TEXTILE CHEMISTRY ASSIGNMENT. 02

Submitted by; NALAKA SAMPATH.

Submitted to; Sir. ARIYASIRI NANDASENA.

Describe the various colourant and the reactions involved, along with the

controls necessary, to deposit an insoluble colourant, with in cellulosic

fibre.

Compare the qualities of each colourations obtained.

Introduction In cellulosic fibres, there are many methods used in dyeing process. When dyeing a fabric from cellulose or any other fibre type, dyestuff should be strictly attached to fibre, to have good fastness property. These dye stuffs make some kind of bonds to get stuck with fibres. Hydrogen bonds, van der walls bonds, and covalent bonds are some of them. To make this type of bonds, first of all dye stuffs should be solubilized in water solution. In dyeing of textile materials, every used dyes are not soluble in aqueous medium. There’re some water insoluble dyes also used in textile dyeing industry. After having some chemical treatments, they take from their water soluble form for further processing. We discus here how to make a dyed fabric from water insoluble colourants, and the reactions involved in that process and their qualities. Following are the mostly used dyes for textile materials.

1. Vat dyes. 2. Sulfur dyes. 3. Disperse dyes. 4. Coupling dyes. 5. Pigments. 6. Reactive dyes. ( Reactive dyes are difference from other

dyes, because water soluble reactive dye molecules react with fibres and then after the dyeing, they become water insoluble. That’s why this dye has taken into water insoluble category. )

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01. Vat dyes Vat dyes are insoluble in water and they can not be used directly for dyeing fabrics. Vat dyes are only possible in dyeing, when they are in their reduced form, which is known as leuco form. For the reduction, it is used sodium hydrosulphite with caustic soda.

When this compound reduced from sodium dithionite, it can be applied to cellulose fibres. But when we look at the indigo’s soluble form (leucoindigo), it’s containing negative charge. The fibres of fabric, we’re going to dye, also contain negative charge. So this dyeing process is not possible under those conditions. There should be a way carry out the dye ions to fibre by reducing the repulsion between dye ion and cellulose molecule. For this purpose we add some sodium salt to the dye bath. Negatively charged dye ions gather around these sodium ions and it makes an affinity to fibre from dye ion. After dye molecules are deposited in fibres, once again they are oxidized to make them water insoluble. Oxidizing reaction is done by oxygen in air. After the dyeing process is end, the dyed materials are rinsed away, then it’s treated with acid, rinsed again, and then it’s soaped at the boiling temperature for the final product.

When dyeing with Indigo vat dye……

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The conditions of dyeing process.

Reduction of insoluble dye molecule is done at room temperature. During the dyeing process temperature is gradually raised about to

80o

C.

The time for dyeing, depend on the temperature, 45mnts for 60 o

C

and 20mnts for 100 o

C. Salt are added to improve absorption of dye molecules.

The qualities of Vat dye. Vat dyes are primary choice where the highest degree of fastness to laundering, weathering and light are required.

02. Sulphur dyes

Sulfur dyes are primarily used for dark colors such as blacks, browns, and dark blues. Same as we discussed in vat dyeing process, in normal state of sulfur dyes are not in water soluble form. It should be converted into reduced form (leuco form), after having a reducing reaction from sodium sulphide or sodium hydrosulphite, to make it water soluble.

After depositing dye molecules in fibres, once again they should be oxidized, to take it into water insoluble state. This reaction can be done by O2 in air or by using oxidizing agents like hydrogen peroxide or sodium bromate in an acidic solution. Same as in vat dyeing, absorption of dye molecules to fibres can be increased by adding some salt to dye bath.

To improve the fastness property to light and washing of sulphur dyes, sometimes it’s added dichromate and copper sulphate contained mixture. The conditions of dyeing process.

The dye has to be treated with a reducing agent and an alkali at around 800C.

Add some salt to improve absorption of dyes to fibres.

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The qualities of sulphur dyes.

Generally have good wash-fastness. Have poor fastness to rubbing. Easy to apply and inexpensive.

03. Disperse dyes. Azo disperse dye is a one of disperse dye, characterized by their presence of one or more azo groups (–N=N–). These azo groups make the linkage of molecule, between two or more aromatic rings. To prepare an azo dye, a primary aromatic amine should be converted in to a diazonium compound. Then prepared diazonium compound should be coupled with phenols, naphthols, or other aromatic amine.

Coupling with those products should be carried out in acidic or basic medium, as they required. Diazotization reactions should be carried out in

temperature, 0 o

C – 5o

C, as those compounds are very unstable.

Disperse dyes are the first invented for dyeing cellulose

acetate fibres. They contain hydrophobic molecule which make these dyes are sparingly soluble in water medium.

Disperse dye molecules have a small polarization, and they contain one or more anthraquinone or azo groups. They don’t have charged in cationic or anionic. Sometimes it can be attached to polar functional group like –NO2 and –CN. This polarity makes those molecules to improve their solubility by bonding dipolar bonds between dye and polymer. The general structure of disperse dye is smaller and co-planer,

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and it’s none ionic in charge. Shape of these dyes makes it easier to slide between tightly packed polymer chains.

Disperse dye’s absorption into the fibre is carried

out at high temperatures and high pressures. Dyes are only can apply to a fibre in their disperse form. To keep them well dispersed state during dyeing, dispersing agents are added while grinding the dye.

The exhaust dyeing with disperse dyes should

be carried out in atmospheric conditions. And also it uses special swelling agent called “carriers”. These carriers help in number of ways in dyeing process. Carriers in fibres swell to open up the sites for dye molecules to enter the fibre easily. Carriers influence the leveling property of dye, and sometimes it help to stability of dispersing agent in dye bath.

