A method for preparing a nucleating agent for heat-resistant transparent polylactic acid

Abstract

This invention discloses a method for preparing a nucleating agent for heat-resistant transparent polylactic acid (PLA), relating to the field of PLA processing aids technology. Specifically, the method includes: S10, adding melamine, lactide, and PLA in a molar ratio between 1:1:1 and 10:10:1 to a pyridine solution; S20, under nitrogen protection, reacting at a temperature of 80–150°C for 2–10 hours with a stirring speed of 100–200 rpm; S30, after the reaction is complete, cooling the reactor to room temperature, precipitating the product in methanol, centrifuging to remove excess water, washing, granulating using a twin-screw extruder, and then drying to obtain the nucleating agent. This invention features a simple preparation process, a short preparation cycle, and a nucleating agent yield of over 95%. The nucleating agent exhibits excellent compatibility with PLA, showing no migration or precipitation. The crystallinity of PLA with the added nucleating agent is above 30%, and the haze of a 300 μm thick sheet remains below 8% after annealing.

Description

Technical Field

[0001] This invention relates to the field of polylactic acid processing aids, specifically a method for preparing a nucleating agent for heat-resistant transparent polylactic acid. Technical Background

[0002] Polylactic acid (PLA) has garnered significant attention in food packaging materials, agriculture, manufacturing, and biomedicine due to its fully biodegradable nature, tensile strength and transparency similar to PET, versatility in various processing methods (casting, thermoforming, injection molding, spinning, etc.), excellent antibacterial and antifungal properties, and superior environmental friendliness. Since 2020, the National Development and Reform Commission and the Ministry of Ecology and Environment have mandated that by 2025, disposable items such as lunch boxes, straws, cutlery, shopping bags, and express delivery bags should gradually replace traditional plastic products with fully biodegradable materials. In food packaging materials, PLA has become the preferred material for replacing PP in disposable food contact products, with a particularly broad market demand for heat-resistant and transparent PLA. Generally, crystalline polylactic acid (PLA) exhibits excellent transparency in its uncrystallized state, but its heat resistance is very poor, with a maximum temperature resistance of only around 55°C. After crystallization, PLA exhibits better heat resistance, reaching around 70-80°C. However, due to the slow crystallization process and the fact that the crystal grains formed during crystallization are larger than the wavelength of visible light, visible light is scattered upon entering the crystalline PLA, increasing its haze, decreasing its transparency, and even causing it to become completely opaque and whitish. This characteristic of PLA makes its market application in heat-resistant transparent materials a bottleneck.

[0003] Currently, the main known method for improving the heat resistance of polylactic acid (PLA) is to add nucleating agents to PLA to increase its crystallization rate and crystallinity. Relatively inexpensive inorganic nucleating agents such as talc, calcium carbonate, and montmorillonite have limited effectiveness in improving PLA crystallization, failing to meet heat resistance requirements and resulting in opaque products. Among organic nucleating agents, TMC-300 and 306 are commercially available with relatively ideal crystallization effects and can achieve heat resistance requirements after addition; however, they are expensive and the resulting products are also opaque. Patent CN110713583B discloses a method for preparing PLA nucleating agent masterbatch using 4,4′-diphenylmethane diisocyanate, benzoyl hydrazine, and PLA through a melt reaction. The preparation method is simple, and the added nucleating agent results in excellent heat resistance for PLA. However, the crystal size formed after PLA crystallization is at the micrometer level, indicating that while adding the nucleating agent achieves heat resistance, it cannot maintain the transparency of the PLA. Patents CN10228212194A and CN114605625A both employ a method of introducing amide groups during the polymerization of polylactic acid (PLA) to prepare hybrid PLA as a nucleating agent. This method can significantly improve the crystallization rate of PLA while maintaining its transparency. However, the preparation method is very complex and has a long synthesis cycle. Patent CN105073887B provides a physically blended composition containing an aminotriazine derivative and PLA. This composition provides PLA with excellent heat resistance and transparency. The aminotriazine derivative acts as a nucleating agent. However, because this nucleating agent is a small molecule and has strong hydrogen bonding, local agglomeration of the nucleating agent can occur during the blending process with PLA. Summary of the Invention

[0004] In view of the above-mentioned technical problems, the purpose of this invention is to prepare a nucleating agent for heat-resistant transparent polylactic acid (PLA). This nucleating agent promotes PLA crystallization and improves its heat resistance while maintaining PLA's good transparency. The nucleating agent has significant advantages such as simple preparation process, short preparation cycle, no migration, no precipitation, biodegradability, and excellent compatibility with PLA.

