JP4530670B2 - Anti-fogging polylactic acid biaxially stretched film and method for producing the same - Google Patents

Description

  The present invention relates to a polylactic acid-based biaxially stretched film for the purpose of packaging aquatic foods containing vegetables and fruits. And a manufacturing method thereof.

  Conventionally, polypropylene and polyethylene terephthalate are known as materials excellent in mechanical strength, heat resistance, and dimensional stability, and biaxially stretched films using these are widely used in the industry. However, when these plastic films are incinerated after being used, if they are incinerated, the amount of heat generated during incineration is high, which may damage the incinerator during the processing. And these plastics remain almost undegradable due to chemical and biological stability. Therefore, with the recent increase in social demand for environmental conservation, there is a demand for a film made of a biodegradable resin that has biodegradability that can be decomposed by microorganisms and that can be composted in compost. Yes.

  Among biodegradable resins, polylactic acid is a plant-derived raw material obtained by polymerizing lactic acid obtained by fermenting various starches, sugars, etc., and is finally recycled on a global scale to carbon dioxide and water. It has begun to be used for various applications as an ideal polymer raw material. Polylactic acid-based stretched films have excellent mechanical properties such as tensile strength, tensile elastic modulus, and impact strength, as well as gloss and transparency, and are expected to expand into fields centered on food packaging.

  When the polylactic acid-based biaxially stretched film is applied to food packaging applications such as vegetables and fruits containing a lot of moisture, long-term antifogging properties of about one week after packaging are strongly required. In general, a polylactic acid-based biaxially stretched film is excellent in water resistance because its surface is hydrophobic. Therefore, moisture tends to adhere to the film surface as minute water droplets. Therefore, when the polylactic acid-based biaxially stretched film is used for packaging foods containing moisture such as vegetables and fruits, the condensed water adheres to the inner surface of the film, causing not only the contents to be difficult to see but also causing decay.

Generally, as a method for imparting antifogging properties to plastic films and sheets, a method of kneading sucrose fatty acid esters or glycerin fatty acid esters or applying them to the surface is employed. However, when the conventional method is applied to a polylactic acid-based biaxially stretched film, the kneading and mixing method not only provides sufficient antifogging properties, but also deteriorates extrudability during film production, film-forming properties, and causes of hydrolysis. It is not practical because it causes a decrease in mechanical properties. Although the initial low-temperature antifogging property can be obtained by applying to the surface, blocking of the films accompanying stickiness of the surface, deterioration of workability at the time of handling, transfer of the antifogging agent to the back surface and accompanying antifogging insufficiency In addition, problems such as a decrease in anti-fogging durability due to insufficient water resistance cannot be avoided. As an example of imparting antifogging properties to a polylactic acid film or sheet, Patent Document 1 discloses an example in which antifogging durability is improved by simultaneously using a specific amount of lubricant and a specific amount of antifogging agent in combination. However, it is necessary to increase the amount of addition in order to obtain sufficient antifogging performance due to the kneading of the antifogging agent, which may impair not only the molecular weight of polylactic acid but also the extrudability and film forming property. There is. Patent Document 2 discloses a film for fruit and vegetable packaging that defines water vapor permeability, and Patent Document 3 discloses an antifogging resin sheet based on a specific mixing ratio of sucrose laurate and a water-soluble resin. There are no examples of polylactic acid-based biaxially stretched films having sufficient antifogging properties and blocking resistance.
JP-A-9-286908 JP 2003-81354 A JP 2000-280410 A

  The present invention is to solve the above-mentioned conventional problems relating to polylactic acid-based films, and to provide a polylactic acid-based biaxially stretched film that is excellent in low-temperature antifogging properties and excellent in blocking resistance and antifogging durability. is there.

  As a result of intensive studies to solve the above problems, the present inventors formed a coating layer comprising a mixture of a specific nonionic surfactant and an aqueous polyurethane resin and / or an aqueous polyester resin on a polylactic acid film. As a result, it has been found that a film excellent in low-temperature anti-fogging property and anti-fogging sustainability can be obtained at the same time, whereby blocking between the films and the back surface migration can be suppressed.

