JP2020151889A - Method for manufacturing polypropylene system vertically uniaxial stretched film - Google Patents

Abstract

To provide a new method for manufacturing the polypropylene system vertically uniaxial stretched film, that maintains good tearability of the film itself, has a uniform film thickness, is excellent in strength, and suppresses a rise in a heat seal temperature.SOLUTION: A film in which difference between maximum thickness and minimum thickness in film thickness is within 20% of average thickness is obtained by setting a melt flow rate (MFR) of polypropylene resin to 1 to 10 g/10 min, melt-extruding a raw material, molding into a sheet with a cooling roll, adjusting crystallinity to 55% or less, and next, uniaxially stretching at a stretch ratio of 1.2 to 3.5 times in a longitudinal direction with a difference in roll speed to form a stretched film, wherein the raw material is a resin composition containing 70 wt% or more of polypropylene system resin that is an isotactic polypropylene that is either a propylene homopolymer, or a random copolymer or a block copolymer obtained by polymerizing propylene and α-olefin other than propylene.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for producing a polypropylene-based longitudinally uniaxially stretched film, and particularly to a method for producing a longitudinally uniaxially stretched film capable of suppressing a heat seal temperature while having linear tearing performance.

Currently, in a bag for packaging an article, a film and contents to be packaged are simultaneously supplied by an automatic wrapping machine, and filling, packaging, and heat-sealing are continuously performed. The performance required for such a packaging bag is that the contents are stably stored without being torn during processing or distribution of automatic packaging. In the packaging bag, if the heat-sealing strength of the sealing portion is low, the sealing portion may be torn during processing or distribution, and in particular, the content is heavy such as shampoo or retort food. If the product is heat-processed after packaging, stronger heat-sealing strength is required.

While strength is required for packaging bags, in recent years, easy-to-open packaging bags have been required. In particular, there is a demand for a packaging bag that can be torn and opened and has a functionality that allows it to be torn straight and easily opened. For example, a laminate obtained by laminating a biaxially stretched film and a non-stretched sealant film, which are often used as a film used for high-strength packaging bags, can be heat-sealed at a relatively low temperature and has low tear strength. The tearing direction is uncertain.

As a countermeasure against such tearability, a film formed by uniaxial stretching has been proposed (see Patent Documents 1 and 2 and the like). Patent Document 1 is a longitudinally tearable laminated film obtained by laminating a film of a propylene-based resin having a melting point higher than this on a heat-sealing layer of various olefin-based resins and stretching the film vertically and uniaxially. Patent Document 2 is a laterally tearable laminated film in which a film of a propylene resin having a melting point higher than this is laminated on a heat seal layer of various olefin resins and stretched laterally uniaxially.

According to the films exemplified in Patent Documents 1 and 2, etc., a certain effect is exhibited in the tearability in the film direction. However, the temperature at which sufficient heat seal strength can be obtained (heat seal temperature) rises due to the orientation due to stretching. As the heat-sealing temperature rises, the packaging film may be thermally deformed during sealing by heat-sealing. As described above, in the existing stretched film, it has been required to suppress the increase in the heat seal temperature.

Further, a film in which an increase in the heat seal temperature of the film is suppressed by lowering the draw ratio has been proposed (see Patent Document 3). However, even if the increase in the heat seal temperature can be suppressed, there is a problem that stretching unevenness occurs and the uniformity of the film thickness is lowered by lowering the stretching ratio.

From such a background, there has been a demand for a sealant film in which the contents can be stably stored, and the sealant film can be easily opened due to tearing and the heat seal temperature can be lowered.

Japanese Patent No. 2518233 Special Fair 8-18416 Gazette Japanese Patent No. 821164

The present invention has been proposed in view of the above circumstances. In a sealant film using polypropylene resin as a main raw material and having tearability by uniaxial stretching, the film thickness is maintained while maintaining the good tearability of the film itself. Provided is a new method for producing a polypropylene-based longitudinally uniaxially stretched film having a uniform shape, excellent strength, and suppressed increase in heat seal temperature.

