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Polypropylene fiber is a synthetic fiber spun from isotactic polypropylene obtained by propylene polymerization. Its trade name in China is polypropylene, which is a high-strength bundled monofilament fiber.
In the early days, propylene polymerization could only produce silicon-free products with low polymerization degree, which belonged to non-crystalline compounds and had no practical value. In 1954, polypropylene was invented by Dr. Nata, and in 1957, the Italian Montecatini Company began industrial production. In the late 1980s, metallocene complexes were successfully manufactured. In 1995, Exxon announced the naming of it as Achieve, creating a new era of PP raw materials for clothing. Most of the large and medium-sized polypropylene devices introduced in China since the 1980s have the ability to produce fiber-grade polypropylene, which can provide raw materials for polypropylene production.
In industry, polypropylene fibers are commonly produced by spinning processes such as melt spinning, film split spinning, short-range spinning and bulk spinning. Its main products include staple fibers, filaments, spunbond and melt-blown nonwovens, film split fibers, filter materials, etc. Polypropylene fiber is widely used in decoration, industry and reinforced composite materials due to its advantages of light texture, low hygroscopicity, high mechanical strength and strong chemical corrosion resistance. In addition, polypropylene fiber can be modified by chemical and physical methods to give it specific properties, such as thermal stability, light stability, antistatic properties, etc.
In the early days, propylene polymerization could only produce silicon-free products with low polymerization degree, which belonged to non-crystalline compounds and had no practical value. In 1954, polypropylene was invented by Dr. Nata, and industrial production began by Montecatini Company in Italy in 1957. After 1964, polypropylene film split fibers for bundling were developed and made into textile fibers and carpet yarns by fibrillation of film.
In the 1970s, short-range spinning technology and equipment improved the production process of polypropylene fibers. At the same time, expanded continuous filaments began to be used in the carpet industry. After 1980, the development of polypropylene and new technologies for manufacturing polypropylene fibers, especially the invention of metallocene catalysts, significantly improved the quality of polypropylene resins. Due to the improvement of its stereoregularity (isotacticity can reach 99.5%), the intrinsic quality of polypropylene fiber has been greatly improved.
In the mid-1980s, polypropylene ultrafine fibers replaced some cotton fibers and were used for textile fabrics and non-woven fabrics. At the same time, polypropylene fine fibers replaced some cotton fibers and were used for textile fabrics and non-woven fabrics. Concrete reinforcement has made important progress in replacing cotton or glass fibers with polypropylene fibers, and the United States and Western Europe have begun to use them in the construction industry. With the development of one-step BCF spinning machines, air deformation machines and composite spinning machines, as well as the emergence and rapid development of non-woven fabrics, the use of polypropylene fibers in decoration and industrial use has been further broadened. In addition, countries around the world are also quite active in the research and development of polypropylene fibers. The popularization and improvement of differentiated fiber production technology has greatly expanded the application field of polypropylene fibers.
In the late 1980s, metallocene complexes were successfully manufactured. In 1995, Exxon announced the name of it as Achieve, creating a new era of PP raw materials for ready-made clothing. In the late 1980s, Western Europe accounted for about 23% of the world's polypropylene fiber production capacity and output, respectively, and was also the main producer and consumer of polypropylene fiber. Western Europe and North America together account for about 48% of the world's supply, and demand also accounts for nearly half of the world's total. In the Western European market, about 40% of polyolefin fiber products are industrial materials. Since the 1980s, most of the large and medium-sized polypropylene plants introduced by China have the ability to produce fiber-grade polypropylene, which can provide raw materials for polypropylene fiber production.
As of 2022, China's polypropylene fiber production will not be able to meet the needs of different levels and fields. There are too many general and low-end products, but few differentiated and high-end products, and the market potential has not been well developed.
