Method for heat-bonding plastic bag and method for manufacturing plastic bag

Abstract

To provide a method for thermally bonding a plastic bag and a method for manufacturing the plastic bag capable of simultaneously achieving bag breakage prevention, sealing, and bonding strength asymptotic to the breaking force of a material.SOLUTION: A method for heat-sealing a plastic bag, comprising sandwiching a heat seal material between a pair of heating bodies and heat-sealing the heat seal material, wherein one of the pair of heating bodies has a linear protrusion having a fine semicircular or trapezoidal cross-sectional shape, and the method comprises pressing the heated linear protrusion toward a sealant of the heat seal material to inject the sealant melted in a temperature zone of cohesive bonding in a band shape along a side end of the linear protrusion to form a mold adhesive band.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to a method for thermally sealing plastic bags and a method for manufacturing plastic bags. Specifically, the present invention relates to a method for thermally sealing plastic bags and a method for manufacturing plastic bags that can simultaneously achieve tear prevention, sealing, and adhesive strength approaching the breaking strength of the material. [Background technology]

[0002] Bags / container packaging (flexible packaging) made from plastic film or sheet materials are made by thermal bonding (hereinafter also referred to as "heat sealing"), which involves pressing a heated metal body against the outer surface of the bag or using the heat generated inside the material by electromagnetic waves or ultrasonic waves, and then sealing the bag after filling it with the contents. As a result of many years of intensive research by the present inventors, technological advances in thermal bonding have been accumulating (see, for example, Patent Documents 1 to 4), expectations for thermal bonding can be summarized in the following two points. (1) Simultaneous achievement of "sealing" and "easy opening" without cutting the inner edge of the heat-sealed surface (2) Obtaining adhesive strength that approaches the material's breaking strength without edge breakage

[0003] (Basic operation of heat sealing technique) Plastic film or sheet materials are used for soft packaging products. Heat sealing techniques that utilize the thermoplasticity of plastic materials are used to make bags and seal packages after they are filled. The heat sealing technique involves pressing a heating element (heat bar) against the outer surface of the plastic material to heat the bonding surface through thermal conduction, or by using electromagnetic waves or ultrasonic waves to generate heat inside the material and heat the area near the bonding surface. The plastic material discussed here refers to a composite material in which a surface material and a sealant are bonded together (lamination).

[0004] Figure 1 shows the heat jaw system, which consists of a pair of heat bars 1-1 and 1-2, which generate heat from heaters 2-1 and 2-2 and press and heat the outer surface of material 3. The automatic heat jaw system operates several dozen times per minute, and is characterized by completing the target operation in just 0.5 to 1.0 seconds. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-313782 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-43988 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-114018 [Patent Document 4] Patent Publication No. 2021-014039 Summary of the Invention [Problem to be solved by the invention]

[0006] (Explanation of heat seal strength) The primary parameter for heat seal completion is the adhesive surface temperature. The thermal adhesive strength (heat seal strength) of heated specimens, with the controlled adhesive surface temperature as a parameter, is measured by a group of tensile tests in which both ends of a 15 mm wide cut specimen are pinched. Figure 2 shows a graph plotting the weld surface temperature on the horizontal axis and the tensile strength (N / 15mm) on the vertical axis. Since heat seal strength changes depending on the heating speed, Figure 2 shows three typical examples of heating speeds: high speed, medium speed, and low speed.

[0007] (Instructions for completing the heat seal surface) Achieving heat seal strength requires adhesion on the order of nanometers. The nanometer-scale minute uneven surface of a material can be sealed by pressing the material that has been softened by heating. ASTM F2029:2000 (the only official standard in the world that uses heating temperature as a parameter in heat sealing techniques) specifies that the pressure to be applied to the heated surface should be 0.1 to 0.4 MPa.

[0008] (Explanation of the establishment of adhesive state on heat-sealed surfaces) When examining a single heated specimen, the effect of heating rate can be ignored if the same heating rate is strictly observed. Heat seal strength begins at the interface where peeling occurs on the adhesive surface, then gradually increases until the adhesive surface becomes a molded, cohesive adhesive state in the molten state. The tensile strength in this state asymptotically approaches the breaking strength of the packaging material.

[0009] The adhesive at the interface uses the peel energy of the adhesive surface to simultaneously realize easy opening and tear resistance, so the heat seal strength is 0.5 to 10 N / 15 mm, which is much smaller than the breaking strength of the packaging material. In reality, to utilize the peel energy function, the adhesive surface needs to have a peel seal width of at least 5 mm.

[0010] Today, the expected functions of the interface adhesion state have been resolved by applying the technologies shown in Patent Documents 1 to 4. In cohesive adhesion, peeling of the adhesive surfaces does not occur, so peeling energy cannot be utilized, and the elongation energy of the material is used to absorb the energy required for bag rupture.

