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“Heat setting” related knowledge full collation

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In the textile dyeing and finishing process, the fabric is subjected to (including physical-mechanical and chemical) a variety of compound effects. So that the product in the external form and structure size changes, some even lost the fabric should have the form, appearance, and style, seriously affect the taking performance. Therefore, to ensure the stability of the external form and size of the fabric is an important criterion to measure the quality of the product.

Heat setting

Heat setting is the process of keeping the fabric at a certain size under proper tension and heating it for a certain time at a certain temperature and then cooling it rapidly. Heat setting can eliminate the existing creases on the fabric, improve the dimensional stability of the fabric, so that it is not easy to remove the creases, and can improve the pilling and surface flatness of the fabric, the strength of the fabric, feel, and dyeing properties also have a certain impact.

The entire heat-setting process can be divided into four stages:

① Heating stage: 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 penetrates into the fiber so that the surface and the interior of the fiber reach the same setting temperature.

③ Transition and molecular adjustment stage: the fiber is under stress

When the setting temperature is reached, the weaker subvalent cross bonds in the fiber structure are destroyed and the fiber molecular chains are reoriented and arranged.

④ Cooling stage: the fabric leaves the stenter before the rapid cooling, so the shape of the fabric is in accordance with the new arrangement of fiber molecules fixed down.

Heat setting mechanism

Synthetic fibers are all thermoplastic, but below the glass transition temperature, the fiber macromolecular chain is in a frozen state, and when the force is applied, only the normal elastic deformation occurs. When the temperature is greater than the glass transition temperature, the molecular chain segments start to move, and the fiber is in a high elastic state, and when the force is applied, high elastic deformation occurs. As synthetic fibers have both crystalline and amorphous areas, so only when the temperature is greater than the melting point and greater than the viscous flow temperature, the fiber macromolecular chain be in the viscous flow state, can produce plastic deformation, otherwise, it is still in the high elastic state. When the synthetic fiber is in the high elastic state, tension is applied to the fiber, and the molecular chain segments are rearranged along the direction of the external force, and new intermolecular forces are established in the new position, maintaining tension and cooling, and the new state is fixed, so as to achieve the purpose of shaping.

Heat setting process

When the fabric is heat-setting processing, it is usually in the dry state with the fabric to apply a certain tension, maintain a certain size, in a high-temperature environment to deal with a certain time to complete. In order to strengthen the role and effect of shaping, reduce the shaping temperature, can also be water as a plasticizer, in the case of fabric containing water or heat treatment in water, so the heat setting process according to whether or not containing plasticizer water can be divided into dry heat setting and wet heat setting two basic processes.

1 Dry heat setting process

The dry heat-setting process is to make the fabric in the dry state, without water for heat treatment. Usually, hot air heating or infrared radiation heating is used.

Hot air stretching and shaping machine shaping

It generally adopts needle plate type stenter, the shaping temperature, and fabric width can be freely controlled, the shaping effect is more satisfactory, it is the most commonly used shaping machine in printing and dyeing factory at present, it is composed of cloth feeding device, super feeding device, edge explorer, stretching device, heating and drying room, cloth discharging device, etc. The heat source can be electric, oil-heated, or gas type.

The temperature of shaping and the type and form of fibers in the fabric are closely related and have a great impact on the type of synthetic fiber blends and the use of fabrics.

Shaping time, depending on the time required for the fabric to reach the temperature of the heat transfer medium, a wide range, from 10s for thin fabrics to 30 ~ the 40s for heavy fabrics; but the actual temperature on the fabric is not easy to determine, so generally more than retaining a relatively large safety factor. However, the shaping time is too long, not only no benefit, and will make the fabric color yellow, feel hard.

Infrared shaping

Long-wave radiation emitted by heat-generating substances, the wavelength range from the red end of the visible spectrum (0.73m) to about 1mm so far, which is infrared. The infrared spectrum is generally divided into three zones, namely, wavelength 0.72 ~ 1.5m near-infrared zone, 1.5 ~ 20m in the infrared zone, and 20 ~ 1000m far-infrared zone, the past has been tested with a wavelength of 1 ~ 4m in the middle infrared radiation on polyester, found that the wavelength of 1 ~ 2m infrared can freely through the fiber, and 3 ~ 3.5m radiation is actually all absorbed by the fiber, Therefore, the shorter wavelength of infrared radiation penetrates into the fiber, but no energy absorption occurs, and the longer wavelength of radiation enters the fiber that gives up all the energy. In this case, only half of the total energy of the radiation reaches the innermost part of the fiber.

