Pigment

Pigments implied general insolubility and complete insolubility in water.

Difference between dye and pigment

The difference between dye and pigment is not a clear one. Most organic pigments are closely related to dyes with respect to their chemical structure and there are dyes which become pigments after application. Vat dye is a dye when used in dyeing but a pigment when used in printing.

	Dye	Pigment
Solubility in water	All dye must be soluble during process	Almost insoluble
Affinity	Possess a specific affinity towards fiber	Have no affinity but used as coating
Chemical nature	Organic and few are metallic	Most are metallic or organometallic.
Application	Through water medium	Through adhesive or binder

Uses of Pigment

0. Pigments are used for coloration of a very broad and diverse number of materials

1. Surface coating for interior, exterior, automotive and other application

2. Paints based on olegoresinous liquid and water emulsion

3. Printing ink for papers (lithographic, rotogravure and flexographic systems (and for other materials such as metal plates, foils, artists and writing material)

4. Coloration of plastics and rubber

5. Textile printing

6. Coloration of manmade fibers by mass pigmentation before fiber formation (dope dyeing) etc.

Required Properties of pigments

0. They should have covering power which is influenced by particle size

1. Should be inert, stable and have long life

2. Should have capability of mixing

3. Good wet fastness, light fastness and abrasion resistance

4. Good resistance to acid, base, perspiration, chlorine, peroxide and gas fading

5. Good solvent resistance (insoluble in water, CCl₄, Cl₂C=CHCl)

6. Suitable brilliance, hardness and stability

7. Suitable characteristics for good dispersion including particle size and distribution, electrical charge (most are negatively charged particle), specific gravity, purity and crystalline structure, conditions of precipitation of the pigments

8. Should be applicable to all fibers.

Physical/Chemical Properties of Pigments

Chemical Structure	Inorganic oxide, salts, organometallic toners, organic insoluble azo pigments, phthalocyanine metal complexes
Physical state	Very important, decreasing particle size increase color value but decreasing hiding power
Particle size	5-7 micron
Density	Sp gravity range from 1.17- 1.37 for most cases
Melting points	Usual range 110 -175°C
Boiling point	Decompose at 195- 345°C. phthalocyanine pigments sublimes at 500°C
Water solubility	Insoluble for all practical purposes.
Other solubility	Inorganic pigments are insoluble in most solvent
Spectra	Very strong and high, though not comparatively sharp peaks

Application Properties of Pigments

Fabric dyed	Any fiber can be dyed by selecting a suitable binder, quality greatly depends on binder used to affix the pigment
Fabrics printed	Any fiber by suitable binder even hard to print polyester blends and glass fibers
Disposable fabrics	Well suited for non woven fabrics
Dischargeability	Some pigments are suitable for discharge printing
Alkali fastness	Poor for organometallic azo toners, good for insoluble azo
Heat resistance	Extremely varied. Some are stable up to 200°C and some up to 300°C. optimum for inorganic pigments
Light fastness	Generally very good. Optimum for inorganic pigments
Wash fastness	Generally good to very good
Useful colors	Diarylide yellows and oranges, Hasna yellow, azoic reds, phthalocyanine blues and greens, carbon black, TiO2 white, violet and browns.
Processes used	Padding for dyeing
Aftertreatment	None required

Classification of pigments

• According to origin

1. Natural/Mineral: Iron ores, clays, chalk etc

2. Synthetic/chemical: white lead, ZnO, TiO₂ and large number of inorganic and organic color

• According to Reactivity

1. Reactive pigment: some pigments on account of the chemical character react with oil, fatty acids and soaps. These are called reactive pigments e.g. ZnO, red lead

2. Inert pigment: TiO₂

• According to Chemical Nature

1. Organic pigment: appx 25% (by wt.) of the world production of organic colorant is accounted for organic pigments. They account for only 4% of total pigment production. Of the total organic pigments production yellow, red and blue tones accounted for 89%.

