Preparation method and device of high-strength and high-elongation regenerated polyester staple fiber

Abstract

The application provides a preparation method and device of high-strength and high-elongation regenerated polyester staple fiber, and the method comprises the following steps: melt extruding a regenerated polyester mixed raw material to obtain primary fibers; performing multi-stage gradient drafting treatment on the primary fibers, which comprises the following steps: pre-oriented drafting the primary fibers at a first temperature with a first drafting multiple; intermediate drafting the primary fibers at a second temperature with a second drafting multiple; main oriented drafting the primary fibers at a third temperature with a third drafting multiple; wherein the first temperature and the third temperature are respectively lower and higher than the glass transition temperature of the regenerated polyester, and the second temperature is between the first temperature and the third temperature; the total drafting multiple is the sum of the first drafting multiple, the second drafting multiple and the third drafting multiple, and the first drafting multiple and the third drafting multiple respectively account for 30%-40% and 15%-30% of the total drafting multiple. The application solves the problem that the existing drafting process is difficult to simultaneously consider high strength and high elongation on the basis of raw material fluctuation.

Description

Technical Field

[0001] This invention relates to the field of polymer material processing technology, and in particular to a method and apparatus for preparing high-strength, high-elongation recycled polyester staple fiber. Background Technology

[0002] With increasing environmental awareness and the development of the circular economy, recycled polyester staple fiber has gained widespread application in textiles, fillings, and nonwovens due to its wide range of raw material sources (such as waste polyester bottle flakes and chemical fiber foam materials) and alignment with the concept of sustainable development. Among these processes, the drafting process, as a key step in imparting mechanical properties to the fiber, applies external force to orient the internal molecular chains of the fiber along the axial direction, thereby improving breaking strength. However, balancing the contradiction between breaking strength and elongation at break, and addressing the processing challenges posed by the uneven intrinsic viscosity of recycled raw materials, remains a core issue for current process optimization.

[0003] Currently, the core principle of the drawing process commonly used in the industry is to apply external force to orient the molecular chains inside the fiber along the axial direction, thereby significantly improving the fiber's breaking strength. To pursue high strength, existing technologies typically employ high-ratio, single-stage, or simple two-stage hot drawing processes. However, when dealing with recycled raw materials, due to their complex origins and uneven viscosity (e.g., a mixture of high-viscosity waste polyester bottle flakes and low-viscosity synthetic fiber foam), uniform drawing parameter settings often struggle to adapt to fluctuations in raw material quality. Although solid-phase thickening (SSP) and other processes have been introduced in some high-end fields to improve raw material quality, technological breakthroughs in the drawing process itself remain relatively limited.

[0004] Although existing drawing processes can effectively improve fiber strength, they often sacrifice elongation at break in pursuit of high strength, meaning that "high strength" and "high elongation" are difficult to achieve simultaneously. Specifically, firstly, while high-ratio, single-stage, or simple two-stage hot drawing can significantly improve the orientation of molecular chains, the molecular chains in amorphous regions are excessively straightened, leading to increased fiber brittleness, decreased elasticity, and a significant reduction in elongation at break. For example, when the total draw ratio exceeds 4.0, the elongation at break is usually difficult to maintain above 15%, failing to meet the application requirements of sewing thread, workwear fabrics, and other applications that demand both high strength and high toughness. Secondly, due to the complex sources and uneven viscosity of recycled raw materials, a uniform drawing process can easily lead to over-orientation or under-orientation of some raw materials, further exacerbating the contradiction between strength and elongation. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method and apparatus for preparing high-strength, high-elongation recycled polyester staple fiber. This method solves the problem that the existing drawing process used in the preparation of recycled polyester staple fiber is difficult to achieve both high strength and high elongation under conditions of raw material fluctuations.

[0006] According to an embodiment of the present invention, a method for preparing high-strength, high-elongation recycled polyester staple fiber includes the following steps: The recycled polyester blend is melted and extruded to obtain nascent fibers; The nascent fibers are subjected to a multi-stage gradient drawing process, which specifically includes: At a first temperature, the nascent fibers are subjected to a first-stage pre-orientation stretching at a first stretching ratio; At a second temperature, the nascent fibers are subjected to a second stage of intermediate stretching at a second stretching ratio; At a third temperature, the nascent fibers are subjected to a third-stage main orientation stretching at a third stretching ratio; Wherein, the first temperature is lower than the glass transition temperature of recycled polyester, the second temperature is between the first temperature and the third temperature, and the third temperature is higher than the glass transition temperature of recycled polyester; the total draw ratio is the sum of the first draw ratio, the second draw ratio and the third draw ratio, and the first draw ratio accounts for 30%-40% of the total draw ratio, and the third draw ratio accounts for 15%-30% of the total draw ratio.

