Water

An abundant supply of clean water is necessary in order to run a dyeing and finishing plant. Dye houses are usually located in areas where the natural water supply is sufficiently pure and plentiful. Knowledge of the impurities and how to remove them is important.

Water Consumption in Textiles

Water is used extensively throughout textile processing operations. Almost all dyes, specialty chemicals, and finishing chemicals are applied to textile substrates from water baths. The amount of water used varies widely in the industry, depending on the specific processes operated at the mill, the equipment used, and the prevailing management philosophy concerning water use.

Textile operations vary greatly in water consumption. Following figure summarizes the water consumption of various types of operations. Wool and felted fabrics processes are more water intensive than other processing subcategories such as wovens, knits, stock, and carpet. Water use can vary widely between similar operations as well. For example, knit mills average 10 gallons of water per pound of production, yet water use ranges from a low of 2.5 gallons to a high of 45.2 gallons.

Water consumption varies greatly among unit processes, as indicated in Figure below. Certain dyeing processes and print afterwashing are among the more intensive unit processes. Within the dye category, certain unit processes are particularly low in water consumption (e.g., pad-batch). Different types of processing machinery use different amounts of water, particularly in relation to the bath ratio in dyeing processes (the ratio of the mass of water in an exhaust dyebath to the mass

of fabric). Washing fabric consumes greater quantities of water than dyeing.

Hardness of water

The presence of Calcium, Magnesium salt of bicarbonates, sulfates, Chlorides in water causes hardness of water. The water containing these salts called hard water.

Hardness is of two types:

• Temporary hardness
• Permanent hardness

The bicarbonate salts of calcium and magnesium are called Temporary hardness because boiling will liberate carbon dioxide and precipitate calcium carbonate. Chloride salts of calcium and magnesium are called Permanent hardness because boiling will not cause a precipitate.

Methods of expressing hardness of water

1. PPM (parts per million)
2. English degree
3. German degree
4. French degree

Potential problem caused by hard water in textile wet processing

The impurities which can seriously impair the quality and reproducibility of the dyeing process are :

• Ca ⁺⁺ and Mg ⁺⁺ can cause “pink cotton” which can be extremely difficult to remove by bleaching
• Fe ⁺⁺⁺ and Cu ⁺⁺ which can catalyse excessive local concentrations of the active peroxy bleach radical during peroxide bleaching of cotton, physical damage and even “pinhole damage” of the fibre.
• Fe ⁺⁺⁺ and Cu ⁺⁺ can cause excessive shade change (eg. red shades move dull and blue)

Fig: Influence of Metal Impurities on Reactive Dyeing

• 

Water Quality

The raw material used in greatest quantity in virtually every stage of textile wet processing is water. The quality of textiles produced by any manufacturing operation which employs wet processes, such as preparation, dyeing, and/or finishing, is profoundly affected by water quality.

Table 1 Dye house water Standard

Characteristic	Permissible Limit
Color	Colorless
Smell	Odorless
pH value	Neutral pH 7–8
Water hardness	< 5dH (6.25eH; 8.95fH; 5.2USA)
Dissolved solids	< 1 mg/l
Solid deposits	< 50 mg/l
Organic substances	< 20 mg/l (KMnO4 consumption)
Inorganic salts	< 500 mg/l
Iron (Fe)	< 0.1 mg/l
Manganese (Mn)	< 0.02 mg/l
Copper (Cu)	< 0.005 mg/l
Nitrate ( NO3-)	< 50 mg/l
Nitrite ( NO2- )	< 5 mg/l

Mordant Dyeing

The term ‘mordant’ is derived from the Latin ‘mordeo,’ which means to bite or to take hole of. Mordant dye have no affinity to textile fiber, they are attached by a mordant, which can be an organic or inorganic substances. The most commonly used mordant is inorganic chromium, so sometimes this dye is called chrome dyes. Other inorganic mordants are Al, Cu, Fe and organic mordant. Tannic acid is rarely used.