  That is, the present invention is a polylactic acid-based biaxially stretched film having a coating layer comprising a nonionic surfactant (A) and an aqueous polyurethane resin and / or an aqueous polyester resin (B) on at least one side, The antifogging characterized in that the mass ratio (A / B) of the nonionic surfactant (A) of the layer to the aqueous polyurethane resin and / or the aqueous polyester resin (B) is 90/10 to 10/90. The essential feature is a conductive polylactic acid-based biaxially stretched film.

  Since the coating layer of the anti-fogging polylactic acid-based biaxially stretched film of the present invention is composed of a specific nonionic surfactant and a water-soluble / water-dispersible polymer, the coating layer has good adhesion to the base film. Because it is strong and suppresses back migration of nonionic surfactants, it has excellent antifogging and blocking resistance at low temperatures, especially antifogging durability, and is suitable as a packaging bag for marine foods containing vegetables and fruits Can be used for

  Embodiments of the present invention will be described below.

  The polylactic acid biaxially stretched film of the present invention refers to a film made of a homopolymer having L-lactic acid as a main structural unit or a polylactic acid resin having L-lactic acid / D-lactic acid as a main structural unit. The polylactic acid-based biaxially stretched film of the present invention promotes oriented crystallization by biaxial stretching and develops a practical strength. Therefore, the molar ratio with L-lactic acid / D-lactic acid (L-lactic acid / D-lactic acid) Preferably satisfies a composition ratio of 100/0 to 94/6 (molar ratio). When D-lactic acid exceeds 6 mol%, the polylactic acid resin does not show a clear melting point and is poor in crystallinity. As a result, the thickness accuracy of the film is remarkably deteriorated, and the orientation crystallization by the heat setting treatment after stretching is difficult to proceed, so that not only the problem that the film is cracked or torn at the time of winding the film occurs, On the surface of secondary processing, breakage due to film tension and troubles due to blocking occur. Moreover, although L-lactic acid may be used independently, the crystallinity is eased and the film forming property is better when D-lactic acid is blended. The L-lactic acid and D-lactic acid may be a copolymer or a blend as long as they are blended in the above proportions.

  The number average molecular weight of the polylactic acid resin is preferably in the range of 50,000 to 300,000, more preferably 80,000 to 150,000. When the number average molecular weight is less than 50,000, the resulting film has insufficient mechanical strength, and also frequently undergoes cutting during the stretching and winding processes, resulting in a decrease in operability. On the other hand, when the number average molecular weight exceeds 300,000, the fluidity at the time of heating and melting becomes poor and the film-forming property is lowered.

  As a polymerization method for obtaining a polylactic acid-based resin, any of a condensation polymerization method and a ring-opening polymerization method can be adopted, and a small amount of chain extender such as a diisocyanate compound or diepoxy is used for the purpose of increasing the molecular weight. Compounds, acid anhydrides and the like may be used.

  A lubricant may be added to the polylactic acid-based resin from the viewpoints of handling in the production process or secondary processing process and film running properties. Lubricants include stable metal oxides such as silica, titanium dioxide, talc, and alumina, stable metal salts such as calcium carbonate, calcium phosphate, and barium sulfate, polymethyl methacrylate, melamine / formaldehyde cured resin, and polyamines such as melamine / benzoguanamine. Examples include so-called organic beads made of an organic resin inert to lactic acid. Any one of these lubricants may be used alone, or two or more may be used in combination.

  In the polylactic acid biaxially stretched film of the present invention, it is necessary to form a coating layer comprising a nonionic surfactant (A) and an aqueous polyurethane and / or an aqueous polyester resin (B) on at least one surface.

