HK1233288B - Thermally adhesive flexible polylactic acid resin composition - Google Patents

Description

Thermally adhesive flexible polylactic acid resin composition

Technical Field

The present invention relates to a flexible polylactic acid resin composition. Specifically, the present invention relates to a heat-sealable flexible polylactic acid resin composition that can be crystallized under industrially feasible conditions and has a low glass transition temperature, a low melting temperature and a low melting enthalpy, excellent film processability such as extrudability, improved storage stability, and good biodegradability.

Background Art

At present, petroleum-based resins such as polyethylene terephthalate (PET), nylon, polyolefins or plasticized polyvinyl chloride (PVC) are widely used in various applications, such as packaging materials. However, such petroleum-based resins are non-biodegradable and therefore cause environmental pollution, for example, by emitting large amounts of greenhouse gases during waste disposal processes. Recently, due to the gradual depletion of petroleum resources, the use of biomass-based resins (usually polylactic acid resins) has been widely considered as an alternative.

In particular, in recent years, people have become increasingly interested in food packaging films made of biodegradable heat-sealable films. To this end, amorphous polylactic acid resins based on optical isomers of a combination of D-lactide monomers and L-lactide monomers have been developed.

However, the amorphous polylactic acid resin that serves as the matrix resin for heat-sealable films may have poor extrudability during film formation, which is caused by the adhesion that may occur at a temperature higher than its glass transition temperature. Due to the unique low cohesion of polylactic acid resin, it shows low adhesion when heat-sealing. Due to its limited storage conditions, it also has the problem of storage instability. Therefore, compared with conventional linear low-density polyethylene (LLDPE) resins that are commonly used as heat-sealable resins, polylactic acid resin has limited applicability due to its poor adhesion, processability and storage stability.

In addition, compared with petroleum-based resins, polylactic acid resins do not have the required mechanical properties. Polylactic acid resins also have the problem of low flexibility when forming films. In order to solve these problems, many methods have been proposed: for example, adding low molecular weight softeners or plasticizers to polylactic acid resins, introducing plasticizers prepared by addition polymerization of polyols based on polyethers or aliphatic polyesters, etc. However, the flexibility of most packaging films prepared from polylactic acid resins by the methods described is still limited. In addition, the plasticizers may ooze out after a period of time and cause the packaging films prepared therefrom to have high haze and low transparency. Therefore, in recent years, it has been proposed to prepare block copolymers by introducing polyurethane polyol repeating units into polylactic acid resins (see Korean Patent Publication No. 2013-0135758) to overcome these problems.

However, such conventional polylactic acid resins still need to be improved in properties desired for film processing, such as glass transition temperature, melting temperature, crystallization properties, etc.; and mechanical properties desired for heat-sealable films, such as thermal adhesiveness, flexibility, and mechanical strength.

Summary of the Invention

Therefore, an object of the present invention is to provide a polylactic acid resin composition that can be crystallized under industrially feasible conditions and has a low glass transition temperature, a low melting temperature and a low melting enthalpy, excellent film processability such as extrudability, improved storage stability and good biodegradability.

According to one aspect of the present invention, a polylactic acid resin composition is provided, comprising a polylactic acid resin containing a hard segment and a soft segment, the hard segment comprising a polylactic acid repeating unit of Formula 1, the soft segment comprising a polyurethane polyol repeating unit, wherein the polyether-based polyol repeating unit of Formula 2 is linearly connected by a urethane bond, wherein the polylactic acid resin comprises 65 wt % to 95 wt % of the hard segment and 5 wt % to 35 wt % of the soft segment based on the weight of the polylactic acid resin, wherein the polylactic acid repeating unit comprises a poly-L-lactic acid repeating unit and a poly-D-lactic acid repeating unit in a molar ratio of 94:6 to 88:12; and wherein, in Formula 1, n is an integer of 700 to 5,000; and in Formula 2, A is a linear or branched alkylene group of 2 to 5 carbon atoms, and m is an integer of 10 to 100:

[Formula 1]

[Formula 2]

Polylactic acid resin composition according to the present invention can be crystallized under industrially feasible conditions, and has low glass transition temperature, low melting temperature and low melting enthalpy.In addition, described polylactic acid resin composition is eco-friendly because its good biodegradability, and demonstrates improved storage stability and excellent film processability such as extrudability.In addition, when described polylactic acid resin composition is processed into film, it shows excellent thermal adhesiveness, flexibility, mechanical strength and anti-blocking property.

DETAILED DESCRIPTION

Hereinafter, a polylactic acid resin composition according to one embodiment of the present invention will be described in detail.

Polylactic acid resin composition

The polylactic acid resin composition contains polylactic acid resin as a main component.

The polylactic acid resin comprises a hard segment and a soft segment, wherein the hard segment comprises a polylactic acid repeating unit of Formula 1, and the soft segment comprises a polyurethane polyol repeating unit, wherein the polyether-based polyol repeating unit of Formula 2 is linearly connected through a urethane bond (-C(=O)-NH-), wherein, in Formula 1, n is an integer from 700 to 5,000; and in Formula 2, A is a linear or branched alkylene group of 2 to 5 carbon atoms, and m is an integer from 10 to 100:

[Formula 1]

[Formula 2]

Preferably, the polylactic acid resin is a block copolymer in which a hard segment is combined with a soft segment, that is, a polylactic acid-based copolymer resin.