Usually disperse dyes cannot give a dark

shades to cellulose acetate fibres from exhaust dyeing. This dyeing process is done at 800 C. But there’s some method to give dark shades from disperse dyes. Dyeing without carrying agent at high temperature and high pressure, is used method to fulfill this necessity.

The conditions of dyeing process. Add a disperse agent during the dyeing to keep dye in dispersing

form. Add a carrier, which act in number of ways, when exhaust dyeing. Dye is absorbed to fibre around 800C, which cannot give dark shade

to cellulose acetate. The diffusion of dyestuff to the fibre is started around 1000C of

temperature. The dyeing without carrier is done at high temperature around

1300C, and high pressure, which can give even a dark shade to the fibre.

The qualities of disperse dye. Fastness to light and washing are generally quite good.

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04. Coupling dyes.

Naphthol dyes and azoic dyes are used dyes in coupling dyes. This type is difference than other dyes, because of its dyeing mechanism. It makes insoluble large molecule, using a two reactive organic component, with in a fibre. The two components used is a diazonium and a naphthol. First of all diazonium salt should be created from aromatic amine.

Diazotization reactions have described under the topic, Disperse dyes.

When coupling dyes, diazonium salt acts as an electrophile in the reaction. A substitution reaction takes place during the process. The pH value of the coupling reaction is quite low. The actual value depends on the component of coupling reaction. And it is mildly acid or neutral. The qualities of coupling dyes.

The dyes which have coupling reaction with in a fibre have good fastness property to washing and rubbing. The final dye molecule come after coupling reaction, is bigger than other types of dyestuffs. So the coupled dyestuff entangle with cellulose polymer. Then it difficult to remove either washing or rubbing.

The light fastness is depended on the coupled component with diazonium salt.

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05. Pigments. Pigments are not dyes but used in textile industry as a colourant, for dyeing and printing. In this dyeing method there’s no actual chemical reaction involved between dye and fabric. In briefly pigments are applied as a layer on fibre, and then they’re bonded to the fibre surface by using with the help of “binders”.

These pigments are insoluble in water and they don’t have an affinity to fibres. (But the affinity is not important character.) Insoluble pigments take as dispersion, they attached to the fibre in this state.

Pigments are maintained as a stable dispersion in aqueous medium, by anionic surfactants. This type of pigment is known as pigment resin color (PRC). These pigments are used in printing. In pigment dyeing, attachment of a cationic polymer to fibre may cause to pigments absorption to the fibre surface. This attachment of cationic polymer is done in pre-processing stage. But this absorption is not a strong one, because bonding occurs between cationic surface of the pre-processing agent and anionic surfactant of the pigment surface. The main disadvantage of pigment is hardening of hand. The chemicals used to bind the pigments to fibre are the reason for this fault. The qualities of pigments.

The fastness property will depend on the degree of strength in pigment and fibre binding.

Pigments have poor rubbing fastness as they’re attached with fibre, only from fibre surface.

Light fastness is generally good for pigments.

06. Reactive dyes. Reactive dyes react with cellulose fibre and they make covalent bond between fibre and dye molecule. Therefore reactive dyes are extremely washing fast. A dye like a direct dye reacts with a trichlorotriazine molecule to form this kind of dyes. The -NH2 group of dye ion react with the –Cl of trichlorotriazine. Then it releases a HCl as a byproduct.

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During the dyeing process, the Cl atom of dye molecule reacts with a primary alcohol group, which attached to the cellulose polymer. Then it combines a dyestuff and cellulose polymer.

Some dyestuffs have low degree of fixation. This occur due to the only having one functional group with dye molecule. To minimize this this situation, dyestuffs are attached to two or more reactive group. Dyestuffs which contain two reactive groups are known as bifunctional dyestuffs, which contain more than two functional groups are known as multifunctional dyestuffs. Temperature, time and pH of the dyeing process depend on the reactive group connected to dyestuff. Each dye class has difference groups or group, so the demanding difference temperature, pH and time for complete fixation. As the reactive dyes are chemically bonded with cellulose fibres, they are difficult to remove from fibre without having a chemical reaction. So they have good washing fastness compared other dyes, and other fastness properties too.

Here is short comparison about the above colourants qualities.

The key to chart; **** - extremely good. *** - good. ** - generally good. * - poor.

Colourant Rubbing fastness

Light fastness

Washing fastness

Vat dyes

Sulphur dyes

Disperse dyes

Coupling dyes

Pigments.

Reactive dyes

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Some stages of dyeing process have listed below.

The dyestuffs in dye bath continually move to the fibre interface.

When they get close to the fibre, the movement of dye molecules becomes slower. The dye solution on the fibre surface is nearly motionless. They make a surface layer on fibre.

Dyestuff’s movement to the fibre surface is depended on diffusion.

The diffusion rate of dye solution is depended on diffusion boundary layer.

To increase diffusion rate, it must be reduce the thickness of diffusion boundary.

The dyestuffs combine with fibre on its surface, by bonding hydrogen bond and van der waals attraction.

There is some repulsion between dye ion and cellulosic fibre, which

have (-) zeta potential. If this repulsion is lower than attraction between and dyestuff,

dyestuffs can migrate to the fibre. In reactive dyeing, it uses salts to improve this.

Dye molecules diffuse from the fibre surface to the interior of fibre.

Diffusion will continue until they come to an equilibrium of dye concentration in fibre and in dye.

Increasing the dyeing temperature can enlarge the diffusion of dye molecules in the interior of fibre. It results to increases the dyeing rate.

When dyeing with reactive dyes, dyestuffs react with fibres during the diffusion.