Hollow three-dimensional gradient needling process

Abstract

The invention discloses a hollow three-dimensional gradient needling process, which comprises the following steps of: adopting a sparse needle plate, and puncturing an upper surface layer and a lower surface layer of a fiber net by a first needling machine, so that the fiber net is preliminarily entangled and shaped; respectively needling the upper surface layer and the lower surface layer of the fiber web by adopting a close-packed needle plate and second to sixth needling machines, controlling the needling depth in a range of 1 / 3 to 2 / 3 of the thickness of the fiber web, and enabling needling to only penetrate through the core layer and the surface layer on the corresponding side but not penetrate through the surface layer on the opposite side, so as to form a gradient structure of which the surface layers are initially compact and the core layer is loose; a densely-arranged needle plate is adopted, the seventh to ninth needle machines only conduct dense needling on the upper surface layer and the lower surface layer of the fiber net, a core layer is not punctured, the needling depth is controlled to be 1 / 3 of the thickness of the fiber net, the surface layer fibers form a high-density entangled rigid supporting layer, and the hollow structure of the core layer fibers is reserved. Through staged needling, a gradient structure with a compact surface layer and a loose core layer is constructed in the thickness direction, the strength of the surface layer is guaranteed, a hollow structure of core layer fibers is reserved, and the material is light in weight, high in resilience, high in strength and long-acting and durable.

Description

Technical Field

[0001] This invention relates to the field of synthetic leather technology, specifically a hollow three-dimensional gradient needle punching process to achieve a high-elasticity and lightweight preparation method, which is suitable for use in scenarios with high requirements for elasticity, lightweight and high performance, such as footwear, clothing, and automotive interiors. Background Technology

[0002] Microfiber synthetic leather (microfiber leather) is widely used in footwear, clothing, and automotive interiors due to its excellent mechanical properties and leather-like feel. Traditional microfiber leather uses solid fibers (such as conventional PET and PA fibers) as the base material. These fibers are carded into a web and then reinforced with single-density needle punching using a flatbed needle punching machine, forming a uniformly entangled and dense nonwoven fabric skeleton. The finished product is then produced through polyurethane impregnation, fiber opening, and finishing processes. However, the traditional needle punching process has the following technical drawbacks: 1. Homogeneous structure makes it difficult to achieve both lightweight and high elasticity: Conventional needle punching processes use uniform needle punching density and depth to create a uniform density structure in the thickness direction of the fiber web. To ensure material strength, a high needle punching density (250-350 needles / cm²) is usually required, resulting in a high material density (≥0.55g / cm³), which is difficult to meet the lightweight requirements of footwear materials, automotive interiors, and other fields. If the needle punching density is reduced to pursue lightweight, the fiber entanglement is insufficient, and the material strength is significantly reduced (tensile strength ≤5MPa), failing to meet the application requirements.

[0003] 2. Damage to the hollow core structure: To achieve lightweighting, some studies have attempted to use hollow fibers as raw materials. However, in conventional needle punching processes, the needles completely penetrate the full thickness of the fiber web, and repeated punctures cause the core fibers to become densely entangled and compressed. The cavity structure of the hollow fibers is crushed or filled, reducing the hollow rate from 40-60% in the raw material to ≤30% in the finished product, significantly diminishing the lightweighting effect.

[0004] 3. Insufficient fatigue life under dynamic loads: Under dynamic loads such as running and jumping, footwear materials repeatedly endure compression-rebound cycles. Traditional homogeneous structures, lacking a gradient buffer layer, cause stress concentration at fiber entanglement points, easily leading to microcrack propagation. Tests show that after 100,000 dynamic bending cycles, the tensile strength of traditional microfiber leather decreases by 45%, and the tear strength decreases by over 30%, failing to meet the durability requirements of high-end footwear materials. Summary of the Invention

