Thermal bonding method for plastic bag and method for producing plastic bag

Abstract

Provided is a thermal bonding method for a plastic bag which includes disposing 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 fine linear protrusion having a semicircular or trapezoidal cross-sectional shape. The method includes pressing the linear protrusion having been heated toward a sealant of the heat seal material, thereby injecting the sealant melted at a temperature in a range for cohesive bonding so that the sealant comes to have a strip shape extending along the lateral edges of the linear protrusion, and forming a mold bonding strip.

Description

Method for thermally bonding plastic bags and method for manufacturing plastic bags

[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 prevent bag breakage, seal, and achieve adhesive strength that approaches the breaking strength of the material.

[0002] Bags / container packaging (flexible packaging) using 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 to be bonded or utilizing the heat generated inside the material by electromagnetic waves or ultrasonic waves, and the bag is then sealed after being filled with the contents. Through many years of intensive research by the inventor, technological advances in thermal bonding have been accumulating (see, for example, Patent Documents 1 to 4), and expectations for thermal bonding can be summarized in the following two points: (1) Simultaneous achievement of "sealing" and "easy opening" without edge breakage on the inside of the thermal bonding surface; (2) Acquisition of adhesive strength without edge breakage that approaches the breaking strength of the material.

[0003] (Basic operation of heat sealing technique) Plastic film and sheet materials are used for flexible packaging products. Heat sealing techniques that take advantage of the thermoplastic properties of plastic materials are used to make bags and seal packages after filling. Heat sealing techniques involve 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 internal heat generation in the material via electromagnetic waves or ultrasonic waves to heat the area near the bonding surface. Here, the plastic material refers to a composite material in which a surface material and a sealant are bonded together (lamination).

[0004] Figure 1 shows a heat jaw system that 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 the 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.

[0005] JP 2007-313782 A JP 2016-43988 A JP 2017-114018 A JP 2021-014039 A

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

[0007] (Explanation of completing the heat seal surface) Completing the heat seal surface requires adhesion on the order of nanometers. The fine nanometer-scale irregularities of the material can be sealed by pressing the material 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, if the same heating rate is strictly observed, the effect of the heating rate can be ignored. The rise in heat-seal strength begins with interfacial adhesion, where peeling occurs on the adhesive surface, and gradually increases until the adhesive surface becomes a molded, cohesive adhesive state in a molten state. The tensile test strength in this state asymptotically approaches the breaking strength of the packaging material.

[0009] The adhesion of the interfacial adhesive zone utilizes 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 far 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 function of the interface adhesion state has been resolved by applying the techniques 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 Sealing Characteristics of Retort Pouch Materials) Figure 3 shows an example of analog output from a tensile test on a specimen (retort pouch material) heated using standard flat-surface compression. Highly thermally conductive aluminum foil is inserted into the surface layer of retort pouches to improve gas barrier properties. This aluminum foil allows the heat flow from the heat bar to escape the system, resulting in a temperature distribution with the highest temperature at the center of the heat-sealed surface, even when heated uniformly. (1) Heating to 146°C results in a mixture of interfacial adhesion (peel seal) and cohesive adhesion on the heat-sealed surface. This heating achieves the highest heat-seal strength and prevents edge tearing. (2) At temperatures below 145°C, the entire surface remains in a peel seal state, preventing edge tearing. (3) At high-temperature adhesive zones above 150°C, the entire surface becomes molten and cohesively bonded, resulting in a strong bond, but unevenly distributed poly sealant beads form at the heat-sealed edges. When the poly beads are formed in a convex shape at the edge, the bag-breaking force becomes a concentrated load at the apex, and pinholes can easily form even with a low bag-breaking force, which then becomes the starting point for bag-breaking. At this point, the heat-seal width of the cohesive adhesive zone no longer affects the break resistance. The relationship between the fine protrusions on the heat-sealed edge and the bag-breaking force (the force that causes pinholes on an uneven heat-sealed edge) is shown in Table 1.

