A porous hollow polyester fiber and a method for preparing the same

Abstract

The application provides a porous hollow polyester fiber and a preparation method thereof, and the preparation method comprises the following steps: raw materials including a PET matrix, a PS pore-forming agent, a functional filler and a solubilizer are used, the solubilizer and the PS pore-forming agent are uniformly mixed, the whole raw materials are melt-blended to obtain a melt, the melt is filtered and extruded to obtain a nascent fiber, the nascent fiber after cooling is stretched to obtain a semi-finished fiber, and the semi-finished fiber is subjected to supercritical treatment to obtain a finished fiber. Through the addition of the functional filler, the application can realize multiple functions such as antibacterial, flame-retardant, far-infrared and negative ion, the porous hollow structure can improve the specific surface area of the fiber, and the functional filler can better play a role. In addition, the polyester fiber has shape long and narrow directional pores, can form a honeycomb structure, and thus has better effects of moisture absorption, moisture conduction, sweat discharge and quick drying.

Description

Technical Field

[0001] This invention relates to the technical field of man-made fiber manufacturing, and more specifically, to a porous hollow polyester fiber and its preparation method. Background Technology

[0002] Polyester is a fiber made from purified terephthalic acid (PTA) or dimethyl terephthalate (DMT) and ethylene glycol (MEG) through esterification or transesterification and then polycondensation. PET is then spun and post-processed to produce fibers.

[0003] Porous hollow polyester fibers have become a focus and hot topic in this field in recent years. The core advantages of porous hollow polyester fibers are their excellent bulkiness and warmth retention. This is because the hollow structure (i.e., "pores") inside the fiber can hold a large amount of air, and air is a poor conductor of heat. This makes the bulkiness and warmth retention of porous hollow fibers far exceed those of solid fibers of the same specifications. Secondly, porous hollow polyester fibers also have enhanced moisture absorption. This is because the micro-slits and pores in the porous hollow fibers can quickly absorb sweat, keeping the skin dry. Finally, for fibers loaded with functional components such as antibacterial agents, anti-mite agents, far-infrared heating materials, and negative ion materials, the porous hollow structure has a larger specific surface area, allowing for a more uniform loading of more functional components to improve the corresponding functional effects of the fiber.

[0004] In existing technologies, methods to achieve a porous, hollow fiber structure at the microscopic level generally include chemical or physical pore formation and special treatment processes during electrospinning. Although the pores of existing porous hollow PET fibers have a moisture-absorbing effect, their moisture-wicking, moisture-dissipating, and perspiration-wicking properties still have room for improvement. Therefore, this invention aims to provide a porous hollow PET fiber with elongated pore shapes and directional channels, which can form a honeycomb structure, enhancing its capillary effect and thus significantly improving the fiber's axial moisture-wicking properties, allowing sweat to be quickly conducted along the fiber and improving moisture dissipation and perspiration wicking. Summary of the Invention

[0005] One of the problems solved by this invention is how to provide a porous hollow polyester fiber with good moisture absorption, moisture wicking, perspiration wicking and quick-drying effects.

[0006] To solve at least one of the above problems, the present invention provides a method for preparing porous hollow polyester fibers, the method comprising: S100 uses raw materials including PET matrix, PS pore-forming agent, functional filler and solubilizer. First, the solubilizer and PS pore-forming agent are mixed evenly. Then, all raw materials are melt-blended to obtain a melt. After the melt is filtered, it is extruded and spun to obtain nascent fibers. S200: Stretch the cooled nascent fibers to obtain semi-finished fibers; S300: Supercritical treatment is performed on the semi-finished fiber to obtain the finished fiber; The ratio of PET matrix to PS pore maker to functional filler to solubilizer is 100:(2-6):(0.5-10):(0.05-1). The supercritical treatment medium includes THF and acetone in a mass ratio of (20-40):100. The temperature conditions for supercritical treatment are 240℃ to 260℃, the pressure conditions are 5MPa to 6MPa, and the supercritical treatment time is within 4 minutes of heat and pressure treatment.

[0007] In the above technical solution, the molecular weight of the PS porogen is from 50,000 g / mol to 80,000 g / mol, the molecular weight of the PET matrix is ​​from 25,000 g / mol to 30,000 g / mol, and the ratio of the molecular weight of the PS porogen to the molecular weight of the PET matrix is ​​from 1.65 to 1.85; and / or the functional filler is at least one or a combination of antibacterial functional filler, mite-removing functional filler, far-infrared functional filler, flame-retardant functional filler, and negative ion functional filler; and / or the solubilizer is a styrene-maleic anhydride random copolymer.

[0008] The preparation method in the above technical solution further includes: drying the raw materials separately before melt blending; and / or using a twin-screw extruder for melt blending, wherein the temperature of the feeding section of the twin-screw extruder is 270°C to 275°C, the temperature of the compression section is 275°C to 280°C, and the temperature of the metering section is 280°C to 285°C.

