JPH0977124A - Packing bag composed of polylactic acid polymer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a packing bag having sufficient practical characteristics and capable of being decomposed under natural environment. SOLUTION: A packing bag is formed by fusing and sealing an oriented crystallized film composed of a crystalline polylactic acid polymer with a wt. average mol.wt. of 100000 or more. In a pref. film, a surface orientation index ΔP is 3.0×10<-3> or more, the difference (ΔHm-ΔHc) between crystal fusion calorie ΔHm at a time of the temp. rise of the film and crystallizing calorie ΔHc generated by crystallization during temp. rise is 20J/g or more and (ΔHm-ΔHc)/ΔHm} is 0.75 or more.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a packaging bag that decomposes in a natural environment, and more particularly to a packaging bag formed by fusing and sealing a film made of a polylactic acid polymer.

[0002]

2. Description of the Related Art Packaging bags, such as food products,
BACKGROUND ART A bag made of a film made of a polyolefin resin such as polyethylene or polypropylene or another thermoplastic resin is known as a bag for storing and packaging clothes, daily necessities and the like. Some of the polyolefin resin films having a good heat-sealing property are produced by so-called fusing sealing.

However, since the above resins are chemically and biologically stable, they remain and accumulate with almost no decomposition even if they are left in a natural environment. Further, there is a problem that not only is it scattered in the natural environment to contaminate the living environment of plants and animals, but when it is buried as garbage, it is hardly decomposed and remains, which shortens the life of the landfill. Therefore, a packaging bag made of a polymer having a spontaneously degrading property that does not cause these problems is desired.

Polylactic acid-based polymers have been attracting attention as polymers having natural degradability. Polylactic acid-based polymer is naturally hydrolyzed in the soil and the original shape does not remain in the soil,
It becomes harmless decomposition products by microorganisms. However, the polylactic acid-based polymer itself has brittleness and is not necessarily suitable as a packaging bag which is required to have flexibility and toughness. Further, since polylactic acid-based polymer is a kind of polyester resin and generally has crystallinity, it is expected that it will be difficult to heat seal the polylactic acid polymer, and therefore heat-sealing bags have not been tried.

[0005]

Means for Solving the Problems The present invention provides a packaging bag made of a polylactic acid-based polymer having a spontaneously degrading property with respect to the above-mentioned problems, the gist of which is a weight average molecular weight of 10 It is a packaging bag formed by fusion-sealing an oriented crystallized film made of more than 10,000 crystalline polylactic acid-based polymers.

The film made of a polylactic acid-based polymer has a plane orientation index ΔP of 3.0 × 10 ⁻³ or more and a heat of crystal fusion ΔH when the film is heated.
The difference (ΔHm-ΔHc) between m and the heat of crystallization ΔHc generated by crystallization during heating is 20 J / g or more, {(ΔHm-
The one having ΔHc) / ΔHm} of 0.75 or more is preferably used.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.
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 within a range that does not impair the effects of the present invention. Can be mixed. As lactic acid,
There are L-lactic acid and D-lactic acid, and typical examples of hydroxycarboxylic acid are glyco-lactic acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 3-hydroxyvaleric acid, 4-hydroxyvaleric acid, 6-hydroxycaproic acid and the like. It can be cited as a specific example.

The constitutional units of the polylactic acid-based polymer include poly (L-lactic acid) whose lactic acid structural unit is L-lactic acid, poly (D-lactic acid) whose structural unit is D-lactic acid, and L-lactic acid. Poly (DL-lactic acid), which is a copolymer of D-lactic acid, and a mixture thereof are also available, but the polylactic acid-based polymer used in the present invention needs to be crystalline, and therefore Is
The composition ratio of L-lactic acid and D-lactic acid is 100: 0 to 94: 6.
Alternatively, it is set to 6:94 to 0: 100. Within this range, a film having high crystallinity can be obtained by heat treatment at a temperature equal to or higher than the crystallization temperature, which will be described later.

As a method for polymerizing the polylactic acid-based polymer, any known method such as a condensation polymerization method and a ring-opening polymerization method can be adopted. For example, in the polycondensation method, L-lactic acid or D-
Polylactic acid having an arbitrary composition can be obtained by directly subjecting lactic acid or a mixture thereof to dehydration and in-situ polymerization.

In the ring-opening polymerization method, polylactic acid can be obtained by using lactide, which is a cyclic dimer of lactic acid, and a catalyst selected as necessary while using a polymerization modifier and the like. Lactide is L-lactide which is a dimer of L-lactic acid, D-lactide which is a dimer of D-lactic acid, and further L-
There is DL-lactide composed of lactic acid and D-lactic acid, and polylactic acid having desired crystallinity can be obtained by mixing and polymerizing these as needed.

