Stack production method

Abstract

This invention involves: preparing an aqueous coating liquid containing a phosphate and a polyhydroxyalkanoate resin having a solid content concentration of 20 wt.% to 60 wt.%, the pH of the liquid being 6.5 to 8; applying the liquid to a surface of a paper base material; and drying the liquid to form a resin layer.

Description

Manufacturing method of laminate

[0001] The present invention relates to a method for producing a laminate or molded article having a paper substrate and a resin layer containing a polyhydroxyalkanoate resin, an aqueous coating liquid for the paper substrate, and the laminate or molded article.

[0002] In recent years, environmental problems caused by discarded plastics have been attracting attention. In particular, marine pollution caused by discarded plastics is serious, and there are high hopes for the widespread use of biodegradable plastics that decompose in the natural environment.

[0003] Various types of biodegradable plastics are known, but among them, poly(3-hydroxybutyrate) resin, a type of polyhydroxyalkanoate resin, is a thermoplastic polyester that is produced and accumulated as an energy storage substance within the cells of many microbial species. Because it is a material that can biodegrade not only in soil but also in seawater, it has attracted attention as a material that can solve the above-mentioned problems.

[0004] A laminate obtained by laminating a layer mainly composed of a polyhydroxyalkanoate-based resin such as a poly(3-hydroxybutyrate)-based resin onto a biodegradable paper substrate is extremely promising from the viewpoint of environmental protection, since both the resin and the substrate are highly biodegradable materials.

[0005] As an example of a method for producing such a laminate, Patent Document 1 discloses a method for producing a laminate sheet by applying an aqueous coating liquid containing a poly(3-hydroxybutyrate) resin to a paper substrate and then heating the coating liquid to a temperature equal to or higher than the melting point of the resin to form a resin layer.

[0006] Japanese Patent Application Laid-Open No. 2022-185708

[0007] According to the method of applying an aqueous coating liquid to a paper substrate described in Patent Document 1, a laminate can be produced in which a resin layer containing a polyhydroxyalkanoate resin as a main component is laminated on a paper substrate. However, it has been found that the laminate obtained by applying such an aqueous coating liquid to a paper substrate may cause discoloration of the paper substrate.

[0008] It was speculated that the discoloration of the paper substrate was due to the acidity of the aqueous coating solution containing the polyhydroxyalkanoate resin, which caused changes in the metal components contained in the paper substrate, resulting in the formation of black spots. Therefore, an attempt was made to add sodium hydroxide to the aqueous coating solution to adjust it to a near-neutral pH, and then apply this aqueous coating solution to the paper substrate to produce a laminate. It was found that this could suppress discoloration of the paper substrate.

[0009] However, aqueous coating solutions containing polyhydroxyalkanoate resins are sometimes used to produce laminates immediately after preparation, but are also sometimes used several days after preparation. It has been found that when used long after preparation, the problem of discoloration of the paper substrate can occur again, even in aqueous coating solutions adjusted to near neutrality by adding sodium hydroxide.

[0010] In view of the above-mentioned current situation, the present invention aims to provide a method for producing a laminate by applying an aqueous coating liquid containing a polyhydroxyalkanoate resin to a paper substrate, which method is capable of suppressing discoloration of the paper substrate regardless of the length of time from preparation to application of the aqueous coating liquid.

[0011] As a result of intensive research into solving the above-mentioned problems, the present inventors discovered that the above-mentioned problems can be solved by adjusting the pH of an aqueous coating liquid containing a polyhydroxyalkanoate resin to 6.5 to 8 using a phosphate instead of sodium hydroxide and applying the liquid to a paper substrate, thereby completing the present invention.

[0012] That is, the present invention relates to a method for producing a laminate comprising a paper substrate and a resin layer formed on at least one surface of the paper substrate, the method comprising the steps of: preparing an aqueous coating liquid containing a polyhydroxyalkanoate resin with a solids concentration of 20% to 60% by weight and a phosphate, and having a pH of 6.5 to 8; and applying the aqueous coating liquid to the surface of the paper substrate and drying it to form a resin layer. The present invention also relates to a method for producing a molded product, the method comprising the steps of producing a laminate by the laminate production method and shaping the laminate. The present invention also relates to an aqueous coating liquid for paper substrates, containing a polyhydroxyalkanoate resin with a solids concentration of 20% to 60% by weight and a phosphate, and having a pH of 6.5 to 8. The present invention also relates to a laminate comprising a paper substrate and a coating layer formed on at least one surface of the paper substrate, wherein the coating layer comprises a polyhydroxyalkanoate resin and 0.01 to 0.90% by weight of a phosphate salt relative to the total amount of the polyhydroxyalkanoate resin, and a molded article comprising the laminate.

[0013] According to the present invention, there is provided a method for producing a laminate by applying an aqueous coating liquid containing a polyhydroxyalkanoate resin to a paper substrate, which method can suppress discoloration of the paper substrate regardless of the length of time from preparation of the aqueous coating liquid to application. Furthermore, according to the present invention, it is possible to suppress a decrease in the molecular weight of the polyhydroxyalkanoate resin contained in the resin layer of the laminate. Therefore, a laminate including a resin layer having desired physical properties can be obtained.

[0014] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

[0015] A manufacturing method according to one embodiment of the present disclosure is for producing a laminate. The laminate includes at least a paper substrate and a resin layer formed on one or both sides of the paper substrate. Because the resin layer contains a polyhydroxyalkanoate resin as a main component, the laminate as a whole can exhibit biodegradability.

[0016] (Paper base material) The paper base material is composed of a sheet mainly made of pulp. The paper base material can be obtained by papermaking a paper stock containing pulp, fillers, various auxiliaries, etc. The type of paper that can be used is not particularly limited, and examples include cup base paper, kraft paper, fine paper, coated paper, tissue paper, glassine paper, and paperboard.

