Written by: Priyadarshini Malshe
Written by: Priyadarshini Malshe
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.
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.
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.
(d) Subsequent dyeing:
It is much more difficult to dye and bond simultaneously if the web is composed of a mixture of different kinds of fibers. In this case and also in many cases when the fiber is homogeneous, dyeing is carried out in a later stage. The nonwoven fabric is then treated as a woven or knitted fabric and is dyed in the traditional ways.
Heavy and high bulk fabrics are dyed continuously since jiggers or dyeing beams, which work in batches, can cope with only small quantities and is therefore not economically feasible. It is however possible to dye light nonwoven fabrics perfectly on dyeing beams.
(e) Cold pad batch dyeing:
This process was patented by Farbwerke Hoechst for the dyeing of bonded webs made from polyamide by the cold pad-batch method. Nonwoven fabrics meant for curtains and table linen produced by the melt spinning or card/cross-laying method and bonded with acrylic acid esters are dyed with acid or metal complex dyes to which acids are added to provide hydrogen bonding together with cold wetting agents to encourage migration. The fabric is then padded, batched and left for 24 hours covered with polyethylene film to be roller burnished. Later it is given a warm rinse followed by soaping and, thereafter, a further rinse.
(f) Continuous dyeing:
The dyeing of heavy nonwoven fabrics is continuous, usually by the conventional pad-steam process followed by steaming to fix the dye. Steaming is usually followed by rinsing and washing.
Due to the increasingly popular use of nonwovens in the home furnishing sector there has been a great expansion of the color range and printing methods. The most commonly employed methods are screen and rotary screen-printing. The nonwoven fabric is placed on the printing backcloth similar to any other fabric and printed with dyestuffs appropriate for the fabric concerned, partially dried, fixed by steaming and washed.
Pigment printing is very important since the pigment binders
bond the fabric even more. The effect is particularly marked in spun-bonded
fabrics. A further consequence is that condensation replaces drying and
steaming. If the thickeners have little body, washing may not be necessary.
(a) Printing of light non-woven bonded fabrics:
Pigments are suitable for all kinds of light, non-woven bonded fabrics. The concentration of dye is high in light fabric printing.
(b) Printing of heavy non-woven bonded fabrics:
The printing paste for the rotary screen-printing of heavy needled fabrics have very different rheological properties from the paste suitable for light fabrics. The printing speed is much lower than when printing light fabrics.
(c) Transfer printing:
In transfer printing, subliming dyestuffs are transferred from a release paper on to the nonwoven bonded fabric with the aid of heat and pressure. Polyester fiber is most suitable for this method.
Nonwovens industry has grown exponentially in the recent past and processing technology for nonwovens has definitely come a long way. Looking forward, growth of nonwovens is expected to come from increased penetration of existing applications in developing market as well as new application areas in developed regions. With still a lot of scope left to implement better and improved processing technology and ongoing researches across the globe at world class institutes, nonwovens industry will continue to see a booming time in future.
About the Author:
Ms. Priyadarshini Malshe is a Masters student in North Carolina State University, USA.