In the polylactic acid resin according to one embodiment of the present invention, the polylactic acid repeating unit of Formula 1 included in the hard segment refers to a repeating unit in which a poly-L-lactic acid repeating unit and a poly-D-lactic acid repeating unit are copolymerized in a specific molar ratio.

The poly-L-lactic acid repeating units may be derived from L-lactide or L-lactic acid, and the poly-D-lactic acid repeating units may be derived from D-lactide or D-lactic acid.

The polylactic acid repeating unit in which the poly-L-lactic acid repeating unit and the poly-D-lactic acid repeating unit are copolymerized may be, for example, a copolymerized repeating unit having an atactic configuration lacking stereoregularity (tacticity) or a heterotactic configuration.

In the polylactic acid repeating unit, the molar ratio of the poly-L-lactic acid repeating unit to the poly-D-lactic acid repeating unit (L:D) is 94:6 to 88:12. When the molar ratio is within this range, a film prepared from the polylactic acid resin composition can have excellent adhesion at a heat sealing temperature of, for example, 100° C. to 130° C. In addition, when the molar ratio is within this range, problems such as clogging of the inlet of the extruder can be prevented, thickness deviation of the molded film can be reduced, and the anti-blocking property of the film can be improved.

Conventional polylactic acid resin prepared by copolymerization of stereoisomers (i.e., poly-L-lactic acid repeating units and poly-D-lactic acid repeating units) in the above-mentioned molar ratio is difficult to crystallize due to the low mobility of the polylactic acid chain. On the contrary, the polylactic acid resin of the present invention can be easily crystallized, wherein the (i) hard segment and (ii) soft segment that give the polylactic acid chain mobility are block copolymerized. Therefore, by preventing the adhesion problem at a temperature higher than its glass transition temperature (Tg) during extrusion or storage, polylactic acid resin can have excellent processability and improved storage stability.

In addition, films prepared from conventional polylactic acid resins exhibit low adhesion during heat sealing due to the low cohesive force of the resin. In contrast, the polylactic acid resin of the present invention can be used to prepare heat-sealable films due to its crystallinity and improved adhesion at temperatures of 100°C to 130°C, in which (i) an amorphous hard segment and (ii) a soft segment having a molecular structure with enhanced cohesive force are block copolymerized.

In addition, the polylactic acid resin of the present invention, which contains polylactic acid repeating units as hard segments, is not only biodegradable like biomass-based resins, but also produces films with excellent mechanical properties. Simultaneously, the polylactic acid resin of the present invention produces films with improved flexibility due to its soft segments. Furthermore, since the hard segments and soft segments combine to form a block copolymer, the exudation of the soft segments can be minimized. Furthermore, the addition of such soft segments prevents the moisture resistance, mechanical properties, heat resistance, transparency, or haze characteristics of the film from deteriorating.

Based on the weight of polylactic acid resin, the polylactic acid resin according to one embodiment of the present invention comprises a hard segment of 65 wt % to 95 wt % and a soft segment of 5 wt % to 35 wt %, preferably a hard segment of 80 wt % to 95 wt % and a soft segment of 5 wt % to 20 wt %, more preferably a hard segment of 82 wt % to 94 wt % and a soft segment of 6 wt % to 18 wt %. When the amount of hard segment is within these ranges, the molecular weight characteristics of the resin can be improved (for example, higher molecular weight and narrower molecular weight distribution). Therefore, the film prepared by the resin can have enhanced mechanical properties in terms of film strength. Meanwhile, when the amount of soft segment is within these ranges, the film prepared by the resin can have good flexibility. In addition, the polyurethane polyol repeating unit can effectively act as an initiator, thereby improving the molecular weight characteristics of the resin.

The polyurethane polyol repeating unit contained in the soft segment has a structure in which the polyether polyol repeating unit of Formula 2 is linearly connected by a carbamate bond (-C(＝O)-NH-). Specifically, the polyol repeating unit based on polyether can be obtained by ring-opening polymerization (copolymerization) of a monomer such as oxyalkylene. The polyol repeating unit based on polyether thus obtained has a hydroxyl group at its end. The terminal hydroxyl group reacts with a diisocyanate compound to form a carbamate bond (-C(＝O)-NH-). In addition, the polyol repeating unit based on polyether is linearly connected by such a carbamate bond, thereby forming a polyurethane polyol repeating unit. Such a polyurethane polyol repeating unit can significantly improve the flexibility of a film prepared from a polylactic acid resin comprising the polyurethane repeating unit as a soft segment. In addition, the polyurethane repeating unit can improve the general properties of a film prepared from a polylactic acid resin comprising the polyurethane repeating unit without compromising the heat resistance, anti-blocking properties, mechanical properties or transparency of the film.

