Modified polylactic acid and method for preparing modified polylactic acid

Abstract

The invention relates to the field of high polymer materials, in particular to modified polylactic acid and a method for preparing the modified polylactic acid.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a modified polylactic acid and a method for preparing the modified polylactic acid. Background Technology

[0002] Polylactic acid (PLA) is synthesized from non-petrochemical products. After use, it can be biodegraded and returned to nature, making it environmentally friendly. It is increasingly used in various packaging materials and is expected to replace some petrochemically synthesized non-degradable polymer packaging materials.

[0003] Polylactic acid (PLA) suffers from low crystallinity, significantly limiting its applications. The most common method to address this is by adding nucleating agents. Currently, nucleating agents are mainly categorized into organic and inorganic agents. Commonly used organic nucleating agents, such as ethylene bis-stearamide (EBS), TMC-306, and TMC-300, are prone to precipitation and are unsuitable for food packaging. Commonly used inorganic nucleating agents, such as talc, montmorillonite, calcium carbonate, and zinc oxide, have poor compatibility with PLA, leading to uneven crystallization and poor product quality. Furthermore, while PLA itself is translucent or milky white, it is prone to yellowing during synthesis, processing, and storage, affecting its applications.

[0004] CN103788356A discloses a method for preparing modified polylactic acid, wherein dicarboxylic acids and polycarboxylic acids are reacted with diamines to prepare polyamide oligomers, which are then mixed and reacted with polylactic acid. Summary of the Invention

[0005] To address the problems existing in the prior art, one objective of this invention is to provide a method for preparing heat-resistant modified polylactic acid (PLA). The method includes reacting lactic acid or oligolactic acid with a diamine at a first temperature, followed by negative pressure treatment at a second temperature to obtain a terminal hydroxylamide compound; reacting lactide with the terminal hydroxylamide compound at a third temperature to obtain low molecular weight PLA; and subjecting the low molecular weight PLA to solid-state polycondensation to obtain the PLA, wherein the first temperature is about 50-150°C, the second temperature is about 90-150°C, and the third temperature is about 130-160°C.

[0006] In another aspect, the present invention also relates to a polylactic acid obtained by the method of the present invention.

[0007] In another aspect, the present invention also relates to a polylactic acid having an amide moiety in its molecule, a molecular weight of about 95,000 or more, and satisfying one or more of the following (1)-(3), preferably all of them: (1) The Vicat softening temperature is approximately 100°C or higher; (2) Yellowness is approximately 3.5 or lower; (3) After one month of storage, the total migration remained at approximately 4 mg / dm³. 2 The following; and / or After 3 months of storage, the total migration was approximately 4 mg / dm³. 2 The following; and / or After 6 months of placement, the total migration was approximately 4.5 mg / dm³. 2 The following; and / or After one year of storage, the total migration was approximately 4.5 mg / dm³. 2 the following; And arbitrarily, (4) Melt index of about 1 to about 10 g / 10 min; and / or (5) The semi-crystallization time is about 1 to about 10 min. Detailed Implementation

[0008] The present invention will now be described in further detail. This description is for illustrative purposes only and is not intended to limit the invention. Those skilled in the art will readily understand other advantages and effects of the invention from the disclosure herein. The invention can also be implemented or applied through other different specific embodiments. Those skilled in the art can make various modifications and changes without departing from the spirit of the invention.

[0009] General definitions and terms

[0010] Unless otherwise stated, all publications, patent applications, patents and other references mentioned herein are incorporated herein in their entirety by way of citation.

[0011] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In case of any conflict, the definitions provided herein shall prevail.

[0012] Unless otherwise stated, all percentages, parts, proportions, etc. are by weight.

[0013] When quantities, concentrations, or other values ​​or parameters are given as ranges, preferred ranges, or preferred upper and lower limits, or specific values, they should be understood as specifically disclosing all ranges formed by pairs of values ​​from any upper or preferred range and any lower or preferred range, regardless of whether the range is disclosed individually. Unless otherwise stated, when a numerical range is referred to herein, the range means including its endpoints and all integers and fractions within that range. The scope of this invention is not limited to the specific numerical values ​​referenced when defining the range.

[0014] When used in conjunction with a numerical variable, the terms "about" or "approximately" generally mean that the value of the variable and all values ​​of the variable are within the experimental error (e.g., within a 95% confidence interval for the mean) or within ±10% of a specified value, or a wider range. When the term "about" is used to define a range, it means that each value within it, including the endpoints, is subject to this definition, although it may be formally defined only at one endpoint.

[0015] The terms “comprising,” “including,” “having,” “containing,” or “involving,” and their other variations herein, are inclusive or open-ended and do not exclude other unlisted elements or method steps. Those skilled in the art will understand that the foregoing terms such as “comprising” encompass the meaning of “consisting of.” The expression “consisting of” excludes any unspecified elements, steps, or ingredients. The expression “substantially constitutes” limits the scope to the specified elements, steps, or ingredients, plus optional elements, steps, or ingredients that do not materially affect the essential and novel features of the claimed subject matter. It should be understood that the expression “comprising” encompasses both the expressions “substantially constitutes” and “consisting of.”

[0016] The term “selected from…” means one or more elements from the groups listed below, selected independently, and may include combinations of two or more elements.

[0017] As used herein, the terms “optional,” “optional,” “optionally,” or “optionally” mean that an event or situation described below may or may not occur, including both the occurrence and non-occurrence of the event or situation.

[0018] As used herein, the terms “one or more” or “at least one” refer to one, two, three, four, five, six, seven, eight, nine or more.

[0019] As used herein, the term “and / or” encompasses both “and” and “or”. For example, “A and / or B” encompasses “A”, “B”, and “A+B”. When this term is used to describe three or more elements, it can indicate the selection of any one of the listed terms, as well as any combination of two or more.

[0020] Furthermore, if the number of components or parts of the present invention is not previously specified, it indicates that there is no limitation on the number of times a component or part may appear (or be present). Therefore, it should be interpreted as including one or at least one, and the singular form of a component or part also includes the plural, unless the value clearly indicates a singular number.

[0021] Unless otherwise stated, the terms "combination thereof" and "mixture thereof" refer to a multi-component mixture of the elements, such as two, three, four, and up to the maximum possible multi-component mixture.

[0022] In this document, terms such as "first," "second," and "third" are used only to identify the elements or products referred to, and are not used to specify the order or quantity of products, unless otherwise stated.

