KR0134938B1 - Biodegradable polymeric film and process for the preparation thereof - Google Patents

Abstract

The present invention melts a polylactic acid having a star-shaped molecular structure at a temperature of 130 to 180 ° C, and melts the obtained melt at a temperature of 50 ° C or more, preferably 50 to 120 ° C or less, preferably 2 to 4 times. A method for producing a biodegradable polylactic acid film characterized by stretching, and a biodegradable polylactic acid film having a tensile strength of 3.5 to 6.0 Kg / mm ² and an elongation of 2.5 to 6.0% produced by this method. Since the polylactic acid film prepared according to the present invention has excellent transparency and very high strength, it can be used as a general-purpose biodegradable material such as a packaging material, as well as a bioabsorbable material, a sustained release matrix of agrochemicals, medicines, pharmaceuticals, and agricultural films. It can be applied widely.

Description

Biodegradable polymer film and preparation method thereof

The present invention relates to a transparent biodegradable polylactic acid film and a method for producing the same. More specifically, the present invention provides excellent mechanical strength obtained by melting a polylactic acid having a star-shaped molecular structure having a lower melting point than a linear polylactic acid, and stretching the obtained melt through a film processing process. The present invention relates to a biodegradable polylactic acid film having good transparency and a method for producing such a film.

Lactic acid is a substance that is widely present in nature and is harmless to animals and plants and the human body, and its polymer, polylactic acid, is easily hydrolyzed in the presence of moisture, so that biodegradation of fracture binders, surgical sutures, staples, and sustained release polymers In addition to being used as a sex medical material, in recent years, it is widely used as a pesticide ingredient for soil treatment, such as herbicides, soil fungicides. Recently, as the problem of environmental pollution caused by plastics is serious, polylactic acid is emerging as an important general-purpose biodegradable polymer material and has been developed as a packaging material, a food container, a coating agent, and the like. In this case, it is essential to obtain a polymer having a high molecular weight because it is essential that the material has a suitable strength. In particular, when used in general-purpose packaging materials and coating materials, there is a demand for a polylactic acid film having excellent mechanical strength and good transparency.

Polylactic acid films are obtained through processing processes such as film blowing or T-die techniques. However, polylactic acid has a disadvantage in that its physical properties drop sharply during processing. The reason for this is because polylactic acid has poor thermal stability, so that molecular weight decreases during processing. Since polylactic acid is a partially crystalline polymer having a melting point of 180 ° C. or less, thermal decomposition easily occurs when polylactic acid is heated to a temperature of 180 ° C. or more, and thus a film obtained after processing has a very weak strength. Accordingly, the present inventors have studied a method for producing a high strength polylactic acid film having no molecular weight decrease during processing. In order to obtain a high strength polylactic acid film, it is required to use high molecular weight polylactic acid as a starting material.

In general, as a method for obtaining high molecular weight polylactic acid, a catalyst such as antimony oxide, antimony fluoride, tin chloride, and the like is added to an oligomer obtained by dehydrating and condensing lactic acid to prepare a cyclic diester by depolymerization reaction, to which octoic acid is added. A method of polycondensing by adding a catalyst such as tin or diethyl zinc is used (see Japanese Patent Publication No. 56-14688). According to this method, however, high molecular weight polylactic acid can be obtained, but the operation is complicated, requires a lot of effort and cost, and is not suitable as a method for producing a general-purpose biodegradable material due to poor productivity. In addition, while the conventional method for producing polylactic acid by dehydration condensation has a simple process, a low molecular weight oligomer of less than 4,000 is obtained, resulting in a film having poor physical properties such as tensile strength.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly examined the polycondensation reaction of lactic acid in order to obtain the high molecular weight polylactic acid which does not have the molecular weight fall at the time of processing, and when changing the molecular structure of polylactic acid into a star-shaped structure, Found that it could be obtained. In general, a star-type polymer is a structure in which a plurality of polymer chains are attached to a polyfunctional material, and the polymer chain has a radiation structure, and is known to have a lower melt viscosity than a straight-chain polymer having the same molecular weight (JEL Roovers). Et al., Macromolecules, 5,385 (1972). Accordingly, the polylactic acid having a star structure has a lower melt viscosity than the straight-chain polylactic acid having the same molecular weight, thereby lowering the processing temperature during processing, thereby preventing the molecular weight decrease of the polylactic acid due to thermal decomposition. The present inventors completed the present invention by discovering that the star-shaped polylactic acid was produced and subjected to film blowing processing, and that the film blowing processing was possible at low temperature, and that the obtained film had excellent mechanical strength and good transparency. It came to the following.

