JP2731813B2 - Manufacturing method of oriented film - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing an oriented film composed of polyvinyl alcohol or a derivative thereof used for a polarizing film or the like. [Prior Art and Problems] Conventionally, a polarizing film used for a liquid crystal display device or the like has a polyvinyl alcohol oriented film as a base material. As a method of manufacturing a polyvinyl alcohol-based polarizing film, a method of making iodine or a dye into an unoriented polyvinyl alcohol film and then stretching the film several times, and a method of making a stretched and oriented polyvinyl alcohol film adsorb iodine or a dye. There is a way. The polyvinyl alcohol film may be oriented by a wet method or a dry method. The wet stretching method will be described with reference to the conceptual drawing of the wet stretching apparatus shown in FIG.
Reference numeral 5 denotes an unoriented polyvinyl alcohol film, which is generally called a raw film. Reference numeral 6 denotes an aqueous solution, and 7 and 8 denote pinch rolls that are driven to rotate at a constant speed. The wet stretching method is a method of stretching by stretching in the longitudinal direction while impregnating a large amount of water into a raw material of polyvinyl alcohol. The tensile stress required for stretching in the longitudinal direction is generated by the peripheral speed ratio of the driven pinch roll of 7.8. However, since the wet stretching method is a stretching stretching between rolls, the distance between stretching (the length in the flow direction from the start to the completion of stretching) is long, and the stretching starting point is not uniform due to the thickness of the raw material. As a result, the film moves in the width direction and in the flow direction during the stretching distance without being stable when viewed in each part of the film. In addition, neck-in is remarkable, and the width of the obtained stretched film is reduced to about 1/2 of the original web. Since the stretching start point is not constant and the neck-in ratio is about 50%, which is an irregular stretching, the wet stretched film has uneven stretching orientation. This polynyl alcohol orientation unevenness is clearly observed with the naked eye under crossed Nicols. This uneven alignment is problematic as color unevenness in the case of an eccentric film on which iodine or a dye is adsorbed. On the other hand, a roll stretching method is known as a dry stretching method. Here, the dry stretching method is a stretching method in which the water content of polyvinyl alcohol is about 10% or less, and the moisture content is greatly different from the wet method of dipping and stretching in an aqueous solution. FIG. 5 shows an inter-roll tensile stretching apparatus. Reference numeral 10 denotes an unoriented polyvinyl alcohol film, and reference numeral 11 denotes a group of driven heating rolls for the purpose of preheating the raw material. Reference numeral 12 denotes a drawing roll group, which is a heating roll to be driven. Numeral 12 causes the polyvinyl alcohol film to be stretched by making a difference in peripheral speed between the roll groups. More specifically, 13
Stretching is performed by increasing the peripheral speed of the 14 rolls compared to the 14 rolls. The stretching ratio is determined by the peripheral speed ratio. Fifteen
Is a cooling or heat treatment roll, which is driven similarly to other rolls. When the polyvinyl alcohol film is oriented by such a dry stretching method between rolls, orientation unevenness also occurs, and when it is used as a polarizing film, color unevenness occurs. This alignment unevenness is caused by the stretching method. In the dry stretching method between rolls, the stretching stress is given at a roll peripheral speed ratio, and in order to facilitate the stretching, the group is heated to a temperature higher than the glass transition point of polyvinyl alcohol by a group of preheating rolls, and then the stretching stress between the stretching rolls is increased. Is given. The stretching stress causes the polyvinyl alcohol film to be stretched uniaxially vertically. When the film-like object is stretched by the tensile force between the rolls, the stretchable portion starts stretching first, and the stretching is sequentially propagated to stretch the entire surface of the film.
