JP2017074750A - Biaxially stretched polyethylene terephthalate-based film and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for stably producing a biaxially stretched polyethylene terephthalate-based film having excellent dimensional stability at a high temperature (dimensional stability in a film longitudinal direction at 150°C and 200°C) and good quality at a low cost and to provide a biaxially stretched polyethylene terephthalate-based film having excellent dimensional stability at a high temperature (dimensional stability in a film longitudinal direction at 150°C and 200°C) and good quality.SOLUTION: There is provided a method for producing a biaxially stretched polyethylene terephthalate-based film which comprises the following steps A to D. Step A: a step of melting and extruding a polyethylene terephthalate-based resin to form a resin sheet, Step B: a vertical stretching step of stretching the resin sheet obtained in the step A in the longitudinal direction, Step C: a step of a lateral stretching step including a step of preheating the film obtained in the step B at a temperature of 140°C or more and 190°C or less and a step of stretching the film after preheating by 5.5 times or more and 7.0 times or less in the width direction at a temperature of 160°C or more and 190°C or less and Step D: a step of thermally fixing the film obtained in the step C at a temperature of 200°C or more and 240°C or less to relax the film in the film width direction.SELECTED DRAWING: None

Description

  The present invention relates to a biaxially stretched polyethylene terephthalate film and a method for producing the same. Specifically, the present invention relates to a biaxially stretched polyethylene terephthalate film excellent in thermal dimensional stability in the longitudinal direction and a method for producing the same.

  A film made of polyester typified by a polyethylene terephthalate resin is excellent in mechanical properties, heat resistance and the like, and is developed for various uses. For example, it is widely used for applications such as packaging applications, electrical insulation applications, optical applications, and magnetic recording applications. Polyester films used for these applications are often required to be resistant to thermal shrinkage and excellent in dimensional stability.

  Methods for reducing the thermal shrinkage of polyethylene terephthalate film include: mixing another thermoplastic resin with polyethylene terephthalate, adding an inorganic compound, mixing crystalline polyester and amorphous polyester, and curing layer. A method of stacking has been proposed. (Patent Documents 1 to 7)

  By the way, the biaxially stretched polyethylene terephthalate resin film is stretched in the longitudinal direction between rolls having a difference in rotational speed, and then stretched in the width direction in a state where the end of the film is gripped in the tenter. Manufactured by being fixed. In this case, as a method of reducing thermal shrinkage, after stretching treatment, a method of loosening stretching tension in the longitudinal direction and width direction, a method of narrowing the interval between clips for gripping the film, a method of relaxing and annealing, a clip For example, a method of cutting and separating the film end from the above has been proposed. In addition, a method for controlling the degree of orientation of each layer of the laminate by heat treatment after biaxial stretching has been proposed. (Patent Documents 8 to 13)

JP-A-2005-75904 JP 2004-195773 A JP 2005-97560 A JP 2004-35720 A JP 2010-18789 A JP 2007-133839 A JP 2004-130594 A JP 2005-335308 A JP 2007-150084 A JP 2007-276190 A JP 2013-75512 Special table 2012-501233 gazette JP 2012-94699 A

  Currently, the films proposed in the above-mentioned patent documents are widely used for packaging applications, electrical insulation applications, optical applications, magnetic recording applications, and the like. However, in recent years, the demand for retort food that can be easily cooked in a microwave oven is expanding, and the packaging is also required to have better heat resistance and lower price. In retort foods, heat sterilization may be performed in a state where food is packaged in the manufacturing process. However, since heat resistance of bacteria has been increasing in recent years, heat sterilization at a higher temperature than before may be required. is there. In addition, with the spread of tablet terminals and the like, there is a demand for optical films that are lighter and less expensive while maintaining heat resistance. In order to satisfy these market requirements, it has become necessary to have excellent dimensional stability, low cost and light weight.

  On the other hand, in any of the patent documents, the main focus is on reducing the thermal contraction rate at a temperature lower than 150 ° C., and dimensional stability at a high temperature has not been maintained.

  Here, in general, in the biaxially stretched film, the thermal contraction rate in the width direction can be arbitrarily controlled by the tension relaxation rate of the film in the transverse stretching step. Therefore, it is considered that imparting dimensional stability at high temperatures is low in technical difficulty. On the other hand, it has been technically difficult to reduce the thermal shrinkage rate at high temperatures in the longitudinal direction of the film for the reasons described below.

  Patent Documents 1 to 7 disclose that another thermoplastic resin or compound is added to polyethylene terephthalate, melt-kneaded, or laminated in order to provide dimensional stability. Here, in order to obtain dimensional stability at a high temperature of 150 ° C. or higher, a compound having excellent dimensional stability at such a high temperature is selected and added in a necessary and sufficient ratio, melt-kneaded, or laminated. Although necessary, such techniques have the potential to impair the properties of polyethylene terephthalate such as film transparency, thinness, and mechanical strength. It has been technically difficult to maintain dimensional stability at high temperatures while maintaining the properties of polyethylene terephthalate such as transparency, thinness, and mechanical strength.