Polypropylene is made by coordination polymerization reaction with propylene as monomer, and its structural formula is:
Polypropylene is composed of macromolecules with carbon atoms as the main chain. According to the different spatial arrangement positions of methyl groups, there are three stereo structures, namely isotactic, syndiotactic and atactic structures. Isotactic polypropylene macromolecules are composed of regular repeating units of the same configuration. The side groups (-CH3) are on the same side of the main chain plane, and each chain link has an asymmetric center in the same stereo position along the molecular chain. This regular structure is easy to crystallize, also known as isotactic polypropylene. Syndiotactic polypropylene is composed of opposite configuration units arranged alternately and regularly, and its side groups (-CH3) are arranged alternately in order up and down on the main chain plane. The molecular chain with this regular stereo structure is also easy to crystallize, also known as syndiotactic polypropylene. The side groups (-CH3) of atactic polypropylene are completely disorderly arranged, and it is an amorphous polymer that is difficult to crystallize, also known as atactic polypropylene.
Analysis of the X-ray diffraction image of isotactic polypropylene shows that its molecular chain is in a three-dimensional helical configuration. This three-dimensional crystallization is not only a regular structure of a single chain, but also has a regular chain stacking at right angles to the chain axis. There are five types of crystal structures of isotactic polypropylene, namely ¦Á, ¦Â, y, 8 and pseudo-hexagonal variants. The most common crystals belong to the single crystal system (a variant). The supramolecular structure of polypropylene can be explained by the "folded chain tassel-shaped microfibril" theory and model. The optimal crystallization temperature of polypropylene is 125~135¡æ. If the temperature is too high, it is not easy to form a crystal nucleus and the crystallization is slow; if the temperature is too low, it is difficult to crystallize due to the difficulty of molecular chain diffusion. The crystallinity of the primary polypropylene fiber is 33%~40%. After post-stretching, the crystallinity rises to 37%~48%. After heat treatment, the crystallinity can reach 65%~75%.
Polypropylene fiber (PP) is made of propylene as raw material. Through directional polymerization, the relative molecular mass, crystallinity (65%) and melting point (158~170¡ãC) are quite high. The isotactic polypropylene fiber that meets the requirements of textile fibers is isotactic polypropylene fiber. The optimal crystallization temperature of polypropylene fiber is 125~135¡æ. If the temperature is too high, it is not easy to form a crystal nucleus and the crystallization is slow; if the temperature is too low, it is difficult to crystallize due to the difficulty of molecular chain diffusion. The crystallinity of the primary polypropylene fiber is 33%~40%. After post-stretching, the crystallinity rises to 37%~48%. After heat treatment, the crystallinity can reach 65%~75%. Its isotactic structure can be expressed as follows:
The molecular structure of polypropylene has three types: isotactic, syndiotactic and atactic. Generally, polypropylene fibers are mostly isotactic polymers. The degree of polymerization of polypropylene staple fibers is generally controlled at 1000~2000, and the degree of polymerization of filaments can be increased to about 5000. The isotacticity of the polymer is generally 85%~97%, and the melting point is 164~170¡ãC. Polypropylene is mostly produced by melt spinning to produce filaments and staple fibers, and the spinning process is similar to that of polyester and nylon. Since the fiber-forming polypropylene has a large relative molecular mass, high melt viscosity, poor fluidity, and is not conducive to spinning, the spinning temperature should be 50~130¡ãC higher than the melting point of polypropylene, that is, the actual melt temperature is 260~300¡ãC. The crystallization rate of polypropylene is relatively fast during the cooling and molding process, so the temperature must be strictly controlled during stretching, and the cooling temperature should be lower than that of polyester and nylon to prevent its excessive crystallization, thereby affecting post-processing and making drafting difficult.
The density of polypropylene fiber is 0.90~0.92g/cm3, which is the lightest among all chemical fibers. Polypropylene fiber has very good strength. The strength of general polypropylene fiber short fiber is 35~53cN/tex. If it is spun into high-strength polypropylene fiber, its strength can reach 75cN/tex. Polypropylene fiber has extremely low hygroscopicity, so the dry and wet strength and elongation at break are almost equal. At an elongation of 3%, the elastic recovery rate of polypropylene fiber can reach 96%~100%. Polypropylene fiber is also very resistant, especially for its long life of repeated bending. It can withstand repeated bending 200,000 times when the breaking angle is 175¡ã.
The biggest advantage of polypropylene fiber is its light texture. It is the lightest variety among common chemical fibers. Its density is 0.9~0.92g/cm3, which is the lightest among all chemical fibers. It is 20% lighter than nylon, 30% lighter than polyester, and 40% lighter than viscose fiber. Therefore, it is very suitable for filling of winter clothing or fabrics of ski suits, mountaineering clothes, etc.
