The purpose of auxiliaries 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 environmental and economical concerns, 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.
Natural sizing compounds, coating materials & thickeners Starch
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, such as acrylic-based compounds, natural sizes that can be decomposed by enzymes 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).
Native starch contains two components, amylose and amylopectin , bound together by hydrogen bonding. Degrees of polymerization and cross-linking vary, yielding a large variety of starches with differing characteristics, including swelling and gelatinization properties. Amylose is a polymer of linear unbranched α-1, 4-glucan, while amylopectin, also a α-1,4-glucan, is additionally highly branched at the C-6-position. The ratio of amylose and amylopectin depends on the source and processing of the respective starch.
Enzymes capable of hydrolyzing starch include α- and β-amylase, amyloglucosidase (glucoamylase) and isoamylase (Uhlig, 1998). Both α- and β-amylases attack the α-1, 4-linkage, but are unable to break the 1,6- branched linkages.
They predominantly produce maltose and dextrins as end products. Glucoamylase liberates glucose from non-reducing ends at α-1,4 and 1,6linkages and generates glucose. Isoamylase is a debranching enzyme, producing mainly maltose.
Modified Starches
The hydroxyl groups in starch can be functionalized to form acetates, ethers or esters to various degrees of substitution. Such modifications have an impact on the gelation and swelling behavior and are useful for printing applications as well as in the food and pharmaceutical industry. Among others, hydroxyethyl-, methyl- and carboxymethyl-starches are important as thickeners. Corresponding watersoluble cellulose derivatives can also be produced for application in thickener formulations (see below).
Gums
Dry roasting of starches at 135-190�C yields British gums. Such starch derivatives are crosslinked in the 1,6 position with decreased degree of polymerization (DP). Other gums that are important for thickening purposes are alginate based (from seaweed) or obtained from other natural polysaccharides (gum arabic, xanthate gum, gum tragacanth, etc.; Miles, 1994). Some of these compounds can also economically be grown by microorganisms. Dextrins (see below) are produced by pyroconversion with very small amounts of acid in the form of a fine spray. Gums and dextrins play an important role as adhesives and binders in the coating industry and as encapsulating compounds.
Xanthan
Structure of cyclodextrin, made from 7 glucopyranose residues
Xanthan gum consists of a β-1,4 linked can main chain with a negatively charged trisaccharide (two man nose units and one glucuronic acid residue) chain at alternating C3 atoms. Because of these substitutions, xanthan is a highly charged polyanion, giving it a rigid-rod structure in solution as a result of repulsion effects. Considering its structure and its high molecular weight, it is not surprising its solution is highly viscous. Low Concentrations are sufficient to prepare excellent thickening agent for printing (Miles,1994) and other purposes.
Alginates
Alginates are polyanionic block copolymers of high molecular weight of α-D-guluronic acid and β-L- mannuronic acid (De Baets et aI., 2002).
Commercial alginates are obtained from seaweed. With the help of enzymes or by chemical means the ratio of guluronic acid and mannuronic acid can be modified and thus their gelation properties tailored to specific applications.
The high viscosity of their solutions and their chemical inertness towards dyes makes them valuable textile printing thickeners.
Dextrin and Cyclodextrin
Dextrins are α-1,6-d-glucopyranosyl polymers, branched through (1,2), (1,3) or (1,4) linkages, while cyclodextrins are cyclic molecules formed by six, seven, eight or, less commonly, nine α-D-(1,4)-linked glucopyranose residues. Cyclodextrins form water-soluble ring structures with numerous hydroxyl groups at the outside. The interior cavity on the other hand is fairly hydrophobic and capable of holding small molecules.
Besides sequestering small molecules, cyclodextrins can serve as amylase inhibitors.
Cellulose derivatives
Water-soluble cellulose ethers are versatile auxiliaries owing to their availability and low toxicity. Carboxymethyl cellulose (MC) and hydroxyethyl celluloses (HEC) are commercially important thickeners, film formers, adhesives and water-retaining agents for the textile, pulp and paper industries (Heinze, 1998). Organic and inorganic cellulose esters are used in coatings, thermoplastic films and resins (e.g. cellulose acetobutyrate) and for textile finishing (e.g. cellulose phosphate for flame retardancy for cotton). Cellulose acetate can also be produced in textile fiber form.
Cellulases are suitable enzymes for the decomposition of cellulose derivatives on condition that the derivatization of the cellulose backbone does not have a significant impact on enzyme recognition (Philipp and Stscherbina, 1992; Glasser et aI., 1994). Whole cellulases, composed of cellobiohydrolases, endocellulases and b-glucosidases, can be applied either individually or in combination with enzymes that focus on other compounds in the thickener or sizing material, such as amylases (starch degradation) or lipases (fat hydrolysis). In enzyme mixtures the requirements for active pH and temperature of all components must be considered for the system to be effective.