[0012]

[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 act as the starting point for breakage. In the case of cohesive adhesion (20 N / 15 mm), even a 5 mm peak width (polyethylene ball size) can become a 7 N / 15 mm bag breakage source. In the case of 70 N / 15 mm, pinholes can easily occur with a 1 mm polyethylene ball size and a 5 N / 15 mm bag breakage source.

[0014] (Actual Measurement of Peel Energy) Figure 3 shows an example of a tensile test pattern for a heat-sealed sample (retort pouch material) heated at various temperatures. Figure 4 shows an example of an evaluation test for the fracture resistance of a heat-sealed sample (retort pouch material). Based on the data shown in Figure 3, the fracture resistance of the interfacial adhesive zone was analyzed using peel energy theory, with heating temperature used 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 test range was integrated, and a graph was created. (1) For cohesively bonded samples, the tensile distance to the fracture point was integrated. (2) For interfacially bonded samples, the integrated value up to the partial fracture point (11 mm) at 146 °C was taken. (3) The integrated value up to the fracture 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 shows approximately four times the tear resistance of 170°C, which is at risk of edge tearing. Looking at the peel initiation point from the edge of the heated surface, at 146°C it begins at approximately 0.8 mm. When heated at 170°C, due to the influence of residual heat, it begins at (-0.5 mm) and reaches the yield point at approximately 1 mm. For reference, heat seal strength data, which is an index used in normal heat seal management, is also shown. From the perspective of peel energy theory, it is clear that there is a problem with the suitability of "heat seal strength" used to evaluate heat seal properties.

[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 was 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 pinhole formation by utilizing adhesive strength near the inherent breaking strength of plastic materials. This is to prevent the molten sealant from extruding as beads from the adhesive surface to the edges, and to achieve precise cohesive adhesion control near the melting temperature (Tm).

[0017] (Analysis of bag rupture mechanism) The causes of composite fracture can be classified into the following situations. (1) The entire material fractures all at once. (2) The adhesive strength (lamination strength) between the sealant and the surface material is involved. (3) The differences in the tensile strength and fracture strength of each constituent material are involved. 1) (Delamination strength) < (Delamination strength of sealant) < (Delamination strength of surface material) → The sealant stretches, and delamination occurs. 2) (Delamination strength) > (Delamination strength) > (Delamination strength) → The surface material stretches, and delamination occurs. (4) After (3)-1), the tensile strength of the sealant exceeds the tensile strength of the surface material → Surface material fractures → Sealant elongates and fractures → End. (5) After (3)-2), the tensile strength of the sealant becomes less than the tensile strength of the surface material → Sealant elongates and fractures → End. In reality, most cases are (4) and (5), so the rupture resistance of composite bags is evaluated based on two factors: (i) the breaking strength of the surface layer material, and (ii) the elongation breaking strength of the sealant. (4) is related to the loss of barrier performance of the surface layer material, and (5) is related to preventing leakage of the contents.

[0018] (How to control bag breakage) The bag breakage force that damages the heat-sealed edges is caused by static stacking compression force during logistics and storage, and dynamic impact and vibration. It is defined as static (compression stress) = (peel force) x (peel length), so preparations should be made so that (bag breakage force) < (compression stress).

[0019] In the case of a quadrilateral bag, the bag rupture force begins 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 peel begins from [(peel force) = (heat seal strength) x peel length] < (bag rupture force), and the peeling of the heat-sealed surface progresses, and when the above relationship becomes equal, the peeling stops. For the next load, if the bag rupture force is below this condition, the peeling will not progress. Bag rupture resistance can be controlled by selecting the heat seal strength and heat seal width.

[0020] However, drop impacts and vibrations during transportation act locally on the heat-sealed edge in a pulse-like manner. Since multiple impacts act individually, there are limitations to utilizing the expansion of the peel surface to improve bag rupture resistance. The current standard, JIS Z 0238:1968 (Test Methods for Heat-Sealed Flexible Packaging Bags and Semi-Rigid Containers), specifies rupture resistance based on the assumption that the bag can absorb two impact loads, so greater resistance improvement is required. It is expected that dynamic rupture resistance will be developed through cohesive bonding (molded bonding) that utilizes the material's inherent rupture strength and elongation.

