Manufacturing method for processed molded products

Abstract

To provide a method for producing a processed molded article containing polylactic acid and having excellent transparency and heat resistance.SOLUTION: A method for producing a processed molded article includes the steps for: preparing a molded article that contains polylactic acid containing an L-lactic acid unit in an amount of 99.0 mol% or more based on the total constituent units of the polylactic acid or a D-lactic acid unit in an amount of 99.0 mol% or more based on the total constituent units of the polylactic acid; and impregnating the molded article with carbon dioxide.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a processed molded body and a method for manufacturing the same.

Background Art

[0002] Polylactic acid is derived from biomass and has biodegradability, so it is attracting great attention as a green material with a low environmental load. However, amorphous polylactic acid has high transparency but has the problem of low heat resistance. On the other hand, crystalline polylactic acid has high heat resistance but has the problem of low transparency. That is, it is difficult to obtain polylactic acid having both heat resistance and transparency, and its applications have been limited.

[0003] Therefore, various studies have been conducted to improve the heat resistance and transparency of polylactic acid. For example, Patent Document 1 shows that a hydroxy group-containing fatty acid amide and a plasticizer having a specific structure are added to polylactic acid. Patent Document 2 shows that talc and a specific plasticizer are added to polylactic acid. Further, Patent Document 3 shows that a crystal nucleating agent containing basic zinc cyanurate and zinc phenylphosphonate is added to polylactic acid. Non-Patent Document 1 describes impregnating polylactic acid with carbon dioxide.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0005]

Non-Patent Document 1

[0006] However, as described in Patent Documents 1-3 above, molded polylactic acid articles using crystal nucleating agents exhibit reduced transparency as their thickness increases, thus requiring further improvement. Furthermore, the technology described in Non-Patent Document 1 had the problem of producing molded articles with significant haze.

[0007] The object of the present invention is to provide a processed molded article containing polylactic acid, having good transparency and good heat resistance, and a method for producing the same. [Means for solving the problem]

[0008] The present invention provides a method for manufacturing the following processed molded articles. A method for producing a processed molded article, comprising the steps of: preparing a molded article containing polylactic acid in which the amount of L-lactic acid units is 99.0 mol% or more of the total constituent units, or the amount of D-lactic acid units is 99.0 mol% or more of the total constituent units; and impregnating the molded article with carbon dioxide.

[0009] The present invention provides the following processed molded articles. A processed molded article containing polylactic acid in which the amount of L-lactic acid units is 99.0 mol% or more of the total constituent units, or the amount of D-lactic acid units is 99.0 mol% or more of the total constituent units, wherein the crystal size of the polylactic acid is 40 nm or less, and the amount of crystal nucleating agent is 0.5 mass% or less. [Effects of the Invention]

[0010] According to the method for producing a processed molded article of the present invention, a processed molded article containing polylactic acid, having good transparency, and good heat resistance can be obtained. [Brief explanation of the drawing]

[0011] [Figure 1] Figures 1A and 1B are schematic cross-sectional views illustrating the manufacturing method of a processed molded product. [Figure 2] Figure 2A is a photograph of the processed molded product made in Example 1, and Figure 2B is a photograph of the processed molded product made in Comparative Example 1. [Modes for carrying out the invention]

[0012] The method for manufacturing a processed molded article and the processed molded article itself will be described in detail below based on specific embodiments. However, the method for manufacturing a processed molded article and the processed molded article are not limited to the embodiments described below.

[0013] 1. Method for manufacturing a processed molded product First, a method for manufacturing a processed molded article according to one embodiment of the present invention will be described. The method for producing the processed molded article of this embodiment includes a step of preparing a molded article containing polylactic acid in which the amount of L-lactic acid units is 99.0 mol% or more of the total constituent units, or the amount of D-lactic acid units is 99.0 mol% or more of the total constituent units (molded article preparation step), and a step of impregnating the molded article with carbon dioxide (carbon dioxide impregnation step).

[0014] As described above, the transparency and heat resistance of polylactic acid are in a trade-off relationship, and conventionally, polylactic acid that sufficiently combines both has not been obtained. In response to this, the present inventors diligently investigated and found that by impregnating a molded body containing polylactic acid of a predetermined composition with carbon dioxide, a processed molded body with high transparency and heat resistance can be obtained. The reason why a processed molded body with good transparency and heat resistance can be obtained with the manufacturing method of this embodiment is thought to be as follows.

