As the first of the five major engineering plastics, PA66 and PA6 account for more than 90% of the entire nylon market. Among them, about 59% of PA66 is used in engineering plastics, and nearly 41% is used to make fibers, of which industrial filament yarn accounts for about 23%.
Photographed at the Huafeng booth
As early as 1939, DuPont first completed the industrial production of nylon 66, and the original nylon 66 fiber was mainly used for socks, that is, the famous nylon socks.
After decades of unremitting efforts and research and development, nylon 66 fiber has been widely used in tire cord fabrics, military parachutes, airbags, clothing textiles and other collars, and clothing textiles involve socks, underwear, gymnastics clothes, casual wear, ski shirts, swimwear and other textile products.
Photographed at the SHEMAR booth
Photographed at the Huafeng booth
So what are the characteristics of nylon 66 industrial yarn? How is it produced? What are the factors influencing performance?
1. Characteristics and application range of nylon 66 fiber
Nylon fiber taken at Huafeng's booth
Nylon 66 has the following advantages.
(1) High strength: Nylon 66 has more strength than other nylon varieties in tensile and bending, and has strong bearing and impact resistance.
(2) Abrasion resistance: Nylon 66 has low friction coefficient and good self-lubrication, which can effectively reduce wear and noise and improve service life.
(3) Heat resistance: The melting point of nylon 66 is as high as 252 °C, and the continuous heat resistance temperature can reach 80~120 °C, which can maintain good performance in high temperature environment.
(4) Chemical resistance: Nylon 66 has good resistance to acids, alkalis and most organic solvents, and is not easy to be corroded or degraded.
(5) Easy processing: Nylon 66 has good fluidity and molding stability, and can be processed into products of various shapes and specifications by injection molding or extrusion.
Table 1: Performance parameters of common industrial yarns
2. Molecular structure characteristics, condensed matter structure characteristics and properties of nylon 66
The molecule of nylon 66 has symmetry, hydrogen bonding can occur between all carbon and nitrogen bonds, while the molecule of nylon 6 does not have symmetry, and some carbon and nitrogen bonds cannot form hydrogen bonds between them, so nylon 66 fiber has higher strength and better mechanical processing performance than nylon 6 fiber.
The crystallization capacity and crystallization rate of nylon 66 are significantly higher than those of nylon 6 (about 20 and 12 times higher, respectively). Therefore, nylon 66 is suitable for high-speed direct spinning, which can effectively improve the mechanical properties and stability of the product, and is mainly suitable for high-quality tires. Nylon 6 usually uses a two-step spinning method, most of which are used for light truck tires, export tires, etc., and a few can be used for high-quality tires.
3. Production process and production equipment of nylon 66 industrial yarn
The production of nylon 66 industrial yarn has two processes: continuous polycondensation direct spinning and intermittent polycondensation solid phase polycondensation spinning. The former is an integrated process, which is made of nylon 66 salt after refining and polymerization to make nylon 66 melt and then spinning, which is technically difficult, but has high efficiency, good stability and good economy.
Nylon 66 industrial filament Source: Hailide
The intermittent method uses nylon 66 chips as raw materials to form nylon 66 industrial yarn through injection, cooling, oil feeding, drafting, winding and other steps. The advantages of this method are that it is convenient for storage and transportation, and the process difficulty is small, but the energy consumption is large and the stability is poor. The following figure shows the flow chart of nylon 66 fiber production.
Figure 1: Flow chart of nylon 66 fiber production equipment
4. The key factors and action rules affecting the production of nylon 66 industrial yarn
4.1 Molecular weight of nylon 66 polymer
The relative molecular mass (M) of a polymer is one of its structural parameters, which can be characterized by relative viscosity (η), with the greater the M, the greater the η. M and η affect the spinning properties and fiber quality of polymers. The control of the quality of the finished nylon 66 fiber depends on the quality of the nylon 66 salt and the polycondensation temperature. Nylon 66 salt has many impurities, which will make the polymer yellow and M reduced. Therefore, it is necessary to control the refining process conditions of nylon 66 salt to ensure that its color is low.
If the η is too high or too low, it will lead to problems such as spinning difficulties and low strength of finished yarn and more wool yarn. When nylon 66 is spindled η when it is controlled between 77~79, the spinning process is stable, and the preparation of high-strength nylon 66 filament can be realized. When the η is raised to 83~85, the silk strip becomes stiff and the spinnability becomes poor; η at 62~64, it will cause the filament to be discontinuous. All things considered, the viscosity of nylon 66 is controlled between 74~79 when spinning, which can achieve better spinnability.
4.2 Water content and extractable content of nylon 66 sections
The moisture content of the polyamide chips has a significant impact on the melting process, which is a reversible equilibrium reaction, as shown in Eq. (1).
+ ′2 ↔ ′ + 2 （1）
The quality of the spinning solution is affected by the moisture and extractable content of the polyamide chips. If the moisture content of the polymer is too low, it will cause the phenomenon of "repolymerization", which will make the relative molecular weight higher and the distribution wider; Too much moisture can form bubbles, causing spinning breaks, material defects, and equipment corrosion. Hydrogen bonds are formed between the water molecule and the amide group, which changes the molecular structure and properties. Therefore, the slices should be dried before spinning, and they are generally controlled below 0.06%.