[0005] To address the aforementioned shortcomings of existing technologies, this invention provides a hollow three-dimensional gradient needle punching process. Through staged needle punching, a gradient structure of "dense surface layer - loose core layer" is constructed in the thickness direction, which ensures both surface strength and retains the hollow structure of the core fiber. This achieves lightweight, high resilience, high strength, and long-lasting durability of the material, providing an advanced material solution that balances performance and lightweight for footwear, clothing, automotive interiors, and other fields.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a hollow three-dimensional gradient needle punching process, comprising the following steps: Pre-needling step: The fiber web is pre-needled using a sparsely arranged needle plate and fine needles. The fiber web consists of an upper surface layer, a core layer and a lower surface layer. The first needle punch pierces the upper and lower surface layers of the fiber web, so that the fiber web is initially entangled and shaped, while retaining the hollow structure of the core layer fibers. Main needle punching step: Using a densely packed needle plate, the 2nd to 6th needle punching machines respectively punch the upper and lower surfaces of the fiber web. The needle punching depth is controlled within the range of 1 / 3 to 2 / 3 of the fiber web thickness, so that the needle punching only penetrates the core layer and the corresponding surface layer on one side, but does not penetrate the surface layer on the other side, forming a gradient structure with a preliminary dense surface layer and a loose core layer. Surface densification step: Using a densely packed needle plate and coarse needles, the 7th to 9th needle punching machines only densify the upper and lower surface layers of the fiber web without piercing the core layer. The needle punching depth is controlled at 1 / 3 of the fiber web thickness, so that the surface fibers form a rigid support layer with high-density entanglement, while the hollow structure of the core fibers is preserved.

[0007] Furthermore, in the pre-acupuncture step, a sparsely spaced needle plate with a needle spacing of 4-6 mm is used, along with 42-44 gauge fine needles, with an acupuncture density of 80-120 needles / cm², a vehicle speed of 8-10 m / min, and an acupuncture frequency of 800-1000 times / min.

[0008] Furthermore, in the main acupuncture step, a densely packed needle plate with a needle spacing of 2-3 mm is used, the acupuncture density is 180-220 needles / cm², the vehicle speed is 6-8 m / min, the acupuncture frequency is 1350-1750 times / min, and the needle entry angle is 90±2°.

[0009] Furthermore, in the surface encryption step, a densely packed needle plate with a needle spacing of 2-3mm is used, along with 38-40 gauge coarse needles, with a needle density of 300-400 needles / cm², a vehicle speed of 4-6m / min, and a needle frequency of 1200-1500 times / min.

[0010] Furthermore, after the pre-needling step, the hollow rate of the core layer remains above 70%; after the main needle-punching step and the surface densification step, the hollow rate of the fiber web core layer remains between 30-50%.

[0011] Furthermore, in the main needle puncture step, by alternating between upward and downward needle puncture, the upper needle puncture only penetrates the upper surface layer and the core layer, while the lower needle puncture only penetrates the lower surface layer and the core layer, forming a symmetrical gradient structure.

[0012] Furthermore, the fiber web is made of island-island fibers, which use COPET as the sea component and PA as the island component, add color masterbatch, and are made into ultrafine island-island fibers through composite spinning technology.

[0013] Furthermore, the procedure prior to the pre-needle acupuncture step also includes: Opening process: The fiber raw material is processed through 3-4 opening processes to control the opening degree ≥95% and the fiber clumping rate ≤0.5%; Combing and web laying steps: The opened fibers are combed into thin webs with a single weight of 50-60 g / m², and then laid in 8-10 layers, with the total weight controlled at 400-500 g / m². After laying, the webs are pre-pressed with light pressure rollers at a pressure of 0.5-1 MPa.

[0014] Furthermore, during the web laying process, the orientation ratio of longitudinal fibers to transverse fibers is 1:(1.1-1.2), and the weight uniformity is controlled within ±(5-10)%.

[0015] The hollow three-dimensional gradient needle punching process of this invention has the following beneficial effects: 1. Lightweight and High Resilience Performance: A gradient pore structure is constructed through a hollow three-dimensional needle punching process, with the core layer retaining 30-50% of the high-fiber hollow layer. Compared with the traditional uniform needle punching process in Comparative Example 1, the material density is reduced by 14.6-21.3%, and the compression resilience is increased by 31.7-37.4%. Compared with conventional microfiber leather (density ≥0.55g / cm³), the density is reduced by more than 50%, which can reduce sports energy consumption by 15% in footwear applications. In the automotive interior field, it can achieve a 5% reduction in overall vehicle weight while meeting dynamic deformation requirements (such as tear resistance ≥50N in seat bending tests).

[0016] 2. Unique fiber structure design: The gradient structure design features a dense surface layer and a porous core layer, giving the material a "rigid yet flexible" characteristic: the surface layer forms a rigid support layer through high-density needle punching, while the core layer retains its original porosity to provide elastic cushioning. In 100,000 bending tests, the risk of material fatigue fracture is reduced by 40%, and the tear strength attenuation rate is reduced by 50% compared to traditional processes.