[0023] The term "number-average molecular weight" or "number-average molar mass" is used in this article. If the molecular weight of the polymer is M... j The mole fraction of the molecule is x j The number of molecules is N j Then the number average molecular weight - M n for - M n =ΣM j x j =ΣM j N j / ΣN j , in - M n The molecular weight can be determined by methods such as end-group determination, gel permeation chromatography, membrane osmosis, vapor osmosis, boiling point rise method, and mass spectrometry. Unless otherwise specified, the molecular weight mentioned herein is the number-average molecular weight. The number-average molecular weight and its distribution of the present invention can be determined, for example, using gel permeation chromatography (GPC) or mass spectrometry.

[0024] Melt flow index (MFI), also known as melt flow rate, is an indicator that measures the melt flowability of polymer materials under specific temperature and pressure. It quantifies the mass (or volume) of material passing through a standard capillary per unit time, reflecting the molecular weight, molecular weight distribution, and molecular chain structure of the polymer chain. The melt flow index can be measured using a melt flow index tester (such as the MFI1211).

[0025] The Vicat softening temperature (VST) is an indicator of the heat resistance of polymer materials. It characterizes the critical temperature at which a material transitions from a rigid solid state to a softened state under specific pressure and heating rate. The Vicat softening temperature can be tested using a heat distortion Vicat softening temperature tester according to GB / T 1633-2000.

[0026] "Yellowness (YI)," also known as the yellowness index, is an optical indicator that quantifies the degree to which the color of a material (especially transparent or translucent materials) leans towards yellow. It measures the material's reflection or transmission characteristics of different wavelengths of visible light, calculating the deviation from "ideal white." This can be used to assess the material's appearance quality, aging rate, and process stability. Yellowness can be measured using a benchtop spectrophotometer, according to GB / T 39822-2021.

[0027] "Half-crystallization time" describes the time required for a crystalline polymer material to reach half of its final crystallinity during the melting and cooling process. It reflects the speed at which the polymer chains transform from a "molten disordered state" to a "crystalline ordered state." Half-crystallization time can be measured using a differential calorimeter (DSC) according to laboratory standard testing (ISO 11357-3).

[0028] Solid-state polycondensation is a method for obtaining polymer products through end-group reactions of small-molecule monomers or oligomers (prepolymers) under conditions below the polymer's melting point and above its glass transition temperature (Tg). It avoids polymer degradation caused by high-temperature melting and can produce polymers with high viscosity, low impurities, and high crystallinity.

[0029] "Negative pressure" refers to a state where the pressure within an environment or system is lower than the external atmospheric pressure. Applying negative pressure can also be called applying a vacuum or drawing a vacuum. Methods of applying negative pressure are well known to those skilled in the art; for example, it can be achieved by reducing the pressure within the environment or system using equipment such as vacuum pumps or fans. The applied negative pressure can be determined based on the required reaction conditions and raw materials. In this invention, the applied negative pressure can be, for example, below approximately 500 Pa, below approximately 400 Pa, below approximately 300 Pa, below approximately 200 Pa, below approximately 150 Pa, or below approximately 100 Pa.

[0030] The method of the present invention

[0031] In one aspect, the present invention provides a method for preparing polylactic acid, the method comprising: Lactic acid or oligolactic acid is reacted with a diamine at a first temperature, and then subjected to negative pressure treatment at a second temperature to obtain a terminal hydroxylamide compound; lactide is reacted with the terminal hydroxylamide compound at a third temperature to obtain low molecular weight polylactic acid; and the low molecular weight polylactic acid is subjected to solid-phase polycondensation to obtain the final polylactic acid product.

[0032] Preparation of terminal hydroxyl amide molecules

[0033] In the method of the present invention, lactic acid or oligolactic acid is reacted with a diamine to prepare a terminal hydroxyl amide molecule.

[0034] Lactic acid is a small-molecule organic acid with the chemical formula CH3-CH(OH)-COOH, which can be polymerized as a monomer to form oligolactic acid. As used herein, "oligolactic acid" refers to an oligomer formed by the linkage of multiple lactic acid molecules. As an example, the degree of polymerization of oligolactic acid can be from about 2 to about 30, such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, etc. The molecular weight of usable oligolactic acid can be from about 162 to about 2000, preferably from about 100 to about 1000, and more preferably from about 300 to about 600.

[0035] In one embodiment, lactic acid can be directly used as a reactant to react with a diamine to generate the corresponding terminal hydroxylamide compound. In another embodiment, oligolactic acid can be used as a reactant to react with a diamine to generate the corresponding terminal hydroxylamide. When using oligolactic acid, it can be used directly or by polymerizing lactic acid into the corresponding oligolactic acid and using it as a reactant. Therefore, the method of the present invention may optionally include a step of synthesizing oligolactic acid from lactic acid.

[0036] Diamine molecules contain two amino groups, which can independently react with lactic acid or oligolactic acid molecules to form amide bonds, thereby obtaining the corresponding hydroxyl-terminated amide compounds. Such amide compounds have a hydroxyl group at their terminal end, and therefore can also be referred to herein as terminal hydroxyl amide (or similar) compounds or prepolymers. These terminal hydroxyl amide compounds can act as initiators in subsequent reactions.

[0037] The reaction of lactic acid or polylactic acid with a diamine may include reacting the lactic acid or polylactic acid with the diamine at a first temperature, followed by applying a negative pressure at a second temperature. The first temperature may be about 50 to about 150°C, preferably about 80 to about 120°C, more preferably about 90 to about 100°C, for example, about 50, about 51, about 52, about 53, about 54, about 55, about 56, about 57, about 58, about 59, about 60, about 61, about 62, about 63, about 64, about 65, about 66, about 67, about 68, about 69, about 70, about... 71, about 72, about 73, about 74, about 75, about 76, about 77, about 78, about 79, about 80, about 81, about 82, about 83, about 84, about 85, about 86, about 87, about 88, about 89, about 90, about 91, about 92, about 93, about 94, about 95, about 96, about 97, about 98, about 99, about 100, about 101, about 102, Approximately 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128. Approximately 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, and 150°C, as well as ranges consisting of any two of these values. Such temperatures are conducive to the full progress of the reaction, to obtaining the desired product, and to suppressing side reactions. For example, this can effectively activate carboxyl and amino groups, allowing the reaction to complete within a reasonable time, while avoiding over-reaction due to high temperatures, as well as preventing the volatilization loss of diamines, inhibiting the self-polymerization / decomposition of lactic acid or polylactic acid, and preventing the oxidation of amino groups. The reaction time at the first temperature can be adjusted according to the amount and type of materials added, typically ranging from 1 min to 2 h, such as 5 min to 1 h, 10 to 30 min, etc.

