Drying (printing inks)
The technical term drying is used in the technical parlance of the printing industry for all processes that lead to the solidification of printing ink films. These include classic drying processes in which water is removed from the paint film, and the expulsion of other liquids such as solvents or mineral oils is also included. From the mechanical engineering side in particular, chemical crosslinking processes ("UV drying") are also referred to as drying. In addition to printing inks, drying is also an important technical aspect for toners , inks in digital printing , for lacquers and for a large part of paints. A correct knowledge of the available drying processes makes it easier to set the desired suitability for use when planning print objects, e.g. B. Abrasion resistance, non-blocking or sensory compatibility.
Evaporation of liquid components
A large proportion of printing inks contain organic solvents . They contain binders and additives and contain finely divided ( suspended ) pigments . The solvents used for printing inks have boiling temperatures between 69 ° C ( n-hexane ) and 98 ° C ( heptane ). They have a low heat of evaporation , so they are easily volatile. Their flammability , environmental hazard and harmful effects require special precautionary measures.
The evaporation of the solvents takes place well below their boiling point by venting, often under mild heating. In the case of smaller systems, the vapors are disposed of by incineration; In the large systems for illustration gravure , they are collected by means of gas scrubbing . The toluene recovered in this way is used again for the production of similar printing inks.
The proportion of solvents in gravure and flexographic printing is around 2/3 of the total formulation. This means that three times the layer thickness is transferred from what is later left over as a print. This is an advantage for colors with bulky pigments such as B. Metallic and effect colors . In paints with water as a liquid component, the proportion of water is around half. They are more suitable for effect pigments than the pasty, highly viscous inks for offset and letterpress , which contain 10–30% thinner.
The remaining amount of ink in the three classic printing processes gravure, flexo and offset printing is around 1 g / m², the layer thickness thus simplified to 1 µm. Similar colors also require similar amounts of pigment per area. Liquid inks (= solvent inks) therefore offer transport advantages if pigments cannot be easily transferred from the printing machine to the substrate. In addition, in their typical printing processes, they are transferred via fewer contact points (e.g. cylinder-cylinder), which is also advantageous.
If water is to be evaporated, a stronger heating of the printing surface is necessary than with z. B. ethyl acetate . In practice, this is achieved with infrared rays, which particularly heat the dark, i.e. heavily colored, areas. This expels water vapor from the paint film - and now virtually hovers over it. If you transport it away quickly, more water vapor will quickly diffuse upwards. This removal of moisture is usually accomplished with heated air. It is a weak heat carrier and exchanger. But warm air greedily absorbs moisture. The combination of IR and warm air has become a standard technical solution.
In order to evaporate higher-boiling mineral oils in web offset heatset , the paper web must be heated up very strongly. In the beginning, flame gases were used directly. In the meantime, however, hot air is used in sophisticated multi-chamber systems with heat recovery, which has dramatically improved the heatset's energy balance.
The heating gases are heated up to 250 ° C, the web by at least 120 ° C. However, this will overdry the paper and typically warp. Despite all the countermeasures, the edge waviness still remains the typical feature of heatset printing, together with the mirror finish caused by the high-gloss chrome-plated cooling rollers behind the dryer.
The evaporation of liquid components is a purely physical mechanism that takes place in a flash. The final state of solidification is practically already reached when a copy, web or sheet comes out of the printing press. With only a few restrictions, the pigment arrangement (i.e. the color shade), the binders and the anchoring in the substrate are ready (rub resistance). Only residual solvents can cause long-term problems such as blocking (sticking) or odors.
Evaporation drying is the typical drying mechanism in gravure, flexographic and inkjet printing. It can also be used in screen printing .
Knocking away liquid components
On absorbent printing materials, there is a way in which the liquid does not have to evaporate, but is sucked into the freshly printed surface, in technical jargon "knocks away". If only the thinner is sucked out of the print and binders and pigments remain on top, the ink film can quickly become tack-free and also solidify to some extent. Chipping is a physical process, typical of offset printing and old letterpress (letterpress).
