WO2009041761A1

WO2009041761A1 - Heat-shrinkable biodegradable film and process for preparation thereof

Info

Publication number: WO2009041761A1
Application number: PCT/KR2008/002501
Authority: WO
Inventors: Kyung-Youn Kim; Sang-Il Kim
Original assignee: Skc Co., Ltd.
Priority date: 2007-09-27
Filing date: 2008-05-02
Publication date: 2009-04-02
Also published as: KR20090032289A

Abstract

Disclosed is a heat-shrinkable biodegradable film comprising 4% to 12% by weight of D-lactic acid, wherein the film exhibits a biodegradability of at least 90%; a shrinkability of 20% to 70% in the primary shrinkage direction (transverse direction), as measured after being dipped in 70°C water for 10 seconds; and a residual stress of 0.05 kgf/mm2 or more, as measured after being dipped in 90°C water for 10 seconds.

Description

HEAT-SHRINKABLE BIODEGRADABLE FILM AND PROCESS FOR PREPARATION THEREOF

FIELD OF THE INVENTION

The present invention relates to a heat-shrinkable, biodegradable film having improved heat shrinkability, mechanical properties, and transparency; and a process for the preparation thereof

BACKGROUND OF THE INVENTION

Heat-shrinkable films made of, e.g., polyvinyl chloride (PVC), polystyrene (PS), or polyethylene terephthalate (PET) film are used in a variety of wrapping or labeling applications and they are required to have satisfactory post-processing properties in terms of sealability and uniform shrinkage, in addition to acceptable properties such as good heat resistance, solvent resistance, weather resistance, printability and transparency.

Such conventional heat-shrinkable films are, however, hampered by a number of problems. For example, a polyvinyl chloride film generates toxic dioxin when burned, and a polystyrene film has poor printability and weak heat resistance. Moreover, they, including polyethylene terephthalate films, are not biodegradable but accumulate in the soil when disposed.

Accordingly, there have been conducted a number of studies to develop a highly degradable aliphatic polyester. Japanese Laid-open Patent Application Nos. Hei 11-187670 and 13-312082 discloses a heat-shrinkable, biodegradable film having good shrinkability, which is made by a process comprising mixing a polyester with a polylactic acid. However, this heat- shrinkable, biodegradable film has the problems of non-uniform shrinking and increased haze. Further, U.S. Patent Publication Nos. 2007/0003774 and 2007/0116909 discloses a heat-shrinkable, biodegradable film comprising a polylatic acid. However, this heat-shrinkable biodegradable film has disadvantages in that the film easily ruptures when subjected to over-drawing and non-uniform shrinkage occurs during labeling. SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a heat- shrinkable, biodegradable film which is environmentally friendly and exhibits uniform shrinkage, as well as good shrinkability, mechanical properties, and transparency.

It is another object of the present invention to provide a process for preparing said heat-shrinkable, biodegradable film. In accordance with one aspect of the present invention, there is provided a heat-shrinkable, biodegradable film containing 4% to 12% by weight of D-lactic acid, wherein the film exhibits a biodegradability of at least 90%; a shrinkability of 20% to 70% in the primary shrinkage direction (transverse direction), as measured after being dipped in 70⁰C water for 10 seconds; and a residual stress of 0.05 kg_f/mm² or more, as measured after being dipped in 90⁰C water for 10 seconds.

In accordance with another aspect of the present invention, there is provided a process for preparing a heat-shrinkable biodegradable film comprising: melt-extruding a poly lactic acid resin containing 4% to 12% by weight of D-lactic acid; casting the extruded resin on a casting roll kept at a temperature of 35°C or less, to produce a sheet; drawing the sheet in the primary shrinkage direction (transverse direction) with a drawing ratio of 3 or more at a temperature ranging from the glass transition temperature of the resin (Tg) to Tg + 40⁰C, to produce a uni-axially oriented film; heat-setting the oriented film at a temperature ranging from Tg + 1O⁰C to Tg + 60⁰C; and cooling the resulting film without subjecting the film to relaxation or with conferring thereon reverse relaxation.