[0011] (Actual thermal adhesive properties of retort pouch materials) An example of analog output from a tensile test on a specimen (retort pouch material) subjected to normal flat compression heating is shown in Figure 3. To improve gas barrier properties, retort pouches have highly thermally conductive aluminum foil inserted into the surface material. This aluminum foil allows the heat flow from the heat bar to flow outside the system, so even when heated uniformly, the heat-sealed surface exhibits a characteristic temperature distribution in which the center is the highest. (1) Heating to 146°C results in a mixture of interfacial adhesion (peel seal) and cohesive adhesion on the heat seal surface. This is the heating temperature required to obtain the best heat seal strength and prevent edge tearing. (2) At temperatures below 145°C, the entire surface is in a peel-sealed state, so the edges of the heat seal do not break. (3) In high-temperature adhesive zones above 150°C, the entire surface becomes molten and cohesively bonded, resulting in a strong bond, but unevenly protruding sealant poly beads form at the heat-sealed edge. When poly beads form in a convex shape at the edge, the bag-breaking force becomes a concentrated load at the apex, easily creating pinholes even with a low bag-breaking force, which then become the starting point for bag breakage. At this point, the heat-seal width of the cohesive adhesive zone no longer affects breakage resistance. Table 1 shows the relationship between the fine protrusions on the heat-sealed edge and the bag-rupturing force (the force that causes pinholes on uneven heat-sealed edges).

[0012] [Table 1]

[0013] Table 1 shows that even on adhesive surfaces with a breaking load of 20 to 70 N / 15 mm, pinholes can easily occur with a localized load of several N, and these pinholes can become the starting point for fracture. In the case of cohesive adhesion (20N / 15mm), even if the pile width (poly ball size) is 5mm, it will become a source of bag breakage at 7N / 15mm. In the case of 70N / 15mm, if the size of the plastic ball is 1mm, pinholes will easily occur with a bag breakage source of 5N / 15mm.

[0014] (Actual measurement of peeling energy) Figure 3 shows an example of a tensile test pattern for a heat-sealed sample heated at various temperatures (example: retort pouch). Figure 4 shows an example of an evaluation test for the resistance to breakage of a heat-sealed sample (example: retort pouch sample). Based on the data in Figure 3, the resistance to breakage of the interfacial adhesive zone was analyzed using peel energy theory, and the calculation results are shown with heating temperature as a parameter. Regarding the peel energy, the measured values ​​at each peel point in the tensile test at each temperature were converted to 1 / 15 = 1 mm, and the peel energy (mJ) at each point was calculated. The verification range was integrated, and a graph was created. (1) For cohesively bonded specimens, the tensile distance to the breaking point was integrated. (2) For the interfacial adhesive specimens, the integral value up to the partial fracture point (11 mm) at 146°C was collected. (3) The integral value up to the breaking point (≒1 mm) at 170°C was set to 1, and the other calculation results were shown as "multiples of peel energy." Heating at 146°C showed approximately four times the breaking resistance of 170°C, where there is a risk of edge breakage. Looking at the starting point of peeling from the edge of the heated surface, at 146°C it starts from approximately 0.8 mm. When heated to 170°C, the residual heat caused the deformation to start at (-0.5 mm) and reach the yield point at about 1 mm. For reference, the heat seal strength data, which is an index for normal heat seal management, is also shown. From the perspective of peel energy theory, the validity of "heat seal strength" used to evaluate heat seal properties is questionable.

[0015] Figure 5 shows an example of a specimen where a tensile test of OPP (biaxially oriented polypropylene) / LLDPE (low-density polyethylene) was stopped midway and edge tearing occurred. The heat-sealed line is established, but it can be seen that pinholes / breaks originating from the heat-sealed edge have occurred at multiple points.

[0016] The inventors attempted to establish a thermal adhesive method that avoids the occurrence of pinholes by utilizing adhesive strength near the inherent breaking strength of plastic materials. This means that the molten sealant is prevented from extruding from the adhesive surface to the edges as poly beads, and precise cohesive adhesion control is achieved near the melting temperature (Tm).

[0017] (Analysis of bag rupture mechanism) The causes of composite failure can be classified into the following situations: (1) The entire body breaks suddenly (2) The adhesive strength (lamination strength) between the sealant and the surface material is related. (3) The difference in tensile strength and breaking strength of each constituent material is related. 1) (Delamination strength) < (Elongation strength of sealant) < (Elongation strength of surface material) → The sealant stretches and delamination occurs. 2) (Sealant tensile strength) > (Surface layer tensile strength) > (Delamination strength) →The surface material stretches and delamination occurs. (4) After (3)-1), the tensile strength of the sealant is greater than the tensile strength of the surface layer. → Surface layer breaks → Sealant stretches and breaks → End (5) After (3)-2), the tensile strength of the sealant becomes less than the tensile strength of the surface layer. →Sealant stretches and breaks →End In reality, most cases fall into the categories of (4) and (5), so the rupture resistance of composite materials is evaluated based on two factors: (i) the breaking strength of the surface material, and (ii) the elongation breaking strength of the sealant. (4) is related to the loss of barrier properties of the surface material, and (5) is related to preventing leakage of contents.

[0018] (Bag breakage control method) The tearing force that damages the heat-sealed edges is caused by static stacking compression forces and dynamic shocks and vibrations during logistics and storage. Since static (compressive stress) is defined as (peeling force) x (peeling length), preparations should be made so that (bag-breaking force) < (compressive stress).