That is, the first invention comprises 70 weights of a polypropylene resin which is an isotactic polypropylene which is either a propylene homopolymer, a random copolymer obtained by polymerizing propylene and α-olefins other than propylene, or a block copolymer. The melt flow rate (MFR) measured in accordance with JIS K 7210-1 (2014) of the polypropylene-based resin was set to 1 to 10 g / 10 min using a resin composition containing% or more as a raw material, and the raw material was melt-extruded. It is formed into a sheet with a cooling roll and the degree of crystallinity measured by the IR method is adjusted to 55% or less, and then uniaxially stretched to a stretching ratio of 1.2 to 3.5 times in the longitudinal direction with a difference in roll speed. To form a stretched film, the difference between the maximum thickness and the minimum thickness in the film thickness measured in accordance with JIS K 7130 (1999) is within 20% of the average thickness. The present invention relates to a method for producing a polypropylene-based longitudinally uniaxially stretched film.

The polypropylene-based longitudinal uniaxial axis according to claim 1, wherein a second invention comprises layered melt extrusion of a plurality of different raw materials selected from the resin compositions at the time of melt extrusion of the raw materials. The present invention relates to a method for producing a stretched film.

A third invention is characterized in that it includes a sealant layer, which is the polypropylene-based longitudinally uniaxially stretched film according to the first or second invention, and a stretched base film having one or more base material layers. It concerns the body.

According to the method for producing a polypropylene-based longitudinally uniaxially stretched film according to the first invention, isotac of either a propylene homopolymer, a random copolymer by polymerization of propylene and α-olefin excluding propylene, or a block copolymer. Using a resin composition containing 70% by weight or more of a polypropylene-based resin which is a tic polypropylene as a raw material, 1 to 10 g of a melt flow rate (MFR) measured in accordance with JIS K 7210-1 (2014) of the polypropylene-based resin. The raw material was melt-extruded at / 10 min, formed into a sheet with a cooling roll, and the crystallinity measured by the IR method was adjusted to 55% or less, and then the draw ratio was 1 in the longitudinal direction due to the difference in roll speed. A stretched film is formed by uniaxially stretching 2 to 3.5 times, and the difference between the maximum thickness and the minimum thickness in the film thickness measured according to JIS K 7130 (1999) is 20 of the average thickness. In order to obtain a film within%, while maintaining the good tearability of the film itself, the film thickness is uniform, the strength is excellent, and the contents can be stably stored, and the heat seal temperature rises. A suppressed film can be obtained.

According to the method for producing a polypropylene-based longitudinally uniaxially stretched film according to a second invention, a plurality of different raw materials selected from the resin compositions are melt-extruded in layers at the time of melt extrusion of the raw materials. It is possible to obtain a film in which an object can be stably stored, and a film in which an increase in heat-sealing temperature is suppressed while maintaining good tearability of the film can be obtained.

According to the laminate according to the third invention, the sealant layer which is the polypropylene-based longitudinally uniaxially stretched film of the first or second invention and the stretched base film having one or more base material layers are provided, and thus stretched. Further functionality derived from the base film can be provided.

It is sectional drawing of the laminated body. It is a schematic diagram which shows the straight-line tear test.

In the production of the longitudinally uniaxially stretched film of the present invention, the molten resin of isotactic polypropylene as a raw material is discharged from an extruder such as a T-die and is formed through stretching between rolls. Therefore, the film produced according to the present invention is a substantially longitudinally uniaxially stretched film in consideration of good tearing in one direction.

A polypropylene resin is mainly used as a raw material for the film produced by the present invention. In particular, an isotactic polypropylene having a composition resin containing 70% by weight or more of either a homopolymer of propylene, a random copolymer of propylene and α-olefin (excluding propylene), or a block copolymer. is there.

