KR0173730B1 - Manufacturing method of biaxially oriented polyester film - Google Patents

Abstract

The present invention relates to a method for producing a biaxially oriented polyester film having excellent thickness uniformity, and specifically, a biaxially oriented polyester comprising melt extruding a polyester resin and stretching an unstretched sheet in a longitudinal direction and a transverse direction. In the method for producing a film, the drawing step is performed in two stages of longitudinal stretching so that the total longitudinal stretching ratio is 3.5 times or more, and the first stage longitudinal stretching is performed so that the crystallization energy of the first stage longitudinal stretching sheets is 20 J / g or more, and the sheet After the second stage longitudinal stretching at a ratio of 2.0 to 3.0 times the glass transition temperature (T _g ) of + 50 ℃ to 140 ℃, and rapidly cooled by a difference of 105 ℃ or more from the second stage longitudinal stretching temperature And transverse stretching at a draw ratio of 3.5 to 5.0 times.

Description

Manufacturing method of biaxially oriented polyester film

1 is a schematic view of a two-stage longitudinal drawing apparatus of a polyester film according to one aspect of the present invention.

The present invention relates to a method for producing a biaxially oriented polyester film having excellent thickness uniformity and surface smoothness.

Polyester film has excellent mechanical strength, heat resistance, electrical insulation, and chemical resistance, and its use has been increased for magnetic recording media, food packaging, and electrical insulators. In particular, the polyester film is excellent in mechanical strength and heat resistance, and its utilization value is very high as a base film in the manufacture of video tape, audio tape, and computer tape. Therefore, the thickness uniformity of the polyester film occupies a very important position in the technical level of the current magnetic recording and reproducing apparatus requiring high magnetic recording density and high smoothness.

In the conventional method for producing a polyester film, when the polyester monomers are polymerized under predetermined reaction conditions such as a constant pressure, temperature, and catalyst to obtain a polyester polymer intermediate product (chip or granule), the moisture content is below a certain level. A method for producing a biaxially stretched polyester film, which is dried up to and then melt-extruded in an extruder and passed through a die to obtain an unstretched sheet, is stretched in the longitudinal direction and stretched in the transverse direction, is disclosed in Japanese Patent Application No. 30-5639. Moreover, the method of improving the intensity | strength and thickness uniformity of a film by uniaxially or biaxially redrawing again after a biaxial stretching process is disclosed by Unexamined-Japanese-Patent No. 54-8672.

On the other hand, it is well known that the longitudinal stretching step of primary stretching the unstretched sheet has a significant effect on the thickness uniformity of the final biaxial culture film. Conventionally, in the longitudinal drawing process, the longitudinal stretching sheet could not be longitudinally stretched at a sufficient high magnification so that the thickness of the longitudinal stretching sheet was uniform. Even if the high longitudinal stretching was performed, the width shrinkage was increased due to excessive orientation crystallization, resulting in poor thickness of both ends of the longitudinal stretching sheet. In addition, there was a limit in producing a polyester film having a good thickness due to breakage and uneven stretching in the lateral stretching process. Therefore, in view of the thickness uniformity of the biaxial culture film as well as the stability of the transverse stretching process, a method of longitudinal stretching at the highest possible magnification while suppressing the orientation determination to the maximum has been desired.

Therefore, in recent years, in order to improve the thickness uniformity of the film compared with the conventional stretching method, the longitudinal multi-stage stretching method has emerged. For example, Japanese Unexamined Patent Publications 48-43772, 50-75, 50-139872, 49-42277, 54-56674, 58-78729, 58-160123, 60-61233 and Japan Special Employment Offices 57-48377, 57-49377, 59 A longitudinal multistage stretching method is described in -36851 and the like. However, these methods cause the inefficiency of the longitudinal stretching apparatus as well as the reduction in thickness uniformity by repeating the cooling and the temperature raising process in the middle of the stepwise stretching. In addition, the longitudinal multistage stretching method described in USP 4,370,291, 4,497,865 and Japanese Patent Laid-Open No. 58-118220 is characterized by the specification of the birefringence of each stage or the limitation of the stretching temperature and the stretching ratio as components of the invention. There is an ambiguous point to realize amorphous high magnification longitudinal stretching and rapid cooling, which are core technologies.

