Printing ink

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Printing inks and additives in a lithographic printing studio

Printing inks are mixtures containing colorants that are transferred to a printing material with the help of a printing forme . Printing inks must form an intensely colored film in thin layers (dry printing ink layers in offset printing and flexographic printing are 1–3 µm thick, whereas in screen printing up to 15 µm). For this purpose they contain colorants , usually inorganic and organic pigments , titanium dioxide or carbon black , as well as solvents or another carrier medium.

The colorants must be embedded in a permanent and mechanically resilient color film on the substrate - many printing inks contain additional binders that coat the pigments .

During the printing process, a transition from the liquid state, in which the printing ink can be distributed and transferred on the printing machine and the printing form , to the dry, solid state on the printing material is necessary. In order to meet this requirement, printing inks contain liquid components that enable the transition from liquid to solid through evaporation, penetration (penetration) into the printing material or through chemical reaction.

Classification of printing inks

Depending on the requirements of the respective printing process , the printing machine technology used and the printing materials (substrates) usually used, very different printing ink formulations are used.

The toner of the electrostatic and the inks of the ink-jet printing processes are no printing inks according to the usual definition. The term printer's ink for (black) printing ink is no longer used in technical terms.

Web offset coldset printing inks (newspaper printing)

Newspapers and newspaper-like advertising material are produced using web offset coldset printing inks. The inks dry when the liquid component is absorbed into the substrate.

Web offset heatset printing inks

Magazines, catalogs, books and advertising leaflets are produced using heatset printing inks. The inks dry by evaporation of mineral oils with the help of thermal energy, hence the name "heatset".

  • 8-15% pigments
  • 25–40% polymers or resins (colophony-modified resins and hydrocarbon resins)
  • 30–45% high-boiling mineral oils (boiling range 250–310 ° C)
  • 2-8% additives

Sheet-fed offset printing inks

Sheet-fed offset printing inks are used to produce books, advertising material, magazines, packaging (e.g. for food or cosmetics), calendars, posters and flyers. The paints dry by setting and oxidative drying ( polymerisation ) of vegetable oils and their derivatives. The majority of the sheet-fed offset printing inks produced in Europe today are mineral oil-free.

  • 10-30% pigments
  • 25–35% polymers or resins (colophony-modified resins, alkyd resins )
  • 30-40% liquid components (e.g. vegetable oil or mineral oil)
  • 5-10% additives

Illustration gravure inks

This printing process produces large print runs of magazines, catalogs and advertising material. The inks dry by the evaporation of toluene or ethyl acetate, which is recovered in the printing system and reused.

Liquid colors (water-based)

In this process, packaging made of paper, cardboard and foils as well as wallpaper, wrapping paper, hygiene paper (tissue) and decor paper are printed. The colors dry by evaporation and absorption of water.

  • 15-25% pigments
  • 10-25% polymers or resins
  • 35-60% water
  • 8-12% additives

Liquid colors (solvent-based)

In this process, packaging made of foils and composite materials as well as decorative foils are printed. The inks dry through the evaporation of solvents that are caught in the printing process.

Radiation curing printing inks

These printing inks are used for special applications in various printing processes. Among other things, packaging, labels, finishing, plastic bottles and tubes are printed. The colors dry through light-induced polymerisation of acrylate oligomers and monomers.

  • 10-30% pigments
  • 40-80% acrylate oligomers and monomers
  • 10-20% additives (including photoinitiators)

General requirements for printing inks

Optical properties

The printing ink should achieve a required color tone (color location, color strength, light / dark) on a substrate. Surface gloss, metallic gloss and interference effects make a printed product appear more valuable. In some cases, gloss effects can also be achieved with paints. The optical properties depend on the printing process, the substrate, the location of the viewer and the light source under which the printed product is viewed. Gloss effects depend on the observer's viewing angle.

Mechanical properties

The mechanical properties of printing inks are also called rheological properties. The large number of different printing processes and substrates require different printing inks. Printing inks for printing processes in which the ink is applied by rollers (offset printing, flexographic printing, gravure printing) require good transport of the ink over the inking rollers.

The color transport is determined by the speed of the color. It is a complex relationship between viscosity, cohesion and adhesion and is measured as tack in printing ink tests.

Chemical and physical properties

The chemical and physical properties largely determine the drying behavior of a printing ink.

The ink setting refers to the penetration of an ink into a printing material. Absorbing colors are colors that consist of a carrier (often a linseed oil derivative) and a pigment as well as a varnish former. In the simplest case, the paint consists only of pigment and linseed oil . The absorption of a printing ink must be precisely defined for high quality printing. The substrate also plays a decisive role here.

