Polylactic acid resin composition and a packaging film

Abstract

The present invention relates to a polylactic acid resin composition useful as a packaging material which has not only improved flexibility but also superior external appearance and superior properties such as mechanical property, transparency, heat resistance, anti-blocking property, workability of a film, and the like, and a packaging film including the same. The polylactic acid resin composition comprises a polylactic acid resin including a hard segment comprising a polylactic acid repeating unit and a soft segment comprising a polyurethane polyol repeating unit in which polyether polyol repeating units are linearly linked via a urethane bond; and a specific content of antioxidant.

Claims

1. A polylactic acid resin composition, comprising: a polylactic acid resin including a hard segment comprising a polylactic acid repeating unit of the following Chemical Formula 1, and a soft segment comprising a polyurethane polyol repeating unit in which structures of the following Chemical Formula 2 are linearly linked to each other via a urethane bond, wherein the urethane bond is bonded directly to each of the structures of Chemical Formula 2; and an antioxidant of 100 to 1500 ppmw per the amount of the monomers for forming the polylactic acid repeating unit: ##STR00003## wherein A is a linear or branched alkylene of 2 to 5 carbon atoms, m is an integer of 10 to 100, and n is an integer of 700 to 5000.

2. The polylactic acid resin composition of claim 1, wherein the polylactic acid resin has a number average molecular weight of 50,000 to 200,000 and a weight average molecular weight of 100,000 to 400,000.

3. The polylactic acid resin composition of claim 1, wherein the polylactic acid resin has a glass transition temperature (Tg) of 25 to 55 C. and a melt temperature (Tm) of 160 to 178 C.

4. The polylactic acid resin composition of claim 1, wherein the urethane bond is formed by a reaction between a terminal hydroxyl group of the structures of Chemical Formula 2 and a diisocyanate compound, and the structures of Chemical Formula 2 are linearly linked via the urethane bond to form the polyurethane polyol repeating unit.

5. The polylactic acid resin composition of claim 4, wherein the polylactic acid resin includes a block copolymer in which a terminal carboxyl group of the polylactic acid repeating unit and a terminal hydroxyl group of the polyurethane polyol repeating unit are linked via an ester bond.

6. The polylactic acid resin composition of claim 5, wherein the polylactic acid resin comprises the block copolymer; and the polylactic acid repeating unit which remains unlinked to the polyurethane polyol repeating unit.

7. The polylactic acid resin composition of claim 4, wherein a molar ratio of the terminal hydroxyl group of the structures of Chemical Formula 2 and the isocyanate group of the diisocyanate compound is 1:0.50 to 1:0.99.

8. The polylactic acid resin composition of claim 1, wherein the structures of Chemical Formula 2 have a number average molecular weight of 450 to 9000.

9. The polylactic acid resin composition of claim 1, wherein the polylactic acid resin comprises 80 to 95 parts by weight of the hard segment and 5 to 20 parts by weight of the soft segment per 100 parts by weight of the polylactic acid resin.

10. The polylactic acid resin composition of claim 1, having a color-b value less than 6.

11. The polylactic acid resin composition of claim 1, wherein the residual monomer content is less than 1 wt % per the weight of the polylactic acid resin.

12. The polylactic acid resin composition of claim 1, wherein the antioxidant is at least one selected from the group consisting of a hindered phenol-based antioxidant, an amine-based antioxidant, a thio-based antioxidant, and a phosphite-based antioxidant.

13. A packaging film, including the polylactic acid resin composition of claim 1.

14. The packaging film of claim 13, having a thickness of 5 to 500 m.

15. The packaging film of claim 13, having a total machine direction and transversal direction Young's modulus of 350 to 750 kgf/mm.sup.2, an initial tensile strength of 10 kgf/mm.sup.2 or higher, a rate of weight loss of 0.01 to 3.0 wt % upon treatment for 1 hr in a 100 C. hot wind oven, a haze of 3% or less, and a light transmittance of 85% or higher.

Description

DETAILS FOR PRACTICING THE INVENTION

(1) The present invention will be explained in detail with reference to the following examples. However, these examples are only to illustrate the invention, and the scope of the invention is not limited thereto.

(2) Definitions of Physical Properties and Measuring Methods:

(3) physical properties stated in the following Examples are defined and measured as follows.

(4) (1) NCO/OH: molar ratio of isocyanate group of diisocyanate compound (e.g., hexamethylene diisocyanate)/terminal hydroxyl group of polyether polyol repeating unit (or (co)polymer) for a reaction to form polyurethane polyol repeating units.