In recent years found a kind of amplitude source, issued by the largest radiation wave is located in the same range of the average absorption area of the material, this kind of amplitude source is called “special selection of emitter”. The wavelength of the radiation source must be compatible with the absorption of the material, such as the wavelength is selected properly, the radiation will be in the fiber material internal radiation out of most of the energy, enhance the vibration of the molecular chain in the fiber, vibration degree is quite large, many cross bonds between molecular chains will be broken, molecular chains in the way of potential energy is very small, automatic alignment. For the synthetic fiber in the general heat-setting equipment, the temperature must be more than 200 ℃ to properly set the shape, while the choice of the infrared emitter, the temperature at 80 ~ 120 ℃ can achieve the purpose. This is the reason why infrared type heat-setting is superior to hot air type heat-setting.

2 Humid heat setting process

Polyamide and polyacrylonitrile fibers and their blended fabrics, certain polyester deformed yarn fabrics mostly use wet heat setting process, the temperature of wet heat setting is lower than dry heat setting.

Water bath setting method

Fabric in 110 ℃ boiling water treatment 0.5 ~ 2h, the method is simple and easy to use, but the shaping effect is poor, shaping the fabric still has a large heat shrinkage rate.

High pressure steam setting method

Shaping in the autoclave, with 110 ~ 135 ℃ high pressure saturated steam treatment fabric 20 ~ 30min, can get a good shaping effect. The method requires special pressure-resistant equipment, production can not be continuous.

Superheated steam setting method

The fabric is treated with superheated steam under atmospheric pressure at a temperature of up to about 130℃. The method can shorten the heat treatment time, high production efficiency, can improve the color vividness, prevent the fiber from yellowing, improve the feel, style, and elasticity of the fabric.

Heat setting influence factors

Synthetic fiber heat setting, mainly affected by temperature, time and tension, and other factors, the reasonable control of these factors is very important to obtain a good heat setting effect.

1 Temperature

The heat shrinkage, mechanical properties, dyeability, whiteness, etc. of the fabric after heat setting are closely related to the set temperature.

Effect of setting temperature on fabric dyeing performance

Acrylic has a distinctive feature when dyeing cationic dyestuff, that is, the dyeing rate increases rapidly after the temperature of the dyeing solution exceeds the glass transition temperature, such as the dyeing rate of 95℃ for 1min exceeds the dyeing rate of 75℃ for 3h. In addition, the dyeing temperature exceeds the glass transition temperature, the fiber is also in the process of wet heat setting. Therefore, in fact, as the wet heat setting temperature of acrylic fabric must be increased, the dyeing rate is also increased accordingly.

The dyeing rate of polyester and its blended fabrics with cotton and viscose fibers also has a certain relationship with the setting temperature before dyeing. When the fabric is dyed at high temperature and high pressure, the setting temperature and the dyeing rate on the fiber are related in a concave curve. When the setting temperature is below 190℃, the dyeing rate decreases due to the increase of the crystallinity of the fiber, which is the lowest point at 190℃. When the temperature rises above 190℃, the crystallinity of the fiber increases continuously, and the grain size increases, but the volume of the amorphous area around a single crystal also increases accordingly and the pores between the grains become larger so that the absorption of dye molecules on the fiber also increases. Especially in the case of high temperature and high-pressure roll dyeing, the dyeing time is long and the dye has enough time to diffuse into the fibers. Therefore, the absorption of dyestuff increases above 190°C with the increase in setting temperature. However, there are some specific dyestuffs that are not sensitive to heat treatment, and their uptake rate changes in a somewhat linear relationship, i.e., the uptake rate is low at high setting temperatures. In the hot-melt dyeing process, the setting temperature also has a certain effect on the dyeing rate.

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The dyeing process of dispersing dyes on polyester is essentially the process of continuous diffusion of dye molecules from the dye solution to the fiber surface and then to the fiber interior. When the degree of diffusion reaches a certain equilibrium state, the dyeing rate of the fiber reaches a state of dynamic equilibrium. With the change of fiber heat and internal molecular structure, this dynamic equilibrium loses its original state and changes to a new dynamic equilibrium state accordingly. As the setting temperature increases, the dyeing rate of dispersing dyestuff on polyester decreases. This is different from the situation in high temperature and high-pressure dyeing. It is generally believed that this is caused by the fact that the fixing time during hot melt dyeing is completed in only a few tens of seconds and the dye molecules do not have enough time to diffuse. It is also believed that this is caused by the fact that the degree of crystalline folding of polyester molecules is increasing at high temperatures and the structure is more compact, making it difficult for the dye molecules to diffuse into the interior of the fiber. In actual production, the dyeing rate of the fabric varies with the specification of the blank, the size of the blending ratio and the nature of different temperature-type disperse dyes.

Effect of setting temperature on the stability of fabric heat shrinkage

The heat shrinkage of acrylic and its blends is different from that of polyester. This is because the heat setting temperature of acrylic is limited by the molecular structure, which is generally 140-160°C in the dry state. The crystallinity is only increased by 3% after setting as measured by X-ray diffraction, but the integrity of the crystal zone is significantly increased. Therefore, after setting acrylic fabric under tension at 140-160°C, the elastic modulus of the fiber decreases, and the elongation increases, resulting in less shrinkage of the fabric in boiling water. As acrylic shaping must be carried out in a relatively small temperature range (if higher than 160 ℃, the fiber to be yellow to scorch and reduce the strength), so the relationship between temperature and shrinkage to the contrast between unshaped and shaped becomes more important, the gap between the two is larger. After shaping the size of the fabric, the stability of the large increases. Blended fabrics containing polyester after heat-setting treatment, its size stability is relatively improved. After the fabric is heat-set, the heat shrinkage stability is improved because of the change in the molecular structure of polyester and the increase in density.

The effect of setting temperature on fabric elasticity

The wrinkle-free performance of synthetic fibers and their blends has a very important relationship with the setting temperature.

Polyester and blended fabrics in a certain range, crease recovery angle increase with the increase of the setting temperature, the crease recovery of the fabric is better than the unshaped fabric. When the shaping temperature of 200 ℃ or more, crease recovery angle with the increase in shaping temperature and reduce, and feel hard, so from the anti-wrinkle performance considerations alone, shaping temperature to less than 200 ℃ is appropriate.

The wet resilience of nylon fabric is significantly improved with the increase of setting temperature and setting time; in addition, under the same setting temperature, the curve rises sharply and the amplitude changes greatly when the setting time is within the 30s, and when the setting time is more than 30s, the curve tends to level off and the change of the angle of return decreases.

The effect of setting temperature on the whiteness of the fabric

Among the factors that affect the whiteness of the fabric when setting, the pH value of the fabric surface before setting is a prominent factor. Fabric surface with alkali, pH value above 8, after heat setting will be yellowish, its degree is related to the amount of fabric surface with alkali. The greater the amount of cloth with alkali, the more serious the degree of yellowing, if the cloth with alkali is not uniform, its yellowing is also uneven. Therefore, the fabric before shaping, in addition to the requirement of whiteness itself evenly, but also to make the amount of alkali with less and evenly, the standard is generally the pH value of the fabric in 8 or less.

In addition to this, the temperature of the setting will also affect the whiteness, no matter what kind of fabric, the whiteness value after setting with the rise of the setting temperature and decline. Among them, the pure viscose fiber fabric is down more than polyester-viscose blended fabrics and pure polyester fabrics. The reason is that the viscose fiber through the heat setting, its water content gradually reduced, part of the fiber dehydration carbonization and yellowing. Polyester-cotton blended fabrics also have this situation. In addition, even at a lower temperature setting, if the time is longer, the same will make the cellulose fiber part of the dehydration and yellowing.

The influence of the control of cooling temperature on the effect of shaping

The higher the cooling temperature, the greater Young’s modulus. Generally speaking, Young’s modulus has a direct relationship with the crease recovery performance of the fabric, high modulus of the fiber must make the fabric has good crease recovery. It can be concluded that: the increase of cooling temperature also helps to improve the crease recovery of the fabric. In the same setting temperature, the higher the cooling temperature corresponds to the larger crease recovery angle, the slower the cooling, the better the crease recovery of the fabric.

The relationship between the temperature of the drying room of the shaping machine and the temperature of the fabric surface

The temperature specified in the heat setting process is usually the temperature actually reached by the fabric matrix, and it is the most important factor in ensuring 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 sizing machine actually only indicates the temperature value reached in the drying room of the sizing machine and does not indicate the actual temperature reached by the fabric body, thus there is a difference between the temperature of the drying room of the machine and the temperature of the fabric body. In the heat setting process, the drying room temperature can generally be controlled fixed, while the main body of the fabric temperature with the fabric fiber, tissue structure, running speed, and other factors are different and changing. As the actual temperature of the fabric surface is difficult to test and show in the actual production, so the process of shaping temperature is often replaced by the machine’s drying room temperature, this fixed drying room temperature and the change in the surface temperature of the fabric body differences, will bring a certain impact on the quality of heat setting. At a certain drying room temperature, the fabric body also needs a warming process, the heating rate is also subject to fabric varieties and tissue structure, running speed and other factors, often the actual temperature of the surface of the fabric body is lower than the temperature of the machine drying room, which is obviously unable to obtain a good stereotypical effect. In order to reasonably solve this problem, usually can take the following measures.

① reduce the moisture content of the fabric, practice shows that the moisture content of the fabric before shaping is controlled at less than 10% is appropriate.

② infrared pre-drying before shaping.

③ improve the drying room temperature, general drying room temperature can be controlled at about 200 ℃.

④ Reasonable control of shaping machine drying room temperature gradient. For the reasonable distribution of the temperature of the heating area before and after the shaping machine, the formation of a temperature gradient. In the temperature control mode generally have before low after high, before high after low, or before and after the same three ways. From the point of view of improving the surface temperature of the fabric body, reduce the fabric heating time, the front low and then high will extend the fabric heating time, while the front high and then low will reduce the fabric high temperature shaping time, so the use of consistent temperature distribution before and after the way more reasonable.

⑤ Reasonable control of speed.

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2.Time

Fabric in the heat setting process, the entire processing time can be divided into the following parts.
(1) heating time: fabric into the shaping machine, the fabric surface is heated to the shaping temperature of the time required.

(2) heat penetration time: heat penetration from the fabric surface to the fabric fiber interior, so that all parts of the fabric fiber are up to the same shaping temperature required time.

(3) fiber macromolecule rearrangement, adjustment time: the fabric body to reach the shaping temperature, fiber macromolecule according to the shaping conditions for the time required for structural adjustment.

(4) cooling time: fabric out of the shaping machine drying room, was cooled down so that the structure of the time required to fix down. Usually, the shaping time referred to in the process does not include cooling time, and the heating time is seen as shaping preparation time. Therefore, the general control of the setting time tends to focus on the heat penetration time and the adjustment time of macromolecular rearrangement.

The heat penetration time of the fabric (including heating time) is closely related to the heating method of the sizing machine, the type of heat source, fiber thermal conductivity, fabric tissue structure moisture content, and other factors. The gas direct combustion method of heating the shaping machine than indirect heating hot air shaping machine, heat transfer efficiency, heating speed, shaping time is short, the same fiber fabric, in the designated equipment, the thicker the fabric, dense, heavy, the higher the moisture content, the longer the required shaping time. Considering the influence of all aspects, practice shows that heating and penetration time takes about 2 to 15s.

The time required for molecular rearrangement adjustment is a very fast process and can be completed within 1 to 2s. Therefore, it is only necessary to ensure that the fabric is uniformly heated to the required setting temperature, after which the molecular rearrangement and adjustment process is extremely fast and the time required is negligible.

Practice shows that the shaping time is too long, not only to improve the stability of the fabric size no obvious effect but also lead to a decline in the whiteness of the fabric, feel the hard, strong loss. In the same shaping temperature, with the extension of the shaping time, the fabric dry heat shrinkage rate decreases, including the warp downtrend is obvious, while the weft to reach a certain time, the shrinkage rate decline is not obvious or even unchanged. General fabric shaping time control in 20 ~ 30s, has been able to achieve the purpose of stabilizing the size, reducing the thermal shrinkage rate.

Fabric by heat treatment, cooling, and curing rate should be moderate, if the cooling time is too short or not enough cooling, easy to cause further deformation of the fabric. The cooling rate is too fast, will produce internal stress, so that the fabric becomes easy to wrinkle, lack body and bone; if the cooling rate is too slow production efficiency is low.

3 Tension

Tension has a certain degree of influence on heat setting quality and product performance indexes (such as heat shrinkage, strength, elongation at break). For synthetic fibers and other thermoplastic fibers, when the fabric in a relaxed state for heat treatment, warp and weft shrinkage rate of up to 5% or more; and in a certain tension on the fabric for heat treatment, due to macromolecular chain elongation along the direction of external force, moving, rearrangement, so that the fiber more dense, higher orientation, once this state is fixed by cooling, fabric shrinkage rate can be significantly reduced, or even to zero, size Stability is fundamentally improved. Therefore, a certain tension is applied to the fabric in the setting process, which helps to improve the setting effect.

Tension under heat setting, need to be in the fabric warp, weft to apply different tension, the size of the tension, depending on product quality requirements, usually in the heat setting process, the warp tension to overfeed rate to express, weft tension to express the amount of fabric stretched. In the sizing equipment, the warp tension is controlled by the mechanical stretching and overfeeding device, and the weft tension is controlled by the needle plate or cloth tongs stenter. When shaping, with the warp overfeed rate increases, the fabric dry heat shrinkage rate decreases, size stability is enhanced, while the weft dry heat shrinkage rate increases with the door width stretching amplitude and increases, size stability with the weft tension increases and decreases. The changes of fabric breaking elongation in warp and weft direction after shaping are different: the weft breaking elongation decreases with the increase of stretching amplitude, while the warp breaking strength becomes larger with the increase of overfeeding rate. Therefore, in order to better improve the fabric’s performance and dimensional stability, the heat-setting treatment should reasonably control the fabric warp overfeed rate and weft stretching amplitude, that is, the tension applied to the fabric warp and weft direction coordination in an appropriate range.


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Business Director at Shanghai Fumao Clothing Co.,LTD

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