[0017] The pulp is not particularly limited, and examples thereof include chemical pulps such as bleached hardwood kraft pulp (LBKP), bleached softwood kraft pulp (NBKP), unbleached hardwood kraft pulp (LUKP), unbleached softwood pulp (NUKP), and sulfite pulp; mechanical pulps such as stone-ground pulp and thermomechanical pulp; wood fibers such as deinked pulp and recycled paper pulp; and non-wood fibers obtained from kenaf, bamboo, hemp, etc. These can be used in appropriate combinations.

[0018] Among these, it is preferable to use chemical pulp or mechanical pulp made from wood fibers, and it is more preferable to use chemical pulp, for reasons such as the fact that foreign matter is less likely to be mixed into the paper, that discoloration is less likely to occur over time when recycled as a waste paper raw material, that the high whiteness results in a good surface appearance when printed, and that the value is particularly high when used as a packaging material. Specifically, it is preferable that the amount of chemical pulp such as LBKP or NBKP in the pulp is 80% or more, and it is particularly preferable that the amount of chemical pulp is 100%.

[0019] The filler is not particularly limited, and examples thereof include inorganic fillers such as talc, kaolin, calcined kaolin, clay, heavy calcium carbonate, light calcium carbonate, white carbon, zeolite, magnesium carbonate, barium carbonate, titanium dioxide, zinc oxide, silicon oxide, amorphous silica, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, barium sulfate, and calcium sulfate; and organic fillers such as urea-formalin resin, polystyrene resin, phenolic resin, and hollow microparticles. Note that fillers are not essential materials and may not be used.

[0020] The various auxiliaries are not particularly limited and include, for example, sizing agents such as rosin, alkyl ketene dimer (AKD), and alkenyl succinic anhydride (ASA), polyacrylamide polymers, polyvinyl alcohol polymers, cationized starch, various modified starches, dry strength agents such as urea-formalin resin and melamine-formalin resin, wet strength agents, retention aids, drainage aids, coagulants, aluminum sulfate, bulking agents, dyes, fluorescent whitening agents, pH adjusters, antifoaming agents, UV inhibitors, anti-fading agents, pitch control agents, slime control agents, etc. These may be selected and used as needed.

[0021] The surface of the paper may be treated with various chemicals. The chemicals are not particularly limited, and examples thereof include oxidized starch, hydroxyethyl etherified starch, enzyme-modified starch, polyacrylamide, polyvinyl alcohol, surface sizing agents, water-resistant agents, water-retaining agents, thickeners, lubricants, etc. Only one type of chemical may be used, or two or more types may be used in combination. Furthermore, these chemicals may be used in combination with pigments.

[0022] The pigment is not particularly limited, and examples thereof include inorganic pigments such as kaolin, clay, engineered kaolin, delaminated clay, heavy calcium carbonate, light calcium carbonate, mica, talc, titanium dioxide, barium sulfate, calcium sulfate, zinc oxide, silicic acid, silicates, colloidal silica, satin white, etc.; organic pigments such as solid, hollow, or core-shell type pigments, etc. Only one type of pigment may be used, or two or more types may be used in combination.

[0023] The basis weight of the paper substrate can be appropriately selected depending on the desired quality and the use of the laminate, but is preferably 40 g / m ２ More than 400g / m ２ Preferably, it is 50 g / m or less. ２ 350g / m or more ２ When the laminate is used for packaging materials such as wrapping paper, paper bags, lids, liner papers, and soft packaging materials, or posters to be used outdoors, the weight is preferably 40 g / m or less. ２ 150g / m or more ２ It is more preferable that the soft packaging material is a packaging material having a density of 40 g / m or less. ２ ~100g / m ２ In addition, when the laminate is used for paper tableware such as paper cups, paper boxes, paper plates, paper trays, etc., or for lids and other paper containers, the thickness is set to 150 g / m. ２ More than 400g / m ２ It is more preferable that:

[0024] The density of the paper substrate can be appropriately selected depending on the desired quality, handling, etc., but is usually 0.5 g / cm ３ 1.0g / cm or more ３ It is preferable that:

[0025] The method for producing the paper base (papermaking) is not particularly limited, and can be carried out by appropriately selecting a known papermaking machine, such as a Fourdrinier papermaking machine, a cylinder papermaking machine, a short wire papermaking machine, or a twin-wire papermaking machine such as a gap former type or a hybrid former type (on-top former type). The pH during papermaking may be in the acidic range (acidic papermaking), pseudo-neutral range (pseudo-neutral papermaking), neutral range (neutral papermaking), or alkaline range (alkaline papermaking). After papermaking in the acidic range, an alkaline agent may be coated on the surface of the paper layer. The paper base may be composed of a single layer, or two or more layers.

[0026] When treating the surface of a paper substrate with a chemical, the method of surface treatment is not particularly limited, and known coating devices such as a rod metering size press, a pond type size press, a gate roll coater, a spray coater, a blade coater, or a curtain coater can be used.

[0027] (Resin layer) The resin layer formed on at least one surface of the paper substrate contains at least a polyhydroxyalkanoate-based resin (hereinafter also referred to as a PHA-based resin). Only one type of PHA-based resin may be used, or two or more types may be used in combination. Furthermore, the resin component contained in the resin layer may be only a PHA-based resin, or may further contain another resin. As the other resin, a biodegradable resin as described below can be used.

[0028] The polyhydroxyalkanoate resin is a general term for polymers containing hydroxyalkanoic acid as a monomer unit. The hydroxyalkanoic acid constituting the PHA resin is not particularly limited, but examples thereof include 3-hydroxybutanoic acid, 4-hydroxybutanoic acid, 3-hydroxypropionic acid, 3-hydroxypentanoic acid, 3-hydroxyhexanoic acid, 3-hydroxyheptanoic acid, and 3-hydroxyoctanoic acid. The PHA resin may be a homopolymer or a copolymer containing two or more types of monomer units.

[0029] The resin layer preferably contains a PHA-based resin in an amount of 50% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and even more preferably 90% by weight or more. By using a PHA-based resin as a main component, the resin layer can exhibit biodegradability.