(5) (2) OHV (KOH mg/g): measured by dissolving the polyurethane polyol repeating unit (or (co)polymer) in dichloromethane, acetylating the repeating unit, hydrolyzing the acetylated repeating unit to generate acetic acid, and titrating the acetic acid with 0.1 N KOH in methanol. It corresponds to the number of terminal hydroxyl groups of the polyurethane polyol repeating units (or (co)polymer).

(6) (3) Mw and Mn (g/mol) and molecular weight distribution (Mw/Mn): measured by applying a 0.25 wt % solution of polylactic acid resin in chloroform, and gel permeation chromatography (manufactured by Viscotek TDA 305, Column: Shodex LF804*2ea). Polystyrene was used as a standard material to determine weight average molecular weight (Mw) and number average molecular weight (Mn). A molecular weight distribution was calculated from Mw and Mn.

(7) (4) Tg (glass transition temperature, C.): measured with a differential scanning calorimeter (manufactured by TA Instruments) while quenching the melted sample and then increasing the temperature of the sample at a rate of 10 C./minute. The Tg was determined from the mid value of tangential line of an endothermic curve and a base line.

(8) (5) Tm (melting temperature, C.): measured with a differential scanning colorimeter (manufactured by TA Instruments) while quenching the melted sample and then elevating the temperature of the sample at a rate of 10 C./min. The Tm was determined from the maximum value of the melt endothermic peak of the crystal.

(9) (6) Residual monomer (lactide) content (wt %): measured by a GC analysis after dissolving 0.1 g of the resin in 4 ml chloroform, adding 10 ml hexane therein, and filtering the same.

(10) (7) Content of polyurethane polyol repeating unit (wt %): the content of polyurethane polyol repeating unit in prepared polylactic acid resin was measured using a 600 MHz nuclear magnetic resonance (NMR) spectrometer.

(11) (8) Pellet color-b: color-b value of the resin chip (pellet) was measured by using Chroma meter CR-410 manufactured by Konica Minolta Sensing Co., and a mean value of five measurements was expressed.

(12) (9) Extrusion state: The polylactic acid resin was extruded at 200 to 250 C. into a sheet phase using a 30 mm single screw extruder equipped with a T die and the extrudated sheet was electrostatically deposited on a casting drum cooled to 5 C. so as to prepare a undrawn sheet. At this time, the melt viscosity of the extrudated sheet was measured using Physica Rheometer (Physica, USA). In detail, while maintaining the initial temperature of the extrudate, a shear force was applied thereto by a 25 mm parallel plate type instrument with a shear rate (1/s) of 1 during which complex viscosity (Pa.Math.s) of the melted resin was measured with the Physica Rheometer. The states of melt viscosity (extrusion states) were evaluated according to the following standards.

(13) : melt viscosity is good enough to perform winding around the cooling drum, : melt viscosity is slightly low and winding is possible although difficult, x: melt viscosity is too low to wind.

(14) (10) Initial tensile strength (kgf/mm.sup.2) MD, TD: A film sample with 150 mm in length and 10 mm in width was conditioned at a temperature of 20 C. and a humidity of 65% RH for 24 hrs, and measured the tensile strength according to ASTM D638 using Universal test machine (manufactured by INSTRON) at a drawing speed of 300 mm/min with the distance of 100 mm between grips. A mean value of five measurements was expressed. MD and TD stand for machine direction and transversal direction of the film, respectively.

(15) (11) Elongation ratio (%) MD, TD: The elongation ratio was determined at the point when the film was torn under the same condition as in the tensile strength test of (10). A mean value of five measurements was expressed. MD and TD stand for machine direction and transversal direction of the film, respectively.

(16) (12) F5 (kgf/mm.sup.2) MD, TD: In the stress-strain curve obtained in the tensile strength test of (10), a tangential value at a stress point of 5% strain was determined, and a stress value at 5% elongation was obtained from the tangential slope. A mean value of five measurements was expressed. MD and TD stand for machine direction and transversal direction of the film, respectively.

(17) (13) F100 (kgf/mm.sup.2) MD: In the stress-strain curve obtained in the tensile strength test of (10), a tangent value at a stress point of 100% strain was determined, and a stress value at 100% elongation was obtained from the tangential slope. A mean value of five measurements was expressed. MD and TD stand for machine direction and transversal direction of the film, respectively.

(18) (14) Young's modulus (kgf/mm.sup.2) MD, TD: The same film sample as in the tensile strength test of (10) was measured for Young's modulus according to ASTM D638 using UTM (manufactured by INSTRON) at a drawing speed of 300 mm/min with a distance between grips of 100 mm. A mean value of five measurements was expressed. As the Young's modulus, particularly, a sum of Young's modulus values measured in machine direction and transversal direction, corresponds to the flexibility of the film, a lower Young's modulus value may indicate higher flexibility. MD and TD stand for machine direction and transversal direction of the film, respectively.

