Heat-sealable paper and packaging materials

Abstract

To provide a heat seal sheet that is environmentally friendly and excellent in water resistance and an inhibitory effect against cracking caused by a fold in a region where heat seal layer is laminated, while keeping heat seal strength.SOLUTION: A heat seal sheet relating to an embodiment of the invention includes a paper substrate having conifer non-bleached kraft pulp as a major component and a heat seal layer laminated on one face of the paper substrate. The paper substrate has a folding strength of 500 times or more and 1,000 times or less in the flow direction and 80 times or more and 200 times or less in the width direction. The heat seal layer has a Bekk smoothness of the laminated surface of 3 seconds or more and 230 seconds or less. The heat seal layer has an ethylene (meth) acrylic acid copolymer alkali neutralized product as a major component. The heat seal layer has a coating weight of 1.5 g / m2 or more and 10.0 g / m2 or less. Preferably, the paper substrate has a content of the conifer non-bleached kraft pulp of 95 mass% or more relative to the whole pulp.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to heat-sealable paper and packaging materials. [Background technology]

[0002] Generally, pillow packaging, widely used for confectionery, pharmaceuticals, and hygiene products, utilizes laminated paper or plastic film made by extruding polyethylene or polypropylene resin onto a base paper. While such laminated paper and plastic film offer excellent strength, water resistance, and moisture resistance, the non-biodegradable plastic film remains, making it unsuitable for recycling as waste paper. Furthermore, if not properly disposed of after use, it can easily become microplastics, contributing to marine pollution and potentially leading to environmental burdens.

[0003] Therefore, in the prior art, packaging paper has been proposed that improves heat seal strength and reduces the amount of plastic used by providing a heat seal layer containing an ionomer on a paper substrate (see Patent Document 1). In addition, a recyclable heat seal sheet has been proposed by providing a heat adhesive layer containing styrene-butadiene copolymer latex and setting the smoothness of the heat adhesive surface to a specific range (see Patent Document 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 6580291 [Patent Document 2] Japanese Patent Publication No. 2018-53400 [Overview of the project] [Problems that the invention aims to solve]

[0005] When heat-sealable paper is opened using a jig in a bag-making machine to facilitate the sealing of contents, creases can form in the areas where the heat-seal layers are laminated. If creases form in the heat-sealable paper, there is a risk of cracking from the creases. However, the folding strength of the areas where the heat-seal layers are laminated in the conventional packaging paper and heat-sealable sheets mentioned above is insufficient and requires further consideration. Furthermore, there is a need for further improvement in the water resistance of heat-sealable paper.

[0006] The present invention was made based on the above circumstances, and aims to provide a heat-sealable paper that does not contain laminated paper or plastic film, which have a high environmental impact, and that ensures heat-seal strength while also being excellent in suppressing cracks caused by folds in the area where the heat-sealable layer is laminated and in water resistance. [Means for solving the problem]

[0007] A heat-sealable paper according to one aspect of the present invention comprises a paper substrate mainly composed of unbleached softwood kraft pulp and a heat-sealable layer laminated on one side of the paper substrate, wherein the paper substrate has a folding strength in the flow direction of 500 to 1,000 times and a folding strength in the width direction of 80 to 200 times, the Beck smoothness of the surface of the heat-sealable layer laminated surface is 3 seconds to 230 seconds, the main component of the heat-sealable layer is an alkali-neutralized ethylene (meth)acrylic acid copolymer, and the coating amount of the heat-sealable layer is 1.5 g / m². 2 More than 10.0g / m 2 The following applies: [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a heat-sealable paper that does not contain laminated paper or plastic film, which have a high environmental impact, while ensuring heat-seal strength, and which has excellent crack suppression effect due to folds in the area where the heat-sealable layer is laminated, as well as excellent water resistance. [Modes for carrying out the invention]

[0009] [Description of Embodiments of the Invention] A heat-sealable paper according to one aspect of the present invention comprises a paper substrate mainly composed of unbleached softwood kraft pulp and a heat-sealable layer laminated on one side of the paper substrate, wherein the paper substrate has a folding strength in the flow direction of 500 to 1,000 times and a folding strength in the width direction of 80 to 200 times, the Beck smoothness of the surface of the heat-sealable layer laminated surface is 3 seconds to 230 seconds, the main component of the heat-sealable layer is an alkali-neutralized ethylene (meth)acrylic acid copolymer, and the coating amount of the heat-sealable layer is 1.5 g / m². 2 More than 10.0g / m 2 The following applies:

[0010] When heat-sealable paper is made into a bag by opening the opening with a jig in a bag-making machine to facilitate the sealing of contents, creases are formed, and there is a risk of cracking occurring at these creases. This heat-sealable paper has a paper base material mainly composed of unbleached softwood kraft pulp, which allows the heat-sealing agent to be appropriately impregnated into the interior of the paper base material, improving the strength of the paper base material itself. This is because unbleached softwood kraft pulp has long and thick fibers, resulting in many voids on the surface of the paper base material, making it difficult for the heat-sealing agent to remain on the surface of the paper base material. Therefore, it is thought that cracks originating from the surface of the paper base material are suppressed. Furthermore, the above paper base material has a folding strength in the flow direction of 500 to 1000 times and a folding strength in the width direction of 80 to 200 times, which improves the crack suppression effect due to creases in the area where the heat-sealing layer is laminated, and also provides excellent suppression of tearing and wrinkling in the bag-making process. By ensuring that the Beck smoothness of the surface of the heat-seal layer laminate of the paper substrate is between 3 seconds and 230 seconds, both heat-seal strength and water resistance can be achieved. Furthermore, the main component of the heat-seal layer of this heat-seal paper is an ethylene (meth)acrylic acid copolymer alkali neutralized product, and the coating amount (in terms of solid content) of the heat-seal layer is 1.5 g / m². 2 More than 10.0g / m 2The following applies: The main component of the heat-seal layer is an alkali-neutralized ethylene (meth)acrylic acid copolymer, resulting in good heat-seal strength and water resistance. Furthermore, the amount of coating of the heat-seal layer within the above range further improves heat-seal strength and water resistance. Therefore, the heat-seal paper ensures heat-seal strength while also being excellent in suppressing cracks caused by folds in the area where the heat-seal layer is laminated and in water resistance. In addition, the heat-seal paper is environmentally friendly as it does not contain laminate paper or plastic film, which cannot be recycled and have a high environmental impact. In this invention, "heat-seal layer" refers to the coating layer on which the heat-sealing process is performed. "Main component" refers to the substance with the highest content among the constituent substances, preferably with a content of 50% by mass or more. "Flow direction" means the papermaking direction (MD), and "width direction" means the direction perpendicular to the above flow direction (CD).

[0011] It is preferable that the content of unbleached softwood kraft pulp in the total pulp of the paper substrate is 95% by mass or more. A content of 95% by mass or more of unbleached softwood kraft pulp further enhances the crack suppression effect caused by folds in the region where the heat-seal layer is laminated.

[0012] It is preferable that the dynamic water absorption test value after 100 seconds in the dynamic water absorption test of the heat seal layer coated surface measured using the "Surface Size Tester EST12.2" manufactured by emtec electoric is 85.0% or more and 97.0% or less. When the dynamic water absorption test value of the heat seal layer coated surface is 85.0% or more and 97.0% or less, the water resistance of the heat seal layer coated surface of the heat seal paper is sufficiently improved, and the water resistance performance can be exhibited. In the above dynamic water absorption test value, a test sample is placed between an ultrasonic transmitter element and a receiver element installed in water, and the amount of water penetration into the paper (dynamic water absorption test value) is obtained from the change in the transmission intensity of ultrasonic waves. In the case of this heat seal paper, it is necessary to conduct quality control in a state close to the actual use conditions, and it is necessary to control the water absorption degree of the instantaneous (that is, dynamic in the state of actually being used as a packaging material) heat seal paper. In view of the fact that higher quality is required for such heat seal paper under such circumstances, in this embodiment, the dynamic water absorption test value is measured.

[0013] The measurement of the dynamic water absorption test value is carried out using the "Surface Size Tester EST12.2" manufactured by emtec electoric. First, a sample piece for measurement (75 mm × 50 mm) is fixed to a folder with double-sided tape and immersed in a test container filled with a solvent (350 ml of water). Here, the folder is immersed between the ultrasonic transmitter element and the receiver element provided in the test container. Measurement is started 0.025 seconds after the sample piece and the solvent come into contact. Since the transmission intensity of ultrasonic waves changes as the solvent penetrates into the sample piece, a graph of the dynamic water absorption test value (%) and the elapsed time (seconds) is displayed on the monitor of the size tester. And the dynamic water absorption test value can be obtained by reading this graph. The ultrasonic frequency for measurement is 2 MHz.