Accordingly, an object of the present invention is to provide a biodegradable polylactic acid film having excellent mechanical strength and good transparency by using a star polymer polylactic acid.

Another object of the present invention to provide a method for producing the biodegradable polylactic acid film.

The object of the present invention is achieved by melting a star polymer polylactic acid and stretching the obtained melt through a film processing process.

According to the present invention, there is provided a method for producing a biodegradable polylactic acid film, characterized in that the star-type polylactic acid is melted at a temperature of 130 to 180 ° C or less, and the obtained melt is drawn at a rate of 5 times or less at a temperature of 50 ° C or more. .

As a method for producing a star polylactic acid which can be used in the method of the present invention, a method of polymerization of lactic acid using a material having a polyfunctional group as an initiator and a method of preparing and coupling a linear polylactic acid may be considered. The latter method is widely used for the production of general star polymers, but the former method is advantageous for the production of the star polylactic acid used in the present invention. The mechanism by which the polylactic acid forms a star structure by the former method is due to the difference in reactivity between the polyhydric alcohol having a polyfunctional group and the lactic acid. In other words, the polyhydric alcohol used in the preparation of the star polylactic acid is a tetrahydric or higher primary alcohol, in which all polyhydric alcohols react with the lactic acid first to form a small star structure. After this reaction, the molecular chain grows due to the reaction between lactic acid while maintaining the star structure, and eventually a star structure polymer is formed.

The star polylactic acid used in the present invention is made by directly polycondensing lactic acid using a polyhydric alcohol having four or more primary hydroxyl groups, and by directly polycondensing the polyhydric alcohol and lactic acid, a star molecular structure is obtained. A high molecular weight biodegradable polylactic acid having a molecular weight of 30,000 or more is produced. More specifically, at least tetravalent polyhydric alcohol is added to the lactic acid and heated under reduced pressure to dehydrate the lactic acid to dehydrate the lactic acid to obtain low molecular weight polylactic acid, which is directly heated to polycondensation under reduced pressure in the presence of a catalyst. Type polylactic acid is provided.

As the monomeric lactic acid, L-, D-, and racemic stereoisomers can be used for the production of the star polylactic acid. Although lactic acid can be used in aqueous solution of lactic acid at various concentrations, it is advantageous to use it at a high concentration in order to improve workability. A concentration of at least 85% is preferred. As the starting material, a lactic acid oligomer which is a low molecular weight polymer of lactic acid can also be used.

The dehydration reaction of lactic acid is carried out by adding polyhydric alcohol to lactic acid and then reacting at least 2 hours, usually 2 to 10 hours at a temperature of 100 to 150 ° C. and a pressure of 350 to 30 mmHg in the absence of a catalyst. Preferably, the water is removed by heating under reduced pressure for 5 to 6 hours while increasing the temperature and the degree of vacuum from 105 ° C and 350mmHg to 150 ° C and 30mmHg in steps. The molecular weight of the polylactic acid obtained at this time is about 2,000-4,000, and is low molecular weight.

The polycondensation reaction is carried out by using a conventional catalyst such as antimony oxide and by a heating and decompression method. Reaction temperature is 150-250 degreeC, Preferably it is 150-200 degreeC, The pressure reduction degree is 30-1mmHg, Preferably you may be 10-1mmHg. The reaction time is 10 hours or more, preferably 10 to 150 hours, more preferably 10 to 100 hours.

Polyhydric alcohols used in the preparation of star polylactic acid are substances having four or more hydroxyl groups. As a result of the repeated experiments, when a substance having four or less hydroxyl groups is used, low molecular weight polylactic acid is obtained because the star-type polylactic acid structure does not develop properly. Although the linear type is preferable for the form of a polyhydric alcohol, what has a cyclic structure can also be used. As the polyhydric alcohol having a linear structure, pentaerythritol, dipentaerythritol, tripentaerythritol, sorbitol, ribitol, polyvinyl alcohol, polyhydroxyethyl methacrylate, polyhydroxypropyl methacrylate, and the like are used. It is possible. Usable polyhydric alcohols having a cyclic structure include glucose, fructose and the like.

As for the polyhydric alcohol, it is preferable to use as much as possible the primary hydroxyl group rather than the secondary hydroxyl group. Most preferred polyhydric alcohols are dipentaerythritol. The polylactic acid produced by polycondensing dipentaerythritol and lactic acid has a star-like structure having four or more branches.