However, the point at which stretching is first started is not uniform in the width direction but is uneven in the thickness and thickness of the raw material, the non-uniformity of the raw material temperature, and a large number of stretching start points occur between the drawing rolls due to the influence of neck-in. . The stretching state in the inter-roll tensile stretching method will be described in more detail with reference to FIG. FIG. 6 is an enlarged view of the extending portion of FIG. 13 is a heat roll rotating at a low speed, and 14 is a heat roll rotating at a high speed. The peripheral speeds of the 13 and 14 hot rolls are V
₁₃ and V ₁₄ , V ₁₄ / V ₁₃ is the stretching ratio (stretching ratio). The film preheated to a temperature equal to or higher than the glass transition point due to the difference in peripheral speed is stretched between rolls 13 and 14. Reference numerals 17 and 18 denote pinch rolls for preventing the film from slipping, which are provided as necessary. In FIG. 6, E is the pinch roll contact point, F is the point where the film separates from the low speed roll, G is the point where the film contacts the high speed roll, and H is the pinch roll contact point. The polyvinyl alcohol 10 preheated by the peripheral speed difference between 13 and 14 is stretched between the point F and the point G unless slippage with the low speed roll 13 occurs. The distance between the points F and G is usually referred to as the distance between stretching. The starting point of stretching (the point where the polyvinyl alcohol is thinned) occurs between the points F and G, and there are several starting points in the width direction. FIG. 7 is a view of FIG. 6 viewed from above, and schematically shows a stretched state in the width direction. Polyvinyl alcohol heated above the glass transition point by the preheating roll group and the low-speed roll 13 is thinned by stretching stress due to a difference in peripheral speed between the low-speed roll 13 and the high-speed roll 14, and neck-in occurs due to tensile stress. In FIG. 7, reference numeral 19 denotes a stretching start point, which is not linear in the width direction but has a wavy shape as shown in the drawing, and the wavy pattern is also fluctuating. Further, in the flow direction, the stretching start point 19 is also moving back and forth at every hour and time. In the inter-roll tensile stretching method in which a film heated above the glass transition point is deformed due to a difference in roll peripheral speed, the formation of a wavy stretching start line has many stretching start points in the width direction of the raw material or in the flow direction. Due to that. The tensile force generated by the peripheral speed difference between the rolls tends to extend the web existing between the point F and the point G. On the other hand, the web between points F and G has non-uniformity such as thickness distribution and temperature distribution, and when tensile force is applied, the most stretchable portion of the web between points F and G Is the first stretching point. There are many stretching start points in the raw material between the point F and the point G, and the stretching progresses in the width direction from each stretching start point, and the entire surface is stretched at the stretching completion point 20. In the inter-roll tensile stretching method, a non-uniformity in the width direction is further increased since a deformation force acting in the width direction due to the tensile force acts at the point F. Specifically, it is conceivable that the film is detached from the low-speed roll 18 unevenly or the tensile force in the width direction is uneven. When the peripheral speed ratio is insufficient in the tensile stretching between rolls of polyvinyl alcohol, a defective phenomenon in which unstretched portions are scattered in an island shape in the stretched film is observed. Is shown. There are many stretching start points between the point F and the point G, and the degree of orientation of the inter-roll tensile stretched film from which the stretching is propagated in the width direction and the length direction from the stretching start point is not uniform. When the stretched film is observed under crossed Nicols, it can be seen that unevenness of the vertical stripe pattern is clearly present in the flow direction. That is, alignment unevenness exists. In the polarizing film, iodine or a dichroic dye is adsorbed on the polyvinyl alcohol alignment film, thereby exhibiting a polarizing performance. If the orientation of polyvinyl alcohol is uneven, the degree of adsorption becomes uneven, and the degree of orientation of iodine / dye becomes uneven, which is a problem as color unevenness and performance unevenness of the polarizing film. In addition, the alignment unevenness of the polyvinyl alcohol film itself is a problem in liquid crystal display applications, and a countermeasure is required. The orientation unevenness observed in a polyvinyl alcohol dry stretched roll-to-roll stretched film also exists in a film obtained by uniaxially stretching another thermoplastic resin film such as polypropylene or polyester by a stretch stretch between rolls.
In other words, it can be said that this is a fundamental drawback of the inter-roll tensile stretching method in which a resin film heated above the glass transition point is stretched by applying a tensile force at a peripheral speed ratio between the rolls. In order to remedy this drawback, a method of reducing the outer diameter of the stretching rolls to reduce the distance between stretchings has been devised, and a stretching method by multi-stage tension stretching between rolls by a plurality of stretching roll groups has been proposed. Irrespective of the tensile stretching, there is uneven alignment. [Means for Solving the Problems] As a result of intensive studies on such problems, the present inventors have found that when stretching polyvinyl alcohol or a derivative thereof uniaxially in a longitudinal direction, the stretching start point is stabilized in a straight line in the width direction, thereby causing uneven alignment. It has been found that a stretched film having an extremely small number of particles can be continuously produced, which has led to the present invention. That is, the present invention, when orienting a film composed of polyvinyl alcohol or a derivative thereof in a longitudinally uniaxial manner,
The unstretched film whose moisture content is adjusted to 1 to 8% is brought into contact with a driving hot roll having a surface temperature of 80 ° C. or more while applying a rearward tension so that the stretching start point of the film is brought into contact with the hot roll. This is a method for producing an oriented film, which is formed immediately before a point or a contact point. A feature of the present invention is that the polyvinyl alcohol film is formed straight in the width direction without changing the stretching start point. The present inventors have proposed PVA as a specific method.
