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Printing Ink Technology
PRINTING INK
Definition: ink is a liquid or paste that contain pigment of dyes and is used to color a surface to produce an image, text and design.
History of Ink: The earliest forms of written communication had been carved cuneiform tablets, used in Sumerian and Babylonian civilizations around 3000 B.C. The earliest use of writing on a paper-like substance (papyrus) with ink took place in ancient Egypt. Around 2500 B.C., black ink was produced by mixing soot and vegetable gums in water; a pen consisted of a reed with a small brush mounted on the end of it. Five thousand years later, the manufacture of printing ink is far more complicated than that of the earliest inks.
Two types of ink:
1. Paste: letter press, offset, screen
2. Liquid: gravure, flexography.
Ingredient of inks:
1. Colorant: (a) pigment (b) dye
A. Pigment used in paste ink.
B. Dye used in liquid ink
2. Binders: varnish or vehicles which consists of oils, resins or alkyds.
3. Carrier substance: solvents
4. Driers
5. Ink additives or modifier
a. Plasticizers
b. Waxes
c. Wetting agents
d. Anti-set off compounds
e. Shortening compounds
f. Reducer
g. Stiffening agent
h. Anti-skinning agents
1. Pigments: pigments made from various solvent such as water, oil, alcohol, and acid like as (sulphur, silica or china clay can be combined with either soda ash or sulfate salts).
a. Organic: soluble dyes, insoluble dyes.
b. Inorganic: naturally occuring, synthetic.
2. Binder: it is bind of pigment to each other.
Vehicle = varnish and additives.
Varnish + resin + drying oil
Vehicle: vehicle is to carry the pigments to the paper surface and protect the image during life span of the print, in adhesion to the paper surface.
Resin: resins are organic compounds and may be blended or chemically combines with ability to be dissolved in some organic solution or natural drying oil in order to improves drying, hardness, scratch resistance and gloss of ink.
Drying oil: oil from animal, vegetable, linseed oil, olive oil which extracted from the seeds of flex plants. These oil absorb atmosphere oxygen and polymerized.
Non-drying oil: it include mineral oil which are high boiling extraction of petroleum.
3. Dries: it is used to accelerate the printing ink after it is transferred on the substrate. Drying oil dried by adding small amount of driers, they are soaps of such metals as cobalt, maganese, lead, cerium or zircomium.
a. Cobalt: disadvantage: discolor tint and white ink. It is liquid form.
b. Maganese and lead dries: these are prepared by grinding (mg) + (lb) in linseed oil varnish. It is paste form.
c. ‘grapho’ or perborate: oxidizer that furnish oxygen.
4. Ink additives (modifiers):
a. Waxes: it is used in vehicle to impart rub resistance to dried ink film. It is reduce set off, improve water resistance, slow drying, reduce tack and viscosity.
b. Anti-skinning agent: agent drying of the ink in litho ink.
c. Antioxidant: prevent form oxygen.
d. Plasticizers: it is high boiling that is low volatility solvent whose main purpose is impart flexibility otherwise the ink. Make resin softer and flexible.
e. Wetting agent: such as fatty acid and alcohol.
f. Anti-set off: prevent from set-off printing problem.
g. Shortening: reduce ink flying or misting. It is a wax.
h. Reducer: ink to make thin.
Element of Ink:
Modern printing inks come in two basic types: liquid inks (which are fluid and watery) and paste inks (which are thick and tacky), and typically comprise three fundamental types of substances: a vehicle, a pigment, and a variety of different types of additive, such as driers.
'Vehicle'. The ink vehicle is the fluid part of the ink that, as its name implies, transports the pigment onto the substrate. The type of vehicle to be used in a particular ink is dependent upon the type of drying system utilized. Inks that dry by absorption utilize non-drying oil vehicles that do not dry by other means be fore they can be absorbending to the paper. Ink that dry via oxidation and or polymerization required drying oil vehicles and paper qualities that do not allow the vehicle to be drained away before oxidation can take place. Inks that dry via evaporation utilize low-boiling-point solvent-resin vehicles. Inks that dry by precipitation require a water-soluble glycol vehicle in which are dissolved water-insoluble resins. When water is added to the vehicle, the glycol is dissolved, but the resin (containing the pigment) is not, and precipitates out onto the surface of the paper. (Such inks are called moisture-set inks.)