[0007] According to another embodiment of the present invention, an apparatus for preparing high-strength, high-elongation recycled polyester staple fiber includes: Melt spinning assembly, used to melt and extrude recycled polyester blends to form nascent fibers; A multi-stage gradient drawing assembly includes a first-stage drawing unit, a second-stage drawing unit, and a third-stage drawing unit. The first-stage drawing unit is used to perform a first-stage pre-orientation drawing of the nascent fiber at a first temperature below the glass transition temperature of the recycled polyester and a first drawing ratio. The third-stage drawing unit is used to perform a third-stage main orientation drawing of the nascent fiber at a third temperature above the glass transition temperature of the recycled polyester and a third drawing ratio. The second-stage drawing unit is used to perform a second-stage intermediate drawing of the nascent fiber at a second drawing ratio between the first and third temperatures. The total draw ratio is the sum of the first draw ratio, the second draw ratio, and the third draw ratio, wherein the first draw ratio accounts for 30%-40% of the total draw ratio, and the third draw ratio accounts for 15%-30% of the total draw ratio.

[0008] Compared with the prior art, the present invention has the following beneficial effects: By performing multi-level gradient drawing treatment on the nascent fibers, the drawing temperature and stretching ratio can be controlled in stages, thereby achieving precise control of the fiber microstructure. This allows the fibers to obtain high orientation (imparting high strength) while retaining the extensibility of some amorphous chain segments (imparting high elongation). This effectively solves the contradiction between high strength and high elongation in traditional processes and avoids problems such as fuzzy fibers and broken fibers caused by improper drawing control.

[0009] By using low-temperature pre-orientation stretching to fix the basic structure of the nascent fiber, and then performing main orientation stretching at high temperature, a multi-level buffer mechanism is formed, which can significantly improve the process's tolerance to fluctuations in the performance of recycled raw materials, effectively reduce defects such as fiber breakage, roller entanglement, and fuzz caused by uneven recycled raw materials, and improve production continuity and finished product qualification rate.

[0010] By performing multi-level gradient stretching on the nascent fibers, the nascent fibers are stretched in a gradient manner, making the stress distribution inside the fibers more uniform and avoiding local stress concentration. This not only improves the effect of subsequent heat setting processes, but also enhances the dimensional stability of the product in downstream processing (such as nonwoven fabric thermoforming), and effectively suppresses shrinkage or deformation caused by stress release. Attached Figure Description

[0011] Figure 1 This is a flowchart illustrating the preparation method of high-strength, high-elongation recycled polyester staple fiber according to an embodiment of the present invention.

[0012] Figure 2 This diagram illustrates the specific steps involved in melt spinning a recycled polyester blend to obtain nascent fibers, as described in an embodiment of the present invention.

[0013] Figure 3 This diagram illustrates the specific steps of the multi-level gradient stretching process in an embodiment of the present invention. Detailed Implementation

[0014] The technical solutions of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0015] Example 1 like Figure 1 As shown in the figure, this invention provides a method for preparing high-strength, high-elongation recycled polyester staple fiber, comprising the following steps: S1. Melt and extrude the recycled polyester blend to obtain nascent fibers; The recycled polyester blend is composed of waste polyester bottle flakes and chemical fiber foam material mixed in a mass ratio of 6:4-8:2. The intrinsic viscosity of the waste polyester bottle flakes is 0.7-0.8 dL / g, and the intrinsic viscosity of the chemical fiber foam material is 0.5-0.6 dL / g.

[0016] Among them, such as Figure 2 As shown, the process of melt spinning the recycled polyester blend to obtain nascent fibers specifically includes: S11. The dried recycled polyester mixture is fed into a screw extruder for melt treatment to obtain polyester melt; S12. The polyester melt is transferred to a filtration system for filtration and then metered by a metering pump. The filtration system includes at least two stages of filtration equipment, and the pore size of the filter media in the filtration equipment decreases progressively. S13. The metered polyester melt is extruded through a spinneret to form a melt stream, and then cooled and solidified by side blowing to form nascent fibers. The spinneret orifice diameter range is 0.15-0.30 mm, the side blowing temperature is 18-25℃, and the cooling rate is 50-100℃ / s.

