JP2005144874A - Method for producing polypropylene film good in tearing and twisting properties - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polypropylene film good in hand-cutting and twisting properties. <P>SOLUTION: In a method for producing the polypropylene film, a resin (B) having a melting point by at least 10°C higher than that of a polypropylene resin layer (A) is laminated on at least one side of the layer (A) so that the thickness of the resin (B) is 5-60% of the total thickness of the polypropylene film and. after being stretched uniaxially, is heat-treated at a temperature which is equal to or higher than a temperature by 10°C lower than the melting point of the resin layer (A) and below the melting point of the layer (B). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a polypropylene drawn film. More specifically, polypropylene stretching is useful as a packaging film or tape film with good tearing and twisting properties while maintaining practical properties without losing water resistance, which is an excellent property of polypropylene stretching films. Related to film.

  Conventionally, cellophane is known as a film having excellent cutting properties. Cellophane is widely used for various packaging materials and adhesive tapes due to its excellent transparency, easy cutting property, twist wrinkle fixability and the like. However, on the other hand, cellophane has a hygroscopic property, and its characteristics fluctuate depending on the season, and it has been difficult to always supply a product of a certain quality. In addition, packaging bags and adhesive tapes made of polypropylene as a base film are used because of the excellent properties such as toughness, water resistance, and transparency of stretched polypropylene films. Although it has excellent characteristics, it has the disadvantages that it is difficult to cut, it is difficult to tear the mouth of the packaging bag, the adhesive tape is difficult to cut, and the twist fixing property is poor, so it cannot be used for twist packaging. It was.

As a method for solving the above drawbacks, a tear film obtained by extruding a composition containing a plurality of incompatible polymers is known. For example, there is a production method in which a linear low density polyethylene and a thermoplastic resin having a specific solubility coefficient are blended at a ratio of 99/1 to 80/20 (weight ratio) and hot melt extruded (for example, patents) Reference 1). In addition, there is an easily tearable film formed by extrusion combining a high melting point resin such as nylon and a low melting point olefin resin such as polyethylene (for example, see Patent Document 2). Furthermore, uniaxially oriented polypropylene films and those using low molecular weight polypropylene resins have been proposed.
Japanese Patent Laid-Open No. 1-153733 Japanese Patent Laid-Open No. 4-19137

  However, in the above prior art, these films have low strength and cohesive force, and greatly reduce handleability and workability. Furthermore, the method of aligning in the uniaxial direction can be easily cut linearly in the alignment direction but difficult to cut in other directions, and the method using a low molecular weight polypropylene resin causes troubles in film breakage in the stretching process. It became easy to do and was not practical.

  The present invention pays particular attention to easy cutting property and twist fixing property among the properties of cellophane, and to obtain a film having these properties and further having excellent properties of polypropylene film, such as moisture resistance and transparency. This has been achieved through research.

  In the present invention, at least one side of the polypropylene resin layer (A) has a melting point higher by 10 ° C. than the melting point of the polypropylene resin layer (A) and has a thickness of 5% or more and 60% or less of the total thickness. The unstretched laminated film in which (B) is laminated is heat-treated at a temperature that is at least 10 ° C. lower than the melting point of the polypropylene resin layer (A) and lower than the melting point of the polypropylene resin layer (B) after at least uniaxial stretching. The present invention relates to a method for producing a polypropylene film having good tearability and twistability.

  That is, in the present invention, after stretching polypropylene laminated films having different melting points, the temperature is 10 ° C. lower than the melting point of the polypropylene (A) having the lower melting point and less than the melting point of the polypropylene (B) having the higher melting point. By carrying out heat treatment at a temperature, the polypropylene (A) layer loses its orientation in the stretching process and constitutes a layer having tearability and twist fixability while maintaining the properties such as heat resistance, transparency and water resistance of the polypropylene resin. In addition, the polypropylene (B) layer is a layered film having two different properties, ie, a layer having excellent properties such as the original heat resistance of the polypropylene film in which orientation is maintained. It has the opposite characteristics of having good tearability and twist fixing property while having characteristics. It found that propylene film is obtained, and have completed the present invention.

  As described above, a polypropylene film having good hand cutting properties and twisting properties is obtained, which proves effective for packaging.

