Printing Ink Basics

Introduction to Printing Inks

Printing inks are formulated from three basic components: a colorant, a vehicle system to carry the colorant, and additives such as waxes and driers. Colorants may be dyes, but are typically pigment-based. They may be in powder form (dry toner), in a concentrated paste dispersion known as a flush, or in a liquid dispersion. Vehicles are made up of oils (petroleum or vegetable), solvents, water, or a combination of these. Most vehicles also contain resins, which serve to bind colorants to the printing surface. Additives can include waxes, driers and other materials that add specific characteristics to an ink or to the dried ink film.

Together, these ingredients produce the key properties of printing ink: visual properties, runnability properties and end-use performance properties.

Visual Properties of Printing Inks

Visual properties of printing inks are a function of the colorants or pigments in relation to the vehicle system used. They include color, transparency or opacity, and gloss. Inks are manufactured in a seemingly endless rainbow of colors. Through careful pigment selection, ink formulators can produce inks that faithfully reproduce the hottest new lipstick color for a cosmetic ad or a classy new car color in an automotive brochure.

But these specialty matches make up only a very small percentage of the total ink produced. By far, the most widely used ink color is black. Then come cyan, magenta and yellow which are used in process printing to create the millions of colors so familiar to us in printed matter.

While the physics of color is a highly sophisticated science, in simplest terms color comes from reflected light. White light contains the entire rainbow of colors. When that light passes through a filter or is separated by a prism or raindrop we see the individual colors in the light spectrum. An ink film acts as a filter on the light reflected from the printed surface, e.g., a red ink film allows the red segment of the reflected spectrum to pass through while blocking the rest of the colors. Because printed surfaces vary in color and in reflectance, they, too, will affect the reflected color. Thus, various ink colors printed individually or "trapped" one on top of the other create different filter effects resulting in different visible colors. Similarly, these same ink colors printed on different substrates will result in visible colors that are different yet.

When we refer to ink color, we are most often speaking of hue or shade—whether the ink is red or blue or green or purple. Secondarily, we might describe its strength or saturation, also termed chroma. Thirdly, we might indicate how light or dark it is--a reference to its purity or value.

The physical and chemical nature of a pigment—the size and even the shape of its particles—contributes to the refractive nature of the colorant, and thus to its hue. The amount of pigment used affects an ink’s color strength, and the type of vehicle used can affect both the hue and the value of the ink color. The color of the vehicle itself, its ability to wet the pigment articles, and even the chemical interaction between the vehicle and pigment can affect the shade or purity. Finally the color of the substrate, and its drying/absorption properties affect the printed color results.

 
The choice of colorants and the degree to which they are dispersed through the vehicle are the most important factors in determining the transparency or opacity of a printing ink.

 
The substrate will also affect the amount of light that is reflected back through the ink film; gloss and matte paper, transparent film and metallic surfaces will have considerably different reflective capacities, making the printed ink film appear more or less opaque accordingly.

Gloss refers to an ink’s own ability to reflect light, and depends upon the lay or smoothness of the ink film on the substrate surface. Generally, the higher the ratio of vehicle to colorant, the smoother the lay, and the higher the gloss. Application of a thicker ink film tends to improve gloss while penetration into the substrate tends to reduce it.

Runnability Properties

Runnability is a term unique to printing. It applies to the trouble-free interaction between the ink and the press, the paper and the press, and finally, the ink and paper. The inks used in the different printing processes require different runnability properties in order to travel in optimal fashion through the press to the substrate. In addition, they must adhere to the substrate and dry properly.

Runnability requirements increase in importance as press speeds increase. Body, temperature stability, length, tack, adhesion and drying all contribute to the runnability of an ink and are primarily a function of the vehicle system used
in the ink.

Body refers to the consistency, stiffness or softness of printing inks. Viscosity is a related term that refers to the flow characteristics of soft or fluid inks. Ink body and viscosity requirements vary widely by printing process. In general, letterpress and offset lithographic inks are fairly thick or "viscous" (much like paste or honey). On press, they move through a series of rollers called the ink train where the action of the rollers spreads the ink into a thin film for transfer to the blanket and/or plate and onto the substrate. Flexographic and gravure printing inks are much more liquid (more like milk), so that they flow easily into and out of the engraved cells on anilox rollers (flexo) and print cylinders (gravure).

 
Temperature stability in a printing ink is important for withstanding the heat generated by the friction that occurs as an ink moves through the rotating rollers and cylinders. If an ink vehicle is not sufficiently stable, the increased temperature can have a deleterious effect on an ink’s body and therefore on its runnability.

 
Yet another quality, length, describes an ink’s tendency to form long threads when stretched or pulled. Long printing inks flow well but form long filaments that have a tendency to sling or mist, especially on high-speed presses. Short printing inks have the consistency of butter and flow poorly. They tend to build up on rollers, plates or blankets. Inks with the best runnability are neither excessively long nor short.

 
Tack is the stickiness of an ink, or the force required to split an ink film between two surfaces. With paste inks, tack is critical to the proper transfer of ink through the ink train to the plate and/or blanket and then to the substrate. Tack also determines whether the ink will pick the surface of the paper; ink tack that is higher than the strength of the paper surface will tear it. Tack affects whether inks will trap properly in multi-color printing. The first-down color should have the greatest tack without picking the stock; succeeding colors usually need progressively less tack for proper trapping. Tack also influences whether an ink prints in sharp lines and images or squashes out on the plate, blanket or substrate.

 
Drying properites of an ink are critical for a number of reasons. The most obvious is that a printed piece cannot be handled or used until the ink has developed film integrity. In addition, however, the way an ink dries can reduce air pollution, improve energy efficiency, and even improve productivity in the pressroom by allowing faster printing and converting.