(19) (15) Wave pattern (horizontal line): Degrees of the wave patterns which are produced due to a difference in melt viscosity when two kinds of resins with different molecular weights or a resin and a plasticizer are compounded and extruded into a film are evaluated on an A4-size film sample according to the following criteria.

(20) : no wave patterns (horizontal lines), : Up to 3 wave patterns (horizontal lines), x: 5 or more wave patterns (horizontal lines).

(21) (16) 100 C. Rate of weight loss (%): A film sample was conditioned for 24 hrs at 23 C. and 65% RH and weighed before heat treatment. Then, it was treated for 60 min in a 100 C. hot wind oven, and again conditioned under the same conditions as in pre-heat treatment, and weighed. Percentages of the pre-treatment weight to the changes between pre- and post-treatment processes were calculated.

(22) (17) Pin hole and anti-bleed-out: After the heat treatment of (15), the surface of the film sample was observed to examine the generation of pin holes. In addition, the bleed-out of the low-molecular weight plasticizer on the film surface was evaluated with tactile sensation on an A4-size film sample according to the following criteria.

(23) : neither pin holes nor bleed-out, : up to 5 pin holes or bleed-out observed, but not serious, x: 5 or more pin holes or serious bleed-out.

(24) (18) Haze (%) and light transmittance (%): A film sample was conditioned for 24 hrs at 23 C. and 65% RH, and the average haze value was measured at three different points according to JIS K7136 using a haze meter (Model Japan NDH2000).

(25) (19) Anti-blocking property: The antistatic surface of a film sample was matched with the print surface by using COLORIT P type stamping of foil (Kurz), and left for 24 hrs at 40 C. under a pressure of 1 kg/cm.sup.2, thereafter the blocking between the antistatic layer and the print surface was observed. Based on the observation, the anti-blocking property of the film between the anti-static layer (layer A) and the print surface of the in-mold transfer foil was evaluated according to the following criteria. Practical performance is guaranteed by at least .

(26) : no changes, : slight surface change (less than 5%), x: defoliated by 5% or higher.

(27) (20) Yellowing coloration of film: after crashing the film sample with a crasher, and carrying out a moisture absorption dry and a crystallization at 120 C., the sample was melted at about 200 C. and made into chips again by a small single screw extruder (Haake Co., Rheomics 600 extruder). The difference of color-b values before/after said film forming process was measured and the yellowing coloration was evaluated according to the following criteria.

(28) : 2 or less, almost no yellowing, : 5 or less, yellowing appeared slightly, x: larger than 5, yellowing appeared heavily.

(29) Materials used in the following Examples and Comparative Examples are given as follows:

(30) 1. Polyether Polyol Repeating Unit (or (Co)Polymer) or Correspondents Thereto PPDO 2.4: poly(1,3-propanediol); number average molecular weight 2400 PPDO 2.0: poly(1,3-propanediol); number average molecular weight 2000 PPDO 1.0: poly(1,3-propanediol); number average molecular weight 1000 PTMEG 3.0: polytetramethylene glycol; number average molecular weight 3000 PTMEG 2.0: polytetramethylene glycol; number average molecular weight 2000 PTMEG 1.0: polytetramethylene glycol; number average molecular weight 1000 PEG 8.0: polyethylene glycol; number average molecular weight 8000 PBSA 11.0: aliphatic polyester polyol prepared by the polycondensation of 1,4-butanediol, succinic acid, and adipic acid; number average molecular weight 11,000

(31) 2. Diisocyanate Compound (or Tri- or Higher Functional Isocyanate) HDI: hexamethylenediisocyanate D-L75: Bayer, Desmodur L75 (TRIMETHYLOL PROPANE+3 toluene diisocyanate)

(32) 3. Lactide Monomer L- or D-lactide: product manufactured by Purac, optical purity of 99.5% or higher

(33) 4. Antioxidants, Etc. TNPP: Tris(nonylphenyl) phosphite U626: Bis(2,4-di-tbutylphenyl)Pentaerythritol Diphosphite S412: Tetrakis[methane-3-(laurylthio)propionate]methane PEPQ: (1,1-Biphenyl)-4,4-Diylbisphosphonous acid tetrakis[2,4-bis(1,1-dimethylethyl)phenyl]ester I-1076: octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate O3: Bis[3,3-bis-(4-hydroxy-3-tert-butyl-phenyl)butanoicacid]glycol ester

(34) A. Preparation of Polylactic Acid Resins A to J

(35) According to the instructions shown in Table 1 below, reactants and catalyst were fed into an 8 L reactor equipped with a nitrogen tube, a stirrer, a catalyst inlet, an effluent condenser and a vacuum system. As the catalyst, dibutyltin dilaurate was used in an amount of 130 ppmw based on the total weight of the reactants. Under a nitrogen atmosphere, a urethane reaction was carried out at 70 C. for 2 hrs, and then 4 kg of L-(or D-) lactide was fed into the reactor, followed by five times of nitrogen flushing.