DETAILED DESCRIPTION OF THE INVENTION

The heat-shrinkable biodegradable film in accordance with one embodiment of the present invention is a uni-axially oriented film containing about 4% to about 12% by weight of D-lactic acid. The film may further comprise an anti-blocking agent, with an optional layer deposited on at least one surface of the film which enhances the heat-resistance of the film.

The inventive film undergoes biodegradation to an extent of 90% or more. As used herein, the term "biodegradability" refers to a ratio of biodegradability of test material to that of standard material (e.g., cellulose) over a same period of time. The Ministry of Environment in Korea regulates that the biodegradability of a particular material relative to that of a standard material should be 90% or more. Therefore, the inventive film qualifies as a biodegradable film, The shrinkability of the inventive film is about 20% to about 70% in the primary shrinkage direction, i.e., transverse direction, as measured after being dipped in about 70⁰C water for about 10 seconds, the range of which is suitable for use as a conventional shrinkable wrapping material or a shrinkable label for a bottle or other container. The inventive film has a residual stress of about 0.05 kg_f/mm² or more, and preferably more than about 0.07 kg_f/mm², as measured after being dipped in about 90⁰C water for about 10 seconds.

When labeling, it typically takes about 10 seconds for a film to pass through a hot air tunnel for shrinking. When the residual stress is lower than about 0.05 kg_f/mm², the stress after shrinking gets close to 0 (zero) so that film relaxation after shrinking may occur to make the film loose.

The inventive film exhibits a heat of fusion (ΔHm) of about 40 J/g or less, preferably about 30 J/g or less. Generally, when the heat of fusion

(ΔHm) of a film is over 40 J/g, the crystallizing and shrinking rates become fast so that non-uniform shrinking occurs, and the increased crystallinity causes the film to become brittle and to rupture easily during shipping, particularly in winter.

The shrinkability of the inventive film in the (longitudinal) direction orthogonal to the primary shrinkage direction (transverse direction) may be about 20% or less, preferably about 10% or less, as measured after being dipped in about 90⁰C water for about 10 seconds.

The inventive film has a haze of about 20% or less, preferably about

10% or less. Consequently, the heat-shrinkable biodegradable film in accordance with the present invention can be effectively used as a shrinkable wrapping material or a shrinkable label for a bottle or other container.

The heat-shrinkable biodegradable film in accordance with one embodiment of the present invention may be prepared by the steps comprising: melt-extruding a poly lactic acid resin containing 4% to 12% by weight of D-lactic acid; casting the extruded resin on a casting roll kept at a temperature of 35⁰C or less, to produce a sheet; drawing the sheet in the primary shrinkage direction (transverse direction) with a drawing ratio of 3 or more at a temperature ranging from the glass transition temperature of the resin (Tg) to Tg + 40⁰C, to produce a uni-axially oriented film; heat-setting the oriented film at a temperature ranging from Tg + 1O⁰C to Tg + 60⁰C; and cooling the resulting film without subjecting the film to relaxation, or conferring thereon reverse relaxation. The poly lactic acid resin that may be used in this invention contains about 4% to about 12% by weight of D-lactic acid, as previously mentioned, preferably about 5% to about 10% by weight of D-lactic acid. When D- lactic acid in the polylactic acid is lower than about 4% by weight, the crystallinity of the resin and the shrinking rate of the film increase, causing the film to be twisted or folded during labeling bottles or other containers; and the film becomes crystallized easily when exposed to heat, generating film whitening phenomena. Whereas, when D-lactic acid in the polylactic acid is greater than about 12% by weight, lower crystallinity and higher shrinkage stress are caused, deteriorating markedly the heat resistance and the uniform shrinkage of film.

The inventive method may further comprise the step of adding an anti-blocking agent to the polylactic resin, prior to melt-extruding the polylactic acid resin.