  Patent Document 8 discloses that the degree of orientation of each layer of the laminate is controlled and dimensional stability is imparted by performing a heat treatment on the heat-set film. Patent Document 9 discloses that a film is once wound after being manufactured and then subjected to a heat treatment while being carried to another process and wound again. However, since heat treatment after stretching in the oven is likely to cause wrinkles due to heat, it is difficult to perform heat treatment uniformly in the width direction, and it is difficult to obtain stable dimensional stability. When trying to obtain dimensional stability at a high temperature of 150 ° C. or higher, heat treatment at a necessary and sufficient high temperature is necessary. However, the higher the temperature, the more likely wrinkles are generated and the uniformity of physical properties is maintained. It was technically difficult. In addition, the heat treatment in a separate process has poor productivity and is expensive.

  Further, Patent Documents 10 and 11 disclose that a relaxation process is performed in the longitudinal direction by narrowing the gripping interval of a member that grips the film after biaxial stretching in order to provide thermal dimensional stability. . Patent Document 12 discloses that simultaneous relaxation is performed in the longitudinal direction and the width direction after biaxial stretching. In order to obtain dimensional stability at a high temperature of 150 ° C. or higher, it is necessary to perform a relaxation treatment at a sufficiently high temperature and at a gripping interval. However, the higher the temperature, the easier the wrinkle is generated in the relaxation process, and the uniformity of physical properties Is technically difficult to maintain.

  In addition, Patent Document 13 discloses a method of inserting a blade at the end of the film after biaxial stretching. In order to obtain dimensional stability at a high temperature of 150 ° C. or higher, it is necessary to perform edge cutting at a necessary and sufficient high temperature. However, the higher the temperature, the lower the film elastic modulus, so that no stretching stress occurs and the cutting is performed. There was a problem that it sometimes breaks easily. Therefore, it has been technically difficult to impart dimensional stability at a high temperature of 150 ° C. or higher.

  In any of the patent documents, it has been technically difficult to impart dimensional stability at high temperatures. In particular, the heat shrinkage rate in the width direction can be arbitrarily controlled by the tension relaxation rate of the film in the transverse stretching process, while the heat shrinkage rate in the longitudinal direction is technically difficult for the reasons described above. there were. However, in order to maintain the balance of dimensional stability at high temperatures, it is important to reduce the thermal shrinkage rate in the longitudinal direction.

  An object of the present invention is to solve the above-mentioned problems, to improve a high-quality dimensional stability (dimensional stability in the film longitudinal direction at 150 ° C. and 200 ° C.) and to provide a good quality biaxially stretched polyethylene terephthalate film. To provide a biaxially stretched polyethylene terephthalate film excellent in quality and a method for stably producing at low cost and excellent in dimensional stability at high temperature (dimensional stability in the film longitudinal direction at 150 ° C. and 200 ° C.). is there.

The representative present invention is as follows.
Item 1.
A method for producing a biaxially stretched polyethylene terephthalate film comprising the following steps A to D.
Step A: A step of melt-extruding a polyethylene terephthalate resin to form a resin sheet,
Step B: A longitudinal stretching step of stretching the resin sheet obtained in Step A in the longitudinal direction,
Step C: A step of preheating the film obtained in Step B at a temperature of 140 ° C. or more and 190 ° C. or less, and a preheated film at a temperature of 160 ° C. or more and 190 ° C. or less and 5.5 times or more in the width direction. A transverse stretching step including a step of stretching by 0 times or less, and a step D: a heat setting step in which the film obtained in the step C is heat-set at a temperature of 200 ° C. to 240 ° C. and relaxed in the film width direction. 2.
In Step B, the film stretched in the longitudinal direction has a value of Nx− (Ny + Nz) / 2 of 0.075 to 0 when the refractive indexes in the longitudinal direction, the width direction, and the thickness direction are Nx, Ny, and Nz, respectively. 0.110, and
Item 2. The method for producing a biaxially stretched polyethylene terephthalate film according to Item 1, wherein the step C is performed after the step of heating the film obtained in the step B at a temperature of 50 ° C to 120 ° C.
Item 3.
A biaxially stretched film made of a polyethylene terephthalate resin, which satisfies the following structural requirements (1) to (4).
(1) The heat shrinkage rate at 150 ° C. for 30 minutes in the film longitudinal direction is 0% to 1%, and the heat shrinkage rate at 200 ° C. for 30 minutes is 2% to 4%.
(2) In the heat shrinkage stress curve, the rising temperature of the heat shrinkage stress in the film longitudinal direction is 150 ° C. or more. (3) The refractive index Nx in the film longitudinal direction is 1.63 or more and 1.64 or less, and the refractive index Ny in the width direction. Is 1.67 or more and 1.70 or less, and the refractive index Nz in the thickness direction is 1.48 or more and 1.49 or less. (4) The crystallite length of the film (−105) plane obtained by wide-angle X-ray diffraction measurement is 71 mm or more. 80 mm or less and the crystallite length of the film (010) plane is 65 mm or more and 75 mm or less.

  According to the production method of the present invention, a biaxially stretched polyethylene terephthalate film having excellent dimensional stability at high temperatures (dimensional stability in the film longitudinal direction at 150 ° C. and 200 ° C.) and good quality can be stably produced at low cost. Can be manufactured. Moreover, the biaxially stretched polyethylene terephthalate film of the present invention is excellent in dimensional stability at high temperature (dimensional stability in the film longitudinal direction at 150 ° C. and 200 ° C.) and quality. Therefore, it is possible to perform post-processing at high temperatures, and is suitable for packaging applications and optical applications that require stable thermal dimensional stability at high temperatures.