The amount of polyhydric alcohol used in the polycondensation reaction is 0.01 to 2% by weight relative to the amount of monomer used in the polymerization reaction. When the amount of the polyhydric alcohol is less than 0.01% by weight, the obtained polymer yields a low molecular weight polylactic acid in which the star structure is not properly developed. It is also impossible to stir the polymerization system when the amount of the polyhydric alcohol is 2% by weight or more, and the obtained polymer is obtained as crosslinked polylactic acid such as elastic rubber. Cross-linked polylactic acid is disadvantageous in that processing is impossible because it is not dissolved at all by a solvent and heat.

Generally as a catalyst used for manufacture of a star polylactic acid, all the well-known catalysts used for the polycondensation reaction of lactic acid can be used. Catalysts used in the polycondensation reaction include tin chloride, cuprous sulfate, cuprous oxide, tin oxide, tetraphenyltin, tin powder, isopropyl titanate, titanium tetrachloride, zinc oxide, antimony oxide, antimony chloride, and lead oxide. Calcium oxide, aluminum oxide, iron oxide, calcium chloride, zinc acetate, paratoluenesulfonic acid, and the like.

The amount of the catalyst added is 0.001 to 1% by weight, preferably 0.01 to 1% by weight based on the amount of the lactic acid monomer. This catalyst is added once to several times during the polymerization reaction.

After the polymerization reaction is initiated, the molecular weight of the polymer gradually increases. The rate of molecular weight increase of the star polylactic acid used in the present invention proceeds significantly faster than the rate of rise of the linear polylactic acid. The molecular weight of the polymer is determined by taking the reaction solution at regular intervals after the start of the reaction and measuring the viscosity in a chloroform solution at 25 ° C., and the melting point is measured by differential scanning calorimetry.

The star polylactic acid polymer used in the present invention has a molecular weight of 30,000 or more, specifically 30,000 to 100,000. Moreover, the obtained polymer is colorless and shows the color nearly white, and melting | fusing point is 145-155 degreeC. Star polymers used in the present invention are described in more detail in patent application No. 93-6920 (April 24, 1993).

The temperature which melt | dissolves a star polylactic acid polymer is 130-180 degreeC. When the temperature at which the polymer is melted is 180 ° C. or higher, polylactic acid readily pyrolyzes, thereby obtaining a film having a very weak strength after crosslinking. In contrast, when the melting temperature is 130 ° C. or lower, a part of the polymer is not melted, thereby obtaining a nonuniform film.

The glass transition temperature of the star polylactic acid is measured by differential scanning calorimetry. The glass transition temperature of the star polylactic acid used for this invention is about 50-53 degreeC. Therefore, the stretching temperature of the molten star polylactic acid is 50 degreeC or more. Stretching temperature is 50-150 degreeC, Preferably it is 50-120 degreeC. The elongation of the film is 5 times or less, preferably 2 to 4 times. When the elongation is more than 5 times, not only the film becomes too thin, but also the film breaks during the stretching operation, resulting in poor workability.

The method for producing a polylactic acid film can be either a film blowing method or a tee-die method. However, the film blowing method is preferable because of its workability and economy.

According to another feature of the invention, there is provided a biodegradable polylactic acid film produced by the method for producing a biodegradable polylactic acid film. Since the polylactic acid film produced by the method of the present invention has excellent transparency and very high strength, it can be used as a general-purpose biodegradable material such as a packaging material, as well as a bioabsorbable material, a sustained-release matrix of agrochemicals or medicines, pharmaceuticals, and agriculture. It can be applied to a wide range of applications such as film.

Hereinafter, the present invention will be described in detail with the following examples and comparative examples.

[Production Example 1]

1,500 g of 90% aqueous L-lactic acid and 1.5 g of dipentaerythritol (DIPET) were placed in a reactor equipped with a thermometer, a condenser, and a nitrogen inlet tube, and under nitrogen stream, the temperature and pressure were reduced from 105 ° C and 350mmHg to 150 ° C and 30mmHg. The dehydration reaction was carried out by heating under reduced pressure while changing stepwise for 6 hours. After extracting about 380g of water, 1g of Sb ₂ O ₃ was added, and the temperature and the reduced pressure were increased to carry out the polycondensation reaction at 3 to 5 mmHg and 185 ° C. As the polymerization proceeded, the viscosity of the reaction system gradually increased. The molecular weight of the polymer was determined by measuring the solution viscosity, and the polymer was taken for each reaction time to measure the molecular weight change. After 72 hours of heating under reduced pressure, the reaction was terminated to obtain 1,060 g of a colorless polymer. The molecular weight was 63,000 when the molecular weight of the polymer was measured after the reaction was completed. In addition, melting | fusing point and glass transition temperature were 148 degreeC and 52 degreeC, respectively.

