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.

The remainder of this article will deal exclusively with inks used in printing.

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 before they can be absorbed into the paper. Inks that dry via oxidation and/or polymerization require 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. (See Vehicle.)

'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. (See Pigment.)

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.

The primary function of the manufacture of printing ink is the dispersion of the pigment (the solid portion) in the vehicle (the liquid portion). The vehicle is prepared first, and contains all the important performance, setting, and consistency properties. Once the vehicle is prepared, it goes through three basic stages of production: mixing, milling, and filtration. Although in many cases mixing is the only stage necessary, the desired end properties of the ink and the nature of the vehicle and of the pigment determine whether the ink will need to undergo milling or filtration.

'Mixing'. At this stage, the pigment, or coloring material—which can be in flushed (i.e., produced by flushing), chip, or pulp form—is added to the vehicle in batch mixers, or vats, which can hold anywhere from 5 to 1,000 gallons. (Only a few types of inks, in particular newsinks, are produced in continuous mixing processes.) The type of ink ultimately produced and the nature of the pigment to be dispersed determine the speed at which the mixture is stirred—anywhere from a few to hundreds of thousands of revolutions per minute. The number of blades and the blade configuration of the mixer also affect the speed at which the mixture is mixed. When pigment materials have been predispersed, ink can be mixed to its desired end-use specifications in a one- or two-step mixing process. Inks that need further milling or grinding need to proceed to the next stage.

'Milling'. The principle of milling an ink slurry (the combination of pigment material and the vehicle) involves exposing the mixture to a greater shearing and mixing force than can be produced in the mixing stage. There are a variety of different machines available for milling the ink slurry, such as a three-roll mill, a ball mill, a sand/shot mill, and a colloid mill. All of these devices expose the ink slurry to a shearing force, either by forcing the slurry through the nips of steel rollers, by spinning the slurry in a cylinder containing steel balls, by forcing the slurry through a layer of special sand or metallic pellets, or by pumping the slurry through a rotor-stator arrangement. There are a variety of other devices used for milling ink, generally based on pumping the slurry through a turbine or high-speed rotor. The nature of the desired ink and the printing process in which the ink will be used determines the nature of the milling and the equipment required. (See Printing Requirements below.)

'Filtration'. Liquid inks frequently undergo filtration to remove dirt, fibers, grit from the mill, and other impurities. The type of filter used can affect the final end properties of the ink, especially its viscosity. For paste inks, filters can also be used. However, changing filters between batches of different colors (so as to prevent color contamination) is time-consuming, so the volume of ink to be filtered must justify the effort. The limitations on filtration, however, include its inability to remove small air bubbles which may have been inadvertently injected into the ink during mixing or milling. In offset lithography, the grittiness of the ink due to trapped air bubbles will not affect print quality due to the dampening system, but other printing methods may have difficulties. Inks that dry by oxidation are more difficult to manufacture using filtration processes, as air bubbles can increase the formation of ink skins. In filtration methods, one problem that can arise is the clogging of the filter, which builds up pressure in the ink and can pull bits of fiber from the filter into the ink, defeating the purpose of filtration.

There are three basic groups of ink properties: optical properties, structural properties, and drying characteristics.

An ink's color is a function of the pigment used, and an ink's other optical properties are primarily determined by the pigment characteristics. An important consideration is color matching, or the ability to precisely duplicate another color. Color matching tests can be done visually under specific lighting conditions, or utilizing a spectrophotometer and computer programs that can match a color wavelength-by-wavelength. Color matching charts and ink mixing formulas and procedures are provided by various companies. (See Color Mixing, Ink.)

An ink's opacity describes how opaque or transparent an ink is, or to what degree the ink allows or prohibits the transmission of light through it and how well the background on which the ink has been printed can be seen. Some inks are required to be opaque; some are required to be transparent. Another important property is the permanence of an ink pigment, or the extent to which an ink will retain its color strength and brightness with time or upon the exposure to light. A pigment's resistance to chemicals, in particular, how well it will retain its permanence or resist bleeding in the presence of acids, alkalis, or other substances is another important consideration—properties which determine on what types of materials an ink can be used. A similar property is the ink's wettability, or the ability of an ink to refrain from bleeding when exposed to water.

An ink's body refers to its consistency, or hardness or softness. An ink's body can be very soft and fluid, such as newsprint and gravure inks, or hard and stiff, like collotype inks. An ink's flow characteristics can be measured in terms of its viscosity, or the degree of its resistance to flow. (See Viscosity.) Some inks also demonstrate a deceptive body, properties called thixotropy and dilatancy. A thixotropic ink is thick and viscous while in its container, but when stress or shear forces are applied, it loses its viscosity and flows quite freely. (It is due to thixotropy that offset presses require so many ink rollers.) The opposite condition is called dilatancy. (See Thixotropy and Dilatancy.)

Another important structural property of ink is its length, which is related to its consistency and describes the ability of an ink to form long stringy filaments. Inks that are long have increased flow characteristics, and form long threads of ink when pulled. Inks that are short flow very poorly and have a kind of buttery consistency. Excessively long and excessively short inks are undesirable. (See Length.) An ink's tack is a measure of how sticky the ink is, or the force required to split the ink film between the plate or blanket and the substrate. Although highly tacky ink is required for many printing applications, an ink's tack should not exceed the paper's surface strength, or tearing can result. (See Tack.)