Accordingly, an object of the present invention is to provide a method for producing a biaxially cultivated polyester film which has solved the above disadvantages of the prior art and has improved thickness uniformity and surface smoothness.

In order to achieve the above object, the present invention provides a method for producing a biaxially oriented polyester film comprising melt extruding a polyester resin and stretching an unstretched sheet in a longitudinal direction and a transverse direction, wherein the stretching step is a total length. Two-stage longitudinal stretching is performed so that the mystery is 3.5 to 5.0 times, but the first stage longitudinal stretching is performed so that the crystallization energy of the first stage longitudinal stretching sheet is 20 to 25 J / g, and 2.0 to 3.0 at the temperature of the following formula (1) The second stage longitudinal stretching is carried out at twice the stretching ratio, and after rapid cooling to the cooling temperature of the following formula (2), it is transversely stretched at the stretching ratio of 3.5 to 5.0 times to provide a method for producing a biaxially oriented polyester film. do.

Formula (1) T _g + 50 ° C 2nd stage longitudinal drawing temperature 140 ℃

Equation (2) Cooling temperature 2nd stage longitudinal drawing temperature-105 degreeC

According to the method of the present invention, the thickness uniformity of the longitudinal stretch sheet is improved by high magnification stretching of 3.5 to 5.0 times the total longitudinal stretch ratio, and the crystallization energy of the first longitudinal stretch sheet is 20 to 25 J / g, By performing the second stage longitudinal stretching at a magnification ratio of 2.0 to 3.0 with the two stage longitudinal stretching temperature in the range of T _g + 50 ° C. to 140 ° C., the width shrinkage of the longitudinal stretch sheet is reduced and the orientation determination proceeds less. The sheet surface smoothness, thickness uniformity and mechanical strength are improved by rapidly cooling the second stage longitudinal stretching temperature by 105 ° C. or more immediately after the second stage longitudinal stretching, and by transverse stretching at a draw ratio of 3.5 to 5.0 times. Breaking and uneven stretching in the transverse stretching process are reduced so that the thickness of the polyester film becomes uniform in the width direction and the longitudinal direction ultimately.

Herein, the crystallization energy of the longitudinally stretched sheet is in the range of 20 to 25 J / g, which means that the energy generated during the crystallization process is at a constant level, which implies that the orientation of the longitudinally stretched sheet is at a constant level. In addition, the longitudinal stretch temperature of the second stage means the surface temperature of the second stage stretching roll. In the present invention, the first stage longitudinal stretching such that the crystallization energy of the first stage longitudinal stretching sheet is 20 to 25 J / g is performed by a combination of the first stage stretching temperature and the stretching ratio and is not limited to any specific stretching temperature and the stretching ratio. No. In the present specification, T _g means glass transition temperature, and in the case of polyester, it is about 67 ° C.

In the longitudinal drawing process, the circumferential speed ratio of each drawing stage is defined as the total longitudinal drawing ratio, and if the total longitudinal drawing ratio is less than 3.5 times, the thickness uniformity of the longitudinal drawing sheet is not sufficient, and the total drawing ratio is more than 5.0 times. If large, breakage in a lateral stretch process is accompanied. If the crystallization energy of the first stage longitudinal drawing sheet is less than 20 J / g, since the orientation crystallization occurs in the first stage drawing process, the first and second longitudinal drawing sheets become wider in shrinkage, and the second stage If the longitudinal stretching temperature is lower than T _g + 50 ° C. of the sheet, the stretching stress is increased, resulting in width shrinkage and orientation determination. If the second stage longitudinal stretching temperature is higher than 140 ° C., uneven stretching causes breakage in the transverse stretching process. And nonuniformity is involved. In addition, if the surface is not rapidly cooled by a difference of 105 ° C. or more from the stretching temperature of the second stage stretching, surface scratches are caused by poor cooling, and molecular orientation is relaxed, resulting in uneven thickness and a decrease in mechanical strength. In this case, the preheating loss in the transverse stretching process is increased, which is inefficient. In addition, the specific gravity of the first stage longitudinal stretch sheet is preferably in the range of 1.335 to 1.35, but when it is larger than 1.35, there is a problem that draw failure and breakage occur.