[0014] It is preferable that the Bek smoothness of the surface of the heat seal layer is 20 seconds or more and 250 seconds or less. When the Bek smoothness of the surface of the heat seal layer is within the above range, the heat seal paper can further improve the heat seal strength and the inhibitory property of blocking of the heat fusion surface. The Bekk smoothness evaluates smoothness based on the air flow rate (air leakage). The Bekk smoothness defined in JIS-P8119 (1998) involves sandwiching the sheet, which is the object to be measured, between an optically flat glass sample table and a rubber pressing plate under a pressure of 100 kPa, and the time required for 10 ml of air to flow through between the glass standard surface of about 10 cm 2 and into a container where the pressure is maintained at a reduced pressure of about 370 mm Hg of the mercury column, which represents the smoothness on the surface of the so-called object to be measured. The Bekk smoothness evaluates the macro smoothness on a relatively wide surface. In the heat-sealing paper, the waviness of the surface can be evaluated by the Bekk smoothness. Therefore, the surface of the heat-sealing layer of the heat-sealing paper can be more appropriately evaluated by the Bekk smoothness.

[0015] The moisture permeability of the heat-sealing paper is preferably 300 g / m 2 ·24 hr or more and 1,200 g / m 2 ·24 hr or less. When the moisture permeability of the heat-sealing paper is within the above range, the moisture in the packaged contents is less likely to become water droplets, and the humidity can be maintained appropriately, so that the quality of the contents can be kept good. Here, the "moisture permeability of the heat-sealing paper" is a value measured in accordance with JIS-Z0208 (1976).

[0016] The packaging material according to another embodiment of the present invention has the heat-sealing paper. Since the packaging material has the heat-sealing paper, while ensuring the heat-sealing strength, it is excellent in the effect of suppressing cracks due to folds in the region where the heat-sealing layer is laminated and water resistance. In addition, since the packaging material does not contain laminated paper or plastic film with a high environmental load, environmental conservation can be achieved.

[0017] [Details of Embodiments of the Present Invention] The following describes in detail a heat-sealable paper according to one embodiment of the present invention. Unless otherwise specified, the amount of each material blended into the paper substrate (absolutely dry amount) refers to the mass ratio of the pulp to the oven-dry mass of the paper substrate. Furthermore, unless otherwise specified, the content of each material blended into the heat-seal layer-forming composition refers to the oven-dry mass ratio of each material to the total mass of the heat-seal layer.

[0018] <Heat sealable paper> The heat-sealable paper has a heat-seal layer on one side of the paper substrate. The heat-seal layer may be a single layer or a multi-layer structure.

[0019] [Paper base material] The paper substrate is obtained by papermaking a slurry containing raw pulp. The paper substrate may be single-layer or multi-layer.

[0020] (Raw pulp) The paper substrate is primarily composed of unbleached softwood kraft pulp (NUKP). By using a paper substrate primarily composed of unbleached softwood kraft pulp, the heat sealant is appropriately impregnated into the interior of the paper substrate, improving the strength of the paper substrate itself. This is because unbleached softwood kraft pulp has long and thick fibers, resulting in many voids on the surface of the paper substrate, making it difficult for the heat sealant to remain on the surface. Therefore, the folding strength of the heat seal paper is higher than that of the paper substrate, which is thought to suppress cracks originating from the surface.

[0021] It is preferable that the content of unbleached softwood kraft pulp in the total pulp of the paper substrate is 95% by mass or more. A content of 95% by mass or more of unbleached softwood kraft pulp further enhances the crack suppression effect caused by folds in the region where the heat-seal layer is laminated.

[0022] Other raw material pulps besides unbleached softwood kraft pulp can be used as the raw material pulp for the paper substrate. Examples of other raw material pulps include virgin pulp other than unbleached softwood kraft pulp, recycled paper pulp, and combinations of these pulps.

[0023] Virgin pulps other than unbleached softwood kraft pulp include, for example, chemical pulps such as bleached softwood kraft pulp (NBKP), bleached hardwood kraft pulp (LBKP), semi-bleached hardwood kraft pulp (LSBKP), semi-bleached softwood kraft pulp (NSBKP), hardwood sulfite pulp, and softwood sulfite pulp; and mechanical pulps (MP) such as stone ground pulp (SGP), pressurized stone ground pulp (TGP), chemigland pulp (CGP), crushed wood pulp (GP), and thermomechanical pulp (TMP). These pulps, chemically or mechanically produced, can be used individually or in combination.

[0024] As recycled paper pulp, for example, disintegrated recycled paper pulp, disintegrated and deinked recycled paper pulp (DIP), disintegrated, deinked and bleached recycled paper pulp, etc., manufactured from brown recycled paper, kraft envelope recycled paper, magazine recycled paper, newspaper recycled paper, flyer recycled paper, office recycled paper, corrugated cardboard recycled paper, white recycled paper recycled paper, Kent recycled paper recycled paper, imitation recycled paper recycled paper, land destiny recycled paper recycled paper, etc., can be used individually or in combination of multiple types.