The MFR (melt flow rate) value of the polypropylene resin as a raw material is in the range of 1 to 10 g / 10 min. When the value of MFR is less than 1 g / 10 min, the fluidity of the resin at the time of heat sealing cannot be said to be sufficient, and the sealing property of the heat-sealed portion cannot be said to be sufficient. On the other hand, if the MFR value exceeds 10 g / 10 min, the flow becomes excessive during the production of the film-forming or the like, which hinders the film-forming or the like. Therefore, a suitable MFR value is in the range of 1 to 10 g / 10 min.

The polypropylene-based resin discharged from an extruder such as a T-die is molded into a sheet while being cooled by a cooling roll to become a sheet. At this time, the crystallinity of the sheet-like material is adjusted to 55% or less. The crystallinity is measured by the IR method. This is because if the crystallinity is higher than 55%, the thickness of the film after stretch molding tends to be uneven, and the heat seal temperature may rise. The crystallinity can be adjusted by adjusting the temperature of the resin when discharged from the T-die or the like, the set temperature of the cooling roll, or the like. Further, in addition to the cooling roll, the cooling effect can be further enhanced by cooling with an air knife, an air chamber, a touch roll, a flex roll, a metal belt, or the like.

Further, when the raw material is discharged from the T-die or the like, a plurality of different polypropylene-based resins among the polypropylene-based resins mentioned above can be discharged as layers. The combination of each polypropylene resin is appropriate. The MFR value of the raw material at this time and the crystallinity after forming the sheet are as described above. For example, a film in which a propylene homopolymer and a random copolymer of propylene and ethylene are combined in a layered manner has heat resistance and heat sealability at a low temperature because the base material layer and the sealant layer have different melting points. Excellent for. In this way, the functionality of the produced film can be improved by combining different polypropylene-based resins to form a layer.

The stretching ratio between rolls is a low stretching ratio of 1.2 to 3.5 times. When the draw ratio is less than 1.2 times, straight tearing (straight tearing property, straight cutting property) derived from uniaxial stretching is less likely to occur. On the other hand, if the draw ratio exceeds 3.5 times, the heat seal temperature may rise too much, resulting in poor appearance or poor sealing performance. In particular, considering that the contents of the packaging bag using the film produced by the present invention are assumed to be heavy objects such as foods and chemicals, the ratio of stretching between rolls is 1.2 to 3.5 times. The draw ratio was low.

Further, the preheating step of heating the sheet before stretching, the stretching step, and the heat setting step of removing the residual strain after stretching are all performed on the heating roll, and the roll temperature is 10 to 80 ° C. lower than the melting point of the extruded sheet. Adjusted to temperature. When the roll temperature is raised to near the melting point of the sheet, the sheet is likely to be cut due to sticking to the roll or winding, and when the temperature is lowered by 80 ° C or more from the melting point, excessive torque is applied to stretching and the sheet is similarly cut. It is easy to occur.

As described above, the crystallinity of the sheet-like material is adjusted to 55% or less. The polypropylene-based resin is a crystalline plastic and has both a crystalline portion and a non-crystalline portion. Since crystalline polymers such as polypropylene resins are neck-stretched, the thickness unevenness tends to be large in a film having a small draw ratio. Therefore, by reducing the proportion of the crystal portion, it is possible to reduce the thickness unevenness even in a film having a low draw ratio.

The thickness of the film is not particularly specified. It is similar to a general film product and has a thickness of 10 to 100 μm. The difference between the thickness of the maximum portion and the thickness of the minimum portion of the film thickness is within 20% of the average thickness. If the thickness of the film is uneven, the heat seal strength may vary in addition to the poor workability. From this, the thickness unevenness of the film shall be within the range where the difference between the maximum thickness and the minimum thickness is within 20% of the average thickness.

The longitudinally uniaxially stretched film described so far is used alone as a packaging material, and is mainly laminated with other films to be processed into various packaging materials. The schematic cross-sectional schematic diagram of FIG. 1 is an example of the laminated body 11. Another film 20 is laminated on one side (particularly, the outside of the packaging material) of the vertically uniaxially stretched film 10. In this way, the laminate 5 is formed as a combination of both films. The method of laminating another film on the longitudinally uniaxially stretched film 10 is not limited, and known methods such as dry laminating, extruded laminating, and hot melt laminating are adopted depending on the purpose. The other film is not particularly limited, and may be a base film having a single layer or a plurality of layers, and a film having an arbitrary function such as gas barrier performance and printing performance can be used.

