JP2001322166A - Method for heat-treating biaxially stretched polyester film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for heat-treating a biaxially stretched polyester film wherein a biaxially stretched polyester film having good flatness and improved thermal dimensional stability can be efficiently obtained. SOLUTION: In the method for heat-treating a biaxially stretched polyester film, during the time while the biaxially stretched polyester film, in the suspended form, is moved from a preheating roll towards a cooling roll, a maximum temperature Tmax defined by the following expression (1) is reached, while the film is subjected to a relaxed heat treatment by excessively feeding the film under conditions that the following expression (2) is satisfied, Tg+50<=Tma<=Tg+150 (1), 3/L<=N<=30/L (2), wherein in the expression (1), Tmax is film maximum temperature ( deg.C) and Tg is secondary transition point temperature ( deg.C), in the expression (2), N denotes the number of vertical wrinkles (1/m)/ unit width of the film, and L denotes the free length of the film (m).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a heat treatment method for a biaxially stretched polyester film, and more particularly to a heat treatment method for a biaxially stretched polyester film by relaxation heat treatment.

[0002]

2. Description of the Related Art Biaxially stretched polyester films exhibit well-balanced performance in heat resistance, mechanical properties, chemical resistance and the like, and are used in various applications. In particular, in applications that are heated during the secondary processing step or in the stage of use by the user, such as membrane switches, FPCs, and heat-developable photographic photosensitive materials, low heat shrink films are demanded.

[0003] In response to these demands, during the manufacturing process of a biaxially stretched polyester film, a high heat setting temperature is set to increase the degree of crystallinity, or a relaxation heat treatment in the longitudinal and transverse directions is performed. I have. However, the on-line processing alone cannot sufficiently reduce the heat shrinkage,
After the production of the biaxially stretched polyester film, further relaxation heat treatment has been carried out offline.

There are various methods for relaxation heat treatment. For example, a method has been proposed in which a film continuously run in a suspended state is relaxed under a specific temperature condition to reduce the heat shrinkage to 0.1% or less. (Japanese Patent Laid-Open No. 1-275031). However, this method reduces the heat shrinkage of the film by 0.1%.
In order to achieve the following, it is necessary to set the tension low just before the film is loosened, and it is difficult to measure such tension stably, and there is a problem in reproducibility. As a method of measuring the tension of the film, a method of measuring the reaction force of the film with a dancer roll or a load cell is often used, but since the accurate tension cannot be measured unless the roll is free, scratches caused by slipping with the roll may be caused. Concerns about entering the film. Therefore, a method of measuring a heat shrinkage rate and adjusting a relaxation rate before taking a product without installing a tension detecting device is adopted. However, this method has a problem that productivity cannot be increased.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and to provide a relaxation heat treatment method for efficiently obtaining a biaxially oriented polyester film having good flatness and excellent thermal dimensional stability. It is to be.

[0006]

The object of the present invention is to allow a biaxially stretched polyester film to reach a maximum temperature Tmax represented by the formula (1) while running from a preheating roll toward a cooling roll in a suspended state, In addition, this is achieved by a heat treatment method for a biaxially stretched polyester film, wherein the film is subjected to a relaxation heat treatment by over-feeding under conditions satisfying the formula (2).

[0007]

Tg + 50 ≦ Tmax ≦ Tg + 150 (1) (In the formula (1), Tmax is the maximum film temperature (° C.), T
g represents the second order transition temperature (° C.) of the polyester. 3 / L ≦ N ≦ 30 / L (2) (in the formula (2), N represents the number of vertical wrinkles per unit width of the film (1 / m), and L represents the free length (m) of the film). .)

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be further described below with reference to the drawings. FIG. 1 is an example of a relaxation heat treatment apparatus representing one embodiment of the present invention. In FIG. 1, 1 is a biaxially stretched polyester film, 2 is a vertical wrinkle of the film, 3 is a preheating roll, 4 is a cooling roll, and 5 is a heat treatment oven.