[0025] (Other additives) Other additives may be added to the paper substrate as needed. Examples of additives include fillers, pigments, sizing agents, binders, oil resistant agents, fluorescent whitening agents, aluminum sulfate, yield enhancers, water drainage enhancers, dry strength enhancers, wet strength enhancers, coloring dyes, coloring pigments, and water-resistant agents, which can be used individually or in combination.

[0026] (Basis weight of paper substrate) The basis weight of the heat-sealing paper is 40.0 g / m². 2 More than 100.0g / m 2 The following is preferable: The basis weight of the paper substrate is 40.0 g / m². 2If the basis weight is less than 100.0 g / m², the bag may not be strong enough. 2 If the weight exceeds this limit, the high rigidity may lead to folding defects during the bag-making process. By keeping the basis weight of the paper substrate within the above range, the heat-sealable paper can be made low in basis weight and lightweight while maintaining rigidity.

[0027] (Thickness of paper substrate) The thickness of the paper substrate of the heat-sealable paper is measured in accordance with the "Paper and cardboard - Test methods for thickness and density" described in JIS-P8118(2014). The lower limit of the paper substrate thickness is preferably 50 μm, and more preferably 60 μm. The upper limit of the paper substrate thickness is preferably 150 μm, and more preferably 120 μm. By having the paper substrate thickness of the heat-sealable paper within the above range, the workability and processability when manufacturing packaging materials using the heat-sealable paper can be improved.

[0028] (Density of paper substrate) The density of the paper base material of the heat-sealable paper is 0.65 g / cm³. 3 More than 0.80g / cm 3 The following is preferable: Since the density of the paper substrate is within the above range, it is possible to maintain folding strength while enabling low basis weight and weight reduction, so the heat-sealable paper has excellent processability.

[0029] (Folding strength in the flow direction) The folding strength in the flow direction is the same as the folding strength in the papermaking direction and is measured in accordance with the "Paper and paperboard - Folding strength test method - MIT testing machine method" described in JIS-P8115 (2001). The lower limit of the folding strength in the papermaking direction is 500 turns, with 600 turns being preferable. The upper limit of the folding strength in the papermaking direction is 1,000 turns, with 900 turns being more preferable. If the folding strength in the papermaking direction is below the lower limit, there is a risk that it will not withstand the tension in the direction of travel during the bag-making process and will tear. On the other hand, if the folding strength in the papermaking direction exceeds the upper limit, the rigidity will be too high, and there is a risk that it will not be able to be folded during the bag-making process and will wrinkle.

[0030] (Folding strength in the width direction) The folding strength in the width direction is the folding strength in the width direction (the direction perpendicular to the flow direction mentioned above). The lower limit for the folding strength in the width direction is 80 turns, with 90 turns being preferred. The upper limit for the folding strength in the width direction is 200 turns, with 140 turns being preferred. If the folding strength in the width direction is below the lower limit, there is a risk of tearing at the point where the jig comes into contact during the bag-making process. On the other hand, if the folding strength in the width direction exceeds the upper limit, the rigidity will be too high, and there is a risk of wrinkles forming instead of being able to be folded during the bag-making process.

[0031] [Heat seal layer] The heat seal layer is laminated on one side of the paper substrate. The heat seal layer is formed by coating one side of the paper substrate with a heat seal layer forming composition.

[0032] The above heat-seal layer is mainly composed of an alkali-neutralized ethylene (meth)acrylic acid copolymer. Because the main component of the heat-seal layer is an alkali-neutralized ethylene (meth)acrylic acid copolymer, it exhibits good water resistance and heat-seal strength. The alkali-neutralized ethylene (meth)acrylic acid copolymer is prepared by neutralizing the carboxyl groups of the acrylic acid in an aqueous dispersion of ethylene (meth)acrylic acid copolymer resin with ammonia while heating and stirring in water. Because the neutralizing agent is easily volatile ammonia, no alkali remains in the dried resin film, which is presumably why sufficient water resistance can be achieved.

[0033] The lower limit of the content of the ethylene (meth)acrylic acid copolymer alkali neutralized product relative to the total solid content of the heat seal layer is 50.0% by mass, preferably 90.0% by mass. If the content of the ethylene (meth)acrylic acid copolymer alkali neutralized product is less than 50.0% by mass, sufficient heat sealability may not be obtained. On the other hand, the upper limit of the content of the ethylene (meth)acrylic acid copolymer alkali neutralized product is 100.0% by mass, preferably 95.0% by mass.

