The Textile Production Process  - 2

There are two types of knitted fabrics: those woven by warp and those by weft. In the warp knits, thousands of threads simultaneously enter the machine, each with its own needle and woven at the same time. Tricot, milanese, raschel and simplex are varieties of this type of knitting. In weft knits, the yarn comes directly into the machine from a cone, spool or other form of packing so that the thread is woven onto a new row on the fabric being created. The yarn can come from one or more feed points, therefore one or more rows can be created at a time.

 

Prior to weaving, fibers are coated with finishes, using chemicals such as starches, gums, plasticizers, penetrants and preservatives. Each manufacturer has its own formulation. There are also less expensive base materials used, such as adhesives, film-forming starches and alcohols. Starches, gums and glues act effectively on hydrophilic natural fibers, but do not produce good results in nylon and other water-resistant fibers.

 

Softeners are used to provide flexibility to the starch coating, and to spread the lubrication through the yarn, which travels through the combs, heddles and harnesses of the loom. The following are used as softeners: tallow, various oils and fats such as olive oil, coconut oil and castor oil, stearin, paraffin, and various oils and synthetic fats.

Untreated fabric, especially tightly woven fabric, almost always contains dirt that is not completely removed by washing processes. The bleaching of the material enhances the removal of the dirt.

 

Most companies that have a bleaching process use hydrogen peroxide (H2O2), which is the most common whitener. Less frequently used are sodium hypochlorite (NaClO) and sodium chlorite (NaClO2). The redox potential of these substances under normal conditions depends greatly on the pH. With hydrogen peroxide, the redox potential is present in both cold and hot processes, providing technical and ecological advantages over NaClO and NaClO2.

The reduction bleaching agents most commonly used are sodium dithionite (Na2S2O4) and thiourea dioxide. The use of these agents requires auxiliary substances that have activators, stabilizers, surfactants, and buffer systems, which control the laundering process to prevent damage to the treated basic fabric and improve its absorbency.

 

Similar to pre-treatment fabric bleaching is done in different ways depending on the material to be treated. 

The following are the most common bleaching processes:

 

Concentrated Bleaching: Diluted solutions used include sodium hypochlorite and hydrogen peroxide, chlorine compounds (sodium or calcium hypochlorite), concentration agents and organic or inorganic sequestering agents such as polyphosphates or ethylenediaminetetraacetic acid (EDTA). EDTA can also be used to bleach linen or rayon in order to prevent insoluble soap film concentrations in the fabric, and to prevent iron ions, which cause a yellowing effect, from impregnating the fabric.

 

Linen Bleaching: Diluted solutions of hydrochloric acid, hydrogen peroxide and alkali are used. 

Rayon Bleaching: Solutions similar to the above are used but with shorter times and lower concentrations of chemicals. 

Silk and Wool Bleaching: Solutions with sulfur dioxide and hydrogen peroxide are used. Chlorine compounds, which cause roughness and yellowing, should not be used.

Dyeing is a process that can generate more pollution because it requires not only the use of dyes and chemicals, but also several special products known as dyeing auxiliaries. These materials are an integral part of the dyeing process (for example, reducing agents for dyeing with vat dyes) which increase the properties of finished products and improve the quality of dyeing, smoothness, firmness, texture, dimensional stability, resistance to light and washing, etc. 

Dyeing auxiliaries are a heterogeneous group of chemical compounds that in general include surfactants, inorganic compounds, polymers and oligomers soluble in water and solubilization agents. Commercial dyeing auxiliaries tend to be preparations that contain several of these compounds.