In the textile dyeing and finishing process, the fabric is subjected to a variety of compound effects (including physical and mechanical, chemical). Make the product change in the external shape and structural size, and some even lose the shape, appearance and style that the fabric should have, which seriously affects the wearing performance. Therefore, ensuring the stability of the outer shape and dimensions of the fabric is an important criterion for measuring product quality.
Heat setting refers to the process of keeping the fabric at a certain size under appropriate tension, heating it at a certain temperature for a certain period of time, and then rapidly cooling it down. Heat setting can eliminate the existing creases on the fabric, improve the dimensional stability of the fabric, make it difficult to produce creases that are difficult to remove, and improve the pilling and surface smoothness of the fabric, and it can improve the strength, feel and feel of the fabric. Dyeing performance also has a certain impact.
The entire heat setting process can be divided into four stages:
① Heating stage: The dry or wet fabric enters the heat setting machine, and the surface of the fabric is heated to the setting temperature.
② Thermal equilibrium stage: Heat energy penetrates into the fiber, so that the surface and the inside of the fiber reach the same setting temperature.
③ Transition and molecular adjustment stage: the fiber is under stress
Next, when the setting temperature is reached, the weaker secondary cross-bonds in the fiber structure are destroyed, and the fiber molecular chains are reoriented and arranged.
④ Cooling stage: The fabric is quickly cooled before leaving the stenter setting machine, so the shape of the fabric is fixed according to the new arrangement of fiber molecules.
Heat setting mechanism
Synthetic fibers have thermoplasticity, but when the glass transition temperature is below, the fiber macromolecular chain is in a frozen state, and only normal elastic deformation can occur when subjected to force. When the temperature is greater than the glass transition temperature, the molecular chain starts to move, the fiber is in a highly elastic state, and high elastic deformation occurs when a force is applied. Because synthetic fibers have both crystalline and amorphous regions, only when the temperature is greater than the melting point and greater than the viscous flow temperature, the fiber macromolecular chain is in a viscous flow state and can produce plastic deformation, otherwise it is still in a high elastic state. When the synthetic fiber is in a highly elastic state, tension is applied to the fiber, which is the peristaltic rearrangement of the molecular chain along the direction of the external force, and establishes a new intermolecular force at a new position, maintains the tension and cools, and the new The state can be fixed, so as to achieve the purpose of stereotypes.
Heat setting process
When the fabric is heat-set, it is usually done by applying a certain tension to the fabric in a dry state, maintaining a certain size, and treating it in a high temperature environment for a certain period of time. In order to strengthen the setting effect and effect, reduce the setting temperature, water can also be used as a plasticizer, and heat treatment in the case of water in the fabric or in water, so the heat setting process can be divided into dry heat setting and moist heat according to whether the water contains plasticizer or not. Two basic crafts for shaping.
1 Dry heat setting process
The dry heat setting process is to heat the fabric in a dry state without water. Usually use hot air heating or infrared radiation heating.
Hot-air stenter setting machine
Generally, a pin-type stenter is used, and the setting temperature and fabric width can be controlled freely, and the setting effect is relatively satisfactory. It is the most commonly used setting machine in printing and dyeing factories at present. , Heating drying room, cloth discharging device, etc. The heat source can be electric heating, oil heating, or gas heating.
The setting temperature is closely related to the type and shape of the fiber in the fabric, and has a great influence on the blending type of synthetic fibers and the use of the fabric.
The setting time depends on the time required for the fabric to reach the temperature of the heat transfer medium. It ranges from 10s for thin fabrics to 30-40s for heavy fabrics; however, the actual temperature on the fabric is not easy to measure, so it is generally reserved for comparison. Big safety factor. However, if the setting time is too long, it will not only be of no benefit, but also will make the fabric color yellow and feel hard.
