JPH0252615B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for producing a biaxially stretched film. More specifically, the present invention relates to a method for producing a biaxially stretched film with improved longitudinal heat shrinkage characteristics at low temperatures. [Prior art] Conventional methods for producing biaxially stretched films that improve heat shrinkage characteristics include the most common tenter heat treatment method (hereinafter referred to as conventional method 1) and the heat treatment method using heated rolls (hereinafter referred to as conventional method 2). )It has been known. However, in the conventional method 1, the clip rail in the tenter is narrowed little by little from the entrance to the exit direction, so that the film is
It is possible to reduce the heat shrinkage rate in the TD direction by providing relaxation (hereinafter referred to as the TD direction), but it is not possible to provide relaxation in the longitudinal direction (hereinafter referred to as the MD direction), so it is not possible to reduce the heat shrinkage rate in the MD direction. I couldn't do it. An improvement to this tenter method is to use a simultaneous MD and TD relaxation type tenter, but this method not only increases the size of the device, but also has problems with productivity and quality, so it has not been put to practical use in high-end applications such as polyester film. do not have. In addition, as an application of conventional method 1, both ends (clip gripping parts) of the film are cut at the rear of the tenter heat treatment zone, and only the central part of the film is relaxed in the MD direction between the tenter and the winder. (For example, Special Publication No. 57-54290)
is proposed. However, with this method, undesirable and unavoidable shrinkage occurs in the TD direction due to the high temperature inside the tenter, and it is extremely difficult to maintain the film cutting device.Furthermore, if the film is thin, wrinkles may occur. If the paper is rolled up in this state, the wrinkles will be fixed, resulting in problems such as quality defects. On the other hand, in conventional method 2, heat treatment using heated rolls can provide relaxation in the MD direction, but when relaxation is applied, the film shrinks more than necessary in the TD direction on the roll, and other properties in the TD direction change. There is a drawback. To prevent this, it is necessary to apply tension in the MD direction, and it has been difficult to reduce the heat shrinkage rate in the MD direction without changing other properties. As described above, in all conventional methods, it has been difficult to improve the heat shrinkage characteristics in the MD direction without changing the characteristics in the TD direction. Furthermore, if the heat shrinkage rate in the MD direction is high, especially in applications where the film is used as a roll, tightening will occur when heat is applied, causing deterioration of the flatness of the film, and in magnetic tape applications, skewing will occur. There were drawbacks such as poor performance. [Object of the Invention] The object of the present invention is to solve the problem of MD, which was difficult with the conventional method.
It is an object of the present invention to provide a method for producing a biaxially stretched film that reduces the heat shrinkage rate in the directions, especially the heat shrinkage rate below 120°C. [Structure of the present invention] The present invention provides a method for producing a biaxially stretched film in which an unstretched film made of a thermoplastic resin is stretched in two axial directions and heat-treated using a tenter. The present invention is characterized by a method for producing a biaxially stretched film, which is provided with a suction roll, and in which the circumferential speed of the suction roll is slower than the transport speed of a film gripping device of a tenter. The unstretched film made of a thermoplastic resin in the present invention is a substantially unoriented film made of a well-known thermoplastic resin such as polyester, polyamide, polyolefin, etc. Among these, an unstretched polyester film is particularly preferred. Stretching in two axial directions refers to stretching by a well-known biaxial stretching method in which the film is stretched in the MD direction and the TD direction, and there may be any number of stretching steps. A tenter is a well-known device that stretches the film in the lateral direction or heat-treats the film while holding both ends of the film with clips or the like. The roll is a well-known roll for taking up the film sent out from the tenter, and preferably one that can be used for cooling. Also, roll 1
Consists of books or more, at least one of which must be a suction roll. The roll may be only a suction roll, but before this suction roll, a cooling process,
For example, it may be desirable to provide a cooling roll as is well known in the art. The suction roll has fine holes on the roll surface and has the function of sucking the gas on the roll surface, and the hole diameter is 1 to 6 mmφ, preferably 1 to 3 mmφ. The diameter of the pores varies depending on the thickness of the film, the temperature of the film at that part, etc., but if it is too large, it is undesirable as it may cause transfer to the pores of the film. In addition, the porosity of fine pores is 1 to 50.
%, preferably 3-20%, more preferably 3-12
% is desirable. The suction roll is. Preferably, the speed is variable. Heat treatment using a tenter means heat treatment in a tenter at a temperature above the stretching temperature and below the melting point of the film while preferably restraining the TD direction (in this case, the TD direction relaxation and heat setting temperature are adjusted in stages). (The method may include a method of increasing the temperature in a stepwise manner, and a method of cooling in one stage or in stages). In the present invention, the film processed in this manner is taken up by at least one suction roll, and the circumferential speed is transferred to the at least one suction roll by a film gripping device (hereinafter abbreviated as a clip) of a tenter. The speed is lower than that and a relaxation process is applied between the tenter and the suction roll. The temperature at the center of the film at the point where it separates from the tenter clip is 40 to 150°C.