  The polypropylene used for the polypropylene resin layer (B) of the present invention is not particularly limited, and examples thereof include polypropylene alone, polyethylene, and a polypropylene copolymer modified with polybutene. The polypropylene used for the polypropylene resin layer (A) is preferably a copolymer having a melting point that is 10 ° C. or more, preferably 20 ° C. or less lower than the melting point of the polypropylene used for the polypropylene resin layer (B).

  Furthermore, the thickness of the polypropylene resin layer (B) is 5% or more and 60% or less, preferably 15% or more and 50% or less of the total thickness. When the thickness of the polypropylene resin layer (B) is less than 5% of the total thickness, the strength of the obtained film is lowered, which impedes practical use. On the other hand, if the thickness of the polypropylene resin layer (B) exceeds 60% of the total thickness, the intended tearability and twistability are lowered. Further, the lamination of the polypropylene resin layer (A) and the polypropylene resin layer (B) may be either a three-layer (B / A / B) or a two-layer (B / A) configuration. Further, the thickness of the stretched film is not particularly limited, although the thickness of the film used in the packaging bag or the adhesive tape which is the intended use of the present invention is 12 μ to 30 μ.

  The polypropylene film of the present invention may contain various known additives such as lubricants, pigments, antioxidants, antistatic agents and the like as long as the effects of the present invention are not impaired.

  The unstretched laminated film here refers to a method of laminating unstretched films that are melted separately at a temperature equal to or higher than the melting point and extruded from a die outlet in a plurality of extruders and the like in a heated state. It is done. As another method, there is a method of melt laminating the other molten film on the surface of one unstretched film. As another method, there is a method of forming an unstretched film by extruding from a die outlet in a state of being laminated by a coextrusion method.

  Next, an example of the manufacturing method of this invention film is demonstrated. Supply hot air-dried polypropylene resin (A) and polypropylene resin (B) to two different extruders, melt and extrude them at a temperature equal to or higher than their melting points, pass through the composite adapter, and make two types of three layers (B / A / B) or two types and three layers (B / A / B) are extruded from a die and cooled and solidified to form an unstretched laminated film.

  In the stretching step, the film can be stretched at an area magnification of about 8 to 50 times, preferably about 10 to 40 times. Further, the stretching method is not limited to uniaxial stretching or biaxial stretching. In the case of biaxial stretching, it can be carried out by simultaneous biaxial stretching method, sequential biaxial stretching method, inflation method or the like. Axial stretching is common.

  As conditions for carrying out sequential biaxial stretching, first, the longitudinal method is stretched about 3 to 8 times between rolls having a peripheral speed difference heated to 100 to 150 ° C., and then using a tenter stretching machine in the width direction. The film is stretched about 4 to 10 times at a temperature of about 140 to 170 ° C.

  This stretched film is heat-treated at a temperature higher than the melting point of the polypropylene resin layer (A) and lower than the melting point of the polypropylene resin layer (B). In this heat treatment, it goes without saying that a relaxation treatment may be performed as necessary.

  Further, the film of the present invention is subjected to corona discharge treatment, plasma treatment, ultraviolet irradiation treatment, flame treatment, etc. to improve the adhesiveness without any trouble with respect to the present invention. May be performed by so-called in-line processing performed in the film manufacturing process, or may be performed by so-called off-line processing in which the manufactured film is processed as a subsequent process.

  The polypropylene resin layer (A) is almost completely collapsed by stretching due to the previous heat treatment, and the tearing and twisting properties of the present invention are obtained, and the polypropylene resin layer (B) maintains the molecular orientation. It is considered that a film having the characteristics of the present invention can be obtained.

  As described above, the present invention provides a target film by a balance between a layer that imparts tearing and twisting properties in which molecular orientation is almost collapsed by heat treatment in a film-forming line, and a layer that has intrinsic properties of polypropylene that maintains molecular orientation. In addition to the advantage that the characteristics can be freely set, there is an advantage that the trouble of breakage in film formation can be prevented because of the existence of the layer maintaining the molecular orientation.

  Hereinafter, the present invention will be described by way of examples. About the evaluation method in an Example and a comparative example, it carried out by the method of (a), (b), (c).

(A) Melting point, crystallization temperature: A single layer film having the same composition as the base layer (A) and the surface layer (B) was prepared, and the obtained film was used in accordance with JIS K-7121. The film was heated by scanning calorimetry, the melting peak temperature was measured to obtain the melting point, cooled, and the crystallization peak temperature was measured to obtain the crystallization temperature.

