Colored polylactic acid oligomer, method for producing the same, and polylactic acid colorant

Abstract

The application discloses a colored polylactic acid oligomer, a preparation method thereof and a polylactic acid colorant, and belongs to the technical field of polymer materials. The colored polylactic acid oligomer is prepared by directly initiating controllable ring-opening polymerization of lactide by using active amino groups in an organic dye, and is composed of linkage of the dye and homogenized polylactic acid molecular chains in a chemical structure, so that the colored polylactic acid oligomer has excellent compatibility and dispersibility in a polylactic acid base material, and thus the base material can still maintain original excellent performance when the colored polylactic acid oligomer is applied to the base material by blending. The homogenized polylactic acid molecular chains in the colored polylactic acid oligomer are linked to auxiliary groups of the dye instead of chromophores, so that the colored polylactic acid oligomer can present the same hue as the corresponding dye, and the color can be adjusted according to the red-yellow-blue (RYB) hue circle law to present diversification, and the original liquid coloring product can present the required color by simple blending.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a colored polylactic acid oligomer, its preparation method, and a polylactic acid colorant. Background Technology

[0002] Polylactic acid (PLA) is a renewable, biodegradable, chemically recyclable, safe, and non-toxic bio-based material. PLA possesses excellent processing properties and can be processed into fibers, films, and engineering structural components through spinning, blown film production, injection molding, and 3D printing, making it an ideal alternative to petroleum-based plastics. In recent years, PLA products have been applied in numerous fields such as packaging, textiles, automobiles, agriculture, and medicine. However, its predominantly white or semi-transparent colors are insufficient to meet the growing consumer demand for high-end, fashionable, and personalized products. For example, the limited color options of PLA products restrict their application in high-end cosmetic packaging, fashion apparel and home textile production, and customized color automotive structural components. While PLA fibers can be colored using disperse dyes in the textile industry, the dye uptake is low, color fastness is poor, requiring the addition of dyeing carriers and auxiliaries. Furthermore, this dyeing method damages the fiber's mechanical properties and is unsuitable for coloring PLA molded products produced through other processing methods.

[0003] Solution coloring technology is a coloring method that involves adding a colorant to a polymer solution or melt to obtain a colored polymer solution or melt, which is then processed directly to produce colored polymer products. Currently, polylactic acid (PLA) solution coloring typically employs a masterbatch method, where the colorant is first incorporated into a carrier to create a masterbatch, which is then applied to PLA solution coloring. For example, patents CN106894103A and CN106633693A disclose solvent-carrier type masterbatches and PET carrier type masterbatches for PLA fiber coloring, respectively, prepared using a solvent-based carrier and modified PET polyester loaded with pigments or dyes. Furthermore, patents CN118531523A, CN108193316A, and CN108193318B disclose PLA carrier type masterbatches for PLA fibers, prepared by directly loading PLA with colorants (pigments), environmentally friendly pigments (pigments and carbon black), and graphene. It is worth noting that these patents all explicitly state the need to add dispersants or excipients such as polyethylene wax to prevent colorant agglomeration and improve its dispersion uniformity and stability. However, the addition of heterogeneous and difficult-to-degrade dispersants or excipients can affect the overall biodegradability of polylactic acid (PLA) products, limit their recycling and recycling, thus shortening their lifespan and undermining their original low-carbon properties. Furthermore, the presence of these heterogeneous components can also limit the use of PLA products in fields requiring high biocompatibility.

[0004] Besides the masterbatch method, some studies have explored chemically bonding colorants to polylactic acid (PLA) molecular chains before solution coloring. For example, patent CN108084412B discloses a pre-spinning colorant for PLA fibers. This involves modifying a hydroxyl-containing pigment (carbon black, aluminum powder, or titanium dioxide) with a silane coupling agent, followed by a solution polymerization reaction between the modified pigment and lactide to produce the PLA fiber pre-spinning colorant. The inventor's subsequent patent CN110129915B discloses the preparation of black PLA fibers based on this type of modified carbon black colorant. However, these two patented technologies only produce black and white PLA products, lacking a range of colors. Furthermore, colorants can be formulated as catalysts for use in the ring-opening polymerization of lactide to achieve coloring of PLA products. For example, patents CN115490837B and CN115612071B, as well as the paper titled "The Combined Synthesis and Coloration of Poly(lactic acid)" in the international journal *Angewandte Chemie International Edition*, all disclose the use of dyes and metal coordination to prepare colored catalysts, which then catalyze the ring-opening polymerization of lactide, ultimately resulting in coloring by chemical bonding to the polylactic acid molecular chain. However, these colored catalysts require specialized manufacturing rather than direct commercial sale. Fluctuations in their production volume and delivery time directly affect the production of solution-colored polylactic acid products. Furthermore, the use of such colored catalysts can lead to high metal residue levels in the synthesized polylactic acid, impacting the biosafety of the final product. It should also be noted that the aforementioned patents and papers require the presence of hydroxyl groups in the reaction system as initiating groups, but under these conditions, the types of dyes that can be selected are limited, making it difficult to meet the diverse color requirements of polylactic acid products. Therefore, there is an urgent need to develop new materials for polylactic acid (PLA) dope coloring that have good compatibility and dispersibility, diverse colors and convenient formulation, controllable synthesis and easy industrial mass production, while not affecting the biodegradability, processing performance, recycling and biosafety of PLA. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a colored polylactic acid (PLA) oligomer, its preparation method, and a PLA colorant. This invention utilizes the ring-opening polymerization of lactide to introduce homogeneous PLA molecular chains composed of repeating dehydrated lactic acid structural units into disperse dyes of different colors, thereby producing colored PLA oligomers. The synthesis is controllable and uses commercially available raw materials. Furthermore, these colored PLA oligomers, due to the presence of homogeneous PLA molecular chains, exhibit excellent compatibility and dispersibility with commercially available PLA without compromising its original biodegradable, processable, recyclable, and biosafety properties. In addition, the colors of these colored PLA oligomers encompass the three primary colors (red, yellow, and blue) and can be further formulated to produce even more colors, thus demonstrating promising application prospects in the field of PLA solution coloring.