[0021] One of the objects 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.

[0022] (Definition of Common Names Used in the Present Invention) Hereinafter, the common name for the adhesion formed by the present invention may be referred to as "mold adhesion."

[0023] In the thermal sealing operation of cohesive adhesive zones of plastic materials, it is desirable to establish the following measures to eliminate edge breakage and utilize the heat seal strength approaching the inherent breaking strength of the material and the elongation characteristics of the material: (1) Identify the defect conditions of the conventional surface crimping / heating method that results in the formation of plastic beads at the heat seal edge. (2) Localize the side edge of the bag into a molded state (see Figure 6). (3) Address this issue in the packaging filling / sealing process and the bag making process. (4) Imitate the surface material as a minute pressure vessel and use it as an injection device. (5) Shape the crimped part of the heat bar into a minute semicircular or trapezoidal linear shape to create a micro injection pump. (6) Reduce the width of the heat seal fin (see Figure 7). (7) Propose a direct method to reduce 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 in area is (4,410 / 17,250) = 26%.

[0024] The objective of the present invention can be achieved through the filling and sealing process and the bag making process: (1) The outer edge 7 of the sealant is finished into a molded state 6 (see Figure 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 sealant joint surface.

[0025] An example of a method for solving the problems of the present invention is as follows: (1) A single fine semicircular or trapezoidal protrusion 8 or 9 is attached to the heating / compression surface of the heat bar (see Figure 9). (2) The outer edge side portion 17 of the sealant is heated to near the melting temperature (Tm). (3) The compression pressure 14 of the semicircular or trapezoidal protrusion is adjusted so that the molten sealant in the fine portion flows and the surface layers come into contact with each other. To prevent over-compression, spacers 19 and 20 are installed on both ends of the heat bar, automatically adjusting the compressed dimension to approximately the equivalent of two layers of surface material. (4) The surface material around the heated area is used as a pressure vessel 18. (5) By heating / compressing the fine semicircular or trapezoidal protrusion, the molten mold mass 15 inside the pressure vessel 18 is injected into the outer edge side portion 17 of the weakly heated area, creating a "molded bond" on the side of the bag body.

[0026] (6) The injection volume can be optimized by changing the dimensions of the semicircular or trapezoidal protrusions. (7) Depending on the thickness of the sealant used, the semicircular or trapezoidal shape can be completed with a base dimension approximately twice the size of the base, so a heat seal fin of 2-3 mm will suffice (see Figure 7). (8) Figure 8 shows a cross-sectional micrograph of an example using semicircular single protrusions (1 and 3 mm). A video of flat compression bonding is also included for comparison. It can be seen that the desired molded mass was generated even with a thin sealant, OPP / LLDPE (20 μm thick). The sealant for retort pouch materials is 50 μm thick. The molded mass generated when a 3 mm single protrusion was selected was sufficient. It was also found that the desired effect could be achieved even with a single protrusion dimension of 1 mm. Weakness of the heat seal edge of the cohesively bonded flat compression specimen was confirmed.

