JP2024067475A - Wrapping Paper - Google Patents

Abstract

[Problem] To provide a packaging paper that combines water resistance, oil resistance, heat sealability and breathability and can be manufactured with a small number of coating cycles. [Solution] A packaging paper with water resistance, oil resistance and heat sealability, characterized in that it has a heat seal layer made of a mixed paper that mixes natural fibers and synthetic fibers, the proportion of synthetic fibers in the total mass of the heat seal layer is 50% or more, one side of the heat seal layer is coated with a coating agent made of paraffin wax, the basis weight of the coating agent before application is 15.0 to 50.0 g/m2, and the heat seal strength of the heat seal layer is 1.20 N/15 mm or more. [Selected Figure] None

Description

The present invention relates to a packaging paper that has water resistance, oil resistance, heat sealability and breathability.

Plastic films are widely used as packaging materials for food and other products. In recent years, efforts have been made to reduce the amount of plastic used in packaging materials in order to reduce the burden on the environment, and the replacement of plastic films with paper materials has been considered. However, paper materials alone have problems in that they are difficult to use as packaging materials because they have low water resistance and oil resistance, and are not heat-sealable.

Various methods for imparting barrier properties and heat sealability to paper materials have therefore been investigated. For example, Patent Documents 1 to 3 describe imparting barrier properties and heat sealability to base paper by laminating a coating layer containing a filler (pigment) and a coating layer containing a resin with heat sealability to the base paper. Patent Document 4 describes an greaseproof paper that is oil-resistant and breathable, which is made by providing an undercoat layer containing paraffin wax on both sides of the base paper and laminating a topcoat layer containing a styrene-acrylic copolymer resin on the undercoat layer on one side.

JP 2022-048970 A JP 2022-024664 A JP 2020-183593 A JP 2020-084368 A

In Patent Documents 1 to 3, the gaps in the base paper are filled with a filler to provide barrier properties, but while this provides high barrier properties, it also lacks breathability, making it difficult for water vapor inside the packaging material to escape to the outside, making it unsuitable for packaging food products that generate water vapor. In addition, the provision of a coating layer containing a filler reduces the transparency of the packaging material, limiting its design potential. Furthermore, providing two or more coating layers as in Patent Documents 1 to 3 also complicates the manufacturing process.

The greaseproof paper in Patent Document 4 is oil-resistant and breathable, so it can also be used as a packaging material for food that generates moisture. However, like Patent Documents 1 to 3, it requires multiple coating layers, which makes the manufacturing process complicated.

Therefore, the present invention aims to provide a packaging paper that combines water resistance, oil resistance, heat sealability and breathability, and can be produced with a small number of coating cycles.

The present invention relates to a packaging paper having water resistance, oil resistance and heat sealability, which has a heat seal layer made of a mixed paper in which natural fibers and synthetic fibers are mixed, the proportion of synthetic fibers in the total mass of the heat seal layer is 50% or more, one side of the heat seal layer is coated with a coating agent made of paraffin wax, the basis weight before coating of the coating agent is 15.0 to 50.0 g/ ^m2 , and the heat seal strength of the heat seal layer is 1.20 N/15 mm or more.

The present invention provides a wrapping paper that combines water resistance, oil resistance, heat sealability, and breathability, and can be produced with a small number of coating cycles.

The wrapping paper according to the embodiment has water resistance, oil resistance, and heat sealability, and has at least a heat seal layer made of a mixed paper made of a mixture of natural and synthetic fibers. A coating agent made of paraffin wax is applied to one side of the heat seal layer to impart water resistance and oil resistance. The wrapping paper may be a single layer of heat seal layer, or may be a laminate with a base layer. In this specification, "wrapping paper" refers to paper after paraffin wax has been applied, and paper before paraffin wax is applied is referred to as "base paper". In addition, when the base layer and heat seal layer are laminated in the base paper, it is preferable that the ratio of the base layer to the mass (basis weight) of the base paper is 30 to 70 mass% from the viewpoint of achieving both heat sealability and oil resistance.

(Base layer)
The base layer may be made of natural fibers or a mixture of natural and synthetic fibers that is wet-laid. The proportion of natural fibers in the raw fibers of the base layer is not particularly limited, but is preferably 50 to 100% by mass of the raw fibers in order to reduce the resin proportion.

