A polylactic acid ultrafine fiber, a method for preparing the same, and a nonwoven fiber membrane obtained thereby

Abstract

The application provides a polylactic acid superfine fiber, a preparation method thereof and a non-woven fiber membrane obtained from the polylactic acid superfine fiber. The polylactic acid superfine fiber is prepared from the following raw materials: 2.5-24.5% of topologically branched polylactic acid, 75-97% of linear polylactic acid and 0.5-2.5% of antioxidant. The application also provides a preparation method of the polylactic acid superfine fiber. The topologically branched polylactic acid is mixed with an equal amount of linear polylactic acid, melt-mixed, and then the remaining linear polylactic acid and antioxidant are added for extrusion and melt spinning to obtain the polylactic acid superfine fiber. The polylactic acid superfine fiber is laid to obtain the non-woven fiber membrane. The branched structure of the topologically branched polylactic acid destroys the chain entanglement structure of the linear polylactic acid, has a molecular chain conformation similar to a 'rolling ball', can significantly improve the rheological property of the melt, the polylactic acid superfine fiber has a small diameter, the non-woven fiber membrane obtained from the polylactic acid superfine fiber has high porosity, small pore size, strong barrier property and good comfort.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ultrafine fibers, in particular to a polylactic acid ultrafine fiber, a preparation method thereof and a non-woven fiber membrane obtained. BACKGROUND

[0002] The non-woven fiber membrane is a kind of non-woven material, also known as non-woven fabric, which is a sheet, web or batt made of fibers arranged in a certain direction or randomly combined by friction, adhesion, bonding or a combination of these methods. The thinner the fiber used in the non-woven fiber membrane, the higher the count, and the better the comfort. Ultrafine fibers have been widely used in non-woven fiber membranes due to their small diameter, large specific surface area and good mechanical and electromagnetic properties.

[0003] Polylactic acid is a biodegradable aliphatic polyester that can be synthesized from renewable crops and ultimately degraded into carbon dioxide (CO2) and water (H2O) and the like, which has little environmental pollution. Its fibers have the advantages of both natural fibers and synthetic fibers, and can be used to make non-woven fiber membranes. However, due to the high melt index of polylactic acid, the rheological properties of the melt are poor, which makes the diameter of the polylactic acid fiber larger, affecting the comfort and insulation performance of the non-woven fiber membrane obtained therefrom.

[0004] In the prior art, plasticizers are usually added to improve the rheological properties of the polylactic acid melt. For example, Chinese patent CN101759970A discloses a hydrolysis-resistant plasticized polylactic acid composition and a film thereof. The composition is composed of 50-89 parts by weight of polylactic acid resin, 10-49 parts by weight of plasticizer and 0.5-5 parts by weight of anti-hydrolysis stabilizer. The composition is mixed and then processed to obtain a film. This polylactic acid composition improves the rheological properties of polylactic acid by adding small or low molecular plasticizers and functional additives. However, this polylactic acid composition cannot balance the rheological properties of the melt and the strength of the melt, and the diameter of the polylactic acid ultrafine fiber obtained therefrom is still large, which reduces the comfort and insulation performance of the non-woven fiber membrane obtained therefrom. SUMMARY

[0005] The purpose of the present application is to provide a polylactic acid ultrafine fiber, a preparation method thereof and a non-woven fiber membrane obtained, which aims to solve the problem that the diameter of the polylactic acid ultrafine fiber obtained from the polylactic acid solution is large due to the poor rheological properties of the polylactic acid solution, thereby reducing the comfort and insulation performance of the non-woven fiber membrane obtained therefrom.

[0006] To solve the above technical problems, the technical solution of the present application is as follows:

[0007] In one aspect, the polylactic acid ultrafine fiber of the present application is prepared from the following raw materials in weight percentage: topologically branched polylactic acid 2.5-24.5%, linear polylactic acid 75-97%, antioxidant 0.5-2.5%.

[0008] The present application organically combines topologically branched polylactic acid and linear polylactic acid. The branched structure of the topologically branched polylactic acid destroys the chain entanglement structure of the linear polylactic acid, forming a "rolling ball-like" molecular chain conformation, which can significantly improve the rheological properties of the melt. Without adding solvents, other lubricants, flow modifiers or inorganic fillers, etc., the polylactic acid ultrafine fiber and its downstream products are completely degradable without secondary pollution. The obtained polylactic acid ultrafine fiber has a small diameter, and the non-woven fiber membrane obtained after laying has a high number of branches, a small pore size, a strong isolation performance, and a good comfort, avoiding the problems of performance degradation and failure due to internal microphase separation. It can be used in gas-liquid multiphase filtration, porous media and biomedical fields.

[0009] As a preferred embodiment, the polylactic acid ultrafine fiber is prepared from the following raw materials in weight percentage: topologically branched polylactic acid 9-19%, linear polylactic acid 80-90%, antioxidant 1-2%. In the present application, the proportion of topologically branched polylactic acid in the total raw materials is 2.5-24.5%, and the proportion of each raw material can be further optimized to make the topologically branched polylactic acid and linear polylactic acid better cooperate. Topologically branched polylactic acid and linear polylactic acid belong to "heterogeneous isomerism", and this homogeneous property ensures excellent compatibility between them.

[0010] As a preferred embodiment, the diameter of the polylactic acid ultrafine fiber is 500 nm-5 μm. The polylactic acid ultrafine fiber of the present application is a solid columnar fiber with smooth or wrinkled surface, and the diameter is in the range of 500 nm to 5 μm, having excellent biocompatibility and biodegradability.

[0011] As a preferred embodiment, the topologically branched polylactic acid has a single number average molecular weight of 1000-10000 g / mol, a melt index of 85-400 g / 10 min, and a topological branching number of 3-16. The molecular weight of the topologically branched polylactic acid of the present application is appropriate, and its melt index is higher than that of linear polylactic acid, and the branching degree is high. The viscosity of this topologically branched polylactic acid is small, and the rheological properties are good, which makes it easier to disperse in linear polylactic acid.

