JP2024081432A - Paper multi-sheet - Google Patents

Abstract

[Problem] To provide a paper mulch sheet that can be fully applied to agricultural crops with long growing periods. [Solution] A paper mulch sheet comprising base paper and a coating layer formed on at least one side of the base paper, the base paper containing 40% by weight or more of wood pulp as papermaking fibers, and the coating layer containing lignin and resin in a weight ratio (lignin:resin) of 30:70 to 99:1. [Selected drawing] None

Description

The present invention relates to a paper mulch sheet used in the agricultural and horticultural fields.

Mulch sheets are agricultural materials that cover the ground around crops or flowers, and are used to improve the quality and yield of crops or flowers by regulating soil temperature and suppressing the growth of weeds. Plastic films such as polyethylene and polyvinyl chloride are generally used as mulch sheets. However, these plastic films are not biodegradable and must be collected and incinerated after use. Furthermore, collection and incineration requires special equipment, a great deal of effort and time, and is problematic in terms of environmental pollution.

In light of this, the use of mulch sheets made of biodegradable materials that can be plowed into the soil after harvesting while maintaining their shape during laying is expanding. For example, Patent Document 1 proposes a mulch sheet made of a resin composition in which powdered activated carbon is dispersed in a biodegradable resin. Patent Document 2 proposes an agricultural mulch sheet that is mainly composed of wood pulp containing 30% to 70% by weight of mechanical pulp and 30% to 60% by weight of chemical pulp, contains 0.05% to 10% by weight of a penetration assistant per pulp, has a basis weight of 50 g/m ² to 120 g/m ² , and has a water permeability of 30 ml/m ² min to 500 ml/m ² min.

JP 2007-68458 A JP 2000-102329 A

Among mulch sheets, those made of biodegradable resin or paper have the advantage that they do not require collection or disposal after use. However, mulch sheets made of biodegradable resin have problems such as deterioration during product storage and entanglement in the rotary when plowing. Paper mulch sheets are thick (heavy) to increase strength, making them difficult to install, and are unsuitable for use on crops with long growing periods, which deteriorate before harvesting due to their fast biodegradation, or have poor resistance to wind and rain. A method has been proposed to extend the biodegradation period by subjecting paper mulch sheets to chemical treatment, but the processing method is complicated, there are issues with productivity, and there is also a problem of a decrease in the strength of the paper.
An object of the present invention is to provide a paper mulch sheet that can be adequately applied to agricultural crops that have a long growing period.

The means for solving the problems of the present invention are as follows.
1. A paper substrate having a base paper and a coating layer on at least one surface of the base paper;
The base paper contains 40% by weight or more of wood pulp as papermaking fibers,
The paper mulch sheet is characterized in that the coating layer contains lignin and resin in a weight ratio (lignin:resin) of 30:70 to 99:1.
2. The paper mulch sheet according to 1., characterized in that the lignin content is 0.3 g/ ^m2 or more.
3. The paper mulch sheet according to 1. or 2., wherein the resin comprises at least one of a polylactic acid-based resin and a polyvinyl alcohol-based resin.
4. The paper mulch sheet according to 1. or 2., characterized in that the base paper has a basis weight of 20 g/ ^m2 or more and 50 g/ ^m2 or less.
5. The paper mulch sheet according to 1. or 2., characterized in that the tear strength in the MD direction measured in accordance with JIS P8116 is 300 mN or more.

The paper mulch sheet of the present invention can be applied to crops that are slow to biodegrade and have long growing periods. The biodegradability of the paper mulch sheet of the present invention can be adjusted by adjusting the blending ratio of wood pulp.

The paper mulch sheet of the present invention comprises a base paper and a coating layer containing lignin coated on at least one surface of the base paper.
(Base paper)
The base paper is a sheet made of papermaking fibers, fillers, various auxiliaries, and the like.
The base paper of the present invention contains at least 40% by weight of wood pulp as papermaking fibers.
The wood pulp may be one or more of bleached hardwood kraft pulp (LBKP), bleached softwood kraft pulp (NBKP), unbleached hardwood kraft pulp (LUKP), unbleached softwood kraft pulp (NUKP), chemical pulp such as sulfite pulp, mechanical pulp such as stone-ground pulp and thermomechanical pulp, waste paper pulp derived from wood pulp, dissolving pulp, mercerized pulp, etc. Among these, NBKP and NUKP are preferred because of their excellent strength, and NUKP is more preferred.