[Creation of uniaxially stretched film]
The longitudinally uniaxially stretched films of Prototype Examples 1 to 25 were melted and kneaded with a resin as a raw material, and substantially uniaxially stretched in the vertical direction using a coextrusion T-die film forming machine and an oven to form a film. The raw materials of Prototype Examples 1 to 8 are 100% by weight of a polymer of polypropylene alone, Prototype Examples 9 to 17 are 100% by weight of a block copolymer of ethylene-propylene, and Prototype Examples 18 to 22 are random copolymers of ethylene-propylene-brene. The polymer was 100% by weight. For Prototype Examples 23 to 25, a polymer of polypropylene alone and a block copolymer of ethylene-propylene were extruded in layers from a T-die to form a film. In each prototype example, the conditions were set to the sheet thickness (μm) before stretching, the crystallinity (%), and the stretching ratio in the table.

[Raw materials used]
The following raw materials were used as the raw material resins for forming each prototype. Raw materials 01 were used in Prototype Examples 1 to 8. Raw materials 02 were used in Prototype Examples 9 to 17. Raw materials 03 were used in Prototype Examples 18 to 22. In Prototype Examples 23 to 25, raw material 01 and raw material 04 were used. The measurement of melt flow rate (MFR) conforms to the A method of JIS K 7210-1 (2014).
(Raw material 01) Homopolypropylene (manufactured by Japan Polypropylene Corporation, trade name "FY6", MFR: 2.5 g / 10 min)
(Raw material 02) Ethylene-propylene block copolymer (manufactured by Japan Polypropylene Corporation, trade name "BC6CB", MFR: 2.5 g / 10 min)
(Raw material 03) Ethylene-propylene-butene random copolymer (manufactured by Japan Polypropylene Corporation, trade name "FW4BT", MFR: 6.5 g / 10 min)
(Raw Material 04) Ethylene-Propylene Random Copolymer (manufactured by Japan Polypropylene Corporation, trade name "WXK1233", MFR: 7.0 g / 10 min)

<Prototype example 1>
The raw material 01 was melted and kneaded, discharged from the T die at 240 ° C., molded into a sheet having a thickness of 42 μm with a cooling roll at 30 ° C., and the crystallinity was adjusted to 51%. The obtained sheet-like material was used as the film of Prototype Example 1.

<Prototype example 2>
Using the raw material 01, the temperature of the resin when discharged from the T-die was set to 240 ° C., the temperature of the cooling roll was set to 30 ° C., and a sheet-like material having a thickness of 60 μm was formed to adjust the crystallinity to 53%. The obtained sheet-like product was preheated at 135 ° C. and rolled with a draw ratio of 1.5 times to obtain a film of Prototype Example 2.

<Prototype example 3>
A film of Prototype Example 3 was obtained in the same manner as in Prototype Example 2 except that the thickness of the sheet-like material was 78 μm, the crystallinity was 54%, and the draw ratio was 2.0 times.

<Prototype example 4>
A film of Prototype Example 4 was obtained in the same manner as in Prototype Example 2 except that the thickness of the sheet-like material was 97 μm, the crystallinity was 55%, and the draw ratio was 2.5 times.

<Prototype example 5>
A film of Prototype Example 5 was obtained in the same manner as in Prototype Example 2 except that the temperature of the cooling roll was 80 ° C., the thickness of the sheet-like material was 101 μm, the crystallinity was 62%, and the draw ratio was 2.5 times.

<Prototype example 6>
A film of Prototype Example 6 was obtained in the same manner as in Prototype Example 2 except that the temperature of the cooling roll was 80 ° C., the thickness of the sheet-like material was 137 μm, the crystallinity was 63%, and the draw ratio was 3.5 times.