(36) Subsequently, the temperature was elevated to 150 C. to completely dissolve the L-(or D-) lactide, and tin 2-ethylhexylate catalyst of 120 ppmw per the total content of the reactants was diluted in 500 ml toluene and the diluted solution was fed into the reactor through the catalyst inlet. Under a nitrogen pressure of 1 kg, the reaction was carried out at 185 C. for 2 hrs, and then phosphoric acid was fed in an amount of 200 ppmw through the catalyst inlet and blended with the reaction mixture for 15 minutes to inactivate the catalyst. After the catalyst deactivation, the vacuum was applied until the pressure reached 0.5 torr to remove unrelated L- (or D-) lactide (about 5 wt % of the initially fed weight). The molecular weight, Tg, Tm, and so on of the obtained resin were measured and given in Table 1.

(37) B. Preparation of Polylactic Acid Resin L

(38) According to the instructions shown in Table 1 below, polyol and 4 kg of L-lactide were fed into an 8 L reactor equipped with a nitrogen tube, a stirrer, a catalyst inlet, an effluent condenser and a vacuum system, followed by five times of nitrogen flushing. Subsequently, the temperature was elevated to 150 C. to completely dissolve the L-lactide, and a dilution of 120 ppmw of the catalyst tin 2-ethylhexylate in 500 ml of toluene was introduced into the reactor through the catalyst inlet. Under a nitrogen pressure of 1 kg, the reaction was carried out at 185 C. for 2 hrs, after which phosphoric acid was fed in an amount of 200 ppmw through the catalyst inlet and blended with the reaction mixture for 15 minutes to inactivate the catalyst. Until the pressure reached 0.5 torr, vacuum was applied to remove unreacted L-lactide. The molecular weight, Tg, Tm, and so on of the obtained resin were measured and given in Table 1.

(39) C. Preparation of Polylactic Acid Resin M

(40) According to the instructions shown in Table 1 below, 6 g of 1-dodecanol and 4 kg of L-lactide were fed into an 8 L reactor equipped with a nitrogen tube, a stirrer, a catalyst inlet, an effluent condenser and a vacuum system, followed by five times of nitrogen flushing. Subsequently, the temperature was elevated to 150 C. to completely dissolve the L-lactide, and a dilution of 120 ppmw of the catalyst tin 2-ethylhexylate in 500 ml of toluene was introduced into the reactor through the catalyst inlet. Under a nitrogen pressure of 1 kg, the reaction was carried out at 185 C. for 2 hrs, after which phosphoric acid was fed in an amount of 200 ppmw through the catalyst inlet and blended with the reaction mixture for 15 minutes to inactivate the catalyst. Until the pressure reached 0.5 torr, vacuum was applied to remove unreacted L-lactide. The molecular weight, Tg, Tm, and so on of the obtained resin were measured and given in Table 1.

(41) D. Preparation of Polylactic Acid Resin O

(42) According to the instruction shown in Table 1 below, PBSA polyol (polyester polyol) and HDI were fed into an 8 L reactor equipped with a nitrogen tube, a stirrer, a catalyst inlet, an effluent condenser and a vacuum system, followed by five times of nitrogen flushing. As a catalyst, dibutyltin dilaurate was used in an amount of 130 ppmw based on the total weight of the reactants. Under a nitrogen atmosphere, a urethane reaction was carried out at 190 C. for 2 hrs, and then 4 kg of L-lactide was fed into the reactor, and completely dissolved at 190 C. in a nitrogen atmosphere. Tin 2-ethylhexylate as an addition polymerization catalyst, and dibutyltin dilaurate as an ester and/or ester amide exchange catalyst were diluted in amounts of 120 ppmw and 1000 ppmw, respectively, based on the total weight of the reactants, in 500 ml of toluene, and added to the reactor. Under a nitrogen pressure of 1 kg, the reaction was carried out at 190 C. for 2 hrs, after which phosphoric acid was fed in an amount of 200 ppmw through the catalyst inlet and blended with the reaction mixture for 15 minutes to inactivate the catalysts. Until the pressure reached 0.5 torr, vacuum was applied to remove unreacted L-lactide (about 5 wt % of the initial amount). The molecular weight, Tg, Tm, and so on of the obtained resin were measured and given in Table 1.

