JPH05286029A - Production of biaxially stretched polyester film - Google Patents

Abstract

PURPOSE:To produce a biaxially stretched polyester film having high mechanical strength in both of its longitudinal and lateral directions and excellent in thermal dimensional stability in its lateral direction with excellent process stability. CONSTITUTION:A biaxially stretched polyester film stretched by 1.5-2.8 times in its longitudinal direction and by 3.5-5.5 times in its lateral direction and subsequently thermally fixed within a temp. range of (Tg + 20 deg.C))-(Tg + 80 deg.C) is again longitudinally stretched within a temp. range of (Tg + 20 deg.C)-(Tg + 80 deg.C) at first so that the sum stretching magnification becomes 4-7 times. Subsequently, stretching start temp. is set to (T1 + 20 deg.C))-(Tm-80 deg.C) and the temp. at the time of the completion of stretching is set to (Tm-70 deg.C)-(Tm-30 deg.C) while temp. is raised stepwise or continuously and the film is again laterally stretched so that the sum stretching magnification in the lateral direction becomes 5.5 times or more and further subjected to slackening treatment of 2-8% in the lateral direction at 100-160 deg.C).

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing a biaxially stretched polyester film, and more specifically,
Manufacture of biaxially stretched polyester film with high mechanical strength in both the machine direction (longitudinal direction of the film) and the transverse direction (film width direction) and excellent thermal dimensional stability in the transverse direction with excellent process stability. It is about how to do.

[0002]

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

By the way, the recent development of the information recording / reproducing apparatus field has been remarkable, and accordingly, the demand for thinning of the base film for magnetic tape has further increased, and the film has not only the strength property in the longitudinal direction but also the transverse direction. The strength characteristics of are also strongly demanded.

As a technique to meet such a demand, there is a method of re-longitudinal stretching of a biaxially stretched film and then re-transverse stretching (for example, JP-A-50-133276 and JP-A-55-22915). Proposed.

[0005]

However, these methods have the problems that film breakage easily occurs during transverse re-stretching, and the production cost increases due to a decrease in productivity. Since the mechanical stability is not sufficient, there is a drawback that width shrinkage occurs in the process of processing the magnetic tape and the yield decreases. In order to reduce the width shrinkage in this processing step, a film having a low lateral heat shrinkage ratio of around 100 ° C is required.

[0006]

Means for Solving the Problems As a result of earnest research on a method for producing a biaxially stretched polyester film having no such defects, the present inventor has found that the mechanical strength in both longitudinal and transverse directions can be increased by selecting certain specific conditions. The present inventors have found that a biaxially stretched polyester film having high dimensional stability in the transverse direction and having high dimensional stability in the transverse direction can be produced with less film breakage during stretching, and has excellent process stability.

That is, according to the present invention, the longitudinal direction is 1.5 to 2.8 times,
After stretching 3.5 to 5 times in the transverse direction (Tg + 20 ° C) to (Tg +
The biaxially stretched polyester film heat-set at a temperature of (80 ° C) is first adjusted so that the total draw ratio is 4 to 7 times at a temperature (T1) of (Tg + 20 ° C) or higher and (Tg + 80 ° C) or lower. Re-longitudinal stretching, then the stretching start temperature (T1 + 20 ℃) or more (Tm
-80 ℃) or less, and the temperature at the end of stretching is (Tm-70 ℃) or more (Tm-30 ℃) while gradually or continuously raising the temperature.
In the following, a method for producing a biaxially stretched polyester film is characterized in that re-lateral stretching is carried out so that the total magnification in the transverse direction becomes 5.5 times or more, and further relaxation treatment is conducted at 100 to 160 ° C in the transverse direction by 2 to 8%. is there.

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

The polyester in the present invention is an aromatic polyester represented by polyethylene terephthalate and polyethylene-2,6-naphthalate, and may be a homopolymer, a copolymer or a blend polymer.
For example, it may be a copolyester obtained by copolymerizing polyethylene terephthalate with 20 mol% or less of a third component. It may be a blend polymer obtained by blending polyethylene terephthalate or copolyester with 20% by weight or less of the third component.

