Printing inks

Colours play an important role in marketing, design and advertising. They can trigger associations and feelings such as relaxation, and they are also perceived as warming or cooling. The selection of colour tones for print media or digital media should therefore be well thought out in order to evoke the desired reaction on the part of the customer or user. Printing inks, which are used especially for the design of print media, can now achieve high-quality printing results. Depending on the demands placed on them, an appropriate choice of printing ink must be made. In the following, you will learn what printing inks are, what requirements they should fulfil and how various influencing factors affect the printing results.

What exactly is printing ink?

Printing inks are mixtures that contain colourants and are transferred to printing media such as paper with the help of printing formes. Special binders are responsible for embedding the colourants in a durable and mechanically resilient film on the medium. In the best case, printing inks should already form an intensive colour film in a thin layer. Together with the printing media, they are among the essential materials in the printing process.

Components of printing ink

From a physical point of view, printing inks consist of a solid, i.e. the colouring agent, which is finely dispersed in a liquid substance (binder). This is also known as a "disperse system". Basically, they consist of four components:

• Colourants (organic and inorganic pigments)
• Binders (resins)
• Solvents (oils)
• Auxiliaries (additives)

Colourants are all colouring components that occur in the form of organic or inorganic pigments. These fine components disperse in the binder, which means that it ensures their extremely fine and even distribution. The solvent serves to transfer the ingredients to the printing form or substrate. Various auxiliaries such as additives optimise the printing ink, i.e. they enable the offset inks to dry and also control product-related properties such as scratch and rub resistance.

Requirements for printing inks and their properties

The viscosity of the printing ink should also depend on the demands made on the printed product. Basically, a distinction can be made between pasty printing inks with high viscosity and thin-bodied printing inks with low viscosity. Furthermore, radiation-curing ink systems must be distinguished from those for digital printing systems (dry and liquid toners). The print products thus determine the choice and, accordingly, the properties of the ink. They can be roughly divided into four groups:

• Packaging
• Advertising
• Catalogues and magazines
• Newspapers and books

In packaging printing, as the first group, inks are expected to have ideal rub resistance, resistance to the filling material and adequate brilliance. If, on the other hand, they are used in advertising, good rub resistance, optimum image reproduction and high gloss are also required. Printing inks in catalogues and magazines must have a high brilliance, be processed quickly and be able to dry. Newspapers and books, on the other hand, require both fast drying and good smudge resistance. In addition to these aspects, certain properties of printing inks are decisive for customers, which can be divided into the following groups:

• Optical properties
• Mechanical properties
• Chemical and physical properties
• Light fastness

Printing inks have good optical properties when they realise the desired colour tone as accurately as possible. Their properties depend on the printing process used, the medium and the existing light source, such as daylight or artificial light, under which the printed product is viewed. Good mechanical properties are particularly important for the different printing processes. For example, printing inks such as offset, flexo and gravure inks, which are applied by rolling, should be able to be transported well over the inking rollers.

The chemical properties of, among other things, the liquid components in the printing inks are also of great importance. These should realise the transition from the liquid to the solid state, for example, when the ink is applied to the printing forme in liquid form and transferred to the medium and then changes to the solid state. Chemical and physical properties thus mainly determine the drying behaviour of printing inks. Fast drying is required when printing books or catalogues, for example, but if they are to penetrate a substrate (also known as "strike-off"), then other, precisely defined chemical and physical properties are required. If packaging has to be printed that is used to store food, hygiene products or even toys, then this requires certain toxicological properties of printing inks. There are also requirements for their physical and chemical resistance, such as rub and smudge resistance as well as resistance to chemical reactions, which is also of great importance in packaging printing. Depending on the application, printing inks should be resistant to solvents, acids, grease, bases, perspiration and much more. Corresponding specifications are precisely defined in standards such as DIN ISO 2836, DIN EN 71, DIN ISO 12040, EN 646 and EU regulations.

In addition to these factors, the light fastness of printing inks is one of the most relevant properties that print shop customers are particularly interested in. UV rays change the structure of binders and pigments, causing inks to fade over time. This is the case with both daylight and artificial light. If printed products such as wallpaper, menus, posters or shop window lettering are continuously exposed to sunlight or artificial light, this should be taken into account.

Factors influencing inks during print production

Depending on which printing process is used in which way, this can have an influence on the printing inks and accordingly on the printing result. We list below how different production methods affect printing inks.