Due to the presence of polylactic acid repeating units and polyurethane polyol repeating units in which a plurality of polyether polyol repeating units are linearly connected by urethane bonds, the polylactic acid resin according to one embodiment of the present invention can significantly improve the flexibility of the film prepared therefrom. In addition, the polylactic acid resin can have desired molecular weight characteristics due to a narrow molecular weight distribution. Due to the presence of relatively large blocks of polylactic acid repeating units, the film prepared by the polylactic acid resin can have excellent mechanical properties, heat resistance and anti-blocking properties. On the contrary, conventional polylactic acid copolymers have problems such as reduced transparency and increased haze value of the film due to the low compatibility between polyester polyol and polylactic acid. These conventional polylactic acid resin copolymers have problems such as poor extrudability of the film and poor mechanical properties, heat resistance and anti-blocking properties due to the wide molecular weight distribution and poor melting characteristics of the copolymer. Such conventional polylactic acid copolymers are prepared by copolymerizing polyether polyol repeating units and polylactic acid repeating units in a branched manner with a trifunctional or higher-functional isocyanate compound, or by copolymerizing polyether polyol repeating units with polylactic acid repeating units and then extending the chain via a urethane reaction. However, these conventional polylactic acid copolymers contain small blocks of polylactic acid repeating units as hard segments. Therefore, due to the wide molecular weight distribution and poor melting properties of the copolymers, they have problems with insufficient heat resistance, mechanical properties, and anti-blocking properties of the films, as well as poor extrudability.

In order to prepare the polyurethane polyol repeating unit, the polyol repeating unit based on polyether is reacted with the diisocyanate compound so that the molar ratio of the terminal hydroxyl group of the polyol repeating unit based on polyether to the isocyanate group of the diisocyanate compound is 1:0.50 to 1:0.99, preferably 1:0.60 to 1:0.90, more preferably 1:0.70 to 1:0.85. The polyurethane polyol repeating unit can act as an initiator for block polymerization with the polylactic acid repeating unit due to the hydroxyl group at its end. When the molar ratio (NCO/OH) of the terminal hydroxyl group to the isocyanate group exceeds 0.99, the number of the terminal hydroxyl group of the polyurethane polyol repeating unit becomes insufficient (for example: OHV <3), making the polyurethane polyol repeating unit unsuitable for acting as an initiator. This can hinder the production of polylactic acid resin with excellent molecular weight characteristics and significantly reduce the polymerization yield. On the other hand, when the molar ratio of terminal hydroxyl groups to isocyanate groups (NCO/OH) is too low, the terminal hydroxyl groups of the polyurethane polyol repeating units become too many (e.g., OHV>21), making it difficult to produce a polylactic acid resin having excellent molecular weight characteristics derived from high molecular weight polylactic acid repeating units.

The polyol repeating unit based on polyether may be a repeating unit of a polyol polymer (copolymer) based on polyether prepared by ring-opening polymerization (copolymerization) of one or more alkylene oxide monomers. Examples of alkylene oxides include ethylene oxide, propylene oxide, butylene oxide, tetrahydrofuran, and the like. Examples of polyol repeating units based on polyether include repeating units of polyethylene glycol (PEG), repeating units of poly(1,2-propylene glycol), repeating units of poly(1,3-propane diol), repeating units of polytetramethylene glycol, repeating units of polybutylene glycol, repeating units of polyols produced by copolymerization of propylene oxide and tetrahydrofuran, repeating units of polyols produced by copolymerization of ethylene oxide and tetrahydrofuran, repeating units of polyols produced by copolymerization of ethylene oxide and propylene oxide, and the like. In order to impart flexibility to the polylactic acid resin film, and in view of its affinity with the polylactic acid repeating unit and water-containing capacity, a repeating unit of poly(1,3-propanediol) or a repeating unit of polytetramethylene glycol is preferred as the polyether-based polyol repeating unit.

The number average molecular weight of the polyol repeating units based on polyethers may be 1,000 to 100,000, preferably 10,000 to 50,000. When the number average molecular weight of the polyol repeating units based on polyethers is too high or too low, the flexibility or mechanical properties of the film prepared from the polylactic acid resin containing the repeating units may be insufficient. Polylactic acid resins with molecular weight characteristics that deviate from the desired number average molecular weight and molecular weight distribution may have insufficient processability or may produce films with degraded mechanical properties.

The diisocyanate compound may be any compound having two isocyanate groups in its molecule. Examples of diisocyanate compounds include 1,6-hexamethylene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 1,3-xylene diisocyanate, 1,4-xylene diisocyanate, 1,5-naphthalene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, 3,3'-dimethyl-4,4'-diphenylmethane diisocyanate, 4,4'-biphenylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, hydrogenated diphenylmethane diisocyanate, etc. In addition, various other diisocyanate compounds well known to those skilled in the art may be used without particular limitation. In order to impart flexibility to the polylactic acid resin film, 1,6-hexamethylene diisocyanate is preferred.

The polylactic acid resin according to one embodiment of the present invention can be a block copolymer in which polylactic acid repeating units in the hard segment are linked to polyurethane polyol repeating units in the soft segment. The terminal carboxyl group of the polylactic acid repeating unit is linked to the terminal hydroxyl group of the polyurethane polyol repeating unit via an ester bond. For example, the chemical structure of the block copolymer can be represented by the following general formula 1:

[General formula 1]

Polylactic acid repeating unit (L)-ester-polyurethane polyol repeating unit (E-U-E-U-E)-ester-polylactic acid repeating unit (L)

wherein E is a polyol repeating unit based on polyether, U is a urethane bond, and ester is an ester bond.

The polylactic acid resin composition of the present invention may include the polylactic acid resin in an amount of 1 wt % or more, specifically 30 wt % or more, 50 wt % or more, 70 wt % or more, or 90 wt % or more.