[0034] The heat-seal layer described above may contain resins other than the ethylene (meth)acrylic acid copolymer alkali neutralized product. Examples of resins other than the ethylene (meth)acrylic acid copolymer alkali neutralized product include polyethylene resins and polypropylene resins. Examples of polyethylene resins include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE), high-density polyethylene (HDPE), ethylene-vinyl acetate copolymer (EVA), carboxylic acid-modified polyethylene, styrene acrylic acid, styrene butadiene latex and their derivatives, as well as mixtures thereof. Examples of polypropylene resins include polypropylene (PP) homopolymer, random polypropylene (random PP), block polypropylene (block PP), chlorinated polypropylene, carboxylic acid-modified polypropylene and their derivatives, as well as mixtures thereof. Among these, ethylene-vinyl acetate copolymer (EVA) is preferred from the viewpoint of heat-seal strength.

[0035] The lower limit for the amount of the heat seal layer applied (in terms of solid content) is 1.5 g / m². 2 Therefore, 2.0 g / m 2 This is preferable. On the other hand, the upper limit for this coating amount is 10.0 g / m². 2 It is 8.0 g / m 2 This is preferable. If the above coating amount does not meet the above lower limit, it may not have sufficient heat seal strength, and when made into a bag, it may not be able to seal properly. If the above coating amount exceeds the above upper limit, the blocking suppression of the heat-seal surface will decrease, and it may become difficult to slip during the bag-making process and prone to tearing. Furthermore, by keeping the amount of the heat seal layer within the above range, the heat seal strength and water resistance can be further improved.

[0036] (Other additives) The heat seal layer of the present invention may contain other additives besides those mentioned above. Other additives may include, for example, water-soluble polymers, adhesives, inorganic pigments, organic pigments, sizing agents, viscosity modifiers, coloring dyes, coloring pigments, water-resistant agents, lubricants, etc., which can be used individually or in combination.

[0037] [Physical properties of heat-sealable paper] (Beck smoothness) The lower limit of the Beck smoothness of the surface of the heat seal layer laminated on the paper substrate is 3 seconds, with 5 seconds being preferred. The upper limit of the Beck smoothness of the surface of the heat seal layer laminated on the paper substrate is 230 seconds, with 180 seconds being preferred. If the Beck smoothness is below the lower limit, the surface of the heat seal paper becomes rough, reducing the contact area and potentially decreasing the heat seal strength. If the Beck smoothness exceeds the upper limit, the blocking suppression effect of the heat fusion surface of the heat seal layer may decrease.

[0038] The lower limit of the Beck smoothness of the surface of the heat seal layer is preferably 20 seconds, and more preferably 23 seconds. The upper limit of the Beck smoothness of the surface of the heat seal layer is preferably 250 seconds, and more preferably 200 seconds. If the Beck smoothness of the surface of the heat seal layer is below the lower limit, the surface of the heat seal paper will become rougher (the contact area will decrease), which may reduce the heat seal strength. If the Beck smoothness of the surface of the heat seal layer exceeds the upper limit, the blocking suppression effect of the heat fusion surface of the heat seal layer may decrease.

[0039] (Folding strength in the width direction of heat-sealable paper) The widthwise folding strength of the heat-seal paper is the folding strength in the widthwise direction (the direction perpendicular to the flow direction mentioned above), and is measured in accordance with the "Paper and cardboard - Folding strength test method - MIT testing machine method" described in JIS-P8115 (2001). The ratio of the widthwise folding strength of the paper substrate to the widthwise folding strength of the heat-seal paper (hereinafter also referred to as the widthwise folding strength improvement rate before and after heat-seal layer formation) is preferably 300% or more, and more preferably 550% or more. If the widthwise folding strength improvement rate before and after heat-seal layer formation is less than 300%, cracks will occur due to folds in the area where the heat-seal layer is laminated, making it unsuitable for packaging applications.

[0040] (Dynamic water absorption test values ​​of the heat-sealed coating surface) The lower limit of the dynamic water absorption test value after 100 seconds in the dynamic water absorption test of the heat-seal layer coated surface measured using the "Surface Size Tester EST12.2" manufactured by emtec electoric is 85.0%, with 90.0% being preferred. The upper limit of the dynamic water absorption test value after 100 seconds in the dynamic water absorption test of the heat-seal layer coated surface is 97.0%, with 95.0% being preferred. If the dynamic water absorption test value after 100 seconds in the dynamic water absorption test of the heat-seal layer coated surface is below the lower limit, the water resistance may decrease. If the dynamic water absorption test value after 100 seconds in the dynamic water absorption test of the heat-seal layer coated surface exceeds the upper limit, it will be necessary to increase the amount of heat sealant applied, which may cause blocking. Furthermore, the dynamic water absorption test values ​​of the heat-seal layer coating surface described above can be achieved by adjusting the blending ratio of the raw material pulp, especially hardwood kraft pulp, adjusting the type and amount of internal sizing agent, adjusting the type and amount of resin used in the heat-seal layer, and adjusting the coating amount.

