JPH0931216A - Poly(lactic acid) sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prepare a poly(lactic acid) sheet excellent in moldability. SOLUTION: A poly(lactic acid) polymer with a weight-average molecular weight ranging from 60,000 to 700,000 is used to produce a poly(lactic acid) sheet having the relation between the heat of crystallization ΔTc and the heat of melting of the crystal ΔHm satisfying (ΔHm-ΔHc)/ΔHm <=0.7, when the sheet is heated to increase its temperature. The poly(lactic acid) polymer with a weight-average molecular weight ranging from 60,000 to 700,000 is used to also give a poly(lactic acid) sheet having no crystallization, point Tc and the melting point Tm.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention is excellent in molding processability,
The present invention relates to a polylactic acid-based sheet having natural degradability.

[0002]

2. Description of the Related Art Blister processed products, food cups, trays, etc. used for display and packaging of various products are
After the resin sheet is obtained, the sheet is generally formed by a thermoforming method such as vacuum forming or pressure forming. Further, in particular, a blister processed product, which has an excellent transparency so that the product inside can be seen through the package, is preferred. Therefore, sheets of polyvinyl chloride, polyethylene terephthalate, polystyrene or the like are used as the resin sheet.

However, the above-mentioned sheet is chemically,
Since it is biologically stable, it remains and accumulates with almost no decomposition even when left in a natural environment. These not only scatter in the natural environment and contaminate the living environment of plants and animals, but also when they are buried as garbage, they remain with almost no decomposition and shorten the life of landfill sites.

Therefore, there is a demand for a material composed of a decomposable polymer that does not cause these problems, and many researches and developments have been conducted. Polylactic acid is known as one of them,
It is considered to be used for various purposes.

[0005]

However, a sheet made of polylactic acid generally has brittleness, and if a blister-processed product or a thin film as described above is to be obtained, it will be cracked during molding. Alternatively, it causes breakage or the like. Therefore, many studies are required to replace the conventional plastic products.

[0006] Therefore, an object of the present invention is to provide a polylactic acid type sheet excellent in molding processability.

[0007]

The gist of the present invention is a sheet made of a polylactic acid-based polymer having a weight average molecular weight of 60,000 or more and 700,000 or less, and crystals when the sheet is heated. A different polylactic acid-based sheet is characterized in that the relationship (ΔHm-ΔHc) / ΔHm between the heat of formation ΔHc and the heat of fusion of crystal ΔHm is 0.7 or less. The gist of the present invention is that the weight average molecular weight is 60,000. As described above, the sheet is made of a polylactic acid-based polymer having 700,000 or less, and the sheet has no crystallization temperature Tc and melting temperature Tm.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The polylactic acid-based polymer used in the present invention is polylactic acid or a copolymer of lactic acid and other hydroxycarboxylic acid, or a mixture thereof, and does not impair the effects of the present invention. Other polymer materials may be mixed within the range. Further, for the purpose of adjusting the molding processability, the physical properties of the sheet or the molded product, a plasticizer, a lubricant, an inorganic filler,
It is also possible to add additives such as ultraviolet absorbers and modifiers.

Examples of lactic acid include L-lactic acid and D-lactic acid, and other hydroxycarboxylic acids include glycolic acid and
Typical examples include 3-hydroxybutyric acid, 4-hydroxybutyric acid, 3-hydroxyvaleric acid, 4-hydroxyvaleric acid, 6-hydroxycaproic acid and the like. As these polymerization methods, it is possible to employ any known method such as condensation polymerization method and ring-opening polymerization method, and further, a small amount of a chain extender for the purpose of increasing the molecular weight, for example, a diisocyanate compound, an epoxy. A compound, an acid anhydride, etc. may be used.

The polylactic acid polymer used in the present invention has a weight average molecular weight of 60,000 or more and 700,000 or less. If the weight average molecular weight is less than 60,000, the molten polymer taken during the sheet forming will flow before solidifying, and it will be difficult to obtain a sheet having a uniform film thickness. Also, the resulting sheet is brittle,
Breaking and cracking easily occur with slight stress and deformation.

On the other hand, when the weight average molecular weight exceeds 700,000, the screw rotation speed is not constant due to high load during sheet molding, and the pressure of the molten polymer (resin pressure) inside the die becomes high, and extrusion by melt fracture or the like occurs. Leads to defects. Further, high shear is applied to the molten polymer at the outlet of the die, which adversely affects the appearance of the sheet.
Although the melt viscosity of the resin can be lowered by increasing the extrusion temperature, polylactic acid is not a preferable method because it is easily decomposed by heat.