  The film of the present invention is made of a polyethylene terephthalate resin. Here, the polyethylene terephthalate resin is a polymer containing ethylene glycol and terephthalic acid as main components. Specifically, it means a resin in which 80 mol% or more of repeating units are composed of ethylene terephthalate. Other dicarboxylic acid components and glycol components may be copolymerized as long as the object of the present invention is not impaired. Examples of other dicarboxylic acid components include isophthalic acid, p-β-oxyethoxybenzoic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-dicarboxybenzophenone, bis- (4-carboxyphenylethane), adipine Examples include acid, sebacic acid, 5-sodium sulfoisophthalic acid, cyclohexane-1,4-dicarboxylic acid, and the like. Examples of the other glycol component include propylene glycol, butanediol, neopentyl glycol, diethylene glycol, bisphenol A and other ethylene oxide adducts, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and the like. In addition, oxycarboxylic acid components such as p-oxybenzoic acid can also be used.

  As a polymerization method for such a polyethylene terephthalate resin (hereinafter simply referred to as PET), a direct polymerization method in which terephthalic acid and ethylene glycol and, if necessary, other dicarboxylic acid components and diol components are directly reacted, and terephthalic acid are used. Any production method such as a transesterification method in which a dimethyl ester (including methyl ester of other dicarboxylic acid if necessary) and ethylene glycol (including other diol components as necessary) are transesterified is used. Can be done.

  The intrinsic viscosity of the polyethylene terephthalate resin is preferably in the range of 0.57 dl / g to 0.7 dl / g, and more preferably in the range of 0.58 dl / g to 0.65 dl / g. When the intrinsic viscosity is lower than 0.57 dl / g, the film is easily torn, and when it is higher than 0.70 dl / g, the increase in filtration pressure is increased and high-precision filtration becomes difficult.

  The resin forming the polyethylene terephthalate resin may contain various additives as required in addition to the polyethylene terephthalate resin. Additives include inorganic lubricants such as titanium dioxide, fine-particle silica, kaolin, and calcium carbonate, and fine-particle materials made of acrylic cross-linked polymers such as acrylic acid, methacrylic acid, acrylic ester, and methacrylic ester And organic lubricants such as cross-linked polymers composed of monomeric monomers. Moreover, a stabilizer, a coloring agent, antioxidant, an antifoamer, an antistatic agent, a ultraviolet absorber etc. may be contained independently as needed, and 2 or more types may be used together.

A method for producing the biaxially stretched polyethylene terephthalate film of the present invention will be described. The manufacturing method of the biaxially stretched polyethylene terephthalate film of the present invention includes the following steps AD.
Step A: A step of melt-extruding a polyethylene terephthalate resin to form a resin sheet,
Step B: A longitudinal stretching step of stretching the resin sheet obtained in Step A in the longitudinal direction,
Step C: A step of preheating the film obtained in Step B at a temperature of 140 ° C. or more and 190 ° C. or less, and a preheated film at a temperature of 160 ° C. or more and 190 ° C. or less and 5.5 times or more in the width direction. A transverse stretching step including a step of stretching by 0 times or less, and a step D: a heat setting step in which the film obtained in the step C is heat-set at a temperature of 200 ° C. to 240 ° C. and relaxed in the film width direction.

  Hereinafter, each step will be described. In addition, although the typical example using the pellet of a polyethylene terephthalate is demonstrated in detail, naturally it is not limited to this.

(Process A: Process of melt-extrusion of polyethylene terephthalate resin to form a resin sheet)
First, it is preferable to dry the film raw material (hot air drying) so that the moisture content is less than 100 ppm. Next, the raw material is supplied to an extruder and melt-extruded into a sheet. Furthermore, the melted sheet is brought into close contact with a rotating metal roll (casting roll) using an electrostatic application method and cooled and solidified to obtain an unstretched PET sheet.

  In addition, it is preferable to perform high-precision filtration in order to remove foreign substances contained in the resin at an arbitrary place where the molten resin is kept at 280 ° C. The filter medium used for high-precision filtration of the molten resin is not particularly limited, but in the case of a stainless steel sintered filter medium, the removal performance of aggregates and high melting point organic substances mainly composed of Si, Ti, Sb, Ge, Cu Excellent and suitable.

  Next, the unstretched sheet obtained by the above method is subjected to sequential biaxial stretching in Step B and Step C, which will be described later, followed by heat treatment.

As disclosed in Patent Documents 8 to 13, methods for improving thermal dimensional stability have been proposed so far by devising the stretching process. However, as described above, in the methods disclosed in Patent Documents 8 to 13, it is difficult to impart stable heating dimensional stability to the film easily and at low cost. In addition, wrinkles and breaks are likely to occur in conventional methods such as heat-treating the film after film formation in an oven or cutting the edges of the film during film formation. It has been technically difficult to suppress the thermal dimensional stability at high temperatures (150 ° C. and 200 ° C.) while suppressing.
In the present invention, the following stretching methods are performed, and the temperature applied to the film in the stretching step and the orientation state of the film are controlled, thereby overcoming such a problem.