Mordant improves the fastness of the dye on the fibre such as water, light and perspiration fastness. The choice of mordant is very important as different mordants can change the final colour significantly. Most natural dyes are mordant dyes and there is therefore a large literature base describing dyeing techniques. Fiber most readily dyed with mordant dyes are the natural protein fibers, particularly wool and sometime synthetic fibers modacrylic and nylon.

It is often noted that when a mordant dye forms a lake with a metal, there is a strong colour change. This is because metals have low energy atoms. The incorporation of these low energy atoms into the delocalised electron system of the dye causes a bathochromic shift in the absorption. It is this delocalised electron system which is fundamentally responsible for colour in dyes. Since different metal atoms have differing energy levels, the colour of the lakes may also differ.

The most commonly used mordant dye is undoubtedly hematein (natural black 1), whose status as a natural product supercedes its mode of dyeing, apparently. Others are eriochrome cyanine R (mordant blue 3) and celestine blue B (mordant blue 14), both used as substitutes for alum hematoxylin but with a ferric salt as the mordant. Alizarin red S (mordant red 3) is valuable for the demonstration of calcium, particularly in embryo skeletons

Reason for so named

Some natural and synthetic dyes can be applied or fixed on wool and other textile fibers with the help of an auxiliary chemical called a mordant. These dyes are therefore called mordant dyes.

The mordant have affinity both for a fiber can be applied by using a mordant.

In wool dyeing, only chromium salts are of importance and hence mordant dyes for wool are usually called chrome dyes.

Classification

On the basis of origin

1. Natural: Alizarine

2. Synthetic: Acid chrome

1. Natural Mordant dye

Alizarine: Alizarine is an example of natural mordant dye. It is obtained from the root of the madder. Alizarine is known as polygenetic mordant dye because it develops a variety of colors on different mordants.

Mordant Color

Al Red

Sn pink

Fe Brown

Cr puce brown

Cu yellowish brown

Haematin: this is extensively used before and only one still in use found from logwood. It yield navy blue or black colors of good fastness with chromium compounds. This is used in nylon and wool.

Recipe: haematin dye: 8-10%

Acetic acid: 1cc/l [pH 4-6]

temperature: 50-90°C

Time: 2hr

As it is time consuming process, the natural mordant dyes are used in lesser extent.

2. Acid mordant dyes:

Acid color + chromium = acid chrome

Few dihydroxy azo dyes could co-ordinate so easily with chromium and they could be dyed as acid dyes and mordanted by aftertreatment with K or Na dichromate. Such dyestuffs are known as the acid mordant dyes & are used extensively for wool & also for polyamide fibers.

They have good wet fastness and most of them possess satisfactory light fastness.

Methods of dyeing:

There are three general methods of application of mordant dyes as described below:

1. Chrome mordant process: two bath process

First bath: mordanting with insoluble chromium hydrate

2^nd bath: dyeing

2. Afterchrome process: two bath/single bath process

First bath: dyeing

2^nd bath: mordanting with chromium

3. Methachrome (or chromate) process

Dye + mordant (dichromate) in same bath

Mechanism of dyeing

[fig]

Fig shows an example of mordant dye and shows the formation of dye mordant complex. The chromium cation has a valency of 6 (i.e. 6 bonds) which represented by six lines toward the chromium cation.

The mordant dye is shown to the Cr cation by three of the six bonds. The other three bonds have molecules of water attached to them. It is thought that the three molecules of water are there as an intermediate step only and will gradually be water replaced by another mordant dye anion. Thus two mordant dye molecules form a complex with the Cr cation to form a lake or a dye chromium complex. The formation of these relatively large complexes results in very good wash fastness of dye.