The polyester in the present invention may contain an agent for improving film properties such as a lubricant, an antistatic agent, a colorant, 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 kinds. Externally added particles include, for example, calcium carbonate, kaolin, silica, titanium oxide, alumina, cross-linked polymer particles, silicon resin particles, and the like, and internally precipitated particles include, for example, a combination of an alkali (earth) metal compound, a phosphorus compound, etc., and polyester. The thing precipitated during manufacture can be mentioned.

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

First, this film is stretched in the longitudinal direction by 1.5 to 2.8.
(Tg + 20 ° C) ~ after stretching, preferably 1.8 to 2.5 times, laterally 3.5 to 5 times, preferably 3.6 to 4.8 times
Heat set at (Tg + 80 ° C) to obtain biaxially stretched film.
Here, Tg is the glass transition point of polyester.

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

A known method can be used for heat fixation, but a tenter method of gripping with a clip is preferable. If the heat-setting temperature is lower than (Tg + 20 ° C), the strength will be largely decreased during re-longitudinal stretching, while if it is higher than (Tg + 80 ° C), the film will be broken during re-longitudinal stretching.

Next, this biaxially stretched film was treated with (Tg + 2
0 ℃) ~ (Tg + 80 ℃), preferably (Tg + 25 ℃) ~
At the stretching temperature (T1) of (Tg + 70 ° C), re-longitudinal stretching is performed so that the total stretching ratio (longitudinal stretching ratio) is 4 to 7 times, preferably 5 to 6 times. If this stretching temperature is lower than (Tg + 20 ℃), the film breakage during re-longitudinal stretching will increase, while (Tg +80
If the temperature is higher than (.degree. C.), the film is excessively crystallized during re-longitudinal stretching, and the number of film breaks in the re-transverse stretching step increases. If the total draw ratio is less than 4 times, the strength in the machine direction is insufficient and the unevenness of the film thickness is deteriorated. On the other hand, if it exceeds 7 times, the film is often broken during the re-longitudinal and transverse re-stretching steps, and stable production cannot be achieved.

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

In this transverse re-stretching, the stretching start temperature is (T1 + 2
The temperature at the end of the stretching is (Tm-70 ° C) or more (Tm-30 ° C) or less while increasing the temperature stepwise or continuously. Here, Tm is the melting point of polyester.

If the stretching start temperature of the transverse re-stretching is lower than (T1 + 20 ° C.), the film is often broken and stable production cannot be performed. On the other hand, if the stretching start temperature is higher than (Tm-80 ° C), or if the stretching end temperature is lower than (Tm-70 ° C), film breakage increases. By stretching while increasing the temperature from (T1 + 20 ℃) or higher (Tm-80 ℃) to (Tm-70 ℃) or higher, there is no film breakage and excellent process stability. The reason for this is that it is possible to prevent rearrangement of the molecular structure in the transverse re-stretching reasonably by suppressing excessive progress of crystallization in the initial stage of stretching, and to raise the temperature in the latter half of stretching, It is considered that the tension of the molecular chains in the film, which has become higher in the transverse orientation as the transverse stretching progresses, is moderately relaxed, and it is possible to stretch at a higher magnification. As a result, the re-longitudinal stretched film is obtained. It is considered that the film can be stably stretched even if it is stretched in the transverse direction again at a total magnification of 5.5 times or more. The temperature at the end of stretching is (Tm-30
If it exceeds (° C.), the strength in the vertical and horizontal directions is greatly reduced, which is not preferable.

The re-transversely stretched film is stretched in the transverse direction at 100 to 160 ° C., preferably 110 to 150 ° C., in order to improve the thermal dimensional stability in the transverse direction, particularly the heat shrinkage ratio around 100 ° C.
Perform 2-8% relaxation treatment. If the treatment temperature is less than 100 ° C., the relaxation effect cannot be obtained, while if it exceeds 160 ° C., the transverse strength is excessively reduced as compared with the thermal shrinkage, which is not preferable. If the relaxation rate is less than 2%, the relaxation effect is small and the reduction in heat shrinkage is insufficient. On the other hand, if it exceeds 8%, the transverse strength is reduced too much. Before performing the relaxation treatment, it may be heat-fixed in a tensioned state by a conventional method, but the improvement of the dimensional stability in the lateral direction is not sufficient only by performing the tension heat-fixing, and the relaxation treatment is required.

