JP2003055484A - Method of storing polylactic acid film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of storing a polylactic acid film, which can control hydrolysis of the film, and also can control reduction of the molecular weight and the mechanical properties of the film to the extent of raising practically no problems as a product. SOLUTION: A polylactic acid film is stored at a temperature lower by 5 deg.C or above than the glass transition temperature of the polylactic acid that constitutes the film.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for storing a polylactic acid film.

[0002]

2. Description of the Related Art The decomposition mechanism of a biodegradable aliphatic polyester resin is generally said to be promoted by microorganisms. However, among biodegradable aliphatic polyesters, polylactic acid is said to have a different decomposition mechanism from other aliphatic polyesters. That is, polylactic acid reacts with water contained in the atmosphere and is first hydrolyzed. Subsequently, it is known that the polylactic acid is decomposed through a two-step decomposition process such that when the weight average molecular weight of polylactic acid is reduced to about 40,000, decomposition by microorganisms starts.

[0003]

When a film is formed from such polylactic acid, it undergoes hydrolysis,
There is a serious problem in product structure that the molecular weight and mechanical properties of the film are likely to be remarkably deteriorated.

In particular, a polylactic acid-based film obtained by adding a plasticizer to a polylactic acid resin for the purpose of softening is
The diffusion of the plasticizer between the polylactic acid molecule chains facilitates the movement of the polylactic acid molecules (chains). As a result, water in the atmosphere is more likely to diffuse inside the polylactic acid, which is more prone to hydrolysis than a non-plasticized polylactic acid film.
The problem is big.

Therefore, the present invention solves such problems and suppresses the hydrolysis of the polylactic acid-based film, and the deterioration of the molecular weight and mechanical properties of the film can be suppressed to such an extent that there is no practical problem as a product. The purpose is to provide.

[0006]

To achieve this object, the present invention is characterized in that a polylactic acid-based film is stored at a temperature lower than the glass transition temperature of polylactic acid forming the film by 5 ° C. or more. To do.

The present invention is also characterized by storing at a relative humidity of 90% RH or less. Further, the present invention is
It is characterized by storing a polylactic acid-based film which is plasticized and has a glass transition temperature of 50 ° C. or lower.

Further, the present invention is characterized in that the retention rate of the weight average molecular weight of the film is 80% or more when two months have passed after the start of storage. That is, the present inventors have found that the present invention can solve the above problems by storing a polylactic acid-based film at a temperature 5 ° C. or more lower than the glass transition temperature of polylactic acid forming the film. It was made based on the.

The storage method of the present invention is particularly suitable for a polylactic acid-based film adjusted to have a glass transition temperature of 50 ° C. or lower by a plasticizing treatment. This point will be described in detail below.

The polylactic acid film consisting only of polylactic acid resin has a glass transition temperature of about 60.degree. Therefore, according to the present invention, it is necessary to store in an atmosphere of 55 ° C or lower. However, room temperature and summer warehouse (about 40 ~
When stored under a temperature condition of about 50 ° C., since the storage temperature is lower than the glass transition temperature of the film, the movement of the polylactic acid molecular chain is slow and the water in the atmosphere is difficult to diffuse inside the polylactic acid. Therefore, almost no decrease in molecular weight due to hydrolysis is observed. Therefore, there is almost no change in the mechanical properties of the product, and it can be handled in the same storage as a general-purpose plastic film such as nylon, polyethylene terephthalate or polypropylene.

On the other hand, a polylactic acid-based film plasticized by adding a plasticizer or the like to a polylactic acid resin for the purpose of softening the polylactic acid film is obtained by diffusing the plasticizer between the polylactic acid molecule chains. (Chain) becomes easy to move. As a result, in addition to lowering the glass transition temperature, water in the atmosphere is more likely to diffuse inside the polylactic acid, which causes hydrolysis more easily than a non-plasticized polylactic acid film. In particular, a polylactic acid-based film plasticized to a glass transition temperature of 50 ° C. or lower causes a problem when stored under the above-mentioned room temperature or the temperature conditions of a warehouse in summer (about 40 to 50 ° C.). .

On the other hand, as in the present invention, when the film is stored at a temperature lower by 5 ° C. or more than the glass transition temperature of the polylactic acid forming the film, it is possible to avoid any such problems. Moreover, the weight average molecular weight retention of the film after 2 months from storage can be maintained at 80% or more.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, as polylactic acid forming a film, poly L-lactic acid whose structural unit is lactic acid is L-lactic acid, poly D-lactic acid whose structural unit is D-lactic acid, and L-lactic acid. Poly DL- which is a copolymer of lactic acid and D-lactic acid
Examples thereof include lactic acid, a mixture thereof, and the like, and those having a number average molecular weight of 80,000 to 150,000 are preferable. Polylactic acid obtained by blending crystalline polylactic acid and amorphous polylactic acid can also be used.

