Stretching method for hollow two-component fiber production

Abstract

The invention relates to a stretching method for hollow two-component fiber production, which comprises the following steps of: preheating and relaxing treatment: preheating and relaxing hollow two-component fiber pre-oriented yarns subjected to spinning and preliminary cooling and curing at a first temperature T1; the first temperature T1 is higher than the glass transition temperature Tg of the two polymer components in the fiber, but lower than the lower crystallization temperature Tc of the two polymer components; two-stage progressive stretching: first-stage stretching: carrying out first-stage stretching at a second temperature T2 at a stretching rate lambda1; second-stage stretching: immediately carrying out second-stage stretching at a third temperature T3 at the total stretching rate lambda; the fibers stretched in the step S2 are subjected to heat setting treatment at the fourth temperature T4, the fourth temperature T4 is higher than the third temperature T3, and the fourth temperature T4 is lower than the low melting point temperature Tm in the two polymer components; and cooling and winding. According to the invention, a hollow structure can be stably maintained, good combination of two-component interfaces is promoted, and collaborative optimization of fiber performance is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of drawing method for hollow bicomponent fiber production. BACKGROUND

[0002] Hollow bicomponent fiber has the advantages of light weight, warm, high loft, special gloss and potential differentiation function due to its unique structure (with hollow cavity, and composed of two different performance polymers), and is widely used in high-grade textiles, filter materials, thermal insulation filling materials and other fields.

[0003] In the fiber production process, drawing (or called drafting) is a key process, which makes the macromolecular chain of fiber oriented along the axial direction, greatly improves the mechanical properties such as strength and modulus of fiber. However, the traditional single-stage hot drawing or simple multi-stage drawing method has obvious defects: (1) Stress field and temperature field do not match: single or narrow temperature window is used in the whole drawing range, which cannot match the viscoelastic response of skin and core two polymer components at different deformation stages. The skin layer may have entered the forced high elastic state and can be stretched, while the core layer is still in the glass state, which leads to the increase of internal shear stress and extrusion of hollow structure; (2) Component interface peeling: the thermal expansion coefficient, viscoelasticity and crystallization behavior of the two polymer components are different. Unreasonable drawing process will cause too much stress at the interface of the two components, leading to interface peeling, which seriously damages the mechanical integrity and uniformity of the fiber; (3) Performance is not uniform: the fiber obtained by the traditional method often has uneven crystalline orientation of skin and core layers, which leads to uneven comprehensive performance (such as strength, elongation, thermal stability) of the fiber, making it difficult to play the synergistic effect of bicomponent.

[0004] Therefore, it is urgent to develop a special drawing method that can stably maintain the hollow structure, promote the good combination of bicomponent interface, and realize the synergistic optimization of fiber performance. SUMMARY

[0005] The present application improves the above-mentioned prior art, that is, the technical problem to be solved by the present application is to provide a drawing method for hollow bicomponent fiber production.

[0006] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is: a drawing method for hollow bicomponent fiber production, comprising the following steps: Step S1, preheating relaxation treatment: pre-oriented hollow bicomponent fiber after spinning and preliminary cooling and solidification is preheated and relaxed at a first temperature T1; the first temperature T1 is higher than the glass transition temperature Tg of the two polymer components in the fiber, but lower than the lower crystallization temperature Tc of the two polymer components; the processing time is t1 seconds; Step S2, Two-stage progressive stretching: The fibers treated in step S1 are subjected to two stages of thermal stretching: Step S21, First-stage stretching: At a second temperature T2, a first-stage stretching is performed with a stretching ratio λ1; the second temperature T2 is higher than the higher glass transition temperature Tg of the two polymer components. Step S22, Interface Co-deformation Zone: The fiber that has passed through step S21 immediately enters an isothermal relaxation channel, with a residence time of 0.05-0.3 seconds; Step S23, Secondary stretching: Immediately at the third temperature T3, perform secondary stretching with a total stretching ratio λ; the third temperature T3 is higher than the second temperature T2, and the third temperature T3 is within the cold crystallization temperature range of the two polymer components; where the total stretching ratio λ = λ1 × λ2, λ2 is the stretching ratio of the secondary stretching, and λ1 < λ2 is controlled. Step S3, High-temperature heat setting: The fiber stretched in step S2 is heat-set at a fourth temperature T4 for t2 seconds; the fourth temperature T4 is higher than the third temperature T3 and lower than the lower melting point temperature Tm of the two polymer components. Step S4, Cooling and Winding: Cool the heat-set fiber to room temperature and wind it to obtain the finished hollow bicomponent fiber.