[0012] As a preferred embodiment, the topologically branched polylactic acid is any one of three-arm star polylactic acid, four-arm star polylactic acid, six-arm star polylactic acid, eight-arm star polylactic acid, and sixteen-arm star polylactic acid. The topologically branched polylactic acid of the present application preferably adopts three-arm and above star polylactic acid, which has a high degree of branching and is more likely to destroy the chain entanglement structure of linear polylactic acid, thereby forming a "rolling ball-like" molecular chain conformation.

[0013] As a preferred embodiment, the number average molecular weight of the linear polylactic acid is 8-15×10 4 g / mol, and the melt index is 30-85 g / 10 min. The linear polylactic acid of the present application adopts conventional linear polylactic acid, which is inexpensive, has multiple sources, is easy to obtain, and is convenient to use.

[0014] As a preferred embodiment, the antioxidant is a mixture composed of a primary antioxidant and a secondary antioxidant in a weight ratio of 0.5-3:1. The addition of the antioxidant in the present application is to improve the antioxidant performance of the polylactic acid ultrafine fibers, make them durable, and improve the service life. Preferably, the primary antioxidant and the secondary antioxidant are used in combination, complement each other, and fully play their roles.

[0015] As a preferred embodiment, the primary antioxidant is a hindered phenolic antioxidant or a hindered amine antioxidant, and the secondary antioxidant is a phosphite antioxidant, a thioester antioxidant, or a thioether antioxidant. The primary antioxidant and the secondary antioxidant of the present application can form hydrogen bond interactions, produce excellent synergistic effects, improve the antioxidant effect, and can also form hydrogen bond interactions with the carbonyl group of the ester bond in polylactic acid; thus, they have good compatibility, low volatility, and high antioxidant efficiency.

[0016] As a preferred embodiment, the hindered phenolic antioxidant is any one of a thio-phenolic antioxidant, a triazine hindered phenolic antioxidant, and a trimer isocyanate hindered phenolic antioxidant, and the hindered amine antioxidant is any one of a naphthylamine, a diphenylamine, a p-diphenylamine, and a quinoline derivative. The hindered phenolic antioxidant of the present application has little effect on the color of polylactic acid, is non-toxic or low-toxic, does not pollute the polylactic acid product, and does not affect its degradability, and can be used for contact with food; the hindered amine antioxidant of the present application has high antioxidant efficiency and has little effect on the degradability of polylactic acid.

[0017] In another aspect, the application provides a method for preparing polylactic acid ultrafine fibers, comprising the following steps: 1) mixing and stirring topologically branched polylactic acid and an equal amount of linear polylactic acid until uniform, melt-kneading at a temperature of 175-190°C and a speed of 30-60 r / min for 7-15 min, cooling, and granulating to obtain modified polylactic acid masterbatch; 2) adding the remaining linear polylactic acid and an antioxidant to the modified polylactic acid masterbatch obtained in step 1), stirring until uniform, extruding at a speed of 80-300 r / min and a temperature of 150-200°C for 2-10 min, and granulating to obtain formula masterbatch; and 3) melt-spinning the formula masterbatch obtained in step 2) at 180-230°C under a gas flow of 30-90 mL / min and drawing at a wind pressure of 10-50 Pa to obtain polylactic acid ultrafine fibers.

[0018] In the method for preparing polylactic acid ultrafine fibers, the topologically branched polylactic acid is used to modify linear polylactic acid, and the branched structure of the topologically branched polylactic acid destroys the chain entanglement structure of the linear polylactic acid. Then, the linear polylactic acid modified by the topologically branched polylactic acid is compounded with linear polylactic acid again to form formula masterbatch. When the formula masterbatch is melt-spun, the topologically branched polylactic acid can form a "rolling ball-like" molecular chain conformation, which can play a role in sliding plasticization and can significantly improve the melt rheological properties of the formula masterbatch. The topologically branched polylactic acid can be well and uniformly dispersed in the linear polylactic acid by stirring without adding solvents, other lubricants, flow modifiers, or inorganic fillers, etc., achieving complete degradation of polylactic acid ultrafine fibers and their downstream products without secondary pollution. Moreover, the modification of linear polylactic acid by the topologically branched polylactic acid uses a melt-kneading processing method, which does not cause adverse consequences such as solvent recovery and environmental pollution. In addition, the formula masterbatch has good processing performance during melt spinning, the melt is uniformly stretched, the obtained polylactic acid ultrafine fibers have a small diameter, the non-woven fiber membrane obtained after laying has a high count, good comfort, and strong isolation performance, and the problems of performance degradation and failure caused by internal micro-phase separation are avoided.

[0019] As a preferred embodiment, in step 1), the topologically branched polylactic acid and the equal amount of linear polylactic acid are both vacuum dried at 70-90°C for 12-24 h. The topologically branched polylactic acid and the linear polylactic acid are both vacuum dried in advance to sufficiently remove the moisture in the topologically branched polylactic acid and the linear polylactic acid, preventing the moisture from affecting subsequent processing. This drying process is convenient, has high drying efficiency, and does not adversely affect the topologically branched polylactic acid and the linear polylactic acid.

[0020] As a preferred embodiment, in the step 2), the modified polylactic acid master batch and the remaining linear polylactic acid are also vacuum dried at 70-90°C for 12-24h in advance. The modified polylactic acid master batch obtained by melt blending the topologically branched polylactic acid and the same amount of linear polylactic acid in the present application also needs to be vacuum dried in advance to remove all moisture in the material, ensuring the smooth progress of the spinning process, thereby improving the comprehensive performance of the polylactic acid ultrafine fibers.