Another major consideration in terms of ink is its drying method, or the means by which the vehicle is removed from the pigment, allowing the pigment to harden and solidify on the surface of the substrate. As we saw earlier, inks dry by oxidation, absorption, polymerization, evaporation, precipitation, or any combination thereof. The suitability of an ink's drying mechanism with a particular substrate and printing process is important in preventing problems such as ink strike-through, ink setoff, and ink chalking. (See Vehicle.)

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.

'Heatset 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.

As printing processes increase in speed and in the ability to print on a wider variety of substrates, new ink formulations must keep pace with new innovations to ensure high print quality. The considerations involved in proper ink formulation include the speed of the printing process, the nature of the printing process, the surface properties of the intended substrate, and the ultimate end-use characteristics of the finished printed piece. As we saw above, each printing process requires inks with specific characteristics to ensure compatibility with press chemistry and mechanics. Ink characteristics such as permanence depend on the end use; newspapers don't neceessarily need to be permanent, but inks used in books do. Chemical resistance is necessary in various types of packaging, a longer degree of permanence is necessary to maintain an attractive appearance for products whose packaging is intended to entice consumers into purchasing them.

In terms of substrates, there are two basic divisions which must be taken into account: paper and non-paper.

'Paper'. An uncoated, unsized, highly-absorbent paper such as newsprint used on high-speed web offset presses requires thin, less viscous inks that dry primarily by absorption; yet, as we have seen, too fluid a vehicle will produce strike-through. Similarly, newsprint (or roto news paper) formulated for high-speed rotogravure printing of newspaper supplements and Sunday magazine sections also requires fluid inks that dry by absorption. Papers which are uncoated (such as bond paper, antique finish paper, and vellum finish paper, for example) have low surface gloss, and high absorbency (depending on the amount of water-resistant sizing added). Inks for printing on uncoated papers are typically moderately viscous paste inks that dry by oxidation or absorption. There is a wide variety of surface features and absorbencies available in uncoated papers, and inks are typically formulated with drying properties and viscosities dictated by what will work best on the paper.

Coated and smooth finish papers and papers that have undergone some degree of calendering or supercalendering are typically glossy and water-repellent, with high degrees of ink holdout. Inks formulated for use on these papers tend to dry by oxidation, although heatset inks are becoming more and more prevalent. To reduce smudging, setoff, and blocking, inks that dry quickly are highly desired for printing on these kinds of papers. The increased quality of these papers also allows the effective use of high-gloss inks to provide a higher-quality printed image. The use of high-speed web presses on these papers also demands that the inks be quick setting.

Multi-color printing processes also impose their own demands on the inks used. (See Printing Ink Defects and Problems below.) Printing hard paperboard and corrugated packaging requires abrasion-resistant and scuff-resistant inks, as well as inks that dry quickly. All the printing processes are employed in the printing of various types of packaging as well, which also places additional demands on the ink formulation. Letterpress and offset lithographic inks utilized in paperboard printing are commonly oxidation-drying inks, and flexographic and gravure inks are commonly absorption-drying and evaporation-drying inks. Glassine papers (such as wax papers used to wrap food products) are highly repellent surfaces, commonly printed using gravure and flexography. Various types of imitation parchment are used to produce high-quality documents, such as diplomas, and are printed using copperplate or letterpress processes.

Various types of parchment are also used for wrapping food products, and inks formulated need to be greaseproof and resistant to other types of materials in the foods. They must also be odorless, and resist bleeding. Decorative papers such as wrapping paper are primarily printed by gravure, flexography, and screen printing, which requires taking into account the ink requirements of the particular process as well as the aesthetic requirements of the end use. Kraft papers used for grocery bags and other such uses are typically printed with flexographic processes, utilizing rapidly-drying inks so as to complete cutting, folding, and bundling in rapid succession without smudging or offsetting.

Non-paper substrates include the following:

'Plastic'. Plastic substrates are frequently used in printing wrappers and other packaging. The important considerations include minimal (or no) absorbency of the ink by the stock, and quickly-evaporating solvent- or water-based inks (printed using flexography) are commonly used. Gravure presses are also commonly used for film packaging. Compatibility of the binder to plasticizing materials in the substrate is also an important consideration, as intermingling of plasticizing materials and ink binding substances can soften the binder, causing smudging, setoff, and blocking. The type of plastic film used—be it cellophane, polyethylene, polypropylene, or other petrochemical substances—is also important. Solvents used in inks also help the ink adhere to the surface of some plastics better than to others, in particular, to cellophane. Often, plastic-coated paper, paperboard, or foils are utilized, and the ink must adhere to both surfaces. In many cases, these "dual-substrates" are used in food wrappers, where solvent-retention by the dry ink film must be avoided, so as to prevent both delamination of the surfaces and leeching of the solvent into the food.

'Metal'. Aluminum sheets or foils are commonly used in various types of packaging, and are printed most commonly with flexography or gravure presses. Often, the foil is covered with a layer of shellac, nitrocellulose, or other material to improve the adhesion of the ink, and frequently thin sheets of foil are laminated on other substrates, such as paper, to