JP2001114352A - Film case - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a film case usable in a microwave oven and processable into a compost. SOLUTION: A film case is composed of a polylactic acid polymer of the surface orientation degree ΔP of 3.0×10-3-30×10-3 and the difference (ΔHm-ΔHc) between the crystal melting calorie ΔHm when the temperature of a film is raised and the crystallization calorie ΔHc generated by the crystallization during the temperature rise is 20 J/g or more and the (ΔHm-ΔHc)/ΔHml is 0.7 or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a film case which can be used in a microwave oven and which can be composted.

2. Description of the Related Art Conventionally, an aluminum foil case has been used as a case for serving side dishes in a lunch box sold at a convenience store or a supermarket. After that, with the development of convenience stores and the like, it became frequent to heat lunches in microwave ovens on the spot, and metal aluminum foil cases were no longer used. In some cases, paper cups were temporarily replaced with paper cups.However, paper-made dishes also had the drawback that, because of their water resistance, side dishes containing a large amount of water could not be served. Plastic films made of polypropylene and polyethylene terephthalate having excellent properties have been developed. However, the sheets used to make the above-mentioned cases are chemically and biologically stable, so they remain and accumulate without being substantially decomposed even when left in a natural environment. Not only pollutes the living environment of the landfill but also remains almost undecomposed when buried as garbage, shortening the life of the landfill. Also,
In convenience stores, a system for composting unsold lunches has been established at each store, but composting cannot be performed for those made of polypropylene or polyethylene terephthalate. Therefore, various packaging materials for lunches are also required to be made of a material that can be composted, and much research and development has been conducted. One of them is known as polylactic acid. It is known that polylactic acid naturally undergoes hydrolysis in soil, does not remain in its original form in soil, and then becomes a harmless degradation product by microorganisms.

However, due to the inherent brittleness of polylactic acid, sufficient strength cannot be obtained even if it is made into a sheet or film. Particularly, heat resistance is not practical. Met. Therefore, the present invention provides a biodegradable film case which has sufficient heat resistance and water resistance, is compatible with a microwave oven, and is capable of being composted.

