Vegetable oil-based inks
These inks are a result of the great aim of environmental policies, sustainable development.
As mineral oil reserves tend to become exhausted, printing ink manufacturers are under pressure to reduce the content of mineral oil in their products in favour of vegetable oils. Another reason to replace mineral oil is the contribution of hydrocarbon emissions to the formation of "summer smog". Mineral oil-free or mineral oil-reduced inks are on the market now.
The substances to replace mineral oil are not vegetable oils themselves, which are fatty acid triesters of the alcohol, glycerol; fatty acid monoesters are employed instead. The rest of the ink remains virtually the same.
The main difference between mineral oil and the fatty acid esters substituting them lies in their evaporation behaviour. While mineral oil evaporates readily, fatty acid esters need much higher temperatures and tend to decompose as heat is applied. Generally, sheet-fed offset litho inks avoiding mineral oils perform equally well as their competitors employing it. With web-offset inks, problems remain. At least, web velocity has to be reduced and the drier temperature must be enhanced in order to achieve sufficient evaporation of the solvent. Typically, the web temperature has to be raised from 120 °C to 150 °C. This is not only expensive, but also a disadvantage to the environment.
A field, where vegetable oil-based inks are very popular, is newspaper printing, particularly in the US. As for newspapers cold-set inks are employed, this is easily understandable. American newspaper printers hold that the use of mineral oil-free inks is not only environmentally friendly, but also increases product quality.
Among the advantages quoted are
- less ink rub-off to the reader's fingers,
- more brillant colours,
- better trapping,
- no misting,
- less ink per area needed,
- less hydrocarbon emissions.
While European inkmakers are used to employ linseed oil as their favourite drying oil, American inks are more soybean oil-based. This is not only because soybean farmers are an important factor of the American economy, but also because soy oil esters are more similar to mineral oil than linseed products. The "soy seal", which is quite popular in the US, says, that the respective ink contains between 7 % (web-offset) and 40 % (black newsink) soy oil. It does not mean that the ink's binder is made of nothing but soy oil.
Although they seem not so convinced, German ink manufacturers (e.g. K+E, Huber) now also sell inks with a reduced content of mineral oil or even without it, for sheet-fed as well as for web offset.
Water based inks
are available for gravure, screen and flexo printing. Rugged packages and newspapers are prominent applications.
The binder usually is an acrylic resin, which was rendered dispersable by converting acidic groups into their salts using amines or ammonia. When drying, the amine is emitted into the air (unpleasant odour !). The resin thus becomes insoluble in water.
Frequently even "water based" inks contain a certain amount of organic solvent, typically ethanol or iso-propanol. This has to be kept in mind, if the inks are to be diluted.
As water needs a large heat of evaporation (2,226 kJ/kg), drying takes more time and possibly more energy as compared to organic solvent inks.
As water shows a high surface tension, difficulties may arise with non-polar stocks.
The biggest advantage of water based inks is the reduction of solvent emissions.
Some physical indices
of water and organic solvents
[kJ / mol]
[kJ / kg]
[mN / m]
Water 40.8 2,226 45.0 72.3 Ethanol 38.7 841 8.3 23.3 i-Propanol 40.5 675 10.5 22.3 Ethyl acetate 32.3 367 2.9 24.4
Waterproof prints from water based inks
are often marketed as environmentally friendly because they do not contain solvents to be emitted from the plant. They have their own risks, however.
The setting process of UV-curing systems is often called "drying", but it should better be described as "hardening", as it is a polymerization reaction.
Carbon-carbon double bonds are partially broken if they absorb a quantum of ultraviolet light; one of the electron pairs forming the bond is transferred into two single electrons, known as radicals. These radicals can start the polymerization reaction. This reaction, however, has to be accelerated by special chemicals called initiators, which are much more effective in forming radicals.
(1) I2 R·
(2) R· + H2C=CHR' --> R-CH2-CHR'·
(3) R-CH2-CHR'· + H2C=CHR' --> R-CH2-CHR'-CH2-CHR'· .
Most popular as monomers and prepolymers are acrylate resins.
Prepolymers Bisphenol A epoxy acrylate where R is Polyester and polyether acrylates Urethane acylates
where R is Y is H or R' is such that OCN - R' - NCO can be TDI, HMDI, IPDI, MDI
UV-curing systems provide dry prints immediately and do not emit solvents into the environment.
There are serious drawbacks, however:
- Acrylate monomers may be irritant to the skin and cause allergies, though the worst offenders have been eliminated.
- Ultraviolet radiation is detrimental to the skin (cf. sunburns) and particularly to the eye; an extra problem is that UV radiation is not visible. If the radiator is visible, there certainly will be UV light, too.
- The UV radiator produces ozone () from atmospheric oxygen (). Ozone is a strong irritant and a suspect carcinogen. This is only possible as oxygen, too, absorbs ultraviolet light. So it decreases the intensity of UV light reaching the ink layer and increases the amount of initiator necessary.
This is undesirable particularly if packages for food are printed, as the initiators may lead to unpleasant odours. Exhaust ventilation close to the radiator is indispensable. Lamps with a nitrogen atmosphere have been developed.
- UV radiators consume a lot of energy (up to 200 W / cm) and hence must be cooled, by air or by water. Delicate substrates may suffer anyway.
- UV-curing systems "dry" if and only if they are irradiated with UV light. If the ink film applied to the printing stock is too thick, it will only harden at the surface, leaving a liquid film underneath.
For the same reason, spills of ink will never solidify but always cause the danger of slipping.
Electron beam curing systems
Electron beam curing systems are basically the same as UV curing ones, save that they do not need photoinitiators. This must not be underestimated, especially for food packagings. The electrons are energy-rich enough to produce free radicals themselves. Typical voltages for the electron sources are 200 to 300 kV.
Such electrons can produce X-rays, which require lavish shielding. This restricts the use of electron beam curing systems to purpose-built equipment. In-depth curing of the ink film is assured with EB curing, as opposed to UV curing. The effect of the electron beam depends only on the density of the material, not on its optical properties.
EB curing devices must be purged with nitrogen.
EB curing is energy-consuming and rather expensive. Hence, it has been limited to special purpose applications.