Optical Brightening Agents (OBA) | Properties of Fluoroscent WhiteningAgents for Textiles

The coloring matter ,whether it is natural or present as a contaminant in the fiber is generally decolorized by different bleaching methods. However the appearance of the textile substrate is somewhat creamish after the bleaching, therefore chemical treatments are become necessary to neutralize the yellow tint of the textile fibers.

There are two methods ,which are generally used for this purpose. By using a blue tinting agent , which absorbs the yellow part of the light and reflected light appears to be of bluish tint. The total light reflected by this mean is less than the total incident light.

By using fluorescent optical brightening agents :-
The OBA s (optical brightening agents ) are most widely used in textiles , paper, detergents and plastics . The optical brightening effect is obtained by the addition of light , which means that the amount of light reflected by the Fluorescent Whitening Agents (also called optical brightener) absorb high energy radiation in the ultraviolet to violet region (330nm-380nm) on the part of characteristic molecules and emit lower energy radiation in blue region in visible spectrum (400nm-450nm), which yields the counteracting the yellowing appearance. FWA should be transparent on the substrate and should not absorb the visible region of the spectrum. The OBAs are effective only when the incident light has a significance proportion (such as daylight) of UV rays. When material treated with OBAs are exposed to UV black light source , it glows in the dark.Anionic OBA’s exhaust on cotton, wool and silk., cationic OBA’s exhaust on acrylic and certain polyesters and nonionic OBA’s are exhaust on all synthetics.

Desired Properties of Fluoroscent Whitening Agents for Textiles Use:
Before selecting an optical brightener for textile application we must look for following properties,
1.it should have good solubility , should not have its own color and good substantivity for the textile substrate under OBA application.
2.OBA’s should have good light as well as wet fastness properties.
3.Its rate of strike on the substarte.
4.Build up and exhaustion properties.
5.Requirement of electrolytes and its sensitivity towards different exhausting agents.
6.Effect of temperature on the exhaustion and build up properties.
7.Application pH range and sensitivity towards change in pH.
8.Effect of water hardness.
9.It should have good leveling and penetrating properties.
10.Should not decompose to colored products on exposure to atmospheric conditions as well as storage , and it should not absorb light in the visible region.
11.it should be compatible and stable with finishing chemicals, auxiliary and process such as heat and temperature.
12.It should be stable and fast to the common oxidative and reductive bleaching chemicals and bleaching systems.

What is Hardness of Water? | Potential Problem Caused by Hard Water inTextile Wet Processing Industry

Hardness of Water:
The presence of Calcium, Magnesium salt i.e bi-carbonates, sulphates, Chloride in water is called causes of hardness of water. The water which contains these salt is called hard water. Soft water is relatively free of calcium and magnesium ions. It produces a rich foamy lather with soap. This is essential for the soap to be an effective emulsifying agent for oils and dirt. With hard water, the soluble sodium salt of soap reacts with the alkaline earth metal ions and precipitates as the useless and undesirable calcium or magnesium soap. The cleaning ability is lost.

Hardness is defined as the presence of soluble calcium and magnesium salts in the water. If these are present in the form of bicarbonates, the hardness is temporary. Heating hard water containing bicarbonates eliminates dissolved carbon dioxide and the causes precipitation of calcium carbonate. Magnesium carbonate is slightly soluble in water but heating will cause its hydrolysis into the much less soluble magnesium hydroxide . Simply boiling and filtering the water therefore eliminates temporary hardness. In regions where water has high temporary hardness, and is used directly without treatment, it is not uncommon to see hot water rinsing and washing baths with a generous crust of chalk (CaCO3) on the inner surfaces. This type of precipitation inside a boiler is also undesirable because the scale reduces the efficiency of heat transfer.

Mg(HCO3)2(aq) =  MgCO3(s) + CO2(g) + H2O

MgCO3(aq) + H2O = Mg(OH)2(s) + CO2(g)

Consequences of Using Hard Water:
The use of hard water in a textile dyeing or finishing mill can have some serious consequences. 
These include:

(1) precipitation of soaps;
(2) redeposition of dirt and insoluble soaps on the fabric being washed – this can cause yellowing and lead to unlevel dyeing and a poor handle;
(3) precipitation of some dyes as calcium or magnesium salts;
(4) scale formation on equipment and in boilers and pipelines;
(5) reduction of the activity of the enzymes used in desizing;
6) decreased solubility of sizing agents;
(7) coagulation of some types of print pastes;
(8) incompatibility with chemicals in finishing recipes.