(B) Hand cutting ability: It was evaluated by a sensory test, and when a tape-like sample having a width of 15 mm was cut by hand, a sample that could be easily cut by hand was indicated by ◯, and a sample that could not be easily cut by hand was indicated by ×.

(C) Twistability: Performed by a sensory test, when a tape-like sample having a width of 30 mm was twisted by hand, a sample that did not return to its original state even when twisted was indicated by ◯, and a sample that could not maintain the twisted state was indicated by ×.

  Example 1 Polypropylene (A) having a melting point of 135 ° C. and polypropylene (B) having a melting point of 163 ° C. were melted by separate extruders at a temperature of 260 ° C., and the melt was joined by a composite adapter, and then from a T-die. Extrusion and quenching with a cooling drum (B / A / B) gave a three-layer unstretched laminated film.

  The unstretched laminated film was first stretched 4.5 times at 130 ° C. in the machine direction and then 8.5 times at 155 ° C. in the transverse direction, and then heat treated at a temperature of 160 ° C. while relaxing 3%. I got the film. The thickness ratio of each B / A / B layer of this film was 2/21/2.

  The film thus obtained could be easily cut in any direction as long as the nails were applied, and when the film was twisted, the twisted state could be maintained. Further, this film had no troubles such as breakage during film formation and slitting, and the productivity was good.

(Example 2)
A 25 μm film was obtained in which only the thickness ratio of each layer of B / A / B was changed to 4/17/4 by the same raw materials and method as in Example 1. The film thus obtained also had a good twist fixing property except that a film having a hand cutting property slightly more resistant than Example 1 was obtained.

(Comparative Example 1)
A 25 μm film was obtained in which only the thickness ratio of each layer of B / A / B was changed to 10/5/10 by the same raw materials and method as in Example 1. The film thus obtained was not cut by hand, and even when the film was twisted, it returned to its original shape and was not twist-fixed.

(Comparative Example 2)
Except for changing the melting point of polypropylene (A) to 155 ° C., a 25 μm film was obtained in the same manner, conditions and thickness ratio as in Example 1. The film thus obtained was not cut by hand, and even when the film was twisted, it returned to its original shape and was not twist-fixed.

(Comparative Example 3)
After stretching with the same raw material, method and thickness ratio as in Example 1, a 25 μm film was obtained that was heat-treated at a temperature of 100 ° C. while relaxing 3%. The film thus obtained was not cut by hand, and even when the film was twisted, it returned to its original shape and was not twist-fixed.

(Comparative Example 4)
After stretching with the same raw material, method and thickness ratio as in Example 1, heat treatment was performed at a temperature of 170 ° C. while relaxing 3%, but no film was obtained by melting.

  Table 1 shows the evaluation results of the films obtained in Examples and Comparative Examples.

  The polypropylene film of the present invention provides a polypropylene film with good hand cutting properties and twistability, and can be used in the field of packaging applications and contributes greatly to the industry.

Claims (3)

  A polypropylene resin layer (B) having a melting point higher by 10 ° C. or more than the melting point of the polypropylene resin layer (A) on at least one surface of the polypropylene resin layer (A) and having a thickness of 5% or more and 60% or less with respect to the total thickness. Tearability characterized by heat-treating an unstretched laminated film laminated with a) at least at a temperature lower by 10 ° C. than the melting point of the polypropylene resin layer (A) and below the melting point of the polypropylene resin layer (B) after uniaxial stretching. And a method for producing a polypropylene film with good twist.   2. The laminated polypropylene film according to claim 1, wherein the polypropylene resin layer (A) and the polypropylene resin layer (B) have a (B) / (A) or (B) / (A) / (B) configuration. A laminated polypropylene-based film, wherein the crystallization temperature of the polypropylene resin layer (A) is 90 to 110 ° C., and the crystallization temperature of the polypropylene resin layer (B) is higher than that of the polypropylene resin layer (A). Laminated polypropylene film.   2. The laminated polypropylene film according to claim 1, wherein the polypropylene resin layer (A) contains a modified polypropylene resin as a main component and the melt index (MI) of the modified polypropylene resin of the polypropylene resin layer (B). Is 5 g / 10 min or more larger than the melt index (MI) of the polypropylene resin of the base material layer (A), and the MI of the modified polypropylene resin of the surface layer (B) is 8 g / 10 min or more. A laminated polypropylene film.