Drying inks

Cold Setting. In this process, also called hot-melt, inks are appHed to a substrate in a molten state and upon cooling form a dry ink film. 

Preformed Two-Piece Metal Containers. Ink vehicles for letterset printing of two-piece aluminum or steel containers are mainly based on special polyester vehicles used in conjunction with melamine cross-linkers. Short cycle ovens which dry inks in 1—5 seconds are now used and operate at temperatures as high as 350 °C. The rheology of these inks must be adjusted to the unique geometry of the press. Desired rheological properties are achieved by the use of additives as weU as extender pigments. 

Vehicles. The soHd pigments are dispersed iato the ink vehicle, which consists of a combination of resia, oil, and solvent. The solvent is absorbed by the paper, leaving a partially dry ink film of resia and oil that biads the pigment to the paper. This film then hardens by oxidation. Oxidation of the vehicle is aided by varnish driers, ie, metallic salts. Cobalt driers are considered the most effective (see Driers and metallic soaps). 

Figure 86 Characteristics of printing processes (d) Methods of drying inks (i) Drying methods (ii) Drying processes...

The technical success of the UV-drying inks may be attributed to the introduction of a number of types of acrylated oligomer and photo-initiator. The broad categories of acrylated oligomers include polyesters, epoxies, urethanes, ethers, and polyacrylates. Photo-initiators generally are derived from compounds that on exposure to UV light cleave into free radicals or abstract hydrogen to form free-radical species. 

An ink must be transformed to a solid from its liquid supply form once on the substrate. This change of state is referred to as an ink drying, and can be physical (absorption or evaporation) or chemical (oxidative or photochemical radiation induced). There are several forms of radiation used to dry ink, including UV, infrared, EB, microwave, and radio frequency. 

In another microemulsion system, ° the colorant is incorporated into the ink as an aqueous pigment dispersion-based inkjet ink composition by formulating the ink to comprise at least one aqueous pigment dispersion and a microemulsion with at least one water-insoluble organic compound, one hydrotropic amphiphile, and water. This ink system was reported to improve waterfastness and bleed control, "" providing a fast drying ink. 

With reference to the thermal characteristics of the substrate, and in light of the proposed use of the printed pattern, there may be a need to match the thermal expansion coefficient of the substrate and the dry ink. In addition, for printing of flexible devices, the flexibility of the printed pattern must be adjusted to that of the substrate. 

My candle reflected on the smeared surface of a flask. When I moved, light fell on a disorderly bookshelf, an empty cage, the mouth of our smallest furnace, my father s desk, which looked as if its surface had been stirred with a giant ladle notebooks left open, the inkstand rusty with dried ink, books piled up anyhow, the contents of his tobacco pouch spilling onto the floor. On the nearby workbench, a pestle was stuck into some dried-up substance, there was a powdering of spilled crystals, bottles were unstoppered, and instruments were dirty. 

While it is evident that the rheology of lithographic inks has a profound effect on printability and press performance, most of the literature records only the rheology of the dry ink. 

In this investigation, a set of six experimental heatset lithographic inks were subjected to a variety of rheological measurements both as a dry ink, as well as after being emulsified with a commercial fountain solution. Determinations of apparent viscosity at 2500 secs l and yield stress at 2.5 secs l were made 25°C from 5 point shear rate/stress curves, and inkometer tacks at 1200 RPM/90°F were also measured. 

The data shows that the second member of each of the three pairs is better in dot resolution and print-ability than the first member. For example, ink A-3 with a rating of 8 was improved over ink A-l with a rating of 7. The dry inks were of roughly equal tack and shortness ratio but the A-3 emulsion was much shorter than the A-l emulsion. This factor plus the higher viscosity of A-3 is the probable reason for the improvement in dot resolution. In series "B", a similar trend was observed with emulsified ink B-5 being shorter than B-l and also of better dot resolution. 

Figure 6. Comparison of apparent viscosities of dry inks and emulsified inks.

Figure 7. Comparison of yield stress (at 2.5 s ) of dry inks and emulsified inks (25°C). Key -------------------, dry ink and--, emulsion.

Another important consideration, particularly in inks, is that the solvent system should be strong enough to redissolve partially-dried ink from the application rolls. This prevents build-up on the rolls and prevents fuzzing of the fine print. There are no tests to define this property other than press runs. 

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