[0027] (9) The characteristics of the "mold bonding" of the present invention and the conventional flat crimping method are illustrated in Figure 10. 1) The heat seal width of the flat crimping method is 10 to 15 mm. When the entire surface is melted and high-pressure bonded, a large amount of paste-like sealant protrudes unevenly onto the heat seal edge, forming poly beads. When the flat crimping width is 10 mm, the heat-sealed area is approximately 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 fracturing. 3) The pinhole generation stress in this state is as shown in Table 1, and even a strong sealant 13 will break at a few N. 4) Flat crimping generates a large amount of molten sealant. Suppressing this is a means to solve 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] The present invention provides the following methods for heat-sealing plastic bags. 1. A method for heat-sealing plastic bags, comprising sandwiching a heat-sealing material between a pair of heating elements and heat-sealing the material, wherein one of the pair of heating elements has linear protrusions with a fine semicircular or trapezoidal cross-section, and the heated linear protrusions are pressed against a sealant of the heat-sealing material, thereby injecting the sealant, melted in a temperature range for cohesive adhesion, in a strip along the side edge of the linear protrusion to form a molded adhesive strip. 2. The 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. 3. The method for heat-sealing plastic bags as 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. The method for thermally sealing plastic bags according to any one of 1 to 3, which comprises adjusting the amount of molten sealant injected by changing the dimensions of the semicircle or trapezoid. 5. A method for manufacturing plastic bags, which comprises manufacturing a plastic bag having the molded adhesive band formed thereon using the method for thermally sealing plastic bags according to any one of 1 to 4.

[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.

[0030]

[0033] Figure 1 illustrates an implementation model (heat jaw method) for thermal bonding (heat sealing). Figure 2 illustrates a model for the development of heat seal strength. Figure 3 illustrates tensile test patterns for heat-sealed specimens heated at various temperatures (example: retort pouch). Figure 4 illustrates an evaluation test for the fracture resistance of heat-sealed specimens (example: retort pouch specimen). Photographs illustrate examples of broken bags caused by OPP / LLDPE plastic beads. Figure 5 illustrates a model for "mold bonding." Figure 6 illustrates the difference between the heat-sealed surfaces of "mold bonding" and flat bonding. Figure 7 illustrates a microscopic photograph (example) of the crimped cross section of a heat-sealed specimen with cohesive bonding. Figure 8 illustrates an operational model for "mold bonding." Figure 9 compares the tensile test characteristics of "mold bonding" and flat crimp bonding. Figure 10 illustrates a tensile test pattern for "mold bonding" of a retort pouch. Figure 11 illustrates differential calculations for the "mold bonding" pattern of a retort pouch. Figure 12 illustrates a fracture photograph of a tensile test of "mold bonding" of a retort pouch material. FIG. 1 is a diagram illustrating the application of "mold bonding" to an OPP / LLDPE film.

[0031] The thermal sealing method for plastic bags and the manufacturing method for plastic bags of the present invention are described in detail below. In this specification, "x to y" represents a numerical range of "greater than or equal to x and less than or equal to y." The upper and lower limits of the numerical ranges can be combined arbitrarily. Furthermore, among the individual embodiments of the aspects of the present invention described below, two or more embodiments that are not mutually exclusive can be combined, and an embodiment combining two or more embodiments is also an embodiment of the aspects of the present invention.

[0032] A method for heat-sealing plastic bags according to one aspect of the present invention includes sandwiching a heat-sealing material between a pair of heating elements and heat-sealing the material, wherein one of the pair of heating elements has linear protrusions with a fine semicircular or trapezoidal cross-sectional shape, and pressing the heated linear protrusions against a sealant of the heat-sealing material to inject the sealant, melted in a temperature range for cohesive bonding, into a strip along the side edge of the linear protrusion to form a molded adhesive strip. This aspect can simultaneously achieve bag breakage prevention, sealing, and adhesive strength approaching the breaking strength of the material. Furthermore, this aspect particularly achieves the following advantages: (1) An accurate heat-sealing method for cohesive bonding (molded bonding) approaching the breaking strength of the plastic material can be achieved, ensuring strong bag breakage resistance; and (2) A specific reduction in the amount of plastic material used, as required by the SDGs, can be achieved.

[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 a state where mold bonding is performed under pressure (an example of pressure bonding of semicircular protrusions), and (c) is a front view illustrating the installation of a spacer for adjusting the pressure.