[0017] S2. Perform multi-stage gradient stretching treatment on the nascent fibers; Among them, such as Figure 3 As shown, the multi-level gradient stretching process specifically includes: S21. At a first temperature, the nascent fibers are subjected to a first-stage pre-orientation stretching at a first stretching ratio; S22. At the second temperature, the nascent fibers are subjected to a second stage of intermediate stretching at a second stretching ratio; S23. At a third temperature, the nascent fibers are subjected to a third-stage main orientation stretching at a third stretching ratio; Wherein, the first temperature is lower than the glass transition temperature of recycled polyester, the second temperature is between the first temperature and the third temperature, and the third temperature is higher than the glass transition temperature of recycled polyester; the total draw ratio is the sum of the first draw ratio, the second draw ratio and the third draw ratio, and the first draw ratio accounts for 30%-40% of the total draw ratio, and the third draw ratio accounts for 15%-30% of the total draw ratio.

[0018] Preferably, the first stage of pre-orientation drawing is oil bath drawing, and the first temperature is 70-80℃; the second stage of intermediate drawing is dry heat drawing, and the second temperature is 80-90℃; the third stage of main orientation drawing is steam drawing, and the third temperature is 90-100℃.

[0019] More preferably, the drawing speed of the first-stage pre-orientation drawing is 50-80 m / min, the drawing speed of the second-stage intermediate drawing is 100-130 m / min, and the drawing speed of the third-stage main orientation drawing is 150-200 m / min.

[0020] In this embodiment, the specific preparation steps for preparing high-performance recycled polyester staple fibers with a breaking strength > 5.0 cN / dtex and a breaking elongation between 45% and 55% are as follows: (1) Raw material preparation and drying: First, waste polyester bottle flakes and chemical fiber foam material are selected as recycled polyester mixed raw materials and mixed evenly at a mass ratio of 6:4. The selected waste polyester bottle flakes have an intrinsic viscosity of 0.75-0.8 dL / g and a melting point of 255-258℃; the chemical fiber foam material has an intrinsic viscosity of 0.55-0.6 dL / g and a melting point of 258-265℃.

[0021] Subsequently, the mixed recycled polyester raw material (100 parts by weight) and 0.1 parts of whitening agent were fed into a vacuum drum drying system for drying. The process parameters of the vacuum drum drying system were set as follows: drying temperature 145℃, steam pressure 0.45MPa, vacuum degree -0.096MPa, and drying time 11 hours.

[0022] Finally, after drying, the moisture content of the recycled polyester blend is tested until it drops to 75 PPM (below the requirement of 80 PPM). At this point, the high standard of moisture control required for subsequent melt spinning is met, effectively preventing PET from undergoing hydrolytic degradation at high temperatures.

[0023] (2) Melt spinning and preparation of nascent fibers: First, the dried recycled polyester blend is fed into a screw extruder through a vacuum-sealed system for melting. The temperatures of each section of the screw extruder are set as follows: 220-240℃ for the feeding section, 250-270℃ for the melting section, 270-280℃ for the metering section, and the spinning box temperature is controlled at 280-295℃. Temperature fluctuations are controlled within ±1℃. The highest temperature zone of the melting spinning screw of the screw extruder (i.e., the third and fourth zones) is always controlled within the preferred range of 280-295℃. At the same time, the residence time of the melt in the high-temperature zone does not exceed 5 minutes to reduce thermal degradation.

[0024] The molten polyester is then passed through a filtration system (the filtration system includes a three-stage filtration device, which includes at least a coarse filter, a fine filter, and a final filter, wherein the coarse filter has a pore size of 60 mm). m, fine filter mesh size is 30 m, terminal filter mesh size is 15 m) Filtration to remove impurities, and after filtration, the filtered polyester melt is precisely metered by a metering pump to obtain accurate data of polyester melt.

[0025] Subsequently, the polyester melt is conveyed to the spinning assembly and extruded through a circular spinneret to obtain a fine melt stream. The spinneret has an orifice diameter of 0.20 mm and the number of orifices is set according to the target fineness (2.22 dtex).

[0026] After the molten stream is extruded from the spinneret, it immediately enters the side-blowing cooling system. The side-blowing cooling system has a side-blowing temperature of 20℃, a wind speed of 0.6m / s, and a cooling rate of 80℃ / s, which enables the molten stream to solidify quickly and form nascent fibers.