[0038] After the reaction at the first temperature, the temperature of the reaction system is adjusted to a second temperature, and a negative pressure is applied. The second temperature can be approximately 90-150°C, preferably approximately 100-120°C, for example, approximately 90, approximately 91, approximately 92, approximately 93, approximately 94, approximately 95, approximately 96, approximately 97, approximately 98, approximately 99, approximately 100, approximately 101, approximately 102, approximately 103, approximately 104, approximately 105, approximately 106, approximately 107, approximately 108, approximately 109, approximately 110, approximately 111, approximately 112, approximately 113, approximately 114, approximately 115, approximately 116, approximately 117, approximately 118, approximately 119, approximately... 120, approximately 121, approximately 122, approximately 123, approximately 124, approximately 125, approximately 126, approximately 127, approximately 128, approximately 129, approximately 130, approximately 131, approximately 132, approximately 133, approximately 134, approximately 135, approximately 136, approximately 137, approximately 138, approximately 139, approximately 140, approximately 141, approximately 142, approximately 143, approximately 144, approximately 145, approximately 146, approximately 147, approximately 148, approximately 149, approximately 150°C, and ranges consisting of any two of these values. The applied negative pressure can be, for example, less than approximately 500 Pa, less than approximately 400 Pa, less than approximately 300 Pa, less than approximately 200 Pa, less than approximately 150 Pa, or less than approximately 100 Pa. This temperature selection effectively removes byproducts generated during the reaction, including water, condensation products from the amidation reaction, and unreacted raw materials such as diamines. It also effectively avoids the residue of low-boiling-point impurities, meeting the high-purity requirements of the food and pharmaceutical industries. Simultaneously, this temperature and negative pressure can promote intermolecular condensation reactions while preventing excessive cross-linking. Furthermore, the negative pressure eliminates oxygen present in the reaction environment, suppressing side reactions. The reaction time at the second temperature can be determined based on the amount and type of reactants, typically ranging from approximately 0.5 to 10 hours, for example, approximately 1 to 5 hours.

[0039] Terminal hydroxyl amide compounds may contain monoamidated products, diamidated products, or combinations thereof. Particularly advantageously, terminal hydroxyl amide compounds contain diamidated products to facilitate the introduction of the amide moiety into subsequent products. For this purpose, the molar ratio of lactic acid or oligolactic acid to diamine can be set to about 2:(1-1.1). Using a ratio of about 2:(1-1.1) allows for a slight excess of diamine, ensuring complete reaction while avoiding side reactions, such as intermolecular crosslinking. Furthermore, applying negative pressure removes water generated during the reaction, as well as unreacted diamine and lactic acid, which is beneficial for subsequent reactions, such as reducing impurities and avoiding side reactions.

[0040] Examples of diamines that can be used include ethylenediamine, propylenediamine, butanediamine, hexanediamine, pentandiamine, decanediamine, p-phenylenediamine, or combinations thereof. Diamines with lower boiling points are preferred, such as ethylenediamine, 1,6-hexanediamine, or combinations thereof.

[0041] The type and molecular weight of the terminal hydroxyl amide compound can be determined by the lactic acid / oligolactic acid and the diamine involved in the reaction. For example, it can be N,N'-di(2-hydroxypropyl)ethylenediamine, N,N'-di(2-hydroxypropyl)hexanediamine, or an amidated form of the corresponding oligolactic acid. In one embodiment, the molecular weight of the terminal hydroxyl amide compound can be from about 200 to about 4000, preferably from about 200 to about 1300, for example about 204, about 260, about 650, or about 1250.

[0042] Preparation of low molecular weight polylactic acid

[0043] The obtained hydroxyl-terminated amide compound can be used as an initiator to react with lactide to obtain low molecular weight polylactic acid (PLA). "Low molecular weight PLA" refers to PLA obtained by reacting lactide with a hydroxyl-terminated amide compound, which contains an amide moiety in its molecule. Therefore, it can also be called amide-modified low molecular weight PLA. Here, "low molecular weight" PLA is relative to the final product PLA, which typically has a molecular weight of approximately 90,000 or higher.

[0044] The inventors unexpectedly discovered that terminal hydroxyl amide compounds have excellent compatibility with polylactic acid (PLA). Unwilling to be bound by any theory, they believe that introducing terminal hydroxyl amide compounds into PLA segments enhances the heat resistance of PLA through a dual effect of inducing microcrystal formation and hydrogen bond stabilization, without causing precipitation and effectively ensuring food contact safety.

[0045] In this step, lactide undergoes a ring-opening polymerization reaction to generate low molecular weight polylactic acid (PLA). This reaction can be carried out in the presence of a catalyst. As mentioned above, the terminal hydroxyl amide compound participates in the reaction as an initiator, and the catalyst can coordinate with the hydroxyl groups therein to form an active intermediate. Under the action of the catalyst, lactide forms an active chain of lactic acid monomers. The ring-opening lactic acid monomers undergo esterification with the hydroxyl groups of the terminal hydroxyl amide to form low molecular weight PLA with chain growth. In this way, the use of nucleating agents can be avoided, further avoiding the precipitation of nucleating agents in the final product, and also avoiding incompatibility problems caused by nucleating agents, thereby improving its stability, crystallinity, and product quality. Therefore, in one embodiment, the method of the present invention does not use a nucleating agent. Such nucleating agents may include organic nucleating agents, such as ethylene bis-stearamide (EBS), TMC-306, TMC-300, polyglycolic acid, polyester, etc.; and inorganic nucleating agents, such as graphite, talc, montmorillonite, calcium carbonate, zinc oxide, lactate, etc.

[0046] The available catalysts include, but are not limited to, one or more of stannous octoate, stannous chloride, stannous oxide, dibutyltin oxide, stannous chloride, stannous lactate, stannous benzoate, dibutyltin dilaurate, zinc oxide, zinc lactate, and zinc acetate, wherein stannous octoate is preferred.

[0047] In the method of this invention, lactide is reacted with the obtained terminal hydroxyl amide at a third temperature to obtain low molecular weight polylactic acid. On one hand, it is necessary to advantageously select the third temperature to obtain suitable low molecular weight polylactic acid for subsequent solid-state polycondensation. Too low a temperature will prevent the polymerization reaction from proceeding sufficiently, failing to obtain a product with the desired molecular weight. Too high a temperature will result in an excessively yellow final product, which is unacceptable. Therefore, in one embodiment, the third temperature can be about 130 to about 160°C, preferably about 140 to about 150°C, for example about 130, about 131, about 132, about 133, about 134, about 135, about 136, about 137, about 138, about 139, about 140, about 141, about 142, about 143, about 144, about 145, about 146, about 147, about 148, about 149, about 150, about 151, about 152, about 153, about 154, about 155, about 156, about 157, about 158, about 159, about 160°C, and a range consisting of any two of these values.