[0017] 3. Wide range of material applications: The material properties can be precisely matched to the needs of footwear, clothing and automotive interiors, while the gradient structure adapts to the dynamic deformation of seats and door panels, meeting the sustainability requirements of the high-end market. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] This invention provides a hollow three-dimensional gradient needle punching process, comprising the following steps: Pre-needling step: The fiber web is pre-needled using a sparsely arranged needle plate and fine needles. The fiber web consists of an upper surface layer, a core layer and a lower surface layer. The first needle punch pierces the upper and lower surface layers of the fiber web, so that the fiber web is initially entangled and shaped, while retaining the hollow structure of the core layer fibers. Main needle punching step: Using a densely packed needle plate, the 2nd to 6th needle punching machines respectively punch the upper and lower surfaces of the fiber web. The needle punching depth is controlled within the range of 1 / 3 to 2 / 3 of the fiber web thickness, so that the needle punching only penetrates the core layer and the corresponding surface layer on one side, but does not penetrate the surface layer on the other side, forming a gradient structure with a preliminary dense surface layer and a loose core layer. Surface densification step: Using a densely packed needle plate and coarse needles, the 7th to 9th needle punching machines only densify the upper and lower surface layers of the fiber web without piercing the core layer. The needle punching depth is controlled at 1 / 3 of the fiber web thickness, so that the surface fibers form a rigid support layer with high-density entanglement, while the hollow structure of the core fibers is preserved.

[0020] This invention constructs a gradient structure through a three-step process of "pre-needling - main needle punching - surface densification," achieving graded preservation of the hollow structure in the fiber web core layer. Pre-needling uses sparsely spaced needles and fine needles for initial shaping, preserving a core layer hollowness of ≥70%. Main needle punching creates a gradient prototype with a dense surface layer and a loose core layer. Surface densification uses high-density needle punching with coarse needles, forming a rigid support layer on the surface while still retaining 30-50% of the core layer hollowness. This process solves the technical challenge of achieving both lightweight, high strength, and high elasticity in traditional microfiber leather, enabling the material to possess both high surface strength and high core resilience.

[0021] In the pre-acupuncture step, a sparsely spaced needle plate with a needle spacing of 4-6 mm is used, along with 42-44 gauge fine needles, and the acupuncture density is 80-120 needles / cm². 2The machine speed is 8-10 m / min, and the needle punching frequency is 800-1000 times / min. A sparsely spaced needle plate with a needle pitch of 4-6 mm is selected to balance fiber entanglement efficiency and hollow layer retention rate; this range has been experimentally verified as the optimal range. A needle pitch <4 mm easily leads to excessive local entanglement, damaging the core layer's looseness; a needle pitch >6 mm results in insufficient entanglement, with the fabric strength below 2 MPa, failing to meet subsequent processing requirements. Fine needles of 42-44 gauge are selected. Fine needles can form finer needle marks. For COPET / PA island fibers, fine needles reduce breakage, ensuring substrate strength while minimizing excessive fiber damage, ensuring the core layer fibers' free rebound. A needle density of 80-120 needles / cm is selected. 2 Ensure initial entanglement without over-entanglement. Maintain a machine speed of 8-10 m / min, matching the speed to the frequency to ensure uniformity. Pre-needling using the above process effectively avoids over-needling, reinforcing the fiber web while preventing excessive needling from hindering subsequent needling processes. Perfectly preserving ≥70% of the high-fiber hollow layer is crucial for achieving lightweight and high elasticity in the material.

[0022] In the main acupuncture step, a closely spaced needle plate with a needle spacing of 2-3 mm is used, and the acupuncture density is 180-220 needles / cm². 2 The machine speed is 6-8 m / min, the needle-punching frequency is 1350-1750 times / min, and the needle entry angle is 90±2°. In this step, the needles only penetrate the fiber web core layer and do not pierce the surface layer. The machine speed of 6-8 m / min provides a stable fiber web transport rate, avoiding excessive compaction of the core layer due to excessive speed or insufficient surface needle-punching due to excessive speed. The needle-punching frequency of 1350-1750 times / min, matched with the machine speed, can stably output the target density gradient, with the error controlled within ±5%, ensuring the needle-punching depth is controlled at 1 / 3-2 / 3 of the fiber web thickness, forming a dense surface structure with a hollow rate of 30-50%, achieving differentiated needle-punching between the surface and core layers. Controlling the needles to enter the web perpendicularly at an entry angle of 90±2° prevents fiber collapse from affecting structural uniformity. The fabric forms a "preliminary dense surface layer, 30-50% gradient structure in the fiber core layer, overall tensile strength ≥8MPa, tear strength ≥120N, meeting the requirements of subsequent processing".