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems, and as a result, completed the present invention. That is, the gist of the present invention is that the plane orientation degree ΔP is 3.
0 × 10 ⁻³ to 30 × 10 ⁻³ , and the difference (ΔHm−ΔHc) between the heat of crystal fusion ΔHm when the film is heated and the heat of crystallization ΔHc generated by crystallization during the temperature rise is 20 J /. g (g) and {([Delta] Hm- [Delta] Hc) / [Delta] Hm} are 0.7 or more.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. The polylactic acid-based polymer used in the present invention is polylactic acid or a copolymer of lactic acid and another hydroxycarboxylic acid, or a mixture thereof, and other polymer materials as long as the effects of the present invention are not impaired. May be mixed. It is also possible to add additives such as plasticizers, lubricants, inorganic fillers and ultraviolet absorbers, and modifiers for the purpose of adjusting the moldability and the physical properties of the sheet or the molded article. Lactic acid includes L-lactic acid and D-lactic acid, and other hydroxycarboxylic acids include glycolic acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 3-hydroxyvaleric acid,
Representative examples include -hydroxyvaleric acid and 6-hydroxycaproic acid. As these polymerization methods, any known methods such as condensation polymerization method and ring-opening polymerization method can be adopted, and further, a small amount of a chain extender for the purpose of increasing the molecular weight, for example, a diisocyanate compound, epoxy Compounds, acid anhydrides and the like may be used. The weight average molecular weight of the polylactic acid-based polymer is 50,000
From 1,000,000 to 1,000,000, practical properties are hardly exhibited when the ratio is less than the above range, and a problem such that the strength of the film cannot be maintained at the time of plastic cup molding occurs. On the other hand, if it exceeds, the melt viscosity becomes too high, resulting in poor moldability. The polylactic acid-based film used in the present invention, after sufficiently drying the above-mentioned polymer to remove moisture, formed into a sheet by a general melt molding method such as an extrusion method, a calendar method, and a press method. , Followed by rapid cooling. Practically, it is preferable that the polymer melt-extruded into a sheet is rapidly cooled by contact with a rotating casting drum (cooling drum).
The temperature of the casting drum is suitably 60 ° C or less. If it is higher than this, the polymer sticks to the casting drum and it becomes difficult to take it off. In addition, crystallization is promoted and spherulites develop and transparency increases. As the temperature decreases, thermoforming becomes difficult. Therefore, it is preferable to rapidly cool the sheet at a temperature of 60 ° C. or less to obtain a substantially amorphous sheet. In addition, in the present invention, in order to greatly improve the inherent brittleness of the polylactic acid-based polymer and prevent cracking during plastic cup molding, the plane orientation degree ΔP of the polylactic acid-based film is set to 3.0 × 10 ^-3 ~
Adjust to 30 × 10 ⁻³ . Here, the degree of plane orientation ΔP represents the degree of orientation in the plane direction with respect to the thickness direction of the film, and is usually calculated by the following formula by measuring the refractive index in three orthogonal directions. ΔP = {(γ + β) / 2} −α (1 where α <β <γ. Here, γ and β are the refractive indices of two orthogonal axes parallel to the film surface, and α is the film thickness The degree of plane orientation ΔP depends on the degree of crystallinity and crystal orientation, but largely depends on the molecular orientation in the film plane. That is, by increasing the molecular orientation in the film plane, particularly in one or two directions, ie, the direction of flow of the film and / or the direction perpendicular thereto, 1.0 × 10
ΔP of ⁻³ or less is defined as 3.0 × 10 3 defined in the present invention.
⁻³ or more. As a method of increasing the plane orientation degree ΔP, in addition to all known stretching methods,
A molecular orientation method using an electric field or a magnetic field can also be employed. Usually, a sheet-like material or a cylindrical material that has been melt-extruded from a T-die, an I-die, a round die, or the like as described above is rapidly cooled by a cooling cast roll, water, pressurized air, or the like, and solidified in a state close to amorphous. Thereafter, a method of uniaxially or biaxially stretching by a roll method, a tenter method, a tubular method, or the like is desirably employed industrially. The stretching conditions of the unstretched polylactic acid-based sheet include a stretching temperature of 50 to 100 ° C., a stretching ratio of 1.5 to 5 times, and a stretching speed of 100% / min to 10,000.
% / Min is generally used, but the appropriate range varies depending on the composition of the polymer and the heat history of the unstretched sheet, and thus can be appropriately determined by checking the value of the plane orientation degree ΔP. By setting the degree of plane orientation ΔP to 3.0 × 10 ⁻³ or more, cracks during case molding can be prevented. The upper limit is practically about 30 × 10 ⁻³ , and if an attempt is made to increase the degree of plane orientation ΔP, stretching becomes unstable or impossible, and
Even if the film can be stretched, it is difficult to form a film case. The degree of plane orientation ΔP is 3.0 × 10 ⁻³ to
In a polylactic acid-based film of 30 × 10 ⁻³ , in order to obtain a practical thermal dimensional property in a microwave oven, a heat of crystal fusion ΔHm when the film is heated and a crystal generated by crystallization during the temperature rise. Difference from the heat of formation ΔHc (ΔHm−ΔH
c) is not less than 20 J / g and ｛(ΔHm−ΔHc) / ΔH
It is important to control m｝ to 0.7 or more. The heat of crystal fusion ΔHm and the heat of crystallization ΔHc are determined by differential scanning calorimetry (DSC) of the film sample, and the heat of crystal fusion ΔHm melts all crystals when the temperature is increased at a rate of 10 ° C./min. The amount of heat needed to
It is determined 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 is determined from the area of the heat generation peak generated during crystallization generated in the process of raising the temperature, and it is important to control (ΔHm−ΔHc) of the film to 20 J / g or more. That is, when (ΔHm−ΔHc) is less than 20 J / g, the thermal dimensional stability of the film case is poor, and the film case is deformed when used. On the other hand, 20 J / g
If it is above, the thermal dimensional stability will be good. The heat of crystal fusion ΔHm mainly depends on the crystallinity of the polymer itself, and takes a large value for a polymer having high crystallinity. Incidentally, in the case of a complete homopolymer of L-lactic acid or D-lactic acid having no copolymerization component, it is 60 J / g or more, and in a copolymer of these two types 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 film with respect to the crystallinity of the polymer. When the heat of crystallization ΔHc is large, the crystallization of the film proceeds in the process of raising the temperature. In other words, this indicates that the crystallinity of the film was relatively low based on the crystallinity of the polymer. Conversely, when the heat of crystallization ΔHc was small, the film had a low crystallinity based on the crystallinity of the polymer. This indicates that the crystallinity was relatively high. Here (ΔHm−Δ
One direction for increasing Hc) is to produce a film having a relatively high degree of crystallinity from a polymer having high crystallinity, and the degree of crystallinity of the film depends on the composition of the polymer. Dependent. Heat of crystal fusion Δ of the polymer itself
In order to make Hm 20 J / g or more, the composition ratio of L-lactic acid and D-lactic acid is in the range of 100: 0 to 94: 6 or 0: 1.
It is good to be in the range of 00-6: 94. Further, in order to reduce the heat of crystallization ΔHc, that is, to increase the degree of crystallinity of the film, it is necessary to select film forming conditions. In order to increase the crystallinity of the sheet in the forming process, particularly in the biaxial stretching by the tenter method, it is useful to increase the stretching ratio to promote oriented crystallization, or to heat-treat after stretching in an atmosphere at a crystallization temperature or higher. is there. In addition, since the crystallization temperature tends to decrease as the degree of plane orientation ΔP increases, in the case of the present invention, the heat treatment is preferably performed at 70 ° C. or more, preferably 90 ° C. to 170 ° C. for 3 seconds or more. The higher the heat treatment temperature and the longer the heat treatment time, the better the thermal dimensional stability. (ΔHm−ΔHc) / Δ
Hm is an index indicating the degree of crystallinity of the sheet. If the value is smaller than 0.7, deformation occurs when using a microwave oven, which is not preferable. In the present invention, the polylactic acid-based film produced as described above is molded into a case to obtain a film case. Although the thickness of the film suitable for the film case is not particularly limited, 0.01 mm to 0.10 mm is preferably used from the application.
Particularly preferably, it is in the range of 0.02 mm to 0.05 mm. The case may be formed by pressing using a normal hydraulic press or a mechanical press. In addition, at the time of molding, in order to improve the moldability, molding may be performed after preheating between the glass transition point Tg (° C.) and the melting point Tm (° C.) of the polylactic acid polymer.