Dyeing procedure of Alizarine dyes

• Boil cotton fabric in solution of 1 part TR oil and

10 part water for 12 hrs

• Dry at 40-60°C

• Treat again with 10°Tw acetate at 60°C for 2hrs

• Dry at 40-60°C

• Again treat with 2 part Sodium phosphate

10 part water at 30-45°C

• Dye 1-1.5% shade with calcium acetate at room temperature for 20 mins

• Wash for 30 min at 70°C

• Soap wash, dry

Dyeing of wool with synthetic mordant:

Dyeing recipe

Dye 1-5%

Acetic acid (80%): 2-5% [pH 4-5]

Glauber salt: 10-25%

H₂SO₄: 1%

L:R= 1:20

Time 45-60 min

Mordanting recipe:

K dichromate: 2%

Temperature: 80-100°C

Time 45 min

M:L = 1:20

Dyeing procedure

• Prepare the dye bath with acetic acid and glauber salt.

• Temperature of the bath is raised to 50-60°C and the goods are entered

• The liquor is brought to boil for 30 min

• H₂SO₄ is now added to complete exhaustion and boiling is continued for another 30 mins

• When dye bath is exhausted and boiling has been continued for long enough for the color to be level, the temperature is allowed to drop and Potassium dichromate is added

• Chroming is continued at boil for 30 min

It is extremely important to make certain that goods are dyed uniformly before the dichromate is added because there will be no further migration afterward. The dye liquor must also be virtually exhausted before mordanting, because of dye remains in solution it will be precipitated on the surface of the fibers in form of its insoluble lake and cause poor rubbing fastness.

Disadvantages:

• Color matching is difficult as the process of mordanting means that the color builds up gradually

• Length periods of applications are both detrimental to protein and polyamide fibers and rather costly.

• Dichromate salts become pollutants once they are discharged into sewerage.

STAIN RESISTANCE

The development of stain repellent general wearing apparel has taken place in response to the consumers’ desire for easy-care garments.

Stain Repellent (or Resistant) Finish: Prevents water and/or oils from penetrating the fabric causing potential aqueous and oily stains to bead up and roll off.

Stain/Soil Release Finish: Enhances the ability of a fabric to release stains during laundering. For a release finish, liquids may not bead up, but usually soak into the fabric.

Combination Repellent/Release Finish: Provides limited stain repellency plus soil release with the objective of overall stain management.

Stain repellants are used on a variety of cotton fabrics from apparel to home furnishings. The main advantage is that the fabrics resist soiling during use. When a spill occurs, it can usually be spot cleaned easily, since the stain is confined to the surface rather than penetrating deep into the fabric.

SOIL RELEASE

Soil release is the term used to describe the cleanability of fabrics by the laundering process. Even though stain resistance finishes made fabrics more resistant to soiling; however, in practice it has been found that soils have a way of penetrating even the best of repellent finishes, the textile item must be cleaned anyway.

Type of Soils

Soils can be defined as unwanted substances at the wrong place. Most common soils fall into one of four categories:

• Water borne stains : Water borne stains are not much of a problem; the stains are soluble in the wash water. Food stains and dried blood, although not water soluble, are responsive to proteolytic enzymes found in most commercial detergents.

• Oil borne stains, Oily soils, e.g. salad oil, motor oil, food grease are particularly difficult to remove from synthetic fabrics such as polyester. The sorption forces between the oils and the synthetic fiber surfaces are so strong that it is virtually impossible to completely remove them by conventional laundering.

• Dry particulate soils: Dry particulate soils such as flour, clay and carbon black are mechanically entrapped in the yarn interstices and reside on the surface of the fiber.

• Composite soils involving oil and grease adsorbed on particulate matter.

How Fabrics are soiled

Soil can be airborne particles that settle by

1. Gravitational forces or are

2. Electrostatically attracted to the fabric.

3. Airborne: Soot is a troublesome airborne particulate that is difficult to remove from fabrics. Drapes, carpets and upholstery are items prone to being soiled by airborne soils.

4. Contact with a dirty surface and they can be ground in by pressure or rubbing.

5. By wicking; liquid soils in contact with fabrics will wick into the structure by capillary action.

6. Redeposition: Soils removed in the laundering process may redeposit back onto the fabric, emulsified oily soils may break out of solution unless the emulsion is well stabilized.

