Laminated polylactic acid film

Abstract

The present invention is a laminated polylactic acid film having a stretched polylactic acid film substrate and a resin layer on at least one surface of the stretched polylactic acid film substrate, wherein the resin layer is formed from a resin-layer-forming material that contains a polyurethane resin and a polyolefin resin. According to the present invention, it is possible to provide a laminated polylactic acid film which has excellent adhesion to a coating layer, such as a printing paint or a hard coat, while having excellent transparency and blocking resistance.

Description

Laminated polylactic acid film

[0001] The present invention relates to a laminated polylactic acid film having good printability and adhesiveness. More specifically, it relates to a laminated polylactic acid film having excellent adhesion in a printing process and further excellent transparency.

[0002] Generally, plastics such as polyolefins, polyesters, and polyamides are used as the base materials for packaging materials and functional films used in foods, pharmaceuticals, industrial products, etc. In recent years, due to the increasing environmental awareness, the development of polylactic acid films made from non-petroleum raw material materials and biodegradable materials has been promoted. However, compared with general polyester films and polyamide films, polylactic acid films have low mechanical strength, heat resistance, etc., so wrinkles occur when winding the film in the film manufacturing process, etc., and when the wound film roll is stored for a long time, blocking occurs where the films stick to each other due to winding tightening over time. Furthermore, since polylactic acid films have high crystallinity due to their molecular structure, the wettability of the surface is low, and the transferability and adhesiveness to various printing paints, etc. are poor.

[0003] Therefore, as a method for suppressing wrinkles generated when winding the film in the film manufacturing process, etc., a method of adding a lubricant or an anti-blocking agent to the polylactic acid film has been proposed (see, for example, Patent Documents 1 and 2). However, although any of these methods can suppress wrinkles, since the polylactic acid film contains a lubricant or an anti-blocking agent, light scattering occurs in the film, and there is a problem that the transparency of the polylactic acid film decreases.

[0004] Also, as a method for improving the mechanical strength and heat resistance of polylactic acid films and imparting lubricity, methods of adding a composition ratio of L-lactic acid and D-lactic acid in polylactic acid and a biodegradable resin other than polylactic acid have also been proposed (see, for example, Patent Documents 3 and 4). However, in these technologies, whitening due to crystallization inside the film and whitening due to poor compatibility between polylactic acid and the added biodegradable resin are likely to occur, and sufficient satisfactory performance has not been obtained yet.

[0005] Therefore, methods have been proposed to impart lubricity and adhesion to the surface of a polylactic acid film by laminating a coating layer containing various resins and lubricants (see, for example, Patent Documents 5 and 6), but these methods have not been able to satisfy the requirements for haze and adhesion.

[0006] Furthermore, a method has been proposed in which a resin layer made of polyurethane is laminated onto a polylactic acid film (see, for example, Patent Documents 7 and 8). However, none of the previously proposed technologies have produced a polylactic acid film that satisfies all of the requirements of adhesion, transparency, and blocking resistance at a high level.

[0007] Japanese Patent Publication No. 2004-331860, Japanese Patent Publication No. 2002-146064, Japanese Patent Publication No. 2004-010900, Japanese Patent Publication No. 2003-170560, Japanese Patent Publication No. Hei 10-120811, Japanese Patent Publication No. 2005-212242, Japanese Patent Publication No. 2023-173579, Japanese Patent Publication No. 2005-194383

[0008] The object of the present invention is to solve the above problems, namely, to provide a laminated polylactic acid film that is mainly composed of biomass-derived and biodegradable polylactic acid, has good adhesion to coating layers such as printing paints and hard coats, and further has good transparency and blocking resistance.

[0009] The inventors, in order to solve the above problems, conducted diligent research and have finally completed the present invention. That is, the present invention is as follows: (1) A laminated polylactic acid film having a stretched polylactic acid film substrate and a resin layer on at least one surface of the stretched polylactic acid film substrate, wherein the resin layer is formed of a resin layer forming material containing a polyurethane resin and a polyolefin resin. (2) The laminated polylactic acid film according to claim 1, wherein the glass transition temperature of the polyurethane resin is 70°C or less. (3) The laminated polylactic acid film according to claim 1 or 2, wherein the polyurethane resin is a polyurethane resin having a polyester skeleton. (4) The laminated polylactic acid film according to any one of claims 1 to 3, wherein the polyurethane resin is a polyurethane resin containing a polydiene skeleton. (5) The surface free energy of the resin layer is 42 mJ / m ２A laminated polylactic acid film according to any one of the following items 1 to 4: (Item 6) A laminated polylactic acid film according to any one of items 1 to 5, containing 0.5% by mass or more and 35% by mass or less of polyolefin resin with respect to 100% by mass of the solid component of the resin layer forming material. (Item 7) A laminated polylactic acid film according to any one of items 1 to 6, wherein the resin layer is formed by an in-line coating method.

[0010] The laminated polylactic acid film of the present invention exhibits excellent adhesion to UV inks and UV hard coats, as well as superior transparency and blocking resistance. Therefore, it can be suitably used as a replacement for conventional plastic films in packaging materials and functional film substrates for food, pharmaceuticals, industrial products, etc. The stretched polylactic acid film substrate of the laminated polylactic acid film is made from non-petroleum raw materials and biodegradable materials, thus making a significant contribution to reducing the burden on the global environment.

[0011] The present invention will be described in detail below.

[0012] (Stretched Polylactic Acid Film Substrate) The stretched polylactic acid film substrate used in the present invention is formed from a film-forming material containing polylactic acid. The polylactic acid is obtained, for example, by ring-opening polymerization of lactide using a compound having a hydroxyl group as an initiator in the presence of a predetermined catalyst. The predetermined catalyst is, for example, tin or aluminum. The polylactic acid preferably has a mass ratio of L-lactic acid (hereinafter referred to as L-form) / D-lactic acid (hereinafter referred to as D-form) of 100 / 0 to 85 / 15, more preferably 100 / 0 to 90 / 10, even more preferably 100 / 0 to 90 / 10, and particularly preferably 100 / 0 to 95 / 5. When the ratio of L-lactic acid (hereinafter referred to as L-form) / D-lactic acid (hereinafter referred to as D-form) is 100 / 0 to 85 / 15, high crystallinity is obtained, which is preferable.

[0013] Polylactic acid may also be copolymerized polylactic acid obtained by copolymerizing hydroxycarboxylic acid components other than lactic acid, dicarboxylic acid components, and glycol components. Examples of hydroxy acid components other than lactic acid include glycolic acid, 3-hydroxypropionic acid, and 6-hydroxycaproic acid (ε-caprolactone). Examples of dicarboxylic acid components include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, sebacic acid, terephthalic acid, isophthalic acid, and orthophthalic acid, with succinic acid, adipic acid, and terephthalic acid being preferred. Examples of glycol components include ethylene glycol, diethylene glycol, propylene glycol, trimethylene glycol, tetramethylene glycol, neopentyl glycol, and 1,6-hexanediol, with ethylene glycol, diethylene glycol, and tetramethylene glycol being preferred. It is preferable that the polymers or oligomers of the dicarboxylic acid component and the glycol component are block copolymerized. In the total components of polylactic acid (total amount of hydroxycarboxylic acid component, dicarboxylic acid component, and glycol component), the lactic acid component is preferably 85 mol% or more, more preferably 90 mol% or more, even more preferably 95 mol% or more, particularly preferably 97 mol% or more, may be 99 mol% or more, or may be 100 mol%.