Inks that use a combination of drying mechanisms, such as quick-set inks, have a portion of the vehicle—a solvent—absorbed first into the paper, and a resin-oil mixture left behind which dries by oxidation and polymerization. Quick-set inks use a resin-oil vehicle. Inks called cold-set inks use a resin-wax vehicle, which is solid at room temperature, is melted by special heated rollers on a press, applied to the paper, then dried by turning back into a solid. Other, lesser-used vehicles include water-soluble gum vehicles, such as in watercolor inks, and photo reactive vehicles, which "set" upon exposure to various types of radiation.
'Pigment'. The pigment is the part of the ink that imparts gloss, color, texture, and other characteristics to the printed image. Pigments can be black pigments (consisting primarily of various types of Carbon Black), white pigments (which are either opaque pigments or transparent pigments), and color pigments which can be produced from either mineral sources (the inorganic color pigments) or from organic derivatives of coal tar (the organic color pigments). Other materials such as metallic powders can be used for various specialty inks.
Printing ink additives include driers, which speed up the drying of inks; bodying agents, which increase the viscosity of an ink; waxes such as microcrystalline, polyethylene, paraffin, beeswax, carnauba wax, and ozokerite are used to prevent such printing defects as ink setoff and blocking, and to increase the ink's scuff resistance. Other materials such as teflon can also be added to help "shorten" an ink. Other compounds can be used to reduce an ink's tack. Lubricants and greases are added to not only reduce tack, but to help the ink distribute on the plate, blanket, or substrate more consistently and uniformly. Reducing oils and solvents such as thinner can be added to increase the ink's setting capacity. Antioxidants and antiskinning agents can be added to keep ink from oxidizing and setting while it is still on the press. Corn starch is added for body and to reduce ink setoff, while surface active agents are used to enhance the dispersion of pigments in the vehicle.
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Ink drying method: The dryer is used to accelerate the printing ink after it is transferred on to the surface. It assist oxidation drying the small amount of dryer with the ink act as catalyst. Assisting chemical change without alternating itself. In the polymerization of the ink film resulting in solidification of the printing ink.
Dryer are compound of cobalt, MN+, and pb has a much slower drying action. Pb is generally purpose dryer. Pb is also quite dryer action in increase with increase temperature. This ink drying faster in hot than in cold weather.
Ink drying mechanism: once the ink has been apply to a substrate. It must be able to dry change from liquid to solids fills quickly. These are following method.
1. Oxidation polymerization: the drying process where the drying oil (linceed, tung etc.) form a hard finish occurs through a process of auto polymerization. In process the unsaturated oxidation. A long chain to get solidify addition of dryer accelerate drying process.
2. Absorption: this involved penetration of ink in to the substrate and the consequent removal of liquid component from the surface this mechanism only operate on porus material such as paper and board. It depend on the lower surface tension.
3. Precipitation: used in letterpress moisture set ink. If an ink with a vehicle consisting of water insoluble resin dissolved in a water miscrible solvent is printed on to paper, water from the paper and the surrounding air diluted the solvent to the point where it can no longer the resin in solution and this resin is precipitated around the pigment to form a dry film of ink. The amount of water in the paper is sufficient to cause rapid precipitation, a jet of stream may be blown on to the wet print. The solvent used in moisture or stream set ink are invariably glycols since these materials are miscible with water.
The vehicle for this type of ink is one of the substance is called DI-lalicol with carbon chain and hydraulic blue and each end the solvent already dissolve with certain type of resin under normal condition.
4. Evaporation drying: process of evaporation is that escaped of molecules from the surface of the liquid into air and away from the liquid. In a bulk of liquid the molecule attract on another in all the direction. In surface of liquid the force attracting the molecule inward are greatest because they are relating the few vapour molecules outside the liquid. It is resulting attraction of three force that give the liquid its surface free energy or surface tension characteristic. In practice the evaporation process is assist by applying heat energy.
By the addition of enter influence:
a. Ultra violet
b. Electron beam
c. Infrared radiation
d. Microwave
5. Radiation induced drying: the two main consideration with their radiation drying and curing assistance are amount of photon energy associate with radiation and absorption characteristics of the ink film. For the radiation drying system relevant absorption ranges are:
UV: 100-380UM.
Visible: 380-780UM
IR: 780-1000 UM
MW: >1000 Um.
a. UV: it shorter wave length and free radicals initiate chain reaction with vehicle and polymeric and hence ink film get solidify.
b. Infrared drying: shorter wavelength lowers viscosity and promotes absorption into pores stock.
c. Electron beam curing: electron accelerate by high voltage short wave length and high energy.
High energy create radicals on collision in molecules in the binder. No photo initiator required. Curing is instantaneous.
d. Microwave drying: microwave interact with polar molecules.