(Heat seal layer)
The heat seal layer is made of a base paper made by wet-processing a mixture of natural and synthetic fibers. The ratio of synthetic fibers in the raw fiber of the heat seal layer is 50% by mass or more and 85% by mass or less. If the ratio of synthetic fibers is less than 50% by mass, the heat sealability required for the packaging material is not sufficiently exhibited, which is not preferable. If the ratio of synthetic fibers exceeds 85% by mass, the paraffin wax is likely to penetrate to the back side of the coated surface, and water resistance and oil resistance are not sufficiently exhibited. If the ratio of synthetic fibers exceeds 85% by mass, the gaps in the heat seal layer become large and the penetration of paraffin wax into the paper decreases, resulting in poor water resistance and oil resistance.

(Natural Fibers)
As the natural fibers, for example, kraft pulp, non-wood pulp, recycled paper pulp, combinations of these pulps, etc. can be used.

Examples of kraft pulp include chemical pulps such as bleached hardwood kraft pulp (LBKP), bleached softwood kraft pulp (NBKP), unbleached hardwood kraft pulp (LUKP), unbleached softwood kraft pulp (NUKP), 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), pressed stone ground pulp (TGP), chemi-ground pulp (CGP), groundwood pulp (GP), and thermomechanical pulp (TMP), which may be used alone or in combination.

Examples of non-wood pulps that can be used include kenaf pulp, abaca pulp, cotton linter pulp, bagasse pulp, hemp pulp, straw pulp, bamboo pulp, etc., either alone or in combination.

Examples of waste paper pulp that can be used include disintegrated waste paper pulp, disintegrated and deinked waste paper pulp (DIP), and disintegrated, deinked and bleached waste paper pulp, which are produced from, for example, brown waste paper, kraft envelope waste paper, magazine waste paper, newspaper waste paper, flyer waste paper, office waste paper, cardboard waste paper, white waste paper, Kent waste paper, imitation waste paper, and land tax waste paper, and can be used alone or in combination.

(Synthetic fiber)
The material of the synthetic fiber used in the base layer is not particularly limited, and may be polyethylene terephthalate (PET), polypropylene (PP), polyethylene (PE), polyvinyl alcohol, vinylon, PLA, PBS, polyhydroxybutyrate (PHB), 3-hydroxybutyrate-3-hydroxyhexanoate copolymer (PHBH), polyamide such as nylon, acrylic resin, or other thermoplastic resin, either alone or in combination. Regenerated fibers such as rayon and lyocell may also be used as the synthetic fiber.

In addition, as the synthetic fiber, a core-sheath structure fiber having a core-sheath structure in which a core is covered by a sheath may be used. The material of the core and the sheath may be any thermoplastic resin selected from the group consisting of polyethylene terephthalate (PET), biopolyester, biopolyolefin, polylactic acid (PLA), polybutylene succinate (PBS), polypropylene (PP), high density polyethylene (HDPE), low density polyethylene (LDPE), polyvinyl alcohol (PVA), polyhydroxybutyrate (PHB), 3-hydroxybutyrate-3-hydroxyhexanoate copolymer (PHBH), nylon, and acrylic. The core and the sheath may be made of the same resin or different resins. In addition, two or more types of core-sheath structure fibers in which one or both of the core and the sheath are made of different materials may be mixed and used.

The material of the synthetic fibers used in the heat seal layer is selected from those mentioned above that have heat sealability. For example, polyethylene or fibers with a core-sheath structure can be suitably used as the synthetic fibers for the heat seal layer. In particular, from the viewpoint of reducing voids in the heat seal layer, it is preferable that the material contains 50% by mass or more of PE fibers (polyolefin hyperbranched fibers).

Additional additives such as pigments, surfactants, waxes, sizing agents, fillers, rust inhibitors, conductive agents, defoamers, dispersants, viscosity regulators, flocculants, coagulants, paper strength improving components, wet strength improving agents, retention improvers, paper powder shedding prevention agents, bulking agents, thickeners, etc. can be added to the raw fiber.

(Coating agent)
Paraffin wax is used as a coating agent to be applied to the heat seal layer. Paraffin wax having a melting point of 45 to 70°C can be suitably used, and paraffin wax having a melting point of 55 to 65°C is more preferable. When the packaging paper is constructed as a single heat seal layer, melted paraffin wax is applied to one side of the base paper (heat seal layer). When the packaging paper is constructed as a laminate of a heat seal layer and a base layer, melted paraffin wax is applied to the surface of the heat seal layer (the side opposite to the base layer). The side coated with paraffin wax is the side that comes into contact with the contents when used as packaging paper.

The paraffin wax applied to the heat seal layer solidifies while filling the gaps between the natural and synthetic fibers that make up the heat seal layer, providing water and oil resistance and preventing the penetration of water and oils from the heat seal surface. When the heat seal layers are butted together and heat sealed, the paraffin wax melts and seeps into the gaps inside the heat seal layer, exposing the synthetic fibers of the heat seal layer to the surface. Therefore, the heat sealability is not hindered by the paraffin wax.