Most organic pigments exhibit a small solubility, typically in polar solvent. All the organic pigments are soluble in one or more of the four chemical: Chloroform (CHCl₃), Methyl alcohol (CH₃OH), Dimethyl formamide (DMF) and concentrated H₂SO₄. Organic pigment consists of:

1. Azo pigment:

• Strong tinctorial strength

• Good alkali resistance

• Excellent brightness

• Cover a wide range with regard to other application properties

• Poor alkali resistance of certain organometallic pigments make them unsuitable for printing

2. Diarylide orange and yellows:

• Extremely bright color

• Inferior light fastness

3. Phthalocyanine

• Blue, greens are dominant shade especially in plastic coloration

• Offer low migration

• Good temperature stability

• Excellent light fastness

• Good heat resistance

• Excellent alkali resistance

• Good solvent resistance

• Used extremely in printing, pad dyeing and dope dyeing

4. Hasna yellow

• Good light fastness

• Have migration tendency

2. Inorganic pigment:

• They account for 96% (by wt.) of total production. More than half of their production volume is accounted for a single production, TiO₂, the most important white pigment

• H₂SO₄ is a good solvent for many inorganic pigments

• They are opaque

• Less expensive

• More weather resistant

• More chemical resistant

• Insoluble in most organic solvents

• Highest degree of light fastness

• Excellent heat resistance

They consist of

1. Salts: Sulfates, carbonates, silicates and chromates of many metal elements like, Ti, Zn, Ba, Pb, Sb, Zr, Ca, Al, Mg, Cd, Fe, Mo, Cr etc.

2. Oxides of Ti, Zn, Ba, Pb, Sb, Zr, Ca, Al, Mg, Cd, Fe, Mo, Cr etc.

3. Metal Complexes: Naturally occurring oxides and silicates

Difference between organic and inorganic pigment

	Organic	Inorganic
Solubility	Soluble in organic solvent	Soluble in inorganic solvent
Tinctorial strength	Higher	Lower
Brightness	Higher	Lower
Purity	Higher	Lower
Transparency	Opaque	Transparent
Weather resistance	Less	More
Chemical resistance	Less	More
Fastness	Good	Excellent
Cost	Expensive	Cheap

Roller Printing

• Engraved roller printing is a modern continuous printing technique developed in the late 19th and early 20th centuries.

• Until the development of rotary screen printing, it was the only continuous technique.

• The high fixed cost of copper rollers, expense of engraving process, and possible distortion of fabric during printing have led to its reduced use, now being less than 5% of the worldwide textile printing market.

• The fine design detail possible with this technique has always been its main advantage.

Main Parts and their functions

• Color Doctor: It is quite essential and is a thin sharp blade of steel that rests on the engraved roller and serves to scrape off color from its surface, leaving only that which rests in the engraving. On the perfect action of this doctor depends the entire success of printing, and as its sharpness and angle of inclination to the copper roller varies with the styles of work in hand. Any roughness, unevenness or the slightest snip in the edge is sufficient to allow color to escape under it, with the result that the finished parts exhibit serious defects in the form of streak or cloudy patches of color.

• Lint Doctor: It does two functions

• Remove any loose fiber, filaments, neps which are attached from the printing cloth and get stuck up to the wet surface of the roller.
• It cleans other colors of a multicolor design which are already printed on the cloth and being still wet impress themselves on the surface of the roller

• Blankets: This is a tightly woven uniform woolen piece of equal thickness and elasticity. It can be removed from the machine, well washed to remove the dried paste, dried and again put on the machine as before. They are resilient and hard wearing.

• Back Grey: Simple pieces of unbleached cloth that are run between the blanket and the cloth to be printed. This is used to absorb the color forced at the back of the printed piece which would otherwise spoil the blanket.

• Furnishers: Color furnishers are usually wooden roller working partly in the color box and partly above. They supply the color to the design rollers.

• Color Box/Tray: they are long, narrow, shallow, trough of word or copper to carry the printing paste. It is capable of being moved backward or forward.

• Color unit: Consists of color box, two doctors and design roller.