[0007] Furthermore, in step S1, the first temperature T1 is a gradient temperature field, and the first temperature T1 smoothly rises from T1a to T1b; wherein, T1a is 10-20℃ higher than the higher glass transition temperature Tg of the two components, and T1b is 5-15℃ lower than the lower initial crystallization temperature Tc of the two components.

[0008] Furthermore, the first temperature T1 is 80℃~120℃, and the processing time t1 is 0.5~3 seconds.

[0009] Furthermore, in step S21, the second temperature T2 is 70℃~90℃, and the first-level stretching ratio λ1 is 1.3~1.8 times; in step S23, the third temperature T3 is 120℃~160℃, and the total stretching ratio λ is 3.5~6.5 times.

[0010] Furthermore, in step S3, the fourth temperature T4 is 150℃~200℃, and the processing time t2 is 0.1~2 seconds.

[0011] Furthermore, in step S3, a two-stage heat setting process is employed, the specific steps of which include: Step S31, High-tension heat setting: At a temperature T4a (slightly higher than T3), maintain high tension and perform a short treatment of 0.1-0.5 seconds, where T4a is higher than T3; Step S32, Low-tension relaxation: At temperature T4b, treat under low tension for 0.5-2 seconds; where T4b ≥ T4a.

[0012] Furthermore, a closed constant-temperature conveying channel is provided between the end of step S1 and the stretching roller in step S21. The temperature inside the constant-temperature conveying channel is maintained at T1b±2℃. A non-contact infrared temperature sensor and an online tension meter are installed inside the constant-temperature conveying channel.

[0013] Furthermore, a pair of preloaded rollers are provided between steps S22 and S23. The linear speed of the pair of preloaded rollers is higher than the end speed of step S22, but lower than the starting speed of the secondary stretching.

[0014] Furthermore, there is a rapid cooling zone between step S23 and step S3. In the rapid cooling zone, room temperature or low temperature airflow is used to instantly cool the fiber, so that the surface temperature of the fiber drops rapidly to below its glass transition temperature Tg.

[0015] Furthermore, the hollow bicomponent fiber has a core-sheath structure, with the sheath being polyester or polyamide and the core being polyolefin or another type of polyester.

[0016] Compared with existing technologies, this invention has the following advantages: It releases internal stress through "preheating relaxation" and achieves gentler, more uniform fiber deformation through "two-stage progressive stretching" and precise temperature control, effectively avoiding localized stress concentration and collapse of the hollow cavities during stretching, resulting in high roundness of the hollow cross-section. Furthermore, by setting the main stretching temperature within the cold crystallization temperature range of the two components, the two polymers crystallize or entangle their molecular chains almost simultaneously near the interface, forming an "interlocked crystallization" or "entanglement-enhanced" interface structure, significantly improving the bonding force between the core and sheath and preventing interface delamination. Moreover, preheating and graded stretching allow the macromolecular chains to orient themselves stepwise and fully, combined with high-temperature heat setting, enabling the fiber to achieve high strength and high modulus while maintaining a suitable elongation at break. The properties of the two components are synergistically utilized, resulting in uniform and stable mechanical properties of the finished fiber. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating the present invention. Detailed Implementation

[0018] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0019] In the description of this invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0020] like Figure 1 As shown, the present invention provides a stretching method for the production of hollow bicomponent fibers, aiming to overcome the defects of existing technologies in stretching hollow bicomponent fibers, which easily leads to hollow collapse, interfacial delamination, and uneven performance. The stretching method includes the following steps: Step S1, Preheating and Relaxation Treatment: The hollow bicomponent fiber pre-oriented yarn, which has been spun and pre-cooled and cured, is preheated and relaxed at a first temperature T1; the first temperature T1 is higher than the glass transition temperature Tg of the two polymer components in the fiber, but lower than the lower crystallization temperature Tc of the two polymer components; the treatment time is t1 seconds; through the preheating and relaxation treatment, the macromolecular chain segments inside the fiber are moderately relaxed, eliminating the internal stress during the spinning process, laying the foundation for subsequent uniform stretching, and preventing hollow deformation caused by internal stress concentration.