FIG. 2 is a sectional view of the film in a suspended state. In FIG. 2, 21 is a biaxially stretched polyester film, 2
2 is a vertical wrinkle of the film.

Running the film in the suspended state as referred to in the present invention means running the film in the direction of gravity from the preheating roll to the cooling roll as shown in FIG. By running in the direction of gravity, the influence of the film's own weight on the processing tension can be reduced. In the suspended state, since the gravity and the traveling direction are the same, the film can be stably transported even under a low tension.

The maximum temperature (Tm) of the film during the relaxation heat treatment
ax) is usually from 120 ° C to 220 ° C, preferably from 135 ° C to 200 ° C, and from 150 ° C to 180 ° C.
C. or lower is more preferable. Below this temperature range, it is difficult to reduce the heat shrinkage. At higher temperatures, the flatness tends to be disturbed, and the objects of the invention cannot be achieved.

For such temperature control, hot air may be blown into the heat treatment oven, or heating may be performed by radiant heat such as an infrared heater. In either method, it is preferable to improve the temperature distribution in the width direction in order to reduce the width distribution of the heat shrinkage. For this purpose, it is preferable to devise a method such as installing a valve inside the nozzle so that the wind speed of the hot air nozzle can be adjusted in the width direction. In radiant heating, it is preferable to arrange a plurality of heaters in the width direction and control them individually. When using hot air, the wind speed is preferably 10 m / s or less in order to minimize flapping of the film.

When the film is rapidly cooled while having wrinkles, the wrinkles are fixed and the flatness of the film is disturbed. In order to prevent this rapid cooling, it is effective to keep the temperature difference between the cooling roll after the heat treatment oven and the film just before touching the cooling roll within a certain range. If the temperature difference between the surface of the cooling roll and the film immediately before touching the cooling roll is within 20 ° C., wrinkles are not fixed and a good flat film can be obtained. More preferably, this temperature difference is within 10 ° C. In order to make the temperature distribution in the width direction of the cooling roll uniform, it is effective to provide a spiral water passage in the roll.

The conditions for obtaining a film having a low heat shrinkage ratio are that not only the temperature but also the film tension be within an appropriate range. Below this range, the film will meander in the roll transport system and cannot be processed stably, and above this range, a film with a low heat shrinkage cannot be obtained. The tension (stress) per unit sectional area may be usually 0.04 to 0.60 MPa, preferably 0.08 to 0.40 MPa, and more preferably 0.10 to 0.20 MPa.

There is a certain relationship between the longitudinal stress of the film and the number of longitudinal wrinkles of the film. The higher the longitudinal stress, the greater the number of longitudinal wrinkles. Also, the number of vertical wrinkles decreases as the free film length (L) between the preheating roll and the cooling roll increases. As a result of conducting experiments on various films, if the number of vertical wrinkles is controlled within the range shown in Equation 2, it is possible to obtain a film having a low heat shrinkage that can be used for films for membrane switches and photosensitive materials. all right. The number of vertical wrinkles per unit width of the film is more preferably in the range of the following formula (3).

[0016]

5 / L ≦ N ≦ 10 / L (3) (where, N is the number of vertical wrinkles per unit width of the film (1 / m), and L is the free length of the film (m). ).)

When heat-treating the biaxially oriented polyester film, the heat treatment time affects the heat shrinkage as well as the temperature and tension. The heat treatment time is preferably maintained at a film maximum temperature Tmax of -20 ° C to Tmax during the heat treatment for 1 to 60 seconds, more preferably 2 to 30 seconds, and particularly preferably 4 to 10 seconds.

The polyester in the present invention is a linear saturated polyester synthesized from an aromatic dibasic acid or an ester-forming derivative thereof and a diol or an ester-forming derivative thereof. As a specific example of polyester,
Examples thereof include polyethylene terephthalate, polyethylene isophthalate, polybutylene terephthalate, poly (1,4-cyclohexylene dimethylene terephthalate), polyethylene 2,6-naphthalenedicarboxylate, and the like. Blends with resins are also included.