[0030] The PHA resin is preferably a poly(3-hydroxybutyrate) resin (hereinafter also referred to as a P3HB resin). A P3HB resin refers to a homopolymer having only 3-hydroxybutyrate units and / or a copolymer containing 3-hydroxybutyrate units and other hydroxyalkanoate units. From the viewpoint of seawater decomposability, it is preferable to include a copolymer containing 3-hydroxybutyrate units and other hydroxyalkanoate units.

[0031] The type of copolymerization in the copolymer is not particularly limited, and may be random copolymerization, alternating copolymerization, block copolymerization, graft copolymerization, etc. Copolymers produced by microorganisms are usually random copolymers.

[0032] The hydroxyalkanoic acid forming the other hydroxyalkanoate unit is not particularly limited, and examples thereof include 4-hydroxybutanoic acid, 3-hydroxypropionic acid, 3-hydroxypentanoic acid, 3-hydroxyhexanoic acid, 3-hydroxyheptanoic acid, and 3-hydroxyoctanoic acid.

[0033] Specific examples of P3HB-based resins include poly(3-hydroxybutyrate) (abbreviation: PHB), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (abbreviation: P3HB3HH), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (abbreviation: P3HB3HV), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (abbreviation: P3HB4HB), and poly(3-hydroxybutyrate-co- Examples of suitable P3HB resins include poly(3-hydroxybutyrate-co-3-hydroxyoctadecanoate) (abbreviation: P3HB3HO), poly(3-hydroxybutyrate-co-3-hydroxydecanoate) (abbreviation: P3HB3HD), and poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-3-hydroxyhexanoate) (abbreviation: P3HB3HV3HH). Among these, PHB, P3HB3HH, P3HB3HV, and P3HB4HB are preferred due to their ease of industrial production. One type of P3HB resin may be used alone, or two or more types may be used in combination.

[0034] The resin layer preferably contains a P3HB-based resin in an amount of 50% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and even more preferably 90% by weight or more. By using a P3HB-based resin as a main component, the resin layer can exhibit biodegradability.

[0035] Among P3HB-based resins, P3HB3HH is particularly preferred from the viewpoints that its melting point and crystallinity can be varied by changing the composition ratio of the repeating units, thereby enabling adjustment of physical properties such as Young's modulus and heat resistance, and that it can be imparted with physical properties intermediate between those of polypropylene and polyethylene. It is also easy to produce industrially and is a physically useful plastic.

[0036] The resin layer preferably contains P3HB3HH in an amount of 50% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and even more preferably 90% by weight or more. By using P3HB3HH as a main component, the resin layer can exhibit biodegradability, particularly seawater degradability.

[0037] The average content ratio of the constituent monomers in the P3HB resin is preferably 3HB units / other hydroxyalkanoate units = 97-70 / 3-30 (mol % / mol %), and more preferably 3HB / 3HH = 94-82 / 6-18 (mol % / mol %). When the average content ratio of other hydroxyalkanoate units in the P3HB resin is 3 mol % or more, good adhesion can be obtained by heat sealing using the resin layer. Furthermore, P3HB resins with an average content ratio of other hydroxyalkanoate units of 30 mol % or less do not have an excessively slow crystallization rate and are relatively easy to produce.

[0038] When a P3HB-based resin contains P3HB3HH, the P3HB-based resin having an average 3HH content of 3 to 30 mol % may be composed of one type of P3HB3HH, or may be composed of a mixture of at least two types of P3HB3HH having different content ratios of constituent monomers, or may be composed of a mixture of at least one type of P3HB3HH and PHB.

[0039] A preferred combination of P3HB3HH or PHB in the mixture is a combination of P3HB3HH having a 3HH unit content of 8 to 25 mol % and P3HB3HH or PHB having a 3HH unit content of less than 8 mol %. By using such a combination, good adhesive strength can be achieved in a short time after heat sealing using the resin layer, even if the heat sealing temperature rises to a temperature at which sufficient adhesion is possible.

[0040] In the above combination, the content of 3HH units in P3HB3HH having a content of 3HH units of less than 8 mol% is preferably 5 mol% or less, more preferably 3 mol% or less, and even more preferably 1 mol% or less. The lower limit of the content of 3HH units in P3HB3HH is not particularly limited, but may be, for example, 0.1 mol% or more.

[0041] The amount of P3HB3HH or PHB containing less than 8 mol% of 3HH units is not particularly limited, but is preferably 0 to 50% by weight relative to the total weight of the P3HB-based resin contained in the resin layer. When used, the amount is preferably 1 to 50% by weight, more preferably 3 to 30% by weight, even more preferably 4 to 20% by weight, and particularly preferably 5 to 15% by weight.

[0042] The average content ratio of each constituent monomer in a P3HB-based resin can be determined by a method known to those skilled in the art, for example, the method described in paragraph

[0047] of WO 2013 / 147139, or by NMR measurement. The average content ratio refers to the molar ratio of 3HB units to other hydroxyalkanoate units in the entire P3HB-based resin contained in the resin layer. When the P3HB-based resin is a mixture containing at least two types of P3HB3HH, or when it is a mixture containing at least one type of P3HB3HH and PHB, the average content ratio refers to the molar ratio of each monomer unit contained in the entire mixture.

[0043] The weight-average molecular weight (hereinafter sometimes referred to as Mw) of the PHA resin contained in the resin layer can be selected as appropriate, but from the viewpoint of achieving both mechanical properties and processability, it is preferably 50,000 to 900,000, more preferably 100,000 to 800,000, and even more preferably 150,000 to 700,000. When the weight-average molecular weight of the PHA resin is 50,000 or more, good mechanical properties are obtained, and when it is 900,000 or less, good adhesiveness can be obtained by heat sealing.

[0044] The weight average molecular weight of the PHA resin can be determined as a molecular weight converted into polystyrene by gel permeation chromatography (GPC) (Shodex GPC-101 manufactured by Showa Denko K.K.) using a polystyrene gel (Shodex K-804 manufactured by Showa Denko K.K.) as a column and chloroform as a mobile phase.

[0045] A specific method for producing P3HB3HH is described in, for example, WO 2010 / 013483. Commercially available P3HB3HH products include Kaneka Biodegradable Polymer Green Planet (registered trademark) manufactured by Kaneka Corporation.

