WO2010038984A2

WO2010038984A2 - Bidirection-shrinkable biodegradable film and process for preparation thereof

Info

Publication number: WO2010038984A2
Application number: PCT/KR2009/005602
Authority: WO
Inventors: Kyung-Youn Kim; Sang Il Kim
Original assignee: SKC Co Ltd
Current assignee: SKC Co Ltd
Priority date: 2008-10-02
Filing date: 2009-09-30
Publication date: 2010-04-08
Anticipated expiration: 2011-04-02
Also published as: WO2010038984A3; KR20100037727A; KR101012167B1

Abstract

A bidirection-shrinkable biodegradable film comprising polylactic acid- based resin containing at least 80 % by weight of the L-lactic acid repeating unit, wherein the film has a shrinkability of 10 to 50 % in both the longitudinal and transverse directions, as measured after being subjected to shrinking in a 100 0C air oven for 10 min; an initial elastic modulus of 50 to 350 kgf/mm2 obtained from a strain-stress curve; and an elongation reduction (ΔE) of 0 to 70 %, as measured after being subjected to shrinking in a 120 0C air tunnel for 10 sec, exhibits improved uniform shrinkability, flexibility, and rupture resistance, which is useful for packaging or wrapping.

Description

BIDIRECTION-SHRINKABLE BIODEGRADABLE FILM AND PROCESS FOR PREPARATION THEREOF

FIELD OFTHE INVENTION

The present invention relates to a bidirection-shrinkable biodegradable film having improved performance characteristics in terms of uniform shrinkability, flexibility, and rupture resistance, and to a process for the preparation thereof.

BACKGROUND OF THE INVENTION

Conventional heat-shrinkable films made of, e.g., polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS)₅ and polyethylene terephthalate (PET) film are used in a variety of wrapping or packaging applications and they are required to have satisfactory performance characteristics in terms of stability during transportation/storage and impact resistance, in addition to satisfactory properties such as shrinkability, printability, transparency, solvent resistance, and heat resistance.

Such conventional heat-shrinkable films are, however, hampered by a number of problems. For example, a polyvinyl chloride film generates toxic pollutants such as dioxin during its incineration, a polystyrene film has poor printability and weak heat resistance, and both the polyethylene and polypropylene have poor heat resistance and unsatisfactory post-processing properties. A polyethylene terephthalate film, on the other hand, has satisfactory properties in terms of heat resistance, decay resistance, and shrinkage uniformity, but generate wastes that are not biodegradable.

Accordingly, there have been conducted a number of studies to develop a highly biodegradable aliphatic polyesters, particularly polylactic acid. Japanese Laid-open Patent Publication No. 2002-113775 discloses a heat-shrinkable film comprising polylactic acid. However, this heat-shrinkable film has the problems of low heat-shrinkability and poor impact resistance. Further, U.S. Patent Publication Nos. 2007/0003774 and 2007/0116909 discloses heat-shrinkable biodegradable polylactic acid-based films prepared by a blown-type method. However, these heat-shrinkable biodegradable films have disadvantages in that the films have poor mechanical properties due to their markedly low strength and poor degree of elongation, and easily rupture during a shrink- wrapping process.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a bidirection- shrinkable biodegradable film having properties suitable for packaging, as well as satisfactory impact resistance, flexibility, and transparency.

It is another object of the present invention to provide a process for preparing said bidirection-shrinkable biodegradable film. In accordance with one aspect of the present invention, there is provided a bidirection-shrinkable biodegradable film comprising a polylactic acid-based resin, wherein the polylactic acid-based resin contains at least 80% by weight of the L- lactic acid repeating unit; and the film has a shrinkability of 10 to 50 % in both the longitudinal and transverse directions, as measured after being subjected to shrinking in a 100 °C air oven for 10 minutes; an initial elastic modulus of 50 to 350 kgf/mm² obtained from a strain-stress curve; and an elongation reduction (ΔE) of 0 to 70 %, as measured after being subjected to shrinking in a 120 ⁰C air tunnel for 10 seconds.

