Thread Content
The main component of cotton fibers is cellulose, and cellulose molecules contain hydrophilic groups known as hydroxyl groups. Cellulose contains many tiny pores and gaps, which possess capillary water-absorbing properties; moreover, the surface of cellulose allows free water to adhere to it, which is why cellulose has a high degree of hydrophilicity. Water exists in cellulose in two forms. One type is called bound water. This type of water is bound firmly to cotton fibers through hydrogen bonds; it is closely linked to the normal physiological activities of other substances within the cotton fibers. Together with those other substances, it determines the properties and utility of the cotton fibers. Under normal conditions, it does not evaporate, nor does it react with enzymes or microorganisms. The water in the capillaries and the water adsorbed on the surface is known as free water. The amount of this water in the fibers maintains a dynamic equilibrium with the moisture content in the surrounding air, under certain external temperature and humidity conditions. When the external temperature is high and humidity is low, free water tends to escape from the cotton fibers, reducing their moisture content. When the external temperature is low and humidity is high, moisture from the outside returns to the cotton fibers, increasing their water content; therefore, free water is the main factor that causes changes in the water content of cellulose. The ability of air to hold water vapor is related to temperature; as air is heated, its capacity to hold water vapor increases rapidly. When the temperature rises from 25°C to 100°C, the air’s capacity to hold water vapor becomes 25 times that at 25°C. When cotton fibers with a high water content are placed in hot air with a low relative humidity, the fibers heat up; the free water within the fibers gains energy and becomes more active. Since the external moisture concentration is lower than that inside the cotton fibers, a large amount of water from the cellulose in the fibers quickly escapes into the hot air, causing the fibers to dry out. When cotton fibers with low moisture content come into contact with humid air with high moisture content, due to the hydrophilicity of the cotton fibers and the fact that their own moisture concentration is lower than that of the surrounding air, water molecules from the humid air enter the cotton fibers in large quantities, causing them to become wet.