Chitosan
Chitosan (copolymer of N-acetylglucosamine and glucosamine, ; De Baets et aI., 2002) is produced from chitin (p-1,4-linked Nacetylglucosamine) by deprotonization, demineralization and partial deacetylation to a product soluble in 1 % acetic acid. Chitosan can be used as a viscosity controlling compound in mixtures with other swelling agents.
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.
Natural sizing compounds, coating materials & thickeners Starch
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, such as acrylic-based compounds, natural sizes that can be decomposed by enzymes 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).
Native starch contains two components, amylose and amylopectin , bound together by hydrogen bonding. Degrees of polymerization and cross-linking vary, yielding a large variety of starches with differing characteristics, including swelling and gelatinization properties. Amylose is a polymer of linear unbranched α-1, 4-glucan, while amylopectin, also a α-1,4-glucan, is additionally highly branched at the C-6-position. The ratio of amylose and amylopectin depends on the source and processing of the respective starch.
Enzymes capable of hydrolyzing starch include α- and β-amylase, amyloglucosidase (glucoamylase) and isoamylase (Uhlig, 1998). Both α- and β-amylases attack the α-1, 4-linkage, but are unable to break the 1,6- branched linkages.
They predominantly produce maltose and dextrins as end products. Glucoamylase liberates glucose from non-reducing ends at α-1,4 and 1,6linkages and generates glucose. Isoamylase is a debranching enzyme, producing mainly maltose.
Modified Starches
The hydroxyl groups in starch can be functionalized to form acetates, ethers or esters to various degrees of substitution. Such modifications have an impact on the gelation and swelling behavior and are useful for printing applications as well as in the food and pharmaceutical industry. Among others, hydroxyethyl-, methyl- and carboxymethyl-starches are important as thickeners. Corresponding watersoluble cellulose derivatives can also be produced for application in thickener formulations (see below).
Gums
Dry roasting of starches at 135-190�C yields British gums. Such starch derivatives are crosslinked in the 1,6 position with decreased degree of polymerization (DP). Other gums that are important for thickening purposes are alginate based (from seaweed) or obtained from other natural polysaccharides (gum arabic, xanthate gum, gum tragacanth, etc.; Miles, 1994). Some of these compounds can also economically be grown by microorganisms. Dextrins (see below) are produced by pyroconversion with very small amounts of acid in the form of a fine spray. Gums and dextrins play an important role as adhesives and binders in the coating industry and as encapsulating compounds.
Xanthan
Structure of cyclodextrin, made from 7 glucopyranose residues
Xanthan gum consists of a β-1,4 linked can main chain with a negatively charged trisaccharide (two man nose units and one glucuronic acid residue) chain at alternating C3 atoms. Because of these substitutions, xanthan is a highly charged polyanion, giving it a rigid-rod structure in solution as a result of repulsion effects. Considering its structure and its high molecular weight, it is not surprising its solution is highly viscous. Low Concentrations are sufficient to prepare excellent thickening agent for printing (Miles,1994) and other purposes.
Alginates
Alginates are polyanionic block copolymers of high molecular weight of α-D-guluronic acid and β-L- mannuronic acid (De Baets et aI., 2002).
Commercial alginates are obtained from seaweed. With the help of enzymes or by chemical means the ratio of guluronic acid and mannuronic acid can be modified and thus their gelation properties tailored to specific applications.
The high viscosity of their solutions and their chemical inertness towards dyes makes them valuable textile printing thickeners.
Dextrin and Cyclodextrin
Dextrins are α-1,6-d-glucopyranosyl polymers, branched through (1,2), (1,3) or (1,4) linkages, while cyclodextrins are cyclic molecules formed by six, seven, eight or, less commonly, nine α-D-(1,4)-linked glucopyranose residues. Cyclodextrins form water-soluble ring structures with numerous hydroxyl groups at the outside. The interior cavity on the other hand is fairly hydrophobic and capable of holding small molecules.
Besides sequestering small molecules, cyclodextrins can serve as amylase inhibitors.
Cellulose derivatives
Water-soluble cellulose ethers are versatile auxiliaries owing to their availability and low toxicity. Carboxymethyl cellulose (MC) and hydroxyethyl celluloses (HEC) are commercially important thickeners, film formers, adhesives and water-retaining agents for the textile, pulp and paper industries (Heinze, 1998). Organic and inorganic cellulose esters are used in coatings, thermoplastic films and resins (e.g. cellulose acetobutyrate) and for textile finishing (e.g. cellulose phosphate for flame retardancy for cotton). Cellulose acetate can also be produced in textile fiber form.
Cellulases are suitable enzymes for the decomposition of cellulose derivatives on condition that the derivatization of the cellulose backbone does not have a significant impact on enzyme recognition (Philipp and Stscherbina, 1992; Glasser et aI., 1994). Whole cellulases, composed of cellobiohydrolases, endocellulases and b-glucosidases, can be applied either individually or in combination with enzymes that focus on other compounds in the thickener or sizing material, such as amylases (starch degradation) or lipases (fat hydrolysis). In enzyme mixtures the requirements for active pH and temperature of all components must be considered for the system to be effective.