[0019] In the case of a quadrilateral bag, the tearing force starts at the point of contact of the inscribed circle, and the peel line expands / progresses in an arc. In the case of a static load such as compression, the peeling starts from [(peel force) = (heat seal strength) x peel length] < (bag breaking force), and as the peeling of the heat seal surface progresses, the peeling stops when the above relationship becomes equal. If the next load is below this condition, peeling will not progress. The resistance to tearing of the bag can be controlled by selecting the heat seal strength and width.

[0020] However, impacts from drops and vibrations during transportation act locally on the heat-sealed edge in a pulse-like manner. Since multiple impacts act individually each time, there are limitations to utilizing the bag rupture resistance achieved by expanding the peel surface. The current standard, JIS Z 0238:1968 (Test method for heat-sealed flexible packaging bags and semi-rigid containers), specifies rupture resistance based on the assumption that two impact loads can be absorbed, so even greater improvements in resistance are required. It is hoped that dynamic rupture resistance will be developed through cohesive bonding (molded bonding) that utilizes the inherent rupture strength and elongation of the material.

[0021] One object of the present invention is to provide a method for thermally sealing plastic bags and a method for manufacturing plastic bags that can simultaneously achieve bag breakage prevention, sealing, and adhesive strength that approaches the breaking strength of the material. [Means for solving the problem]

[0022] (Definition of the common name of the present invention) Hereinafter, the adhesive formed by the present invention may be commonly referred to as "mold adhesive."

[0023] It is desirable to establish a method for heat-sealing plastic materials in cohesive bonded zones that eliminates edge tearing and utilizes the heat seal strength and elongation properties of the material that approach the inherent breaking strength of the material. (1) To clarify the defect conditions of the conventional surface pressing / heating method that causes poly beads to form at the heat seal edge. (2) The end side of the bag is locally molded (see Figure 6). (3) This issue is addressed during the packaging filling and sealing process and the bag making process. (4) The surface material is modeled as a micro pressure vessel and used as an injection device. (5) The heat bar's crimped portion is made into a fine semicircular or trapezoidal linear shape to create a micro injection pump. (6) The width of the heat seal fin is shortened (see Figure 7). (7) We present a direct method for reducing the amount of plastic material used as required by the SDGs. The pouch size in Figure 7 is (115mm x 150mm = 17,250mm 2 ) If the heat seal width is reduced by 9 mm by introducing "mold bonding", the result is (140 mm x 9 mm x 2) + (105 mm x 9 mm x 2) = 4,410 mm 2 The reduction rate is (4,410 / 17,250) = 26%.

[0024] The problem of the present invention can be solved in the filling and sealing process of packaging and the bag making process. (1) The outer edge 7 of the sealant is finished into a molded state 6 (see FIG. 6). (2) A portion of the sealant is locally heated, and a small amount of molten sealant is injected into the area corresponding to the outer edge of the bag, creating a new structure that prevents plastic beads from forming on the mating surface of the sealant.

[0025] An example of a method for solving the problem of the present invention will be described below. (1) A semicircular or trapezoidal fine projection 8 or 9 is provided on the heating / pressure bonding surface of the heat bar (see FIG. 9). (2) The outer edge side surface portion 17 of the sealant is heated to near the melting temperature (Tm). (3) The compression pressure 14 of the semicircular or trapezoidal protrusions is adjusted so that the melted sealant in the fine parts flows and the surface layers come into contact with each other. To prevent excessive pressure, spacers 19 and 20 are installed on both ends of the heat bar, and the compression dimension is automatically adjusted to approximately the width of two surface layers. (4) The surface material around the heated area is used as the pressure vessel 18. (5) By heating / pressing the minute semicircular or trapezoidal protrusions, the molten mold mass 15 in the pressure vessel 18 is injected into the outer edge side portion 17 of the weakly heated portion, making the side of the bag body in a "molded" state.

[0026] (6) The size of the semicircular or trapezoidal protrusion is changed to optimize the injection amount. (7) Depending on the thickness of the sealant used, the heat seal fin can be completed with approximately twice the base dimension of the semicircle or trapezoid, so 2 to 3 mm will suffice (see Figure 7). (8) A cross-sectional micrograph of an example using semicircular single protrusions (1 and 3 mm) is shown in FIG. For comparison, we have attached a video of flat compression bonding. It shows that the desired molded mass is formed even with the thin sealant OPP / LLDPE (thickness 20 μm). The sealant for retort pouch materials is 50 μm thick. When a single protrusion of 3 mm was selected, the resulting molded mass was sufficient. It was found that the desired effect could be obtained even with a single protrusion dimension of 1 mm. Weakness of the heat-sealed edges of the cohesively bonded flat-compression specimens was confirmed.