[0015] First, in order to confirm the relationship between the crystal size and the light scattering property, the inventors prepared a plurality of aqueous solutions in which silica particles having different particle sizes were dispersed, and irradiated visible light thereto. As a result, Mie scattering occurred when the diameter of the silica particles was about 100 nm, but when the diameter of the silica particles reached about 40 nm, it became a state close to Rayleigh scattering. From this result, it is considered that if the diameter of the poly(lactic acid) crystal can be reduced to about 40 nm, its transparency will be increased. Here, when crystallizing poly(lactic acid), it is common to use a crystallization nucleating agent. However, when using a general crystallization nucleating agent to make the diameter of the poly(lactic acid) crystal about 40 nm, it is necessary to make the size of the crystallization nucleating agent several nanometers and disperse them uniformly in the poly(lactic acid). However, such dispersion is very difficult. Further, by heating the poly(lactic acid) containing the crystallization nucleating agent above the glass transition temperature (Tg), crystals can be grown, but the normal heating temperature is 80 to 120 °C, the molecular chains are easy to move, and the crystal size tends to become coarse.

[0016] Therefore, in the present embodiment, the amount of L-lactic acid units or D-lactic acid units in the poly(lactic acid) is set to 99.0 mol% or more. Thereby, the crystallinity of the poly(lactic acid) (molded body) is enhanced, and crystal nuclei are likely to be generated in the poly(lactic acid) by homogeneous nucleation without using a crystallization nucleating agent. Then, in the present embodiment, a molded body containing such poly(lactic acid) is impregnated with carbon dioxide. Thereby, the poly(lactic acid) is plasticized, and the glass transition temperature of the poly(lactic acid) temporarily decreases. As a result, crystallization becomes possible even in a low temperature range where it is originally impossible to achieve both nucleation and crystal growth. At this time, due to the low temperature, the movement of the molecular chains is restricted, the coarsening of the crystal size is suppressed, and a large number of fine crystals (with a diameter of 40 nm or less) are formed. Therefore, it is considered that the transparency of the obtained processed molded body becomes very high. Further, it is considered that sufficient heat resistance can also be obtained by the crystallization of the poly(lactic acid). Hereinafter, each step of the manufacturing method of the processed molded body of the present embodiment will be described.

[0017] (Molded body preparation step) In the molded body preparation step, a molded body containing polylactic acid in which the amount of L-lactic acid units is 99.0 mol% or more, or the amount of D-lactic acid units is 99.0 mol% or more is prepared. The molded body may contain components other than polylactic acid as long as the object and effects of the present embodiment are not impaired. However, the amount of polylactic acid in the molded body is preferably 80% by mass or more, more preferably 90% by mass or more. Further, the amount of the crystal nucleating agent in the molded body is preferably 0.5% by mass or less, and more preferably substantially not contained. When the amount of the crystal nucleating agent is 0.5% by mass or less, the crystal size of polylactic acid is likely to be within the desired range (40 nm or less).

[0018] Here, the polylactic acid may have a structure in which lactic acid is polymerized by an ester bond. Lactic acid has an asymmetric carbon and optical isomers exist. Specifically, L-lactic acid (hereinafter also referred to as "L-form") and D-lactic acid (hereinafter also referred to as "D-form") exist. In general polylactic acid, the L-form and the D-form often coexist. However, in the present embodiment, the amount of either the L-form or the D-form in the polylactic acid is 99.0 mol% or more with respect to all the constituent units of the polylactic acid. The amount of the L-form or the D-form in the polylactic acid is preferably 99.4 mol% or more, more preferably 99.6 mol% or more with respect to all the constituent units of the polylactic acid. When the amount of either the L-form or the D-form in the polylactic acid is 99.0 mol% or more with respect to all the constituent units of the polylactic acid, crystal nuclei are likely to occur as described above.

[0019] The amounts of the L-form and the D-form in the polylactic acid can be adjusted by the raw materials used when synthesizing the polylactic acid. Polylactic acid can be obtained, for example, by ring-opening polymerization of lactide of lactic acid. By adjusting the ratio of L-lactide and D-lactide, the amounts of the L-form and the D-form in the polylactic acid can be adjusted. The ring-opening polymerization of lactide can be carried out by a known method, and it is only necessary to mix a catalyst or a polymerization initiator and lactide and carry out polymerization.

[0020] The weight-average molecular weight of the polylactic acid contained in the molded article is preferably 50,000 to 300,000, and more preferably 70,000 to 250,000. When the weight-average molecular weight of the polylactic acid is 50,000 or higher, the mechanical strength of the resulting processed molded article tends to be higher. On the other hand, when the weight-average molecular weight is 300,000 or lower, it is easier to mold into the desired shape. The weight-average molecular weight of the polylactic acid is measured by gel permeation chromatography and is expressed as a styrene equivalent value.