The anti-blocking agent that may be used in this invention comprises inorganic particles including, for example, at least one of silicon dioxide, calcium carbonate, talc, kaolin and titanium oxide, which has a spherical or flake configuration having an average particle diameter of about 0.05 μm to about 5 μm. The anti-blocking agent may be added in a conventionally effective amount, which can be selected by those of ordinary skill in the art with due consideration of various factors. Preferably, the anti-blocking agent may be employed in an amount ranging from 0.0001% to about 1.0% by weight based on the total weight of the film. In addition, additives including, for example, at least one of charging agent, anti-static agent, UV blocking agent, heat stabilizing agent, and antioxidant may be added to the polylactic acid resin in a normally effective amount, which can be selected by those of ordinary skill in the art with due consideration of various factors, without hampering the object of the present invention.

Then, the polylactic acid resin alone or a mixture thereof with the anti-blocking agent or other additives thus obtained is melt-extruded. In this case, the melt-extruding may be conducted at a temperature above the melting point of the resin. Then, the resulting melt-extruded resin is subjected to a casting process, which is conducted on a casting roll kept at a temperature of about 35⁰C or less, preferably about 30⁰C or less, to produce a sheet.

In the casting process, when the casting roll has a temperature above 35⁰C, the resin may not be rapidly cooled and solidified, thus causing excessive nucleation and crystallization rates.

Optionally, the inventive method may further comprise the step of coating a layer for enhancing heat-resistance on either or both sides of the sheet, prior to drawing the film.

The layer for enhancing heat-resistance may comprise a polyester- based resin or a polyurethane-based resin, which has a melting point higher than that of polylactic acid. The resin for enhancing heat resistance to be coated may be used in a conventional amount, which can be selected by those of ordinary skill in the art with due consideration of various factors. Preferably, it may be employed in an amount ranging from 0.0001% to about 1.0% by weight based on the total weight of the film.

Then, the resulting sheet is subjected to a drawing process, wherein the sheet is uni-axially oriented with a drawing ratio of about 3 or more in the primary shrinkage direction (transverse direction) at a drawing temperature ranging from the glass transition temperature of the resin (Tg) to Tg + 4O⁰C, to produce a uni-axially oriented film.

In this process, when the drawing temperature is lower than Tg, the drawing stress becomes exceedingly high, increasing the frequency of fracture over the entire processes; and, further, the crystallinity of the resulting film becomes high, generating the film whitening phenomena.

Whereas, when the drawing temperature is higher than Tg + 4O⁰C, the crystallinity becomes low, decreasing the shrinkage to about 20% or less in the primary shrinkage direction (transverse direction), and lowering the residual stress to about 0.05 kg_f/mm² or less, as measured after being dipped in about 7O⁰C water for about 10 seconds.

Further, when the sheet is uni-axially oriented with a drawing ratio of less than 3 in the primary shrinkage direction (transverse direction), the crystallinity of the resulting film is insufficient, and, further, the residual stress becomes close to 0 (zero), causing poor shrinkability and uniform shrinkage.

Then, the resulting uni-axially oriented film is subjected to a heat- setting process, which is conducted at a temperature ranging from Tg + 1O⁰C to Tg + 60⁰C. At this time, when the heat-setting is conducted at a temperature lower than Tg + 1O⁰C, crystallization does not occur at all so that the shrinking rate becomes too high. Whereas, when the heat-setting is conducted at a temperature higher than Tg + 6O⁰C, thermal crystallization takes place, not giving required shrinkability. Thereafter, in order to enhance the residual stress of the uni-axially oriented film, the resulting film is subjected to a cooling process in combination with/without normal or reverse relaxation. Preferably, the relaxation is a reverse relaxation, and the reverse relaxation is meant by an additional drawing having a drawing ratio of 0.01 to 2. To the contrary, when a normal relaxation is applied to the film, the molecules oriented by drawing reverts to their original state, and, therefore, it is not helpful to increase the residual stress. The following Examples are now given for the purpose of illustration only, and are not intended to limit the scope of the invention.