The oxidation of the binder is a chemical process that also often contributes to the drying of printing inks . The surface of the printing ink is greatly enlarged compared to the volume due to the thin application on the printing material. The oxygen from the ambient air attacks the double bonds of the binding agent. A chain reaction of the binder components starts , which leads to crosslinking to a 3-dimensional, stable structure within the printed ink layer. In the case of physically drying printing inks, the solidification on the print substrate takes place through evaporation / evaporation of the volatile component of the printing ink or through radiation curing . The volatile components used are water (solvent-free printing inks) or alcohols , esters , ketones (usually for packaging printing inks) , toluene (for illustration gravure printing inks ) or high-boiling mineral oils (web offset inks). Radiation-curing printing inks (offset or flexographic printing inks) are cured by radical polymerization of the unsaturated polymers - triggered by UV rays (ultra violet, wavelength below the visible range between 120 and 380 nm) or EBC ( electron beam curing ). This curing takes place in less than a second. The printing inks are very resistant (cross-linked) and are solvent-free.

The toxicological properties of printing inks are particularly important for the use of printed products for packaging food, but also for toys and hygiene products.

The physical resistance of a printing ink is its resistance to mechanical loads and radiation. Test parameters are therefore, for example, the abrasion / scuff resistance and wipe resistance . The lightfastness is mainly determined by the resistance of the color to UV radiation.

The Chemical resistance is the resistance to chemical reactions with any kind of substances. The chemical resistance is a requirement that is made especially for packaging. Depending on the application are solvent resistance , resistance to grease (butter), acid resistance (eg. As vinegar), base resistance (z. B. detergent), saliva (eg. Children's toys, sweets), sweat, milk, etc. required. These are regulated by corresponding standards and regulations such as DIN ISO 12040, DIN ISO 2836 , DIN EN 71, EN 646 and Regulation (EU) 10/2011.

Dispersion

In addition to the understandable water fastness, the dispersion of the pigments in the printing ink varnish plays an important role in offset printing. Poor dispersion (very fine distribution) leads to pigment agglomerates (pigment lumps) which, at layer thicknesses of around 1 µm, result in a reduction in color strength and gloss due to agglomerates protruding from the layer surface. This also affects transparency. The dispersion takes place on the one hand during the printing ink production and on the other hand additionally in the roller inking unit of the offset printing machine.

Components of printing ink

Printing inks consist of:

Colorants

Colorants are divided into pigments or dyes . Pigments come in the form of inorganic and organic, colored, black or white materials and are practically insoluble in the processing medium. Dyes, on the other hand, lose their crystal / particle structure in the processing medium and dissolve in it.

Effect pigments based on ground metal and mica play a special role .

binder

Binders serve to wet the colorants and enable a permanent bond with the substrate. They are selected based on the printing process , substrate and requirements of the end product. In the paint system, binders are in solvent-dissolved form or in disperse form.

Their fundamental property is the development of the rheology required for the printing process, which enables the ink to be transported from the printing form to the substrate by means of solvents . Furthermore, it enables the formation of a color film when the printing ink dries, which has a decisive influence on the subsequent properties of the print.

Resins

The basic material of the hard resins is rosin (tree resin) from Portugal, Brazil, Mexico or China. Specifically adjusted hard resin is produced through chemical modification. Hard resins form a hard to brittle film on the surface of the substrate.

The base material of liquid resins ( alkyds ) are linseed oil or soybean oil. Printing inks made from liquid resins form a less brittle paint film and the wetting of the pigments during production is made easier.

Ink varnishes

In letterpress and offset printing varnishes are used as binders. In these printing inks, almost all mechanical, physical and chemical end properties are determined by the varnishes.

Mixtures with roughly equal proportions of resins and oils are called varnishes . Resins have a very high molar mass and are therefore in "colloid" form. During the mixing process, they absorb oils and thereby swell up. Vegetable oils (native oils) and mineral oils are used.

Solvent

Organic solvents, water and reactive thinners are used as solvents . Their task is to absorb the ingredients such as pigments, binders or additives in dispersed and / or dissolved form and to enable them to be transported via the printing form onto the substrate.

After the printing process, the solvent escapes from the printed layer through drying / evaporation or absorption; it is partly integrated into the paint film during the polymerization.

Additives

Additives can be used to support the production of the printing inks and to optimize the printing inks, in order to influence specific requirements of the end product or the application. In the production of the printing inks, dispersing additives and defoamers are used as a support; substrate wetting agents, defoamers, waxes and lubricants are used to meet special requirements and to support application.

Manufacture of printing inks

In the production of printing inks, there are roughly two production areas:

  • Production of the pigmented / coloring component : Here, the pigments are dispersed by grinding / rubbing processes to the particle size required to achieve the desired color strength and dissolved as disperse particles in the medium (solvent and / or binder, additive). The product is loosened up by adding binding agents / varnish and delivered to the printer either as a concentrate or as a ready-to-print component.
  • Manufacture of non-pigmented / coloring binder-based products : Here a wide variety of binders are dissolved in the solvent or combined as dispersions (in aqueous systems) and mixed with additives in order to achieve the properties required for the type of printing ink and the final application. These are delivered to the print shop as intermediate products for further internal use in printing ink production as well as waste / varnish components.