It is also possible to use a small amount of a chain extender such as a diisocyanate compound, an epoxy compound or an acid anhydride for the purpose of increasing the molecular weight. Further, for the purpose of adjusting the molding processability and the physical properties of the film, a plasticizer,
It is also possible to add additives such as lubricants, inorganic fillers and ultraviolet absorbers, and modifiers.

It is important that the polymer in the present invention has a weight average molecular weight of 100,000 or more, preferably in the range of 100,000 to 300,000, more preferably in consideration of moldability into a film. It is preferably 150,000 to 250,000.

If the weight average molecular weight is below the above range, the fusing seal strength becomes low and stable sealing becomes difficult. It is considered that this is because the strength of the polymer itself is low and the crystallization rate is high due to the low molecular weight, and irregular crystals are formed in the fusing part to make brittleness remarkable.

It is important that the film used as the material of the packaging bag of the present invention is molecularly oriented and oriented and crystallized. Generally, in sealing by heat, molecular orientation and oriented crystallization are considered to impair the heat sealing property, but in the present invention, in order to obtain strength and dimensional stability against temperature of the entire bag, a certain range of Molecular orientation and crystallization are required.

The polylactic acid polymer film having no molecular orientation is fragile, and it is difficult to obtain a thin film because it has not undergone the stretching treatment for molecular orientation. The spherulites grow over time to make the film brittle. However, there is a problem that whitening is likely to occur. Further, a film having a low degree of crystallization even if it is molecularly oriented shrinks when exposed to a high temperature, so that its use as a packaging bag material is extremely limited. Also, such a film should have good fusing sealability due to its low crystallinity, but the film will suffer heat shrinkage and irregular deformation during fusing sealing, which will impair the seal appearance and seal strength. It may cause a decrease.

With respect to the degree of molecular orientation, the plane orientation index ΔP of the film is preferably 3.0 × 10 ⁻³ or more. Here, the plane orientation index 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 (α <β <
γ) where γ and β are refractive indices of two orthogonal axes parallel to the film surface, and α is a refractive index in the film thickness direction.

ΔP depends on the crystallinity and the oriented crystal, but largely depends on the molecular orientation in the film plane. That is, ΔP can be increased by increasing the molecular orientation in the plane direction of the film, usually the flow direction during film production and / or the direction (width direction) orthogonal thereto.

The most practical method of increasing ΔP is stretching. The preferred conditions will be described below. First, the polylactic acid-based polymer is sufficiently dried and melted in an extruder to obtain an unstretched sheet. The melting temperature is preferably in the range of 140 to 250 ° C. It is preferable that the sheet melt-formed into a sheet is brought into contact with a rotating casting drum (cooling drum) to be rapidly cooled. If the temperature of the casting drum is too high, the polymer sticks to the drum and it becomes difficult to take it back. Also, since crystallization is promoted and spherulites develop and stretching becomes impossible, the glass transition temperature or lower, preferably 50 ° C.
It is preferable to set the temperature below and quench it to obtain a substantially amorphous unstretched sheet.

The stretching method may be either uniaxial stretching, sequential biaxial stretching or simultaneous biaxial stretching, but biaxial stretching is desirable because it is necessary to improve physical properties in both longitudinal and transverse directions for the purpose of use. The stretching ratio of the unstretched sheet is in the range of 1.5 to 5 times in each of the longitudinal (longitudinal) direction and the transverse (width) direction, and the stretching temperature is appropriately selected in the range of 50 to 90 ° C. in can be a plane orientation index ΔP is 1.0 × 10 ^-3 or less is increased to 1.0 × 10 ^-3 or more, to obtain a tough film in thin. The stretching also improves the film thickness uniformity, transparency, and gloss.

The upper limit of ΔP is practically about 30 × 10 ^-3 , and if ΔP is made higher than this, there is a disadvantage that the stretching becomes unstable or impossible. Thus, increasing ΔP increases the film strength, but on the other hand,
The thermal dimensional stability of the film is reduced. Specifically, there are drawbacks such that the film shrinks during the hot summer months, or the film naturally shrinks during storage in a rolled state, resulting in sagging or waviness of the film. Further, the film in the vicinity of the seal portion is contracted and deformed at the time of fusing and sealing, and the seal strength and the seal appearance are impaired.

To prevent this, ΔP is 3.0 × 10
^-3 or more films, the difference (ΔHm-ΔHc) between the heat of crystal melting ΔHm when the film is heated and the heat of crystallization ΔHc generated by crystallization during the heating is 20 J / g.
As described above, it is important to control {(ΔHm-ΔHc) / ΔHm} to be 0.75 or more.