[0009] In the above technical solution, the PET matrix includes: virgin PET chips, which account for 60% to 80% of the total mass of the PET matrix; and recycled PET bottle flakes, which account for 20% to 40% of the total mass of the PET matrix; wherein the recycled PET bottle flakes undergo sorting, crushing, descaling, flotation, washing, drying and solid-state polycondensation treatment.

[0010] In the above technical solution, after S300, the preparation method further includes: S400 involves spraying oil onto the finished fibers, applying tension and hot crimping, high-temperature relaxation drying, and cutting and packaging.

[0011] In the above technical solution, the temperature conditions for supercritical treatment are 240℃ to 245℃, and the pressure conditions do not exceed 5.5MPa; the supercritical treatment time is 2 min to 2.5 min of heat and pressure holding treatment.

[0012] In the above technical solution, the stretching is carried out using three-stage hot stretching; wherein, the temperature of the three-stage stretching is lower than that of the first-stage stretching, and the temperature of the first-stage stretching is lower than that of the second-stage stretching.

[0013] In the above technical solution, the ratio of the third-stage stretching is less than that of the first-stage stretching, and the ratio of the first-stage stretching is less than that of the second-stage stretching.

[0014] In the above technical solution, the temperature for the first stage of stretching is 105℃ to 110℃, and the stretching ratio is 1 to 1.2 times; the temperature for the second stage of stretching is 120℃ to 145℃, and the stretching ratio is 2.5 to 3 times; the temperature for the third stage of stretching is 75℃ to 85℃, and the stretching ratio is 0.2 to 0.4 times.

[0015] In the above technical solution, the temperature for secondary stretching is 130℃ to 135℃, and the stretching ratio is 2.5 to 2.7 times.

[0016] The present invention also provides a porous hollow polyester fiber, which is obtained by the preparation method of any of the above technical solutions.

[0017] Beneficial effects This invention provides a method for preparing porous hollow polyester fibers. The method first uses raw materials including a PET matrix, a PS porogen, functional fillers, and a solubilizer. These are melt-blended to obtain a melt, which is then filtered and extruded to obtain nascent fibers. The cooled nascent fibers are then stretched to obtain semi-finished fibers. Finally, the semi-finished fibers are subjected to supercritical treatment to obtain finished fibers. In the raw materials used in this invention, the PS porogen and the PET matrix are thermodynamically incompatible. During the melt blending process, a small amount of the PS porogen is dispersed as "island"-shaped particles within the "sea"-like matrix of PET. Through hot stretching, the granular PS porogen is elongated into strips at a stretching temperature exceeding its softening temperature. The solubilizer is used to improve the interfacial bonding between the PS porogen and the PET matrix, promote stress transfer between the PET matrix phase and the PS porogen phase, avoid or reduce potential breakage at the interface, and ensure that the fiber material has acceptable tensile strength and elongation at break. Finally, the PS pore-forming agent is extracted using a supercritical fluid extraction process with a mixture of THF and acetone to obtain porous hollow polyester fibers with elongated, oriented channels. The porous hollow polyester fibers of this invention can form a honeycomb structure, exhibiting a stronger capillary effect, thereby enhancing the fiber's axial moisture-wicking properties. This allows sweat to be quickly conducted along the fiber, resulting in excellent moisture absorption, wicking, dissipation, perspiration wicking, and quick-drying effects. Detailed Implementation

[0018] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, a detailed description of specific embodiments of the present invention will be provided below.

[0019] Unless otherwise specified, all reagents and raw materials used in this invention are commercially available. Experimental methods in the following examples that do not specify particular conditions should be performed according to conventional methods and conditions, or as selected in the product instructions.

[0020] This invention aims to provide a method for preparing porous hollow polyester fibers. Polyester fibers possess characteristics such as high strength, wrinkle resistance, shape retention, chemical corrosion resistance, and easy washing and quick drying, thus finding wide application in clothing and home furnishings. In particular, porous hollow polyester fibers, with their excellent warmth retention and breathability, have become a focus and hot topic of research in this field in recent years.

[0021] In existing technologies, porous hollow polyester fibers can generally be obtained through chemical or physical pore initiation, special processing techniques during electrospinning, and pore initiation using shaped spinnerets. Chemical or physical methods refer to creating uniform pores within the matrix fiber by causing a chemical or physical change in the pore-forming agent. For example, Chinese patent application CN121228393A uses sodium bicarbonate as a pore-forming agent, utilizing its thermal decomposition properties to prepare porous hollow polyester fibers. Another example is Chinese patent application CN121228395A, which uses water-soluble polyester as a pore-forming agent, utilizing its water solubility to prepare porous hollow polyester fibers through post-spinning washing.

[0022] The existing technologies described above are characterized by pore-forming agent particles that are typically spherical or irregularly shaped powders. After dissolving or decomposing, they leave behind isolated, nearly spherical cavities. These pores are isotropic, and although they can increase the specific surface area and hygroscopicity of fibers, they are not as efficient as elongated channels in terms of directional moisture conduction.

[0023] In addition, existing technologies also include methods for preparing porous fibers through special processing techniques during electrospinning, such as Chinese patent CN111575917B. However, electrospinning is not suitable for manufacturing textile fibers.