[0021] Step S2, Two-stage progressive stretching: The fibers treated in step S1 are subjected to two stages of thermal stretching, as detailed below: Step S21, First-stage stretching: At a second temperature T2, a first-stage stretching is performed with a stretching ratio λ1; the second temperature T2 is higher than the higher glass transition temperature Tg of the two polymer components. Step S22, Interfacial Co-deformation Zone: After step S21, the fiber immediately enters an isothermal relaxation channel, where it remains for 0.05-0.3 seconds, and the tension drops to near zero. In this region, the molecular chain orientation and interfacial shear stress caused by the previous stage of stretching provide excellent conditions for heterogeneous nucleation. Step S23, Secondary stretching: Immediately at the third temperature T3, perform secondary stretching with a total stretching ratio λ; the third temperature T3 is higher than the second temperature T2, and the third temperature T3 is within the cold crystallization temperature range of the two polymer components; The total stretch ratio λ = λ1 × λ2, where λ2 is the stretch ratio of a single secondary stretch, and λ1 < λ2 is controlled.

[0022] Step S3, High-Temperature Heat Setting: The fiber stretched in step S2 is heat-set at a fourth temperature T4 for t2 seconds. The fourth temperature T4 is slightly higher than the third temperature T3 and lower than the lower melting point temperature Tm of the two polymer components. Through this step, the macromolecular chains are rearranged and crystallized under tension, the tensile deformation is fixed, internal stress is eliminated, the hollow structure is stabilized, and the interfacial bonding of the two components is further enhanced.

[0023] Step S4, Cooling and Winding: Cool the heat-set fiber to room temperature and wind it to obtain the finished hollow bicomponent fiber.

[0024] Specifically, in step S1, the first temperature T1 is a gradient temperature field along the fiber's direction of travel, smoothly increasing from T1a to T1b. T1a is 10-20°C higher than the higher glass transition temperature (Tg) of the two components, while T1b is 5-15°C lower than the lower initial crystallization temperature (Tc). Within this range, the fiber experiences a micro-tension (approximately 5%-15% of the fiber's breaking strength). This step is not simply "preheating." Starting from T1a, the aim is to simultaneously initiate the movement of the chain segments of both components, releasing the non-equilibrium spinning internal stress. As the temperature gradient rises to T1b, under micro-tension, the macromolecular chains begin low-rate pre-orientation, forming a uniform stress distribution network. This provides a uniform "starting point" for subsequent main stretching, completely eliminating the root cause of hollow local collapse due to uneven internal stress. The "gradient heating + micro-tension" approach is the innovative aspect of this step. Specifically, in step S1, the first temperature T1 is 80℃~120℃, and the processing time t1 is 0.5~3 seconds.

[0025] Specifically, in step S21, the second temperature T2 is 70℃~90℃, and the first-level stretching ratio λ1 is 1.3~1.8 times; T2 is set near the temperature corresponding to the maximum deformation rate of the skin layer, while ensuring that the core layer has fully entered the high elastic state; the main purpose of this stage is to allow the skin and core layers to "align" the deformation rhythm at a lower stress level, so that the molecular chains on both sides of the interface can initially establish a cooperative motion mode, rather than pursuing high orientation.

[0026] Specifically, in step S22, this brief pause allows molecular chains at the interface of the two components to diffuse and entangle with each other, and may induce nucleation of one polymer on the surface of the other. This lays the foundation for the formation of an "interpenetrating crystallization" or "co-crystallization interface layer" during subsequent main stretching, and is key to solving the interface delamination problem. Furthermore, this low-tension environment is sufficient to prevent the fiber from becoming excessively thickened due to elastic recoil, while also providing sufficient kinetic conditions for the mutual diffusion and reconstruction of molecular chains on both sides of the interface. Specifically, in step S23, the third temperature T3 is 120℃~160℃, precisely set within the overlapping temperature range of the cold crystallization peaks of the two polymers, with a total stretching ratio λ of 3.5~6.5 times. Main stretching occurs in the overlapping region of the two cold crystallization phases, meaning that the skin and core layers simultaneously enter the crystallization period while undergoing maximum plastic deformation. The coupling between tensile stress-induced crystallization (stress-induced crystallization) and the heterogeneous nucleation effect between the two components causes the crystallization process to occur not only within their respective phases but also at the interface, forming a "crystallization interlocking structure." This structure acts like a "rivet," greatly strengthening the interface. Simultaneously, at this temperature, the polymer modulus is moderate, allowing the hollow cavity to shrink controllably and uniformly under uniform circumferential stress, rather than collapsing.

