Thickener

Thickener:

Thickener is a thick mass which imparts stickiness and plasticity to the print paste so that it may be applied on the fabric surface without bleeding or spreading and be capable of maintain the design out lines.

Function of thickener:

        i.            To give the required viscosity to the printing paste.

      ii.            To hold the ingredients of the print paste on the fabric.

    iii.            To prevent premature reaction between the chemicals contained in the print paste.

Four significant approaches to produce thickeners using:

1.      A low concentration of a polymer of high molecular weight.

2.      A high concentration of material of lower molecular weight or highly branched chain structure.

3.      An emulsion of two immiscible liquids.

4.      A dispersion of a finely divided solid, such as bentonite.

Quality of print paste:

Ø  Print paste stability.

Ø  Properties of the dried thickener film.

Ø  Effect on color yield, such as diffusion, fixation.

Ø  Preparation and removal of the thickener.

Ø  Cost.

Ø  Biological oxygen demand.

Ø  Styles of printing.

Ø  Methods of printing

Ø  Type of material to be printed

Ø  quality of material to be printed

Ø  Compatibility with dyes and chemicals.

Classification of thickeners:

1.      Natural thickeners:

                                i.            Cereals starch: i.e. Marine starch, wheat starch etc.

                              ii.            Plant exudates: i.e. Gum tragacanth, Karaya gum etc.

                            iii.            Root and seeds: i.e. Locust beam gum.

                            iv.            Sea weeds: i.e. Na-alginate.

2.      Synthetic thickeners:

                                i.            Acrylic: i.e. Polyacrylic acid, polyacrylic amides etc.

                              ii.            Vinyl: i.e. poly vinyl alcohol.

3.      Modified natural thickeners:

                                i.            Starch derivatives: i.e. British gum, C.N.C etc.

                              ii.            Cellulose derivatives: i.e. C.M.C , HEC etc.

                            iii.            Gum derivatives: i.e. Meypro gum,indalca etc

Principles of selecting thickening agents:

       I.            The print paste auxiliaries and dyes should be compatible with the thickener and the thickener should be stable. If a cationic dye is added to a thickener with anionic chares the viscosity will be charged and it will produce insoluble complexes. Similarly anionic thickeners can’t be used when metal salts such as Al₂(SO₄) are to be added.

    II.            The p^H of the print paste must be considered because some thickeners like Na-alginate are only usable within a imitated p^H range and form gels when acids or strong alkalis are added.

 III.            Print paste is required to be stable for at least one week. But most natural thickeners are biodegradable. To avoid this hazard phenol or formaldehyde are recommended as preservatives.

 IV.            Good adhesion of the thickeners to fibre is required in order to avoid the less of colorant during mechanical handling. Otherwise particles of colored films may break off, leaving white spots in colored areas, so the thickener film should be flexible and should have good adhesion properties.

    V.            The choice of thickener to use with a particular class of dyestuffs depends on the method of application and on the fabric to be printed. Thus in screen printing chocking of the silk screen fabric can occur if an improper thickener is used. Again in block printing, number of suitable thickener is more limited.

 VI.            thickeners may be classed as having either high or low solid content. Gum tragacanth is a high solid content thickener suitable for screen printing on fine filament rayon fabric.

VII.            The extent to which a thickening agent isremoved in a high speed washing process is a vital consideration in the selection of thickener.

VIII.            The cost and availability of thickener is considered in selection a thickener.

Screen printing

Screen printing:

Screen printing uses the principle of stencil. It is a resist method of printing in the sense that the screen is made to resist penetration of the print paste in areas where printing is not desired. Dye or pigment in a thickened formulation is forced by a blade or roller called a “squeeze” through a permeable screen onto a fabric under math the screen. Screen printing may be done with either flat or cylindrical screens.

Centuries ago, Japanese developed the stenciling technique for textile printing as a fine art, by the 19^th century this printing method spreaded worldwide.

In 1850, French printers introduced the use of silk fabric to provide a continuous support for the paper stencil and that was stretched across a frame. This combination becomes known as a screen. At first screen fabric were made from hydrophilic yarn like cotton, silk, rayon etc afterward from hydrophobic ones like nylon and polyesters.

Strong developed mechanized screen printing process by the introduction of a movable carriage, in which the screens are mounted one at a time. The squeeze was driven by a motor attached to the carriage.

Fully automatic flat screen printing technique was developed by Buser, Stork and Johannes Zimmer in1950’s. These machines print all the colours in a design simultaneously along the top of an endless blanket conveyor belt.

Fully continuous printing by rotary screen was manufactured by AJC de O Barros in 1954. But this rotary screen had a seam line along its circumference.