[0027] (9) The characteristics of the "mold bonding" of the present invention and the conventional flat pressure bonding method are illustrated in Figure 10. 1) The heat seal width for the flat pressure bonding method is 10 to 15 mm. When the entire surface is melted and pressure bonded, a large amount of paste-like sealant spills out unevenly onto the heat seal edge, forming poly beads. If the flat pressure bonding width is 10 mm and the heat bonding area is compared, it will be about 10 times larger (see Figure 10(b)). 2) The protruding poly beads 25 are welded 26 to the surface of the sealant 13. A bag-breaking force acts, and the intersection of the load line 27 and the poly beads becomes the pinhole generation point, leading to a sudden break. 3) The stress required for pinhole generation in this state is as shown in Table 1, and even the durable sealant 13 will break at a few Newtons. 4) Flat pressure bonding generates a large amount of molten sealant. Reducing this amount is a solution to the problem. 5) In the present invention, as shown in FIG. 9, the amount of molten sealant produced is controlled by heating and compressing the material using minute semicircular or trapezoidal protrusions, preferably about 1 mm in size.

[0028] According to the present invention, the following method for thermally sealing plastic bags can be provided. 1. A method for heat-sealing plastic bags, which includes sandwiching a heat-sealing material between a pair of heating bodies and heat-sealing it, wherein one of the pair of heating bodies has linear protrusions with fine semicircular or trapezoidal cross-sectional shapes, and the heated linear protrusions are pressed against a sealant of the heat-sealing material, thereby injecting the sealant, which has been melted in a temperature range for cohesive adhesion, in a band-like shape along the side edges of the linear protrusions, thereby forming a molded adhesive band. 2. A method for heat-sealing plastic bags as described in 1, wherein the heat-sealing material is a composite material containing a surface layer material and a sealant, and the surface layer material of the composite material is used as a pressure vessel during the injection process. 3. The method for heat-sealing plastic bags described in 1 or 2, wherein the diameter of the semicircle is 0.5 to 3 mm and the lower base of the trapezoid is 0.5 to 3 mm. 4. A method for thermally sealing plastic bags described in any one of 1 to 3, which includes adjusting the amount of molten sealant injected by changing the dimensions of the semicircle or trapezoid. 5. A method for producing a plastic bag, comprising producing a plastic bag having the molded adhesive band formed thereon using the method for thermally bonding plastic bags described in any one of 1 to 4. [Effects of the Invention]

[0029] According to the present invention, it is possible to provide a method for thermally sealing plastic bags and a method for manufacturing plastic bags that can simultaneously prevent bag breakage, seal, and achieve adhesive strength that approaches the breaking strength of the material. [Brief explanation of the drawings]

[0030] [Figure 1] FIG. 1 is a diagram illustrating an implementation model (heat jaw method) of thermal bonding (heat sealing). [Figure 2] FIG. 1 is a diagram illustrating a model for the development of heat seal strength. [Figure 3] FIG. 1 is a diagram illustrating the tensile test patterns of heat-sealed specimens heated at various temperatures (example: retort pouch). [Figure 4] FIG. 1 is a diagram illustrating an evaluation test of the fracture resistance of a heat-sealed specimen (example: retort pouch specimen). [Figure 5] This is a photo illustrating an example of a broken bag caused by OPP / LLDPE plastic balls. [Figure 6] FIG. 1 is a diagram illustrating a model of "mold adhesion." [Figure 7] FIG. 1 is a diagram illustrating the difference between the heat seal surfaces of "mold bonding" and flat bonding. [Figure 8] This is a micrograph (example) of the crimped cross section of a heat-sealed specimen with cohesive adhesion. [Figure 9] FIG. 1 is a diagram illustrating a working model of "mold bonding." [Figure 10] FIG. 1 is a diagram for comparing and explaining the characteristics of tensile tests for "mold bonding" and flat pressure bonding. [Figure 11] FIG. 1 is a diagram illustrating the tensile test pattern for "mold adhesion" of a retort pouch. [Figure 12] FIG. 10 is a diagram illustrating the "mold adhesion" pattern differential operation for a retort pouch. [Figure 13] This is a photograph of the fracture of a "mold bond" tensile test of a retort pouch material. [Figure 14] FIG. 1 is a diagram illustrating the application of "mold bonding" to an OPP / LLDPE film. DETAILED DESCRIPTION OF THE INVENTION

[0031] The method for thermally bonding plastic bags and the method for manufacturing plastic bags of the present invention will be described in detail below. In this specification, "x to y" represents a numerical range of "not less than x and not more than y." The upper and lower limits of the numerical ranges can be combined in any way. Furthermore, among the individual embodiments of the aspects of the present invention described below, it is possible to combine two or more embodiments that are not mutually contradictory, and an embodiment that combines two or more embodiments is also an embodiment of the aspects of the present invention.

[0032] A method for thermally sealing a plastic bag according to one aspect of the present invention includes the steps of: A method for heat-sealing a plastic bag, comprising sandwiching a heat-sealing material between a pair of heating elements and heat-sealing the bag, one of the pair of heating elements has linear protrusions having a fine semicircular or trapezoidal cross section; The heated linear protrusion is pressed against the sealant of the heat-sealing material, whereby the sealant melted in a temperature range for cohesive adhesion is injected in a band shape along the side edge of the linear protrusion, thereby forming a molded adhesive band. According to this aspect, it is possible to simultaneously achieve prevention of bag breakage, sealing, and adhesive strength approaching the breaking strength of the material. Furthermore, according to this aspect, the following effects can be obtained in particular. (1) An accurate heat sealing method for cohesive adhesion (mold adhesion) approaching the breaking strength of plastic materials can be perfected, ensuring strong resistance to bag breakage. (2) It will be possible to make concrete reductions in the amount of plastic used, as required by the SDGs.