In accordance with another aspect of the present invention, there is provided a process for preparing the bidirection-shrinkable biodegradable film, comprising the steps of:

(1) melt-extruding a polylactic acid-based feedstock resin containing at least 80% by weight of the L-lactic acid repeating unit, to produce a sheet;

(2) drawing the sheet in both the longitudinal and transverse directions at a drawing ratio of 2 to 7 at a temperature ranging from the glass transition temperature of the feedstock resin (T_g) to T_g+4Q ⁰C, to produce a biaxially-oriented film;

(3) heat-setting the biaxially-oriented film at a temperature ranging from the melting temperature of the feedstock resin (T_m) - 100 ⁰C to T_m-50 ⁰C; and (4) cooling the resulting film without subjecting the film to relaxation, or conferring thereon relaxation.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with one embodiment of the present invention, the bidirection-shrinkable biodegradable film is a biaxially-oriented film comprising a polylactic acid-based resin containing at least 80 % by weight of the L-lactic acid repeating unit, preferably at least 90 % by weight of the L-lactic acid repeating unit. When the L-lactic acid content is lower than 80 % by weight, the crystallinity of the film decreases, which leads to the deteriorated heat resistance, and also to increased shrinking rate, causing non-uniform shrinkage of the film.

The polylactic acid-based resin may further comprise an aliphatic or aliphatic-aromatic polyester resin having a glass transition temperature (T_g) of - 80 ⁰C to 60 ⁰C. The aliphatic or aliphatic-aromatic polyester resin may be prepared by condensation polymerizing a dicarboxylic acid or derivatives thereof with a diol component. The dicarboxylic acid may comprise at least one selected from the group consisting of succinic acid, glutaric acid, malonic acid, oxalic acid, adipic acid, sebacic acid, azelaic acid, and nonandicarboxylic acid. The diol component may comprise at least one selected from the group consisting of ethyleneglycol, 1,3- propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, propylene glycol, 1,4- cyclohexanediol, hexamethylene glycol, polyethylene glycol, triethylene glycol, neopentyl glycol, and tetramethylene glycol. The dicarboxylic acid may further comprises other components of dicarboxylic acids such as terephthalic acid, phthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylic acid, diphenylsulfonic acid dicarboxylic acid, diphenylether dicarboxylic acid, diphenoxyethane dicarboxylic acid, cyclohexane dicarboxylic acid, and a mixture thereof, to the extent they do not adversely affect the biodegradability of the film.

Such aliphatic or aliphatic-aromatic polyester resin may be added to the poly lactic acid-based resin in an amount of 1 to 30 % by weight, preferably 5 to 20 % by weight based on the total weight of the polylactic acid-based resin. When the amount is more than 30 % by weight, the compatibility of the resin decreases so that the transparency of the final film deteriorates, it becomes difficult to form a sheet, and it may easily undergo heat degradation. The bidirection-shrinkable biodegradable film of the present invention has improved properties for use as a variety of packaging or wrapping materials.

The inventive film has a heat shrmkability of 10 to 50 % in both the longitudinal and transverse directions, as measured after being subjected to shrinking in a 100 ⁰C air oven for 10 minutes. When the heat shrinkability of the film is less than 10 %, the film may not adhere to a container tightly and it becomes difficult to apply the film on various shapes of containers. On the other hand, when the shrinkability of the film is more than 50 %, the film may become twisted or undergo folding during shrinking process in a hot air tunnel due to its high shrinkage rate. The inventive film has an initial elastic modulus of 50 to 350 kgf/mm², preferably 150 to 300 kgf/mm² obtained from a strain-stress curve. When the initial elastic modulus of the film is less than 50 kgf/mm², it becomes difficult to conduct such processes as laminating, so that problems occur during printing due to the occurrence of wrinkles or rupture, while when more than 350 kgf/mm², the stiffness of the film becomes too high and the flexibility of the film becomes poor so that the film becomes brittle and easily ruptures.

The inventive film has an elongation reduction (ΔE) of 0 to 70 %, preferably 0 to 50 %, as measured after being subjected to shrinking in a 120 ⁰C air tunnel for 10 seconds. All shrinkable films have built-in stresses that make them go back to the original shape after heat-shrinking. However, when the heating process increases the degree of crystallization, the elongation becomes poor. Accordingly, when the degree of elongation reduction (ΔE) of the film is higher than 70 %, the film may rupture during their conveying step.

