Introduction


The production of nonwoven fabrics is carried out either as a continuous process, with fiber or resin as the input material and a roll of fabric as output, or as a series of batch processes. Nonwoven bonded fabrics are, by definition, textiles and they can be finished in exactly the same way as other textiles such as woven or knitted fabrics. There are many examples of particular methods and types of processing equipment being used for both kinds of fabrics.


Washing


Before any coloration, fabric is treated to have the best possible absorbency, for the purpose of washing is to remove unwanted substances from the fabric. In a wet process a suitable washing machine, using water as the washing medium and occasionally a detergent, intensifies the effects required.


While some anionic washing agents also have the effect of softening the fabric, nonionic agents have the advantage of being universally compatible but are more efficient only at specific temperatures. In all wet and dry processes, the fabric should be subjected to as little tension as possible.


Dyeing


Nonwoven fabrics are colored either plain or patterned when they are to be used for decorative purposes. Examples are in wallpapers or floor coverings, table or bed linen or as furnishing fabrics. The interlinings for shirts or blouses are also colored to match the top fabric. Colors can be divided into dyes and pigments. Dyes have substantivity for fibers, meaning they are attracted from their application media by the fibrous substrate. Pigments are applied from a latex medium. Both dyes and pigments can be applied at various stages of the nonwoven process, starting from the polymer or pulp of fibers prior to web formation.


(a)    Dyeing of polymer:


In certain polymers such as polyester, dyes and pigments can be added as a concentrate to the polymer immediately prior to extrusion. This process is referred to as producer coloration or melt dyeing. The color concentrates are usually pellets or beads that contain a high concentration of dyes or pigments. Acrylic polymer can be "gel dyed" with cationic dyes, which react with the anionic sites in the polymer while the polymer is in the final stages of being formed prior to drying. In rayon, pigments can be introduced to the polymer solution prior to spinning. This is also the case in polypropylene, which has much less affinity to dyes. The advantages of producer coloring are that the web does not have to withstand the rigors of dyeing and the dye fastness is generally superior to dyed webs.


(b) Staple and mass dyeing:


Dyeing and printing are the wet processes, which are time, energy and cost-intensive. Wherever possible, coloring of the web is combined with the wet processes necessary for the bonding, or the fiber is dyed in staple form. Mass dyeing plays an important role in the case of synthetic fibers.


(c)    Dyeing and bonding:


When the web has to be bonded chemically, the dye is also added to the vat containing the bonding agent. The bonding agent may coat the fibers of the web equally, which would make possible the use of finely dispersed pigment dyes. The bonding agent would then adhere to the surface of the fibers and also would exhibit the excellent non-fading properties pigments are noted for. This also improves the rubbing fastness when wet or dry and dye fastness to perspiration and ironing. In the case of bonding agents not applied evenly to all fibers, a dye with affinity to the fibers can be added to the medium containing the bonding agent. Thus even dyeing can be expected despite the uneven distribution of the bonding agent.


If great lengths of web are composed of a single type of fiber, bonding and dyeing can be carried out in a single process without difficulty. For example cotton and viscose webs can be dyed with direct dyes, polyamide webs with acidic dyes and polyester webs with disperse dyes resulting in coloration that is as deep and fast as conventional dyeing. The only consideration is that the pH of the bonding agent be acceptable for the dye.