Example 1

Silicon dioxide having an average particle diameter of 2 μm was dispersed in a polylactic acid resin (available from NatureWorks, LLC, 4042D, D-lactic acid content = 4.5 w.t.%, Tg = 60⁰C) having a melting point of 154⁰C in an amount of 0.07 w.t.% based on the total weight of the final film. The resulting dispersion was dried with a dehumidifying drier at 5O⁰C for 6 hours to remove the moisture therefrom. The dried resin dispersion was melt extruded at 24O⁰C, and casted on a casting roll at 25⁰C, to produce a sheet. 1 w.t.% of a water-dispersant emulsion type acrylic solution was coated to a thickness of 0.5 μm or less on a surface of the sheet. The coated resin thus obtained was drawn with a drawing ratio of 3.7 in the transverse direction at 8O⁰C, without being drawn in the longitudinal direction, to produce a uni-axially oriented film. Then, the resulting uni-axially oriented film was heat set at 85⁰C, and cooled to room temperature without subjecting the film to relaxation, to attain a uni-axially oriented film having a thickness of 50 μm.

Example 2

A first polylactic acid resin (available from NatureWorks, LLC, 4032D, D-lactic acid content = 1.5 w.t.%, Tg = 63⁰C) having a melting point of 154°C and a second amorphous polylactic acid resin (available from

NatureWorks, LLC, 4060D, D-lactic acid content = 11.5 w.t.%, Tg = 62⁰C) were blended at a weight ratio of 30:70, to prepare a third polylactic acid resin (D-lactic acid content = 8.8 w.t.%, Tm = 160⁰C, Tg = 57°C). Then, silicon dioxide having an average particle diameter of 2 μm was dispersed in the third polylactic acid resin in an amount of 0.07 w.t.% based on the total weight of the final film. Then, a uni-axially oriented film having a thickness of 50 μm was produced by the procedure of Example 1 using the third polylactic acid resin thus obtained.

Example 3

A first polylactic acid resin (available from Nature Works, LLC,

4032D, D-lactic acid content = 4.5 w.t.%, Tg = 60⁰C) having a melting point of 154°C and a second amorphous polylactic acid resin (available from Nature Works, LLC, 4060D, D-lactic acid content = 11.5 w.t.%, Tg = 62°C) were blended at a weight ratio of 30:70, to prepare a third polylactic acid resin (D-lactic acid content = 9.6 w.t.%, Tm - 15O⁰C, Tg = 56°C). The third polylactic acid resin thus obtained was melt extruded at 200⁰C, and casted on a casting roll at 18⁰C, to produce a sheet. Then, 1 w.t.% of a water-dispersant emulsion type polyester solution was coated to a thickness of 0.5 μm or less on a surface of the sheet. The coated resin thus obtained was drawn with a drawing ratio of 3.9 in the transverse direction at 7O⁰C, without being drawn in the longitudinal direction, to produce a uni-axially oriented film. Then, the resulting uni-axially oriented film was heat set at 80⁰C, and cooled to room temperature with subjecting the film to 0.3% reverse relaxation, to attain a uni-axially oriented film having a thickness of 50 μm.

Comparative Example 1

A uni-axially oriented film having a thickness of 50 μm was produced by the procedure of Example 1 using a polylactic acid resin (available from Nature Works, LLC, 4032D, D-lactic acid content = 1.4 w.t.%, Tg = 63°C) having the melting point of 170⁰C

Comparative Example 2

The polylactic acid resin used in Example 1 was melt extruded at 24O⁰C, and casted on a casting roll at 25⁰C, to produce a sheet. The sheet thus obtained was drawn with a drawing ratio of 3.7 in the transverse direction at 125⁰C, without being drawn in the longitudinal direction, to produce a uni-axially oriented film. Then, the resulting uni-axially oriented film was heat set at 96⁰C, and cooled to room temperature without subjecting the film to relaxation, to attain a uni-axially oriented film having a thickness of 50 μm.