[0021] The shape of the molded body prepared in this process is not particularly limited and can be appropriately selected depending on the intended use of the processed molded body. For example, it may be in the form of a film, sheet, plate, or flat plate. On the other hand, as long as it is a shape that can be impregnated with carbon dioxide in the impregnation process described later, it may also be a three-dimensional shape. Furthermore, in conventional molded bodies obtained from polylactic acid, increasing the thickness tends to reduce transparency, but with the manufacturing method of the processed molded body of this embodiment, sufficient transparency can be achieved even if the thickness of the molded body (processed molded body) is 500 μm or more.

[0022] The molded product prepared in this process may be a commercially available product as long as the amount of L-isomer or D-isomer in the polylactic acid is within the above range. However, as mentioned above, general polylactic acid often contains less than 99.0 mol% of both L-isomer and D-isomer. Therefore, pellets of polylactic acid with the desired composition may be prepared and molded.

[0023] The method for molding polylactic acid is not particularly limited. For example, as shown in Figure 1A, a pellet 1 of the desired polylactic acid may be prepared and molded using a mold 10 or the like. Alternatively, it may be molded by injection molding, blow molding, extrusion molding, etc. The polylactic acid may be molded into a sheet, and then further subjected to vacuum forming, pressure forming, vacuum pressure forming, etc.

[0024] (Carbon dioxide impregnation process) In the carbon dioxide impregnation process, carbon dioxide is impregnated into the molded body. Specifically, as shown in Figure 1B, the molded body 2 is placed in a pressure vessel 20. Then, gaseous carbon dioxide is supplied into the pressure vessel 20 from a fluid supply device (not shown) connected to the pressure vessel 20. Although liquid or supercritical carbon dioxide may be supplied into the pressure vessel 20, it is preferable to fill the pressure vessel 20 with gaseous carbon dioxide, compress the gas, and convert it into high-pressure gaseous carbon dioxide. Gaseous carbon dioxide is easier to handle than liquid or supercritical carbon dioxide.

[0025] By filling the pressure vessel 20 with carbon dioxide and applying pressure, the molded body 2 is impregnated with carbon dioxide. As a result, the polylactic acid is plasticized, and the glass transition temperature decreases as described above. There is no particular limit to the time for impregnating the molded body 2 with carbon dioxide, but 30 minutes or more is preferred, and 45 minutes or more is more preferred. An impregnation time of 30 minutes or more sufficiently lowers the glass transition temperature of the polylactic acid. On the other hand, there is no particular upper limit to the impregnation time, but from the viewpoint of the manufacturing efficiency of the processed molded body, an impregnation time of 5 hours or less is preferred.

[0026] Furthermore, while there are no particular restrictions on the temperature during carbon dioxide impregnation, it is preferable that it be above the melting point of carbon dioxide and below room temperature. Similarly, there are no particular restrictions on the pressure during carbon dioxide impregnation; it should be above atmospheric pressure. On the other hand, there are no particular upper limits, but the pressure should be appropriately selected depending on the performance of the pressure vessel.

[0027] After a certain period of time, the molded body 2 is removed from the pressure vessel 20, and the carbon dioxide in the polylactic acid is removed to obtain a processed molded body. The removal of carbon dioxide can be performed, for example, by leaving the molded body 2 removed from the pressure vessel 20 at room temperature, but annealing may be performed as needed. The temperature during annealing is preferably between 60°C and 140°C. The annealing time is preferably between 10 and 120 minutes. By performing annealing, carbon dioxide can be removed efficiently.

[0028] 2. Processed molded body A processed molded article according to one embodiment of the present invention will be described. The processed molded article contains polylactic acid in which the amount of L-lactic acid units is 99.0 mol% or more of the total constituent units, or the amount of D-lactic acid units is 99.0 mol% or more of the total constituent units. The processed molded article may contain components other than polylactic acid to the extent that it does not impair the purpose and effect of this embodiment, but it is preferable that the amount of polylactic acid in the processed molded article is 90% by mass or more, and it is more preferable that it is substantially all polylactic acid.

[0029] Furthermore, the amount of nucleating agent in the processed molded product should be 0.5% by mass or less, and it is more preferable that it be substantially absent. By limiting the amount of nucleating agent to 0.5% by mass or less, the crystal size of polylactic acid can be reduced to 40 nm or less.