Chitosan
Chitosan (copolymer of N-acetylglucosamine and glucosamine, ; De Baets et aI., 2002) is produced from chitin (p-1,4-linked Nacetylglucosamine) by deprotonization, demineralization and partial deacetylation to a product soluble in 1 % acetic acid. Chitosan can be used as a viscosity controlling compound in mixtures with other swelling agents.
Further, as a polycation, chitosan binds to polyanions such as anionic dyes, and can improve dyeability if applied as a coating or film forming agent. It is possible to manipulate film stabilities by crosslinking. Chitosan oligomers of DP >30 have antimicrobial properties and thus are useful as wound dressings as well as in finishing of apparel and household goods (Kumar, 2000).
Synthetic auxiliaries for dyeing and finishing General
The function of auxiliaries including compounds that are affected or degraded by enzymes was discussed above. Surface active' substances, salts, oxidizing and reducing agents, and acids and bases also belong to
the category of auxiliaries; however, they are not considered enzymatic substrates although they might influ- ence the effectiveness or mode of action of enzymes.
Electrolytic compounds and pH control substances
For all textile processing steps, water quality and softness, pH and electroIyte content are important considerations. Water hardness is caused by calcium and magnesium sulfates, chlorides (permanent hard- ness) and carbonates (temporary hardness). These salts not only contribute to deposits 56 Textile processing with enzymes on equipment, but also interfere with preparation, dyeing and finishing.
Various techniques are available to soften water on an industrial level, most commonly via ion exchange. A more direct and more expensive approach is the addition of sequestering agents to process water where water softness is crucial. Sequestering agents have functionalities that allow complexing (chelating) of metal ions.
Examples for such chelators are polycarboxylic acids (e.g. oxalic acid), aminopolycarboxylic acids (EDTA, ethyienediaminetetra-acetic acid), sodium polyphosphates (sodium hexametaphosphate, Calgon�), and others.
Besides water softness, the pH of the treatment bath in preparation, dyeing and finishing plays an important role owing to the sensitivity of the textile material to acid or basic conditions on the one hand and the reactivity of dyes and finishing compounds on the other hand. Many processes even require the stabilization of the pH with the help of a buffer system.
Buffer systems consist of an acid and the corresponding salt, for example, acetic acid and sodium acetate for pH 4-5, or a base and the corresponding salt. Buffer systems for any type of pH range can be found in general laboratory reference books (e.g. Shugar and IIinger, 1990). Common acids are organic acids (e.g. sulfuric acid, hydrochloric acid, pH below 2), organic acids (acetic acid, citric acid, pH 4.5), acidic inorganic salts (ammonium sulfate, etc. for pH 6.5-5.5) and mixtures thereof. Alkaline pH ranges are adjusted with common bases (sodium or potassium hydroxide, pH 11 or higher) or basic salts (e.g. carbonates, borax). Great care has to be exercised to make sure that adequate rinsing takes place after each treatment step and that sufficient time is allowed for internal exchange processes within the fiber. Because of adsorption processes, the release of acids, bases or salts can be fairly slow, and the pH of the rinse bath might not represent the realistic pH situation inside the fiber pores.
A large number of dyeing processes afford the addition of common salts, such as sodium chloride or sodium sulfate to enhance dye adsorption and fixation. The amount of these salts can be quite considerable, often 10% of the weight of fiber or more.
The function of these salts is first to help alleviate negative fiber surface charges, thus reducing repulsion between negatively charged dye molecules and the fiber wall.
Second, they support the aggregation of dye molecules inside the fiber pores by making the dye less ionic (for example, in direct dyeing of cellulosics). In some cases, an example being acid dyeing of wool, salts act as retarders.
The smaller salt ions temporarily take the place of the dye at the fiber dyesite. They are then replaced by the larger and slower moving dye molecules, yielding much more uniform dyeing results. Thorough rinsing has to follow any dyeing process, not only to remove unbound and loosely attached surface dye, but also to eliminate any auxiliaries. Even minute amounts of remaining salts can show up as white deposits on dyed goods.
Compounds with whitening effect
High levels of whiteness are desirable for textile materials as well as fundamental for reproducible color shades. Thus, whitening is usually carried out priorto dyeing and finishing. Most commonly this is achieved by the use of oxidizing agents that destroy chromophoric substances. Additionally, fluorescent brightening agents are added that mask yellowing compounds.
Natural cellulosics are in most need of bleaching. Most synthetics are already fairly white; however, if necessary, fluorescent brighteners can be included in the spinning dope. Wool and silk are not routinely bleached.
Common bleaching agents include hydrogen peroxide and chlorine containing compounds (Trotman, 1984; Shore, 1990b). Hydrogen peroxide is preferred over other bleaching agents as it decomposes into oxygen and water without impact on the environment. Bleaching is performed at pH 10.5-11 at boiling temperatures in the presence of stabilizers, sequestrants to control water softness and metal content, and surfactants with detergency.
Stabilizers often consist of polysilicates, acrylates or magnesium salts.