Thickness of inks

If a printing ink is applied to the medium in several layers, then both its intensity and its hue change with an increasing number of layers. As a rule, printing inks become darker with an increasing number of layers, for example, the colour yellow eventually acquires an orange cast. It must therefore be clear before the start of the print job what the ink layer thickness will be in order to select the right ink and achieve the best print result.

Influence of the paper on the ink

Paper exists on the market in hundreds of different types and qualities. The individual papers differ in their surface structure, inherent colouring and ink requirements. These factors have an enormous influence on the colour tone and, accordingly, the print result - especially with offset inks that are transparent. So if you print the same layer thickness on different papers, the resulting colour tones will differ significantly.

Coating - yes or no?

Another factor influencing printing inks is varnishing. If the refraction of light is altered by the coating, the colour tone shifts and the print result is differentiated. This is due to the physics of light refraction, not to chemical changes in the print. An orange print, for example, appears much redder after overprinting than before. So if the colour tones of the printed result are to correspond exactly to those of the original, the influence of the coating on the printing ink must be taken into account and thus the selection made. Likewise, the printing inks must have the required fastness, otherwise further discolouration may occur due to the effect of coating or adhesive ingredients.

Factors influencing colour shades after printing

It is not only during the printing process that changes in colour tone can occur due to printing media, finishing or layer thickness. Even after the process has been completed, some factors should be taken into account that influence the print result.

Changing the colour tones through drying

With classic sheetfed offset inks, drying takes place in two steps. First, the thin oils are absorbed into the pores of the paper surface. The process begins with the first contact between ink and substrate and takes only a few minutes. This results in the first solidification of the ink film. In the second step, sheetfed offset dries through the oxidative cross-linking of vegetable oils. This means that the plant molecules cross-link under the influence of atmospheric oxygen and a cross-linked polymer is formed from the liquid oils. The polymer gives the ink film both gloss and hardness. This process can take a few hours or a small number of days, with the ink film reaching its final structure after a few hours. Since the thickness, pigment wetting and composition of the ink film changes in the second step, this sometimes results in a change in colour tone. For example, sheets fresh from the press will show different colour tones than on the following day.

Influence of ageing on the ink

After a printed product has been produced, it will most likely not remain the same colour forever. This is in part due to the paper, which is affected by ageing processes and can turn yellow or have its optical brighteners decompose. This affects the colour, which takes on a yellowish tinge and loses brightness. Here, very light jewellery colours are particularly affected and show a change in colour tone. Furthermore, the colour fades or discolours if the product is exposed to sunlight for a longer period of time. How much depends on the lightfastness of the pigment used.

Predominant lighting

Of course, the colour appearance also changes with different lighting. Daylight, artificial light, overcast days, neon light or incandescent bulbs, for example, all have an influence because they have completely different radiation spectra. As a result, the human eye perceives the ink in a different way depending on the lighting. Similarly, two printed designs may look the same under the same lighting, but appear different under a different type of lighting, which is also called metamerism. In order to avoid discrepancies in colour assessment due to different lighting, the standard illuminant D50, i.e. medium white daylight, is always used for colour matching and print control.

Four-colour vs. spot colours: the differences

Four-colour printing is also abbreviated with the letters CMYK, where the first three stand for the colour components cyan, magenta and yellow. K, on the other hand, stands for Keycolour, i.e. the black component. Four-colour printing is a subtractive colour model, which means that the more ink is applied, the darker the printed result. Theoretically, cyan, magenta and yellow printed on top of each other should result in black, but instead only a dark brown tone is produced. This is why the black portion is needed to create greater contrasts or to print deep black. The colours are applied one after the other, creating a printing plate for each. Four-colour printing is mainly used for print products and in offset printing. When printed together, the secondary colours red, green and blue are achievable, so a wide colour spectrum can be covered, but not as wide as with RGB: This is a colour space which is used for digital media. Here, red, green and blue represent the physical primary colours (light colours) and form an additive colour model. The more colours are mixed, the lighter the colour tone becomes; all three together in the highest value result in white.

Even though a comprehensive colour spectrum can be covered with CMYK, the use of solid colours such as spot and special colours for neon, metallic colours, gold, silver or opaque white is additionally necessary. These are pre-mixed colours with a distinct colour effect. Colour fluctuations can also be largely avoided with spot colours and colour shades can be realised that cannot be reproduced by four-colour printing.