The polylactic acid repeating units included in the polylactic acid resin composition are not all required to be combined with the polyurethane polyol repeating units to form a block copolymer. At least some of the polylactic acid repeating units may be in the form of a polylactic acid homopolymer resin and are not combined with the polyurethane polyol repeating units. In this case, the polylactic acid resin composition may include a block copolymer and a polylactic acid homopolymer resin that is not combined with the polyurethane polyol repeating units. In addition, the polylactic acid homopolymer resin may include poly-L-lactic acid repeating units and poly-D-lactic acid repeating units in a molar ratio of 94:6 to 88:12. Based on the gross weight of the polylactic acid resin composition, the polylactic acid resin composition of the present invention may include a polylactic acid homopolymer resin in an amount of 1 wt % to 30 wt %.

The polylactic acid resin composition may also contain a phosphorus-based stabilizer and/or antioxidant to prevent the soft segment from being oxidized or thermally degraded. Examples of antioxidants include hindered phenol-based antioxidants, amine-based antioxidants, thio-based antioxidants, phosphite-based antioxidants, and the like. Suitable stabilizers and antioxidants are well known to those skilled in the art.

In addition to the above-mentioned stabilizers and antioxidants, the polylactic acid resin composition may also contain various known additives, such as plasticizers, UV stabilizers, color inhibitors, anti-gloss agents, deodorants, flame retardants, weathering agents, antistatic agents, release agents, antioxidants, ion exchangers, coloring pigments, and inorganic and organic particles, as long as they do not adversely affect the general properties of the polylactic acid resin composition.

Examples of plasticizers include: phthalate-based plasticizers, such as diethyl phthalate, dioctyl phthalate, dicyclohexyl phthalate, etc.; aliphatic dibasic acid ester-based plasticizers, such as di-1-butyl adipate, di-n-octyl adipate, di-n-octyl sebacate, di-2-ethylhexyl azelate, etc.; phosphate-based plasticizers, such as diphenyl 2-ethylhexyl phosphate, diphenyl octyl phosphate, etc.; polyhydroxy carbonate-based plasticizers, such as acetyl tributyl citrate, acetyl tri-2-ethylhexyl citrate, tributyl citrate, etc.; fatty acid ester-based plasticizers, such as methyl acetylic ricinoleate, amyl stearate, etc.; polyol ester-based plasticizers, such as triacetin, etc.; and epoxide-based plasticizers, such as epoxidized soybean oil, epoxidized butyl ester of linseed oil fatty acid, epoxyoctyl stearate, etc.

Examples of the coloring pigment may include inorganic pigments such as carbon black, titanium oxide, zinc oxide, iron oxide, etc.; and organic pigments such as cyanine, phosphine, quinone, perinone, isoindolinone, and thioindigo.

Inorganic particles or organic particles may be used to improve the anti-blocking properties of the film, and examples thereof may include silica, colloidal silica, alumina, alumina sol, talc, mica, calcium carbonate, polystyrene, polymethyl methacrylate, silicon, and the like.

In addition, various additives suitable for polylactic acid resin or films prepared therefrom may be used, and their types and purchase channels are well known to those skilled in the art.

As described above, the polylactic acid resin composition comprises a polylactic acid resin (as a block copolymer) prepared by reacting a polyether polyol repeating unit with a diisocyanate compound to produce a polyurethane polyol repeating unit in which a plurality of polyether polyol repeating units are linearly connected via a urethane bond, and then block copolymerizing the polyurethane polyol repeating unit with a polylactic acid repeating unit. Therefore, compared to conventional polylactic acid copolymers, the polylactic acid resin composition, particularly the polylactic acid resin (as a block copolymer) contained therein, can have a relatively high number average molecular weight and a relatively narrow molecular weight distribution.

The polylactic acid resin composition, particularly the polylactic acid resin (as a block copolymer) contained therein, can have a number average molecular weight (Mn) of 50,000 to 200,000, preferably 50,000 to 150,000, and a weight average molecular weight (Mw) of 100,000 to 400,000, preferably 100,000 to 320,000. When the number average molecular weight and the weight average molecular weight are within these ranges, the film prepared by the resin composition can have excellent processability and mechanical properties such as film strength, etc.

The polylactic acid resin composition, particularly the polylactic acid resin (as block copolymer) included in the composition, can have a molecular weight distribution (Mw/Mn, defined as the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn)) of 1.60 to 2.30, preferably 1.80 to 2.20. When the molecular weight distribution is within these ranges, the polylactic acid resin has a suitable melt viscosity and a characteristic for melt processing (e.g., extrusion), so that film formation has excellent extrudability and processability. The film prepared by the resin has good mechanical properties such as film strength. If the number average molecular weight is too high or the molecular weight distribution is too narrow, the melt viscosity of the resin becomes too high at the processing temperature of the resin extrusion, so that it is difficult to process the polylactic acid resin into a film. On the contrary, if the number average molecular weight is too low or the molecular weight distribution is too wide, the mechanical properties (e.g., strength) of the film may deteriorate and the melt properties of the resin may be poor, e.g., the melt viscosity is low; Therefore, the resin cannot or is difficult to be processed into a film.

The polylactic acid resin composition, particularly the polylactic acid resin (as block copolymer) included in the composition, can have a glass transition temperature (Tg) of 30°C to 50°C, preferably 40°C to 50°C. When the glass transition temperature of the polylactic acid resin composition falls within these ranges, the film prepared by the resin composition can have optimal flexibility and rigidity, and therefore, these films can be used as packaging films. If the glass transition temperature is too low, although the film can have improved flexibility, due to significantly lower rigidity, it may have poor sliding properties, workability, dimensional stability or anti-blocking properties. On the contrary, if the glass transition temperature is too high, the film can have low flexibility and high rigidity, and therefore, it can be easily folded and the folding marks thus formed will not be easy to disappear. In addition, these films show poor adhesion and harsh noise.