[0006] Specifically, the present invention provides the following technical solution:

[0007] The first aspect of the present invention provides a colored polylactic acid oligomer having the structure shown in formula (I):

[0008]

[0009] Where m is 1 or 2, n is 10 to 60, and Dye is an organic dye group.

[0010] The second aspect of the present invention provides a method for preparing colored polylactic acid oligomers as described in the first aspect, wherein an organic dye containing an amino group (-NH2) and lactide are melt-mixed, and under the action of a catalyst, the active amino group in the organic dye molecule directly initiates the controlled ring-opening polymerization of lactide.

[0011] Preferably, the organic dye is anthraquinone or azo disperse dye and has the structure shown in formula (II):

[0012]

[0013] Among them, R1 is one of -H, -CH3, -OPh, -Cl and -Br;

[0014] R2 is one of -H, -NH2, and -OH;

[0015] R3 is -H or -NH2;

[0016] R4 is -H or -OH;

[0017] R5 is one of -H, -NH2, and -NO2.

[0018] Furthermore, the organic dye is an anthraquinone-type or monoazo-type disperse dye containing one or two amino groups, and its specific chemical structural formula is one of the following:

[0019] (Dye 1: 1-Amino-2-phenoxy-4-hydroxyanthraquinone is red); (Dye 2: 1,5-Diamino-2-bromo-4,8-dihydroxy-9,10-anthraquinone is blue);

[0020] (Dye 3: 1,5-Diamino-2-chloro-4,8-dihydroxy-9,10-anthraquinone is blue);

[0021] (Dye 4: 1,4-diaminoanthraquinone is purple);

[0022] (Dye 5: 1-Amino-2-methyl-9,10-anthraquinone is orange);

[0023] (Dye 6:4-aminoazobenzene is yellow);

[0024] (Dye 7: p-Diaminoazobenzene is yellow);

[0025] (Dye 8: 4-(4-nitrophenylazo)aniline is orange).

[0026] Colored polylactic acid (PLA) oligomers can exhibit the colors (CIE hues) inherent in organic dyes. In some preferred embodiments, the organic dye can be selected according to the target color requirements of the PLA oligomer, thereby obtaining red, blue, purple, yellow, or orange PLA oligomers. In this invention, the preferred organic dye is anthraquinone-type or monoazo-type disperse dye containing one or two amino groups. The presence of one or two amino groups as initiating groups enables the prepared colored PLA oligomers to exhibit a linear structure similar to commercially available PLA. Furthermore, it allows for the control of secondary amide content in the colored PLA oligomers at a low level, avoiding the difficulty in controllable synthesis of colored PLA oligomers due to high levels of alkaline secondary amides. It also avoids excessive alkalinity caused by the accumulation of secondary amides during the thermal processing of colored PLA oligomer / PLA blends, which could lead to PLA molecular chain degradation. This ensures good structural stability of the colored PLA oligomers and their solution-colored products during storage and processing. Furthermore, the selected organic dyes are all commercially available and readily available, requiring no additional modification or customization. More importantly, the selected organic dyes encompass the three primary colors of the color wheel: red, yellow, and blue. This not only allows the synthesized polylactic acid oligomers to exhibit the colors inherent to the organic dyes themselves, but also enables the polylactic acid oligomers to display any color contained in the red-yellow-blue (RYB) color wheel by mixing and blending these three primary color polylactic acid oligomers.

[0027] Further, the relative molecular mass of the colored polylactic acid oligomer is greater than 900 g / mol and less than 9000 g / mol. In this invention, the relative molecular mass of the colored polylactic acid oligomer is the sum of the relative molecular mass of the organic dye and the total relative molecular mass of the polylactic acid molecular chain segments. The total relative molecular mass of the polylactic acid molecular chain segments can be adjusted by changing the total number of dehydrated lactic acid repeating units (i.e., the total degree of polymerization), which actually depends on the number of active amino groups in the organic dye and the molar ratio of amino groups to lactide monomers. In ring-opening polymerization, when the molar ratio of initiating groups to polymerizing monomers is too high, the polymer chain growth will exhibit non-steady-state characteristics and significantly enhanced randomness, leading to a wider molecular weight distribution of the polymer and causing significant batch-to-batch differences in the polymerized products. Therefore, this invention optimizes the degree of polymerization of polylactic acid (PLA) molecular chain segments obtained by polymerization initiated by each amino group on the organic dye, i.e., the n value in the structure shown in formula (I) is greater than or equal to 10. Combined with calculations using dye 6, which has the smallest relative molecular mass, the lower limit of the relative molecular mass of the colored PLA oligomer is preferably defined. As the n value increases, the chain growth during ring-opening polymerization becomes more controllable, and the increased length of the PLA molecular chain segments also improves the compatibility of the colored PLA oligomers in the application matrix. However, excessively long PLA molecular chain segments lead to a decrease in the absorption / scattering ratio (K / S value) of the colored PLA oligomers, i.e., a decrease in the depth of the color. Therefore, this invention optimizes the upper limit of the relative molecular mass of the colored PLA oligomers by calculating the molecular weight range of the oligomer concept and dye 2, which has the largest relative molecular mass.

[0028] Specifically, the preparation method of colored polylactic acid oligomers includes the following steps:

[0029] Step (1): Add the selected organic dye and lactide monomer to the reaction flask, then heat to 110-120°C under an inert atmosphere and stir continuously for 20-30 minutes. After the lactide is completely melted and mixed evenly with the organic dye, cool the resulting organic dye / lactide mixture to room temperature.

[0030] Step (2): Add the catalyst to the organic dye / lactide mixture obtained in step (1), and then carry out the lactide ring-opening polymerization reaction under an inert atmosphere, controlling the reaction temperature at 120-130℃ and the reaction time at 24-48 hours.

[0031] Step (3): Cool the crude product obtained in step (2) to room temperature, add dichloromethane to completely dissolve it, then add the resulting solution dropwise to ice-cold methanol to precipitate and separate by vacuum filtration. Repeat this dissolution-precipitation cycle several times, and dry the purified product to constant weight to obtain colored polylactic acid oligomer. Preferably, the purified product is vacuum dried at 40°C to constant weight.