[0014] The preferred glass transition temperature of the polylactic acid used in the stretched polylactic acid film substrate is 40 to 70°C, the melting point is preferably 150 to 180°C, and orientation crystallization is preferably possible. The glass transition temperature and melting point can be obtained by differential scanning calorimeter (DSC) or the like. The presence or absence of crystallinity can be confirmed by the presence or absence of a crystallization peak during the heating process or the cooling process after melting using DSC.

[0015] The reduced viscosity (ηsp / c) of the polylactic acid-containing film-forming material used in this invention is preferably in the range of 1.0 dl / g to 3.0 dl / g, and more preferably in the range of 1.5 to 2.8 dl / g. When the reduced viscosity is 1.0 dl / g or higher, the polylactic acid film can be prevented from tearing. When the reduced viscosity is 3.0 dl / g or lower, the increase in filtration pressure is reduced, making high-precision filtration easier.

[0016] The reduced viscosity (ηsp / c) of the polylactic acid used in the stretched polylactic acid film (and laminated polylactic acid film) of the present invention is preferably 1.0 dl / g or more and 2.5 dl / g or less, and more preferably 1.2 or more and 2.3 or less. When the reduced viscosity is 1.0 dl / g or more, it is preferable because breakage does not occur frequently during the stretching process. When the reduced viscosity is 2.5 dl / g or less, it is preferable because the cutability is good when cutting to a predetermined product width and dimensional defects do not occur. Since the reduced viscosity of polylactic acid tends to decrease when melted, it is preferable to reduce the decrease in reduced viscosity during film manufacturing by thoroughly drying the film-forming material containing polylactic acid and shortening the residence time in the molten state.

[0017] The film-forming material constituting the stretched polylactic acid film substrate used in the present invention may contain one or more additives, such as fluorescent whitening agents, ultraviolet inhibitors, infrared absorbing dyes, heat stabilizers, surfactants, antioxidants, and plasticizers, depending on the purpose of use. As antioxidants, aromatic amine-based and phenol-based antioxidants can be used, and as stabilizers, phosphorus-based (such as phosphoric acid and phosphate esters), sulfur-based, and amine-based stabilizers can be used. Examples of plasticizers include polyethylene glycol, polypropylene glycol, polypropylene glycol-ethylene glycol block copolymers, and lactide addition polymers thereof.

[0018] The film-forming material constituting the stretched polylactic acid film substrate used in the present invention may contain resin components other than polylactic acid as a resin component, but it is preferable that the polylactic acid content in the total resin components of the film-forming material be 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and may be 98% by mass or more, or 100% by mass. In other words, the film-forming material may consist only of polylactic acid.

[0019] The film-forming material of the stretched polylactic acid film substrate used in the present invention may be a blend of polyesters other than polylactic acid. The polyester other than polylactic acid is preferably an aliphatic polyester, such as polybutylene succinate, polybutylene succinate adipate, polybutylene succinate tractate, polybutylene adipate terephthalate, polyethylene succinate, etc.

[0020] Even when copolymerized polylactic acid is used as the film-forming material and / or when polyesters other than polylactic acid are blended, or when polyester components other than lactic acid are included, the lactic acid component is preferably 85 mol% or more, more preferably 90 mol% or more, even more preferably 95 mol% or more, and particularly preferably 97 mol% or more, of the total polyester components (total amount of hydroxycarboxylic acid component, dicarboxylic acid component, and glycol component). Furthermore, even when polyester components other than lactic acid are included, the glass transition temperature, melting point, and reduced viscosity of the copolymerized polylactic acid and the blend are preferably within the above ranges.

[0021] Furthermore, in order to improve the handling properties of the film, such as its slipperiness, windability, and blocking resistance, as well as its abrasion properties, such as its abrasion resistance and scratch resistance, inert particles (lubricant particles) may be included in the stretched polylactic acid film substrate. However, when used as a base film for optical components in industrial products, it is required to maintain high transparency while having excellent handling properties. Specifically, when laminated polylactic acid film is used as an optical component, the total light transmittance of the laminated polylactic acid film is preferably 85% or higher, more preferably 87% or higher, even more preferably 88% or higher, even more preferably 89% or higher, and particularly preferably 90% or higher. Moreover, for optical applications requiring excellent design and high transparency, 92.0% or higher is preferred, 92.5% or higher is even more preferred, and 92.7% or higher is most preferred.

[0022] Furthermore, for high clarity, it is preferable to minimize the amount of inert particles in the stretched polylactic acid film substrate. Therefore, it is preferable to have a multilayer structure in which particles are contained only in the surface layer of the stretched polylactic acid film, or to substantially not contain particles in the stretched polylactic acid film, and instead contain fine particles (lubricant particles) only in the resin layer of the laminated polylactic acid film. In particular, from the viewpoint of transparency, if the stretched polylactic acid film substrate does not substantially contain inert particles, it is preferable to include inorganic and / or heat-resistant polymer particles in the resin layer forming material (coating liquid) to create irregularities on the surface of the resin layer in order to improve the handling properties of the film.

[0023] Furthermore, "substantially free of inert particles" means, for example, in the case of inorganic particles, that when the elements derived from the particles are quantitatively analyzed by fluorescent X-ray analysis, the content is 50 ppm or less, preferably 10 ppm or less, and most preferably below the detection limit, relative to the total amount of stretched polylactic acid film substrate (film forming material). This is because even without actively adding inert particles to the stretched polylactic acid film substrate, contaminants derived from foreign substances, or dirt adhering to the raw resin or the lines and equipment in the film manufacturing process, may peel off and become mixed into the film.

[0024] Furthermore, in order to achieve both high transparency and handling properties, it is also preferable to add inert particles only to the surface layer of the stretched polylactic acid film substrate. For example, in the case of a three-layer structure, it is preferable to contain particles in the outermost layer (layer A in the case of layer A / layer B / layer A) and substantially no particles in the middle layer (layer B).

[0025] The type and content of inert particles contained in the stretched polylactic acid film substrate may be inorganic or organic particles, and are not particularly limited. Examples include metal oxides such as silica, titanium dioxide, talc, and kaolinite, and inorganic particles that are inert to polyester, such as calcium carbonate, calcium phosphate, and barium sulfate. These inert inorganic particles may be used individually or in combination of two or more.