[0024] Finally, existing technologies also employ the technique of creating orifices using irregularly shaped spinnerets. A spinneret is a precision metal plate (commonly made of stainless steel, tantalum, or even platinum) with thousands or even hundreds of thousands of micropores. During spinning, the fluidized polymer (such as polyester melt) is extruded through these micropores under high pressure. The shape of these micropores determines the cross-sectional shape of the liquid stream extruded from the pores. After cooling and solidification, the liquid stream becomes fibers with specific cross-sectional shapes. By changing the geometry of the micropores on the spinneret (e.g., changing from traditional circles to trefoil, cross, C-shape, or honeycomb structures of stacked hexagons), various unique hollow structures can be given to the fibers. While irregularly shaped spinnerets create one-dimensional, regular, large-sized channel structures along the longitudinal direction, their disadvantages include the inability to form dense, small-sized pores, numerous stress concentration points, thin walls, and sharp edges, leading to decreased fiber strength. Furthermore, the melt will experience an "orifice expansion effect" at the exit of the irregular spinneret, which will lead to structural adhesion or collapse, making the fiber forming process difficult.

[0025] Considering the above reasons, in order to obtain a porous hollow PET fiber with narrow and elongated channels, small pore size, and high mechanical strength, this invention uses PS (polystyrene) as a pore-forming agent and uses supercritical extraction to form pores, thereby obtaining a porous hollow PET fiber with a honeycomb structure.

[0026] The preparation method of the present invention specifically includes: S100 uses raw materials including PET matrix, PS pore-forming agent, functional filler and solubilizer. First, the solubilizer and PS pore-forming agent are mixed evenly, and then all raw materials are melted and blended to obtain a melt. After the melt is filtered, it is extruded and spun to obtain nascent fibers. S200: Stretch the cooled nascent fibers to obtain semi-finished fibers; S300: Supercritical treatment is performed on the semi-finished fibers to obtain the finished fibers.

[0027] PET, or polyethylene terephthalate, is a crystalline polymer formed by the condensation polymerization of terephthalic acid and ethylene glycol, and belongs to the category of thermoplastic polymers. PS, or polystyrene, is a polymer synthesized from styrene monomers through a free radical addition polymerization reaction.

[0028] In this invention, PS is used as a pore-forming agent. It is a thermodynamically incompatible system with PET. When a large amount of PET and a small amount of PS are melt-blended, the smaller amount of PS will be dispersed in the "sea" matrix of PET in the form of "island" shaped particles.

[0029] It should be noted that when the PS content is too high or the size is too large, the boundary between the two phases becomes more distinct, and the interfacial bonding force is significantly reduced, which can lead to phase separation and breakage problems in subsequent hot stretching processes. Therefore, controlling the amount and size of PS is essential.

[0030] In this invention, the mass ratio of PET matrix:PS porogen:functional filler:solvent is 100:(2-6):(0.5-10):(0.05-1). Using a smaller amount of PS porogen ensures that the PET fibers have reasonable tensile strength and elongation at break. Furthermore, if the PS particles are too large, large and irregular voids, or even penetrating defects, will be generated after supercritical extraction, affecting the mechanical properties of the fibers. Therefore, this invention preferably uses commercially available PS produced by emulsion polymerization with a particle size between 50 nm and 150 nm as the porogen.

[0031] It is understandable that the role of functional fillers is to endow fibers with corresponding functional properties. For example, functional fillers can be at least one or a combination of antibacterial, anti-mite, far-infrared, flame-retardant, and negative ion functional fillers. For example, inorganic non-metallic oxide nanoparticles or metal particles can be selected as functional fillers, which has the advantage of improving the mechanical strength of PET fibers while imparting antibacterial, anti-mite, and flame-retardant functions. For example, nano-zinc oxide particles, nano-titanium dioxide, nano-copper oxide, nano-silver, and nano-strontium can be used as functional fillers.

[0032] It is understood that, based on functional fillers, common fillers such as silica, alumina, and zirconium oxide can also be added to the raw materials of this invention. Furthermore, the range of functional fillers used is wide, and those skilled in the art can adjust the type and amount of functional fillers according to actual needs. In this embodiment, titanium dioxide is used as an antibacterial functional filler for illustrative purposes. Those skilled in the art can foresee that other functional fillers can be used to obtain other corresponding functional effects.

[0033] This invention uses a styrene-maleic anhydride random copolymer as a solubilizer. Styrene-maleic anhydride copolymer (SMA) is a copolymer obtained by free radical polymerization of styrene and maleic anhydride. The maleic anhydride monomer is randomly attached to the polystyrene backbone, and its molecular structure contains both styrene segments (non-polar, compatible with PS) and maleic anhydride units (polar, containing active anhydride groups). This amphiphilic structure allows it to act as a compatibilizer at the PET / PS interface. In uncompatibilized PET / PS blends, PS forms large, unevenly distributed droplets during melt blending. In PET / PS blends with added compatibilizers, the compatibilizer molecules migrate to the interface between the PET and PS phases, acting as a bridge between the two phases, significantly reducing interfacial tension and making it easier for PS droplets to be sheared and broken into fine particles. After adding the compatibilizer, the particle size of the dispersed PS phase decreases significantly, and the phase interface gradually becomes blurred. Furthermore, during the melt blending process, the broken PS droplets tend to collide and merge again. The compatibilizer forms a "protective film" on the surface of the PS droplets, effectively preventing merging and thus stabilizing the fine "sea-island" structure.

