JP3020723B2 - Method for producing biaxially stretched polyester film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a biaxially stretched polyester film.
Manufactures biaxially oriented polyester films with high mechanical strength in both the vertical (longitudinal direction of the film) and the horizontal (width direction of the film) and excellent thermal dimensional stability in the horizontal direction with excellent process stability. How to do it.

[0002]

2. Description of the Related Art Biaxially stretched polyester films are widely used for base films for magnetic tapes and various other applications because of their excellent strength and dimensional stability.

In recent years, the field of information recording / reproducing equipment has been remarkably developed, and accordingly, demands for thinner base films for magnetic tape have been further increased. Has also been required strongly.

As a technique meeting such a demand, there is a method of stretching a biaxially stretched film again in a longitudinal direction and then stretching it in a transverse direction again (for example, Japanese Patent Application Laid-Open Nos. 50-133276 and 55-22915). Proposed.

[0005]

However, these methods are disadvantageous in that the film is likely to break during re-horizontal stretching, the production cost is increased due to a reduction in productivity, and the produced film is not heat-resistant. Since the mechanical stability is not sufficient, there is a disadvantage that the width shrinks in the process of processing into a magnetic tape, and the yield decreases. In order to reduce the width shrinkage in this processing step, there is a demand for a film having a low heat shrinkage in the lateral direction at around 100 ° C.

[0006]

The inventor of the present invention has conducted intensive studies on a method for producing a biaxially stretched polyester film free of such disadvantages. As a result, by selecting certain specific conditions, the mechanical strength in both the vertical and horizontal directions can be reduced. The present inventors have found that a biaxially stretched polyester film which is high and has excellent transverse dimensional stability can be produced with reduced film breakage during stretching and excellent process stability.

That is, the present invention relates to a method of 1.5 to 2.8 times in the longitudinal direction,
After stretching 3.5 to 5 times in the horizontal direction (Tg + 20 ° C) ~ (Tg +
First, the biaxially stretched polyester film heat-set at a temperature of (80 ° C) is adjusted so that the total stretch ratio becomes 4 to 7 times at a temperature (T1) of (Tg + 20 ° C) or more and (Tg + 80 ° C) or less. Re-longitudinal stretching, and then the stretching start temperature is (T1 + 20 ° C) or more (Tm
-80 ° C) or less, and the temperature at the end of stretching (Tm-70 ° C) or more (Tm-30 ° C) while increasing the temperature stepwise or continuously
The following is a method for producing a biaxially stretched polyester film, wherein the film is stretched again so that the total magnification in the transverse direction becomes 5.5 times or more, and further subjected to a 2 to 8% relaxation treatment in the transverse direction at 100 to 160 ° C. is there.

Here, Tg is the glass transition point of the polyester, and Tm is the melting point of the polyester.

The polyester in the present invention is an aromatic polyester represented by polyethylene terephthalate and polyethylene-2,6-naphthalate, and may be any of a homopolymer, a copolymer and a blend polymer.
For example, a copolyester obtained by copolymerizing 20% by mole or less of a third component with polyethylene terephthalate or the like may be used. Further, a blend polymer in which 20% by weight or less of a third component is blended with polyethylene terephthalate or copolyester may be used.

[0010] The polyester in the present invention may contain an agent for improving film properties, such as a lubricant, an antistatic agent, a coloring agent, a flame retardant, a light-shielding agent, a stabilizer, and an ultraviolet absorber. The lubricant may be either internally precipitated particles or externally added particles, or may be a combination of two or more. As the externally added particles, for example, calcium carbonate, kaolin, silica, titanium oxide, alumina, crosslinked polymer particles, silicon resin particles, and the like, and as the internally precipitated particles, for example, polyester obtained by combining an alkali (earth) metal compound, a phosphorus compound, and the like. Examples thereof include those precipitated during production.

Next, a method for producing a biaxially stretched film according to the present invention will be described. First, after thoroughly drying the polyester raw material, melt extruding with an extruder, a filter,
It is cast on a rotating drum through a die and quenched and solidified. The quenched and solidified film is substantially in an amorphous state and is low or non-oriented.

[0012] First, 1.5 to 2.8 in the longitudinal direction of this film
After stretching, preferably 1.8 to 2.5 times, and 3.5 to 5 times, preferably 3.6 to 4.8 times in the transverse direction, after stretching (Tg + 20 ° C.)
(Tg + 80 ° C) and heat-set to form a biaxially stretched film.
Here, Tg is the glass transition point of the polyester.