Polyester may contain additional fine particles and / or catalyst residues such as lubricants such as calcium carbonate, kaolin, silica, titanium oxide, alumina, crosslinked polystyrene particles, and silicone resin particles in view of film slipperiness and processability. It is preferable to include precipitated fine particles and the like. Further, other additives such as a pigment, a stabilizer, an ultraviolet absorber and the like can be contained as necessary.

The biaxially stretched and heat-set polyester film used in the present invention can be produced by a conventionally known method. For example, after drying the above polyester, it is melted at a polymer melting point (Tm ° C.) to (Tm + 70) ° C., extruded from a die (for example, T-die, I-die, etc.) to a cooling drum, quenched, and cooled to an intrinsic viscosity of 0.35. ~ 0.9
1. An unstretched film of dl / g is obtained, and the unstretched film is longitudinally heated at a temperature of (Tg-10) to (Tg + 70) ° C. (where Tg is the secondary transition temperature of the polyester).
The film is stretched at a magnification of 5 to 5.0 times, and then stretched in a transverse direction with a tenter at a temperature of Tg to (Tg + 70) ° C at a magnification of 2.5 to 5.0 times, and further (Tg + 70) ° C to Tm ° C. It can be manufactured by heat-setting at a temperature of Further, for polyethylene terephthalate film, 190 to 24
It is preferable to heat set at 0 ° C. Heat fixing time is 1-60
Seconds are preferred.

[0021]

The present invention will be further described with reference to the following examples. In addition, each characteristic was measured by the following method.

(1) Method of measuring heat shrinkage at 120 ° C. The support having been subjected to the low tension heat treatment is placed in a measuring direction of 250 mm × width 5 mm.
Cut to 0 mm. Two holes are formed at intervals of 200 mm, and after measuring the humidity at 25 ° C. and 60% RH for 12 hours or more, measurement is performed using a pin gauge (this length is defined as L ₁ ). After this one
It is pressed against a smooth stainless steel plate having a thickness of 10 mm heated to 20 ° C. for 15 seconds. Thereafter, the humidity is adjusted at 25 ° C. and 60% RH for 12 hours or more, and the length is measured again using a pin gauge (this length is referred to as L ₂ ). The thermal dimensional change rate is determined based on the following equation.

[0023]

Equation 4] 120 ° C. Heat shrinkage (%) = 100 × (L 2 -L 1) / L 1 which is measured at 5 points obtained by equally dividing the width direction of the support. The average value is defined as a heat shrinkage of 120 ° C. in the MD direction (longitudinal direction).
The range of the heat shrinkage rate is set.

(2) Heat shrinkage at 150 ° C. A sample having a size of 350 mm in a measuring direction of 350 mm and a width of 50 mm was cut out, and marked at 300 mm intervals near both ends in the longitudinal direction of the sample. And leave for 30 minutes. Take this out and adjust it at room temperature.
Measures the distance between gauge points (this length is L ₃ (mm))
Then, the heat shrinkage is calculated by the following equation.

[0025]

## EQU5 ## Thermal shrinkage (%) at 150 ° C. = 100 × (300−
L ₃₎ / 300

(3) Heat shrinkage at 110 ° C. A sample having a length of 350 mm and a width of 50 mm was cut out from the sample, and marked points were placed at intervals of 300 mm near both ends in the longitudinal direction of the sample. And leave for 30 minutes. Take this out and adjust it at room temperature.
Measures the distance between gauge points (this length is L ₄ (mm))
Then, the heat shrinkage is calculated by the following equation.

[0027]

[Formula 6] 110 ° C. heat shrinkage (%) = 100 × (300−
L ₄ ) / 300

(4) Film flatness A sample having a width of 1000 mm and a length of 2500 mm was cut out, spread out on a flat plate, and was corrugated. The state of the swell was visually evaluated against a limit sample.

(5) Method of Measuring Film Temperature in Oven A hole for measurement is made at a pitch of about 0.5 m at a position on the wall of the oven which is not affected by an internal heater, and the temperature of the internal film is measured therefrom using a radiation thermometer. Was measured. The temperature in the oven was kept from cooling by covering the hole except during the measurement.