(Process B: Longitudinal stretching process of stretching the resin sheet obtained in Process A in the longitudinal direction)
First, an unstretched sheet is stretched in the longitudinal direction. Stretching in the longitudinal direction is preferably carried out in the following range. If the stretching is out of the following range, it may be difficult to obtain good film-forming properties in the subsequent stretching in the width direction.
The stretching temperature is preferably a glass transition temperature to a glass transition temperature + 30 ° C., and the stretching ratio is preferably 2 to 4 times. More preferably, the glass transition temperature to the glass transition temperature + 10 ° C. and the draw ratio of 2.5 to 3.0 times, but the birefringence Nx− (Ny + Nz) / 2 of the film after stretching in the longitudinal direction is 0.075 to The film is preferably stretched under the condition of 0.110. Nx, Ny, and Nz are refractive indexes in the film longitudinal direction, width direction, and thickness direction, respectively.

  The birefringence Nx− (Ny + Nz) / 2 of the film after stretching in the longitudinal direction is preferably in the range of 0.075 to 0.110. More preferably, it is 0.080-0.098, More preferably, it is 0.09-0.095. When Nx− (Ny + Nz) / 2 is lower than 0.075, the strength in the longitudinal direction tends to be inferior due to the low orientation in the longitudinal direction, and the thickness variation in the longitudinal direction tends to increase. When Nx− (Ny + Nz) / 2 is higher than 0.11, the orientation in the longitudinal direction is high, so that orientation crystallization proceeds, and breakage during width direction stretching and uneven thickness in the width direction are likely to occur.

  The method for measuring the birefringence of the film stretched in the longitudinal direction is not particularly limited. Any method such as a method in which a film stretched in the longitudinal direction is sampled by holding it on a roll and measured with an Abbe refractometer, or a method using an on-line birefringence meter can be used.

  Next, it is preferable to heat the film after stretching in the longitudinal direction. The heating temperature is preferably in the range of 50 ° C to 120 ° C. More preferably, it is 70 degreeC-110 degreeC, More preferably, it is 90 degreeC-100 degreeC. By heating at a temperature within the above range, thermal crystallization of the film proceeds sufficiently, and breakage and thickness unevenness during stretching in the width direction are reduced. The heating time is preferably not longer than 1 second, but is not particularly limited as long as the heating time is sufficient for thermal crystallization to proceed.

  As a method for heating the film after stretching in the longitudinal direction, any heating method such as near-infrared heater irradiation, hot air injection, and microwave irradiation can be used.

(Step C: a step of preheating the film obtained in step B at a temperature of 140 ° C. or higher and 190 ° C. or lower, and a preheated film at a temperature of 160 ° C. or higher and 190 ° C. or lower, 5.5 times or more in the width direction 7 A transverse stretching step including a step of stretching by 0 times or less)
Subsequently, a transverse stretching process is performed. In the transverse stretching step, the film is preheated at a temperature of 140 ° C. or higher and 190 ° C. or lower, and the preheated film is stretched at a temperature of 160 ° C. or higher and 190 ° C. or lower in the width direction by 5.5 to 7.0 times. Including the step of.

  In the method for producing a biaxially stretched polyethylene terephthalate film of the present invention, stretching in the width direction is performed within the following range. If it falls outside the following range, it becomes difficult to obtain thermal dimensional stability at high temperatures.

  Conventionally, in order to reduce the thermal contraction rate in the longitudinal direction at a temperature lower than 150 ° C., a method of relaxing the stretching tension after the stretching process, a method of narrowing the interval between clips for gripping the film, a method of relaxing and annealing, etc. Has been proposed. However, in order to reduce the thermal contraction rate in the longitudinal direction at high temperatures (150 ° C. and 200 ° C.), in the present invention, the temperature during stretching in the width direction is sufficiently higher than the crystallization temperature, followed by heat setting. The temperature was very close.

  The effect of the stretching temperature during stretching in the width direction on the thermal shrinkage in the longitudinal direction of the film at high temperatures (150 ° C and 200 ° C) is not clear, but the amorphous and crystalline structure change as follows. I think there is. That is. As for the amorphous part, in the stretching in the width direction at a temperature sufficiently higher than the crystallization temperature, the mobility of the amorphous chain is increased, so that the stress during stretching is lowered. The amorphous portion is oriented in the width direction as the stretching progresses, but since the stress is low, the orientation in the width direction is likely to proceed, and the orientation in the longitudinal direction is unlikely to remain. Therefore, it is considered that the orientation degree of the amorphous chain in the longitudinal direction is lowered in the stretched film.

  On the other hand, the crystal part is considered as follows. That is. In a film stretched in the longitudinal direction, crystals oriented in the longitudinal direction are generally formed. When the film is subjected to stretching in the width direction, in the stretching in the vicinity of the glass transition temperature, the crystallite and lamellar structures are collapsed once in the initial stage of stretching, and a structure oriented in the width direction is formed in the latter stage of stretching. However, in the stretching in the width direction at a temperature sufficiently higher than the crystallization temperature, the crystallite and lamellar structures do not collapse, and the orientation changes by rotating while maintaining the structure. Therefore, it is considered that the crystallite size grows in the longitudinal direction and the width direction in the stretched film.

  Thus, it is considered that the thermal contraction rate in the longitudinal direction is lowered because the amorphous orientation is relaxed in the longitudinal direction and is difficult to shrink and the crystal structure grows and fixes the structure.

