UNIT-2(Technology of Printing-I (NTT-508) TC 3rd Year)

Preparation of fabric for Printing

• Steps of preparation of fabric for printing is almost same as fabric preparation for dyeing.

• Following are the main steps:SingeingDesizingScouringBleachingMercerizing (For cotton fabric)

Singeing

• It is a process of removal of protruding fibres from the surface of fabric using thermal and mechanical energy. Singeing is mostly carried out using gas singeing machine.

• Advantages: 1. Fabric surface become smooth, so the

printing will be sharp. 2. Pilling problem of fabric is reduced.

Desizing• Desizing is a process of removing sizing chemicals from the

fabric. There are four methods for desizing. These are:• Rot steeping ( Treating fabric with hot water and kept for

overnight)• Oxidative desizing (Sodium Hypochlorite: 2.0-5.0 gm/lit. at

room temperature at pH 7.0 or Hydrogen peroxide:3.0-6.0 gm/lit of H2O2 and 7.0-15.0 gm/lit Sodium Hydroxide, 40oC for 12-16 hours)

• Acid steeping (Treating fabric with dilute acids-Sulphuric or Hydrochloric acids)

• Enzymatic desizing (Most common process, Amylase enzyme is used for desizing purpose, Enzyme solution in water with 0.5% to 2.0% on weight of fabric enzyme, Require quantity of Common salt is also added at Neutral pH)

Scouring • It is process to remove all undesirable

impurities (Natural: like dirt, vegetable matter, grease wax etc and Added: like remaining size material, stains etc.)

• Natural Impurities: Based on the composition of natural material like cotton, wool, silk etc.

Bleaching

• This is a process of whitening-fibers, yarns, or fabrics having natural colour.

• It is carried out by using oxidising agents like

• Sodiumhypochlorite (NaOCl)

• Hydrogen peroxide (H2O2)

• Sodium Chlorite (NaClO2)

Bleaching

• Oxidative bleaches oxidize color bodies into colorless compounds. For example, double bonds are known to be oxidize into epoxides which easily hydrolyze into diols.

• The major bleaching agents used in textile preparation are sodium hypochlorite, hydrogen peroxide and sodium chlorite.

MERCERIZATION• Treatment of cotton yarn or fabric with concentrated sodium

hydroxide for a minute or less is called mercerization. • The process of mercerization is named after the John Mercer,

who was the first to discover in 1844, that the treatment of cotton with concentrated sodium hydroxide solution brought about permanent changes in the fibre's properties.

• He claimed that the treatment resulted in significant increase in the tensile strength, water absorbency and dyeability of the fibres. It was also observed by him that treated material underwent considerable shrinkage.

• The excessive shrinkage in the cotton yarn and fabric make this process nonviable in the industry thus it was not commercialized.

• It took more than forty years until it was found by H.A.Lowe that shrinkage could be prevented if the yarns or fabrics were kept under tension during mercerization. When mercerizing is done under tension, the fibre also acquire a silk-like luster.

MERCERIZATION

• This is the process applicable only on cellulosic fibres especially cotton.

• The main purpose of mercerization is to alter the chemical and physical properties of the fibre.

MERCERIZATION

What changes mercerizing bring to cotton?

• Improved luster• Increased ability to absorb dye• Improved reactions with a variety of chemicals• Improved stability of form• Improved strength/elongation• Improved smoothness• Improved hand

Wetting Agents

• A substance is referred to as a wetting agent if it lowers the surface tension of a liquid and thus allows it to spread more easily.

• There are two types of intermolecular forces:• Cohesive Forces: The forces exerted between

molecules holding them together. If cohesive forces are strong, a liquid tends to form droplets on a surface.

• Adhesive Forces: The forces between liquid molecules and a surface. If adhesive forces are strong, a liquid tends to spread across a surface.

• Wetting agents are substances that reduce the surface tension of water to allow it to spread drops onto a surface, increasing the spreading abilities of a liquid. Lowering the surface tension lowers the energy required to spread drops onto a film, thus weakening the cohesive properties of the liquid and strengthening its adhesive properties.

• One example of how wetting agents work is in the formation of micelles. Micelles consist of hydrophilic heads forming an outer layer around lipophilic tails. When in water, the micelles' tails can surround an oil droplet while the heads are attracted to the water.

Wetting Agents

• Dish soap is a great example of a wetting agent. With all the food oils and such on the plate cohesive forces make it difficult for the water to spread and clean the plate. The soap dissolves all theses unwanted particles, exposing a clean surface. The soap also lowers the surface tension of water, allowing it to spread evenly across the entire surface.