If water is present, water hits away first, because it easily wets the pores in paper and cardboard and, due to its low viscosity, can penetrate in fractions of a second. We find it in offset as a dampening solution and in dispersions (water-based inks, varnishes, adhesives) as a liquid carrier, e.g. B. in the dispersion coating at the end of a sheet-fed offset printing machine. The thinner in the pasty inks for letterpress and offset printing consists of mineral oils or fatty acid esters. In comparison to water, both are hardly polar and more viscous (flow more slowly). This means that they do not penetrate the pores of the paper surface as quickly. A few seconds are enough. B. in sheetfed offset that so much thinner has left the ink film that the hard resins in the binding agent fail and form a sufficiently tacky film. If the sheet falls into the delivery , it does not stick to the sheet falling over it and does not stain there either (“does not drop”). The color film shrinks slightly when it is removed. The pigment and binder particles rearrange themselves, which u. a. can lead to color changes in the first half an hour after printing. This can be important for special colors for branded companies.
In newspaper printing, the colors dry purely as they knock away. This is fast enough for the enormous speeds of modern machines, but never leads to halfway abrasion-resistant prints, because coarse pores in uncoated paper and only medium-viscosity thinners (petroleum fractions or vegetable oils) do not allow hard resins and thinners to be separated completely.
In the case of solvent inks and in heatset, the liquids are also absorbed into the substrate; however, evaporation also makes these parts disappear so that they do not appear separately in the balance sheet.
Oxidative filming
From linseed oil , we know that it forms a skin on the air over time on its surface. Here the (double-bonded) oxygen in the air bridges the chain-like oil molecules to form spatial structures. The molecules are esters of glycerine (trihydric alcohol), which is esterified with three long-chain fatty acid molecules . Double bonds in the carbon chains of fatty acids are reactive and can be attacked by oxygen. This chemical process has been used in paint for centuries to make abrasion resistant films. If vegetable oils have enough unsaturated fatty acids for this skin formation, they are still called “drying oils” today.
Oxidative filming is used in modern printing inks for sheet-fed offset and in some places even web offset heatset to strengthen the solidification in the color film. It is accelerated by catalysts ("drying agents", siccatives ) in such a way that, instead of several weeks, a usable, abrasion-resistant ink film is obtained in one or two days. B. can process further bookbinding. The drying oils, nowadays mostly used in their chemically refined and conditioned form as alkyd resins , are inexpensive, wet pigments, keep binding resins finely distributed and help an offset ink in its emulsifying behavior ("litho behavior"). They are therefore also used extensively in printing inks without oxidative drying.
Its numerous disadvantages make oxidative filming an unloved child. The slowness of these chemical reactions is a challenge for developers and users under modern production requirements. A variety of medium and low molecular weight cleavage products cause odor pollution and in some places even impairment of the back of the sheet in the pile. The radical reactions can be delayed or even blocked by a number of risks. In most of the printing inks in which it is used, this drying mechanism only works in a supportive manner, usually in combination with aching. In offset inks for non-absorbent printing materials ("foil inks"), he appears as the only one, but then demands the utmost attention and profound experience from the printer.
Precipitation of high molecular acids
Water-based paints, varnishes and adhesives are referred to as dispersions in technology. Their binders form finely divided emulsions of fat droplets in water like milk. The fat droplets consist of thread-like, very long organic macromolecules (up to 1 million monomer units) that contain acid groups in some places. In a neutral or acidic, watery environment, they behave like waxes. However, if you neutralize the acid groups with a weak alkali such. B. ammonia NH 3 , then these anion groups (acid residue COO-) are sufficient to stabilize a fine distribution as an aqueous emulsion. Dispersion binders are therefore always slightly alkaline. When the paint or varnish film dries, the ammonia dissolved in it disappears with the water , because it is itself gaseous. This means that the base that made the solution slightly alkaline also disappears. The now unusual acid form of the macromolecules has become water-repellent. The print from the water-thinnable paint has become waterproof. This only applies as long as no lye wets it. For practical use, this conversion into the fat form is a significant advantage, especially in packaging printing, but also in advertising printed matter and publications. Applications can be found in gravure and flexographic printing with aqueous inks and varnishes.
