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Title:
PROCESS FOR THE PREPARATION OF BIAXIALLY ORIENTED POLYESTER FILM
Document Type and Number:
WIPO Patent Application WO/1997/018078
Kind Code:
A1
Abstract:
The present invention relates to an improved process for preparing a biaxially oriented polyester film having excellent surface smoothness and uniform thickness and by drawing an amorphous polyester sheet in the longitudinal direction and then in the transverse direction, wherein the longitudinal drawing process is conducted in three successive steps; (i) a diagonal drawing at a total drawing zone length ranging from 10 to 500 mm, 100 to 140 �C and a drawing ratio ranging from 1.1 to 2.0; (ii) a parallel drawing at 100 to 140 �C and a drawing ratio ranging from 1.2 to 3.0; and (iii) a diagonal drawing 100 to 140 �C and a drawing ratio ranging from 1.5 to 3.0.

Inventors:
SEO JEONG WOOK (KR)
SHIM WAN SUP (KR)
Application Number:
PCT/KR1996/000200
Publication Date:
May 22, 1997
Filing Date:
November 13, 1996
Export Citation:
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Assignee:
SKC LTD (KR)
SEO JEONG WOOK (KR)
SHIM WAN SUP (KR)
International Classes:
B29C55/12; B29C55/04; B29C55/06; B29C55/14; B29D7/01; C08J5/18; B29K67/00; B29L7/00; (IPC1-7): B29C55/14; B29D7/01
Foreign References:
US5139727A1992-08-18
US4617164A1986-10-14
US4234532A1980-11-18
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Claims:
What is claimed is
1. A process for preparing a biaxially oriented polyester film which comprises meltextruding a polyester resin to obtain a substantially amorphous sheet, drawing the sheet in the longitudinal direction at a total draw ratio ranging from 3.0 to 6.0 to obtain a longitudinally drawn film, and drawing the longitudinally drawn film in the transverse direction; said longitudinal drawing being conducted in three steps, wherein i) the substantially amorphous sheet is subjected to a first diagonal drawing step at a total drawing zone length of 10500 mm, 100140 °C and a drawing ratio of 1.22.0, to obtain a first longitudinally drawn film; ii) the first longitudinally drawn film is subjected to a parallel drawing at 100140 °C and a drawing ratio of 1.2 3.0 to obtain a second longitudinally drawn film; and iii) the second longitudinally drawn film is subjected to a second diagonal drawing step at 100140 °C and a drawing ratio of 1.53.0 to obtain the longitudinally drawn film.
2. The process of claim 1, wherein the first diagonal drawing step consists of one to three stages of diagonal drawings .
3. The process of claim 1, wherein the transverse drawing ratio ranges from 3.0 to 5.0.
Description:
PROCESS FOR THE PREPARATION OF BIAXIALLY ORIENTED POLYESTER FILM

Field of the Invention

The present invention relates to a high-productivity process for preparing a biaxially oriented polyester film having excellent surface smoothness and uniformity in film thickness .

Background of the Invention

Polyester films generally possess good mechanical strength, heat resistance, electric insulating property and chemical resistance; and therefore, they have been widely used in manufacturing magnetic recording media, food packaging material, electric insulator and the like. Polyester films are particularly useful in preparing video tapes, audio tapes and computer tapes because of their excellent mechanical strength and heat resistance.

Japanese Patent Publication No. 30-5639 discloses a process for preparing a biaxially oriented polyester film. In this method, a polyester resin is dried and melt-extruded through a die, and the amorphous sheet thus obtained is subsequently drawn in the longitudinal direction and then in the transverse direction. Further, Japanese Laid-open Patent Application No. 54-8672 discloses a process for improving the physical strength and the thickness uniformity of a biaxially extended film by conducting further onoaxial or biaxial drawing.

It is well known that the longitudinal drawing process exerts significant influence on the uniformity in thickness of the final biaxially oriented film. In the conventional longitudinal drawing of an amorphous resin sheet, a high drawing ratio may not be employed because of the occurrence of excessive oriented crystallization which causes shrinkage in the film. Therefore, the uniformity in thickness of the

side edge portions of the film becomes poor and the film is liable to break during the transverse drawing step.