The heating at the time of contact with the moisture contained in the base material and the driven hot roll achieved stabilization of the stretching start point and the stretching completion point. The present invention will be described in more detail with reference to FIG. 1, FIG. 2 and FIG. FIG. 1 is a schematic view of the stretching method of the present invention, and FIG. 2 is a side view of FIG. In FIG. 1, reference numeral 1 denotes an unoriented polyvinyl alcohol film, which is called a raw material. Reference numeral 2 denotes a stretched oriented polyvinyl alcohol film, which is referred to as a stretched film. Reference numeral 8 denotes a driving heat roll. Reference numeral 4 denotes a stretching start line formed by this stretching method. In FIG. 2, A, B, C, and D indicate the positional relationship between the driven heat roll 3 and the polyvinyl alcohol film. The line indicated by 4 in FIG. 1 coincides with the point C. The unoriented polyvinyl alcohol comes into contact with the hot roll 3 while being provided with a back tension. At the contact point C, the tensile yield point stress of polyvinyl alcohol sharply decreases. Back tension of unoriented polyvinyl alcohol raw material is C
When the stress at the point is larger than the yield stress, the polyvinyl alcohol is thinned by stretching. Since the stretching start point at this time is point C where the polyvinyl alcohol film is in contact with the hot roll as shown in FIG. 2, the entire stretching start point in the width direction is a horizontal straight line indicated by 4 in FIG. The rapid decrease in the tensile yield point stress at the point C is manifested because the polyvinyl alcohol is heated by the hot roll, but water contained inside the polyvinyl alcohol film has an important function. In other words, the water contained in the polyvinyl alcohol film is heated to a high temperature, and the hydrogen bonds of the polyvinyl alcohol are rapidly cut to significantly reduce the molecular motility, not the decrease in the yield point stress caused by the high temperature of the polyvinyl alcohol. It is thought that it is increasing. In the present invention, it can be said that the rapid change in the tensile yield point stress of polyvinyl alcohol is skillfully utilized by a trace amount of polyvinyl alcohol and heating by a hot roll in order to form the stretching start point straight in the width direction. FIG. 3 shows the relationship between the film temperature of polyvinyl alcohol and the film elongation at points A to D in FIG. The film temperature at the point A shown in t _0. Heat roll surface temperature
It is shown by t _1. Of course, it is necessary to keep t ₀ <t ₁ . The difference between t ₁ and t ₀ requires at least 20 ° C., and the larger the difference, the better. The elongation percentage of the film is indicated by a draw ratio. In FIG. 3, the stretch ratio when the rear tension applied to the raw polyvinyl alcohol film is larger than the tensile yield point stress at point C is indicated by a solid line, and when the rear tension is zero is indicated by a dashed line. At points A and B, the stretching ratio is 1 or a ratio close to 1 because the film temperature is low even when a backward tension is applied. Until the point C is approached, the film temperature does not rise and no stretching occurs. Stretching is started at the moment of contact with the hot roll at the point C, and the stretching is finished almost simultaneously with the start. That is, at the point A, the point B, and the vicinity of the point C before the point C, the temperature of the raw material of the polyvinyl alcohol film is sufficiently lower than the point C, so that the stretching by the back tension does not occur, and the stretching starts significantly at the tensile yield stress. It will only occur at low C points. That is, the stretching start point of polyvinyl alcohol is formed only at point C. (However, strictly speaking, the stretching start point may be formed immediately before the point C or very close to the point C.) The stretching start point of the polyvinyl alcohol film is formed only at the hot roll contact point, and therefore the stretching start point Is fundamentally different from the conventional roll-to-roll tensile stretching method in that the stretching is straight in the width direction, does not fluctuate in the flow direction, and the stretching start point is the stretching end point. Even when the stretched film obtained by the present invention is observed under crossed Nicols, there is almost no unevenness, and there is very little variation in birefringence in the width direction. That is, it is possible to obtain a polyvinyl alcohol longitudinally uniaxially stretched film having extremely little orientation unevenness. The reason that the orientation unevenness is extremely small is that the neck-in between the stretching start point and the stretching end point does not exist because the stretch-in distance does not substantially exist, thereby preventing various strains from being generated in the stretching process, and Since there is no propagation of stretching stress in the oblique direction due to the start point not being scattered in the width direction and length direction (there is no propagation of stretching in the oblique direction, and there is no collision between the stretching portions). It is thought that it prevents irregular molecular orientation, but it has not been completely elucidated. In the present invention, the water content of the unoriented polyvinyl alcohol film is from 1% to 8%, preferably 2%.