The polylactic acid resin composition, particularly the polylactic acid resin (as a block copolymer) contained in the composition, may have a melting temperature (Tm) of 100° C. to 130° C., preferably 110° C. to 120° C. When the melting temperature is within these ranges, the resin is easily processed into a film with improved heat resistance.

The polylactic acid resin composition, particularly the polylactic acid resin (as a block copolymer) contained in the composition, may have a melting enthalpy (ΔHm) of 5 J/g to 30 J/g, preferably 10 J/g to 20 J/g. The melting enthalpy of the resin composition falling within these ranges allows for stable extrusion due to crystallization of the resin and allows the film to have sufficient thermal adhesion at heat sealing temperatures (e.g., 100° C. to 130° C.).

Since the polylactic acid resin composition is prepared to have desired molecular weight characteristics in terms of specific weight average molecular weight and molecular weight distribution, it is easy to process into a film by melt processing such as extrusion. In addition, the film prepared therefrom has good mechanical properties, including strength.

For example, a polylactic acid resin composition containing a polylactic acid resin (as a block copolymer) can be melt-processed, such as extruded, at a temperature of 150°C to 190°C. Within this temperature range, the composition can have a melt viscosity of 1,500 Pa·s to 3,500 Pa·s, preferably 1,700 Pa·s to 3,000 Pa·s. As a result, the polylactic acid resin composition can be easily extruded into films with excellent properties with significantly improved productivity.

As mentioned above, polylactic acid resin composition of the present invention comprises polylactic acid resin as main component, and described polylactic acid resin comprises the optical isomer of the polylactic acid repeating unit of a certain amount and the polyurethane polyol repeating unit as soft segment of a certain amount.Therefore, polylactic acid resin composition has low glass transition temperature, low melting temperature and low melting enthalpy, and can be crystallized under industrially feasible condition (for example, in crystallization chamber 3 hours or shorter).In addition, polylactic acid resin composition is eco-friendly and demonstrates excellent film forming processability (for example, by extruding) and improved storage stability due to its good biodegradability.In addition, the film prepared by polylactic acid resin composition can have excellent thermal adhesiveness, flexibility, mechanical properties and anti-blocking characteristics.

Preparation of polylactic acid resin composition

The following describes a method for preparing a polylactic acid resin composition according to one embodiment of the present invention.

The preparation method of the polylactic acid resin composition may include the following steps: (a) subjecting at least one monomer such as an alkylene oxide to ring-opening polymerization (copolymerization) to obtain a polymer (copolymer) containing polyether-based polyol repeating units; (b) reacting the polymer (copolymer) containing polyether-based polyol repeating units with a diisocyanate compound in the presence of a catalyst to obtain a polymer (copolymer) containing polyurethane polyol repeating units; and (c) polycondensing D-lactic acid and L-lactic acid at a specific molar ratio, or ring-opening polymerization of D-lactide and L-lactide at a specific molar ratio, in the presence of the polymer (copolymer) containing polyurethane polyol repeating units.

As monomers, L-lactic acid and D-lactic acid can be used in a molar ratio (L:D) of 94:6 to 88:12, or L-lactide and D-lactide can be used in a molar ratio (L:D) of 94:6 to 88:12. In addition, 65% to 95% by weight of polylactic acid repeating units (i.e., hard segments) and 5% to 35% by weight of polyurethane polyol repeating units (i.e., soft segments) can be copolymerized.

The polylactic acid resin composition can be prepared by subjecting a polymer (copolymer) comprising a polyether-based polyol repeating unit to a urethane reaction with a diisocyanate compound in the presence of a catalyst to obtain a polymer (copolymer) comprising a polyurethane polyol repeating unit, wherein the polyether-based polyol repeating units are linearly connected through a urethane bond, and then copolymerizing the polyurethane polyol repeating unit with lactic acid (D-lactic acid and L-lactic acid) or lactide (D-lactide and L-lactide) in the presence of a catalyst.

The polylactic acid resin including the block copolymer and having excellent physical properties can be prepared by the above method.

In this method, as described above, by controlling the molar ratio of the polymer (copolymer) containing polyether-based polyol repeating units to the diisocyanate compound, the molecular weight of the polyether-based polyol polymer (copolymer), or the amount of the polymer (copolymer) containing polyurethane polyol repeating units serving as a soft segment, the polylactic acid resin can have desired physical properties, for example, excellent molecular weight properties. Such molar ratio, molecular weight of the polyether-based polyol polymer (copolymer), etc. are as described above.

Hereinafter, a method for preparing a polylactic acid resin composition according to one embodiment of the present invention will be described in more detail.

First, at least one monomer such as an alkylene oxide is ring-opening polymerized (copolymerized) to obtain a polymer (copolymer) containing a polyether-based polyol repeating unit. This step can be performed by conventional polymerization for preparing a polyether-based polyol polymer (copolymer).