The invention of seamless rotary screen of electrodeposited nickel was introduced by Zimmer in 1961 and latter by stork in 1963.

Machines using rod or roller squeeze manufactured by Zimmer was very successful in printing wider substrates e.g. carpets.

Currently rotary screen printing is the major printing method which replaced copper roller printing. These have also been used to print paper for transfer printing process.

Classification of screen printing:

There are four types of screen printing machines/processes-

1.      Hand screen printing.

2.      Semi-automatic screen printing.

3.      Fully automatic screen printing.

4.      Rotary screen printing.

Hand screen printing:

Hand screen printing is restricted in the college of art, small-scale unit and high fashion industries as it is a craft rather than a productive method of printing.

Here the number of color in a design is equal to the no. of screens used in that design. For every color, different print paste is made. Again in case of two or more color design the motif which covers more area in printed portion is applied first and so on. For hand screen printing the following things are required:-

        i.            Screen.

      ii.            Table

    iii.            Bed on the printing table.

    iv.            Back grey.

      v.            Fabric to be printed.

    vi.            Squeeze system.

As here the squeezer is manually driven by hand on the screen, it is called hand screen printing.

Printing is carried out on a flat, solid table covered with a large of resilient felt and a washable blanket (coated with neoprene rubber)

Fabric movement or shrinkage is avoided to maintain registration of the pattern. The fabric to be printed is laid on the table and stuck to be the blanket directly using either a water soluble adhesive. Alternatively the fabric is combined with a back-grey. In the latter  instance an absorbent fabric is stuck to the blanket and the fabric to be printed is pinned down on top of it, Sometimes fabric and back-grey are combined before fixing to the table using an adhesive and a special padding mangle.

Before printing the screens must be positioned carefully on the fabric. The area printed by a screen (screen repeat) must fit exactly alongside the adjacent one. Now print paste is taken on the screen. The printing process consists of forcing this viscous print paste through the open areas of the screen, with a flexible synthetic rubber squeeze. The rubber blade contained in a wooden or metal support is drawn steadily across the screen at a constant angle (about 45°) and pressure. Two strokes of squeezing gives better result. For very even print, 4 strokes are also done.

Advantages:

       I.            less investment cost.

    II.            Not so much necessity of dryer.

 III.            Multicolor design can be produced.

 IV.            Less floor space required.

    V.            Suitable for small scale production.

Disadvantages:

       I.            Slow production.

    II.            Small scale production.

 III.            Labour intensity is large.

 IV.            Fastness properties are not of international standard.

    V.            It is difficult to maintain even penetration of print paste through the screen.

 VI.            Not suitable for large scale production.

Controlling the amount of print paste passing:

1)      The ‘mesh’ (thread/inch) or ‘raster’ (threads/cm) of the screen fabric; generally a coarse mesh allows more paste to pass through than a fine one.

2)      The fraction of open area in the screen fabric, this depends not only on the mesh but also on the yarn diameter and the effect of subsequent treatments e.g. calendering.

3)      The hardness and the cross section of the squeeze blade; a hard rubber squeeze with a sharp cross section is suitable for outlines whereas a soft rounded blade applies more paste and is suitable for blotches.

4)      The hardness of the printing table. If the top of the table is firm a soft squeeze is suitable.

5)      The viscosity of the print paste, thinner paste passes through the screen more readily.

6)      The number of squeeze strokes generally 2-4 strokes are applied.

7)      The squeeze angle and pressure.

8)      The speed of squeeze strokes.

All these variables are taken into consideration along with the nature of design, when printing the chosen fabric.

Printing

Printing:

Printing is a process for applying color to a substrate. However, instead of colouring the whole substrate(cloth, carpet or yarn)  as in dyeing, printing colour is applied only to defined area to obtain the desired pattern. By the term “ Textile printing” we mean the localized application of dyes or pigments and chemicals by any method which can produce particular effect of colour on the fabric according to the design. This involves different technique and different machinery with respect to dyeing, but the physical and chemical processes that take place between the dye and the fiber are analogous to dyeing.

Step of printing:

Grey textiles/raw materials

Preparation of textile materials (singeing, desizing, scouring and bleaching)

Preparation of printing paste

Printing (with a certain style and method)

Drying of the printed fabric (in the drier)

Steaming of the printed fabric ( to transfer dye into fiber, 100-102℃, 15 min in a steamer)

After treatment (soaping of washing)

Steps of printing:

Ø  Color paste preparation: Generally the printing dyes or pigments are insoluble, during printing textiles, the dyes or pigments are not in an aqueous liquor instead, it usually finely dispersed in a printing past, in high concentration.