[0030] Furthermore, the crystal size of polylactic acid in the processed molded article should be 40 nm or less, and preferably 30 nm or less. The crystal size of polylactic acid can be determined by transmission electron microscopy or wide-angle X-ray diffraction. When the crystal size of polylactic acid in the processed molded article is 40 nm or less, it becomes sufficiently smaller than the wavelength of visible light, so Mie scattering is less likely to occur, and the transmittance of visible light is greatly increased.

[0031] Furthermore, the degree of crystallinity of polylactic acid as determined by differential scanning calorimetry is preferably 30-60%, and more preferably 40-60%. When the degree of crystallinity of polylactic acid is 30% or higher, its heat resistance tends to increase. On the other hand, the upper limit of the degree of crystallinity of polylactic acid is usually around 60%.

[0032] The shape of the processed molded body is not particularly limited and can be appropriately selected according to its application. Furthermore, the processed molded body may be obtained by further processing a processed molded body manufactured using the method described above for manufacturing processed molded bodies.

[0033] Furthermore, it is preferable that the transmittance of the processed molded article to light at a wavelength of 400 nm is 80% or higher. Generally, the shorter the wavelength of light, the more likely Mie scattering is to occur, and the lower the transmittance tends to be. Therefore, if the transmittance to light at a wavelength of 400 nm is sufficiently high, it can be said that Mie scattering is less likely to occur across the entire visible light spectrum. And, if the transmittance to light at a wavelength of 400 nm is 80% or higher, the processed molded article can be used for a variety of applications.

[0034] Furthermore, there are no particular restrictions on the use of the processed molded product. The processed molded product can be applied to a wide variety of uses, such as applications requiring transparency, such as disposable products for optical inspection, various containers, and packaging materials. [Examples]

[0035] The present invention will be described below using examples and comparative examples, but the present invention is not limited in any way to the following examples.

[0036] (1) Preparation of polylactic acid Pelletized polylactic acid was prepared for use in the following examples and comparative examples. L-1: Total Corbion Grade L130 L-2: Polylactic acid prepared in Synthesis Example 1 below L-3: NatureWorks 3001D L-4: NatureWorks 3052D D-1: Polylactic acid prepared in Synthesis Example 2 below

[0037] (Synthesis Example 1) 99.0 parts by mass of L-lactide, 1.0 part by mass of D-lactide, a catalyst (tin octylate), and a polymerization initiator (lauryl alcohol) were placed in a reaction vessel equipped with a stirrer. The lactides (L-lactide and D-lactide) were melted at 110°C and stirred for 30 minutes. The temperature was then raised to 160°C, stirring was stopped, and the temperature was lowered to 130°C. After that, the pressure was reduced and the unreacted lactides (L-lactide and D-lactide) were removed by distillation. The resulting mixture was dissolved in chloroform and reprecipitated in methanol to obtain a purified product. IR spectroscopy (infrared absorption spectroscopy) was performed at a wavenumber of 3400 cm⁻¹. -1 After confirming the disappearance of the absorption peaks of the hydroxyl groups before and after the reaction, polylactic acid (L-1) was obtained. The amount of L-lactic acid units in the obtained polylactic acid was 99.0 mol%, and the amount of D-lactic acid units was 1.0 mol%.

[0038] (Synthesis Example 2) Polylactic acid (D-2) was obtained in the same manner as in Synthesis Example 1, except that the ratio of L-lactide to D-lactide was 0.6 parts by mass:99.4 parts by mass. The amount of L-lactic acid units in the polylactic acid obtained by this method was 0.6 mol%, and the amount of D-lactic acid units was 99.4 mol%.

[0039] (2) Forming of polylactic acid and impregnation with carbon dioxide As shown in Figure 1A, each polylactic acid pellet 1 was processed into a circular molded body 2 with a diameter of φ30 mm and a thickness of 1 mm using a mold 10 at a temperature of 200°C and a pressure of 5.0 MPa.

[0040] Next, the molded body 2 was placed inside the pressure vessel 20 as shown in Figure 1B. The pressure vessel was then filled with carbon dioxide, the temperature inside the vessel was set to 0°C, and the pressure was set to 3 MPa. This allowed the carbon dioxide to impregnate the molded body 2. The impregnation time was 2 hours. After that, the molded body was removed from the pressure vessel and annealed at 100°C for 60 minutes under atmospheric pressure.