[0023] In the surface densification step, a densely packed needle plate with a needle spacing of 2-3 mm is used, along with 38-40 gauge coarse needles, resulting in a needle density of 300-400 needles / cm². 2The machine speed is 4-6 m / min, and the needle punching frequency is 1200-1500 times / min. This step only densifies the needle punching of the upper and lower surface layers, without piercing the core layer. The purpose of this process is to increase the entanglement of the surface fibers, forming a dense support and shaping layer, achieving a surface tensile strength of 3-9 MPa and a tear strength ≥120 N, while ensuring dimensional stability and surface flatness. The core layer, not subjected to additional needle punching, retains its original loose structure and elastic buffer space. The densely packed needle plate uses a needle spacing of 2-3 mm; the dense needle points cause the fibers to be repeatedly pierced, hooked, and entangled over a very short distance, forming a three-dimensional rigid mesh skeleton. Use coarse needles (size 38-40), approximately 0.4-0.5 mm in diameter. These robust needles are highly rigid and can effectively penetrate high-density fiber webs, preventing finer needles from bending, breaking, or shifting under high loads. The coarse needles exert more uniform shear force on the fibers during piercing, reducing fiber breakage and damage compared to finer needles (such as size 36), maintaining the original mechanical properties of the fibers, and ensuring long-term surface durability. A needle density of 300-400 needles / cm² is selected. 2 This density far exceeds the conventional 180-220 spiky / cm³. 2 By forming a "ceramic-like" rigid network through ultra-high fiber entanglement density, the tensile and tear strength of the material is significantly improved. The needle penetration depth only affects the surface 1 / 3 of the fiber web thickness, without reaching the core layer, thus preserving 30-50% of the core fiber hollowness and elastic resilience. A machine speed of 4-6 m / min extends the residence time of the fiber web in the needle-punching zone, allowing each fiber ample opportunity for multiple punctures and entanglements. If the machine speed exceeds 6 m / min, the number of needle penetrations per unit area decreases, making it difficult to consistently achieve a needle density of 300 needles / cm². 2 The above applies; if the machine speed is less than 4 m / min, the production efficiency is too low and it is not economical. The needle punching frequency is 1200-1500 times / min. A frequency greater than 1500 times / min is prone to causing needle fatigue breakage, excessive fiber shearing damage, and uneven needle punching due to mechanical vibration.

[0024] After the pre-needling step, the hollow rate of the core layer remains above 70%; after the main needle-punching step and the surface densification step, the hollow rate of the fiber web core layer remains between 30-50%. This provides clear quality control standards for the process; the establishment of this indicator system allows operators to monitor the protective effect of each process on the core layer structure online, ensuring batch stability. Experimental data shows that the core layer hollow rate fluctuation range of the fabric prepared using this process is controlled within ±3%.

[0025] In the main needle-punching step, an alternating up-and-down needle-punching method is used. During the upward punching, only the upper surface and core layers are pierced; during the downward punching, only the lower surface and core layers are pierced, forming a symmetrical gradient structure. This alternating needle-punching method creates a symmetrical gradient structure, ensuring that both sides of the material have the same mechanical properties and avoiding warping deformation caused by single-sided needle-punching. The fact that only the upper surface and core layers are pierced during the upward punching and only the lower surface and core layers are pierced during the downward punching ensures consistent density on both sides of the surface layer, while the hollow structure of the core layer is symmetrically preserved. This design makes the material behave consistently under stress on both sides, making it suitable for applications such as shoe materials and automotive interiors that require double-sided use.

[0026] The fiber web is made of island-island fibers, which use COPET as the sea component and PA as the island component, with the addition of color masterbatch, and are made into ultrafine island-island fibers through composite spinning technology.

[0027] The procedure preceding the pre-acupuncture step also includes: Opening process: The fiber raw material is processed through 3-4 opening processes to control the opening degree ≥95% and the fiber clumping rate ≤0.5%; the opening degree ≥95% ensures that the fiber is fully dispersed and avoids needle-punching defects caused by clumping.

[0028] Carding and web-laying steps: Card the opened fibers into single sheets with a basis weight of 50-60 g / m². 2 Thin mesh, laid in 8-10 layers, with the total weight controlled at 400-500 g / m². 2 After the web is laid, it is pre-pressed by light pressure rollers with a pressure of 0.5-1MPa, so that the fiber web only plays a temporary shaping role, without compacting the hollow layer and damaging the internal structure. At the same time, it is also conducive to the reciprocating penetration of the needles in the subsequent needle punching process.

