Patent Description:
Polylactic acid (PLA) is a plant-derived resin obtained from plants such as corn, and is attracting attention as an environmentally friendly material having biodegradable characteristics and at the same time being excellent in tensile strength and elastic modulus.

Unlike petroleum-based resins conventionally used, such as polystyrene resin, polyvinyl chloride resin and polyethylene, polylactic acid has effects such as prevention of depletion of petroleum resources and suppression of carbon dioxide emission, and thus can reduce an environmental pollution, which is a disadvantage of petroleum-based plastic products. Therefore, as the problem of environmental pollution caused by waste plastics has emerged as a social problem, efforts are being made to expand the scope of application up to product fields where general plastics (petroleum-based resins) were used, such as food packaging materials, containers, and electronic product cases.

Meanwhile, polylactic acid is produced by polymerizing lactic acid produced by microbial fermentation, but direct polymerization of lactic acid produces only low molecular weight polymers. In order to synthesize high molecular weight polylactic acid, a method of polymerizing polylactic acid with a higher molecular weight using a chain coupling agent from polylactic acid with a low molecular weight obtained by direct polymerization of lactic acid is known, but there are disadvantages in that the process is complicated, a coupling agent and an organic solvent are used together, and thus, their removal is not easy.

In the production process of high-molecular-weight polylactic acid commercialized at present, a chemical synthesis method of converting lactic acid into lactide and synthesizing polylactic acid through a ring-opening reaction of lactide is used. However, even in this case, there is a problem that it is difficult to achieve a high molecular weight suitable for commercialization.

The use of tin octylate for preparing polylactic acid polymers is known, for example, from <CIT>.

In the case of a catalyst of Sn(Oct)<NUM>, which is conventionally used to achieve high molecular weight, there was a problem that it has a high viscosity as a liquid catalyst, is difficult to measure accurately, and is weak against oxygen and moisture, so the stability of the catalyst itself is lowered. Particularly, there is a problem that the Sn(Oct)<NUM> catalyst is rapidly decomposed at high temperature, which adversely affects the color of the prepared resin and also lowers the economic efficiency.

Therefore, there is a need to develop a method for preparing a polylactic acid polymer that has excellent physical properties and can easily attain a high molecular weight so that it can be applied to various industries.

It is an object of the present disclosure to provide a method for preparing a polylactic acid polymer that has a desired degree of high molecular weight by using a specific catalyst in a lactide ring-opening polymerization reaction and controlling the rate of change in the molecular weight of the polymer according to changes in polymerization temperature within a specific range.

It is another object of the present disclosure to provide a method for preparing a polylactic acid polymer which facilitates the removal and reuse of the catalyst after completion of the reaction and is excellent in economy by using a catalyst exhibiting excellent stability even under high-temperature polymerization reaction conditions, and also which does not cause changes in physical properties of the polymer due to catalytic decomposition.

In order to achieve the above object, according to the present disclosure, there is provided a method for preparing a polylactic acid polymer, comprising the following step:.

Unless particularly mentioned herein, the term "including" or "comprising" refers to including some element (or component) without any limitation, and should not be construed as excluding addition of other elements (or components).

In the production process for polylactic acid commercialized at present, a chemical synthesis method of synthesizing polylactic acid through a ring-opening reaction of a lactide ring is used. However, there is a problem that it is difficult to achieve a high molecular weight suitable for commercialization. Particularly, in the case of a Sn(Oct)<NUM> catalyst used to achieve a high molecular weight, there was a problem that it has a high viscosity as a liquid catalyst, is difficult to measure accurately, and is weak against oxygen and moisture, so the stability of the catalyst itself is lowered. Particularly, there was a problem that the Sn(Oct)<NUM> catalyst is rapidly decomposed at high temperature, which adversely affects the color of the resin to be produced and also lowers the economic efficiency.

Therefore, the present inventors have found that in the ring-opening polymerization reaction of lactic acid oligomer, a SnHPO<NUM> catalyst having excellent high-temperature stability is used and the reaction conditions are controlled, whereby a polylactic acid polymer having a high molecular weight can be easily prepared without reducing the catalytic activity even under high temperature conditions, and completed the present disclosure. Moreover, the catalyst can be easily removed and reused after the completion of the reaction, and thus is excellent in economy.

According to one embodiment of the present disclosure, the method includes a step of performing a ring-opening polymerization of lactide in the presence of a SnHPO<NUM> catalyst to prepare a polylactic acid polymer.

The SnHPO<NUM> catalyst is tin(II) phosphite, which is a tin (Sn) salt of a phosphite anion (PO<NUM>-). The SnHPO<NUM> catalyst is not decomposed even under high-temperature conditions required for the ring-opening polymerization reaction of lactide, so that it is easy to remove and reuse the catalyst after completion of the reaction, and thus is excellent in economy. In addition, the SnHPO<NUM> catalyst is conveniently used as a solid powder-type heterogeneous catalyst.