The polarized electric field of molecules causing the molecules to oscillate with wave. The resulting friction effect cause the medium to heat-up called dielectric heating.
Cold set- in newspaper: blacks are carbon black in a high-boiling minerals oil = 70% with asphaltic material. Aromatic hydrocarbons = 15%. Mineral oil have been replaced with grade = 5% aromatics, of which less than 0.1% are polycyclic aromatic hydrocarbons (PAHS).
Heat-set in newspaper: heat-set web offset inks are designed to produce high gloss printed image magazines, books. They contain lower-boiling minerals oils that are removed (within 1 sec) as the printed roll (web) passes through a hot-air oven. A typical formulation might be organic pigments (15-36 wt %), hard resins (25-35 wt %), soft resins and drying oils (5-15 wt %), mineral oil (boiling point-240-260C), (25-40 wt %), and additives (5-10 wt %).
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Properties of ink
1. Tack: it is stickness of ink. Tack may be ability of ink to act as an adhesive.
2. Viscosity: the speed of at which various liquid flow will vary widely, the resistance of the liquid to the force trying to move them.
3. Tixo-trophy: it is used in lithography ink.
4. Length: it is property associated with the ability of an ink to flow and form long filament.
5. Transparence & opacity: how many light and dark ink.
Ink term:
1. Baking: away from fountain or handing back.
2. Strike through: soaking of an ink through the paper and discoloring the reverse side of it.
3. Show through: ink visible in back side.
1. Viscosity
2. Thixotropy
3. Yield Value
4. Tack
5. Flow Behavior
Ink transfer is the movement of ink from inking system → plate → blanket (if used) → substrate.
1. Correct Viscosity & Rheology
2. Good Wetting
3. Adhesion
4. Cohesion
5. Surface Characteristics
6. Ink-Water Balance (Offset)
7. Film Thickness Control
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where
η=A⋅eERT\eta = A \cdot e^{\frac{E}{RT}}η=A⋅eRTE
where EEE = activation energy, RRR = gas constant, TTT = temperature (K).
1. Ink Composition
2. Temperature & Humidity
3. Shear Rate (Press Speed)
4. Ink-Water Balance (Offset)
5. Additives
The various classifications of inks are based primarily on their drying methods which, in turn, are based on the vehicle each ink uses.
'Quick-Set Inks'. These types of inks utilize a resin-oil vehicle, consisting of a resin-oil-solvent mixture. The solvent drains very quickly into the substrate leaving the remainder behind to oxidize and polymerize on the surface. Quick-set inks are among the most commonly used in offset lithography and yield extremely good results when printed on enamel paper and cast-coated paper.
'Heat-set inks'. These inks utilize a solvent-resin vehicle that dries primarily by evaporating the solvent from the vehicle, then re-cooling the remaining ink components. Heatset inks accomplish this by utilizing a solvent with a high boiling point, and the ink must then be dried in a special drying oven. Although commonly used, especially in web offset lithography, their drawbacks involve the additional equipment required, such as a drying unit and chill rolls to cool the heated ink.
'Moisture-Set Inks'. These inks utilize a glycol vehicle that dries primarily by precipitation. The pigment and a water-insoluble resin are dissolved in a water-soluble glycol. Upon contact with moisture, the glycol is dissolved, but the resin and pigment are not, and precipitate out of solution onto the surface of the paper.
'Radiation-Curing Inks'. These inks utilize complex vehicles that harden and polymerize upon exposure to radiation, either ultraviolet light (as in UV curing ink), beams of electrons (as in EB curing ink), or infrared light (as in super quick-set infrared ink).
'High-Gloss Inks'. These inks essentially are produced with an additional quantity of varnish, which allows them to dry with a highly glossy appearance. High-gloss inks are dependent upon the properties of the substrate to be truly effective; a high degree of ink holdout is necessary to keep the vehicle from draining into the paper before it can dry by oxidation.
'Metallic Inks'. These inks are used for specialty applications and to produce a printed image with a metallic luster. The pigments used in these inks comprise flakes of metallic powders.
'Magnetic Inks'. These inks were developed for use in banks and are used primarily for printing on MICR (Magnetic Ink Character Recognition) Check Paper and read with MICR equipment. The pigments used in these inks have the ability to be magnetized after printing (or are composed of magnetite, a black, magnetic oxide of iron), and MICR ink and printing must be performed to precise specifications, depending upon the sensitivity of the equipment.