[Production Example 2]

1,040 g of colorless polymers were obtained by the same method as described in Production Example 1, except that 1.5 g of pentaerythritol was used as the polyhydric alcohol. The molecular weight was 53,000 when the molecular weight of the polymer was measured after the reaction was completed. In addition, melting | fusing point and glass transition temperature were 152 degreeC and 53 degreeC, respectively.

[Manufacture example 3]

1,050 g of a colorless polymer was obtained in the same manner as described in Production Example 1, except that 1.5 g of tripentaerythritol was used as the polyhydric alcohol. After the reaction was completed, the molecular weight of the polymer was measured, and the molecular weight was 60,000. In addition, melting | fusing point and glass transition temperature were 146 degreeC and 51 degreeC, respectively.

[Production Example 4]

1,030 g of a colorless polymer was obtained in the same manner as described in Production Example 1, except that 1.5 g of polyhydroxyethyl methacrylate was used as the polyhydric alcohol. After the reaction was completed, the molecular weight of the polymer was measured, and the molecular weight was 42,000. In addition, melting | fusing point and glass transition temperature were 150 degreeC and 53 degreeC, respectively.

Production Example 5

1,010 g of a colorless polymer was obtained by the same method as described in Preparation Example 1, except that 1.5 g of polyhydroxypropyl methacrylate was used as the polyhydric alcohol. After the reaction was completed, the molecular weight of the polymer was measured, and the molecular weight was 40,000. In addition, melting | fusing point and glass transition temperature were 154 degreeC and 53 degreeC, respectively.

[Manufacture example 6]

980 g of a colorless polymer was obtained in the same manner as described in Production Example 1, except that 1.5 g of polyvinyl alcohol was used as the polyhydric alcohol. After the reaction was completed, the molecular weight of the polymer was measured, and the molecular weight was 35,000. In addition, melting | fusing point and glass transition temperature were 149 degreeC and 52 degreeC, respectively.

[Comparative Manufacturing Example 7]

To prepare straight polylactic acid, 720.7 g (5 mol) of L-lactide is placed in a flask with a vacuum coke. To this was added 20 ml (0.001 mol per lactide) of octoic acid diluted in toluene. The flask containing the reactant was vacuumed at 0.01 mmHg for 10 minutes to remove toluene and water, and injected with dry nitrogen. This process was repeated three times. After drying the polymer sufficiently for 20 minutes using a vacuum pump, close the vacuum cock. The flask was immersed in an oil bath of 130 ° C. and subjected to polymerization while stirring. As the polymerization proceeded, the polymer became viscous and solids polymerization was performed for 24 hours while the rotation was stopped when stirring became impossible. The polymerization system was initially a clear gel but turned to a white solid as the reaction proceeded. The polymer recovered after the polymerization showed a white color, and the molecular weight was 69,000 when the molecular weight was measured by solution viscosity analysis. In addition, melting | fusing point and glass transition temperature were 180 degreeC and 55 degreeC, respectively.

Example 1

The star polylactic acid pellets obtained in Production Example 1 were placed in an extruder having a 1 inch wide threaded feeder. The temperature in the extruder was maintained at 165 to 170 ° C to melt the pellet completely. The melted polymer was subjected to film blowing using compressed air using a Rheocord 90 film blowing machine manufactured by Haake, while maintaining the injection temperature at 155 to 160 ° C. The cooling unit temperature of the blowing machine was maintained at 70 ℃, the vertical and horizontal elongation of the blown film was adjusted to three times each. The thickness of the obtained film was 30 microns, and the tensile strength and the elongation were 5.7 Kg / mm ² and 4%, respectively. The star polylactic acid molecular weights before and after the film blowing process were 63,000 and 56,000, respectively.

Example 2

A 30 micron thick transparent film was obtained in the same manner as described in Example 1, except that the star polylactic acid pellets obtained in Production Example 2 were used. The tensile strength and elongation of the obtained film were 5.4 Kg / mm ² and 4.2%, respectively, and the star polylactic acid molecular weights before and after film blowing were 53,000 and 47,000, respectively.

Example 3

A 30 micron thick transparent film was obtained in the same manner as described in Example 1, except that the star polylactic acid pellets obtained in Production Example 3 were used. The tensile strength and elongation of the obtained film were 5.2Kg / mm ² and 3.8%, respectively, and the star polylactic acid molecular weights before and after film blowing were 60,000 and 55,000, respectively.