Next, the plasticizing treatment of polylactic acid will be described. In order to perform plasticization, it is common to add a plasticizer to polylactic acid, but after blending the plasticizer with polylactic acid, the biodegradable aliphatic-aromatic copolyester and biodegradable polyester are further added. It is preferable to blend at least one of the aliphatic aliphatic polyester. With such a composition, flexibility and impact resistance can be imparted to polylactic acid, which is hard and brittle at room temperature.

The plasticizer is not particularly limited as long as it is compatible with polylactic acid, but ether ester type plasticizers and oxyacid ester type plasticizers are bleedouts of the plasticizers into molded articles. It can be preferably used from the viewpoint of bleed-out resistance that suppresses bleeding and plasticization efficiency of the resin composition. Bismethyldiethylene glycol adipate and bisbutyldiethylene glycol adipate can be preferably used as the ether ester plasticizer, and tributyl acetyl citrate and triacetin can be preferably used as the oxyacid ester plasticizer. These plasticizers may be used alone or in combination of two or more.

The mixing ratio of the plasticizer in the resin composition is preferably 1 to 20% by mass. If the blending ratio of the plasticizer is less than 1% by mass, the plasticization of the polylactic acid is insufficient, resulting in poor flexibility. If the blending ratio exceeds 20% by mass, the plasticizer bleeds out and is processed. It becomes inferior in sex. Therefore, the mixing ratio of the plasticizer is preferably 2 to 15% by mass, and 2.5 to 1
It is more preferably 0% by mass.

When polylactic acid is blended with a biodegradable soft component represented by a biodegradable aliphatic-aromatic copolyester, the plasticizer is made of polylactic acid and the biodegradable soft component. It is compatible with both, non-volatile, non-toxic from the viewpoint of environmental issues, and FDA (Foo
d and Drug Administratio
Those that have passed n) are preferred.

The polylactic acid is preferably blended with an inorganic filler which acts as a crystal nucleating agent and a lubricant. That is, by only adding a plasticizer to a resin component containing polylactic acid and a biodegradable aliphatic-aromatic copolyester,
With the plasticization of the resin, the melt tension of the film is lowered and the film-forming property is reduced, and blocking of the film occurs, but by further adding an inorganic filler, it is possible to suppress blocking during film-forming. It is possible to impart slipperiness.

Examples of such inorganic fillers include common inorganic fillers such as talc, silica, calcium carbonate, magnesium carbonate, kaolin, mica, titanium oxide, aluminum oxide, zeolite, clay and glass beads. In particular, talc is preferable because it exhibits the most effect as a crystal nucleating agent for polylactic acid. An organic lubricant may be used in combination with this inorganic filler.

The resin composition composed of polylactic acid includes
Depending on the application, a UV inhibitor, a light stabilizer, an antifogging agent, an antifog agent, an antistatic agent, a flame retardant, a coloring inhibitor, an antioxidant, a filler, a pigment, etc. can be added.

The method for producing the film will be described below with reference to an example. For example, polylactic acid, a biodegradable aliphatic-aromatic copolyester, a plasticizer, and an inorganic filler are blended in a predetermined amount and melt-kneaded by a biaxial kneading extruder to prepare a compound pellet. Then, after drying the compound pellets, a film is formed by an inflation film forming method. That is, the dried compound pellets are put into a single-screw kneading extruder, the molten polymer is pulled up from a round die into a tube shape, and while being air-cooled, it is inflated into a balloon shape at the same time to form a film. It is possible to employ a method in which, after being extruded into a cylindrical shape together with cooling water, it is once folded, pulled up, and then expanded into a balloon shape while being heated to form a film or film.

When the polylactic acid-based film obtained by such a production method is stored, it is stored at a temperature lower by 5 ° C. or more than the glass transition temperature of the polylactic acid forming the film.

As a result, the movement of the molecular chain of polylactic acid during storage is slow, water in the storage atmosphere is difficult to diffuse inside the polylactic acid, and the decrease in molecular weight due to hydrolysis is hardly observed. Therefore, there is an advantage that decomposition by microorganisms is unlikely to start.

Storage at a relative humidity of 90% RH or less is preferable because it is possible to more surely prevent the moisture in the storage atmosphere from diffusing inside the polylactic acid. As described above, when polylactic acid is blended with a biodegradable aliphatic-aromatic copolyester and a plasticizer for plasticization to give flexibility, the glass transition temperature thereof may decrease to 50 ° C. or lower. However, even in such a case, a decrease in molecular weight due to hydrolysis can be prevented by storing at a temperature lower than the glass transition temperature by 5 ° C. or more.