The long-wave radiation emitted by heat-generating materials has a wavelength ranging from the red end of the visible spectrum (0.73m) to about 1mm, which is infrared. Infrared spectrum is generally divided into three regions, namely the near-infrared region with a wavelength of 0.72~1.5m, the mid-infrared region with a wavelength of 1.5~20m and the far-infrared region with a wavelength of 20~1000m. On polyester, it is found that infrared rays with a wavelength of 1~2m can pass through the fiber freely, while the radiation of 3~3.5m is actually all absorbed by the fiber; therefore, although the infrared rays with a shorter wavelength penetrate into the fiber, no energy absorption occurs. The longer-wavelength radiation gives up all the energy when it enters the fiber. In this case, only half of the total energy of the radiation reaches the innermost part of the fiber.
In recent years, a radiation source has been discovered, and the largest radiation wave emitted is located within the same range of the average absorption zone of the processed material. This radiation source is called “specially selected emitter”. The wavelength of the radiation source must be compatible with the absorption of the processed material. If the wavelength is selected properly, the radiation will radiate most of the energy inside the fiber material and enhance the vibration of the molecular chain in the fiber. When the vibration degree is quite large, there are many molecular chains. The cross bond will be broken, and the molecular chain will automatically line up in a way with minimal potential energy. In general heat setting equipment, synthetic fibers must be properly shaped at a temperature above 200°C, while infrared emitters are selected and the temperature is 80~120°C to achieve the goal. This is the reason why infrared heat setting is superior to hot air heat setting.
2 Damp heat setting process
Polyamide and polyacrylonitrile fibers and their blended fabrics, and some polyester textured yarn fabrics mostly use moist heat setting technology, and the temperature of moist heat setting is lower than that of dry heat setting.
Water bath setting method
The fabric is treated in 110°C boiling water for 0.5-2h. This method is simple and easy to implement, but the setting effect is poor, and the fabric still has a large heat shrinkage rate after setting.
The setting is carried out in an autoclave, and the fabric is treated with high-pressure saturated steam at 110-135°C for 20-30 minutes to obtain a good setting effect. This method requires special pressure equipment, and production cannot be carried out continuously.
Superheated steam shaping method
The fabric is treated with superheated steam under normal pressure, and the temperature can reach about 130°C. This method can shorten the heat treatment time, has high production efficiency, can improve the vividness of the color, prevent the yellowing of the fiber, and improve the feel, style and elasticity of the fabric.
Factors affecting heat setting
The heat setting of synthetic fibers is mainly affected by factors such as temperature, time and tension. The reasonable control of these factors is of great significance for obtaining a good heat setting effect.
The heat shrinkage, mechanical properties, dyeability, whiteness, etc. of the fabric after heat setting are closely related to the setting temperature.
The influence of setting temperature on the dyeing performance of fabric
Acrylic fiber has an obvious feature when dyeing cationic dyes, that is, after the temperature of the dye liquor exceeds the glass transition temperature, the dye uptake rate increases rapidly. For example, the dye uptake rate of dyeing at 95°C for 1 min exceeds that of dyeing at 75°C for 3 hours. In addition, the temperature exceeds the glass transition temperature during dyeing, and the fiber is also in the process of moist heat setting. Therefore, in fact, as the damp and heat setting temperature of acrylic fiber fabrics increases, the dye uptake rate also increases accordingly.
There is also a certain relationship between the dye uptake rate of polyester and its blended fabric with cotton and viscose fiber and the setting temperature before dyeing. When the fabric is dyed at high temperature and high pressure, the setting temperature and the dye uptake rate on the fiber are in a concave curve relationship. When the setting temperature is below 190℃, the amount of dye uptake will decrease due to the increase of the crystallinity of the fiber, which is the lowest point at 190℃. When the temperature rises above 190°C, the crystallinity of the fiber continues to increase, and the crystal grain size increases, but the volume of the amorphous region around a single crystal also increases correspondingly, and the pores between the crystal grains become larger, so that the dye molecules in the fiber The amount of absorption on it also increases. Especially in the case of high temperature and high pressure jig dyeing, the dyeing time is long, and the dye has enough time to diffuse into the fiber. Therefore, the absorption of dyes above 190°C increases with the increase of the setting temperature. However, there are some specific dyes that are not sensitive to heat treatment, and their dye uptake changes into a certain linear relationship, that is, when the setting temperature is high, the dye uptake is low. In the process of hot melt dyeing, the setting temperature also has a certain influence on the dye uptake rate.
The dyeing process of disperse dyes on polyester is essentially a process in which dye molecules continue to diffuse from the dye solution to the surface of the fiber, and then to the inside of the fiber. When the degree of diffusion reaches a certain equilibrium state, the dye uptake rate of the fiber reaches a state of dynamic equilibrium. With the change of the fiber’s heating condition and the change of the internal molecular structure, this dynamic balance loses its original state and correspondingly changes to a new dynamic balance state. With the increase of setting temperature, the dye uptake rate of disperse dyes on polyester is continuously decreasing. This is different from the situation during high temperature and high pressure dyeing. It is generally believed that this is because the fixation time during hot melt dyeing is only completed within tens of seconds, and the dye molecules do not have enough time to diffuse. It is also believed that this is caused by the continuous increase in the degree of crystal folding of polyester molecules at high temperatures, and the tighter structure, which makes it difficult for dye molecules to diffuse into the fiber. In actual production, the dye uptake rate of the fabric varies with the specifications of the grey fabric, the size of the blending ratio, and the properties of disperse dyes at different temperatures.
The influence of setting temperature on the thermal shrinkage stability of fabrics
The heat shrinkage of acrylic fiber and its blended fabric is different from that of polyester. This is because the heat setting temperature of acrylic fiber is limited by the molecular structure, and it is generally 140-160°C in the dry state. It is measured by X-ray diffraction that its crystallinity is only increased by 3% after setting, but the integrity of the crystal region is significantly improved. Therefore, after the acrylic fabric is shaped at 140-160°C under tension, the elastic modulus of the fiber decreases, the elongation increases, and the shrinkage of the fabric in boiling water decreases. Since acrylic fiber must be set in a relatively small temperature range (if it is higher than 160℃, the fiber will be changed from yellow to reduce the strength), so the relationship between temperature and shrinkage rate appears in the contrast between unset and set More importantly, there is a big gap between the two. The size and stability of the finalized fabric are greatly improved. After the polyester-containing blended fabric is heat-set, its dimensional stability is relatively improved. After the fabric is heat-set, the reason for the improved thermal shrinkage stability is the change in the molecular structure of polyester and the increase in density.
The influence of setting temperature on fabric elasticity
The anti-wrinkle and non-ironing properties of synthetic fibers and their blended fabrics have a very important relationship with the setting temperature.
Within a certain range of polyester and blended fabrics, the wrinkle recovery angle increases as the setting temperature increases, and the wrinkle recovery of the fabric is better than that of the unshaped fabric. When the setting temperature is above 200℃, the wrinkle recovery angle will decrease with the increase of setting temperature, and the hand feel will become harder. Therefore, considering the anti-wrinkle performance, the setting temperature should be lower than 200℃.
With the increase of setting temperature and setting time, the wet resilience of nylon fabric has been significantly improved. In addition, when the setting time is within 30s at the same setting temperature, the curve rises sharply and the amplitude changes greatly. When the setting time is more than 30s later , The curve tends to be flat, and the change of the return angle decreases.
The influence of setting temperature on fabric whiteness
Among the factors that affect the whiteness of the fabric during setting, the pH value of the cloth surface before setting is a prominent factor. The cloth surface has alkali, and the pH value is above 8. After heat setting, it will turn yellow. The degree is related to the amount of alkali on the cloth surface. The greater the amount of alkali on the cloth, the more serious the degree of yellowing. If the cloth has uneven alkali, the yellowing is also uneven. Therefore, in addition to the uniform whiteness of the fabric before setting, the amount of alkali should be small and uniform. The standard is generally that the pH value of the fabric surface is below 8.
In addition, the temperature during setting will also affect the whiteness. No matter what kind of fabric, the whiteness value after setting will decrease as the setting temperature rises. Among them, the drop of pure viscose fabric is greater than that of polyester viscose blended fabric and pure polyester fabric. The reason is that after the viscose fiber is heat-set, its water content gradually decreases, and part of the fiber is dehydrated and carbonized and turned yellow. The same applies to polyester-cotton blended fabrics. In addition, even if it is set at a lower temperature, if it takes a long time, it will also cause partial dehydration of the cellulose fiber and yellowing.
The influence of cooling temperature control on shaping effect
The cooling conditions after heat setting treatment have a greater impact on the physical and mechanical properties of the shaped fabric. The higher the cooling temperature, the greater the Young’s modulus. Generally speaking, the Young’s modulus is directly related to the wrinkle recovery performance of the fabric, and high modulus fibers will inevitably make the fabric have good wrinkle recovery properties. It can be concluded that the increase of the cooling temperature also contributes to the improvement of the wrinkle recovery of the shaped fabric. At the same setting temperature, a higher cooling temperature corresponds to a larger wrinkle recovery angle. The slower the cooling, the better the wrinkle recovery of the fabric.
The relationship between the temperature of the drying chamber of the setting machine and the temperature of the fabric surface
The temperature specified in the heat setting process usually refers to the temperature that the fabric substrate actually reaches, and it is the most important factor to ensure the quality of the setting. However, the heat setting of synthetic fibers is carried out in a heat setting machine. The temperature indicated by the instrument on the setting machine actually only represents the temperature value reached in the drying chamber of the setting machine, but cannot explain the actual temperature reached by the main body of the fabric. Therefore, there is a difference between the temperature of the machine drying chamber and the main body of the fabric. In the heat setting process, the temperature of the drying chamber can generally be controlled and fixed, while the temperature of the main body of the fabric changes with different factors such as fabric fibers, organizational structure, and running speed. Because the physical temperature of the fabric surface is difficult to test and display in actual production, the process setting temperature is often replaced by the machine’s drying chamber temperature. The difference between the fixed drying chamber temperature and the changing surface temperature of the fabric will cause It has a certain impact on the quality of heat setting. At a certain drying room temperature, the main body of the fabric still needs a heating process, and the heating rate is also limited by factors such as the type of fabric, the structure of the fabric, and the operating speed. Often the actual temperature of the surface of the fabric body is lower than the temperature of the machine drying room. Obviously, it is impossible to obtain a good setting effect. In order to reasonably solve this problem, the following measures can usually be taken:
① Reduce the moisture content of the fabric. Practice shows that it is appropriate to control the moisture content of the fabric below 10% before setting.
② Infrared pre-bake before shaping.
③ Increase the temperature of the drying chamber, generally the temperature of the drying chamber can be controlled at about 200°C.
④ Reasonably control the temperature gradient in the drying chamber of the setting machine. For the reasonable distribution of the temperature of the heating zone before and after the setting machine, a temperature gradient is formed. There are generally three ways of temperature control: low front and high back, high front and low back, or the same before and after. From the perspective of increasing the surface temperature of the fabric body and reducing the heating time of the fabric, low front and high back will extend the heating time of the fabric, while high front and low back will reduce the high temperature setting time of the fabric. Therefore, it is more reasonable to adopt a consistent temperature distribution method.
⑤ Reasonably control the speed of the vehicle.
During the heat setting process of the fabric, the entire processing time can be divided into the following parts:
(1) Heating time: After the fabric enters the setting machine, the time required for the fabric surface to be heated to the setting temperature.
(2) Heat penetration time: the time required for heat to penetrate from the surface of the fabric to the inside of the fabric fiber, so that all parts of the fabric fiber reach the same setting temperature.
(3) Rearrangement and adjustment time of fiber macromolecules: After the main body of the fabric reaches the setting temperature, the time required for the structure adjustment of the fiber macromolecules according to the setting conditions.
(4) Cooling time: the time required for the fabric to be cooled down after leaving the drying chamber of the setting machine to fix the structure. Generally, the setting time referred to in the process does not include the cooling time, and the heating time is regarded as the setting preparation time. Therefore, the general control of the setting time tends to focus on the heat penetration time and the adjustment time of macromolecule rearrangement.
The heat penetration time of the fabric (including heating time) is closely related to factors such as the heating method of the setting machine, the type of heat source, the thermal conductivity of the fiber, and the moisture content of the fabric structure. Compared with the indirect heating type hot air setting machine, the setting machine with gas direct combustion method has higher heat transfer efficiency, faster heating speed, and shorter setting time. The same fiber fabric, on the designated equipment, the thicker, denser, heavier and moisture-containing fabric The higher the rate, the longer the setting time required. Considering all aspects of the impact, practice shows that the heating and infiltration time takes about 2-15s.
The time required for molecular rearrangement adjustment is a very fast process, which can be completed within 1 to 2 seconds. Therefore, it is only necessary to ensure that the fabric is uniformly heated to the required setting temperature, and the subsequent molecular rearrangement and adjustment process is extremely fast, and the required time can be neglected.
Practice shows that too long setting time not only has no obvious effect on improving the dimensional stability of the fabric, but also causes the whiteness of the fabric to decrease, the hand feel is hard, and the strength is lost. At the same setting temperature, with the extension of setting time, the dry heat shrinkage rate of the fabric decreases, and the downward trend in the warp direction is obvious, but after a certain time in the weft direction, the shrinkage rate is not obvious or even unchanged. Generally, the fabric setting time is controlled within 20-30s, which can achieve the purpose of stabilizing the size and reducing the heat shrinkage rate.
After the fabric is heat-treated, the rate of cooling and solidification should be moderate. If the cooling time is too short or the cooling is not enough, it is easy to cause further deformation of the fabric. If the cooling rate is too fast, internal stress will be generated, making the fabric easy to wrinkle and lack of body bone; if the cooling rate is too slow, the production efficiency will be low.
Tension has a certain degree of influence on heat setting quality and product performance indicators (such as heat shrinkage, strength, elongation at break). For thermoplastic fibers such as synthetic fibers, when the fabric is heat-treated in a relaxed state, the warp and weft shrinkage rate can reach more than 5%; and when the fabric is heat-treated under a certain tension, the macromolecular chain elongates in the direction of the external force. Movement and rearrangement make the fibers denser and have a higher degree of orientation. Once this state is cooled and fixed, the shrinkage of the fabric can be greatly reduced, or even reduced to zero, and the dimensional stability is fundamentally improved. Therefore, a certain tension is applied to the fabric during the setting process, which helps to improve the setting effect.
When heat setting under tension, different tensions need to be applied in the warp and weft directions of the fabric. The size of the tension depends on the product quality requirements. Usually in the heat setting process, the warp tension is expressed by the overfeed rate, and the weft tension is expressed by the overfeed rate. The fabric tentering amount is expressed. On the sizing equipment, the warp tension is controlled by mechanical stretching and overfeeding devices, and the weft tension is controlled by the needle board or cloth clip tentering device. When setting, with the increase of the warp overfeed rate, the dry heat shrinkage rate of the fabric decreases and the dimensional stability increases, while the weft dry heat shrinkage rate increases with the increase of the width of the door width, and the dimensional stability It decreases with the increase of weft tension. After setting, the warp and weft breaking elongation of the fabric is different: the weft breaking elongation decreases with the increase of the stretching range, while the warp breaking strength increases with the increase of the overfeed rate. Therefore, in order to better improve the wearability and dimensional stability of the fabric, the warp overfeed rate and the weft stretch range of the fabric should be reasonably controlled during the heat setting process, that is, the tension applied to the warp and weft of the fabric should be coordinated at an appropriate level. In the range.
Circular knitting machines can be used to make a number of different types of clothing and other things.
The hat that is detailed above is a pattern that is easy to create because it is just a single tube. Other clothing that can be created with one of these machines includes polo shirts, t-shirts, skirts, dresses, sweaters, socks, and more.
You can even make pajamas and leggings with a circular knitting machine. Outside of clothing, you can make curtains, towels, blankets, interior linings, and more.