[0026] The inert particles described above preferably have an average particle diameter of 0.1 to 3.5 μm. By setting the average particle diameter within this range, sufficient handling properties can be ensured, and transparency can also be increased. The content of inert particles (especially inorganic particles) is preferably 0.01 to 0.20% by mass relative to the film-forming material. By setting the content within this range, sufficient handling properties can be ensured, and transparency can also be increased.

[0027] The stretched polylactic acid film substrate used in the present invention is obtained by processing a film-forming material containing polylactic acid into an unstretched sheet using various methods, and then stretching it. From the viewpoint of imparting mechanical strength, the stretched polylactic acid film substrate used in the present invention is preferably a stretched film stretched in at least one direction, either the longitudinal (MD) or transverse (TD) direction, and more preferably a biaxially oriented film stretched in both the longitudinal and transverse directions. Any stretching method can be used for the biaxially oriented film, such as simultaneous biaxial stretching or sequential biaxial stretching. One preferred method for stretching in sequential biaxial stretching is to stretch the unstretched film longitudinally in a roll stretcher at a temperature of 50 to 110°C with a stretching ratio of 1.1 to 6.0 times, then stretch it transversely in a tenter stretcher at a temperature of 60 to 140°C with a stretching ratio of 1.1 to 10.0 times, and after stretching, to perform longitudinal relaxation treatment and transverse relaxation treatment of 0.5 to 10% at a temperature of 90 to 180°C. In addition, an in-line coating method is used as the process for forming the resin layer described later, but in the sequential biaxial stretching process described above, the resin layer can be coated onto the film after it has been stretched longitudinally, and then the film can be continuously guided to a tenter stretcher for transverse stretching and heat treatment.

[0028] The stretched polylactic acid film substrate used in the present invention may be a single-layer film (hereinafter also referred to as a polylactic acid layer) formed from a film-forming material containing polylactic acid, or a laminated film in which a polylactic acid layer and other plastic films (two or more types) are laminated together. Alternatively, it may be a laminated film having multiple polylactic acid layers with different compositions of additives, etc. In the case of a laminated film, the type of laminate is not particularly limited as long as there are polylactic acid layers, and the number of layers, lamination method, etc., can be arbitrarily selected from known methods depending on the purpose.

[0029] (Physical Properties of Stretched Polylactic Acid Film Substrate) The thickness of the stretched polylactic acid film substrate of the present invention is preferably 2 μm or more and 500 μm or less, more preferably 15 μm or more and 400 μm or less, and even more preferably 20 μm or more and 250 μm or less. When the thickness of the stretched polylactic acid film substrate is 2 μm or more, the stretched polylactic acid film substrate has minimum rigidity and is easy to handle. Furthermore, when the thickness of the stretched polylactic acid film substrate is 500 μm or less, the transportability of the film when transporting the film on multiple rolls and the handlingability of the manufactured film are improved, making it easier to handle.

[0030] The crystallinity of the stretched polylactic acid film substrate of the present invention is preferably 40% to 90%. It is more preferably 50% to 85%, and even more preferably 55% to 80%. A crystallinity in the range of 40% to 90% is preferable because it improves strength and provides a high modulus of elasticity.

[0031] The tensile modulus of the stretched polylactic acid film substrate of the present invention is preferably 4.0 GPa or higher in both the MD and TD directions. If the tensile modulus in either the MD or TD direction is less than 4.0 GPa, the rigidity of the film is insufficient, resulting in reduced slipperiness when winding the film and making it prone to blocking when the wound film roll is stored for a long period of time. This tensile modulus can be arbitrarily controlled by the stretching conditions and the relaxation treatment after stretching.

[0032] The breaking strength of the stretched polylactic acid film substrate is preferably 75 MPa or higher in both the MD and TD directions. The preferred lower limit for the breaking strength in the MD and TD directions is 100 MPa, a more preferred lower limit is 150 MPa, an even more preferred lower limit is 200 MPa, and an even more preferred lower limit is 220 MPa. A breaking strength of 75 MPa or higher is preferable because it provides sufficient mechanical strength for the film, suppressing defects such as elongation and slippage during the film processing process. Considering manufacturing considerations, the upper limit is considered to be 1000 MPa.

[0033] The elongation at break of the stretched polylactic acid film substrate is preferably 5% or more in both the MD and TD directions. An elongation at break of 5% or more in both the MD and TD directions is preferable because it ensures sufficient mechanical elongation of the film, thereby suppressing defects such as cracking and tearing during the film processing process. Considering manufacturing considerations, the upper limit is considered to be 300%. The upper limit is more preferably 150%, even more preferably 100%, and even more preferably 80%.

[0034] In stretched polylactic acid film substrates, it is preferable that at least one or both of the thermal shrinkage rates in the MD direction and the TD direction are 10.0% or less when heated at 150°C for 30 minutes. When heated at 150°C for 30 minutes, the upper limits of the thermal shrinkage rates in the MD direction and the TD direction are more preferably 8.0% or less, even more preferably 6.0% or less, even more preferably 4.0% or less, particularly preferably 3.0% or less, and most preferably 2.0% or less, independently for the MD and TD directions. A low thermal shrinkage rate facilitates processing such as printing and suppresses appearance defects due to film deformation under high heat. While a low thermal shrinkage rate is preferable, from a manufacturing standpoint, 0.01% is considered the lower limit.

[0035] In stretched polylactic acid film substrates, when heated at 120°C for 30 minutes, it is preferable that at least one or both of the thermal shrinkage rates in the MD direction and the TD direction are 3.0% or less. When heated at 120°C for 30 minutes, the upper limits for the thermal shrinkage rates in the MD direction and the TD direction are, independently of each other, more preferably 2.0% or less, even more preferably 1.6% or less, even more preferably 1.4% or less, particularly preferably 1.2% or less, and most preferably 1.0% or less. A low thermal shrinkage rate facilitates processing such as coating and suppresses appearance defects due to film deformation under high heat. While a low thermal shrinkage rate is preferable, from a manufacturing standpoint, 0.01% is considered the lower limit.

[0036] The haze of the stretched polylactic acid film substrate is preferably 0.3% or less, more preferably 0.2% or less, and particularly preferably 0.1% or less. By keeping it at 0.3% or less, it is possible to suppress a decrease in the design quality of printed materials and a decrease in the accuracy of detecting internal foreign matter, which are drawbacks of functional films.

[0037] (Resin layer) The resin layer in the present invention is laminated on at least one side of the stretched polylactic acid film substrate. The resin layer in the present invention is formed from a resin layer forming material containing polyurethane resin and polyolefin resin.

[0038] Furthermore, the statement "the resin layer is formed by a resin layer forming material containing polyurethane resin and polyolefin resin" is equivalent to "the resin layer contains at least components derived from polyurethane resin and components derived from polyolefin resin" when viewed as the resin layer after the resin layer formation reaction has been completed. In addition, the amounts of polyurethane resin, polyolefin resin, etc. in the resin layer described later can be interpreted as the amounts in the solid content of the resin layer forming material, and can also be interpreted as the amounts of components derived from polyurethane resin and polyolefin resin in the resin layer.