Wetting Agents

Types of Wetting Agents

• There are four main types of wetting agents: anionic, cationic, amphoteric, and nonionic.

• Anionic, cationic, and amphoteric wetting agents ionize when mixed with water.

• Anions have a negative charge, while cations have a positive charge.

• Amphoteric wetting agents can act as either anions or cations, depending on the acidity of the solution.

• Nonionic wetting agents do not ionize in water. A possible advantage for using a nonionic wetting agent is that it does not react with other ions in the water, which could lead to formation of a precipitate.

How to Tell if a Liquid Contains a Wetting Agent

• One method of knowing whether or not a liquid has a wetting agent in it is to spread the liquid on a surface that is coated in grease.

• If the liquid does not contain a wetting agent, then its cohesive forces would overpower adhesive forces, causing the liquid to form droplets on the surface.

• If the liquid does contain a wetting agent, the grease would be dissolved and the surface tension of the liquid would be lowered, causing the adhesive forces to overpower the cohesive forces. This would result in the liquid spreading evenly along the surface.

More cohesive force than adhesive

More adhesive force than cohesive

Wetting Agents

• Another method is to place the liquid in a test tube and observe the liquid's meniscus.

• If the liquid contains a wetting agent, its adhesive forces are stronger than cohesive forces, which means the liquid molecules are more inclined to stick to the surface than other liquid molecules. This results in a concave meniscus.

• If the liquid does not contain a wetting agent and is naturally very cohesive, like mercury, it forms a convex meniscus. This is caused by the fact the molecules of the liquid have a stronger attraction to each other than to the surface of the test tube.

Wetting Agents

ConcaveConvex

More cohesion More Adhesion

Wetting Agents

Uses for Wetting Agents

•Detergents•Fabric softener•Paints•Inks•Anti-fogging•Soil remediation•Foaming agents•Shampoo etc

Wetting Agents

Hygroscopic agents• In printing fixation of dyestuff is carried out by either

steaming or dry heat. Since the amount of moisture in the dried print is less, hygroscopic agents are often added to enable the absorbing of moisture during steaming.

• Optimum quantity of these agents should be used as less quantities will result in poor transfer/fixation of the colour due to improper dissolution, whereas large quantities result in spreading of dye solution beyond the boundaries.

• Examples: Glycerin, ethylene glycol, polyethylene glycol, urea etc.

Dispersing agents• In the printing paste, the dye/pigment has to be

dissolved/disperse in a very small quantity of water. Hence, certain solvents or dispersing agents are used to prevent aggregation of dyestuff molecules in the printing paste.

• Solution aid or hydrotropic agent is added to facilitate the dissolution of the dye in a small quantity of water.

• Commonly used solvents are acetin (triacetates of glycerine), diethylene glycol for vat dyes in leuco and solublised vat form and cellosolve (glycol monoethyl ether) for printing of vat and azoic etc.

A hydrotrope is  a  compound  that  solubilises  hydrophobic  compounds  in  aqueous  solutions. Typically,  hydrotropes  consist  of  a  hydrophilic  part  and a  hydrophobic  part  (like  surfactants) but the hydrophobic part is generally too small to cause spontaneous self-aggregation.

Oxidising agents and oxygen carrier

• An oxidizing agent is generally added to the printing paste of solublised vat, aniline black etc for the final development of colour. The agent should not be active in the printing paste itself so as to prevent, development of colour in the printing paste.

• Therefore, sodium chlorite, potassium chlorite, sodium nitrite, sodium chromate, etc, which are potential oxidizing agents are used in the printing paste and after printing, fabric is treated with an acid for the development of the colour.

• In certain cases, the dye (especially azo dye) are susceptible to a reducing influence during steaming, which may result in loss of colour value. In such instances, sodium m-nitrobenzene sulphonate (alkaline media) and sodium chlorate (acidic media) are used for reactive and disperse dye respectively.

Oxidising agents

Reducing/resisting/discharging agents

• Reducing agents are used in the printing paste of vatting of the vat pigments, fixation of vat pigments onto cotton and resist and discharge style of printing.

• Reducing agents are also used in the after clearing of disperse dyes from the polyester textiles.

• For vatting of vat dye and after clearing process (Reduction clearance or RC), sodium hydrosulhite is mainly used.

List of reducing agents used in resist and discharge printing

• Sodium sulphoxylate formaldehyde• Zinc sulphoxylate formaldehyde• Thiourea dioxide• Stannous chloride• Benzoyl peroxide with sodium chlorite