of.

【0066】

[Comparative Example 1] A polysulfone film (Tg = 190 ° C) having a width of 430 mm and a width of 100 μm was stretched laterally uniaxially with a tenter stretching machine at a stretching temperature of 195 ° C, a stretching ratio of 1.35 times, and a heat setting temperature of 170 ° C.

【0067】

Then, with respect to the obtained retardation film, the R value and (Re) are the same as in Example 1.₄₀/ Re₀) Was evaluated, and the R value was 413.0 nm, (Re₄₀/ Re₀) = 1.26, 0.78, 0.90 Re₄₀/ Re₀ 1.10 (1) It did not have the characteristics of.

【0068】

[Comparative Example 2] A polysulfone film (Tg = 190 ° C) with a width of 430 mm and 100 μm is laterally stretched with a tenter stretcher at a stretching temperature of 190 ° C, a stretching ratio of 1.5 times, a heat setting temperature of 170 ° C, and a heat setting time of 30 sec. It was uniaxially stretched. The theoretical draw ratio a'was calculated to be 1.386 times.

【0069】

And the theoretical reduction rate = (1-1 / a'^1/2) × 100 (%) Then, the theoretical reduction rate in this case was 15.05%.

【0070】

Next, the obtained laterally uniaxially stretched film was shaped into the wavy shape shown in FIG. 3 (setting reduction ratio of 20%), and while maintaining this corrugated shape, both ends of the film in the lateral direction were gripped, and then 195 ° C. , Heat shrinkage treatment was performed for 2 minutes to reduce the vertical dimension.

【0071】

However, wavy wrinkles remained on the film surface and it was impossible to measure.

【0072】

[Effect of the invention]

According to the first aspect of the present invention, it is possible to make the refractive index in the direction orthogonal to the stretching direction and the refractive index in the thickness direction of the polysulfon-based film stretched laterally uniaxially.

【0073】

Further, according to the second aspect of the present invention, it is possible to manufacture a retardation film having a good appearance and having substantially the same refractive index in the stretching perpendicular direction and the thickness direction.

【0074】

Therefore, since the change in the retardation value due to the incident angle in the manufactured retardation film is small, it has the effect of improving the viewing angle characteristic.

[Simple explanation of drawings]

[Figure 1]

Explanatory drawing which shows the waveform of the polysulfone film which concerns on Example.

[Figure 2]

Explanatory drawing which shows the waveform of the polysulfone film which concerns on another Example.

[Fig. 3]

Explanatory drawing which shows the waveform of the polysulfone film which concerns on Comparative Example 2.

[Fig. 4]

The explanatory view for demonstrating the reduction ratio of the polysulfone film which concerns on this invention.

[Fig. 5]

(A) and (B) are explanatory views showing changes in the film due to the stretching treatment.

[Fig. 6]

(A) is an explanatory diagram showing changes in the y-axis and z-axis directions per unit volume when the film is uniaxially stretched a-fold in the x-axis direction, and (B) is a change in the refractive index in each direction due to this stretching. Explanatory drawing which shows.

[Fig. 7]

(A) is an explanatory diagram showing changes in the y-axis and z-axis directions per unit volume when the film is stretched a-fold in the x-axis direction, and (B) is the refractive index in each direction due to this stretching. Explanatory diagram showing the change.

[Explanation of symbols]

p Uniaxial stretch film