[0027] Specifically, in step S3, the fourth temperature T4 is 150℃~200℃, and the processing time t2 is 0.1~2 seconds.

[0028] Specifically, in step S3, a two-stage heat setting process is adopted, and the specific steps include: Step S31, High-tension heat setting: At temperature T4a (slightly higher than T3), maintain high tension and perform a short treatment of 0.1-0.5 seconds, where T4a is higher than T3; the purpose of this step is to quickly fix the highly oriented and highly crystalline morphology. Step S32, Low-tension relaxation: At temperature T4b, treat under low tension for 0.5-2 seconds; where T4b ≥ T4a. This stage allows for limited relaxation of the macromolecular chains, eliminating residual stress, especially interfacial stress, thereby stabilizing the hollow structure and further improving the dimensional stability and toughness of the fiber.

[0029] Specifically, a closed, temperature-controlled conveyor channel is set between the end of step S1 and the stretching roller in step S21. The temperature inside the temperature-controlled conveyor channel is maintained at T1b±2℃. A non-contact infrared temperature sensor and an online tension meter are installed inside the temperature-controlled conveyor channel. The sensor monitors the actual temperature of the fiber before it enters the stretching process in real time. The control system compares the actual temperature with the set value T1b and dynamically fine-tunes the power of the hot plate at the end of step S1. At the same time, it compares the tension detected by the online tension meter with the preset "micro-tension" value and performs closed-loop PID control by adjusting the speed difference of the guide roller. This step ensures that each fiber entering the main stretching process is in the same thermodynamic and stress initial state, which is the fundamental prerequisite for achieving subsequent high-ratio uniform stretching and eliminating "stretching jitter" or "uneven necking". This step provides a stable "baseline" for the entire stretching method.

[0030] Specifically, a pair of preloading rollers is set between steps S22 and S23. The linear velocity of the preloading rollers is higher than the end velocity of step S22, but lower than the starting velocity of the secondary stretching. By controlling this pair of preloading rollers, the fiber tension is linearly and smoothly increased from the relaxation tension to the initial tension required for the main stretching. At the same time, the inlet temperature of the secondary stretching and the temperature at which the fiber leaves the relaxation zone are thermally connected to avoid sudden temperature changes. Through this step, a soft start from "relaxation" to "high-ratio stretching" is achieved, ensuring that the stress distribution along the cross-section of the fiber is uniform at the moment it enters the main stretching zone, thereby ensuring that the hollow cavity shrinks uniformly in subsequent stretching, rather than being "torn apart" or "collapsed".

[0031] Specifically, there is a rapid cooling zone between step S23 and step S3. In the rapid cooling zone, room temperature or low temperature airflow is used to instantly cool the fiber, so that the surface temperature of the fiber drops rapidly to below its glass transition temperature Tg (for example, for PET, rapid cooling to 70-80℃). This step is not the final shaping, but rather "temporarily freezing" the orientation structure obtained by stretching. Its purpose is twofold: (1) locking the deformation: preventing relaxation before entering the heat setting zone; (2) creating a temperature gradient: allowing the fiber to enter the heat setting zone in a relatively "cold" state, and undergoing a "from the surface to the inside" reheating process in the heat setting zone. This is beneficial for the skin layer to be heated and fixed first, forming a "clamping" effect on the core layer and hollow structure, further stabilizing the hollow shape. This is an active use of temperature gradient to strengthen the structure.

[0032] Specifically, hollow bicomponent fibers have a core-sheath structure, with the sheath being polyester (PET) or polyamide (PA) and the core being polyolefin (such as PP, PE) or another type of polyester (such as PBT).

[0033] It should be noted that the stretching process is carried out in an inert gas atmosphere or a vapor atmosphere to reduce fiber oxidation.

[0034] Specifically, an online infrared thermometer and a laser diameter meter are installed at the outlet of step S23 to monitor the fiber temperature and diameter in real time, and the feedback control system dynamically fine-tunes the T3 temperature or stretching roller speed to compensate for fluctuations in raw materials or the environment and ensure stable hollowness.

[0035] Specifically, the entire hot zone is operated in a saturated steam atmosphere. The steam not only serves as a heat transfer medium but also gently swells the amorphous regions on the fiber surface, lowering the glass transition temperature, resulting in more uniform stretching and reduced thermal oxidation.