[0048] On the other hand, it is necessary to control the reaction time of lactide and the terminal hydroxylamide compound to obtain suitable low molecular weight polylactic acid (PLA) and thus a superior final product. The inventors unexpectedly discovered that when the reaction time is too short, the resulting low molecular weight PLA also has an excessively low molecular weight. Even with extended subsequent solid-state polycondensation time, the final PLA product still has a low molecular weight, failing to meet requirements. Simultaneously, excessively long solid-state polycondensation times also lead to excessively high yellowness in the final product. When the polymerization time is too long, the polymerization temperature is high in the later stages of polymerization due to the exothermic reaction, ultimately resulting in excessively high yellowness in the final product. Therefore, in one embodiment, the reaction time of lactide and the terminal hydroxylamide is about 2 to about 3 hours, for example, about 2, about 2.1, about 2.2, about 2.3, about 2.4, about 2.5, about 2.6, about 2.7, about 2.8, about 2.9, about 3 hours, and a range consisting of any two of these values. In a preferred embodiment, the reaction time is about 2.2 to about 2.7 hours, more preferably about 2.5 hours.

[0049] It is also necessary to control the molecular weight of the obtained low molecular weight polylactic acid. The inventors unexpectedly discovered that if the molecular weight is too low, the solid-state polycondensation time needs to be extended in subsequent reactions to obtain the desired product, leading to reduced efficiency, or even failing to obtain the desired polylactic acid despite extended polycondensation time. If the molecular weight is too high, the Tg point of the low molecular weight polylactic acid participating in the reaction will be higher, thus requiring a corresponding increase in the solid-state polycondensation temperature, which in turn increases the degree of yellowing. In a preferred embodiment, the number average molecular weight of the low molecular weight polylactic acid can be about 40,000 to about 80,000, more preferably about 60,000 to about 76,000, particularly about 70,000 to about 76,000, for example about 40,000, about 41,000, about 42,000, about 43,000, about 44,000, about 45,000, about 46,000, about 47,000, about 48,000, about 49,000, about 50,000, about 51,000, about 52,000, about 53,000, about 54,000, about 55,000, and about 56,000. Approximately 57,000, approximately 58,000, approximately 59,000, approximately 60,000, approximately 61,000, approximately 62,000, approximately 63,000, approximately 64,000, approximately 65,000, approximately 65,000, approximately 66,000, approximately 67,000, approximately 68,000, approximately 69,000, approximately 70,000, approximately 71,000, approximately 72,000, approximately 73,000, approximately 74,000, approximately 75,000, approximately 76,000, approximately 77,000, approximately 78,000, approximately 79,000, approximately 80,000, and a range consisting of any two of these values.

[0050] Preparation of polylactic acid products

[0051] In this step, the obtained low molecular weight polylactic acid is subjected to solid-state polycondensation to obtain the final product, polylactic acid. As used herein, solid-state polycondensation is carried out at a temperature above the glass transition temperature (Tg) of the low molecular weight polylactic acid but below its melting point. This temperature selection yields polylactic acid with better crystallinity, lower yellowness, and excellent heat resistance. Simultaneously, this temperature condition also advantageously removes small molecule byproducts (e.g., water or lactic acid). Therefore, in a preferred embodiment, the temperature for solid-state polycondensation is about 100-160°C, preferably about 140-150°C, for example, about 100, about 101, about 102, about 103, about 104, about 105, about 106, about 107, about 108, about 109, about 110, about 111, about 112, about 113, about 114, about 115, about 116, about 117, about 118, about 119, about 120, about 121, about 122, about 123, about 124, about 125, about 126, about 12 7. Approximately 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, approximately 160°C, and a range consisting of any two of these values.

[0052] The solid-state polycondensation time can be affected by factors such as the low molecular weight of polylactic acid and the reaction temperature. Too short a time prevents sufficient reaction and fails to yield a product with the desired molecular weight. Too long a time is also detrimental to the reaction and also fails to yield a product with the desired molecular weight. Advantageously, the solid-state polycondensation time is about 2-7 hours, preferably about 4-6 hours.

[0053] In another embodiment, solid-state polycondensation can be carried out under negative pressure. By applying negative pressure, small molecule byproducts or residues (e.g., water or lactic acid) can be advantageously removed, thereby obtaining a final product with excellent properties. The applied negative pressure can be, for example, less than about 500 Pa, less than about 400 Pa, less than about 300 Pa, less than about 200 Pa, less than about 150 Pa, and preferably less than about 100 Pa.

[0054] The method of the present invention can be used to obtain polylactic acid products with excellent properties, such as heat resistance, low yellowing, high stability, excellent crystallinity, fluidity, and processability.

[0055] As mentioned above, amide segments can be introduced into the final product polylactic acid (PLA) using terminal hydroxyl amide compounds. This can improve the heat resistance of PLA through the dual effects of inducing microcrystal formation and hydrogen bond stabilization. The resulting PLA contains an amide moiety in its molecular chain and can therefore also be called amide-modified PLA.

[0056] The polylactic acid of this invention exhibits excellent heat resistance. In one embodiment, the Vicat softening temperature of the polylactic acid is about 100°C or higher, for example, about 101, about 102, about 103, about 104, about 105, about 106, about 107, about 108°C or higher. In another embodiment, the Vicat softening temperature is below about 125°C, for example, below about 120°C or below about 115°C.

[0057] Compared to existing technologies, the polylactic acid (PLA) of the present invention exhibits significantly lower yellowness. In one embodiment, the PLA of the present invention has a yellowness of about 3.5 or lower, for example, about 3.4, about 3.3, about 3.2, about 3.1, about 3.0, about 2.9, about 2.8, about 2.7, about 2.6, about 2.5, about 2.4, about 2.3, about 2.2, about 2.1, about 2.0 or lower. The method of the present invention obtains low-yellowness PLA without the use of color-changing stabilizers, saving costs while reducing side reactions and impurities. As used herein, color-changing stabilizers are used to prevent color change during the reaction. Such color-changing stabilizers may include phosphoric acid, phosphorous acid, hypophosphite, phosphates, phosphites, hypophosphites, phosphate esters, phosphites, hypophosphites, etc. Therefore, in another embodiment, the method of the present invention does not use color-changing stabilizers.

[0058] Another advantage of the polylactic acid of this invention is its high stability. For example, the total migration of the polylactic acid of this invention remains at about 4 mg / dm³ after one month of storage. 2 The following is an example: approximately 3.6 mg / dm 2 Below, approximately 3.5 mg / dm 2 Below, approximately 3.1 mg / dm 2 The total migration level remained at approximately 4 mg / dm³ after 3 months of placement. 2 The following is an example: approximately 3.4 mg / dm 2 The total migration level remained at approximately 4.5 mg / dm³ after 6 months of placement. 2 The following is an example: approximately 4.1 mg / dm 2 Below, approximately 3.2 mg / dm 2 The following is a summary of the data. Even after a storage period of up to one year, the total migration remained at approximately 4.5 mg / dm³. 2 The following is an example: approximately 3.7 mg / dm 2The following conditions apply. The placement conditions are 25℃ and 50% humidity.