[0021] As a preferred embodiment, in the step 1), the preparation method of the topologically branched polylactic acid is: a) taking lactide monomers, adding a polyol initiator and a catalyst, the molar ratio of hydroxyl groups in the polyol initiator to lactide monomers is 1:7-70, and the molar ratio of the catalyst to lactide monomers is 1-3:1000, to obtain a mixture; b) the mixture obtained in step a) is reacted at 115-130°C for 12-36h under inert gas atmosphere and stirring conditions to obtain a crude product; c) the crude product obtained in step b) is cooled, purified, and vacuum dried at 40-50°C for 24-48h to obtain the topologically branched polylactic acid. The topologically branched polylactic acid of the present application can be purchased directly or synthesized on site; the preparation method of this topologically branched polylactic acid uses lactide monomers as the starting material, and the monomers trigger ring-opening polymerization mechanism under the action of initiators and catalysts, and different hydroxyl content polyol initiators provide chain growth centers, thereby realizing the synthesis of topologically branched polylactic acid with different arm numbers; the catalyst has high catalytic activity, mild reaction conditions, short reaction time, high conversion rate, small catalyst dosage, and no organic solvent is used in the synthesis process, which avoids the yield and cost loss caused by solvent volatilization, has good economic benefits, high controllability of operation, convenient production adjustment, good applicability to different conventional linear polylactic acid, and can effectively promote the green and environmental protection development of non-woven polylactic acid products.

[0022] As a preferred embodiment, the lactide monomer is any one or several of L-lactide, D-lactide and meso-lactide. In the preparation process of the topologically branched polylactic acid of the present application, the lactide monomer can be L-lactide, D-lactide or meso-lactide, and the lactide monomer is sealed and stored in a desiccator before use to ensure that the lactide monomer is not contaminated with moisture during the ring-opening polymerization reaction.

[0023] As a preferred embodiment, the catalyst is any one or several of stannous octoate, stannous chloride, cyclohexanoxystannane, lithium chloride, aluminum isopropylate. The catalyst of the present application is stannous octoate, stannous chloride, cyclohexanoxystannane, lithium chloride, aluminum isopropylate, which can improve the reaction rate and yield, shorten the reaction time, reduce the generation of reaction waste, and can improve the selectivity of the reaction, avoid the occurrence of side reactions, and improve the purity and quality of the product. The ring-opening polymerization reaction of the present application belongs to the coordination-insertion polymerization mechanism, the reaction system is mild and efficient, the side reactions are few, the relative molecular mass distribution of the obtained topologically branched polylactic acid is narrow, and the controlled polymerization of topologically branched polylactic acid with specific structure can be realized; different topologically branched polyols can be used as initiators to synthesize topologically branched polylactic acid with different arm numbers.

[0024] As a preferred embodiment, the initiator is any one or several of glycerol, pentaerythritol, dipentaerythritol, tripentaerythritol, sorbitol, hydroxylated cage polysilsesquioxane, and Boltorn H20. The relative molecular mass of the topologically branched polylactic acid synthesized in the present application is determined by the molar ratio of active hydroxyl groups in the initiator to lactide monomers. The molecular structure and relative molecular mass of the topologically branched polylactic acid can be controlled and adjusted by changing the type of initiator and the feeding ratio of initiator to lactide monomers, and the topologically branched polylactic acid can be synthesized according to the specific attribute parameters of conventional linear polylactic acid on the market, which has excellent product applicability.

[0025] In still another aspect, a non-woven fiber membrane of the present application is obtained by laying the polylactic acid ultrafine fibers obtained by the method for preparing polylactic acid ultrafine fibers according to any one of the above.

[0026] Generally, the polylactic acid ultrafine fibers of the present application are prepared by melting, spinning and stretching the formula master batch in a melt spinning device equipped with a pneumatic drafting device. The master batch is first heated and melted, then extruded from the spinneret by gas extrusion, and stretched under the gas field to form polylactic acid ultrafine fibers. Then, the polylactic acid ultrafine fibers are randomly laid on the laying suction air device to obtain a non-woven fiber membrane. The vertical distance between the spinneret and the upper edge of the pneumatic drafting device is 5-20 cm, and the vertical distance between the lower edge of the pneumatic drafting device and the laying collection device is 5-20 cm.

[0027] Compared with the prior art, the application has the beneficial effects that: the application adopts topological branched polylactic acid to modify linear polylactic acid, the topological branched polylactic acid can form a "rolling ball type" molecular chain conformation, the branched structure can destroy the chain entanglement structure of linear polylactic acid, and play a role of sliding plasticization in the melt processing process, so that the rheological property of the melt can be significantly improved; the two types of polylactic acid belong to "heterogeneous isomerism", and can be uniformly dispersed through melt stirring, without adding solvents, other lubricants, flow modifiers or inorganic fillers and the like, so that the polylactic acid ultrafine fiber and downstream products thereof are completely degraded without secondary pollution; moreover, the melt of the formula master batch has good processing performance in the melt spinning process, and is uniformly stretched; the obtained polylactic acid ultrafine fiber has a small diameter, and the non-woven fiber membrane obtained after laying has a high number of branches, a small pore size, strong isolation performance and good comfort, and the problems of performance degradation and failure caused by internal microphase separation are avoided, and the polylactic acid ultrafine fiber can be used in the fields of gas-liquid multiphase filtration, porous medium and biological medicine. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 It is a schematic diagram of the three-dimensional structure of the spinning equipment used in the application;

[0029] Figure 2 It is a scanning electron microscope photo of the non-woven fiber membrane obtained in Example One;

[0030] Figure 3 It is a scanning electron microscope photo of the non-woven fiber membrane obtained in Example Two;

[0031] Figure 4 It is a scanning electron microscope photo of the non-woven fiber membrane obtained in Example Three;

[0032] Figure 5 It is a scanning electron microscope photo of the non-woven fiber membrane obtained in Example Four;

[0033] Figure 6 It is a scanning electron microscope photo of the non-woven fiber membrane obtained in Example Five;

[0034] Figure 7 It is a scanning electron microscope photo of the non-woven fiber membrane obtained in the control sample One of the application;

[0035] Figure 8 It is a scanning electron microscope photo of the non-woven fiber membrane obtained in the control sample Two of the application;