The coating agent applied to the heat seal layer should not contain fillers (pigments), starch, or other resins (acrylic resins, heat sealable resins, etc.). When a coating agent containing additives such as fillers is used, the additives fill the gaps in the heat seal layer, making it possible to achieve water resistance and oil resistance. However, coating layers containing additives reduce the breathability of the packaging paper, making it unsuitable as packaging paper for food applications where moisture is important.

(Production method)
When the wrapping paper is made of a single heat-seal layer, the base paper can be produced by wet-processing the above-mentioned raw fiber and drying the obtained wet paper. When the wrapping paper is made of a laminate of a heat-seal layer and a base layer, a base paper in which the heat-seal layer and the base layer are laminated can be produced using a multi-layer papermaking machine. The base paper can also be produced by stacking a heat-seal layer made of the above-mentioned raw fiber and a base layer made of the above-mentioned raw fiber separately from the heat-seal layer, and laminating them together by a thermal calendar treatment.

The calendaring process can be carried out using calendaring equipment such as a hard nip calendar, soft calendar, or super calendar. The drying process can also be carried out using calendaring equipment that combines a metal roll and an elastic roll. The metal roll is a heated calendar roll made of cast steel. The elastic roll is a non-heated calendar roll made of materials such as cotton, epoxy resin, special polyester, or aramid.

The surface temperature (heating temperature) of the metal roll during the calendaring process is preferably 180°C or higher and 240°C or lower. If the surface temperature of the metal roll is less than 180°C, the synthetic fibers may not be sufficiently thermally fused, which may lead to a decrease in strength. Also, if the surface temperature of the metal roll exceeds 240°C, the temperature approaches the melting point of the synthetic fibers, and thermal calendaring may not be possible.

The linear pressure in the calendaring process is preferably 30 kg/cm or more and 350 kg/cm or less. From the viewpoint of improving the uniformity of the thickness of the packaging paper, the linear pressure in the calendaring process is more preferably 80 kg/cm or more and 180 kg/cm or less. If the linear pressure in the calendaring process is less than 30 kg/cm, the adhesion between the fibers may be insufficient. Furthermore, if the linear pressure exceeds 350 kg/cm, the bonds between the fibers may be destroyed, and the strength of the packaging paper may be reduced.

Next, a coating agent consisting of molten paraffin wax is applied to the heat seal layer and solidified. The amount of paraffin wax applied is preferably 2.0 to 10.0 g/ ^m2 . If the amount of paraffin wax applied is less than 2.0 g/ ^m2 , water resistance and oil resistance may be insufficient, which is not preferable. Also, if the amount of paraffin wax applied is more than 10.0 g/ ^m2 , the amount of paraffin wax melted during heat sealing increases, which may inhibit heat sealability and result in insufficient heat seal strength, which is not preferable.

(Basis weight: before coating)
The basis weight of the base paper before the coating agent is applied is 15.0 to 50.0 g/ ^m2 . The basis weight is a value measured in accordance with "Paper and paperboard - Method of measuring basis weight" JIS P8124 (2011). If the basis weight is less than 15.0 g/ ^m2 , the air permeability is small, and even if the coating agent is applied, water resistance and oil resistance may not be obtained. In addition, if the basis weight is less than 15.0 g/ ^m2 , the strength required for the packaging paper may not be obtained. If the basis weight is more than 50.0 g/ ^m2 , the stiffness becomes too high, and the processing suitability when packaging using the packaging paper decreases. In addition, heat transfer during heat sealing processing becomes poor, heat sealing strength decreases, and heat sealing processing suitability decreases. The basis weight is the value of the entire base paper. In other words, when the base paper consists of a single heat seal layer, the basis weight of the base paper is the basis weight of the single heat seal layer, and when the base paper is a laminate of a heat seal layer and a base material layer, the basis weight of the base paper is the basis weight of the entire laminate.

(Heat seal strength)
The heat seal strength of the packaging paper after application of the coating agent is 0.80 N/15 mm or more, and preferably 1.50 N/15 mm or more. The heat seal strength is measured by overlapping the heat seal surfaces of the packaging paper with each other, heat sealing for 1 second at a temperature of 180 ° C. and a pressure of 1 kgf / cm ² using a thermal gradient tester (TYPE HG-100 manufactured by Toyo Seiki), cutting the sample into a width of 15 mm, fixing both ends that are not heat sealed to the fixtures of the tensile tester at the top and bottom, forming a T-shape, pulling the heat seal portion up and down, and peeling it off. If the heat seal strength is 1.20 N / 15 mm or more, it is suitable for use as a packaging material.