Fusion (matting of macromolecules)
In a dispersion binder ( polymer dispersion ), the individual droplets consist of up to 10,000 macromolecules that are twisted and intertwined with one another. Due to the movement of heat, they move among each other like snakes in a pit. The droplet boundaries to the aqueous phase limit their movement; they each remain trapped within their droplet. If the water evaporates during the drying process, the droplets come closer and collide. When all the water is gone, the outer boundaries of the droplets disappear. Molecular threads can now penetrate into former neighboring cells and entwine themselves there. This connects the whole mass very closely and is referred to in technology as fusing, glazing. It is characteristic of these diffusion processes that they only take place to a significant extent above certain temperatures. Therefore one chooses for z. B. dispersion paints such binders that melt above about 30 ° C. Immediately after printing, they are dry to the touch and water-resistant. Within the first half an hour there is also an increase in abrasion resistance. B. is superior to the conditions after oxidative drying. The prints can easily be cut, folded or otherwise processed. This drying speed and the good final abrasion resistance are the main reasons that z. B. in sheetfed offset for packaging all and in commercial printing very many machines are equipped with coating units.
Film hardening by irradiation
Acrylic acid is an unsaturated carboxylic acid ; Because of the reactive C = C double bond , the acrylic acid tends to polymerize , but with suitable substituents it can be converted into other substances in a targeted manner. Such compounds can be used as binders in printing inks. The polymerization reaction can be triggered by radiation of high energy intensity. This makes such binders technically valuable: The color can be handled in almost any way without reacting. If it is printed, brief exposure to radiation is sufficient and the print is rock-hard.
The network density is so high, even in the short term, that any further printing can be done without any problems. The state of the freshly printed ink as a liquid is more or less frozen. A unique gloss and unrivaled abrasion resistance are just the most noticeable features. After complete curing, the entire lacquer or paint film is immobilized (rendered immobile) in such a way that migration (migration) can no longer be detected. Such properties are attractive in publications and packaging. The price for this is high - not only as the price of materials for such sophisticated acrylates compared to derivatives of vegetable oils and tree resin. The irradiation systems behind or in the printing press are also complex.
With this reactivity, unhardened ink residues and waste are always hazardous to health and must be properly disposed of as hazardous waste. When working with such binders, strict industrial hygiene standards apply, because the uncured acrylates can irritate and sensitize the skin. The high stability of the binder films also prevents the usual disintegration of other organic molecules: Radiation-hardened prints are not compostable and can no longer be easily converted back into graphic paper during recycling.
The acrylates of the radiation-curable paints are not polymerized; they only become this during radiation curing. So-called acrylic paints are not to be confused with this: They are water-based paints whose acrylates are polymerized into long, thread-like molecules. Acrylic paints are dispersions, which means that they dry through evaporation of water, precipitation of water-insoluble fatty acids and tangling of the thread-like binder molecules.
Hardening by electron beams
Electron beams (cathode rays, corpuscular rays) are a very high-energy type of rays. You are e.g. B. known from the television tube. Where an electron shot in this way hits a molecule, it can start reactions directly, especially in the very active double bonds of certain acrylates. The high energy input during electron beam curing causes a very thorough cross-linking of all available molecules, so there is an extremely low risk of remaining unhealthy residues due to non-cross-linked parts. Oxygen hinders the reaction and must therefore be largely excluded. This costs money because either a vacuum or an inert gas is required. The hardening starts in the binder layer from the inside out. So if residues have remained uncrosslinked, this can be checked on the printing surface. The high price of the irradiation systems and the vacuum device limits electron beam curing to large print runs and high-quality printed products; the procedure is currently used very rarely. As a technical concept, however, it is extremely interesting and was used for a few years. B. used to print drinking boxes that have been printed in gravure printing.
Hardening by ultraviolet
Binders suitable for electron beam curing can also be cured by means of ultraviolet (UV) when photoinitiators are added .
The radiation is traditionally generated by mercury vapor lamps , by excimer gas discharge lamps, and increasingly by light-emitting diodes . Photoinitiators are required because the ultraviolet does not cause the acrylates to polymerize. The photoinitiators absorb the ultraviolet and pass the energy on to the acrylates as a reaction trigger. They do it by z. B. disintegrate to radicals , which in turn attack the double bonds of the acrylates and start the crosslinking, so they are catalysts.