Subsequently, the polymer (copolymer) comprising the polyol repeating unit based on polyether, diisocyanate compound and carbamate reaction catalyst are loaded into the reactor, and carry out carbamate reaction while being heated and stirred.Through this reaction, two isocyanate groups of diisocyanate compound are combined with the terminal hydroxyl group of polymer (copolymer) to form carbamate bond.As a result, a polymer (copolymer) comprising polyurethane polyol repeating unit is formed, wherein the polyol repeating unit based on polyether is linearly connected by carbamate bond.The polyurethane polyol repeating unit serves as the soft segment in polylactic acid resin.The polyurethane polyol repeating unit can be in the form of E-U-E-U-E, wherein the polyol repeating unit (E) based on polyether is linearly connected by carbamate bond (U), and the polyol repeating unit based on polyether is positioned at two ends.

The urethane reaction can be carried out in the presence of a tin catalyst, such as stannous octoate (tin (II) 2-ethylhexanoate), dibutyltin dilaurate, dioctyltin dilaurate, etc. In addition, the urethane reaction can be carried out under typical reaction conditions for preparing polyurethane resins. For example, a diisocyanate compound is reacted with a polyether-based polyol polymer (copolymer) at 70° C. to 80° C. in the presence of a urethane reaction catalyst under a nitrogen atmosphere for 1 to 5 hours to produce a polymer (copolymer) comprising polyurethane polyol repeating units.

Subsequently, a polylactic acid resin composition including a block copolymer according to one embodiment may be prepared by polycondensation of lactic acid (D-lactic acid and L-lactic acid) or by ring-opening polymerization of lactide (D-lactide and L-lactide) in the presence of a polymer (copolymer) including a polyurethane polyol repeating unit.

This polymerization produces polylactic acid repeating units, while the terminal groups of at least some of the polylactic acid repeating units are combined with polyurethane polyol repeating units to produce a block copolymer.

As a result, a block copolymer according to one embodiment of the present invention can be prepared, which is completely different in structure and molecular weight characteristics from a conventional polylactic acid copolymer prepared by chain-extending a prepolymer composed of a polyether polyol and polylactic acid through a diisocyanate compound, or a conventional branched copolymer prepared by reacting such a prepolymer with a trifunctional or higher-functional isocyanate compound.

In particular, the block copolymer according to one embodiment contains polylactic acid repeating units as relatively large blocks (i.e., blocks with a large molecular weight), so that a film made of a polylactic acid resin containing the block copolymer can have a narrow molecular weight distribution and an appropriate Tg, and thus can have excellent mechanical properties and heat resistance.

Meanwhile, the ring-opening polymerization of lactide can be carried out in the presence of a metal catalyst such as an alkaline earth metal, a rare earth metal, a transition metal, aluminum, germanium, tin, or antimony. Specifically, the metal catalyst can be in the form of a carbonate, an alkoxide, a halide, an oxide, or a carbonate/ester. Preferred metal catalysts include stannous octoate, titanium tetraisopropoxide, or aluminum triisopropoxide.

Hereinafter, the effects and functions of the present invention will be described in more detail by way of the following examples. However, these examples are provided for illustrative purposes only, and the scope of the present invention is not limited thereto.

Definitions and measurement methods of physical properties

The physical properties described in the following examples were defined and measured as follows.

(1) NCO/OH: the molar ratio of "isocyanate group (NCO) in the diisocyanate compound/terminal hydroxyl group (OH) in the polyether-based polyol repeating unit" used in the reaction to form the polyurethane polyol repeating unit.

(2) OHV (KOH mg/g): Measured by dissolving polyurethane polyol repeating units in dichloromethane, acetylating the repeating units, hydrolyzing the acetylated repeating units to produce acetic acid, and titrating the acetic acid with 0.1N KOH in methanol. This indicates the number of terminal hydroxyl groups in the polyurethane polyol repeating units.

(3) Mw, Mn, and molecular weight distribution (MWD): Measured by gel permeation chromatography (Viscotek TDA 305, column: Shodex LF804×2 ea.) of a 0.25 wt% solution of polylactic acid resin in chloroform. Polystyrene was used as a standard material for determining the weight average molecular weight (Mw) and number average molecular weight (Mn). Molecular weight distribution (MWD) was calculated as Mw/Mn.

(4) Tg (glass transition temperature, °C): Measured using a differential scanning calorimeter (TA Instruments) by quenching a molten sample and then raising the sample temperature at a rate of 10°C/min. Tg is determined from the midpoint of the baseline and the tangent line on the endothermic curve.

(5) Tm (melting temperature, °C): Measured using a differential scanning calorimeter (TA Instruments) by quenching a molten sample and then raising the sample temperature at a rate of 10°C/min. Tm is determined from the maximum value of the melting endothermic peak of the crystal.

(6) Content of polyurethane polyol repeating units (wt%): The content of polyurethane polyol repeating units in each polylactic acid resin was measured using a 600 MHz nuclear magnetic resonance (NMR) spectrometer.

(7) Melting enthalpy (ΔHm, J/g): Measured using a differential scanning calorimeter (TA Instruments) by quenching a molten sample and then raising the sample temperature at a rate of 10°C/min. The melting enthalpy is determined by integrating the enthalpy above the baseline below the melting endotherm of the crystal.

(8) Melt viscosity and extrudability: To prepare a biaxially oriented film, a polylactic acid resin was extruded into a sheet form at 200° C. to 250° C. using a 30 mm single-screw extruder equipped with a T-die, and the sheet was electrostatically cast onto a cylinder cooled at 5° C. The melt viscosity of the sheet extrudate was measured using a rheometer (Physica, USA). Specifically, a shear force was applied at a shear rate (1/s) of 1 using a 25 mm parallel plate type instrument while maintaining the initial temperature of the extrudate, and the complex viscosity (Pa·s) of the molten resin was measured using the rheometer.