'Fluorescent Inks'. These inks lack permanence, but make use of ultraviolet light to reflect back light in brilliant colors. Limited for many years solely to screen printing, recent innovations and formulations have produced fluorescent inks that can be printed in a variety of ways. Their semi-transparency makes them useful for overprinting on other inks, and fluorescent pink is occasionally printed as a fifth color in four-color printing to enhance skin tones and magentas. When used alone, fluorescent colors need to be printed on white paper, and achieve their best effect when contrasted with darker colors.
'Scuff-Resistant Inks'. Inks that are able to withstand the wear and tear of shipping and handling are available in a variety of grades and formulations.
Each printing process requires ink specially formulated for the mechanics and chemistry of the process.
'Letterpress'. Letterpress uses paste inks whose tack varies according to the speed of the press (though ink of moderate tack is generally preferred), and which typically dry by absorption, oxidation, or evaporation (or a combination of drying methods). The letterpress process, however, is falling into disuse in favor of other printing methods, such as offset lithography and flexography (letterpress now accounts for less than 5% of all printed packaging, for example). The varieties of ink used in letterpress printing are rotary ink, heatset ink, moisture-set ink, water-washable ink, newsink, and job ink. Rotary inks are commonly used in letterpress printing of books, magazines, and newspapers. Book ink is a somewhat fluid ink, and book inks are formulated to be compatible with the surface of the book paper on which it is to be printed. For example, a paper with a high degree of surface hardness requires a fast-drying ink. Rotary inks also include heatset inks. (See Rotary Ink.) Moisture-set inks, as was mentioned earlier, utilize glycol vehicles that set fairly fast and are odor-free, which is why they are frequently used in printing food wrappers and packaging. (See Moisture-Set Ink.) Water-washable inks set very fast and are water-resistant when dry, and are used to print on kraft paper and paperboard. (See Water-Washable Ink.) Newsink, used for printing on newsprint, dries primarily by absorption of the vehicle into the substrate, and consequently needs to have a fluid consistency. Like newsprint—which is made from inexpensive and somewhat low-quality groundwood pulp—newsinks also are made from inexpensive and perhaps less than optimal raw materials. The faster the press, the thinner the ink must be. An ink that is too thick will smudge when the paper is folded or generate ink setoff. An ink that is too thin can soak all the way through the paper, producing a printing defect known as strike-through. Most newspapers, however, although originally printed by letterpress, are now printed using web offset lithography. Job inks have a medium body and a drying process that can be used on as wide a variety of paper as possible. Job inks tend to be a standard default ink in many letterpress print shops, and need to be compatible with many paper types and many types of presses. Letterpress printing processes also use various other types of inks on occasion, such as non-scratch ink that is needed for labels, covers, and other end uses that require a scratch-resistant ink, quick-set inks, and high-gloss inks.
'Offset Lithography'. The suitability of the offset lithographic process for printing on a wide variety of surfaces has resulted in a large number of inks available for the process. Typically, lithographic inks (which are paste inks) are more viscous than other types of inks, and since the ink film is thinner with offset printing, the pigment content must be higher. (Offset presses deposit ink films that are about half the thickness of films deposited by letterpress presses.) And since offset lithography is premised on the fact that oil and water do not mix, inks designed for the process must contain significant amounts of water-repellent materials.
Sheetfed offset presses primarily use quick-set inks, which dry rapidly without the need for additional equipment, such as drying ovens necessary for heatset inks. Some sheetfed offset presses, however, do use various radiation-curing devices, as is needed for super quick-set infrared ink, ultraviolet curing ink, and electron beam curing ink.
Lithographic inks primarily set by a combination of absorption of oil-based vehicle components into the substrate, followed by oxidation and polymerization of the remaining components of the vehicle. Web offset lithographic processes utilize higher press speeds, and consequently need to lay down an ink film more rapidly. The ink must be absorbed into the substrate more quickly to avoid smudging and setoff during folding processes at the end of the press. Hence, web offset inks tend to be more fluid and have less tack than sheetfed lithographic inks. Newsinks have seen improvements recently, especially from soy ink, which is made from the latest development in vegetable oil vehicles, soybean oil. Web presses also utilize heatset inks, which dry as the printed paper web is passed through a high-temperature drying oven. Web presses also utilize radiation-curing methods.
