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.
Heat setting
Heat setting means to set the fabric in the appropriate A process in which a certain size is maintained under 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.
The entire heat setting process can be divided into four stages:
①Heating stage: dry or wet fabrics enter the heat In the setting machine, the surface of the fabric is heated to the setting temperature.
②Thermal equilibrium stage: The heat energy penetrates into the fiber, causing the surface and interior of the fiber to reach the same setting temperature.
③ Transformation and molecular adjustment stage: The fiber is under stress. When the setting temperature reaches the setting temperature, the weaker subvalent cross-bonding in the fiber structure is destroyed, and the fiber molecular chain Reorient the arrangement.
④ Cooling stage: The fabric is rapidly cooled before leaving the tenter setting machine, so that the shape of the fabric is fixed according to the new arrangement of fiber molecules.
Heat setting mechanism
Synthetic fibers They are all thermoplastic, but below the glass transition temperature, the fiber macromolecular chains are in a frozen state, and they 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.
Thermal setting process
The fabric is carried out During heat setting processing, it is usually completed by applying a certain tension to the fabric in a dry state, maintaining a certain size, and processing it in a high temperature environment for a certain period of time. In order to enhance the setting effect and effect and reduce the setting temperature, water can also be used as a plasticizer, and heat treatment is performed when the fabric contains water or in water. Therefore, the heat setting process can be divided into dry heat setting and wet heat according to whether it contains plasticizer water. There are two basic techniques for shaping.
1 Dry heat setting process
The dry heat setting process is to heat-treat the fabric in a dry state without water. Usually hot air heating or infrared radiation heating is used.
Hot air tenter setting machine for shaping
Generally, a pin plate tenter is used. The setting temperature and fabric width can be freely controlled, and the setting effect is relatively satisfactory. It is currently the most commonly used setting machine in printing and dyeing factories. It consists of a cloth feeding device, an overfeeding device, an edge detector, and a stretching machine. It consists of equipment, heating and 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 fibers in the fabric, and has a great impact on the type of synthetic fiber blends and the use of the fabric.
△Setting temperature of different synthetic fiber fabrics
The setting time depends on the fabric reaching heat transfer The time required for the temperature of the medium 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 relatively large safety factors. However, if the setting time is too long, not only is there no benefit, but the color of the fabric will turn yellow and the feel will become hard.
Infrared ray shaping
The heating substance emits Long-wave radiation, the wavelength range is from the red end of the visible spectrum (0.73m) to about 1mm, which is infrared. The infrared spectrum is generally divided into three areas, namely the near-infrared area with a wavelength of 0.72~1.5m, the mid-infrared area of 1.5~20m and the far-infrared area of 20~1000m. In the past, some people have experimented with the mid-infrared radiation with a wavelength of 1~4m. On polyester, it was found that infrared rays with a wavelength of 1~2m can freely pass through the fiber, while radiation rays of 3~3.5m are actually all absorbed by the fiber; therefore, although infrared rays with a shorter wavelength penetrate into the fiber, no energy absorption occurs. Longer wavelength radiation gives up all its energy as 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. The maximum radiation wave emitted is exactly within the same range of the average absorption zone of the material being processed. This radiation source is called a “specially selected emitter”. “. The wavelength of the radiation source must be compatible with the absorption of the material being processed. If the wavelength is properly selected, the radiation will be within the fiber material.The working time can be divided into the following parts:
(1) Heating time: After the fabric enters the setting machine, the time required for the surface of the fabric to be heated to the setting temperature.
(2) Heat penetration time: The time required for heat to penetrate from the surface of the fabric into the interior 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 fiber macromolecules to adjust the structure according to the setting conditions.
(4) Cooling time: After the fabric comes out of the drying chamber of the setting machine, it is cooled down to fix the structural shape. 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.
3 Tension
Tension affects 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