(43) E. Preparation of Polylactic Acid Resin P

(44) According to the instructions shown in Table 1 below, PEG, 3.6 kg of L-lactide, and 0.4 kg of D-lactide were fed into an 8 L reactor equipped with a nitrogen tube, a stirrer, a catalyst inlet, an effluent condenser and a vacuum system, followed by five times of nitrogen flushing. Subsequently, the temperature was elevated to 150 C. to completely dissolve the lactides, and a dilution of 120 ppmw of the catalyst tin 2-ethylhexylate in 500 ml of toluene was fed into the reactor through the catalyst inlet. Under a nitrogen pressure of 1 kg, the reaction was carried out at 185 C. for 2 hrs, after which phosphoric acid was fed in an amount of 200 ppmw through the catalyst inlet and blended with the reaction mixture for 15 minutes to inactivate the catalyst. Until the pressure reached 0.5 torr, vacuum was applied to remove unreacted L-lactide (about 5 wt % of the initial amount). Then, HDI and a dilution of 120 ppmw of the catalyst dibutyltin dilaurate in 500 ml of toluene were introduced through the catalyst inlet into the reactor as shown in Table 1. Under a nitrogen atmosphere, the polymerization was carried out at 190 C. for 1 hr. The molecular weight, Tg, Tm, and so on of the obtained resin were measured and given in Table 1.

(45) F. Preparation of Polylactic Acid Resin Q

(46) According to the instructions shown in Table 1 below, PEG, 3.6 kg of L-lactide, and 0.4 kg of D-lactide were fed into an 8 L reactor equipped with a nitrogen tube, a stirrer, a catalyst inlet, an effluent condenser and a vacuum system, followed by five times of nitrogen flushing. Subsequently, the temperature was elevated to 150 C. to completely dissolve the lactides, and a dilution of 120 ppmw of the catalyst tin 2-ethylhexylate in 500 ml of toluene was introduced into the reactor through the catalyst inlet. Under a nitrogen pressure of 1 kg, the reaction was carried out at 185 C. for 2 hrs, after which phosphoric acid was fed in an amount of 200 ppmw through the catalyst inlet and blended with the reaction mixture for 15 minutes to inactivate the catalyst. Until the pressure reached 0.5 torr, vacuum was applied to remove unreacted L-lactide (about 5 wt % of the initial amount). Then, D-L75 and a dilution of 120 ppmw of the catalyst dibutyltin dilaurate in 500 ml of toluene were introduced through the catalyst inlet into the reactor as shown in Table 1. Under a nitrogen atmosphere, the polymerization was carried out at 190 C. for 1 hr. The molecular weight, Tg, Tm, and so on of the obtained resin were measured and given in Table 1.

(47) G. Examples 1 to 5 and Comparative Examples 1, and 6 to 8: Film Formation

(48) The polylactic acid resins prepared in A to F were dried at 80 C. for 6 hrs under a reduced pressure of 1 torr, and then extruded into a sheet structure using a 30-mm single screw extruder equipped with a T die under the temperature conditions shown in Table 2. The extruded sheets were electrostatically deposited on a casting drum cooled to 5 C. to give unoriented films (undrawn films). They were stretched to 3 times in a machine direction between heating roles under the drawing conditions shown in Table 2. Subsequently, the films were fixed with clips, then stretch to 4 times in a tenter frame, and fixed again in the transverse direction, followed by heat treatment at 120 C. for 60 sec to afford a bi-axially oriented polylactic acid resin film of 20 m thick. The evaluation results of the films are summarized in Table 2.

(49) H. Example 6 and Comparative Examples 2 to 5: Film Formation

(50) The resin compositions or polyols shown in Table 2 were dried at 80 C. for 6 hrs under a reduced pressure of 1 torr, and melt kneaded at 190 C. in a twin screw kneader to give chips of the composition. They were dried at 80 C. for 6 hrs under a reduced pressure of 1 torr, and produced into a bi-axially oriented polylactic acid resin film of 20 m thick in the same manner as in G. The evaluation results of the films are summarized in Table 2.