Engraving of copper rollers

In this method, a heavy copper cylinder (roller) is engraved with the print design by carving the design into the copper. Copper is soft, so once the design is engraved, the roller is electroplated with chrome for durability.

Three different methods of engraving

• By hand with a graver which cuts the metal away

• By etching, in which the pattern is dissolved out in nitric acid; and

• By machine, in which the pattern is simply indented.

Pros and Cons

Advantages:

• Suited for large batches

• Very high speed

• Fine line or sharp line can be easily obtained

• Highest production for single color is 18000 yds/hr and 10000/yds for 12 colors

• Can be used for printing any style

• Can be used for all classes of dye on any fabric for all design

• Repeats do not exist as printing is continuous

Disadvantages

• Changing time is high

• Crush effect is produced

• Engraving the printing roller is expensive operation

Defects in Roller Printing

• Scratches: Due to some hard particles of sand or grit in the color paste. These marks being very fine; they are noticeable only after fully development of the color at the stage of soaping and washing

• Removed by burnishing the rollers

• Snappers: These are in the form of ugly lines printed parallel to the selvedges. They are caused by some substances getting under the color doctor which is lifted up at that part and color escapes unscrapped from its either side giving a dirty and smeared printing. Loose thread from the cloth or dried particle of paste or bad mixing of thickener can cause this defect.

• Can be removed by restraining the color paste.

• Lifts: It occurs at regular interval usually equal to the circumference of the design roller. Some hard particle gets stuck into the engraving which lifts the color doctor and allows the color paste to go ahead unscrapped.

• Removed by pulled out of the particle.

• Scrimps: Due to creases or folds in the cloth. They are visible in the form of a double edge with white unprinted part left over in between, below a crease or fold in the cloth.

• Rollers at the back of the feed and drying arrangements must be right.

• Streaks: this defects manifest in the form of two or more fine lines running parallel to the selvedges either straight (due to scratches on the roller) or in a zigzag (cut in the color doctor) manner.

• Removed by polishing the roller and resharpening the doctor edge

• Scumming: due to proper uncleaning of the design roller surfaces. It spoils the whole cloth due to the printing of one or more colors. It may caused by

1. Rough surface of the roller

2. An uneven doctor edge

3. Doctors not properly adjusted

• Removed by correcting the factor responsible for it i.e. by positioning the roller or correcting the uneven edge of the doctor or setting the doctor right.

• Uneven printing: Due to

1. Uneven pressure on the ends of design rollers, show light and deep printing at the selvedges
2. Uneven lapping of the furnishing roller
3. Uneven addition of the color at the one end of the color tray
4. Greater percentage of insoluble substances in the paste
5. Uneven diameter of the design roller due to constant polishing of one end of the roller

Comparison among roller, flat and rotary screen printing

Feature	Roller	Automatic flat screen	Rotary screen
Minimum run (meters)	10,000	500	1000
Design scope	Excellent	Good	Good to excellent
Size of pattern repeat	Limited by roller diameter of 40 to 46 cm	Large designs possible	Limited by screen diameter of about 64 cm
Heaviness of print	Depends on the depth of engravings, generally less than screen prints	Depends on size of holes in screens, percentage of open area and squeegee pressure
Pressure on fabric	More than in screen printing	Low but depends in squeegee pressure
Production rate	Fast	Slower	Fast
Ease of setting up new designs	Costly and length process	Less costly than rotary screens	Technically more difficult than flat screens

Screen Printing

Screen Printing

Screen printing is arguably the most versatile of all printing processes. It can be used to print on a wide variety of substrates, including paper, paperboard, plastics, glass, metals, fabrics, and many other materials including paper, plastics, glass, metals, nylon and cotton. Some common products from the screen printing industry include posters, labels, decals, signage, and all types of textiles and electronic circuit boards.

Screen Printing Process

• Hand screen

• Semi automatic flat screen

• Rotary Screen

Screens

• Made of cotton, silk, nylon cloth, polyester or metal gauze

• Screen mesh refers to the number of threads per inch of fabric. The more numerous the threads per inch the finer the screen.