[0041] (moisture permeability) The moisture permeability of the heat-sealing paper shall be measured according to JIS-Z0208

[1976] Test method for moisture permeability of moisture-proof packaging materials [cup method], based on condition B. The lower limit of the above moisture permeability shall be 300 g / m². 2 24hr is preferred, and 500g / m² 224h is preferable. Also, the upper limit for the above moisture permeability is 1,200 g / m². 2 24 hours is preferable, (1,000 g / m²) 2 24h is preferable. Because the above moisture permeability is within the above range, moisture in the packaged contents is less likely to turn into water droplets, and the humidity is maintained at an appropriate level, thus preserving the quality of the contents.

[0042] This heat-sealable paper ensures heat-seal strength while also providing excellent crack suppression due to folds in the areas where the heat-sealed layers are laminated, as well as superior water resistance. Furthermore, since this heat-sealable paper does not contain laminate paper or plastic film, which have a high environmental impact, it contributes to environmental protection.

[0043] [Manufacturing method for heat-sealable paper] The method for manufacturing the heat-sealable paper is not particularly limited, but for example, it may include the steps of: making paper from a pulp slurry that will be used as a raw material for a paper substrate; producing a heat-sealable layer-forming composition; and coating at least one surface of the paper substrate with the heat-sealable layer-forming composition.

[0044] In the papermaking process, a raw material slurry containing the aforementioned raw pulp, oil resistant agent, and other additives is processed using a known papermaking machine. Furthermore, the Beck smoothness of the surface of the heat-seal layer laminate of the paper substrate can be easily adjusted by using a machine calender or a soft calender.

[0045] In the process of producing a heat seal layer-forming composition, a heat seal layer-forming composition containing an alkali neutralized ethylene (meth)acrylic acid copolymer as the main component is produced.

[0046] A known coating method can be used for coating the heat seal layer-forming composition, and known coating machines such as a 2-roll size press coater, gate roll coater, blade metering coater, rod metering coater, blade coater, air knife coater, roll coater, brush coater, kiss coater, squeeze coater, curtain coater, die coater, bar coater, and gravure coater can be used.

[0047] For drying the coated heat-seal layer-forming composition, known drying equipment can be used, such as infrared drying equipment, hot air drying equipment, contact dryer drying equipment, etc.

[0048] The heat-sealable paper obtained in this way can be finished using various known finishing devices, such as supercalenders, gloss calenders, soft calenders, and matte calenders, and can be given a product finish as appropriate.

[0049] <Packaging material> Another embodiment of the present invention provides a packaging material having the heat-sealable paper. Because this packaging material has the heat-sealable paper, it ensures heat-seal strength while also providing excellent crack suppression due to folds in the area where the heat-sealable layer is laminated, as well as superior water resistance. Furthermore, since this packaging material does not contain laminated paper or plastic film, which have a high environmental impact, it contributes to environmental protection.

[0050] <Other Embodiments> The present invention is not limited to the embodiments described above, and can be implemented in various modified and improved forms in addition to those described above. [Examples]

[0051] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples. The coating amount, content, and mass % shown in the examples represent the dry solids content or substantial components.

[0052] [Examples 1-7 and Comparative Examples 1-2] (Manufacturing of paper substrates) First, 100% by mass of unbleached softwood kraft pulp was prepared to obtain a pulp slurry. Aluminum sulfate, a yield agent, and a sizing agent were added as additives to this pulp slurry. The obtained pulp slurry was then used to make paper using an on-top type wire screen paper machine. Next, the paper was treated with a soft calender at a pressure of 50 kg / cm and a temperature of 100°C to adjust the Beck smoothness of the surface on which the heat seal layer was laminated. Then, paper substrates for Examples 1 to 7 and Comparative Examples 1 to 2, having the basis weight, thickness, and density listed in Table 1, were obtained. The Beck smoothness of the surface on which the heat seal layer was laminated was measured using the method described later.

[0053] (Lamination of heat-seal layers) Next, a heat-seal layer was formed on one side of the paper substrate to obtain heat-seal paper. The composition and coating amount of the heat-seal layer-forming composition are as shown in Table 1. In addition, ethylene (meth)acrylic acid copolymer alkali neutralized salt was used as the ethylene (meth)acrylic acid copolymer alkali neutralized product for the heat-seal layer-forming composition. The content of the ethylene (meth)acrylic acid copolymer alkali neutralized product relative to the total solids was 100.0% by mass.

