The heat setting process of polyester and nylon fabrics has long been known. The main purpose of heat setting is to improve the thermal dimensional stability of the fabric.
01 The importance of polyester heat setting
On the surface, heat setting processing is not complicated, but in fact, the supramolecular structure of the fiber undergoes profound changes during heat setting, so it will inevitably have a corresponding impact on the mechanical and physical properties and dyeing properties of the fabric. Since it involves the problem of fiber supramolecular structure, high-tech methods and theories must be used to evaluate the heat setting effect and the effects of various influencing factors to achieve the desired purpose.
Although spandex only accounts for a minority of elastic fabrics containing spandex, heat setting is also required in order to ensure that the finished product has good thermal dimensional stability. On the other hand, due to the difficulty of heat-setting technology for spandex-containing elastic fabrics, some companies omit some fabrics that must be heat-set in order to reduce costs, which makes users very embarrassed. For example, some fabrics that are elasticized with spandex Cotton jeans will shrink every time they are washed. After many washes, the legs will just wrap around the legs, which is neither good-looking nor comfortable.
Another example is that after spandex is added to some acrylic shirts, they can only be washed at low temperatures. Once washed in hot water, an acrylic shirt worn by adults can suddenly become suitable for children. For some elastic fabrics containing spandex, the production sequence of the heat setting processing arrangement will have a great impact on the size and specification of the fabric.
Elastic fabrics containing spandex often contain three or more fibers. The different requirements of various fibers must be taken into account during the dyeing and finishing of such fabrics. These add to the complex factors of heat setting processing, such as Wool and silk are not suitable for high-temperature heat-setting processing. In contrast, polyester requires heat-setting at higher temperatures. Some fabrics with special styles, such as the high elasticity of spandex, are planned to use the high resilience of spandex to give the fabric a three-dimensional relief effect (similar to the seersucker effect). Therefore, heat setting processing is a very critical process and must be controlled carefully.
In order to lay the foundation for an in-depth discussion of the heat setting mechanism of spandex-containing elastic fabrics, the basic theory of heat setting is first explained below.
The principle of polyester heat setting: The “minimum energy principle” in general physics can be used to describe the principle of polyester heat setting. Polyester and its textiles (yarns and fabrics, etc.) undergo some degree of change due to mechanical effects such as traction, stretching, and twisting during the textile, dyeing and finishing processes. This deformation occurs due to the glass transition temperature of polyester ( Tg=81℃), so the retained deformation is forced high elastic deformation, which cannot be completely restored even after the external force is removed. The deformation remains on the fiber, which means that the corresponding internal stress is stored in the fiber.
The fiber is placed at a higher energy level and is therefore unstable. During heat setting processing, the fiber is heated to between 170-210°C, which is much higher than Tg. Therefore, internal rotation of macromolecules is easy to occur, and the result of molecular motion must spontaneously tend to the lowest energy state. This process of maintaining size and releasing the energy inside the polyester molecules is described by the term “stress relaxation” in polymer science.
However, when the temperature is above 120℃, another phenomenon will occur in polyester, namely the “crystallization process”. That is to say, the crystallization of polyester will increase, and the proportion of amorphous areas will decrease. Since crystallization is an exothermic process, and the amount of latent heat of crystallization is relatively large, the energy in the polyester is greatly reduced, putting the entire system in a new lowest energy state. It can be seen that there are two reasons for polyester heat setting: polyester reduces the energy in the system and puts them in the lowest energy state. Accordingly, the energy reduction of polyester during the heat setting process can be expressed succinctly by the following formula: crystallization process + stress relaxation process = energy reduction process of polyester.
In order to deepen our understanding of the above statement, let us now discuss the situation of other fibers. For example, as long as cotton, wool, silk and other fibers are stretched and deformed during processing, and if there is no plastic deformation during the deformation, their general elasticity will immediately recover after the external force is removed, and high elasticity will recover quickly. The deformation is also recovered quickly, but the high elastic deformation with a slow recovery speed is often difficult to recover. Even part of the slow elastic deformation is difficult to recover. The most obvious examples are “wrinkles”, “indentations”, etc. that are difficult to automatically eliminate. .
“Wrinkles” and “indentations” are the visual expressions of high elastic deformation and slow elastic deformation with slow recovery. The way to solve this problem in the dyeing factory is to subject the fabric to moist heat treatment (such as fixed-width processing). With the help of moisture and heat, the internal rotation of macromolecules intensifies, and “stress relaxation” is quickly achieved. From a macro perspective, “stress relaxation” is achieved. “Wrinkles” and “indentations” are eliminated.
So why are wool, silk, cotton, viscose and other fibers not heat-set like polyester? Because they do not undergo the above-mentioned crystallization process at high temperatures, heat setting them is superfluous, and high temperatures will also cause thermal damage to such fibers, so steam setting is sufficient for such fibers.
On the other hand, because such fibers do not crystallize but only undergo stress relaxation, they are prone to “wrinkles” and “indentations” during use. If we only perform steaming and length-setting processing of polyester fiber, ��It only undergoes a stress relaxation process. If no crystallization process occurs, its shape retention will definitely not be as good as heat setting processing.
Therefore, when polyester/cotton fabrics are heat-set, crystallization and stress relaxation occur in the polyester, while only stress relaxation occurs in the cotton fiber. If the stress relaxation and crystallization processes are examined from a microscopic perspective, heat setting processing can be divided into the following three processes:
1. When the fabric is heat-set, the fabric is first heated in the box. As the temperature rises, especially after it exceeds the tg of polyester (81°C), the macromolecular segments in the amorphous region of polyester undergo increasingly violent internal rotation. , that is, the active energy of the macromolecular chain segment overcomes the Van der Waals force between macromolecules, and the internal rotation makes the energy in the system tend to the lowest state, that is, all the energy accumulated in the polyester due to textile processing, dyeing and finishing is released. , achieving stress relaxation.
2. In the new stable state, once the molecules or chain segments stabilize, the intermolecular forces begin to rebuild, such as hydrogen bonding forces, van der Waals forces, etc., and then some chain segments enter the crystal lattice and start the crystallization process. , and correspondingly releases the latent heat of crystallization, further reducing the energy of the system and entering a newer low-energy state.
3. When the fabric leaves the heat-setting machine box, the temperature returns to room temperature. Since it is lower than the glass transition temperature of polyester, the macromolecular chain segments are rotated and frozen within, so that the heat-setting effect is maintained.
According to this principle, after the polyester fabric leaves the heating zone of the setting machine, it should be immediately cooled down to below Tg in a flat state, such as using a cold water roller or blowing cold air. Otherwise, if the fabric enters the fabric stacking box above Tg, the fabric will wrinkle again, which means that a new state has begun, forcing the polyester molecules to re-carry out the above three processes in this state, and will cause this wrinkle. The wrinkled state remains, destroying the smoothness of the fabric.
Before the fabric enters the heating zone of the setting machine, overfeeding the fabric will help the polyester release the energy stored inside the fiber and enable the fabric to obtain the required width-count warp and weft density.
Polyester is a thermoplastic fiber. During a series of processing processes such as dyeing, due to multiple mechanical actions and multiple stretches, the original width and coil geometry of the fabric are changed, resulting in deformation and shrinkage, and even horizontal distortion. The strands are skewed, seriously affecting the quality of the product. The main purpose of heat setting is to heat the polyester knitted fabric under tension and bake the fabric at a specified temperature to intensify the thermal motion of the secondary bonds between fiber molecules and molecular chain segments, so that the molecules can be reorganized and arranged. Internal stress is relatively stable. Therefore, the dimensional stability of the fabric can be improved, the anti-wrinkle performance can be enhanced, and the surface finish, strength, anti-pilling and other properties of the fabric can also be improved to a certain extent.
02 Determination of the shaping process of polyester knitted fabrics
The key to the heat setting process of polyester knitted fabrics is to control the setting temperature and setting time. If the heat setting temperature is too low or the time is too short, it can cause defects such as uneven surface, stiffness, and width shrinkage of the fabric, and lose the setting effect; such as If the setting temperature is too high or the time is too long, the fabric will become hard and brittle, the strength will decrease, the elasticity will decrease, and some disperse dyes will sublime, resulting in color difference. In severe cases, the fibers may even melt.
Practice has proven that the appropriate setting temperature for polyester knitted fabrics is 180-210°C, the setting time is 20-90s, and the cooling temperature is about 50°C. And it can be calculated according to the following formula (calculated according to different shaping machine structures):
Setting time (s)=setting length (m)/fabric linear speed (m/min)×60
▊When the setting temperature of various types of polyester knitted fabrics on the Z921 needle plate tenter setting machine is 185-190℃, the linear speed is: polyester warp-knitted mosquito net fabric 10-15m/min, polyester warp-knitted shirt fabric 12-18m/min /min, polyester warp, weft and knitted outerwear fabrics 12-20m/min, polyester yarn-dyed jacquard fabrics 7-10m/min.
In addition, the “tension” and “overfeed” of the fabric should not be ignored when setting the shape. They should cooperate with each other appropriately. If the width of the tenter exceeds the width of the fabric, it will cause an increase in shrinkage and a decrease in strength; if the overfeed is too large or the tension is unequal, it will easily cause weft wavy shapes (commonly known as “fungus edges”) ), otherwise stripes will be produced in the meridional direction. In short, process conditions such as width, tension, and overfeeding must be strictly controlled during the operation.
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