The temperature is preferably 70 to 120°C. Also,
When a cooling roll is provided before the suction roll, the temperature of the film supplied to the suction roll is preferably 80°C or lower, preferably 50°C or lower. The thickness of the film at the point where the film separates from the clip of the tenter (thickness of the biaxially stretched film) is not particularly limited, but is 200μ or less, preferably 80μ or less, more preferably 3~
25μ is desirable. Next, the present invention will be explained based on side views. FIG. 1 is a schematic sectional view showing the main parts of an apparatus to which the present invention is applied, from a tenter to a winder. A heat-treated film 2 is continuously sent out from a tenter 1, and is cooled by a cooling roll 3. The cooling roll 3 is generally transferred so that it comes into contact with a roll through which water or other coolant has passed, but in order to reduce heat shrinkage in a specific temperature range, it is necessary to It is also possible to blow air at a constant temperature in a controlled space near the area. A suction roll 4 is provided after the cooling roll 3. The suction roll 4 has the structure and function as described above. Relaxation, preferably tension, can be controlled between the tenter 1 and the suction roll 4, and this can be done by using two motors to change the speed, or by dividing the drive from one motor into two. do,
A method of changing the speed using a transmission is useful. The edges of the film 2 are cut by an edge cutting device 6, and only the central portion is wound up by a winder 7. The selvedge cut is made by suction roll 4 and winding machine 7.
It is best to cut at a suitable place between the two, especially when
Preferably, the cutting is carried out on a suction roll 4. Further, it is preferable to provide an expander roll 5, which is a well-known wrinkle smoothing roll, between the suction roll 4 and the winder 7 in order to remove wrinkles from the film. Alternatively, the tension may be applied by making the speed of the winder slightly faster than the suction roll. [Effects of the Invention] In the present invention, at least one suction roll is provided after the point where the film leaves the clip of the heat treatment tenter, and the circumferential speed of the suction roll is made lower than the speed of the clip. We were able to obtain the following excellent effects. (a) The heat shrinkage rate of the film at low temperatures in the MD direction, particularly at temperatures below 120°C, was able to be significantly reduced. (b) Furthermore, the heat shrinkage rate in the MD direction could be reduced without affecting the heat shrinkage rate in the TD direction and the strength in the MD direction. [Example] An embodiment of the present invention will be described below based on an example. Example 1 A polyethylene terephthalate film that had been biaxially stretched by a known method was heat-treated at 215°C, and then the temperature of the film at the point where it left the tenter clip was determined.
The film was heated to 100°C and wound through the apparatus shown in FIG.
However, the temperature of the film supplied to the suction roll was 40°C. The thickness of the rolled film is 15μ
It was hot. At this time, the respective speed ratios of tenter speed, suction roll speed, and winder speed are shown in Table 1.
I set it up as shown below. The MD heat shrinkage rate of the obtained film was measured by holding it at a temperature of 80° C. for 30 minutes, and the results were as shown in Table 1. This value was about half of the conventional tenter heat treatment comparative example 1, and the MD heat shrinkage characteristics were significantly improved. Comparative Example 1 A biaxially stretched film similar to that of Example 1 was heat-set using a tenter using a conventional method, and then the edges were cut and wound up. The selvage cutting device is installed just in front of the winder, as in FIG. At this time,
The speed ratio at which no harmful wrinkles were produced during edge cutting and winding was as shown in Table 1, and the heat shrinkage rate when held at 80° C. for 30 minutes was 0.28%. It should be noted that when the speed ratio was set to more than the value shown in Table 1, the film became wrinkled and could not be made into a product. Comparative Example 2 In the method of Comparative Example 1, the cooling roll shown in FIG. 1 was used, followed by a press roll made of a rubber roll. When the speed between the tenter and the press roll was changed using this method to the speed ratio shown in Table 1, a problem occurred in which the slack in the film between the tenter and the press roll was pressed by the press roll, resulting in fixed wrinkles. Example 2 In the same manner as in Example 1, only the speed ratio was further lowered. The MD heat yield was further reduced, and a film with greatly improved MD heat shrinkage properties was obtained. Table 1 shows the results.
It was shown to. At this time, since tension was applied between the suction roll and the winder, there were no wrinkles and the edge cut was stable. Table 1 shows the strength (Young's modulus) of the films of Comparative Example 1 and Example 2, and there was no change. 【table】

[Brief explanation of drawings]

FIG. 1 is a schematic sectional view showing a film manufacturing apparatus to which the present invention is applied, from a tenter to a winder. 1: tenter, 2: film, 3: cooling roll, 4: suction roll, 5: expander roll, 6: edge cutting device, 7: winder.

Claims (1)

[Claims] 1. A method for producing a biaxially stretched film in which an unstretched film made of a thermoplastic resin is stretched in two axial directions and heat treated using a tenter, in which at least one suction roll is provided after the tenter,
A method for producing a biaxially stretched film, characterized in that the peripheral speed of the suction roll is slower than the transport speed of a film gripping device of a tenter.