[0036] Figure 9 It is a melt shear rheological diagram of different formula master batches at 200 DEG C;

[0037] Figure 10 It is a melt shear rheological diagram of different formula master batches at 220 DEG C;

[0038] In the figure: 1 - nitrogen storage tank; 2 - gas flow control valve; 3 - gas flow meter; 4 - gas guide pipe; 5 - barrel; 6 - thermocouple heating ring; 7 - spinneret; 8 - gun type drafter; 9 - iron stand; 10 - screen curtain; 11 - suction fan;

[0039] 1 - Example 1; 2 - Example 2; 3 - Example 3; 4 - Example 4; 5 - Example 5; D1 - Control Sample 1; D2 - Control Sample 2. DETAILED DESCRIPTION

[0040] The technical solutions of the present application will be described below in conjunction with specific embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0041] The polylactic acid ultrafine fiber of the present application is prepared from the following raw materials in weight percentage: topologically branched polylactic acid 2.5-24.5%, linear polylactic acid 75-97%, antioxidant 0.5-2.5%.

[0042] Preferably, it is prepared from the following raw materials in weight percentage: topologically branched polylactic acid 9-19%, linear polylactic acid 80-90%, antioxidant 1-2%.

[0043] Preferably, the diameter of the polylactic acid ultrafine fiber is 500 nm-5 μm.

[0044] Preferably, the number average molecular weight of the topologically branched polylactic acid is 1000-10000 g / mol, the melt index is 85-400 g / 10 min, and the topological branching number is 3-16.

[0045] Preferably, the topologically branched polylactic acid is any one of three-arm star polylactic acid, four-arm star polylactic acid, six-arm star polylactic acid, eight-arm star polylactic acid, and sixteen-arm star polylactic acid.

[0046] Preferably, the number average molecular weight of the linear polylactic acid is 8-15 x 10 4 g / mol, and the melt index is 30-85 g / 10 min.

[0047] Preferably, the antioxidant is a mixture composed of a primary antioxidant and a secondary antioxidant in a weight ratio of 0.5-3:1.

[0048] Preferably, the primary antioxidant is a hindered phenolic antioxidant or a hindered amine antioxidant, and the secondary antioxidant is a phosphite antioxidant, a thioester antioxidant or a thioether antioxidant.

[0049] Preferably, the hindered phenolic antioxidant is any one of a thio-phenolic antioxidant, a triazine hindered phenolic antioxidant, a trimer isocyanate hindered phenolic antioxidant, and the hindered amine antioxidant is any one of a naphthylamine, a diphenylamine, a p-diphenylamine, a quinoline derivative.

[0050] A method for preparing polylactic acid ultrafine fibers according to the present application comprises the following steps:

[0051] 1) Take topologically branched polylactic acid and an equal amount of linear polylactic acid, mix, stir evenly, melt and mix, the mixing temperature is 175-190℃, the mixing speed is 30-60r / min, the mixing time is 7-15min, cool, cut into particles, and obtain modified polylactic acid masterbatch;

[0052] 2) Take the remaining linear polylactic acid and antioxidant, add to the modified polylactic acid masterbatch obtained in step 1), stir evenly, extrude, the extrusion speed is 80-300r / min, the extrusion temperature is 150-200℃, the extrusion time is 2-10min, and granulate to obtain formula masterbatch;

[0053] 3) Melt the formula masterbatch obtained in step 2) at 180-230℃, spin under a gas flow of 30-90mL / min, and draw under a wind pressure of 10-50Pa to obtain polylactic acid ultrafine fibers.

[0054] Preferably, in step 1), the topologically branched polylactic acid and the equal amount of linear polylactic acid are both previously vacuum dried at 70-90℃ for 12-24h.

[0055] Preferably, in step 2), the modified polylactic acid masterbatch and the remaining linear polylactic acid are also previously vacuum dried at 70-90℃ for 12-24h.

[0056] Preferably, in step 1), the method for preparing topologically branched polylactic acid is:

[0057] a) Take lactide monomers, add polyol initiator and catalyst, the molar ratio of hydroxyl groups in the polyol initiator to lactide monomers is 1:7-70, and the molar ratio of catalyst to lactide monomers is 1-3:1000, to obtain a mixture;

[0058] b) React the mixture obtained in step a) under inert gas atmosphere and stirring conditions at 115-130℃ for 12-36h to obtain a crude product;

[0059] c) cooling, purifying, drying at 40-50°C under vacuum for 24-48h, the crude product obtained in step b) to obtain topologically branched polylactic acid.

[0060] Preferably, the lactide monomer is any one or several of L-lactide, D-lactide and meso-lactide.

[0061] Preferably, the catalyst is any one or several of stannous octoate, stannous chloride, cyclohexane tin oxide, lithium chloride, aluminum isopropoxide.

[0062] Preferably, the initiator is any one or several of glycerol, pentaerythritol, dipentaerythritol, tripentaerythritol, sorbitol, hydroxylated cage polysilsesquioxane, Boltorn H20.

[0063] A non-woven fiber membrane of the present application is obtained by laying the polylactic acid ultrafine fibers obtained by the method for preparing polylactic acid ultrafine fibers according to any one of the above.

[0064] The melt spinning equipment used in the preparation of the polylactic acid ultrafine fibers of the present application is the existing conventional melt spinning equipment, as shown in the attached Figure 1 The melt spinning equipment includes a cylinder 5, the outside of which is provided with a thermocouple heating ring 6, which heats the cylinder 5, and the thermocouple heating ring 6 can be set to a heating temperature to fully ensure the melting temperature of the spinning raw material (i.e. melt) in the cylinder 5; the bottom end of the cylinder 5 is connected to a spinneret 7, below which is provided with a pneumatic drafting device, which is a gun-type draft 8, below which is a laying suction air device; the laying suction air device includes a screen curtain 10, which has a suction air 11 vertically passing through its surface. The cylinder 5 and the gun-type draft 8 are fixed and adjusted in relative position by an iron stand 9. The extrusion of the melt raw material adopts a gas extrusion propulsion mode, the cylinder 5 is connected to a gas guide pipe 4, the other end of which is connected to a nitrogen storage tank 1, between the gas guide pipe 4 and the nitrogen storage tank 1 is connected through a gas flow control valve 2, and a gas flow meter 3 is used to monitor the gas flow.