[0046] The resin layer may contain, to the extent that the effects of the invention are not impaired, one or more of the following: resins other than PHA-based resins, adhesives, dispersants or emulsifiers, pH adjusters, inorganic fillers, colorants such as pigments and dyes, odor absorbers such as activated carbon and zeolite, fragrances such as vanillin and dextrin, plasticizers, antioxidants, weather resistance improvers, ultraviolet absorbers, crystal nucleating agents, lubricants, release agents, water repellents, antibacterial agents, and sliding property improvers, etc. However, these are optional components, and the resin layer may not contain these components.

[0047] Resins other than PHA-based resins that can be used in the resin layer are not particularly limited, but are preferably biodegradable resins. Specific examples include aliphatic polyester-based resins such as polycaprolactone, polybutylene succinate adipate, polybutylene succinate, and polylactic acid, and aliphatic aromatic polyester-based resins such as polybutylene adipate terephthalate and polybutylene azelate terephthalate. The blending amount of these resins other than PHA-based resins may be 50 parts by weight or less, 30 parts by weight or less, or 10 parts by weight or less, relative to 100 parts by weight of the PHA-based resin. It may also be 5 parts by weight or less, or 1 part by weight or less.

[0048] The thickness of the resin layer is not particularly limited and can be appropriately determined in consideration of the performance and productivity required of the resin layer, and may be, for example, 0.5 to 100 μm, or 1 to 30 μm.

[0049] (Aqueous Coating Liquid Preparation Process) In the manufacturing method according to this embodiment, first, an aqueous coating liquid containing a PHA-based resin is prepared. The aqueous coating liquid containing a PHA-based resin refers to a liquid in which at least resin particles containing a PHA-based resin are dispersed in water, and can also be referred to as an aqueous suspension or dispersion of a PHA-based resin. Components other than the resin particles may be dissolved or dispersed in the aqueous coating liquid as needed. Applying the aqueous coating liquid to a paper substrate has the advantage that, particularly when a resin layer is directly laminated on the paper substrate, a portion of the aqueous coating liquid penetrates into the paper substrate, which makes it easier to improve the adhesion of the resin layer to the paper substrate.

[0050] The solids concentration of the PHA resin in the aqueous coating liquid is 20% by weight or more and 60% by weight or less. When the solids concentration of the PHA resin in the aqueous coating liquid is within this range, the viscosity of the solution is not too high, making it possible to apply the solution uniformly and maintain the required coating thickness, thereby preventing defects in the coating film. The lower limit of the solids concentration is preferably 30% by weight or more, and more preferably 40% by weight or more.

[0051] The average particle size of the PHA-based resin particles in the aqueous coating liquid may be, for example, 0.1 to 50 μm, preferably 0.5 to 30 μm, and more preferably 0.8 to 20 μm, from the viewpoint of achieving both PHA-based resin productivity and uniformity during application. An average particle size of 0.1 μm or more allows the PHA-based resin to be easily obtained by either microbial production or chemical synthesis. An average particle size of 50 μm or less prevents uneven application. The average particle size of the PHA-based resin particles in the aqueous coating liquid can be calculated using a general-purpose particle size analyzer such as a Microtrac particle size analyzer (manufactured by Nikkiso Co., Ltd., FRA) by adjusting the aqueous coating liquid containing the PHA-based resin to a predetermined concentration and determining the particle size corresponding to 50% of the total particle size in a normal distribution.

[0052] The aqueous coating liquid contains a phosphate together with a PHA resin. By containing the phosphate, the pH of the aqueous coating liquid is stabilized over time, and even if an aqueous coating liquid is used after a certain amount of time has passed between preparation and application, a laminate in which discoloration of the paper substrate is suppressed can be produced.

[0053] Specific examples of the phosphate salt are not particularly limited, and include, for example, tetrapotassium pyrophosphate, disodium dihydrogen pyrophosphate, tetrasodium pyrophosphate, potassium polyphosphate, sodium polyphosphate, potassium metaphosphate, sodium metaphosphate, tripotassium phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, trisodium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, etc. An appropriate phosphate salt may be selected from these salts in consideration of the target pH, and two or more types of phosphate salts may be used.

[0054] The phosphate preferably contains at least a sodium salt and / or a potassium salt, since this makes it easy to control the pH of the aqueous coating liquid containing the PHA resin within the range of 6.5 to 8. In particular, it is preferable to contain disodium hydrogen phosphate and / or potassium dihydrogen phosphate.

[0055] In addition, from the viewpoint of suppressing foaming of the aqueous coating liquid, the phosphate is preferably a sodium salt, and disodium hydrogen phosphate is particularly preferred.

[0056] The amount of phosphate used may be appropriately set so that the pH of the aqueous coating liquid containing the PHA resin falls within the range of 6.5 to 8. Specifically, the total content of phosphate relative to the total amount of PHA resin contained in the aqueous coating liquid is preferably 0.01 to 0.90 wt%. The lower limit is preferably 0.05 wt% or more, more preferably 0.09 wt% or more. The upper limit is preferably 0.70 wt% or less, more preferably 0.50 wt% or less. Setting the total content of phosphate to 0.70 wt% or less also has the effect of suppressing foaming of the aqueous coating liquid.

[0057] The aqueous coating liquid has a pH of 6.5 or more and 8 or less when applied to a paper substrate. A pH of 6.5 or more makes it possible to produce a laminate in which discoloration of the paper substrate is suppressed. Furthermore, a pH of 8 or less makes it possible to suppress a decrease in the molecular weight of the PHA-based resin contained in the resin layer of the laminate. If the pH exceeds 8, the molecular weight of the PHA-based resin is likely to decrease during the heating step during laminate production, which may make it impossible to achieve the desired physical properties of the resin layer.

[0058] Furthermore, since the pH of the aqueous coating liquid is adjusted using phosphate, the pH of the aqueous coating liquid remains stable over time, and discoloration of the paper substrate in the produced laminate can be suppressed even if a certain amount of time (e.g., several days) has passed between the preparation and application of the aqueous coating liquid.

