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An Introduction to Textile Processing Auxiliaries

Although auxiliaries have been a key component of immersion

dyeing processes for many years the precise mode of action of many auxiliaries has not been fully

resolved. This part of the paper discusses the various types of auxiliary available and the nature of

the assistance they provide in immersion dyeing processes, together with both environmental and

financial aspects associated with their use, as well as a discussion of the relationship between

liquor ratio and the use of auxiliaries in immersion dyeing.






The purpose of functional additives is

to facilitate a textile process and/or increase its efficiency. They serve as sizing materials,

lubricants, wetting agents, emulsifiers, agents accelerating or decelerating the dyeing rate,

thickeners, binders, etc. often with considerable overlap in the functions and abilities of a specific

chemical. Compounds used encompass many different chemical classes, some of which are affected by

enzymes and thus can be regarded as substrates, and some of which remain unaffected. Owing to

environment and economical concerns, pre-

treatment auxiliaries
are used as sparingly as possible.






Once the respective process is terminated they are to be removed completely from the treated

material; however, traces could still be present and interfere negatively with subsequent processing

steps.






Sizing compounds and lubricants are applied to yarns before fabric formation to protect the

integrity of the yarns. While increasingly faster weaving processes demand more enduring sizes,

acrylic-based compounds, natural sizes that can be decomposed are still on the market.






Such compounds comprise starch and starch derivatives, as well as soluble. Cellulose derivatives,

with waxes often admixed.Desizing with amylases is one of the oldest enzymatic processes used in the

textile industry. A comprehensive description of the process can be found in Uhlig (1998).






Starch has also been very useful as a thickener in printing pastes and as a component of

adhesives. In printing processes, starches are applied to guarantee a defined design and to avoid

spreading of the printing paste. In the paper industry, starches increase sheet strength and, as

coatings, improve the writing and printing properties of high quality paper.






Dyeing and printing auxiliaries

may be defined as substances that, when applied to a substrate provide color by a process that alters,

at least temporarily, any crystal structure of the colored substances. Such substances with

considerable coloring capacity are widely employed in the textile, pharmaceutical, food, cosmetics,

plastics, photographic and paper industries. The dyes can adhere to compatible surfaces by solution,

by forming covalent bond or complexes with salts or metals, by physical adsorption or by mechanical

retention. Dyes are classified according to their application and chemical structure, and are composed

of a group of atoms known as chromophores, responsible for the dye color. These chromophore-containing

centers are based on diverse functional groups, such as azo, anthraquinone, methine, nitro,

arilmethane, carbonyl and others. In addition, electrons withdrawing or donating substituents so as to

generate or intensify the color of the chromophores are denominated as auxochromes. The most common

auxochromes are amine, carboxyl, sulfonate and hydroxyl.






It is estimated that over 10,000 different dyes and pigments are used industrially and over 7 x

105 tons of synthetic dyes are annually produced worldwide. Textile materials can be dyed using batch,

continuous or semi-continuous processes. The kind of process used depends on many characteristics

including type of material as such fiber, yarn, fabric, fabric construction and garment, as also the

generic type of fiber, size of dye lots and quality requirements in the dyed fabric. Among these

processes, the batch process is the most common method used to dye textile materials.






In the textile industry, up to 200,000

tons of these dyes are lost to effluents every year during the dyeing and finishing operations, due to

the inefficiency of the dyeing process. Unfortunately, most of these dyes escape conventional

wastewater treatment processes and persist in the environment as a result of their high stability to

light, temperature, water, detergents, chemicals, soap and other parameters such as bleach and

perspiration. In addition, anti-microbial agents resistant to biological degradation are frequently

used in the manufacture of textiles, particularly for natural fibers such as cotton. The synthetic

origin and complex aromatic structure of these agents make them more recalcitrant to biodegradation.

However, environmental legislation obliges industries to eliminate color from their dye-containing

effluents, before disposal into water bodies.






The textile industry consumes a substantial amount of water in its manufacturing processes used

mainly in the dyeing and finishing operations of the plants. The wastewater from textile plants is

classified as the most polluting of all the industrial sectors, considering the volume generated as

well as the effluent composition. In addition, the increased demand for textile products and the

proportional increase in their production, and the use of synthetic dyes have together contributed to

dye wastewater becoming one of the substantial sources of severe pollution problems in current times.






Textile wastewaters are characterized by extreme fluctuations in many parameters such as chemical

oxygen demand (COD), biochemical oxygen demand (BOD), pH, color and salinity. The composition of the

wastewater will depend on the different organic-based compounds, chemicals and dyes used in the dry

and wet-processing steps. Recalcitrant organic, colored, toxicant, surfactant and chlorinated

compounds and salts are the main pollutants in textile effluents.






In addition, the effects caused by other pollutants in textile wastewater, and the presence of

very small amounts of dyes (

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