Vat Dye

Vat dyes are used in the dyeing and printing of all types of cellulosic fibre, and also of blends of cotton with polyester. In their coloured form they are insoluble pigments, so their application depends on reversible reduction–oxidation (redox) reactions (page 42). In the dyebath the pigment is converted into a water-soluble form using a strongly alkaline solution of a powerful reducing agent. This forms the sodium ‘leuco’ compound of the dye, which is soluble in water but often different in colour from the original pigment. It is then allowed to dye the cellulose in this water-soluble form; once exhaustion is completed the leuco compound is oxidised to the coloured form.

The large majority of vat dyes are based on the anthraquinonoid or the indigo (or thioindigo)
chromophores; indigo, one of the oldest dyes still in use, remains popular through the wide use of
indigo-dyed denim. Several of the anthraquinone dyes are complex polycyclic quinones (Appendix
1), and they all possess two carbonyl groups ( C=O) linked by alternate single and double bonds in
a conjugated chain. This molecular arrangement is responsible for the easily reversible redox
reactions on which the application of vat dyes depends.

Printing

Modern procedures for printing textile goods may be traced back to the block printing of silks in ancient China. In this method a wooden block with a raised pattern on the surface was dipped into the printing colorant and then pressed face down on to the fabric. The desired pattern was obtained by repeating the process using different colours. Printing by brushing colorant through thin metal stencils and the transfer of illustrations to the printed page from engraved rollers in a printing press were also widespread by the fifteenth century.

Block printing remained a practical proposition until the roller printing machine was invented by James Bell in 1783. This enabled six colours to be printed at a rate equivalent to that of 40 hand-block printers. The success of the machine depended on the hard rollers, each of which bore an engraving (i.e. an intaglio engraving, in which the depth of the recess on the roller determines the intensity of the print produced) corresponding to a particular colour component of the design. The machines were capable of continuously printing six different colours in sequence, with the rollers pressed against the fabric.

PRINTING PASTE

In textile printing the printing paste on the fabric may be regarded as a miniature localised dyebath containing the dye and all the additives necessary for the coloration process. Thus print pastes for reactive dyes will contain alkali, those for vat dyes will contain reducing agent, and so on. Dye diffuses into the fibre from the paste, aided either by a rise in temperature and absorption of water within a steamer, or by the fibre becoming a more favourable environment as the paste becomes affected by the heat of a dry fixation process. To a certain extent the nature of the fixation process to be used influences the type of additive mixed with the print paste.

Textile printing pastes are usually water-based, but for paper printing a solvent base may be chosen since for this purpose rapid drying through evaporation is more important than substrate penetration. Compared with a dyeing process on a similar fabric, dyes of lower substantivity and high water solubility are often chosen, so as to ease the final washing-off process. The viscosity of the print paste needs careful attention to ensure that it flows smoothly from the source of application on to the fabric. Too high a viscosity impedes transfer and penetration, and if the paste does not spread evenly on the fabric the surface appearance will be unsatisfactory. Most thickening agents are polymeric in nature, and the overall chemical composition of the paste must be compatible with the dye fixation mechanism. For example, there is little point in using a polysaccharide thickening agent (such as starch) for reactive dyes, because of its chemical similarity

Softener

Softening agents are applied to textiles to improve their hand, drape, cutting and sewing qualities. An effective softener must be readily dispersible in rinse water and rapidly absorbed so that uniform deposition on the fabric can occur within a relatively short treatment time and generally, exhaustion should take place in about 5min for the softener to be effective and economically usable.

 

Fabric softener (also called fabric conditioner) is used to prevent static cling and make fabric softer. It is available as a liquid or as dryer sheets. Popular brand names include Downy (Lenor), Snuggle, Bounce, Comfort and Sta‐Soft.

It is work by coating the surface of the cloth fibers with a thin layer of chemicals; these chemicals have lubricant properties and are electrically conductive, thus making the fibers feel smoother and preventing buildup of static electricity. Other functions are improvements of iron glide during ironing, increased resistance to stains, and reduction of wrinkling.

REASONS FOR USING SOFTENER

• As the textile material goes under various mechanical and chemical processes that make the surface of the material harsh. For example, Removal of natural oil and waxes by scouring and bleaching. 
• Resin finishing of textile material also imparts some degree of harshness.Soaping of textile material also add harsh feeling to the material. 

DESIRABLE PROPERTIES OF TEXTILE SOFTENER   

• It should be easy to handle. 
• It should have good compatibility to other chemicals. 
• It should not affect the shade of the material. 
• It should not affect the fastness of dyed material. 
• It should not cause any yellowing effect on dyed and finished material. 
• It should be stable to high temperature. 
• It should be non volatile by water vapor.  
• It should be non toxic and non caustic.   
• It should be easily bio degradable. 

MECHANISMS OF SOFTENING EFFECT

Softeners provide their main effects on the surface of the fabrics. Small softener molecules,  in addition, penetrate the fiber and provide an internal plasticization of the fiber forming polymer by reducing of the glass transition temperature. The physical arrangement of the usual softener molecules on the fiber surface is important. Depending on the ionic nature of the softener molecule and the relative hydrophobicity of the fiber surface, cationic softeners orient themselves with their positively charged ends toward the partially negatively charged fabrics (zeta potential), creating a new surface of hydrophobic carbon chain that provide the characteristics excellent softening and

seen with cationic softeners. Anionic softeners, on the other hand, orient themselves with  their negatively charged ends repelled away from the negatively charged fiber surface. This  leads to higher hydrophilicity, but less softening than with cationic softeners.   The orientation of non-ionic softeners depends on the nature of the fiber surface, with the  hydrophilic portion of the softener being attracted to hydrophilic surfaces and the  hydrophobic portion being attracted to hydrophobic surface. 

 

CLASSIFICATION OF SOFTENER 

1. Cationic softeners
2. Anionic softeners   
3. Non ionic softeners 
4. Amphoteric softeners 
5. Reactive softeners 
6. Silicone softeners 
7. Anti­ozone Softener 
8. Urethane Softeners
9. Macro softener
10. Micro softener 

Roving

Roving is done in the roving machine. The task of this machine is to transform the sliver coming from the drawframe into roving. It is present in the carded ring spinning cycle and in the combed ring spinning cycle, in the first case it is found following the post-carding drawframe (one or two drawing steps), while in the second case after the post-combing drawframe.

Transforming the sliver into roving occurs in a continuous manner through three stages:

• Drawing
• Twisting 
• Winding

Task Of Roving Frame

1. Attenuation- drafting the sliver into roving 
2. Twisting the drafted strand 
3. Winding the twisted roving on a bobbin 

The Factors Affecting Roving Strength are as follows 

• The length, fineness, and parallelisation of fibres
• The amount of twist and compactness of the roving
• The amount of twist and compactness of the roving