During the textile dyeing and finishing process, fabrics are subject to a variety of complex effects (including physical, mechanical, and chemical). This causes changes in the external shape and structural dimensions of the product, and some even lose the shape, appearance and style that the fabric should have, seriously affecting the wearing performance. Therefore, ensuring the stability of the external shape and size of the fabric is an important criterion for measuring product quality. Author: Zeng Linquan
1 Introduction to heat setting
Heat setting is It refers to a process in which the fabric is maintained at a certain size under appropriate tension, heated at a certain temperature for a certain period of time, and then cooled rapidly. Heat setting can eliminate existing wrinkles on the fabric, improve the dimensional stability of the fabric, make it less likely to produce creases that are difficult to remove, and improve the pilling and surface smoothness of the fabric, which has a positive impact on the fabric’s strength, feel and Dyeing performance also has a certain impact.
2 Heat setting mechanism
Synthetic fibers are all thermoplastic, but when the temperature is below the glass transition temperature , the fiber macromolecular chain is in a frozen state, and can only undergo general elastic deformation when subjected to force. When the temperature is greater than the glass transition temperature, the molecular chain segments begin to move, and the fiber is in a highly elastic state. When subjected to force, highly elastic deformation occurs. Since 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 chains are in a viscous flow state and can produce plastic deformation, otherwise they are still in a highly elastic state. When the synthetic fiber is in a high elastic state, applying tension to the fiber causes the molecular chain segments to wriggle and rearrange along the direction of the external force, and establish new intermolecular forces at new positions to maintain tension and cool. The state is fixed to achieve the purpose of finalization.
3 Factors affecting heat setting
When synthetic fibers are heat-set, It is mainly affected by factors such as temperature, time and tension. Reasonable control of these factors is of great significance to obtain good heat setting effects.
Temperature
Thermal shrinkage and mechanical properties of fabrics after heat setting , dyeability, whiteness, etc. are closely related to the setting temperature.
1. The influence of setting temperature on the dyeing properties of fabrics
Acrylic fiber has some problems when dyeing with cationic dyes An obvious feature is that after the dye solution temperature exceeds the glass transition temperature, the dye uptake rate increases rapidly. For example, the dye uptake rate after dyeing at 95°C for 1 minute exceeds the dye uptake rate at 75°C for 3 hours. In addition, the temperature during dyeing exceeds the glass transition temperature, and the fiber is also in the process of moist heat setting. Therefore, in fact, as the wet heat setting temperature of acrylic fabrics increases to a certain extent, the dye uptake rate also increases accordingly.
There is also a certain relationship between the dyeing rate of polyester and its blended fabrics with cotton and viscose fiber and the setting temperature before dyeing. When the fabric is dyed at high temperature and high pressure, the relationship between the setting temperature and the dye uptake rate of the dye on the fiber is a concave curve. When the setting temperature is below 190°C, due to the increase in crystallinity of the fiber, the amount of dye absorbed is reduced, reaching the lowest point at 190°C. When the temperature rises above 190°C, the crystallinity of the fiber continues to increase, and the size of the crystal grains increases. However, the volume of the amorphous zone 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 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 dye absorption increases as the setting temperature increases above 190°C. However, there are also some specific dyes that are not sensitive to heat treatment, and their dye uptake changes in a certain linear relationship, that is, when the setting temperature is high, the dye uptake is low. In the hot-melt dyeing process, the setting temperature also has a certain impact on the dye uptake rate.
The dyeing process of disperse dyes on polyester is essentially a process in which dye molecules continuously diffuse from the dye solution to the surface of the fiber, and then diffuse into the interior of the fiber. When the degree of diffusion reaches a certain equilibrium state, the dyeing rate of the fiber reaches a dynamic equilibrium state. As the heating conditions of the fiber change and the internal molecular structure changes, this dynamic balance loses its original state and changes into a new dynamic balance state accordingly. As the setting temperature increases, the dye uptake rate of disperse dyes on polyester continues to decrease. This is different from the situation during high temperature and high pressure dyeing. It is generally believed that this is due to the fact that 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. Some people also believe that this is because the degree of crystalline folding of polyester molecules continues to increase at high temperatures, and the structure becomes tighter, making it difficult for dye molecules to diffuse into the fiber. In actual production, the dye uptake rate of fabrics varies depending on the specifications of the gray fabric, the blending ratio, and the properties of different temperature disperse dyes.
2. The influence of setting temperature on the thermal shrinkage stability of fabrics
The thermal shrinkage of acrylic and its blended fabrics is different from that of polyester. This is because the heat setting temperature of acrylic fiber is limited by the molecular structure, which is generally 140~160℃ in dry state. It was measured by X-ray diffraction that the crystallinity only increased by 3% after setting, but the integrity of the crystal region was significantly improved. Therefore, acrylic fabric is fixed at 140~160℃ under tension.��The time required for the form to become fixed. Usually the shaping time referred to in the process does not include the cooling time, and the heating time is regarded as the shaping preparation time. Therefore, the general control of setting time often focuses on the heat penetration time and macromolecule rearrangement adjustment time.
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 hot air setting machine, the setting machine heated by gas direct combustion has higher heat transfer efficiency, faster heating speed and shorter setting time. For the same fiber fabric, on the designated equipment, the thicker, denser and heavier the fabric will be, the more moisture it will contain. The higher the rate, the longer the setting time required. Taking all influences into consideration, practice shows that the heating and penetration time takes about 2 to 15 seconds.
The time required for molecular rearrangement and adjustment is a very fast process and can be completed within 1 to 2 seconds. Therefore, it is only necessary to ensure that the fabric is evenly heated to the required setting temperature, and the subsequent molecular rearrangement and adjustment process is extremely fast, and the time required can be ignored.
Practice shows that if the setting time is too long, it will not only have no obvious effect on improving the dimensional stability of the fabric, but will also lead to a decrease in the whiteness of the fabric, a hard feel, and a loss of strength. At the same setting temperature, as the setting time increases, the dry heat shrinkage of the fabric decreases, with an obvious downward trend in the warp direction, but after a certain period of time in the weft direction, the shrinkage decreases not significantly or even remains unchanged. Generally, the fabric setting time is controlled at 20 to 30 seconds, which can achieve the purpose of stabilizing the size and reducing the thermal 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 will easily cause further deformation of the fabric. If the cooling rate is too fast, internal stress will be generated, making the fabric prone to wrinkles and lack of body. If the cooling rate is too slow, the production efficiency will be low.
Tensile
The influence of heat setting quality and product performance indicators (such as thermal shrinkage, strength, Elongation at break) all have a certain degree of influence. For thermoplastic fibers such as synthetic fibers, when the fabric is heat treated in a relaxed state, the shrinkage rate in the warp and weft directions can reach more than 5%; when the fabric is heat treated under a certain tension, due to the elongation of the macromolecular chains in the direction of the external force, Movement and rearrangement make the fibers denser and more oriented. Once this state is fixed by cooling, the fabric shrinkage can be greatly reduced, or even reduced to zero, and the dimensional stability is fundamentally improved. Therefore, applying a certain tension to the fabric during the setting process will help 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 during the heat setting process, the warp The axial tension is expressed by the overfeed rate, and the weft tension is expressed by the fabric tenter amount. On the setting equipment, the warp tension is controlled by mechanical stretching and overfeeding devices, and the weft tension is controlled by needle plates or clip tenter devices. During shaping, as the warp overfeed rate increases, 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 stretching amplitude, and the dimensional stability Decreases with increasing weft tension. After setting, the changes in the breaking elongation of the fabric in the warp and weft directions are different: the weft breaking elongation decreases with the increase of the tensile amplitude, while the warp breaking strength increases with the increase of the overfeed rate. Therefore, in order to better improve the wearing performance and dimensional stability of the fabric, the warp overfeed rate and weft stretch amplitude of the fabric should be reasonably controlled during heat setting treatment, that is, the tension applied to the fabric in the warp and weft directions should be coordinated at an appropriate In the range. </p
