Properties And Principle Of Mercerizing Fiber

　　 Cotton fibre Changes after the action of concentrated alkali. The fiber diameter increases and becomes round, the longitudinal natural distortion rate changes (80% → 14.5%), the cross section changes from waist to ellipse, or even round, and the cell cavity shrinks to a point. If appropriate tension is applied, the fiber roundness increases, the original wrinkles on the surface disappear, and the surface smoothness and optical performance are improved (the reflection of light changes from diffuse reflection to more directional reflection), The intensity of reflected light is increased, and the fabric shows silky luster. The change of fiber morphology in the fabric is the main reason for producing luster, and tension is the main factor for improving luster.

Microstructure. The crystallinity ↓ (70% → 50%) and the amorphous area ↑ make the hydroxyl groups that were not accessible in water become accessible, so the adsorption and chemical reaction properties of the fiber to dyestuff are improved. In addition, due to the change of fiber morphology after mercerizing, the light scattering on the surface and inside is reduced, so the dyeing depth is also increased when dyeing with dyes of the same concentration. After the fiber swells, the hydrogen bond between macromolecules is broken. Under the effect of tension, the macromolecules tend to be arranged neatly, which improves the orientation degree. At the same time, the uneven deformation of the fiber surface is eliminated, reducing the weak links. The fiber can share the external force evenly, thus reducing the fracture phenomenon caused by stress concentration. In addition, the expanded and rearranged fibers cling to each other and have cohesive force, which also reduces the factor of fracture caused by macromolecular slippage.

Changes in molecular structure. After the cotton fiber swells in the concentrated alkali solution, the hydrogen bonds between the macromolecular chains are broken down, relieving the internal stress stored in the fabric. Through stretching, the macromolecules are oriented and arranged, and new molecular bonds are established at new positions, and the intermolecular force is greater than before the swelling. Finally, under tension, the hydrogen bond between the aligned fibers is fixed (in a more natural and stable state). At this time, the fibers are in a lower energy state, so the size is stable.

Mercerizing principle, Mercerizing It is a complex process, and there are two theories to explain the reason why cotton fiber swells violently in concentrated lye. Hydration theory. Caustic soda reacts with natural cellulose (cellulose) to produce alkali cellulose, which mainly includes two types: alcohol compound: molecular compound (addition compound): both products are unstable and hydrolyzed into hydrated cellulose after washing, After dehydration and drying, it becomes mercerized cellulose (cellulose II). The change of cellulose in the whole process is shown as follows: the reason why cotton fiber undergoes severe irreversible swelling after being treated with concentrated NaOH is that sodium ions are small in volume and can enter the crystal zone of fiber; At the same time, Na is an ion with strong hydration ability. There are as many as 66 water molecules around a Na, which forms a hydration layer. When Na enters into the fiber and combines with fiber, a large amount of water is also brought in, causing severe swelling. Because it can enter the crystal region, swelling is irreversible. This swelling is affected by temperature: swelling is also affected by NaOH concentration and neutral salt.

According to Donnen's membrane equilibrium theory, swelling is the result of osmotic pressure. Assuming that cellulose is a weak base, sodium salt can be formed in caustic soda solution, cellulose Ionization of sodium salt generates immovable cellulose anion Cell-O?, There are still movable Na and OH - in the solution. If there is NaCl, there is also Cl?, The fiber surface is regarded as having a semi permeable membrane property. These ions are distributed according to certain conditions. When equilibrium is reached, the inside and outside of the membrane must be electrically neutralized.