CN112064126A

CN112064126A - Preparation method of polylactic acid short fiber

Info

Publication number: CN112064126A
Application number: CN202010847054.7A
Authority: CN
Inventors: 陈中碧; 杨阳; 谢贤勇
Original assignee: Anhui Fengyuan Bio Fiber Co Ltd
Current assignee: Anhui Fengyuan Bio Fiber Co Ltd
Priority date: 2018-09-14
Filing date: 2018-09-14
Publication date: 2020-12-11
Also published as: CN109234820A; CN109234820B

Abstract

The invention provides a preparation method of polylactic acid short fiber, which takes fiber-grade polylactic acid (PLA) as a raw material and prepares the polylactic acid (PLA) short fiber through the processes of vacuum drying, screw extrusion melting, spinning, cooling, bundling, drafting, heat treatment, tow curling and cutting. The method has simple process route, and the polylactic acid (PLA) short fiber prepared after batch industrialization has excellent performance, the linear density of the short fiber is 0.56 dtex-6.67 dtex, the comprehensive index completely meets the requirements of yarns for knitting and weaving, and the polylactic acid (PLA) short fiber has better market application value.

Description

Preparation method of polylactic acid short fiber

The invention is a divisional application of 201811076224.5-a preparation method of polylactic acid short fiber

Technical Field

The invention relates to the technical field of short fiber preparation, in particular to a preparation method of polylactic acid (PLA) short fiber.

Background

Polylactic acid (PLA) fiber is one of the green fibers with the greatest development prospect at present, and has the advantages of environmental protection, no toxicity, antibiosis, flame retardation, allergy prevention, degradability and renewable raw materials. The polylactic acid fiber has excellent physical characteristics and wide application, has high dry and wet strength and initial modulus, good dimensional stability, integrates the comfort of cotton, the draping of viscose, the strength of terylene and the hand feeling of real silk into a whole, has excellent wearability, and is widely applied to the fields of clothing, fabrics, underwear, home textiles, sanitary materials, health care products and the like.

The polylactic resin can be prepared by ring-opening polymerization of lactide or direct polymerization of lactic acid. Lactic acid can be obtained by fermenting starch, and the starch has wide sources, and renewable natural plants such as sweet potato, corn and other grains can be used as raw materials. In addition, the waste polylactic acid products can be rapidly decomposed into carbon dioxide and water in nature, and the environment-friendly effect is very good. Therefore, as an ideal green fiber, polylactic acid has a wide application prospect of sustainable development, and the development and application of the fiber are one of approaches for solving white pollution, and undoubtedly have good social benefits and development prospects.

At present, the production line for producing the polylactic acid short fibers in China is basically based on the original polyester short fiber production line, the equipment of the production line is non-professional equipment, and the process route and technical parameters are adjusted according to the polyester, so that the product quality is unstable, and the real industrialization cannot be achieved. In addition, the characteristics of the polylactic acid chip are obviously different from those of the polyester, so that the melt spinning process of the polylactic acid chip is obviously different from that of the polyester. Polylactic acid can be made into fibers by solution spinning and melt spinning. At present, dichloromethane, trichloromethane and toluene are commonly used as solvents for solution spinning, but the solvents are toxic, the spinning environment is severe, the process is complex, the solvent is difficult to recover, and the industrial production of the solvent is limited. Polylactic acid can also be melt-spun using melt-spinning equipment, and therefore, melt-spinning is more promising and becomes the mainstream of industrial production. The melt direct spinning technology is mainly adopted abroad, the technological process and technological parameters are different from the slicing method adopted by the invention, the polylactic acid is mostly processed into plastics, the research on the production technology of fiber spinning is less, and particularly, the industrialized production of the polylactic acid fiber is basically blank at home.

Disclosure of Invention

In view of the defects of the prior art, the invention aims to provide a preparation method of polylactic acid short fiber.

The invention also aims to provide the polylactic acid short fiber prepared by the preparation method.

The invention provides a preparation method of polylactic acid short fiber, which comprises the steps of pre-spinning and post-spinning, wherein the pre-spinning step comprises the following steps: extruding and melting the dried fiber-grade polylactic acid slices by a screw rod at 235-250 ℃, filtering the melt, and then spinning by a spinning box assembly, wherein the temperature of the spinning box is 235-250 ℃; spinning, cooling and forming to obtain nascent fiber, bundling and oiling the nascent fiber, and dropping the nascent fiber into a barrel;

the post-spinning step comprises: and after the nascent fiber is collected, conducting yarn guiding, oil immersion, primary drawing, oil bath drawing, secondary drawing, steam water bath, tertiary drawing, tension heat setting, cooling spraying, drawing, yarn folding, steam ironing, curling, conveying, relaxation heat setting, drawing cutting and packaging are sequentially carried out.

Since polylactic acid contains ester bonds, the ester bonds are hydrolyzed under high water content and high temperature, thereby causing degradation of molecular quality. In addition, the water content in the polylactic acid slices can form bubbles during spinning, the bubbles are mixed in the melt stream, spinning floating filaments, broken filaments and broken filaments are easily generated, and the polylactic acid slices with high water content are extruded by a screw to easily cause ring-shaped blocking materials, so the slices are dried before melt spinning so as to strictly control the water content of the slices.

After the optimization process, the fiber-grade polylactic acid slices in the pre-spinning step are determined to enter a rotary drum for vacuum drying at the temperature of 95-115 ℃ for 10-12 h, and the water content of the dried slices is less than or equal to 0.03 per thousand.

When the polylactic acid melt is melted and extruded through the micropores of the spinneret plate by a screw extruder, if the melt contains gel particles, impurities and the like with larger sizes, the micropores can be blocked, the micropore is abnormal, thick silk appears, defects and broken ends are generated, and the product quality is seriously influenced, and the polylactic acid spinning temperature is lower than the polyester spinning temperature, so that in the preparation method provided by the invention, 5 layers of filter screens are adopted during melt filtration, the filtration precision is 20-25 mu m, the filter screens are composite filter screens of 50-200 meshes, and the filter sand is 20-mesh sea sand. The high requirements of polylactic acid (PLA) fibers on the quality uniformity and purity of the melt are met, the product quality is stable, and the service cycle of the assembly is ideal.

The filtering precision and the selected sea sand mesh number are comprehensively considered according to the spinning component specification and the spinning pressure control parameter. If other specifications of sea sand are adopted, such as 40 meshes, the spinning pressure fluctuation is large, and the index of the primary fiber is unqualified.

The micro-pores of the spinneret plate are fine, the configuration mesh number of the component filter and the filter screen is increased, the flow resistance of the melt in the component is increased, the flow performance is poor, and meanwhile, the quality of the melt is required to be uniform and the generation of condensed particles is reduced, so that the spinning temperature control is very important. In the invention, the temperature of the screw and the spinning box body is controlled to be 235-250 ℃, the melt fluidity is good, the high elastic deformation relaxation time can be shortened, the melt bulking effect can be reduced, and the melt fracture can be prevented, but the spinning temperature is not too high, otherwise the melt is easy to degrade seriously, and the normal spinning can not be ensured.

The invention controls the air pressure of circular blowing to be 200 pa-800 pa, the air temperature to be 18-24 ℃, and the air speed to be 1.2 m/s-2.5 m/s during cooling forming. The high or low wind speed results in high strand cross-sectional unevenness (DVC). The wind speed is too low, the cooling and solidifying speed difference on the section of the nascent fiber is small, and the self-rolling effect is poor; the wind speed is too high, the yarn strip disturbance is large, and spinning and drafting are not facilitated. Too high wind speed can lead the pre-orientation degree of the nascent fiber to be higher, thereby generating transition drafting in the drafting, leading to the increase of broken filaments, broken ends and winding rollers, leading the drafting process not to be normally carried out, and leading the contraction capacity of macromolecules to be reduced, so that the curling and the fluffiness of the fiber are reduced on the contrary.

In the pre-spinning step of the preparation method, when the pre-spinning is carried out on bundling oiling, the concentration of the oil agent is 0.8-2.0%. In the post-spinning step, the concentration of the oil solution is 1.0-2.0% during oil bath drafting, and the concentration of the oil solution is 2.0-5.0% during cooling spraying. Polylactic acid fibers have a low moisture regain, a low dielectric constant, and a high friction coefficient, and therefore, it is necessary to use an oil agent. The oil agent mainly comprises a surfactant and can form an oriented adsorption layer, namely an oil film, on the surface of the chemical fiber. The oil film can reduce the surface resistance of the fiber and increase the conductive effect, and the oil film isolates the fiber and has certain affinity to the fiber, so that the fiber can generate certain concentration without scattering. The oil film also gives the fibres a certain smoothness. Therefore, the oil agent generally has the functions of smoothness, static resistance, bundling or cohesion and the like, and is used in the front-end spinning production process.

In the pre-spinning step of the preparation method, the spinning speed is 800 m/min-1500 m/min when the barrel falls. The spinning speed mainly affects the degree of pre-orientation of the as-spun fibers. Similar to other polymer materials, the higher the spinning speed, the higher the degree of orientation, and the lower the post-draw ratio. On the other hand, when the high-temperature spinning melt is extruded out of the spinneret orifice, the high-temperature spinning melt can be contacted with oxygen and moisture in the air to be subjected to thermal degradation and hydrolysis, so that the relative molecular mass is reduced; as the spinning speed becomes higher, the time for which the spinning melt contacts air is shortened, the time for thermal degradation and hydrolysis to occur is shortened, and so the relative molecular mass loss is smaller. Therefore, a higher spinning speed is beneficial to ensuring the quality of the polylactic acid fiber, and an excessively high spinning speed can cause excessively short crystallization time of the polylactic acid, incomplete crystallization and easy breakage of the polylactic acid. Therefore, the selection of the spinning speed can be considered according to the overall process of front and back spinning. In a proper range, a higher spinning speed can be selected, so that the product quality can be ensured, and the yield can be improved.

Drawing of nascent fibers is a process in which molecular segments are oriented in the axial direction of the fiber under the action of an external force and strength and other mechanical properties are improved. This requires oil bath drawing to maintain the filament at a temperature sufficient to provide sufficient mobility of the molecular segments being drawn. When the temperature is lower, the high polymer can generate certain orientation by the forced action of mechanical force, but the internal stress of the fiber is large, the stretching point is unstable, the tow is whitish and hot, and even broken filaments and broken ends can be generated, and the high polymer is easy to wind a roller. When the temperature is too high, the plastic deformation of the tows is greatly increased, the tows are easy to break, meanwhile, the crystallization speed is increased due to the too high temperature, the tensile stress is increased, and the tows are easy to break during driving. Therefore, the better oil bath temperature is higher than the glass transition temperature of the fiber, so as to promote the movement of molecular chain segments, reduce the tensile stress and be beneficial to the smooth drawing. Therefore, in the post-spinning step, the temperature is 40-50 ℃ when the oil is soaked; oil bath drafting is carried out at the temperature of 70-80 ℃; when in steam water bath, the temperature is 85-100 ℃.

The draft ratio is selected to be between the maximum draw ratio of the as-spun fiber and the natural draw ratio, because if the draft ratio is less than the natural draw ratio, the neck of the drawn fiber is not extended to the whole fiber, and inevitably contains more undrawn yarn, and such fiber has no practical value, and when the draft ratio reaches the maximum ratio, the fiber is broken. When the draft multiple is smaller, the difference of the microstructure of the fiber section is not enough to be pulled open, and the curling effect cannot be fully reflected; if the draft multiple is too large, the axial arrangement homogenization of macromolecules can be accelerated, the self-rolling capability is weakened, and the draft multiple of the crimped fiber is not constant in the drafting process. On the premise of meeting the quality, the drafting speed of the first, second and third drafting of the post-spinning is 20-150 m/min, and the total drafting multiple is 3.0-5.0. The invention adjusts the proportion of each level of drafting and the total drafting multiple according to different titer of the tows, prevents the fiber from being damaged by stretching, and ensures the uniform stability of the drafting.

In the post-spinning step of the preparation method provided by the invention, when the fabric is subjected to tension heat setting, the heat setting temperature is 80-130 ℃, and the heat setting time is 5.2-13.2 s; when the heat setting is relaxed, the heat setting temperature is 50-70 ℃, and the heat setting time is 15-20 min. Because the internal microstructure of the nascent fiber has difference, the macromolecular plastic deformation exists after stretching, after the external force disappears, the high elastic deformation relaxes and generates different shrinkage effects to twist around the axial direction, and the helical curling is formed, but at the moment, the internal structure of the fiber is not fixed, the dimensional stability is poor, the crystallinity is low, and the relaxation time of the high elastic deformation at low temperature is long, so that the relaxation time of the macromolecule must be shortened by a heat setting mode, the crystallinity of the fiber is improved, and the strength and the physical and mechanical properties of the fiber are improved.

In the post-spinning step of the preparation method provided by the invention, a crimper is utilized to perform mechanical crimping, and during crimping, the speed of the crimper is 63.0-157.5 m/min, the main pressure of a crimping wheel is 0.1-0.15 MPa, and the back pressure of the crimping wheel is 0.02-0.05 MPa. To improve the crimping effect, the fibers need to be ironed before crimping. When the steam ironing is carried out, the temperature is 80-100 ℃. The surface of general fiber is smooth, and the cohesive force between fibers is small, so that the fiber is not beneficial to textile processing. The fiber is subjected to chemical, physical or mechanical crimping deformation processing to endow the fiber with certain crimp, so that the cohesion of the fiber can be effectively improved, and meanwhile, the bulkiness and elasticity of the fiber are increased.

The invention provides the polylactic acid short fiber prepared by the preparation method.

The invention designs a technological route suitable for polylactic acid spinning aiming at the characteristics of polylactic acid slices, can effectively control the problems of lumps and defects generated by the degradation of small monomers of polylactic acid molecular chain segments in products, and greatly improves the post-spinning drying and shaping effects. The polylactic acid short fiber produced by the process steps has good evenness, and the variation coefficient CV of evenness can reach below 15%; the length of the polylactic acid short fiber is controllable, the fiber length can be controlled in a cutting process, and fibers with different lengths, such as 31mm, 33mm, 35mm, 38mm and the like, can be obtained by controlling the distance between cutting knives in a cutting knife disc; various product indexes such as strength, elongation at break and the like can be adjusted according to needs, and the obtained product can completely meet the application requirements in the fields of downstream woven and non-woven fabrics and the like. In addition, the process can also be used for producing hollow fibers with 2 holes, 4 holes, 6 holes, 9 holes and the like, wherein the hollow fibers have high hollowness, good compression resilience, heat retention, fluffiness and air permeability and can be used as various filling materials. The method has simple process route, the polylactic acid short fiber prepared after batch industrialization has excellent performance and stable product quality, the linear density of the short fiber is 0.56 dtex-6.67 dtex, the comprehensive index completely meets the requirements of yarns for knitting and weaving, and the method has good market application value.

Detailed Description

Unless otherwise defined, all terms of art used hereinafter have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the present invention.

Unless otherwise specified, the reagents and materials used in the present invention are commercially available products or products obtained by a known method. The fiber grade polylactic acid chips used in the examples were purchased from Nature works, USA. The linear density test adopts the GBT 14335-.

Example 1: preparation of polylactic acid short fiber

(1) Step of pre-spinning

Drum drying: adding the polylactic acid slices into a vacuum drum dryer, drying for 12h at 95 ℃, and measuring the water content of the dried raw material to be 0.026%.

Melt spinning: and conveying the dried raw materials into a screw extruder for heating and melting, and filtering and spinning the melt through a spinning assembly to form nascent fibers. The temperature of the screw and the spinning box is 239 ℃ during heating and melting; when filtering the melt, a composite filter screen with the filtering precision of 20-25 mu m and the filter screen of 50-200 meshes is selected, and the filter sand adopts 20-mesh sea sand.

And (3) cooling and forming: and cooling and forming the nascent fiber by circular air blowing. The air pressure of the circular blowing is 220Pa, the air temperature is 19 ℃, and the air speed is 1.2 m/s;

bundling, oiling and barrel dropping: and (3) oiling and bundling the cooled and formed nascent fiber by an oiling machine, winding by a traction machine and a feeding machine, and feeding into a silk containing barrel. The concentration of the oiling oil agent is 1.0%, and the spinning speed is 1200m/min when the spinning tank falls.

(2) Post-spinning step

Bundling, wire guiding, oil immersion and drafting: and (3) after the nascent fiber obtained by pre-spinning is collected, the nascent fiber is drawn sequentially through a yarn guide machine, an oil immersion groove, a first drawing machine, an oil bath drawing groove, a second drawing machine, a steam water bath box and a third drawing machine so as to be crystallized in an oriented mode and improve the physical and mechanical properties. The draft was 3.5 times. The temperature of the oil immersion groove is 45 ℃, the speed of the first drafting machine is 22.9m/min, the temperature of the oil bath drafting groove is 70 ℃, the speed of the second drafting machine is 72.0m/min, the temperature of the steam water bath box is 90 ℃, and the speed of the third drafting machine is 80.0 m/min. The oil solution concentration of the oil bath drawing tank is 1.8%.

Tension heat setting, cooling spraying, steam ironing and curling: and (3) carrying out heat setting on the drafted nascent fiber through a tension heat setting machine, then oiling and cooling the drafted nascent fiber through a cooling spray tank, sending the drafted nascent fiber into a steam preheating box through a traction machine and a yarn stacking machine for steam ironing, and then sending the drafted nascent fiber into a crimping machine for crimping and forming yarn pieces. The tension heat setting temperature is 105 ℃, the heat setting time is 9.9s, the concentration of cooling spray oil is 3.5%, the steam ironing temperature is 85 ℃, the speed of the crimping machine is 84.0m/min, the main pressure of the crimping wheel is 0.11MPa, and the back pressure of the crimping wheel is 0.025 MPa.

And (3) relaxation heat setting: and sending the curled tows into a relaxation heat setting machine for relaxation setting. The temperature of the relaxation and the setting is 55 ℃, and the setting time is 19 min.

Dragging, cutting off and packaging: and (3) feeding the loosened and shaped tows into a cutting machine through a traction tensioner for cutting, and packaging by a baling machine to obtain a 6.67dtex polylactic acid short fiber finished product.

Example 2: preparation of polylactic acid short fiber

(1) Step of pre-spinning

Drum drying: adding the polylactic acid slices into a vacuum drum dryer, drying for 11 hours at 105 ℃, and measuring the water content of the dried raw material to be 0.024%.

Melt spinning: and conveying the dried raw materials into a screw extruder for heating and melting, and filtering and spinning the melt through a spinning assembly to form nascent fibers. The temperature of the screw and the spinning box is 244 ℃ during heating and melting; when filtering the melt, a composite filter screen with the filtering precision of 20-25 mu m and the filter screen of 50-200 meshes is selected, and the filter sand adopts 20-mesh sea sand.

And (3) cooling and forming: and cooling and forming the nascent fiber by circular air blowing. The air pressure of the circular blowing is 310Pa, the air temperature is 21 ℃, and the air speed is 1.6 m/s;

bundling, oiling and barrel dropping: and (3) oiling and bundling the cooled and formed nascent fiber by an oiling machine, winding by a traction machine and a feeding machine, and feeding into a silk containing barrel. The concentration of the oiling oil agent is 1.5%, and the spinning speed is 1100m/min when the spinning barrel falls.

(2) Post-spinning step

Bundling, wire guiding, oil immersion and drafting: and (3) after the nascent fiber obtained by pre-spinning is collected, the nascent fiber is drawn sequentially through a yarn guide machine, an oil immersion groove, a first drawing machine, an oil bath drawing groove, a second drawing machine, a steam water bath box and a third drawing machine so as to be crystallized in an oriented mode and improve the physical and mechanical properties. The draft was 4.5 times. The temperature of the oil immersion groove is 47 ℃, the speed of the first drafting machine is 22.2m/min, the temperature of the oil bath drafting groove is 73 ℃, the speed of the second drafting machine is 90.0m/min, the temperature of the steam water bath box is 92 ℃, and the speed of the third drafting machine is 100.0 m/min. The oil solution concentration of the oil bath drawing tank is 1.6%.

Tension heat setting, cooling spraying, steam ironing and curling: and (3) carrying out heat setting on the drafted nascent fiber through a tension heat setting machine, then oiling and cooling the drafted nascent fiber through a cooling spray tank, sending the drafted nascent fiber into a steam preheating box through a traction machine and a yarn stacking machine for steam ironing, and then sending the drafted nascent fiber into a crimping machine for crimping and forming yarn pieces. The tension heat setting temperature is 115 ℃, the heat setting time is 7.9s, the concentration of cooling spray oil is 3.8%, the steam ironing temperature is 88 ℃, the speed of the crimping machine is 105.0m/min, the main pressure of the crimping wheel is 0.12MPa, and the back pressure of the crimping wheel is 0.03 MPa.

And (3) relaxation heat setting: and sending the curled tows into a relaxation heat setting machine for relaxation setting. The temperature of the relaxation and the setting is 60 ℃, and the setting time is 16 min.

Dragging, cutting off and packaging: and (3) feeding the loosened and shaped tows into a cutting machine through a traction tensioner for cutting, and packaging by a baling machine to obtain a 1.67dtex polylactic acid short fiber finished product.

Example 3: preparation of polylactic acid short fiber

(1) Step of pre-spinning

Drum drying: adding the polylactic acid slices into a vacuum drum dryer, drying for 12 hours at 115 ℃, and measuring the water content of the dried raw material to be 0.02%.

Melt spinning: and conveying the dried raw materials into a screw extruder for heating and melting, and filtering and spinning the melt through a spinning assembly to form nascent fibers. The temperature of the screw and the spinning box is 249 ℃ during heating and melting; when filtering the melt, a composite filter screen with the filtering precision of 20-25 mu m and the filter screen of 50-200 meshes is selected, and the filter sand adopts 20-mesh sea sand.

And (3) cooling and forming: and cooling and forming the nascent fiber by circular air blowing. The air pressure of the circular blowing is 550Pa, the air temperature is 22 ℃, and the air speed is 2.1 m/s;

bundling, oiling and barrel dropping: and (3) oiling and bundling the cooled and formed nascent fiber by an oiling machine, winding by a traction machine and a feeding machine, and feeding into a silk containing barrel. The concentration of the oil agent during oiling is 2.0 percent, and the spinning speed during falling into the barrel is 1000 m/min.

(2) Post-spinning step

Bundling, wire guiding, oil immersion and drafting: and (3) after the nascent fiber obtained by pre-spinning is collected, the nascent fiber is drawn sequentially through a yarn guide machine, an oil immersion groove, a first drawing machine, an oil bath drawing groove, a second drawing machine, a steam water bath box and a third drawing machine so as to be crystallized in an oriented mode and improve the physical and mechanical properties. The draft was 5.0 times. The temperature of the oil immersion groove is 50 ℃, the speed of the first drafting machine is 26.0m/min, the temperature of the oil bath drafting groove is 78 ℃, the speed of the second drafting machine is 117.0m/min, the temperature of the steam water bath box is 98 ℃, and the speed of the third drafting machine is 130.0 m/min. The oil solution concentration of the oil bath drawing tank is 1.5%.

Tension heat setting, cooling spraying, steam ironing and curling: and (3) carrying out heat setting on the drafted nascent fiber through a tension heat setting machine, then oiling and cooling the drafted nascent fiber through a cooling spray tank, sending the drafted nascent fiber into a steam preheating box through a traction machine and a yarn stacking machine for steam ironing, and then sending the drafted nascent fiber into a crimping machine for crimping and forming yarn pieces. The tension heat setting temperature is 125 ℃, the heat setting time is 6.1s, the concentration of cooling spray oil is 4.5%, the steam ironing temperature is 92 ℃, the speed of the crimping machine is 136.5m/min, the main pressure of the crimping wheel is 0.15MPa, and the back pressure of the crimping wheel is 0.05 MPa.

And (3) relaxation heat setting: and sending the curled tows into a relaxation heat setting machine for relaxation setting. The temperature for relaxation and setting is 68 ℃, and the setting time is 15 min.

Dragging, cutting off and packaging: and (3) feeding the loosened and shaped tows into a cutting machine through a traction tensioner for cutting, and packaging by a baling machine to obtain a 0.89dtex polylactic acid short fiber finished product.

The above examples are only for describing the preferred embodiments of the present invention, and are not intended to limit the scope of the present invention, and various modifications and improvements made to the technical solution of the present invention by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A preparation method of polylactic acid short fiber is characterized by comprising the steps of pre-spinning and post-spinning, wherein the pre-spinning step comprises the following steps: extruding and melting the dried fiber-grade polylactic acid slices by a screw rod at 235-250 ℃, filtering the melt, and then spinning by a spinning box assembly, wherein the temperature of the spinning box is 235-250 ℃; spinning, cooling and forming to obtain nascent fiber, bundling and oiling the nascent fiber, and dropping the nascent fiber into a barrel;

the post-spinning step comprises the following steps of sequentially collecting the primary fibers: first drafting, second drafting, third drafting, tension heat setting at 105-130 deg.C, cooling and spraying, steam ironing at 80-100 deg.C, curling, relaxation heat setting at 50-70 deg.C, and cutting.

2. The method of claim 1, wherein the dried fiber-grade polylactic acid slices in the pre-spinning step have a moisture content of 0.03% o or less.

3. The preparation method according to claim 1, wherein in the pre-spinning step, during melt filtration, the melt filtration has a filtration precision of 20 μm to 25 μm, the filter screen is a composite filter screen of 50 meshes to 200 meshes, and the filter sand is 20 meshes of sea sand.

4. The preparation method according to claim 1, wherein in the pre-spinning step, when cooling and forming are carried out after spinning, the air pressure of circular blowing is 200Pa to 800Pa, the air temperature is 18 ℃ to 24 ℃, and the air speed is 1.2m/s to 2.5 m/s.

5. The preparation method according to claim 1, wherein in the pre-spinning step, when bundling and oiling, the concentration of the oil agent is 0.8-2.0%; and/or the spinning speed is 800 m/min-1500 m/min when the fiber falls into the barrel.

6. The process according to any one of claims 1 to 5, wherein in the post-spinning step, the draft speed of the first, second and third drafts is 20 to 150m/min, and the total draft ratio is 3.0 to 5.0.

7. The method according to any one of claims 1 to 5, wherein in the post-spinning step, before the first drawing, the fiber is subjected to oil immersion at a temperature of 40 ℃ to 50 ℃;

and/or, passing through an oil bath drafting groove with the temperature of 70-80 ℃ and the oil concentration of 1.0-2.0% between the first drafting and the second drafting;

and/or passing through a steam water bath with the temperature of 85-100 ℃ between the second drafting and the third drafting.

8. The method according to any one of claims 1 to 5, wherein in the post-spinning step, the tension heat setting time is 5.2s to 9.9 s; and/or the time for relaxation and heat setting is 15-20 min.

9. The production method according to any one of claims 1 to 5, wherein in the post-spinning step, the crimper speed is 63.0m/min to 157.5m/min, the crimping wheel main pressure is 0.1MPa to 0.15MPa, and the crimping wheel back pressure is 0.02MPa to 0.05MPa at the time of crimping.

10. Polylactic acid short fiber prepared by the preparation method of any one of claims 1 to 9.