The mechanism of bleaching most likely follows a radical route (Zeronian and Inglesby, 1995). In the presence of metal ions that act as peroxide activators, fiber damage is possible as radicals can attack the fiber polymer instead of the chromophore of the colorant. Small amounts of hydrogen peroxide that might still be held back by the fiber after bleaching have to be removed to avoid interference later on with dyes or finishes.
Besides by chemical means, this step can also be performed enzymatically with catalase, an oxidoreductase that catalyzes the breakdown of hydrogen peroxide to water (Tzanov et aI., 2002) .
The bleaching effect of sodium chlorite strongly depends on the pH value. The reaction occurs most rapidly at low pH and higher temperatures. In commercial operations bleaching is performed at pH 3.5-6 for cotton and temperatures around 80�C. Toxic chlorine dioxide is produced at lower pH; above pH 9 the bleaching effect is insignificant.
Sodium hypochlorite bleaches at pH values above 11 in a buffered system. The active chlorine content should be determined before use.
Severe oxidative fiber damage can be expected if the pH falls below 9 with formation of hydrochloric acid, which will reduce the pH despite the buffer system. Further, hypochlorite decomposes upon storage or exposure to light, and an antichlor after-treatment with reducing agents following treatment with chlorite and hypochlorite might be necessary to remove chlorine traces from the fiber.
Owing to the possible encounter of significant problems with hypochlorite and chlorite, hydrogen peroxide has advanced to the favored bleaching agent in commercial wet-processing operations nowadays. Still, chlorine-containing agents are sometimes applied to bleach bast fibers, such as flax or jute.
Compounds intended to affect interfacial properties Surfactants
Surface-affecting substances (surfactants) are a very important group of textile auxiliaries. They find use as wetting agents, softeners, detergents, emulsifiers and defoaming agents, to name just a few applications.
Commercial products rarely contain a pure compound, but rather mixtures of a range of surfactants to tailor their properties to the tasks in demand (Flick, 1993).
Surfactants generally consist of a hydrophilic part, providing water solubility, and a hydrophobic part, creating a link to non-aqueous media. Based on the nature of the hydrophilic portion of the molecule they are classified as follows (Broze, 1999):
Anionic surfactants: negatively charged groups (e.g. sulfates, carboxylates, phosphates or sulfonates) are associated with the hydrophobic part of the molecule. These surfactants are important wetting agents and detergents. Owing to the fact that many substrates are also negatively charged, anionic surfactants do not firmly adhere to such surfaces and impede redeposition of soil.
Nonionic surfactants: polar but without actual charge, solubilization properties in non-ionic surfactants are usually provided by incorporation of ethoxy units into their structure (alcohols, ethers, esters, etc.). Nonionic surfactants can be mixed with any other group of surfactants and are fairly insensitive to water hardness. Most commonly, they are blended with anionic surfactants for increased detergency or used alone as emulsifiers.
Cationic surfactant: these surfactants carry a positively charged group, commonly a quaternary ammonium group, associated with the hydrophobic portion of the molecule. Often, these compounds are additionally ethoxylated. Being positively charged, cationic surfactants adsorb more firmly to negatively charged substrates. Their major application is in softeners and emulsifiers.
Amphoteric (zwitterionic) surfatants: these surfactants contain both anionic and cationic groups in the structure and thus behave as cationic or anionic compounds dependent the respective pH. Common structures are betaines, amino acid derivatives and imidazoline derivatives. Their electric point does not necessarily at pH 7.
Although they seem to have a great application potential, they are the Ieast important commercially.
The hydrophobic portion in all types of surfactant consists of fairly long-chained linear saturated or unsaturated alkanes, derived from fats or oils. The chain length lies between 8 to 18 carbon atoms (e.g. stearate, palmitate, oleate, linoleate). Aromatic moieties and/or alkyl-substituted groups are also common.
Surfactants added in increasing amounts to water orient themselves at the interface of water/air with their hydrophilic parts towards the water and the hydrophobic parts pointing into the air. At a specific concentration (critical micelle concentration), when the entire water surface is covered by surfactant molecules, more or less ordered aggregations of surfactant molecules form in the bulk of the solution (micelles).
In a micelle the polar hydrophilic parts of the surfactant molecules are oriented towards the water, the hydrophobic parts towards the interior of the micelle (in oil instead of water, their orientation is reversed). The hydrophobic center of the micelle can thus interact with hydrophobic compounds of the system, such as insoluble dyes, finishes, oils, etc., fulfilling solubilizing, emulsifying and dispersing tasks (Datyner, 1993).
Enzyme reactions on textile materials have been performed in the presence the various types of surfactants and their effect studied. The reports, however, often provide controversial results. (Helle et aI.,1993; Kaya et aI., 1995; eda et aI., 1994).