That is, below this condition, there is a problem that the thermal dimensional stability of the film becomes poor and the film naturally shrinks during storage. In addition, when the packaging bag is used, a phenomenon such as a change in size occurs depending on the temperature, and usage conditions and applications are greatly restricted.

ΔHm and ΔHc are obtained by differential scanning calorimetry (DSC) of the film sample, and ΔHm and ΔHc are
Hm is the amount of heat required to melt all the crystals when the temperature is raised at a rate of 10 ° C./min, and is determined from the area of the endothermic peak due to crystal melting which appears near the crystal melting point of the polymer. ΔHc is determined from the area of the exothermic peak generated during crystallization that occurs during the temperature rise process.

ΔHm mainly depends on the crystallinity of the polymer itself, and takes a large value in a polymer having a large crystallinity. Incidentally, the complete homopolymer of L-lactic acid or D-lactic acid without copolymerization is 60 J / g or more, and in the copolymer of these two kinds of lactic acid, ΔHm changes depending on its composition.

ΔHc is an index related to the crystallinity of the film at that time with respect to the crystallinity of the polymer, and when ΔHc is large, the crystallization of the film progresses during the temperature rising process, that is, the crystal of the polymer. The crystallinity of the film was relatively low based on the property.

1 for increasing (ΔHm-ΔHc)
One direction is to make a film with a relatively high degree of crystallinity from a polymer with a high crystallinity. The crystallinity of the film depends to a large extent on the composition of the polymer, and the ΔHm of the polymer itself should be 20 J / g or more as described above.
It is important that the composition ratio of L-lactic acid and D-lactic acid is within the range of 100: 0 to 94: 6 or within the range of 0: 100 to 6:94.

In order to reduce ΔHc, that is, in order to increase the crystallinity of the film, it is necessary to select the film processing conditions. In order to increase the crystallinity of the film in the molding process step, particularly in the biaxial stretching by the tenter method, it is useful to increase the stretching ratio to promote oriented crystallization, and to perform heat treatment after stretching in an atmosphere at a crystallization temperature or higher.

The larger the ΔP, the lower the crystallization temperature tends to be. In the case of the present invention, the thermal dimension is at least 70 ° C., preferably 90 to 170 ° C. for 3 seconds or more. Stability can be imparted. Within this range, the higher the heat treatment temperature and the longer the heat treatment time, the better the thermal dimensional stability.

The thickness of the film used as the packaging bag of the present invention is not particularly limited, but is selected within the range of about 10 to 500 μm depending on the application. The packaging bag of the present invention is formed by fusing and sealing the polylactic acid polymer film having the above-mentioned strength and thermal dimensional stability.
The fusing seal is a sealing method in which the cut portion is fused at the same time when the film is cut by heat, and two films are fused and cut by a heating wire or a fusing blade having an acute tip. An elastic body such as silicone rubber is preferably arranged on the surface facing the heating wire or the fusing blade.

The fusing seal has a neat appearance because the fusion-bonded portion (seal portion) has no width and is linear, and the fusion is limited to a very narrow local portion, so that the polymer in that portion is sufficiently filled. It can be melted and has less adverse effect on film properties. Unlike the polyester resin such as polyethylene terephthalate, the polylactic acid-based polymer was able to obtain an unexpectedly high seal strength even if the film was crystallized by fusing. The fusing and sealing temperature varies depending on the melting point of the polymer and the fusing and sealing time, but is preferably in the range of 180 to 400 ° C.

In manufacturing the packaging bag, conventionally known methods can be applied without any trouble. For example, send out a long oriented crystallization film as a stack of two, or send it out as a half-folded film that is folded in half from the center, and perform a fusing seal with a rotating ring-shaped sealing blade, a vertically moving heating wire or a fusing blade, etc. You can Further, the packaging bag can be preliminarily formed with one side opening and then the contents can be filled later, or the contents can be filled simultaneously with the bag making. In addition, if the amount is small, it can be packaged while making a bag manually with a so-called L-type sealer.

The form of the packaging bag is usually three sides (before storing the contents) or four sides (after storing the contents) around the bag by fusing and sealing, but the opening side for storing the contents has A removable adhesive can also be used. It is also possible to make the film into a tubular shape by fusing and sealing it in the lengthwise direction, and twist it by 90 ° so that the fusing seal comes to the center.