On the other hand, when the second stage longitudinal draw ratio of 2.0 to 3.0 times and the second stage longitudinal stretch are not satisfied, the total longitudinal draw ratio is 3.5 times to 5.0 times in the longitudinal drawing process and the crystallization energy cannot be set to 20 to 25 J / g. This will occur.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

1 is a schematic diagram of a two-stage longitudinal drawing apparatus of a polyester film according to one aspect of the present invention. Rolls 1-7 of FIG. 1 are preheat rolls, rolls 8 are 1st draw rolls, rolls 9 are 2nd draw rolls, rolls 10-11 are cooling rolls, and 8 ', 9', 10 'are rolls. Is roll roll. After preheating the polyester unstretched sheet (F) obtained by melt-extruding a polyester resin through a T-die through a roll 1 to 7, according to a conventional method, the circumferential speed difference between rolls 8 and 9 The first stage stretching is performed, and the second stage stretching and rapid cooling are performed between the rolls 9 and 10 to produce the longitudinal stretch sheet F '.

The longitudinally stretched sheet F 'as described above is laterally stretched at a draw ratio of 3.5 to 5.0 times in a conventional transverse stretching apparatus and heat-set according to a conventional method to prepare a biaxially oriented polyester film of the present invention.

Hereinafter, examples of the present invention will be described. However, these are merely provided to aid the understanding of the present invention, and the present invention is not limited by these examples.

Example 1

An unstretched sheet is obtained by melt extrusion of a polyester chip having an intrinsic viscosity of 0.63 dl / g at a sheet forming speed of 70 m / min at 280 ° C. through a die. In the longitudinal drawing apparatus of FIG. 1, the roll temperature of No. 1-3 is 100 ° C, the roll temperature of 4-7 is 120 ° C, the roll temperature of 8-9 is 125 ° C, and the unstretched sheet is 1.8 times between rolls 8 and 9. After the first stage longitudinal stretching, 2.0 times the second stage longitudinal stretching between the rolls 9 and 10, the longitudinal stretch sheet is rapidly cooled by the 10th cooling roll in which the cooling water at 20 ° C. circulates at a flow rate of 300 l / min. Then, the film was laterally stretched at a lateral stretch ratio of 4.3 times and then heat-set by a conventional method to obtain a biaxially oriented polyester film having a thickness of 14 µm.

Example 2

In Example 1, the sheet forming speed was 77.5 m / min, the 8th roll temperature was 120 degreeC in the longitudinal drawing apparatus of FIG. 1, 1.8 times of 1st stage longitudinal stretch ratio, 2.1 times of 2nd stage longitudinal stretch ratio, and It carried out similarly to Example 1 except having set the 10th cooling roll temperature to 18 degreeC.

Example 3

In Example 1, the sheet forming speed was 52.5 m / min, the roll temperature No. 9 was 130 ° C. in the longitudinal drawing apparatus of FIG. 1, the first stage longitudinal draw ratio was 2.0 times, the second stage longitudinal draw ratio was 2.4 times, and It carried out similarly to Example 1 except having set the 10th cooling roll temperature to 15 degreeC.

Comparative Example 1

In Example 1, in the longitudinal drawing apparatus of FIG. 1, the roll temperature of 1-7 is 90 ° C, the roll temperature of 8 is 110 ° C, the roll temperature of 9-11 is 20 ° C, and between rolls 8 and 9 The same procedure as in Example 1 was carried out except that the first stage longitudinal draw ratio was 4.0 times.

Comparative Example 2

In Example 1, the sheet forming speed was 77.8 m / min, the roll stretching apparatus of FIG. 1 was set at 4 ° C roll temperature to 100 ° C, the 8th roll temperature to 105 ° C, and the 9th roll temperature to 110 ° C. The same procedure as in Example 1 was conducted except that the first stage longitudinal draw ratio was 1.8 times and the second stage longitudinal draw ratio was 1.8 times.