  In this invention, a nonionic surfactant (A) is a component required in order to express antifogging property. As the nonionic surfactant (A), polyoxyethylene alkyl ether, polyoxyethylene fatty acid ester, polyoxyethylene fatty acid amide, polyoxyethylene sorbitan fatty acid ester, polyglycerin fatty acid ester, sucrose fatty acid ester and the like are relatively water-soluble. From the viewpoint of anti-fogging performance, polyoxyethylene sorbitan fatty acid ester, polyglycerin fatty acid ester, and sucrose fatty acid ester are preferable. In the polyoxyethylene sorbitan fatty acid ester, the number of carbon atoms of the fatty acid is preferably C12 to C22 and the number of moles of ethylene oxide added is 5 to 20 mol. More preferred are polyoxyethylene sorbitan laurate and polyoxyethylene sorbitan oleate added with 20 mol of ethylene oxide. These polyoxyethylene sorbitan fatty acid esters may be used alone or in combination of two or more. The polyglycerol fatty acid ester preferably has a fatty acid carbon number of C12 to C18 and a polyglycerol polymerization degree of 4 to 10, and decaglycerol of a polymerization degree of 10 is particularly preferable. The ester composition preferably has a monoesterification rate of 60 mol% or more from the viewpoint of water solubility and coatability. More preferred are decaglycerin laurate and decaglycerin oleate. These polyglycerin fatty acid esters may be used alone or in combination of two or more. The sucrose fatty acid ester is preferably a sucrose laurate, sucrose oleate, or sucrose stearate whose fatty acid has C12 to C18 carbon atoms. The ester composition preferably has a monoesterification rate of 60 mol% or more from the viewpoint of water solubility and coatability. These sucrose fatty acid esters may be used alone or in combination of two or more.

  The above nonionic surfactants may be used alone or in combination as long as they satisfy water solubility or water dispersibility. Nonionic surfactants particularly preferably used in the present invention are polyoxyethylene sorbitan monolaurate (20 mol addition of ethylene oxide) and decaglycerin monolaurate.

  In the present invention, the water-based polyurethane resin and / or water-based polyester resin (B) used in combination with the nonionic surfactant (A) is a polylactic acid biaxial compound. Necessary as a binder resin for firmly adhering to the surface of the stretched film, whereby the blocking effect and anti-fogging durability of the coating layer can be improved.

  In the present invention, the aqueous polyurethane resin and / or aqueous polyester resin (B) refers to a water-soluble or water-dispersible resin. The aqueous polyurethane resin is not particularly limited, and examples thereof include polyester-based polyurethane, polyether-based polyurethane, polyurethane polyurea resin, and prepolymers thereof. As a specific example of such a polyurethane resin, a self-emulsifying polyurethane resin having a hydrophilic group such as carboxylate or sulfonate introduced into the main chain or side chain of the polyurethane resin is preferable. A small amount of a crosslinking agent such as an isocyanate compound, an epoxy compound, or an amino resin may be added for the purpose of improving the adhesion to the polylactic acid-based biaxially stretched film, the water resistance of the coating, and the heat resistance. Examples of the aqueous polyester resin include various polyester resins and modified products thereof. Specific examples of such an aqueous polyester resin include terephthalic acid, isophthalic acid, trimellitic acid, pyromellitic acid, 2-sulfoisophthalic acid, 5-sulfoisophthalic acid, succinic acid, adipic acid, sebacic acid, dodecanedioic acid. And other diol components such as ethylene glycol, neopentyl glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, cyclohexanedimethanol, and ethylene oxide adduct of bisphenol A. Examples include reactants, and modified products such as acrylic resins and epoxy resins are also included.

  In the present invention, the mass ratio (A / B) between the nonionic surfactant (A) and the aqueous polyurethane resin and / or the aqueous polyester resin (B) needs to be 90/10 to 10/90. Yes, preferably 80/20 to 40/60. By setting the mass ratio within this range, adhesion to the polylactic acid biaxially stretched film, antifogging properties, and blocking resistance of the coating can be satisfied. When the blending ratio of the aqueous polyurethane resin and / or the aqueous polyester resin (B) is less than 10%, the antifogging durability is insufficient because of poor water resistance. Moreover, in a film roll, a nonionic surfactant tends to transfer to the opposite side of the coating layer, resulting in variations in antifogging performance and blocking. On the other hand, if it is higher than 90%, the antifogging effect becomes poor.