[0033] An embodiment of this aspect will be described with reference to Fig. 9. Fig. 9 is an explanatory diagram of an operating model of "mold bonding." In Fig. 9, (a) is a side cross-sectional view showing a standby state of mold bonding, (b) is a side cross-sectional view showing the mold bonding during compression bonding (an example of compression bonding of semicircular protrusions), and (c) is a front view illustrating the installation of a spacer for adjusting the compression pressure.

[0034] One embodiment of this aspect can be carried out according to the following (1) to (7). (1) A fine single projection having a semicircular 8 or trapezoidal 9 cross section is provided on the heating / pressure bonding surface of the heat bar (Fig. 9(a)). In Fig. 9(a), the case where the cross section of the fine single projection is semicircular 8 is shown by a solid line, and the case where the cross section is trapezoidal 9 is shown by a dashed line. The heat sealing device used in this embodiment is equipped with a pair of heating elements arranged to face each other. Here, one heating element is composed of a heat bar body 10 and a semicircular 8 or trapezoidal 9 fine single protrusion attached to the surface of the heat bar body 10 facing the other heating element. The fine single protrusion is provided along the longitudinal direction of the heat bar body 10. The fine single protrusion can be formed by micro-machining the surface of a heat bar made of a commonly used metal such as brass, copper, aluminum, or stainless steel. The other heating element is constituted by a heat bar main body 11. The surface of the heat bar main body 11 facing one heating element is wider than the fine single protrusions. The material for the member constituting the contact surface of the heat bar main body 11 with the heat sealing material (surface layer materials 12-1, 12-2 and sealant 13) can be, for example, a resin. Resins that do not soften even at the heating temperatures used during heat sealing can be suitably used, and examples of such resins include fluororesins (e.g., polytetrafluoroethylene) and polyimide resins. Specific examples of such resins include Teflon and Kapton (both registered trademarks of DuPont). Furthermore, the member constituting the contact surface does not necessarily have to be an elastic body (e.g., an elastic body with a Shore hardness of 40A to 90A, such as silicone rubber or fluororubber) as described in Patent Document 2. The heat-sealing material placed between the pair of heaters is composed of two composite materials. Each composite material is a laminate including a surface layer material and a sealant. The two composite materials are placed with the sealant sides facing each other. The thickness of the surface layer material in one sheet of composite material can be set as appropriate, for example, 10 to 3,000 μm, preferably 20 to 2,000 μm. The surface layer material may be a single layer or a laminate of two or more layers. The surface layer material can also be referred to as a layer other than the sealant in the composite material, and can also be referred to as a "substrate layer." The thickness of the sealant in one sheet of composite material can be set as appropriate, for example, from 10 to 3,000 μm, and preferably from 20 to 2,000 μm.

[0035] At least one of the pair of heated heating bodies is moved so that the heat seal material (surface materials 12-1, 12-2 and sealant 13) is sandwiched between the pair of heating bodies. This allows the heated fine single projections to be pressed against the sealant of the heat seal material. At this time, the molten sealant is injected in a strip shape along the side edge of the fine single projections, forming a mold adhesive strip made of the mold mass 15 (Figure 9(b)). Specifically, this is as follows.

[0036] (2) The outer edge side surface portion 17 of the sealant is heated to near the melting temperature (Tm). The mold mass 15 formed by the fine single projection 8 is generated on the bag side (inside the bag) and on the outer edge side of the bag. In this embodiment, the mold mass generated on the bag side can be utilized. The "outer edge side portion 17 of the sealant" is the boundary portion between the bonded and non-bonded portions of the sealant 13 of the surface materials 12-1 and 12-2 on the inside of the bag, and is the portion of the molded mass 15 formed on the bag side that is located on the inside of the bag. The outer edge side surface portion 17 of the sealant is preferably heated to a temperature near the melting temperature (Tm) of the sealant, for example, in the range of Tm±5 to 10°C, preferably Tm to Tm+10°C, and more preferably Tm to Tm+5°C.

[0037] (3) The compression pressure 14 of the semicircular or trapezoidal projections is adjusted so that the molten sealant in the fine portions (portions compressed by the fine single projections) flows and the surface materials come into contact with each other. At this time, it is preferable to install spacers 19, 20 on both ends of the heat bar (between the heat bar bodies 10, 11) to automatically prevent excessive pressurization, and to satisfy the following condition (i) or (ii) (see FIG. 9(c)). Condition (i): Spacer height H [mm] ≒ (thickness of one surface layer [mm]) × 2 + (height of one protrusion h [mm]) Condition (ii): Spacer height H [mm] × α = (thickness of one surface layer [mm]) × 2 + (height of one protrusion h [mm]) Here, α is 0.9 to 1.1. In addition to or instead of the embodiment in which a spacer is provided (or the distance between the pair of heating bodies is adjusted when the pair of heating bodies is closest to each other), the crimping pressure 14 may be adjusted. The compression by the pair of heating bodies is driven by an air cylinder, and the crimping pressure 14 can be adjusted by the driving pressure of the air cylinder. Excessive pressure can be prevented by providing spacers (or adjusting the distance between the pair of heating bodies when they are closest to each other) or by adjusting the crimping pressure 14. Here, "excessive pressure" refers to, for example, applying pressure to the heat seal material so that the thickness becomes thinner than the combined thickness of the two surface layers of the heat seal material.