Accordingly, there have been developed multi-step longitudinal drawing processes for preparing films having improved uniformity in thickness over the conventional process. For example, Japanese Laid-open Patent Application Nos. 48-43772, 50-75, 50-139872, 49-42277, 54-56674, 58- 78729, 58-160123 and 60-61233, and Japanese Patent Publication Nos. 57-48377, 57-49377 and 59-36851 disclose multi-step longitudinal drawing processes. However, these methods require insertion of cumbersome quenching and heating steps between the drawing steps, and moreover, the uniformity in thickness of the final film product is often not satisfactory. Further, USP 4,370,291 discloses a two-step longitudinal drawing process wherein two separate drawing zones are utilized while controlling birefringence values of the film as well as the drawing temperatures and ratios at different stages of the process. However, nip rolls are required in order to block the drawing stress at the first parallel longitudinal drawing step. Therefore, the longitudinal drawing equipment becomes complicated and there exists the problem that the film tends to adhere to the roll surface.

Summary of the Invention

Accordingly, it is an object of the present invention to provide a high-productivity process for preparing a biaxially oriented polyester film having improved surface smoothness and thickness uniformity.

In accordance with one aspect of the invention, there is provided a process for preparing a biaxially oriented polyester film which comprises melt-extruding a polyester resin to obtain a substantially amorphous resin sheet, drawing the sheet in the longitudinal direction at a total draw ratio ranging from 3.0 to 6.0 to obtain a longitudinally drawn film, and drawing the longitudinally drawn film in the

transverse direction; said longitudinal drawing being conducted in three steps, wherein i) the substantially amorphous sheet is subjected to a first diagonal drawing step at a total drawing zone length of 10-500 mm, 100-140 °C and a drawing ratio of 1.2-2.0, to obtain a first longitudinally drawn film; ii) the first longitudinally drawn film is subjected to a parallel drawing at 100-140 °C and a drawing ratio of 1.2-3.0 to obtain a second longitudinally drawn film; and iii) the second longitudinally drawn film is subjected to a second diagonal drawing step at 100-140 °C and a drawing ratio of 1.5-3.0 to obtain the longitudinally drawn film.

Brief Description of Drawings

Fig. 1 is a schematic illustration of the three-step longitudinal drawing process in accordance with one embodiment of the present invention.

Fig. 2 shows another scheme for the three-step longitudinal drawing process in accordance with another embodiment of the present invention.

Fig. 3 illustrates a scheme for a conventional three- step longitudinal drawing process.

Detailed Description of the Invention

The term "parallel drawing" means that a sheet is drawn from the point where the film leaves a roll to the point where the film is brought into contact with a next roll, wherein two rolls rotate in the same direction.

The term "diagonal drawing" means that a sheet is drawn from the point where the film leaves a roll to the point where the film is brought into contact with a next roll, wherein two rolls rotate in the reverse direction. The term "oriented crystallization" means that random polymer chains in an amorphous sheet are oriented and crystallized by the action of stretching the sheet.

In accordance with the present invention, there is provided a process for preparing a biaxially oriented polyester film which comprises melt-extruding a polyester resin to obtain a substantially amorphous resin sheet, drawing the sheet in the longitudinal direction at a total draw ratio ranging from 3.0 to 6.0 to obtain a longitudinally drawn film, and drawing the longitudinally drawn film in the transverse direction; said longitudinal drawing being conducted in three steps, wherein i) the substantially amorphous sheet is subjected to a first diagonal drawing step at a total drawing zone length of 10-500 mm, 100-140 °C and a drawing ratio of 1.2-2.0, to obtain a first longitudinally drawn film; ii) the first longitudinally drawn film is subjected to a parallel drawing at 100-140 °C and a drawing ratio of 1.2-3.0 to obtain a second longitudinally drawn film; and iii) the second longitudinally drawn film is subjected to a second diagonal drawing step at 100-140 °C and a drawing ratio of 1.5-3.0 to obtain the longitudinally drawn film. The first diagonal drawing step may consist of one to three stages of diagonal drawings.

The above three-step longitudinal drawing process of the present invention provides a polyester film having improved thickness uniformity and suppresses excessive oriented crystallization of the film. The first diagonal drawing step of the inventive longitudinal drawing process induces some surface crystallization and orientation of polymer chains in the film, which effectively prevents the film from adhering to the roll surface. The subsequent parallel drawing does not produce scratches on the film due to blocking of the drawing stress, and the second diagonal drawing step minimizes the shrinkage in the film width, while enhancing the thickness uniformity and mechanical strength of the longitudinally drawn film. Each roll surface is coated with teflon, silicon rubber, ceramic or the like in order to minimize the film adhesion to the roll surface.