Soil Removal

The adhesion between particulate soil and the fiber depends on the location within the fabric structure, the forces of attraction between the soil and fiber, and the area of contact. The greater the area of contact, the more difficult it is to break the adhesive bond. Fine particles have a greater area of contact. The tighter the fabric, the smaller are the interfiber voids which make also make the outward transport more difficult.

Roll-up Mechanism

Oily-soil removal will depend on the three phase boundary interaction that occurs in the detergent solution. The roll-up mechanism first postulated by Adams argues that for removal to take place, the surface forces generated at the three phase boundary of fiber/detergent solution/oily soil results in progressive retraction of the oil along the fiber surface until it assumes a contact angle of 180 degrees. The various phases of the roll-up mechanism is shown in figure

Soil Release Chemicals

1. Acrylic Soil Release Finishes

The chemical composition of acrylic SR finishes may be generalized as follows:

1. Polymethacrylic Acid PMAA

Poly(methacrylic) acid is completely water soluble and functions as a soil release finish. However the proper amount of cross-linking is necessary before the finish to functions properly.

2. Methacrylic Acid - Ethyl Acrylate Co-Polymers

Co-polymers of methacrylic or acrylic acid and ethyl acrylate have been found to be particularly useful as soil release agents. A particularly good combination for soil release is 70% methacrylic acid and 30% ethyl acrylate.

Practical Considerations and Fabric Properties

1. About 6 to 10% acrylic soil release agent is needed to give good results. The polymeric films are stiff and brittle, giving the fabric a stiff and harsh hand. Being brittle and stiff, the finish tends to cause dusting, excessive needle and sewing thread breakage.

2. Most of the finish is lost after the first wash; however, the small amount remaining is effective for many launderings. The fabric is considerably softer after washing.

3. Excellent soil release results can be obtained when the optimum conditions are met. It is the most effective finish against dirty motor oil.

4. The finish is temperamental. It takes precise condition at the finishing plant to give reproducible results.

5. The finish is cost-effective for work clothing when dirty motor oil release is a significant quality.

2. Fluorochemical Soil Release

A unique block co-polymer, developed by the 3M company (Scotchgard Brand Dual-Action Fabric Protector) combines oil repellency with soil release. The hybrid polymer backbone is comprised of segments based on polyoxyethylene united with segments containing long-chain perfluoroaliphatic groups. Figure shows the structure of the H portion (the hydrophilic portion), the F portion ( the perfluoroaliphatic portion) and the block co-polymer. The individual segments alone do not confer effective soil release; however, when combined into a single molecule, the new composition is effective both as a soil release agent and an oil repellent finish.

Figure Fluorochemical Soil Release Agent

Recipe

A suggested formulation for a woven fabric is shown below (percent on weight of bath):

6% - 8% Fluorochemical repellent

5% Extender (optional)

0.2% Non-rewetting, wetting agent

5% Glyoxal (DMDHEU) resin

1.5% MgCl₂ catalyst

Extender: An extender is less expensive aliphatic or wax water repellent that is used to boost performance and help reduce the amount of fluorochemical needed.

Non-rewetting, wetting agent: A non-rewetting, wetting agent will then evaporate or “flash off” during curing. If a regular wetting agent is used, it may remain on the fabric after curing and interfere with water repellency.

3. Hydrophilic Soil-Release Finishes for 100% Polyester

Effective soil release finishes have been developed for 100% polyester fabrics which are best applied during the dye cycle and are often called Exhaustible SR finishes.

These finishes work best on loosely structured, textured polyester fabrics. Fabrics made from continuous filament or spun 100% polyester yarns are not responsive to these finishes.

Water quickly penetrates treated fabrics and is transported away from the source. This quality has been promoted as improved summer comfort, the ability to adsorb and wick away body perspiration. The finish is not effective at all on polyester/cotton blends. The finish imparts good soil anti-redeposition protection to treated fabrics and a modest measure of antistatic protection.