JP6251891B2 - Method for producing low warpage multilayer film - Google Patents

Description

The present invention is a multilayer film used as a packaging material for foods and the like formed by a coextrusion water-cooled inflation method, and has a low warpage multilayer film that can exhibit excellent handling properties with little warpage at a cutting portion. It is related with the manufacturing method .

Conventionally, a multilayer film in which different kinds of resins are laminated is widely used as a packaging material for food and industrial parts. Many such multilayer films use a polyamide resin layer as the outermost layer because they have good gloss and slipperiness on the film surface, are easy to print and have high oxygen barrier properties. On the other hand, when a resin film is made into a bag or when an opening is sealed, since a simple and reliable heat sealing method is generally used, the innermost layer of the film has a low melting point and is transparent. Polyolefin resins having good heat sealability are often used.
And the cylindrical film obtained by a water-cooled inflation molding method is bagged by welding the part cut | disconnected perpendicularly | vertically with the shaping | molding direction, and is provided for utilization.

  The polyamide resin and the polyolefin resin are generally used as a multilayer film via an adhesive resin layer because they are difficult to adhere to each other when coextruded by a water-cooled inflation molding method. It is also known to add an ethylene-vinyl alcohol copolymer layer in order to provide a higher oxygen barrier property.

However, in the case of a multilayer film in which the outermost layer is a polyamide resin layer and the innermost layer is a polyolefin resin layer, the cylindrical film formed by the co-extrusion water-cooled inflation method has a molding direction of the bag material cut to a certain length. There was a phenomenon in which the cut surface was warped to the outer layer side so that the cut surface turned up.
When such warpage occurs, when the bags are stacked in a bag making process or the like in which the cut portions of the bag material are heat-sealed, the vertical side that is the opening side swells and a large number is stacked. In addition, there is a problem in that the warped portion is caught even when the contents are filled, resulting in a problem in the supply of the bag, and there is a problem that the handleability of the film is inferior.

  Here, the low warpage refers to a warp that does not cause the above-described troubles in using a cylindrical film formed by water-cooling inflation as a bag. In addition, as a standard of the degree of warping that does not hinder the work, a warpage degree index (curved radius) described later is at least 15 mm or more.

Thus, a combination of a highly hygroscopic nylon layer and a low hygroscopic nylon layer has been proposed as a multilayer film that hardly warps (see, for example, Patent Document 1). In addition, as shown in Patent Document 2, the present applicant developed a film in which a material having rubber elasticity was laminated to prevent warpage in order to use molding distortion, and filed a patent application first. However, since the films described in any of the patent documents use expensive materials, there remains a problem that the cost increases.
Other known structures include a low-warp multi-layer film in which polyamide is used for the intermediate layer that is symmetric with the outer layer of polyamide and polyolefin, and the outer layer and polyolefin are used for the inner layer. Therefore, there is a problem that it is difficult to handle because heat care needs attention.

JP-A-7-227944 JP 2013-22893 A

As a result of examining the cause of the warping phenomenon, the present inventor speculated that the warping phenomenon occurs due to the following reasons in addition to environmental effects such as temperature and humidity.
FIG. 6 is an explanatory diagram for explaining the cause of the warping phenomenon in the conventional example. In FIG. 6, 1 is a polyamide resin layer on the outer layer side, 2 is an olefin resin layer on the inner layer side, and 3 is an adhesive resin layer for bonding these resin layers.
As shown in FIG. 6, when a multilayer film is produced by the coextrusion water cooling inflation method, the coextruded resin in a molten state is extruded from a circular die and then cooled by contacting the outer surface of the cylindrical film with cooling water. Solidify. At that time, the outermost polyamide resin layer 1 is in a state of moisture absorption by the cooling water in direct contact (left figure). Next, it is considered that the polyamide resin layer 1 in a moisture-absorbing state is in a moisture absorption state higher than the equilibrium water absorption rate, and moisture is gradually released to reach the equilibrium water absorption rate (center diagram). As a result, the polyamide resin layer 1 decreases in volume and generates a contracting force as indicated by an arrow (right diagram). It is presumed that the warp is developed toward the outside by the action of the contraction force.

Based on the above knowledge, the present invention solves the conventional problems, eliminates the occurrence of warping due to moisture release of the polyamide resin layer, and can exhibit excellent handleability in the bag making process and filling process. The present invention has been completed for the purpose of providing a method for producing a low warpage multilayer film.

In order to solve the above-mentioned problems, the invention of claim 1 is directed to a multilayer film produced by a co-extrusion water-cooled inflation method, comprising a multilayer film comprising at least two layers of a polyamide resin layer as an outermost layer and an olefin resin layer as an innermost layer. The roll body around which the tube film after film formation is heated is subjected to heat treatment, and a shrinkage force is generated by releasing the residual strain of the olefin-based resin layer. It is a method for producing a low warpage multilayer film characterized in that it is balanced with the shrinkage force of the polyamide resin layer to be shrunk by releasing into the atmosphere.