As the stretching method, known methods such as sequential biaxial stretching and simultaneous biaxial stretching can be used. Stretching temperature is Tg ~
(Tg + 60 ° C.) is preferable. This longitudinal stretching ratio is 1.
If it is lower than 5 times, the longitudinal strength of the product film is insufficient, while if it is higher than 2.8 times, the film breaks during re-longitudinal stretching increases. Further, if the transverse stretching ratio is lower than 3.5 times or higher than 5 times, the film breakage during the vertical stretching is increased.

Although a known method can be used for heat fixing, a tenter method in which a clip is used for holding is preferable. If the heat setting temperature is lower than (Tg + 20 ° C.), the strength is significantly reduced during the re-longitudinal stretching. On the other hand, if the temperature is higher than (Tg + 80 ° C.), the film will break more during the re-longitudinal stretching.

Next, this biaxially stretched film is treated with (Tg + 2
0 ° C)-(Tg + 80 ° C), preferably (Tg + 25 ° C)-
The film is stretched again at a stretching temperature (T1) of (Tg + 70 ° C.) so that the total stretching ratio (longitudinal stretching ratio) becomes 4 to 7 times, preferably 5 to 6 times. If the stretching temperature is lower than (Tg + 20 ° C.), the film breakage in the re-longitudinal stretching increases, while
C.), the crystallization of the film proceeds excessively during the re-longitudinal stretching, and the film breakage in the re-lateral stretching step increases. When the total stretching ratio is less than 4 times, the strength in the longitudinal direction is insufficient, and the unevenness of the thickness of the film is deteriorated. On the other hand, when the ratio exceeds 7 times, the film is frequently broken in the re-longitudinal and re-horizontal stretching steps, and stable production cannot be performed.

The re-stretched film is then re-stretched in the transverse direction so that the total stretch ratio becomes 5.5 times or more. If the total stretching ratio is less than 5.5 times, the strength in the transverse direction becomes insufficient.

In the re-lateral stretching, the stretching start temperature is set to (T1 + 2
0 ° C) or more and (Tm-80 ° C) or less, and the temperature at the end of stretching is (Tm-70 ° C) or more and (Tm-30 ° C) or less while increasing the temperature stepwise or continuously. Here, Tm is the melting point of the polyester.

If the stretching start temperature of the transverse re-stretching is lower than (T1 + 20 ° C.), the film is liable to be broken and cannot be produced stably. On the other hand, when the stretching start temperature is higher than (Tm-80 ° C.) or when the stretching end temperature is lower than (Tm-70 ° C.), the film breakage increases. By stretching while raising the temperature from (T1 + 20 ° C) or higher to (Tm-80 ° C) or higher to (Tm-70 ° C) or higher, there is no film breakage and excellent process stability. The reason is that the crystallization is prevented from progressing more than necessary in the initial stage of stretching so that the molecular structure can be easily rearranged in the transverse stretching, and by increasing the temperature in the latter half of stretching, the temperature can be increased. It is considered that the tension of the molecular chains in the film whose lateral orientation has increased with the progress of the lateral stretching is moderately moderated, and that it is possible to achieve a higher stretching ratio. It is considered that the film can be stretched stably even if the film is stretched in the transverse direction again at a total magnification of 5.5 or more. The temperature at the end of stretching is (Tm-30
C.), the strength in the vertical and horizontal directions is greatly reduced, which is not preferable.

[0019] The re-stretched film is treated at 100-160 ° C, preferably 110-150 ° C, in order to improve the thermal dimensional stability in the transverse direction, in particular the thermal shrinkage at around 100 ° C.
Perform 2-8% relaxation. If the treatment temperature is lower than 100 ° C., the relaxation effect cannot be obtained. If the relaxation rate is less than 2%, the relaxation effect is small and the decrease in heat shrinkage is insufficient, while if it exceeds 8%, the decrease in transverse strength is too large. Before the relaxation treatment, heat-setting may be performed in a tensioned state by a conventional method. However, simply performing the tension-heating fixing does not sufficiently improve the dimensional stability in the lateral direction, and thus requires the above-described relaxation treatment.

The biaxially stretched polyester film thus obtained preferably has a thickness of 5 to 12 μm, more preferably 6 to 10 μm.