[0038] (4) The load on the fine single protrusion is 20 to 30 N / 10 mm. This load corresponds to 0.15 to 0.2 MPa for surface pressure bonding of a 15 mm width, and is not a particularly large operating force.

[0039] (5) The surface material around the heated area is used as the pressure vessel 18. That is, the heat seal material is a composite material containing a surface layer material and a sealant, and it is preferable to use the surface layer material of the composite material as a pressure vessel during injection.

[0040] (6) By heating / pressing the fine single protrusion, the molten sealant in the pressure vessel is injected into the outer edge side portion 17 of the sealant in the weakly heated portion, and the side of the bag body is put into a "molded adhesive" state. Here, the "weakly heated area" refers to an area shifted to the side from the tip of the fine projection. Although the sealant in this area is preheated by a pair of heating elements, it is heated less strongly (heat rises more slowly due to heating) than the sealant in the area pressed by the tip of the fine projection, and since it is not cohesively bonded, no polymer balls are formed. The mold adhesive band thus formed is in the form of a band along the side edge of the linear projection (fine single projection), and its width (band width) is, for example, 0.5 to 3 mm, preferably 1 to 2 mm.

[0041] (7) By changing the dimensions of the semicircular or trapezoidal shape, the amount of molten sealant injected can be adjusted. The size is determined depending on the thickness of the sealant and the desired heating speed, but the size is preferably in the range of 0.5 to 3 mm, particularly 0.25 to 1.5 mm. Here, the above dimensions can be applied as diameters in the case of semicircles, and as lower bases in the case of trapezoids.

[0042] In the example of FIG. 9, the cross-sectional shape of the linear protrusion is mainly semicircular, but the cross-sectional shape of the linear protrusion may be trapezoidal 9 as shown by the dashed line in FIG. 9(a). In the case of the trapezoid 9, it is preferable that its upper base (the side that forms the contact surface with the heat seal material) is shorter than its lower base (the side on the heat bar body 10 side). For example, if the length of the lower base of the trapezoid 9 is 100%, the length of the upper base is 20 to 80%. Each of the two interior angles (base angles) at both ends of the lower base of trapezoid 9 is preferably an acute angle, particularly preferably 45 to 80°. The two base angles may be the same or different. It should be noted that the term "trapezoid" includes not only a trapezoid but also a trapezoid with rounded corners at both ends of the upper base.

[0043] In this embodiment, it is essential that the temperature during heat sealing be set within the temperature range of cohesive adhesion (also referred to as the "temperature region"). This imparts fluidity to the sealant melted within the temperature range of cohesive adhesion, causing injection and forming a molded adhesive. As described above, the temperature range of cohesive adhesion is a temperature near the melting temperature (Tm) of the sealant, for example, in the range of Tm ±5 to 10°C, preferably Tm to Tm + 10°C, and more preferably Tm to Tm + 5°C. The molded adhesive zone can achieve strong adhesion and sealing, and can reliably seal even when the filler is liquid, for example. In contrast, the technology described in Patent Document 2 sets the heat-sealing temperature within the temperature range for interfacial adhesion (the temperature range for forming a peel seal). This peel seal achieves the goal of easy opening. In this case, a temperature range lower than the melting temperature (Tm) of the sealant is applied. Even if the sealant softens in this temperature range, it does not have enough fluidity to cause injection, and a molded adhesive zone is not formed.

[0044] A method for manufacturing a plastic bag according to one aspect of the present invention includes manufacturing a plastic bag having a molded adhesive band formed thereon using a thermal sealing method for a plastic bag according to one aspect of the present invention. According to this embodiment, the resulting plastic bag can simultaneously achieve bag breakage prevention, sealing, and adhesive strength approaching the breaking strength of the material. [Example]

[0045] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0046] (Example 1): Confirmation of injection function using a semicircular single protrusion (thin plastic material) Using the method shown in Figure 9, a "mold-bonded" specimen was prepared under the following conditions. <Specimen materials> Thin plastic material: OPP (biaxially oriented polypropylene) / LLDPE (low density polyethylene) 20μm, Tm of sealant LLDPE: 100~115℃ <Heating and compression conditions> ·Heating: Equilibrium temperature heating, 114℃, 1 second Single protrusion; semicircular cross section, diameter 1mm Crimping pressure: 100mm / 300N = 30N / 10mm