(51) TABLE-US-00001 TABLE 1 Resin A B C D E F L M O P Q PPDO 2.4 (g) 378.8 542.8 PPDO 2.0 (g) PPDO 1.0 (g) 209.5 PTMEG 3.0 (g) 386.9 PTMEG 2.0 (g) 755.5 PTMEG 1.0 (g) 184.8 PEG 8.0 (g) 2400 800 800 PBSA 11.0 (g) 800 HDI (g) 13.1 21.2 30.5 15.2 44.4 17.1 9.5 10.1 D-L75 (g) 14.9 NCO/OH 0.6 0.8 0.9 0.50 0.70 0.45 0.8 0.7 0.65 OHV (KOHmg/g) 10 6 4 20 6 22 47 3 5.5 5.5 TNPP (g) 4 U626 (g) 2 3 6 0.1 3 PEPQ (g) 4 S412 (g) 2 I-1076 (g) 1 O3 (g) 2 L-Lactide (g) 4000 4000 4000 4000 4000 4000 4000 3600 3600 D-Lactide (g) 4000 4000 400 400 Antioxidant 1000 1000 1000 1500 1500 25 0 750 0 0 0 Content (ppmw) IV (dl/g) 0.95 1.35 1.52 0.64 0.92 0.58 0.2 1.55 Mn (1,000, 75 122 148 60 70 48 14 128 65 60 55 g/mol) Mw (1,000, 148 245 315 115 149 90 26 295 185 150 215 g/mol) MWD 1.97 2.01 2.13 1.92 2.13 1.88 1.86 2.30 2.85 2.50 3.91 Tg ( C. ) 49 42 54 55 31 37 15 65 18 22 17 Tm ( C.) 170 168 172 173 164 167 130 176 85, 145 142 165 Color b 4 3 2 5 6 8 5 4 13 6 6 PU polyol 10% 10% 6% 5% 17% 13% 39% 0% 18% 18% 17% repeating unit Content (wt %) Residual 0.45 0.4 0.3 0.65 0.55 0.5 8 0.3 2.5 1.2 1.5 Monomer Content (wt %)

(52) As shown in the Table 1, resins AE were polylactic acid resins (block copolymers) which were prepared by reacting poly(1,3-propanediol) having a molecular weight of 1000 to 2400 or polytetramethylene glycol having a number average molecular weight of 1000 to 3000 with 1,6-hexamethylene diisocyanate at a molar ratio of NCO/OHV of 0.5 to 0.99 to give a polyurethane polyol repeating unit (or (co)polymer) in which polyether polyol repeating units, such as poly(1,3-propanediol), were linearly linked, and using the same as a soft segment and as an initiator for the polymerization of a hard segment. Furthermore, the polylactic acid resin is polymerized in the presence of a specific content of antioxidant, it can be recognized that the resin exhibits low color-b value because of suppressed yellowing and the residual monomer content is low.

(53) In the polylactic acid resins, the polyurethane polyol repeating unit (or (co)polymer) was found to have an OHV of from 3 to 20, so that they could act as an initiator for the polymerization of polylactic acid repeating units. In addition, the final polylactic acid resins A to E had a weight average molecular weight of 100,000 to 400,000, a molecular weight distribution of 1.80 to 2.15, Tg of 25 to 55 C., and Tm of 160 to 178 C. Given these thermal parameters, the resin can be prepared into chips, and they alone can be produced into films, as the resins exhibit a suitable melt viscosity at a film extrusion temperature, e.g., 200 C. or higher. Furthermore, it was recognized that yellowing was scarcely observed due to low residual lactide content in the resin less than 1 wt % and low color-b value less than 6.

(54) In contrast, it was recognized that resin F in which the content of the amount used of the polyurethane polyol repeating unit (or (co)polymer), the soft segment, was less than 5 wt % showed Tg higher than 55 C. In addition, it was recognized that its color-b value was relatively high because the molecular weight was not sufficient and the antioxidant content, which was 25 ppmw, was lower than the amount of the monomer (lactide) used for forming the polylactic acid repeating unit.

(55) And, resin L was the polylactic acid resin prepared by directly utilizing a poly(1,3-propandiol) having a molecular weight of 2000 and a polyethylene glycol having a molecular weight of 8000 as an initiator for the ring-opening polymerization of L-lactide, without a urethane reaction. In this case, however, the OHV of the initiator was too high to obtain a polylactic acid resin with a desired weight average molecular weight. Furthermore, it was recognized that resin L includes much residual lactide and its Tg was just 15 C. and had low polymerization conversion because it did not include antioxidant. In addition, it was recognized that the resin was too low in melt viscosity to be produced into a film alone at a film extrusion temperature of 200 C. or more.

(56) Resin M was the polylactic acid resin prepared by a ring opening polymerization of L-lactide using a small amount of 1-dodecanol as an initiator according to a conventional preparation method of a polylactic acid resin, without introducing a soft segment (polyurethane polyol repeating unit). This polylactic acid resin alone could be produced into a film at a film extrusion temperature of 200 C. or higher. However, it was found to have a molecular weight distribution of as large as 2.30 which is very broad.

