JP6377355B2 - Method for producing stretched film - Google Patents

Description

  The present invention relates to a method for producing a stretched film.

  When producing a stretched film, a method of preparing a film as a material and stretching the prepared film is used. As a method of stretching the film, the film is held in a heating furnace while holding both ends of the film with clips. There is known a simultaneous biaxial stretching method in which heating and stretching are simultaneously performed in the length direction and the width direction by clips that are conveyed and gripped at both ends of the film in a heating furnace.

  In such a simultaneous biaxial stretching method, in the heating furnace, the film is stretched by heating to the necessary stretching ratio by pulling the film in the length direction and the width direction. In addition, a large stress is applied to both ends of the film, which is the part gripped by the clip, resulting in tears at both ends of the film and where the thickness of the film is thin. It may break.

  On the other hand, for example, in Patent Document 1, in order to prevent breakage of the film at the time of heat stretching by simultaneous biaxial stretching, both ends held by the clips are made thicker than the center portion of the film before heat stretching. Thus, a technique for reinforcing is disclosed.

JP-A-11-105131

  However, in the technique of Patent Document 1 above, the film for heat stretching is formed by melt extrusion of a thermoplastic resin with a molding die, so that the thickness of a part of the film is reduced during melt extrusion. Therefore, there is a problem that the thinned portion is torn when the heat stretching is performed and the entire film is broken.

  That is, in a thermoplastic resin film melt-extruded from a molding die, it is called neck-in that extends in the length direction and narrows the film width after being melt-extruded and taken up by a cooling roll or the like. The phenomenon occurs. Such neck-in is considered to occur as follows. That is, the thermoplastic resin melt-extruded from the molding die has a thermoplastic resin adjacent to each other at the center in the width direction of the film, so that the flow direction of the thermoplastic resin is limited, Plane extension is performed along a predetermined surface inside the thermoplastic resin, whereby shrinkage in the width direction is suppressed, and shrinkage is mainly performed in the thickness direction. On the other hand, the thermoplastic resin melt-extruded from the forming die has no thermoplastic resin adjacent to the outer side surface at the portion that becomes both ends in the width direction of the film, so that the thermoplastic resin flows freely. Then, it extends uniaxially around a predetermined axis inside the thermoplastic resin, and thereby contracts in the width direction in addition to the thickness direction. Therefore, in the formed film, the boundary part between the width direction center part and the width direction both ends will be dented in the thickness direction by the difference in the shrinkage | contraction form of a thermoplastic resin, and thickness will become thin. And when such a film is heat-stretched, there is a problem that cracks are generated at the thin boundary portion, which makes it easy to break the entire film.

  The present invention has been made in view of such a situation, and in producing a stretched film by heating and stretching the film, it is possible to prevent the film from being broken, and to provide a stretched film excellent in productivity and quality. It aims at providing the manufacturing method of the stretched film which can be obtained.

  By adjusting the thickness of the boundary portion formed between the central portion and both end portions of the film with respect to the average thickness of the central portion of the film, the present inventors can The inventors have found that the object can be achieved and have completed the present invention.

That is, according to the present invention, alone only acrylic resin, after melt extrusion from the molding die, and solidified by cooling by引取Ru a roll, before stretching to form a pre-stretch film made of only an acrylic resin film Forming a stretched film by heating and stretching the pre-stretched film in at least one direction, wherein the stretched film is formed in the pre-stretched film forming step. The central portion of the front film is contracted toward the specific surface by the planar extension that extends along the specific surface located in the central position in the thickness direction of the pre-stretch film or in the vicinity of the central position, and the stretching By uniaxial extension in which both ends of the front film extend around a specific axis passing through the center of the both ends or near the center position, By contracting around a serial particular axis, the minimum thickness of the boundary portion formed between said central portion and the both end portions and t _b, the average thickness of the central portion in the case of the t _c, The film before stretching is formed such that a ratio “t _b / t _c ” between the minimum thickness t _{b of the} boundary portion and the average thickness t _c of the central portion is 0.75 or more. A method for producing a stretched film is provided.

In the production method of the present invention, the maximum thickness of the end portions in the case of a t _e, the ratio "t _{e /} t _c" between the maximum thickness t _e of the end portions and the average thickness t _c of the central portion, It is preferable to form the pre-stretch film in the pre-stretch film forming step so as to be in the range of 1.0 to 2.0.
In the production method of the present invention, wherein when the slit width of the outlet of the molding die was t _s, the ratio "t _s and the average thickness t _c of the slit width t _s and the central portion of the outlet of the molding die It is preferable to form the pre-stretch film in the pre-stretch film forming step so that “/ t _c ” is 8.0 or less.
In the production method of the present invention, it is preferable that the heat stretching of the pre-stretched film in the stretching step is performed by simultaneous biaxial stretching that simultaneously stretches in the length direction and the width direction of the pre-stretched film.
In the manufacturing method of this invention, it is preferable that the draw ratio with respect to the extending | stretching direction of the heat stretching of the said film before extending | stretching in the said extending process shall be 3 times or less.
In the production method of the present invention, it is preferable to perform the heat stretching of the pre-stretched film in the stretching step so that the thickness of the central portion of the stretched film after the heat stretching is in the range of 15 to 50 μm .