Sequestering Agents | Functions of Sequestering Agents in Dyeing |Characteristics of Sequestering Agents

Introduction:
Sequestering agent is a dyeing auxiliaries which is used during dyeing for removing hardness of water.Sequestering agents combine with calcium and magnesium ions and other heavy metal ions in hard water. They form molecules in which the ions are held so securely (sequestered) that they can no longer react.

Sequestering agent

The most undesirable impurities in Fibre, Common salt, Glauber salt, Caustic Soda and Soda ash are the di- and tri-valent cations, e.g., Ca++, Mg ++ Cu ++, Fe+++ etc. These ions increase hardness of the process bath and generate iron oxides in the bath. Calcium and Magnesium reacts with alkali and precipitates as a sticky substance on the textile material, which creates patchy dyeing and discoloration of the fibre. The ferric oxide with cellulose and creates small pinhole on the fibres also damages the machinery by scale formation in the nozzles and base.

To overcome these deleterious effects in the scouring and bleaching bath adequate amount of sequestrant must be used. Sequestrants prevent di-and tri-valent metal ions, e.g., Cu++, Fe +++ , Mn ++, Ca++, Mg++ etc from interfering with the chemical processing of the textile material. It prevents catalytic damage of cellulosic fibres in bleaching hath during hydrogen peroxide bleaching.

Thus, unwanted metal salts cause a lot of problems in processing. Now, with the focus on minimising costs and maximising efficiency, consistency and fastness are two important parameters that every dyer would like to achieve first time. This reduces reprocessing costs, making him competitive.

The dyer has to use a suitable sequestering agent in the process, wherever it is required. Selection of the right sequestering agent is very important. First and foremost, the sequestering agent should chelate offending metal ions under the given condition and should form a stable complex, which does not decompose over a prolonged processing period.

Now a days many other types of sequestering agents are widely used in textile processing. The three main stages in which sequestering agents are used are

1. Pretreatment
2. Bleaching
3. Dyeing

There are some main type of commercial sequestering agents are:

1. Aminocarboxylic acid base products
2. Phosphates and Phosphonates
3. Hydroxy carbroxylates
4. Polyacrylates
5. Sugar acrylates

1. Aminolycarboxylates
In aminopolycarboxylates, it is assumed that one molecule of sequestering agent complexes with one ion of metal. Depending upon the pH of the medium, i e acidic, neutral or alkaline, the preferential sequestering order or each product could change.

Some of the characteristics of some of these sequestering agents are summarised as below:

EDTA: Good sequestering agent for calcium and magnesium at alkaline pH but no sequestering agents on Fe3+ at alkaline pH. Not stable with oxidising agents. Low solubility in acidic medium.

NTA: Sequestering of Fe3+ only at acidic pH but sequestering of Cu2+ between pH 3 to 12. Low solubility in acidic medium. Not stable with oxidising agents.

DTPA: Good sequestering action Fe3+ under alkaline pH but complexes with alkaline earth salts are less stable than EDTA. slightly more resistant to oxidising agents. Low solubility in acidic medium.

2. Phosphates and Phosphonates
These sequestering agents are divided in two broad classes:

• Inorganic polyphosphates such as sodium hexameta phosphate (SHMP), sodium polyphosphate, sodium tripolyphosphate, sodium trimeta phosphate, sodium pyrophosphates
• Phosphonated aminopolycarboxylates such as EDTMP, DETMP, ATMP, HEDP, DTPMP

Inorganic phosphates work under specific conditions and work as sequestering agents by converting troublesome metal ions into water soluble complex by a process of ion exchange.

Phosphates of aminopolycarboxylic acids or phosphonates are derivatives of phosphorous acid and are characterised by a C-P bond, which has stronger hydrolytic stability than the P-O-P bond of polyphosphates. This type of sequestering agent has emerged as a major class of sequestering agent, since these possess more features than mere chelation. These characteristics are: Threshold effect, i e inhibition of precipitation of CaCO3, CaSo4 with sub-stiochiometric quantities of inhibitor

1. Corrosion inhibition
2. Resistance to hydrolysis
3. Deflocculation i.e. stability effect on colloidal suspensions

Compared to popular amino polycarhoxylic acid based sequestering agents, these phosphonates based sequestering agents have a high chelation ratio. Apart from better chelation value or better chelation ratio, these phosphonates also have better iron chelation than EDTA and NTA.