[0032] Furthermore, in step (1), the molar ratio of organic dye to lactide monomer is 1:5 to 60. The molar ratio of organic dye to lactide monomer directly affects the relative molecular mass of colored polylactic acid oligomers and the length of polylactic acid molecular chain segments therein. This invention sets this molar ratio within a preferred range, which can reduce the instability and randomness of chain growth during ring-opening polymerization, ensure the controllability of colored polylactic acid oligomer synthesis, and at the same time maintain the depth of color (K / S value) of the colored polylactic acid oligomers at a high level.

[0033] Further, the catalyst mentioned in step (2) is at least one of stannous octoate, stannous chloride, zinc lactate, zinc chloride, and zinc acetate. The preferred catalyst of this invention catalyzes the ring-opening polymerization of lactide through a coordination-intercalation mechanism. This catalytic reaction system has high selectivity, good controllability, and few side reactions, and can prepare colored polylactic acid oligomers with controllable relative molecular mass, narrow distribution, high purity, and acyclic structure. The preferred catalyst of this invention has good biocompatibility, wherein zinc is an essential trace element for the human body, and zinc ions at the catalyst amount are essentially non-toxic and harmless. In addition, stannous catalysts are common catalysts in the industrial synthesis of polylactic acid, and their safety at the catalyst amount has been certified by the U.S. Food and Drug Administration (FDA). A more preferred catalyst is stannous octoate.

[0034] Furthermore, the molar ratio of catalyst dosage to added lactide monomer in step (2) is 1.1–1.3:1000. The preferred catalyst of this invention exhibits high catalytic activity, thus requiring a small amount of catalyst in the ring-opening polymerization system. Even at approximately one-thousandth of the molar equivalent of lactide monomer, this preferred catalyst effectively improves monomer conversion, shortens reaction time, and increases polymerization yield. Moreover, the relatively low dosage of catalyst in this invention, after subsequent multiple dissolution-precipitation treatments, can further reduce the metal ion content in the polymer, ensuring the biocompatibility of the polymer.

[0035] Further, the lactide monomer is at least one selected from L-lactide, D-lactide, and meso-lactide. The lactide monomer of this invention can be obtained directly through commercial channels or prepared by means of methods commonly used by those skilled in the art. More preferably, the monomer is L-lactide.

[0036] Furthermore, the inert atmosphere includes, but is not limited to, nitrogen, argon, and helium.

[0037] A third aspect of this invention provides a polylactic acid (PLA) colorant, comprising at least one colored PLA oligomer of the color described in the first aspect or a colored PLA oligomer prepared according to the preparation method described in the second aspect. Preferably, the PLA colorant consists of 0-100 wt.% red PLA oligomer, 0-100 wt.% yellow PLA oligomer, and 0-100 wt.% blue PLA oligomer. By adjusting the content of the red, yellow, and blue PLA oligomers, the PLA colorant can be made to exhibit any color contained in the Red-Yellow-Blue (RYB) color wheel, thereby coloring the PLA with the corresponding color. A fourth aspect of this invention provides a method of using the PLA colorant as described in the third aspect, specifically: adding the PLA colorant to PLA by solution blending and / or melt blending to achieve in-liquid coloring, wherein the amount of PLA colorant added is 0.001-5 wt.%, preferably 1-3 wt.%. The colored polylactic acid oligomers described in this invention have the same dehydrated lactic acid repeating unit structure as commercially available polylactic acid, and this homogeneous structural feature ensures excellent compatibility between the two in the organic solvent dissolved state, the heated molten state, and the room temperature solid state. The organic solvents used in the solution blending process include, but are not limited to, dichloromethane and trichloromethane, and the melt blending temperature can be set according to temperature settings commonly used by those skilled in the art.

[0038] In this invention, the optional polylactic acid includes, but is not limited to, NatureWorks' Ingeo TM Series, TotalEnergies Corbion The invention relates to a series of polylactic acid (PLA) products, including the Zhejiang Haizheng Biomaterials REVODE series, the Anhui Fengyuan FY series, and the Jilin COFCO JSC series. The colored PLA oligomers described in this invention can be blended into PLA to achieve solution coloring, and this solution-colored blend can maintain the excellent properties of PLA itself, meeting the needs of injection molding, spinning, blown film, 3D printing, chemical recycling, and other applications.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0040] This invention provides a colored polylactic acid (PLA) oligomer, structurally composed of disperse dyes chemically bonded to homogeneous PLA molecular chains. The presence of these homogeneous molecular chains enables it to exhibit excellent compatibility and dispersibility in commercially available PLA without the need for heterogeneous additives or excipients, thus ensuring that blending it with commercially available PLA substrates does not compromise the original overall biodegradability and processability of the substrate. Furthermore, thanks to the homogeneity of the molecular chain structure, the chemical recycling method for this colored PLA oligomer is the same as that for commercially available PLA, therefore, blends of this colored PLA oligomer with commercially available PLA still maintain excellent recycling characteristics and low-carbon properties.

[0041] The synthesis method of colored polylactic acid (PLA) oligomers used in this invention offers excellent controllability. All raw materials are commercially available, making it more suitable for industrial-scale production. Furthermore, the catalyst used does not bond to the colored PLA oligomers, and after purification, the residual metal content is extremely low, ensuring excellent biocompatibility with commercially available PLA blends. More importantly, the homogeneous molecular chains in the colored PLA oligomers are bonded to the auxochrome amino groups of disperse dyes without affecting the chromophores of the disperse dyes. This allows the colored PLA oligomers to exhibit the same hue as the disperse dyes, and different colored oligomers can be mixed and matched according to the Red-Yellow-Blue (RYB) hue cycle, thus demonstrating a diverse color advantage in the application of PLA solution coloring. Attached Figure Description

[0042] Figure 1 The reaction equation is for the colored polylactic acid oligomer prepared in Example 1.

[0043] Figure 2 The 1H NMR spectrum of the colored polylactic acid oligomer prepared in Example 1 ( 1 H NMR).

[0044] Figure 3 Fourier transform infrared (FTIR) spectrum of the colored polylactic acid oligomer prepared in Example 1.