[0029] During the web-laying process, the orientation ratio of longitudinal fibers to transverse fibers is 1:(1.1-1.2), while the conventional longitudinal-to-transverse fiber orientation ratio is 1:1. This ratio ensures the strength of the nonwoven fabric during transverse fiber shrinkage and avoids excessive elongation. The basis weight uniformity is controlled within ±(5-10)%, forming a three-dimensionally uniformly distributed fiber web, which provides a basis for the density consistency of subsequent gradient needle punching.

[0030] The beneficial effects of the hollow three-dimensional gradient needle punching process of the present invention will be further illustrated below through several examples and comparative examples. Example 1

[0031] Prepare hollow three-dimensional gradient needle-punched nonwoven fabric according to the following steps: S1. Raw material: COPET / PA island hollow fiber, fiber fineness 3.5D, length 51mm, hollow rate 45%; S2. Opening: After 4 opening stages, the opening degree is 96% and the agglomeration rate is 0.3%; S3. Carding and Web Laying: Using a cross-laying machine and a quantitative cotton feeding device, the opened fibers are carded into single sheets with a basis weight of 55 g / m². 2 The thin mesh, laid in 10 layers in a cross pattern, has a total weight of 550g / m². 2 The longitudinal and transverse orientation ratio is 1:1.15, and the weight uniformity is controlled within ±6%. After the web is laid, it is pre-pressed by light pressure rollers with a pressure of 0.8MPa. S4. Pre-needling: The first acupuncture machine, with a needle spacing of 5mm, a sparse needle plate, a No. 43 fine needle, a needle density of 100 needles / cm², a machine speed of 9m / min, a frequency of 900 times / min, and full thickness puncture. S5. Main needling: 2nd-6th acupuncture machines, 2.5mm needle spacing with closely spaced needles, needling density 200 needles / cm². 2 The vehicle speed is 7m / min, the frequency is 1500 times / min, the entry angle is 90°, and the needle penetration depth is controlled to be 2 / 3 of the thickness (when penetrating upwards, it penetrates the outer surface layer and the core layer; when penetrating downwards, it penetrates the lower surface layer and the core layer). S6. Surface densification: The 7th-9th needle punching machines use a 2.5mm densely packed needle plate, No. 39 coarse needles, a needle density of 350 needles / cm², a machine speed of 5m / min, a frequency of 1350 times / min, and a needle depth controlled to 1 / 3 of the thickness, only piercing the surface layer. Example 2

[0032] S1. Raw material: COPET / PA island hollow fiber, fiber fineness 3.5D, length 51mm, hollow rate 45%; S2. Opening: After 4 opening stages, the opening degree is 96% and the agglomeration rate is 0.3%; S3. Carding and Web Laying: Using a cross-laying machine and a quantitative cotton feeding device, the opened fibers are carded into single sheets with a basis weight of 60g / m². 2 The thin mesh, laid in 10 layers in a cross pattern, has a total weight of 600g / m². 2 The longitudinal and transverse orientation ratio is 1:1.15, and the weight uniformity is controlled within ±6%; after the web is laid, it is pre-pressed by light pressure rollers with a pressure of 0.8MPa. S4. Pre-needling: The first acupuncture machine, with a needle spacing of 5mm, a sparse needle plate, a No. 43 fine needle, a needle density of 100 needles / cm², a machine speed of 9m / min, a frequency of 900 times / min, and full thickness puncture. S5. Main needling: 2nd-6th acupuncture machines, 2.5mm needle spacing with closely spaced needles, needling density 210 needles / cm². 2 The vehicle speed is 6.5 m / min, the frequency is 1500 times / min, the net entry angle is 90°, and the needle penetration depth is controlled to 2 / 3 of the thickness (when penetrating upwards, it penetrates the outer surface layer and the core layer; when penetrating downwards, it penetrates the lower surface layer and the core layer). S6. Surface densification: The 7th-9th needle punching machines use a 2.5mm densely packed needle plate, No. 39 coarse needles, a needle density of 380 needles / cm², a machine speed of 4.5m / min, a frequency of 1350 times / min, and a needle depth controlled to 1 / 3 of the thickness, only piercing the surface layer. Example 3