(Heat seal strength ratio before and after application of coating agent)
The heat seal strength ratio calculated by the following formula is preferably 50% or more. If the heat seal strength ratio is 50% or more, the heat sealability is maintained even after applying paraffin wax to the heat seal layer, making it suitable for use as a packaging material.
Heat seal strength ratio = heat seal strength of packaging paper after coating with coating agent / heat seal strength of base paper before coating with coating agent x 100 (%)

(Air permeability)
The air permeability of the wrapping paper is preferably 10 to 3600 seconds. The air permeability is a value measured in accordance with JIS P 8117 (2009). If the air permeability value is within this range, when the wrapping paper is used for packaging food that generates water vapor, the generated water vapor can be released to the outside.

(Water repellency, oil repellency)
The water repellency of the wrapping paper is preferably R6 or more, and the oil repellency (kit value) of the wrapping paper is preferably 3 or more. The water repellency is the value of the heat-sealed surface measured in accordance with JAPAN TAPPI No. 68, and the oil repellency is the value of the heat-sealed surface measured in accordance with JAPAN TAPPI No. 41. If the water repellency and oil repellency are within these ranges, the paper has good water resistance and oil resistance, and is suitable for packaging foods that generate water vapor or foods that contain fats and oils.

As described above, the packaging paper according to the present embodiment is constructed by applying a coating agent made of paraffin wax to a base paper including a heat seal layer made by papermaking raw fibers including natural fibers and synthetic fibers, the ratio of synthetic fibers being 50% by mass or more. By applying paraffin wax to the heat seal layer having heat sealability, the gaps in the heat seal layer are filled, and water resistance and oil resistance are exhibited. In addition, since the coating agent applied to the heat seal layer is made of paraffin wax and does not contain additives such as fillers, the gaps in the heat seal layer are not excessively filled by additives, and the breathability of the base paper is maintained. In addition, the packaging paper according to the present embodiment is more transparent than conventional packaging paper coated with a coating agent containing fillers, and does not impair the design as a packaging material. In addition, since the packaging paper according to the present embodiment can be manufactured by applying the coating agent once to one side of the base paper, the manufacturing process and raw materials used are fewer than conventional packaging paper that is coated multiple times, and the productivity is excellent. In addition, since the basis weight of the base paper before the coating agent is applied is 15.0 to 50.0 g/ ^m2 , the strength required for packaging paper and processing suitability during packaging are also good.

Therefore, this embodiment makes it possible to realize a packaging paper that combines water resistance, oil resistance, heat sealability, and breathability, and can be produced with a small number of coating cycles.

The following describes specific examples of the present invention. However, the present invention is not limited to these examples.

First, raw material fibers containing pulp fibers and/or synthetic fibers in the ratios shown in Table 1 were wet-laid and dried to produce base papers according to Examples 1 to 4 and 6 to 10 and Comparative Examples 1 to 6. The base papers according to Examples 1 to 4 and 6 to 10 and Comparative Examples 1, 2, 5 and 6 were made using a multi-layer papermaking machine. For Example 5, the raw material fibers shown in Table 1 were used to make the base layer and the heat seal layer separately, and then the base layer and the heat seal layer were bonded together by a thermal calendaring process to produce the base paper. Note that the base paper according to Example 1 had a basis weight of 15.85 g/m ² for the base layer and a basis weight of 15.85 g/m ² for the heat seal layer.

Next, a coating agent having the composition shown in Table 1 was applied to the surface of the heat seal layer and solidified to obtain packaging papers according to Examples 1 to 10 and Comparative Examples 1 to 6. The paraffin wax used in the coating agent was paraffin wax (Paraffin Wax-135, melting point 59°C) manufactured by Nippon Seiro Co., Ltd., and the acrylic resin used was Harvil B-7 manufactured by Daiichi Toryo Seizo Co., Ltd. The amount of coating agent applied was as shown in Table 2.

The blending ratios of the raw fiber and the coating agent shown in Table 1 are based on mass. Polyolefin hyperbranched fiber was used for the PE fiber. PET core-sheath, PP core-sheath, PLA core-sheath, and PET/PE core-sheath shown in Table 1 refer to core-sheath fiber with a core and sheath made of PET, core-sheath fiber with a core and sheath made of PP, core-sheath fiber with a core and sheath made of PLA core-sheath, and core-sheath fiber with a core made of PET and a sheath made of PE, respectively. Bonnell M.V.P (H400) manufactured by Mitsubishi Chemical Corporation was used as the acrylic-based fiber.

