1、 Fiber characteristics and Applications
1.1 characteristics of aramid fiber
1.1.1 good mechanical characteristics
Aramid is a flexible polymer material. Its low stiffness and high elongation characteristics make it the same spinnability as ordinary fibers. It can be processed into various fabrics or non-woven fabrics by conventional spinning machines. Moreover, aramid fabrics are wear-resistant and tear resistant, and can be widely used in special fields such as labor protection and military products production.
1.1.2 excellent flame retardant and heat resistance
Aramid fiber is a kind of flame-retardant fiber, and its limiting oxygen index LOI value is ≥ 28%, which belongs to flame-retardant fiber. Therefore, aramid fabric will not burn, melt or produce droplets in the air, and will not emit harmful gases, nor support combustion, and has self extinguishing property.
1.1.3 stable chemical properties
Aramid fiber can be used for a long time at a high temperature of 220 ℃ without aging. The effectiveness of its electrical and mechanical properties can be maintained for 10 years, and its dimensional stability is excellent.
1.1.4 radiation resistance
Aramid fiber resistance α、β The radiation performance of X-ray and ultraviolet light is very excellent. For example, in β When the radiation amount accumulates to 1000mrad, its intensity remains basically unchanged.
1.1.5 application of aramid fiber
Aramid fiber can be used in all aspects of the national economy, such as aerospace, electromechanical, construction, automobile, sporting goods, etc. it is mainly used in labor protection (fire protection clothing, fire boots, fire bags, etc.), special equipment combat clothing (high-performance military clothing such as flight clothing, radiation protection clothing, chemical warfare clothing, etc.). It is a new high-tech synthetic fiber with ultra-high strength, high modulus, high temperature resistance, acid and alkali resistance Light weight and other characteristics. The morphological structure of aramid fiber is shown in Figure 1.
1.2 properties of flame retardant viscose fiber
1.2.1 permanent flame retardancy
There is almost no flame when the flame-retardant viscose fiber burns, and it goes out in a very short time. It will neither continue to burn nor smolder.
1.2.2 excellent thermal insulation performance
Flame retardant viscose fiber can effectively prevent high-temperature heat radiation transmission. The fiber will not shrink or deform under high temperature.
1.2.3 arc resistance and antistatic property
Flame retardant viscose fiber maintains the original natural antistatic property of viscose fiber.
1.2.4 moisture absorption and drainage
Clothes made of flame-retardant viscose fiber are comfortable and breathable. High temperature environment is conducive to sweat discharge.
1.2.5 excellent dyeability
Flame retardant viscose fiber has the characteristics of easy coloring, bright dyeing and good color fastness of natural fiber.
1.2.6 green environmental protection
Flame retardant viscose fiber takes regenerated natural plants as raw materials, and the waste can be naturally degraded, which meets the requirements of environmental protection. Phosphorus flame retardant also meets the requirements of EU flame retardant halogen-free.
1.2.7 application of flame retardant viscose fiber
Flame retardant viscose fiber is a new high-tech fiber material with flame retardant and droplet resistance. Compared with synthetic polyester flame retardant fiber, it not only ensures the excellent physical properties of the fiber, but also realizes the characteristics of low smoke, non-toxic, no odor, no melting and dripping. With the promotion of fire-retardant mandatory regulations at home and abroad and the improvement of people's awareness of fire safety, the application scope of fire-retardant textiles is expanding day by day, extending to home, underwear and other fields. The requirements for fire-retardant textiles are also extended from simple protection to the pursuit of better comfort, ventilation and performance. See Figure 2 for the morphological structure of flame retardant viscose fiber.
1.3 main physical indexes of aramid fiber and flame retardant viscose
See Table 1 for the main physical indexes of aramid fiber and flame retardant viscose.
2、 Technical requirements for spinning process
2.1 preparation of pigments
Customers require color spinning, so they need to dye aramid fiber. Carrier dyeing method is adopted, and the dyeing process is similar to that of polyester. The principle of choosing carrier is: better dyeing effect, less dosage, non-toxic and odorless. After pretreatment, add carrier hps-505 to aramid fiber at 50 ℃, run for 10 minutes, raise the temperature to 70 ℃, add cationic dye, run for 10 minutes, add sodium nitrate (sodium chloride) and run for 10 minutes, then raise the temperature to 130 ℃ at 1 ℃ / min and keep it warm for 60 minutes, and lower the temperature to 75 ℃ at 1 ℃ / min, color matching, liquid drainage and cleaning (see Figure 3 for aramid dyeing process). Flame retardant viscose is blended with raw white fiber and dyed aramid.
Fig. 3 aramid dyeing process
2.2 treatment of raw materials
Because the specific resistance of aramid fiber after dyeing is large, the phenomenon of static electricity is serious, and the fiber pretreatment is improper, it is easy to wrap around the cylinder, draw flowers, not leave the sliver, wrap the roller, etc., so the raw materials need to be cured, add a certain amount of antistatic agent and water, and strictly implement the curing time (it must be ensured that the curing time is more than 16 hours, and placed in an environment of 28 degrees and 65% relative humidity).
Flame retardant viscose is a White Masterbatch cellulose fiber. In order to avoid the formation of white knots during carding, this fiber cannot be cured with water. After curing, aramid fiber is blended with flame retardant viscose.
2.3 process flow
Because the static electricity of aramid and viscose fibers is large and fluffy, and the fiber length is consistent, in order to reduce fiber damage during spinning, a short process spinning process is adopted. The process flow is:
Bc262 and wool (automatic cotton feeder) → A186 carding → fa306 head doubling → fa306 two doubling → fa306 three doubling → fa306 four doubling → fa458 roving → FA506 spinning → automatic winding
2.4 problems and technical parameters in the spinning process
2.4.1 technological measures for carding process
Because aramid and flame-retardant viscose fibers are chemical fibers, with good fiber uniformity, less impurities and serious electrostatic phenomenon, the carding process must ensure less damage, mainly loosening, and try to reduce the increase of knots caused by backflowers. It should mainly solve the problems of difficult carding and blocking inclined pipes. In order to improve the fiber transfer effect, the cylinder cover spacing is 0.35, 0.35, 0.30, 0.30, 0.35mm, the sliver weight is 17.5g/5m, and the spacing between the upper and lower rollers is 0.20mm, so as to improve the smoothness of each channel and reduce blocking and breaking. The speed of cylinder is 330r/min, the speed of take-up roller is 780r/min, the speed of cover plate is 81mm/min, and the speed of large pass is 17r/min. After the above process is put into operation, the neps of raw sliver spun are 4 grains /g, the clarity of cotton web is good, and the appearance of raw sliver is smooth and delicate.
2.4.2 technological measures for drawing process
Due to the slight static electricity of aramid fiber and flame retardant fiber, it is necessary to focus on solving the phenomenon of winding roller cots in production. The main measures are:
(1) WSN coating for drawing cots is better than acid treated cots.
(2) Control the speed on the low side, control the bell mouth on the small side, and improve the sliver holding force.
(3) The full cylinder length is changed from 2km to 1.6km to reduce the friction between the sliver and the surface of the upper sliver.
The drawing adopts four-way merging, the pulling speed of the front roller is 190m/min, the bell mouth is 2.4mm, the weight of cooked strips is 19.5g/5m, and the roller spacing is 9 × 20mm。
2.4.3 technical measures for roving process
Reasonable selection of roving twist factor should not only improve the holding force between fibers, but also prevent roving from "hard head". After optimization, the twist factor is set as 71. When spinning, the spinning tension is too small, so as to reduce the accidental elongation of the yarn.
The roving weight is 5.5g/10m, the total draft is 7.09 times, the front roller speed is 180r/min, and the roller spacing is 10 × twenty-six point five × 3lmm。
2.4.4 technical measures for spinning process
The total draft of spinning is 30.83 times, the rear draft is 1.32 times, the twist coefficient is 376, the speed of the front roller is 183r/min, and the roller spacing is 18 × 24mm, jaw spacer block 3.0mm.
After testing, the spinning indexes are: weight deviation +1.6%, evenness CV value 14.34%, details 5 /km, coarse knots 55 /km, neps 145 /km, single yarn breaking strength 17.58cn/tex, strength CV value 9.8%.
In terms of yarn quality, except for a little more neps, other indicators are good. We believe that the main reason for the relatively large number of neps is the relative slippage between fibers and the kink of yarn drafting, which needs to be overcome.
3、 Other technical requirements
Because aramid fiber is very fluffy, elastic, high elongation, and easy to accumulate static electricity during processing; Flame retardant viscose fiber has low wet strength, poor wear resistance, poor cohesion between fibers, difficult sliver forming, loose sliver, and high end breakage rate during spinning. The following measures should be taken in the spinning process:
(1) When spinning aramid, it is necessary to maintain a high environmental humidity to avoid electrostatic accumulation. The relative humidity of carding is 65%, drawing is 62%, and spinning is 58%.
(2) Because the length of aramid fiber is 44mm, the twist of roving is not easy to be too high. With a small ration and a small drafting multiple, the spinning twist factor is appropriately increased, generally controlled at 370.
(3) The winding tension is controlled to be too small, and the chemical fiber splicer is used. The joint strength must ensure that it reaches 85% of the original yarn strength, and the appearance quality is good.
(4) The sun fastness of aramid fiber after dyeing is only grade 3, which needs further technical research.