  According to the present invention, when producing a stretched film by heat-stretching the film, a stretched film production method capable of appropriately performing heat-stretching and obtaining a stretched film excellent in productivity and quality is provided. Can be provided.

FIG. 1 is a diagram for explaining a method of producing a pre-stretch film. FIG. 2 is a view for explaining neck-in of a melt-extruded thermoplastic resin. FIG. 3 is a view for explaining shrinkage of the melt-extruded thermoplastic resin. FIG. 4 is a diagram illustrating an example of the thickness of the unstretched film with respect to the position in the width direction. FIG. 5 is a diagram for explaining a method of stretching a pre-stretched film by a simultaneous biaxial stretching method in a stretching step. FIG. 6 is a graph showing the results of measuring the thickness of the pre-stretched film and stretched film produced in Examples and Comparative Examples with respect to the width direction position.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The method for producing a stretched film according to the present embodiment includes a pre-stretch film forming step of forming a pre-stretch film by melt-extruding a thermoplastic resin with a molding T-die, and the pre-stretch film in the length direction and A stretching step of heating and stretching in the width direction.

<Film forming step before stretching>
The pre-stretching film forming step is a step of obtaining the pre-stretching film 100 by melt-extruding a thermoplastic resin from a T die. Here, FIG. 1 is a figure for demonstrating the film formation process before extending | stretching.

  In the pre-stretching film forming step, first, a thermoplastic resin is supplied to the T dice 220 through the feed block 210 in a state of being melted by heating.

  In the present embodiment, the feed block 210 is connected to a melt extruder (not shown) for melt-extruding a thermoplastic resin. The melt extruder is not particularly limited, and either a single screw extruder or a twin screw extruder can be used. In this embodiment, the thermoplastic resin is supplied to the feed block 210 by being melt-extruded at a temperature equal to or higher than the melting point (melting) temperature by a melt extruder.

  In the present embodiment, the thermoplastic resin may be selected according to the intended use of the stretched film. For example, acrylic resin (PMMA), cyclic olefin copolymer (COC), polycarbonate (PC), One type of polyester terephthalate (PET) or the like can be used alone, or a mixed resin in which two or more types are mixed can be used.

  In the T die 220, the thermoplastic resin supplied from the feed block 210 is widened in the width direction by the manifold 221 provided in the T die 220, and is thereby extruded from the die slip 222 into a sheet shape.

  Next, as shown in FIG. 1, the extruded sheet-like thermoplastic resin is continuously taken up by the touch roll 230 and the cooling roll 240, and sandwiched to cool and solidify, thereby obtaining the film 100 before stretching.

  And in this embodiment, the produced film 100 before extending | stretching is wound up by the film winding roll (not shown) before extending | stretching, and, thereby, the film 100 before extending | stretching can be obtained continuously. .

  In the pre-stretched film 100 obtained in this way, a phenomenon called neck-in that shrinks in the width direction after being melt-extruded from the die slip 222 of the T-die 220 and taken up by the cooling roll 240 is present. Occur.

  Here, FIG. 2 is a view showing a cross section of the die slip 222 of the T-die 220 and the pre-stretching film 100 formed in the present embodiment. The relationship with the width of the film 100 is shown. In the present embodiment, when the pre-stretch film 100 is formed, the thermoplastic resin is melt-extruded by the width of the die slip 222 by the T-die 220, but after being melt-extruded until taken up by the cooling roll 240. 2, a neck-in that shrinks in the width direction occurs as shown by the arrows in FIG.

  In such a neck-in, the thermoplastic resin melt-extruded from the T-die 220 contracts in the direction of the arrow shown in FIG. 2, that is, the direction at which the center of the unstretched film 100 is indicated by the arrow ( It is generated by contracting in the direction (thickness direction and width direction) indicated by the arrows. Then, the thermoplastic resin melt-extruded from the T-die 220 contracts by neck-in so that the cross-sectional shape becomes as shown in FIG.