As described above, the weight average molecular weight of the polylactic acid polymer used in the present invention is 60,000 or more and 700,000 or less in view of the melt viscosity, the extrusion stability, the physical properties of the sheet and the like.

After removing the water content of the polylactic acid-based polymer, melt extrusion is performed to form a sheet. The preferable melting temperature is preferably selected appropriately depending on the composition or molecular weight of the polylactic acid polymer, but usually 140 ° C. to 25 ° C.
It is in the range of 0 ° C.

The polylactic acid-based polymer melt-formed into a sheet is preferably brought into contact with a rotating casting drum (cooling drum) to be rapidly cooled. When the temperature of the casting drum is high, the polymer sticks to the casting drum and cannot be removed. Furthermore, crystallization is promoted and spherulites develop, and as described later, thermoforming and stretching cannot be performed.

The polylactic acid-based sheet of the present invention has a heat of crystallization ΔHc and a heat of fusion of crystal ΔHm when the sheet is heated.
The relationship (ΔHm−ΔHc) / ΔHm is 0.7 or less. (ΔHm-ΔHc) / ΔHm is an index showing the crystallinity of the sheet, and a sheet having the above value of more than 0.7 is liable to cause an appearance defect such as whitening and thickness deviation even if thermoforming or stretching is performed. .

Heat of fusion of crystal ΔHm, heat of crystallization ΔHc
Is obtained by differential scanning calorimetry (DSC) of the sheet sample, and the heat of crystal fusion ΔHm is the heating rate 1
It is the amount of heat required to melt all the crystals when the temperature is raised at 0 ° C./min, and is calculated from the area of the endothermic peak due to crystal melting that appears near the crystal melting point of the polymer. The heat of crystallization ΔHc can be obtained from the area of the exothermic peak generated during crystallization that occurs during the temperature rising process.

The heat of crystal fusion ΔHm mainly depends on the crystallinity of the polymer itself, and takes a large value in a polymer having a large crystallinity. By the way, the complete homopolymer of L-lactic acid or D-lactic acid has 60 J / g or more, and in the copolymer of these two kinds of lactic acid, the heat of crystal fusion ΔHm changes depending on the composition ratio.

The heat of crystallization ΔHc is an index relating to the crystallinity of the sheet at that time with respect to the crystallinity of the polymer,
When the amount of heat of crystallization ΔHc is large, crystallization of the sheet proceeds during the temperature rising process. That is, it means that the crystallinity of the sheet was relatively low based on the crystallinity of the polymer. On the contrary, when the heat of crystallization ΔHc is small, it means that the crystallinity of the sheet was relatively high based on the crystallinity of the polymer.

In order to reduce (ΔHm-ΔHc) / ΔHm, it is necessary to use a polymer having low crystallinity. L
In the copolymerization of polylactic acid composed of -lactic acid and D-lactic acid, the crystallinity differs depending on the composition ratio of both. A polymer that does not crystallize can be obtained depending on the composition ratio.

The different polylactic acid-based sheets of the present invention do not have a crystallization temperature Tc and a melting temperature Tm. That is,
The polylactic acid-based sheet of the present invention does not crystallize. The non-crystallized polylactic acid-based sheet can be thermoformed or stretched into a desired shape.

A polylactic acid-based sheet that does not crystallize can be obtained by bringing the composition ratio of L-lactic acid and D-lactic acid close to each other. L-lactic acid and D- which constitute a polylactic acid-based polymer
The composition ratio of L-lactic acid to D-lactic acid is usually 90:10 to 10:90, and more preferably 80:20 to 20:80, though it depends on the order of arrangement with lactic acid. It can be obtained by using coalescence.

[0022]

EXAMPLES Examples will be shown below, but the present invention is not limited thereto. The measured values shown in the examples were measured and calculated under the following conditions.

(1) Tg, Tc, Tm, (ΔHm-ΔHc) / ΔHm A differential scanning calorimeter DSC-7 (manufactured by Perkin Elmer Co., Ltd.) was used to prepare a film sample (10 mg) according to JIS-K7122.
Based on the temperature rise rate of 10 ° C / min
From the mologram, the glass transition temperature Tg, the crystallization temperature Tc, the melting temperature Tm, and the heat of crystal melting ΔHm and the heat of crystallization ΔH.
c was calculated and (ΔHm−ΔHc) / ΔHm was calculated.

