JPH0125695B2 - - 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 manufacturing method that makes the physical properties of a biaxially stretched film made of thermoplastic polyester such as polyethylene terephthalate and polyethylene 2-6 naphthalate uniform in the width direction. Polyester biaxially stretched films are used for a variety of industrial applications, and among them, for applications such as photography, drafting, and magnetic disks, physical properties in both vertical and horizontal directions, especially linear expansion coefficient, humidity expansion coefficient, and thermal contraction coefficient, must be balanced. It is desirable that the It is also desirable that every part of the product film be homogeneous. 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 lateral stretching is restrained by the clip, the restraining force is relatively weak in the center of the film. As a result, the stretching orientation of the central portion is delayed in time and position due to the shrinkage stress. If a straight line is virtually drawn in the width direction on the film surface before lateral stretching, this straight line becomes a concave curve in the film traveling direction during lateral stretching. This phenomenon is called bowing, and due to this bowing, the film has differences in physical properties (heterogeneity: especially linear expansion coefficient, humidity expansion coefficient, thermal expansion coefficient, etc.) between the center and both sides in the width direction. This is the cause of non-uniform shrinkage rate. When the physical properties at the center of the film are balanced vertically and horizontally, a main axis of orientation is formed at the edge of the film that is tilted further vertically with respect to the bowing line, and the coefficient of temperature expansion and coefficient of humidity expansion in the direction of this main axis becomes smaller. , each value in the direction perpendicular to the principal axis becomes larger. Several methods have been proposed to eliminate such differences in physical properties in the width direction. However, none of them have reached a satisfactory stage. For example,
Although Japanese Patent No. 1588 discloses a method of transverse stretching and longitudinal stretching, it is not an essential countermeasure. Unexamined Japanese Patent Publication 1986
Japanese Patent No. 73978 proposes a method of using a nip roll between the transverse stretching step and the heat treatment step, but this method tends to cause surface scratches on the film, creating another problem. Although simultaneous biaxial stretching can eliminate this bowing phenomenon (Japanese Unexamined Patent Publication No. 137076/1983), it cannot be applied to sequential biaxial stretching. The present inventors have discovered that the bowing phenomenon can be reduced to the extent that there is no actual damage by combining the biaxial stretching conditions and the heat treatment conditions, and have thus arrived at the present invention. That is, in the present invention, when subjecting a thermoplastic polyester film to sequential biaxial stretching and heat treatment, (a) a substantially amorphous polyester unstretched film is stretched at a stretching ratio of 3.0 to 4.0 times in its longitudinal direction; (b) The uniaxially stretched film is uniaxially stretched at a stretching ratio of 3.0 to 4.0 times in a temperature range equal to or higher than the glass transition temperature, and Stretching is carried out in the width direction by selecting a stretching ratio that is 0.83 to 1.43 times the stretching ratio in the longitudinal direction, (c) the biaxially stretched film is then cooled to below the glass transition temperature, and (d) the film is A first heat treatment is performed at a temperature T ₁ in the range of 200 to 240°C, and (e) the film is then stretched in the width direction by 1 to 20% at a temperature T ₂ in a second heat treatment zone (provided that T ₁ ≧ ( _f ) Further, _the film is heated in the _third heat treatment zone.
A method for producing a thermoplastic polyester film comprising maintaining the temperature at _T3 in the temperature range of 100-200°C. The present invention will be explained. Substantially amorphous polyester refers to one in which an unstretched sheet having a crystallinity of less than 0.2, preferably 0.05 or less can be obtained by film casting. In the present invention, polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalene dicarboxylate, etc. can be used as the thermoplastic polyester. Of course, copolyesters with slightly low crystallinity are also included,
The above-mentioned polyester includes a composition film capable of suppressing the bowing phenomenon, which is the gist of the present invention, even if 15% by weight or less of an organic or inorganic compound or other polymer is added thereto. As for stretching conditions in the longitudinal direction, the stretching ratio is 3.0~
It is essential that the stretching temperature and speed be within the range of 4.0 times.
It is necessary to select conditions that fall within the range of 0.2 to 0.3. Here, the crystallinity is expressed by equation (1). 0.30＞ρx−ρmin／ρmax−ρmin＞0.20……(1)
Equation (where ρx: Density after longitudinal stretching ρmax: Theoretical density when completely crystallized ρmin: Density when completely amorphous) In longitudinal stretching, the range of formula (1) should be observed, and the If the density is large and deviates from formula (1), it will be easy to break during the transverse stretching process, and if the density after longitudinal stretching is small and deviates from formula (1), the Boig reduction effect will be reduced. In the case of polyethylene terephthalate, the preferred density range after longitudinal stretching is ρmax=