[0034] One embodiment of this aspect can be implemented according to the following steps (1) to (7). (1) Semicircular 8 or trapezoidal 9 fine single-line projections are attached to the heating / compression surface of the heat bar ( FIG. 9( a)). In FIG. 9( a), the solid lines indicate the semicircular 8 cross-section of the fine single-line projections, while the dashed lines indicate the trapezoidal 9 cross-section. The heat-sealing device used in this embodiment includes a pair of heating elements arranged to face each other. One heating element is composed of a heat bar body 10 and semicircular 8 or trapezoidal 9 fine single-line projections attached to the surface of the heat bar body 10 facing the other heating element. The fine single-line projections are arranged along the longitudinal direction of the heat bar body 10. The fine single-line projections can be formed by microfabrication on 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 composed of a heat bar body 11. The surface of the heat bar body 11 facing one of the heaters is wider than the fine single protrusions. The material of the heat bar body 11 that contacts the heat seal 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 are suitable, such as fluororesins (e.g., polytetrafluoroethylene) and polyimide resins. Specific examples of such resins include Teflon and Kapton (both registered trademarks of DuPont). Furthermore, the material that constitutes the contact surface does not necessarily have to be an elastomer (e.g., an elastomer with a Shore hardness of 40A to 90A, such as silicone rubber or fluororubber) as described in Patent Document 2. Here, the heat seal material disposed between the pair of heaters is composed of two composite materials. Each composite is a laminate including a surface layer material and a sealant. The two composite materials are disposed with their sealant sides facing each other. The thickness of the surface layer material in one 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 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 appropriately, but is, for example, 10 to 3,000 μm, preferably 20 to 2,000 μm.

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

[0036] (2) The outer edge side portion 17 of the sealant is heated to near the melting temperature (Tm). The molded mass 15 formed by the fine single protrusions 8 is generated on the bag side (inside the bag) and on the outer edge side of the bag. In this embodiment, the molded mass formed 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 sealants 13 of the surface layer 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 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 protrusions is adjusted so that the molten sealant in the fine portions (portions compressed by the fine single protrusions) flows and the surface layers come into contact with each other. In this case, it is preferable to install spacers 19, 20 at both ends of the heat bar (between the heat bar bodies 10, 11) to automatically prevent excessive pressure and 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 h [mm] of the single protrusion) Condition (ii): Spacer height H [mm] × α = (thickness of one surface layer [mm]) × 2 + (height h [mm] of the single protrusion) Here, α is 0.9 to 1.1. Note that the compression pressure 14 may be adjusted in addition to or instead of the embodiment in which a spacer is installed (or the distance between the pair of heating bodies is adjusted when they are closest to each other). Compression by the pair of heating elements 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 installing a spacer (or adjusting the distance between the pair of heating elements when they are closest to each other) or by adjusting the crimping pressure 14. Here, "excessive pressure" refers to, for example, pressurizing the heat seal material so that it is thinner than the combined thickness of the two outer 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 layer 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 the pressure vessel during injection.

[0040] (6) By heating / compressing the fine single projections, the molten sealant in the pressure vessel is injected into the outer edge side portion 17 of the sealant in the weakly heated portion, bringing the side of the bag body into a "molded bond" state. Here, the "weakly heated portion" refers to a region shifted laterally from the tip of the fine single projection, and although the sealant in this region is preheated (preheated) by the pair of heating elements, it is heated more weakly (the temperature rise due to heating is slower) than the sealant in the region pressed by the tip of the fine single projection, and since it is not cohesively bonded, no polymer beads are formed. The molded bonded bands formed in this way are band-shaped along the side edges of the linear projections (fine single projections), and their width (band width) is, for example, 0.5 to 3 mm, preferably 1 to 2 mm.

[0041] (7) The injection amount of molten sealant can be adjusted by changing the dimensions of the semicircular or trapezoidal shape. The dimensions depend on the thickness of the sealant and the desired heating rate, but a range of 0.5 to 3 mm, and particularly 0.25 to 1.5 mm, is preferred. Here, the above dimensions can be applied as the diameter in the case of a semicircular shape, and as the base in the case of a trapezoidal shape.