[0039] (1) Polyurethane resin The resin layer forming material of the present invention contains a polyurethane resin, thereby improving the adhesion to the functional layer laminated on the resin layer. The polyurethane resin is not particularly limited, but as constituent components, it includes at least a polyol component and a polyisocyanate component, and may further include a chain extender and a chain terminator as required. Further, the polyurethane resin may be a heat-reactive type. When using a heat-reactive type polyurethane resin, for example, a polyurethane resin in which the terminal isocyanate group is blocked with an active hydrogen group can be mentioned. The polyurethane resin is preferably a water-soluble or water-dispersible polyurethane or the like.

[0040] Examples of the polyol component include polyester polyol, polyether polyol, polycarbonate polyol, polyolefin polyol, acrylic polyol, etc.

[0041] Examples of the polyester polyol include polyester polyols obtained from the reaction of polyvalent carboxylic acids or their acid anhydrides with polyhydric alcohols, and polyester polyols obtained by ring-opening polymerization of cyclic ester compounds with glycols such as ethylene glycol. Examples of the polyvalent carboxylic acids include malonic acid, succinic acid, adipic acid, sebacic acid, fumaric acid, maleic acid, terephthalic acid, isophthalic acid, etc. Examples of the polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, neopentyl glycol, 1,6-hexanediol, etc. Examples of the cyclic ester compounds include lactide, ε-caprolactone, etc.

[0042] Examples of the polyether polyol include polyethylene glycol, polypropylene glycol, polyethylene propylene glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, and the like. Examples of the polycarbonate polyol include a polycarbonate polyol using 1,6 - hexanediol and / or 1,5 - pentanediol as the main diol component. Here, "main" means that the diol component is used at 60 mol% or more, more preferably 70 mol% or more, of the diol components.

[0043] Examples of the polyolefin polyol include those in which the ends of polyolefins mainly composed of polyisoprene, polybutadiene, etc. are modified with hydroxyl groups, and further those that are hydrogenated.

[0044] Examples of the acrylic polyol include poly(meth)acrylates into which hydroxyl groups are introduced at the ends.

[0045] The polyol component preferably has a number average molecular weight of 500 or more, more preferably 700 or more, and still more preferably 1000 or more. Also, the number average molecular weight is preferably 20000 or less, more preferably 15000 or less, and still more preferably 10000 or less. First, the average molecular weight can be measured, for example, by gel permeation chromatography (GPC) using polystyrene as a standard substance. In the case of a polyester polyol, for example, 0.03 g of the resin is dissolved in 10 ml of tetrahydrofuran, and using a GPC - LALLS apparatus low - angle light scattering photometer LS - 8000 (manufactured by Tosoh Corporation, tetrahydrofuran solvent, reference: polystyrene), it can be measured at a column temperature of 30°C, a flow rate of 1 ml / min, and using columns (such as Shodex KF - 802, 804, 806 manufactured by Showa Denko KK).

[0046] Examples of the polyisocyanate component include aromatic diisocyanates such as tolylene diisocyanate and diphenylmethane-4,4-diisocyanate, aromatic aliphatic diisocyanates such as xylylene diisocyanate, alicyclic diisocyanates such as isophorone diisocyanate and 4,4-dicyclohexylmethane diisocyanate and 1,3-bis(isocyanate-methyl)cyclohexane, aliphatic diisocyanates such as hexamethylene diisocyanate and 2,2,4-trimethylhexamethylene diisocyanate, or polyisocyanates obtained by pre-adding these compounds, either individually or in combination, with trimethylolpropane or the like.

[0047] Examples of the chain extender include glycols such as ethylene glycol, diethylene glycol, 1,4-butanediol, neopentyl glycol, and 1,6-hexanediol; polyhydric alcohols such as glycerin, trimethylolpropane, and pentaerythritol; diamines such as ethylenediamine, hexamethylenediamine, and piperazine; amino alcohols such as monoethanolamine and diethanolamine; thiodiglycols such as thiodiethylene glycol; or water.

[0048] The polyurethane resin preferably contains a polyester skeleton. The polyester skeleton is preferable because it has the same ester skeleton as polylactic acid, which is the main component of the stretched polylactic acid film substrate, and has a similar molecular structure, resulting in excellent adhesion to the film substrate. The polyester skeleton can be introduced into the polyurethane resin, for example, by using a polyester polyol as the polyol component. When a polyester polyol is used as the main component of the polyol component, the proportion of polyester polyol in the polyol component used in the polyurethane resin is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more.

[0049] Furthermore, it is preferable that the polyurethane resin contains a polydiene skeleton. Examples of polydiene skeletons include polybutadiene skeletons and polyisoprene skeletons. The polydiene skeleton may also contain components copolymerizable with diene monomers such as styrene and acrylonitrile as monomer units. The presence of a polydiene skeleton in the polyurethane resin makes it a flexible resin and results in a resin layer with excellent impact resistance. Therefore, even when a hard layer such as a hard coat layer is laminated on top of the resin layer as a functional layer, high adhesion with the hard coat layer can be achieved. Moreover, generally, when a laminated film with a hard layer such as a hard coat layer is cut or punched, the hard coat layer may crack at the cut portion, resulting in a poor appearance of the cut surface and defects in the final product due to generated dust. However, in the structure of the laminated polylactic acid film of the present invention, by using a polyurethane resin having a polydiene skeleton as one of the materials forming the resin layer, the occurrence of these defects can be suppressed.

[0050] One method for introducing a polydiene skeleton into polyurethane resin is to add a polydiene compound having hydroxyl groups at at least one end, particularly both ends, as a polyol component, chain extender, or chain length inhibitor during the manufacturing of the polyurethane resin. Examples of polydiene compounds having hydroxyl groups at the ends include hydroxyl-terminated liquid polybutadiene such as Poly bd (registered trademark) manufactured by Idemitsu Kosan Co., Ltd. and NISSO-PB G series manufactured by Nippon Soda Co., Ltd., and hydroxyl-terminated liquid polyisoprene such as Poly ip (registered trademark) manufactured by Idemitsu Kosan Co., Ltd.

[0051] When the polyurethane resin contains a polydiene skeleton, the proportion of the polydiene skeleton in the polyurethane resin (total of polyol component and polyisocyanate component, and further including chain extender and chain length inhibitor as needed) is preferably 1% by mass or more, preferably 3% by mass or more, and more preferably 5% by mass or more. Furthermore, the above proportion is preferably 80% by mass or less, preferably 70% by mass or less, more preferably 60% by mass or less, even more preferably 50% by mass or less, most preferably 40% by mass or less, and may also be 30% by mass or less. By setting it within the above range, good adhesion with the stretched polylactic acid film substrate and functional layers such as the hard coat layer can be achieved.