[0059] Furthermore, the polylactic acid of this invention also possesses excellent crystallinity, flowability, processability, and mechanical properties, making it suitable for applications in fields such as food and medical products. In another embodiment, the melt index of the polylactic acid is about 1 to about 10 g / 10 min, for example, about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9, about 10 g / 10 min, and about 3.7, about 4.2, about 5.6, about 4.7, about 6.4, about 6.5 g / 10 min, etc. In yet another embodiment, the semi-crystallization time of the polylactic acid is about 1 to about 10 min, for example, about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9, about 10 min, and about 3.5, about 3.7, about 3.8, about 4.0, about 4.2 min.

[0060] In another aspect, the polylactic acid of the present invention has a molecular weight of about 95,000 or more, for example, about 96,000 or more, about 97,000 or more, about 98,000 or more, about 99,000 or more, about 100,000 or more, about 101,000 or more, about 102,000 or more, about 103,000 or more, about 104,000 or more, about 105,000 or more, about 106,000 or more, about 107,000 or more, about 1 The molecular weight of polylactic acid (PLA) can advantageously be below approximately 200,000. Such a molecular weight gives the PLA material of the present invention excellent mechanical strength, excellent processability, and controllable degradation.

[0061] The polylactic acid of this invention has particular advantages in possessing the heat resistance, low yellowness, high stability, and molecular weight requirements described herein. Therefore, it can meet the Vicat softening temperature, yellowness, total migration, and molecular weight requirements described above. It also preferably further meets the melt index and semi-crystallization time parameters described above.

[0062] Accordingly, in another aspect, the present invention also relates to a polylactic acid prepared by the method of the present invention. In one embodiment, the properties of the polylactic acid are as described above.

[0063] Polylactic acid products

[0064] In another aspect, the present invention also relates to a modified polylactic acid that contains an amide moiety in its molecule and has the properties described above.

[0065] In one embodiment, the present invention also relates to a modified polylactic acid that contains an amide moiety in its molecule and satisfies one or more of the following (1)-(3), preferably all of them:

[0066] (1) The Vicat softening temperature is about 100°C or higher, such as about 101, about 102, about 103, about 104, about 105, about 106, about 107, about 108°C or higher. Alternatively, the Vicat softening temperature is below about 125°C, such as below about 120°C or below about 115°C.

[0067] (2) Yellowness is about 3.5 or lower, such as about 3.4, about 3.3, about 3.2, about 3.1, about 3.0, about 2.9, about 2.8, about 2.7, about 2.6, about 2.5, about 2.4, about 2.3, about 2.2, about 2.1, about 2.0 or lower.

[0068] (3) At 25°C and 50% humidity, the total migration amount remained at approximately 4 mg / dm³ after one month of storage. 2 The following; and / or At 25°C and 50% humidity, the total migration remained at approximately 4 mg / dm³ after 3 months of storage. 2 The following; and / or At 25°C and 50% humidity, the total migration remained at approximately 4.5 mg / dm³ after 6 months of storage. 2 The following; and / or At 25°C and 50% humidity, the total migration amount remained at approximately 4.5 mg / dm³ after one year of storage. 2 the following.

[0069] Optionally, the polylactic acid satisfies one or more of the following, preferably all of them: (4) Melt index is about 1 to about 10 g / 10min. For example, about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9, about 10 g / 10min, and about 3.7, about 4.2, about 5.6, about 4.7, about 6.4, about 6.5 g / 10min, etc.

[0070] (5) The semi-crystallization time is about 1 to about 10 min. For example, about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9, about 10 min, and about 3.5, about 3.7, about 3.8, about 4.0, about 4.2 min.

[0071] In one embodiment, the amide portion is covalently bonded to the polylactic acid portion of the modified polylactic acid. Therefore, the polylactic acid of the present invention can also be amide-modified polylactic acid.

[0072] In one embodiment, the modified polylactic acid of the present invention has a molecular weight of about 95,000 or more, for example, about 98,000 or more, about 100,000 or more, about 108,000 or more, about 110,000 or more, about 113,000 or more, about 114,000 or more, about 120,000 or more, about 125,000 or more, or about 130,000 or more. Alternatively, the molecular weight of the polylactic acid is about 200,000 or less.

[0073] Beneficial effects

[0074] This invention provides a method for preparing polylactic acid (PLA) and PLA products. The inventors unexpectedly discovered that terminal hydroxyl amide compounds have excellent compatibility with PLA. Introducing these compounds into PLA chain segments can improve the heat resistance of PLA without causing precipitation, effectively ensuring safety for applications in the food and medical fields.

[0075] In existing technologies, polyamide oligomers with multiple end groups are added as crosslinking agents during the polymerization process. Under polycondensation conditions, dicarboxylic acids and polycarboxylic acids (e.g., oxalic acid, malonic acid, citric acid, pyromellitic acid, pyromellitic anhydride, trimellitic anhydride, and ethylenediaminetetraacetic acid) react with diamines to obtain polyamide oligomers, which are then reacted with polylactic acid. Such methods require additional steps, use more complex raw materials, produce more byproducts and side reactions, and are difficult to control the properties and impurities of the final product. They may even require stabilizers to prevent discoloration. In contrast, the method of this invention reacts diamines with lactic acid / oligolactic acid to obtain terminal hydroxyl amide compounds, directly and simply introducing the amide moiety into low molecular weight polylactic acid, followed by solid-state polycondensation to obtain a high-performance polylactic acid product. The method of this invention has fewer steps, simpler conditions, fewer side reactions and impurities, and obtains a high-performance product without the need for stabilizers.

[0076] Yellowing is one of the key performance indicators in the processing and application of polylactic acid (PLA), directly affecting the commercial value of the product's appearance. PLA itself is translucent or milky white, but it is prone to yellowing during synthesis and processing. When a large amount of lactic acid monomers or oligolactic acid are present during the reaction, they are easily degraded in subsequent reactions, releasing small molecules and initiating chain breakage, leading to increased yellowing. Furthermore, excessively high processing temperatures can result in significant yellowing. The inventors of this invention prepare PLA by combining lactide ring-opening polymerization with solid-state polycondensation, effectively reducing the reaction temperature and solving the problem of amide segments easily causing yellowing of PLA at high temperatures, affecting the performance of the final product. This also allows for the acquisition of higher molecular weights that meet usage requirements. Furthermore, residual reactants and byproducts can be removed by applying negative pressure, ensuring the acquisition of a product with excellent performance. Another advantage of this method is that it eliminates the need for nucleating agents, improving product stability, compatibility, and quality.