The inventive film may have an impact absorption energy of 5 kgf-cm or higher, preferably 5 to 30 kgf-cm. When the impact absorption energy is lower than 5 kgf-cm, the film may easily rupture during stages of transporting, treating and storing.

The inventive film has an Elmendorf tear strength of 3.0 kgf/μm or higher, preferably 3.0 to 10.0 kgf/μm. When the Elmendorf tear strength is lower than 3.0 kgf/μm, a rupture area may rapidly propagate over the packaging. Generally, the film that has low tear rate after rupture is regarded as having excellent flexibility and impact resistance.

The inventive film has a haze of 30 % or less, preferably 20 % or less. When the haze is higher than 30 %, the transparency of the film becomes significantly low so that it becomes unsuitable for transparent packaging.

The bidirection-shrinkable biodegradable film of the present invention may be prepared by the steps comprising: (1) melt-extruding a feedstock resin to produce a sheet; (2) drawing the sheet in both the longitudinal and transverse directions to produce a biaxially-oriented film; (3) heat-setting the biaxially-oriented film; and

(4) cooling the resulting film.

The feedstock resin that may be used in the present invention essentially comprises the polylactic acid resin containing at least 80 % by weight of the L- lactic acid repeating unit. Also, the feedstock resin may further comprise an aliphatic or aliphatic-aromatic polyester resin having a glass transition temperature of -80 to 60 ⁰C in an amount of 1 to 30 % by weight, based on the total weight of the feedstock resin.

The feedstock resin may further comprise other additives such as a cross- linking agent, antioxidant, heat stabilizer, UV absorber, anti-blocking agent, inorganic lubricant, and antistatic agent, to the extent they do not adversely affect the film properties.

The sheet obtained in the melt-extruding process is then subjected to a drawing process in both the longitudinal and transverse directions on a casting roll kept at a temperature of 0 to 30 ⁰C, preferably 10 to 20 ⁰C. When the casting roll temperature in the drawing process is too high, the crystallization rate may become unsatisfactory.

Also, it is preferred that the drawing step is carried out at a temperature ranging from the glass transition temperature of the feedstock resin (T_g) to T_g + 40 ⁰C. When the drawing temperature is lower than T_g, the drawing stress becomes exceedingly high, increasing the frequency of fracture over the process, and further, the crystallinity of the resulting film becomes high, generating the whitening phenomena in the film. When the drawing temperature is higher than T_g+40 ⁰C, an adhesion between a roll and a film easily occur, deteriorating uniform shrinkage of the film due to a poor flatness. The drawing ratio may be 2 to 7, preferably 3 to 5 in both the longitudinal and transverse directions. When the drawing ratio is lower than 2, the crystallinity derived from a stress may become low, causing a low shrinkage and unsatisfactory uniform shrinkage.

The resulting biaxially-oriented film is then subjected to a heat-setting process, which is conducted at a temperature lower than the melting temperature (T_1n) of the feedstock resin by 50 to 100 ⁰C, preferably by 50 to 80 ⁰C. When the heat-setting process is conducted at a temperature lower than the lower limit (T_m- 100⁰C), thermal crystallization may occur, not giving sufficient shrinkability, and the crystallinity of the resulting film becomes high, deteriorating the flexibility and the impact resistance of the film. When the heat-setting process is conducted at a temperature higher than the higher limit (T_m-50°C), the crystallization does not occur and the shrinking rate becomes too high, so that it becomes difficult to handle the resulting film in converting processing.

Thereafter, the resulting film is subjected to a cooling process without subjecting the film to relaxation, or conferring thereon relaxation. The relaxation may be conferred on the film at a relaxation rate of 0.01 to 5 %, which is capable of recrystallizing the molecules oriented by drawing. If the cooling process is conducted with a reverse relaxation, there arises the problem that only the longitudinal shrinkability becomes high.

