1. Principle of wrinkle formation
Cotton fiber is composed of cellulose macromolecules linked by β-2D glucose residues through 1,4 glycoside bonds. Each glucose ring in the cellulose molecule retains three free hydroxyl groups that can form hydrogen bonds. When the fiber is subjected to external force, the molecular chains are arranged neatly in the crystalline area with high regularity, forming more hydrogen bonds, and can jointly withstand the external force. Therefore, under the action of external forces that do not exceed the elastic limit, generally only small reversible deformation occurs, that is, ordinary elastic deformation. In the amorphous region with low regularity, most of the hydroxyl groups are in a free state and fewer hydrogen bonds are formed. When subjected to external force during washing or wearing, the cellulose molecules undergo a certain deformation along the direction of the external force. The basic structural unit Relatively slipping, the hydroxyl groups will generate new hydrogen bonds and rearrange themselves in new positions. When the external force is removed, the system creeps back. If the resistance generated by the newly formed hydrogen bonds is greater than the restoring force, the system deformation cannot be restored, and permanent deformation occurs. Due to the diversity of hydrogen bond arrangements, various morphological changes occur. The macroscopic manifestation of this uneven and irreversible deformation is the wrinkles of the fabric.
In addition, when cotton fibers swell in water, the cross-sectional area increases by more than 40%, while the length does not change much. Due to the interweaving of warp and weft yarns, uneven shrinkage and uneven recovery during drying are also one of the reasons for the formation of wrinkles.
2. Anti-wrinkle mechanism
There are two main mechanisms for anti-wrinkle finishing of fabrics: covalent cross-linking and resin fixation.
Covalent cross-linking uses a multifunctional compound containing two or more functional groups as a cross-linking agent, which reacts with the hydroxyl groups on adjacent molecular chains in the fiber to form a bridged covalent bond. The cross-linking reaction often requires a catalyst. Resin fixation coats the polymer material around the cellulose molecules or deposits it between the fiber molecules, thereby limiting the deformation of the molecular chains or the relative slippage between the molecular chains. In this way, when the system is subjected to external forces, covalent cross-linking or resin fixation not only reduces the deformation of cellulose macromolecules, but more importantly stabilizes the original hydrogen bond structure and reduces the possibility of forming new hydrogen bonds. properties, improving the ability to recover from deformation, thereby achieving the purpose of anti-wrinkle.
It can be seen from the mechanism of anti-wrinkle finishing that if the textile has formed a certain shape during the anti-wrinkle finishing operation, it will tend to maintain that shape after anti-wrinkle finishing, such as trouser lines and skirt pleats. Therefore, the method and mechanism of durable press finishing are similar to those of anti-wrinkle finishing.
If the cellulose is fixed in a swollen state, the cross-linked fibers will be in a relaxed state after drying, so wrinkles can still appear. However, when exposed to water, the fibers can only reach the original fixed state when they are swollen and the wrinkles disappear. This property is called “wet wrinkle resistance”.
It is worth noting that cross-linking of cellulose will not only worsen the feel, but also reduce the tensile strength. This is because cross-linking limits the movement and internal rotation of cellulose molecules, which can easily cause stress concentration and breakage under the action of external forces. Of course, anti-wrinkle finishing, especially acid-catalyzed systems, will also cause the strength of textiles to decrease.
In addition, reducing the force between fiber molecules and increasing the smoothness and softness of the fiber can also play an anti-wrinkle role. The main anti-wrinkle mechanism of silicone and liquid ammonia treatment is this effect.
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