Comparative Example 3

In Example 1, in the longitudinal drawing apparatus of FIG. 1, the roll temperature of 1-7 is 90 ° C, the roll temperature of 8 is 100 ° C, the roll temperature of 9 is 150 ° C, the first stage longitudinal draw ratio is 2.0 times, and the second However, it carried out similarly to Example 1 except having made the draw ratio 2.0 times.

Comparative Example 4

In Example 1, the sheet forming speed was 52.5 m / min, the roll stretching apparatus of FIG. 1 was 110 ° C. for the 4-7 roll temperature, the roll temperature was 120 ° C. for the 8th roll, and the roll temperature was 125 ° C. for the 9th roll. The same procedure as in Example 1 was conducted except that the first stage longitudinal draw ratio was 1.5 times and the second stage longitudinal draw ratio was 3.5 times.

Comparative Example 5

In Example 1, it carried out similarly to Example 1 except having set the 10th cooling roll temperature to 25 degreeC in the longitudinal drawing apparatus of FIG.

Comparative Example 6

In Example 1, it carried out similarly to Example 1 except having set the sheet forming speed to 82.4 m / min, and making the lateral stretch ratio 3.4 times.

Comparative Example 7

In Example 1, it carried out similarly to Example 1 except having set the sheet forming speed to 53.8 m / min, and making lateral draw ratio 5.2 times.

The conditions of the above examples and comparative examples, the crystallization energy and specific gravity of the longitudinally stretched sheets, the number of breaks in the transverse stretching process, the surface smoothness and the thickness uniformity of the finally obtained film were measured and shown in Table 1 below.

[Assessment Methods]

1. Crystallization energy: measured using a differential calorimeter (DSC, Perkin-Elmer) at a temperature increase rate of 20 ℃ / min.

2. Specific gravity: Specific gravity of the film was measured by the density gradient tube according to ASTM D1505.

3. Number of breaks: The number of breaks that occurred during the 72 hours transverse stretching process of the polyester film was measured.

Thickness uniformity: The thickness was measured at intervals of 20 mm in the transverse direction using a thickness gauge (manufactured by Annitsu Co., Ltd.) and expressed as a thickness deviation that is a difference between the maximum value and the minimum value.

Surface smoothness: The presence or absence of surface scratches in the longitudinal direction on the film surface in the light of fluorescent lamps was visually evaluated.

As shown in the above table, the polyester film of Example 1-3 prepared by the manufacturing method of the present invention has surface smoothness and thickness uniformity more than that of the polyester film of Comparative Example 1-5 without the manufacturing method of the present invention. It can be seen that the productivity is good because the number of breaks during the transverse operation is small.

Therefore, the manufacturing method of the present invention can be used for the production of polyester film suitable as a base film of video recording, audio recording, and computer magnetic recording media requiring high magnetic recording density, high smoothness and high driving stability. .

Claims (2)

In the method for producing a biaxially oriented polyester film comprising melt extruding a polyester resin and stretching the unstretched sheet in the longitudinal and transverse directions, the stretching step is such that the total longitudinal stretch ratio is in the range of 3.5 to 5.0 times. Perform two stage longitudinal stretching, but perform the first stage longitudinal stretching so that the crystallization energy of the first stage longitudinal stretching sheet is in the range of 20 to 25 J / g, and at a temperature of the following formula (1) at a ratio of 2.0 to 3.0 times the second stretching ratio, However, after longitudinal stretching and rapid cooling to the cooling temperature of the following formula (2), transverse stretching is carried out at a draw ratio of 3.5 to 5.0 times; producing a biaxially oriented polyester film; Formula (1) T _g + 50 ° C 2nd stage longitudinal drawing temperature 140 ℃ Equation (2) Cooling temperature 2nd stage longitudinal drawing temperature-105 degreeC Where T _g is the glass transition temperature of the sheet. The method according to claim 1, wherein the specific stretching sheet has a specific gravity of 1.335 to 1.35.