[0059] The lower limit of the pH of the aqueous coating liquid is preferably 6.7 or more, more preferably 6.8 or more, and the upper limit is preferably 7.8 or less, more preferably 7.5 or less.

[0060] The aqueous coating liquid may not contain an emulsifier, but preferably contains one to stabilize the coating liquid. Examples of emulsifiers include anionic surfactants such as sodium lauryl sulfate and sodium oleate, cationic surfactants such as lauryl trimethylammonium chloride, nonionic surfactants such as glycerin fatty acid esters and sorbitan fatty acid esters, polyvinyl alcohol derivatives such as polyvinyl alcohol, carboxy-modified polyvinyl alcohol, sulfonated polyvinyl alcohol, and ethylene-modified polyvinyl alcohol, cellulose derivatives such as methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, and hydroxypropyl cellulose, starch derivatives such as starch, oxidized starch, and etherified starch, and water-soluble polymers such as chitin, chitosan, casein, and gum arabic. These may be used alone or in combination of two or more. Among these, polyvinyl alcohol is preferred because it facilitates industrial production of the coating liquid.

[0061] The amount of emulsifier added is not particularly limited, but is preferably 1 to 10% by weight based on the solid content of the PHA resin. When the amount of emulsifier added is 1% by weight or more, the stabilizing effect of the emulsifier tends to be easily obtained, and when it is 10% by weight or less, deterioration of physical properties, coloration, etc. due to the inclusion of an excess emulsifier in the PHA resin can be avoided.

[0062] The aqueous coating liquid contains an aqueous solvent. The aqueous solvent may be water alone or a mixed solvent of water and an organic solvent. The organic solvent is preferably an organic solvent that is compatible with water. The concentration of the organic solvent is preferably equal to or lower than the solubility of the organic solvent in water.

[0063] Examples of organic solvents compatible with water include alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, pentanol, hexanol, and heptanol; ketones such as acetone and methyl ethyl ketone; ethers such as tetrahydrofuran and dioxane; nitriles such as acetonitrile and propionitrile; amides such as dimethylformamide and acetamide; dimethyl sulfoxide, pyridine, and piperidine. Among these, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, acetone, methyl ethyl ketone, tetrahydrofuran, dioxane, acetonitrile, and propionitrile are preferred because they are easily removable. Furthermore, methanol, ethanol, 1-propanol, 2-propanol, butanol, and acetone are more preferred because they are easily available. Furthermore, methanol, ethanol, and acetone are particularly preferred.

[0064] The content of water in the entire aqueous solvent contained in the aqueous coating liquid is preferably 5% by weight or more and 100% by weight or less, more preferably 10% by weight or more, even more preferably 30% by weight or more, still more preferably 50% by weight or more, particularly preferably 70% by weight or more, and most preferably 90% by weight or more, from the viewpoints of the stability of the aqueous coating liquid, its coatability to a paper substrate, its drying property after coating, and the like.

[0065] When the PHA resin is produced by a microorganism, the aqueous coating liquid may contain impurities derived from the bacterial components of the microorganism. Depending on the application of the laminate, such impurities may be present as long as they do not impair the physical properties. However, in applications requiring a high-purity PHA resin, it is preferable to reduce the impurities as much as possible. From this perspective, the amount of protein in the aqueous coating liquid is preferably 30,000 ppm or less, more preferably 15,000 ppm or less, even more preferably 10,000 ppm or less, and particularly preferably 7,500 ppm or less, relative to the total amount of the PHA resin contained in the aqueous coating liquid.

[0066] The method for producing an aqueous coating liquid containing a PHA resin is not particularly limited, but may include first producing a PHA resin in the cells of a microorganism, then crushing the microbial cells containing the PHA resin in an aqueous dispersion state, centrifuging the aqueous dispersion of the PHA resin obtained by crushing to obtain a precipitate, washing the precipitate with water and, if necessary, with methanol, and finally adding appropriate amounts of water and phosphate to obtain an aqueous coating liquid containing a PHA resin with a desired solids concentration.

[0067] Alternatively, an aqueous coating liquid containing a PHA resin can be obtained by crushing the microbial cells to obtain an aqueous dispersion of a PHA resin, washing the dispersion appropriately, and then spray-drying the dispersion to obtain a PHA resin powder, dispersing the powder in water, and adding a phosphate. Alternatively, an aqueous coating liquid can be obtained by obtaining a PHA resin powder, dispersing the powder in water, and adding a phosphate.

[0068] The stage at which the phosphate is added is not particularly limited, and the phosphate may be added to an aqueous dispersion of a PHA-based resin, or the PHA-based resin may be added to and dispersed in water in which the phosphate has been dissolved in advance.

[0069] The obtained aqueous coating liquid may be subjected to a step of applying mechanical shear to separate partially aggregated PHA resin particles from each other. By applying mechanical shear, aggregates are substantially eliminated, and an aqueous coating liquid containing a PHA resin with a uniform particle size can be obtained. The mechanical shear of the aqueous coating liquid can be performed using, for example, a stirrer, a homogenizer, ultrasonic waves, etc.

[0070] The aqueous coating liquid described above also constitutes one aspect of the present disclosure. The aqueous coating liquid can be used to form a resin layer on a paper substrate by applying it to the paper substrate and drying it.

[0071] (Coating step) Next, the aqueous coating liquid described above is applied to the surface of the paper substrate to form a coating film. The coating method is not particularly limited, and known methods can be used as appropriate. Specifically, spraying, scattering, slit coater, air knife coater, roll coater, bar coater, comma coater, blade coater, screen printing, gravure printing, etc. can be used. Before applying the aqueous coating liquid, a step of subjecting the paper substrate to surface treatment such as corona treatment may be carried out.

[0072] The coating amount of the PHA-based resin is not particularly limited and can be determined appropriately in consideration of the performance and productivity required for the resin layer. Specifically, the coating amount is 1.0 g / m2 in dry weight. ２ 80g / m or more ２ Preferably, the amount is 5.0 g / m or less. ２ 60g / m or more ２ More preferably, 10 g / m or less ２ 50g / m or more ２ When the coating amount of the PHA resin is within the above range, defects such as pinholes can be prevented, the resin layer can have sufficient strength to withstand use, and functions such as water resistance and oil resistance can be efficiently exhibited.