The bidirection-shrinkable biodegradable film prepared by the process of the present invention has improved performance characteristics in terms of shrinkability, impact resistance, flexibility, and transparency. Therefore, the packaging materials comprising same exhibit superior impact resistance, flexibility, and rupture resistance.

EXAMPLE

The following Examples are intended to further illustrate the present invention without limiting its scope.

Polylactic acid resins available from Nature Works, LLC, i.e., 4042D (L- lactic acid content: 95.5 wt%, T_m: 150⁰C, T_g: 6O⁰C) or 4032D (L-lactic acid content: 98.5 wt%, T_m: 170⁰C, T_g: 62⁰C) were employed in the following examples and comparative examples.

Example 1

A silicon dioxide powder having an average particle diameter of 2 μm was dispersed in a polylactic acid resin (available from Nature Works, LLC, 4042D) to a content of 0.07 wt% based on the total weight of the final film. The resulting dispersion was dried with a hot air drier at 110 ⁰C for 2 hours to remove the moisture therefrom.

The dried resin dispersion was melt-extruded at 240 ⁰C to produce a sheet. The sheet was drawn on a casting roll kept at 25 ⁰C at a draw ratio of 3.0 in the longitudinal direction at 75 ⁰C and at a draw ratio of 4.0 in the transverse direction at 90 ⁰C, to produce a biaxially-oriented film.

Then, the biaxially-oriented film was heat set at 80 ⁰C₅ and cooled without subjecting the film to relaxation, to attain a bidirection-shrinkable biodegradable film having a thickness of 20 μm.

Example 2

A silicon dioxide powder having an average particle diameter of 2 μm was dispersed in a polylactic acid resin (available from Nature Works, LLC, 4032D) to a content of 0.07 wt% based on the total weight of the final film. The resulting dispersion was dried with a hot air drier at 110 ⁰C for 2 hours to remove the moisture therefrom.

The dried resin dispersion was melt-extruded at 240 ⁰C to produce a sheet. The sheet was drawn on a casting roll kept at 25 ⁰C at a draw ratio of 2.5 in the longitudinal direction at 75 ⁰C and at a draw ratio of 3.7 in the transverse direction at 90 ⁰C₅ to produce a biaxially-oriented film.

Then, the biaxially-oriented film was heat set at 90 ⁰C, and cooled without subjecting the film to relaxation, to attain a bidirection-shrinkable biodegradable film having a thickness of 20 μm.

Example 3

The procedure of Example 1 was repeated except that a mixture of polylactic acid resin (available from Nature Works, LLC, 4032D) and ρoly(butylene adipate- co-terephthalate) resin (PBAT, available from Ire chemical, LTD, G8060) blended in a weight ratio of 90:10 was used as the feedstock resin, to obtain a bidirection- shrinkable biodegradable film having a thickness of 20 μm. Comparative Example 1

The dried resin dispersion was melt-extruded at 240 ⁰C to produce a sheet. The sheet was drawn on a casting roll kept at 25 ⁰C at a draw ratio of 3.0 in the longitudinal direction at 80 ⁰C and at a draw ratio of 4.0 in the transverse direction at 90 ⁰C, to produce a biaxially-oriented film.

Then, the biaxially-oriented film was heat set at 150 ⁰C, and cooled without subjecting the film to relaxation, to attain a bidirection-shrinkable biodegradable film having a thickness of 20 μm.

Comparative Example 2

The dried resin dispersion was melt-extruded at 240 ⁰C to produce a sheet. The sheet was drawn on a casting roll kept at 25 ⁰C at a draw ratio of 1.5 in the longitudinal direction at 60 ⁰C and at a draw ratio of 5.0 in the transverse direction at 100 ⁰C, to produce a biaxially-oriented film.

Then, the biaxially-oriented film was heat set at 50 ⁰C, and cooled the film with conferring thereon reverse relaxation, to attain a bidirection-shrinkable biodegradable film having a thickness of 20 μm. Comparative Example 3

A polylactic acid resin (available from Nature Works, LLC, 4032D) and a polybutylene succinate resin (PBS, available from IRE chemical. LTD., G4560) were blended at a weight ratio of 40 : 60, to prepare a feedstock resin. The resulting feedstock resin was dried with a dehumidifying drier at 70 ⁰C for 5 hours to remove the moisture therefrom.