As described above, according to the present invention, since the decrease in the molecular weight due to hydrolysis can be prevented, the weight average molecular weight retention of the film at the time of 2 months after the start of storage is 8
It is possible to easily achieve 0% or more.

[0026]

EXAMPLES Next, the present invention will be specifically described based on Examples. However, the invention is not limited to only these examples. In addition, the measurement of various physical property values in the following Examples and Comparative Examples was performed by the following methods. (1) Glass transition temperature of film: PERKIN EL
Temperature rising rate 20 ° C./mi by MER DSC-7
The temperature was raised from 0 ° C. with n and the change in the baseline, which is considered to be the glass transition of polylactic acid, was measured. (2) Weight average molecular weight (Mw): By gel permeation chromatography (GPC), polystyrene was used as a standard substance, and polylactic acid in THF melt was converted into Styra.
The measurement was performed using a gel HR column and an Ultrastyra gel column, and a refractometer as a detector. (3) Weight average molecular weight retention rate: Mw of the film before storage
And the Mw of the film after being stored for 2 months, it was determined by the following calculation formula. (Mw of film after 2 months) /
(Mw of film before storage) × 100 (%) (4) Tensile strength: Measured according to JIS K-7127. (5) Tensile strength retention: The tensile strength of the film before storage and the tensile strength of the film after storage for 2 months were calculated by the following formula. (Strength of film after 2 months) / (strength of film before storage) × 100 (%) (6) Tensile elongation: Measured according to JIS K-7127. (7) Tensile elongation retention: The tensile elongation of the film before storage and the tensile elongation of the film stored for 2 months were calculated by the following formula. (Elongation of film over time) / (Elongation of film before storage) × 100 (%)

[Production of Film 1] Crystalline polylactic acid (L-lactic acid = 98.8 mol%, D-lactic acid = 1.2 mol%, weight average molecular weight 200,000, Cargill Dow: Nature Works) , Amorphous polylactic acid (L
-Lactic acid = 90 mol%, D-lactic acid = 10 mol%, weight average molecular weight 200,000, and a blending ratio of 70/30 mass% with Cargill Dow: Nature Works) were blended. In addition, both of these polylactic acids were compounded so that the compounding ratio of the aliphatic-aromatic copolyester having a glass transition temperature of -30 ° C (Ecoflex F manufactured by BASF) was 60/40% by mass. . Further, bismethyldiethylene glycol adipate (MXA manufactured by Daihachi Chemical Co., Ltd.) as a plasticizer is 8% by mass based on 100% by mass of polylactic acid and aliphatic-aromatic copolyester.
In addition to being blended, talc as an inorganic filler (Upu HST-0.5 manufactured by Hayashi Kasei Co., average particle size 2.75)
μm) was blended in an amount of 15% by mass based on 100% by mass of polylactic acid and the aliphatic-aromatic copolyester. Then, these raw materials were melt-kneaded by a twin-screw extrusion kneader (manufactured by Japan Steel Works, model number TEX44α) to prepare a polylactic acid-based compound raw material at an extrusion temperature of 230 ° C.

Then, using this polylactic acid-based compound raw material, a single screw extruder having a screw diameter of 45 mm equipped with a circular die having a diameter of 100 mm was used,
Melt extrusion was performed at a set temperature of 190 ° C. The molten resin composition discharged from the die was expanded by air pressure and simultaneously air-cooled by an air ring while being molded into a tube shape. Thus, a film formed to have a film thickness of 30 μm and a film folding width of 250 mm was processed with a pair of pinch rolls installed on the die at 20 m /
It was collected at a speed of min. After a cooling time of about 7 seconds, the tubular film was nipped by a pinch roll, and a length of 100 m was wound by a winder.
Film formation of such a composition was carried out in an environment where the temperature was controlled at 25 to 30 ° C.

The glass transition temperature (T
g), weight average molecular weight (Mw) and tensile strength and elongation values are shown in Table 1.
Shown in. MD represents the machine direction, and TD represents the direction orthogonal to the machine direction.

[0030]

[Table 1]

[Production of Film 2] Crystalline polylactic acid (L
-Lactic acid = 98.8 mol%, D-lactic acid = 1.2 mol%, weight average molecular weight 200,000, Cargill Dow, Inc .: Nature Works), and screw diameter 90 equipped with a T-die
Melt extrusion was performed at a preset temperature of 230 ° C. using a mm single-screw extruder. This produced the 220-micrometer-thick unstretched film. Then preheat temperature 85 ° C, stretching temperature 80
℃, heat set temperature 90 ~ 125 ℃, surface magnification 10 times,
A simultaneous biaxially stretched film having a thickness of 25 μm was produced. Table 1 shows the glass transition temperature, the weight average molecular weight, and the tensile strength and elongation of the obtained film 2.