[0073] (Drying step) After forming the coating film, the coating film is heated and dried to form a resin layer. This drying step may be a step aimed only at evaporating the aqueous medium contained in the aqueous coating liquid, but may also be a step aimed at fusing the PHA-based resin particles contained in the aqueous coating liquid together to increase the uniformity of the resin layer, in addition to the evaporation.

[0074] When the drying step is intended only to evaporate the aqueous medium, the heating temperature of the coating film in the drying step is not particularly limited, but may be a temperature lower than the melting point of the PHA resin. Specifically, the upper limit of the heating temperature may be less than 130°C and may be 125°C or lower. The lower limit of the heating temperature is not particularly limited, but may be, for example, 70°C or higher, preferably 90°C or higher, and more preferably 100°C or higher. The heating time at this time is not particularly limited, but may be, for example, 10 seconds to 10 minutes, and preferably about 30 seconds to 5 minutes.

[0075] On the other hand, when the drying step is intended to evaporate the aqueous medium and fuse the PHA-based resin particles together, the heating temperature of the coating film in the drying step is preferably equal to or higher than the melting point of the PHA-based resin. By heating at such a temperature, at least a portion of the PHA-based resin melts, and when the molten portion cools after heating, the resin particles fuse together, integrating the resin components and forming a more uniform resin layer. This improves the adhesion of the resin layer to the paper substrate and the water resistance and oil resistance of the resin layer.

[0076] The heating temperature in this case is preferably 10 to 40°C above the melting point of the PHA resin, and more preferably 20 to 30°C above. Specifically, the heating temperature is preferably 160°C or higher, more preferably 165°C or higher, and particularly preferably 170°C or higher. The heating temperature is also preferably 200°C or lower. If the heating temperature is 200°C or lower, excessive drying of the paper substrate and thermal decomposition of the PHA resin can be avoided. The melting point of the PHA resin mentioned above refers to the highest peak top temperature in the crystalline melting curve in differential scanning calorimetry.

[0077] This drying step can be carried out using a known heating method, such as hot air heating, infrared heating, ultrasonic irradiation, microwave heating, roll heating, or hot plate heating, which can be used alone or in combination of two or more.

[0078] After the drying step, a humidity control step may be carried out to adjust the moisture content of the dried paper substrate, thereby reducing curling of the paper substrate that occurs during the drying step.

[0079] In this step, it is preferable to increase the moisture content of the paper substrate by spraying water onto the surface of the paper substrate. The sprayed water may contain additives such as humectants such as glycerin and propylene glycol, various fragrances, and preservatives.

[0080] The temperature of the sprayed water is not particularly limited, and may be, for example, about 10 to 50°C, or may be room temperature (about 10 to 30°C) without temperature control.

[0081] The amount of water used can be appropriately determined taking into consideration the moisture content of the paper substrate after the drying process and the moisture content of the target paper substrate. The moisture content of the target paper substrate is not particularly limited, but may be, for example, about 5 to 8%, or 6 to 7%. However, this humidity adjustment process does not necessarily have to be performed.

[0082] [Laminate] The laminate itself that can be produced by this embodiment also constitutes one aspect of the present disclosure. The laminate includes a paper substrate and a coating layer formed on at least one surface of the paper substrate. The coating layer is the resin layer described above, and includes a polyhydroxyalkanoate resin and 0.01 to 0.90 wt % of a phosphate, based on the total weight of the polyhydroxyalkanoate resin. Such a laminate in which a resin layer containing a polyhydroxyalkanoate resin also contains a predetermined amount of phosphate has not been reported to date.

[0083] The coating layer may be laminated directly onto the paper substrate or may be laminated via another layer, but it is preferably laminated directly onto the paper substrate.

[0084] The coating layer may be the outermost layer in the laminate, and in this case, the coating layer may function as a heat seal layer, a water-resistant layer, and / or an oil-resistant layer.

[0085] Furthermore, another layer may be laminated on the coating layer. In this case, the coating may function as an anchor coating layer between the paper substrate and the other layer. The other layer is not particularly limited and may be another resin layer or an inorganic layer.

[0086] The surface of the paper substrate opposite to the side on which the coating layer is laminated may be the outermost layer, or may have any other layer laminated thereon. The layer may be a layer that can correspond to the resin layer, a different resin layer, or an inorganic layer.

[0087] [Molded Product] The laminate that can be produced by this embodiment can be molded into a predetermined shape to form a molded product (hereinafter also referred to as the "present molded product"). The molded product includes the laminate and has a desired size and shape. The present molded product is formed from a laminate including a resin layer containing a PHA-based resin, and is therefore advantageous in various applications.

[0088] The present molded article is not particularly limited as long as it contains the present laminate, and examples thereof include paper, film, sheet, tube, plate, rod, container (e.g., bottle container), bag, part, etc. From the viewpoint of measures against marine pollution, the present molded article is preferably a packaging bag, a lid material, or a container such as a cup or a tray.

[0089] In one embodiment of the present disclosure, the present molded article may be the present laminate itself, or may be a product obtained by subjecting the present laminate to secondary processing.

[0090] Because the laminate has been subjected to secondary processing, the molded article containing it can be suitably used as various packaging container materials such as shopping bags, various bags, food and confectionery packaging materials, cups, trays, cartons, etc. (in other words, in various fields such as food, cosmetics, electronics, medicine, and pharmaceuticals.) Because the laminate contains a resin that has high adhesion to substrates and good heat resistance, it is more suitable as a container for holding liquids, particularly containers for holding hot contents such as cups for food and drink such as instant noodles, instant soup, and coffee, and trays for prepared meals, boxed lunches, and microwaveable foods.