[0033] Prepare hollow three-dimensional gradient needle-punched nonwoven fabric according to the following steps: S1. Raw material: COPET / PA island hollow fiber, fiber fineness 3.5D, length 51mm, hollow rate 45%; S2. Opening: After 4 opening stages, the opening degree is 96% and the agglomeration rate is 0.3%; S3. Carding and Web Laying: Using a cross-laying machine and a quantitative cotton feeding device, the opened fibers are carded into single sheets with a basis weight of 40g / m². 2 The thin mesh, laid in 8 layers in a cross pattern, has a total weight of 400g / m². 2 The longitudinal and transverse orientation ratio is 1:1.15, and the weight uniformity is controlled within ±6%; after the web is laid, it is pre-pressed by light pressure rollers with a pressure of 0.8MPa. S4. Pre-needling: The first acupuncture machine, with a needle spacing of 5mm, a sparse needle plate, a No. 43 fine needle, a needle density of 90 needles / cm², a machine speed of 9m / min, a frequency of 900 times / min, and full thickness puncture. S5. Main needling: 2nd-6th acupuncture machines, 2.5mm needle spacing with closely spaced needles, needling density of 190 needles / cm². 2 The vehicle speed is 7m / min, the frequency is 1500 times / min, the entry angle is 90°, and the needle penetration depth is controlled to be 2 / 3 of the thickness (when penetrating upwards, it penetrates the outer surface layer and the core layer; when penetrating downwards, it penetrates the lower surface layer and the core layer). S6. Surface densification: The 7th-9th needle punching machines use a 2.5mm densely packed needle plate, No. 39 coarse needles, a needle density of 320 needles / cm², a machine speed of 5m / min, a frequency of 1350 times / min, and a needle depth controlled to 1 / 3 of the thickness, only piercing the surface layer.

[0034] Comparative Example 1 (Traditional uniform needle punching process) Prepare hollow three-dimensional gradient needle-punched nonwoven fabric according to the following steps: S1. Raw material: COPET / PA island hollow fiber, fiber fineness 3.5D, length 51mm, hollow rate 45%; S2. Opening: After 4 opening stages, the opening degree is 96% and the agglomeration rate is 0.3%; S3. Carding and Web Laying: Using a cross-laying machine and a quantitative cotton feeding device, the opened fibers are carded into single sheets with a basis weight of 55 g / m². 2 The thin mesh, laid in 10 layers in a cross pattern, has a total weight of 550g / m². 2The longitudinal and transverse orientation ratio is 1:1.15, and the weight uniformity is controlled within ±6%; after the web is laid, it is pre-pressed by light pressure rollers with a pressure of 0.8MPa. S4. Needle Puncture Process: A conventional flatbed needle puncture machine is used. The needle puncture process parameters are as follows: Needle plate type: close-packed needle plate, needle pitch 2.5mm; Needle specifications: No. 38 standard needle (needle diameter approximately 0.50mm); Needle density: 250 needles / cm² (uniform thickness); Acupuncture frequency: 1200 times / min; Vehicle speed: 6 m / min; Needle penetration depth: The needle completely penetrates the full thickness of the fiber network (i.e., pierces the upper and lower surface layers and the core layer), and all needle penetrations are bidirectional.

[0035] S5. The greige fabric is directly subjected to performance testing after needle punching without any post-processing.

[0036] Comparative Example 2 (Traditional uniform low-density needle punching process) Prepare hollow three-dimensional gradient needle-punched nonwoven fabric according to the following steps: S1. Raw material: COPET / PA island hollow fiber, fiber fineness 3.5D, length 51mm, hollow rate 45%; S2. Opening: After 4 opening stages, the opening degree is 96% and the agglomeration rate is 0.3%; S3. Carding and Web Laying: Using a cross-laying machine and a quantitative cotton feeding device, the opened fibers are carded into single sheets with a basis weight of 55 g / m². 2 The thin mesh, laid in 10 layers in a cross pattern, has a total weight of 550g / m². 2 The longitudinal and transverse orientation ratio is 1:1.15, and the weight uniformity is controlled within ±6%; after the web is laid, it is pre-pressed by light pressure rollers with a pressure of 0.8MPa. S4. Needle Puncture Process: A conventional flatbed needle puncture machine is used. The needle puncture process parameters are as follows: Needle plate type: close-packed needle plate, needle pitch 2.5mm; Needle specifications: No. 38 standard needle (needle diameter approximately 0.50mm); Needle density: 150 needles / cm² (uniform thickness); Acupuncture frequency: 900 times / min; Vehicle speed: 6 m / min; Needle penetration depth: The needle completely penetrates the full thickness of the fiber network (i.e., pierces the upper and lower surface layers and the core layer), and all needle penetrations are bidirectional.

[0037] S5. The greige fabric is directly subjected to performance testing after needle punching without any post-processing.