  However, the extreme decrease in stretching stress, on the other hand, tends to cause brittle fracture and stretching unevenness in the stretching process. Therefore, in order to obtain good stretchability, after stretching in the longitudinal direction, the film is subjected to heat treatment separately from the preheating in the tenter (before the preheating step in Step C). The heat treatment introduces thermal crystals into the film, and the crystals crosslink between the molecular chains. Therefore, it is considered that good stretchability can be obtained by suppressing breakage and unevenness due to extreme reduction of the stretching stress.

(About the preheating process in process C)
First, the preheating temperature is set to 140 ° C. or higher and 190 ° C. or lower. Preferably, they are 150 degreeC or more and 180 degrees C or less, More preferably, they are 170 degreeC or more and 180 degrees C or less. Moreover, it is preferable that the temperature difference with extending | stretching temperature is the range of the extending | stretching temperature -20 degreeC at the time of width direction extending | stretching-extending | stretching temperature 0 degreeC at the time of width direction extending | stretching. More preferably, the stretching temperature is −10 ° C. during stretching in the width direction to −0 ° C. during stretching in the width direction. If the temperature is less than 140 ° C., preheating is insufficient, and the entire film tends to be whitened or broken. If the temperature exceeds 190 ° C., stretch unevenness or brittle fracture tends to occur due to excessive preheating.
By setting the preheating temperature to 140 ° C. or higher and 190 ° C. or lower, the thermal shrinkage rate in the longitudinal direction at high temperatures (150 ° C. and 200 ° C.) can be reduced. The mechanism is considered as follows. When the preheating temperature is in the above range, the film is sufficiently heated to a temperature higher than the crystallization temperature, and thus the mobility of the amorphous chain is increased. As the heating in the preheating step is sufficient, the stretching in the width direction is performed in a state where the amorphous chain is more relaxed. Therefore, it is considered that the relaxation of amorphous chains and the growth of crystallites in the stretching process in the width direction are promoted, and the thermal contraction rate in the longitudinal direction at high temperatures (150 ° C. and 200 ° C.) is reduced.

(About the process of extending in the width direction in process C)
Subsequently, the stretching temperature is preferably 160 ° C. or higher and 190 ° C. or lower. More preferably, it is 170 degreeC or more and 190 degrees C or less, More preferably, it is 180 degreeC or more and 185 degrees C or less. When the stretching temperature is less than 160 ° C., the entire film is whitened due to insufficient temperature. In addition, amorphous chain relaxation and crystallite growth do not occur sufficiently. On the other hand, when the stretching temperature exceeds 190 ° C., the stretching stress is lowered due to excessive temperature, and stretching unevenness is likely to occur. In addition, since the crystal structure is easily melted by heat, sufficient crystallite growth is not observed. In either case, it becomes difficult to maintain thermal dimensional stability at high temperatures (150 ° C. and 200 ° C.).

Moreover, it is preferable that a draw ratio shall be 5.5 times or more and 7.0 times or less. More preferably, they are 6.0 times or more and 7.0 times or less, More preferably, they are 6.0 times or more and 6.5 times or less. When the draw ratio is less than 5.5 times, a stretch residue is generated and the quality is impaired. 5.5 times or more is preferable because the thickness unevenness in the width direction is reduced, and in addition, the strength in the width direction is imparted. When the draw ratio is 7.0 times or less, it is preferable from the standpoint of achieving tear resistance in the width direction, and in addition, it is preferable because breakage is suppressed.
By setting the draw ratio to 5.5 times or more and 7.0 times or less, the thermal shrinkage rate in the longitudinal direction at high temperatures (150 ° C. and 200 ° C.) can be reduced. The mechanism is considered as follows. When the draw ratio is in the above range, it is considered that in the drawing process in the width direction, the orientation of the amorphous chain in the width direction is likely to be strong, and the orientation in the longitudinal direction is easily relaxed. The crystallites are rotated and oriented in the width direction while maintaining the structure, but it is considered that the size of the oriented crystallites is likely to grow during the stretching process. Therefore, it is considered that the relaxation of amorphous chains and the growth of crystallites in the stretching process in the width direction are promoted, and the thermal contraction rate in the longitudinal direction at high temperatures (150 ° C. and 200 ° C.) is reduced.

(Step D: Heat setting step in which the film obtained in Step C is heat-set at a temperature of 200 ° C. to 240 ° C. and relaxed in the film width direction)
After stretching in the width direction, the film is then heat-set. In the heat setting step, the film is heat fixed at a temperature of 200 ° C. or higher and 240 ° C. or lower and relaxed in the width direction by 2% to 8%.
The temperature for heat setting is more preferably 210 ° C. or higher and 240 ° C. or lower, and further preferably 230 ° C. or higher and 240 ° C. or lower. If it is less than 200 ° C., the thermal crystallization of the film does not proceed sufficiently and the structure is not fixed, so that the effect of the high-temperature stretching treatment cannot be sufficiently obtained. When the temperature exceeds 240 ° C., the structure is melted because it is close to the melting point, and brittle fracture is likely to occur. Moreover, the relaxation rate in the width direction of the film is not particularly limited, and an arbitrary rate can be set, but 2% to 8% is preferable. In the case of this application, although the relaxation process of the film longitudinal direction in a heat setting process may be performed, it is not necessarily required.

  The film of the present invention is produced by the above method, but is not limited to the above specifically disclosed method as long as it is within the scope of the technical idea. In producing the film of the present invention, it is important to perform high-precision control within a limited range of transverse stretching and heat setting based on the above technical idea.