The economic advantage is offset by a few disadvantages: Photoinitiators are not networked, so they remain capable of migration. They often smell unpleasant due to impurities (wash liquor odor of the UV varnishes). The curing takes place in longer cascades and the final crosslinking takes time so that the prints come out of the machine so that they can be cut and folded, but the process is not yet complete.
Hardening deficits tend to occur in the film and not on the surface. Particularly dark pigmented colors are not penetrated by the ultraviolet.
UV curing can be found in many areas of technology, not just in paper printing. In the furniture industry u. a. Paper webs printed with wood grain laminated on chipboard and then covered with a UV-curable varnish. There are UV offset printing machines (waterless offset) that have been specially developed for printing CDs and DVDs, but also for plastic cards. In large-format inkjet printing , too , UV inks and varnishes are used in sometimes very thick layers (e.g. 20 µm instead of 1 µm in offset).
A side effect of the ultraviolet emitters is the generation of heat and the creation of ozone . Working with gas discharge lamps therefore use z. B. wavelength-selective reflectors and filters in order to emit only the necessary wavelengths.
Reinforced networking
In solvent-based and aqueous printing inks, there is occasionally a requirement for particularly high robustness of the prints. Examples are certain bottle labels and sterilizable packages for medical items. Additional crosslinking components can increase the film strength here. In a type of polyurethane formation, polyols in the binder are crosslinked with polyisocyanates while the paint is drying . These hardener components can only be mixed in shortly before the ink is printed in order to avoid premature reactions - they are two-component systems.
Thermally induced polymerization
In UV-curing printing inks, photoinitiators start the crosslinking reactions through the formation of radicals. There are also substances that break down into radicals by themselves at a certain temperature and can thus be used as chain starters. The binders correspond exactly to those of the UV inks. With the radical formers, it is possible to print the colors in a stable manner and to heat them on the way to the display, e.g. B. with infrared emitters to reach the starting temperature of the radical generator. Then the printed sheets harden in the stack after the offset printing machine. This technique is widely used in the plastics and rubber industry ( vulcanization ).
Solidification of a melt
Substances such as a hot-melt adhesive are also suitable as binders in a paint or a toner. Examples are powder and liquid toners in electrophotographic printing ( laser printer , indigo digital printing machine ).
Special inkjet printers process waxy paints that are melted, sprayed on and then cooled.
Individual evidence
- ↑ Helmut Kipphan (Ed.): Handbuch der Printmedien , 1st edition, Springer, Heidelberg 2000, ISBN 3-540-66941-8 , p. 177.
- ↑ RH Leach and RJ Pierce: The Printing Ink Manual , Fifth Edition, Blueprint 1993, ISBN 0-948905-81-6 , p. 332.
- ^ Helmut Teschner: Print & Media Technology . 12th edition. Fellbach 2005, ISBN 3-86522-384-2 . P. 652.
- ^ RH Leach and RJ Pierce: The Printing Ink Manual , Fifth Edition, Blueprint 1993, ISBN 0-948905-81-6 , p. 353.
- ↑ RH Leach and RJ Pierce: The Printing Ink Manual , Fifth Edition, Blueprint 1993, ISBN 0-948905-81-6 , p. 9.
- ^ RH Leach and RJ Pierce: The Printing Ink Manual , Fifth Edition, Blueprint 1993, ISBN 0-948905-81-6 , p. 652.
- ↑ Helmut Kipphan (Ed.): Handbuch der Printmedien , 1st edition, Springer, Heidelberg 2000, ISBN 3-540-66941-8 , p. 182.
- ↑ Helmut Kipphan (Ed.): Handbuch der Printmedien , 1st edition, Springer, Heidelberg 2000, ISBN 3-540-66941-8 , p. 180.
- ↑ Helmut Kipphan (Ed.): Handbuch der Printmedien , 1st edition, Springer, Heidelberg 2000, ISBN 3-540-66941-8 , p. 719.