Comparative Example 3

The polylactic acid resin used in Example 2 was melt extruded at 24O⁰C, and casted on a casting roll at 25⁰C, to produce a sheet. Then, 1 w.t.% of a water-dispersant emulsion type polyester solution was coated to a thickness of 0.5 μm or less on a surface of the sheet. The coated resin thus obtained was drawn with a drawing ratio of 2.8 in the transverse direction at

55⁰C, without being drawn in the longitudinal direction, to produce a uni- axially oriented film. Then, the resulting uni-axially oriented film was heat set at 6O⁰C, and cooled to room temperature with subjecting the film to 0.2% relaxation, to attain a uni-axially oriented film having a thickness of 50 μm.

The uni-axially oriented films prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were each examined to determine the properties shown below. The results are shown in Table 1.

(1) Heat characteristics (Tg, Tm, ΔHm)

A Perkin-Elmer DSC-7 differential scanning calorimeter was used to accurately measure the values of melting point (Tm), glass transition (Tg), and heat of fusion (ΔHm). The temperature was programmed at a rate of 10°C/min to measure the heat characteristics of the film specimen; the glass transition temperature (Tg, ⁰C) was determined based on the initial endothermic variation; the crystallizing temperature (Tc, ⁰C), on the peak point of the exothermic curve appeared after Tg; and the melting point of the film (Tm, ⁰C), on the peak point of the endothermic curve appeared after Tc. Further, the heat of fusion (ΔHm, J/g) of the film was determined based on the peak area under the endothermic curve. (2) Polarimeter

The D-lactic acid content (w.t.%) was measured with a JASCO P- 1020 automatic polarimeter equipped with a sodium lamp as a light source at a wavelength of 589 nm.

(3) Shrinkability (%)

A film specimen of 200 mm (length) X 15 mm (width) was obtained by cutting in line with the primary shrinkage direction (transverse direction), and the film specimen was heat-treated by dipping into 7O⁰C water for 10 seconds. The shrinkability in the primary shrinkage direction

(transverse direction) was calculated as below:

(length before heat treatment - length after heat treatment)

Shrinkability (%) = X lOO length before heat treatment

(4) Residual stress

A film specimen of 110 mm (length) X 15 mm (width) was obtained by cutting in line with the primary shrinkage direction (transverse direction), and both ends of the film specimen were fastened to a fixed frame having a length of 100 mm. The film specimen thus prepared was dipped in 9O⁰C water for 10 seconds, and then the residual stress was calculated according to the following equation:

Shrink force after 10 sec (kg_f) Residual stress after 10 sec (kg/mm ) = _{Unit area of film (mm2)}

(5) Haze

The haze of a film specimen was measured with a hazemeter (Model: SEP-H, Nihohn Semitsu Kogaku, Japan) using a C-light source.

(6) Kinetic friction coefficient (μk)

The kinetic friction coefficient was measured in conformity to ASTM D 1894, Figure 1 (C). (7) Biodegradability (%)

The biodegradability of a film specimen was evaluated according to KS M3100-1 (2003), and the ratio of biodegrability value of the film specimen and that of a standard material over a period of 180 days was calculated according to the following equation:

Biodegradability of film specimen

Biodegradability (%) = X 100

Biodegradability of standard material

(8) Stability of prepared film The stability of a prepared film was evaluated as below to determine the fracture, if any and the flatness thereof.

When neither fracture nor staining was found, and the flatness of the film was excellent, it was rated "O" (good); when no fracture but white residue and staining were found, it was rated "Δ" (relatively good); and when the film production was interrupted for a long period of time due to the fracture of the finally prepared film, it was rated "^x" (poor).