 
In most cases, the first phase of ink drying is setting; immediately upon being applied to the stock, the liquid portion of the ink begins to evaporate into the air or to penetrate the stock, causing the ink to thicken. Setting is followed by actual drying via one or more possible mechanisms: absorption, oxidation, evaporation, or polymerization. The specific mechanism is determined by the relationship between the printing process itself, the ink vehicle system, and the substrate.

 
Inks that are applied to an absorbent substrate such as newsprint or corrugated board dry by absorption. The liquid portion of the ink penetrates the substrate, leaving an ink film on the surface. Depending upon the printing process, this ink film may undergo additional drying procedures.

In oxidation, components in the ink’s oils chemically combine with oxygen in the atmosphere to form a semisolid or solid ink film. It often occurs in combination with absorption. Oxidation can be accelerated by the use of driers in the ink formulation or by the application of heat or infrared radiation to the printed piece.

 
Since non-porous substrates such as plastic films and glass cannot absorb ink vehicles, they require inks that dry either through evaporation or by polymerization (e.g., radiation curing). In the former, vehicle solvents evaporate, leaving resins and other materials behind to bind the pigments to the substrate. Evaporation from the inks must be rapid enough for complete drying, but not so rapid as to cause instability while the inks are still running on press.

 
In radiation curing, all of the components in the ink remain on the surface of the substrate, but are polymerized into a hard film by the use of ultra-violet light or electron beam energy to trigger a chemical reaction. UV-curable inks require the presence of a photo-initiator, while EB-curable formulations do not.

Adhesive characteristics of printing inks are critical as well. For successful printing, an ink must adhere to the substrate for the life span of the product, whether it’s on paper, board, film, foil, plastic, metal, glass or rubber. Here again the vehicle system is most responsible for an ink’s adhesive properties, although colorants can have an effect depending upon their chemical compatibility with the vehicle system and the ratio of pigment to vehicle.

As with drying, adhesion can occur in several ways. On absorbent substrates, vehicle penetration is key. On non-absorbent surfaces the vehicle resin’s ability to form a continuous film and its chemical affinity for the substrate are most important. Ink solvents provide the wetting and flow-out of ink to give the continuous film necessary for good adhesion. Solvents can also soften substrates such as PVC to promote both physical and chemical bonding. Selection or formulation of the proper vehicle is essential to proper adhesion.

End Use Properties

Inkmakers not only have to consider all of the conditions that have to be met for successful print production, they must also formulate a product that takes into consideration the finishing process and the end use of the printed article.

Folding cartons, for example, may be printed, diecut, scored, folded, glued and delivered in one continuous operation. These inks must be formulated to dry quickly to an especially tough, rub-resistant surface in order to withstand this physical converting process as well as the ultimate filling and shipping.

Paper napkins, towels, wallpaper and the like often are “creped” or embossed with a design either during or after printing. Other printed surfaces may be laminated. Beer or food containers may undergo sterilization or pasteurization. Inks for posters require lightfastness.

Some bread wrappers and milk containers are waxed after printing to make the paper moisture resistant. Inks for this purpose must be formulated with pigments and vehicles that will not bleed in hot wax.

Selected waxes, lubricants, gums and starches are used together with pigments and vehicles to lend these and other decorative and functional properties to inks.

Because most printed articles are subject to rub or abrasion over time, all printing inks are formulated for some degree of abrasion resistance. Resistance can come from the resin used in the vehicle, the level of pigment dispersed in the ink, or the method used for dispersion. Surface and slip compounds such as waxes can be added to provide additional abrasion resistance as they move to the surface of the dried ink film.

Heat resistance in printing inks is important where pasteurization or heat sealing procedures will be used. The vehicle system must not soften nor the color decompose when exposed to high temperatures.

Lightfastness of an ink is important in poster printing, art reproduction, labels, and point of purchase displays. It is a function of ambient conditions, length of exposure, substrate and ink film thickness, but primarily the pigments used.

Pigments and resins have varying degrees of resistance to acids, alkalis, oils, fats, detergents and other substances and must be selected carefully. Inks used on cartons or labels must be able to withstand contact with the contents of the packages. Where food is involved, compounds must be chosen that won’t affect the food’s flavor.

Finally, an ink exposed to the weather or saltwater will have to use colorants that resist fading under the physical and chemical attack by these elements.

Good Information Produces Good Printing Ink Formulations

At times, printing problems may occur when an ink manufacturer is not fully informed about the particular substrate to be printed. Others can be traced to pressroom operations. Still others occur in the final applications. Regardless of their source, many of these problems can be avoided by the proper formulation of printing inks or corrected through modifications of inks on press.

As we have seen, printing ink ingredients can make inks transparent or opaque, glossy or dull, metallic or fluorescent, light-fast or rub-proof, heat resistant or chemical resistant. Printing inks for soap wrappers and detergent cartons can be alkali and soap resistant, and inks for outdoor displays can be light and water-resistant. Printing inks for shipping containers can be scuff and scratch resistant, and inks for food packages can be odorless. Printing nks on liquor labels can be alcohol resistant, and inks for boilable or microwaveable food containers can resist high temperatures.

Printing inks can be properly formulated to avoid quality problems such as picking, poor trapping, strike through, set-off, dot distortion and tinting.

 
Having the right ink formulation is key to the success of a printing job. It should also be apparent that creating the right ink formulation depends upon having sufficient information to select the proper combination of colorants, vehicles and additives. The ink formulator must have comprehensive information on the printing and finishing process, the substrate and the end use of the printed product.