[0077] The polylactic acid of this invention has excellent heat resistance, low yellowing, and high stability, and can meet molecular weight requirements. In addition, it also has excellent crystallinity, flowability, and processability, and has a wide range of applications, such as packaging materials (including food and industrial packaging), biodegradable materials, biomedicine, 3D printing and other fields. Example

[0078] The present invention will now be described in further detail with reference to specific embodiments.

[0079] It should be noted that the following embodiments are merely examples to clearly illustrate the technical solutions of the present invention, and are not intended to limit the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here, and any obvious variations or modifications derived therefrom are still within the protection scope of this invention. Unless otherwise specified, the instruments, equipment, and reagents used herein are commercially available or can be prepared in-house.

[0080] Example 1

[0081] 90 g of lactic acid was placed in a flask, heated to 90 °C, and stirred. 31 g of ethylenediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa. The vacuum period was 1 h, yielding a terminal hydroxyl amide compound.

[0082] 3000 g of L-lactide, 1.5 g of the terminal hydroxylamide compound prepared above, and 3 g of stannous octoate were added to a reactor, heated to 140 °C, and reacted at 140 °C-150 °C for 2.5 h to obtain low molecular weight polylactic acid.

[0083] Take 2000 g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation, keep the vacuum pressure below 100 Pa, and vacuum time for 5 h to obtain the product polylactic acid.

[0084] Example 2

[0085] Take 300 g of lactic acid and place it in a flask. Heat it to 150°C and stir it. Perform a vacuum pressure operation to control the molecular weight to 300, and obtain oligolactic acid.

[0086] 150 g of oligolactic acid was placed in a flask, heated to 90 °C, and stirred. 16 g of ethylenediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa. The vacuum period was 1 h, yielding a terminal hydroxyl amide compound.

[0087] 3000g of L-lactide, 4.5g of the terminal hydroxyl amide molecule prepared above, and 3g of stannous octoate were added to a reactor, heated to 140℃, and reacted at 140℃-150℃ for 2.5h to obtain low molecular weight polylactic acid.

[0088] Take 2000 g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation, keep the vacuum pressure below 100 Pa, and vacuum for 5 h to obtain the product polylactic acid.

[0089] Example 3

[0090] Take 300g of lactic acid and place it in a flask. Heat it to 150℃ and stir it. Perform a vacuum pressure operation to control the molecular weight to 600, and obtain oligolactic acid.

[0091] Take 150g of the oligolactic acid prepared above and place it in a flask. Heat the flask to 90℃ and stir. Slowly add 8g of ethylenediamine. After reacting for 10 min, adjust the temperature to 110℃ and apply a vacuum. The vacuum pressure should be below 150 Pa. Apply the vacuum for 1 h to obtain the terminal hydroxyl amide compound.

[0092] 3000g of L-lactide, 9g of the terminal hydroxyl amide compound prepared above, and 3g of stannous octoate were added to a reactor, heated to 140℃, and reacted at 140℃-150℃ for 2.5 h to obtain low molecular weight polylactic acid.

[0093] Take 2000g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation, keep the vacuum pressure below 100Pa, and vacuum time for 5h to obtain the product polylactic acid.

[0094] Example 4

[0095] 90 g of lactic acid was placed in a flask, heated to 90 °C, and stirred. 60 g of 1,6-hexanediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa, and the vacuum time was 1 h, yielding a terminal hydroxyl amide compound.

[0096] 3000 g of L-lactide, 1 g of the terminal hydroxyl amide compound prepared above, and 3 g of stannous octoate were added to a reactor, heated to 140 °C, and reacted at 140 °C-150 °C for 2.5 h to obtain low molecular weight polylactic acid.

[0097] Take 2000 g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation, keep the vacuum pressure below 100 Pa, and vacuum time for 5 h to obtain the product polylactic acid.

[0098] Example 5

[0099] 90 g of lactic acid was placed in a flask, heated to 90 °C, and stirred. 60 g of 1,6-hexanediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa, and the vacuum time was 1 h, yielding a terminal hydroxyl amide compound.

[0100] 3000 g of L-lactide, 2 g of the terminal hydroxyl amide molecule prepared above, and 3 g of stannous octoate were added to a reactor, heated to 140℃, and reacted at 140℃-150℃ for 2.5 h to obtain low molecular weight polylactic acid.

[0101] Take 2000g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation, keep the vacuum pressure below 100Pa, and vacuum time for 8h to obtain the product polylactic acid.

[0102] Example 6

[0103] 90 g of lactic acid was placed in a flask, heated to 90 °C, and stirred. 60 g of 1,6-hexanediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa, and the vacuum time was 1 h, yielding a terminal hydroxyl amide compound.

[0104] 3000 g of L-lactide, 3 g of the terminal hydroxylamide compound prepared above, and 3 g of stannous octoate were added to a reactor, heated to 140 °C, and reacted at 140 °C-150 °C for 2.5 h to obtain low molecular weight polylactic acid.

[0105] Take 2000g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation, keep the vacuum pressure below 100Pa, and vacuum time for 5 hours to obtain the product polylactic acid.

[0106] Comparative Example 1

[0107] 90 g of lactic acid was placed in a flask, heated to 90 °C, and stirred. 60 g of 1,6-hexanediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa, and the vacuum time was 1 h, yielding a terminal hydroxyl amide compound.

[0108] 3000g of L-lactide, 2g of the terminal hydroxyl amide compound prepared above, and 3g of stannous octoate were added to a reactor, heated to 140℃, and reacted at 140℃-200℃ for 6 h to obtain polylactic acid.

[0109] Comparative Example 2

[0110] 5000 g of L-lactide and 3 g of stannous octoate were added to a reactor, the temperature was raised to 140℃, and the reaction was carried out at 140℃-200℃ for 6 h to obtain polylactic acid.

[0111] 3000g of the polylactic acid prepared above and 3g of ethylene bis-stearamide were subjected to twin-screw extrusion at 200℃ to obtain the product polylactic acid.

[0112] Comparative Example 3

[0113] 5000g of L-lactide and 3g of stannous octoate were added to a reactor, the temperature was raised to 140℃, and the reaction was carried out at 140℃-200℃ for 6 h to obtain polylactic acid.

[0114] Comparative Example 4

[0115] 90 g of lactic acid was placed in a flask, heated to 90 °C, and stirred. 60 g of 1,6-hexanediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa, and the vacuum time was 1 h, yielding a terminal hydroxyl amide compound.

[0116] 3000g of L-lactide, 2g of the terminal hydroxylamide compound prepared above, and 3g of stannous octoate were added to a reactor, heated to 140℃, and reacted at 140℃-150℃ for 1 h to obtain low molecular weight polylactic acid.