[0042] In the example of FIG. 9, the cross-sectional shape of the linear protrusion is primarily semicircular. However, as shown by the dashed line in FIG. 9(a), the cross-sectional shape of the linear protrusion may also be a trapezoid 9. In the case of a trapezoid 9, its upper base (the side that forms the contact surface with the heat seal material) is preferably shorter than its lower base (the side on the heat bar main 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 the trapezoid 9 is preferably an acute angle, and particularly preferably 45 to 80°. The two base angles may be the same or different. Note 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 to set the heat-sealing temperature within the temperature range for cohesive adhesion (also referred to as the "temperature range"). This imparts fluidity to the sealant melted within the temperature range for cohesive adhesion, resulting in injection and the formation of a molded adhesive. As described above, the temperature range for cohesive adhesion can be a temperature near the melting temperature (Tm) of the sealant, for example, within 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, enabling reliable sealing 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 that forms 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 used. Furthermore, even if the sealant softens within this temperature range, it does not have the fluidity required for injection, and a molded adhesive zone is not formed.

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

[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) A "mold adhesion" specimen was prepared under the following conditions using the method shown in Figure 9. <Specimen material> Thin plastic material: OPP (biaxially oriented polypropylene) / LLDPE (low density polyethylene) 20 μm, Tm of LLDPE sealant: 100 to 115°C <Heating and compression conditions> Heating: equilibrium temperature heating, 114°C, 1 second Single protrusion: semicircular cross section, diameter 1 mm Compression pressure: 100 mm / 300 N = 30 N / 10 mm

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

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

[0049] (Example 3): Application of "Mold Bonding" to Retort Pouches The thermal bonding of retort pouches is subject to HACCP (Hazard Analysis and Critical Control Point) regulations, and requires the most advanced handling in heat sealing techniques. The material composition is PET / AL / CPP 50 μm, with a Tm of 170°C. Using the method shown in Figure 9, "mold bonding" specimens were 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, 1 mm diameter Inherent breaking strength of material: 66 N / 15 mm

[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 graph in Figure 11 lists the characteristics of different heating temperatures as follows: ・◆145°C: Tensile test response of interfacial adhesion (peel seal). It shows adhesive strength (20-30N / 15mm) and is not uniform. ・●150°C: The adhesive width is 2mm. Although it is "molded adhesive," it is a mixture of interfacial and cohesive adhesives, so it broke at a tension of 2.7mm. ・●160°C-175°C: This shows the adhesive state in the target range of "molded adhesive." The response to tensile load is a combination of elongation of the material itself and fracture of the surface material. The bag rupture mechanism corresponds to (4) defined in "(Bag rupture mechanism analysis)" above.

[0052] (Analysis of test results) *The result at 145°C shows the peel pattern of flat crimping, with the entire surface peeling and sealed. *At 150°C and 160°C for "molded adhesive," the "molded adhesive" is incomplete, and the adhesive easily breaks due to the peel force, eliminating the effectiveness of "molded adhesive." *At 160°C to 175°C for flat crimping, a tensile test pattern comparable to that of "molded adhesive" is obtained, but the heat seal edge has a curved finish and pinholes are observed. *At 170°C to 175°C for "molded adhesive," a tensile test pattern shows a smooth rise. At 170°C to 175°C for "molded adhesive," the heat seal edge has increased breaking strength. The sudden change in tensile strength occurs due to sealant rupture, and the expected results can be observed. 175°C exceeds the Tm of 170°C, so 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 15 mm width, or when a slight deviation occurs 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 judged to have been 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 from visual observation and the differential value. 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 material 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 interfacial adhesion is [(adhesion strength) x (peel length)]. The resistance to rupture of "mold adhesion" is [(adhesion strength) x (elongation)]. Peel length can be adjusted by the heat seal width, but elongation length is an inherent property of the material.

