Softness of fabric depends upon several factors, these are:
1. Composition of the Fiber: The more crystalline region a fiber has, the less soft will be the fabric. Thus viscose is more soft than cotton as it has less crystalline region than cotton.
2. Softness of Yarn: It depends upon the twist. The higher the twist, the less soft will be the yarn and hence less soft will be the fabric.
3. Weave: Loose weave give more softness to the fabric than closer weave. Thus a fabric made with satin or twill weave will be more softer than that made with plain weave.
Chemical softening is done by using chemicals which act as a lubricating agent and permits sliding of the fibers in the fabric. However, washing can eliminate these chemicals. Therefore they are applied during the final stage of the treatment.
The most common type of Chemical Softeners are:
1. Non Ionic softeners: These are less efficient than anionic or cationic softeners, but they can withstand the effects of hard water. They can also sustain themselves in acid and basic environments and thus are most suitable for normal washing.
2. Anionic Softeners: These are very good and give the fabric a full hand. However they cannot sustain hard water and acid environment. They can also cause yellowing at certain temperatures.
3. Cationic Softeners: They are amongst the best of the softeners. However they can cause dye toning. Also they can affect the color fastness to light.
4. Silicone Based Softeners: These are insoluble in water and therefore must be applied on the fabric after dissolution in organic solvents. They have good fastness to washing. They create a lubricating film on the surface and give fabric a velvety silky hand.
5. Reactive Softeners: These products have to be cross linked and provide permanent softness and water repellency.