Polypropylene has very low hygroscopicity, almost no hygroscopicity, and its regain under normal atmospheric conditions is close to zero. Therefore, when used for clothing fabrics, it is often blended with fibers with high hygroscopicity. Fine denier polypropylene has a strong wicking effect, and water vapor can be removed through the capillaries in the fiber. After being made into clothing, the clothing is more comfortable, especially ultra-fine polypropylene fiber, which can transfer sweat faster due to the increased surface area, so that the skin remains comfortable. Because the fiber does not absorb moisture and has a small shrinkage rate, polypropylene fabrics are easy to wash and dry quickly.
Polypropylene fiber has extremely low hygroscopicity, so its dry, wet strength and breaking strength are almost equal, which is better than nylon and is particularly suitable for making fishing nets, ropes and filter cloths. The strength of polypropylene fiber increases with decreasing temperature and decreases with increasing temperature, and the degree of decrease exceeds that of nylon. Due to the low melting point of polypropylene fiber, the strength decreases more at high temperatures, and sufficient attention should be paid during dyeing and finishing. Polypropylene fiber has high strength, good elongation at break and elasticity. Polypropylene fiber also has good wear resistance, especially long life of repeated bending resistance, which is better than other synthetic fibers.
Polypropylene fiber is better than other synthetic fibers in terms of acid resistance, cracking and other chemical agents. Polypropylene fiber has good chemical corrosion resistance. Except for concentrated nitric acid and concentrated caustic soda, polypropylene fiber has good resistance to acid and alkali, so it is suitable for use as filter material and packaging material. Polypropylene fiber is slightly less stable to organic solvents.
The disadvantages of polypropylene fiber are: it is difficult to dye, has poor hygroscopicity, and its regain is almost "0" under standard atmospheric conditions; it has poor light resistance, is easy to age, and is not resistant to dry heat. When heated in dry conditions (such as temperatures exceeding 130¡ãC), it will crack due to oxidation, but it has good resistance to wet heat; it will not deform when boiled in boiling water for several hours.
Among synthetic fibers, polypropylene has the worst hygroscopicity and dyeability. Polypropylene has very little hygroscopicity, almost no hygroscopicity, and its moisture regain under normal atmospheric conditions is close to zero. Therefore, when used for clothing fabrics, it is often blended with fibers with high hygroscopicity.
It is very difficult to dye polypropylene macromolecules, and ordinary dyes cannot color them. Dyeing with disperse dyes can only produce very light colors, and the dyeing fastness is very poor. To improve the dyeing performance of polypropylene, graft copolymerization, solution coloring, metal compound modification and other methods can be used.
Polypropylene fiber is a thermoplastic fiber with a low melting point (165~173¡ãC), and the softening point temperature is 10~15¡ãC lower than the melting point, so it has poor heat resistance. During dyeing and finishing and use, attention should be paid to controlling temperature changes to avoid plastic deformation. To improve its stability, a certain amount of antioxidant can be added during spinning.
In the field of polypropylene fiber modification, the main research and development directions include achieving fiber fineness, enhancing its dyeability, and giving it multiple functional properties such as flame retardant, UV resistance, antibacterial and antistatic.
Fibers with a single filament linear density (dtex) less than 1 (dpf < 1) are called fine polypropylene fibers. The fabrics made of it are not only soft and comfortable to the touch, but also give full play to the unique wicking effect after fineness, so that sweat is discharged from the inside to the outside along the longitudinal direction of the fiber. It has the function of moisture conduction and perspiration removal, and is breathable, smooth and non-sticky. In addition, its light weight and other characteristics have excellent wearing properties that other fiber fabrics do not have.
The development of fine-specialized polypropylene fibers focuses on two aspects: one is the production of raw polypropylene, and the other is the research on the correlation between fine-specialized polypropylene technology and structure. The study found that the relative molecular mass and distribution of polypropylene have an important influence on the structure and properties of winding yarn. Adding a small amount of cooling masterbatch (adding chemical degradation agent) can effectively improve the fluidity of polypropylene, making it easy for the primary fiber to form a quasi-crystalline or mixed-crystalline structure that is conducive to post-stretching.