[0065] Example One

[0066] A method for preparing polylactic acid ultrafine fibers of the present application includes the following steps:

[0067] 1) The raw materials are weighed according to the following weight percentage: topologically branched polylactic acid 2.5%, linear polylactic acid 97%, antioxidant naphthylamine 0.5%;

[0068] The topologically branched polylactic acid has a single number average molecular weight of 1000 g / mol, a melt index of 200 g / 10 min, and a topological branching number (total number of arms) of 16.

[0069] The number average molecular weight of the linear polylactic acid is 8 x 10 4 g / mol, and the melt index is 85 g / 10 min;

[0070] 2) Take the topologically branched polylactic acid and an equal amount of linear polylactic acid, respectively, vacuum dry at 70℃ for 24h, mix, stir evenly, add to the preheated torque rheometer for melt mixing, the mixing temperature is 175℃, the mixing speed is 30r / min, the mixing time is 15min, cool, granulate, get modified polylactic acid master batch;

[0071] 3) Take the remaining linear polylactic acid and antioxidant naphthylamine, vacuum dry the linear polylactic acid and the modified polylactic acid master batch obtained in step 2) at 70℃ for 24h, mix, and add antioxidant naphthylamine, stir evenly, add to the twin-screw extruder for extrusion, the screw speed is 80r / min, the extrusion temperature is 150℃, the extrusion time is 10min, granulation, get melt modified composite formula master batch;

[0072] 4) The melt modified composite formula master batch obtained in step 3) is melt spun on the melt spinning equipment shown in the accompanying drawings, the melt temperature is 180℃, the spinning is performed under the air flow of 30mL / min, and the drawing is performed under the air pressure of 10Pa, to obtain polylactic acid ultrafine fibers. Figure 1

[0073] Example two

[0074] A method for preparing polylactic acid ultrafine fibers according to the present application comprises the following steps:

[0075] S1 Preparation of topologically branched polylactic acid

[0076] a) Take monomer - L-lactide, initiator - pentaerythritol, catalyst - stannous octoate, the molar ratio of hydroxyl groups in the initiator to lactide monomers is 1:17.35, and the molar ratio of stannous octoate to lactide monomers is about 1:1000, add the initiator and catalyst to the lactide monomers in turn to obtain a mixed system;

[0077] b) The mixed system obtained in step a) is subjected to vacuum-nitrogen filling treatment, and the cycle is 3 times, then the mixed system obtained in step a) is subjected to oil bath reaction at 120℃ under the conditions of nitrogen atmosphere and magnetic stirring for 24h to obtain a crude product;

[0078] c) The crude product obtained in step b) is cooled, dissolved in dichloromethane, purified by ice methanol precipitation, and vacuum dried at 50℃ for 48h to obtain topologically branched polylactic acid;

[0079] ​S2 according to the following weight percentage of raw materials: topological branched polylactic acid obtained in step S1 20%, linear polylactic acid 77.5%, antioxidant 2.5%;

[0080] The topological branched polylactic acid obtained in step S1 is a four-arm star polylactic acid, and the number average molecular weight of the single arm is 2500 g / mol; the number average molecular weight of the linear polylactic acid is 8 x 10 4 g / mol, and the melt index is 85 g / 10 min; the antioxidant is a mixture of a main antioxidant, thio-phenol antioxidant bis(3,5-di-tert-butyl-4-hydroxybenzyl) sulfide, and an auxiliary antioxidant, thio-ester antioxidant bis-dipropionate, with a weight ratio of 3:1;

[0081] S3 takes topological branched polylactic acid and an equal amount of linear polylactic acid, respectively, and is vacuum dried at 90℃ for 12h, mixed, stirred uniformly, added to a preheated torque rheometer for melt mixing, the mixing temperature is 190℃, the mixing speed is 60r / min, the mixing time is 7min, cooled, granulated, and the modified polylactic acid master batch is obtained;

[0082] S4 takes the remaining linear polylactic acid and antioxidant, and the linear polylactic acid and the modified polylactic acid master batch obtained in step S3 are vacuum dried at 90℃ for 12h, respectively, mixed, and the antioxidant is added, stirred uniformly, added to a twin-screw extruder for extrusion, the screw speed is 300r / min, the extrusion temperature is 200℃, the extrusion time is 2min, and the granulation is performed to obtain the melt modified composite formula master batch;

[0083] S5 melts the melt modified composite formula master batch obtained in step S4 on the melt spinning equipment shown in the accompanying drawings, the melting temperature is 230℃, the spinning is performed under the air flow of 90mL / min, the drawing is performed under the air pressure of 50Pa, and the polylactic acid ultrafine fiber is obtained. Figure 1

[0084] Example Three

[0085] A preparation method of the polylactic acid ultrafine fiber of the present application comprises the following steps:

[0086] S1 preparation of topological branched polylactic acid

[0087] a) take monomer-lactide, initiator-bisquintanol, catalyst-stannous chloride, the molar ratio of hydroxyl in the initiator to lactide monomer is 1:11.1, and the molar ratio of stannous chloride to lactide monomer is about 3:1000, the initiator and catalyst are sequentially added in the lactide monomer, and a mixed system is obtained;

[0088] ​b) vacuum-nitrogen treatment is performed on the mixture system obtained in step a) for 3 cycles, and then the mixture system obtained in step a) is subjected to oil bath reaction at 115℃ under nitrogen atmosphere and magnetic stirring for 36h to obtain a crude product;

[0089] c) the crude product obtained in step b) is cooled, dissolved in dichloromethane, purified by ice methanol precipitation, vacuum dried at 40℃ for 24h to obtain the topologically branched polylactic acid;

[0090] S2: raw materials are weighed according to the following weight percentage: 15% of the topologically branched polylactic acid obtained in step S1, 84% of linear polylactic acid, and 1% of antioxidant;

[0091] The topologically branched polylactic acid obtained in step S1 is a six-arm star polylactic acid with a number average molecular weight of 1600g / mol; the linear polylactic acid has a number average molecular weight of 1.5×10 5 g / mol and a melt index of 30g / 10min; the antioxidant is a mixture of primary antioxidant, i.e., antioxidant p-diphenylamine, and auxiliary antioxidant, i.e., phosphite antioxidant, with a weight ratio of 2:1;

[0092] S3: the topologically branched polylactic acid and an equal amount of linear polylactic acid are vacuum dried at 80℃ for 18h, mixed and stirred uniformly, added to a preheated torque rheometer for melt mixing, the mixing temperature is 180℃, the mixing speed is 40r / min, the mixing time is 10min, cooled, pelletized to obtain a modified polylactic acid master batch;

[0093] S4: the remaining linear polylactic acid and antioxidant are vacuum dried at 80℃ for 18h, mixed, and the antioxidant is added, stirred uniformly, added to a twin-screw extruder for extrusion, the screw speed is 200r / min, the extrusion temperature is 180℃, the extrusion time is 6min, pelletized to obtain a melt modified composite formulation master batch;

[0094] S5: the melt modified composite formulation master batch obtained in step S4 is subjected to melt spinning on a melt spinning device as shown in the accompanying drawings, the melt temperature is 200℃, the spinning is performed under a gas flow of 60mL / min, the drawing is performed under a wind pressure of 30Pa to obtain polylactic acid ultrafine fibers. Figure 1

[0095] Example Four

[0096] A method for preparing polylactic acid ultrafine fibers according to the present application comprises the following steps:

[0097] S1: preparing topologically branched polylactic acid

[0098] ​a) taking monomer - L-lactide, initiator - glycerol, catalyst - cycloalkyltin oxide, the molar ratio of hydroxyl group in initiator to lactide monomer is 1:7, the molar ratio of cycloalkyltin oxide to lactide monomer is about 2:1000, adding initiator and catalyst in lactide monomer in turn to obtain a mixed system;

[0099] b) vacuumizing and nitrogen-filling the mixed system obtained in step a) for 3 cycles, then the mixed system obtained in step a) is reacted at 130℃ under nitrogen atmosphere and magnetic stirring for 12h in an oil bath to obtain a crude product;

[0100] c) cooling the crude product obtained in step b), dissolving in dichloromethane, purifying by ice methanol precipitation, vacuum drying at 45℃ for 36h to obtain topologically branched polylactic acid;

[0101] S2 taking raw materials according to the following weight percentage: topologically branched polylactic acid obtained in step S1 24.5%, linear polylactic acid 75%, antioxidant 0.5%;

[0102] wherein the topologically branched polylactic acid obtained in step S1 is three-arm star polylactic acid with a number average molecular weight of 1000g / mol; the linear polylactic acid has a number average molecular weight of 1.0x10 5 g / mol and a melt index of 55g / 10min; the antioxidant is a mixture of primary antioxidant - triazine hindered phenolic antioxidant 6-(4-hydroxy-3,5-di-tert-butylphenylamino)-2,4-di-n-octylthio-1,3,5-triazine and secondary antioxidant - thioester antioxidant pentaerythritol tetrakis(3-laurylthiopropionate) with a weight ratio of 1:1;

[0103] S3 taking topologically branched polylactic acid and an equal amount of linear polylactic acid, vacuum drying at 80℃ for 20h respectively, mixing and stirring uniformly, adding to a preheated torque rheometer for melt mixing, the mixing temperature is 185℃, the mixing speed is 50r / min, the mixing time is 12min, cooling, granulating to obtain modified polylactic acid master batch;

[0104] S4 taking the remaining linear polylactic acid and antioxidant, vacuum drying the linear polylactic acid and the modified polylactic acid master batch obtained in step S3 at 80℃ for 20h respectively, mixing and adding antioxidant, stirring uniformly, adding to a twin-screw extruder for extrusion, the screw speed is 150r / min, the extrusion temperature is 160℃, the extrusion time is 8min, granulating to obtain melt modified composite formulation master batch;

[0105] S5 adding the melt modified composite formulation master batch obtained in step S4 to a film blowing machine to obtain a melt modified composite film; Figure 1The melt spinning equipment shown is used for melt spinning, the melt temperature is 215℃, the spinning is performed under a gas flow of 40mL / min, the drawing is performed under a wind pressure of 40Pa, and polylactic acid ultrafine fibers are obtained.

[0106] Example Five

[0107] A method for preparing polylactic acid ultrafine fibers according to the present application comprises the following steps:

[0108] S1. Preparation of topologically branched polylactic acid

[0109] a) monomer - meso-lactide, initiator - pentaerythritol, catalyst - aluminum isopropoxide, the molar ratio of hydroxyl groups in the initiator to lactide monomers is 1:69.4, and the molar ratio of aluminum isopropoxide to lactide monomers is about 2:1000, the initiator and the catalyst are sequentially added to the lactide monomers to obtain a mixed system;

[0110] b) the mixed system obtained in step a) is subjected to vacuum-nitrogen filling treatment, and the cycle is repeated for 3 times, then the mixed system obtained in step a) is subjected to oil bath reaction at 125℃ under the conditions of nitrogen atmosphere and magnetic stirring for 36h to obtain a crude product;

[0111] c) the crude product obtained in step b) is cooled, dissolved in dichloromethane, and purified by ice methanol precipitation, and vacuum dried at 45℃ for 36h to obtain topologically branched polylactic acid;

[0112] S2. The raw materials are weighed according to the following weight percentage: 10% of the topologically branched polylactic acid obtained in step S1, 88% of linear polylactic acid, and 2% of antioxidant;

[0113] The topologically branched polylactic acid obtained in step S1 is a four-arm star-shaped polylactic acid with a number average molecular weight of 10000g / mol; the linear polylactic acid has a number average molecular weight of 8×10 4 g / mol and a melt index of 85g / 10min; the antioxidant is a mixture of primary antioxidant - antioxidant diphenylamine and auxiliary antioxidant - phosphite antioxidant in a weight ratio of 0.5:1;

[0114] S3. The topologically branched polylactic acid and an equal amount of linear polylactic acid are vacuum dried at 80℃ for 20h, mixed, stirred uniformly, and added to a preheated torque rheometer for melt mixing, the mixing temperature is 180℃, the mixing speed is 40r / min, the mixing time is 10min, and the mixture is cooled, granulated, and a modified polylactic acid master batch is obtained;

[0115] S4 Take the remaining linear polylactic acid and antioxidant, and vacuum dry the linear polylactic acid and the modified polylactic acid masterbatch obtained in step S3 at 80℃ for 20h respectively. Mix them, add antioxidant, stir evenly, and add them to a twin-screw extruder for extrusion. The screw speed is 250r / min, the extrusion temperature is 180℃, the extrusion time is 4min, and granulation is performed to obtain melt modified composite formulation masterbatch.

[0116] S5. The melt-modified composite masterbatch obtained in step S4 is then applied to the attached... Figure 1 Melt spinning is performed on the melt spinning equipment shown, with a melting temperature of 190℃, and the fibers are spun under an airflow of 80mL / min and drawn under an air pressure of 20Pa to obtain polylactic acid microfiber.

[0117] Experiment 1

[0118] The polylactic acid microfibers obtained in Examples 1 to 5 of the present invention were randomly laid on a web-laying and suction device to obtain a nonwoven fiber membrane, which was used as an experimental sample.

[0119] Haizheng polylactic acid in the appendix Figure 1 Melt spinning was performed on the melt spinning equipment shown, with a melting temperature of 190℃. The fibers were spun under an airflow of 80 mL / min and drawn under an air pressure of 20 Pa to obtain polylactic acid fibers. The fibers were then randomly laid on a web-laying and suction device to obtain a nonwoven fiber membrane, which served as a control sample 1.

[0120] A hydrolysis-resistant plasticized polylactic acid composition was prepared according to the method disclosed in Example 1 of Chinese Patent CN101759970A mentioned in the background art. (See attached...) Figure 1 Melt spinning was performed on the melt spinning equipment shown, with a melting temperature of 190℃. The fibers were spun under an airflow of 80 mL / min and drawn under an air pressure of 20 Pa to obtain polylactic acid fibers. The fibers were then randomly laid on a web-laying and suction device to obtain a nonwoven fiber membrane, which served as control sample two.

[0121] The experimental sample, control sample 1, and control sample 2 were observed using a scanning electron microscope (MVE0352891782) manufactured by Phenom GmbH, Germany. (See attached image.) Figures 2-6 It can be seen that the nonwoven fiber membrane (experimental sample) obtained by this invention is composed of smooth, solid columnar fibers. The fibers in the nonwoven fiber membrane are arranged randomly with a uniform diameter distribution. Specifically, the average fiber diameter in Example 1 is 4.9 μm, in Example 2 it is 1.8 μm, in Example 3 it is 2.5 μm, in Example 4 it is 0.5 μm, and in Example 5 it is 3.5 μm. Figure 7It can be seen that the fibers in the non-woven fiber film (control sample one) obtained from Haizheng polylactic acid 210 exhibit flat columnar structure, smooth surface, and an average diameter of about 6.9 μm; the fibers in the non-woven fiber film (control sample two) obtained from the composite of the background art exhibit flat columnar structure, but a little particles appear on the surface of the fibers, and the average diameter of the fibers is higher, reaching 12.4 μm. Figure 8 It can be seen that the fibers in the non-woven fiber film (control sample one) obtained from Haizheng polylactic acid 210 exhibit flat columnar structure, smooth surface, and an average diameter of about 6.9 μm; the fibers in the non-woven fiber film (control sample two) obtained from the composite of the background art exhibit flat columnar structure, but a little particles appear on the surface of the fibers, and the average diameter of the fibers is higher, reaching 12.4 μm.

[0122] In the present application, the average diameter of the fibers is measured by using Nano Measurer software to measure the diameters of 100 fibers randomly selected from the scanning electron microscope image and with clear outlines, and then the average diameter of the fibers is calculated.

[0123] Experiment 2

[0124] The melt modified composite master batch for preparing polylactic acid ultrafine fibers obtained from the examples one to five of the present application, linear polylactic acid Haizheng 210 (control sample one), and the hydrolysis-resistant plasticized polylactic acid composition (control sample two) prepared according to the method disclosed in example one of the Chinese patent CN101759970A mentioned in the background art were respectively placed in a dynamic shear rheometer of MCR302 type produced by Anton Paar to perform viscosity test.

[0125] The melt complex viscosity of the modified composite master batch for preparing polylactic acid ultrafine fibers obtained from the examples one to five of the present application is significantly lower than that of the linear polylactic acid Haizheng 210 (control sample one) and the hydrolysis-resistant plasticized polylactic acid composition (control sample two) at 200℃ and 220℃. Figure 9 and Figure 10 It can be seen that, whether at 200℃ or at 220℃, the melt complex viscosity of the modified composite master batch for preparing polylactic acid ultrafine fibers obtained from the examples one to five of the present application is significantly lower than that of the linear polylactic acid Haizheng 210 (control sample one) and the hydrolysis-resistant plasticized polylactic acid composition (control sample two).

[0126] According to the commonly used processing temperature (200℃ and 220℃) and the machine speed (10 rad / s, i.e. 100 r / min) of polylactic acid fibers, the melt complex viscosities in the tables 2 and 3 are summarized and listed in table 1. Figure 9 and Figure 10

[0127] It can be seen from table 1 that the complex viscosities of the melt modified composite master batch for preparing polylactic acid ultrafine fibers obtained from the examples one to five of the present application at 200℃ and 220℃ are significantly lower than those of the linear polylactic acid Haizheng 210 (control sample one) and the hydrolysis-resistant plasticized polylactic acid composition (control sample two); therefore, the topologically branched polylactic acid of the present application can significantly improve the rheological properties of the melt.

[0128] Table 1 performance test results of different polylactic acid fibers

[0129] ​

[0130] Compared with the prior art, the application has the beneficial effects that: the application adopts topological branched polylactic acid to modify linear polylactic acid, the topological branched polylactic acid can form a "rolling ball type" molecular chain conformation, the branched structure can destroy the chain entanglement structure of linear polylactic acid, and play a role in sliding plasticization in the melt processing process, so that the rheological properties of the melt can be significantly improved; the two types of polylactic acid belong to "heterogeneous isomerism", and can be uniformly dispersed through melt stirring, without adding solvents, other lubricants, flow modifiers or inorganic fillers, etc., realizing the complete degradation of polylactic acid ultrafine fibers and downstream products without secondary pollution; moreover, the processing performance of the formula masterbatch melt is good in the melt spinning process, and the stretching is uniform; the obtained polylactic acid ultrafine fibers have small diameters, the nonwoven fiber membrane obtained after laying has high count, small pore size, strong isolation performance and good comfort, and the problems of performance decline and failure caused by internal microphase separation are avoided, and can be used in gas-liquid multiphase filtration, porous media and biological medical fields.

[0131] The above only describes the preferred embodiments of the application and is not intended to limit the application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the application shall be included in the protection scope of the application.

Claims

1. A method for producing polylactic acid ultrafine fibers, characterized by, The method comprises the following steps: 1) the raw materials are weighed according to the following weight percentage: topologically branched polylactic acid 2.5-24.5%, linear polylactic acid 75-97%, antioxidant 0.5-2.5%; The topologically branched polylactic acid is any one of three-arm star polylactic acid, four-arm star polylactic acid, six-arm star polylactic acid, eight-arm star polylactic acid, and sixteen-arm star polylactic acid. The antioxidant is a mixture of primary antioxidants and auxiliary antioxidants in a weight ratio of 0.5-3:

1. The primary antioxidant is a hindered phenolic antioxidant or a hindered amine antioxidant. The auxiliary antioxidant is a phosphite antioxidant, a thioester antioxidant, or a thioether antioxidant. 2) The topologically branched polylactic acid and an equal amount of linear polylactic acid are vacuum dried at 70-90 ℃ for 12-24 h, mixed, stirred uniformly, melt mixed, the mixing temperature is 175-190 ℃, the mixing speed is 30-60 r / min, the mixing time is 7-15 min, cooled, and granulated to obtain a modified polylactic acid master batch; 3) The remaining linear polylactic acid, antioxidant, and the modified polylactic acid master batch obtained in step 2) are mixed, the antioxidant is added, stirred uniformly, extruded, the extrusion speed is 80-300 r / min, the extrusion temperature is 150-200 ℃, the extrusion time is 2-10 min, and granulated to obtain a formula master batch; 4) The formula master batch obtained in step 3) is melted at 180-230 ℃, spun under a gas flow of 30-90 mL / min, and drawn under a wind pressure of 10-50 Pa to obtain polylactic acid ultrafine fibers.

2. The method according to claim 1, wherein: In step 1), the raw materials are weighed according to the following weight percentage: topologically branched polylactic acid 9-19%, linear polylactic acid 80-90%, and antioxidant 1-2%.

3. The method according to claim 1, wherein: The diameter of the polylactic acid ultrafine fibers is 500 nm-5 μm.

4. The method according to claim 1, wherein: The topologically branched polylactic acid has a number average molecular weight of 1000-10000 g / mol, a melt index of 85-400 g / 10 min, and a topological branching number of 3-16.

5. The method according to claim 1, wherein: The linear polylactic acid has a number average molecular weight of 8-15 x 10 4 g / mol and a melt index of 30-85 g / 10 min.

6. The method according to claim 1, wherein: The hindered phenolic antioxidant is any one of a thio-phenolic antioxidant, a triazine hindered phenolic antioxidant, and a triisocyanate hindered phenolic antioxidant. The hindered amine antioxidant is any one of a naphthylamine, diphenylamine, p-diphenylamine, and a quinoline derivative.

7. The method of producing polylactic acid ultrafine fibers according to any one of claims 1 to 6, characterized by, In step 1), the method for preparing the topologically branched polylactic acid is: a) taking lactide monomers, adding polyol initiator and catalyst, the molar ratio of hydroxyl in polyol initiator to lactide monomers being 1:7-70, the molar ratio of catalyst to lactide monomers being 1-3:1000, to obtain a mixture; b) reacting the mixture obtained in step a) at 115-130 ℃ under inert gas atmosphere and stirring for 12-36 h to obtain a crude product; c) cooling the crude product obtained in step b), purifying, vacuum drying at 40-50 ℃ for 24-48 h to obtain topologically branched polylactic acid.

8. The preparation method of polylactic acid ultrafine fibers according to claim 7, characterized in that: the lactide monomers are any one or several of L-lactide, D-lactide and meso-lactide.

9. The preparation method of polylactic acid ultrafine fibers according to claim 7, characterized in that: the catalyst is any one or several of stannous octoate, stannous chloride, cyclohexanoato tin, lithium chloride and aluminum isopropoxide.

10. The preparation method of polylactic acid ultrafine fibers according to claim 7, characterized in that: the initiator is any one of glycerol, pentaerythritol, dipentaerythritol, tripentaerythritol, sorbitol, hydroxylated cage polysilsesquioxane and Boltorn H20.

11. A non-woven fiber membrane, characterized in that: the polylactic acid ultrafine fibers obtained by the preparation method of polylactic acid ultrafine fibers according to any one of claims 1-10 are laid to obtain a non-woven fiber membrane.

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