The biaxially stretched polyethylene terephthalate film of the present invention has a refractive index Nx in the film longitudinal direction of 1.63 to 1.64, a refractive index Ny in the width direction of 1.67 to 1.70, and a thickness direction. The refractive index Nz is preferably 1.48 or more and 1.49 or less.
By setting the orientation of the film in the above range, it is possible to obtain a structure in which the orientation degree of the amorphous chain in the longitudinal direction is lowered while the orientation degree of the amorphous chain in the width direction is increased. That is, the orientation of the amorphous chain in the longitudinal direction is relaxed and the structure is less likely to be thermally contracted.

  In addition, the biaxially stretched polyethylene terephthalate film of the present invention has a crystallite length of 71 to 80 mm on the film (−105) plane obtained by wide-angle X-ray diffraction measurement, and a crystallite length of 65 mm on the film (010) plane. It is preferable that it is 75 or less. More preferably, the crystallite length of the (−105) plane is 71 to 77 mm and the crystallite length of the (010) plane is 65 to 73 mm. More preferably, the crystallite length of the (−105) plane is 71 to 73 mm and the crystallite length of the (010) plane is 65 to 68 mm. By setting the crystallite length of the film in the above range, a structure in which crystallites grow in the width direction and the longitudinal direction can be obtained. That is, the crystallites fix the amorphous chain and have a structure that is difficult to heat shrink.

  Here, the (−105) plane is a plane substantially perpendicular to the molecular chain of the PET crystallite, and the crystallite length reflects the crystallite size in the direction parallel to the molecular chain. The (010) plane is a plane substantially parallel to the molecular chain of the PET crystallite, and the crystallite length reflects the crystallite size in the direction perpendicular to the molecular chain. In the film stretched in the width direction, since the PET molecular chain is probably oriented in the width direction, the (−105) plane well reflects the crystallite size in the width direction, and the (010) plane is in the longitudinal direction. It is thought to reflect the crystallite size well.

  In the biaxially stretched polyethylene terephthalate film produced based on the above technical idea, it is preferable that the heat shrinkage rate at 150 ° C. for 30 minutes in the film longitudinal direction is 0% to 1%. More preferably, it is 0% to 0.8%, and further preferably 0% to 0.75%. Moreover, it is preferable that the thermal contraction rate in 200 degreeC 30 minutes is 2 to 4%. More preferably, it is 2-3%, More preferably, it is 2-2.5%.

The thermal shrinkage rate decreases as the stress acting during shrinkage is lower. Therefore, it is desirable that the shrinkage stress at high temperature is low in the heat shrinkage stress curve in the film longitudinal direction. Therefore, in the heat shrinkage stress curve, the rising temperature of the stress in the film longitudinal direction is preferably 150 ° C. or higher, more preferably 155 ° C. or higher, and further preferably 160 ° C. or higher. The higher the stress rise temperature, the better. However, it is difficult to achieve 200 ° C. or higher because of the possibility of melting during the stretching process from the viewpoint of production, and the practical upper limit is 190 ° C. .
If it is out of the above range, the thermal dimensional stability becomes poor, and the thermal dimensional stability at high temperatures required for packaging applications and optical applications is not maintained.

  Next, the effect of this invention is demonstrated using an Example and a comparative example. First, the evaluation method of the characteristic values used in the present invention is shown below.

[Evaluation method]

(1) Intrinsic viscosity (IV)
The polyethylene terephthalate resin was pulverized and dried, and then dissolved in a mixed solvent of parachlorophenol / tetrachloroethane = 75/25 (weight ratio). Using a Ubbelohde viscometer, the flow time of a solution having a concentration of 0.4 (g / dl) at 30 ° C. and the flow time of a solvent alone were measured, and from these time ratios, the Huggins constant was determined using the Huggins equation. The intrinsic viscosity was calculated on the assumption that the viscosity was 0.38.

(2) Refractive index It measured based on JISK7142. The refractive index was measured with NaD line light by an Abbe refractometer. The mounting liquid was methylene iodide, and the refractive index in the longitudinal direction (Nx), the refractive index in the width direction (Ny), and the refractive index in the thickness direction (Nz) were measured. The measurement was performed at the center in the film width direction.

(3) Thermal contraction rate at 150 ° C., thermal contraction rate at 200 ° C. Measured in accordance with JIS C 2318-1997 5.3.4 (dimensional change). The film was cut into a width of 10 mm and a length of 190 mm with respect to the direction to be measured (film longitudinal direction), marked at intervals of 10 mm, and the mark interval (A) was measured. Next, the film was placed in an oven in an atmosphere of 150 ° C., subjected to heat treatment at 150 ° C. ± 3 ° C. for 30 minutes under no load, and the interval (B) between the marks was measured. The thermal shrinkage rate at 150 ° C. was determined from the following formula.
Thermal contraction rate (%) = (A−B) / A × 100
Further, the film was put in an oven in an atmosphere at 200 ° C. in the same manner, and heat-treated at 200 ° C. ± 3 ° C. for 30 minutes under no load to obtain a heat shrinkage rate of 200 ° C. The measurement was performed at the center in the film width direction.