[0036] Example 1, taking the stretching of PET (sheath) / PP (core) hollow bicomponent fibers as an example: 1. Preheating relaxation: The spun POY yarn is contacted on a 100℃ (T1) hot roller for 1 second (t1). 2. Two-stage progressive stretching: At 85℃ (T2), a first-stage stretching is performed at 1.5 times (λ1); then immediately imported into a 140℃ (T3) hot box for the main stretching (i.e., the second-stage stretching), so that the total stretching ratio reaches 4.5 times (λ=4.5, then λ2=3.0). 3. High-temperature heat setting: Heat setting is performed on a hot plate at 180℃ (T4) for 0.5 seconds (t2); 4. Cooling and winding: Winding after air cooling.

[0037] Example 2, taking the stretching of PA6 (sheath) / PET (core) hollow bicomponent fibers as an example: 1. Preheating and relaxation: 110℃, 1.5 seconds; 2. Two-stage progressive stretching: First stage stretching: 80℃, 1.8 times; Second stage stretching: 155℃ hot box, total stretching ratio 5.0 times; 3. High-temperature heat setting: 195℃, 0.3 seconds.

[0038] 4. Cooling and winding.

[0039] Comparative Example Traditional single-stage hot stretching is used: the same POY yarn is stretched 4.5 times at 140℃ in one go, and then heat-set at 180℃ for 0.5 seconds.

[0040] The fibers obtained in Examples 1 and 2 and the comparative examples were tested, and the results are shown in the table below.

[0041] As can be seen from the table above, Examples 1 and 2, which employ the method of the present invention, are significantly superior to the comparative examples of traditional single-stage stretching in terms of hollowness retention rate, monofilament strength, interfacial bonding force (peel strength), and dimensional stability (low boiling water shrinkage).

[0042] The advantages of this invention are: (1) Good hollow structure retention: The internal stress is released by "preheating relaxation" and the fiber deformation is made more gentle and uniform by "multi-stage progressive stretching" and precise temperature control, which effectively avoids local stress concentration and collapse of the hollow cavity during the stretching process and the roundness of the hollow section is high. (2) Strong interfacial bonding: By setting the main stretching temperature (T3) within the cold crystallization temperature range of the two components, the two polymers are prompted to crystallize or entangle their molecular chains almost simultaneously near the interface, forming an interfacial structure of "crystallization interlocking" or "entanglement enhancement", which significantly improves the bonding force between the core and the skin and prevents interfacial peeling. (3) Excellent comprehensive fiber performance: Preheating and graded stretching enable the macromolecular chains to be oriented stepwise and fully, and combined with high-temperature heat setting, the fiber obtains high strength and high modulus while maintaining a suitable elongation at break. The properties of the two components are synergistically utilized, and the mechanical properties of the finished fiber are uniform and stable; (4) Strong process controllability: The temperature, multiplier and time parameters of each step are clear, which makes it easy to achieve precise control on industrial production lines. It has good reproducibility and is suitable for large-scale production. (5) The roundness (roundness > 0.9) and retention rate (> 95%) of the hollow cavity are extremely high, and the final size of the hollow cavity can be designed within a certain range by adjusting the stretching ratio and temperature of S23; (6) Synergistic super-strengthening of performance: The fiber not only has high strength, but its toughness, fatigue resistance and anisotropic properties are also optimized simultaneously. For example, the ternary synergy of the obtained fiber in terms of strength-modulus-fracture work is significantly better than that of traditional methods.

[0043] If this invention discloses or relates to components or structural parts that are fixedly connected to each other, then, unless otherwise stated, a fixed connection can be understood as: a fixed connection that can be detached (e.g., using bolts or screws), or a fixed connection that cannot be detached (e.g., riveting, welding). Of course, a fixed connection can also be replaced by an integral structure (e.g., manufactured in one piece using a casting process) (except where it is obviously impossible to use an integral molding process).

[0044] In addition, unless otherwise stated, the terms used in any of the technical solutions disclosed in this invention to indicate positional relationships or shapes include states or shapes that are similar to, close to, or approximate with those states or shapes.