It is desirable to be in the range of at least 5%. Moisture content is 1
% Or less, the tensile yield point stress value of polyvinyl alcohol at the hot roll contact point is close to the breaking stress value, making it difficult to produce a continuous stretched film. When the water content is 8% or more, the running polyvinyl alcohol film starts tensile stretching with neck-in at a temperature equal to or lower than the tensile yield point stress value of polyvinyl alcohol at the hot roll contact point to form stretching unevenness. (Tensile stretching occurs at point A in FIG. 2, etc.) On the other hand, the surface temperature of the hot roll in the present invention is 80 ° C. or higher, preferably 100 ° C. or higher. When the surface temperature of the hot roll is 80 ° C. or less, the tensile yield point stress value at the hot roll contact point becomes close to the breaking stress value, and it becomes difficult to produce a continuous stretched film. The higher the surface temperature of the heat roll, the more the stretching of the polyvinyl alcohol film with a lower back tension value is possible, which is preferable in view of the area of the processing area. The surface material of the heat roll may be any material as long as it does not cause slippage with the polyvinyl alcohol film, but a metal or ceramic material is preferable. The surface roughness of the heat roll is preferably as close to mirror finish as possible. The schematic diagrams shown in FIG. 1 and FIG. 2 are the simplest conceptual views of the apparatus for easy understanding of the stretched state of the present invention, but a pinch roll may be provided on the hot roll after stretching.
Providing a guide roll at or just before the hot roll contact point is a preferable method, and it is more preferable that the guide roll has a cooling action for preventing an increase in the raw material temperature.
In the installation of the guide roll, the gap between the guide roll and the heat roll is preferably equal to or larger than the thickness of the raw material. By using the method for producing an oriented film according to the present invention, a film having a draw ratio of 2 or more can be produced very stably. [Effects of the present invention] The longitudinally uniaxially oriented film comprising polyvinyl alcohol and its derivative produced in the present invention has very little orientation unevenness, and the dyed film obtained by adsorbing iodine or a dichroic dye on this oriented film has a color unevenness. And a polarizing film with very little unevenness in performance. [Examples] Hereinafter, the present invention will be specifically described with reference to Examples. Example-1 An unoriented polyvinyl alcohol film having a thickness of 75 μm and having a moisture content adjusted to 3.7% (Kurarevinylon: VF-9P75)
R) was contacted with an outer diameter of 250 mm, a surface length of 700 mm, a surface temperature of 130 ° C., a surface roughness of 0.1 s, and a metal roll driven by hard chromium plating with a surface tension of 130 kg. By keeping the back tension constant, a continuous elongated stretched film could be obtained, and the polyvinyl alcohol was thinned to 15 μm. The stretching start point of the polyvinyl alcohol was at the hot roll contact point, and was straight in the width direction. The orientation unevenness of the obtained stretched film was observed under crossed Nicols, but the orientation unevenness was extremely small. The birefringence Δn of the obtained stretched film was 56 points (560 mm width) in 1 cm steps in the width direction.