(2) Weight average molecular weight Mw The molecular weight was compared with that of standard polystyrene using gel permeation chromatography HLC-8120GPC (manufactured by Tosoh Corporation) under the following measurement conditions.

Chromatographic column: Shim-Pack series (manufactured by Shimadzu Corporation) Solvent: Chloroform Sample solution concentration: 0.2 wt / vol% Sample solution injection amount: 200 μl Solvent flow rate: 1.0 ml / min Pump column Detector temperature: 40 ° C. (3) Extrudability A 30 mmφ single-screw small extruder was used to perform melt extrusion from the T die. The manufacturing conditions are as follows. The extrusion temperature was changed according to the molecular weight.

T-die Lip width: 200 mm Lip gap: 0.6 mm Screw full flight L / D: 25 (4) Folding resistance A sheet is cut into a strip shape with a length of 30 mm and a width of 10 mm, and the length of the strip. Both ends in the direction were bent and the folding condition of the sheet was examined. For one type of sheet, the number of samples was n = 10. The case where all the pieces did not break was indicated as ⊚, the case where 1 to 2 pieces failed to break was marked as ○, the case where 3 to 4 pieces failed to break was marked as Δ, and the case where 5 or more pieces were broken was marked as ×. The practical level is ◯ or higher.

(5) Moldability A PLAVAC-FE36PH type (a thermoforming machine manufactured by Sanwa Kogyo Co., Ltd.) was fitted with a polylactic acid sheet cut into 150 mm × 150 mm, and a molding temperature (60 ° C. to
It was preheated to within 90 ° C). Then, the moldability was evaluated by lifting three types of dies 1 to 3 described below from below the sheet, forming a vacuum in the dies, and forming various blister.

The evaluation is indicated by ⊚ when molding is extremely easy, by ◯ when molding is possible, by Δ when molding is possible but the appearance is bad such as whitening or thickness deviation, and when almost molding is not possible. The practical level is ◯ or higher.

FIGS. 1 to 3 show a sectional view (A) and a bottom view (B) of the used dies 1 to 3, respectively. Mold 1 shown in FIG.
Is substantially dish-shaped, the mold 2 shown in FIG. 2 is a relatively shallow cup having a square bottom surface, and the mold 3 shown in FIG. 3 is a cup having a deep bottom surface. Only when using the mold of Figure 3,
The sheet was pressed by using a plug having substantially the same shape as the die to push the sheet up to 3 mm from the bottom of the die.

(Example 1) Polylactic acid having a composition ratio of L-lactic acid and D-lactic acid of about 98: 2 and a weight average molecular weight of 73,000 was used while sending dry air to remove water.
After drying at 0 ° C for 3 hours, it was extruded from a T-die at 190 ° C with a 30 mmφ single-screw extruder and rapidly cooled with a casting roll (roll temperature 56 ° C) to a thickness of about 20.
A 0 μm transparent sheet was obtained. Extrudability was good.

(Examples 2 and 3) Weight average molecular weights 180,000 and 620,0 having the same composition as in Example 1
00 polylactic acid was extruded from the T-die at 210 ° C. and 230 ° C., respectively. Casting at 56 ℃,
A transparent sheet of about 200 μm was obtained. Extrudability was good.

Example 4 Polylactic acid having a ratio of L-lactic acid to D-lactic acid of 80:20 and a weight average molecular weight of 140,000 was extruded at 200 ° C. in the same manner as in Example 1 to obtain 54 ° C.
And a transparent sheet of about 200 μm was obtained. Extrudability was good.

(Comparative Example 1) Polylactic acid having the same composition as in Example 1 and having a weight average molecular weight of 40,000 was extruded from a T die at 190 ° C and taken out by a casting roll at 56 ° C. Was too low, the pick-up was not stable, and the sheet width fluctuated. The obtained sheet was transparent.

(Comparative Example 2) A weight average molecular weight of 750,000 polylactic acid having the same composition as in Example 1 was added to 230-2.
Although it was attempted to extrude at 40 ° C. and cast at 56 ° C., the melt viscosity was high, the discharge amount was not stable, and a sheet with uniform width and thickness could not be obtained. In order to further reduce the melt viscosity, I tried to extrude it at 260 ° C or higher,
Foaming occurred in the sheet due to thermal decomposition, unevenness was observed on the surface, and the appearance of the sheet was not good.