  Here, FIG. 3 is a view for explaining the shrinkage of the melt-extruded thermoplastic resin. In the present embodiment, the thermoplastic resin melt-extruded from the T-die 220 is, as shown in FIG. 3, the thermoplastic resin due to the presence of the adjacent thermoplastic resin in the portion that becomes the central portion 110 of the unstretched film 100. Thus, the thermoplastic resin contracts in the thickness direction as indicated by the arrows due to the planar extension extending along the surface α located at or near the center in the thickness direction. On the other hand, the thermoplastic resin melt-extruded from the T-die 220 has a thermoplastic resin adjacent to the outer side surfaces of both end portions 120 at the portions to be the both end portions 120 of the unstretched film 100 as shown in FIG. Since the thermoplastic resin does not exist, the thermoplastic resin flows relatively freely. As a result, the uniaxial extension extending about the axis β passing through the center of the both ends 120 or near the center position causes the width in addition to the thickness direction as shown by the arrows. It also shrinks in the direction. Thereby, between the center part 110 and the both ends 120, the boundary part 130 of the shape dented in the thickness direction is formed by the difference in the shrinkage | contraction form of a thermoplastic resin.

  Therefore, in the pre-stretched film 100 formed by the method shown in FIG. 1, the boundary portion 130 between the central portion 110 and both end portions 120 is particularly thin as shown in FIG. In addition, FIG. 4 is a figure which shows an example of the result of having measured the thickness with respect to the position of the width direction about the film 100 before extending | stretching.

  Here, when the thickness of the boundary portion 130 of the formed pre-stretching film 100 is too thin with respect to the thickness of the central portion 110, the boundary where the thickness is thin when the pre-stretching film 100 is heat stretched in the stretching step. There is a problem that cracks are likely to occur in the portion 130 and heating and stretching cannot be performed appropriately.

On the other hand, in this embodiment, as shown in FIG. 4, the average thickness of the central portion 110 is set to t _c for the unstretched film 100 formed by melt extrusion using a T-die 220 and drawing by a cooling roll 240. When the minimum thickness of the boundary 130 is t _b , the ratio of these thicknesses “t _b / t _c ” is adjusted to 0.75 or more to heat the unstretched film 100 as will be described later. When stretching, the boundary 130 can be effectively prevented from cracking, and the productivity of the stretched film can be improved.

Note that the average thickness t _c of the central portion 110 shown in FIG. 4 is an average value of the thickness of the portion where the thickness of the central portion 110 is stable. For example, the thickness is ± 5 with respect to the center of the central portion 110. It can be set as the average value of the thickness in a region within 10%. As the minimum thickness t _b of the boundary portion 130, of the two minimum thickness of the boundary portion 130 of the pre-stretch film 100, a more thinner thickness.

<Extension process>
The stretching step is a step of heating and stretching the pre-stretching film 100 obtained by the pre-stretching film forming step in the length direction and the width direction. Here, FIG. 5 is a figure for demonstrating an extending process. In the present embodiment, in the stretching step, the unstretched film 100 is sent out from the above-described unstretched film winding roll, and the length direction and the width direction are held while the unstretched film 100 is held by the clip 310 as shown in FIG. The film 100 before stretching is heated and stretched by a simultaneous biaxial stretching method in which the film is stretched simultaneously.

  Specifically, in the stretching step, the unstretched film 100 is continuously fed out from the unstretched film winding roll, the unstretched film 100 is gripped at regular intervals using a plurality of clips, and each of the clips 310 is stretched before stretching. The film 100 is transported into the stretching furnace 320, and in the stretching furnace 320, the pre-stretching film 100 is pulled in the length direction and the width direction by each clip 310 to be stretched. At this time, the unstretched film 100 is transported while being held by the clip 310, so that it passes through the stretching furnace 320, and is stretched in the pre-tropical zone in the stretching furnace 320. The pre-film 100 is preheated to a temperature that is higher by about 10 to 30 ° C. than the glass transition temperature of the thermoplastic resin that constitutes the film, and is then kept in the drawing zone in the drawing furnace 320 by the clip 310. It is pulled in the length direction and the width direction and stretched in the length direction and the width direction. And a stretched film can be obtained by cooling and solidifying in the cooling heat fixed zone following this. And the stretched film can be obtained continuously by opening the clip 310 and winding up with a roll.

  In the present embodiment, a pair of guide rails are provided for moving the clip 310 so as to pass through the drawing furnace 320. The pair of guide rails are respectively installed at the position of the clip 310 that holds the upper side of the pre-stretching film 100 shown in FIG. 5 and the position of the clip 310 that holds the lower side. They are parallel, separated from each other in the width direction of the pre-stretching film 100 in the stretching band, and parallel to each other in the cooling heat fixing band. Alternatively, in the cooling heat fixing band, the distance between the pair of guide rails in the cooling heat fixing band is determined in consideration of the shrinkage when the stretched film heated and stretched in the stretching band is solidified. On the basis of the width of the stretched film on the side, it may be approximated by several percent in the width direction. In the present embodiment, the clip 310 that holds the unstretched film 100 moves along such a guide rail so that the unstretched film 100 can be conveyed and stretched.