[0034] It should also be noted that, since the nascent PET fibers containing PS need to be thermally stretched to change the shape or form of the PS pore-forming agent dispersed in the PET, making it elongated, the tensile stability of the PET / PS blended fibers needs to be considered.

[0035] The key to improving tensile stability lies in matching the viscosity ratios of PS and PET to avoid PS phase breakage or interfacial debonding. When the viscosity ratio of the PS dispersed phase to the PET matrix phase is too small, PS is easily over-sheared and broken during melt extrusion; when the viscosity ratio is too large, PS is prone to interfacial stress concentration during stretching, leading to debonding.

[0036] Since the viscosity of both PS and PET is positively correlated with their molecular weight, this invention requires selecting PS and PET raw materials with compatible viscosity and molecular weight. Preferably, the molecular weight of the PS porogen is 50,000 g / mol to 80,000 g / mol, the molecular weight of the PET matrix is ​​25,000 g / mol to 30,000 g / mol, and the ratio of the molecular weight of the PS porogen to the molecular weight of the PET matrix is ​​1.65 to 1.85. More preferably, the molecular weight of the PS porogen is 50,000 g / mol to 55,000 g / mol, and the molecular weight of the PET matrix is ​​30,000 g / mol. Matching the molecular weights of PET and PS ensures that, during stretching, the PS phase can be uniformly elongated into microfibers without being sheared or broken.

[0037] After introducing the raw materials used in this invention, the process method employed in this invention will be described below. It can be understood that the inventive point of this invention lies in the supercritical extraction of stretched PET fibers containing PS porogen to obtain PET fibers with multiple oriented, elongated pores inside.

[0038] Therefore, one of the research focuses of the process method of this invention is the study of the stretching process, and another focus is the investigation of parameters such as the medium, temperature, pressure, and time of the supercritical treatment. This allows for the control of the pore shape, size, and porosity of PET fibers, balancing the fiber's moisture absorption and conductivity with its tensile strength and elongation at break, while avoiding defects such as phase separation and microcracks.

[0039] The primary purpose of the stretching process is to orient the PET to improve fiber strength. Simultaneously, the stretching process elongates spherical or granular PS islands into strips or even microfibers. The stretching ratio is a key parameter determining the shape of the PS islands and subsequent pores, as it determines the aspect ratio of the pores after extraction. An excessively high stretching ratio may lead to excessive deformation or even breakage of the PS phase into smaller particles, or microcracks at the PET / PS interface, affecting subsequent uniform extraction. An excessively low stretching ratio will fail to obtain the desired PET fibers, and the PS islands will not form elongated pores. Therefore, this invention selects a total stretching ratio of 3.5 to 4.5 times, and employs multi-stage stretching based on this ratio, allowing for better control over the shape and size of the PS islands and pores.

[0040] Specifically, the present invention employs a three-stage thermal stretching method; wherein the temperature of the three-stage stretching is lower than that of the first-stage stretching, and the temperature of the first-stage stretching is lower than that of the second-stage stretching. The stretching ratio of the three-stage stretching is lower than that of the first-stage stretching, and the stretching ratio of the first-stage stretching is lower than that of the second-stage stretching.

[0041] More preferably, the temperature for the first stage of stretching is 105°C to 110°C, and the stretching ratio is 1 to 1.2 times; the temperature for the second stage of stretching is 120°C to 145°C, and the stretching ratio is 2.5 to 3 times; and the temperature for the third stage of stretching is 75°C to 85°C, and the stretching ratio is 0.2 to 0.4 times.

[0042] More preferably, the temperature for the first stage of stretching is 105°C to 110°C, and the stretching ratio is 1 to 1.2 times; the temperature for the second stage of stretching is 130°C to 135°C, and the stretching ratio is 2.5 to 2.7 times; and the temperature for the third stage of stretching is 75°C to 85°C, and the stretching ratio is 0.2 to 0.4 times.