[0052] Polyurethane resins containing a polydiene skeleton preferably contain a polydiene compound having hydroxyl groups at both ends as a polyol component, and further preferably contain each of the aforementioned polyol components. In particular, polyurethane resins having a polydiene skeleton and a polyester skeleton are preferred. Polyurethane resins having a polydiene skeleton and a polyester skeleton are thought to exhibit high adhesion because, in addition to mitigating stress during peeling due to their high impact resistance, they have an ester skeleton and good affinity with polylactic acid, which is the base material. Examples of polyurethane resins containing a butadiene skeleton and a polyester skeleton include NeoSticker 200, NeoSticker 400, NeoSticker 700, and NeoSticker 1200 manufactured by Nikka Chemical Co., Ltd.

[0053] Furthermore, in polyurethane resins having a polydiene skeleton and a polyester skeleton, when polyester polyol and a polydiene compound having hydroxyl groups at both ends are used as the polyol component, the preferred proportion of polyester polyol in the polyol component and the proportion of the polydiene compound having hydroxyl groups at both ends are adjusted so as not to exceed 100% by mass, so that their respective proportions are consistent. For example, in the polyol component, the proportion of the polydiene compound having hydroxyl groups at both ends is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, while preferably 80% by mass or less, more preferably 70% by mass or less, even more preferably 60% by mass or less, particularly preferably 50% by mass or less, and may be 40% by mass or less, or 30% by mass or less. On the other hand, the proportion of polyester polyols is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, based on polyols other than polydiene compounds having hydroxyl groups at both ends (the proportion obtained by subtracting polydiene compounds having hydroxyl groups at both ends from the total polyol components), while the upper limit may be 100% by mass.

[0054] The glass transition temperature of the polyurethane resin in the present invention may be 70°C or lower, but may also be 50°C or lower, more preferably 30°C or lower, more preferably 15°C or lower, even more preferably 0°C or lower, particularly preferably -10°C or lower, and most preferably -20°C or lower. The glass transition temperature is preferably -100°C or higher, more preferably -80°C or higher, and even more preferably -60°C or higher. By setting the temperature within the above range, good adhesion with the stretched polylactic acid film substrate and functional layers such as the hard coat layer can be achieved.

[0055] In the present invention, the polyurethane resin content in the resin layer (or resin layer forming material) is preferably 45% by mass or more, more preferably 55% by mass or more, even more preferably 65% ​​by mass or more, even more preferably 70% by mass or more, particularly preferably 75% by mass or more, and most preferably 80% by mass or more, relative to 100% by mass of the solid component of the resin layer forming material. The polyurethane resin content is preferably 99% by mass or less, more preferably 95% by mass or less, and even more preferably 90% by mass or less. By setting it within the above range, good adhesion can be achieved. The solid component of the resin layer is the same as the solid component of the coating liquid used when forming the resin layer, but if volatile components are generated due to curing reactions after coating, etc., these volatile components are removed and the remaining components remain in the resin layer.

[0056] (2) Polyolefin resin The polyolefin resin used in the resin layer in the present invention is not particularly limited, but examples include low-density polyethylene, high-density polyethylene, modified polyethylene, polypropylene, and modified polypropylene waxes. Since polyolefins are hydrophobic, when used in an aqueous solvent, they are thought to orient themselves on the surface of the coating liquid and help form a smooth coating film. Furthermore, it is thought that the transparency will be improved by increasing the smoothness of the coating film. In addition, since polyolefin resins have a low surface free energy, adding them to the coating liquid can lower the surface free energy of the formed resin layer surface and improve blocking resistance. Suitable polyolefin resins include water-soluble or water-dispersible polyolefins.

[0057] The weight-average molecular weight of the polyolefin resin is preferably 1,000 or more and 40,000 or less. A molecular weight of 1,000 or more allows for good dispersibility in the solvent. A molecular weight of 40,000 or less allows for good dispersibility in the coating solution, as well as good film-forming properties during coating and drying, improving the appearance and transparency of the coating. Furthermore, when the polyolefin resin is unevenly distributed on the surface, improved transparency and blocking resistance can be expected, and a molecular weight of 40,000 or less is preferable because uneven distribution on the surface is more likely to occur. From the above viewpoint, the lower limit of the weight-average molecular weight of the polyolefin resin is more preferably 1,500 or more, even more preferably 2,000 or more, and may also be 2,500 or more, or 3,000 or more. The upper limit of the weight-average molecular weight of the polyolefin resin is more preferably 30,000 or less, even more preferably 25,000 or less, and may also be 20,000 or less, or 15,000 or less. The weight-average molecular weight of polyolefin resins can be measured, for example, by dissolving the polyolefin resin in a chlorine-based solvent such as 1,2,4-trichlorobenzene or o-dichlorobenzene at 130 to 170°C and measuring it using GPC within the aforementioned temperature range. Suitable columns include the PLgel series from Agilent Technologies and the TSKgel® HHR series from Tosoh Corporation, which can be selected according to the temperature and molecular weight range. Polystyrene can be used as the standard substance.

[0058] In the present invention, the polyolefin resin content in the resin layer (or resin layer forming material) is preferably 0.5% by mass or more and 35% by mass or less, relative to 100% by mass of the solid components of the resin layer forming material. More preferably, it is 1% by mass or more, even more preferably 2% by mass or more, and particularly preferably 3% by mass or more. The upper limit is more preferably 30% by mass or less, even more preferably 25% by mass or less, particularly preferably 20% by mass or less, and most preferably 15% by mass or less. By setting it within the above range, it becomes easier to achieve both improved transparency and blocking resistance and adhesion.

[0059] In this invention, the resin layer forming material may contain resins other than polyurethane resin and polyolefin resin in order to improve adhesion. Such resins are not particularly limited, but examples include polyester resin, polyolefin resin, and acrylic resin.

[0060] (3) Crosslinking agent The resin layer forming material of the present invention may contain a crosslinking agent. The crosslinking agent of the present invention is not particularly limited, but examples include urea-based, epoxy-based, melamine-based, isocyanate-based, oxazoline-based, and carbodiimide-based compounds. In addition, catalysts and the like may be used as appropriate to promote the crosslinking reaction.

[0061] The content of the crosslinking agent in the resin layer forming material is preferably 50% by mass or less, more preferably 0.5% by mass or more and 50% by mass or less, based on 100% by mass of the solid components of the resin layer forming material. Furthermore, 2% by mass or more and 50% by mass or less is preferred. More preferably 5% by mass or more and 30% by mass or less. By keeping it within the above range, the flexibility and strength of the resin in the resin layer can be improved, and good adhesion can be maintained at room temperature and under high temperature and high humidity conditions. The amount of crosslinking agent can be appropriately determined depending on the type of crosslinking agent, but based on 100% by mass of the total of the crosslinking agent and polyurethane resin, the crosslinking agent is preferably 3% by mass or more, more preferably 5% by mass or more, preferably 55% by mass or less, more preferably 50% by mass or less, and even more preferably 45% by mass or less.