  In the present embodiment, the pre-stretching film 100 is stretched in the stretching zone in the stretching furnace 320 using the clip 310 that moves along such a guide rail. That is, in the stretching zone in the stretching furnace 320, the clip 310 that holds the unstretched film 100 is moved so as to spread in the width direction along the guide rail, and at the same time, the interval between the clips 310 is increased. Thus, the unstretched film 100 is pulled in the length direction and the width direction as indicated by arrows in FIG. Thereby, the film 100 before extending | stretching is heat-stretched until it becomes a draw ratio required in the length direction and the width direction. And the film 100 before extending | stretching is heat-stretched, Then, it is cooled and solidified in the cooling heat fixing zone in the stretching furnace 320, and is wound up by a roll installed outside the stretching furnace 320. Thus, a stretched film can be obtained continuously.

  In the present embodiment, it is possible to obtain a stretched film by making the stretching step and the pre-stretching film forming step a consistent continuous line (step).

  In this embodiment, when the pre-stretch film 100 is stretched by heating, the stretch ratio with respect to the stretching direction is preferably within 3 times, more preferably within 2.5 times, and even more preferably within 2 times. . Thereby, the fracture | rupture of the film 100 before extending | stretching during heat-stretching can be prevented more effectively, and the heat-stretching of the film 100 before stretching can be performed appropriately.

  Moreover, in this embodiment, the thickness of the part of the center part 110 of the stretched film obtained by heat-stretching the film 100 before extending | stretching becomes like this. Preferably it is 15-50 micrometers, More preferably, it is 20-40 micrometers. By controlling the thickness of the central portion 110 in the stretched film within the above range, it is possible to more effectively prevent the pre-stretching film 100 from being broken during the heat stretching and appropriately heat-stretch the pre-stretching film 100. .

  Furthermore, in this embodiment, you may cut and remove the part of the both ends 120 about the stretched film obtained by heat-stretching the film 100 before extending | stretching as needed. Thereby, the part of the both ends 120 with especially thick thickness in a stretched film can be removed, and the thickness of the whole stretched film can be equalize | homogenized.

  As described above, in this embodiment, a stretched film is obtained by forming the unstretched film 100 made of a thermoplastic resin in the pre-stretching film forming step and heating and stretching the pre-stretched film 100 in the stretching step. be able to.

Here, in the present embodiment, when the pre-stretch film 100 is formed by the pre-stretch film forming step, the ratio “t _b ” between the average thickness t _c of the central portion 110 and the minimum thickness t _b of the boundary portion 130. The thickness of the unstretched film 100 is adjusted so that “/ t _c ” is 0.75 or more. Thereby, when heat-stretching the film 100 before extending | stretching at an extending | stretching process, generation | occurrence | production of the crack in the boundary part 130 with thin thickness can be prevented effectively, and productivity of a stretched film can be improved.

  In addition, when the film 100 before extending | stretching is heat-drawn, the boundary part 130 among the films 100 before extending | stretching has low extending | stretching stress required for extending | stretching by thinness, and will be preferentially extended | stretched. As the stretching proceeds at the boundary portion 130, the stretching stress at the boundary portion 130 gradually increases, and when the stretching stress necessary for stretching at the central portion 110 is reached, the central portion 110 is also stretched following the boundary portion 130. Become so. At this time, if the thickness of the boundary portion 130 is too thin with respect to the central portion 110, the boundary portion 130 breaks before the central portion 110 starts to be stretched while the boundary portion 130 is being stretched. End up. Further, if the thickness of the boundary portion 130 is too small with respect to the central portion 110, the impact when releasing the unstretched film 100 from the clip 310 and the obtained stretched film after heating and stretching as shown in FIG. Cracks are also generated in the boundary portion 130 due to the stress at the time of winding on the roll.

  Here, conventionally, as a method for preventing breakage of the film during heat stretching by simultaneous biaxial stretching, a method of forming both end portions of the film before heat stretching thicker than the central portion is known. However, when the film to be stretched is produced by melt extrusion using the T-die 220, the boundary formed between the center portion and both end portions of the film even if both end portions of the film are thickened as described above. As shown in FIG. 3, the thickness of the portion is reduced, and there is a problem that a crack occurs at such a boundary portion when the film is heated and stretched.