3. Hydroxy Carboxylic Acids
Organic compounds that have several hydroxylic groups often have the property of preventing precipitation of bi and trivalent metal cationis in an alkaline medium. Some of the well known products in this category are:

Citric acid, Tartaric acid, Gluconic acid and Oxalic acid.

These are less important sequestering agents, compared to aminocarboxylic acid or phosphonates. Gluconic acid/sodium gluconate has been found to be an effective chelating agent for iron under alkaline conditions.

4. Polyacrylates
Polyacrylates are effective dispersants, with mild chelation values and protective colloid properties. The chelation values of polyacrylates have no demetallising effect on metal containing dyestuffs. They are completely non foaming.

They are very suitable as dyebath conditioners, soaping agents and washing aids. Being non surface active agents they are easily rinsable and thus reduce the quantity of water required for removing their traces from the substrates, unlike all surfactants. The typical chelation values offered by polyacrylates do not come close to the chelation values offered by amino polycarboxylates or the phosphonates. This problem has been overcome by development of sugar acrylates.

5. Sugar Acrylates
Sugar acrylates have sequestering values as high as amino polycarboxylates or the phosphonates. They are biodegradable, effective components in cellulosic fabric pretreatment during desizing, scouring, bleaching and mercerising. These products are characteristed by good chelation values from the acidic to the alkaline range and from temperatures of 45 to 115øC. They also exhibit no demetalising effect on metal-containing dyestuffs and are non-foaming. They are ideally recommended in pretreatment for desizing, scouring and bleaching and as dyebath conditioners during the cellulosic dyeing.

Factors to be taken into consideration while selecting a sequestering agent for the process :

1. Stability Constant: 

As chelation is a reversible reaction, the equilibrium is dependent on the process pH and the concentration of the metal ions and chelating agent, which react together to form a chelate. The stability of the metal complex is expressed in terms of its stability constant. If we represent chelation of metal ion, Mm+ with sequestering agent, An- as: Mm+ + An- MA(m-n) then the stability constant is Ks = MA (m-n) (Mm+) (An-)

A high value of Ks indicates high sequestering effect. For example, in the case of aminopolycarboxylates, the stability constant for same metal iron increases in the order NTA, EDTA, DTPA.

In the case of metal ions, the stability constant increases in the order.

From the above information it can be deduced that the NTA-Mg2+ complex has the least stability, whereas DTPA - Fe3+ has the highest stability. Thus, in a process, if more than one metal ion is present, the metal ion having the highest stability will be chelated preferentially. If chelating agent is present in sufficient quantity, the metal with the highest stability constant will be chelated completely, followed by the next metal ion in te order given above. Even after chelation is complete in this order, if additional metal impurity, with metal having a higher stability constant, is introduced, then this metal ion will displace low stability constant metal ions from the complex. For example, Fe3+ displaces Ca2+ from a Ca2+ chelating agent complex. Of course, the chelating agent should be capable of chelating Fe3+ under given conditions.

2. The pH of the Process: 

The pH of the system will influence the formation of the chelation complex. For example, NTA, EDTA cannot chelate Fe3+ under alkaline conditions, whereas DTPA can. HEDP can chelate Fe3+ up to pH 12, and so also gluconic acid.

3. Demetalisation:  

This property is particularly important for dyeing and printing with premetallised dyes - for example, some direct, reactive and premetallised metal complex dyes. If Ni2+, Cu2+, Cr3+, Co2+ or Fe3+ is present in premetallised dyes, these could be preferentially chelated ahead of Ca2+ and Mg2+, due to the higher stability constant of these metal ions. Therefore pretrials in the lab are required to establish the suitability of the chelating agent, and also to arrive at the optimum concentration for the given process, when premetallised dyes are to be used.

4. Other Features:  

Stability of chelate to prolonged process periods, dispersing properties, crystal-growth inhibition, effect on equipment, etc. are also to be considered when selecting a commercial sequestering agent.