[0117] Take 2000g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation, keep the vacuum pressure below 100Pa, and vacuum time for 5h to finally obtain the product polylactic acid.

[0118] Comparative Example 5

[0119] 90 g of lactic acid was placed in a flask, heated to 90 °C, and stirred. 60 g of 1,6-hexanediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa, and the vacuum time was 1 h, yielding a terminal hydroxyl amide compound.

[0120] 3000g of L-lactide, 2g of the terminal hydroxylamide compound prepared above, and 3g of stannous octoate were added to a reactor, heated to 140℃, and reacted at 140℃-150℃ for 1.5h to obtain low molecular weight polylactic acid.

[0121] Take 2000g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation, keep the vacuum pressure below 100Pa, and vacuum time for 5h to finally obtain the product polylactic acid.

[0122] Comparative Example 6

[0123] 90 g of lactic acid was placed in a flask, heated to 90 °C, and stirred. 60 g of 1,6-hexanediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa, and the vacuum time was 1 h, yielding a terminal hydroxyl amide compound.

[0124] 3000g of L-lactide, 2g of the terminal hydroxylamide compound prepared above, and 3g of stannous octoate were added to a reactor, heated to 140℃, and reacted at 140℃-150℃ for 2h to obtain low molecular weight polylactic acid.

[0125] Take 2000g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation, keep the vacuum pressure below 100Pa, and vacuum time for 5h to finally obtain the product polylactic acid.

[0126] Comparative Example 7

[0127] 90 g of lactic acid was placed in a flask, heated to 90 °C, and stirred. 60 g of 1,6-hexanediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa, and the vacuum time was 1 h, yielding a terminal hydroxyl amide compound.

[0128] 3000g of L-lactide, 2g of the terminal hydroxyl amide compound prepared above, and 3g of stannous octoate were added to a reactor, heated to 140℃, and reacted at 140℃-190℃ for 3.5h to obtain low molecular weight polylactic acid.

[0129] Take 2000g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation, keep the vacuum pressure below 100Pa, and vacuum time for 5h to finally obtain the product polylactic acid.

[0130] Comparative Example 8

[0131] 90 g of lactic acid was placed in a flask, heated to 90 °C, and stirred. 60 g of 1,6-hexanediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa, and the vacuum time was 1 h, yielding a terminal hydroxyl amide compound.

[0132] 3000g of L-lactide, 2g of the terminal hydroxylamide compound prepared above, and 3g of stannous octoate were added to a reactor, heated to 140℃, and reacted at 140℃-150℃ for 1.5h to obtain low molecular weight polylactic acid.

[0133] Take 2000g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation, keep the vacuum pressure below 100Pa, and vacuum time for 15h to finally obtain the product polylactic acid.

[0134] Comparative Example 9

[0135] 90 g of lactic acid was placed in a flask, heated to 90 °C, and stirred. 60 g of 1,6-hexanediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa, and the vacuum time was 1 h, yielding a terminal hydroxyl amide compound.

[0136] 3000 g of L-lactide, 6 g of the terminal hydroxyl amide compound prepared above, and 3 g of stannous octoate were added to a reactor, heated to 140 °C, and reacted at 140 °C-150 °C for 2.5 h to obtain low molecular weight polylactic acid.

[0137] Take 2000 g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation with a vacuum pressure below 100 Pa, and vacuum time of 8 h to obtain the product polylactic acid.

[0138] Comparative Example 10

[0139] 90 g of lactic acid was placed in a flask, heated to 90 °C, and stirred. 60 g of 1,6-hexanediamine was slowly added. After reacting for 10 min, the temperature was adjusted to 110 °C, and a vacuum was applied. The vacuum pressure was below 150 Pa, and the vacuum time was 1 h, yielding a terminal hydroxyl amide compound.

[0140] 3000 g of L-lactide, 9 g of the terminal hydroxyl amide compound prepared above, and 3 g of stannous octoate were added to a reactor, heated to 140 °C, and reacted at 140 °C-150 °C for 2.5 h to obtain low molecular weight polylactic acid.

[0141] Take 2000g of the low molecular weight polylactic acid prepared above, control the temperature at 140℃, perform a vacuum operation, keep the vacuum pressure below 100Pa, and vacuum time for 8h to obtain the product polylactic acid.

[0142] Table 1 below lists the molecular weights of the intermediate products for each preparation example.

[0143] Test case

[0144] Number average molecular weight (M n ): Tested using gel permeation chromatography (GPC).

[0145] Melt flow index: Tested using a melt flow indexer (MFI1211), preheated for 5 minutes, and measured the number of grams of melt flowing out within 10 minutes at 190℃ and a load of 2.16 kg.

[0146] Vicat softening temperature: Tested using a heat distortion Vicat softening point temperature tester, according to GB / T 1633-2000.

[0147] Yellowness: Tested using a benchtop spectrophotometer, according to GB / T 39822-2021.

[0148] Semi-crystallization time: Tested using differential calorimetry (DSC) according to laboratory standard test (ISO 11357-3).

[0149] The final products obtained from the examples and comparative examples were tested, and the results are listed in Table 2 below.

[0150] The results above show that the polylactic acid prepared by this invention has excellent heat resistance, with a Vicat softening temperature exceeding 100°C; it also exhibits very low yellowness, all below 3; and a molecular weight exceeding 95,000. Furthermore, it demonstrates excellent crystallinity, flowability, processability, and mechanical properties. In particular, the products of Examples 1, 3, and 5 have Vicat softening temperatures exceeding 104°C, even lower yellowness, and molecular weights exceeding 110,000.

[0151] In contrast, in Comparative Example 1, polylactic acid (PLA) was obtained by reacting lactide with a terminal hydroxyl amide compound without solid-state polycondensation. The reaction temperature was higher, resulting in a significantly increased yellowness of the final product. In Comparative Example 2, the polymerization of lactide into PLA required ethylene bis-stearamide as a nucleating agent, and a higher temperature necessitated twin-screw extrusion to obtain the final product, resulting in poor heat resistance and significantly higher yellowness. Comparative Example 3 shows that direct polymerization of lactide into PLA requires higher reaction temperatures and times to obtain the final product, which exhibits poor heat resistance, yellowness, and crystallinity.