(9) Labeling characteristic The labeling characteristic of a prepared film was determined using a hot air tunnel system having a plurality of hot air nozzles positioned at the upper and lower portion of the side wall, in which the hot air nozzles °an be opened or closed when needed, and the tunnel system can rotate while a testing container passes through the tunnel. The inner temperature of the tunnel was kept at 80⁰C, and the flowing rate of hot air was 0.10 m/s.

Further, in order to measure the uniformity of the shrinkability, grid lines were printed on the film, and the edge portions of the film was sealed using a solvent, to attain a test label.

The label thus prepared was placed over a 1.5 L beverage bottle to cover the top portion 2 cm above the shoulder of the PET beverage bottle. The labeled PET beverage bottle was then forced to pass through the hot air tunnel system, and the degree of shrinkage of the label was evaluated first. Then, the shrinked PET beverage bottle was filled with 80⁰C water, and the appearance of the label was evaluated as below.

When the label was underwent uniform shrinkage so that its appearance was satisfactory, it was rated "O" (good); when the the appearance of the shrunk label was poor, but the appearance of the label became good after filling the PET beverage bottle with 8O⁰C water, it was rated "Δ" (relatively good); and when the shrinkage of the label occurred non-uniformly to create craters, and the appearance was not sufficiently improved even after filling the PET beverage bottle with 8O⁰C water, it was rated "X" (poor).

TABLE 1

As can be seen from Table 1 above, the inventive films showed good required properties, such as shrinkability, uniform shrinkage, etc., whereas the films that fall out the scope of the present invention showed deteriorated properties.

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 heat-shrinkable biodegradable film comprising 4% to 12% by weight of D-lactic acid, wherein the film exhibits a biodegradability of at least 90%; a shrinkability of 20% to 70% in the primary shrinkage direction (transverse direction), as measured after being dipped in 7O⁰C water for 10 seconds; and a residual stress of 0.05 kg/mm² or more, as measured after being dipped in 90⁰C water for 10 seconds.

2. The heat-shrinkable biodegradable film of claim 1, wherein the heat of fusion (ΔHm) of the film is 40 J/g or less.

3. The heat-shrinkable biodegradable film of claim 1, wherein the film exhibits a shrinkability of 20% or less in the longitudinal direction (orthogonal to the primary shrinkage direction), as measured after being dipped in 90⁰C water for 10 seconds.

4. The heat-shrinkable biodegradable film of claim 1, which further comprises an anti-blocking agent.

5. The heat-shrinkable biodegradable film of claim 1, which further comprises a layer for enhancing heat resistance on at least one surface of the film.

6. The heat-shrinkable biodegradable film of claim 4, wherein the antiblocking agent is selected from the group consisting of silicon dioxide, calcium carbonate, talc, kaolin, and titanium oxide.

7. The heat-shrinkable biodegradable film of claim 5, wherein the layer for enhancing heat resistance comprises a polyester-based resin or a polyurethane-based resin.

8. The heat-shrinkable biodegradable film of claim 1, wherein the film exhibits a haze of 20% or less.

9. A process for preparing a heat-shrinkable biodegradable film comprising: melt-extruding a polylactic acid resin containing 4% to 12% by weight of D-lactic acid; casting the extruded resin on a casting roll kept at a temperature of 35⁰C or less, to produce a sheet; drawing the sheet in the primary shrinkage direction (transverse direction) with a drawing ratio of 3 or more at a temperature ranging from the glass transition temperature of the resin (Tg) to Tg + 4O⁰C, to produce a uni-axially oriented film; heat-setting the oriented film at a temperature ranging from Tg + 1O⁰C to Tg + 60⁰C; and cooling the resulting film without subjecting the film to relaxation, or conferring thereon reverse relaxation.

10. The process of claim 9, which further comprises the step of adding an anti-blocking agent to the polylactic acid, prior to melt-extruding the polylactic acid.

11. The process of claim 9, which further comprises the step of coating a layer for enhancing heat resistance on at least one surface of the sheet, prior to drawing the film.

12. A wrapping material comprising the heat-shrinkable biodegradable film of claim 1.