<Prototype example 7>
A film of Prototype Example 7 was obtained in the same manner as in Prototype Example 2 except that the thickness of the sheet-like material was 201 μm, the crystallinity was 59%, and the draw ratio was 5.0 times.

<Prototype example 8>
A film was prepared in the same manner as in Prototype Example 2 except that the temperature of the cooling roll was 80 ° C., the thickness of the sheet was 1580 μm, the crystallinity was 68%, and the stretching ratio was 5.0 times, and the film was further stretched with a tenter. The film was stretched in the lateral direction at a magnification of 8.0 times to obtain a film of Prototype Example 8 which is a biaxially stretched film having a total stretching ratio of 40.0 times.

<Prototype example 9>
The film of Prototype Example 9 was prepared in the same manner as Prototype Example 1 except that the raw material 02 was used, the thickness of the sheet-like material was 41 μm, and the crystallinity was 45%.

<Prototype example 10>
Using the raw material 02, a film of Prototype Example 10 was obtained in the same manner as in Prototype Example 2 except that the thickness of the sheet-like material was 78 μm, the crystallinity was 45%, and the draw ratio was 2.0 times.

<Prototype example 11>
The raw material 02 was used, and the temperature of the cooling roll was 80 ° C., the thickness of the sheet was 81 μm, the crystallinity was 56%, and the draw ratio was 2.0 times. I got a film.

<Prototype example 12>
Using the raw material 02, a film of Prototype Example 12 was obtained in the same manner as in Prototype Example 2 except that the thickness of the sheet-like material was 97 μm, the crystallinity was 45%, and the draw ratio was 2.0 times.

<Prototype example 13>
Using the raw material 02, the temperature of the cooling roll was 80 ° C., the thickness of the sheet was 102 μm, the crystallinity was 56%, and the draw ratio was 2.5 times. I got a film.

<Prototype example 14>
Using the raw material 02, a film of Prototype Example 14 was obtained in the same manner as in Prototype Example 2 except that the thickness of the sheet-like material was 118 μm, the crystallinity was 45%, and the draw ratio was 3.0 times.

<Prototype example 15>
Using the raw material 02, a film of Prototype Example 15 was obtained in the same manner as in Prototype Example 2 except that the thickness of the sheet-like material was 138 μm, the crystallinity was 45%, and the draw ratio was 3.5 times.

<Prototype example 16>
Using the raw material 02, a film of Prototype Example 16 was obtained in the same manner as in Prototype Example 2 except that the thickness of the sheet-like material was 158 μm, the crystallinity was 47%, and the draw ratio was 4.0 times.

<Prototype example 17>
Using the raw material 02, a film of Prototype Example 17 was obtained in the same manner as in Prototype Example 2 except that the thickness of the sheet-like material was 204 μm, the crystallinity was 46%, and the draw ratio was 5.0 times.

<Prototype example 18>
Using the raw material 03, the temperature of the resin when discharged from the T-die was set to 230 ° C., the thickness of the sheet-like material was 42 μm, and the crystallinity was set to 38%. It was made into a film.

<Prototype example 19>
Using the raw material 03, the temperature of the resin when discharged from the T-die was set to 230 ° C., the temperature of the cooling roll was set to 30 ° C., and a sheet-like material having a thickness of 81 μm was formed to adjust the crystallinity to 41%. The obtained sheet-like product was preheated at 100 ° C. and rolled with a draw ratio of 2.0 times to obtain a film of Prototype Example 19.

<Prototype example 20>
A film of Prototype Example 20 was obtained in the same manner as in Prototype Example 19 except that the thickness of the sheet-like material was 102 μm, the crystallinity was 41%, and the draw ratio was 2.5 times.

<Prototype example 21>
A film of Prototype Example 21 was obtained in the same manner as in Prototype Example 19 except that the thickness of the sheet-like material was 118 μm, the crystallinity was 40%, and the draw ratio was 3.0 times.