On the other hand, according to the present embodiment, the average thickness t _c of the central portion 110 and the minimum of the boundary portion 130 of the unstretched film 100 formed by being melt-extruded by the T-die 220 and then pulled by the cooling roll 240. By adjusting the ratio “t _b / t _c ” to the thickness t _b within the above range, when the film 100 before stretching is heated and stretched, the occurrence of cracks at the boundary portion 130 can be effectively prevented, The productivity of the film can be improved.

In the present embodiment, the ratio “t _b / t _c ” between the average thickness t _c of the central portion 110 and the minimum thickness t _b of the boundary portion 130 may be 0.75 or more as described above. , Preferably 0.8 or more, more preferably 0.9 or more.

In the present embodiment, the ratio “t _b / t _c ” between the average thickness t _c of the central portion 110 and the minimum thickness t _b of the boundary portion 130 is adjusted to the above range for the pre-stretch film 100 to be formed. There are no particular restrictions on the method used, for example, a method using a resin having a lower extensional viscosity as the thermoplastic resin, a method of adjusting the slit width of the die slip 222 of the T die 220, and the method of using the T die 220 and the cooling roll 240. A method of reducing the distance, a method of reducing the take-up speed of the pre-stretching film 100 by the cooling roll 240, or the like can be used alone or in combination.

In this embodiment, among these methods, the type of applicable thermoplastic resin is not limited, and the slit width of the die slip 222 is adjusted from the viewpoint of not reducing the production efficiency of the pre-stretching film 100. It is preferable to use the method to do. At this time, when the slit width of the die lip 222 was _{t s,} the ratio _"t s / _{t c"} of the average thickness _{t c} of the slit width _{t s} and the central portion 110 of the die lip 222, preferably It adjusts so that it may be 8.0 or less, More preferably, it is 6.0 or less, More preferably, it is 5.0 or less. Thereby, the thickness of the unstretched film 100 obtained by melt extrusion with the T die 220 can be made more uniform, and the ratio “t _b ” between the average thickness t _c of the central portion 110 and the minimum thickness t _b of the boundary portion 130. / T _c ”can be appropriately adjusted to the above range.

In the present embodiment, the pre-stretched film 100 to be formed, the ratio _"t b / _{t c"} with minimum thickness _{t b} of the average thickness _{t c} and the boundary portion 130 of the central portion 110 as described above the In addition to adjusting to the range, by adjusting the maximum thickness of the both end portions 120 so as to be moderate, it is possible to more effectively prevent the pre-stretching film 100 from being broken during the heat stretching.

Specifically, in the case of forming the pre-stretch film 100, as shown in FIG. 4, the maximum thickness of the end portions 120 when the t _e, the average of the maximum thickness t _e and the central portion 110 of the end portions 120 the ratio of the thickness _{t c "t} e / _{t c",} preferably 1.0 to 3.0, more preferably 1.0 to 2.0, more preferably adjusted to 1.0-1.5. Here, the maximum thickness t _e of the ends 120, of the thickness of the end portions 120 of the pre-stretched film 100 (the one end portion and another end portion in the width direction), and more thicker thickness. Incidentally, with respect to the average thickness _{t c} of the central portion 110, when the maximum thickness _{t e} of the end portions 120 is too thick, the unstretched film 100 obtained melt extrusion by a T die 220, the touch roll 230 and the chill roll When the two ends 120 are too thick when pinched by 240, the pressure concentrates on both ends 120 and the pressure is not uniformly transmitted to the entire unstretched film 100, and the thickness of the unstretched film 100 varies, and this is heated. The thickness of the stretched film obtained by stretching also tends to vary. On the other hand, with respect to the average thickness t _c of the central portion 110, when the maximum thickness t _e of the end portions 120 is too thin, the time of pre-stretched film 100 is melt extruded by T die 220 is neck-in at both ends 120 tends to increase the pulling force of the thermoplastic resin, whereby the thickness of the boundary portion 130 becomes thinner, and the pre-stretching film 100 is likely to break during the heat stretching.

  In the present embodiment, it is preferable to smooth the side surfaces of both end portions 120 before heating and stretching the pre-stretching film 100 formed by the pre-stretching film forming step. By smoothing the side surfaces of both end portions 120 of the pre-stretching film 100, when the pre-stretching film 100 is heated and stretched by pulling the both end portions 120 of the pre-stretching film 100 in the stretching step, Concentration of local stress due to roughness can be prevented, generation of tears at both ends 120 can be prevented, and productivity of the stretched film can be improved.