[0005] Specifically, the present invention provides:

[0006] A method for preparing a nucleating agent for heat-resistant transparent polylactic acid includes the following steps:

[0007] S10, melamine, lactide, and polylactic acid are mixed in a molar ratio between 1:1:1 and 10:10:1 and added to the pyridine solution.

[0008] Under S20 and nitrogen protection, the reaction temperature is 100-150℃, the reaction time is 3-10h, and the stirring speed is 100-200rpm.

[0009] S30. After the reaction is complete, the reactor is cooled to room temperature, and the product is precipitated in methanol. After centrifugation to remove dryness, washing, granulation by a twin-screw extruder, and then drying, the nucleating agent is obtained.

[0010] Preferably, the nucleating agent prepared by this method has a yield of over 95%.

[0011] Preferably, the chemical structure of the nucleating agent is selected from one or more mixtures of the following structures:

[0012]

[0013] Where n represents 500 to 2000.

[0014] Preferably, the polylactic acid is an optically active polylactic acid with a number-average molecular weight of 80,000 to 250,000. Preferably, the mass ratio of the total mass of the raw materials to pyridine is 1:50.

[0015] Preferably, the extruder in S30 is a twin-screw extruder with at least one vacuum device, an aspect ratio of 40 to 50, an extrusion temperature of 170 to 200°C, and a screw speed of 100 to 250 rpm.

[0016] Preferably, the drying temperature in S30 is 60-80℃ and the drying time is 6-10h.

[0017] Beneficial effects

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] (1) The raw materials required for the preparation of the nucleating agent involved in this invention are inexpensive and widely available. In particular, polylactic acid and lactide are environmentally friendly materials. The nucleating agent prepared in the end is also biodegradable, low-carbon and environmentally friendly. The preparation formula is simple. The selected pyridine is both a solvent and a catalyst, and there is no need to add other catalysts containing heavy metals. The preparation process is simple, the preparation cycle is short and the yield is high.

[0020] (2) The nucleating agent involved in this invention promotes the crystallization of polylactic acid (PLA) while maintaining its transparency. Its addition does not degrade PLA during processing or mixing, nor does it cause PLA matrix degradation that affects the final material properties. Structurally, the nucleating agent is extremely similar to PLA and exhibits excellent compatibility. During processing, the nucleating agent is easily dispersed and will not agglomerate, thus maintaining the final transparency of the material. The amide and hydroxyl groups at the ends of the nucleating agent molecules readily form hydrogen bonds with PLA, acting as crystal nuclei to induce PLA molecules to align on the surface of the nucleating agent molecules, promoting crystallization and forming fine grains, which facilitates the transmission of visible light and thus provides transparency.

[0021] (3) The nucleating agent involved in this invention is pure after being washed with methanol, extruded and devolatile, and then dried. The nucleating agent itself is a macromolecular polymer with no volatility, and there will be no food safety issues such as migration and precipitation.

[0022] (4) The nucleating agent involved in this invention is an extruded granulation material, which is convenient for mixing formulas in downstream production, improving production efficiency, and compared with traditional powder nucleating agents, it is dust-free and suitable for clean workshops. Detailed Implementation

[0023] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] The polylactic acid (PLA) material used in this embodiment is FY601 PLA produced by Anhui Fengyuan Futailai PLA Co., Ltd., with a melting point of 175℃, a glass transition temperature of 60℃, and a melt index of 9g / 10min. Other raw materials, solvents, and nitrogen were commercially available. The reactor used was a stainless steel reactor from Shanghai Yikai Instrument Co., Ltd., the centrifuge was a centrifuge from Shanghai Xingda Electromechanical Co., Ltd., the drying equipment was a DZF vacuum drying oven from Shanghai Jinghong Experimental Equipment Co., Ltd., the extrusion equipment was an APEX RXT52 co-rotating twin-screw extruder from Nanjing Ruiya Extruder, the internal mixing torque rheometer RTOI-55 / 20 was from Guangzhou Putong Experimental Analytical Instrument Co., Ltd., the crystallization performance testing equipment was a Mettler Toledo differential scanning calorimeter DSC3, the haze testing equipment was a Shanghai Yitian Precision Instruments WGT-S, and the melt index testing equipment was a Shanghai Sierda Scientific Instruments Co., Ltd. RL-Z1B1.