The SnHPO<NUM> catalyst may be contained in an amount of <NUM> ppmmol to <NUM>,<NUM> ppmmol, preferably, <NUM> ppmmol to <NUM> ppmmol, <NUM> ppmmol to <NUM> ppmmol, <NUM> ppmmol to <NUM> ppmmol relative to the lactide. Within the above range, the ring-opening polymerization can be promoted and at the same time, the formation of by-products can be suppressed.

The ring-opening polymerization can be preferably performed at <NUM> or more, <NUM> to <NUM> or <NUM> to <NUM>. Even if the SnHPO<NUM> catalyst is used in the above temperature range, the polymerization reaction can be easily performed without changing the activity of the catalyst. Moreover, by performing the ring-opening polymerization in the above temperature range, a polylactic acid polymer having a desired high molecular weight can be easily formed and the generation of by-products can be minimized. Further, a polylactic acid polymer having a desired high molecular weight can be easily prepared without decomposing the SnHPO<NUM> catalyst and without changing the color of the polymer. Meanwhile, when the reaction temperature is less than <NUM>, the activity of the ring-opening polymerization may be lowered.

More preferably, the ring-opening polymerization can be performed at <NUM> or more, <NUM> or more, <NUM> or more, or <NUM> or less. Within the above range, a polylactic acid polymer having a desired high molecular weight can be easily prepared without the above-mentioned problems.

The ring-opening polymerization reaction is performed in the presence of a SnHPO<NUM> catalyst. Particularly, the rate of change in the weight average molecular weight defined by the following Equation <NUM> has a positive value: <MAT>.

The rate of change is the rate of change in the weight average molecular weight of polylactic acid prepared by increasing the ring-opening polymerization reaction temperature by <NUM>, and the rate of change may be interpreted as an index explaining the degree of decomposition of the catalyst at high temperature. The fact that the rate of change has a positive value means that the weight average molecular weight of the polymer increases as the polymerization reaction temperature increases. Theoretically, it means more than <NUM>%, but practically, it can mean about <NUM>% or more, about <NUM>% or more, or about <NUM>% or more.

In the case of Sn(Oct)<NUM> conventionally used in the ring-opening polymerization, there was a problem that since the catalyst is decomposed within the high polymerization temperature of the ring-opening reaction, the weight average molecular weight of the polymer is rather decreased, which makes it difficult to realize the desired high molecular weight. Therefore, the ring-opening polymerization reaction according to the present disclosure have solved such problems while the rate of change in the weight average molecular weight has a positive value as described above.

Preferably, the rate of change in the weight average molecular weight may be <NUM>% to <NUM>%, <NUM>% to <NUM>% or <NUM>% to <NUM>%. Even when the polymerization reaction is performed at a high temperature within the above range, it is suitable for realizing excellent high molecular weight without decomposing the catalyst, and is economical because the catalyst can be recycled.

The measurement of the weight average molecular weight of the polymer will be described in more detail in Experimental Examples described later.

On the other hand, if necessary, before the ring-opening polymerization, the polymerization reactor is replaced with an inert condition. Specifically, after fastening the reactor, a vacuum-argon purge is performed about three times, so that the inside of the reactor can be replaced with an inert condition.

Further, if necessary, before the ring-opening polymerization, the lactide and SnHPO<NUM> catalyst may be each independently pretreated at <NUM> to <NUM> and <NUM> mbar to <NUM> mbar for <NUM> hour to <NUM> hours. Oxygen, moisture, impurities, etc. inside the lactide and SnHPO<NUM> catalyst may be removed through the pretreatment step.

According to one embodiment of the present disclosure, there is provided a polylactic acid polymer prepared according to the above-mentioned preparation method.

The weight average molecular weight (Mw) of the polylactic acid polymer may be <NUM>,<NUM> to <NUM>,<NUM>, preferably <NUM>,<NUM> to <NUM>,<NUM>.

The number average molecular weight (Mn) of the polylactic acid polymer may be <NUM>,<NUM> to <NUM>,<NUM>, preferably, <NUM>,<NUM> to <NUM>,<NUM>.

The polydispersity index (PDI) of the polylactic acid polymer may be <NUM> to <NUM>, preferably, <NUM> to <NUM>.

(yellow index) of the polylactic acid polymer is <NUM> to <NUM>, preferably <NUM> to <NUM>, or <NUM> to <NUM>.

Methods for measuring the weight average molecular weight, the number average molecular weight, the polydispersity index and the Y. will be described in detail in Experimental Examples described later.

According to yet another embodiment of the present disclosure, there is provided an article including the polylactic acid polymer.

As described above, the method for preparing a polylactic acid polymer according to the present disclosure provides a method for preparing a polylactic acid polymer that has a desired degree of high molecular weight by using a specific catalyst in a lactide ring-opening polymerization reaction and controlling the rate of change in the molecular weight of the polymer according to changes in polymerization temperature within a specific range.

Also, it is possible to prepare a polylactic acid polymer which facilitates the removal and reuse of the catalyst after completion of the reaction and is excellent in economy by using a catalyst exhibiting excellent stability even under high-temperature polymerization reaction conditions, and also which does not cause changes in physical properties of the polymer due to catalytic decomposition.

In addition, the polylactic acid polymer prepared according to the above method has a high molecular weight and is excellent in color change stability.