[0027] (3) Multi-level gradient stretching treatment: To balance high strength and high elongation, this preparation method adopts a three-stage gradient drawing process, with the total drawing ratio set at 3.4 times.

[0028] 1) First stage: Pre-oriented oil bath drawing (below the glass transition temperature Tg of recycled polyester) After being bundled, the nascent fibers first enter a first-stage oil bath drawing machine at a drawing speed of 60 m / min. The oil bath drawing machine is equipped with an oil bath tank for pre-orienting the nascent fibers. The temperature inside the oil bath tank is strictly controlled at 70-75°C, which is lower than the glass transition temperature of the recycled polyester (preferably 80°C). Under these conditions, the fibers are pre-oriented at a first draw ratio (1.2 times). This stage of drawing accounts for 30%-40% of the total draw ratio (in this example, 1.2 / 3.4 ≈ 35.3%), aiming to initially stretch the fibers, promote the initial orientation of the amorphous molecular chains, and prepare for subsequent main drawing.

[0029] 2) Second stage: intermediate dry heat stretching The pre-stretched fibers are then fed into a second-stage dry heat drawing machine at a speed of 120 m / min. The temperature is strictly controlled between the first-stage oil bath temperature and the third-stage steam temperature (preferably 85°C). Under these conditions, the fibers undergo further intermediate orientation at a second draw ratio (0.7 times). This stage of drawing accounts for 15%-30% of the total draw ratio (in this example, 0.7 / 3.4 ≈ 20.6%), serving as a buffer between the preceding and following stages and preventing internal structural defects in the fibers caused by sudden changes in draw ratio.

[0030] 3) Third stage: Main orientation steam drawing (above the glass transition temperature Tg of recycled polyester) After intermediate drafting, the fibers enter the third-stage steam drafting machine at a drafting speed of 180 m / min. The steam drafting machine is equipped with a steam chamber for main orientation drafting of the pre-oriented fibers. The temperature inside the steam chamber is set at 95-100°C, which is higher than the glass transition temperature of recycled polyester (preferably 95°C). This allows the molecular chains to obtain sufficient energy for rearrangement and slippage. Under this high-temperature and high-humidity environment, the fibers are subjected to main orientation drafting at a third draft ratio (1.5 times). This is a crucial step in achieving high fiber strength, ensuring that the macromolecular chains are highly oriented along the fiber axis, forming a stable crystalline structure.

[0031] (4) Post-processing and finished product: First, the drawn fiber bundles are fed into a crimping machine to increase the cohesion between fibers. Then, they are sent to a relaxation heat setter for setting. This heat setter has three sections: a drying zone at 110℃, a heat setting zone at 160℃, and a cooling zone at 60℃, with a total setting time of 28 minutes. The relaxation heat setter aims to eliminate internal fiber stress, stabilize fiber morphology, and further improve fiber crystallinity. Finally, the fibers are cut to a length of 51mm and packaged to obtain the final product.

[0032] (5) Performance test results: The finished fiber was tested according to GB / T 14337 standard and compared with existing technology (CN105937064A). The key performance indicators are shown in the table below:

[0033] Results Analysis: By employing optimized raw material ratios and precise drying processes, especially a three-stage gradient drawing process (low-temperature oil bath drawing → intermediate dry-heat drawing → high-temperature steam main drawing), where the first-stage pre-orientation drawing accounts for 35% of the total drawing and is below the glass transition temperature of recycled polyester, and the second-stage main orientation drawing is above the glass transition temperature of recycled polyester, the target range of tensile strength > 5.0 cN / dtex and elongation at break stable within 45%-55% was successfully achieved. Recycled polyester staple fibers with high tensile strength and elongation at break were prepared, overcoming the technical challenge of achieving both high strength and high elongation in traditional products, and resolving the contradiction of "high strength with low elongation, and high elongation with low strength" in conventional recycled polyester fibers.

[0034] Example 2 Another embodiment of the present invention provides an apparatus for preparing high-strength, high-elongation recycled polyester staple fiber, comprising: Melt spinning assembly, used to melt and extrude recycled polyester blends to form nascent fibers; A multi-stage gradient drawing assembly includes a first-stage drawing unit, a second-stage drawing unit, and a third-stage drawing unit. The first-stage drawing unit is used to perform a first-stage pre-orientation drawing of the nascent fiber at a first temperature below the glass transition temperature of the recycled polyester and a first drawing ratio. The third-stage drawing unit is used to perform a third-stage main orientation drawing of the nascent fiber at a third temperature above the glass transition temperature of the recycled polyester and a third drawing ratio. The second-stage drawing unit is used to perform a second-stage intermediate drawing of the nascent fiber at a second drawing ratio between the first and third temperatures. The total draw ratio is the sum of the first draw ratio, the second draw ratio, and the third draw ratio, wherein the first draw ratio accounts for 30%-40% of the total draw ratio, and the third draw ratio accounts for 15%-30% of the total draw ratio.

[0035] Furthermore, the first-stage drawing unit is an oil bath drawing machine, which is equipped with an oil bath tank for pre-orienting the nascent fibers. The third-stage drawing unit is a steam drawing machine, which is equipped with a steam box for main orientation drawing of the pre-oriented fibers.

[0036] In this embodiment, the preparation device mainly consists of the melt spinning assembly and the multi-stage gradient drawing assembly, which are connected sequentially according to the process flow, wherein: (1) Melt spinning assembly: including raw material drying system (vacuum drum dryer), screw extruder (temperature control accuracy ±2℃), filtration system (three-stage filtration equipment), metering pump, spinning box, spinneret and side blowing cooling device. The spinneret adopts a circular hole spinneret with a single hole diameter between 0.15-0.30mm (preferably 0.20mm) to adapt to different fineness requirements. An annular side blowing device is set below the spinneret, with a wind temperature control accuracy of ±1℃ and a center wind speed adjustable range of 0.5-1.0m / s to ensure uniform cooling and solidification of the melt stream.

[0037] (2) Multi-level gradient drawing assembly: It consists of three drawing units arranged in gradient configuration connected in series, with the following specific structure: 1) First-stage drawing unit: Specifically, it is an oil bath drawing machine connected after the melt spinning assembly. It includes a set of traction rollers and an oil bath tank. The oil bath tank is filled with oil agent, and its temperature is precisely maintained at 70-75℃ by a closed-loop control system. Squeeze rollers are provided at the inlet and outlet of the oil bath tank to control the amount of liquid carried by the fiber and prevent oil agent splashing.

[0038] 2) Second stage drawing unit: Specifically, it is a dry heat drawing machine, located between the first and third stages. It includes a set of heating rollers and a sealed heat preservation box. The heating rollers are electrically heated and their surface temperature is controlled by PID to be 80-85℃. The heat preservation box is equipped with ventilation holes to discharge the trace amounts of water vapor generated by fiber heating.

[0039] 3) Third-stage drawing unit: Specifically, it is a steam drawing machine, located after the second-stage drawing unit. It includes a set of traction rollers and a steam box. The steam box inlet is connected to the workshop steam main pipe, and the temperature inside the box is stably controlled at 90-100℃ through a pressure reducing valve and a temperature sensor. Fiber channel openings are provided at both ends of the steam box, and sealing felt is provided to reduce heat and steam loss.

[0040] Using the apparatus described in this embodiment, a continuous production test was conducted according to the process parameters provided in Embodiment 1. After 24 hours of stable operation, samples of the finished fiber were taken for testing, and the results are as follows: Fracture strength: average value is 5.4 cN / dtex, with a maximum deviation of ±0.15 cN / dtex within the batch.

[0041] Elongation at break: average value is 50.8%, maximum deviation within batch is ±2.0%.

[0042] Fineness unevenness (CV value): 1.8%, far below the industry standard (usually 3%), indicating uniform fiber thickness and high equipment operating stability.

[0043] The breakage rate was 0.02 times / ton, indicating that the fiber traction force was well matched during the three-stage gradient drafting process, and there was no frequent fiber breakage caused by excessive or uneven drafting.