[0091] The secondary processing can be carried out in the same manner as conventional resin-laminated paper or coated paper, i.e., using various bag-making machines, filling and packaging machines, etc. Processing can also be carried out using machines such as paper cup forming machines, punching machines, box making machines, etc. In these processing machines, known techniques can be used to bond the present laminate, such as heat sealing, impulse sealing, ultrasonic sealing, high-frequency sealing, hot air sealing, and frame sealing.

[0092] Furthermore, in order to improve the physical properties of the present molded article, it can also be composited with a molded article made of a material different from the present molded article (for example, fiber, thread, rope, woven fabric, knitted fabric, nonwoven fabric, paper, film, sheet, tube, plate, rod, container, bag, part, foam, etc.). These materials are also preferably biodegradable.

[0093] The following items list preferred aspects of the present disclosure, but the present invention is not limited to them. [Item 1] A method for producing a laminate including a paper substrate and a resin layer formed on at least one surface of the paper substrate, the method comprising the steps of: preparing an aqueous coating liquid containing a polyhydroxyalkanoate resin with a solids concentration of 20% to 60% by weight and a phosphate, and having a pH of 6.5 to 8; and applying the aqueous coating liquid to the surface of the paper substrate and drying it to form a resin layer. [Item 2] A method for producing a laminate according to item 1, wherein the phosphate includes at least one of a sodium salt and a potassium salt. [Item 3] A method for producing a laminate according to item 1, wherein the phosphate includes at least one of disodium hydrogen phosphate and potassium dihydrogen phosphate. [Item 4] A method for producing a laminate according to any one of items 1 to 3, wherein the total content of the phosphate is 0.01 to 0.90% by weight based on the total amount of the polyhydroxyalkanoate resin. [Item 5] A method for producing a laminate according to any one of Items 1 to 4, wherein the polyhydroxyalkanoate resin comprises a copolymer containing 3-hydroxybutyrate units and other hydroxyalkanoate units. [Item 6] A method for producing a molded article, comprising the steps of producing a laminate by the method according to any one of Items 1 to 5, and shaping the laminate. [Item 7] An aqueous coating liquid for a paper substrate, comprising a polyhydroxyalkanoate resin having a solids concentration of 20% by weight to 60% by weight and a phosphate, and having a pH of 6.5 to 8. [Item 8] The aqueous coating liquid for a paper substrate according to Item 7, wherein the phosphate comprises at least one of a sodium salt and a potassium salt. [Item 9] The aqueous coating liquid for a paper substrate according to Item 7, wherein the phosphate comprises at least one of disodium hydrogen phosphate and potassium dihydrogen phosphate. [Item 10] The aqueous coating liquid for paper substrates according to any one of Items 7 to 9, wherein the total content of the phosphate is 0.01 to 0.90% by weight based on the total amount of the polyhydroxyalkanoate resin.[Item 11] The aqueous coating liquid for paper substrates according to any one of Items 7 to 10, wherein the polyhydroxyalkanoate resin comprises a copolymer containing 3-hydroxybutyrate units and other hydroxyalkanoate units. [Item 12] A laminate comprising a paper substrate and a coating layer formed on at least one surface of the paper substrate, wherein the coating layer comprises a polyhydroxyalkanoate resin and 0.01 to 0.90% by weight of a phosphate salt relative to the total amount of the polyhydroxyalkanoate resin. [Item 13] A molded product comprising the laminate according to Item 12.

[0094] The present invention will be specifically explained below with reference to examples, but the technical scope of the present invention is not limited to these examples.

[0095] The substances used in the examples and comparative examples are listed below. [Polyhydroxyalkanoate-based resin] PHA-based resin: poly[3-hydroxybutyrate-co-3-hydroxyhexanoate] (average content ratio 3HB / 3HH=89 / 11 mol% / mol%, weight average molecular weight: 240,000 g / mol) Produced in accordance with the method described in WO 2018 / 070492.

[0096] [Additives] Disodium hydrogen phosphate (Na ２ HPO ４ ): Potassium dihydrogen phosphate (KH) manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. ２ P.O. ４ ): Manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. Sodium hydroxide (NaOH): Manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

[0097] The evaluation methods used in the examples and comparative examples are as follows: [Evaluation of pH of aqueous suspension] pH was measured using a JIS type I glass electrode pH meter (LAQUAact / manufactured by HORIBA).

[0098] [Discoloration of Laminate] The laminate was visually inspected to check for discoloration of the paper substrate. No discoloration: ○ Discoloration: ×

[0099] [Molecular Weight Change] The paper substrate was removed from the laminate, and the weight average molecular weight (weight average molecular weight after heating) of the resulting resin pieces was measured. The weight average molecular weight was determined as a polystyrene-equivalent molecular weight by gel permeation chromatography (GPC) (Shodex GPC-101 manufactured by Showa Denko K.K.) using a polystyrene gel (Shodex K-804 manufactured by Showa Denko K.K.) as a column and chloroform as a mobile phase. The molecular weight retention rate after heating was calculated based on the following formula, and a molecular weight retention rate of 90% or more was evaluated as ○, and a molecular weight retention rate of less than 90% was evaluated as ×. Formula: Molecular weight retention rate after heating (%) = weight average molecular weight of the resin pieces / weight average molecular weight of the PHA-based resin before heat treatment × 100

[0100] [Example 1] Na ２ HPO ４ 0.788% by weight based on the solid content of the PHA resin in the aqueous suspension, KH ２ P.O. ４ was added to the aqueous suspension so that the amount was 0.067 wt % based on the PHA resin solid content, and then the PHA resin was added while stirring, and the mixture was stirred for 10 minutes at 12,000 rpm using a PRIMIX Corporation MARK II 2.5 homomixer to disperse the PHA resin in water, obtaining an aqueous suspension (i) containing a PHA resin at a solid content concentration of 50 wt %. Within one day after the preparation of the aqueous suspension (i), the pH was measured, and a laminate (i) was prepared according to the following method. The weight-average molecular weight of the resin pieces obtained from the laminate (i) was measured, and the molecular weight retention rate was calculated.