  The method for smoothing the side surfaces of both end portions 120 of the unstretched film 100 is not particularly limited, but a method of trimming a predetermined width from both side surfaces of both end portions 120 with a cutter, a method for polishing the end portions of both end portions 120, For example, a method of heat-pressing the end portions of the both end portions 120 can be used. The smoothing of the side surfaces of both end portions 120 reduces the unevenness of the side surfaces of both end portions 120, and when the pre-stretch film 100 is pulled in the length direction, stress is not concentrated on a part of both end portions 120. Just go to

  When trimming the both ends 120 of the unstretched film 100 with a cutter, any cutter can be used as long as it can smoothly smooth the side surfaces of both ends 120 by trimming. You can use a rotary shear cutter that continuously rotates while rubbing the blade and lower blade and cut by shearing, or a laser cutter that uses a solid laser, semiconductor laser, liquid laser, gas laser, etc. From the viewpoint that the stress applied to the pre-stretching film 100 can be reduced and the occurrence of cracks in the pre-stretching film 100 during trimming can be prevented, a laser cutter is preferable.

  In addition, when trimming the both end portions 120 of the unstretched film 100, it is preferable to trim the both end portions 120 while heating. Thereby, the side surfaces of both end portions 120 can be made smoother, and breakage of the pre-stretching film 100 when the pre-stretching film 100 is heated and stretched can be more appropriately prevented.

  Moreover, in the example mentioned above, as shown in FIG. 5, as a method of heating and stretching the pre-stretching film 100, a simultaneous biaxial stretching method in which the pre-stretching film 100 is heated and stretched in both the length direction and the width direction is used. Although the example used is shown, in this embodiment, you may use the method of uniaxially stretching the film 100 before extending | stretching only to a length direction.

  In this case, the heat stretching in the length direction of the unstretched film 100 can be performed in the same manner as the simultaneous biaxial stretching method shown in FIG. That is, the film 100 before stretching is conveyed into the heating furnace 320 while being gripped by the clip 310, and then heated and stretched only in the length direction by the clip 310 holding the film 100 before stretching in the heating furnace 320. Can be used.

  In this embodiment, in both cases where simultaneous biaxial stretching is performed in the length direction and width direction, and when uniaxial stretching is performed only in the length direction, the unstretched film 100 is clipped 310 as shown in FIG. By stretching while gripping with, it is possible to improve the productivity of the stretched film compared to the conventional sequential biaxial stretching method, and the obtained stretched film has excellent quality can do.

  The conventional sequential biaxial stretching method is a method in which the pre-stretching film 100 produced by the method shown in FIG. 1 is first heat stretched in the length direction and then heat stretched in the width direction. In the sequential biaxial stretching method, the film 100 before stretching is heated and stretched in the length direction by being conveyed by a plurality of rolls, and then the width of the film 100 while being held by the clip 310 as shown in FIG. Heat stretch in the direction.

  Here, the stretching in the length direction of the pre-stretching film 100 in the sequential biaxial stretching method is specifically performed as follows. That is, according to the sequential biaxial stretching method, the pre-stretching film 100 is preheated to the glass transition temperature of the thermoplastic resin constituting the pre-stretching film 100 while being transported by a plurality of preheated preheated rolls. The heated unstretched film 100 is continuously transported by a cooling roll while keeping heat by an infrared heater or the like. At this time, by making the conveyance speed by the cooling roll faster than the conveyance speed by the pre-tropical roll, a tension is generated between the pre-tropical roll and the cooling roll. Is stretched in the length direction to a necessary stretching ratio.

  Here, in the sequential biaxial stretching method, when the pre-stretching film 100 is stretched in the length direction, the surface of the pre-stretching film 100 comes into contact with the preheating roll and the cooling roll. Scratches may occur on the surface, and the appearance quality of the obtained stretched film may be deteriorated. Further, in the sequential biaxial stretching method, when the film 100 before stretching is heated and stretched in the length direction, since the both ends 120 of the film 100 before stretching are not fixed with clips or the like, the film 100 before stretching is heated. There exists a possibility that it may shrink | contract in the width direction and productivity of a stretched film may fall.

  On the other hand, according to this embodiment, the film 100 before stretching is stretched in the length direction by using the simultaneous biaxial stretching method described above or the method of uniaxial stretching only in the length direction described above. (That is, by using a method of stretching in the length direction while holding the pre-stretching film 100 with the clip 310 as shown in FIG. 5), it is possible to avoid contact with the roll. The stretched film obtained by heating and stretching the pre-stretched film 100 can improve the appearance quality by reducing the scratches on the surface, and is particularly suitable for use in optical films and the like that have strict requirements on the appearance quality. it can. Furthermore, according to this embodiment, since the film 100 before extending | stretching is hold | gripped with the clip 310 when extending | stretching the film 100 before extending | stretching to a length direction, about the film 100 before extending | stretching, the shrinkage | contraction of the width direction by a heat | fever is carried out. This can be prevented and the productivity of the stretched film can be improved.

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

<Example 1>
An acrylic resin (glass transition temperature Tg ₁ : 123 ° C., elongation at break at room temperature: 5%) was prepared as a thermoplastic resin for forming the pre-stretch film 100.

  Here, for the prepared thermoplastic resin, the glass transition temperature was measured by differential scanning calorimetry (DSC), and the elongation at break was measured by a tensile tester (manufactured by Orientec Co., Ltd., model number: RTC-1210A). The same applies to Example 2 and Comparative Example 1 below.

Next, using the prepared thermoplastic resin, as shown in FIG. 1, a pre-stretch film 100 was produced under the following conditions. Here, the produced unstretched film 100 had an overall width of about 310 mm. Then, when the pre-stretched film 100 manufactured was measured thickness, the average thickness _{t c} of the central portion 110 is 160 .mu.m, the minimum thickness _{t b} of the boundary portion 130 is 128 .mu.m, the maximum thickness _{t e} of the both end portions 120 in 290μm The thickness ratio “t _b / t _c ” was 0.8, “t _e / t _c ” was 1.81, and “t _s / t _c ” was 5.0. The results are shown in FIG. Here, in FIG. 6A and FIGS. 6B and 6C described later, the thickness corresponding to the position in the width direction of the unstretched film 100 is shown. In addition, as shown to FIG. 6 (A), the boundary part 130 of the film 100 before extending | stretching was formed in the position of each about 40 mm from the edge part of the width direction of the film 100 before extending | stretching.
T-die 220 outlet width direction dimension: 380mm
Slit width t _s of die slip 222: 0.8 mm
Distance between T dice 220 and cooling roll 240: 60 mm
Take-up speed of cooling roll 240: 5 mpm

Next, the obtained unstretched film 100 is gripped by a clip 310 and, as shown in FIG. 5, is stretched by heating in the length direction and the width direction under the following conditions by a simultaneous biaxial stretching method, and then by a roll. A stretched film was obtained by winding. In this example, the film 100 before stretching did not break while the film 100 before stretching was heated and stretched. Further, when the thickness of the stretched film obtained was measured, the thickness of the portion corresponding to the boundary portion 130 was comparatively thick at 30 μm or more, and the product effective width (region having a thickness of 40 μm or more at the central portion 110) was 390 mm. A stretched film secured relatively widely could be obtained. The results are shown in FIG.
Stretcher entrance speed: 1 mpm
Outlet speed of stretching machine: 2 mpm
Stretch ratio: 100% in length direction x 100% in width direction (twice in length direction x double in width direction)
Clip 310 gripping position: position 15 mm from the end of the film 100 before stretching Pretropical temperature, distance: 140 ° C., 350 mm
Stretch zone temperature, distance: 140 ° C., 500 mm
Cooling heat fixing temperature, distance: 90 ° C, 700mm

<Example 2>
In making the pre-stretch film 100, except that was enlarged slit width t _s of die lip 222 to 1.2mm, with the pre-stretch film 100 and a stretched film in the same manner as in Example 1, measuring the thickness did. The result of having measured thickness about the film 100 before extending | stretching and a stretched film is shown to FIG. 6 (B).

In Example 2, the produced unstretched film 100 has an average thickness t _c of the central portion 110 of 160 μm and a minimum thickness t _b of the boundary portion 130 of 120 μm, and the ratio of these thicknesses “t _b / t _c ”. but 0.75, _"t s / _{t c"} was 7.5. Moreover, in Example 2, compared with Example 1 mentioned above, the boundary part 130 of the film 100 before extending | stretching before heat drawing becomes thin, and, as a result, the stretched film after heat drawing is shown in FIG. As shown, the product effective width (region having a thickness of 40 μm or more in the central portion 110) was reduced.

  However, also in Example 2, as in Example 1, while the film 100 before stretching was heated and stretched, the film 100 before stretching did not break, and a stretched film excellent in quality was continuously produced. We were able to.

<Comparative Example 1>
While increasing the extrusion amount of the thermoplastic resin by the T dice 220 and increasing the take-up speed of the cooling roll 240 to 15 mpm, the film 100 before stretching and the stretched film were obtained in the same manner as in Example 1, and the thickness was increased. It was measured. The result of having measured thickness about the film 100 before extending | stretching and a stretched film is shown in FIG.6 (C).

In Comparative Example 1, the produced unstretched film 100 has an average thickness t _c of the central portion 110 of 158 μm and a minimum thickness t _b of the boundary portion 110 of 110 μm, and the ratio of these thicknesses “t _b / t _c ”. Was 0.70.