(5) Thermal contraction stress It measured using the TMA / SS6100 type | mold thermomechanical analyzer by Seiko Instruments. The film was cut to a width of 2 mm and a length of 30 mm with respect to the direction to be measured (film longitudinal direction). Subsequently, the film was installed in the apparatus, and the lower load at the time of measurement was set to 1.0763 mN. The assembly L control mode was selected, and the temperature was increased from room temperature to 250 ° C. at a rate of 20 ° C./min. In the obtained heat shrinkage stress curve, the rise temperature of the heat shrinkage curve is defined as the temperature at the intersection of the baseline before the heat shrinkage stress curve rises and the tangent at the point where the slope becomes maximum after the heat shrinkage stress rises. . The measurement was performed at the center in the film width direction.

(6) Crystallite length The crystallite length was measured by the transmission method using an X-ray diffractometer RINT2500 manufactured by Rigaku Corporation. The measurement conditions are as follows. Target: Cu, output: 40 kv 200 mA, optical system: 1 mmφ pinhole corriometer, horizontal 1/2 °, vertical 2 °.
About 5 films were cut out and overlapped with the direction of the longitudinal direction and the width direction aligned to make the total thickness in the range of 90 μm to 120 μm. The superposed film was placed in the apparatus in such a direction that the longitudinal direction was perpendicular to the ground, and X-rays were irradiated perpendicularly to the film surface. Next, the crystal peak intensity of the (−105) plane and the crystal peak intensity of the (010) plane with respect to the crystal lattice spacing were measured by 2θ / θ scan. For each measurement surface, the half width of the peak was calculated in the obtained crystal peak curve, and the X-ray diffraction angle when the peak was highest was calculated. The full width at half maximum and the X-ray diffraction angle were substituted into Scherrer's equation “ACS = kλ / βcos θ” to calculate the apparent crystallite length ACS. Here, k is the correction constant, λ is the X-ray wavelength, β is the square root of the value obtained by subtracting the square of the broadening constant of the apparatus from the square of the half width, and θ is the X-ray diffraction angle.

(7) Thermal wrinkle determination method The following silicone coating solution was applied to one side of the obtained film by a die coating method with a processing tension of 10 kg / m applied, and dried in an oven at 150 ° C.
(Silicone coating solution)
Curable silicone (KS847H, Shin-Etsu Chemical) 100 parts by mass Curing agent (CAT PL-50T, Shin-Etsu Chemical) 2 parts by weight Methyl ethyl ketone / xylene / methyl isobutyl ketone 898 parts by mass The sample was cut from a sheet, extended to a length of 5 m on a flat table, and the light from the fluorescent lamp was reflected on the coated surface, and the presence or absence of thermal wrinkles was confirmed by the following evaluation method.
○: Thermal wrinkles are not seen at all and are good.
(Triangle | delta): Although a heat wrinkle was not seen on the whole surface, a heat wrinkle was seen partially.
X: Thermal wrinkle was confirmed on the entire surface.

(8) Stretchability Film formation was performed continuously for 20 minutes, and the number of breaks in the middle was measured.
○: No breakage △: Breakage occurs but film can be collected ×: Breakage occurs frequently and film collection is difficult

(9) Quality Two polarizing plates were placed in a crossed Nicol pattern on a white light source, and the film obtained in each example was placed between them. A 180 W meta-harassment transmission light was used as the light source. Stretching unevenness was visually observed from the film appearance seen through the crossed Nicols. Further, the appearance of whitening was visually observed from the appearance of the obtained film held under a fluorescent lamp.
○: The contrast when passing through crossed Nicols is good, and uneven stretching and unstretched parts are not visible. In addition, whitening under fluorescent lights is not visible.
(Triangle | delta): Stretching nonuniformity, an unstretched part, and whitening were seen in the range which does not exceed 50% in the total area of the film to observe.
X: Stretching unevenness, unstretched portion, or whitening was observed in a range exceeding 50% of the total area of the film to be observed.

  EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited by these Examples.

Example 1
The polyethylene terephthalate resin was dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours and then supplied to the extruder. The raw material supplied to the extruder is melt-extruded in a sheet form from the T-die at a resin temperature of 280 ° C for the melting section, kneading section, polymer tube, gear pump, and filter of the extruder, and 275 ° C for the subsequent polymer tube. It was. In addition, a stainless steel sintered filter medium (nominal filtration accuracy: 95 μm of 10 μm particles) was used for each of the filters. The temperature of the T die was controlled so that the temperature of the extruded resin was 275 ° C.

  The sheet was stretched 3.0 times at 105 ° C. in the longitudinal direction. Next, the film after stretching in the longitudinal direction was heated at 100 ° C. for 1 second. The heated film is guided to a tenter, preheated at 170 ° C., stretched 6.0 times in the width direction at 180 ° C., and then heat treated at 230 ° C. while performing a 7% relaxation treatment to a thickness of 20 μm. A biaxially stretched polyethylene terephthalate film was obtained. The obtained film was a film of good quality without any stretching unevenness or thermal wrinkle. The birefringence of the film after stretching in the longitudinal direction was measured by an Abbe refractometer after the film was sampled by being held on a roll.

(Example 2)
A biaxially stretched polyethylene terephthalate film having a thickness of 20 μm was obtained in the same manner as in Example 1 except that the preheating temperature in the tenter was 150 ° C. and the stretching temperature in the width direction was 160 ° C. A slight whitening was observed in a part of the obtained film.

(Example 3)
A biaxially stretched polyethylene terephthalate film having a thickness of 20 μm was obtained in the same manner as in Example 1 except that heating after stretching in the longitudinal direction was not performed. Several breaks occurred during the stretching process, but the film was collected.