The higher the ratio of wood pulp to the total papermaking fibers of the base paper, the faster the biodegradation will be; however, the paper mulch sheet of the present invention has a coating layer that contains lignin, which makes it possible to suppress biodegradation even with a high wood pulp content.
In the paper mulch sheet of the present invention, from the viewpoints of biodegradation rate and cost, the blending ratio of wood pulp to the total papermaking fibers is preferably 50% by weight or more, more preferably 60% by weight or more. The base paper can be made of 100% by weight of wood pulp, but if the blending ratio of wood pulp is too high, biodegradation may become rapid, so the blending ratio of wood pulp is preferably 95% by weight or less, more preferably 90% by weight or less.

The papermaking fiber used in combination with wood pulp is not particularly limited, and may include, for example, non-wood pulp such as bast fibers such as flax, kenaf, paper mulberry, and mitsumata, hard fibers such as bagasse, bamboo, and esparto, seed hair fibers such as cotton pulp, and leaf sheath and leaf fibers such as Manila hemp pulp and sisal hemp pulp; and synthetic fibers such as polylactic acid fibers, regenerated cellulose fibers, polybutylene succinate fibers, polyethylene succinate fibers, polybutylene succinate-adipate fibers, polybutylene succinate-carbonate fibers, polycaprolactone fibers, and polyvinyl alcohol fibers. The synthetic fibers may be either fibers made of a single material or composite fibers having a core and a sheath. The synthetic fibers are preferably biodegradable fibers, and more preferably polylactic acid fibers and composite fibers having a core of polylactic acid and a sheath of polybutylene succinate.
When polylactic acid fibers are used, the fineness is preferably 0.01 dtex or more and 3.0 dtex or less, more preferably 0.05 dtex or more and 1.5 dtex or less, and the fiber length is preferably 2 mm or more and 10 mm or less, more preferably 3 mm or more and 8 mm or less.

The base paper may contain known internal additives. Because the paper mulch sheet is laid on the ground, it is preferable that the base paper contains a wet strength agent. It is preferable that the wet strength agent is contained in an amount of 0.5 to 3 parts by weight per 100 parts by weight of papermaking fiber.

The method for producing the base paper (papermaking) is not particularly limited, and the paper base material can be produced by making paper using a known Fourdrinier former, on-top hybrid former, gap former machine, etc., using an acidic papermaking, neutral papermaking, or alkaline papermaking method. The base paper may be one layer or may be composed of two or more layers.

The basis weight of the base paper of the present invention is not particularly limited, but can be, for example, 15 g/ ^m2 or more and 80 g/ ^m2 or less. From the viewpoint of strength, the basis weight of the base paper is preferably 18 g/ ^m2 or more, and more preferably 20 g/ ^m2 or more. From the viewpoint of weight reduction, the basis weight of the base paper is preferably 60 g/ ^m2 or less, more preferably 50 g/ ^m2 or less, even more preferably 45 g/ ^m2 or less, and even more preferably 35 g/ ^m2 or less.

(Coating layer)
The coating layer is formed by coating on at least one surface of the base paper. The coating layer may contain lignin and resin in a weight ratio (lignin:resin) of 30:70 to 99:1, and may contain additives such as external papermaking chemicals such as surface strength agents and sizing agents, colorants, surfactants, dispersants, thickeners, water retention agents, defoamers, and water repellents. In addition, the coating layer may contain one or more chemicals such as insecticides, miticides, nematicides, fungicides, plant growth regulators, and fertilizers.
Lignin, along with cellulose and hemicellulose, is a major component of woody plants. Although lignin is decomposed by white-rot fungi, the decomposition is slow. By forming a coating layer on the paper mulch sheet, the decomposition rate can be slowed down.
The lignin is not particularly limited as long as it can be dissolved and applied, but lignin sulfonic acid or a salt thereof is preferred because it is easy to prepare the coating material. Examples of the salt of lignin sulfonic acid include calcium salt, magnesium salt, and sodium salt. As the lignin sulfonic acid or a salt thereof, commercially available products such as Sanex M-100, Sanex P321, Sanex 252, Sanex P252, Sanex SCL, Sanex SCP, Sanex FDL, Vanillex N, Pearlex NP, and Pearlex DP (all manufactured by Nippon Paper Industries Co., Ltd.) can be used.

In the present invention, the coating layer contains lignin and resin in a weight ratio (lignin:resin) of 30:70 to 99:1. If the amount of lignin is less than this range, biodegradation may not be slowed down sufficiently. On the other hand, if the amount of lignin is more than this range (less resin), the adhesion of the lignin to the base paper may decrease, and the base paper may biodegrade faster than when the amount of resin is within the above range, despite the high lignin content. A weight ratio of lignin to resin of 35:65 to 95:5 is more preferable.