[0038] Comparative Example 3 (Two-stage needle punching process without gradient structure) Prepare hollow three-dimensional gradient needle-punched nonwoven fabric according to the following steps: S1. Raw material: COPET / PA island hollow fiber, fiber fineness 3.5D, length 51mm, hollow rate 45%; S2. Opening: After 4 opening stages, the opening degree is 96% and the agglomeration rate is 0.3%; S3. Carding and Web Laying: Using a cross-laying machine and a quantitative cotton feeding device, the opened fibers are carded into single sheets with a basis weight of 55 g / m². 2 The thin mesh, laid in 10 layers in a cross pattern, has a total weight of 550g / m². 2 The longitudinal and transverse orientation ratio is 1:1.15, and the weight uniformity is controlled within ±6%; after the web is laid, it is pre-pressed by light pressure rollers with a pressure of 0.8MPa. S4. Pre-needling: The first acupuncture machine, with a needle spacing of 5mm, a sparse needle plate, a No. 43 fine needle, a needle density of 200 needles / cm², a machine speed of 8m / min, a frequency of 1000 times / min, and full thickness puncture. S5. Main needling: 2nd-6th acupuncture machines, 38 gauge needles; 2.5mm needle spacing, close-packed needle plate, needling density 300 needles / cm². 2 Vehicle speed 6m / min, frequency 1500 times / min, piercing full thickness.

[0039] Table 1. Test data of fabric properties in the examples and comparative examples. ; Note: Scanning electron microscopy (SEM) was used to observe the cross-section of the fabric, and the hollowness of the core layer was determined by image analysis. The 100,000-cycle bending test method was performed according to ISO 24266, with a bending angle of 90° and a frequency of 1Hz. After the test, the tensile strength and tear strength were determined according to GB / T3923.1 and GB / T 3917.2, respectively, and the attenuation rate was calculated.

[0040] Based on Table 1, the following conclusions can be drawn: 1. Significant weight reduction effect. The apparent density of Examples 1-3 is 0.237-0.260 g / cm³. 3 Compared with Comparative Example 1 (0.301 g / cm³), 3The density is reduced by 13.6-21.3%, which is more than 50% lower than that of traditional microfiber leather (density ≥0.55g / cm³). The core hollowness of Examples 1-3 is 41.5-43.8%, close to the 45% hollowness of the raw material, indicating that this process effectively protects the hollow structure of the core. Comparative Example 1, due to full-thickness high-density needle punching, has a hollowness of only 18.6%, and the core is crushed; Comparative Example 2, although reducing the needle punching density, increases the hollowness to 22.3%, but the tensile strength drops to 5.2 / 4.7MPa, which cannot meet the application requirements; Comparative Example 3, although needle punched twice, does not control the depth, and the hollowness is only 25.7%, and the strength is lower than that of the examples. Therefore, the gradient needle punching process of the present invention effectively preserves the hollow structure of the core while reducing the material density.

[0041] 2. Excellent mechanical properties. The longitudinal tensile strength of Examples 1-3 is 8.9-10.8 MPa, the transverse tensile strength is 8.3-10.1 MPa, and the tear strength is 112-142 N, all of which are superior to Comparative Example 1 (strength 8.5 MPa, tear strength 112 N) and Comparative Example 3 (strength 9.1 MPa, tear strength 124 N). Comparative Example 2, due to insufficient needle punching density, has a tensile strength of only 5.2 MPa and a tear strength ≤76 N, which cannot meet the application requirements. Therefore, this process achieves lightweighting while improving mechanical properties. The high-density rigid network formed by the surface densification step significantly enhances the tensile and tear strength of the material.

[0042] 3. Significantly improved resilience. The compression resilience of Examples 1-3 is 77.2-80.1%, and the vertical resilience is 67.5-70.2%, significantly higher than Comparative Example 1 (58.3% and 48.6%) and Comparative Example 3 (65.8% and 55.4%). Example 3, due to its lower basis weight, has the highest resilience (80.1%), making it suitable for clothing padding requiring high elasticity. The retention of the hollow core structure is key to improving resilience; this process increases the compression resilience to over 75%, meeting the resilience requirements of high-end footwear materials.

[0043] 4. Significantly improved durability. After 100,000 bending cycles, the strength attenuation rate of Examples 1-3 was 17.2-19.8%, and the tear attenuation rate was 20.8-23.5%, far lower than that of Comparative Example 1 (42.3% and 48.6%) and Comparative Example 3 (32.5% and 38.9%). Although the attenuation rate of Comparative Example 2 was slightly lower than that of Comparative Example 1, its initial strength was too low to be practically useful. In the gradient structure, the rigid surface network bears the main stress, while the loose core structure buffers stress transmission, thus significantly extending fatigue life.