[0045] Figure 4 Gel permeation chromatogram (GPC) of the colored polylactic acid oligomer prepared in Example 1.

[0046] Figure 5 The International Commission on Illumination (CIE) chromaticity diagram of the colored polylactic acid oligomer prepared in Example 1.

[0047] Figure 6 The image shows a dichloromethane solution of the colored polylactic acid oligomer prepared in Example 1 and its stability assessment.

[0048] Figure 7 The reaction equation is for the colored polylactic acid oligomer prepared in Example 6.

[0049] Figure 8 The image shows a dichloromethane solution of the colored polylactic acid oligomer prepared in Example 10 and its stability assessment.

[0050] Figure 9 The image shows a dichloromethane solution of the colored polylactic acid oligomer prepared in Example 11 and its stability assessment.

[0051] Figure 10 The image shows a physical picture of the product obtained by coloring commercially available polylactic acid with the colored polylactic acid oligomer solution prepared in Example 1, and the product after injection molding.

[0052] Figure 11 The image shows a physical picture of the product obtained by coloring commercially available polylactic acid with the colored polylactic acid oligomer dope solution prepared in Example 2, and the product spun into a filament. Detailed Implementation

[0053] The technical solution of the present invention will be further described in detail below with reference to several preferred embodiments and accompanying drawings. These embodiments are implemented based on the technical solution of the invention, providing detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments. It should be understood that the specific embodiments described herein are merely used to clearly and completely explain the technical solution of the present invention and are not intended to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0054] It should be noted that all raw materials and reagents used in this invention are available from conventional commercial sources, and there are no special restrictions on the manufacturers from which they are purchased. Here, for the purpose of effective comparative analysis between the examples and comparative examples, in the application of colored polylactic acid oligomer solution coloring, the commercially available polylactic acid involved is exemplarily selected as NatureWorks' Ingeo... TM 4032D, and the amount of colored polylactic acid oligomer added in the blend is exemplarily set to 3% of the mass of commercially available polylactic acid.

[0055] Example 1

[0056] This embodiment provides a red polylactic acid oligomer, which is prepared by direct ring-opening polymerization of L-lactide initiated by the amino group in 1-amino-2-phenoxy-4-hydroxyanthraquinone (red dye), and the specific preparation steps are as follows:

[0057] First, 5 mmol of 1-amino-2-phenoxy-4-hydroxyanthraquinone and 100 mmol of L-lactide monomer were weighed and added to a reaction flask, with a molar ratio of organic dye to L-lactide monomer of 1:20. The mixture was then heated to 115°C under a nitrogen atmosphere and stirred continuously for 25 minutes until the L-lactide monomer was completely melted and homogeneously mixed with the dye. The resulting mixture was then cooled to room temperature. Next, 0.12 mmol of stannous octoate catalyst was added to the aforementioned dye / L-lactide mixture, with a molar ratio of catalyst to lactide monomer of 1.2:1000. The mixture was then subjected to a ring-opening polymerization reaction at 125°C for 24 hours under a nitrogen atmosphere. After the reaction was completed, the crude product was cooled to room temperature and dichloromethane was added to dissolve it completely. The resulting solution was then added dropwise to 20 times the volume equivalent of ice-cold methanol to precipitate and separated by filtration. This dissolution-precipitation cycle was repeated 3 times. Finally, the purified product was dried under vacuum at 40°C to constant weight to obtain the colored polylactic acid oligomer in this example.

[0058] The synthesis reaction equation for the colored polylactic acid oligomer provided in this embodiment is as follows: Figure 1 As shown, and its chemical structure is obtained through 1 H NMR test ( Figure 2 ) and FTIR test ( Figure 3 ) to conduct analysis. From Figure 2 It can be seen that the colored polylactic acid oligomers prepared in this embodiment... 1 The 1H NMR spectrum contained characteristic peaks of methyl and methine protons belonging to polylactic acid (PLA) molecular chains, as well as characteristic peaks belonging to protons on the aromatic rings of organic dyes, confirming the successful preparation of the colored PLA oligomer. Furthermore, calculations showed that the yield of this colored PLA oligomer (red) was 94.7%. Figure 3 It can be seen that the colored polylactic acid oligomer prepared in this embodiment has the same infrared characteristic peaks as commercially available polylactic acid, including the C=O stretching vibration peak (1758 cm⁻¹). -1 ), COC asymmetric stretching vibration peak (1180 cm⁻¹) -1 ), COC symmetric stretching vibration peak (1081 cm⁻¹) -1 The asymmetric bending vibration peak of methyl CH (1455 cm⁻¹) -1 ) and symmetrical bending vibration peak (1359cm) -1 The asymmetric stretching vibration peak of methyl CH (2997 cm⁻¹) -1 ) and symmetrical stretching vibration peak (2946cm) -1This indicates that the prepared colored polylactic acid oligomers have a homogeneous structure with commercially available polylactic acid, meaning they both contain repeating dehydrated lactic acid structural units. Furthermore, the infrared spectra of the colored polylactic acid oligomers also reveal distinct NH and OH stretching vibration regions (3650–3200 cm⁻¹). -1 ), CH stretching vibration peak in benzene ring (3070 cm⁻¹) -1 ) and the out-of-plane bending vibration region of CH in the substituted benzene ring (857-790 cm⁻¹) -1 This confirms the presence of the characteristic structure of the organic dye. Furthermore, the infrared spectrum of the colored polylactic acid oligomer shows distinct characteristic peak regions of amide I and II bands (1640-1500 cm⁻¹). -1 ), and at 3454cm -1 The presence of a significant NH stretching vibration peak in the aromatic secondary amide indicates that the amino group in the organic dye reacts with lactide to form an amide group.

[0059] Figure 4 The GPC curve of the colored polylactic acid oligomer prepared in this embodiment is shown. No obvious bimodal or tailing phenomenon is observed, indicating that the synthesis and preparation of this colored polylactic acid oligomer has good controllability. From the CIE chromaticity diagram (… Figure 5 As can be seen, the colored polylactic acid oligomer prepared in this embodiment is red, consistent with the color of the organic dye used. This indicates that the introduction of polylactic acid molecular chain bonding did not affect the characteristics of the chromophore in the organic dye. It also reflects the scientific validity and effectiveness of the strategy of preparing colored polylactic acid oligomers by initiating the ring-opening polymerization of lactide using the auxochrome amino group in the organic dye. Figure 6 It can be seen that the colored polylactic acid oligomer prepared in this embodiment has excellent solubility in dichloromethane. The resulting red solution contains no solid particles or precipitates and remains in its original state after standing at room temperature for 24 hours. This reflects the excellent structural stability of the colored polylactic acid oligomer and also indicates that the organic dye and polylactic acid molecular chain segments are chemically bonded rather than physically blended.

[0060] Example 2

[0061] This embodiment provides a yellow polylactic acid oligomer, most of which is the same as the preparation steps in Example 1, except that the organic dye is 4-aminoazobenzene (yellow dye).

[0062] Example 3

[0063] This embodiment provides a blue polylactic acid oligomer, most of which is the same as the preparation steps in Example 1, except that the organic dye is 1,5-diamino-2-chloro-4,8-dihydroxy-9,10-anthraquinone (blue dye).

[0064] Example 4

[0065] This embodiment provides a blue polylactic acid oligomer, the preparation steps of which are mostly the same as those in Example 1, except that: the organic dye is 2 mmol of 1,5-diamino-2-bromo-4,8-dihydroxy-9,10-anthraquinone (blue dye), the molar ratio of organic dye to L-lactide monomer is 1:60, and the molar ratio of catalyst to lactide monomer is 1.3:1000.

[0066] Example 5

[0067] This embodiment provides a yellow polylactic acid oligomer, most of which is the same as the preparation steps in Example 1. The difference is that the organic dye is 20 mmol of 4-aminoazobenzene (yellow dye), the molar ratio of organic dye to L-lactide monomer is 1:5, and the molar ratio of catalyst to lactide monomer is 1.1:1000.

[0068] Example 6

[0069] This embodiment provides a purple polylactic acid oligomer, which is prepared by direct ring-opening polymerization of D-lactide initiated by the amino group in 1,4-diaminoanthraquinone (purple dye), and the specific preparation steps are as follows:

[0070] First, 1,4-diaminoanthraquinone (4 mmol) and D-lactide monomer (120 mmol) were weighed and added to a reaction flask, with a molar ratio of organic dye to D-lactide monomer of 1:30. The mixture was then heated to 120°C under an argon atmosphere and stirred continuously for 20 minutes until the D-lactide monomer was completely melted and uniformly mixed with the dye. The resulting mixture was then cooled to room temperature. Next, stannous chloride catalyst (0.132 mmol) was added to the aforementioned dye / D-lactide mixture, with a molar ratio of catalyst to lactide monomer of 1.1:1000. The mixture was then subjected to a ring-opening polymerization reaction at 120°C for 48 hours under an argon atmosphere. After the reaction was complete, the crude product was cooled to room temperature and dichloromethane was added to dissolve it completely. The resulting solution was then added dropwise to 20 times its volume equivalent of ice-cold methanol to precipitate the product, which was then separated by filtration. This dissolution-precipitation cycle was repeated three times. Finally, the purified product was vacuum dried at 40°C to constant weight to obtain the colored polylactic acid oligomer of this example. The synthesis reaction equation for the colored polylactic acid oligomer provided in this example is as follows: Figure 7 As shown.

[0071] Example 7

[0072] This embodiment provides an orange polylactic acid oligomer, which is prepared by direct ring-opening polymerization of D-lactide initiated by the amino group in 1-amino-2-methyl-9,10-anthraquinone (orange dye), and the specific preparation steps are as follows:

[0073] First, 3 mmol of 1-amino-2-methyl-9,10-anthraquinone and 120 mmol of D-lactide monomer were weighed and added to a reaction flask, with a molar ratio of organic dye to D-lactide monomer of 1:40. The mixture was then heated to 120°C under an argon atmosphere and stirred continuously for 30 minutes until the D-lactide monomer was completely melted and homogeneously mixed with the dye. The resulting mixture was then cooled to room temperature. Next, 0.156 mmol of zinc lactate catalyst was added to the aforementioned dye / D-lactide mixture, with a molar ratio of catalyst to lactide monomer of 1.3:1000. The mixture was then subjected to a ring-opening polymerization reaction at 130°C for 48 hours under an argon atmosphere. After the reaction was completed, the crude product was cooled to room temperature and dichloromethane was added to dissolve it completely. The resulting solution was then added dropwise to 20 times the volume equivalent of ice-cold methanol to precipitate and separated by filtration. This dissolution-precipitation cycle was repeated 3 times. Finally, the purified product was dried under vacuum at 40°C to constant weight to obtain the colored polylactic acid oligomer in this example.

[0074] Example 8

[0075] This embodiment provides a yellow polylactic acid oligomer, which is prepared by direct meso-lactide ring-opening polymerization initiated by the amino group in p-diaminoazobenzene (yellow dye), and the specific preparation steps are as follows:

[0076] First, 2 mmol of p-diaminoazobenzene and 100 mmol of meso-lactide monomer were weighed and added to a reaction flask, with a molar ratio of organic dye to meso-lactide monomer of 1:50. The mixture was then heated to 110°C under an argon atmosphere and stirred continuously for 20 minutes until the meso-lactide monomer was completely melted and homogeneously mixed with the dye. The resulting mixture was then cooled to room temperature. Next, 0.12 mmol of zinc chloride catalyst was added to the aforementioned dye / meso-lactide mixture, with a molar ratio of catalyst to lactide monomer of 1.2:1000. The mixture was then subjected to a ring-opening polymerization reaction at 120°C for 36 hours under an argon atmosphere. After the reaction was completed, the crude product was cooled to room temperature and dichloromethane was added to dissolve it completely. The resulting solution was then added dropwise to 20 times the volume equivalent of ice-cold methanol to precipitate and separated by filtration. This dissolution-precipitation cycle was repeated 3 times. Finally, the purified product was dried under vacuum at 40°C to constant weight to obtain the colored polylactic acid oligomer in this example.