The resin plays a role in adjusting the adhesion of lignin to the base paper. Resins that are generally blended as binders in coating liquids can be used without any particular restrictions, and for example, one or more of polylactic acid resins, polyvinyl alcohol resins, acrylic resins, starches, cellulose ethers, styrene-butadiene copolymers, methyl methacrylate-butadiene copolymers, vinyl acetate resins, etc. can be used. Of these, one or more of polylactic acid resins and polyvinyl alcohol resins are preferred because they are biodegradable.

There are no particular limitations on the method of applying the coating material for forming the coating layer to the base paper, and coating can be performed using known coating equipment and coating systems. Examples of coating equipment include blade coaters, bar coaters, nip coaters, roll coaters, air knife coaters, reverse roll coaters, curtain coaters, spray coaters, size press coaters, and gate roll coaters. Examples of coating systems include water-based coating using a solvent such as water, and solvent-based coating using a solvent such as an organic solvent, with water-based coating being preferred.

In the present invention, the coating amount of the coating layer on one side is preferably 0.8 g/ ^m2 or more and 10 g/ ^m2 or less in dry weight. If the coating amount is less than 0.8 g/ ^m2 , the biodegradation inhibitory effect may not be fully exhibited. On the other hand, if it is more than 10 g/ ^m2 , the drying load during coating increases and the paper mulch sheet becomes heavy. The coating amount of the coating layer is more preferably 1.0 g/ ^m2 or more, even more preferably 1.1 g/ ^m2 or more, and more preferably 8 g/ ^m2 or less, and even more preferably 5 g/ ^m2 or less.

In terms of inhibiting biodegradation, the paper mulch sheet of the present invention preferably has a lignin content of 0.3 g/ ^m2 or more in dry weight. The lignin content is preferably 0.35 g/ ^m2 or more in dry weight, and more preferably 0.4 g/ ^m2 or more. There is no particular upper limit to the lignin content, but it is, for example, about 5 g/ ^m2 . Even if the lignin content is increased beyond this, the biodegradation inhibition effect saturates and hardly improves, and the raw material cost increases, resulting in high costs.

(Paper multi-sheet)
The paper mulch sheet has a base paper and a coating layer containing lignin formed by coating on at least one side of the base paper. The paper mulch sheet may have coating layers on both sides of the base paper, or may have layers other than the base paper and the coating layer.
The paper mulch sheet preferably has a (dry) tensile strength in the MD direction of 15 N/15 mm or more and a wet tensile strength in the MD direction of 5 N/15 mm or more, as measured in accordance with JIS P8118, in order to prevent breakage during the cultivation period. This (dry) tensile strength is more preferably 16 N/15 mm or more, and even more preferably 17 N/15 mm or more. This wet tensile strength is more preferably 6 N/15 mm or more, and even more preferably 7 N/15 mm or more.
The paper multi-sheet preferably has a tear strength in the MD direction of 300 mN or more, as measured in accordance with JIS P8116, from the viewpoint of preventing breakage during installation work. This tear strength is more preferably 400 mN or more, and even more preferably 500 mN or more.

In order to inhibit the growth of weeds, it is preferable that the light transmittance of paper mulch sheets is 4.5% or less for both wavelengths of 660 nm and 450 nm. A paper mulch sheet whose transmittance satisfies this condition can inhibit the germination of weeds, and even if they germinate, they cannot photosynthesize efficiently, thereby inhibiting the growth of weeds. The light transmittance of both wavelengths of 660 nm and 450 nm is more preferably 4.0% or less, even more preferably 3.0% or less, even more preferably 2.5% or less, and even more preferably 2.0% or less.

The present invention will be described in more detail below with reference to examples, but the configuration of the present invention is not limited thereto.
The obtained base paper and paper mulch sheet were evaluated by the following measurement methods. The results are shown in Table 1.

(Evaluation method)
Basis Weight and Thickness Basis weight was measured in accordance with JIS P8124.
According to JIS P8118, the pressure between the pressure surfaces was set to 100 kPa, and the thickness of one sheet was measured.
Lignin content The coating amount was calculated from the basis weight before and after coating.
The lignin content was calculated from the coating amount and the weight ratio of lignin to the total solid content in the coating material.