If the extractables content is too low, it will lead to polymer depolymerization and an increase in monomers and oligomers; If the extractable content is too high, the relative molecular weight distribution will be broadened and the viscoelasticity of the melt will decrease. Monomers and oligomers are also affected by equipment, processes, etc. Generally, the extractable content is controlled at about 0.3%~0.5%.
Source: Tianchen Qixiang
4.3 Spinning box temperature
The melt viscosity of nylon 66 polymer is greatly affected by temperature. When the temperature is too low, the fluidity of the melt becomes poor, and spinning is difficult to achieve; At too high temperatures, the polymer will pyrolyze and cross-link, forming bubbles and gels, resulting in broken ends and floating filaments. In order to ensure the quality of the spinning fluid, it is necessary to control the appropriate temperature range. When the spinning box temperature drops to 300°C, the η of nylon 66 polymer can be maintained at 83 and has little effect on the flow properties of the melt. Therefore, the spinning box temperature is determined to be (300±1)°C.
4.4 Spinning assembly filter media and filter screen
The spinning assembly is composed of sealing materials, filter materials, distribution plates, spinnerets and other components. Its main function is to mix the melt from the metering pump evenly, filter it, and spray the filaments through the spinneret. The pressure and filtration effect of the spinning module have an important impact on the melt extrusion quality, and the pressure of the module is mainly determined by the structure of the filter layer and the melt properties of the module. In order to ensure the smooth extrusion of the melt, the pressure of the component must be increased appropriately. Generally, 20~22MPa is used.
The main function of the spinning filter media and filter screen is to filter impurities and gels in the melt. The commonly used filter materials are sea sand + composite metal mesh and metal sand + metal mesh. Compared with sea sand, metal sand has a longer service life, more uniform physical property indexes, and can ensure that the melt temperature is consistent. In addition, the sharp corner structure of the metal sand can puncture air bubbles, block foreign matter and condensed particles, and improve filtration accuracy. It can effectively reduce the number of broken wires and improve production efficiency.
After the melt is extruded through the spinneret, the elastic potential energy and static pressure energy are released, resulting in the expansion of the diameter. This expansion causes a change in the uniformity of the melt and reduces the spinning quality. The smaller the aperture of the spinneret port, the more severe the expansion; The larger the length-diameter ratio of the spinneret, the weaker the expansion. The length-to-diameter ratio of the spinneret for nylon 66 industrial filament yarns is usually 4:1.
4.5 Side blowing cooling process and pump supply
After the spinning melt is ejected from the spinneret, it is stretched by winding and heat transfer to the environment. When the temperature drops, the melt polymer crystallizes, forming a solidification point. The position of the curing point affects the physical properties of the filament, and the cooling conditions need to be strictly controlled. The cooling device controls the parameters such as wind speed, air temperature, air volume, wind pressure, humidity and other parameters to control the fiber curing point, and the cooling rate is too fast or too slow, which will lead to fluctuations in spinning quality.
Different spinning speeds correspond to different cooling conditions, and high-speed spinning requires lower air temperature and higher wind speed. The air temperature of high-speed spinning is 18~20°C, which is 4~10°C lower than that of conventional spinning, because the contact time between the wire strip and the cooling medium is shortened. The wind speed has a great influence on the uniformity of the spinning sliver, and too large or too small will reduce the uniformity of the primary fiber. The wind speed of high-speed spinning is generally 0.7~0.8m/s, and the humidity is 75%~85%.
In addition, according to the different deners of the production denier, the process parameter requirements for the opposite side blowing are also different.
4.6 spinning cluster point
The spinning bunch point has an important impact on the structure and quality of the fiber. If the bunching point is too close to the spinneret, the primary fiber will not be sufficiently cooled, and the premature bunching will cause the filament to shake, which is easy to cause filament winding and breakage. If the bunching point is too far away from the spinneret, the flow stability of the fiber will be reduced, the uniformity of the fiber will be affected, the phenomenon of wool and broken ends will be aggravated, and the winding molding will be affected. Usually the distance between the bunching point and the spinneret during the spinning process is 5~6m.
4.7 Wetting and oiling process
After the primary fiber is cooled, it needs to be wet and oiled to achieve the equilibrium moisture content of the winding (3.0%~3.5%) and prevent problems such as static electricity and wool silk. The dosage of wetting and oiling should be appropriate, too much or too little will affect the quality of the winding. The oiling rate of high-speed spinning is generally 0.4%~0.6%.
The selection of oil type should not only meet the characteristics of lubrication, hugging, antistatic, etc., but also match the processing technology of the fiber. In the process of stretching and texturing, the heating environment of polyamide filament is often about 200 °C, so the heat resistance of the selected oil should reach more than 200 °C.
4.8 spinning speed
The spinning speed has a certain effect on the structure and tensile properties of the fiber. When the spinning speed is less than 2000m/min, the maximum stretch ratio of the fiber will decrease significantly with the increase of spinning speed. When the spinning speed is higher than 2000m/min, the maximum stretching ratio is hardly affected by the change of spinning speed. When the spinning speed reaches 4000m/min, the orientation and crystallinity of the wire strip are stable at a high level, the expansion and deformation of the wire strip after moisture absorption has been significantly improved, and the quality of the package forming has been greatly improved.
4.9 Winding process
Winding is the last processing of primary fibers, and the winding process parameters include tensile ratio, winding overfeed, winding angle, winding head contact pressure, etc. The magnitude of the tension is directly proportional to the tensile ratio and winding angle, and inversely proportional to the winding overfeed. Generally, the overfeed rate is controlled at 1.6%~1.8%, and the winding angle is controlled between 6°~8°. If the winding tension is too large or too small, it will affect the broken end of the silk strip, the forming and hardness of the silk and the silk cake. Too much or too little contact pressure of the winding head will affect the hardness of the bobbin, and the winding contact pressure is usually controlled at 120N.
4.10 Stretching process
The magnitude of the tensile tension has a significant impact on the stability and structure of the fiber during the stretching process, which mainly depends on the coefficient of friction between the fiber and the surface of the stretching roll and the wrapping angle of the fiber on the roll. The greater the coefficient of friction and the larger the wrapping angle of the fiber, the greater the tensile tension. If the coefficient of friction is too large, it will cause the fibers to appear hairy.
The tensile speed also affects the tensile tension, and its effects are more complex. According to the viscoelastic principle of polymers, increasing the tensile speed will lead to an increase in tensile tension and tensile stress.
The tensile tension of a fiber is affected by the tensile speed and temperature. As the tensile velocity increases, the tensile tension increases slowly, but the thermal effect also increases, resulting in a decrease in tensile stress. Therefore, an optimal stretching speed exists. The tensile temperature should be higher than the glass transition temperature (40~60°C) and lower than the softening temperature (185~215°C). Molecular chains are more easily stretched, straightened, and recrystallized at high temperatures, enhancing fiber strength. The tensile factor should increase with the increase of temperature, and be lower than the elongation at break and greater than the natural tensile factor.
4.11 Network degree
The netting degree refers to the number of fibers that can withstand interweaving and hugging in each meter of filament, and increasing the netting degree can increase the holding force between monofilaments. In order to avoid the phenomenon of loose monofilament and splitting of nylon 66 industrial filament, it must be given a certain degree of network during the manufacturing process.
The network degree has an impact on the physical properties and unwinding performance of the industrial yarn. The higher the network degree, the better the physical properties of the industrial filament, and the lower the unwinding anomaly rate. However, if the network is too high, the fiber will break, wool and other phenomena, which will affect the physical properties of the finished product; If the network is too low, the frequency of broken ends will increase, and subsequent processing will be difficult. The optimal network degree range is 6~10 pcs/m.
4.12 Influence of spinning environmental conditions
4.12.1 Ambient temperature between spinning
In the production process of nylon 66 industrial yarn, in order to make the undrawn yarn have good tensile properties, the high-temperature polymer melt needs to be cooled and cooled. The ambient temperature between spinning will have an effect on the cooling of the fibers. If the ambient temperature is too high, it will lead to a decrease in the cooling rate of the fiber, an increase in the crystallization time, which will lead to a decrease in stretchability and an increase in wool and broken ends. The optimal ambient temperature at the spinning site is 23~25°C.
4.12.2 Ambient humidity in the spinning room
The humidity between spinning also has an effect on the cooling of the fibers. Nylon 66 has good hygroscopicity, and proper humidity can eliminate static electricity, increase the holding force of the tow, and reduce the jitter of the tow. Too low or too high humidity will affect the uniformity and stability of the formed fibers. The humidity between spinning is optimally 56%~60%, and the spinnability is good.
4.12.3 Ambient temperature between winding
The temperature between winding can have an impact on the spinnability of the fiber. The high temperature between winding will lead to an increase in the temperature between spinning, which will cause wool and broken ends to occur in the spinning process. The most suitable temperature between winding is 18~19°C, and the waste wire rate is stable at about 2%. When the temperature exceeds 19°C, the waste rate rises to about 3%.
4.12.4 Ambient humidity in the winding room
Humidity between winding has an effect on the length and spinnability of the yarn. If the humidity is too high, it will not only affect the forming of the package, but also reduce the spinnability and increase the waste rate. The most suitable humidity between winding is 65%~66%, and the waste wire rate is stable at about 2%.
5. The best process conditions for the production of nylon 66 industrial yarn
Table 2 shows the optimal process conditions for the production of nylon 66 industrial yarn.
6. The promotion prospect of nylon 66 fiber
In the past, the development of nylon 66 fiber was restricted by the shortage of upstream raw material adiponitrile, insufficient domestic adiponitrile production capacity, and high import costs. However, as domestic enterprises overcome the technical difficulties of adiponitrile and build adiponitrile production capacity on a large scale, the cost of nylon 66 fiber is expected to drop significantly, and the market potential is expected to be fully utilized.