The composition ratio and weight average molecular weight of L-lactic acid and D-lactic acid of the polylactic acid used in Examples 1 to 4 and Comparative Examples 1 and 2, sheet extrudability, Tg and Tc of the obtained sheets,
Tm and (ΔHm-ΔHc) / ΔHm were measured to evaluate folding resistance and formability. The overall evaluation was performed by combining the results of folding endurance and formability. Particularly excellent is indicated by ⊚, above practical level is indicated by ◯, below practical level is indicated by Δ, and particularly inferior is indicated by ×. The results are shown in Table 1.

[0036]

[Table 1] (Example 5) The sheet obtained in Example 2 was placed in a hot air circulator for 8 hours.
Heat treatment is performed by leaving it at 0 ° C for about 20 minutes,
The crystallinity (ΔHm-ΔHc) / ΔHm was increased. The sheet was slightly whitened.

Example 6 A sheet prepared in the same manner as in Example 2 except that the speed of the casting roll was changed to 400 μm, and the sheet was stretched at 70 ° C. in the lengthwise and widthwise directions 1.5 times each. To increase the crystallinity. The thickness of the obtained film is about 170 μm.

(Comparative Example 3) By leaving the sheet obtained in Example 2 in a hot air circulator at 110 ° C for about 15 minutes,
Heat treatment was applied to increase the crystallinity (ΔHm-ΔHc) / ΔHm. The sheet has turned white.

Comparative Example 4 The 400 μm sheet obtained in Example 6 was stretched at 70 ° C. by a factor of 2.5 in both length and width to increase the crystallinity. The thickness of the obtained film is about 6
0 μm.

The composition ratio of L-lactic acid and D-lactic acid of the polylactic acid used in Examples 5 and 6 and Comparative Examples 3 and 4, and Tg, Tc, Tm and (ΔHm-ΔHc) / ΔHm of the obtained sheets were calculated. Table 2 shows the measured folding resistance, moldability, and comprehensive evaluation.

[0041]

[Table 2] As is clear from Table 1, Examples 1 to 4, which are the polylactic acid-based sheets of the present invention, have good extrudability, and thus the width and thickness of the sheets are uniform. Further, it is a polylactic acid-based sheet which is excellent in folding resistance and moldability and is excellent in molding processability in general. In Example 4, the crystallization temperature Tc and the melting temperature T
It is a polylactic acid type sheet that does not have m and has a crystallinity (ΔH
m−ΔHc) / ΔHm is zero.

On the other hand, Comparative Example 1 in which the weight average molecular weight is out of the range of the present invention is a brittle sheet in which extrusion failure occurs and the resulting sheet has poor folding resistance. Moldability is relatively good, but it does not have sufficient practical strength as a sheet for moldability. Similarly, Comparative Example 2 having a weight average molecular weight outside the range of the present invention has uneven width and thickness, and cannot be used as a sheet for moldability.

In Example 5 shown in Table 2, a polylactic acid-based sheet is heat-treated, and in Example 6, it is stretched.
For this reason, the degree of crystallinity (ΔHm-ΔHc) / ΔHm is increased, which is not suitable for a deep cup such as the mold 3, but is excellent in other performances such as the molds 1 and 2. It is fully available for shallow cups.

On the other hand, in Comparative Examples 3 and 4, (ΔHm-ΔHc)
/ ΔHm exceeds 0.7, the moldability is poor, and it cannot be used as a sheet for moldability.

[0045]

As described above, since the polylactic acid-based sheet of the present invention is excellent in molding processability, the blister,
It enables the use of polylactic acid in various fields such as containers.

[Brief description of drawings]

FIG. 1 is a sectional view (A) and a bottom view (B) of a mold 1 used in an embodiment.

FIG. 2 is a sectional view (A) and a bottom view (B) of the mold 2 used in the embodiment.

FIG. 3 is a sectional view (A) and a bottom view (B) of a mold 3 used in the embodiment.

Claims (2)

[Claims] 1. A sheet made of a polylactic acid-based polymer having a weight average molecular weight of 60,000 or more and 700,000 or less, and
The relationship (ΔHm-ΔHc) / ΔHm between the heat of crystallization ΔHc and the heat of crystal melting ΔHm when the temperature of the sheet is raised is 0.
A polylactic acid-based sheet characterized by being 7 or less. 2. A sheet made of a polylactic acid-based polymer having a weight average molecular weight of 60,000 or more and 700,000 or less, and
The polylactic acid-based sheet is characterized in that the sheet does not have a crystallization temperature Tc and a melting temperature Tm.