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

[0021]

EXAMPLES The present invention will be described in detail below with reference to examples. The method of measuring the film characteristics in the present invention is as follows.

(1) A sample film having a Young's modulus of 10 mm width and a gap of 100 mm between chucks was made 20 by Tensilon (UTM-III L) manufactured by Toyo Baldwin.
Calculated according to a conventional method from the initial gradient of the stress-strain curve obtained by pulling at 10 mm / min at 65 ° C and 65% RH.

(2) Heat Shrinkage The sample film, the length of which has been measured in advance, is heat-treated by keeping it for 30 minutes in an air constant temperature bath kept at 100 ° C. without tension, and the length after cooling is measured. .. Then, the ratio of the shrinkage amount due to the heat treatment to the length before the heat treatment is expressed as a percentage, and is referred to as the heat shrinkage ratio.

[0024]

[Examples 1 and 2 and Comparative Examples 1 to 5] An unstretched sheet of polyethylene terephthalate obtained by melt extrusion and rapid solidification by a conventional method was first stretched 2.3 times in the longitudinal direction at 100 ° C and then transversely 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. This biaxially stretched film was again stretched 2.4 times in the machine direction at 120 ° C., and then transversely re-stretched under the conditions shown in Table 1, and further at 120 ° C. in the transverse direction.
% Relaxation treatment. Table 1 shows the physical properties of the obtained biaxially stretched film.

[0025]

[Table 1]

[0026]

Example 3 and Comparative Examples 6 to 9 Biaxially stretched films obtained by stretching and heat-setting in the longitudinal and transverse directions of the first stage in the same manner as in Example 1 were subjected to the conditions shown in Table 2. Re-longitudinal stretching, followed by stretching start temperature 175 ℃, stretching end temperature 210 ℃, draw ratio
Re-transverse stretching as 1.5 times, then 5% in transverse direction at 140 ℃
Relaxed.

The characteristics of the obtained biaxially stretched film are shown in Table 2.

[0028]

[Table 2]

[0029]

[Example 4 and Comparative Examples 10 to 13] After re-longitudinal stretching, the same procedure as in Example 1 was carried out, and then re-lateral stretching was carried out with a stretching start temperature of 160 ° C, an end temperature of 210 ° C and a stretching ratio of 1.5 times. The obtained biaxially stretched film was subjected to a relaxation treatment under the conditions shown in Table 3.

The characteristics of the obtained biaxially stretched film are shown in Table 3.

[0031]

[Table 3]

[0032]

INDUSTRIAL APPLICABILITY According to the present invention, the biaxially stretched polyester film is re-longitudinal, re-horizontally stretched and relaxed under a specific condition to have high strength in both longitudinal and transverse directions and good thermal dimensional stability in the transverse direction. Film, for example, Young's modulus in the length and width of 600k
This enables stable production of biaxially stretched films having a heat shrinkage of g / mm ² or more and a transverse heat shrinkage at 100 ° C 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, especially for long-time recording magnetic tapes.

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Claims (1)

[Claims] 1. The vertical direction is 1.5 to 2.8 times, and the horizontal direction is 3.5 to 2.8 times.
After stretching 5 times, the biaxially stretched polyester film heat-set at a temperature of (Tg + 20 ℃) to (Tg + 80 ℃) is first heated to (Tg + 20 ℃) or higher (Tg + 80 ℃) or lower (T1). Re-longitudinal stretching to obtain a total stretch ratio of 4 to 7 times, then set the stretching start temperature to (T1 + 20 ° C) or higher (Tm-80 ° C) or lower, and raise the temperature stepwise or continuously. The temperature at the end of stretching is (Tm-70 ° C) or higher (Tm-30 ° C) or lower,
Re-transverse stretching is performed so that the total magnification in the transverse direction is 5.5 times or more,
A method for producing a biaxially stretched polyester film, which further comprises a relaxation treatment in the transverse direction at 2 to 8% at 100 to 160 ° C.