[0047] (Example 2): Confirmation of injection function using a semicircular single protrusion (retort pouch) Using the method shown in Figure 9, "mold bonded" specimens were prepared under the following conditions and evaluated. <Specimen materials> Retort pouch: PET (polyethylene terephthalate) / AL (aluminum) / CPP (non-oriented polypropylene) 50 μm, Tm of CPP sealant: 170°C <Heating and compression conditions> ·Heating: Equilibrium temperature heating, 170℃, 2 seconds ·Single protrusion: semicircular cross section, diameter 3mm Crimping pressure: 100mm / 300N = 30N / 10mm

[0048] 8 shows micrographs of Examples 1 and 2 using semicircular single protrusions (1 mm and 3 mm). For comparison, a photograph of flat surface compression bonding (flat surface adhesion) is also shown. It can be seen that the desired molded mass is formed even with a thin sealant of 20 μm OPP / LLDPE. The reinforcement effect of the heat-sealed edge is not observed in the flat-pressed specimen. The sealant for retort pouch materials is 50 μm thick. When a single protrusion of 3 mm was selected, sufficient molding mass was generated. It was found that the desired effect could be obtained even with a protrusion of 1 mm.

[0049] (Example 3): Application of "mold bonding" to retort pouches The thermal sealing of retort pouches is subject to HACCP (Hazard Analysis and Critical Control Point) regulations, and requires the most advanced response in the operation of heat sealing techniques. The material composition is PET / AL / CPP 50 μm, and Tm is 170°C. Using the method shown in Figure 9, a "mold-bonded" specimen was prepared under the following conditions. For reference, specimens were also prepared using the conventional "flat crimping" method, which does not use a single protrusion. <Heating conditions> Heating: Equilibrium temperature Heating: Each temperature shown in Figure 11, 2 seconds Single protrusion: semicircular cross section, diameter 1mm - Material specific breaking strength: 66N / 15mm

[0050] The measurement results of the tensile test patterns for the conventional "flat pressure bonding" method and the "mold bonding" method of the present invention are shown in Figure 11. Figure 12 shows the differential value calculation results of the tensile test data, and Figure 13 shows the fracture state of the surface layer material in the tensile test.

[0051] The characteristics according to the heating temperature from the graph in Figure 11 are listed below. 145℃: Tensile test response of interfacial adhesion (peel seal). Adhesion strength (20-30N / 15mm) is not uniform. 150°C: The bond width is 2 mm. Although it is a "mold bond," it breaks at a tension of 2.7 mm because it is a mixture of interfacial and cohesive bonding. 160°C to 175°C: This shows the adhesive state in the target range for "mold bonding." The response to tensile load is a combination of elongation of the material itself and fracture of the surface layer. The bag rupture mechanism corresponds to (4) defined in "(Bag rupture mechanism analysis)" above.

[0052] (Analysis of test result characteristics) *The result at 145°C is a peeling pattern for flat pressure bonding, with the entire surface being peeled and sealed. *At temperatures of 150°C and 160°C for "mold bonding," the "mold bonding" is incomplete, and the bonded parts easily break due to the peeling force, so the effectiveness of the "mold bonding" is not seen. *The characteristics of flat pressure bonding at ◆160℃~◆175℃ are that a tensile test pattern similar to that of "molded bonding" is obtained, but the heat seal edge has a curved finish and pinholes are observed. *The tensile test pattern for "Mold Adhesion" between 170°C and 175°C shows a smooth rise. The heat-sealed edge fracture strength of "Mold Adhesion" between 170°C and 175°C is enhanced. The sudden change in tensile strength occurs due to sealant fracture, a result that can be observed as expected. Since 175°C exceeds the Tm of 170°C, signs of high-temperature thermal denaturation are observed. The tensile test results for heated specimens of cohesive adhesive zones vary, but when a uniform load is applied across a 15mm width, or when there is a slight deviation and delamination or peeling occurs from the side edges, the tensile test values ​​decrease. In this case, if no fracture occurs in the "mold adhesive" part of the delaminated sealant, it is considered to be the desired result.

[0053] ●The tensile test pattern at 170°C was differentiated with respect to the tensile length to evaluate the characteristics of the progress of the tensile test. The fracture point of the surface layer material was identified by visual observation and differential values. In the test specimen, an inflection point was observed at the tensile length (initial elongation length) of [3 mm / 60 mm (initial length of specimen)], and it was determined that fracture of the surface layer occurred after this elongation. As shown in Figure 13, the response after a tensile length of 3 mm is the elongation characteristic of the sealant alone.

[0054] The resistance to rupture of the interfacial bond is [(adhesion strength) x (peel length)]. The resistance to bag breakage for "mold bonding" is [(adhesive strength) x (elongation)]. The peel length can be adjusted by the heat seal width, but the elongation length is an inherent property of the material.

[0055] A comparison was made between integral calculations of the peel seal at 145°C and the "mold adhesive" pattern at 170°C. The heat seal strength (N / 15 mm) was converted to adhesive strength (N / 1 mm), and the pull length and peel length were applied and integrated. The results are shown in Figure 12. ◆The integral value at 145℃ is the heating condition at which the material exhibits its maximum fracture resistance. ●The integral value of "mold adhesion" at 170℃ exceeds the peel energy at ◆145℃ in all areas. The analysis results in Figure 11 show that the surface layer fractures after stretching approximately 3 mm. The sealant does not break, but the fracture of the surface layer poses a risk of compromising the gas barrier properties. When evaluating the fracture resistance in this situation, the fracture resistance of the surface layer until fracture is equivalent to 3.7 mm for a peel seal, confirming the superiority of "mold adhesion."