(Comparative Example 1)
Polyethylene terephthalate resin was extruded in the same manner as in Example 1 to obtain a resin sheet. The sheet was stretched 3.0 times at 105 ° C. in the longitudinal direction. Next, the film stretched in the longitudinal direction is guided to a tenter, preheated at 90 ° C., stretched 4.0 times in the width direction at 100 ° C., and then heat treated at 230 ° C. while performing 7% relaxation treatment. A biaxially stretched polyethylene terephthalate film having a thickness of 20 μm was obtained.

(Comparative Example 2)
Polyethylene terephthalate resin was extruded in the same manner as in Example 1 to obtain a resin sheet. The sheet was stretched 3.0 times at 105 ° C. in the longitudinal direction. Next, the film stretched in the longitudinal direction is guided to a tenter, preheated at 90 ° C., stretched 4.0 times in the width direction at 100 ° C., and then heat treated at 230 ° C. while performing 7% relaxation treatment. A biaxially stretched polyethylene terephthalate film having a thickness of 20 μm was obtained. The obtained polyethylene terephthalate was subjected to a relaxation treatment of 3% in the longitudinal direction in a drying furnace at 190 ° C. In the obtained film, fine unevenness and whitening were observed on the entire surface. Furthermore, since fine wrinkles were observed on the entire surface, there was no need to evaluate by a thermal wrinkle determination method.

(Comparative Example 3)
Polyethylene terephthalate resin was extruded in the same manner as in Example 1 to obtain a resin sheet. The sheet was stretched 3.0 times at 105 ° C. in the longitudinal direction. Next, the film after stretching in the longitudinal direction was heated at 100 ° C. for 1 second. The heated film was guided to a tenter, preheated at 170 ° C., stretched 4.0 times in the width direction at 180 ° C., and then heat treated at 230 ° C. while performing 7% relaxation treatment. Several breaks occurred during the stretching process, making it difficult to collect the film. Further, even when a part of the film was able to be collected, when the film was observed through crossed Nicols, a fine stretch residue was generated over the entire surface. Therefore, subsequent physical property evaluation was impossible.

(Comparative Example 4)
Polyethylene terephthalate resin was extruded in the same manner as in Example 1 to obtain a resin sheet. The sheet was stretched 3.0 times at 105 ° C. in the longitudinal direction. The film stretched in the longitudinal direction was guided to a tenter, preheated at 90 ° C., stretched 6.0 times in the width direction at 100 ° C., and then heat treated at 230 ° C. while performing 7% relaxation treatment. Many breaks occurred, and no film was obtained.

  Table 1 shows the film forming conditions of Examples 1, 2, and 3 and Comparative Examples 1, 2, 3, and 4. Table 2 shows the physical property evaluation results of the films obtained in Examples 1, 2, and 3 and Comparative Examples 1, 2, 3, and 4.

  Since the biaxially stretched polyethylene terephthalate film of the present invention is a film excellent in thermal dimensional stability at high temperatures, it is used as a component of optical equipment such as retort food packaging and tablet terminals that require lightweight and heat resistance. Because it is suitable and can be used in a wide range of application fields, it contributes greatly to the industry.

Claims (3)

A method for producing a biaxially stretched polyethylene terephthalate film comprising the following steps A to D.
Step A: A step of melt-extruding a polyethylene terephthalate resin to form a resin sheet,
Step B: A longitudinal stretching step of stretching the resin sheet obtained in Step A in the longitudinal direction,
Step C: A step of preheating the film obtained in Step B at a temperature of 140 ° C. or more and 190 ° C. or less, and a preheated film at a temperature of 160 ° C. or more and 190 ° C. or less and 5.5 times or more in the width direction. A transverse stretching step including a step of stretching by 0 times or less, and a step D: a heat setting step in which the film obtained in the step C is heat-set at a temperature of 200 ° C. to 240 ° C. and relaxed in the film width direction. In Step B, the film stretched in the longitudinal direction has a value of Nx− (Ny + Nz) / 2 of 0.075 to 0 when the refractive indexes in the longitudinal direction, the width direction, and the thickness direction are Nx, Ny, and Nz, respectively. 0.110, and
The manufacturing method of the biaxially-stretched polyethylene terephthalate film of Claim 1 which performs the process of the process C, after passing through the process of heating the film obtained at the process B at the temperature of 50 to 120 degreeC. A biaxially stretched film made of a polyethylene terephthalate resin, which satisfies the following structural requirements (1) to (4).
(1) The heat shrinkage rate at 150 ° C. for 30 minutes in the film longitudinal direction is 0% to 1%, and the heat shrinkage rate at 200 ° C. for 30 minutes is 2% to 4%.
(2) In the heat shrinkage stress curve, the rising temperature of the heat shrinkage stress in the film longitudinal direction is 150 ° C. or more. (3) The refractive index Nx in the film longitudinal direction is 1.63 or more and 1.64 or less, and the refractive index Ny in the width direction. Is 1.67 or more and 1.70 or less, and the refractive index Nz in the thickness direction is 1.48 or more and 1.49 or less. (4) The crystallite length of the film (−105) plane obtained by wide-angle X-ray diffraction measurement is 71 mm or more. 80 mm or less and the crystallite length of the film (010) plane is 65 mm or more and 75 mm or less.