In case when the total longitudinal drawing ratio is

below 3.0, the uniformity in the film thickness becomes poor, while when total drawing ratio is over 6.0, the film easily breaks in the subsequent transverse drawing. If the longitudinal drawing is not conducted in the above-mentioned three steps in succession, there tend to appear the problems that the longitudinally-drawn film becomes excessively crystallized and less uniform in thickness, and also that the ■drawing equipment becomes complicated and the process becomes less efficient due to the requirement of cooling-heating cycles.

The first diagonal drawing step of the inventive process may not be replaced with a parallel drawing step, for the parallel drawing would induce significant shrinkage in the film width and reduce the uniformity in the film thickness in the transverse direction.

Fig. 1 shows a scheme for three-step longitudinal drawing process in accordance with an embodiment of the present invention. Roll 1 is a pre-heating roll; rolls from 2 to 4 are drawing rolls; roll 5 is a chill roll; rolls 3' and 4' are nip rolls; and 1 1 is the drawing zone length of the first longitudinal drawing. An amorphous sheet(F) is subjected to the first longitudinal drawing step, i.e., a diagonal drawing step conducted between rolls 2 and 3; a parallel drawing corresponding to the second longitudinal drawing step is conducted between rolls 3 and 4; and in the third longitudinal drawing step, a second diagonal drawing is carried out between rolls 4 and 5 to obtain a longitudinally- drawn sheet(F' ) .

Fig. 2 is another scheme for the three-step longitudinal drawing process in accordance with another embodiment of the present invention wherein the first longitudinal drawing step consists of three-stage diagonal drawings. Roll 1 is a pre¬ heating roll; rolls from 2 to 6 are drawing rolls; roll 7 is a chill roll; rolls 5' and 6' are nip rolls; and 1 1 1 12 and 1 13 are drawing zone lengths of the first, second and third stages in the first longitudinal drawing step. An amorphous sheet(F) is subjected to the first longitudinal drawing step,

which corresponds to three-stage diagonal drawings between roll 2 and 3, roll 3 and 4, and then roll 4 and 5; the second longitudinal drawing step is a parallel drawing conducted between rolls 5 and 6; and the third longitudinal drawing step corresponds to a second diagonal drawing carried out between rolls 6 and 7 to obtain a longitudinally-drawn sheet(F' ) .

The following Examples are given for the purpose of illustration only and are not intended to limit the scope of the invention.

Example 1

A polyester composed predominantly of polyethylene terephthalate and having an intrinsic viscosity of 0.63 dl/g was melt-extruded through a die at a molding rate of 60 m/min to form an amorphous sheet and a biaxially oriented film was prepared using the drawing device shown in Fig. 1 as follows. Referring to Fig. 1, rolls 1 and 2 were maintained at 110 °C, and rolls 3, 4 and 5 were kept at 120 °C, 125 °C and 25 °C, respectively. The drawing zone length between rolls 3 and 4 was 40 mm. The amorphous sheet obtained above was subjected to a three-step longitudinal drawing process: the sheet was first subjected to a diagonal drawing step between rolls 2 and 3 at a drawing ratio of 1.3; followed by the second step, a parallel drawing between rolls 3 and 4 at a drawing ratio of 1.5; and in the third step, another diagonal drawing was conducted between rolls 4 and 5 at a drawing ratio of 2.3 to produce a longitudinally drawn film. Subsequently, the longitudinally-drawn film was drawn at a ratio of 4.0 in the transverse direction, and then, thermally fixed at 220 °C to obtain a biaxially oriented polyester film having a thickness of 14 μm.

Example 2

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the temperature of rolls 1 and 2 was 120 °C, while rolls 3 and 4 were kept at 125 °C and 130 °C, respectively; and the drawing ratios of the three successive longitudinal drawing steps were 1.4, 1.6, and 2.0, respectively.

Example 3

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the temperature of rolls 1 and 2 was 125 °C, while rolls 3 and 4

were kept at 130 °C and 135 °C, respectively; and the drawing ratios of the three successive longitudinal drawing steps were 1.3, 1.6, and 2.16, respectively.

Example 4

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the drawing zone length of the first longitudinal drawing between roll 3 and 4 was 200 mm.