• The usual mesh of screen employed for cotton and silk printing is 80 threads per inch.

• The finer the screen the sharper are the outlines but more effort is needed to force the printing paste through the screen.

Screen Frames

• There are two types of screen frames, metal and wood.

• Screen frames for commercial use are usually made of steel, or a lighter metal, with a hollow cross section to provide rigidity with minimum weight.

• Screen frames are usually 26" x 55" (measured externally) and 23" x 52" (measured internally) for printing 45" wide cloth

Screen Fabric

There are two types of threads for screen fabric:

• Monofilament - single strands weaved into fabric

• Primarily used in commercial printing and other applications

• Advantage: Monofilament is easier to clean than multifilament

• Multifilament - multiple strands wound together like a rope, then weaved into fabric.

• Primarily used in textile printing.

• Disadvantage: ink tends to build up on screen, more difficult to clean. Monofilament mesh has become the industry standard.

Screen Fabric Types

• Silk - multifilament weave

• loses toughness with frequent use

• reclaiming chemicals containing bleach or chlorinated solvents destroy the silk

• Today silk is primarily used for printing art, not commercial use as before

• Nylon - multifilament or monofilament

• good for stretching

• compared to polyester, lacks stability

• less rigid than polyester

• unsuitable for closely registered colors

• Polyester - multifilament or monofilament (calendared monofilament polyester, metallized monofilament polyester)

• primary material used in commercial screen printing

• Polyester is strong and stable when stretched

• Other screen materials - carbonized polyester

• glass

• wire mesh

• stainless steel

Screen Preparation

Photochemical method is most widely used for preparing the screen. This is based on the principle that when a coating of a solution of ammonium dichromate-gelatine or ammonium dichromate-polyvinyl alcohol is dried and exposed to light, Insolubilisation takes place

Other method for screen preparation is lacquer and laser screen.

Photochemical method

• Coat the flat screen with light-sensitive polymer, and dry it in the dark.

• Position a positive transparency of the pattern on the polymer-coated screen.

• Expose the screen to ultraviolet light. Ultraviolet light rays pass through the transparent (non-pattern) areas of the transparency on to the screen and harden the polymer.

• Wash the screen in warm water to remove the polymer from the unexposed (pattern) areas of the screen through which the printing paste will pass.

• Dry the screen

Preparation of Sensitising solution

Sensitising solution may be prepared as follows:

(1) Chrome-Gelatine Solution

Solution A 200 g Pure gelatine

500 g Boiling water

Total 700 g

Solution B 70 g Ammonium dichromate

150 g Boiling water

80 g Liquor Ammonia

Total 300 g

Solution A and Solution B are mixed in a dark room.

(2) Chrome-Polyvinyl Alcohol Solution

600 g Polyvinyl alcohol (15% solution)

120 ml Ammonium dichromate (33% solution)

240 ml Cold water

1 litre with cold water

Squeegee system

• Rubber Squeegee

• Double Squeegee

• Magnetic rod Squeegee

Rubber Squeegee

• These vary in Shore hardness from 55 (soft) to 70 (hard). Softer blades give a heavier print. The edge shape of the rubber blades is chosen to suit requirements.

• Round ones [Figure (a)] suit, for example, wool and fleece fabrics, where a heavy print is needed to penetrate the fibrous surface.

• Long, tapered edges [Figure (c)] are used when penetration is not important as on flat and woven fabrics.

• The stubby edge [Figure (b)] is good for one-stroke printing on interlock. The chisel shape

• [Figure (d)] is used to flood the screen with printing paste while the screen is raised in preparation for the print stroke when only one print stroke is to be used.

Double Squeegee
This system is easier to make than a single squeegee, which must be lifted over the pool of print paste at the end of each stroke.

Magnetic rod Squeegee

A
rolling rod (a) or a pair of rods (b) is moved by a driven electromagnet moving under the printing blanket. The diameter of the single rod is small enough to allow print paste to flow over and round it at the end of a pass. The twin rods form a well of paste, the volume of which depends on rod spacing and diameter.