In the present invention, the coating amount after drying of the coating layer on the polylactic acid film is preferably 0.01 to 1 g / m ² . If it is less than 0.01 g / m ² , the antifogging effect is poor, and if it exceeds 1 g / m ² , no significant change is observed in the antifogging performance. Thick coating is not preferred.

  In the present invention, any known coating method can be applied as a method of applying the aqueous solution or aqueous dispersion on the surface of the polylactic acid film. For example, a gravure roll method, a Meyer bar method, a reverse roll method, and an air knife method are applied independently or in combination. The film to be coated is preferably an unstretched film after melt-extrusion of the polymer and casting on a cooling roll, or a uniaxially stretched film after stretching in the machine direction, but may be a biaxially stretched film. When drying in an oven after coating, it should be performed under temperature conditions that do not cause thermal deformation of the unstretched film, uniaxially stretched film, and biaxially stretched film, and preferably not exceed 100 ° C. Further, after coating, drying may be performed by using preheating, stretching, and heat setting zones in a stretching machine.

  In the present invention, it is preferable to use a so-called in-line coating method in which the aqueous solution or aqueous dispersion is applied to an unstretched film or a uniaxially stretched film, and a biaxially stretched film is obtained through a stretching heat treatment. That is, the aqueous solution or water dispersion is applied to an unstretched polylactic acid film, and then stretched in a biaxial direction, or the polylactic acid film stretched only in a uniaxial direction, the aqueous solution or water dispersion. It is preferable to use an in-line coating method in which a liquid is applied and subsequently stretched in a direction perpendicular to the initial stretching direction. This method can reduce the thickness of the coating layer and can improve the adhesion to the substrate film. Further, since only one step is required, the cost can be reduced, which is very useful, but a post-coating method for applying to a biaxially stretched film may be used.

  Examples of the method for producing the polylactic acid-based biaxially stretched film of the present invention include a T-die method, an inflation method, a calendar method, and the like. As an example of the production method of the T-die method, a polylactic acid polymer, a polylactic acid resin composition containing a plasticizer and a lubricant in an appropriate amount as needed is supplied to an extruder hopper, and the extruder has a cylinder temperature of 180 to 250, for example. C., T-die temperature is 200 to 250.degree. C., melt kneaded and extruded, and cooled with a cooling roll controlled to 20 to 40.degree. C. to obtain an unstretched sheet having a thickness of 100 to 500 .mu.m.

  As a biaxial stretching method of the unstretched sheet, any of a coaxial biaxial stretching by a tenter method and a sequential biaxial stretching method by a metal roll and a tenter may be used. For example, when the unstretched film is formed into a film by the sequential biaxial stretching method, the obtained unstretched film is stretched in the longitudinal direction at a roll surface temperature of 50 to 80 ° C. in the longitudinal direction by the rotation speed ratio of the drive roll, Subsequently, the film is continuously stretched in the transverse direction at a stretching temperature of 70 to 100 ° C. The draw ratio is not particularly limited, but considering the mechanical properties of the film, at least the longitudinal draw ratio is preferably 2.5 times or more, and the surface magnification is preferably 8 times or more. If the longitudinal and lateral draw ratios are less than 2.5, sufficient mechanical properties cannot be obtained, resulting in poor practicality. The upper limit of the draw ratio is not particularly limited, but if it exceeds 8 times, film breakage tends to occur. Therefore, the length and width are preferably 2.5 to 8.0 times, and the longitudinal draw ratio is 2. It is preferable that it is 5 to 5.0 times and the transverse draw ratio is 2.5 to 8.0 times. After the above stretching process is performed, a heat setting process is performed at a temperature of 100 to 150 ° C., and a heat relaxation process is performed under a condition of a relaxation rate of 2 to 8%.

  The polylactic acid-based biaxially stretched film of the present invention is obtained by laminating a coating layer of a mixture comprising an antifogging agent and an aqueous resin on at least one side. In order to increase the surface, a surface treatment such as corona discharge treatment, plasma treatment, ozone treatment, flame treatment, or the like may be applied in advance to the coated surface side of the film. Among these, the corona discharge treatment is most preferable from the viewpoint of simplicity. It is preferable that the surface tension be 40 mN / m or more by these surface treatments.

