Heatsetting

Heat setting is a term used in the textile industry to describe a thermal process usually taking place in either a steam atmosphere or a dry heat environment. The effect of the process gives fibers, yarns or fabric dimensional stability and, very often, other desirable attributes like higher volume, wrinkle resistance or temperature resistance. Very often, heat setting is also used to improve attributes for subsequent processes.

Heat setting can eliminate the tendency of undesirable torquing. At the winding, twisting, weaving, tufting and knitting processes, the increased tendency to torquing can cause difficulties in processing the yarn. When using heat setting for carpet yarns, desirable results include not only the diminishing of torquing but also the stabilization or fixing of the fiber thread. Both twist stabilization and stabilization of frieze effect are results of the heat setting process. Heat setting benefits staple yarns as well as bulked continuous filament (BCF) yarns. Heat setting often causes synthetic fibers to gain volume as well. This volume growth is commonly described as "bulk development". All processes using temperature and/or moisture to give textiles one of the above-mentioned attributes are known as heat setting. The term "thermal fixation" is used less frequently. In the carpet industry, the process is exclusively called "heat setting".

The crinkle tendency is due to the technological conditions of the spun yarn production and the physical fiber properties. Above all, the "technological conditions of the spun yarn production" means the turning moment of the thread. A twisted thread will always try to twist when it hangs freely between two fixed points in the form of a loop. In doing this, it gives up a part of its original twist which becomes spirals whose twisting direction is opposite to the original twist direction. This development of twist in the opposite direction occurs as the twisted yarn attempts to reach equilibrium.

Twisting in the opposite direction is due to the tensions resulting from the yarn twisting that Mueller indicated in the diagram of tension and pressure. The total tension acting against the twisting is increased in relation to increased twisting due to the increasing tension and pressure of the bundle of fibres in the yarn. It may become so strong that the thread core buckles when it can no longer withstand the compressive strains. The yarn curls, meaning that the yarn tries to reach a state of equilibrium in which twists in the opposite direction from the original twist direction balance the yarn's torque. These twists are also called negative twists. In this state of equilibrium, the inner torsional tensions cancel each other out. The thread always buckles at a spot where the cross section is small due to the unevenness of the thread. During the spinning process this spot took up more twists and is therefore subjected to higher inner tensions, which ultimately break the thread core. Although thicker yarns are less twisted than fine ones, the inner tension rises opposite to the yarn size. Smaller yarn is more weakened by steaming. Further positive aspects of steaming are the reduction of curling and, at the same time, the setting of the physical properties of closeness and extension imparted to the yarn by twisting.

There are completely different behaviors depending on the kind of yarn material. Much is known about the steaming of woolen yarns but more research is needed on the steaming behaviour of artificial fibers and cotton.

As soon as steam enters, the yarns quantity of moisture rises at once, caused by the heating of the yarn and by steam condensation. According to Speakmann the following phenomena can be seen in the stretched woolen fiber: The cystine side chains are subjected to a hydrolysis at the sulphur bridge, where cystine is dissolved into cysteine and a not yet isolated sulphonic acid.

An ionization can be seen at the bridges that were produced from salt liberation. Due to the increase of temperature in the fibers during steaming an oscillation of the molecules is produced which leads to the bursting of the hydrogen bridges; now residual valencies are set free which are able to saturate with the dipole water. The water then acts like a lubrication between the individual molecules. Thus the bonds of the main chains between each other are dissolved by the side chains, the individual polypeptide chains can shift against each other and the tensions find their equilibrium (see illustration 4). When the steaming of the yarn is continued, new side chains are formed between the individual components of the main chains. When finally the yarn is dried, i.e. the moisture balance occurring within the yarn, salt is liberated again and hydrogen bridges are formed. Now the individual polypeptide chains can no longer be shifted against each other and the fibers regained their former closeness, however without having remarkable tensions inside.

The yarn or doubled yarn twist is set. Of course, the morphological structure of the fibers must be considered when equalizing the tensions by steaming. Since the woolen fiber very quickly gets the temperature for breaking up the hydrogen bridges and the steam for hydrolysing the cystine bridges, a relatively quick twist modification is possible which roughly corresponds to the values of an autoclave moderated yarn; however, the steaming quality of the Steamatic steaming process is much better with reference to the evenness of moisture absorption.