Fundamental characteristics of screen printing

• In screen printing process – hydrodynamic pressure is built up in the print paste between the squeegee and the screen surface through which the paste is passed.

• The hydrodynamic pressure appears to be inversely proportional to the radius of the pore i.e. Hydrodynamic pressure  1/rn (n<2)

Here the pore radius greatly affects the amount of paste flowing through screen; Hydrodynamic pressure is also proportional to the viscosity of paste.

• The percentage of open area of the screen also plays a role. More open screens allow more paste to pass.

• The fabric is to be printed forms a three dimensional structure with the screen where the absorbency of the fibers and penetration capacity between yarn also affect the uptake of the paste.

• The usual hexagonal openings are larger at the outside of the screen than at the inside, the capillarity and surface tension forces etc. result in a printing with actually more color deposited in the areas between holes than opposite holes.

Flat bed screen printing

• An automated version of the older hand operated silk screen printing

• The flat-bed screen process is a semi-continuous, start-stop operation.

• F
  or each color in the print design, a separate screen must be constructed or engraved

• Fabric glued to blanket

• Screens rise and fall

• Printing done while screen in down position

• Rod or blade squeegee system

• Up to four strokes possible

• Productivity is in the range of 15-25 yards per minute.

• The design repeat size is limited to the width and length dimensions of the flat screen.

• Currently accounted for apprx. 15-18% of printed fabric production worldwide

• Slow process

Factors affecting Print paste passing through the screen

• The ‘mesh’ (threads per inch) of the screen fabric

• The fraction of open area in the screen fabric, this not only depends on the mesh but also on the yarn diameter and the effect of subsequent treatments, such as calendaring

• The hardness and cross section of the squeegee blade; a hard rubber squeegee with a sharp cross section is suitable for outlines, whereas soft, rounded blade applies more paste and is suitable for blotches

• The hardness of the printing table, if the top of the table is firm a soft squeegee is probably necessary, whereas with a resilient table surface a harder squeegee is preferable.

• The viscosity of printing paste

• The number of squeegee strokes; from two to four strokes are usually applied

• The speed of the squeegee stroke

Flat bed to Rotary screen

• Modification of flat bed screen printing: from semi continuous to continuous, low productivity to high productivity.

• Quality of end result.

• Amount of color that can be applied. Note that the screen area consisting of holes is smaller in rotary screens than in flat screens.

• Evenness of color.

• Ability to produce fine lines and half-tones. Half-tones are tone graduations within one colored area.

• More compact than flat screen machines for the same number of colors in the pattern

• Rotary screen machines are highly productive, allow for the quick changeover of patterns, have few design limitations, and can be used for both continuous and discontinuous patterns

• Typical speeds are from 50-120 ypm (45-100 mpm)

• High investment cost and the machines are generally not profitable for short yardages of widely varying patterns.

• Controls approximately 65% of the printed fabric market worldwide

Rotary screen printing

In basic operation, rotary screen and flat screen-printing machines are very similar. Both use the same type of in-feed device, glue trough, rotating blanket (print table), dryer, and fixation equipment. The process involves initially feeding fabric onto the rubber blanket. As the fabric travels under the rotary screens, the screens turn with the fabric.

Print paste is continuously fed to the interior of the screen through a color bar or pipe. As the screen rotates, the squeegee device pushes print paste through the design areas of the screen onto the fabric. As in flat-bed screen printing, only one color can be printed by each screen. After print application, the process is the same as flat screen printing.

Estimates indicate that this technique controls approximately 65% of the printed fabric market worldwide.

Defects on screen printing

• Out of registration – pattern out of fit.

• Glue streaks – from the rubber blanket.

• Color smear.

• Color out – from a lack of print paste.

• Creased fabric.

• Pinholes in any screen.

• Damage to the screen leading to misprints.

• Lint on the fabric causes pick-off.

• The prints may come out lighter in the middle and deeper towards the selvedges. This occurs when too much cloth is steamed in one batch or when the cloth is very thick.