[0043] The reason for using the above stretching temperature and stretching ratio is that the purpose of the first-stage stretching is preheating and pre-stretching, so a stretching temperature slightly higher than the PS softening temperature is used to initiate PS deformation. At this temperature, PS begins to soften, and the spherulites begin to elongate slightly along the axial direction. The stretching ratio should not be too fast at this stage, and sufficient response time needs to be given to PS to prevent excessive instantaneous interfacial stress. The second-stage stretching is the main stretching, during which PS achieves the transformation from ellipsoids to slender microfibers. This invention uses a relatively high second-stage stretching temperature and a relatively low second-stage stretching ratio. At this temperature, the PET matrix undergoes significant hardening and orientation, effectively transferring stress to the PS phase, elongating it into a slender shape with a high aspect ratio, and also promoting the growth of PET lamellar crystals. In this temperature range, PS is in a highly elastic state, which is easily elongated. The tertiary stretching function used in this invention is for low-temperature shaping and relaxation. At a temperature of 75°C to 85°C, the PET fibers fix their orientation structure and release internal stress to avoid channel collapse due to stress retraction during subsequent extraction. Considering the need to avoid distortion or breakage of PS during low-temperature stretching, which could disrupt the connectivity of the channels, the stretching ratio of the third-stage stretching is relatively small.

[0044] Regarding the selection of the extraction medium, a good solvent for PS needs to be chosen. Aromatic hydrocarbons, halogenated hydrocarbons, ketones, and ethers all provide good solvents for PS (e.g., benzene, toluene, xylene, chloroform, dichloromethane, chlorobenzene, dichlorobenzene, tetrahydrofuran, ethyl acetate, dimethylformamide, nitrobenzene, etc.). Considering the low efficiency and incomplete extraction issues of commonly used solvent immersion extraction methods in existing technologies, this invention employs supercritical extraction. Among the aforementioned good solvents for PS, this invention selects THF (tetrahydrofuran) as the supercritical medium. THF is not only a good solvent for PS, but also, since stretched PET has a highly crystalline structure, supercritical THF will not affect the structure and strength of the PET fibers. It should be noted that although THF cannot dissolve PET, the co-solution system of THF and acetone can cause slight swelling of PET. This slight swelling of PET facilitates the diffusion of the supercritical fluid within the PET fibers, resulting in more uniform and thorough PS extraction. However, the supercritical treatment time needs to be carefully monitored to ensure a reasonable level of PS extraction while avoiding any impact on the strength of the PET fibers. Preferably, the medium used in the supercritical treatment of the present invention comprises THF and acetone in a mass ratio of (20-40):100.

[0045] It should also be noted that styrene-maleic anhydride copolymers also have good solubility in THF. Therefore, during the extraction process, the styrene-maleic anhydride copolymer solubilizer can also be carried away by THF. In this case, the present invention chooses to use a smaller amount of solubilizer and PS, first mixing the solubilizer and PS pore-forming agent evenly, and then melting and blending all the raw materials to obtain a melt. This ensures that the solubilizer acts between the two phases and avoids leaving uncontrollable pores of quantity and size in the PET fibers after the solubilizer is extracted.

[0046] When selecting a medium for supercritical fluid extraction, the thermal stability of PET fibers must be considered. Acetone is added in this invention to induce slight swelling of the PET and to lower the temperature and pressure of the supercritical process through a co-solution system. Experimental data shows that PET experiences significant weight loss after heat treatment at 260°C for 10 minutes. Therefore, even for only a few minutes, the extraction temperature should be kept below 250°C as much as possible.

[0047] The supercritical temperature of THF is 268°C and the supercritical pressure is 5.19 MPa, while the supercritical temperature of acetone is 235°C and the supercritical pressure is 4.7 MPa. Acetone can lower the supercritical conditions of the co-solution system, and compared to other media with similarly low supercritical conditions (such as ethanol), acetone does not damage the PET matrix fibers. For the mixed supercritical medium of THF and acetone, preferably, the supercritical treatment temperature is 240°C to 260°C, the pressure is 5 MPa to 6 MPa, and the supercritical treatment time is within 4 minutes of holding at the same temperature and pressure. More preferably, the supercritical treatment temperature is 240°C to 245°C, the pressure does not exceed 5.5 MPa, and the supercritical treatment time is 2 minutes to 2.5 minutes of holding at the same temperature and pressure.

[0048] Finally, it should be noted that, in order to recycle PET, this invention can also use an appropriate amount of recycled PET as raw material. Since PET recycling and processing are relatively mature existing technologies, this invention will briefly describe the recycling and processing process.

[0049] Optionally, the PET matrix of the present invention includes: virgin PET chips, wherein the virgin PET chips account for 60% to 80% of the total mass of the PET matrix; and recycled PET bottle flakes, wherein the recycled PET bottle flakes account for 20% to 40% of the total mass of the PET matrix; wherein the recycled PET bottle flakes are subjected to sorting, crushing, descaling, flotation, washing, drying and solid-state polycondensation treatment.

[0050] The key to reusing recycled PET bottle flakes lies in overcoming the challenges of inconsistent composition, high impurity levels, and decreased viscosity of the recycled materials. Therefore, the main processing techniques include: Sorting and Cleaning: The recycled "bottle bricks" are sorted manually or automatically to separate non-PET bottles and discolored bottles (blue / green bottles are usually handled separately). Then, through processes such as tearing, label removal (removing bottle labels), hot alkaline washing, and flotation separation (removing HDPE / PP from bottle caps), impurities, foreign objects, residual liquids, and adhesives are removed to obtain pure PET bottle flakes.