(6) Method of Measuring Film Tension A guide roll with a load cell was installed at a position from the oven to the cooling roll, and the tension was measured. The guide roll was configured to be in contact with the film only when measuring the tension and to retract when not in use.

(7) Secondary transition temperature (Tg) A sample (10 mg) was set in a DSC device (differential scanning calorimeter) manufactured by PerkinElmer, and the sample was heated at a temperature of 300 ° C. for 5 minutes.
After melting for 1 minute, the sample was quenched in liquid nitrogen, and the quenched sample was heated at a rate of 10 ° C./min to measure the secondary transition temperature Tg.

Example 1 A pellet of polyethylene terephthalate (Tg: 70 ° C.) having an intrinsic viscosity of 0.65 dl / g (measured at 35 ° C. in an o-chlorophenol solution) was used as one pellet.
After drying at 80 ° C. for 5 hours, the mixture was supplied to an extruder heated to 270 to 300 ° C. and formed into a sheet by an extrusion die. Further, this film was adhered and solidified on a cooling drum having a surface temperature of 25 ° C. by static electricity to obtain an unstretched film. Next, the unstretched film is heated by a heating roll group at 80 to 100 ° C., stretched longitudinally by 3.4 times in one step in a longitudinal direction, cooled by a roll group at 20 to 50 ° C., and subsequently led to a tenter. While holding both ends of the film with clips,
It was preheated in a hot-air atmosphere heated to 90 ° C., and was stretched transversely 3.6 times in a hot-air atmosphere of 95 ° C.

The film thus biaxially stretched is continuously subjected to a heat treatment at 235 ° C. as it is in a tenter. After the heat treatment, the film is once cooled to 180 ° C., and is heat-fixed in a zone where hot air is not blown off. Is cut off, the take-off speed is reduced by 1.5%, relaxed, gradually cooled from 180 ° C. to 110 ° C., taken out from the tenter, and the film naturally cooled to 90 ° C. at room temperature is taken up by a take-up roll at 75 ° C. The film cooled to room temperature to 50 ° C. was wound. The thermal shrinkage at 150 ° C. of the obtained biaxially stretched film having a thickness of 100 μm was 0.4% in the vertical direction and 0.1% in the horizontal direction. When the heat shrinkage at 110 ° C. was measured, it was 0.2% in the vertical direction and 0.05% in the horizontal direction.

The obtained biaxially stretched film is 1500 mm
It was slit to a width and subjected to relaxation heat treatment by the method of the present invention shown in FIG. By adjusting the length of the oven 5 to 4 m, the transport speed of the film 1 at 15 m / min, and the temperature of the preheating roll, the temperature of the film immediately after leaving the preheating roll 3 is 125 ° C., and the temperature of the infrared heater provided in the oven The maximum temperature of the film in the oven 5 was set to 150 ° C., and the film temperature immediately before the cooling roll 4 was set to 120 ° C. When measuring the film temperature through the temperature measurement hole from outside the oven with a radiation thermometer,
The time for the film to be in the range of 130-150 ° C. in the oven was 3.5 seconds. The relaxation rate of the film was 0.46%, and the number of vertical wrinkles in the film at the outlet of the oven was 5 (N = 5). Thereafter, the film was cooled by a cooling roll 4 having a surface temperature of 110 ° C.

The thermal shrinkage of the obtained heat-treated film at 150 ° C. was 0.06% in the vertical direction, 0.04% in the horizontal direction, and 120%.
° C heat shrinkage is 0.015% in the vertical direction and -0.0 in the horizontal direction.
27% (negative value indicates expansion rate), good
The flatness was also at a good level.

Example 2 A biaxially stretched polyethylene terephthalate film was obtained in the same manner as in Example 1. The obtained biaxially stretched film is slit into a width of 1500 mm,
The relaxation heat treatment was performed by the method of the present invention shown in FIG. By adjusting the length of the oven 5 to 2.5 m, the transport speed of the film 1 at 7 m / min, and the temperature of the preheating roll, the film temperature immediately after leaving the preheating roll 3 is set to 100 ° C., and the infrared heater provided in the oven The maximum temperature of the film in the oven 5 by adjusting the temperature of the
The film temperature Tf1 at 105 ° C. immediately before the cooling roll 4 exiting the oven was 105 ° C. The time during which the film is in the range of 130-150 ° C. in the oven is 6.0
Seconds. The relaxation rate of the film was 0.44%, and the number of vertical wrinkles observed at the exit of the oven at that time was 7 (N = 7). Thereafter, the film is heated to a surface temperature of 90.
The cooling roller 4 was brought into contact with a cooling roller of 4 ° C. and cooled. The 150 ° C heat shrinkage of the obtained heat-treated film was 0.07% in the vertical direction, 0.03% in the horizontal direction, and the heat shrinkage in the vertical direction was 0.07% in the vertical direction.
20% and -0.028% in the horizontal direction were good, and the flatness was also at a good level.

Comparative Example 1 A biaxially stretched polyethylene terephthalate film was obtained in the same manner as in Example 1. Thereafter, a relaxation heat treatment was performed in the same manner as in Example 1 except that the relaxation rate was set to 0.25% and the number of vertical wrinkles of the film at the outlet of the oven was set to 12. 1 of the heat-treated film obtained
The 50 ° C. heat shrinkage was 0.20% in the vertical direction and −0.0 in the horizontal direction.
The heat shrinkage rate at 01% and 120 ° C. was 0.080% in the vertical direction and −0.070% in the horizontal direction, and was incompatible with the product standard. Further, the product after the heat treatment was fixed with vertical wrinkles, and the flatness was extremely poor.

Comparative Example 2 A biaxially stretched polyethylene terephthalate film was obtained in the same manner as in Example 1. Thereafter, a relaxation heat treatment was performed in the same manner as in Example 1 except that the relaxation rate was 0.56% and the number of longitudinal wrinkles of the film at the outlet of the oven was one or two. The film meandered, and stable heat treatment could not be performed. The thermal shrinkage at 150 ° C of the obtained heat-treated film was 0.07% in the vertical direction, 0.02% in the horizontal direction, and 120 ° C.
The heat shrinkage is 0.011% in the vertical direction and 0.027 in the horizontal direction.
However, the variation was large depending on the measurement position, the flatness was disturbed, and the product was unsuitable.

[0039]

According to the method for producing a biaxially oriented polyester film of the present invention, a biaxially oriented polyester film having good flatness and excellent thermal dimensional stability can be efficiently obtained.

[Brief description of the drawings]

FIG. 1 is an example of a relaxation heat treatment apparatus representing one embodiment of the present invention.

FIG. 2 is a cross-sectional view of the film in a suspended state.

[Explanation of symbols]

 1: Biaxially stretched polyester film 2: Film longitudinal wrinkle 3: Preheating roll 4: Cooling roll 5: Heat treatment oven 21: Biaxially stretched polyester film 22: Film longitudinal wrinkle.

Claims (2)

[Claims] 1. While running a biaxially stretched polyester film in a suspended state from a preheating roll to a cooling roll, the polyester film is allowed to reach a maximum temperature Tmax represented by the formula (1),
And a heat treatment method for a biaxially stretched polyester film, wherein the film is subjected to a relaxation heat treatment by oversupplying under conditions satisfying the formula (2). Tg + 50 ≦ Tmax ≦ Tg + 150 (1) (In the equation (1), Tmax is the film maximum temperature (° C.), T
g represents the second order transition temperature (° C.) of the polyester. 3 / L ≦ N ≦ 30 / L (2) (in the formula (2), N represents the number of vertical wrinkles per unit width of the film (1 / m), and L represents the free length (m) of the film). .) 2. The temperature of the running biaxially stretched polyester film is set to Tmax-2 when the film is subjected to relaxation heat treatment.
The method for heat-treating a biaxially stretched polyester film according to claim 1, wherein the temperature is maintained in a range of 0 (° C) to Tmax (° C) for 1 to 60 seconds.