1.457, ρmin=1.335, 1.372>ρx>1.359. If inorganic additives are included, it is natural that the specific gravity should be calculated excluding them. Resins containing endothermic pigments can easily achieve this preferred range. Next, the stretch ratio in the transverse direction (width direction) is in the range of 3.0 to 4.0, and the ratio of longitudinal stretch ratio/horizontal stretch ratio is in the range of 0.7 to 1.2 (the ratio of transverse stretch ratio/longitudinal stretch ratio is 1.43 to 4.0). 0.83). When the stretching ratio is within this range, the film exhibits mechanical and thermal properties that are well-balanced in the vertical and horizontal directions, and
Its absolute value is also high and excellent properties can be expressed. Further, uniformity in the width direction of the film can also be maintained. The film that has been biaxially stretched is immediately cooled to below its glass transition temperature. It is important in the present invention that the biaxially oriented state is frozen. If the film is left in a temperature range above the glass transition temperature, bowing will occur at that stage and non-uniformity will inevitably occur. The heat treatment of the film is performed in at least three heat treatment stages. Regarding the heat treatment section, each condition must be specified for 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 and re-stretching in the width direction are performed in accordance with the order of the present invention. In other words, after the transverse stretching, the resin passes through a section in which the ambient temperature is lowered to below the glass transition temperature of the resin used, preferably for at least 0.2 seconds, and then the process of transversely stretching again in the range of 1% to 20%. It is important to include a step in which the subsequent temperature does not exceed the maximum temperature up to that point. Even if separately regulated conditions are inserted between these steps, it is within the scope of the present invention as long as the effect of reducing the widthwise physical property difference (homogenization effect) is not impaired. After the process of the present invention, it is possible 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, if necessary. 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 that occurs 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. The film in that part tends to be relaxed, and the amount of bowing tends to become smaller due to the longitudinal shrinkage stress added during toe-out. This toe-out treatment tends to increase the heat shrinkage rate in the lateral direction, but if necessary, it may be necessary to add toe-in (narrowing of the width of the clip rail in the lateral direction; lateral relaxation treatment) in the subsequent heat treatment section. This can be solved by means. Next, the invention will be further explained with reference to examples. Example 1 After melting polyethylene terephthalate and extruding it through a die slit and forming it into a film on a quenching drum, the film between rolls having different circumferential speeds was ignited with a silicon carbide heating element, and the density after stretching was
1.362 and stretched 3.6 times in the longitudinal direction at 90℃
A biaxially stretched film was obtained by stretching 3.5 times in the transverse direction at a temperature of . First, blow cold air onto the film to bring the surface temperature of the film to 60°C immediately after horizontal stretching, and then
Heat treated in an atmosphere of 230℃ (T ₁ ) and then further heated to 215℃
(T ₂ ), re-stretched by 10% in the transverse direction, and then subjected to a third heat treatment in an atmosphere of 100°C (T ₃ ) while shrinking by 3% in the transverse direction, and the film was removed from the clip. was removed to obtain a 75μ biaxially stretched film. Comparative Example 1 In Example 1, the density after longitudinal stretching was 1-357
A 75μ biaxially stretched film was obtained in the same manner as in Example 1, except that the infrared heater strength was lowered so that Comparative Example 2 In Example 1, cooling after lateral stretching was stopped and T ₂
A biaxially stretched film was obtained at a lateral stretching ratio of 3.8 times without performing lateral re-stretching in an atmosphere of . Comparative Example 3 A film longitudinally stretched under the longitudinal stretching conditions of Comparative Example 1 was subjected to transverse stretching heat treatment under the conditions of Comparative Example 2 to obtain a 75μ biaxially stretched film. A comparison of these physical properties is shown in the table below.

[Table] From the results, it is clear that the method of the present invention exhibits a bowing prevention effect and uniformity of physical properties.

Claims (1)

[Claims] 1. When producing a thermoplastic polyester film by biaxial stretching and heat treatment, a substantially amorphous unstretched polyester film is stretched in the longitudinal direction at a stretching ratio of 3.0 to 4.0 times. Uniaxially stretched and the crystallinity of the film after uniaxially stretched
The uniaxially stretched film has a stretching ratio of 3.0 to 4.0 times in the temperature range above the glass transition temperature, and a stretching ratio of 0.83 to 0.3 in the longitudinal direction.
Stretching is performed in the width direction by selecting a stretching ratio of 1.43 times, and then the biaxially stretched film is cooled to below the glass transition temperature, and the film is subjected to a first heat treatment at a temperature T ₁ in the range of 200 to 240°C. Subsequently, in the second heat treatment zone, the film is stretched in the width direction by 1 to 20% at a temperature T ₂ (however, the temperature T ₂ is selected under the condition of T ₁ ≧T ₂ >T ₃ ), and further A method for producing a thermoplastic polyester film, which comprises maintaining the film at a temperature _T3 in the temperature range of 100 to 200°C in a three-stage heat treatment zone.