Oxidizing and Reducing Used in Sulpher Dye | Reducing Steps of SulpherDyes | Oxidation Step of Sulpher Dye

Some oxidizing and reducing agent used in Sulpher Dyeing

Oxidizing  Agent	Reducing Agent
Potassium Dichromate(K2Cr2O7)	Sodium Sulphide (Na2S)
Acetic Acid(CH3COOH)	Sodium Hydro sulphide(NaHSO3)
Sodium Perborate	Thioglycolic acid
Sodium per carborate	Thio Salicylic acid
Sodium peroxide(NaO2)	Pseudo thiohydantion
Hydrozen peroxide(H2O2)

Reducing Steps of Sulpher Dyes:

Reducing step is the most important in the application of Sulpher dyes. Unless the dyes are converted in to the completely soluble form, the full colour value cannot be achieved. The solubility of reduced Sulpher dyes varies appreciably from dye to dyes. For example yellow Sulpher dyes are the last soluble dyes (i.e. most insoluble dyes)

The reducing agents are used for Sulpher dyes are:-

1. Na-Sulphide.
2. Na-Hydro Sulphide.
3. Thioglycolic Acid.
4. Pseudo-Thio-Hydantion.

But Na Sulphide is the most widely used reducing agent for dissolving Sulpher dyes. They may be replaced by Na-Hydrosulphide in some cases. But the latter is a more powerful reducing agent. So, over reducing of the dye may take place, leading to a product having lower affinity for cellulosic fibres. Hence lower colour yields are obtained along with wastage of dyestuff. In some cases lower wash fastness results if Na-Hydrosulphide is used. It also decreases the life time of the vessel.

For dissolving the Sulpher dyes, reducing agent is taken according to the weight of dye. In practice, they are taken at same weight. For example, 10 gm dye is mixed with 10 gm reducing agent. Then the mixture is boiled after adding necessary amount of water to get a good solution. For this wooden vessel or enameled iron vessel are used but not iron vessels. Because iron reacts with Sulpher.
 

Oxidation Step of Sulpher Dye:

After dyeing the reduced water soluble form of the dyes have to be converted in to the original water insoluble form by oxidation. 

The commonly used oxidizing agents are-

1. Potassium dichromate
2. Na-Perborate.
3. Na-percarborate.
4. Na-peroxide.

The method of oxidizing selection plays an important role in the development of correct shades & their optimum fast5ness properties. Rinsing off the dyeing should be done rapidly to remove as much as Na-Sulphide as possible. If even after washing some Na-sulphide or Soda-ash stays on fabric, we use CH3-COOH to neutralize them. 

Using of different oxidizing agents may give following results-

1. Use of perborate or percartborate in presence of acetic acid gives brighter shade.
2. Treating dyed material with dichromate without rinsing reduces colour losses but causes dull shade.

The chemistry of oxidization is as below:-

Dye-S-S-Dye +2[H] = Dye- SH + HS-Dye

Dye- SH + HS-Dye +[O] = Dye-S-S-Dye +H2O

Assistants for Dyeing with Reactive Dyes | Dyeing Assistants ofReactive Dye

Assistants used for dyeing with reactive dyes: The following assistants are used in dye bath for dyeing with reactive dyes.

1) Salt:
As a salt NaCl is used widely. The salts do the following things-
- Salts are used to increase the affinity of dye to fibre.
- It decreases the hydrolysis rate of dyes.
- It neutralizes the electro negativity of fibre surface when immersed in solution.
- It puts extra energy to push the dye inside the fibre polymer i.e. increase absorption of dye.

The amount of salt and used depends upon the shade to be produced.

For light shade – 10gm/ L salt is used.
For medium shade – 20 gm/ L salt is used.
For dark shade – 30gm/L salt is used.

2) Alkali:
Alkali is used for the following purposes:
- Alkali is used to maintain proper pH in dye bath and thus to create alkaline condition.
- Alkali is used as a dye-fixing agent.
- Without alkali no dyeing take place.
- The strength of alkali used depends on the reactivity of dyes.
- As strong alkali caustic soda is used to create pH 12.5-12.
- As medium alkali soda ash (Na2CO3) is used to create pH 11-12 when dye is of medium reactivity.
- As weak alkali NaHCO3 is used to create pH 10-11 when dye is highly reactive.

3. Urea: 

Urea is used in continuous method of dying. It helps to get required shade of dye. To get dark shade more urea is used and to get light shade less amount of urea is used.

4. Soaping: 

By soaping, the extra color is removed from fibre surface. Thus washing fastness is improved. Soaping increases the brightness and stability of the dye.