  The thickness of the polylactic acid-based biaxially stretched film of the present invention is not particularly limited, and may be appropriately set depending on the application, required performance, price and the like. Generally, a thickness of about 10 to 50 μm can be exemplified. Furthermore, for the purpose of improving the printability, laminating property, coating suitability, etc. of the film, the coated surface may be subjected to corona discharge treatment again.

  The polylactic acid-based biaxially stretched film of the present invention may be added or surface-coated with a pigment, an antioxidant, a plasticizer, an ultraviolet absorber, a lubricant, a crystal nucleating agent, an antistatic agent, etc., as required. Can do.

  EXAMPLES Next, although an Example demonstrates this invention, this invention is not restrict | limited by the following Example. In addition, the measuring method and evaluation method used for the Example and the comparative example were implemented with the following method.

(1) Anti-fogging property and anti-fogging durability 300 ml of hot water at 50 ° C. is put in a 500 ml glass container, sealed with the coated surface of the film inside, and left in a refrigerator at 5 ° C. Observed over time. The antifogging property was evaluated in the following four stages. The standing time was 1 hour, 8 hours, 1 day, 3 days, and 7 days.
A: The surface is uniform and transparent, and there is no adhesion of water droplets.
○: The surface is transparent and there is no adhesion of water droplets, but it appears uneven and distorted.
Δ: Large water droplets partially adhere to the surface.
X: A fine water droplet adheres to the whole surface, and the bottom cannot be seen.

(2) Blocking resistance The coated and non-coated surfaces of the film were overlapped and treated for 24 hours at 40 ° C. × 80% RH under a load of 10 kg / cm ² , and then the peel force between the films was measured. From the peel load, blocking resistance was evaluated according to the following criteria.
A: Peeling load less than 10 g B: Peeling load 10 g or more and less than 30 g Δ: Peeling load 30 g or more and less than 100 g X: Peeling load 100 g or more

(3) Adhesion to the film substrate After 100 cuts were cut into a grid pattern on the coated surface with a cutter, a peel test was conducted using a Nichiban adhesive tape, and the peel status of the coated layer was judged according to the following criteria. did.
◎: No peeling part ○: Less than 20 peeling parts Δ: 20 or more and less than 80 peeling parts ×: 80 or more peeling parts

(4) Comprehensive evaluation The above-described characteristic values were combined and evaluated according to the following criteria.
◎: Very good ○: Good △: Somewhat problematic ×: There is a problem

Example 1
60% by mass of polyoxyethylene sorbitan monolaurate (A-1) (Sorbon T-20 manufactured by Toho Chemical Co., Ltd.) as a nonionic surfactant and a self-emulsifying polyurethane resin (B-1) (as a water-dispersed polyurethane resin) To 100 parts by mass of a mixture consisting of 40% by mass of Dainippon Ink & Chemicals Hydran KU-400SF), 10 parts by mass of an amino resin (Dainippon Ink & Chemicals Becamine PMN) is added, and the total solid content is A water dispersion was prepared by adjusting the concentration with water to 5 mass%.

  100 parts by mass of a polylactic acid resin (manufactured by Cargill Dow Polymer Co., Ltd., L-lactic acid / D-lactic acid = 98.5 / 1.5 (molar ratio), melting point 165 ° C., number average molecular weight 105,000), antiblocking agent A resin composition blended with 0.1 part by mass of amorphous silica (Silicia 310P manufactured by Fuji Silysia Chemical Co., Ltd., average particle size: 1.4 μm) is melt extruded at a T die temperature of 230 ° C. with a 90 mmφ single screw extruder. Then, the film was closely contacted and rapidly cooled to a cast roll whose temperature was controlled at 35 ° C. to obtain an unstretched sheet having a thickness of 300 μm.