[0051] Solid-phase polycondensation (SSP): After multi-stage cleaning and drying, the PET flakes enter a solid-phase polycondensation reactor where a polycondensation catalyst is added for solid-phase polycondensation treatment. This is because PET undergoes hydrolysis and thermal degradation during processing and use, leading to molecular chain breakage and a significant reduction in intrinsic viscosity, making it impossible to directly spin high-strength fibers. Through solid-phase polycondensation under high temperature, high vacuum, and nitrogen protection, the residual active end groups in the PET chips undergo a condensation reaction, reconnecting the molecular chains and increasing their viscosity to meet spinning requirements.

[0052] If recycled PET bottle flakes are used, the melt spinning process is basically the same as that of virgin polyester staple fiber, and the melt spinning method is also used. However, because recycled PET bottle flakes have a wide molecular weight distribution and high end carboxyl content, their thermal stability is worse. Therefore, the melting temperature needs to be precisely controlled to prevent further degradation.

[0053] Of course, it is understandable that recycled PET flakes cannot completely remove all impurities and pigments, resulting in a general color difference (yellowish, grayish), poor whiteness and purity. Therefore, recycled PET flakes should be used with caution in high-end, light-colored textiles.

[0054] The above are the main process steps for obtaining the finished fiber. It is understandable that in actual production, after obtaining the finished fiber, it still needs to undergo post-processing such as oiling, crimping, drying, cutting, and packaging. The fibers prepared and tested in the examples below are all finished fibers. The post-processing technology is a relatively mature existing technology; therefore, the post-processing technology will be briefly described below.

[0055] After obtaining the finished fiber, the post-processing required mainly includes: Spray oiling: Spray oiling is performed on the stretched and perforated PET fibers. Silicone oil-based oils can be used and applied evenly through atomizing nozzles. The oiling rate is controlled at 0.25%-0.50% (based on the weight of PET fibers) to give the fibers excellent bundle properties, smoothness and antistatic properties, making them suitable for subsequent textile processing.

[0056] Tension-heat crimping: After oiling, the filament bundles are kept under a certain tension and then crimped by a mechanical crimping machine using heat and pressure to form two-dimensional wavy crimps, increasing the bulkiness of the fibers.

[0057] High-temperature relaxation drying: The crimped fiber bundles are sent into a hot air circulating drying oven and subjected to flow drying (i.e., free shrinkage in a relaxed state) at a temperature of 170℃ to 180℃ for about 30 minutes. This step can eliminate the internal stress of PET fibers, allowing the pore structure to finally relax and stabilize, while fixing the crimped shape, and giving the PET fibers a stable boiling water shrinkage rate.

[0058] Cutting and Packaging: Quantitative cutting involves feeding the dried continuous fiber bundle into a cutting machine and cutting it according to product specifications (e.g., 38mm, 51mm, 76mm). The cut fibers are then pneumatically conveyed into an automatic packaging machine and packaged into neat fiber bales according to a preset weight (e.g., 200kg / bale). Sampling inspections are conducted on the finished fiber bales; once all bales pass inspection, they are stored according to specifications. Example 1

[0059] This embodiment prepared a series of PET fibers, and the preparation process is as follows.

[0060] S1. Raw material pretreatment and batching The PET matrix, PS porogen, titanium dioxide functional filler, and styrene-maleic anhydride random copolymer solubilizer were weighed according to a mass ratio of 100:3:2:0.5. The PET matrix had a molecular weight of 30,000 g / mol, the PS porogen had a molecular weight of 50,000 g / mol, the PS porogen had a particle size D90 of 150 nm, and the titanium dioxide had a particle size D50 of 30 nm. The PET matrix was then transferred to a vacuum drying chamber and continuously dried for 8 to 10 hours at a temperature of 120°C to 140°C and a vacuum level below -0.095 MPa, reducing the moisture content of the mixed PET matrix to below 30 ppm. The titanium dioxide functional filler and the styrene-maleic anhydride random copolymer solubilizer were then precisely dried separately at 80-100°C to prevent powder agglomeration and the influence of moisture.

[0061] S2, blending and melting The dried and qualified PS pore-forming agent and styrene-maleic anhydride random copolymer solubilizer are first mixed evenly, and the mixture and PET matrix are respectively conveyed to the main feed hopper through a feeding system. At the same time, the dried titanium dioxide functional filler is injected into the conveying pipeline of the main feed hopper at a constant rate through a high-precision variable frequency feeding device, so as to achieve online precise blending of all raw materials.

[0062] The blended raw materials are fed into two parallel co-rotating twin-screw extruders, A and B. Under the conveying, shearing, and mixing action of the screws, the materials are uniformly heated to 275-285℃ and completely melted, forming a homogeneous slurry. Specifically, the feeding section temperature of the co-rotating twin-screw extruder is 270℃ to 275℃, the compression section temperature is 275℃ to 280℃, and the metering section temperature is 280℃ to 285℃.