[0041] (3) Evaluation • Measurement of crystallinity The degree of crystallinity of the processed circular press sheet (processed molded body) was determined by differential scanning calorimetry (DSC). A sample of 5-8 mg was measured from the circular press sheet, placed in an aluminum pan, and loaded into the DSC measurement section. The sample was then heated to 220°C at a rate of 10°C / min. The crystallinity enthalpy (ΔHc) and fusion enthalpy (ΔHm) were measured, and (ΔHm - ΔHc) × 100% was defined as the degree of crystallinity. The results are shown in Table 1.

[0042] • Measurement of crystal size A circular press sheet was cut using the ultrathin sectioning method in two directions: (a) parallel to the radial direction and perpendicular to the sheet surface (axial direction), and (b) parallel to the radial direction and perpendicular to the circumferential direction. Observation samples were prepared, stained with ruthenium tetroxide, and the cut surfaces were photographed at 120,000x magnification using a transmission electron microscope. If crystals were not present as island components, the crystal size was determined using the Scherrer formula by wide-angle X-ray diffraction.

[0043] On the other hand, when crystals existed as island components, the obtained photographs were loaded into image processing software, 10 island components were randomly selected, and image processing was performed to calculate the size of the island components as shown below. The major axis (la) and minor axis (lb) of the island components appearing in the cross-section of (a) were determined, and the major axis (lc) and minor axis (ld) of the island components appearing in the cross-section of (b) were determined. Furthermore, the shape coefficients I = (average value of la + average value of lb) / 2 and J = (average value of lc + average value of ld) / 2 were used for the island components, and the crystal size was set to (I + J) / 2. The results are shown in Table 1.

[0044] • Measurement of heat resistance temperature The heat resistance temperature of the processed circular press sheet (processed molded body) was determined by dynamic viscoelasticity measurement. Specifically, a rectangle measuring 14 mm in length, 3 mm in width, and 1 mm in thickness was cut from the circular press sheet and fixed to the measurement section of a dynamic viscoelasticity measuring device. The temperature at which the storage modulus decreased to 180 MPa at a heating rate of 10°C / min was defined as the heat resistance temperature. The results are shown in Table 1.

[0045] • Transmittance and scattering rate of light with a wavelength of 400 nm The transmittance and scattering rates of light at a wavelength of 400 nm were measured using an ultraviolet-visible spectrophotometer equipped with an integrating sphere. The scattering rate was calculated as (light flux of scattered components / total light flux transmitted) × 100%. The results are shown in Table 1. Photographs of the processed circular press sheets (processed molded products) from Example 1 and Comparative Example 1 are shown in Figures 2A and 2B.

[0046] [Table 1]

[0047] As shown in Table 1 above, when the amount of L-lactic acid units relative to the total constituent units of polylactic acid was 99.0 mol% or more (Examples 1 and 2), or when the amount of D-lactic acid units relative to the total constituent units of polylactic acid was 99.0 mol% or more (Example 3), the degree of crystallinity exceeded 40%, and the heat resistance temperature was high. Furthermore, the transmittance of light at a wavelength of 400 nm was high, and the scattering rate of this light was low, indicating that both high heat resistance and high transparency were achieved. As shown in Table 1, this is thought to be due to the crystal size being 40 nm or less and the degree of crystallinity being high.

[0048] On the other hand, when the amounts of both L-lactic acid units and D-lactic acid units in polylactic acid were less than 99.0 mol% (Comparative Examples 1 and 2), even when molded articles were produced or impregnated with carbon dioxide under similar conditions, the degree of crystallinity was low and the crystal size did not become sufficiently small. As a result, the heat resistance temperature was low, and the transmittance of light at a wavelength of 400 nm was also low. [Industrial applicability]

[0049] The method for manufacturing a processed molded article according to this embodiment is useful for producing processed molded articles that can be applied to a wide variety of uses, such as disposable products for optical inspection, various containers, and packaging materials. [Explanation of Symbols]

[0050] 1 pellet 2 Molded body 10 molds 20 Pressure vessels

Claims

1. A step of preparing a molded article containing polylactic acid in which the amount of L-lactic acid units is 99.0 mol% or more of the total constituent units, or the amount of D-lactic acid units is 99.0 mol% or more of the total constituent units, and which is in the form of a film, sheet, plate, or flat plate, A step of impregnating the molded body with carbon dioxide, The process of impregnating the molded body with carbon dioxide is followed by the process of annealing the molded body at a temperature of 60°C to 140°C. It has, To obtain a processed molded article having a transmittance of 80% or more for light with a wavelength of 400 nm. A method for manufacturing a processed molded product.

2. In the process of impregnating the molded body with carbon dioxide, the impregnation time is 30 minutes or more. A method for manufacturing a processed molded article according to claim 1.

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