<Prototype example 22>
A film of Prototype Example 22 was obtained in the same manner as in Prototype Example 19 except that the thickness of the sheet-like material was 160 μm, the crystallinity was 40%, and the draw ratio was 4.0 times.

<Prototype example 23>
The raw material 01 and the raw material 4 were discharged in layers from the T-die to prepare a film of Prototype Example 23 in the same manner as in Prototype Example 1 except that the thickness of the sheet-like material was 40 μm and the crystallinity was 49%.

<Prototype example 24>
Raw material 01 and raw material 4 are discharged from the T-die in layers, the temperature of the resin when discharged from the T-die is 240 ° C., the temperature of the cooling roll is 30 ° C., and a sheet-like material having a thickness of 80 μm is formed and crystallized. The degree of crystallization was adjusted to 52%. The obtained sheet-like product was preheated at 110 ° C. and rolled to obtain a film of Prototype Example 24 at a draw ratio of 2.5 times.

<Prototype example 25>
A film of Prototype Example 25 was obtained in the same manner as in Prototype Example 24 except that the thickness of the sheet-like material was 120 μm, the crystallinity was 52%, and the draw ratio was 5.0 times.

[Sheet thickness]
When forming the longitudinally uniaxially stretched film of each prototype, the thickness (μm) of the sheet before being stretched was measured.

[Crystallinity]
The crystallinity (%) of the sheet-like material formed in the form of a sheet by a cooling roll was determined during the film formation of the longitudinally uniaxially stretched film of each prototype. The crystallinity was determined by using the Fourier transform infrared spectroscopic analyzer Spectrum Two manufactured by PerkinElmer, and using the following formula (i) by IR method (Koichi Kinda and Reikichi Iwamoto, "Material Analysis by Infrared Method-Basics and Applications". -”, Kodansha Spectroscopy, August 1986, pp. 214-215). In formula (i), A ₉₉₈ , A ₉₇₄ and A ₉₂₀ mean the absorbances at wavenumbers 998 cm ^-1 , 974 cm ^-1 and ₉₂₀ cm ^-1 , respectively.

[Stretching ratio]
It is a stretch ratio at the time of making a film of each prototype.

[Film thickness]
For each of the prototype examples, the thickness at 20 points was measured along the flow direction (MD) of the film formation, and the average thickness (t) was measured in accordance with JIS K 7130 (1999). In addition, the thickness (minimum thickness) (min) of the thinnest part and the thickness (maximum thickness) (max) of the thickest part in the film of each prototype are measured, and the difference (R) {(R). = (Max)-(min)} was calculated. The thickness difference (R) between the maximum thickness and the minimum thickness was divided by the average thickness (t) to obtain a percentage.

[Heat seal temperature]
The heat seal start temperature was measured for the longitudinally uniaxially stretched film of each prototype in accordance with JIS Z 1713 (2009). At this time, the longitudinal direction of the measurement piece (width 50 mm, length 250 mm) was defined as the stretching direction of the film. Then, the heat seal layers of the two test pieces were overlapped with each other, and a heat inclination tester (heat seal tester) manufactured by Toyo Seiki Seisakusho Co., Ltd. was used to set the heat seal pressure to 0.34 MPa and the heat seal time to 1. It was set to 0 seconds. At this time, a PET film (thickness 12 μm) for preventing fusion was sandwiched between the hot plate of the heat sealer and the test piece film. Then, heat-sealing was performed under the condition that the temperature was inclined (increased) by 5 ° C. After heat sealing, the test piece was cut out with a width of 15 mm. The test piece fused by heat sealing was opened at 180 °, and the unsealed portion was sandwiched between chucks by a tensile tester (EZ-SX) manufactured by Shimadzu Corporation, and the sealed portion was peeled off. Then, the temperature at the time when the heat seal strength reached 3N was determined.