[0025] Comparative Example 1

[0026] Melamine, lactide, and polylactic acid were added to 20 kg of pyridine solution in a molar ratio of 5:0:1. Under nitrogen protection, the reaction temperature was 120°C, the stirring speed was 150 rpm, and the reaction time was 8 h. After the reaction was completed, the mixture was cooled to room temperature, and the solution containing the product was added dropwise to methanol to precipitate the product. After centrifugation to remove the precipitate, the product was washed again with methanol. This washing-centrifugation-dehydration process was repeated three times. The product was then directly extruded through an extruder set at 190°C and 150 rpm. The resulting particles were dried in a vacuum oven at 80°C for 7 h to obtain the nucleating agent, with a yield of 97%.

[0027] Comparative Example 2

[0028] Melamine, lactide, and polylactic acid were added to 20 kg of pyridine solution in a molar ratio of 0:5:1. Under nitrogen protection, the reaction temperature was 120 °C, the stirring speed was 150 rpm, and the reaction time was 8 h. After the reaction was completed, the mixture was cooled to room temperature, and the solution containing the product was added dropwise to methanol to precipitate the product. After centrifugation to remove the precipitate, the product was washed again with methanol. This washing-centrifugation-dehydration process was repeated three times. The product was then directly extruded through an extruder set at 190 °C and 150 rpm. The resulting particles were dried in a vacuum oven at 80 °C for 7 h to obtain the nucleating agent, with a yield of 96%.

[0029] Example 1

[0030] Melamine, lactide, and polylactic acid were added to 20 kg of pyridine solution in a molar ratio of 1:1:1. Under nitrogen protection, the reaction was carried out at 120°C with a stirring speed of 150 rpm for 10 h. After the reaction was completed, the mixture was cooled to room temperature, and the solution containing the product was added dropwise to methanol to precipitate the product. After centrifugation to remove the precipitate, the product was washed again with methanol. This washing-centrifugation-removal process was repeated three times. The product was then directly extruded through an extruder set at 190°C and 150 rpm. The resulting particles were dried in a vacuum oven at 80°C for 7 h to obtain the nucleating agent, with a yield of 96%.

[0031] Example 2

[0032] Melamine, lactide, and polylactic acid were added to 20 kg of pyridine solution in a molar ratio of 5:1:1. Under nitrogen protection, the reaction was carried out at 120°C with a stirring speed of 150 rpm for 10 h. After the reaction was completed, the mixture was cooled to room temperature, and the solution containing the product was added dropwise to methanol to precipitate the product. After centrifugation to remove the precipitate, the product was washed again with methanol. This washing-centrifugation-removal process was repeated three times. The product was then directly extruded through an extruder set at 190°C and 150 rpm. The resulting particles were dried in a vacuum oven at 80°C for 7 h to obtain the nucleating agent, with a yield of 97%.

[0033] Example 3

[0034] Melamine, lactide, and polylactic acid were added to 20 kg of pyridine solution in a molar ratio of 1:5:1. Under nitrogen protection, the reaction was carried out at 120°C with a stirring speed of 150 rpm for 10 h. After the reaction was completed, the mixture was cooled to room temperature, and the solution containing the product was added dropwise to methanol to precipitate the product. After centrifugation to remove the precipitate, the product was washed again with methanol. This washing-centrifugation-removal process was repeated three times. The product was then directly extruded through an extruder set at 190°C and 150 rpm. The resulting particles were dried in a vacuum oven at 80°C for 7 h to obtain the nucleating agent, with a yield of 96%.

[0035] Example 4

[0036] Melamine, lactide, and polylactic acid were added to 20 kg of pyridine solution in a molar ratio of 1:1:1. Under nitrogen protection, the reaction temperature was 140℃, the stirring speed was 100 rpm, and the reaction time was 6 h. After the reaction was completed, the mixture was cooled to room temperature, and the solution containing the product was added dropwise to methanol to precipitate the product. After centrifugation to remove the precipitate, the product was washed again with methanol. This washing-centrifugation-removal process was repeated three times. The product was then directly extruded through an extruder set at 190℃ and 150 rpm. The resulting particles were dried in a vacuum oven at 80℃ for 7 h to obtain the nucleating agent, with a yield of 97%.