Furthermore, extrudability was evaluated according to the following criteria.

◎: Good melt viscosity and constant discharge pressure

Δ: The melt viscosity is slightly lower, but the discharge pressure is constant

×: The discharge pressure is not constant and the extrusion film is poor

(9) Film thickness deviation (%): The thickness of the stretched film was measured by a digital thickness gauge (Mitutoyo, Japan).

(10) Initial Tensile Strength (kgf/ ^mm² ) MD, TD: Film samples 150 mm in length and 10 mm in width were conditioned at 20°C and 65% RH for 24 hours, and the tensile strength was measured using a universal testing machine (UTM, Instron) at a pulling speed of 300 mm/min with a 100 mm inter-grip distance according to ASTM D638. The average of five measurements is shown. MD and TD represent the machine direction and transverse direction of the film, respectively.

(11) Elongation (%) MD, TD: The elongation was measured at the time the film was torn under the same conditions as the tensile strength test in (10). The average value of 5 measurements is shown. MD and TD represent the machine direction and transverse direction of the film, respectively.

(12) Young's modulus (kgf/ ^mm² ) MD, TD: The Young's modulus of the same film sample as in the tensile strength test in (10) was measured using a universal testing machine (UTM, Instron) with a grip distance of 100 mm and a pulling speed of 300 mm/min according to ASTM D638. The Young's modulus, specifically the sum of the Young's modulus values measured in the machine direction and the transverse direction, represents the flexibility of the film. The lower the Young's modulus, the higher the film flexibility. MD and TD represent the machine direction and transverse direction of the film, respectively.

(13) Anti-blocking properties: A COLORIT P-type (Kurz) printed surface was placed on the antistatic surface of the film sample and then allowed to stand at 40°C for 24 hours under a pressure of 1 kg/ ^cm² . The blocking between the antistatic surface and the printed surface was observed. The anti-blocking properties of the film between its antistatic surface and the printed surface of the in-mold transfer foil were evaluated according to the following criteria. A score of at least 0 ensured practical performance.

◎: No change

○: Slight surface change (5% or less)

×: Peeling greater than 5%

(14) Thermal Adhesion: A heat-sealable film sample was subjected to thermal bonding at a temperature of 100° C. to 130° C. under a pressure of 2 kgf/cm ² and a bonding time of 2 seconds, and then the bonded film was peeled off at a bonding strength of (gf/15 mm). Thermal adhesion was evaluated according to the following criteria.

◎: Properly bonded, and no delamination was observed in the bonded area

Δ: Properly bonded, but delamination was observed in the bonded area

×: No combination

Base material

The materials used in the following examples and comparative examples are as follows:

(1) Polyol repeating units based on polyether or its counterpart

-PPDO 2.0: poly(1,3-propylene glycol), number average molecular weight 2,000

-PPDO 2.4: poly(1,3-propylene glycol), number average molecular weight 2,400

-Lauryl alcohol

(2) Diisocyanate compound—HDI: Hexamethylene diisocyanate

(3) Lactide monomers—L-lactide and D-lactide: from Purac

(4) Antioxidants, etc.

-TNPP: tris(nonylphenyl) phosphite

-U626: Bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite

-PEPQ: (1,1'-biphenyl)-4,4'-diylbisphosphonite tetrakis[2,4-bis(1,1-dimethylethyl)phenyl]ester

-S412: Tetrakis[methane-3-(laurylthio)propionate]methane

-I-1076: Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate

-O3: Bis[3,3-bis-(4'-hydroxy-3'-tert-butyl-phenyl)butyrate]ethylene glycol ester

Example: Preparation of polylactic acid resins A to E

The reactants and catalyst listed in Table 1 below were loaded into an 8L reactor equipped with a nitrogen line, a stirrer, a catalyst inlet, an effluent condenser, and a vacuum system. Dibutyltin dilaurate was used as a catalyst in an amount of 80 ppm based on the total weight of the reactants. Under a nitrogen atmosphere, a urethane reaction was carried out at 70°C for 2 hours. Afterwards, a total of 4 kg of L-lactide and D-lactide, as listed in Table 1, were introduced, followed by five nitrogen purges.

Then, the reaction mixture was heated to 150°C to completely dissolve L-lactide and D-lactide. The catalyst 2-ethylhexanoate tin diluted in 100mL toluene was fed to the reactor at a concentration of 100ppm based on the total weight of the reactants. Under a nitrogen pressure of 1kg, the reaction was carried out at 185°C for 2 hours, followed by addition of 200ppm of phosphoric acid through the catalyst inlet and mixing for 15 minutes to deactivate the residual catalyst. Then, vacuum was applied to the reactor until the pressure reached 0.5 torr to remove unreacted L-lactide and D-lactide (about 5% by weight of the initial feed weight). The physical properties of the resulting resin are measured and shown in Table 1.

Comparative Example: Preparation of Polylactic Acid Resins F to J

The steps of Examples for preparing polylactic acid resins A to J were repeated except that D-lactide was not added or the amount of the reactants was not within the range specified in the present invention as shown in Table 1. The molecular weight, Tg, Tm, ΔHm, etc. of the obtained resins were measured and shown in Table 1.