[0152] As shown in Comparative Examples 4-6, when the reaction time between lactide and the terminal hydroxylamide compound is too short, the resulting low molecular weight polylactic acid (PLA) has an excessively low molecular weight, leading to a significantly lower molecular weight of the final product. While this allows for lower yellowness in the subsequent solid-state polycondensation at a lower temperature, the product's molecular weight is also low, resulting in poor heat resistance and crystallinity. In Comparative Example 7, the reaction time between lactide and the terminal hydroxylamide compound is longer and the temperature is higher, resulting in a significantly higher yellowness of the product. In Comparative Example 8, the reaction time between lactide and the terminal hydroxylamide compound is shorter, requiring a longer solid-state polycondensation time to complete, resulting in a product with high yellowness but also failing to meet the molecular weight standard. Similarly, in Comparative Example 9, the long preparation time for PLA resulted in a product with poor heat resistance and a molecular weight that also fails to meet the standard. Likewise, the low molecular weight PLA in Comparative Example 10 has a low molecular weight; achieving a lower yellowness would necessitate a longer solid-state polycondensation time, resulting in PLA with unsatisfactory heat resistance and molecular weight.

[0153] In general, when preparing low molecular weight polylactic acid (PLA), a short polymerization time results in an excessively low molecular weight PLA, even with extended solid-state polycondensation time. Excessive solid-state polycondensation leads to higher polymerization temperatures in the later stages due to the exothermic reaction, resulting in a higher yellowness of the product. Conversely, an excessively long polymerization time results in an excessively high molecular weight PLA and a higher reaction temperature, both of which also contribute to a higher yellowness of the product.

[0154] Based on the above results, the polylactic acid sample prepared by this invention has the characteristics of good crystallinity, low yellowness, and excellent heat resistance. By first performing ring-opening polymerization and then solid-state polymerization, polylactic acid can achieve a high molecular weight, which meets production requirements.

[0155] Stability test

[0156] Samples from Examples 1, 3, and 5, and Comparative Examples 2 and 3 were placed at 25°C and 50% humidity for 0 months, 1 month, 3 months, 6 months, and 12 months, respectively, and their migration data were measured. The results are shown in Table 3.

[0157] According to the migration data shown in Table 3, in the polylactic acid sample of Comparative Example 2, the added additives began to precipitate after prolonged storage, resulting in a significant increase in total migration. Similarly, the total migration of the polylactic acid sample of Comparative Example 3 was also significantly higher than that of the polylactic acid sample of this application after prolonged storage. In contrast, the total migration of the polylactic acid sample prepared by this invention remained very low even after prolonged storage. Even after storage for up to one year, its total migration remained less than 5 mg / dm³. 2 (e.g., 4.5 mg / dm) 2 Below, even 3.7 mg / dm 2 The following (items) meet the requirements for production and use.

[0158] The above description is merely a specific embodiment of the present invention and is not intended to limit the scope of the present invention. Any equivalent transformations made using the present invention, or direct or indirect applications in other related technical fields, are included within the protection scope of the present invention.

Claims

1. A method for preparing polylactic acid, characterized in that, The method includes Lactic acid or oligolactic acid is reacted with diamine at a first temperature, and then subjected to negative pressure treatment at a second temperature to obtain a terminal hydroxyl amide compound. The lactide was reacted with a terminal hydroxylamide compound at a third temperature to obtain low molecular weight polylactic acid. The low molecular weight polylactic acid is subjected to solid-phase polycondensation to obtain the polylactic acid. in The first temperature is approximately 50-150°C. The second temperature is approximately 90-150°C. The third temperature is approximately 130-160°C.

2. The method according to claim 1, characterized in that, The oligolactic acid has a molecular weight of about 162 to about 2000, preferably about 100 to about 1000, more preferably about 300 to about 600; and / or The low molecular weight polylactic acid has a number average molecular weight of about 40,000 to about 80,000, preferably about 60,000 to about 76,000.

3. The method according to claim 1 or 2, characterized in that, The diamine includes ethylenediamine, propylenediamine, butanediamine, hexamethylenediamine, pentanediamine, decanediamine, p-phenylenediamine, or combinations thereof, preferably ethylenediamine, 1,6-hexamethylenediamine, or combinations thereof; and / or The lactide includes L-lactide, D-lactide, or a combination thereof; and / or The reaction of lactide with the terminal hydroxyl amide compound is carried out in the presence of a catalyst, said catalyst including stannous octoate, stannous chloride, tin oxide, dibutyltin oxide, stannous chloride, stannous lactate, stannous benzoate, dibutyltin dilaurate, zinc oxide, zinc lactate, zinc acetate or combinations thereof, preferably stannous octoate.

4. The method according to any one of claims 1-3, characterized in that, The first temperature is about 80 to about 120°C, more preferably about 90 to about 100°C; and / or The second temperature is approximately 100-120°C; and / or The third temperature is approximately 140-150°C; and / or The reaction time of lactide with the terminal hydroxyamide compound is about 2 to about 3 hours, preferably about 2.2 to about 2.7 hours, and more preferably about 2.5 hours.

5. The method according to any one of claims 1-4, characterized in that, The solid-phase polycondensation is carried out under negative pressure.

6. The method according to any one of claims 1-5, characterized in that, The molar ratio of lactic acid or oligolactic acid to diamine is approximately 2:(1-1.1).

7. The method according to any one of claims 1-6, characterized in that, The method does not use nucleating agents and / or color-changing stabilizers.

8. The method according to any one of claims 1-7, characterized in that, The polylactic acid has at least one of the following (1)-(4), preferably all of them: (1) The Vicat softening temperature is approximately 100°C or higher; (2) Yellowness is approximately 3.5 or lower; (3) At 25°C and 50% humidity, the total migration amount remained at approximately 4 mg / dm³ after one month of storage. 2 The following; and / or At 25°C and 50% humidity, the total migration amount after 3 months of storage was approximately 4 mg / dm³. 2 The following; and / or At 25°C and 50% humidity, the total migration after 6 months of storage was approximately 4.5 mg / dm³. 2 The following; and / or At 25°C and 50% humidity, the total migration after one year of storage is approximately 4.5 mg / dm³. 2 the following; (4) The molecular weight is approximately 95,000 or higher. And arbitrarily, (5) Melt index of about 1 to about 10 g / 10 min; and / or (6) The semi-crystallization time is about 1 to about 10 min.

9. A polylactic acid obtained by the method of any one of claims 1-8.

10. A polylactic acid comprising an amide moiety in its molecule, having a molecular weight of about 95,000 or more, and satisfying one or more of the following (1)-(3), preferably all of them: (1) The Vicat softening temperature is approximately 100°C or higher; (2) Yellowness is approximately 3.5 or lower; (3) After one month of storage, the total migration remained at approximately 4 mg / dm³. 2 The following; and / or After 3 months of storage, the total migration was approximately 4 mg / dm³. 2 The following; and / or After 6 months of placement, the total migration was approximately 4.5 mg / dm³. 2 The following; and / or After one year of storage, the total migration was approximately 4.5 mg / dm³. 2 the following; And arbitrarily, (4) Melt index of about 1 to about 10 g / 10 min; and / or (5) The semi-crystallization time is about 1 to about 10 min.

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