The dried resin was melt-extruded at 220 ⁰C to produce a sheet. The sheet was drawn on a casting roll kept at 25 ⁰C at a draw ratio of 2.8 in the longitudinal direction at 80 ⁰C and at a draw ratio of 5.5 in the transverse direction at 105 ⁰C, to produce a biaxially-oriented film.

Then, the biaxially-oriented film was heat set at 60 ⁰C, and cooled the film with conferring thereon reverse relaxation, to attain a bidirection-shrinkable biodegradable film having a thickness of 20 μm.

Performance Test

The each of the polyester films prepared in Examples 1 to 3 and Comparative Examples 1 to 3 was examined to determine the following properties thereof. The results are shown in Table 1.

(1) Film composition

Deuteriochloroform (CDCl₃) was blended with trifluoroacetic acid at a weight ratio of 4 : 1. A film sample was dissolved in the resulting solvent to obtain a solution, which was assessed with H-NMR (JSM-LA300, available from JEOL Ltd., Japan) to obtain a film composition.

(2) Polarimeter The L-lactic acid content (wt%) was measured with an automatic polarimeter (P- 1020, available from Jasco Inc., Japan) equipped with a sodium lamp as a light source at a wavelength of 589 nm.

(3) Heat characteristics

Differential scanning calorimeter (DSC-7, available from Perkin-Elmer Inc.) analysis was performed at a temperature programming rate of 10 °C/min. The glass transition temperature (T_g) was determined from the first peak in the heat absorption curve. The next peak in the curve corresponded to the crystallization temperature (T₀), and the third heat absorption peak, to the melting temperature (T_n,)

(4) Initial elastic modulus

The initial elastic modulus (kgf/mm²) was measured according to ASTM D

882 using a 100 mm x 15 mm film sample at an elongation rate of 200 rnm/min and an interval between chucks of 50 mm with a universal tester (UTM 4206-001, available from Instron Inc.) The lower initial elastic modulus indicates the higher flexibility.

(5) Heat shrinkage (%)

A film specimen of 200 mm (length) x 15 mm (width) was obtained by cutting in line with the primary shrinkage direction, the film specimen was maintained in a 100 ⁰C air oven for 10 minutes, and the change in the film length was measured. The shrinkability in the primary shrinkage direction was calculated by:

Shrinkability (%) = (Length before heat treatment - Length after heat treatment ) / Length before heat treatment x 100 (6) Elongation reduction (ΔE₅ %)

The degree of elongation before shrinkage was measured according to ASTM D 882 using a 100 mm x 15 mm film sample at an elongation rate of 200 mm/min and an interval between chucks of 50 mm with a universal tester (UTM 4206-001, available from Instron Inc.) Using samples obtained in (11) (see below), the degree of elongation after shrinkage was measured by the same method. The elongation reduction (%) was calculated by: Elongation Reduction (%) = (Degree of elongation before shrinkage -

Degree of elongation after shrinkage) / Degree of elongation before shrinkage x 100

(7) Impact absorption energy

The impact absorption energy was measured according to ASTM D3420 using a film impact tester (available from Toyoseki Co., Ltd., Japan) equipped with a hemispheric pendulum tip having a diameter of 0.5 inch. The film sample was placed on a sample holder having a 50 mm diameter round hole. The impact absorption energy of each sample was measured 10 times and an average value thereof was recorded.

(8) Elmendorf tear strength

The Elmendorf tear strength (kgf/μm) was measured according to ASTM D 1922 using an Elmendorf tear tester, which evaluates the strength at which the film tears by the pendulum released from a pre-determined height.

(9) Haze

The haze of a film specimen was measured with a hazemeter (SEP-H, available from Nihon Semitsu Kogaku Co., Ltd., Japan) using a C-light source.

(10) Stability of film

The stability of the each film was evaluated as below to determine the characteristics of a fracture and a smoothness thereof.