Experimental Example: Preparation of Thermal Adhesive Film

At least one of the polylactic acid resins A to J was dried at 80° C. for 6 hours under a reduced pressure of 1 Torr and then extruded into a sheet form using a 30 mm single-screw extruder equipped with a T-die under the temperature conditions shown in Table 2. The extruded sheet was electrostatically stretched onto a cylinder cooled to 5° C. to obtain a non-oriented film.

The resulting non-oriented film was stretched 3x in the machine direction (MD) between heated rollers under the drawing conditions shown in Table 2. The uniaxially oriented film was secured with clips and then stretched 4x in the transverse direction using a tenter. The film was then secured in the transverse direction and heat-treated at 120°C for 60 seconds to obtain a biaxially oriented polylactic acid resin film. The evaluation results of the film are summarized in Tables 2 and 3.

[Table 1]

[Table 2]

[Table 3]

As shown in Table 1 above, resins A to E prepared according to the present invention (L-lactide/D-lactide = 94/6 to 88/12, soft segment content (polyurethane polyol repeating unit) = 5% to 35% by weight) have a Tg of 50°C or less, a Tm of 130°C or less, and a melting enthalpy (ΔHm) of 20 J/g or less, and also exhibit excellent molecular weight characteristics. As shown in Table 2 above, resins A to E exhibit uniform extrudability without any clogging at the inlet during extrusion, indicating that these resins can be crystallized under industrially feasible conditions.

In contrast, as shown in Table 1 above, resins F to J falling outside the scope of the present invention had Tm of 130° C. or higher, melting enthalpy (ΔHm) of 20 J/g or higher, and poor extrudability due to clogging at the inlet during extrusion, as shown in Table 2.

As shown in Table 2, films A to E prepared according to the present invention had a tolerable thickness deviation of about ±5%, excellent anti-blocking properties, and good general film properties such as tensile strength, elongation, Young's modulus, etc. As shown in Table 3, these films exhibited excellent thermal adhesion in the temperature range of 100° C. to 130° C.

However, films F and G prepared from resins F and G outside the scope of the present invention had very poor thermal adhesion. Films H and I prepared from resins H and J had poor thickness deviation and anti-blocking properties. In addition, film J using resins I and J showed poor thermal adhesion. Films K and L, in which an excessive amount of a resin outside the scope of the present invention (resin G) was mixed, had poor thermal adhesion even though they contained a resin within the scope of the present invention (resin A or E).

Claims (12)

1. A polylactic acid resin composition comprising: A polylactic acid resin containing hard segments and soft segments, wherein the hard segments comprise polylactic acid repeating units of Formula 1, and the soft segments comprise polyurethane polyol repeating units, wherein polyether-based polyol repeating units of Formula 2 are linearly linked by urethane bonds, wherein, based on the weight of the polylactic acid resin, the polylactic acid resin comprises 65% to 95% by weight of the hard segments and 5% to 35% by weight of the soft segments, wherein the polylactic acid repeating units comprise poly-L-lactic acid repeating units and poly-D-lactic acid repeating units in a molar ratio of 94:6 to 88:12; and wherein, in Formula 1, n is an integer from 700 to 5,000; and in Formula 2, A is a straight-chain or branched alkylene group of 2 to 5 carbon atoms, and m is an integer from 10 to 100: [Formula 1] [Equation 2] The polylactic acid resin has a glass transition temperature (Tg) of 30°C to 50°C and a melting temperature (Tm) of 100°C to 130°C. 2. The polylactic acid resin composition according to claim 1, wherein the polylactic acid resin comprises 80% to 95% by weight of the hard segments and 5% to 20% by weight of the soft segments. 3. The polylactic acid resin composition according to claim 1, wherein the poly-L-lactic acid repeating unit and the poly-D-lactic acid repeating unit are derived from L-lactide and D-lactide, respectively. 4. The polylactic acid resin composition according to claim 1, wherein the urethane bond is formed by reacting the terminal hydroxyl group of the polyether-based polyol repeating unit with the isocyanate group of the diisocyanate compound. 5. The polylactic acid resin composition according to claim 4, wherein the molar ratio of the terminal hydroxyl group of the polyether-based polyol repeating unit to the isocyanate group of the diisocyanate compound is from 1:0.50 to 1:0.99. 6. The polylactic acid resin composition according to claim 1, wherein the number average molecular weight of the polyether-based polyol repeating unit is from 1,000 to 100,000. 7. The polylactic acid resin composition according to claim 1, wherein the polylactic acid resin is a block copolymer in which the hard segment is combined with the soft segment. 8. The polylactic acid resin composition according to claim 7, wherein the terminal carboxyl group of the polylactic acid repeating unit included in the hard segment is bonded to the terminal hydroxyl group of the polyurethane polyol repeating unit via an ester bond to form the block copolymer. 9. The polylactic acid resin composition according to claim 1, wherein the polylactic acid resin composition comprises 1% to 30% by weight of polylactic acid repeating units not bound to the polyurethane polyol repeating units. 10. The polylactic acid resin composition according to 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. 11. The polylactic acid resin composition according to claim 1, wherein the glass transition temperature (Tg) is 40°C to 50°C and the melting temperature (Tm) is 100°C to 130°C. 12. The polylactic acid resin composition according to claim 1, wherein the melting enthalpy (ΔHm) is 10 J/g to 20 J/g.