Tensile Strength (Dry) Tensile strength was measured in accordance with JIS P8118.
The wet tensile strength was measured in accordance with JIS P8118 after the test sample, immersed in water, was sandwiched between filter paper to remove excess water adhering to the surface.
Tear Strength The tear strength was measured in accordance with JIS P8116.
Light transmittance The light transmittance (%) of a test sample was measured using a spectrophotometer (UV-160A, Shimadzu Corporation) for red light (660 nm) and blue light (450 nm), which are absorbed in large amounts during photosynthesis in plants.

Soil decomposition: 250 g of home gardening soil was placed in a 1.4 L Tupperware (soil moisture content: 20-30%). A sample piece for evaluation (width 16 cm, length 10 cm) was placed in a draining net, its weight was measured, and then it was placed on the soil surface, covered with 250 g of gardening soil, and buried. The Tupperware with the lid closed was placed in an environmental tester (Silvery Emperor, Kato Co., Ltd.) at 40°C to investigate decomposition in soil. After one week, the sample was taken out, the surface soil was washed off with running water, and the sample was air-dried, and then its weight was measured.
Soil decomposition was calculated as the reduction rate according to the following formula:
Reduction rate (%) = weight after test / weight before test × 100

"Example 1"
70 parts by weight of softwood unbleached kraft pulp (NUKP) with Canadian Standard Freeness (CSF) of 400 ml was blended with 30 parts by weight of polylactic acid fiber (Miracle Fiber KK-PL, manufactured by Yamato Boseki Co., Ltd., core polylactic acid, sheath polybutylene succinate, 2.4 dtex x 5 mm) to prepare raw fiber. A wet strength agent was added to the raw fiber at 2% by weight based on the fiber to prepare a paper stock, which was then made into a base paper with a target basis weight of 30 g/ ^m2 .
A paint made by mixing lignin (Sunex P321, magnesium lignin sulfonate, manufactured by Nippon Paper Industries Co., Ltd.) and polylactic acid resin (Rezem W-990, manufactured by Chukyo Yushi Co., Ltd.) in a solids weight ratio of 70:30 was applied to the base paper by size press impregnation coating, and the paper was dried at 110°C to form a coating layer, thereby obtaining a paper mulch sheet.

"Example 2"
A paper mulch sheet was obtained in the same manner as in Example 1, except that the blending ratio of lignin to polylactic acid resin was 53:47.
"Example 3"
A paper mulch sheet was obtained in the same manner as in Example 1, except that the blending ratio of lignin to polylactic acid resin was 40:60.
"Example 4"
A paper mulch sheet was obtained in the same manner as in Example 1, except that the blending ratio of lignin to polylactic acid resin was 90:10.
"Example 5"
A paper mulch sheet was obtained in the same manner as in Example 1, except that a polyvinyl alcohol resin (Poval 25-88KL, manufactured by Kuraray Co., Ltd.) was used instead of the polylactic acid resin.

"Comparative Example 1"
A paper mulch sheet was obtained in the same manner as in Example 1, except that only lignin was applied (the compounding ratio of lignin to polylactic acid resin was 100:0).
"Comparative Example 2"
A paper mulch sheet was obtained in the same manner as in Example 1, except that no coating layer was formed.

The paper mulch sheets obtained in Examples 1 to 5 of the present invention, although containing a high content of wood pulp, are slow to decompose in soil and can be applied to agricultural crops with long growing periods.
The paper mulch sheet obtained in Comparative Example 1, in which the coating layer did not contain resin, biodegraded faster than, for example, the paper mulch sheet obtained in Example 3, which had a lower lignin content. This is presumably because the absence of resin resulted in poor adhesion of lignin to the base paper, and the lignin was more likely to peel off from the base paper.

Claims (5)

A base paper and a coating layer on at least one surface of the base paper,
The base paper contains 40% by weight or more of wood pulp as papermaking fibers,
The paper mulch sheet is characterized in that the coating layer contains lignin and resin in a weight ratio (lignin:resin) of 30:70 to 99:1. 2. The paper mulch sheet according to claim 1, characterized in that the lignin content is 0.3 g/ ^m2 or more. The paper mulch sheet according to claim 1 or 2, characterized in that the resin contains one or more of a polylactic acid-based resin and a polyvinyl alcohol-based resin. 3. The paper mulch sheet according to claim 1, wherein the base paper has a basis weight of 20 g/ ^m2 or more and 50 g/ ^m2 or less. The paper mulch sheet according to claim 1 or 2, characterized in that the tear strength in the MD direction measured in accordance with JIS P8116 is 300 mN or more.