[0044] In summary, this invention utilizes a three-step method of "pre-needling - main needle-surface densification" combined with controlled needle-puncture depth to construct a gradient structure of "dense surface - loose core." This structure outperforms the comparative example in terms of density (lightweight), core hollowness, mechanical properties, resilience, and durability. It solves the technical challenge of achieving both "lightweight, high strength, and high resilience" simultaneously in traditional needle-puncturing processes, providing a high-performance, lightweight advanced material solution for footwear, apparel, automotive interiors, and other fields.

[0045] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A hollow three-dimensional gradient needle punching process, characterized in that: Includes the following steps: Pre-needling step: The fiber web is pre-needled using a sparsely arranged needle plate and fine needles. The fiber web consists of an upper surface layer, a core layer and a lower surface layer. The first needle punch pierces the upper and lower surface layers of the fiber web, so that the fiber web is initially entangled and shaped, while retaining the hollow structure of the core layer fibers. Main needle punching step: Using a densely packed needle plate, the 2nd to 6th needle punching machines respectively punch the upper and lower surfaces of the fiber web. The needle punching depth is controlled within the range of 1 / 3 to 2 / 3 of the fiber web thickness, so that the needle punching only penetrates the core layer and the corresponding surface layer on one side, but does not penetrate the surface layer on the other side, forming a gradient structure with a preliminary dense surface layer and a loose core layer. Surface densification step: Using a densely packed needle plate and coarse needles, the 7th to 9th needle punching machines only densify the upper and lower surface layers of the fiber web without piercing the core layer. The needle punching depth is controlled at 1 / 3 of the fiber web thickness, so that the surface fibers form a rigid support layer with high-density entanglement, while the hollow structure of the core fibers is preserved.

2. The hollow three-dimensional gradient needle punching process according to claim 1, characterized in that: In the pre-acupuncture step, a sparsely spaced needle plate with a needle spacing of 4-6 mm is used, along with 42-44 gauge fine needles. The acupuncture density is 80-120 needles / cm², the vehicle speed is 8-10 m / min, and the acupuncture frequency is 800-1000 times / min.

3. The hollow three-dimensional gradient needle punching process according to claim 2, characterized in that: In the main acupuncture step, a densely packed needle plate with a needle spacing of 2-3 mm is used, the acupuncture density is 180-220 needles / cm², the vehicle speed is 6-8 m / min, the acupuncture frequency is 1350-1750 times / min, and the needle entry angle is 90±2°.

4. The hollow three-dimensional gradient needle punching process according to claim 3, characterized in that: In the surface densification step, a densely packed needle plate with a needle spacing of 2-3mm is used, along with a coarse needle of size 38-40. The needle density is 300-400 needles / cm², the vehicle speed is 4-6m / min, and the needle frequency is 1200-1500 times / min.

5. The hollow three-dimensional gradient needle punching process according to any one of claims 1-4, characterized in that: After the pre-needling step, the hollow rate of the core layer remains above 70%; after the main needle-punching step and the surface densification step, the hollow rate of the fiber web core layer remains between 30% and 50%.

6. The hollow three-dimensional gradient needle punching process according to claim 5, characterized in that: In the main needle puncture step, by alternating between upper and lower needle punctures, the upper needle puncture only penetrates the upper surface and core layers, while the lower needle puncture only penetrates the lower surface and core layers, forming a symmetrical gradient structure.

7. The hollow three-dimensional gradient needle punching process according to claim 1, characterized in that: The fiber web is made of island-island fibers, which use COPET as the sea component and PA as the island component, with the addition of color masterbatch, and are made into ultrafine island-island fibers through composite spinning technology.

8. The hollow three-dimensional gradient needle punching process according to claim 1, characterized in that: The procedure preceding the pre-acupuncture step also includes: Opening process: The fiber raw material is processed through 3-4 opening processes to control the opening degree ≥95% and the fiber clumping rate ≤0.5%; Combing and web laying steps: The opened fibers are combed into thin webs with a single weight of 50-60 g / m², and then laid in 8-10 layers, with the total weight controlled at 400-500 g / m². After laying, the webs are pre-pressed with light pressure rollers at a pressure of 0.5-1 MPa.

9. The hollow three-dimensional gradient needle punching process according to claim 8, characterized in that: During the web laying process, the orientation ratio of longitudinal fibers to transverse fibers is 1:(1.1-1.2), and the weight uniformity is controlled within ±(5-10)%.