[0063] S3, spinning and forming The homogenized melt is pumped through a precision metering pump and then enters a high-mesh, multi-layer filter to remove impurities. The clean melt is then distributed to a spinneret with 0.25mm orifices. After being extruded from the micropores of the spinneret, the melt initially forms nascent fibers under the combined effect of surface tension and rapid cooling. The extruded filaments are rapidly cooled in a side-blowing cooling system and then wound and bundled, with 100 filaments per bundle.

[0064] S4, Stretching The stretching workshop temperature was maintained at approximately 22℃ and the relative humidity at approximately 55%. A multi-roll stretching unit was used, and the cooled nascent fibers underwent three stages of stretching sequentially. The nascent fibers first remained on the preheating rollers for 2 to 3 seconds to ensure uniform temperature penetration into the fiber interior. An infrared thermometer was used to monitor the surface temperature of each roller in real time, with fluctuations controlled within ±1℃. The temperature for the first stage of stretching was 105℃, with a stretch ratio of 1.1. The temperature and stretch ratio for the second stage of stretching are shown in Table 1. The temperature for the third stage of stretching was 80℃, with a stretch ratio of 0.4. The stretched fibers were then dried in a vacuum drying oven.

[0065] Performance testing According to GB / T14344, the breaking strength and elongation at break of a series of PET fiber samples in this embodiment were tested. It should be noted that the temperature for the first-stage tensile testing was selected based on the softening temperature of PS, the temperature for the third-stage tensile testing needed to be below 100℃, and the temperature and ratio for the second-stage tensile testing had the most decisive comprehensive impact on the distribution morphology of the PS pore-forming agent and the strength of the PET fibers. This embodiment focuses on the effect of changes in second-stage tensile conditions on the strength of PET fibers; specific results are shown in Table 1.

[0066] The test results in Table 1 show that, regardless of the stretch ratio, it is difficult to obtain PET fibers with ideal strength at stretching temperatures below 130℃. This may be due to the PS pore-forming agent not being effectively stretched and deformed. However, overall, at stretching temperatures below 130℃, high stretch ratios help improve the strength of PET fibers. In the stretching temperature range of 130℃ to 135℃, higher strength PET fibers can be obtained, with medium-low stretch ratios of 2.5 or 2.7 yielding higher strength fibers. In the stretching temperature range of 140℃ to 145℃, low stretch ratios can actually produce higher strength PET fibers.

[0067] Table 1 Example 2

[0068] In this embodiment, the PET fiber sample 8 obtained in Example 1 is subjected to supercritical treatment, and the treatment method is as follows.

[0069] S1. Ensure that the stretched PET fibers are loose and place them into the sample holder inside the supercritical extraction vessel, ensuring that there are gaps between the fibers to avoid tight stacking. The filling amount should be less than 1 / 3 of the vessel volume.

[0070] Close the reactor, tighten the sealing bolts, introduce a small amount of low-pressure nitrogen into the reactor for purging, and perform an airtightness check to ensure that the entire high-pressure system is leak-free.

[0071] S2. Inject the pre-mixed extraction medium into the reactor using a high-pressure metering pump. Start the heating system, set the target temperature, and increase the temperature at a rate of 5°C / min to 10°C / min. First, heat to near the target temperature, then slowly increase the pressure, maintaining the pressure inside the reactor within the target range by controlling the medium flow rate and monitoring the pressure. Start timing when both temperature and pressure conditions are met. Refer to Table 2 for the extraction medium ratio and time. The extraction temperature is 250°C, and the extraction pressure is 6 MPa.

[0072] S3. After the holding and pressure maintenance time is reached, immediately open the pressure relief valve to quickly depressurize the system to atmospheric pressure. Rapid pressure release helps the extraction medium carry the dissolved PS away from the fiber interior, while also preventing the dissolved PS from redepositing within the pores. After depressurization, allow the reactor to cool naturally to a safe temperature. Remove the fiber, wash and dry it to obtain a porous PET fiber sample.

[0073] Table 2 Performance testing After obtaining porous PET fiber samples 1 to 12, the removal rate of PS (and a small amount of solubilizer) in each sample was verified by thermogravimetric analysis (TGA). Among them, the DTG curves of samples 4, 5, 6, 10, 11, and 12 showed a shoulder peak next to the main peak. This test result indicates that if the amount of THF added in the medium is too high or pure THF is used, effective PS extraction cannot be achieved at a relatively low temperature of 250℃, resulting in residual PS in the PET fibers. Although no shoulder peak appeared next to the main peak of the DTG curves of samples 1 and 7, their TG curves showed an initial decomposition temperature slightly lower than that of samples 2, 3, 8, and 9, indicating that a small amount of residual PS in samples 1 and 7 underwent premature thermal decomposition. Example 3

[0074] In this embodiment, the PET fiber sample 8 obtained in Example 1 was subjected to supercritical treatment, and the treatment method was the same as in Example 2. The difference was that the extraction medium used in this embodiment was THF and acetone with a mass ratio of 30:100, and the treatment time was 2 min or 2.5 min. The temperature and time conditions for supercritical treatment are shown in Table 3, and the extraction pressure was 5.5 MPa.