[0062] The resin layer forming material can preferably be a polyurethane resin, a polyolefin resin, and, if necessary, a crosslinking agent, dissolved or dispersed in water or an organic solvent (for example, an aqueous solution containing less than 50% by mass of alcohol, alkyl cell solver, ketone, or ether).

[0063] In the present invention, it is preferable to contain lubricant particles in the resin layer (or resin layer forming material). Examples of lubricant particles include (1) inorganic particles such as silica, kaolinite, talc, light calcium carbonate, heavy calcium carbonate, zeolite, alumina, barium sulfate, carbon black, zinc oxide, zinc sulfate, zinc carbonate, titanium dioxide, zirconium dioxide, tin oxide, satin white, aluminum silicate, diatomaceous earth, calcium silicate, aluminum hydroxide, hydrated halloysite, magnesium carbonate, and magnesium hydroxide; and (2) organic particles such as acrylic or methacrylic, vinyl chloride, vinyl acetate, nylon, styrene / acrylic, styrene / butadiene, polystyrene / acrylic, polystyrene / isoprene, polystyrene / isoprene, methyl methacrylate / butyl methacrylate, melamine, polycarbonate, urea, epoxy, urethane, phenol, diallyl phthalate, and polyester.

[0064] By incorporating lubricant particles, the film can be made to have lubricity, which can suppress the occurrence of wrinkles when winding the film during the film manufacturing process, and the blocking that occurs when the wound film roll is stored for a long period of time, due to the film becoming tighter over time.

[0065] The average particle size of the lubricant particles is not particularly limited, but from the viewpoint of maintaining the transparency of the film, particles with an average particle size of 1 to 1000 nm are preferred, and those with an average particle size of 1 to 500 nm are more preferred. The average particle size is the average particle size measured using a laser diffraction particle size distribution analyzer (SALD-7500, manufactured by Shimadzu Corporation). Two or more types of particles with different average particle sizes may be used as the particles.

[0066] The content of the lubricant particles is preferably 0.5% by mass or more and 30% by mass or less based on 100% by mass of the solid component of the resin layer forming material. The content of the lubricant particles is more preferably 1% by mass or more, and even more preferably 2% by mass or more. The content of the lubricant particles is more preferably 25% by mass or less, and even more preferably 20% by mass or less. By setting the content within the above range, it becomes easier to obtain blocking resistance, as well as good scratch resistance, transparency of the resin layer, and coating film strength.

[0067] The resin layer forming material may also contain a surfactant to improve leveling during coating and to degas the coating liquid. The surfactant can be cationic, anionic, or nonionic, but silicone-based, acetylene glycol-based, or fluorine-based surfactants are preferred. These surfactants are preferably included in the resin layer to an extent that does not impair adhesion with the functional layer laminated on the resin layer.

[0068] The resin layer forming material may contain various additives to impart other functionalities, provided that these additives do not impair adhesion to the functional layer. Examples of such additives include fluorescent dyes, fluorescent whitening agents, plasticizers, ultraviolet absorbers, pigment dispersants, antifoaming agents, defoaming agents, preservatives, and antistatic agents.

[0069] In the present invention, a method for providing a resin layer on a stretched polylactic acid film substrate is to apply a coating solution containing a resin layer-forming material dissolved or dispersed in a solvent to the stretched polylactic acid film and then dry it. As the solvent, water or a mixture of water and a water-soluble organic solvent is preferred from the viewpoint of environmental issues, and the amount of water in the coating solution is preferably 50 to 95% by mass, and particularly preferably 60 to 90% by mass.

[0070] In the present invention, the solid content concentration of the resin layer forming material (coating liquid) that forms the resin layer is preferably 0.5 to 35% by mass, and particularly preferably 1.0 to 15% by weight.

[0071] Any known method can be used to apply the coating solution to the polylactic acid film. Examples include the reverse roll coating method, gravure coating method, kiss coating method, die coater method, roll brush method, spray coating method, air knife coating method, wire bar coating method, pipe doctor method, impregnation coating method, curtain coating method, and the like. These methods can be used individually or in combination for coating.

[0072] The method for forming the resin layer is not particularly limited, and conventionally known methods such as coating methods can be used. Among coating methods, preferred methods include coating after manufacturing the stretched polylactic acid film substrate (offline coating method) and coating during the manufacturing process of the stretched polylactic acid film substrate (in-line coating method). However, the in-line coating method is preferred because it improves adhesion between the substrate film and the resin layer, and minimizes deterioration of the mechanical properties of the substrate film during manufacturing and reduces thermal wrinkles. In the case of the in-line coating method performed during the manufacturing process of the stretched polylactic acid film substrate, the drying and heat treatment conditions during coating depend on the coating thickness and the conditions of the equipment, but it is preferable to immediately send the film to the stretching process in a perpendicular direction after coating and dry it in the preheating zone or stretching zone of the stretching process, and in such cases, it is usually preferable to set the temperature to around 50 to 120°C. Furthermore, the heat treatment process after stretching depends on the required mechanical properties of the substrate film and the conditions of the equipment, but it is preferable to perform heat treatment at a temperature of 130°C or higher from the viewpoint of improving the adhesive strength between the substrate film and the resin layer.

[0073] In the in-line coating method, the resin layer is formed by applying the coating solution to an unstretched or uniaxially stretched polylactic acid film, drying it, stretching it at least uniaxially, and then performing a heat treatment.

[0074] In the present invention, the thickness of the final resin layer is not particularly limited, but is preferably 20 nm or more and 200 nm or less, more preferably 150 nm or less, even more preferably 120 nm or less, and even more preferably 100 nm or less. If the thickness of the resin layer is less than 20 nm, the effect on lubricity required in the present invention is almost lost. On the other hand, if the thickness of the resin layer exceeds 200 nm, haze increases and transparency decreases.

[0075] In this invention, the surface free energy γs of the final resin layer is 42 mJ / mJ / m ２ The following is preferable: Surface free energy γs is 42 mJ / m ２ The following measures improve resistance to blocking, but they also make it less likely for wrinkles to form when winding the film or for blocking to occur when the film roll is stored for a long period of time, which is undesirable.

[0076] (Laminated Polylactic Acid Film) The laminated polylactic acid film of the present invention can be used with any thickness depending on the desired purpose and application, such as mechanical strength and transparency. The thickness is not particularly limited, but is preferably 2 μm to 500 μm, more preferably 15 μm to 400 μm, and even more preferably 20 μm to 250 μm. If the thickness is too thin, handling is likely to be poor. On the other hand, if the thickness is too thick, not only are there cost issues, but when stored wound in a roll, poor flatness due to curling is likely to occur.

[0077] The haze of the laminated polylactic acid film of the present invention is preferably 5.0% or less, more preferably 3.0% or less, and most preferably 2.0% or less. If it is 5.0% or more, the design quality of printed materials laminated with UV inks, etc., decreases, the transparency of functional films such as UV hard coat layers decreases, and the accuracy of detecting internal foreign matter, which is a defect, decreases.