O Good: neither fracture nor staining was found, and the smoothness of the film was excellent;

Δ Decent: no fracture but white residue or staining was found; and x Poor: the film production process was interrupted due to the fracture of the final film or the film adhered to the roll.

(11) Shrinking characteristic

The shrinking characteristic of the each film was determined using a hot air tunnel system equipped with a plurality of hot air nozzles at the upper and lower portion of the side wall, in which the hot air nozzles can be opened or closed and a testing container can rotate during passing through the tunnel. In order to measure the uniformity of the shrinkability, grid lines were printed on the film, which was sealed over a 1.5 L beverage bottle to cover the top 2 cm portion above the shoulder of the PET beverage bottle. The PET beverage bottle being labeled was then forced to pass through a 120 ⁰C air tunnel system for 10 seconds, and the degree of shrinkage of the label was evaluated as below: O Good: the label was undergone uniform shrinkage so that its appearance was satisfactory;

Δ Decent: the shrinkage of the label was not uniform; and x Poor: the shrinkage was not uniform so that the label did not adhere to the bottle. Table 1

As shown in Table 1, the inventive films showed superior properties over those of the films that fall out the scope of the present invention in terms of shrinkability, impact resistance, flexibility, and transparency.

While the embodiments of the subject invention have been described and illustrated, it is obvious that various changes and modifications can be made thereto by those of ordinary skill in the art without departing from the spirit and scope of the present invention which should be limited only by the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A bidirection-shrinkable biodegradable film comprising polylactic acid-based resin, wherein: the polylactic acid-based resin contains at least 80 % by weight of the L-lactic acid repeating unit; and the film has a shrinkability of 10 to 50 % in both the longitudinal and transverse directions, as measured after being subjected to shrinking in a 100 ⁰C air oven for 10 minutes; an initial elastic modulus of 50 to 350 kgf/mm² obtained from a strain-stress curve; and an elongation reduction (ΔE) of 0 to 70 %, as measured after being subjected to shrinking in a 120 ⁰C air tunnel for 10 seconds.

2. The bidirection-shrinkable biodegradable film of claim 1, which further comprises an aliphatic or aliphatic-aromatic polyester resin having a glass transition temperature of -80 to 60 ⁰C in an amount of 1 to 30 % by weight, based on the total weight of the polylactic acid-based resin.

3. The bidirection-shrinkable biodegradable film of claim 1, which has an impact absorption energy of 5 kgf-cm or higher.

4. The bidirection-shrinkable biodegradable film of claim 1, which has an Elmendorf tear strength of 3.0 kgf/μm or higher.

5. The bidirection-shrinkable biodegradable film of claim 1, which has a haze of 30 % or less.

6. A process for preparing the bidirection-shrinkable biodegradable film of claim 1, comprising the steps of:

(1) melt-extruding a polylactic acid-based feedstock resin containing at least 80 % by weight of the L-lactic acid repeating unit, to produce a sheet;

(2) drawing the sheet in both the longitudinal and transverse directions at a drawing ratio of 2 to 7 at a temperature ranging from the glass transition temperature of the feedstock resin (T_g) to T_g+40 ⁰C, to produce a biaxially- oriented film;

(3) heat-setting the biaxially-oriented film at a temperature ranging from the melting temperature of the feedstock resin (T_1n) - 100 ⁰C to T_m-50 ⁰C; and

(4) cooling the resulting film without subjecting the film to relaxation, or conferring thereon relaxation.

7. The process for preparing a bidirection-shrinkable biodegradable film of claim 6, wherein the drawing ratio in step (2) is in the range of 3 to 5.

8. The process for preparing a bidirection-shrinkable biodegradable film of claim 6, wherein the feedstock resin further comprises an aliphatic or aliphatic-aromatic polyester resin having a glass transition temperature of -80 to 60 ⁰C in an amount of 1 to 30 % by weight, based on the total weight of the feedstock resin.

9. The process for preparing a bidirection-shrinkable biodegradable film of claim 6, wherein the drawing process of step (2) is carried out on a casting roll kept at a temperature ranging from 0 to 30 ⁰C.

10. A wrapping material comprising the bidirection-shrinkable biodegradable film of claim 1.