[0101] (Method for Producing Laminate) A paper substrate (Oji Material Co., Ltd., bleached kraft paper "Kujira", basis weight 50 g / m) ２ , paper thickness 70μm, density 0.7g / cm ３ ) using a bar coater, a dry coating film of 10 g / m ２ The aqueous suspension was applied to the substrate so that the thickness of the coated layer was 180° C. and then heated at 180° C. for 2 minutes to obtain a laminate.

[0102] Furthermore, the aqueous suspension (i) was prepared and then left to stand for 11 days in an environment of 20°C to 25°C to obtain an aqueous suspension (ii). After preparing the aqueous suspension (ii), the pH was immediately measured, and a laminate (ii) was prepared according to the method described above. The weight average molecular weight of resin pieces obtained from the laminate (ii) was measured, and the molecular weight retention rate was calculated.

[0103] The results of the pH of the aqueous suspension (i) and the aqueous suspension (ii), the discoloration of the laminate (i) and the laminate (ii), and the change in molecular weight of the resin layer in each laminate are shown in Table 1.

[0104] [Examples 2 to 4] Na ２ HPO ４ , and K.H. ２ P.O. ４ Aqueous suspensions (i) and (ii) and laminates (i) and (ii) were prepared in the same manner as in Example 1, except that the amount of each of the components added was changed as shown in Table 1. Measurements and evaluations were then carried out, and the results are shown in Table 1.

[0105] [Examples 5 and 6] Na ２ HPO ４ , and K.H. ２ P.O. ４ Aqueous suspensions (i) and (ii) and laminates (i) and (ii) were prepared in the same manner as in Example 1, except that the amount of the PHA-based resin added and the solids concentration of the PHA-based resin contained in the aqueous suspensions were changed as shown in Table 1. Measurements and evaluations were then carried out, and the results are shown in Table 1.

[0106] [Comparative Example 1] Na ２ HPO ４ , and K.H. ２ P.O. ４ Aqueous suspension (i) and aqueous suspension (ii), and laminate (i) and laminate (ii) were prepared in the same manner as in Example 1, except that no additive was added. Measurements and evaluations were carried out, and the results are shown in Table 1.

[0107] [Comparative Examples 2 to 3] Na ２ HPO ４ , and K.H. ２ P.O. ４Aqueous suspension (i) and aqueous suspension (ii), and laminate (i) and laminate (ii) were prepared in the same manner as in Example 1, except that NaOH was added in place of HCl in the amounts shown in Table 1. Measurements and evaluations were then carried out, and the results are shown in Table 1.

[0108]

[0109] Table 1 shows that the PHA-based resin-containing aqueous suspensions of Examples 1 to 6, in which the pH was controlled in the range of 6.5 to 8 by adding phosphate, had a stable pH over time, and that the laminates produced using these aqueous suspensions suppressed both discoloration of the paper substrate and changes in the molecular weight of the resin, regardless of the length of time after the suspension was produced.

[0110] On the other hand, in Comparative Example 1, in which the aqueous suspension was prepared without adding an alkaline component, discoloration of the paper substrate in the laminate occurred both within one day after preparation and after 11 days. Furthermore, in Comparative Examples 2 and 3, in which the pH of the aqueous suspension was adjusted by adding sodium hydroxide instead of phosphate, discoloration of the paper substrate in the laminate was not observed when aqueous suspension (i) was used within one day after preparation, but discoloration of the paper substrate was observed in the laminate prepared using aqueous suspension (ii) after 11 days after preparation. This is presumably because the pH of aqueous suspension (ii) dropped to less than 6.5. Additionally, in Comparative Example 3, the pH of aqueous suspension (i) within one day after preparation was too high at 8.8, resulting in a significant decrease in the molecular weight of the resin contained in the laminate prepared using this suspension.

Claims

1. A method for producing a laminate comprising a paper substrate and a resin layer formed on at least one surface of the paper substrate, the method comprising the steps of: preparing an aqueous coating liquid containing a polyhydroxyalkanoate resin with a solids concentration of 20% to 60% by weight and a phosphate, and having a pH of 6.5 to 8; and applying the aqueous coating liquid to the surface of the paper substrate and drying it to form a resin layer.

2. The method for producing a laminate according to claim 1, wherein the phosphate includes at least one of a sodium salt and a potassium salt.

3. The method for producing a laminate according to claim 1, wherein the phosphate comprises at least one of disodium hydrogen phosphate and potassium dihydrogen phosphate.

4. The method for producing a laminate according to any one of claims 1 to 3, wherein the total content of the phosphates is 0.01 to 0.90% by weight based on the total amount of the polyhydroxyalkanoate resin.

5. The method for producing a laminate according to any one of claims 1 to 3, wherein the polyhydroxyalkanoate resin comprises a copolymer containing 3-hydroxybutyrate units and other hydroxyalkanoate units.

6. A method for producing a molded body, comprising the steps of producing a laminate by the method according to any one of claims 1 to 3, and shaping the laminate.

7. An aqueous coating liquid for paper substrates, comprising a polyhydroxyalkanoate resin having a solids concentration of 20% by weight or more and 60% by weight or less, and a phosphate, and having a pH of 6.5 to 8.

8. The aqueous coating liquid for paper substrates according to claim 7, wherein the phosphate comprises at least one of a sodium salt and a potassium salt.

9. The aqueous coating liquid for paper substrates according to claim 7, wherein the phosphate comprises at least one of disodium hydrogen phosphate or potassium dihydrogen phosphate.

10. The aqueous coating liquid for paper substrates according to any one of claims 7 to 9, wherein the total content of the phosphates is 0.01 to 0.90% by weight based on the total amount of the polyhydroxyalkanoate resin.

11. The aqueous coating liquid for paper substrates according to any one of claims 7 to 9, wherein the polyhydroxyalkanoate-based resin comprises a copolymer containing 3-hydroxybutyrate units and other hydroxyalkanoate units.

12. A laminate comprising a paper base material and a coating layer formed on at least one surface of the paper base material, the coating layer comprising a polyhydroxyalkanoate resin and 0.01 to 0.90% by weight of a phosphate based on the total amount of the polyhydroxyalkanoate resin.

13. A molded article comprising the laminate according to claim 12.

Images