Testing of printing inks

When testing printing inks, the specific parameters for the respective printing process and color-relevant parameters for the printed end product are examined. Examples of specific parameters are the viscosity , the solids content , the color location , the color strength or the pH value . Examples of color-relevant parameters for the printed end product are abrasion , gloss , bond strength, splitting resistance and crease resistance.

Order of color

In all printing processes, the printing ink is applied to the substrate (application) in two steps. First, the printing ink is applied to the printing form . The printing ink is then transferred to the printing material. The application and dosage of the printing ink on the printing forme and the transfer from the printing forme to the printing material vary depending on the printing process.

Print finishing

Print finishing represents the treatment of the substrate after the actual printing. It contributes to better durability and a better appearance of the printed material. Print finishing is mainly used when a certain rub resistance is to be achieved. In order to carry out this process, the printing ink must be completely dry. It is also important that the substrate is free of abrasion protection pastes, silicones and waxes . It should be noted that due to the subsequent treatment of the printing material, color tones can be changed.

Lacquers

Varnishes serve to protect the printed surface, to optimize required properties such as abrasion and to achieve special design effects (e.g. spot varnishing). Paints are available based on all known systems (water-based, solvent-based, UV-based).

Dispersion varnishes

The dispersion varnishes are applied to the substrate in the printing machine via a coating unit. Due to their high water content, dispersion varnishes are also called water-based varnishes. A polymer dispersion made from modified acrylates and a resin solution (varnish) form the film. Drying by evaporation of the main part of the water. There is neither an unpleasant smell nor yellowing. Another component are waxes (abrasion resistance and lubricity) and wetting agents. Dispersion varnishes are mainly used in offset printing to make the substrate more durable, to give it a high-quality appearance and as protection.

economy

The historical letterpress ink was black and was mainly produced by soot . For a long time, this was associated with lower costs compared to color printing . Modern printing inks are highly complex mixtures of substances, so that prices have converged over the course of technical developments.

In 2011, worldwide sales of printing inks were around $ 20 billion. While the ink consumption of classic print media is falling, the demand for packaging is increasing. The printing ink market is currently growing by 2-3% per year. The USA consumes 31%, Europe 28% and Japan 25% of the printing inks produced.

See also

literature

  • Blana, Hubert: The production . Munich: KG Saur Verlag, 1998. ISBN 3-598-20067-6
  • Scheper, Hans Jürgen: Examination knowledge printing technology . Itzehoe: Verlag Beruf und Schule, 2005. ISBN 3-88013-623-8
  • Ulrich Zorll (Ed.): Römpp Lexikon. Lacquers and printing inks . Thieme, Stuttgart 1998, ISBN 3-13-776001-1
  • Printing Ink Handbook, 7th Edition 2017, National Association of Printing Ink Manufacturers (NAPIM)

Web links

Wiktionary: Printing ink  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ a b R.H. Leach, RJ Pierce: The Printing Ink Manual . 5th edition. BLUEPRINT, 1993, ISBN 978-0-948905-81-0 .
  2. ↑ Dr. Bernd Th. Grande: Drying mechanisms in printing inks. (PDF) Retrieved February 24, 2017 .
  3. ↑ a b c d e f g Lecture 4: Diversity of printing inks - structure and requirements. Retrieved February 24, 2017 .
  4. ↑ Position paper Toluene - Facts and Data Website of the Association of the German Paint and Printing Ink Industry. Accessed on February 10, 2017.
  5. ↑ Dr. Bernd Th. Grande: Rheology for Printing Inks. (PDF) Retrieved February 24, 2017 .
  6. ↑ KBA glossary of printing inks. (PDF) Retrieved February 24, 2017 .
  7. ↑ Regulation (EC) No. 1935/2004 on materials and objects that are intended to come into contact with food , accessed on February 24, 2017
  8. ↑ Directive 2009/48 / EC on the safety of toys , accessed on February 24, 2017
  9. ^ Fogra Forschungsgesellschaft Druck ev - Scouring the printing ink. Retrieved February 24, 2017 .
  10. ↑ Willy Herbst, Klaus Hunger: Industrial Organic Pigments . Wiley-VCH, ISBN 978-3-527-62496-6 .
  11. ↑ Ulrich Poth: Synthetic binders for coating systems . 1st edition. Vincentz Network, ISBN 978-3-86630-878-7 .
  12. ↑ Ceresana: Market study printing inks - world . Retrieved May 21, 2013.