Example 4

A 30 micron thick transparent film was obtained in the same manner as described in Example 1, except that the star polylactic acid pellets obtained in Production Example 4 were used. The tensile strength and elongation of the obtained film were 4.5 Kg / mm ² and 5.3%, respectively, and the star polylactic acid molecular weights before and after film blowing were 42,000 and 35,000, respectively.

Example 5

A 30 micron thick transparent film was obtained in the same manner as described in Example 1, except that the star polylactic acid pellets obtained in Production Example 5 were used. The tensile strength and elongation of the obtained film were 4.3Kg / mm ² and 5.6%, respectively, and the star polylactic acid molecular weights before and after film blowing were 40,000 and 34,000, respectively.

Example 6

A 30 micron thick transparent film was obtained in the same manner as described in Example 1, except that the star polylactic acid pellets obtained in Production Example 6 were used. The tensile strength and elongation of the obtained film were 3.8 Kg / mm ² and 2.5%, respectively, and the star polylactic acid molecular weights before and after film blowing were 35,000 and 31,000, respectively.

Comparative Example 1

In Preparation Example 1 the star polylactic acid pellets were placed in an extruder with a 1 inch wide threaded feeder. The temperature in the extruder was maintained at 200-230 ° C. and the pellets were completely melted. The molten polymer was maintained at an injection temperature of 190 to 220 ° C. and subjected to film blowing using compressed air. The cooling unit temperature of the blowing machine was maintained at 70 ℃, the vertical and horizontal elongation of the blown film was adjusted to three times each. The thickness of the obtained film was 30 microns, the tensile strength and the elongation were 0.3Kg / mm ² and 2%, respectively, and the star polylactic acid molecular weights before and after film blowing were 63,000 and 16,000, respectively.

Comparative Example 2

The linear polylactic acid pellets obtained in Comparative Production Example 7 were placed in an extruder with a 1 inch wide threaded feeder. The temperature in the extruder was maintained at 200-230 ° C. and the pellets were completely melted. The molten polymer was maintained at an injection temperature of 190 to 220 ° C. and subjected to film blowing using compressed air. The cooling unit temperature of the blowing machine was maintained at 70 ℃, the vertical and horizontal elongation of the blown film was adjusted to three times each. The thickness of the obtained film was 30 microns, the tensile strength and the elongation were 0.5Kg / mm ² and 2.5%, respectively, and the molecular weights of the linear polylactic acid before and after film blowing were 69,000 and 18,000, respectively.

Claims (10)

A biodegradable poly characterized in that the polylactic acid having a star-shaped molecular structure prepared from lactic acid and a polyhydric alcohol of at least 4 is melted at a temperature of 130 to 180 ° C., and the resulting melt is drawn up to 5 times or less at a temperature of 50 ° C. or more. Process for producing lactic acid film. The method of claim 1 wherein the lactic acid is selected from the group consisting of L-lactic acid, D-lactic acid, DL-lactic acid and lactic acid oligomers prepared from polylactic acid. The polyhydric alcohol of claim 1, wherein the polyhydric alcohol is selected from the group consisting of pentaerythritol, dipentaerythritol, tripentaerythritol, polyhydroxyethyl methacrylate, polyhydroxypropyl methacrylate, and polyvinyl alcohol. Way. The method according to claim 1, wherein the polylactic acid has a molecular weight of 30,000 or more, a melting point of 145 to 155 ° C and a glass transition temperature of 50 to 53 ° C. The method according to claim 1, wherein the stretching temperature and the stretching ratio are 50 to 120 ° C. and 2 to 4 times, respectively. A biodegradable polylactic acid film having a tensile strength of 3.5-6.0 Kg / mm ² and an elongation of 3.5-6.0% prepared according to the method of claim 1. A biodegradable polylactic acid film having a tensile strength of 3.5-6.0 Kg / mm ² and an elongation of 3.5-6.0% prepared according to the method of claim ² . A biodegradable polylactic acid film having a tensile strength of 3.5-6.0 Kg / mm ² and an elongation of 3.5-6.0% prepared according to the method of claim 3. A biodegradable polylactic acid film having a tensile strength of 3.5-6.0 Kg / mm ² and an elongation of 3.5-6.0% prepared according to the method of claim 4. A biodegradable polylactic acid film having a tensile strength of 3.5-6.0 Kg / mm ² and an elongation of 3.5-6.0% prepared according to the method of claim 5.