Foam control substances
Foam control is a very important issue for various textile processes, such as scouring, dyeing and printing, and processing with the goal of economic, low water pick-up. For a foaming system to be effective both foam antifoam agents are necessary to control the liquid drainage rate from the film walls. The interfacial tension between the foaming and defoaming compound needs to be manipulated. Anionic and non-polar surfactants can ion as both types; however, fats, waxes, fatty acids and oils, long-chain alcohols and polyglycols, polyalkylsiloxanes and their block copolymers with poly(oxyethylene) are more efficient as defoamers.
Foam application with controlled foam stabilization and collapse can be created with a blend of anionic and nonionic surfactants and foam stabilizers, such as poly(vinyl alcohol), poly(acrylic acids), polysaccharides and cellulose derivatives.
Foam breakers are compounds that destroy foam, while foam inhibitors are made to prevent foam from being formed. Foam breakers quickly drain liquid and drastically reduce the surface tension at interfaces. They often consist of metal carboxylates in oil dispersion. Common formulations for effective foam inhibitors are water-soluble silicone glycol chemicals (see below), silica dispersed in water, or fluorinated alcohols and acids.
Such compounds replace the elastic surface film by a more brittle 'film, so that the increase in surface tension caused by expansion is counterbalanced (Slate, 1998).
Softening agents
Besides cationic surfactants, often mixed with non-ionic surfactants, polysiloxanes are frequently used for fabric softening. Siloxanes are usually non-durable, but can be made durable by modification and addition of functional groups, followed by crosslinking.
Other permanent types include reactive N-methylol derivatives of fatty acids and chlorotriazines, similar to reactive groups in fiber-reactive dyes. Some of these softeners are commonly applied together with easy- care finishes. Finishing compounds that render the textile material less hydrophilic by either coating the fiber surface and/or crosslinking and thus closing up the amorphous areas can present a barrier for enzymatic access.
Silicones and flourochemicals
Silicones are compounds that can act as defoamers, soil repellants or lubricants. Fluorochemicals are especially useful as water- and stain-repellants. Both chemical classes encompass a wide range of compounds and are valuable for various purposes.
Synthetic sizes and thickeners
A large group of synthetic 'sizes and thickening agents are acrylic-based polymers, either linear or crosslinked in structure. If used for sizing, they often remain on the fabric to add to the hand properties and softness of the textile material. Besides, complete' desizing is often problematic (Lewin and Sello, 1983a; Lewin and Pearce, 1998).
Poly(vinyl alcohol) used as a synthetic size or thickener has the advantage of being easily recyclable and reusable as it can be removed by dissolving in hot water (Reife and Freeman, 1996). Polyacrylates swell in hot water and need sufficient mechanical impact to be completely removed from the fabric.Polyester-based sizes are broken down by hot alkaline solutions; however, insoluble oligomers may remain on the fabric (Shore, 199Gb). Copolymers of methyl methacrylate are soluble in organic solvents.
Their application and removal occurs in non-aqueous media. If completed in a closed system, they are valued as environmentally benign; however, the machinery necessary needs modification from standard equipment to accommodate the process with sol~ vents other than water.
Crosslinking resins
A large number of cellulosic fabrics, especially cotton and cotton in blends with polyester, are finished with easycare or wrinkle-resistant finishes.
Compounds used for this process form crosslinked networks involving the cellulosic hydroxyl functional groups on the fiber surface ,as well as in the accessible fiber interior, thus providing dimensional stability by fixing the structure in a specific state. Coloration of the textile material has to be performed prior to crosslinking because otherwise the amorphous areas become partially or completely inaccessible. If carried out on dyed material, crosslinking leads to improved wet fastness, locking the dye molecules in place.
The function of auxiliaries including compounds that are affected or degraded by enzymes was discussed above. Surface active' substances, salts, oxidizing and reducing agents, and acids and bases also belong to
the category of auxiliaries; however, they are not considered enzymatic substrates although they might influ- ence the effectiveness or mode of action of enzymes.
Electrolytic compounds and pH control substances
For all textile processing steps, water quality and softness, pH and electroIyte content are important considerations. Water hardness is caused by calcium and magnesium sulfates, chlorides (permanent hard- ness) and carbonates (temporary hardness). These salts not only contribute to deposits 56 Textile processing with enzymes on equipment, but also interfere with preparation, dyeing and finishing.
Various techniques are available to soften water on an industrial level, most commonly via ion exchange. A more direct and more expensive approach is the addition of sequestering agents to process water where water softness is crucial. Sequestering agents have functionalities that allow complexing (chelating) of metal ions.
Examples for such chelators are polycarboxylic acids (e.g. oxalic acid), aminopolycarboxylic acids (EDTA, ethyienediaminetetra-acetic acid), sodium polyphosphates (sodium hexametaphosphate, Calgon�), and others.
Besides water softness, the pH of the treatment bath in preparation, dyeing and finishing plays an important role owing to the sensitivity of the textile material to acid or basic conditions on the one hand and the reactivity of dyes and finishing compounds on the other hand. Many processes even require the stabilization of the pH with the help of a buffer system.
Buffer systems consist of an acid and the corresponding salt, for example, acetic acid and sodium acetate for pH 4-5, or a base and the corresponding salt. Buffer systems for any type of pH range can be found in general laboratory reference books (e.g. Shugar and IIinger, 1990). Common acids are organic acids (e.g. sulfuric acid, hydrochloric acid, pH below 2), organic acids (acetic acid, citric acid, pH 4.5), acidic inorganic salts (ammonium sulfate, etc. for pH 6.5-5.5) and mixtures thereof. Alkaline pH ranges are adjusted with common bases (sodium or potassium hydroxide, pH 11 or higher) or basic salts (e.g. carbonates, borax). Great care has to be exercised to make sure that adequate rinsing takes place after each treatment step and that sufficient time is allowed for internal exchange processes within the fiber. Because of adsorption processes, the release of acids, bases or salts can be fairly slow, and the pH of the rinse bath might not represent the realistic pH situation inside the fiber pores.
A large number of dyeing processes afford the addition of common salts, such as sodium chloride or sodium sulfate to enhance dye adsorption and fixation. The amount of these salts can be quite considerable, often 10% of the weight of fiber or more.
The function of these salts is first to help alleviate negative fiber surface charges, thus reducing repulsion between negatively charged dye molecules and the fiber wall.
Second, they support the aggregation of dye molecules inside the fiber pores by making the dye less ionic (for example, in direct dyeing of cellulosics). In some cases, an example being acid dyeing of wool, salts act as retarders.
The smaller salt ions temporarily take the place of the dye at the fiber dyesite. They are then replaced by the larger and slower moving dye molecules, yielding much more uniform dyeing results. Thorough rinsing has to follow any dyeing process, not only to remove unbound and loosely attached surface dye, but also to eliminate any auxiliaries. Even minute amounts of remaining salts can show up as white deposits on dyed goods.
Compounds with whitening effect
High levels of whiteness are desirable for textile materials as well as fundamental for reproducible color shades. Thus, whitening is usually carried out priorto dyeing and finishing. Most commonly this is achieved by the use of oxidizing agents that destroy chromophoric substances. Additionally, fluorescent brightening agents are added that mask yellowing compounds.
Natural cellulosics are in most need of bleaching. Most synthetics are already fairly white; however, if necessary, fluorescent brighteners can be included in the spinning dope. Wool and silk are not routinely bleached.
Common bleaching agents include hydrogen peroxide and chlorine containing compounds (Trotman, 1984; Shore, 1990b). Hydrogen peroxide is preferred over other bleaching agents as it decomposes into oxygen and water without impact on the environment. Bleaching is performed at pH 10.5-11 at boiling temperatures in the presence of stabilizers, sequestrants to control water softness and metal content, and surfactants with detergency.
Stabilizers often consist of polysilicates, acrylates or magnesium salts.
The mechanism of bleaching most likely follows a radical route (Zeronian and Inglesby, 1995). In the presence of metal ions that act as peroxide activators, fiber damage is possible as radicals can attack the fiber polymer instead of the chromophore of the colorant. Small amounts of hydrogen peroxide that might still be held back by the fiber after bleaching have to be removed to avoid interference later on with dyes or finishes.
Besides by chemical means, this step can also be performed enzymatically with catalase, an oxidoreductase that catalyzes the breakdown of hydrogen peroxide to water (Tzanov et aI., 2002) .
The bleaching effect of sodium chlorite strongly depends on the pH value. The reaction occurs most rapidly at low pH and higher temperatures. In commercial operations bleaching is performed at pH 3.5-6 for cotton and temperatures around 80�C. Toxic chlorine dioxide is produced at lower pH; above pH 9 the bleaching effect is insignificant.
Sodium hypochlorite bleaches at pH values above 11 in a buffered system. The active chlorine content should be determined before use.
Severe oxidative fiber damage can be expected if the pH falls below 9 with formation of hydrochloric acid, which will reduce the pH despite the buffer system. Further, hypochlorite decomposes upon storage or exposure to light, and an antichlor after-treatment with reducing agents following treatment with chlorite and hypochlorite might be necessary to remove chlorine traces from the fiber.
Owing to the possible encounter of significant problems with hypochlorite and chlorite, hydrogen peroxide has advanced to the favored bleaching agent in commercial wet-processing operations nowadays. Still, chlorine-containing agents are sometimes applied to bleach bast fibers, such as flax or jute.
Compounds intended to affect interfacial properties Surfactants
Surface-affecting substances (surfactants) are a very important group of textile auxiliaries. They find use as wetting agents, softeners, detergents, emulsifiers and defoaming agents, to name just a few applications.
Commercial products rarely contain a pure compound, but rather mixtures of a range of surfactants to tailor their properties to the tasks in demand (Flick, 1993).
Surfactants generally consist of a hydrophilic part, providing water solubility, and a hydrophobic part, creating a link to non-aqueous media. Based on the nature of the hydrophilic portion of the molecule they are classified as follows (Broze, 1999):
Anionic surfactants: negatively charged groups (e.g. sulfates, carboxylates, phosphates or sulfonates) are associated with the hydrophobic part of the molecule. These surfactants are important wetting agents and detergents. Owing to the fact that many substrates are also negatively charged, anionic surfactants do not firmly adhere to such surfaces and impede redeposition of soil.
Nonionic surfactants: polar but without actual charge, solubilization properties in non-ionic surfactants are usually provided by incorporation of ethoxy units into their structure (alcohols, ethers, esters, etc.). Nonionic surfactants can be mixed with any other group of surfactants and are fairly insensitive to water hardness. Most commonly, they are blended with anionic surfactants for increased detergency or used alone as emulsifiers.
Cationic surfactant: these surfactants carry a positively charged group, commonly a quaternary ammonium group, associated with the hydrophobic portion of the molecule. Often, these compounds are additionally ethoxylated. Being positively charged, cationic surfactants adsorb more firmly to negatively charged substrates. Their major application is in softeners and emulsifiers.
Amphoteric (zwitterionic) surfatants: these surfactants contain both anionic and cationic groups in the structure and thus behave as cationic or anionic compounds dependent the respective pH. Common structures are betaines, amino acid derivatives and imidazoline derivatives. Their electric point does not necessarily at pH 7.
Although they seem to have a great application potential, they are the Ieast important commercially.
The hydrophobic portion in all types of surfactant consists of fairly long-chained linear saturated or unsaturated alkanes, derived from fats or oils. The chain length lies between 8 to 18 carbon atoms (e.g. stearate, palmitate, oleate, linoleate). Aromatic moieties and/or alkyl-substituted groups are also common.
Surfactants added in increasing amounts to water orient themselves at the interface of water/air with their hydrophilic parts towards the water and the hydrophobic parts pointing into the air. At a specific concentration (critical micelle concentration), when the entire water surface is covered by surfactant molecules, more or less ordered aggregations of surfactant molecules form in the bulk of the solution (micelles).
In a micelle the polar hydrophilic parts of the surfactant molecules are oriented towards the water, the hydrophobic parts towards the interior of the micelle (in oil instead of water, their orientation is reversed). The hydrophobic center of the micelle can thus interact with hydrophobic compounds of the system, such as insoluble dyes, finishes, oils, etc., fulfilling solubilizing, emulsifying and dispersing tasks (Datyner, 1993).
Enzyme reactions on textile materials have been performed in the presence the various types of surfactants and their effect studied. The reports, however, often provide controversial results. (Helle et aI.,1993; Kaya et aI., 1995; eda et aI., 1994).
Foam control substances
Foam control is a very important issue for various textile processes, such as scouring, dyeing and printing, and processing with the goal of economic, low water pick-up. For a foaming system to be effective both foam antifoam agents are necessary to control the liquid drainage rate from the film walls. The interfacial tension between the foaming and defoaming compound needs to be manipulated. Anionic and non-polar surfactants can ion as both types; however, fats, waxes, fatty acids and oils, long-chain alcohols and polyglycols, polyalkylsiloxanes and their block copolymers with poly(oxyethylene) are more efficient as defoamers.
Foam application with controlled foam stabilization and collapse can be created with a blend of anionic and nonionic surfactants and foam stabilizers, such as poly(vinyl alcohol), poly(acrylic acids), polysaccharides and cellulose derivatives.
Foam breakers are compounds that destroy foam, while foam inhibitors are made to prevent foam from being formed. Foam breakers quickly drain liquid and drastically reduce the surface tension at interfaces. They often consist of metal carboxylates in oil dispersion. Common formulations for effective foam inhibitors are water-soluble silicone glycol chemicals (see below), silica dispersed in water, or fluorinated alcohols and acids.
Such compounds replace the elastic surface film by a more brittle 'film, so that the increase in surface tension caused by expansion is counterbalanced (Slate, 1998).
Softening agents
Besides cationic surfactants, often mixed with non-ionic surfactants, polysiloxanes are frequently used for fabric softening. Siloxanes are usually non-durable, but can be made durable by modification and addition of functional groups, followed by crosslinking.
Other permanent types include reactive N-methylol derivatives of fatty acids and chlorotriazines, similar to reactive groups in fiber-reactive dyes. Some of these softeners are commonly applied together with easy- care finishes. Finishing compounds that render the textile material less hydrophilic by either coating the fiber surface and/or crosslinking and thus closing up the amorphous areas can present a barrier for enzymatic access.
Silicones and flourochemicals
Silicones are compounds that can act as defoamers, soil repellants or lubricants. Fluorochemicals are especially useful as water- and stain-repellants. Both chemical classes encompass a wide range of compounds and are valuable for various purposes.
Synthetic sizes and thickeners
A large group of synthetic 'sizes and thickening agents are acrylic-based polymers, either linear or crosslinked in structure. If used for sizing, they often remain on the fabric to add to the hand properties and softness of the textile material. Besides, complete' desizing is often problematic (Lewin and Sello, 1983a; Lewin and Pearce, 1998).
Poly(vinyl alcohol) used as a synthetic size or thickener has the advantage of being easily recyclable and reusable as it can be removed by dissolving in hot water (Reife and Freeman, 1996). Polyacrylates swell in hot water and need sufficient mechanical impact to be completely removed from the fabric.Polyester-based sizes are broken down by hot alkaline solutions; however, insoluble oligomers may remain on the fabric (Shore, 199Gb). Copolymers of methyl methacrylate are soluble in organic solvents.
Their application and removal occurs in non-aqueous media. If completed in a closed system, they are valued as environmentally benign; however, the machinery necessary needs modification from standard equipment to accommodate the process with sol~ vents other than water.
Crosslinking resins
A large number of cellulosic fabrics, especially cotton and cotton in blends with polyester, are finished with easycare or wrinkle-resistant finishes.
Compounds used for this process form crosslinked networks involving the cellulosic hydroxyl functional groups on the fiber surface ,as well as in the accessible fiber interior, thus providing dimensional stability by fixing the structure in a specific state. Coloration of the textile material has to be performed prior to crosslinking because otherwise the amorphous areas become partially or completely inaccessible. If carried out on dyed material, crosslinking leads to improved wet fastness, locking the dye molecules in place.
Crosslinking of dry fibers, on the other hand, yields short bridges, provides excellent wrinkle recovery and fairly high tensile strength losses. Hand builders are often added to improve the harsh feel of these finished goods.
The selection of compounds explored for this process is very large and is documented in thousands of publications and patents. A major group of crosslinking resins is based on urea and melamine formaldehyde precondensates.
Examples of common resins are dimethyloldihydroxyethylene urea , dimethylolethy-Iene urea (DMEU) and dimethylolpropy-Iene urea (DMPU). Newer compounds include polyfunctional carbamates, 4alkoxypropylene ureas and N-methylolacrylamide derivatives (Vigo, 1994). Catalysts for these finishes are most often inorganic acids or salts, which can cause a drop in DP of the fiber polymer owing to the sensitivity of cellulosics to acidic conditions.
Formaldehydefree compounds include multifunctional carboxylic acids, such as 1,2,3,4-butanetetra- carboxylic acid (STCA,), citric acid and maleic acid (Raheel, 1998). Their finishing effect is somewhat less in most cases.
Flame-retardant finishes
Flammability of textile materials has always been a major problem and numerous attempts have been made to develop effective finishes to improve flame retardancy for all types of textile fibers (Lewin and Pearce, 1998).
The approach taken involves delaying ignition, reducing the amount of flammable gases during a fire, increasing the amount of charred material formed, and enhancing the capability of a material to withdraw from the source of combustion (thermoplastic fibers).
Depending on the fiber type, flame-retardant compounds can be applied as topical finishes, grafted onto the fiber, copolymerized during fiber formation, or incorporated into the spinning dope of synthetic fibers. The durability and effectiveness of these finishes vary.
Commercially available compounds are based on a variety of inorganic metal salts (e.g. antimony, titanium zirconium), on boric acid and its salts, phosphoric acid and its salts, organo-phosphates and halogen containing compounds. Combinations of different compounds are found to have a synergistic effect (Lin and Zheng, 2002)
Originally published in: New Cloth market, September-2010
The selection of compounds explored for this process is very large and is documented in thousands of publications and patents. A major group of crosslinking resins is based on urea and melamine formaldehyde precondensates.
Examples of common resins are dimethyloldihydroxyethylene urea , dimethylolethy-Iene urea (DMEU) and dimethylolpropy-Iene urea (DMPU). Newer compounds include polyfunctional carbamates, 4alkoxypropylene ureas and N-methylolacrylamide derivatives (Vigo, 1994). Catalysts for these finishes are most often inorganic acids or salts, which can cause a drop in DP of the fiber polymer owing to the sensitivity of cellulosics to acidic conditions.
Formaldehydefree compounds include multifunctional carboxylic acids, such as 1,2,3,4-butanetetra- carboxylic acid (STCA,), citric acid and maleic acid (Raheel, 1998). Their finishing effect is somewhat less in most cases.
Flame-retardant finishes
Flammability of textile materials has always been a major problem and numerous attempts have been made to develop effective finishes to improve flame retardancy for all types of textile fibers (Lewin and Pearce, 1998).
The approach taken involves delaying ignition, reducing the amount of flammable gases during a fire, increasing the amount of charred material formed, and enhancing the capability of a material to withdraw from the source of combustion (thermoplastic fibers).
Depending on the fiber type, flame-retardant compounds can be applied as topical finishes, grafted onto the fiber, copolymerized during fiber formation, or incorporated into the spinning dope of synthetic fibers. The durability and effectiveness of these finishes vary.
Commercially available compounds are based on a variety of inorganic metal salts (e.g. antimony, titanium zirconium), on boric acid and its salts, phosphoric acid and its salts, organo-phosphates and halogen containing compounds. Combinations of different compounds are found to have a synergistic effect (Lin and Zheng, 2002)
Originally published in: New Cloth market, September-2010
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