The obtained unstretched sheet was stretched 3.0 times in the longitudinal direction at a preheating roll of 65 ° C. and a stretching roll of 75 ° C., and then the aqueous dispersion was applied so that the coating amount after stretching was 0.05 g / m ^2. After coating with the Mayer bar coat, the film was stretched 4.0 times in the transverse direction at a stretching temperature of 85 ° C. in the tenter, and then heat treated at 140 ° C. with a transverse relaxation rate of 5%. A sequential biaxially stretched film was obtained. Table 1 shows the physical properties of the obtained film.

Examples 2-4
A biaxially stretched film was obtained in the same manner as in Example 1 except that the nonionic surfactant, aqueous polyurethane resin, and aqueous polyester resin shown in Table 1 were used and the mass ratio and coating amount were changed.

Example 5
Polylactic acid resin (L-lactic acid / D-lactic acid = 96.0 / 4.0 (molar ratio), melting point 150 ° C., number average molecular weight 105,000, manufactured by Cargill Dow Polymer Co., Ltd.) and nonionic systems shown in Table 1 A sequential biaxially stretched film was obtained in the same manner as in Example 1 except that the surfactant was used and the mass ratio and the coating amount were changed.

Comparative Example 1
A sequential biaxially stretched film was obtained in the same manner as in Example 1 without providing a coating layer composed of a nonionic surfactant and an aqueous polyurethane resin and / or an aqueous polyester resin.

Comparative Example 2
Biaxial stretching in the same manner as in Example 1 except that anionic surfactant (A-4) (alkyl sulfonic acid sodium salt, Chemistat 3033N manufactured by Sanyo Chemical Industries, Ltd.) was used instead of the nonionic surfactant. A film was obtained.

Comparative Examples 3-4
Using the nonionic surfactant shown in Table 1, a biaxially stretched film was obtained in the same manner as in Example 1 except that the mass ratio was changed.

Comparative Example 5
Using polyethylene oxide (B-3) (manufactured by Meisei Chemical Co., Ltd., Alcox R-150, molecular weight 150,000) and the nonionic surfactant shown in Table 1 as the aqueous resin, the mass ratio and the coating amount are changed. Except for this, a biaxially stretched film was obtained in the same manner as in Example 1.

  The polylactic acid biaxially stretched film of the present invention represented by the examples is excellent in long-term antifogging property for about one week, and at the same time, is excellent in blocking resistance and substrate adhesion.

  On the other hand, the polylactic acid type biaxially stretched film that does not satisfy the present invention represented by the comparative example has insufficient antifogging property, or causes poor substrate adhesion and deterioration of blocking properties between films.

Claims (4)

A polylactic acid biaxially stretched film having a coating layer comprising a nonionic surfactant (A) and an aqueous polyurethane resin and / or an aqueous polyester resin (B) on at least one side, wherein the coating layer is a nonionic system The anti-fogging polylactic acid type two characterized in that the mass ratio (A / B) of the surfactant (A) to the aqueous polyurethane resin and / or the aqueous polyester resin (B) is 90/10 to 10/90. Axial stretched film. The nonionic surfactant (A) is at least one nonionic surfactant selected from polyoxyethylene sorbitan fatty acid ester, polyglycerin fatty acid ester, and sucrose fatty acid ester. The antifogging polylactic acid biaxially stretched film as described. An aqueous solution or aqueous dispersion comprising a nonionic surfactant (A) and an aqueous polyurethane resin and / or aqueous polyester resin (B) is applied to at least one surface of an unstretched polylactic acid film, and then biaxially. The method for producing an antifogging polylactic acid biaxially stretched film according to claim 1, wherein the film is heat-fixed and heat-set. Applying an aqueous solution or aqueous dispersion comprising a nonionic surfactant (A) and an aqueous polyurethane resin and / or an aqueous polyester resin (B) to at least one surface of a polylactic acid film stretched only in a uniaxial direction, 3. The method for producing an antifogging polylactic acid biaxially stretched film according to claim 1 or 2, wherein the film is stretched in a direction perpendicular to the first stretching direction and thermally fixed.