Example 5

A biaxially oriented polyester film was prepared in accordance with a procedure similar to that of Example 1 using the drawing device shown in Fig. 2 wherein: the temperature of rolls 1 to 4 shown in Fig. 2 was 110 °C, while rolls 5, 6 and 7 were kept at 120 °C, 125 °C and 25 °C, respectively. The first longitudinal drawing step was conducted by a three-stage diagonal drawing process, the first stage having a drawing zone length of 40 mm between rolls 2 and 3 at a drawing ratio of 1.15; the second stage having a drawing zone length of 40 mm between rolls 3 and 4 at a drawing ratio of 1.15, and the third stage having a drawing zone length of 40 mm between rolls 4 and 5 at a drawing ratio of 1.21. Parallel drawing corresponding to the second longitudinal drawing step was carried out between rolls 5 and 6 at a ratio of 1.5; and as the third longitudinal drawing step, diagonal drawing was carried out between rolls 6 and 7 at a ratio of 1.88.

Comparative Example 1

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the temperature of rolls 1 and 2 (shown in Fig. 1) was 90 °C.

Comparative Example 2

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the temperature of roll 3 (shown in Fig. 1) was 90 °C.

Comparative Example 3

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the temperature of roll 4 (shown in Fig. 1) was 95 °C.

Comparative Example 4

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the temperature of roll 4 shown (in Fig. 1) was 150 °C.

Comparative Example 5

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the sheet molding rate was 92.3 m/min.; and that the total longitudinal drawing ratio was 2.93 wherein the drawing ratios of the three successive drawing steps were 1.3, 1.5 and 1.5, respectively.

Comparative Example 6

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the sheet molding rate was 42 m/min.; and that the total longitudinal drawing ratio was 6.48 wherein the drawing ratios of the three successive drawing steps were 1.5, 1.8 and 2.4, respectively.

Comparative Example 7

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the drawing ratios of the three successive longitudinal drawing steps were 1.0, 2.0 and 2.25, respectively.

Comparative Example 8

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the drawing ratios of the three successive longitudinal drawing steps were 1.8, 2.0 and 1.25, respectively.

Comparative Example 9

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the drawing zone length between rolls 2 and 3 (shown in Fig. 1) was 600 mm.

Comparative Example 10

A biaxially oriented polyester film was prepared in accordance with the procedure of Example 1 except that the first longitudinal drawing between rolls 2 and 3 was a parallel drawing and the drawing zone length between rolls 2 and 3 was 140 mm.

Test Example

The drawing ratio, the drawing zone length, the degree of shrinkage of the film width, the frequency of film breakage during the transverse drawing process, the uniformity in thickness and the appearance of scratches on the film surface were measured according to the following methods and the results are shown in Table 1.

1. Drawing Ratio in Longitudinal Direction

'12

Drawing Ratio(e ι ) =

M wherein i is a drawing stage from 1 to 3; S ., is the speed of the roll having the starting point, of the longitudinal drawing; and S l2 is the speed of the roll having the end point of the longitudinal drawing.

Total Drawing Ratio in = e. X e 2 X the Longitudinal Direction

2. Drawing Zone Length

Drawing Zone Length(1.,.) Length between the point where the film leaves a roll and the point where the film is brought into contact with a next roll

Total Drawing Zone Length in the First Stepd.,) - Σ 1.

wherein j is a drawing stage from 1 to 3 in the first longitudinal drawing step.

3. Degree of Shrinkage in Film Width w

Degree of Shrinkage = (1- ) X 100(%) w.

wherein W g is the film width after leaving a drawing roll;

W. is the film width before contacting a drawing roll.

Degree of Breakage

Degree of breakage was estimated by the number of

breakages occurring during the transverse drawing process (72 hours ) .

A: below 1, B: from 2 to 4,

C: from 5 to 8, D: over 9

5. Uniformity in Thickness

The thickness of a film was measured at 20 mm intervals in the transverse direction by using a thickness tester (Amrits, Japan). The variation in the sheet thickness was listed in Table 1.

6. Scratches formed on Film Surface

The scratches formed on the film surface were evaluated by holding the film against a fluorescent light for visual inspection.

X: no scratch observed O: scratches observed

Table 1

a: drawing ratio in the longitudinal direction b: total drawing ratio in the longitudinal direction c: drawing zone length in the first longitudinal drawing step

As shown in Table 1, biaxially oriented polyester films obtained in Examples 1 to 5 in accordance with the present invention have superior surface smoothness and uniform thickness compared to the polyester films obtained in Comparative Examples 1 to 10. Also, the reduced number of breakages occurring during the transverse drawing step shows that the productivity of the inventive process is high.