It was measured. Table 1 shows Δ measured with a Nippon Kogaku polarizing microscope.
n shows the results. On the other hand, the color unevenness of the dyed film obtained by dyeing a stretched film obtained with a long length with a dichroic dye (C.I.Direct Black 17, trade name: Japanol First Black Deconc, manufactured by Sumitomo Chemical Co., Ltd.) is visually observed. However, color unevenness was extremely small. The water content of the unoriented polyvinyl alcohol film was measured after the initial weight measurement (W ₀ ) in a hot air dryer at 100 ° C. for 1 hour, and after the weight measurement (W ₁ ) after drying (W ₀ −W ₁ ) / W ₁ × 10
It was calculated with 0. Example 2 Using the same polyvinyl alcohol film and a hot roll as in Example 1, stretching was performed in the same manner at a hot roll surface temperature of 100 ° C., followed by dyeing and observation. First result
As shown in the table, alignment unevenness under crossed Nicols and color unevenness after dyeing were extremely small. Comparative Example-1 The same stretching was attempted at a hot roll surface temperature of 75 ° C. using the same polyvinyl alcohol film and hot roll as in Example 1. Stretching was not started even when a back tension of 160 kg was applied, so when the back tension was further increased, the film was cut at the hot roll contact point. That is, stretching was impossible. Comparative Example 2 The same stretching as in Example 2 was attempted except that the moisture content was adjusted to 0.5%. As in the case of Comparative Example-1, stretching was not started, and film cutting occurred at the hot roll contact point. Comparative Example 3 Stretching was performed in the same manner as in Example 2 except that the moisture content was adjusted to 9%. When the back tension is applied, the running of the unoriented polyvinyl alcohol film with neck-in is unstable before contact with the hot roll (a state where running is stopped intermittently, which is generally referred to as "shakuri"). And no stretching at the hot roll contact points. When the back tension was further increased, the unoriented polyvinyl alcohol raw material was cut far in front of the hot roll contact point. Comparative Example-4.5 A stretched film was produced by a wet stretching method using the unoriented polyvinyl alcohol film used in Example-1. A pair of drive pinch rolls was provided before and after the 40 ° C. water bath, and a wet stretched polyvinyl alcohol oriented film of 32 μm and 21 μm was obtained while changing the peripheral speed ratio. Table 2 shows the results. The obtained stretched film had a remarkable neck-in, and when observed under crossed Nicols, a clear vertical streak-like pattern was observed, indicating a remarkable orientation unevenness. The variation of Δn was also large. The color unevenness of the dyed film dyed in the same manner as in Example 1 was also remarkable. Comparative Example-6 The unoriented polyvinyl alcohol film used in Example-1 was slit-finished with 550 mm width on both ears, and then a stretched film was produced by a dry roll-to-roll tensile stretching method. The apparatus was a multi-stage roll stretching machine manufactured by Japan Steel Works. The preheating roll is an ordinary general longitudinal stretching in which four metal rolls having an outer diameter of 230 mmφ and a surface length of 600 mm and two stretching rolls having the same specifications are used. The stretching rolls are each provided with a pinch roll for preventing slippage. Both the pre-heating roll and the stretching roll were stretched at 140 ° C. at a mechanical magnification (stretching roll peripheral speed ratio) of 3.3 times to obtain a stretched film. When this stretched film was observed under crossed Nicols, unstretched portions were scattered in an island shape on the film surface. Comparative Example -7 The same polyvinyl alcohol as Comparative Example 6 was stretched at a roll temperature of 140 ° C and a mechanical magnification of 4.0 times using the same longitudinal stretching apparatus to obtain a stretched film free of unstretched. The results are shown in Table 3. When the obtained stretched film was observed under crossed Nicols, a clear vertical streak-like pattern was observed, and the orientation unevenness was significant. The variation of Δn was also large. Example-
The color unevenness of the dyed film dyed in the same manner as in No. 1 was also remarkable.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1, FIG. 2, and FIG. 3 are diagrams for explaining the concept and effects of the present invention in detail. FIG. 4 is a view for explaining a conventional wet stretching method, FIG. 5, FIG.
FIG. 7 is a view for explaining a conventional dry stretching method between rolls. In the figures, 1.5, 10 are unoriented raw materials, 2, 9, 16 are stretched films, 3 is a hot roll used in the present invention, 4 is a stretching start line formed in the present invention, 7 and 8 Is a driving pinch roll in wet stretching, and 13 and 14 are stretching rolls in dry stretching between rolls.

Claims (1)

(57) [Claims] In producing an oriented film by longitudinally and uniaxially stretching an unoriented film made of polyvinyl alcohol or a derivative thereof, using an unoriented film adjusted to a water content of 1 to 8% as the unoriented film; A method for producing an oriented film comprising polyvinyl alcohol or a derivative thereof, wherein the film is stretched longitudinally and uniaxially by being brought into contact with a single heated roll having a surface temperature of 80 ° C. or more while being applied.