[0021]

The present invention will be described below in detail with reference to examples. In addition, the measuring method of the film characteristic in this invention is as follows.

(1) A sample film having a Young's modulus of 10 mm width and a chuck-to-chuck distance of 100 mm was applied to a Toyo Baldwin Co., Ltd. Tensilon (UTM-III L) for 20 minutes.
It is calculated according to a conventional method from the initial slope of a stress-strain curve obtained by pulling at 10 mm / min at 65 ° C. and 65% RH.

(2) Heat Shrinkage The sample film whose length has been measured in advance is subjected to a heat treatment in an air thermostat kept at 100 ° C. for 30 minutes without tension, and the length after cooling is measured. . Then, the ratio of the amount of shrinkage due to the heat treatment to the length before the heat treatment is expressed as a percentage, and is defined as the heat shrinkage.

[0024]

Examples 1 and 2 and Comparative Examples 1 to 5 An unstretched sheet of polyethylene terephthalate obtained by melt-extrusion and quenching and solidification by a conventional method was stretched 2.3 times at 100 ° C. in the longitudinal direction, and then transversely stretched at the same temperature. The film was stretched 4.0 times in the direction and then heat-set at 120 ° C. to obtain a biaxially stretched film. The biaxially stretched film was stretched again by a factor of 2.4 in the longitudinal direction at 120 ° C., and then stretched in the transverse direction again under the conditions shown in Table 1.
% Relaxation treatment. Table 1 shows the physical properties of the obtained biaxially stretched film.

[0025]

[Table 1]

[0026]

EXAMPLE 3 AND COMPARATIVE EXAMPLES 6-9 A biaxially stretched film obtained by stretching the first stage in the longitudinal and transverse directions and heat setting in the same manner as in Example 1 was obtained under the conditions shown in Table 2. Stretching again in the longitudinal direction, then stretching start temperature 175 ° C, stretching end temperature 210 ° C, stretching ratio
Re-stretch as 1.5 times, then 5% in the transverse direction at 140 ° C
Relaxed.

Table 2 shows the properties of the obtained biaxially stretched film.

[0028]

[Table 2]

[0029]

Example 4 and Comparative Examples 10 to 13 The same procedure as in Example 1 was repeated up to the longitudinal stretching, and then the transverse stretching was performed at a stretching start temperature of 160.degree. C., an end temperature of 210.degree. The obtained biaxially stretched film was subjected to a relaxation treatment under the conditions shown in Table 3.

Table 3 shows the properties of the obtained biaxially stretched film.

[0031]

[Table 3]

[0032]

According to the present invention, the biaxially stretched polyester film is subjected to re-longitudinal, re-horizontal stretching and relaxation treatment under specific conditions, so that the strength in both the longitudinal and transverse directions is high and the thermal dimensional stability in the transverse direction is good. Film, e.g. 600k vertical and horizontal Young's modulus
This enables stable production of a biaxially stretched film having a g / mm ² or more and a heat shrinkage at 100 ° C. in the transverse direction of 1.0% or less. The biaxially stretched polyester film produced by the method of the present invention is useful as a base film for magnetic recording applications, particularly for magnetic tapes for long-time recording.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-22915 (JP, A) JP-A-55-27211 (JP, A) JP-A-58-205735 (JP, A) JP-A-63-205 60731 (JP, A) JP-A-48-7059 (JP, A) JP-A-50-133276 (JP, A) JP-A-2-202925 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B29C 55/02-55/16 C08J 5/18 CFD

Claims (1)

(57) [Claims] [Claim 1] 1.5 to 2.8 times in the vertical direction and 3.5 to 2.8 times in the horizontal direction.
After stretched 5 times, the biaxially stretched polyester film heat-set at a temperature of (Tg + 20 ° C) to (Tg + 80 ° C) is first heated to a temperature (T1) not lower than (Tg + 20 ° C) and not higher than (Tg + 80 ° C). The longitudinal stretching is performed again so that the total stretching ratio becomes 4 to 7 times, and then the stretching start temperature is set to (T1 + 20 ° C) or more and (Tm-80 ° C) or less, and the temperature is increased stepwise or continuously. Set the temperature at the end of stretching to (Tm-70 ° C) or more and (Tm-30 ° C) or less,
Re-stretching so that the total magnification in the horizontal direction is 5.5 times or more,
A method for producing a biaxially stretched polyester film, which further comprises a 2-8% relaxation treatment in the transverse direction at 100-160 ° C.