(57) Also, resin O was the polylactic copolymer which was prepared by employing a polyurethane formed from a polyester polyol repeating unit, such as PBSA, instead of the polyether polyol repeating unit, as a soft segment while copolymerizing the polyurethane with lactide in the presence of a ring opening polymerization catalyst, an ester exchange catalyst, and/or an ester amide exchange catalyst. In this polylactic copolymer, the polyurethane was randomly introduced in small segment sizes and copolymerized with the polylactic acid repeating unit during the ester and/or ester amid exchange reaction. Resin O had as wide a molecular weight distribution as 2.85, and its Tg was low and its Tm was relatively low as well. Furthermore, resin O did not include an antioxidant and thus it was recognized that the residual lactide content was relatively high and the color-b value was considerably high.

(58) Finally, resins P and Q were a polylactic copolymer (P) or a branched copolymer (Q) which were prepared by addition polymerization of polyether polyol repeating units with lactide to form a prepolymer and then by subjecting the prepolymer to chain extension with a diisocyanate compound (copolymer P) or to a reaction with a tri-functional isocyanate compound (copolymer Q), respectively. Resins P and Q had as wide a molecular weight distribution as 2.50 and 3.91, and their Tg were low and their Tm were relatively low as well. Furthermore, resins P and Q did not include an antioxidant and thus it was recognized that the residual lactide content was relatively high and the color-b value was considerably high.

(59) TABLE-US-00002 TABLE 2 Example Comparative Example 1 2 3 4 5 6 1 2 3 4 5 6 7 8 Resin 1 A B C D E E 50 M F 40 L 40 PDO PBSA O P Q (wt %) 100 100 100 100 100 100 10 10 100 100 100 Resin 2 M 50 M60 M 60 M 90 M 90 (wt %) Extrusion 220 230 240 200 200 240 240 200 200 200 200 200 200 240 Temp. ( C.) Melt Visco. 1100 1600 2100 580 1000 1400 2000 450 250 1200 1400 1400 1200 1800 (Pa .Math. s) Extrusion X X X X state Drawing 81 80 80 70 80 80 80 80 80 80 80 80 80 80 Temp. ( C.) Drawing 20 20 20 30 20 20 20 20 20 20 20 20 20 20 Time (sec) Drawing 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 Ratio Film Thick. 20 20 20 21 20 20 20 20 20 20 20 20 20 20 (um) Initial 10 15 18 10 12 17 20 7 2.5 15 9 7 6 14 Tensile Strength (kgf/mm.sup.2) MD Initial 13 20 25 14 14 22 26 12 3.1 18 10 8 7 17 Tensile Strength (kgf/mm.sup.2) TD Sum of 23 35 43 24 26 39 46 19 5.6 33 19 15 13 31 Tensile Strength (kgf/mm.sup.2) Elongation 117 140 120 144 160 137 124 114 152 145 135 212 210 85 Rate (%) MD Elongation 70 70 75 78 98 89 86 53 89 66 98 105 98 65 Rate (%) TD F5(kgf/mm.sup.2) 5.3 8 10 5 4.8 9.4 9.8 5.1 1.5 8.7 7.9 5 6 11 MD F5 (kgf/mm.sup.2) 8.1 10 11 7.7 7.8 12 12 9.4 2.1 11 9.8 6.5 6.8 13 TD F100 (kgf/ 8.1 15 16 6.7 12 17 17 7.9 1.8 5.6 6.1 4.2 4.5 8.8 mm.sup.2) MD Young's 236 230 330 212 180 242 386 312 179 338 327 150 160 302 Modulus (kgf/mm.sup.2) MD Young's 295 280 418 319 235 300 460 418 241 419 412 165 175 355 Modulus (kgf/mm.sup.2) TD Sum of 531 510 748 531 415 542 846 730 420 757 739 315 335 657 Young's Modulus (kgf/mm.sup.2) Wave X X X X pattern Pin hole X X X X 100 C. Rate 0.2 0.2 0.2 0.3 0.4 0.3 0.2 0.2 6 5.1 5.5 7.2 3.8 4.7 of weight loss (%) Bleed-out X X Haze (%) 0.2 0.2 0.2 0.3 0.3 0.2 0.7 0.5 0.7 10 14 2.1 1.1 1.8 Transmittance 94 94 94 94 93 94 94 88 87 89 81 84 84 85 (%) Anti- X X X X blocking Property Yellowing X X X X X X Coloration

(60) As shown in the Table 2, the films of Examples 1 to 5 were prepared from the polylactic acid resin compositions of the present invention including a specific content of the antioxidant and the polylactic acid resins which included the soft segment (polyurethane polyol repeating unit) in an amount of 5 to 20 wt % and had the properties of low color-b value, a weight average molecular weight of 100,000 to 400,000, a molecular weight distribution of 1.80 to 2.15, and Tm of 160 to 178 C. Furthermore, the film of Example 6 was prepared by using the composition in which the polylactic acid resin of the present invention (resin E), a general polylactic acid resin (resin M), and the antioxidant were mixed together.