[Production of Film 3] The same production process as in the case of the film 1 was adopted, but the aliphatic-aromatic copolyester was not used and the blending ratio of the plasticizer was polylactic acid 10.
The content was changed to 20% by mass with respect to 0% by mass, and the compounding ratio of the inorganic filler was changed to 25% by mass with respect to 100% by mass of polylactic acid. Other than that, the compound raw material was produced in the same manner as in the case of the film 1.

Then, from this compound raw material, under the same conditions as in the case of the film 1, a film thickness of 30 μm,
A film formed to have a film folding width of 250 mm was obtained. However, the set temperature of the extruder was changed to 175 ° C.

Table 1 shows the glass transition temperature, weight average molecular weight, and tensile strength / elongation of the obtained film.

(Example 1) The above film 1 (Tg = 3)
5 ° C.) in a constant temperature and humidity chamber under the condition of 30 ° C. × 80% RH, and the weight average molecular weight (Mw) retention rate, tensile strength retention rate, and tensile elongation of the film after 2 months have elapsed. The retention rate was calculated from each measured value. The obtained results are shown in Table 2.

[0036]

[Table 2]

Example 2 Film 1 (Tg = 35 ° C.)
Was left in a constant temperature and humidity chamber under the conditions of 20 ° C. and 65% RH.
And other than that, it carried out similarly to Example 1, and calculated the retention rate of various physical properties. The obtained results are shown in Table 2.

Example 3 Film 3 (Tg = 20 ° C.)
Was left in a constant temperature and humidity chamber under conditions of 10 ° C. and 70% RH.
And other than that, it carried out similarly to Example 1, and calculated the retention rate of various physical properties. The obtained results are shown in Table 2.

Example 4 Film 2 (Tg = 59 ° C.)
Was left in a constant temperature and humidity chamber under the conditions of 40 ° C. and 90% RH.
And other than that, it carried out similarly to Example 1, and calculated the retention rate of various physical properties. The obtained results are shown in Table 2.

Comparative Example 1 Film 1 (Tg = 35 ° C.)
Was left in a constant temperature and humidity chamber under the conditions of 40 ° C. and 70% RH.
And other than that, it carried out similarly to Example 1, and calculated the retention rate of various physical properties. The obtained results are shown in Table 2.

Comparative Example 2 Film 3 (Tg = 20 ° C.)
Was used. Otherwise, in the same manner as in Comparative Example 1, 4
After leaving it in a constant temperature and humidity room under the condition of 0 ° C. × 70% RH, the retention rate of various physical properties was calculated. Table 2 shows the obtained results.
Shown in.

In Examples 1 to 4, the weight average molecular weight retention rate, the tensile strength retention rate, and the tensile elongation retention rate of the film after storage for 2 months were all kept at 80% or more, and the product condition was very good. It was

In Comparative Example 1, the weight average molecular weight retention of the film after storage for 2 months was 60%, and the tensile elongation retention was 40 /.
It was significantly reduced to 70%, and the product condition was poor. In Comparative Example 2, the weight average retention rate of the film after 2 months of storage was 2
The tensile elongation retention rate was 3%, and the tensile elongation retention rate was 10/5%, which was extremely brittle and had no commercial value.

[0044]

According to the present invention, a polylactic acid-based film is stored at a temperature 5 ° C. or more lower than the glass transition temperature of the polylactic acid forming the film, so that two months have passed since the storage. The weight average molecular weight retention of the subsequent film can be maintained at 80% or more, and this point is particularly effective in the case of a polylactic acid-based film whose glass transition temperature is adjusted to 50 ° C. or less by using a plasticizer or the like. Therefore, it is possible to prevent quality deterioration due to physical properties of the film represented by molecular weight and mechanical properties.

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Claims (4)

[Claims] 1. A method for storing a polylactic acid-based film, which comprises storing the polylactic acid-based film at a temperature lower than the glass transition temperature of polylactic acid forming the film by 5 ° C. or more. 2. The method for storing a polylactic acid-based film according to claim 1, which is stored at a relative humidity of 90% RH or less. 3. The method for storing a polylactic acid-based film according to claim 1, wherein the polylactic acid-based film that has been plasticized and has a glass transition temperature of 50 ° C. or lower is stored. 4. The polylactic acid according to claim 1, wherein the weight average molecular weight retention of the film is 80% or more when two months have passed after the start of storage. Method for storing base film.