[0075] Table 3 Performance testing The purpose of this embodiment is to test the lower limit of the supercritical temperature for PS removal using a 30:100 mass ratio of THF and acetone as the extraction medium at a relatively low pressure (5.5 MPa). After obtaining porous PET fiber samples 13 to 20, the removal rate of PS (and a small amount of solubilizer) in each sample was verified by thermogravimetric analysis (TGA). Both DTG and TG curves showed that almost no PS residue was found in samples 15, 16, 19, and 20. This indicates that, with the current medium ratio, a supercritical temperature below 240°C is insufficient for effective PS residue removal.

[0076] According to GB / T 14344, the breaking strength and elongation at break of samples 13 to 20 in this embodiment were tested, and the test results are shown in Table 4. According to GB / T 6503, the moisture regain of samples 13 to 20 was tested and calculated, and the results are shown in Table 4. According to GB / T 21655.1, the wicking height of fabrics woven from samples 13 to 20 after 20 standard washes was tested and calculated, and the results are shown in Table 4. The test results indicate that, overall, the porous structure reduces fiber strength. Furthermore, residual PS further affects fiber strength, which may be because residual PS, being a thermodynamically incompatible system with PET, affects the elongation characteristics of PET.

[0077] Table 4 Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A method for preparing porous hollow polyester fibers, characterized in that, The preparation method includes: S100. Using raw materials including PET matrix, PS pore-forming agent, functional filler and solubilizer, the solubilizer and PS pore-forming agent are first mixed evenly, and then all raw materials are melted and blended to obtain a melt. The melt is filtered and then extruded and spun to obtain nascent fibers. S200: The cooled nascent fibers are stretched to obtain semi-finished fibers; S300. The semi-finished fiber is subjected to supercritical treatment to obtain the finished fiber. Wherein, by mass ratio, PET matrix:PS pore maker:functional filler:solvent = 100:(2-6):(0.5-10):(0.05-1); the media used in the supercritical treatment include THF and acetone with a mass ratio of (20-40):100; the temperature conditions of the supercritical treatment are 240℃ to 260℃, the pressure conditions are 5MPa to 6MPa, and the time of the supercritical treatment is within 4 minutes of heat and pressure holding treatment.

2. The preparation method according to claim 1, characterized in that, The PS porogen has a molecular weight of 50,000 g / mol to 80,000 g / mol, the PET matrix has a molecular weight of 25,000 g / mol to 30,000 g / mol, and the ratio of the molecular weight of the PS porogen to the molecular weight of the PET matrix is ​​1.65 to 1.85; and / or The functional filler is at least one or a combination of antibacterial functional filler, mite-removing functional filler, far-infrared functional filler, flame-retardant functional filler, and negative ion functional filler; and / or The solubilizer is a styrene-maleic anhydride random copolymer.

3. The preparation method according to claim 1, characterized in that, The preparation method further includes: drying the raw materials separately before melt blending; and / or The melt blending is carried out using a twin-screw extruder, wherein the temperature of the feeding section of the twin-screw extruder is 270°C to 275°C, the temperature of the compression section is 275°C to 280°C, and the temperature of the metering section is 280°C to 285°C.

4. The preparation method according to claim 1, characterized in that, The PET matrix includes: Native PET chips, wherein the native PET chips account for 60% to 80% of the total mass of the PET matrix; Recycled PET bottle flakes, wherein the recycled PET bottle flakes account for 20% to 40% of the total mass of the PET matrix; The recycled PET bottle flakes undergo sorting, crushing, label removal, flotation, washing, drying, and solid-phase polycondensation.

5. The preparation method according to claim 1, characterized in that, The stretching process employs a three-stage hot stretching method; wherein the temperature of the three-stage stretching is lower than that of the first-stage stretching, and the temperature of the first-stage stretching is lower than that of the second-stage stretching.

6. The preparation method according to claim 5, characterized in that, The ratio of the third-stage stretching is less than the ratio of the first-stage stretching, and the ratio of the first-stage stretching is less than the ratio of the second-stage stretching.

7. The preparation method according to claim 6, characterized in that, The temperature for the first stage of stretching is 105°C to 110°C, and the stretching ratio is 1 to 1.2 times; the temperature for the second stage of stretching is 120°C to 145°C, and the stretching ratio is 2.5 to 3 times; the temperature for the third stage of stretching is 75°C to 85°C, and the stretching ratio is 0.2 to 0.4 times.

8. The preparation method according to claim 7, characterized in that, The secondary stretching temperature is 130°C to 135°C, and the stretching ratio is 2.5 to 2.7 times.

9. The preparation method according to any one of claims 1 to 8, characterized in that, The temperature conditions for the supercritical treatment are 240℃ to 245℃, and the pressure conditions do not exceed 5.5MPa; the time for the supercritical treatment is 2 min to 2.5 min of heat and pressure holding treatment.

10. A porous hollow polyester fiber, characterized in that, The porous hollow polyester fiber is obtained by the preparation method described in any one of claims 1 to 9.

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