[0054] [Comparative Example 3] Heat-sealable paper was prepared in the same manner as in Example 1, except that a metal salt (ionomer) of ethylene (meth)acrylic acid copolymer was used instead of the alkali neutralized ethylene (meth)acrylic acid copolymer in the heat-seal layer forming composition.

[0055] [Comparative Example 4] A single-layer laminated paper containing polypropylene film was produced. The base paper is made from commercially available kraft paper (basis weight 65.0 g / m²). 2 ) was used. The polypropylene film was produced by extruding and laminating commercially available polypropylene resin using a pelletizer, and then laminating it onto a base paper to a thickness of 20 μm on only one side.

[0056] The heat-sealable paper obtained in the manner described above was evaluated in various ways.

[0057] (Beck smoothness of the heat-sealed layer) The Beck smoothness of the heat-seal layer laminated surface, the non-laminated surface of the heat-seal layer, and the surface of the heat-seal layer on a paper substrate was measured in accordance with JIS-P8119 (1998).

[0058] (Dynamic water absorption test value) The dynamic water absorption test value after 100 seconds was determined in a dynamic water absorption test of a heat-sealed coated surface using the emtec electronics "Surface Size Tester EST12.2". First, a sample piece (75 mm x 50 mm) was fixed to a holder with double-sided tape, and this holder was submerged between the ultrasonic transmitting and receiving elements in a test container filled with 350 ml of water. Measurement began 0.025 seconds after the sample piece came into contact with the water, and the dynamic water absorption test value after 100 seconds was read from the graph of the dynamic water absorption test value and elapsed time displayed on the size tester's monitor. The ultrasonic frequency used for measurement was 2 MHz.

[0059] (Moisture permeability of heat-sealing paper) The moisture permeability of the heat-sealing paper in the examples and comparative examples was measured under condition B in accordance with JIS-Z0208 (1976) Method for testing moisture permeability of moisture-proof packaging materials [cup method].

[0060] (Folding strength in the flow direction) The "folding strength in the flow direction (longitudinal direction) (number of turns)" was measured in accordance with "Paper and paperboard - Folding strength test method - MIT testing machine method" described in JIS-P8115 (2001), which measures the folding strength in the flow direction, which is the papermaking direction.

[0061] (Folding strength in the width direction) The "folding strength in the width direction (number of turns)" was measured in accordance with the "Paper and cardboard - Folding strength test method - MIT testing machine method" described in JIS-P8115 (2001), which measures the folding strength in the width direction (the direction perpendicular to the flow direction mentioned above). Then, the bending strength in the width direction was measured before and after the formation of the heat seal layer, and the improvement rate [%] of the bending strength after the formation of the heat seal layer compared to the bending strength in the width direction before the formation of the heat seal layer was calculated.

[0062] (Evaluation of folds during bag making) For each example and comparative example, a commercially available automatic bag-making machine was used to process individual bags from heat-sealable paper, and the folds during bag making were evaluated. The evaluation results were assessed using the following three-level criteria. An evaluation of A or B indicates a good suppression effect on fold formation during bag making. A: The individual packaging has no wrinkles or tears at the bottom or edges, and the appearance is good. B: There are wrinkles at the folds on the bottom and edges of the individual packaging, but this does not affect its usability. C: The individual packaging has wrinkles and tears at the folds on the bottom and edges, which is a level that affects its usability.

[0063] (Heat seal strength) Using a thermal gradient testing machine (manufactured by Toyo Seiki Seisakusho Co., Ltd.), the sealant pressure was 2 kgf / cm². 2 After processing under conditions of a sealing time of 2 seconds and a sealing temperature of 140°C, the peel strength of the heat-sealed portion was measured using a load cell type tensile testing machine.

[0064] (water resistance) The obtained heat-sealable paper was folded at the corners with the heat-sealed side facing inward to create a box-shaped container without a lid, and then colored water was poured inside. The condition was checked after 24 hours, and the water resistance was evaluated according to the following three criteria. A rating of A or B indicates good water resistance. A: The water has not permeated the inside of the container. B: Water has permeated the inside of the container, but it is not leaking from the outside surface of the container. C: Water is leaking from the outer surface of the container.

[0065] Table 1 shows the evaluation results for each example and comparative example.