[0078] (Radiation-cured layer laminated polylactic acid film) The laminated polylactic acid film of the present invention can be used as a radiation-cured layer laminated film by applying a radiation-curable coating liquid or printing an ink onto its resin layer surface and curing the coating liquid or ink by irradiating it with radiation. Examples of the aforementioned radiation include visible light, ultraviolet rays, X-rays, and electron beams, with ultraviolet rays and electron beams being preferred. Specific examples of radiation-cured layers include UV hard coat layers and UV ink layers.

[0079] The material used for the radiation-curable layer is not particularly limited, and any resin compound that polymerizes and / or reacts upon irradiation with radiation can be used. Examples of such curable resins include melamine-based, epoxy-based, acrylic-based, silicone-based, and polyvinyl alcohol-based curable resins, but photocurable acrylic-based curable resins are preferred in terms of obtaining high surface hardness or optical design. Examples of such acrylic-based curable resins include polyfunctional (meth)acrylate monomers and acrylate oligomers, and examples of acrylate oligomers include polyester acrylate, epoxy acrylate, urethane acrylate, polyether acrylate, polybutadiene acrylate, and silicone acrylate. By mixing these radiation-curable resins such as acrylic-based curable resins with a reaction diluent, a photopolymerization initiator, a sensitizer, etc., a coating composition for forming the UV hard coat layer, UV ink layer, etc., can be obtained.

[0080] As described above, the laminated polylactic acid film of the present invention is made from non-petroleum raw materials and biodegradable materials, thus contributing to the reduction of environmental impact. Furthermore, it has good adhesion to printing paints and maintains good adhesion even in humid and hot environments, making it easy to replace common plastics such as polyolefins, polyesters, and polyamides. It can be suitably used as a packaging material or a base material for functional films used in food, pharmaceuticals, industrial products, etc.

[0081] The present invention will be described in detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[0082] (Evaluation Method) The following methods were used to measure (A) the glass transition temperature and (B) the average particle size of the lubricant particles. In addition, the film properties of the laminated polylactic acid films obtained in Examples 1 to 7 and Comparative Examples 1 to 3 were measured and evaluated by the following methods (1) to (3). The results are shown in Table 1. Furthermore, radiation-cured laminated films (UV hard coat laminates) were manufactured from the laminated polylactic acid films obtained in each example, and the film properties of each were measured and evaluated by the following method (4). The results are shown in Table 1.

[0083] (A) Glass transition temperature In accordance with JIS K7121-1987, a differential scanning calorimeter (Seiko Instruments, DSC6200) was used to heat 10 mg of a stretched polylactic acid film substrate resin sample at a rate of 5°C / min over a temperature range of 25 to 300°C, and the extrapolation glass transition onset temperature obtained from the DSC curve was defined as the glass transition temperature.

[0084] (B) Average particle size of lubricant particles The dispersion containing lubricant particles was diluted with deionized water so that the lubricant particle content was 0.05% by mass, and the particle size distribution was measured using a laser diffraction particle size analyzer (SALD-7500, Shimadzu Corporation). The average value of the particle size distribution was calculated and defined as the average particle size.

[0085] (1) Haze In accordance with JIS K7136, the haze of stretched polylactic acid film substrates, laminated polylactic acid films, and UV hard coat laminates was measured using a turbidimeter (Nippon Denshoku, NDH2000). For laminated polylactic acid films, those with a haze of 3.0% or less were considered transparent, and those with a haze of 2.0% or less were judged to have particularly good transparency. Similarly, for UV hard coat laminates, those with a haze of 0.7% or less were considered transparent, and those with a haze of 0.6% or less were judged to have particularly good transparency.

[0086] (2) Surface free energy γs After leaving the laminated polylactic acid film samples in an atmosphere of 50% relative humidity for 24 hours, the contact angles of distilled water and diiodomethane one minute after dropping them onto the resin layer were measured using a FACE contact angle meter (Kyowa Interface Chemical Co., Ltd., CA-X type). Five measurements were taken for each sample, and the average of the three measurements (excluding the maximum and minimum values) was used as the contact angle. The surface free energy γs was then calculated from the contact angles of distilled water and diiodomethane.

[0087] (3) Overlap the front and back surfaces of two film samples of block-resistant laminated polylactic acid film and apply 1 kgf / cm² to them. ２ After applying pressure to the two films in a 60°C atmosphere for 24 hours, they were peeled off, and the peeling state was judged according to the following criteria: A: No transfer of the coating layer, and easy peeling. B: A peeling sound is produced, and the coating layer has partially transferred to the mating surface. C: The two films are stuck together and cannot be peeled off, or even if they can be peeled off, the base film is cleaved. Films with a blocking resistance rank of A or B were considered to have blocking resistance, and those with a rank of A were judged to be particularly good.

[0088] <Physical Properties of Radiation-Cured Layered Polylactic Acid Film> <Manufacturing of UV Hard Coat Laminate> 100 parts by mass of UV-curable urethane acrylate resin (Beamset 577, manufactured by Arakawa Chemical Industries, Ltd., solid content concentration 100% by mass) and 5 parts by mass of photopolymerization initiator (Omnirad 184, manufactured by IGM Resins, solid content concentration 100% by mass) were diluted with MEK / toluene (=1:1) solution to prepare a UV hard coat coating solution with a solid content of 40% by mass. This UV hard coat coating solution was applied to the resin layer of the laminated polylactic acid film using a reverse gravure coater to a thickness of 4 μm after drying. Then, it was dried with hot air at 80°C for 30 seconds, and immediately afterwards, ultraviolet irradiation (200 mJ / cm²) was performed using an electrodeless lamp (H-bulb, manufactured by Fusion Co., Ltd.) ２ ) was performed to obtain a UV hard coat laminate.

[0089] (4) Adhesion to the UV Hard Coat Layer The UV hard coat laminate obtained above was subjected to 100 grid-like cuts made using a cutter guide with a gap of 2 mm, penetrating the UV hard coat layer and reaching the stretched polylactic acid film substrate. Next, cellophane adhesive tape (Nichiban Co., Ltd., No. 405; 24 mm wide) was applied to the grid-like cut surfaces and rubbed with an eraser to ensure complete adhesion. After that, the cellophane adhesive tape was peeled vertically from the UV hard coat layer surface five times, and the number of grids that peeled off from the UV hard coat layer surface of the UV hard coat laminate was visually counted, and the adhesion between the UV hard coat layer and the laminated polylactic acid film was determined using the following formula. Note that grids that were partially peeled off were also counted as peeled grids, and were ranked according to the following criteria. Adhesion (%) = (1 - number of peeled squares / 100) × 100 A: 100-95% B: 94-80% C: 79-60% D: 59-0% Products with an adhesion rank of A to C to this UV hard coat layer were considered to have good adhesion, with those with a rank of A being judged to have particularly good adhesion.