[0045] Any component provided by this invention can be assembled from multiple individual components or can be a single component manufactured by a one-piece molding process.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the present invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. A stretching method for the production of hollow bicomponent fibers, characterized in that: Includes the following steps: Step S1, Preheating and Relaxation Treatment: The hollow bicomponent fiber pre-oriented yarn that has been spun and pre-cooled and cured is preheated and relaxed at a first temperature T1; the first temperature T1 is higher than the glass transition temperature Tg of the two polymer components in the fiber, but lower than the lower crystallization temperature Tc of the two polymer components; the treatment time is t1 seconds. Step S2, Two-stage progressive stretching: The fibers treated in step S1 are subjected to two stages of thermal stretching: Step S21, First-stage stretching: At a second temperature T2, a first-stage stretching is performed with a stretching ratio λ1; the second temperature T2 is higher than the higher glass transition temperature Tg of the two polymer components. Step S22, Interface Co-deformation Zone: The fiber that has passed through step S21 immediately enters an isothermal relaxation channel, with a residence time of 0.05-0.3 seconds; Step S23, Secondary stretching: Immediately at the third temperature T3, perform secondary stretching with a total stretching ratio λ; the third temperature T3 is higher than the second temperature T2, and the third temperature T3 is within the cold crystallization temperature range of the two polymer components; The total stretch ratio λ = λ1 × λ2, where λ2 is the stretch ratio of the second stretch, and λ1 < λ2 is controlled. Step S3, High-temperature heat setting: The fiber stretched in step S2 is heat-set at a fourth temperature T4 for t2 seconds; the fourth temperature T4 is higher than the third temperature T3 and lower than the lower melting point temperature Tm of the two polymer components. Step S4, Cooling and Winding: Cool the heat-set fiber to room temperature and wind it to obtain the finished hollow bicomponent fiber.

2. The stretching method for producing hollow bicomponent fibers according to claim 1, characterized in that: In step S1, the first temperature T1 is a gradient temperature field, and the first temperature T1 smoothly rises from T1a to T1b; wherein, T1a is 10-20℃ higher than the higher glass transition temperature Tg of the two components, and T1b is 5-15℃ lower than the lower initial crystallization temperature Tc of the two components.

3. A stretching method for producing hollow bicomponent fibers according to claim 1 or 2, characterized in that: The first temperature T1 is 80℃~120℃, and the processing time t1 is 0.5~3 seconds.

4. The stretching method for producing hollow bicomponent fibers according to claim 1, characterized in that: In step S21, the second temperature T2 is 70℃~90℃, and the first-level stretch ratio λ1 is 1.3~1.8 times; in step S23, the third temperature T3 is 120℃~160℃, and the total stretch ratio λ is 3.5~6.5 times.

5. The stretching method for producing hollow bicomponent fibers according to claim 1, characterized in that: In step S3, the fourth temperature T4 is 150℃~200℃, and the processing time t2 is 0.1~2 seconds.

6. The stretching method for producing hollow bicomponent fibers according to claim 1, characterized in that: In step S3, a two-stage heat setting process is adopted, and the specific steps include: Step S31, High-tension heat setting: At a temperature T4a (slightly higher than T3), maintain high tension and perform a short treatment of 0.1-0.5 seconds, where T4a is higher than T3; Step S32, Low-tension relaxation: At temperature T4b, treat under low tension for 0.5-2 seconds; where T4b ≥ T4a.

7. The stretching method for producing hollow bicomponent fibers according to claim 1, characterized in that: A closed, temperature-controlled conveying channel is provided between the end of step S1 and the stretching roller in step S21. The temperature inside the temperature-controlled conveying channel is maintained at T1b±2℃. A non-contact infrared temperature sensor and an online tension meter are installed inside the temperature-controlled conveying channel.

8. The stretching method for producing hollow bicomponent fibers according to claim 1, characterized in that: A pair of preloaded rollers are provided between steps S22 and S23. The linear speed of the pair of preloaded rollers is higher than the end speed of step S22, but lower than the starting speed of the secondary stretching.

9. The stretching method for producing hollow bicomponent fibers according to claim 1, characterized in that: There is a rapid cooling zone between step S23 and step S3. In the rapid cooling zone, room temperature or low temperature airflow is used to instantly cool the fiber, so that the surface temperature of the fiber drops rapidly to below its glass transition temperature Tg.

10. A stretching method for producing hollow bicomponent fibers according to claim 1, characterized in that: The hollow bicomponent fiber has a core-sheath structure, with the sheath being polyester or polyamide and the core being polyolefin or another type of polyester.

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