The heat treatment after the film formation is preferably carried out in the glass transition temperature higher than the polyamide resin, the invention of claim 2 it.

The invention according to claim 1 is a multilayer film manufacturing method for manufacturing a multilayer film comprising at least two layers of a polyamide resin layer as an outermost layer and an olefin resin layer as an innermost layer by a coextrusion water-cooled inflation method. Heat treatment is applied to the roll body wound with the tube film after film formation, and the shrinkage force is generated by releasing the residual strain of the olefin resin layer, and the shrinkage force releases moisture to the atmosphere. Since the shrinkage force of the polyamide resin layer to be shrunk is balanced, it is possible to efficiently produce a multilayer film that does not warp at a low cost.

In the invention which concerns on Claim 2 , since heat processing after film forming is performed at the temperature more than the glass transition point of a polyamide resin, generation | occurrence | production of curvature can be suppressed exactly by simple heat processing.

It is an expanded sectional view of the film manufactured by this invention. It is explanatory drawing for demonstrating reduction of the film curvature of this invention. It is explanatory drawing explaining the state of the film wound up continuously used as a shape when relieving the residual distortion of a film. It is explanatory drawing which shows a curl evaluation method. It is explanatory drawing which measures a curvature radius. It is explanatory drawing for demonstrating the generation | occurrence | production cause of the film curvature in a prior art example.

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a basic multilayer film composed of three layers produced by a coextrusion water-cooled inflation method, wherein 1 is a polyamide resin layer located on the outermost side, and 2 is an olefin resin located on the innermost side. Is a layer. Since the olefin resin layer 1 and the polyamide resin layer 2 are difficult to adhere to each other when coextrusion molding is performed, the olefin resin layer 1 and the polyamide resin layer 2 have a structure in which they are laminated via an adhesive resin layer 3. In addition, since this adhesive resin layer 3 is a general thing like adhesive polyethylene (adhesive PE), it abbreviate | omits in the following description.

  FIG. 2 is an explanatory diagram for explaining the cause of the current low warpage phenomenon. In FIG. 2, 1 is a polyamide resin layer on the outer layer side, 2 is an olefin resin layer on the inner layer side, and 3 is an adhesive resin layer that adheres these resin layers.

The polyamide resin layer 1 is generally known as a general-purpose resin having high strength and high heat resistance, and excellent in oxygen barrier properties and surface glossiness, and is suitable for the outermost layer of a bag.
Specifically, copolymer nylon such as polyamide 6 (PA6) and polyamide 6.66 (PA6.66) is mainly used.

The olefin-based resin layer 2 has a melting point lower than that of the polyamide resin layer 1, is excellent in low-temperature heat sealability, and is excellent in transparency, and is suitable for the innermost layer of the bag.
Specific examples include low density polyethylene (LDPE), linear low density polyethylene (L-LDPE), ethylene-vinyl acetate copolymer (EVA), and ionomer resin.

As described above, the present inventor presumed that, in the conventional case, the outermost polyamide resin layer 1 shrinks by releasing the cooling water, thereby generating curl (warping) toward the outside.
Therefore, in the present invention, the residual strain is released by the shrinkage force of the polyamide resin layer 1 that is intended to shrink by releasing moisture absorbed from the cooling water during molding into the atmosphere and the heat treatment after film formation. The present invention has been completed from the idea that curling can be prevented if the shrinkage of the generated olefin resin layer 2 is balanced.

  Thus, in the present invention, the shrinkage force of the polyamide resin layer 1 and the shrinkage force of the olefin-based resin layer 2 generated by releasing residual strain are balanced and balanced, As described above, only the polyamide resin layer 1 contracts and curls outwardly is accurately suppressed.

FIG. 2 is an explanatory diagram illustrating a process in which the shrinkage force of the polyamide resin layer 1 and the shrinkage force of the olefin-based resin layer 2 are balanced.
As shown in the left diagram of FIG. 2, the film formed by the coextrusion water-cooled inflation method has a shrinkage force that tends to shrink by releasing moisture absorbed from the cooling water being formed into the polyamide resin layer 1 into the atmosphere. Will occur and will be warped outward. Therefore, when this film is subjected to a predetermined heat treatment to release the residual strain, a shrinkage force is generated in the olefin-based resin layer 2 as shown in the central view of FIG. As shown in the right diagram of FIG. 2, if the shrinkage force generated in the olefin resin layer 2 and the shrinkage force of the polyamide resin layer 1 are balanced, the film will not be warped. .