[0090] The resin, crosslinking agent, and lubricant particles used in the resin layer are as follows, and the composition of the coating solution is shown in Table 1.

[0091] (A-1: Water-based polyurethane resin) NeoSticker 400 (manufactured by Nikka Chemical Co., Ltd., solids content 37% by mass) Polyurethane resin containing polybutadiene and polyester skeletons: Glass transition temperature -45°C (A-2: Water-based polyurethane resin) Hydran AP-201 (manufactured by DIC Corporation, solids content 23% by mass) Polyurethane resin containing polyester skeleton: Glass transition temperature 10°C (A-3: Water-based polyurethane resin) Hydran AP-40F (manufactured by DIC Corporation, solids content 23% by mass) Polyurethane resin containing polyester skeleton: Glass transition temperature 55°C

[0092] (B-1: Water-based polyolefin resin: polyethylene-based) AQUACER 552 (manufactured by BYK, solids content 35% by mass) (B-2: Water-based polyolefin resin: polyethylene-based) Zyxen L (manufactured by Sumitomo Seika Co., Ltd., solids content 25% by mass)

[0093] (C-1: Aqueous polyester resin) Dimethyl terephthalate (95 parts by mass), dimethyl isophthalate (95 parts by mass), ethylene glycol (35 parts by mass), neopentyl glycol (145 parts by mass), zinc acetate (0.1 parts by mass), and antimony trioxide (0.1 parts by mass) were charged into a reaction vessel in a stainless steel autoclave equipped with a stirrer, thermometer, and partial reflux condenser, and a transesterification reaction was carried out at 180°C for 3 hours. Next, 5-sodium sulfoisophthalic acid (6.0 parts by mass) was added, and an esterification reaction was carried out at 240°C for 1 hour, followed by a polycondensation reaction at 250°C under reduced pressure (10-0.2 mmHg) for 2 hours to obtain copolymer polyester resin (A) with a number average molecular weight of 19,500 and a softening point of 60°C. In a reactor equipped with a stirrer, thermometer, and reflux device, 30 parts by mass of the polyester resin (A) and 15 parts by mass of ethylene glycol n-butyl ether were added and heated at 110°C, and the resin was dissolved by stirring. After the resin was completely dissolved, 55 parts by mass of water were gradually added to the polyester solution while stirring. After the addition, the liquid was cooled to room temperature while stirring to prepare a milky white polyester aqueous dispersion (C-1) with a solid content of 30% by mass. The glass transition temperature of the polyester aqueous dispersion (C-1) was 62°C.

[0094] (D-1: Silica particles) MP4540M (manufactured by Nissan Chemical Industries, solid content 40% by mass) The average particle size of the lubricant particles was 450 nm according to the evaluation method described above. (D-2: Silica particles) Snowtex (registered trademark) 30L (manufactured by Nissan Chemical Industries, solid content 30% by mass) The average particle size of the lubricant particles was 45 nm according to the evaluation method described above.

[0095] (Other) Silicone-based surfactant (solid content concentration 100% by mass)

[0096] (Examples 1-7, Comparative Examples 1-3) Total Corbion's poly-L-lactic acid PLA L175 (mass ratio of L-lactic acid to D-lactic acid: 99 / 1) was used as the polylactic acid for the polylactic acid film substrate. The poly-L-lactic acid (L175) was dried under reduced pressure at 120°C for 6 hours (1 Torr), then melted at 220°C using an extruder, and the molten resin was extruded from the T-die into a sheet. The sheet was then brought into close contact with a cooling roll heated to 50°C to obtain an unstretched sheet with a thickness of 500 μm. The obtained unstretched sheet was guided to a roll-type stretcher and stretched 3.0 times in the longitudinal direction at 80°C using the difference in peripheral speed of the rolls. On one side of the obtained uniaxially oriented film, a resin layer-forming material prepared in the proportions shown in Table 1 (coating solution: the values ​​for aqueous resin, crosslinking agent, and lubricant particles in Table 1 are all mass percent of the solution) was applied by the fountain coat method at a rate of 5.0 g / m². ２ The film was adjusted to the desired consistency and then applied. Subsequently, this inline-coated uniaxially oriented film was continuously fed into a tenter stretcher, preheated to 70°C, stretched 4.5 times transversely at 75°C, heat-set at 150°C, then relaxed by 3% at 120°C, trimmed at both ends with shear blades, and the laminated film was wound around a 6-inch diameter cylindrical core made of polypropylene to obtain the laminated polylactic acid films of Examples 1-7 and Comparative Examples 1-3. The glass transition temperature of the stretched polylactic acid film substrate, measured using the evaluation method described above, was 58°C. The haze of the stretched polylactic acid film substrate was 0.1%. The properties of the obtained laminated polylactic acid films are shown in Table 1.

[0097] The laminated polylactic acid films of Examples 1 to 7 exhibited good adhesion to the hard coat layer, satisfied transparency (haze) and blocking resistance, and Examples 1, 4, and 7 showed particularly excellent adhesion to the hard coat layer, with no cracking at cuts in the carter.

[0098] On the other hand, Comparative Example 1, lacking polyurethane resin, exhibited poor adhesion to the hard coat layer. The glass transition temperature of Comparative Example 1 was that of polyester resin. Comparative Example 2, lacking olefin resin and exhibiting high surface free energy, resulted in blocking. Comparative Example 3, with its polyurethane resin having a polybutadiene structure, achieved high adhesion to the hard coat layer, but lacking polyolefin resin, it failed to satisfy the transparency requirement.

[0099]

[0100] The laminated polylactic acid film of the present invention contributes to reducing environmental impact because it is made from non-petroleum raw materials and biodegradable materials. Furthermore, it exhibits excellent transparency and adhesion, similar to general plastics such as polyolefins, polyesters, and polyamides, and can be suitably used as a packaging material or base material for functional films used in food, pharmaceuticals, industrial products, etc.

Claims

1. A laminated polylactic acid film having a stretched polylactic acid film substrate and a resin layer on at least one surface of the stretched polylactic acid film substrate, wherein the resin layer is formed of a resin layer forming material containing a polyurethane resin and a polyolefin resin.

2. The laminated polylactic acid film according to claim 1, wherein the glass transition temperature of the polyurethane resin is 70°C or lower.

3. The laminated polylactic acid film according to claim 1 or 2, wherein the polyurethane resin is a polyurethane resin having a polyester skeleton.

4. The laminated polylactic acid film according to any one of claims 1 to 3, wherein the polyurethane resin is a polyurethane resin containing a polydiene skeleton.

5. The surface free energy of the resin layer is 42 mJ / m ２ The laminated polylactic acid film according to any one of the following claims 1 to 4.

6. A laminated polylactic acid film according to any one of claims 1 to 5, wherein the resin layer forming material contains 0.5% by mass or more and 35% by mass or less of polyolefin resin based on 100% by mass of the solid components.

7. The laminated polylactic acid film according to any one of claims 1 to 6, wherein the resin layer is formed by an in-line coating method.

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