JPS6243856B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a polyester film by sequential biaxial stretching and multistage heat treatment. More specifically, the present invention relates to an improved method for producing a biaxially stretched film made of a thermoplastic polyester such as polyethylene terephthalate or polyethylene 2-6 naphthalate, with uniform physical properties in the width direction. Polyester biaxially stretched films are used for a variety of industrial purposes, and among them, for use in photography, drafting, magnetic disks, etc., physical properties in both the vertical and horizontal directions, especially humidity expansion coefficient,
It is desirable that the coefficient of thermal expansion and the coefficient of thermal contraction are balanced. Further, it is desired that the product film be uniform in all parts. However, it has been extremely difficult to make the physical properties of the product film uniform in the width direction using the usual sequential biaxial stretching method, that is, longitudinal stretching followed by tenter transverse stretching. The reason for this is that both ends of the film are gripped inside the tenter,
Although the shrinkage stress in the longitudinal direction due to horizontal stretching is restrained by the clip, the restraining force is relatively weak at the center of the film. As a result, the center portion moves due to the shrinkage force, and the horizontal straight line drawn on the film surface before the horizontal stretching becomes concavely curved in the film traveling direction. This phenomenon is called bowing, and this bowing is caused by differences in the physical properties of the film in the width direction (heterogeneity: especially humidity expansion coefficient,
This is the cause of non-uniformity in thermal expansion coefficient. When the physical properties in the longitudinal and lateral directions at the center of the film are balanced, the main axis of orientation is formed at the edge of the film, which is further inclined in the longitudinal direction with respect to the bowing line.
The humidity expansion coefficient and temperature expansion coefficient in the direction of the main axis become smaller, and the values in the directions perpendicular to the main axis become larger. Several methods have been proposed to eliminate such differences in physical properties in the width direction. However, none of them are satisfactory. For example, Japanese Patent Publication No. 37-1588 discloses a method of transverse stretching and longitudinal stretching, but this is not an essential countermeasure. JP-A No. 50-73978 proposes a method of using nip roll between the transverse stretching process and the heat treatment process, but this tends to cause surface scratches on the film, and the surface scratches limit the film's use. . Furthermore, JP-A No. 51-80372 and JP-A No. 54-137076 are related to bowing reduction technology, but simultaneous biaxial stretching as a solution is not universally applicable, and moreover, it is not applicable to sequential biaxial stretching. Can not. As a result of intensive research, the inventors of the present invention elucidated the process by which bowing occurs, found a means to suppress this bowing, and arrived at the present invention. That is, the present invention provides a method for heat-treating a thermoplastic polyester film that has been uniaxially stretched in the longitudinal direction after stretching it in the transverse direction at a temperature higher than the glass transition temperature of the polyester. Then, (b) the temperature T ₁ of the first heat treatment section is set to 200℃≧T ₁ ≧240℃, and (c) the temperature T ₂ of the second heat treatment section is set such that T ₁ ≧T ₂ >T ₃ (however, , T ₃
(d) toe out 1% to 20% (the end of the tenter crimp rail widens) in the second heat treatment zone while maintaining the temperature _T3 in the third heat treatment zone to 100℃≦T. _3. A method for producing a polyester film characterized by maintaining the temperature at ≦200°C. The polyester used in the present invention has polyethylene terephthalate, polyethylene 2-6 naphthalate, etc. as its main component, but it also contains a few percent or less of an inorganic material, a mixture of less than 30% of other polymers, or a polymer with other components. It also includes copolymers formed by coalescence. Stretching ratio in length and width. Although there are no restrictions on the stretching ratio, a range in which the physical properties in the longitudinal and lateral directions can be balanced (a range in which the longitudinal and lateral stretching ratio is close to 1) can be preferably selected. Regarding the heat treatment section, each condition must be specified for the above three sections. Even if the tenter has four or more heat treatment sections, it does not depart from the scope of the present invention as long as the heat treatment temperature and the re-stretching in the width direction are performed in accordance with the order of the present invention. That is, after lateral stretching, the film passes through a section where the ambient temperature is lowered to below the glass transition temperature of the resin used, preferably for at least 0.2 seconds;