[0077] Example 9

[0078] This embodiment provides an orange polylactic acid oligomer, which is prepared by direct meso-lactide ring-opening polymerization initiated by the amino group in 4-(4-nitrophenylazo)aniline (orange dye), and the specific preparation steps are as follows:

[0079] First, 10 mmol of 4-(4-nitrophenylazo)aniline and 100 mmol of meso-lactide monomer were weighed and added to a reaction flask, with a molar ratio of organic dye to meso-lactide monomer of 1:10. The mixture was then heated to 110 °C under argon atmosphere and stirred continuously for 30 minutes until the meso-lactide monomer was completely melted and uniformly mixed with the dye. The resulting mixture was then cooled to room temperature. Next, 0.12 mmol of zinc acetate catalyst was added to the aforementioned dye / meso-lactide mixture, with a molar ratio of catalyst to lactide monomer of 1.2:1000. The mixture was then subjected to a ring-opening polymerization reaction at 130 °C for 36 hours under argon atmosphere. After the reaction was completed, the crude product was cooled to room temperature and dichloromethane was added to dissolve it completely. The resulting solution was then added dropwise to 20 times the volume equivalent of ice-cold methanol to precipitate and separated by filtration. This dissolution-precipitation cycle was repeated 3 times. Finally, the purified product was dried under vacuum at 40°C to constant weight to obtain the colored polylactic acid oligomer in this example.

[0080] Example 10

[0081] This embodiment provides an orange polylactic acid oligomer, which is prepared by dissolving the red polylactic acid oligomer prepared in Example 1 and the yellow polylactic acid oligomer prepared in Example 2 in dichloromethane, and then mixing them. The molar ratio of the red polylactic acid oligomer to the yellow polylactic acid oligomer in the mixture is 1:1.

[0082] Figure 8 The figure illustrates the preparation process and stability of the orange polylactic acid (PLA) oligomer in this embodiment. As can be seen from the figure, after standing at room temperature for 24 hours, no precipitates or exudates were observed in the orange PLA oligomer solution prepared in this embodiment. This reflects the excellent compatibility between the different colored PLA oligomers prepared based on the strategy of this invention, and the absence of adverse interactions between them, resulting in the excellent stability of the colored PLA oligomer solution in this embodiment, which maintains its original state even after 24 hours at room temperature. More importantly, the color of the colored PLA oligomer prepared by mixing red and yellow PLA oligomers in this embodiment satisfies the color matching rules of the three primary colors in the Red, Yellow, and Blue (RYB) color wheel, i.e., red and yellow are mixed to produce orange. This reflects that the colored PLA oligomers prepared by this invention can be mixed and matched like ordinary organic dyes in solution coloring applications, thereby preparing PLA products with the desired color.

[0083] Example 11

[0084] This embodiment provides a purple polylactic acid oligomer, which is prepared by dissolving the red polylactic acid oligomer prepared in Example 1 and the blue polylactic acid oligomer prepared in Example 3 in dichloromethane, and then mixing them. The molar ratio of the red polylactic acid oligomer to the blue polylactic acid oligomer in the mixture is 1:1.

[0085] Figure 9 This illustrates the preparation process and stability of the purple polylactic acid oligomer in this embodiment. Figure 8 Similarly, the colored polylactic acid oligomer solution prepared in this embodiment is uniform and stable, confirming that the colored polylactic acid oligomers prepared by the strategy of this invention have excellent compatibility and no adverse interactions. Furthermore, the color of the colored polylactic acid oligomers prepared in this embodiment also satisfies the color matching rules of the three primary colors in the Red-Yellow-Blue (RYB) color wheel, that is, red and blue are mixed to produce purple, further demonstrating the application advantages of the colored polylactic acid oligomers prepared by the strategy of this invention in achieving coloring of whole-chromatographic stock solutions.

[0086] Comparative Example 1

[0087] Most of the preparation steps of Comparative Example 1 and Example 1 are the same, except that the amount of 1-amino-2-phenoxy-4-hydroxyanthraquinone (red dye) is changed to 0.474 g (1.43 mmol), and the molar ratio of organic dye to lactide monomer is 1:70.

[0088] Comparative Example 2

[0089] Most of the preparation steps of Comparative Example 2 and Example 2 are the same, except that the amount of 4-aminoazobenzene (yellow dye) is changed to 7.89 g (40 mmol), and the molar ratio of organic dye to lactide monomer is 1:2.5.

[0090] Comparative Example 3

[0091] Most of the preparation steps of Comparative Example 3 and Example 1 are the same, except that the organic dye is 1,4,5,8-tetraaminoanthraquinone, which contains 4 amino groups.

[0092] Application Example 1

[0093] When applying the original solution for coloring, take 0.09g of the colored polylactic acid oligomers prepared in Examples 1, 2, 3, 4, 5, 7, 9, 10, 11, and Comparative Examples 1 and 2, respectively, and 3g of commercially available polylactic acid, and dissolve them in 20mL of dichloromethane. The amount of colored polylactic acid oligomers added is 3wt.% of the polylactic acid. Stir and mix at room temperature for 6 hours, and then pour the resulting solution into a mold until it is completely dry to obtain the colored polylactic acid oligomer / commercially available polylactic acid blended coloring product.

[0094] from Figure 10 It can be seen that the colored polylactic acid oligomer solution prepared by solution blending in Example 1, which is used to color commercially available polylactic acid, has good thermoplasticity and can be injection molded into the required sample strips. This indicates that the blending of colored polylactic acid oligomer does not affect the original processing performance of commercially available polylactic acid.

[0095] Figure 11 The product obtained by coloring commercially available polylactic acid with colored polylactic acid oligomer dope prepared by solution blending in Example 2 has good spinnability and can be melt-spun into colored fibers, indicating that the product obtained after adding colored polylactic acid oligomer blend still retains the original processing performance of commercially available polylactic acid.

[0096] In addition, the amount of colored polylactic acid oligomers prepared in Example 1 was changed to 0.03 g and 0.15 g, and solution blending was carried out according to the above method to obtain colored polylactic acid oligomer / commercially available polylactic acid blended colored products.

