Heat setting
Heatsetting (including heat-setting ) is a term used in the textile industry and refers to the thermal process, which usually runs in a steam atmosphere or dry heat, to produce fibers , yarns and fabrics dimensional stability and other desirable properties such. B. greater volume, resistance to wrinkles or increased temperature resistance to give.
target
Heatsetting is very often used to give the yarns better properties for subsequent processes. After being freshly spun, cabled or twisted, yarns often tend to have an increased tendency to "curl". The heatset process can influence or eliminate this property. Particularly in the processing stages of winding, twisting , weaving , tufting and knitting , a strong tendency to curl can lead to processing difficulties. In the case of yarns for the carpet industry , in addition to reducing the tendency to curl , one would also like to stabilize or fix the fiber composite. This applies to both staple fibers and continuous fibers (BCF). With synthetic fibers, there is also the effect that the yarn gains volume. One speaks of an increase in the bulge or bulge. In addition, there is a not insignificant amount of effect yarns, among other things for the carpet industry, z. B. for the production of Frieze carpets , Shaggy carpets, Trackles carpets or texture carpets, which are subjected to the heatset process. These yarns are usually deformed into arcs, kinks and curls by mechanical texturing . Usually this is in a so-called stuffer box (Engl. Stuffing box , twinrollbox ). In order to permanently stabilize these mechanical deformations, a heatset process follows after the shaping. All processes with which one would like to give a textile material one of the aforementioned properties with the help of temperature and / or humidity are called heatset, heatsetting or set processes. The term thermosetting is less common . In the carpet industry in particular, one speaks exclusively of heatsetting.
Causes of the curl tendency
The tendency to curl, i.e. the reason for the disintegration of the fiber structure in cut yarns (e.g. cutpile carpets), is based on the technological conditions of the production of webs and the physical fiber properties. The thread torque is primarily understood under the technological conditions of web production. The aim of a twisted thread is to twist itself together when it is hung freely between two fixed points in the form of a loop. He gives off a part of his turns, which show up in turns whose direction of rotation is opposite to the original direction for reasons of balance. The reason for this is the tension caused by the twist of the yarn, which Müller stated in the known distribution. As the rotation increases, the total tension counteracting the rotation increases as a result of the increasing tensile and compressive stress on the fibers in the yarn package. It can become so large that the thread core can no longer cope with the compressive stresses that occur and buckles. The yarn curls d. H. By making a number of twists in the opposite direction - which are also known as negative twists - the yarn package strives for a state of equilibrium in which the internal torsional stresses cancel each other out. The thread always kinks at a point where there is a small cross-section due to the unevenness. This has absorbed a greater number of turns in the spinning process and is therefore subject to higher internal tensions that ultimately break the core of the thread. Although stronger yarns are less twisted than finer ones, the internal tension increases in the opposite direction to the yarn count. The heatsetting weakens this tension the more the yarn count is coarser. Another task of the heat setting, in addition to reducing the crimp ability, is to simultaneously fix the physical properties of strength and elongation transferred to the yarn by the twist.
Chemical processes
Depending on the yarn material you are dealing with, completely different processes take place. The steaming of woolen yarns has been studied most closely, while the steaming of synthetic fibers and cotton has been less studied.
Wool
After the steam has entered, the moisture content of the yarns increases suddenly as a result of yarn heating and steam condensation. According to Speakmann, the following phenomena are triggered in the stretched wool fiber : The cystine side chains are subjected to hydrolysis at the sulfur bridge, the cystine being converted into cysteine and a not yet isolated sulfonic acid. '
1. Hydrogen bond between peptide groups (polar bond)
2. Cystine bridge (covalent bond)
3. Salt bridge between two amino acids (ionic bond)
4. Hydrophobic interaction between a valine and an isolyein residue (apolar bond)
The dashed ellipse represents the area from which water is displaced by the hydrophobic interaction.
Ionization takes place at the bridges created by salt formation. As a result of the temperature increase in the fibers during steaming, the molecules vibrate, causing the hydrogen bonds to burst; this exposes residual valences that are able to saturate with the dipole water. The water then acts as a kind of lubricant between the individual molecules. The side chains break the bonds between the main chains, the individual polypeptide chains can shift against each other and the tensions are balanced. As the yarn continues to steam, new side chains are formed between the individual components of the main chain. During the subsequent drying of the yarn, i. In other words, the moisture balance that takes place within the yarn again results in salt formation and the formation of hydrogen bonds. The individual polypeptide chains can then no longer be shifted relative to one another and the fibers have regained their old strength without, however, exhibiting greater internal tensions. The twine or twist is fixed. The morphological structure of the fibers must of course also be taken into account when compensating for tension through damping. As the temperature for breaking the hydrogen bonds and the water vapor for hydrolysis for the cystine bridges are available to the wool fiber very quickly, the twist can be reduced relatively quickly, which corresponds approximately to the values of an autoclave-calmed yarn; The steaming quality of the Sewimatic steaming process is significantly better in terms of the evenness of moisture absorption.
Synthetic fiber
In the case of synthetic fibers , we distinguish between two fiber areas, the crystalline (ordered) area and the amorphous (disordered) area. In crystalline fiber areas, physical forces of attraction act between the closely parallel polymers. These forces acting across the fiber axis make up the strength of a fiber. If tension is applied to the fiber, these forces prevent the fiber from tearing. The amorphous fiber areas, on the other hand, act as joints of the fibers. They are responsible for the flexural strength of the fiber. In addition, the amorphous fiber areas allow z. B. the penetration of water or dye.
What happens now during the steaming process. As the fiber heats up, its molecules begin to vibrate. The increase in vibration, which can be influenced by the level and duration of the heating, cancels the electrical binding forces in the fiber; first in the amorphous areas, later in the crystalline areas and lastly in the polymers. As with wool, the tensions introduced by the spinning process are now released. When the fiber dries or cools down, the binding forces build up again without any internal tension.
| Type of fiber areas |
cause |
|---|---|
| crystalline | → tensile strength → elasticity |
| amorphous | → flexibility → water absorption → dyeability |
The problem with synthetic fibers is that the decrease in binding forces only occurs between the so-called glass transition temperature - the beginning of the transformation of the solid (solidified) amorphous fiber areas into a viscoelastic one; easily deformable state - and the softening point (the crystalline fiber areas also change to the viscoelastic state) takes place and this is in a relatively high temperature range for synthetic fibers.
| material | Glass transition temperature | Softening temperature |
|---|---|---|
| polyester | 80 ... 85 ° C | 230 ... 240 ° C |
| Polyamide 6 | 80 ... 85 ° C | 180… 200 ° C |
| Polyamide 66 | 90 ... 95 ° C | 220 ... 235 ° C |
| Polypropylene | (−10) −0 ° C | 160 ... 165 ° C |
This fact also explains why wool blends mixed with synthetic fibers are more difficult to calm than pure wool. A calming of synthetic fibers is only possible over a temperature range of 85 to 95 ° C. Pure wool, on the other hand, can be soothed very well at these temperatures.
cotton
Cotton plays a subordinate role with regard to composite steaming, and the exact physical or chemical process in the fiber is not known. Therefore cotton can be neglected in the considerations.
Application in the carpet industry
Especially with yarns for the carpet industry and here again for carpets that are cut ( cut pile ), the reduction of internal tension contributes to a significant increase in quality.
Carpet division
There are two basic shapes for carpets. On the one hand there is the loop pile and on the other hand the cut carpet. The heatset process is particularly important for cut-pile carpets and the associated variants (Saxony, Shag, Frieze ).
Tip definition
When you cut a thread, the ends twist up like a string or rope and a kind of “brush” is created. And that is exactly what you want to avoid with a cut carpet and not have under any circumstances. In addition to a poorer appearance, such a carpet would also have a shorter lifespan and, what is even more interesting, as scientific studies have shown, also physiological disadvantages for the "walker". The carpet is not as elastic and does not cushion the user's step as well as a carpet that has been bedded. A carpet made from thermoset yarn is therefore of higher quality. As a rule, a hatched carpet can be recognized by its grainy structure, in technical jargon one also speaks of a “pinpoint tip definition”, ie an appearance similar to needle points.
Common heatsetting processes
Several heatset processes are known in the textile industry. The most important are presented here.
Autoclave process
The oldest process is the autoclave process . This is mostly a discontinuous process. With the autoclave, a distinction is made between systems that work with vacuum and those that only work with pressure. The textile material is introduced into the autoclave in the form of coils or strands. Since almost all autoclaves are exposed to pressure, they are generally cylindrical in shape and set up in a horizontal position. There are autoclaves that are loaded and unloaded from one side, but also those that are loaded from one side and unloaded from the other. Autoclaves that are set up in a vertical position are less common.
Steamatic process
There are so-called composite dampers for automation in the spin-package composite (English link spinning ). The world's first process was the so-called steamatic process by Resch. In this process the heatsetting process takes place between a ring spinning machine and the winding machine. As soon as the ring spinning machine has finished the spinning process, the finished spinning heads are automatically fed to the compound damper. There the spinning cops are steamed in a vacuum process and dried again in seconds. The bobbins are then fed to the winding machine, where they are then rewound into a cheese.
For the carpet industry are two continuous process application: the Power-Heat-set process (developed from the Süssen -Heatset process) and the TVP method, derived from the autoclave technology.
Power heat set process
The power heat set process, also known as the Süssen process, originated in the early 1970s and was one of the first continuous heat setting processes worldwide. The process was revolutionary in that it was the first to be operated not with saturated steam under pressure, but with a superheated steam-air mixture under atmospheric pressure. Due to the treatment process (overheated steam-air mixture), completely new collections of carpets were created. In the power heat set process, the surface of the fibers or yarns is slightly oxidized by the oxygen in the air and the higher temperatures. This fine oxidation layer later makes the finished carpet less sensitive to dirt. The dirt particles adhere less well to the fibers.
TVP procedure
In addition to the power heat set process, there is also the TVP process, which is also one of the continuous processes. In the TVP process, the yarns are placed on a belt and fed through a lock into a pressure tunnel, which can be up to 15 meters long, where they are subjected to the heat setting process using saturated steam. At the end of the tunnel, the yarn is fed back out of the heatsetting tunnel through a lock. The still hot and moist yarn is then dried and cooled and fed to the winding process. Up to 36 threads can be processed on the system.
Process illustration using the example of the power heat set process
In the power heat set process, superheated steam is used in an open system under atmospheric pressure. All common materials that are used in the carpet industry are processed. These mainly include polyamide 6, polyamide 66, polypropylene , acrylic , PET , polyester and wool .
The untreated yarn is placed in the form of bobbins (up to 48 pieces) in a creel. The yarn is drawn off the bobbins at a speed of up to 700 m / min and fed into the heatset process. There are two basic transport options for the yarn, on the one hand by laying the yarn in a circle or flat like an “8” on a belt and feeding it to the heatset process, or by winding it on ropes that are arranged as a polygon Feed heatset process. In the case of Frieze yarn, only the type of tape transport is selected. Frieze yarns are produced with the help of a special storage chamber in the Twinrollbox . The heatset process is carried out at temperatures of 110 ° C to 200 ° C in a steam-air mixture. After the heatset process, the yarn is cooled down again and wound into bobbins with the help of a winding machine. The power heat set process usually consists of 6 working lines with 8 threads each (ends). With this process a daily production of approx. 10.5 tons can be achieved.
literature
- Uniform effects when steaming yarn. In: textile company. 1981, p. 29.
- H.-J. Henning, Cl. Sustmann: Investigations into the vacuum steaming of wool yarns. In: Melliand textile reports. 1966, p. 530
- Jens Holm Dittrich, Paul Naefe, Johann Kreitz: Method for reducing the twist of wool yarns by means of short-term steaming. In: Melliand textile reports. 1986, p. 817
- Jens Holm Dittrich, Attila Bereck, Günter Blanckenburg: Investigations into the yellowing of woolen yarns during steaming '. In: Melliand textile reports. 1983
- Jens Holm Dittrich, Gesine Töpert: Causes of the yellowing of sliver bumps and packages during HF drying. In: Melliand textile reports. 1988, p. 288.
- Oskar Becker: Tension threads in wool yarn. In: Melliand textile reports. 1977, p. 97
- H. Kranz GmbH & Co .: Process for fixing yarns. Patent application DP 3601099.5.
- W. Schefer: Change of wool through heat treatments in the finishing area. Federal Materials Testing and Research Institute, St. Gallen.
- Hans Erich Schiecke: Wool as a textile raw material. Schiele & Schön publishing house.
- K. Kröll: drying technology. Volume II / I, Springer Verlag, Heidelberg / Berlin / New York.
- Peter Toggweiler, Simon Gleich, Freddy Wanger, F. Steiner: Improving the quality of cotton yarns conditioned with Contexxor. In: Melliand textile reports. No. 9, 1995.