[0033]

EXAMPLES Examples will be described below, but the present invention is not limited to these examples. In addition, various measured values were obtained by performing measurement under the following conditions. (1) Plane orientation index ΔP The refractive index (α, β,
γ) was measured and calculated by the following equation. ΔP = {(γ + β) / 2} −α (α <β <
γ) γ: Maximum refractive index in the film plane β: Refractive index in the in-plane direction of the film orthogonal to it α: Refractive index in the film thickness direction (2) Weight average molecular weight Mw Under the following measurement conditions, gel permeation chromatography Graffiti HLC-8120GPC (manufactured by Tosoh Corporation) was used to determine the molecular weight by comparison with standard polystyrene.

Chromatography 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 / detection Chamber temperature: 40 ° C (3) (ΔHm-ΔHc) and (ΔHm-ΔHc) /
ΔHm Perkin-Elmer DSC-7 was used to determine the heat of crystal fusion ΔHm and the heat of crystallization ΔHc from the thermogram when 10 mg of the film sample was heated at a heating rate of 10 ° C./min based on JIS-K7122. It was calculated.

(4) Fusing Seal Strength A 10 mm × 100 mm long strip-shaped sample was cut out from the film so that the fusing seal portion was perpendicular to the sample length direction at the central portion, and the Intesco Universal Testing Machine Model 205 was used. Using, distance between check 40mm, pulling speed 100mm
The tensile test is performed at the speed of 1 / min, and the stress (k
g / cm) was read.

(5) Heat resistance A film sample measuring 100 mm × 100 mm is heated at 80 ° C.
After being left in the constant temperature bath for 5 hours, the (dimensional) change was measured, and the shrinkage rate (%) was calculated.

(6) Impact resistance Hydroshot high-speed impact tester HTM-1 type (Co., Ltd.)
The impact resistance was measured using Shimadzu Corporation). 100m
A sample piece cut out into m × 100 mm was fixed with a clamp, a weight was dropped at the center of the sample piece to give an impact, and the breaking energy when the sample was broken was read. Measurement temperature is 23
The falling velocity of the falling weight is 3 m / sec.

Experimental Example 1 Polylactic acid having a composition ratio of L-lactic acid and D-lactic acid of 98: 2, a glass transition temperature Tg of 58 ° C., a melting temperature Tm of 175 ° C. and a weight average molecular weight of 180,000. The polymer is 2 with a 60mmφ single-screw extruder.
Extruded from a T-die at 10 ° C and quenched with a casting roll (roll temperature 58 ° C) to a thickness of about 200 μm
An unstretched sheet of transparent polylactic acid polymer was obtained.

Using a tenter manufactured by Mitsubishi Heavy Industries, Ltd., the sheet was stretched and heat-treated under the conditions shown in Table 1 to obtain a polylactic acid film (thickness: 29 μm). The heat treatment time is 3
It was 0 seconds.

The film was fused and sealed using a side weld bag making machine HK-65V manufactured by Totani Giken Kogyo Co., Ltd. at a fusing seal bar set temperature of 350 ° C. and a number of shots of 180 sheets / min, and then 280 mm × 320 mm. I made a bag. The performance of the film and bag is shown in Table 2.

(Experimental Examples 2 to 4) Using the polylactic acid polymer having the composition shown in Table 1, a film having a thickness of 29 μm was obtained under the conditions shown in Table 1. Then, a bag was obtained in the same manner as in Experimental Example 1. The performance of each film and bag is shown in Table 2.

[0042]

[Table 1]

[0043]

[Table 2]

Film No. in which the film is oriented and crystallized 1
The bag using the film No. 2 had high fusing seal strength, and was excellent in both film strength and heat resistance. On the other hand, No. 1 which is neither oriented nor crystallized. No. 2 has a melt-sealing strength to some extent, but has a drawback that the film itself has poor impact resistance, and it is oriented but has a low degree of crystallization. No. 3 is not practical as a packaging bag because it has a problem that the thermal dimensional stability is poor and that the periphery of the heating portion shrinks at the time of fusing and sealing to cause wrinkles. Moreover, No. 1 having a low weight average molecular weight. No. 4, a practical fusing seal strength could not be obtained.

[0045]

According to the present invention, a packaging bag having a practically sufficient film strength, thermal dimensional stability and sealing strength can be obtained from a polylactic acid polymer having degradability.

Claims (2)

[Claims] 1. A packaging bag obtained by fusing and sealing an oriented crystallization film made of a crystalline polylactic acid-based polymer having a weight average molecular weight of 100,000 or more. 2. A film made of a polylactic acid-based polymer having a plane orientation index ΔP of 3.0 × 10 ⁻³ or more, and a heat of crystal melting ΔHm when the film is heated and crystallization during heating. Difference (ΔHm-Δ) from the heat of crystallization ΔHc caused by
Hc) is 20 J / g or more, {(ΔHm-ΔHc) / ΔH
The packaging bag according to claim 1, wherein m} is 0.75 or more.