The study of the relationship between the crystal structure of fine polypropylene winding yarn and its post-stretching performance shows that the presence of secondary crystal structure is conducive to improving spinnability and post-stretching performance, and the higher the relative content of secondary crystal to ¦Á crystal in the winding yarn, the better the post-stretching performance; the sample with secondary crystal structure as the main body can reach 0.55dpf and 6.02cN/dtex in fiber single fiber fineness after full stretching, which is an ultra-fine polypropylene filament with excellent mechanical properties.
The oxygen limit index of polypropylene fiber is only 19%~20%. The combustion releases a lot of heat, the flame propagates quickly, and is accompanied by smoke and dripping, which can easily cause large-scale fires. At the same time, a large amount of smoke and toxic gases will be released during the combustion process, causing asphyxiating gases, which pose a huge threat to human life safety, thus limiting the wide application of polypropylene fiber. At present, the flame retardant modification of polypropylene fiber is to select flame retardants and polypropylene to pre-make flame retardant masterbatches, which are blended with polypropylene chips in proportion during spinning. When burning, carbonaceous coke is formed on the surface of polypropylene fiber to prevent contact with oxygen to achieve the purpose of flame retardancy.
The flame retardant used for flame retardant polypropylene fiber should generally meet the following requirements:
(1) Have good thermal stability (£¾260¡ãC) during polypropylene processing.
(2) Have good compatibility with polypropylene and will not leach or migrate.
(3) The release of toxic gases should be reduced to an acceptable level.
(4) It should be highly efficient and the amount added should be as small as possible (generally the mass fraction should be less than 10%) to reduce production costs and reduce its impact on the mechanical properties of the fiber.
Currently, most polypropylene fibers are colored by pre-spinning coloring. At the same time, some dyeable polypropylene fiber technologies have been developed, including grafting polymers or monomers containing dye affinity groups onto polypropylene molecular chains through graft copolymerization to make them dyeable, and destroying and reducing the tight aggregation structure between polypropylene macromolecules through blending spinning, so that polymers containing dye affinity groups are mixed into polypropylene fibers, so that some submicroscopic discontinuous points with high interfacial energy are formed in the fibers, so that the dye can smoothly penetrate into the fibers and combine with the dye affinity groups.
Compared with the blending method, it is currently a practical method for manufacturing dyeable polypropylene fibers. The main products include the following:
(1) Mordant dye-dyable polypropylene fibers.
(2) Basic dye-dyable polypropylene fibers.
(3) Disperse dyes can dye polypropylene fibers.
(4) Acid dyes can dye polypropylene fibers, among which acid dyes can dye polypropylene fibers and are extremely promising.
Currently, the main way to prepare antistatic polypropylene fibers is to add conductive tin oxide, zinc oxide, polyaniline, carbon nanotubes, etc. to polypropylene fibers to make conductive polypropylene fibers, and use corona discharge to achieve antistatic properties of polypropylene fibers. Tin oxide, zinc oxide, etc. are good semiconductors. Studies have shown that when 20% of conductive powder tin oxide is mixed into the fiber, the volume resistivity of the fiber is 10¡ã¦¸¡¤cm, which is 9 orders of magnitude lower than that of pure polypropylene fiber, and has a good antistatic effect. When 5% of zinc oxide derivatives are added, the resistivity of the fiber is 10¡ãC¡¤cm. Polyaniline and carbon nanotubes also have relatively good conductive effects. After polyaniline and carbon nanotubes are mixed into polypropylene by blending, it can have a certain conductive function.
Some people use a two-component structure to add antistatic agents to the cortex and control the conductive network morphology through drawing and annealing processes to prepare high-conductivity high-strength polypropylene composite fibers. The conductivity of the prepared polypropylene composite fiber is as high as 275S/m, and the tensile strength is about 500MPa. This is a polypropylene fiber with extremely high conductivity obtained by melt spinning. The content of multi-walled carbon nanotubes in the outer layer of the fiber is about 5% (mass fraction), and the carbon nanotubes in the entire system are only 0.5% (mass fraction). It is characterized in that the outer layer is a copolymer polypropylene conductive composite material with a low melting point filled with multi-walled carbon nanotubes, and the core layer is an unfilled high-melting point homogeneous polypropylene.
In order to improve the ultraviolet aging resistance of polypropylene fibers, some organic or inorganic ultraviolet shielding agents or ultraviolet absorbers are usually added to polypropylene slices, and polypropylene fibers with ultraviolet aging resistance are obtained by melt spinning. Studies have shown that adding inorganic shielding agents to polypropylene can significantly improve the anti-ultraviolet radiation ability of polypropylene fibers, and has little effect on its main wearing properties. Ultrafine titanium dioxide, ultrafine zinc oxide, antibacterial zeolite (ZA), nano calcium carbonate, etc. also show obvious light shielding properties for polypropylene. Nano titanium dioxide not only has ultraviolet absorption and shielding functions, but also has antibacterial effects. Due to its many excellent functions and properties such as stable chemical properties, good thermal stability, non-toxicity and unique optical effects, it is widely used in the field of fiber textiles.
The melt-blown process mainly uses high-speed hot air (310~374¡æ) to stretch the molten polymer into ultra-fine fibers with a length of 25~100 mm and an average diameter of less than 4 microns. Then it is cooled and solidified on the receiving device to form a fiber web. The fibers in the fiber web are intertwined, and the residual heat of the polymer after spinning and the hot air of stretching make the fibers hot-melt and bonded, and they are directly consolidated together to become non-woven fabrics.
Film split spinning includes steps such as film forming, uniaxial stretching, heat setting and fiber splitting. Film forming includes flat film extrusion method and blown film method. Flat film extrusion method uses a T-type die to extrude a flat film and cool it to obtain a film; blown film method uses a ring die to extrude the melt and blow it to expand it to obtain a film. There are three methods for uniaxial stretching, including infrared, hot plate and hot roller stretching. Heat setting can use the same heating equipment as stretching, and the setting temperature should be 5~10¡ãC higher than the stretching temperature to reduce the boiling water shrinkage. There are two methods for splitting fibers: cutting and tearing. Cutting is to cut the film into flat strips and then stretch it, while tearing is to reduce the transverse strength by stretching and make a mesh or continuous filament.
Short-range spinning technology is a new process route with a shorter process flow than conventional spinning, a direct connection between the spinning process and the stretching process, an increased number of spinnerets, and a reduced spinning speed. The entire production line can be shortened to about 50m, and all from slice input to fiber packaging are continuous, producing short fibers with a monofilament linear density of 1~200dtex. It has the advantages of small footprint, high output, low cost, easy operation, easy and rapid development and strong adaptability. With the continuous development of short-range spinning, there have been breakthroughs in technology and equipment. For example, the height of the machine has been compressed from three layers to one layer. This technology is mainly used to produce polypropylene, and can also be used for polyester and nylon production.
The production process of bulk spinning has two steps and one step. The two-step method is to first wind the spun silk into a roll, and then stretch, deform and wind it: the one-step method integrates spinning, drawing and deformation. Not only are the processes continuous, but also all processes are completed on one unit. It occupies a small area, has a high degree of automation, stable product quality and low cost. The one-step process is currently the most widely used.
Monofilament is superior to mesh fiber in improving the shrinkage resistance of concrete. The main reason is that monofilament is finer than mesh fiber, its dispersion degree in concrete is higher, and the distance between fibers is smaller, which improves the continuity of concrete material medium and makes it more integrated, so that the concrete can well transmit and consume the tensile stress generated by the base, formwork and steel bars during the concrete pouring process, and improve the crack resistance of concrete.
Mesh fiber is a mesh structure formed by multiple fiber monofilaments interlaced with each other. The transverse connection between fiber monofilaments is destroyed by the kneading and friction of concrete mixing, forming a monofilament or mesh structure and fully opening, so as to achieve the effect of evenly dispersing tens of millions of fibers per cubic meter of concrete.
Fabrics made of polypropylene fiber are easy to burn, accompanied by burning dripping, which limits its scope of use. The flame retardant modification of polypropylene fiber is mainly through fabric flame retardant finishing and blending flame retardant modification. Fabric flame retardant finishing is to use flame retardants containing reactive groups such as carbon-carbon double bonds or hydroxymethyl and multifunctional compounds (cross-linking agents) with similar reactive groups to copolymerize on polypropylene fiber fabrics to form polymers and fix them on the fabrics. Disadvantages: Due to the high crystallinity of isotactic polypropylene and the lack of reactive groups in the macromolecular chain, it is difficult for flame retardant molecules to diffuse into the fiber or chemically combine with it. It is difficult to give fabrics flame retardancy by finishing methods, and the feel is poor. Therefore, it is generally used for products that are washed less frequently, such as carpets.
By selecting high molecular weight and high isotactic polypropylene raw materials, starting from improving the extension degree and crystallinity of the macromolecular chain, and rationally controlling the spinning, stretching, and heat treatment processes, high-strength and high-modulus polypropylene fibers can be obtained. High-strength polypropylene fibers have great competitive potential in the field of industrial fibers. In addition to its excellent mechanical properties and chemical resistance, it also has obvious technical and economic advantages such as low investment in production equipment, cheap raw materials, and low energy consumption in the production process. The annual sales volume of high-strength polypropylene fibers abroad continues to increase. High-strength polypropylene fibers can be used as filter fabrics for various industrial slings, safety nets in the construction industry, safety belts for automobiles and sports, marine cables, metallurgy, chemical industry, food and sewage treatment industries, geotextiles for strengthening dams, reservoirs, railways, highways and other projects, tarpaulins for automobiles and tourism, as well as high-pressure water pipes and industrial sewing threads and other industrial fields.
Short-range spinning is a new process route with a shorter process flow than conventional spinning, in which the spinning process is directly connected to the stretching process, the number of spinneret holes is increased, and the spinning speed is reduced. Short-range spinning equipment can use masterbatch to blend with conventional chips to produce colored fibers. The extruder, the box and the pipeline are greatly shortened, which can reduce the layer height and the resistance to transporting the melt. All transmissions are controlled by microcomputers to achieve full automation, so that the output and quality meet the design requirements. In recent years, short-distance spinning has developed greatly. The height of the machine has been compressed from three layers to one layer, and the length has been shortened from 100m to nearly 50m. All the processes from slice input to fiber packaging have been continuous. For example, the polypropylene staple fiber equipment produced by the German Automation Equipment Company has an output of 450~2400 kg/h and a linear density of 1.5~200dtex. It is said that the equipment is mainly used to produce polypropylene, and can also be used for polyester and nylon production.
The equipment used for bulked filaments is to connect several processes together and assemble them on a main machine. This can greatly reduce the floor space and complete the functions of each process on one machine. This is the development trend of BCF in recent years. The BCF production process is: slice conveying ¡ú screw extrusion ¡ú spinning ¡ú stretching ¡ú deformation ¡ú winding.
Polypropylene BCF is to process the unstretched polypropylene yarn into BCF filament by stretching, deformation or re-networking. The filament is a three-dimensional curled filament with fluffy, elasticity, and good hand feel, giving people a full and soft feeling. The filament can be made into a certain linear density according to different uses, such as: 1500~3500 dtex for carpets, 1100~2600dtex for furniture fabrics, and 550~770 dtex for decorative fabrics.
The traditional process of BCF filament production is discontinuous, and the connection between the previous and subsequent processes requires a large number of rollers for round-trip transportation and reversing. This process not only occupies a large area, has high noise, and has a large end loss, but also consumes time and energy, so the cost is relatively high. Nowadays, the use of high-tech BCF has innovated the above shortcomings, not only making each process continuous, but also completing each process on a single unit, and using microcomputer control to improve the degree of automation, reduce noise, and improve the quality of the finished product.
The spinning process of polypropylene fiber includes melt spinning, film splitting spinning, short-range spinning and bulk spinning. Melt spinning is used to produce filaments and staple fibers, and its principle and equipment are similar to those of polyester and polyamide fibers. Film splitting spinning includes film forming, uniaxial stretching, heat setting and fiber splitting processes, among which film forming includes flat film extrusion and blown film making methods, and fiber splitting mainly includes two methods: cutting and tearing. Short-range spinning is a new process with short process flow, continuous, small footprint, high output and low cost, which can produce staple fibers. The bulk spinning production process includes two-step method and one-step method. The one-step method has the advantages of good continuity, small footprint, high degree of automation, stable product quality and low cost. With the development of production technology, new spinning processes continue to emerge, which improves the production efficiency and product quality of polypropylene fibers.
Melt spinning is a spinning method that u
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