Ø  Printing: After suitable preparation of printing paste then it is applied to the substrate using different technique.

Ø  Fixation: After printing the printed textile material needed to fixation by dried. The prints are fixed mainly with steam or hot air.

Ø  After treatment: After fixation the specimen is needed to after treatment.

Printing paste and its ingredients:

Printing paste is viscous solution of printing ingredients referred below that is used for textile printing. The main three printing ingredient given below:

        i.            Dyestuff or pigments: Reactive, Disperse, Vat etc.

      ii.            Thickeners: Thickener can be defined as a substance used to increase the viscosity of a print paste or other fluid, which is controlled the disperse of dyes/pigments and contain the definite design.

    iii.            Chemicals and water: During printing many chemical is necessary such as wetting agents, reducing agents, oxidizing agents, dispersing agents, solvents, acids, alkalis, hygroscopic agents, catalyst and carries.

Printing pastes contain a thickening agent & other auxiliaries. According to their function, the classification given below:

        i.            Reducing agents(e.g. formaldehyde sulphoxylates, tin(‖)chloride, sodium dithionite, thiourea dioxide)

      ii.            Oxidizing agents(e.g. sodium chlorate, hydrogen peroxide, m-nitrobenzene sulphonate)

    iii.            Substances with a hydrotropic effect, like urea.

    iv.            For discharge printing, Discharging agents needed such as anthraquinone.

      v.            Resist for reactive resist printing such as sulphonated alkanes.

    vi.            Dye solubilisers, which are polar organic solvents like glycerine, butyl glycol, ethylene glycol etc.

  vii.            Defoamers( e.g. esterssilicon compounds etc)

Thickeners:

A thickener is a colorless, viscous paste made with one or more thickening agents.

Four significantly different approaches may be used to produced thickeners, using:

Ø  A low concentration of high relative molecular mass(r.m.m.).

Ø  A high concentration of a material of lower r.m.m. or highly branched chain structure.

Ø  An emulsion of two immiscible liquid, similar to the emulsions used as cosmetic creams or a foam of air in a liquid a dispersion of a finely divided solid.

Selection of thickener:

Ø  Viscosity

Ø  Print paste stability

Ø  Good adhesion of the dried thickener film

Ø  Minimum effect on color yield

Ø  Ease of removal

Ø  Acceptable cost

Important of viscosity:

There are two essential reasons for the importance of the viscosity and hence the flow, of a print paste.

First: It affects the amounts of paste applied.

Secondly: The spread of paste, on the surface of the textile material and into its structure.

The upper limit of the viscosity is determined by the flatness of the fabric surface and the condition of the printing process. The lower limit also depends on the process conditions, but is mainly determined sharpness of print.

Examples of natural thickeners:

1.      Starch and its derivatives(rice, potato, wheat etc):

                 i.            Provides high color yield

               ii.            Susceptibility to crush and poor levelness

2.      British gums:

                    i.            Good stability to alkali

                  ii.            Used for printing vat dyes

                iii.            For resist print, it is used

3.      Locust bean gum:

                    i.            Required 45℃ or higher for complete dispersion in water

                  ii.            P^H has little effect on the viscosity over the p^H range 3-11

4.      Guar gum:

                    i.            Can be dispersed in cold water.

5.      Alginates:

-          Source in brown phaeophycae seaweeds

-          Very important because of their ready solubility

-          Every after high temperature fixation treatments.

They are especially important for pastes of reactive dyes because the extent of interaction is very small.

The stability is good between p^H 4 and 10.

 Alginate pastes are compatible with a wide range of materials, including starch and tragacanth.

6.      Gum Arabic:

Gum Arabic has been used as an adhesive more than as a thickening agent.

7.      Gum tragacanth:

It is less sensitive to additions of electrolyte.

Sequence of printing:

       I.            Printing with pigment:

Print   à  Dry   à   Curing(At 150℃ for 30s to 5 min)

    II.            Print with dye:

Print   à  Dry   à  Steaming/afeing(At 100-200℃ for 10s to 60 min)  à  After washing.

Style of printing:

Ø  Direct style

Ø  Discharge

White discharge

Color discharge

Ø  Resist style

Ø  Flock style

Ø  Crimp/Crepon style

Ø  Burn out style

Method of printing:

1)      Block printing

2)      Stencil printing

3)      Roller printing

4)      Screen printing

–Hand screen printing

–Semi automatic flat screen

–Rotary screen

5)      Transfer printing

–Flat bed

–Continuous transfer

–Vacuum transfer

6)      Digital inject printing