EXAMPLES Examples will be shown below, but the present invention is not limited to these examples. The measurement values shown in the examples were calculated by measuring under the following conditions.・ Degree of plane orientation ΔP The refractive index (α, β,
γ) was measured and calculated by the following equation (1). ΔP = {(γ + β) / 2} −α 1 (where α <β <γ. Here, γ and β are biaxial refractive indices parallel to the film surface, and α is a film thickness direction. (ΔHm−ΔHc) / ΔHm Using a differential scanning calorimeter DSC-7 (manufactured by PerkinElmer), 10 mg of a film sample was subjected to JIS-K7122.
When the temperature is raised at a rate of 10 ° C./min.
From the gram, the heat of crystal fusion ΔHm and the heat of crystallization ΔHc
Was calculated, and (ΔHm−ΔHc) / ΔHm was calculated. -Suitability for microwave oven About half of the water in the film case was put, and heated in a microwave oven with a rated high-frequency output of 500 W for 5 minutes. The deformation state after heating was determined from the dimensional change in the diameter of the upper opening. The dimensional change rate is expressed by the following two equations. Dimensional change rate (%) = {(BA) / A} × 100 (2, where diameter before test: A (mm) diameter after test: B
(Mm). ) In addition, evaluation: ：: 0 to 10% = no problem at all in practical use,
： １: 11 to 20% = no problem in practical use, Δ: 21 to 30
% = Can be used depending on the conditions; X: 31% or more; (Examples 1 to 4) In a composition ratio of L-lactic acid and D-lactic acid as shown in Table 1, polylactic acid having a weight average molecular weight of about 200,000
Using a 0 mmφ uniaxial extruder, the sheet was extruded at 200 ° C., and the thickness was set so that the width after stretching was 300 mm and the thickness after stretching was 0.04 mm, thereby producing a sheet. The unstretched sheet was sequentially stretched by a biaxial stretching machine under the conditions shown in Table 1 to obtain a desired stretched film. ΔP of the obtained stretched film of polylactic acid was measured by the above-mentioned method to obtain ΔP. Further, ΔHm and ΔHc were obtained by the measurement according to the method described above. A film case having a bottom diameter of 45 mm, an upper opening diameter of 70 mm, and a height of 30 mm was formed from the polylactic acid-based film. (Comparative Example) As in the example, L-lactic acid and D-
A polylactic acid having a weight-average molecular weight of about 200,000 in a composition ratio with lactic acid was heated to 200 ° C. using a 90 mmφ uniaxial extruder.
Extruded with a width of 300mm and a thickness of 0.04m after stretching
The sheet was prepared by setting the thickness so as to be m. ΔP of the obtained stretched film of polylactic acid was measured by the above-mentioned method to obtain ΔP. Also, when measured by the above-described method, Δ
Hm and ΔHc were obtained. From the polylactic acid-based film, a film case having a bottom diameter of 45 mm and a height of 30 mm was formed. As is clear from Table 1, Examples 1, 2, and 3 satisfying the conditions of the present invention are excellent in moldability and microwave oven suitability. Also, in Example 4, (△ Hm- △ Hc) was small, so that it was inferior to microwave oven suitability but not at a level where it could not be used. On the other hand, in Comparative Example 1, (△ Hm- △ Hc) / △
Since Hm was small, the suitability for a microwave oven was poor, and it was not usable. Comparative Example 3 satisfies (ΔHm−ΔHc) and (ΔHm−ΔHc) / ΔHm, but could not be formed due to cracking at the time of case molding because ΔP was small. (Example 5) Using the film case prepared in Example 1 as a sample, water, humus, and nutrients were put in a household poster at 60 ° C, and two weeks later, rapid decomposition began, and four weeks later Later, the weight retention was less than 10%. (Comparative Example 4) As a comparative example, a compost treatment was performed under the same conditions as in Example 5 using a film case made of OPP.
Even after a week, the physical property retention was 100%, and there was no decomposition at all.

As described above, the polylactic acid-based plastic cup of the present invention can be heated in a microwave oven and can be composted.

[Table 1]

Claims (1)

[Claims] 1. The degree of plane orientation ΔP is 3.0 × 10 ⁻³ to 30.
× 10 ⁻³ , and the difference (ΔHm−ΔHc) between the heat of crystal fusion ΔHm when the film is heated and the heat of crystallization ΔHc generated by crystallization during the temperature rise is 20 J / g or more and ｛(ΔHm− A film case comprising a polylactic acid-based polymer having ΔHc) / ΔHm｝ of 0.7 or more.