[0044] In summary, the device structure of "melt spinning assembly + multi-stage gradient drawing assembly (oil bath-dry heat-steam)" provided in this embodiment is highly synergistic with the specific process parameters of Embodiment 1 (especially the drawing ratio distribution and temperature control), successfully realizing the stable and large-scale production of high-strength and high-elongation recycled polyester staple fiber. The process has good repeatability and excellent product quality. The device has a compact structure and high control precision, and has significant industrial application value.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing high-strength, high-elongation recycled polyester staple fiber, characterized in that, Includes the following steps: The recycled polyester blend is melted and extruded to obtain nascent fibers; The nascent fibers are subjected to a multi-stage gradient drawing process, which specifically includes: At a first temperature, the nascent fibers are subjected to a first-stage pre-orientation stretching at a first stretching ratio; At a second temperature, the nascent fibers are subjected to a second stage of intermediate stretching at a second stretching ratio; At a third temperature, the nascent fibers are subjected to a third-stage main orientation stretching at a third stretching ratio; Wherein, the first temperature is lower than the glass transition temperature of recycled polyester, the second temperature is between the first temperature and the third temperature, and the third temperature is higher than the glass transition temperature of recycled polyester; the total draw ratio is the sum of the first draw ratio, the second draw ratio and the third draw ratio, and the first draw ratio accounts for 30%-40% of the total draw ratio, and the third draw ratio accounts for 15%-30% of the total draw ratio.

2. The method for preparing high-strength, high-elongation recycled polyester staple fiber as described in claim 1, characterized in that, The first stage of pre-orientation stretching is oil bath stretching, and the first temperature is 70-80℃; the second stage of intermediate stretching is dry heat stretching, and the second temperature is 80-90℃; the third stage of main orientation stretching is steam stretching, and the third temperature is 90-100℃.

3. The method for preparing high-strength, high-elongation recycled polyester staple fiber as described in claim 1 or 2, characterized in that, The drawing speed of the first-stage pre-orientation drawing is 50-80 m / min, the drawing speed of the second-stage intermediate drawing is 100-130 m / min, and the drawing speed of the third-stage main orientation drawing is 150-200 m / min.

4. The method for preparing high-strength, high-elongation recycled polyester staple fiber as described in claim 1, characterized in that, The recycled polyester blend is made by mixing waste polyester bottle flakes and chemical fiber foam material in a mass ratio of 6:4-8:

2.

5. The method for preparing high-strength, high-elongation recycled polyester staple fiber as described in claim 4, characterized in that, The intrinsic viscosity of the waste polyester bottle flakes is 0.7-0.8 dL / g, and the intrinsic viscosity of the chemical fiber foam material is 0.5-0.6 dL / g.

6. The method for preparing high-strength, high-elongation recycled polyester staple fiber as described in claim 1, characterized in that, The process of melt spinning the recycled polyester blend to obtain nascent fibers specifically includes: The dried recycled polyester mixture is fed into a screw extruder for melt treatment to obtain polyester melt; The polyester melt is transferred to a filtration system for filtration and then metered by a metering pump. The filtration system includes at least two stages of filtration equipment, and the pore size of the filter media in the filtration equipment decreases progressively. The metered polyester melt is extruded through a spinneret to form a melt stream, and then cooled and solidified by side blowing to form nascent fibers. The spinneret orifice diameter ranges from 0.15 to 0.30 mm, the side blowing temperature is 18-25°C, and the cooling rate is 50-100°C / s.

7. An apparatus for preparing high-strength, high-elongation recycled polyester staple fiber, employing the method for preparing high-strength, high-elongation recycled polyester staple fiber according to any one of claims 1-6, characterized in that, include: Melt spinning assembly, used to melt and extrude recycled polyester blends to form nascent fibers; A multi-stage gradient drawing assembly includes a first-stage drawing unit, a second-stage drawing unit, and a third-stage drawing unit. The first-stage drawing unit is used to perform a first-stage pre-orientation drawing of the nascent fiber at a first temperature below the glass transition temperature of the recycled polyester and a first drawing ratio. The third-stage drawing unit is used to perform a third-stage main orientation drawing of the nascent fiber at a third temperature above the glass transition temperature of the recycled polyester and a third drawing ratio. The second-stage drawing unit is used to perform a second-stage intermediate drawing of the nascent fiber at a second drawing ratio between the first and third temperatures. The total draw ratio is the sum of the first draw ratio, the second draw ratio, and the third draw ratio, wherein the first draw ratio accounts for 30%-40% of the total draw ratio, and the third draw ratio accounts for 15%-30% of the total draw ratio.

8. The apparatus for preparing high-strength, high-elongation recycled polyester staple fiber as described in claim 7, characterized in that, The first-stage drawing unit is an oil bath drawing machine, which is equipped with an oil bath tank for pre-orienting the nascent fibers. The third-stage drawing unit is a steam drawing machine, which is equipped with a steam box for main orientation drawing of the pre-oriented fibers.

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