<FIG> shows the results of XRD analysis of the powder-type SnHPO<NUM> catalyst obtained in Preparation Example <NUM>.

Hereinafter, embodiments of the present disclosure will be explained in detail with reference to examples. However, the following examples are for illustrative purposes only, and the detailed description of the present disclosure is not limited by these examples.

A <NUM>-neck round flask was prepared in an oil bath, and <NUM> of phosphorous acid (H<NUM>PO<NUM>) was charged into the flask, and then dissolved by raising the temperature to <NUM> while refluxing under nitrogen substitution conditions. Then, while raising the temperature of the flask to <NUM>, <NUM> of tin(II) oxide was added divisionally over <NUM> to <NUM> times and dissolved.

When the viscous mixture in the flask changes to a transparent state, the oil bath turned off and the temperature was slowly lowered to room temperature (<NUM>±<NUM>). When water was added to the flask, a solid was precipitated. It was washed with methanol at <NUM>±<NUM> and water at <NUM>±<NUM> to remove residual H<NUM>PO<NUM>, which was vacuum-dried to obtain a solid powdery catalyst. Then, the obtained powdery catalyst was subjected to XRD analysis, and the results are shown in <FIG>.

No peak of tin oxide existed on the XRD pattern, thus confirming that the catalyst was synthesized. PLA catalyst SnHPO<NUM> single phase was observed, and tin oxide and H<NUM>PO<NUM> crystal phases used for synthesis were not observed. The grain size of SnHPO<NUM> catalyst was about <NUM>~<NUM>.

After setting a <NUM> lab-scale glass reactor, Ar condition was maintained overnight and the inside of the reactor was replaced with Ar. After the Viton O-ring was installed in a <NUM> beaker, lactide was added at about <NUM> to <NUM>% (about <NUM>) based on the level of the beaker. After fastening the reactor, it was maintained at <NUM> mbar for <NUM> minute using a diaphragm pump and a vacuum controller to proceed the leak test. Then, vacuum-argon purge was performed about <NUM> times using a Schlenk line, and the inside of the reactor was replaced with an inert condition and finally replaced with Ar condition.

The catalyst septum was opened, and <NUM> ppm mol of the SnHPO<NUM> powder catalyst of Preparation Example <NUM> (converted to lactide charging amount) (about <NUM>) was weighed and added. The reaction mixture was heated to <NUM> (lower mantle <NUM>) for about <NUM> hours under vacuum conditions using a Schlenk line to remove impurities. After replacing the reactor with Ar, the temperature was raised to <NUM> (lower mantle <NUM>, upper mantle <NUM>). After the reaction temperature reached <NUM>, the reaction proceeded for <NUM> hours to prepare a polylactic acid polymer.

A polylactic acid polymer was prepared in the same manner as in Example <NUM>, except that in Example <NUM>, the polymerization reaction was performed at a temperature of <NUM>.

Polylactic acid polymers were prepared in the same manner as in Examples <NUM> to <NUM>, except that in Examples <NUM> to <NUM>, Sn(Oct)<NUM> catalyst was used in the content of <NUM> ppm mol instead of SnHPO<NUM> catalyst.

The characteristics of the polymers prepared in Examples and Comparative Examples were evaluated as follows.

The weight average molecular weight, number average molecular weight, and polydispersity index of the polymers prepared in the Examples and Comparative Examples were measured by gel permeation chromatography (GPC, Tosoh ECO SEC Elite), and the results are shown in Table <NUM> below.

For the polymers prepared in Examples and Comparative Examples, a pellet specimen was prepared with a twin-screw extruder, and the Y. (Yellow Index) of the prepared specimen was measured according to ASTM E <NUM> [D65/<NUM>], and the results are shown in Table <NUM> below.

As can be seen in Table <NUM>, it could be confirmed that in the case of Examples <NUM> to <NUM>, as the ring-opening polymerization reaction temperature increases, the weight average molecular weight of the synthesized polymer increases. Thereby, it could be confirmed that it exhibits excellent activity without decomposing SnHPO<NUM> catalyst even when the polymerization reaction proceeds at high temperature.

On the other hand, it could be confirmed that in the case of Comparative Examples <NUM> to <NUM>, as the ring-opening polymerization temperature increases, the weight average molecular weight of the synthesized polymer decreases, which decomposes the catalyst and significantly lowers its activity at high temperatures.

Claim 1:
A method for preparing a polylactic acid polymer, the method comprising:
performing a ring-opening polymerization of lactide in the presence of a SnHPO<NUM> catalyst to prepare a polylactic acid polymer,
wherein a rate of change in a weight average molecular weight defined by the following Equation <NUM> has a positive value: <MAT>
M(t) is the weight average molecular weight of the polylactic acid polymer prepared according to the ring-opening polymerization reaction at a temperature of t°C,
M(t+<NUM>) is the weight average molecular weight of the polylactic acid polymer prepared according to the ring-opening polymerization reaction at a temperature of t+<NUM>, and
t is <NUM> to <NUM>,
wherein the average molecular weight is measured using GPC as indicated in the description.