Further, for Prototype Examples 2 to 8, the difference (ΔT) in heat seal temperature from the unstretched Prototype Example 1 was determined. Similarly, for Prototype Examples 10 to 17, the difference in heat seal temperature from the unstretched Prototype Example 9 was obtained, for Prototype Examples 19 to 22, the difference in heat seal temperature from Prototype 18 and for Prototype Examples 24 and 25 were obtained. The difference in heat seal temperature from that of Prototype Example 23 was determined. An example in which the difference in heat seal temperature (ΔT) was 25 ° C. or lower was designated as “A”, and an example in which the difference was larger than 25 ° C. was designated as “B”.

[Tearing direction]
A straight-line tear test was conducted to determine the quality of easy-opening based on the good tearability of the longitudinally uniaxially stretched film of each prototype. The straight-line tear test will be described with reference to the schematic diagram of FIG. The longitudinally uniaxially stretched film 10 is cut into a rectangular shape having a long side 51 of 300 mm in the flow direction (MD) and a short side 52 of 100 mm in the width direction (TD) during film formation to obtain a test film 50. A 100 mm notch 30 is made inside the test film 50 from the center of the short side 52 parallel to the long side, and a straight line 40 is drawn parallel to the long side 5 mm to the left and right of the notch 30. Then, the left and right ends 31 and 32 of the notch 30 of the test film 50 were pulled at a tensile speed of 6 m / min to tear the test film 50. If the tear was inside the two straight lines, it was marked with "◯", and if it protruded outside the straight line, it was marked with "x".

Tables 1 to 6 show the measurement results and evaluation results of each prototype. From the top column of the table, sheet thickness (μm), crystallinity (%), draw ratio, average thickness t (μm), thickness difference R, thickness difference ratio (%), heat seal start temperature (° C) ), The difference in the heat seal start temperature from the unstretched prototype example, its evaluation, and the determination of the tear direction are shown.

Prototype Examples 1 to 8 shown in Tables 1 and 2 are examples in which homopolypropylene is used as a raw material. First, the trial examples 2 to 4 and the trial examples 5 and 6 are compared and examined. Each prototype is a film having a low draw ratio in the range of 1.5 to 3.5 times. In the evaluation of the film thickness in the prototype examples 2 to 4, the unevenness of the film thickness is small, and in the trial examples 5 and 6, the unevenness of the thickness is large. It is considered that this is due to the difference in crystallinity when each prototype is a sheet-like product before stretching. The crystallinity of Prototype Examples 2 to 4 is 53 to 55%, whereas that of Prototype Examples 5 and 6 is 62% and 63%, respectively. Further, with respect to Prototype Examples 7 and 8 having a high degree of crystallinity, if the stretching ratio was stretched at a high stretching ratio of 5 times or 40 times, the thickness unevenness was reduced.

From these facts, it was found that when the film is stretched at a low stretching ratio, unevenness in the thickness of the film can be suppressed by setting the crystallinity low. Further, according to the comparison between Prototype Examples 2 to 4 and Prototype Examples 7 and 8, when the stretching ratio is set to a low stretching ratio, the heat seal start temperature becomes low, and Prototype Examples 2 to 4 are heat-sealed in a low temperature range. Can be done. For Prototype Examples 5 and 6, since a film having a uniform thickness could not be produced, the start temperature of the heat seal could not be measured and could not be evaluated, and was set to "-(measurable)". The same applies to the tear direction.

Next, Prototype Examples 9 to 17 shown in Tables 3 and 4 are examples in which an ethylene-propylene block copolymer is used as a raw material. According to the comparison between Prototype Examples 10 and 12 and Prototype Examples 11 and 13, similar to the results shown in Tables 1 and 2 above, Prototype Examples 11 and 13 having a high crystallinity when the draw ratios are the same are films. In the evaluation of the thickness, the unevenness of the film thickness became large, and the unevenness of the film thickness became small in the prototype examples 10 and 12 having a low crystallinity. Similarly, in the prototype examples 14 and 15 having a low crystallinity, the unevenness of the film thickness was reduced. When the draw ratio exceeds 3.5 times (4.0 times or more) as in the prototype examples 16 and 17, the heat seal start temperature rises by 25 ° C. or more as compared with the non-stretched prototype example 10. I understand. For Prototype Examples 11 and 13, since it was not possible to produce a film having a uniform thickness, the start temperature of the heat seal could not be measured and could not be evaluated, so it was set as "-(not measurable)". The same applies to the tear direction.