After that, preferably in an atmosphere kept at 200℃ or higher.
Processes that pass for more than 0.2 seconds and then 1%
It is important to include the step of horizontally stretching again in the range of ~20% and the step of keeping the subsequent temperature from exceeding the maximum temperature up to that point, and between these steps, separately regulated Even if the conditions are inserted, they are included in the scope of the present invention as long as they do not impair the effect of reducing the difference in physical properties in the width direction (homogenization effect). After the process of the present invention, it is possible, if necessary, to further toe-in (narrowing the tip of the tenter rail width; loosening treatment) or to loosen the film in the longitudinal direction after it is released from the clip. Cooling to below the glass transition temperature after lateral stretching according to the present invention prevents bowing in the lateral stretching stage. In order to prevent bowing caused by shrinkage stress in the longitudinal direction during lateral stretching, it is effective to lower the temperature of the film after stretching and to keep its mobility and deformability low. Next, when the film is heat-treated while being held in the clip, bowing is likely to occur due to the action of the frozen longitudinal shrinkage stress. As a result, the film in that area is likely to move, and the longitudinal shrinkage stress applied during toe-out causes the bowing to become smaller. This toe-out treatment tends to increase the lateral heat shrinkage rate somewhat, but if necessary, measures such as adding toe-in (re-stretching) in the subsequent heat treatment section can be taken. Next, the invention will be further explained with reference to examples. Example 1 Polyethylene terephthalate was melt-extruded and formed into a film on a quenching drum, and then stretched 3.6 times in the longitudinal direction by a group of rolls with different circumferential speeds.
A biaxially stretched film is made by stretching the film 3.5 times in the transverse direction at a temperature of 3.5°C, and first blows cold air to bring the surface temperature of the film immediately after the horizontal stretching to 48°C, then to 230°C.
After heat treatment in an atmosphere of (T ₁ ) and further stretching in the transverse direction by 10% in an atmosphere of 218℃ (T ₂ ),
After heat treatment in an atmosphere of 100°C (T ₃ ), the film was removed from the clip to obtain a 75μ biaxially stretched film.
The thickness of the biaxially stretched film before heat treatment was 81μ. Comparative Example 1 In Example 1, the second stage heat treatment section (T ₂ = 218
After stretching at a transverse stretching ratio of 3.8 times without re-stretching (toe-out) at 100° C.), a 75 μm biaxially stretched film was obtained under the same temperature conditions as in Example 1. Comparative Example 2 In Example 1, cooling after lateral stretching was not performed,
A biaxially stretched film of 75μ was obtained under the same conditions as in Example 1, except that the film temperature was the measured value of 135°C. Comparative Example 3 A biaxially stretched film of 75 μm was obtained under the same conditions as in Example 1 except that T ₁ =110°C, T ₂ =230°C, and T ₃ =100°C. A comparison of these physical properties is shown in the table below.

[Table] From the results, it became clear that the method of the present invention showed remarkable effects.

Claims (1)

[Claims] 1. When a thermoplastic polyester film that has been uniaxially stretched in the longitudinal direction is stretched in the transverse direction at a temperature higher than the glass transition temperature of the polyester and then heat-treated, the film immediately after the transverse stretching is first cooled to a temperature lower than the glass transition temperature, and then the first film is In the heat treatment section, the film is heat treated at a temperature T ₁ within the temperature range of 200 to 240°C, and in the second heat treatment section, the film is stretched by 1 to 20% in the lateral direction while T ₁ ≧T ₂ > T ₃ . 1. A method for producing a polyester film, which comprises heat-treating the film at a satisfactory temperature T ₂ and then maintaining the film at a temperature T ₃ of 100° C. or more and less than 200° C. in a third heat treatment section.