Thus, in order to balance the shrinkage force of the polyamide resin layer 1 and the shrinkage force of the olefin resin layer 2, for example, as shown in FIG. 3, a roll body in which the tube film after film formation is wound up 10 may be subjected to heat treatment (annealing treatment).
In addition, the tube film can be cut to form a sheet, the sheet can be wound up into a roll, the sheet can be laminated, and the like can be subjected to heat treatment.
In this heat treatment, the treatment temperature and the treatment time are the points. When the temperature is low, the shrinkage force of the polyamide resin layer 1 is not balanced and the warpage cannot be eliminated. On the other hand, the higher the treatment temperature, the easier the residual strain is released, and the amount of shrinkage increases. If the temperature is too high, the shrinkage force cannot be balanced and the warpage may be inwardly warped.

As a result of various experiments on the treatment temperature, when polyamide 6 is used, heat treatment at a temperature higher than the glass transition point (50 ° C.) has a high effect of reducing the degree of warp, and the crystal spheres of polyamide 6 are elongated. As a result, the frictional resistance of the surface is reduced, and it has been confirmed that the surface is more suitable as a packaging bag. In addition, the heating of the temperature within plus 30 degrees C is preferable rather than a glass transition point. If the temperature is higher than this, internal warping may occur.
On the other hand, the heat treatment can be efficiently performed on the roll body 10 wound up continuously. However, since plastic has low thermal conductivity, it needs to be processed for a sufficient time in order to sufficiently transmit the temperature to the winding core and release the residual strain.

<Film forming equipment>
A water-cooled coextrusion inflation molding machine comprising three extruders and one circular three-layer die was used. Further, a box type hot air dryer (PO-80) manufactured by Matsui Seisakusho was used as a heat treatment furnace.

<Configuration of multilayer film>
(1) Composition of each layer A three-layer water-cooled coextruded tube film having the composition shown in Table 1 below was molded. The film was compared with the one that was not heat-treated.

<Processing conditions>
(1) Temperature conditions (a) First layer PA6 ... 240 ° C
(B) Second layer Adhesive PE ... 190 ° C
(C) Third layer L-LDPE ... 200 ° C
(D) Circular die: 240 ° C
(2) Molding conditions (a) Blow ratio 0.9
(B) Cooling water temperature 23 ° C
(C) Film drying temperature in the film forming process 50 ° C
(D) Film thickness 0.07mm
(3) Heat treatment conditions (a) Heating temperature Unheated (room temperature), 40 ° C., 50 ° C., 60 ° C., 70 ° C. (b) Heating time 24 hours (c) Heating target Continuously winding the film Executed with rolled product (roll body)

<Evaluation of curling properties>
(1) Evaluation method of curling property Heat treatment was performed in a state in which the formed tube-shaped film was continuously wound (roll body), the cut portions at both ends were removed, and a test piece for evaluation was collected. As shown in FIG. 4, the curl evaluation method at that time draws a 10 × 10 cm square at the center of the film, cuts the diagonal line, and measures the degree of warpage of each bisected triangle. As shown in FIG. 5, the curving radius r (mm) of the curled film was measured to evaluate the degree of warpage. The smaller the curvature radius r (mm), the higher the degree of warping.
The evaluation environment was 25 ° C. and 50% relative humidity.
(2) Evaluation results Table 2 shows the measurement results of the presence or absence of warpage and the bending radius r (mm).

As is apparent from the above examples, when the coextruded film obtained by the water-cooled inflation molding method comprising polyamide and polyethylene is heat-treated by the method of the present invention, the film at this time is warped in the molding direction. There was no occurrence, and it was confirmed that a low warpage multilayer film exhibiting excellent handleability was obtained.
However, when the film is continuously wound, both side cut portions at the beginning and end of the winding are severely curled. This difference is considered to be the difference between the both-side cut portion where the shrinkage during the heat treatment is in an open state and the continuous portion that can be held in a tension state.

1 Polyamide resin layer 2 Olefin resin layer 3 Adhesive resin layer 10 Roll body

Claims (2)

A multilayer film manufacturing method for manufacturing a multilayer film comprising at least two layers of a polyamide resin layer as an outermost layer and an olefin resin layer as an innermost layer by a co-extrusion water-cooled inflation method, and winding the tube film after film formation The polyamide resin layer which is subjected to heat treatment on the rolled body and generates a shrinkage force by releasing the residual strain of the olefin resin layer, and the shrinkage force tries to shrink by releasing moisture into the atmosphere. A method for producing a low-warp multilayer film, characterized in that the shrinkage force is balanced. The manufacturing method of the low curvature multilayer film of Claim 1 which performs the heat processing after film forming at the temperature more than the glass transition point of a polyamide resin.

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