[0037] Example 5

[0038] Melamine, lactide, and polylactic acid were added to 20 kg of pyridine solution in a molar ratio of 5:1:1. Under nitrogen protection, the reaction temperature was 140℃, the stirring speed was 100 rpm, and the reaction time was 6 h. After the reaction was completed, the mixture was cooled to room temperature, and the solution containing the product was added dropwise to methanol to precipitate the product. After centrifugation to remove the precipitate, the product was washed again with methanol. This washing-centrifugation-removal process was repeated three times. The product was then directly extruded through an extruder set at 190℃ and 150 rpm. The resulting particles were dried in a vacuum oven at 80℃ for 7 h to obtain the nucleating agent, with a yield of 96%.

[0039] Example 6

[0040] Melamine, lactide, and polylactic acid were added to 20 kg of pyridine solution in a molar ratio of 1:5:1. Under nitrogen protection, the reaction temperature was 140 °C, the stirring speed was 100 rpm, and the reaction time was 6 h. After the reaction was completed, the mixture was cooled to room temperature, and the solution containing the product was added dropwise to methanol to precipitate the product. After centrifugation to remove the precipitate, the product was washed again with methanol. This washing-centrifugation-dehydration process was repeated three times. The product was then directly extruded through an extruder set at 190 °C and 150 rpm. The resulting particles were dried in a vacuum oven at 80 °C for 7 h to obtain the nucleating agent, with a yield of 97%.

[0041] Example Characterization Test

[0042] The nucleating agents from Comparative Examples 1, 2, and Examples 1-6 were added at 10 wt% of the polylactic acid matrix and mixed at 190°C for 7 minutes to obtain modified polylactic acid. The modified polylactic acid was sandwiched between two 150 mm × 150 mm × 2 mm steel plates with polyimide films on their surfaces and hot-pressed at 190°C and 10 MPa pressure, then quenched to form amorphous polylactic acid films. One film each with a thickness of 300 μm and 150 μm were prepared.

[0043] The polylactic acid after the above mixing was tested for melt index using a melt indexer. The test conditions were 190℃ and 2.16kg. The test results are summarized in Table 1.

[0044] The non-isothermal crystallization properties of the above-mentioned polylactic acid compound were tested using a scanning calorimeter: Under N2 atmosphere, 7 mg of sample was heated from 30 °C to 200 °C at a rate of 10 °C / min and held at an isothermal temperature for 5 min; then the temperature was lowered from 200 °C to 30 °C at a rate of 10 °C / min; and then the temperature was raised from 30 °C to 200 °C at a rate of 10 °C / min. The crystallinity of the material was calculated based on the enthalpy of crystallization during the cooling process.

[0045] The amorphous membrane was cut into 50mm × 50mm square pieces and annealed on a hot plate at 110℃ for 30 minutes. The haze of the heat-treated membrane was then evaluated. The haze at three different points was measured and the average value was calculated. The test results are summarized in Table 1.

[0046] Table 1. Test results of crystallization properties, haze, heat resistance and melt index of nucleating agents in the examples and comparative examples.

[0047]

[0048]

Claims

1. A method for preparing a nucleating agent for heat-resistant transparent polylactic acid, characterized in that, Includes the following steps: S10, melamine, lactide, and polylactic acid are mixed in a molar ratio between 1:1:1 and 10:10:1 and added to a pyridine solution, wherein the polylactic acid is optically active and has a number average molecular weight of 80,000 to 250,000. Under S20 and nitrogen protection, the reaction temperature is 100~150℃, the reaction time is 3~10h, and the stirring speed is 100~200rpm; S30. After the reaction is completed, the reactor is cooled to room temperature, the product is precipitated in methanol, centrifuged to remove dryness, washed, granulated by a twin-screw extruder and then dried to obtain the nucleating agent. The nucleating agent is a mixture with the following structure: , , , , where n represents 500~2000.

2. The method for preparing the nucleating agent in heat-resistant transparent polylactic acid as described in claim 1, characterized in that, The nucleating agent prepared by this method has a yield of over 95%.

3. The method for preparing the nucleating agent in heat-resistant transparent polylactic acid as described in claim 1, characterized in that, The total mass ratio of melamine, lactide, and polylactic acid to pyridine is 1:

50.

4. The method for preparing the nucleating agent in heat-resistant transparent polylactic acid as described in claim 1, characterized in that, The extruder in S30 is a twin-screw extruder with at least one vacuum device, an aspect ratio of 40 to 50, an extrusion temperature of 170 to 200°C, and a screw speed of 100 to 250 rpm.

5. The method for preparing a nucleating agent in heat-resistant transparent polylactic acid as described in claim 1, characterized in that, The drying temperature in S30 is 60-80℃, and the drying time is 6-10h.

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