[0097] Application Example 2

[0098] For the application of solution coloring, 0.9g of the colored polylactic acid oligomers prepared in Examples 6 and 8 and 30g of commercially available polylactic acid were taken and premixed by stirring at room temperature. The amount of colored polylactic acid oligomers added was 3 wt.% of the polylactic acid. The mixture was then vacuum dried at 75°C for 24 hours. Then, the dried premix was added to a torque rheometer preheated to 180°C and melt-blended at this temperature at a mixing speed of 45 rpm for 8 minutes. After extrusion, cooling, and pelletizing, the colored polylactic acid oligomer / commercially available polylactic acid blended coloring product was obtained.

[0099] Test characterization:

[0100] The colored polylactic acid oligomers prepared in the above examples and comparative examples, as well as the products obtained in application examples 1 and 2, were characterized by testing, as follows:

[0101] Chemical structure characterization: The colored polylactic acid oligomer was dissolved in deuterated chloroform, and its structure was then analyzed using a JNM-ECZ600R / S1 nuclear magnetic resonance spectrometer. 1 ¹H NMR spectra. The methylene proton peak (4.3 ppm) attached to the terminal hydroxyl group and the methylene proton peak (5.2 ppm) not attached to the terminal hydroxyl group in the polylactic acid molecular chain were analyzed and identified. The integrated areas I of the two peaks were obtained by integration. 5.2 and I 4.3 The number of repeating dehydrated lactic acid units (total degree of polymerization) in colored polylactic acid oligomers is equal to the ratio of the integral areas of the two peaks (Ig). 5.2 / I 4.3Add one. The NMR calculation value of the relative molecular mass of colored polylactic acid oligomers is equal to the relative molecular mass of the dehydrated lactic acid unit multiplied by the total degree of polymerization plus the relative molecular mass of the organic dye.

[0102] Color performance testing: The (L*, a*, b*) values, CIE hue, color difference (△E), and color depth (K / S value) of the samples were tested using a Datacolor 850 spectrophotometer.

[0103] The relative molecular mass and total degree of polymerization of the colored polylactic acid oligomers prepared in Examples 1-11 and Comparative Examples 1-3 are shown in Table 1. The color performance indicators of the colored polylactic acid oligomers prepared in Examples 1-11 and Comparative Examples 1-2 are shown in Table 2. The color performance indicators of the products obtained by coloring the prepared colored polylactic acid oligomers from Examples 1-11 and Comparative Examples 1-2 are shown in Table 3.

[0104] Table 1

[0105]

[0106] As can be seen from the data in Table 1, the colored polylactic acid oligomers prepared in each embodiment and Comparative Examples 1-2 can all be identified by NMR characterization, with methyl and methylene proton peaks belonging to the polylactic acid molecular chain segments. Furthermore, the actual relative molecular mass and total degree of polymerization of the colored polylactic acid oligomers can be calculated using the integral area ratio of the proton peak connected to the terminal hydroxyl group to the corresponding non-terminal hydroxyl group connected proton peak. This indicates that within the required range of monomer type, gas atmosphere, catalyst type and dosage, reaction temperature, and time, the lactide ring-opening polymerization reaction used in this invention can proceed smoothly, thereby achieving the preparation of colored polylactic acid oligomers. A horizontal comparison of the theoretically designed values ​​of the relative molecular mass and total degree of polymerization of the colored polylactic acid oligomers in Examples 1-11 with their corresponding NMR calculation values ​​reveals that the differences between the two sets of data for each embodiment are small. This indicates that within the required organic dye / lactide monomer molar ratio range, the preparation of colored polylactic acid oligomers based on the lactide ring-opening polymerization reaction has excellent controllability.

[0107] Comparing Example 1 with Comparative Example 1, it can be found that when the molar ratio of organic dye to lactide monomer exceeds the upper limit of the range required by this invention, the preparation of colored polylactic acid oligomers still exhibits good controllability. However, excessively high relative molecular mass and total degree of polymerization negatively impact its color performance indicators, as detailed in the data analysis in Tables 2 and 3. Comparing Example 2 with Comparative Example 2, it can be found that when the molar ratio of organic dye to lactide monomer is lower than the lower limit of the range required by this invention, the theoretical design values ​​of the relative molecular mass and total degree of polymerization of the prepared colored polylactic acid oligomers differ significantly from the NMR calculation values. The relative molecular mass deviation rate can reach 375%, indicating that the randomness of chain growth in ring-opening polymerization increases sharply under this condition, resulting in poor controllability. Therefore, the rationality and effectiveness of this invention in terms of the structural design, preparation scheme, and synthesis requirements of colored polylactic acid oligomers are evident. Comparing Example 1 with Comparative Example 3, it can be found that the synthesis yield of Comparative Example 3 is extremely low, approximately 3%, significantly lower than the 94.7% synthesis yield in Example 1. After polymerization in Comparative Example 3, repeated dissolution-precipitation purifications were performed, but it was almost impossible to collect enough product for characterization. This reflects that excessive amino groups in organic dyes can lead to a strong alkalinity in the polymerization system, which affects the controllable ring-opening polymerization of lactide. This further demonstrates the scientific validity and rationality of the requirement for the number of amino groups in organic dyes in this invention.

[0108] Table 2

[0109]

[0110] As can be seen from the data in Table 2:

[0111] The CIE hues of the colored polylactic acid oligomers prepared in Examples 1-9, determined by spectrophotometer measurements of (L*, a*, b*) values, are consistent with the hues of the organic dyes used in the corresponding examples. This indicates that the introduction of homogenized polylactic acid molecular chains into organic dyes via amino bonds does not affect the chromophore characteristics and hue of the organic dyes. This reflects the scientific validity and rationality of the strategy of using amino groups in organic dyes to initiate the ring-opening polymerization of lactide to prepare colored polylactic acid oligomers. The CIE hues of the colored polylactic acid oligomers prepared in Examples 10-11 are consistent with the hues of the corresponding colors in the Red, Yellow, and Blue (RYB) color wheel. This indicates that the color mixing of the colored polylactic acid oligomers in this invention is the same as that of ordinary colors and also satisfies the color mixing rules of the three primary colors in the RYB color wheel, confirming the effectiveness of color change achieved through mixing.