[0066] [Table 1]

[0067] As shown in Table 1, the paper substrate is made primarily of unbleached softwood kraft pulp, the paper substrate has a folding strength in the flow direction of 500 to 1,000 cycles, a folding strength in the width direction of 80 to 200 cycles, a Beck smoothness of the surface of the heat seal layer laminated surface of 3 seconds to 230 seconds, and the coating amount of the heat seal layer, which is mainly composed of ethylene (meth)acrylic acid copolymer alkali neutralized product, is 1.5 g / m². 2 More than 10.0g / m 2 Examples 1 to 7, described below, showed high folding strength in both the flow direction and the width direction, and exhibited good dynamic water absorption test values, heat seal strength, and water resistance. Furthermore, it was found that the folding strength in the width direction was significantly improved by the formation of the heat seal layer. As a result, the appearance of the folds during bag making was excellent. On the other hand, Comparative Examples 1 and 2, in which the paper substrate did not contain unbleached softwood kraft pulp, and the paper substrate had a folding strength in the flow direction of less than 500 cycles and a folding strength in the width direction of less than 80 cycles, exhibited inferior heat seal strength. Furthermore, the appearance of the folds during bag making was noticeably poor. Furthermore, Comparative Example 3, in which the heat seal layer did not contain an alkali neutralized ethylene (meth)acrylic acid copolymer and whose main component was a metal neutralized salt of ethylene acrylic acid copolymer, exhibited inferior water resistance. Comparative Example 4 is made of polypropylene (PP) laminated paper, which is undesirable from the standpoint of environmental pollution. Furthermore, because heat-sealable paper has low moisture permeability, it is considered that using it as packaging material may reduce the freshness of the contents.

[0068] The results above demonstrate that this heat-sealable paper ensures heat-seal strength while also providing excellent crack suppression due to folds in the areas where the heat-sealed layers are laminated, as well as superior water resistance. Furthermore, since this heat-sealable paper does not contain laminate paper or plastic film, which have a high environmental impact, it contributes to environmental protection. [Industrial applicability]

[0069] The heat-sealable paper of the present invention is environmentally friendly, ensures heat-seal strength, and exhibits excellent crack suppression due to folds in the areas where the heat-sealable layers are laminated, as well as superior water resistance, making it suitable for packaging materials.

Claims

1. A paper base material primarily composed of unbleached coniferous kraft pulp, A heat seal layer laminated on one side of the above paper substrate and It is equipped with, The above paper substrate has a folding strength in the flow direction of 500 to 1,000 cycles, and a folding strength in the width direction of 80 to 200 cycles. The Beck smoothness of the surface of the heat seal layer laminate is 3 seconds or more and 230 seconds or less. The main component of the heat seal layer described above is solely ethylene (meth)acrylic acid copolymer alkali neutralized product. The content of the ethylene (meth)acrylic acid copolymer alkali neutralized product relative to the total solid content of the above heat seal layer is 90.0% by mass or more. The coating amount of the above heat seal layer is 1.5 g / m². ２ 10.0g / m or more ２ The following is heat-sealable paper.

2. A paper substrate mainly composed of unbleached softwood kraft pulp, A heat seal layer laminated on one side of the above paper substrate and It is equipped with, The bending strength in the width direction is 240 times or more. The Beck smoothness of the surface of the heat seal layer laminate is 3 seconds or more and 230 seconds or less. The main component of the heat seal layer is an alkali neutralized ethylene (meth)acrylic acid copolymer. Heat-sealable paper having a coating amount of the above-mentioned heat-seal layer of 1.5 g / m² or more and 10.0 g / m² or less.

3. The bending strength in the flow direction is 604 times or more and 610 times or less, The heat-sealable paper according to claim 1 or claim 2, wherein the folding strength in the width direction is 453 times or more and 1208 times or less.

4. The heat-sealable paper according to any one of claims 1 to 3, wherein the content of unbleached softwood kraft pulp in the total pulp of the paper substrate is 95% by mass or more.

5. The heat-seal paper according to any one of claims 1 to 4, wherein the dynamic water absorption test value after 100 seconds in the dynamic water absorption test of the coated surface of the heat-seal layer, measured using the "Surface Size Tester EST12.2" manufactured by emtec electric, is 85.0% or more and 97.0% or less.

6. The heat-seal paper according to any one of claims 1 to 5, wherein the Beck smoothness of the surface of the heat-seal layer is 20 seconds or more and 250 seconds or less.

7. Moisture permeability of 300 g / m ２ ・1,200g / m for 24hr or more ２ - Heat-sealable paper according to any one of claims 1 to 6, wherein the heat retention period is 24 hours or less.

8. A packaging material having heat-sealable paper according to any one of claims 1 to 7.

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