[0055] A comparison of integral calculations was performed for the peel seal at 145°C and the mold bonded pattern at 170°C. The heat seal strength (N / 15 mm) was converted to adhesive strength (N / 1 mm), and the integration was performed using the tensile length and peel length. The results are shown in Figure 12. The integral value at 145°C is the heating condition under which the material exhibits its highest fracture resistance. The integral value for mold bonded at 170°C exceeds the peel energy at 145°C across the entire range. The analysis results in Figure 11 show that the surface layer fractured after approximately 3 mm of elongation. While the sealant did not fracture, fracture of the surface layer poses a risk of compromising the gas barrier properties. When evaluating fracture resistance under these conditions, the fracture resistance until fracture of the surface layer was equivalent to 3.7 mm for the peel seal, confirming the superiority of mold bonded.

[0056] (Example 4): Confirmation of application of "mold adhesion" to OPP / LLDPE film This specimen (material inherent breaking strength: 48 N / 15 mm) is the most widely available general-purpose material on the market. The sealant is thin at 20 μm. A test was conducted to confirm the suitability of "mold adhesion" for this thin material. Specimens were prepared under the following conditions using the method shown in Figure 9, and tensile tests were conducted on the conventional flat pressure bonding method and the "mold adhesion" 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 projection: semicircular cross section, 1 mm diameter

[0057] (Analysis of Test Result Characteristics) (1) The flat-compression temperature of 112°C indicates interfacial adhesion. The tensile test pattern shows peeling characteristics. (2) The flat-compression temperature of 114°C and above indicates a cohesive adhesive zone. The tensile test pattern is distorted, with frequent edge breakage. This situation is explained in Figure 10, and an example is shown in Figure 5. (3) Edge breakage is prevented even with "molded adhesion" at 112°C. (4) At 114°C and 116°C, a heat seal strength of 21 N / 15 mm is achieved, ensuring complete "molded adhesion." (5) At 118°C, the effect of overheating is evident. (6) Differentiating the response at 114°C reveals an inflection point between 1.7 and 2.0 mm of tension. It can be seen that sealant failure begins after this tension distance, with complete failure occurring at 2.5 mm of tension. (7) It was found that even with a thin sealant material (20 μm), a high level of hermetic adhesion is possible by “mold adhesion,” which is superior to flat pressure bonding.

[0058] The expected functions of general-purpose plastic packaging are as follows: (1) The adhesive strength approaches the breaking strength of the applied packaging material, preventing pinholes and breakage of the heat-sealed edge due to compression and impact during distribution. (2) A guaranteed seal is established. (3) Consumers desire an easy-to-open, pick-and-pinch system without the need for tools such as scissors. This adhesive mechanism relies on the thermoplasticity of plastic materials. Depending on the heating temperature, the adhesive surface of the plastic material changes from an interfacial adhesive, where the adhesive surface remains, to a cohesive adhesive, where the adhesive surface disappears, resulting in a molded state. The inventions in Patent Documents 2 and 3 have simultaneously achieved both "sealing" without breaking the heat-sealed edge and "easy opening." However, a method for controlling adhesive strength equivalent to the breaking strength of the material based on precise logic has not been achieved. This invention successfully created a molten sealant injection function at a fine heat-sealing operation site, thereby achieving "molded adhesion" on the outer edge of the bag, preventing breakage of the polypropylene balls. This method can be completed even with a heat-sealed surface as thin as 3 mm. The SDGs set deadlines and require a reduction in the use of plastic packaging, and the present invention specifically addresses this requirement.

[0059] Although several embodiments and / or examples of the present invention have been described in detail above, those skilled in the art will readily be able to make numerous modifications to these exemplary embodiments and / or examples without substantially departing from the novel teachings and advantages of the present invention. Accordingly, these numerous modifications are within the scope of the present invention. The contents of all documents cited in this specification and of the applications from which this application claims priority under the Paris Convention are incorporated by reference in their entirety.

Claims

1. A method for heat-sealing plastic bags, comprising sandwiching a heat-sealing material between a pair of heating bodies and heat-sealing the material, wherein one of the pair of heating bodies has linear protrusions with a fine semicircular or trapezoidal cross-section, 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 to form a molded adhesive band.

2. The method for heat-sealing plastic bags according to claim 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 according to claim 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 heat-sealing plastic bags according to any one of claims 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 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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