The most important criterion for offset inks, however, is their insolubility, as they must resist bleeding in the presence of the water-based press dampening systems. Problems with the drying of offset inks that dry by oxidation include emulsification of the fountain solution into the ink. An excessive amount of dampening solution (or one with a high pH) can impede proper ink drying, and the use of papers with a low pH also has a deleterious effect on ink-drying properties. (See Acid Paper and Alkaline Paper.) Lithographic processes are also well-suited to printing on surfaces other than paper. Lithographic inks used for printing on metals (such as the printing of cans and other metallic packaging) contain synthetic resin varnishes that dry in high-temperature ovens. Letterset inks and waterless inks are also available for recent developments in waterless offset printing processes.
'Flexography'. Flexographic presses typically use liquid inks that possess low viscosity and dry primarily by evaporation of the vehicle. Flexographic presses use either water inks (typically on non-absorbent substrates such as polyolefins and laminated surfaces and, in the past, on various types of paperboard) or solvent inks (for use on surfaces such as cellophane). Water-based ink vehicles are composed of ammonia, protein (solubilized by amine), casein, shellac, esterified fumarated rosins, acrylic copolymers, or mixtures thereof. They have a high degree of printability, perform well on the press, and clean up easily. Water-based inks are used extensively in flexographic newspaper printing as they are almost totally smudgeproof. Water-based flexographic inks, however, have a longer drying time on less absorbent substrates and a low degree of gloss. Water-based inks are undergoing further research and development due to the desire to decrease the dependence on solvent-based flexographic inks, which contribute to air pollution. The vehicle for solvent-based inks is a solvent-resin mixture, formulated to suit the surface to be printed, as well as the press plate and other parts of the press it will be in contact with. Incompatible solvents can distort and damage the rubber flexographic plates. The solvent is made up of an alcohol—ethyl, propyl, or isopropyl. To produce optimal resin solubility, glycol ethers, aliphatic hydrocarbons, acetates or esters may be added. These additives also contribute to the desired viscosity and drying speed. The resins themselves must be chosen with care, as they affect the end properties of the ink. Typical resins used in flexographic inks include acrylics, cellulose esters, nitrocellulose, polyamides, modified rosins, and ketone resins.
'Gravure'. Unlike most inks produced for other printing processes, gravure inks comprise a pigment, a binder to keep the pigment uniformly dispersed and to bind the pigment to the surface of the substrate, and a solvent to dissolve the binder and eventually evaporate away in the drying phase. Depending on the solvent used and what it is capable of dissolving, a wide variety of materials may be used as binders. They are chosen according to the end properties desired, such as gloss, resistance to water or other substances, flexibility, etc. Some binders, such as film formers, dissociate themselves from their solvents rapidly after printing, which enables the ink to dry quickly. Finishing operations such as rolling, diecutting, etc., can be performed immediately as is the case with types of wrapping and packaging. In rotogravure printing, the most important considerations in terms of solvents are their dissolving of the film-forming resins, the rate at which they dry, whether or not they have deleterious effects on previously-printed ink (as in multi-color jobs), their toxicity, and whether they release harmful vapors. Pigment particles must also be more finely ground than in other printing processes, lest damage be incurred by the gravure cylinder. As part of the effort to reduce the usage of solvent-based inks, water-based gravure inks are being developed, but have not yet met with resounding success.
'Screen Printing'. Screen process printing requires paste inks that are thick and able to print sharply through the screen. They must also perform well under the action of a squeegee. The binder added to screen process ink must be compatible with the surface on which it will be printed. The solvents used should also not be overly volatile, as excessively early evaporation would cause the remaining ink components to clog the screen. Screen inks typically utilize a drying oil vehicle.
'Ink-Jet Printing'. The inks used in ink jet printers—typically used for computer printouts, labels, etc.—consist of dyes mixed with a highly fluid vehicle or carrier that form very small drops, can pick up an electrical charge, and can be deflected properly to fall in the right place for the formation of a printed character or image.
'Copperplate and Die Stamping'. Copperplate printing is commonly used to print stamps, bank notes, securities, and other high-quality decorative applications. These processes utilize a somewhat viscous, heavy ink that allows the designs etched in the printing plate to be completely filled in, much like in gravure printing. The vehicles for these inks utilize light litho oils and fluid resins mixed with low-volatility solvents that evaporate very slowly.
'Electrostatic Printing'. Also called xerography, electrostatic printing is commonly found in photocopying machines and computer laser printers. The "ink" used in these processes—commonly referred to as toner—consists of a fine, dry powder coated with the desired color imparted by a colored resin binder. The important consideration is not only particle size, but also electrical properties, as electrostatic printing works by attracting particles electrostatically to a charged drum, the point of attraction on the drum being the printing areas.
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Advantages:
Types: