JP3520589B2 - Method for producing biaxially oriented polyamide film - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a biaxially oriented polyamide film by a longitudinal and transverse sequential biaxial stretching method. More specifically, the present invention relates to a method for producing a biaxially oriented polyamide film having uniform physical, chemical and physicochemical properties in the width direction. 2. Description of the Related Art Conventionally, biaxially oriented polyamide films have excellent properties such as toughness, high gas barrier properties, pinhole resistance, transparency, and easy printability.
It is widely used as a bag packaging material for liquid foods such as konjac, hamburger, miso, ham, etc., aquatic foods, frozen foods, retort foods, pasty foods, meat and marine products. In general, as a method for producing a biaxially oriented film, a vertical and horizontal sequential biaxial stretching method is known, and this is also used for a polyamide film. [0003] However, it is known that the longitudinal and horizontal sequential biaxial stretching method tends to vary in physical properties in the film width direction. The reason for this is that in the tenter in the transverse stretching step, longitudinal stress due to transverse stretching and heat in the longitudinal direction cause contraction stress in the longitudinal direction. By the way, while both ends of the film are gripped and restrained by the clip, the central portion of the film is weakly affected by the gripping means, and the restraining force is weak. Therefore, the running speed of the central portion of the film is slower than that of both ends gripped by the clip due to the influence of the above-described stress, and thus the physical properties vary in the width direction. Polyamide film used for bag packaging material has a large variation in physical properties in the width direction, for example, an oblique difference in boiling water shrinkage ratio, which causes a twisting phenomenon in a heat treatment after bag making, causing serious trouble. Become. In order to avoid this problem, it is effective to reduce the above-mentioned stress by lowering the magnification of longitudinal stretching, but this not only causes another problem of impairing the strength in the longitudinal direction, but also reduces the production. This leads to a decrease in speed, which is not preferable for industrial production. An object of the present invention is to provide a method for producing a biaxially oriented polyamide film which reduces variations in physical properties in the width direction of a film generated in a transverse stretching step in a sequential biaxial stretching method. More specifically, an object of the present invention is to provide a method for producing a polyamide film by biaxial stretching, which reduces variations in physical properties such as uneven thickness and oblique difference in boiling water shrinkage ratio in the film width direction without lowering the longitudinal stretching ratio. Means for Solving the Problems In view of the above problems, the present inventors have made intensive studies and as a result, have reached the present invention. That is, the present invention relates to a method for successively biaxially stretching a polyamide film obtained by stretching a substantially unoriented polyamide sheet in the longitudinal direction and then stretching it in the transverse direction. ) +20] ° C. or more, [low-temperature crystallization temperature (Tc) of polyamide + 20] ° C.
At the following temperature, it is divided into two stages, a former stage and a latter stage, so that the total longitudinal stretching ratio is 3 times or more, and the latter stage longitudinal stretching uses a ceramic roll having a roll roughness Ra ≦ 0.2. To a method for producing a biaxially oriented polyamide film. The method of the present invention is applied to the production of a polyamide film, and is particularly suitable for producing a biaxially oriented film from a polyamide containing nylon 6 as a main component. As the polyamide, for example, nylon 6, a copolymer obtained by copolymerizing nylon 6 with a small amount of nylon salt or the like, nylon 6
And a nylon salt or the like. Examples of the nylon salt include a nylon salt of hexamethylenediamine and adipic acid or isophthalic acid, and a nylon salt of metaxylylenediamine and adipic acid.
The polyamide may contain a small amount of various known additives, such as various antiblocking agents, antistatic agents and stabilizers, as long as the properties are not impaired. According to the present invention, a biaxially oriented polyamide film is obtained by stretching a substantially unoriented polyamide sheet longitudinally in two steps, followed by transverse stretching, and heat setting. More specifically, in the longitudinal stretching of the substantially unoriented polyamide sheet, the first-stage stretching is performed,
Subsequently, the second-stage stretching is performed using a ceramic roll having a roll roughness Ra ≦ 0.2. Then, it is stretched in the horizontal direction and further heat-set. That is, after drying the above-mentioned polyamide raw material, it is melt-extruded by an extruder, cast from a die on a rotating drum and rapidly cooled and solidified to obtain a polyamide sheet. This polyamide sheet is in a substantially unoriented state. This sheet is first treated with the glass transition temperature (T
g) The first stage of longitudinal stretching is performed at a temperature not lower than +20] ° C. and not higher than the low-temperature crystallization temperature (Tc) of the raw polyamide (Tc) +20] ° C. Here, the low-temperature crystallization temperature (Tc) is a crystallization temperature caused by heating from a glassy state. When the longitudinal stretching is performed at a temperature lower than (Tg of raw material polyamide + 20) ° C., necking is caused and unevenness in thickness tends to increase. On the other hand, if the stretching is performed at a temperature exceeding (Tc + 20 of the raw material polyamide) ° C., thermal crystallization proceeds, and it is easy to break in the transverse stretching, which is not preferable. A more preferred stretching temperature is (Tg of raw polyamide + 30) ° C. to (Tg of raw polyamide).
c + 10) ° C. If the stretching ratio (running speed of the film after stretching / running speed of the film before stretching) in the first-stage longitudinal stretching is too low, the stretching effect cannot be obtained, and if it is too high, the orientation crystallization proceeds. If it is too much, the stretching stress in the second-stage stretching described later becomes too high, resulting in breakage or in transverse stretching. From this viewpoint, the stretching ratio in the first-stage longitudinal stretching is preferably 1.1 to 2.9 times. A more preferred stretching ratio is 1.5 to 2.5 times. For the first-stage longitudinal stretching, a known longitudinal stretching method such as hot roll stretching or infrared radiation stretching can be used. After the first-stage longitudinal stretching, the second-stage longitudinal stretching is subsequently performed. For the second-stage longitudinal stretching, hot roll stretching is used. One of the features of the present invention is to use a ceramic roll having a roughness Ra ≦ 0.2 in the second-stage longitudinal stretching. Ra
If a roll having a size of> 0.2 is used, the film is stretched while the film is slipping on the roll, so that the film surface is undesirably scratched. In addition, since the stretching start point on the roll becomes uneven in the width direction and the stretching start point pulsates, thickness unevenness occurs in the longitudinal and transverse directions of the film flow, which is not preferable. More preferably, a ceramic roll satisfying Ra ≦ 0.05 is used. That is, in the second-stage longitudinal stretching, regardless of the thickness profile in the width direction of the film subjected to the first-stage longitudinal stretching, the film is stretched in a state where the film is linearly adhered in the width direction on a roll. In addition, uniform heating and stretching in the width direction can be performed. Here, Ra is
Center line average roughness is the average height of irregularities (unit = μ)
m), which is a value defined by JIS B0601. The second-stage longitudinal stretching ratio is set so that the total longitudinal stretching ratio is 3.0 times or more. When the ratio is less than 3.0 times, the variation in physical properties in the width direction of the biaxially oriented film is reduced, but the strength in the vertical direction is reduced. If the longitudinal stretching ratio is too large, the effect of reducing variations in physical properties in the width direction of the biaxially oriented film may not be exhibited. Considering this, the preferable total longitudinal stretching ratio is 3.0 to 3.0.
It is 3.8 times, and more preferably 3.3 to 3.6 times. The stretching temperature in the second-stage longitudinal stretching is also (Tg + 20 of raw polyamide) ° C. to (Tc + 20 of raw polyamide).
° C. The stretching temperature is (Tg + 2 of raw material polyamide)
If the temperature is lower than 0) ° C., the stretching stress increases, and the film tends to be broken by transverse stretching. If the temperature exceeds (Tc + 20) of the raw material polyamide, the thickness unevenness increases. More preferably, (Tg of raw material polyamide + 25) ° C. to (Tc of raw material polyamide + 10)
° C. [0011] The uniaxially oriented film thus obtained is then stretched in the transverse direction using a stenter. Here, if the transverse stretching temperature is too low, the transverse stretchability may deteriorate (breakage may occur), while if too high, the thickness unevenness tends to increase. From such a point, the transverse stretching temperature is 100
C. to less than the melting point is preferable, and 100 C. to 180 C. is more preferable. In addition, from the viewpoint of securing the strength in the lateral direction, the stretching ratio is preferably 3.0 times or more, and more preferably 3.5 times or more. The biaxially oriented film stretched in this way is heat-set and wound up. As described above, according to the method of the present invention,
By performing the longitudinal stretching in two stages and performing the second-stage stretching using a ceramic roll having a roll roughness Ra ≦ 0.2, a biaxially oriented polyamide film having small variations in physical properties in the width direction is obtained. be able to. The reason is,
Not only the effect of reducing the stretching stress by dividing the longitudinal stretching into two stages can be obtained, but also the roll roughness Ra in the second stage stretching.
Since the film is stretched in a state in which the film is in close contact with the roll using a ceramic roll of ≦ 0.2, the film can be uniformly heated on the roll in the film width direction. As a result, a linear stretching start point can be obtained in the width direction, and the orientation of the uniaxially oriented film becomes uniform in the width direction. In addition, since the structure of the uniaxially oriented film before the horizontal stretching is made gentle by the sheet orientation relaxing action after the first-stage stretching in the roll adhesion process, the formation of the horizontal orientation which is exhibited at the time of the horizontal stretching becomes easy, and It is considered that the stretching property was improved by reducing the transverse stretching stress. Hereinafter, the present invention will be described in detail with reference to Examples, but it goes without saying that the present invention is not limited to the following Examples. In addition, the film temperature, physical property values, and characteristics used in Examples and Comparative Examples are measured and defined as follows. Glass transition temperature (Tg) and low-temperature crystallization temperature (Tc) An unoriented polyamide sheet is frozen in liquid nitrogen, thawed under reduced pressure, and heated at a rate of 10 ° C. /
Measured in minutes. Film temperature (stretching temperature) The temperature in the longitudinal stretching is measured by a radiation thermometer I manufactured by Minolta Co., Ltd.
The temperature of the film was measured using R-004. For the temperature in the transverse stretching, the temperature of the film was measured using a radiation thermometer RHP3 manufactured by Raytec Japan Co., Ltd. The biaxially oriented polyamide film having uneven thickness is cut into strips of 1 m × 5 cm each in the vertical and horizontal directions, and the thickness shape is measured using a thickness gauge K306C manufactured by Anritsu Electric Co., Ltd. The thickness unevenness per meter is calculated by the following formula, and this is repeated five times.
The average value was regarded as thickness unevenness. [Equation 1] A biaxially oriented polyamide film having an oblique difference in boiling water shrinkage is cut into 21 cm square samples from a position (end) 40% of the full width to the left and right from the center of the full width. Draw a circle with a diameter of 20 cm centering on the center of each sample, draw straight lines passing through the center of the circle in 45 ° and 135 ° directions when the vertical direction is 0 °, measure the diameter in each direction, process It will be the previous length. The sample is heat-treated in boiling water for 30 minutes, taken out, removed of water adhering to the surface, and air-dried. After air drying,
The diameter in each direction is measured and defined as the length after processing. The boiling water shrinkage is calculated using the following equation. [Equation 2] 45 ° and 135 when the vertical direction is 0 °
The absolute value of the difference in the boiling water shrinkage in the ° direction was determined, and the average value at both ends was defined as the oblique difference in the boiling water shrinkage. The surface layer of the biaxially oriented polyamide film wound into a hygroscopic design strain roll is removed, a sample of the entire width is cut out from the inside, and a vacuum dryer adjusted to 30 ° C. × 1.0 mmHg or less is immediately used. The sample was dried for at least 6 hours. Take it out,
Immediately after adjusting for 24 hours or more in a 20 ° C. × 30% RH environment using a desiccator, draw a circle mark with a diameter of 20 cm from the center of the full width to the left and right at 40% of the full width (end). 0 °, 45 °, 9 when the direction is 0 °
Straight lines passing through the center of the circle are drawn in the directions of 0 ° and 135 °, and the diameter in each direction is measured, which is defined as the length before processing. Then, after adjusting the sample for 24 hours in an environment of 30 ° C. × 80% RH, the diameter of the mark in each direction is measured, and the measured length is defined as the length after processing. The moisture elongation is calculated using the following equation. (Equation 3) The absolute value of the difference between the moisture absorption elongation in the 0 ° and 90 ° directions when the vertical direction is 0 ° and the absolute value of the difference in the moisture absorption elongation in the 45 ° and 135 ° directions are determined. The average value of five measurements was defined as the vertical difference and the oblique difference in the moisture elongation. Moisture absorption elongation Vertical and oblique differences are less than 0.5% for 4 points, 0.5%
3 points when the value is less than 1.0% or less, 2 points when the value is 1.0% or more and less than 1.5%, and 1 point when the value is 1.5% or more.
The evaluation was made in 4 grades of 6 to 7 points of ○, 4 to 5 points of Δ, and 2 to 3 points of X, and this evaluation was defined as moisture absorption pattern distortion. The film was biaxially stretched sequentially for 2 hours under the same conditions as described in Example 1. As soon as the film was broken, the film was formed and stretched, and the number of breaks was examined. Example 1 Nylon 6 pellets [relative viscosity (RV) = 2.8] were vacuum-dried, supplied to an extruder, melted at 260 ° C., extruded from a T-die into a sheet, and subjected to direct current heating. A voltage was applied, and the sheet was electrostatically brought into close contact with the cooling roll, and cooled and solidified to obtain a 200 μm-thick non-oriented sheet. The Tg of this sheet was 40 ° C. and the Tc was 68 ° C. The sheet is first pre-heated at a temperature of 50 ° C. and then stretched at a temperature of 75 ° C.
The first-stage longitudinal stretching was performed at a temperature of ℃ and a stretching ratio of 1.7 times.
Next, the second-stage longitudinal stretching was performed using a ceramic roll having a roll roughness Ra of 0.05 so that the total stretching ratio was 3.4 times at a stretching temperature of 70 ° C. Subsequently, the sheet was continuously guided to a stenter, stretched four times at 130 ° C., heat-set at 210 ° C. and subjected to a 5% transverse relaxation treatment, cooled, and both edges were cut and removed. An axially oriented polyamide film was obtained. Table 1 shows the film formation state, physical properties and properties of the film at this time. Example 2 A biaxially oriented polyamide film was obtained in the same manner as in Example 1 except that the first-stage stretching temperature in the longitudinal stretching was 85 ° C. Example 3 A biaxially oriented polyamide film was obtained in the same manner as in Example 1 except that the second-stage stretching temperature in the longitudinal stretching was changed to 80 ° C. Example 4 The roll used for the second-stage longitudinal stretching was changed to a roll roughness Ra.
A biaxially oriented polyamide film was obtained in the same manner as in Example 1 except that the ceramic roll was 0.1. Comparative Example 1 The roll used for the second-stage longitudinal stretching was prepared by using a roll roughness Ra.
A biaxially oriented polyamide film was obtained in the same manner as in Example 1 except that a ceramic roll of = 0.3 was used. Comparative Example 2 A biaxially oriented polyamide film was obtained in the same manner as in Example 1 except that the first-stage stretching temperature in the longitudinal stretching was changed to 92 ° C. Comparative Example 3 A biaxially oriented polyamide film was obtained in the same manner as in Example 1 except that the second-stage stretching temperature in the longitudinal stretching was 90 ° C. Comparative Example 4 The procedure of Example 2 was repeated except that a ceramic roll having a roll roughness Ra = 0.3 was used for the subsequent stretching roll.
An axially oriented polyamide film was obtained. Comparative Example 5 The procedure of Example 3 was repeated except that a ceramic roll having a roll roughness Ra = 0.3 was used for a subsequent stretching roll.
An axially oriented polyamide film was obtained. Table 1 shows film forming conditions and film evaluation results in Examples and Comparative Examples. [Table 1] The biaxially oriented polyamide film obtained by the longitudinal and transverse sequential stretching obtained by the production method of the present invention has no film breakage, small thickness unevenness, oblique difference in boiling water shrinkage ratio, and moisture absorption pattern distortion. Can be smaller.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-8-197620 (JP, A) JP-A-7-290565 (JP, A) JP-A-8-174563 (JP, A) JP-A-56-197 56827 (JP, A) JP-A-55-21258 (JP, A) JP-A-58-128820 (JP, A) JP-A-51-49266 (JP, A) (58) Fields investigated (Int. ⁷ , DB name) B29C 55/14

Claims (1)

(57) [Claims 1] A sequential sheet obtained by stretching a substantially unoriented polyamide sheet in the longitudinal direction and then in the transverse direction.
In a method for producing a polyamide film by axial stretching,
Longitudinal stretching is performed using [glass transition temperature of polyamide (Tg) +2
0] ° C or higher, [low temperature crystallization temperature (Tc) of polyamide +
20] At a temperature of not more than ° C., two stages of a former stage and a latter stage are performed so that the total longitudinal stretching ratio is 3 times or more, and the latter stage longitudinal stretching uses a ceramic roll having a roll roughness Ra ≦ 0.2. A method for producing a biaxially oriented polyamide film, comprising: