How is the plasticity of wool?

　During processing, wool is often subjected to various external forces such as stretching, crimping, and twisting, which cause the fibers to change their original shape. Because wool has excellent elasticity, it tends to return to its original form, thereby generating various internal stresses within the fibers. These internal stresses gradually diminish and eventually disappear over a relatively long period of time. This phenomenon often poses difficulties in the processing of wool products and is one of the factors contributing to dimensional and shape instability during both processing and use. The plasticity of wool refers to its ability, under moist and warm conditions, to rapidly dissipate internal stresses and to adopt new shapes in response to external forces; after cooling or drying, these new shapes can be permanently retained.
　　The plasticity of wool is closely related to the conformational changes in its polypeptide chains as well as the breaking and reformation of secondary bonds among these chains. When wool fibers subjected to tensile stress are treated briefly in hot water or steam, and then the external force is removed while allowing the fibers to contract in steam, they can shrink to a length shorter than their original length. This phenomenon is known as "over-shrinking." The underlying reason for this phenomenon is that the combined effects of external tension and moisture and heat cause the conformation of the polypeptide chains to change, leading to the disruption of existing secondary bonds. However, since the treatment duration is relatively short, new secondary bonds have not yet had sufficient time to form at the newly altered positions, allowing the polypeptide chains to freely contract and thus resulting in over-shrinking. If wool fibers under tensile stress are treated for a slightly longer period in hot water or steam, and then the external force is removed, the fibers will no longer fully return to their original length. Yet, when subjected to even higher temperatures, these fibers can still contract—a phenomenon referred to as "temporary setting." The cause of this temporary setting lies in the fact that although the secondary bonds have been disrupted, not all of them have yet been fully reestablished at their new positions, leaving the bonds insufficiently stable. As a result, the fibers can only achieve temporary stabilization and remain capable of shrinking again under appropriate conditions. If elongated wool fibers are treated for an even longer period—say, 1 to 2 hours—in hot water or steam, after the external force is removed and even subsequent steam treatments fail to induce any further significant contraction. This phenomenon is termed "permanent setting." The reason for permanent setting is that, due to the prolonged treatment, once the secondary bonds have been disrupted, new, stable secondary bonds have already formed at the altered positions, thereby stabilizing the conformation of the polypeptide chains. Consequently, the fibers become permanently set and unable to revert to their original shape after being deformed.
　　The shaping of wool fabrics leverages the plasticity of wool fibers: under specific conditions of temperature, humidity, and external force applied for a certain duration, the fabric undergoes treatment that causes the breakage and reformation of secondary bonds between fiber chains, thereby achieving stable dimensions and shape. During the dyeing and finishing processes, operations such as fulling, steaming, applying voltage, and width-setting drying all contribute to the shaping of wool fabrics. Whether these shaping effects are "temporary" or "permanent" depends on the specific conditions and outcomes of the shaping process; there is no clear-cut boundary between the two.
　　Ironing woolen garments also takes advantage of wool’s plasticity: under the combined effects of moisture, heat, and pressure, the garments become smooth and wrinkle-free, and the creases formed can last for a relatively long time.