(61) All of the films of Examples 1 to 6 were found to have an initial tensile strength of 10 kgf/mm.sup.2 or higher in both machine direction and transverse direction, which indicates excellent mechanical properties. In addition, they exhibited a total Young's modulus in both machine direction and transverse direction of 750 kgf/mm.sup.2 or less, which reflects excellent flexibility. This optimized range of total Young's modulus was helpful in maintaining a suitable level of stiffness. Also, they were found to be superior in various physical properties including transparency, haze, anti-blocking property, and heat resistance as demonstrated by a rate of weight loss of 3 wt % or less after treatment for 1 hr in a 100 C. hot wind oven, a haze of 5% or less, and a light transmittance of 90% or higher. Furthermore, the films of Examples 1 to 6 had good appearance and were superior in thermal stability, and the color-b change (yellowing coloration) was not severe even after the film extrusion process.

(62) In contrast, the film of Comparative Example 1 which was prepared from general polylactic acid resin M exhibited a total Young's modulus in both machine direction and transverse direction of exceeding 750 kgf/mm.sup.2, so that the flexibility was too insufficient to use the film as a packaging film. In addition, the extrusion state of the film of Comparative Example 3 made from resins M and L together was poor, because of large difference of melt viscosity between the two resins. Wave patterns were also found in the final film. Furthermore, the appearance of the film was poor due to pin holes on the film generated by high content of the residual lactide, and the excessively low Tg of resin L caused a problem to the anti-blocking property. The initial tensile strength, the transparency, and the yellowing coloration were poor as well.

(63) And, in Comparative Examples 4 and 5, the films were formed by just compounding poly(1,3-propanediol) having a number average molecular weight of 2400 and an aliphatic polyester polyol having a number average molecular weight of 11,000 prepared by a polycondensation of 1,4-butanediol, succinic acid, and adipic acid with resin M as plasticizing components, without using the polyurethane polyol repeating unit, the soft segment of the resin. The films of Comparative Examples 4 and 5 had high haze and were poor in yellowing coloration because of the incomplete dispersion of the plasticizing components in the resin, and it was recognized that the plasticizing components bleed out from the surface of the film after time passes.

(64) In addition, resin F of Comparative Example 2 has low molecular weight and thus it could not be extruded into a film. However, it was possible to carry out a film extrusion by compounding the same with general polylactic acid resin M which had no soft segment but the extrusion state was poor and wave patterns were also found in the final film because of large difference of melt viscosity between the two resins. Initial tensile strength and transmittance of the films were also poor due to this. In addition, it was recognized that partial yellowing colorations occurred during the film formation due to the low antioxidant content.

(65) And, the film of Comparative Example 6 was formed of a copolymer including a polyester polyol repeating unit and having a wide molecular weight distribution. This film exhibited relatively good flexibility because polyurethane components responsible for flexibility were randomly introduced as small segment units. Nonetheless, it was difficult to be formed to the film because it exhibited a blocking problem as well as poor heat resistance due to low Tg and Tm, as the polylactic acid repeating units were introduced in relatively small sizes as well. In addition, the film was high in haze with low transparency due to low compatibility between the polyester polyols and the polylactic acids, both responsible for the flexibility. A wide molecular weight distribution appeared due to the ester and/or ester amide exchange reaction during the preparation of the resin incurred non-uniform melt properties, and deterioration in the film extrusion state and mechanical properties.

(66) The films of Comparative Examples 7 and 8 were formed of the resins which were prepared by addition polymerizing of polyether polyol with lactide to form a prepolymer and then by subjecting the prepolymer to urethane reaction with diisocyanate or tri- or higher functional compounds. These resins also had a wide molecular weight distribution and the polyether polyol repeating units in the resins were linearly linked via urethane bonds but it did not satisfy the structural characteristics of the present invention including the polylactic acid repeating units of relatively high molecular weight as the hard segments in addition. These films were also found to exhibit non-uniform melt viscosity and poor mechanical properties. Furthermore, since the block characteristics of the hard segment and the soft segment of the resin were deteriorated and the resin had low Tm and Tg, the resin had low heat resistance followed by difficulties in forming into a film due to a blocking problem.

(67) In addition, the films of Comparative Examples 6 to 8 exhibited quite poor external appearance in the film state due to high residual lactide content and relatively high color-b value, and 100 C. rate of weight loss was commercially inadequate. Furthermore, since the films of Comparative Examples 6 to 8 were required of using excessive catalysts in the preparation process of the resins, degradation of the polylactic acid resins were induced in the film formation or the use. Thus, they were poor in the yellowing coloration of the films and generated pin holes and a significant weight change at high temperatures, exhibiting poor stability.