The basis weight, thickness, density, heat seal strength and air permeability of the base paper (before coating) and wrapping paper (after coating) of each Example and Comparative Example, as well as the oil resistance (kit value) and water repellency of the heat seal surface of the wrapping paper, were measured as follows: The coating amount of the coating agent shown in Table 2 was calculated from the difference in basis weight before and after coating.
(1) Basis weight was measured in accordance with JIS P 8124 (2011).
(2) The thickness was measured in accordance with JIS P 8118 (2014) at a measurement pressure of 100 kPa.
(3) Density was measured in accordance with JIS P 8118 (2014).
(4) Heat seal strength was measured in accordance with JIS Z 0238 (1998) using a sample obtained by overlapping two pieces of packaging paper with their heat seal surfaces facing each other and heat sealing the pieces for one second at 180°C and 1 kgf/ ^cm2 . A sample with a measured heat seal strength of 1.20 N/15 mm was deemed to have heat sealability.
(5) Air permeability was measured in accordance with JIS P 8117 (2009). The value was measured for one substrate.
(6) Oil repellency (kit value) was measured in accordance with JAPAN TAPPI No. 41. If the kit value was 3 or more, it was determined that the sample had oil resistance.
(7) Water repellency was measured in accordance with JAPAN TAPPI No. 68. If the water repellency was R6 or higher, it was determined that the sample had water resistance.

Table 2 shows the evaluation values of the packaging paper for each example and each comparative example.

All of the packaging papers according to Examples 1 to 10 had water resistance, oil resistance, and heat sealability. In addition, the packaging papers according to Examples 1 to 10 had air permeability of 10 to 3600 seconds, and had air permeability suitable for packaging food that generates water vapor. In addition, because the water resistance and oil resistance were achieved by applying paraffin wax that does not contain fillers as a coating agent, the packaging papers according to Examples 1 to 10 could be manufactured with a small number of coatings.

In contrast, the packaging papers of Comparative Examples 1 and 2 used a coating agent containing an acrylic resin, so the acrylic resin in the coating layer did not melt at the heat sealing temperature, hindering the heat sealing and resulting in insufficient heat sealing properties. In addition, the acrylic resin filled the gaps in the base paper, increasing the air permeability and failing to provide the breathability required for food packaging applications.

The wrapping paper in Comparative Example 3 had a heat seal layer made only of synthetic fibers and did not contain natural fibers, so the gaps in the heat seal layer were large and the coating material leaked through to the back of the coated surface, resulting in poor water and oil resistance.

The wrapping paper in Comparative Example 4 was made using only natural fibers, and therefore had no heat sealability and was not suitable for use as a packaging material.

The packaging paper in Comparative Example 5 had a low basis weight of the base paper before the coating agent was applied, so the air permeability was low and the coating agent leaked through to the back side of the coated surface, resulting in poor water and oil resistance.

The packaging paper in Comparative Example 6 had a high basis weight of the base paper before the coating agent was applied, and also had a high proportion of the base layer made of pulp fibers, so it had poor thermal conductivity during heat sealing and insufficient heat seal strength under the same conditions.

The present invention can be used as packaging paper, and is particularly suitable for packaging foods that generate water vapor or foods that contain fats and oils.

Claims (6)

The heat seal layer is made of a mixed paper containing natural fibers and synthetic fibers,
The proportion of the synthetic fiber in the total mass of the heat seal layer is 50% or more,
A coating agent made of paraffin wax is applied to one surface of the heat seal layer,
The coating agent has a basis weight before coating of 15.0 to 50.0 g/ ^m2 ;
A wrapping paper characterized in that the heat seal strength of the heat seal layer is 1.20 N/15 mm or more. The wrapping paper according to claim 1, characterized in that the application amount of the paraffin wax is 2.0 to 10.0 g/ ^m2 . The wrapping paper according to claim 1 or 2, characterized in that the heat seal strength of the heat seal layer after the coating agent is applied is 50% or more of the heat seal strength of the heat seal layer before the coating agent is applied. The wrapping paper according to claim 3, characterized in that the air permeability measured in accordance with JIS P 8117 (2009) is 10 to 3600 seconds. The wrapping paper according to claim 3, characterized in that a base layer made by papermaking raw fibers containing natural fibers is laminated on the other side of the heat seal layer. The water repellency measured in accordance with JAPAN TAPPI No. 68 is R6 or more;
The wrapping paper according to claim 3, which has a kit value of 3 or more as measured in accordance with JAPAN TAPPI No. 41.