Prototype Examples 18 to 22 shown in Table 5 are examples in which an ethylene-propylene-butene random copolymer is used as a raw material. Even if the film raw material was changed to ethylene-propylene-butene random copolymer, the same results as those shown in Tables 1 to 4 above were shown. From these results, it was found that an increase in the heat seal start temperature can be suppressed when the draw ratio is less than 4.0 times, preferably 3.5 times or less. The lower limit of the draw ratio is considered to be 1.2 times as long as the tear direction is ensured. Further, when the crystallinity exceeds 56%, the unevenness of the film thickness becomes remarkable, so that the upper limit of the crystallinity is considered to be 55%.

Table 6 shows an example in which homopropylene and ethylene-propylene random copolymers are layered and used as raw materials. The raw materials used above tended to be similar to those of one type of prototype. Comparing Prototype 24 and Prototype 25, the heat seal start temperature of Prototype 25 having a draw ratio of 5.0 times is higher than that of Prototype 24 having a draw ratio of 2.5 times. .. Further, since the crystallinity of Prototype Examples 24 and 25 is 52%, the unevenness of the film thickness is small.

[Summary]
By verifying a series of prototype examples, it was possible to produce a film having a tearing direction by adopting a longitudinally uniaxially stretched film with little unevenness in film thickness. Therefore, it is suitable for various packaging material applications. By setting the draw ratio of the film to a low draw ratio, it is possible to suppress an increase in the heat seal temperature while maintaining the strength of the film, so that the production is easy and economical. In particular, the uniaxially stretched film having a low draw ratio was able to eliminate the drawback that the film thickness tends to be uneven. Therefore, since the longitudinally uniaxially stretched film produced by the production method of the present invention has a tearing direction while maintaining strength, it also has easy-opening properties while reliably holding the contents.

As described above, the longitudinally uniaxially stretched film produced by the production method of the present invention has little unevenness in film thickness even at a low draw ratio, and has good tearability while having strength. Further, since the draw ratio is low, it is possible to suppress an increase in the heat seal temperature, which is economical.

10 Vertical uniaxially stretched film 11 Laminated body 20 Other film 30 Notch 40 Straight line 50 Test film 51 Long side 52 Short side MD Vertical uniaxially stretched film flow direction during film formation TD Vertical uniaxially stretched film width during film formation direction

Claims (3)

A resin composition containing 70% by weight or more of a polypropylene-based resin which is an isotactic polypropylene of either a propylene homopolymer, a random copolymer obtained by polymerizing propylene and α-olefin excluding propylene, or a block copolymer is used as a raw material. age,
The melt flow rate (MFR) measured in accordance with JIS K 7210-1 (2014) of the polypropylene resin was set to 1 to 10 g / 10 min.
The raw material is melt-extruded and
It is molded into a sheet with a cooling roll and the crystallinity measured by the IR method is adjusted to 55% or less.
Next, a stretched film was formed by uniaxially stretching the stretch ratio in the longitudinal direction to 1.2 to 3.5 times with a difference in roll speed.
A polypropylene-based longitudinally uniaxially stretched film characterized by obtaining a film in which the difference between the maximum thickness and the minimum thickness in the film thickness measured in accordance with JIS K 7130 (1999) is within 20% of the average thickness. Production method. The method for producing a polypropylene-based longitudinally uniaxially stretched film according to claim 1, wherein a plurality of different raw materials selected from the resin compositions are melt-extruded in layers at the time of melt extrusion of the raw materials. A laminate comprising a sealant layer, which is a polypropylene-based longitudinally uniaxially stretched film obtained by the production method according to claim 1 or 2, and one or more layers of a base film.

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