[0112] Comparing Example 1 with Comparative Example 1, it can be found that when the molar ratio of organic dye to lactide monomer exceeds the upper limit of the range required by this invention, the hue of the prepared colored polylactic acid oligomers deviates significantly. In Comparative Example 1, the organic dye is red, while the corresponding colored polylactic acid oligomer is pink. Comparing Example 2 with Comparative Example 2, it can be found that when the molar ratio of organic dye to lactide monomer is lower than the lower limit of the range required by this invention, the hue of the prepared colored polylactic acid oligomers does not deviate, but the data analysis in Table 1 confirms its poor controllability in synthesis. Therefore, the data analysis in Table 2 further confirms the rationality and effectiveness of this invention in terms of the structural design, synthesis scheme, and color adjustment of colored polylactic acid oligomers.

[0113] Table 3

[0114]

[0115]

[0116] As can be seen from the data in Table 3:

[0117] The colored polylactic acid oligomers prepared in Examples 1-9, when applied to solution coloring via different blending methods, exhibited minimal color differences, with ΔE values ​​all less than 0.8 (ΔE < 1, meaning color difference is imperceptible to the naked eye). This reflects the excellent compatibility and uniform dispersion of the colored polylactic acid oligomers in commercially available polylactic acid substrates. Furthermore, the CIE hue of the solution-colored products remained consistent with the hue of the colored polylactic acid oligomers and their corresponding dyes used. Simultaneously, the solution-colored products all displayed good color depth, with the vast majority exhibiting K / S values ​​exceeding 10. This confirms the effectiveness and significance of the solution coloring application of the colored polylactic acid oligomers of this invention. The solution-colored products obtained in Examples 10-11 also showed minimal color differences and significant color depth, and their CIE hues remained consistent with the hues of the colored polylactic acid oligomers used. This demonstrates that the colored polylactic acid oligomers of this invention, even after color adjustment as needed, still possess excellent solution coloring performance.

[0118] Comparing Example 1 with Comparative Example 1, and Example 2 with Comparative Example 2, it can be found that when the molar ratio of organic dye to lactide monomer exceeds the upper limit of the range required by this invention, the solution-colored application performance of the prepared colored polylactic acid oligomer decreases, which is similar to the analysis results of the data in Table 2. The CIE hue of the solution-colored product obtained in Comparative Example 1 differs significantly from the hue of the corresponding dye, and the color difference of the product increases with a significant decrease in color depth, reflecting the rationality of this invention in terms of the structural design and relative molecular mass range requirements of the colored polylactic acid oligomer.

[0119] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that 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. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A colored polylactic acid oligomer, characterized in that, It has the structure shown in equation (I): Formula (I) Where m is 1 or 2, n is 10~60, and Dye is a residue of an organic dye group. The organic dye has the following structural formula: or Among them, R1 is one of -H, -CH3, -OPh, -Cl and -Br; R2 is one of -H, -NH2, and -OH; R3 is -H or -NH2; R4 is -H or -OH; R5 is one of -H, -NH2, and -NO2.

2. The colored polylactic acid oligomer according to claim 1, characterized in that, The relative molecular mass of the colored polylactic acid oligomer is greater than 900 g / mol and less than 9000 g / mol.

3. A method for preparing the colored polylactic acid oligomer according to claim 1, characterized in that, When an amino-containing organic dye and lactide are melt-mixed, the active amino group in the organic dye molecule directly initiates the controlled ring-opening polymerization of lactide under the action of a catalyst. The organic dye is an anthraquinone or azo disperse dye and has the structure described in the following formula: or Among them, R1 is one of -H, -CH3, -OPh, -Cl and -Br; R2 is one of -H, -NH2, and -OH; R3 is -H or -NH2; R4 is -H or -OH; R5 is one of -H, -NH2, and -NO2.

4. The method for preparing colored polylactic acid oligomers according to claim 3, characterized in that, The organic dye has one of the following chemical structural formulas: ， ， ， ， ， ， ， 。 5. The method for preparing colored polylactic acid oligomers according to claim 3, characterized in that, The preparation steps are as follows: Step (1): Add the selected organic dye and lactide monomer to the reaction flask, and then heat to 110~120 ℃ under an inert atmosphere and stir continuously for 20~30 minutes. After the lactide is completely melted and mixed evenly with the organic dye, cool the resulting organic dye / lactide mixture to room temperature. The molar ratio of organic dye to lactide monomer is 1:5~60. Step (2): Add the catalyst to the organic dye / lactide mixture obtained in step (1), and then carry out the lactide ring-opening polymerization reaction under an inert atmosphere. Control the reaction temperature to be 120~130 ℃ and the reaction time to be 24~48 hours. The molar ratio of the amount of catalyst to the amount of lactide monomer added is 1.1~1.3:1000. Step (3): Cool the crude product obtained from step (2) to room temperature, add dichloromethane to dissolve it completely, then add the resulting solution dropwise to ice-cold methanol to precipitate and filter to separate. Repeat the dissolution-precipitation cycle several times, and dry the purified product to constant weight to obtain colored polylactic acid oligomer.

6. The method for preparing colored polylactic acid oligomers according to claim 3, characterized in that, The catalyst is at least one of stannous octoate, stannous chloride, zinc lactate, zinc chloride, and zinc acetate.

7. The method for preparing colored polylactic acid oligomers according to claim 3, characterized in that, The lactide monomer is at least one of L-lactide, D-lactide, and meso-lactide.

8. A polylactic acid colorant, characterized in that, The colored polylactic acid oligomer according to any one of claims 1 to 2 includes at least one color, or the colored polylactic acid oligomer prepared by the preparation method according to any one of claims 3 to 7, wherein the color is red, blue, purple, yellow or orange.

9. The polylactic acid colorant according to claim 8, characterized in that, The polylactic acid colorant consists of 0~100 wt.% red polylactic acid oligomer, 0~100 wt.% yellow polylactic acid oligomer and 0~100 wt.% blue polylactic acid oligomer.

10. The polylactic acid colorant according to claim 8, characterized in that, The method of using polylactic acid colorant is as follows: the polylactic acid colorant is added to polylactic acid by solution blending and / or melt blending to achieve original solution coloring, and the amount of polylactic acid colorant added is 0.001~5 wt.%.

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