[0056] (Example 4): Confirmation of the application of "mold adhesion" to OPP / LLDPE film This specimen (material specific breaking strength: 48N / 15mm) is a general-purpose material that is most commonly available on the market. The sealant is thin, at 20μm. Tests were conducted to confirm the suitability of "mold adhesion" for this thin material. Specimens were prepared under the conditions below using the method shown in Figure 9, and tensile tests were performed on the conventional flat pressure bonding method and the "mold bonding" method of this invention. The results are shown in Figure 14. <Heating conditions> Heating: Equilibrium temperature heating, each temperature shown in Figure 14 Heating time: 1 second Single protrusion; semicircular cross section, diameter 1mm

[0057] (Analysis of test result characteristics) (1) The flat pressure bonding temperature of 112°C indicates the interfacial adhesion state. The tensile test pattern shows the peeling characteristics. (2) The temperature of 114°C for flat pressure bonding is a cohesive adhesive zone. The tensile test pattern is disordered and there are many edge cuts. This situation is explained in Figure 10, and an example is shown in Figure 5. (3) Edges are protected from chipping even during mold bonding at 112°C. (4) At temperatures of 114°C and 116°C, a heat seal strength of 21N / 15mm can be achieved, ensuring complete "mold bonding." (5) At 118°C, the effects of overheating are evident. (6) When the response at 114°C was differentiated, an inflection point was observed at a pulling distance of 1.7 to 2.0 mm. It can be seen that the sealant began to break after this pulling distance, and reached complete failure at 2.5 mm of pulling. (7) It was found that even with thin sealant materials (20 μm), a highly sealed bond can be achieved by “mold bonding,” which is superior to flat pressure bonding. [Industrial Applicability]

[0058] The expected functions of general-purpose packaging using plastic materials are as follows: (1) The adhesive strength is close to the breaking strength of the applied packaging material, so pinholes and breakage of the heat-sealed edge do not occur due to compression or impact during distribution. (2) Sealing assurance is established. (3) Consumers want easy-to-open packages that can be picked up without using tools such as scissors. This adhesion mechanism relies on the thermoplastic properties of the plastic material. The adhesive surface of the plastic material changes from an interfacial adhesive where the adhesive surface remains to a cohesive adhesive in a molded state where the adhesive surface disappears depending on the heating temperature. The simultaneous achievement of "sealing" without tearing of the heat-sealed edge and "easy opening" has been achieved in the inventions of Patent Documents 2 and 3. However, a method based on precise logic for controlling adhesive strength equivalent to the breaking strength of the material has not yet been achieved. The present invention has succeeded in creating a molten sealant injection function at the fine heat-sealing operation site, and applying "mold bonding" to the outer edge of the bag, which prevents the plastic balls from breaking. This method can achieve a heat seal surface of about 3 mm. The SDGs set a deadline for reducing the use of plastic packaging. This invention specifically addresses this requirement. [Explanation of symbols]

[0059] 1-1: Heat Bar (1) 1-2: Heat Bar (2) 2-1: Heater (1) 2-2: Heater (2) 3: Material 4-1: Surface material (1) 4-2: Surface material (2) 5: Sealant 6: Mold mass 7: Side edge 8: Semicircular protrusion 9: Trapezoidal protrusion 10: Heat bar body (1) 11: Heat bar body (2) 12-1: Surface material (1) 12-2: Surface material (2) 13: Sealant 14: Crimping pressure 15: Mold block 16: Bag body 17: Outer edge side part 18: Pressure vessel 19: Spacer (1) 20: Spacer (2) 21: Heat seal surface 22: Heat seal surface 23: Heat seal edge 24: Heat seal edge 25: Poly ball 26: Pinhole occurrence point 27: Load line

Claims

1. A method for heat-sealing a plastic bag, comprising sandwiching a heat-sealing material between a pair of heating elements and heat-sealing the bag, One of the pair of heating bodies has linear protrusions having a fine semicircular or trapezoidal cross section, the method comprising pressing the heated linear protrusions against the sealant of the heat seal material, thereby injecting the sealant, melted at a temperature range for cohesive adhesion, in a band shape along the side edges of the linear protrusions to form a mold adhesive band.

2. the heat seal material is a composite material including a surface layer material and a sealant, In the injection, the surface layer material of the composite material is used as a pressure vessel. The method for thermally sealing plastic bags according to claim 1.

3. The diameter of the semicircle is 0.5 to 3 mm, The lower base of the trapezoid is 0.5 to 3 mm. The method for thermally sealing plastic bags according to claim 1 or 2.

4. 3. The method for thermally sealing plastic bags according to claim 1, further comprising adjusting the amount of molten sealant injected by changing the dimensions of the semicircle or trapezoid.

5. A method for manufacturing a plastic bag, comprising manufacturing a plastic bag having the molded adhesive band formed thereon using the method for thermally sealing a plastic bag according to any one of claims 1 to 4.

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