In Comparative Example 1, in the prepared film 100 before stretching, the minimum thickness t _b of the boundary portion 130 was too thin with respect to the average thickness t _c of the central portion 110. Cracks occurred in the boundary portion 130 of the front film 100, the breakage of the pre-stretch film 100 occurred frequently, and the productivity of the stretched film was reduced. Here, in Comparative Example 1, the frequency of pre-stretching film 100 at the time of heat stretching is reduced by changing the temperature of the pre-tropical zone and the stretch zone during heat stretching from 140 ° C. to 150 ° C. Although the obtained stretched film had a very small minimum thickness of about 8 μm in the portion corresponding to the boundary portion 130, the stress at the time of releasing the clip 310 from the stretched film after heat stretching. In addition, due to the stress when winding the obtained stretched film on a roll, a crack was generated in a portion corresponding to the boundary portion 130, and the stretched film was broken.

As described above, the ratio “t _b / t _c ” of the minimum thickness t _b of the boundary portion 130 to the average thickness t _c of the central portion 110 is 0.75 or more for the pre-stretch film 100 before performing the heat stretching. In Examples 1 and 2, when the pre-stretched film 100 was heated and stretched, it was possible to suppress breakage of the pre-stretched film 100, so that it was possible to obtain a stretched film excellent in quality, and production of a stretched film It was possible to improve the performance. In particular, Examples 1 and 2, with respect to the average thickness _{t c} of the central portion 110, since the ratio of the slit width _{t s} of die lip 222 _"t s / _{t c"} was 8.0 or less, in FIG. 6 (A) As shown, the obtained stretched film had a uniform thickness and excellent quality.

On the other hand, as described above, the ratio “t _b / t _c ” of the minimum thickness t _b of the boundary portion 130 to the average thickness t _c of the central portion 110 is 0.75 for the unstretched film 100 before heat stretching. In Comparative Example 1, which was less than the above, during the heat stretching of the pre-stretched film 100, the pre-stretched film 100 was frequently broken, resulting in poor stretched film productivity.

DESCRIPTION OF SYMBOLS 100 ... Film before extending | stretching 110 ... Center part 120 ... Both ends 130 ... Boundary part 210 ... Feed block 220 ... T dice 230 ... Touch roll 240 ... Cooling roll 310 ... Clip 320 ... Drawing furnace

Claims (6)

Alone only acrylic resin, after melt extrusion from the molding die, and solidified by cooling by引取Ru a roll, a pre-stretch film-forming step of forming a pre-stretch film composed only of acrylic resin,
A stretching process for forming a stretched film by heating and stretching the pre-stretched film in at least one direction, and a method for producing a stretched film,
In the pre-stretching film forming step, the specific surface is obtained by planar extension in which a central portion of the pre-stretched film extends along a specific surface located near or in the middle of the thickness direction of the pre-stretched film. And the both ends of the pre-stretched film are contracted about the specific axis by uniaxial extension that is centered on the specific axis passing through the center of the both ends or near the center position. In the case where the minimum thickness of the boundary portion formed between the center portion and the both end portions is t _b and the average thickness of the center portion is t _c ,
The film before stretching is formed such that a ratio “t _b / t _c ” between the minimum thickness t _{b of the} boundary portion and the average thickness t _c of the central portion is 0.75 or more. A method for producing a stretched film. The maximum thickness of the end portions in the case of a _{t e,} the ratio of the maximum thickness _{t e} of the end portions and the average thickness _{t c} of the central portion _"t e / _{t c"} is 1.0 to 2.0 The method for producing a stretched film according to claim 1, wherein the pre-stretch film is formed in the pre-stretch film forming step so as to be in the range. Wherein when the slit width of the outlet of the molding die was t _s, the ratio "t _{s /} t _c" of the average thickness t _c of the slit width t _s and the central portion of the outlet of the molding die, 8 The method for producing a stretched film according to claim 1 or 2, wherein the pre-stretch film is formed in the pre-stretch film forming step so as to be 0.0 or less.   The heat stretching of the film before stretching in the stretching step is performed by simultaneous biaxial stretching that simultaneously stretches in the length direction and width direction of the film before stretching. A method for producing a stretched film.   The method for producing a stretched film according to any one of claims 1 to 4, wherein a stretch ratio with respect to a stretching direction of the heat stretching of the pre-stretched film in the stretching step is 3 times or less.   6. The heat stretching of the pre-stretched film in the stretching step is performed so that the thickness of the central portion of the stretched film after the heat stretching is in the range of 15 to 50 μm. The manufacturing method of the stretched film of description.